ISSN 1831-9424 (PDF) ISSN 1018-5593 (Printed)
New advanced high strength steels by the quencing and partitioning (Q&P) process (NEWQP)
Research and Innovation
EUR 27552 EN
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EUROPEAN COMMISSION Directorate-General for Research and Innovation Directorate D — Key Enabling Technologies Unit D.4 — Coal and Steel E-mail:
[email protected] [email protected] Contact: RFCS Publications European Commission B-1049 Brussels
European Commission
Research Fund for Coal and Steel New advanced high strength steels by the quencing and partitioning (Q&P) process (NEWQP)
P. Rodriguez-Calvillo, J. M. Cabrera CTM Av. Bases de Manresa 1, ES-08242 Manresa
R. G. Thiessen Thyssenkrupp Kaiser-Wilhelm-Strasse 100, Postfach 47161, DE-47166 Duisburg
J. Molina, I. Sabirov, Y. Cui, I. Diego de Calderon, G. Xu IMDEA Profesor Aranguren sn, ES-28040 Madrid
L. Kestens, R. Petrov, D. De Knijf, J. Van Poucke RUG Sint Pietersnieuwstraat 25, BE-9000 Gent
C. Föjer OCAS Jonh Kennedylaan 3, BE-9060 Zelzate
A. Di Schino CSM Via di Castel Romano 100, IT-00128 Rome
P. Mecozzi, J. Siestma, M. J. Santofimia TUDELFT Stevinweg 1, NL-2628 Delft Grant Agreement RFSR-CT-2011-00017 1 July 2011 to 31 December 2014
Final report Directorate-General for Research and Innovation
201
EUR 27552
EN
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ISBN 978-92-79-53367-9 ISBN 978-92-79-53368-6
ISSN 1018-5593 ISSN 1831-9424
© European Union, 2014 Reproduction is authorised provided the source is acknowledged. Printed in Luxembourg Printed on white chlorine-free paper
doi:10.2777/951031 KI-NA-27-552-EN-C doi:10.2777/104771 KI-NA-27-552-EN-N
1. TABLE OF CONTENTS 1. TABLE OF CONTENTS 3 2. FINAL SUMMARY 5 3. SCIENTIFIC AND TECHNICAL DESCRIPTIOIN OF THE RESULTS 13 3.1. Objectives of the project 13 3.2. Comparison of initially planned activities and work accomplished 14 5.3. Description of activities and discussion 15 3.3.1. WP1. Project Co-ordination 15 3.3.1.1. Task 1.1. Coordinating Project Implementation 15 3.3.1.2. Task 1.2. Coordinating Dissemination and Exploitation 15 3.3.2. WP2. Material Design, Realization and Delivery 17 3.3.2.1. Task 2.1. Alloys Strategy and Design 17 3.3.2.2. Task 2.2. Casting, Hot and Cold rolling of material. 19 3.3.2.3. Task 2.3. Q&P Annealing Cycles 21 3.3.3. WP3. Microstructure Characterisation 25 3.3.3.1. Task 3.1. Quantitative description of the microstructure 25 3.3.3.1.1. Task 3.1.1. Analysis of morphology, grain size and volume fractions of phases 25 I. Microstructural characterization of hot/cold rolled material 25 II. Microstructural characterization of Q&P heat treated material 26 (a) Microstructural characterization techniques on Q&P in general 26 (b) Evolution of the microstructures during partitioning with SEM 27 3.3.3.1.2. Task 3.1.2. Texture and compositional investigations 28 3.3.3.1.3. Task 3.1.3 Characterisation of carbide precipitation 30 3.3.3.1.4. Task 3.1.4 Characterisation of the retained austenite 31 3.3.3.1.5. Task 3.1.5 Thermal stability of the retained austenite by magnetic measurements 34 3.3.3.1.6. Task 3.1.6. Microstructure evaluation of the industrial feasibility 35 3.3.3.2.Task 3.2.Characterization of the kinetics of the QP process. 37 (a) Volume fractions of martensite formed in the first and second quench 37 (b) Competing reactions during partitioning 37 (c) Continuous-Cooling-Transformation diagrams 38 3.3.4. WP4. Analysis of Mechanical Properties 41 3.3.4.1. Task 4.1. Deformation Behaviour, Fracture, Fatigue and Formability of Primary Material and Annealed Q&P Steels 41 3.3.4.1.1. Task 4.1.1. High temperature deformation behavior of as-cast material 41 3.3.4.1.2. Task 4.1.2. Room temperature mechanical behaviour of hot-strip material and annealed Q&P steel 42 3.3.4.1.3. Task 4.1.3. Fracture and fatigue behavior of the QP processed steels 45 (a) Fracture behaviour of QP steels 45 (b) Fatigue behaviour of QP steels 46 3.3.4.1.4. Task 4.1.4. Formability of QP steels 47 3.3.4.2. Task 4.2. Effect of the microstructure on the tensile properties of Q&P steels 47 3.3.4.2. Task 4.3. Technical positioning of QP steels vs. other steel families 49 3.3.5. WP5. Modelling of Microstructure Development 51 3.3.5.1. Task 5.1 Phase-field and Mixed-Mode Cellular Automata modelling 51 3.3.5.2. Task 5.2 Ginzburg-Landau model for the mixed microstructure of retained austenite and martensite in the Q&P steels 58 (a) Development of models 58 (b) The framework of (semi-)quantitative modelling 59 (I) Thermodynamic database of martensite and pseudo-ternary systems 59 (II) The methodology of (semi-) quantitative MT modelling 59 (c) Inertia dynamic model of single-crystal martensite 60 3.3.5.3. Task 5.3 Integration of the Phase Field / MMCA model and the Ginzburg-Landau Model. Validation of the models against microstructural observations 62 3
(a) The inertia GL model of polycrystalline MT model 62 (b) Coupling of three fields, i.e. grain orientation, elastic and composition fields for the Q&P process 63 (c) Link processing parameters and microstructure evolution 63 3.3.6. WP6. Industrial feasibility study 65 3.3.6.1. Task 6.1 Processability grades via hot rolling + cold rolling + Q&P 65 3.3.6.1.1. Task 6.1.1. Hot and cold rolling 65 3.3.6.1.2. Task 6.1.2 Q&P process 66 3.3.6.2. Task 6.2 Analysis of weldability and galvanised 66 3.3.6.2.1. Task 6.2.1. Analysis of weldability of Q&P steels 66 3.3.6.2.2. Task 6.2.2. Analysis of Galvanised Q&P steels 69 3.3.6.3. Task 6.3. Simulation of forming process for automotive components 70 3.3.6.4. Task 6.4. Life Cycle of the 3rd generation of AHSS via Q&P treatments 75 3. 4. CONCLUSIONS 79 3. 5. EXPLOITATION AND IMPACT OF THE RESEARCH RESULTS 81 4. List of figures 83 5. List of tables 87 6. List of acronyms and abbreviations 89 7. List of references 91
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2. FINAL SUMMARY Main Objectives The main objectives of this project were the following: 1. To gain knowledge and a better understanding of/about the mechanisms underlying and controlling the microstructural changes during the Q&P process, as well as the role of the microstructural characteristics of the Q&P steels in the control of the mechanical properties. 2. To develop new advanced high strength grades for their application in the automotive sector with improved mechanical properties of strength, ductility and strain hardening in comparison with existing steels. In this regard, the so-called “Quenching and Partitioning” (Q&P) process on cold rolled steels will be used. 3. To improve the industrial applicability of the Q&P process in terms of compositions, treatments and properties as formability, galvanisability and weldability to develop a controlled and reproductive process to manufacture such materials. 4. To create methods for future development of new Q&P grades, allowing faster and cheaper product development as well as a reduction of the trial and error procedures. All the main objectives have been fulfilled along the project. WP1. Project Co-ordination Task 1.1 Coordinating project implementation Setting-up and maintaining a platform for communication within the project allowing to share the information between the WPs with different levels of confidentiality (http://newqp.ctm.com.es/). During the project 6 face to face meeting have been organised and other 6 online to regularly monitoring the overall performance, time and costs of the project. Task 1.2 Coordinating dissemination and exploitation The activities of organization 2 workshops for dissemination purposes were carried out in September 2013, Seville, Spain, and in September 2014, Darmstadt, Germany, within the EUROMAT2013 and MSE 2014, respectively. Up to date 7 papers in international journals has been published and 20 contributions to international conferences. WP 2. Material design, realization and delivery The design and realization/production of steel grades will contribute to the development of the 3rd generation of advanced high strength steels (AHSS), with improved mechanical properties of ductility and strength in comparison with existing AHSS-grades by application of Q&P. Task 2.1. Alloys strategy and design. A literature study concerning restrictions of composition, influence of elements, hardenability, growth rate and rate of nucleation of carbides and austenite stabilization was performed. Based on this study and thermodynamic information as Thermocalc and TTT-software, six chemical compositions were proposed given the boundary condition of carbon concentrations between 0.15-0.20 (CSM), 0.20-0.30 (OCAS) and 0.30-0.45wt% (TKSE). One iteration of the chemical composition of TKSE was done after feedback from WP4 indicating that the increased carbon contents in TKSE1 and 2 compared to compositions studied by OCAS brought little to no advantage for the mechanical properties and only very limited applicability in industry (cfr. welding results in WP6). Therefore, a new cast with a lower C-content (TKSE3) was produced. Task 2.2. Casting, Hot and Cold rolling of material. Casting, hot and cold rolling of all grades was performed in the lab facilities of CSM, OCAS and TKSE. The hot/cold rolled material was distributed to other partners for further investigations. Task 2.3. Q&P Annealing Cycles The heat treatment cycles were designed based on thermodynamic data to predict the AC3-temperatures, experimental CCT-diagrams to determine the critical cooling rate to avoid ferrite/pearlite and bainite formation, empirical equations to estimate the Ms-temperature and Koistinen-Marburger relations to predict the quenching temperatures. Variations of the quenching temperatures and partitioning cycles (temperatures and times) were applied to study their influence on the mechanical properties. The Q&P heat treatments were conducted in different simulators: a dilatometer Bähr DIL805A/D in CTM, a 5
Gleeble and MDA-simulator (Multipurpose Dynamic Annealing) in OCAS and VATRON Multi-Pass annealing simulator in TKSE. WP3. Microstructure Characterisation In WP3 of the project, the main focus was microstructural characterization of the steel products during processing. Knowledge of the microstructure and how the processing parameters affect this is essential for an optimal Q&P design. Besides, by knowing the link between microstructure and mechanical properties, a microstructure can be designed which would result in desired mechanical properties. This microstructure can then be related with the heat treatment parameters necessary to produce the steel. Three tasks were addressed in WP3: Task 3.1 Quantitative description of the microstructure, Task 3.2 Characterization of the kinetics of Q&P process, Task 3.3 Interrelation with other work packages. These tasks will be discussed separately addressing the project objectives, the results obtained and their usefulness and conclusions and possible applications. It has to be mention that several attempts were carried out to characterized the QP microstructures via 3D-EBSD, nevertheless the austenite induced transformation during sample preparation (either via ion-milling or mechanical polishing) and the reduced scanned areas made these analysis not suitable for the current works. Task 3.1. Quantitative description of the microstructure The objective of task 3.1 was to document the microstructure changes in the selected alloys of interest after hot/cold rolling and after the final heat treatment. Microstructure parameters were characterized at multiple scales ranging from LOM to TEM. The resolution of LOM was too low to characterize the microstructures. SEM on the other hand was proven to be a successful technique for characterizing the HT cycle qualitatively. However EBSD is required to analyse the RA morphology, and FM and TM fractions. XRD was observed to estimate the RA fraction highest from all techniques due to its best resolution and statistical representation. Besides, an estimation of the chemical stability (based on the C-content of RA) could be made with XRD. The influence of the processing parameters on the microstructural evolution (RA-fraction and its %C content) was related with the obtained mechanical properties. The microstructural evolutions during partitioning revealed that more austenite was stabilized at higher PT and longer Pt and that more FM was present at higher QT. This is in the case of ideal partitioning without competing reactions. Decomposition of RA was investigated with DSC (carbide precipitation in martensite) and dilatometry (bainite formation and austenite decomposition). Partitioning below 300°C resulted in the formation of ε-carbides depleting the available carbon for partitioning. Partitioning between 370°C (the bainite start temperature, Bs) and 400°C was found to be the optimum partitioning temperature range, with enhanced C-diffusivity without competing reactions. Above 400°C, decomposition of austenite and tempering of martensite forming cementite occurred, decreasing strongly the RA fraction and total elongation. These temperature ranges of austenite decomposition were also observed with magnetic measurements. Three types of RA could be detected: blocky, lamellar and film-like. Blocky austenite is easy detectable with SEM, EBSD at the prior austenite grain boundaries but is also believed to transform at low strains whereas lamellar interlath RA is believed to contribute to ductility at higher strains due its higher stability for martensitic transformation. Film-like austenite is only detectable with TEM and TKD. This austenite is hence most stable compared to other morphologies and is believed to remain stable or transform at very high strains. The Q&P microstructure with the best mechanical properties consists of retained austenite and tempered martensite without FM. FM alters the strain distribution in the microstructure hereby reducing the retained austenite stability against martensitic transformation. Besides, the locations between the hard FM constituent and the matrix appear as potential crack nucleation sites. The strength is controlled mainly by the tempered martensite and ductility is improved if the retained austenite fraction and its Ccontent increase and if the presence of FM can be avoided by a proper Q&P heat treatment design. The weldability and galvanisability of the selected compositions was tested. The Q&P material with 0.25C could be successfully welded without post-weld heat treatment. The mechanical Chisel test on the 0.28C material illustrated that this composition is less suited for welding. Both steels were electrogalvanized with success.
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Task 3.2.Characterization of the kinetics of the QP process. The kinetics of phase transformations and competing reactions were studied with dilatometry. The area where bainite formation competed with partitioning could be identified with an isothermal holding below 370°C.The partitioning kinetics was studied with XRD by following the retained austenite fraction and its C-content for samples partitioned at consecutive times. From these measurements it was observed that C-enrichment occurs after austenite stabilization and that the partitioning time needs to be chosen wisely to have a combination of high RA fraction and sufficient C-content (>1%) to have a good mechanical stabilization. Martensitic tempering was studied with DSC resulting in precipitation of epsilon-carbides below 300°C and cementite above 400°C. Additionally, the CCT diagrams were made with the dilatometer as a first step in designing the Q&P heat treatment cycle (to determine the critical cooling rates). The image quality of EBSD measurements was proven to be a reliable criterion to distinguish between fresh and tempered martensite since the higher dislocation density of FM makes the Kikuchi pattern more diffuse resulting in a lower image quality compared to tempered martensite. WP4. Analysis of Mechanical Properties Task 4.1. Deformation Behaviour, Fracture, Fatigue and Formability of Primary Material and Annealed Q&P Steels The modelling of the flow curves as a function of steel chemistry presented good correlation with the experimental data. The stress relaxation experiment allowed calculating the recrystallized fraction at a given time. No evidence of high temperature precipitation was found. The outcomes of the analysis of high temperature deformation behaviour can be used for selection of optimum hot rolling parameters. The samples for TKSE1 (0.32C-2.5Mn-2Si) showed a significant increase in tensile strength (>1900 MPa) with an expected (yet still acceptable) decrease in elongation. TKSE2 (0.4C-2.5Mn-1Si), for twostep annealing, was in the range 1550 – 1650 MPa for tensile strength and an elongation greater than 8 %. However, some samples show an unexpected brittleness and do not achieve tensile strengths above 1400 MPa. The negative results for TKSE2 become more pronounced when the “1-step” annealing results were considered. Some TKSE2 samples, on the other hand, exhibited an extremely brittle behaviour. After that the chemistry for TKSE-alloys was optimized. A new steel grade TKSE3 (0.28C2.5Mn-2Si) was produced by TKSE. Two types of annealing cycles were carried out, namely 1- & 2step. The materials with a 1-step annealing exhibited significantly higher tensile strengths (up to 1943 MPa) and reduced ductility (8%). There was negligible difference between the two annealing cycles (quench temperature was either 240°C or 280°C), and very little difference between samples taken longitudinal or transverse to the rolling direction. The samples annealed with a two-step annealing cycle exhibited a noticeable difference in yield stress, depending on the quench temperature. With an increase in the quench temperature from 200°C to 280°C, the yield stress decreased from approximately 1250 MPa to 875 MPa. The OCAS1 (0.25C-3Mn-1.5Si) steel grade having a carbon content of 0.25%, tensile strength was in the range of ~1300 MPa to ~1750 MPa and elongation to failure was in the range of ~5% to ~14%. Tensile strength tended to decrease with increasing partitioning temperature and partitioning time whereas ductility showed the opposite trend due to the increasing volume fraction of RA. Similar tendencies were observed when partitioning temperature increased from 300 to 400oC. However, a further increase of the partitioning temperature up to 450oC led to degradation of both strength and ductility. A good combination of high strength ~1750 MPa and moderate ductility ~9% was obtained when the OCAS2 (0.3C-3Mn-1.5Si) steel grade is partitioned at 300oC. CSM steels grades showed much lower level of mechanical strength after all QP cycles compared to OCAS and TKSE steel grades. CSM1 (0.15C-2.5Mn-1.5Si) stayed with lower strength values, below 1200 MPa, whereas CSM2 (0.15C-3Mn-1.5Si) did not exceed 1250 MPa. The strength level was reduced with partitioning time and increased when the partition temperature is lowered. In terms of ductility both steels presented similar tendency, being reduced with increasing Pt and with the decrease of PT, while it seemed to be not sensible to the QT. Larger values of ductility were obtained for the CSM2 steel grade. The analysis of the mechanical properties revealed that the best mechanical properties might be obtained by quenching to 335ºC followed by partitioning at 350oC for 100 s. The results of this WP demonstrate that local fracture initiation toughness Ji is slightly increased in the sample with the highest volume fraction of RA. Total crack growth resistance of Q&P steels is significantly improved with increasing volume fraction of RA. Although this effect is modest at high volume fractions of RA. Matrix conditions (volume fraction of tempered and fresh martensite) can play 7
important role in fracture behaviour of QP steels. So it is concluded that for this type of multiphase material, the arrangement of different phases, such as ductile and brittle components, as well as stability of RA phase play a key role for determining the fracture behaviour. The FLCs in CSM2, OCAS1 and TKSE3 steel are comparable with the ones of commercial steels with similar mechanical properties. The material with higher mechanical properties (TKSE3) has the highest fatigue resistance (FS) at 2.106 cycles. As the ultimate strength decreases, the fatigue resistance does too, as can be seen with the other two steels, OCAS1 and CSM1, respectively. Task 4.2. Effect of the microstructure on the tensile properties of Q&P steels The mechanical stability of RA was studied by in-situ X-ray diffraction measurements during tensile tests by TU Delft. OCAS1, OCAS2 and TKSE3 steel grades were chosen for this investigation. The experiments showed that an increase in Pt resulted in enhanced stability of RA during tensile deformation and, even though QT had strong effect on volume fraction of RA in the QP steels, its effect on the stability of RA was not that significant In addition the orientation dependence of the austenite-to-martensite transformation during uniaxial tensile testing was modelled by UGENT using the phenomenological theory of martensite crystallography and the mechanical driving force. It was validated experimentally by means of electron backscatter diffraction measurements on a pre-defined zone of a QP steel during interrupted tensile tests. A close match is obtained between the predictions of the model and the experimental observations Task 4.3. Technical positioning of QP steels vs. other steel families Most of the QP steels generated in frame of NewQP project showed much higher tensile strength and acceptable ductility compared to current 3rd generation steels. The OCAS1 steel presents an excellent combination of high mechanical properties, good formability and response to fatigue among all QP steels. If this material is compared with a commercial steel of similar mechanical properties, as DP1180, the QP steel has a higher capacity for uniaxial deformation, even with a smaller thickness. WP5. Modelling of Microstructure Development Task 5.1 Phase-field and Mixed-Mode Cellular Automata modelling In the WP5.1 a multiphase field modelling was applied to describe the microstructure development during rapid cooling to different quenching temperatures and the carbon partitioning between the carbon supersaturated ferrite and the austenite present at the quenching temperature at different partitioning conditions. In the used PF approach the microstructure of a polycrystalline system is represented by a set of phase field parameters which are continuous function of both time and space; the phase field parameters have constant value in the bulk of the grain and change continuously across a diffuse grain boundary region of finite width. The phase field parameters are then phenomenological variables used to indicate which phase is present at a particular position in the system and distinguish coexisting phases with a different lattice structure, orientation and strain. The microstructural evolution as a function of time is implicitly given by the evolution of the phasefield variables, obtained by solving numerically a set of phase field equations. The phase-field equations are derived from thermodynamic principles and contain a number of model parameters, which can be related to physical properties like interfacial energy and mobility and the driving force of the transformation. In literature there is also a second type of phase-field modelling where the phase field variable are related to microscopic parameters, like the local composition, or refer to the crystal symmetry relations between coexisting phases. The model has been applied to a variety of solid-state phase transformations that involve a symmetry reduction, as the martensitic transformations. A phase field model belonging to this category was developed to describe the martensitic transformation during cooling in WP5.2. Although the phenomenological PFM applied in the WP5.1 is not suitable to describe the martensitic transformation, by treating the martensite as acicular ferrite, the obtained microstructure at different quenching temperature reproduces quite well the experimental austenite/ martensite microstructure formed below Ms. The simulation of the carbon partitioning process during holding at a select partitioning temperature, for a given relative fraction and morphology of austenite and ferrite at the quenching temperature, provided information on the local carbon content in austenite at different times 8
at the partitioning temperature. Applying a criterion to calculate the fraction of retained austenite at room temperature from the local austenite carbon content before the final quenching, the phase field model was able to predict the fraction of RA after the final quenching as function of the QT, PT and Pt. The experimental RA fraction at the end of the Q&P process and the value predicted by 2D PFM simulation are significantly different. When 2D simulations are compared with 3D simulations, it becomes evident that 2D simulations give an overestimation of the retained austenite fraction at the end of the Q&P process since carbon has more space for diffusing in 3D than in 2D space. Consequently carbon easily homogenises within the austenite grain in a 3D space reducing its fraction in this phase especially in large austenite area Task 5.2 Ginzburg-Landau model for the mixed microstructure of retained austenite and martensite in the Q&P steels Task 5.2 is aimed to develop a physical model based on the Ginzburg-Landau (GL) theory for describing the formation of the rich microstructure during the Q&P process via definition of the strain and the composition as field variables. Sticking to the schedule, four research activities have been carried out in WP5.2: 1) Imposing with appropriate energetics and dynamics, a nonlinear GL model for martensite transformation (MT) was developed, which is capable of describing the intricate microstructure features in MT comprising of, e.g. characteristic twin boundaries, habit planes and orientation relationship, etc. 2) The in-house codes based on the proposed model, and represented the formation of mixed microstructure was developed e.g. the retained austenite and martensite in the Q&P steels. 3) An integrated computational tool (i.e. model and in house codes) was initiated, which is competent to investigate displacive-diffusive coupled phase transformation that hosts two significant events in the Q&P process, i.e. the formation of martensite during quenching and the subsequent diffusion of carbon in partitioning. 4) To realize (semi-)quantitative modelling, we have proposed the thermodynamic description of the martensitic phase in the spirit of the CALPHAD technique; created the thermo-kinetic database with a Fe-M-C pseudo-ternary approximation for counting the alloying effects equivalent to the original multicomponent database; and consolidated the thermodynamic and diffusion computational engine and database directly into our code. The achievements in Task5.2 include 1) The nonlinear GL model was in conjunction with experimental investigation, including nanoindentation determination of elastic constants, laser confocal characterization of interface mobility, and was integrated with the assessed pseudo-ternary thermo-kinetic database that can be generalized to the majority of microstructure modelling and alloy design. 2) The simulation results performed on our developed FORTRAN code well represent the microstructure evolution during the martensitic transformation, and the simulated spatiotemporal distribution of carbon concludes that the coupling between eigenstrain of MT and composition governs the diffusion from martensite to austenite. The new GL model for MT, competent not only to twin martensite but also slip martensite, is believed to surpass the existing models in microstructure simulation by a clearer perspective of physics because it has been kernelled with continuum mechanics that breaks through the limit of microstructure evolution and provides the capacity for predicting mechanical properties. More importantly and practically, it allows prediction of the correct trends and pathway of phase transformations associated with the Q&P processing that could reasonably narrow down the processing window. Particularly, the proposed conceptual (semi-)quantitative (both thermodynamics and kinetics) GL modelling is applicable to provide insight into such important events of MT in steels as inertia, damping, undercooling depth etc. and of the partitioning process as partial, para- and ortho-equilibrium. Task 5.3 Integration of the Phase Field / MMCA model and the Ginzburg-Landau Model. In this task, the nonlinear GL model and developed the in-house codes to investigate the MT occurring in polycrystals, the subsequent partitioning, and the elastic properties of polycrystals by coupling the crystal orientation degrees of freedom, elastic strain and compositions were widened. The activities cover the development of physical-sound description of these three types of materials behaviours in individual spatial and temporal scales, especially introducing the crystal symmetries of order parameters 9
(OPs) and strain compatibility into the integrated model, and implementing and encoding the PF model of grain evolution in polycrystals (Task 5.1) explicitly into the final GL code in Task 5.2. By doing so, the important effect of cooling rate on the formation of martensites, the variants selections and the constitutive response of polycrystals of Q&P steels to the uniaxial loading was qualitatively investigated. The model and simulation well represent the coupling between eigenstrain and grain orientations, for specific, the martensitic variants in individual grains collectively tune their optimal directions to align with grain orientation but preserve the intragranular texturing directions of twinning. Be capable of retrieving the spatial distribution of stress and/or strain and screening out the candidates for the novel steels, our model and simulation, have been proven to physically coincide with the recoverable strain theory proposed by Bhattacharya rather than simply replicate the response of the domain morphology to external loading. In the scientific point of view, the developed model is extendable to a geometrically description for polycrystalline MT e.g., by utilizing the finite strain theory to describe the elasticity, as such, the crystallographic orientations originating from MT texturing and the grain texturing can be addressed. The new GL model developed is therefore believed to surpass other available models for microstructure simulation and is capable of predicting mechanical properties. WP6. Industrial feasibility study Task 6.1 Processability grades via hot rolling + cold rolling + Q&P Although the casting was briefly studied, the emphasis for the feasibility of industrial production was put on the investigation of hot- and cold-rolling properties and the required annealing parameters. The hot rolling parameters were easily achievable in the hot-strip mills of the participating industrial partners, similar to the cold rolling parameter although the cold-rolling the TKSE3 (0.28C-2.5Mn-2Si) chemistry presented significant challenges and led to occasional tearing of the rolled strip. The actual soaking temperatures implemented in this project were close to the estimated upper limit of industrial furnaces. The cooling rates for all chemistries also appear to be within the estimated ranges of industrial furnaces, but it should be noted here that the necessary cooling rate for CSM2 was 50°C/s down to the quenching temperature might not be always possible. The required quench temperature also provides some challenges. The estimated lowest quench temperature for TKSE and CSM furnaces would not be low enough to produce the TKSE3 and OCAS1 grades. Some adaptations of the cooling section would need to be implemented before these grades could be produced in these furnaces. Task 6.2 Analysis of weldability and galvanised The weldability of TKSE1 and TKSE2 is severely restricted while the remaining concepts exhibit a minimum weldability. Although typical weldability studies are generally carried out with 1.5 mm sheet, this thickness was only available for TKSE3. These results should, however, be treated with caution as the number of samples and available geometries could not satisfy the requirements for an industrial classification. Samples from the studied materials as well as one standard cold-rolled surface were subjected to galvanizing trials in a ZnSO4 electrolyte at approximately 55°C under 50 A/dm2. The adhesion of the zinc layer was evaluated with the lap shear test according to NBN EN1465. The results indicated that all of the studied materials can be electro-galvanized Task 6.3. Simulation of forming process for automotive components Laboratory studies can provide critical initial indications of material properties, but it is difficult to gain an impression of how these materials could perform in an actual automotive component when only small samples can be produced. For this extrapolation to larger scale application, finite element modelling with detailed material models can provide the necessary support. Material models have been developed for TKSE3, OCAS1 and CSM2 based on the results of specific tests conducted in WP4. Several geometries are being considered for the simulation of component application, such as the bumper and a floor panel. Aspects such as spring-back and likelihood of failure during forming were also considered. Components such as bumpers can be formed with steels OCAS1 and CSM2, but the TKSE3 grade is not advised for these pieces. These pieces can be manufactured with a DP1200 steel grade. The car floor has a geometry with small radii and none of the three analysed material have sufficient ductility to prevent failure during the forming process. More ductile steels are required for these pieces, such as DP800 or lower. The thin sheet studied shows instabilities that generate wrinkles on the piece. 10
The shock absorbers may be formed with the three materials, it was observed that under pure bending condition, steels withstand high levels of deformation, significantly exceeding the FLC information. This is because the strains in the thickness of the sheet have a large gradient, passing from traction to compression, from outer radius to the inner radius of the bend. Task 6.4. Life Cycle of the 3rd generation of AHSS via Q&P treatments. Simplified Life Cycle Assessment (LCA) has been performed following the precepts set out in ISO 14040 series and UNE 150041 with the help of Simapro software (CTM). The assemblies for LCA study have been built using Simapro Developer v8.0.4.28 coupled to excel datasheet. ReCiPe midpoint for Europe, v1.04 has been used as an impact characterization method. Greenhouse Gas Emissions as main impact category and Freshwater eutrophication, Human toxicity, Freshwater ecotoxicity, Marine ecotoxicity and Metal depletion have been the impact categories analysed. The study focusses on three scenarios: • The analysis of the effect of alloying elements in different steels used in the project • A comparison of thermal treatment of Q&P and AHSS steels from laboratory and pilot plant data • The evaluation of environmental impact of three automotive components build with Q&P steels from data provided by numerical simulations Results from laboratory and pilot scale data, and from numerical simulations of body car components shown that the impact of steel used is higher than the quench and partitioning thermal process. In this regards, the use of alloying elements is critical, especially for metal depletion category, which needs a more detailed analysis and considerations before the new QP steels reach the market. Moreover, within the thermal process, the gas used for cooling is the hot spot in terms of environmental impact. Considering all evaluations performed and tests carried out, LCA studies performed have demonstrated that the impact of NewQP steels for major impact categories are not significantly higher than alternative AHSS steels.
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3. SCIENTIFIC AND TECHNICAL DESCRIPTION OF THE RESULTS 3. 1. OBJECTIVES OF THE PROJECT The detailed objectives for each specific Work Package are the following: WP1. Project Co-ordination 1. Smooth implementation of the project characterised by/via: • Good (fluid & timely) communication within the project as a whole, and within and between WPs, as well as with the European Commission. • Regular, and when necessary, ad-hoc opportunities for the partners to meet and discuss (kickoff and periodic project progress meetings) about the content and the progress of the project. • Compliance of each partner with their obligations under this project (performance, time, cost) and subsequent successful on-time delivery of all project reports and other necessary documents (Technical, Mid-Term Technical Implementation, Final technical Reports, as well as the Publishable report and the required Financial Statement). 2. Optimal dissemination and exploitation of project results to targeted audiences and stakeholders characterised by/via: • Good project visibility by disseminating the project results to the scientific & industrial community, and to the public. • Proper indication of the commercial potential of project results to the European industry and developing a proper IPR strategy. • Proper indication of further research activities that can be conducted after the end of the project, based upon the results obtained. WP2. Material Design, Realization and Delivery • To design isolate chemistries with a theoretical affinity for the Q&P process. • To produce lab-scale melts with the required optimised chemistries. • To carry out the hot- and cold-rolling. • To realise a systematic optimisation of final mechanical properties via annealing parameter matrices WP3. Microstructure Characterisation • To quantitatively describe the microstructures at different preliminary production stages and after various Q&P treatments using light optical microscopy (LOM), XRD, SEM, EBSD, (incl. 3D- EBSD), TEM (TEM-EBSD). • To describe the kinetics of the phase transformations and carbon partitioning during the Q&P process by accurate dilatometric and calorimetric tests. • To give appropriate feedback for the optimisation of alloy design, analysis of the properties and modelling. • To evaluate the industrial feasibility for the formability, galvanising and welding process. WP4. Analysis of Mechanical Properties • Determination of the high temperature deformation behaviour of the selected chemistries of steels. • To evaluate the room temperature deformation behaviour at macro- and micro- scales during tensile testing to gain a better understanding of the relationship between local microstructure and local properties resulting from the thermo-mechanical processing parameters of Q&P steels. • To study the fracture, fatigue and formability behaviour of the Q&P steels • Obtaining an accurate, reliable and statistically representative set of data for mechanical properties of the new steel grades after various Q&P processes. • Determination and selection of the chemistry-process combinations leading to improved mechanical properties. These combinations will be studied more in detail in WP5 in order to obtain a better phenomenological understanding of the mechanical behaviour. 13
WP5. Modelling of Microstructure Development • To develop a model based on the phase-field theory and the Mixed-Mode Cellular Automata method for the microstructure development during the Q&P process. • To develop a model based on the Ginzburg-Landau (GL) theory to describe the formation of the rich microstructure during the Q&P process by defining the strain and the composition as the field variables. • To integrate both models and validate them against microstructural observations in WP3 and WP4, thus creating new methods for design and optimization of the Q&P-processes. WP6. Industrial feasibility study • Evaluation of industrial feasibility of the results • To position all obtained results vs. the typical technical feasibilities of existing processing lines • To obtain a guidance for potential future optimisations and developments 3. 2. COMPARISON OF INITIALLY PLANNED ACTIVITIES AND WORK ACCOMPLISHED There have not been encountered major deviations from the originally planned activities.
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3. 3. DESCRIPTION OF ACTIVITIES AND DISCUSSION 3.3.1. WP1. Project Co-ordination 3.3.1.1. Task 1.1 Coordinating project implementation The activities regarding this task are related to the creation and maintenance of the communication platform between the partners with different levels of confidentiality (http://newqp.ctm.com.es/) and the scheduling and organization of the different progress meetings, 6 face to face and 6 online. Additionally the overall performance of the project, technically and economical, was periodically evaluated in these meetings before the required reports to the EU commission. 3.3.1.2.Task 1.2 Coordinating dissemination and exploitation The activities of organization 2 workshops for dissemination purposes were carried out in September 2013, Seville, Spain, and in September 2014, Darmstadt, Germany, within the EUROMAT2013 and MSE 2014, respectively. The workshop organized by CTM in 2013 included a general overview of the NewQP project that was also printed as poster at the venue place of the workshop, open presentations related to the different WPs and a round table of scientific and technical discussions. Fig. 1.1 shows (a) the poster overview of the project, (b) general view of the assistance during the oral presentation and (c) round table.
Figure 1. 1. (a) Poster overview of the NEWQP RFCS project, assistance to the workshop talks and (c) round table. The workshop organized by OCAS and UGENT in 2014 invited to give an key note talk to Prof. E. Moors from Colorado School of Mines, reputed international expert on the physical metallurgy of the Quench and Partitioning of steels, under the title of “Austenite Stabilization through Quenching and Partitioning” Additionally talks were given by the partners of the project with significant attention from the audience. The dissemination of the results of the project is illustrated by the participation in 5 international conferences and publishing works in 7 international journals. Furthermore several ongoing works are been prepared for submission to international journals regarding fracture, fatigue and formability of QP steels. The results of the project are not considered suitable for patenting and are more related to the scientific community where previous literature exists.
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3.3.2. WP 2. Material design, realization and delivery 3.3.2.1. .Task 2.1. Alloys strategy and design. A detailed analysis of the literature regarding the mechanical properties and the deformation behaviour of steels most appropriate for quenching and partitioning treatment showed that: - The extraordinary mechanical properties of the Q&P steels can be tailored via alloying design and adjusting Q&P processing parameters. - No clear microstructure-property relationship in the Q&P steels can be outlined from the analysis of literature at this stage. More detailed microstructure analysis (size of phase constituents, their distribution, their orientation with respect to the applied load, etc.) should be performed to establish clear microstructure-properties relationship. - The role of retained austenite in mechanical behaviour of the Q&P steels still remains unclear. A correlation between volume fraction of retained austenite and mechanical properties is reported in some publications, whereas no any correlation is observed in other works. - There have been no publications on fracture behaviour of the Q&P steels up to date. Chemical compositions adequate for Q&P heat treatments were designed by means of literature research and thermodynamic databases. An important literature review dealing with restrictions of composition, influence of elements, hardenability, growth rate and rate of nucleation of carbides and austenite stabilization was performed.
Figure 2. 1. Schematic diagrams of the Q&P process, producing ferrite/austenite/martensite microstructures, as appropriate, from homogeneous austenite. Ci, Cγ and Cm represent the carbon contents of the initial alloy, austenite, and martensite, respectively, and QT and PT are the quenching and partitioning temperatures, respectively [2]. Quench and Partitioning (Q&P), as a new steel processing concept, was first proposed in 2003 [1]. As schematically presented in Fig. 2. 1, the Q&P processing involves: 1. Quenching austenite to a temperature (QT) between the martensite start (Ms) martensite finish (Mf) temperatures resulting in formation of martensite and untransformed RA; 2. Partitioning treatment either at or above the quench temperature (PT) resulting in carbon diffusion from supersaturated martensite into RA and stabilization of the RA at room temperature. A significant body of research into microstructural evolution during Q&P process can be found in literature [1-27]. A comprehensive review of the mechanisms controlling microstructural changes during the application of the Q&P process has been published very recently [3]. There are fewer publications focused on the mechanical properties of the Q&P steels and microstructure-properties relationship in these materials. Thermodynamic calculations were carried out to determine a suitable chemistry given the boundary condition of carbon concentrations between 0.15-0.20 (CSM), 0.20-0.30 (OCAS) and 0.30-0.45 wt% (TKSE) by using ThermoCalc®, FactSage®, Dictra® and an internally developed calculation tool at TKSE which combines several from the literature and in-house databanks. It has to be mention that the computation of the Ac3-temperature is based on a regression analysis on a large set of experimental data at ThyssenKrupp Steel Europe. The Acm-temperature has been computed according to the equation given by Lee, et al. (Seok-Jae Lee, Young-Kook Lee; Thermodynamic Formula for the Acm 17
Temperature of Low Alloy Steels; ISIJ International 2007, 47, 769). For the bainitic-start temperature the model of Steven, et al. has been applied (W. Steven and A.G. Haynes: J. Iron Steel Institute 183 (1956), 349-359). The computed martensitic-start temperature has been derived from equations given in the following sources (Sugimoto et al., ISIJ Intl. Vol.40 , No.9 (2000), pp.902-908.; J. Wang, S. Van der Zwaag, Metall. Trans. 32 (2001) 1527; Meyer, et al., Mat. Sci. Tech. Vol. 18, No.10 (2002) 11211132.; Perlade et al.; Mat. Sci. Eng. A356 (2003) 145-152). The selection of appropriate alloying elements was based on three main requirements: 1. That the chemistry has a sufficiently low critical cooling rate when cooling from a fully austenitic microstructure in order to avoid ferrite, bainite or carbide formation. By adding Mn, Ni or Cr the bainite transformation can be shifted to longer times. 2. Carbide precipitation and bainite formation should be avoided during the partitioning stage. It is a well-established fact that cementite precipitation can be suppressed by Si or Al additions. 3. The stabilization of austenite during final quenching is needed which requires that the carbon content of the austenite must be high enough. Manganese is effective in fulfilling the first requirement [32]. However, high concentrations result in segregation banding [33] and therefore the Mn-content was limited to 3wt.%. According to literature [7, 34-36], 1.5wt.% Si is adequate to suppress cementite formation during partitioning since it changes the carbide precipitation kinetics from carbon diffusion control to silicon diffusion control, away from the carbide interface [37]. The carbon content was varied for the different institutions CSM (0.15wt%C), OCAS (0.25-0.30wt%C) and TKSE (0.30-0.45wt%C). Different concentrations of these elements influence the Time Temperature Transformation (TTT) diagram [38], cfr. Fig.2. 2.
Figure 2. 2. TTT diagrams modelled with mucg-83 [38] software for different compositions: 0.25C – 2.5Mn – 1.5Si (wt.%) by dotted lines; 0.25C – 3Mn – 1.5Si (wt.%) by small stripes; 0.3C – 3Mn – 1.5Si (wt.%) by the full line. Increasing the Mn-concentration from 2.5 to 3wt% shifts the ferrite/pearlite transformation to the right and the bainite onset temperature down from 440 to 370°C. The influence of increasing the carbon content from 0.25 to 0.3wt% is similar; however, the effect is less pronounced. Therefore, the combination of 3wt% Mn with 0.25wt% C fulfils the requirements of high hardenability, bainite suppression during partitioning and austenite stabilization [7, 34-36]. Based on these data, the compositions in Table 2. 1 were proposed for casting and further processing. The actual realized compositions are given in Table 2. 2. Table 2. 1. Proposed chemical compositions suitable for Q&P. Variant C * Si Mn Cr Al CSM1
LowCref1
0.15 1.5 2.5
-
0.03
CSM2
lowCref2
0.15 1.5 3.0
-
0.03
0.25 1.5 3.0
-
0.03
OCAS1 MedCref1
18
OCAS2 MedCref2
0.30 1.5 3.0
-
TKSE1
HigherC+Cr
0.35 2.0 2.5
0.20 0.03
TKSE2
HigherC+CrlowSi 0.40 1.0 2.5
0.20 0.03
TKSE3
MedC_Iteration
-
0.28 2.0 2.5
0.03
0.03
An iteration of the chemical composition of TKSE was done after feedback from WP4 indicating that the increased carbon contents in TKSE1 and 2 compared to compositions studied by OCAS brought little to no advantage for the mechanical properties and only very limited applicability in industry (cfr. welding results in WP6). Therefore, a new cast TKSE3 was produced. Table 2. 2. Actual realized chemical compositions studied in the project. C
Si
Mn
Al
P
S
N
Cr
V
Mo
Ti
Nb
B
CSM1
0.16
1.52
2.49
0.03
-
CSM2
0.15
1.52
2.98
0.03
-
OCAS1
0.25
1.5
2.82
0.023
0.002
0.002
0.015
0.003
0.002
0.0008
0.004
0.0003
OCAS2
0.3
1.6
2.83
0.034
0.005
0.001
0.016
0.003
0.002
0.001
0.005
0.0003
TKSE1
0.32
1.97
2.45
0.005
0.003
0.004
0.0015
0.2
0.006
0.002
0.002
0.005
0.0004
TKSE2
0.42
0.96
2.49
0.009
0.005
0.004
0.0017
0.2
0.007
0.004
0.004
0.005
0.0004
TKSE3.1
0.276
1.88
2.53
0.027
0.004
0.004
0.0019
0.15
0.004
