Effects of Finish Rolling Temperature on Microstructure and Properties in Nb-Ti Low Alloying Bainite Steel

The effects of finish rolling temperature on microstructures and properties were investigated in TMCP (thermo-mechanical controlled processing) bainite steel using optical microscope, transmission electron microscope, EDS analysis and mechanical testing equipments. The results show that strength and toughness improve respectively as finish rolling temperature decrease. A lot of proeutectiod ferrite grains form along boundaries and strength and toughness of steel plate decreases after finish rolling at 815 ℃ with the reason of temperature drop into austenite and ferrite double phases zone before accelerate cooling. Excellent properties can be obtained by finish rolling temperature is 870 ℃. Yield and tensile strength are 805 MPa and 1 005 MPa respectively. Average charpy impact energy is 197 J. The microstructures are bainite, martensite and Nb-Ti contained dispersive precipitates with the size of 30-50 nm.