Effect of contact pressure on IHTC and the formability of hot-formed 22MnB5 automotive parts

The interfacial heat transfer coefficient (IHTC) between the steel blank and die directly affects temperature distribution during hot forming of advanced high-strength steel automobile parts and further affects martensite microstructure distribution and formability. In this paper, an experimental platform is established and Beck's nonlinear inverse estimation method is employed to solve the IHTC and investigate the rules of IHTC following contact pressure. This paper demonstrates that the IHTC increases with the increase in contact pressure. An actual hot-formed automotive B-pillar was obtained in the experiment. Results show that a high contact pressure corresponds to a high IHTC, which promotes fast blank cooling on the fillet corner area. By contrast, a low contact pressure results in low IHTC and cooling rate on the side wall area. Thus, a significant temperature difference induces asynchronous martensite transformation between the fillet corners and the side walls, where crack is more likely to occur. As a solution, local oil coating is used on fillet corners with high contact pressure before hot forming to improve the formability and avoid cracks. This finding indicates that local oil coating can effectively weaken the heat transfer performance, balance the temperature field in the different areas of the part, and consequently achieve better formability for the hot forming process.