Investigation On The Local Partitioning Treatment Of Hot Stamping Parts Of High Strength Steel Sheet

Parts treated by hot stamping process are used more and more frequently in the automotive industry because of their ultra-high strength(nearly about 1500MPa),unfortunately,their plasticity is poor(elongation rate 5% to 8%).In order to improve the plasticity of the part treated by hot stamping at the expense of reduce the strength slightly,in this thesis,traditional hot stamping process integrated with Q&P(quenching and carbon partitioning)treatment will be investigated so as to improve the comprehensive performance of overall or local parts.Meanwhile,to improve the thermal response and heating efficiency of Q&P process in the partitioning process,a new partition process is proposed,namely,partitioning through the hot air flow.Different regions of the same part may have different mechanical properties demand,that is,the differential distribution of mechanical properties of parts.Tailored properties can be achieved through TTP(tailored tempering process),so do the local partitioning process with hot stamping.The plasticity of the parts improves while maintaining high tensile strength(1300MPa).In this thesis,ultra-high strength steel 30CrMnSi2 Nb is used as experimental research.Concretely,several conclusions can be got in this research.Numerical simulation is used to simulate and analyze the temperature field of U-shaped part during the partitioning stage in the partitioning device so as to optimize the structure of the partitioning device.And the final structure of the device is designed according to the result of numerical simulation.Besides,actual measurement of the temperature field of U-shaped part during the partitioning stage verifies the feasibility of the device designed in this method.The maximum temperature difference is 15?,and it is more efficient than partitioning in furnace.Using ultra-high strength steel for automobile 30CrMnSi2 Nb as materials,plate parts treated in this new partitioning device with different partitioning parameters after hot stamping and quenching stage.The results show that the newly proposed process improve the plasticity of the part compared with direct hot stamping,what's more,the maximum product(about 18GPa%)of tensile strength and elongation comes when the partitioning temperature is 420?and the partitioning time is 60 s.Result of Experiments with U-shaped parts show that the mechanical properties of different parts of the parts is not uniform,the tensile strength of side wall is 100 MPa lower than that of other region.This is due to the complex contact conditions during forming and quenching process,results in a non-uniform quenching temperature field after quenching process(temperature of side wall is 60? higher than other region).The adoption of cold pre-forming before hot stamping with Q&P process can diminish the inhomogeneous distribution of mechanical properties of the final part.Concretely,the maximum tensile strength difference decrease to about 40 MPa.Another purpose of the partitioning device proposed in this thesis is to obtain tailored performance.And experimental study show that gradient mechanical properties exist in different regions of the part.In the partition region,the tensile strength is about 1350 MPa,the elongation and PSE is about 13% and 18GPa%.In the high strength region,the tensile strength is about 1800 MPa,the elongation and PSE is about 6.5% and 10GPa%...