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

Tetsu-to-Hagané Vol. 96 (2010), No. 6

  • Numerical Study on Metal/Slag Drainage Rate Deviation during Blast Furnace Tapping

    pp. 353-362

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    DOI:10.2355/tetsutohagane.96.353

    Remarkable deviation of metal fraction in the liquid drained out of blast furnace tap hole has been occasionally observed between the operated tap holes and/or tapping time stages. Introducing the concept of low permeability zone whose wall, due to the difference in two liquid phases' viscosity and/or wettabilitiy to coke particle, allows for metal to permeate freely but for slag not to permeate, the liquid drainage behaviors are examined by furnace hearth mathematical model simulations.
    The deviation of metal fraction between two operated tap holes is materialized under the hypothesis that furnace heath is divided into two sections by planar vertical low permeability wall (VLPW). While, the variation of time series change in liquid metal fraction during tapping operation is reproduced by hypothesizing the formation of cylindrical low permeability wall (CLPW) which concentrically parts the furnace hearth into center and peripheral area.
    Since the results of calculation for VLPW or CLPW formation indicate the notable raise of liquid level in furnace which could influence on abrupt increase of blowing pressure, the effectiveness of several operational optimization is assessed, resulting in suggestive conclusion that increasing the initial tap hole diameter is the most effective.
  • Diffusion Simulation of Liquid-phase Formation between High-carbon Iron-based Filler and Steel

    pp. 363-370

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    DOI:10.2355/tetsutohagane.96.363

    A plane-to-plane bonding by the rapid reaction with liquid phase enables a simplified forming of hollow or complex steel parts. The time for disappearance of transient liquid is absolutely short if a high-carbon filler material controls the Fe–C eutectic liquid at bonding interfaces. Taking advantage of multi-component diffusion simulation, this paper aims to investigate how the behavior of eutectic liquid depends on the constituent in ferrous high-carbon (FHC) foil. Due to the rapid diffusion of carbon in austenite, the FHC foils with dispersed cementite or graphite failed to maintain a prescribed C composition and was predicted to yield a liquid only in the midst of foil. The monophase cementite (θ) foil, on the other hand, resulted in a liquid layer thicker than the initial foil thickness, and that ensures a dissolution of alloying elements in liquid from joined steels.
    It was possible to synthesize the monophase θ filler by substituting a part of Fe with Cr. Using the Cr-substituted θ filler, microstructures have been analyzed in the quenched samples during bonding of 0.21C–0.8Mn–0.25Si–0.22Cr steels at 1453K. Although the lowest temperature at which a liquid phase was observed was far lower than the simulation, Mn and Si dissolved in liquid layer have been confirmed. The liquid completely disappeared within 180 s and however, the segregation of Cr would remain severe.
  • Effect of Preadsorption of Polyethylene Glycol on the Appearance and Morphology of Electrogalvanized Steel Sheets

    pp. 371-377

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    DOI:10.2355/tetsutohagane.96.371

    Zn electrodeposition was performed galvanostatically on a steel sheet at 1500 A/m2 in an unagitated sulfate solution at 40°C to investigate the effect of preadsorption of polyethylene glycol (PEG) on the lightness and morphology of Zn. The lightness of deposited Zn was increased by the preadsorption at coating masses of 10–80 g/m2, except in the initial stage of the deposition. The overpotential for Zn deposition was increased by the preadsorption, and its polarization effect was maintained even when Zn deposition proceeded. The platelet crystals of Zn were smaller and more random in growth direction when Zn deposited on the cathode preadsorbed by PEG. The decrease in size of platelet crystals of Zn with the preadsorption is attributed to both the increase in overpotential for Zn deposition and the decrease in epitaxial growth of Zn. The orientation of {0001} Zn basal plane decreased because of an increase in overpotential for Zn deposition by the preadsorption of PEG. The lightness of deposited Zn increased in spite of a decrease in orientation of the {0001} plane with the preadsorption. This indicates that the lightness of deposited Zn was more easily affected by the decease in size of Zn platelet crystals than by the crystal orientation of Zn.
  • Effect of Quenching Rate on Hardness and Microstructure of Hot-Stamped Steel

    pp. 378-385

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    DOI:10.2355/tetsutohagane.96.378

    The effect of cooling rate on hardness and microstructure of the hot-stamped boron steel with 0.2 mass% carbon was investigated. After sheet specimen with a thickness of 1.6 mm or 1.2 mm was heated up to 900°C for 4 min, it was press formed and simultaneously quench hardened with dies, or water-quenched. Simulated hot-stamping test was also carried out at various cooling rates. The Vickers hardness of quenched specimens was measured. The microstructure on the cross-section of quenched specimens was observed with optical microscope and transmission electron microscope. The microstrucure of hot-stamped specimen was composed of auto-tempered-martensite and was softer than water-quenched specimen which consisted of lath-martensite. Tempered martensite was distinguished from bainite by observation of cementite precipitation morphology. Cooling rate below the Ms point affects hardness significantly, even if cooling rate is higher than the upper critical cooling rate. Decrease in hardness caused by auto-tempering was formulated with the tempering parameter in which was taken account of integration of tempering effect.
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  • The Effect of Ti Addition on Phase Equilibria among Ni (A1), Ni3Al (L12) and Ni3V (D022) Phases

    pp. 386-391

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    DOI:10.2355/tetsutohagane.96.386

    The effect of Ti addition on phase equilibria among Ni (A1), Ni3Al (L12) and Ni3V (D022) phases at 950°C was investigated by means of TEM/EDS analysis on heat-treated alloys. The three-phase coexisting region of A1+L12+D022 was found to exist around the composition of Ni–4Al–19V (at.%) in the Ni–Al–V ternary system. By addition of Ti into the ternary system the three-phase coexisting region was shifted to the Ni rich side. Ti partitioned most into the L12 phase and least into the A1 phase. These results suggest that the addition of Ti stabilizes the L12 phase against D022 and A1 phases.
  • Effects of Si and Cr on Bainite Microstructure of Medium Carbon Steels

    pp. 392-399

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    DOI:10.2355/tetsutohagane.96.392

    Effects of Si and Cr contents on bainite microstructure of medium carbon steels have been examined in order to obtain further information for good strength–ductility balanced spring steel. Four practical medium carbon steels, S55C, SUP9, SUP7 and SUP12 were isothermally transformed at temperatures between 300°C and 500°C after austenitized at 1000°C. The microstructure was observed by means of scanning electron microscopy and transition electron microscopy. While carbide was found rapidly with bainite transformation in S55C without Si and Cr, carbide precipitation was suppressed by the increase of Si and Cr concentration in SUP9, SUP7 and SUP12. The fraction of retained austenite was increased with increasing of Si and Cr at the intermediate stage of bainite transformation because of the increase of C concentration in retained austenite. Particularly, Si addition promoted carbide-free bainitic ferrite and leading to larger amounts of retained austenite by carbon enrichment.
  • The Effect of Over-Carburizing on the Fatigue Strength at Edges of Vacuum-Carburized Samples

    pp. 400-405

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    DOI:10.2355/tetsutohagane.96.400

    The objective of this reseach is to study the effect of over-carburizing on the strength at edges of vacuum-carburized (VC) samples. The relationship between the edge angles and fatigue strength was investigated using developed bending tests in which stress maximized at the top of edges. It was found that the strength of atmosphere carburized (AC) samples rarely depends on edge angles. The fatigue strength of VC samples with edge angle near 180° is higher than that of AC ones. The fatigue strength of VC samples decreases sharply with decreasing edge angle and becomes lower than that of AC samples for the edge angles smaller than 90°, because of the over-carburizing at the edge caused by the overlapping of the carbon diffusion field. In order to improve fatigue strength of VC samples with sharp edges it is suggested that VC conditions are changed so that carbon content on planar surface and edges is lower thus lowering the degree of over-carburization.
  • Effect of Tempering Temperature on Stretch-flangeability of Maltensitic Steels

    pp. 406-413

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    DOI:10.2355/tetsutohagane.96.406

    The effect of microstructures on the stretch-flangeability has been investigated using high-strength martensitic steels tempered at various temperatures. For the index of the stretch-flangeability, hole-expanding ratio λ was measured using plate specimens with a hole introduced by a punching process. Tensile tests were also carried out using the same tempered specimens. λ increased with increasing tempering temperature, and took the maximum value at 773K, and then decreased in the specimens tempered at temperatures higher than 773K. On the other hand, in the tensile tests, total elongations or local elongations increase with increasing tempering temperature up to 973K, and decreased in the specimen tempered at 1073K where typical dual phase microstructure was observed. Microstructural observations around the holes formed by punching process suggest that the maximum value of λ is determined by the balance between the recovery enhancing ductility and the growth of cementite size causing void formation during the punching process. It is to be noted that grain refinement due to severe plastic deformation during punching process was observed around the holes, which might contribute to enhancing the stretch-flangeability.
  • Influence of Microstructure on Yield Strength of Ferrite–Pearlite Steels

    pp. 414-423

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    DOI:10.2355/tetsutohagane.96.414

    The mechanical properties of two-phase steels are mainly controlled with the volume fraction of the harder phase. In the previous reports, however, some different relationships between the mechanical properties and pearlite volume fraction have been demonstrated for ferrite–pearlite steels. This means that the relationship must be changed sensitively to experimental conditions so that the influence of the volume fraction of pearlite on the mechanical properties of hot-rolled ferrite–pearlite steels were experimentally investigated using Fe–C alloy with variation of carbon content.
    Yield strength of the air-cooled samples after hot-rolling gradually increased with the increase of pearlite volume in lower volume fraction range while dramatically in higher range. The water-cooled samples exhibited higher yield strength than that of the air-cooled samples especially in the lower pearlite volume fractions. These results indicated that the mechanical properties of the ferrite–pearlite steels having the lower pearlite fraction are determined by properties in ferrite area. This is attributed to sparse distribution of pearlite; almost all ferrite grains are surrounded by the other ferrite grains.
    The yield strength of ferritic steels consisted of grain refinement strengthening, precipitation strengthening by carbides and solid-solution strengthening of carbon. The solid-solution strengthening varied with change in the amount of carbon in solution and grain refinement strengthening also raised with increase in Hall–Petch coefficient by grain boundary segregation of carbon during aging.
    These results confirmed that the aging condition of ferrite is important for alloy designing of ferrite–pearlite steels.

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