Method for producing a press-hardened laser welded steel part and press-hardened laser welded steel part
A method for producing a part includes providing a first and a second precoated sheet ( 1,2 ), butt welding the first and second precoated sheets ( 1 ) to obtain a blank ( 15 ), and heating the blank ( 15 ) to a heat treatment temperature at least 10° C. lower than the full austenitization temperature of the weld joint ( 22 ) and at least 15° C. higher than a minimum temperature T min : T min ( ° C . ) = AC 3 ( WJ ) - α IC max 1 0 0 ( Ac 3 ( W J ) - 6 7 3 - 40 × Al ) . where Ac3(WJ) is the full austenitization temperature of the weld joint ( 22 ) α IC max = ( 1 - ( 1 + ρ ) ( max ( 1 ; ρ ) T s 2 - 3 5 0 ) ( 1 - β ) ( ρ T s 2 + T s 1 ) + β ( 1 + ρ ) ( 3 1 3 0 C F W + 750 ) - 3 5 0 × ( 1 + ρ ) ) × 100 , where Ts 1 and Ts 2 are the ultimate tensile strengths of the strongest and the weakest substrate after press-hardening C FW is the carbon content of the filler material β is the proportion of filler material ρ is the ratio between the thicknesses of the weakest and the strongest substrate The method also includes holding the blank ( 15 ) at the heat treatment temperature for a time between 2 and 10 minutes; and press-forming the blank ( 15 ) into a part and cooling.
1 . A press-hardened laser welded steel part comprising:
a first coated steel part portion and a second coated steel part portion, each coated steel part portion comprising a steel substrate, at least one of the first coated steel part portion and the second coated steel part portion having, on at least one of its main faces, an aluminum-containing coating comprising at least 30% by weight of aluminum,
the first coated steel part portion having a first thickness and the second coated steel part portion having a second thickness, the steel substrate of the first coated steel part portion having an ultimate tensile strength greater than an ultimate tensile strength of the steel substrate of the second coated steel part portion, and the product of the first thickness by the ultimate tensile strength of the first coated steel part portion being greater than the product of the second thickness by the ultimate tensile strength of the second coated steel part portion;
the first and second coated steel part portions being joined by a weld joint, the weld joint having an aluminum content comprised between 0.5 wt. % and 1.25 wt. %, and the microstructure of the weld joint comprising martensite and/or bainite and a fraction of intercritical ferrite comprised between 15% and a maximum intercritical ferrite fraction−5%, the maximum intercritical ferrite fraction being defined using the following formula:
α
IC
m
ax
=
(
1
-
(
1
+
ρ
)
(
max
(
1
;
ρ
)
Ts
2
-
3
5
0
)
(
1
-
β
)
(
ρ
Ts
2
+
Ts
1
)
+
β
(
1
+
ρ
)
(
3
1
3
0
C
FW
+
7
5
0
)
-
3
5
0
(
1
+
ρ
)
)
×
100
,
where
Ts 1 is the ultimate tensile strength of a stronger one of the two steel substrates after press-hardening, in MPa,
Ts 2 is the ultimate tensile strength of a weaker one of the two steel substrates after press-hardening, in MPa,
β is the proportion of a filler material added to a weld pool, comprised between 0 and 1,
C FW is the carbon content of the filler material, in wt. %,
ρ is the ratio between the thickness of the coated steel part portion comprising the weaker steel substrate and the thickness of the coated steel part portion comprising the stronger steel substrate (ρ=t 2 /t 1 ), and
the steel substrate of at least one of the first and the second coated steel part portions having a mainly martensitic and/or bainitic microstructure.
2 . The press-hardened laser welded steel part according to claim 1 , wherein the ratio between the ultimate tensile strength of the steel substrate of the first coated steel part portion and the ultimate tensile strength of the steel substrate of the second coated steel part portion is greater than or equal to 1.2.
3 . The press-hardened laser welded steel part according to claim 1 , wherein, for at least one of the first and the second coated steel part portions, the steel of the steel substrate comprises, by weight:
0.1
%
≤
C
≤
0.5
%
0.5
%
≤
Mn
≤
3
%
0.1
%
≤
Si
≤
1
%
0.01
%
≤
Cr
≤
1
%
Ti
≤
0.2
%
Al
≤
0.1
%
S
≤
0.05
%
P
≤
0.1
%
B
≤
0.01
%
a remainder being iron and impurities resulting from manufacturing.
4 . The press-hardened laser welded steel part according to claim 3 , wherein, for at least one of the first and the second coated steel part portions, the steel of the steel substrate comprises, by weight:
0.15
%
≤
C
≤
0.25
%
0.8
%
≤
Mn
≤
1.8
%
0.1
%
≤
Si
≤
0.35
%
0.01
%
≤
Cr
≤
0.5
%
Ti
≤
0.1
%
Al
≤
0.1
%
S
≤
0.05
%
P
≤
0.1
%
B
≤
0.005
%
the remainder being iron and impurities resulting from manufacturing.
5 . The press-hardened laser welded steel part according to claim 1 , wherein, for at least one of the first and the second coated steel part portions, the steel of the steel substrate comprises, by weight:
0
.
0
40
%
≤
C
≤
0.1
%
0.7
%
≤
Mn
≤
2.
%
Si
≤
0.5
%
S
≤
0.005
%
P
≤
0.03
%
0.01
%
≤
Al
≤
0.07
%
0.015
%
≤
Nb
≤
0.1
%
Ti
≤
0.08
%
N
≤
0.009
%
Cu
≤
0.1
%
Ni
≤
0.1
%
Cr
≤
0.2
%
Mo
≤
0.1
%
Ca
≤
0.006
%
,
a remainder being iron and impurities resulting from manufacturing.
6 . The press-hardened laser welded steel part according to claim 1 , wherein, for at least one of the first and the second coated steel part portions, the steel of the steel substrate comprises, by weight:
0
.24
%
≤
C
≤
0.38
%
0.4
%
≤
Mn
≤
3
%
0.1
%
≤
Si
≤
0.7
%
0.015
%
≤
Al
≤
0.07
%
0
%
≤
Cr
≤
2
%
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
%
0.0001
%
≤
P
≤
0.025
%
wherein the titanium and nitrogen contents satisfy the following relationship:
Ti
/
N
>
3.42
,
and the carbon, manganese, chromium and silicon contents satisfy the following relationship:
2.6
C
+
Mn
5
.
3
+
Cr
1
3
+
Si
1
5
≥
1.1
%
,
the steel optionally comprising one or more of the following elements:
0
.05
%
≤
Mo
≤
0.65
%
0.001
%
≤
W
≤
0.3
%
0.0005
%
≤
Ca
≤
0.005
%
a remainder being iron and impurities inevitably resulting from manufacturing.
7 . Press-hardened laser welded steel part according to claim 1 , wherein, for at least one of the first and the second coated steel part portions, the steel of the steel substrate comprises, by weight:
0
.06
%
≤
C
≤
0.1
%
1.4
%
≤
Mn
≤
1.9
%
0.2
%
≤
Si
≤
0.5
%
0.01
%
≤
Al
≤
0.07
%
0.04
%
≤
Nb
≤
0.06
%
3.
4
×
N
≤
Ti
≤
8
×
N
0.02
%
≤
Cr
≤
0.1
%
0.0005
%
≤
B
≤
0.004
%
0.001
%
≤
S
≤
0.009
%
a remainder being iron and impurities resulting from manufacturing.