Steel plate for thick-walled steel pipe, method for manufacturing the same, and thick-walled high-strength steel pipe

A steel plate for a thick-walled steel pipe with a CTOD value at −10° C. of 0.30 mm or more in a heat affected zone formed as a result of performing girth welding, the steel plate having a chemical composition consisting of, by mass %,

• C:
• 0.030% or more and 0.10% or less,
• Si:
• 0.05% or more and 0.50% or less,
• Mn:
• 1.00% or more and 1.80% or less,
• P:
• 0.015% or less,
• S:
• 0.005% or less,
• Mo:
• 0.20% or less,
• Nb:
• 0.01 or more and 0.05% or less, and
• optionally, one or more of Al:
• 0.005% or more and 0.1% or less, Cu:
• 1.00% or less, Ni:
• 1.00% or less, Cr:
• 0.50% or less, and V:
• 0.05% or less;
• and
• the balance being Fe and inevitable impurities, in which
• Pcm* (%) (=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/2+V/10,
• where atomic symbols respectively represent the contents (mass %) of the corresponding alloying chemical elements in the equation, and where the symbol of an alloying chemical element which is not contained is assigned a value of 0) is 0.20 or less, wherein a fraction of bainite in a base material of the steel plate in terms of area ratio is 50% or more, and wherein a fraction of island martensite (MA) in terms of area ratio, which is generated in a coarse-grain region reheated in a dual-phase temperature range when the steel plate is subjected to multilayer welding performed with a heat input per pass of 5 kJ/cm or more and 70 kJ/cm or less is 5.0% or less.