Equilibrium between sulfur and titanium in liquid iron
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INTRODUCTION
D E C R E A S E D strength, ductility, toughness, and corrosion resistance can often be attributed to sulfur in steels. With the need for improved properties it has become apparent that sulfur must be controlled in iron and steelmaking processes. Sulfur can be removed from steel during processing in the blast furnace, the iron ladle, the steelmaking furnace, the steel ladle, and the tundish. While the majority of the sulfur is removed in these facilities, a small amount will be retained in the finished product. To minimize the deleterious effects of the remaining sulfur, metals such as calcium, rare earths, titanium, and zirconium are added to the steel prior to casting to provide sulfide shape control. Sulfide shape control is achieved when alloying elements added to the steel react with sulfur to produce spherical and dispersed nonmetallic inclusions which are not deformed upon plastic working. These inclusions, unlike the manganese sulfide "stringers", allow nearly isotropic properties in the steel product, providing more reliable forming operations. In order to optimize the sulfide shape control process, an understanding of the physical chemistry of sulfide shape control is required. The effectiveness of titanium for sulfide shape control has long been known. In their review series in the 1930's, Portevin and Castro ~ discussed the ability of titanium to modify the sulfide morphology in steels. Titanium, in solution with manganese, provided a finely dispersed (Mn, Ti) S inclusion which was less deformable at steel rolling temperatures than manganese sulfide. Sims, Saller, and Boulger2 reported that the desirable Type I sulfide inclusions (rounded and dispersed morphology) occurred in cast steels treated with up to 0.0025 wt pct titanium. Additional titanium resulted in undesirable Type II sulfide inclusions (grain boundary film) which were deleterious to the steel properties. Laboratory and mill trials performed by Lichy, Duderstadt, and Samways 3 showed that titanium was an effective sulfide shape control element in aluminum-killed AISI 1065 steel only when additions of greater than 4 pounds per ton were made. Studies of the physical chemistry of the liquid Fe-Ti-S system have focused on characterizing the interaction between sulfur and titanium in liquid iron solutions. Work by Fishel, Roe, and Ellis 4 identified the equilibrium sulfide as TiS at steelmaking temperatures. FRANCIS M. DONAHUE, III is a Member of the Technical Staff, Watkins-Johnson Company, Palo Alto, CA. ROBERT D. PEHLKE is Professor, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2136. Manuscript submitted February 11, 1987. METALLURGICAL TRANSACTIONS B
Work by Ejima, Suzuki, Harada, and Sanbongi 5 on the Fe-Ti-S system confirmed the equilibrium sulfide to be TiS at 1600 °C. This study also measured the titanium-sulfur interaction parameter and the titanium sulfide solubility product and reported their values to be - 0 . 1 8 and 0.575, respectively. In a study of a number of Fe-S al
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