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

Tetsu-to-Hagané Vol. 78 (1992), No. 10

  • Modernization of Japanese Steel Industry Based on My Experience

    pp. 1505-1511

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  • Approach to Automotive Weight Reduction

    pp. 1512-1516

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  • Mechanical Properties of Quasicrystals

    pp. 1517-1522

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  • Kinetic Analysis of Reduction of Hematite Powder to Magnetite with CO-CO2 Gas Mixtures in a Batch-type Fluidized Bed

    pp. 1523-1530

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    Hematite powders were reduced to magnetite with CO-CO2 gas mixtures in a batch-type fluidized bed under the conditions of 700900°C in temperature, 0.150.23 mm in particle size, 2.67×10-5 m3/s (STP) in gas flow rate. The reduction curves were analyzed based on the bubble assemblage model and the perfect mixing model. It was concluded from the results of analysis that the bubble assemblage model was more suitable for the analysis. In those models, the unreacted-core models for sphere or plate were used for the reduction model of single particle. The rate parameters included in the unreacted-core model were determined by using the reduction data. The value of chemical reaction rate constant did not vary with the model for sphere or plate. However, the value of intraparticle diffusion coefficient in the model for plate was smaller than ones in the model for sphere.
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  • Confinement of Molten Metal Puddle in a Twin Roll Caster by Use of an Electromagnetic Dam Combining a Solid Dam

    pp. 1531-1537

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    A combined electromagnetic dam to confine molten metal puddle in a twin roll caster is newly proposed. It is composed of the solid dam of an inside type and the electromagnetic dam which makes use of the electromagnetic force performed by the direct electric field imposed on metal along the casting direction and the direct magnetic field applied between rolls. In the solid dam, ferromagnetic material is buried in order to smoothly transfer the magnetic flux from one roll to the other through it and electrodes are attached separately along the side face of the solid dam for concentrating the electric current density in the gap between the dam and the roll. The electromagnetic force produced by the combined dam is much larger than that of only the previously proposed electromagnetic dam where both the magnetic flux and the electric current are imposed, but there is no solid dam.
    The experimental works were carried out to find how much height of molten metal is confined by the combined electromagnetic dam. It was found that the holding height of meniscus is much higher than that of the previous works of the electromagnetic dam and can be controlled by adjusting the imposed electric current. A mathematical model of the combined electromagnetic dam was developed to predict the holding height of meniscus from the given electric and magnetic fields and verified by the experimental data.
  • Effect of Chemical Composition on Apparent Viscosity of Semi-solid Metals

    pp. 1538-1545

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    Stirring experiments of Al, Cu and Fe semi-solid alloys on continuous cooling were carried out to investigate the effect of chemical composition and stirring conditions on the apparent viscosity of semi-solid metals.
    The apparent viscosity of every alloy increased with increasing solidification rate and decreasing shear rate and the critical fraction solid for fluidity increased with decreasing solidification rate and increasing shear rate. The equation for apparent viscosity ηa of semi-solid alloy is proposed as follows,
    ηaLa{1+αρmC1/3γ-4/3/2[1/ƒS-1/(0.72-βC1/3γ-1/3)] }(Pa·s)
    α=2.03×102(X/100)1/3
    β=19.0(X/100)1/3
    where ηLa is the apparent viscosity of liquid, ρm is the density of alloy, C is solidification rate, γ is shear rate and ƒS is fraction solid. The α, β values depended on chemical composition of alloys and became larger with increasing solute content X. These results were explained by the shape parameter value of suspended particles and the trapped liquid volume in suspended particles in semi-solid metals.
  • Effect of Micro-defects on Surface Brightness of Cold Rolled Stainless Steel Strip

    pp. 1546-1553

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    To produce cold rolled stainless steel strip efficiently, high speed rolling is attempted in cluster type rolling mill which has small diamater work rolls, or in tandem cold rolling mill which has large diameter work rolls.
    To attain the high speed rolling, surface brightness of the stainless steel strip must be improved.
    In this study, the effect of micro-defects on surface brightness is examined by actual and laboratoryscale rolling mills, and mechanisms of micro-pits occurring in cold rolling are discussed.
    The results obtained are as follows:
    (1) The surface brightness is strongly affected by micro surface defects. The brightness improved with decreasing micro-defects.
    (2) Micro surface defects are sorted to four types such as micro-pits occurred from surface roughness on the mother sheet, oil-pits formed in cold rolling, grooves formed by intergranular corrosion in pickling, and scratches due to surface roughness of roll.
  • Analysis of Vibration for Sendzimir Mill in Cold Rolling of Stainless Steel Strips

    pp. 1554-1560

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    Both high speed rolling and heavy reduction rollling have been attempted in order to improve rolling efficiency for stainless steel strips. On the other hand, surface quality and gauge accuracy requirements are becoming increasingly strict. Mill vibration has become an important technical problem, since it reduces rolling efficiency and is detrimental to surface quality and gauge accuracy.
    The vibration of cluster-type mills such as the 20-high Sendzimir mill is complicated due to their multi-roll structure. In order to clarify the vibration phenomenon in the Sendzimir mill, two simulation models were developed to calculate the natural frequencies of Sendzimir mill vibration, and the frequencies of mill vibration during the cold rolling of stainless steel strip were measured. Two distinct vibration modes were found to be nealy 700 Hz.
  • Corrosion Behavior of Aluminized Steel Sheet under Exhaust Gas Condensate Environment in Automotive Muffler

    pp. 1561-1568

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    The followings are main results of investigation of corrosion behavior of aluminized steel sheet under exhaust gas condensate in automotive muffler.
    (1) In neutral or weakly alkaline solution of NH4 including Cl-, NO3-, SO2-4, CO2-3, CH3COO- and HCOO- which are found in exhaust gas condensate, corrosion of aluminized low carbon steel sheet is enhanced by galvanic corrosion between coating layer and base steel at the site of coating defect.
    (2) Galvanic corrosion above is influenced substantially by the corrosion characteristic of coating layer and base steel. In the solution including Cl-, SO2-4, CH3COO- or HCOO- ion individually, corrosion of aluminized steel is accelerated by corrosion of base steel. Especially under Cl- rich condition this phenomenon is remarkable. In the solution including CH3COO- or HCOO- ion, corrosion is accelerated under lower pH corrosion condition which corresponds to ion dissociation at high temperature, but it is restrained by formation of passivation film on base steel in higher pH corrosion condition at room temperature.
  • The Effect of Chromium in Substrate of Aluminized Steel Sheet on Corrosion Behavior under Exhaust Gas Condensate in Automotive Muffler

    pp. 1569-1576

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    The following are main results of investigation about the effect of chromium of substrate on corrosion behavior of aluminized steel sheet in neutral or weakly alkaline solution of NH4+ which contains Cl-, SO2-4, CH3COO- and HCOO- which are found in exhaust gas condensate.
    (1) Corrosion resistance of aluminized steel is enhanced if the substrate contains more than 5 wt% of chromium. One reason is cathodic corrosion protection of coating layer at coating defects, and the other is improvement in corrosion resistance of base steel itself.
    (2) Improvement in corrosion resistance of base steel is due to the covering effect of Cr2O3 oxide film on steel in the ambient air and the effect of being passivated in each solution which contains (NH4)2SO4, CH3COONH4 or HCOONH4 individually. Substrate chromium steel has more stable cathodic protection than low carbon steel due to these films on the surface.
    The effect grows as chromium content increases and is remarkable especially in case of 11 wt% Cr steel.
    (3) In the synthetic condensate solution including various ions passivation is generated on the surface of 11 wt% Cr steel even in presence of substantial Cl- ion.
    Therefore aluminized steel sheet with substrate containing more than 11 wt% Cr is judged to show practically excellent corrosion performance for the reasons described in (2).
  • Reaction of Zinc Corrosion at Under-film Corrosion Tips of Coatings

    pp. 1577-1584

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    The study on corrosion reaction of zinc at under-film corrosion tips of coatings was carried out.
    It was found that the following coating corrosion reaction in wet & dry and wet environments occured.
    (1)Anodic reaction: Zn→Zn2++2e-
    (2)Cathodic reaction: 1/2O2+H2O+2e-→2OH-
    (3)Formation of simonkollite(ZnCl2·4Zn(OH)2): 5Zn2++2Cl-+8OH-→ZnCl2·4Zn(OH)2
    (4)Overall reaction: 5Zn + 5H2O+5/2O2+2Cl-→ZnCl2·4Zn(OH)2+2OH-
    On the other hand, it was made clear that the layer of ZnCl2·4Zn(OH)2 formated by the coating corrosion retains weak alkali (pH=89) environment with in-situ pH measurement. The zinc phosphate film on the ZnCl2·4Zn (OH)2 layer exists as undissolved and stable layer with EPMA analysis.
    It is considered that the ZnCl2·4Zn(OH)2 layer is the corrosion protective film with respect to the corrosion of steel substrate because steel substrate has no corrosion under the ZnCl2·4Zn(OH)2 layer.
  • Formation of Hardfacing Clad by Laser Cladding with Blown Powder

    pp. 1585-1592

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    Formation of hardfacing clad layers by laser cladding with blown powder and their wear properties have been investigated. Co base hardfacing alloys, and niobium carbides or chromium carbides mixed Co base alloys were deposited on to mild steel substrates with varying laser power and traverse speed under constant amount of powder feeding per unit bead length.
    Dilution and area of melted substrate linearly increased with heat input per unit bead length. Melting efficiency for substrate melt in the laser clad was approximately one-sixth of that in laser surface melting, since the substrate was melted indirectly by heat from melting clad layer. This is the reason why low dilution can be obtained easily in the wide range process variables. Dilution in the carbide mixed clads was lower than that in Co base alloy clad, since melting of the former may require higher heat input than that of the latter.
    In Co base alloy clads, wear resistance enhanced with increasing hardness of clad layers. In the carbide mixed clads, significantly higher wear resistance was obtained than that estimated from their matrix hardness because of high hardness of carbide particles.
  • Estimation of Long Term Isothermal Temper Embrittlement in 21/4Cr-1Mo Steel from Chemical Compositions

    pp. 1593-1600

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    Step cooling and isothermal aging tests at 425, 450 and 475°C, for 54000, 20000 and 5000 h, respectively, were performed on 21/4 Cr-1Mo steels with various amount (in mass pct.) of Si, Mn, Cu, Ni and impurities. The relationship between the step cooling-and the isothermal temper-embrittlement and chemical compositions was discussed.
    The results are summarized as follows.
    1) The shift in 50% fracture appearance transition temperature and 40ft-1b transition temperature by the step cooling treatment (ΔFATTsc, Δ40TTsc) have good correlation to a embrittling factor K2=(2Si+Mn+Cu+Ni)·X, where X=(10P+5Sb+4Sn+As)×102. Therefore, the reduction of amount of Si, Mn, Cu, Ni and impurities is effective to reduce the susceptibility to temper embrittlement.
    2) The equations, which can accurately estimate the shift in 50% fracture appearance transition temperature and 40ft-lb transition temperature by isothermal temper embrittling at T°C for th (ΔFATTiT, Δ 40TTiT) from K2 and X, were derived.
    3) Assuming that the life of peterochemical plants is 20 years at 425°C and the required toughness for forgings is 40TT≤10 or 38°C, chemical compositions of a 300 mm thich 21/4 Cr-1Mo steel forging shall be controlled as follows.
    40TT≤10°C……X≤13.0, K2≤14.3
    40TT≤38°C……X≤17.5, K2≤19.3
  • Metallurgical Factors Affecting Creep Cavitation Behavior and Rupture Ductility of Cr-Mo-V Steel Forging

    pp. 1601-1608

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    In order to clarify metallurgical factors affecting creep rupture ductility of Cr-Mo-V steel, creep tests for the specimens with actual rotor materials differing in quenching temperature and also in impurity content were carried out at 550°C-210 MPa and 575°C-160 MPa. Creep properties were compared and also nucleation-growth behavior of creep cavities during creep and rupture ductility were investigated. The obtained results are as follows:
    (1) The high-purity materials have the best rupture ductility among as-heat-treated materials quenched at 940-970°C and show rupture elongation higher than 10%. Other low-purity materials show rupture elongation lower than 10%. The decrease in ductility corresponds to the increase in impurity content.
    (2) The rupture ductility of the re-heat-treated rotor materilas quenched at 1025°C decreases greatly to less than half the value of as-heat-treated materials in (1). The decrease in ductility corresponds to grain growth of prior-austenite grains.
    (3) The metallurgical factors that affect rupture ductility remarkably are (a) the pre-austenite grain size corresponding to rotor quenching temperature, and (b) the amounts of impurity, and the smaller (a) is and the lower (b) is, the higher is the rupture ductility.
    (4) Consequently a parameter is proposed so that rupture ductility of Cr-Mo-V steel can be obtained, combining both the above-mentioned (a) and (b) factors. This creep ductility parameter, defined as the Y-factor, is expressed as:
    Y=0.34[P]+3.8[S]+8.4[Sn]+12.0[As]+5.6[Sb]+2.6G.S.
    where [P], [S], [Sn], [As], and [Sb] are the contents of respective elements (wt.%), and G.S. is grain size (mm).
    Rotor materials of Y>0.5 show notch-sensitive behavior, and so the Y-factor is effective in judging the notch-sensitive/notch-insensitive property.
  • Effect of Ni and B on the Hot Workability of High Mo Austenitic Stainless Steels

    pp. 1609-1616

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    Effects of Ni content and B addition on the hot workability of austenitic stainless steels containing 4.56.2%Mo for sea water applications have been investigated by hot tensile tests and physical analyses of fractured surfaces. Optimum Ni contents were 22% for 25Cr-4.5Mo steel and 17% for 20Cr-6.2Mo steel respectively, since 34% δ ferrite remaining in the solidification structure was effective to improve the ductility at temperatures between 1000°C and 1150°C. The addition of 0.0020.004% B was also effective in the improvement of hot workability of the steels containing δ ferrite less than 4%. It was clarified by Auger Electron Spectrometer and Ion Micro Analyser that the hot workability was greatly affected by the degree of S segregation at grain boundaries, which could be decreased by the existence of small amount of δ ferrite and the segregation of B at grain boundaries.
  • A Concept of High Temperature Alloy Design Based on Creep Curve Data Analysis

    pp. 1617-1623

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    Creep curve analysis based on the modified θ concept provides three material constants, α, A and B, which can specify the creep process of alloys quantitatively: α is the rate constant of deformation, A is small when strain hardening is significant, and B is large when microstructural degradation is substantial. In order to utilize these material constants for high temperature alloy design, the following problems must be solved: the correlation between the material constants and the important creep properties (minimum creep rate εm and rupture life tr), and the method to control the material constants.
    The creep properties are related to the material constants by the following simple equations:
    εm=2α√AB, tr=[ln(εr/B)]/α
    These equations contain only one more parameter (rupture strain εr) in addition to the three material constants. This simplicity is of practical benefit in alloy design on the basis of the modified θ concept. The material constants can be controlled by several metallurgical means. For example, reduction of diffusivity in matrix and precipitation of second phase lower the rate constant α, resulting in the improvement of both εm, and tr. Coarsening of grains in matrix and stabilization of precipitates reduce the weakening constant B, and consequently improve mainly εm. Though the present knowledge of the material constants control is still limited, further creep curve analysis on various other materials will enable us to construct a comprehensive alloy design system.

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