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

Tetsu-to-Hagané Vol. 97 (2011), No. 5

  • Influence of Hydrogen Content on Post Combustion in Converter

    Goro Okuyama, Toshio Yamada, Yasuo Kishimoto, Kazuhiro Yamamoto, Naoki Hayashi, Hiroshi Yamashita

    pp. 245-251

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    DOI:10.2355/tetsutohagane.97.245

    Abstract

    In this study, numerical simulation of high temperature combustion in CO–H2 mixture with oxygen jet has been conducted to consider post combustion in a converter for steel making process. In the numerical model, a detailed reaction model and mixture fraction model were used. The flammability characteristics and flame structure were compared between both calculation results. The influence of H2 content in the mixture and the reaction mechanism of the combustion were discussed. In mixture fraction model, the post combustion ratio decrease with an increase of H2 content, but the influence of H2 content on post combustion ratio is small. On the other hand, in the detailed reaction model, the influence H2 content on post combustion ratio is bigger, and the post combustion ratio takes its maximum when the molar hydrogen concentration is 2%. When the molar hydrogen concentration is 20% in CO–H2 mixture, CO2 is decomposed around the edge of combustion zone, where the endothermic reaction occurs. Resultantly, the temperature in the downstream region is reduced. In the CO–H2 combustion, water vapor is also decomposed by the shift reaction through CO+H2O→CO2+H2.

  • Effect of Mixed Gas Top Blowing on Decarburization and Manganese Evaporation of Steel Melt in Low Carbon Content Range

    Yoshiei Kato, Takeshi Suzuki, Yasuo Kishimoto

    pp. 252-258

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    DOI:10.2355/tetsutohagane.97.252

    Abstract

    Experimental and theoretical studies have been carried out to clarify the effects of mixed gas top blowing on decarburization and manganese evaporation of steel melt in low carbon content range.
    20 kg scale of induction furnace was used for decarburization experiment. Oxygen utilization efficiency for decarburization increased with an increase in inert gas flow rate and a decrease in oxygen flow rate of mixed gas top blowing. Decarburization rate below carbon content of 0.03 mass% depended on CO partial pressure caused by mixed gas of oxygen and inert gas. To explain these results, an index, IM for decarburization was introduced. The IM is found to be inversely proportional to the oxygen utilization efficiency for decarburization in the low carbon content range, and there is a good correlation between the IM and oxygen content in steel melt.
    Unexplained manganese content, which seems to depend on its evaporation, decreased with an increase in inert gas ratio of top blowing to oxygen. Manganese evaporation rate was introduced according to Hertz–Knudsen–Langmuir's equation which showed an increase with increasing temperature. Assuming that manganese evaporation occurs at the fire spot caused by top blowing during the final stage of refining, the temperature of the fire spot was estimated to reach 2890K for oxygen top blowing, and decrease to 2475K for mixed gas blowing of (Ar+N2)/O2=2/1.

  • The Change in Composition of Inclusions in Molten Steel during Simultaneous Addition of Ca alloy and CaO–Al2O3 flux

    Mitsuhiro Numata, Yoshihiko Higuchi

    pp. 259-265

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    DOI:10.2355/tetsutohagane.97.259

    Abstract

    In order to examine the effects of calcium addition method on inclusion composition, the variations of the inclusion composition were measured during simultaneous addition of calcium alloy and CaO–Al2O3 flux to the molten steel by 180 kg scale experiments. Fifty four grams CaSi alloy mixed with 13.5 g CaO–Al2O3 flux was added 10 times at the interval of 1 min and the samples of molten steel were sampled during the experiments. The composition of the molten steel and the composition of the inclusions were analyzed. From the experiments and chemical reaction kinetic calculation, the effects of CaO–Al2O3 composition in flux on the changes in the inclusion composition were examined. The results obtained are summarized as follows;
    (1) Calcium concentration in molten steel increased with increasing CaO concentration in CaO–Al2O3 flux.
    (2) The CaO concentration in inclusion at which CaS concentration began to increase was changed by the composition of CaO–Al2O3 flux.
    The results of the reaction rate calculation considering the reaction of calcium in molten steel and flux showed a good agreement with the experimental results.
    (3) The reaction path of the variation of inclusion concentration was changed as a result of the reaction among calcium in molten steel, inclusion and flux.
    (4) It was suggested that an inclusion composition could be optionally controlled by adding CaSi alloy and CaO–Al2O3 flux at the same time.

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  • Two-dimensional Defect Map for the Deformed Pure Iron Samples by Positron Probe Microanalyzer

    Masanori Fujinami, Yuji Kawashima, Kyousuke Yanagi, Satoshi Jinno, Masahito Uchikoshi, Minoru Isshiki, Shigeru Suzuki

    pp. 266-272

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    DOI:10.2355/tetsutohagane.97.266

    Abstract

    We have developed positron probe microanalyzer (PPMA) for obtaining two-dimensional map of open-volume type defects. The primary positron beam with 4 mm diameter was guided in a static magnetic field, followed by focusing to the Ni(100) foil with 150 nm thick for the brightness enhancement. The remoderated positron beam re-emitted from the Ni foil was accelerated and focused to the specimen by the magnetic objective lens. The focused beam diameter was estimated to be 14.5 μm. PPMA has been applied to investigate the defect behavior in a plastic deformation for polycrystalline high-purity Fe and Fe–0.5mass%Cu alloy samples, and Doppler broadening of the positron annihilation line has been used to map the defect distribution. The results showed that the defects were heterogeneously introduced by deformation and the concentration of the defects was high in a necking region. This can be attributed to the inhomogeneous deformation in the polycrystalline sample. A formation of vacancy clusters in the vicinity of the fractured point is suggested by a large change in Doppler broadening of the positron annihilation line. It has been, further, proven that Cu precipitation in Fe–0.5Cu alloys by annealing is enhanced by the defects induced by a plastic deformation.

  • Development of Simultaneous Ion-Exclusion/Anion-Exchange Chromatographic Determination of Ammonium, Nitrite and Nitrate Ions and Its Application to High-Salt Content Sample

    Masanobu Mori, Takahiro Hironaga, Tatsuya Satori, Hideyuki Itabashi, Nobutake Nakatani, Daisuke Kozaki, Kazuhiko Tanaka

    pp. 273-278

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    DOI:10.2355/tetsutohagane.97.273

    Abstract

    Simultaneous chromatographic separation of nitrate ion (NO3−), nitrite ion (NO2−) and ammonium ion (NH4+) was obtained using single injection, single column and a detector (conductivity). The principle is based on selective ion-exclusion effect for NH4+ and anion-exchange effects for NO3− and NO2− on an OH−-formed basic anion-exchange resin column (Tosoh TSKgel Super IC-AZ) with basic eluent (2 mol/m3 KOH). The present system could determine NH4+, NO2− and NO3− in 500-fold diluted artificial seawater sample in 40 min at 0.6 mL/min of flow rate, by standard addition method. Also, this method was applicable to nitrogen analysis in artificial seawater with iron matrix, by pre-column packed with an iminodiacetate chelating resin in order to remove iron or calcium ions.

  • Influences of Si and Al on Mechanical Properties of Low Alloy TRIP-assisted Multiphase Steel Sheets

    Tatsuya Nakagaito, Hiroshi Matsuda, Saiji Matsuoka, Yasushi Tanaka

    pp. 279-287

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    DOI:10.2355/tetsutohagane.97.279

    Abstract

    Effects of Si and Al on the mechanical properties of low alloy TRIP-assisted steels were investigated especially focusing on characteristics of retained austenite and work hardening behavior of ferrite. 0.17%C–1.5%Si–Mn and 0.17%C–1.5%Al–Mn steels, of which the microstructures consist of a ferrite matrix with dispersed bainite, retained austenite and martensite, were used in the study. Higher elongation was obtained with increase in carbon content of retained austenite between the steels of the same chemical compositions. However, the change in ductility can not be explained only by the stability of retained austenite. The 1.5% Si added steel exhibits higher ductility than the 1.5% Al added steel in spite that the stability and the volume fraction of retained austenite are almost the same. The ferrite matrix was highly work-hardened in the 1.5% Si steel than in the 1.5% Al steel. Considering the fact that Si addition lowers stacking fault energy and depresses cross slip, high work-hardening rate of ferrite phase may be caused by suppressing dynamic recovery of dislocations during tensile straining in the Si added steels. The results strongly suggest that not only the characteristics of retained austenite but also the work-hardening behavior of ferrite phase affects the ductility of low alloy TRIP-assisted steels.

  • Absorption of Hydrogen in High-Strength Low-Alloy Steel during Tensile Deformation in Gaseous Hydrogen

    Koichi Takasawa, Ryoji Ishigaki, Yoru Wada, Rinzo Kayano

    pp. 288-294

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    DOI:10.2355/tetsutohagane.97.288

    Abstract

    Absorption of hydrogen in a high-strength nickel–chromium–molybdenum steel during tensile deformation in 0.5 MPa gaseous hydrogen was examined using a thermal desorption analysis method. The tensile strength of the specimen was varied in the range from 1214 to 947 MPa by heat treatment. The dislocation density of the specimens was measured by X-ray diffractometry after tensile testing in a hydrogen atmosphere. The hydrogen content absorbed during tensile deformation increased with increasing tensile strain in proportional elastic range until just before yielding. The yield stress was defined as 0.2% proof stress in this work. At the same tensile strains, the hydrogen content of lower-strength specimens was larger than that of higher-strength specimens. The dislocation density gradually decreased until just before yielding, corresponding to the proportional increase of hydrogen content to the tensile strain. This implies that the hydrogen absorption behavior during tensile deformation in gaseous hydrogen is related to the motion of mobile dislocations initially contained in the specimens. The activation energy for desorption of hydrogen absorbed during tensile deformation did not depend on the strength of the steel. This indicates that the trap sites of hydrogen atoms created through the tensile deformation were the same regardless of the strength levels.

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  • Effect of Carbon, Nitrogen and Nickel on Long-term Creep Rupture Strength of 10 Cr Steels Containing Boron

    Masahiko Arai, Kentaro Asakura, Hiroyuki Doi, Hirotsugu Kawanaka, Toshihiko Koseki, Toshiaki Horiuchi

    pp. 295-304

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    DOI:10.2355/tetsutohagane.97.295

    Abstract

    Microstructure and precipitates of creep ruptured specimens were investigated in order to understand the effects of carbon (C), nitrogen (N) and nickel (Ni) on long-term creep rupture strength of 10Cr heat-resistant steels containing boron (B). The low-N steels showed higher creep rupture strength than the high-N steels. In long-term creep rupture region such as over 10,000 h at 650°C, the deterioration of creep rupture strength was not observed in the low-N, high-C steel. On the other hand, the creep rupture strength of the low-N, low-C steel dropped to the strength level of the high-N steels. The addition of N to the B containing steels promoted the recovery of microstructure. The formation of coarse BN in the high-N steels led to the decrease of the amount of effective B that dissolved in M23(C,B)6 and suppressed its coarsening. From EDS analysis of precipitates, the fraction of M23C6 in the low-C steels was less than that in the high-C steels, while the fraction of coarse Laves phase in the low-C steels was more than that in the high-C steels. In the low-N, low-C steel, the coarsening of precipitates caused the deterioration of the creep rupture strength after 10,000 h exposure. Ni lowered Ac1 and Ac3 transition temperature, but it did not affect the fraction of precipitates according to the calculation of thermodynamic equilibrium using Thermo-Calc. It is concluded that C and N are more effective to the stability of microstructure than Ni.

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  • Oxidation Behavior of β+γ TiAl Alloys with an Addition of Cr at Elevated Temperatures

    Kouichi Niinobe, Shinsaku Endou, Naoyuki Takiyama

    pp. 305-314

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    DOI:10.2355/tetsutohagane.97.305

    Abstract

    Oxidation behavior of β+γ TiAl alloys with an addition of Cr was evaluated by cyclic oxidation tests at temperatures ranging from 1073 to 1173K under the atmospheric environment, comparing with that of a binary Ti–49Al alloy. The scales formed on the Cr added alloys didn't spall in the oxidation test at 1073K, even though that of Ti–49Al alloy spalled markedly. Ti–44Al–8Cr and Ti–45Al–9Cr with high Al and high Cr contents showed little scale spallation up to 1123K and the mass gains were found to be much smaller. To the contrary, the mass gains of Ti–40Al–7Cr and Ti–42Al–6Cr with low Al and low Cr contents were significantly large, similar to that of Ti–49Al. The scales formed during the oxidation test were composed predominately of five layers from outside to the TiAl substrate, i.e., TiO2/Al2O3/TiO2+Al2O3/Al2O3/α2-Ti3Al. Even in the Cr added alloys, Cr2O3 wasn't detected by X-ray diffraction analysis and scanning electron microscopy observation. It is found in Ti–44Al–8Cr and Ti–45Al–9Cr that the oxide layers of Al2O3 and α2-Ti3Al, which were formed in the vicinity of the scale/substrate interface, are significantly thin compared with those of the other specimens. This appears to influence the excellent oxidation behavior in the Cr added alloys with high Al and high Cr contents.
  • © 2011 The Iron and Steel Institute of Japan

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