The Ladle Metallurgy Furnace (LMF) in secondary steelmaking processing allows (1) the adjustment of the chemical composition by ferroalloy addition, (2) temperature control by electrical reheating (3) homogenization of chemical composition by argon stirring (4) desulphurization and (5) control of inclusions produced during steel deoxidation. The solid oxide inclusions not only cause nozzle clogging during the continuous casting process and reduce production efficiency, but they are also harmful to the properties of the steel product. In the case of aluminum killed steel, calcium treatment of steel is typically employed to modify the solid alumina inclusions or magnesium aluminate inclusions to liquid or partially liquid calcium aluminates. Injected calcium in form of calcium wire creates calcium bubbles. A portion of calcium dissolves into the steel and diffuses into the steel bulk. However, during the dissolution process some of the injected calcium reacts, close to the injection plume, with dissolved sulfur and oxygen in the steel to form calcium sulfide and calcium oxide inclusions [1]. During ladle treatment of aluminum killed steel the inclusions which form initially are typically alumina. As the dissolved oxygen content of the steel is reduced to very low levels magnesium may be reduced from the slag and subsequently react with the inclusions resulting in a shift towards higher contents of MgO and formation of magnesium aluminate spinel. Magnesium may originate from the top slag, ferro alloys or refractories. Therefore, magnesium aluminate spinels are almost always present before calcium injection and any consideration of calcium treatment should also consider the modification of spinel in addition to alumina. The current work develops a fundamental kinetic model to describe the evolution of the inclusion population during ladle treatment of aluminum killed steels. The model builds on previous work in the author’s laboratory predicting the kinetics of slag metal reactions during ladle treatment and the transformation of alumina to magnesium aluminate spinel. The model addresses the modification of alumina inclusions by calcium and considers mass transfer of species to the inclusion-steel interface and diffusion within the calcium aluminate phases formed on the inclusion. The dissolution of calcium from calcium bubbles into the steel and formation of oxide and sulfide inclusions at the plume is coupled with the kinetic model for inclusion modification. It is found that rate of supply of calcium to the inclusions controls the overall rate of transformation. The inclusion-steel kinetic model is then coupled with the previously developed steel-slag kinetic model. The coupled inclusion-steel-slag kinetic model is applied to the chemical composition changes in molten steel, slag, and evolution of inclusions in the ladle. The result of calculations is found to agree well with industrial heats for species in the steel as well as inclusions during Ca treatment. The kinetic model is further extended to model the modification of spinel inclusions by calcium treatment. Coupling the model for inclusions modification to a multi component kinetic model for the slag-steel reactions in the ladle furnace allowed prediction of the change of average composition of inclusions which was subsequently verified using plant data from ArcelorMittal Dofasco operations. Good agreement between the experimental and calculated average composition of inclusions was obtained for most of the industrial heats. Finally, a sensitivity analysis of the coupled kinetic model was performed to compare the effect of the different processing conditions including sulphur content, stirring, total oxygen, slag composition and reoxidation, on the evolution path of inclusions. / Thesis / Doctor of Philosophy (PhD)

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