Abstract: Continuous annealing hot-dip galvanizing process of a 780 MPa grade dual phase (DP) steel was studied by using hot-dip galvanizing simulation testing machine. Effects of four different heat treatment processes as direct quenching, quenching after slow cooling, post-galvanized martensite process and pre-galvanized martensite process on the microstructure and tensile properties of the steel were compared and analyzed. The results show that the slow cooling process after heating in the two-phase region can improve the hardenability of the untransformed austenite and obtain stable island-like martensite. A small amount of tempered martensite is obtained by the pre-galvanized martensite process. The Nb can improve the tempering stability of martensite and improve the mechanical properties of hot-dip galvanized DP steel. There is no significant change in precipitated phases of the steel after the post-galvanized martensite process and the pre-galvanized martensite process, the Nb-rich precipitates are prone to heterogeneous nucleation and precipitation at the interface of TiN precipitated firstly. The tensile strength of the DP steel obtained by the pre-galvanized martensite process is 867 MPa, and the elongation is 21.3%, which is close to the tensile strength (878 MPa) and elongation (24.0%) of the post-galvanized martensite process. However, the yield strength of the DP steel obtained by the pre-galvanized martensite process (564 MPa) is significantly reduced compared to the post-galvanized martensite (611 MPa). Therefore, the pre-galvanized martensite process can effectively reduce the yield ratio and improve the formability of the dual phase steel while ensuring its strength and plasticity.

Key words: dual phase steel, hot-dip galvanizing, martensite, continuous annealing