Thermodynamics of nozzle blockage in continuous casting of calcium-containing steels
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I.
INTRODUCTION
STEELMAKERS worldwide are developing and using ladle injection metallurgy as a means of refining steel and modifying the inclusion morphology in steel. At Inland's No. 1 Electric Furnace and Billet Caster Shop, the main interest in ladle injection metallurgy has been to try to develop a process for producing and sequence casting aluminum fine grain steels through metering nozztes. Previously, these steels could not be cast because solid alumina precipitates built up on the nozzle walls, eventually blocking the flow of steel through the nozzle. However, as suggested by Farrell and Hilty I and Saxena, et al., 2 and subsequently demonstrated in laboratory experiments by Faulring, Farrell, and Hilty,3 calcium can complex the alumina to form lower melting point calcium aluminates that do not plug nozzles. On the basis of that work, full-scale experiments were undertaken at Inland using ladle injection of calciumsilicide to control the chemistry of the oxides. Preliminary experiments were carried out to test the reliability of the injection equipment and to begin developing practices to prevent nozzle blockage by aluminum-killed steel. A few of these initial heats were made with silicon-killed steel (without aluminum) to test the injection equipment. The equipment used, practices followed, and results of these experiments have been described as part of a previous paper. 4 The results of these preliminary experiments were interesting--and disturbing. Despite apparently successful injection of the calcium-silicide, all of the heats with >0.010 pct aluminum plugged. In addition, several heats with no aluminum addition, but injected with calciumsilicide, also plugged. The results therefore challenged us to determine the cause of plugging in both types of heats and to develop a theory of plugging to guide further development of the process. We therefore calculated from thermodynamics the conditions that govern the formation of oxides and sulfides from iron containing silicon, aluminum, calcium, oxygen, and sulfur, and compared the results of the
HOWARD M. PIELET and DEBANSHU BHATTACHARYA are Senior Research Engineers with Inland Steel Research Laboratories, 300l East Columbus Drive, East Chicago, 1N 46312. Manuscript submitted May 26, 1983.
METALLURGICALTRANSACTIONS B
thermodynamic calculations to analyses of inclusions found in steel removed from the blocked tundish nozzles. The results showed that, in the aluminum-killed steel the calcium content was too high, so that solid calcium sulfides plugged the nozzles. In steels without aluminum, the calcium content was also too high, so that solid calcium silicates and calcium sulfides plugged the nozzles. This paper presents and discusses the thermodynamic calculations, as well as the analyses of the inclusions, and then compares the two. II.
THERMODYNAMICS
A. Deoxidation by Calcium in the Presence of Aluminum: Activities
When iron containing oxygen is deoxidized with both aluminum and calcium (i.e., in an Fe, _QO,A1, Ca system), the equilibrium deoxidatio
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