Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 88 (2002), No. 6

  • Development of Sample Preparation Techniques for Analyses of Trace Elements in Steel

    pp. 285-291

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    Precise and accurate determination of trace elements in steel is required in order to estimate the relationship between the material properties and the amounts of trace elements. This paper deals with the sample preparation including sample decomposition, matrix separation, and preconcentration, which are indispensable for the trace analysis.
    Several decomposition techniques such as single test tube decomposition, acid digestion in a pressure bomb, and microwave digestion were applied to the steel sample. The contamination, which was caused by the large amounts of acids used and the atmosphere, was decreased by these techniques. In addition, simple operation was achieved.
    For the matrix separation and pre-concentration of trace elements in steel, several methods including solvent extraction, chromatographic separation, precipitation separation, and vapor phase separation have been developed. Some of the techniques can be applied to the pre-concentration of multiple elements, and are applicable for the simultaneous analysis by ICP-AES and/or ICP-MS. Furthermore, flow injection analyses coupled with these pre-concentration techniques were developed and the good repeatability and simple operation were achieved.
  • Coanda Effect on Bubbling Jet in a Mercury Bath

    pp. 292-299

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    Water model experiments have been carried out on the Coanda effect in air-water two-phase flows. It is however not clear whether the results are applicable to gas-molten metal two-phase flows. The objective of this study is to investigate the characteristics of bubbling jets subjected to the Coanda effect in a mercury bath. Two types of Coanda effects were treated. One is the attachment of a single bubbling jet to the side wall of a vessel. The other is the coalescence of two bubbling jets. Gas hold-up was measured with a two-needle electro-resistivity probe to determine the attachment length and the coalescence length measured from the nozzle exit. Empirical equations were proposed for the two lengths.
  • Effect of Initial [Si] Content on the Lime Efficiency on the Dephosphorization

    pp. 300-305

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    Hot metal dephosphorization slag is in the state in which solid and liquid coexist. However, in the previous reaction analysis, the contribution of solid phase is not taken into consideration in many cases because of the lack of thermodynamic and kinetic theory understanding. In this research, dephosphorization examination was conducted. In this examination the solid phase existing during the reaction was changed by changing initial concentration of Si in the hot metal. And relationship between type of solid phase in slag and dephosphorization efficiency was investigated.
    The results are summarized as follows: The range of Δ [Si] exists where dephosphorization efficiency is improved. The dephosphorization reaction in a solid and liquid coexistence slag is greatly influenced by the solid phase that precipitates in the slag. Dephosphorization efficiency improves in the 2CaO·SiO2 crystallizing range. This improvement can be explained by concentration of phosphorus in the solid phase.
  • Deoxidation of Molten Iron with Magnesium Vapour Produced In-situ by Aluminothermic Reduction of Magnesia

    pp. 306-313

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    A novel method of deoxidation of molten iron is proposed with magnesium vapour produced in-situ by aluminothermic reduction of magnesia. A reaction model is developed to examine the rate controlling mechanism and highly efficient deoxidation with magnesium vapour. It is shown that, in order to achieve the high efficiency, control of the magnesium vapour pressure in the injected bubble is essential.
    In the experiments, the pressure of Mg vapour in the injected bubble was changed with pellet mass, pellet charging method, Ar carrier gas flow rate, size of MgO powder in the pellet and reduction temperature of pellet. It is found that the calculated results are well consistent with the experimental results. In the early period of the experiment, the Mg pressure in the bubble is high, so that the deoxidation rate is controlled by the mass transfer of oxygen in the melt. In the following period, the Mg pressure becomes lower, and the deoxidation rate is controlled by the mass transfer of Mg vapour in the bubble and oxygen in the melt. The experimental results show that the deoxidation efficiency increased with increasing Ar carrier gas flow rate and decreasing reduction temperature of pellet. Dividing the pellet charging into several portions is also effective to increase the deoxidation efficiency. It is shown that all these changes in the operating conditions are concerned with the control of the Mg vapour pressure in the injected bubble for improving the deoxidation process.
  • Estimation of Rolling Parameters by Signal Processing of Measured Data

    pp. 314-319

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    Mill modulus and plastic modulus of material are important parameters for operating automatic gage control stably. In this paper, estimation of the parameters by processing actual rolling data is studied. The parameters are calculated according to amplitudes of rolling force change and exit thickness change witch synchronize with a frequency of roll eccentricity. Discrete Fourier Transform (DFT) is used for analyzing amplitudes of the waves, but spectrums by DFT are affected by a cutting-off period from sampled time-base waves. A method that filters the spectrums and inverses them is proposed to improve accuracy of amplitude analysis. The effect of the method is confirmed by simulation with a sinusoidal wave. Secondly, the optimum cutting-off period is selected to maximize signal-to-noise ratio by calculating actual measured data during rolling. Finally using the above method, mill modulus and plastic modulus are estimated on three coil data rolled with a reversing cold mill. As a result, it has been shown that amount of scatter of estimated values is small.
  • Analysis of Chemical State for Alloying Elements in Iron Rust

    pp. 320-325

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    A binary-phase potential-pH diagram has been investigated to evaluate the chemical stability of various kinds of complex iron rust (Fe-X), that is a corrosion-resistance principle of low alloy steel. It was found that there are the following types of corrosion-resisting elements in the rust: (1) iron substitution type (Ni), (2) oxide formation type (Al), (3) metallic type (Ru), and (4) oxygen-acid salt type (WO4). XPS and TEM analysis have been conducted on the rust formed on the low alloy steel in saline environment. The physical analysis agrees with potential-pH diagrams. The iron substitution type and the oxide formation type elements make complex spinel oxides with iron. In the corrosion tests, steels added with Ni or Al had high corrosion resistance. Thus it is possible to obtain high corrosion resistance by the creation of complex spinel such as Fe2NiO4 and FeAl2O4 in an inner layer.
    On the other hand it was found that the metallic type and the oxygen-acid salt type elements were not absorbed into the rust. In particular the oxygen-acid salt elements were excluded from the iron rust and concentrated into the defects of the rust. It is suggested that insoluble salts like FeWO4 are formed on the base metal in the defects to act as an anodic inhibitor. Thus the addition of a small quantity of W gives high corrosion resistance.
    In this way the penetration of Cl ions can be prevented by the complex spinel in an inner layer and the oxygen-acid salt in the defects. Thus the high corrosion resistance by the addition of these elements can be understood from the potential-pH diagram and the physical analysis.
  • Creep Damage Development of a 1.25Cr-0.5Mo Steel Weldment Exposed to Long Term Service

    pp. 326-333

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    Most of mechanical problems take place at weldment especially for the components operated in the creep regime. In the present paper, creep damage development of a service exposed 1.25Cr-0.5Mo steel weldment containing the service induced damage at HAZ associated with high temperature operation for 23 years has been examined. The components fabricated from a 1.25Cr-0.5Mo steel tend to suffer premature Type III damage at the early stage of operations. Though the similar tendency has been observed in the current work, the ultimate failure mode has been predominantly Type IV. The Type IV failure was generated in a relatively short period of time by applying spirally notched specimens. The transition of a failure mode from ductile transgranular rupture at a parent material to brittle intergranular cracking at Intercritical HAZ (ICZ) took place with the increase in time to failure. And the difference in the susceptibility to damage at HAZ between two materials (a pipe fabricated from plates and a forged flange) constituting welds was found. Despite lower creep strength of a pipe parent, HAZ generated on the pipe side has been almost immune to both Type III and Type IV damage in the testing conditions examined, suggesting that the creep life of weldment is not necessarily determined by creep strength of parent materials.
  • Measurement and Analysis of Work Hardening of Sheet Steels Subjected to Plane-strain Tension

    pp. 334-339

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    A novel experimental method for precisely measuring the stress-strain relations of sheet metals subjected to plane-strain tension has been developed. A hydraulically servo-controlled biaxial tensile testing apparatus and newly-devised cruciform specimens have been used. The geometry of the cruciform specimen has been determined using FEM analysis so that the stress distribution in the gage section of the specimen becomes as uniform as possible. By measuring the principal strain components of the specimen by strain gages and controlling the nominal strain rates using an electrical feed-back circuit, we have been successful to realize plane-strain tension tests of sheet metals up to a tensile strain of 0.1. The minimum (width) strain has been confirmed to be almost zero within the resolution of strain measurement, ±22 με. The measurement error of the maximum principal flow stress has been estimated to be under 2% according the FEM analysis. Stress-strain curves for IF steel (SPCEN) and 370 MPa-level rephosphorized steel (SPFC370) under plane-strain tension have been measured and compared with those predicted by the Hill quadratic and Hosford yield criteria. It has been found that the stress-strain curves calculated by the Hosford yield criterion are generally in good agreement with the experimental ones during early work hardening stages (less than 0.02-0.03 tensile strains). However, the difference between the experimental and predicted flow stresses increases gradually with increasing the tensile strain. The plane-strain flow stresses predicted by the Hill quadratic yield criterion are tend to be larger than the observed ones for the whole strain range irrespective of materials.
  • Behavior of Fluorine Dissolution from Hot Metal Pretreatment Slag

    pp. 340-346

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    Based on the results for the time dependence of F, Ca, Si and Al amount dissolved from hot metal pretreatment slags, the behavior of fluorine dissolution has been examined. It was found that the amount of fluorine dissolved from hot metal pretreatment slag depends on CaO/SiO2 mass ratio and fluorine content in slag. This trend is obvious in the case of synthetic slags. Fluorine ion in solution is incorporated into CaO-SiO2-H2O and CaO-SiO2-Al2O3-H2O gels during the formation of these gels. Therefore, it was found difficult to estimate the amount of fluorine in solution from the amount of F-containing mineral phases and slag compositions.
  • Immobilization of Fluorine Dissolved from Hot Metal Pretreatment Slag

    pp. 347-354

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    The immobilization of fluorine dissolved from hot metal pretreatment slag has been studied by using synthesized compounds such as calcium aluminates, calcium silicates, calcium aluminum silicates and calcium aluminum ferrites. Fluorine ion can be immobilized strongly by 3CaO · Al2O3, and moderately by 3CaO ·SiO2. The F immobilization capability of calcium aluminum silicates decrease with increasing SiO2 content. Fluorine is effectively immobilized by calcium aluminum ferrites due to low hydration rate. The immobilization of fluorine proceeds effectively by using a secondary refining slag with low contents of SiO2 and total Fe.
  • Calibration of Dilute Moisture Meter by Coulometry

    pp. 355-357

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