Influence of Cooling Methods on Hot Stamping Microstructure and Properties of B1800HS Ultrahigh Strength Steel

The B1800HS ultrahigh strength steel was heated to 930 ℃ and held for 5 min to ensure complete austenitization. It was then air cooling to the forming temperature （750 ℃） and placed in the mold for holding pressure for 2 s. It was subsequently air cooling, oil cooling, die cooling, and water cooling to room temperature （20 ℃）. The effects of cooling methods on the microstructure and mechanical properties and the strength and plasticity mechanism of the steel were studied. The results show that the microstructures of the samples under air cooling, oil cooling, die cooling, and water cooling （with increasing cooling rates in that order） were ferrite + granular bainite, granular bainite + martensite, upper bainite + granular bainite + martensite, and full martensite, respectively. Increasing the cooling rate could significantly refine the martensite laths and improve the tensile strength, but the loss of plasticity was relatively obvious. The samples under water cooling exhibited the highest tensile strength, reaching 1 930 MPa, but the percentage elongation after fracture was only 5.0%. Under oil cooling, the granular bainite in the sample was small in size and evenly distributed, which could partition and refin the martensite structure, resulting in the best balance between plasticity and strength. Compared with those uner water cooling, the tensile strength under oil cooling decreased only slightly, while the percentage elongation after fracture increased by 102%, and the product of strength and plasticity was the highest at 17 412 MPa·%, indicating the optimal comprehensive mechanical properties.