Thermodynamics of MnO-SiO 2 -Al 2 O 3 -MnS Liquid Oxysulfide: Experimental and Thermodynamic Modeling
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-AL2O3-MNS–TYPE molten oxysulfide inclusion is often observed in semi-killed steel by Si/Mn deoxidation. Such type of inclusions has practical importance because (1) precipitation of solid MnS from the molten oxysulfide inclusion may induce the Mn-depleted zone near the inclusion during reheating of structural steel, which is one mechanism of intragranular acicular ferrite formation,[1] and (2) MnS inclusions associated with soft oxide (or oxysulfide) inclusions evenly dispersed in re-sulfurized steel for free machining act as lubricant between machining tool and the steel chips during high-speed machining.[2] Estimation of solubility of MnS is of practical importance, in particular for free-machining steel, because the appropriate balance in the amounts between liquid oxysulfide (as a lubricant) and solid MnS (as a chip breaker) is necessary for stable machining. Therefore, accurate control of evolution of such inclusions in steels is important, and it requires
YE-JIN KIM, Graduate Student, HENRI GAYE, formerly Professor, HAE-GEON LEE, Professor, and YOUN-BAE KANG, Assistant Professor, are with the Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology, Pohang, Republic of Korea. Contact e-mail: [email protected] DAE-HEE WOO, formerly Graduate Student, Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology, Pohang, Republic of Korea, is now with Technical Research Laboratories, POSCO Ltd, Kwangyang, Cheonnam, Republic of Korea. Manuscript submitted October 1, 2010. Article published online March 15, 2011. METALLURGICAL AND MATERIALS TRANSACTIONS B
information on thermodynamics and phase equilibria of the MnO-SiO2-Al2O3-MnS system. The importance of such kinds of information on inclusion control in different grades of steels has recently been discussed.[3,4] Furthermore, sulfide dissolved in liquid aluminosilicate oxide exhibits a displacement reaction between cations and anions,[5] which is interesting to analyze by solution thermodynamics. So far, very few studies have been performed previously, primarily on the sulfide capacity of MnO-SiO2Al2O3 molten oxide,[6,7] and no information on phase equilibria has been known for the MnO-SiO2-Al2O3MnS system. Recently, Woo et al.[8,9] performed a phase diagram study in the MnO-SiO2-Al2O3-MnS system as well as in its subsystems (MnO-MnS, MnO-SiO2-MnS, MnO-Al2O3-MnS) employing high-temperature equilibration followed by quenching and electron probe microanalysis (EPMA), and assisted by differentialthermal analysis (DTA) and the X-ray diffraction technique (XRD). In this work, it was found that the solubility of MnS in liquid oxysulfide strongly depends on the composition of the liquid oxysulfide. Increasing the SiO2 concentration in liquid MnO-SiO2-MnS oxysulfide rapidly decreases the solubility of MnS. Moreover, adding Al2O3 in the oxysulfide further decreases the solubility of MnS. The solubility of MnS in the liquid oxysulfide will be dependent on the thermodynamic properties of the liquid oxysulfide as well
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