Synchrotron x-ray diffraction and crystal plasticity modeling study of martensitic transformation, texture development, and stress partitioning in deep-drawn TRIP steels

The micromechanics of the formability of a lean duplex TRIP steel was investigated using Synchrotron X-Ray Diffraction (S-XRD) measurements and Crystal Plasticity Finite Element Modeling (CPFEM). Specifically, the effect of the ferrite phase on the reduction of stress concentrations in the martensite phase and the influence of the austenite texture on the distribution of the martensite phase in the deep-drawn duplex TRIP steel were studied in comparison to a TRIP steel case. A series of deep-drawing processes were carried out to examine the sheet formability at ambient temperature, followed by S-XRD evaluations of the phase fraction, texture, and the residual-stress distributions in the deep-drawn cups. The macroscopic residual stress and its partitioning among constituent phases were studied using both S-XRD and CPFEM. In the deep-drawn TRIP steel, large tensile hoop residual stresses concentrated in the strain-induced α’ martensite phase, correlating well with the cracking phenomenon observed. Furthermore, the initial austenite texture influenced the martensite transformation kinetics during the deep-drawing process, resulting in a heterogeneous distribution of the martensite phase fractions around the circumference of the deep-drawn cups, which, in turn, caused an orientation-dependent cracking behavior. In the deep-drawn duplex TRIP steel, the tensile hoop residual stresses in the α’ martensite phase were significantly reduced due to a favorable load partitioning to the ferrite phase, resulting in a better formability.