62 Advanced materials in automotive engineering (b) c (d) (e) (f) (g (h) () ND RD 2μm 4.3 SEM microstructures of (a),(d),(g),cold-rolled specimen,(b), (e),(h),specimens annealed at 600C after cold-rolling and (c),(f), (i),specimens annealed at 650C after cold-rolling.The cold-rolling reductions are (a),(b),(c),85%,(d),(e),(f),91%and (g),(h),(i),94%. Observed from TD. shaped martensite in Fig.4.3 suggested that large plastic strains including shear strain components were introduced to the ferrite grains. On the other hand,the martensite islands were also deformed to some extent in the cold-rolling and showed diamond shapes.Table 4.2 summarises the cold-rolling reduction and the mean thickness ratio,t/to,of the martensite islands in the microstructures measured using the OM of the hot-rolled sheet and the SEM micrograph of the cold-rolled specimens. Here,t and to are the mean intersect lengths of the martensite islands along ND after and before cold-rolling,respectively.The reduction of the martensite islands was much smaller than the reduction of the specimen, which indicated that a larger strain was introduced to the ferrite grains. By TEM analysis in the previous study,s large local misorientations in the deformed martensite regions were also confirmed in spite of the smaller strain in martensite.Ueji et al.24 have reported that 50%cold-rolled low-carbon martensite exhibited fine lamellar structure involving large misorientations. which was equivalent to the microstructure in SPD processed steels.This is thought to be attributed to the complex and fine microstructure of the Woodhead Publishing Limited,2012
62 Advanced materials in automotive engineering © Woodhead Publishing Limited, 2012 shaped martensite in Fig. 4.3 suggested that large plastic strains including shear strain components were introduced to the ferrite grains. On the other hand, the martensite islands were also deformed to some extent in the cold-rolling and showed diamond shapes. Table 4.2 summarises the cold-rolling reduction and the mean thickness ratio, t/t0, of the martensite islands in the microstructures measured using the OM of the hot-rolled sheet and the SEM micrograph of the cold-rolled specimens. Here, t and t0 are the mean intersect lengths of the martensite islands along ND after and before cold-rolling, respectively. The reduction of the martensite islands was much smaller than the reduction of the specimen, which indicated that a larger strain was introduced to the ferrite grains. By TEM analysis in the previous study,8 large local misorientations in the deformed martensite regions were also confirmed in spite of the smaller strain in martensite. Ueji et al. 24 have reported that 50% cold-rolled low-carbon martensite exhibited fine lamellar structure involving large misorientations, which was equivalent to the microstructure in SPD processed steels. This is thought to be attributed to the complex and fine microstructure of the (a) (d) (g) (b) (e) (h) (c) (f) (i) ND RD 2 µm 4.3 SEM microstructures of (a), (d), (g), cold-rolled specimen, (b), (e), (h), specimens annealed at 600°C after cold-rolling and (c), (f), (i), specimens annealed at 650°C after cold-rolling. The cold-rolling reductions are (a), (b), (c), 85%, (d), (e), (f), 91% and (g), (h), (i), 94%. Observed from TD
Nanostructured steel for automotive body structures 63 Table 4.2 Cold-rolling reduction and mean thickness ratio of the specimens and the martensite islands of the cold-rolled UFG-FC specimens Cold-rolling reduction t/to of the specimen Mean t/to of the of the specimen(%) martensite islands 85 0.15 0.47 91 0.09 0.32 94 0.06 0.27 as-transformed martensite,also involving a high density of dislocations. They also showed equiaxed UFG microstructure after annealing of the 50% cold-rolled specimen with a single phase of martensite.It can be concluded therefore that the strain applied to martensite in the present study was not very large but probably enough to introduce large local misorientations and to form UFG microstructure through subsequent annealing. Figure 4.3 (b),(e)and (h)show the microstructures of the cold-rolled sheets after annealing at 600C.In the 85%cold-rolled and annealed specimen (Fig.4.3 (b)),both equiaxed fine ferrite grains and elongated ferrite grains located in an arc-like row (in the lower part of Fig.4.3(b)) were observed.The fine equiaxed ferrite grains seemed to be formed by continuous coarsening of the finely subdivided regions in the cold-rolled microstructure together with recovery.8 It was difficult to distinguish clearly which area in Fig.4.3 (b)was originally ferrite or martensite,but according to the supposed mechanism discussed before,the ultrafine ferrite grains could originate from both ferrite and martensite.On the other hand, some coarse grains in an arc-like row seen were probably formed mainly by recovery of the ferrite regions such as the elongated and bent ferrite seen in the upper and left part of Fig.4.3(a).Those ferrite regions in the cold-rolled microstructure seem to be deformed to a smaller plastic strain because of the relatively low density of surrounding martensite islands. Also the fact that the coarse ferrite grains in the annealed microstructure contained few cementite particles suggests that they were originally ferrite When the cold-rolling reduction was higher than 91%,the specimens were filled mostly with equiaxed ultrafine ferrite grains(Fig.4.3 (e)and (h)). This seemed to be because the spacing of the martensite islands as seen in Fig.4.3(d)and (g)decreased due to larger cold-rolling reductions.When the specimens were annealed at 650C.significant grain growth occurred in all the specimens.Fine cementite particles dispersed in ferrite grains were also observed in all annealed specimens in Fig.4.3.Figure 4.4 shows the relationship between the annealing temperature and mean ferrite grain size measured on the SEM micrographs by the mean intersection method along the RD and ND.The larger the cold-rolling reduction was,the smaller the obtained ferrite grain size became. Woodhead Publishing Limited,2012
Nanostructured steel for automotive body structures 63 © Woodhead Publishing Limited, 2012 as-transformed martensite, also involving a high density of dislocations. They also showed equiaxed UFG microstructure after annealing of the 50% cold-rolled specimen with a single phase of martensite. It can be concluded therefore that the strain applied to martensite in the present study was not very large but probably enough to introduce large local misorientations and to form UFG microstructure through subsequent annealing. Figure 4.3 (b), (e) and (h) show the microstructures of the cold-rolled sheets after annealing at 600 °C. In the 85% cold-rolled and annealed specimen (Fig. 4.3 (b)), both equiaxed fine ferrite grains and elongated ferrite grains located in an arc-like row (in the lower part of Fig. 4.3 (b)) were observed. The fine equiaxed ferrite grains seemed to be formed by continuous coarsening of the finely subdivided regions in the cold-rolled microstructure together with recovery.8 It was difficult to distinguish clearly which area in Fig. 4.3 (b) was originally ferrite or martensite, but according to the supposed mechanism discussed before, the ultrafine ferrite grains could originate from both ferrite and martensite. On the other hand, some coarse grains in an arc-like row seen were probably formed mainly by recovery of the ferrite regions such as the elongated and bent ferrite seen in the upper and left part of Fig. 4.3 (a). Those ferrite regions in the cold-rolled microstructure seem to be deformed to a smaller plastic strain because of the relatively low density of surrounding martensite islands. Also the fact that the coarse ferrite grains in the annealed microstructure contained few cementite particles suggests that they were originally ferrite. When the cold-rolling reduction was higher than 91%, the specimens were filled mostly with equiaxed ultrafine ferrite grains (Fig. 4.3 (e) and (h)). This seemed to be because the spacing of the martensite islands as seen in Fig. 4.3 (d) and (g) decreased due to larger cold-rolling reductions. When the specimens were annealed at 650 °C, significant grain growth occurred in all the specimens. Fine cementite particles dispersed in ferrite grains were also observed in all annealed specimens in Fig. 4.3. Figure 4.4 shows the relationship between the annealing temperature and mean ferrite grain size measured on the SEM micrographs by the mean intersection method along the RD and ND. The larger the cold-rolling reduction was, the smaller the obtained ferrite grain size became. Table 4.2 Cold-rolling reduction and mean thickness ratio of the specimens and the martensite islands of the cold-rolled UFG-FC specimens Cold-rolling reduction t/t0 of the specimen Mean t/t0 of the of the specimen (%) martensite islands 85 0.15 0.47 91 0.09 0.32 94 0.06 0.27
64 Advanced materials in automotive engineering 0-85% 昌e △91% 0-94% 5 550 600 650 700 Annealing temperature,TC 4.4 The relationship of annealing temperatures and mean ferrite grain sizes measured by the mean intersect method of UFG-FC steels. The sizes of the equiaxed ultrafine ferrite grains shown in Fig.4.3(b),(e) and(h)were not so different.However,due to the existence of the elongated and arc-like ferrite grains the average ferrite grain size in the specimen 85% cold-rolled and annealed at 600C(Fig.4.3(b))was slightly larger than that in other specimens having larger cold-rolling reductions(Fig.4.3 (e)and (h)).On the other hand,the effect of the annealing temperature on the ferrite grain size was more significant as shown in Fig.4.4.The 85%cold-rolled specimens annealed at 600C and 625C contained some recovered ferrite grains as shown in Fig.4.3(b).The minimum grain size of the homogeneous grain structures obtained was 0.43 um in the specimen 94%cold-rolled and then annealed at 600C(Fig.4.3 (h)). The UFG ferrite formed throughout the specimens by large cold-rolling reductions followed by low annealing temperatures.In order to clarify the process of fine-grain formation,Fig.4.5 shows the microstructure of a specimen that was 94%cold-rolled and subsequently annealed at 525C for 120 seconds. The dotted lines roughly delineate the former martensite islands that had already changed to fine-grained ferrite and cementite.In the ferrite matrix, UFG ferrite also formed along the wavy microstructure in the vicinity of the former martensite.Finer equiaxed grains of ferrite were seen around the former martensite islands.The result suggests that a larger strain was introduced into the ferrite/martensite interface regions through cold-rolling. Therefore,in order to reduce the cold-rolling reductions required,it seems to be effective to decrease the spacing of martensite islands in the starting microstructure before cold-rolling. Woodhead Publishing Limited,2012
64 Advanced materials in automotive engineering © Woodhead Publishing Limited, 2012 The sizes of the equiaxed ultrafine ferrite grains shown in Fig. 4.3 (b), (e) and (h) were not so different. However, due to the existence of the elongated and arc-like ferrite grains the average ferrite grain size in the specimen 85% cold-rolled and annealed at 600 °C (Fig. 4.3 (b)) was slightly larger than that in other specimens having larger cold-rolling reductions (Fig. 4.3 (e) and (h)). On the other hand, the effect of the annealing temperature on the ferrite grain size was more significant as shown in Fig. 4.4. The 85% cold-rolled specimens annealed at 600 °C and 625 °C contained some recovered ferrite grains as shown in Fig. 4.3 (b). The minimum grain size of the homogeneous grain structures obtained was 0.43 mm in the specimen 94% cold-rolled and then annealed at 600 °C (Fig. 4.3 (h)). The UFG ferrite formed throughout the specimens by large cold-rolling reductions followed by low annealing temperatures. In order to clarify the process of fine-grain formation, Fig. 4.5 shows the microstructure of a specimen that was 94% cold-rolled and subsequently annealed at 525 °C for 120 seconds. The dotted lines roughly delineate the former martensite islands that had already changed to fine-grained ferrite and cementite. In the ferrite matrix, UFG ferrite also formed along the wavy microstructure in the vicinity of the former martensite. Finer equiaxed grains of ferrite were seen around the former martensite islands. The result suggests that a larger strain was introduced into the ferrite/martensite interface regions through cold-rolling. Therefore, in order to reduce the cold-rolling reductions required, it seems to be effective to decrease the spacing of martensite islands in the starting microstructure before cold-rolling. 85% 91% 94% 550 600 650 700 Annealing temperature, T/°C Mean ferrite grain size, d/mm 2 1.5 1 0.5 0 4.4 The relationship of annealing temperatures and mean ferrite grain sizes measured by the mean intersect method of UFG-FC steels
Nanostructured steel for automotive body structures 65 ND RD 1μm 4.5 SEM microstructure of UFG-FC steel that was 94%cold-rolled and annealed at 525C. 4.3.2 Mechanical properties of nanostructured ferrite- cementite steel sheets The tensile properties of nanostructured ferrite-cementite steels were investigated using a high-speed servo-hydraulic material test system produced by Saginomiya Inc.equipped with a special load-sensing block.25.26 With this machine,stress-strain (s-s)curves at a wide range of strain rates from quasi-static to dynamic deformations can be obtained.Figure 4.6 shows the appearance of the prepared tensile specimen having a gauge length of 6mm and a width of 2mm. The tensile direction was parallel to RD.Tensile tests were operated at various strain rates ranging from 10-s to 10s-at room temperature.Total elongation of the specimens was measured from the difference in the gauge length before and after testing.Fabricating conditions,microstructures and quasi-static mechanical properties measured at a strain rate of 102s-are summarised in Table 4.3. UFG-FCA,B,C and FCM specimens were prepared by 91%cold-rolling of the same hot-rolled sheet as shown in Fig.4.2 and subsequent annealing at 620C,635C,670C and 700C for 120 seconds,respectively.The mean ferrite grain sizes indicated in Table 4.3 were calculated by the EBSD (electron backscatter diffraction)data measured on the TD sections.UFG- FC steels showed microstructures of ultrafine ferrite and cementite,while the FCM specimen showed a microstructure composed of ferrite,cementite and martensite.FCM specimen contained about 14%of martensite in the Woodhead Publishing Limited,2012
Nanostructured steel for automotive body structures 65 © Woodhead Publishing Limited, 2012 4.3.2 Mechanical properties of nanostructured ferritecementite steel sheets The tensile properties of nanostructured ferrite-cementite steels were investigated using a high-speed servo-hydraulic material test system produced by Saginomiya Inc. equipped with a special load-sensing block.25,26 With this machine, stress–strain (s–s) curves at a wide range of strain rates from quasi-static to dynamic deformations can be obtained. Figure 4.6 shows the appearance of the prepared tensile specimen having a gauge length of 6mm and a width of 2mm. The tensile direction was parallel to RD. Tensile tests were operated at various strain rates ranging from 10–2s –1 to 103 s –1 at room temperature. Total elongation of the specimens was measured from the difference in the gauge length before and after testing. Fabricating conditions, microstructures and quasi-static mechanical properties measured at a strain rate of 10–2s –1 are summarised in Table 4.3. UFG-FC A, B, C and FCM specimens were prepared by 91% cold-rolling of the same hot-rolled sheet as shown in Fig. 4.2 and subsequent annealing at 620 °C, 635 °C, 670 °C and 700 °C for 120 seconds, respectively. The mean ferrite grain sizes indicated in Table 4.3 were calculated by the EBSD (electron backscatter diffraction) data measured on the TD sections. UFGFC steels showed microstructures of ultrafine ferrite and cementite, while the FCM specimen showed a microstructure composed of ferrite, cementite and martensite. FCM specimen contained about 14% of martensite in the ND RD 1 µm 4.5 SEM microstructure of UFG-FC steel that was 94% cold-rolled and annealed at 525 °C
66 Advanced materials in automotive engineering 0 27 4.6 Schematic drawing of the test piece for dynamic and quasi-static tensile tests. microstructure.The Acl transformation temperatures of UFG-FC steel measured by a dilatometer was approximately 700C,so that the FCM specimen contained both ferrite and austenite during the annealing at 700C and the austenite transformed to martensite during the subsequent water-cooling. Figure 4.7 shows s-s curves at strain rates of 10-2,102 and 103 s-1.Figure 4.7 (a).(b),(c)and (d)correspond to the test results of UFG-FC steels A. B,C and the FCM steel listed in Table 4.3. Both UFG-FC and FCM steels showed the yield drop phenomenon.The flow stress increased whereas the uniform elongation significantly decreased as the ferrite grain size in the UFG-FC specimens decreased(Fig.4.7(a). (b)and (c)).The same behavior has been reported for UFG IF steel sheets prepared by ARB and the annealing process.18 On the other hand,as shown in Fig.4.7(d),the work hardening increased when martensite was introduced in the microstructure. The flow stress significantly increased when the strain rate increased in UFG-FC steels.In order to investigate the strain rate dependence of the flow stress,the difference in tensile flow stress at 5%nominal strain between the strain rates of 103 s-and 10-2 s-,Ao,was evaluated.Figure 4.8 shows the Woodhead Publishing Limited,2012
66 Advanced materials in automotive engineering © Woodhead Publishing Limited, 2012 microstructure. The Ac1 transformation temperatures of UFG-FC steel measured by a dilatometer was approximately 700 °C, so that the FCM specimen contained both ferrite and austenite during the annealing at 700 °C and the austenite transformed to martensite during the subsequent water-cooling. Figure 4.7 shows s–s curves at strain rates of 10–2, 102 and 103 s–1. Figure 4.7 (a), (b), (c) and (d) correspond to the test results of UFG-FC steels A, B, C and the FCM steel listed in Table 4.3. Both UFG-FC and FCM steels showed the yield drop phenomenon. The flow stress increased whereas the uniform elongation significantly decreased as the ferrite grain size in the UFG-FC specimens decreased (Fig. 4.7 (a), (b) and (c)). The same behavior has been reported for UFG IF steel sheets prepared by ARB and the annealing process.18 On the other hand, as shown in Fig. 4.7 (d), the work hardening increased when martensite was introduced in the microstructure. The flow stress significantly increased when the strain rate increased in UFG-FC steels. In order to investigate the strain rate dependence of the flow stress, the difference in tensile flow stress at 5% nominal strain between the strain rates of 103 s–1 and 10–2 s–1, Ds, was evaluated. Figure 4.8 shows the 10 6 2 27 4.6 Schematic drawing of the test piece for dynamic and quasi-static tensile tests