A study on the multi-phase transformation kinetics of ultra-high-strength steel and application in thermal-mechanical-phase coupling simulation of hot stamping process

It is a significant issue to deeply understand the phase transformation kinetics and further predict the multi-phase compositions of hot stamping part with finite element method (FEM). As for BR1500HS ultra-high-strength steel, a time-temperature-transformation (TTT) test schedule from austenitizing temperature (1100 °C) to different transforming temperatures (380-750 °C) was conducted on Gleeble 3800 machine. On the basis of microstructure observations, ferrite+pearlite region (600-740 °C), bainite region (420-600 °C) and martensite region (lower than 420 °C) were distinguished. According to the acquired dilatometric curves, the starting and ending TTT curves were fitted. Subsequently, Johnson-Mehl-Avrami type kinetics equation and Magee's equation were respectively solved to describe the diffusional and non-diffusional transformation kinetics as follows: the diffusional transformation efficiency increases to a maximum value followed by a gradual decrease till 100% with time extending; the non-diffusional transformation degree gradually increases with decreasing temperature without time influence. Eventually, the multi-phase transformation kinetics were applied to the construction of the thermal-mechanical-phase dynamic coupling finite element (FE) model of hot stamping process and analysis of the multi-phase evolution and distribution in a hot stamping part. Eventually, the hot stamping process experiment was conducted and proved that the simulation results were effective.