Papers by Author: Gyo Sung Kim

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Authors: Yi Gil Cho, Young Roc Im, Gyo Sung Kim, Heung Nam Han
Abstract: A finite element model was developed to simulate the deformation, temperature and phase transformation behavior in high carbon steels. The heat capacity of each phase and the heat evolution due to phase transformation were obtained from the thermodynamic analysis of S45C, 50CrV4 and SK85 steels. Phase transformation kinetics of the steels were derived from continuous cooling experiments. An additivity technique was applied to a modified Johnson-Mehl-Avrami equation to analyze continuous cooling curve. To predict the strain due to TRansformation Induced Plasticity (TRIP), a variant selection model for diffusionless transformation and an accelerative creep model for diffusional transformation were adopted. In order to calculate the deformation behavior, the elastic strain, the volumetric strain due to thermal contraction and phase transformation, the plastic strain and the TRIP strain were taken into account. Using the finite element model developed in this study, the temperature-phase-deformation behavior of the high carbon steels was calculated.
Authors: Se Jong Kim, Yi Gil Cho, Dong Woo Suh, Sung Joon Kim, Gyo Sung Kim, Heung Nam Han
Abstract: In general, plastic strain occurs over a certain stress, called yield stress. However, it has been reported that the permanent strain could happen during boundary migrating even under the extremely slight externally applied stress. In this study, we performed dilatometry experiments under the various compressive stresses and measured the amount of recrystallization and growth induced permanent strain. A new empirical constitutive equation was suggested to describe the recrystallization and growth induced plasticity. This equation was verified by comparing the calculated values with dilatometric experimental data under the various compressive stresses.
Authors: Kyoo Young Lee, Gyo Sung Kim, Leo Kestens
Abstract: By applying a double cold rolling and annealing treatment, the evolution of the α and γ fiber components differed from the ones observed in conventional processing. This is attributed to the difference of the initial texture. An increased reduction of stored energy of the {111}<112> component was found by monitoring the change of the stored energy during annealing, indicating that the onset of the nucleation stage of recrystallization by relaxation and annihilation of dislocations occurred mainly on the {111}<112> component with its higher stored energy. The detailed texture evolution of the double cold rolled specimen during 2nd annealing is described by coupling the theory of oriented nucleation and orientation pinning, which is experimentally confirmed by OIM scan.
Authors: No Jin Park, Gyo Sung Kim, Moo Young Huh
Authors: Ohjoon Kwon, Kyoo Young Lee, Gyo Sung Kim, Kwang Geun Chin
Abstract: The body design with light weight and enhanced safety is a key issue in the car industry. Corresponding to this trend, POSCO is developing various automotive steel products with advanced performance. Conventional advanced high strength steels such as DP and TRIP steels are now expanding their application since the steels exhibit higher strength and ductility than those of conventional solution and precipitation strengthened high strength steels. Efforts have been made to enhance the mechanical performance of these steels such as ductility, hole expansion ratio, deep drawability, etc. Current research is focused on development of extra- and ultra-AHSS. Extra-AHSS are designed to utilize nano-scale retained austenite embedded in fine bainite and martensite. Ultra-AHSS are designed to have austenite as the major phase, and the ductility is enhanced primarily by continuous strain hardening generated during forming. These steels including extra- and ultra-AHSS are believed to be the next generation automotive steels which will replace the existing high strength steels due to their extremely high strength and ductility combinations.
Authors: Kyoo Young Lee, Young Roc Im, Leo Kestens, Gyo Sung Kim
Abstract: The microstructural evolution and the softening behavior of hot rolled and 60% cold rolled 0.85wt% carbon pearlitic steels during spheroidization annealing have been investigated by using the textural and microstructural information contained in the Orientation Imaging Microscopy (OIM) scans. The local boundary energy map, recently suggested by the present authors, is used to monitor the changes of stored plastic strain energy distribution in ferrite during the annealing process, which shows that the spheroidization process of cementite is finished before the completion of recrystallization of the 60% cold-rolled high carbon pearlitic steel.
Authors: Kyoo Young Lee, Gyo Sung Kim, Kwang Geun Chin, Leo Kestens
Abstract: The evolution of texture and stored energy was studied for Ti bearing and Ti-Nb bearing IF high strength steel undergoing a double cold rolling and annealing treatment by using Orientation Imaging Microscopy. Ti bearing IF high strength steel showed a similar texture evolution of the α and γ fiber components to that of IF steel. On the contrary, Ti-Nb bearing IF high strength steel displayed a different texture evolution from the one observed in the former steels. The difference in texture and stored energy evolution between both high strength steels seemed to be affected by the segregation behavior of solute alloying elements such as P.
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