Method for fabricating steel sheet for press hardening, and parts obtained by this method
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.
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
)
Δ
≥
0.01
with the depth Δ expressed in microns, and
the contents Ni max and Ni nom expressed in percentages by weight.
24. A press hardened part according to claim 22 , wherein a mechanical strength Rm is greater than or equal to 1800 MPa.
25. A press hardened part according to claim 22 , further comprising an aluminum or aluminum-based alloy, or a zinc or a-based alloy resulting from diffusion between the steel substrate and a precoat, during a thermal treatment of press hardening.
26. A structural or reinforcing part for an automotive vehicle comprising:
a press hardened part according to claim 22 .