To correlate the phase transformation and mechanical behavior of QP steel sheets

The Quenching and Partitioning steels (QP steel) have received attention due to their deformation-induced martensitic transformation (DIMT) behavior. However, this DIMT behavior makes the stress-strain response of QP steel more complex to predict. In this paper, a comprehensive study of the DIMT behavior of QP980 steels was carried out in which an extended martensitic transformation kinetics model was further developed to incorporate the stress state, temperature and strain rate. To further correlate the microstructure evolution and macromechanical behavior, a modified plastic hardening equation considering martensitic transformation strengthening, thermal softening and strain rate strengthening effects was proposed. The impact of temperature and strain rate on the stress-strain response (first-order effect) of QP steels has been decoupled into two parts: (i) the thermal and strain-rate effect on traditional strain hardening and (ii) the temperature- and strain rate- dependent DIMT behavior on martensitic transformation-induced hardening. The correlation between the microstructure volume fraction and yield surface evolution (second-order effect) in stress space was investigated. The kinematic hardening behavior was also studied to explore the Bauschinger effect with different phase transformation rates. This extended model, therefore, provides a good description of the complex stress state-, strain rate- and temperature-dependent mechanical behavior of QP steels.