Method for making a steel part of multiphase microstructure

1. A process for manufacturing a part made of steel having a multiphase microstructure, said microstructure comprising ferrite and being homogeneous in each region of said part, which process comprises:

cutting a blank from a strip of steel, the composition of which comprises, in % by weight:

0.05≦C≦0.50%

0.50≦Mn≦3.0%

0.001≦Si≦3.0%

0.005≦Al≦3.0%

Mo≦1.0%

Cr≦1.50%

Ni≦2.0%

Cu≦2.0%

P≦0.10%

S≦0.05%

Ti≦0.20%

V≦1.0%,

the balance of the composition being iron and impurities resulting from smelting; the microstructure of the steel, after the part has been cooled, is a TRIP multiphase microstructure comprising ferrite, residual austenite and optionally martensite and/or bainite;

optionally, said blank undergoes prior cold deformation;

said blank is heated so as to reach a soak temperature T s above Ac1 but below Ac3 and held at this soak temperature T s for a soak time t s adjusted so that the steel, after the blank has been heated, has an austenite content equal to or greater than 25% by area;

said heated blank is transferred into a forming tool so as to hot-form said part; and

the part is cooled within the tool at a cooling rate V such that the microstructure of the steel, after the part has been cooled, is a multiphase microstructure, said microstructure comprising ferrite and being homogeneous in each region of said part.

2. The process as claimed in claim 1 , wherein the microstructure of the steel, after the part has been cooled, is a multiphase microstructure with a ferrite content equal to or greater than 25% by area.

3. The process as claimed in claim 1 , wherein

the blank is held at the soak temperature T s for a soak time t s adjusted so that the steel, after heating, has an austenite content between 25 and 75% by area; and the microstructure of the steel, after the part has been cooled, is a multiphase microstructure comprising ferrite and either martensite, or bainite, or both martensite and bainite.

4. The process as claimed in claim 3 , wherein the steel comprises, in % by weight:

0.08≦C≦0.15%

1.20≦Mn≦2.00%

0.01≦Si≦0.50%

0.005≦Al≦1.0%

0.001≦Mo≦0.10%

Cr≦0.50%

P≦0.10%

Ti≦0.15%

Nb≦0.15%

V≦0.25%,

the balance of the composition being iron and impurities resulting from smelting.

5. The process as claimed in claim 3 , wherein the soak time t s is between 10 and 1000 s.

6. The process as claimed in claim 3 , wherein the cooling rate V is greater than 10° C./s.

7. The process as claimed in claim 3 , wherein the multiphase structure of the steel, after said part has been cooled, comprises 25 to 75% ferrite by area and 25 to 75% martensite and/or bainite by area.

8. The process as claimed in claim 1 , wherein the steel comprises, in % by weight:

0.10≦C≦0.30%

0.60≦Mn≦2.0%

0.01≦Si≦2.0%

0.005≦Al≦3.0%

Mo≦0.60%

Cr≦1.50%

Ni≦0.20%

Cu≦0.20%

P≦0.10%

S≦0.05%

Ti≦0.20%

V≦0.60%,

the balance of the composition being iron and impurities resulting from smelting.

9. The process as claimed in claim 1 , wherein the soak time t s is between 10 and 1000 s.

10. The process as claimed in claim 1 , wherein the cooling rate V is between 10 and 200° C./s.

11. The process as claimed in claim 1 , wherein, after the part has been cooled, the multiphase microstructure of the TRIP steel consists, in % by area, of ferrite with a content equal to or greater than 25%, of 3 to 30% residual austenite and optionally of martensite and/or bainite.

12. The process as claimed in claim 1 , wherein the forming operation is a deep-drawing operation.

13. The process as claimed in claim 1 , wherein the steel strip is coated with a metal coating before being cut to form a blank.

14. The process as claimed in claim 13 , wherein the metal coating is a coating of zinc or a zinc alloy.

15. The process as claimed in claim 13 , wherein the metal coating is a coating of aluminum or an aluminum alloy.

16. A process for manufacturing a part made of steel having a multiphase microstructure, said microstructure comprising ferrite and being homogeneous in each of the regions of said part, which process comprises:

cutting a blank from a strip of steel, the composition of which comprises, in % by weight:

0.01≦C≦0.50%

0.50≦Mn≦3.0%

0.001≦Si≦3.0%

0.005≦Al≦3.0%

Mo≦1.0%

Cr≦1.5%

P≦0.10%

Ti≦0.20%

V≦1.0% and

optionally, one or more elements such as:

Ni≦2.0%

Cu≦2.0%

S≦0.05%

Nb≦0.15%,

the balance of the composition being iron and impurities resulting from the smelting;

optionally, said blank undergoes prior cold deformation;

said blank is heated so as to reach a soak temperature T s above Ac1 but below Ac3 and held at this soak temperature T s for a soak time t s adjusted so that the steel, after the blank has been heated, has an austenite content equal to or greater than 25% by area;

said heated blank is transferred into a forming tool so as to hot-form said part, wherein the forming operation is a deep-drawing operation; and

the part is cooled within the tool at a cooling rate V such that the microstructure of the steel, after the part has been cooled, is a multiphase microstructure, said microstructure comprising ferrite and being homogeneous in each of the regions of said part.

17. The process as claimed in claim 16 , wherein the microstructure of the steel, after the part has been cooled, is a multiphase microstructure with a ferrite content equal to or greater than 25% by area.

18. The process as claimed in claim 16 , wherein the composition of the steel comprises, in % by weight:

0.01≦C≦0.25%

0.50≦Mn≦2.50%

0.01≦Si≦2.0%

0.005≦Al≦1.5%

0.001≦Mo≦0.50%

Cr≦1.0%

P≦0.10%

Ti≦0.15%

Nb≦0.15%

V≦0.25%,

the balance of the composition being iron and impurities resulting from the smelting; the blank is held at the soak temperature T s for a soak time t s adjusted so that the steel, after heating, has an austenite content between 25 and 75% by area; and the microstructure of the steel, after the part has been cooled, is a multiphase microstructure comprising ferrite and either martensite, or bainite, or both martensite and bainite.

19. The process as claimed in claim 18 , wherein the steel comprises, in % by weight:

0.08≦C≦0.15%

1.20≦Mn≦2.00%

0.01≦Si≦0.50%

0.005≦Al≦1.0%

0.001≦Mo≦0.10%

Cr≦0.50%

P≦0.10%

Ti≦0.15%

Nb≦0.15%

V≦0.25%,

the balance of the composition being iron and impurities resulting from the smelting.

20. The process as claimed in claim 18 , wherein the soak time t s is between 10 and 1000 s.

21. The process as claimed in claim 18 , wherein the cooling rate V is greater than 10° C./s.

22. The process as claimed in claim 18 , wherein the multiphase structure of the steel, after said part has been cooled, comprises 25 to 75% ferrite by area and 25 to 75% martensite and/or bainite by area.

23. The process as claimed in claim 16 , wherein the steel strip is coated with a metal coating before being cut to form a blank.

24. The process as claimed in claim 23 , wherein the metal coating is a coating of zinc or a zinc alloy.

25. The process as claimed in claim 23 , wherein the metal coating is a coating of aluminum or an aluminum alloy.