Study on the Microstructure, Properties, and Strengthening Mechanisms of Hot-Stamped Steel Produced by Multi-Mode Continuous Casting and Rolling Production

In response to the growing demand for high-performance automotive materials, the multi-mode continuous casting and rolling (MCCR) process presents a promising solution to enhance production efficiency while reducing environmental impact. This study investigates the effects of three cooling methods—water quenching (WQ), gas quenching, and oil quenching—on the microstructure, mechanical properties, and strengthening mechanisms of hot-stamped MCCR steel by scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The findings indicate that higher cooling rates significantly increase both yield and tensile strengths while preserving elongation, with WQ conditions yielding optimal mechanical properties, achieving a yield strength of 1207 MPa and tensile strength of 1902 MPa. Additionally, higher cooling rates suppress auto-tempering, leading to greater dislocation density, intensified lattice distortion, and enhanced residual stress. These structural transformations are critical in determining grain boundary characteristics; higher cooling rates increase martensitic variant selection, reduce the proportion of high-angle grain boundaries (HAGBs), yet maintain the highest HAGB density in WQ conditions due to the refinement of packets and blocks. Quantitative assessments reveal that dislocation strengthening primarily contributes to yield strength variations at lower cooling rates, while dislocation and grain refinement strengthening mechanisms jointly govern yield strength at higher cooling rates.