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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 82 (1996), No. 2

  • Effect of MgO Contents on the Dissolution Rate of Alumina into Molten CaO-SiO2-Al2O3-MgO Slags

    pp. 99-104

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    Dissolution rates of sintered alumina into molten CaO-SiO2-9mass%Al2O3-MgO(CaO/SiO2=0.80) slags were measured by a rotating specimen method. The dissolution rates into slags containing 5, 10, and 15mass% MgO were measured at 1773, 1823 and 1873K, and the revolution speed varied from 100 to 600 rpm. The microstructures of specimens after experiments were examined by an electron probe microanalyzer(EPMA), and the dissolution mechanism was discussed.
    The dissolution rate of alumina increased with increasing revolution speed and temperature. The dissolution rate of alumina into the slags containing more than 10mass% MgO were increased more rapidly with increasing revolution speed and temperatures, compared with that into the slag containing 5mass% MgO.
    The dissolution rate of alumina decreased with increasing MgO content up to 15mass%. According to the EPMA analysis, a deposition of MgO ·Al2O3 spinel particles was observed in slag adjacent to the alumina specimen. The spinel particles formed a layer in slags containing more than 10mass% MgO, and the thickness of the layer was increased with increasing MgO content in slags. These morphological differences of the spinel layers suggest that the spinel layers suppress the flow of slag components, and the dissolution rate decreases with increasing MgO content. Moreover, the morphology of spinel layer was changed with revolution speed, this morphological change is considered to be the reason why the dependence of the dissolution rate on revolution speed differs by the MgO content in slag.
  • Experimental and Parametric Analyses of Heat Transfer within Two-layered Packed Beds of Iron and Graphite in a High-frequency Induction Furnace

    pp. 105-110

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    This paper describes an experimental and theoretical study on heat transfer within two-layered packed beds of iron and graphite in a high-frequency induction furnace. An unsteady, two-dimensional mathematical model for the induction furnace was developed in cylindrical coordinates and was verified by the experimental results. Temperature dependence of physical properties of packing materials such as magnetic transformation of iron was needed to be taken into account in the model. The experimental results showed that iron spheres near the wall were heated most quickly.
    Parametric analysis was performed by using this mathematical model to investigate optimum operation conditions. The influence of supplied current, induced frequency, voidage and diameter of graphite particles and layer height ratio of iron and graphite on the heating rate of iron spheres was assessed. As a result, the heating rate of iron spheres was found to be significantly dependant on the supplied current, the frequency and the layer height ratio. Interestingly, small graphite particles were well heated although their diameter is less than the skin depth in the furnace. These results suggest the possibility of dense packing of graphite near the wall in the high-frequency induction furnace for efficient heating.
  • Computer Simulation of Pore Formation in Iron-ore Sintercake

    pp. 111-115

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    Behavior of pore and bond formation in sintercake on iron ore sintering has been studied by means of computer simulation in which melt flow phenomenon is taken into account. Simulations have revealed that pore size increases with increasing coke content and increasing melt fluidity. On the other hand, the excess of them results in a huge void and dense and massive clusters below which an unfired region is formed. These results are confirmed by comparing with the direct observation by X-ray CT.
  • Mechanism of Decarburization Reaction of Stainless Steel in Reduced Pressure and the Effect of Stirring Condition

    pp. 117-122

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    The most suitable stirring condition has been investigated on the basis of the mechanism of the decarburization in VOD.Results are summerized as follows :
    (1) A decarburization model considering each reaction sites as hot spot, free surface, internal, bubble and slag was constructed. As a results, it became possible to quantify the contribution ratio of each reaction site in decaburization rate at each [C] region in VOD.
    (2) Decarburization rate became larger with increasing stirring energy. And it was larger at a single tuyer than bouble tuyers according to the defference of the plume eye area.
  • Dephosphorization of High Manganese Molten Iron Treated with BaO-BaCl2-Mn0 Flux and Oxidation by MnO2

    pp. 123-128

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    Dephosphorization process of high manganese molten iron with BaO-BaCl2-MnO flux by oxidation of MnO2, includingslag recycle system, was investigated in a laboratory scale test.
    By using BaO-BaCl2 flux of 100kg/t and MnO2 of 10-15kg/t, dephosphorization yield of 70% was achieved at 1400°C, if the molten iron with manganese content of 18 mass% has carbon content of 2 mass%. To get satisfactory dephosphorization, (BaO)/(BaCl2) mass% ratio in the slag should be controlled to 50/50-70/30, MnO content in the slag to 20 mass%, and (BaO)/(SiO2) mass% ratio should be larger than 15. Phosphate capacity of the slag was calculated to be 1022.9-1023.3.
    Also, it is considered possible to use the slag repeatedly for dephosphorization by subsequent reduction of P2O5 in BaO-BaCl2 slag with carbon at 1600°C.
  • Removal of Copper from Carbon Saturated Molten Iron by Using FeCl2

    pp. 129-134

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    Possibility of removal of copper from molten iron by using FeCl2 is experimentally examined in a small size experimental apparatus. Pulverized FeCl2 is injected with nitrogen gas into molten iron saturated with carbon. The iron chloride is first vaporized in molten iron and then reacts with copper alloyed in molten iron. 0.7 and 0.06% of copper contents are decreased to 0.5 and 0.04%, respectively, under 230hPa in partial pressure of FeCl2 at 1450°C.
    Rate equation can be written as an apparent first order reaction for the copper contents:
    -log[%Cu]t/[%Cu]o={exp(-3.72 × 103/T+1.04)}t, where T is temperature in Kelvin and t is time in minute.
    No effect of mass of specimen on the reaction rate is observed under the experimental condition. Rate determining step of the reaction is estimated as diffusion process of copper in the molten metal. It is estimated that the minimum ratio of mass of FeCl2 to molten iron is 18.5kg-FeCl2/1t-iron when 0.5% of the copper content change to 0.1%.
  • Elimination of Copper from Molten Steel by Ammonia Gas Blowing

    pp. 135-140

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    Based on the finding that blowing of NH3 gas onto molten pure copper at atmospheric pressure causes its evaporation of enormously high rate, attempts have been made on elimination of copper dissolved in molten steel by NH3 gas blowing under reduced pressure. Typical behaviors of molten steel by NH3 gas are boiling, splashing and in a violent case slopping from the crucible.
    Evaporation from liquid pure copper by top-blowing of NH3 was observed by the laser-light sheet method and it was confirmed that a large amount of copper smoke takes off from the molten surface attacked by NH3 gas. The condensates of the smoke were fine copper particles.
    The decuprization experiments have been conducted to measure the rate at which the copper level in the molten steel is reduced by using a vacuum induction furnace and blowing NH3 gas through a vertical water-cooled copper nozzle. For comparison, the rate of copper elimination from a small molten steel droplet has also been determined. From these experiments the rate constant for copper elimination from a flat and smooth surface of molten steel was the order of 2 × 10-3m·min-1 at 1900 K and the pressure range of 10010000Pa.
    Because the boiling phenomena are accompanied by an immense increase in the surface area from which volatile species are able to evaporate, NH3 gas blowing may be useful to reduce the decuprization time. Nitrogen concentration in the molten steel increases with increasing the flow rate of NH3 gas. It was reduced down to the initial level by vacuum degassing after stopping NH3 gas blowing.
  • Effects of the Shape of δ-Phase on the Growth Rate of γ-Phase during Peritectic Reaction in Fe-C System

    pp. 141-146

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    Peritectic reaction process in iron-carbon system has been computer-simulated based on the diffusion-controlled mechanism by using the direct finite difference method to investigate the effects of the shape of δ-phase on the growth rate of γ phase. When the shape is sphere, γ phase grows the fastest, with cylinder and plate following in that order. Model experiments performed by using a solid-liquid diffusion couple method indicated that the results of the simulation are correct.
  • Analysis of Precipitates and Precipitation Behavior in Nb-Ti Bearing Steels

    pp. 147-152

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    The method for isolation and determination of TiN and (Ti, Nb)(C, N) precipitated in Nb-Ti bearing steels has been investigated in the cooperative research of the Precipitate Analysis Subcommittee of Iron and Steel Analysis Committee of Iron and Steel Institute of Japan. Also, the precipitation behavior of TiN and (Ti, Nb)(C, N) in the steels has been studied. The results are summarized as follows ; (1) TiN, (Ti, Nb)(C, N) and other precipitates were extracted by potentiostatic or galvanostatic electrolysis in 10% acetylacetone-1% tetramethylammonium chloride-methanol electrolyte. (2) Only the TiN in the above mentioned precipitates was insoluble and remained by 60°C-10% brominemethanol solution treatment. Consequently, the amount and composition of TiN and (Ti, Nb)(C, N) could be determined quantitatively by analysis of the residues extracted by using the method of (1) and the residues after this treatment.(3) The size of TiN was several micrometers, the amount of TiN did not change with heating temperatures between 1000 and 1250°C, and the atomic ratio of Ti to N was stoichiometrically one to one. (4) Total amount of Ti and Nb in the precipitates decreased with an increase in heating temperature, because the small size of (Ti, Nb)(C, N) dissolved. (5) Both lattice constant and composition changes in (Ti, Nb)(C, N) showed that Nb and C dissolved preferably into the matrix and the composition of the precipitates approached to that of TiN with an increase in heating temperature. (6) Such behavior of precipitates agreed well with rough calculation from the solubility products.
  • Deboronization Phenomena in Fe-Ni Alloys

    pp. 153-158

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    Effects of atmosphere, holding time and temperature on deboronization phenomena at heating of Fe-36%Ni and Fe-78%Ni alloys have been investigated. The results obtained are as follows:
    (1) A considerable amount of boron removal was observed during the heat treatment at 1050°C in air. Neither argon nor hydrogen (dew point -40°C) atmosphere could eliminate the deboronization.
    (2) Boron concentration profiles after deboronization of Fe-36%Ni alloys containing 126ppm B or less in air at the temperature higher than 1050°C are explainable by diffusion controlled mechanism.
    (3) The diffusion coefficient of boron in Fe-Ni alloys may be represented by the following equations,
    DFe-36%Ni(cm2/s)=4.6×10-3e-95, 000/RT,
    DFe-78%Ni(cm2/s)=6.5×10-3e-98, 000/RT,
    (4) In Fe-Ni alloys, the subscale composed of metal/oxide mixed zone and intergranular oxide is observed. The subscale zone in 36%Ni alloy is far wider than that in 78%Ni alloy. Oxidized boron is solved in the oxide particles of the subscale. Boron concentration is the most depleted at the boundary of subscale and matrix, and increases gradually with increase of the distance from the boundary into the matrix.
  • Lattice Constants and Compositions of the γ" Phase Precipitated in a Ni-15Cr-8Fe-6Nb Alloy

    pp. 159-163

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    The lattice constants of γ and γ"phases and the composition of γ"phase in a nickel-base superalloy, a modified NCF 3 type alloy (X-750M), are investigated by using an X-ray diffractometer and an analytical electron microscope with an energy dispersive X-ray (EDX) system. The measurement on the γ"phase is carried out using γ"precipitates extracted from the aged alloy. The relationship between the lattice constants and the composition of the γ"precipitates is discussed in detail. The lattice constant of the γ phase decreases and those of the γ" phase increase with aging time. The change in the lattice constants in various aging conditions suggests a change in the composition of the γ and γ" phases. The γ"phase is composed of nickel, niobium, chromium and iron. While the content of nickel in the γ"phase increases with increasing aging time and reaches at a constant composition, 72mol%, after 72ks aging at 993K, the contents of chromium and iron decrease with increasing aging time and reach at constant values, 6.5mol% and 3mol%, respectively, after 72ks aging. The content of niobium maintains constant value of 18mol% regardless of aging time. A non-stoichiometric composition of γ"phase suggests that the normal sites of nickel and niobium atoms are occupied by chromium and/or iron atoms. If nickel and niobium atoms occupy normal sites regardless of the sites of chromium and iron atoms, it is difficult to explain the change in lattice constants of the γ" phase with aging time. If a part of nickel (including chromium and iron) and niobium atoms can be replaced each other site, it can be explained well.
  • Effects of Nitriding Temperature on Gas Nitriding Property of Steels for Nitriding

    pp. 164-169

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    Effects of nitriding temperature on gas nitriding property of SACM645 steels were investigated in order to develop a new nitriding process. In this investigation, TEM observation was performed to study surface hardening mechanism in gas nitriding.
    Effective case depth increased with nitriding temperature was raised. On the other hand, surface hardness decreased as nitriding temperature increased. Precipitation of complex nitride (Al, Cr) N which has f.c.c structure was observed in the nitrided layer. It is proved that, surface hardness increased at lower nitriding temperature because precipitates became fine and were inducing large coherent strain on matrix. In examining a few patterns of temperature profiles in nitriding, it is concluded that the nitriding process with continuous heating showed better nitriding property than other ordinary nitriding processes. The reasons for this are as follows, i) high surface hardness obtained at lower temperature in the early nitriding stage is maintained in higher temperature, ii) enough amount of nitrogen diffuse into the steel during continuous heating.
  • Diffusible Hydrogen Behavior and Delayed Fracture of Zinc-electrogalvanized High Strength Steel

    pp. 170-175

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    Diffusible hydrogen behavior in zinc electrogalvanized high strength steel has been investigated by use of a newly developed hydrogen content determining method for better understanding of delayed fracture of electrogalvanized high strength steel.
    For as-electrogalvanized high strength steel, there are two peaks on the hydrogen evolution rate curve at 200°C and 350°C from room temperature to 600°C. After electrogalvanizing and baking at 200°C for 4 hours, 200°C peak becomes very low but 350°C peak shows still the same as before. Hydrogen at 200°C peak is diffusible and relates to delayed fracture of high strength steel. Hydrogen at 350°C peak is undiffusible and does not relate to delayed fracture. Hydrogen diffusion remarkably falls down by electrogalvanized zinc layer. In as-electrogalvanized high strength steel with 8μ zinc layer, diffusible hydrogen content is about 0.23ppm and decreases to 0.03ppm after baking at 200°C for 4 hours.
    In as-electrogalvanized and carburized high strength bolt with 8μ zinc layer, there are two peaks on the evolution rate curve at 150°C and 327°C. The former is diffusible hydrogen and its content is 0.69ppm. This diffusible hydrogen decreases to 0.10 ppm after baking at 190°C for 5 hours. It is investigated to show the diagram of relationships about baking time-diameter of bolt-change of initial hydrogen content.
    Diffusion coefficient of carburized high strength M8 bolt with 8μ zinc layer at 190°C is about 2.36 × 10-45.91 × 10-4 mm2/sec.

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