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

Tetsu-to-Hagané Vol. 97 (2011), No. 1

  • The Change of Composition of Inclusions in Molten Steel during Ca and Mg Addition

    pp. 1-6

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    DOI:10.2355/tetsutohagane.97.1

    In order to examine the mechanism of the generation of MgO in CaO–MgO–Al2O3 inclusion during Ca addition, the variations of inclusion composition and molten steel composition were measured during and after FeMg and CaSi addition to 2 kg heats of Al killed molten steel at 1873K. From experimental studies, the equilibrium reaction was discussed by the relationship between Ca concentration in molten steel and MgO concentration in inclusion and apparent equilibrium constant of magnesium deoxidation was obtained.
    The results obtained are summarized as follows;
    (1) The addition pattern of calcium and magnesium affected composition variation path, although it had no effects on final composition of inclusion.
    (2) The boundary condition of formation and disappearance of MgO in Ca–Al–O inclusion was given by Ca concentration and Mg concentration in molten steel.
    (3) The reaction of MgO formation in Ca–Al–O inclusion was expressed as follows.
    CaO·Al2O3 in inclusion+Mg=MgO·Al2O3 in inclusion+Ca
    (4) The apparent equilibrium constant log KMg was estimated to be −7.2 by thermodynamic calculation.
    log KMg=aMg·aO=−7.2
  • Metallurgical Analysis of Iron Nails for Roofing Tiles of the Mieidou of the Senjuji Temple

    pp. 7-11

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    DOI:10.2355/tetsutohagane.97.7

    The macro- and micro-structure of the iron nails for roofing tiles of the Mieidou at the Senjuji temple in Mie, produced in the Edo period, have been investigated using an optical microscope and an SEM for the elucidation of the nails production method. The object of this work is to obtain materials science data of the Japanese old nails for roofing tiles. The test pieces were mainly cut at the longitudinal direction and partially cut at the horizontal direction for the confirmation of carburizing method. The average chemical compositions are as follows; 0.03 mass%C, 0.06 mass%Si, Mn<0.01 mass%, 0.031 mass%P, S<0.005 mass%, Cu<0.01 mass% and Ti<0.005 mass%.
    The matrix structure is mainly equiaxed ferrite grain, whose grain sizes were from 20 to 100 μm. Moreover, we confirmed that the only one side of the nails was carburized clearly at the depth from 0.3 to 1.0 mm. These results show that the forged pieces of the nails were cut from a carburized plate and forged individually.
  • Reduction of Oxide Scales Formed on Low Carbon Steel Sheet in Synthesized Combustion Gas

    pp. 12-18

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    DOI:10.2355/tetsutohagane.97.12

    Reduction of oxide scale, which formed on low carbon steel sheet at 973K for 4.5 ks in 10.9 kPa CO2–14.5 kPa H2O–bal. N2, was studied at 673–1073K in pure H2. Reduction was slow at 673K and it became fast up to 823K. Between 823–873K, however, the reduction became slower than that at lower temperatures and showed a minimum at 873K. Then the reduction became faster with increasing temperature above 873K. The temperature at which the reduction was the slowest coincides with that of transformation between magnetite and wüstite. Surface appearance of scales was different in these two temperature regions; surface cracks increased with reduction time at low temperature, whereas surface sintering proceeded with the time at high temperatures. The reason for the sudden reduction manner change around 873K was considered in terms of morphology, type and porosity of the oxide scales. It is concluded that the formar two can be the reason for the sudden change of reaction manner.
  • Influence of Si Addition to the Coating Bath on the Growth of Al–Fe Alloy Layers in Hot-dip Zn–Al–Mg Alloy-coated Steel Sheets

    pp. 19-25

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    DOI:10.2355/tetsutohagane.97.19

    The influence of Si addition to the coating bath on the growth of Al–Fe alloy layers formed in the interface between the coating and the steel substrate was studied utilizing a Zn–Al–Mg coating bath with 10–11 mass% Al content. While the Al–Fe intermetallic compound rapidly grew in the interface between the coating and the steel substrate when dipped into a coating bath of Zn–10mass%Al–3mass%Mg at 550°C, the growth was extremely suppressed by the addition of 0.2 mass% Si to the coating bath. TEM observation revealed that, in the interface between the coating and the substrate the Si addition resulted in the uniform formation of very fine grains with a size of 20–30 nm in diameter of the Fe2Al5 phase with Si and Zn in a solid solution. The mechanism of the Si addition was postulated that the large amount of Fe in the substrate may be dissolved easily into the liquid phase in the case of a ternary Fe–Al–Zn system, whereas in the case of a quaternary Fe–Al–Zn–Si system, the dissolution of Fe into the liquid phase is significantly suppressed by the presence of very fine and thin Fe2Al5 containing Si in the interface between the coating and the substrate. Thus, the growth rate of Fe2Al5 was extremely reduced.
  • Microstructure and Mechanical Properties of Austempered Medium Carbon Steels

    pp. 26-33

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    DOI:10.2355/tetsutohagane.97.26

    Microstructures and mechanical properties for medium carbon steels including practical spring steels austempered at 400°C and 300°C were investigated. The larger strain was applied, the larger the fraction of retained austenite became martensite through strain induced transformation. 0.2% proof stress was increased by the increase of the fraction of bainitic ferrite. Uniform elongation was increased by increasing the fraction of retained austenite. Good strength–ductility balance with tensile stress over 1800 MPa for high strength spring was achieved in austempered SUP12 at 300°C composed of three phases of retained austenite, martensite and lower bainite.

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