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

A cold-rolled steel sheet with a strength greater than 1400 MPa with an elongation at break greater than 8%, with a composition of steel, by weight, comprising:

• 0.10%≤C≤0.25%
• 1%≤Mn≤3%
• Al≥0.010%
• 1.2%≤Si≤1.8%
• S≤0.015%
• P≤0.1%
• N≤0.008%,
• it being understood that
• 1.2%≤Si+Al≤3%,
• Mo≤0.25%, and
• Cr≤1.65%,
• it being understood that
• Cr+3Mo≥0.3%,
• the composition optionally comprising:
• 0.05%≤V≤0.15%
• B≤0.005%
• Ti in an amount so that Ti/N≥4 and Ti≤0.040%,
• a balance of the composition consisting of iron and inevitable impurities resulting from processing,
• a microstructure of the steel comprising 45 to 65% bainite, the remainder consisting of islands of martensite and residual austenite.

A cold-rolled steel sheet with a strength greater than 1600 MPa with an elongation at break greater than 8%, with a composition of steel, by weight, comprising:

• 0.10%≤C≤0.25%
• 1%≤Mn≤3%
• Al≥0.010%
• 1.2%≤Si≤1.8%
• S≤0.015%
• P≤0.1%
• N≤0.008%,
• it being understood that
• 1.2%≤Si+Al≤3%,
• Mo≤0.25%
• Cr≤1.65%,
• it being understood that
• Cr+3Mo≥0.3%, and
• 0.0005≤B≤0.005%
• the composition optionally comprising:
• 0.05%≤V≤0.15% and
• Ti in an amount so that Ti/N≥4 and Ti≤0.040%,
• a balance of the composition consisting of iron and inevitable impurities resulting from processing,
• a microstructure of the steel comprising 15 to 45% bainite, the remainder consisting of martensite and residual austenite.