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

Tetsu-to-Hagané Vol. 96 (2010), No. 10

  • Control of Swirl Motion of Bottom Blown Bubbling Jet Using an Immersion Cylinder

    pp. 579-585

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    DOI:10.2355/tetsutohagane.96.579

    Gas injection techniques have extensively been used in the steelmaking industry. When gas is injected into a reactor, for example, through a bottom nozzle at a relatively high gas flow rate, a jet of a mixture of gas and liquid is formed above the nozzle exit. This jet, called a bottom blown bubbling jet, swirls under a certain gas injection condition. The appearance of the swirl motion of bubbling jet is not expected in the steelmaking industry, as it causes dangerous splashing and slopping of molten steel. In this study, we focus on the control of the swirl motion using an immersion cylinder. The cylinder is immersed concentrically from the bath surface after a steady swirl motion is observed. Empirical equations are proposed for identifying the boundary of the regime in which the swirl motion survives even if the cylinder is immersed in the bath.
  • Influence of Hydrogen on Reaction Behavior of Sinter Under a Blast Furnace Simulated Condition

    pp. 586-591

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    DOI:10.2355/tetsutohagane.96.586

    In order to further reduce CO2 emission in ironmaking, utilization of a large amount of hydrogen to a blast furnace would be essential in the near future. In this work, influence of half substitute of hydrogen to bosh gas on reaction behavior of sinter under a blast furnace simulated condition was examined until 1400°C and investigated together with List model.
    Reduction of sinter finalized around 1100°C in the condition of H2 substitute, while the reduction retarded around these temperatures in the condition without H2 substitute, being likely because low melting slag containing residual FeO was formed.
    The condition of H2 substitute provided higher carbon contents in sinter after reduction than the condition without H2 substitute. This is likely because hydrogen promoted carburization to solid iron with CO gas.
    The condition of H2 substitute improved softening melting behavior of sinter by nearly 70°C, being because the condition inhibited the formation of the low melting slag.
    From analysis based on List model it was evaluated that an operation with half substitute of hydrogen to a basic bosh gas gave less coke rate 346 kg/THM and less BF production 7945 t/d compared with 461 kg/THM and 8999 t/d in a basic operation.
  • Fountain Pyrometer for Strip Temperature Measurement in Cooling Banks of the Hot Strip Mill

    pp. 592-600

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    DOI:10.2355/tetsutohagane.96.592

    Sumitomo Metal Industries, Ltd. (SMI) developed a new temperature measurement method (Fountain pyrometer) and associated control system, for hot strip cooled by water in cooling banks. Use of the Fountain pyrometer and control system achieved very precise temperature control of hot strip in the run-out table.
    Recently, high tensile steel has been applied to lightweight cars as a measure to reduce CO2. In order to stabilize material quality of a hot strip including high tensile steel, the temperature control in run-out table is important. But the accuracy of the conventional control was not good enough because the temperature of a hot strip couldn't be measured in cooling banks that had much cooling water, which disturbs thermal radiation from the hot strip surface and the stable measurement of hot strip temperature.
    SMI has developed Fountain pyrometer, which uses water purge to stabilize the optical pass of thermal radiation from hot strip surface. It was confirmed that Fountain pyrometers are reliable to measure hot strip temperature even in cooling banks where much cooling water exists. The range for Fountain pyrometer is above 360°C and its responsiveness is less than 20 ms. SMI also developed a new control system to use the Fountain pyrometers. Use of Fountain pyrometers and the control system achieved very precise temperature control of hot strip in the run-out table.
  • Tension Reference Optimization in Automatic Gauge and Tension Control for a Tandem Cold Mill

    pp. 601-607

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    DOI:10.2355/tetsutohagane.96.601

    During acceleration and deceleration in cold rolling, variation in friction between a strip and the rolls can adversely affect strip thickness accuracy. Though automatic gauge and tension control is being introduced, constant tension references are insufficient for adequate control over a wide range of rolling speeds. Therefore, we have optimized tension references which vary depending on rolling speed. In order to limit maximum tension to an acceptable value, the optimization problem is composed in a multi level way by using an approximate surface. Moreover, the lowest level optimization problem is decomposed into sub-problems to obtain a solution stably and quickly. Experimental results with a 5-stand tandem cold mill showed a decrease in off-gauge length.
  • Mechanism of Acicular Ferrite Formation in Low Carbon Steel Ti–B Weld Metals with Different Oxygen Levels

    pp. 608-613

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    DOI:10.2355/tetsutohagane.96.608

    Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view to understand the formation mechanism for acicular ferrite microstructure in mild and low alloy steel weld metals.
    The samples with different oxygen content in Ti–B weld metal were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from some inclusions and acicular ferrite had Kurdjumov–Sachs orientation relationship with austenite matrix. The inclusions contributing to acicular ferrite nucleation were multi-phase type consisting of amorphous phase and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and analysis of EDS that the Ti-enriched layer was TiO. The acicular ferrite had Baker–Nutting orientation relationship with TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO.
    Therefore, it is considered that the TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit.
  • Change in Hydrogen Desorption Characteristics of Mod.9Cr–1Mo Steel due to Creep

    pp. 614-619

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    DOI:10.2355/tetsutohagane.96.614

    Hydrogen is usually trapped by vacancy, dislocation, grain boundary, precipitate and void in steels and it is easily released from these trap sites by heating the steels. However, the desorption temperature ranges vary according to the kinds of trap sites and they can be separated in principle, because the binding energy between hydrogen and each trap site is different. It seems that this interesting nature of hydrogen is available for determining microstructural changes and defect formation processes during creep. In this study, the change in hydrogen desorption characteristic due to creep was investigated to discuss the applicability of this technique as a nondestructive procedure for evaluating creep damage accumulation of high Cr ferritic steels. The gauge and grip portions of creep ruptured specimens with different rupture times were charged with hydrogen by means of cathodic electrolysis. Subsequently, the thermal desorption spectroscopic (TDS) analysis was applied to those hydrogen-charged samples for measuring hydrogen desorption curve. Experimental results revealed that a clear peak appeared on the hydrogen desorption curve at around 65°C for the as-tempered. The peak temperature and the peak height of the gauge portion increased significantly resulting in the increase in amount of desorbed hydrogen as the creep rupture time increased, although those of the grip portion showed almost no difference. These changes in hydrogen desorption characteristic seemed to be attributable to the formation of geometric damages such as a creep cavity and microcrack rather than the microstructural changes, because the re-normalizing had almost no influence on the hydrogen desorption curve of the gauge portion.
  • Effect of Cobalt and Boron on Long-term Creep Rupture Strength of 12Cr Cast Steels

    pp. 620-628

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    DOI:10.2355/tetsutohagane.96.620

    Microstructure of creep ruptured specimens is investigated in order to understand the effect of Co and B on long term creep rupture strength of 12Cr cast steels. The specimen with 2.5% Co contains no delta-ferrite and has better tensile and impact properties than Co free specimens including about 1 vol% delta-ferrite. In short-term creep rupture region, Co containing steel has shown better strength than Co free steel, while in long term region such as over 7000 h at 923K, Co free steel has shown better strength. The addition of Co promoted recovery of microstructure. The difference between 20 ppm B and 50 ppm B steels in short-term creep rupture property was not observed, while in long-term region, 50 ppm B steel has a tendency to show better strength. From ATE method, it is considered that some of B have been included in carbide such as M23(CB)6, and retarded coarsening of carbide. According to calculation of thermodynamic equilibrium, solid solution content of W and Mo was reduced by Co addition. Therefore the thermodynamic stabilization of microstructure associated with the solid solution strengthening of W and Mo has decreased and the formation of Laves phase has been enhanced. It has been concluded that improvement of stabilization and enhancement of formation of Laves phase by 2.5% Co addition is one of the reasons of the deterioration of long-term creep rupture strength.
  • Characteristic of Proton Conductor Prepared by Spark Plasma Sintering in the Simulated Coke Oven Gas

    pp. 629-635

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    DOI:10.2355/tetsutohagane.96.629

    The SrZr0.9Y0.1O3-α of proton conductor was prepared by normal sintering and SPS (spark plasma sintering) methods in order to separate pure hydrogen gas from the simulated coke oven gas (COG) at high temperature. The SrZr0.9Y0.1O3-α obtained by normal sintering at 1853K for 10 h and SPS at 1773K for 3 min were found to be single phase of perovskite structure. The relative densities of SrZr0.9Y0.1O3-α obtained by SPS at 1673 and 1773K were over 95%. Furthermore, the relative density increased with sintering temperature and time of SPS. The proton conductivity of SrZr0.9Y0.1O3-α of SPS increased with sintering temperature, and was higher than one of normal sintering in dry and wet 10% H2–90% Ar gases atmosphere. On the other hand, the proton conductivity of SrZr0.9Y0.1O3-α of SPS in the dry and wet simulated COG gas atmosphere was equivalent to one in 50% H2–50% Ar gases atmosphere. Furthermore, the structure of SrZr0.9Y0.1O3-α was chemically stable in the dry and wet simulated COG gas atmosphere. The applied voltage (Vappl) at 1273K in the wet simulated COG gas atmosphere using SrZr0.9Y0.1O3-α was equivalent to one in wet 50% H2–50% Ar gases atmosphere, but the electrode polarization (Vappl-iR) was higher. Finally, the hydrogen gas was separated from the wet simulated COG gas atmosphere.
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