The advancement in computational thermodynamics can help researchers to test their hypotheses regarding complex steelmaking operations in a more quantified manner. The main aim of the current work was to use develop a kinetic model that can predict changes in steel, slag and inclusions during ladle refining and use this model as a tool to develop better understanding of the steelmaking process itself. The important reactions during ladle refining are: steel-refractory reaction, slag-refractory reaction, flotation of inclusions to slag, steel-inclusion reaction, steel-slag reaction and inclusions originating from slag. The chemical reactions between two phases were considered to be mass transfer controlled. The macro-processing feature in FactSage was used to do multiple equilibrium calculations and calculate the change in steel, slag and inclusion composition. Targeted experiments and industrial trials were conducted to find model parameters. For laboratory experiments, the rate of magnesium-transfer to oxide inclusions in steel due to steel-crucible and steel-slag reaction was studied. It was concluded that the presence of spinel layer on MgO crucible at the steel-crucible reaction can help in significantly reduce the rate of Mg pick-up due to steel-crucible reaction. For industrial trials, a comparison between the rate of steel-slag reaction and inclusion flotation rate showed that the steel-slag reaction could be significantly slowed due to slag inhomogeneity. The kinetic model was also used to identify artifacts in steel and slag sampling during ladle refining. One of the main limitations of the kinetic model was the over-prediction of calcium pick-up in steel due to steel-slag reaction. Induction furnace experiments were conducted using MgO, ZrO2 and CaO crucible with different slag composition and silicon concentration to study the extent of calcium pick-up due to steel-slag and steel-crucible reactions. The steel-CaO crucible equilibrium experiment was used to estimate Ca-O interaction parameter. The equilibrated steel was reoxidized with known amount of oxygen to allow all the dissolved calcium to precipitate as oxide inclusions. Inclusion analysis of sample taken after reoxidation was used to estimate dissolved calcium in steel. The measured dissolved calcium was used to estimate Ca-O interaction parameter. A private database for liquid steel was created in FactSage and used for kinetic modeling of laboratory scale steel-slag-crucible experiments. The use of private database for kinetic model helped in avoiding excess calcium pick-up in steel due to steel-slag reaction. However, the model and database should be tested for conditions where significant calcium pick-up is experimentally observed. In the present work, the inclusion removal was assumed to be first order reaction with fixed rate constant. In practice, the inclusion removal is expected to be a more complicated process of agglomeration and flotation. Similarly, the steel-inclusion reactions were considered in equilibrium for each time step of calculation. Sometimes, the composition difference inside single inclusions was found. Some characterization tools were used that could be useful in future to study the agglomeration of inclusions and composition differences inside single inclusion. The agglomeration behavior of inclusions at the steel-argon interface inside confocal laser scanning microscope was compared to the agglomeration in bulk samples from laboratory and industrial steel samples. The size and morphology of inclusion clusters were studied using X-ray micro CT. The composition and morphology of single inclusion was studied using focused ion beam methods: Ga-FIB instrument and plasma-FIB instrument.

The main part of this work has been a literature survey, reviewing scientifical reports forinformation on how steel cleanness is evaluated today, and also how the steel cleanness is related tothe fatigue performance of clean steels.

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Twenty Four samples of xenoliths and country rocks from the 1961 lava flow of Calbuco volcano have been studied. Fourteen samples have been analyzed for major elements and P, Ni, Ba, Cr, V, Zr, Sc, Y, and Sr. Five of these samples were further analyzed for Sm, Nd, Sr, and Pb isotope ratios. Seventeen samples were studied under the microscope and three samples were analyzed by microprobe for their pyroxene compositions. Based on petrographic studies xenoliths were divided into three groups. Fine grained xenoliths (groups I and II) probably formed from metamorphosed MORB-like basalts, whereas coarse grained xenoliths (group III) were apparently derived from cumulate minerals that crystallized from the Calbuco magma. The fine grained xenoliths were probably entrained in magma at intermediate levels of the crust, near the stability limit of amphibole to form pyroxene and plagioclase. In the coarse grained xenoliths amphibole that formed at depth dehydrated as the xenoliths were brought to the surface. The country rocks are apparently unrelated to the xenoliths.

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Magnesium aluminate spinel inclusions are a concern in the steelmaking industry since these particles affect the processing and the properties of steel. During the refining of low carbon aluminum killed steel in the ladle furnace; the initial alumina inclusions shift their composition towards higher contents of MgO and eventually they become magnesium aluminate spinel inclusions. This research developed a kinetic model for the transformation of alumina inclusions to spinel in liquid steel. The aspects of simultaneous deoxidation and of solid state cation counterdiffusion were addressed in the model. Coupling the model for spinel inclusions to a kinetic model for the slag-steel reactions in the ladle furnace allowed verifying the modeled concentrations in the inclusions with the plant data measurements of ArcelorMittal Dofasco operations. Good agreement between the experimental and calculated Mg contents in the inclusions was obtained for most of the industrial heats analyzed. Finally, a sensitivity analysis of the coupled kinetic model was performed to compare the effect of the different processing conditions and mass transfer rates on the amount of Mg and spinel in the inclusions. Several results from this work indicate that the rate limiting step on the formation of magnesium aluminate spinel inclusions is the supply rate of dissolved [Mg] from the slag-steel reaction; the supply of [Mg] is in turn controlled by the changes at the slag-steel interface. / Thesis / Master of Applied Science (MASc)

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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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