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

Tetsu-to-Hagané Vol. 82 (1996), No. 3

  • Feasibility of Symbiotic Ironworks

    pp. 177-184

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    The concept of zero emission of waste material in the manufacturing process has been focussed worldwide for more efficient utilization of fossil fuels and environmental protection. In this study, background and feasibility of symbiotic ironworks based on this concept were discussed from the state-of-the-art technology in catalyst and heat storage/ transportation. One possibility is ironworks with methanol industry, in which charged coke is partially replaced by natural gas and the hydrogen-enriched blast furnace offgas(BFG) is used as raw material for methanol synthesis. The results of systematic analysis demonstrated that this system leads to not only exergy saving but also less emission of greenhouse gas(GHG). The key technology is to develop a catalyst for BFG of CO2-CO-H2 system. Another one is "urban-symbiotic ironworks", where excess outflow energy from ironworks is efficiently employed as a utility for household affairs. The use of latent heat and reaction heat is being planned in this system for thermal energy recovery and transportation. Possible energy recovery processes for the waste heat remaining in the ironmaking industry was reviewed, together with the detailed concept of 'urban-symbiotic ironworks' as an energy supplier. In particular, the use of phase change materials(PCMs)and of hydrogen storage alloy as the new technologies was fundamentally promising.
  • Bubble and Liquid Flow Behavior in a Bath Agitated by Porous Nozzle Bubbling

    pp. 185-190

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    Air was injected through a centric wooden porous nozzle into a water bath. More than 1000 bubbles in a mean diameter of approximately 3 mm were generated during one second. The radial dispersion of the bubbles in the course of rising in the bath, mean bubble diameter, and mean bubble rising velocity were determined using a camera and a high-speed video camera. The liquid motion was driven mainly by the buoyancy force of bubbles. The mean velocity components, root mean square values of turbulence components, Reynolds shear stress and so on were measured using a two-channel laser Doppler velocimeter.
    These results were compared with those for a centric single-hole bottom nozzle to clarify the effects of bubble size on the above-mentioned bubble and liquid flow characteristics. In an axial region away from the nozzle where the buoyancy force of bubbles plays an important role, the radial dispersions of bubbles for the two kinds of nozzles were the same. The axial (vertical) mean velocity and turbulence production were significantly dependent on the size of bubbles. Larger bubbles promoted the turbulence production in the vertical bubbling jet.
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  • Refining Control Model for Stainless Steel

    pp. 191-196

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    The refining control model for stainless steel was developed. At the time of developing this model, the refining index that was concerned with distributing oxygen to C and Cr was derived. The following points were clarified :
    1) The refining control model that was composed of the mass balance and the heat balance in the converter and made it possible to estimate the composition of molten steel, the amount and composition of slag, and temperature of molten steel was developed.
    2) The refining index (DOS) in which the bulk region as a reaction site in addition to the fire spot regions of top and bottom blowing was taken into consideration, was derived.
    3) This model was applied to the actual YAWATA 145t LD-OB converter and it became possible to estimate [%C], [%Cr], temperature of molten steel, and the amount and the composition of slag accurately.
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  • Improvement of Surface Quality of Continuously Cast Metals by Imposing Intermittent High Frequency Magnetic Field and Synchronizing the Field with Mold Oscillation

    pp. 197-202

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    In order to improve the surface quality of continuously cast steel, two new casting processes are developed. One is the imposition of an intermittent high frequency magnetic field from the outside of a mold and the other is a synchronizing imposition of the intermittent high frequency magnetic field with mold oscillation are developed. To distinguish the characteristics of the processes, the meniscus dynamic behavior was investigated in molten gallium as simulator of molten steel and the meniscus deformation was numerically analyzed. The surface quality was examined in the cast tin used as simulator of steel. The imposition of the high frequency magnetic field reduces the effect of mold oscillation on dynamic pressure in a flux channel between molten metal and a mold and suppresses the deformation on meniscus shape.
    The intermittent high frequency magnetic field suppresses the surface wave motion generated by mold oscillation. When the timing of the magnetic field imposition is synchronized with the period including the lowest position in the stroke of mold oscillation, the better surface quality is obtained than that synchronized with the period including the highest position. The amount of electric current required to get the same surface quality decreases in the order of the imposition of continuous high frequency magnetic field, the imposition of intermittent high frequency magnetic field and the synchronizing imposition.
  • Intensive Cooling Method of Slab Just After Exit of Mold by High Speed Water Film

    pp. 203-207

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    In order to improve cooling and supporting of slab strand just after the exit of mold in higher casting speed, a double stage mold which consits of indirect cooling zone through cupper plate and direct cooling zone by high speed water film was designed. The efficiency of the mold was tested on the experimental continuous casting machine with 0.17 mass% carbon steel. The size of slab was 240mm in width and 80mm in thicness. The solidified shell temperature of the strand was measured in casing, and then, heat transfer coefficients between the strand surface and the water film were estimated at the temperature more than 1273K.
    Conseqently, it was found that the cooling efficiency of high speed water film was much higher than that of conventional spray cooling.
    The surface of the cast slab was smooth and has no defects, even through the intensive cooling by high speed water film.
  • Characterization of Oxide Layers on SUS410Ti Stainless Steel Grown in Low Oxygen Pressures by Raman Spectroscopy

    pp. 208-213

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    Raman spectroscopy was applied to the study on the transient oxidation of the Fe-14Cr-0.4Mn-0.6Ti ferritic stainless steel in Ar gases at 1123K. It was found that in the thermal oxidation under low oxygen pressure, the minor alloying constitution of manganese and titanium resulted in the formation of thin oxide layers. During the early stage of oxidation, the double layer structure, which the outer and the inner were identified as MnCr2O4 and silicon oxide respectively, was formed on the grain of the substrate and Ti2O3 at the grain boundary. As a result of outer diffusion of titanium with reduction of Cr, Mn and Si oxides, the oxide layer produced for more than 600s showed mainly Ti2O3. In addition, the frequency shifts of Raman spectra showed that the compressive and tensile stresses existed in the oxide layers, MnCr2O4 and Ti2O3, respectively. These residual stresses in the oxide layers were released by the increase of porosity with oxidation time. It is shown that the frequency shifts of Raman spectra induced by residual stresses in the oxide layers reflect the low porosity of the oxide layers which results in the decrease of the metal dissolution in high temperature deaerated pure water at 488K.
  • Formation Mechanism of Folding in Multi-Layer Welds with Single-Bevel Groove

    pp. 214-219

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    Very small flaws are occasionally observed in the root of the penetration beads (uranami beads) of multi-layer welds with single-bevel groove. The formation mechanism of these flaws has been studied to elucidate whether the flaws are defects or not toward the increased severity of safety requirement under earth-wide enviromental issues.
    The results show that the flaw is not a weld crack but a folding to be grown up from a buckled weld zone at the root of non-symmetric throats in the first layer of penetration beads, where stresses are concentrated by the thermal contractions of weld metals after the second layers. It is demonstrated to prevent effectively the formation of foldings by the introduction of newly developed welding method.
  • Development of Synthetic Ester Based Rolling Oil for Tin Gauge Cold Rolling

    pp. 220-225

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    In case when a tin gauge sheet steel is rolled with higher speed than 6000fpm, the rolling oil is usually composed of natural fatty oils such like beef tallow or palm oil with good lubricity and high oil film strength. Since these oils have a high pour point, their mixture with iron fines forms a scum which is difficult to remove and accumulates around the mill housing. Furthermore, this scum must be a nuisance of poor working environment leading to high risk of fire.
    This paper is concerning with design and development of synthetic ester based rolling oil suitable to tin gauge cold rolling.
    Considering that the tin gauge rolling oil needs the optimal viscosity characteristics and molecular structure, a new oil was designed to have a combination of trimethyrol-propane and fatty acid mainly consisting of C12.
    As the new oil has a lower pour point, low deterioration rate and fine lubricity, the scum accumulation is reduced and the working environment is improved withtout loss of high oil film strength.
  • Corrosion Resistance of Various Materials in Molten Metal

    pp. 226-231

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    Corrosion resistance of various materials such as iron and steel, cermet, cermet-coated steel, ceramics and ceramicscoated steel was evaluated in molten zinc, molten aluminum-zinc alloy and molten aluminum at 470, 600, 700°C, respectively. These temperatures are commonly used in continuous molten hot dip-plating line.
    The results are summarized as follows :
    ( 1 ) All iron and steel samples tested were corroded in metal baths. But the corrosion amount was found to depend on sample composition and increased in the order Al > Al-Zn> Zn bath. Especially, the samples were severely corroded in the agitated Al bath.
    ( 2 ) All cermet and cermet-coated steel samples tested were corroded. Corrosion amount varied with the composition of metal bath, composition and amount of bonding metal for ceramic particles.
    ( 3 ) All ceramics and ceramics-coated steel samples tested were neither corroded nor markedly reacted with molten metals. However, vanadium carbide film was partially crazed and peeled off when tested in the agitated Al bath.
  • Formation of Retained Austenite and Effect of Retained Austenite on Elongation in Low Carbon Hot-Rolled High Strength Steels

    pp. 232-237

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    Austenite stabilization behavior and effect of retained austenite on elongation of Fe-0.2%C-1.5%Si-1.51.7%Mn hotrolled high strength steels were investigated in the mill-scale test. The results are as follows ;
    (1)A large amount of retained austenite (maximum_??_15vol%) remained in steels as-hot-rolled.
    (2)The two metallurgical processes (ferrite transformation and bainite transformation) are important for the austenite stabilization. Especially ferrite transformation plays an important role to obtain retained austenite in 0.2%C steels. The volume fraction of retained austenite is in proportion to VPF/dPF (VPF ; volume fraction of polygonal ferrite, dPF ; grain diameter of polygonal ferrite).
    (3)TS×T.El value increases with the volume fraction of retained austenite. It is to be noted that the TS×T.El values of 2500030000MPa·% were obtained, which are higher than those of conventional steels.
    (4)TS×T.El value was improved by increasing of the plastic stability of retained austenite.
  • Grain Boundary Migration and it's Mechanism of Fe-3wt%Si Solid Solution Bicrystals

    pp. 238-243

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    The migration kinetics of the (221)Σ9 coincidence and random grain boundaries in Fe-3wt%Si solid solution bicrystals was experimentally studied at temperatures between 1200 and 1625K by a capillarity technique. The driving force and the mobility for the grain boundary migration was calculated from the experimental results. On the basis of the calculations, the mechanism of grain boundary migration was discussed.
    There are two different mechanisms of the grain boundary migration for both the grain boundaries depending on the annealing temperature and the driving force. At lower temperatures, the migrating rate is small and the mobility is about 4 times larger for the (221)Σ9 grain boundary than the random grain boundary. At these temperatures, the activation energies of the grain boundary migration for both the grain boundaries are almost the same and coincide with that of the intrinsic diffusion of Si in αFe. Thus, the rate controlling process of the grain boundary migration seems to be the dragging of solute atmosphere formed along the migrating grain boundary. On the other hand, at higher temperatures, the migrating velocity is large and the mobility is greater for the random grain boundary than for the (221) Σ9 grain boundary. The activation energies of the grain boundary migration for both the grain boundaries are about 3/4 of those of the self diffusion in αFe. Therefore, the grain boundary migration at these temperatures is considered to be controlled by the grain boundary diffusion. The transition temperature between the two different mechanisms decreases with increasing driving force. This may be due to breaking away from the solute atmosphere for the migrating grain boundary.
  • A New Observation Method of σ Phase in Austenitic Stainless Steels

    pp. 244-248

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    In order to establish an appropriate process for observation of distribution of σ-phase particles in austenitic stainless steels, which enables observation of a large numbers of specimens in a short time, extraction replica methods combined with image-processing technique were explored. Obtained results are as follows:
    (I) For σ-phase in SUS316H steel, any of the following three extraction method works satisfactorily : (1) electrolytic etching by 10%acetylacetone-1%HCl-metanol followed by extraction by the same electrolyte ; (2) etching by 10%Br2 -ethanol solution followed by extraction by Villella's solution : (3) etching by Villella's solution followed by extraction by the same solution.
    (II) For σ-phase in SUS304H and SUS321H steels, the third method above is suitable.
    (III) For exclusive observation of the σ-phase particles from other coexisting M23C6, TiC and Laves phase particles, image-processing is useful.
  • Relationship between Creep Deformation Behavior and Structural Change in High Strength 12Cr Steel

    pp. 249-254

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    The creep curves of 12Cr steel tested at 873948K up to about 3000h were analized using the modified θ projection concept based on the following equation. Subsequently, the correlations between all three parameters obtained and the structural degradation were investigated.
    ε = A{1-exp (-αt)}+B{exp (αt) }-1
    where ε is the strain, t is the time, and A, B and α are the parameters determined by curve fitting.
    Both values of strain hardening parameter A and weakening parameter B increased with lowering the stress or raising the temperature, i.e., to the long term condition. The degree of their increases depended on the amount of recovery in tempered martensite occurred during creep. For the parameter α related to the recovery rate, the structural degradation brought about the lowering of stress exponent. Such behavior of all parameters corresponded with vanishing the VN finely precipitated during tempering and creep testing.
    The estimation curve for creep rupture life at 873K showed good agreement with the experimental data up to about 10 times the longest rupture data. In addition, this method could predict long term rupture lives with high accuracy compared with Larson-Miller parameter method.
  • High Temperature Properties of Thermo-mechanically Treated γ+α2 Type Titanium Aluminides

    pp. 255-260

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    The effects of microstructure morphorgy and the rolls of α2(Ti3Al) secondary phase on high temperature tensile property and creep rupture strength was investigated in structure controlled titanium aluminides. Three types of microstructure such as equi-axed, duplex and nearly lamellar were obtained by thermo-mechanical treatments performed in the α +γ phase field. As the volume fraction of α2 phase increased from Ti-50.8at%Al to Ti-48.5at%Al, with decreasing Al content tensile strength at high temperature was improved by the strength of theα2 phase itself and the stabilized lamellar structure which severely obstructed deformation. On the other hand, specimens with the stabilized lamellar structure in Ti-48.5at%Al showed little tensile elongation as little as 5% even at 1173K. The stabilized lamellar structure by α2 phase also interferes with recrystalization during deformation, leading to a significant increase in creep rupture strength.

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