Austenitic stainless steel and method for producing austenitic stainless steel

Abstract

Provided is an austenitic stainless steel which enables both a reduction in load of working during production and an increase in strength of an end product and can be produced with high productivity. The austenitic stainless steel: contains more than 0.03% and not more than 0.15% of C, 0.1-2.0% of Si, 0.3-2.5% of Mn, not more than 0.04% of P, not more than 0.015% of S, 3.0-6.0% of Ni, 16.0-18.5% of Cr, 1.5-4.0% of Cu, and 0.005-0.25% of N, in percent by mass, and the other part composed of Fe and an inevitable impurity; includes not less than 20% by volume of an austenite phase, a Cu-rich phase having a number density of not less than 1.0×103 μm−3 and a long diameter of not more than 30 nm, and a remaining part composed of a deformation-induced martensite phase and an inevitably formed phase; and has an Md30 value of 10.0-80.0.

1 . An austenitic stainless steel, comprising more than 0.03% and not more than 0.15% of C, not less than 0.1% and not more than 2.0% of Si, not less than 0.3% and not more than 2.5% of Mn, not more than 0.04% of P, not more than 0.015% of S, not less than 3.0% and less than 6.0% of Ni, not less than 16.0% and not more than 18.5% of Cr, not less than 1.5% and not more than 4.0% of Cu, and not less than 0.005% and not more than 0.25% of N, in percent by mass, and the other part composed of Fe and an inevitable impurity, the austenitic stainless steel including (i) not less than 20% by volume of an austenite phase, (ii) a Cu-rich phase having a number density of not less than 1.0×10 3  μm −3  and a long diameter of not more than 30 nm, and (iii) a remaining part composed of a deformation-induced martensite phase and an inevitably formed phase,
 the austenitic stainless steel having an Md 30  value of not less than 10.0 and not more than 80.0 as represented by expression (1) below, 
 
       
         
           
             
               
                 
                   
                     
                       Md 
                       
                         3 
                         ⁢ 
                         0 
                       
                     
                     = 
                     
                       551 
                       - 
                       
                         462 
                         ⁢ 
                         
                           ( 
                           
                             C 
                             + 
                             N 
                           
                           ) 
                         
                       
                       - 
                       
                         9.2 
                           
                         Si 
                       
                       - 
                       
                         8.1 
                         Mn 
                       
                       - 
                       
                         29 
                         ⁢ 
                         Ni 
                       
                       - 
                       
                         10.6 
                           
                         Cu 
                       
                       - 
                       
                         1 
                           
                         3.7 
                           
                         Cr 
                       
                       - 
                       
                         18.5 
                           
                         Mo 
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
       wherein in expression (1) above, a content, in percent by mass, of each element contained in the austenitic stainless steel is substituted into a corresponding element symbol, and 0 is substituted into an element symbol of an element not added to the austenitic stainless steel. 
     
     
         2 . The austenitic stainless steel as set forth in  claim 1 , further comprising at least one selected from the group consisting of: not more than 1.0% of Mo; not more than 1.0% of W; not more than 0.5% of V; not less than 0.0001% and not more than 0.01% of B; not more than 0.8% of Co; not more than 0.1% of Sn; not more than 0.03% of Ca; not more than 0.03% of Mg; not more than 0.5% of Ti; not more than 0.5% of Nb; not more than 0.3% of Al; not more than 0.5% of Sb; not more than 0.5% of Zr; not more than 0.03% of Ta; not more than 0.03% of Hf; and not more than 0.2% of a rare earth metal, REM, in percent by mass. 
     
     
         3 . The austenitic stainless steel as set forth in  claim 1 , having an average crystal grain size of not more than 10.0 μm. 
     
     
         4 . A method for producing an austenitic stainless steel, the austenitic stainless steel containing more than 0.03% and not more than 0.15% of C, not less than 0.1% and not more than 2.0% of Si, not less than 0.3% and not more than 2.5% of Mn, not more than 0.04% of P, not more than 0.015% of S, not less than 3.0% and less than 6.0% of Ni, not less than 16.0% and not more than 18.5% of Cr, not less than 1.5% and not more than 4.0% of Cu, and not less than 0.005% and not more than 0.25% of N, in percent by mass, and the other part composed of Fe and an inevitable impurity, the austenitic stainless steel having an Md 30  value of not less than 10.0 and not more than 80.0 as represented by formula (1) below,
 said method comprising a finishing annealing step of carrying out finishing annealing at a temperature of not lower than 750° C. and not higher than 980° C.,   wherein in a case where a peak temperature in the finishing annealing step is not lower than 850° C., time during which heating is carried out at not lower than 850° C. is not more than 30 seconds,   
       
         
           
             
               
                 
                   
                     
                       Md 
                       
                         3 
                         ⁢ 
                         0 
                       
                     
                     = 
                     
                       551 
                       - 
                       
                         462 
                         ⁢ 
                         
                           ( 
                           
                             C 
                             + 
                             N 
                           
                           ) 
                         
                       
                       - 
                       
                         9.2 
                           
                         Si 
                       
                       - 
                       
                         8.1 
                         Mn 
                       
                       - 
                       
                         29 
                         ⁢ 
                         Ni 
                       
                       - 
                       
                         10.6 
                           
                         Cu 
                       
                       - 
                       
                         1 
                           
                         3.7 
                           
                         Cr 
                       
                       - 
                       
                         18.5 
                           
                         Mo 
                       
                     
                   
                 
                 
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
       wherein in expression (1) above, a content, in percent by mass, of each element contained in the austenitic stainless steel is substituted into a corresponding element symbol, and 0 is substituted into an element symbol of an element not added to the austenitic stainless steel. 
     
     
         5 . The austenitic stainless steel as set forth in  claim 2 , having an average crystal grain size of not more than 10.0 μm.