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

Tetsu-to-Hagané Vol. 94 (2008), No. 3

  • Simulation of Gas Gap Formation between Substrate Surface and Molten Steel During Initial Solidification

    pp. 73-79

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    DOI:10.2355/tetsutohagane.94.73

    In order to evaluate the motion of molten steel just before its solidification, contact conditions between the molten steel and the mold surface at a time when the molten steel contacts the mold surface were analyzed. Conditions of gas-gap formation on the interface between the molten steel and the mold were simulated by preparing a simplified dynamic model in which physical-property values, such as, depths of unevenness of the mold surface, gas absorptions into the molten steel, heat flux, and various other conditions were systematically varied. The time needed for the molten steel to penetrate to the bottom of a depression 100 μm deep in the mold surface was estimated to be 0.03 s. It was also estimated that the molten-steel penetration time is significantly influenced by the depth of depressions in the mold surface, gas absorptions into the molten steel, and heat fluxes, whereas the effects of viscosity and surface tension of the molten steel are insignificant.
  • Separation and Preconcentration of Trace Amounts of Bismuth in Iron-digested Solution by Ion-exchange Adsorption/Ion-pair Elution Followed by Graphite Furnace Atomic Absorption Spectrometric Determination

    pp. 80-85

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    DOI:10.2355/tetsutohagane.94.80

    A unique preconcentration method “ion-exchange preconcentration followed by ion-pair elution” was applied to separation and preconcentration of trace amounts of bismuth in a digested solution of iron and steel. Bismuth(III) was derived to anionic iodo complex prior to preconcentration. The bismuth complexes were adsorbed on cotton impregnated with capriquat through ion-exchange reaction, followed by being recovered as ion-pair of capriquat by elution with methanol. Since iron ions in the digested solution were reduced into divalent state with ascorbic acid, which also works as an anti-oxidized agent of iodide ions, matrix irons did not form iodide complexes, leading to no adsorption on the cotton. More than 99.99% iron could be removed by rinse with a solution containing 0.8 mol dm−3 ascorbic acid and 0.5 mol dm−3 hydrochloric acid. The described preconcentration could be successfully combined with graphite furnace atomic absorption spectrometry (GF-AAS) due to the removal of capriquat with ashing stage in the GF-AAS protocol. Combination of 10-fold preconcentration and GFAAS measurement allowed us to determine trace amounts of bismuth in a digested solution of iron with a detection limit (3σ) of 4.4×10−9 mol dm−3, which corresponds to 4.6 ppb in iron and steel. The combined method was successfully applied to determination of bismuth in certified reference materials of iron and steel.
  • XAFS Observation to Investigate Possible Change in Crystal Structure of Cu Precipitates in Iron during Plastic Deformation

    pp. 86-90

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    DOI:10.2355/tetsutohagane.94.86

    It is well known that Cu particles precipitate during aging from super-saturated ferrite Fe initially as bcc structure, then transform to stable fcc structure as aging progresses to over-aging. It is also well known that the structure transformation goes through intermediate structures such as 3R and 9R. Several previous investigations have suggested that plastic deformation enhances structure transformation of meta-stable precipitates. However, no direct experimental evidence to show this hypothesis has been observed. SR-X-ray absorption fine structure (XAFS) is a useful method to analyze crystal structure of Cu precipitates in Fe. XAFS profiles were compared before and after plastic deformation. A possibility of transformation in crystal structure was observed when Cu precipitates were most likely meta-stable condition.
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  • Long Term Efficiency and Stability of MX Precipitation Strengthening of High Chromium Steel

    pp. 91-98

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    DOI:10.2355/tetsutohagane.94.91

    Applicability of high Cr steels to a main structural material in fast breeder reactors (FBR) has been explored to enhance the economical competitiveness of the FBR power plants. V and Nb are believed to improve the high temperature strength of high Cr steels by precipitating as carbides and/or nitrides, namely MX fine particles, although the long-term effectiveness and stability of such a dispersion strengthening mechanism has not fully been understood yet. A series of trial melts controlling V and Nb contents are produced and aging tests are conducted to investigate the long-term stability of the MX strengthening mechanism. VX dispersion strengthening in high Cr steels bearing with V has been found to be stable even after aging for 12000 h at 600°C, which is equivalent to the expected FBR operation condition, 700000 h at 550°C, because the aspect ratio and chemical compositions of VX particles do not change much with aging. MX strengthening with Nb, and with both Nb and V, on the other hand, seems to be unstable, because the number density of MX particles decreases with aging. Z-phase is found to be stable and fine as other MX particles, suggesting that it may contribute to the dispersion strengthening as well as MX in the FBR conditions.
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  • Creep Curve Analysis Ω with Method and Effect of Creep Strengthening Mechanism on Ω Value in 9Cr Ferritic Heat-resistant Steels

    pp. 99-105

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    DOI:10.2355/tetsutohagane.94.99

    Ω method is effective for an assessment of creep life time. In this study, creep curves of 9Cr ferritic heat-resistant steels were analyzed by the Ω method. Samples were four types of 9Cr steel, which were fundamental 9Cr steel, solid solution hardened steel containing W, precipitation hardened steel with MX and precipitation hardened steel with MX and Laves phase. The comparison between rupture time in creep tests and creep life time assessed by the Ω method indicated that the life assessment of the Ω method was sufficiently effective in the 9Cr steels. In addition, we investigated the dependence of Ω value on stress applied in creep test. In the case of the precipitation hardened steels, Ω value had the strong dependence on stress. This dependence indicated that Ω value has relevance to creep deformation mechanism.

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