The Oxygen Steelmaking process is used to refine pig iron produced in the blast furnace to produce liquid steel for further refining in secondary steelmaking processes. The main advantages of the process are its autogenous nature, wherein the heat is generated through the refining reactions itself, and the refining is completed in a relatively short time (typically 15-25 mins). Achieving the desired end-point composition of refined steel is essential to avoid re-blows, which lead to delays in downstream processes and an increase in steel production costs. Improving process control through regular monitoring and a better understanding of the process is thus very critical. Multiple reaction interfaces are formed between various phases (slag, metal, gas), at extremely high temperatures and this makes the monitoring of the process through sampling and observation difficult and expensive. Consequently, mathematical modelling has been used as a tool to improve the understanding of the process and propose developments in operation. Numerous models have been developed in the past; however, these models do not address several open questions regarding the detailed reaction mechanisms and the contributions from different reaction zones inside the Basic Oxygen Furnace. The current work aimed to fill this gap. In this work, four prominent reaction zones, namely; impact, slag-metal bulk, cavity, and emulsion zones were identified. A more mechanistic approach involving process variables has been used to decrease the level of empiricism. With regards to the impact and the slag-metal bulk zones, the velocity of flow of metal (or surface-renewal) at the interfaces of these zones are calculated by taking into the momentum induced by the top-jets and bottom-stirring plumes. This study found that these zones contribute negligibly to overall refining in the oxygen steelmaking process. In the case of the emulsion zone, a very rigorous description of all aspects (external and internal decarburization, bloating behavior, and trajectory) pertaining to the life cycle of a single metal droplet in slag has been achieved. The emulsion zone is found to contribute 5 to 75 % of decarburization during various times of blow. The cavity zone model represents the first reported effort to predict the refining behavior of metal droplets that are exposed to oxygen jets within the lance cavities. The model incorporated the mass transfer, reaction equilibria, and kinetics of the reactions. It is predicted that this zone plays a critical role in the removal of silicon and FeO formation in the early part of the blow and removal of carbon throughout the blow. Several significant insights with regards to improvement in the operation of the oxygen steelmaking process are derived from each sub-models. The integration of these models will guide the steelmaker to improve their practices so that they can achieve better consistency in the end-point composition of refined steel and reduce re-blows. / Thesis / Doctor of Philosophy (PhD)

Iron- and steel production is a material- and energy intensive industrial activity. The production of one tonne of steel commonly results in some 400 kilograms of residual materials such as metallurgical slags, dusts, sludge and scales generated in the processes. Much work is continuously devoted to finding possible ways of using residual materials and minimising landfilled volumes. As these materials often contain considerable amounts of valuable elements such as iron, coal, manganese and calcium, it may be profitable to use them to replace virgin raw materials or to sell them as products that may be useful in other industries and/or processes.    The thesis is based on case studies that exemplify how the use of process integration, through system analysis, can assist in assessing effects and opportunities for different concepts for increased material efficiency in Nordic ore-based steelmaking systems. The process integration approach taken for this research work was the simulation of a specific iron- and steel production system and the use of an optimisation tool for the evaluation of an extended system including the symbiosis between four steel plants.   Three different cases were studied including: system effects of increased magnesium oxide content in the lime raw material, investigation of the prospects for vanadium enrichment and slag reduction concept and a study of the logistics perspective for a joint residual material upgrading and recycling venture between four steel plants. The analysed cases present possibilities to improve the material efficiency by: •      Enhanced recovery of residual materials; •      Upgrading of residual materials to products; •      Specific elements recovery; •      Decreased use of virgin raw material; •      Improved quality of residual materials; •      Decreased amounts of materials placed in long-term storage or landfills.   From the results of the cases studied, the best scenarios and potential gains by enhanced material efficiency is presented. In the case of system effects of increased magnesium oxide content in the lime raw material, the issue was mainly to obtain maximum usage of metallurgical slags without compromising the quality of the main product. The calculated possibility of increased slag recirculation enabled further a decreased consumption of iron ore pellet and limestone. Regarding the investigation of the vanadium enrichment and slag reduction concept, the best scenario markedly increased the internal slag recirculation in order to enrich the vanadium content in the slag for ferrovanadium production. By the vanadium enrichment and recovery concept, considerable amounts of vanadium would be made useful instead of ending up in long-term storage. The study of a shared Nordic concept for residual materials upgrading and use demonstrated the potential for upgrading the materials to a direct reduced iron product for recovery in blast furnace. The concept showed high potential for significantly reducing the amount of material being long-term stored or deposited to landfill and thus a potential step towards achieving the zero-waste philosophy target.   None of the concepts for enhanced material efficiency studied in this thesis work has been implemented; however, they are still of relevance for the Nordic steel industry.

Fundamental research is required to support the commercialization of 3rd Generation Advanced High Strength steels (3G AHSS). Mid-manganese 3G AHSS steels can contain up to 11wt% manganese and are expensive if traditional ferroalloying practices are used; reductive alloying is a promising alternative. This study has researched the fundamental science behind possible processing methods. Silicothermic reduction of MnO from slag was studied. The reaction is fast but can be blocked by a stagnant layer of SiO bubbles cutting the rate of reaction by one order of magnitude. A theoretical model for mixed mass transport control was tested against original experimental data. Across nine datasets, the mass transfer coefficient for metal species, kMetal, was 2.3∙10-4m/s and the slag mass transfer coefficient, kSlag, was 6.7∙10-4m/s. In real industrial systems, gas blockage should not have an effect because stirring will dislodge these bubbles. Carbothermic reduction is dramatically different and has been qualitatively documented in this work. The reaction occurs in two stages: the first approximately three times faster than the second. The first stage is characterized by internal CO nucleation and growth and is rate-limited by the formation and growth of these CO bubbles. The second stage occurs along the metal interface and shows that the slag and metal are essentially separated by an intermediary gas phase. This reaction is controlled by decomposition of metal oxides at the gas-slag boundary, decomposition of CO2 at the gas-metal boundary, and transport of CO2 across the gas bubble; this mechanism is nearly identical to the carbothermic reduction of FeO. Reductive alloying can be utilized with the silicothermic reduction process to obtain high levels of manganese in steel but the carbothermic reduction may be too slow to be a viable process. / Thesis / Doctor of Philosophy (PhD) / 3rd Generation Advanced High Strength steels (3G AHSS) are a promising opportunity to produce steels with improved mechanical properties. These steels are alloyed with up to 11wt% manganese; traditional alloy additions are added as ferroalloys which may not be the most economical solution to achieve the required concentrations of manganese. Reductive alloying is a potential method for achieving high concentrations of manganese in the metal. By adding manganese oxide to slag, and reductants like carbon or silicon to the molten metal, manganese can be reduced from slag to metal. This work has determined the kinetics (rate of reaction) during the silicothermic and carbothermic reduction of manganese oxide from slag. The silicothermic reduction of manganese oxide is fast and can achieve high levels of manganese in the metal. The carbothermic reduction is much slower with questionable viability.

Thermal spray coatings applied with high velocity techniques produce dense, industrial quality coatings with strong adhesion and minimal decomposition. This thesis reports on investigations of splat-substrate interactions for both solid and molten splats. Specifically, individual particles were studied to see how the particle is altered during the spray coating process, how they bond to the substrate and the role of surface oxides. Investigations of NiCr particles high velocity air fuel (HVAF) thermally sprayed onto different materials found that soft substrates predominantly had deeply penetrating solid particles, whereas harder substrates resisted particle penetration and had a higher percentage of molten splats. This effect is caused by particle kinetic energy converted into heat during plastic deformation. The percentage of particle kinetic energy converted into heat is proportional to substrate hardness. It was also discovered that during the coating process the oxide is not removed or altered in composition, but becomes redistributed over a larger surface area due to the plastic deformation of the substrate. During this process, small scale redistribution and penetration of the oxide material by the incoming particle occurs. These results support the idea that successful bonding can occur only when the surface oxide on the substrate and on the coating material has been disturbed (for solid splats) or disrupted (for molten splats). To date, our knowledge of solid splat bonding processes within thermal spray coatings has been very subjective where mechanical and chemical bonding has been expected to contribute. In this thesis, the splat-substrate interface was investigated with focused ion beam (FIB) microscopy, cross-sectional SEM and cross-sectional TEM. For solid NiCr splat HVAF coatings, the discovery of interfacial formations, together with no evidence of chemical bonding across the particle-substrate interface suggest that mechanical bonding is the dominant bonding mechanism for solid splat coatings; where as chemical bonding only plays a role when splats and/or substrate become molten. / GNS Science

Thermal spray coatings applied with high velocity techniques produce dense, industrial quality coatings with strong adhesion and minimal decomposition. This thesis reports on investigations of splat-substrate interactions for both solid and molten splats. Specifically, individual particles were studied to see how the particle is altered during the spray coating process, how they bond to the substrate and the role of surface oxides. Investigations of NiCr particles high velocity air fuel (HVAF) thermally sprayed onto different materials found that soft substrates predominantly had deeply penetrating solid particles, whereas harder substrates resisted particle penetration and had a higher percentage of molten splats. This effect is caused by particle kinetic energy converted into heat during plastic deformation. The percentage of particle kinetic energy converted into heat is proportional to substrate hardness. It was also discovered that during the coating process the oxide is not removed or altered in composition, but becomes redistributed over a larger surface area due to the plastic deformation of the substrate. During this process, small scale redistribution and penetration of the oxide material by the incoming particle occurs. These results support the idea that successful bonding can occur only when the surface oxide on the substrate and on the coating material has been disturbed (for solid splats) or disrupted (for molten splats). To date, our knowledge of solid splat bonding processes within thermal spray coatings has been very subjective where mechanical and chemical bonding has been expected to contribute. In this thesis, the splat-substrate interface was investigated with focused ion beam (FIB) microscopy, cross-sectional SEM and cross-sectional TEM. For solid NiCr splat HVAF coatings, the discovery of interfacial formations, together with no evidence of chemical bonding across the particle-substrate interface suggest that mechanical bonding is the dominant bonding mechanism for solid splat coatings; where as chemical bonding only plays a role when splats and/or substrate become molten. / GNS Science

Thermal spray coatings applied with high velocity techniques produce dense, industrial quality coatings with strong adhesion and minimal decomposition. This thesis reports on investigations of splat-substrate interactions for both solid and molten splats. Specifically, individual particles were studied to see how the particle is altered during the spray coating process, how they bond to the substrate and the role of surface oxides. Investigations of NiCr particles high velocity air fuel (HVAF) thermally sprayed onto different materials found that soft substrates predominantly had deeply penetrating solid particles, whereas harder substrates resisted particle penetration and had a higher percentage of molten splats. This effect is caused by particle kinetic energy converted into heat during plastic deformation. The percentage of particle kinetic energy converted into heat is proportional to substrate hardness. It was also discovered that during the coating process the oxide is not removed or altered in composition, but becomes redistributed over a larger surface area due to the plastic deformation of the substrate. During this process, small scale redistribution and penetration of the oxide material by the incoming particle occurs. These results support the idea that successful bonding can occur only when the surface oxide on the substrate and on the coating material has been disturbed (for solid splats) or disrupted (for molten splats). To date, our knowledge of solid splat bonding processes within thermal spray coatings has been very subjective where mechanical and chemical bonding has been expected to contribute. In this thesis, the splat-substrate interface was investigated with focused ion beam (FIB) microscopy, cross-sectional SEM and cross-sectional TEM. For solid NiCr splat HVAF coatings, the discovery of interfacial formations, together with no evidence of chemical bonding across the particle-substrate interface suggest that mechanical bonding is the dominant bonding mechanism for solid splat coatings; where as chemical bonding only plays a role when splats and/or substrate become molten. / GNS Science

Thermal spray coatings applied with high velocity techniques produce dense, industrial quality coatings with strong adhesion and minimal decomposition. This thesis reports on investigations of splat-substrate interactions for both solid and molten splats. Specifically, individual particles were studied to see how the particle is altered during the spray coating process, how they bond to the substrate and the role of surface oxides. Investigations of NiCr particles high velocity air fuel (HVAF) thermally sprayed onto different materials found that soft substrates predominantly had deeply penetrating solid particles, whereas harder substrates resisted particle penetration and had a higher percentage of molten splats. This effect is caused by particle kinetic energy converted into heat during plastic deformation. The percentage of particle kinetic energy converted into heat is proportional to substrate hardness. It was also discovered that during the coating process the oxide is not removed or altered in composition, but becomes redistributed over a larger surface area due to the plastic deformation of the substrate. During this process, small scale redistribution and penetration of the oxide material by the incoming particle occurs. These results support the idea that successful bonding can occur only when the surface oxide on the substrate and on the coating material has been disturbed (for solid splats) or disrupted (for molten splats). To date, our knowledge of solid splat bonding processes within thermal spray coatings has been very subjective where mechanical and chemical bonding has been expected to contribute. In this thesis, the splat-substrate interface was investigated with focused ion beam (FIB) microscopy, cross-sectional SEM and cross-sectional TEM. For solid NiCr splat HVAF coatings, the discovery of interfacial formations, together with no evidence of chemical bonding across the particle-substrate interface suggest that mechanical bonding is the dominant bonding mechanism for solid splat coatings; where as chemical bonding only plays a role when splats and/or substrate become molten. / GNS Science

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