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A new low-nickel (6.0-7.5%Ni) steel plate for LNG storage tanks has been developed with performance equivalent to 9% Ni steel, which has been used for LNG storage tanks for several decades. This excellent performance equivalent to 9% Ni steel was achieved by reducing the amount of added nickel, optimizing the chemical composition, and by applying the latest Thermo-mechanical controlled process (TMCP) technology to the steel plate, in order to contribute to cost savings and natural resource savings. Developed steel has been practically applied through joint research with customers and committees, and the expansion of its applications is progressing.

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Offshore structure steel with high strength of YS550MPa has been investigated. As for offshore structure steel, high toughness in welded joints is required in addition to that in base metal. TMCP type steel of up to YS420MPa grade is used widely, and up to YS500MPa grade is reported in some papers. However, steel of higher strength grade with good toughness and weldability will be beneficial to structures in strict conditions. To reach the YS550MPa requirement, hardening effect by Cu precipitation was utilized. Steel plates were designed with micro-alloyed low C-Mn-Cu-Ni-Cr-Mo system. The combination of the copper precipitation and TMCP technology can increase strength without deteriorating toughness and weldability. Heat treatment for Cu precipitation was carried out to optimize the balance of strength and toughness of the base metal. The developed steel also shows good HAZ CTOD toughness up to 76.2mm thickness in several welding conditions including after PWHT. The newly developed steel has the possibility to increase the flexibility to design large-sized structures.