Effect of Final Cooling Temperature in Controlled Rolling and Cooling Process on Microstructure and Tensile Properties of High Strength Building Steel

A high strength building steel was treated by controlled rolling and cooling process, and the effect of final cooling temperature (350-650 ℃) on the microstructure and tensile properties at room temperature of the steel was studied. The results show that the microstructure of the test steel after controlled rolling and cooling at final cooling temperature of 650 or 550 ℃ was bainite ferrite+martensite-austenite (M-A) island; the microstructure at final cooling temperature of 450 ℃ were still bainite ferrite+M-A island, but the content of M-A island was lower than that at final cooling temperature of 650 or 550 ℃; when the final cooling temperature was 350 ℃, the microstructure was mainly lath bainite ferrite, and a small amount of film-like M-A island distributed among local laths. The yield strength, tensile strength and yield ratio of the test steel increased with decreasing final cooling temperature, while the percentage elongation after fracture exceeded 16% at the final cooling temperature of 350 ℃ or 450 ℃ or 550 ℃. The tensile properties of the test steel met the requirements of 780 MPa grade high strength low yield ratio building steel when the final cooling temperature was 450 ℃; at this time the fine and round M-A island dispersed in the bainite ferrite, which was helpful to obtain high strength and plasticity and low yield ratio of the test steel.