Patent ID: 8317945

Claim:
A steel plate, having a composition comprising: carbon (C) in a content of 0.1 percent by mass to 0.16 percent by mass (hereinafter contents will be simply expressed in “%”), silicon (Si) in a content of 0.05% to 0.35%, manganese (Mn) in a content of 0.9% to 1.6%, aluminum (Al) in a content of 0.01% to 0.06%, molybdenum (Mo) in a content of 0.13% to 0.3%, boron (B) in a content of 0.0005% to 0.002%, and at least one of chromium (Cr) in a content of 0.3% or less and vanadium (V) in a content of 0.07% or less, iron and inevitable impurities, the steel plate having an F value defined by the following expression (1) and satisfying the following condition: 3.20≦(F value)≦4.50, the steel plate having a tempered bainite microstructure in which the average equivalent area diameter of grains surrounded by high-angle boundaries with a difference in orientation between two grains of 15° or more is 4 μm or less, and the steel plate having a tensile strength of 585 MPa or more: F value= 9.4×[Mo]+8.1×[V]+4.7×[Cr] (1) wherein [Mo], [V] and [Cr] represent contents (percent by mass) of Mo, V, and Cr, respectively.