<p>Steel refining via ladle treatment is critical to final product quality in the steel manufacturing process. The process of ladle refining serves to assist in the removal of non-metallic inclusions, which can impact steel product fatigue strength, impact toughness, and corrosion resistance. While the steelmaking industry has in place best practices for the process, it remains costly to performing trial and error testing on the ladle. In addition, an understanding of the flow phenomena within the ladle during operation can provide industry with key knowledge necessary to improve the efficiency and throughput of the process.</p> <p> </p> <p>The method by which this research aims to address this is through the development of a comprehensive computational fluid dynamics (CFD) model of the steelmaking ladle. Such a model, capable of predicting the inclusion removal process and flow patterns within the ladle, would serve to provide the necessary information to advance steelmaking efficiency and improve product quality. A full scale unsteady state three dimensional CFD model has been developed to predict removal of inclusion during gas-stirring in a ladle. The Eulerian-Eulerian model was used to simulate the multiphase flow, the Population Balanced Model (PBM) has been used to describe the inclusion distribution. The phenomena of bottom-blow argon bubble coalescence and breakup were considered. </p> <p> </p> <p>Additionally, a model has been developed to predict inclusion removal during operation. For the inclusion removal model, the CFD-PBM coupled method has been proposed to investigate the inclusion behavior. This includes representing phenomena such as inclusion-bubble collision, inclusion removal by attachment to the ladle refractory, and inclusion capture by slag floating on the surface of the melt. The unified computational model for simulation of fluid flow and inclusion removal was validated against industry measurements provided by Nucor Steel. </p> <p> </p> <p>Using this CFD model and a ladle geometry and set of baseline conditions provided by Nucor Steel, studies were carried out to examine flow development, gas bubble distribution, and inclusion removal. Examining the impacts of inclusion size on removal rate indicated that larger inclusions are removed faster. This agreed with both industry expectations and data found in published literature. In addition, the model predicts that bubble-inclusion collision are primarily responsible for 99% inclusion removal in a gas-stirred ladle.</p>

The present thesis deals with the development of a new type of dynamic model for metallurgical reactors. It also covers some of the theoretical aspects of steelmaking that is necessary to include in such an application. The thesis consists of modeling work, high temperature experiments and cold model experiments. Two different aspects of slags in the oxygen steelmaking were investigated. In the first study, slag samples were equilibrated with copper at 1923K in order to study their capacities in capturing phosphorous. Some of the samples were liquid-solid mixtures. The solid phases in these samples were identified by SEM analysis. The identified phases were found to agree well with Thermocalc calculations while the amount of solid fractions didn’t. The phosphorous distribution between the different phases was examined. The phosphate capacities of the samples were evaluated. The MgO content didn’t show any appreciable impact on the phosphate capacity. Furthermore the activities of FeO in the liquid slag samples were calculated and were found to deviate positively from ideality. In the second study the foaming height of CaO-SiO2-FeO slags by the reaction with hot metal was investigated. It was found that the foaming height increased with increasing FeO content up to 20-25%. The foaming height was seen to decrease with increased viscosity. The present results indicated that simply using foaming index for converter slag might lead to wrong conclusion. Simulation experiments using cold model at room temperature were conducted. Cold model experiments were carried out in order to study the penetration depth due to an impinging gas jet on the surface of a liquid metal. The liquid alloy Ga-In-Sn was used to simulate steel. And an HCl solution was used to simulate the slag. A comparison with predictions of existing models was made and a new model parameter was suggested. The observation of the movement of metal droplets generated by the gas jet was also made. The low velocity of droplets suggested that the turbulent viscosity played important role and the droplets could have long resident time in the slag. Furthermore a study of the effect of gas flow rate on homogenization and inclusion removal in a gas stirred ladle was carried out. Both industrial trials and cold model experiments were conducted. As an auxiliary tool CFD was used to predict the mixing times and was found to agree well with both the model experiments and industrial data. The increase of flow rate of inert gas would not improve the mixing substantially at higher flow rates. The water model study showed also that the gas flow rate had negligible effect on the rate of inclusion removal. Both the experiments and CFD calculation strongly suggested that low gas flow rate should be applied in the ladle treatment. Lastly a new approach to a dynamic process model of 300 ton BOF converter was made. The main feature was to utilize the velocity vectors obtained by CFD simulation. In the standalone model, the steel melt domain was sliced into 1000 cells. Based on the imported velocity vectors from the CFD calculation, the mass transfer of carbon and phosphorus was calculated taking into account the slag metal reactions. The mass exchange between slag and metal was considered to be dominated by the metal droplet formation due to oxygen jet. The convergence of the model calculation and the promising comparison between the model prediction and the industrial data strongly suggested that the proposed approach would be a powerful tool in dynamic process control. However, more precise descriptions of other process aspects need to be included before the model can be practically employed in a dynamic controlling system. / <p>QC 20120829</p>

A importância de um escoamento adequado em distribuidores de lingotamento contínuo, quando o objetivo é produzir aços limpos (clean steels), vem sendo largamente discutida e estudada nas últimas décadas. Esses estudos usualmente são realizados em modelos físicos e/ou numéricos, visto que em unidades industriais são de difícil execução. Nos modelos físicos, os estudos como a simulação da remoção de inclusões para diferentes condições de escoamento tem se mostrado bastante importante para o entendimento do escoamento no distribuidor e seus respectivos artifícios favorecedores de flotação de inclusões. Este trabalho tem como objetivo estudar o comportamento da remoção de inclusões em um modelo físico do distribuidor de lingotamento contínuo de tarugos frente à modificação de escoamento no distribuidor. O ensaio proposto para quantificar a remoção de inclusões envolve a utilização de micropartículas de polietileno para simulação das inclusões sólidas de alumina e peneira metálica como método de captura de inclusões passantes pelo veio do distribuidor. Para fornecer suporte a este estudo, foram realizados estudos de qualificação do escoamento por meio do ensaio de linhas de fluxo e de quantificação dos tempos de residência e frações de volumes característicos por meio do ensaio de distribuição de tempos de residência. A técnica utilizada se mostrou confiável sob o ponto de vista experimental. Como resultados do comportamento da remoção de inclusões frente aos parâmetros estudados, têm-se especificamente para o distribuidor estudado que: o aumento da velocidade de lingotamento, em média, aumentou o porcentual de inclusões passantes de 11,61 (1,8 m/min) para 17,45% (2,6 m/min); a presença do tubo longo, em média, aumentou o porcentual de inclusões passantes de 2,79 (sem tubo longo) para 27,03% (com tubo longo); a presença das barreiras, em média, diminuiu o porcentual de inclusões passantes de 16,16 (sem barreiras) para 13,65% (com barreiras). / When the objective is to produce clean steels, the appropriated melt flow in the tundish is very important. It has been widely discussed and studied by researchers in the lasts decades. Physical and/or mathematical models are used to perform these studies because is very difficult to do it in industrial plants. In the physical models, the studies as the simulation of inclusion removal to different conditions of fluid flow have showed to be very important to understand the tundish and its respective tools to improve the inclusion removal. This work aims to study the behavior of inclusion flotation in a physical model of continuous casting tundish of steel billets in different kinds of fluid flow. The method proposed to quantify the inclusion removal involves the utilization of microparticles of polyethylene to simulate solid alumina inclusion and a metallic sieve in the strand as a method to catch the non-floated inclusions. The quantification of inclusion removal has the support of two others experiments to qualify de flow pattern by visual observation using dye tracer and quantify the residence time distribution of the model using acid tracer. The method used in this work to quantify inclusion removal showed to be reliable from an experimental point of view. As results of the behavior of the inclusion removal, the speed of ingoting increased the non-floated inclusions from 11.61 (1.8 m/min) to 17.45% (2.6 m/min), the presence of the submerged inlet shroud increased the non-floated inclusions from 2.79 (without) to 27.03% (with) and the presence of the dams the non-floated inclusions decreased from 16.16 (without) to 13.65% (with).

A importância de um escoamento adequado em distribuidores de lingotamento contínuo, quando o objetivo é produzir aços limpos (clean steels), vem sendo largamente discutida e estudada nas últimas décadas. Esses estudos usualmente são realizados em modelos físicos e/ou numéricos, visto que em unidades industriais são de difícil execução. Nos modelos físicos, os estudos como a simulação da remoção de inclusões para diferentes condições de escoamento tem se mostrado bastante importante para o entendimento do escoamento no distribuidor e seus respectivos artifícios favorecedores de flotação de inclusões. Este trabalho tem como objetivo estudar o comportamento da remoção de inclusões em um modelo físico do distribuidor de lingotamento contínuo de tarugos frente à modificação de escoamento no distribuidor. O ensaio proposto para quantificar a remoção de inclusões envolve a utilização de micropartículas de polietileno para simulação das inclusões sólidas de alumina e peneira metálica como método de captura de inclusões passantes pelo veio do distribuidor. Para fornecer suporte a este estudo, foram realizados estudos de qualificação do escoamento por meio do ensaio de linhas de fluxo e de quantificação dos tempos de residência e frações de volumes característicos por meio do ensaio de distribuição de tempos de residência. A técnica utilizada se mostrou confiável sob o ponto de vista experimental. Como resultados do comportamento da remoção de inclusões frente aos parâmetros estudados, têm-se especificamente para o distribuidor estudado que: o aumento da velocidade de lingotamento, em média, aumentou o porcentual de inclusões passantes de 11,61 (1,8 m/min) para 17,45% (2,6 m/min); a presença do tubo longo, em média, aumentou o porcentual de inclusões passantes de 2,79 (sem tubo longo) para 27,03% (com tubo longo); a presença das barreiras, em média, diminuiu o porcentual de inclusões passantes de 16,16 (sem barreiras) para 13,65% (com barreiras). / When the objective is to produce clean steels, the appropriated melt flow in the tundish is very important. It has been widely discussed and studied by researchers in the lasts decades. Physical and/or mathematical models are used to perform these studies because is very difficult to do it in industrial plants. In the physical models, the studies as the simulation of inclusion removal to different conditions of fluid flow have showed to be very important to understand the tundish and its respective tools to improve the inclusion removal. This work aims to study the behavior of inclusion flotation in a physical model of continuous casting tundish of steel billets in different kinds of fluid flow. The method proposed to quantify the inclusion removal involves the utilization of microparticles of polyethylene to simulate solid alumina inclusion and a metallic sieve in the strand as a method to catch the non-floated inclusions. The quantification of inclusion removal has the support of two others experiments to qualify de flow pattern by visual observation using dye tracer and quantify the residence time distribution of the model using acid tracer. The method used in this work to quantify inclusion removal showed to be reliable from an experimental point of view. As results of the behavior of the inclusion removal, the speed of ingoting increased the non-floated inclusions from 11.61 (1.8 m/min) to 17.45% (2.6 m/min), the presence of the submerged inlet shroud increased the non-floated inclusions from 2.79 (without) to 27.03% (with) and the presence of the dams the non-floated inclusions decreased from 16.16 (without) to 13.65% (with).

A importância de um escoamento adequado em distribuidores de lingotamento contínuo, quando o objetivo é produzir aços limpos (clean steels), vem sendo largamente discutida e estudada nas últimas décadas. Esses estudos usualmente são realizados em modelos físicos e/ou numéricos, visto que em unidades industriais são de difícil execução. Nos modelos físicos, os estudos como a simulação da remoção de inclusões para diferentes condições de escoamento tem se mostrado bastante importante para o entendimento do escoamento no distribuidor e seus respectivos artifícios favorecedores de flotação de inclusões. Este trabalho tem como objetivo estudar o comportamento da remoção de inclusões em um modelo físico do distribuidor de lingotamento contínuo de tarugos frente à modificação de escoamento no distribuidor. O ensaio proposto para quantificar a remoção de inclusões envolve a utilização de micropartículas de polietileno para simulação das inclusões sólidas de alumina e peneira metálica como método de captura de inclusões passantes pelo veio do distribuidor. Para fornecer suporte a este estudo, foram realizados estudos de qualificação do escoamento por meio do ensaio de linhas de fluxo e de quantificação dos tempos de residência e frações de volumes característicos por meio do ensaio de distribuição de tempos de residência. A técnica utilizada se mostrou confiável sob o ponto de vista experimental. Como resultados do comportamento da remoção de inclusões frente aos parâmetros estudados, têm-se especificamente para o distribuidor estudado que: o aumento da velocidade de lingotamento, em média, aumentou o porcentual de inclusões passantes de 11,61 (1,8 m/min) para 17,45% (2,6 m/min); a presença do tubo longo, em média, aumentou o porcentual de inclusões passantes de 2,79 (sem tubo longo) para 27,03% (com tubo longo); a presença das barreiras, em média, diminuiu o porcentual de inclusões passantes de 16,16 (sem barreiras) para 13,65% (com barreiras). / When the objective is to produce clean steels, the appropriated melt flow in the tundish is very important. It has been widely discussed and studied by researchers in the lasts decades. Physical and/or mathematical models are used to perform these studies because is very difficult to do it in industrial plants. In the physical models, the studies as the simulation of inclusion removal to different conditions of fluid flow have showed to be very important to understand the tundish and its respective tools to improve the inclusion removal. This work aims to study the behavior of inclusion flotation in a physical model of continuous casting tundish of steel billets in different kinds of fluid flow. The method proposed to quantify the inclusion removal involves the utilization of microparticles of polyethylene to simulate solid alumina inclusion and a metallic sieve in the strand as a method to catch the non-floated inclusions. The quantification of inclusion removal has the support of two others experiments to qualify de flow pattern by visual observation using dye tracer and quantify the residence time distribution of the model using acid tracer. The method used in this work to quantify inclusion removal showed to be reliable from an experimental point of view. As results of the behavior of the inclusion removal, the speed of ingoting increased the non-floated inclusions from 11.61 (1.8 m/min) to 17.45% (2.6 m/min), the presence of the submerged inlet shroud increased the non-floated inclusions from 2.79 (without) to 27.03% (with) and the presence of the dams the non-floated inclusions decreased from 16.16 (without) to 13.65% (with).

The demand for higher quality steel and higher production rates in the production of steel slabs is ever increasing. These slabs are produced using a continuous casting process. The molten steel flow patterns inside the components of the caster play an important role in the quality of these products. A simple yet effective design method that yields optimum designs is required to design the systems influencing the flow patterns in the caster. The tundish is one of these systems. Traditionally, experimental methods were used in the design of these tundishes, making use of plant trials or water modelling. These methods are both costly and time consuming. More recently, Computational Fluid Dynamics (CFD) has established itself as a viable alternative to reduce the number of experimentation required, resulting in a reduction in the time scales and cost of the design process. Furthermore, CFD provides more insight into the flow process that is not available through experimentation only. The CFD process is usually based on a trial-and-error basis and relies heavily on the insight and experience of the designer to improve designs. Even an experienced designer will only be able to improve the design and does not necessarily guarantee optimum results. In this thesis, a more efficient design methodology is proposed. This methodology involves the combination of a mathematical optimiser with CFD to automate the design process. The methodology is tested on a four different industrial test cases. The first case involves the optimisation of a simple dam-weir configuration of a single strand caster. The position of the dam and weir relative to inlet region is optimised to reduce the dead volume and increase the inclusion removal. The second case involves the optimisation of a pouring box and baffle of a two-strand caster. In this case, the pouring box and baffle geometry is optimised to maximise the minimum residence time at operating level and a typical transition level. The third case deals with the geometry optimisation of an impact pad to reduce the surface turbulence that should result in a reduction in the particle entrainment from the slag layer. The last case continues from the third case where a dam position and height is optimised in conjunction with the optimised impact pad to maximise the inclusion removal on the slag layer. The cases studies show that a mathematical optimiser combined with CFD is a superior alternative compared to traditional design methods, in that it yields optimum designs for a tundish in a continuous casting system. / Thesis (PhD (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

The quality of aluminium cast billets is highly dependent on the cleanliness of the material. Hydro Extruded Solutions are casting billets in Sjunnen that are delivered for extrusion within the same company. In order to produce extruded profiles of high quality and also maintain the pressing tools, it is a good method to keep the billets as free from inclusions as possible. A fluxing agent in form of salt compounds is used to purify the material during melting, but also to protect it from further oxidation. Another task of fluxing agents is to minimize the aluminium amount in the dross phase. The purpose of this project was to investigate if the fluxing agent that is used by the cast house is having any impact on the quality of the billets. Therefore, for that purpose, samples with a variating amount of fluxing agent were processed by the cast house in order to be investigated. These samples were investigated with the Optical Microscopy-method in order to determine the area fraction of non-metallic inclusions inside them. The results did not reveal any significant difference in the quality of the different samples since most samples obtained a low OM-index after the investigation. That can be explained by the type of scrap that was used since new scrap is usually very clean and contains smaller amounts of inclusions. / Kvaliteten hos aluminiumgöt är starkt beroende av renheten i materialet. Hydro Extruded Solutions gjuter rundgöt i Sjunnen, som sedan levereras för strängpressning inom samma företag. För att producera strängpressade profiler av hög kvalitet och även upprätthålla pressverktygen, är en bra metod att hålla göten så fria från inneslutningar som möjligt. Ett flussmedel, i form av saltföreningar, används för att rena materialet under smältningen, men också för att skydda den från vidare oxidation. En annan uppgift som flussmedel har är att minimera mängden aluminium i slaggfasen. Syftet med detta projekt var att undersöka om flussmedelet som används av omsmältverket har någon inverkan på götkvaliteten. Därför, för detta ändamål har prover med en varierande mängd av flussmedel tillverkats i omsmältverket för att bli undersökta. Dessa prover undersöktes med den Optiskt Mikroskopiska-metoden för att bestämma areafraktionen av icke-metalliska inneslutningar inuti dem. Resultaten avslöjade inte någon signifikant skillnad i kvaliteten hos de olika proverna eftersom de flesta prover erhöll ett lågt OM-index efter undersökningen. Detta kan förklaras av vilken typ av skrot som användes, då process-skrot oftast är väldigt rent och består av mindre mängder innseslutningar.

The present thesis was to study non-metallic inclusions during vacuum degassing in ladle treatment. Emphasize was mostly given to liquid CaO-Al2O3 inclusions. A series of industrial experiments were carried out at Uddeholm Tooling AB, Hagfors, Sweden. To gain an insight into the industrial findings, laboratory investigations were also performed. Large number of steel samples were collected and examined. Liquid calcium aluminate inclusions with low SiO2 and high SiO2 were often found with spinel inclusions before vacuum degassing. Laboratory experiments showed that spinel would react with the dissolved Ca in the liquid steel forming calcium aluminate inclusions. This laboratory results agreed with the industrial observation that spinel phase was quite often found in the center of the calcium aluminate phase. After vacuum degassing, most of the inclusions were calcium aluminate liquid inclusions. When dissolved Al level was low, 2 types of liquid calcium aluminate inclusions with considerably different SiO2 contents were found to coexist even at the end of the process. In view of the lack of the thermodynamic data for SiO2 activities in the low silica region, thermodynamic measurements were conducted in the CaO-Al2O3-SiO2-MgO system. The experimental results could reasonably explain the coexistence of the two types of the liquid oxide inclusions. While the total number of inclusions decreased during vacuum degassing, the number of bigger inclusions (&gt;11.3 μm) increased generally in used ladles. This finding was in accordance with the previous studies, wherein, ladle glaze was found to be responsible for the supply of bigger inclusions. The behaviors of several types of inclusions in liquid steel were examined using a laser scanning confocal microscope (LSCM). While alumina inclusions tended to impact on each other, agglomerate and grow very quickly, none of the other types of inclusions, such as spinel and calcium aluminate was observed to agglomerate. The results of LSCM study agreed well with the industrial observation. Examination on a huge number of inclusions did not show any indication of impact and physical growth of the inclusions, except the alumina inclusions. The removal of inclusions around open-eye in a gas-stirred ladle was experimentally studied by a cold model with oil and water. Most of the simulated inclusions were brought up to the oil phase by gas-water plume. Inclusion removal into oil layer took place when the inclusions passed through the sphere-bed of the oil layer around the open-eye. A calculation showed that the contribution of metal-gas plume in inclusion removal was much larger than that of buoyancy mechanism. The results of the industrial experiments revealed that the inclusions distribution strongly depended on stirring condition. When a ladle was stirred by both gas and induction, inclusion removal near slag layer was significant. / QC 20100803

In order to considerably improve the quality of steel products, efforts are being made to increase the cleanness of steel. It is known that the size, type and distribution of non-metallic inclusions (NMIs) in metals significantly influence their mechanical properties. Within the frame of the Collaborativ Research Centre 920 'Multifunc-tional filters for metal melt filtration – a contribution to zero defect materials.”, carbon-bonded alumina filters for steel melt filtration has been investigated. In the present thesis, the interactions between coated carbon-bonded alumina filters and a steel melt were investigated in more detail, with the porous coatings being based on alumina. The time-depended behaviour was evaluated by immersing coated filters for different dwell times. After the immersion tests, the microstructure of the filter surface and the NMIs remaining in the solidified steel were examined to comprehend the interactions. The here presented results imply that carbon of the carbon-bonded alumina sub-strate took part in the filter–steel interactions. The presence of dissolved carbon at the filter–steel melt interface is essential to promote alumina dissolution and pre-cipitation processes. Thereby, the melt was locally supersaturated with aluminium, which reacted with dissolved oxygen to form secondary corundum. During these interactions, a liquid oxide film was formed directly at the ceramic surface and provided nuclei for heterogeneous nucleation of secondary corundum. After immersion during cooling, a characteristic layer built-up formed at the filters surface. All alumina-based coated filters contributed to the filtration of especially alumina-based NMIs, and outperformed the uncoated carbon-bonded alumina filter. During the first experimental trials, it became obvious that the thermal shock resistance of the alumina coating has to be enhanced. For this purpose, a material combination was investigated which so far has not been used as a coating material to the author’s knowledge: alumina-zirconia-titania in the ration 95 : 2.5 : 2.5. This material is known for its excellent thermal shock resistance. Thereby, the influence of zirconia or titania doping of the coating were considered. The addition of titania enhanced wetting of this filter by the steel melt. As a result, alumina inclusions of the steel melt were modified: they were more in number, but distinctly smaller compared to trials without filter or the immersion of the other filter types. Especially, the decreased average area of the alumina inclusions is interesting because the particle size of NMIs strongly influences the fatigue life of a steel product. The deformability of a steel product, however, is determined by the amount of NMIs. Thus, the modification of alumina inclusions by adding titania to the filter coating might present a way to tailor these inclusions depending on the product’s application.