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

Tetsu-to-Hagané Vol. 90 (2004), No. 8

  • Electrochemical Analysis on Reaction between Iron and Slag by AC Impedance Method

    pp. 533-537

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    In order to reveal the kinetic behavior of the anodic reaction between FeO-CaO-SiO2(-P2O5) slags and solid iron, the interfacial impedance was measured by using AC impedance method, and factors on kinetics of the electrode reaction such as Warburg parameter and charge transfer resistance were determined.
    The measurements were made in the frequency range from 0.01 Hz to 20 kHz at 1623K under an argon gas atmosphere by applying 60 mV DC potential between a working electrode and a counter electrode of iron crucible.
    The Warburg parameter and the charge transfer resistance were of the order 10-7Ω m2 s-1/2 and 10-710-8 Ω m2, respectively. Both of them decreased with the increase of the CaO content in slag when the FeO content was constant and decreased with the addition of P2O5 to slag. This result shows that the mass transfer behaves as a rate-control step in the dissolution reaction of iron into FeO-CaO-SiO2(-P2O5) slag, as the charge transfer resistance is rather small.
  • Slip Velocities of Solid Particles in Turbulently Agitated Liquid

    pp. 538-545

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    Separation of nonmetallic inclusion by flotation is the most important method for the clean steel production. In order to clarify the capability of inclusion separation in tundish or mould for continuous caster. In the present study, the terminal velocities of glass beads (density ρ=2410 kg/m3) and silica balloons (ρ=580 kg/m3) were measured in a mechanically agitated water vessel. Firstly the axial distribution of particle concentration in the agitated water was measured from a scattered light intensity obtained by a silicon photodiode. The ratio of particle terminal velocity to turbulent diffusivity, Vt/Dt, was derived from the axial distribution of particle concentration by comparing with the theoretical distribution obtained by the equation of balance between flotation/sedimentation flux and diffusion flux. And then, the value of Dt was measured by the complete mixing time of the vessel and consequently the value of Vt was calculated from the ratio, Vt/Dt. Finally the values of Vt, were correlated with several variables like particle diameter, densities of particle and water, energy-dissipation rate, etc., and a dimensionless correlation of drag coefficient was derived. From this correlation, relation between the velocity of ascending Al2O3 particle in liquid steel and particle diameter were estimated at various values of energy-dissipation rate.
  • Influence of Limestone and Coke Breeze Distribution in the Quasi-particle on Permeability during Sintering and Sinter Quality

    pp. 546-553

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    To improve sinter productivity and quality, the effect of secondary admixing coke breeze and limestone in the drum mixer on the permeability and the reduction characteristics were investigated by the sintering pot and tablet tests. The following results were obtained.
    (1) The secondary admixing technique to coke breeze enhanced permeability at moisture zone with increasing the quasi-particle size and strength, and the secondary admixing technique to limestone enhanced permeability at melting zone by the improvement of melt fluidity.
    (2) The sinter reducibility was improved by secondary admixing limestone and coke breeze, because of increasing micro pore originated from relict ore.
    (3) The above improvements were decreased with extending the coating time, because of destruction of quasi-particles due to intrusion of secondary admixing material inside the granules.
  • Formation of Fine Ferrite Grains during Hot Deformation of Cu Bearing Steel

    pp. 554-561

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    In order to refine the microstructure of low carbon steels, effect of copper addition was investigated in terms of super-cooling of austenite, dynamic austenite to ferrite transformation, and dynamic recrystallization of ferrite.
    Large decrease in transformation starting temperature was observed by copper addition. This is caused by the segregation of copper to austenite grain boundaries during prolonged austenitizing treatment. Fine ferrite grain structure in size of 2.1 μm was obtained by the deformation of super-cooled austenite at the reduction of 50% at 1043K in 2 mass% Cu bearing steel whose Ar3 temperature was decreased to 1008K by austenitizing for 14.4 ks at 1223K. This grain refining process has two stages. In the first stage, the austenite was transformed to ferrite through a deformation induced dynamic transformation (DIDT). The DIDT ferrite was characterized by fine cementite particles which cannot be observed in usual ferrite formed by thermal transformation. Volume fraction of the DIDT ferrite increases with increasing the degree of deformation. In the next stage, the ferrite grains were further refined and equiaxed with increasing the degree of deformation. A possibility that a dynamic recrystallization of DIDT ferrite occurs during the deformation was shown and this results in the marked grain refinement of ferrite.
  • Atmospheric Corrosion Monitoring in Each Part of Model Structure Using AC Impedance Method

    pp. 562-566

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    AC impedance method has been applied to evaluate the corrosion resistance in each part of the steel model structure exposed to outdoor environment. The corrosion sensors, which were made of the same materials as the steel model structure, were installed to each part of the model structure. The impedances at 10 kHz and 10 mHz were continuously measured between the steel model structure and the corrosion sensors, and the corrosion rate was obtained by subtracting the impedance at 10 kHz from at 10 mHz.
    The corrosion rate and the solution resistance in each part of the steel model structure were greatly influenced by the environmental factors. While rain and dew condensation were observed, the low solution resistance and the high corrosion rate were measured in all parts of the steel model structure, indicating that the corrosion progresses due to the wetness of the surface. From the corrosion loss in each part of the steel model structure, the horizontal parts showed larger corrosion loss than the perpendicular parts, and the tendency was stronger in flange part than in roof part. The change in impedance and the corrosion loss suggested that the corrosion behaviors in each part of the steel model structure are different.
  • Rafted Structure of γ' Phase in Polycrystalline Nickel-based Superalloy, IN-100, Subjected to Creep Deformation

    pp. 567-573

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    The correlation between the direction of rafted γ' plates and the orientation of crystal grains in a polycrystalline nickel-based Superalloy, IN-100, was investigated through the microstructural observation of creep ruptured and creep interrupted specimens at 1273K, 98-216 MPa. TEM observation showed that the rafted γ' plates formed as parallel to the {001} plane independent the orientation of crystal grains. The aspect ratio of γ' phase was measured in many grains with different orientations. The rate of rafting was supposed to decrease with increasing the orientation between the stress axis and the [001] direction of the grain, θ. The time at which the aspect ratio reaches to the maximum value becomes longer with decreasing the stress applied on {001} plane. Consequently, it was confirmed that the formation of rafted γ' is not only defined by the creep testing time.
  • Early Stage of Deformation in a Bainitic Steel

    pp. 574-580

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    Tensile deformation behavior at an early stage in a bainitic steel has been examined by detecting formation of surface relief, and analyzed stereographically. The variants of bainite could be identified by both morphology and crystallography. The surface relief was detected in a bainite-variant at an early stage of deformation. It could be analyzed that the variant with surface relief has many slip systems of larger Schmid factors compared with those that other variants have, resulting in the early stage of deformation of bainite being largely dependent on Schmid factor.
  • Development of TRIP-aided Cold Rolled Steel Sheets with the Excellent Stretch-flange Formability

    pp. 581-587

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    To develop the TRIP-aided cold rolled steel sheet with excellent stretch-flange formability, cold rolling of 0-50% was employed on the hot bands having martensite or ferrite-martensite microstrure, followed by intercritical annealing and austemper treatment. When hot band of martensite microstructure was subjected to cold rolling up to 20% and subsequent heat-treatment, the cold rolled steel sheet has relatively high stretch-flange formability. This may be caused by the uniform fine microstructure composing of annealed martensite lath matrix and interlath retained austenite. If hot band of ferrite-martensite-microstructure with 60 vol% ferrite was subjected to cold rolling up to 20% and subsequent heat-treatment, the TRIP-aided cold rolled steel sheet achieved higher stretch-flange formability than that of martensite microstructure. In this case, the ferrite phase is considered to suppress the plastic deformation of martensite and prevent martensite lath sub-structure vanishing during cold rolling. Then finally the microstructure containing the fine uniform matrix and filmy lath-like retained austenite obtains after annealing.
  • Suppression of Delamination in Hyper-eutectoid Steel Wires by Multi Skin-pass Drawing

    pp. 588-592

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    The effect of multi skin-pass drawing on mechanical properties of high carbon steel wires are investigated. 1.0mass%C-0.2mass%Si-0.4mass%Mn-20ppmB steel was used in this study. The tensile strength change in wet drawing was not different between the filament with single skin pass draft condition and that of multi skin-pass draft condition. However torsion property was drastically changed. Delamination did not occur between 0.287 mm and 0.213 mm in diameter under the multi skin-pass draft, which strength was over 4200 MPa. From DSC curve, the first peak (70100°C peak) was observed only in multi skin-passed filament. By analyising Kissinger plot, the activation energy of the first peak was about 90kJ/mol which was close to that of carbon atoms lattice diffusion in ferritic iron. The effect of multi skin-pass on torsion property change is considered originating from alleviation of dynamic strain aging by carbon atoms desorption from dislocation located ferrite/cementite boundaries.
  • Analytical Conditions for Quantitative Analysis of Lead in Steels in Direct-current or Radio-frequency Glow Discharge Optical Emission Spectrometry

    pp. 593-597

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    Optimum analytical lines as well as the discharge conditions for lead determination in steel samples were investigated in direct-current and radio-frequency glow discharge optical emission spectrometry (d.c. and r.f. GD-OES). Two emission lines: Pb I 405.784 nm and Pb II 220.356 nm, were compared on their analytical performance in d.c. GD-OES, indicating that the Pb II 220.356-nm line should be selected as the analytical line because the emission intensity is much larger than that of the Pb I 405.784-nm line. Several iron lines were also measured for correcting variations in the sampling amounts so that the emission intensities of Pb II 220.356 nm can be estimated more accurately. Atomic and ionic iron lines, such as Fe I 344.374 nm and Fe II 254.874 nm, having similar excitation energies to Pb II 220.356 nm, were suitable for the internal standard line, because their emission intensities well follow the intensity of the Pb II line when the sampling rate and the excitation conditions are changed. In r.f. GD-OES, a bias-current introduction method was employed for enhancing the emission intensity of the Pb I 405.784-nm line. By conducting of the bias-current of 33 mA at the r.f. power of 80 W, the emission intensities were 12 times larger than those obtained with the conventional plasma. This effect contributes to Pb determination with a higher detection ability in r.f. GD-OES.

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