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

Tetsu-to-Hagané Vol. 87 (2001), No. 8

  • Control of Intensity and Distribution of Compression Waves Excited by Alternating Electromagnetic Force

    pp. 521-528

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    Compression waves have a lot of useful functions such as refinement of solidified structures and promotion of reaction rate. In order to extend the effective use of these functions, the control of intensity and distribution of compression waves in a liquid metal is indispensable. The intensity and the distribution of compression waves excited by the simultaneous imposition of a static magnetic field and an alternating current were studied. Transmission and reflection coefficients of magneto-acoustic waves at the interface between two media set in a high static magnetic field were calculated as a function of their physical properties. Moreover, the theoretical predictions on the distribution and the intensity of the compression waves in a liquid metal were derived based on electromagnetic field theory and compressible fluid dynamics for the system composed of a liquid metal and a solid. The intensities of the compression waves in a liquid metal were also measured under the different experimental conditions and compared with the theoretical predictions. Based on these results, the effect of operating parameters such as the frequency of an alternating current and the thickness of a solid layer on the distribution and the intensity of the compression waves were discussed.
  • Water Model Experiment on Mold Powder Entrapment around the Exit of Immersion Nozzle in Continuous Casting Mold

    pp. 529-535

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    The problem of mold powder entrapment in molten steel in a continuous casting mold continues to be a serious obstacle to the producing of clean steel. Four types of the mechanisms of mold powder entrapment have been proposed on the basis of water model experiments. These mechanisms, however, cannot fully explain the size distribution of mold powder in the finished steel products. In this study, another type of entrapment mechanism was proposed. Namely, when the molten steel flow crosses the immersion nozzle, the pressure on the forward stagnation point increases, while that on the rear stagnation point decreases. As a result, the mold powder moves downwards along the rear surface of the immersion nozzle. If the mold powder reaches one of the ports of the immersion nozzle, it is sucked into the discharging molten steel flow, which causes significant mold powder entrapment. In order to ascertain the validity of this proposed mechanism, water model experiments were carried out using various kinds of silicone oils and salt water. The results of the model experiments revealed the possibility that the mold powder entrapment based on the mechanism proposed in this study takes place in the real continuous casting mold.
  • Tribological Behavior between Commercially Pure Titanium and High Speed Steel at High Temperature

    pp. 536-543

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    It is said that in hot-rolling titanium surface defects are apt to occur by seizures between rolls and products. Authors have examined the effects of temperatures and lubricants on the tribological behavior of titanium strip surfaces contact with rolls using a hot rolling and sliding wear tester with commercially pure titanium disks and a high speed steel ones. Consequently in cases of water spray without lubricant oil, fine cracks due to hard layers by oxidation decrease with lower temperature, but the coefficient of friction increases because oxide scales become thin and then metals are easily in contact. The coefficient of friction at 650°C doubles in comparison with that of 850°C and foil-like overlaps occur. Addition of lubricant oil reduces the coefficient of friction at 650°C by half and smooth surfaces are obtained. The smooth surface is thought to be due to the lubrication effects of oxide-scales as well as the lubricant.
  • Demonstration Test of New Corrosion-resistant Boiler Tube Materials in High Efficiency Waste Incineration Plant

    pp. 544-551

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    In order to develop a 9.8 MPa/500°C highly efficient waste incineration boiler with 30% electric efficiency, application of highly corrosionresistant materials for superheater and corrosion-resistant coatings for waterwall are indispensable. Corrosion life of six conventional and six newly developed superheater tube materials and two waterwall coatings were evaluated for two years in the NEDO waste incineration pilot plant. As the results of the test, an excellent durability of more than six years for Ni-Cr-Mo alloy seamless and composite tubes of tertiary superheater, for Ni-Cr-Fe alloy tubes and cermet spray coating of secondary superheater and for alloy625 weld overlay and NiCrSiB alloy HVOF coatings of waterwall were clarified. Also the corrosion rate equation capable of corrosion life estimation was suggested by multiple regression analysis. Particularly, an addition of Mo to Ni base alloy was effective to prevent corrosion in severe corrosive conditions. Furthermore, the effect of corrosion factors that important to design the plant, i.e. soot blower effect, gas/metal temperature, time dependence of corrosion thickness loss etc. were clarified quantitatively.
  • Effects of Carbon and Sulfur Contents on Hot-ductility of Medium Carbon Steel

    pp. 552-556

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    The effects of carbon and sulfur contents on the hot-ductility of laboratory melted medium carbon steels were investigated.
    At a strain rate of 2 × 10-3s-1, 0.16% C-steel showed the ductility trough at 800°C, which was slightly higher than the starting temperature of Ar3 transformation. The microstructure observation revealed that the filmy ferrite covered the austenite grain boundaries. This suggests that the deformation induced transformation occurred. Decreasing C content down to below 0.10% improves the hot-ductility around the temperature range of 750850°C and also raise the temperature of ductility trough. The change in hot-ductility with decrease in C content may be attributed to the change in morphology of filmy ferrite at grain boundaries.
    The ductility of 0.16%C-0.008%S-steel around the temperature range of 750850°C was markedly improved by prolonging the holding time for over 20 min at the temperature above Ar3 while ultra low S-steel (0.16%C-0.0003%S) showed the ductility trough without isothermal holding. The recovery of hot-ductility can be attributable to re-precipitation and coarsening of the MnS precipitates in the filmy ferrite at grain boundaries, which decrease the void initiation site in the filmy ferrite.
  • Improvement of Fatigue Strength of High Strength Hot Rolled Steel Sheet for Truck Frame Use

    pp. 557-563

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    Weight reduction of a truck body brings about increase of load capacity and diminution of fuel consumption. The weight reduction is feasible in terms of the application of high strength steel sheet to body. Especially, the application to truck frame is important because it is heavy. The required properties of steel sheet for truck frame use are press formability and fatigue strength. However, investigations on fatigue strength are relatively few, and the value of fatigue strength is not sufficient. Therefore, the fatigue strength of high strength steel sheet for truck frame use was investigated in this study.
    The fatigue strength of steel sheet with scale is significant for truck frame use. It increases with the decrease in the surface roughness of steel sheet. The fatigue strength of sheared edge is also important. It increases with the decrease in the roughness of sheared edge. In case of the microstructure including pearlite and coarse carbides, these phases increase the roughness of sheared edge through the connection of micro-voids arising around them. Accordingly, the reduction of these phases is significant for improving the fatigue strength of sheared edge. Based on these findings, three types of the 780 MPa grade high strength hot rolled steel sheet were manufactured. From this result, it was suggested that the effect of the roughness reduction on improving the fatigue strength was stable.
  • Development of High- and Low-pressure Rotor Forgings for High Temperature Steam Turbine

    pp. 564-569

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    The 2.25CrMoV steel is applied to conventional combined high-pressure (HP) and low-pressure (LP) rotor forging, and it has excellent creep properties and toughness at 538°C conventional steam temperature. However, at 566°C steam temperature, it is required to improve the properties of the rotor forgings. The effects of chemistry on toughness and creep rupture strength have been investigated on the 9CrMoV base materials for the purpose of developing a new HP-LP rotor material for 566°C steam temperature. Thus 9.8Cr1.4Mo1.0NiVNbN steel, named HLP9F, with higher toughness and creep rupture strength compared with conventional 2.25CrMoV steel, has been developed. A trail HP-LP rotor forging with the diameter of LP section of 1800 mm and that of HP section of 1200 mm has successfully been manufactured through the VCD process from 68 t ingot. In spite of a large ingot, neither chemical composition change nor eutectic Nb (C, N) formation could be recognized. The rotor, furthermore, exhibited satisfactory quality in toughness and creep rupture strength. This rotor material is suitable to the HPLP rotor material for advanced combined cycle steam turbine.

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