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

Tetsu-to-Hagané Vol. 80 (1994), No. 1

  • Combustion Characteristics of Single Pulverized Coal

    pp. 1-6

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    In order to make clear the combustion behavior of pulverized coal(PC), the combustion characteristics of a single PC in the mixture of oxygen and nitrogen gases were measured by using the combustion apparatus equipped with CO2 LASER, high speed camera, image analyzer and pyrometer.
    The ignition time of PC was not dependent on the chemical composition of PC, but the burning time and the maximum volume expansion of PC was dependent. Therefore the burning time can be estimated from the composition of PC. The ignition time can be shorten with increase of the irradiating density of LASER, and the burning time can be shorten with increase of oxygen concentration in the mixture of oxygen and nitrogen. The burning time was not dependent on the irradiating density of LASER.
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  • Combustion Mechanism and Improvement of Combustion Efficiency of Single Pulverized Coal

    pp. 7-12

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    For keeping stable operation in PCI (pulverized coal injection) operation of blast furnace (BF), it is important to make clear the combustion mechanism of pulverized coal (PC). In this paper, combustion behavior of a single PC particle was investigated. Improvement of combustion efficiency of PC can be made by effective oxygen supply, because the combustion rate was controlled by oxygen diffusion.
    As a new method to improve combustion efficiency, addition of oxidizing agent, was investigated. In present work, KMnO4 solution was used as the oxidizing agent. It was confirmed that the burning time can he shorten with the increase of KMnO4 concentration. The addition of oxidizing agent which improves PC combustion can bring great advantage for larger size PC injection.
  • The Solubility of CaO·Cr2O3 in the CaO-CaF2 System Associated with Dephosphorization of Fe-Cr-C Melts

    pp. 13-17

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    The solubility of chromium oxide in the CaO-CaF2 system has been determined at 1773K by equilibrating the flux with a CaO·Cr2O3 pellet at the oxygen partial pressure of 2.28×10-7 Pa. The solubility of Cr2O3 varies from 0.80 to 1.28 mass%in the flux with increasing the CaO content from 15.6 to 20.6 mass%.
    In case of the dephosphorization of Fe-Cr-C melts with CaO bearing fluxes, the prevailing oxygen partial pressure of the system is controlled by the reaction between CaO and Cr2O3 forming CaO·Cr2O3. The phosphorus distribution ratio between Fe-Cr-Csatd. alloys and CaO-SiO2-CaF2 fluxes saturated with CaO·Cr2O3, CaO and 3CaO·SiO2 has been measured at 1573K. The phosphorus and chromium distribution ratios decrease with increasing chromium content of the metal from 1 to 13 mass%.
  • Fluid Flow Phenomena in a Cylindrical Bath Agitated by Top Lance Gas Injection

    pp. 18-23

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    Fluid flow phenomena in a cylindrical bath agitated by gas injection from a submerged top lance were investigated on the basis of cold model experiments. A swirl motion of liquid, which was similar to rotary sloshing, was observed under a certain blowing condition. The condition was experimentally clarified and subsequently the following experiments were made for cases without swirl motion. The vertical migration distance of gas from the lance exit was measured using an electro-resistivity probe and an empirical correlation of the migration distance was proposed as a function of the modified Froude number. The axial and radial velocity components of liquid phase were measured for a water-air system using a two-dimensional Laser Doppler Velocimeter. Applicability of the previous correlations of axial mean velocity proposed for gas injection through a centric bottom nozzle to the present case was examined. An electrochemical method was used to measure the mass transfer coefficient between a platinum flat plate and aqueous FeSO4 solution. In the axial region below the bubble plume, the so-called dead water region was observed.
  • Theoretical Study on the Separation of Inclusion Particles by Pinch Force from Liquid Steel Flowing in a Circular Pipe

    pp. 24-28

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    The pinch force which is one of the most general electromagnetic forces in the metallurgical field can be generated by impressing an electrical current in a liquid metal.In this force field, electrically nonconductive particles suspended in a liquid metal will receive a force in the opposite direction to the pinch force, and be squeezed out from the liquid metal.
    In the present study, this principle was applied to the separation of nonmetallic inclusion particles from liquid steel.The separation efficiency η of inclusion particles from a circular pipe flow of liquid steel was thought to be a function of the following nondimensional pararneters:VR(=Vpt/Wm), C1(=μeIrms2/ρν2), DR(=dp/2r1), Re=(r1wm/ν), Z=(z/r1), and r1/δ.The plug-flow model and the particle-trajectory model were used for calculating η. The results obtained by both models showed that η was a function of VR, (CIDR2/Re)Z and r1/δ. The particle-tralectory model showed a srnaller value of η than the plug-flow model.The value of η calculated by the former model increased with increasing(CIDR2/Re)Z and decreased with increasing VR. Although η did not change with r1/δ when r1/δ<1, it decreased gradually with increasing r1/δin the region ofr1/δ>1.
    The values of η for a channel induction heater installed in a continuous casting tundish were estimated to discuss the practicability of the present inclusion separator.It was found that η was greater than 95% for inclusion particles with the diameter greater than60μm.
  • Effect of Slag on Decarburization Reaction of Stainless Steel Melt

    pp. 30-35

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    During the refining of stainless steel melt, changes of slag condition and composition, especially with chromium oxide, affect the decarburization rate. The experiments were carried out to evaluate the effect of top slag state (liquid and/ or solid), and composition on the decarburization of stainless steel melt using 15kg induction furnace. Results were as follows:
    1. In the range of high carbon content of melt of more than 0.5mass%, decarburization rate depends on (Cr2O3) content in slag, but is unaffected by addition of (MgO) and (Al2O3) less than 20mass%.
    2. While, for the melt of carbon content less than 0.5mass%, decarburization rate becomes less than 1/4 of that in high carbon range (>0.5mass%) and is not affected by slag composition.
    3. With increase of the activity of (Cr2O3) in slag, the lower decarburization limit of carbon content by slag is decreased.
  • Effect of Casting Atmosphere on the Formation of Strip Surface in Twin Roll Casting Process

    pp. 36-42

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    In order to elucidate effects of casting atmosphere on formation of strip surface in twin roll casting process, an experiment of the direct casting of SUS304 austenitic stainless steel under Ar, air and He gas atmospheres was performed using a laboratory scale twin roll caster. Furthermore, heat transfer from molten metal to casting roll was theoretically analyzed by taking account of gas film between the roll and metal.
    In the experiment, surface wrinkles of strip were observed under a critical casting velocity. This critical casting velocity decreased in order of Ar, air and He gases. This would come from that fluctuation of the overall heat transfer coefficient in the case of He gas is smaller than Ar and air gases. In the case of air atmosphere, it has been very difficult to obtain better surface quality of strip because of formation of fine surface cracks. The control of casting atmosphere has been found to be very important for better surface quality of strip. It has been also considered that the thickness of the gas film between roll and metal under a steady state is related to the thickness of gas boundary layer on the roll surface, which is estimated by a boundary layer theory.
  • Development of Soft Reduction Technology Using Crown Rolls for Improvement of Centerline Segregation of Continuously Cast Bloom

    pp. 42-47

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    In order to improve centerline segregation of continuously cast bloom, effective technology of soft reduction with crown roll has established. The centerline segregation is improved remarkably by application of optimum conditions of this technology and combination of promotion of equiaxed solidification and proper soft reduction is able to decrease the degree of segregation much more.
    The optimum rate of reduction of this technology is analyzed with calculation of solidification of continuously cast bloom and analyses of deformation of bloom by soft reduction and the calculated results were compared with experimental results.
    Furthermore mathematical models were developed to analyze the relation between reduction force and amount of reduction. The effects of profile of reduction roll and number of reduction roll etc. on above relation and behavior of load transfer in segment for soft reduction were clarified by calculations with these mathematical models.
  • Effects of Cu and Ti-precipitates on Recrystallization Texture of Cu-precipitation Hardening Ultra Low Carbon Cold-rolled Steel Sheet

    pp. 48-53

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    The mechanism of improved r-value of continuously annealed steel sheets by high temperature hot-coiling in Cubearing ultra low C interstitial-free (IF) steel has been investigated by using the hot-bands with different morphologies of precipitates obtained by various heat-treatment in the temperature range of 500°C to 800°C. By the ODF analysis of recrystallization texture, it was clarified that the γ-fiber texture (<111>//ND) extremely developed by hot-coiling at the temperature ranging from 700 to 750°C. TEM observations also revealed that the complex-precipitates composed of Ti-carbosulfide and Cu were observed only in the same range of hot-coiling temperature. Improvement of r-value by high temperature hot-coiling is caused by the development of γ-fiber texture by precipitation of Cu combined with coarse Ti-carbosulfides which avoids the detrimental influence of fine pre-precipitation of Cu in matrix.
  • Precipitation Behavior of Sulfides in Ti-added Ultra Low-carbon Steels in Austenite

    pp. 54-59

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    The influences of S and Ti contents as well as slab reheating temperature on the precipitation behavior of sulfides in hot-rolled bands in Ti-added ultra low-carbon steels have been investigated. The results obtained are as follows : ln the specimens with the same 0.02% Ti content, Ti4C2S2 decreases and TiS increases in number as S content increases while in the specimens with the same 0.008% S content, TiS decreases and Ti4C2S2 increases in number as Ti content increases. The number of TiS increases when the slab reheating temperature is high.
    The solubility products of TiS and Ti4C2S2 in austenite have been also experimentally determined as follows:
    log [Ti][S]=-3252/T-2.01
    log[Ti][C]0.5[S]0.5=-5208/T-0.78
    The experimental results above agree well with the results calculated using these solubility products.
    The reliability of solubility products determined in this study are checked and it is confirmed that the solubility products in this study is in good agreement with the published experimental results.
  • α+γ and γ Phases Annealing in Ultra Low-carbon Sheet Steels

    pp. 60-65

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    α+γ andγphases annealing in Ti and Nb-added ultra low-carbon sheet steels with and without Mn, P, and/or Cr has been investigated.
    In high-strength steels with Mn, P and/or Cr, BH increases as the annealing temperature increases, whereas YP-El after aging at 100°C for one hour clearly decreases. It is speculated that the coexistence of BH and nonaging properties stems from the high dense dislocation introduced into ferrite matrix by γ→α transformation. In contrast, YP-El in mild steel without Mn, P. and/ or Cr increases as BH increases because dislocation density in not high enough to provide nonaging property.
    In mild steel, r-value distinctly decreases and texture is randomized by γ phase annealing, while r-value increases and {111} component develops by intercritical annealing. On the contrary, in high-strength steel, recrystallization texture developed in α phase is assumed to be inherited even after α→γ→α transformation takes Place. Apparently, variant is distinctly selected when. at least γ transforms into α in high-strength sheet steels. The variant selection in γ→α transformation is speculated to be caused by residual stress introduced by α→γ transformation.
  • Effects of Strengthening Mechanisms on Fatigue Properties of Ferrite-Pearlite Hot-Rolled Sheet Steel

    pp. 66-71

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    In order to develop hot-rolled sheet steel with excellent formability and fatigue properties, the effects of strengthening mechanisms on the fatigue properties of ferrite-pearlite hot-rolled sheet steels were studied. Tests were conducted on the hot-rolled steels having tensile strength 500 to 600 MPa which were strengthened by solid solution, precipitation, increasing dislocation density and pearlite volume fraction and grain-refining. The following results are obtained :
    (1)Fatigue limit and endurance ratio depend on strengthening mechanism. Endurance ratio is increased by solidsolution and precipitation strengthening, while not by increasing dislocation density and increasing pearlite volume fraction.
    (2)Fatigue limit of the strengthened material can be related with the fatigue limit of base material, strengthening mechanism, and the increment of tensile strength by each strengthening mechanism.
    (3)Though the stress amplitude at strain-controlled fatigue test, that is the resistance of material against cyclic deformation, is dependent on strengthening mechanisms, it is linearly related with fatigue limit independent of strengthening mechanisms.
    (4)This fact suggests that the resistance of ferrite phase against crack initiation is proportional to its resistance against cyclic movement of dislocation, and that, therefore it is more effective to use the strengthening mechanisms which prevent the dislocation movement in ferrite phase under cyclic deformation for improving fatigue strength.
  • Lattice Constants of γ and γ″ Phases and γ″/γ Lattice Mismatches in Ni-15Cr-8Fe-6Nb Alloy

    pp. 72-77

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    The lattice parameters of γ and γ″ phases and the γ″/γ lattice mismatches in a nickel-base superalloy, a modified NCF 3 type alloy, were investigated by X-ray diffraction. The measurment on the γ″ phase was carried out using γ″ precipitates extracted from the alloy. The relationship between the morphology of the γ″ phase and the lattice parameters of the γ and γ″ phases and the γ″/γ lattice mismatches was discussed in detail.
    With increasing aging time, the lattice parameter of the γ phase decreased and those of the γ″ phase increased. The former after a certain aging time reached at a constant value at each aging temperature. The changes in the lattice parameters in various aging conditions suggest a change in the composition of the γ and γ″ phases. The γ″/γ lattice mismatch in the direction normal to the plane of the γ″ plate, i.e. parallel to the c axis was nearly twice greater than that in the plane. The dependence of the morphology of the γ″ phase on the lattice parameters, the γ″/γ lattice mismatches and the axis ratio c0 /a0 for the γ″ phase was not clear. The γ″ phase precipitated at 1073K in a square plate-like morphology was incoherent with the matrix in the direction of the c axis, while that precipitated at up to 1033K in a near disc-shaped morphology was maintained to be coherent.
  • Influence of Annealing Conditions on Surface Characteristics of Invar Alloy for Shadow Mask

    pp. 78-82

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    Surface characteristics of invar alloy for shadow mask in color picture tube were investigated by changing annealing conditions after cold rolling-such as soaking temperature and atmospheric gas composition. Results obtained are as follows:
    1) Invar tends to show grain boundary oxidation by small amount of H2O in the annealing atmosphere.
    2) Grain boundary oxidation is due to Si, Mn, and Cr which are commonly contained in invar and more active than main components (Fe and Ni).
    3) However, behavior of each impurity depends upon annealing conditions. In case of less oxidizing annealing atmosphere, grain boundary oxide is mainly composed of mangano-silicate. On the other hand, mangano-chromic precipitatesat grain boundaries are observed in more oxidizing conditions.
    4) Higher silicon content leads to severer grain boundary oxide precipitation.
  • Effects of Thermo-mechanical Treatment and Al Content on the Microstructure and Room Temperature Ductility in γ+α2 Type Binary Titanium Aluminides

    pp. 83-88

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    Thermo-mechanical treatment conditions have been investigated on γ+α2, type titanium aluminides to obtain a uniformly fine grained structure which is expected to exhibit a high ductility at room temperature. A higher working reduction, followed by a heat treatment at the temperature at which volume fractions of the γ and α phases are equal to each other is required to obtain the fine duplex microstructure consisting of the equi-axed γ and lamellar grains. The room temperature tensile elongation in such fine duplex microstructures strongly depends on the volume fraction of the α2 phase. A higher Al content results in a high tensile elongation, especially 7.8% in the stoichiometric composition where small amount of α2 are contained. The maximum elongation of 7.8% is substantially higher than those formerly reported.
  • Production and Technology of Iron and Steel in Japan during 1993

    pp. N4-N4

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