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High manganese steel (KHMN30L) as a cryogenic nonmagnetic material has been developed by Kawasaki Steel Corporation, which is designed for structural material for superconducting magnet in particle accelerator system. This steel satisfies the following requirements for the present use. 1) Low magnetic permeability: its relative magnetic permeability is lower than 1.002 throughout the range between 1.9 K and room temperature, and shows little temperature dependency which is the result of the highly elevated Neel temperature controlled by alloying composition design. 2) Low thermal expansion: its integrated contraction from room temperature to 4.2 K is as small as 0.18%. 3) Appropriate mechanical properties: yield strength and tensile strength can be adjusted to the desirable value by the manufacturing process condition without deteriorating physical properties. With these excellent properties, this steel is being supplied for nonmagnetic lamination of the cold mass of the LHC (Large Hadron Collider) superconducting dipole magnet, which is under construction by CERN.