Papers by Author: Ki Soo Kim

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Authors: Nuria Sánchez Mouriño, Roumen H. Petrov, Jin Ho Bae, Ki Soo Kim, Leo Kestens
Abstract: The phase transformation and the final microstructure were studied in a pipeline steel grade API-X80 by carrying out a number of physical simulations of the industrial hot rolling schedules. The deformation and the cooling parameters were simulated by means of hot torsion and dilatometry experiments. Torsion deformations in the same range as in the hot rolling schedule were applied in a multi-deformation cycle at various temperatures in the austenite region. Subsequently the following parameters were varied with respect to a reference status: the reheating temperature from 900 to 1200°C, the deformation step from 0.6 to 0.15 von Misses strain, the strain rate from 1 to 10 s-1, the inter-pass time from 0.4 to 2 s, the deformation temperature from 1,100 to 850°C, the cooling rate from 0.1 to 100°C/s and the cooling stop temperature from 650 to 25°C. The transformation product microstructures were observed with optical microscopy, scanning electron microscopy and electron backscatter diffraction. The experimental data were used to study the microstructure evolution of none-deformed austenite and highly deformed austenite (Von Misses strain of 3.2), and the corresponding CCT diagrams were constructed. The detailed microstructure characteristics obtained from the present work as well as the data from the CCT diagrams for undeformed and deformed austenite could be used to optimize the mechanical properties, strength and toughness of pipeline steel grades by thermo-mechanical control process.
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Authors: Tae Kwon Ha, Chang Hoon Lee, Ki Soo Kim
Abstract: The effect of initial microstructure, cold reduction ratio, and annealing temperature on the spherodization rate of SK85 high carbon steel sheet was investigated. High carbon steel sheet fabricated by POSCO was soaked at 800oC for 2 hr in a box furnace and then treated at 570oC for 5 min in a salt bath furnace followed by water quenching to obtain a fine pearlite structure. Cold rolling was conducted on the sheets of fine pearlite by reduction ratios of 20, 30, and 40 % and heat treatment for spheroidization was carried out at 600 and 720oC for the various time intervals from 0.1 to 32 hrs. Area fraction of spheroidized cementite was measured with an image analyzer as a function of cold reduction ratios and duration times.
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Authors: Jin Ho Bae, Shi Hoon Choi, Ki Soo Kim, Ki Bong Kang
Abstract: This study has been conducted to analyze the effect of texture and microstructure on the anisotropy of yield strength and Charpy fracture toughness of an X80 line pipe steel. The texture and microstructure were investigated by X-ray diffractometer and electron backscattered diffraction (EBSD). The yield strength and impact energy were measured along 0o (longitudinal), 30o and 90o (transverse) to the rolling direction. It was found that the microstructure of the developed steel consisted of fine acicular and polygonal ferrite with small pearlite and martensite or retained austenite (MA constituents). The major components of textures were {332}<113> and {113}<110> orientations. In order to investigate the effect of both morphological and crystallographic texture on yield strength anisotropy, the prediction of the plastic property was carried out by using a viscoplastic self-consistent (VPSC) polycrystal model. The predicted anisotropy of yield strength with VPSC model assuming ellipsoidal grain shape was in a good agreement with experimental observation. EBSD results showed that the density of {001} cleavage planes of Charpy specimen, 30 degree to rolling direction, was the highest compared with that of other specimens. Therefore, the highest susceptibility to the cleavage fracture, i.e. increased ductile-brittle transition temperature, can be seen in the 30 degree direction.
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