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

Tetsu-to-Hagané Vol. 96 (2010), No. 2

  • Intensification of Bubble Disintegration and Dispersion by Mechanical Stirring in Gas Injection Refining

    pp. 57-63

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    DOI:10.2355/tetsutohagane.96.57

    Water model experiments were performed for developing highly efficient gas injection refining processes. Mechanical stirring was applied to disintegrate the injected bubbles and to disperse them widely in the bath. The bubble disintegration and dispersion were investigated by changing rotation mode (direction of rotation), rotation speed and blade size of the impeller, and gas flow rate. Forward rotation of the impeller induced a stable tangential flow and could not disperse bubbles in the bath due to formation of a vortex around the impeller shaft. The tangential flow was suppressed by forward-interrupt rotation, which could reduce the vortex formation to some degree. However, the forward-interrupt rotation could not disperse the bubbles widely in the bath. Forward–reverse rotation could prevent the vortex formation completely and create a turbulent and strong shear stress field, which intensified the bubble disintegration and dispersion in the bath. Higher impeller rotation speed and larger blade length in the forward–reverse rotation could enhance the bubble disintegration more intensely, and make the dispersed bubbles smaller and the bubble dispersion zone wider. The bubble size tended to be larger at higher gas flow rates. However, its dependence on the gas flow rate became smaller at higher impeller rotation speed.
  • In-Situ Observation of Martensite Transformation of Low Carbon High Alloy Steel Using High Temperature Laser Scanning Confocal Microscopy and X-ray Diffraction by Synchrotron Radiation

    pp. 64-69

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    DOI:10.2355/tetsutohagane.96.64

    Martensite transformation of Low carbon high alloy steel during cooling was in-situ observed by using high temperature laser scanning confocal microscopy and X-ray diffraction by Synchrotron radiation. The crystallography of martensite and retained austenite structure were analyzed by electron back scattered diffraction patterns (EBSD). From the test results it could be concluded that the austenite phase is stress- relieved by twin deformation when the martensite transformation is occurred.
  • Effects of Nitrogen Concentration and Plastic Deformation on Small Angle Neutron Scattering for Austenitic Stainless Steels

    pp. 70-75

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    DOI:10.2355/tetsutohagane.96.70

    The small angle neutron scattering (SANS) profile has been found to be influenced by nitrogen addition and plastic deformation for austenitic stainless steels. In solution-treated specimens, the slope of plots for SANS intensity versus the magnitude of scattering vector, q, becomes larger than −4 in the Porod region, suggesting the influence of short range N ordering. The change in scattering intensity profile with tensile deformation was in situ measured and it is found that SANS intensity becomes larger with increasing of plastic strain except a q region around 0.6 nm−1 for N bearing steel, where the intensity decreases in the beginning and then increases with increasing of plastic strain. This temporary decrease in SANS intensity is confirmed for cold rolled specimens. Severe deformation given by swaging to a N free steel brings larger increase in SANS intensity than a N bearing steel. The influence of plastic deformation on SANS intensity is larger in austenitic steels than in ferritic steels.
  • Phenomenological Aspect of Hydrogen Enviroment Embrittlement of SNCM439 Steels

    pp. 76-83

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    DOI:10.2355/tetsutohagane.96.76

    The hydrogen environment embrittlement of SNCM439 steels (Ni–Cr–Mo steels) was investigated in 20-MPa hydrogen gas at room temperature at different tensile strengths ranging from 800 to 1200 MPa by heat treatments. The results of tensile tests revealed that hydrogen environment embrittlement was observed as decrease of ductility, whereas the tensile strength of the specimens did not change. Although the uniform elongation of the specimens was the same in air and hydrogen, local contraction decreased with increasing tensile strength in hydrogen. In contrast to the specimens with smooth surface, the notch tensile strength of the specimens having a tensile strength of over 1000 MPa remarkably decreased. The analysis of fractures in the specimens revealed that a crack was initiated from the external of the specimens in hydrogen, whereas it was initiated from the center of the specimens in air. The fracture mode was the same in both the tensile tests with notched and notchless specimens, and the presence of hydrogen induced quasi-cleavage fracture. It was observed that intergranular fracture was dominant in highly embrittled materials, and quasi-cleavage fracture was ahead of intergranular fracture. Even in the notched specimens whose intergranular fracture was dominant, quasi-cleavage fracture was observed beneath the notch. These behaviors indicate that the susceptibility to hydrogen embrittlement was more remarkable in specimens with high tensile strength, and the large plastic deformation required for the formation of quasi-cleavage fracture induced hydrogen penetration and hydrogen environment embrittlement.
  • Stretch-flangeability of Ultra High-strength Low Alloy TRIP-aided Sheet Steels with Mixed Structure Matrix of Bainitic Ferrite and Martensite

    pp. 84-92

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    DOI:10.2355/tetsutohagane.96.84

    The microstructure, retained austenite characteristics, tensile properties and stretch-flangeability of ultra high-strength 0.2%C–1.5%Si–1.5%Mn (mass%) TRIP-aided bainitic ferrite cold-rolled sheet steel, “TBF steel”, were investigated for automotive applications. When isothermally held at temperatures less than martensite-start temperature for 300–3000 s after annealing or austenitizing, the TBF steel possessed mixed matrix structure of bainitic ferrite and martensite, with retained austenite of about 4 vol%. The TBF steel achieved the tensile strength higher than 1400 MPa and hole-expanding ratio of 40%. The good combination of tensile strength and hole-expanding ratio was mainly caused by highly carbon–enriched retained austenite and softened matrix structure composing of bainitic ferrite and martensite.
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  • Effects of Nb and O Contents on Microstructures and Mechanical Functionalities of Biomedical Ti–Nb–Ta–Zr–O System Alloys

    pp. 93-100

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    DOI:10.2355/tetsutohagane.96.93

    Ti–30Nb–10Ta–5Zr–0.3O (TNTZO) alloy, which is simplified chemical composition similar to that of biomedical Ti–29Nb–13Ta–4.6Zr alloy, and the same system alloys of TNTZO with different oxygen (O) and niobium (Nb) contents such as Ti–30Nb–10Ta–5Zr–0.5O, Ti–30Nb–10Ta–5Zr–0.6O alloys, and Ti–27Nb–10Ta–5Zr–0.3O, Ti–28Nb–10Ta–5Zr–0.3O and Ti–29Nb–10Ta–5Zr–0.3O alloys are fabricated by powder metallurgy processing, in which it undergoes severe deformation during swaging. Changes in the deformation behaviors of these alloys caused by small changes in the O and Nb contents are investigated in this study. The stress–strain curve of TNTZO changes nonlinearly with the strain and is characterized by a small stress hysteresis, indicating that TNTZO shows a nonlinear recovery behavior during unloading. This deformation in the alloy is a result of a typical elastic deformation as well as stress-induced martensitic transformation. However, with an increase in the O content, as in the case of alloys with 0.5O and 0.6O, the stress–strain curves change almost linearly with the strain and exhibit a linear recovery behavior during unloading without stress hysteresis. In such alloys, deformation is mainly caused by the typical elastic deformation. On the other hand, with a decrease in the Nb content, the stress–strain curves show two steps with an increase in the strain, and deformation is accompanied by stress hysteresis during both loading and unloading. This deformation is mainly caused by stress-induced martensitic transformation and reverse transformation.

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