발간논문

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Present study was carried out to understand the thermodynamic behavior of BaO-B₂O₃ slag system at high temperature by chemical equilibration technique. Also, thermodynamic properties of Ag-Ba binary system at high temperature was measured to carry out the above mentioned investigation by the same technique. Thermodynamic behavior of Ba in Ag melts shows negaive deviation from ideal solution and can be expressed quantitatively by using α function concept as follows : logγ_(Ra)=2.63(1-X_(Ba))² α=logγ_(Ba)/(1-X_(Ba))²=2.3-7700/T BaO activity was increased with increasing the mole fraction of BaO and shows the negative deviation from the ideal sloution model. Thermodynamic properties of BaO shows similar behavior with CaO system. The thermodynamic properties of BaO in BaO-B₂O₃ slag system could be related with the stability of BaO-B₂O₃ melts which was considered to be stable compound at high temperature as a 3BaO·B₂O₃ composition. also, comparison between BaO and CaO activity and optical basicity was carried out to confirm relative strength of BaO basicity to CaO. BaO system shows higher optical basicity than CaO system at the same activity of those basic oxide, This fact may imply that BaO was more effective oxide for the refining reagents because of strong basicity.

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A study has been made of the effects of frequency and bath temperature on the pit size, structure of 99.98% aluminum foil AC etched in 1.5M HCl solution and its capacitance after anodization. The maximum capacitance was obtained at a certain frequency value, f_M, which was shifted to a higher value with increasing bath temperature. Below f_M, the foils were macro-thinned, resulted in low capacitane, and above f_M, they showed very irregular etched morphology. The etch pit size decreased with increasing etching frequency. The frequency effect could be explained using galvanodynamic voltammetry polarization curves.

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A New technique for determining 2-phase boundaries and 3 apexes of 3-phase triangle internary system is presented based on the convexified Gibbs free energy surface. Computer program which calculates equilibria over whole composition plane and any temperature ranges and graphics software which plots isoplethic phase diagram as well as isothermal were developed. This method was successfully applied to Fe-Cr-C system which has 10 phases in cluding 5 carbides.

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New refining processes have been developed to produce high quality wire rod steels. The utilization of a ladle furnace(LF) allowed decrease of blow end temperature, and hence a better dephosphorization efficiency in BOF operation, and ladle slag control process could be practicized for effective inclusion removal from the molten steel. The steel cleanness was improved through the application of ladle slag control practices. The average total oxygen content in cast products could be reduced to the extent of about l0ppm for Al-killed bearing steel and about 14ppm for Si-killed tire cord steel. In case of tire cord steel, oxide inclusions were modified to those of low melting point.

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In alloy systems of the WCrCo and MoCrCo steels, the secondary hardening and fracture behavior were studied. The decrease in hardness was very small up to 500℃ tempering temperature before the Co addition led to the clear secondary hardening at 550℃. Also, the peak hardness was higher than the as-quenched hardness. Since the Co addition may increase the driving force for formation of the M₂C type carbides and inhibit the softening by delaying the recovery of the dislocation structure, a substantial secondary hardening can be produced by the fine precipitation of M₂C type carbides from such a dislocation structure. The Co addition may degrade the impact toughness by means of the lowering of intrinsic toughness of matrix, the precipitation hardening of M₂C type carbides and the retention of coarse cementite. In the WCrCo steel, the lower hardeness and the higher impact toughness at relatively high test temperatures may result from the less dense precipitation of M₂C type carbides.

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In alloy systems of the WCr and MoCr steels, which contains both the W or Mo, M₂C type carbide forming elements, and the Cr, the secondary hardening and fracture behavior were studied. The extent of secondary hardening was very weakened and the secondary hardening temperature range was lowered in the MoCr steel, as compared to the ternary Mo steels. Furthermore, the secondary hardening did not occur in the WCr steel. Because the Cr addition stabilizes the cementite of M₃C type, the effective precipitation of fine M₂C carbides cannot be achieved. In the WCr steel the coarse cementite is retained at higher temperatures probably due to the additive stabilizing effect of the W. Although the Cr addition has no merit in the secondary hardening itself, however, it eliminated the secondary hardening embrittlement, which was observed in the Mo and W steels in the absence of Cr.

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The effects of compressive strain and strain rate on the strain-induced martensitic transformation and mechanical properties of Fe-20%Mn alloy were investigated. At low strain rates of 2×10^(-3)/sec and 2x10^(-1)/sec, the amounts of ε and α′ martensite increased with strain at the expense of γ. The increase was the largest at low strains up to 8%. At the highest strain rate of 6x10³/sec, the amount of retained γ increased due probably to the stabilization of γ by adiabatic heating during testing. The alloy showed high strain hardening especially in low strain range where most of the strain induced transformation occurred. The strain rate did not have an effect on strength when strain rates were low. When the strain rate increased to 6×10³/sec, the true stress increased at all strain levels. However, high strain rate did not increase the hardness. It seems that during a dynamic compression test an additional strengthening mechanism may work to increase the true stress. We believe that the dislocation drag effect may be responsible far the increase in stress during the high-rate compression test.

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