Microscopic investigation of the strain rate hardening for autobody steel sheet

Most of auto-body steel sheets show the strain rate hardening during high strain rate deformation. The high strain rate properties of steel sheets have been studied for accurate crashworthiness of vehicles since mechanical properties at high strain rates show different characteristics such as flow stress and fracture elongation from those at the quasi-static state. This paper deals with microscopic investigation of the strain rate hardening for auto-body steel sheets such as SPCC and TRIP steel in viewpoints of deformation texture and dislocation behavior. Tensile tests for various strain rates are performed in the rage of 10-2 to 102/sec which locally happens in real vehicle crash. Specimens are extracted on the same strain position to investigate the microscopic behavior of a deformed material. The extracted specimen is investigated by using the electron backscattered diffraction (EBSD) to examine the texture evolution such as size, shape change and aspect ratio of grains. Transmission electron microscopy (TEM) is employed to demonstrate dislocation structures which contain the change of dislocation density and dislocation behavior.