Effects of ferrosilicon alloy, Si content of steel, and slag basicity on compositions of inclusions during ladle furnace refining of Al-killed steel

To obtain solid Al₂O₃ or MgO·Al₂O₃ inclusions in ladle furnace (LF) refining process and achieve ultra-low total oxygen content in steel through subsequent Ruhrstahl Heraeus degassing, the effects of ferrosilicon alloy, Si content of steel, and slag basicity on the compositions of inclusions during LF refining were investigated in Al-killed steel. Al₂O₃ inclusions could be transformed into CaO–Al₂O₃–MgO–CaS inclusions after adding ferrosilicon alloy in the LF refining process as this alloy contains some CaSi alloy impurities. The addition of all ferrosilicon alloys required for the steel in the tapping process could eliminate the influence of Ca in ferrosilicon alloy on the compositions of inclusions. Si in liquid steel had a significant influence on the compositions of inclusions during LF refining when CaO–Al₂O₃–SiO₂–MgO slag with high basicity of 7.0 was used. This was because [Ca] produced by the reaction of CaO and [Al] could be consumed more readily by SiO₂ in Si-free steel than in Si-containing steel, which was confirmed by the difference of total calcium content between Si-free and Si-containing steels. As a result, Al₂O₃ and MgO·Al₂O₃ inclusions were retained in Si-free steel, whereas calcium aluminate inclusions were found in Si-containing steel. For CaO–Al₂O₃–SiO₂–MgO slag with low basicity of 2.8, Al₂O₃ and MgO·Al₂O₃ inclusions were obtained after LF refining in Si-containing steel when all ferrosilicon alloys required for the steel were added in the tapping process. This was because the reaction of CaO and [Al] was weak, and residual [Ca] in the steel could be rapidly consumed by SiO₂ in low-basicity slag owing to the low activity of CaO and high activity of SiO₂, leading to a low total calcium content of 0.0003% in Si-containing steel.