Calcium Modification of Inclusions via Slag/Metal Reactions
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THERE have been a number of recent studies applying slag/metal reaction models to ladle refining of steels. These models either solve systems of rate equations,[1–4] or they use the macro processing feature in FactSage to perform repeated equilibrium calculations.[5–9] All models rely on thermodynamic data that come in the form of equilibrium constants and interaction coefficients from handbooks,[10,11] or FactSage databases. Modeling behavior of reactive species such as Ca and Mg is a key feature of these models, since these species can modify the composition of non-metallic inclusions.
CHENGSONG LIU is with the The State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, P.R. China and also with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: [email protected] DEEPOO KUMAR, BRYAN A. WEBLER, and PETRUS CHRISTIAAN PISTORIUS are with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted June 11, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS B
Slag and refractory are commonly mentioned sources of Ca and Mg that can modify inclusion compositions. The Mg modification of Al2O3 inclusions can be attributed to both MgO-containing slag and MgO-containing refractories[12–16] and the presence of impurities in MgO-containing refractories can significantly increase dissolved Mg due to steel/refractory interactions.[6,17] Ca modification of Al2O3 inclusions is frequently accomplished by intentional Ca additions[18–23]; however, Ca pickup by the steel without intentional addition has also been observed. The pickup of Ca has been attributed to three factors: (1) slag/metal mass transfer,[24–26] (2) Ca contamination in ferrosilicon,[27] and (3) entrained slag droplets.[28] The extent of inclusion modification due to each source cannot easily be determined. Recent laboratory-scale studies of reactions between liquid Fe-Al[29] and Fe-Cr-Al[30] alloys with CaO (sat.)-MgO (sat.)-Al2O3 slags have attempted to isolate the extent of modification due to slag/metal reactions. It was observed in both studies that initially Al2O3 inclusions were rapidly modified to (MgO)Æ(Al2O3) spinels and MgO. Little Ca modification was observed until reaction times were long (so inclusion concentrations were low and the amount of Ca transferred to them was measurable) and Al contents were approximately 1 wt pct, higher than that needed for
deoxidation. Similar results were observed for steels in contact with a CaO-CaF2-Al2O3-MgO slag.[31] However, many of the previously mentioned kinetic models and experimental results predicted a much greater extent of Ca modification. The comparisons are challenging depending on whether plant data or laboratory-scale data are used, but it is worthwhile to examine the thermodynamics of Ca in liquid Fe alloys to test if the predictions are consistent with observations or if observed calcium levels might be due to other so
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