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남기우(K. W. Nam),황석환(S. H. Hwang),김대용(D. Y. Kim),이문용(M. Y. Lee),이상문(S. M. Lee) 대한기계학회 2012 大韓機械學會論文集A Vol.36 No.10
핫 스탬핑은 오스테나이트 변태 온도 이상에서 프레스 성형 후 급속히 냉각되는 고강도 부품을 제조하는 성형 방법이다. 매우 적은 양의 보론 성분을 가지고 있는 보론강은 핫 스탬핑에 사용되는 재료 중의 하나이다. 본 연구의 목적은 열처리 조건에 따르는 기계적 성질과 에릭슨 커핑 시험에 의하여 성형성을 조사하는 것이다. 다양한 온도에서 다이퀜칭은 대기 시간을 달리하여 실시하였다. 1173 K-0s에서 퀜칭 후 TWB는 1203 MPa의 인장 강도를 얻었다. 이것은 모재 인장강도(1,522 MPA)의 79 %이다. 금형 온도(298, 523, 673 K)에 따른 보론강 TWB의 성형성은 차이가 크지 않았다. 그러나 성형 속도가 증가함에 따라 성형성이 감소하는 것을 확인할 수 있었다. The hot-stamping technique is a forming method used for manufacturing high-strength parts, in which a part is cooled rapidly after press forming above the austenite transformation temperature. Boron steel, which contains a very small amount of boron, is one of the materials used for hot stamping. The purpose of this study is to investigate the mechanical properties of boron steel according to the heat-treatment conditions and the formability by using an Erichsen cupping test. Die quenching from various temperatures was conducted for different elapsed heat-treatment times. Laser-welded boron steel after quenching at 1173 K-0 s has a tensile strength of 1203 MPa. This is 79% of the tensile strength of the base metal (1522 MPa). The formability of boron steel was not significantly different from that at the mold temperature. However, it decreased with increasing forming speed. These properties provide practical information for the use of boron steels for hot stamping.
압력용기용강의 피로균열전파 및 지연거동에 관한 확률통계적 연구
김선진(S.J. Kim),남기우(K.W. Nam),김부안(B.A.Kim) 한국해양공학회 1995 韓國海洋工學會誌 Vol.9 No.1
The purpose of the present study is to investigate the statistical characteristics of m and C in the fatigue crack propagation law, da/dN=C(ΔK)<SUP>m</SUP> and to studies on the randomness of fatigue crack propagation and retardation behavior. Fatigue tests were performed on 32 CT specimens of SPV50 steel under the same one condition. First, the value of m and C were determined for each specimen, and all the data were analyzed statistically. Second, the material's resistance to fatigue crack propagation is modeled as a stochastic process, which varies randomly along the crack path. The statistical analysis of the material resistance is performed with the data obtained by constant load controlled tests. Finally, retardation behavior was examined experimentally by using a CT specimen, and a retardation parameters were analyzed statistically.
MATLAB/SIMULINK 를 이용한 자동 변속기 유압 제어 시스템의 동적 모델링
김기우(K.W.KIM),장재덕(J.D.JANG) 한국자동차공학회 1998 한국자동차공학회 춘 추계 학술대회 논문집 Vol.1998 No.11_2
To develop the Automatic Transmission (in short : A/T) which is characterized by excellent shift quality and performance, an evaluation tool for a new hydraulic control system is needed. To do this, the dynamic modeling using a MATLAB/SIMULINK are performed and simulation results are compared with test results. In addition, the design of a new pressure control system employing 3 way pressure control solenoid valve is discussed. Simulation and test results show valuable feasibility of the design methodology suggested in this study.<br/>
음향방출법에 의한 고강도 구조용 내화강의 열화특성에 관한 연구
강창룡,김현수,남기우,김부안 한국열처리공학회 2000 熱處理工學會誌 Vol.13 No.5
Demand for new nondestructive evaluations is growing to detect tensile crack growth behavior to predict long term performance of materials and structure in aggressive environments, especially when they are in non-visible area. Acoustic emission technique is well suited to these problems and has drawn a keen interests because of its dynamic detection ability, extreme sensitivity and location of growing defects. In this study, we investigated the strength of fire resistance steel for frame structure by tensile test after degradation treatment and analysed acoustic emission signals obtained from tensile test with time frequency analysis methods. In the T and TN specimens(under 600℃-10min) consisting of ferrite and pearlite structure, most of acoustic emission events were produced near yield point, mainly due to the dislocation activities during the deformation. However, B specimen under 600℃-10min had a two peak which was attribute to the presence of martensite phase. The first peak is before yield point and the second after yield point. The sources of second acoustic emission peak were the debonding of martensite-martensite interface and the micro-cracking of brittle martensite phase. In 600℃-30min to 700℃-60min, many signals were observed before yield point and were decreased after yield point.