The Effect of Annealing and Aging Temperature on the Microstructure and Properties of an 800 MPa Grade Dual-Phase Steel with High Formability

To develop automotive steel with a higher strength–ductility balance, an 800 MPa grade Nb-Ti microalloyed dual-phase steel with high elongation was designed. Continuous annealing tests were conducted using a continuous annealing simulation testing machine. The effects of annealing temperature and aging temperature on the microstructure and mechanical properties were investigated through SEM, EBSD, TEM, and tensile testing. The results indicate that the microstructure of the steel primarily consists of ferrite, martensite, and a small amount of retained austenite. As the annealing temperature rises, the martensite content increases, and the retained austenite content first increases and then decreases. Therefore, the tensile strength and elongation initially increase and then decrease, while the yield strength gradually decreases. As the aging temperature rises, the martensite content decreases, while the tempering degree of martensite increases and the retained austenite content rises. Therefore, the tensile strength gradually decreases, and the yield strength and elongation gradually increase. The optimal comprehensive performance was achieved with an annealing temperature of 830 °C and an aging temperature of 330 °C, resulting in a tensile strength of 915 MPa, a yield strength of 470 MPa, and an elongation of 24.4%. This represents a 28.4% increase in elongation compared to conventional dual-phase steels of the same strength grade. The strength–ductility product reaches 22.3 GPa%.