Study On Bainite Phase Transformation And Microstructure Of Medium Carbon Alloy Steel

Due to the good strength-ductility,the carbide-free bainite becomes the alternative steel for the third generation advanced high-strength steel.Long heat treatment cycles have been a major obstacle to the development of carbide-free bainitic steels.To this end,This paper proposes method to combining composition optimization with pre-generated martensite-induced bainite transformation to solve this problem.In this paper,two kinds of medium carbon alloy steels 28Mn3Si1.4 and 28Mn2Si1.6 were studied.The bainite transformation kinetics of the experimental steel was studied by thermal expansion.By studying the microstructure and mechanical properties of the heat treated samples,the Ingredients-Process-Organization-Performance Relationship was established of experimental steel.The experiment draws the following conclusions.(1)Steel 28Mn2Si1.6 austempered at a temperature above the Ms point,the transformed products is carbide-free bainite and retained austenite,and the retained austenite transform to martensite when cooled to room temperature.With the decrease of austempered temperature,the incubation period of bainite transformation is shortened and the transformation speed is accelerated,the amount of transformed bainite is increased,and the Ms of martensite is lowered in the cooling process.(2)When 28Mn3Si1.4 steel and 28Mn2Si1.6 steel are austempered below the Ms temperature,a part of martensite is formed during the cooling process,then bainite transformation occurs during the isothermal process.With the austempered temperature decreases,the volume fraction of the pre-generated martensite structure increases,the incubation period of the bainite transformation shortens,the bainite transformation speed decreases,and the time required for the completion of the bainite transformation is shortened.With the austempered temperature decreases,the volume fraction of retained austenite in the structure decreases,the strength increases,and the plasticity decreases.(3)When steel 28Mn3Si1.4 queching to the temperature below Ms and then heated to a higher temperature.First,a part of martensite is formed during the cooling process,and then bainite transformation occurs during heating and isothermal process.With the isothermal temperature increases,the transition termination time of the carbide-free bainite transformation is gradually shortened,when the isothermal temperature is higher than the Ms temperature,bainite transformation is incomplete.(4)When steel 28Mn3Si1.4 austempered at a temperature below the Ms point,the transformation speed of bainite is very low.Extending the isothermal time helps eliminate secondary martensite and reduce the strength and improve the plasticity.As the isothermal temperature decreases,the retained austenite content in the fully transformed martensite+bainite structure decreases,the strength of the steel increases,but the plastic toughness does not change much.The mechanical properties of steel 28Mn3Si1.4 austempered at 280? for 6 h:Rp0.2=1030 MPa,Rm=1490 MPa,A=15.8%,Akv=26J,Rm×A=23542 MPa%.(5)When steel 28Mn2Si1.6 austempered at a temperature below the Ms point,the bainite transformation speed is accelerated.Mechanical properties of steel 28Mn2Si1.6 austempered at 280?:Rp0.2=990 MPa,Rm=1590 MPa,A=14.6%,Akv=36 J,Rm×A=23214 MPa%;austempered at 330?:Rp0.2=850 MPa,Rm=1430 MPa,A=17.3%,Akv=25 J,Rm×A=24739 MPa%;when steel 28Mn2Si1.6 quenched to 280? then heated to 330? abtain the best mechanical properties:Rp0.2=1000 MPa,Rm=1540 MPa,A=17.6%,Akv=32 J,Rm×A=27104 MPa%.