The Japan Atomic Energy Agency (JAEA) has developed a cryogenic structural steel to be used in large superconducting magnets for a fusion machine, and achievements from this development work will be utilized in the International Thermonuclear Experimental Reactor (ITER). Low carbon and boron-added JK2 (JK2LB), which has high strength, fracture toughness at 4K and thermal contraction from room temperature to 4K lower than that of conventional 316LN steel, has been developed as a conduit material for the ITER Central Solenoid (CS) conductor, which is based on ordinary JK2. In order to achieve the ITER requirements (0.2% yield strength ≥1,000 MPa, fracture toughness KIC(J) ≥130 MPa√m) of CS conduit material, chemical components such as carbon, nitrogen and boron, were optimized. In addition, since the CS will be operated under load cycles of 6 x 104 with a maximum principal stress of 490 MPa, fatigue crack growth assessment of the CS jacket was performed. As a result, JK2LB with a nitrogen content of 0.2%, boron of 15-40ppm and low carbon was produced to achieve the strength and fracture toughness requirements. Concerning the welding of JK2LB, a filler wire of JK2LB with a low nitrogen content of 0.13% was developed and fracture toughness of more than 130 MPa√m was confirmed in a weld metal. Measured fatigue crack growth rates of the base and weld metal at 4 K are low enough to achieve the operation cycle of the CS coil.
