| Recently,"Industry 4.0" proposed by Germany and "Made in China 2025" proposed by China have placed green manufacturing on the key position while emphasizing intelligent manufacturing.The steel industry and the automotive industry are also increasingly receiving attention as the energy consumption giants.The automobile industry is devoting itself to the study of automotive lightweight.With the increasing sales and possession of automobiles,environmental pressure and the shortage of non-renewable energy have become huge problems in our country,This greatly promoted the development of advanced high strength automotive steel.The traditional first generation automotive steel and the second generation automotive steel have been unable to meet the development requirements of the future automobile,so the development of new generation automotive steel and new materials has become the focus of current research.Based on the traditional automobile steel,the third generation automotive steel?-TRIP780 is taken as the research object in this paper.Through the design of new components and the optimization of heat treatment process,the strength and ductility of automotive steel can be further improved to meet the development trend of lightweight and"Made in China 2025".The main work and achievements in scientific research are as follows:(1)Composition design of ?-TRIP780 steel.On the basis of traditional TRIP steel and dual phase steel,the design of alloy composition can improve the strength and ductility,and reduce the density of steel.The compositional design of ?-TRIP780 steel using A1 instead of Si,with different gradient C,Mn,Si and Al content,through the comparative experiments to select the best composition system.Addition of 3%stable ferrite Al can retain the ?-ferrite and achieve high ductility.In bainite transformation,Al can inhibit the precipitation of cementite and improve the stability of retained austenite.(2)Phase transformation dynamics of ?-TRIP780 steel.The main tasks include the calculation and determination of phase transition point of each experimental steel,the critical annealing temperature and phase fraction,and the bainite transformation temperature and phase fraction.Among them,the austenite of 40%-60%can be formed when the critical zone temperature at 800?-900? heat 3min,bainite transformation complete when the bainite transformation temperature is 380?-420? holding 4-8min.During this process,25%-30%of bainite is produced with the remaining 20%-25%of the stable retained austenite.(3)Development and optimization of heat treatment process for ?-TRIP780 steel.According to the research results of critical zone and bainite phase transformation,develop the heat treatment process system of ?-TRIP780 steel,carry on the mechanical property test and the microstructure analysis,and study the mechanical steel performance difference and influence factors.Further optimization of the critical zone and bainite zone temperature and holding time,to improve the mechanical properties of ?-TRIP780 steel.(4)Study on continuous annealing of 8-TRIP780 steel and mechanical stability of retained austenite.According to the preliminary heat process optimization,continuous annealing treatment to simulate industrial production,making the continuous annealing process with the actual production situation,analyze the mechanical properties and microstructure.It is concluded that the yield strength of DT1 is?550MPa,the tensile strength is?810MPa,the total elongation is?35%,and the mechanical properties are stable and can be actually produced.The mechanical stability of austenite in-TRIP780 steel was studied,and the content and morphology of retained austenite in the process of deformation were analyzed.(5)Study on formability of ?-TRIP780 steel.The elastic modulus and hole-expanding ratio of ?-TRIP780 steel were measured.The elastic modulus of the newly designed?-TRIP780 steel is?190GPa.The hole expanding ratio of the experimental steel is?32%.Study on the effect of Al on elastic modulus of ?-TRIP780 steel. |
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