Development of 7%Ni-TMCP steel plate for LNG storage tanks? (concept of development and properties of 10, 25 and 40 mm Thick 7%Ni steel plate)

LNG storage tanks made of 9%Ni steel plate have safely been operated at the many LNG export and import terminals in the world for the last half-century. Over the years extensive improvement and enhancement have been achieved on the 9%Ni steel plate in terms of toughness in steel to withstand the cryogenic temperatures under an operating condition with LNG. This paper reports research and development of the 7%Ni - TMCP steel plate. As nickel is an expensive and valuable rare metal, it was aimed to reduce nickel content to 7% to save investment costs for constructions of LNG storage tanks. Inferiority of the lower nickel content was compensated by adjusting chemical contents of Mn, Si, Cr etc. and by Thermo-Mechanical Controlled Process (TMCP) with an intermediate heat treatment. In order to evaluate fitness of the 7%Ni - TMCP steel plate and its weld for LNG storage tanks a series of testing was conducted. Three different kinds of plate thicknesses of 40, 25 and 10 mm were chosen to run large scale fracture toughness tests such as duplex ESSO tests, cruciform wide plate tests as well as small scale tests. Those mechanical tests showed excellent quality of the steel and demonstrated safety of the steel which was considered the same safety level as the conventional 9%Ni steel plate. Hence, it is considered that the 7%Ni - TMCP steel plate warrants serious consideration for use in LNG storage tanks.