Due to the demands for expanding liquefied natural gas (LNG), an increase in LNG storage tank constructions is now expected. For over 50 years, 9%Ni steel with excellent mechanical properties at a cryogenic temperature under -162°C has been used as inner-tank material for LNG storage tanks. As nickel is an expensive and a rare metal, a reduction of the nickel content in steel can save construction costs of LNG storage tanks.
A new steel plate for LNG storage tanks has been developed by optimizing the chemical composition and applying the latest thermomechanical control process (TMCP) technology. TMCP is a production process wherein the rolling temperature and cooling rate after rolling are controlled, thereby enabling the refining of the microstructure. For the application of the new steel to LNG storage tanks, the brittle crack initiation properties and propagation properties of the base plate and welded joint were analyzed.
The new steel has a finer microstructure and larger amount of retained austenite than conventional 9%Ni steel. It was demonstrated through various large-scale fracture tests that the new steel has excellent performance equivalent to conventional 9%Ni steel. The new steel plate (7.0%–7.5%Ni) has already been adopted in an aboveground LNG storage tank in Senboku Terminal 1 of Osaka Gas Corporation. This tank, which has a capacity of 230,000 m3 and is the largest of its kind in the world, was constructed in November 2015. It was confirmed that weldability and formability of new steel in the construction of LNG storage tanks are also equivalent to 9%Ni steel. The LNG storage tank has been safely operating till this day.
The newly developed steel (6.0%-7.5%Ni) was standardized in JIS, ASTM, ASME, and API. Furthermore, in addition to Class 9 of ASTM, Class 10 having higher strength was specified. The new steel can be used in LNG storage tanks all over the world.