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

Tetsu-to-Hagané Vol. 105 (2019), No. 11

  • Intra-Particle Water Migration Dynamics during Iron Ore Granulation Process

    pp. 1033-1041

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    DOI:10.2355/tetsutohagane.TETSU-2019-056

    The influence of iron ore properties, such as ore type, mineralogical texture, and particle size, on the intra-particle water migration dynamics were evaluated using immersion method. When immersed, ores were reached 68–78% of their final saturation in first 60 s and then approached final saturation slowly. It typically took up to 1×105 s to reach final saturation. Compared with the initial and final saturation water contents of 2.8–4.0 mass% in the case of Brazilian ores, Australian ores showed higher water contents of 5–6.4 mass% due to more porous structure. While the final saturation water content was partially explained by the porosity and total pore volume of ores, the kinetics of water migration should be considered to explain the saturation curve of different ores. In terms of mineralogical texture, porous texture showed higher final saturation water contents than dense texture. Finer particles showed higher final saturation water contents than coarser particles. A revised migration model was introduced to explain the effect of pore size distribution and trapped air. It was revealed that water migration proceeds more readily in the finer pores due to the larger capillary force, which is needed to overcome the trapped air. The water migration in the coarser pores is restrained due to the weak capillary force against trapped air, resulting in lower degree of saturation at equilibrium. Compared with Australian ores, Brazilian ores showed a lower degree of saturation due to their higher proportion of coarse pores.
  • Composition Change of Inclusions in High Carbon Steel before and after Addition of Aluminum

    pp. 1042-1049

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    DOI:10.2355/tetsutohagane.TETSU-2019-037

    The effect of Al addition in the middle of the metal-slag reaction on the formation of MgO·Al2O3 spinel-type inclusions was investigated by laboratory-scale experiments and a kinetic model calculation in order to reduce the spinel-type inclusions in high-carbon steel. As results of the experiments, the total Mg content in the steel and average content of MgO in the inclusions were relatively low before Al addition, and spinel-type inclusions were hardly formed. After Al addition, spinel-type inclusions formed when CaO/SiO2 and CaO/Al2O3 in the slag were high, and the total Mg content in the steel and average MgO content in the inclusions were also higher. On the other hand, formation of spinel-type inclusions was suppressed at lower CaO/SiO2 and CaO/Al2O3 in the slag. Therefore, the experimental results indicated that addition of Al at the midpoint in the reaction and control of the slag composition were effective for suppression of spinel-type inclusions. However, spinel-type inclusions formed soon after Al addition in slag with higher CaO/SiO2 and higher CaO/Al2O3. To evaluate the effect of midpoint addition of Al on the actual process, a kinetic model calculation under virtual conditions was carried out. According to the calculation, the increase in the content of Mg in the steel under actual-scale conditions was slower than that in the laboratory, and formation of spinel-type inclusions could be avoided.
  • Evaluation of the Performance of Nanofluid as Quenching Coolant

    pp. 1050-1058

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    DOI:10.2355/tetsutohagane.TETSU-2019-035

    Nanofluid is a liquid in which nanometer-sized particles are dispersed in base liquid. It is known that the critical heat flux and the wall superheat at the minimum heat flux in pool boiling are improved in nanofluids. In this research, performance of silica-water nanofluid as quenching coolant is explored experimentally since the above-mentioned parameters play important roles in this application. First, we investigated the immersion cooling of high-temperature test piece in the nanofluid; here, the test piece was cylindrical in shape and made of Inconel 718 or SUS304. It was confirmed that the test piece is cooled faster in the nanofluid than in distilled water. It was also found that non-uniformity of temperature in the test piece during quenching is mitigated in the nanofluid. This indicates that the silica nanofluid is considered a promising coolant to avoid the occurrence of hardening crack during quenching. Finally, Vickers hardness test was done for the Inconel 718 test piece. It was shown that the hardness tends to increase with an increase in the cooling rate even under the high cooling rate of about 1000 K/min although the difference of hardness was not noticeable between the experiments using the distilled water and the silica nanofluid as the quenching coolant.
  • Mechanism of Improvement of Mechanical Properties of Cu-contained Low Alloy Steel by Intercritical Quenching

    pp. 1059-1069

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    DOI:10.2355/tetsutohagane.TETSU-2019-024

    Better balance of strength and toughness is a strong demand for the ASTM A707 5 L grade steel as offshore structural material. In our previous study, therefore, inter-critical quenching from dual-phase of ferrite and austenite region, which is called lamellarizing (L) treatment, brought a clear improvement of balance between strength and toughness of Cu containing low alloy steel based on A707 5 L grade. However it is important to investigate the effects of C and Cu amount and microstructure before the quenching on mechanical properties of L treated Cu-contained low alloy steel in order to clarify the mechanism of improvement of the properties by the L treatment.From present study, the investigation revealed that C act on strength of harder phases that were inherited the transformed γ phase region in L treatment in the C rage from 0.01 to 0.05 mass%. Moreover, the steel was given better balance of strength by optimized L temperature which was (AC3-15) K. Additionally, Cu was useful to adjust balance of strength and toughness during tempering by aging. However, Cu precipitates were not observed after tempering in the steel was added 0.61 mass% Cu. Hence, the results suggest that the steel need to be contained over 1.0 mass% of Cu amount in order to make good use of Cu. Microstructure before L treatment had an effect upon optimized L temperature. The temperature was (AC3-15) K in the case of granular bainitic ferrites (αB) that was granular structure but it was the middle between AC1 and AC3 in the bainitic ferrites (α°B) and martensite (α’m). From results of in-situ EBSD (Electron Back Scatter Diffraction), the transformed γ phase did not show γ memory effect when microstructure before L treatment was granular structure. On the other hand, it had the γ memory effect in the case of acicular structure. It is inferred that the difference between the granular and the acicular structure of optimized L temperature results from generating behavior of the transformed γ phase during the lamellarizing.
  • Effect of Crystal Textures and Microstructure on the Toughness of the Stretch Reduced Steel Tube in Austenite and Ferrite Two-phase Region

    pp. 1070-1079

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    DOI:10.2355/tetsutohagane.TETSU-2019-044

    The hot stretch reducing process of steel tubes has been attracting attention as an effective means to achieve both high strength and excellent formability by developing the {011}<100> texture reducing in austenite and ferrite (α/γ) two-phase region. However, it has been known that, in the case of control-rolled steel plate, the textures developed by the hot rolling in the α/γ region closely relate with the brittle fracture.In this study, the quantitative relationship between textures and toughness of the steel tubes reduced in the α/γ region was investigated. The increase of the DBTT (Ductile Brittle Transition Temperature) due to the stretch reducing in the α/γ region is observed only when the carbon content is below 0.06%. In contrast, the increase of DBTT is negligible small due to grain refinement when the carbon content is in the range from 0.12% to 0.18%. The increase of the DBTT in the low carbon steel tubes is attributed to the increment of coarse and brittle grains with high orientation factor for brittle fracture cos2θ by the stretch reducing in the α/γ region. Accordingly, the DBTT shows a strong dependence on both the area fraction of the brittle crystal grains Af (cos2θ≧0.9) and maximum grain size rather than average grain size.
  • Effect of Crystallographic Orientation, Grain Shape, and Elastic Anisotropy on X-ray Stress Factor in Polycrystals with Crystallographic Textures

    pp. 1080-1089

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    DOI:10.2355/tetsutohagane.TETSU-2019-052

    The effects of grain shape, crystallographic texture, and elastic anisotropy on the X-ray stress factor in polycrystals were studied. Polycrystal models with <110> fiber textures, comprised of single crystals exhibiting four different elastic anisotropy factors, were prepared. Then, the effects of the degree of <110> orientation, elastic anisotropy, and shape of the grains on the X-ray stress factor were examined using a self-consistent method consisting of the effective-medium approximation and Eshelby’s inclusion method. Also, the effects of the degree of <110> orientation and elastic anisotropy on the X-ray stress factors were analyzed using the Voigt and Reuss approximations. The analyses using the self-consistent method revealed that the degree of <110> orientation and elastic anisotropy of single crystal affect the dependence of the X-ray stress factor on the aspect ratio of spheroidal grains. The comparison of the analyses using the self-consistent method, Voigt and Reuss approximations revealed that there is a correlation between the aspect-ratio dependency of the X-ray stress factor and the difference between the X-ray stress factors calculated using the Voigt and Reuss approximations; when the difference between the X-ray stress factors calculated using the Voigt and Reuss approximations is large, the effect of the aspect ratio of the spheroidal grain on the X-ray stress factor is large.
  • Effect of Magnetic Transitions on the Formation of the Thermal Vacancy in αFe

    pp. 1090-1097

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    DOI:10.2355/tetsutohagane.TETSU-2019-041

    In the processes of precipitations and phase transformations, thermal vacancies play an important role through diffusions of atoms. Due to magnetic transitions, the thermal vacancy fraction becomes smaller in the ferromagnetic state comparing to the paramagnetic state. In this work, the effect of magnetic transitions on the vacancy formation was examined using Inden model for the magnetic excess Gibbs energy, which has been widely applied in the CALPHAD-type thermodynamic assessments. In the present work, the effect of magnetic transitions on SFeMag/R and HFeMag is estimated to be 0 ~ –0.5 and 0 ~ 0.06 eV, respectively, The differences between ferromagnetic and paramagnetic states of αFe are +0.06 eV for the enthalpy of vacancy formation, and −0.435 R for the entropy of vacancy formation.

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