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

Tetsu-to-Hagané Vol. 85 (1999), No. 6

  • Damping Behavior of Disturbance on Liquid Metal Surface Submerged in an Intermittent Alternating Magnetic Field

    pp. 427-431

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    In several material processes, stability of the free surface of a molten metal is important for producing high quality materials. The imposition of magnetic field on a molten metal was introduced as a efficacious mean controlling the free surface of a molten metal. Several investigators have verified that static and high frequency magnetic fields enable the suppression of the disturbance given on the free surface of a molten metal. However, it is not clear whether an intermittent alternating magnetic field has the suppression function as well as the static and high frequency magnetic fields.
    In this work, the possibility of the suppression of the disturbance on free surface through the periodical wave motion excited by the intermittent alternating magnetic field has been explored in an experiment. The surface behavior of the molten gallium disturbed by the impact of a steel ball has been observed under the continuous or the intermittent alternating magnetic field. It has been found that the damping period of the disturbance under the imposition of the intermittent alternating magnetic field is shorter than those under the continuous alternating magnetic field and no magnetic field.
  • Bubble and Liquid Flow Characteristics in Interacting Two Water-air Vertical Bubbling Jets in a Cylindrical Vessel

    pp. 432-438

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    Two bubbling jets generated from two nozzles placed a short distance off on the same horizontal plane pulled each other and merged into a larger scale bubbling jet. This phenomenon is caused through the Coanda effect. The bubble and liquid flow characteristics above the merging position were measured with an electroresistivity probe and a laser Doppler velocimeter, respectively. The results were compared to those for a bubbling jet of the same total gas flow rate generated from a single nozzle. Gas holdup, mean bubble rising velocity, the axial mean velocity and the root-mean-square values of three turbulence components of water flow for the merged bubbling jet agreed with their respective values for the bubbling jet generated from the single nozzle. On the other hand, bubble frequency and mean bubble chord length for the two nozzle injection differed from those for the single nozzle injection. This fact means that when the distance from the nozzle tip to the bath surface is much larger than the distance from the nozzle tip to the merging position, mixing in the bath is not expected to be enhanced by the two nozzle injection, whereas metallurgical reactions between bubbles and liquid can be enhanced because the interfacial area increases.
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  • Formation of Partially Reduced Iron by Heating Iron Ore-coal Char Composite Pellet in Oxygen Bearing Gas Flow: Reactions, Swelling Rate and Crushing Strength

    pp. 439-446

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    A single composite pellet composed of iron ore and coal char was reacted in an oxygen bearing nitrogen flow to very efficiently compensate the heat consumed by the strongly endothermic reaction, C+CO2=2CO. But the reactions are very complicated because oxygen not only generates heat by the combustion of carbon but also re-oxidizes reduced iron ore. In this study, the experiments were conducted principally using a single pellet containing 18 mass% coal char over the temperature range from 1273K to 1473K and O2 concentration range from pure nitrogen to 40 pet oxygen. The reduction started as the pellet was heated over about 1200K and reached the maximum net fraction, and followed by a rather predominant re-oxidation. The maximum reduction increased with increasing the pellet diameter and the temperature, and decreasing oxygen partial pressure, showing about 0.30. In this study, also, the swelling rate and crushing strength as the most important process parameter were also watched along the reactions. As the reactions proceeded, first, the crushing strength decreased to the minimum value, about 0.6 N common for all the experiments, at about 120 s. Thereafter, it turned to the increase and ultimately reached the individual maximum value at about 500 s, which is dependent on temperature. The drop in the swelling rate coupled with the rise in the crushing strength. That is, the densification of pellet, i.e. decrease in swelling rate resulted in the strengthening of the pellet.
  • Reaction Model for the Formation of Partially Reduced Iron by Heating Iron Ore-coal Char Composite Pellet in Oxygen Bearing Gas Flow

    pp. 447-451

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    A basic research on the production of partially reduced iron by heating iron ore-coal char composite pellet was conducted by using a pellet in oxygen bearing gas flow. Oxygen in the reaction gas generates heat by combusting carbon to very efficiently compensate the heat consumed by the strongly endothermic reaction of carbon C+CO2=2CO, while oxygen also re-oxidized reduced iron. A simple non-isothermal model for the reaction was made to simulate the very complicated phenomena. We approximated the reduction of iron oxides and the gasification of carbon to be of first-order one-step reaction, respectively. Further we assume that the shrinking-core model is applicable to the penetration of oxygen into the pellet under the diffusion limit of oxygen in the gas film and the re-oxidized product shell. By applying the reaction model, we succeeded in simulating the experimental results.
  • Smelting Reduction Rate of Fine Wustite Particles in a CO Gas-conveyed Bed

    pp. 452-459

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    Using a laboratory scale fine particles-gas conveyed bed, the reduction rates of liquid wustite with CO gas were measured. CO-CO2 mixtures having various flow rates and compositions were flowed downward through a cylindrical reactor maintained at a constant temperature of 1723 to 1823K. A batch of pure spherical wustite particles (mean dia.: 48.5 μm) was concurrently fed into the reactor at a small constant rate and reduced in a hot zone.
    The reduction process was found to proceed in such a manner that metallic iron particles were enclosed inside a wustite droplet. Rate analysis was made of one dimensional mass balance equations for particles and gas in a steady moving bed under an isothermal condition using the reaction rate for a single particle taking the shrinkage into consideration. Under relatively small reducing potentials, it was concluded that the major fraction of overall reaction resistance is attributable to chemical reaction. However, under higher reducing potentials, the reduction process was estimated to include some mass transfer resistances within the liquid oxide phase. From the temperature dependence of forward chemical reaction rate constants, the activation energy was evaluated to be 90.6 kJ/mol.
  • Substitute Function for Mold Oscillation Replaced by Imposition of an Intermittent Alternating Magnetic Field in a Continuous Caster

    pp. 460-465

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    In order to develop a mold oscillation-less continuous casting process, i.e. to replace the mold oscillation by an intermittent alternating magnetic field in a continuous caster, a model experimental work has been conducted using molten tin and gallium with low melting point, densities of which are as large as molten steel. The behavior of the mold flux penetrating into a flux channel between the molten metal and the mold and the process forming oscillation marks due to the mold oscillation were visualized. It was found that mold flux can penetrate periodically under the imposition of the intermittent alternating magnetic field even if without mold oscillation. The measured penetration depth of the mold flux agrees well with model predictions. Finally, the molten tin was continuously cast in a cold crucible type copper mold under the imposition of an intermittent alternating magnetic field instead of mold oscillation. It is noticed that molten tin can be successfully cast with the imposition of the magnetic field, but can not without it.
  • Evaluation for Lubricity of Lubricant by Simulation Testing Machine Developed Newly in Hot Rolling

    pp. 466-470

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    Recently, rolls made of high speed steel (HSS) which are superior to wear and abrasion resistance are used in many hot rolling mills. However, some troubles are occurred due to by the use of the HSS roll. To solve these problems, new lubricants are developed and the lubricities of them are estimated by using the two-disk testing machine or the pin-on-disk testing machine. But the evaluation obtained by these testing machines are different from one of the actual rolling processing. In order to evaluate the lubricity of lubricant in hot rolling, the simulation testing machine in the laboratory is modified from one for cold rolling. The evaluation performance of the simulation testing machine is investigated into using the SPHC workpiece strip having dimensions of a width of 20 mm and a thickness of 2 mm at a hot rolling temperature of 1030°C. From the experimental results, the lubricity of lubricant in hot rolling can be evaluated by the coefficient of friction measured. The effect of the additive on the coefficient of friction can be also confirmed. The obtained results are as follows; (1) When the mineral oil and one with oiliness additives are used as lubricants, the coefficient of friction increases rapidly with increasing rolling distance. (2) When the oils with the extreme-pressure additives are used, the coefficient of friction is steady over a rolling distance of 1000 mm.
  • Origin of Fine Ferrite Grain Structure of Sheet Steel Hot-rolled in Ferrite Region without Lubrication

    pp. 471-478

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    The structural variation in the thickness direction of ultra-low C sheet steel rolled in the ferrite region without lubrication is analyzed with EBSD technique as well as conventional measuring method. (1) A nonuniform microstructure through the thickness is formed due to the additional shear strain introduced by frictional force between the rolls and the material, and the fine grains with the size of 1.0 to 1.5 microns are observed near the surface where the shear strain is maximum. Since the dislocation density of these grains is relatively low and these grains are surrounded by high-angle boundaries having a misorientation angle over 15°, these grains might be recrystallized ones. (2) The microstructure observed in the etched steel with SEM are changed continuously from the deformed grains to the subgrains and the recrystallized grains with increasing shear strain. On the other hand, the size of the subgrains consisting of substructure is 0.5 to 1.0 microns by means of EBSD, and it is independent of shear strain. (3) The misorientation angle between the adjacent subgrains is increased with increasing shear strain, and high-angle boundaries are formed near the surface. (4) These experimental results suggest that the fine recrystallized grains might have formed in such a way that the subgrains are rotated continuously and the misorientation angle between the adjacent subgrains is increased over 15° with increasing shear strain, and the rearrangement of dislocations occurs during and/or after rolling, resulting in the formation of recrystallized grains.
  • Effect of Repeating Load and Dynamic Strain Aging on the Delayed Failure Strength of High Strength Steels

    pp. 479-485

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    Delayed failure tests of notched specimens of 18Ni-maraging steel, precipitation hardening stainless steel (JIS: SUS630) and Cr-Mo low alloyed steel (JIS: SCM435), which were aged or tempered to high strength levels, were carried out in 3% NaCl aqueous solution under static and superposed repeating load. Moreover, the effects of static and dynamic strain aging at 573 K or 673 K on the delayed failure strength were investigated on the Cr-Mo steel.
    The superposition of repeating load drastically decreases the delayed failure strength and the lower limit values of apparent stress intensity factor, under which no crack initiation occurs within 360 ks, for every steel. For Cr-Mo steel, the dynamic strain aging at 673 K is effective to increase the delayed failure strength not only under static load but also under repeating load, while the static strain aging at 673 K and the dynamic strain aging at 573 K has no improving effect.
  • Effect of B on the Spheroidizing Behavior of Cementite in Fe-0.8mass%C Alloy

    pp. 486-491

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    The effect of B, with the addition of 0.01 and 0.1 mass%, on the spheroidizing behavior of cementite in Fe-0.8mass%C alloy was investigated.
    The addition of B accelerated the softening by spheroidizing annealing. This effectiveness was enhanced with the increase in the solution treatment temperature and B content. Spheroidal cementites were most refined when 0.01 mass% B was added. This was attributed to the increase in the number density of nucleation sites which arose from fine cementites equilibrium at solution temperature and coarse cementites precipitated from liquid phase. When the amount of B was increased to 0.1 mass%, the effect of coarse cementites precipitated from liquid phase became significant. This resulted in the increase in the number of coarse spheroidal cementites with the diameter greater than 10 μm, and the decrease in the number of fine spheroidal cementites, supposedly through Ostwald growth mechanism.
  • Improvement of Ω Method to Estimate Creep Behavior of a Mod.9Cr-1Mo Steel

    pp. 492-499

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    In order to predict creep behavior of a Mod.9Cr-1 Mo steel, an equation for logarithmic creep giving below has been exploited to describe pronounced primary creep.
    ε=(1/Ω*)ln(Ω*ε*0t+1),
    whereε*0 is the initial strain rate and Ω*is the gradient between logarithm of strain rate and true strain in a primary creep stage. Differentiation of the above equation with respect to time yields the following equation:
    ε=lnε*0-Ω*ε.
    Designating a primary creep duration by t*, creep life tr can be expressed by the following equation:
    tr=t*+e-Ωε*/(Ωε0),
    where t* is given below:
    t*=(eΩ*ε*-1)/(Ω*ε*0).
    Since relatively good correlations were seen between ε*0 and ε0 and between Ω*and Ω as below, the total creep life under a given condition can be described as a function of Ω and ε0.
    ε*0=0.138·ε0.7250,
    and
    Ω* /Ω=AΩ* Ω·σ0-nΩ*Ω·exp(QΩ* Ω/RT),
    where AΩ* Ω is temperature and stress independent constant, nΩ*/Ω is the stress exponent of Ω*/Ω, and QΩ*/Ω is the temperature-dependent parameter of Ω*/Ω. They are, respectively, 2.82×103/S, 2.0±0.1 and 21±1kJ/mol. Furthermore, since there is a correlationship between Ω and ε0, creep life tr can be expressed by a single parameter out of four parameters, ε*0, ε0, Ω* and Ω.

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