Non-equilibrium Grain Boundary Segregation And Kinetics Model Of Nb In Niobium Microalloyed Steel During Isothermal Temperature

In this paper,the diffusion coefficients of niobium-vacancy complex calculated by the three methods were compared.The differences of the diffusion coefficients with temperature in different methods and their effects on the dynamics of non-equilibrium grain boundary segregation are discussed.In theory,the research object is Fe-Nb binary alloy,according to the non-equilibrium grain boundary segregation kinetic model,the effects of isothermal temperature,matrix niobium content,and austenite grain size on non-equilibrium grain boundary segregation dynamics are discussed.When cooling from1200?solid solution state to a lower isothermal temperature,the diffusion coefficient increases continuously with the increase of the isothermal temperature,the critical time is smaller,the solid solution temperature difference become smaller,the maximum segregation of niobium at the grain boundary,is smaller,the time for the grain boundary niobium element content to return to the equilibrium grain boundary segregation concentration is shorter.During the isothermal cooling from 1200? to 1000?,with the increase of the matrix niobium content,the maximum segregation range of the grain boundary niobium increases linearly,while the increase of the niobium content has no effect on the critical time of isothermal temperature non-equilibrium grain boundary segregation.The critical time of non-equilibrium grain boundary segregation gradually increases with the increase of the original Austrian grain size.In the experiment,the non-equilibrium grain boundary segregation behavior of low niobium and high niobium alloys at 1200? for 5 minutes after solid solution at 1000? for isothermal temperature was studied,and the microstructures obtained by different isothermal temperature times of samples were determined by light microscope and scanning electron microscope.Determine the original austenite grain boundary and ferrite grain boundary of the sample according to the original austenite grain boundary corrosion and EBSD,and then use EPMA to quantitatively detect the niobium element in the original austenite grain boundary,to obtain the change of the grain boundary niobium element at various time in the isothermal temperature process.The low niobium alloy at1000? isothermal temperature process,isothermal 15 minutes,the grain boundary niobium segregation amount is detected to be 0.15wt%,which is the maximum segregation amount of isothermal temperature process,that is,the critical time of niobium segregation process at 1000? is about 15 minutes.The experimental results are in good agreement with the variation of the niobium grain boundary segregation fitted by the non-equilibrium grain boundary segregation model.According to the experimental results,the dimensionless physical quantity?of the grain boundary segregation of niobium element is 1.13×10~5.During the time when the amount of niobium segregation in the grain boundary reaches the maximum,the ferrite grain boundary is the smoothest,the intragranular precipitates are the smallest,the grain size is the smallest,and the original austenite grain growth is the slowest.The non-equilibrium grain boundary segregation of niobium can reduce the jagged grain boundaries of ferrite,inhibit the precipitation of precipitates,hinder the growth of austenite,and refine the crystal grains.When the niobium content reaches 0.2wt%,the austenite grain size will not grow in the isothermal temperature of 1000?.