| Carbide-free bainite steel has good plastic-toughness cooperation,and has become the third-generation advanced high-strength steel candidate.However,due to the long heat treatment period of carbide-free bainite,a combination of composition optimization and preformed martensite-induced bainite transformation is proposed in this paper.In this dissertation,two kinds of alloy steels,medium-carbon alloy steel 41MnSiCr steel and high-carbon alloy steel 50MnSiCr steel,are used as research objects.The thermal expansion behavior of bainite transformation of the experimental steel was investigated,and the micro structure and properties of the heat treated samples were characterized.The composition-process-structure-property relationship of the experimental steel was established.The following results were obtained.(1)When austenitizing 41MnSiCr steel is subjected to isothermal quenched at different temperatures above the Ms point,the structure at room temperature is carbide-free bainite and secondary martensite transformed during isothermal cooling.With the decrease of the isothermal quenched temperature,the incubation period and completion time of the bainite transformation are shortened,and the maximum transformation rate during the bainite transformation is reduced.After the austenizing of 41MnSiCr steel,the best strong plasticity is obtained after isothermal quenched at 360?×0.5 h.At this time,the austenite content in the structure is 21.2%,the yield strength is 876 MPa,the tensile strength is 1305 MPa,and the elongation is 24.9%.(2)When austenitizing 41MnSiCr steel after isothermal treatment at different temperatures below the Ms point,a part of martensite is formed during cooling,and then bainite transformation occurs during isothermal process.As the isothermal temperature decreases,the pre-formed martensite content increases,and the bainite transformation rate decreases,but the time required to complete the bainite transformation is shortened because the bainite transformation variable is reduced;the volume fraction of retained austenite in the structure decreases,The tensile strength of steel increases and its plasticity decreases.After the austenizing of 41MnSiCr steel,the best strong plasticity is obtained after isothermal quenched at 300?× 0.5 h.At this time,the austenite content in the structure is 17.2%,the yield strength is 937 MPa,the tensile strength is 1318 MPa,and the elongation is 21.3%.(4)41MnSiCr steel is austenitized and then quenched to a temperature below the Ms point,and then heated to 400? for 1 h.During the cooling process,a part of martensite is formed,and then the bainite transformation occurs during the heating and holding process.As the quenched termination temperature decreases,the time required for the termination of carbidefree bainite transformation gradually decreases.The 41MnSiCr steel is austenitized after quenched to 210? and heated in a 400? furnace for 1 h to obtain the best strong plasticity.At this time,the residual austenite content in the structure is 10.6%,the yield strength is 1302 MPa,and the tensile strength is 1602 MPa,elongation 17.8%.(3)When the 50MnSiCr steel is austenitized and quenched to a temperature above the Ms point for isothermal treatment,the room temperature structure is carbide-free bainite formed during the isothermal process and martensite formed during the isothermal cooling process.As the isothermal temperature decreases,the martensite content decreases and the retained austenite content decreases.When 50MnSiCr steel is isothermally quenched at 330?,the steel obtains the best comprehensive mechanical properties.the yield strength is 905 MPa,the tensile strength is 1470 MPa,and the elongation is 19.0%.50MnSiCr steel is tempered at 360? and tempered at+400?.At this time,the austenite content in the structure is 27.5%and the yield strength 957 MPa,tensile strength 1421 MPa,elongation 27.7%.(4)After austenitizing,the 50MnSiCr steel is quenched to a temperature below the Ms point and then heated and maintained at 400? for 1 hour.Martensite and bainite transformation occur during the treatment.Martensite is formed during quenched and cooling.Generated during secondary heating and holding.With the decrease of quenched temperature,the content of martensite increases,and the bainite transformation incubation period prolongs the transformation rate and decreases.With the decrease of quenched temperature,the yield strength and tensile strength of the samples gradually increased.After increasing the quenched temperature to 400? for 1h,high plasticity is obtained when the quenched temperature is 210?.At this time,the austenite content in the structure is 23.1%,the yield strength is 1143 MPa,the tensile strength is 1646 MPa,and the elongation is 19.1%;Ultra-high strength is obtained when the quenched termination temperature is 120?.At this time,the austenite content in the structure is 15.6%,the yield strength is 1595 MPa,the tensile strength is 1853 MPa,and the elongation is 13.7%?... |
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