Study On Microstructure And Mechanical Properties Of Dual-phase Steel By Fast Heat Treatment

With the development of energy-saving and emission-reduction as well as increase of safety demand,modern automobile are using lots of high performance sheet steel with higher strength and thinner thickness,in order to reduce the vehicle weight and improve the collision performance.In high speed collision,the automobile sheet needs good crumple energy absorption ability,which means good plasticity.Specifically saying,the automobile steel must has super high strength,together with excellent elongation.In general,increase the strength,the elongation will decrease.Therefore,it is emergent and of great practical meaning to develop the AHSS both with high strength and good plastic.This thesis uses DP780 dual-phase steel as research object,the static CCT diagram and CHT diagram have been determined by the dilatomter.Using single factor variables,the continuous annealing of DP780 steel was simulated by salt bath furnace.Analyze the microstructure with Optical Microscopy,Field Emission Scanning Electron Microscopy,Transmission electron microscopy and so on.Test the mechanical properties of DP780 with electronic universal material testing machine.The Ac 1,Ac3 points of DP780 at the experimental condition are determined by considering the CHT curve and the salt bath furnace heating rate,namely 720 ?,890 ?respectively.The continuous annealing of DP780 steel was simulated by salt bath furnace.Samples were soaked 15 s in the furnace of 910 ?,and put into the low temperature furnace with temperature 700 ?,650 ?,630 ?,600 ? respectively and cooled time(5 s-30 s)or cooled time(15 s-45 s)respectively in the air,in order to cool to intercritical zone to obtain the proeutectoid ferrite,and then quenched into water of room temperature.In this study,effect of cooling rate and cooling time in precooling section on the microstructure and mechanical properties of DP780 cold-rolled steel was studied.The high heating rate of salt bath furnace can provide great driving force for austenite nucleation,and short austenizing time can prevent austenitic coarsening,that can effectively obtain fine austenite grain size.In the cooling process,austenite transform to fine proeutectoid ferrite and lath martensite.The research results turn out as follows:with the increase of cooling rate or the increase of cooling time,the amount of proeutectoid ferrite transformed from austenite get more,the original austenite goes down,so the number of martensite transformed from original austenite after water quenching gets down.with the increase of cooling rate or the increase of cooling time,tensile strength decrease,elongation increase.In the experiments of precooling in the furnace(quenching after),with the parameters of 630 ?×20 s?630 ?×15 s?600 ?×10 s,or precooling 25 s in the air,the samples gain good mechanical properties with an average number of tensile strength over 1000 MPa and elongation over 13%,the tensile strength is respectively 1054MPa,1045 MPa,1029 MPa and 1007 MPa,and the elongation is respectively 13.7%,13.0%,13.8%and 13.5%.