Calcium deoxidation equilibrium in liquid iron
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I.
INTRODUCTION
C A L C I U M alloys are used to control the composition of the remaining oxide and sulfide inclusions, for example, to ensure a good flowability of aluminum-killed steel and to improve deformability. Although experimental studies on calcium deoxidation equilibrium have been made by several investigators, the solubility product of calcium oxide, [mass pct Ca] 9 [mass pct O] (-- 2 x 10 sin), and the equilibrium constant, Kca, for the reaction CaO (s) = C_aa + O_ ( - 10 -6 to 10 -m[2-6]) are observed to be several orders of magnitude greater than those calculated from the reliable thermodynamic data (9.3 x 10-1~, tTl 4.6 x 10-~1, tSl and 1.45 x 10-1ttsJ). This discrepancy was discussed based on the experimental errors involved in (1) oxygen and calcium analysis, (2) emf measurement by an oxygen sensor, and (3) thermodynamic values for the Gibbs free energy of formation of CaO. However, recently Turkdogan t81 pointed out, based on the independent check on the freeenergy data, that the standard free energy of CaO given in the compiled thermodynamic tables is correct within perhaps 10 to 30 kJ/g-mol. Gustafsson and Mellberg 141 emphasize the importance of the relationship between the interaction parameter and the equilibrium constant in the case of high-mutual affinity of solutes with oxygen. This implies that a particular value of an interaction coefficient is based on the specific value of the equilibrium constant. Furthermore, the applicability of only the first-order interaction parameter is limited to a certain range of composition. The purpose of this study is to obtain the equilibrium constant, Kca, and the first-order interaction parameter, e0Ca (eOa) in the equilibrium experiments between CaOA1203 slags and liquid iron, using lime and alumina crucibles at 1873 K. The previous results obtained in the equilibrium experiments using CaO-SiO2, I91 CaOA1203, [t~ CaO-SiO2-Al203, [1H and CaO-A1203-Mgu" ,--1~21 slags are also used for the estimation of the values for Kca and e ca (ec~
TAKASHI KIMURA, formerly Graduate Student, Department of Metallurgical Engineering, Tohoku University, is with Kashima Steel Works, Sumitomo Metal Industries Ltd., Kashima, Japan. HIDEAKI SUITO, Professor, is with the Institute for Advanced Materials Processing, Tohoku University, Sendai, Japan. Manuscript submitted November 19, 1992. METALLURGICAL AND MATERIALS TRANSACTIONS B
II.
EXPERIMENTAL
A. Apparatus and Procedure A vertical resistance furnace with heating bars of LaCrO3 was used. Triple reaction tubes (outer A1203 tube: ~b60 x ~b50 x 850 mm; middle graphite tube: 4~48 x qb44 x 700 mm; inner AlzO3 tube: ~b42 x ~b35 x 800 ram) were used in order to prevent the reaction of the outer reaction tube with calcium vapor, resulting in a hole by the formation of liquid calcium aluminate. The flow rate of argon gas, which was dehydrated through magnesium perchlorate as well as phosphorous pentoxide and deoxidized by Ti (973 K) and Mg (723 K), was chosen as 0.2 to 1.0 1/min. Twenty-five grams of high-purity electrolytic ir
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