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

Tetsu-to-Hagané Vol. 101 (2015), No. 12

  • Effects of Hydrogen and Nitrogen Gas Mixture on Nitrogen Absorption Rate in Low Carbon Steel Melt

    pp. 627-635

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    DOI:10.2355/tetsutohagane.TETSU-2015-026

    To clarify the effects of blowing a H2-N2 mixture onto the surface of molten steel containing various oxygen contents on the absorption reaction of nitrogen in the molten steel, experimental studies were carried out using a 20 kg induction furnace. Blowing of the H2-N2 mixture accelerates the nitrogen absorption rate because the oxygen concentration at the gas-metal interface is decreased by the reducing effect of the hydrogen gas. The apparent chemical reaction rate of nitrogen absorption in the present work was evaluated at almost the same reaction rate as that of desorption of nitrogen in previous works. Furthermore, a mathematical model for the nitrogen absorption and desorption reactions in a RH degasser was developed in order to estimate the contribution of each nitrogen reaction site during decarburization and killing treatment with injection of pure N2 gas or the H2-N2 mixture in the molten steel. Using the mathematical model, it was estimated that a larger increase in the nitrogen concentration during decarburization treatment could be achieved by applying a 30% H2-N2 mixture to the injection gas in the RH degasser than by applying pure N2 gas.
  • Removal of Cu in Carbon Saturated Iron by Sulfurization via Ag Phase

    pp. 636-644

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    DOI:10.2355/tetsutohagane.TETSU-2015-039

    The copper distribution ratio between the Na2S flux and silver, LCu(flux-Ag) (=[mass% Cu](in flux) / [mass% Cu](in Ag)), was measured at 1473 K in order to know the copper capacity of Na2S flux. As the greatest value, LCu(flux-Ag) = 42 was obtained. By combining the LCu(flux-Ag) value with the distribution ratio of copper between the silver and the carbon-saturated iron, LCu(Ag-Fe), the distribution ratio of copper between the Na2S flux and carbon-saturated iron, LCu(flux-Fe), is derived to be 330 at 1473 K. Moreover, the sulfurization removal of copper in iron silver phase into Na2S flux has been tried at 1473 K. Silver can keep iron from being sulfurized, which enables to maintain the high copper capacity of the Na2S flux. For this reason, the LCu value increases with an increase of sulfur potential. Copper removal proceeds at lower 0.1mass%Cu, and the Cu content decreased to 0.06mass%. The silver phase prevents the sulfur dissolution into the iron, and the sulfur content of iron can be kept lower ( [mass%S]in Fe-C < 0.007-0.02).
  • Kinetics of Iron Carburization via Slag Containing Sulfur at 1723 K

    pp. 645-652

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    DOI:10.2355/tetsutohagane.TETSU-2015-035

    Kinetics of iron carburization via CaO-SiO2-Al2O3 slag containing sulfur at 1723 K was investigated. The simulated blast-furnace (B.F.) slag (CaO/SiO2=1) containing sulfur and high basicity slag (CaO/SiO2=7.6) containing sulfur were prepared. The rate of carburization of iron through the high basicity slag containing sulfur was higher than the rate of carburization of iron through the simulated blast-furnace slag containing sulfur. Furthermore, the rate of carburization of iron through the slag containing sulfur was higher than the rate of carburization of iron through the slag without sulfur. On the other hand, the rates in the middle stage of carburization of iron through the slag containing sulfur were k=6.15×10–5 mol/m2·s (the simulated blast-furnace slag) and k=1.35×10–4 mol/m2·s (the high basicity slag), and were much higher than the other stages, and were influence by the existence of sulfur in the slag. The rates in the last stage of carburization of iron through the slag containing sulfur were k=2.95×10–5 mol/m2·s (the simulated blast-furnace slag) and k=6.21×10–5 mol/m2·s (the high basicity slag), and were closed to the rates carburization of iron through the slag without sulfur, and were not influenced by the existence of sulfur in the slag.
  • Effect of Natural Gas Injection into Blast Furnace on Combustion Efficiency of Pulverized Coal

    pp. 653-660

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    DOI:10.2355/tetsutohagane.TETSU-2015-052

    Reducing CO2 emission in ironmaking process is pressing issue. Low RAR (reducing agent rate) operation of the blast furnace and the utilization of hydrogenous reducing agent are effective to reduce CO2 emission. In this study, the influence of the hydrogenous reducing agent on the combustibility of the pulverized coal was examined by using a small scale combustion furnace. As a result, the combustibility of the pulverized coal was improved by simultaneous injection of the pulverized coal and the hydrogenous reducing agent. Furthermore, the fundamental study about the effect of natural gas (CH4) injection point on the combustibility of the pulverized coal was conducted by experiment using above mentioned small scale combustion furnace and by three-dimensional numerical analysis for further high efficiency. In the case of the relative position of CH4 injection point and the pulverized coal injection point being near, the ignition point of the pulverized coal came closer to lance tip. Especially, in the case of CH4 injection point being upstream in blow pipe about 0 to 20 mm from the pulverized coal injection point, the fastest ignition of the pulverized coal was confirmed by experimental and calculation results.
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  • Generation Mechanism of Unsteady Bulging in Continuous Casting-1 –Development of Measurement Method for Amount of Unsteady Bulging in Continuous Casting–

    pp. 661-666

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    DOI:10.2355/tetsutohagane.TETSU-2015-031

    Fluctuation of inter-roll bulging in a commercial continuous casting machine was measured in consideration of the fluctuation of the relative distance from the segment to the ground, and compared to the cycle and volume fluctuation of the mold level. The following results were obtained.
    1. The amount of segment fluctuation was much smaller than the amount of inter-roll bulging. Therefore, segment fluctuation did not affect the measured results of inter-roll bulging.
    2. Inter-roll bulging and the mold level fluctuated with the same cycle, and this cycle corresponded to the cycle calculated from the roll pitch and casting speed. Therefore, it was confirmed that the value measured by an ultrasonic range finder in this study was unsteady bulging.
    3. The amplitude of the mold level converted from the amount of fluctuation of inter-roll bulging corresponded to the actual mold level. Therefore, the amount of fluctuation of inter-roll bulging measured in this study was considered reasonable.
    In addition, the unsteady bulging shape was estimated.
  • Generation Mechanism of Unsteady Bulging in Continuous Casting-2 –FEM Simulation for Generation Mechanism of Unsteady Bulging–

    pp. 667-672

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    DOI:10.2355/tetsutohagane.TETSU-2015-032

    In the continuous casting of steel, unsteady bulging contribute to degradation of the slab quality. It has been reported that unsteady bulging is promoted by uneven solidified in the mold, but the effect of uneven solidified on unsteady bulging had not been clarified. In this study, a FEM (finite element model) simulation was constructed. Shell deformation was calculated by an elasto-plastic analysis assuming that the slab moves between the rolls, considering time dependency. The bulging value and mold level fluctuation, which change corresponding to the solidified shell thickness, ferrostatic pressure and roll pitch, were obtained.
    In the simulation results, the shell is deformed by ferrostatic pressure. The bulging shell pushes out under the rolls in the thickness direction, and unsteady bulging causes. While the shell is passing through the same pitch rolls, unsteady bulging becomes larger. When the solidified shell is uneven, stress concentrates on the thinner portions. The stress concentration accelerated the unsteady bulging even at the same average shell thickness. Based on this result, an operational index for suppressing unsteady bulging by reducing unevenness of the solidified shell is proposed.

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