Papers by Author: Chan Gyung Park

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Authors: H.S. Park, J.S. Kang, J.Y. Yoo, Chan Gyung Park
Abstract: Low carbon (~0.34 at.%) pipeline steels with high strength and high toughness are widely used for the transportation of petroleum and natural gas. These pipelines usually experience UOE (bending by U press, forming by O press, Expansion) piping followed by coating process at 250°C, which usually cause strain aging phenomenon with discontinuous yielding in service. In order to understand the major cause of the strain aging in these low carbon pipeline steels, the dislocations associated with solute carbon atoms have been investigated in three different types of steels (plate, pipe, and coated pipe). In-situ TEM heating up to 250°C for 30min did not cause any change of structure in both steel plate and pipe. However, APT results revealed the segregation of carbon atoms alone dislocation lines in ferrite region of both UOE pipes and coated pipes. It is believed that this solute segregation along the dislocations is the major cause in the case of strain aging of low carbon pipeline steels.
Authors: Young Won Chang, Chong Soo Lee, Chan Gyung Park
Authors: Chan Gyung Park, Chong Soo Lee, Young Won Chang
Authors: Yo Sep Yang, Jong Gu Bae, Chan Gyung Park
Abstract: In this study, the effects of drawing strain and annealing at low temperature on the mechanical properties have been investigated. The mechanical properties, such as hardness and strength and fatigue, were performed. The fatigue tests of hyper eutectoid steel wires were performed by the Hunter-type tester, specially designed for ultra thin wires (<0.2mmφ), under the bending stress. The mechanical properties of the steel wires increased with increase of annealing temperature, and the increase was more significant with higher drawing strain. Also, as annealing temperature increased, the hardness and tensile strength gradually increased up to 200 °C and then dramatically decreased at above temperature. However, bending fatigue limit, including fatigue life, showed the some different phenomena with increase of annealing temperature. These different behaviors depending on annealing temperature were discussed in terms of microstructural parameter changes such as pearlite morphology, rearrangement and coalescence of the dislocations.
Authors: Yo Sep Yang, S.Y. Park, Hyun Jo Jun, Chan Gyung Park, S.H. Lim, D.Y. Ban
Abstract: Effects of microstructural parameters on fatigue resistance (σFL) of the steel tire cords have been investigated experimentally from microscopic points of view. At first, microstructural parameters depending on carbon content have been identified by using transmission electron microscopy (TEM). The fatigue resistance of the steel tire cords depending on carbon content has been measured by using the Hunter rotating beam tester under the bending stress of 900 to 1500 MPa. The fatigue resistance was improved with increasing the carbon content from 0.7, 0.8 to 0.9 wt. % C, due to variations of microstructural parameters, such as lamellar spacing (λp), cementite thickness (tc), and volume fraction (Vc) of cementite. As the carbon content increased, the lamellar spacing and the cementite thickness decreased, while the volume fraction of cementites increased. The effects of microstructure on fatigue resistance have been discussed in terms of the microstructural parameters mentioned above.
Authors: J.S. Kang, Y. Huang, C.W. Lee, Chan Gyung Park
Abstract: Effects of deformation at austenite region and cooling rate on the microstructure and mechanical properties of low carbon (0.06 wt. % C) high strength low alloy steels have been investigated. Average grain size decreased and polygonal ferrite transformation promoted with increasing deformation amount at austenite region due to increase of ferrite nucleation site. Microstructure was also influenced by cooling rate resulting in the development of a mixture of fine polygonal ferrite and acicular ferrite at 10°C/s cooling rate. Discontinuous yielding occurred in highly deformed specimen due to the formation of polygonal ferrite. However, small grain size of highly deformed specimen caused lower ductile-to-brittle transition temperature than slightly deformed specimen.
Authors: K.M. Lim, Moon Chul Kim, Chan Gyung Park
Authors: Soo Ho Park, Kyu Young Kim, Yong Deuk Lee, Chan Gyung Park
Authors: J.S. Kang, S.S. Ahn, C.Y. Yoo, Chan Gyung Park
Abstract: In the present study, focused ion beam (FIB) technique was applied to make site-specific TEM specimens and to identify the 3-dimensional grain morphologies of bainitic microstructure in low carbon HSLA steels such as granular bainite, acicular ferrite and bainitic ferrite. Granular bainite consisted of fine subgrains and 2nd phase constituents like M/A or pearlite located at grain and subgrain boundaries. Acicular ferrite was characterized by an aggregate of ramdomly orientated and irregular shaped grains. The high angle boundaries between adjacent acicular ferrite grains caused by intragranular nucleation during continuous cooling process. Bainitic ferrite revealed uniform and parallel lath structure within the prior austenite grain boundaries and its’ packet size could effectively decreased by the formation of intragranular acicular ferrite.
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