Influence of auto-tempering on mechanical properties and microstructure of 22MnB5 hot stamping steel by discontinuous cooling process

In this paper, thermal simulation testing machine (Gleeble3500) was used to investigate the mechanical properties and microstructure evolution of the 22MnB5 hot stamping steel under different interrupt quenching temperatures and cooling rates to explore the auto-tempering behaviour during discontinuous cooling process. The results demonstrated that the auto-tempering was distinctly promoted with the elevated interrupt quenching temperature and decreasing cooling rate. The tensile strength and hardness significantly decreased. The elongation of the test steel increased first and then slightly decreased ultimately increased. The test steel had the optimal combination of mechanical properties when quenched to 350 degrees C and then cooled to room temperature with cooling rate of 8 degrees C/s. The product of tensile strength and elongation was 24.34 GPa%. The auto-tempering effects led to the reduction of martensite variants and smallangle grain boundaries. The variation of the amount of small-angle grain boundaries and hardness were greater at 0.5 degrees C/s, compared to that of 15 degrees C/s.