Equilibria between the rare earth elements, oxygen and sulfur, in molten iron
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I.
INTRODUCTION
THE rare
earth elements are strong deoxidizers and desulfurizers. In liquid steels treated by rare earth elements, rare earth oxides and sulfides and oxysulfides, such as RE2OzS, are formed. The rare earth oxysulfides are found more often in steel ~-4 than the sulfides or oxides. Nevertheless, little work has been done on the equilibrium of RE-O-S in molten steel until recently. Considerable research has dealt with the problems on the equilibrium constant of low-valency rare earth sulfide in molten iron, but there is almost no information on the determination of equilibrium constant for high-valency rare earth sulfides. Highly sensitive radioassay was adopted to determine the amount of dissolved RE and S in the iron specimens by means of electrolyzing the specimens in organic electrolyte at low temperature to remove the rare earth inclusions. The oxygen activity in molten iron was measured by a solidelectrolyte oxygen-probe made of ZrOz(MgO) tube so as to determine accurately the equilibrium constants of RE-O-S and RE-S.
II.
EXPERIMENTAL METHOD
A. Charge Material Electrolytic iron containing about 170 ppm oxygen was used as the base material in the experiments. In the determination of RE-S equilibria, the iron was deoxidized to about 10 ppm with high purity calcium. The oxygen activity was measured by a ZrO2(MgO) solid electrolyte oxygen probe. The iron was cooled down under argon and its top layer removed before use. The FeS used was chemically pure. In equilibrium experiments involving high valency rare earth sulfides, the [S]/[O] ratio in molten iron should be between 500 and 700, in order to prevent the formation of rare earth oxysulfide and ensure the formation of the equilibrium product RE2S 3. Actually this condition
QIYONG HAN, Professor, and CHANGXIANG XIANG and SIFU YANG, Lecturers, are with the Department of Physics and Chemistry of Metals, Beijing University of Iron and Steel Technology, Beijing, China. YUANCHI DONG, Lecturer, is with the Department of Metallurgy, East China Institute of Metallurgy, Maanshan, China. DONG CHEN, Engineer, is with the Hebei Institute of Metallurgy, Shijia Zhuang, China. Manuscript submitted May 29, 1987. METALLURGICAL TRANSACTIONS B
was fulfilled by maximum deoxidation of the iron melt prior to the addition of FeS. It has been reported that the [S]/[O] ratio in molten iron should be between 10 and 100 so as to ensure the formation of the equilibrium product RE202S. 5 In our experiment, the [S]/[O] ratio was around 15 before RE was added. A number of experiments have shown that in the molten iron [S]/[O] = 10 - 100.
B. Experimental Procedure Experiments were carried out in a carbon tube furnace under the protection of argon of 99.99 pct purity and iron melted in an MgO crucible. After the [S]/[O] ratio had been adjusted to the values given above, the radioactive isotope ~41Ce (14~ 147Nd, or 9Oy) was added to the bath in batches until sufficient RE2OzS could form and adhere to the MgO crucible wall and float to the surface of the molten iron. When eq
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