Thermodynamics of Complex Sulfide Inclusion Formation in Ca-Treated Al-Killed Structural Steel
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AMONG all the steel grades in the market, low-alloy structural steel de-oxidized by aluminum has a high market share. To satisfy diverse customer requirements and to be competitive, structural steel with high strength, good formability, excellent weldability, outstanding fatigue and corrosion resistance, admirable toughness, and favorable machinability must be offered.[1] The properties are significantly affected by the characteristics of the non-metallic inclusions, such as their size, shape, distribution, and composition. Long strip-shaped sulfide inclusions in particular are detrimental to the corrosion resistance,[2,3] machinability,[4] toughness, ductility, and isotropy of the toughness and ductility of structural steel.[1,5] Removing sulfur from liquid steel to the ‘‘zero content’’ level is not only impossible but also not profitable. Therefore, it is critical to modify the inclusions by adding different modifiers to the liquid steel. Calcium alloy is a popular modifier for sulfide spheroidization and alumina liquefaction.[6–9] In Ca-treated Al-killed steel, complex sulfide inclusions are often observed; among these inclusions, single-phase (Mn,Ca)S solid solution and oxide-sulfide duplex, which consists of an oxide core and a sulfide ring, can be detected.[10–13] In the many theoretical investigations of inclusion formation in Ca-treated Al-killed steel, the activity of CaS associated with the oxide phase has often been
YIN-TAO GUO and GU-JUN CHEN, Ph. D. Students, SHENGPING HE, Associate Professor, and QIAN WANG, Professor, are with the College of Materials Science and Engineering, Chongqing University, Chongqing 400044, China. Contact e-mail: heshp@cqu. edu.cn Manuscript submitted April 24, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS B
assumed to be a constant, such as 1,[14] because of the very low solubility of CaS (2 to 5 wt pct) in calcium aluminate at 1823 K (1550 C)[15] and the lack of MnS inclusion in general structural steel above the liquidus temperature. However, MnS, resulting from the segregation and enrichment of sulfur element in the liquid portion, starts to precipitate in the interdendritic space; subsequently, CaS and MnS can dissolve each other to generate a solid solution of (Mn,Ca)S.[16] Therefore, the activity of CaS varies as a function of the molar fraction of CaS (XCaS) in (Mn,Ca)S and cannot be treated as a constant. It is thus critical to develop a thermodynamic model for predicting the activities of CaS and MnS in a (Mn,Ca)S solid solution, which would be beneficial in understanding the formation mechanism of the sulfide and/or oxide-sulfide duplex inclusions during solidification. As CaS and MnS have similar crystal structures (NaCl-type rock salt structure), Jiang et al.[17,18] treated (Mn,Ca)S as an ideal solid solution to simplify the thermodynamic model. However, Lu et al.[19] disagreed with the ideal solution model and proposed a regular solution model to predict the activity of CaS in a (Mn,Ca)S solution by assuming a symmetrical miscibility gap. Piao et al.[20] developed another soli
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