Showing papers in "Metallurgical and Materials Transactions B-process Metallurgy and Materials Processing Science in 2003"

Heat-transfer and solidification model of continuous slab casting: CON1D

Abstract: A simple, but comprehensive model of heat transfer and solidification of the continuous casting of steel slabs is described, including phenomena in the mold and spray regions. The model includes a one-dimensional (1-D) transient finite-difference calculation of heat conduction within the solidifying steel shell coupled with two-dimensional (2-D) steady-state heat conduction within the mold wall. The model features a detailed treatment of the interfacial gap between the shell and mold, including mass and momentum balances on the solid and liquid interfacial slag layers, and the effect of oscillation marks. The model predicts the shell thickness, temperature distributions in the mold and shell, thickness of the resolidified and liquid powder layers, heat-flux profiles down the wide and narrow faces, mold water temperature rise, ideal taper of the mold walls, and other related phenomena. The important effect of the nonuniform distribution of superheat is incorporated using the results from previous three-dimensional (3-D) turbulent fluid-flow calculations within the liquid pool. The FORTRAN program CONID has a user-friendly interface and executes in less than 1 minute on a personal computer. Calibration of the model with several different experimental measurements on operating slab casters is presented along with several example applications. In particular, the model demonstrates that the increase in heat flux throughout the mold at higher casting speeds is caused by two combined effects: a thinner interfacial gap near the top of the mold and a thinner shell toward the bottom. This modeling tool can be applied to a wide range of practical problems in continuous casters.

Calciothermic reduction of titanium oxide and in-situ electrolysis in molten CaCl2

Abstract: A concept for calciothermic direct reduction of titanium dioxide in molten CaCl2 is proposed and experimentally tested. This production process consists of a single cell, where both the thermochemical reaction of the calciothermic reduction and the electrochemical reaction for recovery of the reducing agent, Ca, coexist in the same molten CaCl2 bath. A few molar percentages of Ca dissolve in the melt, which gives the media a strong reducing power. Using a carbon anode and a Ti basket-type cathode in which anatase-type TiO2 powder was filled, a metallic titanium sponge containing 2000 ppm oxygen was produced after 10.8 ks at 1173 K in the CaCl2 bath. The optimum concentration of CaO in the molten CaCl2 was 0.5 to 1 mol pct, to shorten the operating time and to achieve a lower oxygen content in Ti.

A Transient Simulation and Dynamic Spray Cooling Control Model for Continuous Steel Casting

Abstract: A two-dimensional heat-transfer model for transient simulation and control of a continuous steel slab caster is presented. Slab temperature and solidification are computed by the model as a function of time-varying casting speed, secondary spray cooling water flow rates and temperature, slab thickness, steel chemistry, and pouring and ambient temperatures. Typically, the solidification path, temperature-solid fraction relationship, is prescribed. However, if these data are not available, a microsegregation solidification model that approximates the effects of steel chemistry and cooling rate is incorporated in the caster model. Measured slab surface temperatures recorded from an operating caster are compared with predictions from the transient model. These demonstrate that the model typically can predict the temperature response at the slab surface within 30 °C. Results of several simulations are given to demonstrate the effects of changing casting conditions on the slab thermal profile, end of liquid pool, and solidification end point. A control methodology and algorithm suitable for online control of a continuous casting machine is described, and the ability to control the surface temperature profile by dynamically adjusting secondary spray cooling flow rates is demonstrated by simulation. Results from a preliminary version of the model that is capable of running in real time are presented and are compared with the slower, but more realistic, version of the model.

Viscosity of blast furnace type slags

Abstract: The effect of MgO, TiO2, or Fe2O3 on the viscosity of 40CaO-40SiO2-20Al2O3 (mass pct) slags has been measured by the rotating crucible viscometer. Viscosity of these quaternary slags decreased with an increase in the content of additive oxide. At the same content of additive oxide, the viscosity decreases from MgO, TiO2 to Fe2O3. In addition, the effect of SiO2 or Al2O3 on the viscosity of 26.1CaO-73.9Fe2O3 (mass pct) (CF) and 14.9CaO-85.1Fe2O3 (mass pct) (CF2) slags has been measured. Viscosity of calcium ferrite slags increased with increasing SiO2 or Al2O3 content. Al2O3 was found to be more effective for increasing the viscosity at the same content of the additive oxide.

Calciothermic reduction of titanium oxide in molten CaCl2

Abstract: Titanium oxides were reduced to metallic titanium using the liquid calcium floating on the molten CaCl2. A part of Ca dissolved into CaCl2 and reacted with TiO2 settled below CaCl2. The by-product CaO also dissolved by about 20 mol pct into CaCl2, which was effective in reducing the oxygen concentration in the obtained Ti particles. The compositional region in the Ca-CaCl2-CaO system was examined for the less oxygen contamination in Ti and the better handling in leaching. A large amount of the residual calcium oxidized the titanium powder in leaching. The metallic Ti powder less than 1000 mass ppm oxygen could be obtained only for 3.6 ks using 5 to 7 mol pct Ca-CaCl2 at 1173 K. The powder was slightly sintered like sponge, and contained approximately 1500 ppm Ca. The anatase phae, the intermediate product in the refining process of TiO2, could be also supplied as raw material as well as rutile.

Modeling transient slag-layer phenomena in the shell/mold gap in continuous casting of steel

Abstract: Mold-slag friction and fracture may cause heat-transfer variations in continuous casting, which leads to steel shell temperature and stress variations, resulting in surface cracks Analytical transient models of liquid slag flow and solid slag stress have been coupled with a finite-difference model of heat transfer in the mold, gap, and steel shell to predict transient shear stress, friction, slip, and fracture of the slag layers The models are validated by comparing with numerical models and plant measurements of mold friction Using reported slag-fracture strength and time-temperature-transformation (TTT) diagrams, the models are applied to study the effect of casting speed and mold-powder viscosity properties on slag-layer behavior between the oscillating mold wall and the solidifying steel shell The study finds that liquid-slag lubrication would produce negligible stresses A lower mold-slag consumption rate leads to high solid friction and results in solid-slag-layer fracture and movement below a critical value Crystalline slag tends to fracture near the meniscus and glassy slag tends to fracture near the mold exit A medium casting speed may be the safest to avoid slag fracture, due to its having the lowest critical lubrication consumption rate The high measured friction force in operating casters could be due to three sources: an intermittent moving solid slag layer, excessive mold taper, or mold misalignment

Physicochemical aspects of titanium slag production and solidification

Abstract: The titanium-rich slags produced by ilmenite smelting are unusual in several respects, including their low viscosity, high electrical conductivity, propensity to solidify predominantly as one phase (pseudobrookite), and reactivity (with oxidizers) in the liquid and solidified form. The low viscosity is related to the unexpectedly “basic” behavior of Ti4+; the high conductivity is likely through an electron transfer mechanism involving the Ti4+-Ti3− pair; the oxidation behavior is caused by the presence of a significant amount of Ti3+. Possible reasons for the nearly single-phase structure of the solidified slag are considered, and it is concluded that the most likely cause is the presence of a eutectic groove close to the M3O5 (pseudobrookite) composition. This conclusion was tested by examining the compositions and microstructure of slag samples from a pilot smelter. The results show that the slags do contain a small amount of rutile, in line with predictions.

Estimation and modeling of parameters for direct reduction in iron ore/coal composites: Part II. Kinetic parameters

Abstract: The published approaches to the mathematical modeling of rates of reduction, of coal gasification, and of devolatilization of coal in iron/ore coal composites are reviewed and critically analyzed. The effect of different parameters on the overall process is discussed. The concepts of a local rate of reduction and gasification and an integrated rate of reduction are introduced, and the rate-controlling steps in each are reviewed. Current approaches to modeling coal pyrolysis are also described. This review, with the estimates, data, and analysis related to modeling rates of reduction, gasification, and pyrolysis, should prove useful to researchers developing models of coalbased iron ore reduction and related processes.

Kinetics and mechanism of carbothermic reduction of magnesia

Abstract: The reaction between MgO and graphite powders under flowing argon atmosphere was studied using a dynamic thermogravimetric method. In the temperature range 293 to 1973 K, the effects of compacting pressure, magnesia/carbon ratio, heating rate, Ar carrier-gas flow rate, and CO-partial pressure were investigated. An experimentally determined reaction mechanism was proposed and discussed. The reduction process could be divided into two stages. The first stage includes the direct reaction between MgO and graphite particles and partial gas-solid reaction at relatively low temperature (below 1750 K). The overall reaction rate depends on the solid phase-boundary reaction between magnesia and carbon particles. The second stage is the gas-solid reaction between CO and MgO, which determines the overall reaction rate. The apparent activation energies of the two stages were estimated to be 208.29 and 374.13 kJ/mol, respectively.

Dusts, scale, slags, sludges... Not wastes, but sources of profits

Abstract: Historically, the steel industry has focused on the need for and the many benefits of recycling steel that is discarded either in its own or in its customers’ manufacturing processes, as well as in recovery and reuse of steel scrap that arises after the product has served its intended purpose. In fact, modern steelmaking relies on the use of recycled iron units for at least half of its production.

Numerical analysis of metal transfer in gas metal arc welding

Abstract: The present article describes a numerical procedure to simulate metal transfer and the model will be used to analyze the transport processes involved in gas metal arc welding (GMAW). Advanced Computational fluid dynamics (CFD) techniques used in this model include a two-step projection method for solving the incompressible fluid flow; a volume of fluid (VOF) method for capturing free surface; and a continuum surface force (CSF) model for calculating surface tension. The electromagnetic force due to the welding current is estimated by assuming several different types of current density distribution on the free surface of the drop. The simulations based on the assumption of Gaussian current density distribution show that the transition from globular to spray transfer mode occurs over a narrow current range and the size of detached drops is nonuniform in this transition zone. The analysis of the calculation results gives a better understanding of this physical procedure. Comparisons between calculated results and experimental results are presented. It is found that the results computed from the Gaussian assumption agree well with those observed in experiments.

Crystal growth in liquid steel during secondary metallurgy

Abstract: Morphology of nonmetallic inclusions depends on their crystallographic structure, the growth conditions, and the presence of impurities. Inclusions were extracted from industrial aluminum-killed steel samples and investigated under high-resolution scanning electron microscopy. In this article, the morphology of these aluminum oxide inclusions, including their surface features, is approached from the viewpoint of crystal growth. Commonly, aluminum oxide inclusions are considered to be corundum, but some inclusion shapes prove that other aluminum oxide polymorphs are present as well.

Laminar-turbulent transition in an electromagnetically levitated droplet

Abstract: During experiments on the MSL-1 (first microgravity science laboratory) mission of the space shuttle (STS-83 and STS-94, April and July 1997), a droplet of palladium-silicon alloy was electromagnetically levitated for viscosity measurements. For the nondeforming droplet, the resultant magnetohydrodynamic (MHD) flow inside the drop can be inferred from motion of impurity particulates on the surface. In the experiments, subsequent to melting, Joule heating produces a continuous reduction of viscosity of the fluid resulting in an acceleration of the flow with time. These observations indicate formation of a pair of co-rotating toroidal flow structures inside the spheroidal drop that undergo flow instabilities. As the fluid temperature rises, the amplitude of the secondary flow increases, and beyond a point, the tracers exhibit noncoherent chaotic motion signifying emergence of turbulence inside the drop. Assuming that the observed laminar-turbulent transition is shear-layer type, the internal structure of the toroidal loops is used to develop a semiempirical correlation for the onset of turbulence. Our calculations indicate that the suggested correlation is in modest agreement with the experimental data, with the transition occurring at a Reynolds number of 600.

Marangoni flow at the gas/melt interface of steel

Abstract: Direct observation with a scanning laser microscope was made to determine the direction and velocity of surface flow of steel melt in the vicinity of the solid/melt (S/M) interface. During solidification, a fast solutal Marangoni flow moving away from the S/M interface was confirmed to exist on steel melts containing oxygen and sulfur of 10 to 105 ppm. Even in such a low range of oxygen and sulfur content, the solutal Marangoni flow can be very fast, carrying inclusion particles up to the free surface along the S/M interface. During heating and holding, however, a thermal Marangoni flow combined with convective flow generated a reverse flow directed toward the S/M interface. These features have important relevance to inclusion entrainment and solute segregation during the solidification of steel.

Solution-chemistry analysis of ammonium bicarbonate consumption in rare-earth-element precipitation

Abstract: A solution-chemistry analysis is applied to estimating the consumption of ammonium bicarbonate in the recovery of rare-earth (RE) elements from leachates of weathered clays. The theoretical analysis shows that a two-step process is needed for recovering RE from the leachates of the weathered clays by precipitation using ammonium bicarbonate. The first step is a precipitation at solution pH 5 to remove impurities such as Fe and Al. The second step is to precipitate RE by adjusting the solution pH above 8. The consumption of ammonium bicarbonate was found to depend on the concentration of RE elements and impurities in the leachates. The total amount of ammonium bicarbonate consumption for the entire process was determined experimentally, and the results showed an excellent agreement with that calculated based on solution-chemistry analysis. The decomposition of H2CO3 was identified as one of the main causes of ammonium bicarbonate overdose, accounting for up to 41 pct in comparison to 20 pct consumption for the removal of impurities. The amount of ammonium bicarbonate required in terms of the NH4HCO3: RE2O3 (RE oxides) molar ratio was found to be 4:1 for maximal RE recovery. An overall RE recovery around 90 pct was achieved with a product purity being about 90 pct.

Studies of the wetting characteristics of liquid iron on dense alumina by the X-ray sessile drop technique

Abstract: In the present work, the reaction between a molten iron drop and dense alumina was studied using the X-ray sessile-drop method under different oxygen partial pressures in the gas atmosphere. The changes in contact angles between the iron drop and the alumina substrate were followed as functions of temperature and varying partial pressures of oxygen in the temperature range 1823 to 1873 K both in static and dynamic modes. The results of the contact angle measurements with pure iron in contact with dense alumina in extremely well-purified argon as well as under different oxygen partial pressures in the gas atmosphere showed good agreement with earlier measurements reported in the literature. In the dynamic mode, when argon was replaced by a CO-CO2-Ar mixture with a well-defined PO, in the gas, the contact angle showed an initial decrease followed by a period of nearly constant contact angle. At the end of this period, the length of which was a function of the P-O2 imposed, a further steep decrease in the contact angle was noticed. An intermediate layer of FeAl2O4 was detected in the scanning electron microscope (SEM) analysis of the reacted substrates. An interesting observation in the present experiments is that the iron drop moved away from the site of the reaction once the product layer covered the interface. The results are analyzed on the basis of the various forces acting on the drop.

Observation of Calcium Aluminate Inclusions at Interfaces between Ca-Treated, Al-Killed Steels and Slags

Abstract: The evolution of Al2O3-CaO inclusions on molten steel surfaces and at molten steel/slag interfaces was observed in-situ through a confocal scanning laser microscope (CSLM) equipped with a gold-image furnace. Depending on the slag chemistry, some of the initially liquid inclusions evolved into irregular Al2O3 or SiO2-enriched inclusions during the separation across the interface. Inclusions were found to cluster at specific locations at the steel/slag interface. Unlike capillary-depression-driven clustering, which is observed on molten steel surfaces, a weak repulsive force opposes fluid-flow-driven clustering at the steel/slag interface. After clustering, the irregular solid inclusions were observed to agglomerate to form large aggregates.

Static and dynamic energy absorption of Al foams produced by the sintering and dissolution process

Abstract: This article investigates the effects of the processing conditions of the novel sintering and dissolution process (SDP), including sintering temperature, sintering time, Mg addition, and cell size, on the capacities of the as-processed Al foams for static and dynamic energy absorption. While higher temperatures generally promote better bonding between the Al particles during sintering and, therefore, improved energy-absorption capacities, there exist optimum sintering times and cell-size ranges for producing Al foams with the best energy-absorption characteristics. The optimum sintering time increases when the sintering temperature or the relative density is decreased. The addition of a small amount of Mg powder to the Al/NaCl compact can enhance the sintering markedly and increase the energy-absorbing capacity of the foam by up to 50 pct. For any Al foam, the energy absorbed in the static condition is much greater than that in the dynamic condition. The mechanisms of these effects are also discussed.

Bubble Bursting Phenomenon in Gas/Metal/Slag Systems

Abstract: Iron droplets can be ejected into the surrounding atmosphere or entrained into the slag phase when gas bubbles pass through the metal surface or the metal/slag interface. The phenomena occurring during passage of single bubbles through the free surface and the interface were investigated by using the in-situ X-ray transmission technique. The mass of droplets ejected into the atmosphere attained a maximum value at a certain bubble size, which depended on the surface tension of the iron melt. Bubble bursting on the free surface of iron melt ejected numerous fine iron droplets called “film droplets” and a few much larger jet droplets. Two different groups of iron droplets were also observed as entrained in the slag due to bubble passage through the iron/slag interface, although the physical phenomena are to some extent different from bubble bursting to the gas phase.

Mathematical model of COREX melter gasifier: Part I. Steady-state model

Abstract: The COREX melter gasifier is a countercurrent reactor to produce liquid iron. Directly reduced iron (DRI), noncoking coal, and other additives are charged to the melter gasifier at their respective temperatures, and O-2 is blown through the tuyeres. Functionally, a melter gasifier is divided into three zones: a moving bed, fluidized bed, and free board. A model has been developed for the moving bed, where the tuyere region is two-dimensional (2-D) and the rest is one-dimensional (1-D). It is based on multiphase conservation of mass, momentum, and heat. The fluidized bed has been treated as 1-D. Partial equilibrium is calculated for the free board. The calculated temperature of the hot metal, the top gas, and the chemistry of the top gas agree with the reported plant data. The model has been used to study the effects of bed height, injection of impure O-2, coal chemistry, and reactivity on the process performance.

Influence of ash on interfacial reactions between coke and liquid iron

Abstract: Interfacial reactions occurring between molten iron and carbonaceous materials are of great significance in the steel industry, and specifically, the reaction of iron with metallurgical coke is one of the key phenomena occurring during blast furnace ironmaking. Major operating parameters such as hot metal composition will be directly influenced by the reactions occurring between liquid iron and coke. In the current investigation, the interfacial reactions occurring between coke and liquid iron were studied at a temperature of 1550 °C using the sessile drop method to further the understanding of the fundamental reactions occurring at the interface between coke and iron. The formation of interfacial reaction products was observed, and time-dependent reactions were identified. The transfer of elements such as carbon, sulfur, and silicon was determined. The reduction of silica was determined as having a major influence on the transfer of both silicon and carbon into liquid iron.

Nucleation of bubbles on a solidification front—experiment and analysis

Abstract: The heterogeneous nucleation of bubbles on an advancing solidification front during the freezing of water containing a dissolved gas has been experimentally and analytically studied. The formation of bubbles resulting from supersaturation of liquids is commonly encountered in different fields such as heat transfer, manufacturing, and bioscience. In this work, the sizes of nucleating bubbles and the concentration profiles of dissolved oxygen and carbon dioxide gases in the water ahead of the solidification front have been measured. From successful comparisons between the measured and predicted critical radii of nucleating bubbles and distributions of dissolved gas content, the phenomena of heterogeneous nucleation in a binary weak solution during the freezing process are quantitatively confirmed. The results show that an increase in gas content at the solidification front in the liquid decreases the free-energy barrier and critical radii of bubbles that are formed on the solidification front. The sizes of the critical radii decrease and the number of nucleating bubbles increase in the early stage of solidification. As the solidification rates decrease at longer times, the content of the dissolved gas in the liquid on the advancing interface decreases and the critical radii of nucleating bubbles increase.

Mathematical modeling of aluminum evaporation during electron-beam cold-hearth melting of Ti-6Al-4V ingots

Abstract: A mathematical model was developed to describe the kinetics of aluminum evaporation during the electron-beam cold-hearth melting of titanium alloys. The analysis treated the diffusion of aluminum to the surface of the melt, chemical reaction between the melt and vapor phases at the surface, and the transport of aluminum into the vacuum chamber. The kinetics equation was combined with appropriate mass and energy balance equations to determine the effect of melting rate, the electron-beam power input to the melt pool in the hearth and the mold, and charge chemistry on the composition of the cast ingot. The model was validated by comparison to measurements for Ti-6Al-4V processed under a wide range of conditions.

Delta-Ferrite Recovery Structures in Low Carbon Steels

Abstract: The development of delta-ferrite recovery substructures in low-carbon steels has been observed in-situ utilizing laser scanning confocal microscopy (LSCM). Well-developed sub-boundaries with interfacial energies much smaller than that of delta-ferrite grain boundaries formed following transformation from austenite to delta-ferrite on heating. It is proposed that transformation stresses associated with the austenite to delta-ferrite phase transformation generate dislocations that subsequently recover into sub-boundaries by a process of polygonization. Experimental evidence in support of this proposal was found in a ferritic stainless steel. Thermal cycling through the high-temperature delta-ferrite/austenite/delta-ferrite phase transformation leads to the development of a well-defined recovery substructure, which, in turn, modifies the low-temperature austenite decomposition product from Widmanstatten to polygonal ferrite, with a commensurate change in hardness.

Modeling of the Solidification Process in a Continuous Casting Installation for Steel Slabs

Abstract: The development of a computational simulation system for modeling the solidification process in a continuous casting facility for steel slabs is discussed. The system couples a module for solving the direct problem (the calculation of temperatures in the steel strand) with an inverse analysis module that was developed for evaluating the steel/mold heat fluxes from the information provided by thermocouples installed in the continuous casting mold copper plates. In order to cope with the non-uniqueness of the inverse analysis, a priori information on the solution, based on the consideration of the problem physics, is incorporated. The stability of the system predictions are analyzed and the influence of the first trial used to start the evaluation procedure is discussed. An industrial case is analyzed.

Effect of a catalyst on heterogeneous nucleation in pure and Fe-Ni alloys

Abstract: In order to elucidate the nature of the heterogeneous nucleation, a differential scanning calorimetry (DSC) thermal analysis of pure Fe and Fe-Ni alloys (Ni content: 1.0 to 29.3 mass pct) containing TiN, Al2O3, and Ti2O3 was conducted. Then, special attention was paid to the difference in the phase of the primary crystal nucleated by the triggering effect of a catalyst (nucleating agent). The solidification and transformation mode appearing during cooling in these alloys is classified into three cases: F mode, FA mode, and A mode. The change of modes and the critical undercooling (ΔT) depend on the kind of catalyst used as well as the chemical composition (Ni content). In addition, in spite of the kind of primary crystal, the value of ΔT is always small in the order of TiN, Al2O3, and Ti2O3. As a matter of fact, only TiN has a practical effect as a catalyst on the triggered nucleation of the primary crystal of the δ phase. None of them has a practical effect on the nucleation of the primary crystal of the γ phase.

The effect of salt-phase composition on the rate of soda-ash roasting of chromite ores

Abstract: The formation of a liquid phase during the early stages of the roasting reaction is a common problem in the sodium chromate manufacturing process. The molten salt phase, which is primarily constituted of a binary mixture of Na2CrO4 and Na2CO3, creates major operational problems such as the granulation and blocking of the kilns. In addition to the operational problems, it was observed that the molten salt also affects the transport of oxygen toward the reaction interface. The mechanism of the soda-ash roasting reaction has been analyzed for improving the yield of sodium chromate. It was observed that the conversion efficiency of the roasting process changed dramatically, depending on the origin and the type of the chromite ores used. Thermal and scanning electron microscopic analyses of the products of roasting were carried out to establish the reaction mechanism. It was observed that the presence of silicates in the chromite ores interferes with the formation of sodium chromate involving the binary Na2CO3-Na2CrO4 liquid. The roasting reaction proceeds in a certain crystallographic direction in the chromite spinel in the presence of a nonsilicate molten salt, whereas a complete dissolution of chromite appears to take place in the binary liquid containing silicate phases present in the ore.

Modeling of ingot development during the start-up phase of direct chill casting

Abstract: Direct chill (DC) casting is a core primary process in the production of aluminum ingots. However, its operational optimization is still under investigation with regard to a number of features, one of which is the issue of curvature at the base of the ingot. Analysis of these features requires a computational model of the process that accounts for the fluid flow, heat transfer, solidification phase change, and thermomechanical analysis. This article describes an integrated approach to the modeling of all the preceding phenomena and their interactions.