Effect of Original Microstructure on Microstructure and Mechanical Properties of High Strength Steel WHF1500H during Hot Forming

The press hardening steel sheets WHF1500H with different original structures produced by compact strip production (CSP) line in Wuhan Iron and Steel Ltd. were austenitized at 950 °C for 5 min, and then hot stamped and quenched by using the flat die. The microstructure and mechanical properties were investigated by optical microscope, transmission electron microscope and universal testing machine. The results show that the microstructure after stamping and quenching is fully composed of lath martensite. The mechanical properties of the steel samples with different original structures are different after hot stamping and quenching, and this difference is smaller than that before hot stamping and quenching. When the original structure consists of ferrite and pearlite, the austenite grains after austenitization are fine and uniform. In addition, the martensite structure obtained after hot stamping and quenching is also uniform and fine, leading to higher mechanical properties. With the increase of the strength of the original steel, when the original structure is martensite, the austenite grains after austenitization are coarse, and the martensite structures obtained after quenching is also coarse, and thereby decreasing the strength.