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Fe-0.1C-(V, Nb) 제어압연강의 미세조직 및 기계적 특성
조경목 ( Kyung Mox Cho ),김수영 ( Su Young Kim ),박인애 ( In Ae Park ),이영중 ( Young Jung Lee ),강남현 ( Nam Hyun Kang ) 대한금속재료학회 ( 구 대한금속학회 ) 2007 대한금속·재료학회지 Vol.45 No.5
Following the trend of industrial high strength and toughness structural steel manufacturing, it is evolving and increasing needs of basic research and development efforts. One of the examples is, due to the high cost of vanadium, the replacement of Fe-V steel to Fe-Nb steel. However, there still exist difficulties for commercial production of Fe-Nb steel because of poor impact toughness of this steel comparing with Fe-V steel. In this study, a fundamental investigation was carried out to analyze the microstructure, tensile properties and impact toughness of Fe-Nb steel as a function of the rolling conditions with no post heat treatment. The steel samples were prepared with three different compositions, i.e., Fe-V steel (Fe-0.05V-0.001Nb), Fe-V-Nb steel (Fe-0.014V-0.03Nb), Fe-Nb steel(Fe-0.003V-0.033Nb). The steels were deformed with controlled rolling conditions, namely start rolling temperature(SRT) 1150℃ for the all experiment, but two different finish rolling temperature(FRT); 950℃ and 860℃. Ferrite grain size decreased and thus impact toughness was measured higher at FRT of 860℃ than at FRT 950℃ treated steels. Even though the impact toughness of Fe-Nb steel was quite low due to inhomogeneous distribution of ferrite grain size, Fe-V-Nb steel exhibited impact toughness improved with controlled rolling at FRT 860℃. Application of the controlled rolling process showed one of the possibilities to replace Fe-V steel with Fe-V-Nb and/or Fe-Nb steel.
변형구배 결정소성 유한요소해석법을 이용한 니켈기 다결정 합금의 Hall-Petch 관계 모델링
최윤석,조경목,남대근,최일동,Choi, Yoon Suk,Cho, Kyung-Mox,Nam, Dae-Geun,Choi, Il-Dong 한국재료학회 2015 한국재료학회지 Vol.25 No.2
A strain-gradient crystal plasticity constitutive model was developed in order to predict the Hall-Petch behavior of a Ni-base polycrystalline superalloy. The constitutive model involves statistically stored dislocation and geometrically necessary dislocation densities, which were incorporated into the Bailey-Hirsch type flow stress equation with six strength interaction coefficients. A strain-gradient term (called slip-system lattice incompatibility) developed by Acharya was used to calculate the geometrically necessary dislocation density. The description of Kocks-Argon-Ashby type thermally activated strain rate was also used to represent the shear rate of an individual slip system. The constitutive model was implemented in a user material subroutine for crystal plasticity finite element method simulations. The grain size dependence of the flow stress (viz., the Hall-Petch behavior) was predicted for a Ni-base polycrystalline superalloy NIMONIC PE16. Simulation results showed that the present constitutive model fairly reasonably predicts 0.2%-offset yield stresses in a limited range of the grain size.
Sn-Ag-Cu-X 무연솔더로 솔더링 된 접합부의 진동파괴 거동
진상훈,강남현,조경목,이창우,홍원식,Jin, Sang-Hun,Kang, Nam-Hyun,Cho, Kyung-Mox,Lee, Chang-Woo,Hong, Won-Sik 대한용접접합학회 2012 대한용접·접합학회지 Vol.30 No.2
Environmental and health concerns over the lead have led to investigation of the alternative Pb-free solders to replace commonly used Pb-Sn solders in microelectronic packaging application. The leading candidates for lead-free solder alloys are presently the near eutectic Sn-Ag-Cu alloys. Therefore, extensive studies on reliability related with the composition have been reported. However, the insufficient drop property of the near eutectic Sn-Ag-Cu alloys has demanded solder compositions of low Ag content. In addition, the solder interconnections in automobile applications like a smart box require significantly improved vibration resistance. Therefore, this study investigated the effect of alloying elements (Ag, Bi, In) on the vibration fatigue strength. The vibration fatigue was conducted in 10~1000Hz frequency and 20Grms. The interface of the as-soldered cross section close to the Cu pad indicated the intermetallic compound ($Cu_6Sn_5$) regardless of solder composition. The type and thickness of IMC was not significantly changed after the vibration test. It indicates that no thermal activities occurred significantly during vibration. Furthermore, as a function of alloying composition, the vibration crack path was investigated with a focus on the IMCs. Vibration crack was initiated from the fillet surface of the heel for QFP parts and from the plating layer of chip parts. Regardless of the solder composition, the crack during a vibration test was propagated as same as that during a thermal fatigue test.
탄소섬유 강화 Cu 기지 금속 복합재료의 Squeeze Cast 조직 및 내마멸특성
김남수,지동철,조경목,박익민 ( Nam Soo Kim,Dong Chul Chi,Kyung Mok Cho,Ik Min Park ) 한국주조공학회 1992 한국주조공학회지 Vol.12 No.3
N/A A carbon fiber(CF) reinforced Cu-10%Sn alloy matrix composite was successfully fabricated by squeeze casting method employing preheated graphite mold and proper process controlling factors. The matrix solidification microstructure of the Cu-10% Sn /CF composite reveals α-dendrite and α+δ eutectoid. To compare the squeeze cast Cu-10%Sn /CF compostie with PM route fabricated Cu-graphite composites for electric contact material, mechanical wear and electrical arc wear tests were performed. Mechanical wear rate of the Cu-10%Sn /CF is much lower than that of the Cu-graphite composite. Weight loss with a variation of contact number in electrical arc wear tests shows a similar trend between the squeeze cast Cu-10%Sn /CF and PM Cu-graphite composites.
논문 : Al 첨가 TWIP강에서의 지연파괴에 대한 변형유기 마르텐사이트 변태의 영향
김영우 ( Young Woo Kim ),강남현 ( Nam Hyun Kang ),박영도 ( Young Do Park ),최일동 ( Il Dong Choi ),김교성 ( Gyo Sung Kim ),김성규 ( Sung Kyu Kim ),조경목 ( Kyung Mox Cho ) 대한금속재료학회 ( 구 대한금속학회 ) 2008 대한금속·재료학회지 Vol.46 No.12
For the advanced high strength steels (AHSS), high-manganese TWIP (twinning induced plasticity) steels exhibit high tensile strength (800-1000 MPa) and high elongation (50-60%). However, the TWIP steels need to be understood of delayed fracture following the cup drawing test. Among the factors to cause delayed fracture, i.e, martensite transformation, hydrogen embrittlement and residual stress, the effects of martensite transformation (γ→ε or γ→α`) were investigated on the delayed fracture phenomenon. Microstructural phase analysis was conducted for cold rolled (20, 60, 80% reduction ratio) steels and tensile deformed (20, 40, 60% strain) steels. For the Al-added TWIP steels, no martensite phase was found in the cold rolled and tensile deformed specimen. But, the TWIP steels with no Al addition indicated the martensite transformation. The cup drawing specimens showed the martensite transformation irrespective of the Aladdition to the TWIP steel. However, the TWIP steel with no Al exhibited the larger amount of martensite than the case of the TWIP steel with Al addition. For the reason, it was possible to conclude that the Al addition suppressed the martensite transformation in TWIP steels, therefore preventing the delayed fracture effectively. However, it was interesting to note that the mechanism of delayed fracture should be incorporated with hydrogen embrittlement and/or residual stress as well as the martensite transformation.
Q&P와 AM강의 잔류오스테나이트 분율과 안정도에 따른 인장특성 거동
변상호,오창석,남대근,김영석,강남현,조경목,Byun, Sang-Ho,Oh, Chang-Suk,Nam, Dae-Geun,Kim, Young-Seok,Kang, Nam-Hyun,Cho, Kyung-Mox 한국재료학회 2009 한국재료학회지 Vol.19 No.6
The effects of Quenching and Partitioning (Q&P) and Annealed Martensite (AM) heat treatment on the microstructure and tensile properties were investigated for 0.24C-0.5Si-1.5Mn-1Al steels. The Q&P steels were annealed at a single phase ($\gamma$) or a dual phase (${\gamma}+{\alpha}$), followed by quenching to a temperature between $M_s$ and $M_f$. Then, enriching carbon was conducted to stabilize the austenite through the partitioning, followed by water quenching. The AM steels were intercritically annealed at a dual phase (${\gamma}+{\alpha}$) temperature and austempered at $M_s$ and $M_s{\pm}50^{\circ}C$, followed by cooling in oil quenching. The dual phase Q&P steels showed lower tensile strength and yieldyield strength than those of the single phase Q&P steels, and tThe elongation for the dual phase Q&P steel was partitioning 100s higher than that of that for the single phase Q&P steels as the partitioning time was less than 100s up to partitioning 100s. For AM steels, the tensile/yield strength decreased and the total elongation increased as the austempering temperature increased. The stability of the retained austenite controlled the elongation for Q&P steels and the volume fraction of the retained austenite controlled the elongation for AM steels.