In high temperature processes, physico-chemical properties of the phases involved have proved useful in identifying the mechanisms responsible for process problems and product defects. However, recently mathematical models have developed to the stage where they can now be used to identify these mechanisms. These models require reliable values for the thermo-physical properties for the various phases involved in the process. The thermo-physical properties of slags are very dependent upon slag structure. The factors affecting slag structure are outlined and the ways in which structural factors affect various physical properties are described. Finally, the manner in which the following properties (liquidus temperature; viscosity, surface and interfacial tension; thermal conductivity and enthalpy) can be manipulated to optimise process control and product quality is demonstrated for various industrial processes.

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The density of blast furnace slag bearing TiO₂ has been investigated using the dispensed drop method at 1673 K under the Argon atmosphere (Pressure ≈1.2 atm). The influence of slag basicity and the TiO₂ content on the density was analyzed according to the measurements. It was found that the density of the slag increases with increasing the TiO₂ content from 0% to 23%, once the TiO₂ content exceed of 23%, the density decreases. The reason of this transition can be attributed to the reduce of the coordination numbers of Ti–O, Ca–O and Mg–O by means of the molecular dynamics simulation. For the influence of the increase of basicity from 0.9 to 1.2, the density slightly rises from 2.83 g/cm³ to 2.92 g/cm³. The empirical formulas for the density which relates TiO₂ content and basicity are developed. The prediction model for the density by the partial molar volume method was also discussed, indicating that the calculated values can reach a good quantitative agreement with the measured values without the apparent change of the coordination numbers of the cations with O.

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The viscosities of the industrial and synthetic iron blast furnace slags have been measured using a custom designed rotating bob apparatus. This advanced apparatus enables control of the gas atmosphere and rapid quenching of the samples on completion of the viscosity measurements. The experiments were carried out using Mo spindle and crucible under ultra-high purity Ar flow. The microstructures and phase compositions of the quenched slag samples were determined by Electron Probe X-ray Microanalysis (EPMA) after the viscosity measurements.
It was found that the viscosities of the industrial slags are lower than those of the corresponding synthetic slags made from pure Al₂O₃, CaO, MgO and SiO₂. The difference in viscosities between industrial slags and synthetic slags will provide useful indications when applying the results of the synthetic slags to the real iron blast furnace slags. The activation energy of both industrial and synthetic slags is 174 kJ/mol and increases with decreasing MgO concentration. Replacement of (CaO+SiO₂) or CaO by MgO can decrease the slag viscosity. The viscosity of iron blast furnace slag at 1500°C can be estimated by V = 0.005+0.0262[SiO₂]+0.0184[Al₂O₃]–0.0172[CaO]–0.0244[MgO] which is derived from the present and previous data.

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The structure-viscosity relationship of the low-silica (SiO₂ ≤ 10 wt%) calcium aluminosilicate melts, which represent the secondary refining ladle slag systems, was investigated by employing the rotating-cylinder viscosity measurement in conjunction with the Raman spectroscopy measurement for linking the macroscopic thermophysical property and molecular (ionic) structural information. Furthermore, the influence of CaF₂ on the structure-property relationship was explored. The viscosity of low-silica calcium aluminosilicate melts decreased with increasing both CaO/Al₂O₃ and CaO/SiO₂ ratios. However, the effect of the former on the viscosity of low-silica calcium aluminosilicate melts was larger than that of the latter. By employing the Neuville’s structure model, in which the silicate structural units with various NBO, i.e. Qⁿ_Si are located at the boundary of the AlO₄ aluminate, and the Raman scattering data of the glass samples, it was demonstrated that the aluminate and silicate units are more effectively modified by increasing the CaO/Al₂O₃ ratio at fixed silica content. The addition of small amounts of CaF₂ (~5 wt%) to the low-silica calcium aluminosilicate melts decreased the viscosity of the melts. From the analysis of Raman scattering data, the liberation of SiO₄^4– (Q⁰_Si) units from the AlO₄ aluminate structure by addition of CaF₂ was understood. However, the effect of CaF₂ addition on the viscosity became less discernible at higher CaF₂ content (≥ 10 wt%) region, where the F ions simply substitute for the non-bridging oxygen ions in AlO₄ tetrahedra.

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We investigated the changes induced in the viscosities of CaO–SiO₂–Al₂O₃–K₂O melts (CaO/SiO₂ molar ratio = 0.68 ± 0.04, Al₂O₃ content = 13.4 ± 0.6 mol%) with the addition of K₂O in amounts of 0–17.4 mol% for temperatures of 1673–1873 K using the rotating crucible method. The viscosity increased with an increase in the K₂O content when K₂O/Al₂O₃ molar ratio < 0.7. On the other hand, the viscosity decreased with the addition of K₂O for K₂O/Al₂O₃ molar ratio > 0.9. The maximum values of the viscosities were noticed in the range of 0.7 < K₂O/Al₂O₃ molar ratio < 0.9. This behavior could not be explained on the basis of the polymerization degree of the aluminosilicate network structure. The ¹⁷O magic angle spinning nuclear magnetic resonance (MAS NMR) spectrum of the 31.6CaO-44.2SiO₂-13.4Al₂O₃-10.8K₂O (mol%) glass suggested that K⁺ ions preferentially compensate the negative charge of AlO₄, while the Ca²⁺ ions primarily create non-bridging oxygens (NBOs). The viscosity data and the ¹⁷O MAS NMR spectrum also suggested that the increase in viscosity for K₂O/Al₂O₃ molar ratio < 0.7 was because of the increase in the average bond strength of the aluminosilicate framework, which was attributable to the substitution of Ca²⁺ ions by K⁺ ions at the charge compensator sites. The activation energies for viscous flow indicated that the NBOs bonded with K⁺ ions, forming NBO-K species, for K₂O contents greater than those corresponding to the viscosity maxima. Thus, the viscosities of aluminosilicate melts are indicative of the average bond strengths of the melts.

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The viscosity of SiO₂–Na₂O–NaF melts was measured by using a rotating crucible viscometer developed by the authors. The basicity (= C_Na2O/C_SiO2 in mol%) of the melts was 0.67, 0.82, and 1.00. The concentrations of NaF were 5, 10, and 15 mol%. As a result, the viscosity of the melts showed a good Arrhenius type linearity in any melts. It was found that the sample loss occurred in highly basic composition region due to a prolonged measurement at high temperature. The viscosity of the melts decreased with increasing the concentration of NaF, and the composition dependence is similar to that for SiO₂–CaO–CaF₂ melts. Viscosity decreasing ability of NaF is higher than that of Na₂O within the composition region investigated in this study. Transitions of activation energy of viscous flow are different between Na₂O addition and NaF addition. This implies that the mechanism of enhancement of fluidity is different between Na₂O and NaF.

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The effect of SiO₂/Al₂O₃ and TiO₂/SiO₂ ratios on the viscosity of the TiO₂–MnO–SiO₂–Al₂O₃ welding flux system was studied using the rotational spindle method. The viscosity was measured at the fully liquid region of the melt from 1773 K to the break temperature at approximately 1623 K. Higher TiO₂/SiO₂ ratio resulted in lower viscosity in the temperature range of interest. The SiO₂ dominant slag system showed higher viscosity values than the Al₂O₃ dominant slag in the present TiO₂ based system until a SiO₂/Al₂O₃ ratio of 1.61. Fourier transform infrared (FTIR) results revealed the [SiO₄]^4–-tetrahedral symmetric stretching vibrations and the [AlO₄]^5–-tetrahedral asymmetric stretching vibration became weaker with higher TiO₂/SiO₂, which was also apparent with the increase in the extended basicity ((TiO₂+MnO)/(SiO₂+Al₂O₃)). The Al–O–Si bending vibration and [SiO₄]^4–-tetrahedral symmetric stretching vibrations were also pronounced with higher SiO₂/Al₂O₃ ratio. From the X-ray photoelectron spectroscopy (XPS) results, the bridged oxygen (O⁰) slightly decreased with higher TiO₂/SiO₂ and increased with higher SiO₂/Al₂O₃.

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Initial wetting phenomena and spreading property of CaO–SiO₂ slag on MgO single and poly-crystal refractory substrates are investigated using sessile drop technique and a high-speed camera (1500 frame/s). By observing and analyzing the spreading rate, change of the contact angle, and the composition change in slag and refractory, we found that the initial wetting is non-reactive and controlled by viscous friction.

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The slags and fluxes found in modern steelmaking convertors all contain finely dispersed gas phases, which are generated by the refining reaction used to decarburize the molten iron. The frothing effect that is often generated as a result of these gasses can often prove to be a fatal obstacle in the efficient operation of the converter. In the present study, a simulated slag foam was produced by dispersing N₂ bubbles in silicone oil. The effect of varying the volume fraction and bubble size of the dispersed gas phase, the shear rate, and the viscosity of the liquid phase, was then systematically investigated by measuring the viscosity of the N₂ bubble dispersed silicone oil with a rotating viscometer. This found that the relative viscosity is increased as the volume fraction of the gas phase is increased, ultimately transitioning from a Newtonian to pseudo-plastic fluid at higher gas phase rates. In addition, an empirical model for the viscosity of the slag foam was developed by modifying the Einstein-Roscoe equation, with this model capable of reproducing the variation in relative viscosity with various gas phase rates, shear rates, and bubble sizes.

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The effects of the agitation field and the crystal morphology on the primary and secondary crystallization temperatures of CaO–SiO₂–CaF₂ supercooled melts were systematically investigated by measuring the electrical capacitance.
Two-stage decreases in electrical capacitance were observed for most samples investigated. These stages correspond to the primary crystallization and the eutectic reaction resulting from the compositional change in the supercooled melts associated with the primary crystallization, respectively.
The primary crystallization of dendritic CaO·SiO₂ was affected by the agitation in the same manner as reported previously, but the crystallization of faceted 3CaO·2SiO₂·CaF₂ revealed little dependence on the agitation. This indicates that the effects of the agitation are different for different morphologies of the primary crystalline phase because of shifts in the rate-determining step of crystallization.

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Using a transient hot-wire method, the thermal conductivity of the CaO–SiO₂–B₂O₃ mold flux system was measured. The effects of temperature, BO_1.5 concentration and basicity on the thermal conductivity were considered, along with structural investigation by Raman spectroscopy. It was found that the addition of boron oxide caused both a decrement and increment of thermal conductivity, depending on the basicity. These conflicting effects on thermal conductivity were considered to be caused by the following two different behaviors in the oxide melts. Boron oxide is incorporated into silicate networks at a lower basicity, while it tends to form borate networks at higher CaO/SiO₂ ratios. In the case of basicity dependency, thermal conductivity initially decreases or remains constant with increasing CaO/SiO₂ ratio in regions of low basicity, but increases when the CaO/SiO₂ ratio is higher than 1.15. Due to the incorporated state of boron oxide in silicate networks at low basicity, the thermal conductivity is likely to be predominantly affected by the silicate networks. However, at a relatively high CaO/SiO₂ ratio, an increase in chain-type metaborate was observed through Raman spectroscopy; this structural change in borate being responsible for the increment in thermal conductivity with higher basicity. Finally, the apparent activation energy of thermal conductivity was calculated, and was found to be reduced by the addition of boron oxide.

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Thermal diffusivity and conductivity of oxide scales formed on steel plates are essential to control the cooling rate of the steel in the hot-rolling process. A flash method was applied to measure the thermal diffusivity of oxide scales. Sample specimens were commercial hot-rolled plates on which about 25 μm-thick oxide scales were formed. The results of the flash measurement with the analysis for two-layered sample provided an average thermal diffusivity value of 7.3 × 10^–7 m²s^–1 for the oxide scale. This value produces a thermal conductivity value of 3.8 W m^–1 K^–1. The future application of this measurement and analysis technique was also discussed to derive the thermal diffusivity and conductivity of oxide scales formed in actual hot-rolling processes.

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The primary-slags formation behaviors of sinter, lump ores and integrated burdens were studied to explore the high temperature interaction between sinter and lump ores in blast furnace. The results showed that the softening and melting properties of lump ores, viscosity and fluidity index of primary-slags of lump ores would be improved by the high temperature interaction. Particularly, the high temperature interaction was influenced by the chemical composition and porosity of iron ore samples. The high temperature interaction between sinter and lump ore L-2 (a typical limonite lump ore from Australia) was much stronger. Though the high temperature properties of lump ore L-2 was much worse than lump ore L-1 (a typical hematite lump ore from South Africa), the high temperature properties of integrated burden B (consist of sinter and lump ore L-2) was not worse than that of integrated burden A (consist of sinter and lump ore L-1) due to the strong high temperature interaction. In addition, the suitable MgO/Al₂O₃ weight ratio was calculated based on the assumption that the only phases of slags were akermanite and gehlenite, which showed a decreasing tend with the increase of CaO/SiO₂ weight ratio.

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Surface tension of molten SUS304 and SUS316 stainless steels was measured by an oscillating droplet method using electromagnetic levitation furnace to investigate the influence of oxygen partial pressure, P_O2, of atmosphere on surface tension and its temperature coefficient. We successfully measured the surface tension of molten stainless steels over a wide temperature range including an undercooling condition under Ar-He-5 vol.%H₂ and Ar–He mixed gases. Although the surface tension of molten SUS304 stainless steel decreased with increasing temperature under the Ar-He-5 vol.%H₂ mixed gas, the surface tension showed a boomerang temperature dependence when P_O2 was fixed at 10^–2 Pa under the Ar–He mixed gas; and surface tension increased and then decreased with increasing temperature. When the surface tension of molten SUS316 stainless steel was measured under the Ar-He-5 vol.%H₂ and Ar–He mixed gases, the surface tension increased with increasing temperature regardless of the atmospheric gas due to the large sulfur content. The uncertainty of the measurement was evaluated as less than 5% in this study.

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The surface tension of 430 stainless steel was measured using an electromagnetic levitation (EML) method at temperatures of 1707–2000 K, under a 5 vol% H₂–He atmosphere. For comparison, the surface tension was also measured using a constrained drop method; specifically the advanced sessile drop method. At 1823 K, the surface tension of the 430 stainless steel was estimated from the electromagnetic levitation and the constrained drop methods to be 1.802 and 1.614 N/m, respectively. A subsequent analysis of oxygen content showed that the former contained ~7 ppm oxygen, whereas the latter had 60 ppm. It was therefore considered that the observed difference in measurements was the result of a contamination by oxygen. Furthermore, the EML experimental results were found to be close to the theoretically calculated values for the Fe–Cr–Si system. Consequently, for complex multi-component commercial steels such as the 430 stainless steel, the levitation method is recommended for the measurement of surface tension.

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A new technique to study the kinetics of the carburization and decarburization processes of liquid iron is proposed. A liquid iron droplet was electromagnetically levitated in a CO–CO₂ gas mixture during carburization and decarburization, and its surface oscillation was continuously recorded using a high-speed camera. The surface tension varied depending on each elementary step in the carburization and decarburization processes. This behavior was caused by transient adsorption and desorption of oxygen on the surface of the liquid iron accompanied by CO and CO₂ gases. The kinetics of the carburization process were discussed and the conclusion was drawn that the desorption rate of oxygen adatoms in the form of CO₂ was predominantly controlled by the gas-phase mass transfer.

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Surface tension of liquid Ag–Cu binary alloys has been measured contactlessly using the technique of electromagnetic levitation in combination with the oscillating drop technique. The measurements were performed at temperatures above the melting point of alloys. The surface tension values were obtained from an analysis of the frequency spectra of droplet oscillations recorded with a CMOS-camera at 400 fps.
The alloy samples covered the entire composition range. The surface tension data can be described by linear functions of temperature with negative slopes. The new experimental data were compared to the corresponding theoretical values calculated combining the Butler equation with the ideal and subregular solution models, respectively. The agreement with the experimentally obtained data is excellent for the isothermal surface tension and quite reasonable for their temperature coefficients.

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The molar volumes of binary M–Si (M=Fe, Ni, and Ge) alloys in a liquid state were obtained from their densities measured using electromagnetic levitation coupled with a static magnetic field. We have systematically analyzed the molar volume as a function of composition. Although the concentration dependence of the molar volume of the liquid Ge–Si alloys obeys a linear law which is indicative of an ideal mixture, in comparison, that of the Ni–Si alloys shows a large discrepancy from the linear law as well as the Fe–Si system. The interaction between the constituent atoms was discussed with a view to analyzing the partial molar volume.

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Hydrogen ingressed into steels has been treated as one of detrimental factors causing the hydrogen-induced damages or failures and its concentrations is generally measured by a hot extraction system calibrated with references. High concentration references above 50.00 wppm is proposed to solve the bottlenecks of conventional references such as thermal instability and lower hydrogen concentrations at room temperature; by varying a mixing amount of titanium hydride as a hydrogen source with its balancing iron bead, the synthesized reference has freely controlable hydrogen concentration and also showed a good thermal stability at room temperature. Hydrogen residue in the iron bead was controlled less than 0.13 wppm by an optimized procedure including wet cleaning and thermal soaking at 300°C for 3 minute. Hydrogen weight fraction in the titanium hydride was calculated by 0.04 based on the assumption of the stoichiometric composition (or TiH₂). Hot extraction measurements and theoretical calculations of the hydrogen concentrations were done for the hydride-containing iron beads. Both data showed a good linear correlation each other and a validity of the proposed concept is confirmed emprically.

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In order to properly interpret the thermal conductivity of molten silicate slags, several different measurement methods are briefly reviewed, along with a discussion as to their respective advantages and limitations. In addition, a large number of thermal conductivity values measured by these methods are assessed, in order to evaluate their dependence on composition and temperature. The behavior of the thermal conductivity of molten silicate slags was found to show good agreement with well-focused analyses into its microstructure. Moreover, with an improved understanding of the microstructure of silicate slags, there is an apparent co-relation between thermophysical properties such as thermal conductivity, viscosity, and so forth.

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