Dispersion of Fine Oxide and Sulfide in Steel Formed by Stable and Metastable Monotectic Reaction

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The morphology of TiS and CrS in steels formed during solidification and the modification of this morphology by the addition of some alloying elements were investigated by means of optical and scanning electron microscopy. The thermodynamic analyses of phase equilibria of the Fe-Ti-S and Fe-Cr-S ternary systems were also carried out. The results obtained are as follows :1. The morphologies of TiS can be classified into four types ; (i) fine-particle (ii) globular (iii) rod-like and (iv) plate-like sulfides, which depends on the Ti/S ratio and formation process.2. The morphology of TiS in the Fe-0.5%Ti-0.3%S base alloy is the rod-like sulfide while additions of Mn and Zr change the morphology from rod-like to globular. The morphology of sulfide is discussed in terms of eutectic and monotectic-type phase diagrams in the Fe-sulfide quasi-binary system.3. The solubility product of TiS in steel can be determined.log[Ti]γ[S]γ=7.74-17820/T, log[Ti]α, δ[S]α, δ=8.07-17171/Tlog[Ti]liq[S]liq=7.09-12447/T4. The sulfide morphologies in the Fe-Cr-S ternary alloy are calssified into three types ; (i) cell wall (ii) fine particle and (iii) globular sulfides. Fine particle sulfide morphology is the result of the remelting or the eutectioid reaction attendant on the phase transformation of matrix from δ to γ. The morphology of sulfides changes from the cell wall and the fine particle type to the globular type with increasing Cr content.5. Additions of Mn, Ti or Zr to Fe-Cr-S alloys lead to the formation of MnS, TiS or ZrS with the globular, the rod-like and the polyhedral morphologies respectively, which are due to the sulfide formation tendency of the additions and their effect on the nature of the phase diagram.6. The solubility product of CrS in steel at 1%Cr level is determined.log[Cr]liq[S]liq=3.89-4500/T, log[Cr]δ[S]δ=3.35-6550/Tlog[Cr]γ[S]γ=3.43-7307/T, log[Cr]α[S]α=5.32-9068/T