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

Cold-rolled and annealed ferrite-martensite dual phase (DP) steel sheets are a useful material for automotive applications because of their excellent balance of strength and ductility. Although few research papers have examined the crystallography and microstructure of lath martensite in DP steel, the characteristics of lath martensite have been investigated in single-phase martensitic steel. In the present study, the crystallography and microstructure of lath martensite in a ferrite-martensite dual phase were studied using electron microscopy and electron diffraction analysis, and the crystallographic orientation relationship between the ferrite and martensite was analyzed. The main results are as follows: (1) The lath martensite in DP steel consists of some number of packets, and these packets consist of blocks. This structure is the same as the microstructure of single-phase lath martensite. (2) The habit planes of the martensite laths in a packet tend to be parallel to the close-packed plane of the adjacent ferrite grain, whose fraction is about 30%.