Effect Of Rare Earth On NbC Dissolution And Precipitation Behaviors In Microalloy Steels

Through the combination of microalloying and controlled rolling and cooling technology,the strength and toughnes of steel can be improved by the interaction of carbonitride precipitation and deformation recrystallization.Amongst Ti,Nb,V,B,rare earth(RE)and other microalloying elements,Nb has been widely used in the high strength low alloy(HSLA)steel owning to its significant inhibition of austenite grain growth,improvement of non-recrystallization temperature,refinement of phase transformation microstruture,and improvement of the strenghth and toughness of steel.Due to the purification,corrosion resistance,antioxidant properties in the steel,RE elements caused widespread concern in the material research field.In China,RE resources are rich in Baiyunebo mine.In Baotou Iron and Steel Company,a certain amount of residual RE were found in the production of steel,in which the raw material is from the Baiyunebo mine,its content meet the micro-alloy content requirements.How to play the beneficial role of RE,not only the role of RE alloying need to focus on,but also the interaction between RE and micro-alloying elements should be concerned.However,due to the the special nature of physical and chemical properties of RE in steel and the limitations of research methods,the microalloy mechanism of RE in steel is still lack of systematic research.The formation and dissolution enthalpy of the stable second phase of La?Ce?Y in the Fe-based alloy were calculated based on the dilute solid solution,and the curve of solubility dependent on the temperature of the RE were obtained.According to the internal friction evlovement of Fe-RE alloy in the process of heating and cooling,at the high temperature zone,there will be damping peak of solute atoms segregation in the grain boundary when the RE atoms are dissolved in the matrix.When the RE content increases exceed the maximum solid solution,it will react with other alloying elements to form compounds.Furthermore,the RE atom occupancy tendency and the strengthening and toughening effect on grain boundary are investigated from the electronic structure level.The effect of RE on the dissolution behavior of NbC in the austenitizing state was analyzed by using the interaction coefficient of RE with other microalloy elements and solid solubility product of NbC,and the precipitation kinetics model of NbC in the austenite region was established.In addition,the isothermal precipitation experiment of austenite region was designed.Combined with the results of microstructures and hardness tests,the results show that the solid solution product of NbC in austenite increase,the dissolution temperature decrease,and the dissolution of NbC in austenite promote after the RE addition.Based on the isothermal and hot deformation experiments,the precipitation behavior of NbC in the steels with and without RE addition was characterized quantitatively and qualitatively by the physical and chemical phase analysis combining the scanning electron microscopy(SEM).The effect of RE on the precipitation kinetics of NbC in microalloyed steels was discussed by JMA model according to the change of the second phase precipitation peak in the Differential Thermal Analysis(DSC).The results show that the activation energy of NbC in the y-Fe increased from 75.32kJ/mol to 90.15kJ/mol in the presence of RE,while in the ?-Fe reduced from 176.98kJ/mol to 65.47kJ/mol.In addition,the interaction between RE and Nb atom,and the effect of RE on diffusion energy of Nb atom were analyzed by using the first principle.Furthmore,the mechanism of RE on NbC precipitation in austenite and ferrite region was explained in detail.The influence of La on the interdiffusion behavior of Fe-Nb was discussed by means of a series of diffusion couple experiments.Consequentely,the diffusion coefficient of Nb in ferrite under the action of La was obtained.The results show that the diffusion coefficient of Nb in ferrite increases with the RE addition.Based on the dislocation nucleation mechanism,the nucleation and precipitation model of NbC in ferrite was established.T The experimental and calculated results show that the addition of RE will accelerate the precipitation of NbC in the ferrite region and increase the precipitation density and dispersion strengthening effect.The nonrecrystallization temperature of the steels was determined by the average flow stress experiment based on the series of hot deformation process.The influence mechanism of RE on the deformation recrystallization behavior was discussed from the relationship of the recovery and recrystallization under the action of deformation storage.On this basis,the volume fraction and distribution of NbC of the strain induced precipitation,in consideration of the pinning and drag effect,the recrystallization isothermal growth kinetics was predicted.Based on the above theoretical and experimental research,the feasibility and economy of the steel production by Baiyunebo containing RE iron ore were evaluated,and the composition design and thermo mechanical controlled process(TMCP)process design of RE alloy were provided.