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

Tetsu-to-Hagané Vol. 94 (2008), No. 6

  • Mould Lubrication and Control of Initial Solidification Associated with Continuous Casting of Steel

    pp. 189-200

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    DOI:10.2355/tetsutohagane.94.189

    In continuous casting of steel, it is important to provide stable lubrication for a solidifying shell that is kept as uniform as possible in an oscillating mould, because this improves not only productivity of the process but also surface quality of cast slabs. In the first part of the present article, mould lubrication is reviewed with an emphasis on infiltration of mould flux in the channel between a solidifying shell and a mould. Theoretical understandings of the phenomenon are described in comparison with experimental and empirical knowledge in casting operation. Next, development of uneven solidifying shell is discussed from not only thermo-mechanical behavior of the shell but nucleation and growth during initial solidification at cast surface. This reveals importance of heat transfer control between the shell and mould. For this reason, the discussions are extended to crystallization of mould flux film that provides stable reduction in the heat transfer. Effect of the crystallization on conductive and radiative heat transfer through mould flux film is discussed as well as change in thermal resistance at interface between the flux film and mould wall. Then, recent studies on crystallization of cuspidine (3CaO·2SiO2·CaF2) are also referred. Finally, current knowledge of mould flux infiltration, control of initial solidification and heat transfer in continuous casting mould is briefly summarized.
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  • Formulation of Oxidation Reaction Rate for Deposit Carbon in a Coke Oven Chamber by Thermogravimetry

    pp. 201-206

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    DOI:10.2355/tetsutohagane.94.201

    Deposit carbon on the wall disturbs a stable operation in a coke-oven battery while it plays an important role to seal up the liner bricks between a coking chamber and a combustion chamber. Therefore it is important to control the thickness of deposit carbon. Thus, it is necessary to estimate oxidation reactivity of deposit carbon.
    In this study, the thermogravimetry was carried out and the oxidation reaction rate was formulated for deposit carbon.
    In the measurement, deposit carbon samples were heated up to 1273 K at four different heating rates using thermogravimetric analyzer (TGA). It was found the deposit carbon has lower oxidation reactivity than coal char and coke and higher oxidation reactivity than graphite material.
    In the formulation, the activation energy and the reaction mechanism model were determined by Friedman method and by master-plot method, respectively. The evaluated equation well predicted experimental data on the oxidation reaction rate of deposit carbon.
  • Superplastic Behavior in Magnesium Alloy of AZ91 Grain Refined by Isothermal Cross-rolling

    pp. 207-214

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    DOI:10.2355/tetsutohagane.94.207

    Thin magnesium alloy sheets of AZ91 by using isothermal cross-rolling have been prepared in present study. These sheets were annealed around the temperature between 0% and 100% recrystallization. X-ray pole figure analysis (XPFA) by Schulz's reflection method was used in order to investigate the texture of annealed sheets. As a result of XPFA, a large anisotropic texture was reduced compared to unidirectional rolled AZ91 sheets. Constant initial strain tensile tests were carried out at elevated temperatures and at several strain-rates. The m value over about 0.3 was observed at testing temperature of 573 to 623K at strain rate of 1.0×10−4 to 2.5×10−4 s−1, and at testing temperature of 648K at strain rate of 1.0×10−3 to 2.5×10−3 s−1. The temperature indicated the peak in elongations moved largely to lower temperature region. The activation energy required for superplastic deformation was calculated to be 73–76 kJ·mol−1, which was almost the same value of the activation energy for self-diffusional grain boundary coefficient in magnesium.
  • Evaluation of Delayed Fracture Characteristics of High Strength Steel based on CSRT Method

    pp. 215-221

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    DOI:10.2355/tetsutohagane.94.215

    A conventional strain rate technique (CSRT) to evaluate the delayed fracture characteristics of high strength steels has been proposed. The critical “maximum stress—diffusible hydrogen concentration” at the delayed fracture initiation point near the notch tip is thought to be a material constant, which was originally demonstrated using the SSRT (slow strain rate technique) test method. The SSRT method takes hours to complete the test and uses a special test machine, which causes difficulty and complication. Therefore, a simple and conventional test technique, CSRT test method for delayed fracture was investigated. The crosshead speed is around 1 mm/min, so that the stress induced diffusion of hydrogen is negligible. The results obtained are as follows.
    (1) Since the stress induced hydrogen diffusion does not take place during the CSRT test, it is necessary to introduce the amount of hydrogen in the specimen, corresponding to the accumulated hydrogen at the vicinity of the notch tip region in the SSRT test. The electrochemical hydrogen charging conditions were established to introduce a wide range of hydrogen contents into the specimens.
    (2) A unique relationship between the maximum stress at the vicinity of the notch tip and hydrogen contents was obtained irrespective of the notch configuration using the CSRT test and FEM stress analysis. Therefore, it can be said that this relation is the material constants for delayed fracture.
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  • Damage Model for Simulating the Effect of Material Properties on Ductile Crack Growth Resistance

    pp. 222-230

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    DOI:10.2355/tetsutohagane.94.222

    This study develops a damage model for characterizing the ductile crack growth on the bases of observation of micro-void nucleation for recent structural steels.
    The resistance to ductile crack initiation is controlled by a local strain at the tip of a notch or a fatigue pre-crack of a specimen. On the other hand, ductile crack extension is controlled by a stress triaxiality dependent critical strain for ductile failure obtained by tension tests of circumferentially notched round-bar specimens. In steels used, the main process for ductile failure is nucleation of micro-voids, whose size is in the order of 1 μm, that are generated at the final stage of ductile failure. According to these observations, a damage model is proposed for simulating the ductile crack extension.
    This model enables the prediction of the crack growth resistance in terms of CTOD for fatigue pre-cracked specimens with two different crack depth ratios a0/W, where a0 is the initial crack length and W is the specimen width.

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