Austenitic stainless steel and method of manufacturing austenitic stainless steel

Abstract

An austenitic stainless steel with good strength and ductility is provided. An austenitic stainless steel has a chemical composition of, in mass %: 0.005 to 0.060% C; 0.20 to 1.20% Si; 4.0 to 8.0% Mn; 12.0 to 15.0% Ni; 19.0 to 24.0% Cr; 1.0 to 4.0% Mo; 0.05 to 0.40% Nb; 0.05 to 0.40% V; 0.20 to 0.50% N; up to 0.050% Al; and other elements, the tensile strength being not lower than 800 MPa, the braking elongation being not lower than 35%, the steel satisfying the following expressions, (1) and (2): (1) 0.7×Nb≤[Nb]≤0.30; and (2) 20×[Nb]/D≥0.050. In expressions (1) and (2), the Nb content in the chemical composition represented in mass %, the amount of Nb determined through analysis of an electrolytic extraction residue represented in mass %, and the crystal grain size represented in μm are substituted for “Nb”, “[Nb]”, and “D”, respectively.

1 . An austenitic stainless steel having a chemical composition of, in mass %:
 0.005 to 0.060% C;   0.20 to 1.20% Si;   4.0 to 8.0% Mn;   12.0 to 15.0% Ni;   19.0 to 24.0% Cr;   1.0 to 4.0% Mo;   0.05 to 0.40% Nb;   0.05 to 0.40% V;   0.20 to 0.50% N;   up to 0.050% Al;   0 to 3.0% Cu;   0 to 0.50% Co;   0 to 0.0050% Ca;   0 to 0.0050% B;   0 to 0.10% W;   0 to 0.10% Ta; and   balance Fe and impurities,   the impurities including:   up to 0.030% P;   up to 0.010% S; and   up to 0.015% O,   a tensile strength being not lower than 800 MPa,   a braking elongation being not lower than 35%,   an amount of Nb determined through analysis of an electrolytic extraction residue satisfying expression (1) below,   the amount of Nb determined through analysis of the electrolytic extraction residue and a crystal grain size satisfying expression (2) below:   
       
         
           
             
               
                 
                   
                     
                       
                         0.7 
                         × 
                         Nb 
                       
                       ≤ 
                       
                         [ 
                         Nb 
                           
                         ] 
                       
                       ≤ 
                       0.3 
                     
                     , 
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
         
           
             and 
           
         
         
           
             
               
                 
                   
                     
                       
                         20 
                         × 
                         
                           
                             [ 
                             Nb 
                               
                             ] 
                           
                           / 
                           D 
                         
                       
                       ≥ 
                       
                         
                           0 
                           . 
                           0 
                         
                         ⁢ 
                         50 
                       
                     
                     , 
                   
                 
                 
                   
                     
                       ( 
                       2 
                       ) 
                     
                   
                 
               
             
           
         
         in expressions (1) and (2), the Nb content in the chemical composition represented in mass %, the amount of Nb determined through analysis of the electrolytic extraction residue represented in mass %, and the crystal grain size represented in μm are substituted for “Nb”, “[Nb]”, and “D”, respectively. 
       
     
     
         2 . The austenitic stainless steel according to  claim 1 , wherein the austenitic stainless steel is a solid member. 
     
     
         3 . The austenitic stainless steel according to  claim 2 , wherein the solid member is a round bar. 
     
     
         4 . The austenitic stainless steel according to  claim 3 , having an outer diameter not smaller than 25 mm. 
     
     
         5 . The austenitic stainless steel according to  claim 1 , wherein the chemical composition includes one or more elements selected from the group consisting of, in mass %:
 0.01 to 3.0% Cu;   0.01 to 0.50% Co;   0.0001 to 0.0050% Ca;   0.0001 to 0.0050% B;   0.01 to 0.10% W; and   0.001 to 0.10% Ta.   
     
     
         6 . The austenitic stainless steel according to  claim 1 , used as a material in equipment for storing or transporting high-pressure hydrogen gas or liquid hydrogen. 
     
     
         7 . A method of manufacturing the austenitic stainless steel according to  claim 1 , comprising:
 performing a primary heat treatment on a steel material having said chemical composition at 1180 to 1280° C.;   after the primary heat treatment, performing cold working on the steel material with a reduction in area not lower than 5% and lower than 20%; and,   after the cold working, performing a secondary heat treatment on the steel material at a temperature of 1000 to 1180° C.   
     
     
         8 . The method of manufacturing the austenitic stainless steel according to  claim 7 , wherein the temperature for the secondary heat treatment is 1050 to 1160° C.