Heat-resistant austenitic stainless steel and a production process thereof

Abstract

To provide a heat-resistant austenitic stainless steel having high-temperature strength and sag-resistance capable of resisting working temperatures of not less than 550° C. as well as being low in cost, and a production process thereof. The steel consistent with the present invention contains not more than 0.1 wt % C, less than 1.0 wt % Si, 1.0 wt % to 10.0 wt % Mn, not more than 0.03 wt % P, not more than 0.01 wt % S, 0.01 wt % to 3.0 wt % Cu, 7.0 wt % to 15.0 wt % Ni, 15.0 wt % to 25.0 wt % Cr, 0.5 wt % to 5.0 wt % Mo, not more than 0.03 wt % Al, 0.4 wt % to 0.8 wt % N, and the remainder substantially consisting of Fe and unavoidable impurities.

1 . A heat-resistant austenitic stainless steel comprising: 
 not more than 0.1 wt % C;    less than 1.0 wt % Si;    1.0 wt % to 10.0 wt % Mn;    not more than 0.03 wt % P;    not more than 0.01 wt % S;    0.01 wt % to 3.0 wt % Cu;    7.0 wt % to 15.0 wt % Ni;    15.0 wt % to 25.0 wt % Cr;    0.5 wt % to 5.0 wt % Mo;    not more than 0.03 wt % Al;    0.4 wt % to 0.8 wt % N, and optionally    at least one element selected from the group consisting of:    not more than 1.0 wt % W; and    not more than 5.0 wt % Co;    at least one element selected from the group consisting of:    0.03 wt % to 0.5 wt % Ti;    0.03 wt % to 0.5 wt % Nb; and    0.03 wt % to 1.0 wt % V;    at least one element selected from the group consisting of:    0.001 wt % to 0.010 wt % B; and    0.01 wt % to 0.10 wt % Zr;    at least one element selected from the group consisting of:    0.001 wt % to 0.010 wt % Ca;    0.001 wt % to 0.010 wt % Mg, and the remainder substantially consisting of Fe and unavoidable impurities.    
     
     
         2 . The heat-resistant austenitic stainless steel according to  claim 1 , wherein PN is not less than 60 when expressed by the following equation:  
           PN= 2.4Mn—Cu−0.6Ni+3Cr+0.8Mo (wt %).  
     
     
         3 . The heat-resistant austenitic stainless steel according to  claim 2 , wherein the heat-resistant austenitic stainless steel is subjected to solution treatment, cold working at a cold working ratio of 40% to 70%, and aging treatment at 400° C. to 650° C. for not less than one minute.  
     
     
         4 . The heat-resistant austenitic stainless steel according to  claim 3 , wherein primary hardness at room temperature is not less than 45 HRC.  
     
     
         5 . The heat-resistant austenitic stainless steel according to  claim 4 , wherein 
 room-temperature hardness after 400-hour heat treatment at 600° C. is 45 HRC, and    room-temperature hardness after 400-hour heat treatment at 700° C. is 40 HRC.    
     
     
         6 . The heat-resistant austenitic stainless steel according to  claim 5 , wherein a residual stress ratio after a 50-hour relaxation test at 700° C. is not less than 25%.  
     
     
         7 . The heat-resistant austenitic stainless steel according to  claim 4 , wherein a residual stress ratio after a 50-hour relaxation test at 700° C. is not less than 25%.  
     
     
         8 . The heat-resistant austenitic stainless steel according to  claim 3 , wherein 
 room-temperature hardness after 400-hour heat treatment at 600° C. is 45 HRC, and    room-temperature hardness after 400-hour heat treatment at 700° C. is 40 HRC.    
     
     
         9 . The heat-resistant austenitic stainless steel according to  claim 8 , wherein a residual stress ratio after a 50-hour relaxation test at 700° C. is not less than 25%.  
     
     
         10 . The heat-resistant austenitic stainless steel according to  claim 3 , wherein a residual stress ratio after a 50-hour relaxation test at 700° C. is not less than 25%.  
     
     
         11 . The heat-resistant austenitic stainless steel according to  claim 2 , wherein primary hardness at room temperature is not less than 45 HRC.  
     
     
         12 . The heat-resistant austenitic stainless steel according to  claim 2 , wherein 
 room-temperature hardness after 400-hour heat treatment at 600° C. is 45 HRC, and    room-temperature hardness after 400-hour heat treatment at 700° C. is 40 HRC.    
     
     
         13 . The heat-resistant austenitic stainless steel according to  claim 2 , wherein a residual stress ratio after a 50-hour relaxation test at 700° C. is not less than 25%.  
     
     
         14 . A production process of a heat-resistant austenitic stainless steel comprising the-steps of: 
 applying solution treatment to the heat-resistant austenitic stainless steel according to  claim 2;     providing cold-working at a cold working ratio of 40% to 70% to the heat-resistant austenitic stainless steel subjected to the solution treatment; and    applying aging treatment at temperatures of 400° C. to 650° C. for not less than one minute to the heat-resistant austenitic stainless steel subjected to the cold working.    
     
     
         15 . The heat-resistant austenitic stainless steel according to  claim 1 , wherein the heat-resistant austenitic stainless steel is subjected to solution treatment, cold working at a cold working ratio of 40% to 70%, and aging treatment at 400° C. to 650° C. for not less than one minute.  
     
     
         16 . The heat-resistant austenitic stainless steel according to  claim 15 , wherein primary hardness at room temperature is not less than 45 HRC.  
     
     
         17 . The heat-resistant austenitic stainless steel according to  claim 15 , wherein 
 room-temperature hardness after 400-hour heat treatment at 600° C. is 45 HRC, and    room-temperature hardness after 400-hour heat treatment at 700° C. is 40 HRC.    
     
     
         18 . The heat-resistant austenitic stainless steel according to  claim 15 , wherein a residual stress ratio after a 50-hour relaxation test at 700° C. is not less than 25%.  
     
     
         19 . The heat-resistant austenitic stainless steel according to  claim 1 , wherein primary hardness at room temperature is not less than 45 HRC.  
     
     
         20 . The heat-resistant austenitic stainless steel according to  claim 1 , wherein 
 room-temperature hardness after 400-hour heat treatment at 600° C. is 45 HRC, and room-temperature hardness after 400-hour heat treatment at 700° C. is 40 HRC.    
     
     
         21 . The heat-resistant austenitic stainless steel according to  claim 1 , wherein a residual stress ratio after a 50-hour relaxation test at 700° C. is not less than 25%.  
     
     
         22 . A production process of a heat-resistant austenitic stainless steel comprising the steps of: 
 applying solution treatment to the heat-resistant austenitic stainless steel according to  claim 1;     providing cold-working at a cold working ratio of 40% to 70% to the heat-resistant austenitic stainless steel subjected to the solution treatment; and    applying aging treatment at temperatures of 400° C. to 650° C. for not less than one minute to the heat-resistant austenitic stainless steel subjected to the cold working.