A 490 yield-stress steel for liquified-gas vessels

摘要

For the construction of a 60,000-m~3 vessel for liquified earth gas we used a newly developed 490-MPa yield-stress structural steel. The steel has a microstructure of distorted ferrite and pearlite with an intercept grain size of approximately 2 μm. The steel is low-carbon and low-alloyed, has a low-carbon equivalent and is welded without preheating. Approximately 50% of the yield stress is due to the strengthening effect of the grain size. The upper-shelf notch toughness is approximately 250 J, and the transition temperature for ductile to brittle fracture is below-80°C. The steel has a superior resistance to hydrogen embrittlement. After applying the NACE test with a tensile stress of 80% of the yield stress in a solution rich in H_2S the reduction of area was above 60%. In contrast it was only approximately 20% with a 350-MPa yield-stress steel that had a microstructure of polygonal ferrite and pearlite and a grain size of 20-30 μm. One vessel has been in operation since 1999 in an ecologically sensitive area, another is being built, and several others are planned.