Abstract: Effects of the soaking temperature and over-aging temperature during continuous annealing on microstructure and mechanical properties of C-Si-Mn-Al steel DH780 were investigated by using the Gleeble-3500 thermal simulator, and the microstructure and mechanical properties under different processes were compared and analyzed. The results show that when the soaking temperature is 750 ℃, the martensite content in the microstructure after cooling is about 32%, and there is still obvious banded structure. As the soaking temperature increases, the carbon content of austenite in the two-phase region gradually decreases, and the morphology and proportion of the ferrite and martensite formed by cooling also change. When the soaking temperature is 820 ℃, the microstructure after cooling is composed of about 40% martensite and a small amount of retained austenite and bainite, and ultimately, the tensile strength shows a downward trend, while the yield strength shows an upward trend. As the over-aging temperature increases, the carbon atoms in the martensite diffuse to the grain boundaries, the degree of over-saturation decreases, the high-density dislocations decrease, the retained austenite and part of the martensite decompose, and when the over-aging temperature is higher than 310 ℃, it leads to a downward trend in the final tensile strength and the change of the yield strength is not significant. When the soaking temperature is 800 ℃ and the over-aging temperature is 320 ℃, the tensile strength is 847 MPa and the yield strength is 328 MPa, which can obtain dispersed martensite and basically eliminate the banded structure. The C-Si-Mn-Al steel DH780 can obtain relatively good comprehensive properties.

Key words: DH780 steel, soaking temperature, over-aging temperature, mechanical properties