| Low carbon bainitic steels, having excellent combination of high strength, toughness and weldability, have developed in recent years. The strength has been improved through combination of TMCP and solid solution strengthening, precipitation strengthening of microalloy elements such as Cu, Nb, Ti, Mo and B. Low carbon bainitic steels can been widely applied in pipelines, engineering machinery, sea platforms, bridge and ship fabrication. Most of them are designed by adding of several microalloy elements such as Cu, Nb, Ti, Mo, Ni, Cr, and B, which remarkably increase production cost. Mn-Nb-B low carbon bainitic steels with few alloy elements and low cost have been investigated in this paper. Continues cooling transformation and hot deformation behaviors of Mn-Nb-B low carbon bainitic steels were analyzed. The microstructure and mechanical properties of the steels with different TMCP processes were analyzed through hot rolling, and the potential properties were further exploited. Furthermore, the effect of QT and DQ processes on the microstructure and mechanical properties of a low cost800MPa grade high strength steel was investigated. The main works are as follows:(1) The continuous cooling experiments of Mn-Nb-B low-carbon bainitic steels with different Boron content were employed to study the intermediate transformation temperature and microstructures. The results show that the products of phase transformation are, in order, ferrite, pearlite, granular bainite and lath bainite. The phase transformation start temperatures are decreased gradually with the increasing of Boron content. When the Boron content is low than16ppm, the intermediate transformation temperatures are decreased significantly with the increasing of Boron content. The intermediate transformation temperatures are decreased slowly with the increasing of Boron content, as the Boron content is higher than16ppm.(2) The hot deformation behavior of Mn-Nb and Mn-Nb-B low-carbon bainitic steels were studied, the effect of Boron on recrystallization of austenite was analyzed. Zener-Hollomon parameter was introduced to confirm the effects of deformed temperature and strain rate on dynamic recrystallization. The dynamic recrystallization activation energies of two steels are341kJ/mol and362kJ/mol, and the static recrystallization activation energy of two steels are294.3kJ/mol and301.7kJ/mol respectively. The dynamic recrystallization activation energy is increased by the addition of Boron. The Boron left the softening curves of low carbon Mn-Nb steel, and retards its static recrystallization.(3) Effects of deformation and cooling parameters on microstructures of Mn-Nb-B and Mn-Cr-Nb-B steels were investigated. The results indicate that the microstructures of investigated steels are quasi-polygonal ferrite, granular bainite and lath bainite in turn with decreasing of finished cooling temperatures. The formation of lower temperature microstructure is promoted and the final microstructure is refined as cooling rate is increased. When finished cooling temperature is decreased or cooling rate is increased, volume fraction of the M/A islands is decreased with smaller size and changed from irregular block to slim rod and granular shape. Addition of Cr the hardenability of low carbon Mn-Nb-B steel is significantly increased and the formation of lath bainite can be promoted at the same process.(4) Microstructure and mechanical properties of Mn-Nb-B low carbon bainitic steels with different rolling process parameters were investigated. The results show that when the finished rolling temperature, finished cooling temperature and cooling rate is860～790℃,590～440℃and about15℃/s, respectively, the microstructure is characterized by pronounced granular bainite, the mechanical properties of investigated steel can meet the requirement of600MPa grade. When the finished rolling temperature, finished cooling temperature and cooling rate is about810℃,550～450℃and about25℃/s, respectively, the microstructure is consist of fine granular bainite, acicular ferrite and a certain amount of lath bainite, the mechanical properties of investigated steel can satisfy the requirement of700MPa grade.(5) The microstructures of Mn-Nb-B low carbon bainitic steels were analyzed. The results show that the grain size of granular bainite and M/A island size can be refined, and low temperature impact toughness is improved through increasing cooling rate. The lath of acicular ferrite has large angle grain boundary and higher misorientation, which can effectively prevent crack propagation, and consequently improve the low temperature impact toughness. While in lath bainite low angle boundary is displayed between the laths, which deteriorates the toughness. For Mn-Nb-B low carbon bainitic steel, the microstructure consist of granular bainite, acicular ferrite and lath bainite can increase strength without lose toughness.(6) The quenching and tempering processes of800MPa grade low cost high strength steel were researched. The optimal process parameters for investigated steel was quenched at930℃and tempered at530℃. And the final microstructure is consist of tempered bainite and tempered martensite.(7) The different quenching processes of800MPa high strength steel were performed. The results show that tensile strength and yield strength of HR-DQ and CR-DQ steels are obviously higher than those of RQ steel. The impact energy at-20℃is decreased drastically in HR-DQ steel while decreased slightly in CR-DQ steel. Compared to that of RQ steel, CR-DQ steel exhibits excellent combination of strength and toughness. Therefore, CR-DQ process can be used to replace RQ process to produce800MPa grade steels.(8) The effect of direct quenching temperatures and finish quenching temperatures on microstructure and mechanical properties of800MPa grade high strength steel was investigated. The results show that when the steel is direct quenched at the range of840～890℃after controlled rolling, the tensile strength, yield strength, impact energy at-20℃and elongation are about900MPa,863-875MPa,174-199J, and18.3-19%, respectively. And the tested steel possesses stable mechanical properties and displays excellent combination of strength and toughness. When finish quenching temperature changes from450℃to room temperature, the strength is increasing gradually, while impact energy at-20℃is decreasing significantly as temperature below350℃. |
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