Crystallographic Analysis of Lath Martensite in Ferrite-Martensite Dual Phase Steel Sheet Annealed after Cold-Rolling

The martensite steels produced by quenching and tempering has been used for machine structural use. Martensite in ferrite-martensite dual-phase steels (hereinafter, DP steels) also has important roll in strengthening of the automobile steel sheets. Therefore the importance of analyzing the microstructure and their mechanical properties of the martensite has been increased year by year. Many researches of microstructure analyses of the martensite have been carried out.1,2) Especially in recent years, the microstructure and crystallography of martensite using electron back scatter diffraction (hereinafter, EBSD) were reported.3) Among several kinds of martensite morphologies, the lath martensite consisting of fine martensite crystals with a thickness of around 0.2 μm is frequently used in practical steels. Figure 1 shows a schematic illustration of the microstructure of the lath martensite in low carbon steels. In the lath martensite structure, the grain boundaries of austenite are inherited invariably because of its diffusionless transformation characteristics, and the prior austenite grain is divided into packets, which are groups of laths having the same habit plane. A packet consists of several blocks, each of which in turn consists of groups of laths having similar crystal orientations. However, previous researches on the martensitic microstructure described above concerned only fully martensitic Crystallographic Analysis of Lath Martensite in Ferrite-Martensite Dual Phase Steel Sheet Annealed after Cold-Rolling