Abstract: The testing steel was cooled using ultra fast cooling (UFC) after two stage hot rolling， and the effects of UFC final cooling temperatures on the microstructure and mechanical properties of the high strength bridge steel were investigated. The results showed that the microstructural characteristics of the testing steel were obviously affected by the UFC final cooling temperatures. The main part of the microstructure of the testing steel changed from the granular bainite to the lath bainite gradually and the M/A size was significantly refined with the decrease of UFC final cooling temperatures. The effects of UFC final cooling temperatures on mechanical properties of the testing steels was also revealed. For the UFC final cooling temperature of 236℃， the yield strength， tensile strength， yield ratio， impact energy with the testing temperature of -40℃ and elongation were 745MPa， 961MPa， 078， 1665J and 168%， respectively. The balance of strength， toughness and plasticity was achieved and lower yield ratio was also gained.

Key words: bridge steels, ultra fast cooling, yield ratio, microstructure, mechanical properties