Effect of Silicon on the Desulfurization of Al-Killed Steels: Part I. Mathematical Model
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INTRODUCTION
THERE is an increasing demand for clean steel low in sulfur. Sulfur negatively affects the workability of steel, and lowers room-temperature ductility; hence, the stringent requirements can be for a few parts per million of residual sulfur in final steel product. The desulfurization of liquid steel with basic slags proceeds according to the reaction: ðCaOÞ þ ½S ¼ ðCaSÞ þ ½O
½1
where ( ) indicates the slag phase and [ ] indicates the steel phase. It is clear from Eq. [1] that high lime activity (high basicity) in the slag and a low oxygen potential at the slag/steel interface will drive the desulfurization reaction in the forward direction.[1] There are several options for sulfur removal in integrated plants. Highly reducing conditions in the blast furnace (and large slag volumes) are conducive to desulfurization, but the blast furnace is generally operated with a slag of lower basicity, limiting the extent of sulfur removal. Hot metal can be desulfurized by injection of reagents with high affinity of sulfur (such as magnesium or calcium carbide). Oxygen steelmaking, being an oxidation pro-
DEBDUTTA ROY, former Graduate Student, in the Department of Materials Science & Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, is now a Research Engineer at Saint-Gobain Abrasives, Worcester, MA. PETRUS CHRISTIAAN PISTORIUS, Professor, and RICHARD J. FRUEHAN, US Steel Professor, are with the Department of Materials Science & Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University. Contact e-mail: [email protected]. Manuscript submitted October 2, 2012. METALLURGICAL AND MATERIALS TRANSACTIONS B
cess, provides little desulfurization despite the use of highly basic slags. Over 50 pct of the steel in the U.S. is produced in electric arc furnaces, for which ladle desulfurization is the only method to lower the sulfur content of liquid steel significantly. In aluminum-killed steels, ladle desulfurization occurs by transfer of sulfur and aluminum (dissolved in the bulk steel) to the slag/steel interface where these react with lime from the slag; the result is that sulfur dissolves in the slag as CaS and aluminum as alumina (Reaction [2]): 3ðCaOÞ þ 3½S þ 2½Al ¼ ðAl2 O3 Þ þ 3ðCaSÞ
½2
Lade desulfurization (Reaction [2]) is driven in the forward direction if the lime activity in the slag and the initial aluminum content of the liquid steel are high, the activity of alumina is low, and the activity coefficient of CaS is low.[2] However, the oxygen potential at the interface can also be influenced by the reduction of reducible oxides from the slag by aluminum; hence, reducible oxides hinder the desulfurization reaction by consuming the aluminum available at the slag–metal interface.[1] Recent industrial observations suggested that increased silicon levels improve ladle desulfurization of an aluminum-killed steel. At first glance, it is not clear why silicon would affect desulfurization since silicon does not take part in rea
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