METHOD FOR PRODUCING PLASMA-SHIELDED ELECTRIC RESISTANCE WELDED STEEL PIPE

摘要

To utilize electric resistance welded steel pipes as high-grade steel pipes such as oil well pipes, the qualities of the electric resistance welded steel pipes, in particular, the components of a plasma operating gas in plasma-shielded welding were fundamentally reexamined to reduce penetrator defects. As a result, an effect achieved by adding no or a minor amount of hydrogen, said effect having been ignored by conventional findings, was newly discovered and an optimum plasma operating gas composition was successfully obtained. Provided is a method for producing a plasma-shielded electric resistance welded steel pipe, said method comprising, in shaping a steel sheet into a pipe and welding end faces thereof, applying a voltage to a plasma operating gas that is supplied between a cathode and an anode of a plasma gun to generate a plasma and then shielding the vicinity of a weld part by the plasma, characterized in that the components of the plasma operating gas comprise, in terms of vol%, 70-90% inclusive of N 2 and the balance being Ar and inevitable impurities. Also provided is a method for producing a plasma-shielded electric resistance welded steel pipe, characterized in that the plasma operating gas contains 1 vol% or less of H 2 .