Strain rate sensitivity of ultra-high strength hot stamping steel

The tensile test of an ultra-high strength hot stamping steel was tested using the CMT5105 electronic universal testing machine and HTM 16020 electro-hydraulic servo high-speed material testing machine. The impacts of the hot stamping parts were simulated at strain rates range of 10^-3-10³ s^-1. The results show that in the low strain rate (10^-3-10^-1 s^-1), the strain rate sensitivity of the tested steel is not very high, and the steel strength and elongation change little with an increase of strain rate. In the high strain rate stage (10⁰-10³ s^-1), the strain rate sensitivity of the steel is very high, and the steel strength and elongation increase with strain rate. The strain rate sensitivity of the tensile strength is higher than the yield strength mainly because of the adiabatic temperature rise phenomenon and the strain working phenomenon that simultaneously occur during the high strain rate stage. The elongation after necking decreases with an increase of strain rate, mainly because of the local inhomogeneous deformation of the martensite at the high strain rate. The impact energy absorption capacity of the experimental steel increases with strain rate, and is more sensitive at the uniform elongation. Compared with the low strain rate stage, the average fracture diameter of the dimple in the high strain rate stage is smaller, and its depth is deeper; this is related to the fragmentation of the martensite grains region in the high strain rate stage. Scanning elec-tron microscope and transmission electron microscope images reveal that the grains are elongated at high strain rate stage and some microvoids are present in the stress-concentrated regions. Moreover, the fragmentation phenomenon can be found in part of region at the 10³ s^-1 strain rate.