Dynamic CCT Behaviour of 600 MPa Grade Hot-Rolled Dual Phase Steel and Optimization of Hot-Rolled Coiling Process

The chemical composition of medium temperature coiling type hot-rolled dual phase steel was designed by appropriately reducing the content of chromium and molybdenum elements on basis of the composition of traditional 600 MPa grade dual phase steel. The dynamic continuous cooling transformation behaviour of the test steel was analyzed by thermo-simulation. The hot-rolled coiling process was optimized according to the thermo-simulation results, and a trial industrial production was conducted. The microstructure and properties before and after optimization were studied. The results show that when the cooling rate was greater than 15℃·s^-1, the microstructure of the test steel mainly consisted of ferrite, martensite and a small amount of bainite; with the increase of cooling rate, bainite changed from granular to lath-shape, and martensite content increased. The grains of hot-rolled dual phase steel produced by optimal medium temperature coiling whole intensive cooling process was finer than that before optimization, and martensite content was higher. The yield strength, tensile strength and hardness of hot-rolled dual phase steel produced by optimal process were all higher than those before optimization; percentage elongation after fracture and hole expansion rate were both lower than those before optimization, but still satisfied the standard requirement, indicating that trial-produced hot-rolled dual phase steel had good comprehensive mechanical properties.