Abstract: A Nb and Ti microalloyed low-carbon complex phase steel was designed, and the evolution of microstructure, precipitates and mechanical properties of the hot-rolled complex phase steel during annealing and galvanizing was studied by means of SEM, TEM, mechanical properties test. Moreover, the industrial trial production of 800 MPa hot-rolled galvanized complex phase steel was carried out. The results show that, the microstructure of the hot-rolled complex phase steel is mainly composed of ferrite, martensite and M/A island. During annealing and galvanizing, the martensite and M/A island are decomposed to form high-temperature tempered martensite, and the density of glissile dislocation in ferrite is decreased, and the nano-scale composite carbides of Nb, Ti and Mo are precipitated at the same time, resulting in a decrease of tensile strength, a significant increase of yield strength and hole expansion ratio. The microstructure of the hot-rolled galvanized complex phase steel is mainly composed of ferrite and high-temperature tempered martensite. The yield strength, tensile strength, elongation after fracture and bake hardening value are 769 MPa, 852 MPa, 14.5% and 43 MPa, respectively, and the hole expansion ratio reaches 53%, indicating excellent mechanical properties and local formability.

Key words: hot-rolled galvanizing, complex phase steel, microstructure, mechanical properties, hole expansion ratio