Investigation of microstructure evolution and recrystallization refinement of Usibor®2000 press hardening steel sheets during deep drawing process

The flow behavior and drawability of commercial press hardening steel Usibor®2000 were investigated at different forming temperatures using tensile test and Swift test in this study. The drawing performance of the alloy in room temperature forming, warm forming and hot forming processes was compared, and a comprehensive analysis of the microstructure and crystallographic texture characteristics was conducted. The results show that the press hardening process at 860 ℃ achieves uniform grain refinement and excellent formability in a short time due to the significant softening effect of dynamic recrystallization, with a recrystallized grain fraction reaching 58.7%. The tensile strength decreases significantly at 860 ℃, while the total elongation and limiting drawing ratio increased by 17 and 16.5%, respectively. The alloy exhibits excellent drawability due to the deep drawing in the hot stamping process of strong < 110 > α//RD textures in the transformed and < 111 > γ//RD textures in the untransformed grains. A mixture of martensite (70.22%), ferrite (16.21%) and bainite (13.57%) was obtained during PH process, realizing the ideal mechanical properties. The ultrafine grain structure was obtained by thermoforming process while maintaining the good formability of ultrahigh strength parts. These findings can serve as a useful reference for improving the forming process of 2 GPa press hardening steel.