Effect of Final Temperature of Ultra Fast Cooling after Rolling on Yield Ratio of 780 MPa Constructional Steel

The effect of final temperature of ultra fast cooling after rolling on microstructure and yield ratio was studied in laboratory by a φ 450 mm hot-rolling mill and an ultra fast cooling system. The results show that the strength and yield ratio of the tested steel increased when final cooling temperature varies from 630 ℃ to 360 ℃, and microstructure passed through the transformation of composite phases microstructure including bainite ferrite matrix and M/A island phase to the single lath bainite microstructure, and the yield ratio increased to 0.94 from 0.69. When final cooling temperature was about 550 ℃, the yield strength was 650 MPa, the tensile strength was 955 MPa, the yield ratio was 0.68, and the elongation was about 19.2%.