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

Tetsu-to-Hagané Vol. 82 (1996), No. 6

  • Effect of Thermal Resistance in a Sphere on Apparent Convective Heat Transfer Coefficient

    pp. 459-464

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    The apparent convective heat transfer coefficient between a spherical particle and a surrounding fluid was investigated by the techniques of numerical analysis and dimensional analysis in case of a considerable heat transfer resistance in the particle.
    The ratio of the apparent Nusselt number, Nu', to the true Nusselt number, Nu, was found to be related with Biot number, Bi, by the following equation.
    Nu'/Nu= 1/n·Bi+1
    In this equation, the value of n was as follows (1) n was 0.2 in case of a constant heat flux between the particle and the fluid, and (2) n varied with Bi number in case of a constant fluid temperature.
  • Thermodynamics of Ni-Ca-O and Ni-Ta-Ca-O Melts in Equilibrium with Solid CaO

    pp. 465-470

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    The deoxidation equilibrium of calcium in liquid nickel has been measured by the sampling technique at the temperature ranges from 1873 to 1973 K using calcia crucibles. The effect of tantalum on the activity coefficient of oxygen in liquid nickel have been also investigated in order to understand the effect of the alloying element on the activity coefficient of oxygen for the production of superalloys.
    The effect of temperature on the equilibrium constants, Kca (= aca ·ao/acao), for the deoxidation reaction in liquid nickel, i.e:
    CaO(s) =Ca+O
    was found to be :
    logKca = - 19430/T + 5.35
    while the temperature dependence of the deoxidation product, K'ca ( = [%Ca] [%O]), and the effect of calcium on the activity coefficient of oxygen in liquid nickel were determined by the expressions :
    logK'ca = logKca - (-75900/T +28.7) ( [%Ca] +2.51 [%O])
    logf ca O=(-75900/T + 28.7) [%Ca] 0.01 < mass%Ca < 0.13
    The results below 0.01mass% calucium obtained at 1973K have been discussed.
    Further, the effect of tantalum on the activity coefficients of oxygen and calcium in liquid nickel-tantalum alloys at 1873 K were given by :
    logfTa O + logfTa Ca = - 0.12 [%Ta] mass%Ta <4.
  • In-Situ Observation of Grain-Boundary Migration in Fe-3wt%Si Bicrystals by Synchrotron X-Radiation Topography

    pp. 471-474

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    In order to examine the grain-boundary migration mechanism, two high purity Fe-3wt%Si bicrystals were produced.One has a (114)Σ9 coincidence boundary, and the other has a random boundary. In-situ observation of grain-boundary migration was conducted by synchrotron X-radiation dynamic topography.
    It was found that macroscopic grain-boundary migration was recognised at the temperature above 1170K. From the results of isothermal annealing(12701320K), it was clarified that the mobility of Σ9 coincidence boundary was higher than that of the random boundary.
  • Analysis of Heat Transfer in Porous Solid Using Image Data Characterization

    pp. 475-479

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    Much time was required to estimate the effective thermal diffusivity by the calculation with use of the image data. The reduction method of calculation time for the estimation of effective thermal diffusivity has been developed. The thinning method and the characterization of image data are used for the reduction. The calculation time is reduced to 1/230. The effective thermal diffusivity measured using the LASER flash method agrees well with that calculated.
    x

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  • New Prediction Model of Fluidity of Blended Coal Based on Viscous Behavior of Suspension

    pp. 480-485

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    A new model for estimating the maximum fluidity (MF) of blended coal was developed. MF is the common logarithm of maximum fluidity with the Gieseler plastometer. This model is characterized by the viscous behavior of a suspension. In the model, the heated coal during carbonization was regarded as a suspension composed of melted coal, non melted coal, and the solidified coke.
    The viscosity of the suspension (η) was evaluated by Mori and Ototake's equation, which included the viscosity of the solvent (ηL) and the volume fractions of solid (S). The volume fraction of solid at the MF temperature was assumed as the total inert value of the coal. The temperature dependence of ηL was determined by Andrade's equation. The viscosity of the suspension of blended coal were calculated from the values of each single coal. The relation between fluidity and viscosity was obtained by Morotomi et al. using the standard substance of polybutene.
    The calculated fluidity of two component coal with a small overlap in the melting temperature range agreed well with the measured value. Good agreement with measured values supported the validity of this model.
  • Gas Flow Calculation with a Turbulence Model in a Packed Bed

    pp. 486-491

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    A turbulence model of the gas flow in a packed bed was proposed involving the turbulent kinetic energy transport equation and a turbulent mixing length related to the packing structure. Empirical coefficients in the model have been determined on measured radial effective diffusion rates in a packed beds. The Peclet numbers for both heat and mass transfer predicted with the turbulence model are in good agreement with measured data over a wide range of the Reynolds number. Two case studies showed that the packed bed generates intense turbulence in the gas flow and enhances radial diffusion.
  • Stochastic Model of Flow and Dispersion of Fine Particles in a Packed Bed

    pp. 492-497

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    The turbulent dispersion of coal particle is a particularly important factor influencing the combustion efficiency of pulverised coal in the blast furnace raceway. A stochastic approach in modelling the particle dispersion was implemented for simulating particle trajectories in both free space and a packed bed. A new model for the interaction between particles and a packed bed has been developed and incorporated in a two-dimensional mathematical model of pulverised coal combustion.
    Validation work of the model were completed for a round free jet and a packed bed. Intense particle dispersion and mixing were observed in the packed bed. Along with the large slip velocity between gas and particles, the intense mixing suggests a higher burning rate in a packed bed than in a free space.
  • Inclusion Separation from Molten Steel in Tundish with Rotating Electromagnetic Field

    pp. 498-503

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    A new process was devised for promoting inclusion separation from the molten steel in the tundish. The process utilizes electromagnetic force to rotate molten steel in a cylindrical tundish. The centrifugal force caused by the rotational flow promotes separation of inclusions. This tundish is referred to as the Centrifugal Flow Tundish (the CF tundish).
    Industrial plant tests carried out at Chiba works showed that the CF tundish has an excellent deoxidation performance estimated at 0.17-0.25 min-1 as deoxidation rate constant.
    The mechanism of inclusion separation in the CF tundish is also discussed. Centripetal force and large turbulence energy caused by rotational flow accelerate inclusion separation. Furthermore, residence time distribution of molten steel in a tundish is improved and it promotes slag removal during ladle exchange.
    This process is successfully used in commercial production of stainless steel slabs with high quarity.
  • Thermal Diffusivities of Continuous Casting Mold Fluxes for Steel in the Glassy and Crystalline States

    pp. 504-508

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    Laser-flash method has been applied to determine thermal diffusivities of mold fluxes containing iron-, zirconium- and titanium-oxides for continuous casting of steel in temperature range between room temperature and 800K for glassy samples and between room temperature and 1000K for crystalline samples. Thermal diffusivity of glassy samples was found to be 4.6±0.5×10-7m2/s, slightly higher than 4.0±0.5×10-7m2/s of liquid samples, and insensitive to the variation of temperature and concentration within the range presently investigated. When crystallized, the thermal diffusivity was increased by about 40% over that of glassy samples. The diffusivity was decreased with increasing temperature, exhibiting only 20% increment of that of the glassy ones at temperatures beyond 800K. The apparent absorption coefficients of crystalline samples estimated from the measured spectral transmissibity were over 2400m-1, indicating their opacity. Mold flux film was collected from the bottom part of mold for casting of steel and about 50mass% of which was confirmed by X-ray diffraction to be in crystalline state. The present results suggest that the change in radiative component due to absorption by the crystalline part of mold flux is one of the responsible factors in controlling the heat transfer through a continuous casting flux film between the mold and solidified shell.
  • TLP Bonding of Hastelloy X with Ion Plating Filler

    pp. 509-513

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    The transient liquid phase bonding (TLP bonding) has been studied on a Hastelloy X whose surface is ion-plated with a self-melted tertiary alloy of a Ni-15mass% Cr-B alloy. The ion-plated coat filler contains about 2 to 4mass% boron as a melting point depressant. The thickness of the filler metal is from 3 to 12μm. It was found that the diffusion zone is narrow and the joint strength is insufficient when the content of boron is low and the thickness of a filler is thin. However, the joint strength increases sharply when the thickness reaches about 7μm. These facts were discussed on the basis of microstructural observation, the distribution of diffusion elements and the isothermal solidification time of a thin filler.
  • The Effect of Nitrogen Content on Creep Rupture Strength of Low Carbon Type 316 Austenitic Stainless Steel

    pp. 514-519

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    The influence of nitrogen addition on creep rupture strength of low carbon Type 316 austenitic stainless steel has been investigated.
    The creep rupture strength of this steel remarkedly increased with increasing nitrogen content and in the steels with more than about 0.085mass%N, the rupture strengths were almost equivalent to or higher than those of comercial Type 316 austenitic stainless steels. It is thought that its beneficial effect is due to the solid solution hardening controlled by Mo-N atomic pairs in the matrix. Increasing the nitrogen content from 0.085 to 0.12mass% suppressed the precipitation of intermetallic phases such as sigma(FeCr) and Laves(Fe2Mo), but accelerated the precipitation of the Cr2N.
    From the result of analysis of creep deformation behavior using the modified θ projection concept for the steel with 0.085%N, the strengthening effect made into molybdenum and nitrogen solutes can give the highest creep resistance, but in a longer term condition, the lowering in strength was occurred by the precipitation of Laves phase, which was related to the removal of molybdenum in solution and to the precipitation behavior of Laves phases.
  • Metallurgical Factors Affecting the Fluting Resistance of Continuous Annealed Tinplate

    pp. 520-525

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    Effects of annealing temperature and primary cooling rate in continuous annealing on the fluting resistance of soft temper tinplate were experimentally investigated. Based on its result, the influence of grain size, solute carbon and carbide morphology affecting the fluting resistance was studied. Main results are the following:
    The more rapid the cooling rate is and the higher the annealing temperature is, the better the fluting resistance is improved. However, the dependency of the fluting resistance on cooling rate becomes small with the annealing temperature. Therefore, high temperature annealing is most effective and rapid cooling is second most effective to keep the fluting resistance acceptable under the normal reduction rate of temper rolling. These results mean that coarsening of grains is most effective, and the reduction of solute carbon is second most effective especially in the case of insufficient grain coarsening. Materials with fine and dispersed morphology of carbide show better fluting resistance than those with aggregated carbide on grain boundaries even if the grain size and solute carbon content are the same with each other.
  • Effects of Alloying Elements on the Creep Rupture Time of High Cr Ferritic Steel Weldments

    pp. 526-531

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    The effects of precipitation strengthening by V and Nb additions and the effects of N and W additions on the creep rupture time (CRT) of heat affected zone (HAZ) in the weldment of high Cr ferritic steels were studied by welding heat cycle simulation.
    The CRT of HAZ was slightly shorter than that of base metal, even in the steel without V and Nb addition. This can be explained by the coarsening of grain size in HAZ.
    As the base metal is more strengthened by V and Nb containing nitrides (V, Nb) N, the CRT of HAZ drops more. The CRT of HAZ of stronger base metal never becomes lower than that of weaker base metal. Although the increase in nitrogen content improves the CRT of base metal, the CRT of HAZ is improved less than that of base metal. These results can be related with the state of(V, Nb)N precipitation by the local climb mechanism.
    The W addition promotes the strengthening effects of(V, Nb)N precipitation, by increasing the solution of Nb during welding heat cycle. Thus, when the CRT of base metal is improved by W addition, the ratio of CRT in HAZ to that in base metal is kept nearly the same as that of steel without W addition.
  • Strengthening Mechanisms of Ti and Nb Bearing Steel Plates Produced by Thermomechanical Control Process

    pp. 532-537

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    Strengthening mechanisms of Ti and Nb bearing steel plates produced by thermomechanical control process have been investigated. The strengthening of Ti bearing steel plate is induced by precipitation of Ti carbide, ferrite grain refining, and increase in bainite volume fraction. The strengthening of Nb bearing steel plate owes to precipitation of Nb carbonitride, ferrite grain refining, and increase in bainite volume fraction. The major factor of strengthening of Ti and Nb bearing accelerated cooled steel plate is the increase in bainite volume fraction.
  • Effect of Microstructure on Creep Fatigue Properties for Type 316 Austenitic Stainless Steels

    pp. 538-543

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    For the structural materials of Fast Breeder Reactor, the modified type 316 ; 316MN has been developed. In this steel, improvement of creep fatigue endurance was achieved by reducing carbon and adding nitrogen. In this study, with conventional SUS316 (0.05%C-0.03%N) and 316MN (0.01%C-0.07%N) produced by commercial product line, changes of creep fatigue properties at high temperature after aging, stress-aging and fatigue were investigated in consideration of microstructure change. Moreover, quantitative evaluation of damage by microstructure change during creep fatigue was tried with remaining ductility for SSRT (Slow Strain Ratetensile Test). In result, it was certified that 316MN had excellent stability of microstructure at high temperature against aging, stress-aging and fatigue. Therefore, the creep fatigue properties of 316MN were little damaged by aging and stress-aging. Furthermore, after fatigue, , the remaining ductility of 316MN was able to hold approximately three times as much as that of conventional SUS 316.
    x

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  • Technology for Producing Semi-solid Metal by Shearing and Cooling Roll Method

    pp. 544-549

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    Semi-solid processing is believed to be effective at improving the quality of various properties of the materials that are processed, i. e., reduction of segregation, surface crack, porosity, etc. To realize these improvements, it will be necessary to produce semi-solid metal slurry with a high fraction solid and fine primary particles. A basic study was carried out in order to produce semi-solid metal having high fraction solid at high cooling rate. The study used shearing and cooling roll method to examine the controllability of the discharge velocity of semi-solid metal, the fraction solid, the solidified structure, and the quality of the semi-solid metal slurry. The results of the study have clarified the following points.
    1) The discharge velocity of semi-solid metal slurry can be controlled by changing the clearance of the discharge port and the number of revolutions of the roll, thereby making it possible to produce slurry with a desired fraction solid.
    2) Slurry with a high fraction solid (greater than 0.3) and very fine primary particles can be produced.
    3) The conditions necessary for producing fluid slurry with a high fraction solid have been clarified.

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