Thick steel plate for structural pipes or tubes, method of producing thick steel plate for structural pipes or tubes, and structural pipes and tubes

A steel plate for structural pipes or tubes, comprising:

• a chemical composition that contains, in mass %,
• C:
• 0.030% to 0.100%,
• Si:
• 0.01% to 0.50%,
• Mn:
• 1.50% to 2.50%,
• Al:
• 0.080% or less,
• Mo:
• 0.05% to 0.50%,
• Ti:
• 0.005% to 0.025%,
• Nb:
• 0.005% to 0.080%,
• N:
• 0.001% to 0.010%,
• O:
• 0.005% or less,
• P:
• 0.010% or less,
• S:
• 0.0010% or less, and
• a balance consisting of Fe and inevitable impurities, with the chemical composition having a carbon equivalent Ceq as defined by the following Expression (1) of 0.42 or more:
• Ceq=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5 (1),
• where each element symbol indicates content in mass % of the element in the steel plate and has a value of 0 if the element is not contained in the steel plate;
• and
• a microstructure that is mainly composed of bainite, an area fraction of the bainite being 90% or more,
• wherein
• the steel plate has a plate thickness of 38 mm or more, and
• the steel plate satisfies a set of conditions including:
• a tensile strength being 705 MPa or more and 825 MPa or less;
• a Charpy absorption energy vE−20° C. at 20° C. at its mid-thickness part being 100 J or more;
• a variation in Vickers hardness in a plate thickness direction ΔHV10,t being 50 or less;
• and
• a variation in Vickers hardness in a plate widthwise direction ΔHV10,c being 50 or less.