| 摘要 |
PROBLEM TO BE SOLVED: To provide a thick steel plate having tensile strength of 625 MPa or more and excellent in elongation and toughness.SOLUTION: The thick steel plate has a chemical composition including, by mass%, C:0.03-0.18%, Si:0.01-1.0%, Mn:0.5-2.0%, sol.Al:0.001-0.1%, Cu:0-1.0%, Ni:0-1.0%, Cr:0-1.0%, Mo:0-1.0%, V:0-0.1%, Nb:0-0.1%, Ti:0-0.1%, B:0-0.01%, Ca:0-0.01%, REM:0-0.02%, Mg:0-0.01%, Zr:0-0.1% and the balance Fe with impurities. P, S, N and O in the impurities are P:0.02% or less, S:0.01% or less, N:0.01% or less and O:0.005% or less. A microstructure in the 1/4 thickness of the plate thickness is a composite structure of a soft structure and a hard structure. The area ratio of the soft structure is less than 70%, and the average particle size thereof is less than 10 μm. The average length of the hard structure in a minor axis direction is less than 5 μm, and the average aspect ratio thereof is less than 10. |
