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,2012Nanostructured 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