High-strength steel plate and producing method therefor

1. A high-strength steel plate comprising the following composition:

0.18 to 0.23 mass % of C;

0.1 to 0.5 mass % of Si;

1.0 to 2.0 mass % of Mn;

0.020 mass % or less of P;

0.010 mass % or less of S;

greater than 0.5 mass % and equal to or less than 3.0 mass % of Cu;

0.25 to 2.0 mass % of Ni;

0.003 to 0.10 mass % of Nb;

0.05 to 0.15 mass % of Al;

0.0003 to 0.0030 mass % of B;

0.006 mass % or less of N; and

a balance composed of Fe and inevitable impurities,

wherein a weld crack sensitivity index Pcm is calculated by Pcm=[C]+[Si]/30 +[Mn]/20 +[Cu]/20 +[Ni]/60 +[Cr]/20 +[Mo]/15 +[V]/10 +5 [B], and is 0.39 mass % or less, where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are the concentrations (mass %) of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B, respectively,

an Ac 3 transformation point is equal to or less than 850° C.,

a percentage value of a martensite structure is equal to or greater than 90%,

a yield strength is equal to or greater than 1300 MPa, and

a tensile strength is equal to or greater than 1400 MPa and equal to or less than 1650 MPa,

a prior austenite grain size number Nγ is calculated by Nγ=−3+log 2 m using an average number m of crystal grains per 1 mm 2 in a cross section of a sample piece,

if the tensile strength is less than 1550 MPa, the prior austenite grain size number Nγ and the tensile strength satisfy the formula Nγ≧([TS]−1400 )×0.006+7.0 , and if the tensile strength is equal to or greater than 1550 MPa, the prior austenite grain size number Nγ and the tensile strength satisfy the formula Nγ([TS]-1550 )×0.01+7.9, where [TS] (MPa) is the tensile strength, and

wherein the prior austenite grain size number Nγ is 11 or less.

2. The high-strength steel plate according to claim 1 , further comprising one or more kinds selected from the group consisting of:

0.05 to 1.5 mass % of Cr;

0.03 to 0.5 mass % of Mo; and

0.01 to 0.10 mass % of V.

3. The high-strength steel plate according to claim 1 or 2 , wherein Cu is greater than 1.0% and equal to or less than 3.0 mass %.

4. The high-strength steel plate according to claim 1 or 2 , wherein a thickness is equal to or greater than 4.5 mm and equal to or less than 25 mm.

5. A producing method for a high-strength steel plate, the method comprising:

heating a slab having the following composition:

0.18 to 0.23 mass % of C;

0.1 to 0.5 mass % of Si;

1.0 to 2.0 mass % of Mn;

0.020 mass % or less of P;

0.010 mass % or less of S;

greater than 0.5 mass % and equal to or less than 3.0 mass % of Cu;

0.25 to 2.0 mass % of Ni;

0.003 to 0.10 mass % of Nb;

0.05 to 0.15 mass % of Al;

0.0003 to 0.0030 mass % of B;

0.006 mass % or less of N; and

a balance composed of Fe and inevitable impurities,

wherein a weld crack sensitivity index Pcm is calculated by Pcm=[C]+[Si]/30+[Mn]/20+[Cu]/20+[Ni]/60+[Cr]/20+[Mo]/15+[V]/10+5[B], and is 0.39 mass % or less, where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], and [B] are the concentrations (mass %) of C, Si, Mn, Cu, Ni, Cr, Mo, V, and B, respectively,

and an Ac 3 transformation point of 850° C. or less, to 1100° C. or greater;

performing hot rolling in which a cumulative rolling reduction is equal to or greater than 30% and equal to or less than 65% in a temperature range of equal to or less than 930° C. and equal to or greater than 860° C. and the rolling is terminated at a temperature of equal to or greater than 860° C., thereby producing a steel plate having a thickness of equal to or greater than 4.5 mm and equal to or less than 25 mm;

reheating the steel plate at a temperature of equal to or greater than 20° C. greater than the Ac 3 transformation point and equal to or less than 870° C. after cooling;

performing accelerated cooling to 200° C. or less under a cooling condition in which an average cooling rate at a plate thickness center portion of the steel plate during cooling from 600° C. to 300° C. is equal to or greater than 20° C/s; and

performing tempering in a temperature range of equal to or greater than 200° C. and equal to or less than 300° C., thereby producing a high-strength steel plate having a prior austenite grain size number Nγ of 11 or less as calculated by Nγ=−3 +log 2 m using an average number m of crystal grains per 1 mm 2 in a cross section of a sample piece.

6. The method according to claim 5 , wherein the slab further comprises one or more kinds selected from the group consisting of:

0.05 to 1.5 mass % of Cr;

0.03 to 0.5 mass % of Mo; and

0.01 to 0.10 mass % of V.

7. The method according to claim 5 or 6 , wherein Cu is greater than 1.0% and equal to or less than 3.0 mass %.