Activities in MnO-SiO 2 -AI 2 O 3 slags and deoxidation equilibria of Mn and Si
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I.
INTRODUCTION
IN the production of high-quality steels, the knowledge of the equilibrium between inclusions and steel is indispensable for the precise control of the composition of inclusions. In particular, the activities in the MnO-SiO,-AI20~ system are of essential importance in Mn-Si killed steels. Sharma and RichardsonUl measured the activities of MnO in this system with high MnO content at 1923 K by using a gas-slag-Pt equilibration technique. Fujisawa and Sakao ul measured the activities of MnO and SiO, in this system at 1823 K along the liquidus line saturated with alumina. By using the ternary Gibbs-Duhem relation, they[31 evaluated the activities of MnO, SiO,, and AI,O3 in the liquid region at 1823 and 1923 K, based on the MnO iso-activity curves obtained by Sharma and Richardson. However, there are no reported activity values in this system in the SiO:-rich region, which are required for a clear understanding and the precise control of the inclusions composition. In the present study, the activities of MnO and SiO~_ along the liquidus line in the MnO-SiO~-ALO3-Fe,O (1.2 to 6.7 mass pet) system, which is saturated with mullite and alumina, were determined at 1823 and 1873 K by using a slag-metal equilibration technique. On the basis of the activities of MnO reported by Sharma and Richardson m coupled with the present results, the MnO iso-activity curves were re-evaluated. Then, the activities of SiO, and A1203 were estimated from the MnO iso-activity curves by using the ternary Gibbs-Duhem relation, coupled with the present activities of SiO: along the liquidus line. Furthermore, the control of inclusions composition in Si-Mn killed steels is discussed based on the equilibrium between inclusion and steel with respect to Si, Mn, A1, and O. II.
EXPERIMENTAL
A. Procedure The master slags were prepared by premelting the mixture of reagent grade Mn20_~, SiO2, and AI,,O~ in a Pt cru-
HIROKI OHTA, Research Associate, and H[DEAKI SUIT(), Professor. are with the Institute for Advanced Materials Processing, Tohoku University, Sendal 980, Japan. Manuscript submitted May 23. 1995,
METALLURGICAL AND MATERIALS TRANSACTIONS B
cible under air. High-purity electrolytic iron (30 g) starting with approximately 500 to 1000 mass ppm oxygen and MnO-SiO._-AI20: slag (7 g) was melted in an Alto 3 crucible at 1823 and 1873 K under deoxidized Ar atmosphere. An appropriate amount of Fe-t mass pet AI, Fe-76 mass pct Si, and Fe-10 to 20 mass pct Mn alloys was added by dropping afterward. Then, the melts were stirred over a period of 2 to 5 hours by an AI20 ~ rod for 90 seconds at 30minute intervals. In previous slag-metal equilibrium experiments using CaO-AI20~,~< CaO-Al203-MgO,m and CaO-AIzO3-SiO2 slags, t63 the time for attainment of equilibrium was confirmed to be within 1 hour. In the present work, the equilibrium time was chosen as 2 hours for the AlzO3-saturated slags and as 4 hours for the mullite-saturated slags with high viscosity. To eliminate oxide inclusions by flotation, the melts were unstirred for at least 1 h
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