발간논문

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Through the microscopic observation of slab surface layer and the variation test of mold powder and Ar gas flow rate purged into submerged entry nozzle, the formation mechanism of pin hole in the surface layer of slab was verified. By changing casting variables, such as casting width, immersion depth, pouring angle of submerged entry nozzle and strand withdrawal speed, the effect of stream velocity on pin hole formation was investigated. Ar gas is the main source of pin hole, which is formed due to entrappment of Ar gas by solidifying meniscus hook. When the stream velocity in meniscus increases or exothermic mold powder is used, pin hole formation is reduced due to hotter meniscus, which retards the hook formation.

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The cyclic softening during high temperature Low Cycle Fatigue (LCF) test of a Ni-base superalloy Rene 80 was studied. The LCF tests were conducted with total-strain (0.6~0.9%) control mode at 760℃, 871℃ and 982℃ using high temperature extensometer. The electrolytic extraction was carried out to observe the microstructural change of γ`. The alloy showed different cyclic softening with temperature and strain range. The quantity of cyclic softening increased with the test temperature and decreased with the strain range. The extent of cyclic softening was analyzed by the theories of strengthening. The theoretical prediction based on the precipitate hardening alone underestimated whereas the one based on the coherency hardening overestimated the amount of cyclic softening. Therefore the actual cyclic softening mechanism was attributed to the combination of the two mechanisms.

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Mass transfer between two immiscible liquids, oil and water, simulating slag and steel respectively, was measured in a scale model of ladle. The transferred species was thymol, which has an equilibrium partition ratio between oil and water similar to that of sulfur transfer between slag and metal. The mass transfer rate was measured as a function of commonly encountered design parameters used in ladle steelmaking. Mixing times in the presence of an oil layer were also measured and interpret ed from the standpoint of two phase mass transfer. The dependence of mass transfer on gas flow rate allows one to define three distinct regimes of gas flow rate, each exhibiting a different dependence. The critical gas flow rate at the boundary between the first and the second flow rate regimes, was found to be a function of the position of gas injector, the slag/metal volume ratio, the densities of two phases, and the interfacial tension. The data allowed these critical gas flow rates to be correlated against system parameters.

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Microstructures of TiAl alloys can he divided into four groups; near-γ, duplex, nearly-lamellar and fully-lamellar. Depending on heat treatment temperature and time, these structures can be formed. The duplex microstructure, the finest microstructure, can be formed by heat treatment in an α+γ field. In this study, heat treatment conditions for the formation of duplex microstructures were studied. The TiAl alloys were cast in a plasma arc melting system. The effects of recrystallization time in the α+γ field and the addition of a third element, such as Mn or Zr on the microstructures were investigated. The fully-lamellar microstructure which has smooth grain boundaries resulted in low fracture strain. Recrystallization time in the α+r field decreased with the addition of Mn, but increased with the addition of Zr.

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A recovery process on pure alumina power from the wasted aluminum etching solution of electrolytic condenser works was studied. In order to remove the impure cations such as Fe^(3+) and Cu^(2+) from the etching solution, solvent extraction was performed, and then, Al^(3+) was precipitated by purging HCl into the solution. Thermal behaviors of the recovered materials were tasted and X-ray diffraction patterns were obtained. Finally, the recovered materials and the calcined one were observed by scanning electron microscope. As the results, more than 95% of Fe^(3+) and Cu^(2+) were removed by the solvent extraction and the precipitated phase by HCl purging was found as AlCl₃·6H₂O. The recovery ratio of aluminum component from the solution was higher than 99.3% after 40minutes of HCl purging at the purging rate of 7.2㎖/sec.. The AlC₃·6H₂O was transformed to powdery α-Al₂O₃ by calcination of the temperature about 1200℃, and the heat of phase transformation from calcined non-crystalline from to crystalline α-Al₂O₃ was about 13.7㎉/㏖.

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In order to estimate the contribution of smelting reduction rate by carbon in slag, the reduction rate of FeO was measured by using a mass spectrometer technique. Series of experiments were carried out with wide ranges of FeO content to illuminate the dependence of rate limiting step on FeO content in slag. Reacting procedure can be divided into three regions; incubation, steady state reaction, and transition period. Since rates at incubation and transition periods are changed by reacting condition, rate at steady state period is generally used to represent the overall reduction rate. The reduction rate of FeO was increased with increasing FeO content in slag. The dependence of reduction rate on FeO content can be expressed as follow : r=4.4 × 10^(-5) a^(1.34)_(FeO) or r=1.67×10^(-7)(wt%FeO)^(1.34) The activation energy was measured to be 60.6kcal/mol for l0wt%FeO experiment. From the results of present study, chemical reaction at the interface between gas and carbon would be a rate limiting step for high FeO content. However role of FeO mass transfer in slag became important with decreasing FeO content. Consequently, the reaction mechanism was changed from chemical reaction at carbon surface to mixed controlling step of chemical reaction and mass transfer of FeO in slag by decreasing FeO content. Appling present results for FeO reduction rate by solid carbon to the pilot operation of smelting reduction process, the contribution of reduction rate by solid carbon was estimated to be about 20-30%.

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