Patent ID: 9499873
Filing Date: 2016-11-22
CPC Classification: C21D,C22C,Y10T

Claim Text:
1. A steel plate having a chemical composition comprising, in terms of mass %: C—: 0.07 to 0.10%; Si—: 0.01 to 0.10%; Mn—: 0.5 to 1.5%; Ni—: 0.5 to 3.5%; Cr—: 0.1 to 1.5%; Mo—: 0.1 to 1.0%; V—: 0.005 to 0.070%; Al—: 0.01 to 0.10%; B—: 0.0005 to 0.0020%; N—: 0.002 to 0.010%; P—: 0.006% or less; S—: 0.003% or less; Cu—: 0 to 1%; Nb—: 0 to 0.05%; Ti—: 0 to 0.020%; Ca—: 0 to 0.0030%; Mg—: 0 to 0.0030%; REM-: 0 to 0.0030%; and the remainder including Fe and impurities, an α value defined by Expression 1 is 0.13 to 1.0 mass %, a β value defined by Expression 2 is 8.45 to 15.2, a yield strength is 670 to 870 N/mm a tensile strength is 780 to 940 N/mm a grain is defined as an area surrounded by a boundary in which a misorientation is 30° or more and which is identified by performing an orientation analysis using an electron beam backscatter diffraction pattern analysis method, a grain size is defined as an equivalent circle diameter of the grain, an average grain size is defined as a grain size at which a cumulative frequency is 90% when a frequency distribution of the grain size is cumulated from a small grain size side, the average grain size at mid-thickness of the steel plate is 35 μm or less, and a plate thickness is 25 to 200 mm, and C, Si, P, Mn, Cu, Ni, Cr, and Mo indicate the amounts of C, Si, P, Mn, Cu, Ni, Cr, and Mo in terms of mass %, respectively.