발간논문

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The effects of coiling temperature on the texture development and mechanical properties of a 1wt% copper containing extra low carbon steel were investigated. An experimental alloy was vacuum induction melted, forged and then hot rolled. Coiling process was simulated by annealing at 500℃ or 700℃ for 1 hour and was followed by furnace cooling. After the coiling simulation, specimens were cold rolled with reduction ratio of 80% or 90%. ε-Cu precipitates in ferrite matrix are rarely observed in specimens coiled at 500℃ and coiling does not affect the development of {111} recrystallization texture. However fine ε-Cu particles are precipitated in α-Fe matrix when coiled at 700℃ and the precipitates remarkably increase the intensities of α-fiber texture components during cold rolling, retard the recrystallization during annealing and suppress the development of {111} recrystallization texture. Coiling temperature does not affect the tensile properties of 1wt% copper containing steel but, in specimens coiled at 700℃, suppression of {111} recrystallization texture results in lower mean plastic strain ratio.

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Numerical simulations including the fluid flow, conduction and radiation heat transfer have been performed using the geometry of real furnace for the single-crystal growth by heat exchanger method. The finite difference method based on the control volume approach and SIMPLE algorithm were used to solve the momentum and energy equations. Almost all parts of the furnace including the heater, insulating materials and crucible were considered in the calculation domain and the latent heat was accounted by an iterative heat evolution method. Silicon with low thermal conductivity was selected as a model material in order to compare the results with the previous report on the copper single-crystal growth. The effects of cooling rate of the heater, crucible material, crucible shape and melt weight on single-crystal growth were investigated together with the role of natural convection in melt. The optimum process conditions such as the critical cooling rate and the critical ratio of the height to the radius of crystal for the silicon single-crystal growth by heat exchanger method were determined. According to the simulations, among many parameters crucible shape was a dominant processing parameter to control single crystal growth when a thermal conductivity of the specimen was low. The lower the thermal conductivity of base material of crucible was then that of the specimen, the less the region of poly crystal formed at the edge of the crystal was.

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Al₂O₃ thick film was deposited into the steel pipe to increase the mechanical properties by thermit process. Al₂O₃ film was formed by exothermic reaction of Fe₃O₄ and Al powder. The porosity and the thickness of Al₂O₃ thick film were increased with increasing the Al₂O₃ content. However, the hardness of film is decreased with increasing the Al₂O₃ content as a heat adsorbent. Resulted in increasing of the thickness and the microhardness of the specimen. The increased compaction pressure of the forming.

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Hot rolling was carried out on the extruded 6061 Al alloy composite reinforced with 10%(Al₂O₃)p along a direction perpendicular to the extrusion direction of the composite. The variations of the Young`s modulus as a function of the rolling ratio were measured along the longitudinal and the transverse directions using the standard sonic resonance test method. The results showed that the Young`s modulus increases with increasing rolling ratio. Qualitative analyses were made on the observed experimental results based on the microstructural changes induced by the rolling and annealing operation.

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High carbon ferro-manganese melt has been decarburized by blowing oxygen-argon mixture gas. Fraction of oxygen and flow rate of the gas mixture were changed to find out the optimum blowing schedule for the decarburization process. At initial blowing period, decarburization proceeds rapidly because of high carbon concentration. However, when carbon concentration decreases below about 2%, decarburization is degraded and simultaneously soidation of manganese occurs tremendously. An empirical equation has been proposed to analyze relationship between decarburization and oxidation of manganese in melt. Medium carbon ferro-manganese was made successfully by injection of oxygen-argon mixture without excessive oxidation of manganese.

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Equilibrium experiments between gas and slag were carried out to understand the thermodynamic behavior of nitrogen in CaO-SiO₂-CaF₂ slag system at 1600℃. The solubility of nitrogen in this slag system increased as the oxygen potential decreased and as the reaction temperature increased. The values of the nitride capacity has a minium at bout 2.0 slag basicity having higher values in both more acidic and basic region. This may be explained by two mechanisms for nitrogen dissolution ; incorporated nitride ion and free nitride ion state. In slag with 2.0 basicity or less, MgO content increased the nitride capacity slightly at higher slag basicity, however, nitride capacity decreased with MgO content. The effects of BaO to substitute CaO on nitride capacity showed similar behaviors as MgO. These complex relationship between basicity of slag and nitride capacity is explained by using optical basicity. It was found that nitride capacity and optical basicity had a clear relationship even in the different basic oxide systems.

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Hot ductility of 18Cr-8Ni stainless steels(type 304) containing high nitrogen was investigated in terms of fracture initiation mechanisms during high temperature deformation. The effects of nitrogen, sulphur and δ-ferrite contents on fracture mode changes were determined. It was found that, with the addition of nitrogen, the major fracture initiation mode at 1100℃ changed from brittle cracking along δ/γ interfaces to ductile cavity or micro-void formations along the austenite grain boundaries. It is due to decrease in residual δ-ferrite content with increase in nitrogen content. TEM analyses of the hot deformed microstructure showed that the transition from homogeneous dislocation tangled structure to planar array of dislocations occurred with the increase in nitrogen content. This planar slip behavior markedly reduced hot ductility. The fracture initiation mechanism was also found to be largely affected by the content of sulphur impurity, particularly in the high-nitrogen-containing steels. In these steels, a small increase in sulphur content resulted in intergranular fracture with micro-void formation along austenite grain boundaries at the temperatures below 1200℃.

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The objective of the present study is to investigate the adiabatic shear banding behavior in a tungsten heavy alloy penetrator. The penetrator was highly deformed at high strain rate by high speed impact, and after the high speed impact testing, metallurgical observation of the impacted penetrator specimen was conducted using an optical microscope and a scanning electron microscope. Heavily elongated tungsten grains and reaction products such as tungsten oxides were observed on the specimen surface presumably due to the local temperature rise occurring during high speed impact. A few adiabatic shear bands including elongated tungsten grains were observed in the regions near the surface cracks, and their width was wide in comparison to the shear band in armor plates. The cracks had trends to propagate along the shear bands, but sometimes changed their propagation path along the interfaces between adhering tungsten grains. These findings suggested that the minimization of the interfacial area of tungsten grains was required in order to improve the penetration performance of the tungsten heavy alloy.

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The growth behaviors of MoSi₂ diffusion layer formed by the reaction of the chemical vapor deposited Si with Mo substrate have been investigated in the temperature range between 850℃ and 1400℃ using coldwall horizontal reactor and SiCl₄-H₂system. The thickness of MoSi₂ diffusion layer was linearly increased with the square root of total gas flow rate of reactants and with the reactants ratio(SiCl₄/H₂) of 0.09 but constant over 0.09 due to etching process of Si by HCl at the deposition temperature of 1100℃. The growth of MoSi₂ diffusion layer on Mo substrate obeyed the parabolic rate law and rate constant was 2.26 ×10^(-5) ㎝ sec^(1/2) at 1100℃. The formation process of MoSi₂ diffusion layer was controlled by gas phase diffusion of reactants through a boundary layer to Mo substrate over 1000℃ and activation energy for the growth of it was 102 kJ/mole. But the rate determining step was solid diffusion of Si into it below 900℃ because activation energy for the growth of it was 223 kJ/mole, which was similar to that for diffusion of Si in MoSi₂

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Damping capacity and hardness were investigated with respect to deformation degree for an austenitic stainless steel (STS 304L). The damping capacity showed the maximum values at 10% and 20% in reduction of thickness for the room temperature and the liquid nitrogen temperature specimens, respectively. It was considered that the main damping source of the austenitic stainless steel was the stacking fault boundary which consists of partial dislocations in austenite. The specimens cold-rolled at room temperature exhibited higher hardness than the specimens cold-rolled at liquid nitrogen temperature at the same amount of α` martensite. This suggested that hardening for room temperature rolling was attributed to α` martensite, stacking fault, and dislocation in austenite formed during deformation, and that strain hardening for liquid nitrogen temperature rolling was caused by α` martensite and stacking fault in austenite produced during deformation. After the specimens that had been cold-rolled were aged below As temperature, the hardness increased at all deformation degrees. After aging, the damping capacity decreased at low deformation degrees, but didn`t change at high deformation degrees.

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