The Regulation On Residual Austenite In Al-Containing Medium-Carbon Bainite And The Research On Its Strength And Ductility

As the society developing,Medium-carbon bainitic steel is needed to be of better performance.The strength and ductility of medium-carbon bainitic steel can be further improved by partially replacing Si with Al.The thesis is based on the two samples which comprise 1.48Si or 0.26Si-1.1Al after the isothermal process.The research mainly depends on the microstructure and tensile properties of the two sets of samples.In addition,0.26Si-1.1Al samples which performs better on strength and ductility are selected to be processed under isothermal processes of different temperature(300?,350? and 400?)as systematic research.The tensile strength of 0.26Si-1.1Al samples are tested.It is worthy mentioning that the product of strength and ductility of 0.26Si-1.1Al samples under isothermal process at 350 ? for 4 hours reaches 31.6 GPa%.Tensile strength of the samples reaches 1036 MPa while elongation of the samples is 30.5%.The researches about mediumcarbon bainite show that product of strength and ductility of medium-carbon bainite ranges from 20 GPa% to 25 GPa%.At the same time,elongation of the samples is under 20%.It is found that the elongation of the 0.26Si-1.1Al samples is significantly higher than that of the 1.48 Si samples,so does product of strength and ductility.This results from that the partial substitution of Al element for Si element can increase the free energy of transformation from austenite to bainite and increase the starting temperature of bainite transformation.And it promotes the bainite transformation process of the samples and increases the driving force of the bainite transformation.At the same time,it can accelerate the diffusion of C atoms and inhibit the precipitation of carbides.Thus,it enhances the carbon content of the residual austenite and pushes the formation of film residual austenite of finer size which is more stable.The film residual austenite can lead cracks to branch and prevent the cracks from further expanding when the sample is deformed.It also delays the necking of the samples.So it can greatly increase the elongation of the 0.26Si-1.1Al samples.The product of strength and ductility can be greatly increased.It is found that there is higher bainite transformation rate of the samples under the 400? isothermal process.The 300? and 350? are lower than the martensite starting temperature.The bainite transformation of the samples under the 300? and 350? isothermal process is accompanied by martensite during the isothermal process.The volume fraction of bainite of this two sets of samples gets slightly lower.At the same time,the strength of the three sets of samples are above 1000 MPa.As the isothermal temperature decreases,the bainite lath and the residual austenite in the medium-carbon aluminum-containing bainite are getting finer,such as film residual austenite that is more stable.In this case,the size of residual austenite in the samples processed by the isothermal process of 300? and 350? is smaller than that of the samples processed by the isothermal process of 400?.Therefore,the ductility of the samples under the isothermal process of 300? and 350? is greater than that of the samples under the isothermal process of 400?.