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

Tetsu-to-Hagané Vol. 98 (2012), No. 12

  • Behavior of Heat Transfer to Molten Metal by Burner Combustion with Addition of Raw Material Particles

    pp. 627-633

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    DOI:10.2355/tetsutohagane.98.627

    In order to increase the amount of chromium ore in smelting reduction furnace, it is important to increase the heat supply to the furnace. Many techniques have been developed with the aim of increasing post -combustion. However, the heat transfer efficiency of post -combustion to the molten metal is low. This is also a factor in reduced refractory life. In this study, experiments with a 4 ton furnace and a numerical simulation were conducted to consider a method of promoting heat transfer from burner combustion heat to the metal. The behavior of heat transfer from the burner to the metal by feeding heated particles of raw materials through the flame was investigated. It was found that:
    1) The temperature increment of the metal is increased by feeding of heated particles of raw materials through the burner flame.
    2) The quantity of heat transferred from the flame to the metal increases as the feeding rate of heated particles is increased.
    3) From the results of the numerical simulation, both the particles temperature and the flame temperature decrease as the particle feeding rate increases. However, the total sensible heat of all particle raw materials increases.
    4) As the feeding rate of heated particles increases, heat transfer from the flame by radiation and convection decreases, and heat transfer by the sensible heat of the heated particles increases.
    As a result, the particles heated by the flame function as a medium of heat transfer from the flame to the molten metal.
  • Effect of Slag Melting Behavior on Metal-Slag Separation Temperature in Powdery Iron, Slag and Carbon Mixture

    pp. 634-640

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    DOI:10.2355/tetsutohagane.98.634

    Utilization of self reducing pellet in blast furnace is one of the effective technologies to mitigate the CO2 emissions in the steel industry. However, there are not sufficient researches on how the pellet behaves around cohesive zone. Therefore, purpose of this study is to clarify the effect of slag melting behavior on metal-slag separation behavior. In order to simulate the behavior around cohesive zone, electrolytic iron powder, carbon powder, and synthetic slag were prepared as reduced iron, residual carbon and slag components, respectively. They were well mixed as given mass ratios decided from the composition of the self reducing pellet. Different kinds of slag compositions were adopted to change properties, such as melting temperature and viscosity. The mixtures were pressed into tablets and used as experimental samples. “In-situ” observations of metal-slag separation behavior in the mixtures during constant rate heating were done by a laser microscope combined with infra-red furnace and metal-slag separation temperatures were decided. Following results were obtained. The metal-slag separation behavior was dominated largely by agglomeration behavior of liquid phase of iron. The separation surely occurred when both phases of iron and slag change to liquid phases. At the same time, the separation could also occur even if a small amount of solid phases still remained in the mixture. In this experimental condition, the effect of slag melting temperature on the separation was larger than the effect of slag’s viscosity.
  • Influence of Steel Grade on Oxidation Rate of Molten Steel in Tundish

    pp. 641-649

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    DOI:10.2355/tetsutohagane.98.641

    The influence of steel grade on the oxidation rate of molten steel in tundish was studied by conducting oxidation experiments on the Ti and Ti-Al deoxidized molten steel and comparing the obtained oxidation rates with that of the Al deoxidized molten steel as measured in a previous report. In the still state, the oxidation rate of the Ti deoxidized molten steel is faster than those of the Ti-Al deoxidized molten steel and the Al deoxidized molten steel, showing dependence on the steel grade. This means that in the still state, while the oxidation rates of the Ti-Al and Al deoxidized molten steel are controlled by the mass transfer of oxygen in the oxide film, the oxidation rate of the Ti deoxidized molten steel is controlled by the mass transfer of O2 gas in the gas phase because the surface is not covered with oxide films. In addition, in the stirred state, the oxidation rates of the Ti and Ti-Al deoxidized molten steel become faster than that of the Al deoxidized molten steel in the region where the O2 gas partial pressure exceeds 10 kPa. This dependence on the steel grade can be explained by the mechanism of accelerating the mass transfer in the gas phase due to active iron evaporation in the Ti and Ti-Al deoxidized molten steel, in which the surface disturbance is larger than that in the Al deoxidized molten steel.
  • Bubble Formation into Flowing Liquid Through a Refractory Having Bad Wettability and Inclusion Removal in Tundish by Ar Bubbles

    pp. 650-657

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    DOI:10.2355/tetsutohagane.98.650

    The injection method of Ar gas into the molten steel through porous plug is usually applied to the removing process of the nonmetallic inclusions. It is commonly confirmed by many studies on this method that the availability of the injected Ar gas increases with the decreasing bubble diameter. In this paper, the influence of the pore diameter, flow rate of Ar gas, the wettability between liquid and porous plug and the streaming velocity of liquid on the bubble diameter is experimentally examined and the obtained results are analytically discussed. When Ar gas is injected into the stationary liquid through the wettable porous plug the bubble diameter increases with the increasing pore diameter and flow rate of Ar gas. In case of the wettability is bad the bubble diameter grows up greater and shows almost constant value regardless of the injection conditions. To make this grown bubble fine it is effective to give a stream to the liquid. The experimental results were applied to the manufacturing equipment aiming to make the molten steel clean in tundish. This test showed that the Ar gas bubble by the streaming molten steel has a removing ability of the nonmetallic inclusions in molten steel.
  • Formation of Fine Protrusions on Steel Sheets and Wires by Sputter Etching

    pp. 658-666

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    DOI:10.2355/tetsutohagane.98.658

    Six kinds of steels with various thickness smaller than 3mm and SUS304 steel wires with 0.5 and 0.2 mm diameter were etched by argon ion sputtering at a radio frequency power of 100-350W for 0.9-19.8ks. Cone-shaped protrusions with various sizes smaller than 5μm were formed on the surface of SK120, SUS304 and SKD61 steel sheets. These protrusions were formed from the bottom of pillar-shaped carbides that precipitate perpendicular to the surface. When the sheet thickness is very small, the conical protrusions were not formed, but the column- or ringshaped protrusions with various sizes smaller than 1μm were formed on the surface of SUS304, SUS316, SUS316L and SKD61 steels. The reason will be that the precipitation of fine pillar-shaped carbides occurs densely due to a small temperature gradient in the thickness direction and the conical protrusions quickly decay to the column- or ring-shaped protrusions. Also for the SUS304 wire, the fine column- or ringshaped protrusions were formed over whole of the surface due to a roundabout of plasma. The sheets and wires with not only the cone-shaped protrusions but also the column- or ring-shaped ones will give wide and flexible applicability to machine parts and structures revealing many functions.
  • Change in Dislocation Mobility with Ni Content in Ferritic Steels and Its Effect on Brittle-to-Ductile Transition

    pp. 667-674

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    DOI:10.2355/tetsutohagane.98.667

    In order to elucidate the mechanism behind the decrease in the brittle-to-ductile transition (BDT) temperature with the addition of Ni, impact tests and tensile tests were performed at various test temperatures from 130K to 320K with Ni added ultra-low carbon steels. The dependence of absorbed impact energy on temperature and the Ni content indicates that the BDT temperature was decreased with the increasing Ni content, which suggests that the dislocation mobility at low temperatures was increased with the Ni content. The yield stress which is also influenced by the dislocation mobility was decreased at low temperatures while it was increased at room temperature with the Ni content. The values of the activation volume and the effective stress were measured at several temperatures, and then the dependence of the activation energy for dislocation gliding on Ni content was obtained by extrapolating the relation between temperature and the multiplication of the activation volume and the effective stress to 0K. The activation energy was found to decrease with the increasing Ni content, which suggests that the dislocation mobility was increased with the addition of Ni. Discrete dislocation dynamics simulation was also performed in order to calculate the dependence of fracture toughness on temperature and the Ni content, and it was clarified that the BDT temperature is decreased by increasing dislocation mobility.

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