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

摘要

A high strength electric resistance welded steel pipe having a yield strength of 450 MPa or more and excellent resistance to softening for a long period in an intermediate temperature range and a method for manufacturing the steel pipe are provided. Specifically, a steel pipe is made using a common method for manufacturing an electric resistance welded steel pipe and made from a hot rolled steel sheet manufactured by heating and soaking a steel material having a chemical composition containing, by mass%, C: 0.026% or more and 0.084% or less, Si: 0.10% or more and 0.30% or less, Mn: 0.70% or more and 1.90% or less, Al: 0.01% or more and 0.10% or less, Nb: 0.001% or more and 0.070% or less, V: 0.001% or more and 0.065% or less, Ti: 0.001% or more and 0.033% or less, Ca: 0.0001% or more and 0.0035% or less, in which the condition that Pcm is 0.20 or less is satisfied, at a temperature of higher than 1200°C and 1280°C or lower for 90 minutes or more, by performing hot rolling under conditions such that the hot rolling reduction in an un-recrystallization temperature range is 20% or more, by cooling the hot rolled steel sheet, in terms of the temperature of a central portion in the thickness direction, down at an average cooling rate of 7°C/sec. or more and 299°C/sec. or less in a temperature range of 780°C to 620°C, by subsequently performing a heat treatment under conditions in which a cumulative holding time in a temperature range of 480°C to 350°C is 2 hours or more and 20 hours or less. Subsequently, an online heat treatment is performed under conditions such that the whole of the wall thickness is heated up to a temperature of 800°C or higher and 1150°C or lower and then cooled, in terms of the temperature of a central portion in the wall thickness direction, at an average cooling rate of 7°C/sec. or more and 299°C/sec. or less in a temperature range of 780°C to 620°C, and further subjected to a treatment in which a cumulative holding time in a temperature range of 500°C to 360°C is 2 seconds or more and 200 seconds or less.