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

1. A method of manufacturing a plug used to pierce and roll a metal material, comprising the steps of:

preparing a plug material; and

manufacturing a plug including an oxide scale layer having a thickness of from 500 μm to 1000 μm and having inner scale layer formed on the surface of the plug material and outer scale layer formed on said inner scale layer by thermally treating said prepared plug material 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. from 6 hours to 25 hours and cooling said thermally treated plug material at a cooling rate of from 25° C./hr to 150° C./hr.

2. The method of manufacturing a plug according to claim 1 , wherein in said step of manufacturing the plug including the oxide scale layer, said plug is thermally treated in a heat treatment atmosphere that contains at least 2.0 vol. % oxygen.

3. The method of manufacturing a plug according to claim 1 , wherein in said step of manufacturing the plug including said oxide scale layer, said plug is thermally treated at a heat treatment temperature from 950° C. to 1000° C.

4. The method of manufacturing a plug according to claim 2 , wherein in said step of manufacturing the plug including said oxide scale layer, said plug is thermally treated at a heat treatment temperature from 950° C. to 1000° C.

5. The method of manufacturing a plug according to claim 1 , further comprising the step of removing the outer scale layer in said oxide scale layer.

6. A method of manufacturing a metal pipe, comprising the steps of:

preparing a plug material;

manufacturing a plug including an oxide scale layer having a thickness of from 500 μm to 1000 μm and having inner scale layer formed on the surface of the plug material and outer scale layer formed on said inner scale layer by thermally treating said prepared plug material 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. from 6 hours to 25 hours and cooling said thermally treated plug material at a cooling rate of from 25° C./hr to 150° C./hr

removing the outer scale layer in said oxide scale layer; and

manufacturing a metal pipe by piercing and rolling a metal material using the plug removed of said outer scale layer.

7. A plug used to pierce and roll a metal material, comprising:

a base material; and

an oxide scale layer having a thickness of from 500 μm to 1000 μm and formed on the surface of said base material by heat treatment 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. from 6 hours to 25 hours and cooling said thermally treated plug material at a cooling rate of from 25° C./hr to 150° C./hr.

8. The plug according to claim 7 , wherein said oxide scale layer is formed by heat treatment in a heat treatment atmosphere that contains at least 2.0 vol. % oxygen.

9. The plug according to claim 8 , wherein said oxide scale layer is formed by heat treatment at a heat treatment temperature from 950° C. to 1000° C.

10. A plug used to pierce and roll a metal material, comprising:

a base material;

an inner oxide scale layer formed on the surface of the base material; and

an outer oxide scale layer formed on said inner scale layer and including one or more pores that extend along the surface of said base material, wherein in a section of said outer scale layer having a width of 1000 μm, the LPmax defined by the maximum length of a virtual line parallel to the surface of said base material that overlaps the one or more pores in said outer scale layer is at least 500 μm.