Modification of Inclusions in Molten Steel by Mg-Ca Transfer from Top Slag: Experimental Confirmation of the ‘Refractory-Slag-Metal-Inclusion (ReSMI)’ Multiphase Reaction Model

High-temperature experiments and Refractory-Slag-Metal-Inclusion (ReSMI) multiphase reaction simulations were carried out to determine the effect of the ladle slag composition on the formation behavior of non-metallic inclusions in molten steel. Immediately after the slag-metal reaction, magnesium migrated to the molten steel and a MgAl₂O₄ spinel inclusion was formed due to a reaction between magnesium and alumina inclusions. However, the spinel inclusion changed entirely into a liquid oxide inclusion via the transfer of calcium from slag to metal in the final stage of the reaction. Calcium transfer from slag to metal was more enhanced for lower SiO₂ content in the slag. Consequently, the spinel inclusion was modified to form a liquid CaO-Al₂O₃-MgO-SiO₂ inclusion, which is harmless under steelmaking conditions. The modification reaction was more efficient as the SiO₂ content in the slag decreases.     

Effect of Acid-Soluble Aluminum on the Evolution of Non-metallic Inclusions in Spring Steel

The content of acidic soluble aluminum in molten steel ([Al]_s) is of significance to the control of total oxygen (TO), the formation of non-metallic inclusions, and the improvement of the surface quality of billets. Industrial trials and thermodynamic calculations were performed to study the effects of [Al]_s content on the TO and the evolution of non-metallic inclusions in 60Si2Mn-Cr spring steel that was deoxidized by Si-Mn ((low aluminum process (LAP)) and Si-Mn-Al (high aluminum process (HAP)). The results show that the [Al]_s contents in billets are within 0.0060 to 0.0069 mass pct in the LAP and 0.016 to 0.055 mass pct in the HAP. The TO content at each station of the LAP is higher than that in the HAP; the inclusions of billets were mainly of the CaO-Al₂O₃-SiO₂ type in the former, and of the CaO-Al₂O₃-MgO and CaS-Al₂O₃-MgO types in the latter. A tendency is found that the higher the [Al]_s, the easier it is to deviate from the low melting point region of the inclusion distribution and the larger the size of the inclusions. The relationships between [Al]_s and the melting point of the oxide inclusions and the Al₂O₃ content in the oxide inclusions are also discussed in terms of experiment and calculation.     

Studies on dissolution kinetics of dolime in electrothermal magnesium slag

The electrothermal process of magnesium metal production is a promising route, where large sized internally heated reactor is used for magnesium production resulting in less energy and labour intensive and high space-time yield process. However, the dissolution behavior of dolime in the electrothermal slag has been found critical for the process optimization. In this paper, the dissolution kinetics of the dolime in the slag was discussed. Quaternary slag (CaO-Al₂O₃-SiO₂-MgO) was prepared having basicity CaO/SiO₂ ≥ 1.8 and Al₂O₃/SiO₂ ≥ 0.26 for dolime dissolution studies by static hot dip method. Prior to the experiments, FactSage calculations were carried out varying temperatures and slag compositions. In the kinetic studies, dolime particles 10–15 mm size was added in slag melted at 1450, 1500 and 1550°C and samples were taken at various time intervals. The chemical analysis of slag sample was carried out to investigate the dissolution kinetics to establish the rate expression. The activation energy for the process was calculated for different models used in study and was found to be in the range of 130–270 kJ/mol. SEM analysis was done for surface analysis of reacted particles. This study would be helpful in optimizing the dolime charging rate during pilot scale trials for electrothermal magnesium production at CSIR-NML, Jamshedpur.     

Reduction of CaO and MgO Slag Components by Al in Liquid Fe

This study documents laboratory-scale observations of reactions between Fe-Al alloys (0.1 to 2 wt pct Al) with slags and refractories. Al in steels is known to reduce oxide components in slag and refractory. With continued development of Al-containing Advanced High-Strength Steel (AHSS) grade, the effects of higher Al must be examined because reduction of components such as CaO and MgO could lead to uncontrolled modification of non-metallic inclusions. This may lead to castability or in-service performance problems. In this work, Fe-Al alloys and CaO-MgO-Al₂O₃ slags were melted in an MgO crucible and samples were taken at various times up to 60 minutes. Inclusions from these samples were characterized using an automated scanning electron microscope equipped with energy dispersive x-ray analysis (SEM/EDS). Initially Al₂O₃ inclusions were modified to MgAl₂O₄, then MgO, then MgO + CaO-Al₂O₃-MgO liquid inclusions. Modification of the inclusions was faster at higher Al levels. Very little Ca modification was observed except at 2 wt pct Al level. The thermodynamic feasibility of inclusion modification and some of the mass transfer considerations that may have led to the differences in the Mg and Ca modification behavior were discussed.     

Evaluation of Secondary Steelmaking Slags and Their Relation with Steel Cleanliness

Based on data provided from an industrial plant and FactSage commercial software use, a study of secondary refining slags and inclusion cleanliness was performed. Six heats of two slag series, namely, A and B, with average chemical composition (wt pct) of 43.00CaO-25.90SiO₂-12.96Al₂O₃-18.13MgO for series A and 49.98CaO-23.88SiO₂-10.11Al₂O₃-11.99MgO-4.03CaF₂ for series B, were used for the study. Both series used DIN 38MnS6 modified steel. The effective viscosity, solid fraction, composition of the liquid fraction, and slag saturation degree in MgO (calculated through thermodynamic software) were related to the experimental results obtained for the inclusion cleanliness. The B slags showed lower effective viscosity than the A slags, due to their high liquid fraction. Regarding the capacity of slags in the inclusion removal, slag B5 resulted in the lowest inclusion density and was considered as the best choice among the slags studied. The inclusion species formed using B slags are constituted especially of CaO-Al₂O₃-SiO₂ and MgO-Al₂O₃ and are Al₂O₃ rich. The presence of sulfide-type inclusions (AlMnS and CaS) were more pronounced among A slags.     

Behavior of MgO· Al2O Based Inclusions in Alloy Steel During Refining Process

The transformation of MgO ? Al₂O₃ based inclusions in alloy steel during refining has been studied by industrial trials. Besides Factsage software is used to study the formation and modification of spinel inclusions in alloy steel using calcium treatment during refining process. The results show that the transformation sequence of inclusions is: MgO ? Al₂O₃→CaO-Al₂O₃-MgO complex inclusions→MgO ? Al₂O₃, and under present experimental condition, in order to avoid forming MgO ? Al₂O₃ inclusions the content of dissolved Ca in the molten steel has to reach 1×10^?6. Also the results show that when more calcium was added to molten steel, the content of Al₂O₃ and MgO will be lower. Besides, increasing the content of CaO in the inclusions will increase even if the content of SiO₂ changes little.     

In situ studies of agglomeration between Al2O3–CaO inclusions at metal/gas,metal/slag interfaces and in slag

Abstract

Studies of inclusion behaviour at the metal/slag interface is of great importance for the steel industry in order to achieve better control of both the size and amount of the inclusions, as well as improving the steel quality and the casting process. In this work agglomeration of liquid Al₂O₃–CaO particles at both steel/argon gas and steel/slag interfaces was studied with a confocal scanning laser microscope. In addition, agglomeration of liquid Al₂O₃–CaO–SiO₂ inclusions present in the slag was investigated. The results showed that liquid inclusions more easily agglomerated to semiliquid inclusions than to liquid inclusions. Moreover, the agglomeration of liquid particles was found to be improved remarkably when the particles were present in the slag compared to when they were in the steel/slag interface.     

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.     

Dissolution Mechanism of Indium in CaO-Al2O3-SiO2 Slag at Low Silica Region

The solubility of indium was measured in the low-silica region (<20?mass pct SiO₂) of the CaO-Al₂O₃-SiO₂ system by a thermochemical equilibration technique. The dissolution mechanism of indium into the CaO-Al₂O₃-SiO₂ slag at 1773?K (1500?°C) under a reducing atmosphere was elucidated. The indium solubility increases in the calcium silicate-based flux and decreases in the calcium aluminate-based flux with increasing oxygen partial pressure. Also, the solubility was found to decrease initially with increasing slag basicity until the basicity reached a critical level after which the solubility increases. This behavior is believed to indicate that the indium dissolution mechanism changes according to the basicity of the slag.     

Effect of CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO slags on the formation of MgO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> inclusions in ferritic stainless steel

A thermodynamic equilibrium between the Fe-16Cr melts and the CaO-Al₂O₃-MgO slags at 1823 K as well as the morphology of inclusions was investigated to understand the formation behavior of the MgO-Al₂O₃ spinel-type inclusions in ferritic stainless steel. The calculated and observed activities of magnesium in Fe-16Cr melts are qualitatively in good agreement with each other, while those of aluminum in steel melts exhibit some discrepancies with scatters. In the composition of molten steel investigated in this study, the log (X _MgO/X _{Al 2}O₃) of the inclusions linearly increases by increasing the log [a _Mg/a _Al ² ·a _O ² ] with the slope close to unity. In addition, the relationship between the log (X _MgO/X _{Al 2}O₃) of the inclusions and the log (a _MgO/a _{Al 2}O₃) of the slags exhibits the linear correlation with the slope close to unity. The compositions of the inclusions are relatively close to those of the slags, viz. the MgO-rich magnesia-spinel solid solutions were formed in the steel melts equilibrated with the highly basic slags saturated by CaO or MgO. The spinel inclusions nearly saturated by MgO were observed in the steel melts equilibrated with the slags doubly saturated by MgO and MgAl₂O₄. The spinel and the Al₂O₃-rich alumina-spinel solid solutions were formed in the steel melts equilibrated with the slags saturated by MgAl₂O₄ and MgAl₂O₄-CaAl₂O₄ phases, respectively. The apparent modification reaction of MgO to the magnesium aluminate inclusions in steel melts equilibrated with the highly basic slags would be constituted by the following reaction steps: (1) diffusion of aluminum from bulk to the metal/MgO interface, (2) oxidation of the aluminum to the Al³⁺ ions at the metal/intermediate layer interface, (3) diffusion of Al³⁺ ions and electrons through the intermediate layer, and (4) magnesium aluminate (MgAl₂O₄ spinel, for example) formation by the ionic reaction.     

Simultaneous Modification of Alumina and MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Inclusions by Calcium Treatment During Electroslag Remelting of Stainless Tool Steel

Calcium modification of both alumina and MgO·Al₂O₃ inclusions during protective gas electroslag remelting (P-ESR) of 8Cr17MoV stainless steel and its effect on nitrides and primary carbides were studied by analyzing the transient evolution of oxide and sulfide inclusions in the P-ESR process. The oxide inclusions that were not removed during P-ESR without calcium treatment were found to retain their original state until in as-cast ingot. Calcium treatment modified all MgO·Al₂O₃ and alumina inclusions that had not been removed in the P-ESR process to liquid/partially liquid CaO-Al₂O₃-(MgO) with uniformly distributed elements, in addition to a small proportion of partially modified inclusions of a CaO-MgO-Al₂O₃ core surrounded by a liquid CaO-Al₂O₃. The modification of low-MgO-containing MgO·Al₂O₃ inclusions involves the preferential reduction of MgO from the MgO·Al₂O₃ inclusion by calcium and the reaction of calcium with Al₂O₃ in the inclusion. It is the incomplete/complete reduction of MgO from the spinel by calcium that contributes to the modification of spinels. Alumina inclusions were liquefied by direct reaction with calcium. Calcium treatment during P-ESR refining also provided an effective approach to prevent the formation of nitrides and primary carbides in stainless steel through modifying their preferred nucleation sites (alumina and MgO·Al₂O₃ inclusions) to calcium aluminates, which made no contribution to improving the steel cleanliness.     

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.     

Assessment of Oxygen Control and Its Effect on Inclusion Characteristics during Electroslag Remelting of Die Steel

Deoxidation during electroslag remelting of S136 die steel was experimentally studied. The characteristics of inclusions in the electrode and ESR ingots were determined by image analyzer and SEM‐EDS. The results show that the oxygen content can be reduced from 89 ppm in the electrode to the lowest (12 ppm) in the ingot only when protective Ar gas remelting in combination with specially designed slag deoxidation treatment were employed simultaneously. The proportion of the oxygen combined as oxide inclusions increases with decreasing the total oxygen content in ESR ingot. The original inclusions in the electrode are mainly large (Mn,Cr)S and the large inclusions in the form of Al₂O₃ core surrounded by an outer sulfide layer, besides a few pure Al₂O₃ inclusions. After ESR process, while only pure Al₂O₃ inclusions with the size of about 1 µm were observed in ESR ingots. The large inclusions in the electrode were removed during ESR process. With higher oxygen content in the ingot, the contents of inclusions and large inclusions would be relatively higher. The results from industrial experiments have confirmed the availability of the present oxygen control technique. The mechanisms of oxygen behavior and control as well as inclusion evolution during ESR process were proposed based on experimental results along with thermodynamic analysis.     

Kinetics of reduction of lead oxide in liquid slag by carbon in iron

The reduction of lead oxide in dilute solution in CaO-Al₂O₂-SiO₃ slag by carbon dissolved in iron was investigated using a composite crucible as a container so as to exclude graphite from the system. The variables studied to elucidate the reaction mechanism were pressure inside the crucible, carbon content of the metal, lead oxide concentration in slag, and slag composition. The experimental results are best explained by postulating the existence of a gas film at the slag metal interface. It is suggested that the rate controlling step for the lead oxide reduction by carbon is a chemical reaction at the gas/slag interface. The rate constant for up to 3 wt pct PbO in the slag and 2.0 to 4.3 pct C in iron at 1400 °C as calculated from the present study is 4.6 x 10^-4 mol/cm²/min/atm.     

Refractory–Slag–Metal–Inclusion Multiphase Reactions Modeling Using Computational Thermodynamics: Kinetic Model for Prediction of Inclusion Evolution in Molten Steel

The refractory–slag–metal–inclusion multiphase reaction model was developed by integrating the refractory–slag, slag–metal, and metal–inclusion elementary reactions in order to predict the evolution of inclusions during the secondary refining processes. The mass transfer coefficient in the metal and slag phase, and the mass transfer coefficient of MgO in the slag were employed in the present multiphase reactions modeling. The “Effective Equilibrium Reaction Zone (EERZ) Model” was basically employed. In this model, the reaction zone volume per unit step for metal and slag phase, which is dependent on the ‘effective reaction zone depth’ in each phase, should be defined. Thus, we evaluated the effective reaction zone depth from the mass transfer coefficient in metal and slag phase at 1873 K (1600 °C) for the desulfurization reaction which was measured in the present study. Because the dissolution rate of MgO from the refractory to slag phase is one of the key factors affecting the slag composition, the mass transfer coefficient of MgO in the ladle slag was also experimentally determined. The calculated results for the variation of the composition of slag and molten steel as a function of reaction time were in good agreement with the experimental results. The MgAl₂O₄ spinel inclusion was observed at the early to middle stage of the reaction, whereas the liquid oxide inclusion was mainly observed at the final stage of the refining reaction. The content of CaO sharply increased, and the SiO₂ content increased mildly with the increasing reaction time, while the content of Al₂O₃ in the inclusion drastically decreased. Even though there is slight difference between the calculated and measured results, the refractory–slag–metal multiphase reaction model constructed in the present study exhibited a good predictability of the inclusion evolution during ladle refining process.     

Evolution of CaO–MgO–Al2O3–CaS–(SiO2) inclusions in H13 die steel during electroslag remelting process

The effect of Al–Mg alloy addition on the cleanliness and CaO–MgO–Al₂O₃–CaS–(SiO₂) inclusions during electroslag remelting of H13 die steel with low oxygen content was investigated by experimental study and thermodynamic calculation. The results show that the oxygen content of consumable electrode (15?×?10^?6) was invalid to be reduced after Al–Mg alloy addition during protective gas electroslag remelting (P-ESR) process. In the case of Al–Mg alloy addition during P-ESR process, the oxygen content pick-up and silicon loss of remelted ingot were avoided, and sulphur content was further decreased. CaS content of complex inclusions were decreased significantly after P-ESR process. The type of inclusions was not changed during P-ESR process with Al–Mg alloy addition, except the increase in the concentrations of CaS in individual inclusion. The original oxide inclusions inconsumable electrode transformed to liquid state during P-ESR, and original CaS inclusions were eliminated. The CaS portion in CaS-bearing complex inclusions is the inclusion that formed on the surface of oxide inclusion during the cooling of liquid steel process in liquid metal pool.     

Formation of Low-Melting-Point Inclusions in Al-Deoxidized Steel Refined by High-Basicity Calcium Aluminate Slag in ZrO2 Crucible Experiments

ZrO₂ crucible experiments were carried out in the laboratory to study the formation of low-melting-point inclusions in steel, during which aluminum deoxidization and a high-basicity calcium aluminate refining slag (CaO/SiO₂: 6 to 8, Al₂O₃ 40 to 45 pct) were used. Four experiments were done with different slag/steel reaction times (30, 60, 90, and 180 minutes). It was found that inclusions were mainly composed of CaO-Al₂O₃-ZrO₂ with very limited SiO₂, in spherical morphology and with sizes mainly less than 5 μm. They can be classified into two types according to the ZrO₂ content. The first type contained much lower ZrO₂, whereas a much higher level of ZrO₂ was detected in the other type. An evolution of inclusions with the reaction time was observed and studied. The obtained results indicated that chemical compositions of inclusions were widely scattered in the CaO-Al₂O₃-ZrO₂ phase diagram after 30 min reaction. However, the composition of inclusions became much more uniform and concentrated in low-melting-temperature regions at 60, 90, and 180 minutes, which would be favorable to prevent nozzle clogging and fatigue problems. Compared with the authors’ previous results obtained in MgO crucible experiments, it was found that low-melting-temperature inclusions can be targeted in shorter time in ZrO₂ crucible experiments, without any degradation of cleanliness.     

Dissolution of alumina particles in CaO-Al2O3-SiO2-MgO slags

Abstract

The dissolution behaviour of alumina particles in CaO-Al₂O₃-SiO₂-MgO slags was studied by confocal scanning laser microscopy. Three different slag systems were investigated to evaluate the effect of changing the MgO content in a high silica tundish slag, and the effect of decreasing the silica content when dissolving the particles in a ladle slag. A particle of 80 mum diameter dissolved in 50 s in a simulated ladle slag but needed 100 s to dissolve in a high silica slag at the same temperature. When the silica content was decreased, a decrease in the alumina particle dissolution time was noted. During dissolution of alumina particles in a slag containing MgO, the alumina particle reacts with the slag to form an MgAlO₄ layer. No reaction layer was observed during the dissolution of alumina particles in slags that did not contain MgO. This work suggests that the dissolution time of large alumina particles is significant, increasing with particle size and with decreased temperature.     

Calculation and Analysis of Sulfide Capacities for CaO-Al2O3-SiO2-MgO-TiO2 Slags

The empirical models of sulfide capacity calculated by traditional optical basicity do not consider the charge compensation of alkaline metal ions to Al³⁺ in the molten slags, so that the deviations between the calculated values and measured ones of sulfide capacity are inevitable. The relation between sulfide capacity and the corrected optical basicity put forward by Mills considering the charge compensation was investigated. Combined with the relation between sulfide capacity and temperatures, a novel and accurate calculation model of sulfide capacity was proposed, which was applied to calculate the sulfide capacities of CaO-Al₂O₁-SiO₂-MgO and CaO-Al₂O₁-SiO₂-MgO-TiO₂ systems, where the sum of the CaO and MgO concentrations in the slags must be not lower than the Al₂O₃ concentration. It was also found that the calculated values were in a good agreement with the measured values, and the mean deviations were 2. 57% and 2. 65%, respectively.     

Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CaO-Al2O3, Al2O3-SiO2, and CaO-Al2O3-SiO2 systems

All available thermodynamic and phase diagram data have been critically assessed for all phases in the CaO-Al₂O₃, Al₂O₃-SiO₂, and CaO-Al₂O₃-SiO₂ systems at 1 bar pressure from 298 K to above the liquidus temperatures. All reliable data for the binary systems have been simultaneously optimized to obtain, for each system, one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasichemical model was used for the slag. With these binary parameters and those from the optimization of the CaO-SiO₂ system reported previously, the quasichemical model was used to predict the thermodynamic properties of the ternary slag. Two additional small ternary parameters were required to reproduce the ternary phase diagram and ternary activity data to within experimental error limits. The calculated optimized phase diagram and thermodynamic properties are self-consistent and are the most reliable currently available estimates of the true values.