The viscosities of a set of silicone oils containing different size ranges of charcoal or paraffin particles as well as the viscosities of silicone oil foams were measured at room temperature in order to determine the effect of dispersed phase on the viscosity of a liquid and its effect on foaming ability. The effective viscosity of the samples increased with volume fraction of the second phase. The foaming ability was improved by the presence of the particles. The improved foaming effect was for the most part not a result of the increased viscosity. No connection between the particle size and the effective viscosity could be determined. On the other hand particle morphology and the particle size distribution had effect on the effective viscosity. The viscosity data were compared with a number of existing equations for the estimation of effective viscosity. Einstein-Roscoe equation is suitable for two-phase mixtures containing globular particles with narrow particle size distribution and low interfacial tension. New mathematical models are required for effective viscosity prediction, where the suspending phase viscosity, effect of the interfacial tension, as well as the particle morphology should be taken in consideration.

The present work deals with some aspects of slags related to secondary metallurgy in the steelmaking process. More specifically the focus is given to sulfur and nitrogen in ladle slags. Even though slags have been fairly well-researched in the past, the available data for these elements in typical ladle slag compositions is rather scarce. In some cases the available data is in discordance. There are also inconsistencies between the literature data and what is commonly observed in the industrial processes.

Sulfide capacities were measured at steelmaking temperatures, 1823–1873 K, in ladle slags. The data was found to be in reasonable agreement with the industrial process norms. The sulfide capacity was found to increase with the basic oxides CaO and MgO; and decrease with the acidic components Al₂O₃ and SiO₂. The sulfide capacity was also found to increase with temperature.

The dependence of sulfide capacity on the oxygen partial pressure, for slags containing multivalent elements, was investigated experimentally at 1873 K with a slag containing vanadium oxide. A strong dependence of oxygen partial pressure was observed. The sulfide capacity increase by more than two orders of magnitude when the oxygen partial pressure was increased from 4.6×10^-16 atm to 9.7×10^-10 atm.

The nitrogen solubility and the effect of carbon was investigated in typical ladle slags and the CaO–MgO–SiO₂ system at 1873 K. Carbon increases the nitrogen solubility substantially. In the absence of carbon, the nitrogen solubility is extremely low. Low concentrations of cyanide was detected in the carbon saturated slag. This was much lower than the total nitrogen content and formation of cyanide cannot explain the large increase.

The possibility of removing sulfur with oxidation from used ladle slag was investigated experimentally at 1373–1673 K. The sulfur removal of mostly solid slag was found to be a slow process, and would not suitable for industrial practice. At 1673 K the slag was mostly liquid and more than 85% of the sulfur was removed after 60 min of oxidation in pure oxygen atmosphere.

Experiments were conducted to investigate the possibility of removing sulfur from used ladle slag by oxidation. Slag samples (solid, two-phase mixture, and liquid with a small fraction of solid MgO particles) were subjected to an oxygen-rich atmosphere in the temperature range 1373–1673 K. The sulfur removal from the samples of solid and two-phase mixture was found to be a slow process due to the slow diffusion. The sulfur removal was found to have little dependence on temperature in the range 1373–1573 K. When the slag was mostly liquid (at 1673 K), the sulfur removal was significantly increased. More than 85 % of the sulfur could be removed after 60 min of oxidation in pure oxygen. An increase in oxygen partial pressure was found to increase the desulfurization slightly. Increasing the Al2O3 content in the slag decreased the degree of sulfur removal.

An investigation of the nitrogen pickup of liquid steel from ladle slag after vacuum degassing was made. Nitride capacities, C-N, of a number of ladle slags were determined at controlled nitrogen and oxygen potentials at 1873K. The nitride capacities in the composition range studied were found to be very low. In accordance with the literature, the nitride capacity was found to increase with increasing SiO2 content. Industrial trials were performed. The nitrogen content of the steel was determined before and after vacuum degassing as well as after the waiting period. Three different trends of the variation of nitrogen content in the steel were observed. Both the laboratory study and the industrial trials revealed that the transfer of nitrogen from slag to steel was not the reason for nitrogen pickup in the steel subsequent to vacuum degassing.

The solubility of calcium in liquid iron at 1823 K and 1873 K (1550 A degrees C and 1600 A degrees C) as a function of calcium potential was studied experimentally. The measurements were performed using a closed molybdenum holder in which liquid calcium and liquid iron were held at different temperatures. The results indicate a linear relationship between the activity of calcium, relative to pure liquid calcium, and the mole fraction of dissolved calcium in liquid iron, with a negligible temperature dependency in the ranges studied. The activity coefficient of calcium in liquid iron at infinite dilution, was calculated as 1551.

The main focus of the present work was to develop a sampler to obtain the physical description of the slag-metal interface in the presence of bulk flow. Industrial trial experiments were carried out in the ladles at Uddeholm Tooling, Hagfors, Sweden using the designed sampler. Samples of the slag-metal interface were successfully taken in both gas stirring and induction stirring modes. The similarities of the appearance as well as the micrographs of the samples suggested that the slag-metal interfaces in the two stirring modes were very similar except in the open eye area. In the case of both stirring modes, metal droplets were found in the slag bulk. On the other hand, no appreciable amount of slag was detected in the metal bulk. Further systematic investigation using a large number of samples is required to obtain a quantitative description of the interface.

In the present work, the deoxidation of liquid steel with aluminum wire injection in a gas-stirred ladle was studied by mathematical modeling using a computational fluid dynamics (CFD) approach. This was complemented by an industrial trial study conducted at Uddeholm Tooling AB (Hagfors, Sweden). The results of the industrial trials were found to be in accordance with the results of the model calculation. In order to study the aspect of nucleation of alumina, emphasis was given to the initial period of deoxidation, when aluminum wire was injected into the bath. The concentration distributions of aluminum and oxygen were calculated both by considering and not considering the chemical reaction. Both calculations revealed that the driving force for the nucleation of Al2O3 was very high in the region near the upper surface of the bath and close to the wire injection. The estimated nucleation rate in the vicinity of the aluminum wire injection point was much higher than the recommended value for spontaneously homogeneous nucleation, 10(3) nuclei/(cm(3)/s). The results of the model calculation also showed that the alumina nuclei generated at the vicinity of the wire injection point are transported to other regions by the flow.

The present work aimed at determining the major source for hydrogen pick-up of the steel after vacuum degassing but before casting. Samples of slag and metal were taken at different stages during ladle treatment at SSAB. Hydrogen increase after vacuum treatment was observed. Moisture contents of the industrial slags were analysed and their water capacities were calculated. It could be seen that the hydrogen increase was correlated to the amount of moisture in the slag and the water capacity. The study showed that the slag containing most water was also the heat having the largest hydrogen increase. The slag with most water had the highest water capacity. It could be concluded that the major source for hydrogen increase after degassing was due to the slag-metal reaction.

The aim of the present work was to clarify the mechanisms of hydrogen removal during vacuum degassing. The main reason for this was because the primary source of hydrogen pick-up in steel-making is the moisture in the furnace atmosphere and the raw material charged into the ladle furnace. Previous studies showed that the presence of hydroxyl ions in the ladle slag results in hydrogen transfer from the slag back into the steel bath. The main focus of this thesis was therefore to gain deeper knowledge of the ladle slag and its properties. For this purpose a number of slag compositions were examined in order to clarify whether these slags were single liquids at 1858 K. 14 out of 27 compositions in the Al₂O₃ CaO MgO SiO₂ system was completely melted. These results were in disagreement with the existing phase diagrams.

Water solubility measurements were carried out by employing a thermo gravimetric technique. The temperature was found to have negligible effect on the water solubilities. The experimental results showed that the water capacity values varied between 1x10³ and 2x10³ in the majority of the composition range. However, for compositions close to CaO saturation the water capacity value could reach higher than 3x10³. The experimental determined water capacity was further used to develop a water capacity model for the quaternary slag system Al₂O₃ CaO MgO SiO₂. The model was constructed by considering the affects of the binary interactions between the cations in the slag on the capacity of capturing hydroxyl ions. The model calculations agreed well with the experimental results as well as with the literature data.

An attempt was made to develop a preliminary process model for dehydrogenation by using the results from CFD calculation. For this purpose industrial sampling was made during vacuum treatment. The hydrogen concentrations decreases fast in the initial stages of the degassing, but is slowed down in the final stage. The model calculations fit the initial stage of the dehydrogenation process well. In the final stage of the process the predicted values are somewhat lower than the plant data. The results from the model prediction showed that a dynamic process model could be satisfactorily constructed using the results from CFD calculation.

The present work aimed at determining how big impact hydroxyl ions in the slag have on the final hydrogen concentration in the liquid steel. It was found that the effect is of less importance regarding the final concentration of the metal after the degassing treatment.

A thermo-gravimetric technique was used to determine the hydrogen solubilities of some Al₂O₃-CaO-MgO-SiO₂ quaternary slags. The focus of the work was to determine the water capacities in slags having lower SiO₂ concentrations, which were relevant to industrial practices. The majority of the experiments were carried out in the temperature interval 1747 to 1827 K with water pressure of 157 mbar. The reliability of the experiments was confirmed using a quenching technique. The temperature was found to have a negligible effect on the water solubilities. The composition of the slag did not seem to significantly affect the water capacity. However, as the slag composition approached CaO saturation, a considerable increase of the solubility was noticed.

The focus of the present work was to develop a water capacity model for the quaternary slag system Al2O3-CaO-MgO-SiO2. In the model, a silicate melt was considered to consist of two ion groupings, viz. cation grouping and oxygen ion. The water capacity of a melt is supposed to depend on the interactions between the cations in the presence of oxygen ions. These interactions were determined on the basis of the experimentally measured water solubility data. Only binary interactions were employed in the model. For the system CaO-SiO2, disagreement in the literature data was found. Since the interaction between Ca2+ and Si4+ would play an important role, experiments were carried out to determine the water capacities of some CaO-SiO2 slags. For this purpose a thermogravimetric method was employed. Iso-lines of water capacities at constant MgO contents were predicted by the model and compared with the experimental data from literature. The model calculations agreed well with the experimental results.

Sulfide capacity measurements of slag with MgO content up to 18 mass% were carried out at 1713, 1743 and 1773 K to obtain reliable data for the blast furnace process.  In the measurement, slag is equilibrated with copper at controlled oxygen partial pressure for 24h. The sulfide capacities are calculated based on the sulfur analyses for both slag and copper. 

A number of slag compositions in the Al₂O ₃-CaO-MgO-SiO₂ quaternary system was investigated. The Al₂O₃, CaO, and SiO₂ oxide powders were calcinated at 1073 K for 24 hours before being mixed in an agate mortar. The oxide mixtures were further pressed into small pellets and sealed in a glass bottle. A horizontal platinum furnace and platinum crucibles were used and each slag composition was investigated four times to confirm the reproductibility of the process. Efforts were made to carry out the experiments with identical quenching speed. The slags were quenched from 1793 K to examine the liquidity of the slag samples at lower temperatures. It was revealed that the quenched samples were multi-phase mixtures instead of melting at the experimental temperature under the influence of the phase diagrams.

In order to gain an insight into the mixing behaviour of slag and metal under different stirring conditions, samples of slag/metal interface were collected from an industrial 65 ton ladle furnace using a new sampling technique. Different stirring conditions and sampling positions in the ladle were chosen. Complementary cold model experiments were also performed. The experimental results strongly suggested that the shear force between the two liquids played an important role in slag-metal mixing. Evidence for the existence of slag droplets each surrounded by a very thin metal film was found in the samples taken under both gas stirred and induction stirred conditions. This observation strongly suggested that slag-metal system behaved similarly as oil-water system, wherein, the shear force owing to the bulk flow led to a packed 'sphere bed' of oil droplets each coated by a thin water film above the water bath. On the basis of the experimental results, preliminary mathematical models were developed to describe the rate of mass exchange at the slag/metal interface.

In the present study, the dissolution mechanism and rate of lime, limestone and dolomite in converter slag was studied. Lime dissolution in stagnant slag was studied first and dissolution of lime, limestone and dolomite under forced convection were carried out by new experimental setup.

Dissolution of different CaO samples into stagnant converter slags was carried out in a closed tube furnace at 1873K. In the case of CaO-‘FeO’-SiO₂ slag, the dissolution of CaO rod in the stagnant slag was retarded after the initial period (2 minutes). A dense layer of 2CaO∙SiO₂ was found to be responsible for the total stop of the dissolution. It could be concluded that constant removal of the 2CaO∙SiO₂ layer would be of essence to obtain high dissolution rate of lime. In this connection, it was found necessary to study the dissolution of lime in moving slag.

In order to obtain reliable information of lime dissolution under forced convection, the commonly used rotating rod method was examined. Both CFD calculation and cold model experiments showed evidently that the mass transfer due to radial velocity introduced by forced convection was zero if the rod was centrally placed in a cylindrical container. A new experimental design was therefore developed. A cube was placed in the crucible and stirred by Mo rod along with slag. The whole system could be quenched in order to maintain the state of the system at high temperature. A linear relationship between normalized length and time was obtained for lime dissolution. Different lime samples showed big difference in dissolution rate. It was found that the main mechanism of CaO dissolution in slag was due to the removal of 2CaO∙SiO₂ layer.

Decomposition and dissolution of limestone and dolomite in slag at 1873 K were studied. The decomposition was carried out both in argon and in slag under argon atmosphere. The decomposition process was simulated using Comsol. The results showed evidently that the decomposition of limestone and dolomite was controlled mostly by heat transfer.

It was also found that the decomposition of limestone product: CaO had very dense structure, no matter the sample was decomposed in slag or in argon. The slow decomposition and the dense CaO layer would greatly hinder the dissolution of lime in the slag. The present results clearly indicate that addition of limestone instead of lime would not be beneficial in converter process.

Discontinuous 2CaO∙SiO₂ layer along with MgO∙Fe₂O₃ particles was found on the surface of the dolomite sample. Some 2CaO∙SiO₂ islands were found in the vicinity of the sample in the slag, which revealed therefore that the dissolution was dominated by the peeling-off of the layer of 2CaO∙SiO₂-MgO∙Fe₂O₃ mixture. 2CaO∙SiO₂, (Mg, Fe)O_ss along with super cooled liquid phases were found inside dolomite sample close to the surface. 2CaO∙SiO₂ phase was replaced gradually by 3CaO∙SiO₂ towards the centre of the decomposed sample.

The mechanism of dolomite dissolution in converter slag was studied and found to proceed by two steps: decomposition of dolomite into CaO and MgO and dissolution of the product into the slag. The first step was found to be controlled by heat transfer, which was confirmed by theoretical calculation. The decomposed dolomite had many pores, which allowed easy slag penetration. A discontinuous 2CaO.SiO2 layer along with MgO.Fe2O3 particles was found on the surface of the samples. Some 2CaO.SiO2 islands were found in the vicinity of the samples in the slag, revealing that the dissolution was dominated by peeling-off of the layer of 2CaO.SiO2- MgO.Fe2O3 mixture. 2CaO.SiO2, (Mg, Fe)Oss along with super cooled liquid phases were found inside dolomite samples close to the surface. 2CaO.SiO2 phase was replaced gradually by 3CaO.SiO2 towards the centre of the decomposed sample. While addition of small amounts of dolomite directly into the slag could be considered in steelmaking converter, too much would result in slow dissolution and waste of the raw materials.

Dissolution of different CaO cubes under force convection in liquid CaO-"FeO"-SiO2 slag was studied at 1873 K (1600 A degrees C). A linear relationship between normalized lengths and time was obtained after the experiment. It was evidently observed that the removal of the interface layer(s) including 2CaO center dot SiO2 by shear stress was the main mechanism for the dissolution. A stirring rate of approximately 100 rpm was found to be efficient to remove the interface layer(s). The limes with different structures had different dissolution rates.

The applicability of rotating rod technique in the study of lime dissolution in slag was investigated. Both computational fluid dynamic (CFD) and cold model experiments showed that the mass transfer due to radial velocity introduced by forced convection was zero if the rod was long. The mass transfer by forced convection was also less important in comparison with natural convection and diffusion when the rod was half length of the height of the bath. This finding was in accordance with the criteria put forward by the original work that the method could only be applicable when a thin disk (instead of rod) with big diameter and big liquid bath were used. To study the lime dissolution by forced convection a new experimental technique was developed. A cube was placed in the slag that was eccentrically stirred. The whole system, viz. the sample along with the slag could be quenched. The new technique could study the effect of forced convection on the dissolution. The microscopic study on the quenched slag-lime samples could reveal the dissolution mechanism successfully.

Dissolution of different CaO samples into molten synthetic 'FeO'-SiO2 and 'FeO'-SiO2-CaO slags was carried out in a closed tube furnace at 1873K. The slag was kept stagnant. It was found that the dissolution rate was very fast when CaO rod was dipped into 'FeO'-SiO2 slag. In the case of 'FeO'-SiO2-CaO slag, the dissolution of CaO rod in the stagnant slag was retarded after the initial period (2 minutes). Only less than 16 percent CaO reacted with the slag, irrespective of the type of lime. Three phase-regions were identified in the reacted part of the lime rod by SEM-EDS analysis. The formation of these regions was explained thermodynamically. A dense layer of 2CaO center dot SiO2 was found to be responsible for the total stop of the dissolution. It could be concluded that constant removal of the 2CaO center dot SiO2 layer would be of essence to obtain a high dissolution rate of lime. In this connection, it was found necessary to study the dissolution of lime in moving slag to reach a reliable conclusion regarding the relevance of the reactivity obtained by water ATSM test to the real reactivity of lime in high temperature slag.

Decomposition and dissolution of limestone in slag at 1873 K (1600 A degrees C) were studied. The limestone samples were in the shape of cubes (11 mm x 11 mm x 11 mm approximately). The decomposition was carried out both in argon and in slag under argon atmosphere. In order to gain an insight into the phenomenon of slow decomposition, the decomposition process of CaCO3 was simulated using Comsol. The results showed evidently that the decomposition of calcium carbonate was controlled mostly by heat transfer. It was also found that the decomposition product CaO had very dense structure, whether the sample was decomposed in slag or in argon. The slow decomposition and the dense CaO layer would greatly hinder the dissolution of lime in the slag. The present results clearly indicate that the addition of limestone instead of lime would not be beneficial in the converter process.

The present thesis focuses on the interaction between refractory and liquid steel. The aim of this work is to understand the interaction behavior between refractory and liquid steel regarding steel cleanliness. The effect of different refractories on different inclusions in Al-killed steel was studied in a furnace. The sintering mechanism of filler sand were also investigated in laboratory. In the industrial trials, the attachments of different oxides on the walls of submerged entry nozzle (SEN) were discussed in the cases of high strength low alloy steel (HSLA) and ultra-low carbon steel (ULC).

It is found that the effect of alumina and spinel refractory on all the three types of inclusions is very little, while MgO refractory influences the inclusions depending on the activity of dissolved oxygen in liquid steel. At low oxygen level, alumina inclusions could transform into spinel inclusions with the help of MgO refractory, while the effect on spinel and calcium aluminate inclusions is not evident. On the other hand, when the activity of dissolved oxygen is high enough, the evolution of spinel inclusions from alumina inclusions could not be seen.

The reaction between chromite and silica grains leading to liquid formation is the main mechanism for the sintering of filler sand. The factors viz. steel composition, silica size and content, operation temperature and process holding time have a strong influence on the sintering of the filler sand. Smaller size and higher content of silica in sand, steel grades containing higher Mn and Al contents, higher temperature and longer holding time would result in serious sintering. The choice of the sand needs to take those factors into account.

The results show that solid alumina particles are always agglomerated on the inner wall of SEN in the case of ULC steel. The top slag with high FeO and MnO contents is considered as the main reason of this kind of attachments. The removal of slag might be a good method to avoid the attachments. In the case of HSLA steel, liquid calcium aluminate inclusions could attach on the inner wall of SEN as well. The smoothness of the inner wall of the SEN holds the key of liquid attachments. In addition, the attachment situation on the outer wall of SEN depends on the operations. The oxygen entrainment through the mold powder would result in the formation of plate-like alumina attachments. The control of reoxidation due to oxygen entrainment would help to avoid this situation.

In the present work the effects of temperature and holding time on the sintering of ladle filler sand are studied. Laboratory experiments are carried out using pellets made of chromite based filler sand and two steel grades containing different contents of Mn and Al. It is found that the liquid steel plays a major role in the sintering behavior. The results also show that the amount of liquid phase in the sintered sand pellets increases with the increase of temperature and holding time. The Al₂O₃ content increases substantially in the chromite phase (spinel), especially in the region close to the liquid phase, when the temperature is high enough or when the holding time is long enough. Higher content of dissolved Al would accelerate the formation of the alumina-rich chromite.

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.

In the present study the foaming of CaO-SiO2-FeO slags is investigated. The foaming height induced by the reaction between slag and hot metal were carried out. It was found that the foaming height increases with increased 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.

Slag is of great importance for dephosphorisation in converter steelmaking. In the present study, slag samples were equilibrated with copper at 1923 K to study their capacities in capturing phosphorous. The phosphate capacities of the slags were calculated. Some of the samples were liquid-solid mixtures. The solid phases in these samples were identified by SEM analysis, and the identified phases were found to agree well with Thermocalc calculations. On the other hand, the fractions of the solid phase did not agree with the Thermocalc calculation. Phosphorous distributions in the different phases were examined, and the phosphate capacities of the samples were evaluated. The MgO content did not show any appreciable impact on the phosphate capacity. The activities of FeO in the pure liquid slag samples were calculated and found to deviate positively from ideality.

A new approach was made to model the dephosphorisation process in a 300 tons basic oxygen furnace converter with three argon gas inlets. The main feature of the new approach was to utilise the velocity vectors obtained by computational fluid dynamics (CFD) simulation in a standalone model. The CFD simulation was carried out using commercial software COMSOL Multiphysics. In the standalone model, the steel melt domain was sliced into 1000 cells. The calculated velocity vector in each cell was assumed constant. Based on the imported velocity vectors from the CFD calculation, the mass transfer of carbon and phosphorus was calculated by 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 the 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. As a preliminary step, the model only simulated the process after the formation of slag-metal-gas emulsion. Note that the present work is intended to establish a structure of the model. More precise descriptions of other process aspects need to be included before the model can be practically employed in a dynamic controlling system.

To study the penetration depth in the case of a gas jet impinging on the surface of liquid steel, cold model experiments were carried out using a liquid alloy GaInSn, which had similar physical properties as liquid steel. A HCl solution was used to simulate the top slag. The top phase was found to have appreciable effect on the penetration depth. Comparison of the experimental data with the predictions of the existing models indicated that most the model predictions deviated from the experimental results at higher lance heights and gas flow rates. New model parameter was suggested based on the present experimental data. The observation of the formation and movement of metal droplets generated by the gas jet was also made. The velocity of the droplet was found to be at a level only about 1% of the terminal velocity. This low velocity suggested that the turbulent viscosity played important role and the droplets could have long resident time in the slag.

In the present study, the effect of gas flow rate on homogenization and inclusion removal in a gas stirred ladle was investigated. Both industrial trials and cold model experiments were conducted. CFD calculation was also carried out as an auxiliary tool. The mixing times predicted by CFD simulation agreed well with both the model experiments and industrial data. 99% mixing could be achieved in about 2-3 minutes. The increase of flow rate of inert gas would not improve the mixing substantially, while the mixing time decreased somewhat with the increase of gas flow rate. The water model study showed also that the gas flow rate had a negligible effect on the rate of inclusion removal. Both the experiments and CFD calculation strongly suggested that a low gas flow rate should be applied in the ladle treatment.

A two dimensional axisymmetric model was developed to predict the heat flux in a steelmaking ladle during the teeming process. The model predicts dynamically the flow fields in both liquid phase and gas phase along with the movement of the liquid upper surface. The model also predicts the temperature distributions in the liquid metal, gas phase and all layers in the ladle wall. Industrial measurements using infrared radiation camera inside the ladle after teeming and at the wall outside the ladle during the whole process were carried out. The model predictions were found to be in agreement with the measured data. It was found that the heat transfer to the surrounding atmosphere and the conductivity of the highly insulating layer were the most important factors for the heat loss. The decrease of the thickness of the working lining was found to have limited effect on the total heat flux.

In the present thesis a study on the thermal state of steelmaking ladles was undertaken. The transient hot wire method was verified for thermal conductivity measurements on metallurgical slags and applied to ladle slag measurements. Temperature measurements on ladles in an industrial environment were carried out. The emissivities of the outer and inner shells of steelmaking ladles were investigated. Two dynamic models were developed to predict the heat transfer and fluid flow in a preheating and teeming ladle. The gathered thermal conductivity values for ladle slag were used to study the effect of the slag layer on the top surface of the melt on heat transfer and fluid flow in a teeming ladle.

In the first stage, the transient hot-wire method was verified to measure the thermal conductivity of metallurgical slags at steelmaking temperatures. A numerical model was developed, cold model experiments were conducted and test measurements using a high temperature experimental setup were carried out. To minimize natural convection and to obtain more reliable measurements, the crucible diameter, the hot-wire diameter, the applied current, the position of the wire in the crucible and the cooling on the upper surface of the crucible were studied. Investigations into the choice of sheathing material of the circuit exposed to the slag were also made. It was found that only certain materials were suitable for slag measurements depending on slag composition and temperature. The electrical resistivity of the hot wire was measured to make the thermal conductivity calculation more reliable. The wire diameter also played a major role due to the heat generation per surface area. The thermal conductivity should be derived from the values measured during the first seconds. In this initial stage, the effect of the natural convection as a function of the wire position in the crucible, the cooling on the top surface and the diameter of the crucible are negligible. A compromise has to be made in choosing the electrical current, since higher current results in higher sensitivity but at the same time in more natural convection.

In the second stage, the thermal conductivities of four different ladle slags were measured at 1773 K, 1823 K, 1873 K and 1923 K using the transient hot wire method. Very good reproducibility was obtained. The thermal conductivity did not vary substantially with the variation of slag composition at 1873 K and 1923 K, at which the slag samples were all entirely liquid. The thermal conductivities were low. It was found that the precipitation of solid phase resulted in a considerable increase of thermal conductivity.

In the third stage, a two dimensional model was developed in order to predict the temperature distribution in the ladle wall during the preheating process. The model calculated the heat transfer and the velocity field in the gas phase inside the ladle as well as the heat transfer in the solid walls during the preheating process. Measurements of the temperature profiles in an industrial ladle were carried out using an infrared thermography. The measurements were made both inside and outside the ladle. The model predictions were found to be in reasonably good agreement with the measured temperatures. It was found that the preheating time could be minimized when the working lining became thinner. The effect ofthe distance between the lid and the ladle was also studied by the model. The results indicated that there was no significant temperature change on the upper side wall of the ladle. On the lower side wall and bottom the temperature changed slightly. The temperature difference in the lower part of the ladle could be explained by the larger flame distance from the bottom layer.

In the fourth stage, a two dimensional axisymmetric model was developed to predict the heat flux in a steelmaking ladle during the teeming process. The model predicts dynamically the flow fields in both the liquid phase and the gas phase along with the movement of the liquid upper surface. The model also predicts the temperature distributions in the liquid metal, gas phase and all layers in the ladle wall. Again, industrial measurements were performed using an infrared thermography, both inside the ladle after teeming and at the wall outside the ladle during the whole process sequence. The model predictions were found to be in agreement with the measured data. It was found that the heat transfer to the surrounding atmosphere and the conductivity of the highly insulating layer were the most important factors for the heat loss. The decrease of the thickness of the working lining was found to have limited effect on the total heat flux.

In the fifth and final stage, the effect of the slag layer on the top surface of the melt, on fluid flow and on heat transfer in a teeming ladle was investigated theoretically. The two dimensional axisymmetric model developed in the fourth stage was used. To predict the effect of the slag layer a stationary heat conduction boundary condition including thermal conductivity and slag layer thickness was employed. Different calculations with differing thermal conductivity values for the slag layer were carried out. The calculations showed that the effect of the slag layer was insignificant. This could be explained by the similarity of the thermal conductivity of slag and gas phase.

To predict the temperature distribution in the ladle wall during the preheating process a two dimensional model was developed. The model calculated the heat transfer and the velocity field in the gas phase inside the ladle as well as the heat transfer in the solid walls during the preheating process. Measurements of the temperature in an industrial lade were carried out using an infrared radiation (IR) camera. The measurements were made inside and outside the ladle. The model predictions were found to be in reasonably good agreement with the measured temperatures. It was found that the preheating time could be minimized when the working lining became thinner. The effect of the distance between the lid and the ladle was also studied by the model. The results indicated that there was no significant temperature change on the upper side wall of the ladle. On the lower side wall and bottom the temperature changed slightly. The temperature difference in the lower part of the ladle could be explained by the larger flame distance from the bottom layer.

In order to apply the hot wire method for metallurgical slags at steelmaking temperatures, a numerical model was developed, cold model experiments were conducted and test measurements using a high temperature experimental setup were carried out. To minimize natural convection and obtain more reliable measurements, the crucible diameter, the hot-wire diameter, the applied current, the position of the wire in the crucible, and the cooling on the upper surface of the crucible were studied. Investigations into the choice of sheathing material of the circuit exposed to the slag were also made. It was found that only certain materials were suitable for slag measurements depending on slag composition and temperature. The electrical resistivity of the hot wire was measured to make the thermal conductivity calculation more reliable. The wire diameter also played a major role, because of the heat generation per surface area. The thermal conductivity should be derived from the values measured during the first seconds. In this initial stage, the effect of the natural convection as a function of the wire position in the crucible, the cooling on the top surface, and the diameter of the crucible are negligible. A compromise has to be made in choosing the electrical current, since higher current results in higher sensitivity but at the same time in more natural convection.

Thermal conductivities of four different ladle slags were measured at 1773 K, 1823 K, 1873 K, and 1923 K (1500 A degrees C, 1550 A degrees C, 1600 A degrees C, and 1650 A degrees C) using the transient hot wire method. Very good reproducibility was obtained. The thermal conductivity did not vary substantially with the variation of slag composition at 1873 K and 1923 K (1600 A degrees C and 1650 A degrees C), at which the slags were all entirely liquid. The thermal conductivities were low. It was found that the precipitation of solid phase resulted in considerable increase of thermal conductivity.

The present thesis deals with some aspects concerning secondary metallurgy of steel where there is little or very inconsistent information in the literature. More specifically, it is devoted to the studies on high temperature phase equilibria in the Al2O3-CaO-MgO-SiO2 system, the formation of ladle glaze and the thermodynamics of magnesium in liquid iron.

First, the solidification of different slags on MgO based refractories was studied in order to reveal the mechanism behind the formation of “ladle glaze”. The formation of the slag glaze layer was studied by dipping MgO rods, dense or porous, into liquid slags at 1873 K. The rods were thereafter cooled at a predetermined rate. From a later SEM-EDS microscopy, it was found that the initial slag composition had the most profound effect on the phases found in the solidified slag layer. It was found that the type of MgO rod used and cooling speed had a minor impact on the morphology on the solidified samples. In addition, the slags used in the study were equilibrated at 1773 K, 1673 K and 1573 K in order to get an understanding of the equilibrium phases and their relationship during cooling. On the basis of the experimental results, the mechanism regarding entrainment of exogenous inclusions from the refractory lining was also discussed.

Secondly, phase diagram studies in the high basicity region of the Al2O3-CaO-MgO-SiO2 system were performed using the quench technique followed by EPMA analysis. The main focus in the study was to find the liquidus surfaces for MgO and CaO saturation at 1773 and 1873 K. Based on the experimental data, phase diagrams for the 25, 30 and 35 mass percent alumina sections were constructed for silica contents generally less than 20 mass percent.. The results generally agreed very well with previous, well established phase diagrams. In addition, the activities of MgO, CaO and Al2O3 were estimated using the phase diagram information.

At last, the thermodynamics of magnesium in liquid iron at 1823 K were studied. In a pre-study, the thermodynamics of Ag-Mg solutions were studied, necessary for the Fe-Mg system. For the Ag-Mg system, two different experimental techniques were used; the vapor pressure method and the gas equilibration technique. The temperature range of the Ag-Mg study was 1573 to 1823 K. It was found that the excess Gibbs energy of this system can be described quite well with a sub-regular solution model. In the Fe-Mg study, the partition of Mg between liquid iron and liquid silver were studied at 1823 K. Using the results from the pre-study, the activity coefficient of Mg in liquid iron and the self-interaction parameter were determined at 1823 K.

The thermodynamics of magnesium in liquid iron was determined at 1823 K (1550 degrees C). For this purpose, liquid iron was equilibrated with Ag-Mg alloys in a semienclosed molybdenum vessel. From the partition of magnesium between iron and silver, the activity coefficient of Mg and the self-interaction parameter e(Mg)(Mg) were determined.

Activity measurements of magnesium in liquid Ag-Mg solutions were carried out at 1573, 1673. 1773 and 1823 K using two different techniques. While most measurements were carried out using the vapor pressure method at 1573 and 1673 K, a number of measurements were made at 1773 and 1873 K using the gas equilibration technique. The latter method was used mostly for obtaining Ag-Mg solution at very low Mg contents. It was found that magnesium had a negative deviation from ideality in Ag-Mg solutions. The excess Gibbs energy of Ag-Mg solutions seemed to have a quite weak temperature dependency in the temperature region studied. The excess Gibbs energy of Ag-Mg solution can be described as;

The formation of a slag glaze layer on dense and porous MgO rods was studied by dipping MgO rods into liquid slag at 1873 K and thereafter cooling the rods at a predetermined cooling rate. Three different slag compositions and three different cooling rates were employed. It was found that the phases formed upon cooling were mostly dependent on slag composition and to a minor extent on the cooling rate. The initially liquid slag was transformed into crystalline phases for all the samples except the ones terminated at 1573 K and one of the samples with high cooling rate. In addition, the three slags were equilibrated at 1773, 1673 and 1573 K in order to get an understanding of the equilibrium phases and their relationship during cooling. On the basis of the experimental results, the mechanism regarding entrainment of exogenous inclusions from the refractory lining was also discussed.

The liquidus in the high basicity region in the Al2O3(30 mass%)-CaO-MgO-SiO2 system were determined experimentally at 1773 and 1873 K using the quench technique followed by EPMA analysis. Based on the experimental data, a phase diagram of the Al2O3(30 mass%)-CaO-MgO-SiO2(< 20 mass%) section was constructed for 1773 and 1873 K. The solubilities of 2CaO center dot SiO2 and 3CaO center dot SiO2 at 1773 K were found to be considerably higher in comparison with the existing phase diagram. Even the solubility of MgO at 1873 K was found to be somewhat higher. In addition, the activities of MgO, CaO and Al2O3 at 1773 K were estimated using the phase diagram information.

Liquidus in the Al(2)O(3)(25 mass pct)-CaO-MgO-SiO(2)(<20 mass pct) and Al2O3(35 mass pct)-CaO-MgO-SiO(2)(<20 mass pct) systems were determined experimentally in the high-CaO-containing region at 1873 K (1600 degrees C). For the Al(2)O(3)(35 mass pct)-CaO-MgO-SiO(2)(<20 mass pct) system, liquidus data were also determined for 1773 K (1500 degrees C). The equilibrating and quenching technique with subsequent electron probe microanalyzer (EPMA) microanalysis were employed. Based on the data, liquidus lines were constructed for the 25 and 35 mass pct alumina planes at silica contents generally below 20 mass pct. The current results showed a slightly lower solubility of CaO and a higher solubility of MgO at 1873 K (1600 degrees C) for the 25 mass pct Al(2)O(3) section compared with the existing phase diagram. At 1773 K (1500 degrees C), the result showed a slightly lower solubility of both CaO and MgO in the 35 mass pct Al(2)O(3) section compared with the existing phase diagram. In addition, the activities of MgO, CaO, and Al(2)O(3) were estimated at 1773 K and 1873 K (1500 degrees C and 1600 degrees C) using the phase diagram information.

In the present thesis a study of the mechanisms of hydrogen control pickup during ladle treatment was undertaken. Previous studies showed that the presence of hydroxyl ions in the ladle slag resulteds in hydrogen transfer from the slag back into the steel bath. The main focus of the present work was therefore to gain deeper knowledge of the ladle slag, its properties and impact on hydrogen concentration in the liquid steel. For this purpose a number of slag compositions were examined in order to clarify whether these slags were single liquids at 1858 K. 14 out of 27 compositions in the Al₂O₃-CaO-MgO-SiO₂ system werewas completely melted, while the rest had solid shape present . These results were in disagreement with the existing phase diagram.

Water solubility measurements were carried out by employing a thermo gravimetric technique. The temperature was found to have negligible effect in the water solubilities. The experimental results showed that the water capacity values varied between 1·10³ and 2·10³ in the majority of the composition range. However, for compositions close to CaO saturation the water capacity value could reach higher than 3·10³. The experimental determined water capacity was further used to develop a water capacity model for the quaternary slag system Al₂O₃-CaO-MgO-SiO₂. The model was constructed by considering the effects of the binary interactions between the cations in the slag on the capacity of capturing hydroxyl ions. The model calculations agreed reasonably well with the experimental results as well as with the literature data.

The water capacity model was used in the last part of the present thesis in order to determine the major source for hydrogen pick-up of the steel after vacuum degassing but before casting. For this purpose, samples of slag and metal were taken at different stages ofduring ladle treatment at SSAB. Hydrogen increase after vacuum treatment was observed. Moisture contents of the industrial slag were analysed and the water capacities of the slags were calculated. It could be seen that the hydrogen increase was correlated to the amount of moisture in the slag and the water capacity. The study showed that the slag containing most water was also the heat having the largest hydrogen increase. The slag with most water had the highest water capacity. It could be concluded that the major source for hydrogen coming back into the steel was due to the slag-metal reaction.

A tentative process model to predict the final contents of hydrogen and nitrogen after tundish process was attempted. More work is needed to improve the model.