Study On Effect Of Heat Treatment On The Microstructure And Properties Of Medium Carbon Alloy Steel

The microstructure of carbide-free bainite steel and quenching-partitioning steel are both complex phase structures of bainitic ferrite(lath msrtensite)+retained austenite,which are the alternative steels of the third generation of advanced high strength steel,especially the latter has higher strength-plasticity advantage.Aiming at the development of advanced high strength and high plasticity steels with strength surpass 1600 MPa,two kinds of medium carbon alloy steel of 34SiMn2CrNiMo and 39SiMn2CrNiMo were designed.The phase transition behavior during Q&P treatment was analyzed through thermal expansion simulation experiment,and the microstructure characterization and mechanical properties test of samples treated with Q&P and austempering.The microstructure transformation law and the relationship between microstructure and mechanical properties of the experiment steel were analyzed,and the relationship between composition-process-microstructure and properties was established.The results were as follows:(1)When the supercooled austenite of 34SiMn2CrNiMo steel was treated at an isothermal temperature above Ms temperature,due to insufficient driving force,the phase transition was incomplete,and the transformation amount of carbide-free bainite was small.In the end,there were a large number of secondary martensite and a small amount of retained austenite in the microstructure,resulting in high strength,poor plasticity and poor comprehensive mechanical properties of the experimental steel.When 34SiMn2CrNiMo steel was treated at an isothermal temperature below Ms temperature,the presence of primary martensite promoted the isothermal transformation of bainite.During the isothermal process,the retained austenite is rich in carbon,which improves its stability.The final structure is primary martensite,carbide-free bainite,reatined austenite and a small amount of secondary martensite.The comprehensive mechanical properties of 34SiMn2CrNiMo steel were the best after austempering at 300? for 6 h.The retained austenite content is 10.6%,the tensile strength is 1494 MPa,the elongation is 18.9%,and the products of strength and elongation is 28.2 GPa%.(2)The quenching interruption temperature determines the phase ratio in the microstructure of the experimental steel after Q&P treatment,which has a great influence on the mechanical properties.The retained austenite content increased first and then decreased with the increase of quenching interruption temperature,which was consistent with the theoretical prediction,but the austenite content was slightly lower than the theoretical prediction.When the quenching interruption temperature of 34SiMn2CrNiMo steel and 39SiMn2CrNiMo steel are 170?180? and its comprehensive mechanical performance is the best.(3)Partitioning time and temperature also have great influence on the microstructure and properties of steel after Q&P treatment.After quenching 34SiMn2CrNiMo steel to 180? and partitioning at 400?,with the extension of the partitioning time,the content of retained austenite first increased and then decreased,reaching the maximum value at 10 min.At 450?,with the extension of the partitioning time,the retained austenite content decreased,and the carbon content retained austenite also decreased gradually.After quenching 39SiMn2CrNiMo steel to 120? and partitioning at 400?,with the extension of the partitioning time,the yield strength increased,the tensile strength decreased and the retained austenite content remained stable within the first l0min as the distribution time was prolonged at 400?.Elongation decreased with the extension of partitioning time.(4)The best heat treatment process of 34SiMn2CrNiMo steel is:austenitization temperature is 900?,then air cooling to 170?,then slowly heated in the air furnace to 400?,the sum of temperature rise and partitioning time is 1 h.At this time,the retained austenite content in the experimental steel is 21.7%,the tensile strength is 1607 MPa,the elongation is 24.9%,and the products of strength and elongation is 40 GPa%.Or after 900? austenitization,air cooling to 180?,and then heating in a salt bath furnace to 400? and partitioning for 5 min.At this time,the retained austenite content in the experimental steel is 18.9%,the tensile strength is 1722 MPa,the elongation is 21.9%,and the products of strength and elongation is 37.7 GPa%.(5)The optimum heat treatment process of 39SiMn2CrNiMo steel is:air cooling to 120?after austenitization at 900?,and then slowly heating to 400? in the air furnace.The sum of heating and holding time is 1 h.At this time,the retained austenite content in the microstructure of the experimental steel is 24.3%,the tensile strength is 1692 MPa,the elongation is 23.0%,and the products of strength and elongation is 38.9 GPa%.After the austenitization at 900?,it was air-cooled to 120?,and then heated in a salt bath furnace to 400? for 5 min.At this time,the retained austenite content in the experimental steel is 22.2%,the tensile strength is 1800 MPa,the elongation is 22.3%,and the products of strength and elongation is 40.1 GPa%.