발간논문

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The optimized condition for preventing metal delivery tube from freezing up during atomization was evaluated by analyzing the experimental data of gas atomizations which were carried out on various conditions. The optimum condition was established by controlling the protrusion length and the temperature of metal delivery tube. It was observed that powder shape is affected by the collision of particles. The probability of collision among particles can be controlled by changing atomization variables such as protrusion length of metal delivery tube, nozzle slit size and the condition of atomizing chamber. So it is possible to control the powder shape by changing the atomizing variables. It was also shown that the average particle size decrease with increasing atomizing pressure.

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An investigation was conducted to examine the effects of microstructural parameters on the cracking behavior, often occurred in the top parts of the deep drawn cups of hot rolled formable steel. Particular emphasis was placed on the role of grain boundary carbides in the carcking phenomenon. Detailed microstructural analyses of the cracked region showed that a number of voids initiated at grain boundary carbides were observed to form intergranular cracks. These grain boundary carbides were identified as Mn-containing Fe₃C carbides which might be associated with the formation of the band structure. In the microstucture of the hot rolled steel, film or bulk-type carbides along grain boundaries, together with banded ferrite-pearlite structures, were commonly observed, and were thought to initiate cracks during deep drawing process. Thus the formation of grain boundary carbides and band structures must be minimized to prevent cracking. A1 killing process, homogenization treatment and low temperature coiling treatment are suggested as possible methods for minimizing the amount of grain boundary carbides.

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This study aims at investigating the microstructural modification in hot rolled formable steels in order to monitor the amount and shape of grain boundary carbides. The steels were deoxidized with aluminum and aluminum-silicon addition, hot rolled, and coiled at the temperature range between 600℃ and 650℃. Detailed microstructural analyses of the hot rolled steels showed that film- or bulk-type grain boundary carbides, together with banded pearlite structures, were observed and that the morphology of these carbides were varied with processing variables, e.g., killing procedure and coiling temperature. In the Al killed steels, the carbide shape had a trend of changing from film-type to bulk-type, which might be beneficial to the improvement of elongation and consequently deep drawability. In addition, as the coiling temperature was increased, the amount of band structure and bulk-type carbides tended to increase. These findings suggest that deep drawability can be improved by modifying the carbide morphology in the microstructures.

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The effects of melt holding time and various melt treatment conditions such as phosphorus treatment, simultaneous treatment after remelting of phosphorus treated alloy, and strontium treatment one hour after phosphorus treatment on the simultaneous refinement of primary and eutectic silicon in hypereutectic Al-Si alloy were studied. In case of simultaneous treatment with 0.04%P and 0.05%Sr, the size of primary silicon particles increased with melt holding time and started to decrease after one hour and modified microstructure was maintained during melt holding time for four hours. In case of strontium treatment after remelting of phosphorus treated alloy, the size of primary silicon particles increased with melt holding time and started to decrease after 30 minutes and eutectic Si modification was achieved during melt holding time for four hours. Strontium treatment after remelting of phosphorus treated alloy showed the smallest primary Si particle size among the melt treatment conditions.

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To find out the critical strain value for recrystallization, strain distribution in cylindrical specimen after hot compression test was simulated by Finite Element Method(FEM) and evaluated by observing the microstructure of hot compressed specimen. FEM analysis results show that effective strain distribution depends on the position in specimen. Core and edge region of specimen shows higher effective strain than other region. Especially the magnitude of strain at core is twice larger than the mean strain value. Mode of effective strain distribution also changed with the magnitude of applied strain and localized phenomena. In hot compression test, the shape change in barreling region and recrystallized state of austenite grains were investigated. The calculated result of strain distribution and microstructure observation result of austenite were correlated to find out the critical strain for recrystallization of austenite, because the shape of hot compresses specimen was well coincident with that of FEM calculation. In case of 304 stainless steel, 17% strain was required was for recrystallization at 1050℃. For practical application, one or two hot compression test instead of dozen of hot compression tests is enough to obtain the critical strain for recrystallization by simulating the strain distribution in specimen.

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The influence of tensile hold time on the low cycle fatigue life far AISI 316 stainless steel is investigated at 873K. Using the test results, the characteristics of the life prediction model which is based on the cavitation damage under creep-fatigue condition is discussed. From this study, it can be found that the introduction of tensile hold time reduces fatigue life and the major cause of the reduction in fatigue life is the cavities which are formed continuously with cycles on grain boundaries. The predicted fatigue life is in good agreement with the experimentally observed one for AISI 316 stainless steel. Investigating the relationship between the new cavity nucleation factor, P´, and the distribution of grain boundary carbides, it is found that the value of P´ is strongly related to the characterstics of the grain boundary carbides and is increased as the number of grain boundary carbides is increased.

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Microstructure and magnetic properties of CoCrPt/Cr thin films were investigated as a function of sputtering condition during the deposition of CoCrPt thin films. Strong separation of Co grains was observed with increasing argon sputtering pressure due to the enhanced scattering of argon ions with sputtering particles, thereby resulting in the reduced exchange coupling and also the increased coercivity by a factor of four. Plane and cross sectional TEM indicated that most Co grains sputtered at high pressure were separated by 10∼20Å thick boundary and share their growth on bigger Cr grains. The Co grain separation decreased with the application of substrate bias and disappeared by -200V bias. The bias sputtered CoCrPt thin films decreased their grain size and increased epitaxial growth on Cr layer and also (1010) texture. The slight decrease in squareness of bias sputtered films resulted from the residual stress in the films. The required magnetic field to saturate the film significantly increased with increasing substrate bias.

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The purpose of this study is to investigate the high temperature deformation behavior of Ti_(50) Al_(49)Cr₁ and Ti_(50)Al_(47)Cr₃ intermetallic compounds. The alloys were prepared by vacuum induction melting process using calcia crucible. Flow stresses at high temperature could be well explained by the hyperbolic relations and Zener-Hollomon parameter. With increasing Cr content, the activation energy of deformation also increased significantly. Deformability maps were obtained as a function of temperature, strain rate and flow stress. Higher Cr content in the alloy resulted in larger volume fraction of lamellar grains, the greater inhomogeneities in deformation, and higher flow stresses. Dynamic recrystallization is observed to start along the boundaries of lamellar structure at 1000℃

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In STS303 austenitic stainless steel, the effects of chemical composition, boron addition and deformation condition on hot ductility and/or machinability, were investigated. Hot-tensile test was carried out at the temperature range of 950℃∼1150℃. Optical microscopy and scanning electron microscopy were used to investigate the occurrence of cracks during hot deformation. Also the shape of chips during machining, was investigated at the turning speed of 50m/min-200m/min with 1.0, 1.5, 2.0㎜ in the depth of cut for selected compositions. Hot ductility of STS303 is increased with increasing the Mn/S ratio, while the total oxygen content has little influence on hot ductility. Boron added steels showed a superior hot ductility and s machinability equivalent to the type of 303 austenitic stainless steel.

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The mechanical properties of extruded bar, which was made of Fe-4.1, 4.7 and 4.9 wt%C alloy powders, were investigated in relation to the size, volume fraction of primary cementite and of eutectic cementite and distribution of temper carbon. Since the content of temper carbon generated from cementite increases as the size of primary cementite decreases, the tensile strength of extruded bar was deteriorated from 852MPa to 739MPa. The yield strengths of extruded bars are 392, 549, 528MPa and its their elongations are 4.4, 1.0, 0.8% when they contain 4.1, 4.7, 4.9 wt%C, restpectively. The yield strength improvement of hypereutectic extruded bar compared to hypoeutectic extruded bar is due to the larger volume fraction of cementite. Considering the hypereutectic extruded bar, the decreased yield strength of extruded bar with 4.9 wt%C is explained by the content and irregular distribtuion of temper carbon.

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