발간논문

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Liquid phase sintering of cast iron powder compacts added 0-5% Cu, or the same contents Cu and 0.4% phosphor, at 1100℃ or 1120℃ in H_2 atmosphere was studied. In the case of copper addition, the maximum sintered density of 6.3g/㎤ and the maximum tensile strength of 300㎫ were obtained at 2% copper. On the contrary, in the case of both copper and phosphor addition, the sintered density decreased with increasing copper content because of compacts swelling in pressing direction. This phenomenon might be due to the bad wettability of P-containing liquid phase on the surface of cast iron powder.

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Hot rolled ultra-low carbon steel strips containing Ti and P have occasionally broken tranversely to rolling direction in the process of pickling. An investigation for this failure was carried out by means of microscopy, mechanical tests and Auger electron spectroscopy. It was found that the failure occurred due to the combination of (1) The phosphorous segregation on ferrite grain boundaries, (2) the impact energy such as tension hunting, and (3) the surface defects which played a role as a notch. The phosphorous segregation was proved to be a major cause for the failure which occurred along the grain boundary. As the amount of phosphorous segregation exponentially increases with an increase in coiling temperature, it must be controlled below 650℃ to prevent the occurrence of the failure.

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TiAl intermetallic compounds are regarded as one of the most feasible future elevated temperature structural materials, because of their high elevated temperature strength and oxidation resistance. However, it has been pointed out that they are difficult to fabricate due to their poor ductility at room temperature, which restricts their applications. In this study, attempts to improve the room temperature ductility of TiAl intermetallic compounds were made via the addition of Mn and designing heat treatments, which can refine grain size, shorten slip length and control microstructure and phase constitution. Maximum strain was obstained at the composition of Ti-47.6Al-2.7Mn(at.%) heat created at 1623 K for 30 hours. This can be rationalized by the fact that the addition of Mn tends to lower volume fraction of α₂, promote formation of γ phase containing twins, change axial ratio (c/a), refine grain size and shape lamellar morphology. Peak hardness value was obtained with the heat treatment at 1623 K for 30 hours. This increased hardness might be explained in terms of the increased volume of the α₂ phase and decreasing grain size by the addition of Mn.

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An investigation was carried out on the possibility whether metal powder injection moulding using sieve size powders instead of ultrafine carbonyl powders could be applied to the fabrication of high performance Ni-Mo-steel sintered parts. The debinding was performed by employing two step debinding process. The higher the green density and the shorter the presintering time, the higher sintered density of the injection moulded parts could be obtained in Fe-5Ni-3Mo-0.2B-0.4P alloy steel. The injection moulded parts with the relative sintered density above 97% could be obtained by following three step process; debinding, presintering at 650℃ for Omin, sintering at 1230℃ for 1h. The hardness and UTS of the sintered MIM part at 1230℃ for 1 and 3h were 40HRc, 272±19MPa and 44HRc, 458±9MPa, respectively.

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A computer simulation model of various smelting reduction processes for the production of hot metal was developed by using energy and material balance concept. this model can predict the coal flux, oxygen consumption and the volume, temperature, and composition of off gas. This model was comprehensive in that take account all of the important variables including composition of materials and operation factors. Four different concepts for smelting reduction processes were considered. A sensitivity analysis of the critical variables for four different concepts was performed including content of sulfur and phosphorus. The major critical variables for each concept can be explained by using thes analysis of energy and material balance. The major conclusions were as follows; 1. The most important variables for in-bath smelting process were post-combustion ratio, high HTE and refractory protection technology. 2. In the case of high pre-reduction processes, gas utilization and anti-sticking technologies were required for the attractive process. 3. The quality of hot metal by smelting reduction process was almost the same level of that of BF. But sulfur content may be higher than BF type.

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Al-10wt% Ti alloy was rapidly solidified by N₂gas atomization process, and the resulting alloy powders were consolidated by hot extrusion with an extrusion ratio of 17 : 1. The characteristics of the powders and the microstructures of the powders and extrudates have been examined by using a combination of particle size analyser, X-ray diffractometer, and optical, scanning electron and transmission electron microscopes. At-10wt% Ti powders show a spherical shape with mean particle size of 50㎛, and a microstructure of equiaxed α-Al grain of 2-3㎛ with Al₃Ti particles. Powders smaller than 44㎛ consist of α-Al, Ll₂metastable Al₃Ti and DO_(22) equilibrium Al₃Ti. With increasing th powder size, metastable Al₃Ti particles disappears and equilibrium Al₃Ti particles with a petal like shape become dominant. The metastable and equilibrium Al³,Ti particles show following orientation relationships with the surrounding α-Al matrix :［100]_(Al)//［001］DO_(22)-Al₃Ti and［010］Ll₂-Al₃Ti;［100］_(A1)//［100］Ll₂-Al₃Ti and［010］_(Al)//［010］Ll₂-Al₃Ti Extrudates show a uniform show a uniform distribution of equilibrium Al₃Ti particles in α-Al matrix with grain size of 2㎛

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The swelling and alloying behaviour of ferrous metal powder compacts have been studied using TMA(thermomechanical analysis) and optical microscope. Binary Fe-Cu compacts containing 3wt% Cu and up to 7wt% Cu were prepared from blends of Fe and Cu powders, and Cu coated Fe powders respectively. The TMA observations indicated that penetration of Cu-liquid phase into Fe particle and grain boundary leads to swelling of the powder compacts during heating and early stage of isothermal sintering at 1150˚C. Those swelling can be suppressed by using Cu coated Fe powder. The alloying process was accompanied by grain boundary penetration of Cu-rich liquid phase.

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The magnetic properties of rapidly solidified powders, hot pressed and die upset bulk magnet of Nd-Fe-B base alloy were investigated. Rapidly solidified powders were manufactured by crushing the melt spun ribbons. The effects of melt spinning condition on the magnetic properties of powders were evaluated. Hot pressing was adopted to consolidate rapidly solidified powders. Fully dense isotropic magnets were produced at the compacting temperature higher than 720℃. The best magnetic properties were obtained in the hot pressed magnet at the compacting temperature of 750℃ within the studied range. The effect of hot pressing time on the magnetic properties was insignificant. Die upset magnet showed a magnetic anisotropy due to the alignment of C axis in Nd₂Fe_(14) crystal parallel to the die upsetting direction.

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The effects of heating temperature and cooling rate on the tensile properties of continuously air cooled medium carbon micro-alloyed steels have been investigated. Transformation start temperature measured from inflection point of cooling curves has been found to decrease with increasing heating temperature and cooling rate and to be more sensitive to Mn contents when cooling rate is fast. Prior austenite grain size has been measured to abnormally grow in the range of 1050℃∼1150℃ and rapidly grow above 1150℃. It has been found that proof strength and tensile strength increase with increasing heating temperature and increase with increasing cooling rate up to 250℃/min. and remain relatively unchanged in the range of 250℃∼400℃/min. The regression equations for proof strength and tensile strength have beer, calculated by using the multi-regression analysis package and it is expected to be useful for the prediction of tensile properties of air cooled mudium carbon micro-alloyed steels.

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Cast structures of Ti-Al alloys containing 44, 48, and 52 at.% Al, respectively, were studied after casting at different cooling rates. In 44 at.% Al alloy, slowly cooled structure consisted of fully lamellar grains of α₂and γ whereas in fast cooled one lamellar formation was considerably suppressed, so that supersaturated α₂phases with A.P.D. (Anti-Phase Domain) structures were predominantly formed. In 48 at.% Al alloy dendritic lamellar grains were the major constituent with gamma phase as interdendritic segregates. Lamellar grains became finer with faster cooling. Dendritic lamellar grains transformed from primary α were distributed within the matrix of γ in 52 at.% Al alloy. Isolated γ islands were observed inside of the lamellar grains in slowly cooled 52 at.% Al alloy. Primary phase during solidification seems to be β in 44 at.% Al alloy, while α seems to be a major primary phase in 48 at.% Al alloy.

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