Patent ID: 7935197

Claim:
A high strength steel plate having a yield strength of 448 MPa or higher, consisting of 0.02 to 0.08 mass % C, 0.01 to 0.5 mass % Si, 0.5 to 1.8 mass % Mn, 0.01 mass % or less P, 0.002 mass % or less S, 0.05 to 0.5 mass % Mo, 0.005 to 0.04 mass % Ti, 0.5 mass % or less Cr and 0.07 mass % or less Al, 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 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 0.7 to 2, and having a metallic structure of (i) a ferrite phase, (ii) a bainite phase and (iii) in a total volume percentage of 5% or smaller, one or more phases selected from the group consisting of martensite, pearlite and cementite, the ferrite phase containing precipitates of complex carbides having a particle size of 5 nm or smaller and containing Ti and Mo, the bainite phase having an area percentage of 10 to 65% and the ferrite phase is a minimum of 30 volume %, the steel plate having a value of Ceq of 0.36 or less, wherein Ceq is defined as follows: Ceq =C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5, wherein C is the mass % of carbon, Mn is the mass % of manganese, Cu is the mass % of copper, Ni is the mass % of nickel, Cr is the mass % of chromium, Mo is the mass % of molybdenum and V is the mass % of vanadium.