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

Tetsu-to-Hagané Vol. 99 (2013), No. 3

  • Contraction Behavior of Inertinite in Coal and Formation Mechanism of Crack around Inertinite Texture in Coke

    pp. 175-184

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    DOI:10.2355/tetsutohagane.99.175

    Coke strength is mainly determined by pores and cracks that cause the fracture of cokes. Cracks around inertinite textures form in the contracting process after re-solidification. In this study, inertinite and reactive concentrated coals were prepared by the density fractionation and the differences in contraction behaviors of them were measured. Moreover, by the use of the measurement results, the thermal stress and the thermal strain energy around inertinite textures were calculated and the formation mechanism of cracks was investigated. The results were follows.
    (1) The contraction rate of inertinite is found to be 5-6 points lower than that of reactive (vitrinite and exinite) within the extent of coking or semi-soft coking coal usually used in coke making plants.
    (2) At the boundary of inertinite and reactive textures, the maximum tensile stress is generated in the tangential direction.
    (3) It is estimated that the critical inertinite size that cracks may form around inertinite textures is about 0.1mm when the temperature reaches 950°C.
  • Scrap-melting Operation by Shaft Furnace using all Blast Furnace Coke and Steel Scrap

    pp. 185-190

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    DOI:10.2355/tetsutohagane.99.185

    In the scrap-melting operation of shaft furnaces, an operation technology with use of 100% small size blast furnace coke for solid fuel instead of typically utilizing foundry coke has been established. An operation technology with use of 100% steel scrap without using return scrap containing 3-4 % carbon for ferrous raw material, and 70% or more of steel scrap consists of shredder scrap, has been established as well.
    This technology was first developed with one-stage tuyere operations using only lower tuyeres. Next two-stage tuyere operations using lower and upper tuyeres has been developed to explore more efficient operation.
    In the two-stage tuyere operation, the number of pipes, the diameter, and the protruding length of the upper tuyeres were examined as parameters, an operation technology with high efficiency was established, which enabled us to achieve high productivity and low coke ratio.
  • Effect of Silicon Content in Steel on the formation of Titanium Nitride and Solidification Structure

    pp. 191-197

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    DOI:10.2355/tetsutohagane.99.191

    TiN precipitates in the molten steel act as effective heterogeneous nucleation sites for delta-ferrite during solidification. Some reports indicate that Si increases the activity of that of Ti. In this work, effect of Si content on the formation of TiN in the molten steel and on the solidification structure of steel ingots was investigated experimentally. And thermodynamic calculations for the formation of TiN and heat-transfer solidification analysis were conducted. Ratio of equiaxed grain structure of an ingot of 11mass%Cr-0.13mass%Ti-0.0080mass%N steel of which primary crystal was delta-ferrite increased with increase in Si content. Thermodynamic calculations suggested that Si increased the activity of that of Ti in the molten steel and increased TiN before solidification. Positive large values of eTiSi can explain the experimental results in this work, but negative small value can not. Heat-transfer solidification calculations indicate that increase in Si content can make it easy to cause columnar to equiaxed transition similar to very low superheat in the mold.
  • Effect of Sulfur Concentration on Inclusion Entrapment in Solidifying Shell

    pp. 198-205

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    DOI:10.2355/tetsutohagane.99.198

    In order to clarify the effect of compositions on the surface quality of ultra low carbon steel slabs, a model experiment investigating the effect of compositions on entrapment of inclusions was performed using molten steel with various concentrations of S, which is a surface active element between molten steel and inclusions, while varying the velocity of the molten steel flow at the solidification interface to obtain clear changes in the interfacial tension between the inclusions and the molten steel.
    As a result, entrapment of inclusions was enhanced with increasing S concentration, and the effect of S concentration on entrapment of inclusions depended on the velocity of the molten steel flow at the solidification interface.
    In the case of a large inclusion, the effect of S concentration on entrapment of inclusions is considered to be affected by the various thicknesses of the concentration boundary layer.
  • Blistering Behavior during Oxide Scale Formation on Steel Surface

    pp. 206-213

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    DOI:10.2355/tetsutohagane.99.206

    Blistering occurs when oxide scale is swollen during oxidation. Blistered scale causes surface defect problems when it is rolled. Present study investigated the nucleation and growth behavior of blistering when steel is oxidized at high temperature. The following conclusions are drawn. Blistering phenomenon has the nucleation and growth process. At the nucleation stage scale is delaminated at the scale/metal interface. The gas compositions inside blisters at this stage are CO, CO2, and N2. The steel surface inside blisters is oxidized while the stage changes from nucleation to growth. At the growth stage, the separated steel surface from the scale is not oxidized.
  • Influence of Temperature and Oxygen Content of Oxidation Atmosphere on Initial Oxidation of Si Containing Steels

    pp. 214-220

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    DOI:10.2355/tetsutohagane.99.214

    Oxidation behavior of Si containing steels in relatively low temperature was investigated. Oxidation rate of 1.5%Si-steel was determined by Si oxide formed at steel/scale interface. Two kinds of Si oxide formed in investigated oxidation condition; SiO2 and Fe2SiO4. In the case that SiO2 formed, the oxidation rate of the steel was suppressed and growth rate of Si inner oxide was also suppressed. SiO2 transformed into Fe2SiO4 at higher temperature and the oxidation rate became larger. The transformation temperature of Si oxide was influenced by oxygen content of the atmosphere. SiO2 is more stable than Fe2SiO4 at higher temperature in higher oxygen content. This transformation behavior is well explained by thermodynamic calculation.
  • Influence of Fe Oxidation on Selective Oxidation Behavior of Si and Mn Added in High Strength Sheet Steel

    pp. 221-227

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    DOI:10.2355/tetsutohagane.99.221

    In the process of hot dip galvanizing of Si and Mn added high tensile strength sheet steels, the selective surface oxidation of Si and Mn causes coating defects. One of the promising methods overcoming this problem is oxidation-reduction process. The steel surface exposed to an oxidizing atmosphere will react primarily by forming mainly an iron oxide, which can be reduced by hydrogen in the following reduction process. It has been explained that due to the formation of pure iron, good wettability can be obtained. However, the mechanism of the suppression of selective surface oxidation has not been clarified in detail yet.
    In order to reveal this mechanism, present study focused on both Mn and Fe oxidation behavior on the way of the oxidation-reduction process of the cold rolled sheet steel contained 0.25mass%Si-1.8mass%Mn. The surface and cross-sectional analysis were performed by using secondary electron microscopy and transmission electron microscopy. The selective surface oxidation behavior was investigated by glow discharge spectroscopy.
    The main results obtained are as follows. First, Even if the soaking was continued after Fe oxide's reduction finished, the selective surface oxidation of Mn was suppressed.
    Second, Mn was trapped as the internal oxide under Fe oxide layer as reduction was processed. Moreover, the depletion of solute Mn was observed in the matrix.
    From these results, the depletion of solute Mn supposes to suppress the outer diffusion of Mn during the soaking. Therefore, even after Fe oxide's reduction completes, the selective surface oxidation of Mn is suppressed.
  • Effect of P Addition on the Texture Formation Attributable to Preferential Dynamic Grain Growth in Fe-3mass%Si during High-Temperature Uniaxial Compression Deformation

    pp. 228-235

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    DOI:10.2355/tetsutohagane.99.228

    In order to clarify the effect of solutes on texture formation during high-temperature deformation of BCC solid solution alloys, the texture and microstructure formations in Fe-3.1mass%Si (Fe-Si) and Fe-3.2mass%Si-0.12mass%P (Fe-Si-P) during high-temperature uniaxial compression deformation are studied. The analysis of true stress-true strain curves indicates that the stress exponents at 1173K for Fe-Si-P and Fe-Si are 3.4 and 3.8, respectively. This suggests the addition of phosphorous affects the motion of dislocations. Texture having high axis densities at <001> and <111> are formed both in Fe-Si and Fe-Si-P by the deformation up to a desired strain of 1.0. The volume fraction of <001> oriented region increases and that of <111> oriented region decreases with decreasing strain rate and increasing temperature. This tendency is more obvious in Fe-Si-P than Fe-Si. Microstructural observations by EBSD measurements reveal that the notable increase of the volume fraction of <001> oriented region is attributable to the preferential growth of <001> oriented grains. The growth of <001> oriented grains begins earlier and becomes more active in Fe-Si-P with increasing strain than in Fe-Si. The results suggest that the acceleration of the preferential growth of <001> in Fe-Si-P relates to the dislocation structure affected by the solute atmosphere.
  • Reduction of Delayed Fracture Susceptibility of Tempered Martensitic Steel through Increased Si Content and Surface-Softening

    pp. 236-244

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    DOI:10.2355/tetsutohagane.99.236

    Improvement of the surface layer as well as the microstructure has been needed to develop high-strength steels, since delayed fracture cracks initiate in the surface layer. In the present study, two approaches were taken to reduce the delayed fracture susceptibility of tempered martensitic steel with tensile strength of 1450 MPa. One was by increasing the Si content, which was intended to improve the microstructure. The other was by a surface-softening treatment, which was for improving the surface layer. Delayed fracture susceptibility was evaluated by conducting tensile tests and constant load tests in a NH4SCN aqueous solution. It was found that increasing the Si content from 0.2 mass% to 1.88 mass% prevented intergranular fracture and reduced delayed fracture susceptibility. One reason for this improvement is that the Fe3C particle size on prior-γ grain boundaries and in the matrix decreases with increasing Si content, which implies that Si stabilizes dislocation structures. When the surface strength of surface-softened steel specimens was lowered to 1150 MPa, delayed fracture susceptibility was reduced further. This is attributed to not only a reduction of the Vickers hardness of the surface layer but also a reduced hydrogen concentration at the surface layer. The rearrangement and annihilation of dislocations and also the spheroidizing and coarsening of Fe3C particles at the surface layer subjected to a high tempering temperature lead to a reduction of the hydrogen concentration at the surface layer.
  • Effect of Tempering Temperature on the Bendability of Martensitic Steels

    pp. 245-253

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    DOI:10.2355/tetsutohagane.99.245

    Effect of tempering temperature on bendability of ultra-high-strength steel sheets with fully martensitic structure having tensile strength ranging from 1100 to 1650 MPa was investigated. In this study, cold-rolled steel sheets with carbon content ranging from 0.15 to 0.4% were quenched from austenite region and subsequently tempered at various temperatures below 500°C. Bendability seriously deteriorated at tempering temperature around 300°C when the tempered martensite embrittlement occurs. Under the same tensile strength level, steel sheets tempered below 200°C were superior in bendability to those tempered at even over 400°C when the ductility recovered. Minimum bending radius was well correlated with the elongated cementite particle number density in the tempered martensite matrix. In the cross section of the bent specimens, cracks along the elongated cementite particles were frequently observed. These results suggested that deterioration in bendability in steels tempered at higher temperature over 200°C was caused by the acceleration of crack propagation during bending deformation due to the elongated cementite precipitates. On the other hand, bendability of fully martensitic steels could not be clarified by the uniform elongation obtained by a tensile testing. Uniqueness of the material deformation behavior in bending was also discussed from the viewpoint of plastic working theory.
  • A Hysteresis Loss Modeling of Grain Oriented Electrical Steel Sheet

    pp. 254-259

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    DOI:10.2355/tetsutohagane.99.254

    To establish a new model that can explain influence of various factors on hysteresis loss of electrical steel sheet, the hypothesis was formed that hysteresis loss accompanied by magnetic wall motion was expressed as a function of the volume of the area where magnetic walls moved (V) and the mobility of magnetic wall. V was expressed as a function of magnetostriction, magnetostriction constant, magnetic flux density, saturated magnetic flux density and the angle between the axis of easy magnetization and the direction of magnetic field when grain oriented electrical steel sheet was magnetized parallel to <100>, <110> and <111>. This model of V was verified by measuring hysteresis loss and magnetosriction of grain oriented electrical steel sheet under stress and revealing that hysteresis loss was proportional to 1.6th power of V.
  • Effect of Carbonated Steelmaking Slag on the Growth of Benthic Microalgae

    pp. 260-266

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    DOI:10.2355/tetsutohagane.99.260

    Calcium oxide in steelmaking slag may increase the pH when the slag is applied to marine environments such as the creation of an artificial tidal flat. Since sharp increases in pH are supposed to have negative impacts on the growth of benthic microalgae, carbonation of the steelmaking slag surface with CO2 is effective in alleviating the increase in pH and turbidity of seawater attributed to the formation of magnesium hydroxide. Even though the pH was increased by the non-carbonated slag, growth of Nitzschia laevis was significantly enhanced, when compared to that of the carbonated slag. In batch experiments (without N.laevis), the amounts of dissolved silicate, phosphate and iron from the non-carbonated slag were higher than those from the carbonated slag, indicating that the release of these elements might be the main factor responsible for enhancing the growth of this algal species. However, the released phosphate from the non-carbonated slag was initially low, a fact that could be attributed to suppression of phosphate dissolution under higher pH condition. Therefore, this study elucidated that the nutrients released from the non-carbonated slag enhanced the growth of N.laevis well, whereas sharp increase in pH may have inhibited the growth directly. On the contrary, even though the carbonated slag was effective in alleviating the pH increase, it was less effective in enhancing the growth of this algal species because released nutrients from the carbonated slag were low.

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