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Dynamic recrystallization of ferrite in a low carbon steel during hot rolling in the (F+A) two-phase range
Pergamon
ScriptaMetallurgicaet Materialia,Vol. 31, No. 9, pp. 1193-1196,1994 Copyright©1994ElsevierScienceLtd Printedin the USA. All rights reserved 0956-716X/94 $6.00+ 00
DYNAMIC RECRYSTALLIZATION OF FERRITE IN A LOW CARBON STEEL DURING HOT ROLLING IN THE (F+A) TWO-PHASE RANGE R. Z. W a n g ' and T. C. Lei Department of Metals and Technology, Harbin Institute of Technology, Harbin 150001, P.R. China. * Now Working in the Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China ( R e c e i v e d March 16, 1994) ( R e v i s e d J u n e 15, 1994) Introduction The dynamic restoration mechanisms of metals during hot working have been one of the main topics in the past two decades (1-9). In most cases, it has been found that dynamic recovery is the sole dynamic restoration mechanism in ferrite (a-iron) due to its high stacking fault energy (1-5). Dynamic recrystallization is found to occur only in high purity ferrite and in ferritic interstitial free steels (9,10). In the latter case, the nucleation of dynamic recrystallization takes place via the bulging of grain boundaries (9,10). However, all of the above work has been done on single-phase ferrite, and the dynamic restoration behavior of ferrite in coarse two-phase strucures (e.g., ferrite-austenite two-phase structure in a low carbon steel) is still unclear. In a previous study on the hot deformation of low carbon steel in the ferrite-austenite (F+A) two-phase range (11), the authors found that the nucleation mechanism of dynamic recrystallization in the austenite phase was greatly influenced by the presence of F / A phase boundaries. In this paper, the dynamic restoration of ferrite in (F+A) two-phase structures and the role of F / A phase boundaries were investigated. Experimental The material used in this study was steel 1015 (with 0.17 C, 0.34 Si, 0.46Mn, 0.022 S, and 0.025 P, in wt. - % ) . Bar specimens 4 ~ 14 mm in thickness, 14 mm in width, and 80mm in length, were austenitized at 920 *C for 20 min and cooled at 1 °C / s to 800 °C , where they were held for 30 min to obtain an equilibrium ferrite-austenite two-phase structure. Then they were hot rolled on a laboratory rolling mill at a strain rate of 10 s-~. Specimens were water quenched immediately after hot rolling. The final thickness was 2.5 mm. Samples for optical and transmission electron microscopic observations were prepared from longitudinal sections of the rolled specimens. Marshall's etchant was used to reveal the sub-boundaries of the ferrite phase (12). Thin foils were observed in a Philips CM 12 transmission electron microscope. Local crystallographic orientations were determined from the Kikuchi diagrams obtained using a microdiffraction technique. Results and Discussion The volume fraction and average grain size of the ferrite phase in the tested steel at 800 ~ are 49% and 14 #m, respectively, as described previously (8). The optical micrographs in Fig. 1 show the substructural development in the ferrite with increasing rolling reduction. The sub-boundaries are found to be randomly distributed within the ferrite grains at a reduction of 30% (Fig. la), indicating that dynamic recovery of the ferrite is at its developing stage. At a rolling reduction of 50%, perfectly developed and equiaxed subgrains form in the ferrite (Fig. lb), which indicates that the ferrite phase has reached the steady state stage of dynamic recovery. TEM observations confirm the above results, as shown in Fig. 2a. The average subgrain size at this stage is 1.1/~m, and the misorientations across the subgrain boundaries are in the range 0.5--2.5 ° (as marked in Fig. 2a). When the 1193
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rolling reduction is further increased to 70%, the ferrite subgrains maintain an equiaxed shape and show little change in dimensions, although the original ferrite and austenite phases are elongated along the rolling direction (Fig. lc). In all the three stages described above, no dynamically recrystallized grains are found to form through bulging of the original ferrite grain boundaries. Therefore, it can be concluded that, during the hot rolling of (F+A) two-phase structures at relatively low rolling reductions, ferrite undergoes dynamic recovery, which is the same as in the case of single phase ferrite. Experiments reveal that, when the rolling reduction is further increased, the misorientation across the ferrite subgrain boundaries does not remain constant, but gradually increases with strain. The optical micrograph in Fig. ld shows that the ferrite phase is severely deformed at a rolling reduction of 85%, and consists of only one layer of subgrains in some places (indicated by arrows). Such single-layer subgrains can be seen more clearly in the TEM photograph shown in Fig. 2b. The mean subgrain size in Fig. 2b is 1.8 pm, and the misorientations across the sub-boundaries are in the range 5 ~ 12 ° The positive and negative signs in Fig. 2b reflect the changing directions of the Kikuchi pole across a given sub-boundary, and the alternations in sign indicate that the subgrains are rotated relative to each other in a regular way. The misorientations in Fig. 2b are at least three times those in Fig. 2a, where the rolling reduction is 50%, while the subgrain size in the former is less than two times that in the latter. Evidently, the large misorientations in Fig. 2b are not caused by the coalescence of subgrains, but by the continuous rotation of subgrains during rolling. Fig. 2c and d give two other examples of single-layer ferrite subgrains; the misorientations across the sub-boundaries are as high as 10 ° and 20~ 40 ° respectively, the latter falling in the high angle grain boundary range. That is, a kind of dynamic recrystallization has taken place in the ferrite phase at this stage. In single-phase ferrite, it has been known that the misorientations across subgrain boundaries remain unchanged on further straining when the stable state of dynamic recovery has been reached (1-4). But in the hot deformation of the (F+A) two-phase structure discussed here, the misorientation across sub-boundaries continues to increase with strain until the sub-boundaries are changed into high angle grain boundaries, while the subgrain size undergoes no significant increase. This is similar to the so-called continuous dynamic recrystallization often observed in aluminum alloys (13), and it is obviously different from the discontinuous dynamic recrystallization found in high purity ferrite and in ferritic interstitial free steels where dynamic recrystallization nucleates via the bulging of ferrite grain bounderies (9,10). The reason for continuous dynamic recrystallization in the ferrite is mainly the presence of the F / A phase boundaries. At high strains, especially when the ferrite phase is deformed to a thickness of only one or two subgrains, the ferrite subgrain boundaries are strongly pinned by the F / A interfaces, and dynamic recovery of the ferrite cannot continue by the unravelling and reknitting of sub-boundaries. These constrained sub-boundaries absorb dislocations continuously on further straining; as a result, the misorientation gradually increases with strain, and continuous dynamic recrystallization takes place. This process also differs from the geometric dynamic recrystallization discussed by McQueen et al. on severely hot deformed aluminum alloys (5), where the sub-boundaries were replaced discontinuously by pre-existing grain boundaries. Summary When a low carbon steel is hot rolled in the ferrite-austenite two-phase range, dynamic recovery occurs in the ferrite phase at relatively low strains. At high strains, the dynamic recrystallization of ferrite takes place via the continuous dynamic recrystallization mechanism due to the constraints caused by the presence of ferrite / austenite phase boundaries. References 1. H.J. McQueen and J. J. Jonas, in R. J. Arsenault (ed.), Plastic Deformation of Materials, Treatise on Materials Science and Technology, vol.6, P.393, Academic Press (1975). 2. H. J. McQueen, Metall. Trans. 8A, 807 (1977). 3. H. J. McQueen and J.J. Jonas, J. Applied Metalworking, 3, 233 (1984). 4. E. Inoue and T. Sakai, J. Japan Inst. Metals, 55, 286 (1991).
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5. H. J. McQueen, E. Evangelista and M. E. Kassner, Z. Metallkde. 82, 336 (1991). 6. T. Sakai and J. J. Jonas, Acta Metall. 32, 189(1984). 7. W. Roberts, in G. Krauss (ed.), Deformation, Processing and Structure, p.109, American Society for Metals, Metals Park, OH (1983). 8. R.Z. Wang and T. C. Lei, Mater. Sci. Eng. A165, 19 (1993). 9. G. Glover and C. M. Sellars, Metall. Trans. 4, 765 (1973). 10. A. Najafi-Zadeh, J.J. Jonas and S. Yue, Metall. Trans. 23A, 2607 (1992). 11. R. Z. Wang and T. C. Lei, Scr. Metall. Mater. 28,725 (1993). 12. W, A. Marshall, J. Electrodep. Tech. Soc. 28, 27 (1952). 13. S. J. Hales and T. R. McNelley, Acta Metall. 36, 1229 (1988).
Fig.1 Optical micrographs of the (F+A) two-phase structures produced when steel 1015 was quenched after rolling at 800°C showing the dependence of the ferrite substructure on reduction in thickness a: 30%; b: 50%; c: 70%; d: 85%
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Fig.2 TEM photographs of the (F+A) two-phase structures obtained by quenc hing after rolling at 800U illustrating the occurence of continuous dynamic recrystallization in the ferrite. The numbers specify the misorientations across the sub-boundaries a: thickness reduction 50%; b-d: 85%
Vol. 31, No. 9
ScriptaMetallurgicaet Materialia,Vol. 31, No. 9, pp. 1193-1196,1994 Copyright©1994ElsevierScienceLtd Printedin the USA. All rights reserved 0956-716X/94 $6.00+ 00
DYNAMIC RECRYSTALLIZATION OF FERRITE IN A LOW CARBON STEEL DURING HOT ROLLING IN THE (F+A) TWO-PHASE RANGE R. Z. W a n g ' and T. C. Lei Department of Metals and Technology, Harbin Institute of Technology, Harbin 150001, P.R. China. * Now Working in the Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, P.R. China ( R e c e i v e d March 16, 1994) ( R e v i s e d J u n e 15, 1994) Introduction The dynamic restoration mechanisms of metals during hot working have been one of the main topics in the past two decades (1-9). In most cases, it has been found that dynamic recovery is the sole dynamic restoration mechanism in ferrite (a-iron) due to its high stacking fault energy (1-5). Dynamic recrystallization is found to occur only in high purity ferrite and in ferritic interstitial free steels (9,10). In the latter case, the nucleation of dynamic recrystallization takes place via the bulging of grain boundaries (9,10). However, all of the above work has been done on single-phase ferrite, and the dynamic restoration behavior of ferrite in coarse two-phase strucures (e.g., ferrite-austenite two-phase structure in a low carbon steel) is still unclear. In a previous study on the hot deformation of low carbon steel in the ferrite-austenite (F+A) two-phase range (11), the authors found that the nucleation mechanism of dynamic recrystallization in the austenite phase was greatly influenced by the presence of F / A phase boundaries. In this paper, the dynamic restoration of ferrite in (F+A) two-phase structures and the role of F / A phase boundaries were investigated. Experimental The material used in this study was steel 1015 (with 0.17 C, 0.34 Si, 0.46Mn, 0.022 S, and 0.025 P, in wt. - % ) . Bar specimens 4 ~ 14 mm in thickness, 14 mm in width, and 80mm in length, were austenitized at 920 *C for 20 min and cooled at 1 °C / s to 800 °C , where they were held for 30 min to obtain an equilibrium ferrite-austenite two-phase structure. Then they were hot rolled on a laboratory rolling mill at a strain rate of 10 s-~. Specimens were water quenched immediately after hot rolling. The final thickness was 2.5 mm. Samples for optical and transmission electron microscopic observations were prepared from longitudinal sections of the rolled specimens. Marshall's etchant was used to reveal the sub-boundaries of the ferrite phase (12). Thin foils were observed in a Philips CM 12 transmission electron microscope. Local crystallographic orientations were determined from the Kikuchi diagrams obtained using a microdiffraction technique. Results and Discussion The volume fraction and average grain size of the ferrite phase in the tested steel at 800 ~ are 49% and 14 #m, respectively, as described previously (8). The optical micrographs in Fig. 1 show the substructural development in the ferrite with increasing rolling reduction. The sub-boundaries are found to be randomly distributed within the ferrite grains at a reduction of 30% (Fig. la), indicating that dynamic recovery of the ferrite is at its developing stage. At a rolling reduction of 50%, perfectly developed and equiaxed subgrains form in the ferrite (Fig. lb), which indicates that the ferrite phase has reached the steady state stage of dynamic recovery. TEM observations confirm the above results, as shown in Fig. 2a. The average subgrain size at this stage is 1.1/~m, and the misorientations across the subgrain boundaries are in the range 0.5--2.5 ° (as marked in Fig. 2a). When the 1193
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rolling reduction is further increased to 70%, the ferrite subgrains maintain an equiaxed shape and show little change in dimensions, although the original ferrite and austenite phases are elongated along the rolling direction (Fig. lc). In all the three stages described above, no dynamically recrystallized grains are found to form through bulging of the original ferrite grain boundaries. Therefore, it can be concluded that, during the hot rolling of (F+A) two-phase structures at relatively low rolling reductions, ferrite undergoes dynamic recovery, which is the same as in the case of single phase ferrite. Experiments reveal that, when the rolling reduction is further increased, the misorientation across the ferrite subgrain boundaries does not remain constant, but gradually increases with strain. The optical micrograph in Fig. ld shows that the ferrite phase is severely deformed at a rolling reduction of 85%, and consists of only one layer of subgrains in some places (indicated by arrows). Such single-layer subgrains can be seen more clearly in the TEM photograph shown in Fig. 2b. The mean subgrain size in Fig. 2b is 1.8 pm, and the misorientations across the sub-boundaries are in the range 5 ~ 12 ° The positive and negative signs in Fig. 2b reflect the changing directions of the Kikuchi pole across a given sub-boundary, and the alternations in sign indicate that the subgrains are rotated relative to each other in a regular way. The misorientations in Fig. 2b are at least three times those in Fig. 2a, where the rolling reduction is 50%, while the subgrain size in the former is less than two times that in the latter. Evidently, the large misorientations in Fig. 2b are not caused by the coalescence of subgrains, but by the continuous rotation of subgrains during rolling. Fig. 2c and d give two other examples of single-layer ferrite subgrains; the misorientations across the sub-boundaries are as high as 10 ° and 20~ 40 ° respectively, the latter falling in the high angle grain boundary range. That is, a kind of dynamic recrystallization has taken place in the ferrite phase at this stage. In single-phase ferrite, it has been known that the misorientations across subgrain boundaries remain unchanged on further straining when the stable state of dynamic recovery has been reached (1-4). But in the hot deformation of the (F+A) two-phase structure discussed here, the misorientation across sub-boundaries continues to increase with strain until the sub-boundaries are changed into high angle grain boundaries, while the subgrain size undergoes no significant increase. This is similar to the so-called continuous dynamic recrystallization often observed in aluminum alloys (13), and it is obviously different from the discontinuous dynamic recrystallization found in high purity ferrite and in ferritic interstitial free steels where dynamic recrystallization nucleates via the bulging of ferrite grain bounderies (9,10). The reason for continuous dynamic recrystallization in the ferrite is mainly the presence of the F / A phase boundaries. At high strains, especially when the ferrite phase is deformed to a thickness of only one or two subgrains, the ferrite subgrain boundaries are strongly pinned by the F / A interfaces, and dynamic recovery of the ferrite cannot continue by the unravelling and reknitting of sub-boundaries. These constrained sub-boundaries absorb dislocations continuously on further straining; as a result, the misorientation gradually increases with strain, and continuous dynamic recrystallization takes place. This process also differs from the geometric dynamic recrystallization discussed by McQueen et al. on severely hot deformed aluminum alloys (5), where the sub-boundaries were replaced discontinuously by pre-existing grain boundaries. Summary When a low carbon steel is hot rolled in the ferrite-austenite two-phase range, dynamic recovery occurs in the ferrite phase at relatively low strains. At high strains, the dynamic recrystallization of ferrite takes place via the continuous dynamic recrystallization mechanism due to the constraints caused by the presence of ferrite / austenite phase boundaries. References 1. H.J. McQueen and J. J. Jonas, in R. J. Arsenault (ed.), Plastic Deformation of Materials, Treatise on Materials Science and Technology, vol.6, P.393, Academic Press (1975). 2. H. J. McQueen, Metall. Trans. 8A, 807 (1977). 3. H. J. McQueen and J.J. Jonas, J. Applied Metalworking, 3, 233 (1984). 4. E. Inoue and T. Sakai, J. Japan Inst. Metals, 55, 286 (1991).
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5. H. J. McQueen, E. Evangelista and M. E. Kassner, Z. Metallkde. 82, 336 (1991). 6. T. Sakai and J. J. Jonas, Acta Metall. 32, 189(1984). 7. W. Roberts, in G. Krauss (ed.), Deformation, Processing and Structure, p.109, American Society for Metals, Metals Park, OH (1983). 8. R.Z. Wang and T. C. Lei, Mater. Sci. Eng. A165, 19 (1993). 9. G. Glover and C. M. Sellars, Metall. Trans. 4, 765 (1973). 10. A. Najafi-Zadeh, J.J. Jonas and S. Yue, Metall. Trans. 23A, 2607 (1992). 11. R. Z. Wang and T. C. Lei, Scr. Metall. Mater. 28,725 (1993). 12. W, A. Marshall, J. Electrodep. Tech. Soc. 28, 27 (1952). 13. S. J. Hales and T. R. McNelley, Acta Metall. 36, 1229 (1988).
Fig.1 Optical micrographs of the (F+A) two-phase structures produced when steel 1015 was quenched after rolling at 800°C showing the dependence of the ferrite substructure on reduction in thickness a: 30%; b: 50%; c: 70%; d: 85%
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Fig.2 TEM photographs of the (F+A) two-phase structures obtained by quenc hing after rolling at 800U illustrating the occurence of continuous dynamic recrystallization in the ferrite. The numbers specify the misorientations across the sub-boundaries a: thickness reduction 50%; b-d: 85%
Vol. 31, No. 9