The knowledge of Cr₂O₃ activity in slag is necessary to decarburize chromium-containing special steel while minimizing the oxidation loss of chromium. In the present study, firstly, the phase relationship in the CaO-SiO₂-Cr₂O₃ system was determined at 1573 K. Subsequently, the Cr₂O₃ activities in two- and three-phase coexisting slags at 1573 K were measured by equilibrating molten copper with oxide phases under a stream of Ar + H₂ + CO₂ gas mixtures, and the Gibbs energy changes of the formations of CaCr₂O₄ and Ca₃Cr₂Si₃O₁₂ were derived. It was found that the present results were compatible with the phase diagram and the literature data reported at higher temperature than 1573 K.

For a better thermodynamic understanding of the optimization of molten slag in the stainless-steel refining process, the solubility of chromium oxide in the CaO–SiO₂–Cr₂O₃ slag system was investigated. A chemical equilibrium technique was employed in which molten slag and solid Cr₂O₃ pellets were equilibrated under a regulated oxygen partial pressure (P_O2) lower than 10⁻¹¹ atm at 1873 K. The mass ratio of CaO to SiO₂ (X_CaO/X_SiO2), as an index of slag basicity, was varied from 0.5 to 1.5. A CO–CO₂–Ar gas mixture was used, and the oxygen partial pressure was precisely controlled by evaluating possible contamination by oxygen that was inevitably introduced into the gas mixture system. A zirconia oxygen sensor was used to directly measure the oxygen partial pressure for this evaluation. The solubility of chromium oxide increased with decreasing slag basicity and oxygen partial pressure. Accordingly, slag design is a huge prospect toward achieving desirable refining conditions.

The chemical states of Fe and Cr in CaO-SiO₂-MgO based oxide glasses prepared assuming actual chromium steel slag with different compositions and different melting conditions were investigated using X-ray absorption near edge structure (XANES) measured by X-ray absorption spectroscopy (XAS), and the valence ratios of each metal were determined. The results showed that the valence ratios changed depending on the oxygen partial pressure during melting, the basicity (CaO/SiO₂ weight ratio), and the coexistence of Fe and Cr. It was also found that Fe was a two-component system with divalent and trivalent valences coexisting, while Cr had divalent, trivalent, and hexavalent valences, but only divalent and trivalent or trivalent and hexavalent valences coexisted. In particular, the effect of coexistence is complicated by the chemical states of Fe and Cr. The effect of MgO addition is not as large as that of metal oxide coexistence, but it is not only as a basic oxide. The usefulness of direct and quantitative analysis of metal chemical states by XAS was demonstrated.

Controlling inclusion content in high chromium steel is very important to prevent submerged entry nozzle from clogging in continuous casting and avoid the negative impacts of inclusions on steel properties. Therefore, effects of temperature and content of elements on phase stability diagram should be clarified in chromium bearing steel. However, the effect of chromium content on boundaries of MgO, MgO∙Al₂O₃ and Al₂O₃ in phase stability diagram are much different among the researchers. The direction of boundaries shift is affected by chromium content differently. Temperature dependencies of deoxidation equilibrium constants below 1873 K are also scattered. Calcium, which is used to avoid the negative effect of MgO∙Al₂O₃ inclusion, enlarges liquid region in phase stability diagram. However, the region replaced by liquid oxide is understood differently in low alloyed steel and high chromium steel. In TiO_x-Al₂O₃-MgO system inclusion, commercial thermochemical software predicts that boundaries of Ti₂O₃, Ti₃O₅, Al₂O₃ and TiO_x-Al₂O₃ shift toward lower titanium content in high chromium steel. However, the calculated phase stability diagrams vary among studies even in liquid iron or low alloyed steel. Therefore, equilibrium experiments under various conditions and reliable technique of thermodynamic calculation with high accuracy are desired.

The aluminum deoxidation equilibrium in molten Fe-10 to 40 mass%Cr-8 mass%Ni and Fe-18 mass%Cr-8 to 30 mass%Ni alloys was experimentally determined at 1 873 K and 1 773 K to obtain the thermodynamic parameters at both temperatures, corresponding to the refining and casting processes, respectively. Thermodynamic analysis on Al deoxidation was carried out based on the sub-regular solution model using a Redlich–Kister type polynomial. Fe–Al, Ni–Al, Cr–Al and Fe–Cr–Ni interaction parameters were obtained from experimental results and a thermodynamic assessment. Using these parameters, the Al deoxidation equilibrium over the complete composition range of the Fe–Ni alloy and in more than 50 mass%Fe of the Fe–Cr and Fe–Cr–Ni alloys can be calculated for the temperature ranges of both of the refining and casting processes.

Demands for cleanliness of high chromium steel have been increasing. In steel refining process, aluminum is usually added in molten steel as a deoxidizing agent. As a result, such inclusions as alumina (Al₂O₃) and spinel (MgO∙Al₂O₃) are formed, which cause fatigue failures and surface defects. Therefore, it is important to understand the conditions of the inclusions which form in high chromium steel, and to reduce their harmful effects on steel qualities. In this work, to begin with, thermodynamic conditions of MgO and MgO∙Al₂O₃ formation in Fe-17 mass%Cr molten steel at 1873 K were investigated. The results showed that MgO is more stable in high chromium steel than in plain steel. The boundary of the stable condition of MgO and MgO∙Al₂O₃ shifts toward higher Al and lower Mg contents in high Cr steel. This cause is judged to be the effect of thermodynamic interaction between Cr and Mg. The interaction parameter of Cr on Mg was estimated to be 0.040 so that the boundary of stable condition of MgO and MgO∙Al₂O₃ can be explained. Moreover, phase stability diagram of Fe-Cr-Al-Ca-Mg-O system at 1873 K was developed to estimate the effect of chromium on the stable condition of MgO, MgO∙Al₂O₃ and CaO-MgO-Al₂O₃(l). Subsequently, the variations of inclusions which formed in Fe-17 mass%Cr molten steel were also investigated at 1873 K. The variations of inclusions in molten Fe-Cr steel were reasonably explained by considering the stable conditions of MgO and MgO∙Al₂O₃ investigated in this work.

Dolomite refractories are widely used in the refining process of clean steel and are considered potential sources of Mg and Ca that form MgO·Al₂O₃ spinel and CaO-containing inclusions. In this study, dolomite refractories were immersed into Al-killed molten steel with either 0.05% Al or 0.25% Al. The dissolution behavior of Mg and Ca from the dolomite refractory was studied, and the inclusion transformation behavior was observed. The results revealed that MgO in the dolomite refractory was reduced by Al in the molten steel, and the Mg content depended on the Al content. On the contrary, CaO barely dissolved into the molten steel even though the Al content increased. After immersion in both the low Al (0.05% Al) and high Al (0.25% Al) steels, an interfacial layer consisting of solid MgO and liquid phase CaO–Al₂O₃–MgO was formed on the surface of the rods. The initial Al₂O₃ inclusions gradually changed into Al₂O₃ saturated MgO–Al₂O₃ spinel after 60 min in low-Al steel; but were quickly transformed into MgO-saturated MgO–Al₂O₃ spinel in high Al steel. No CaO-containing inclusions were detected in the molten steel regardless of the immersion time and Al content.

Deoxidation equilibria of Fe–Mn–Al melt with Al₂O₃ or MnAl₂O₄ were measured at 1773 K. Composition of melts doubly-saturated with Al₂O₃ and MnAl₂O₄ were also measured using a crucible comprising these two phases at 1873 or 1773 K. Equilibria with each solid oxide were analyzed using Wagner’s Interaction Parameter Formalism (WIPF). In the case of Al₂O₃ saturation, Al deoxidation curve at 1773 K was similar in shape to that at 1873 K, and the equilibrium oxygen content was approximately 1/3 of that at 1873 K. The deoxidation equilibria were reproduced using WIPF at the composition range above 0.1 mass%Al by using −0.32 as e_O^Al and 10^−13.4 as the equilibrium constant of Al₂O₃ dissolution reaction, both of which were determined through analysis of measured results for Fe–(20 to 30) mass% Mn melt. In the case of MnAl₂O₄ saturation, accurate values of equilibrium constant were not obtained because of the relatively significant influence of oxygen analysis error. On the contrary, using compositions doubly-saturated with Al₂O₃ and MnAl₂O₄, valid values of the equilibrium constant of MnAl₂O₄ dissolution reaction, 10^−15.4 and 10^−17.7 at 1873 and 1773 K, respectively, could be determined.

Morphology and composition of inclusions change with temperature. However, besides the temperature conditions during steelmaking or continuous casting, other factors contributing to changes in the morphology and composition of inclusions during solidification are still unknown. In this study, the formation of complex inclusions in Si-Mn deoxidized steel after isothermal holding at the solid-liquid equilibrium temperature (T_S) of steel was investigated.

The typical inclusions found in the alloy were MnO-SiO₂ based, spherically shaped and homogeneously distributed. With isothermal holding at the solid-liquid equilibrium temperature of steel, formation of a secondary SiO₂-rich inclusion phase occurred. The changes in the composition of the inclusions depended on the manganese and silicon contents in the metal.

The general mechanism of inclusion formation observed in this study can be divided into three steps: 1) the formation of primary MnO-SiO₂ inclusions above the liquidus temperature when the steel is in a completely molten state as a result of the deoxidation process; 2) the nucleation of secondary inclusions as the molten steel becomes supersaturated with the solute elements while holding at the solid-liquid equilibrium temperature; and 3) the growth and coalescence of inclusions due to natural convection in the molten alloy. From this, the inclusions formed in Si-Mn deoxidized alloys held isothermally at the solid-liquid equilibrium temperature were of three types: primary MnO-SiO₂ inclusions, secondary SiO₂ inclusions and complex inclusions with both primary MnO-SiO₂ inclusions and precipitated secondary SiO₂ inclusions.

With the emerging significance of creating an acicular ferrite microstructure to provide an optimum set of properties in steel, MnS precipitation behavior on a MnO-SiO₂ inclusion at the solid-liquid coexistence temperature was experimentally investigated and thermodynamically elucidated in this study. Using a direct method of forming inclusions, alloy samples with varying sulfur concentrations [Fe-1.1Mn-0.10Si-0.05O-S (initial mass %); 0.005 to 0.031 initial mass % S] were prepared by holding at the solid-liquid coexistence temperature for 1 hour.

In samples with less than 0.011 mass % sulfur, the formation of a MnO-SiO₂ inclusion with a SiO₂-rich precipitate was observed. Formation of SiO₂ was described as a consequence of silicon enrichment in the liquid phase, which, under appropriate thermodynamic conditions, homogeneously precipitated and later on coalesced with the primary MnO-SiO₂ phase. On the other hand, in samples with more than 0.022 mass % sulfur, heterogeneous precipitation of MnS along the boundary of the primary MnO-SiO₂ inclusion and the alloy matrix was observed. Also, the SiO₂-rich phase was found to disappear with increasing sulfur addition. Since the likelihood of heterogeneous nucleation is higher than homogeneous nucleation, it was assumed that MnS precipitation on the surface of the primary MnO-SiO₂ and prevented the secondary SiO₂-rich inclusion from coalescing with the existing MnO-SiO₂ inclusion. This was also further validated for solute enrichment in the liquid phase, wherein MnS precipitation temperature was found to shift to higher temperatures in alloys with higher sulfur content.

Phase equilibria in Fe-Mn-S and Fe-Cr-S ternary systems at 1843 K were investigated experimentally, respectively. The main characteristic of these two systems at 1843 K was confirmed to be a wide miscibility gap between two liquid phases: molten metal alloy phase and molten sulfide phase. Through metal/sulfide equilibrium method, activity of constituents in sulfide phase were determined in Fe-Mn-S and Fe-Cr-S systems separately. The activity curves of constituents in sulfide phase were estimated by utilizing regular solution model.

Phase equilibria in Fe-Cr-Mn-S quaternary system at 1843 K were investigated experimentally. Two liquid phases: molten metal alloy phase and molten sulfide phase were in equilibrium in this system at 1843 K. The equilibrium relations between molten metal alloy and sulfide phases were experimentally measured. By using metal/sulfide equilibrium method, activity of constituents in molten MnS-CrS-FeS sulfide phase were determined. By utilizing regular solution model, activity curves of constituents in sulfide phase were estimated.

Thermodynamic property of solid MnS-CrS-FeS system was determined based on the combination between thermodynamic properties of liquid MnS-FeS, CrS-FeS, and MnS-CrS-FeS phase determined by the authors and reported phase diagram of MnS-FeS, CrS-FeS and MnS-CrS system. The determined parameters were verified by comparison with experimental results of equilibrium relationship between metal/sulfide in Fe-Cr-Mn-S system at 1793K. By utilizing the determined parameters, phase equilibria involving sulfide phase in liquid Fe-Cr-Mn-S system was established. According to the phase equilibria information controllability of MnS-CrS-FeS sulfide phase in typical stainless steel during solidification was evaluated.

The mechanism of monotectic sulfide formation in free-machining ferritic stainless steel (SUS430F) was investigated. Monotectic sulfides were observed in SUS430F ingots with Si-Mn deoxidation. Eutectic sulfide was found in Al deoxidized ingot. The selection of sulfide morphology can be predicted by the thermodynamic calculation using FactSage software. It was thought that the decrease in sulfide activity by dissolved oxygen as an oxy-sulfide was contributed to the monotectic sulfide formation. Microsegregation of sulfur and oxygen during solidification reduces the interfacial energy between liquid and sulfides and promotes the nucleation of sulfides. Thermodynamic stability was considered to determine the selection of sulfide morphology in SUS430F.

The contact angles between three non-metallic inclusion-type oxide substrates, viz. Al₂O₃, MgO, and MgO·Al₂O₃, and molten Fe and molten Fe-based stainless steel (Fe-Cr-Ni alloy) were measured using the sessile drop method in Ar atmosphere at 1873 K. The contact angles between molten Fe and oxide substrates ranged between 111° and 117°, while that between molten Fe-Cr-Ni alloy and substrates ranged between 103° and 105°. The angles between the alloy and each of the substrates were smaller than the corresponding values for Fe, which was attributed to the superior wettability of molten Fe-Cr-Ni alloy on the substrates. The wettability of the molten materials is related to the interfacial tension between the molten metals and each substrate. Thus, the interfacial tension between the molten metals and the non-metallic substrates was quantitatively evaluated using Young’s equation and the measured contact angles; the interfacial tension for molten Fe ranged from 1.862 to 2.781 N·m⁻¹, while that for molten Fe-Cr-Ni alloy ranged from 1.513 to 2.286 N·m⁻¹. Owing to the higher reactivity between molten Fe-Cr-Ni alloy and the substrates, the interfacial tension and energy between them were lower than those between molten Fe and the substrates.

The contact angle between molten Fe-Al alloy with 0.03, 0.3, and 3 mass% Al composition, and Y₂O₃ matrix oxide substrate with 0.002, 0.32, and 1 SiO₂ activity was measured using sessile drop method in Ar atmosphere at 1873 K, and the interfacial tension was evaluated. The contact angle and interfacial tension between the molten Fe-0.3 Al alloy and the Y₂Si₂O₇ + SiO₂ (a_SiO2 = 1) substrate decreased over time during 60 s after the molten alloy was dropped onto the substrate. The decrease of the contact angle was 20°, and that of the interfacial tension was 628 mN・m⁻¹ Conversely, the other contact angles and the other interfacial energies were almost stable during the same period. The decrease of the contact angles ranged between 0° and 7°, and that of the interfacial tensions ranged 4 and 195 mN・m⁻¹. By observing the wetting behavior for 60 min, it was recognized that the interfacial reaction between the Fe-Al alloy and the oxide substrate was the redox reaction between Al composition in the alloy and SiO₂ composition in the substrate, composed of SiO₂ decomposition reaction and Al₂O₃ formation reaction between oxygen absorbed at the interface and Al composition in the alloy. In addition, it was indicated from the interfacial tension dependence on SiO₂ activity that the medium SiO₂ volume slag for the molten low-Al steel and the low SiO₂ volume slag for the molten high-Al steel were effective in preventing the small droplets of molten slag into the molten steel.