Phase transformation and plastic behavior of QP steel sheets: Transformation kinetics-informed modeling and forming limit prediction

Abstract

A previous study on quenching and partitioning (QP) steel showed that the transformation-induced plasticity (TRIP) effect strongly depends on the stress triaxiality and Lode angle parameters during the plastic deformation. However, only the impact of stress states on the linear hardening response (first-order effect) was investigated, assuming its yield and plastic flow evolution (second-order effect) follow isotropic behavior for both tension and compression. The high-order effects of phase transformation on plasticity are still elusive and difficult to incorporate owing to the inconsistent usage of the equivalent strain in kinetics law and constitutive modeling. In this study, the hardening law, considering martensitic transformation, was utilized in conjunction with a strength differential (SD) effect-induced asymmetric yield function based on the proposed work-conjugate framework. The contradiction where the previous model correlates the martensitic transformation law based on the von Mises strain and the mechanical responses based on the normalized plastic work-per-unit material (or the work-conjugate equivalent strain) was eliminated. This elimination imposed the phase transformation influence into size evolution of the yield surface for QP980 because the effective stress directly correlated with the martensitic transformation using the work-conjugate equivalent strain. One can use any yield function in conjunction with this work-conjugate framework to correlate the martensitic transformation and its impact on the mechanical behavior for both first- and second-order effects. Thus, the yield surface of QP980 steel was stably normalized through asymmetric function under different plastic strains and deformation modes. The SD effect of QP980 steel has proven to be anisotropic and the martensitic transformation has negligible impact on the shape evolution of its yield surface. The pressure-dependent phenomenon in the plastic flow behavior of the employed QP steel suggested that its volume change during the plastic deformation can be neglected. The correlation between the microstructure volume fraction, evolving yield surface, and the stress–strain response under nonlinear loadings (Bauschinger effect) was predicted accurately. The forming limit imperfection analysis served as a typical application, where the phase transformation was further correlated with the localized necking phenomenon. The framework to understand the impact of microstructure evolution in QP steel sheets on its final forming limits was provided, suggesting that an optimized strain path design will greatly enhance the accumulated plastic limit before the localized necking.