TECHNICAL FIELD
[0001] The present invention relates to an Al-based
alloy plated steel material having excellent postcoating
corrosion resistance.
BACKGROUND ART
[0 0 02] Various kinds of rustproof materials are
widely used in various fields in order to reduce an
environmental load, cut down life-cycle cost, and
further ensure safety. Their use examples include
small components such as electronic components, home
electric appliances, automobiles, construction
materials, and further large structures such as
infrastructure facilities.
[0003] A plated steel material is one of the
rustproof materials, and especially a Zn-plated steel
material is often used. Main reasons why the Znplated
steel material is often used are that it is
relatively low-priced and has a sacrificial
anticorrosive action on a base iron, and that a
corrosion rate of Zn itself in an atmospheric
environment is low. Examples of a plated steel
material other than the Zn-plated steel material
include an Al-plated steel material. However, the
Al-plated steel material has problems that it does
not have a sacrificial anticorrosive action on a base
- 1 -
iron due to an oxide coating film existing on a
surface of its Al plating layer, and that it is poor
in post-coating corrosion resistance since its
chemical conversion treatability is insufficient due
to the oxide coating film existing on the surface of
the Al plating layer. On the other hand, since Al
itself is lower in corrosion rate than Zn, it is
thought that the application range widens if
sacrificial corrosion resistance and post-coating
corrosion resistance can be ensured in the Al plating
layer.
[0004] Therefore, arts for improving corrosion
resistance of an Al plating layer have been proposed..
For example, Japanese Laid-open Patent Publication No
2003-34845 (Patent Literature 1) discusses that, by
making 0.5 to 10% Mg contained in Al plating, a
sufficient amount of a chemically converted coating
film is generated, so that post-coating corrosion
resistance improves. However, adding an active
element such as Mg to the Al plating increases
activity of the Al plating layer itself, so that bare
corrosion resistance (corrosion resistance in an
uncoated state) deteriorates on the contrary.
[0005] Further, Japanese Laid-open Patent
Publication No. 2007-302982 (Patent Literature 2)
describes that applying Zn, a compound of Zn, or the
like on a surface of an Al plating layer improves
post-coating corrosion resistance. However, an
effect of improving post-coating corrosion resistance
- 2
is still insufficient since adhesion of Zn or the Zn
compound to plating is not sufficient.
CITATION LIST
PATENT LITERATURE
[0006] Patent Literature 1: Japanese Laid-open
Patent Publication No. 2003-34845
Patent Literature 2: Japanese Laid-open Patent
Publication No. 2007-302982
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007] In consideration of the aforesaid problems,
it is an object of the present invention to provide
an Al-based alloy plated steel material having
sacrificial corrosion resistance and more excellent
in post-coating corrosion resistance than
conventional Al-plated steel materials.
SOLUTION TO PROBLEM
[0008] As a result of studious studies for
overcoming the aforesaid problems, the present
inventors have found out that it is possible to
realize an Al-based alloy plated steel material
having a sacrificial anticorrosive effect and being
more excellent in post-coating corrosion resistance
than conventional Al-based plated steel materials, by
providing a layer containing ZnO on a surface of an
Al-based alloy plating layer, making Fe and Si
contained in the Al-based alloy plating layer, and
further forming a composite oxide layer including Zn
and Al between the layer containing ZnO and the Al-
_ 3 _
based alloy plating layer. The gist of the present
invention is as follows.
[0009] (1) An Al-based alloy plated steel material
having excellent post-coating corrosion resistance,-
the Al-based alloy plated steel material including: a
steel material and a coating layer formed on a
surface of the steel material, wherein the coating
layer
contains, in mass%, not less than 10% nor more
than 50% Fe and not less than 3% nor more than 15% Si,
and
include s:
an Al-based alloy plating layer formed on the
surface of the steel material;
a layer containing ZnO and formed above a surface
of the Al alloy plating layer; and
a ZnAl204 layer with a thickness of not less than
0.05 fim nor more than 2 {j. m, formed between the Al
alloy plating layer and the layer containing ZnO.
(2 ) The Al-based alloy plated steel material
having excellent post-coating corrosion resistance
according to claim 1, wherein a surface roughness Ra
of the coating layer is within a range of not less
than 1 ism nor more than 5 M m.
ADVANTAGEOUS EFFECTS OF INVENTION
[0010] As described above, according to the Al-based
alloy plated steel material of the present invention,
it has sacrificial corrosion resistance, and its
post-coating corrosion resistance can be far more
- 4 -
improved than that of conventional Al-plated steel
materials. Accordingly, it is applicable to home
electric appliances, automobiles, construction
materials, and further large structures such as
infrastructure facilities, and thus its industrial
contribution is very large.
DESCRIPTION OF EMBODIMENTS
[0011] Hereinafter, an embodiment of the present
invention will be described in detail. An Al-based
alloy plated steel material of this embodiment
includes a steel material and a coating layer formed
on a surface of the steel material. The coating
layer includes: an Al-based alloy plating layer
formed on the surface of the steel material; a layer
containing ZnO and formed above a surface of the .A1
alloy plating layer; and a ZnAl204 layer formed
between the Al alloy plating layer and the layer
containing ZnO. In the description of the embodiment
and examples, the layer containing ZnO is referred to
as a "ZnO-containing layer").
[0012] The Al-based alloy plating layer needs to
contain Fe. Fe has an effect of imparting
sacrificial corrosion resistance to the Al-based
alloy plating layer itself and an effect of
increasing a surface roughness of the Al-based alloy
plating layer to exhibit an anchoring effect, thereby
improving post-coating corrosion resistance. The
content of Fe in the Al-based alloy plating layer
needs to be not less than 10% nor more than 50% in
- 5 -
mass%. When the content of Fe is less than 10%, the
effect of imparting sacrificial corrosion resistance
and the effect of increasing the surface roughness of
the Al-based alloy plating layer are weak, which is.
not preferable... Further, when the content of Fe is
over 50%, the Al-based alloy plating layer itself
becomes brittle and becomes poor in plating adhesion,
which is not preferable.
[0013] Further, in view of more improving plating
a-dhesion, the Al-based alloy plating layer needs to
contain Si within a range of not less than 3% nor
more than 15% in mass%, in addition to Fe. By making
Si contained in the Al-based alloy plating layer, it.
is possible to suppress the growth of a Fe-Al alloy
layer to improve plating adhesion. Incidentally,
when the content of Si is less than 3%, this effect
becomes weak, and when the content is over 15%,
plating adhesion deteriorates on the contrary.
[0014] Further, the surface roughness Ra of the Albased
alloy plating layer is preferably not less than
1 p. m nor more than 5 jit m. The surface roughness Ra
is a surface shape parameter defined by JIS B 0601.
When the surface roughness Ra is less than 1 /im,
post-coating corrosion resistance becomes poor due to
an insufficient anchoring effect. When the surface
roughness Ra is over 5 iim, irregularities become too
great, which is a cause to generate a variation in
film thickness, leading to deterioration in corrosion
resistance on the contrary. Incidentally, in the Al-
6
based alloy plated steel material of this embodiment,
the surface roughness Ra of the Al-based alloy
plating layer is reflected in a surface roughness of
the coating layer. Therefore, the surface roughness
Ra of the, coating layer falls within the range of not
less than 1 \x m nor more than 5 /zm.
[0015] Further, the coating layer needs to have the
ZnAl204 layer immediately on the Al-based alloy
plating layer, that is, between the ZnO-containing
layer and the Al-based alloy plating layer. The
ZnAl20 4 layer has an effect of strengthening adhesion
of the Al-based alloy plating layer and the steel
material to the ZnO-containing layer to improve postcoating
corrosion resistance. The ZnAl204 layer needs
to have a thickness of not less than 0.05 (i m nor, more
than 2 /x m. When the thickness is less than 0.05 /x m,
the effect of increasing adhesion to improve postcoating
corrosion resistance becomes weak. Further,
when the thickness is over 2 /im, the ZnAl204 layer
itself becomes brittle to easily peel off. Note that
the thickness of the ZnAl204 layer can be measured in
such a way that, after an arbitrary section of the
Al-based alloy plating layer of this embodiment is
cut out, it is buried in a resin and polished, and
this arbitrary section is observed by a scanning
electron microscope.
[0016] The Al-based alloy plated steel material
needs to have the ZnO-containing layer on its
uppermost surface. The ZnO-containing layer has an
_ 7 _
effect of imparting chemical conversion treatability,
and also serves as a supply source of Zn for forming
the ZnAl2C>4 layer which is necessary for improving
corrosion resistance of the Al-based alloy plated
steel material .of this embodiment. An amount of the
ZnO-containing layer is not particularly specified,
but when a Zn amount of the ZnO-containing layer is
less than 0.4 g/m2, it becomes difficult to form a
sufficient amount of the ZnAl204 layer. On the other
hand, when the Zn amount is over 5 g/m2, the ZnAl204
layer becomes poor in adhesion to the Al-based alloy
plating layer to come off or weldability is likely to
deteriorate. Therefore, the amount of the ZnOcontaining
layer is preferably such an amount that
the Zn amount becomes not less than 0.4 g/m2 nor more
than 5 g/m2'
[0017] A component, a form, and so on of the steel
material being a base material of the Al-based alloy
plated steel material are not limited at all. The
component may be a soft steel material, or may be a
steel material containing a strengthening, element
such as Si or Mn. Further, the form may be a thin
sheet, a thick sheet, a steel pipe, a section steel,
or a molding.
[0018] Next, a method of manufacturing the Al-based
alloy plated steel material having excellent postcoating
corrosion resistance of this embodiment will
be described.
The method of manufacturing the Al-based alloy
plated steel material of this embodiment includes: a
step of. forming the Al-based alloy plating layer on
the steel material; a step of forming the ZnOcontaining
layer on the surface of {immediately on).
the Al-based alloy plating layer; and a step of
forming the ZnAl204 layer between the Al-based alloy
plating layer and the ZnO-containing layer.
Hereinafter, the steps will be described.
[0019] (Step of Forming Al-based Alloy Plating
tayer)
As the method of forming the Al-based alloy
plating layer, conventionally used hot dipping or the
like can be employed. Further, a type of the hot
dipping may be any of a redox balance process, a
total oxidation process, an internal oxidation
process, a flux process, a pre-plating process, and
the like. In order to make Fe contained in the Albased
alloy plating layer, Fe may be mixed in advance
in an Al-based alloy plating bath containing S_i, or
after Al-based alloy plating containing Si is applied,
Fe contained in the steel material may be diffused
into the Al-based alloy plating layer by heating the
steel material. A deposition amount of the Al-based
alloy plating layer is preferably not less than 3 0
g/m2 nor more than 200 g/m2 per surface. When the
deposition amount is less than 30 g/m2, corrosion
resistance becomes rather poor, and when it is over
200 g/m2, there is a concern about the peeling of the
plating.
9
[0020] In order for the surface roughness Ra of the
Al-based alloy plating layer to be not less than 1 & m
nor more than 5 jim, the steel material on which the
Al-based alloy plating layer is formed is heated up.
to not lower than 850°C nor higher than 10 0 0°C in a
region of not lower than 600°C nor higher than 1000°C
at a heating rate of 1°C/second or more and less than
50°C/second. Alternatively, in order for the surface
roughness Ra of the Al-based alloy plating layer to
be not less than 1 jim nor more than 5 // m, the
surface roughness Ra of the surface of the steel
material on which the Al-based alloy plating layer is
not yet formed may be controlled. However, the
method of controlling the surface roughness Ra of the
surface of the steel material involves a possibility
that the surface roughness Ra of the Al-based alloy
plating layer varies depending on the plating
deposition amount. Therefore, the method of
controlling the surface roughness Ra of the Al-based
alloy plating layer by heating is preferable. As
previously described, the surface roughness Ra is the
surface shape parameter defined by JIS B 0601.
Further, a method to measure and evaluate the surface
roughness Ra is not particularly specified, and any
conventionally and generally used method may be used,
and for example, the method defined by JIS B 0633 may
be adopted.
Further, in the heating process when the surface
roughness Ra is controlled, the diffusion of Fe into
10 -
the Al-based alloy plating layer may be
simultaneously performed.
[0021] (Step of Forming ZnO-containing Layer)
An example of a method of forming the ZnOcontaining..
layer is a method in which a coating
liquid is prepared by mixing a predetermined organic
binder in a suspension containing ZnO, and the
coating liquid is applied on the surface of the Albased
alloy plating layer. As the suspension
containing ZnO, one in which a ZnO powder is
dispersed in a dispersion medium such as water is
preferably used. Further, examples of the
predetermined organic binder are a polyurethane-based
resin, a polyester-based resin, an acrylic resin, and
a silane coupling agent. Further, silica may be
included in organic binder components. These organic
binders are preferably water-soluble so that they can
be mixed with the ZnO suspension. The coating liquid
thus obtained is applied on the surface of the Albased
alloy plating layer and is dried.
[0022] As another method, a method in which a ZnO
powder or a powder containing a solid content such as
a predetermined organic binder is applied by a powder
coating method may be used.
[0023] The total content of the aforesaid organic
binder components is desirably about 5 to 30% in mass
ratio to ZnO. When the content of the binder
components is less than 5% in mass ratio, a binder
effect cannot be sufficiently obtained and
- 11 -
accordingly the applied film is likely to peel off.
In order to stably obtain the binder effect, the
content of the binder components is more preferably
10% or more in mass ratio. On the other hand, when
the content of the binder components is over 30% in
mass ratio, smell is noticeably generated at the time
of the heating, which is not preferable.
[0024] (Step of Forming ZnAl204 Layer)
To form the ZnAl204 layer between the ZnOuontaining
layer and the Al-based alloy plating layer,
the ZnO-containing layer is first formed on the
surface of the Al-based alloy plating layer.
Thereafter, the resultant is heated in a range of
600°C or lower at a heating rate of over 2 5°C/second
and 100°C/second or less under an air atmosphere,. is
heated in a range of over 600°C and 10 0 0°C or lower at
a heating rate of 1°C/second or more and less than
50°C/second, and is subjected to heating whose
ultimate temperature is within a range of not lower
than 850°C nor higher than 1000°C. That is, with 600°C
being a boundary, the heating rate is made different.
Thereafter, an air cooling step or a cooling step at
a rate equal to or more than that of the air cooling
is performed.
[0025] Thus, with 600°C being the boundary, the
heating rate for the temperature range of 600C or
lower and the heating rate for the temperature range
of over 600°C are made different. By such a step, ZnO
reacts with Al in the Al-based alloy plating layer,
- 12 -
so that the ZnAl204 layer is formed between Al in the
Al-based alloy plating layer and the ZnO-containing
layer. Further, according to such a step, it is
possible for the formed ZnAl204 layer to have an aimed
thickness of not less than 0.05 jim nor more than 2 \x
m. Incidentally, a reason why the ZnAl204 layer with
the predetermined thickness is formed by such a step
is not clear, but a possible reason is as follows.
Specifically, in the range of 600°C or lower, when the
heating rate is less than 25 °C/second, the Al-based
alloy plating layer itself is excessively oxidized,
and when the heating rate is over 100°C /second, the
organic binder burns insufficiently to remain on the.
surface of the Al-based alloy plating layer.
Conseguently, the reaction with ZnO by the following
heating becomes insufficient, so that the ZnAl204
layer is not sufficiently generated. On the other
hand, in the range of over 60 0°C, when the heating
rate is less than 1°C/second, the ZnAl204 layer is
excessively generated and accordingly becomes brittle
to easily peel off, and when the heating rate is 50°C
/second or more, the ZnAl204 layer is not sufficiently
generated, leading to poor corrosion resistance. It
is thought that a reason why 600°C is thus a point of
change of the heating rate is associated with the
fact that the formation of the ZnAl204 layer is
influenced by a surface state of the Al-based alloy
plating layer up to 600°C, and the formation of the
ZnAl204 layer progresses especially when the
13 -
temperature exceeds 6 0 0°C. Another possible reason is
that, by changing the heating rate at 6 0 0 °C, a minute
crack is formed on the surface of the moderately
generated Al-based alloy plating layer, so that the
formation of the ZnAl204 is promoted. Incidentally,
in this embodiment, the heating rates for the range
of 60 0°C or lower and for the range of over 600°C and
1000°C or lower both include the range over 25°C
/second and less than 5 0°C . However, even when the
heating is performed at a heating rate within this
overlapping range, the heating rate for the range of
6 0 0°C or lower and the heating rate for the range of
over 600°C and 1000°C or lower are made different. In
this case, the heating rate for the range of over
6 0 0°C and 10 0 0°C or lower is preferably lower than the
heating rate for the range of 600 °C or lower. Further,
this heating process may also serve as the heating
process for controlling the surface roughness Ra of
the Al-based alloy plating layer and the heating
process for diffusing Fe into the Al-based alloy
plating layer.
[0026] Incidentally, the Al-based alloy plating
layer is preferably oxidized by heating in the
atmosphere before the ZnO-containing layer is formed.
As such a heating process, a process of heating the
Al-based alloy plating layer at 300 to 6 0 0 °C for 30
seconds to 10 minutes under an air atmosphere can be
employed, for instance. By performing such a heating
process, a sufficient amount of an Al203 coating film
- 14 -
is formed on the surface of the Al-based alloy
plating layer, so that a reaction of the Al203 layer +
the ZnO-containing layer —» ZnAl204 more easily
progresses. This heating process may also serve as.
the heating process for controlling the surface
roughness Ra of the Al-based alloy plating layer and
the heating process for diffusing Fe into the Albased
alloy plating layer.
[0027] As described hitherto, according to the Albased
alloy plated steel material of this embodiment,
its post-coating corrosion resistance can be far more
improved than that of conventional Al-plated steel
materials, owing to the sacrificial anticorrosive
effect of Zn. Further, owing to the presence of the
ZnAl204 layer, workability can be enhanced.
EXAMPLES
[0028] Next, examples of the present invention will
be described in detail.
First, Al-based alloy plating layers were formed
on cold-rolled steel materials with a 1.2 mm sheet
thickness having the components specified, in Table 1
by a hot dipping method. Table 1 is a table
presenting the components, other than Fe, of the
cold-rolled steel materials. The hot dipping was
performed on a line of a non-oxidizing furnace -
reducing furnace type. Then, after plating, a
plating deposition amount was adjusted to 40 g/m2 per
surface by a gas wiping method. Thereafter, the
resultants were cooled and subjected to a zero-
- 15 -
spangle treatment. The composition of a plating bath
was set to Al - 10% Si, and the temperature of the
plating bath was set to 660°C.
[0029] [Table 1]
[0030] A coating liquid in which a ZnO suspension
and a binder including a urethane-based resin were
mixed was applied on surfaces of the thus fabricated
A1-based alloy plating layers by a roll coater, and
the resultants were heated at about 8 0°C to be dried.
Note that a mass ratio of the urethane-based resin to
ZnO was set to 20%. An application amount of the
coating liquid was set so that a Zn amount became 1.0
g/m2. Thereafter, they were heated under the
conditions presented in Table 2 and air-cooled in an
air atmosphere, whereby Fe was diffused into the Albased
alloy plating layers and also ZnAl20 4 layers
were formed between ZnO-containing layers and the Albased
alloy plating layers. Thereafter, post-coating
corrosion resistance and workability were evaluated
as examples of performance of Al-based alloy plated
steel materials by the following methods.
[0031] (Post-coating Corrosion Resistance)
The fabricated Al-based alloy plated steel
materials were each cut into a 70 X 150 mm size,
whereby samples were fabricated. Then, after the
fabricated samples were subjected to alkaline
degreasing, they were subjected to a chemical
- 16 -
conversion treatment by PALBOND SX35 (manufactured by
Nihon Parkerizing Co., Ltd.), following the recipe of
the maker, and further a cationic electrodeposition
paint (POWERNICS 110: manufactured by NIPPON PAINT .
Co., Ltd.) was applied with a 15 // m. thickness, and
they were cross-cut. Then, these samples were
subjected to a 300-cycle test by the M610 method
defined by Japanese Automobile Standards Organization
{JA S O ) . Post-coating corrosion resistance was
evaluated based on the following criteria. 1
indicates rejection, and 2 and 3 indicate acceptance.
[0032] (Swell of Coating Film)
1 : over 0.5 mm
2: 0.2 to 0.5 mm
3: less than 0.2 mm
[0033] (Workability)
The fabricated Al-based alloy plated steel
materials were each cut into a 30 X 70 mm size, and
were subjected to a 9 0-degree bending test with a 1
mm radius of curvature. Thereafter, they were bent
back, tapes were pasted on bent portions,. and plating
peeling widths after the tapes were peeled off were
measured. Workability was evaluated based on the
following criteria. 1 indicates rejection, and 2 and
3 indicate acceptance.
[0034] (Peeling Width of Plating or Coating Film)
1: over 5.0 mm
2: 2.0 to 5.0 mm
3: less than 2.0 mm
- 17 -
[0035] The results are presented in Table 2. The
results, have turned out that the examples which fall
within the ranges of the present invention are good
both in workability and post-coating corrosion
resistance, but comparative examples which fall out
of the ranges of the present invention are poor in
workability or post-coating corrosion resistance.
[0036] [Table 2]
[0037] As is seen in the comparative examples No. 30,
31, when the heating rate for higher than 600 °C and
1000°C or lower is not within a range of 1°C/second or
- 18 -
more and less than 50°C/second, post-coating corrosion
resistance is low. Further, as is seen in the
comparative examples No. 2 8, 2 9, when the heating
rate for 60 0 °C or lower is not within a range of over
25°C/second and 100°C/second or less,, post-coating
corrosion resistance is low. Further, No. 32 is a
result when the heating was performed without the
heating rate being changed and it is poor in postcoating
corrosion resistance. A possible reason for
these is that a ZnAl204 layer with a thickness of not
less than 0.05 fim. nor more than 2 /im is not formed
between the ZnO-containing layer and the Al-based
alloy plating layer because the heating conditions
after the plating fall out of the ranges of the
present invention.
[003 8] Hitherto, the preferred embodiment and the
examples of the present invention have been described,
but the present invention is not limited to these
embodiments, and various modifications and changes
can be made therein within the scope of its spirit.
INDUSTRIAL APPLICABILITY
[0039] As described above, according to the Al-based
alloy plated steel material of the present invention,
its post-coating corrosion resistance can be far more
improved than that of conventional Al-plated steel
materials, and therefore it is applicable to home
electric appliances, automobiles, construction
materials, and further, large structures such as
infrastructure facilities, and its industrial
- 19 -
contribution is very large.

We claim
[Claim 1] (Amended) An Al-based alloy plated steel
material having excellent post-coating corrosion
resistance, the Al-based alloy plated steel material
comprising:
a steel material and a coating layer formed on a
surface of the steel.material, wherein the coating
layer includes:
an Al-based alloy plating layer containing, in
mass%, not less than 10% nor more than 50% Fe and not
less than 3% nor more than 15% Si, and formed on the
surface of the steel material;
a layer containing ZnO and formed in an uppermost
surface; and
•a ZnAl204 layer with a thickness of not less than
0.05 (im nor more than 2 /z m, formed directly on the
Al alloy plating layer and in close contact with the
layer containing ZnO.
[Claim 2] The Al-based alloy plated steel material
having excellent post-coating corrosion resistance
according to claim 1, wherein a surface roughness Ra
of the coating layer is within a range of not less
than 1 ji m nor more than 5 p, m.