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

Tetsu-to-Hagané Vol. 94 (2008), No. 8

  • Water Model Experiment on the Behavior of an Argon Bubble Rising Near the Immersion Nozzle

    pp. 271-277

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    DOI:10.2355/tetsutohagane.94.271

    Attack of a large argon bubble onto the meniscus in the continuous casting mold is considered to be one of main causes for the mold powder entrapment. The immersion nozzle is poorly wetted by molten steel and, accordingly, an argon bubble rising near the immersion nozzle attaches preferably to it. Its rising velocity has a significant effect on the mold powder entrapment. Water model experiments are carried out to reveal the rising velocity of an argon bubble rising along the immersion nozzle. Water and air are used as the models for molten steel and argon gas, respectively. The immersion nozzle is modeled by a cylindrical rod made of acrylic resin. Contact angles chosen are 77°, 110°, and 144°. An air bubble attaches to a rod of the contact angle of 144°. The rising velocity of the bubble is much greater than that of an air bubble rising away from the rod. It decreases with an increase in the rod diameter. The drag coefficient also is obtained for estimating the velocity of an argon bubble rising along the immersion nozzle.
  • Carburizing and Decarburizing of Iron Nails for Roofing Tiles by Smithing

    pp. 278-283

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    DOI:10.2355/tetsutohagane.94.278

    Purpose of this study for the possibility of development from mild steel or soft iron to steel, to smith using iron nails for roofing tiles with low concentration of the carbon as source material.
    It analyzed to the change of carbon concentration and the structural change of a metal structure in iron made in each process of the smith. The smith similar to making the Japanese sword after the iron nail had been made detailed like the plate.
    The analysis used the furnace-combustion infrared absorption method for the determinate of the carbon, and used an optical microscopy and the EPMA analysis for the analysis of a metal structure.
    As a result, that was rose up to about 2.6% of the carbon in the mass of iron after carburizing from about 0.1–0.3% of the carbon in iron nail of source material. At the same time, graphite flakes extracted in metal structure, that it was the gray cast iron. The concentration of the carbon decreased to about 0.6% in the iron material after smithed, and then graphite flakes were lost.
    It was confirmed to be able to adjust the concentration of the carbon, which develop from iron with low concentration of the carbon to steel.
  • Estimation of the Center Porosity Closure Behavior by Hydrostatic Integration Parameter and Process Design of Forging and Rolling

    pp. 284-289

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    DOI:10.2355/tetsutohagane.94.284

    Steel bars and billets may have center porosities caused by contraction during castng. It is important to optimize not only casting conditions but conditions of subsequent forging and rolling in order to decrease the center porosity.
    Forging and rolling, the two representative processes to produce steel bars, show different deformation behaviors, productivity and cost. But no study explained the porosity closure in the both processes consistently with an identical parameter. In this study, the authors discussed the integration of the hydrostatic stress, Gm, which was usually used in free forging, as a parameter to describe the closure of the porosities. The parameter can be calculated from plastic strain and stress by CAE analysis.
    It was found that the cross-sectional area of the porosity was in proportion with the hydrostatic integration Gm in single-pass rolling of plasticine, when a plasticine billet was repeatedly rolled under an H–V sequence, however, the summation of Gm did not show a clear threshold for the porosity closure. The hydrostatic integration was modified as Gm+ by subtracting a coefficient C, to reproduce the result of the plasticine test. In this case, the value C was 0.024, and the threshold of the summation of Gm+ was 0.25. The new parameter Gm+ has been successfully used to develop new pass schedules with larger Gm+ for a new rolling mill.
  • High-temperature Aluminizing Treatment of Stainless Steels Using Aluminum Foil

    pp. 290-298

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    DOI:10.2355/tetsutohagane.94.290

    The alloy layer formation during high-temperature aluminizing at diffusion temperature TD from 1073 to 1273K in air was investigated for austenitic, ferritic and martensitic stainless steels. The void accumulation was observed in FeAl and αFe interface in high-temperature aluminized carbon steel, while in all stainless steels that were not observed. The alloy layers observed in stainless steels at TD=1073K for the diffusion time t=0.6 ks were Fe2Al5 and Al8Cr5. With increasing TD and t, FeAl layer and αFe layer appeared. As TD and t increased further, Fe2Al5 disappeared, and finally become only two layers FeAl and αFe.
  • Effect of Microstructural Change on Creep Rupture during Creep Deformation in 18Cr–9Ni–3Cu–Nb–N Steel

    pp. 299-304

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    DOI:10.2355/tetsutohagane.94.299

    18Cr–9Ni–3Cu–Nb–N steel has the highest creep strength in the austenitic heat-resistant steels. It is expected as the component of the future thermal power plants. In order to clarify the mechanism of creep strengthening and creep rupture for the 18Cr–9Ni–3Cu–Nb–N steel, the creep test and the microstructural observations using electron microscopy were carried out. Seamless pipe was used as the sample because the initial microstructure for the sample conformed to the one for steels in the plants. From the TEM observation, fine NbX and Cu particles were formed in the austenite grains. The pinning effect by these particles is effective for the creep strengthening. In the regard to the microstructural change during the creep deformation, the crystal orientation rotations were observed using SEM-OIM. In the crystal grains rotated during the creep deformation, ‹001› or ‹111› was oriented to the tensile direction. Many boundaries were cracked due to these crystal rotations. This means that the creep rupture is occurred by the difference of the plastic deformation behavior between the grains oriented ‹001› to the tensile direction and the grains oriented ‹111› to it.
  • Effects of Carbon and Silicon on Static/Dynamic Mechanical Properties of 780MPa Grade Dual Phase Steel

    pp. 305-312

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    DOI:10.2355/tetsutohagane.94.305

    Various types of high strength steel have been developed to improve the impact safety and reduce the weight of cars. For the steels used in anti-crash equipments, mechanical properties, especially, at high strain rates such as 103/s is of importance. In this study, 0.110%C–1.44%Si–1.29Mn–0.65%Cr–0.29%Mo containing hot rolled dual phase steel with 780 MPa grade in tensile strength was employed as a base steel and the effects of carbon and silicon on static (strain rate: 10−3/s)/ dynamic (strain rate: 103/s) mechanical properties of the dual phase steel were investigated. Carbon and silicon contents were changed in a range of 0.076–0.190% and 1.44–2.39%, respectively. Grain size of the steels was varied by hot rolling reduction: 53% (named coarse grain process), 73% (middle grain process) and 88% (fine grain process). Dynamic absorbed energy up to 10% tensile strain had a linear relationship with tensile strength, regardless of microstructures, i.e., neglecting carbon and silicon contents, and hot rolling conditions. All absorbed energy to fracture had a close relationship with tensile strength–ductile balance parameter (tensile strength×total elongation), reflecting microstructural change through chemical and rolling conditions. All the processed 0.190% C steels, and the fine grain processed 1.93% Si and 2.39% Si steels showed the highest all absorbed energy of all the steels tested. The 0.190% C steel was characterized by almost 100% martensite with some content of retained austenite, and the 1.93% Si and 2.39% Si steels were fine grained ferrite+martensite. It was found that carbon improves all absorbed energy through increase in volume fraction of martensite and silicon raises it through solid solution hardening of ferrite matrix.
  • Hierarchical Characterization by EBSD and Neutron Diffraction on Heterogeneous Deformation Behavior of a Martensitic Steel

    pp. 313-320

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    DOI:10.2355/tetsutohagane.94.313

    A novel combined technique of neutron diffraction and electron back scattering diffraction was applied to examine hierarchical deformation behavior of 18 mass% Ni martensitic steel. In-situ neutron diffraction experiment during tensile deformation demonstrated that intergranular stress was generated. EBSD analysis suggested that slip bands terminated not only at block boundaries, but also sub-block boundaries at a relatively small strain. In many cases, slip bands crossing sub-block boundaries were zigzagged. With increasing strain, sub-block became unclear and then block boundaries worked as a main barrier for dislocation gliding. Such kind of heterogeneous plastic flow in differently oriented hkl blocks seems to be a possible reason for the intergranular stresses.
  • Evolution of Dislocation Structure and Fatigue Crack Behavior in Fe–Si Alloys during Cyclic Bending Test

    pp. 321-330

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    DOI:10.2355/tetsutohagane.94.321

    The evolution of dislocation structures was investigated by TEM in Fe–Si alloys with 0, 0.5 and 1.0 mass% Si during a cyclic bending test in conjunction with fatigue crack behavior. The addition of Si increased the fatigue strength. The evolution of dislocation structures was significantly influenced by the Si addition. Namely, in the steel without Si the dislocation cell structure develops, whereas in the steel with 1 mass% Si the vein structure develops which is considered to lead to increased fatigue strength. The dislocation cell structure observed in the steel without Si is postulated to be caused by the easy cross slip of dislocations during cyclic deformation, whereas the vein structure developed in the steels with Si is inferred to be caused by the difficulty in cross slip due to the decrease in stacking fault energy. Furthermore, the Si added steel shows a characteristic structure in a manner such that the dislocations are free in approximately 0.5 μm zones along grain boundaries. The examinations of the fatigue fracture surface revealed that the transgranular fracture takes place in steel without Si, whereas in steel with 1 mass% Si many intergranular cracks were observed just beneath the top surface. The intergranular cracks in the 1 mass% Si steel were thought to be caused by the fact that a) strains are dispersed within grains owing to the vein structure and b) micro cracks are initiated and propagated along grain boundaries due to the dislocation free zones.

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