Research On Microstructure Evolution And Mechanical Properties Of C-Si-Mn-Cu Cold Rolled Q&P980 Steel

With the consideration of the automobile safety,lightweight has become an inevitable trend in automobile industry.Q&P steel,with ultra-high strength and good ductility,can reduce the weight of automobile body under the premise of ensuring safety,and has broad prospects.In this paper,copper element was added to steel on the basis of traditional C-Si-Mn components.The microstructure evolution and the relationship between microstructure and mechanical properties during the Q&P process were carried out by thermodynamic simulation,dilatometric simulation and practical heat treatment experiments.The results are as follows:(1)In the ortho-equilibrium condition,the temperature of A1 and A3 are 651? and 871?,respectively.The cementite precipitation temperature is 696?,the NbC precipitation temperature is 1188? and the Cu precipitation temperature is 733?.(2)The Ac1 temperature measured by static dilatometer is 670 ?,Ac3 is 887 ?,Ar1 is 512?,Ar3 is 748?.Meanwhile,the temperature of Ms and Mf at 50?/s cooling rate are 360? and 180?,respectively.Furthermore,The critical cooling rate of bainite transformation is about 5?/s.During the quenching and partitioning process,the obvious bainite transformation can be observed with isothermal partitioning of 400?,whilst the corresponding transformation rate is much larger than those of other treatment experiments.The optimum partitioning temperature of the experimental steel is 400?.(3)When the annealing temperature increased from 725 ? to 860?,the tensile strength firstly increased and then decreased.When the experimental steel was annealed at 760?,the tensile strength was 1207 MPa.The yield strength of experimental steel decreased firstly followed by increasing,which varied from 555 to 956 MPa.The PSE and elongation had the same trend of change.Both of them decreasd firstly followed by increasing and then decreased.Meanwhile,the PSE of 26362 MPa·%with superior total elongation of 24.8%was obtained in the sample annealed at 725?.The microstructure was composed of ferrite,fresh martensite and retained austenite.Moreover,the volume fraction of RA was 24%with the carbon concentration of 1.26%.(4)When the quenching temperature increased from 180? to 300?,the tensile strength increased as a whole;the yield strength decreased firstly followed by increasing,the overall trend of PSE and elongation were the same,both of them increasd firstly followed by decreasing and then increased finally.When the quenching temperature was 200?,the tensile strength of the experimental steel was 1124 MPa,the yield strength was 779 MPa,the elongation was 22.4%,the PSE was 25178 MPa·%,and the optimal soft-hard phase ratio was obtained.The results were consistent with the optimum quenching temperature(193.5?)calculated by CCE model.(5)The tensile strength of the experimental steel gradually decreased with the prolongation of partitioning time,the elongation increased firstly and then remained basically constant.The change of yield strength was not obvious.When the partitioning time was 300s,the tensile strength of the experimental steel was 1098 MPa,the yield strength was 588 MPa,the elongation was 20%,and PSE reached 21960 MPa·%.When the partitioning time was 300s,the partitioning process had basically completed.