Experimental and Modeling Study of Phase-Specific Flow Stress Distribution in Intercritically Annealed Quenching and Partitioning Steels

Abstract

To meet the increasing demand and stringent requirements of automotive structural steels, intercritically annealed quenching and partitioning (QP) steels are attracting significant attention owing to their excellent strength–plasticity balance. However, to date, limited reports have focused on the correlation between the microstructure and strength of intercritically annealed QP. In this study, the mechanical behaviors of QP steels with different Si contents were investigated by developing a physical-based mechanical model based on microstructural characterizations. In situ neutron diffraction was used to analyze the evolution of the phase constitution. Si content influenced the phase transformation behavior of the test steel. In the early stages of deformation, Si-strengthened steel exhibited lower retained austenite (RA) stability and faster transformation kinetics. The variation in the RA volume fraction with the deformation was fitted using a segmented exponential function. Based on the microstructure and strengthening mechanisms, a mechanical model considering grain refinement during phase transformation was proposed. The model was validated using intercritically annealed QP steels with different Si contents. The transformation-induced plasticity effect, that is, the contribution of RA to the strength, was discussed from two perspectives. Deformation-induced martensite (DIM) exhibited a significant work-hardening rate owing to the high solid solution strengthening by carbon and the high dislocation density. The residual RA after the DIM transformation exhibited a non-negligible stress distribution. Particularly, the grain boundary density and dislocations increased with strain, strengthening the remaining RA.