Method for making a steel part of multiphase microstructure

1. A hot deep drawn part made of steel having a homogeneous multiphase microstructure throughout said part, said microstructure comprising ferrite, wherein the part made of steel is obtained by a process comprising:

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

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

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 deep drawing tool so as to hot-deep draw 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 throughout said part.

2. The part 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 part as claimed in claim 1 , 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.

4. The part 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 the smelting.

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

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

7. The part 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 part as claimed in claim 1 , wherein the steel 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 the 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.

9. The part as claimed in claim 8 , 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 the smelting.

10. The part as claimed in claim 8 , wherein the soak time t s is between 10 and 1000 s.

11. The part as claimed in claim 8 , wherein the cooling rate V is between 10 and 200° C./s.

12. The part as claimed in claim 8 , 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.

13. The process as claimed in claim 1 , wherein the steel strip is coated beforehand 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 based on zinc or a zinc alloy.

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

16. A land motor vehicle comprising the steel part as claimed in claim 1 .

17. The part as claimed in claim 1 , wherein the steel comprises, in % by weight, at least one of the elements selected from:

Ni≤2.0%,

Cu≤2.0%,

S≤0.05%,

Nb≤0.15%.

18. The part as claimed in claim 1 , wherein the blank was subject to cold deformation.

19. The part as claimed in claim 1 , wherein a dispersion in the tensile strength Rm within a cooling rate range varying from 10 to 100° C./s is less than 25%.

20. The part as claimed in claim 1 , wherein the cooling rate is from 10° C./s to 200° C./s.

21. The part as claimed in claim 1 , wherein the microstructures include martensite and residual austenite.

22. The part as claimed in claim 1 , wherein the microstructures include from 3 to 30% residual austenite.

23. The part as recited in claim 1 , wherein the part is a TRIP steel.

24. The part as claimed in claim 1 , wherein the microstructures include from 0 to 48% martensite.

25. The part as claimed in claim 1 , wherein the microstructures include from 0 to 5% bainite.