Study On Phase Transformation Behavior And Microstructure And Properties Under Heat Treatment Process Of Q690 High Strength Steel

In this thesis,C-Mn steel with 0.05wt.%C and 0.15wt.%C and microalloyed steel containing Mo and Nb were selected as study objects.The effects of continuous cooling phase transformation behavior,tissue transformation law under isothermal conditions,experimental steels'microstructure and properties under heat treatment process were systematically studied.In addition,the welding properties were experimentaled and analyzed,in order to provide experimental data for industrial production.The main research contents and results of this thesis are as follows:(1)Through continuous cooling transformation experiments,it was found that for C-Mn steel with 0.05wt.%C,when the cooling rate was increased from 0.25?/s to 5?/s,the percentage of pearlite reduced from 5.61%to 1.14%.For the C-Mn steel with 0.15wt.%C,when the cooling rate was increased from 0.25?/s to 10?/s,the pearlite and ferrite and bainite in microstructures were completely substituted by bainite.For the microalloyed steel containing Mo and Nb,when the cooling rate exceeded 2?/s,bainite structure was obtained.(2)Through the isothermal treatment experiment,it was found that the phase transition temperature of C-Mn steel with 0.05wt.%C was 550?580?,and the proportion of polygonal ferrite was the highest when isothermal temperature was 600?.When the isothermal temperature was 450?,the C-Mn steel with 0.15wt.%C and the microalloyed steel containing Mo and Nb were consisted by bainite completely.(3)The heat treatment process showed that,when the C-Mn steel with 0.15wt.%C was quenched off-line from 900? and then tempered at 400?,the yield strength and tensile strength were 961MPa and 1022MPa,the elongation was 15.5%,and the impact energy at-20? was 195J.When the microalloyed steel containing Mo and Nb quenched off-line from 900? and then tempered at 300?,the yield strength and tensile strength were 817MPa and 967MPa,the elongation was 17.8%,and the impact energy at-20? was 239J.(4)Welding heat simulation experiments showed that the impact energy of CMn steel with 0.05wt.%C at-20? was 132J,and for the C-Mn steel with 0.15wt.%C,the impact energy at-20? was 96J,and the impact energy of microalloyed steel containing Mo and Nb at-20? was 65J.All three experimental steels had good weldability.For the C-Mn steel with 0.05wt.%C,the lowest hardness was 148HV and obtained at 2mm from the central of heat affected zone.For the C-Mn steel with 0.15wt.%C and microalloyed steel containing Mo and Nb,the lowest hardness at central of heat affected zone were 181HV and 292HV,respectively.