Influence of stress on martensitic transformation and mechanical properties of hot stamped AHSS parts

Non-isothermal tension and compression tests of 22MnB5 boron steel were carried out in this study. How different stress state influences the martensitic transformation of advanced high strength steel (AHSS) parts was analyzed. The analysis reveals that the martensitic transformation starting temperature (Ms) changes with different stress states. Specifically, the Ms temperature rises with increasing tensile stress, however, it rises first and then drops with increasing compressive stress. Moreover, a higher initial forming temperature leads to a higher Ms temperature under the same stress. Simulation of an actual hot-formed AHSS B-pillar together with the microscopic metallography, hardness and martensitic content shows that in higher tensile stress dominated area, the martensitic content and hardness are usually higher than in other areas. Although the stress can promote the Ms temperature, a lower cooling rate may lead to less martensite fraction.