HIGH-STRENGTH ELECTRIC-RESISTANCE-WELDED STEEL PIPE OF EXCELLENT LONG-TERM SOFTENING RESISTANCE IN INTERMEDIATE TEMPERATURE RANGES, AND METHOD FOR PRODUCING SAME

摘要

Provided are a high-strength electric-resistance-welded steel pipe of yield strength 450 MPa or higher and excellent long-term softening resistance in intermediate temperature ranges, and a method for producing the same. More specifically, an electric-resistance-welded steel pipe is made by a regular method for forming an electric-resistance-welded pipe where the material is a hot-rolled steel strip obtained by subjecting a steel material of a composition comprising, by mass, C: 0.026-0.084%, Si: 0.10-0.30%, Mn: 0.70-1.90%, Al: 0.01-0.10%, Nb: 0.001-0.070%, V: 0.001-0.065%, Ti: 0.001-0.033%, and Ca: 0.0001-0.0035%, and having a Pcm satisfying 0.20 or less, to the following steps: heat-soaking for 90 min or longer at a temperature ranging from greater than 1200°C to 1280°C; hot rolling at a rolling rate 20% or higher in an un-recrystallized temperature range; cooling, after conclusion of the rolling, at a mean cooling rate of 7-299°C/s at a temperature of 780-620°C in the thickness-wise middle part of the plate; and subsequently adjusting the cumulative residence time in the temperature range 480-350°C to 2-20 h. The entire thickness of the material of the plate near an electric resistance weld section is heated inline to a temperature in the range of 800-1150°C, the plate is cooled so that the mean cooling rate at 780-620°C in the thickness middle part temperature is in the range of 7-299°C is carried out, and the cumulative residence time at 500-360°C is adjusted to 2-200 s.