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

Tetsu-to-Hagané Vol. 84 (1998), No. 5

  • Development of Tensile Test Method for High-density Coal under Carbonization

    pp. 321-326

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    A new tensile test method under carbonization for high-density coal that would be used in coke making process in next century was developed to estimate the appearance mechanism of coke strength. This method can clarify the appearance mechanism of coke strength under carbonization. We obtained the result that the tensile strength and Young's modulus become large in proportion to the final temperature. In case of large heating rate, the tensile strength and Young's modulus were large. From the microscope observation, coal particles agglomerated to each other among the softening temperature and the strength increased with an increase of heating rate. We also estimated factors related with the appearance mechanism of the strength using a coal pyrolytic model, which was previously developed by authors. The results showed that the large strength is caused by the progress of coke graphitization and an increase of internal tar contributing to sticking on each particle.
  • Kinetic Analysis on Initial Reduction Stage of Wustite Containing CaO with CO-CO2-Ar Gas Mixture

    pp. 327-332

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    Dense wustite plates containing CaO have been reduced at 1273K with CO-CO2-Ar gas mixtures which have a chemical potential to form metallic iron. In the case that the samples were reduced with low Pco and high α(=PCO2/Pco) gas mixtures, only the oxygen content of the samples decreased and metallic iron was not formed in the early stage of the reduction. A mixed chemical reaction/solid state diffusion limited model was applied to this stage. The reduction rate r was expressed by r= k'·(1-α/Ks)· Pco (kg-oxygen/m2·s),
    where k' and Ks are the apparent chemical reaction rate constant and the PCO2/Pco ratio in the reducing gas mixture in equilibrium with the surface wustite, respectively. The k' and inter-diffusion constant D were estimated by comparison of the iron ion concentration profile in the sample obtained from the lattice parameter measurement with that calculated from the model. The k' increased markedly with CaO content and was dependent on Ks as expressed by k'= A·Ksn (kg-oxygen/m2·s·Pa),
    where A and n are CaO-dependent constants. The D was in the range of 2.86.4×10-10m2/s. The mass change of the sample during the reduction was explained reasonably by the model. Remarkable increase in the chemical reaction rate shortened the reduction time up to the metallic iron formation.
  • Effect of Stirring and Slag Condition on Reoxidation on Molten Steel

    pp. 333-338

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    The kinetics of reoxidation mechanism with slag in Al killed steel were investigated in laboratory experiments at 1873K in MgO crucibles with 10kg steel. The CaO-SiO2-Al2O3-FeO-MnO slags were used to study the effect of (FeO+MnO), slag volume, Ws and stirring gas flow rate, Q on [sol.Al] reoxidation rate. Experimental results were evaluated by a coupled reaction model.
    Apparent rate constants of [sol.Al], αA1 increased with increase of (FeO+MnO) and Ws. Mass transfer coefficients in metal, slag and reaction rate coefficient for reduction of SiO2 were estimated to be 2.5×10-6-6.0×10-6(m/s), 1.0×10-5-2.5×10-5 (m/s) and 3.2×10-3 (kg/m2·s), respectively.
    Mass transfer coefficients in metal, km obtained by parameter fitting method under the various conditions of Q showed kmU1.5, where U is mean surface velocity calculated using two dimensional flow analysis. This relationship was also obtained from turbulent flow model under the assumption that friction velocity has linearity with surface velocity.
    Calculated [sol.Al] in 250t ladle showed a good agreement with observed one during killing before casting. Calculated results showed that [sol.Al] in the ladle rapidly decreased in the last period during casting because slag volume ratio to molten steel increased during casting.
  • Effect of Ni Precoating and Organic Additives on the Morphology of Zinc Electrodeposit

    pp. 339-344

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    The effect of Ni precoating and organic additives on the morphology of zinc electrodeposit was investigated by using the polycrystal commercial low carbon Al-killed steel sheet as a base material and the sulfate bath under the Ni precoating weight of 10-200mg/m2 and adding organic compound of 5-1000ppm to bath. (1) Without reference to plating conditions, zinc electrodeposits grow heteroepitaxially at preferred orientations of (0002), (1013) and (0004) Zn of which angle between Zn basal plane and the sheet surface plane is less than 45°, zinc electrodeposits grow randomly at preferred orientations' of (1012), (1011) (1010) and (1120) Zn of which angle between Zn basal plane and the sheet surface plane is larger than 45°.(2) When surface condition of sheet is changed by Ni precoating, fine zinc electrodeposits grow randomly from initial stage of plating, so that surface of zinc electrodeposits is smooth. (3) When organic compound is added to bath, the morphology of zinc electrodeposit changes according to the amount of additive.
    When the amount of additive is small, the morphology of zinc is same as that under Ni precoating condition. When the amount of additive is large, Zn basal plane grows vertically to the sheet surface plane and surface of zinc electrodeposit is rough because of high overvoltage of electrodeposit.
  • Effect of Surface Oxide Structure on the Descalability of Cold-rolled and Annealed SUS304

    pp. 345-350

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    In order to obtain good descalability of cold rolled and annealed Type 304 stainless steels, oxidation behavior and scale structure of the steels were investigated. The oxide scale consisted of four layers. The outer first layer was composed of Fe2SiO4 and Fe3O4, the second layer was (Fe, Mn) Cr2O4, the third layer was Cr2O3, and the fourth layer adjacent to substrate was amorphous oxide, SiO2. The third layer became thick and the Si content in the fourth layer increased with the rise of annealing temperature. The thick third layer provided a large Cr depletion zone at the surface layer of substrate. Thus, annealing at higher than 1125°C to develop the Cr depletion zone is effective to obtain good descalability in Type 304 stainless steels.
  • Peculiar Microstructure Formed by Rolling Contact Fatigue in Bearing Steels

    pp. 351-356

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    A peculiar microstructure formed in a case-hardened steel has been examined by transmission electron microscopy. The microstructure is called a bright etched region (BER) because it appears bright when etched with Nital. BER consists mainly of very fine grains, the average size of which is a few tens nanometers. An amorphous region is observed near a crack which is formed in BER. The observation indicates that, during rolling contact fatigue, a coarse grain of martensite is transformed into polycrystals with very fine grains, and then into an amorphous region, from which a crack initiates.
  • Room Temperature Recrystallization and Ultra Fine Grain Refinement of an SUS316L Stainless Steel by High Strain Powder Metallurgy Process

    pp. 357-362

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    The high strain powder metallurgy (HS-PM) process, which is a novel and the most efficient non-equilibrium powder metallurgy process, is applied to an SUS 316L austenitic stainless steel. The HS-PM process is a powder metallurgy process combining mechanical milling, heat treatment and sintering processes, and enables one to produce an ultra-fine grain structure. In the case of the SUS 316L stainless steel, room temperature recrystallization and recovery of an austenite phase take place because of the increased high angle grain boundary area and the existence of excess vacancies, which are stored during the milling process. Very fine ferrite grains are formed in the early stage of the milling and an ultra fine (α+γ) microduplex structure is formed at the end. In the case of higher energy milling, almost fully ferritic nanograin structure with an average grain size of approximately 20nm is formed. The ultra fine (α+γ) microduplex structure in the HS-PM processed powder accelerates precipitation of σ phase in the sintering process. The sintered compacts with a very fine (γ+σ) microduplex structure show an extremely high strength, i.e., more than three times higher 0.2% proof stress than the annealed parent material, without any severe depression in the elongation.
  • Effects of the Amount of Retained Austenite on the Microstructures and Mechanical Properties of a Precipitation Hardening Martensitic Stainless Steel

    pp. 363-368

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    The influences of the amount of retained austenite on the microstructure and the mechanical properties were investigated for the martensitic stainless steel whose chemical composition was Fe-1.8Cu-15.9Cr-7.3Ni-1.2Mo-0.08Nb-low C, N (wt%).
    The main results obtained are as follows ; (1) Microstructures of all specimens after aging (753K ×14.4ks) are lath martensitic with film like retained austenite dispersed among the lathes. The reverted austenite phase dose not appear. The ε-Cu phase which contributes to the precipitation hardening can not be observed clearly. (2) The mechanical properties are linearly approximated to the amount of retained austenite, so that the following equations are obtained.: 0.2%Y.S. (MPa)=1192.3-13.6×γ%, T.S. (MPa) = 1250.1-9.3×γ%, E.L. (%) =12.16 + 0.43×γ%, R.A. (%) = 64.25+0.14×γ%, A.E. (J) = 72.5 + 0.8×γ% (3) The balance between the tensile strength and ductility is improved with increment of the amount of austenite. It is in the same level as the microduplex stainless steel and the low carbon martensitic stainless steel with the excellent ductility and toughness, although inferior to the work hardening austenitic stainless steel.
  • Improvements of Room Temperature Tensile Properties in Cast TiAl-Fe-V-B Alloy by Microstructural Control

    pp. 369-374

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    A gamma TiAl alloy, Ti-46.7Al-1.3Fe-1.1V-0.35B (mol%) was developed as a casting material for turbine components, replacing superalloys. This alloy is characterized by the addition of Fe, V and B which improve castability. Analysis of X-ray diffraction patterns revealed the presence of the γ phase, the α2 phase and the β phase. The β phase exists inherent in these alloying additions. This work has focused on the optimization of heat treatment conditions and metallurgical features to improve the room temperature tensile properties though understanding of the β phase in this alloy. On the basis of these experimental results, Fe and oxygen had a significant effect on the presence of the β phase. It was found that Fe works as strong β stabilizers and causes a peak in amount of the β phase at approximately 1250°C. We discussed the homogenizing conditions, followed the HIP treatment, to decrease the amount of the β phase. As a result, a heat treatment process has been developed to produce TiAl-Fe-V-B alloy with the duplex structure, which lead to a tensile strength of 550MPa and elongation of 1.8% at room temperature.
  • Growth Kinetics of Intergranular Allotriomorph Ferrite in V-containing Steel

    pp. 375-380

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    Growth kinetics of intergranular ferrite in a V-N steel has been investigated from the measurement of isothermal transformation kinetics. When compared with a V-C steel, the start of intergranular ferrite transformation starts earlier on isothermal transformation in the V-N steel, whereas its growth is retarded. The growth rates of the intergranular ferrite are under the parabolic rate rule for both steels with a smaller rate constant for the V-N steel. When the rate constants are ascribed to the supersaturation of carbon at the ferrite/austenite interface, the carbon concentrations are estimated to be 0.29 and 0.35 mass% for the V-N and V-C steels, respectively. Johnson-Mehl-Avrami plot of the transformation kinetics indicates that the ferrite nucleation rate is larger for the V-N steel. It is discussed that the precipitation of intragranular ferrite in the V-N steel is facilitated owing to the retardation of the intergranular ferrite growth in the steel.
  • Effects of Aging Temperature on the Microstructures and Mechanical Properties of a Precipitation Hardening Martensitic Stainless Steel Containing Retained Austenite

    pp. 381-386

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    The influences of aging temperature on the microstructure formation and the mechanical properties were experimentally investigated for the Fe-1.8Cu-15.9Cr-7.3Ni-1.2Mo-0.08Nb-low C, N steel which contained about 10% retained austenite after solution treatment.
    The main results obtained are as follows; The hardness change with aging is given from the balance of the precipitation hardening due to ε-Cu phase and the softening due to the formation of reverted austenite. The excellent balances among the strength/ductility and the strength/toughness are obtained for the specimens aged at 813K and 853K for 14.4ks., where the lamella structure consisting of the martensite and the reverted austenite phases are formed. The process of inverse transformation of austenite is mainly controlled by diffusion of Ni-atoms, so that the Ni-atoms are concentrated to the reverted austenite phase in twice of the alloy composition. The ε-Cu precipitates which contributes to the precipitation hardening hardly coarsen even at high temperature, for an instance 10nm in diameter for the specimen aged at 813K for 14.4ks.
  • Influence of Cementite Particle Size Distribution on Machinability and Tool Life of High Carbon Cr-bearing Steels

    pp. 387-392

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    A study has been made on the relation between the distribution of cementite (Fe3C) and the wear of tool in high carbon chromium bearing steels. Two kinds of materials with different state of cementite were prepared by utilizing the thermo-calc program. The particle size and content of cementite in these materials were controlled by heat-treatment. In the first stage, machinability was estimated by using the materials of three kinds with a different particle diameter of cementite. In the second of the experiment, the influence of interparticle distance of cementite on the tool life was evaluated using the cutting tool of TiC and SiC. The amounts of tool wear were increased with increasing particle size and the areal ratio of cemented carbide. The amounts of tool wear were also increased as the interparticle distance became short. The tool life of TiC tool was longer than that of SiC tool in cutting the same chemical composition of materials. Surface roughness (Rmax) of the material was influenced by the particle diameter and interparticle distance of cementite. An experimental equation was proposed to estimated the tool life from interparticle distance of the precipitated particle.
  • Selection of Test Sands for Abrasive Wear of High-chromium Cast Irons

    pp. 393-398

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    Dry sand/rubber wheel tests are used for low-stress abrasive wear. Test sands have to be selected to simulate an actual abrasive. But, hardness, roundness and size of sands have usually been a major concern. The present paper, restricting to abrasive wear in coal pulverizing mill, describes an experimental study on the selection of test sands based on similarity in abrasivity against white cast irons. Single-scratch tests were performed using a rubber wheel tester. The width to depth ratio of a groove, groove area and load on a single grain were measured or calculated for five kinds of martensitic white cast irons scratched by various sands. They were compared with those by hard particles extracted from coal. Two commercially available sands were selected, having similar abrasivity with larger and smaller sands extracted from coal. The characteristics of grooves such as the groove area were correlated with the wear rate in conventional sand/rubber wheel tests. Demonstrating strong dependence of the wear resistance on the size of sands, it was concluded that both two selected sands have to be used for rubber wheel wear tests as far as abrasive wear for coal pulverizing mill is concerned.

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