Steel sheet with high mechanical strength, ductility and formability properties, production method and use of such sheets

1. A cold rolled and annealed steel sheet, the composition of which, expressed in per cent by weight, comprises:

0.15%≦C≦0.25%

1.8%≦Mn≦3.0%

1.2%≦Si≦2%

Al≦0.10%

0%≦Cr≦0.50%

0%≦Cu≦1%

0%≦Ni≦1%

S≦0.005%

P≦0.020%

Nb≦0.015%

Ti≦0.020%

V≦0.015%

Co≦1%

N≦0.008%; and

B≦0.001%

whereby Mn+Ni+Cu≦3%;

the remainder of the composition consisting of iron and the inevitable impurities resulting from processing,

a microstructure of the composition including, in area proportions, 5 to 20% polygonal ferrite, 10 to 15% residual austenite, 5 to 15% martensite, the balance being bainite in the form of laths and including, between these laths, inter-lath carbides such that a number N of the inter-lath carbides larger than 0.1 micrometer per unit of surface area is less than or equal to 50000/mm 2 ,

the steel sheet having a mechanical strength greater than or equal to 1000 MPa and a uniform elongation greater than or equal to 12%.

2. The steel sheet as recited in claim 1 , wherein the composition includes, expressed in per cent by weight, 0.18%≦C≦0.22%.

3. The steel sheet as recited in claim 1 , wherein the composition includes, expressed in per cent by weight, 2%≦Mn≦2.6%.

4. The steel sheet as recited in claim 1 , wherein the composition includes, expressed in per cent by weight, 1.4%≦Si≦1.8%.

5. The steel sheet as recited in claim 1 , wherein the composition includes, expressed in per cent by weight, 0%≦Cr≦0.35%.

6. The steel sheet as recited in claim 1 , wherein a V-bend angle at which cracking occurs is greater than or equal to 90°.

7. The steel sheet as recited in claim 1 , further comprising a zinc or zinc-alloy coating.

8. A method for fabricating the cold rolled and annealed steel sheet as recited in claim 1 , the method comprising the steps of:

obtaining the steel sheet recited in claim 1 ;

casting the steel sheet in the form of a semi-finished product;

bringing the semi-finished product to a temperature T rech which is greater than 1150° C. to obtain a reheated semi-finished product;

hot rolling the reheated semi-finished product to obtain a hot-rolled sheet, the temperature at the end of the hot rolling T f1 being greater than or equal to 850° C.;

coiling the hot rolled sheet at a temperature T coil to obtain a coiled, hot-rolled sheet, T coil being in a range from 540 to 590° C.;

cooling the coiled, hot-rolled sheet to an ambient temperature;

performing a batch annealing of the coiled, hot-rolled sheet so a mechanical strength for the coiled, hot-rolled sheet is less than or equal to 1000 MPa;

uncoiling and pickling the annealed, coiled, hot-rolled sheet to obtain a hot-rolled sheet suitable for cold rolling;

cold rolling the hot-rolled sheet suitable for cold rolling at a rate of reduction in a range from 30 to 80%, to obtain a cold-rolled sheet;

annealing the cold rolled sheet by reheating the cold rolled sheet at a rate Vc which is from 2 to 6° C. per second to a temperature T 1 which is between Ac3-15° C. and Ac3-45° C. for a period of time t 1 which is greater than or equal to 50 seconds;

cooling the cold-rolled sheet by subjecting the sheet to a first cooling step at a rate of cooling from 50 to 1500° C./s, then to a second cooling step such that the temperature at the end of the second cooling step T fr is between a temperature T fr1 equal to Ms-20° C. and a temperature T fr2 equal to MS+40° C.;

holding the cold-rolled sheet in a temperature range from T fr1 to T fr2 for a length of time t 2 between 100 and 1000 seconds;

cooling the cold-rolled sheet at a rate V R2 from 3 to 25° C. per second to the ambient temperature to obtain a cold-rolled and annealed sheet.

9. The fabrication method as recited in claim 8 , wherein the temperature Tfl is greater than or equal to 900° C.

10. The fabrication method as recited in claim 8 , wherein the step of batch annealing includes a hold at a temperature Tm and a time tm such that:

2.29×Tm(° C.)+18.6×tm( h )≧1238,

and the hold temperature Tm is greater than 410° C.

11. The fabrication method as recited in claim 8 , wherein the temperature T 1 is between 790° C. and 820° C.

12. The fabrication method as recited in claim 8 , further comprising the step of coating the annealed, cold-rolled sheet with zinc or a zinc alloy.

13. The fabrication method as recited in claim 8 , wherein the sheet is annealed at a hold temperature T base in a range from 150° C. to 200° C. for a hold time t base in a range from 10 h to 48 h.

14. A ground-based motor vehicle comprising:

a cold-rolled and annealed sheet as recited in claim 1 .

15. A ground-based motor vehicle comprising:

a cold-rolled and annealed sheet fabricated by the method recited in claim 8 .

16. A method of fabricating a ground-based motor vehicle comprising:

incorporating into the motor vehicle at least one cold rolled and annealed sheet recited claim 1 .

17. The fabrication method recited in claim 8 further comprising the step of:

incorporating the cold-rolled and annealed sheet into a motor vehicle.