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Synthesis of Resonator Filters With Arbitrary Topology Using Hybrid Method
Manseok Uhm,Sangho Nam,Jeongphill Kim IEEE 2007 IEEE transactions on microwave theory and techniqu Vol.55 No.10
<P>This paper presents a new synthesis method for resonator filters of arbitrary topology using an evolutionary hybrid method. This method consists of a Levenberg-Marquardt algorithm for a local optimizer and genetic algorithm for a global optimizer, respectively. Unlike the conventional hybrid method in which the local optimization is performed after finding appropriate initial values from global optimization, the local optimizer in the proposed method is used as a genetic-algorithm operator to prevent trapping in local minima of the cost function. This method can provide fast convergence and good accuracy to find the final solution from initial population generated by a random number and the known value for the filters with stringent requirements. In addition, multiple coupling matrices to meet the given requirement can be obtained from the initial population based on a random number. Resonator filters with asymmetric eight-pole configurations for single and dual passbands are synthesized using the current method for validation. Excellent agreement between the response computed from characteristic polynomials and the response computed from couplings is obtained from the proposed method.</P>
박기태(Gitae Park),엄상호(Sangho Uhm),이창희(Changhee Lee) 대한용접·접합학회 2021 대한용접·접합학회지 Vol.39 No.3
Medium Mn TRIP steels have a dual-phase matrix of ferrite and austenite containing intentionally redistributed manganese and carbon contents to obtain stable austenite at room temperature. However, regardless of similar and dissimilar weldment, coarse-grained HAZ and FZ undergo martensitic transformation during heating above the Ac₃ temperature and then rapid cooling to room temperature. Then the high retained austenite content is sharply reduced, resulting in deterioration of mechanical properties with increased brittleness. Various ways to improve weldability have been proposed, and the paint-baking heat treatment process during automotive production can drastically improve the tensile properties of martensite, even when conducted at low temperature. This paper highlights the main characteristics and microstructural changes in the resistance spot welding of medium Mn TRIP steel, which is expected to be used as a next-generation automotive steel sheet.
중망간강 용접부 미세조직 해석과 상변태 강화를 통한 인성 향상
박기태(Gitae Park),엄상호(Sangho Uhm),정보영(Boyoung Jeong) 대한용접·접합학회 2021 대한용접학회 특별강연 및 학술발표대회 개요집 Vol.2021 No.11
전세계적 환경규제 강화, 자동차 연비 향상 요구로 인해 자동차의 주요 소재인 철강 소재, 특히 차체 및 외장 판넬에 주로 사용되는 차량용 강판에 새로운 변화가 요구되고 있다. 세계 주요 철강업체들은 차체의 고강도화 및 차량경량화를 위해, 변태유기소성 현상을 이용하는 다양한 강종을 개발하여 차체에 적용해왔다. 변태유기소성 현상은 잔류오스테나이트 기지상이 소성가공에 의해 마르텐사이트로 상변태되면서 강재의 연신율을 크게 향상시키는 현상이며, 이와 더불어 가공경화에 의한 강도 향상을 동시에 유도할 수 있기 때문에, 강종의 변태조직 제어는 차세대 자동차강판 연구의 핵심이라 할 수 있다. 중망간강은 다량의 망간을 활용하여 잔류오스테나이트의 안정성과 함량을 증가시켜 변태유기소성 현상을 이용하여 연성을 극대화시킨 강종이다. 강재 내에 상온에서 안정적인 페라이트 및 오스테나이트의 2개의 상을 얻기 위해서 생산공정 중 열처리를 통한 페라이트-오스테나이트간 합금원소 재분배가 필수적이나, 용접 후 용융부 및 열영향부에서 나타나는 인성 저하 문제는 강재의 활용에 있어서 제약이 되고 있다. 본 연구에서는, 최근 이러한 문제점을 해결하기 위한 여러 합금설계 및 후속 공정 변수 제어 방안들에 대한 연구를 소개하고, 중망간강의 미세조직 변화와 인성 향상의 상관관계에 대해 고찰하고 논의하고자 한다.
Gitae Park,Kwangjoong Kim,Youngchai Lee,Sangho Uhm,Changhee Lee 대한금속·재료학회 2022 METALS AND MATERIALS International Vol.28 No.5
The kinetics of low-temperature tempering in a dissimilar resistance spot weldment between medium Mn transformationinducedplasticity (MT) steel and dual-phase (DP) steel were investigated to interpret the change of failure mode after apaint-baking process. After the isothermal heat treatments with various soaking conditions, the Johnson–Mehl–Avrami (JMA)model describing nucleation and growth kinetics was adopted. The results indicated that the tempering kinetics parameterswere different for each area of the welds. Especially in the coarse-grained heat-affected zone (CGHAZ) in the MT steel,carbon diffusion from martensite to austenite acts as a tempering mechanism at an earlier stage of tempering. Thereafter,the tempering mechanism changes to epsilon carbide precipitation and coarsening. In comparison, the change was relativelyslow or absent in the DP steel CGHAZ and fusion zone because of its lower kinetics parameters. Based on the results ofthis study, ways to improve the cross-tension strength of entire welds in automotive applications of MT steel are discussed.