| 摘要 |
PURPOSE:To obtain an electric welded steel pipe of which the tensile strength in the periphery of weld zones is equal to or higher than that in the other parts without degrading toughness and weldability in the production of an electric welded steel pipe having strength equivalent to or higher than API5LX-X52 by cooling the electric welded zones forcibly right after the heat treatment of said zones. CONSTITUTION:A steel strip contg., by weight %, <=0.10 C, 0.8-2.0 Mn, 0.01- 0.10Al, and >=1 kind among 0.01-0.10 Nb, 0.01-0.15V, 0.01-0.10 Ti and consisting of the balance Fe with unavoidable impurities is used as a blank material 1. This is made into an electric welded steel pipe 1A which is heated and annealed locally in its weld zones up to an austenitizing temp. by a post annealer 6. Right after said annealing, the parts around the weld zones are forcibly cooled at >=30 deg.C/sec cooling rate with a force cooler 7. Thus the grain fining effect and hardening effect around the weld zones are obtained, and the decrease in tensile strength is prevented. |
