METHOD FOR MANUFACTURING METAL PIPE
In this method, the inner surface of a metal pipe containing, by mass percent, 20 to 55% of Cr and 20 to 70% of Ni is subjected to mechanical treatment, the metal pipe is subjected to heat treatment such as to be held in a temperature range of 1050 to 1270° C. for 0.5 to 60 minutes, and thereby an oxide scale layer consisting mainly of Cr is formed on at least the inner surface of the metal pipe. A metal pipe excellent in carburization resistance and coking resistance in a carburizing gas environment can be obtained by the method.
1 . A method for manufacturing a metal pipe containing, by mass percent, 20 to 55% of Cr and 20 to 70% of Ni, wherein
the inner surface of the metal pipe is subjected to mechanical treatment;
the metal pipe is subjected to heat treatment such as to be held in a temperature range of 1050 to 1270° C. for 0.5 to 60 minutes; and thereby
an oxide scale layer consisting mainly of Cr is formed on at least the inner surface of the metal pipe.
2 . The method for manufacturing a metal pipe according to claim 1 , wherein the metal pipe has a chemical composition consisting of, by mass percent, C: 0.01 to 0.6%, Si: 0.1 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less, Cr: 20 to 55%, Ni: 20 to 70%, N: 0.001 to 0.25%, O (oxygen): 0.02% or less, the balance being Fe and impurities.
3 . The method for manufacturing a metal pipe according to claim 2 , wherein
the metal pipe further contains at least one selected from the elements, by mass percent, given in the following items (a) to (g):
(a) Cu: 5% or less
(b) Co: 5% or less
(c) At least one selected from Mo: 3% or less, W: 6% or less, and Ta: 6% or less
(d) One or two selected from Ti: 1% or less and Nb: 2% or less
(e) At least one selected from B: 0.1% or less, Zr: 0.1% or less, and Hf: 0.5% or less
(f) At least one selected from Mg: 0.1% or less, Ca: 0.1% or less, and Al: 1% or less
(g) At least one selected from Y: 0.15% or less and Ln group elements: 0.15% or less.
4 . The method for manufacturing a metal pipe according to claim 1 , wherein the metal pipe has a rib-shaped protrusion on the inner surface of pipe.
5 . The method for manufacturing a metal pipe according to claim 2 , wherein the metal pipe has a rib-shaped protrusion on the inner surface of pipe.
6 . The method for manufacturing a metal pipe according to claim 3 , wherein the metal pipe has a rib-shaped protrusion on the inner surface of pipe.