High strength thick-walled elecric-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 method for manufacturing a high-strength thick-walled electric-resistance-welded steel pipe for a deep-well conductor casing, the method comprising:

• continuously rolling a hot-rolled steel plate with a roll forming machine to form an open pipe having a generally circular cross section;
• butting edges of the open pipe;
• electric-resistance-welding a portion where the edges are being butted while pressing the butted edges to contact each other by squeeze rolls to form an electric-resistance-welded steel pipe;
• subjecting the electric-resistance-welded portion of the electric-resistance-welded steel pipe to in-line heat treatment;
• and
• reducing a diameter of the electric-resistance-welded steel pipe by rolling,
• wherein:
• the hot-rolled steel plate is manufactured by:
• heating to soak a steel at a heating temperature in the range of 1150° C. to 1250° C. for 60 minutes or more,
• hot-rolling the steel with a finishing delivery temperature of 750° C. or more,
• after completion of the hot rolling, subjecting the hot-rolled steel plate to accerelated cooling such that an average cooling rate in a temperature range of 750° C. to 650° C. at the center of plate thickness ranges from 8° C./s to 70° C./s, and
• coiling the hot-rolled steel plate at a coiling temperature in the range of 580° C. to 400° C.,
• the hot-rolled steel plate has a composition comprising:
• 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 %, and
• Fe and incidental impurities.