Papers by Author: Sung Kyu Kim

Abstract: High Mn steels demonstrate an exceptional combination of high strength and ductility due to their high work hardening rate during deformation. The microstructure evolution and work hardening behavior of Fe18Mn0.6C1.5Al TWIP steel in uni-axial tension were examined. The purpose of this study was to determine the contribution of all the relevant deformation mechanism : slip, twinning and dynamic strain aging. Constitutive modeling was carried out based on the Kubin-Estrin model, in which the densities of mobile and forest dislocations are coupled in order to account for the continuous immobilization of mobile dislocations during straining. These coupled dislocation densities were also used for simulating the contribution of dynamic strain aging on the flow stress. The model was modified to include the effect of twinning.

Abstract: The present study focused on the design and testing of new 3rd generation TWIP steels to find reduced Mn content alternatives to the existing high Mn FeMnC and FeMnAlC alloy systems. In order to investigate the effect of nitrogen addition, 12Mn0.6C-N was examined and 18Mn0.6C-N steel was used as a reference. Effects of nitrogen and manganese on the microstructure and mechanical properties were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy and tensile testing. The deformation microstructures of 12Mn0.6C-N were mixtures of twinned austenite and 5~10% ε-martensite. Only deformation twinning was observed in 18Mn0.6C-N during the deformation due to its higher stacking fault energy. 18Mn0.6C-N steel exhibited higher strength and elongation than 12Mn0.6C-N steel. The effect of the strain rate on the tensile behavior of both materials was also examined. While 18Mn0.6C-N showed clear negative strain rate sensitivity, 12Mn0.6C-N did not show a clear relationship between flow stress and strain rate. The effect of annealing temperature on the tensile behavior and microstructure was also examined.