Process for manufacturing steel sheet having very high strength, ductility and toughness characteristics, and sheet thus produced
The invention relates to a hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, an R e /R m ratio of less than 0.75 and an elongation at break of greater than 10%, the composition of which contains, the contents being expressed by weight: 0.10%≦C≦0.25%; 1%≦Mn≦3%; Al≧0.015%; Si≦1.985%; Mo≦0.30%; Cr≦1.5%; S≦0.015%; P≦0.1%; Co≦1.5%; B≦0.005%; it being understood that 1%≦Si+Al≦2%; Cr+(3×Mo)≧0.3%, the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of the steel consisting of at least 75% bainite, residual austenite in an amount equal to or greater than 5% and martensite in an amount equal to or greater than 2%.
1. A hot-rolled steel sheet having a tensile strength of greater than 1200 MPa, a yield strength/tensile strength ratio R e /R m of less than 0.75 and an elongation at break of greater than 10%, the composition of which comprises, the contents being expressed by weight:
0.10%≦C≦0.25%
1%≦Mn≦3%
Al≧0.015%
Si≦1.985%
Mo≦0.30%
0.3%<Cr≦1.5%
S≦0.015%
P≦0.1%
Co≦1.5%
B≦0.005%
and wherein
1%≦Si+Al≦2%
Cr+(3×Mo)≧0.3%,
the balance of the composition consisting of iron and inevitable impurities resulting from smelting, the microstructure of said steel consisting of at least 75% per unit area of bainite, residual austenite in an amount equal to or greater than 5% per unit area of and martensite in an amount equal to or greater than 2% per unit area of and containing martensite/residual austenite islands, wherein the number N MA per unit area of said martensite/residual austenite islands having a maximum size L max greater than 2 microns and having an elongation factor (maximum size L max /minimum size L min ) less than 4 is less than 14 000/mm 2 , wherein the steel is obtained by continuous cooling steps after hot rolling.
2. The steel sheet according to claim 1 , the composition of said steel comprises, the content being expressed by weight:
0.10%≦C≦0.15%.
3. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
0.15%<C≦0.17%.
4. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
0.17%<C≦0.22%.
5. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
0.22%<C≦0.25%.
6. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
1%≦Mn≦1.5%.
7. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
1.5%<Mn≦2.3%.
8. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
2.3%<Mn≦3%.
9. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
1.2%≦Si≦1.8%.
10. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
1.2%≦Al≦1.8%.
11. The steel sheet according to claim 1 , characterized in that the composition of said steel comprises, the content being expressed by weight:
Mo≦0.010%.
12. The steel sheet according to claim 1 , wherein the carbon content of the residual austenite is greater than 1% by weight per unit area of.
13. The steel sheet according to claim 1 , comprising carbides between the bainite laths, wherein the number N of said interlath carbides of size greater than 0.1 micron per unit area is equal to 50 000/mm 2 or less.
14. The steel sheet according to claim 1 , wherein the steel undergoes cooling at a primary cooling rate and a secondary cooling rate.
15. The steel sheet according to claim 14 , wherein the primary cooling rate is from 50 to 90° C./s.
16. The steel sheet according to claim 15 , wherein the secondary cooling rate is from 0.8 to 2.0° C./min.
17. The steel sheet according to claim 15 , wherein the secondary cooling rate is greater than 2.0° C./min and does not exceed 600° C./min.