Effects of Austenitizing and Quenching Parameters on Microstructures and Mechanical Properties of Press Hardened WHT1500HF Steel

The effect of austenitizing temperature and the cooling rate during quenching on the microstructure and mechanical properties of press hardening steel were investigated. The as-quenched microstructure and prior austenite grain size were characterized by optical microscopy and transmission electron microscopy. The mechanical properties were evaluated by uniaxial tensile tests. The results showed that the ultimate tensile strength and yield strength values were the highest and the formability index value was excellent for the water-quenched samples compared to the oil-quenched samples or the die-quenched samples. After quenching at different cooling rates, the microstructure was fully martensite, in which the martensite exhibited lath morphology. As the cooling rate increased, the martensite laths became finer and the martensite packets size decreased. The martensite laths for the specimens with the highest cooling rate were finer than for those of specimens with lower cooling rates, and there was a larger number of fine needle-like ϵ-Fe2C in the matrix of martensite laths. For the WHT1500HF steel, the optimum austenitizing temperature is 900°C, which resulted in a mean austenitic grain size of 9.4 μm. Therefore, the properties of the hot stamping steel can be improved through using an optimal austenitizing temperature and increasing the quenching rates by changing the physical die design.