Thermodynamics of oxygen and nitrogen in liquid iron equilibrated with CaO- SiO 2 -Al 2 O 3 slags
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I.
INTRODUCTION
F U N D A M E N T A L knowledge on thermodynamic properties in CaO-rich CaO-SiO2-AI203 slags is required to understand the behaviors of oxygen, sulfur, and nitrogen as well as the phase control of inclusions in the ladle refining of liquid steel. For this reason, a number of investigations concerning sulfide capacities, ~,2,3~ sulfur distribution ratios, t41 and activities of the components ts-8] in CaO-SiO2-A1203 slags have been carried out. One of the authors recently measured nitride capacities t9am in binary CaO-SiO2 and CaO-A1203 systems and nitrogen distribution ratios between these slags and liquid iron, t~l'~2~ using gas-slag and slag-metal equilibration techniques, respectively. On the basis of these results, the values for activities of SiO2 and A1203 were estimated. It was found, however, that the activities of SiO2 in CaO-SiO2 slags t12~ at 1923 K and those of A1203 in CaO-A1203 slags m~ saturated with a lime crucible at 1823, 1873, and 1923 K were inconsistent with those reported by Rein and Chipman, tTJ which have been widely used in the thermodynamic discussion of ladle refining processes. The present work was a continuation of previous investigations in CaO-AI203/111 and CaO-SiO2 I121 systems. The main objective was to measure nitrogen distribution ratios between CaO-SiO2-A1203 slags and liquid iron at 1823 and 1873 K in lime and alumina crucibles and to evaluate activities of SiO2 and A1203 coupled with the values for nitride capacity obtained in present and previous gas-slag experiments, t91
II.
EXPERIMENTAL
An Fe-0.0004 - 28 mass pet Si master alloy was prepared in an arc furnace. Oxygen content in this master alloy was analyzed to be in the range of 50 to 70 mass ppm. In most of the experiments, an Fe-Si master alloy (25 g) and CaO-SiO2-A1203 slags (7 g), in which A1N was initially added, were melted in lime and alumina FUMIHIKO TAMURA, formerly Graduate Student with the Department of Metallurgical Engineering, Tohoku University, is with Iron & Steel Research Laboratories, Kobe Steel Ltd., Kakogawa, Japan. HIDEAKI SUITO, Professor, is with the Institute for Advanced Materials Processing, Tohoku University, Sendai, Japan. Manuscript received March 9, 1992. METALLURGICAL TRANSACTIONS B
crucibles at 1823 and 1873 K. In some of the experiments (ACS-4, CAS-5, and CAS-8 in Figure 1), a pure iron starting with approximately 1000 mass ppm oxygen and CaO-SiO2-A1203 slag were melted at 1873 K, with an appropriate amount of Fe-10 mass pet Si alloy added by dropping afterward. Then the melts were stirred by an AIzO3 rod. Details of the apparatus, procedure, chemical analysis, and observation of inclusion followed herein have already been given in previous articles, t9-~31Nitride capacity was measured for two slag compositions (CAS-5 and CAS-9 in Figure 1) in a similar manner to a previous gas-slag equilibration technique, tm~
III.
RESULTS AND DISCUSSION
The chemical compositions of slag and metal phases are given in Table I. The contents of acid-soluble and insoluble A1 were determi
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