Method of manufacturing plug used to pierce and roll metal material, method of manufacturing metal pipe and plug used to pierce and roll metal material

摘要

A plug material having a prescribed shape is prepared, the prepared plug material is thermally treated in a heat treatment atmosphere that contains at least 1.0 vol. % oxygen at a heat treatment temperature of at least 950° C. and less than 1050° C. and thus a plug having an oxide scale layer 30 on the surface is manufactured. According to the manufacturing method, pores PO that extend along the surface SF of the plug material are formed under outer scale layer 20 in the oxide scale layer 30, which allows cracks to be more easily propagated in the outer scale layer 20. Therefore, the outer scale layer 20 is more easily peeled off than the conventional one.