발간논문

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The roll bite deformation and heat transfer during hot rolling of commercial austenitic stainless steels were investigated by means of finite element simulation of hot rolling process. Particular attention was focused on the thermo-mechanical factors which are responsible for the formation of surface crack during hot rolling. It was found that a severe concentration of tensile stress and a large temperature drop developed on the workpiece surface at the roll exit region, particulary under rough rolling conditions. The high surface tensile stress at roll exit was caused by the constrained plastic deformation in surface layer at roll exit due to the surface chilling effect and the friction between roll and workpiece. The formation of surface crack could be attributed to the surface tensile stress and the loss of ductility of surface layer due to the temperature drop at roll exit region. The stainless steals containing high nitrogen, which exhibited a higher temperature dependency of flow stress, were more susceptible to surface crack formation during hot rolling. Higher possibility of surface cracking was also predicted in a slower roll speed and,%or in sticking friction condition.

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The microstructural evolution during a hot forging of Alloy 718 has been studied using compression tests and FEM simulation. The microstructures developed during a hot forging varied depending on dynamic recrystallization. The Zener-Holloman parameter(Z) measured as a function of temperature(T) and strain rate(ε˙) was given by Z=ε˙exp(447.706/RT). The size of dynamically recrystallized grains decreased as Z increased. The critical strain(ε_0) for the initiation of dynamic recrystallization was obtained from compression test and FEM simulation. The detailed variation of microstructures due to dynamic recrystallization could effectively be predicted at various locations in a forged pancake.

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Damping capacity of tempered martensite of an Fe-0.7wt.%C steel was investigated as a function of tempering temperature up to 600℃. The damping capacity abruptly decreased up to 200℃, while increased from 200 to 400℃, and again decreased with further tempering temperature over 400℃. These experimental results were well explained by the number of precipitates(carbon atoms or carbides) per unit length of dislocation and the number of dislocations available for damping source.

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