Study On TMCP Process Of780MPa Grade Hot-rolled Dual Phase Wheel Steel

Due to excellent comprehensive properties, Hot-rolled dual phase steel has been widely used in many fields, such as automobiles, construction, shipping and petroleum and so on. However, the cost of Cr-Mo hot-rolled dual-phase steel produced by traditional process is high, the control of process parameters for Mn-Si hot-rolled dual-phase steel is difficult. Above steels-grade are limited in the range of550-600MPa. As automobile wheels are one of the extremely important components for the automobiles and bear complex alternating loads when vehicles are running, upgrading and improving on material and performance for commercial vehicle wheel are urgent in order to meet the increasing demand of automobile lightweight.According to the performance and low cost requirements for commercial vehicle wheel, the method was proposed to develop the low cost and high performance of commercial vehicle wheel steel about DP780grade, by adding microalloy element Ti and combining TMCP process based on low content of C and high content of Si-Mn. The main works and results of this paper are as follows:(1) Dynamic recrystallization behavior of tested steel were investigated by single pass compression experiment, on mathematical model of dynamic recrystallization and deformation resistance was regressed. Dynamic recrystallization activation energy of Tested steel is Q=383.97kJ/mol, the mathematical model had good precision.(2) Static/dynamic CCT curves of tested steel were drew by Austenite continuous cooling transformation experiment, the effects of cooling rate and deformation on phase transition behavior were investigated. Deformation promote phase transition of ferrite and pearlite, enlarge critical cooling rate of ferrite and bainite, and critical cooling rate was15℃/s of martensite transition.(3) TMCP process of tested steel were studied by hot rolling experiment, the effects of finishing temperature, relaxation temperature, relaxation time, and coiling temperature on structure and property were analysed. F+M were appeared at the low temperature coiling (100-350℃) condition, whereby tensile strength was from780MPa to830MPa and yield ratio was below0.70, meanwhile, the elongation ws about19%, F+B were appeared at the medium temperature coiling(400-500℃) condition, tensile strength was about760MPa and yield ratio was0.82, simultaneously, elongation was23%.(4) Research on the performance of cold bending, chambering, impact by the forming and impact experiment. At the condition of d=a, bending angle=180°, the results show that no matter low temperature coiling and medium temperature coiling, cold bending property was perfect; chambering rate was around40%under low temperature coiling, and chambering rate of medium temperature coiling was around60%.(5) Through the analysis for precipitation and the counting of strengthening mechanism, the contributor for strength of tested steel were investigated. The temperatures of a large number of TiC precipitated was700-730℃, and the size of TiC were lower than10nm. The strength mechanism of tested steel were solution strengthening, dislocation strengthening, fine-grain strengthening, precipitation strengthening and phase transition strengthening.(6) Comprehensive analysis above results of experiments, the theory foundation was prepared for the next research by obtaining optimum rolling process parameters.