Abstract: A 1180 MPa high strength complex phase steel in Cr-Mo-Nb-Ti-B series was designed and developed. From the perspective of microstructure control, bainite was introduced to make up for the high strength difference between the soft and hard phases between ferrite and martensite, the effects of soaking temperature and overaging temperature on mechanical properties and microstructure of the complex phase steel during continuous annealing were studied by means of Gleeble-3500 thermal simulation test machine, tensile test machine and optical microscope. The results show that when the soaking temperature is from 720 ℃ to 840 ℃, the bainite and martensite contents gradually increase with the increase of temperature, and the tensile strength and yield strength increase continuously as a whole, but the tensile strength and yield strength decrease when the temperature exceeds 840 ℃. With the increase of overaging temperature, the tensile strength decreases monotonically, and the yield strength fluctuates at first and then decreases gradually. When the soaking temperature is 790 ℃ and the overaging temperature is 280 ℃, the microstructure of the continuous annealed plate is a complex phase structure of ferrite, bainite and martensite, which makes the steel have better formability, the bending property and hole-expanding property are also greatly improved compared with the same grade dual-phase steel products.

Key words: ultra high strength steel, complex phase steel, annealing process, performance stability, formability