Effect of Magnesia-Carbon Refractory on the Kinetics of MgO·Al₂O₃ Spinel Inclusion Generation in Extra-Low Oxygen Steels

MgO-base refractory and MgO bearing slag both have the potential to supply Mg to the molten steel and then prompt the spinel generation. In this article, the effect of MgO-C refractory on the kinetics of spinel transformation was investigated on a laboratorial scale by inserting a MgO-C refractory rod into the Al-killed molten steel. With the refractory/steel reaction time increasing from 1 to 10 minutes, inclusions of Al₂O₃ gradually degraded into MgO·Al₂O₃ spinel and the high MgO content inclusion was finally equilibrium with the MgO-C refractory. This interaction process involved Mg supply reactions, Mg transfer in molten steel, and spinel generation reactions. Although MgO-C refractory could supply Mg into the molten steel through MgO reduction reaction both by Al in the melt and by carbon in the refractory, it was found that the Mg mainly came from MgO reduction by the carbon in the refractory. Mg transfer in molten steel was set as the rate controlling step of spinel generation according to theoretical analysis. A mathematic model was developed based on this rate controlling step, and the model calculation agreed well with the experimental results. The Mg diffusion rate was obtained by the regression of the experimental results as 5 × 10⁻⁴ m/s. The mechanism of MgO·Al₂O₃ generation was clarified, and the reaction between dissolved Mg and Al₂O₃ inclusions occurred first and then the extra dissolved Mg reacted with dissolved Al to generate MgO·Al₂O₃.