HIGH-STRENGTH THICK-WALLED ELECTRIC-RESISTANCE-WELDED STEEL PIPE HAVING EXCELLENT LOW-TEMPERATURE TOUGHNESS, AND METHOD FOR MANUFACTURING SAME

摘要

There is provided a high-strength thick-walled electric resistance welded steel pipe having excellent low-temperature toughness and excellent HIC resistance and having a yield strength of 400 MPa or more. Steel is heated to and held at a temperature in the range of 1200°C to 1280°C. The steel has a chemical composition consisting of C: 0.025% to 0.084%, Si: 0.10% to 0.30%, Mn: 0.70% to 1.80%, controlled amounts of P, S, Al, N, and O, Nb: 0.001% to 0.065%, V: 0.001% to 0.065%, Ti: 0.001% to 0.033%, and Ca: 0.0001% to 0.0035% on a mass percent basis and the remainder being Fe and incidental impurities, and satisfies Pcm of 0.20 or less. The steel is hot-rolled at a rolling reduction of 20% or more in an unrecrystallization temperature range. After the completion of the hot-rolling, the steel is cooled to a finish cooling temperature of 630°C or less at a cooling rate in the range of 7°C/s to 49°C/s and is coiled at 400°C or more and less than 600°C to form a hot-rolled steel strip. The hot-rolled steel strip is subjected to roll-forming and electric resistance welding to produce an electric resistance welded steel pipe. The welded portion of the electric resistance welded steel pipe is then subjected to a heat treatment in which the welded portion is heated to a temperature in the range of 800°C to 1150°C over the total wall thickness and is then cooled to 630°C or less at an average cooling rate in the range of 7°C/s to 49°C/s. The base steel portion and the electric resistance welded portion of the resulting high-strength thick-walled electric resistance welded steel pipe have excellent low-temperature toughness and excellent HIC resistance.