A Kinetic Model for Modification of MgAl 2 O 4 Spinel Inclusions During Calcium Treatment in the Ladle Furnace
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ALUMINUM deoxidized steel forms solid alumina inclusions in the ladle furnace. These solid inclusions are known to create challenges during casting. Various practices are used for elimination of inclusions, such as transporting them to slag phase by argon stirring.[1] However, some alumina inclusions remain in the steel melt. It is well known that these alumina inclusions can transform to magnesium aluminate spinel inclusions, MgAl2O4, by reaction of alumina with dissolved magnesium from top slag or ladle refractories or from magnesium contamination of aluminum or ferroalloy additions.[2,3] Magnesium aluminate spinel inclusions have a tendency to adhere to the inner wall of the submerged entry nozzle which negatively affects productivity of the process.[4] They also adversely affect properties of the final steel.[5] There is some disagreement in the literature on the effectiveness of spinel modification by calcium treatment. While earlier study proposed that the modification of spinel by calcium would be less effective than
YOUSEF TABATABAEI, KENNETH S. COLEY, and GORDON A. IRONS are with the Department of Materials Science and Engineering, Steel Research Centre, McMaster University, 1280 Main Street West, Hamilton, ON, L8S 4L7 Canada. Contact e-mail: [email protected] STANLEY SUN is with ArcelorMittal Global R&D-Hamilton, 1330 Burlington Street East, Hamilton, ON, L8N 3J5 Canada. Manuscript submitted April 7, 2018.
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pure alumina,[6] more recent study indicates that, under reducing conditions, spinel modification by Ca treatment is easier than for alumina inclusions.[7,8] Kang et al.[9] studied the formation of liquid calcium aluminate inclusions from MgAl2O4 spinel in ladle treatment by thermodynamic calculations and laboratory experiments. Using a stability diagram, they showed that when the spinel phase was in contact with the liquid steel containing calcium, it would become thermodynamically unstable and liquid magnesium calcium aluminate forms. It was also found that an extremely low activity of calcium, less than 2 ppm, is sufficient for formation of liquid oxide although, concentration gradients of Ca were observed in the formed liquid phase.[9] Pretorius et al.[8] on evaluation of metal samples from industrial heats proposed that effective modification of spinel inclusions is possible and the mechanism is the preferential reduction of the MgO in the spinel to dissolved Mg in the steel followed by reaction of the residual Al2O3 from the spinel and resultant CaO to form completely liquid inclusions. Verma et al.[7] also showed that the addition of calcium is effective in spinel modification. Pretorius et al.[8] further suggested that calcium modification of spinel inclusions is somewhat easier for low-calcium aluminum-killed (LCAK) steels with very low oxygen content. However, Yang et al.,[10] from thermodynamic analysis of industrial data, have concluded that spinel inclusions can be modified into liquid inclusions only if the dissolved calcium content in the steel is
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