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黃昞轍 건국대학교 1988 論文集 Vol.4 No.-
The purpose of this study is to develope the algorithm for the establishment of ten-day optimal opera titan policy of the Soyang multi-purpose dam. The Soyang dam should be operated for the downstream writer supply, hydropower generation and flood control puposes. The water demand series for '86, '91, 2001 years are selected out of future ten-day inflow data which is simulated from the past ten-day average inflow of Soyang dam site. The ten-case possible basis water levels in each inflow series are established in order that Soyang dam could supply 22% of the water demand of Han River water system, (24.4% '91-2001 year). Therefore, An atternative which can satisfy the objective function of system based on the rule curve in the extreme is determined as an optimal operation policy from this study.
고인성 API X70 라이파이프강의 파괴특성 : I. 압연조건-미세조직-저온인성의 상관관계
황병철,이성학,김영민,김낙준,유장용 대한금속·재료학회 2004 대한금속·재료학회지 Vol.42 No.3
This study is concerned with the effects of microstructure on fracture properties of a high-toughness API X70 pipeline steel. Twelve kinds of steel specimens were fabricated by vacuum-induction melting and hot-rolling, and their microstructures were varied by the rolling conditions. Charpy V-notch (CVN) impact test and drop-weight tear test (DWTT) were conducted on the rolled steel specimens in order to analyze low-temperature properties. Charpy impact test results indicated that the energy transition temperature (ETT) was below -100oC when the finish cooling temperature range was 350~500oC, implying excellent low-temperature toughness. The ETT increased because of the formation of bainitic ferrite and martensite at low finish cooling temperatures and because of the increase in effective grain size due to the formation of coarse ferrites at high finish cooling temperatures. Most of the specimens also showed excellent DWTT properties as the percent shear area well exceeded 85%, irrespective of finish rolling temperatures or finish cooling temperatures, although a large amount of inverse fracture area occurred at some finish cooling temperatures.
준안정 오스테나이트계 Fe-18Cr-10Mn-N 합금의 연성-취성 천이 거동에 미치는 Cu와 Ni의 영향
황병철,Hwang, Byoungchul 한국재료학회 2013 한국재료학회지 Vol.23 No.7
The influence of Cu and Ni on the ductile-brittle transition behavior of metastable austenitic Fe-18Cr-10Mn-N alloys with N contents below 0.5 wt.% was investigated in terms of austenite stability and microstructure. All the metastable austenitic Fe-18Cr-10Mn-N alloys exhibited a ductile-brittle transition behavior by unusual low-temperature brittle fracture, irrespective of Cu and/or Ni addition, and deformation-induced martensitic transformation occasionally occurred during Charpy impact testing at lower temperatures due to reduced austenite stability resulting from insufficient N content. The formation of deformation-induced martensite substantially increased the ductile-brittle transition temperature(DBTT) by deteriorating low-temperature toughness because the martensite was more brittle than the parent austenite phase beyond the energy absorbed during transformation, and its volume fraction was too small. On the other hand, the Cu addition to the metastable austenitic Fe-18Cr-10Mn-N alloy increased DBTT because the presence of ${\delta}$-ferrite had a negative effect on low-temperature toughness. However, the combined addition of Cu and Ni to the metastable austenitic Fe-18Cr-10Mn-N alloy decreased DBTT, compared to the sole addtion of Ni or Cu. This could be explained by the fact that the combined addition of Cu and Ni largely enhanced austenite stability, and suppressed the formation of deformation-induced martensite and ${\delta}$-ferrite in conjunction with the beneficial effect of Cu which may increase stacking fault energy, so that it allows cross-slip to occur and thus reduces the planarity of the deformation mechanism.
저탄소 보론강의 경화능에 미치는 Mo 및 Cr 함량의 영향
황병철,서동우,Hwang, Byoungchul,Suh, Dong-Woo 한국재료학회 2013 한국재료학회지 Vol.23 No.10
The hardenability of low-carbon boron steels with different molybdenum and chromium contents was investigated using dilatometry, microstructural observations and secondary ion mass spectroscopy (SIMS), and then discussed in terms of the segregation and precipitation behaviors of boron. The hardenability was quantitatively evaluated by a critical cooling rate obtained from the hardness distribution plotted as a function of cooling rate. It was found that the molybdenum addition was more effective than the chromium addition to increase the hardenability of boron steels, in contrast to boron-free steels. The addition of 0.2 wt.% molybdenum completely suppressed the formation of eutectoid ferrite, even at the slow cooling rate of $0.2^{\circ}C/s$, while the addition of 0.5 wt.% chromium did this at cooling rates above $3^{\circ}C/s$. The SIMS analysis results to observe the boron distribution at the austenite grain boundaries confirmed that the addition of 0.2 wt.% molybdenum effectively increased the hardenability of boron steels, as the boron atoms were significantly segregated to the austenite grain boundaries without the precipitation of borocarbide, thus retarding the austenite-to-ferrite transformation compared to the addition of 0.5 wt.% chromium. On the other hand, the synergistic effect of molybdenum and boron on the hardenability of boron steels could be explained from thermodynamic and kinetic perspectives.
고인성 라인파이프강의 Drop-Weight Tear Test시 발생되는 비정상 파괴양상에 대한 연구
황병철,이성학,김영민,김낙준,유장용,우종수 대한금속재료학회 2003 대한금속·재료학회지 Vol.41 No.5
In this study, various types of drop-weight tear test (DWTT) were conducted on a high-toughness line-pipe steel in order to investigate abnormal fracture appearance occurring in the region impacted by a hammer. A pressed notch or a chevron notch was introduced into DWTT specimens, some of which had a back slot designed to decrease the inverse fracture area. After the DWTT, percent shear area, 85% shear appearance transition temperature, area of inverse fracture surface, hardness and work hardening exponent of the hammer-impacted region were measured. The results indicated that the shear area of the pressed notch DWTT specimens was larger than that of chevron notch DWTT specimens. In the hammer-impacted region of all the DWTT specimens, abnormal inverse fracture having a cleavage fracture mode appeared, and its area fraction was correlated well with Vickers hardness and work hardening exponent(n) measured by an indentation test. The formation of this inverse cleavage fracture raised the fracture propagation transition temperature by decreasing the shear area measured in accordance of the API 5L3 specification, which was confirmed by relating to the Charpy V-notch test data of pre-strained steel specimens.
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.