Thermodynamic aspects of steel reoxidation behavior by the ladle slag system of CaO-MgO-SiO 2 -Al 2 O 3 -Fe t O-MnO-P 2
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I. INTRODUCTION
RECENTLY, the demand for ultra-low-carbon steel with high cleanliness has been progressively increasing. To obtain high-purity steel, the deoxidized products should be completely removed and the reoxidation of aluminum with air, slag (reducible oxides such as FetO, MnO, and SiO2), and refractory in ladle and tundish should be prevented effectively. The high oxygen potential in the ladle slag may cause reoxidation of the elements in liquid iron such as Al and Ti, resulting in contamination of steel. Especially, in the case of ultra-low-carbon steel produced by decarburization and subsequent deoxidation processes in RH vessel, the oxygen content in liquid iron is extremely low. Therefore, FetO and MnO in the refining slag can bring about reoxidation of Al easily. Therefore, it is necessary to avoid reoxidation reaction by the ladle slag for the stable production of ultra-lowcarbon steel with high cleanliness. The oxygen potential of slag is related directly to the activity of FetO in slag, but it has been estimated so far by the amount of FetO and MnO in slag for convenience. The slag components such as CaO, MgO, Al2O3, SiO2, P2O5, etc. may affect the activities of FetO and MnO by the interaction, and the activity coefficients of FetO and MnO may also be dependent on the basicity and other properties of slag. Therefore, it is not accurate to describe the oxygen potential of slag only in terms of the amount of FetO and MnO in slag. In this work, the thermodynamic equilibria between the liquid iron containing Mn and P and CaO-MgO-SiO2-Al2O3P2O5-MnO-FetO slag system have been carried out in a MgO or Al2O3 crucible at 1873 K. The activities of FetO and MnO were discussed from the standpoint of steel cleanliness and SEON-HYO KIM, Associate Professor, and BO SONG, Research Associate, are with the Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Kyungbuk, Korea. Manuscript submitted August 24, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
calculated by using the regular solution model proposed by Lumsden.[1] The comparison between the measured and calculated activities of FetO and MnO was discussed in order to derive a conversion equation related to the measured and calculated activities. II. EXPERIMENTALS The master slags were prepared by premelting the mixture of reagent grade CaCO3, Al2O3, SiO2, and MgO in a graphite crucible, then crushed and baked at 1100 8C for 24 hours for decarburization. A vertical resistance furnace with heating elements of LaCrO3 was used. The experimental apparatus is shown in Figure 1. The metal/slag equilibria with respect to P and Mn were approached from the slag to metal side in the experiments; P was added to the slag as a form of NH4H2PO4. An argon gas flow rate of 200 mL/min was used in the furnace. The argon was passed through, in order, sulfuric acid, P2O5, Mg(ClO4)2, sponge Ti heated at 620 8C, and Mg(ClO4)2 again. Thirty or forty grams of high-purity electrolytic iron containing approximately 500 to 1000 ppm oxy
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