Effects of FeO and CaO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Ratio in Slag on the Cleanliness of Al-Killed Steel

The slag composition plays a critical role in the formation of inclusions and the cleanliness of steel. In this study, the effects of FeO content and the C/A (CaO/Al₂O₃) ratio in the slag on the formation of inclusions were investigated based on a 10-minute slag–steel reaction in a MgO crucible. The FeO content in the top slag was shown to have a significant effect on the formation of MgO·Al₂O₃ spinel inclusions, and critical content exists; when the initial FeO content in the slag was less than 2 pct, MgO·Al₂O₃ spinel inclusions formed, and the T.O (total oxygen) was 20 ppm; when the initial FeO content in the slag was more than 4 pct, only Al₂O₃ inclusions were observed and the T.O was 50 ppm. It was clarified that the main source of Mg for the MgO·Al₂O₃ spinel inclusion formation was the top slag rather than the MgO crucible. In addition, the cleanliness of the steel increased as the initial FeO content in the top slag decreased. As regards the effects of the C/A ratio, the MgO amount in the observed inclusions gradually increased, whereas the T.O content decreased gradually with the increasing C/A ratio. Slag with a composition close to the CaO-saturated region had the best effect on the inclusion absorption.     

Thermodynamics on the formation of spinel (MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript>) inclusion in liquid iron containing chromium

The stability diagram of MgO, spinel solid solution (MgO·(Al _X Cr_1−X )₂O₃), and sesquioxide solid solution ((Al _Y Cr_1−Y )₂O₃) as a function of Mg, Al, and O contents at a constant chromium content (18 mass pct) in liquid iron is drawn at 1873 K. The interaction parameters between Mg and other solutes (Al, Cr, Ni, Ti, Si, and C) are determined by the experimental method, which assures equilibrium between Mg vapor and liquid iron, were applied to calculate the diagram. Titanium deoxidation is not recommended for the prevention of spinel formation, because Ti accelerates Mg dissolution from refractory or slag due to its high affinity for Mg (e _Mg ^Ti = − 0.64). The standard Gibbs free energies of formation for the three inclusions (periclase, spinel, and sesquioxide solid solutions) and the tielines between two solid solutions were calculated with the aid of the regular solution model and the thermochemical F*A*C*T database computing system, respectively. The phase stability regions and oxygen content in steel for the current Fe-Mg-Al-Cr (18 mass pct)-O system are compared with those of the previous non-Cr system. Detailed information on the spinel composition according to Mg and Al contents is also available from the present stability diagram.     

Amphoteric behavior of alumina in viscous flow and structure of CaO-SiO<Subscript>2</Subscript> (-MgO)-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> slags

The viscosity of CaO-SiO₂ (-MgO)-Al₂O₃ slags was measured to clarify the effects of Al₂O₃ and MgO on the structure and viscous flow of molten slags at high temperatures. Furthermore, the infrared spectra of the quenched slags were analyzed to understand the structural role of Al₂O₃ in the polymerization or depolymerization of silicate network. The Al₂O₃ behaves as an amphoteric oxide with the composition of slags; that is, the alumina behaves as a network former up to about 10 mass pct Al₂O₃, while it acts as a network modifier, in parts, in the composition greater than 10 mass pct Al₂O₃. This amphoteric role of Al₂O₃ in the viscous flow of molten slags at the Newtonian flow region was diminished by the coexistence of MgO. The effect of Al₂O₃ on the viscosity increase can be understood based on an increase in the degree of polymerization (DOP) by the incorporation of the [AlO₄]-tetrahedra into the [SiO₄]-tetrahedral units, and this was confirmed by the infrared (IR) spectra of the quenched slags. The influence of alumina on the viscosity decrease can be explained on the basis of a decrease in the DOP by the increase in the relative fraction of the [AlO₆]-octahedral units. The relative intensity of the IR bands for the [SiO₄]-tetrahedra with low NBO/Si decreased, while that of the IR bands for [SiO₄]-tetrahedra with high NBO/Si increased with increasing Al₂O₃ content greater than the critical point, i.e., about 10 mass pct in the present systems. The variations of the activity coefficient of slag components with composition indirectly supported those of viscosity and structure of the aluminosilicate melts.     

Thermal diffusivity measurements of some synthetic CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> slags

In an attempt to systematize the knowledge of the heat conduction of liquid silicates, the effective thermal diffusivities of some synthetic slags containing CaO, Al₂O₃, and SiO₂ have been measured, using the three-layer laser-flash method on a differential scheme in the temperature range of 1625 to 1825 K. The effective thermal diffusivities measured, which are a combination of the phononic and photonic heat-transfer mechanisms, were found to increase with increasing temperature for all the presently investigated slags. The slag compositions were chosen in such a way that the changes in the effective thermal diffusivities would reflect the changes in the structure of the slags. It was observed that, at a CaO/Al₂O₃ molar ratio of 4.42, an increase of the SiO₂ content had very little effect on the effective thermal diffusivity values. On the other hand, addition of SiO₂ to a slag with the CaO/Al₂O₃ molar ratio of 2.59 resulted in a significant increase in the effective thermal diffusivity. The addition of Al₂O₃ to slags with a constant CaO/SiO₂ molar ratio resulted in a marked increase in the effective thermal diffusivity. Both these trends indicate that there might be an influence of the network formation in silicate melts on the effective thermal diffusivity.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Concentration on Density and Structure of (CaO-SiO<Subscript>2</Subscript>)-xAl<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag

The effect of Al₂O₃ concentration on the density and structure of CaO-SiO₂-Al₂O₃ slag was investigated at multiple Al₂O₃ mole percentages and at a fixed CaO/SiO₂ ratio of 1. The experiments were conducted in the temperature range of 2154 K to 2423 K (1881 °C to 2150 °C) using the aerodynamic levitation technique. In order to understand the relationship between density and structure, structural analysis of the silicate melts was carried out using Raman spectroscopy. The density of each slag sample investigated in this study decreased linearly with increasing temperature. When the Al₂O₃ content was less than 15 mole pct, density decreased with increasing Al₂O₃ content due to the coupling of Si (Al), whereas above 20 mole pct density of the slag increased due to the role of Al³⁺ ion as a network modifier.     

Effect of ferrosilicon addition on the composition of inclusions in 16Cr-14Ni-Si stainless steel melts

The silicon deoxidation equilibrium between the 16Cr-14Ni-1.5Mn-Si melts and the CaO-SiO₂-8MgO-5CaF₂ (basicity=1.8) slag at 1743 K was investigated to understand the effect of aluminum and silicon contents on the composition of inclusions. Therefore, the ferrosilicon alloys with different aluminum content were chosen based on the preceding objective. In addition, the phase stability diagram of the inclusions was computed using commercial thermodynamic software based on the Gibbs energy minimization principles. The content of MnO in the inclusions sharply decreases with increasing silicon content when the steel melts were deoxidized by the ferrosilicon alloys containing high aluminum (FeSi-H). The content of SiO₂ in the inclusions slightly increases with increasing silicon content when the FeSi-L is used, while a maximum value is shown at [Si]=1.5 pct when the FeSi-H is used. The content of MgO in the inclusions increases by increasing the content of silicon, regardless of the kinds of ferrosilicon alloys. The use of the FeSi-L as a deoxidizer could suppress the formation of Al₂O₃ in the inclusions, while the content of Al₂O₃ increases with increasing silicon content when the FeSi-H is used. When the FeSi-H is used as a deoxidizer, the inclusions are the glassy type with the composition of Mn-silicates at [Si]≤1.3 pct, while the Mg(Ca)-silicates with the composition of the forsterite phase are observed in the steel composition of [Si]=3.3 pct. When the steel melts were deoxidized by the FeSi-L alloys, the inclusions are the glassy-type Mn-silicates at [Si]=0.8 pct, while the Mn-silicates containing the cristobalite phase are observed at [Si]=1.5 to 2.4 pct. In the composition of [Si]=3.3 pct, the Mg-silicates with the composition of the rhodonite phase are observed. The log(X _SiO2/X _MnO) of the inclusions linearly increases by increasing the log [a _Si · a _O / a _Mn] with the slope close to unity when the FeSi-L is used as a deoxidizer, while the slope of the line is about 2 times greater than that of the expected value when FeSi-H is used. The log (X _MgO/X _MnO) of the inclusions linearly increases by increasing the log [a _Mg/a _Mn] with slopes greater than the expected value of unity.     

The Viscous Behavior of FeO<Subscript>t</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> Copper Smelting Slags

Understanding the viscous behavior of copper smelting slags is essential in increasing the process efficiency and obtaining the discrete separation between the matte and the slag. The viscosity of the FeO_t-SiO₂-Al₂O₃ copper smelting slags was measured in the current study using the rotating spindle method. The viscosity at a fixed Al₂O₃ concentration decreased with increasing Fe/SiO₂ ratio because of the depolymerization of the molten slag by the network-modifying free oxygen ions (O²⁻) supplied by FeO. The Fourier transform infrared (FTIR) analyses of the slag samples with increasing Fe/SiO₂ ratio revealed that the amount of large silicate sheets decreased, whereas the amount of simpler silicate structures increased. Al₂O₃ additions to the ternary FeO_t-SiO₂-Al₂O₃ slag system at a fixed Fe/SiO₂ ratio showed a characteristic V-shaped pattern, where initial additions decreased the viscosity, reached a minimum, and increased subsequently with higher Al₂O₃ content. The effect of Al₂O₃ was considered to be related to the amphoteric behavior of Al₂O₃, where Al₂O₃ initially behaves as a basic oxide and changes to an acidic oxide with variation in slag composition. Furthermore, Al₂O₃ additions also resulted in the high temperature phase change between fayalite/hercynite and the modification of the liquidus temperature with Al₂O₃ additions affecting the viscosity of the copper smelting slag.     

Modification and Minimization of Spinel(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>·<Emphasis Type="Italic">x</Emphasis>MgO) Inclusions Formed in Ti-Added Steel Melts

High-melting-point inclusions such as spinel(Al₂O₃·xMgO) are known to promote clogging of the submerged entry nozzle (SEN) in a continuous caster mold. In particular, Ti-alloyed steels can have severe nozzle clogging problems, which are detrimental to the slab surface quality. In this work, the thermodynamic role of Ti in steels and the effect of Ca and Ti addition to the molten austenitic stainless steel deoxidized with Al on the formation of Al₂O₃·xMgO spinel inclusions were investigated. The sequence of Ca and Ti additions after Al deoxidation was also investigated. The inclusion chemistry and morphology according to the order of Ca and Ti are discussed from the standpoint of spinel formation. The thermodynamic interaction parameter of Mg with respect to the Ti alloying element was determined. The element of Ti in steels could contribute to enhancing the spinel formation, because Ti accelerates Mg dissolution from the MgO containing refractory walls or slags because of its high thermodynamic affinity for Mg ( e_\textMg^\textTi = - 0. 9 3 3). ( {e_{\text{Mg}}^{\text{Ti}} = - 0. 9 3 3}). Even though Ti also induces Ca dissolution from the CaO-containing refractory walls or slags because of its thermodynamic affinity for Ca ( e_\textCa^\textTi = - 0.119 ), \left( {e_{\text{Ca}}^{\text{Ti}} = - 0.119} \right), dissolved Ca plays a role in favoring the formation of calcium aluminate inclusions, which are more stable thermodynamically in an Al-deoxidized steel. The inclusion content of steel samples was analyzed to improve the understanding of fundamentals of Al₂O₃·xMgO spinel inclusion formation. The optimum processing conditions for Ca treatment and Ti addition in austenitic stainless steel melts to achieve the minimized spinel formation and the maximized Ti-alloying yield is discussed.     

Electrical conductivity of NaF-AlF<Subscript>3</Subscript>-CaF<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> melts

The electrical conductivity of NaF-AlF₃-Al₂O₃ melts with a CaF₂ concentration of 5 wt % is measured at a continuously varying cell constant when the molar cryolitic ratio CR = [NaF]/[AlF₃] changes from 1.2 to 2.0 [1, 2]. The experimental data are used to obtain a regression equation to describe the dependence of the electrical conductivity of the melts under study on CR, the alumina content, and temperature {χ] = f(CR, [Al₂O₃], T)}.     

Formation of CaS on Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO inclusions during solidification of steels

In this article, the effect of CaS formation on the evolution of Al₂O₃-CaO inclusions in low-carbon Al-killed and Ca-treated steel during the solidification process is investigated through high-temperature confocal scanning laser microscopy (CSLM). The inclusions started as mostly liquid-globular inclusions that did not agglomerate with each other on the melt surface but during solidification were seen to change shape into an irregular morphology. The shape change was found to be due to the reaction between the Al₂O₃-CaO inclusions with the dissolved S and Al in the melt, resulting in the formation of dense CaS shells around the inclusions. The melt composition during solidification, estimated from the observed solid δ-front advance rate, was compared to the thermodynamic limit for CaS precipitation. The observed growth rate of the CaS shell was found to initially increase with decreasing temperature because of the higher, solid δ-front advance rates at lower temperatures, which results in higher rates of S and Al partitioning. Once CaS had precipitated, the inclusions were found to form agglomerates on the melt surface because of fluid flow, initially, and later, the capillary depression.     

A quasi-chemical viscosity model for fully liquid slags in the Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-CaO-‘FeO’-SiO<Subscript>2</Subscript> system

A structurally based viscosity model for fully liquid silicate slags has been proposed and applied to the Al₂O₃-CaO-‘FeO’-SiO₂ system at metallic iron saturation. The model links the slag viscosity to the internal structure of melts through the concentrations of various anion/cation structural units (SUs). The concentrations of structural units are equivalent to the second nearest neighbor bond concentrations calculated by the quasi-chemical thermodynamic model. This viscosity model describes experimental data over the entire temperature and composition range within the Al₂O₃-CaO-‘FeO’-SiO₂ system at metallic iron saturation and can be extended to other industrial slag systems.     

The sulphide capacity of CaO-SiO2-Al2O3-MgO(-FeO) smelting reduction slags

The effects of MgO and FeO contents on the sulphide capacity of Corex slags were investigated at 1773 K using gas/slag equilibrium technique as a fundamental study for stabilising Corex operational conditions. The gradual substitution of MgO for CaO at a fixed basicity [B = {(%CaO)+(%MgO)}/(%SiO₂)] decreased the sulphide capacities of CaO-SiO₂-Al₂C₃-MgO slags. The addition of FeO into the CaO-SiO₂-Al₂O₃-MgO slags at the fixed (%CaO)/(%SiO₂) ratio, MgO and Al₂O₃ contents significantly increased the sulphide capacities. The sulphide capacity decreased according to the increasing Al₂O₃ content at the fixed (%CaO)/(%SiO₂) ratio, MgO and FeO content. Based on the previously reported and present values of sulphide capacities, the sulphide capacity as a function of slag composition on the mole fraction base at 1773 K was expressed by the formula of log C_S = X_CaO – 3.89 X_SiO2 – 3.18 X_Al2O₃ – 0.55 X_MgO + 2.43 X_FeO – 2.61. In addition, the relationship between the sulphide capacity and optical basicity could be represented as the formula of log C_S = 12.51 A – 12.24 and the theoretical optical basicity of FeO was found be 0.94 from the correlation. The effect of FeO on the sulphur distribution ratio was estimated using the present sulphide capacity values and the oxygen activity in liquid iron, which could be determined with the help of Fe/FeO equilibrium. FeO activity in slag was well described by the quadratic formalism based on the regular solution model. The sulphur distribution ratio according to FeO content varies in an opposite way, compared with that of the sulphide capacity.     

Dissolution Behavior of Mg from Magnesia-Chromite Refractory into Al-killed Molten Steel

Magnesia-chromite refractory materials are widely employed in steel production, and are considered a potential MgO source for the generation of MgO·Al₂O₃ spinel inclusions in steel melts. In this study, a square magnesia-chromite refractory rod was immersed into molten steel of various compositions held in an Al₂O₃ crucibles. As the immersion time was extended, Mg and Cr gradually dissolved from the magnesia-chromite refractory, and the Mg and Cr contents of the steel melts increased. However, it was found that the inclusions in the steel melts remained as almost pure Al₂O₃ because the Mg content of the steel melts was low, approximately 1 ppm. On the surface of the magnesia-chromite refractory, an MgO·Al₂O₃ spinel layer with a variable composition was formed, and the thickness of the MgO·Al₂O₃ spinel layer increased with the immersion time and the Al content of the steel melts. At the rod interface, the formed layer consisted of MgO-saturated MgO·Al₂O₃ spinel. The MgO content decreased along the thickness direction of the layer, and at the steel melts interface, the formed layer consisted of Al₂O₃-saturated MgO·Al₂O₃ spinel. Therefore, the low content of Mg in steel melts and the unchanged inclusions were because of the equilibrium between Al₂O₃-saturated MgO·Al₂O₃ layer and Al₂O₃. In addition, the effects of the Al and Cr contents of the steel melts on the dissolution of Mg from the magnesia-chromite refractory are insignificant.     

Thermodynamics of MnO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MnS Liquid Oxysulfide: Experimental and Thermodynamic Modeling

The activities of MnO and MnS in a MnO-SiO₂-Al₂O₃(or AlO_1.5)-MnS liquid oxysulfide solution were investigated by employing the gas/liquid/Pt-Mn alloy chemical equilibration technique under a controlled atmosphere at 1773 K (1500 °C). Also, the sulfide capacity, defined as C _S = (wt pct S)(pO₂/pS₂)^1/2, in MnO-SiO₂-Al₂O₃ slag with a dilute MnS concentration was obtained from the measured experimental data. As X _SiO2/(X _MnO + X _SiO2) in liquid oxysulfide increases, the activity coefficient of MnO decreases, while that of MnS first increases and then decreases. As X(AlO_1.5) in liquid oxysulfide increases, the activity coefficient of MnS increases, while no remarkable change is observed for the activity coefficient of MnO. The behavior of the activity coefficient of MnS was qualitatively analyzed by considering MnO + A _x S _y (SiS₂ or Al₂S₃) = MnS + A _x O _y (SiO₂ or Al₂O₃) reciprocal exchange reactions in the oxysulfide solution. The behavior was shown to be consistent with phase diagram data, namely, the MnS saturation boundary. Quantitative analysis of the activity coefficient of the oxysulfide solution was also carried out by employing the modified quasichemical model in the quadruplet approximation.     

The Thermodynamic Behavior of Copper in the CaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> and BaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag System

The Cu solubility was measured in the CaO-B₂O₃ and BaO-B₂O₃ slag systems to understand the dissolution mechanism of Cu in the slags. The Cu solubility had a linear relationship with oxygen partial pressure in the CaO-B₂O₃ slag system, which corresponds with previous studies. Also, the Cu solubilities in slag decreased with increasing the slag basicity, which value of slope was close to –0.5 in logarithmic form. From the results of experiment, the Cu dissolution mechanism established as follows:

Temperature dependence of the refractive index of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Na<Subscript>2</Subscript>O-SiO<Subscript>2</Subscript> melts: Role of electronic polarizability of oxygon controlled by network structure

Refractive indexes for the Al₂O₃-Na₂O-SiO₂ system have been measured using an ellipsometer for a wavelength of 632.8 nm over a wide temperature range (1100 to 1800 K). Two kinds of sample were used: xAl₂O₃-(40-x)Na₂O-60SiO₂ and yAl₂O₃-yNa₂O-(100-2y)SiO₂, where x ranged between 6 and 20 mol pct and y between 12.5 and 25 mol pct. In the former samples, the temperature coefficient of refractive indexes changed from negative to positive on increasing the concentration of Al₂O₃. In the latter samples, the refractive indexes increased monotonically with decreasing concentration of SiO₂, and the temperature coefficient was always positive. It has been found that the temperature dependence of refractive indexes in these melts is determined by the coefficient of thermal expansion, which would be relevant to the degree of polymerization of the melts. In addition, the electronic polarizability of oxygen derived from the refractive indexes increased with increasing temperature in each melt. This suggests that the basicity of the alumino-silicate melts increases as temperature increases. The positive temperature coefficient of the electronic polarizability of oxygen can be attributed to an increase in the distance between cation and oxygen ion due to thermal expansion. The dependence of the electronic polarizability of oxygen on the concentration of Al₂O₃ has also been discussed in terms of the electronic polarizabilities of three types of oxygen contained in the melts. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.     

Modeling of the solubilities of NiO/NiAl<Subscript>2</Subscript>O<Subscript>4</Subscript> and FeO/FeAl<Subscript>2</Subscript>O<Subscript>4</Subscript> in cryolite melts at 1300 K

Experiments to measure the solubilities of NiO/NiAl₂O₄ and FeO/FeAl₂O₄ were performed, and the results confirmed existing literature values. The solubilities of NiAl₂O₄ and FeAl₂O₄ in Al₂O₃-saturated cryolite melts at 1300 K were modeled thermodynamically in terms of the Ni-containing complexes Na₂NiF₄ and Na₄NiF₆, and the Fe-containing solutes FeF₂, Na₂FeF₄, and Na₄FeF₆. The experimental solubility data were fitted to multiple simultaneous equilibria. Equilibrium constants and ΔG _f ⁰ values for the formation reactions of the these solutes were thereby estimated. The solubilities of NiO/NiAl₂O₄ and FeO/FeAl₂O₄ and solute distributions in Al₂O₃-undersaturated cryolite melts were calculated for a number of melt compositions from the present model. The existence of several competitive solute species is inherent to highly buffered ionic cryolite solutions where the traditional log-log methodology had previously failed to identify dominant single solutes. In such solutions, individual solutes of oxides are not likely to dominate over a wide composition range so that a more global modeling is required. The principal solute species identified in the present study exhibit reasonable three-dimensional (3-D) anion geometries.