Thermodynamic Analysis And Experimental Study On The Effect Of Rare Earth Yttrium On Inclusions In T4003 Stainless Steel

Stainless steel is widely used in industrial production and daily life due to its excellent corrosion resistance,so it is very important to improve the corrosion resistance of stainless steel.Non-metallic inclusions in steel are one of the important factors affecting corrosion resistance of stainless steel.The addition of rare earth elements into stainless steel can optimize the composition,distribution,quantity and size of inclusions,so as to improve the performance of stainless steel.In this paper,T4003 ferritic stainless steel is taken as the research object.The influence of rare earth element yttrium content on non-metallic inclusions in T4003 stainless steel has been studied by means of computational simulation and experimental analysis.The effect of yttrium content on corrosion resistance of stainless steel is further discussed,and the optimum yttrium content is summarized by comparing with the calculated results.The precipitation law of TiN inclusions in the experimental steel is obtained by the combination of thermodynamic software Factsage calculation and Gibbs free energy change calculation,and a certain amount of Mg O-Al₂O₃ and Ca O-TiO₂ composite oxide inclusions are formed in the steel.Y₂O₃ and Y₂O₂S inclusions are formed in the steel after the addition of yttrium,but Y₂S₃ and YS are not formed.The addition of yttrium can also transform the original inclusions into Y₂O₃ and Y₂O₂S inclusions.According to the results of mismatch calculation,it can be found that both Mg Al₂O₄ and Ca TiO₃ can be used as nucleation cores,which can effectively induce TiN precipitation.These two oxides can be modified by adding yttrium into steel,and the resulting Y₂O₃ and Y₂O₂S can also become effective nucleation cores.Through the experimental analysis of inclusions formation and transformation,it is found that the inclusions of T4003 stainless steel include the heterogeneous composite inclusions with irregular shape Mg Al₂O₄ and Ca O-TiO_x as the core and the homogeneous TiN inclusions with clear edges without adding yttrium..Mn S and Ca S are precipitated on the outside of TiN of some inclusions.With the increase of yttrium content in steel,the core gradually transforms into spherical or ellipsoidal Y₂O₃ and Y₂O₂S.The modification path of oxides in inclusions is:Mg Al₂O₄-Ca O-TiO_x→Mg Al₂O₄-Y₂O₃-（Ca O-）TiO_x→Y₂O₃-TiO_x→Y₂O₃-Y₂O₂S and Y₂O₃ and Y₂O₂S.In general,with the increase of yttrium content in steel,the average size of inclusions in steel decreases first and then increases,and the number density first increases and then decreases.The immersion corrosion experiment shows that the pitting corrosion resistance of the yttrium-containing stainless steel is first optimized and then deteriorated with the yttrium content increasing gradually.In the experiment,stainless steel containing 70 ppm yttrium has the best pitting resistance.It is found from the in-situ observation of inclusion immersion corrosion that the corrosion order of the experimental steel is:steel matrix,yttrium inclusions,and TiN-containing inclusions.The dynamic potential polarization test shows that adding appropriate amount of rare earth yttrium can improve the electrochemical corrosion resistance,self-corrosion potential and pitting potential of stainless steel.The self-corrosion potential and pitting potential of stainless steel containing 70 ppm yttrium reached the maximum values of-1.0198 V_SCE and 0.1274 V_SCE,respectively.With the gradual increase of yttrium content,the polarization resistance value also increases first and then decreases.The polarization resistance reaches a maximum value of 2080.7Ω·cm² in 70 ppm yttrium stainless steel sample.