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

Tetsu-to-Hagané Vol. 86 (2000), No. 2

  • Numerical Modeling of Grinding Rate in Granular Flow of Brittle Materials

    pp. 73-78

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    In an iron-making blast furnace, an operation with pulverized coal injection (PCI) causes an accumulation of powder by crushing between coke particles and decreases permeability through the coke bed.
    It is so difficult to measure a mount of coke powders, so we developed a model to simulate a grinding rate with movements of particles of brittle material such as coke using Discrete Element Method (DEM).
    In order to estimate the performance of our simulation model, we carried out the mutual friction of grinding test with two gypsum areas, and derived the grinding rate equation for the simulation model from the experimental results. The grinding rate of gypsum particles in a rotational drum was measured and calculated with various porosities of gypsum and the rotational speed of the drum.
    As a result, the difference between predicted and measured grinding rate was larger in the case that the particle porosity was small (i.e. particle is hard) and a rotational speed of drum is high because we ignored the body breakage at contact between particles. On the other hand, the calculated grinding rate was the same as experimental one when the particle porosity was large (i. e. particle is soft.).
    It is concluded that DEM is useful tool for the prediction of grinding rate of particles. We expect the developed model to predict the grinding rate of coke particles in a raceway. However we might consider not only friction but also the body breakage in the simulation model to determine the correct grinding rate.
  • The Structural Analysis of the Rapid Heating Treated Coal Using High Temperature in-situ NMR Imaging

    pp. 79-85

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    A new carbonization process with rapid preheating and coke discharging at medium temperature has been developed in Japan. The result of this process shows that even when non or slightly coking coal is by 50wt% the coking property is improved and a coking coke with cold strength usable at blast furnace can be manufactured with the new carbonization process. The mechanism for the improvement of the coking property was examined by coal properties with using mainly in-situ NMR imaging. To monitor the dynamical changes in coals with temperature, an in-situ method must be used, therefore, we have applied Single-Point-Imaging and have carried out the first systematic in-situ variable-temperature NMR imaging study of coals between 25 and 500°C with our newly developed high temperature imaging probe and systems. It has been clarified that the macromolecular structure of coal is relaxed by the rapid heat treatment and in addition there is a close relation in hydrogen bond and relaxation of molecular structure of coal. The fact that rapid heating treatment of coal raised on DI150 is at almost 6 point in the treatment of temperature at 380°C, compared to the raw sample and slow heat sample is mainly caused by the decreasing of unsoftened area in coal. Rapid heat treatment is that the cohesion structure of coal molecule is relaxed by rapid heat and the kinetic property of molecule increases. And further, a bridge forming reaction generated by slow heating is suppressed and high mobility component increases and as the above results, a softening and melting phenomenon is accelerated as a time of coking. Finally, we would like to propose the mechanism for the improvement of the coking property during this rapid heat treatment, which have the improvement of coal properties.
  • Effects of Carbon and Ferrite-stabilizing Elements on Austenite Grain Formation for Hypo-peritectic Carbon Steel

    pp. 86-91

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    In order to investigate the formation process from δ dendrite to γ grain in a hypo-peritectic carbon steel, an unidirectionally solidifying carbon steel sample is quenched from a temperature above the solidus temperature. Three zones of primary dendrite, fine columnar grain and coarse columnar grain are observed in the sample. The dendritic zone consists of (L+δ) phase, the fine columnar grain is both phases of (L+δ+γ) and (δ+γ), and the coarse columnar grain is γ phase. The width of the fine columnar grain agrees with the primary arm spacing of the δ dendrite and it is the same result as that for a hyper-peritectic carbon steel. The γ grain boundary in the zone of the fine columnar grain forms at the skeleton of the δ dendrite, while that for a hyper-peritectic carbon steel forms at the position between the δ dendrites. Since transition from the fine columnar grain to the coarse one occurs immediately after the disappearance of δ phase, the δ phase prevents the coarsening of the fine columnar grain. A length of the zone of the fine columnar grain decreases with increasing carbon concentration. The length extends according to the addition of ferrite-stabilizing elements. The extended effect becomes stronger in the sequence of Nb>V>Mo, while Ni as an austenite-stabilizing element have no influence on the length.
  • Temperature and Deformation of Strip Caster's Mold Roll

    pp. 92-98

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    The purpose of this paper is to discuss the temperature and deformation of strip caster's mold roll. At first roll temperature was calculated by one-dimensional analysis using heat transfer coefficient derived from cooling water temperature increase and roll back temperature of strip caster's roll of pilot plant. Two-dimensional analysis was carried out more over. It was found that the temperature unevenness of mold roll surface is small due to the cooling water jacket set up cyclical. Next, the influence of the mold roll temperature was examined on sleeve thickness and welding thickness and on sleeve material and welding material. It was found that the higher temperature and lower temperature was raised as increase of sleeve thickness and welding thickness but temperature difference was decreased. Using the results of two-dimensional thermal analysis, the deformation of roll by thermal expansion was calculated by non-linear analysis. The predicted strip profile derived from roll surface profile was compared with the profile of a strip cast by pilot plant. And so flat strip could be cast.
  • Recent Development of Motor Speed Control System and It's Application to Seamless Pipe Mill

    pp. 99-104

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    Recent progress of main motor speed control apparatus using power electronics devices applied for rolling mills especially seamless pipe mills have contributed Recent to the advancement of quality of products. Both quick response and precise control of motor speed are the main advantage of recent motor speed control apparatus.
    The overview of this development and its application to seamless pipe mills in Wakayama steel works including new seamless pipe mill are described.
  • Microstructures of Heavily-deformed High Carbon Steel Wires

    pp. 105-110

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    The influence of heavy drawing (true strain4.0) on the structures of pearlitic steel wires containing 0.9mass% of carbon has been investigated. As for ferrite region, typical dislocation structures are observed in the lightly deformed wires and cell structures are observed in the heavily deformed wires. After heavy drawing, cementite has polycrystal structures and are decomposed partially. The diameter of the cementite particles are less than 5 nm and the amount of the decomposition is strongly affected by strain and half of the cementite is decomposed when true strain is 4.0.
    The free carbon which comes from cementite decomposition is insoluble in ferrite and it might be segregated at dislocations both in ferrite and at cementite/ferrite boundaries.
  • Effect of Co and Ni on the Electrochemical Behavior of Rust Formed on the Low Alloy Steels in Cyclic Wet and Dry Environment

    pp. 111-115

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    The iron rust phase formed on low alloy steels has been analyzed by using EPMA, XPS, TEM and alternating current (AC) impedance methods after cyclic wet and dry corrosion test using 0.5mass% NaCl solution. The steel containing Co or Ni from l to 3mass% showed higher corrosion resistance than carbon steel in the test. Cobalt was identified as its trivalent state (Co3+) in the rust of Co-bearing steel by EPMA and TEM, which means that Co is incorporated mainly into FeOOH. On the other hand, nickel was identified as bivalent state (Ni2+) in the rust of Ni-bearing steel by XPS and TEM, which implies that Ni is involved in the formation of spinel oxide in rust. By AC impedance it is demonstrated that the resistance of the rust (Rrust) corresponds to the structural factor of the rust of steels. The high value of Rrust of Co-bearing steel indicated resistance of its FeOOH-type rust against the penetration of Cl ion. The high Rrust value was observed mainly at the early stage of corrosion cycles. The Rrustvalue of Ni-bearing steel increases with increasing amount of rust on steel, which implies that Ni takes part in the conversion of spinels into dense and fine structure in the inner layer of rust layer.
  • Electrodeposition Behavior and Properties of Iron-group Metal Alloys with W from Ammoniacal Citrate Baths

    pp. 116-122

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    Electrodeposition behavior of iron-group metal alloys with W was studied in ammoniacal citrate baths of various compositions at 50°C under galvanostatic conditions. The wear and corrosion resistances of iron-group metal alloys with W were also investigated by sliding wear test of ring-on-plate type at 300°C and dipping test in HNO3, H2SO4 and HF solutions. The results obtained were as follows:
    (1) The relationship between the alloy composition and the cathode current efficiency showed the existence of two characteristic alloy compositions, i.e., the one appeared at a maximum partial current efficiency of W and the other corresponded to the limiting W content.
    (2) Electrodeposition behavior of iron-group metal alloys with W was explainable by the mechanism that the intermediate W4+ oxide electrochemically formed was reduced by atomic hydrogen held on freshly deposited iron-group metals.
    (3) The wear resistance of iron-group metal alloys with W was improved with increasing W content in the deposit and showed the best performance at 3040 mass% W. When W content in the deposit was high, hardness of the film was increasaed to prevent adhesive wear between the film and press ring. Hardening of the film due to the frictional heat also seemed to be contributed to the improvement of wear resistance.
    (4) The corrosion resistances of the films were improved with increasing W content in the deposit, especially in HNO3 solutions.
  • Effects of Alloying Elements and Microstructure on Corrosion Resistance and Hardness of Martensitic Stainless Steels

    pp. 123-130

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    In the application field of self-drilling-tapping screws and joint parts like a high strength steel pin etc., high corrosion resistance as well as high strength property are required even in martensitic stainless steels, because commercial martensitic stainless steels have a large disadvantage in terms of corrosion resistance in comparison with austenitic stainless steels like SUS304. In this investigation, effects of alloying elements and microstructures on both of hardness and corrosion resistance in martensitic stainless steels are examined to improve the corrosion resistance of commercially applied high strength martensitic stainless steels. The results obtained are as follows.
    (1) Corrosion resistance of martensitic stainless steels is improved to the level as same as SUS304 by suppressing the formation of δ-ferrite which leads to the promotion of Cr-carbide precipitation, and also by increasing the P. I. value; Pitting Index (=Cr+3%Mo+16%N) to 18 or more.
    (2) On the hardening of martensite, carbon has 2.5 times larger effect than nitrogen, hence the hardness of as-quenched martensite can be linearly plotted to the value; (2.5C+N).
    (3) Carbon contributes the significant hardening of martensite through the promotion of introduction of transformation twin and the refining of block structure.
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  • Effect of Microstructural Change on Creep Deformation Behaviour and Long-term Creep Strength of 1Cr-0.5Mo Steel

    pp. 131-137

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    Effects of the initial microstructure and its changes on creep deformation behaviour of a 1Cr-0.5Mo steel have been investigated, in order to understand a mechanism of complex creep deformation behaviour which shows several local minima in creep rate. Sigmoidal inflections are observed on stress vs. time to rupture curves and stress vs. minimum creep rate curves at 823 and 873K. Under the stress conditions lower than 100MPa, creep rate vs. time curves indicate inflection at the same time of a tertiary creep stage.
    Significant decrease in creep rupture strength due to prior ageing for 500h at 873K has been observed at stress conditions higher than 100MPa. However, no difference in creep rupture strength between pre-aged and un-aged steels has been observed in the stress conditions lower than 100MPa. The inflection of the creep rate vs. time curves has disappeared by ageing prior to creep test.
    High density of dislocations and many fine carbide particles are observed within ferrite grain of the un-aged steel. On the other hand, coarse needle-like carbides and very low density of dislocations are observed within ferrite grain of the pre-aged steel. Because of the similar decrease in the number of dislocations and precipitate occurring in the un-aged steel during creep deformation, no difference in microstructural morphology is observed for the un-aged and pre-aged steels after creep for 200h at 873K-88MPa. It has been considered that the effect of precipitation strengthening have disappeared during ageing for 500 h at 873K prior to creep test and creep deformation for 200h at 873K-88MPa.
    It has been concluded that complex creep deformation behaviour and sigmoidal inflection of stress vs. time to rupture curve are caused by decrease in creep strength due to microstructural change followed by advent of inherent creep strength.

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