Study On Grain Refinement Of Structural Steel During Deformation At Critical Temperature

Steel is the indispensable material of choice for global economic development and social civilization progress.After long-term development,the demand structure for the performance of steel materials has changed across industries,steel metallurgy from the skill into the level of engineering science.It is urgent to study ways to improve the comprehensive performance of structural steel.Combining theory and experiment,this thesis makes an in-depth study of the grain refinement behavior of 20,20Cr,60 and60Si2Mn steels with high temperature at 730-1000?,strain rate at 0.01-10s^-1 and strain variable at 0.1-1 in the process of high temperature deformation.And,the deformation behavior and deformation mechanism of structural steel in the process of thermal deformation are analyzed,and the rhemaniological stress equations of four kinds of steel are established based on the Arrhenius model.At the same time,the thermal processing diagram based on dynamic material model is established and analyzed,which provides the guiding basis for determining the parameters of the thermal process.First of all,the thermal deformation behavior of 20,20Cr,60 and 60Si2Mn steels is studied by thermal simulator,and the true stress-strain curve under different deformation conditions is compared and analyzed.The results show that rheumial stress and its peak increase with the decrease of deformation temperature and the increase of strain rate.With the increase of carbon content and alloy element Cr,the value of peak strain increases,that is,C and Cr are beneficial to improve the critical response variable of dynamic recrystallization.Taking 60Si2Mn steel as an example,the hardness test of different deformation areas is carried out to obtain the law of influence of uneven deformation on its mechanics performance.Secondly,a mathematical model based on Arrhenius type hyperbolic sine equation was established,and the flow stress equations of 20,20Cr,60 and 60Si2Mn steels at true strain 0.5 were concluded.The activation energy Q and structure factor A increased with the increase of alloy elements.The parameters in the model were fitted by polynomial fitting,and the strain compensation mathematical equation was established.The predicted values and experimental values of 20,20Cr,60 and 60Si2Mn steel were verified,and their correlation coefficients were 0.98125,0.98721,0.9835 and 0.99234,respectively,with high accuracy.Then,by analyzing the thermal processing diagram of 60 steel,the energy dissipation rate increases with the deformation temperature increases or the strain rate decreases,and the non-instability zone is mainly concentrated in the low temperature low strain rate zone(730-800?,0.01-0.1s^-1)and the medium-high temperature strain rate zone(850-1000?,0.1-1s^-1).Finally,the microscopic tissues of 20,20Cr,60 and 60Si2Mn steels under different deformation conditions were observed.The results show that with the decrease of deformation temperature,the driving force of phase change is enhanced,which promotes the phase change of Australis to ferrite,and the grain is smaller when deformed near the critical temperature.At the same time,as the Australacne temperature decreases,the volume fraction of ferrite increases.