Effect of post rolling stress on phase transformation behavior of microalloyed dual phase steel

The slow phase transformation of microalloyed dual phase steel makes the nonuniform stress and temperature fields during the post rolling cooling process have a significant impact on the phase transformation process. Given the relatively slow phase transformation of DP780 steel within the microalloyed dual phase steel series, the influence of stress on the phase transformation behavior of DP780 steel was investigated. To quantify the nonuniform thermal and stress conditions in the steel coil, a thermo-mechanical coupled finite element model of the hot-rolled strip cooling process was established. Based on the simulation data, DP780 steel was chosen as the research material, and Gleeble 3500 thermal simulation equipment was used for experimental validation. The thermal expansion curves were analyzed through regression to establish the dynamic model of DP780 steel phase transformation under stress. Subsequently, metallographic analysis was conducted to determine phase transformation type and grain size of DP780 steel. The results confirmed that the stress promotes the occurrence of semi-diffusion-type bainite transformation. Furthermore, an appropriate level of stress facilitates the growth of bainitic grains, while the increased stress inhibits the growth of ferritic grains.