Gyubaek An

Development Trends of Steel Plates for Ship Building and Off-Shore Construction and It`s Weldability

최근 세계 인구의 증가와 생활경제의 성장으로 생활 수준이 향상되고 있으며, 세계적으로 대규모의 해상활동 이 급격하게 증가 하고 있는 추세이다. 선박수요 또한 눈에 띠게 증가 하고 있으며, 2008년 말 시점에서 세계 선박수주량은 42백만CGT를 넘었으며, 향후 100백만 CGT를 넘을 것으로 기대하고 있다. 선박은 대량의 강재가 용접에 의하여 조립된 대형 용 접구조물이다. 따라서 조선소에서는 종래의 용접효율을 향상시키기 위하여 각종의 조립방법을 제안하여 적용하 고 있으며, 강재측면에서도 이러한 요구에 부합하여 신 강종의 개발 및 새로운 용접재료의 개발이 이루어지고 있다. 또한 최근 선박의 대형화에 따라서 고강도 극후물 재가 요구되고 있으며, 최근에는 항복강도가 460 MPa 급인 TMCP 고강도강이 개발되어 선박건조에 적용되고 있다. 한편, 이러한 고강도 극후물강재의 적용에 따라서, 해 상운송에 대한 안전.환경의식을 높이기 위하여 IMO (International Maritime Organization: 국제해사기관)를 중심으로 선박관련 규제가 강화 되고 있다. 이러한 환경 변화와 함께 선박에 적용되는 강재에 대한 요구조건 또 한 더욱 엄격하게 적용 하고 있는 것이 현실이다. 뿐만 아니라, 세계선급협회에서는 새로운 고강도강의 출현에 따른 안전성의 평가와 보다 안전한 선박의 건조를 위한 새로운 선급규정의 제정을 검토 중에 있으며, 특히 항복 강도가 460MPa급인 TMCP 고강도강의 실선 적용에 따 른 고강도의 파괴안전성, 피로강도 문제 등에 많은 관심 을 가지고 NK, GL, ABS 등의 선급협회를 중심으로 신 강재에 대한 새로운 선급규정이 만들어 지고 있다. 한편, 해양구조물의 경우는 최근 두 가지 측면의 환경 변화에 대응해서 발전 해 나가고 있다. 첫째 근해의 석 유 및 천연가스 자원의 고갈에 따른 심해저의 자원 개 발이 증가하고 있다. 따라서 해양구조물의 형태가 고정 식에서 부유식으로 변화하고, 이에 따라 해양구조물의 경량화 및 대형화 요구와 함께, 요구되는 강재 또한 고 강도화 되어서 YS 345Mpa 급 강재뿐만 아니라 YS 420 및 460Mpa급 강재까지 사용되고 있다. 둘째 극저온 지역에서의 자원개발이 증가하고 있다. 따라서 사용되는 강재는 높은 저온 인성이 요구되며, 특히 모재의 경우는 -60°C에서, 용접열영향부의 경우는 -40°C에서의 CTOD (Crack Tip Opening Displacement) 특성이 요구되고 있 다. 최근에는 일부 극지방에 사용되는 강재의 경우 용접 열영향부에 대해서 까지도 -60°C의 CTOD 특성을 요구 하고 있다. 본 논문에서는 이처럼 조선 및 해양 구조물에 사용되 는 강재가 사용 환경의 변화에 따라서 고인성 고강도강 재가 요구되므로, 조선 및 해양구조용 강재의 지금까지 개발현황 및 최근 이슈가 되는 용접부에서 요구되는 특 성에 대하여 기술하고, 다양한 강재의 개발과 함께 개발 된 용접재료의 개발현황 및 요구특성에 대하여 소개 하 고자 한다.

Fracture Toughness of Thick Steel Plate for Ship Building

조선업을 포함한 다양한 산업 분야에서 후판 강재의 수요량의 증가와 사용범위의 폭이 넓게 되고 있다. 후판 의 사용에 따라서 강재의 파괴인성에 문제가 여러 측면 에서 제기 되고 있는 실정이다. 특히, 최근 선박 산업에 서는 컨테이너선으 대형화에 따라서 H/C을 비롯한 여 러 부분에서 극후판이 사용되고 있으며, 조선산업에서 용접이 차지하는 비중은 약 40%정도에 달하고 있다. 주 로 파괴인성은 이러한 용접에 의해서 더욱 취약하게 나 타나고 있으며, 선박의 모든 연결부는 용접프로세스를 사용하고 있으며, 선체구조물의 용접부 및 모재의 안전 성평가는, 선체구조물의 안전성과 직결된다. 선체에 용 접프로세스를 적용한 이래 제2차 세계대전 중 미국에서 건조된 전용접 전시표준함의 용접부 취성파괴에 의한 손상으로 많은 연구가 수행되어 모재 및 용접재료의 개 발과 연구가 진행되었고, 이를 통해 여러 가지 기준을 제시할 수 있는 단계가 되었다. 최근, 이러한 기준은 고강도 강재의 개발, 선박건조의 생산성 향상을 위한 용접프로세스 개발(대입열용접), 컨 테이너선의 초대형화에 따른 극후판 강재의 사용에 따 라서, 일부 선급협회(현재는 NK:일본선급협회)에서는 극후판 대입열용 TMCP강에 대하여 취성균열 전파 정 지 특성에 대하여 문제를 제기 하고 있으며, 용접부 건 전성평가방법의 재검토가 요구되고 있는 실정이다. 따라 서 여기서는 선체구조물의 용접부 취성파괴 발생의 원 인과, 파괴인성평가법 및 특성에 대해 기술 하고자 한 다.

Ductile Crack Initiation Criterion with Mismatched Weld Joints Under Dynamic Loading Conditions

Brittle failure of high toughness steel structures tends to occur after ductile crack initiation/propagation. Damages to steel structures were reported in the Hanshin Great Earthquake. Several brittle failures were observed in beam-to-column connection zones with geometrical discontinuity. It is widely known that triaxial stresses accelerate the ductile fracture of steels. The study examined the effects of geometrical heterogeneity and strength mismatches (both of which elevate plastic constraints due to heterogeneous plastic straining) and loading rate on critical conditions initiating ductile fracture. This involved applying the two-parameter criterion (involving equivalent plastic strain and stress triaxiality) to estimate ductile cracking for strength mismatched specimens under static and dynamic tensile loading conditions. Ductile crack initiation testing was conducted under static and dynamic loading conditions using circumferentially notched specimens (Charpy type) with/without strength mismatches. The results indicated that the condition for ductile crack initiation using the two parameter criterion was a transferable criterion to evaluate ductile crack initiation independent of the existence of strength mismatches and loading rates.

The Effect of Welding-Pass Grouping on the Prediction Accuracy of Residual Stress in Multipass Butt Welding

The residual stress analysis of a thick welded structure requires a lot of time and computer memory, which are different from those in thin welded structure analysis. This study investigated the effect of residual stress due to welding-pass grouping as a way to reduce the analysis time in multipass thick butt welding joint. For this purpose, the parametric analysis which changes the number of grouping passes was conducted in the multipass butt weld of a structure with a thickness of 25 mm and 70 mm. In addition, the residual stress by thermal elastoplastic FE analysis is compared with the results by the neutron diffraction method for verifying the reliability of the FE analysis. The welding sequence is considered in order to predict the residual stress more accurately when using welding-pass grouping method. The results of the welding-pass grouping model and half model occurred between the results of the left/right of the full model. If the total number of welding-pass grouping is less than half of that of welding pass, a large difference with real residual stress is found. Therefore, the total number of the welding-pass grouping should not be reduced to more than half.

Prediction and Welding sequence of Minimum Welding Deformationin Large Steel Block Welding

In order to minimize the effect of weld deformation on the preceding equipment during Pre-erection joint welding, the optimal welding sequence by unit load method was derived. And the weld deformation was predicted by the equivalent load method considering the constraint according to the derived welding sequence. For this purpose, the restraint coefficient was derived by conducting experiments and elastic FEM analysis on various welded specimens in order to derive the restraint of butt welds. In addition, the welding sequence that can minimize the welding deformation was derived by calculating the constraint by the unit load method in steel block structure. According to the derived welding procedure, the welding deformation near the preceding equipment during the pre-erection joint welding was predicted within 4 mm by using the equivalent load method considering the constraint. Specifically, the deformation in the lower part with the block supports occurred about 2 mm, and the deformation of 3~4 mm occurred at the upper part of block.

A Basic Study on Brittle Crack Propagation Path with Ultra Large Steel Plate Weld Joints

As the recession in shipbuilding industry became longer, domestic shipbuilders focused on the need for high value added and high technology, and strived to build safer vessels. There are LNG carriers and container ships that require representative technologies. Both ship types are rapidly increasing ship size in the 2000s. Recently, there have been the increase of ship size and the development of oil and gas in arctic region. These trends have led to the requirements such as high strength, good toughness at low temperature and good weldability for prevent of brittle fracture at service temperature. Studies on unstable fracture have mainly been conducted to evaluate the brittle crack stopping properties of the high strength steel welded joints through large scale fracture tests, and studies on the difficulty of stopping brittle cracks in welded joints. It is known that BCA (Brittle Crack Arrest) steel has recently been developed to prevent unstable fracture if cracks propagate toward the BCA steel even if brittle cracks occur. Therefore, it is considered that the ship can be dried more easily if a technology capable of inducing the brittle crack to the BCA steel is developed. In this study, we describe a technique for securing unsafe fracture safety by inducing cracks toward the BCA steel, which has excellent brittle crack stopping ability, regardless of the welding process when brittle cracks occur in the superstructure of super large container ships.

Characteristic and Measurement Technology of Inner Welding Residual Stresses in Thick Steel Structures

Abstract Recent keywords of the heavy industries are large-scale structure and productivity. Especially, the sizes of the commercial vessels and the offshore structures have been gradually increased to deliver goods and explore or produce oil and natural gas in the Arctic. High heat input welding processes such as electro gas welding (EGW) have been widely used for welding thick steel plates with flux-cored arc welding (FCAW),especially in the shipbuilding industries. Because high heat input welding may cause the detrimental effects on the fracture toughness of the welded joint and the heat affected zone, it is essential to obtain the sufficienttoughness of welded joint. There are well known that the fracture toughness like CTOD, CVN, and KIC were very important factors in order to secure the safety of the structures. Furthermore, the welding residual stress should be considered to estimate the unstable fracture in both EGW and FCAW. However, there are no references on the welding residual stress distribution of EGW and FCAW with thick steel plates. Therefore the welding residual stresses were very important elements to evaluate the safety of the welded structure. Based on the measurement results, the characteristics of residual stress distribution through thicknesswere compared between one-pass electron gas welding and multi-pass flux-cored arc welding. The longitudinal residual stress in the multi-pass flux-cored arc welding is tensile through all thicknesses in the welding fusion zone. Meanwhile, longitudinal residual stress of EGW is tensile on both surfaces and compressive at the insideof the plate. The magnitude of residual stresses by electron gas welding is lower than that by flux-cored arc welding.Key Words : Welding residual stress, Thick steel plate, Heat input, Fracture toughnessISSN 2466-2232Online ISSN 2466-2100

Design for avoid unstable fracture in shipbuilding and offshore plant structure

Recently, there have been the increase of ship size and the development of oil and gas in arctic region. These trends have led to the requirements such as high strength, good toughness at low temperature and good weldability for prevent of brittle fracture at service temperature. There has been the key issue of crack arrestability in large size structure such as container ship. In this report for the first time, crack arrest toughness of thick steel plate welds was evaluated by large scale ESSO test for estimate of brittle crack arrestability in thick steel plate. For large structures using thick steel plates, fracture toughness of welded joint is an important factor to obtain structural integrity. In general, there are two kinds of design concepts based on fracture toughness: crack initiation and crack arrest. So far, when steel structures such as buildings, bridges and ships were manufactured using thick steel plates (max. 80~100mm in thickness), they had to be designed in order to avoid crack initiation, especially in welded joint. However, crack arrest design has been considered as a second line of defense and applied to limited industries like pipelines and nuclear power plants. Although welded joint is the weakest part to brittle fracture, there are few results to investigate crack arrest toughness of welded joint. In this study, brittle crack arrest designs were developed for hatch side coaming of large container ships using arrest weld, hole, and insert technology.