| 摘要 |
PROBLEM TO BE SOLVED: To provide a thick steel plate having high strength of 690 MPa or more, excellent in super low-temperature toughness at -196°C or lower in Ni steel having a Ni content of 5.0 to 7.5% (especially super low-temperature toughness in a C direction) and capable of achieving brittle fracture surface ratio at -196°C of ≤10%.SOLUTION: The thick steel plate contains predetermined elements in the steel and has an average circularity A of an inclusion having an equivalent circle diameter of above 1.0 μm present in the steel of 1.8 or less, a volume fraction (V) of a retained austenite phase present at -196°C of 2.0 to 12.0%, and B value represented by the formula (1) satisfies 1.3 or more. B=V/A (1) |
