Method for fabricating steel sheet for press hardening, and parts obtained by this method

Patent metadata

Field	Value
Publication number	US-9845518-B2
Application number	US-201715610084-A
Country	US
Kind code	B2
Filing date	May 31, 2017
Priority date	Jul 30, 2014
Publication date	Dec 19, 2017
Grant date	Dec 19, 2017

Abstract

The present invention provides a rolled steel sheet, for press hardening, for which the chemical composition includes, with contents expressed by weight, 0.24%≦C≦0.38%, 0.40%≦Mn≦3%, 0.10%≦Si≦0.70%, 0.015%≦Al≦0.070%, 0%≦Cr≦2%, 0.25%≦Ni≦2%, 0.015%≦Ti≦0.10%, 0%≦Nb≦0.060%, 0.0005%≦B≦0.0040%, 0.003%≦N≦0.010%, 0.0001%≦S≦0.005%, 0.0001%≦P≦0.025%, it being understood that the titanium and nitrogen content satisfy: Ti/N>3.42, and that the carbon, manganese, chromium and silicon content satisfy:
2.6 ⁢ C + Mn 5.3 + Cr 13 + Si 15 ≥ 1.1 ⁢ % ,
with the chemical composition optionally including one or more of the following elements: 0.05%≦Mo≦0.65%, 0.001%≦W≦0.30%, 0.0005%≦Ca≦0.005%, with the remainder made up of iron and inevitable impurities coming from preparation. The sheet includes a nickel content Ni surf at any point of the steel near the surface of said sheet over a depth Δ, such that Ni surf >Ni nom , where Ni nom designates the nominal nickel content of the steel, and such that Ni max designates the maximum nickel content within Δ:

( Ni max + Ni nom ) 2 × ( Δ ) ≥ 0.6
and such that:

( Ni max - Ni nom ) Δ ≥ 0.01 ,
with the depth Δ expressed in microns and the Ni max and Ni nom contents expressed in percentages by weight.

First claim

What is claimed is:

1. A rolled steel sheet for press hardening comprising:
a chemical composition including, with contents expressed by weight:
0.24%≦C≦0.38%;
0.40%≦Mn≦3%;
0.10%≦Si≦0.70%;
0.015%≦Al≦0.070%;
0%≦Cr≦2%;
0.25%≦Ni≦2%;
0.015%≦Ti≦0.10%;
0%≦Nb≦0.060%;
0.0005%≦B≦0.0040%;
0.003%≦N≦0.010%;
0.0001%≦S≦0.005%; and
0.0001%≦P≦0.025%;

the titanium and nitrogen contents satisfying Ti/N>3.42;
the carbon, manganese, chromium and silicon contents satisfying:

2.6
⁢

⁢
C

+

Mn
5.3

+

C
⁢

⁢
r

13

+

S
⁢

⁢
i

15

≥

1.1
⁢
%

a remainder of the chemical composition being made up of iron and inevitable impurities resulting from processing;
a nickel content Ni surf at any point of the steel near a surface of the sheet over a depth Δ, such that Ni surf >Ni nom , wherein Ni nom is a nominal nickel content of the steel;
a maximum nickel content Ni max within Δ being:

(

Ni
max

+

Ni
nom

)

2

×

(
Δ
)

≥
0.6

,

and:

(

Ni
max

+

Ni
nom

)

Δ

≥
0.01

with the depth Δ expressed in microns and Ni max and Ni nom expressed in percentages by weight.

2. Steel sheet according to claim 1 , wherein the chemical composition includes at least one of the following elements, by weight:
0.05%≦Mo≦0.65%;
0.001%≦W≦0.30%; and
0.0005%≦Ca≦0.005%.

3. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.32%≦C≦0.36%;
0.40%≦Mn≦0.80%; and
0.05%≦Cr≦1.20%.

4. Steel sheet according to claim 1 , wherein the chemical composition thereof includes, by weight:
0.24%≦C≦0.28%; and
1.50%≦Mn≦3%.

5. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.50%≦Si≦0.60%.

6. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.30%≦Cr≦0.50%.

7. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.30%≦Ni≦1.20%.

8. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.30%≦Ni≦0.50%.

9. Steel sheet according to claim 1 , wherein the chemical composition thereof comprises, by weight:
0.020%≦Ti.

10. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.020%≦Ti≦0.040%.

11. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.15%≦Mo≦0.25%.

12. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.010%≦Nb≦0.060%.

13. Steel sheet according to claim 1 , wherein the chemical composition includes, by weight:
0.030%≦Nb≦0.050%.

14. Steel sheet according to claim 3 , wherein the chemical composition includes, by weight:
0.50%≦Mn≦0.70%.

15. Steel sheet according to claim 3 , further comprising a ferritic-pearlitic microstructure.

16. Steel sheet according to claim 1 , wherein the sheet is a hot rolled sheet.

17. Steel sheet according to claim 1 , wherein the sheet is a cold rolled and annealed sheet.

18. Steel sheet according to claim 1 , wherein the sheet is precoated with a metal layer of aluminum or aluminum alloy or aluminum-based alloy.

19. Steel sheet according to claim 1 , wherein the sheet is precoated with a metal layer of zinc or zinc alloy or zinc-based alloy.

20. Steel sheet according to claim 1 , wherein the sheet is precoated with at least one coat of inter-metallic alloy including aluminum and iron and wherein the precoating does not include free aluminum, of phase τ 5 of type Fe 3 Si 2 Al 12 , and τ 6 of type Fe 2 Si 2 Al 9 .

21. Steel sheet according to claim 20 , wherein the at least one coat of inter-metallic alloy includes silicon.

22. A part obtained by press hardening a steel sheet of the chemical composition according to claim 1 with a martensitic or martensitic-bainitic structure.

23. A press hardened part according to claim 22 , comprising a nominal nickel content Ni nom , wherein the nickel content Ni surf in the steel near the surface is greater than Ni nom over a depth Δ, and a maximum nickel content Ni max within Δ is:

(

Ni
max

+

Ni
nom

)

2

×

(
Δ
)

≥
0.6

,

and:

(

Ni
max

+

Ni
nom

Assignees

• Arcelormittal

Inventors

• Cobo Sebastian
• Puerta Velasquez Juan David
• Beauvais Martin
• Vinci Catherine

Classifications

• C21D8/00

  Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment (hardening articles or materials formed by forging or rolling with no further heating beyond that required for the formation C21D1/02) · CPC title

• C21D9/46

  for sheet metals · CPC title

• C23C2/12

  Aluminium or alloys based thereon · CPC title

• C21D2211/009

  Pearlite · CPC title

• C21D1/06

  Surface hardening · CPC title