Effect of annealing process on microstructure and properties of ultra-high strength automotive steel

Automobile steel not only requires lightweight body, but also requires good formability. Therefore, it is of great engineering significance to develop high strength and high formability automobile steel. Medium manganese steel is a typical representative of this kind of steel. In this paper, the effects of annealing temperature and holding time on the microstructure, mechanical properties and formability of cold rolled medium manganese steel were studied. The results show that under the two-phase zone direct annealing process, the microstructure of the experimental steel is mainly composed of ferrite, austenite and martensite. With the increase of annealing temperature, the yield strength decreases first and then increases, while the tensile strength increases. The elongation, strength-ductility product and residual austenite content increase first and then decrease. The content of favorable texture 111 and 110 increases with the increase of annealing temperature. The comprehensive properties of the experimental steel are the best when annealed at 630 ℃. The tensile strength is 1 144 MPa, the yield strength is 847 MPa, the elongation is 34.4%, the product of strength and elongation is 39.4 GPa·%, and the volume fraction of retained austenite is 23.68%. At the optimal annealing temperature of 630 ℃, with the increase of holding time, the average value of the cupping of the experimental steel increases first and then decreases, and reaches the maximum when the holding time is 15 min. The cold bending experiments with different bending angles and bending radius were carried out on the samples annealed at 630 ℃ for 15 min. The results show that the developed 1 000 MPa grade ultra-high strength automobile steel has no cracks during 0° cold bending and has excellent cold forming performance.