Effect of Ce-Y Composite Addition on the Inclusion Evolution in T91 Heat-Resistant Steel

This study investigates the influence of rare earth elements Ce and Y on the evolution of inclusions in T91 steel by melting experimental steels with varying Ce-Y contents in a vacuum induction melting furnace. The results show that the inclusions in the steel without rare earth are mainly composed of Mg-Al-O oxides, (Nb, V, Ti)(C, N) carbonitrides, and composite inclusions formed by carbonitrides coated oxides, and all of them have obvious edges and corners. Upon the addition of different concentrations of Ce and Y, the oxygen content in the steel significantly decreased, and the inclusions were modified into spherical rare earth oxides, sulfides, and oxy-sulfides. Additionally, no large-sized primary carbonitrides were observed. The average size of the inclusions was reduced from 2.8 μm in the non-rare-earth-added steel to 1.7 μm and 1.9 μm with rare earth addition. Thermodynamic analysis indicates that the possible inclusions precipitated in the steel with varying Ce contents include Ce₂O₃, Ce₂O₂S, Y₂O₃, Y₂S₃, and CeS. With the increase in Ce content, the rare earth inclusions Y₂S₃, Y₂O_3, and CeS can be transformed into Ce₂O₂S and Ce₂O₃. There are two kinds of reactions in the process of high-temperature homogenization: one is the internal transformation reaction of inclusions, which makes Y easier to aggregate in the inner layer, and the other is the reaction of Y₂S₃→CeS and Y₂O₃ + Y₂S₃→Ce₂O₂S due to the diffusion of Ce in the matrix to the inclusions. Combined with the mismatch analysis, it can be seen that Al₂O₃ has the best effect on the heterogeneous nucleation of carbonitrides during the solidification of molten steel. Among the rare earth inclusions, only Ce₂O₃ may become the nucleation core of carbonitrides, and the rest are more difficult to form heterogeneous nucleation. Therefore, by Ce-Y composite addition, increasing the Y/Ce ratio can reduce the formation of Ce₂O₃, which can avoid the precipitation of primary carbonitride and ultimately improve the dispersion strengthening effect. This study is of great significance for understanding the mechanism of rare earth elements in steel and provides theoretical guidance for the composition design and industrial trial production of rare earth steel.