Method for producing martensitic stainless steel tubes

Abstract

Martensitic stainless steel tubes are produced by optimizing the chemical composition of steel for billets (C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, N: 0.01-0.05%, P: not more than 0.020% and S: not more than 0.010%) and the relation between the soaking temperature for pierced-and-rolled tubing material and the section area reduction rate in final rolling. By this method, delayed fracture cracking in impact-receiving portions of tubes in martensitic stainless steel s such as 13% Cr tubes can be inhibited and the generation of inner-surface flaws can be prevented.

1 . A method for producing martensitic stainless steel tubes, comprising the steps of: 
 using a billet having a chemical composition containing C: 0.15-0.22%, Si: 0.1-1.0%, Mn: 0.10-1.00%, Cr: 12.00-14.00%, N: 0.01-0.05%, P: not more than 0.020% and S: not more than 0.010%, each in mass %, the balance being Fe and impurities;    subjecting said billet to piercing-and-rolling, followed by further elongation-rolling, thus yielding a tubing material; and    soaking thus-obtained tubing material and subjecting the same to final rolling under such conditions that a reheating temperature T (° C.) in soaking prior to the final rolling and a section area reduction rate R (%) in final rolling satisfy the relation represented by an inequality formula (1) given below:        T> 44.4 ×ln ( R )+821  (1).    
   
   
       2 . The method for producing martensitic stainless steel tubes as set forth in  claim 1 , wherein the chemical composition of said billet includes, in lieu of a part of Fe, one or more of V: not more than 0.200%, Nb: not more than 0.200%, Ti: not more than 0.200% and B: not more than 0.0100%, each in mass %.  
   
   
       3 . The method for producing martensitic stainless steel tubes as set forth in  claim 1 , wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Ni: not more than 0.5% and Cu: not more than 0.25%, each in mass %.  
   
   
       4 . The method for producing martensitic stainless steel tubes as set forth in  claim 2 , wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Ni: not more than 0.5% and Cu: not more than 0.25%, each in mass %.  
   
   
       5 . The method for producing martensitic stainless steel tubes as set forth in  claim 1 , wherein the chemical composition of said billet further included, in lieu of part of Fe, one or more of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %.  
   
   
       6 . The method for producing martensitic stainless steel tubes as set forth in  claim 2 , wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %.  
   
   
       7 . The method for producing martensitic stainless steel tubes as set forth in  claim 3 , wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %.  
   
   
       8 . The method for producing martensitic stainless steel tubes as set forth in  claim 4 , wherein the chemical composition of said billet further includes, in lieu of part of Fe, one or more of Al: not more than 0.1% and Ca: not more than 0.0050%, each in mass %.