A Multilayer Model for Alumina Inclusion Transformation by Calcium in the Ladle Furnace
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INTRODUCTION
INCLUSION control in steelmaking practice relies upon the choice of deoxidants and the top slag, stirring by argon gas, and calcium treatment. The latter modifies solid alumina inclusions to liquid spherical calcium aluminates. This process has been widely used in low-carbon aluminum-killed (LCAK) steels to alleviate submerged entry nozzle clogging by solid alumina inclusions during continuous casting. The presence of alumina inclusions has unfavorable effects not only on the production efficiency[1] but also on steel mechanical properties.[2] Calcium treatment is one of the most difficult practices in steelmaking because (a) calcium boils and has low solubility in the steel, forming bubbles that are not completely dissolved in the steel; (b) calcium reacts strongly with oxygen and sulfur,[3] so these elements in the steel must be well controlled; and (c) there is a narrow window of liquid calcium aluminate composition at steelmaking temperatures (Figure 1). Since the early days of calcium treatment in the 1990s, great improvements have been made in the practical aspects YOUSEF TABATABAEI, KENNETH S. COLEY, and GORDON A. IRONS are with the Department of Materials Science and Engineering, Steel Research Centre, McMaster University, Hamilton, ON L8S4L7, Canada. Contact email: [email protected] STANLEY SUN is with ArcelorMittal Global R&D-Hamilton, Hamilton, ON L8N3J5, Canada. Manuscript submitted June 16, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS B
so most plants can control calcium treatment on a day-to-day basis, but it is a sensitive process.[4] Nevertheless, our fundamental understanding of the mechanism and kinetics of the transformation is not established and discrepancies exist in the literature in terms of the mechanism and rate-controlling step.[5–9] Lu et al.[5] were the first to model the kinetics of formation of oxide and sulfide inclusions during and after calcium injection based on their experiments in 40-kg steel heats. They assumed fast diffusion within the inclusions and fast reaction at the interfaces and developed a mathematical model for inclusion evolution, which offered reasonable agreement with the experimental data. Later, Ito et al.[7] employed an unreacted shrinking core model for alumina transformation. They assumed three different possible rate-determining steps: (a) mass transfer in the melt, (b) mass transfer in the product layer, and (c) chemical reaction at the interface. By comparing the calculated results against experimental data. they concluded that diffusion through the product layer was the rate-controlling step. However, they did not include the dissolution rate of calcium in their analysis. Higuchi et al.[8] also developed a mathematical model of the kinetics of inclusion transformation similar to that developed by Lu et al.[5] with the additional consideration of the chemical reaction rate between inclusions and the melt. In their case, mass transport of calcium from the bulk to the surface of particles in the melt was ignored. Ye et al.[9] proposed that the reaction
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