Phase transformation testing and modeling for hot stamping of boron steel considering the effect of the prior austenite deformation

Hot stamping of boron steels is a thermal-mechanical and phase transformation coupled process. The phase transformation of boron steels during continuous cooling is a complex process since the phase transformation is tightly coupled with plastic deformation during the hot stamping process. In the present work, the effects of hot deformation amount, deformation temperature, and strain rate on phase transformation are investigated by using a comprehensive set of methods including in-situ isothermal tensile tests combined with dilatometry and ex-situ microstructure characterization. The dilatation curves, phase fraction evolution curves, continuous cooling transformation (CCT), and deformed continuous cooling transformation (DCCT) diagrams are constructed. Finally, based on the testing results, a new non-isothermal kinetic model of phase transformation are proposed to introduce the effect of the austenite deformation on transformation during the continuous cooling. The proposed model is validated by testing Vickers hardness of DCCT specimens and hot stamping W-channel parts. The results demonstrate that the deformation accelerates the diffusive phase transformation, causing the CCT curves to move in the direction of shorter incubation time. But the acceleration of diffusion transformation is rapidly saturated as the plastic strain increases. Lower deformation temperature and higher strain rate are more favorable to the diffusion phase transformation, and the higher the critical cooling rate of the martensite transformation is needed. The proposed kinetic model not only accurately predicts the final phase fraction and hardness of the DCCT specimen, but also the distribution of mechanical properties of the hot stamping W-channel parts.

Automated summary

The study investigated the effects of hot deformation amount, deformation temperature, and strain rate on phase transformation in boron steels, proposing a new non-isothermal kinetic model to describe the process. Results showed that deformation accelerates diffusive phase transformation, but its effect rapidly saturates with increasing plastic strain. Lower deformation temperature and higher strain rate are more favorable to diffusion phase transformation.