• Corrosion Science and Technology
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• v.16 no.1
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• pp.15-22
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• 2017

An oxidation-reduction scheme is an alternative approach for improving the galvanizability of advanced high-strength steel in the continuous hot-dip galvanizing process. Here, we investigated the effect of oxygen partial pressure ($P_{O_2}$) on the oxidation behavior of a transformation-induced plasticity steel containing 1.5 wt% Si and 1.6 wt% Mn during heating to and holding for 60 s at $700^{\circ}C$ under atmospheres with various $P_{O_2}$ values. Irrespective of $P_{O_2}$, a thin amorphous Si-rich layer of Si-Mn-O was formed underneath the Fe oxide scale (a $Fe_2O_3/Fe_3O_4$ bilayer) in the heating stage. In contrast to Si, Mn tended to segregate at the scale surface as $(Fe,Mn)_2O_3$. The multilayered structure of $(Fe,Mn)_2O_3/Fe_2O_3/Fe_3O_4$/amorphous Si-Mn-O remained even after extended oxidizing at $700^{\circ}C$ for 60 s. $Fe_2O_3$ was the dominantly growing oxide phase in the scale. The enhanced growth rate of $Fe_2O_3$ with increasing $P_{O_2}$ resulted in the formation of more Kirkendall voids in the amorphous Si-rich layer and a less Mn segregation at the scale surface. The mechanisms underlying the absence of FeO and the formation of Kirkendall voids are discussed.

• Applied Microscopy
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• v.47 no.1
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• pp.43-49
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• 2017

A new method was introduced to distinguish the ferrite, bainite and martensite in transformation induced plasticity (TRIP) steel by using electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). EBSD is a very powerful microstructure analysis technique at the length scales ranging from tens of nanometers to millimeters. However, iron BCC phases such as ferrite, bainite and martensite cannot be easily distinguished by EBSD due to their similar surface morphology and crystallographic structure. Among the various EBSD-based methodology, image quality (IQ) values, which present the perfection of a crystal lattice, was used to distinguish the iron BCC phases. IQ values are very useful tools to discern the iron BCC phases because of their different density of crystal defect and lattice distortion. However, there are still remaining problems that make the separation of bainite and martensite difficult. For instance, these phases have very similar IQ values in many cases, especially in deformed region; therefore, even though the IQ value was used, it has been difficult to distinguish the bainite and martensite. For more precise separation of bainite and martensite, IQ threshold values were determined by a correlative TEM analysis. By determining the threshold values, iron BCC phases were successfully separated.

• Korean Journal of Metals and Materials
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• v.46 no.10
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• pp.627-633
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• 2008

The austenitic Fe-Mn alloys have received considerable attention as a possible candidate for the automotive structural materials due to their high strength and high formability with high elongation. This research investigates the effect of alloying elements on the phase transformation, deformation behavior and mechanical properties in high Mn steels for the development of a high strength high ductility steel. The mechanical stability of austenitic phases is very important for high ductility and it depends largely on the composition of carbon, manganese and aluminum. The dominant deformation mode shifts from TRIP to TWIP mode as the amount of C, Mn and Al is increased. Especially, even a small amount of Al addition facilitates significantly TWIP deformation due to the increase of stacking fault energy in Fe-Mn alloys, this leads to increase the ductility and also decrease the crack sensitivity.

• Transactions of Materials Processing
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• v.10 no.6
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• pp.465-470
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• 2001

Hole flanging experiments are performed on flat circular plates with a hole in the center and the flangeability and fracture behaviors of TRIP steels and ferrite-Bainite duplex steels were examined. In the hole flanging, deformation by lip and petalling occurs when plates are struck by punches of various shapes and high circumferential strains induced in the target material cause radial cracking and the subsequent rotation of the affected plate material in a number of symmetric petals. In all cases, failure of the plate was due to lip fracture that results from multiple localized neckings that take place around the hole periphery where straining is most severe and a somewhat regular pattern was observed in a fracture shape. The neck characteristics in flange formation and the transition from the lip to petal mode at which fracture occurs were compared with two materials.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.349-356
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• 2004

자동차의 외관품질과 관련된 외판재용 고강도강으로는 내 dent성을 향상을 위해서 IF-HSS강과 BH강의 사용량이 크게 증가하였으며, 최근에는 490MPa급 DP강이 적용되기 시작하고 있다. 그리고 내판 판넬의 경우에는 고가공성 을 갖는 고강도강의 개발으로 고강도강의 사용량이 늘어나고 있다. 내판재 중 승객의 안전과 관련된 멤버, 필라와 같은 구조부재는 중간 정도의 강도를 갖는 고강도강을 주로 적용되어 왔으나, 최근 차체경량화 요구의 증가로 590MPa급 이상 고강도강이 적용되기 시작하였으며, 특히 고속변형에서 에너지 흡수능이 우수한 TRIP (Trans-formation Induced Plasticity)강 및 DP(Dual Phase)강에 대한 관심이 크게 증대되고 있다. 저속충돌에서 차체를 보호하는 범퍼보강재는 고강도화가 빠르게 진행되어, 현재는 석출경화강에 변태 조직 강화를 더한 780MPa급 이상의 초고강도강을 주로 사용하고 있으며, 1370MPa급 까지 적용하고 있다.

• Proceedings of the Korean Society for Technology of Plasticity Conference
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• 2004.05a
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• pp.341-348
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• 2004

Recently deveoped HSS of POSCO and commerciallizing activities were introduced. 590DP, 590FB, 780TRIP. In this year, another three type of hot rolled AHSS will be developed and try to commerciallize with auto makers. 780FB, 780DP and 980CP. We have future plan to develope ultra high strength steels 980TR, 1170CP.

• Journal of the korean Society of Automotive Engineers
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• v.18 no.5
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• pp.24-37
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• 1996

자동차용 열연강판은 그 용도에 따라 매우 다양한 제품특성이 요구되고 있다. 즉 고강도 및 고강성, 우수한 피로특성, 용접성 및 내식성 등이 요구되고 있다. 이중에서도 고강도화는 향후 자동차의 경량화와 더불어 안전성을 동시에 확보하기 위해 매우 필요한 기술이다. 고강도화에 가장 많은 노력이 이루어졌던 wheel용 강재의 경우, 기존에는 60kg급 강재가 주종을 이루었으나, 일본등에서는 80kg급 강재의 시험생산 및 실차 test가 진행되는 등 소재공급사와 자동차제조사와의 공동노력에 의해 많은 기술개발이 이루어져 왔다. 더우기 96년 6월 일본철강연맹 규격에는 자동차용 열간압연 강판으로 80kg급 강재가 등록이 됨으로서 향후 고강도강의 소요가 크게 증가될 전망이다. 또한 근래 상용화가 활발하게 진행되고 있는 열연 TRIP강의 경우 기존의 개념을 넘어서는 고강도 및 고연성, 우수한 가공선과 더불어 우수한 피로특성을 보이고 있어 향후 동 소재의 사용확대가 크게 기대되고 있다. 그러나 이러한 좋은 철강소재의 개발과 이의 적용은 강재제조기술과 더불어 이의 적정 가공기술 그리고 독자적인 설계기술등이 연계되어야만 비로소 이루어질 수 있다. 이를 위해 소재공급사와 자동차제조사간에 지속적인 기술개발을 위한 공동노력이 이루어져야 되리라 생각된다.

• KIEE International Transactions on Power Engineering
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• v.2A no.4
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• pp.153-159
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• 2002

Power transformer protective relay should block the tripping during magnetizing inrush and rapidly operate the tripping during internal faults. Recently, the frequency environment of power system has been made more complicated and the quantity of 2nd frequency component in inrush state has been decreased because of the improvement of core steel. And then, traditional approaches will likely be maloperate in the case of magnetizing inrush with low second harmonic component and internal faults with high second harmonic component. This paper proposes a new relaying algorithm to enhance the fault detection sensitivities of conventional techniques by using a fuzzy logic approach. The proposed fuzzy based relaying algorithm consists of flux-differential current derivative curve, harmonic restraint, and percentage differential characteristic curve. The proposed relaying was tested with relaying signals obtained from EMTP simulation package and showed a fast and accurate trip operation.

• Proceedings of the Computational Structural Engineering Institute Conference
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• 2010.04a
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• pp.504-507
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• 2010

본 논문에서는 저온용 오스테나이트계 스테인리스강(ASS)의 온도 및 변형률 속도의 영향을 고려한 통합 구성 방정식 및 손상 모델을 제안하였다. 저온 영역에서, 304L ASS의 온도 및 변형률 속도별 인장 실험을 시행하였다. 그 결과, 변형 유기 마르텐사이트 상변태에 의해 상변태 유기 소성(TRIP)이 저온에서 현저히 나타났으며 온도 및 변형률 속도의 영향이 지대하였다. 실험 결과를 바탕으로 ASS의 저온 거동 및 특성을 규명하여 수치 모델에 반영하였다. 저온에서 일어나는 2차 경화 현상을 표현하기 위해, Bodner/Partom 점소성 구성 방정식을 수정하고 Tomita/Iwamoto 변형 유기 상변태 모델을 구성 방정식에 적용시켰다. 저온 연성 파단 현상을 표현하기 위해, Bodner/Chan 손상모델을 수정하여 접목시켰다. 제안된 모델을 유한요소 프로그램에 탑재시키고, 온도 및 변형률 속도 의존 재료 정수를 결정하였다. 저온 영역에서, 온도 및 변형률 속도별 재료 거동을 시뮬레이션하고 이를 실험 결과와 비교 및 검증하였다.

• Proceedings of the KIPE Conference
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• 2001.07a
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• pp.288-291
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• 2001

In steel plant, load torque of mill driver motor is changed periodically because working state and idle state are repeated and load current of working state is necessarily higher than rated motor current. The over current limiter is one of the basic thermal protection method from over heating. In this paper, we analyzed the structure of over current limiter for motor and motor driver systems, developed over current limiter with same operation and structured warning system of action of over current limiter. As using this warning system, we can avoid abrupt plant stop by over current limiter in mill driver and lessen producing loss by plant stop. The developed warning system of action of over current limiter is exactly inspected by computer simulation md analysis of acquired data.