Effect of Silica in Slag on Inclusion Compositions in 304 Stainless Steel Deoxidized with Aluminum

Experiments were carried out to determine the effect of silica in the slag of CaO-SiO2-Al2O3-MgO-F system on the formation of MgO·Al2O3 spinel inclusion in 304 stainless steel deoxidized with Al. Immediately after the addition of Al into the molten steel, alumina clusters formed. Simultaneously, reduction of MgO in the slag occurred to raise Mg content in the steel. This resulted in the change in inclusion composition to MgO·Al2O3 spinel which did not further change. In the previous experiments with CaO-Al2O3-MgO-F slag, however, spinel inclusions changed to MgO or liquid CaO-Al2O3-MgO system. The difference in behavior was caused by the existence of silica in the slag. Silica in the slag was considered to prevent the extensive reduction of MgO or CaO in the slag by Al to supply soluble Mg or Ca into the molten steel. A stability diagram of inclusions corresponding to Mg and Al contents in the steel was calculated employing available thermodynamic data. The inclusion compositions experimentally obtained well agreed with the diagram. This implies that spinel inclusions were the most stable in the molten 304 stainless steel deoxidized with Al under the presence of 10 mass% silica in the slag. As a result, it was concluded that silica in the slag enhanced the formation of spinel inclusions.