| 摘要 |
A thick steel plate having a steel composition that comprises, in mass%, 0.03 to 0.20% of C, 0.1% or less of Si, 0.5 to 2.2% of Mn, P, S, 0.005 to 0.05% of Nb, 0.005 to 0.03% of Ti, 0.005 to 0.08% of Al, 0.0075% or less of N, optionally at least one component selected from Cu, Ni, Cr, Mo, V, B, Ca and REM, and a remainder made up by Fe and unavoidable impurities, wherein the microstructure is a structure mainly composed of a worked ferrite, and the Charpy ductile-to-brittle transition temperature at a part positioned at a depth of (1/2 of the thickness (t) of the plate) + 6 mm as observed in the thickness direction is -40°C or lower. A steel material (slab) having the above-mentioned composition is heated to a temperature of 1000 to 1200°C, then rolled at a cumulative rolling reduction ratio of 30% or more in such a temperature range that a thickness center part of the steel material (i.e., a center part of the steel material as observed in the thickness direction) can be in an austenite recrystallization temperature range, then subjected to a first cooling procedure at a cooling rate of 15°C/s or less until the temperature of the thickness center part reaches a temperature point Ar3 or lower, then rolled at a cumulative rolling reduction ratio of 40% or more, then subjected to a second cooling procedure to 600°C or lower at a cooling rate of 4°C/s or more, and then optionally tempered at a temperature equal to or lower than a temperature point Ac1. |
