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O desgaste em revestimentos refratários é um dos problemas críticos encontrados em ambientes siderúrgicos, que limita a produtividade do processo devido a necessidade de paradas para manutenção nos equipamentos industriais, em favor da substituição do revestimento refratário em regiões desgastadas. Para identificar e caracterizar os mecanismos de desgaste gerados pela ação do banho metal/escória na microestrutura de materiais refratários utilizados como revestimento de panelas de aço de aciaria elétrica, o objetivo deste trabalho foi dividido em três etapas. A primeira etapa é denominada de estudo \"Post-Mortem\", que consiste na amostragem dos tijolos refratários da panela de aço, ao qual foram preparados para análises físico-químicas e microestruturais para a identificação dos mecanismos de desgastes atuantes. A segunda etapa consistiu na preparação dos tijolos novos (mesma composição química dos tijolos post mortem) de maneira a avaliar seu comportamento em relação ao ataque por escória e/ou metal em ensaios termoquímicos e analisar os resultados obtidos com os tijolos \"Post - Mortem\". A terceira etapa foi baseada nos resultados obtidos das investigações das etapas anteriores, ao qual, foram propostos novos tijolos comerciais com diferentes formulações, em parceria com uma indústria brasileira de materiais refratários (Togni refratários), que atenda melhor as exigências das aciarias elétricas. Os resultados que foram apresentados por esta tese, comprovaram que os revestimentos próximos ao ideal, seria os refratários com matriz de cromo (amostra AC_1) e matriz de carbono (amostra AC_2). / Wear on linning refractory is the critical problems found in steelmaking environments, which limits process productivity due to the need for maintenance shutdowns in industrial equipments, in favor of replacing the refractory lining on wear regions. The objective of this work was to identify and characterize the wear mechanisms generated by the action of the metal/slag bath in the microstructure of refractory materials used as coating of steel pans of electric steelmaking, in order to prolong the campaign time of steel pans. This work was divided into three steps. The first step is called a \"Post-Mortem\" study, which consists of the sampling of the refractory bricks of the steel ladle, to which they were prepared for physicochemical and microstructural analyzes to identify the mechanisms of active wear. The second step consisted in the preparation of the new bricks in order to evaluate their behavior in relation to the slag attack in thermochemical tests and to analyze the results obtained with the \"Post - Mortem\" bricks. Based on the results of these initial investigations, new commercial bricks with different formulations were proposed, in partnership with a Brazilian refractory materials industry (Togni refractory), which better meets the requirements of electric steel mills. The results presented by this thesis are innovative and have characteristics in terms of technological contribution in the steel and refractory industries.

O desgaste em revestimentos refratários é um dos problemas críticos encontrados em ambientes siderúrgicos, que limita a produtividade do processo devido a necessidade de paradas para manutenção nos equipamentos industriais, em favor da substituição do revestimento refratário em regiões desgastadas. Para identificar e caracterizar os mecanismos de desgaste gerados pela ação do banho metal/escória na microestrutura de materiais refratários utilizados como revestimento de panelas de aço de aciaria elétrica, o objetivo deste trabalho foi dividido em três etapas. A primeira etapa é denominada de estudo \"Post-Mortem\", que consiste na amostragem dos tijolos refratários da panela de aço, ao qual foram preparados para análises físico-químicas e microestruturais para a identificação dos mecanismos de desgastes atuantes. A segunda etapa consistiu na preparação dos tijolos novos (mesma composição química dos tijolos post mortem) de maneira a avaliar seu comportamento em relação ao ataque por escória e/ou metal em ensaios termoquímicos e analisar os resultados obtidos com os tijolos \"Post - Mortem\". A terceira etapa foi baseada nos resultados obtidos das investigações das etapas anteriores, ao qual, foram propostos novos tijolos comerciais com diferentes formulações, em parceria com uma indústria brasileira de materiais refratários (Togni refratários), que atenda melhor as exigências das aciarias elétricas. Os resultados que foram apresentados por esta tese, comprovaram que os revestimentos próximos ao ideal, seria os refratários com matriz de cromo (amostra AC_1) e matriz de carbono (amostra AC_2). / Wear on linning refractory is the critical problems found in steelmaking environments, which limits process productivity due to the need for maintenance shutdowns in industrial equipments, in favor of replacing the refractory lining on wear regions. The objective of this work was to identify and characterize the wear mechanisms generated by the action of the metal/slag bath in the microstructure of refractory materials used as coating of steel pans of electric steelmaking, in order to prolong the campaign time of steel pans. This work was divided into three steps. The first step is called a \"Post-Mortem\" study, which consists of the sampling of the refractory bricks of the steel ladle, to which they were prepared for physicochemical and microstructural analyzes to identify the mechanisms of active wear. The second step consisted in the preparation of the new bricks in order to evaluate their behavior in relation to the slag attack in thermochemical tests and to analyze the results obtained with the \"Post - Mortem\" bricks. Based on the results of these initial investigations, new commercial bricks with different formulations were proposed, in partnership with a Brazilian refractory materials industry (Togni refractory), which better meets the requirements of electric steel mills. The results presented by this thesis are innovative and have characteristics in terms of technological contribution in the steel and refractory industries.

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.

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)

This is a thesis includes both experimental and modeling studies for high temperature slag/metal reaction system. / Bloating of metal droplets in emulsion is an important phenomenon in BOF steelmaking in controlling the kinetics of refining. This bloating controls the kinetics by mainly increasing the residence time (from ~¼th of a second to ~10-15 seconds) of the droplets in emulsion and the slag/metal surface (~5-6 times) area. The bloating behavior is determined by the decarburization kinetics. This work aims to develop fundamental understanding of the bloating phenomena through series of experiments and mathematical modeling to explore various factors affecting the kinetics of decarburization. An experimental study on varying the droplet carbon concentration, slag FeO concentration and basicity evidenced mixed controlled kinetics including transport of oxygen in the slag, interfacial (slag/metal) chemical reaction, nucleation and growth of CO bubbles. A mathematical model including these kinetic steps was developed. The model was able to demonstrate the partitioning of oxygen at the slag/metal interface into external (at the slag/metal interface) and internal (within droplet) decarburization in presence of the surface-active element sulfur. The model was developed using a single data set and validated for a wide range of experimental conditions. The model showed excellent agreement with experimental data for most of the reaction period but failed to predict a premature shutdown for droplets reacting with low conductivity slag. In order to understand this discrepancy, the slag ionic and electronic conductivity were varied which showed a premature shutdown of decarburization reaction with low conductivity slag and continuation of the reaction to the thermodynamic limit with high conductivity slag. A mechanism of generation of local electric field by accumulation of charge at the slag/metal interface was proposed to explain the premature shutdown of the reaction for low basicity slags. In all experiments with low conductivity slag sulfur was observed to delay the onset of internal decarburization. However, this effect was diminished or disappeared completely with high conductivity slag. This observation motivated additional experiments to study the competitive adsorption of oxygen and sulfur at the slag/metal interface both through experiments and modelling. It was shown that for low conductivity slag, sulfur poisoning inhibited reaction at the surface whereas for the high conductivity slags the faster transport of oxygen allowed oxygen to compete with sulfur for adsorption sites creating pathways for oxygen into the droplet. By including the possibility of competitive adsorption in the model it was possible to predict the behavior of high sulfur droplets in conductivity slags where the only modification to the model was to change the mass transfer coefficient as appropriate to the higher conductivity. Extension of this study to include silicon in the droplet showed significant effect on decarburization both in delaying bloating as well as increasing peak rate of decarburization. / Thesis / Candidate in Philosophy

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)

Comments and Discussion : Minoru Sawai

Low phosphorus content steels are essential for steel applications where high ductility is required, such as thin sheets, deep drawn, pipelines and automobile exteriors. In the past, phosphorus control was not considered a big challenge in steel production in the US because iron ores with low phosphorus contents were readily available and considered cheap. However, in the last decade, the iron ore price has risen by roughly 400% and lower cost iron sources generally have higher phosphorus content. In integrated steel plants, phosphorus removal usually takes place during the oxygen steelmaking process (OSM) but in Japan a intermediate step for hot metal dephosphorization is commonly used. There are various types of OSM furnaces but the most widely used remains the top-blown Basic Oxygen Furnace (BOF). The BOF slag can be recycled to a sinter plant or directly to the blast furnace, ultimately increasing the phosphorus input in the process. In order to meet new demands for phosphorus control, it is necessary to improve our understanding on the thermodynamics and kinetics of the phosphorus partitioning reaction between slag and metal melts during steelmaking. Therefore, the present work has been divided in three strongly correlated sections: phosphorus equilibrium between metal and slag; analysis of plant data; and observations of the reaction kinetics. Phosphorus equilibrium between liquid metal and slag has been extensively studied since the 1940's. It is well known that CaO and FeO are the main slag constituents that help promote dephosphorization. On the other hand, dephosphorization decreases with temperature due to the endothermic nature of the reaction. Many correlations have been developed to predict the phosphorus partition ratio as a function of metal and slag composition as well as temperature. Nevertheless, there are still disagreements in the laboratory data and the equilibrium phosphorus partition can be predicted with an uncertainty of a factor of up to 5. The first part of the present work focuses on generating more reliable equilibrium data for BOF-type slags by approaching equilibrium from both sides of the reaction. The experimental results were combined with two other sets of data from different authors to produce a new correlation that includes the effect of SiO2 on the phosphorus partition coefficient, LP . Although the quantification of phosphorus equilibrium is extremely important, most industrial furnaces do not operate at equilibrium, usually due to liquid slag formation, kinetics and time constraints. Thus, it is important to know how close to equilibrium different furnaces operate in order to suggest optimal slag compositions to promote dephosphorization. The present work analyzed four large sets of data containing the chemical compositions of both slag and metal phase as well as the tapping temperature of each heat. Each set of data corresponded to different furnaces: one AOD (Argon Oxygen Decarburization), two top-blown BOFs and one Q-BOP or OBM. It was found that the bulk slag composition can greatly \mask" the data due to solid phases coexisting with the liquid slag. The author used the software package FactSage to estimate the amount of solids in the slag and liquid slag composition. It was found that the AOD is the reactor closest to equilibrium, followed by the Q-BOP (OBM) and the two top-blown BOFs. It was noted that the stirring conditions and slag composition are two key variables to enable optimum phosphorus removal. Also, over saturating the slag with CaO and MgO does not seem to benefit the process to any extent. Lastly, interesting observations on the behavior of small metal droplets reacting with slag are presented and discussed. It was found that dynamic interfacial phenomena at the metal-slag interface is likely to play a significant role in the kinetic behavior of the system, due to the exchange of surface active elements, such as oxygen, which dramatically lowers the interfacial tension and cause spontaneous emulsification. Although this phenomenon has been studied, actual quantification of changes in interfacial area remain a challenge. The author developed an experimental method to enable better quantification of spontaneous emulsification and two sets of experiments were carried. One with an Fe containing 0.2 wt.% P and another in a P-free system where pure iron was oxidized. It was found that phosphorus did not play a role in spontaneous emulsification and it was rapidly removed before the onset of dynamic interfacial phenomena. Emulsificaion was maybe caused by de-oxidation of the metal after phosphorus removal took place and the metal became super saturated with oxygen by an unknown reason. The estimated surface area rapidly increases by over an order of magnitude during the beginning and intermediate periods of the reaction. The metal drop breaks into hundreds of small droplets, effectively emulsifying the metal into the slag. With time, the surface area decreases and the metal droplets coalesce. Similar results were observed for an Fe droplet being oxidized. Spontaneous emulsification takes place regardless of the direction of oxygen transfer and the changes in surface area are similar for both cases. The last chapter describes the industrial relevance of the present work, summarizes the findings, revisits the hypotheses and presents potential future work where further research is encouraged.

This research project investigated the option of steelmaking with the ESS furnace by using computational modelling to estimate steady state decarburisation rates. It focused on understanding metallurgical phenomena that would dictate refining rates of molten pig iron with iron ore. The results obtained are aimed at designers and potential users of the furnace technology to improve their understanding of the expected steady-state process behaviour. A mass-and-energy-balance model with a decarburisation sub-model was developed to estimate feed material requirements for steady state operation. Modelling and simulation results suggest that it may not be possible to produce steel under the conditions proposed. However, the furnace still holds potential if ideal operational conditions are understood and applied. Modelling also gave insight into which areas areas of concern, such as bubble formation in the furnace’s channel induction heaters, and necessity for a well designed refractory lining to contain heat and allow the process to operate at a stable condition under the conditions proposed. Keywords: ESS furnace, steelmaking, metallurgical analysis, modelling, mass and energy balance, decarburisation kinetics / Dissertation (MEng)--University of Pretoria, 2020. / Materials Science and Metallurgical Engineering / MEng / Unrestricted