Tetsu-to-Hagané
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ONLINE ISSN: 1883-2954
PRINT ISSN: 0021-1575

Tetsu-to-Hagané Vol. 87 (2001), No. 1

  • Microstructure of Iron Containing Oxide Particle Obtained by CO Reduction of Wustite Pellets Consisting of MgO or CaO Solid Solution

    pp. 1-6

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    CO reduction of wustite pellets dissolving MgO or CaO was carried out in order to clarify the effect of solutes on reduction rate and the microstructure of oxide particle that were precipitated in the reduced iron, as well as the morphologies and microstructure of reduced iron phase were observed with X-ray diffraction and microscope. The results are summarized as follows.
    The reduction rate of wustite pellets increased with an increase in CaO contents. Although the reduction rate of wustite pellets dissolving MgO also increased, the acceleration effect of MgO was smaller than that of CaO.
    The particle size of MgO and CaO were 300-500 nm and 20-40 nm, respectively. They were larger than the particle size obtained by H2 reduction. The total porosity of the reduced pellet increased with an increase in CaO content or MgO content, because the sintering of reduced iron grains was prevented by addition of CaO particle or MgO particle.
    The magnesium concentration at the reaction interface in wustite side increased to approximately 5-6 times that of bulk content, and the calcium concentration also increased to about 1.5 times.
    From the discussion of mass transfer in wustite, it can be said that the oxide particles size should be depending on the ratio of vacancy diffusion to chemical reaction rate.
  • Analysis of the Stirring Process of Water Bath Contained in a Rotating Cylindrical Vessel with Particle Imaging Velocimetry

    pp. 7-13

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    The instantaneous velocity of laminar flow in a water bath contained in a cylindrical vessel, being initially at rest and then suddenly set in rotation, was measured with particle imaging velocimetry (PIV). The period from the start of rotation to the moment at which the flow in the bath reaches steady state is defined as the flow establishment time. The results for the establishment time were compared with previous empirical equations derived on the basis of velocity measurements with laser Doppler velocimetry (LDV), and an applicable range of the equations was evaluated. The measured values of the flow establishment time were underestimated by the equations. This is because the equations were obtained in a higher Reynolds number range. An empirical equation for the flow establishment time was newly proposed.
  • X-ray Fluoroscopic Observation of Slag Foaming

    pp. 14-20

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    Slag foaming caused by slag/metal reaction in a graphite crucible was observed with X-ray fluoroscopic apparatus in order to make clear the effect of bubble size on the foam height and the distribution of bubbles in slag. It was observed that the foam layer was formed when CO bubbles evolved at slag/metal interface became less than approximately 2 mm in diameter. The foam height increased with the decrease of bubble size even in case of almost the same gas evolution rate. The size of bubbles decreased with the increase of iron oxide content in slag and the decrease of sulphur content. It is considered that the bubbles become smaller when slag is wettable with metal. Therefore, the physical properties of slag/metal interface also affect the foam height besides the surface tension or viscosity of slag because they change the bubble size.
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  • Development of the Continuous Dephosphorization and Decarburization Process Using BOF

    pp. 21-28

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    The authors proposed a new refining process. In this process, the hot metal is dephosphorized and decarburized continuously with intermediate deslagging after dephosphorization in the same converter. The slag after decarburization is remained in the converter and reused to the dephosphorization of the next charge, so as to minimize the amount of exhausted slag. The experiments with 8-ton scale converter were performed to make clear the properties of this process. The high productivity was confirmed, because the dephosphorization rate could be increased with the high oxygen blowing rate and the addition of iron ore. The lime consumption was relatively low due to the dephosphorization with low basicity slag and hot recycling of decarburization slag to the next dephosphorization. The proper conditions, such as slag composition in dephosphorization, the rate of oxygen supply, bottom stirring and the intermediate deslagging rate, were also discussed by the basis of experimental results.
  • Material Evaluation and Thermal Stress Analysis of Side Dam for Twin Roll Type Strip Caster

    pp. 29-36

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    This paper discusses the strip caster's side dam. Material evaluation and thermal stress analysis of side dam were carried out to get longer duration of side dam. At first, refractory of side dam was evaluated. Three kinds of refractory were dipped into molten stainless steel and were observed. Also ceramic component of side dam and crack distribution observed on refractory were checked after 10-ton castings.
    Thermal analysis and thermal stress analysis of side dam were carried out to estimate the cause of this side dam cracking. Side dam was preheated from front side and back side before casting to reduce thermal stress caused by the touch of molten steel on the side dam surface and to prevent the growth of solidified shell on it. Temperature and thermal stress of side dam during preheating before casting were evaluated and adequate preheating way for preventing cracking was checked and the positions estimated at high stress were compared with the results after 10 ton casting side dam. It is possible to explain the cause of side dam cracking by the results of thermal stress analysis and to reduce these thermal stress cracking by adopting adequate preheating way of side dam.
  • Modeling of Shearing Deformation Characteristics of Flange in Stretch-type and Shrink-type Flanging of Sheet Metals

    pp. 37-42

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    The defective phenomena of dimensional accuracy, cracking, and wrinkling, etc. are generated in the flanging process of press forming such as the panel parts of car body. In this study, the experimental model forming and the analysis are done to clarify the characteristics of shearing deformation, which causes a defective phenomenon of dimensional accuracy of flange height in stretch-type, shrink-type and reverse-type flanging of sheet metals. There is a close relationship between the amount of shearing deformation and the profile curvature (1/R) of flange. An analytical modeling of shearing deformation of flange has been led, and the amount of shearing strain can be calculated by introducing the equivalent curvature(1/ρ0).
  • Evaluation on the Protective Ability by the Potential of Steel with Rust Layer Related to the Composition of the Rusts Formed on the Weathering Steel Bridges Using Ternary Diagram

    pp. 43-48

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    The rust layers formed on the weathering steel bridges exposed for 2.5 to 18 years in both of mountain and seashore atmospheric environments in Japan have been characterized by the composition analyzed by X-ray diffraction (XRD) and evaluated on the protective ability by the potential (ERL/S) of steels with rust layers corresponding to the corrosion rates. The fraction of rust constituents is newly described in the ternary rust diagram which consists of γ*; γ-FeOOH(L)+β-FeOOH(B)+Fe3O4, X-ray amorphous substance (Am) and goethite(α-FeOOH)-type oxyhydroxide (G). Exposure term dependence of the rust composition leads to the schematic progress of long-term growth of the protective rust layer in mountain atmosphere, rust layer is mainly composed of Am and the remainder; G rich (Stage III, 2≤α/γ*) via L rich (Stage I, α/γ*<1) and G, L less(Stage II, 1≤α/γ*<2). ERL/S has been related to the change of rust composition and increased with decreasing atmosphere corrosion rate of the weathering steel. The presence of deicing salt increases B and reduces the ratio of α/γ* and ERL/S. Therefore, α/γ* is a good parameter for evaluating the protective ability of the rust layer in mountain area. However, evaluation by α/γ* is restricted to the application to the rust layer in seashore area. In conclusion, both of α/γ* and ERL/S are necessary to evaluate the protective ability of the rust layer against atmospheric corrosion in various environments.
  • Isothermal Transformation Behavior in 12%Cr-0.3%C Steel

    pp. 49-54

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    In order to get fine grain of large martensitic stainless steels products, it is available to use the isothermal transformation before the austenitizing. When 12%Cr-0.3%C steel is subjected to the full solution treatment and following isothermal transformation at 900-1020K, it is observed that the microstructures are affected by the isothermal ageing temperature. The microstructure transformed isothermally at 1020K is the full eutectoid structure where carbides precipitate homogeneously. On the other hand, the microstructure transformed at 900K is the mixed heterogeneous structure: the eutectoid structure where carbides precipitate densely around the edge of prior austenite grains and the ferrite structure where carbides hardly precipitate in the center of prior austenite grains. These phenomena are concerned with the decreasing in carbon content in the untransformed austenite as the eutectoid transformation progresses. The amount of decreasing in carbon at 900K is larger than at 1020K, which causes the shortage of carbon in austenite in the last period of isothermal transformation. In the case of the isothermal transformation at 900K, the reason of why there are the ferrite structures with no carbide in the center of prior austenite grains is that the massive transformation is induced by the shortage of carbon in untransformed austenite.
  • Effect of Microstructure on Fracture Characteristics of Ti-6Al-2Sn-2Mo-2Zr-2Cr-Si Alloy

    pp. 55-62

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    Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti-62222S) is highly expected to be widely used for structural aircraft materials because Ti-62222S shows both characteristics of α type alloys with excellent high temperature strength and creep resistance, and α+β type alloys with high fracture toughness and high strength. The intermetallics, Ti3Al, and silicides, precipitated in Ti-62222S during aging treatments decrease the strength and fracture toughness. However, the effect of each intermetallics on fracture characteristics of Ti-62222S is not yet clearly understood. Therefore, the fracture characteristics of Ti-62222S with only Ti3Al or only silicides precipitated by aging treatment were investigated with relating the matrix microstructure.
    0.2% proof stress, tensile strength and elongation of the as-received material are much better than those of the aged materials. 0.2% proof stress and tensile strength of the material with only Ti3Al precipitated by aging treatment are better than those of the material with only silicides precipitated by aging treatment, while elongation of the material with only silicides is better than that of the material with only Ti3Al. Static fracture toughness of the material with only silicides is better than that of the material with only Ti3Al. The intergranular fracture appears in the material with only Ti3Al. Coarsening of Widmanstätten α structure and increasing ductility of β phase during aging is considered to be effective for increasing static fracture toughness.
  • Computer Simulation of the Yield Point Phenomena Based on the Work-softening Model

    pp. 63-69

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    The yield point phenomena in low carbon steel have been explained by the Cottrell or the Johnston-Gilman theory in terms of microscopic variables such as dislocation density, dislocation velocity and the interaction energy between dislocation and solute atom. However, the lower yield stress and the amount of the Lüders strain (stretcher strain) seems not to have been predicted by the above theories. In the present work we tried to explain the phenomena quantitatively by a phenomenological (rheological) model using macroscopic variables such as work-hardening rate and the strain rate sensitivity of flow stress which we can be measured by tensile test. In the model a new internal stress was introduced which is supposed to be work (strain)-softened and age-hardened. Numerical calculation showed that the model can predict the values of the upper and lower yield stress and the amount of the Lüders strain, and the effect of strain rate on these characteristics.
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