Effect of cooling rates after annealing on the microstructure and properties of 1000 MPa grade automobile steel for cold forming

Abstract The effects of different cooling rates (0.05 °C s⁻¹, 0.1 °C s⁻¹, and 0.2 °C s⁻¹) on the microstructure and mechanical properties of 1000 MPa grade automobile steel for cold forming after two-phase annealing were studied. The microstructure of the experimental steel was observed by SEM and TEM, and its mechanical properties were tested by a universal tensile testing machine. The results showed that by increasing the cooling rate of two-phase annealing, more massive retained austenite, more uniform and fine ferrite, better elongation and higher ultimate tensile strength of steel can be obtained, so as to obtain better production of tensile strength and total elongation (product of tensile strength and elongation, PSE). The final result shows that after the test steel is quenched at 800 °C + 10 min and annealed in the two-phase region at 690 °C + 10 min, the faster the cooling rate, the better the mechanical properties. The mechanical properties of the steel plate are the best when the cooling rate reaches 0.2 °C s⁻¹, and PSE can reach 27.44 GPa·%.