Activities in CaO-SiO 2 -Al 2 O 3 slags and deoxidation equilibria of Si and Al
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I.
INTRODUCTION
IN ladle refining processes, the CaO-SiO2-Al2O3 based slags with high CaO/SiO2 ratio are commonly used for the production of high-quality steels. In the production of clean steels such as a tire cord steel,[1] these slags with low CaO/SiO2 ratio are also used for the control of the inclusions composition after Si-Mn deoxidation processes. Thermodynamic analysis for the refining reactions in these processes requires a precise knowledge of the activities of Al2O3 and SiO2 over a wide range of liquid composition. Rein and Chipman[2] determined the activities of SiO2 in the entire liquid region of the CaO-SiO2-Al2O3 system at 1823 and 1873 K from the measurements of the silicon distribution between slags and Fe-Si-C alloys saturated with either graphite or silicon carbide under Pco 5 1 atmosphere. On the basis of these activities of SiO2, they calculated the activities of AlO1.5 and CaO using the method of Schuhmann.[3] In this calculation, the activities in the CaO-AlO1.5 binary system were taken as the starting points with the aid of the known free energies of the interoxide compounds and the accepted phase diagram. However, the activities of AlO1.5 and CaO in the SiO2-rich region are not reported, and in addition, the activities of AlO1.5 calculated at 1873 K are given only at limited slag compositions. Kay and Taylor[4] obtained the activities of SiO2 at 1723, 1773, and 1823 K in the entire liquid region of this system by measurements of the equilibrium CO pressure in the reaction SiO2 1 3C 5 SiC 1 2CO, and the values at 1873 K were also estimated by extrapolation. The activities of Al2O3 and CaO at 1823 K calculated by the method of Schuhmann are reported in the entire liquid region. Ozturk and Fruehan[5] determined the isoactivity lines for SiO2 at 1798 and 1873 K at low SiO2 concentrations by using the method of Rein and Chipman.[2] The activities of CaO at limited slag compositions at 1873 K were determined from HIROKI OHTA, Research Associate, and HIDEAKI SUITO, Professor, are with the Institute for Advanced Materials Processing, Tohoku University, Sendai 980, Japan. Manuscript submitted January 2, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS B
the measurements of sulfide capacity by Kalyanram et al.[6] It can be summarized from these previous results that the activities of SiO2 obtained by Rein and Chipman[2] and Kay and Taylor[4] agree fairly well with each other, whereas those of Al2O3 and CaO are inconsistent to a significant degree. In a series of our previous slag-metal equilibrium experiments[7–10] at 1823 and 1873 K, the activities of SiO2 in the CaO-SiO2-Al2O3 slags were determined from the reaction Si 1 2O 5 SiO2 (s), in which the activities of oxygen were estimated from the values for the nitrogen distribution ratio (LN), coupled with those for the nitride capacity (CN) obtained in independent gas-slag equilibrium experiments.[11] As a result, the activities of SiO2 were found to be in satisfactory agreement with those by Rein and Chipman[2] and Kay and Taylor.[4] The reasons that
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