발간논문

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For the production of the clean steels, liquid steel transfer in practice from electric furnace through tundish to continuous casting mold has been studied. Samples of the slag and liquid steel were taken simultaneously from each sequential step especially including the tundish at three different times and their chemical compositions were analyzed. The metal specimens were examined under an optical microscope and the electron optics and their nonmetallic inclusions were investigated in terms of area fractions of the inclusions by an image analyzer. The results are as follows : Open stream transfers of the liquid steel from ladle to tundish and presumably from tundish to mold were found to be contributed greatly to reoxidation of metal by air. So was the reoxidation of the metal in the tundish particularly at early stage without protection by the synthetic slag. The slag carryover from the ladie to tundish towards the end of metal transfer was found to be obviously detrimental to the cleanliness of the steel.

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Using a semi-batch reactor(ID, 7.9㎝; height, 20㎝), experiments have been made on the characteristics of reduction of k₂TaF_7, with Na to produce condenser grade tantalum powders. The reaction temperature was controlled by the feed rate of \!a. The effects of stirring of the reactant solution, the amount of diluent and the reaction temperature were investigated on the particle size and production yield of the tantalum powder. The purity of the tantalum powder was 99.8% and the particle size ranged from 3.45 to 7.15㎛ in terms of the Fisher size.

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We have obtained a dense and unidirectionally solidified Bi-Pb-Sr-Ca-Cu-O superconductor thick film by line heating. When the maximum line heating temperature was above 870℃, it was divided into a (001) textured l0K phase and a Bi free phase solidified in scanning direction. After post heat treatment at 860℃ for 12h, it became a dense and homogenious superconductor(Tc=65K) by diffusion between two phases.

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The solubility and diffusion behaviors of hydrogen in dual phase steel have been investigated by using electrochemical permeation experiment in dual phase steel. With increasing volume fraction of martensite, the diffusion coefficient of hydrogen decreased in both the annealed and quenched ones, whilst the solubility of the both specimens increase. The hydrogen solubility in step quenched specimen with 84.83% martensite volume fraction is larger than that of the intercritical annealed specimen with 84.83% martensite volume fraction

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The structure and magnetic anisotropy of Pd/Co multilayer thin films prepared by thermal evaporation were studied. The designed periodicities of the Pd/Co films well agreed with the calculated values based on the perfect superlattice model within 10%. The films with large periodicities(△$gt;15Å) showed several satellite peaks. The main peak positions of the films with thicker Pd layer than Co layer shifted to 2θ of fcc Pd(111) position. The calculated strain of Co at the interface was proportional to the perpendicular magnetic anisotropy energy(Ku), so that the perpendicular magnetic anisotropy was originated from the magnetoelastic surface anisotropy due to the misfit strain as well as the magnetocrystalline interface anisotropy.

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The effect of hydrogen on crack propagation of dual phase steel has been studied by hydrogen charging. The results were summerized as follows 1) The stress intensity value of dual phase steels in stage 1 increased with increasing martensite volume fraction. 2) The crack growth rate of dual phase steels in stage 1 was decreased with increasing martensite volume fraction. 3) The stress intensity value and crack growth rate were reduced by the hydrogen charging. 4) The fracture appearance of the hydrogen charged materials was predominantly intergranular fracture.

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Evolution of microstructure in a rapidly solidified Al-Fe-V-Si alloy has been investigated by TEM. The as-cast, microstructure varies depending on casting conditions and also through the thickness of melt-spun ribbon. Microstructure of the completely zone A ribbon consistis of the microcellular precipitation of the mocroquasicrystalline phase, with the globular particles of clustered mcroquasicrystalline phase near the air side of the ribbon. For the zone A and zone B mixed ribbon, on the other hand, intercellular particle is composed of silicide phase, but the formation of globular particles of microquasicrystalline phase(somewhat decomposed) is again observed near the air side of the ribbon. During heating, as observed by annealing studies and by in-situ hot-stage TEM studies, intercellular microquasicrystalline particles are quite nustable and decompose at a very fast rate, resulting in a coarse distribution of silicide particles. This decomposition of the intercellular microquasicrystalline particles assists the decomposition of glebular partioles of clustered microquasicrystalline phase in completely zone A ribbon, resulting in a very coarse distribution of silicide particles after annealing. For the zone A and zone B mixed ribbon, decomposition of globular particles of clustered microquasicrystalline phase results in the precipitation of silicide particles along the cell boundaries.

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In compacting the melt-spun Nd_(14)Fe_(76)Co₄B_0 and Nd_(10.3)Fe_(79)Co₂Zr_(1.5)B_7 magnetic powders, the difference in composition induces a different behavior of closed packing rate as a function of aspect ratio of the powders. The Nd_(10.5)Fe_(79)Co₂Zr_(1.5)B_7 alloy having a low Co/Fe ratio(low density) shows the better green density to have an enhanced closed packing rate. An empirical power equation relating the green density with the compacting pressure was obtained such as ρ(g/㎤) =5.2∼5.6 × p^(0.04◎∼0.06◎)(ton/㎠). The Nd_(14)Fe_(76)Co₄B_6 alloy having a high Nd/Fe ratio possesses much finer grain size(50∼60㎚) than that of Nd_(0.5)Fe_(79)Co₂Zr_(1.5)B_7 alloy and shoves the higher coercivity(iHc=14∼15kOe). The higher Nd/Fe ratio in the melt-spun Nd-Fe-Co-B alloy, where the domain wall pinning mechanism was found to be predominant., assists the formation of Nd-rich grain boundary phase acting as a pinning site. The grain boundary ranges over 12∼16Å thick in the Nd-Fe-Co-B alloy while it ranges over 8∼12Å thick in the Nd-Fe-Co-Zr-B alloy.

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A series of blow formings was performed to develop a forming limit diagram of a superplastic 8090 Al-Li alloy sheet. The forming limit showed minimum in major strains at. a plane strain state, and increased with increasing the absoulte value of minor strains. It was found that the grain size and volume fraction of cavities were samall, and they had little effect on the forming limit, while diffuse necking and localized necking had some influence. The minor effect of the cavitation was considered to be due to the back pressure and somewhat high strain rate during forming.

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An investigation has been performed to find the optimum production route for producing microhomogenous W-Cu composite powder by hydrogen reduction of WO₃-CuO oxide mixture. The optimum reduction condition was determined not only by TG and X-ray diffraction analysis of the phase decomposition occuring during reduction of WO₃-CuO mixture but also by investigating the powder characteristics and the mixing homogeneity of coreduced W-Cu powder. As a result, two stage reduction process has been established, consisting of the stepwise isothermal heat treatment at low (200℃∼300℃) and high temperatures(800℃). The experimental results revealed that the two stage reduction process is more effective than the one stage process for improving the refinement and the mixing homogeneity of W -Cu powder. This is mainly due to the fact chat tile first step isothermal reduction heating at low temperature efficiently inhibits Cu segregation and minimize the formation of W-Cu aggregates due to sintering of Cu powder.

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