Steel plate for high-strength and high-toughness steel pipes and method for producing steel plate

A steel plate for high-strength and high-toughness steel pipes, the steel plate having a chemical composition containing, by mass %,

• C:
• 0.03% or more and 0.08% or less,
• Si:
• more than 0.05% and 0.50% or less,
• Mn:
• 1.5% or more and 2.5% or less,
• P:
• 0.001% or more and 0.010% or less,
• S:
• 0.0030% or less,
• Al:
• 0.01% or more and 0.08% or less,
• Nb:
• 0.010% or more and 0.080% or less,
• Ti:
• 0.005% or more and 0.025% or less, and
• N:
• 0.001% or more and 0.006% or less, and further containing, by mass %, at least one selected from
• Cu:
• 0.01% or more and 1.00% or less,
• Ni:
• 0.01% or more and 1.00% or less,
• Cr:
• 0.01% or more and 1.00% or less,
• Mo:
• 0.01% or more and 1.00% or less,
• V:
• 0.01% or more and 0.10% or less, and
• B:
• 0.0005% or more and 0.0030% or less, with the balance being Fe and inevitable impurities,
• wherein the steel plate has a microstructure in which an area fraction of ferrite at a ½ position of a thickness of the steel plate is 20% or more and 80% or less and deformed ferrite constitutes 50% or more and 100% or less of the ferrite,
• wherein separations that occur in a fractured surface of a test piece of the steel plate have a separation index (SI−55° C.) of 0.10 mm−1 or more provided that the test piece is subjected to a DWTT test (Drop Weight Tear Test) at a test temperature of −55° C., the separation index being defined by formula (1):
• SI−55° C. (mm−1)=ΣLi/A (1)
• where ΣLi:
• a total of lengths (mm) of separations having a length of 1 mm or more existing in an evaluation region (A) of the test piece for the DWTT test,
• A:
• an area (mm2) of the evaluation region of the test piece for the DWTT test, the evaluation region being a region excluding a first portion and a second portion in the test piece, the first portion having a dimension extending from a press notch side to the evaluation region, the second portion having a dimension extending from a drop weight impact side to the evaluation region, the dimension of the first portion and the dimension of the second portion each being equal to a thickness, t, of the test piece (in a case that the thickness t<19 mm) or each being 19 mm (in a case that the thickness t≥19 mm),
• wherein the steel plate has a tensile strength of 625 MPa or more, and a Charpy impact absorbed energy at −55° C. (vE−55° C.) of 160 J or more,
• wherein the steel plate has a percent ductile fracture of 85% or more, as determined by a DWTT test at −55° C.