Patent ID: 8147626

Claim:
A method for manufacturing a high strength steel plate having a yield strength of 448 MPa or higher, comprising the steps of: hot rolling, under the conditions of a heating temperature of 1000 to 1300° C. and of a finishing temperature of 750° C. or higher, a steel slab consisting essentially of 0.02 to 0.08% C, 0.01 to 0.5% Si, 0.5 to 1.8% Mn, 0.01% or less P, 0.002% or less S, 0.05 to 0.5% Mo, 0.005 to 0.04% Ti, and 0.07% or less Al, by mass, optionally containing one or both of 0.005 to 0.05 mass % Nb and 0.005 to 0.1 mass % V, optionally containing at least one element selected from the group consisting of 0.5 mass % or less Cu, 0.5 mass % or less Ni, 0.5 mass % or less Cr and 0.0005 to 0.005 mass % Ca, and a balance of Fe, wherein [C/(Mo+Ti)] as a ratio of C content to the sum of Mo and Ti contents by atom percentage is 0.5 to 3; applying an accelerated cooling to the hot rolled steel plate to a cooling stop temperature of 300 to 600° C. at a cooling rate of 5° C./s or higher; and reheating the steel plate immediately after cooling thereof to a temperature of 550 to 700° C. at a heating rate of 0.5° C./s or higher, the steel plate having substantially a two phase microstructure of ferrite and bainite, the ferrite containing precipitates of complex carbides having a particle size of 10 nm or smaller and containing Ti and Mo.