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 CASINGFOR DEEP WELLS

摘要

Provided is an electric resistance welded steel pipe that has high strength, high toughness, and excellent post-welding heat treatment resistance. A hot-rolled steel sheet comprising the following is used as a starting material: a composition containing 0.01-0.12% of C, 0.05-0.50% of Si, 1.0-2.2% of Mn, 0.03% or less of P, 0.005% or less of S, 0.001-0.10% of Al, 0.006% or less of N, 0.010-0.100% of Nb, and 0.001-0.050% of Ti; and a structure that comprises 90% or more by volume of a main phase that is a bainitic ferrite phase and 10% or less (including 0%) by volume of a second phase, in which the average particle size of the bainitic ferrite phase is 10 µm or less and extremely fine Nb deposits having a particle size of less than 20 nm are dispersed at a rate that, when calculated in terms of Nb, surpasses 75% by ratio (%) of the total amount of Nb. The starting material is formed to have a substantially circular cross section by roll forming and subsequently subjected to electric resistance welding in order to obtain an electric resistance welded pipe, the electric resistance welded section of the electric resistance welded pipe is subjected to inline heat treatment, and the result is subjected to diameter reduction rolling so that the deviation from roundness of a steel pipe end section is 0.6% or less.