Microstructure simulation and mechanical property prediction of boron steel during hot stamping

Based on the Johnson-Mehl-Avrami type kinetics equations and the Koistinen-Marburger equation,a fully coupled thermomechanical-metallurgical finite element model was established for a door anti-collusion bumper of boron steel 22 Mn B5 in hot stamping. The characteristics of temperature,microstructure and hardness distribution on the door anti-collusion bumper were investigated. The effects of holding force and holding time on the microstructure and mechanical properties of door anti-collusion bumper hot stamped parts were also analyzed. Simulation results indicate that the cooling rate of the top is 137.3℃·s^-1 and the cooling rate of the side wall is 69.8℃·s^-1. Different cooling rates lead to different microstructure and hardness distributions on the door anti-collusion bumper. With the increasing of holding force,the holding time when the door anti-collusion bumper gets 95% martensite can be shorten,and this can speed up the production efficiency. In addition,we analyzed the microstructure and hardness at the top and side wall of the door anti-collusion bumper. Experimental results show that when the holding time is 10 s,the top and side wall transfer to fully lath martensite,and the hardness at the top is about 508 HV,higher than that at the side wall of 474 HV.