ISIJ International
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ONLINE ISSN: 1347-5460
PRINT ISSN: 0915-1559

ISIJ International Vol. 33 (1993), No. 5

  • Kinetics of the Oxidation and Reduction of Synthetic Ilmenite

    pp. 523-528

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    DOI:10.2355/isijinternational.33.523

    The kinetics of the oxidation and reduction of synthetic ilmenite was studied thermogravimetrically in the light of kinetic theory for heterogeneous noncatalytic reactions. It was found that the oxidation and reduction proceeded topochemically and their kinetic parameters were determined according to the unreacted core shrinking model. The mineral composition of the partially oxidized and/or reduced ilmenite was examined by means of characteristic X-ray analysis, the results of which showed that there were two phases in the oxidized ilmenite which separated each other. One of them was rich in titanium and the other was rich in iron. There were two phases too separating completely in the reduced ilmenite, one of which was the metallic phase consisting of almost pure iron and the other was an oxide phase mainly composed of titanium dioxides. The composition of the titanium-rich phase in the oxidized ilmenite was between the composition of Fe2O3·TiO2 and that of Fe2O3·3TiO2.
  • Multiparticle Dissolution Kinetics of Carbon in Iron-Carbon-Sulphur Melts

    pp. 529-538

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    DOI:10.2355/isijinternational.33.529

    The derivation of a model of the dissolution of carbon particles following their injection into an iron bath is presented. The model is based on the dissolution of single particles in complete contact with the liquid iron and predicts the change in bath carbon concentration under continuous injection conditions. The model takes into account rate limitation by mass transport, chemical or mixed control.
    The model predictions are shown to be consistent with the results of laboratory studies of the injection of high purity graphite into iron when mass transport limitations dominate. This is so for both pure iron/carbon alloys and those containing up to 1.0% dissolved sulfur. If a carbon source such as petroleum coke is substituted for graphite the experimental results deviate from the model predictions. This experimental result suggests that factors other than mass transport can be significant in limiting the rate of dissolution of non-graphitic carbon into liquid iron.
    The experimental and industrial implications of the model are discussed and future plans for extension of the model are presented.
  • Thermodynamic Simulation on the Behavior of Recycling Elements in the Iron Bath Smelting Reduction Process

    pp. 539-548

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    DOI:10.2355/isijinternational.33.539

    A computer program system was developed to simulate the smelting reduction ironmaking process. This system, combining the database MALT with the phase equilibrium calculation software ChemSage-derived from the widely well-known SOLGASMIX, can be expected to deal with problems encountered in metallurgical processes. As an example, the behaviors of some recycling elements, such as Na, K, Zn and S, in the smelting reduction process have been investigated from the thermodynamic point of view in case of the dust being recycled. The simulation results indicate that Zn is most influential on sulphur behavior among Zn, Na and K, and ZnS is the prevailing form of the deposited sulphur when there is zinc existing in the system. Zn can effectively prevent sulphur from entering the outlet gas. Alkaline components, especially potassium K, accumulate as carbonates in the system. At a high post combustion ratio of the gas generating from the smelting reduction furnace, or at a lower temperature of Cooler 1, FeS deposition takes place and results in an appreciable amount of sulphur leaving the system in the outlet gas.
  • Sulphur Partition between CaO-SiO2-Ce2O3 Slags and Carbon-saturated Iron

    pp. 549-555

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    DOI:10.2355/isijinternational.33.549

    A slag-metal equilibrium study was carried out to investigate the effect of rare earth oxides on the sulphur partition between CaO-SiO2 slags and carbon-saturated iron at 1500°C. The sulphur partition was increased with increase in Ce2O3 concentration in the slag. The oxygen potential of the system was found to be controlled by the Fe-FeO equilibrium. Sulphide capacities of CaO-SiO2-Ce2O3 slags measured in the present study agreed well with the values predicted by the optical basicity method. It was tentatively concluded that Ce2O3 decreases the activity coefficient of SiO2 in the CaO-SiO2-Ce2O3 slag.
  • Mathematical Modeling of Inclusion Transport and Removal in Continuous Casting Tundishes

    pp. 556-566

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    DOI:10.2355/isijinternational.33.556

    Three-dimensional mahematical models are currently being used successfully to model liquid steel flow and turbulence behavior in continuous casting tundishes. Traditionally, this information is used to calculate the residence time distribution (RTD) of liquid metal in the given turdish configuration. The RTD curve provides the effectiveness of a tundish to produce cleaner steel in an indirect and qualitative manner. Recently, some computational models have been developed to predict inclusion trajectories and their rate of flotation in a semi-quantitative manner. In the present work, a model that addresses the inclusion transport and removal phenomena from the molten metal has been developed. The model examines three modes (flotation to the surface, coalescence of particles to form larger inclusions, and sticking to the solid surfaces) of inclusion density reduction from molten steel in the tundish. The effect of various flow control devices, such as dams, weirs and baffles with holes on each of these inclusion reduction modes was investigated and their inclusion removal efficiencies were compoared. The role of different flow control devices in producing cleaner steel has been discussed.
  • Fe-Cr Melt Nitrogenation When Exposed to Nitrogen Plasma

    pp. 567-576

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    DOI:10.2355/isijinternational.33.567

    The nitrogen absorption/desorption for pure iron and Fe-C alloys have been investigated in detail in levitated melts. However, limited study seems to be made while molten Fe-Cr alloy is exposed to nitrogen plasma. Nitrogen plasma offers an attractive means to nitrogenise Fe-Cr alloys in view of rapid absorption to higher nitrogen content. Several workers have reported that sulphur in the melt renders higher nitrogen. Industrially melt with higher nitrogen with sulphur may not be attractive. The experimental condition of present study solves this problem. Melts were made to observe the effect of arc current, plasma gas composition, surface active elements (SAE) in melt on melt nitrogen content. It was noted that the nitrogen was first absorbed upto certain maximum limit [Nmax] followed by its desorption on continued plasma exposure may be due to nitrogen bubble formation. The maximum nitrogen level in melts could be enhanced when rate of absorption in plasma arc zone was much higher with low desorption occurring in non-plasma arc zone of the melt. The use of higher melt temperature and low SAE in melt rendered higher absorption rate. The slower desorption rate could be obtained by maintaining lower SAE and temperature in melts. The nitrogen absorption in plasma arc zone followed first order reaction rate, however, desorption was probably depended on bubble formation frequency.
  • Iron Droplet Formation Due to Bubbles Passing through Molten Iron/Slag Interface

    pp. 577-582

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    DOI:10.2355/isijinternational.33.577

    Iron suspension is formed in slag when gas bubbles pass through molten iron/slag interface. This paper proposes a model to explain iron droplet formation and to estimate thereby the behavior of the droplets in slag.
    When a gas bubble passes through molten iron/slag interface, an iron film is considered to be formed on the bubble surface. It was predicted that the film reached a constant thickness within 0.5 s at the longest. The probable range of the mass of the iron droplet formed from the film was evaluated with the thickness and the surface area of the film. The experimental values of the mass of droplet fell within the predicted range. The movement of the iron droplets in the slag phase was estimated taking the surface energies of bubble and iron film as energy source. The penetration distance and the residence time of droplets in the slag phase were evaluated from 1 to 6 cm and from 1 to 0.1 s, respectively. These small values suggest possible high speed recirculation of iron droplets between the iron and slag phases through the interface.
  • A Solid-Liquid Diffusion Couple Study of a Peritectic Reaction in Iron-Carbon System

    pp. 583-587

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    DOI:10.2355/isijinternational.33.583

    δ-iron and melted high carbon steel with carbon contents of the solvus and liquidus lines, respectively, in an iron-carbon equilibrium phase diagram, were held in contact with each other at 1696 K. The thickness of the γ-phase formed between the δ-phase and the liquid phase was measured. The relationship between the thickness x (μm) and the holding time t (s) was found to be x=85.7 t0.50.
    The distribution of the carbon concentration over those three phases was also measured. The results showed a steep gradient of the carbon concentration in the γ-phase and an equilibrium conjugation relationship at both the δ/γ and γ/liquid interfaces.
    These findings regarding the growth rate of the γ-phase and the distribution of the carbon concentration were both in good agreement with the results of a simulation of the peritectic reaction based on a diffusion-controlled mechanism.
  • The Development of a Mathematical Model to Predict Composition Distribution in Casting Slab and Intermix Slab Length during Ladle Changeover Period and Its Verification by Physical Model

    pp. 588-594

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    DOI:10.2355/isijinternational.33.588

    It is desirable to be capable of determining the composition distribution in casting slab and the intermix slab length during the ladle changeover period of the continuous casting operation. A 3-D mathematical model has been developed based on the SOLA-SURF technique and the K-ε two equations turbulence model to simulate the fluid flow and mass transport phenomena in tundishes during this period, which in turn determines the composition distribution in the casting slab. The free surface which varies during the ladle change over period was treated by a kinematic equation of height function rather than assuming it to be constant and flat. The model was designed to be executable on an IBM compatible 386 or 486 personal computer. An irregular mesh system was also employed to handle the actual dimensions of the ladle nozzle, tundish, and submerged nozzle to the casting mold. The mathematical model was then verified with the measurement data obtained from a full scale water model. The comparision results are rather satisfactory. It is shown from this study that the quantity of the residual fluid in the tundish when ladle changeover occurs affects the composition distribution in the transitional slab and thus the intermix slab length.
  • Mathematical Modelling of Single Roll Continuous Steel Strip Caster Based on Fluid Flow and Heat Transfer Considerations

    pp. 595-604

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    DOI:10.2355/isijinternational.33.595

    The Single Roll Continuous Strip Casting Process has been quantitatively analysed using a mathematical model based on fluid flow and heat transfer considerations. The process is divided into four distinct zones: (1) liquid metal reservoir, (2) liquid metal pool, (3) solid strip zone, and (4) caster drum. Model equations are formulated using a control volume approach and setting up equations representing balances of mass, momentum and energy for these various zones. These equations, which are coupled by the thermophysical properties and various interfaces, are solved using an iterative finite difference technique. It has been possible to simulate the process and predict the effect of various process parameters on the process performance using the model. The parameters examined include: (1) liquid steel head in the tundish, (2) speed of rotation of the caster drum, (3) superheat of melt in the tundish, (4) gap between the caster drum and the tundish, (5) cooling conditions prevailing at the inner surface of the drum, (6) drum geometry, and (7) drum material. While the speed of rotation of the caster drum and the physical dimensions of the liquid metal pool affect the process strongly, the cooling conditions prevailing at the inner surface of the drum only marginally affect the process as far as the final strip thickness is concerned. These, however, along with the drum material affect the temperature distribution in the drum which may have a direct bearing on the microstructure of the product.
  • Static Recrystallization in Austenite and Its Influence on Microstructural Changes in C-Mn Steel and Vanadium Microalloyed Steel at the Hot Strip Mill

    pp. 605-614

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    DOI:10.2355/isijinternational.33.605

    By using torsional test, microstructural changes in commercially available C-Mn and vanadium microalloyed steel were studied and changes in austenite on rolling through roughing and finishing mills were estimated on the basis of preliminary static recrystallization determinations. A method for determining the temperature at which recrystallization starts to be inhibited in microalloyed steel involving experimental measurements of the activation energy was developed. The influence of the activation energy on changes in austenite brought about by rolling at an ordinary hot strip mill was established by torsional simulation of various thermal cycles, both at a roughing and at a finishing mill. Austenite in C-Mn steel was found to be impossible to harden under these conditions–not even on rolling at a finishing mill at temperatures close to Ar3–as a result of its activation energy being constant at all temperatures. On the other hand, hardening of austenite in microalloyed steel was readily accomplished at a finishing mill at temperatures below the critical recrystallization temperature (915°C) as the activation energy increases sharply below such a temperature.
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  • Plasma-carburizing of Tungsten with a C3H8-H2 Mixed Gas

    pp. 615-617

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    DOI:10.2355/isijinternational.33.615

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