극저온 환경에서의 고 Mn강 용접부의 파괴안전성 향상에 관한 연구

Abstract

Recent trends in shipbuilding and offshore industries are huge increase in structure size, and the environment for exploration, production of oil and natural gas is becoming more severe. Therefore, high performance steel is required by these industrial trends. Recently, the operation of these ships faces environmental pollution problems. The issue of environmental change such as global warming caused by greenhouse gas emission has been increasingly regulated. IMO(International Maritime Organization) has begun to regulate ship fuels for environmental protection. Therefore it is difficult to use continuously bunker-C oil in working ship. As these environmental regulations have been strengthened, the use of LNG(Liquid Natural Gas) as a clean fuel has increased. As a result, demands for LNG fuel ships and LNG carriers are rapidly increasing. Cryogenic steels used in LNG storage tanks should have excellent strength and low temperature fracture toughness in cryogenic environments. LNG tanks are made of conventional cryogenic steels such as aluminum-alloy, SUS 304 and 9%-Ni steel. These steels have disadvantages that they are expensive and difficult to weld. Therefore, the demand for the development of new usable materials in cryogenic environments has been increased. Domestic steel companies have developed high Mn steel considering economic feasibility and strength. The high Mn steels had developed with some advantages which were decreasing construction cost and easy to weld. In this study, The evaluation for applying the newly developed high Mn steel to real structures including LNG storage tank was carried out as follows. Firstly, the micro structure and chemical composition, and the mechanical properties were evaluated by tensile test and charpy impact test on the base material and FCAW, SAW joints. Secondly, The fracture toughness was evaluated with CTOD(Crack Tip Opening Displacement) test and the possibility of brittle fracture was confirmed. Through this result, it was confirmed that brittle fracture did not occur in high Mn steel and ductile fracture occurred in spite of cryogenic condition. Getting CTOD value is relatively difficult experiment because it requires a lot of time and cost. Thus, the CTOD value is estimated by conventional research results more easily. In addition, a new estimated equation for the fracture toughness of high Mn steel was proposed to get more exact value of fracture toughness. Finally, loading-unloading compliance method was conducted to different scale of 30mm thick specimen of high Mn steel welded joint to evaluate the unstable ductile fracture. The unstable ductile fracture was not occurred at least in this results. As a result of evaluating fracture safety of high Mn steel as cryogenic materials, safety is secured from the fracture mechanics point of view.