Cooling process of high strength and low yield ratio steel plates for buildings

Effect of cooling process on microstructure and mechanical properties of structural steel for buildings was studied in laboratory by a MMS-300 thermal simulator and a 450 mm mill.The results show that the steel with microstructure of single lath bainite has high strength,but the yield ratio is up to 0.94.The steel with microstructure composed of bainite ferrite matrix(soft phase)and fine martensite-austenite(M-A,hard-phase)exhibits higher strength and lower yield ratio.When the final cooling temperature is 550 ℃,the steel has the optimum microstructure and mechanical properties:yield strength is 650 MPa,tensile strength is 955 MPa and yield ratio is 0.68.