Martensitic stainless steel, part made in said steel and method for manufacturing same

Abstract

A martensitic stainless steel is provided, wherein its composition is: trace amounts≦C≦0.030%; trace amounts≦Si≦0.25%; trace amounts≦Mn≦0.25%; trace amounts≦S≦0.020%; trace amounts≦P≦0.040%; 8%≦Ni≦14%; 8%≦Cr≦14%; 1.5%≦Mo+W/2≦3.0%; 1.0%≦Al≦2.0%; 0.5%≦Ti≦2.0%; 2%≦Co≦9%; trace amounts≦N≦0.030%; trace amounts≦O≦0.020%; the remainder being iron and impurities resulting from the steelmaking; and its martensitic transformation beginning temperature Ms is calculated by the formula Ms (° C.)=1302−28Si−50Mn−63Ni−42Cr−30Mo+20Al−12Co−25Cu+10[Ti−4(C+N)]  (1) wherein the contents of the various elements are expressed in weight percentages, is greater than or equal to 50° C., preferably greater than or equal to 75° C. A part made in this steel and its manufacturing method are also provided.

1 - 28 . (canceled) 
     
     
         29 : A martensitic stainless steel, comprising a composition, in weight percentages:
 trace amounts≦C≦0.030%;   trace amounts≦Si≦0.25%;   trace amounts≦Mn≦0.25%;   trace amounts≦S≦0.020%;   trace amounts≦P≦0.040%;   8%≦Ni≦14%;   8%≦Cr≦14%;   1.5%≦Mo+W/2≦3.0%;   1.0%≦Al≦2.0%;   0.5%≦Ti≦2.0%;   2%≦Co≦9%;   trace amounts≦N≦0.030%;   trace amounts≦O≦0.020%;   
       the remainder being iron and impurities resulting from the steelmaking; 
       and its martensitic transformation beginning temperature Ms calculated by the formula
     Ms  (° C.)=1302−28Si−50Mn−63Ni−42Cr−30Mo+20Al−12Co−25Cu+10[Ti−4(C+N)]  (1)
 
 
       wherein the contents of the different elements are expressed in weight percentages, is greater than or equal to 50° C., preferably greater than or equal to 75° C. 
     
     
         30 : The martensitic stainless steel according to  claim 29  wherein trace amounts≦C≦0.010%. 
     
     
         31 : The martensitic stainless steel according to  claim 29  wherein trace amounts≦Si≦0.10%. 
     
     
         32 : The martensitic stainless steel according to  claim 29  wherein trace amounts≦Mn≦0.10%. 
     
     
         33 : The martensitic stainless steel according to  claim 29  wherein trace amounts≦S≦0.005%. 
     
     
         34 : The martensitic stainless steel according to  claim 29  wherein trace amounts≦P≦0.020%. 
     
     
         35 : The martensitic stainless steel according to  claim 29  wherein 11.3%≦Ni≦12.5%. 
     
     
         36 : The martensitic stainless steel according to  claim 29  wherein 8.5%≦Cr≦10%. 
     
     
         37 : The martensitic stainless steel according to  claim 29  wherein 1.5≦Mo+W/2≦2.5%. 
     
     
         38 : The martensitic stainless steel according to  claim 29  wherein 1.0%≦Al≦1.5%. 
     
     
         39 : The martensitic stainless steel according to  claim 29  wherein 1.10%≦Ti≦1.55%. 
     
     
         40 : The martensitic stainless steel according to  claim 29  wherein 2.5%≦Co≦6.5%. 
     
     
         41 : The martensitic stainless steel according to  claim 40  wherein 2.50%≦Co≦3.50%. 
     
     
         42 : The martensitic stainless steel according to  claim 29  wherein trace amounts≦N≦0.0060%. 
     
     
         43 : The martensitic stainless steel according to  claim 29  wherein trace amounts≦O≦0.0050%. 
     
     
         44 : The martensitic stainless steel according to  claim 29  wherein 1.05%≦Al≦2.0%. 
     
     
         45 : The martensitic stainless steel according to  claim 44  wherein 1.05%≦Al≦1.5%. 
     
     
         46 : The martensitic stainless steel according to  claim 29  wherein Creq/Nieq≦1.05, with
   Cr eq.=Cr+2Si+Mo+1.5Ti+5.5Al+0.6W 
   Ni eq.=2Ni+0.5Mn+30C+25N+Co+0.3Cu 
 
     
     
         47 : The martensitic stainless steel according to  claim 29  wherein the proportion of delta ferrite in its microstructure is less than or equal to 1%. 
     
     
         48 : A method for manufacturing a part in martensitic stainless steel comprising:
 preparing a semi-finished product in steel having the composition according to  claim 29  by one of the following methods:
 a liquid steel having the composition according to  claim 29 , is prepared and from the liquid steel, an ingot is cast and solidified and is transformed into a semi-finished product by at least one hot transformation; 
 a sintered semi-finished product in a steel having the composition according to  claim 29  is prepared by power metallurgy; 
   achieving a complete solution heat treatment of the semi-finished product in the austenitic domain, at a temperature comprised between 800 and 940° C.;   quenching of the semi-finished product down to a final quenching temperature of less than or equal to −60° C.; and   carrying out an ageing between 450 and 600° C. for 4 to 32 h.   
     
     
         49 : The method according to  claim 48  wherein the quenching of the semi-finished product is performed down to a final quenching temperature of less than or equal to −75° C. 
     
     
         50 : The method according to  claim 48  wherein an ingot is cast and solidified, and between the solidification of the ingot and the solution heat treatment of the semi-finished product, homogenization of the ingot or of the semi-finished product is carried out at 1,200-1,300° C. for at least 24 h. 
     
     
         51 : The method according to  claim 48  wherein between the quenching and the ageing, a cold transformation of the semi-finished product is achieved. 
     
     
         52 : The method according to  claim 48  wherein the quenching is carried out in two steps, in two different quenching media. 
     
     
         53 : The method according to  claim 52  wherein the first quenching step is carried out in water. 
     
     
         54 : The method according to  claim 48  wherein the liquid steel is prepared by a dual treatment by melting in vacuo, the second treatment in vacuo being an ESR or VAR re-melting treatment. 
     
     
         55 : A martensitic stainless steel part, wherein it was prepared by the method according to  claim 48 . 
     
     
         56 : The martensitic stainless steel part according to  claim 55  wherein the martensitic stainless steel part is an aeronautical structural part.