| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength steel plate excellent in low temperature toughness of a high heat input welding heat-affected zone and also excellent in material uniformity in the steel plate; and a method for manufacturing the same.SOLUTION: A high strength steel plate includes, by mass, 0.030-0.150% C, ≤0.10% Si, 1.00-2.00% Mn, each content of P, S, Al, Ti, N, Ca, O is specified, and each content of Ca, O, S satisfies a specific formula, and further includes, if required, one or more of B, V, Nb, Ni, Cu, Cr, Mo, W, Mg, Zr and REM, and the balance of Fe and inevitable impurities. In the steel plate, the metal structure includes substantially a ferrite and bainite structure, unevenness of the hardness of the steel plate in the direction of plate thickness is &Dgr;HV 50 or less and unevenness of the hardness of the steel plate in the direction of plate width is &Dgr;HV 50 or less. The steel having the steel composition is heated to a specific temperature, subjected to hot rolling at a steel plate surface temperature of ≥700°C and ≤900°C, and then subjected to first cooling from a temperature on the surface of the steel plate of ≥700°C at a cooling rate of ≥20°C/s and ≤100°C/s on the surface of the steel plate to a temperature on the surface of the steel plate of ≥400°C and ≤600°C, and then subjected to second cooling to a temperature of ≤400°C at a cooling rate of ≥4°C/s in average of the steel plate. |
