| With the wide application of hot stamping technology of high strength steel in the automotive industry,higher performance is required for hot stamping parts,as for anti-collision parts,such as the B-pillar,door beams and bumper,die quenched martensitic structural components have been shown to provide excellent intrusion resistance.But exhibit only limited ductility,which may limit the overall impact performance of the component.The introduction of lower strength and more ductile tailored properties within some regions of a hot stamped component has the potential to improve impact performance.The high strength region ensures the safety of passengers,local low strength regions can absorb collision energy by plastic deformation.High energy consumption was the main factors restricting widely use of the hot stamping technology.Conventional furnace has the disadvantages of large volume,high energy consumption and low heating efficiency.Resistance heating system is relatively simple,convenient control,fast heating and high efficiency,which is an ideal heat source for hot forming of sheet metal.Heated and cooled segment die and resistance heating device were developed.Hot stamping U parts with tailored property has been realized,the advantages of self-resistance heating were analyzed by comparing with the furnace heating method,and the tailored property of the stamping part were studied by experiment.Firstly,the influence of die temperature on the microstructure and properties of 22MnB5 boron steel sheet was analyzed with plate mold.With the increasing of die temperature,the yield strength and tensile strength decrease,while the elongation increases gradually.When the die temperature is lower than 250?,the microstructure is mainly lath martensite,the tensile strength is greater than 1200 MPa,and the elongation is greater than 5%;When the mold temperature is higher than 450?,the microstructure is mainly ferrite+pearlite+carbide,the tensile strength is greater than 600 MPa,and the elongation is greater than 15%.The mechanical properties of high strength zone and low strength zone of hot stamping parts are satisfied respectively.Secondly,using heated and cooled segment die,the distribution of microstructure and property at transition zone were analyzed with the method of numerical simulation and experiment.With the increase of heated die temperature,the hardness difference between high strength zone and low strength zone increases gradually,and the average hardness of high strength zone is about 500 HV,the hardness of low strength zone is gradually decreasing with the increase of heated die temperature.When the temperature of heated die is between 200~500?,and the hardness is within 450HV~250HV.The width of the transition zone is about 60 mm,which is located in the air gap and is inclined to the cold die side.The microstructure of the high strength side is mainly martensite,and the bainite structure appears gradually during the transition to the low strength zone.The low strength region is mainly composed of ferrite and carbide.The elongation of self resistance heating is higher than that of furnace heating,but the tensile strength decreases slightly,there are some ferrite phase distributed in martensite lath at the high strength zone,and more carbide precipitates exist at the low strength zone.Finally,the deformation behavior and energy absorption property of pipe shaped parts with gradient performancwere analyzed.With the increase of the hot mould temperature,the axial crushing load gradually decreases,and the total energy of crushing and absorption decreases.The crushing deformation of the original base metal and the whole cold die quenched parts begin at the middle of the specimen,while the deformation of the forming parts with the gradient property concentrates in the low strength area.The main failure modes are weld joint cracking,and the phenomena of tearing occurring at specimens with higher strength. |
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