Effects of <i>α</i>+<i>γ</i> Intercritical and <i>γ</i> Single-Phase Annealing on Martensitic Texture Evolution in Dual-Phase Steel Sheets

Abstract

Dual-phase (DP) steel sheets composed of soft ferrite and hard martensite phases are typical advanced high strength steel sheets applicable to a variety of automobile parts. The crystallite texture of steel sheets is an important factor which influences the press formability. However, the texture of the martensite itself in DP steel sheets has not been discussed, since the texture was generally measured by the X-ray diffraction method, which does not distinguish the texture of martensite from that of ferrite. The objective of this study is to investigate the texture evolution behavior of each compromising phase; especially the martensite phase, in DP steel sheets by a newly-developed analysis method using Electron Back-Scatter Diffraction (EBSD). The chemical composition of the steel used was 0.088%C-1.23%Si-2.29%Mn-0.093%Ti (mass%). The two sequent annealing was conducted, changing the second annealing temperature, both in the intercrtical region and in the γ single-phase region. The obtained DP microstructures were controlled to have the same volume fraction of martensite of approximately 40%. The overall texture including martensite after the intercritical annealing was similar to the texture before 2nd-annealing, while the texture after the γ single-phase annealing became weak. The new analysis technique using OIM clearly revealed that the discriminate textures from only martensite were close to, but slightly weaker than those of ferrite under the two annealing conditions.