Materials Science Forum Vols. 638-642 (2010) pp 2573-2578 Online available since 2010/Jan/12 at www.scientific.net © (2010) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.638-642.2573
The microstructure change during modeling of conventional and thermo-mechanical rolling of S355 steel bars G. Stradomski1a, H. Dyja1b, Z. Stradomski2c Czestochowa University of Technology, 1Faculty of Process and Materials Engineering and Applied Physics, 1Institute of Modelling and Automation of Plastic Working Processes, 2Institute of Materials Engineering a email:
[email protected], be-mail:
[email protected], ce-mail:
[email protected] Keywords: normalising rolling, microstructure change, allotriomorphic ferrite
Abstract. Substantial differences – mainly in plasticity – were found based on statistical analysis of hot rolled bars mechanical properties. Investigations presented in the paper were related to the possibility of modification of continuous bar mills used now in order to improve and stabilise plastic properties determined by the energy of breaking. The paper presents results of laboratory investigations representing processes of conventional and normalising rolling of bars from S355 steel. The experimental analysis of both rolling processes comprised assessment of actual changes occurring in the microstructure of bars, deformed acc. to suggested parameters corresponding to conventional and normalising rolling. The investigations included also the assessment of accelerated cooling after rolling influence on the microstructure of finished products. 1.
Introduction
Rolled products, in particular long products, are an important and significant part of overall steel products output [1-5]. Modern technologies of bars rolling in continuous shape mills feature a very small, equal around 100°C, difference between the temperature of charge and the temperature of product rolling end, what makes ensuring homogeneous mechanical and plastic properties on product’s length and cross-section difficult [6-8]. The usability of a structural material, apart from mechanical and plastic parameters, is determined in particular by the impact strength (more often the energy of breaking in J) determined, depending on conditions of its use, at ambient temperatures or lower, from -20°C to -60°C. Material’s grain size and its in homogeneity is the parameter particularly strongly affecting the value of energy of breaking as well as the nature of steel transition into brittle state. S355 steel, which is the subject of investigations, is a material widely used for welded structures. Statistical analysis of mechanical properties for 465 random selected smooth bars made of S355 steel, 24÷60 mm in diameter, produced between January 2004 and May 2007, is presented in figure 1. From the data specified it is clear that at very high stability of Rm and Re, around 4% of products did not meet requirements related to elongation A5 and also there was a high instability of plasticity parameter determined by the energy of breaking (KV). The amount of products, which did not meet requirements of the lowest category – J0 – was around 12%, which met category J0 – around 31%, category K0 – around 32% and L0 nearly 25%. Such results indicate a high instability of product’s mechanical properties, but at the same time existing possibilities to increase requirements, i.e. to obtain satisfactory impact strengths at lower temperatures.
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a)
b)
c)
d)
Figure 1. Bar charts of mechanical properties of 465 analysed products: a) Re, b) Rm, c) A5, d) Kv
2. Selection of materials The steel was smelted in an electric arc furnace of 97 Mg capacitance and then subject to VOD secondary metallurgy and argon refining. The steel was cast from a tundish in a continuous caster to continuous strands, 145x180 mm in cross-section. After heating in a walking-beam furnace the strands were rolled to smooth bars in a continuous shape mill. The temperature of rolling start was around 1130°C, while of rolling end around 1050°C. Optical microscopes Neophot 21 and Zeiss Axiovert 25 were used for microscopic analysis microscope and transmission electron microscope Philips CM20 (200kV). Metallographic microsections used in microstructure investigations were etched with Mi1F reagent (nital). Quantitative analysis of grain size and microstructure components was carried out using a comparative method, acc. to ISO 643:2003 standard and using ImagePro Plus 3.0. Table 1. Chemical composition of tested steels
material of research
Chemical composition S Cr Ni
C
Mn
Si
P
0,20
1,33
0,27
0,013
0,004
0,08
0,09
Cu
Al
N2
0,24
0,2
-
3. Results Physical modeling of conventional and normalising bars rolling was carried out by means of Gleeble 3800 physical processes simulator. Results of investigations allowed evaluating the actual scope of changes occurring in the microstructure of bars deformed acc. to parameters suggested in this study. Prior to deformation the specimens were heated at a rate of 8,6°C/s and held at 1035°C for 15 seconds to homogenise the temperature distribution across the specimen. After finished process of
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deformation the specimens were cooled using four cooling rates: 3°C/s, 4,3°C/s 6,7°C/s and 9,4°C/s. Figure 2 presents the diagram of specimens deformation during physical modeling of both rolling processes. The applied draft values and periods of breaks between individual deformations corresponded to parameters existing in an actual process of strip rolling in rolling stands nine to twelve (table 2). Figures 3 and 4 present selected TEM microstructures of specimens deformed acc. to conditions presented in figure 2 and figures 5 and 6 nital etched microsections. a)
1035°C
b)
1035°C 980°C
1030°C 930°C
1025°C
900°C
9,4°C/s
6,7°C/s
4,3°C/s
3°C/s 9,4°C/s
6,7°C/s
4,3°C/s
3°C/s
Figure 2. Diagrams of specimens deformation during physical modelling of bars rolling: a) acc. to conventional technology, b) taking into account normalising rolling
Table 2. Process parameters used during simulation of conventional rolling and suggested normalising rolling of bars Rolling stand no
Pause time [s]
Mean temperature [°C]
ε [%]
ε& [s-1]
Conventional rolling process 9
0,96
1041
37
23,18
10
1,48
1036
21
22,90
11 12
1,13
1029
23
21,50
ε [%]
ε& [s-1]
37
23,18
Normalising rolling process Pause time [s] Rolling stand Time without Mean temperature [°C] no accelerated cooling accelerate cooling 9 0,36 0,60 983 10
0,30
1,18
931
21
22,90
11 12
0,30
0,83
898
23
21,50
Figures 3 and 4 presents results of the thin foil investigations made with use of transmission electron microscope of material after both physical modelling rolling processes.
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b)
Figure 3. Specimen after physical modelling of conventional rolling a) ferrite and pearlite area with bainite grain magnification 7500x, b) magnification of bainite fragment from fig. 3a, magnification 15000x, TEM a)
b)
Figure 4. Specimen after physical modeling of normalising rolling process a) cooled at a rate of 3°C/s ferrite and pearlite area 7500x, b) cooled at a rate of 6,7°C/s ferrite and pearlite area 7500x,TEM Results of the thin foil investigations, of material after physical modelling of conventional rolling processes showed converence of obtained structure with industrial material. Such results indicate that the physical modelling process was made correctly. In the specimens beside the dominant ferrite and pearlite structure were observed areas of bainite. Presented in figures 4a and 4b microstructures obtained after physical modelling of normalising rolling, show the specimens cooled 3°C/s and 6,7°C/s. The first cooling rate is the modelling of air-cooling and the second should guaranty to obtain grain size reduction of ferrite and pearlite structure, without presence of bainite. a)
b)
Figure 5. S355 steel microstructure after physical modelling of conventional rolling process a) specimen cooled at a rate of 3°C/s magnification 100x, b) specimen cooled at a rate of 6,7°C/s magnification 100x. Nital etched
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Figure 6. S355 steel microstructure after physical modelling of normalising rolling process a) specimen cooled at a rate of 3°C/s magnification 100x, b) specimen cooled at a rate of 6,7°C/s magnification 100x. Nital etched The size for grains exceeding standard number 10 was determined by means of the formula presented in the ISO 643:2003 standard. The table 3 present results of grain size after deformed acc. to parameters suggested in this study. Table 3.Specifies grain sizes in specimens cooled at various rates determined by a comparative method and their percentage fractions in the microstructure Sample Speed of continuous cooling after the Number of grain size standard and its number rolling process [°C/s] percentage share modelling of conventional rolling process 1 3 9,5 – 80% 10 – 20% 2 4,3 9,5 – 65% 10 – 35% 3 6,7 9,5 – 40% 10 – 60% 4 9,4 9,5 – 20% 10 – 80% modelling of normalising rolling process 5 3 10 – 50% 11,5 – 50% 6 4,3 11,5 – 40% 12 – 60% 7 6,7 12 – 30% 12,5 – 70% 8 9,4 12 – 10% 12,5 – 70% 13 – 20% Based on results of investigations presented on figure 6 in Table 3 it is possible to state that additional grain refinement was achieved by the application of accelerated continuous cooling after the rolling. An increase in cooling rate up to 9,4°C/s resulted in achieving grain corresponding to value of standard 13. However accelerated heat removal from bars volume after rolling is not always favourable from bars practical properties point of view. It was necessary to determine the optimum range of cooling rates which guarantee grain refinement but which at the same time do not cause origination of undesirable structures such as bainite. As a result of investigations carried out it has been found that 6,7°C/s cooling rate (fig. 6b) is optimal for the studied bars which may result in obtaining class 12,5 grain and an idiomorphic ferrite, guaranteeing high mechanical and plastic properties. The steel studied features very high sensitivity to the cooling rate after rolling and by the application of cooling rates between 4,3°C/s and 9,4°C/s it is possible to obtain not only highly diversified size of actual grains, but also a change of ferrite morphology with the existence of allotriomorphic ferrite with traces of Widmannstätten structure idiomorphic ferrite and bainite.
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4. Conclusions
–
–
–
–
Based on investigations carried out the following conclusions may be drawn and observations made: The range of temperatures and deformation rates applied during normalising rolling guarantees reducing the actual grains in bars from the studied steel from the range of around 13,3 – 11 µm (80% of coarse grain) to the range of 11 – 6,65 µm (fractions of around 50%), The combination of technology of S355 steel bars normalising rolling with accelerated continuous cooling of finished products after the rolling enables achieving nearly 300% reduction of the grain size (to class 12,5 – around 4,7 µm). The optimal continuous cooling rate for finished products amounts to approximately 6,7°C/s, The change of hypoeutectoid ferrite morphology into an idiomorphic form, obtained using bars cooling at the rate of approximately 6,7°C/s is the factor, which has a decisive influence on mechanical properties and in particular on plastic properties of S355 steel bars. These two factors – grain refinement and obtained ferrite morphology should guarantee higher categories of S355 steel bars plasticity determined in impact tests, It has been shown that applying higher cooling rates reaching 9,4°C/s it is possible to obtain further grain refinement even up to class 13 (3,9 µm). However such heat removal rate results in origination of bainitic structure in the bars, which has adverse impact on plastic properties, in particular on the impact strength of the studied steel.
REFERE CES
[1] Raport: Polska Metalurgia w Unii europejskiej . Wydawnictwo Naukowe Akapit, Komitet Metalurgii PAN, Kraków (Report: Polish Metalurgy in UE, the Publishing House Akapit, Comitee of Metalurgy of Polish Science Academy Krakow) 2006, pp. 58. [2] Garbarz B., Szulc W., Rębiasz G.: Prognozy rozwoju popytu i podaży na rynku stalowych wyrobów hutniczych w Polsce (Forecast of development of demands and supply on the steel products market), Hutnik-Wiadomości Hutnicze, No 3(74), 2007, pp. 125-132. [3] Dudała R.: Duże będą jeszcze większe (Big become bigger), Nowy Przemysł, No 2(106, 2007), pp. 54-56. [4] Gabryś H. L.: Dobry rok. gorsze perspektywy (Good year. Worst perspectives), Nowy Przemysł. No 4(84), 2005, pp. 33-35. [5] Dudała R.: W oczekiwaniu na poprawę (In attendance for better), Nowy Przemysł, Raport Specjalny, 2005, pp. 136-141. [6] Stradomski G.. Dyja H.: Wpływ zabiegu wyżarzania normalizującego na strukturę stali S355J2G3, VII Międzynarodowa Konferencja Naukowa, Nowe technologie i osiągnięcia w metalurgii i inżynierii materiałowej (The influence of normalisation on the structure of S355J2G3 steel, VII Internation Scientific Conference New technologies and achievements in metallurgy and materials engineering), Seria Metalurgia nr 58, Częstochowa, 2006, pp. 546550. [7] Stradomski G., Rydz D., Dyja H.: Optymalizacja struktury stali w gatunku S355 w aspekcie poprawy właściwości plastycznych, VIII Międzynarodowa Konferencja Naukowa. Nowe technologie i osiągnięcia w metalurgii i inżynierii materiałowej (The optimalisation of S355 steel structure in aspects of higher plastic properties, VIII International Scientific Conference New technologies and achievements in metallurgy and materials engineering), Seria Metalurgia nr 61, Częstochowa, 2007, pp. 231-235. [8] Stradomski G., Rydz D., Dyja H., Stradomski Z.: Changes of microstructure after physical modeling of conventional and normalizing rolling of bars from carbon – manganese steel, Steel research international, Special Edition, Volume 1 2008, pp. 440-446.
THERMEC 2009 doi:10.4028/www.scientific.net/MSF.638-642 The Microstructure Change during Modeling of Conventional and ThermoMechanical Rolling of S355 Steel Bars doi:10.4028/www.scientific.net/MSF.638-642.2573 References [1] Raport: Polska Metalurgia w Unii europejskiej . Wydawnictwo Naukowe Akapit, Komitet Metalurgii PAN, Kraków (Report: Polish Metalurgy in UE, the Publishing House Akapit, Comitee of Metalurgy of Polish Science Academy Krakow) 2006, pp. 58. [2] Garbarz B., Szulc W., Rbiasz G.: Prognozy rozwoju popytu i poday na rynku stalowych wyrobów hutniczych w Polsce (Forecast of development of demands and supply on the steel products market), Hutnik-Wiadomoci Hutnicze, No 3(74), 2007, pp. 125-132. [3] Dudaa R.: Due bd jeszcze wiksze (Big become bigger), Nowy Przemys, No 2(106, 2007), pp. 54-56. [4] Gabry H. L.: Dobry rok. gorsze perspektywy (Good year. Worst perspectives), Nowy Przemys. No 4(84), 2005, pp. 33-35. [5] Dudaa R.: W oczekiwaniu na popraw (In attendance for better), Nowy Przemys, Raport Specjalny, 2005, pp. 136-141. [6] Stradomski G. Dyja H.: Wpyw zabiegu wyarzania normalizujcego na struktur stali S355J2G3, VII Midzynarodowa Konferencja Naukowa, Nowe technologie i osignicia w metalurgii i inynierii materiaowej (The influence of normalisation on the structure of S355J2G3 steel, VII Internation Scientific Conference New technologies and achievements in metallurgy and materials engineering), Seria Metalurgia nr 58, Czstochowa, 2006, pp. 546-550. [7] Stradomski G., Rydz D., Dyja H.: Optymalizacja struktury stali w gatunku S355 w aspekcie poprawy waciwoci plastycznych, VIII Midzynarodowa Konferencja Naukowa. Nowe technologie i osignicia w metalurgii i inynierii materiaowej (The optimalisation of S355 steel structure in aspects of higher plastic properties, VIII International Scientific Conference New technologies and achievements in metallurgy and materials engineering), Seria Metalurgia nr 61, Czstochowa, 2007, pp. 231-235. [8] Stradomski G., Rydz D., Dyja H., Stradomski Z.: Changes of microstructure after physical modeling of conventional and normalizing rolling of bars from carbon – manganese steel, Steel research international, Special Edition, Volume 1 2008, pp. 440446.
