Inverse characterization method for mechanical properties of strain/strain-rate/temperature/temperature-history dependent steel sheets and its application for hot press forming

In order to measure the flow curves of steel sheets at high temperatures, which are dependent on strain and strain rate as well as temperature and temperature history, a tensile test machine and specimens were newly developed in this work. Besides, an indirect method to characterize mechanical properties at high temperatures was developed by combining experiments and its numerical analysis, in which temperature history were also accounted for. Ultimately, a modified Johnson-Cook type hardening law, accounting for the dependence of hardening behavior with deterioration on strain rate as well as temperature, was successfully developed covering both pre- and post-ultimate tensile strength ranges for a hot press forming steel sheet. The calibrated hardening law obtained based on the inverse characterization method was then applied and validated for hot press forming of a 2-D mini-bumper as for distributions of temperature history, thickness and hardness considering the continuous cooling transformation diagram. The results showed reasonably good agreement with experiments