Development Of590MPa Grade Low Yield Ratio Construction Steel

Steel structure has received extensive attention in engineering application for its excellent earthquake-resistant capability and its adaptation of residence industrialization. However, the drawbacks such as low strength, high yield ratio, thin gauge and poor earthquake-resistance and weldability restrict its popularization. Therefore, it’s of realistic significance to develop thick construction plate with high strength and low yield ratio as well as good earthquake-resistance.In this paper, on basis of the project "Reduced Rolling Technology of Saving Type Steel Products" in the National Key Science and Technology Supporting Program for the11th Five-Year Plan, a cost saving chemical composition design was adopted in terms of compound addition of Nb, V and Ti avoiding addition of expensive alloying elements such as Cu, Cr, Ni, Mo, etc. Recrystallization and transformation behavior, microstructure evolution and influence of microstructure on mechanical properties under certain processing conditions such as TMCP-L-T, N-L-T, DL-T and HOP were investigated. And the research results have been applied to instruct industrial production. The chief original work and results are as follows:(1) The recrystallization and transformation behavior were studied, and the regulation of recrystallization and effect of deformation and cooling on microstructure was explored. The activation energy of dynamic recrystallization for peak and steady status is323.164kJ/mol and224.244kJ/mol respectively. And the thermal deformation equation and kinetics models of static recrystallization were also obtained through regression. The continuous cooling experimental results show that the γâ†'α transformation temperature decreases as cooling rate increases and proeutectoid ferrite was refined with nucleation sites transferring from grain boundary to grain surface. In addition, deformation enlarges the diffusive transformation region and narrows the transformation region such as acicular ferrite, granular bainite and lath bainite.(2) A new low-cost and short-flow processing (TMCP-L-T) was proposed to produce590MPa grade construction steel. Microstructural evolution and influence of microstructure on mechanical properties were investigated, and industrial trial was carried out. The result shows that as quenching temperature increases, yield strength increases monotonously and tensile strength affected by both volume fraction and strength of tempered martensite firstly increases slightly and then decreases, which makes yield ratio increase. The optimal quenching temperature ensuring both strength and toughness and low yield ratio is760℃. The result of industrial trial shows that the developed590MPa grade construction plate possesses good match of strength and toughness with low yield ratio of0.74, completely meeting the requirement of SA440.(3) A low-cost processing (N-L-T) for low compression ratio was proposed to produce590MPa grade construction steel. Microstructural evolution and influence of microstructure on mechanical properties were investigated, and industrial trial was carried out. The result shows that as tempering temperature increases, tensile strength decreases monotonously while yield strength firstly increases then decreases, and toughness, ductility and Z-direction property improves constantly. The optimal tempering temperature is500℃considering requirement of strength, toughness and yield ratio. The result of industrial trial shows that yield ratio of590MPa grade large gauge plate(90mm) is0.69as produced by N-L-T processing, which is the lowest among high performance construction plates.(4) A low-cost and short-flow processing (DL-T) was proposed to produce590MPa grade construction steel. Microstructural evolution and influence of microstructure on mechanical properties were investigated. The result shows that high quenching rate ensures the ferrite-martensite microstructure. As direct quenching temperature increases, both yield strength and tensile strength increase but elongation decreases. When quenching temperature increases to750℃, yield strength increases sharply, making yield ratio increase from0.76to0.82. As tempering temperature increases, yield strength firstly increases then decreases due to the precipitation of microalloying elements. While, tensile strength decreases monotonously and yield ratio increases. At relatively low tempering temperature, yield plateau is short, ensuring low yield ratio and good match of strength and toughness. From the results, it is found that the mechanical properties of590MPa grade construction steel can be satisfied when cooling rate reaches30℃/s, quenching temperature ranges from600～700℃and tempering temperature is500℃. And the mechanical properties of780MPa grade construction steel can be satisfied when the quenching temperature reaches750℃ (5) Mechanisms of microstructural evolution under the HOP processing were originally explored. The result indicates that low finish cooling temperature is benefit for shear transformation so that guarantees high hardness. The holding time after cooling mainly affects the transformation volume fraction. For short holding time, a certain part of transformation occurs during the subsequent heating process. For long holding time, transformation time is sufficient, so that recovery, enrichment of carbon into austenite and cementite precipitation happens during the subsequent heating process. As heating rate during tempering increases, dislocation density increases and carbides are refined, which help to the increasing of hardness. As tempering time increases, cementite precipitates from the carbon supersaturated bainite and dislocation density decreases as well, leading to the hardness decreasing.(6) The effect of processing parameters on microstructure and properties for the hot rolling+HOP was originally systematically explored, and an alternative processing was proposed to take place of the HOP processing. The result shows that as tempering time increases, volume fraction of M-A constituents firstly increases then decreases and yield ratio increases. Increasing volume fraction of M-A constituents helps to increase tensile strength but deteriorated toughness follows. Therefore, an optimal way to ensure both low yield ratio and good toughness is to obtain certain amount of M-A constituents to reduce yield ratio and to refine their size and increase its dispersity at the same time for good toughness. It is foreseeable that low yield ratio and high toughness can be realized through the control of carbon enrichment and dislocation recovery in rapid heating process. In the new processing, multiple phases containing ferrite, acicular ferrite and a small part of lath bainite was obtained by fast cooling to450℃and holding for6min. Tiny and dispersed M-A constituents with volume fraction of8.1%ensures high tensile strength so that the low yield ratio and good toughness was realized. Therefore, an alternative processing for HOP is provided to produce high strength steel with low yield ratio and good toughness.