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 Y 2 O 2 S and Y 2 O 3 being more easily formed inclusions in steel at 1873 K than YS and Y 2 S 3 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, Al 2 O 3 -SiO 2 -TiO 2 and MnS inclusions. The addition of 0.011% Y can eliminate large inclusions and form square Y 2 O 3 and complex Y 2 O 3 -Y 2 O 2 S inclusions with Y 2 O 2 S in the core and surrounding Y 2 O 3 . When the Y content increased to 0.019%, the size of Y 2 O 3 decreased, and isolated spherical Y 2 O 2 S formed. The addition of 0.031% Y can further decrease the size of Y 2 O 2 S, and Y 2 O 3 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 Y 2 O 2 S → Y 2 O 3 with Y contents in the range of 0.01−0.02%, YS → Y 2 O 2 S → Y 2 O 3 with Y contents in the range of 0.02−0.06% and Y 2 S 3 → YS → Y 2 O 2 S → Y 2 O 3 with Y contents in the range of 0.06−0.08%.