Abstract: Dual phase steels with three strength levels of 590, 780 and 980 MPa were achieved simultaneously by regulating the isothermal temperature of a low-carbon Si-Mn-Cr steel under different heat treatment processes. The results show that the martensite content in the final microstructure is increased with the increase of isothermal temperature, and with a consequent increase in the tensile and yield strengths and a decrease in the total elongation of the tested steel. Compared with the intercritical isothermal process, the inclusion of the overaging step requires a higher isothermal temperature and a higher fraction of martensite in the microstructure to achieve the same strength. As the overaging temperature increases, the isothermal temperature required to reach the same strength level further increases and the martensite fraction further increases. Dual phase steels with three different strength levels of 590, 780 and 980 MPa are prepared, respectively, by using the identical composition with both heat treatment processes. However, the 980 MPa steel after the intercritical isothermal-overaging process has nearly full martensite microstructure, which does not meet the microstructure characteristics of the actual dual phase steel.

Key words: dual phase steel, microstructure, mechanical properties, overaging, isothermal temperature