9Ni steel is widely used in LNG storage tanks due to its good low temperature toughness. However, nickel alloy has many shortcomings, such as fluctuating nickel price, and unstable  supply and demand . Therefore, it is an important development direction of LNG storage tank steel to realize the development of Ni-reduced steel plate without reducing performance by optimizing the chemical composition design and the advanced manufacturing process. In 2014, Japan developed 7Ni steel through TMCP and microalloying technology. It is reported that its strength and toughness were equivalent to 9Ni steel. At present, the comprehensive properties of 7Ni steel developed by domestic steel plants have also reached the level of 9Ni steel .

Properties of 7Ni steel

Compared with 9Ni steel, the Ni content of 7N steel is reduced by 2%, which usually reduces the toughness of steel. For 7Ni steel, the low temperature toughness and strength are generally compensated by reducing the content of Si and adding a small amount of Cr, Mo and other alloying elements .

The 7Ni steel has the equivalent tensile properties of 9Ni steel, and reaches the level of 9Ni according to the data released by the steel plants .

The 7Ni  steel has the  same impact properties as 9Ni steel .

According to the acceptance criteria of the above standards, the performance requirements of 7Ni steel are equivalent to 9Ni steel .

Domestic and international standards

7Ni steel has been included in the JIS standards, and graded as SL7N590, as well as in the ASTM standards with the grade of SA553 Type III. In 2023, 7Ni steel is included in Steel Plate, Sheet and Strip for Pressure Equipments - Part 4: Nickel-alloy Steels with Specified Low Temperatures Properties (GB/ T 713-4), with the grade of 06Ni7DR, which will come into effect on March 1, 2024.

Case of 7Ni steel

The No.5 LNG storage tank of the first plant of Osaka Senboku Gas Manufacturing Institute,  a 7Ni  steel LNG  storage tank developed by Nippon Steel, was completed at the end of November 2015. Since then, 7Ni steel has been used in the construction of No.3 LNG tank (Aichi-ken) of Chita-Midorihama Plant of Toho Gas and LNG storage tank (Fukushima-Ken) of Soma LNG Base, which

was completed in 2017.

Study on the Application of 7Ni Steel in Marine LNG Storage Tank

At present, the materials for marine LNG storage tanks specified by the International Maritime Organization (IMO) include 9Ni steel, austenitic stainless steel, aluminum alloy,  36Ni  steel  and  high  manganese austenitic steel. For the metallic materials other  than  those  mentioned  above,  the approval of the competent authority may be obtained in consideration of their possible design loads, properties and intended uses .

At the 102nd  session, the IMO Maritime Safety Committee, aware of the possible need for other metallic materials, approved the Circular Guidelines for the Accreditation of Alternative Metallic Materials for Cryogenic Environments for Ships Carrying Liquefied Gases in Bulk and Ships Using Gases or Other Low Flash Fuels (IMO MSC.1/Circ.1622) as guidance for the application of metallic materials outside the IMO Code. Considering that 7Ni steel has not been included in IMO rules, approval by the competent authority in accordance with IMO MSC.1/Circ.1622 is required for its application in marine LNG storage tanks .

T he  compet ent  authority  can  add or delete the above-mentioned test items according to their possible design load, performance and intended use .

Services provided by CCS

CCS  has  accepted  the  government authorization of more than 60 major shipping countries or regions in the world, including the Chinese government. Thus, CCS can evaluate the  application of 7Ni  steel in specific projects according to the customer's application .