Research On Hot Forming Process Improvement Of Automobile High Strength Steel And Its Application

High strength steel hot forming technology can solve the problem of reducing weight of vehicles and improving crash safety at the same time. And it will be of great importance in manufacturing energy saving, environmental protection and safety vehicles. In this thesis, hot forming process improvement of high strength steel22MnB5and its application in automotive bodies are studied through experiments and numerical simulations.Basic process parameters improvement experiments were conducted to determine the influence of heating temperature, soaking time and cooling rate on mechanical properties of the quenched material. For22MnB5with thickness of1.6mm, the optimized heating temperature range is900℃/s～950℃, the optimized soaking time range is2min-4min, and the optimized cooling rate range is36℃/s～173℃/s, both high strength and good plasticity can be obtained.U-shaped and complex shaped hot forming experiments, ABAQUS and Dynaform finite element simulations were conducted to investigate the hot forming process improvement method. Effects of the rapid cooling process on the cooling rate, the quenching uniformity, the macrostructure, the mechanical property, the springback, and the formability were obtained. With improved process, the hot blank was rapidly cooled to the appropriate temperature and then be formed and quenched. While the forming temperature zone is600℃～700℃, the average cooling rate is above30℃/s, the tensile strength is higher than1500MPa with the martensitic microstructure, and the springback and the formability improved significantly at the same time.Hot forming door beams were manufactured with the improved process, and their material properties, formability, flexural properties and energy absorption performances were tested. For the door beam formed at650℃, the maximum bending load is14.58kN and the maximum dynamic impact energy absorption is2.25kJ, the side impact resistance improved. Hot forming parts were used to improve the design of the body side structure, and side impact simulations were conducted based on LS-DYNA. While the original2.5mm thick door beam was replaced by the1.6mm thick hot forming door beam and the original1.5mm thick B-pillar was replaced by the1.2mm thick hot forming B-pillar, the invasive displacement, the intrusive velocity, the body deformation and the energy absorption were equal or better than the original case. The vehicle weight was reduced by3.7kg. The crash safety can be improved and the weight can be reduced at the same time.