Papers by Author: Jin Ho Bae

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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.
3068
Authors: Orlando León García, Roumen H. Petrov, Jin Ho Bae, Leo Kestens, Ki Bong Kang
Abstract: The dependence of Charpy toughness on the microstructure and texture of two industrial grades of API –X80 pipeline steel with a thickness of 15 and 17 mm, respectively, were studied in this work. Standard size Charpy samples with a long axis orientated at 0, 22.5, 45, 67.5 and 90° with respect to the rolling direction of the plate were tested at different temperatures varying from -196°C to 20°C. Microstructure and texture of the plates were studied by means of electron backscattering diffraction (EBSD) and XRD. The highest value of the absorbed energy (440 J) was measured at room temperature in the samples oriented with the long axis parallel to the rolling direction and the minimum values for the same temperature were observed in the samples enclosing an angle of 45° to RD (340J). Statistical model parameters obtained from Charpy impact tests were quantitatively correlated to microstructural parameters by means of a linear regression model with the aim to describe the Brittle-Ductile Transition Curve in terms of microstructural features. Based on dedicated experiments the Brittle Fracture Factor (BFF) parameter was introduced, which quantifies the fraction of the {100} planes oriented parallel to the crack propagation front. It was found that the Charpy toughness anisotropy of the plates in the lower shelf region can not be related only to their crystallographic texture, but to the microstructural anisotropy characterized by the grain size variations i.e. the density of grain boundaries in the different directions. Also the spatial distribution of the 2nd phase plays a determining role in the fracture behavior.
840
Authors: Roumen H. Petrov, Orlando León García, Nuria Sánchez Mouriño, Leo Kestens, Jin Ho Bae, Ki Bong Kang
Abstract: The variations of in plane Charpy toughness anisotropy as a function of the microstructure and texture of an industrial grade of API –X80 pipeline steel was studied. Standard size Charpy samples with a long axis orientated at 0, 22.5, 45, 67.5 and 90° with respect to the rolling direction of the plate were tested at different temperatures varying from -196°C to 20°C. Microstructure and texture of the plates were investigated by means of electron backscattering diffraction (EBSD), XRD and the recently developed 3D EBSD technique. The spatial grain shape orientation distribution was examined on samples which were cut from the middle thickness of an industrial rolled plate by means of 3D EBSD and following grain shape reconstruction and approximation of the grain shape with ellipsoids. It was found that the experimentally observed 3D microstructures could well be correlated to the anisotropy of the measured Charpy impact toughness of the steel for the Charpy samples. The Charpy toughness anisotropy of the plates in the transition region where both ductile and brittle fractures take place can be related to the microstructural anisotropy characterized by the grain shape orientation and the spatial distribution of the 2nd phase.
1429
Authors: Jin Ho Bae, Seong Jun Park, Kyu Hwan Oh, Chang Gil Lee, Sung Joon Kim
449
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.
531
Authors: Jin Ho Bae, Gyun Heo, Seung Taik Oh, Eun Joo Shin, Baek Seok Seong, Chang Hee Lee, Kyu Hwan Oh
215
Authors: Roumen H. Petrov, Orlando León García, J.J.L. Mulders, Ana Carmen C. Reis, Jin Ho Bae, Leo Kestens, Yvan Houbaert
Abstract: The microstructural anisotropy together with the crystallographic texture of an industrial grade of X70 pipeline steel is studied by means of the 3D-EBSD technique known also as EBS3 which was recently developed by FEI. Samples of size 8x10x3mm³ were cut from the middle thickness of an industrial rolled plate and after special sample preparation have been studied in a Nova 600 dual beam scanning electron microscope equipped with a field emission gun and HKL Channel 6 EBSD data collection software for crystallographic orientation, which allows multiple sectioning of the sample in automatic mode and, afterwards reconstruction of both the 3D microstructure and texture of the examined volume. Three scanned zones of different volumes that varied between 15x10x27 4m³ and 16x14x6 4m³ have been examined and the results for the crystallographic orientation, grain shape and grain shape orientation are discussed together with the data for the anisotropy of the Charpy impact toughness of the material.
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