Influence Of Tailored Tempering Factors On The Mechanical Properties Of Boron Steels Within Hot Stamping

Hot stamping components can achieve high levels of strength and hardness over conventional cold stamping ones, allowing for vehicle weight reduction while maintaining the same crashworthiness. However, it brought problems in energy absorption during crash and rework operations after forming. It may be desired to create regions of the component with softer and more ductile microstructures. Hence, one of the forming techniques-Tailored tempering process is proposed which represents a significant improvement for automotive load distribution is proposed. It has been suggested that the martensite transformation for hot stamping is driven by cooling rates within the tooling during forming and quenching. It is possible to control the final microstructure and mechanical properties by regulating the cooling rate of the blank.Three factors influencing tailored tempering process of22MnB5are investigated:tooling temperature, contact pressure, air gap. The experimental tooling used in the experiments is designed and manufactured by our team. The blank is formed with a segmented tooling with local heating and cooling zones. Cartridge heaters are installed into sections of the tooling allowing it to reach the maximum temperature of500℃. By controlling the cooling rate, the blank possesses very high strength and hardness for the heating zone and relative lower strength and higher ductility for the cooling zone. Then we get a component with tailored mechanical properties. The conclusions are as follows:(1) Tooling temperature:Cooling zones are not influenced by the tooling temperatures. It is possible to insulate the heat exchange between heating and cooling zones with the air gap. The results show a very promising trend to control the Vickers hardness and tensile strength of the heating zones using different tool temperature. Vickers hardness is reduced49.7%from463HV at tooling temperature of100℃to233HV at500℃and with the tensile strength reduction of47.3%from1400MPa to763MPa. When the temperature reaches400℃, the cooling rates decrease below approximately30℃/s to promote the formation of bainite structure.(2) Contact pressure:As the contact pressure increases, the cooling rates increase slightly and the hardness and strength without obvious changes. When the tooling temperature exceeds330℃, the elongation at fracture of the tensile specimen shows a slight decrease with increasing pressure; Both the hardness and strength increase linearly in dependency of the cooling rate.(3) Air gap:The range of transition zone increases with the increasing air gap.Numerical simulations of tailored tempering process are developed using a commercial finite element code (LS-DYNA) to predict the temperature-time profiles, as-formed Martensite volume fraction and Vickers hardness. The models are able to capture the correct trends with respect to the heating tooling temperature; However, the predicted Vickers hardness of the soft region is over-predicted by approximately23%in comparison to the experimental measurements. The models need further correction and modification.