Microstructure and mechanical properties of 30SiMnCrB5 hot stamping steel

A C-Si-Mn-Cr-B hot stamping steel was designed to improve its comprehensive properties. The continuous cooling transformation (CCT) curves and phase transformation rules of 30SiMnCrB5 hot stamping steel were measured and studied by thermal dilatometry. The microstructure morphology and mechanical properties of 30SiMnCrB5 steel sheet after rolling, annealing and hot stamping simulated were analyzed by scanning electron microscopy, transmission electron microscopy and tensile testing. The orientation relationship between martensite variants and parent phases in the hot-stamped steel sheet was determined by pole figure contouring. It is found that the critical cooling rate of 30SiMnCrB5 hot stamping steel is 5℃·s^-1 with good hardenability which inhibits the formation of pearlite and bainite effectively. The hardness of full martensite is more than 600HV. The microstructure of the hot-stamped steel sheet is composed of lath martensite and retained austenite, which accounts for 6% to 8% and distributes between martensite laths by film. The strength and ductility of the hot-stamped steel sheet are matched well with a tensile strength of about 1800MPa and a total elongation up to 10%. The orientation relationship between martensite variants and parent phases in the hot-stamped steel sheet is closer to the N-W orientation relationship, which the 12 variants do not appear wholly in prior austenite.