Method for manufacturing metal pipe

Abstract

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.