U.S. patent application number 15/748033 was filed with the patent office on 2018-08-09 for steel sheet coated with a metallic coating based on aluminum.
This patent application is currently assigned to ARCELORMITTAL. The applicant listed for this patent is ARCELORMITTAL. Invention is credited to Christian ALLELY, Joost DE STRYCKER, Tiago MACHADO AMORIM, Krista Godelieve Oscar VAN DEN BERGH.
Application Number | 20180223409 15/748033 |
Document ID | / |
Family ID | 54015125 |
Filed Date | 2018-08-09 |
United States Patent
Application |
20180223409 |
Kind Code |
A1 |
ALLELY; Christian ; et
al. |
August 9, 2018 |
Steel Sheet Coated with a Metallic Coating based on Aluminum
Abstract
A steel sheet coated with a metallic coating is provided. The
steel sheet includes from 2.0 to 24.0% by weight of zinc, from 7.1
to 12.0% by weight of silicon, optionally from 1.1 to 8.0% by
weight of magnesium, and optionally additional elements chosen from
Pb, Ni, Zr, or Hf, the content by weight of each additional element
being less than 0.3% by weight, the balance being aluminum and
optionally unavoidable impurities and residual elements. The ratio
Al/Zn is above 2.9.
Inventors: |
ALLELY; Christian; (Metz,
FR) ; MACHADO AMORIM; Tiago; (Longeville Les Metz,
FR) ; DE STRYCKER; Joost; (Zele, BE) ; VAN DEN
BERGH; Krista Godelieve Oscar; (Sint-Gillis-Waas,
BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ARCELORMITTAL |
Luxembourg |
|
LU |
|
|
Assignee: |
ARCELORMITTAL
Luxembourg
LU
|
Family ID: |
54015125 |
Appl. No.: |
15/748033 |
Filed: |
July 8, 2016 |
PCT Filed: |
July 8, 2016 |
PCT NO: |
PCT/IB2016/000977 |
371 Date: |
January 26, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 21/10 20130101;
C23C 2/12 20130101; C23C 2/00 20130101; C23C 2/40 20130101 |
International
Class: |
C23C 2/12 20060101
C23C002/12; C23C 2/40 20060101 C23C002/40; C22C 21/10 20060101
C22C021/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 30, 2015 |
IB |
PCT/IB2015/001281 |
Claims
1-25. (canceled)
26. A steel sheet coated with a metallic coating, the metallic
coating comprising: 2.0 to 24.0% by weight of zinc; 7.1 to 12.0% by
weight of silicon; and a balance being aluminum, unavoidable
impurities and residual elements; and a ratio of Al/Zn being
greater than 2.9
27. The steel sheet according to claim 26, further comprising from
1.1 to 8.0% by weight of magnesium
28. The steel sheet according to claim 26, further comprising
additional elements chosen from Pb, Ni, Zr, or Hf, the content by
weight of each additional element being less than 0.3% by
weight.
29. The steel sheet according to claim 26, wherein the ratio Al/Zn
is less than or equal to 8.5.
30. The steel sheet according to claim 29, wherein the ratio Al/Zn
is between 3.0 and 7.5.
31. The steel sheet according to claim 30, wherein the ratio Al/Zn
is between 4.0 and 6.0.
32. The steel sheet according to claim 26, wherein a microstructure
of the metallic coating comprises an Al--Zn phase.
33. The steel sheet according to claim 26, wherein the coating
comprises from 10.0 to 20.0% by weight of zinc.
34. The steel sheet according to claim 26, wherein the coating
comprises from 10.0 to 15.0% by weight of zinc.
35. The steel sheet according to claim 26, wherein the coating
comprises from 8.1 to 10.0% by weight of silicon.
36. The steel sheet according to claim 26, wherein the coating
comprises from 3.0 to 8.0% by weight of magnesium.
37. The steel sheet according to claim 36, wherein the coating
comprises from 3.0 to 5.0% by weight of magnesium.
38. The steel sheet according to claims 26, wherein a
microstructure of the metallic coating comprises a Mg.sub.2Si
phase.
39. The steel sheet according to claim 26, wherein a microstructure
of the metallic coating comprises a MgZn.sub.2 phase.
40. The steel sheet according to claim 26, wherein an amount of
aluminum is greater than 71% by weight.
41. The steel sheet according claim 26, wherein an amount of
aluminum is greater than 76% by weight.
42. The steel sheet according to claim 26, wherein a thickness of
the metallic coating is between 5 and 50 .mu.m.
43. The steel sheet according to claim 42, wherein the thickness is
between 10 and 35 .mu.m.
44. The steel sheet according to claim 42, wherein the thickness is
between 12 and 18 .mu.m.
45. The steel sheet according to claim 42, wherein the thickness is
between 26 and 31 .mu.m.
46. The steel sheet according to claim 26, wherein the coating does
not comprise elements selected among Cr, Mn, Ti, Ce, La, Nd, Pr,
Ca, Bi, In, Sn and Sb or combinations thereof.
47. A part comprising: the coated steel sheet according to claim 26
formed into a part by hot-forming or cold-stamping.
48. The part according to claim 47, wherein the part is formed by
hot-forming and cold-stamping.
49. The part according to claim 47, wherein the part is a press
hardened steel part having a variable thickness.
50. The part according to claim 49, wherein the variable thickness
is produced by a continuous flexible rolling process.
51. The part according to claim 47, wherein the part is a tailored
rolled blank.
52. The part according to claim 47, wherein the part is a front
rail, a seat cross member, a side sill member, a dash panel cross
member, a front floor reinforcement, a rear floor cross member, a
rear rail, a B-pillar, a door ring or a shotgun.
53. An automotive vehicle comprising: the part according to claim
47.
Description
[0001] The present invention relates to a steel sheet coated with a
metallic coating. The invention is particularly well suited for the
manufacture of automotive vehicles.
BACKGROUND
[0002] Zinc-based coatings are generally used because they allows
for a protection against corrosion thanks to barrier protection and
cathodic protection. The barrier effect is obtained by the
application of a metallic coating on steel surface. Thus, metallic
coatings prevent the contact between steel and corrosive
atmosphere. The barrier effect is independent from the nature of
coating and substrate. On the contrary, sacrificial cathodic
protection is based on the fact that zinc is a metal less noble
that steel. Thus, if corrosion occurs, zinc is consumed
preferentially to steel. Cathodic protection is essential in areas
where steel is directly exposed to corrosive atmosphere, like cut
edges where surrounding zinc will be consumed before steel.
[0003] However, when press hardening process is performed on such
zinc coated steel sheets, for example by hot-stamping, microcracks
are observed in steel which spread from the coating. Additionally,
the step of painting of some hardened parts coated with zinc
necessitates sanding operations before phosphatation due to the
presence of a weak layer of oxides at the part surface.
[0004] Other metallic coatings usually used for the production of
automotive vehicle are aluminum and silicon based coatings. There
is no microcrack in steel when press hardening process is performed
due to the presence of an intermetallic layer Al--Si--Fe. Moreover,
they have a good aptitude for painting. They allow for a protection
by barrier effect and can be welded. However, they do not allow for
a cathodic protection or they have a very low cathodic
protection.
[0005] The patent application EP1225246 discloses a Zn--Al--Mg--Si
alloy-plated material wherein the coating comprises, in terms of
weight %, Al: at least 45% and no greater than 70%, Mg: at least 3%
and less than 10%, Si: at least 3% and less than 10%, with the
remainder Zn and unavoidable impurities, wherein the Al/Zn ratio is
0.89-2.75 and the plating layer contains a bulky Mg.sub.2Si phase.
It also discloses a Zn--Al--Mg--Si alloy-plated steel material
wherein the coating comprises, in terms of weight %, Al: at least
45% and no greater than 70%, Mg: at least 1% and less than 5%, Si:
at least 0.5% and less than 3%, with the remainder Zn and
unavoidable impurities, wherein the Al/Zn ratio is 0.89-2.75 and
the plating layer contains a scaly Mg.sub.2Si phase. These specific
coatings show unpainted corrosion resistance and edge creep
resistance at cut edge sections after painting.
[0006] However, the fabrication of specific Mg.sub.2Si phases,
scaly or bulky, is complex. Indeed, it depends on the size and on
the ratio of the short diameter mean size with respect to the long
diameter of Mg.sub.2Si phases, as observed with a 5.degree.
polished cross-section. The size is affected most predominantly by
the cooling rate after hot-dip plating. Moreover, the fabrication
of Mg.sub.2Si phases also depends on the quantity of Mg and Si.
[0007] From an industrial point of view, Mg.sub.2Si phases can be
difficult to obtain because of these specifics criteria. Therefore,
there is a risk that the desired Mg.sub.2Si phase is not
obtained.
SUMMARY OF THE INVENTION
[0008] The purpose of the invention is to provide a coated steel
sheet easy to form having a reinforced protection against
corrosion, i.e. a sacrificial cathodic protection in addition to
barrier protection, before and after the forming.
[0009] In terms of sacrificial protective corrosion,
electrochemical potential has to be at least 50 mV more negative
than the potential of steel, i.e. a maximum potential of -0.78V
with respect to a saturated calomel electrode (SCE). It is
preferable not to decrease the potential at a value of -1.4V/SCE,
even -1.25V/SCE which would involve a fast consumption and would
finally decrease the period of protection of steel.
[0010] A steel sheet coated with a metallic coating comprising from
2.0 to 24.0% by weight of zinc, from 7.1 to 12.0% by weight of
silicon, optionally from 1.1 to 8.0% by weight of magnesium, and
optionally additional elements chosen from Pb, Ni, Zr, or Hf, the
content by weight of each additional element being less than 0.3%
by weight, the balance being aluminum and optionally unavoidable
impurities and residuals elements, wherein the ratio Al/Zn is above
2.9.
[0011] The invention also provides parts made from the steel sheet
coated with the metallic.
[0012] The invention further provides a coated part for the
manufacture of an automotive.
BRIEF DESCRIPTION OF THE DRAWING
[0013] To illustrate the invention, various embodiments and trials
of non-limiting examples will be described, particularly with
reference to the following FIGURE:
[0014] FIG. 1 illustrates one corrosion cycle corresponding to 168
hours of the norm VDA 233-102.
DETAILED DESCRIPTION
[0015] Other characteristics and advantages of the invention will
become apparent from the following detailed description of the
invention.
[0016] Any steel can be advantageously used in the scope of the
invention. However, in case steel having high mechanical strength
is needed, in particular for parts of structure of automotive
vehicle, steel having a tensile resistance superior to 500 MPa,
advantageously between 500 and 2000 MPa before or after
heat-treatment, can be used. The weight composition of steel sheet
is preferably as follows: 0.03%.ltoreq.C.ltoreq.0.50%;
0.3%.ltoreq.Mn.ltoreq.3.0%; 0.05%.ltoreq.Si.ltoreq.0.8%;
0.015%.ltoreq.Ti.ltoreq.0.2%; 0.005%.ltoreq.Al.ltoreq.0.1%;
0%.ltoreq.Cr.ltoreq.2.50%; 0%.ltoreq.S.ltoreq.0.05%;
0%.ltoreq.P.ltoreq.0.1%; 0%.ltoreq.B.ltoreq.0.010%;
0%.ltoreq.Ni.ltoreq.2.5%; 0%.ltoreq.Mo.ltoreq.0.7%;
0%.ltoreq.Nb.ltoreq.0.15%; 0%.ltoreq.N.ltoreq.0.015%;
0%.ltoreq.Cu.ltoreq.0.15%; 0%.ltoreq.Ca.ltoreq.0.01%;
0%.ltoreq.W.ltoreq.0.35%, the balance being iron and unavoidable
impurities from the manufacture of steel.
[0017] For example, the steel sheet is 22MnB5 with the following
composition: 0.20%.ltoreq.C.ltoreq.0.25%;
0.15%.ltoreq.Si.ltoreq.0.35%; 1.10%.ltoreq.Mn.ltoreq.1.40%;
0%.ltoreq.Cr.ltoreq.0.30%; 0%.ltoreq.Mo.ltoreq.0.35%;
0%.ltoreq.P.ltoreq.0.025%; 0%.ltoreq.S.ltoreq.0.005%;
0.020%.ltoreq.Ti.ltoreq.0.060%; 0.020%.ltoreq.Al.ltoreq.0.060%;
0.002%.ltoreq.B.ltoreq.0.004%, the balance being iron and
unavoidable impurities from the manufacture of steel.
[0018] The steel sheet can be Usibor.RTM.2000 with the following
composition: 0.24%.ltoreq.C.ltoreq.0.38%;
0.40%.ltoreq.Mn.ltoreq.3%; 0.10%.ltoreq.Si.ltoreq.0.70%;
0.015%.ltoreq.Al.ltoreq.0.070%; 0%.ltoreq.Cr.ltoreq.2%;
0.25%.ltoreq.Ni.ltoreq.2%; 0.020%.ltoreq.Ti.ltoreq.0.10%;
0%.ltoreq.Nb.ltoreq.0.060%; 0.0005%.ltoreq.B.ltoreq.0.0040%;
0.003%.ltoreq.N.ltoreq.0.010%; 0.0001%.ltoreq.S.ltoreq.0.005%;
0.0001%.ltoreq.P.ltoreq.0.025%; it being understood that the
contents of titanium and nitrogen satisfy Ti/N>3.42; and that
the contents of carbon, manganese, chromium and silicon
satisfy:
2.6 C + Mn 5.3 + Cr 13 + Si 15 .gtoreq. 1.1 % ##EQU00001##
the composition optionally comprising one or more of the following:
0.05%.ltoreq.Mo.ltoreq.0.65%; 0.001%.ltoreq.W.ltoreq.0.30%;
0.0005%.ltoreq.Ca.ltoreq.0.005%, the balance being iron and
unavoidable impurities from the manufacture of steel.
[0019] For example, the steel sheet is Ductibor.RTM.500 with the
following composition: 0.040%.ltoreq.C.ltoreq.0.100%;
0.80%.ltoreq.Mn.ltoreq.2.00%; 0%.ltoreq.Si.ltoreq.0.30%;
0%.ltoreq.S.ltoreq.0.005%; 0%.ltoreq.P.ltoreq.0.030%;
0.010%.ltoreq.Al.ltoreq.0.070%; 0.015%.ltoreq.Nb.ltoreq.0.100%;
0.030%.ltoreq.Ti.ltoreq.0.080%; 0%.ltoreq.N.ltoreq.0.009%;
0%.ltoreq.Cu.ltoreq.0.100%; 0%.ltoreq.Ni.ltoreq.0.100%;
0%.ltoreq.Cr.ltoreq.0.100%; 0%.ltoreq.Mo.ltoreq.0.100%;
0%.ltoreq.Ca.ltoreq.0.006%, the balance being iron and unavoidable
impurities from the manufacture of steel.
[0020] Steel sheet can be obtained by hot rolling and optionally
cold rolling depending on the desired thickness, which can be for
example between 0.7 and 3.0 mm.
[0021] The invention provides a steel sheet coated with a metallic
coating comprising from 2.0 to 24.0% by weight of zinc, from 7.1 to
12.0% by weight of silicon, optionally from 1.1 to 8.0% by weight
of magnesium, and optionally additional elements chosen from Pb,
Ni, Zr, or Hf, the content by weight of each additional element
being less than 0.3% by weight, the balance being aluminum and
optionally unavoidable impurities and residuals elements, wherein
the ratio Al/Zn is above 2.9. Metallic coatings according to the
invention have a high sacrificial protection.
[0022] Preferably, the metallic coating does not comprise elements
selected among Cr, Mn, Ti, Ce, La, Nd, Pr, Ca, Bi, In, Sn and Sb or
their combinations. In another preferred embodiment, the metallic
coating does not comprise any of the following compounds: Cr, Mn,
Ti, Ce, La, Nd, Pr, Ca, Bi, In, Sn and Sb. Indeed, without willing
to be bound by any theory, it seems that when these compounds are
present in the coating, there is a risk that the properties of the
coating, such as electrochemical potential, are altered, because of
their possible interactions with the essential elements of the
coatings.
[0023] Preferably, the ratio Al/Zn is below or equal to 8.5.
Preferably, the ratio Al/Zn is between 3.0 and 7.5, advantageously
between 4.0 and 6.0. Without willing to be bound by any theory, it
seems that if these conditions are not met, there is a risk that
the sacrificial protection decreases because zinc rich phases are
not in sufficient amount in the coating.
[0024] In a preferred embodiment, the coating layer further
comprises an Al--Zn phase.
[0025] Advantageously, the metallic coating comprises from 10.0 to
20.0%, preferably from 10.0 to 15.0%, by weight of zinc.
[0026] Preferably, the metallic coating comprises from 8.1 to 10.0%
by weight of silicon.
[0027] Advantageously, the coating comprises from 3.0 to 8.0% by
weight of magnesium, preferably, from 3.0 to 5.0% by weight of
magnesium. Without willing to be bound by any theory, it has been
found that the addition of magnesium in the above range further
improve the anti-corrosion properties.
[0028] Preferably, the microstructure of said coating comprising a
Mg.sub.2Si phase. In another preferred embodiment, the
microstructure of said coating further comprises a MgZn.sub.2
phase.
[0029] Advantageously, the amount of aluminum is above 71%,
preferably above 76%, by weight.
[0030] The coating can be deposited by any methods known to the man
skilled in the art, for example hot-dip galvanization process,
electrogalvanization process, physical vapour deposition such as
jet vapor deposition or sputtering magnetron. Preferably, the
coating is deposited by hot-dip galvanization process. In this
process, the steel sheet obtained by rolling is dipped in a molten
metal bath.
[0031] The bath comprises zinc, silicon, aluminum and optionally
magnesium. It can comprise additional elements chosen from Pb, Ni,
Zr, or Hf, the content by weight of each additional element being
less than 0.3% by weight. These additional elements can improve
among others ductibility, coating adhesion on the steel sheet.
[0032] The bath can also contain unavoidable impurities and
residuals elements from feeding ingots or from the passage of the
steel sheet in the molten bath. Residual element can be iron with a
content up to 3.0% by weight.
[0033] The thickness of the coating is usually between 5 and 50
.mu.m, preferably between 10 and 35 .mu.m, advantageously between
12 and 18 .mu.m or between 26 to 31 .mu.m. The bath temperature is
usually between 580 and 660.degree. C.
[0034] After the deposition of the coating, the steel sheet is
usually wiped with nozzles ejecting gas on both sides of the coated
steel sheet. The coated steel sheet is then cooled. Preferably, the
cooling rate is above or equal to 15.degree. C.s.sup.-1 between the
beginning of the solidification and the end of the solidification.
Advantageously, the cooling rate between the beginning and the end
of the solidification is superior or equal to 20.degree.
C.s.sup.-1.
[0035] Then, a skin-pass can be realized and allows work hardening
the coated steel sheet and giving it a roughness facilitating the
subsequent shaping. A degreasing and a surface treatment can be
applied in order to improve for example adhesive bonding or
corrosion resistance.
[0036] Then, the coated steel sheet according to the invention can
be shaped by any method known to the man skilled in the art, for
example cold-stamping and/or hot-forming.
[0037] In a preferred embodiment, the part is obtained by
cold-stamping. In this case, the coated steel sheet is cut to
obtain a blank and then cold-stamped in order to obtain a part.
[0038] In another preferred embodiment, the part coated is obtained
by a press hardening process including the hot-forming. In this
case, this method comprises the following steps: [0039] A) the
provision of a steel sheet pre-coated with a metallic coating
comprising from 2.0 to 24.0% by weight of zinc, from 7.1 to 12.0%
by weight of silicon, optionally from 1.1 to 8.0% by weight of
magnesium, and optionally additional elements chosen from Pb, Ni,
Zr, or Hf, the content by weight of each additional element being
less than 0.3% by weight, the balance being aluminum and
unavoidable impurities and residuals elements, wherein the ratio
Al/Zn is above 2.9, [0040] B) the cutting of the coated steel sheet
to obtain a blank, [0041] C) the thermal treatment of the blank at
a temperature between 840 and 950.degree. C. to obtain a fully
austenitic microstructure in the steel, [0042] D) the transfer of
the blank into a press tool, [0043] E) the hot-forming of the blank
to obtain a part, [0044] F) the cooling of the part obtained at
step E) in order to obtain a microstructure in steel being
martensitic or martensito-bainitic or made of at least 75% of
equiaxed ferrite, from 5 to 20% of martensite and bainite in amount
less than or equal to 10%.
[0045] Indeed, after, the provision of steel sheet pre-coated with
the metallic coating according to the present invention the cutting
to obtain a blank. A thermal treatment is applied to the blank in a
furnace under non protective atmosphere at an austenitization
temperature Tm usually between 840 and 950.degree. C., preferably
880 to 930.degree. C. Advantageously, said blank is maintained
during a dwell time tm between 1 to 12 minutes, preferably between
3 to 9 minutes. During the thermal treatment before the
hot-forming, the coating forms an alloy layer having a high
resistance to corrosion, abrasion, wear and fatigue.
[0046] After the thermal treatment, the blank is then transferred
to a hot-forming tool and hot-formed at a temperature between 600
and 830.degree. C. The hot-forming comprises the hot-stamping and
the roll-forming. Preferably, the blank is hot-stamped. The part is
then cooled in the hot-forming tool or after the transfer to a
specific cooling tool.
[0047] The cooling rate is controlled depending on the steel
composition, in such a way that the final microstructure after the
hot-forming comprises mostly martensite, preferably contains
martensite, or martensite and bainite, or is made of at least 75%
of equiaxed ferrite, from 5 to 20% of martensite and bainite in
amount less than or equal to 10%.
[0048] A coated part according to the invention can thus obtained
by cold or hot forming but also by any suitable combination of
cold-stamping and hot-forming.
[0049] In a preferred embodiment, the part is a press hardened
steel part having a variable thickness, i.e. the press hardened
steel part of the invention can have a thickness which is not
uniform but which can vary. Indeed, it is possible to achieve the
desired mechanical resistance level in the zones which are the most
subjected to external stresses, and to save weight in the other
zones of the press hardened part, thus contributing to the vehicle
weight reduction. In particular, the parts with non-uniform
thickness can be produced by continuous flexible rolling, i.e. by a
process wherein the sheet thickness obtained after rolling is
variable in the rolling direction, in relationship with the load
which has been applied through the rollers to the sheet during the
rolling process.
[0050] Thus, within the conditions of the invention, it is possible
to manufacture advantageously vehicle parts with varying thickness
in order to obtain for example a tailored rolled blank.
Specifically, the part can be a front rail, a seat cross member, a
side sill member, a dash panel cross member, a front floor
reinforcement, a rear floor cross member, a rear rail, a B-pillar,
a door ring or a shotgun.
[0051] For automotive application, after phosphating step, the part
is dipped in an e-coating bath. Usually, the thickness of the
phosphate layer is between 1 and 2 .mu.m and the thickness of the
e-coating layer is between 15 and 25 .mu.m, preferably less than or
equal to 20 .mu.m. The cataphoresis layer ensures an additional
protection against corrosion.
[0052] After the e-coating step, other paint layers can be
deposited, for example, a primer coat of paint, a basecoat layer
and a top coat layer.
[0053] Before applying the e-coating on the part, the part is
previously degreased and phosphated so as to ensure the adhesion of
the cataphoresis.
[0054] The invention will now be explained in trials carried out
for information only. They are not limiting.
EXAMPLES
[0055] For all samples, steel sheets used are 22MnB5. The
composition of the steel is as follows: C=0.2252%; Mn=1.1735%;
P=0.0126%, S=0.0009%; N=0.0037%; Si=0.2534%; Cu=0.0187%;
Ni=0.0197%; Cr=0.180%; Sn=0.004%; Al=0.0371%; Nb=0.008%;
Ti=0.0382%; B=0.0028%; Mo=0.0017%; As=0.0023% and V=0.0284%.
[0056] All coatings were deposited by hot-dip galvanization
process. All coatings have a thickness of 15 .mu.m.
Example 1
Cut Edge Potential Test
[0057] Trials 1 to 4 were prepared and subjected to an
electrochemical potential test.
[0058] A test consisting in measuring the cut edges potential of
coated steel sheet was realized. To this end, each steel sheet was
dipped in a solution comprising 2.43% by weight of sodium sulfate
and 0.1% by weight of sodium chloride. A saturated calomel
electrode (SCE) was also immersed into the solution. The coupling
potential of cut edges was measured. Results are shown in the
following Table 1:
TABLE-US-00001 Coupling Coating Thickness potential Trials Al Si Zn
Mg (.mu.m) (V/SCE) 1* 81 9 10 -- 15 -0.84 2* 77 9 10 4 15 -0.84 3*
73 9 10 8 15 -0.84 4 91 9 -- -- 15 -0.625 *examples according to
the invention.
[0059] Trials according to the invention (Trials 1 to 3) have a
lower coupling potential than a coating comprising aluminum and 9%
by weight of silicon. Coupling potentials of Trials 1 to 3 are
under -0.78V/SCE as required.
Example 2
Cut Edge Corrosion Test
[0060] Trials 5 to 12 were prepared and subjected to a corrosion
test to evaluate the cut edge protection of the coated steel
sheets.
[0061] All trials were dipped in a solution comprising 2.43% by
weight of sodium sulfate and 0.1% by weight of sodium chloride
during 50 hours. The presence of corrosion on cut edges of coated
steel sheet was observed with the naked eye: 0 means excellent, in
other words, there is little or no corrosion and 5 means very bad,
in other words, there are is a lot of corrosion on the cut edges.
Results are shown in the following Table 2:
TABLE-US-00002 Coating Thickness Trials Al Si Zn Mg (.mu.m)
Corrosion 5* 86 9 5 -- 15 2 6* 81 9 10 -- 15 1.5 7* 71 9 20 -- 15 1
8* 77 9 10 4 15 0 9* 73 9 10 8 15 0 10* 67 9 20 4 15 0 11* 63 9 20
8 15 0 12 91 9 -- -- 15 5 *examples according to the invention.
[0062] Trials 5 to 11 have very good protection against corrosion
on the cut edges of coated steel sheet. By contrast, Trial 12 does
not show enough corrosion resistance on the cut edges.
Example 3
Electrochemical Behavior Test
[0063] Trials 13 to 16 were prepared and subjected to an
electrochemical potential test.
[0064] A test consisting in measuring the electrochemical potential
of the coated steel surface sheet was realized. Steel sheets and
coatings were separated and dipped in a solution comprising 5% by
weight of sodium chloride at pH 7. A saturated calomel electrode
(SCE) was also immersed into the solution. The coupling potential
of the surface was measured over time. Results are shown in the
following Table 3:
TABLE-US-00003 Coupling Coating Thickness potential Trials Al Si Zn
Mg (.mu.m) (V/SCE) 13* 81 9 10 -- 15 -0.98 14* 77 9 10 4 15 -0.98
15* 73 9 10 8 15 -0.99 16 0.2 -- 99.8 -- 7 -1.00 *examples
according to the invention.
[0065] Trials 13 to 15 are sacrificial such as zinc coating.
Coupling potential are under -0.78V/SCE as required.
Example 4
Corrosion Test
[0066] Trials 17 to 20 were prepared and subjected to a corrosion
test to evaluate the protection of the coated steel sheets.
[0067] A test, consisting in submitting coated steel sheet to
corrosion cycles according to the norm VDA 233-102, was realized.
At this end, trials were put in a chamber wherein an aqueous
solution of sodium chloride of 1% by weight was vaporized on trials
with a rate of flow of 3 mLh.sup.-1. The temperature varied from 50
to -15.degree. C. and the humidity rate varied from 50 to 100%.
FIG. 1 illustrates one cycle corresponding to 168 hours, i.e. one
week.
[0068] The presence of corrosion on coated steel sheet was observed
by naked eyes: 0 means excellent, in other words, there is little
or no corrosion and 5 means very bad, in other words, there is a
lot of corrosion. Results are shown in the following Table 4:
TABLE-US-00004 Coating Thickness Number of cycles Trials Al Si Zn
Mg (.mu.m) 1 5 7 10 15 20 17* 81 9 10 -- 15 0 0 0.5 1 3 4 18* 77 9
10 4 15 0 0 0 0 0 0 19* 73 9 10 8 15 0 0 0 0 0 0 20 0.2 -- 99.8 --
7 0 2 4 ND ND ND *examples according to the invention, ND: not
done.
[0069] Trials 17 to 19 show excellent protection against corrosion,
in particular when the coating comprises magnesium (Trials 18 and
19).
Example 5
Corrosion Test on Scratched Trials
[0070] Trials 21 to 24 were prepared and subjected to a corrosion
test to evaluate the protection of the coated steel sheets.
[0071] Firstly, all trials were scratched on a width of 0.5, 1 and
2 mm. then, all trials were submitted to corrosion cycles according
to the norm VDA 233-102 represented in FIG. 1.
[0072] The presence of corrosion on coated steel sheet around
scratches was observed by naked eyes: 0 means excellent, in other
words, there is little or no corrosion around scratch and 5 means
very bad, in other words, there is a lot of corrosion around
scratch. Results are shown in the following Table 5:
TABLE-US-00005 Thickness Coating (.mu.m) Number of cycles Trials Al
Si Zn Mg 15 1 2 3 4 5 6 21* 81 9 10 -- 15 0 0 0.5 1 2 3 22* 77 9 10
4 15 0 0 0 0 0 0 23* 73 9 10 8 15 0 0 0 0 0 0.5 24 0.2 -- 99.8 --
10 0 0 0 1 2 3 *examples according to the invention.
[0073] Trials according to the invention (Trials 21 to 23) have an
excellent protection against corrosion, in particular when the
coating comprises magnesium (Trial 22 and 23).
Example 6
Corrosion Test on Heat Treated and Scratched Trials
[0074] Trials 25 to 28 were prepared and subjected to a corrosion
test to evaluate the protection of the coated steel sheets after
austenitization treatment.
[0075] All trials were cut in order to obtain a blank. Blanks were
then heated at a temperature of 900.degree. C. during a dwell time
varying between 5 and 10 minutes. Blanks were transferred into a
press tool and hot-stamped in order to obtain parts. Then, parts
were cooled to obtain a hardening by martensitic transformation.
All trials were submitted to 6 corrosion cycles according to the
norm VDA 233-102 represented in FIG. 1.
[0076] The presence of corrosion on coated steel sheet around
scratches was observed by naked eyes: 0 means excellent, in other
words, there is little or no corrosion around scratch and 5 means
very bad, in other words, there is a lot of corrosion around
scratch. Results are shown in the following Table 6:
TABLE-US-00006 Dwell time Coating Thickness (min) Trials Al Si Zn
Mg (.mu.m) 5 10 25* 71 9 20 -- 15 1 1 26* 77 9 10 4 15 0.5 0.5 27*
73 9 10 8 15 2 3 28 91 9 -- -- 15 5 5 *examples according to the
invention.
[0077] Trials 25 to 27 show good protection against corrosion
compared to the coating comprising aluminum and silicon (Trial
28).
Example 7
Electrochemical Behavior Test
[0078] Trials 29 to 40 were prepared and subjected to an
electrochemical potential test after austenitization treatment.
[0079] All trials were cut in order to obtain a blank. Blanks were
then heated at a temperature of 900.degree. C. during a dwell time
of 5 minutes. Blanks were transferred into a press tool and
hot-stamped in order to obtain parts. Then, parts were cooled to
obtain a hardening by martensitic transformation.
[0080] A test consisting in measuring the electrochemical potential
of the coated steel surface sheet was realized. Steel sheets and
coatings were separated and dipped in a solution comprising 5% by
weight of sodium chloride at pH 7. A saturated calomel electrode
(SCE) was also immersed into the solution. The power of sacrificial
protection, also called galvanic coupling, was measured over time.
In other words, it has been assessed how long the coating remains
sacrificial in these conditions. Results are shown in the following
Table 7:
TABLE-US-00007 Galvanic Coating Thickness coupling Trials Al Si Zn
Mg (.mu.m) (hours) 29 88 2 10 -- 15 0 30 83 2 15 -- 15 0 31 80 5 15
-- 15 0 32* 81 9 10 -- 15 16 33* 77 9 10 4 15 45 34* 73 9 10 8 15 7
35* 76 9 15 -- 15 26 36* 83 9 15 2 15 84 37* 71 9 20 -- 15 140 38*
67 9 20 4 15 91 39* 63 9 20 8 15 14 40 91 9 -- -- 15 0 *examples
according to the invention.
[0081] Trials 32 to 39 according to the present invention are and
remain sacrificial protection over time.
* * * * *