The possible decomposition of metastable austenite during the partitioning process in the high end quenching and partitioning （Q＆P） steels is somewhat neglected by most researchers. The effects of primary martensit...The possible decomposition of metastable austenite during the partitioning process in the high end quenching and partitioning （Q＆P） steels is somewhat neglected by most researchers. The effects of primary martensite and alloying elements including manganese, cobalt and aluminum on the isothermal decomposition of austenite during typical Q＆P process were studied by dilatometry. The transformation kinetics was studied systematically and resulting microstruc tures were discussed in details. The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries. This effect can be eliminated by a flash heating which recovered dislocations. Co addition significantly promoted the bainite transformation during partitioning while A1 and Mn suppressed the isothermal bainite transformation. The bainite transformation played an important role in carbon distribution during partitioning, and hence the amount and stability of austenite upon final quenching. The bainite transformation during partitioning is an important factor in optimizing the microstructure in Q＆P steels.展开更多
Three kinds of high Co Ni secondary hardening steels with different Ni contents were studied. The nanoscale austenite layers formed at the interface of matensite laths were observed. Both observation and diffusion kin...Three kinds of high Co Ni secondary hardening steels with different Ni contents were studied. The nanoscale austenite layers formed at the interface of matensite laths were observed. Both observation and diffusion kinetic simulation results showed that both Ni and Co did not obtain enough time to get the equilibrium content in this system. The Ni content in austenite layers decreased significantly, and Co content increased slightly with the decrease of Ni content in overall composition. The austenite stability was estimated by Olson-Cohen model, in which both chemical and mechanical driving force could be calculated by equilibrium thermodynamic and Mohr＇s circle methods, respectively. Simulation and mechanical test results showed that The decrease of Ni content in austenite layers would cause the change of austenite stability and decrease the fracture toughness of the steels. When the Ni content in the overall composition was lower than 7 wt. %, the Ni content in y phase would be lower than 20 wt.%. And the simulation value of M; （stress induced critical martensite transformation temperature） would be up to 80 ℃, which was about 60℃ higher than room temperature. Based on the analysis, the Ni content in the overall composition of high Co Ni secondary hardening steels should be higher than 8 wt. % in order to obtain a good fracture toughness.展开更多
The design of high-strength steel has long been discussed in the field of metal structural materials.To further increase the strength of common high-strength steel and further decrease the cost for production,three di...The design of high-strength steel has long been discussed in the field of metal structural materials.To further increase the strength of common high-strength steel and further decrease the cost for production,three direct-quench hot rolled steels were designed and fabricated.The rolling and coiling processes were set based on continuous cooling transformation curves.In addition,the effect of the coiling temperature on the tensile properties was discussed to further guide the optimization of the process.It was found that compared with granular bainite,lower bainite probably has more advantages for both the strength and low temperature impact toughness of direct-quench hot rolled steels.Through a process of tailoring the morphology of bainite and controlling the grain boundary precipitation,the newly designed direct-quench hot rolled steels showed greatly improved strength and acceptable ductility/toughness compared with traditional quenched and tempered steels.展开更多
基金financial support from National Natural Science Foundation of China(Grant No.51574080)the National Key R&D Program of China(No.2017YFB0304201)
文摘The possible decomposition of metastable austenite during the partitioning process in the high end quenching and partitioning （Q＆P） steels is somewhat neglected by most researchers. The effects of primary martensite and alloying elements including manganese, cobalt and aluminum on the isothermal decomposition of austenite during typical Q＆P process were studied by dilatometry. The transformation kinetics was studied systematically and resulting microstruc tures were discussed in details. The results suggested that the primary martensite decreased the incubation period of isothermal decomposition by accelerating the nucleation process owing to dislocations especially on phase and grain boundaries. This effect can be eliminated by a flash heating which recovered dislocations. Co addition significantly promoted the bainite transformation during partitioning while A1 and Mn suppressed the isothermal bainite transformation. The bainite transformation played an important role in carbon distribution during partitioning, and hence the amount and stability of austenite upon final quenching. The bainite transformation during partitioning is an important factor in optimizing the microstructure in Q＆P steels.
基金financially supported by National Basic Research Programs of China (No.2015CB654802 and No.2015GB118001)National Natural Science Foundation of China(Grant No.51471094)
文摘Three kinds of high Co Ni secondary hardening steels with different Ni contents were studied. The nanoscale austenite layers formed at the interface of matensite laths were observed. Both observation and diffusion kinetic simulation results showed that both Ni and Co did not obtain enough time to get the equilibrium content in this system. The Ni content in austenite layers decreased significantly, and Co content increased slightly with the decrease of Ni content in overall composition. The austenite stability was estimated by Olson-Cohen model, in which both chemical and mechanical driving force could be calculated by equilibrium thermodynamic and Mohr＇s circle methods, respectively. Simulation and mechanical test results showed that The decrease of Ni content in austenite layers would cause the change of austenite stability and decrease the fracture toughness of the steels. When the Ni content in the overall composition was lower than 7 wt. %, the Ni content in y phase would be lower than 20 wt.%. And the simulation value of M; （stress induced critical martensite transformation temperature） would be up to 80 ℃, which was about 60℃ higher than room temperature. Based on the analysis, the Ni content in the overall composition of high Co Ni secondary hardening steels should be higher than 8 wt. % in order to obtain a good fracture toughness.
基金financially supported by the National Natural Science Foundation of China(Grant Nos.51722101 and U1808208)National Key Research and Development Program(Grant Nos.2017YFB0304402 and 2017YFB0703001)the financial support provided by the Joint Project of Benxi Iron and Steel Group Co.,(KJB2016004).
文摘The design of high-strength steel has long been discussed in the field of metal structural materials.To further increase the strength of common high-strength steel and further decrease the cost for production,three direct-quench hot rolled steels were designed and fabricated.The rolling and coiling processes were set based on continuous cooling transformation curves.In addition,the effect of the coiling temperature on the tensile properties was discussed to further guide the optimization of the process.It was found that compared with granular bainite,lower bainite probably has more advantages for both the strength and low temperature impact toughness of direct-quench hot rolled steels.Through a process of tailoring the morphology of bainite and controlling the grain boundary precipitation,the newly designed direct-quench hot rolled steels showed greatly improved strength and acceptable ductility/toughness compared with traditional quenched and tempered steels.
