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

A cold-rolled and annealed steel sheet with a strength greater than 1200 MPa, the composition the composition of which comprises, the contents being expresses by weight:

• 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.