Chemical Potentials of Oxygen in Four‐Phase Assemblages of the System CaO‐P2O5‐CaF2‐FexO

In steelmaking processes, because of environmental requirements and health considerations, there is a strong incentive to reduce slag volume. The key to meet this requirement is the better understanding of phosphorus removal, which relies on the knowledge of the thermodynamic properties of slags and fluxes used for dephosphorization. In this study, the liquidus compositions of the four‐phase assemblages in the quaternary system CaO‐P₂O₅‐CaF₂‐Fe_xO were determined at 1573K by employing electron probe microanalysis. Measurements were also made on the Fe_xO activities at temperatures between 1523K and 1673K by employing an electrochemical technique involving stabilized zirconia electrolyte.     

Activities of FexO in CaO‐(Na2O)0.1875(K2O)0.0625(Al2O3)0.25(SiO2)0.5‐FexO Pseudo‐Ternary Slags

Activities of Fe_xO in CaO‐(Na₂O)_0.1875(K₂O)_0.0625(Al₂O₃)_0.25(SiO₂)_0.5‐Fe_xO pseudo‐ternary slags were determined at 1673K by using a solid‐oxide galvanic cell; The substitution of (Na₂O)_0.1875(K₂O)_0.0625(Al₂O₃)_0.25(SiO₂)_0.5 for CaO has an effect of raising the Fe_xO activities.     

The Activities of FexO in {CaO‐SiO2‐Al2O3‐MgO‐FexO} Slags at 1723 K

In Japanese steelworks, hot metal is now produced by scrap melting process. With this process removal of sulphur is very much handicapped because of very high sulphur levels (0.04 to 0.09 pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such disadvantages, the authors explored on the phase diagrams of {CaO‐SiO₂‐Al₂O₃‐MgO} slags, and this research revealed that those slags at 35 wt%‐Al₂O₃ would be good candidates as reagents for the removal of sulphur from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, activities of Fe_xO were determined by using solid‐state electrochemical cells incorporating MgO‐stabilized zirconia and Mo + MoO₂ reference electrode.     

Thermodynamic behaviours of manganese and phosphorus between CaO‐MgOsat‐SiO2‐Al2O3‐FetO‐MnO‐P2O5 ladle slag and liquid iron

The thermodynamic equilibria of manganese and phosphorus between liquid iron and CaO‐MgO_Sat‐SiO₂‐Fe_tO‐MnO‐P₂O₅‐Al₂O₃ (0–33%) ladle slag have been investigated at 1873 K from the viewpoint of Mn and P yields for the production of high‐strength steels. The equilibrium distribution ratios of Mn and P were found to increase with increasing Fe_tO content; however, these ratios vary with basicity, but they do this the other way round. The addition of alumina into slag at a fixed basicity and Fe_tO content decreases both the equilibrium manganese and phosphorus distributions. The equilibrium distribution ratios were discussed in terms of the variation of activity coefficients of Fe_tO, MnO and PO_2.5, according to the slag basicity and Al₂O₃ content. The quantitative contributions of basicity and (%Fe_tO + %MnO) on L_Mn and L_P were empirically determined and their usefulness was discussed with the aid of plant data: To improve Mn and P yields in the practical RH operation, it is strongly recommended that Fe‐Mn and Fe‐P alloys be added after Al deoxidation treatment inducing relatively high Al₂O₃ in slag and maintaining low Fe_tO content. In addition, a ladle slag composition for the targeted Mn and P contents in liquid iron was substantially estimated using the empirical relationships.     

Thermodynamic assessment of CaO‐SiO2‐Al2O3‐MgO‐Cr2O3‐MnO‐FetO slags for refining chromium‐containing steels

The slag system of CaO‐SiO₂‐Al₂O₃‐MgO‐Cr₂O₃‐MnO‐Fe_tO relevant to refining chromium‐containing steels such as bearing steel is thermodynamically assessed at 1873 K. The activity coefficient of Fe_tO shows an initially rapid increment followed by a gradual reduction according to Cr₂O₃ content at a constant basicity, and decreases with increasing slag basicity. γ_MnO is decreased abruptly by increasing Cr₂O₃ content and thereafter, maintains a nearly constant level. From the standpoint of inclusion control, the Cr₂O₃ presence in ladle refining slags is thermodynamically harmful in that it minimizes the inclusion level by inducing the increment of γ_FetO even though Cr₂O₃ exists in extremely small amounts. However, it is beneficial in that it diminishes AI reoxidation by decreasing γ_MnO. The presence of carbon in slag decreases γ_FetO and γ_MnO, which turns out to be favourable for the reduction of Al reoxidation. The thermodynamic equilibria of chromium and manganese are quantified in terms of Fe_tO and Cr₂O₃ content as well as slag basicity by using multiple regression analysis. L_Cr and L_Mn are increased by the presence of Cr₂O₃, indicating a low recovery efficiency of Cr and Mn in the treatment of ferroalloy addition. In determining L_S values, Cr₂O₃ is not so important as the basicity of slags.     

The Effect of Carbon in Slag on Steel Reoxidation by CaO‐SiO2‐Al2O3‐MgO‐MnO‐FetO Slags

An equilibrium study was carried out at 1873K to ascertain the effect of carbon in CaO‐SiO₂‐Al₂O₃‐MgO‐MnO‐Fe_tO slag systems on their Fe_tO and MnO activity coefficients, representing the slags’ thermodynamic potential for steel reoxidation. Both γf e_to and γm no showed not only a sharp increment but also a simultaneous slow decrement by increasing carbon content in slag, suggesting opposite roles of the carbon according to its stable forms. XPS (X‐ray photoelectron spectroscopy) was introduced to clarify the stable forms of carbon in slag. XPS results prove that carbon dissolves in slag as carbonate, and carbide ions under oxidizing and reducing atmospheres, respectively. It was concluded that carbonate ions increase γf e_to and γm no , but that carbide decreases them. This paper suggests an application method of the present results to actual ladle refining processes, in order to enhance steel cleanliness with maintaining (Fe_tO + MnO) in slag to some allowable amount.     

Synthesis and Characterization of Fe2MoO4 as a Precursor Material for Mo Alloying in Steel

Iron molybdate (Fe₂MoO₄) has been studied as a new potential precursor for Mo additions in high alloy steel processing. Fe₂MoO₄ was synthesized by high temperature reactions between MoO₃, FeO_x and carbon by holding the mixture first for 23 hours at 873 K and then for 16 hours at 1373 K. The Fe₂MoO₄ syntheses were carried out with pure reagents as well as commercial grade materials supplied by steel industry. A thermodynamic analysis of the stabilities of the various phases in the Fe‐Mo‐O‐C quaternary was carried out. The synthesis processes, leading to the Fe₂MoO₄ formation from the precursors and further reduction by carbon were studied with the aid of thermogravimetric analysis (TGA), high‐temperature X‐ray diffraction (HT‐XRD) and evolved gas analysis by gas chromatography (GC). The maximum temperature in the case of all the experiments was 1373 K. It was found that the reactions between the precursor components start already above 873 K. The precursor mixture from commercial grade materials offers an economically advantageous process route with high Mo yield in steel.     

Viscosities of the Quaternary Al2O3‐CaO‐MgO‐SiO2 Slags

Viscosities of some quaternary slags in the Al₂O₃‐CaO‐MgO‐SiO₂ system were measured using the rotating cylinder method. Eight different slag compositions were selected. These slag compositions ranging in the high basicity region were directly related to the secondary steel making operations. The measurements were carried out in the temperature range of 1720 to 1910 K. Viscosities in this system and its sub‐systems were expressed as a function of temperature and composition based on the viscosity model developed earlier at KTH. The iso‐viscosity contours in the Al₂O₃‐CaO‐MgO‐SiO₂ system relevant to ladle slags were calculated at 1823 K and 1873 K for 5 mass% MgO and 10 mass% MgO sections. The predicted results showed good agreement with experimental values and the literature data.     

A thermodynamic study of SrO + SrCI2 + FexO fluxes by means of solid-oxide galvanic cell

Electrochemical measurements of the solid-oxide galvanic cell, Mo / Mo + MoO₂ // ZrO₂(MgO) // Fe + (SrO + SrCI₂ + Fe_xO) / Ag / Fe, have been made at temperatures of 1473 and 1623 K in order to obtain the activities of Fe_xO in SrO + SrCI₂ + Fe_xO fluxes. The Fe_xO activities showed a significant dependence upon the molar ratio of SrO/SrCI₂; within homogeneous liquid region the substitution of SrCI₂ for SrO has an effect of raising the Fe_xO activities. From these activity data, the phase diagrams of the system SrO + SrCI₂ + Fe_xO have been drawn at 1473 and 1623 K. The temperature dependence of the activity coefficient of Fe_xO at a fixed SrO/SrCI₂ molar ratio can be expressed by the formula R T In constant.     

Sulphide Capacities of CaO‐SiO2‐Al2O3‐MgO Slags

In Japanese steelworks, hot metal is now being produced by a scrap melting process. With this process, removals of sulphur is very much handicapped because of very high sulphur levels (0.04‐ to 0.09‐ pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such handicaps, the authors explored on the respective phase diagrams. These explorations revealed that {CaO‐SiO₂‐Al₂O₃‐MgO} slags with Al₂O₃ contents of 30‐ to 35‐pct by weight would be good candidates as reagents for sulphur removal from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, sulphide capacities were determined through gas/slag equilibrium technique. The experimental results suggest that there would be, at least, a “window” to remove sulphur from high sulphur hot metal as relatively low temperatures.     

A thermodynamic study of the system CaO + Al2O3 + FexO at 1673 K

Electrochemical measurements of the solid-oxide galvanic cell: Mo/Mo + MoO₂/ZrO₂(MgO)/(CaO + Al₂O₃ + Fe_xO) + Fe(s) + Ag/Fe have been conducted at 1673 K in order to obtain the activities of Fe_xO) in CaO + Al₂O₃ + Fe_xO slags. By using the activity data for Fe_xO, the isothermal section of the phase diagram for the system CaO + Al₂O₃ + Fe_xO was derived.     

Oxidation State of Titanium in CaO‐SiO2‐TiOx Slags at 1873K

Oxidation state of titanium was determined in CaO‐SiO₂‐TiO_x slags in the composition range 25‐53 percent CaO, 27‐46 percent SiO₂, 10‐55 percent TiO_x at 1873K using gas equilibration method. In the experiments, slags with different titanium oxide contents were equilibrated with a known carbon monoxide and carbon dioxide ratio. The results were used to determine the Ti³⁺ and Ti⁴⁺ contents as well as the activity coefficient ratio of corresponding oxides in the slag. The dependence of the activity coefficient ratio as a function of oxygen partial pressure was determined.     

Chemical potentials of components of the system CaO + P2O5 + FexO at 1673 K

Electromotive force (emf) measurements were conducted with a solid oxide galvanic cell of the type Mo/Mo + MoO₂/ZrO₂ (MgO)/Fe (s) + Fe_xO (in slag)/Ag/Fe at 1673 K in order to obtain the activities of Fe_xO in CaO + P₂O₅ + Fe_xO ternary slags. By using the Gibbs-Duhem integration, the activities of P₂O₅ and CaO were also obtained.     

A thermodynamic study of the system FexO + AI2O3 + SiO2 at 1673 K

Electrochemical measurements of the solid-oxide galvanic cell Mo/Mo + MoO₂/ZrO₂(MgO)/Fe + (Fe_xO + A1₂O₃ + SiO₂)_slag/Ag/Fe have been made at 1673 K in order to obtain the activities of Fe_xO in Fe_xO + A1₂O₃ + SiO₂ slags. Activities of A1₂O₃ and SiO₂ were also determined by virtue of Gibbs-Duhem integration. By using the activity data, the free energies of formation of hercynite and mullite were also obtained. Leave of absence from the Steelmaking Research Section, Iron and Steel Research Laboratories, Kobe Steel Ltd.     

Activities of Phosphorus in Copper‐Iron Liquid Alloys Saturated with Copper‐Iron Solid Solutions

In order to determine the activities of phosphorus and iron in liquid {Cu‐Fe‐P} alloys, the two coexisting phases of liquid {Cu‐Fe‐P} alloys + <Cu‐Fe‐P> solid solutions were brought into equilibrium with a mixture of Al₂O₃ + AlPO₄ + Fe_xAl₂O₄ at temperatures of 1416K and 1526K. The oxygen partial pressures were measured with the aid of a solid‐oxide galvanic cell of the type: (+)Mo / Mo + MoO₂/ ZrO₂(MgO) / {Cu‐Fe‐P} + <Cu‐Fe‐P> + <Al₂O₃> + <AlPO₄> + <FeAl₂O₄> / Fe(‐) The equilibrium reactions underlying the experiments can be expressed by 2[P]_cu + (5/2) (O₂) + <Al₂O₃> = 2 <AlPO₄> and x[Fe]_Cu + (1/2) (O₂) + <Al₂O₃> = <Fe_xAl₂O₄> The Henrian activity coefficient referred to 1 wt pct solution in pure liquid copper could be well expressed by the formula log f_P° = (4.46±0.40) ‐ (8.67±0.59)/(T/K). The iron activities referred to pure solid iron could be formulated as log a_Fe =‐ (0.37 ± 0.12) + (500 ±200) /(T/K).     

A thermodynamic study of SrO + FexO and BaO + FexO liquid slags by disposable electrochemical oxygen probes

An electrochemical technique involving magnesia-stabilized zirconia as the solid electrolyte and a mixture of Mo + MoO₂ as the reference electrode for the measurements of the activities of Fe_xO in the SrO + Fe_xO and BaO + Fe_xO systems at 1673 K. Measurement of oxygen potentials established at the slag electrode by disposable solid state cells. Expression of the thermodynamic properties of SrO + Fe_xO and BaO + Fe_xO by the binary subregular model.     

Phase Equilibrium of the System CaO‐P2O5‐SiO2 between 1473 K and 1673 K

Phase equilibrium was investigated in the ternary system of CaO‐P₂O₅‐SiO₂ at 1473K, 1573K and 1673K.     

Composition Control of CaO‐MgO‐Al2O3‐SiO2 Inclusions in Tire Cord Steel ‐ a Thermodynamic Analysis

The effect of oxide component content on the low melting point zone (LMP) in the CaO‐MgO‐Al₂O₃‐SiO₂ system has been analysed using FactSage software. The contents of dissolved elements [Si], [Mg], [O] and [Al] in liquid steel in equilibrium with the LMP inclusions in the CaO‐MgO‐Al₂O₃‐SiO₂ system have been calculated. The results show that the CaO‐MgO‐Al₂O₃‐SiO₂ system has the largest LMP zone (below 1400°C) when the Al₂O₃ content is 20% or the MgO content is 10%. The LMP zone becomes wide with the increase in CaO content (within the range of 0~30 mass%) and the decrease in SiO₂ (from 25 to 5 mass%). To obtain the LMP (below 1400°C) inclusions, the [Mg], [Al] and [O] contents must be controlled within the range of 0.2~2 ppm, 1.0~2.0 ppm and 60~100 ppm, respectively.     

The Effect of MgO on the Viscosity of the CaO‐SiO2‐20 wt%Al2O3‐MgO Slag System

The viscosities of CaO‐SiO₂‐20 wt%Al₂O₃‐MgO slags (CaO/SiO₂ = 1.0–1.2, wt%MgO = 5–13) were measured to estimate the effect of MgO on the viscous behaviour at elevated temperatures. The slag viscosity at 1773 K decreased with increasing MgO contents, which was typical of a basic oxide component at relatively low basicity (CaO/SiO₂) of 1.0. The FT‐IR spectroscopic analysis of the slag structure seems to verify this behaviour. However, an unexpected contradiction with the temperature dependence was observed above 10 wt%MgO and above CaO/SiO₂ of 1.2. Although the apparent activation energy was expected to decrease with additions of the basic oxide component MgO, the apparent activation energy increased. This unexpected behaviour seems to be related to the change in the primary phase field correlating to the phase diagram corresponding to the slag composition. Therefore, in order to understand the viscosity at both high Al₂O₃ and MgO, not only should the typical depolymerization of the slag structure with high MgO content be considered but also the primary phases of which the molten slag originates.     

Surface Structure of Wustite observed using Scanning Tunneling Microscopy

The surface structure of polycrystalline wustite was observed by means of scanning tunnel microscopy (STM). Wustite (Fe_1‐xO) samples were prepared from oxidation of iron plates under different oxygen partial pressures. The non‐stoichiometry numbers (1‐x) of Fe_1‐xO were 0.912, 0.901 and 0.887, respectively. Fe_1‐xO surface is composed of many terraces like stairs. The planes are covered with mesh. The mesh size is about 1.3 nm, corresponding to the double lattice constant of Fe_1‐xO, and independent of x. There are some depressions in the mesh randomly distributed and the mesh around the depressions is deformed.