Interfacial phenomena between fluxes for continuous casting and liquid stainless steel

Abstract

Casting powders melt on the surface of the liquid metal forming a liquid slag layer. Samples taken during casting revealed convective flows in the flux layer and mass exchange with the liquid metal. It is demonstrated that concentrations of certain elements are considerably higher at the phase boundary than in the bulk of the metal and slag phase. Disturbances of interfacial tension produced by mass and charge transfer evidently cause strong shearing forces which act in parallel with the phase boundary. These forces induce convective movements in the flow boundary layer. Convective flows next to the interface between two liquids have been studied in laboratory experiments using various liquids. The results show that the movement velocity of volume elements next to the interface (due to disturbances of interfacial tension) are dependent on liquid layer thickness and on liquid properties. A new dimensionless number describing this manner of convective flow and suitable for evaluation of experimental results is introduced. Its contribution to the total mass transfer will be shown. A dimensionless function describing the relation between convective flows in the slag layer and mass transport is theoretically developed. Coefficients of this function for Ti transfer into the flux layer have been determined empirically.     

Mass Exchange at the Metal‐Mould Flux Interface

Mass exchange between liquid steel and the mould flux leads to the oxidation of some elements in the steel and the reduction of slag components. In the continuous casting process, where metal initially solidifies at the metal‐slag interface, reaction products generated there can be cached by the growing solidification front. The disruptions of the interface promote the entrapment of flux particles additionally. They can cause surface defects with quality damages of the cast products as a consequence. These investigations are focused on the contact area between liquid mould flux and metal. The samples were taken from the mould during the continuous casting process. The results show that layers up to approximately 300 nm from the slag/metal interface represent a specific reaction space. The transport of oxygen near the interface and the charge transfer is explained on the basis of the ionic character of the slag. Sodium in this layer on the slag side plays a special role in the oxygen transport besides of the iron ions. The mass transfer rate of oxygen crossing the interface is calculated from measured data and conclusions concerning the stability of the interfacial tension are made.     

Numerical Analysis of Fluctuation Behavior of Steel/Slag Interface in Continuous Casting Mold with Static Magnetic Field

Utilizing ANSYS CFX commercial software and volume fraction of fluid （VOF） model, fluctuation behav- ior of steel/slag interface was numerically simulated in continuous casting mold with static magnetic field, and the influence of metal jet characteristics on the behavior of steel/slag interface was investigated. The results indicated that the behavior of steel/slag interface is similar at different process parameters, which is closely related to the characteristic of the flow field. The steel/slag interface has an obvious trough characteristic, which can be divided in- to three zones： frontal valley zone, back valley zone and horizontal zone~ as the magnetic flux density increases, the fluctuation of liquid level increases firstly and then decreases, and a reasonable magnetic flux density can make steel/ slag interface obtain a relatively flat interface, which can prevent slag from being entrapped into liquid steel. For a thin slab continuous casting process, when the casting speed is 4 m/min, a reasonable magnetic flux density is about 0.5 T, and the interfacial fluctuation is weaker. No matter the position of magnetic field is horizontal or vertical, for different operating parameters, there is a corresponding reasonable magnetic field position where the steel/slag inter- face fluctuation can be properly controlled and slag entrapment can be prevented.     

Mass transfer of sulfur from liquid iron into lime-saturated CaO-Al2O3-MgO-SiO2 slags

Laboratory experiments on the kinetics of sulfur transfer from aluminium-deoxidized liquid iron into lime-saturated CaO-Al₂O₃-MgO-SiO₂ slags were carried out at 1600°C in MgO crucibles with 1500 g iron and 180 to 250 g slag. The mass-transfer coefficients of sulfur were determined under defined flow conditions of liquid metal induced by gas stirring. A square-root dependence of measured rate constants on flow velocity of metal was found. This is interpreted as normal liquid-liquid mass transfer caused by the forced convection of gas stirring. It could be described by boundary-layer theory applied to the metal-slag interface. With increasing sulfur contents of metal mass-transfer coefficients became larger. This is interpreted as mass transfer by interfacial convection superimposed to normal mass transfer. Interfacial convection was confirmed by microscopic observation of quenched metal-slag interface samples. If the sulfur content in liquid iron exceeds a critical value, calcium sulfide crystals precipitate in slag at the slag-metal interface. The precipitation inhibits generation of interfacial convection.     

Influence of slag and foam characteristics on reduction of FeO-containing slags by solid carbon

The present study reports experimental results on the reduction of FeO in molten CaO-SiO₂-Al₂O₃-MgO-FeO slags by solid carbon in an extended-arc plasma reactor. The reduction reaction was found to be controlled by mass transport of FeO in liquid slag. The CO gas generated stirs the bath to establish a convective mass transport system. CO also causes foaming. An analysis using dimensionless numbers provides correlations between the rate constant, k, as well as the foaming index, Σ, with some properties of the slag such as viscosity, surface tension, and density. A correlation between k and Σ is also developed using these parameters for slag characteristics.     

Modelling molten flux layer thickness profiles in compact strip process moulds for continuous thin slab casting

Abstract

The important functions promoted by powdered flux added over the liquid steel surface in continuous casting moulds are strongly affected by the thickness of the liquid layer that forms as a result of the heat absorbed. The present work discusses the results of a three-dimensional steady state model, developed to represent the coupled fluid flow and heat transfer phenomena that determine thickness profiles of the liquid flux layer. Since the laminar flow of the liquid slag layer depends on the shearing imposed on it by the turbulent motion of the liquid steel beneath it, and since additionally this motion is strongly influenced by the flow characteristics of the steel stream poured into the mould through the submerged entry nozzle (SEN), separate turbulent flow models for the liquid steel in the SEN and the mould were also developed. The consistency among the models and their accuracy was judged by comparing thickness and temperature flux profiles measured in plant against predicted ones; the comparison showed good agreement. The effects of casting speed, mould width, and flux viscosity and heat of melting on the liquid layer thickness were investigated. The last variable was found to exert the most marked influence. Different from conventional casting moulds, where the liquid layer thickness increases with increasing casting speed, in compact strip process moulds the thickness remains almost constant with increasing casting speed. This difference is well accounted for by the model, which suggests that this behaviour stems from the different slag flow patterns generated in straight, wide moulds and in thin moulds having a central upper funnel shaped section.     

Heat transfer and fluid flow phenomena in electroslag refining

A mathematical formulation has been developed to represent the electromagnetic force field, fluid flow and heat transfer in ESR units. In the formulation, allowance has been made for both electromagnetically driven flows and natural convection; furthermore, in considering heat transfer the effect of the moving droplets has been taken into account. The computed results have shown that the electromagnetic force field appears to be the more important driving force for fluid motion, although natural convection does affect the circulation pattern. The movement of the liquid droplets through the slag plays an important role in transporting thermal energy from the slag to the molten metal pool, although the droplets are unlikely to contribute appreciably to slag-metal mass transfer The for-formulation presented here enables the prediction of thermal and fluid flow phenomena in ESR units and may be used to calculate the electrode melting rates from first principles. While a detailed comparison has not yet been made between the predictions based on the model and actual plant scale measurements, it is thought that the theoretical predictions are consistent with the plant-scale data that are available.     

Model Investigations on the Stability of the Steel-Slag Interface in Continuous-Casting Process

In the continuous-casting mold, the mold powder in contact with the liquid steel surface forms a liquid slag layer. The flow along the steel-slag interface generates shear stress at the interface, waves, and leads to fingerlike protrusions of liquid slag into steel. Reaching a critical flow velocity and thereby shear stress, the protrusions can disintegrate into slag droplets following the flow in the liquid steel pool. These entrained droplets can form finally nonmetallic inclusions in steel material, cause defects in the final product, and therefore, should be avoided. In the current work, the stability of a liquid-liquid interface without mass transfer between phases was investigated in cold model study using a single-roller driven flow in oil-water systems with various oil properties. Applying the similarity theory, two dimensionless numbers were identified, viz. capillary number Ca and the ratio of kinematic viscosities ν ₁/ν ₂, which are suitable to describe the force balance for the problem treated. The critical values of the dimensionless capillary number Ca^* marking the start of lighter phase entrainment into the heavier fluid, are determined over a wide range of fluid properties. The dimensionless number ν ₁/ν ₂ was defined as the ratio of kinematic viscosities of the lighter phase ν ₁ and heavier phase ν ₂. The ratios of kinematic viscosities of different steel-slag systems were calculated using measured thermophysical properties. With the knowledge of thermophysical properties of steel-slag systems, Ca^* for slag entrainment as a function of v ₁/v ₂ is derived. Assuming no reaction between the phases and no interfacial flow, slag entrainment should not occur under the usual casting conditions.     

Flow of molten slag and iron at 1500 °C to 1600 °C through packed coke beds

The blast furnace dripping zone is of great importance to the mass transfer of elements such as sulfur, carbon, and silicon, to and from the liquid metal phase. To understand mass transfer in the dripping zone, not only mass-transfer reactions and kinetics should be known, but the flow phenomena and process dynamics should be understood as well. The flow of hot metal and slag in the dripping zone was studied in experiments, in which liquid slag and metal trickled through a packed coke bed at 1500 °C to 1600 °C. The results indicate that slag and iron flow concurrently in a funicular type of flow. The iron flows through the core of the voids in the bed and is enveloped by slag, which flows filmwise in between the coke and the iron. This mode of flow allows for a large contact area between slag and iron, through which mass can be transferred. While flowing, the liquid can only pass and access a void, if and when the fluid capillary pressure at the void neck can be overcome. As a result, liquid droplets collect into rivulets. These rivulets flow down, along the accessible voids, using only a part of the available volume. The residence times of the fluids in the bed depend partly on the length of the pathway and are a function of the bed structure, the void neck distribution, and the stochastics of the flow. During flow, slag may react with coke, thus changing the distribution of the slag composition, and its sulfur capacity. In addition, the residence time distribution of the slag and the liquid holdup change as a result of these reactions. Holdup and residence time distribution of the liquids as measured in the experimental setup could not be modeled quantitatively, most likely due to the doubly distributed nature (in space and in time) of the model parameters, induced by reactions between slag, coke, and liquid metal.     

Utilization of Multiphase Fluxes for the Dephosphorization of Hot Metal

Although steelmaking slags have been usually treated and studied as homogeneous liquids, they are actually mixtures of a liquid and solids in practical processes. CaO‐based refining flux that does not contain fluxing agents such as CaF₂ inevitably forms a heterogeneous slag in normal cases, and hence, it is defined as a “multiphase flux.” Efficient utilization of this type of flux would decrease the consumption of resources and the emission of CO₂, and thus, would reduce the load on the environment. Metallurgical studies on multiphase fluxes are limited, however, the physical chemistry and reaction kinetics of the same are important for the development of advanced refining processes. The reaction mechanism of dephosphorization using a multiphase flux at hot metal temperatures was investigated in this study. The reaction of a P₂O₅‐containing slag with solid CaO was studied by immersing a CaO disc in the slag. A CaO‐FeO layer was formed near the interface, and a solid solution of Ca₂SiO₄‐Ca₃P₂O₈ was observed in this layer. The Fe‐P‐Si alloy reacted with calcium ferrites at 1673 K, and the samples were analysed by XMA. The same solid solution (Ca₂SiO₄‐Ca₃P₂O₈) was observed near the slag‐metal interface, which suggests that the phosphorus removed from the metal gets concentrated in the solid phase. The experimental results were reproduced with a kinetic simulation model. The simulation program was also applied to the reaction of the CaO‐FeO droplet in a hot‐metal bath.     

Applications of Computational Fluid Dynamics (CFD) in iron- and steelmaking: Part 2

Abstract

After presenting a review of some applications of computational fluid mechanics (CFD) to ironmaking processes in Part 1, the authors now explore the use and extent of CFD in steelmaking and steel casting processes. Steelmaking processes generally include the basic oxygen furnace, electric arc furnace or equivalent, the ladle and continuous casting and incorporating a tundish and moulds. All these steelmaking processing steps involve highly coupled complex transport phenomena. The use of CFD to model such processes has been an active area of research for the last three decades. Many models have been developed to predict mixing behaviour, slag foaming, gas–liquid interactions, multiphase flows, as well as heat and mass transfer aspects. In the present review, the role of CFD in modelling steelmaking operations is reviewed, discussed and critiqued.     

Submerged liquid slag injection in steel melt: scaleup study using cold model and dimensional analysis

Abstract

Submerged injection of solid flux powder is used in the steel industry to eliminate impurities in an economical way. The efficiency of such an injection process is limited by the fact that only a fraction of the injected particles penetrate into the liquid melt, while the majority remain as bubble encapsulated solids, causing poor heat and mass transfer. Therefore, liquid slag injection can be considered a potential alternative technique in the refining of steel to improve the efficacy of mass transfer in such a process. In the present work, liquid slag injection in a steel melt has been simulated by means of laboratory scale cold model experiments in which, water, paraffin oil and benzoic acid have been used as low temperature analogues for liquid steel, slag and impurities, respectively. Through dimensional analysis it is observed that the modified Froude number can be considered as a criterion for scaling up such a process from a bench scale to a full scale system. A regression analysis has also been carried out to correlate the dimensionless mass transfer rate constant with the relevant dimensionless numbers, namely, dimensionless gas velocity, Froude number, aspect ratio and non-dimensional lance depth.     

Modeling of Electromagnetic Field and Liquid Metal Pool Shape in an Electroslag Remelting Process with Two Series-Connected Electrodes

A three-dimensional finite-element model has been developed to understand the electromagnetic field and liquid metal pool shape in an electroslag remelting (ESR) process with two series-connected electrodes. The magnetic vector potential is introduced into the Maxwell’s equations, and the nodal-based method is used to solve a three-dimensional harmonic electromagnetic field. The heat transfer of the solidifying processes of ingot is modeled by a source-based enthalpy method, and the Joule heating is included in an inner source. The results show the main part of the current flows through the slag cap and a little enters into ingot in a two-series-connected electrode ESR system. As the interaction of self-induced and mutual-induced of two electrodes occurs, the skin effect is significantly suppressed by the neighbor effect. A symmetrical pattern of magnetic flux density in a two-series-connected electrode ESR system is displayed. The magnetic flux density between two electrodes is reinforced and reduced at the outside of two electrodes. The maximum Joule heat power density is located at the interface of slag and electrodes, and it decreases with an increase of the electrode immersion depth. The averaged Joule heat power density increases when slag cap thickness is reduced. With the increase of ingot height, the liquid metal pool shape changes from arc shaped to “V” shaped. When the ingot height is more than the diameter in the ESR processes, the liquid metal pool shape is constant.     

FTSC薄板坯连铸中碳钢保护渣研究及开发

针对唐钢FTSC薄板坯连铸所产生的板坯裂纹、表面夹渣、卷渣及漏钢现象进行保护渣生产试验研究与理论分析。研究结果表明,组分变化对保护渣熔化温度和粘度等指标有着重要的影响。通过实验室对保护渣组份及其变化对其性能影响的研究,结合唐钢FTSC薄板坯连铸自身特点,设计出保护渣基本配方,且在不断试验改进中,最终开发出适合唐钢薄板坯连铸用中碳钢保护渣C2.     

Dynamic and equilibrium interfacial phenomena in liquid steel-slag systems

The equilibrium interfacial energy between a liquid iron alloy and a liquid slag is a key physical parameter in the design of steel-refining processes as high interfacial energies are desired to avoid emulsification of slag in steel and the creation of casting defects. During a chemical reaction between a liquid iron alloy droplet and a liquid slag, it is possible to observe by X-ray photography a number of dynamic interfacial phenomena such as droplet flattening, interfacial turbulence, and spontaneous emulsification that can potentially lead to serious processing problems. These dynamic phenomena have been studied during reactions between Fe-Al and Fe-Ti alloys and silica-containing slags, and the presence of significant interfacial disturbance has been observed during the times of high reaction rate between the slag and the metal. It is suggested that interfacial chemical reactions induce Marangoni and natural convection at the slag-metal interface. This interfacial flow gives rise to interfacial waves due to a Kelvin-Helmholtz instability. The waves grow, become unstable, and lead to spontaneous emulsification of slag in steel and steel in slag. Experiments using industrial samples and controlled laboratory tests have indicated that this phenomenon may be more common than once thought and could lead to some serious problems in the processing of steel alloys containing high quantities of aluminum and/or titanium. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Heat transfer and fluid flow phenomena in electroslag refining

A mathematical formulation has been developed to represent the electromagnetic force field, fluid flow and heat transfer in ESR units. In the formulation, allowance has been made for both electromagnetically driven flows and natural convection; furthermore, in considering heat transfer, the effect of the moving droplets has been taken into account. The computed results have shown that the electromagnetic force field appears to be the more important driving force for fluid motion, although natural convection does affect the circulation pattern. The movement of the liquid droplets through the slag plays an import-ant role in transporting thermal energy from the slag to the molten metal pool, although the droplets are unlikely to contribute appreciably to slag-metal mass transfer. The for-formulation presented here enables the prediction of thermal and fluid flow phenomena in ESR units and may be used to calculate the electrode melting rates from first principles. While a detailed comparison has not yet been made between the predictions based on the model and actual plant scale measurements, it is thought that the theoretical predictions are consistent with the plant-scale data that are available. Presently on leave from Institute of Chemical Engineering and Technology, Punjab University, Lahore-1, Pakistan.     

低碳钢薄板坯高速连铸保护渣研究与优化

 针对低碳钢薄板坯高速连铸过程中保护渣液渣层过薄、黏结报警频发、铸坯表面纵裂纹过多等问题,在充分考虑高拉速下低碳钢凝固收缩特性的基础上,确定了保护渣润滑与传热性能的优化方向并开展了工业试验。将保护渣碱度从1.10提高到1.30,Li₂O质量分数从0.57%提高到1.06%,Na₂O质量分数从5.48%提高到8.16%,碳质量分数由7.71%降低到6.72%。对2种保护渣的流变性能和渣膜3层结构进行了深入研究,发现优化后保护渣渣膜中的液渣层比例增加,渣膜润滑系数α增大;同时,渣膜中的结晶层比例也有一定程度的提高,渣膜热阻系数β增大,从而使保护渣的润滑性能和控制传热能力均得到改善。从矿相分析结果看出,保护渣碱度的提高在一定程度上会促进硅灰石的析出,导致渣膜结晶率提高、热阻增大,进而起到控制传热的目的。生产实践表明,在拉速提高后,使用新型保护渣基本避免了黏结和裂纹的产生,生产效率和铸坯质量均得到显著提高。     

Effect of casting parameters on permeability in lower part of blast furnace

Abstract

Maintenance of adequate permeability in the lower zone of a blast furnace is crucial for stable and efficient furnace operation. Permeability in the lower zone is influenced by the changing levels of hot metal and slag in addition to other operational factors. Thus, accumulation of liquids in the hearth and inadequate drainage will lead to deterioration in permeability, thereby limiting wind acceptance and furnace productivity. Therefore, the knowledge of the liquid level in the hearth and factors influencing drainage would be helpful for ensuring high permeability. Effort has been made in the present study to analyse the effect on liquid level of casting parameters such as casting rate (CR), production rate, gun up to knock out time (GKT), slag delay, cast duration (CD) and number of casts (NC). The relationships between casting parameters, liquid level and permeability resistance in the lower zone have been derived mathematically based on material balance. From known casting parameters, the liquid levels have been estimated. The prediction of liquid levels by the model was in good agreement with the furnace data on permeability resistance. In order to maintain high permeability in the lower zone, the optimum values of GKT, NC and CR for different production rates have been suggested to the plant.