Thermodynamics of iron alumino-chromite spinel inclusions in steel

The effect of chromium on the oxygen concentration of iron melts in equilibrium with various spinel reaction products has been determined. Alumina crucibles were used and experiments were performed at 1550, 1600, and 1650°C. Thermodynamic relationships between the equilibrium concentrations of chromium and oxygen in the iron melts have been established for chromium concentrations ranging up to 20 wt pct. Results from X-ray and electron microprobe analyses for the composition of the deoxidation products, together with solute activity relationships, indicate that the composition of the equilibrium spinel phase changes progressively from iron aluminate in the absence of chromium, through a series of aluminate-chromite solid solutions, FeO (Al _x Cr_1−x )₂O₃, (<0.5 pct chromium), to a complex chromite spinel, Fe₂Cr₇O₁₂, (0.5 to 3 pct chromium), and finally chromium oxide, Cr₃O₄ (>3 pct chromium). Deoxidation diagrams have been constructed and the effects of small amounts of alloying elements on the deoxidation behavior of aluminum interpreted in terms of buffered reactions which maintain oxygen concentrations in the melt at levels in excess of those normally associated with aluminum killed steel in equilibrium with alumina alone.     

Quantitative evaluation of inclusion in deoxidation of Fe-10 mass pct Ni alloy with Si, Ti, Al, Zr, and Ce

The composition, size, number, and morphology of the inclusions in deoxidation of Fe-10 mass pct Ni alloys with 0.2 mass pct M (M=Si, Ti, Al, Zr, and Ce) containing mostly 60 to 130 mass ppm oxygen were studied as a function of holding time at 1873 K. It was found that the initial primary inclusions contained FeO and the FeO content decreased with an increase of deoxidation power and holding time. The mean spatial diameter of inclusions tends to increase with increasing holding time, but did not show the systematic trend with respect to the deoxidation power. The number of particle sections per unit area decreased with decreasing deoxidation power and increasing holding time. In the absence of stirring of melt, the growth rate of the mean spatial diameter of inclusions and the removal rate of particle sections per unit area decreased rapidly with increasing holding time and approached a constant after 10 minutes. The morphology of inclusions was found to be spherical, polyhedral (except Si), and irregular including cluster, and the mean spatial diameter of these inclusions increased with increasing holding time.     

Thermodynamics of iron-manganese aluminate spinel inclusions in steel

The effect of manganese on the residual oxygen concentrations of liquid iron in equilibrium with alumina saturated iron-manganese aluminate spinel solid solutions was investigated at temperatures of 1550, 1600, and 1650°C. The relationship between the equilibrium concentrations of manganese and oxygen in iron melts containing up to 6 wt pct manganese has been established. The compositions of the spinel deoxidation products, (Fe _x Mn _j-x ) O · Al₂O₃, which were formed during equilibration with the iron melts were determined with electron microprobe and neutron activation analysis. From these results, new thermodynamic data pertaining to galaxite formation reactions have been derived and their implications with respect to the deoxidation of aluminum semikilled, silicon free, steels have been discussed.     

Kinetics of oxide formation in liquid copper and silver

The process of formation of oxide phases in liquid copper and liquid silver has been studied by pumping oxygen into the liquid alloy with the use of a solid-electrolyte electrochemical cell. The oxygen potential at the electrolyte-metal interface is monitored simultaneously by the electrochemical cell. A wide range of effects were observed ranging from the very rapid formation of a layer of oxide at the electrolyte-metal interface to what appears to be homogeneous nucleation of the oxide in the metal. The results may help to explain some of the difficulties that sometimes have been observed in using this type of cell to measure the oxygen potential of a liquid metal. The results indicate that a supersaturation ratio of about 9 is necessary for homogeneous nucleation of iron oxide (most probably Fe₃O₄(s)) in liquid copper containing 0.01 to 0.07 pct iron. The interfacial tensionσ _{Cu-Fe 3}O₄ is calculated to be 0.74 J/m². In experiments with higher concentrations of deoxidant (0.2 pct Fe in Cu and 0.2 to 0.4 pct Ni in Ag) equilibrium precipitation of oxides apparently predominates over homogeneous nucleation for the experimental conditions employed. A mathematical model which partly explains the different effects observed is presented.     

Optimization of Intensified Silicon Deoxidation

For demanding wire applications steel cleanliness should be very high and the inclusions inevitably found in steel should be harmless. This means strict control of inclusions' size, quantity, and composition, pursuing deformable inclusions in rolling conditions. Primary inclusions are formed during steel treatment in the ladle. Most of these are removed to the ladle slag or on the lining. However, the rest of the inclusions still remain through the successive process stages, and some new inclusions are formed during casting and solidification. Conventionally, deformable inclusions are produced by Si–Mn deoxidation resulting in MnO–SiO₂–Al₂O₃ inclusions. This leaves, however, the oxygen content too high for demanding applications. In order to get really clean steel, the Si deoxidation needs to be strengthened by lowering the activity of SiO₂ forming in steel. This can be done by bringing the steel in intimate contact with a slag containing SiO₂–MnO–Al₂O₃ and additionally CaO and some MgO. With this kind of intensified Si deoxidation it is possible to produce steels with low oxygen content having inclusions that will elongate at rolling. In this paper thermodynamic examination of potential slag systems and compositions to equilibrate with steels having medium carbon and high silicon were scrutinized. The optimal slag composition for producing low‐O steels with deformable inclusions was evaluated by using FactSage thermodynamic calculation program. The lowest SiO₂ activities at the region in which slag is still liquid at 1400°C, can be found when slag composition is approximately 36–40 wt% SiO_2, 6–18 wt% Al₂O_3, 30–40 wt% CaO, 6–8 wt% MgO, and 2–4 wt% MnO. Industrial heats using intensified Si deoxidation and slag based inclusion engineering were produced in a steel mill with 60 tons heat size. Inclusions and slag compositions were in satisfactory accordance with the theoretical examinations, though some scattering was discovered.     

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.     

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.     

Inclusion Characterization in Aluminum-Deoxidized Special Steel with Certain Sulfur Content Under Combined Influences of Slag Refining,Calcium Treatment,and Reoxidation

Inclusions in Al-killed steel with [S] of about 0.0060 to 0.0070 mass pct were characterized and discussed, evaluating the combined effects of basic slag refining and Ca treatment in ladle, together with reoxidation of liquid steel in casting tundish. Inclusions were changed from Al₂O₃ to MgO-Al₂O₃ spinel and then to MgO-Al₂O₃-CaO during basic slag refining. After Ca treatment, many (MgO-Al₂O₃)?+?CaS inclusions were formed, featuring the coexistence of MgO-Al₂O₃ and CaS to form a dual-phased structure. In the following Ar blowing, the number density of (MgO-Al₂O₃)?+?CaS inclusions and pure CaS increased obviously, which implied that [Ca] preferentially reacted with [S] rather than [O] in steel. Reoxidation in casting tundish caused the pickup of oxygen in steel, and the rise of total oxygen (T.O) was 0.0002 mass pct; even 55t steel has been poured. As a result, the content of CaO in inclusions increased and MgO-Al₂O₃-CaO inclusions were formed again. Thermodynamic calculations revealed that the driving force was strong for the formation of CaS-based inclusions. Higher carbon content in steel would help to reduce oxygen content while enhancing the activity of [S] in steel, which further stabilized the existence of CaS-based inclusions. Therefore, inclusions were mostly the solid (MgO-Al₂O₃)?+?CaS dual-phase ones, without the formation of liquid calcium aluminates. Contents of CaS and CaO in inclusions were affected by the [mass pct S]/[mass pct O] ratio, which was calculated as about 4.58 K and 5.34 K at 1873 K and 1823 K, respectively. This finding implied that lower oxygen was not favorable to prevent the solid inclusions in the calcium treatment of high carbon special steel.     

A thermodynamic study of silica-saturated iron silicate slags in equilibrium with liquid copper

The thermodynamic properties of silica-saturated iron silicate slags in equilibrium with liquid copper have been studied from oxygen partial pressure measurements in the temperature range from 1490 to 1580 K by means of a solid electrolyte galvanic cell. The following cells were used: Pt, Ni-NiO/O⁼/slag-Cu(l), Cr₂O₃, Pt; Pt, Fe-FeO/O⁼/slag-Cu(Fe sat.), Fe. A strong correlation was found between oxygen pressure and the copper content of the slag; the copper content increased from less than 1 pct near iron saturation to about 4 pct at an oxygen partial pressure of 7.2 x 10^?3 Pa. A similar correlation was found between the ferric iron/total iron ratio and the oxygen pressure. The oxygen content in liquid copper decreased with increasing iron content in liquid copper and increased slightly near iron saturation. This behavior could be explained qualitatively by using the standard free energy of formation of FeO and the activities of components.     

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.     

Control of MgO·Al2O3 Spinel Inclusions during Protective Gas Electroslag Remelting of Die Steel

The effect of calcium treatment and/or aluminum-based deoxidant addition on the oxygen control and modification of MgO·Al₂O₃ spinel inclusions during protective gas electroslag remelting (P-ESR) of H13 die steel with low oxygen content was experimentally studied. It is found that all the inclusions in the consumable electrode are MgO·Al₂O₃ spinels, besides a few MgO·Al₂O₃ spinels surrounded by an outer (Ti,V)N or MnS layer. After P-ESR refining combined with proper calcium treatment, all the original MgO·Al₂O₃ spinels in the electrode (except for the original MgO·Al₂O₃ spinels having been removed in the P-ESR process) were modified to mainly CaO-MgO-Al₂O₃ and some CaO-Al₂O₃ inclusions, both of which have a low melting point and homogeneous compositions. In the case of only Al-based deoxidant addition, all the oxide inclusions remaining in ESR ingots are MgO·Al₂O₃ spinels. The operation of Al-based deoxidant addition and/or calcium treatment during P-ESR of electrode steel containing low oxygen content is invalid to further reduce the oxygen content and oxide inclusions amount compared with remelting only under protective gas atmosphere. All the original sulfide inclusions were removed after the P-ESR process. Most of the inclusions in ESR ingots are about 2 μm in size. The mechanisms of non-metallic inclusions evolution and modification of MgO·Al₂O₃ spinels by calcium treatment during the P-ESR process were proposed.     

On Physico‐Chemical and Technical Limits in Clean Steel Production

Great progresses in steel cleanliness have been attained during the last decades. In the measures of oxygen the lowest levels are approaching 5 ppm in O_tot i.e. close to the thermodynamic limit of Al deoxidation. As the thermodynamics of the reaction system is firmly anchored to the steel chemistry and thus to the properties of steel and the final product, it is useful to examine the thermodynamic constraints for selected steels in the refining and casting processes. In most steels Al‐O equilibrium determines the limit of deoxidation. Calculations by applying thermodynamic software showed that in selected “common” steels the equilibrium oxygen content varied from below 5 ppm up to 30 ppm or higher depending on the aluminium content, interaction effect of other alloying elements and temperature. For lower oxygen content there are several possibilities. The first one is to use stronger deoxidizing additions like Ca, Zr, Ce etc. However, they can be problematic as substitutes as they produce inclusions which influence steel properties. The second way is to intensify the deoxidizing power of certain elements e.g. Si by bringing the steel in intimate contact with a proper slag with low SiO₂ activity in ladle treatment with violent stirring. In the calculation example it was possible to decrease oxygen content from 30 ppm to the level below 10 ppm in C/Mn/Si steel at very low Al level. The third potential but unused process is vacuum deoxidation especially for medium and high carbon steels. Equilibrium oxygen contents below 1 ppm are thermodynamically easily attainable. But in practice the vacuum treatment should be designed to intensify the carbon‐oxygen reaction on the top surface of the stirred steel liquid. Also the eventual reactions with refractory materials should be suppressed.     

A Reaction Between High Mn-High Al Steel and CaO-SiO2-Type Molten Mold Flux: Part I. Composition Evolution in Molten Mold Flux

In order to elucidate the reaction mechanism between high Mn-high Al steel such as twin-induced plasticity steel and molten mold flux composed mainly of CaO-SiO₂ during continuous casting process, a series of laboratory-scale experiments were carried out in the present study. Molten steel and molten flux were brought to react in a refractory crucible in a temperature range between 1713 K to 1823 K (1440 °C to 1550 °C) and composition evolution in the steel and the flux was analyzed using inductively coupled plasma atomic emission spectroscopy, X-ray fluorescence, and electron probe microanalysis. The amount of SiO₂ in the flux was significantly reduced by Al in the steel; thus, Al₂O₃ was accumulated in the flux as a result of a chemical reaction, 4[Al] + 3(SiO₂) = 3[Si] + 2(Al₂O₃). In order to find a major factor which governs the reaction, a number of factors ((pct CaO/pct SiO₂), (pct Al₂O₃), [pct Al], [pct Si], and temperature) were varied in the experiments. It was found that the above chemical reaction was mostly governed by [pct Al] in the molten steel. Temperature had a mild effect on the reaction. On the other hand, (pct CaO/pct SiO₂), (pct Al₂O₃), and [pct Si] did not show any noticeable effect on the reaction. Apart from the above reaction, the following reactions are also thought to happen simultaneously: 2[Mn] + (SiO₂) = [Si] + 2(MnO) and 2[Fe] + (SiO₂) = [Si] + 2(FeO). These oxide components were subsequently reduced by Al in the molten steel. Na₂O in the molten flux was gradually decreased and the decrease was accelerated by increasing [pct Al] and temperature. Possible reactions affecting the Al₂O₃ accumulation are summarized.     

Thermodynamic assessment of the Al deoxidation reaction in liquid iron

The deoxidation reaction of aluminum in liquid iron has been investigated thermodynamically using Al₂O₃ crucible at 1873 K under Ar atmosphere as a fundamental study for the accurate control of inclusions in the ladle refining process. In addition to the equilibrium constant log K_A1 for the aluminum deoxidation reaction, the first-order and second-order interaction parameters between aluminum and oxygen were experimentally determined in the concentration range of aluminum up to 1 %. The temperature dependence of the equilibrium constant and the first-order interaction parameter e^A1 was also obtained: log K_A1 = 12.32 - 47400/T, e^A1 = 15.57 - 36500/T. The equilibrium relation between aluminum and oxygen contents in the aluminum deoxidized iron by applying interaction parameters and the equilibrium constant determined in this work satisfies fairly well the equilibrium data over the whole concentration range of aluminum considered.     

Measurement of FeO activity and solubility of MgO in smelting slags

In bath smelting, the FeO activity of the slag must be known to predict the equilibrium of slag-metal reactions and for effective control of the rate of reduction in the system. Also, knowledge of the solubility of MgO in these slags is useful for reducing refractory consumption. A series of measurements of the FeO activity in simulated bath smelting slags (CaO-SiO₂-Al₂O₃-MgO_sat-FeO) were conducted by the electromotive force (EMF) technique. The influence of the slag composition on the relationship between the FeO activity coefficient and FeO content was studied. It has been found that the measured FeO activity coefficient decreases with increasing FeO content in the slag and increases slightly with increasing slag basicity, which is defined as (CaO + MgO)/(SiO₂ + Al₂O₃) on a mole fraction basis. The measured values of the FeO activity coefficient are in reasonable agreement with previously published data. The solubility of MgO was also measured and found to rang from 16 to 30 pct and decrease with increasing basicity.     

Interaction between refractory crucible materials and the melted NiTi shape-memory alloy

Attempts have been made to quantify the amount of contaminants absorbed by liquid metal from commercial ZrO₂-, Al₂O₃-, and SiC-base crucibles used for vacuum melting of Ni-45 wt pct Ti alloy. The molten alloy was held under vacuum for 90 minutes at 1450 °C to become homogenized. Reactions between the liquid metal and the crucible were investigated by visual observation, chemical analysis, scanning electron microscopy (SEM) image processing, and X-ray mapping. The relative degree of contamination declined in the following sequence: commercially pure SiC>SiC-5 wt pct Al₂O₃-5 wt pct SiO₂>slurry cast alumina>recrystallized alumina>zircon type A>oxygen deficient high-purity zirconia. Thermodynamic calculations showed a difference between the equilibrium and the experimental data, indicating that except for commercially pure SiC crucible, the amount of the crucible elements entering the melt is greater than the calculated equilibrium values. This discrepancy seems to be due to the immersion into the melt of the undissolved chemical compounds formed due to the reactions between the crucible and the liquid phase.     

A model for the silicon-manganese deoxidation of steel weld metals

From an analytical and theoretical study of flat and out-of-position gas metal arc (GMA) C-Mn steel welds containing varying additions of silicon and manganese, we conclude that the buoyancy effect (flotation obeying Stokes’ law) does not play a significant role in the separation of oxide inclusions during weld metal deoxidation. Consequently, the separation rate of the particles is controlled solely by the fluid flow pattern in the weld pool. A proposed two-step model for the weld metal deoxidation reactions suggests that inclusions formed in the hot, turbulent-flow region of the weld pool are rapidly brought to the upper surface behind the arc because of the high-velocity flow fields set up within the liquid metal. In contrast, those formed in the cooler, less-turbulent flow regions of the weld pool are to a large extent trapped in the weld metal as finely dispersed particles as a result of inadequate melt stirring. The boundary between “hot” and “cold” parts for possible inclusion removal is not well defined, but depends on the applied welding parameters, flux, and shielding gas composition. As a result of the intricate mechanism of inclusion separation, the final weld metal oxygen content depends on complex interactions among the following three main factors: (1) the operational conditions applied, (2) the total amount of silicon and manganese present, and (3) the resulting manganeseto-silicon ratio. The combined effect of the latter two contributions has been included in a new deoxidation parameter, ([pct Si][pct Mn])^−0.25. The small, negative exponent in the deoxidation parameter indicates that control of the weld metal oxygen concentrations through additions of silicon and manganese is limited and that choice of operational conditions in many instances is the primary factor in determining the final degree of deoxidation to be achieved.     

Effects of Reoxidation of Liquid Steel and Slag Composition on the Chemistry Evolution of Inclusions During Electroslag Remelting

Electroslag remelting (ESR) is increasingly used to produce some varieties of special steels and alloys, mainly because of its ability to provide extreme cleanliness and an excellent solidification structure simultaneously. In the present study, the combined effects of varying SiO₂ contents in slag and reoxidation of liquid steel on the chemistry evolution of inclusions and the alloying element content in steel during ESR were investigated. The inclusions in the steel before ESR refining were found to be oxysulfides of patch-type (Ca,Mn)S adhering to a CaO-Al₂O₃-SiO₂-MgO inclusion. The oxide inclusions in both the liquid metal pool and remelted ingots are CaO-Al₂O₃-MgO and MgAl₂O₄ together with CaO-Al₂O₃-SiO₂-MgO inclusions (slightly less than 30 pct of the total inclusions), which were confirmed to originate from the reduction of SiO₂ from the original oxide inclusions by dissolved Al in liquid steel during ESR. CaO-Al₂O₃-MgO and MgAl₂O₄ are newly formed inclusions resulting from the reactions taking place inside liquid steel in the liquid metal pool caused by reoxidation of liquid steel during ESR. Increasing the SiO₂ content in slag not only considerably reduced aluminum pickup in parallel with silicon loss during ESR, but also suppressed the decrease in SiO₂ content in oxide inclusions. (Ca,Mn)S inclusions were fully removed before liquid metal droplets collected in the liquid metal pool.     

Effect on cleanliness of molten steel with different refining slag systems for low alloy ship plate

To study the effect of refining slag on the compositions of molten steel and inclusions, the reaction between ship plate steel and different slag systems (slag A-CaO/Al₂O₃: 1.0, SiO₂: 5 mass-% and slag B-CaO/Al₂O₃: 1.5, SiO₂: 9 mass-%) was investigated by laboratory-scale high-temperature equilibrium experiments. Results showed that the desulphurisation capacity of the two slags was very similar, and the average sulphur content for both was 0.002?mass-% in steel, but the deoxidation capacity of slag B was slightly higher than slag A. The amount of inclusions <5?μm was more in steel balanced with slag B than in that balanced with slag A. To generate inclusions smaller than 5?μm, and spherical liquid CaO–MgO–Al₂O₃–SiO₂ inclusions, and to decrease the T[O] (total oxygen) content in steel, refining slag composition should be: CaO/Al₂O₃ ～ 1.5, and SiO₂ ～ 9?mass-%. Slag system optimisation can reduce or even eliminate calcium treatment during production. T[O] content in the slab could be controlled to 15–21?ppm, with typical micro-inclusions of ≤10?μm, and CaO–Al₂O₃–MgO and CaO–Al₂O₃–CaS systems in molten steel.