Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2015"

Hydrogen Embrittlement Understood

Abstract: The connection between hydrogen-enhanced plasticity and the hydrogen-induced fracture mechanism and pathway is established through examination of the evolved microstructural state immediately beneath fracture surfaces including voids, “quasi-cleavage,” and intergranular surfaces. This leads to a new understanding of hydrogen embrittlement in which hydrogen-enhanced plasticity processes accelerate the evolution of the microstructure, which establishes not only local high concentrations of hydrogen but also a local stress state. Together, these factors establish the fracture mechanism and pathway.

Microstructural Characterization and Mechanical Performance of Hot Work Tool Steel Processed by Selective Laser Melting

Abstract: Microstructural characterization of hot work tool steel processed by selective laser melting was carried out. The findings shed light on the interrelationship between processing parameters and the microstructural evolution. It was found that the microstructure after layer-wise processing partially consists of metastable-retained austenite which transforms to martensite in a subsequent tensile test. This improves the mechanical properties of the hot work tool steel enabling direct application.

Numerical Simulation of the Interaction Between Supersonic Oxygen Jets and Molten Slag–Metal Bath in Steelmaking BOF Process

Abstract: The impinging of multiple jets onto the molten bath in the BOF steelmaking process plays a crucial role in reactor performance but is not clearly understood. This paper presents a numerical study of the interaction between the multiple jets and slag–metal bath in a BOF by means of the three-phase volume of fluid model. The validity of the model is first examined by comparing the numerical results with experimental measurement of time-averaged cavity dimensions through a scaled-down water model. The calculated results are in reasonably good agreement with the experimental data. The mathematical model is then used to investigate the primary transport phenomena of the jets-bath interaction inside a 150-ton commercial BOF under steelmaking conditions. The numerical results show that the cavity profile and interface of slag/metal/gas remain unstable as a result of the propagation of surface waves, which, likely as a major factor, governs the generation of metal droplets and their initial spatiotemporal distribution. The total momentum transferred from the jets into the bath is consumed about a half to drive the movement of slag, rather than fully converted as the stirring power for the metal bath. Finally, the effects of operational conditions and fluid properties are quantified. It is shown that compared to viscosity and surface tension of the melts, operating pressure and lance height have a much more significant impact on the slag–metal interface behavior and cavity shape as well as the fluid dynamics in the molten bath.

Three-Dimensional Modeling of Flow and Thermochemical Behavior in a Blast Furnace

Abstract: An ironmaking blast furnace (BF) is a complex high-temperature moving bed reactor involving counter-, co- and cross-current flows of gas, liquid and solid, coupled with heat and mass exchange and chemical reactions. Two-dimensional (2D) models were widely used for understanding its internal state in the past. In this paper, a three-dimensional (3D) CFX-based mathematical model is developed for describing the internal state of a BF in terms of multiphase flow and the related thermochemical behavior, as well as process indicators. This model considers the intense interactions between gas, solid and liquid phases, and also their competition for the space. The model is applied to a BF covering from the burden surface at the top to the liquid surface in the hearth, where the raceway cavity is considered explicitly. The results show that the key in-furnace phenomena such as flow/temperature patterns and component distributions of solid, gas and liquid phases can be described and characterized in different regions inside the BF, including the gas and liquids flow circumferentially over the 3D raceway surface. The in-furnace distributions of key performance indicators such as reduction degree and gas utilization can also be predicted. This model offers a cost-effective tool to understand and control the complex BF flow and performance.

Effect of Inclusions’ Behavior on the Microstructure in Al-Ti Deoxidized and Magnesium-Treated Steel with Different Aluminum Contents

Abstract: To clarify the precipitation behavior of beneficial inclusions and mechanism of their effects on microstructure, the effect of aluminum content on inclusion’s characteristics and their influence on the refinement of microstructure in Al-Ti complex deoxidized magnesium-treated steels were systematically investigated based on experiment and calculation. The results showed that due to the dual effects of Ti and Mg deoxidation, a large amount of finely dispersed Al2O3-TiO x -MgO inclusions in low aluminum steel with a complex multilayer or mosaic structure were formed, whereas a relatively smaller amount of Al2O3-MgO inclusions with the simple bundle structure were observed in high aluminum steel. The Al2O3-TiO x -MgO core oxides are more conducive to the precipitation of multiple manganese sulfides with thinner thickness on their local surfaces. Thus, the inclusion deformation, which mainly depends on the surface manganese sulfides layer, is smaller in low aluminum steel than that in high aluminum steel. Complex inclusions in low aluminum steel can pin austenite grain boundaries and induce interlocking acicular ferrite effectively. In addition to the small size and chemical composition of inclusions, the complex structure of oxides and the precipitation of multiple MnS on their surface are important to the nucleation of interlocking AFs on inclusions in Ti-deoxidized Mg-treated steel. The AFs quantity is much more, and the grain size is more uniform in low aluminum steel than that in high aluminum steel.

Fluid Flow Characteristics and Porosity Behavior in Full Penetration Laser Welding of a Titanium Alloy

Abstract: In this paper, a computational fluid mechanics model is developed for full penetration laser welding of titanium alloy Ti6Al4V. This has been used to analyze possible porosity formation mechanisms, based on predictions of keyhole behavior and fluid flow characteristics in the weld pool. Numerical results show that when laser welding 3 mm thickness titanium alloy sheets with given laser beam focusing optics, keyhole depth oscillates before a full penetration keyhole is formed, but thereafter keyhole collapses are not predicted numerically. For lower power, lower speed welding, the fluid flow behind the keyhole is turbulent and unstable, and vortices are formed. Molten metal is predicted to flow away from the center plane of the weld pool, and leave a gap or void within the weld pool behind the keyhole. For higher power, higher speed welding, fluid flow is less turbulent, and such vortices are not formed. Corresponding experimental results show that porosity was absent in the melt runs made at higher power and higher welding speed. In contrast, large pores were present in melt runs made at lower power and lower welding speed. Based on the combination of experimental results and numerical predictions, it is proposed that porosity formation when keyhole laser welding may result from turbulent fluid flow behind the keyhole, with the larger the value of associated Reynolds number, the higher the possibility of porosity formation. For such fluid flow controlled porosities, measures to decrease Reynolds number of the fluid flow close to the keyhole could prove effective in reducing or avoiding porosity.

Effect of P2O5 and FetO on the Viscosity and Slag Structure in Steelmaking Slags

Abstract: The present paper investigates the influence of P2O5 and FetO on the viscosity and structure of steelmaking slags. An understanding of the viscous behavior and structure of FetO-bearing smelting slags is essential to control the dephosphorization in steelmaking process and to efficiently recycle the phosphorus from steelmaking slags. It is found that the viscosity of CaO-SiO2-Al2O3-MgO-FetO-P2O5 slags slightly increases with increasing P2O5 content, while the viscosity decreases with increasing FetO content. The degree of the polymerization of quenched slags, determined from Raman spectra, is found to increase with increasing P2O5 content and decrease with increasing FetO content. It is also noted that the peaks of Raman spectra between 800 and 1200 cm−1 were nearly absent at the FetO content of 22.46 wt pct; whereas according to 29Si MAS-NMR and FTIR analysis, it is clearly seen that the [SiO4]-tetrahedra-related peaks existed even for the same slag. This may confirm that small quantities of extra-framework iron species can absorb the Raman scattering and damp the Raman signal intensity and the presence of FetO in the slag does not necessarily eliminate [SiO4]-tetrahedra.

Kinetics of the Reduction of Hematite Concentrate Particles by Carbon Monoxide Relevant to a Novel Flash Ironmaking Process

Abstract: A novel ironmaking process is under development at the University of Utah to produce iron directly from iron oxides concentrates by the gas–solid flash reaction using gaseous fuels and reductants. This process will reduce energy consumption and minimize carbon dioxide emissions. Having investigated the hydrogen reduction kinetics of magnetite and hematite concentrate particles relevant to the novel flash ironmaking process, the carbon monoxide reduction kinetics of hematite concentrate particles (average particle size 21 µm) was determined in the temperature range 1473 K to 1623 K (1200 °C to 1350 °C) under various carbon monoxide partial pressures. At 1623 K (1350 °C) and residence time 5 seconds, the reduction degree of hematite concentrate particles was more than 90 pct under a pure carbon monoxide. This is slower than reduction by hydrogen but still significant, indicating that CO will contribute to the reduction of hematite concentrate in the flash process. The kinetics of CO reduction separately from hydrogen is important for understanding and analyzing the complex kinetics of hematite reduction by the H2 + CO mixtures. The nucleation and growth rate equation with the Avrami parameter n = 1.0 adequately described the carbon monoxide reduction kinetics of hematite concentrate particles. The reduction rate is of 1st order with respect to the partial pressure of carbon monoxide and the activation energy of the reaction was 231 kJ/mol, indicating strong temperature dependence. The following complete rate equation was developed that can satisfactorily predict the carbon monoxide reduction kinetics of hematite concentrate particles and is suitable for the design of a flash reactor $$ \frac{{{\text{d}}X}}{{{\text{d}}t}} = 1.91 \times 10^{7} \times e^{{\frac{ - 231000}{\text{RT}}}} \times \left( {p{\text{CO}} - \frac{{p{\text{CO}}_{2} }}{K}} \right) \times (1 - X), $$ where X is the fraction of oxygen removed from iron oxide, R is 8.314 J/mol K, T is in K, p is in atm, and t is in seconds.

Population Balance Modeling of Polydispersed Bubbly Flow in Continuous-Casting Using Multiple-Size-Group Approach

Abstract: A population balance model based on the multiple-size-group (MUSIG) approach has been developed to investigate the polydispersed bubbly flow inside the slab continuous-casting mold and bubble behavior including volume fraction, breakup, coalescence, and size distribution. The Eulerian–Eulerian approach is used to describe the equations of motion of the two-phase flow. All the non-drag forces (lift force, virtual mass force, wall lubrication force, and turbulent dispersion force) and drag force are incorporated in this model. Sato and Sekiguchi model is used to account for the bubble-induced turbulence. Luo and Svendsen model and Prince and Blanch model are used to describe the bubbles breakup and coalescence behavior, respectively. A 1/4th water model of the slab continuous-casting mold was applied to investigate the distribution and size of bubbles by injecting air through a circumferential inlet chamber which was made of the specially-coated samples of mullite porous brick, which is used for the actual upper nozzle. Against experimental data, numerical results showed good agreement for the gas volume fraction and local bubble Sauter mean diameter. The bubble Sauter mean diameter in the upper recirculation zone decreases with increasing water flow rate and increases with increasing gas flow rate. The distribution of bubble Sauter mean diameter along the width direction of the upper mold increases first, and then gradually decreases from the SEN to the narrow wall. Close agreements between the predictions and measurements demonstrate the capability of the MUSIG model in modeling bubbly flow inside the continuous-casting mold.

Macrosegregation During Electroslag Remelting of Alloy 625

Abstract: A numerical model of electroslag remelting is used to examine the effects of ingot diameter, mushy zone permeability, process current levels, and initial composition on macrosegregation in alloy 625. Composition variations are made within the standard ranges for alloy 625 to alter the solutal contribution to buoyancy-driven flows and macrosegregation. Average steady-state macrosegregation and radial composition distributions are compared to identify processing conditions that best ameliorate this defect. Also, a novel approach to evaluate macrosegregation of an ingot is used, comparing the composition distribution to the alloy specification. As expected, increasing the ingot size from 51 cm (20 in.) to 76 cm (30 in.) increases the overall segregation, especially at the centerline. The segregation tends to decrease with decreasing interdendritic liquid velocity and sump depth. Processing ingots with a low current and an initial composition in the low end of the specification range is the best choice to reduce macrosegregation.

Structural Roles of Boron and Silicon in the CaO-SiO 2 -B 2 O 3 Glasses Using FTIR, Raman, and NMR Spectroscopy

Abstract: The present paper provided not only a deep insight of network structures of borosilicate glasses but also a basic linkage between the network structures and the viscous flow behaviors of many borosilicate melts. The structures of a ternary system of CaO-SiO2-B2O3 were characterized using Fourier transformation infrared (FTIR), Raman, and magic angular spinning nuclear magnetic resonance spectroscopy. The results of FTIR and Raman spectra complementally verified that the main Si-related units were SiO4 tetrahedral with zero, one, two, and three bridging oxygens [Q0(Si), Q1(Si), Q2(Si), and Q3(Si)]; the added B2O3 leaded to an increase of Q3(Si) at the cost of Q0(Si) and Q2(Si), and therefore an increasing degree of polymerization (DOP) was induced. Additionally, the 11B NMR spectra demonstrated that the dominant B-related groups were BO3 trigonal and BO4 tetrahedral, while an increasing B2O3 content facilitated the existence of BO4 tetrahedral. Moreover, there was a competitive effect between the enhanced DOP and the presence of BO3 trigonal and BO4 tetrahedral in the networks, which therefore resulted in a decreasing viscosity of borosilicate melts in numerous studies.

Visualization of Liquid Metal Two-phase Flows in a Physical Model of the Continuous Casting Process of Steel

Abstract: We present an experimental study concerned with investigations of the two-phase flow in a mock-up of the continuous casting process of steel. A specific experimental facility was designed and constructed at HZDR for visualizing liquid metal two-phase flows in the mold and the submerged entry nozzle (SEN) by means of X-ray radioscopy. This setup operates with the low melting, eutectic alloy GaInSn as model liquid. The argon gas is injected through the tip of the stopper rod into the liquid metal flow. The system operates continuously under isothermal conditions. First results will be presented here revealing complex flow structures in the SEN widely differing from a homogeneously dispersed bubbly flow. The patterns are mainly dominated by large bubbles and large-area detachments of the liquid metal flow from the inner nozzle wall. Various flow regimes can be distinguished depending on the ratio between the liquid and the gas flow rate. Smaller gas bubbles are produced by strong shear flows near the nozzle ports. The small bubbles are entrained by the submerged jet and mainly entrapped by the lower circulation roll in the mold. Larger bubbles develop by coalescence and ascend toward the free surface.

Effect of Al2O3 on the Viscosity and Structure of CaO-SiO2-MgO-Al2O3-FetO Slags

Abstract: The present paper provided a fundamental investigation on the effect of Al2O3 on the viscosity and structure of CaO-SiO2-MgO-Al2O3-FetO slags for the purpose of efficiently recycling the valuable elements from the steelmaking slags. The results show that the viscosity of CaO-SiO2-Al2O3-MgO-FetO slags slightly increases with increasing Al2O3 content. The degree of the polymerization (DOP) of quenched slags, determined from Raman spectra and magic angle spinning–nuclear magnetic resonance, is also found to increase with increasing Al2O3 content. It can be deduced that the increasing DOP can promote the formation of gehlenite phase (Ca2Al2SiO7), thus facilitating the formation of higher phosphorous (or vanadium) contained solid solution (n′Ca2SiO4·Ca3((P or V)O4)2). As Al2O3 content increases up to a specific value, the charge compensating ions which present near [AlO4]-tetrahedra and [FeO4]-tetrahedra are not fully supplied due to the scarcity of Ca2+. In this case, the existing Fe3+ in the melt cannot completely form [FeO4]-tetrahedra and part of Fe3+ would form [FeO6]-octahedra to substitute Ca2+ to modify the slags.

Effects of B 2 O 3 and BaO on the Crystallization Behavior of CaO-Al 2 O 3 -Based Mold Flux for Casting High-Al Steels

Abstract: The non-reactive lime–alumina-based mold flux has been proposed to overcome the aluminum introduced slag/steel interaction problems during the casting of high aluminum bearing steels In this article, a new mold flux with CaO/Al2O3 (C/A) ratio of 2 was designed, and the effects of BaO substituting for CaO to replace B2O3 on the mold flux crystallization behaviors have been investigated through the using of single hot thermocouple technology and double hot thermocouple technology (DHTT) The results suggested that BaO shows the stronger tendency to inhibit crystallization of mold flux comparing with B2O3; however, the synergic effect of the combination of BaO and B2O3 tends to improve the mold flux crystallization in this non-reactive mold flux system The analysis of the crystallized phases for different samples and the crystallization behaviors under simulated thermal gradient through DHTT were also conducted to support the reliability of above conclusions

Effect of SiO 2 on the Crystallization Behaviors and In-Mold Performance of CaF 2 -CaO-Al 2 O 3 Slags for Drawing-Ingot-Type Electroslag Remelting

Abstract: The crystallization characteristics of CaF2-CaO-Al2O3 slags with varying amounts of SiO2 were experimentally studied. The effects of slag crystallization behaviors on the horizontal heat transfer and lubrication performance in drawing-ingot-type electroslag remelting (ESR) were also evaluated in terms of as-cast ingots surface quality and drawing-ingot operation. The results show that increasing SiO2 addition from 0 to 6.8 mass pct strongly suppresses the crystallization of ESR type CaF2-CaO-Al2O3 slags. The crystallization temperature of the studied slags decreases with the increase in SiO2 addition. The liquidus temperatures of the slags also show a decreasing trend with increasing SiO2 content. In CaF2-CaO-Al2O3-(SiO2) slags, faceted 11CaO·7Al2O3·CaF2 crystals precipitate first during continuous cooling of the slag melts, followed by the formation of CaF2 at lower temperatures. 11CaO·7Al2O3·CaF2 was confirmed to be the dominant crystalline phase in the studied slags. CaF2-CaO-Al2O3 slags with a small amount of SiO2 addition are favorable for providing sound lubrication and horizontal heat transfer in mold for drawing-ingot-type ESR, which consequently bring the improvement in the surface quality of ESR ingot and drawing-ingot operating practice as demonstrated by plant trials.

Microstructural Development and Technical Challenges in Laser Additive Manufacturing: Case Study with a 316L Industrial Part

Abstract: Additive manufacturing (AM) brings disruptive changes to the ways parts, and products are designed, fabricated, tested, qualified, inspected, marketed, and sold. These changes introduce novel technical challenges and concerns arising from the maturity and diversity of today’s AM processes, feedstock materials, and process parameter interactions. AM bears a resemblance with laser and electron beam welding in the so-called conduction mode, which involves a multitude of dynamic physical events between the projected feedstock and a moving heat source that eventually influence AM part properties. For this paper, an air vent was selected for its thin-walled, hollow, and variable cross section, and limited size. The studied air vents, randomly selected from a qualification batch, were fabricated out of 316L stainless steel using a 4 kW fiber laser powder-fed AM system, referred to as construction laser additive direct (CLAD). These were systematically characterized by microhardness indentation, visual examination, optical and scanning electron microscopy, and electron-back-scattering diffraction in order to determine AM part suitability for service and also broadly discuss metallurgical phenomena. The paper then briefly expands the discussion to include additional engineering alloys and further analyze relationships between AM process parameters and AM part properties, consistently utilizing past experience with the same powder-fed CLAD 3D printer, the well-established science and technology of welding and joining, and recent publications on additive manufacturing.

Simulation on Decarburization and Inclusion Removal Process in the Ruhrstahl–Heraeus (RH) Process with Ladle Bottom Blowing

Abstract: To enhance the refining efficiency of the Ruhrstahl–Heraeus (RH) process, the ladle bottom blowing was employed in RH degasser and a numerical method was employed to investigate the decarburization and inclusion removal in RH with ladle bottom blowing. The results showed that the decarburization rate in RH with ladle bottom blowing is greater than that in traditional RH. The larger mass fraction of carbon at the recirculation zone under up snorkel disappears because of the gas bubbles from ladle bottom blowing in an RH degasser. For RH with ladle bottom blowing, the decarburization at argon bubble surface accounts for the majority of the removed carbon, and it is approximately two times greater than that in the inner site of the vacuum chamber. Besides, the inclusion removal rate in RH with ladle bottom blowing is greater than that in traditional RH, and the maximum inclusion characteristic radius is much less in RH with ladle bottom blowing than that in traditional RH. Besides, the accumulation of inclusions in ladle between sidewall and up snorkel and the recirculation zone under up snorkel, which can be found in traditional RH, disappears in RH with ladle bottom blowing. For RH with ladle bottom blowing, the average number density of inclusions decreases more drastically than that in traditional RH and the average terminal number density of inclusions is much smaller than that in traditional RH.

Stability Diagram of Mg-Al-O System Inclusions in Molten Steel

Abstract: In the current study, the stability diagrams of Mg-Al-O system in molten steel are calculated using two methods. After comparing the result of connecting iso-oxygen contours of different phases (iso-oxygen contours method) and calculating the border lines of different phases (border lines method), the former method is more accurate and popular. Particularly, the detailed calculation procedures and connection line principles of stability diagram are exhibited. The effects of interaction coefficient, temperature, and activity of oxides on the stability diagram are also discussed. With the currently reported method, stability diagrams of various inclusions in molten steel can be calculated to predict the formation of inclusions.

Mixing Phenomena in a Bottom Blown Copper Smelter: A Water Model Study

Abstract: The first commercial bottom blown oxygen copper smelting furnace has been installed and operated at Dongying Fangyuan Nonferrous Metals since 2008. Significant advantages have been demonstrated in this technology mainly due to its bottom blown oxygen-enriched gas. In this study, a scaled-down 1:12 model was set up to simulate the flow behavior for understanding the mixing phenomena in the furnace. A single lance was used in the present study for gas blowing to establish a reliable research technique and quantitative characterisation of the mixing behavior. Operating parameters such as horizontal distance from the blowing lance, detector depth, bath height, and gas flow rate were adjusted to investigate the mixing time under different conditions. It was found that when the horizontal distance between the lance and detector is within an effective stirring range, the mixing time decreases slightly with increasing the horizontal distance. Outside this range, the mixing time was found to increase with increasing the horizontal distance and it is more significant on the surface. The mixing time always decreases with increasing gas flow rate and bath height. An empirical relationship of mixing time as functions of gas flow rate and bath height has been established first time for the horizontal bottom blowing furnace.

Dynamic Modeling of LD Converter Steelmaking: Reaction Modeling Using Gibbs’ Free Energy Minimization

Abstract: Slag–metal emulsion plays an important role in the oxidation kinetics of metalloids in oxygen steelmaking. The importance of droplet generation rate, droplet size, and its residence time in the slag–metal emulsion on the overall reaction kinetics has become evident in recent times. Residence times of the droplets are strongly dependent on the decarburization rate, the CO bubbles giving a buoyant force to the droplets. The present work aims at developing a mathematical model for predicting the composition evolutions of the slag and the metal phases as the blow proceeds in an LD converter. The process dynamics are modeled by dividing the LD convertor into three separate continuous stirred tank reactors. Oxidation reactions are assumed to be primarily taking place at the interface between the slag and the metal phases in the emulsion. Among the different mass transfer and reaction steps controlling the kinetics, the mass transfer of FeO in the slag phase and that of the metalloids within the metal droplet are assumed to be rate-controlling. For a Fe-C-X (X = Mn, Si etc.) droplet, simultaneous removal of elements have been modeled by Gibbs’ free energy minimization at the slag–metal interface. Effects of droplet size, mass transfer coefficient, and initial carbon content on the mean residence time of metal droplets in the slag–metal emulsion have also been identified. Mixing in the metal phase is simulated in terms of metal exchange rate and the reactor weight ratio between the upper and the lower parts of the bath.

Formation Mechanism of SiO 2 -Type Inclusions in Si-Mn-Killed Steel Wires Containing Limited Aluminum Content

Abstract: The origin, formation mechanism, and evolution of SiO2-type inclusions in Si-Mn-killed steel wires were studied by pilot trials with systematical samplings at the refining ladle, casting tundish, as-cast bloom, reheated bloom, and hot-rolled rods. It was found that the inclusions in tundish were well controlled in the low melting point region. By contrast, MnO-SiO2-Al2O3 inclusions in the as-cast bloom were with compositions located in the primary region of SiO2, and most CaO-SiO2-Al2O3-MnO inclusions lied in primary phase region of anorthite. Therefore, precipitation of SiO2 particles in MnO-SiO2-Al2O3 inclusions can be easier than in CaO-SiO2-Al2O3-MnO inclusions to form dual-phase inclusions in the as-cast bloom. Thermodynamic calculation by the software FactSage 6.4 (CRCT-ThermFact Inc., Montreal, Canada) showed that mass transfer between liquid steel and inclusions resulted in the rise of SiO2 content in inclusions from tundish to as-cast bloom and accelerated the precipitation of pure SiO2 phase in the formed MnO-SiO2-Al2O3 inclusions. As a result, the inclusions characterized by dual-phase structure of pure SiO2 in MnO-SiO2-Al2O3 matrix were observed in both as-cast and reheated blooms. Moreover, the ratio of such dual-phase SiO2-type inclusions witnessed an obvious increase from 0 to 25.4 pct before and after casting, whereas it changed little during the reheating and rolling. Therefore, it can be reasonably concluded that they were mainly formed during casting. Comparing the evolution of the inclusions composition and morphology in as-cast bloom and rolled products, a formation mechanism of the SiO2-type inclusions in wire rods was proposed, which included (1) precipitation of SiO2 in the formed MnO-SiO2-Al2O3 inclusion during casting and (2) solid-phase separation of the undeformed SiO2 precipitation from its softer MnO-SiO2-Al2O3 matrix during multipass rolling.

A Dynamic Mesh-Based Approach to Model Melting and Shape of an ESR Electrode

Abstract: This paper presents a numerical method to investigate the shape of tip and melt rate of an electrode during electroslag remelting process. The interactions between flow, temperature, and electromagnetic fields are taken into account. A dynamic mesh-based approach is employed to model the dynamic formation of the shape of electrode tip. The effect of slag properties such as thermal and electrical conductivities on the melt rate and electrode immersion depth is discussed. The thermal conductivity of slag has a dominant influence on the heat transfer in the system, hence on melt rate of electrode. The melt rate decreases with increasing thermal conductivity of slag. The electrical conductivity of slag governs the electric current path that in turn influences flow and temperature fields. The melting of electrode is a quite unstable process due to the complex interaction between the melt rate, immersion depth, and shape of electrode tip. Therefore, a numerical adaptation of electrode position in the slag has been implemented in order to achieve steady state melting. In fact, the melt rate, immersion depth, and shape of electrode tip are interdependent parameters of process. The generated power in the system is found to be dependent on both immersion depth and shape of electrode tip. In other words, the same amount of power was generated for the systems where the shapes of tip and immersion depth were different. Furthermore, it was observed that the shape of electrode tip is very similar for the systems running with the same ratio of power generation to melt rate. Comparison between simulations and experimental results was made to verify the numerical model.

Establishing Mathematical Models to Predict Grain Size and Hardness of the Friction Stir-Welded AA 7020 Aluminum Alloy Joints

Abstract: In this study, response surface methodology in conjunction with a central composite design was applied to predict the grain size and hardness of friction stir-welded AA 7020 aluminum alloy joints. For this purpose, three welding parameters, including tool rotational speed, traverse speed, and tool axial force, at five levels and 20 runs were considered. In order to validate the predicted models, the analysis of variance was performed. Hardness and microstructural features of the joints were investigated using microhardness test and optical microscopy, respectively. In addition, the influences of friction stir welding parameters on grain size and hardness of the joints were examined thoroughly. The analysis of variance results revealed that the developed models were significant and accurate to predict the responses. Furthermore, with increasing the heat input, the hardness of the joints decreased, where the grain size increased continuously. In addition, the optimized condition for achieving the lowest grain size and highest hardness of the joints was reached as 800 rpm, 125 mm/min and 8 kN.

Improvement of Resistance of MgO-Based Refractory to Slag Penetration by In Situ Spinel Formation

Abstract: MgO-in situ spinel substrate was prepared at 1773 K (1500 °C) using colloidal alumina suspension and coarse MgO as raw materials. While the addition of 10 mass pct colloidal alumina had limited effect, addition of 20 mass pct colloidal alumina into the MgO matrix improved greatly the resistance of the substrate against the slag penetration at 1873 K (1600 °C). The improvement was found to be mainly related to the formation of solid phases, CaO·Al2O3 and CaO·MgO·Al2O3 at the grain boundaries due to slag-spinel reaction. Putting the reacted substrate into contact again with new slag revealed no appreciable new slag penetration. The results showed a potential solution to improve the resistance of MgO-based refractory to slag penetration and to improve steel cleanness.

A Mathematical Modeling Study of Tracer Mixing in a Continuous Casting Tundish

Abstract: A mathematical model based on a water model was developed to study the tracer mixing in a single strand tundish. The mixing behavior of black ink and KCl solution was simulated by a mixed composition fluid model, and the data were validated by water modeling results. In addition, a model that solves the scalar transport equation (STE) without any physical properties of the tracer was studied and the results were compared to predictions using the density-coupled model. Furthermore, the mixing behaviors of different amounts of KCl tracers were investigated. Before the model was established, KCl tracer properties such as the KCl molecule diffusion (KMD), the water molecule self-diffusion (WSD) in KCl solution, and the KCl solution viscosity (KV) were evaluated. The RTD curve of 250 mL KCl for the KMD case was closer to the water modeling results than that of the case implemented with only density. Moreover, the ensemble average deviation of the RTD curves of the cases implemented with KMD + WSD, KMD + KV, and KMD + WSD + KV to the KMD case is less than 0.7 pct. Thus, the water self-diffusion and KV were neglected, while the KCl density and KMD were implemented in the current study. The flow pattern of black ink was similar to the STE result i.e., the fluid flowed upwards toward the top surface and formed a large circulating flow at the outlet nozzle. The flow behavior of the 100, 150, and 250 mL KCl cases exhibited a strong tendency to sink to the tundish bottom, and subsequently flow through the holes in the dam. Thereafter, it propagated toward the outlet nozzle. Regarding the KCl tracer amount, the tracer concentration propagated to the outlet nozzle much faster for the larger amount case than for the smaller amount cases. However, the flow pattern for the 50 mL KCl case was somewhat different. The fluid propagated to the top surface which acted like black ink during the initial injection, and subsequently the fluid flowed throughout the holes at a much slower pace. The breakthrough time and peak concentration of RTD curves of model predictions and water modeling results showed a good agreement (all difference within 12.5 pct) for the 100, 150, and 250 mL KCl cases.

Viscosity of CaO-MgO-Al 2 O 3 -SiO 2 -TiO 2 Melts Containing TiC Particles

Abstract: The present study reports an experimental investigation into the effect of solid suspension on the viscosity of molten slags by the rotating-cylinder method. The viscosities of CaO-MgO-Al2O3-SiO2-TiO2 melts containing different volume percentages of TiC particles (1.0 μm) are measured at different temperatures. It is found that the Arrhenius law was obeyed for both the melts with and without TiC particles. The viscosity for the TiC-containing melt is found to increase with the addition of the particles, but the effect appears to be much greater than estimated by the Einstein–Roscoe type equation, $$ \eta = \eta_{0} (1 - 1.35f)^{ - 2.5} $$ , where η and η o are the viscosities of the two-phase mixture and the solid-free melt, and f is the volume fraction of solid particles in the melt.

Influence of Sludge Particles on the Tensile Properties of Die-Cast Secondary Aluminum Alloys

Abstract: The effects of sludge intermetallic particles on the mechanical properties of a secondary AlSi9Cu3(Fe) die-casting alloy have been studied Different alloys have been produced by systematically varying the Fe, Mn, and Cr contents within the composition tolerance limits of the standard EN AC-46000 alloy The microstructure shows primary α-Al x (Fe,Mn,Cr) y Si z sludge particles, with polyhedral and star-like morphologies, although the presence of primary β-Al5FeSi phase is also observed at the highest Fe:Mn ratio The volume fraction of primary compounds increases as the Fe, Mn, and Cr contents increase and this can be accurately predicts from the Sludge Factor by a linear relationship The sludge amount seems to not influence the size and the content of porosity in the die-cast material Furthermore, the sludge factor is not a reliable parameter to describe the mechanical properties of the die-cast AlSi9Cu3(Fe) alloy, because this value does not consider the mutual interaction between the elements In the analyzed range of composition, the design of experiment methodology and the analysis of variance have been used in order to develop a semi-empirical model that accurately predicts the mechanical properties of the die-cast AlSi9Cu3(Fe) alloys as function of Fe, Mn, and Cr concentrations

Splash Distribution in Oxygen Steelmaking

Abstract: The study of splashing is important in understanding oxygen steelmaking. Splashing creates large interfacial area between reacting surfaces and thereby directly affects the kinetics of steelmaking reactions. In the present cold modeling work, a study of splashing has been carried out for various lance heights and gas flow rates. Sampling of droplets has been done in both radial positions and vertical positions across the bath to investigate the effect of sampling positions in estimation of the droplet generation rate. A novel approach has been developed to estimate the droplet generation rate. The results of the study have been compared with previous investigations. Results show that positioning of sampling is a critical issue and can affect the estimation of droplets present in the emulsion. This study also demonstrates quantitatively how much the droplet generation rate is reduced when the cavity mode changes from splashing to penetrating for different Blowing numbers.

In Situ Synchrotron X-ray Study of Ultrasound Cavitation and Its Effect on Solidification Microstructures

Abstract: Considerable progress has been made in studying the mechanism and effectiveness of using ultrasound waves to manipulate the solidification microstructures of metallic alloys. However, uncertainties remain in both the underlying physics of how microstructures evolve under ultrasonic waves, and the best technological approach to control the final microstructures and properties. We used the ultrafast synchrotron X-ray phase contrast imaging facility housed at the Advanced Photon Source, Argonne National Laboratory, US to study in situ the highly transient and dynamic interactions between the liquid metal and ultrasonic waves/bubbles. The dynamics of ultrasonic bubbles in liquid metal and their interactions with the solidifying phases in a transparent alloy were captured in situ. The experiments were complemented by the simulations of the acoustic pressure field, the pulsing of the bubbles, and the associated forces acting onto the solidifying dendrites. The study provides more quantitative understanding on how ultrasonic waves/bubbles influence the growth of dendritic grains and promote the grain multiplication effect for grain refinement.

Effects of MgO and Al2O3 Addition on Redox State of Chromium in CaO-SiO2-CrOx Slag System by XPS Method

Abstract: The effects of MgO and Al2O3 on the redox state of chromium in CaO-SiO2-CrO), system have been investigated at 1873 K (1600 degrees C) under Ar-CO-CO2 atmosphere and analyzed by means of X-ray phot ...