Study On Microstructure Evolution And Mechanical Properties Of Q690qENH Steel

In the present paper, the key parameters of controlled rolling and controlled cooling were determined via the study on the behaviors of hot deformation and continuous cooling transformation of Q690qENH steels. The effects of cooling modes on microstructure and mechanical properties of Q690qENH steels were also investigated at a higher finishing temperature. The main results as follows:1. The recrystallization behaviors of Q690qENH steels were investigated by one pass and double pass thermal simulation experiment. The results showed the softening of tested steels was controlled by the dynamic recovery and the no recrystallization temperature is about950℃. The dynamic recrystallization activation energy and static recrystallization activation energy were determined and the models of hot deformation were also established.2. The continuous cooling transformation of Q690qENH steels were investigated by thermal simulation experiment of CCT. The results showed that the bainite was gained with the cooling rate ranging from0.5℃/s to30℃/s and the lath bainite transformation was restrained after compression deformation.3. The effects of cooling models on microstructure and mechanical properties of Q690qENH steels were also investigated at a higher finishing temperature.(1) The yield strength is higer than690MPa, the elongation higher than15%, the CVN impact energies higher than120J at-40℃, the yield ratio lower than0.9at the cooling model AC+UFC+AC, when the cooling finish temperatures are lower than450℃. And these mechanical properties satisfy the need of Q690qENH. Its yield strength/tensile strength is about876MPa/1010MPa=0.87and its low-temperature toughness is best, when the colling finish temperatures decrease to400℃(2) The yield strength is higer than800MPa, the elongation higher than14%, the CVN impact energies higher than160J at-40℃, the yield ratio lower than0.85at the cooling model AC+ACC+AC. when the cooling finish temperatures are lower than430℃. And these mechanical properties satisfy the need of Q690qENH. The yield ratio is decreased effectively when the cooling finish temperatures of ACC are decreased to400℃.(3) The microstructure of tested steels is fine, and the yield strength is about800MPa, the tensile strength higher than1000MPa, the elongation higher than15%at the cooling model TMCP+H+Q, when the holding time is up to4min and6min. In addition, the yield ratios of tested steel are all lower than0.8, the needs of high strength and low yield ratio are satisfied.