| 摘要 |
Provided is an electric resistance-welded steel pipe with excellent internal pressure leak resistance and toughness of the electric resistance-welded section. A hot-rolled steel sheet of a composition comprising, in mass%, C: 0.025-0.168%, Si: 0.10-0.30%, and Mn: 0.60-1.90%, and comprising Ca and at least one species selected from among Nb, V and Ti so as to satisfy Pcm not being more than 0.20 is continuously roll-formed cold to form a tubular body. When doing so, a tapered groove, the tapered section of which is 10-80% of the steel sheet thickness, is provided on the width direction edges of the steel sheet. The edges of the tubular body are abutted against each other and electric resistance-welded to form a pipe. Ultrasonic waves are transmitted to the electric resistance-welding surface so that beam width is in the range of 0.1-4.0 mm. Reflected waves are used for ultrasonic flaw detection by an ultrasonic flaw detection device using an array probe and the soundness of the electric resistance-welded section is confirmed. After inspection or prior to same, a seam annealing process of heating to 850-1150°C and cooling at a cooling rate of 20-200°C/s is performed on the electric resistance-welded section. As a result, an electric resistance-welded steel pipe is obtained in which both the base metal section and the electric resistance-welded section have a structure with a pseudopolygonal ferrite phase of a 10 µm or less mean particle size as the main phase, internal pressure sealing properties and toughness of the electric resistance-welded section are excellent, and yield strength is at least 400 MPa. |
