ZINC-PLATED STEEL SHEET HAVING SUPERIOR BLACKENING
RESISTANCE AND CORROSION RESISTANCE AND
METHOD FOR PRODUCING SAME
TECHNICAL FIELD
[0001]
The present invention relates to a zinc-plated steel
10 sheet having superior blackening resistance and corrosion
resistance, and more particularly, to a zinc-plated steel
sheet that can be used in home appliance, construction
material, civil engineering, mechanical, automotive,
furniture and container applications. In addition, the
15 present invention also relates to a method for producing
that zinc-plated steel sheet.
20
BACKGROUND ART
[0002]
Zinc-plated steel sheets having superior corrosion
resistance are widely used in applications such as home
appliances, construction materials or automobiles. In
addition, a Zn-Al-Mg-Si-plated steel sheet described in
Patent Document 1 is known as a technology for further
25 enhancing the corrosion resistance of zinc-plated steel
sheets.
[0003]
On the other hand, these zinc-plated steel sheets
are susceptible to the formation of white rust caused by
30 corrosion of zinc, and technologies are knovm that
consist of carrying out chromate treatment or other
chemical conversion treatment in order to inhibit this.
Although zinc-plated steel sheets that have undergone
chromate treatment are resistant to the formation of
35 white rust, on the other hand, they have the problem of
the plated surface becoming discolored to a grayish black
color when exposed to the atmosphere for a long period of
- 2 -
time. This discoloration phenomenon may hereinafter be
referred to as blackening. Blackening occurs
particularly prominently in Zn-Al-based alloy-plated
steel sheets or Zn-Al-Mg-based alloy-plated steel sheets
5 in which Al or Mg has been added to the zinc plating.
[0004]
Patent Documents 2 to 6 describe technologies for
improving blackening resistance of zinc-plated steel
sheets. Patent Document 2 discloses a technology
10 consisting of treating a zinc-plated steel sheet with a
solution in which Ni ions or Co ions are mixed. Patent
Document 3 discloses a technology consisting of treating
a zinc-plated steel sheet with a chromate treatment
solution that contains nitrate ions and has a specific
15 composition. Patent Document 4 discloses a technology
consisting of coating an aqueous treatment solution
containing 1 g/1 to 100 g/1 of molybdate ion as Mo, 0.2
to 2 of, as the weight ratio of P/Mo, phosphate ions,
0.03 to 0.3 of, as the weight ratio of Co/Mo, cobalt ions
20 and 1 g/1 to 300 g/1 of oxycarboxylic acid onto a zincplated
steel sheet at 10 g/m2 to 120 g/m2 as the adhered
amount of Mo. Moreover, Patent Document 5 discloses a
technology for a plating layer structure having a zinc
phosphate treatment layer on a zinc plating layer, and
25 having a portion adhered Hi th 0.1 mg/m2 to 500 mg/m2 of Ni
interposed between the zinc plating layer and the
phosphate treatment layer. Patent Document 6 discloses
that blackening resistance of a hot-dipped Zn-Al-Mg
alloy-plated steel sheet can be improved by forming a
30 phosphate coating on the surface of the hot-dipped Zn-AlMg
alloy-plated steel sheet and forming a chemical
conversion treatment coating obtained by crosslinking a
specific aqueous fluorine-containing resin on the
phosphate coating.
35 [0005]
As vlill be subsequently described, a paint film is
used in the present invention that contains flake-like
- 3 -
aluminum particles. Patent Document 7 discloses a
precoated metal sheet imparted Hith a metallic appearance
by a paint film containing an aluminum pigment, Hherein
the surface of the aluminum pigment is coated to prevent
5 blackening of the pigment caused by contact between the
aluminum pigment and the underlying metal sheet.
[0006]
On the other hand, although unrelated to blackening
resistance, Patent Document 8 describes a paint
10 composition containing thin film, flake-like aluminum
that is used to impart steel sheets and the like with a
plated appearance having a superior aesthetic appearance.
Patent Document 9 describes a technology for preventing
discoloration of a paint film or other decreases in
15 aesthetic appearance of a coated steel sheet obtained by
dispersing aluminum particles in a paint film, by
inhibiting elution of aluminum from the paint film in a
strongly alkaline environment.
20 Prior Art Documents
Patent Documents
[0007]
Patent Document 1:. JP 3179446 B
Patent Document 2: JP S59-177381 A
25 Patent Document 3: JP H10-18048 A
Patent Document 4: JP 2001-158972 A
Patent Document 5: JP 2006-225737 A
Patent Document 6: JP 2012-077322 A
Patent Document 7: WO 2013/065354
30 Patent Document 8: JP 2000-136329 A
Patent Document 9: JP 2011-194872 A
DISCLOSURE OF THE INVENTION
35 Problems to be Solved by the Invention
[0008]
As was previously described, although Zn-Al-Mg-based
5
- 4 -
alloy-plated steel sheets have recently been developed
for use as zinc-plated steel sheets having superior
corrosion resistance, these steel sheets in Hhich Al and
Mg have been added to the zinc plating conversely have
the significant problem of blackening. Since Zn-Al-Mg-Si
alloy-plated steel sheets having superior corrosion
resistance in particular have superior corrosion
resistance for a long period of time and are resistance
to the formation of 1·1hite rust, there is a growing desire
10 to use these steel sheets at locations such as exterior
panels that are visible from the outside in building
material structures or home appliances and the like.
There is also a growing demand for inhibiting blackening
in Zn-Al-Mg-Si alloy-plated steel sheets due to the
15 increasing emphasis being placed on the design of
architectural structures and home appliances in recent
years in particular.
[0009)
According to the technologies described in Patent
20 Documents 2 to 6, the blackening resistance of Zn-Al-Mgbased
alloy-plated steel sheets, including Zn-Al-Mg-Si
alloy-plated steel sheets, can be improved to a certain
degree. Ho\'/ever, these technologies are merely
technologies for inhibiting blackening of zinc-plated
25 steel sheets during the time they are stored in a
Harehouse or transported prior to being used by a
construction material manufacturer, home appliance
manufacturer or automobile manufacturer or other user
foll01'1ing the production of the zinc-plated steel sheets
30 by a steel manufacturer. Consequently, even if these
technologies for imparting resistance to blackening were
applied, there was the problem of the occurrence of
blackening after a long period of time had elapsed
following assembly of the zinc-plated steel sheets into
35 building material structures, home appliances or
automobile parts and use of those products and parts.
[0010)
- 5 -
On the other hand, although the technology described
in Patent Document 7 prevents blackening of an aluminum
pigment contained in a paint film of a precoated steel
sheet, it does not prevent blackening of the surface of
5 the underlying zinc plating of the precoated steel sheet
\"/hen exposed to the atmosphere for a long period of time.
[0011]
With the foregoing in viel"l, an object of the present
invention is to provide a Zn-Al-Mg-Si alloy-plated steel
10 sheet that is capable of demonstrating both corrosion
resistance and blackening resistance over a long period
of time.
Means for Solving the Problems
15 [0012]
As a result of conducting extensive studies to solve
the aforementioned problems, the inventors found that, by
providing a coating film containing aluminum (Al) on a
plating layer on the surface of a Zn-Al-Mg-Si alloy-
20 plated steel sheet so that the Al in the coating film
does not contact the plating layer, and by making the
coverage rate of Al, l·lhich is defined as the ratio of the
area of the portion of the plating layer concealed by the
Al in the coating film to the total area of the observed
25 field of view "'hen observing the coating film containing
Al from the direction perpendicular to the surface
thereof, to be 75% to 100%, corrosion resistance and
long-term blackening resistance are ensured.
[0013]
30 The invention of the present application was
completed on the basis of these findings, and the gist of
the present invention is as indicated belo\"1.
(1] A zinc-plated steel sheet having superior
blackening resistance and corrosion resistance,
35 comprising:
a steel sheet,
a Zn-Al-Mg-Si alloy plating layer formed on the
- 6 -
surface of the steel sheet, and
a coating film containing Al formed on the plating
layer;
characterized in that the Al contained in the
5 coating film containing Al is separated from the plating
layer by the presence of an insulating substance, and
the coverage rate of Al, which is defined as the
ratio of the area of the portion of the plating layer
concealed by the Al in the coating film to the total area
10 of the observed field of vievT when observing the coating
film containing Al from the direction perpendicular to
the surface thereof, is 75% to 100%.
[2] The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance described
15 in [1], characterized in that the coating film containing
Al is composed of an insulating substance containing
flake-like Al particles, and the Al particles are not
present within a range of at least 0.5 fUll from the
interface between the coating film containing Al and the
20 plating layer.
[3] The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance described
in [2], characterized in that the average particle
diameter of the Al particles is 5 ~lm to 30 ~uu and the
25 aspect ratio thereof is 20 or more.
[4] The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance described
in [1], characterized in that the coating film containing
Al is composed of at least two layers, consisting of an
30 intermediate layer formed with an insulating substance
and an Al metal layer, in that order starting from the
plating layer side.
[5] The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance described
35 in [4], characterized in that the Al metal layer is
composed of an aggregate of flake-like Al particles.
[6] The zinc-plated steel sheet having superior
- 7 -
blackening resistance and corrosion resistance described
in [1], characterized in that the insulating substance is
a resin.
[7] The zinc-plated steel sheet having superior
5 blackening resistance and corrosion resistance described
in [6], characterized in that the resin is a polyester
resin crosslinked Hith a melamine compound.
[8] The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance described
10 in [7], characterized in that the glass transition
temperature Tg of the polyester resin is -20°C to 70°C and
the number average molecular \•Ieight thereof is 15,000 to
25,000.
[9] The zinc-plated steel sheet having superior
15 blackening resistance and corrosion resistance described
in [1], characterized in that the thickness of the
coating film containing Al is 2 fll1l to 10 fll11·
[10] The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance described
20 in [1], characterized by having a clear resin coating
film on the coating film containing Al.
[11] The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance described
in [10], characterized in that the thickness of the clear
25 resin coating film is 0. 2 pm to 20 ~un.
[12] The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance described
in [ 1], ,characterized in that the zinc plating layer
contains 0.01% by Height to 60% by Height of Al, 0.001%
30 by Height to 10% by Height of Mg and 0. 001% by "Ieight to
2% by weight of Si.
[13] A method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance described in [2], characterized by coating a
35 zinc plating layer on the surface of the steel sheet "1i th
a coating material containing flake-like Al particles and
- 8 -
an insulating substance in a solvent, the viscosity
thereof under conditions of a shear velocity of 1 s-1 as
measured with a rotational viscometer at 25° being 150
mPa·s to 1500 mPa·s, and the viscosity thereof at a shear
5 velocity of 10, 000 s-1 as measured t·li th a rotational
viscometer at 25°C being 50 mPa ·s to 150 mPa ·s, foll01-1ed
by heating the steel sheet to a peak metal temperature of
180°C to 230°C at a heating rate of 5°C/s to 70°C/s in an
induction heating furnace to form the coating film
10 containing Al.
[14] The method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance described in [13], characterized in that the
coating material is prepared by mixing flake-like Al
15 particles with 100 parts by weight of an aqueous
emulsion-type polyester resin solid and 10 parts by
weight to 30 parts by ~Ieight o~f a melamine compound solid
as a crosslinking agent.
[15] The method for producing the zinc-plated steel
20 sheet having superior blackening resistance and corrosion
resistance described in [13], characterized in that the
viscosity of the coating material is adjusted using a
viscosity modifier.
[16] The method for producing the zinc-plated steel
25 sheet having superior blackening resistance and corrosion
resistance described in [15], characterized in that 0.2
parts by weight to 10 parts by weight of a surfactant
composeti•·ma·iT!ly of a urethane-modified polyether based on
100 parts by ~reight of a dispersion of the aqueous
30 emulsion-type polyester resin is used for the viscosity
modifier.
[17] The method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance described in [13], characterized in that Al
35 particles having an average particle diameter of 5 ~Lm to
30 ~ and aspect ratio of 20 or more are used for the Al
5
- 9 -
particles.
[18] A method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance described in [4], characterized by:
(a) forming an intermediate layer of an insulating
substance on a zinc plating layer on the surface of a
steel sheet follm1ed by forming an Al metal layer thereon
by a plating method, or
(b) coating a zinc plating layer on the surface of a
10 steel sheet with a liquid material for forming an
intermediate layer of an insulating substance, spraying
flake-like Al particles onto the liquid material, and
then allowing the liquid material to solidify to form an
intermediate layer of an insulating substance and an Al
15 metal layer thereon.
20
[19] The method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance of [18], characterized in that the plating
method is vacuum deposition plating.
Effects of the Invention
[0014]
According to the present invention, a novel Zn-AlMg-
Si-based zinc-plated steel sheet can be provided that
25 is provided with long-term blackening resistance in
addition to superior corrosion resistance inherently
possessed by Zn-Al-Mg-Si-based zinc-plated steel sheets.
MODE FOR CARRYING OUT THE INVENTION
30 [0015]
The following provides an explanation of embodiments
of the present invention.
The zinc-plated steel sheet of the present invention
is composed by being provided with a steel sheet, a Zn-
35 Al-Mg-Si alloy plating layer formed on the surface of the
steel sheet, and a coating film containing Al formed on
the Zn-Al-Mg-Si alloy plating layer.
- 10 -
[0016]
There are no particular limitations on the steel
sheet, and a hot-rolled steel sheet, cold-rolled steel
sheet or other ordinary steel sheet can be used. There
5 are also no particular limitations on the type of steel,
and for example, Al-killed steel, ultra-loH carbon steel
to Hhich has been added Ti or Nb and the like, or high
tensile steel to.Hhich has been added thereto elements
such as P, Si or Mn, can be used.
10 [0017]
The Zn-Al-Mg-Si alloy plating layer is a plating
layer formed on the surface of the steel sheet. This
plating layer is a plating layer composed of 0.1% by
Height to 60% by 1·1eight of Al, 0. 001% by 1·1eight to 10% by
15 1·1eight of Mg and 0. 001% by 1·1eight to 2% by v1eight of Si,
Hith the remainder consisting of Zn and incidental
impurities.
[0018]
If the Al content of the Zn-Al-Mg-Si alloy plating
20 layer is less than 0.01% by Height, the effect of
improving corrosion resistance of the plated steel sheet
attributable to the addition of Al is not demonstrated,
Hhile if the content exceeds 60% by Height, the effect of
improving corrosion resistance ends up being saturated.
25 The Al content is preferably 1% by Height to 60% by
weight and more preferably 5% by weight to 60% by weight.
[0019]
If the Mg content of the Zn-Al-Mg-Si alloy plating
layer is less than 0.001% by weight, the effect of
30 improving corrosion resistance of the plated steel sheet
attributable to the addition of Mg is not demonstrated,
v1hile if the content exceeds 10% by weight, the Mg is not
completely melted in the plating bath and rises to the
surface of the bath in the form of an oxide (typically
35 referred to as dross), thereby resulting in the risk of a
poor appearance or the formation of portions that are not
plated (typically referred to as non-plated portions)
- 11 -
caused by adherence of oxides to the plating surface
layer when zinc plating is carried out in this plating
bath. The Mg content is preferably 1% by Height to 5% by
weight and more preferably 1% by weight to 4% by weight.
5 [0020]
If the Si content of the Zn-Al-Mg-Si alloy plating
layer is less than 0.001% by weight, the effect of
improving corrosion resistance is not demonstrated. In
addition, if the Si content is less than 0.001% by
10 weight, there is increased susceptibility to the
formation of oxides containing Zn, Mg or Al (typically
referred to as dross). On the other hand, if the Si
content exceeds 2% by weight, the Si is not completely
melted in the plating bath and rises to the surface of
15 the bath in the form of an oxide (typically referred to
as dross), thereby resulting in the risk of a poor
appearance or the formation of portions that are not
plated (typically referred to as non-plated portions)
caused by adherence of oxides to the plating surface
20 layer l·lhen zinc plating is carried out in this plating
bath. Dross may be slightly formed even at a Si content
of about 1% by weight depending on the case. The Si
content is preferably 0. 01% by 1·1eight to 1% by weight and
more preferably 0.01% by 1·1eight to 0.5% by weight.
25 [0021]
The adhered amount of the Zn-Al-Mg-Si alloy plating
layer on one side of the steel sheet is preferably 10 g/m2
or more from the vie1•1point of corrosion resistance, and
preferably 350 g/m2 or less from the viev1point of
30 processability.
[0022]
The zinc-plated steel sheet of the present invention
having superior blackening resistance and corrosion
resistance is provided 1vith a coating film containing Al
35 on the Zn-Al-Mg-Si alloy plating layer. This coating
film is important in terms of improving blackening
resistance.
- 12 -
[0023]
Plated steel sheets having a Zn-Al-Mg-Si alloy
plating layer to Hhich has been added Al and Mg are kno\'ln
to be susceptible to the occurrence of blackening. The
5 main cause of blackening is oxidation of the plating
layer surface. Simply covering the Zn-Al-Mg-Si alloy
plating layer Hith a coating film composed mainly of a'
film forming material such as a resin is not sufficient
for preventing permeation of oxygen in the air and is
10 therefore ineffective for preventing blackening. In the
present invention, blackening can be effectively
prevented by causing Al to be present in the coating film
that covers the plating layer. Since Al forms a stable
Al oxide in the surface layer in the atmosphere, it is an
15 extremely stable metal. Consequently, by providing an
Al-containing coating film on the surface of the Zn-AlMg-
Si alloy-plated steel sheet, the Zn-Al-Mg-Si alloy
plating layer becomes resistant to blackening over a long
period of time. In addition, blocking of the pathv1ay by
20 which oxygen permeates the coating film by Al present in
the coating film also greatly contributes to prevention
of blackening of the plating layer.
[0024]
It is important that the Al in the coating film that
25 covers the plating layer conceal the surface of the
plating layer as much as possible from the vieHpoint of
blocking the permeation pathv1ay of oxygen. Consequently,
in the present invention, the coverage rate of Al, 1·1hich
is defined as the ratio of the area of the portion of the
30 plating layer concealed by the Al in the coating film to
the total area of the observed field of view when
observing the coating film containing Al from the
direction perpendicular to the surface thereof, is
required to be 75% to 100%. Al coverage rate is
35 preferably as high as possible, and is therefore
preferably 85% or more and more preferably 95% or more,
for example. •
- 13 -
[0025]
Since Al surface oxides are substances that are
nobler than the Zn contained in the plating layer of the
Zn-Al-Mg-Si alloy-plated steel sheet, they easily cause
5 contact corrosion betHeen different metals in a state in
Hhich they are in contact l'li th each other. Therefore, in
the present invention, the Al in the coating film and the
plating layer are required to be separated by an
insulating substance in order to prevent contact betv1een
10 the Al in the coating film and the surface of the plating
layer. The interval between the Al in the coating film
and the plating layer is required to be 0. 5 ~lm or more
and is preferably 1. 0 ~lm or more. If the interval is
less than 0.5 ~~, insulating effects are not obtained.
15 If the interval exceeds 3.0 ~' not only do insulating
effects become saturated, but it becomes difficult to
-form such a large interval per se. The interval is most
preferably of the order of 0.5 ~to 1.5 ~·
[0026]
20 Flake-like Al particles can be used for the Al in
the coating film that covers the plating layer. Flakelike
Al particles are preferable since they are able to
easily form a coating film containing Al by coating the
plating layer with a coating material in which they are
25 dispersed. Flake-like Al particles having an average
particle diameter of 5 ~~ to 30 ~ and aspect ratio
(ratio of average particle diameter/thickness) of 20 or
more can be used for the flake-like Al particles. If the
average particle diameter is less than 5 ~~, the Al
30 coverage rate is more likely to be less than 75% thereby
diminishing the effect of concealing the plating layer.
If the average particle diameter exceeds 30 ~lm, a portion
of the Al particles end up being outside the coating film
as a result of being excessively large, thereby resulting
35 in the formation of an irregular surface and the risk of
a poor appearance. If the aspect ratio is less than 20,
- 14 -
the Al coverage rate is more likely to be less than 75%.
Although there are no particular limitations thereon, the
upper limit of the aspect ratio is preferably less than
300. Al particles having an aspect ratio of greater than
5 300 are difficult to produce and difficult to acquire.
[0027]
Average particle diameter is obtained by measuring
the major axis and minor axis of a single arbitrary
aluminum particle, defining the average of the sum
10 thereof as the average particle diameter of a single Al
particle, or in other words, defining in the manner of
[average particle diameter of single Al particle] =
[(major axis+ minor axis)]/2, and using the average of
measuring 100 arbitrary Al particles as the average
15 particle diameter. The average particle diameters of
individual Al particles can be measured by magnifying the
Al particles with an optical microscope or electron
microscope. In addition, average particle diameter can
also be determined by determining cumulative weight
20 distribution using a sieve or a laser diffraction-type
particle size distribution analyzer based on the
principle of laser diffraction. Average particle
diameter may also be determined based on cumulative
weight distribution, by calculating the particle diameter
25 at 50% in the cumulative weight (typically referred to as
average particle diameter D50). In the present
invention, the average particle diameter of 100 particles
as measured l·li th a microscope or the particle diameter at
50% in the cumulative weight can be used as average
30 particle diameter.
[0028]
In addition, the average thickness of Al particles
required to determine aspect ratio in the present
invention can be the Al particle average thickness
35 defined as the average thickness of 100 arbitrary Al
particles determined by measuring the thickness of an
arbitrary cross-section of an arbitrary Al particle by
- 15 -
observing with an optical microscope or electron
microscope (typically, the dimension in the direction
perpendicular to the plane in Hhich the aforementioned
major axis and minor axis are measured).
5 [0029]
In this manner, aspect ratio is defined as [aspect
ratio] ~ [Al particle average particle diameter measured
as described above]/[Al particle average thickness
measured as described above] .
10 [0030]
The Al-containing coating film on the plating layer
can be formed Hith a coating material containing flakelike
Al particles, for example, and in a coating film
formed in this manner, the Al particles are oriented in a
15 direction parallel or nearly parallel to the underlying
plating layer, are dispersed in a continuous phase formed
by an insulating substance in the form of a film forming
component in the coating material, and are gathered in
the upper portion of the coating film. Only the
20 continuous phase is present in the lmqer portion of the
coating film, and Al particles are separated from the
surface of the plating layer by forming the previously
described interval.
25
30
[0031]
The coating film containing Al may be composed of at
least tHo layers consisting of an intermediate layer
formed 1·1ith an insulating substance and an Al metal layer
in that order starting from the plating layer side.
[0032]
The Al metal layer in this case can be formed in the
form of an aggregate of flake-like Al particles on the
intermediate layer formed using an insulating substance
on the plating layer, or may be formed as a continuous Al
layer by plating. In the case of an aggregate of flake-
3'5 like Al particles, the previously described flake-like Al
particles can be used. In contrast to aggregates of
flake-like Al particles being present Hith the particles
- 16 -
dispersed in a continuous resin phase in the coating film
formed by a coating material containing flake-like Al
particles as previously explained (although resin is
present between adjacent particles and contact is
5 possible among particles at the upper portion of the
coating film having a higher particle concentration,
there is extremely little contact, if any, among
particles at the lower portion of the coating film having
a lower particle concentration), aggregates of flake-like
10 Al particles are formed by using the resin of the
intermediate layer as an adhesive or binder, and are
fixed in position by the resin that fills the intervals
therebetHeen "'hile making mutual contact. As a result
thereof, such aggregates of flake-like Al particles can
15 be included in the "Al metal layer" in the present
invention.
[0033]
In the case where the coating film containing Al is
composed of at least hlo layers consisting of an
20 insulating substance intermediate layer and an Al metal
layer, the thickness of the intermediate layer may be 0.5
~tm or more and the thickness of the Al metal layer may be
1.5 ~to 9.5 ~· If the thickness of the intermediate
layer is less than 0.5 ~~, insulating effects are not
25 obtained and corrosion resistance is inferior. The
thickness of the intermediate layer is more preferably
0.5 ~to 3 ~and even more preferably 0.5 ~to 1.5 ~~.
If the thickness exceeds 3 ~~, further coating is not
required since insulating effects are saturated. If the
30 thickness of the Al metal layer is less than 1.5 ~~,
concealment of the l01·1er plating layer is inadequate,
Hhile if the thickness exceeds 9.5 Jlm, there is the risk
of processability becoming inferior. The thickness of
the Al metal layer is preferably 2.5 ~to 9.5 ~and
35 more preferably 3.5 ~ to 9.5 ~~.
[0034]
- 17 -
The Al material in the coating film that contains Al
may be composed of pure Al or an Al alloy composed mainly
of Al. Commonly known Al alloys can be used for the Al
alloy.
5 [0035]
A resin is preferable for the insulating substance
in the coating film containing Al. The insulating
substance fulfills the role of preventing corrosion of
the plating layer attributable to contact between
10 different metals, i.e., bet1-1een Al in the coating film
and the plating layer, and the insulating substance
having an insulation resistance of 10 Q or more is
preferable.
[0036]
15 In the case where the coating film containing Al is
formed with a resin having flake-like Al particles
dispersed therein, the resin is preferably a polyesteJ;'~
resin crosslinked vii th a melamine compound. This
polyester resin preferably has a glass transition
20 temperature Tg of -20°C to 70°C and a number average
molecular weight of 15,000 to 25,000. If the glass
transition temperature Tg is lower than -20°C, there is
the risk of a decrease in the adhesion of processed
portions of the coating film layer containing Al. If the
25 glass transition temperature Tg is higher than 70°C, there
is the risk of a decrease in the processability of the
coating film layer containing Al and the formation of
cracks in the coating film layer during processing. Tg
is more preferably 0°C to 50°C. The glass transition
30 temperature Tg can be determined by measuring the coating
film resin with a differential scanning calorimeter
abbreviated as DSC or a thermomechanical analyzer
abbreviated as TMA. In the case 1·1here the number average
molecular weight of the polyester resin is less than
35 15,000, there is the risk of a decrease in processability
of the coating film and the formation of cracks in the
- 18 -
coating film during processing. If the number average
molecular weight exceeds 25,000, there is the risk of the
occurrence of streak-like coating defects commonly
referred to as ribbing during coating, or defects such as
5 decreases in coverage rate caused by difficulty in
uniformly dispersing the Al particles, due to excessively
high viscosity when in the form of a coating liquid.
Number average molecular 1·1eight can be measured by a
commonly known method such as gel permeation
10 chromatography abbreviated as GPC.
[0037]
15
There are no particular limitations on the resin of
the intermediate layer in the case 1·1here the coating film
containing Al is composed of at least t1-1o layers
consisting of an intermediate layer and an Al metal layer
thereon, and a commonly kno1-1n resin can be used.
Examples of resins that can be used include polyester
resin, epoxy resin, urethane resin, acrylic resin and
melamine resin. Ho-;·rever 1 since there are many cases in
20 which zinc-plated steel sheets are used after forming and
processing, polyester resin or urethane resin having
superior processability is more preferable. Epoxy resin
is also preferable since it demonstrates superior
adhesion with metal. When an intermediate layer is
25 formed by applying a coating liquid obtained by
dissolving such a resin in a solvent or emulsifying and
dispersing it in water or a solvent, workability during
production is improved thereby making this more
effective. In addition, if a curing agent such as
30 melamine or isocyanate is added to these resins to obtain
a thermosetting resin, adhesion betv1een the Al metal
layer and Zn-Al-Mg-Si alloy plating layer is enhanced,
thereby making this more preferable. If the resin is the
same as that of the coating film layer containing Al,
35 adhesion with the coating film layer containing Al is
superior, thereby making this more preferable.
[0038]
- 19 -
In the case where the coating film containing Al is
composed of an intermediate layer and an Al metal layer
thereon, a layer separate from these can also be present.
For example, in the case of forming the Al metal layer by
5 a plating method, a layer can be provided between the
intermediate layer and Al metal layer that is effective
for enhancing adhesion of plating to the intermediate
layer.
[0039]
10 An additive such as a pigment, aggregate or rust
preventive can be added, as needed, to the resin that
composes the film coating containing Al. The addition of
a pigment or aggregate is more preferable since, in
addition to enhancing the strength of the coating film,
15 it also enhances adhesion bet1-1een Al and the Zn-Al-Mg-Si
alloy plating layer. In addition, the addition of a rust
preventive is more preferable since it improves corrosion
resistance of the Zn-Al-Mg-Si alloy plating layer. The
amount of additive added may be suitably determined so as
20 not to be disadvantageous for the coating film of the
present invention.
[0040]
The thickness of the coating film containing Al is
preferably within the range of 2 11m to 10 ~un. In the
25 case 11here the film thickness is less than 2 ~un, there is
the risk of the coverage rate being less than 75% as a
result of being unable to completely conceal the plating
layer. If the film thickness exceeds 10 ~un, there is the
risk of processability becoming inferior. Film thickness
30 can be measured by observing a cross-section with an
optical microscope or electron microscope.
[0041]
As 1vas previously explained, a portion in which Al
particles dispersed in a continuous resin phase are not
35 present (portion consisting of resin only), or an
intermediate layer composed of resin independent from the
5
- 20 -
Al metal layer, is located on the side of the coating
film containing Al that contacts the plating layer. The
thickness thereof is as was previously explained.
[0042]
In the zinc-plated steel sheet of the present
invention, a clear resin coating film can be provided on
the coating film containing Al. In the case where the
coating film containing ~l has an Al metal layer in
particular, the clear resin coating film is able to
10 improve the fingerprint resistance thereof. The clear
resin coating film also has the effect of smoothing the
surface of the plated steel sheet by filling in surface
irregularities on the surface of the coating film
containing Al.
15 [0043]
There are no particular limitations on the type of
clear resin, and a commonly knu~·ln resin can be used,
examples of which include polyester resin, epoxy resin,
urethane resin, acrylic resin and melamine resin.
20 However, since there are many cases in which zinc-plated
steel sheets are used after forming and processing,
polyester resin or urethane resin having superior
processability is more preferable. Epoxy resin is also
preferable since it demonstrates superior adhesion with
25 metal (in this case, Al exposed on the surface of the
coating film containing Al). If a curing agent such as
melamine or isocyanate is added to the clear resin to
obtain a ther.mosetting resin, hardness of the coating
film is improved resulting in superior scratch
30 resistance, thereby making this more preferable.
[0044]
A rust preventive can also be added to the clear
resin as necessary. The addition of a rust preventive is
preferable since it improves corrosion resistance of the
35 coating film containing Al. Moreover, the addition of
pigment or aggregate enhances the strength of the clear
resin coating film and enhances the adhesion with the
5
- 21 -
coating film containing Al, thereby making this more
preferable.
[0045]
The thickness of the clear resin coating film layer
is preferably v1ithin the range of 0. 2 Jlffi to 20 pm. If
the thickness is less than 0.2 ~~, the effect on
fingerprint resistance may diminish, while if the
thickness exceeds 20 pm, there is the possibility of the
occurrence of a coating film defect typically referred to
10 as boiling Hhen a solvent-dissolved type or emulsiondispersed
type of resin coating liquid is applied
foll01·1ed by drying and curing.
[0046]
In the case of using Al particles in the coating
15 film containing Al, there are cases in which the Al
particles may slightly diffuse and disperse into the
clear resin coating film thereabove depending on the
production method. In this case, the portion to which
the Al particles have diffused is regarded as the coating
20 film containing Al, and the thickness extending to that
portion (distance from the surface ·of the plating layer)
can be taken to be the thickness of the coating film
containing Al. Thus, in this case, the thickness of the
clear resin coating film refers to the thickness of the
25 portion where Al particles are not present (portion
consisting of resin only) .
[0047]
···rn ·tire ·present invention, the thicknesses of the
plating layer, coating film containing Al, insulating
30 resin intermediate layer, Al metal layer and clear resin
coating film as well as the interval between the Al in
the coating film and the plating layer and the like can
be determined by embedding the zinc-plated steel sheet
produced in resin, and observing a cross-section exposed
35 by grinding in the direction of thickness with an
electron microscope or the like.
- 22 -
[0048]
In the zinc-plated steel sheet of the present
invention, a kno\·ln chemical conversion treatment may be
carried out on the surface of the Zn-Al-Mg-Si alloy
5 plating layer. Examples of applicable chemical
conversion treatment include chromate treatment,
phosphoric acid-based treatment, silica-based treatment,
Mo-based treatment, Co-based treatment, Ni-based
treatment and Zr-based treatment. In an embodiment in
10 v1hich an Al metal layer is located on the surface of the
coating film containing Al and a clear resin coating film
is further provided thereon, the aforementioned chemical
conversion treatment can also be carried out on the Al
metal layer.
15 [0049]
20
The folloHing provides an explanation of a method
for producing the zinc-plated steel sheet of the present
invention.
[0050]
The zinc-plated steel sheet of the present invention
in Hhich the coating film containing Al is composed of an
insulating substance that contains flake-like Al
particles, and Al particles are not present Hithin a
range of at least 0. 5 fUU from the interface bet1-1een the
25 coating film containing Al and the aforementioned plating
layer, can be produced by coating the zinc plating layer
on the surface of the steel sheet vii th a coating material
that.con;t;i3.ins.flake-like Al particles and an insulating
substance in a solvent, has a viscosity under conditions
30 of a shear velocity of 1 s-1 as measured l·lith a rotational
viscometer at 25°C of 150 mPa·s to 1500 mPa·s, and has a
viscosity at a shear velocity of 10,000 s-1 as measured
1·1ith a rotational viscometer at 25°C of 50 mPa ·s to 150
mPa·s, followed by heating the steel sheet to a peak
35 metal temperature of 180°C to 230°C at a heating rate of
5°C/s to 70°C/s in an induction heating furnace to form
- 23 -
the aforementioned coating film containing Al.
[0051]
The inventors found that, when a solution is
prepared in which flake-like Al particles are dispersed
5 in a solution of a resin, which is an insulating
substance, (which is also referred to as a ''coating
liquid" or "coating material" hereinafter) and the
viscosity thereof is adjusted to specific conditions, the
viscosity thereof is controlled in a step of drying and
10 baking of the coating liquid applied to the plated steel
sheet serving as the base material and the flake-like Al
particles rise to the upper layer portion of the coating
film formed due to convection of the coating liquid,
1·1hile a lower layer portion appears that does not contain
15 Al particles. Namely, the inventors found that, by
adjusting the viscosity of a coating liquid containing
flake-like Al particles and an insulating substance in
the form of a resin or emulsified resin in a solvent as
measured 1·1ith a rotational viscometer at 25°C to 150 mPa ·s
20 to 1500 mPa·s under conditions of a shear velocity of 1s-1
and to 50 mPa·s to 150 mPa·s under conditions of a shear
velocity of 10,000 s-1
, follo1-1ed by coating the coating
liquid directly onto a zinc plating layer and heating to
a peak metal temperature of 180°C to 230°C at a heating
25 rate of 5°C/s to 70°C/s in an induction heating furnace to
form a coating film, the Al particles rise to the upper
layer portion of the coating film in the drying and
bakirrg'"S"tep following the start of heating, and as a
result of the coating film ultimately being cured while
30 in that state, Al particles are not present 1-1ithin a
range of at least 0. 5 ~un from the interface bet1-1een the
coating film and the plating layer, thereby resulting in
the formation of a coating film having a unique
configuration in which Al particles are only present in
35 the upper layer portion.
[0052]
- 24 -
In this case, an aqueous emulsion-type polyester
resin can be preferably used for the resin serving as the
insulating substance. Moreover, adhesion with the Zn-AlMg-
Si alloy plating layer can be enhanced by using, as a
5 crosslinking agent, a melamine compound to obtain a
thermosetting resin. Examples of aqueous emulsion-type
polyester resins include members of the Vylonal® series
manufactured by Toyobo Co., Ltd. Examples of melamine
compounds serving as crosslinking agents include members
10 of the Cymel® Series manufactured by Cytec Industries
Inc. In general, 10 parts by weight to 30 parts by
weight of melamine crosslinking agent in terms of the
solid content thereof can be used to 100 parts by weight
of aqueous emulsion-type polyester resin in terms of the
15 solid content thereof. The amount of melamine
crosslinking agent used is more preferably 10 parts by
weight to 20 parts by 1·1eight based on 100 parts by_ weight
of aqueous emulsion-type polyester resin in terms of the
solid content thereof.
20 [0053]
Preparation of the coating material can be carried
out by adding an aqueous emulsion-type polyester resin, a
melamine crosslinking agent and flake-like Al particles
(as previously explained) to an aqueous solvent such as
25 water or alcohol follo"1ed by stirring. The blending
ratio of the aqueous emulsion-type polyester resin and
melamine crosslinking agent is as previously described.
The ineorp0rated amount of flake-like Al particles may be
determined according to the properties of the particles
30 used and the film thickness of the coating film formed.
In particular, the incorporated amount of Al particles
increases relatively as film thickness increases (the
thickness of the portion "'here Al particles are present
becomes relatively larger than the interval between the
35 Al in the coating film and the plating layer). On the
other hand, the interval bet1•een the Al in the coating
film and the plating layer varies depending on the
- 25 -
viscosity conditions and drying/baking conditions of the
coating material. Thus, the amount of flake-like Al
particles incorporated in the coating material may be
determined experimentally in consideration of these
5 requirements. However, coating materials prepared in
this manner do not normally satisfy the aforementioned
viscosity conditions. A viscosity modifier can be used
so that the viscosity of the coating material satisfies
the aforementioned conditions.
10 [0054]
A phenomenon in 1·1hich a liquid such as a coating
liquid demonstrates high viscosity in the region of low
shear velocities and low viscosity in the region of high
shear velocities is typically referred to as the shear
15 thinning effect. A highly concentrated liquid containing
granular fine particles is typically used as a dense
dispersion system in order to impart a shear thinning
effect to a liquid. It is publicly said that as a result
of using a dense dispersion system, the inter-particle
20 distance bet1-1een fine particles added to the coating
liquid becomes shorter, and attraction between particles
increases, resulting in the appearance of the shear
thinning effect. However, differing from granular fine
particles, inter-particle distance is unli~ely to become
25 shorter even if flake-like metal particles having a
comparatively high specific gravity are added at a high
concentration as in the present invention. In addition,
due to the h~,gh specific gravity, if the amount added is
increased in an attempt to further reduce inter-particle
30 distance, there is increased susceptibility to the
occurrence of particle settling, thereby making this
unsuitable.
[0055]
A technique is known for controlling the viscosity
35 of the same liquid at different shear velocities by
adding a specific additive. Additives typically referred
to as rheology control agents are used in this case. In
- 26 -
the case of adding such a rheology control agent to a
coating liquid, the rheology control agent reacts
slightly with the resin in the coating liquid and causes
it to form a net1wrk phase in the coating liquid.
5 However, the aforementioned required coating liquid
viscosity conditions cannot be satisfied by adding an
ordinary rheology control agent to the coating liquid
used in the present invention.
10
[0056]
In the present invention, a specific viscosity
modifier can be used so that the viscosity of the coating
liquid satisfies the aforementioned conditions. The
viscosity modifier used in the invention is a type of
substance differing from ordinary rheology control agents
15 in that it does not react 1o1ith the resin in the coating
liquid and the molecular terminal chains thereof are
bound to the resin in the coating liquid Hith a weak
bonding force such that the chains are adsorbed thereto.
Examples of this substance include a surfactant mainly
20 composed of a urethane-modified polyether, and for
example, "SN-Thickener 629N" manufactured by San Nopco
Ltd. may be referred to. Since the amount of viscosity
modifier added in the present invention varies according
to such factors as the type of resin or type of solvent
25 used in the coating liquid, the amount added may be
suitably selected as necessary. More specifically, in
the present invention, it is necessary to form the
coat~ng film containing Al so that Al particles are not
present within a range of at least 0.5 )lil\ from the
30 interface between the coating film and the plating layer,
and the amount of viscosity modifier used is determined
so as to satisfy this condition. In general, the
viscosity modifier can be used in an amount of 0.2 parts
by weight to 10 parts by weight based on 100 parts by
35 1·1eight of a dispersion of the aqueous emulsion-type
polyester resin. If the amount used is less than 0.2
parts by 1·1eight, it becomes difficult to obtain the
- 27 -
required coating film viscosity conditions, while if the
amount used exceeds 10 parts by weight, there is the risk
of gelling of the aqueous emulsion-type polyester resin.
The preferable added amount of the viscosity modifier is
5 0. 2 parts by weight to 1. 0 part by v1eight based on 100
parts by weight of a dispersion of the aqueous emulsiontype
polyester resin.
[0057]
As a result of forming a coating film by coating a
10 coating liquid for 1-1hich viscosity has been adjusted
directly onto a zinc plating layer followed by heating to
a peak metal temperature of 180°C to 230°C at a heating
rate of 5°C/s to 70°C/s and completing curing of the resin
in the coating liquid, a coating film is formed in v1hich
15 Al particles are not present 1-1ithin a range of at least
0. 5 ~un from the interface bet1-1een the coating film and
the plating layer, thereby resu±ting in Al particles only
being present in the upper layer portion. Heating is
required to be carried out in an induction heating
20 furnace. The reason for this is that induction heating
is advantageous for obtaining a coating film having a
configuration in which the distribution of Al particles
is concentrated on one side (farthest side from plating
layer) by inhibiting the settling of Al particles to the
25 plating layer surface that have risen to the upper
portion of the coating liquid on the plating layer due to
Marangoni convection as Hill be subsequently described,
by the ·fl"m-l ·o·f···sol vent that evaporates from the plating
layer side to the coating liquid surface side as a result
30 of heating from the side of the base material of plated
steel sheet by induction heating. If the heating rate is
less than 5°C/s, it becomes difficult to generate
Marangoni convection due to the sl01·1 heating rate, or Al
particles do not easily float upv1ard due to a slo1-1
35 convection velocity, and there is the risk of the
presence of the Al particles within a range of 0.5 pm
- 28 -
from the interface between the coating film and the
plating layer, resulting in inferior corrosion
resistance. In addition, if the heating rate exceeds
70°C/s, the coating material ends up curing while in a
5 boiled state in the solvent drying step, due to the
excessively rapid heating rate, thereby resulting in the
risk of the formation of coating defects (typically
referred to as boiling) in which remnants of air bubbles
caused by boiling remain in the coating film. In
10 addition, since heating time becomes short if the heating
rate is excessively rapid, there is increased likelihood
of the coating film surface layer being slightly uncured,
thereby causing peeling of the coating film. If the peak
metal temperature is lower than 180°C, the coating film is
15 not completely cured resulting in the risk of the coating
film surface not being dried, and if the coating film
surface layer is uncured_+undried), this can cause
peeling of the coating film. If the peak metal
temperature exceeds 230°C, the coating material ends up
20 curing while in a boiled state in the solvent drying
step, thereby resulting in the risk of the formation of
coating defects (typically referred to as boiling) in
~1hich the remnants of air bubbles caused by boiling
remain in the coating film. Moreover, the coating film
25 becomes hard due to progression of curing of the coating
film caused by baking at a high temperature, thereby
increasing susceptibility to the formation of cracks or
peelil"l"g"·Oi:" 't::lie coating film caused by processing. Since
zinc-plate steel sheets are typically used after
30 processing, processed portions 1·1here cracking or peeling
occurs in the coating film due to processing are likely
to become a source of corrosion.
[0058]
Although the detailed mechanism of the formation of
35 the aforementioned coating film having a unique
configuration has not been elucidated, the inventors have
- 29 -
presumed it to be as indicated below. When a coating
liquid is coated onto the plating layer surface of a base
material of plated steel sheet at, in general, room
temperature or a slightly higher temperature (such as a
5 temperature of the order of 30°C or 40°C) and heating is
started for the purpose of drying and baking, regions of
different temperatures soon appear in the coating liquid,
Marangoni convection occurs and the coating liquid begins
to flow. During the initial stage of this. , coating liquid
10 flow, namely when under conditions in which shear
velocity is comparatively high, Al particles rise to the
upper portion of the coating liquid on the plating layer
because the viscosity of the coating liquid is low, and
after the drying and baking step proceeds, convection
15 subsides and shear velocity decreases, a film is formed
in which Al particles are not contained in the lower
portion since the Al particles no longer settle due to
the high viscosity of the coating liquid.
[0059]
20 In this manner, in the present invention, by using a
coating liquid for which viscosity has been adjusted so
as to satisfy the aforementioned specific conditions, the
movement of Al particles by Marangoni convection, v1hich
is generated in a process in which a coating liquid
25 coated onto a plating surface is heated for drying and
baking, is controlled, thereby causing the Ai .particles
to be concentrated in the upper portion of the coating
liquid during the initial stage of heating. As a result
of continuing to heat further while maintaining this
30 state, the solvent of the coating liquid is vaporized and
the resin is cured resulting in the formation of a
coating film in which Al particles are not present within
a range of at least 0. 5 ~un from the interface with the
plating layer.
35 [0060]
The coating film having the unique configuration of
the present invention in which Al particles are not
- 30 -
present within a range of at least 0. 5 'un from the
interface 1-1ith the plating layer is realized by heat
treatment at a high temperature range providing a peak
metal temperature of 180°C to 230°C. On the other hand,
5 in the present invention, although it may seem strange at
first glance, the use of a coating liquid for forming
such a coating film is required that satisfies a
viscosity condition defined by measured values (at
different shear velocities} at 25°C. However, the
10 inventors found that, by using a coating material that
satisfies this viscosity condition and forming a coating
film under the aforementioned drying and baking_
conditions, a coating film is obtained in 1-1hich Al
particles are actually not present 1-1ithin a range of at
15 least 0. 5 ,un from the interface Hi th the plating layer
(see the Examples}.
[0061]
The zinc-plated steel sheet according to the present
invention, in which a coating film containing Al is
20 composed of at least two layers consisting of an
intermediate layer formed l·li th an insulating substance
and an Al metal layer in that order starting from the
plating layer side, can be produced by:
(a} forming an intermediate layer of an insulating
25 substance on a zinc plating layer on the surface of a
steel sheet followed by forming an Al metal layer thereon
by a plating method, or
(b) coating a zinc plating layer on the surface of a
steel sheet 1-1ith a liquid material for forming an
30 intermediate layer of an insulating substance, spraying
flake-like Al particles onto the liquid material, and
then allov1ing the liquid material to solidify to form an
intermediate layer of an insulating substance and an Al
metal layer thereon.
35 [0062]
A commonly knovm insulating substance can be used
- 31 -
for the insulating substance of the intermediate layer
both in the case of producing the zinc-plate steel sheet
according to the aforementioned (a) and in the case of
producing according to the aforementioned (b), and
5 examples of insulating substances that can be used
include polyester resin, epoxy resin, urethane resin,
acrylic resin and melamine resin. Since there are many
cases in which zinc-plate steel sheets are used after
forming and processing, a polyester resin or urethane
10 resin having superior processability is more preferable.
Epoxy resin is also preferable due t? its superior
adhesion to metal. When the intermediate layer is formed
by applying a coating liquid obtained by dissolving such
a resin in a solvent or emulsifying and dispersing it in
15 water or a solvent, workability during production is
improved thereby making this more effective. In
addition, if a curing agent such as melamine or
isocyanate is added to these resins to obtain a
thermosetting resin, adhesion bet1veen the Al metal layer
20 and Zn-Al-Mg-Si alloy plating layer is enhanced, thereby
making this more preferable.
[0063]
The intermediate layer of the insulating resin can
be easily formed on the Zn-Al-Mg-Si alloy plating layer,
25 by using a coating material of the type in which the
insulating resin is dissolved in a solvent such as paint
thinner or of the emulsion type in which it is dispersed
in 1·1ater ~.,.and after applying the coating material to the
plating layer, drying and baking it, or by melting the
30 insulating resin at a temperature lov1er than the melting
point of Zn, and coating the Zn-Al-Mg-Si alloy plating
layer therewith, for example.
[0064]
In the case of forming the Al metal layer using the
35 method described in (a) above, the Al metal layer can be
formed on a preliminarily formed intermediate layer of an
insulating resin by a commonly known plating method such
- 32 -
as vacuum deposition plating, electroplating or nonelectrolytic
plating. Among these plating methods,
vacuum deposition plating is used preferably. Hot dip
galvanization is not suitable since the plating layer
5 ends up melting as a result of the melting temperature of
Al being higher than the melting temperature of the Zn
1·1hich is the main component of Zn-Al-Mg-Si alloy plating.
[0065]
In the case of forming the Al metal layer using the
10 method described in (b) above, the Al metal layer can be
formed in the form of an aggregate of flake-like Al
particles by applying a coating material for forming the
intermediate layer (the same coating material as that
previously explained for the method of (a)) to the zinc
15 plating layer, spraying flake-like Al particles onto the
applied coating material, and then solidifying the
coating material of the intermediate layer by heating.
Here, Al particles can be fixedly adhered on the
insulating coating film layer by using the resin of the
20 intermediate layer as an adhesive or binder.
[0066]
The blending ratio of the resin and crosslinking
agent, heating conditions for drying and baking the
coating film and the like are the same as those explained
25 with respect to the method for forming a coating film
with a coating liquid having Al particles dispersed
therein both in the case of using the method described in
the aforementioned ,(a) and the method described in the
aforementioned (b).
30 [0067]
In the case of forming a clear resin coating film on
the coating film containing Al, it can be formed in such
a v1ay that a coating liquid of the type in Hhich an
ordinary clear resin as previously exemplified is
35 dissolved in a solvent such as paint thinner, or that of
the emulsion type in Hhich the resin is dispersed in
1·1ater, is coated onto the coating film containing Al
- 33 -
followed by drying and baking, or a clear resin is melted
at a temperature lower than the melting of Zn, and the
resultant melt is coated onto the coating film containing
Al.
5 [0068]
In the present invention, there are no particular
limitations on the method used to apply the coating
material when forming the coating film containing Al and
the clear resin coating film, and a method ordinarily
10 used in the coating of steel sheets can be used. For
example, a coating method using a roll coater or curtain
coater can be used preferably. A method ordinarily used
in the coating of steel sheets can also be used for
drying and baking the coating material.
15
Examples
[0069]
The following provides a more detailed explanation
of the present invention through examples thereof. It
20 goes without saying that the present invention is not
limited to the follm'ling examples.
[0070]
(1. Formation of Plating Layer)
A cold-rolled steel sheet having a thickness of 1 rnrn
25 1;as subjected to hot-dip plating by immersing for 3
seconds in a Zn-Al-Mg-Si plating bath at 450°C to which
various types of metal had been added, and a Zn-Al-Mg-Si
plat.itlg ... l.ay.e.r was formed on the steel sheet by adjusting
the plated amount to 90 g/m2 per side by N2 wiping.
30 [0071]
(2. Formation of Coating Film Containing Al
Particles)
Al powder was dispersed in a mixed solution of
aqueous emulsion-type high molecular weight polyester and
35 melamine resin, and pure \'later and an additive containing
urethane-modified polyether (SN-Thickener 629N)
manufactured by San Nopco Ltd. were suitably added to the
- 34 -
resulting dispersion to prepare a coating liquid for
which viscosity during a shear velocity of 1 s-1 (low
shear viscosity) and viscosity during a shear velocity of
10,000 s-1 (high shear velocity) were adjusted at 30"C.
5 Vylonal® MD-1480 (number average molecular weight:
15,000, Tg: 20"C), Vylonal® MD-1220 (number average
molecular weight: 15,000, Tg: 67"C), Vylonal® MD-1100
(number average molecular weight: 20,000, Tg: 40"C),
Vylonal® MD-1985 (number average molecular weight:
10 25,000, Tg: -20"C), Vylonal® MD-1335 (number average
molecular v1eight: 8, 000, Tg: 4"C) and Vylonal® MD-1500
(number average molecular weight: 8,000, Tg: 77"C)
manufactured by Toyobo Co., Ltd. 1·1ere used for the
aqueous emulsion-type high molecular weight polyester.
15 Cymel® 303 manufactured by Mitsui Cytec Ltd. was used for
the melamine resin. "Sap 561PS" manufactured by Showa
Aluminum Powder K.K. (average particle diameter: 16 Jllll,
aspect ratio: 20 or more), "Sap 2173SW" manufactured by
Showa Aluminum Pov1der K.K. (average particle diameter: 6
20 [lm, aspect ratio: 20 or more), "Sap 720N" manufactured by
Shm·1a Aluminum Powder K. K. (average particle diameter: 30
[lm, aspect ratio: 20 or more) and "Aluminum Powder"
reagent manufactured by Kanto Chemical Co., Inc., which
were sized for particle diameter using sieves of
25 different mesh sizes follm·1ed by extraction of only fine
particles (average particle diameter: 20 ~~. aspect
,-Jrratio:
less than 20), were used for the Al particles.
The polyester resin and melamine resin \•/ere blended at a
Height ratio of polyester resin solid fraction to
30 melamine resin solid faction of 100:20, and the Al
particles were added so that the weight ratio of
polyester resin to Al particles 1·1as 100:15. The coating
liquid was coated onto the plating layer of the steel
sheet with a curtain coater, and the resin was cured by
35 heating in an induction heating furnace to a prescribed
- 35 -
peak metal temperature at a heating rate of 50°C/s
followed by cooling with 1·1ater to form a coating film
containing Al. Subsequently, the total thickness of the
coating film, the thickness of the portion of the coating
5 film where Al particles are not present (portion
consisting of resin only), and the Al coverage rate were
measured. The film thickness was measured by embedding
the zinc-plated steel sheet in resin, observing an
arbitrary longitudinal cross-section exposed by grinding
10 at a magnification of 500X using a scanning electron
microscope (SEM), and averaging the measured values
obtained from five arbitrary sections. The Al coverage
rate 1·1as determined as the area ratio at which Al is
detected in all fields of view when an arbitrary plane of
15 the coating film was analyzed by EMPA and Al 1·1as
subjected to elemental mapping in a field of view
magnified at 100X. This method is hereinafter referred
to as the "particle dispersion method".
In addition, samples 1·1ere also prepared in the
20 particle dispersion method 1·1hile changing the amount of
Al particles added and added amount of melamine resin as
necessary.
[0072]
(3. Formation of Coating Film Containing Al Metal
25 Layer by Plating)
Vylon® 29CS manufactured by Toyobo Co., Ltd., which
is an amorphous polyester resin that is dissolved in.an
organic solvent consisting of a mixture of cyclohexanone
and Solvesso, and Vylonal® MD-1220 manufactured by Toyobo
30 Co., which is a water-dispersed high molecular Height
polyester, were used for resin coating liquids. The
insulating layer serving as an intermediate layer
produced using Vylon® 29CS is hereinafter referred to as
the "solvent type", while the insulating layer produced
35 using Vylonal® MD-1220 is hereinafter referred to as the
"water-dispersed type". A melamine resin sold under the
- 36 -
tradename Cymel® 303 manufactured by Mitsui Cytec
Industries Inc. was added, as a curing agent, to these
resin coating liquids so that weight ratio of the
polyester resin solid fraction to the melamine resin
5 solid fraction was 100:20. Moreover, with respect to the
"solvent type" coating liquid, Catalyst TM600, v1hich is
an acidic catalyst manufactured by Mitsui Cytec
Industries Inc., was added at 0.5% by weight to a mixed
solution of the polyester resin and the melamine resin.
10 A catalyst was not added to the ."water-dispersed type"
coating liquid.
[0073]
The resin coating liquid 1·1as coated onto the
previously prepared plating layer with a bar coater
15 followed by drying and curing under conditions of a peak
metal temperature of 200°C in an air-heating furnace and
cooling with water to form an insulating layer on the ·plating
layer.
20
25
[0074]
Next, an Al metal layer 1-1as formed by deposition
plating Al onto the insulating layer with a vacuum
deposition plating apparatus. This method is hereinafter
referred to as the "deposition method".
[0075]
Film thickness and coverage rate were measured as
previously described for the coating films containing an
Al metal layer obtained by plating that were formed in
thispmanneer.
[0076]
30 (4. Formation of Coating Film Containing Al Metal
Layer Composed of Al Particles)
Flake-like Al particles were sprayed onto an
intermediate layer in the form of an insulating layer
formed in the manner described above to form an Al metal
35 film. The Al metal layer composed of Al particles was
produced by coating a coating liquid for the insulating
layer produced in the manner described above onto the
- 37 -
plating layer v1ith a bar coater, follm1ed by uniformly
spraying the Al particles on the coating film prior to
drying Hith a sieve, and then drying and curing the
coating film in an air-heating furnace under the
5 conditions providing a prescribed peak metal temperature.
In the present examples, particles obtained by drying
"Sap 561PS" aluminum paste manufactured by Showa Aluminum
P01·1der K .K. to form particles (average particle diameter:
16 f1ID) Here used for the Al particles. This method is
10 hereinafter referred to as "particle spraying".
[0077]
(5. Formation of Clear Resin Coating Film)
A resin coating liquid prepared as the coating
liquid for an intermediate layer in the form of an
15 insulating layer explained in the formation of a coating
film containing an Al metal layer by plating was coated
onto the Al metal layer v1i th a bar coater, follo1-1ed by
drying and curing in an air-heating furnace under the
conditions providing a peak metal temperature of 230°C and
20 then cooling Hith 1·1ater to form a clear resin coating
film layer.
[0078]
Zinc-plated steel sheets were produced in the manner
described above. The details of the zinc-plated steel
25 sheets produced are shoHn in Tables 1 to 4.
[0079]
...
[Table 1]
No. Hot-dip
galvanized layer
composition
(wt%)
Al Mg Si Formation Res. in Amt. of Type of
Method TyPe Melamine A1
Added Particles
(parts by
weight
' based on
100 parts
by weight of
polyester)
1)-l 11 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1)-2 11 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1)-3 11 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1) -4 11 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1)-5 11 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1) 6 11 3 0.2 Particle MD-HBO 20 Sap561PS
Dispersion
1}-7 11 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1)-8 11 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1)-9 1 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1} -10 7 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1) -11 20 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1) -12 55 3 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1) 13 11 0. 8 0.2 Particle MD-1480 20 Sap561PS
Dispersion
1)-14 11 1 0.2 Particle MD-1-480 20 Sap561PS
Dispersion
1)-15 11 5 0.2 Particle M0-1-480 20 Sap561PS
Dispersion
Table 1
Coating Film Containing Al
I
Al Coating Coating Heating
Particle Liquid Liquid Rate
Aspect Low High during
Ratio Shear Shear Baking
Viscosity Viscosity (°C/S)
(mPa ·s) (mPa ·s)
>20 1000 50 7
>20 1000 100 7
>20 160 50 7
>20 160 140 7
>20 1000 100 7
>20 1000 100 7
>20 1000 100 7
>20 1000 100 7
>20 1000 100 7
>20 1090 100 7
>20 1000 100 7
~20 1000 100 7
>20 1000 100 7
>20 1000 100 7
>20 1000 100 7
Peak Metal Film
Temperature Thickness
during I '"'I
Baking
("C)
210 4
I
210 7
210 4
210 4
210 4
210 2
210 10
210 20
210 4
210 4
210 4
210 4
210 4
210 4
210 4
Thickness
of Region
o=: Coating
Film Lower
Portion
where Al
Particles
are not
Present
I '"'I
1
3
1
0.5
1.5
0.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
A1
Coverage
Rate ('0)
85
85
85
85
85
75
95
95
85
85
85
85
85
85
85
w
co
I!J!:.JJE.
1)-16 11 10 0.2 Particle MD-1480 20
Dispersion
1) -17 11 3 0 Particle MD-1480 20
Dispersion
1)-18 11 3 0.02 Particle MD-1480 20
Dispersion
1)-::.9 11 3 0.2 Particle MD-1480 20
Dispersion
1)-20 11 3 1 Particle MD-H80 20
Dispersion
1)-21 11 3 2 Particle MD-1480 20
Dispersion
1)-22 0.05 0. 005 0 Particle MD-1480 20
Dispersion
1)-23 0.1 0.001 0 Particle MD-1'.1J.80 20
Dispersion
1)-24 0,2 0.001 0 Particle MD-1480 20
Disoersion
1) -25 0 0 0.1 Particle MD-1480 20
Dispersion
1} -26 11 0.5 0 Particle MD-1480 20
Dispersion
1)-27 11 3 0.2 Particle MD 1480 20
Dispersion
1) -28 11 3 0.2 Particle MD-1480 20
DisPersion
1)-29 11 3 0.2 Particle MD-1480 20
Disoersion
1)-30 11 3 0.2 Particle MD-1480 20
Dispersion
1)-31 11 3 0.2 Particle MD-1480 20
Dispersion
1) -32 11 3 0.2 Particle MD-1480 20
Disoersion
1) -33 11 3 0.2 Particle MD-1480 20
Disoersion
1) -34 11 3 0.2 Particle MD-1480 20
Dispersion
1)-35 11 3 0.2 Particle MD-1480 20
Dispersion
[0080]
Sap561PS >20 1000 100 7
I
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 80 40 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 5
Sap561PS >20 1000 100 70
Sap561PS >20 1000 100 2
Sap561PS >20 1000 100 100
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
Sap561PS >20 1000 100 7
210 4
210 4
210 4
210 4
210 4
210 4
210 4
210 4
210 4
210 4
210 2
210 1
210 4
210 4
210 4
210 4
180 4
150 4
230 4
250 4
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0
0.8
0.7
1.5
0.2
1.5
1.5
1.5
1.5
1.5
85
85
85
85
85
85
85
85
85
85
75
60
85
85
85
85
85
85
85
85
w
"'
I!JCCI:
[Table 2]
No. Hot-dip
galvanized layer
composition
{wt%)
A1 Mg Si Formation Method Coating Amt. of
Liquid Melamine
'• Type Added
(Resin (parts by
Type) weight
based on
100 parts
by weight of
polyester)
2)-1 11 3 0.2 Particle Dispersion MD-1220 20
2)-2 11 3 0.2 Particle Dispersion MD-1100 20
2)-3 11 3 0.2 Particle Dispersion MD-1985 20
2)-4 11 3 0.2 Particle Dispersion MD-1335 20
2)-5 11 3 0.2 Particle Dispersion MD-1500 20
2)-6 11 3 0.2 Particle Dispersion MD-1480 5
2)-7 11 3 0.2 Particle Dispersion MD-1480 10
2) 8 11 3 0.2 Particle Dispersion MD-lLIBO 30
2)-9 11 3 0.2 Particle Dispersion MD-1480 40
2)-10 11 3 0.2 Particle Dispersion MD-1480 20
2) -11 11 3 0.2 Particle Dis·persion MD-1480 20
2)-12 11 3 0.2 Particle Dispersion MD-1480 20
[0081]
Table 2
Coating Film Containing Al
Type of A1 Coating
A1 Particle Liquid
Particles Aspect Low
Ratio Shear
Viscosity
(mPa ·s)
sap561PS >20 1000
Sap561PS >20 1000
Sap561PS '2:20 1000
Sap561PS >20 1000
Sap561PS >20 1000
Sap561PS >20 1000
Sap561PS >20 1000
Sap561PS >20 1000
Sap561PS >20 1000
Sap2173PS ~20 1000
Sap720N >20 1000
Aluminum <20 1000
Powder
reagent
Coating Fil:n
Liquid Thickness
High l>m>l
Shear
Viscosity
(mPa ·s)
100 4
100 4
100 4
100 4
100 4
100 4
100 4
100 4
100 4
100 4
100 4
100 4
Thickness
of Region
of Coating
Film Lower
Layer Portion
where Al
Particles
are not
Present
l>m>l
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
A1
Coverage
Rate I~ I
85
85
85
85
85
85
85
85
85
75
95
60
~
0
' ll:li:E
[Table 3]
No. Hot dip galvanized layer Insulating Intermediate Layer
composition (wt%)
Al Mg Si Type Film
Thickness
I fUll)
3)-1 1 3 0.2 . Solvent 1
3) -2 7 3 0.2 Solvent 1
3)-3 11 3 0.2 ' Solvent 1
3) -4 20 3 0.2 Solvent 1
3)-5 11 1 0.2 Solvent 1
3)-6 11 5 0.2 Solvent 1
3) -7 11 10 0.2 Solvent 1
3) 8 11 3 0 Solvent 1
3)-9 11 3 0. 001' Solvent 1
3)-10 11 3 2 Solvent 1
3) -11 11 3 5 Solvent 1
3)-12 11 3 0.2 Solvent 0.5
3)-13 11 3 0.2 Solvent 1.5
3)-14 11 3 0.2 Solvent 3
3)-15 11 3 0.2 Solvent 1
3)-16 11 3 0.2 Solvent 1
3) -17 11 3 0.2 Solvent 1
3)-18 11 3 0.2 Solvent 1
3)-19 11 3 0.2 Solvent 1
3)-20 11 3 0.2 Solvent 1
3)-21 11 3 0.2 Solvent 1
3)-22 11 3 0.2 Solvent 1
3) 23 11 3 0.2 Solvent 1
3)-24 11 3 0.2 Solvent 1
3) -25 11 3 0.2 Water dispersed 1
3 I -26 11 3 0.2 None -
3)-27 11 3 0.2 None -
Table 3
Al Metal Layer
Formation Method Film Al Coverage
Thickness Rate I% I
luml
Deposition 4 100
De osition 4 100
Deposition 4 100
Deposition 4 100
Deoosition 4 100
Deoosition 4 100
Deposition 4 100
Deposition 4 100
Deposition 4 100
De osition 4 100
Dep_osi tion 4 100
Deposition 4 100
Deposition 4 100
Deposition 4 100
Deposition 1.5 75
De osition 3 85
Deposition 5 100
Deposition 10 100
Deposition 20 100
Deposition 5 100
De osition 1.5 75
Deposition 1.5 75
Deposition 1.5 75
Deposition 1.5 75
Depgsi tion 1.5 75
None - 0
Deposition 4 100
Clear Resin Coating
Film Layer
Type Film
Thickness
luml
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solve:-Jt l
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
Solvent 1
None
Solvent 0.2
Solvent 5
Solvent 20
Solvent 25
water dispersed 1.5
None
Solvent 1
"f-''
I!J!I!Ii
[0082]
[Table 4]
No. Hot-dip galvanized layer
composition (wt%)
A1 Mg Si
41-1 1 3 0.2
4)-2 7 3 0.2
41-3 11 3 0.2
4 I -4 20 3 0.2
4 I -5 11 1 0.2
41-6 11 5 0.2
41-7 11 10 0.2
41-8 11 3 0
41-9 11 3 0. 001
41-10 11 3 2
4 I -11 11 3 5
41-12 11 3 0.2
4)-13 11 3 0.2
41-14 11 3 0.2
41-15 11 3 0.2
41-16 11 3 0.2
4) -17 11 3 0.2
4 I -18 11 3 0.2
41-19 11 3 0.2
41-20 11 3 0.2
41-21 11 3 0.2
41-22 11 3 0.2
4)-23 11 3 0.2
4)-24 11 3 0.2
41-25 11 3 0.2
Insulating Intermediate Layer
Type Film
Thickness
('"'1\l
Water-dispersed 1
Water-disoersed 1
Water-dispersed 1
Water-disoersed 1
Water-dispersed 1
Water-dispersed 1
Water-dispersed 1
Water-dispersed 1
Water dispersed 1
Water-disoersed 1
Water-dispersed 1
Water-dispersed 0.5
Water dispersed 1.5
Water-dispersed 3
Water-dispersed 1
Water-dispersed 1
Water-dispersed 1
Water-dispersed 1
Water-dispersed 1
Water dispersed 1
Water-dispersed 1
Water-dispersed 1
Water-dispersed 1
Solvent 1
None -
Table 4
Al Metal Layer
Formation Method Film Al Coverage
Thickness Rate (~)
I>Unl
Particle spraying 4 100
Particle Sora ina 4 100
Particle S rayinq 4 100
Particle Spraying 4 100
Particle Spra inq 4 100
Particle S ra inq 4 100
Particle S rayin 4 100
Particle Spra inq 4 100
Particle S rayinq 4 100
Particle Soravino 4 100
Particle Spravinq 4 100
Particle S rayin 4 100
Particle Sprayinq 4 100
Particle Sprayinq 4 100
Particle Spraying 1.5 75
Particle Sora inq 3 85
Particle S rayin 5 100
Particle Spraying 10 100
Particle Spravinq 20 100
Particle S rayinq 1.5 75
Particle Spraying 1.5 75
Particle Sprayinq 1.5 75
Particle S rayin 1.5 75
Particle Soravinq 1.5 75
Particle Sprayinq 4 100
Clear Resin Coating
Film La er
Type Film
Thickness
l"ml
Water-dispersed 1
Water-di.:;persed 1
Water-dispersed 1
Water-dispersed 1
Water-disoersed 1
Water disoe.;::sed 1
Water-dis ersed 1
Water-dispersed 1
Water-dispersed 1
Water dispersed .
Water-dispersed l
Water-dis ersed 1
Water-dispersed 1
Water-dispersed 1
Water-dis ersed 1
Water dispersed 1
Water-disoersed 1
Water-disoersed 1
Water dispersed 1
None -
Water-dispersed 0.2
Water-dispersed 5
Water-dis ersed 20
Solvent 1.5
Water-dispersed 1
"N '
I!JG::I:
- 43 -
[0083]
The following evaluation tests were carried out on
the zinc-plated steel sheets produced. Furthermore, in
each of the tests, tests were carried out using the side
5 having the coating film containing Al for the evaluated
side.
[0084]
(I. Processability Test)
A cupping test 1-1as carried out under conditions of
10 an indentation depth of 8 mm according to the method
described in the "Cupping Test" of JIS K 5600-5-2. The
test was carried out under conditions such that the
evaluated side was outside the cup, and a test typically
referred to as a tape peeling test was carried out in
15 which tape is affixed to and then peeled from a processed
portion after testing.
[0085]
Follo~o1ing completion of the test, damage to the
surface of the portion from which the tape 1·1as peeled was
20 observed visually, and the case in which there was no
damage 1-1hatsoever 1-1as evaluated as A, the case in 1-1hich
peeling of the Al metal layer at the processed portion
1-1as less than 20% in terms of the area ratio 1-1as
evaluated as B, the case in Hhich peeling of the Al metal
25 layer at the processed portion Has 20% to 50% in terms of
the area ratio was evaluated as C, and the case in which
peeling of the Al metal layer at the processed portion
exceeded 50% in "terms of the area ratio 1·1as evaluated as
D.
30 [0086]
(II. Corrosion Resistance Test)
The produced zinc-plated steel sheets 1-1ere cut to a
size of 70 mrn wide x 150 mrn long, a cut extending to the
steel sheet substrate Has provided on the evaluated side,
35 and the end surfaces of the cross-sections on all four
sides 1·1ere sealed with tape to produce corrosion
resistance test samples. A salt 1-1ater spraying test 1-1as
- 44 -
carried out according to the method described in section
9.1 of JIS K 5400. The salt water was sprayed so as to
contact the evaluated side. The duration of the test was
240 hours.
5 [0087]
Follovling completion of the test, the maximum amount
of swelling on one side of the cuts was measured, and the
case in v1hich the amount of swelling was 3 mm or less was
evaluated as A, the case in which the amount of swelling
10 was more than 3 mm to 4 mm or less was evaluated as AB,
the case in which the amount of swelling was more than 4
mm to 5 mm or less was evaluated as B, the case in which
the amount of s1·1elling was more than 5 mm to 10 mm or
less was evaluated as C, and the case in which the amount
15 of swelling exceeded 10 mm was evaluated as D.
[0088]
(III. Blackening Resistance Test)
The produced zinc-plated steel sheets were cut to a
size of 70 mm vlide x 150 mm long. The cut sheets were
20 subjected to an exposure test in which the steel sheets
1·1ere exposed for 6 months on a coastline in Kimitsu City,
Chiba Prefecture, Japan, the color tone of the steel
sheets before and after the exposure test was measured
1o1i th a spectrophotometer, and the L * value representing
25 lightness of the CIE color system (L*a*b* color system)
v1as measured. Those sheets for which 1'1L*o:;5 based on the
equation I'lL* = [L* value before testing] - [L* value
after ···testi·ng] ·'We're evaluated as A, those sheets for
v1hich 5<1'1L*o:;1o Here evaluated as B, those sheets for
30 which 10<1'1L*o:;15 ~rere evaluated as C, and those sheets for
which 15<1'11*~20 \•I ere evaluated as D.
[0089]
(IV. Fingerprint Resistance)
After adhering a fingerprint to, the side to be
35 evaluated by pressing 1·1i th the index finger, the case in
which the fingerprint did not adhere at all was evaluated
- 45 -
as A, the case in 1-1hich the fingerprint 1·1as adhered but
was able to be removed by 1-1iping with a cloth Has
evaluated as B, the case in which the fingerprint was
adhered but the remaining fingerprint was difficult to
5 visually confirm after ~Viping 1-1ith a cloth was evaluated
as C, and the case in 1-1hich the fingerprint 1-1as unable to
be removed at all even after ~Viping 1-1ith a cloth was
evaluated as D.
10
(V. Appearance Test)
The side to be evaluated was observed visually and
evaluated for the presence or absence of visual defects.
[0090]
The following provides a detailed description of the
evaluation results (see Tables 5 to 8). Evaluation
15 criteria for the examples of the invention and
comparative examples were such that they were evaluated
as comparative examples without exception if either of
corrosion resistance or blackening resistance \•Jas
evaluated as D. On the other hand, those that were
20 evaluated as C for only one of the evaluation parameters
while others were evaluated as B or better were
considered to be examples of the present invention.
Moreover, those that were evaluated as D or C for
fingerprint resistance, but were evaluated as B for both
25 corrosion resistance and blackening resistance were
considered to be examples of the present invention.
[0091]
[Table 5]
No. Processability Corrosion Blackening
Resistance Resistance
1)-1 A A B
1)-2 A A B
1)-3 A A B
1)-4 A B B
1)-5 A A B
1)-6 A B c
1) 7 A A A
1)-8 c A A
1)-9 A AB B
1)-10 A A B
1) -11 A A B
1)-12 A A B
1) -13 A AB B
1)-14 A A B
1)-15 A A B
1)-16 A A B
1) -17 A c B
1)-18 A AB B
1)-19 A A B
1)-20 A A B
1)-21 A A B
1)-22 A D B
1)-23 A D B
1)-24 A D B
1)-25 A D A
1)-26 A D c
1)-27 A A D
1)-28 A A A
1)-29 A A A
1) 30 A D D
1)-31 A A B
1)-32 A A B
1) 33 A A B
1) 34 A A B
1)-35 A A B
[0092]
Table 5
Fingerprint Appearance
Resistance
A Good
A Good
A Good
A Good
A Good
A Good
A Good
c Good
A Good
A Good
A Good
A Good
A Good
A Good
A Good
A Poor platinq appearance due to dross
A Verv poor platinq appearance due to dross
A Good
A Good
A Sliqhtl poor platinq appearance due to dross
A Poor latin a earance due to dross
A Good
A I Good
A Good
c Good
A Good
A Good
A Good
A Good
A Good
A Coating defects due to boiling,
uncured coating film surface layer
A Good
A Sliqhtl uncured coatinq film surface layer
A Good
A Coating defects due to boiling,
crackinq of coatinq film layer when bent at 180°C
Remarks
Example of invention
E>:ample of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Exam le of invention
Example of invention
Example of invention
Exam le of invention
Example of invention
Example of invention
Example of invention
Ex~~ple of invention
Example of invention
Comparative example
Exam le of invention
Example of invention
Example of invention
Example of invention
Com arative exam le
Comparative example
Comparative example
Comparative example
Comparative example
Com arative example
Examole of invention
Example of invention
Com arative example
Comparative example
Example of invention
Com arative exam~1e
Exam le of invention
Comparative example
"'" Go
I!DI
[Table 6]
No. Processability Corrosion Blackening
Resistance Resistance
21-1 c A B
2)-2 B A B
2)-3 B A B
2)-4 c A B
21-5 c A B
2)-6 c A B
21-7 A A B
2)-8 B A B
2)-9 c A B
21-10 A A c
2) -11 A A A
2) -12 A A D
[0093]
Table 6
Fingerprint Appearance
Resistance
A Good
A Good
A Good
A Good
A Good
A Good
A Good
A Good
A Good
A Good
A Sli ht surface irre ularities
A Good
Remarks
Example of invention
E:-:ample of invention
Example of invention
Example of invention
Example of invention
Example of invention
Exa~ple of invention
Example of invention
Example of invention
Example of invention
Exam le of inven~ion
Co~?rative examole
"--"l
[.!]I
[Table 7]
No. Processability Corrosion Blackening
Resistance Resistance
3) 1 A B A
3)-2 A A A
3)-3 A A A
3)-4 A A A
3) 5 A B A
3)-6 A A A
3)-7 A A A
3)-8 A c A
3)-9 A A A
31 10 A A A
3) -11 A A A
3)-12 A B A
3)-13 A A A
3)-14 c A A
3) -15 A A c
3)-16 A A B
3)-17 A A A
3)-18 A A A
3)-19 c A A
3)-20 A A A
3)-21 A A c
3) 22 A A c
3) -23 A A c
3)-24 A A c
3)-25 A A c
3)-26 A A D
3)-27 A D A
[0094]
Table 7
Fingerprint Appearance
Resistance
B Good
B Good
B Good
B Good
B Good
B Good
B Good
B Ver ocr latin a earance due to dross
B Good
B Poor platinq appearance due to dross
B Poor olatinq a earance due to dross
B Good
B Good
B Good
B Good
B Good
B Good
B Good
B Good
D Good
c Good
A Good
A Good
A Boilinq in clear resin coatinq film
A Good
D Good
B Good
Remarks
Examo1e of invention
Exam le of invention
Example of invention
Example of invention
Exam le of invention
Example of invention
Example of invention
Com arative examE_le
Example of invention
Example of invention
Example of invention
Exam le of invention
Exam le of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Exam le of invention
Comparative example
Example of invention
Example of invention
Example of invention
Example of invention
Comparative example
Comparative example
"00'
llll:L
[Table 8]
No, Processability Corrosion
Resistance
4) -1 A B
4) -2 A A
4) 3 A A
4) -4 A A
41 -s A B
4 I -6 A A
4) -7 A A
4)-8 A B
4 I -9 A A
4)-10 A A
4) -11 A A
41-12 A B
4) 13 A A
4) -14 c A
4) -15 A A
4) -16 A A
4) -17 A A
4) -18 A A
4) -19 c A
41-20 A A
41-21 A A
4)-22 A A
41-23 A A
4)-24 A A
4) 25 A D
Blackening Fingerprint
Resistance Resistance
A B
A B
A B
A B
A B
A B
·, A B
A B
A B
A B
' A B
A B
A B
A B
c B
B B
A B
A B
A B
c D
c c
c A
c A
c A
A B
Table 8
Appearance
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Good
Remarks
Example of invention
Example of inventio~
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Exam le of invention
Ex~~le of invention
Example of invention
Example of invention
Example of invention
Example of invention
Example of invention
Comoarative exam~le
Ex~~p~e of invention
Example of invention
Example of invention
Example of invention
Comparative example
"\0'
llEI
- 50 -
[0095]
Tables 5 and 6 indicate evaluation results for zincplated
steel sheets produced according to the
aforementioned "particle dispersion method". The
5 inventions of the present application of Nos. 1)-1 to 1)-
24 and Nos. 2)-1 to 2)-11 demonstrated superior
evaluation results for each of the parameters of
processability, corrosion resistance, blackening
resistance and fingerprint resistance.
10 [0096]
The examples of the present invention of Nos. 1)-1
to 1)-4 tended to have decreased corrosion resistance if
the region of the coating film lower portion in which Al
was not present was 0.5 pm. In addition, in the
15 comparative example of No. 1)-26, since the region of the
coating film lm·1er portion in which Al was not present
was 0 pro,_ or in other 1·10rds, the Al material and the
plating layer were in contact with each other, corrosion
resistance was inferior, thereby making this unsuitable.
20 [0097]
25
The example of the present invention of No. 1)-6
demonstrated a low Al coverage rate of 75% since the
thickness of the Al metal coating film layer was thin at
2 pro, and blackening resistance tended to decrease. In
addition, the comparative example of No. 1)-27
demonstrated inferior blackening resistance since the
thickness of the Al metal coating film layer was even
thinne-r ·c~nci··tht! coverage rate ~;as less than 75%, thereby
making this unsuitable. The example of the present
30 invention of No. 1)-8 demonstrated decreased
processability since the thickness of the Al metal
coating film layer was excessively thick at 20 ~un.
[0098]
In the example of the present invention of No. 1)-
35 16, appearance was slightly poor due to the presence of
dross attributable to oxides resulting from oxidation of
- 51 -
part of Mg, which 1·1as not mel ted in the plating bath,
adhering to the plating layer since the amount of Mg
added in the hot-dip galvanized layer was high at 10% by
v1eight. Visual defects caused by dross tend to be
5 disapproved from the viewpoint of design properties at
locations that are visibly conspicuous (such as exterior
panels of home appliances or buildings). However, since
visual defects caused by dross are merely the result of
oxides adhering to the plating layer, steel sheets having
10 such visual defects can still be used as plated steel
sheet products provided there are no problems in terms of
their performance.
[0099]
In the example of the present invention of No. 1)-
15 21, appearance was slightly poor due to the presence of
dross attributable to oxides resulting from oxidation of
part of Si, which was not melted in the plating bath,
adhering to the plating layer since the amount of Si
added in the hot-dip galvanized layer was high at 2% by
20 weight. In addition, appearance \'las also slightly poor
due to dross in the example of the present invention of
No. 1)-20 in l'lhich the amount of Si added 1·1as 1% by
1veight. Visual defects caused by dross tend to be
disapproved from the viel'lpoint of design properties at
25 locations that are visibly conspicuous (such as the
exterior panels of home appliances or buildings).
H01·1ever, since visual defects caused by dross are merely
the r~pult of oxides adhering to the plating layer, steel
sheets having such visual defects can still be used as
30 plated steel sheet products provided there are no
problems in terms of their performance.
[0100]
The comparative example of No. 1)-25 was unsuitable
due to inferior corrosion resistance since Al and Mg \'lere
35 not contained in the zinc plating layer.
[0101]
The comparative example of No. 1)-30 demonstrated
- 52 -
inferior corrosion resisFance since the heating rate when
baking the coating liquid to form the Al metal coating
film layer was sl0\'1 at less than 5°C/s, thereby making it
difficult for Al particles to rise and causing Al
5 particles to be present v1i thin a range of 0. 5 ~tm from the
interface between the coating film and plating layer.
The comparative example of No. 1)-31 demonstrated the
occurrence of the coating defect referred to as boiling,
since in this example, the coating material ends up
10 curing while boiling in the solvent drying step and the
remnants of air bubbles caused by boiling remain in the
coating liquid, due to the heating rate when baking the
coating film to form the Al metal coating film layer of
higher than 70°C/s. Moreover, since heating time is short
15 due to the slow heating rate, the coating film surface
layer becomes slightly uncured, causing the fingers to
slightly stick to the coating film~--surface layer (feel
sticky) 1·1hen touched. The uncured coating film surface
layer causes the coating film to peel, thereby making
20 this example unsuitable. The coating film surface layer
of comparative example of No. 1)-33 was slightly uncured
since the peak metal temperature when baking the coating
liquid to form the Al metal coating film layer was below
180°C, thereby causing fingers to slightly stick to the
25 coating film surface layer (feel sticky) 1·1hen touched.
The uncured coating film surface layer causes the coating
film to peel, thereby making this example unsuitable. In
the comparative example of No. 1)-35, the coating defect
referred to as boiling vias occurred since the coating
30 material ended up being cured 1-1hile boiling in the
solvent drying step and the remnants of air bubbles
caused by boiling remain in the coating liquid, due to
the peak metal temperature when baking the coating liquid
to form the Al metal coating film layer of above 230°C.
35 Moreover, since the coating material was baked at such a
high temperature, the coating film became hard as curing
- 53 -
of the coating film proceeded, thereby resulting in the
formation of cracks or peeling in the coating film when
subjected to bending processing at 180 degrees. Since
zinc-plated steel sheets are typically used after
5 processing, there is the risk the occurrence of corrosion
at processed portions Hhere the coating film has become
cracked or peeled due to processing, Hhich significantly
decreases product value, thereby making this example
unsuitable.
10 [0102]
In the comparative example of No. 2)-12, the aspect
ratio of the Al particles was less than 20 and Al
coverage rate v1as also low, thereby resulting in inferior
blackening resistance and making this example unsuitable.
15 [0103]
Since Vylonal® MD-1335 having a lo1-1 number average
molecular v1eight (number average molecular weight: 8, 000,
Tg: 4°C) was used in the example of the present invention
of No. 2)-4 and Vylonal® MD-1500 having a high Tg (number
20 average molecular \•Ieight: 8, 000, Tg: 77°C) \•las used in the
example of the present invention of No. 2)-5,
processability tended to decrease in these examples.
Since Vylonal® MD-1220 having a some1-1hat high Tg (number
average molecular Height: 15,000, Tg: 67°C) v1as used in
25 the example of the present invention of No. 2)-1 and
Vylonal® MD-1985 having a somev1hat high number average
molecular 1-1eight and somewhat low Tg (number average
molecular v1eight: 25,000, Tg: -20°C) 1·1as used in the
example of the present invention of No. 2)-3,
30 processability tended to decrease in these examples.
[0104]
In the example of the present invention of No. 2)-
11, a portion of the Al particles ended up being outside
the coating film since the average particle diameter of
35 the Al particles v1as large at 30 ~tm, thereby resulting in
the slight formation of surface irregularities. Despite
- 54 -
this, since surface irregularities are merely the result
of Al particles present in the coating film being outside
the coating film, steel sheets having such surface
irregularities can still be used as plated steel sheet
5 products provided there are no problems in terms of their
performance.
[0105]
Table 7 indicates evaluation results for zinc-plated
steel sheets produced according to the aforementioned
10 "deposition method". As shown in Table 7, examples of
the present invention of Nos. 3)-1 to 3)-25 demonstrated
superior evaluation results for each of the parameters of
processability, corrosion resistance, blackening
resistance and fingerprint resistance.
15 [0106]
The examples of the present invention of Nos. 3)-1
to 3)-13 and Nos. 3)-15 to 3)-25 demonstrated superior
processability since the insulating layer (intermediate
layer) consisted of resin.
20 [0107]
Although the example of the present invention of No.
3)-14, in which the thickness of the insulating layer
(intermediate layer) exceeded 1.5 ~m, demonstrated
somewhat inferior processability in comparison with other
25 examples of the present invention in which the insulating
coating film layer was 1. 5 ~un or less, it demonstrated
favorable results for parameters other than
process·ability.
30
·[0108]
Although the example of the present invention of No.
3)-20, in which a clear resin coating film was not formed
on the Al metal coating film layer, demonstrated inferior
fingerprint resistance than other examples of the
invention having a clear resin coating film, it
35 demonstrated superior results for evaluation parameters
other than fingerprint resistance.
[0109]
- 55 -
In the comparative example of No. 3)-21, in which
the thickness of the clear resin coating film was 0.2 pm,
the evaluation result for fingerprint resistance was at a
lower limit level of C.
5 [0110]
In the example of the present invention of No. 3)-
24, although all of the evaluation results were
favorable, since the clear resin coating film was
comparatively thick at greater than 25 pro, coating
10 defects referred to as boiling tended to occur in the
coating film during the course of drying and curing after
the application of the clear resin coating film.
[0111]
In the comparative example of No. 3)-8, in which the
15 Si content in the hot-dip galvanized layer 11as less than
0.001% by Height, corrosion resistance tended to be
some1·1hat inferior_ to other examples of the invention in_
which Si v1as added at 0. 001% by v~eight or more, and
appearance defects of the plating attributable to dross
20 occurred extensively, thereby making this example
unsuitable. In the invention of the present application
of No. 3)-10, in which the Si content in the hot-dip
galvanized layer was 2% by \·Ieight, slight dross formation
\·las observed. Ho\'lever, since the dross formation was
25 only slight, the level of quality of this example is
considered to not present a problem in terms of actual
use.
[0112]
In the example of the present invention of No. 3)-
30 11, in which the Si content exceeded 2% by weight,
although the appearance of the plating tended to be
inferior, due to the formation of dross, to other
examples of the invention, all other evaluation
parameters were superior.
35 [0113]
In the invention of the present application of No.
3)-24, the coating defect referred to as boiling occurred
- 56 -
since the thickness of the clear film coating was thick
at 25 ~un. Boiling refers to a coating defect in which
remnants of the solvent boiled in the step for drying and
baking the coating film remain in the form of craters,
5 and occurs easily when the coating film is thick.
Consequently, it is preferable that boiling not occur
since products having poor appearance tend to be
disapproved from the vie1·1point of design properties at
locations that are visibly conspicuous (such as the
10 exterior panels of home appliances or buildings),.
Ho1-1ever, although the appearance of such products is
unattractive, they can still be used 1-1ithout problems
provided there are no problems in terms of their
performance.
15 [0114]
On the other hand, the comparative example of No.
3)-26 demonstrated inferior blackening resistance since
an Al metal coating film layer was not formed on the ZnAl-
Mg-Si alloy plating layer. The comparative example of
20 No. 3)-27 was unsuitable due to inferior corrosion
resistance since it was not provided 1-1ith an insulating
coating film layer composed of an insulating substance
betHeen the hot-dip galvanized layer and the Al metal
layer.
25 [0115]
Table 8 indicates results of evaluation tests for
zinc-plated steel sheets produced according to the
aforement,it:me-d-,.'~particle spraying method". Examples of
the present invention of Nos. 4)-1 to 4)-19 and Nos. 4)-
30 21 to 4)-24 were superior for each of processability,
corrosion resistance, blackening resistance and
fingerprint resistance.
INDUSTRIAL APPLICABILITY
35 [0116]
According to the present invention, a Zn-Al-Mg-Sibased
zinc-plated steel sheet can be provided that
- 57 -
realizes both superior corrosion resistance and
blackening resistance for a long period of time. As a
result, a Zn-Al-Mg-Si-based zinc-plated steel sheet that
does not require coating, is inexpensive and superior in
5 corrosion resistance can be applied as an exterior panel
of construction materials and home appliances, and users
of zinc-plated steel sheets can omit some of the
production steps and reduce the production cost of their
products. Thus, the present invention can be said to be
10 an invention that demonstrates an extremely high level of
industrial value.
5
10
- 58 -

CLAIMS
1. A zinc-plated steel sheet having superior
blackening resistance and corrosion resistance,
comprising:
a steel sheet,
a Zn-Al-Mg-Si alloy plating layer formed
on the surface of the steel sheet, and
a coating film containing Al formed on the
plating layer;
characterized in that the Al contained in
the coating film containing Al is separated from the
plating layer by the presence of an insulating substance,
and
the coverage rate of Al, which is defined
15 as the ratio of the area of the portion of the plating
layer concealed by the Al in the coating film to the
total area of the observed field of view when observing
the coating film containing Al from the direction
perpendicular to the surface thereof, is 75% to 100%.
20 2. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
to claim 1, characterized in that the coating film
containing Al is composed of an insulating substance
containing flake-like Al particles, and the Al particles
25 are not present "'ithin a range of at least 0. 5 IJffi from
the interface between the coating film containing Al and
the plating layer .
. 3. . .. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
30 to claim 2, characterized in that the average particle
diameter of the Al particles is 5 pm to 30 run and the
aspect ratio thereof is 20 or more.
4. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
35 to claim 1, characterized in that the coating film
containing Al is composed of at least two layers,
consisting of an intermediate layer formed with an
- 59 -
insulating substance and an Al metal layer, in that order
starting from the plating layer side.
5. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
5 to claim 4, characterized in that the Al metal layer is
composed of an aggregate of flake-like Al particles.
6. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
to claim 1, characterized in that the insulating
10 substance is a resin.
7. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
to claim 6, characterized in that the resin is a
polyester resin crosslinked with a melamine compound.
15 8. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
to claim 7, characterized in that the glass transition
temperature Tg of the polyester resin is -20°C to 70°C and
the number average molecular 1-1eight thereof is 15,000 to
20 25,000.
9. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
to claim 1, characterized in that the thickness of the
coating film containing Al is 2 ~ to 10 ~·
25 10. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
to claim 1, characterized by having a clear resin coating
film on ··the coating film containing Al.
11. The zinc-plated steel sheet having superior
30 blackening resistance and corrosion resistance according
to claim 10, characterized in that the thickness of the
clear resin coating film is 0.2 pm to 20 pm.
12. The zinc-plated steel sheet having superior
blackening resistance and corrosion resistance according
35 to claim 1, characterized in that the zinc plating layer
contains 0.01% by weight to 60% by weight of Al, 0.001%
- 60 -
by weight to 10% by weight of Mg and 0. 001% by \'Ieight to
2% by weight of Si.
13. A method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
5 resistance according to claim 2, characterized by coating
a zinc plating layer on the surface of the steel sheet
with a coating material containing flake-like Al
particles and an insulating substance in a solvent, the
viscosity thereof under conditions of a shear velocity of
10 1 s-1 as measured with a rotational viscometer at 25°
being 150 mPa·s to 1500 mPa·s, and the viscosity thereof
at a shear velocity of 10, 000 s-1 as measured with a
rotational viscometer at 25°C being 50 mPa·s to 150 mPa·s,
follm·1ed by heating the steel sheet to a peak metal
15 temperature of 180°C to 230°C at a heating rate of 5°C/s
to 70°C/s in an induction heating furnace to form the
coating film containing A~.
14. The method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
20 resistance according to claim 13, characterized in that
the coating material is prepared by mixing flake-like Al
particles with 100 parts by weight of an aqueous
emulsion-type polyester resin and 10 parts by weight to
30 parts by weight of a melamine compound as a
25 crosslinking agent.
15. The method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance accqrding to claim 13, characterized in that
the viscosity of the coating material is adjusted using a
30 viscosity modifier.
16. The method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance according to claim 15, characterized in that
0.2 parts by weight to 10 parts by weight of a surfactant
35 composed mainly of a urethane-modified polyether based on
100 parts by v1eight of the aqueous emulsion-type
- 61 -
polyester resin is used for the viscosity modifier.
17. The method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance according to claim 13, characterized in that •
5 Al particles having an average particle diameter of 5 ~nn
to 30 ~un and aspect ratio of 20 or more are used for the
Al particles.
18. A method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
10 resistance according to claim 4, characterized by:
15
(a) forming an intermediate layer of an
insulating substance on a zinc plating layer on the
surface of a steel sheet followed by forming an Al metal
layer thereon by a plating method, or
(b) coating a zinc plating layer on the
surface of a steel sheet with a liquid material for
forming an intermediate layer of an insulating substance,
spraying flake-like Al particles onto the liquid
material, and then allowing the liquid material to
20 solidify to form an intermediate layer of an insulating
substance and an Al metal layer thereon.
19. The method for producing the zinc-plated steel
sheet having superior blackening resistance and corrosion
resistance according to claim 18, characterized in that
25 the plating method is vacuum deposition plating.