Research On The Process Of Low-cost Third-generation Automotive Steel

The third-generation automotive steel is committed to ensuring that the strong plastic product meets the actual use requirements while reducing costs,and ensuring that the material is easy to form in the actual factory.In this thesis,through the control of the composition design and heat treatment process,the strong plastic product of automobile steel can reach more than 30Gpa·%while the cost is reduced,and the influence of different cold rolling reduction rates on the forming performance is studied.At the same time,the intermediate annealing in the cold rolling process in actual production was researched to provide theoretical guidance for actual production.Therefore,the third-generation automotive steel（0.12C-5.2Mn）is hot rolled to 4.5mm and then cold rolled.The final thickness of the cold rolled is 1.0mm,1.2mm,1.5mm,2.0mm,and the cold rolling reduction rate is 79%,73%,66%,55%.The phase transition point of the test steel was tested with an automatic phase change instrument.The morphology and phase distribution were observed by SEM,and the texture was measured by XRD and the austenite content was calculated.At the same time,the mechanical properties are measured by universal testing machine equipment.The research content and results are as follows:After the test,the Ac₁ of the sample is 585°C,and the Ac₃ is 755°C.The CCT curve is drawn,which provides a theoretical basis for subsequent experiments.The effects of different annealing time and annealing temperature on the microstructure and properties of 66%and 55%cold rolled steels were studied by austenite reverted transformation（ART）annealing and two-phase zone direct annealing.The austenite,ferrite and martensite structures were obtained through direct annealing in the two-phase zone and ART annealing.At low temperature or short-term holding time,ART annealing can still observe the austenite boundary after quenching,and there are unrecovered cold-rolled elongated grains directly annealing in the two-phase zone.Under the two processes,the grains of the samples gradually grow with the increase of temperature and time,the austenite content first increases and then decreases,the elongation first increases and then decreases,the yield strength generally shows a downward trend,and the product of strength and ductility first increases and then decreases.The difference is that with the increase of temperature and time,the tensile strength of ART annealing gradually increases,and the tensile strength of direct annealing in the two-phase region first decreases and then increases.At the same time,there is delamination on the tensile fracture surface of the specimen directly annealing in the two-phase zone.There is no delamination phenomenon in the tensile fracture of the ART annealing process.In addition to the elongation,the direct annealing in the two-phase zone has higher yield strength,tensile strength and strong plastic product than the ART annealing process.The best process for ART annealing is 800℃-5min/630℃-10min,and the best process for direct annealing in the two-phase zone is630℃-10min.By comparing the structure and properties of the two sets of samples after the heat treatment of the intermediate annealing and the non-intermediate annealing during the rolling process,it can be concluded that the grain size of the intermediate annealed samples during the rolling process is significantly increased,the yield strength is significantly reduced,and the tensile strength relatively high.This also led to the low yield ratio of the intermediate annealing samples during the rolling process,that is,better forming properties.And its yield platform in the tensile curve is shorter,and the surface quality of the specimen is higher after molding.The microstructure and mechanical properties of the third-generation automobile steel with 66%,73%and 79%cold rolling reduction are similar after direct annealing at 630℃-10min.However,as the reduction rate increases,{111}and{110}favorable texture components increase,{100}unfavorable texture components decrease,and the plastic strain ratio r gradually increases,indicating that as the reduction rate increases,and the molding performance of the material is improved.