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

Tetsu-to-Hagané Vol. 97 (2011), No. 8

  • Effect of Changes in Sulfur and Oxygen Concentration on Change in Nitrogen Concentration in Liquid Steel During CaO Powder Blowing under Reduced Pressure

    pp. 407-415

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    DOI:10.2355/tetsutohagane.97.407

    To examine the change of the rate of nitrogen desorption with sulfur concentration and low oxygen concentration change, the rates of nitrogen desorption from molten steel were measured with the blow of CaO powder or argon gas onto the molten steel through a top lance under the reduced pressure. 15 kg of electrolytic iron was melted molten in a MgO crucible by a high frequency induction furnace and the temperature of the melt was held at 1873K. Aluminum, FeS and other alloying elements were added and composition of molten steel was kept at 0.001–0.19%Al, 0.0007–0.0064%S, 0.002–0.006%N, 0.5%Mn and 0.2%Si. CaO powder was blown onto molten steel with argon gas for 20 min under the reduced pressure of about 670 Pa and metal samples were taken during CaO powder blowing in order to clarified the removal rate of sulfur and nitrogen. The rate constant of desulfurization was maintained during CaO powder blowing, however, the apparent rate constant of nitrogen desorption increased gradually. A mathematical model of nitrogen desorption was presented with the consideration of the chemical reaction of aluminum and sulfur in molten steel with CaO flux, respectively. The calculated nitrogen concentration by the model were in good agreement with the experimental values. It was estimated that the apparent rate constant of nitrogen desorption was increased as a result of decrease of oxygen and sulfur concentration in moleten steel by the reaction between aluminum in molten steel and CaO flux.
  • Phosphorus Separation from Steelmaking Slag by High Temperature Reduction with Mechanical Stirring

    pp. 416-422

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    DOI:10.2355/tetsutohagane.97.416

    The effects of temperature (1473–1873K) and slag composition (CaO/SiO2=1.2–4.0) on reduction of (FexO) and (P2O5) in steelmaking slag by carbon with mechanical stirring were investigated in laboratory-scale experiments. The results were summarized as follows.
    1) The reduction ratios of (FexO) and (P2O5) in steelmaking slag obtained in this research were 0–98% and 3–98%, respectively. These values were highly dependent on both slag basicity and the reduction temperature.
    2) (P2O5) was reduced by carbon and concentrated in iron, which was obtained by reduction of (FexO). Higher reduction ratios of (P2O5) exceeding 50% were obtained under the condition that the activity of (FexO) in the slag was less than 0.01. The (P2O5) reduction ratio decreased as the equilibrium phosphorus partition ratio between slag and metal calculated by slag composition and temperature increased.
    3) From the phosphorus mass balance before and after experiments, 1–29 mass% of unidentified phosphorus was confirmed. The ratio of unidentified phosphorus increased as the equilibrium P2 gas partial pressure calculated with an equation for the gaseous dephosphorization reaction increased. The mechanism of (P2O5) reduction in steelmaking slag was discussed based on the results of phosphorus distribution and observation of the reduced slag by EPMA analysis.
  • Numerical Simulation on Inclusion and Bubble Entrapment in Solidified Shell in Model Experiment and in Mold of Continuous Caster with DC Magnetic Field

    pp. 423-432

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    DOI:10.2355/tetsutohagane.97.423

    A model experiment investigating entrapment of inclusions and bubbles on the solidified shell was performed using molten steel, and the conditions for inclusion and bubble entrapment and mechanism of entrapment were studied. The results were applied to the flow behavior in the casting mold of a continuous caster.
    1. At the solid–liquid interface, entrapment of inclusions is greatly reduced by the existence of a low velocity flow, e.g., 0.05 m/s.
    2. The above-mentioned interfacial flow velocity dependency of inclusion entrapment is considered to be largely influenced by changes in the thickness of the concentration boundary layer, which depend on the interfacial flow velocity. Specifically, bubbles and inclusions which enter the concentration boundary layer are drawn to the solid–liquid interface by a suction force which is several orders larger than the Saffman's force.
    3. In addition to the above-mentioned suction force, the so-called cleaning effect is determined by fluid-dynamic forces such as drag force, etc. which act on particles, and furthermore, by resident time of particles at the solid–liquid interface, which depends on the solidification rate.
    4. In a FC mold with a 2-stage electromagnetic brake, flotation of bubbles entrained in the jet flow from the nozzle is accelerated with the large DC magnetic field. This is attributed to the braking effect of the DC field on the nozzle jet and the upward flow by the buoyancy of the bubbles. As a result, the interfacial flow velocity can be normalized by increasing the strength of the magnetic field, and entrapment of large bubbles and inclusions can be reduced.
  • Development of Anti-entrapment Mould Flux which Crystallizes into Melilite as a Main Phase

    pp. 433-440

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    DOI:10.2355/tetsutohagane.97.433

    Two characteristics, high basicity and high viscosity, are required for the anti-entrapment mould flux. However, it had been difficult to apply the high basicity and high viscosity mould flux for a round billet casting because of heat flux fluctuations in the mould.
    We supposed that the heat flux fluctuations were caused by unstable crystallization of the flux film between the mould and the solidified shell. Assuming that the contaminating constituents which do not compose the main crystal phase bring the unstable crystallization of the flux film, we have researched an adequate main crystal for the high basicity and high viscosity mould flux which has proper composition and melting point. As a result, there was no proper single crystal, but we have found an adequate combination of two crystals, gehlenite and akermanite. These two crystals have similar constructions each other to make a solid solution, melilite. Melilite behaves as a single phase crystal as well as it can take any compositions between gehlenite and akermanite. These properties of melilite diminish restraints on mould flux design.
    Consequently, we have successfully developed the anti-entrapment mould flux for the round billet casting, achieving a result to reduce mould flux defects effectively.
  • A Mechanism of Improvement in the Corrosion Resistance of Ferritic Stainless Steels by Cu Addition

    pp. 441-449

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    DOI:10.2355/tetsutohagane.97.441

    A mechanism of improvement in the corrosion resistance of ferritic stainless steels by Cu addition was studied by field exposure tests, XPS analysis of passive films and electrochemical evaluation.
    Corrosion area ratio after a 1-month field exposure test was the smallest with 0.4%Cu addition among 21%Cr–0~2.0%Cu stainless steels, and chromium concentration of the passive films was higher with the Cu-bearing stainless steels than the Cu-free stainless steel. After an 18-month field exposure test, chromium concentration of the passive films of high Cr and Cu bearing stainless steels was higher than that of low Cr and Cu bearing stainless steels. This result suggested an existence of a synergistic effect between Cr and Cu.
    Cu in ferritic stainless steels accelerates cathodic reaction and reduces anodic reaction when the steels dissolve in the active potential region. Therefore, Cu in the steels increases the open circuit potential at the passivation stage, and the increase of open circuit potential promotes the enrichment of chromium concentration of passive films on the Cu-bearing ferritic stainless steels. The cycle of dissolution and passivation strengthens the passive films on the Cu-bearing stainless steels.
    On the other hand, the 1-month field exposure test revealed corrosion area ratio of stainless steels bearing more than 1% Cu was larger than that of a 0.4%Cu-bearing stainless steel. Stainless steels containing more than 1% Cu contain ε-Cu in matrix. It is considered that precipitation of ε-Cu deteriorate the corrosion resistance of these stainless steels.
  • Influence of Strain Rate on TRIP Effect in SUS301L Metastable Austenite Steel

    pp. 450-456

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    DOI:10.2355/tetsutohagane.97.450

    In order to investigate the effect of strain rate on TRIP effect in metastable austenitic steels, tensile deformation behavior of a JIS-SUS301L steel was examined at wide range of strain rates between 103/s and 10−3/s, in addition of the comparison with data of stable austenitic steel (JIS-SUS310S). Concerning of the SUS301L steel, the 0.2% proof stress increased and the uniform elongation decreased with increasing of strain rate. The tensile strength decreased with an increase in strain rate between 10−3/s and 101/s and then increased again at strain rate more than 101/s. The significant difference between the strain rate dependences on tensile strength and uniform elongation of SUS301L and SUS310S was clarified especially at strain rates between 10−3/s and 100/s. As far as the effect of strain rate on stress-induced martensitic transformation behaviors is concerned, the volume fraction of stress-induced martensite at the same true strain decreased with increasing of strain rate. These stress-induced transformation behaviors were associated with the tensile strength and uniform elongation of SUS301L steel between 10−3/s and 100/s. The conditions of stress-induced martensitic transformation in order to obtain better uniform elongation at a high strain rate of 100/s were discussed by the calculations using the Weng secant method.

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