Method of forming a hot stamped coated steel product

1. A method of forming a hot stamped coated steel product, comprising:

providing a precoated steel strip, the precoated steel strip including a base steel having a first side and a second side, and a precoating on at least one of the first side and the second side, the precoating being made of aluminum or an aluminum alloy;

heating the precoated steel strip in a furnace, wherein the precoated strip experiences a temperature rise at a heating rate (V c ) between 4° and 12° C./s, wherein V c is a mean heating rate between 20 and 700 C;

removing the heated precoated steel strip from the furnace; and then

hot stamping the precoated strip to deform the precoated steel strip into a hot stamped coated steel product, the hot stamped coated steel product having a coating including, proceeding from the base steel outwards:

(a) an interdiffusion layer,

(b) an intermediate layer,

(c) an intermetallic layer, and

(d) a superficial layer,

wherein layers (a), (b), (c), and (d) each have a different mean composition from one another, wherein the coating on the hot stamped coated steel product has a thickness greater than 30 micrometers, wherein the interdiffusion layer has a first mean iron composition, the intermediate layer has a second mean iron composition less than the first mean iron composition;

wherein the intermetallic layer has a third mean iron composition, the third mean iron composition less than the first mean iron composition and greater than the second mean iron composition, and the superficial layer has a fourth mean iron composition, the fourth mean iron composition being less than the third mean iron composition.

2. The method of claim 1 , wherein said heating the precoated steel strip in a furnace including heating to between 750° C. and 1200° C., wherein the precoated strip experiences the temperature rise at the heating rate (V c ) between 4° and 12° C./s.

3. The method of claim 1 , wherein said heating the precoated steel strip in a furnace includes heating to between 880° C. and 940° C., wherein the precoated strip experiences the temperature rise at the heating rate (V c ) between 4° and 12° C./s, the hot stamped coated steel product having a martensitic structure.

4. The method according to claim 1 , further comprising cooling at a rate of more than 30° C./s, wherein the cooling rate is defined as the mean rate between the exit of the heated blank from the furnace, down to 400° C.

5. The method according to claim 4 , wherein an elapsed time between said removing of the heated precoated strip from the furnace and said hot stamping is less than 10 seconds.

6. The method according to claim 4 , wherein the hot stamping includes hot stamping at a strain higher than 10% and wherein the cooling is cooling at a rate higher than 50° C./s to form a fully martensitic hot stamped coated steel product.

7. The method according to claim 2 , wherein the hot stamping includes hot stamping at a strain higher than 10% and wherein the cooling is cooling at a rate higher than 50° C./s to form a fully martensitic hot stamped coated steel product.

8. The method according to claim 3 , wherein the hot stamping includes hot stamping at a strain higher than 10% and wherein the cooling is cooling at a rate higher than 50° C./s.

9. The method according to claim 8 , wherein an elapsed time between said removing of the heated precoated strip from the furnace and said hot stamping is less than 10 seconds.

10. The method according to claim 1 , wherein less than 10% of layer (c) is present at an extreme surface of the hot stamped coated steel product.

11. The method according to claim 1 , wherein the layers (c) and (d) are quasi continuous by occupying at least 90% of a level corresponding to each said layer.

12. The method according to claim 1 , wherein said layer (a) has a thickness of 17 micrometers or less.

13. The method according to claim 1 , wherein said layer (a) has a thickness of 15 micrometers or less.

14. The method according to claim 1 , wherein said layer (a) has a thickness less than 10 micrometers.

15. The method according to claim 1 , wherein the base steel of the precoated steel strip consists of the following components by weight based on total weight:

0.15%<carbon<0.5%;

0.5%<manganese<3%;

0.1%<silicon<0.5%;

0.01%<chromium<1%;

titanium<0.2%;

aluminum<0.1%;

phosphorus<0.1%;

sulfur<0.05%;

0.0005%<boron<0.08%;

a remainder being iron and impurities inherent in processing.

16. The method according to claim 1 , wherein the base steel of the precoated steel strip consists of the following components by weight based on total weight:

0.15%<carbon<0.25%,

0.8%<manganese<1.8%,

0.1%<silicon<0.35%,

0.01%<chromium<0.5%,

titanium<0.1%

aluminum<0.1%

phosphorus<0.1%

sulfur<0.05%

0.002%<boron<0.005%,

a remainder being iron and impurities inherent in processing.

17. The method according to claim 1 , wherein a thickness t p of said precoating on the provided precoated steel strip is from 20 to 33 micrometers at every location on at least one of said first and second sides.

18. The method according to claim 1 , wherein a thickness t p of said precoating on the provided precoated steel strip is from 20 to 33 micrometers at every location on both said first and second sides.

19. The method according to claim 1 , wherein said aluminum or aluminum alloy precoating on the provided precoated steel strip is an aluminum alloy precoating comprising from 8% to 11% silicon by weight, from 2% to 4% iron by weight, a remainder being aluminum and impurities inherent in processing.

20. The method according to claim 19 , wherein the aluminum alloy precoating comprises from 9% to 10% silicon by weight.

21. The method according to claim 1 , wherein the interdiffusion layer comprises the following components by weight based on total weight: 86 to 95% Fe, 4 to 10% Al, and 0 to 5% Si.

22. The method according to claim 21 , wherein the intermediate layer comprises the following components by weight based on total weight: 39 to 47% Fe, 53 to 61% Al, and 0 to 2% Si.

23. The method according to claim 22 , wherein the intermetallic layer comprises the following components by weight based on total weight: 62 to 67% Fe, 30 to 34% Al, and 2 to 6% Si.

24. The method according to claim 22 , wherein the superficial layer comprises the following components by weight based on total weight: 39 to 47% Fe, 53 to 61% Al, and 0 to 2% Si.

25. The method according to claim 1 , wherein the intermediate layer comprises the following components by weight based on total weight: 39 to 47% Fe, 53 to 61% Al, and 0 to 2% Si.

26. The method according to claim 1 , wherein the intermetallic layer comprises the following components by weight based on total weight: 62 to 67% Fe, 30 to 34% Al, and 2 to 6% Si.

27. The method according to claim 1 , wherein the superficial layer comprises the following components by weight based on total weight: 39 to 47% Fe, 53 to 61% Al, and 0 to 2% Si.

28. The method according to claim 1 , wherein the hot stamped coated steel product has a mechanical strength in excess of 1000 MPa.

29. The method according to claim 1 , wherein the hot stamped coated steel product has a mechanical strength in excess of 1500 MPa.

30. The method according to claim 1 , wherein the hot stamped coated steel product has a yield strength in excess of 1200 MPa.