Role of hard martensite phase prior to cold-rolling on microstructure evolution after annealing in ferritic stainless steel

To improve the surface quality of 16mass%Cr ferritic stainless steel sheets processed by cold-rolling and subsequent annealing, the role of the hard martensite phase, in particular, the hardness ratio k between martensite and ferrite prior to cold-rolling on microstructure evolution was investigated by varying the k value through tempering specimens at different temperatures after intercritical annealing. In the specimen with the high ratio k (k>2.0), the heterogeneous deformation in ferrite around the hard martensite becomes prominent and the areas with the high KAM value are locally limited with less development of cold-rolling texture. The specimen with the small ratio k (k<2.0), however, shows rather uniform deformation structure, because tempered martensite deforms easily. Therefore, the recrystallized grain with the specimen k>2.0 becomes fine, because the presence of hard martensite provides the nucleation site of recrystallization leading to fragmenting the elongated {100}<011>colony structure, and the orientation is randomized. Consequently, the presence of the hard martensite phase prior to cold-rolling is effective for preventing the ridging of ferritic stainless steel sheets.