발간논문

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The effect of hot rolling condition on the mechanical properties, microstructure, find HIC susceptibility was investigated for microal1oyed steels which were prtpared in the mill lines as well as in the la bo ratory. The results are as follows. (1) The microstructure of hot-rolled steels was hardly affected by the change of the finishing temperature. But by lowering the coiling temperature, the polygonal ferritic and pearlitic structures were changed to the bainitic stucture. (2) With lowering the coiling temperature, the tensile and yield strengths were increased by more than 10 ㎏.f/㎟, while total elongation was decreased by 10%. However, the effect of the coiling temperature on the impact tou ghness was very little. (3) The most important factor affecting the initiation of HIC was the morphology of non-metallic inclusions. Elongated MnS inclusions as well as large and/or clustered oxides rendered the steels very crack sensitive. (4) Cracks propagated along the pearlitic bands or low temperature transformed phases developed in the center-segregated regions of C.C.slabs. HIC susceptibility was decreased to some extent as center-segregation was decreased. (5) The propagation of HIC was reduced when the formation of the low temperature transformed structure and/or the growth of the lamellar pearlite was supressed by proper cooling control.

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Precipitation and ripening behaviours of metastable phases in Al-2.33wt%Li-0.32wt% Zr alloy are studied by employing TEM. The δ`, Al₃Zr and composite phases are observed at the as-quenched state. Activation energy for the growth of matrix δ` phases is 31.5㎉/㏖ and the growth rate is affected by addition of Zr. We argue that the composite phase can be formed during quenching and aging by impingement of previously existing δ` and A1₃Zr phases followed by preferential coarsening of δin contact with the A1₃Zr, leading to encapsulation of the A1₃Zr by δ`. The Al₃Zr phase accelerates the nucleation and growth of δ` phase, and improves the mechanical properties such as hardness.

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The equilibrium segregation of solute atoms to free surfaces was studied in the Ni-Cu binary alloy. The surface structures were assumed as TLK(Terrace-Ledge-Kink) model and the regular solution theory was used to define atomic interactions. The Cu segregation follows the Gibbsian equilibrium segregation behavior and its amount increases as the Miller index of surface increases. For the law index planes as (111), (200) and (220), the Cu concentration profiles decrease monotonically, as the Miller indices increase but they become complicated because of the ledges and kinks on the terrace. These ledges and kinks locally change the number of broken bonds at each atomic sites, which influences the solute segregation behavior. The Cu concentrations at each sites around them could be calculated using TLK model. The surface free energy is reduced by the solute segregation and its reduction is even larger at low temperatures.

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The role of carbide was studied in continuous annealing process of extra low carbon Al-killed steels. 2-stage annealing has a1so been attempted to clarify the correlation between behavior of carbide and development of recrystallization texture. Effect of carbide on the development of (111) recrystallization texture is greatly divided into two different way. At the early stage of recrystallization Mn-C dipole, closed by dissolution of carbide, plays a role of inhibitor against nucleation. On the other hand, at the stage of grain growth, not only recrystallized grain boundaries but sub-boundaries are prevented from migrating by fine spherical cementite, [Fe, Mn]₃C. This carbide provides preferred precipitation sites for solute carbon during aging.

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The effect of interlamellar spacing on microstructural stability, tensile behavior and fracture mode at high temperature has been studied for unidirectionally solidified Al-CuAl₂ eutectic. The results of isothermal heat treatments showed the higher coarsening rate at 520℃ than at 450℃. It was observed that the decrease of interface area(Δ5) was inversely proportional to the square of the interlamellar spacing(λ²). Therefore, it can be seen that the microstructural stability at high temperature improves as the interlamellar spacing increases. The tensile properties at high temperature improved as the interlamellar spacing increased, which is opposite behavior at room temperature. The Al-CuAl₂eutectic exhibited brittle fracture mode below the temperature at which the reinforcing materials softened but ductile fracture mode above the temperature.

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A mathematical model of fluid flow in a induction-stirred ladle was developed with the use of 2-D computational scheme together with modeling of turbulent viscosity. The electromagnetic body force was calculated from the approximation of Maxwell equation. The numerical results show that fluid flow in induction-stirred ladle is generally uniform in contrast with gas-stirred ladle regardless of direction and magnitude of magnetic force. But there is difference in kinetic ernergy map in direction of magnetic force and velocity magnitude is not exactly proportional to the magnitude of magnetic force.

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In this work the effects of different thermomechanical treatments(TMT) on the superplasticity of 7075 Al-alloy, and cavitation by superplastic deformation have been studied. The adapted TMT are conventional method by Paton and some other methods, which modified the rolling process. However, any other convenient methods, which bring better superplasticity than the conventional method, could not be found. Superplasticity of the studied 7075 alloy are affected more strongly by the distribution of secondary particles than the grain refinement. Fracture in superplastic deformation are caused by cavitation. The preferential sites of cavitation are Fe-rich or Si-rich secondary phase at the grain bound-aries. Coarse precipitates and Cr-including phases also helped cavitation.

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The effects of TiO₂content in flux and the %Ti/(%Al)＾(4/3) ratios in the electrodes on titanium recovery of STS 321 with ESR process were studied. The Ti recovery could be increased by using the flux with TiO₂during ESR. The required amount of TiO₂was increased when the %Ti/(%Al)＾(4/3) ratio in electrode was increased. The excess TiO₂however, promted the oxidation of Ti. The Ti recovery was increased with the decreased ration of %Ti/(%Al)＾(4/3).

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Effects of grain boundary segregation of phosphorus (P) on the stress corrosion cracking (SCC) and on the SCC mechanism of mild steal in hot 55% Ca(NO₃)₂ soultion have been investigated by using the slow strain rate and constant load methods. The SCC susceptibility was examined as a function of applied potential, strain rate and test temperature. The extent of P segregation effect on SCC is greatly affected by the kinetics of dissolution of passivating film developed on the specimen surface depending upon the SCC test condition. That is, the effect of P segregation at grain boundaries is revealed markedly in the presence of stab1e passive films on the specimen surfaces. This result is discussed in terms of the stability of passivating films as a function of applied potential, strain rate, test temperature and loading method. The IGC susceptibility seams to be necessary for the occurrence of SCC susceptibility in case of the P-doped mild steel. From the experimental SCC results and SEM fractography, the SCC mechanism of mild steel proved to be strain-induced anodic dissolution irrespective of grain boundary segregation of P.

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The major impurities of the native cold and the crude gold produced as a byproduct of copper, lead, or bismuth smelting etc. are copper and silver. It is impossible to refine the native gold containing silver more than 8% by electrolytic refining or aqua regia treatment. In this study, in order to reduce the silver content in Au-Ag alloy by the pyrometallurgical technique, Cuprous Chloride Treatment was carried out, in which copper was substituted for silver in Au Ag alloy. The results obtained were as follows : 1. The silver in Au-Ag alloy was removed more than 90% by cuprous chloride treatment. 2. The substitution ratio for silver in Au-Ag alloy was enhanced by increasing the ratio of Cu₂Cl₂/Ag but unaffected by increasing the ratio of CaCl₂/Cu₂Cl₂. 3. Initially, the content of silver in Au-Ag alloy decreased rapidly as the temperature increased. However, the substitution ratio for silver decreased as the temperature increased after 25 minutes.

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