| 摘要 |
PROBLEM TO BE SOLVED: To provide a high-strength thick steel plate over 690 MPa being a Ni steel containing about 530-7.5% Ni, which has an excellent cryogenic toughness at or below -196°C (especially the cryogenic toughness in C direction) and which can achieve a percent brittle fracture at -196°C≤10%.SOLUTION: A thick steel plate includes a predetermined steel composition, satisfies a condition that a volume fraction of residual austenite phase present at -196°C, also satisfies N≤200 pieces/mm, where N is a number density of inclusions with an equivalent circle diameter over 1.0 μm present in the steel plate, and also satisfies that a value A represented by formula (1) below is 11.5 or less. There is represented by the formula (1): A=V+0.012×&pgr;×N. |
