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

Tetsu-to-Hagané Vol. 102 (2016), No. 4

  • Interaction between Alumina Inclusions in Molten Steel Due to Cavity Bridge Force

    pp. 187-195

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    DOI:10.2355/tetsutohagane.TETSU-2015-072

    The formation and extinction behavior of cavity bridges has been experimentally evaluated by allowing the cylindrical particles simulating inclusions to approach and separate off in mercury and in molten steel. An interaction model of spherical alumina particles close to the actual inclusion shape due to cavity bridge force has been developed on the basis of the experimental results. Using this interaction model, the processes of agglomeration and separation of alumina inclusions in molten steel have been analyzed, and the agglomeration force due to cavity bridge force has been discussed in comparison with different agglomeration forces that are derived from the van der Waals force in molten steel and the capillary force on the surface of molten steel. When two isospherical alumina inclusions are approaching each other in molten steel, a large agglomeration force of 1.54 d·σFe (d: the diameter of alumina inclusions, σFe: the surface tension of molten steel.) is generated by the cavity bridge formation from the interparticle surface distance of 0.07 d, and then the agglomeration force also gradually increases to reach the maximum value of 1.88 d·σFe in complete contact state. Conversely, when two isospherical alumina inclusions in molten steel are separated from the contact state, a large agglomeration force of 0.92 d·σFe and above is maintained until the cavity bridge extinction in the interparticle surface distance of 0.12 d, whereas the agglomeration force gradually decrease from 1.88 d·σFe. In addition, it is assumed that alumina inclusions in aluminum deoxidized molten steel principally agglomerate and coalesce on the basis of the agglomeration force derived from very strong cavity bridge force in comparison with the van der Waals force in molten steel and the capillary force on the surface of molten steel, and coarse alumina clusters are thus formed in molten steel.
  • Solid/liquid Mixing Pattern and its Comparison with Liquid/liquid One in a Mechanically- stirred Vessel

    pp. 196-201

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    DOI:10.2355/tetsutohagane.TETSU-2015-100

    Solid/liquid mixing pattern was investigated and compared with the liquid/liquid one in a mechanically-stirred vessel. The cold model experiment was carried out to make clear the effect of operating factors such as volumetric ratio of solid particles to liquid, rotation speed, impeller position, etc. on the mixing pattern. The solid/liquid mixing pattern was observed visually and the vortex depth of solid/liquid or gas/liquid interface was measured with a ruler. It was categorized into 3 types as well as the liquid/liquid mixing pattern. I: the region where solid particles have no dispersion, II: the region where some of the solid particles disperse into liquid, III: the region where almost all of the solid particles disperse into liquid. The solid/liquid mixing pattern transits from I to II, and from II to III as the impeller depth decreased and the rotation speed increased. The transition of I-II shifted to a higher rotation speed in cases of smaller volumetric ratio of solid to liquid, larger particles diameter, larger density difference between solid-liquid and smaller liquid viscosity. The transition of II-III shifted to the higher rotation speed in cases of smaller impeller diameter and larger liquid viscosity, and showed independency on volumetric ratio of solid to liquid, particles diameter and density difference between solid and liquid. Multi regression analysis on the transition of I-II showed that the calculation agreed with the measurement. Dimensionless correlation equation on the transition of II-III also showed a good agreement between calculation and measurement and it was adaptable to liquid/liquid system.
  • Mechanism of Formation of Streak-shaped Defects on Ultra-low Carbon IF Steel for Automobile Outer Panels after Press Forming and Influence of Slab Reheating Temperature before Hot Rolling and Sb-addition on Defects

    pp. 202-208

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    DOI:10.2355/tetsutohagane.TETSU-2015-061

    Streak-shaped defects were infrequently observed on ultra-low carbon IF cold-rolled steel sheets for automotive outer panels after press-forming. The defects occurred due to heterogeneous remaining of elongated grains on the steel surface layer. Elongated grains are hard to recrystallize due to the pinning effect of small precipitates on grain boundaries. These precipitates consist mainly of Ti(NC), TiS and AlN. In order to eliminate the streak defect, lower excess Ti, lower temperature slab reheating and Sb-addition to the steel are efficient. These countermeasures result in a drastic reduction of the defect ratio. This improvement is considered to be obtained due to the reduction of small precipitates of Ti(CN), TiS and AlN in the steel surface layer.
  • Effects of Nitrided Surface Layer and Microstructure on Press Formability in Commercially Pure Titanium Sheet Annealed in Nitrogen Gas Atmosphere

    pp. 209-218

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    DOI:10.2355/tetsutohagane.TETSU-2015-079

    The effects of nitride layer and microstructure on the Erichsen value of commercially pure titanium JIS-class1 sheets were evaluated after cold rolling followed by annealing at 700~860°C both in nitrogen gas and in vacuum in order to clarify the controlling factors independently.
    Irrespective of the annealing methods, grain size increased with the increase in the annealing temperature; however, after the maximum grain size by annealing at 810°C, grain size decreased with the increase in the temperature, due to the retardation effect of grain growth by the presence of the β phase. The Erichsen value was confirmed to increase with the increase in grain size; however, even if grain size is the same, the titanium sheets annealed in the two phase (α+β) region exhibit inferior forming properties such as the Erichsen value, elongation and n-value than those annealed in the single α phase. The hard Fe rich region formed along α grain boundaries, which is originated from β to α phase transformation during cooling, is considered to impair the forming properties.
    Furthermore, the Erichsen value was evidently improved by annealing in nitrogen gas due to the formation of the nitride layer, which contributes to lowering the friction coefficient. However, the thick nitride layer formed at temperatures higher than 840°C no longer has the beneficial effect, because the thick nitride layer tends to easily fracture leading to fine cracks in the sheet surface. Annealing the sheets in the temperature range of 810~830°C for 30 s in nitrogen gas was the optimum condition for the best press formability.
  • Influence of Gas Injection Pipe on CO2 Decomposition by CaCl2-CaO Molten Salt and ZrO2 Solid Electrolysis

    pp. 219-225

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    DOI:10.2355/tetsutohagane.TETSU-2015-025

    The electrochemical decomposition of carbon dioxide to form carbon and oxygen gas was studied in CaCl2-CaO molten salt. The water model experiment was carried out to study on the influence of the tip shape of the pipe, the pipe diameter and wettability to the gas injection pipe for the bubble shape in molten salt. Bubbles were formed in both conditions of horizontal and oblique tip with good wettability of the gas injection pipe. The specific surface area of bubble using oblique tip of the pipe was smaller than horizontal tip. On the other hand, slug flows appeared with poor wettability. In the hot mode experiment, the current density was measured, and the carbon dioxide gas concentration decreased. The carbons were detected from the sample after the experiment. The decrease of carbon dioxide gas concentration using oblique tip were more remarkable than the case using horizontal tip with the same pipe. It is likely that the form of carbon dioxide gases in the molten salt were bubbles shape.
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