발간논문

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In an attempt to investigate the stabilization of austenite and its effects on the mechanical properties of DP steel, the variation in the volume fraction of retained austenite and stress-strain curves were analysized in accordance with the variations in alloy composition, annealing temperature, cooling rate, prior structure and isothermal holding time below Ms temperature. The amount of retained austenite increased maximum 80% when 1% nickel was added, and more austenite was retained from martensite structure than from as hot-rolled structure. When specimens were isothermally treated below Ms temperature the amount of retained austenite was more than doubled. Stress-strain curves were analyzed according to the Hollomon and Crussard-Jaoul equations. Isothermal holding below Ms temperature resulted in the increases in 0.2% yield strength and elongation, decrease in tensile strength, and, particulary, larger work hardening rates in third stage compared to second stage. This peculiar work hardening behavior was interpreted as the result of deformation induced transformation of retained austenite which presents in larger amount in this specimen.

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In determining non-metallic inclusion rating in steel, the efficiency and reliability of conventional manual method and computer aided automatic method using image analyzer have been investigated. To see the effect of shape change of the inclusions on the cleanliness value based on existing test standards, the test coupons examined were taken from continuously casted slab, hot rolled plate and from cold rolled sheet where some inclusions were extremely elongated. Results show that the cleanliness values determined by two methods agree fairly well for both samples with globular oxide inclusions and with properly elongated inclusions. For samples with extremely elongated inclusions, however, the cleanliness value processed by image analyzer tends to be smaller than that of manual count. It appears that the automatic method gives faster and more reliable cleanliness value since the image analyzer determines the value in area fraction instead of in point fraction.

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In order to discuss on the mechanism of massive transformation, the effect of the cooling rate on massive transformation start temperature, massive transformation peak temperature and massive transformation finish temperature, and the free energy change accompanied with massive transformation of Fe-Ni binary alloy, have been studied by means of Differential Thermal Analysis (DTA). The results obtained are as follows. 1) The λtype heat evolution attributed to massive transformation, has been observed in the DTA curve. Also, the serration of heat evolution has been observed in the DTA curie scanned below a certain cooling rate. It has been found that the mechanism of massive transformation has been explained by the occurance of serration in the heat evolution. 2) Massive transformation start temperature has been constant, independent on the cooling rate. However, massive transformation peak temperature and massive transformation finish temperature have decreased with the cooling rate, and its behavior has distinguished into the three region. 3) Massive transformation start temperature has decreased with Ni content in Fe-Ni binary alloy, however the free energy change accompanied with massive transformation has increased with Ni content in it. 4) Massive transformation has not occurred but maitensite transformation has occurred at a cooling rate below 2.5×10^(-1)˚K/s.

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SiC fiber reinforced Al composite has been fabricated by a technique based on the liquid metal hot pressing method. In this experiments SiC fiber orientation was varied from 0˚to 90˚with respect to the tensile axis. Ambient temperature tensile and impact test were performed on as fabricated and thermally treated composites. Ultimate tensile strength exhibits maximum values at 0˚which continually decreases to a minimum at 90˚Impact strength also decreases with increase in fiber orientation angle. Fracture mode after tensile and impact tests was observed by scanning electron microscopy. The distribution of constituents at the interaction zones between Sic fiber and matrix was analysed by SEM and WDS.

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In this second paper the fiber texture is dealt with. Experimental pole figures of iron and copper wires obtained by neutron diffraction method were analyzed using the computer program TXFIB to test analytical accuracies of Bunge`s method. For samples of widely differing texture sharpness, Jo＝0.005∼0.78, analytical errors and R factors resulted from various number of input data and degree of series expansion are compared and results are discussed. And, observed features of hot-and cold-drawn textures and some recrystalization textures of the samples are summarized.

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