Model of Inclusion Evolution During Calcium Treatment in the Ladle Furnace
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THE requirements for the production of superior-quality steel for specific applications have led to the development of various secondary steelmaking processes. Refining of the steel before casting is carried out in ladles via several operations, namely deoxidation, desulphurization, alloy addition, removal of inclusions, and control of inclusion shape, size, number, and composition. Ineffective elimination and/or modification of nonmetallic inclusions during secondary treatment of steel can cause nozzle blockage during continuous casting[1] and quality issues in the cast product.[2,3] Calcium treatment is the most common approach to modify nonmetallic inclusions. Ca additions modify solid alumina to globular liquid CaO-Al2O3 (CAx) inclusions and magnesium aluminate spinel inclusions to calcium-magnesium aluminates. This results in not only improved castability, but also minimization of inclusion-related surface defects, enhancing the machinability of the final product at high
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 St W., Hamilton, ON, L8S4L7, Canada. Contact e-mail: [email protected] STANLEY SUN is with the ArcelorMittal Global R&D-Hamilton, 1330 Burlington St E., Hamilton, ON, L8N3J5, Canada. Manuscript submitted November 1, 2017.
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cutting speeds and decreasing of the susceptibility of high-strength low-alloy and pipeline steels to hydrogen-induced cracking.[4] Ca treatment can also be utilized for desulphurization to very low levels,[5] but it may also result in the formation of deleterious CaS inclusions. In summary, calcium treatment is effective in alleviating nozzle clogging caused by alumina inclusions, but the treatment should be done cautiously. Calcium is usually introduced to the steel by steel-clad Ca wire injection. The boiling point of calcium (1480 C) is lower than steelmaking temperature (~ 1600 C), and thus when calcium is added to liquid steel, calcium bubbles form, from which some calcium dissolves into the steel. Actually, most of the injected calcium escapes to the atmosphere and the recovery in industry is usually less than 30 pct.[6] The dissolved calcium reacts with dissolved oxygen, sulfur, and alumina inclusions by the following reactions: ½Ca þ ½O ! CaO;
½1
½Ca þ ½S ! CaS;
½2
1 2 ½Ca þ ½O þ x þ Al2 O3 ! CaO ðAl2 O3 Þx þ ½Al: 3 3 ½3 In spite of the fact that since the 1990s, many studies have been conducted to understand the mechanism and
kinetics of alumina inclusion modification by calcium,[7–11] some uncertainties remain in the literature. Previous work by the authors developed a fundamental multi-layer growth model[12] and concluded that, for the case of a fixed calcium content in the steel, the rate of supply of calcium to the alumina inclusion is the rate-controlling step for modification. This work also showed that changing the fixed concentration of dissolved calcium had a profound effect o
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