Method of producing a phosphatable part from a sheet coated with an aluminum-based coating and a zinc coating
A steel sheet coated with an aluminum-based coating and a second zinc coating having a thickness less than or equal to 1.1 μm is provided. A method for preparing the coated steel sheet, a method for preparing a press-hardened part from the steel sheet, a press-hardened part, and the use of the press-hardened part are also provided.
1 . A coated steel sheet for press hardening comprising:
a steel sheet;
an aluminum-based coating; and
a zinc coating over the aluminum-based coating, the zinc coating having a thickness less than or equal to 1.1 μm, the zinc coating being primarily zinc;
the coated steel sheet having a weight composition as follows:
0.20%≤C≤0.25%,
0.15%≤Si≤0.35%,
1.10%≤Mn≤1.40%,
0%≤Cr≤0.30%,
0%≤Mo≤0.35%,
0%≤P≤0.025%,
0%≤S≤0.005%,
0.020%≤Ti≤0.060%,
0.020%≤Al≤0.060%, and
0.002%≤B≤0.004%,
a balance of the composition being iron and unavoidable impurities resulting from processing.
2 . A coated steel sheet for press hardening comprising:
a steel sheet;
an aluminum-based coating; and
a zinc coating over the aluminum-based coating, the zinc coating having a thickness less than or equal to 1.1 μm, the zinc coating being primarily zinc the coated steel sheet having a weight composition as follows:
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%,
0.020%≤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%,
amounts of titanium and nitrogen satisfying Ti/N>3.42, and
amounts of carbon, manganese, chrome, and silicon satisfying
2.6
C
+
Mn
5.3
+
Cr
13
+
Si
15
≥
1.1
%
,
a balance of the composition being iron and unavoidable impurities resulting from processing.
3 . The coated steel sheet according to claim 2 , further comprising one or more of the following elements by weight:
0.05%≤Mo≤0.65%,
0.001%≤W≤0.30%, and
0.0005%≤Ca≤0.005%.
4 . A method for preparing the coated steel sheet according to claim 1 , wherein the zinc coating is performed by cementation, by electroplating, or by sonic jet vapor deposition under vacuum.
5 . The method according to claim 4 wherein when the zinc coating is deposited by cementation or electroplating, said zinc coating being directly deposited on the aluminum-based coating.
6 . The method according to claim 5 , wherein the zinc coating deposited by cementation is carried out by immersion or by spraying.
7 . The method according to claim 5 , wherein when the zinc coating is deposited under vacuum by sonic jet vapor, a thin layer of stainless steel is deposited between the aluminum-based coating and the zinc coating.
8 . A method for preparing a press-hardened part coated with a phosphatable coating comprising the sequential steps of:
A) providing the coated steel sheet for press-hardening according to claim 1 ;
B) cutting the coated steel sheet obtained in step A) to obtain a blank; then
C) heating the blank at a temperature T 1 in a range from 840° C. to 950° C. to obtain a completely austenitic structure in the steel;
D) transferring the blank to a press;
E) hot-pressing the blank to obtain a part;
F) cooling the part obtained in step E) to obtain a martensitic or a martensitic-bainitic structure in the steel composed of at least 75% equiaxed iron, an amount of martensite greater than or equal to 5% and less than or equal to 20%, an amount of bainite less than or equal to 10%.
9 . The method according to claim 8 , wherein step C) is implemented for a time t 1 from 3 to 12 minutes in an inert atmosphere or an atmosphere comprising air.
10 . The method according to claim 8 , wherein during step E), hot-pressing is carried out at a temperature T 2 ranging from 600° C. to 830° C.
11 . A hardened part obtained by the method according to claim 8 , comprising:
a ZnO layer; and
a phosphate crystal layer on the ZnO layer obtained after an additional phosphatation step G).
12 . The hardened part according to claim 11 , wherein a degree of phosphate crystal coverage on the surface of the part is greater than or equal to 90%.
13 . The hardened part according to claim 12 , wherein the degree of phosphate crystal coverage on the surface of the part is greater than or equal to 99%.
14 . The hardened part according to claim 11 , further comprising a cataphoresis layer on the phosphate crystal layer.
15 . The press hardened part according to claim 11 further comprising a cataphoresis layer on the phosphate crystal layer.
16 . A motor vehicle comprising:
the press-hardened part according to claim 11 .
17 . A method of manufacturing motor vehicle parts comprising:
preparing the press hardened part according to claim 11 .