Process for manufacturing steel sheet having high tensile strength and ductility characteristics, and sheet thus produced

Abstract

The invention relates to a hot-rolled steel sheet having a tensile strength greater than 800 MPa and an elongation at break greater than 10%, the composition of which comprises, the contents being expressed by weight: 0.050%≦C≦0.090%, 1%≦Mn≦2%, 0.015%≦Al≦0.050%, 0.1%≦Si≦0.3%, 0.10%≦Mo≦0.40%, S≦0.010%, P≦0.025%, 0.003%≦N≦0.009%, 0.12%≦V≦0.22%, Ti≦0.005%, Nb≦0.020% and, optionally, Cr≦0.45%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of the sheet or the part comprising, as a surface fraction, at least 80% upper bainite, the possible complement consisting of lower bainite, martensite and residual austenite, the sum of the martensite and residual austenite contents being less than 5%.

1 . Hot-rolled steel sheet or part having a tensile strength greater than 800 MPa and an elongation at break greater than 10%, the composition of which comprises, the contents being expressed by weight:
   0.050%≦C≦0.090%     1%≦Mn≦2%     0.015%≦Al≦0.050%     0.1%≦Si≦0.3%     0.10%≦Mo≦0.40%     S≦0.010%     P≦0.025%     0.003%≦N≦0.009%     0.12%≦V≦0.22%     Ti≦0.005%     Nb≦0.020%   and, optionally,
   Cr≦0.45% 
   
     the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of said sheet or said part comprising, as a surface fraction, at least 80% upper bainite, the possible complement consisting of lower bainite, martensite and residual austenite, the sum of the martensite and residual austenite contents being less than 5%. 
   
   
       2 . Steel sheet or part according to  claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
   0.050%≦C≦0.070%.   
   
   
       3 . Steel sheet or part according to  claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
   0.070%<C≦0.090%.   
   
   
       4 . Steel sheet or part according to any one of  claims 1  to  3 , characterized in that the composition of said steel comprises, the content being expressed by weight:
   1.4%≦Mn≦1.8%.   
   
   
       5 . Steel sheet or part according to any one of  claims 1  to  4 , characterized in that the composition of said steel comprises, the content being expressed by weight:
   0.020%≦Al≦0.040%.   
   
   
       6 . Steel sheet or part according to any one of  claims 1  to  5 , characterized in that the composition of said steel comprises, the content being expressed by weight:
   0.12%≦V≦0.16%.   
   
   
       7 . Steel sheet or part according to any one of  claims 1  to  6 , characterized in that the composition of said steel comprises, the content being expressed by weight:
   0.18%≦Mo≦0.30%.   
   
   
       8 . Steel sheet or part according to any one of  claims 1  to  7 , characterized in that the composition of said steel comprises, the content being expressed by weight:
   Nb≦0.005%.   
   
   
       9 . Steel sheet or part according to any one of  claims 1  to  8 , characterized in that the composition of said steel comprises, the content being expressed by weight:
   0.20%≦Cr≦0.45%.   
   
   
       10 . Steel sheet or part according to any one of  claims 1  to  9 , characterized in that said sheet or said part is coated with a zinc-based or aluminium-based coating. 
   
   
       11 . Steel part with a composition and a microstructure according to any one of  claims 1  to  9 , characterized in that it results from a process that includes heating at a temperature T of between 400 and 690° C., then warm-drawing in a temperature range of between 350° C. and (T-20° C.) and then cooling down to ambient temperature. 
   
   
       12 . Welded assembly produced from at least one steel sheet or part according to any one of  claims 1  to  11 , characterized in that at least one sheet or part is welded by a high-energy-density beam. 
   
   
       13 . Process for manufacturing a hot-rolled steel sheet having a tensile strength greater than 800 MPa and an elongation at break greater than 10%, in which:
 a steel with the composition according to any one of  claims 1  to  9  is provided;   a semi-finished product is cast from this steel;   said semi-finished product is heated to a temperature above 1150° C.;   said semi-finished product is hot-rolled to a temperature T ER  in a temperature range in which the microstructure of the steel is entirely austenitic so as to obtain a sheet;   said sheet is cooled in such a way that the cooling rate V c  is between 75 and 200° C./s; and then   said sheet is coiled at a temperature T coil  of between 500 and 600° C.   
   
   
       14 . Process for manufacturing a hot-rolled steel sheet according to  claim 13 , characterized in that the end-of-rolling temperature T ER  is between 870 and 930° C. 
   
   
       15 . Process for manufacturing a hot-rolled steel sheet according to  claim 13  or  14 , characterized in that the cooling rate V c  is between 80 and 150° C./s. 
   
   
       16 . Manufacturing process in which a sheet manufactured according to any one of  claims 13  to  15  is pickled, then optionally skin-passed and then coated with zinc or a zinc alloy or else with aluminium or an aluminium alloy. 
   
   
       17 . Process for manufacturing a steel sheet according to  claim 16 , characterized in that said coating is carried out continuously by hot-dip coating. 
   
   
       18 . Process for manufacturing a warm-drawn part, characterized in that:
 a steel sheet according to any one of  claims 1  to  10 , or manufactured by a process according to any one of  claims 13  to  17 , is provided; then   said sheet is cut so as to obtain a blank; then   said blank is partly or completely heated to a temperature T of between 400 and 690° C., where it is maintained for a time of less than 15 minutes so as to obtain a heated blank; then   said heated blank is drawn at a temperature of between 350 and T-20° C. in order to obtain a part; and then   said part is cooled down to ambient temperature at a rate V′ c .   
   
   
       19 . Manufacturing process according to  claim 18 , characterized in that the rate V′ c  is between 25 and 100° C./s. 
   
   
       20 . Use of a hot-rolled steel sheet according to any one of  claims 1  to  10 , or manufactured by a process according to any one of  claims 13  to  19 , for the manufacture of structural parts or reinforcing elements in the automotive field.