Deoxidation equilibria of calcium and magnesium in liquid iron
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INTRODUCTION
IN ladle furnace practices, the optimum compositions of CaO-SiO2-Al2O3-MgO slags are especially critical in order to meet the demand for ultraclean steels. In Al-strong deoxidation process, it is found that the contents of Ca and Mg in liquid steel, which have significant effects on inclusions compositions, tend to increase as a result of slag-metal and/or refractory-metal reactions. Therefore, for a clear understanding of inclusions formation, knowledge of thermodynamics on the deoxidation equilibria with Ca and Mg is of crucial importance. Although numerous attempts were made to determine the solubilities of CaO and MgO in liquid Fe, there are considerable disparities between the results of different investigators.[1–12] Kimura and Suito[13] studied the Ca-O equilibrium at 1873 K under normal pressure by the method of immersing a pure Fe capsule containing Ca metal into liquid Fe, which was equilibrated with CaO-Al2O3 slags in Al2O3 and CaO crucibles. Based on these results and the data for CaO-Al2O3,[14] CaO-SiO2,[15] CaO-SiO2-Al2O3,[16] and CaO-Al2O3-MgO[17] slags obtained by a slag-metal equilibration technique, the first-order interaction coefficient, e Ca O , was estimated and coupled with the apparent equilibrium constant, K'Ca, for the reaction CaO (s) 5 Ca 1 O in three different ranges of {[mass pct Ca] 1 2.51[mass pct O]}.[13] Thereafter, using the reciprocal and conversion relationships of the interaction coefficients between mass percent and mole-fraction scale along with the equilibrium constant KCa obtained from the thermodynamic data, Cho and Suito[18] determined the firstCa,O order and second-order (r Ca ) interaction coefficients O and r O in two different ranges of {[mass pct Ca] 1 2.51[mass pct O]}. Using the contents of Mg and O in liquid Fe equilibrated with CaO-Al2O3-MgO slags at 1873 K, Inoue and Suito[17] determined the equilibrium constant, KMg, for the reaction MgO (s) 5 Mg 1 O and the first-order interaction coefficient, e Mg O . However, their contents of Ca in liquid Fe are relatively high, and their data points are limited to a certain HIROKI OHTA, Research Associate, and HIDEAKI SUITO, Professor, are with the Institute for Advanced Materials Processing, Tohoku University, Sendai 980-77, Japan. Manuscript submitted January 3, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
range of metal composition. Therefore, additional experiments, in which the influence of Ca on the estimation of the e Mg O value was eliminated to the utmost, were carried out by Ohta and Suito.[19] Unfortunately, the scatter of their data was found to be still large so that only the e Mg O value could be reevaluated, although the results seem to be explained by using the first-order and second-order interaction coefficients Mg,O (r Mg ). O and r O In the present work, the equilibrium experiments between CaO-SiO2-Al2O3-MgO slags and liquid Fe were carried out at 1873 K using CaO and MgO crucibles. Based on these results and previous experimental results,[13–17,20] the firstorder and second-order
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