Steel plate having yield strength of 670 to 870 N/mm

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,

wherein

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 2 ,

a tensile strength is 780 to 940 N/mm 2 ,

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

wherein

α=C+6×Si+100×P  Expression 1

β=0.65×C 1/2 ×(1+0.64×Si)×(1+4.10×Mn)×(1+0.27×Cu)×(1+0.52×Ni)×(1+2.33×Cr)×(1+3.14×Mo)  Expression 2, 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.

2. The steel plate according to claim 1 , wherein the chemical composition includes, in terms of mass %:

Mn—: 0.7 to 1.2%;

Ni—: 0.8 to 2.5%;

Cr—: 0.5 to 1.0%;

Mo—: 0.35 to 0.75%;

V—: 0.02 to 0.05%;

Al—: 0.04 to 0.08%; and

Cu—: 0.2 to 0.7%.

3. The steel plate according to claim 1 , wherein the plate thickness of the steel plate is 50 to 150 mm.

4. The steel plate according to claim 1 , wherein

when a stress relief annealing is performed on the steel plate with a holding temperature of 560° C., a holding time of h hours defined by Expression 3 and Expression 4, a rate of temperature increase within a temperature range of 425° C. or more of 55° C./hour or less, and a rate of temperature decrease within the temperature range of 425° C. or more of 55° C./hour or less, a Charpy absorbed energy at −40° C. is 100 J or more,

when t≧ 50, h=t/ 25  Expression 3,

when t<50, h=2  Expression 4, and

t indicates the plate thickness of the steel plate in terms of mm and h indicates the holding time in unit of hour.

5. The steel plate according to claim 2 , wherein the plate thickness of the steel plate is 50 to 150 mm.

6. The steel plate according to claim 1 , wherein the chemical composition includes, in terms of mass %:

Mn: 0.5 to 0.88%.