Equilibria between rare earth elements and sulfur in molten iron
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INTRODUCTION
W I T H the development of low-alloyed high strength steels and super high-strength steels, the amounts of deleterious impurities such as sulfur, oxygen, etc. should be as low as possible.~'2 Rare earth elements are strong deoxidizers, but there is not yet general agreement about their desulfurizing ability in molten iron, though much work has been done in this respect. The equilibrium constants of Ce-S and La-S in molten iron cited in the literature are shown in Table I. From Table I it can be seen that the equilibrium constants of RES = [RE] + [S] obtained by different investigators differ by three orders of magnitude. There may be several reasons: first, the earlier investigators used chemical methods to determine the rare earth and sulfur contents in the metal and it can be rather difficult for rare earth sulfides to float completely out from the molten iron because of their comparatively high specific gravity. As a result, their analytical results may have included some RE and S which existed in the form of nonmetallic inclusions. Such methods may also give rise to erroneous results owing to the extremely low contents of the rare earth elements at equilibrium caused by their strong desulfurizing abilities. Second, high values of K may have been obtained because not enough time was available in their experiments for the system to reach equilibrium; ~~also, it seems to be a shortcoming that the reaction products were not identified in these experiments. With the help of a radioassay technique of high sensitivity, the present authors redetermined the Ce-S and La-S equilibrium constants. Also studied was the Nd-S equilibrium, on which no previous work seems to have been published. The related interaction coefficients are also given.
QIYONG HAN, Associate Professor, and YUANCHI DONG, XIAN FENG, CHANGXIANG XIANG, and SIFU YANG, lecturers, are with the Department of Physics and Chemistry of Metals, Beijmg Umversity of Iron and Steel Technology, Beijmg, China. Manuscript submitted August 2, 1984. METALLURGICALTRANSACTIONS B
II.
EXPERIMENTAL METHOD
A. Charge Material Electrolytic iron with an oxygen content of about 170 ppm was melted in a carbon tube furnace in argon and deoxidized with metallic calcium (99.9 pct purity) weighing 0.5 pct of the iron and held for a period of 0.5 hour. After this treatment, the oxygen activity measured by solid electrolyte sensors made of ZrO2(MgO) tube was about 5 to 10 ppm. The iron was ready for use after cooling down in argon and removal of its top layer which was rich in deoxidation products. Chemically pure FeS was used as the source of sulfur. The ratio of a,/ao in liquid iron was kept above 100 to prevent the formation of rare earth oxysulfide.18'19
B. Equilibrium Procedure The experimental method used is similar to that in the RE-O equilibrium experiment. 2~ The sulfur-containing iron material was melted in a MgO crucible under an argon atmosphere with a purity of 99.9 pct in a carbon tube furnace. After the experimental temperature has been reached, metallic
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