HIGH-STRENGTH THICK-WALLED ELECTRIC RESISTANCE WELDED STEEL PIPE FOR CONDUCTOR CASING FOR DEEP WELL, PRODUCTION METHOD THEREFOR, AND HIGH-STRENGTH THICK-WALLED CONDUCTOR CASING FOR DEEP WELL

摘要

A high-strength high-toughness electric-resistance-welded steel pipe having high resistance to post-weld heat treatment is provided. A hot-rolled steel plate serving as a material is subjected to roll forming so as to have a generally circular cross section and is subjected to electric resistance welding to form an electric-resistance-welded steel pipe. The electric-resistance-welded portion of the electric-resistance-welded steel pipe is then subjected to in-line heat treatment. The electric-resistance-welded steel pipe is then subjected to reducing rolling such that the circularity of an end portion of the steel pipe is 0.6% or less. The hot-rolled steel plate serving as a material has a composition containing C: 0.01% to 0.12%, Si: 0.05% to 0.50%, Mn: 1.0% to 2.2%, P: 0.03% or less, S: 0.005% or less, Al: 0.001% to 0.10%, N: 0.006% or less, Nb: 0.010% to 0.100%, and Ti: 0.001% to 0.050% and has a structure composed of 90% or more by volume of a bainitic ferrite phase and 10% or less (including 0%) by volume of a second phase, the bainitic ferrite phase having an average grain size of 10 µm or less, the structure containing fine Nb precipitates having a particle size of less than 20 nm dispersed in a base material portion, the ratio (%) of the fine Nb precipitates to the total amount of Nb being 75% or less on a Nb equivalent basis. The electric-resistance-welded steel pipe can have high strength and toughness and maintain strength through post-weld heat treatment including heating to a temperature of 600°C or more. The electric-resistance-welded steel pipe is less likely to be broken while placed as a deep-well conductor casing.