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Hwang, Byoungchul,Lee, Sunghak,Kim, Yong Chan,Kim, Nack J.,Shin, Dong Hyuk Elsevier 2006 Materials science & engineering. properties, micro Vol.441 No.1
<P><B>Abstract</B></P><P>Microstructural development of adiabatic shear bands formed in ultra-fine-grained low-carbon steels fabricated by equal channel angular pressing (ECAP) was investigated in this study. Dynamic torsional tests were conducted on four steel specimens, two of which were annealed after ECAP, using a torsional Kolsky bar. The ECAP’ed specimen consisted of fine equiaxed grains of 0.2μm in size, which were slightly coarsened and had an equiaxed shape after annealing. Some adiabatic shear bands were observed at the gage center of the dynamically deformed torsional specimen, and their width was narrower in the ECAP’ed specimen than in the 1-h annealed specimen. Detailed transmission electron microscopic analysis on adiabatic shear bands indicated that very fine equiaxed grains of 0.05–0.2μm in size were developed within the adiabatic shear band, and that cell structures were formed in the shear band flank by partitioning elongated ferrites. These phenomena were explained by dynamic recovery and recrystallization due to the highly localized plastic deformation and temperature rise occurring in the shear band. The temperature rise in the shear band formation process was estimated to be above 540°C by observing spheroidized cementites inside pearlite grains.</P>
황병철 ( Byoungchul Hwang ),심재혁 ( Jae-hyeok Shim ),이명규 ( Myoung-gyu Lee ),이준호 ( Joonho Lee ),정준호 ( Jun-ho Jung ),김보성 ( Bo-sung Kim ),원성빈 ( Sung-bin Won ) 대한금속재료학회(구 대한금속학회) 2016 대한금속·재료학회지 Vol.54 No.12
Since reconstruction of old town in Korea requires high-rise and seismic design construction, many attentions have been paid to high strength seismic reinforced steel bar. In the present paper, technical developments and trends are summarized for developing next-generation seismic reinforced steel bar of grade 700 MPa. Steelmaking process requires high energy efficiency and refining ability. Effects of alloying elements are explained, and alloy design based on computational thermodynamics is introduced. On the other hand, it is considered that grain size refinement by the controlled rolling and low temperature transformation structures formed by the accelerated cooling are effective to obtain acceptable mechanical properties with high strength. Finite element simulation analysis is also useful to understand plastic deformation by rolling, internal and external heat transfer, and corresponding phase transformation of austenite phase to various low-temperature transformation structures. †(Received September 20, 2016; Accepted September 26, 2016)
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.
대기 플라즈마 용사법으로 제조된 Mo 및 Mo 혼합 코팅층의 미세조직과 마모특성
황병철,안지훈,이성학 대한금속재료학회 2003 대한금속·재료학회지 Vol.41 No.3
The objective of this study is to investigate the correlation between the microstructure and wear resistance of various molybdenum blend coatings applicable to automotive parts. Five types of molybdenum spray powders, one of which was pure molybdenum powders and the others were blend powders of molybdenum powders mixed with brass, bronze, and aluminum alloy powders, were sprayed on a low-carbon steel substrate by atmospheric plasma spraying. Microstructural analysis of the coatings showed that they consisted of curved lamellar structures formed by elongated splats, while hard precipitates formed during spraying were relatively homogeneously distributed in the molybdenum matrix. The wear test results revealed that the blend coatings showed the better wear resistance than the pure molybdenum coating because they contained a number of hard precipitates. In particular, the molybdenum coating blended with bronze and aluminum alloy powders showed the excellent wear resistance, together with the excellent wear resistance of a counterpart material, resulting from the presence of hard precipitates and their homogeneous wear. Therefore, in order to improve the overall wear properties with consideration of the wear resistance of a counterpart material, appropriate spray powders should be blended with molybdenum powders to form hard precipitates in the coatings of this study.
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 Ⅲ. Charpy 충격특성, DWTT, CTOA의 상관관계
황병철,신상용,이성학,김낙준,강기봉 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.9
Correlation of Charpy V-notch (CVN) impact properties, drop-weight tear test (DWTT) properties, and crack tip opening angle (CTOA) values of a high-toughness API X70 pipeline steel was investigated in this study. Two-specimen CTOA test (TSCT) was conducted on the rolled steel specimens to measure CTOA, and the test results were related with the CVN and DWTT data to find correlations between them. The CVN total energy density showed almost 1:1 linear correlation with the DWTT initiation energy density. The TSCT test results indicated that the specimens rolled in the single phase region had the larger CTOA as well as the higher CVN and DWTT energies than those rolled in the two phase region because their microstructures were composed of acicular ferrites and fine polygonal ferrites. The measured CTOA showed better correlation with the DWTT propagation energy than with the CVN or DWTT total energy as it was related with the crack propagation speed. Since the value of Sin(2CTOA) was linearly proportional to the DWTT energy, it could be used as a useful parameter to reliably estimate the CTOA.
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)
0.15C-2.0Mn-0.2Si 2상 조직강의 준정적 및 동적 변형특성에 미치는 페라이트 결정립 크기와 마르텐사이트 부피분율의 영향
황병철,T. Cao,신상용,이성학,김성준 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.7
Effects of ferrite grain size and martensite volume fraction on quasi-static and dynamic deformation behavior of dual phase steels were investigated in this study. Dynamic torsional tests were conducted on six steel specimens, which had different ferrite grain size and martensite volume fraction, using a torsional Kolsky bar. Under a dynamic loading condition, maximum shear stress and fracture shear strain increased with decreasing the ferrite grain size and increasing the martensite volume fraction. Observation of the deformed area beneath fracture surface after the dynamic torsional test indicated that adiabatic shear bands of 5-15 μm in width were formed along the shear stress direction; voids or microcracks initiated at martensite/ferrite interfaces below the shear band. The width of the shear band decreased with increasing ferrite grain size and with decreasing martensite volume fraction. These phenomena were then analyzed by introducing concepts of critical shear strain. (Received April 7, 2004)
황병철,이성학,김영민,김낙준,안성수 대한금속재료학회 2004 대한금속·재료학회지 Vol.42 No.3
This study is concerned with the effects of microstructure on fracture properties of high-toughness API X70 pipeline steels. Three kinds of steels were fabricated by varying alloying elements such as C, Cu, and Mo, and their microstructures were varied by the rolling conditions such as finish rolling temperature and finish cooling temperature. Charpy V-notch (CVN) impact test and press notch (PN) drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature fracture properties. Charpy impact test results indicated that the specimens rolled at the γ single phase region of the steel having the reduced C and Mo content had high upper shelf energy (USE) and low energy transition temperature (ETT) because of the appropriate formation of quasi-polygonal or polygonal ferrite and the decreased fraction of martensite-austenite constituent. Most of the specimens rolled at γ region also showed excellent DWTT properties as the percent shear area well exceeded 85%, irrespective of finish cooling temperatures, while their USE was higher than that of the specimens rolled at the (α+γ) two phase region. Thus, overall fracture properties of the specimens rolled at rolled at γ region were better than those of the specimens rolled at (α+γ) region, considering both USE and DWTT percent shear area.