Process for manufacturing steel sheet

1. A process for manufacturing a hot-rolled steel sheet having a tensile strength greater than 800 MPa and an elongation at break greater than 10% comprising the steps of:

providing a steel with a composition that 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%; and

Nb≤0.020%;

the balance of the composition comprising iron and inevitable impurities resulting from smelting;

casting a semi-finished product from the steel;

heating the semi-finished product to a temperature above 1150° C.;

hot rolling the semi-finished product to obtain a steel sheet at an end of rolling temperature T ER in a temperature range so microstructure of the steel is entirely austenitic;

cooling the steel sheet at a cooling rate V c from 75 to 200° C./s; and

coiling the steel sheet at a temperature T coil from 500 to 600° C.,

a microstructure of the steel sheet consisting of, as a surface fraction, at least 80% upper bainite, lower bainite, martensite and residual austenite, a sum of the martensite and residual austenite contents being less than 5%.

2. The manufacturing process as recited in claim 1 , wherein the microstructure does not include any ferrite.

3. The manufacturing process as recited in claim 1 , wherein the end of rolling temperature T ER is from 870 to 930° C.

4. The manufacturing process as recited in claim 1 , wherein the cooling rate V c is from 80 to 150° C./s.

5. The manufacturing process as recited in claim 1 , further comprising the step of skin pass rolling the steel sheet.

6. The manufacturing process as recited in claim 5 , further comprising the step of coating the steel sheet with zinc, zinc alloy, aluminum or aluminum alloy.

7. The manufacturing process as recited in claim 6 , wherein the step of coating is hot dip coating.

8. The manufacturing process as recited in claim 5 , wherein deformation from the skin pass rolling is less than 1%.

9. The manufacturing process as recited in claim 1 , further comprising the steps of:

cutting the steel sheet to obtain a blank;

heating the blank, partially or completely, to a temperature T from 400 to 690° C. for a time less than 15 minutes;

drawing the heated blank at a temperature from 350 to T−20° C. to obtain a part; and

cooling the part down to ambient temperature at a further cooling rate of V′ c .

10. The manufacturing process recited in claim 9 , wherein the further cooling rate V′ c is from 25 to 100° C./s.

11. The manufacturing process recited in claim 9 , wherein the part has a thickness of 1 to 5 mm.

12. The manufacturing process recited in claim 9 , further comprising the step of laser welding the part.

13. The manufacturing process recited in claim 1 , wherein the steel composition further includes, as a percentage expressed by weight, Cr≤0.45%.

14. The manufacturing process as recited in claim 1 , wherein the elongation at break is greater than 10% in both the rolling direction and transverse direction.

15. The manufacturing process as recited in claim 1 , wherein the steel sheet has a thickness of 1 to 5 mm thick.

16. The manufacturing process recited in claim 1 , further comprising the step of laser welding the steel sheet.

17. The manufacturing process recited in claim 1 , wherein the elongation at break is from 17 to 23%.

18. The manufacturing process recited in claim 1 , wherein the microstructure is at least 90% upper bainite.

19. The manufacturing process recited in claim 1 , wherein the tensile strength varies by 10 MPa or less.

20. A process for manufacturing a hot-rolled steel sheet having a tensile strength greater than 800 MPa and an elongation at break greater than 10% comprising the steps of:

providing a steel with a composition that 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%; and

Nb≤0.020%;

the balance of the composition comprising iron and inevitable impurities resulting from smelting;

casting a semi-finished product from the steel;

heating the semi-finished product to a temperature above 1150° C.;

hot rolling the semi-finished product to obtain a steel sheet at an end of rolling temperature T ER in a temperature range so a microstructure of the steel is entirely austenitic;

cooling the steel sheet; and

coiling the steel sheet,

a microstructure of the steel sheet consisting of, as a surface fraction, at least 80% upper bainite, lower bainite, martensite and residual austenite, a sum of the martensite and residual austenite contents being less than 5%, the microstructure not including any ferrite.

21. A process for manufacturing a hot-rolled steel sheet having a tensile strength greater than 800 MPa and an elongation at break greater than 10% comprising the steps of:

providing a steel with a composition that comprises, the contents being expressed by weight:

0.050%≤C≤0.090%;

1%≤Mn≤2%

0.015%≤A≤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%; and

Nb≤0.020%;

the balance of the composition comprising iron and inevitable impurities resulting from smelting;

casting a semi-finished product from the steel;

heating the semi-finished product;

hot rolling the semi-finished product to obtain a steel sheet at an end of rolling temperature T ER in a temperature range so a microstructure of the steel is entirely austenitic;

cooling the steel sheet at a cooling rate V c from 75 to 200° C./s; and

coiling the steel sheet,

a microstructure of the steel sheet consisting of, as a surface fraction, at least 80% upper bainite, lower bainite, martensite and residual austenite, a sum of the martensite and residual austenite contents being less than 5%, the microstructure not including any ferrite.