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

Tetsu-to-Hagané Vol. 103 (2017), No. 4

  • Softening Behaviors of Acid and Olivine Fluxed Iron Ore Pellets in the Cohesive Zone of a Blast Furnace

    pp. 175-183

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    DOI:10.2355/tetsutohagane.TETSU-2016-097

    A ferrous burden loses its permeability in the cohesive zone of a Blast Furnace (BF) which has an effect on the flow of reducing gases. Iron ore pellets with various chemical compositions have different softening properties. Due to the occurrence of numerous simultaneous phenomena the clarification of different variables is difficult. In this study the effect of Reduction Degree (RD) on the softening behavior of individual acid and olivine fluxed iron ore pellets was experimentally tested under inert conditions. The acid pellet softened rapidly at 1150°C and reached about 40% contraction at 1200°C. The olivine fluxed pellet softened gradually in the range of 1150 and 1350°C and reached 30-35% contraction. The RDs of 50-70% for acid and 50-65% for olivine fluxed pellet had no significant effect on the softening behavior. However, the highest contraction-% was reached with the lowest RD. The results indicate that softening of the pellets is caused by the softening of the pellet core. The early softening of the acid pellet was attributed to high SiO2 content and formation of fayalite slag with high wüstite solubility. The superior properties of the olivine fluxed pellet were attributed to the low SiO2 content and favorable effects of fluxes to prevent wüstite dissolution. FactSage V6.4 -software and its FToxid-database was used to compute the phase equilibrium of a pre-reduced pellet with a quaternary FeO-SiO2-CaO-MgO system in the core region. The computed phase equilibrium provided additional information about the effects of different components in the phase equilibrium.
  • Effect of Mn Addition on Scale Structure of Nb Containing Ferritic Stainless Steel

    pp. 184-193

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    DOI:10.2355/tetsutohagane.TETSU-2016-090

    The effect of the Mn addition on the oxidation resistance of high purity Nb containing 19% Cr ferritic stainless steels has been studied using specimens isothermally oxidized in air at temperatures from 1073 K to 1273K. The structures of the scale and the scale/metal interface were investigated in detail by means of FE-SEM, SEM-EBSD and FE-TEM.The addition of Mn was confirmed to impair the oxidation resistance. Moreover, the scale structure was significantly affected by the addition of Mn. After oxidizing, 1%- Mn steel in air for 720 ks at 1123 K, 5-μm thick oxidation scale formed. The Mn rich (Mn, Cr)3O4 spinel formed in the upper most layer following MnCr2O4 just above 3-μm thick Cr2O3 scale. Beneath the Cr2O3 scale, another MnCr2O4 spinel layer formed. The grain size of Cr2O3 of 1% Mn steel was surprisingly fine with approximately 200 nm and it turned out to be much smaller than that of the Mn-free steel where the 2-μm thick Cr2O3 monolayer consisting of the similar size of Cr2O3 formed. Such a difference is postulated to be related to the oxidation resistance. These effects of the Mn addition are inferred to stem from the fast diffusion of Mn through the grain boundaries of the very fine Cr2O3 scale. Furthermore, the scale of 1% Mn steel has a tendency to be bent into folds after long oxidation.
  • Influence of Induction Heating Condition on Quenched Structure of JIS-SUJ2 Steel

    pp. 194-200

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    DOI:10.2355/tetsutohagane.TETSU-2016-058

    When JIS-SUJ2 steel is hardened by induction heating, high temperature austenitizing is applied in order to achieve high productivity. In this case, SUJ2 is quenched from the single-phase austenite zone. As a result, the properties of SUJ2 may be different from the conventional furnace process involving SUJ2 quenched from the two-phase austenite / cementite zone. In this study, we experimentally investigated the relation between the amount of undissolved carbide, austenitizing temperature, and duration. Additionally, we developed a prediction formula for undissolved carbide in the single-phase austenite zone. The formula is consists of the Arrhenius equation and the Kolomogorov-Johnson-Mehl-Avrami equation. Subsequently, the effect of austenitizing temperature and amount of undissolved carbide on hardness, retained austenite content, prior-austenite grain size, and martensite block size were investigated.
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  • Effect of Loading Method and Mean Stress on Fatigue Strength of Super Duplex Stainless Steel

    pp. 201-207

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    DOI:10.2355/tetsutohagane.TETSU-2016-103

    As a first step toward building a database of fatigue strength of super duplex stainless steel S32750, fatigue tests have been conducted by three methods, axial loading, plane bending and rotating bending, and with some different values of stress ratio R ranging from –1 to 0.4. The fatigue limits from the bending tests turned out appreciably higher than those from the axial loading. This is attributed to a lower stress amplitude than the nominal value designed in the bending tests, which results from partial yielding in the surface region. The influence of the partial yielding on the stress amplitude has been corrected for by calculating the non-linear stress distribution inside the specimen by following the method proposed by Koe et al. (1983). The corrected values of the fatigue strength from the bending tests match those from the axial loading. With this correction to both the mean stress and the stress amplitude, the effects of varying mean stress on the fatigue limit follows modified Goodman’s law, and the fatigue strengths at shorter times form a universal S-N diagram by employing the equivalent stress amplitude proposed by Smith, Watson and Topper (1970), except for a single set of data from a test with a high mean stress, R=0.4.

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