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황병철,이한상,김양곤,이성학,이대식 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.7
In this study, a microstructural investigation was conducted on the side cracking phenomenon occurring in low-carbon steel plates during hot rolling. Particular emphasis was placed on the role of iron oxides and pearlite band structure in the side cracking. Detailed microstructural analyses of the hot-rolled plates showed that the pearlite band structure in the side region was considerably slanted to the surface, while that in the central region was parallel to the surface. Small cracks were often observed when the iron oxide layer formed by inhomogeneous oxidization along the slanted pearlite band structure in the side region was intruded into the plate interior during hot rolling. In addition to these small cracks, a few large cracks were also found when some protrusions formed on the plate surface were decarburized, folded, and then intruded into the plate interior. Thus, in order to prevent the side cracking, 1) the minimization of the segregation of C and Mn in slabs by the sufficient homogenization, 2) the parallel arrangement of the pearlite band structure in the side region by the appropriate control of the hot rolling process, 3) the minimization of the iron oxide formation by the shortest time exposed to high temperatures, and 4) the careful control of the spacing between steel plates and side guide rolls were suggested. (Received January 17, 2004)
황병철,김양곤,이성학,김낙준,유장용 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.4
This study is concerned with the effects of microstructure on inverse fracture in the hammer-impacted region during drop-weight tear test(DWTT) of three high-toughness API X70 pipeline steels. Pressed notch or Chevron notch DWTT and Charpy V-notch impact test were conducted on the rolled steel specimens, and the results were discussed in comparison with the data obtained from tensile and Charpy impact tests of pre-strained specimens. In the hammer-impacted region of the DWTT specimens, cleavage-type inverse fracture appeared, and its fracture area was correlated well with upper shelf energy (USE) obtained from the Charpy impact test and with microstructure. The steel specimens having higher USE or having coarse polygonal ferrite had the larger inverse fracture area than those having lower USE or having fine acicular ferrite because of the larger compressive pre-strain induced by work hardening of the hammer-impacted region. These results were confirmed by relating to the Charpy impact test data of pre-strained steel specimens. (Received January 19, 2004)
고인성 라인파이프강의 인성과 천이온도의 분석 Ⅰ. Charpy 충격특성과 유효결정립도
황병철,김양곤,이성학,김영민,김낙준,유장용 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.9
This study is concerned with the effects of microstructure on Charpy V-notch (CVN) impact properties of a high-toughness API X70 pipeline steel. Six kinds of steel specimens were fabricated by varying hot-rolling conditions, and their microstructures and CVN properties were investigated. In addition, their effective grain sizes were characterized by the electron back-scatter diffraction (EBSD) analysis. The Charpy impact test results indicated that the specimens rolled in the single phase region had the higher upper shelf energy (USE) and the lower energy transition temperature (ETT) than the specimens rolled in the two phase region because their microstructures were composed of acicular ferrites and fine polygonal ferrites. The decreased ETT in the specimens rolled in the single phase region could be explained by the decrease in the overall effective grain size due to the presence of acicular ferrite having smaller effective grain size. On the other hand, CVN properties of the specimens rolled in the two phase region were deteriorated as they contained a small amount of martensites or cementites.
Kim, Yang Gon,Hwang, Byoungchul,Lee, Sunghak,Lee, Chul Won,Shin, Dong Hyuk Elsevier 2009 Materials science & engineering. properties, micro Vol.504 No.1
<P><B>Abstract</B></P><P>Dynamic deformation and fracture behavior of ultra-fine-grained pure copper fabricated by equal channel angular pressing (ECAP) was investigated. The 1-pass ECAP’ed specimen consisted of fine dislocation cell structures, which were changed to very fine, equiaxed subgrains of 300–400nm in size with increasing number of ECAP pass. The dynamic torsional test results indicated that maximum shear stress increased with increasing number of pass, but that the rate of the strength increase was higher in the 4-pass or 8-pass specimen than in the 1-pass specimen. This was because boundaries of subgrains formed during the 4-pass or 8-pass ECAP were more stabilized and high angled. Most of the ultra-fine-grained pure copper specimens were not fractured because adiabatic shear bands were hardly formed as the pure copper specimens were ductile and had high thermal conductivity, although a weak adiabatic shear band was observed in the 8-pass specimen. These findings suggested that the grain refinement by the ECAP was effective in strengthening of pure copper, and that the ECAP’ed pure copper could be used without much loss in fracture resistance under dynamic loading.</P>
ECAP으로 제조된 초미세립 5083 Al 합금의 동적 변형거동
김양곤,황병철,이성학,김우겸,신동혁 대한금속재료학회 2005 대한금속·재료학회지 Vol.43 No.1
Dynamic deformation behavior of ultra-fine-grained aluminum alloys fabricated by equal channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests were conducted on four aluminum alloy using a torsional Kolsky bar, and then the test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear band formation. The ECAP'ed specimens consisted of a considerable amount of second phase particles, which were refined and had an equiaxed shape as the ECAP pass number increased. The dynamic torsional test results indicated that maximum shear stress increased, while fracture shear strain remained constant, with increasing ECAP pass number. Observation of the deformed area beneath the fracture surface after the dynamic torsional test indicated that a number of voids initiated mainly at second phase particle/matrix interfaces, and that the number of voids increased with increasing pass number. Adiabatic shear bands of 100-150μm in width were formed in the as-extruded and 1-pass specimens having coarser second phase particles, while they were hardly formed in the 4-pass and 8-pass specimens having finer second phase particles. The possibility of the adiabatic shear band formation was explained by concepts of absorbed deformation energy and void initiation. (Received October 29, 2004)
[진동 소음부문] 탄매의 점진적인 배출을 위한 소음기의 구조 개선에 관한 연구
송용희(Yonghee Song),이유엽(Youyub Lee),황원걸(Wongul Hwang),기창두(Changdoo Kee),최병철(Byoungchul CHoi),양기영(Giyoung Yang) 한국자동차공학회 2001 한국자동차공학회 춘 추계 학술대회 논문집 Vol.2001 No.11_2
The muffler must be redesigned to get a maximum noise reduction and a minimum flow loss. The object of this paper is to redesign an exhaust muffler to prevent the gradual discharge of particulate while keeping the performance of present exhaust system for heavy truck. We found the problems of present model from experiment and analysis about noise and flow, and we redesigned the muffler to reduce the particulate's accumulation while keeping the level of noise. We noticed the improvement of TL. performance and back pressure from experiment and analysis. We think that the redesigned model well decrease the inner noise in a car and reduce the gradual discharge of particulate.