High strength thick-walled electric-resistance-welded steel pipe for deep-well conductor casing, method for manufacturing the same, and high strength thick-walled conductor casing for deep wells

A high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing, the steel pipe having a chemical composition consisting of:

• C:
• 0.01% to 0.12%, by mass %;
• Si:
• 0.05% to 0.50%, by mass %;
• Mn:
• 1.0% to 2.2%, by mass %;
• P:
• 0.03% or less, by mass %;
• S:
• 0.005% or less, by mass %;
• Al:
• 0.001% to 0.10%, by mass %;
• N:
• 0.006% or less, by mass %
• Nb:
• 0.010% to 0.100%, by mass %;
• Ti:
• 0.001% to 0.050%, by mass %;
• optionally at least one Group selected from group consisting of A and B:
• Group A:
• at least one selected from the group consisting of:
• V:
• 0.1% or less, by mass %,
• Cr:
• 0.5% or less, by mass %,
• Ni:
• 1.0% or less, by mass %, and
• B:
• 0.0030% or less, by mass %, and
• Group B:
• at least one selected from the group consisting of:
• Ca:
• 0.0050% or less, by mass %, and
• REM:
• 0.0050% or less, by mass %;
• and
• Fe and incidental impurities,
• wherein the steel pipe has:
• (i) an electric-resistance-welded portion that has a tensile strength TS of 625 MPa or more and an absorbed energy vE−40 in a Charpy impact test at a test temperature of −40° C. of 27 J or more, and (ii) a base material portion that has a yield strength YS of 555 MPa or more, a tensile strength TS of 625 MPa or more and an absorbed energy vE−40 in a Charpy impact test at a test temperature of −40° C. of 27 J or more, and the base material portion has a yield strength YS of 555 MPa or more and a tensile strength TS of 625 MPa or more after post-weld heat treatment performed at a temperature of 600° C. or more,
• the steel pipe has a structure in an electric-resistance-welded portion of the steel pipe, the structure composed of 90% or more by volume of a bainitic ferrite phase as a main phase and 10% or less by volume of a second phase, the bainitic ferrite phase having an average grain size of 10 μm or less, and the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in the base material portion, a ratio of fine Nb precipitates to a total amount of Nb being 20% or more and 75% or less on a Nb equivalent basis, and
• a circularity of an end portion of the steel pipe defined by formula (1) being 0.6% or less,
• circularity (%)={(maximum outer diameter mmϕ of steel pipe)−(minimum outer diameter mmϕ of steel pipe)}/(nominal outer diameter mmϕ)×100 formula (1).