Process for manufacturing cold-rolled and annealed steel sheet with a very high strength, and sheet thus produced

Abstract

The invention relates to a cold-rolled and annealed steel sheet with a strength greater than 1200 MPa, the composition of which comprises, the contents being expressed by weight: 0.10%≦C≦0.25%, 1%≦Mn≦3%, Al≧0.010%, Si≦2.990%, S≦0.015%, P≦0.1%, N≦0.008%, it being understood that 1%≦Si+Al≦3%, the composition optionally comprising: 0.05%≦V≦0.15%, B≦0.005%, Mo≦0.25%, Cr≦1.65%, it being understood that Cr+3Mo≧0.3%, Ti in an amount such that Ti/N≧4 and Ti≦0.040%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of the steel comprising 15 to 90% bainite, the remainder consisting of martensite and residual austenite.

1 . A cold-rolled and annealed steel sheet with a strength greater than 1200 MPa, the composition by weight comprising:
 0.10%≦C≦0.25%   1%≦Mn≦3%   Al≧0.010%   Si≦2.990%   S≦0.015%   P≦0.1% and   N≦0.008%,   it being understood that   1%≦Si+Al≦3%,   the composition optionally comprising:   0.05%≦V≦0.15%   B≦0.005%   Mo≦0.25%   Cr≦1.65%,   it being understood that   Cr+3Mo≧0.3%, and   Ti in an amount so that Ti/N≧4 and Ti≦0.040%,   
       the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of said steel comprising 15 to 90% bainite, the remainder consisting of islands of martensite and residual austenite. 
     
     
         2 . The steel sheet as claimed in  claim 1 , with an elongation at break greater than 10%, wherein:
 Mo<0.005%   Cr<0.005% and   B=0%,   
       the microstructure of said steel comprising 65 to 90% bainite, the remainder consisting of islands of martensite and residual austenite. 
     
     
         3 . The steel sheet as claimed in  claim 1 , wherein it contains:
 Mo0.25% and   Cr1.65%,   it being understood that   Cr+3Mo≧0.3%, and   B=0%,   
       the microstructure of said steel comprising 65 to 90% bainite, the remainder consisting of islands of martensite and residual austenite. 
     
     
         4 . The steel sheet as claimed in  claim 1 , with a strength greater than 1400 MPa and an elongation at break greater than 8%, wherein it contains:
 Mo≦0.25% and   Cr≦1.65%,   it being understood that   Cr+3Mo≧0.3%,   
       the microstructure of said steel comprising 45 to 65% bainite, the remainder consisting of islands of martensite and residual austenite. 
     
     
         5 . The steel sheet as claimed in  claim 1 , with a strength greater than 1600 MPa and an elongation at break greater than 8%, wherein it contains:
 Mo≦0.25% and   Cr≦1.65%,   it being understood that   Cr+3Mo≧0.3%,   
       the microstructure of said steel comprising 15 to 45% bainite, the remainder consisting of martensite and residual austenite. 
     
     
         6 . The steel sheet as claimed in  claim 1 , wherein the composition by weight of said steel comprises:
 0.19%≦C≦0.23%.   
     
     
         7 . The steel sheet as claimed in  claim 1 , wherein the composition by weight of said steel comprises:
 1.5%≦Mn≦2.5%.   
     
     
         8 . The steel sheet as claimed in  claim 1 , wherein the composition by weight of said steel comprises:
 1.2%≦Si≦1.8%.   
     
     
         9 . The steel sheet as claimed in  claim 1 , wherein the composition by weight of said steel comprises:
 1.2% Al≦1.8%.   
     
     
         10 . The steel sheet as claimed in  claim 1 , wherein the composition by weight of said steel comprises:
 0.05%≦V≦0.15%   0.004≦N≦0.008%.   
     
     
         11 . The steel sheet as claimed in  claim 1 , wherein the composition by weight of said steel comprises:
 0.12%≦V≦0.15%.   
     
     
         12 . The steel sheet as claimed in  claim 1 , wherein the composition by weight of said steel comprises:
 0.0005≦B≦0.003%.   
     
     
         13 . The steel sheet as claimed in  claim 1 , wherein the average size of said islands of martensite and residual austenite is less than 1 micron, and the average distance between said islands is less than 6 microns. 
     
     
         14 . A process for manufacturing a cold-rolled steel sheet with a strength greater than 1200 MPa and an elongation at break greater than 10%, in which:
 a steel is provided having a composition as claimed in  claim 2 ; then   a semifinished product is cast from this steel; then   said semifinished product is brought to a temperature greater than 1150° C.; then   said semifinished product is hot-rolled so as to obtain a hot-rolled sheet; then   said sheet is coiled; then   said hot-rolled sheet is pickled; then   said sheet is cold-rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet; and then   said cold-rolled sheet is reheated at a rate V c , between 5 and 15° C./s up to a temperature T 1  between Ac3 and Ac3+20° C., and held there for a time t 1  between 50 and 150 s, then said sheet is cooled at a rate V R1  greater than 40° C./s but below 100° C./s down to a temperature T 2  between (M S −30° C.) and (M S +30° C.), said sheet is maintained at said temperature T 2  for a time t 2  between 150 and 350 s and then it is cooled at a rate V R2  of less than 30° C./s down to the ambient temperature.   
     
     
         15 . A process for manufacturing a cold-rolled steel sheet with a strength greater than 1200 MPa and an elongation at break greater than 8%, in which:
 a steel is provided having a composition as claimed in  claim 1 , the Mo and Cr contents being Mo≦0.25% and Cr≦1.65%, it being understood that: Cr+3Mo≧0.3%; then   a semifinished product is cast from this steel; then   said semifinished product is brought to a temperature greater than 1150° C.; then   said semifinished product is hot-rolled so as to obtain a hot-rolled sheet; then   said sheet is coiled; then   said hot-rolled sheet is pickled; then   said sheet is cold-rolled with a reduction ratio of between 30 and 80% to obtain a cold-rolled sheet; and then   said cold-rolled sheet is reheated at a rate V c  between 5 and 15° C./s up to a temperature T 1  between Ac3 and Ac3+20° C., and held there for a time t 1  between 50 and 150 s, then said sheet is cooled at a rate V R1  greater than 25° C./s but below 100° C./s down to a temperature T 2  between B S  and (M S −20° C.), said sheet is maintained at said temperature T 2  for a time t 2  between 150 and 350 s and then it is cooled at a rate V R2  of less than 30° C./s down to the ambient temperature.   
     
     
         16 . The manufacturing process as claimed in  claim 14  wherein the temperature T 1  is between Ac3+10° C. and Ac3+20° C. 
     
     
         17 . (canceled) 
     
     
         18 . The manufacturing process as claimed in  claim 15 , wherein the temperature T 1  is between Ac3+10° C. and Ac3+20° C. 
     
     
         19 . A structural part or reinforcing element in the automotive field comprising the cold-rolled and annealed steel sheet as claimed in  claim 1 . 
     
     
         20 . A structural part or reinforcing element in the automotive field comprising the cold-rolled steel sheet manufactured by the process as claimed in  claim 14 . 
     
     
         21 . A structural part or reinforcing element in the automotive field comprising the cold-rolled steel sheet manufactured by the process as claimed in  claim 15 .