Study On Martensite Microstructure And Properties Of Low-carbon Low-alloyed Steel After Rolling And Annealing

In this dissertation,the ultra fine grained and nano-grained steel sheets were gottenby80%warm rolling,93.6%and65%cold rolling and low temperature annealing from300°C-650°C to lath martensite structure of09MnNiDR steel, Q235steel and15CrMnMoV steel respectively. By comparing the thermal stability of the rolled andannealed three steels we studied the effects of the alloying elements on the thermalstability of the steel.The microstructure of the prepared samples were studied by optical (OM),transmission electron microscopy(TEM),and the mechanical property were studied bymicrohardness test machine and Gleeble3500thermo-mechanical simulation machine.The results show that temperature has little effects on the microstructure andproperties to the prepared09MnNiDR steel samples when the annealing temperature isless than450°C, the degree of the recrystallization is very small, the recrystallizationvolume fraction is less than10%when annealing for60min, there is little change inhardness and tensile strength,the steel has good thermal stability. But it has great effectson the microstructure and properties,when the annealing temperature is more than500°C,the degree of the recrystallization is increasing obviously, the hardness and tensile strengthof09MnNiDR steel decreased significantly,the thermal stability becomes poor. We canget ultra-fine grain steel of09MnNiDR with the mean size from24.4nm-1.2μm.whenannealing at400°C-600°C, the tensile strength of the prepared09MnNiDR steeldecreased from1642MPa to511MPa and the hardness decreased from424HV to230HV when the annealing temperature increased from350°C to600°C.The alloying elements, especially Mo,V and Cr has large effects on the thermalstability to the alloyed steel. In this paper the thermal stability of the15CrMnMoVA steelis the best, its structure becomes instability at the temperature of520°C. the followed isthe09MnNiDR steel,its structure becomes instability at the temperature of450°C, theQ235steel has the worst stability, the microstructure of it becomes instability at thetemperature of350°C, this proves that alloying elements can improve the thermal stability of the steel.