Effect of Heat Treatment Process on the Mechanical Properties of 40MnBNbVTiMo Hot-Stamped Steel and Industrial Validation

This study developed a novel hot-stamped steel 40MnBNbVTiMo for automotive application, which achieves ultra-high strength by microalloying and hot stamping process optimization. The continuous cooling transformation (CCT) curve is established, revealing that Ac1, Ac3, Ms, and Mf are 765.2 °C, 812.2 °C, 314 °C, and 190 °C, respectively, and the martensitic critical cooling rate is approximately 1 °C/s. The effects of the austenitizing temperatures (890 °C, 910 °C, 930 °C) on the microstructure and mechanical properties are systematically examined. Optimal performance is achieved at 910 °C for 5 min, with a tensile strength of 2201 MPa, an elongation of 6%, and refined prior austenite grains (7.47 µm, grain size level 11.4). TEM results showed a martensitic matrix with high dislocation density and uniformly dispersed (Nb,Ti) C and (Nb,Ti,V) C precipitates, ranging from 75 nm to 400 nm, which provide strong precipitation strengthening and grain refinement. The mechanical performance and the optimized heat treatment process are validated through numerical simulations and forming tests conducted on the door anti-collision beam part; the mechanical properties of Rp0.2, Rm, and At are 1480 MPa, 2287.9 MPa, and 4.96%, respectively.