not possible manually (based on complex length measurements) ..... Figure 6.73: Thermal cycle applied to AR_DP800_2 steel to study 3D ..... Cooling was, either natural air cooling, or accelerated cooling (in boiling water for ...... 1503 6000 1173 858 ..... Ac ). No nucleation occurs during this transformation. All the nuclei are ...
1.
Table of contents
1.
TABLE OF CONTENTS.......................................................................................3
2.
ABSTRACT .........................................................................................................5
3.
FINAL SUMMARY ...............................................................................................7
3.1
Objectives of the pr oject ......................................................................................................................... 7
3.2
Compar ison of initially planned activities and wor k accomplished ................................................... 7
3.3
Descr iption of activities and discussion ................................................................................................. 8
3.4
Conclusions ............................................................................................................................................ 10
3.5
Exploitation and impact of the r esear ch r esults ................................................................................. 11
3.6
Publications / confer ence pr esentations r esulting fr om the pr oject.................................................. 12
4.
LIST OF FIGURES AND TABLES.....................................................................13
4.1
Figur es.................................................................................................................................................... 13
4.2
Tables ..................................................................................................................................................... 19
5.
LIST OF REFERENCES ....................................................................................21
6.
APPENDICES: ...................................................................................................23
6.1 Pr esentation of the mater ials and pr ocessing par ameter s ................................................................. 23 6.1.1 Composition of the steels ......................................................................................................................23 6.1.2 Industrial process parameters ................................................................................................................25 6.1.3 Pilot simulation process parameters, High Cr DP 600 and 800 steels, LowCr DP 800 steel, Si added TRIP steel............................................................................................................................................................27 6.2 Segr egation maps and evolution along the pr ocess ............................................................................ 32 6.2.1 Mo added DP 800 steel .........................................................................................................................32 6.2.2 High Cr DP600 and DP800 steels .........................................................................................................39 6.2.2.1. Segregation maps from slab to cold rolled and annealed material (heat #1, VA DP600_1)........39 6.2.2.2. Laboratory Rolling Simulation (heat #3, VA_DP800_1) ............................................................43 6.2.3 Low Cr DP800 steel (AR_DP800_1)....................................................................................................45 6.2.3.1. Composition mapping (Mn) by EPMA .......................................................................................45 6.2.3.2. Complementary analysis for the methodology of segregation index measurements ..................49 6.2.3.3. EPMA characterization of C segregation ....................................................................................51 6.3 Quantitative char acter ization of banded micr ostr uctur es: new pr ocedur es. .................................. 52 6.3.1 Standard practice...................................................................................................................................52 6.3.2 Acquiring and preparing the images for measurement..........................................................................53 6.3.3 Measurement methods. .........................................................................................................................57 6.3.3.1. Adapted standard practice............................................................................................................57 6.3.3.2. Method based on progressive erosion..........................................................................................58 6.3.3.3. Method based on meangreying (local analysis of vicinity banded phase content) ......................59 6.3.3.4. Comparison of the methods. ........................................................................................................60 6.4 Effect of pr ocess par ameter s on the hot r olled band.......................................................................... 61 6.4.1 High Cr DP600 800 steels, Industrial trials...........................................................................................61 6.4.1.1. Impact of industrial coiling temperature, heat #1 (va_DP600_1)................................................61 6.4.1.2. Impact of industrial coiling temperature, heat #4 (va_DP800_2)................................................62 6.4.2 High Cr DP600 800 steels, Pilot simulations material ..........................................................................64 6.4.2.1. Impact of finishing and coiling temperature, heat #2 (va_DP600_2) ..........................................64 6.4.2.2. Impact of finishing and coiling temperature, heat #3 (va_DP800_1) ..........................................66
3
6.4.3 6.4.4 6.4.5 6.4.6 6.5
Low Cr DP 800 steels (AR_DP 800 1, 2), Pilot simulations material...................................................69 Si added TRIP 800 steels (AR_TRIP 800 1), Pilot simulations material ..............................................73 Comparative analysis on the effect of cooling conditions on the steel hardness...................................75 Reheating simulations and experiments. ...............................................................................................76 Effect of cold r olling on the initial hot r olled micr ostr uctur e ........................................................... 78
6.6 Detailed mechanisms of phase tr ansfor mation dur ing inter cr itical annealing................................ 81 6.6.1 Kinetics for microstructure evolution....................................................................................................81 6.6.2 Detailed quantification of microstructure evolution during soaking.....................................................85 6.7
Effect of hot r olled micr ostr uctur e on the final (cold r olled and annealed) pr oduct micr ostr uctur e: r efinement and band for mation ........................................................................................................... 89 6.7.1 Bands in the microstructure ..................................................................................................................89 6.7.1.1. Mo added DP800 grade (OCAS_DP_800_1) ..............................................................................89 6.7.1.2. High Cr DP800 grade (VA_DP_800_1)......................................................................................91 6.7.1.3. Progressive bands reappearance during soaking (ARSA_DP800_1 steel) Dilatometric samples95 6.7.2 Refinement of microstructure................................................................................................................96 6.7.2.1. ARSA_DP800_1 steel. Dilatometric samples .............................................................................96 6.7.2.2. VA_DP600_1 steel ......................................................................................................................99
6.8 Effect of hot r olled micr ostr uctur e on the final pr oduct: tensile pr oper ties.................................. 101 6.8.1 Low Cr DP 800 steel (ARSA_DP800_1 et 2) .....................................................................................101 6.8.2 Si added TRIP steels (AR_TRIP800_1)..............................................................................................103 6.8.3 High Cr DP600-800 steels ..................................................................................................................104 6.8.3.1. Impact of coiling temperature on mechanical properties of cold rolled and annealed material (fully industrial produced), heat #1 (va_DP600_1) ......................................................................................104 6.8.3.2. Impact of coiling temperature on mechanical properties of cold rolled and annealed material (laboratory hot&cold rolling and annealing), heat #2 (va_DP600_2) ..........................................................105 6.8.3.3. Annealing simulations of laboratory hot rolled samples from heat #3 (VA DP800 1)..............107 6.8.3.4. Recristallization behavior of laboratory hot rolled samples of heat #3 (VA DP800 1) .............109 6.9
Effect of hot r olled micr ostr uctur e on the final pr oduct: in-use pr oper ties, for mability and damage r esistance.............................................................................................................................................. 110 6.9.1 Mo added DP 800 steel (OCAS_DP800_1), Bending tests on industrial material..............................110 6.9.2 Mo added DP 800 steel (OCAS_DP800_1), Bending tests on laboratory simulations .......................111 6.9.2.1. Process parameters ....................................................................................................................111 6.9.2.2. Tensile properties ......................................................................................................................112 6.9.2.3. Microstructures..........................................................................................................................112 6.9.2.4. Bending test after coiling simulation + CA simulation .............................................................115 6.9.2.5. Bending test after laboratory simulation to suppress the bands.................................................119 6.9.3 Low Cr DP 800 steels (AR_DP800_1), fracture resistance with notched specimen and hole expansion 121 6.9.3.1. Preliminary results on industrial DP600 grades.........................................................................121 6.9.3.2. Forming and damage resistance tests after laboratory simulation to suppress the bands ..........124
6.10 Modelling of the band for mation in the hot r olled str ips................................................................. 128 6.10.1 Presentation of the model, from solidification to ferrite nucleation ...............................................128 6.10.2 Application of the model, from solidification to ferrite nucleation to dual phase and TRIP steels 130 6.10.3 Extension of modelling to ferrite growth in segregated and non segregated area ..........................143 6.11 Modelling of the micr ostr uctur e evolution dur ing annealing ......................................................... 148 6.11.1 Dissolution of Pearlite ....................................................................................................................148 6.11.2 Ferrite-to-Austenite Transformation: Continuous heating .............................................................150 6.11.3 Ferrite-to-Austenite Transformation: intercritical soaking.............................................................152 6.11.4 Banded ferrite and pearlite microstructure .....................................................................................152 6.11.5 Homogeneous ferrite and pearlite microstructure ..........................................................................155 6.11.6 Experimental validation of the model on ARSA_DP800_1 and VA_DP600_1 steels...................156 6.11.6.1. Continuous heating transformation............................................................................................157 6.11.6.2. Intercritical annealing ................................................................................................................158
7.
COPY OF THE TECHNICAL ANNEX..............................................................159
4
2.
Abstract
The objective of this project is to study the effect of hot rolling scheme on cold rolled high strength steels. Two metallurgical points are addressed: banded structures and microstructure refinement. The detrimental effect of microstructural banding on in-use properties was demonstrated and quantified. The underlying cause of microstructural banding is segregation in the form of alternating high and low substitutional element layers. For the typical C content of 600MPa grades, the occurrence of microstructural banding is very low and becomes of prime importance for 800MPa grades. In the hot rolled state, microstructural banding does not always appear in steel with compositional gradient. At high cooling rates and/or low coiling temperatures, the underlying pattern of segregation can be hidden behind a more uniform bainitic or martensitic microstructure. In the same time, low coiling temperature and high cooling rate induces an increase in the tensile strength of the materials, linked with a slight refinement of the microstructure and a change from pearlite to bainite and martensite as a second phase. Even if the accelerated cooling after hot rolling and a low coiling temperature are efficient in order to minimize the microstructural banding of the hot rolled strip, this only hides the microchemical banding, which tends to reappear during the intercritical annealing. By applying classical industrial continuous annealing or galvanizing scheme, bands are present in the final state and the improvement made on the hot rolled microstructure is lost. Only a slight increase of the mechanical properties, due to microstructure refinement is kept.
5
3.
Final summary
3.1
Objectives of the project
The general objective of this collaborative project is to define the best hot rolling scheme in order to produce high strength cold rolled steels. Two main metallurgical points will be addressed: banded structures and refinement of the microstructure. The importance of the hot rolling scheme on the final microstructure of cold rolled steels lies in two major points. A decrease in coiling temperature and an increase in cooling rate can lead to: Ü The suppression of banded microstructures by limitation of carbon diffusion during phase transformation in the hot band Ü The refinement of microstructure, especially the hard second phase, which induces an increase in the strength. By refining the microstructure, higher strength can be reached with lower alloying element content. This possible decrease in alloying elements has two great advantages: Ü a lower susceptibility to band formation and Ü a decrease in rolling forces (hot rolling and cold rolling) which allows the production of thinner cold rolled strips. The detailed objectives of the proposal can be described as follows: ‚ ‚ ‚ ‚ ‚ ‚ ‚
3.2
To relate and quantify the effect of hot rolling parameters on the microstructure with respect to carbides (density, morphology and composition) and banded structure (type of phases, size, …) To establish, by a model coupling phase transformations and diffusion, the critical cooling rate and coiling temperature needed in order to avoid the formation of the band structure. To study the effect of cold rolling on the initial hot rolled microstructure with respect to carbides and banded structure To examine the detailed mechanisms of phase transformation during intercritical annealing of various initial cold rolled microstructures To study the effect of heating rate and soaking temperature on the kinetics of austenite formation and on the microstructure after intercritical annealing To develop models for the kinetics of phase transformation and the description of microstructure evolution To apply classical cooling paths after annealing of different initial microstructure to quantify the effect of final microstructure: suppression of banded structure, refinement of microstructure and improvement in mechanical properties (tensile properties, formability, resistance to damage) Comparison of initially planned activities and work accomplished
No deviations from the initially planned activities have to be reported. This project progressed regularly in conformity with the time schedule. Seven meetings gathering all the partners were held for during the project. In parallel, face to face meetings were organized for specific topics (bands characterization, modelling) when necessary.
7
3.3
Description of activities and discussion
The first decision made was to work on industr ial mater ials, in order to be representative of chemical segregations originated from the continuous casting. Three partners supplied different high strength steels, mainly DP and TRIP steels and they were distributed among the other partners. Two DP grades were studied (600MPa and 800MPa). For the highest grade, different metallurgical alloying concept (Mo added, High Cr and Low Cr) were used. For the TRIP steel, only an 800 grade based on Si-Mn concept was tested in the frame of the project. Industrial slabs were subsequently hot rolled, cold rolled and annealed. We both used industr ial r olling and pilot r olling. The last one enabled us to study, one by one, the effect of process parameters, on a more accurate basis (see App. 6.1). As a preliminary task, the evaluation of chemical segr egations in the mater ials was necessary. It was performed by Electron Probe Micro Analysis on all the materials used in the project. This was done at different stages of the process, from the slab to the final cold rolled and annealed sheet. It was demonstrated that the origin of banded structure in cold rolled and annealed material is clearly shown to be the Manganese (and other substitutional elements) enrichments between dendrites during solidification of slab (see App 6.2). As one of the major goals of our project was linked to microstructural banding, specific interest was put on the quantitative char acter ization of banded str uctur e, new procedures were developed and compared, leading to the following advantages and disadvantages list (see App. 6.3).
Adapted AST method
r close to simple notions - counting r normally easy to compare to manual methods fi not always very discriminant fi very sensitive to grains linking
Erosion of the “matrix”
r not too sensitive to grains linking r good discrimination r relatively fast finot possible manually (based on area measurements) fi arbitrary choice of the “decay” distance
Meangreying
r rather insensitive to grains linking r fairly good discrimination r potentially more detailed information analysis r not possible manually (based on complex length measurements) fi arbitrary choice of the vicinity thresholds fi time-consuming
These procedures were used during the project to quantify the effect of pr ocess par ameter s on the microstructural banding. First of all, hot r olling par ameter s (cooling rate, coiling temperature and finishing temperature) were varied in order to quantify their effect on the microstructural banding, grain refinement and mechanical properties (see App. 6.4). The increase in cooling rates generally leads to a microstructural refinement, and to a decrease in microstructural banding aspect. This is particularly true for the low coiling temperatures. A strong increase in the yield strength and tensile strength with decreasing coiling temperature can be determined, the uniform and total elongation is decreasing with decreasing coiling temperature. This is due to the change in the microstructure from a Ferrite-Pearlite microstructure to a Ferrite-Bainite microstructure for lower coiling temperatures. The lower finishing temperatures yield to a slightly more pronounced band formation.
8
It was especially noticed and confirmed that when simulating hot rolling by pilot simulation, starting material should come from slab and not from roughing bars. In the latter case, chemical segregations are decreased during reheating and their impact on bands is stringly reduced. Concer ning the effect of cold r olling, only those samples with hot rolled bands were found to have banding problems after cold rolling. Quantitative characterisation of banding revealed that cold rolling significantly increases the anisotropy index and reduces the mean free path spacing of bands (see App. 6.5). The evolution during cold rolling is classical: the pearlite islands are elongated, with ferrite flowing in between, and the cementite lamellae of the pearlite are bowed and occasionally broken. The effect of the hot r olled micr ostr uctur e on the final (cold r olled and annealed) pr oduct microstructure were studied in terms of: r efinement and band for mation (See App. 6.7), tensile pr oper ties (See App. 6.8) in-use pr oper ties, for mability and damage r esistance (See App. 6.9) Even if the variations in the hot rolling process produce a significant change in banding the hot rolled microstructure, this effect is annihilated in the annealed state. The microchemical segregation is still present, and restores banding during the intercritical annealing. A slight microstructure refinement is observed in the cold rolled and annealed state is low coiling temperature and high cooling rate are used in the hot rolling process (See App. 6.7). The mechanical properties of the cold rolled and annealed material show an interesting dependency from coiling temperature. With decreasing coiling temperature increasing yield and tensile strength can be observed, elongation is constant or decreases only a little. This increased is probably linked to the slight microstructure refinement (See App. 6.8). Different tests were used to characterize the in use properties as formability and resistance to damage (Bending, hole expansion, notched tensile specimen). The negative effect of bands was demonstrated and quantified. In particular, specific heat treatments were designed to suppress or strongly reduce the chemical segregation and finally the bands in the cold rolled and annealed microstructure. As a consequence, the resistance to damage was strongly (more than 25%) improved (See App. 6.7). Finally, models were developed to describe the previous phenomenon, especially the band for mation dur ing hot r olling and the phase tr ansfor mation dur ing inter cr itical annealing, as schedule in the technical annex of this project (see App. 6.10 and 6.11) A model for predicting the conditions for ferrite/pearlite band prevention in dual phase and TRIP steels during hot rolling has been developed. The competition between processing parameters such as the austenitisation time and temperature, the transformation temperature and microchemical segregation wavelength is explored. The effects of alloy composition in the tendency to form ferrite/pearlite bands are quantified. A simple formula combining processing parameters and compositions for describing band formation is presented. The calculations show that the most prominent factor for preventing banding is the control of the microchemical wavelength. In addition to C and Mn, Al and Si concentrations have shown to play a smaller but significant role in band formation behaviour (see App. 6.10). Concerning the austenitisation of microstructures composed of ferrite and pearlite, two different transformations are involved: pearlite dissolution and ferrite-to-austenite transformation. Pearlite dissolution takes place by nucleation and growth processes. While ferrite-to-austenite transformation takes place by the growth of high carbon austenite grains formed during the previous transformation. This phenomenon were successfully described, taking into account the various possible morphologies of the microstructure before annealing (see App. 6.11).
9
3.4
Conclusions
The most important conclusions from the research project are summarized below. The underlying cause of microstructural banding is compositional segregation mainly in the form of alternating high and low manganese layers. Other substitutional elements play also an important role but they are present at a lower level in multiphase DP and TRIP high strength steels. Carbon content is a key point for this phenomenon because microstructural banding occurs after carbon redistribution from the areas with low substitutional element content towards the areas with high substitutional element content. It was shown that for the typical C content of 600MPa grades, the occurrence of microstructural banding is very low and becomes of prime importance for 800MPa grades. The evolution of segregation pattern was analysed and quantified, from the slab to the cold rolled and annealed sheet. Increasing deformation of the slab segregation structure forms more and more a banded structure. Manganese rich regions – between secondary dendrite arms – are elongated to bands. This chemical inhomogeneity leads after hot rolling to a microstructure of Ferrite and Pearlite, in which the Pearlite can be found on Manganese rich regions. In the hot rolled state, microstructural banding does not always appear in steel with compositional gradient. At high enough cooling rates and/or low enough coiling temperatures to form bainite or martensite in both high and low solute bands, the underlying pattern of segregation can be hidden behind a uniform bainitic or martensitic microstructure. By contrast, at cooling rates slower than the critical velocity to form bainite in both bands, and/or at coiling temperatures too high to form bainite in both bands, microstructural banding appears to be due to the effects of alloy chemistry on the nucleation and growth of ferrite and pearlite. In the same time as these parameters may induce a suppression of microstructural banded structure, low coiling temperature and high cooling rate induces an increase in the tensile strength of the materials, linked with a slight refinement of the microstructure and a change from pearlite to bainite and martensite as a second phase. Even if the accelerated cooling after hot rolling and a low coiling temperature are efficient in order to minimize the microstructural banding of the hot rolled strip, this only hides the microchemical banding, which tends to reappear during the intercritical annealing. By applying classical industrial continuous annealing or galvanizing scheme, bands are present in the final state and the improvement made on the hot rolled microstructure is lost. Only a slight increase of the mechanical properties, due to microstructure refinement is kept. It was not the aim of this project to study the effect of new annealing cycles to suppress the banded structure in the cold rolled and annealed state. However, some experimental results obtained in the project suggest that a low temperature annealing in the intercritical range or a full austenitic annealing (analogy with hot rolled microstructure) could be interesting way to reduce microstructural banding after cold rolling and annealing. The interest of reducing the microstructural banding was demonstrated by looking at specific in-use properties linked to formability and resistance to damage. Detailed microstructural analysis showed that damage forms and propagates along the bands of hard consitituents (martenstite) in cold rolled and annealed multiphase steels. Dedicated experiments were performed to strongly reduce the chemical banding and therefore the final microstructural banding. They confirmed the link between chemical and microstructural banding. The presence of bands in the final microstructure does not change the tensile properties but leads to a sensible improvement (more than 25%) of the resistance to damage and formability, measure on hole expansion tests and notched tensile specimen. The computer model for band formation in hot rolled microstructure combines the effects of solute segregation due to solidification, the diffusion of the segregated components during homogenisation and the nucleation of ferrite in regions possessing different concentration values during controlled cooling after hot rolling. A simple algebraic formula was fitted on the model results and is available to quantify the effect of composition, reheating temperature and time, bands wavelength on the occurrence of microstructural banding. The model confirm the experimental results showing that low ferrite
10
transformation temperature (linked for instance to high cooling rate) is favourable to reduce microstructural banding. 3.5
Exploitation and impact of the research results
The conclusions drawn in this project have already different industrial applications: ‚
Detrimental effects of microstructural banding on in use properties were quantified. This confirm the importance one should put on this topics when developing new high strength steels
‚
Quantitative procedures for microstructural banding characterization were developed and validated. They are explained in the report and available for all RFCS partners.
‚
The evolution of chemical segregation from the slab to the final products were checked and quantified. As this parameter is of first order for microstructural banding, the importance of reducing the segregation in the slab is now quantified. The results support the strong effort put to reduce the chemical segregation during solidification in the continuous caster.
‚
It was demonstrated that microstructural banding suppression in the hot rolled stage was possible but not maintained after cold rolling and annealing. This supports again the necessity to reduce chemical banding in the early stage of the process.
‚
The model developed gives orientation for product development at the hot rolled stage without banded structure. The quantitative link between chemical composition, segregation and transformation temperature on the run out table was given in a straightforward equation.
In a whole, the results of this project give clear indication of the importance of each process parameters and composition effect on the microstructural banding. Each European steelmaker will find in the report detailed information to help product development in very high strength steels. By using the results obtained, one can find guidelines to improve the quality of very high strength steels. Both process parameters and composition effects were studied and quantified. By a steady and continuous development of new products, for example in the automotive market, steel will keep the leading place compared to competitors.
11
3.6
Publications / conference presentations resulting from the project
P. E. J. Rivera Díaz del Castillo, J. Sietsma, S. van der Zwaag: Metallurgical and Materials Transactions A 35A (2004) 425-433. P. E. J. Rivera Díaz del Castillo, S. van der Zwaag: Steel Research int, 75 (2004) 711-715. W. Xu, P. E. J. Rivera Díaz del Castillo, S. van der Zwaag: FERRITE/PEARLITE BAND PREVENTION IN DUAL PHASE AND TRIP STEELS, PART I – and PART II, ISIJ in press. F.G. Caballero, H. Mathy, A. García-Junceda, C. Capdevila y C. García de Andrés, “Nuevo Método Cuantitativo para la Caracterización de Microestructuras Bandeadas o Preferentemente Orientadas”. 3ª Jornada Nacional de Metalografía y Caracterización Microestructural, INASMET, San Sebastián, España, 2004. ISBN: 84-95520-02-8. F. G. Caballero, A. García-Junceda, C. Capdevila, C. García de Andrés, “Recocido Intercrítico de Aceros Duales: Microestructuras Bandeadas”, X Congreso Nacional de Tratamientos Térmicos y de Superficie, Eds. A. Domínguez Rodríguez, J.A. Odriozola Gordón, D. Gómez García, F. Gutiérrez Mora, Sevilla, 2005, ISBN: 84-933135-1-3. F.G. Caballero, D. San Martín, C. Capdevila, A. García-Junceda and C. García de Andrés, “Modeling of the Microstructural Evolution during Intercritical Annealing of Steels with a Ferrite and Pearlite Initial Microstructure”, The Minerals, Metals & Materials Society, 2004, in press. F.G. Caballero, A. García-Junceda, C. Capdevila, and C. García De Andrés, “Microstructural Banding in Dual Phase Steels: Can it be Suppressed?” submitted to Metallurgical and Materials Transactions A in November 2005. F.G. Caballero, A. García-Junceda, C. Capdevila and C. García de Andrés, “Evolution of Bands during Intercritical Annealing of Dual Phase Steels”, ICAMMP 2006 to be help in Khargpur, India, February 2006.
12
4. 4.1
List of figures and tables Figures
Figure 6.1: Hot rolling of strips of heat #3, left: finishing and coiling temperature versus strip length (strip: 636630), right: temperature on the run out table Figure 6.2: Rolling Technology Centre (RTC) in CRM-Gent Figure 6.3: Example of parameters recording during hot rolling (2567 of VA_DP_600_2 steel) Figure 6.4: Comparison of the microstructure of the slab at ¼ (left) and ½ thickness (right) (macro) Figure 6.5: “Macro” segregation at ¼ thickness Figure 6.6: “Macro” segregation at ½ thickess Figure 6.7: “Micro”segregation at ¼ thickness Figure 6.8: “Micro”segregation at ½ thickness Figure 6.9: Correlation between different measured elements Figure 6.10: Optical microscopy. Coiling Temperature 655°C, transverse direction, ½ thickness. Figure 6.11: Composition picture relative to the Figure 6.10 Figure 6.12: Optical microscopy. Coiling Temperature 655°C, transverse direction, ¼ thickness. Figure 6.13: Composition map relative to the figure 6.12. Figure 6.14: linescan at half-thickness, CT 655°C. Figure 6.15: Principle of Concentration Mapping Figure 6.16: Sampling of the slab Figure 6.17: Overview of 6 CM analyses at 6 different depths from surface to center. Note: different resolutions (see the 500µm markers).Grey level coding: % by mass (scale captions in each element column top right) Figure 6.18: Concentration maps of the hot rolled strips versus local coiling temperature, longitudinal samples Figure 6.19: Concentration maps of the cold rolled and annealed strips versus local coiling temperature, longitudinal samples Figure 6.20: Hot rolling parameters and reduction rates of samples Figure 6.21: concentration maps and micrographs of hot rolled samples, evolution of bands Figure 6.22: Position of the plates for Mn segregation measurement, from the outer edge to the center of the slab Figure 6.23: Evolution of Mn profile from the center to the edge of the slab Figure 6.24: Mn macrosegregation in the center of the slab Figure 6.25: Effect of spot size (2 or 7µm) on the Mn profile measurements in AR_DP800_1 rough bar at ¼ thickness Figure 6.26: Comparison between classical procedure (SR: 1.32, lower lines) and another procedure based on absolute max and min level (SR: 1.4) Figure 6.27: Distribution function of Mn content and definition of maximum and minimum level (AR_DP800_1, rough bar, spot size = 2µm) Figure 6.28: Qualitative comparison of optical micrograph (bottom) and EPMA C mapping (top) showing the correspondence of carbon content and pearlite bands Figure 6.29: Quantitative comparison of grey level obtained by image analysis (upper curve) and C content deduced from EPMA (lower curve) showing the correspondence of carbon content and pearlite bands Figure 6.30: Quantitative comparison of grey level obtained by image analysis (middle curve), C content (lower curve) and Mn content (upper curve) deduced from EPMA showing the role of Mn segregation in the formation of bands.
13
Figure 6.31: Illustration of the counting of particle interceptions (N) and boundary intersections (P) for an oriented microstructure. T indicates a tangent hit and E indicates that the grid line ended within the particle (1) Figure 6.32 Pearlite banding in ferritic matrix – Original and thresholded images. Figure 6.33: Pearlite banding in ferritic matrix – scrapped and closed images. Figure 6.34: Pearlite banding in ferritic matrix – Median filtered and resulting thresholded images. Figure 6.35: Image preparation by various degrees of closure (detail of image 7). Figure 6.36: Example of particle counts adapted to the standard practice. Figure 6.37: Microstructural aspect (picral etch) of the 2 steels for preliminary banding characterization. Figure 6.38: Definition of an “erosion-based” anisotropy index. Figure 6.39: Banding characterised by local analysis of vicinity pearlite content. Figure 6.40: Mechanical properties of the hot rolled strip versus local coiling temperature, transverse samples Figure 6.41: Microstructure of the hot rolled strip versus local coiling temperature, transverse samples, Le Pera etching Figure 6.42: Microstructure of hot rolling of strip: 965363, CT=500°C (LePera etching) Figure 6.43: Microstructure of hot rolling of strip: 965369, CT=600°C (LePera etching) Figure 6.44: Comparison of the microstructure laboratory (CRM, Nital etching) and industrial (voestalpine, Le Pera etching) hot rolled samples, longitudinal Figure 6.45: Microstructures of hot rolled bands: VA_DP_600_2 coiled at 600°C. Cooling rates: 2.5°C/s (2567), 15°C/s (2568), 50°C/s (2608), 300°C/s (2606) Figure 6.46: Microstructures of hot rolled bands: VA_DP_600_2 coiled at 500°C. Cooling rates: 2.5°C/s (2569), 15°C/s (2570), 50°C/s (2607), 300°C/s (2605) Figure 6.47 Effect of coiling temperature and cooling rate (moderate) on banding of VA_DP_600_2. Figure 6.48: AI and Y12 versus coiling temperature (VA analysis) Figure 6.49: Effect of coiling temperature and finish rolling temperature on banding of VA_DP_800_1 cooled at 15°C/s. (CRM analysis) Figure 6.50: Influence of finishing and coiling temperatures on hot rolled microstructure in VA_DP800_1 steel (CENIM analysis)
Figure 6.51: Microstructure of the laboratory hot rolled samples of heat #3, LePera etching (CR = 15°C/s) Figure 6.52: Microstructures of hot rolled bands: VA_DP_800_1 cooled at 300°C/s. Cooling temperatures: 600°C (4054), 400°C (4056), 200°C (4058). Figure 6.53: Microstructures of hot rolled bands: AR_DP_800_1 cooled at 2.5°C/s. Coiling: 650°C (3096 – 3992), 500°C (3102). For 3992: pretreatment at 1300°C. Figure 6.54: Effect of coiling temperature and pretreatment at 1300°C on banding of AR_DP_800_1cooled at 2.5°C/s Figure 6.55- Light optical micrographs and scanning electron micrographs of hot rolled specimens parallel to the rolling direction. a) Sample S1: CR=7 ºC/s, CT=500 ºC; b) Sample S3: CR=60 ºC/s, CT=500 ºC; c) Sample S10: CR=7 ºC/s, CT=650 ºC; and d) Sample S12: CR=60 ºC/s, CT=650 ºC. B is bainite, M is martensite, F is ferrite and P is pearlite. Figure 6.56: Microstructures of hot rolled bands: AR_TRIP_800_1 cooled at 2.5°C/s. Coiling: 700°C (2564), 600°C (2561), 500°C (2560). Figure 6.57: Effect of coiling temperature on banding of AR_TRIP_800_1 cooled at 2.5°C/s. Figure 6.58: Microstructures of hot rolled bands: AR_TRIP_800_1 coiled at 650°C (600 for 2849). Cooling: 2.5°C/s (3913 – 3994), 50°C/s (2849). For 3994: pretreatment at 1300°C. Figure 6.59: Effect of cooling rate on banding of AR_TRIP_800_1 coiled at 600°C. Figure 6.60: Effect of pretreatment at 1300°C on banding of AR_TRIP_800_1 and AR_DP_800_1 cooled at 2.5°C/s and coiled at 650°C. Figure 6.61: Microstructures of hot rolled bands: AR_TRIP_800_1 coiled at 500°C.
14
Accelerated cooling: 15°C/s (2847), 50°C/s (2848), 300°C/s (2562). Figure 6.62: Influence of the cooling conditions on the as hot rolled hardness. Figure 6.63: AR_DP_800_1 – Diffusion simulations (TUDelft program). Figure 6.64: VA_DP_800_1 – Microstructures of the transfer bars after reheating simulations (60 min). Figure 6.65: Patchworking and anamorphosis: schematic view. Figure 6.66: Intermediate and final image from “inverse rolling” the specimen illustrated at figure 6.64a. Figure 6.67: Light optical micrographs and scanning electron micrographs of 68% cold rolled specimens parallel to the rolling direction. a) Sample S1: CR=7 ºC/s, CT=500 ºC; b) Sample S3: CR=60 ºC/s, CT=500 ºC; c) Sample S10: CR=7 ºC/s, CT=650 ºC; and d) Sample S12: CR=60 ºC/s, CT=650 ºC. Figure 6.68: Effect of cold rolling on the AR_TRIP_800_1 microstructure (C.R. 2.5°C/s – Tcoil 650°C) Figure 6.69: Optical micrographs from intercritical annealed samples (a) at 750 ºC for 1 s; (b) at 750 ºC for 20 s; (c) at 750 ºC for 100 s; (d) at 800 ºC for 1 s; (e) at 800 ºC for 20 s; (f) at 800 ºC for 100 s; (g) at 850 ºC for 1 s; (h) at 850 ºC for 20 s; (i) at 850 ºC for 100 s; Initial microstructure: cold rolled sample S10 (CR=7 ºC/s, CT=650 ºC). LePera reagent. Figure 6.70: Scanning electron micrographs corresponding to the beginning of the austenitisation process in hot rolled and cold rolled samples. 2 pct Nital etching solution. Intercritical temperature: 750 ºC; soaking time: 1 s Figure 6.71: Evolution of austenite volume fraction during intercritical annealing in a) hot and b) cold rolled material. Sample S1 (CR=7 ºC/s, CT=500 ºC); Sample S3 (CR=60 ºC/s, CT=500 ºC); Sample S10 (CR=7 ºC/s, CT=650 ºC); and Sample S12 (CR=60 ºC/s, CT=650 ºC). Figure 6.72: Evolution of carbon content in austenite during intercritical annealing of hot (HR) and cold (CR) rolled material Figure 6.73: Thermal cycle applied to AR_DP800_2 steel to study 3D morphology of bands. Figure 6.74: 3D morphology of the bands after soaking Figure 6.75: Annealing cycle to study the effect of annealing temperature on microstructure refinement Figure 6.76: Annealing cycle to study the effect of annealing temperature on microstructure refinement Figure 6.77: Effect of soaking temperature on the distribution of martensite islands size. Figure 6.78: Annealing cycle to study the effect of hot rolled microstructure on microstructure refinement Figure 6.79: effect of hot rolled microstructure on the kinetics for austenite transformation during intercritical annealing Figure 6.80: effect of hot rolled microstructure on microstructure grain size during intercritical annealing Figure 6.81: Optical and electron micrographs of cold rolled and annealed samples of OCAS_DP800_1 steel in transverse direction. 2 pct Nital etching solution Figure 6.82: Annealed (770 and 820 °C) OCAS_DP_800_1 sheets. Effect of hot rolled conditions on banding. Figure 6.83: Laboratory (a) hot dip galvanizing (HDG) and (b) continuous annealing cycle (CA) simulations. Figure 6.84: Optical micrographs of HDG samples in longitudinal direction of VA_DP800_1 steel. LePera reagen Figure 6.85: Optical micrographs of CA samples in longitudinal direction of VA_DP800_1 steel. Le Pera regent. Figure 6.86 VA_DP_800_1 after CA cycle: effect of hot rolling parameters on banding. Figure 6.87: VA_DP_800_1 after HDG cycle: effect of hot rolling parameters on banding. Figure 6.88: Evolution of microstructural banding during intercritical annealing: Anisotropy index a) at 750 ºC and b) at 800 ºC of soaking temperature; and c) mean edge-to-edge spacing of the bands, n . Sample S1 (CR=7 ºC/s, CT=500 ºC); Sample S3 (CR=60 ºC/s, CT=500 ºC); Sample S10 (CR=7 ºC/s, CT=650 ºC); and Sample S12 (CR=60 ºC/s, CT=650 ºC).
15
Figure 6.89: Evolution of microstructural banding during intercritical annealing in sample S3 (CR=60 ºC/s, CT=500 ºC): a) 750 ºC, 20 s; b) 750 ºC, 100 s; c) 800 ºC, 1 s; and d) 800 ºC, 20 s; Figure 6.90: Evolution of the ferrite grain size during the intercritical annealing of a hot rolled S10 sample in ARSA_DP800_1 steel. The austenite volume fraction is indicated in red. Figure 6.91- Evolution of the austenite grain size during the intercritical annealing of (a) hot rolled and (b) cold rolled S1 sample Figure 6.92: Evolution of the austenite grain size during the intercritical annealing of (a) hot rolled and (b) cold rolled S3 sample in ARSA_DP800_1 steel Figure 6.93: Evolution of the austenite grain size during the intercritical annealing of (a) hot rolled and (b) cold rolled S10 sample in ARSA_DP800_1 steel Figure 6.94: Evolution of the austenite grain size during the intercritical annealing of (a) hot rolled and (b) cold rolled S12 sample in ARSA_DP800_1 steel Figure 6.95: Evolution of the austenite grain size during austenite formation of (a) hot rolled and (b) cold rolled samples in ARSA_DP800_1 steel. Figure 6.95 bis: Optical and electron micrographs of cold rolled and annealed samples of VA_DP600_1 steel in transverse direction. 2 pct Nital etching solution Figure 6.96: Effect of coiling temperature on the strength and ductility of cold rolled and annealed AR_DP800_1 steel. Annealing temperature: 800°C Figure 6.97: Effect of coiling temperature on the strength and ductility of cold rolled and annealed TRIP 3 steel. Figure 6.98: Mechanical properties of the hot dip galvanized strips versus local coiling temperature, longitudinal samples Figure 6.99: Microstructure of the cold rolled and annealed strips versus local coiling temperature, longitudinal samples, Le Pera etching Figure 6.100: cycles for annealing simulation Figure 6.101: mechanical properties after HDG and CA simulation versus coiling temperature (laboratory hot and cold rolled samples) Figure 6.102: AI and Y12 versus coiling temperature (laboratory hot and cold rolled samples after HDG and CA simulation) Figure 6.103: applied cycles studying recristallization behavior Figure 6.104: recristallization behavior versus finishing and coiling temperature Figure 6.105: Results of bending test after coiling at 585°C and soaking at 820°C Figure 6.106: Annealing simulation performed after coiling simulations Figure 6.107: Overview of the samples after nital etching. Simulated coiling temperature is indicated below the corresponding picture Figure 6.108: Microstructure after coiling and continuous annealing simulation (Le Pera etching). Simulated coiling temperature is indicated below the corresponding picture. Note the scales are the same. Figure 6.109: Illustration of the bending procedure Figure 6.110: View of a sample after bending test. Description of the “width of a sample” to describe the geometry when crack occurs Figure 6.111: Overview of the samples after the samples after bending test during measurement by the CASA system Figure 6.112: Characterization of the stop of the appearance of crack in banding test Figure 6.113: Comparison between the curvatures determined by equation 1 and the space between the dies. Figure 6.114: Pictures of the surface at the end of the bending test Figure 6.115 Pictures after bending test. Nital etching to show the phases / bands. Figure 6.116: Example of damage found in the material at some distance from the tip of crack Figure 6.117: Thermal cycles used to produce equivalent material with and without bands.
16
Figure 6.118: Microstructures after full processing of the material without bands (left) and with bands (right). Picral +Na2S2O3 etching. Figure 6.119: Sample geometry for continuous annealing simulation, tensile test and bending test Figure 6.120: Results of the bending test Figure 6.121: microstructure of the cold rolled and annealed DP600 steels used to qualify the fracture procedure. Figure 6.122: Notched specimen and procedure to quantify the resistance to damage. Figure 6.123: Comparison of experiment results for D51 and B2121 DP steels in notched tensile testing Figure 6.124: Damage characterization in D51 steel under the fracture surface. Figure 6.125: Damage characterization in D51 steel on the fracture surface. Figure 6.126: Annealing simulations to study the effect of bands on the forming properties Figure 6.127: Hole expansion test, principle and geometry Figure 6.128: Mechanical properties of reheated (3992) and non reheated (3096) ARSA_DP_800_1 steel. Figure 6.129: Forming properties of reheated (3992) and non reheated (3096) ARSA_DP_800_1 steel: a) hole expansion testing, b) energy deduced for tearing tests on notched specimens. Figure 6.130: Optical micrographs at ¼ thickness of hot dip galvanized specimens in ARSA_DP800_1 and ARSA_TRIP800_2 steels. LePera reagent
Figure 6.131: Austenite grain size in hot dip galvanized specimens of ARSA_DP800_1 Figure 6.132: Ferrite grain size in hot dip galvanized specimens of ARSA_DP800_1 Fig. 6.133: Progress in nodal microchemical concentration of OCAS_DP800_1 across microchemical bands for 1473 K and n=50 om. Fig. 6.134: Effect of microchemical wavelength variation in band prevention plots for the studied grades at an austenitisation time of 3600 s. The solid lines are obtained from the numerical computations and the dotted lines result from analytical equation (TT). Fig. 6.135: Effect of austenitisation time variation in band prevention plots for the studied grades with a microchemical wavelength of 25 om. The solid lines are obtained from the numerical computations and the dotted lines result from employing analytical equation (TT). Fig. 6.136: Variation of transformation temperature for concentrations normalised to the grades given in Table 6.46: Austenitisation temperature of 1373 K and an austenitisation time of 3600 s. The microchemical wavelength was taken as 75 om. Fig 6.137: Example of cooling curves for VA_DP600_1 steel case C Fig. 6.138: Transformation behaviour of OCAS_DP800_1 case A (DP1). Fig. 6.139: Transformation behaviour of VA_DP600_1 case B (DP2). Fig. 6.140: Transformation behaviour of VA_DP600_1 case C and D (DP2). Fig. 6.141: Transformation behaviour of AR_DP800_1 case E, F, G (DP3). Fig. 6.142: Transformation behaviour of AR_TRIP800_1 case H (TRIP3). Figure 6.143: Schemas of the cells used in the not-segregated (left) and the segregated areas (right). In the last case, the external boundary condition is not a zero flux condition. Figure 6.144: Microstructures after thermal treatment to simulate hot-rolling. The coiling temperatures are 780, 730, 680, 630 and 450°C (from left above to right bottom) (Nital Etching). Figure 6.145: Fraction of ferrite in function of the temperature for the hot-rolling simulations. The dots indicate the segregated areas, while the continue lines indicate the bulk composition behaviour. Figure 6.146: Fraction of ferrite in function of time. The dots indicate the segregated areas, while the continue lines indicate the bulk composition behaviour. Figure 6.147: Comparison of the amount of second phase in the bands (simulated) and the anisotropy index (experimental) as a function of the coiling temperature
17
Figure 6.148: Evolution of the carbon concentration gradient (a) during ferrite-to-austenite transformation under continues heating condition; (b) carbon concentration gradient inside austenite at the beginning of intercritical soaking Figure 6.149: Scheme of a banded ferrite and pearlite microstructure Figure 6.150: Evolution of the carbon concentration gradient inside austenite grains during (a) the first stage and (b) the second stage of the isothermal transformation of austenite. Figure 6.151: Scheme of an homogeneous distribution of pearlite nodules in a ferritic matrix Figure 6.152: (a), (b) Austenite carbon content and (c), (d) parabolic growth rate constant during ferrite to austenite transformation at a continuous heating of 5ºC/s Figure 6.153: Experimental validation of austenitisation model under continuous heating conditions. Heating rate of 5 ºC/s Figure 6.154: Experimental validation of intercritical annealing. Heating rate of 5 ºC/s. Soaking temperature of 750 ºC in ARSA_DP800_1 steel and 765 ºC in VA_DP600_1 steel Figure 6.155: (a) Evolution of the carbon concentration inside austenite grains during intercritical annealing. (b) Experimental validation of intercritical annealing model. Heating rate of 5ºC/s. Soaking temperature of 800 ºC in ARSA_DP800_1 steel
18
4.2
Tables
Table 6.1: Composition of industrial Mo added DP Steel (wt %) Table 6.2: Chemical analysis of high Cr industrial heat #1-4 (mass %) Table 6.3: Chemical composition (wt %) of low Cr DP and Si TRIP steels Table 6.4: Hot rolling conditions Table 6.5: Reheating and hot rolling parameters of industrial heat #1 to 4 Table 6.6: Hot dip galvanizing parameters of industrial produced strips, heat #1 (VA_DP600_1) and #3 (VA_DP800_1) Table 6.7: Hot rolling parameters Table 6.8: description of the scanning condition for the EPMA analysis Table 6.9: Average composition at two different thicknesses in the slab Table 6.10: Characteristic distances at the different steps Table 6.11: Evaluation of n versus production route Table 6.12: Segregation ratio (Mn) deduced from the profile measured in the transverse direction (from the edge to the center) Table 6.13: Segregation ratio (Mn) deduced from the profile measured in the transverse direction (from the edge to the center): Rough bar. AR_DP800_1 Table 6.14: Segregation ratio (Mn) deduced from the profile measured in the transverse direction (from the edge to the center): After finishing, AR_DP800_1 Table 6.15: Segregation ratio (Mn, Si) deduced from the profile measured in the transverse direction (from the edge to the center) in the rough bar (spot size 2µm) Table 6.16: Comparison of min. max. value and segregation index with the two methods (AR_DP800_1, rough bar, spot size = 2µm) Table 6.16: Measurements performed with the adapted ASTM method on the 2 preliminary specimens. Table 6.17: Measurements performed with the erosion-based method on the 2 preliminary specimens. Table 6.18: Mechanical properties of hot rolled strips, industrial heat #4 (L longitudinal and Q transverses test direction) Table 6.19: AI and Y412 of hot rolled strips, industrial heat #4 (L longitudinal) Table 6.20: Mechanical property of laboratory (CRM) and industrial (voestalpine) hot rolled strip, heat #2 (VA_DP600_2) Table 6.21: Hot Rolling conditions of ARSA_DP800_1 steel Table 6.22: Characterisation of banded ferrite-pearlite microstructures in hot and cold rolled samples. Sample S10: CR=7 ºC/s, CT=650 ºC. Effect of cold rolling will be detailed in the next chapter Table 6.23: Mechanical properties of hot rolled simulation AR_DP800_1 (DP3) and AR_DP800_2 (DP4) steels. Table 6.24: Characterisation of banded ferrite-pearlite microstructures in hot and cold rolled samples. Sample S10: CR=7 ºC/s, CT=650 ºC Table 6.25: Heating critical temperatures Table 6.26: Coiling and soaking temperatures of OCAS_DP800_1 steel Table 6.27: Annealed OCAS_DP_800_1 sheets – banding characterisation. Table 6.28: Amount of second phase according to the different process parameters Table 6.29: Laboratory hot dip galvanizing (HDG) and continuous annealing cycle (CA) simulations in the studied steels Table 6.30: Hot and cold rolling parameters of VA_DP600_1 steel Table 6.31: Laboratory hot dip galvanizing (HDG) and simulations in the studied steels Table 6.32: Characterisation of cold rolled and annealed samples of VA_DP600_1 steel Table 6.33: Mechanical properties of cold rolled and annealed simulation AR_DP800_1 steel.
19
Table 6.34: Mechanical properties of cold rolled and annealed simulation AR_DP800_1 steel. Table 6.35: Microstructural characterization and mechanical properties of cold rolled and annealed simulation TRIP 3 steel. Table 6.36: Mechanical property of full laboratory processed strips and industrial produced strip, heat #2
Table 6.37: Mechanical properties of industrial material Table 6.38: Results of tensile testing (average of 3 tests). Tensile tests were achieved after coiling and annealing simulations Table 6.39: Amount of bainite and martensite as a function of the temperature of coiling simulation measured after Le Pera etching at ½ and ¼ thickness Table 6.40: Individual values for the bending tests Table 6.41: Comparison of the fraction of martensite in the samples with and with bands Table 6.42: Tensile properties of the samples with/without band structure Table 6.43: Chemical composition of DP600 steels Table 6.44: Hot rolling parameters Table 6.45: Characterisation of banded ferrite-martensite microstructures in ARSA_DP800_1 Table 6.46: Composition of studied grades Table 6.47: Processing parameters for the steels used to check the model Table 6.48: Characterization of samples after thermal simulation of hot rolling Table 6.49: Simulation parameters Table 6.50: Microstructural parameters, input of the model
20
5.
List of References
1. E 1268-99: Standard Practice for Assessing the Degree of Banding or Orientation of Microstructures. Published July 1999. 2. D.W. Hetzner. “Quantitative Assessment of Banding by Automatic Image Analysis”. Microstructural Science, Vol 24 (1996), pp. 211-225. 3. J. Komenda, R. Sandström. “Assessment of Pearlite Banding Using Automatic Image Analysis: Application to Hydrogen-Induced Cracking”. Materials Characterization 31 (1993), pp. 143-153 4. E.E. Underwood, Quantitative Stereology, Addison-Wesley Publishing Co., Reading, MA, 1970: 73-75. 5. A. Roosz, Z. Gacsi, and E.G. Fuchs, Acta Metall., 31, 1983: 509-517. 6. F.S. LePera, Metallography, 12 (1979), 263-268. 7. G.R. Speich, V.A. Demarest, R.L. Miller, ‘Formation of Austenite During Intercritical Annealing of Dual-Phase Steels’, Metallurgical Transactions A, 12 (8): 1419-1428 1981. 8. K.W. Andrews, JISI, 203, (1965), 721-727. 9. M. De Meyer, J. Mathieu, and B.C. De Cooman, Materials Science and Technology, 18, 2002: 1121-1132. 10. MTDATA: Phase diagram calculation software. Teddington: National Physical Laboratory; 2003. 11. C. García de Andrés, M.J. Bartolomé, C. Capdevila, D. San Martín, F.G. Caballero, V. López, ‘Metallographic techniques for the determination of the austenite grain size in medium-carbon microalloyed steels’, Materials Characterization, 46, 2001: 389-398. 12. P. G. Bastien: Journal of the Iron and Steel Institute (1957) 281-291. 13. R. Großterlinden, R. Kawalla, U. Lotter, H. Pircher: Steel Research 63 (1992) 331-336. 14. J. D. Verhoeven: Journal of Materials Engineering and Performance 9 (2000) 286-296. 15. S. E. Offerman, N. H. van Dijk, M. T. Rekveldt, J. Sietsma, and S. van der Zwaag: Materials Science and Technology 18 (2002) 297-303. 16. P. E. J. Rivera Díaz del Castillo, J. Sietsma, S. van der Zwaag: Metallurgical and Materials Transactions A 35A (2004) 425-433. 17. MTDATA: Metallurgical and Thermochemical Databank, National Physical Laboratory, Teddington, Middlesex, United Kingdom, 1995.
18. M. Avrami, “Kinetics of Phase Change II”, J. Chem. Phys., 8 (1940), 212-224. 19. R.T. De Hoff and F.H. Rhines, Quantitative Stereology, (New York, McGraw-Hill, 1968), 93. 20. F.G. Caballero, C. Capdevila and C. García de Andrés, An attempt to establish the variables that most directly influence the austenite formation process in steels, ISIJ International, 43, (2003), 726-735.
21
6.
Appendices:
The technical work performed in the project is now detailed. The whole project (see technical annex) was divided in tasks as followed: Task 1: Supply of the material for the investigations in the laboratory Task 2: Hot rolling: experiments and characterization Task 3: Prediction of microstructural banding in hot rolling Task 4: Cold rolling: experiments and characterization Task 5: Dilatometry experiments for transformation during intercritical annealing Task 6: Modelling of microstructural evolution during intercritical annealing Task 7: Annealing simulations and evaluation of mechanical properties For the sake of clarity, the results will be presented in eight chapters, defined as follow 6.1: Presentation of the materials and processing parameters (Task 1) 6.2: Segregation maps and evolution along the process (new task) 6.3: Quantitative characterization of banded structure, new procedures (nex task) 6.4: Effect of process parameters on the hot rolled band (Task 2) 6.5: Effect of cold rolling on the initial hot rolled microstructure (Task 4) 6.6: Detailed mechanisms of phase transformation during intercritical annealing (Task 5) 6.7: Effect of hot rolled microstructure on the final (cold rolled and annealed) product microstructure: refinement and band formation (Task 7) 6.8: Effect of hot rolled microstructure on the final product : tensile properties (Task 7) 6.9: Effect of hot rolled microstructure on the final product : in-use properties, formability and damage resistance (Task 7) 6.10: Modelling of the band formation in the hot rolled strips (Task 3) 6.11: Modelling for the kinetics of phase transformation during annealing (Task 6)
6.1
Presentation of the materials and processing parameters
We first present the informations concerning the steel studied in this project and the major process parameters used for hot rolling, cold rolling and annealing. 6.1.1
Composition of the steels
Industrial multiphase very high strength steels were supplied for this project by Voest Alpine and ARCELOR. The major part of the work was performed on DP-type steels. Some complementary results were obtained on TRIP-type steels. No laboratory heats were used, in order to have representative results with respect to the chemical segregation. Concerning DP steels, all materials are based on Cr and (or) Mo alloyed DP steels. The major differences lies in: ‚ the Cr and Mo content ‚ the grades (in term of maximal strength) achievable with these compositions.
23
Mo added DP800 steel A DP steel has been delivered to OCAS from SIDMAR plant (composition in table 6.1). The slab thickness was 220mm. Table 6.1: Composition of industrial Mo added DP Steel (wt %) Steel OCAS_DP800_1
C 0.14
Mn Si Cr 1.49 0.13 0.37
Al 0.03
Ti -
P -
S -
Mo 0.20
Nb -
N 0.0050
High Cr DP600 and DP800 steels Table 6.2 gives the chemical analysis of the industrially produced material at Voestalpine. Crude steel production was performed on a conventional LD based route. After adjusting the chemical analysis casting took place in a continuous casting machine with two strands, the slab thickness is 210 mm. Heat #1 and #2 are standard production of DP600 grades via hot dip galvanizing at Voestalpine. Heat #3 and #4 are industrial trail productions for a DP800 grade. The aim of heat #4 was to reduce Carbon with increased Manganese content and some Niobium addition to reduce grain size and therefore minimizing band formation. Table 6.2: Chemical analysis of high Cr industrial heat #1-4 (mass %) Al Cr Mo Nb Ti N heat # C Si Mn P S 1 0,065 0,11 1,41 0,009 0,003 0,051 0,741 0,008 0,004 0,001 0,0055
VA_DP600_1
Al Cr Mo Nb Ti N heat # C Si Mn P S 2 0,071 0,09 1,24 0,008 0,006 0,048 0,737 0,008 0,003 0,001 0,0054
VA_DP600_2
heat # Al Cr Mo Nb Ti N C Si Mn P S 3 0,159 0,18 1,68 0,013 0,001 0,050 0,730 0,007 0,003 0,002 0,0036
VA_DP800_1
heat # Al Cr Mo Nb Ti N C Si Mn P S 4 0,133 0,18 1,75 0,009 0,002 0,053 0,730 0,006 0,025 0,003 0,0040
VA_DP800_2
Low Cr DP 800 steels and Si added TRIP steel Industrial materials were also selected and taken from the production route by ARCELOR RESEARCH. Three products, two DPs and one TRIP, were chosen. Their chemical composition is given in the following table (Table 6.3). In order to follow the evolution of chemical segregation along the industrial route, materials were taken from the production at different stages of the process: After casting (slab): thickness = 250mm After roughing: thickness = 30mm After finishing: thickness = 3mm
24
Table 6.3: Chemical composition (wt%) of low Cr DP and Si TRIP steels Steel ARSA_DP800_1 ARSA_DP800_2 ARSA_TRIP800_2
6.1.2
C 0.15 0.124 0.22
Mn 1.9 1.4 1.73
Si 0.2 0.35 1.77
Cr 0.2 0.2 0.021
Al 0.03 0.035 0.036
Ti 0.025 0.006
P 0.015 0.016 0.015
S 0.002
Industr ial pr ocess par ameter s
For High Mo and High Cr DP steels only, industrial trials were performed. Mo added CrMn DP steel The DP steel has been hot rolled at the SIDMAR facilities with two different coiling temperatures. The final thicknesses are 3.2 and 4mm. The temperature at the end of the hot rolling was 900°C. The coiling temperatures were 585°C and 655°C. The cooling rates can be estimated between 15 and 19 °C/s. A summary of the hot rolling conditions is shown in table 6.4. Table 6.4: Hot rolling conditions End HR Temperature Cooling rate Coiling Temperature Thickness
900°C 15-19°C/s 655-585°C 3.2-4mm
High Cr DP600 and DP800 steels After reheating in a pusher-type furnace, the slabs were hot rolled at a finishing temperature of roughly 900°C. The coiling temperature was varied between 500 to 600°C. The reheating and hot rolling conditions and the dimensions of the hot strips are given in Table 6.5.
25
Table 6.5 : Reheating and hot rolling parameters of industrial heat #1 to 4 heat #
grade
Heat
Slab
Strip
1 1 1 1 1 1
DP600 DP600 DP600 DP600 DP600 DP600
812775 812775 812775 812775 812775 812775
1 2 3 21 22 23
563524 563526 554483 563525 554482 563523
2
DP600
814710
2
584401 1228 1099 882 614
601 604
2,70
3 3 3
DP800 DP800 DP800
911903 911903 911903
22 3 2
639463 1215 1123 873 564 638184 1232 1113 881 565 636630 1223 1099 881 558
595 607 595 604 589 596
1,89 2,39 3,20
4 4
DP800 DP800
834957 834957
1 22
965363 1223 1092 910 528 965359 1232 1106 908 596
512 510 604 599
2,85 3,99
SRT FT RghT
slab reheating temperature finishing temperature temperature after roughening
SRT °C 1251 1254 1195 1249 1207 1253
RghT °C 1122 1115 1105 1117 1103 1125
CTA CTM CTE HS thickn
FT °C 893 894 894 894 894 891
CTA °C 631 616 610 556 588 526
CTM CTE HS thickn °C °C mm 599 647 2,70 601 605 2,70 551 591 2,69 554 553 2,69 503 535 2,70 499 543 2,70
coiling temperature head end coiling temperature middle coiling temperature tail end hot strip thickness
639463 900
900
900
FT 639463 638184 636630
850
temperature [°C]
800
700
600
500 0
100
200
300
400
500
600
700
800
900
1000
1100
800
800
CR ~ 80 – 100
750
Ks-1
750
700
700
650
650
600
600 0
1200
850
2
4
6
8
10
CT
12
time [s] 0 CS\Head\finTemp
1 CS\Head\coilTemp
Figure 6.1: Hot rolling of strips of heat #3, left: finishing and coiling temperature versus strip length (strip: 636630), right: temperature on the run out table Figure 6.1 shows exemplary the hot rolling conditions, left: finishing and coiling temperature versus coil length, right: strip temperature versus time on the run out table. The cooling from the finishing temperature (~900°C) to coiling temperature (~600°C) is done in two steps; first step: cooling from finishing temperature to approx. 700°C with a cooling rate of 80-100 Ks-1 followed by a second step: cooling to coiling temperature with a cooling rate of approx. 10 Ks-1. The hot rolling was done at different thicknesses variing from 2 up to 4 mm. Cold rolling and hot dip galvanizing on HDG2 line of voestalpine was performed, the parameters applied are given in Table 6.6. The cooling rate during hot dip galvanizing varied from 20 Ks-1 to 40 Ks-1.The different hot and cold rolling thickness yield to different cold reduction rates: 0.63/0.50/0.44.
26
Table 6.6: Hot dip galvanizing parameters of industrial produced strips, heat #1 (VA_DP600_1) and #3 (VA_DP800_1) heat #
Strip all
TS °C 792
tS s 65
Tjet °C 501
TZn °C 458
DG % 0,7
CR thickn mm 1,2
1 2
584401
790
67
502
470
0,7
1,2
3 3 3
639463 638184 636630
841 835 848
35 48 76
490 507 500
454 455 460
0,50 0,55 0,49
0,71 1,20 1,79
TS
soaking temperature
tS
soaking time
Tjet
gas jet temperature
TZn DG CR thickn
Zinc pot temperature skinpass reduction cold rolled thickness
During industrial production the following samples have been taken: ‚ ‚ ‚ ‚ ‚
Slab Transfer bar Hot strip Full hard Hot dip galvanizing
The hot strip samples and the samples after hot dip galvanizing have been characterized by the means of tensile testing. Micrographs were taken from all samples, concentration mapping was performed of all samples except full hard samples. From the transfer bar (heat #2 and #3) specimens of the dimension 130 (rolling direction) x 110 x 30 mm³ were machined and sent to CRM for laboratory hot rolling experiments. 6.1.3
Pilot simulation pr ocess par ameter s, High Cr DP 600 and 800 steels, LowCr DP 800 steel, Si added TRIP steel
Besides industrials testings, a lot of laboratory experiments and pilot simulations were performed in the course of the project. We report in this paragraph the major part of hot rolling simulations but, some complementary simulations (hot rolling and cold rolling) will be described for specific purpose in the following chapters. Hot rolling was performed in the Rolling Technology Centre (RTC) in CRM-Gent (Figure 6.2). It is operating a unique rolling line dedicated to the simulation of the hot rolling processes. The line (started in Jan 2002) comprises two complementary computer controlled hot rolling stands operating simultaneously. The line layout, that combines 10 reheating and coiling furnaces and powerful cooling equipments (from 15 to 800°C/s on a 3mm thick hot strip), allows a flexible and accurate simulation of a great variety of rolling processes for the production of flat and long products. The stands are equipped with multiple measuring systems, allowing for instance to record parameters such as entry/exit temperature, force, thickness gage, current, speed (see example at figure 6.3).
27
Figure 6.2: Rolling Technology Centre (RTC) in CRM-Gent
28
Hot Rolling HR Number :
2567g
4 Passes : Remarks : width : 110 mm N° 1 2 3 4
Thickn meas t 17 17.47 10 10.53 5.5 6.07 3.5 3.86
Speed 40 -40 40
Temp 1066 1066 1031 928
Force 80 88 115 137
1200
P2
P3
P1 1000
Temperature (°C)
800
600
400
200
0 60
70
80
90
Tim e (s ec)
200
P4
1100
Force
1000
Tin 900
100
800
Tout T (°C)
Force (tons)
150
700 50 600 0 2000
3000 t (msec) 4000
500 5000
Figure 6.3: Example of parameters recording during hot rolling (2567 of VA_DP_600_2 steel)
29
Before hot rolling, the 30 mm blocks were soaked in a laboratory reheating furnace for various times and temperatures. The first trials were performed with reheating for 1h at 1200 °C (Table 6.7). Subsequently, lower temperatures (Table 6.7 – continued) were used in order to avoid biasing by high temperature diffusion. In some cases, in order to voluntarily favour that diffusion, a preliminary stay at higher temperature (1300°C) was also used. The rolling was performed in 4 passes in the pilot rolling line (2 rolling stands). Some variations of the reduction sequence occurred (see table), but the finished thickness was always between 3 and 4 mm. The majority of the rollings were finished at temperatures higher than 900°C, but, and specially for the low reheating temperatures, finish rolling temperatures between 800 and 900°C were also realised. The table shows the various cooling rates and coiling temperatures which were applied to the plates. Cooling was, either natural air cooling, or accelerated cooling (in boiling water for 15°C/sec, and in Misting Jet Cooling Unit for 50 to 300°C/sec). Coiling was simulated in a furnace which was allowed a natural cooling after plate introduction. After conditioning, part of the plates were delivered to the suppliers in the as hot-rolled state, or after cold rolling, while part of them has only been submitted to characterisation, and discarded from further consideration because of only minor variations in regard to other rollings. In some selected cases, duplicate or even triplicate rollings were realised, in order to provide a sufficient quantity of plates for subsequent variations in the final treatment. Table 6.7: Hot rolling parameters
Reheat.
Sequence
Tfr
2508 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
910
Internal (setup)
2509 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
915
Internal (setup)
2510 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
935
Internal (setup)
2511 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
900
Internal (setup)
2560 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
998
Air (2.5)
500
CR - sent to AR
2561 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
987
Air (2.5)
600
Micr. charact.
2562 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
986
UFC (300)
500
CR - sent to AR
2563 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
993
UFC (300)
600
Micr. charact.
2564 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
987
Air (2.5)
700
Micr. charact.
2565 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
922
Air (2.5)
700
Micr. charact.
2566 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
910
BWC (15)
700
Micr. charact.
2567 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
928
Air (2.5)
600
CR - sent to VA
2568 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
900
BWC (15)
600
CR - sent to VA
2569 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
930
Air (2.5)
500
Micr. charact.
2570 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
918
BWC (15)
500
Micr. charact.
2605 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
920
UFC (300)
500
CR - sent to VA
2606 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
915
UFC (300)
600
CR - sent to VA
2607 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
925
UFC (50)
500
CR - sent to VA
2608 VA_DP_600_2
1200°C 1h
30/17/10/5.5/3.5
930
UFC (50)
600
CR - sent to VA
2846 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
993
BWC (15)
600
Micr. charact.
2847 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
975
BWC (15)
500
Micr. charact.
2848 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
960
UFC (50)
500
Micr. charact.
2849 AR_TRIP_800_1
1200°C 1h
30/20/12/7/3.8
955
UFC (50)
600
Micr. charact.
n°
Steel
30
C.R.(°C/
Tcoil
Destination
n°
Steel
Reheat.
Sequence
Tfr (°C)
C.R.(°C/ s)
Tcoil (°C)
2864 -
1100°C 45m
30/17/10/5.5/3
850
Internal (setup)
2865 -
1100°C 45m
30/17/10/5.5/3
865
Internal (setup)
3096 AR_DP_800_1
1100°C 1h
30/16/9/5/3
840
Air (2.5)
650
CR - sent to AR
3097 VA_DP_800_1
1100°C 1h
30/16/9/5/3
795
BWC (15)
650
HR - sent to VA
3098 VA_DP_800_1
1100°C 1h
30/16/9/5/3
845
BWC (15)
650
HR - sent to VA
3099 AR_DP_800_1
1100°C 1h
30/16/9/5/3
845
Air (2.5)
600
Micr. charact.
3100 VA_DP_800_1
1100°C 1h
30/16/9/5/3
795
BWC (15)
600
HR - sent to VA
3101 VA_DP_800_1
1100°C 1h
30/16/9/5/3
840
BWC (15)
600
HR - sent to VA
3102 AR_DP_800_1
1100°C 1h
30/16/9/5/3
840
Air (2.5)
500
CR - sent to AR
3103 VA_DP_800_1
1100°C 1h
30/16/9/5/3
795
BWC (15)
500
Micr. charact.
3104 VA_DP_800_1
1100°C 1h
30/16/9/5/3
845
BWC (15)
500
HR - sent to VA
3725 VA_DP_800_1
1150°C 1h
30/16/9/5/3
840
BWC (15)
600
HR - sent to VA
3726 VA_DP_800_1
1150°C 1h
30/16/9/5/3
880
BWC (15)
600
HR - sent to VA
3727 VA_DP_800_1
1150°C 1h
30/16/9/5/3
880
BWC (15)
650
HR - sent to VA
3728 AR_DP_800_1
1100°C 1h
30/16/9/5/3
845
UFC (60)
500
Micr. charact.
3729 AR_DP_800_1
1100°C 1h
30/16/9/5/3
860
UFC (60)
650
Micr. charact.
3731 VA_DP_800_1
1100°C 45m
30/16/9/5/3
860
BWC (15)
500
HR - sent to VA
3733 VA_DP_600_2
1300 1h + 1100 30m
30/16/9/5/3
3910 VA_DP_800_1
1100°C 45m
30/16/9/5/3
890
BWC (15)
600
HR - sent to VA
3911 VA_DP_800_1
1100°C 45m
30/16/9/5/3
900
BWC (15)
400
HR - sent to VA
905
BWC (15)
400
HR - sent to VA
Air (2.5)
650
CR - sent to AR
Air (2.5)
Destination
Internal (setup)
3912 VA_DP_800_1
1100°C 45m
30/16/9/5/3
3913 AR_TRIP_800_1
1100°C 1h
30/20/12/7/3.8
3992 AR_DP_800_1
1300 1h + 1100 30m
30/16/9/5/3
890
Air (2.5)
650
CR - sent to AR
30/16/9/5/3
850
UFC (60)
500
Rol : NOK Cool : NOK
30/20/12/7/3.8
950
Air (2.5)
650
CR - sent to AR
30/20/12/7/3.8
940
UFC (60)
500
Cool NOK ( 900°C Cooling Rate ~20°C/s Coiling T 600°C, 400°C, 120°C - 90 minutes then air cooling In table 6.23, the mechanical properties at the hot rolled stage are presented for the two DP steels, AR_DP800_1 (DP3) and AR_DP800_2 (DP4). A decrease in coiling temperature, from 600°C to 120°C, induces:
72
o o
A continuous increase in the yield and ultimate tensile strength. A progressive change in the microstructure : the ferritic matrix becoming more and more lath like towards bainite, the second phase changing from pearlite to martensite
Concerning the ductility, a minimum level is detected for 400°C coling temperature. Table 6.23: Mechanical properties of hot rolled simulation AR_DP800_1 (DP3)and AR_DP800_2 (DP4) steels. Nuance
Coil temp.
YS (MPa)
UTS (MPa)
El %
UEl %
P%
Phases
445
580
25,2
13,9
2,1
463
603
22,3
12,7
2
Ferrite+ Pearlite
638
725
6,3
3,1
0
568
688
14,1
6,8
0
655
960
12,6
6,9
0
Acicular Ferrite +
546
738
16,9
9,3
0
Martensite
430
557
24,8
14,0
1,8
432
565
15,1
8,7
0,7 / 1,9
545
693
13,5
6,4
0
488
629
15,4
8,6
0
618
769
16,4
9,0
0
547
773
11,2
8,0
0
600°C
DP 3
400°C
120°C
600°C
DP 4
400°C
120°C
6.4.4
Ferrite / Bainite + Martensite
Acicular Ferrite + fine Pearlite Acicular Ferrite + Bainite Acicular Ferrite Bainite / Martensite
Si added TRIP 800 steels (AR_TRIP 800 1), Pilot simulations mater ial
The effect of coiling temperature at low cooling rate is not appreciable (figures 6.56 and 6.57). The major effect is a coarsening of the microstructure (grains but also pearlite lamellae) when coiling at 700 °C.
Figure 6.56: Microstructures of hot rolled bands: AR_TRIP_800_1 cooled at 2.5°C/s. Coilin: 700°C (2564), 600°C (2561), 500°C (2560).
73
AIgrim
AIeros
AIASTM 1.50 1.40
1.60
1.60
1.50
1.50
1.40
1.40
1.30
1.30
1.30 1.20
1.20
1.20
1.10
1.10
1.10
1.00 500
1.00 500
550
600
650
700
550
600
650
700
Tcoil (°C)
Tcoil (°C)
1.00 500
550
600
650
700
Tcoil (°C)
Figure 6.57: Effect of coiling temperature on banding of AR_TRIP_800_1 cooled at 2.5°C/s.
Figure 6.58: Microstructures of hot rolled bands: AR_TRIP_800_1 coiled at 650°C (600 for 2849). Cooling: 2.5°C/s (3913 – 3994), 50°C/s (2849). For 3994: pretreatment at 1300°C. AIgrim 1.7 1.6 1.5 1.4 1.3 1.2 1.1 1 1
10
100
C.R. (°C/s)
Figure 6.59: Effect of cooling rate on banding of AR_TRIP_800_1 coiled at 600°C. The beneficial effect of increasing the cooling rate, even moderately, is clearly evidenced at figures 6.58 and 6.59. The most remarkable effect is probably the one which is induced by a preliminary treatment at 1300°C before reheating at 1100°C before hot rolling. The effect was evidenced with a dual phase grade, but is enhanced with this TRIP composition (figure 6.60). This is in good agreement with the model concerning band attenuation.
74
For the highest cooling rates, and for coiling temperatures as low as 500 °C, the band appearance is generally considerably attenuated, with the exception of the mid-thickness position where the central segregation still induces some faint bands (figure 6.61).
AIASTM
AR_DP_800_1
AR_DP_800_1
AIer os
AR_TRIP_800_1
3.0
3.0
2.5
2.5
2.0
2.0
1.5
1.5
AR_TRIP_800_1
3.0
AIgr im
AR_DP_800_1 AR_TRIP_800_1
2.5
1.0
2.0 1.5 1.0
1.0
1100
1300+1100
1100
1300+1100
1100
1300+1100
Figure 6.60: Effect of pretreatment at 1300°C on banding of AR_TRIP_800_1 and AR_DP_800_1 cooled at 2.5°C/s and coiled at 650°C.
Figure 6.61: Microstructures of hot rolled bands: AR_TRIP_800_1 coiled at 500°C. Accelerated cooling: 15°C/s (2847), 50°C/s (2848), 300°C/s (2562). 6.4.5
Compar ative analysis on the effect of cooling conditions on the steel har dness
Among the parameters related to hot rolling, two of them influence very significantly the hardness of the hot rolled plates: the cooling rate after hot rolling, and the coiling temperature. This is shown in figure 6.62, where the hardness has been measured with a Vickers indenter under a load of 5 Kg. As expected, the hardness always decreases when the coiling temperature is increased, what is consistent with the nature of the microconstituents. Logically, the influence of cooling rate (higher cooling rates producing harder steel) is globally larger for low coiling temperatures. For the highest coiling temperatures, the hardness remains between 160 and 190 HV5 for cooling rates ranging from 2.5 to 60 °C/s. Generally, for the intermediate cooling rates (15 to 60 °C/s), this parameter has not a strong effect. So, in selecting plates for further processing, cooling conditions in that range do not induce eligible variants. The highest cooling rate (300°C/s) gives very distinct microstructures and hardness. It can also been appreciated that AR_DP_800_1 cooled at 60°C/s and VA_DP_800_1 cooled at 15°C/s behave quite similarly.
75
Hot rolled plates - Hardness 450
A R_DP _800_1- 2.5°C /s
400
A R_DP _800_1- 60°C /s A R_TRIP _800_1- 2.5°C /s
HV5
350
A R_TRIP _800_1- 15°C /s A R_TRIP _800_1- 50°C /s
300
A R_DP _800_1- 300°C /s
250
VA _DP _800_1- 15°C /s
200
VA _DP _800_1- 300°C /s
150 400
450
500
550
600
650
700
Tcoil (°C)
Figure 6.62: Influence of the cooling conditions on the as hot rolled hardness. 6.4.6
Reheating simulations and exper iments.
The degree of banding in laboratory rolled specimens (starting from specimens cut off at the roughing stage) was suspected to be inferior to what occurs in industrial practice. It must be remembered that the segregation wavelength has been reduced by a factor of 7.5 between a 250 mm thick slab and a 30 mm thick roughing plate. It is the reason why, for AR_DP_800_1 steel, simulations and experiments were designed to evaluate the effect of that reduction on segregation attenuation during reheating, and hence the possibility to have a better fitting to the industrial process. In a first step, the group “diffusion programs” of the model set from TU Delft was used to examine the impact of time and temperature on the segregation attenuation during reheating before hot rolling. The wavelength of banding in the roughing stage was estimated by metallography to be about 30 µm. It was then decided to fix the corresponding value in the slab at 250 µm. Figure 6.63 reports the simulation results, in terms of the evolution with reheating time of the segregation ratio for Mn and Cr (Si evolution is very close to the latter).
Figure 6.63 : AR_DP_800_1 – Diffusion simulations (TUDelft program). In the slab condition, reheating at 1200°C keeps to a very large extent the degree of segregation, and more for Mn than for Cr. Due to the shortest diffusion distances necessary in the case of plates at roughing stage, the segregation is considerably reduced after 1 hour at 1200°C, and even annihilated for Cr. By lowering the reheating temperature down to 1100°C, the model predicts that a more reasonable amount of segregation is preserved, thus keeping closer to the industrial process in terms of segregation (but not of austenitic grain size or precipitation dissolution!).
76
In view of validating the conclusions of those simulations, reheating experiments were performed on some blocks of that steel in the roughening state. We admit that the as-received specimen exhibits a microsegregation which is an anamorphosis of the state prevailing during slab reheating. After reheating for 1 hour at 1100 and 1200°C, the specimens were given a further intercritical heat treatment, including a 10 min stay in the intercritical region (720°C), and then allowed to cool slowly. Visually, the specimen reheated at 1100°C exhibits a significantly less banded structure than the as-received one, and the attenuation strongly increases for reheating at 1200°C. Local microanalysis of band and interband regions reveals that Mn segregation has been attenuated by a factor of 4 in that last case, what is in good agreement with the simulation results. In order to reinforce that analysis, and in preparation to further hot rolling (to keep enough segregation when reheating transfer bars), the same heat treatments were applied to VA_DP_800_1 transfer bars, including a simulation of reheating at 1300°C. The general aspect of the bands at mid-thickness is reported at figure 6.64. When the reheating temperature is increased, the thinnest bands tend to disappear, or to merge, and finally fade out to a large extent. In view of retrieving the features in the slab solidification which are to be associated with the microstructural banding, the following set of image acquisition and manipulation was performed, as described in figure 6.65. For a given position over the thickness of the transfer bar (about 1 mm in the following case), a set of 10 adjoining micrographs were taken, and assembled, so realising a patchwork of 10 frames. While doing so, each image was given a size reduction in the rolling direction, corresponding to the ratio of the slab and the transfer bar thickness (about 1/7). Finally, a second anamorphosis of the same ratio was performed, because while the slab is elongated, its thickness is also reduced by the same amount, giving a total deformation of about 50. The described steps are illustrated at figure 6.66. The scale markers are related to the dimensions in the transfer bar state for the left image, and to the dimensions of the original slab in the right one. a
1100°C
b
1200°C
c
1300°C
Figure 6.64: VA_DP_800_1 – Microstructures of the transfer bars after reheating simulations (60 min).
77
w
7xh
+
h
w/7
Figure 6.65: Patchworking and anamorphosis: schematic view.
Figure 6.66: Intermediate and final image from “inverse rolling” the specimen illustrated at figure 6.64-a. In the intermediate step, a significant attenuation of the band appearance can be noticed at 1200°C in some regions, while others keep well-defined bands. At 1300°C, the specimen remains with only much more diffuse regions with enhanced carbide-containing regions. The wavy appearance of the bands, at best visible in specimen treated at 1100°C, is also characteristic. In the final version (right image), cells are perceptible in the ½ thickness position, the size of which ranges from 0.3 to 1 mm. It could correspond to equiaxed solidification grains, with solutes being rejected to boundaries and giving rise, after alignment by hot rolling, to the wavy microstructural bands. The situation is more confuse near to the surface. 6.5
Effect of cold rolling on the initial hot rolled microstructure
Work was mainly performed on ARSA_DP800_1 steel, hot rolling simulations are described in Table 6.21. After hot rolling, samples were cold rolled with approximately a 65 pct of final reduction. Figure 6.67 shows the microstructure of cold rolled samples in ARSA_DP800_1 steel. The ferrite grains and the pearlite colonies are elongated, and deformation bands are present in the ferrite. The amount of pearlite in sample S10 exhibiting a banded ferrite and pearlite microstructure remains invariable after cold rolling (See results on VP, in Table 6.24). Likewise, pearlite colonies present a lamellar structure as scanning electron micrographs revealed (See an example in Figure 6.67c). Cementite lamellae do not seem to be evidently fragmented and irregularly spaced. The determination of the morphological parameters of pearlite before and after cold rolling, listed in Table 6.24, demonstrated that deformed pearlite exhibits finer interlamellar spacing, u2. and higher area per unit
78
S PP
volume of colonies interface, v , i.e. smaller pearlite colonies. This refinement is related to the approach of neighbouring cementite lamellae due to cold work, more noticeable in transverse section.
Figure 6.67: Light optical micrographs and scanning electron micrographs of 68% cold rolled specimens parallel to the rolling direction. a) Sample S1: CR=7 ºC/s, CT=500 ºC; b) Sample S3: CR=60 ºC/s, CT=500 ºC; c) Sample S10: CR=7 ºC/s, CT=650 ºC; and d) Sample S12: CR=60 ºC/s, CT=650 ºC.
79
Table 6.24: Characterisation of banded ferrite-pearlite microstructures in hot and cold rolled samples. Sample S10: CR=7 ºC/s, CT=650 ºC VP
u2"*om)
S vPP x10-3 *om-1)
AI
n` "*om)
Longitudinal
0.30 ‒ 0.02
0.19 ‒ 0.02
616 ‒ 127
1.8 ‒ 0.2
26 ‒ 7
Transversal
0.32 ‒ 0.02
0.18 ‒ 0.03
556 ‒ 102
1.7 ‒ 0.3
24 ‒ 2
Longitudinal
0.35 ‒ 0.02
0.16 ‒ 0.03
813 ‒ 323
6.3 ‒ 1.0
11 ‒ 1
Transversal
0.35 ‒ 0.02
0.10 ‒ 0.01
911 ‒ 336
2.7 ‒ 0.4
11 ‒ 1
Hot rolled sample
Cold rolled sample
Regarding microstructural banding, only those samples with hot rolled bands were found to have banding problems after cold rolling. Quantitative characterisation of banding revealed that cold rolling significantly increases the anisotropy index, AI, and reduces the mean free path spacing of bands, n` (See results in Table 6.24). AI values measured in longitudinal cold rolled samples were found to be much higher that those measured in transversal samples. Such difference was not detected in the corresponding hot rolled sample. On the other hand, n` value gives us an idea of the distance between bands. In this sense, it is not surprising that n` value is reduced during cold rolling. During a subsidiary experiment, the effect of cold rolling (65%) was observed for AR_TRIP_800_1 grade, starting from a hot rolled strip cooled at 2.5°C/s and coiled at 650°C (strip 3913). The microstructures are represented at figure 6.68 (optical and scanning electron microscopies). The evolution during cold rolling is classical for such a ferrite-pearlite steel: the pearlite islands are elongated, with ferrite flowing in between, and the cementite lamellae of the pearlite are bowed and occasionally broken.
Figure 6.68: Effect of cold rolling on the AR_TRIP_800_1 microstructure (C.R. 2.5°C/s – Tcoil 650°C)
80
6.6
Detailed mechanisms of phase transformation during intercritical annealing
6.6.1
Kinetics for micr ostr uctur e evolution
Intercritical annealing must be carried out between Ac1 and Ac3 temperatures. The austenite formation temperatures were therefore determined using an Adamel Lhomargy DT1000 high-resolution dilatometer in ARSA_DP800_1 steel. Cylindrical specimens 12 mm in length and 2 mm in diameter were heated at a rate of 5 ºC/s to 1000 ºC and then cooled at 1 ºC/s. The formation of austenite during heating was detected by monitoring the relative change in length of the specimen with temperature. Table 6.25 shows the Ac1 and Ac3 temperatures of hot rolled and cold rolled samples. Dilatometric samples of hot and cold rolled material were also intercritical annealed at three different temperatures (750, 800 and 850 ºC) for different times (1, 20 and 100 s) before gas quenching. The heating rate selected for the intercritical annealing experiments was 5 ºC/s.
Samples S1 S3 S10 S12
Table 6.25: Heating critical temperatures Hot rolled material Cold rolled material Ac1 (ºC) Ac3 (ºC) Ac1 (ºC) Ac3 (ºC) 738 850 734 846 740 842 737 849 743 860 734 856 745 844 738 853
Austenite, which is formed during intercritical annealing, transforms to martensite during quenching. Thus, the progress of austenitisation is determined throughout the evolution of the volume fraction of martensite. Specimens from dilatometric experiments were polished in the usual way for metallographic examination. LePera’s reagent was used to reveal martensite formed during quenching. The quantitative measurement of martensite volume fraction was carried out by point-counting method. Figure 6.69 shows microscopic evidences of how austenite formation occurs in cold rolled sample S10 (CR=7 ºC/s, CT=650 ºC) throughout optical micrographs from intercritical annealing specimens at different temperatures and times. LePera’s reagent reveals pearlite and ferrite as darker phases in the microstructure, whereas martensite formed during quenching appears as lighter regions in the micrographs. When a specimen contains more than 60% martensite, contrast between ferrite and martensite begins to degrade [Ref 6]. In that case, ferrite appears as lighter grey regions (See micrographs in Figures 6.69.e, f and g). Microstructure in Figure 6.69.a is formed mainly of ferrite, pearlite and some grains of martensite. At this quench-out time, the pearlite-to-austenite transformation has already started. Once pearlite dissolution has finished annealed microstructure consists of a mixture of ferrite and martensite (Figures 6.69.b and 6.69.c). Increasing the soaking temperature from 750 ºC to 800 ºC resulted in lower amounts of ferrite in the microstructure (Figures 6.69 .d, e and f). This is due to the larger amount of austenite formed at higher temperatures, which transforms into martensite on quenching. At 850 ºC, when the intercritical soaking is reached, a few ferrite grains remain untransformed (Figure 6.69.g). But, it takes less that 20 s to completed the transformation to austenite, as a consequence a fully martensitic microstructure is formed on quenching (See Figures 6.69.h and i). At 750 ºC and 1 s of soaking time, austenitisation process has already started for all the initial microstructures. Figure 6.70 shows electron micrographs corresponding to the beginning of the transformation in annealed samples. It is clear from those micrographs that in samples consisting on ferrite and pearlite (samples S1 and S10), the nucleation of austenite takes place inside pearlite preferentially at the points of intersection of cementite with the edges of the pearlite colony (See Figures 6.70.e and 6.70.f). Likewise, in samples mainly formed by bainite (samples S3 and S12), austenite nucleates at the interface between the plates of ferrite in the sheaves of bainite (Figures 6.70.c and 6.70.d). Moreover, carbides at grain boundaries and inside ferritic grains are an important nucleation site for austenite in cold rolled samples.
81
Figure 6.69: Optical micrographs from intercritical annealed samples (a) at 750 ºC for 1 s; (b) at 750 ºC for 20 s; (c) at 750 ºC for 100 s; (d) at 800 ºC for 1 s; (e) at 800 ºC for 20 s; (f) at 800 ºC for 100 s; (g) at 850 ºC for 1 s; (h) at 850 ºC for 20 s; (i) at 850 ºC for 100 s; Initial microstructure: cold rolled sample S10 (CR=7 ºC/s, CT=650 ºC). LePera reagent. Ferrite recrystallisation was completed during heating in all cold rolled samples tested (See Figure 6.70). In all the cases when the intercritical annealing stage is reached, the ferrite grains are fully recrystallised. On the other hand, spherodisation of the deformed pearlite occurs concurrently with ferrite recrystallisation during annealing of the cold rolled samples (see Figure 6.70.f). That process has not been observed in the corresponding hot rolled sample (See Fig. 6.70.e). Finally, it seems that the number of austenite nuclei at this temperature is higher in the cold rolled material than in the hot rolled material. This must be related to the size of the initial microstructure.
82
a) S1-HR
b) S1-CR
c) S3-HR
d) S3-CR
e) S10-HR
f) S10-CR
g) S12-HR
h) S12-CR
Figure 6.70: Scanning electron micrographs corresponding to the beginning of the austenitisation process in hot rolled and cold rolled samples. 2 pct Nital etching solution. Intercritical temperature: 750 ºC; soaking time: 1 s
83
Figure 6.71 compares the kinetics of austenite formation in cold rolled specimens with those of the corresponding un-deformed microstructures. The mechanism of austenite formation in the cold rolled steel would be controlled by the same mechanism as in the un-deformed microstructures, namely, carbon diffusion in austenite as concluded by Speich et al. [Ref 7]. Thus, the volume fraction of austenite increases with increasing soaking temperature and time. Likewise, the rate of austenite formation in both cases is quite similar. It seems that cold rolling does not increase significantly the kinetics of the austenite formation during intercritical annealing. Moreover, this figure suggests that the initial microstructure barely affects the kinetics of austenite formation. It was just observed that austenitisation process has finished at 850 ºC and 1 s of soaking time in S1 and S3 as hot and cold rolled material, and S12 as hot rolled material, whereas \10 % of ferrite is still un-transformed in S10 as hot and cold rolled material, and S12 as cold rolled material at the same conditions. These kinetics results are also in accordance with dilatometric data listed in Table 6.25. The results represented in Figure 6.71 that describes the evolution of the ferrite-to-austenite transformation during intercritical annealing were fitted to the JMA equation
]
ln}ln 1 / *1 / f
+_
1 / f i ? exp( /k © t n )
. If
i the transformation follows the JMA equation, a plot of versus ln t must result in a single straight line with a slope equal to n and an intersection with the y axis equal to lnk. These calculations were only carried at a temperature of 800 ºC since nucleation events still occur at 750 ºC. In that case, it is not possible to study separately the growth kinetics of austenite in ferrite. Particularly low values of the time exponent n were found for all the initial microstructures, around 0.35. For a purely carbon controlled planar growth of the austenite a value of 0.5 is expected. The activation energy related to the austenite formation could not be calculated since there were not enough experimental data to determine the dependence of the k parameter.
850 ºC
100
850 ºC
100
800 ºC
800 ºC 80
A usten ite Vo lu me Fraction
A usten ite Vo lu me Fraction
80
S1 S3
60
S10 S12 40
750 ºC 20
S1 S3
60
S10 S12
40
750 ºC 20
0
0 0
20
40
60 Time, s
a) Hot rolled material
80
100
0
120
20
40
60 Time, s
80
100
120
b) Cold rolled material
Figure 6.71: Evolution of austenite volume fraction during intercritical annealing in a) hot and b) cold rolled material. Sample S1 (CR=7 ºC/s, CT=500 ºC); Sample S3 (CR=60 ºC/s, CT=500 ºC); Sample S10 (CR=7 ºC/s, CT=650 ºC); and Sample S12 (CR=60 ºC/s, CT=650 ºC). Figure 6.72 shows the evolution of carbon content in the austenite during intercritical annealing in hot and cold rolled specimens calculated from the measured martensitic start temperature and using the equation [Ref 8, 9]
M s ? 539 / 423%C / 30.4% Mn / 12.1%Cr / 7.5% Si - 30% Al which takes into account the influence of Mn, Cr, Si and Al. For the calculations the concentration of those alloying elements were set equal to those of the steel composition. As expected, the austenite
84
0.8
0.6
Austenite carbon content, wt-%
Austenite carbon content, wt-%
carbon content decreases with time as ferrite-to-austenite transformation progresses. Thus, at 750 ºC and 1 s, when a few grains of austenite have been only formed, the austenite carbon content is close to the eutectoid carbon content of the steel (0.66%C according to MTDATA [Ref 10] calculations). Likewise, the austenite carbon content approaches the initial carbon content of the steel (0.15 wt-%) as transformation finishes.
750 ºC-CR 800 ºC-CR
0.4
850 ºC-HR 850 ºC-CR
0.2
0 0
20
40
60
80
100
0.8
0.6
750 ºC-CR 800 ºC-HR 800 ºC-CR
0.4
850 ºC-CR 0.2
0 0
120
20
40
Austenite carbon content, wt-%
Austenite carbon content, wt-%
0.8
750 ºC-HR
0.6
750 ºC-CR 800 ºC-HR
0.4
800 ºC-CR 850 ºC-HR
0.2
850 ºC-CR
0 20
80
100
120
b) Specimen S3
a) Specimen S1
0
60
Time, seconds
Time, seconds
40
60
80
100
0.8
0.6
750 ºC-HR 750 ºC-CR
0.4
800 ºC-CR 850 ºC-HR 850 ºC-CR
0.2
0 0
120
20
40
60
80
100
120
Time, seconds
Time, seconds
d) Specimen S12
c) Specimen S10
Figure 6.72: Evolution of carbon content in austenite during intercritical annealing of hot (HR) and cold (CR) rolled material 6.6.2
Detailed quantification of micr ostr uctur e evolution dur ing soaking
Preliminary studies were conducted to characterize the 3D morphology of bands in cold rolled and annealed steels. A DP steel was selected (AR_DP800_2) to conduct the experiments. Thermal cycle applied to simulate band apparition is described on figure 6.73. Annealing is interrupted by quenching to facilitate the metallographical observation. The microstructure obtained is therefore a mixture of ferrite and martensite. 100s - 760°C
Heating rate 20°C/s
Quenching Figure 6.73: Thermal cycle applied to AR_DP800_2 steel to study 3D morphology of bands. Metallographical observations are presented on figure 6.74. Observations were performed in three perpendicular planes: parallel to the surface (rolling) plane, perpendicular to the rolling direction,
85
perpendicular to the transverse direction. Bands mainly appear in the plane perpendicular to the transverse direction. In the rolling plane, bands appear more or less as equiaxe zones. In the plane perpendicular to the rolling direction, the microstructure appears the most homogeneous. As a conclusion, we suggest to observe in the plane perpendicular to the transverse direction for band characterization. This was applied for all characterization in this project for bands characterization.
section parallel to the rolling direction
tr
plane parallel to the surface at ¼ thickness
an pl sv an er e se
rolling direction
Figure 6.74: 3D morphology of the bands after soaking Experiments were performed to carefully study the microstructure refinement during annealing on cold rolled industrial AR_DP800_2 steel. The annealing simulation cycles are presented on figure 6.75. We only study the effect of soaking treatment. Variations of cooling paths are not the aim of this RFCS project. Experiments are performed on the BÄHR plastodilatometer. In some treatments, annealing is interrupted by quenching to facilitate the metallographical observation. The microstructure obtained is therefore a mixture of ferrite and martensite. Metallographical observations were performed in the plane perpendicular to rolling direction where the microstructure appears the most homogeneous.
100s - 760°C or 850°C 20°C/s Heating rate
420°C, 20s
20°C/s
Quenching
Figure 6.75: Annealing cycle to study the effect of annealing temperature on microstructure refinement The effect of annealing temperature on grain size is presented on figure 6.76. In this case, the annealing time is 100s and the annealing temperature is 760, 790, 830 and 850°C. Test is interrupted by quenching after soaking. We measured the evolution of mean ferritic and austenitic grain size (from the martensite islands size). At 850°C, annealing is performed in the austenitic phase, which explains the null value for ferritic grain size. Increase in the soaking temperature induces a decrease in ferritic grain size, due the austenite progressive growth. Both constituents are finer in average when the annealing temperature is lower.
86
These experiments explain why, to understand the effect of hot rolling parameters, detailed investigations of the experiments presented in the previous paragraph have been performed on the low temperature annealed steels, to limit the effect of austenite growth on our interpretation. intercritical holding : grain size
12
grain size (µm )
10
Dferrite Dautenite
8 6 4 2 0 750
760
770
780
790
800
810
820
830
840
850
860
870
tem perature (°C)
Figure 6.76: Annealing cycle to study the effect of annealing temperature on microstructure refinement It is also important to note the strong heterogeneity in martensite islands size in the annealed steel. The cooling parameters were fixed to 20°C/s for the cooling rate and 20s at 420°C for the galvanizing simulation. Two soaking temperatures were tested: 760°C and 850°C (austenitic annealing). Results are presented on figure 6.77. Martensite islands size range from less than 1µm2 up to more than 10µm2. The number of small islands is always higher and controls the average grain size. However, the soaking temperature has a clear effect on the distribution. Especially, austenitic soaking increases the heterogeneity. Microstructure shows a much higher numbers of big islands in the microstructures. 3
% marten site
2.5
850°C-20°C/s-20s à 420°C
2
760°C-20°C/s-20s à 420°C
1.5
1
0.5
0 1
2
3
4
5
6
7
8
9
10
>10
(µm²)
Figure 6.77: Effect of soaking temperature on the distribution of martensite islands size.
87
Similar experiments were conducted on AR_TRIP800_1, with two different hot rolled microstructure The hot rolling parameters differ by the coiling temperature (550°C and 400°C). The cooling rate applied was 20°C/s. Hot bands were cold rolled (70% level) before annealing simulations. The annealing simulation cycles are presented on figure 6.78. Experiments are performed on the BÄHR plastodilatometer. Annealing is interrupted by quenching after cooling at 650°C to facilitate the metallographical observation. The microstructure obtained is therefore a mixture of ferrite and martensite. Metallographical observations were performed in the plane perpendicular to rolling direction where the microstructure appears the most homogeneous.
Temperature :770°C, 800°C or 830°C Time : 0s, 20s, 60s or 120s Cooling rate : 400°C/s, 20°C/s, 4,4°C/s 650°C
4.4°C/s 650°C 18°C/s
quench He
Figure 6.78: Annealing cycle to study the effect of hot rolled microstructure on microstructure refinement Concerning the kinetics for austenite transformation during intercritical annealing, results are presented on figure 6.79. Three temperatures are reported. For these high temperatures, austenite transformation is very rapid and occurs mainly during heating. The kinetics for phase transformation does not depend, in this case, on the initial hot rolled microstructure. The evolution of austenitic and ferritic grain size is presented on figure 6.80. Once again, differences in hot rolled microstructure do not prove to have any effect on the microstructure size during annealing. In the range of this study, we can conclude that the variations in hot rolled microstructure will not have any major effect on the final microstructure size after annealing
austenite %
90 80
830°C
70
800°C
60
770°C
50 40
coiling 550°C
30
coiling 400°C
20 10 0 0
20
40
60 80 holding tim e (s)
100
120
14
Figure 6.79: effect of hot rolled microstructure on the kinetics for austenite transformation during intercritical annealing
88
6
grains size (µm)
5 4
austenitic grains size ferritic grains size 550°C 400°C 550°C 400°C
3 2 1 0 760
780
800
820
840
intercritical hoding tem perature Figure 6.80: effect of hot rolled microstructure on microstructure grain size during intercritical annealing 6.7
Effect of hot rolled microstructure on the final (cold rolled and annealed) product microstructure: refinement and band formation
6.7.1
Bands in the micr ostr uctur e
6.7.1.1.Mo added DP800 grade (OCAS_DP_800_1) Cold rolled and annealed samples of OCAS_DP800_1 steel were supplied. Table 6.26 lists coiling and annealing temperatures of the specimens supplied. Sample 167 comes from a steel with a slightly lower chromium content (0.25 wt-%) than that in OCAS_DP800_1 steel. The final thicknesses are 1.5 and 1.7mm. The annealing temperatures were 820 or 770°C. The cooling rate was more or less constant at 30°C/s. The overaging section temperature was 280°C, +/-20°C. Table 6.26: Coiling and soaking temperatures of OCAS_DP800_1 steel Samples 129 143 174 167
Coiling Temperature (ºC) 655 585 585 655
Soaking Temperature (ºC) 820 820 770 770
Cold and annealed specimens of were ground and polished using standardised techniques for metallographic examination. A 2 pct Nital etching solution was used to reveal the microstructure by optical and electron microscopy. All the samples consist of ferrite, martensite and tempered martensite in bands. Figure 6.81 shows optical and electron micrographs of all the samples in transverse direction.
89
Hv- 5kg = 247
1
a) Sample 129. LOM Hv- 5kg = 258
b) Sample 129. SEM 2
c) Sample 143. LOM Hv- 5kg = 254
d) Sample 143. SEM 4
e) Sample 174. LOM Hv- 5kg = 234
f) Sample 174. SEM 6
g) Sample 167. LOM
h) Sample 167. SEM
Figure 6.81: Optical and electron micrographs of cold rolled and annealed samples of OCAS_DP800_1 steel in transverse direction. 2 pct Nital etching solution The 3 types of banding characterisation have been applied to the specimens. The results are included in table 6.27 for AIASTM and AIeros, and in figure 6.82 for all of them.
90
Table 6.27: Annealed OCAS_DP_800_1 sheets – banding characterisation ASTM - based ¼ thickness
Nr (TCoil/TSoak)
n (µm) 5.2 5.3 4.8 5.2
A.I.
129 (655/820) 143 (585/820) 167 (655/770) 174 (585/770)
½ thickness
1.62 1.72 1.64 1.81
n (µm) 4.8 6.4 5.1 5.6
A.I. 1.83 1.61 1.67 1.62
Erosion - based ¼ thickness
½ thickness
A.I.
LV (µm)
A.I.
LV (µm)
1.51 1.72 1.59 1.59
2.8 3.1 3.0 2.3
1.73 1.77 1.76 1.57
2.8 4.4 3.0 3.2
770
770
AIASTM
2.00
820
AIg r i m
5.00
820
770
AIer o s
2.00
820
4.50 4.00 3.50 1.50
3.00
1.50
2.50 2.00 1.50 1.00
1.00 550
600
650
Tcoil (°C)
700
1.00 550
600
650
T coil (°C)
700
550
600
650
700
Tcoil (°C)
Figure 6.82: Annealed (770 and 820 °C) OCAS_DP_800_1 sheets. Effect of hot rolled conditions on banding. The general tendency is that the anisotropy index is larger at ½ thickness than at ¼ thickness, but it is not as clear as with rot rolled specimens. The soaking temperature has apparently no effect on the anisotropy index. The same conclusion applies to the coiling temperature. The wavelength of banding shows a slight tendency to increase when the coiling temperature is reduced. These partial results tend to prove that, even if the variations in the hot rolling process produce a significant change in banding the hot rolled microstructure, this effect is annihilated in the annealed state. The microchemical segregation is still present, and restores banding during the intercritical annealing. The solution is rather to be found at the level of the annealing cycle itself, which was out of the scope of the present research. Other researches are more directly concentrated on that aspect. The fractions of second phase have been determined as indicated in table 6.28. A higher annealing temperature leads to a much higher fraction of second phase. Table 6.28: Amount of second phase according to the different process parameters CT655 CT585 An820 25% 27% An770 13% 15%
6.7.1.2. High Cr DP800 grade (VA_DP_800_1) VASL has performed annealing simulations (HDG and CA cycles) on the cold rolled sheets from the hot rolled strips detailed in chapter 4, table 6.7 (FRT 800°C – 850°C, CR 15°C/s, CT 500-650°C). Annealing cycles are described on figure 6.83 and table 6.29.
91
(a)
(b)
Figure 6.83: Laboratory (a) hot dip galvanizing (HDG) and (b) continuous annealing cycle (CA) simulations. Table 6.29 Laboratory hot dip galvanizing (HDG) and continuous annealing cycle (CA) simulations in the studied steels HDG conditions CA conditions Ts, ºC ts, s Tjet, ºC Tzn, ºC t, s Ts, ºC ts, s v1, ºC/s v2, ºC/s Tag, ºC tag, s Steel VA_DP800_1 800 100 470 465 40 800 100 10 40 270 400 Figures 6.84 and 6.85 show optical micrographs of specimens after HDG and CA simulations, respectively of VA_DP800_1 steel. HDG samples present a banded ferrite-bainite-martensite microstructure, whereas CA samples consist on ferrite and tempered martensite.
(a) 3098 (FT=850 ºC, CT= 650 ºC)
(b) 3101 (FT=850 ºC, CT= 600 ºC)
92
(c) 3104 (FT=850 ºC, CT= 500 ºC)
(d) 3097 (FT=800 ºC, CT= 650 ºC)
(e) 3100 (FT=800 ºC, CT= 600 ºC)
Figure 6.84: Optical micrographs of HDG samples in longitudinal direction of VA_DP800_1 steel. LePera reagent
As expected, the higher the soaking temperature, the higher the total volume fraction of martensite. However, results suggest that the coiling temperature does not significantly affect the kinetics of the austenite formation during annealing. This is consistent with results observed in ARSA_DP800_1 steel.
(a) 3098 (FT=850 ºC, CT= 650 ºC)
(b) 3101 (FT=850 ºC, CT= 600 ºC)
93
(c) 3104 (FT=850 ºC, CT= 500 ºC)
(d) 3097 (FT=800 ºC, CT= 650 ºC)
Figure 6.85: Optical micrographs of CA samples in longitudinal direction of VA_DP800_1 steel. Le Pera regent.
(e) 3100 (FT=800 ºC, CT= 600 ºC)
The general tendency, illustrated at figure 6.48 and 6.49 for the hot rolled strips, to lower the banding severity by coiling at lower temperatures, is not so clear after cold rolling and annealing, but can be observed, mainly for the CA cycle (figures 6.86 and 6.87). The bands seem to be somewhat attenuated after annealing if the finish rolling temperature (hot rolling) is low. Nevertheless, this tendency is not clear for all the analysis. Finally, n` values are quite related to the volume fraction of pearlite/martensite present in the banded microstructure. The lower the volume fraction of martensite the higher the mean edge-to-edge spacing of the bands. Tf 800
AIASTM
2
Tf 850
Tf800
AIer o s
2
Tf850
1.5
1.5
1.5
1
1
1
0.5 500
550
600
650
700
0.5 500
550
Tcoil (°C)
600
650
700
0.5 500
Tf800
AIg r i m
2
550
600
Tf850
650
700
T coil (°C)
Tcoil (°C)
Figure 6.86 VA_DP_800_1 after CA cycle: effect of hot rolling parameters on banding. Tf 800
AIASTM
2.5
Tf 850
Tf 800
AIer o s
2.5
Tf 850
2
2
2
1.5
1.5
1.5
1
1
1
0.5 500
550
600 Tcoil (°C)
650
700
0.5 500
550
600 Tcoil (°C)
650
700
0.5 500
Tf800
AI g r i m
2.5
550
600 T coil (°C)
Tf850
650
Figure 6.87: VA_DP_800_1 after HDG cycle: effect of hot rolling parameters on banding.
94
700
6.7.1.3.Progressive bands reappearance during soaking (ARSA_DP800_1 steel) Dilatometric samples Cold rolled bands do not vanish during intercritical annealing, since austenite formation starts in the carbon-rich regions featuring pearlite. Thus, martensite bands will form during quenching in the regions previously occupied by pearlite. However, it is not clear if martensite bands will reappear during intercritical annealing once banding was eliminated by fast cooling during hot rolling. Figure 6.88 shows the evolution of the anisotropy index, AI, and the mean edge-to-edge spacing of the bands, n`, of cold rolled samples during intercritical annealing of ARSA_DP800_1 steel (see § 6.6.1 for details on the experiments). Both parameters characterise the degree of martensite banding on longitudinal sections. Those samples with a non-banded microstructure present a AI value of one. That is the case of samples annealed at 850ºC consisted mainly of martensite whose AI values have not been included here.
Figure 6.88: Evolution of microstructural banding during intercritical annealing: Anisotropy index a) at 750 ºC and b) at 800 ºC of soaking temperature; and c) mean edge-to-edge spacing of the bands, n . Sample S1 (CR=7 ºC/s, CT=500 ºC); Sample S3 (CR=60 ºC/s, CT=500 ºC); Sample S10 (CR=7 ºC/s, CT=650 ºC); and Sample S12 (CR=60 ºC/s, CT=650 ºC). Only samples S1 and S10, both slowly cooled during rolling, present microstructural banding at 750ºC, more severe in sample S10 initially consisted of ferrite-pearlite bands. Martensite bands did not appear in samples S3 and S12, both rapidly cooled during rolling, at the same soaking temperature. However, all the samples that originally did not present banding (samples S1, S3 and S12), exhibit a banded martensitic microstructure after an intercritical annealing at 800 ºC (See micrographs in Figure 6.89). At this temperature the AI value increases as the transformation proceeds with time, approaching an AI value close to that measured on the initial hot rolled banded ferrite and pearlite microstructure (about 2). This is not surprising since hot rolled pearlite bands roughly resemble original manganese segregation. On the other hand, n` values decrease as transformation proceeds (Figure 6.88.c) since bands of austenite get closer.
95
In general, samples rapidly cooled during hot rolling stage (samples S3 and S12), consisting mainly of bainite and martensite, exhibit the lowest degree of banding after intercritical annealing. It is also remarkable that, annealed samples at 750 ºC for 100 s present much less severe banding problem that those annealed at 800 ºC for 1 s (See micrographs in Figures 20.b and 20.c for comparison), although the volume fraction of austenite formed in both cases is quite similar (around 35%). Therefore, increasing the cooling rate during hot rolling, and using low intercritical annealing temperatures and longer soaking time is possible to permanently eliminate microstructural banding in cold rolled dual phase steels.
Figure 6.89: Evolution of microstructural banding during intercritical annealing in sample S3 (CR=60 ºC/s, CT=500 ºC): a) 750 ºC, 20 s; b) 750 ºC, 100 s; c) 800 ºC, 1 s; and d) 800 ºC, 20 s; 6.7.2
Refinement of micr ostr uctur e
6.7.2.1.ARSA_DP800_1 steel. Dilatometric samples The Evolution of the ferrite and austenite grain size during the intercritical annealing was studied on hot and cold rolled samples in ARSA_DP800_1 steel. Ferrite and austenite grain size was determined on optical micrographs with the help of an image analyser. Results are analysed in terms of mean values of the equivalent circle diameter AGS=(4A/r)1/2 with A the grain area. Austenite grain boundaries were revealed using an etching reagent based on saturated aqueous picric acid plus a wetting agent [Ref 11]. Likewise, ferrite grain boundaries were revealed using a 2 pct Nital solution. Figure 6.90 shows the evolution of the ferrite grain size during the intercritical annealing of a hot rolled sample with a banded ferrite and pearlite initial microstructure (sample S10; cooling rate of 7 ºC/s and coiling temperature of 650 ºC). Results at 850 ºC are not presented since ferrite-to-austenite transformation has been already completed at that temperature. Results suggest that the average ferrite grain size does not increase significantly during intercritical annealing.
96
Ferrite Grain Size (S10)
Relativ e Frequ enc y (% )
35 30 25 20 15 10 5 0
100s
35 30 25 20 15 10 5
2-4
4-6
750 ºC
6-8
8-10
Diameter (
10-12 12-14
>14
0-2
4-6
0.15%
m)
6-8
8-10
Diameter (
D = (8.1± 2.6)om
10-12 12-14
35 30 25 20 15 10 5
35 30 25 20 15 10 5
0
>14
0-2
2-4
4-6
800 ºC
6-8
8-10
10-12 12-14
D = (8.4± 3.5)om
35 30 25 20 15 10 5
>14
0-2
2-4
4-6
36%
Diameter ( m)
6-8
8-10
2-4
4-6
10-12
12-14
8-10
10-12
12-14
>14
32% D = (6.5± 2.4)om
40 35 30 25 20 15 10 5 0 0-2
>14
2-4
4-6
6-8
8-10
Di ameter
76%
Diam eter ( m)
6-8
Diameter ( m)
0
0
0-2
24%
m)
40 Relative Frequenc y (%)
40
2-4
D = (9.2± 3.2)om
40
0 0-2
Relati ve Fr equen cy (%)
D= (9.6± 3.0)om
40
Relati ve Fr equen cy (%)
Relativ e Frequ enc y (%)
20 s
D = (7.1± 3.6)om
Rel ative Fr equ ency (%)
1s 40
CR=7 ºC/s; CT=650 ºC
10-12
12-14
>14
83%
m)
Figure 6.90: Evolution of the ferrite grain size during the intercritical annealing of a hot rolled S10 sample in ARSA_DP800_1 steel. The austenite volume fraction is indicated in red. On the other hand, Figures 6.91-94 show the evolution of the austenite grain size during the intercritical annealing of hot and cold rolled samples with different initial microstructure. The austenite volume fraction is indicated in the graphs. Specimen S1 (7 ºC/s of cooling rate and 500 ºC of coiling temperature) consists of ferrite and some fine randomly dispersed pearlite, less than 5 pct. Specimens S3 and S12 (60 ºC/s of cooling rate) are mainly formed by bainite, martensite and some ferrite. Microstructure of specimen S3 also presents some isolated fine pearlite nodules. Specimen S10 (7 ºC/s of cooling rate and 650 ºC of coiling temperature) consists of a banded ferrite-pearlite microstructure. More precise details of the initial microstructures tested were previously reported.
4-6
6-8
8-10
10-12
0-2
0.44%
Diameter ( m)
50 40 30 20 10
4-6
6-8
8-10
10-12
0
0-2
50 40 30 20 10
4-6
6-8
8-10
10-12
>12
Rel ati ve Fr equ en cy (%)
50 40 30 20 10
4-6
6-8
8-10
10-12
0-2
850 ºC
2-4
4-6
6-8
8-10
Diameter ( m)
10-12
>12
100%
8-10
10-12
>12
28%
50 40 30 20 10
40 30 20 10
2-4
4-6
6-8
8-10
10-12
>12
90%
Diameter ( m)
2-4
4-6
6-8
8-10
Diameter ( m)
10-12
>12
100%
2-4
4-6
6-8
8-10
>12
100 90 80 70 60 50 40 30 20 10 0
2-4
4-6
6-8
8-10
10-12
0-2
40 30 20 10
4-6
6-8
8-10
Diameter ( m)
10-12
4-6
6-8
8-10
0-2
70 60 50 40 30 20 10
0-2
>12
100%
850 ºC
(a)
2-4
4-6
6-8
8-10
Diameter ( m)
10-12
>12
100%
10-12
>12
2-4
4-6
6-8
100 90 80 70 60 50 40 30 20 10 0
0-2
2-4
8-10
10-12
>12
Diameter ( m)
4-6
6-8
D ? *1 . 0 ‒ 0 . 3 +o m
8-10
0-2
2-4
4-6
6-8
8-10 8-10
50 40 30 20 10 0
0-2
2-4
4-6
6-8
8-10
Diameter ( m)
10-12
>12
100%
10-12 10-12
33%
>12 >12
84%
Diameter ( m)
D ? *3 . 6 ‒ 1 . 3 +o m 60
>12
D ? *1 . 9 ‒ 0 . 9 +o m
100 90 80 70 60 50 40 30 20 10 0
77%
70
10-12
Diameter ( m)
30% D ? *1 . 6 ‒ 0 . 7 +o m
100 90 80 70 60 50 40 30 20 10 0
36%
0
2-4
2-4
Diameter ( m)
>12
Diameter ( m)
100 s
D ? *0 . 8 ‒ 0 . 3 +o m
D ? *3 . 0 ‒ 1 . 1 +o m
50
0-2
100 90 80 70 60 50 40 30 20 10 0
9% D ? *1 . 2 ‒ 0 . 5 +o m
0-2
D ? (5 . 9 ‒ 2 . 9 ) o m
60
10-12
Diameter ( m)
800 ºC
0 0-2
0-2
750 ºC
50
0-2
D ? ( 5 .8 ‒ 2 .6 ) o m
60
D ? *0 . 8 ‒ 0 . 2 +o m
D ? ( 5 .1 ‒ 2 .0 ) o m
87%
0
0
6-8
60
>12
Diameter ( m)
D ? ( 5 .3 ‒ 2 .7 ) o m
60
2-4
28%
Diameter ( m)
4-6
0
0-2
Rel ati ve Fr equ en cy (%)
800 ºC
2-4
2-4
Diameter ( m)
0
0-2
10
20%
D ? (5 .2 ‒ 2 .1) o m
60
20
0
>12
Diameter ( m)
D ? (3 . 9 ‒ 1 . 5 ) o m
60
2-4
30
100 90 80 70 60 50 40 30 20 10 0
Rel ati ve Fr equ en cy (%)
10
>12
Rel ati ve Fr equ en cy (%)
2-4
Relati ve Fr equ en cy (%)
Rel ati ve Fr equ en cy (%)
750 ºC
Rel ati ve Fr equ en cy (%)
20
0
0-2
40
Relative Fr equ en cy (%)
0
D ? ( 4 .0 ‒ 1 .6 ) o m
50
D ? *3 . 8 ‒ 1 . 5 +o m Rel ati ve Fr equ en cy (%)
10
30
60
Rel ati ve Fr equ en cy (%)
20
40
CR=7 ºC/s; CT=500 ºC
20 s
Rel ati ve Fr equ en cy (%)
30
50
1s Rel ati ve Fr equ en cy (%)
40
Austenite Grain Size (S20 CR)
Relati ve Fr equ en cy (%)
50
100 s
D ? ( 3 .6 ‒ 1 .6 ) o m
Relative Fr equ en cy (%)
60 Rel ati ve Fr equ en cy (%)
60 Relati ve Fr equ en cy (%)
CR=7 ºC/s; CT=500 ºC
20 s
D ? ( 2 .3 ‒ 0 .9 ) o m
Rel ati ve Fr equ en cy (%)
1s
Rel ati ve Fr equ en cy (%)
Austenite Grain Size (S1)
70 70 60 60 50 50 40 40 30 30 20 20 10 10 00
0-2 0-2
2-4 2-4
4-6 4-6
6-8 6-8
8-10 8-10
Diameter ( m)
10-12 10-12
>12 >12
100%
(b)
Figure 6.91: Evolution of the austenite grain size during the intercritical annealing of (a) hot rolled and (b) cold rolled S1 sample in ARSA_DP800_1 steel
97
10 10 00
0-2 0-2
2-4 2-4
4-6 4-6
0.45%
Diameter (( m) m) Diameter
50 50 40 40 30 30 20 20 10 10
800 ºC
4-6 4-6
6-8 6-8
8-10 8-10
10-12 10-12
50 50 40 40 30 30 20 20 10 10 00
>12 >12
0-2 0-2
31%
Diameter (( m) m) Diameter
2-4 2-4
4-6 4-6
40 40 30 30 20 20
10 10
850 ºC
6-8 6-8
0-2 0-2
2-4 2-4
4-6 4-6
8-10 8-10
8-10 8-10
10-12 10-12
10-12 10-12
30 30 20 20
10 10 4-6 4-6
100%
Diameter (( m) m) Diameter
2-4
4-6
750 ºC
25% 25%
40 40 30 30 20 20 10 10
0-2 0-2
2-4 2-4
4-6 4-6
6-8 6-8
8-10 8-10
10-12 10-12
6-8
8-10
10-12
0-2
>12
2-4
4-6
1.3%
Diameter ( m)
D ? *0 . 7 ‒ 0 . 2 +o m
100 90 80 70 60 50 40 30 20 10 0
0-2
>12 >12
2-4
4-6
800 ºC
91% 91%
Diameter (( m) m) Diameter
6-8 6-8
8-10 8-10
10-12 10-12
D ?? *55..88 ‒‒ 22..99+oom m D
60 60
40 40
2-4 2-4
0-2
>12 >12
50 50
85% 85% D ?? *55..33 ‒‒ 22..77+oom m D
0-2 0-2
10-12 10-12
D ?? **55..22 ‒‒ 22..55++oom m D
60 60
00
>12 >12
50 50
>12 >12
8-10 8-10
6-8
8-10
10-12
>12
0-2
2-4
4-6
26%
Diameter ( m)
6-8
8-10
10-12
>12
30%
Diameter ( m)
6-8
8-10
10-12
D ? *1 . 1 ‒ 0 . 7 +o m
100 90 80 70 60 50 40 30 20 10 0
0-2
>12
2-4
4-6
30%
Diameter ( m)
6-8
8-10
10-12
>12
D ? *1 . 6 ‒ 1 . 0 +o m
100 90 80 70 60 50 40 30 20 10 0 0-2
2-4
87%
Diameter ( m)
4-6
6-8
8-10
10-12
>12
91%
Diameter ( m)
9S3HR
60 60
00
6-8 6-8
Diameter (( m) m) Diameter
Relativ Relativ Frequ enc (% RelativeeeFrequ Frequenc encyyy(% (%)))
50 50
4-6 4-6
6-8 6-8
Diameter (( m) m) Diameter
Rel Rel ati ve Fr equ en cy (%) Relati ative ve Fr Frequ equen ency cy (%) (%)
Rel Rel ati ve Fr equ en cy (%) Relati ative ve Fr Frequ equen ency cy (%) (%)
60 60
2-4 2-4
10 10
8S3HR D ?? *55..11 ‒‒ 22..33+oom m D
0-2 0-2
20 20
19% 19%
D ? *4.7 ‒ 2.0 +om
7S3HR
00
30 30
00
>12 >12
D ? *0 . 8 ‒ 0 . 2 +o m
100 90 80 70 60 50 40 30 20 10 0
6S3HR 6S3HR
60 60
Rel Rel ati ve Fr equ en cy (%) Relati ative veFr Frequ equen ency cy (%) (%)
Rel Rel ati ve Fr equ en cy (%) Relati ative ve Fr Frequ equen ency cy (%) (%)
D ?? **44..00 ‒‒ 11..55++o om m D
2-4 2-4
10-12 10-12
5S3HR 5S3HR
60 60
0-2 0-2
8-10 8-10
Diameter ( m)
4S3HR 4S3HR
00
6-8 6-8
Rel ati ve Fr equ en cy (%)
20 20
Rel ati ve Fr equ en cy (%)
30 30
>12 >12
Rel ati ve Fr equ en cy (%)
10-12 10-12
100 s
D ? *0 . 8 ‒ 0 . 2 +o m
100 90 80 70 60 50 40 30 20 10 0
50 50 40 40 30 30 20 20 10 10
0-2 0-2
2-4 2-4
4-6 4-6
100% 100%
Diameter (( m) m) Diameter
60
6-8 6-8
8-10 8-10
10-12 10-12
50 40 30 20 10
4-6
850 ºC
100% 100%
Diameter (( m) m) Diameter
2-4
6-8
8-10
10-12
50 40 30 20 10
50 40 30 20 10 0
0-2
>12
2-4
4-6
100%
Diameter ( m)
D ? *3 . 2 ‒ 1 . 3 +o m
60
0
0-2
>12 >12
D ? *3 . 3 ‒ 1 . 3 +o m
60
0
00
>12 >12
D ? *2 . 6 ‒ 1 . 2 +o m
Relativ e Frequ enc y (% )
8-10 8-10
40 40
D ? *0 . 6 ‒ 0 . 2 +o m
100 90 80 70 60 50 40 30 20 10 0
Rel ati ve Fr equ en cy (%)
6-8 6-8
50
CR=60 ºC/s; CT=500 ºC
20 s
Relativ e Frequ enc y (% )
750 ºC
4-6 4-6
D ?? **33..11‒‒ 11..22++oom m D
60 60
Rel ati ve Fr equ en cy (%)
2-4 2-4
40 40
Rel Rel ati ve Fr equ en cy (%) Relati ative veFr Frequ equen ency cy (%) (%)
0-2 0-2
D ?? **33..77 ‒‒ 11..22++oom m D
50 50
Rel ati ve Fr equ en cy (%)
40 40 30 30 20 10 10
1s
3S3HR 3S3HR
60 60
R R el ati ve Fr equ en cy (%) Rel elati ative veFr Frequ equen ency cy (%) (%)
R R el ati ve Fr equ en cy (%) Rel elati ative veFr Frequ equen ency cy (%) (%)
60 60 50 50
00
100 s
2S3HR 2S3HR
D ?? **11..77 ‒‒ 00..55++o om m D
90 90 80 80 70 70
Austenite Grain Size (S22 CR)
CR=60 ºC/s; CT=500 ºC
20 s
Rel Rel ati ve Fr equ en cy (%) Relati ative veFr Frequ equen ency cy (%) (%)
1s
1S3 HR HR 1S3
Rel ati ve Fr equ en cy (%)
Austenite Grain Size (S3)
6-8
8-10
10-12
0-2
>12
2-4
100%
Diameter ( m)
4-6
6-8
8-10
10-12
>12
100%
Diameter ( m)
(b)
(a)
Figure 6.92: Evolution of the austenite grain size during the intercritical annealing of (a) hot rolled and (b) cold rolled S3 sample in ARSA_DP800_1 steel
20 15 10
30 25 20 15 10 5
5
2-4
4-6
6-8
8-10 10-12 >12
0-2
Diameter ( m)
750 ºC
4-6
0.15% D ? *5.5 ‒ 2. 3+om
40 35 30 25 20 15 10
4-6
800 ºC
6-8
Diameter (
8-10
10-12
10-12
2-4
4-6
24%
30 25 20 15
>12
0-2
>12
D ? *7.3 ‒ 2.7+om
35
6-8
8-10
10-12
>12
0-2
32%
Diameter ( m)
35
25 20 15
5 0
5 2-4
4-6
6-8
8-10
10-12
Diameter ( m)
>12
6-8
8-10 10-12
Diameter
2-4
4-6
76%
6-8
8-10
10-12
20 10
10
25 20 15 10
5
35 30 25 20 15
2-4
4-6
6-8
8-10
Diameter
5 0-2
850 ºC
2-4
4-6
6-8
8-10
10-12
0-2
>12
2-4
4-6
88%
Diameter ( m)
6-8
8-10
10-12
100%
Diameter ( m)
4-6
6-8
Diameter
8-10
10-12
>12
100%
m)
50 40 30 20 10 0 0-2
20 10 4-6
6-8
Diameter
8-10
10
10-12
8-10
4-6
6-8
8-10
Diameter
>12
30% D = (2.8 ± 1.1) om
50 40 30
10-12
20 10 0-2
>12
2-4
84%
m)
4-6
6-8
8-10
60
91% D = (4.7 ± 1.8) om
60
50 40 30 20 10
0-2
2-4
4-6
6-8
8-10
10-12
50 40 30 20 10 0
>12
0-2
2-4
4-6
100%
m)
Diameter
10-12 >12
m)
Di ameter
D = (4.2 ± 1.7) om
>12
10-12
m)
0 2-4
97%
m)
6-8
60
0 2-4
4-6
Diameter
20
31%
m)
2-4
23%
m)
D = (2.6 ± 1.1) om
0-2
30
0-2
2-4
70 60
>12
30
0
0-2
10-12
40
D = (3.8 ± 1.4)om
850 ºC
0
>12
8-10
50
>12
40
5
0
0
10-12
50
10
6-8
60
Relativ e Frequ enc y (%)
15
30
4-6
D = (1.3 ± 0.6) om
90 80
0
800 ºC
D ? *8.3 ‒ 3.2+om
40
35
2-4
Diameter
30
0-2
>12
83%
Diameter ( m)
Relative Frequency (% )
20
10 0
0
0-2
D ? *8.0 ‒ 3. 0+om
40 Relative Frequency (% )
Relative Frequency (% )
25
30 20
0-2
40
Relativ e Frequ enc y (%)
D ? *7.3 ‒ 2.4+om
35 30
50 40
D = (2.1 ± 0.9) om
60
40
70 60
4.5%
m)
100 s
D = (1.3 ± 0.6) om
90 80
>12
50
0 0-2
4-6
60
30
10
2-4
750 ºC
D ? *8.2 ‒ 2. 6+om
40
10
36%
m)
8-10
40
0 2-4
6-8
Diameter ( m)
5 0-2
2-4
D = (1.4 ± 0.5) om
90 80 70 60 50 40 30 20 10 0
0
0
Relative Frequency (% )
0-2
D ? *5..9 ‒ 2. 3+om
35
Relativ e Frequ enc y (% )
25
Rel ati ve Fr equ en cy (%)
10 0
Relative Frequency (% )
35 30
Relativ e Frequ enc y (%)
20
D ? *5. 2‒ 2.3+om
40
CR=7 ºC/s; CT=650 ºC
20 s Relativ e Frequ enc y (% )
30
1s Relativ e Frequ enc y (% )
50 40
40
Relativ e Frequ enc y (% )
D ? *2...3 ‒ 0.9+om Relative Frequency (% )
60
Relative Frequency (% )
Relative Frequency (% )
70
Austenite Grain Size (S29 CR)
CR=7 ºC/s; CT=650 ºC 100 s
20 s
Relative Frequency (% )
1s
Relativ e Frequ enc y (% )
Austenite Grain Size (S10)
6-8
Diameter
8-10 10-12
>12
100%
m)
(b)
(a)
Figure 6.93: Evolution of the austenite grain size during the intercritical annealing of (a) hot rolled and (b) cold rolled S10 sample in ARSA_DP800_1 steel
Diameter
8-10
10-12
>12
2-4
0. 12%
m)
4-6
6-8
Diameter
8-10
30 20 10 0
0-2 0-2
50 40 30 20 10
4-6
6-8
Di ameter
8-10
10-12
>12
50 40 30 20 10
4-6
6-8
8-10
10-12
0-2
850 ºC
2-4
4-6
6-8
Di ameter
8-10 10-12 m)
>12
100%
27%
D ? *5.0 ‒ 1.8 +o m
40 30 20 10
>12
0-2
2-4
50 40 30 20 10
4-6
6-8
Di ameter
D ? *5.9 ‒ 2.2 +om
8-10
10-12
50 40 30 20 10
0-2
2-4
4-6
6-8
Di ameter
8-10 m)
10-12
>12
100%
4-6 4-6
6-8 6-8
Diameter ameter Di
8-10 10-12 10-12 8-10 m) m)
>12 >12
2-4 2-4
4-6 4-6
6-8 6-8
Diameter ameter Di
8-10 10-12 10-12 8-10
>12 >12
m) m)
37% D ?? *33..00 ‒‒11..33+oom m D
70 70 60 60 50 50 40 40 30 30 20 20 10 10 00
0-2
2-4
4-6
6-8
Di ameter
8-10 m)
10-12
>12
850 ºC
100%
2-4 2-4
4-6 4-6
6-8 6-8
Diameter ameter Di
8-10 10-12 10-12 8-10 m) m)
>12 >12
90%
100 100
Rel ati ve Fr equ en cy (%)
2-4 2-4
4-6 4-6
6-8 6-8
Diameter Diameter
8-10 10-12 10-12 8-10
D ?? **00..88‒‒00..22++oom m D
90 90 80 80 70
70 60 60
50 50 40 40 30 30 20 20 10 10
D ?? **22..33‒‒11..11++oom m D
100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 00 0-2 0-2
2-4 2-4
4-6 4-6
6-8 6-8
Diameter ameter Di
8-10 10-12 10-12 8-10 m) m)
2-4 2-4
50 50 40 40 30 30 20 20 10 10
6-8 6-8
8-10 8-10 m) m)
10-12 10-12
>12 >12
31% D ?? **22..55‒‒11..22++oom m D
100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 00
0-2 0-2
2-4 2-4
80%
60 60
4-6 4-6
Diameter Diameter
>12 >12
D ?? **33..66 ‒‒11..44++oom m D
70 70
0-2 0-2
>12 >12
24%
m) m)
4-6 4-6
6-8 6-8
8-10 10-12 10-12 >12 >12 8-10
Diameter ameter m) m) Di
86%
D ?? **44..77‒‒11..66++oom m D
70 70 60 60 50 50 40 40 30 30 20 20 10 10 00
00
0-2 0-2
100 s
D ?? **00..77 ‒‒ 00..22++oom m D
00
0-2 0-2
2.7% D ?? *11..11‒‒ 00..55+oom m D
0-2 0-2
D ? *6.4 ‒ 2.2+om
60
2-4 2-4
100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 00
800 ºC
>12
94%
m)
0
0
0
m) m)
0-2 0-2
750 ºC
>12 >12
50
88%
m)
60 Relativ e Frequ enc y (% )
60
2-4
Di ameter
D ? *5.3 ‒ 2.0 +om
8-10 8-10 10-12 10-12
0
0-2
34%
m)
6-8 6-8
60
Relativ e Frequ enc y (% )
800 ºC
2-4
4-6 4-6
Di Diameter ameter
0 0-2
2-4 2-4
23% Relativ e Frequ enc y (% )
Relativ e Frequ enc y (% )
40
10 10
>12
D ? *4.3 ‒ 1.7 +om
60
50
10-12
m) m)
D ? *3.1 ‒ 1.3+om
60
20 20
00
0-2
20 s 100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 00
Relative Frequency (%)
6-8
CR=60 ºC/s; CT=650 ºC
Relative Frequen cy (%)
4-6
Rel ati ve Fr equ en cy (%)
2-4
30 30
D ?? *00..66 ‒‒ 00..22+oom m D
Rel ati ve Fr equ en cy (%)
10
40 40
100 100 90 90 80 80 70 70 60 60 50 50 40 40 30 30 20 20 10 10 00
Re lative Frequency (%)
20
Rel ati ve Fr equ en cy (%)
30
0
750 ºC
Relativ e Frequ enc y (% )
40
1s
50 50
Relative Frequency (%)
20 10 0
50
D ? *2.1 ‒ 1.1+om
60 60 Relativ e Frequ enc y (% )
40 30
100 s
D ? *2.5 ‒ 0.9 +om
60
0-2
Relat iv e Frequ enc y (% )
20 s
D ? *1.5 ‒ 0.4 +om Relativ e Frequ enc y (% )
Relativ e Frequ enc y (% )
1s 90 80 70 60 50
Austenite Grain Size (S31 CR)
CR=60 ºC/s; CT=650 ºC
Relative Frequency (%)
Austenite Grain Size (S12)
0-2 0-2
2-4 2-4
4-6 4-6
6-8 6-8
Diameter ameter Di
8-10 10-12 10-12 8-10 m) m)
>12 >12
100%
0-2 0-2
2-4 2-4
4-6 4-6
6-8 6-8
8-10 10-12 10-12 >12 >12 8-10
m) Diameter ameter m) Di
100%
(b)
(a)
Figure 6.94: Evolution of the austenite grain size during the intercritical annealing of (a) hot rolled and (b) cold rolled S12 sample in ARSA_DP800_1 steel
98
Results on the evolution of the AGS during austenite formation in Figure 6.95 indicate that the initial microstructure slightly affects the final AGS. It seems that samples cooled at 60 ºC/s and/or coiled at 500 ºC during hot rolling always exhibited finer austenite grain during annealing than those initially cooled at 7 ºC/s and coiled at 650 ºC. Moreover, bearing in mind that the number of austenite nuclei at the beginning of transformation is higher in the cold rolled material than in the hot rolled material, it is not surprising that cold rolled samples present finer austenite grain during transformation than those hot rolled. Beside their different nucleation kinetics, significant differences on the rate of transformation were not found in hot and cold rolled samples. It seems that the number of nuclei at the beginning of transformation is lower in hot rolled material, but they grow faster than those in cold rolled material. Therefore, deformation during cold rolling does not play an important role on austenite kinetics formation, but it does in the final austenite grain size.
(a)
(b)
Figure 6.95: Evolution of the austenite grain size during austenite formation of (a) hot rolled and (b) cold rolled samples in ARSA_DP800_1 steel.
6.7.2.2.VA_DP600_1 steel Cold and annealed specimens of VA_DP600_1 steel (see Table 6.30 and 6.31 for process conditions), longitudinal and transverse to the hot rolling direction were ground and polished using standardised techniques for metallographic examination. A 2 pct Nital etching solution was used to reveal the microstructure by optical and electron microscopy. Cold rolled and annealed samples present a microstructure consisting of ferrite, martensite and some bainite, as shown in Figure 6.95 bis. The volume fraction of bainite and martensite was estimated by a systematic manual point counting procedure on electron micrographs. The average diameter of martensite grains, dm, was measured on electron micrographs by means of the intercept method. The results listed in Table 6.32 suggest that the coiling temperature does not significantly affect the annealing microstructure. Table 6.30: Hot and cold rolling parameters of VA_DP600_1 steel Samples
SRT (°C)
RghT (ºC)
FT (ºC)
CTA (ºC)
CTM (ºC)
CTE (ºC)
3526 3525 3523
1254 1249 1253
1115 1117 1125
894 894 891
616 556 526
601 554 499
605 553 543
HS thickness (mm) 2.70 2.69 2.70
CS thickness (mm) 1.15 1.15 1.15
SRT: slab reheating temperature; RghT: temperature after roughening; FT: finishing temperature; CTA: coiling temperature head end; CTM: coiling temperature middle; CTE: coiling temperature tail end; HS thickness: hot strip thickness; CS thickness: cold strip thickness
99
Table 6.31: Laboratory hot dip galvanizing (HDG) and simulations in the studied steels HDG conditions Steel Ts, ºC ts, s Tjet, ºC Tzn, ºC t, s VA_DP600_1 792 65 501 458 40 Table 6.32: Characterisation of cold rolled and annealed samples of VA_DP600_1 steel Specimen Vm Vb dm (om) 3523
0.11 ‒ 0.04
0.04 ‒ 0.01
2.2 ‒0.2
3525
0.09 ‒ 0.05
0.04 ‒ 0.02
2.3 ‒ 0.3
3526
0.11 ‒ 0.04
0.02 ‒ 0.01
1.8 ‒ 0.2
Hv- 5kg = 167
2
Hv- 5kg = 168
7
Hv- 5kg = 168
8
a) 3523. LOM
b) 3523. SEM
c) 3525. LOM
e) 3526. LOM
d) 3525. SEM
f) 3526. SEM
Figure 6.95 bis: Optical and electron micrographs of cold rolled and annealed samples of VA_DP600_1 steel in transverse direction. 2 pct Nital etching solution
100
6.8
Effect of hot rolled microstructure on the final product: tensile properties
6.8.1
Low Cr DP 800 steel (ARSA_DP800_1 et 2)
For pilot simulation, the hot rolling parameters are the followings: Reheating T: 1245°C - 45 min (Argon atmosphere in the furnace) Finish Rolling T > 900°C Cooling Rate ~20°C/s Coiling T 600°C, 400°C, 120°C - 90 minutes then air cooling After final cooling, the strips are cold rolled with a cold rolling level of 67%. Finally, annealing simulation are performed. The heating rate is 5°C/s. For 800°C soaking temperature, three holding times were tested: 30, 60 and 90s. For the 30s holding time, two other soaking temperatures were tested (770°C and 830°C). After soaking, the cooling rate applied is 20°C/s up to room temperature. In tables 6.33 and 6.34, the mechanical properties at the cold rolled and annealed stage are presented. Major results can be summarized as follows: For both DP steels, there is a clear loss of ductility for a 400°C coiling temperature. Figure 6.96 illustrates this point for AR_DP800_1 steel annealed for different times at 800°C. This applies also to 770°C annealing treatment and 830°C annealing treatment. Mechanical properties show also an interest of very low coiling temperature. For an equivalent ductility, an increase in ultimate tensile strength is measured. The effect is more important for AR_DP800_1 steel (+50MPa) than for AR_DP800_2 steel (10-20MPa). It applies only for the lowest annealing temperature (770°C and 800°C) Table 6.33: Mechanical properties of cold rolled and annealed simulation AR_DP800_1 steel. Nuance
Temp. Coil.
600°C
YS (MPa)
UTS (MPa)
El%
U. El %
P%
r
n
YS/TS
800°C 30s
383
867
16,5
13,5
0
0,87
0,201
0.44
17
800°C60s
409
881
16,1
12,8
0
0,80
0,193
0.46
16
800°C 90s
414
897
15,6
12,4
0
0,80
0,194
0.46
18
770°C 30s
407
921
14,0
11,7
0
0,68
0,209
0.44
830°C 30s
436
884
16,3
12,8
0
0,80
0,186
0.49
800°C 30s
397
846
12,4
9,5
0
0,85
0,198
0.47
22
0,196
0.48
23 23
Annealing Cycle
800°C 60s DP3
400°C
120°C
403
846
11,4
8,8
0
0,83
%M
800°C 90s
398
844
11,7
9,3
0
0,70
0,192
0.47
770°C 30s
392
882
11,0
9,2
0
0,59
0,205
0.44
830°C 30s
434
871
12,8
10,4
0
0,63
0,185
0.50
800°C 30s
411
924
15,8
12,4
0
0,80
0,196
0.44
15 21 20
800°C 60s
429
933
16,5
12,5
0
0,96
0,189
0.46
800°C 90s
452
948
14,5
11,9
0
0,86
0,182
0.48
770°C 30s
416
948
13,9
12,0
0
0,80
0,179
0.44
830°C 30s
460
928
14,5
11,9
0
0,82
0,198
0.50
101
G
Table 6.34: Mechanical properties of cold rolled and annealed simulation AR_DP800_1 steel. Nuance
Temp. Coil.
600°C
DP4
400°C
YS (MPa)
UTS (MPa)
800°C 30s
300
695
23,6
16,9
800°C 60s
293
696
22,6
15,9
800°C 90s
304
708
22,3
15,7
Annealing Cycle
U. El %
P%
r
n
YS/TS
%M
G
0
1,07
0
1,10
0,188
0.43
12
12
0,184
0.42
13
0
1,11
12
0,179
0.43
15
11
770°C 30s
297
706
21,7
16,1
0
0,98
0,186
0.42
830°C 30s
323
712
23
16,1
0
1,02
0,181
0.45
800°C 30s
300
687
21,6
16,3
0
0,90
0,188
0.43
14
13
800°C 60s
302
700
20,6
15,3
0
0,93
0,177
0.43
15
13
800°C 90s
307
707
21,3
15,0
0
0,93
0,174
0.43
15
13
770°C 30s
394
697
18,8
15,4
0
0,97
0,189
0.56
830°C 30s
319
710
22,2
16,0
0
1,01
0,181
0.45
0,195
0.47
13
15
15
14
15
14
800°C 30s
180°C
El%
331
700
23,3
17
0
1,05
800°C 60s
310
717
20,5
16,7
0
0,99
0,181
0.43
800°C 90s
310
724
19,9
15,2
0
1,10
0,181
0.43
770°C 30s
315
720
22,7
16,6
0
0,92
0,193
0.44
830°C 30s
319
710
22,2
16,0
0
0,94
0,186
0.45
950
UTS (MPa)
930 910 890
Coiling T 600°C
870
Coiling T 400°C
850 830
X
X
X
30s
60s
90s
Coiling T 120°C
810 790
Time (s) 20
El%
18 16 14 12
X X
X
10 30s
60s
90s
time (s)
Figure 6.96: Effect of coiling temperature on the strength and ductility of cold rolled and annealed AR_DP800_1 steel. Annealing temperature: 800°C
102
6.8.2
Si added TRIP steels (AR_TRIP800_1)
For pilot simulation, the hot rolling parameters are the followings: Reheating temperature 1200°C, 45 mn. Finish rolling temperature 900°C Coiling 550°C (industrial route), 400°C, 20°C After final cooling, the strips are cold rolled with a cold rolling level of 70%. Finally, annealing simulation are performed with the following process parameters: Heating rate Soaking Cooling rate Overageing
10°C/s 800°C, 90s 20°C/s 400°C during 10s, 100s et 500s
In table 6.35, the effect of coiling temperature on the mechanical properties and on the microstructure is reported. The results are plotted on figure 6.97 to depict the effect of overageing time during annealing on the mechanical properties. Table 6.35: Microstructural characterization and mechanical properties of cold rolled and annealed simulation TRIP 3 steel. Trajet Ref
Coiling 550°C + CR + Anneal.
1
Coiling 400°C + CR + Anneal.
2
Coiling 20°C + CR Anneal.
toa s) YS (MPa) UTS (MPa) 10 360 874 100 365 816 500 416 749 10 377 897 100 370 819 500 413 771 10 387 913 100 366 826 500 439 777
El% 16,1 23,4 20,5 13,5 20,3 19,9 11,5 19,9 16,6
UEl% 13,1 18 15,9 11,3 16,6 15,5 9,3 15,4 14,8
TS.El% %RA %M %B 14071 8 16 1 19094 12 5 3 15355 16 0 4 12110 8 15 2 16626 14 5 5 15343 16 1 7 10500 7 18 2 16437 16 0 3 12898 16 0 5
The main conclusions of these experiments are: ‚ Concerning the effect of overageing time: a better compromise is obtained for 100s, which corresponds to optimized austenite enrichment in carbon and stabilisation of this constituent. ‚ Concerning the effect of hot rolled microstructure (coiling temperature) : A slight degradation of the optimum product UTSxEl is observed when the coiling temperature is decreased. This degradation is mainly due to a loss in ductility. The ultimate tensile strength being independent on the hot rolled microstructure. 25
900
Bob400°C
20
800
Bob20°C
700 0
200
400
TEl%
Bob550°C Référence
TS (MPa)
1000
15 10 0
600
Time at 400°C (s)
200
400
600
Time at 400°C (s)
Figure 6.97: Effect of coiling temperature on the strength and ductility of cold rolled and annealed TRIP 3 steel.
103
6.8.3
High Cr DP600-800 steels
6.8.3.1.Impact of coiling temperature on mechanical properties of cold rolled and annealed material (fully industrial produced), heat #1 (va_DP600_1) The characterization of the mechanical properties of the fully industrial produced cold rolled and hot dip galvanized material showed only a small dependency from the coiling temperature (Figure 6.98). 600
Rp0.2; Rm [MPa]
550
Rp02 Rm
500
300
540
560
580
600
620
640
CT [°C]
a) Yield point (Rp0.2) and Tensile Strength (Rm) 30 28
elongation [%]
26
Ag A
24 22 20 18 16 540
560
580
600
620
640
CT [°C]
b) Uniform (Ag) and Total Elongation (A80) Figure 6.98: Mechanical properties of the hot dip galvanized strips versus local coiling temperature, longitudinal samples The microstructure of the hot dip galvanized samples was investigated after Le Pera etching. As in the hot rolled samples almost no banded structure can be seen (Figure 6.99). The martensite grains show a slight banded formation.
104
a) BdNr.: 563526, CTlocal=610°C
c) BdNr.: 563523, CTlocal=540°C
b) BdNr.: 563525, CTlocal=550°C
Figure 6.99: Microstructure of the cold rolled and annealed strips versus local coiling temperature, longitudinal samples, Le Pera etching
6.8.3.2.Impact of coiling temperature on mechanical properties of cold rolled and annealed material (laboratory hot&cold rolling and annealing), heat #2 (va_DP600_2) From the laboratory hot rolled samples (CRM hot rolling) cold rolling was performed at voestalpine. After laboratory cold rolling with a cold reduction of approximately 63% a standard hot dip galvanizing and continuous annealing cycle was applied using MULTIPAS at voestalpine (annealing parameters given in Figure 6.100).
105
TS, tS
TS, tS
CRSC TQ
TGJC
Temperature
Temperature
CRGJ
TZinc
HR
CRRJ HR tOA
TOE
TOS
CRFJ
Time
HDG Simulation Time TS soaking temperature 800°C TGJC gas jet temperature 470°C TZinc zinc temperature 465°C
CA Simulation TS soaking temperature 800°C TQ quench temperature 680°C TOS overageing start temperature 270°C TOE overageing end temperature 270°C
HR heating rate 5 Ks-1 CRGJ cooling rate gas jet -20 Ks-1 CRFJ cooling rate final jet -6 Ks-1
HR heating rate 5 Ks-1 CRSJ cooling rate slow jet 10 Ks-1 CRRJ cooling rate rapid jet 40 Ks-1
tS soaking time 100 s tGJZn time exit gas jet - zinc pot 40 s
tS soaking time 100 s tOA overageing time 400 s
Figure 6.100: cycles for annealing simulation The mechanical properties of the full laboratory processed material was tested, the results are given in Table 6.36. The mechanical properties of the industrial hot dip galvanized strip are given, too. The properties of the laboratory produced and industrial produced samples show a good correspondence. Only the yield point of the industrial produced strip is significantly higher due to skin passing during industrial production; the mechanical properties of the laboratory produced samples were tested without prior skin passing. On this low grade DP steel, there is no effect of coiling temperature and cooling rate after hot rolling on HDG or CA strip mechanical properties.
106
Table 6.36: Mechanical property of full laboratory processed strips and industrial produced strip, heat #2 sample
rem
HT
CR
°C
K/s
cycle
LL
a
Rp02
Rm
Ag
A
r10
r18
n4-6
n10-18
mm
MPa
MPa
%
%
1
1
1
1
2567
air cooling
600°C
2.5
HDG
L
1.35
246
498
19.0
27.3
0.95
0.84
0.258
0.195
2568
boiling water
600°C
15
HDG
L
1.32
254
502
18.9
27.2
0.93
0.82
0.260
0.197
2607
UFC 1
500°C
50
HDG
L
1.40
250
506
18.9
27.0
0.92
0.80
0.250
0.189
2608
UFC 1
600°C
50
HDG
L
1.43
254
506
18.8
26.8
0.90
0.80
0.248
0.187
2605
UFC 2
500°C
300
HDG
L
1.39
246
501
17.3
24.7
0.90
2606
UFC 2
600°C
300
HDG
L
1.37
258
510
19.3
27.8
0.94
0.87
0.255
0.252 0.193
584401
industrial
600
100
HDG
L
1.16
284.8
508
19.2
31.9
0.96
0.92
0.221
0.178
2568
boiling water
600°C
15
CA
L
1.32
240
538
19.1
27.5
0.92
0.86
0.253
0.192
2607
UFC 1
500°C
50
CA
L
1.40
242
545
18.2
25.9
0.88
0.78
0.245
0.185
2608
UFC 1
600°C
50
CA
L
1.41
240
544
20.3
31.1
0.94
0.92
0.245
0.187
2605
UFC 2
500°C
300
CA
L
1.33
246
549
19.0
26.8
0.90
0.83
0.239
0.181
2606
UFC 2
600°C
300
CA
L
1.42
243
533
16.3
22.9
0.93
0.242
6.8.3.3.Annealing simulations of laboratory hot rolled samples from heat #3 (VA DP800 1) To further study the impact of finishing and coiling temperature on the mechanical properties and microstructure of cold rolled and annealed material, laboratory simulation of a typical hot dip galvanizing (HDG) and a continuous annealing (CA) cycle was performed (cycles see Figure 6.100) on laboratory cold rolled samples of higher DP grade, thickness 1.2 mm (cold reduction rate: ~67%). After annealing the mechanical properties were tested and the microstructure was investigated, including the AI and Y12 evaluation. Results are given in Figure 6.101 and Figure 6.102. The mechanical properties show an interesting dependency from coiling temperature: With decreasing coiling temperature increasing yield and tensile strength can be observed, elongation is constant or decreases only a little.
107
1100
800
HDG
750
0,75 0,70
1050
700
0,65
FT=800°C FT=850°C
600 550 500
Rp0,2 / Rm [1]
1000
Rm [MPa]
Rp0,2 [MPa]
650
950
900
450
FT=800°C FT=850°C
850 400 350 500
550
600
650
700
0,50 0,45
FT=800°C FT=850°C
0,35 450
500
HT [°C]
550
600
650
700
450
500
16
16
0,22
14
14
0,20
12
12
0,18
10
10
6 4
600
8 6
FT=800°C FT=850°C
0,12
FT=800°C FT=850°C
0,10
FT=800°C FT=850°C
2
0
700
0,14
4
2
650
0,16
n 46 [1]
8
550
HT [°C]
HT [°C]
A 80 [%]
A g [%]
0,55
0,40
800 450
0,60
0,08
0 450
500
550
600
650
700
450
500
HT [°C]
550
600
650
700
450
500
HT [°C]
550
600
650
700
HT [°C]
a) HDG simulation 1100
800
CAL
750
0,75 0,70
1050
700
0,65
Rm [MPa]
Rp0,2 [MPa]
600 550 500
Rp0,2 / Rm [1]
1000
650
950
900
450 400
FT=800°C FT=850°C
850
FT=800°C FT=850°C
350 500
550
600
650
700
0,50 0,45
FT=800°C FT=850°C
0,35 450
500
HT [°C]
550
600
650
700
450
500
16
16
0,22
14
14
0,20
12
12
0,18
10
10
6 4
600
650
700
FT=800°C FT=850°C
0,16
n 46 [1]
8
550
HT [°C]
HT [°C]
A 80 [%]
A g [%]
0,55
0,40
800 450
0,60
8 6
0,14 0,12
4
FT=800°C FT=850°C
2 0
0,10
FT=800°C FT=850°C
2
0,08
0 450
500
550
600
HT [°C]
650
700
450
500
550
600
HT [°C]
650
700
450
500
550
600
650
700
HT [°C]
b) CA simulation Figure 6.101: mechanical properties after HDG and CA simulation versus coiling temperature (laboratory hot and cold rolled samples) A decreasing AI and Y12 with decreasing coiling temperature is not that clearly pronounced compared to the microstructure of hot strip samples. The impact of finishing temperature is not strong, either.
108
0,40
2,0
HDG
1,9
1,8
FT=800°C
1,4
0,15
1,3 0,10
FT=890°C industrial
FT=800°C
0,20
0,25
1,5 1,4
FT=800°C
0,20
FT=800°C
0,15
1,3
FT=890°C industrial
0,10 1,2
0,05
0,05
1,1
0,00
1,0
FT=850°C
FT=850°C
1,6
AI
Y12
AI
0,30 1,7
0,25
1,5
1,1
0,35
FT=850°C
FT=850°C
1,6
1,2
CA
1,8 0,30
1,7
0,40
2,0
0,35
Y12
1,9
1,0
0,00
500 525 550 575 600 625 650
500 525 550 575 600 625 650
500 525 550 575 600 625 650
500 525 550 575 600 625 650
CT / °C
CT / °C
CT / °C
CT / °C
Figure 6.102: AI and Y12 versus coiling temperature (laboratory hot and cold rolled samples after HDG and CA simulation)
6.8.3.4.Recristallization behavior of laboratory hot rolled samples of heat #3 (VA DP800 1) To study the impact of coiling temperature and finishing temperature on the cold rolled annealed properties and microstructure recristallization experiments were performed, applied cycles see Figure 6.103. The investigation of the samples is in progress, the mechanical properties are shown in Figure 6.104. TS soaking temperature
temperature
WQ
WQ
WQ
WQ
WQ
WQ
740°C ( c + i"."fi
