| As the most convenient vehicle and important means of conveyance, automobiles have already entered all areas of human social and economic life. The development trend of new-generation automobiles is energy conservation, reduction of consumption, environmental protection and security. Therefore, use of high strength and super high strength steel is the development trend of steels used for automobiles in the future. Currently, most of HSLA steel’s strength used in automobile industry is below500MPa and far from meeting the requirement of weight reduction. So improve the strength and toughness have a great practical significance. The microalloying technology and controlled rolling controlled cooling (TMCP) technology are the main means of HSLA steel’s strengthening and toughening.In this thesis, the high temperature deformation behavior was studied through single-pass compression experiments on the Gleeble-2000thermal simulation testing machine, and the influence of alloying elements and TMCP parameters to the HSLA steel’s microstructure and properties were studied through the hot rolling experiments on the ΦD450mm mill in the RAL laboratory. The best alloying elements and hot rolling parameters were obtained. The main conclusions of the study are as follows:(1) Under the strain rate of0.1s-1and strain0.7, The true stress and true strain curve of the plain C-Mn experimental steel is work hardening type at the temperature of800℃、850℃、870℃, dynamic recovery type at the temperature of900℃, and dynamic recrystallization type at the temperature of950℃、1000℃. So the experimental steel’s dynamic recrystallization temperature is more than900℃(2) Alloying element Cr had solid solution strengthening effect and increased the hardenability. Compared with plain C-Mn steel, adding0.22%Cr made the experimental steel’s strength increase by142MPa. Alloying element Ti had strong precipitation strengthening effect and medium grain refinement effect, when adding0.061%Ti to the experimental steel, its yield strength increased by169MPa. The experimental steel with 0.238%Cr and0.0799%Ti, compared with the steels which only have Cr or Ti, the strength and hardness were higher.(3) The experimental steel with0.0654%Ti and0.002%B, for the alloying element B restrained the eutectoid ferrite transformation and pearlite transformation, increased hardenability significantly, compared with the steel which only contain Ti, its yield strength increase by26MPa. The experimental steel with0.47%Cr and0.04Nb, for the alloying element Nb had strong grain refinement effect, compared with the steel with0.0238%Cr and0.0799%Ti, its yield strength increased by41MPa.(4) The experimental steel which contained Cr, Ti, V and B had good combination mechanical properties for the influence of various alloying elements, not only improved the strength and plastic was also significantly improved.(5) The influence of different TMCP parameters to the low-alloy steels was studied. The finishing temperature decreased from870℃to800℃made yield strength and tensile strength of the experimental steel which contained Ti and B decreased separately to564.79MPa and701MPa. The main reason was that lower finishing temperature reduced the precipitation strengthening effect. To the other two steels which contained Cr, Ti, V, B and Cr, Nb, lower finishing temperature increased the strength for the enhanced grain strengthening effect. Yield strength increased separately to669.2MPa and684.2MPa and tensile strength increased separately to773.4MPa and774.43MPa.(6) The coiling temperature decreased from600℃to550℃increased the strength of the experimental steel which contained Cr, Ti, V, B. Yield strength and tensile strength separately increased from669.2MPa and773.4MPa to737.84MPa and848.17MPa. Elongation decreased from22.6%to18.9%. Hardness increased to235.3HV. The main reasons were lowering coiling temperature refined ferrite grains and increased bainite content.(7) While deformation amount increased, it promoted the ferrite transformation, the strength of the experimental steel decreased a little. While cooling rate increased, it was beneficial to the grain refinement, increased the bainite content. The experimental steel’s strength increased.(8) Through different alloy composition designed and researched on TMCP technology, the best alloy composition and hot rolling parameters were obtained. The experimental steel which contains0.46%Cr,0.11%Ti,0.07%V,0.002%B, in the conditions of finishing temperature800℃, coiling temperature600℃, cooling rate13℃/s, had the best comprehensive performance. Yield strength669MPa, tensile strength773MPa, elongation22.6%, and fully match the requirement of the high strength steels for automobiles. |
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