Method for producing high-strength steel parts with improved ductility, and parts obtained by said method

Patent metadata

Field	Value
Publication number	US-11976342-B2
Application number	US-202217943561-A
Country	US
Kind code	B2
Filing date	Sep 13, 2022
Priority date	Jun 1, 2017
Publication date	May 7, 2024
Grant date	May 7, 2024

Abstract

A rolled steel sheet, for press hardening is provided, having a chemical composition where Ti/N>3.42, and the carbon, manganese, chromium and silicon contents satisfy:
2.6 C + Mn 5.3 + Cr 13 + Si 15 ≥ 1.1 % .
The sheet has a nickel content Ni surf at any point of the steel in the vicinity of the surface over a depth Δ, such that: Ni surf >Ni nom , Ni nom denoting the nominal nickel content of the steel, and such that, Ni max denoting the maximum nickel content within Δ:

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

( Ni max - Ni nom ) Δ ≥ 0.01
and the surface density of all of the particles D i , and the surface density of the particles D (>2 μm) larger than 2 micrometers satisfy, at least to a depth of 100 micrometers in the vicinity of the surface of said sheet:

D i +6.75 D (>2 μm) <270

D i and D (>2 μm) being expressed as number of particles per square millimeter, and said particles denoting all the oxides, sulfides, and nitrides, either pure or combined such as oxysulfides and carbonitrides, present in the steel matrix.

First claim

The invention claimed is:

1. A part obtained by press hardening a steel sheet,
the steel sheet having a chemical composition comprising, with contents being expressed by weight:
either 0.24%≤C≤0.38% and 0.40%≤Mn≤3%,

or 0.38%<C≤0.43% and 0.05%≤Mn<0.4%

0.10%≤Si≤1.70%

0.015%≤A1≤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%

the titanium and nitrogen contents satisfying:
Ti/N>3.42,

and the carbon, manganese, chromium, and silicon contents satisfying:

2.6

C

+

Mn
5.3

+

Cr
13

+

Si
15

≥

1.1
%

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%

a remainder being iron and unavoidable impurities resulting from the processing, said sheet containing a nickel content Ni surf at any point of the steel in the vicinity of the surface of said sheet to a depth Δ, such that:
Ni surf >Ni nom ,
Ni nom denoting the nominal nickel content of the steel,
and such that, Ni max denoting the maximal nickel content within Δ:

(

Ni
max

+

Ni
nom

)

2

×

(
Δ
)

≥
0.6

,

and such that:

(

Ni
max

+

Ni
nom

)

Δ

≥
0.01

the depth Δ being expressed in micrometers,
the Ni max and Ni nom contents being expressed in percentages by weight,
the press hardened part having a martensitic or martensitic-bainitic structure, a mechanical strength Rm greater than or equal to 1800 MPa, and the surface density of all the particles D i and the surface density of the particles larger than 2 micrometers D (>2μm) satisfy, at least to a depth of 100 micrometers in the vicinity of the surface of said press hardened part:
D i +6.75 D (>2μm) <270

D i and D (>2 μm) being expressed as the number of particles per square millimeter, and said particles denoting all oxides, sulfides, nitrides, pure or combined such as oxysulfides and carbonitrides, present in the steel matrix.

2. The press hardened part as recited in claim 1 wherein the chemical composition includes, by weight:
0.39%≥C≥0.43%

0.09%≥Mn≥0.11%.

3. The press hardened part as recited in claim 2 wherein the chemical composition includes, by weight:
0.95%≤Cr≤1.05%.

4. The press hardened part as recited in claim 2 wherein the chemical composition includes, by weight:
0.48%≤Ni≤0.52%.

5. The press hardened part as recited in claim 2 wherein the chemical composition includes, by weight:
1.4%≤Si≤1.70%.

6. The press hardened part as recited in claim 1 wherein the chemical composition includes, by weight:
0.95%≤Cr≤1.05%.

7. The press hardened part as recited in claim 1 wherein the microstructure is ferritic-perlitic.

8. The press hardened part as recited in claim 1 wherein said sheet that is press hardened into the press hardened part is a hot-rolled sheet.

9. The press hardened part as recited in claim 1 wherein said sheet that is press hardened into the press hardened part is a cold-rolled and annealed sheet.

10. The press hardened part as recited in claim 1 wherein said sheet that is press hardened into the press hardened part is precoated with an aluminum or aluminum alloy or aluminum-based metallic layer.

11. The press hardened part as recited in claim 1 wherein said sheet that is press hardened into the press hardened part is precoated with a zinc or zinc alloy or zinc-based metallic layer.

12. The press hardened part as recited in claim 1 wherein said sheet that is press hardened into the press hardened part is precoated with one or more layers of intermetallic alloys containing aluminum and iron, and optionally silicon, the precoating not containing free aluminum, phase τ 5 of type Fe 3 Si 2 Al 12 , and phase τ 6 of type Fe 2 Si 2 Al 9 .

13. The press hardened part as recited in claim 1 wherein the part has at least a bending angle of more than 50° in a direction of rolling.

14. The press hardened part as recited in claim 1 wherein the manganese, phosphorus, chromium, molybdenum and silicon contents satisfy
[455Exp(−0.5 [Mn+25P])+[390Cr+50Mo]+7Exp(1.3Si)][6−1.22×10 −9 σy 3 ][C SCC ]≥750

σy being the yield strength which is between 1300 MPa and 1600 MPa,
and C SCC being equal to 1 for an uncoated sheet, and equal to 0.7 for a coated sheet.

15. The press hardened part as recited in claim 1 wherein the manganese, phosphorus, chromium, molybdenum, and silicon contents satisfy:
[455Exp(−0.5 [Mn+25P])+[390Cr+50Mo]+7Exp(1.3Si)][6−1.22×10 −9 σy 3 ][C SCC ]≥1100.

16. The press hardened part as recited in claim 1 wherein the part contains a nominal nickel content characterized in that the nickel content Ni surf of the steel in the vicinity of the surface is greater than Ni nom to a depth Δ, and in that, Ni max denoting the maximum nickel content within Δ:

(

Ni
max

+

Ni
nom

)

2

×

(
Δ
)

≥
0.6

,

and in that:

Assignees

• Arcelormittal
• Arcelor Mittal

Inventors

• Cobo Sebastian
• Allely Christian
• Beauvais Martin
• Aouafi Anis
• Lucas Emmanuel

Classifications

• C21D8/02 Primary

  during manufacturing of plates or strips (C21D8/12 takes precedence) · CPC title

• C21D2211/008

  Martensite · CPC title

• C21D2211/002

  Bainite · CPC title

• C23C2/28

  Thermal after-treatment, e.g. treatment in oil bath · CPC title

• C23C2/0224

  Two or more thermal pretreatments · CPC title