• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2009년 추계학술발표대회
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• pp.197-201
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• 2009

A lot of work is carried out concerning to acicular ferrite formation in the weld metal of high strength and low-alloy steel. Those results are suggesting that oxides that contain titanium elements provides nucleation site of intragranular ferrite, referred as acicular ferrite. Thus, when intragranular ferrite is expected to form in heat-affected zone, oxide containing titanium element should be formed in the steel. However, normal steel is deoxidized by using aluminum element (Al-killed steel) with little oxygen content. It means almost oxygen is deoxidized with aluminum elements. In the present work, in order to form the acicular ferrite in the heat affected zone, with the same concept in the case of weld metal, the steel deoxidized with titanium element (titanium killed-steel) is prepared and the acicular ferrite formation is observed in detail by using laser-conforcal microscopy technique. The confocal technique makes it possible that the morphological change along the phase transformation from austenite to ferrite is in-situ tracked. Thus, the inclusion that stimulated the ferrite nucleation could be directly selected from the observed images, in the HAZ of the Ti-killed steel. The chemical composition of the selected inclusion is analyzed and the nucleation potential is discussed by changing the nucleation site with boron element. The potency for the ferrite nucleation is summarized and the existence of effective and ineffective manganese sulfide for nucleation is made clear.

• Smart Structures and Systems
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• 제24권1호
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• pp.111-125
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• 2019

Metallic shape memory alloys present fascinating physical properties such as their super-elastic behavior in austenite phase, which can be exploited for providing a structure with both a self-centering capability and an increased ductility. More or less accurate numerical models have been introduced to model their behavior along the last 25 years. This is the reason for which the literature is rich of suggestions/proposals on how to implement this material in devices for passive and semi-active control. Nevertheless, the thermo-mechanical coupling characterizing the first-order martensite phase transformation process results in several macroscopic features affecting the alloy performance. In particular, the effects of day-night and winter-summer temperature excursions require special attention. This aspect might imply that the deployment of some devices should be restricted to indoor solutions. A further aspect is the dependence of the behavior from the geometry one adopts. Two fundamental lacks of symmetry should also be carefully considered when implementing a SMA-based application: the behavior in tension is different from that in compression, and the heating is easy and fast whereas the cooling is not. This manuscript focuses on the passive devices recently proposed in the literature for civil engineering applications. Based on the challenges above identified, their actual feasibility is investigated in detail and their long term performance is discussed with reference to their fatigue life. A few available semi-active solutions are also considered.

• Corrosion Science and Technology
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• 제23권1호
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• pp.11-19
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• 2024

A Fe-Ni Bainitic steel as a weathering steel application was developed by combining its excellent mechanical properties and corrosion resistance in maritime environments. Nickel concentration (0.4-3 wt%) and inverted austempering multi-step (IAM) process were primary determinants of the microstructure of the Fe-Ni Bainitic steel. The initial austempering steel was performed at 300 ℃ for 600 seconds to obtain a partly bainitic transformation. The steel was heated again for 1800 s at 450 ℃. The microstructure was comprised of ferrite, a blocky martensite/austenite island, and a homogeneous lath-shape bainite structure with widths ranging from 4.67 to 6.89 ㎛. The maximum strength, 1480 MPa, was obtained with 3 wt% nickel. In this study, corrosion behavior was investigated utilizing potentiodynamic and electrochemical impedance spectroscopy (EIS) tests. A higher nickel content in Fe-Ni Bainitic steel refined the grain size, improved the bainite fraction, lowered the corrosion rate to 0.0257 mmpy, and increased the charge transfer of film resistance to 1369 Ω.

• 한국재료학회지
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• 제10권6호
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• pp.409-414
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• 2000

3%C-10%Cr-5%Mo-5%W, 3%C-10%V-5%Mo-5%W 및 3%C-17%Cr-3%V의 3종류 고합금백주철에 있어서 열처리후의 기지조직의 변태거동을 연구하였다. 15kg 용량의 고주파 유도용해로에 선철, 고철, Fe-Cr, Fe-V, Fe-Mo 및 Fe-W 등을 장입시켜 용해시킨 후 슬래그를 제거시키고 $1550^{\circ}C$에서 Y-block의 펩 주형에 주입시켰다. 적당한 크기로 절단한 시편을 진공분위기하에$950^{\circ}C$에서 5시간동안 우선 균질화처리를 실시하였다. 그 후 다시 이 시편을 $1050^{\circ}C$에서 2시간동안 오스테나이징시킨 후 강제공냉을 행하였다. 강제공냉된 시편을 $300^{\circ}C$에서 3시간 유지시킨 후 템퍼링을 실시하였다. 주방상태에서의 기지조직은 3합금 공히 퍼얼라이트이었으며 강제공냉후에는 마르텐사이트 및 오스테나이트로 변태하였다. 또한 기지조직내에 무수히 많은 이차탄화물 입자들의 석출되었다. 템프링 후 일부 오스테나이트 및 마르텐사이트는 템퍼드마르텐사이트로 변태하였다. 변태하였다.

• 한국재료학회지
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• 제12권10호
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• pp.776-783
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• 2002

Austenitic stainless steel (STS304) has been carburized using glow discharge plasma and its microstructure, wear resistance and corrosion property have been investigated. A repeat boost-diffuse carburizing was used as an effective plasma carburizing method. The effective case depth of the plasma carburized specimens was increased with the carbon concentration at the surface area. The specimens prepared by 3 hours plasma carburizing under $600^{\circ}C$ did not have the standard hardness for the effective case depth, but the specimen prepared by 11 hours plasma carburizing at $500^{\circ}C$ had nearly the same hardness with the specimen plasma carburized for 3 hours at $800^{\circ}C$. The wear resistance increased with temperature but the corrosion properties of the specimens prepared over $600^{\circ}C$ decreased rapidly due to the grain boundary sensitization. However, the specimen plasma carburized for 11 hours at $500^{\circ}C$ had nearly the same wear resistance with the specimen plasma carburized for 3 hours at $800^{\circ}C$ without deterioration of corrosion property. This could be resulted from the fact that the microstructure of the specimen plasma carburized for 11 hours at $500^{\circ}C$ was composed of martensite and austenite, because a partial martensite transformation was occurred only in the specimen plasma carburized for 11 hours at 50$0^{\circ}C$.

• 한국해양공학회지
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• 제26권3호
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• pp.55-60
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• 2012

This study was carried out to investigate the effect of the deformation temperature in high manganese austenitic stainless steel. ${\alpha}$'-martensite was formed with a specific direction by deformation. The volume fraction of the deformation induced martensite was increased by increasing the degree of deformation and decreasing the deformation temperature. With the increase in the deformation, the hardness and tensile strength were increased, while the elongation was rapidly decreased at the initial stage of the deformation, and then gradually decreased. The hardness and tensile strength were increased and the elongation was decreased with adecrease in the deformation temperature. The hardness and tensile strength were strongly controlled by the volume fraction of martensite, but the elongation was controlled by the transformation behavior of the deformation induced martensite.

• 대한기계학회논문집A
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• 제36권3호
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• pp.319-324
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• 2012

이산화탄소에 대한 규제가 강화되면서 환경 보호를 위하여 차량의 경량화를 요구하고 있다. 이를 위하여 다른 재질, 두께 및 강도를 갖는 소재를 맞춤 블랭크로 제조하여 활용하는 TWB 기술과 오스테나이트 변태온도 이상으로 승온, 스탬핑 ��칭하는 핫스탬핑용 보론강을 이용한 열간 성형 기술을 접목함으로써 경량화 및 고안전성을 확보할 수 있다. 따라서 본 연구에서는 레이저 용접된 보론 강판의 열처리 온도를 달리하여 다이��칭 조건에 따른 기계적 특성을 조사하였다. 레이저 용접한 보론 강판을 다이 ��칭한 후 인장 시험한 결과, 최대인장강도 1454 MPa, 연신율은 6 %로 나타났으며, 이것은 용접하지 않은 모재 열처리재 인장강도, 1522 MPa의 95.5 %이다. 이 값은 핫스탬핑용 보론강 용접재의 실용적인 데이터로 활용될 수 있을 것이다.

• 한국가스학회지
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• 제24권5호
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• pp.39-46
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• 2020

본 논문에서는 파이프라인에서 사용되는 API 5L 철강의 중심선 편석에 대해 다루고자 한다. Mn/S와 Nb 그리고 C는 파이프 두께 방향으로의 중심선에서 편석되는 요소로 알려져 있다. Mn은 일반적으로 긴 점성상을 이루는 S를 동반한다. 중심선의 미세조직은 MnS와 산소를 포함한 Nb/Ti로 구성되어 있는데 이 중심선 지역의 편석효과를 OM, SEM/EDS 및 마이크로비커스 경도기로 분석하였다. Mn, Nb 및 C는 오스테나이트가 페라이트 또는 마르텐사이트로 변태하는 것을 억제시키는 원소이다. 이러한 원소들은 마르텐사이트의 한 종류인 베이나이트 미세조직을 만들 수 있는데 이는 용융 및 열처리에서 얻어지는 중심선과 매트릭스 간의 원소 편석과는 다르다.

• 한국주조공학회지
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• 제32권2호
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• pp.104-108
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• 2012

The effect of heat treatment on mechanical properties of 0.4C-2.3Si(wt%) steel with bainitic ferrite matrix were investigated. This steel has been synthesized intergrating concepts from TRIP(Transformation Induced Plasticity) steel & Austempered Ductile Cast Iron(ADI) technology. The low alloy medium carbon (0.4 %C) steel with high silicon (2.3 %Si) was initially annealed for 60 min at $800^{\circ}C$, $820^{\circ}C$ and $840^{\circ}C$ respectively in the intercritical region and then subsequently austempered at various temperatures at $260^{\circ}C$, $320^{\circ}C$ and $380^{\circ}C$ for 30 min in a salt bath. The mechanical properties were measured by using a tensile test. A detailed study of the microstructure of this steel after heat treatment was carried out by means of electron back scattering diffraction (EBSD) technic. In this study, a new low alloy steel with high strength (780~1,050MPa) and exceptionally high ductility (20~40%) was obtained.

• 대한기계학회논문집A
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• 제35권4호
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• pp.383-391
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• 2011

최근 차체 경량화를 위한 여러 가지 방법 가운데 고온에서 가공하여 성형성을 확보하고, 이후 열처리를 통하여 고강도를 가질 수 있는 프레스 열간 성형 및 핫스탬핑 기술이 각광을 받고 있으며, 이에 따라 핫스탬핑용 재료인 보론 첨가 강판의 수요도 늘어나고 있는 추세이다. 본 연구에서는 핫스탬핑용보론 첨가 강판의 우수한 경화능을 보이는 임계 최적 열처리 조건을 파악하고자, Al+Si 코팅이 된 보론강판을 각기 다른 온도로 열처리하여 수냉 조건에 따른 기계적 특성 및 조직을 관찰하였다. 또한, 이 결과를 토대로 선별된 기계적 특성이 우수한 보론강 시험편과 일반 냉연 강판인 SPRC 340과 SPRC 590 2종류의 상대재를 활용하여 특정 점용접 조건에서 용접을 실시하고, 보론강 및 상대재의 기계적 특성 변화에 따른 용접 후의 기계적 특성 및 조직, 그리고 코팅층 유무에 따른 특성 변화를 연구하였다.