발간논문

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The rapid heating rate by the irradiation of CO₂laser on the C-steel induced the increase of A_f temperature by about 250˚K. The desirable laser power density for the surface-hardening of medium C-steel for 5 second varied from 1.7×10³W/㎠ to 4.1×10³W/㎠ depending upon the hardened thickness. The calculated temperature gradient in the hardened layer was about 160˚K/min. Thus the desired thickness can be controlled by the selection of appropriate laser power density. When cold-worked medium C-steel was irradiated with a relatively higher laser power, the softened region with minium hardness was located between the hardened layer and the matrix. Meanwhile if the laser power was not enough to transform the laser-irradiated area into austenite, the softening of structure resulted from the relief of work-hardening. Although the turn-off of the laser irradiation caused the self-quenching of low C-steel, the cooling in air was not rapid enough to reach the critical value at which the martensite could be obtained.

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A study has been made of solid solubility of chromium in aluminium, variation of hardness with chromium contents and thermal stability in the Al-Cr alloy produced by the Atomization-Splat Quenching method and of the effects of manganese up to 1.6% on the properties mentioned above of the Al-4.5 Cr-Mn alloy. At the cooling rate of 10^5-10^6 K˚/sec. the solubility of chromium was extended to above 5.5% for the Al-Cr system. The hardness of the Al-Cr alloy increased linearly with chromium contents, an empirical equation of which may be expressed as Hv＝36.2＋12.5 Cr (%) and that of the Al-4.5Cr-Mn alloy also increased linearly with manganese contents. The average thermal stability of the Al-Cr alloys was estimated to be 0.65. And the manganese addition lowered the decomposition-start temperature of the solid solution, but didn`t affect the thermal stability of the Al-Cr alloy.

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The effects of heat treatment paths and chromium addition on tensile properties in austenite dual phase steel obtained by isothermal heat treatment in the two phase region directly from the austenite region and intercritical dual phase steel obtained by intercritical annealing were investigated. As a specimen Fe/1.1.Mn/0.12C and Fe/1.0Mn/1.1Cr/0.12C steels were used. It was found that austenite dual phase steel had a increased volume fraction of martensite than the intercritical dual phase steel over the same time of heat treatment. Chromium addition decreased the volume fraction of martensite at the same isothermal heat treatment temperature. The isolated coarse martensite particles were obtained with little effect of chromium addition in austenite dual phase steel, but in the case of intercritical dual phase steel, coarse and finely dispersed martensite particles were observed with the addition of chromium caused by initial structure before intercritical annealing. As for the effect of chromium on tensile properties, total elongation of austenite dual phase steel was remarkably decreased with low tensile strength, but tensile strength increased and total elongation decreased slightly in intercritical dual phase steel. In Fe/1.0Mn/1.1Cr/0.12C steel, the total elongation of intercritical dual phase steel was excellent than that of austenite dual phase steel due to the fact that the mean size of martensite was so small and the number of martensite particles per unit area were remarkably increased at the same volume fraction.

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The formability of commercial 444 stainless steel sheets containing 18% Cr and 2% Mo of 0.5 or 0.6mm in thickness was evaluated by analyzing their tensile properties such as yield strength, tensile strength, plastic strain ratio, strain hardening exponent, strain rate sensitivity, uniform elongation and total elongation, which were all measured at different angles to the rolling direction, forming limit curve (FLC), strain distribution and crystallographic texture. The FLC level of 444 stainless steel was considerably lower than that of 304 stainless steel and was slightly lower than those of general low carbon steels (Keeler-Goodwin band). The theoretical R-value calculated from the measured texture using the Lee method was in good agreement with the experimental R-value.

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The various analytical expressions for the free energies of pure metals, binary and ternary solution phases were discussed. The methods of obtaining parameters in the expressions from experimental data were also described in detail. The methods were used to optimize thermodynamic data of eight binary systems and activities of components of the Ag-Sn-Pb system at 1073˚K, the Bi-Cd-Pb and Sn-Cd-Pb systems at 773˚K. The correction terms added to the ternary model equations such as the Kohler, Toop, and Muggianu equations improved the calculated values within the experimental error, in which the correlation coefficients were larger than 0.98 for the three ternary systems. The published activities of Ag and Pb in the Ag-Sn-Pb were found to be erroneous because wrong equations were used. The corrected activities were calculated using the above method without the Darken integration.

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In hot metal, effects of temperature, basicity, and oxygen source on the reaction of desiliconization and demanganization were studied. The results are as follows; 1. The temperature of hot metal decreased slowly because of the reaction of desiliconization and demanganization. 2. Lower degree of desiliconization and demanganization was obtained at 1450℃ compared with 1350℃. 3. For the effective desiliconization and reducing Mn-loss, it was desirable that the basicity was about 1.0. 4. It was considered that the desiliconization treatment of hot metal should be conducted by mixture oxidizer (gas-solid) to reduce the Mn loss.

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