Mechanism and kinetics of transformation of alumina inclusions in steel by calcium treatment

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Helsinki University of Technology publications in materials science and metallurgy, Teknillisen korkeakoulun materiaalitekniikan julkaisuja, 180
In steelmaking, commonly used calcium treatment has the benefit of modifying inclusion composition, and the shape and size of these inclusions are also adjusted. Two of the main advantages of calcium treatment are not only the improvement of the castability but also the improvement of the final properties of the steels' machinability, toughness and surface quality. The effects of calcium are mainly based on its strong ability to form sulphides and oxides. In aluminium deoxidised steels, the inclusion population will generally include alumina inclusions and maybe some silicates and manganese sulphides. After calcium treatment, the inclusions are restricted mainly to calcium aluminates (CaO-Al2O3) and the sulphur in the steel is associated with these inclusions as calcium sulphide. The topic of the present study concerns the basic mechanism and kinetics of the transformation process of alumina inclusions in steel when calcium is introduced into the steel e.g., by wire feeding or powder injection. To clarify the mechanisms, several types of model experiments were performed in laboratory furnaces and on an industrial scale. Reactions between only Al2O3 and CaO were studied in a laboratory furnace. The phases formed during the reaction between Al2O3 and CaO were examined by SEM-EDS, and a discussion of the proposed reaction sequence of Al2O3⇒CA6⇒CA2⇒CA⇒CAx (liquid) was based on the experimental observations and thermodynamic equilibrium examinations. The kinetics of the reaction of calcium with alumina inclusions were simulated by performing Ca treatments in an induction furnace for 8 kg steel deoxidised with aluminium. Results were compared with observations of inclusion transformation in a real steelmaking process. Based on the results, a kinetic model was proposed.
alumina inclusions, calcium treatment, mechanism, kinetics, steel
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