Deformation Mode and Strain Path Dependence of Martensite Phase Transformation in QP980 Steel

The martensite phase transformation dependence upon deformation modes and strain paths in QP980 steel formed into a T-shape panel was studied through combination of a 3D digital image correlation (DIC) and neutron diffraction. The T-shape emulates the rocker-end of a generic automotive component B-pillar. QP980 belongs to the third-generation advanced high strength steel, which is chosen for two reasons: (1) deformation-induced phase transformation from austenite to martensite occurs in QP980; (2) the initial retained austenite volume fraction (RAVF) of approximately 12% enables the examination of martensite phase transformation at various deformation levels before fracture. Strain fields obtained from DIC enable an investigation of the effects of linear, bi-linear, and nonlinear strain paths together with deformation modes, such as tension, plane strain, biaxial tension and equibiaxial tension. The measured RAVF values from neutron diffraction were extracted from ten specific locations on the formed T-shape panels, which provide the martensite phase transformation percentage at every forming depth. The coupled results between strain field and RAVF reveal significant martensite phase transformation dependence on deformation mode and strain path before fracture. The most phase transformation generally occurs under biaxial tension, while the least happens under plane strain deformation. Moreover, six different forming cases are included in this study with various draw and stretch depths to investigate the forming effects on the martensite phase transformation.