Method for producing high-strength steel parts with improved ductility, and parts obtained by said method
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.
1. A rolled steel sheet, for press hardening, 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%≤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%
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:
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 surface density of all the particles D 1 and the surface density of the particles larger than 2 micrometers D (>2 μm) satisfy, at least to a depth of 100 micrometers in the vicinity of the surface of said sheet:
D i +6.75D (>2 μm) <270
D 1 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 steel sheet as recited in claim 1 wherein the composition includes, by weight:
3 9%≤C≤0.43%
0.09%≤Mn≤0.11%.
3. The steel sheet as recited in claim 2 wherein the composition includes, by weight:
0.95%≤Cr≤1.05%
4. The steel sheet as recited in claim 2 wherein the composition includes, by weight:
48%≤Ni≤0.52%.
5. The steel sheet as recited in claim 2 wherein the composition includes, by weight:
1.4%≤Si≤1.70%.
6. The steel sheet as recited in claim 1 wherein the composition includes, by weight:
0.95%≤Cr≤1.05%.
7. The steel sheet as recited in claim 1 wherein the microstructure is ferritic-perlitic.
8. The steel sheet as recited in claim 1 wherein wherein said sheet is a hot-rolled sheet.
9. The steel sheet as recited in claim 1 wherein said sheet is a cold-rolled and annealed sheet.
10. The steel sheet as recited in claim 1 wherein said sheet is precoated withan aluminum or aluminum alloy or aluminum-based metallic layer.
11. The steel sheet as recited in claim 1 wherein said sheet is precoated with a zinc or zinc alloy or zinc-based metallic layer.
12. The steel sheet as recited in claim 1 wherein said sheet 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 typeFe 2 Si 2 Al 9 .
13. A method for manufacture of the rolled steel sheet as recited in claim 1 , the method comprising successive steps of:
producing a liquid steel to which manganese, silicon, niobium, and chromium are added, the additions being made in a vacuum chamber, then
desulfurizing the liquid metal without increasing its nitrogen content, then
adding titanium, said additions being made so as to obtain a liquid metal 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%≤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%
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, then casting a semi-finished product, then
heating said semi-finished product to a temperature between 1250° C. and 1300° C. for a holding period at this temperature between 20 minutes and 45 minutes, then
hot rolling said semi-finished product to a rolling end temperature TFL between 825° C. and 950 ° C., to obtain a hot-rolled sheet, then
coiling said hot-rolled sheet at a temperature between 500° C. and 750° C. to obtain a hot-rolled and coiled sheet, then
pickling the oxide layer formed in the previous steps.