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An experimental study was undertaken to quantify the rate of DRI decarburization in a steelmaking slag using the constant–volume pressure increase technique. Experiments were conducted by dropping DRI pellets into molten slag at temperatures from 1500°C to 1600°C. Further experiments were carried out in which the DRI pellets were preheated while the slag temperature remained constant. The effect of initial carbon content and preheating temperature of the DRI on the reaction rate was investigated. The decarburization of DRI appears to comprise of two stages; reaction with the FeO of DRI, followed by further decarburization through the iron oxide of slag. Carbon has a significant effect on the kinetics of both stages while the preheating temperature mainly influences the rate of the reaction between FeO and carbon inside the pellet.

An experimental study was undertaken to quantify the rate of DRI decarburization in a steelmaking slag using the constant–volume pressure increase technique. Experiments were conducted by dropping DRI pellets into molten slag at temperatures from 1500°C to 1600°C. Further experiments were carried out in which the DRI pellets were preheated while the slag temperature remained constant. The effect of initial carbon content and preheating temperature of the DRI on the reaction rate was investigated. The decarburization of DRI appears to comprise of two stages; reaction with the FeO of DRI, followed by further decarburization through the iron oxide of slag. Carbon has a significant effect on the kinetics of both stages while the preheating temperature mainly influences the rate of the reaction between FeO and carbon inside the pellet.

Water models using perspex have been built to study the fluid flow and recirculation patterns developed in the sump of a steel continuous casting machine and the influences these have on the behaviour of inclusions. An experimental method has been devised to simulate the behaviour of inclusions in the sump and to study the apportionment of the input flux of inclusions between the molten mould powder layer and the strand. The method entails the uses of finely dispersed coloured paraffin oil in the inlet stream together with a floating colourless paraffin layer on the top of the water in the model mould to simulate the molten powder layer on top of the molten steel. A theoretical model has been formulated which relates the inclusion separation in the sump to the fluid flow there. The inclusion removal ratio in the sump for a given continuous casting machine can be predicted using this theoretical model. The model, using the properties of liquid steel and practicable casting speeds, demonstrates that the removal of inclusions of small size (< 40 um) from the mould sump is less than 5% efficient. Inclusion agglomeration plays an important role in inclusion removal. It has been shown that deep submersion of the SEN enhances the agglomeration of inclusion particle. Under certain conditions, for example, the average particle diameter in the meniscus region has been found to be as much as three times its value at the SEN nozzle. The use of fine alumina flakes or small air bubbles, together with a plane light source, has been found to be very successful in studying the fluid flow patterns developed in three-dimensional models. Employing this method, the fluid flow patterns developed on different planes within the model mould have been viewed and recorded photographically. The photographs so obtained have helped to explain the results obtained for the removal of inclusions. The fluid flow patterns developed when small outside diameter nozzles with deep SEN submerged depths are used have been found to be of benefit to the removal of inclusions. Increasing the SEN submerged depth promotes inclusion agglomeration and hence increases the inclusion removal ratio. Reducing the nozzle outside diameter and the casting speed increases the inclusion removal ratio in the sump. But the infleunces of these latter changes are not very strong, so that inclusion removal consideration need not influence the design strategies used for the casting speed and nozzle outside diameter. The SEN port angle has a little effect on the inclusion removal when using deep SEN submerged depth. Although argon stream introduced into the tundish nozzle stream can protect the nozzle blockage, it is not beneficial to the inclusion removal in the sump.

A room temperature water model of a tundish was design, constructed and operated. The model was equipped with a steam heating system that simulates that simulates the tundish plasma heating systems operated by some of the more modem continuous casting plants. Similarity between steam heating in the water model and plasma heating in the tundish has been established. A dimensionless criterion was developed to validate the simulation experiments and its represented by the plasma heating number. Using this similarity criterion plasma heating can be simulated by steam heating in an appropriately designed water model. A theoretical dispersion model has been formulated for the flow through the tundish and the parameters in this model determined from the results obtained from residence time distribution measurements. A conductivity method was used, a highly conducting species being injected at the inlet point and changes in conductivity monitored at the exit. Measurements were also made of the changes in temperature at the exit resulting both from changes in temperature of the inlet stream and from the use of steam heater system. A stable inverse heat conduction method has been developed in which the measured and estimated temperature are analysed in terms of a steady components of short duration. A finite difference method has been used to predict the effect on a thermocouple temperature of the deviatory components of the liquid steel temperature. The incorporation of these predictions into look-up tables has allowed an algorithm to be developed thet can deduce the current deviatory component of the steel temperature from the thermocouple response.

The hydration of calcium oxide and its control by superficial recarbonisation have been investigated by suspending a single pellet into an atmosphere of controlled humidity or carbon dioxide content for hydration and recarbonisation respectively. The reaction rate was monitored using a computer aided gravimetric method. The computerised system allowed the simultaneous measurement of the temperature and weight of a reacting pellet. It has been shown that the hydration reaction takes place on a sharp definite interface between the product layer and the unreacted core. A theoretical model for the hydration rate has been derived on the basis of the unreacted core model by introducing the effect due to the sample swelling. The model was solved and compared to experimental results data. Quite reasonable values were obtained for the chemical reaction rate constant and the effective diffusion coefficient through the product shell. Results suggested that gas phase mass transfer, porediffusion, and chemical reaction were all contributing resistances in the model, that is, the process was under mixed control. On the other hand, the results obtained clearly indicate that the recarbonated film increases the hydration resistance of the lime core.

Please read the abstract in the section 00front of this document / Dissertation (M Eng (Electronic Engineering))--University of Pretoria, 2007. / Electrical, Electronic and Computer Engineering / unrestricted

Undesirable solid oxides and sulphides, known as inclusions, are an inherent product of the steelmaking process that can clog the steel flow during casting. Caster operators can break up the clogged material to improve flow, but this reduces steel quality and yield. Modifying these inclusions using calcium treatment and its effects on industrial processing conditions are the focus of this study. To effectively improve calcium treatment, experimental and industrial data are required to develop models to predict inclusion behaviour. The advantage of experimental testing is that the precise control of chemistry, temperature, alloying elements, and sampling can help identify the effects of each variable. The difficulty, however, is correlating experimental results to real-time industrial processing, where these variables are difficult to control individually. In industry, indirect observations of caster behaviour may give an idea of the effectiveness of the treatment, but this is after the fact. In this study, liquid steel samples were taken at different processing steps without any modification to the process itself to ensure normal shop conditions. The inclusion analysis was conducted using an automated Scanning Electron Microscope (SEM) and AZtech feature analysis software to determine inclusion characteristics, including population, composition, and size. This study shows 8 sequential calcium-treated steel heats in an industrial setting and their associated clogging behaviour during a cast. The study indicated that a few heats with undermodified alumina inclusions immediately or subsequently affected the casting process leading to a severe clogging event. / Thesis / Master of Applied Science (MASc)

This study involves the experimental and modelling work of high temperature reactions pertaining to steelmaking. / Certain non-metallic inclusions are known to cause deleterious effects in steel products and affect the production efficiency of steelmaking if not controlled. Most of these are oxide inclusions formed during deoxidation in refining processes, especially in the ladle metallurgy furnace (LMF), an understanding of which is essential for process control. The composition of these inclusions changes subsequently while interacting with other phases like slag, alloy additions, and refractories. The efficient removal and composition control of inclusions are important variables to consider for any steelmaker. Moreover, desulphurisation of steel is another aspect that needs attention since excess sulphur can precipitate sulfide inclusions while cooling. Hence, tracking the composition evolution of different phases in a ladle furnace is beneficial for the steelmaking industry. Previous researchers in the authors’ laboratory developed a model that could be used to predict the kinetics of steel-slag-inclusion reactions in aluminium (Al) killed steel. The current work focusses on developing a kinetic model that can be used to describe the inclusion evolution during ladle treatment of silicon-manganese (Si-Mn) killed steel. For this, first, the formation of complex oxides in Si-Mn-killed steel was analyzed using a mathematical model of nucleation and growth of particles in melts. The results revealed that spontaneous nucleation of complex oxides occur during alloy additions, resulting in different compositions of oxide nuclei, based on the local supersaturation conditions. Sensitivity analysis with different parameters was carried out to understand the influence of physicochemical variables on the model. Following this, two kinetic models were built: 1) average inclusion composition tracking method; and 2) multi oxide inclusion (MOI) composition tracking method. The latter approach included the thermodynamics and kinetics of each inclusion formation and could further incorporate the total inclusion number density variation in steel. The MOI model can be used to predict the changes in not only the average inclusion composition but also the type of inclusions precipitating in steel. Following this, laboratory deoxidation experiments were carried out using FeSi and FeMn to understand the inclusion behaviour post alloy additions. The MOI model showed good potential in simulating these laboratory deoxidation experiments and was then coupled with a previously developed steel-slag model to simulate actual ladle refining reactions. The calculated results were compared with different industrial data and showed good agreement with what is observed in reality, showing the success of this new approach. Similar to previous investigations, the rate-determining step could be attributed to the availability of solutes in steel (from slag or alloys) along with their transport to the steel-inclusion interface. The overall model was also able to simulate the desulphurization behaviour in steel. The effects of different processing conditions such as [Al], [O] content, reoxidation, and stirring conditions, were also examined and discussed through a parametric analysis. / Dissertation / Candidate in Philosophy