US2025354239A1PendingUtilityA1
Method for fabricating steel sheet for press hardening, and parts obtained by this method
Est. expiryJul 30, 2034(~8 yrs left)· nominal 20-yr term from priority
C21D 8/00C22C 38/14C22C 38/12C21D 2211/002C21D 9/0068C21D 1/06B21B 3/02C23F 17/00C22C 38/58C22C 38/54C22C 38/50C22C 38/48C22C 38/44C22C 38/002C22C 38/001C21D 8/0278C21D 8/0263C21D 1/18B21D 22/022C21D 9/46C23C 2/261C23C 2/29C21D 8/02C22C 38/40C23C 2/40C23C 2/28C23C 2/12C21D 2211/009C21D 2211/008C21D 2211/005C21D 1/673C23C 2/06C21D 8/0226C22C 38/08C22C 38/06C22C 38/04C22C 38/02C22C 38/38C21D 8/0273C21D 8/0236C22C 38/18C22C 38/32C21D 8/005
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Claims
Abstract
The present invention provides fabrication methods for cold rolled, precoated and press hardened steel sheets, 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:2.6C+Mn5.3+Cr13+Si15≥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.
Claims
exact text as granted — not AI-modifiedWhat is claimed is:
1 . A fabrication method for a hot rolled steel sheet, comprising the steps of:
casting an intermediate product with a chemical composition including, with contents being expressed by weight, either 0.32%≤C≤0.36% and 0.40%≤Mn≤0.8% and 0.5%≤Cr≤1.2% or 0.24%≤C≤0.28% and 1.5%≤Mn≤3% and 0%≤Cr≤2%; the chemical composition further including
0.1
%
≤
Si
≤
0.7
%
;
0.25
≤
Ni
≤
2
%
;
0.015
%
≤
Ti
≤
0.1
%
;
0
%
≤
Nb
≤
0.06
%
;
0.0005
%
≤
B
≤
0.004
%
;
0.003
%
≤
N
≤
0.01
%
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
+
Cr
13
+
Si
15
≥
1.1
%
;
and
a remainder of the chemical composition being made up of iron and inevitable impurities resulting from processing;
reheating the intermediate product to a temperature between 1250° C. and 1300° C. for a hold time between 20 and 45 minutes;
hot rolling the intermediate product until an end of rolling temperature, ERT, between 825° C. and 950° C. to obtain a hot rolled sheet;
coiling the hot rolled sheet at a temperature between 500° C. and 750° C. to obtain a hot rolled and coiled sheet; and
pickling an oxide layer over a nickel enriched layer formed during the preceding steps.
2 . A fabrication method for a cold rolled and annealed sheet, comprising the steps of:
supplying a hot rolled sheet, coiled and pickled, fabricated by the method according to claim 1 ; and cold rolling the hot rolled, coiled and pickled sheet to obtain a cold rolled sheet; and annealing the cold rolled sheet at a temperature between 740° C. and 820° C. to obtain a cold rolled and annealed sheet.
3 . The fabrication method as recited in claim 1 wherein the intermediate product is a slab between 200 mm and 250 mm thick.
4 . The fabrication method as recited in claim 2 wherein the intermediate product is a slab between 200 mm and 250 mm thick.
5 . The fabrication method as recited in claim 1 wherein the hot rolled sheet is between 1.5 mm and 4.5 mm thick.
6 . The fabrication method as recited in claim 2 wherein the hot rolled sheet is between 1.5 mm and 4.5 mm thick.
7 . The fabrication method as recited in claim 6 wherein the cold rolling is performed at a reduction rate between 30 and 70%.
8 . The fabrication method as recited in claim 2 wherein the hot rolled sheet is pickled in an acid bath with an inhibitor so as to remove the oxide layer but leave the nickel enriched layer in place.
9 . The fabrication method as recited in claim 2 wherein the hot rolled sheet is pickled in an acid bath with an inhibitor so as to remove the oxide layer.
10 . The fabrication method as recited in claim 2 wherein the cold-rolled and annealed sheet is subjected to the further following method steps:
cutting the sheet to obtain a blank;
heating the blank to a temperature between 810° C. and 950° C. to obtain a fully austenitic structure in the steel;
transferring the blank inside a press;
hot stamping the blank to obtain a part;
holding the part inside the press to obtain a hardening by martensitic transformation of the austenitic structure.
11 . The fabrication method as recited in claim 10 wherein the holding the part step includes cooling the part, at temperatures from 750° C. to 400° C., at a cooling speed of between 180° C./s and 210° C./s.
12 . The fabrication method as recited in claim 10 wherein the heating of the blank is at 850° C. and 950° C. for five minutes in a furnace under an atmosphere with a dewpoint of −5° C.
13 . The fabrication method as recited in claim 2 wherein the cold-rolled and annealed sheet is subjected to the further following method step: continuous hot-dip coating in a bath comprising aluminum or an aluminum or aluminum-based alloy.
14 . The fabrication method as recited in claim 2 wherein the cold-rolled and annealed sheet is subjected to the further following method step: continuous hot-dip coating in a bath comprising zinc or a zinc or zinc-based alloy.
15 . The fabrication method as recited in claim 1 wherein the pickling provides an area enriched with nickel on or near a surface of the hot rolled and coiled sheet.
16 . The fabrication method as recited in claim 15 wherein the area enriched with nickel is on the surface of the hot rolled and coiled sheet.
17 . The fabrication method as recited in claim 1 wherein the hot rolled and coiled sheet has a nickel content that varies over a depth of the hot rolled and coiled sheet.
18 . The fabrication method as recited in claim 1 wherein a nickel content of the hot rolled and coiled sheet is such that
(
Ni
max
+
Ni
nom
)
2
×
(
Δ
)
≥
0.6
wherein Δ is a depth near a surface of the hot rolled and coiled sheet, Ni nom is a nominal nickel content of the hot rolled and coiled sheet, Ni max designates a maximum nickel content within the depth, the depth expressed in microns and the Ni max and Ni nom expressed in percentages by weight.
19 . The fabrication method as recited in claim 1 wherein a nickel content of the hot rolled and coiled sheet is such that
(
Ni
max
-
Ni
nom
)
Δ
≥
0.01
,
wherein Δ is a depth near a surface of the hot rolled and coiled sheet, Ni nom is a nominal nickel content of the hot rolled and coiled sheet, Ni max designates a maximum nickel content within the depth, the depth expressed in microns and the Ni max and Ni nom expressed in percentages by weight.
20 . The fabrication method as recited in claim 1 wherein a nickel content of the hot rolled and coiled sheet is such that
(
Ni
max
+
Ni
nom
)
2
×
(
Δ
)
≥
0.6
,
and
(
Ni
max
-
Ni
nom
)
Δ
≥
0.01
,
wherein Δ is a depth near a surface of the hot rolled and coiled sheet, Ni nom is a nominal nickel content of the hot rolled and coiled sheet, Ni max designates a maximum nickel content within the depth, the depth expressed in microns and the Ni max and Ni nom expressed in percentages by weight.
21 . The fabrication method as recited in claim 1 wherein a nickel content at a point near a surface of the hot rolled and coiled sheet is greater than a nominal nickel content of the hot rolled and coiled sheet.
22 . The fabrication method as recited in claim 1 , wherein the chemical composition includes 0.015%≤Ti≤0.04%.
23 . The fabrication method as recited in claim 22 , wherein the chemical composition includes 0.015%≤Ti≤0.036%.
24 . The fabrication method as recited in claim 1 , wherein the chemical composition includes 1.19%≤Ni≤2%.
25 . The fabrication method of claim 10 , wherein after the holding step, the part has a mechanical strength Rm greater than or equal to 1800 MPa.
26 . The fabrication method of claim 10 , wherein, after the holding step, a diffusible hydrogen content over the part is less than or equal to 0.16 ppm.
27 . The fabrication method of claim 1 , wherein the hot rolled steel sheet comprises precipitates of TiN, Nb(CN) or (Ti, Nb)(CN).Join the waitlist — get patent alerts
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