Effect of Coiling Temperature and Cooling Rate on Microstruture and Tensile Properties of Niobium Containing Hot-Rolled Dual Phase Steel

Niobium containing dual phase steel hot-rolled under the same process parameters was cooled to 610 ℃ by primary water cooling at a cooling rate of 60 ℃·s^-1 or to 461, 434, 410 ℃ by two-stage water cooling at different cooling rates and coiled. The effects of the coiling temperature and cooling rate on the microstructure and tensile properties of the hot-rolled dual phase steel were analyzed. The results show that the microstructures of the hot-rolled dual phase steel under the four processes were all mainly composed of ferrite and martensite, and bainite appeared at the coiling temperature of 461 ℃ and below. With the decrease of coiling temperature, the content of martensite decreased, and the bainite presented dispersed granular, aggregated granular and lath shape successively. The yield strength of the hot-rolled dual phase steel increased, the tensile strength first decreased and then increased, the yield ratio increased, and the precentage elongation after fracture first increased and then decreased. When two-stage water cooling at primary and secondary cooling rates of 53,79 ℃·s^-1 to 461 ℃ and coiling, the tensile properties of the hot-rolled dual phase steel were the best, with the average yield strength, average tensile strength, and average precentage elongation after fracture of 459 MPa, 591 MPa, and 35.63%, respectively.