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

Tetsu-to-Hagané Vol. 84 (1998), No. 8

  • Control of the Particle Size Distribution of Coke Breakage Product by Means of Coal Crushing

    pp. 541-546

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    Coke samples were produced by using of electrical coke test oven (300 kg charge). Packing density was varied (Series 1). And Crushing weight ratio (<3 mm) of A coal which had high inertinite content ratio was varied, and packing density of blended coal containing A coal was varied in those tests (Series 2). Produced coke samples were investigated by a drum tester according to the JIS method. Size distributions of coke breakage products smaller than 15 mm sieve size were investigated after the drum test. The results are summarized as follows.
    (1) There was a close correlation between weight of coke breakage products smaller than 1 mm sieve size and packing density of charged coal.
    (2) There was a close correlation between weight of coke breakage products (115 mm sieve size) and weight ratio (<3 mm) of crushed A coal in blended coals.
    These results show that the primary factor which controls the weight of coke breakage products is different in each breakage particle size fraction. Based on these results, size distribution of coke breakage products generated by mechanical impact can be controlled by the conditions of crushing operation of coals.
  • The Effect of the Carbon Content on the Latent Heat of Phase Transformation of Iron and Steel

    pp. 547-552

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    The latent heat and the temperature of phase transformations in iron and hypo-eutectoid steel have been measured by using differential scanning calorimetry. For hypo-eutectoid carbon steel (0.10%0.69%), the shapes of the latent heat in heating and cooling conditions approximately agree qualitatively with the carbon content of steel. The major increasing of the latent heat occurred in the carbon content between 0.17% C and 0.26% C. At the higher carbon contents, the latent heat is almost saturated, The latent heat of A3 transformation of iron, at heating process is measured to be 16 kJ/kg and at cooling, it is 19 kJ/kg. Relation between the temperature of phase transformation and the carbon content of steel is examined and make it clear.
  • Constitutive Equation Describing Tertiary Creep of a Modified 9Cr-1Mo Steel

    pp. 553-558

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    Creep tests has been conducted of a Mod.9C-1Mo steel to describe the creep behavior on a basis of the omega method. The test conditions were in the range from 107.9 to 196.1 MPa and from 873 to 923 K. The stress and temperature dependence of the Ω value was expressed in the form:
    Ω=AΩ·σο-nΩ·exp(QΩ/RT),
    where AΩis the constant independent of temperature and stress, σο is the initial stress, nΩ is the stress exponent for Ω, QΩis the parameter describing the temperature dependence of Ω, respectively. The magnitudes of AΩ, nΩ, QΩare 0.15/s, 1.46 and 97.8 kJ/mol, respectively.
    The studies on the effect of prestrain showed that the magnitude of Ω was substantially independent of the amount of prestrain. The measured creep life was compared with the predicted one based on the omega method. It was revealed that the calculated creep life was longer than the measured one by a factor of 1.5 to 3.1 with stress and temperature. This discrepancy was ascribed to the fact that the fraction of primary creep was relatively large in the whole creep life of a Mod.9Cr-1 Mo steel. However, the calculated creep life coincides with the measured one when the specimens were prestrained up to 4%. This was due to the decrease of the fraction of primary creep with increasing strain.
  • Fatigue Strength of TRIP-aided Bainitic Sheet Steels

    pp. 559-565

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    Fatigue strength and crack initiation-propagation behavior of TRIP-aided bainitic sheet steels which are associated with the transformationinduced plasticity (TRIP) of retained austenite were investigated for the automotive applications. The steels composing of bainitic ferrite lath matrix and the retained austenite films completed the highest fatigue limit of several high-strength dual-phase type and bainitic type of steels. The threshold value of the stress intensity factor range (ΔKth) was lower than those of the other dual-phase type of steels, although the crack propagation rate at a high stress intensity factor range agreed well with those of the other steels. It was concluded that the retained austenite films suppressed a micro-crack initiation and propagation in the matrix and a long crack propagation due to "stress relaxation" and "block effect" resulting from the strain-induced transformation, because the mean interfilm space of retained austenite is smaller than a plastic zone of crack tip.
  • Discussion of the Solid Solution Hardening Mechanism of Phosphorus in Steel

    pp. 566-572

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    Phosphorus (P), which is widely used as solid solution hardening element in ultra-low carbon interstitial free (IF) steel, often exhibits unpreferable increment of yield strength. In this study, the influence of phosphorus on yielding characteristics in IF steel by focusing the segregation behavior of P to grain boundary. Higher P addition shows normally higher yield elongation compared to its lower addition, leading to relatively high yield strength when the steel is subjected to conventional recrystallization annealing. On the other hand, when additional heat treatment at 700°C is employed, the yield strength of the high P added steel can be significantly reduced. This corresponds to the reduction of the k-value of the Hall-Petch equation. This phenomena is well related to the segregation characteristics of P at grain boundary. Segregated P may induce to a sort of structural change of the grain boundary of steel, resulting in lowering the yield strength. The equilibrium estimation of the segregated P content by considering the grain diameter can lead to considerably good agreement with the experimental data obtained in this study.
  • Relationship between Creep Rupture Properties and Microstructures of Type 316 Stainless Steels with Different Carbon and Nitrogen Contents

    pp. 573-579

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    It is quite important to understand the relationship between creep rupture properties and microstructures for estimating of long term creep behaviour of fast reactor structural materials. For this purpose, the microstructural changes of a newly developed low carbon medium nitrogen Type 316(316FR) stainless steel has been investigated comparing with medium carbon low nitrogen Type 316(SUS316) steel. Creep rupture tests were conducted at 550°C and 600°C up to 40000 h. The microstructures of specimens ruptured and also those aged were observed with electron microscope, and solute concentrations were assessed by analyzing extract residues of aged specimens. 316FR has higher rupture strength and ductility than those of SUS316 for long time creep. Laves phase precipitates mainly on the grain boundaries at 550 and 600°C and also in matrix after 10000 h creep at 600°C for 316FR. The Laves phase on the grain boundaries and in matrix does not decrease rupture ductility but that in matrix may result in loss of rupture strength due to solute molybdenum depletion. Solid solution hardening by nitrogen is effective for a long period of time, because of extremely small amount of nitrides. On the other hand, large amount of carbides precipitate on the grain boundaries and in matrix in SUS316. They cause loss of the rupture strength due to the decreasing in solute carbon content. Precipitation of carbides on the grain boundaries results in the decrease in ductility because of the grain boundary embrittlement.
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  • Residual Creep Life Assessment by Change of Martensitic Lath Structure in Modified 9Cr-1Mo Steels

    pp. 580-585

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    Mod.9Cr-1 Mo steel has a martensitic lath structure. Recovery of the lath structure takes place in the course of creep. Microstructural degradation due to the recovery results in the acceleration of creep rate and the subsequent failure of a specimen. Change of lath width during creep of the steel was quantitatively investigated to propose a residual life assessment methodology based on the recovery process.
    Since the steel was tempered at 1053 K, the lath structure is thermally stable at the testing temperatures (848 K923 K). However, recovery of lath structure readily takes place during creep, indicating that the recovery is induced by creep deformation. Lath width d increases with creep strain and saturates to a value ds determined by creep stress. The increase of d is faster at a higher stress and temperature. A normalized change in lath width, Δdds, was introduced to explain the variation of lath growth rate with creep stress and temperature. Δd is the change in lath width from the initial value d0, and Δds is the difference between ds and d0. Δd/ Δds is uniquely related to creep strain ε and the relationship is independent of creep stress as well as creep temperature. This Δdds-ε relationship obtained by an accelerated creep test at a higher temperature or stress is applicable to any creep condition including service conditions of engineering plants.
    Creep strain can be evaluated from the measurement of Δdds based on the Δdds-ε relationship. A creep curve under any creep condition can readily be calculated by creep data of the steel. Combining these information one can assess residual life of a structural component made of the steel.
  • Development of High Modulus Steel Based on Thermodynamic Phase Equilibrium between TiB2 and Ferrite

    pp. 586-591

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    A new TiB2-reinforced steel with an enormously high Young's modulus has been developed for enabling a leaner design of automobile parts. This article describes the high modulus steel (HMS) with an emphasis on its microstructure and potential elastic properties based on the assessed phase equilibria in Fe-Cr-Ti-B system.
    The concept was to increase the isotropic Young's modulus by the optimum combination of reinforcing particles and steel matrices, and thermodynamic considerations strongly supported the most effective contribution of TiB2 to this purpose.
    Calculated phase diagrams located the narrow two-phase region of ferrite (α)+ TiB2 along the pseudo-binary system of (Fe-17Cr)-TiB2 in a wide temperature range. The proved phase stability and the small solubility of iron/chromium in TiB2 were considered most responsible for maintaining its own high modulus of 540 GPa in the steel matrix.
    In conventional powder metallurgy techniques, TiB2 particles were successfully dispersed in the matrix using commercial TiB2 powders, or synthesized through the in-situ reaction of ferro-titanium and ferro-boron powders.
    The measured Young's modulus of the HMS agreed well with the theoretically predicted value, and at 46 vol% TiB2, the specific modulus reached twice of those of conventional steels owing to its reduced density.
  • Effect of Isothermal Holding in Austenite Region on Proeutectoid Ferrite Growth Rate in a V-C-N Steel

    pp. 592-597

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    The mechanism of retarded growth rate of intergranular proeutectoid ferrite in a V-C-N steel has been investigated by examining the effect of holding in austenite region during cooling from the solutioning prior to the isothermal transformation at 600°C. The growth rate increases and the transformation start time decreases by increasing the holding temperature, while a C-curve behaviors appears. The growth rate can be ascribed to the supersaturation of C at the transformation interface. THERMO-CALC calculation indicates that substantial precipitation of V(C, N) takes place in the V-C-N steel and the fraction of C in the precipitate decreases with the increasing holding temperature. Analysis of the transformation kinetics in terms of the Johnson-Mehl-Avrami equation shows a two-step process, suggesting decrease in the ferrite nucleation rate by the holding. It is concluded that absorption of C into the precipitate during the incubation period decreases the supersaturation of C at the transformation interface.
  • A Consideration on Dynamics of R & D and Possibility of Its Application to Steel Technology

    pp. 598-602

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    Relationship between R&D resource input and R&D output is dynamic in nature. If this dynamics is understood well by researchers and engineers, it can help them plan and perform R&D effectively. There have been several proposed dynamic models, but they are intended for planning and analysis for managerial purposes and not necessarily describe and predict dynamics. It is shown in this paper that, based on simple hypotheses, a simple qualitative equation describing R&D dynamics can be obtained where basic variable describing technological competitiveness is taken as technological stock. The hypotheses are in the following: ( 1 ) Relative increment in technological stock (ΔS/S) is proportional to relative increment in cumulative resource input (ΔR/R). ( 2 ) Technological stock decreases with time at a constant rate. ( 3 ) There is a certain delay time for the resource input to produce result. The resulting equation is as follows:
    S=R(t-γ)αet
    where α is an efficiency parameter, β is a decay or obsolescence parameter, and γ is delay time. For examples, this equation together with another minor hypothesis can describe well-known favorable effects of focused resource investment, switching of R&D subject, and top-heavy investment. The meanings of these parameters are also discussed in relation to the efficiency in R&D. The model can also be applied to various case studies of resource input and R&D output.
  • Combustion and Reaction of Coarse-sized Pulverized Coal in Blast Furnace

    pp. 603-608

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    In order to establish the coarse-sized pulverized coal injection technology in the blast furnace, the behavior of pulverized coal with the characteristic diameter of 350450μm has been investigated concerning the pyrolysis, combustion, gasification and accumulation, by use of two kinds of model experiment; one is the vertical tower type reactor called the combustion model and the other is the blast furnace type reactor called the hot model.
    The coarse-sized coal is split and the small-sized coal is generated in the combustion model experiment. As a result, the combustion efficiency of coarse-sized coal is 10% lower than that of small-sized coal with the characteristic diameter of 50 μm.
    As the coarse-sized unburnt char is gasified preferentially against coke in the lower part of the hot model, char is little accumulated in the model and little flown out of the top.
    In the actual operation at Muroran No.2 blast furnace with 150 kg/t-pig injection of coarse-sized coal, the stable operation has been performed in which the permeability deterioration and deadman temperature drop are not occurred, and the unburnt char is little flown out of the furnace top.

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