Process for manufacturing sheet of austenitic stainless steel having high mechanical properties and sheet thus obtained

Abstract

The invention relates to a hot-rolled sheet made of austenitic stainless steel, the chemical composition of which comprises, the contents being expressed by weight: 0.015%≦C≦0.030%, 0.5%≦Mn≦2%, Si≦2%, 16.5%≦Cr≦18%, 6%≦Ni≦7%, S≦0.015%, P≦0.045%, Al≦0.050%, 0.15%≦Nb≦0.31%, 0.12%≦N≦0.16%, the Nb and N contents being such that: Nb/8+0.1%≦N≦Nb/8+0.12%, optionally: Mo≦0.6%, 0.0005%≦B≦0.0025%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting.

1 . A hot-rolled sheet comprising austenitic stainless steel, the product P (R p0.2  (MPa)×uniform elongation (%)) of which is greater than 21000 MPa. % and the chemical composition of which comprises, the contents being expressed by weight:
 0.015%≦C≦0.030% 
 0.5%≦Mn≦2% 
 Si≦2% 
 16.5%≦Cr≦18% 
 6%≦Ni≦7% 
 S≦0.015% 
 P≦0.045% 
 Al≦0.050% 
 0.15%≦Nb≦0.31% 
 0.12%≦N≦0.16% 
 the Nb and N contents being such that: 
 Nb/8+0.1%≦N≦Nb/8+0.12% 
 optionally: 
 0.0005%≦B≦0.0025% 
 Mo≦0.6%, 
 the balance of the composition consisting essentially of iron and inevitable impurities resulting from the smelting. 
 
     
     
         2 . The hot-rolled sheet according to  claim 1 , wherein the niobium and nitrogen contents of said steel, expressed by weight, are such that:
 0.20%≦Nb≦0.31%   0.12%≦N≦0.16%.   
     
     
         3 . The hot-rolled sheet according to  claim 1 , having a yield strength R p0.2  greater than 650 MPa, wherein the mean austenitic grain size of said steel is less than 6 microns, the non-recrystallized surface fraction is between 30 and 70% and the niobium is completely in the form of precipitates. 
     
     
         4 . The hot-rolled sheet according to  claim 1 , having a uniform elongation greater than 45%, wherein the niobium is not completely precipitated. 
     
     
         5 . A process for manufacturing a hot-rolled sheet comprising austenitic stainless steel, the yield strength R p0.2  of which is greater than 650 MPa, comprising:
 heating a semi-finished product comprising steel having the composition according to  claim 1  to a temperature of between 1250° C. and 1320° C.; and then   rolling said semi-finished product with an end-of-rolling temperature below 990° C. and a cumulative reduction ratio ε on the last two finishing stands of greater than 30%.   
     
     
         6 . The manufacturing process according to  claim 5 , wherein the steel of the semi-finished product comprising steel comprises niobium and nitrogen in the following amounts, expressed by weight:
 0.20%≦Nb≦0.31%   0.12%≦N≦0.16%,   and said semi-finished product is rolled with an end-of-rolling temperature below 970° C.   
     
     
         7 . A process for manufacturing a hot-rolled sheet comprising austenitic stainless steel, the uniform elongation of which is greater than 45%, comprising:
 heating a semi-finished product comprising steel having the composition according to  claim 1  to a temperature of between 1250° C. and 1320° C.; and then rolling said semi-finished product with an end-of-rolling temperature above 1000° C.   
     
     
         8 . A process for manufacturing a hot-rolled sheet comprising austenitic stainless steel, the product P (R p0.2  (MPa)×uniform elongation (%)) of which is greater than 21000 MPa. %, comprising:
 heating a semi-finished product made of a steel having the composition according to  claim 1  to a temperature of between 1250° C. and 1320° C.; and then hot-rolling said semi-finished product. 
 
     
     
         9 . (canceled) 
     
     
         10 . The hot-rolled sheet of  claim 1 , the balance of the composition consisting of iron and inevitable impurities resulting from the smelting.