| The complex microstructure of low carbon martensite and rich carbon austenite can be obtained by quenching and partitioning treatment so as to obtain good strength plasticity combination.In this study,two kinds of medium carbon alloy steels were designed to develop high strength plasticity and ultra-high-strength steels.The relationship between the microstructure transformation and the microstructure-performance of the experimental steels was studied by means of the microstructure characterization and mechanical properties testing and combined with thermal dilatation experiment analysis of the samples after quenching and partitioning treatment in different processes.The effects of process parameters and chemical composition on the microstructure and performance were studied.The results of this study are as follows:(1)The dilatation behavior of Steel 28Mn3Si1.4 and Steel 28Mn2Si1.6 during austenitizing and quenching to different temperatures below Ms,then heating to 380～450℃,then cooling to room temperature was studied by means of full-automatic metal transformation instrument.Among them,the varying degrees volume dilatation of Steel 28Mn3Si1.4 occurs during heating and holding process,while Steel 28Mn2Si1.6 has a small volume dilatation in the heating process and a volume contraction in the holding process.The analysis shows that the precipitation of carbides in martensite and the decomposition reaction of austenite in Steel 28Mn3Si1.4 destroyed the partition reaction due to insufficient Si content.The partition reaction was weakened by the martensite decomposition reaction at 450℃ after the increase of Si content in Steel 28Mn2Si1.6.(2)After austenitizing,Steel 28Mn3Si1.4 and Steel 28Mn2Si1.6 quenched to different temperatures below Ms,and then heated to 450℃ for 5 min.The retained austenite content in Q&P microstructure of Steel 28Mn3Si1.4 is significantly lower than that of Steel 28Mn2Si1.6,resulting in lower yield strength,tensile strength,elongation and impact absorbing energy than that of the latter.(3)After austenitizing,Steel 28Mn3Si1.4 and Steel 28Mn2Si1.6 quenched to 140℃and 160℃ respectively,and then partitioned in salt bath at 380℃ and 400℃ respectively.With the partitioning time prolonging from 5 min to 40 min,the retained austenite content in their micro structures increases slowly at about 10%and 12%,respectively.The impact absorbing energy of Steel 28Mn3Si1.4 decreases from 27 J to 8 J,and its yield strength,tensile strength and elongation are stable at 1200 MPa,1400 MPa and 18%respectively.The yield strength,tensile strength,elongation and impact absorbing energy of Steel 28Mn2Si1.6 are stable at 1240 MPa,1580 MPa,18%and 33 J,respectively.(4)After austenitizing,Steel 28Mn3Si1.4 and Steel 28Mn2Si1.6 quenched to 140℃and 160℃ respectively,and then partitioned in salt bath at 450℃.With the partitioning time prolonging from 5 min to 40 min,the retained austenite content in Steel 28Mn3Si1.4 decreases from 10%to 3%,the yield strength decreases from 1100 MPa to 900 MPa,the tensile strength decreases from 1400 MPa to 1270 MPa,and the elongation decreases from 18%to 15%,the impact absorbing energy is stable at about 20 J.The content of retained austenite in Steel 28Mn2Si1.6 decreases from 18%to 11%,resulting in the decrease of yield strength from 1190 MPa to 900 MPa,the decrease of elongation from 20%to 16%,and the stabilization of tensile strength and impact absorbing energy at 1600 MPa and 20 J,respectively.(5)The comprehensive mechanical properties of Steel 28Mn3Si1.4 are the best when austenitizing,then quenched to 140℃ and then heated to 450℃ for 5 min.Its austenite content is 10.4%,yield strength is 1102 MPa,tensile strength is 1416 MPa,elongation is 18.0%,impact absorbing energy is 16 J，and the product of strength and elongation is 25.5 GPa·%.The comprehensive mechanical properties of Steel 28Mn2Si1.6 are the best when austenitizing,then quenching to 160℃ and then heating to 450℃ for 5 min.Its austenite content is 18.0%,yield strength is 1190 MPa,tensile strength is 1607 MPa,elongation is 20.2%,impact absorption energy is 25 J,and the product of strength and elongation can reach 32.5 GPa·%. |
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