Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2021"

Formation Mechanism of AlN Inclusion in High-Nitrogen Stainless Bearing Steels

Abstract: The existence of angular and hard AlN inclusions would seriously deteriorate the service life of high-nitrogen stainless bearing steels (HNSBSs). In this work, the formation mechanism of AlN inclusion in HNSBSs under as-cast, annealing and austenitizing states was systematically investigated by microstructure observation and thermodynamic, kinetic analyses. The results showed that the concentration product of Al and N could exceed the critical solubility of AlN inclusion at liquidus temperature with the Al content higher than 0.050 wt pct, which led to the formation of AlN inclusions about 1 to 5 μm (equivalent diameter) in liquid steel. Based on the ‘Clyne-Kurz’ model, AlN inclusion could form at the solidifying front due to the enrichment of N in the residual liquid steel with the Al content higher than 0.030 wt pct. Besides, the precipitation of Cr2N and the extremely low diffusion coefficient of Al in α phase restrained the precipitation of AlN during annealing at 1023 K. However, AlN and AlN-MnS composite inclusions less than 0.6 μm could precipitate during austenitizing at 1323 K with the Al content higher than 0.006 wt pct, which was the critical Al content to avoid AlN formation in HNSBSs after melting, solidification, and heat treatment processes.

CFD Study of Hydrogen Injection in Blast Furnaces: Tuyere Co-injection of Hydrogen and Coal

Abstract: Hydrogen is a carbon-free clean energy and a potential fuel to mitigate CO2 emission in ironmaking blast furnaces (BFs) where the co-injection of hydrogen/coal is one of the most promising and feasible technologies. In this article, a 3D steady-state industrial-scale CFD model is improved and used for investigating the co-injection of hydrogen/coal in BFs. The model involves gas–particle-solid flow, heat and mass transfer related to the chemical reactions of hydrogen, coal and coke. This model has been validated against the measurements in terms of gas distribution, temperature and burnout. Several injection schemes of the co-injection of hydrogen/coal are designed under the conditions of constant bosh gas volume. The typical in-furnace phenomena, including the interaction between hydrogen and coal, are described, and the effects of the hydrogen injection rate on the co-injection of hydrogen/coal are analyzed. The simulation results indicate that hydrogen combustion enhances the devolatilization of coal, but hinders the volatiles combustion. It is found that, as the hydrogen rate increases, the raceway volume-averaged temperature increases and the raceway peak temperature increases and then decreases; both the raceway surface-averaged burnout and final burnout increase. Such different responses of them to hydrogen injection rates indicate the importance of 3D modeling study. In addition, the higher hydrogen injection rate increases the molar fraction of reducing gas components (H2 and CO) in the coke bed. The model provides a cost-effective tool for the design, optimization and industrialization of the co-injection of hydrogen and coal.

Role of Direct Aging and Solution Treatment on Hardness, Microstructure and Residual Stress of the A357 (AlSi7Mg0.6) Alloy Produced by Powder Bed Fusion

Abstract: Applying additive manufacturing (AM) technologies to the fabrication of aluminum automotive components, with an optimized design, may result in improved vehicle light weighting. However, the post-process heat treatment of such alloys has to be customized for the particular AM microstructure. The present study is aimed at investigating the effect of different heat treatments on the microstructure, hardness and residual stress of the A357 (AlSi7Mg0.6) heat-treatable alloy produced by laser-based powder bed fusion (LPBF, also known as selective laser melting). There are two major issues to be addressed: (1) relieving the internal residual stress resulting from the process and (2) strengthening the alloy with a customized heat treatment. Therefore, stress-relief annealing treatment, direct aging of the as-built alloy and a redesigned T6 treatment (consisting of a shortened high-temperature solution treatment followed by artificial aging) were examined. Comparable hardness values were reached in the LPBF alloy with optimized direct aging and T6 treatments, but complete relief of the residual stress was obtained only with T6. Microstructural analyses also suggested that, because of the supersaturated solid solution, different phenomena were involved in direct aging and T6 treatment.

Effects of Interphase Forces on Multiphase Flow and Bubble Distribution in Continuous Casting Strands

Abstract: A three-dimensional computational model, combining the large eddy simulation (LES) turbulent model, VOF multiphase model for air phase, and discrete phase model (DPM) for injected bubbles, was established to evaluate the effects of drag force, lift force, pressure gradient force, virtual mass force, and wall lubrication force on the fluid flow and spatial distribution of bubbles in a continuous casting (CC) strand. The effect of lift force and wall lubrication force on the fluid flow was achieved via a user defined subroutine (UDF). The contribution of interphase forces was quantitatively evaluated using UDF. The appropriate interphase force model was determined by comparing the predicted fluid flow and bubble distribution in the CC strand with the measured results one using particle image velocimetry (PIV). The interphase force had a significant effect on the spatial distribution of bubbles and the flow field near the meniscus. The drag force and buoyancy force were the dominant ones at low turbulent kinetic locations. Moreover, the magnitude of the lift force, pressure gradient force, and virtual mass force was increased sharply at high turbulent kinetic locations, approximately one to two orders greater than that of the buoyancy force.

Numerical Study on Desulfurization Behavior During Kanbara Reactor Hot Metal Treatment

Abstract: The hot metal desulfurization in Kanbara Reactor (KR) metal treatment was simulated in this study via a transient-coupled 3D numerical model of the two-phase flow, heat transfer, and particle motion desulfurization processes. The KR impeller stirring was described via the multiple reference frame model. The volume of fluid approach was employed to capture the air-hot metal interface. The particle motion and aggregation were defined by the two-way coupled Euler–Lagrangian method. A desulfurization kinetic model was simultaneously introduced to represent the sulfur mass transfer rate. The effect of the initial diameter of desulfurizing agent (DA) particles on the desulfurization efficiency was quantitatively assessed. The lowest sulfur content was observed in the impeller vicinity and the highest one in the inactive colder liquid metal at the vessel bottom. With the DA particle initial diameter reduction from 3.0 to 0.5 mm, the overall desulfurization rate was increased from 83.2 to 97.1 pct. Insofar the specific surface area of smaller particles exceeded that of larger ones, they had higher motion velocities and heating rates, creating a greater reactivity for the desulfurization. However, desulfurization at the vessel bottom was only slightly enhanced using smaller DA particles, so the overall improvement did not exceed 20 pct. Further enhancement is envisaged by refining the impeller design and particle-adding technique.

The Utilization of Bauxite Residue with a Calcite-Rich Bauxite Ore in the Pedersen Process for Iron and Alumina Extraction

Abstract: Metallurgical grade alumina is produced worldwide through the well-known Bayer process, which unavoidably generates bauxite residue (BR, also known as red mud) in almost equal amounts to alumina. This study aims the valorization of BR through a smelting-reduction process to obtain calcium aluminate slags that can be a proper feed for alumina recovery via the Pedersen process. It investigates the thermodynamics and characteristics of the slags and pig iron produced from mixtures of BR, a bauxite beneficiation byproduct, and lime. In this context, the evolution of the different phases in the slags is studied with advanced analytical techniques and thermodynamic calculations. According to the results, a CaO/Al2O3 mass ratio within 1.3 to 1.4 in the slags can yield more Al2O3-containing leachable phases, such as CaO·Al2O3 and 12CaO·7Al2O3. The cooling dictates the amount and the characteristics of these phases, and the slower cooling rate yields improved slag characteristics. The distribution of the elements between the slag and metal phases shows that iron is separated, and the majority of the P, Cr, Ni, and V are distributed in the produced pig iron, while S, Ti, and Si are mostly concentrated in the slags.

Calculating the Susceptibility of Carbon Steels to Solidification Cracking During Welding

Abstract: Existing experimental results of weldability tests show the susceptibility of carbon steels to solidification cracking varies significantly with the C content. To analyze the effect of the C content on the susceptibility, equilibrium solidification of binary Fe-C alloys was assumed as an approximation in view of the rapid diffusion of the interstitial solute C in Fe. First, the curve of the equilibrium freezing temperature range vs. the C content was plotted and compared with the experimental results, but the agreement was not good. Then, the susceptibility index, i.e., |dT/d(fS)1/2| near (fS)1/2 = 1 (T: temperature; fS: fraction solid) recently proposed for Al alloys was tried. The curve of the susceptibility index vs. the C content was calculated. The curve agreed well with the experimental results of crack susceptibility tests of carbon steels in welding.

A Numerical Study on the Operation of the H 2 Shaft Furnace with Top Gas Recycling

Abstract: The breakthrough route involving a reduction shaft furnace operated with pure hydrogen gas (here called H2-SF) and the electric arc furnace is widely accepted as one of the most viable future alternatives for industrial-scale production of primary steel with minor CO2 emissions. It has been clarified that the largest portion of the total energy for the entire route is consumed by the H2-SF operation, but this unit has not yet received much attention and should therefore be explored. For this, a mathematical model of a reduction shaft furnace is presented in this paper, where a set of simulations were also performed to shed more light on the operation of the H2-SF equipped with a top gas recycling system. The results show that a high gas feed rate is required for guaranteeing a smooth H2-SF operation due to the strong heat demand. An increase in the feed temperature of the gas or in furnace height can reduce the required gas feed. However, an excessive length may conversely result in an increase in the total energy consumption. The model and its results are expected to be helpful for gaining a better understanding of the complex processes in and constraints of the H2-SF.

Thermodynamic Evaluation of Element Transfer Behaviors for Fused CaO-SiO2-MnO Fluxes Subjected to High Heat Input Submerged Arc Welding

Abstract: Submerged arc welding has been performed by employing fused CaO-SiO2-MnO fluxes of varying MnO and CaO contents on EH36 shipbuilding steel grade. Transfer levels of O, Si, and Mn between fluxes and weld metals have been quantified and evaluated from thermodynamic perspectives. The results show that both slag-metal and gas-slag-metal equilibrium considerations are capable of placing limits on the direction and amount of element transferred between fluxes and weld metals.

CFD Modeling and Analysis of Particle Size Reduction and Its Effect on Blast Furnace Ironmaking

Abstract: Particle size reduction inevitably occurs inside ironmaking blast furnaces (BFs) but is not studied under BF conditions due to the lack of an effective tool. This paper simulates this phenomenon inside a 5000-m3 commercial BF using a recently developed three-dimensional computational fluid dynamics (CFD) process model. In particular, the sinter reduction degradation and coke gasification are modeled for considering the size reduction. By incorporating this information in the BF model, the in-furnace states and global performance are evaluated with respect to sinter reduction degradation index (RDI) under different conditions, e.g., keeping a constant blast rate or gas pressure drop. The results show that with increasing RDI, the bed permeability deteriorates, and there exists a critical value of RDI beyond which the decreased bed permeability leads to a significant drop in productivity for a given gas pressure drop. However, this problem can be mitigated substantially by charging more coke particles at the furnace periphery under the conditions considered. The proposed approach offers an extended capability to model and control BF operations.

Electrical Conductivity, Viscosity and Structure of CaO-Al 2 O 3 -Based Mold Slags for Continuous Casting of High-Al Steels

Abstract: The four-electrode method was employed to examine the electrical conductivity and viscosity of CaO–Al2O3-based mold slags designed especially for continuous casting of high-Al steels. The effect of equal mass substitutions of SiO2 by Al2O3 and CaO by BaO on the electrical conductivity and viscosity of the mold slags was investigated in combination with the microstructure analysis using Raman spectroscopy. The equal mass substitutions of SiO2 by Al2O3 and CaO by BaO in the molten slags caused the electrical conductivity to decrease and the viscosity to increase, which was attributed to the increased polymerization degree of the melts and the depletion of the cations. The polymerization degree of the melts increased with Al2O3 replacing SiO2 because of the generation of Al-related networks by the charge compensation effect of Al3+. The equal mass substitution of CaO by BaO decreased the number of available network modifiers and accelerated the structural complexity of the [SiO4] tetrahedra. Meanwhile, Ba2+ would preferentially promote the [AlO4] tetrahedra configuration because it has a stronger ability of charge compensation than Ca2+. So the polymerization degree of slag melts increased with BaO replacing CaO. Nonlinear equations between the electrical conductivity and the polymerization degree of the molten slags were obtained to accurately predict the electrical conductivity of the melts at different temperatures in the current slag compositions range.

Ferrosilicon-Based Calcium Treatment of Aluminum-Killed and Silicomanganese-Killed Steels

Abstract: Calcium-containing ferrosilicon—FeSi(Ca)—does give much higher calcium yield than calcium disilicide during calcium treatment of steel under laboratory conditions, in line with industrial observations. Upon calcium treatment of both aluminum-killed and silicomanganese-killed steel, a new population of small calcium-rich inclusions forms. The results indicate that pre-existing (deoxidation) inclusions dissolve into the liquid steel after calcium addition, rather than directly reacting with the added calcium.

Generation of Reverse Meniscus Flow by Applying An Electromagnetic Brake

Abstract: A numerical study is presented that deals with the flow in the mold of a continuous slab caster under the influence of a DC magnetic field (electromagnetic brakes (EMBrs)). The arrangement and geometry investigated here is based on a series of previous experimental studies carried out at the mini-LIMMCAST facility at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR). The magnetic field models a ruler-type EMBr and is installed in the region of the ports of the submerged entry nozzle (SEN). The current article considers magnet field strengths up to 441 mT, corresponding to a Hartmann number of about 600, and takes the electrical conductivity of the solidified shell into account. The numerical model of the turbulent flow under the applied magnetic field is implemented using the open-source CFD package OpenFOAM®. Our numerical results reveal that a growing magnitude of the applied magnetic field may cause a reversal of the flow direction at the meniscus surface, which is related the formation of a “multiroll” flow pattern in the mold. This phenomenon can be explained as a classical magnetohydrodynamics (MHD) effect: (1) the closure of the induced electric current results not primarily in a braking Lorentz force inside the jet but in an acceleration in regions of previously weak velocities, which initiates the formation of an opposite vortex (OV) close to the mean jet; (2) this vortex develops in size at the expense of the main vortex until it reaches the meniscus surface, where it becomes clearly visible. We also show that an acceleration of the meniscus flow must be expected when the applied magnetic field is smaller than a critical value. This acceleration is due to the transfer of kinetic energy from smaller turbulent structures into the mean flow. A further increase in the EMBr intensity leads to the expected damping of the mean flow and, consequently, to a reduction in the size of the upper roll. These investigations show that the Lorentz force cannot be reduced to a simple damping effect; depending on the field strength, its action is found to be topologically complex.

Oxidation Kinetics of Impurities in Metallurgical-Grade Silicon Melt by O 2 Blowing Refining Process

Abstract: Oxygen blowing refining is an effective method for the removal of the main Al and Ca metallic impurities from metallurgical-grade silicon (MG-Si). However, the removal of impurities in silicon melt is affected and restricted by the kinetics of the refining process. In this work, the oxidation mechanism and kinetics of impurity removal from silicon melt by oxygen blowing refining were investigated experimentally. The limiting step of the impurity removal is the diffusion of impurities in the Si melt. The kinetic model and equation related to the apparent rate constant and the mass transfer coefficient of impurity were established. Examination of the distribution and morphology of the impurities at the slag-silicon interface showed that oxygen blowing refining can achieve a strong impurity removal effect and obtain high-quality MG-Si. Based on the experimental results for impurity Al removal, the apparent rate constant (kAl) and the mass transfer coefficient (βAl) were obtained as 3.83×10−4 s−1 and 1.04×10−5 m s−1, respectively, while the silicon loss rate reaches 5.24 pct for 90 minutes of oxygen blowing refining.

Effect of CaCl 2 Addition on the Melting Behaviors of CaO-SiO 2 -FeO x Steelmaking Slag System

Abstract: The melting behavior of BOF slag (CaO-SiO2-FeOx) is one of the key factors for dephosphorization during the converter process. Laboratory experiments using mixed chemical reagents and pre-melted slags were carried out to study the effect of the slag compositions on its hemispheric melting point, and the phases in the slag system were observed at high temperatures. The results reveal that only a small amount of CaCl2 can decrease the hemispheric melting points of low basicity slags (w(CaO)/w(SiO2) ≤ 1.0); while an excessive amount of CaCl2 will increase the hemispheric melting point of the slag system. In the case of higher basicity slag (w(CaO)/w(SiO2) ≥ 1.5), the addition of CaCl2 will increase the ratio of solid phase in slag, which results in the rise of hemispheric melting point. Even though, the positive effect of CaCl2 on the dissolution of CaO is still obvious, higher slag basicity generally leads to a higher hemispheric melting point of the CaCl2-added slags except for a few cases, and the content of FeOx has an evident impact on the hemispheric melting point of this slag system. A certain amount of FeOx in BOF slag is very necessary.

Numerical Simulation on Refractory Wear and Inclusion Formation in Continuous Casting Tundish

Abstract: The formation and removal of exogenous inclusions in a real-size two-strand tundish is simulated by the proposed unsteady 3D comprehensive numerical model of the respective fluid-structure interaction, which takes into account the impacting and washing effects on the refractory wear. A large eddy simulation is employed to describe the molten steel vortex flow. Thus, the thermal profiles of the molten steel and refractory lining are constructed. One-way coupled unsteady Euler-Lagrange approach is adopted to estimate the detachment and motion of the exogenous inclusion. The inclusion’s Reynolds number is utilized for evaluating the inclusion separation at the refractory lining after formation and at the upper surface of the molten steel. At a 1.2 m/min casting speed, 49 and 38 pct of exogenous inclusions are created at the turbulent inhibitor inner bottom and long nozzle inner wall, respectively. In contrast, only 13 pct of new inclusions are produced at all other inner walls. About 80 pct of newly generated inclusions are then trapped by free surfaces, 78 pct of which are removed at the first free surface. The initial diameter of exogenous inclusions ranges from 13 to 48 μm. The removal ratio of exogenous inclusions in the tundish first grows from 61 to 80 pct, with the casting speed rising from 1.0 to 1.2 m/min and then drops to 63 pct after the further casting speed rise to 1.4 m/min.

A Comprehensive Investigation on the Microstructure and Thermal Conductivity of CaO-Al 2 O 3 Based Mold Slags: Equilibrium Molecular Dynamics Simulations

Abstract: The microstructure and thermal conductivity of the CaO-Al2O3-based (CA-based) mold slags are crucial for casting high aluminum steel. Herein, the microstructure and thermal conductivity of the CA-based mold slags were systematically investigated by using molecular dynamics (MD). Moreover, the effects of the CaO/Al2O3 mole (C/A) ratio and CaO/SiO2 mole (C/S) ratio on the microstructure and thermal conductivity of CA-based mold slags were studied. The results showed that the Al-O structure plays a skeleton role in the CA-based mold slags. The thermal conductivity of molten CA-based mold slags increases first and then decreases with increasing C/A ratio. The thermal conductivity of molten CaO-Al2O3-SiO2-MgO-Na2O (CASMN) slag is affected by the combination of short- and medium-range order structures.

Hydrometallurgical Recycling of Red Mud to Produce Materials for Industrial Applications: Alkali Separation, Iron Leaching and Extraction

Abstract: Red mud is a polymetallic waste generated during Bayer's process of alumina production. High alkalinity (pH > 11), multiple elements, and micron-sized particles make red mud recycling energy-intensive and challenging. The following work presents a hydrometallurgical flowsheet for separation of different red mud elements and recovery of high purity Fe (II) product using cost-effective reagents, energy-efficient processes, and minimal waste generation. Red mud preprocessing was carried out by mild hydrochloric acid wash (1 M, 13 pct pulp density, 40 °C 15 minutes), followed by leaching of hematite from neutralized red mud in oxalic acid (2 M, 10 pct pulp density, 95 °C, 2.5 hours). UV light-assisted photochemical reduction of oxalic leach solution of red mud separated more than 98 pct Fe in the form of ferrous oxalate (purity more than 99 pct) within 5 hours. The process's material balance shows a overall recovery of more than 85 pct Fe value as solid ferrous oxalate of high purity and concentrating titanium oxide in the residue and aluminum in the leaching solution.

Molecular Dynamics Simulations of Melt Structure Properties of CaO–Al2O3–Na2O Slag

Abstract: Molecular dynamics (MD) simulations have been used to study the effect of Na ions on the structure properties of CaO–Al2O3–Na2O slag. The short- and medium-range structures of CaO–Al2O3–Na2O in this study are consistent with existing data. Through the replacement of Ca2+ ions with Na+ ions in CaO–Al2O3–Na2O slag, the structure of the AlO4 tetrahedron is stabilized as the proportion of AlO4 tetrahedron in the melt increases and the average values of the O–Al–O bond angle are closer to those of an ideal tetrahedron. The changes in the melt structure show that Ca2+ ions mainly play a role in modifying the network, while Na+ ions mainly play a role in the charge compensation of the AlO4 tetrahedron; thus, as more Na+ ions replace Ca2+ ions added to the melt, the charge compensation ability in the melt is enhanced, and the network modification ability is weakened. Part of the weak non-bridge oxygen (NBO) structures in the form of Al–NBO–Ca are transformed to strong bridge oxygen (BO) structures in the form of Al–BO–Al, and the microstructure of the melt gradually becomes complicated, which provides a reasonable explanation for the mechanism for the increase of macroscopic viscosity in CaO–Al2O3–Na2O slag with high Al content.

Assessment of Weld Metal Compositional Prediction Models Geared Towards Submerged Arc Welding: Case Studies Involving CaF2-SiO2-MnO and CaO-SiO2-MnO Fluxes

Abstract: Submerged arc welding has been performed by utilizing CaF2-SiO2-MnO and CaO-SiO2-MnO fluxes over a wide range of compositions and basicity index values. Contents of essential elements, including O, Si, and Mn, in the weld metal, are predicted by employing the basicity index model, slag–metal equilibrium model, and gas–slag–metal equilibrium model. Capabilities of each model to predict weld metal compositions have been evaluated from thermodynamic perspectives. The results show that the basicity index model is capable of predicting the variation trend of O content with MnO addition, but fails to differentiate O levels when fluxes with same basicity index are applied. The slag–metal model overestimates the contents of Si and Mn due to underestimated O level or overestimated oxide activity. The gas–slag–metal equilibrium model, on the other hand, offers better prediction accuracy for O content than the basicity index model, and is able to differentiate the O content of the weld metals produced by fluxes with varying formulas but same basicity index. Furthermore, when the gas–slag–metal equilibrium model is applied, the prediction error for Si and Mn contents is significantly reduced as compared to the slag–metal equilibrium model. Thermodynamic calculation data indicates that the consideration of gas formation, which essentially controls the predicted flux O potential and oxide activity, is necessary to improve the overall prediction accuracy.

Effect of Cerium on the Behavior of Primary Carbides in Cast H13 Steels

Abstract: Different amounts of elemental cerium were added into H13 ingots to investigate the effect of cerium content on primary carbides’ behavior. The enrichment of the elemental Cr, Mo, V, and C in the last-to-solidify region was identified by EPMA. The enrichment of alloy elements is the main reason for the precipitation of primary carbides during solidification. As the increase of cerium content, the size of the last-to-solidify region decreases first and then increases. We found a huge difference between the 2D and the 3D observations of primary carbides. The number density of the primary carbides appeared higher, while their size was smaller in the 2D observation. In the 3D observation, as the cerium content increases, the primary carbides’ size also decreases first and then increases, while the morphology of the primary carbides changes from dendritic structure to sheet structure and then to dendritic structure again. The cerium content has little influence on the thermodynamic precipitation process of primary carbides. The enrichment of the elemental cerium in the last-to-solidify region observed by TOF-SIMS accelerates the nucleation of the austenite, further leading to smaller last-to-solidify region size. The smaller last-to-solidify region size provides less space for the further growth of the primary carbide, resulting in the smaller primary carbide size.

Three-Dimensional Macrosegregation Model of Bloom in Curved Continuous Casting Process

Abstract: In the current study, a three-dimensional semi-coupling macrosegregation model was established for a curved bloom continuous casting strand. The fluid flow, heat transfer, and solidification were calculated firstly with the effect of the mold electromagnetic stirring (M-EMS) and final electromagnetic stirring (F-EMS). Then, the transport of the solute was solved after the fluid flow reached a steady state. Predicted values of the magnetic induction intensity, temperature, and content of carbon agreed well with the measured ones, respectively. The negative segregation in the subsurface of the bloom was generated due to the rotational flow induced by M-EMS. The rotational flow pattern was formed along the cross section of the F-EMS zone. The maximum tangential velocity 0.012 m/s was located near the solidification front. The distribution of the carbon was uniform and the concentration of the carbon was about 0.205 pct in the molten steel. From the position 11.1 to 20.1 m beneath the meniscus, the liquid fraction at the centerline of the bloom decreased gradually to 50 pct and the concentration of carbon increased from 0.2 to 0.24 pct. The concentration of the carbon at the centerline of bloom increased sharply to 0.29 pct at the solidification end.

Evolution of Inclusions in Si-Mn-Killed Steel During Ladle Furnace (LF) Refining Process

Abstract: To further clarify the evolution of inclusions in Si-Mn-killed steel grades, industrial samples were taken during the LF (ladle furnace) refining process, and laboratory experiments were carried out to investigate the effects of alloy additions, refining slags, and refractory materials on the evolution of inclusions. It is found that the inclusions transform along the route of “MnO-SiO2-based inclusions → CaO-MnO-SiO2-based inclusions → CaO-SiO2-based inclusions”, and a small amount of MgO and Al2O3 are contained in these inclusions. The increase of CaO content in inclusions is generally caused by the reduction of MnO and SiO2 (especially MnO) by dissolved Ca in liquid steel, while the dissolved Al and Mg in liquid steel could also increase Al2O3 and MgO in the inclusions. Both alloys and refining slag supply very limited Ca to liquid steel, which make it difficult to transform the MnO-SiO2-based inclusions into CaO-SiO2-based inclusions. The use of CaO-containing refractory is the main reason to cause the generation of CaO-MnO-SiO2-based inclusions in this study, and this kind of refractory is not recommended for tire cord and saw wire steel grades.

Kinetic Prediction for the Composition of Inclusions in the Molten Steel During the Electroslag Remelting

Abstract: A mathematical model coupled with the penetration theory, the ion and molecule coexistence theory, and thermodynamic equilibrium was proposed for predicting the composition evolution of inclusions in the molten steel during the electroslag remelting process. The model was used to evaluate the transformation of composition of inclusions in a plain carbon steel and the mechanism of the transformation of inclusions was accurately revealed, which was mainly the mass transfer of aluminum through steel/slag reactions. The rate of the transformation of inclusions composition was the lowest in the metal pool, while that in the slag pool was the fastest which was due to the acceleration of reactions by higher temperature and faster fluid flow. Inclusions with smaller diameter had faster transformation rate, but had less content of Al2O3 in the final composition. The size of droplet showed little influence on the transformation of composition of inclusions. When the content of Al2O3 in the slag increased from 20 to 50 wt pct, the calculated content of Al2O3 in the final inclusions increased from 79 to 90 wt pct, approximately. The low content of Al2O3 in the slag was beneficial to the removal of aluminum in the steel, while the high content of Al2O3 in the slag increased the content of total aluminum in the steel.

Direct Alloying of Molten Steel with Vanadium Slag as a Substitute of Ferrovanadium: Self-reduction Behavior of Vanadium Slag Briquette with Aluminum Dross Addition

Abstract: A direct alloying technology of molten steel by the vanadium slag briquette via self-reduction reaction was proposed. From the perspective of selective reduction, a theoretical analysis on the variation of equilibrium compositions including slag and metal phases was carried out based on the Gibbs energy minimization principle. The self-reduction behavior of vanadium slag briquettes with aluminum dross addition and slag-metal separation was experimentally investigated under various conditions, including the reduction temperature, briquette basicity and reductant amount. The results demonstrated that increasing the reduction temperature and briquette basicity could promote the deep reduction of vanadium slag and coalescence of metal droplets in molten slag, resulting in an increase in reduction degrees of FeO, MnO, Cr2O3 and V2O5. Excessive addition of aluminum dross with an A/O molar ratio > 0.67 would significantly increase the slag viscosity and deteriorate the fluidity of slag because of the introduction of Al2O3 and MgO, which caused an obvious decrease in the reduction degrees of FeO, MnO, Cr2O3 and V2O5 and difficulty in coalescence of metal droplets. The solid compound of magnesia-alumina spinel (MgAl2O4) was found to be precipitated from liquid slag, which strongly affects the slag viscosity and settling velocity of the metal droplets in slag phase. Under the optimal conditions, a reduction temperature of 1600 °C, briquette basicity of 2.0, Al/O molar ratio of 0.67 and reduction degrees of FeO, Cr2O3, MnO and V2O5 in vanadium slag could reach the highest values of 99.05, 81.94, 84.02 and 89.83 pct, respectively, and metal droplets could be well agglomerated and separated from slag phase.

Numerical and Experimental Study of the Meniscus Vortex Core in a Water Model of Continuous Casting Mold

Abstract: Slag entrainments in a continuous casting mold result in serious defects in steel products. Most previous studies concentrated on the magnitude of meniscus velocity or transient roll cell motion to investigate the cause of slag entrainment. However, slag entrainment is more directly connected with the meniscus vortex, which accompanies a meniscus deformation (e.g., vortex dimple or core). Most previous numerical studies on the meniscus vortex were conducted using a single-phase flow analysis; thus, the effect of meniscus deformation was never examined. This study conducts a multiphase flow simulation by employing a free-surface-capturing technique, and succeeds in reproducing the meniscus vortex core in the mold. The core depth and onset condition for the meniscus vortex obtained by the present numerical study show good agreements with the experimental results. The vorticity distribution on the mold meniscus and meniscus vortex patterns according to flow asymmetry are also dealt with herein.

A Review of Ferroalloy Tapping Models

Abstract: Tapping is an important furnace operation in the ferroalloy industry and poses a number of complex and coupled challenges of both practical and economical importance. Owing to the hazardous high-temperature conditions surrounding the tap hole, the application of various modeling techniques allows for development and acquisition of both scientific and engineering knowledge of the process through physical or numerical proxies. In this review, earlier work on modeling of ferroalloy tapping is summarized and main principles of the tapping process and multiphase interaction of slag and metal are discussed and summarized. The main focus is on drainage of slag and alloys, but some attention will also be given to metal loss, metal overflow and health, safety and environment. Our review shows that although considerable progress has been made in computational capability over the last decades, However, it is clear that research and development in the field of ferroalloy furnace tapping remains at a relatively nascent stage. The most progress up to date has happened in the area of so called reduced-order models. Such models are robust and simple, and may be easily fitted to process data from a particular operation in order to develop tailored solutions. Such models are more easily combined with software and instruments, ultimately enabling improved automation, process control and ultimately improved tapping consistency.

Interface-Resolved Simulation of Bubbles–Metal–Slag Multiphase System in a Gas-Stirred Ladle

Abstract: A grid-filtered scale interface-resolved volume of fluid (VoF) model coupling a sub-grid scale large eddy simulation (LES) was used to directly simulate bubble behavior and the evolution of multiphase interfaces and free surfaces in a bubble–metal–slag multiphase system. The model has been applied to an industrial 150-ton gas-stirred ladle. The results show transient behavior of the slag eye, with large variations over time of the eye size. Bubble detachment frequency and rise characteristics (including coalescence and breakup) directly affect the size, position and variation of the slag eye. The effects of the argon flow rate, slag thickness, slag viscosity, and slag–steel interfacial energy were investigated; the latter two have little effect. The calculated average slag eye size agrees with previously reported results for industrial ladles. A new correlation for the time-averaged slag eye area, based on the modified Froude number, is proposed for industrial ladles.

Investigation on Slag–Metal-Inclusion Multiphase Reactions During Electroslag Remelting of Die Steel

Abstract: Experimental and theoretical studies have been carried out to investigate the effects of slag composition on the MgO·Al2O3 inclusion in ingot during the electroslag remelting (ESR) process with a focus on developing a mass-transfer model to understand the evolution mechanism of MgO·Al2O3 inclusion. H13 die steel was used as the electrode and remelted with two different kinds of slags by using a 30-kg ESR furnace. The inclusion compositions and contents of magnesium, silicon, and aluminum along the axial direction of product ingots were analyzed. On the basis of the unreacted core model as well as the penetration and film theories, the theoretical model developed in this work well elucidates the kinetics of slag–metal-inclusion reactions revealing the mechanism of inclusion evolution during the ESR process. The calculation results obtained from the model agree well with the experimental results. The model indicates that the inclusions of the outer MnS layer, which surrounds the MgO·Al2O3 core in the electrode, are disintegrated and removed during the metal film formation process at the tip of the electrode in the ESR furnace. The more CaO there is in the slag, the higher the aluminum and magnesium in the ingot and the lower the silicon. The concentration of MgO in the MgO·Al2O3 inclusion increases with the increase of CaO/SiO2 in the slag. The aluminum in the electrode has little effect on the MgO·Al2O3 inclusion compositions in the final product ingots.

Reduction of Manganese Ores in CO-CO2 Atmospheres

Abstract: The reduction of higher manganese oxides to MnO in the prereduction zone of a ferromanganese furnace is largely decisive for the overall energy efficiency and climate gas emissions for the overall production process. Kinetics of the gas-solid reaction between the ore and CO(g) in the ascending furnace gas is dependent on the ore characteristics. The aim of this study was to elucidate the reduction steps during prereduction of two commercial manganese ores, Comilog and Nchwaning, in CO-CO2 atmosphere. Ore samples in various size fractions were exposed to isothermal and non-isothermal temperature regimes in various gas atmospheres utilizing a thermogravimetric furnace. Samples were characterized by titration, XRF, XRD, and SEM analyses. Comilog is a high oxygen ore, where the majority of the ore is constituted by various tetravalent manganese minerals. When heated in reducing atmosphere, MnO2-oxides were reduced to MnO in a single step at temperatures below 550 °C. The rate was dependent on the particle size and CO-concentration. When the temperature reached 550 °C, any present MnO2 rapidly decomposed to Mn2O3, which further continued to reduce to MnO. Nchwaning ore was found to be mainly constituted by Mn2O3-oxides, calcite and hematite. Manganese- and iron-oxides were found to reduce at similar temperature ranges, however the manganese oxide reduction was initiated prior to the iron oxides in smaller particle sizes (< 4 mm). Trivalent manganese oxides reduced to MnO in a single step, and the reduction of iron oxides subsided with the formation of wustite. Carbonates decomposed at temperature range 800 °C to 1000 °C. Both ores obtained a promoted reaction rate with decreasing particle size and increased pctCO in CO-CO2 atmosphere.