Effect of yttrium addition on inclusion formation in 0.5C-1Cr-0.2Mo-0.1V spring steel: thermodynamic and experimental analyses.

The effects of yttrium (Y) addition on the formation of inclusions in 0.5C-1Cr-0.2Mo-0.1V spring steel were studied via thermodynamic calculations and scanning electron microscope (SEM) coupled with energy dispersive spectrum (EDS). According to thermodynamic calculations of Gibbs free energy, [Y] reacts with [O] and [S] but not with [C] in molten steel, with Y₂O₂S and Y₂O₃ being more easily formed inclusions in steel at 1873 K than YS and Y₂S₃ when the Y content is less than 0.06%. The experimental results show that the inclusions in spring steel without Y addition are mainly large irregular aluminum silicate, Al₂O₃-SiO₂-TiO₂ and MnS inclusions. The addition of 0.011% Y can eliminate large inclusions and form square Y₂O₃ and complex Y₂O₃-Y₂O₂S inclusions with Y₂O₂S in the core and surrounding Y₂O₃. When the Y content increased to 0.019%, the size of Y₂O₃ decreased, and isolated spherical Y₂O₂S formed. The addition of 0.031% Y can further decrease the size of Y₂O₂S, and Y₂O₃ is replaced by YS. A good consistency was observed between the calculated result of the Gibbs free energy and the experimental result. Moreover, a three-dimensional stability diagram of precipitation under equilibrium conditions at 1873 K is established to predict the formation sequence of Y-containing inclusions with different Y contents. The formation sequences are Y₂O₂S → Y₂O₃ with Y contents in the range of 0.01−0.02%, YS → Y₂O₂S → Y₂O₃ with Y contents in the range of 0.02−0.06% and Y₂S₃ → YS → Y₂O₂S → Y₂O₃ with Y contents in the range of 0.06−0.08%. [ABSTRACT FROM AUTHOR]