O fluxante é uma escória sintética que influencia na qualidade superficial do aço e na estabilidade do processo de lingotamento contínuo. Este produto é aplicado diretamente sobre o aço líquido na região do molde de cobre refrigerado a água e atua diretamente no resfriamento primário do aço. O fluxante tem as propriedades físico-químicas adaptadas para cada tipo de aço e também para as condições de lingotamento. Na superfície do aço líquido, o fluxante funde e forma uma poça líquida, atuando como isolante térmico, protegendo o aço da reoxidação e absorvendo inclusões principalmente de Al2O3. A poça líquida escoa, lubrificando e controlando a transferência de calor na interface entre o molde e a pele de aço em solidificação. O problema de qualidade superficial do aço, quando relacionado ao fluxante, se resume a alarmes de colamento, trincas de quina, marcas de oscilação profundas e trincas longitudinais, sendo este último um problema particular do aço médio carbono. Neste trabalho, foram analisados diferentes fluxantes baseados inicialmente no fluxante comercial aplicado no lingotamento contínuo de placas de aço médio carbono. Todos os fluxantes foram desenvolvidos com composições químicas similares. O objetivo foi avaliar o impacto da substituição de fontes de matérias-primas em diferentes composições de modo a avaliar as propriedades físico-químicas com base no fluxante comercial de referência. Como resultado, de todas as propriedades físico-químicas, foi a temperatura de cristalização que sofreu a alteração mais significativa. / The mould flux is a synthetic slag which influences the quality of steel surface and also the stability of the continuous casting process. This product is applied in direct contact with the surface of liquid steel in the region of the water cooled cupper mold and acts directly on the primary cooling of steel. The mould flux has the physical and chemical properties adapted for each type of steel as weel as casting conditions. On the surface of liquid steel, the mould flux melts and forms the liquid pool, working as an insulator, protecting the steel against reoxidation and also absorbing mainly Al2O3 inclusions. The liquid pool flows, lubricating and controlling the heat transfer in the interface between the mold and the steel shell during solidification. The surface quality issues when related with mould flux can cause alarms of breakout, corner cracks, deep oscillation marks, longitudinal cracks that is a particular problem in medium carbon steel grade. In this study, different mould fluxes were investigated and compared to a commercial mould flux applied to the continuous casting of slabs in the medium carbon steel grade. All mould fluxes were developed with a similar chemical composition. The main goal was to evaluate the impact of changing raw material sources on the physicochemical properties of the different mould fluxes and the results was compared with a commercial standard mould flux. As result, variations in the crystallization temperatures were observed more than others properties analyzed.

O fluxante é uma escória sintética que influencia na qualidade superficial do aço e na estabilidade do processo de lingotamento contínuo. Este produto é aplicado diretamente sobre o aço líquido na região do molde de cobre refrigerado a água e atua diretamente no resfriamento primário do aço. O fluxante tem as propriedades físico-químicas adaptadas para cada tipo de aço e também para as condições de lingotamento. Na superfície do aço líquido, o fluxante funde e forma uma poça líquida, atuando como isolante térmico, protegendo o aço da reoxidação e absorvendo inclusões principalmente de Al2O3. A poça líquida escoa, lubrificando e controlando a transferência de calor na interface entre o molde e a pele de aço em solidificação. O problema de qualidade superficial do aço, quando relacionado ao fluxante, se resume a alarmes de colamento, trincas de quina, marcas de oscilação profundas e trincas longitudinais, sendo este último um problema particular do aço médio carbono. Neste trabalho, foram analisados diferentes fluxantes baseados inicialmente no fluxante comercial aplicado no lingotamento contínuo de placas de aço médio carbono. Todos os fluxantes foram desenvolvidos com composições químicas similares. O objetivo foi avaliar o impacto da substituição de fontes de matérias-primas em diferentes composições de modo a avaliar as propriedades físico-químicas com base no fluxante comercial de referência. Como resultado, de todas as propriedades físico-químicas, foi a temperatura de cristalização que sofreu a alteração mais significativa. / The mould flux is a synthetic slag which influences the quality of steel surface and also the stability of the continuous casting process. This product is applied in direct contact with the surface of liquid steel in the region of the water cooled cupper mold and acts directly on the primary cooling of steel. The mould flux has the physical and chemical properties adapted for each type of steel as weel as casting conditions. On the surface of liquid steel, the mould flux melts and forms the liquid pool, working as an insulator, protecting the steel against reoxidation and also absorbing mainly Al2O3 inclusions. The liquid pool flows, lubricating and controlling the heat transfer in the interface between the mold and the steel shell during solidification. The surface quality issues when related with mould flux can cause alarms of breakout, corner cracks, deep oscillation marks, longitudinal cracks that is a particular problem in medium carbon steel grade. In this study, different mould fluxes were investigated and compared to a commercial mould flux applied to the continuous casting of slabs in the medium carbon steel grade. All mould fluxes were developed with a similar chemical composition. The main goal was to evaluate the impact of changing raw material sources on the physicochemical properties of the different mould fluxes and the results was compared with a commercial standard mould flux. As result, variations in the crystallization temperatures were observed more than others properties analyzed.

Due to the high efficiency and productivity of continuous casting process, this method has been the most employed process to produce steel in past decades. The need to improve and optimize the finished product made it essential to gain more knowledge about the process, types of defects that may occur and the reasons for them. Moreover, the solutions for reducing the shortcomings in continuous casting process have been an intriguing subject to study. Many attempts have been done in order to reach this goal. Understanding, determining and optimizing the mould flux slag properties is especially important, since it plays an important and significant role in this process. For this, it is of outmost importance to acquire more knowledge about different properties of mould flux powders. Hence, there has been a world wide effort to measure and model the properties of mould flux properties, such as liquidus and solidus temperatures, heat capacity, enthalpy, thermal expansion, density, viscosity, electrical conductivity, surface tension and thermal conductivity. This thesis presents a brief review on continuous casting process, mould flux powder and its properties and characteristics. Furthermore, it focuses on the thermo-physical properties of mould fluxes. In present work, different industrial mould flux powders have been analyzed to measure their viscosity, break temperature, physical properties such as density, flowablity of powder, slag structure and chemical composition. The experimental data have been compared to some of the most commonly used models such as Riboud model, Urbain model, Iida model and KTH model.

Mould powders are synthetic slags that contain mixtures of silica (SiO2), lime (CaO), sodium oxide (Na2O), fluorspar (CaF2), and carbon (C). When heated to elevated temperatures these powders liquefy and float on the liquid steel in the mould. Mould oscillation helps the liquid flux to penetrate the tiny gap between the mould and the newly formed solid steel shell. In this position the liquid flux partially solidifies against the water cooled mould, while a small portion of the flux remains liquid next to the steel shell to provide lubrication between the moving parts. Effective horizontal heat transfer in the mould is critical for solidifying the liquid steel inthe mould. This process is largely influenced by the thickness and the nature of the flux layer that infiltrates the mould/shell gap. When casting titanium stabilised stainless steels the alloying element reacts with the molten flux, ultimately changing the behaviour of the flux. During the casting process, titanium from the liquid steel reacts with the molten flux producing solids at high temperatures known as perovskite (CaTiO3). Research has shown that perovskite reduces the lubrication capabilities of casting fluxes leading to detrimental effects on product quality while posing a serious threat of machine damage (breakout). The focus of this study is to investigate the effect of titanium pickup on the solidification nature of mould flux and the consequences on horizontal heat transfer. To achieve this, an experimental setup was constructed to simulate the behaviour of mould flux during continuous casting. Analyses of the test flux indicated that the liquid flux closest to the cold side (mould) instantly froze to produce a glassy solid structure. Closer to the hot side (steel shell), solid particles such as perovskite, cuspidine (Ca4Si2O7F2), olivine (Ca,Mg,Mn)2SiO4 and nepheline (Na2O.Al2O3.(SiO2)2) could be identified. Similar solid particles were also found in a slag rim sample taken during the industrial casting of 321- titanium stabilised stainless steel using SPH-KA1 mould powder. Further investigations of the crystalline flux layers showed the entrapment of many tiny gas bubbles during solidification. This porous structure acted as a thermal heat barrier limiting horizontal heat transfer. Experimental testing on 3.0 and 6.0mm flux thickness revealed that the overall thermal conductivity of mould flux decreased as the flux porosity increased. Larger amounts of gas entrapment (in the solid flux structure) resulted in higher thermal resistances which ultimately reduced the heat transfer capabilities of the flux. A second heat barrier, which has a far more dominating effect on the overall heat transfer, is created on mould surface during flux solidification. This thermal contact resistance is also found to be the result of entrapped gas bubbles. Experimental results concluded that the effect of titanium pickup on heat transfer is primarily overshadowed by the larger effect of the thermal contact resistance that is formed during mould flux solidification. The contact resistance in combination with gas entrapment in the solid crystalline structure is considered to be the key factors preventing horizontal heat transfer during continuous casting. / Dissertation (MEng (Metallurgical Engineering))--University of Pretoria, 2006. / Materials Science and Metallurgical Engineering / MEng / unrestricted

<p>The present dissertation describes part of the efforts directed towards the development of computational tools to support process modeling. This work is also a further development of the Thermoslag software developed in the Division of Materials Process Science, KTH.</p><p>The essential parts of the thesis are</p><p>a) development of a semi-empirical model for the estimation of the molar volumes/densities of multicomponent slags with a view to incorporate the same in the model for viscosities and</p><p>b) further development of the viscosity model for application towards fluorid- containing slags, as for example, mould flux slags.</p><p>The model for the estimation of molar volume is based on a correlation between the relative integral molar volume of a slag system and the relative integral molar enthalpies of mixing of the same system. The integral molar enthalpies of the relevant systems could be evaluated from the Gibbs energy data available in the Thermoslag software. The binary parameters were evaluated from experimental measurements of the molar volumes. Satisfactory correlations were obtained in the case of the binary silicate and aluminate systems. The model was extended to ternary and multi component systems by computing the molar volumes using the binary parameters. The model predictions showed agreements with the molar volume data available in literature. The model was used to estimate the molar volumes of industrial slags as well as to trace the trends in molar volume due to compositional variations. The advantage of the present approach is that it would enable prediction of molar volumes of slags that are compatible with the thermodynamic data available.</p><p>With a view to extend the existing model for viscosities to F--containing slags, the viscosities of mould flux slags for continues casting in steel production have been investigated in the present work. The measurements were carried out utilizing the rotating cylinder method. Seven mould fluxes used in the Swedish steel industry and the impact of Al₂O₃pick up by mould flux slags on viscosities were included in the study. The results showed that even relatively small additions of Al₂O₃ are related with a significant increase in viscosity</p>

The present dissertation describes part of the efforts directed towards the development of computational tools to support process modeling. This work is also a further development of the Thermoslag software developed in the Division of Materials Process Science, KTH. The essential parts of the thesis are a) development of a semi-empirical model for the estimation of the molar volumes/densities of multicomponent slags with a view to incorporate the same in the model for viscosities and b) further development of the viscosity model for application towards fluorid- containing slags, as for example, mould flux slags. The model for the estimation of molar volume is based on a correlation between the relative integral molar volume of a slag system and the relative integral molar enthalpies of mixing of the same system. The integral molar enthalpies of the relevant systems could be evaluated from the Gibbs energy data available in the Thermoslag software. The binary parameters were evaluated from experimental measurements of the molar volumes. Satisfactory correlations were obtained in the case of the binary silicate and aluminate systems. The model was extended to ternary and multi component systems by computing the molar volumes using the binary parameters. The model predictions showed agreements with the molar volume data available in literature. The model was used to estimate the molar volumes of industrial slags as well as to trace the trends in molar volume due to compositional variations. The advantage of the present approach is that it would enable prediction of molar volumes of slags that are compatible with the thermodynamic data available. With a view to extend the existing model for viscosities to F--containing slags, the viscosities of mould flux slags for continues casting in steel production have been investigated in the present work. The measurements were carried out utilizing the rotating cylinder method. Seven mould fluxes used in the Swedish steel industry and the impact of Al2O3 pick up by mould flux slags on viscosities were included in the study. The results showed that even relatively small additions of Al2O3 are related with a significant increase in viscosity / QC 20101123

Effects of mould flux contaminant on scale-steel adhesion and hydraulic descaling of scale formed on slabs were investigated. In this investigation, stainless steel type 304 (austenitic with 18% Cr and 8% Ni) and specific mould fluxes were used when growing the scale on contaminated samples under simulated industrial reheating conditions, with subsequent high pressure water hydraulic descaling. The basic hypothesis was that the steel-scale adhesion depends on the microstructure of different phases present in the scale, the segregation of specific elements at the interface and the interfacial morphology of the scale after reheating. It was found that mould flux contaminant decreases scale-steel adhesion and therefore improved the descaling effectiveness significantly compared to non contaminated stainless steel. The descaling effectiveness of contaminated and uncontaminated slab was dependent to the presence of metal free paths (chromite layers along the austenite grains boundaries) and the presence of unoxidized metal in the scale due to nickel enrichment at the interface. Compared to the uncontaminated samples, the descaling of contaminated samples was efficient which could be due to the fact that some mechanisms which increase scale– steel adhesion (notably nickel enrichment at the interface) were considerably reduced. For all contaminated samples, the descaling effectiveness after visual observation were close to 100% and it was found that mould flux type 832 ( low basicity) gave a high descaling efficiency with better steel surface quality after descaling compared to mould fluxes type 810 and RF1. / Dissertation (MSc)--University of Pretoria, 2009. / Materials Science and Metallurgical Engineering / unrestricted