Note: Descriptions are shown in the official language in which they were submitted.
REFERENCE TO PATENTS, APPLICATIONS AND PUBLICATIONS
PERTINENT TO THE INVENTION
As far as we know, there is available the following
prior art document pertinent to the present invention:
"Plating & Surface Finishing", March 1989,
pp. 62-69.
The contents of the prior art disclosed in the
above-mentioned prior art document will be discussed
hereafter under the heading of the "BACXGROUND OF THE
INVENTION".
FIELD OF THE INVENTION
The present invention relates to an electrogalvanized
steel sheet having two electroplating layers and excellent
in antifriction upon press-forming, corrosion resistance
and painting finish.
BACKGROUND OF THE INVENTION
In general, the body of an automobile is exposed
to a corrosive environment, and particularly to a severe
corrosive environment in a coastal area or a cold area
where an automobile tends to come into contact with a
substance containing chlorine ions havina a violent
corrosivity.
.J ~
An electrogalvanized steel sheet is conventionally
widely used as ~ steel sheet for an automobile body having
an excellent corrosion resistance even in such a severe
corrosive environment.
The conventional electrogalvanized steel sheet has
however the following prohlems:
(1) The zinc electroplating layer of the electrogalvanized
steel sheet, having a relatively low hardness, is
deformed upon the press-forming of the electrogalvanized
steel sheet, thus increasing a contact area between the
zinc electroplating layer and -the pressing portion of
a press. In other words, the electrogalvanized steel
sheet has a frictional coefficient higher than that of
the other steel sheets such as a cold-rolled steel sheet
or a zinc alloy electroplated steel sheet. When press-
forming the electrogalvanized steel sheet, therefore,
cracks mav be produced in the zinc electroplating layer
thereof. As is clear from the above description, the
conventional electrogalvanized steel sheet is poor in
antifriction (hereinafter referred to as the "problem
l"); and
(2) To ensure a sufficient corrosion resistance, the
electrogalvanized steel sheet has a relatively large
plating weight per surface of the steel sheet, i.e.,
the zinc electroplating layer of the electrogalvanized
2 ~
steel sheet has a relatively large thickness~ When
applying an electropainting to the electrogalvanized
steel sheet having such a relatively thick zinc electro-
plating layer to form a painting film on the surface
thereof, bubbles tend to occur in the painting film
which lead to a defect having a recess or a projection
in the painting film. The electrogalvanized stee~ shee-t
having been subjected to the electropainting, is further
subjected to a finish painting to form a finish painting
film on this painting film. The above-mentioned defect
having a recess or a projection exerts an adverse effect
even on the finish painting film, thus deteriorating
the appearance of the painted electrogalvanized steel
sheet. As is evident from the above description, the
conventional electrogalvanized steel sheet is poor in
painting finish (hereinafter referred to as the "problem
2").
It is a conventional practice, as a means for
solving the problem 1, to apply a high-viscosity lubricant
oil onto the surface of the electro~alvanized steel sheet
prior to press-forming the electrogalvanized steel sheet to
improve antifriction of the electrogalvanized steel sheet.
Application of the high-viscosity lubricant oil
onto the surface of the electrogalvanized steel sheet as
described above poses however the following problems:
t'~ J
(a) The high~viscosity lubricant oil contaminates the
working place; and
(b) It is necessary to remove the high-viscosity lubricant
oil applied onto the surface of the electrogalvanized
steel sheet prior to applying painting thereto. This
removing operation is not however easy. Complete
removal of the high-viscosity lubricant oil re~uires
much time and labor.
With regard to the frictional coefficient of an
electxogalvanized steel sheet, the "Plating & Surface
Finishing", March 1989, pp. 62-69, teaches as follows
(hereinafter referred to as the "prior art"):
(i) Application of a conventional anticorrosive oil
onto the surface of the zinc electroplating layer
having crystals oriented along the <0001> plane, leads
to a relatively large frictional coefficient thereof
of 0.19; and
(ii) Application of a conventional anticorrosive oil
onto the surface of the zinc electroplating layer
having crystals oriented along the <lOlX> plane (where,
X is 1, 2, 3 or 4), on the other hand, results in a
small frictional coefficient thereof of 0.13.
Apart from the above-mentioned problems resulting
from application of the high-viscosity lubricant oil, the
-- 5 --
electrogalvanized steel sheet applied with the high-
viscosity lubricant oil on the surface thereof has a small
frictional coefficient of 0.11. If the orientation of the
crystals of the zinc electroplating layer along the <lOlX>
plane (where X is 1, 2, 3 or 4) as taught by the prior art
can be main-tained, an antifriction of the same order as in
the application of the high-viscosity lubricant oil would
be available by the application of the conventional
anticorrosive oil which is easy to remove, onto the surface
of the electrogalvanized steel sheet.
~ owever, the crystal orientation of the zinc
electroplating layer of the electrogalvanized steel sheet
depends upon electroplating conditions, and among others,
upon an electric current density. As a result, it is
inevitable to alter the plating conditions in response to
the width, for example, of the steel sheet to be electro-
plated. In the manufacture of the electrogalvanized steel
sheet in an industrial scale, it is practically impossible
to maintain the orientation of the crystals of the zinc
electroplating layer along the <lOlX> plane (where, X is
1, 2, 3 or 4).
A means to solve the prob~em 2 has not as yet been
proposed.
Under such circumstances, there is a strong demand
for the development of an electrogalvanized steel sheet
-- 6 --
S ~
~ ~J ~ -f .s ~f ;~J
excellent in antifriction, corrosion resistance and painting
finish, but such an electrogalvanized steel sheet has not
as yet been proposed.
SUMMARY OF THE INVENTION
An object of the present invention is therefore
to provide an electrogalvanized steel sheet having two
electroplating la~ers and excellent in antifriction,
corrosion resistance and painting finish.
In accordance with one of the features of the
present invention, there is provided an electrogalvanized
steel sheet having two electroplatina layers and excellent
in antifriction, corrosion resistance and painting finish,
characterized by comprising:
a steel sheet;
a zinc electroplating layer formed on at least one
surface of said steel sheet, said zinc electroplating
layer having a center-line mean roughness (Ra) of up to
1.5 ,um and a plating weight within a range of from 25 to
150 g/m2 per surface of said steel sheet; and
. a zinc zlloy electroplating layer formed on said
æinc electroplating layer, said zinc alloy electroplating
layer comprising zinc and at least one element selected
from the group consisting of cobalt, manganese, nickel,
iron and chromium, said zinc alloy electroplating layer
containing said at least one element in an amount ~ithin
a range of from 3 to 99 wt.% relative to said zinc alloy
electroplating layer, and said zinc alloy electroplating
5 layer having a plating weight within a range of from 1 to
20 g/m2 per surface of said steel sheet.
The term "center-line mean roughness" ~Ra) as used
herein means a value of the surface rouyhness as expressed
by the following formula:
Ra = L J ¦f(x)¦ dx
where, L : measuring length, and
f(X) : roughness curve.
BRIEF DESCRIPTION OF THE DRAWI~IGS
Fig. 1 is a graph illustrating, in the case where
an electrogalvanized steel sheet having a zinc electro-
plating layer as a single layer on each of the both surfaces
thereof is subjected to an electropainting, the relationship
between a defect occurrence ratio and a center-line mean
roughness (Ra) of the zinc electroplating layer;
Fig. 2 is a graph illustrating, for the electro-
galvanized steel sheet of the present invention having a
zinc electroplating layer formed on the surface of the
steel sheet and a zinc-cobalt alloy electroplating layer
~3 i ~ . J
formed on the zinc electroplating layer, the relationship
between a frictional coefficient of the electrogalvanized
steel sheet and a cobalt content in the zinc-cobalt alloy
electroplating layer; and
Fig. 3 is a schematic front view illustrating
an apparatus for measuring a frictional coefficient~
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
From the above-mentioned point of view, extensive
studies were carried out to develop an electrogalvanized
steel sheet having two electroplating layers and excellent
in antifriction, corrosion resistance and painting finish.
As a result, the following findings were obtained:
An electrogalvanized steel sheet having two electroplating
layers and excellent in antifriction, corrosion resistance
and painting finish is available by the following steps:
(1) forming a zinc electroplating layer on at least one
surface o a steel sheet;
(2) limiting a center-line mean roughness (Ra) of the
zinc electroplating layer to up to 1.5 ~m;
0 (3~ limiting a plating weight of the zinc electroplating
layer within a range of from 25 to 150 g/m2 per
surface of the steel sheet;
(4) forming a zinc alioy electroplating layer comprising
zinc and at least one element selected from the group
3 ~
consisting of cobal-t, manganese, nickel, iron and
chromium, on the zinc electroplating layer;
(5) limiting a content of the above-mentioned at least
one element in the zinc alloy electroplating layer
within a range of from 3 to 99 wt~ relative to the
zinc alloy electroplating layer; and
(6) limiting a plating weight of the zinc alloy electro-
plating layer within a range of from 1 to 20 g/m2 per
surface of the steel sheet.
The present invention was made on the basis of
the above-mentioned findings. The electrogalvanized steel
sheet of the present invention having two electroplating
layers and excellent in antifriction, corrosion resistance
and painting finish is described below with reference to
the drawings.
The electrogalvanized steel sheet of the present
invention excellent in antifriction, corrosion resistance
and painting finish comprises a steel sheet, a zinc
electroplating layer formed on at least one surface of the
steel sheet and a zinc alloy electroplating layer formed
on the zinc electroplating layer.
The zinc electroplating layer has a function of
imparting an excellent corrosion resistance to the electro-
galvanized steel sheet.
-- 10 --
s~ ,3
The center-line mean roughness (Ra) of the zinc
electroplating layer exerts an important effect on painting
finish of the electrogalvanized steel sheet. The effect
of the center-line mean roughness lRa) of the zinc electro-
plating layer exerting on painting finish of the electro-
galvanized steel sheet is described below with reference
to Fig. 1.
Fig. 1 is a graph illustrating, in the case where
an electrogalvanized steel sheet having a zinc electro-
plating layer as a single layer on each of the both surfaces
thereof is subjected to an electropainting, the relationship
between an occurrence ratio of bubbles and a center-line
mean roughness (Ra) of the zinc electroplating layer.
More particularly, each of a plurality of steel sheets for
automobile hood having a center-line mean roughness (Ra)
of 0.~ ,um was electrogalvanized to form a zinc electro-
plating layer having a plating weight of 60 g/m2 per
surface of the steel sheet on each of the both surfaces
of each steel sheet. Then, each of these steel sheets,
having the zinc electroplating layer formed on -the both
surfaces thereof, was subjected to a conventional electro-
painting to form a painting film on the zinc electro-
plating layer, thereby preparing a plurality of ~u-tomobile
hoods. Then, the relationship between a ratio of the
number of automobile hoods having bubbles in the painting
, ~a~2
films thereof to the total number of the thus pxepared
automobile hoods, i.e., an occurrence ratio of bubbles in
the painting film, on the one hand, and a center-line
mean roughness (Ra) of the zinc plating layer, on the other
hand, was investigated. In Fig. 1, the ordinate represents
the occurrence ratio of bubbles in the painting film, and
the abscissa represents the center-line mean roughness (Ra)
of the zinc electroplating layer~
As is clear from Fig. 1, when the center-line mean
roughness (Ra) of the zinc elec-troplating layer is up to
1.5 ,um, the occurrence ratio of bubbles in the painting
film is 0%. When the center-line mean roughness (Ra) is
over 1.5 ,um, on the other hand, the occurrence ratio of
bubbles in the painting film sharply increases, and a
center-line mean roughness (Ra) of 2.5 ,um results in an
occurrence ratio of bubbles in the painting film of even
10%.
As described above, bubbles are produced in the
painting film when the center-line mean roughness (Ra) of
the zinc electroplating layer i5 over 1.5 ~m. The reason
of this is estimated to be as follows:
(a) A hydrogen qas produced during the electropainting
remains on the surface of the zinc electroplating
layer under the effect of the increased center-line
mean roughness (Ra) of the zinc electroplating layer.
- 12 -
,?
This hydrogen gas is confined in the painting film
and expands during the process of baking of the
painting film, thus forming bubbles in the painting
film; and
(b) Piror to applying the electropainting to the electro-
galvanized steel sheet, a phosphate film is formed
on the surface thereof to improve paint adhesion.
During the baking of the painting film after electro-
painting, water is eliminated from phosphate crystals
of the phosphate film. Water thus eliminated is
confined in the painting film and vaporized to form
bubbles in the painting film~
As is clear from the above description, the
production of bubbles in the painting film during the
electropainting of the electrogalvanized steel sheet having
a zinc electroplating layer as a single layer on each of
the both su~faces thereof, is prevented by limiting the
center-line mean roughness (Ra) of the zinc electroplating
layer to up to 1.5 ~um.
In the electrogalvanized steel sheet of the
present invention, a zinc alloy electroplating layer is
formed on the zinc electroplating layer formed on the
surface of the steel sheet. Since this zinc alloy
electroplating layer has a plating weight within a range
of from 1 to 20 g/m2 per surface of the steel sheet as
- 13 -
described later! i-e., the zinc alloy electroplating layer
has a small average thickness, the surface roughness of the
zinc electroplating layer exerts an important effect on the
surface roughness of the zinc alloy electroplating layer
formed thereon. Therefore, the above-mentioned relation-
ship, in the case where the electrogalvanized steel sheet
having a zinc electroplating layer as a single laver on
each of the both surfaces thereof is subjected to the
electropainting, between the occurrence ratio of bubbles in
the painting film and the center-line mean roughness (Ra)
of the zinc electroplating layer applies also to the zinc
electroplating layer on which the zinc alloy electroplating
layer has been formed, in the electrogalvanized steel sheet
of the present invention.
Therefore, the center-line mean roughness (Ra)
of the zinc electroplating layer of the electrogalvanized
steel sheet of the present invention should be limited
to up to 1.5 ~m.
In order to limit the center-line mean roughness
(Ra) of the zinc electroplating layer to be formed on the
surface of the steel sheet to up to 1.5 ~m, it suffices
to appropriately alter the electroplating conditions for
forming the zinc electroplating layer. The plating weight
of the zinc electroplating layer is described later.
Since, when the zinc electroplating layer has a relatively
-- 1~1 --
2 ~ 3 ~
small thickness, the surface roughness of the steel sheet
exerts an effect on the surface roughnèss of the zinc
electroplating layer formed thereon, the center-line mean
roughness (Ra) of the steel sheet should be limited to up
to 1.5 ~um by grinding the surface thereof.
The plating weight of the zinc electroplating
layer exerts an important effect on corrosion resistance
and antifriction of the electrogalvanized steel sheet.
With a plating wèight of the zinc electroplating layer of
under 25 g/m2 per surface of the steel sheet, an excellent
corrosion resistance cannot be imparted to the electro-
galvanized steel sheet. With a plating weight of the zinc
electroplating layer of over 150 g/m2 per surface of the
steel sheet, on the other hand, zinc crystals of the zinc
electroplating layer become coarser. Such coarsening of
zinc crystals poses the following problems: It is impossible
to form a uniform zinc allov electroplating layer having a
relatively small thickness as described later on the zinc
electroplating layer; in other words, it is impossible to
cover the entire surface of the zinc electroplating layer
with the zinc alloy electroplating layer and part of the
surface of the zinc electroplating layer is exposed, thus
making it impossible for the zinc alloy electroplating
layer to fully display the function thereof described
later of impartiny an excellent antifriction to the
- 15 ~
electrogalvanized steel sheet. The plating ~ei~ht of the
zinc electroplating layer should therefore be limited
within a range of from 25 to 150 g/m2 per surface of the
steel sheet.
The zinc alloy electroplating layer formed on
the zinc electroplating layer has a function of imparting
an excellent antifriction to the electrogalvanized steel
sheet.
The zinc alloy electroplating layer comprises
zinc and at least one element selected from the group
consisting of cobalt, manganese, nickel, iron and chromium.
The content of the above-mentioned at least one
element selected from that group in the zinc alloy
electroplating layer exerts an important effect on
antifriction and chipping resistance of the electro-
galvanized steel sheet. The effect of the content of
the above-mentioned at least one element selected from
that group in the zinc alloy electro~lating layer
exerting on antifriction of the electrogalvanized steel
sheet, i5 described below ~ith reference to Fig. 2.
Fig. 2 is a graph illustrating, for the electro-
galvanized steel sheet of the present invention having a
zinc electroplating layer formed on the surface of the
steel sheet and a zinc-cobalt alloy electroplating layer
formed on the ~inc electroplating layer, the relation-
- 16 -
2 ~ .3 ~ ^~ 2
ship between a frictional coefficient of the electro-
galvanized steel sheet and a cobalt content in the zinc-
cobalt alloy electroplating layer. More specifically,
for an electrogalvanized steel sheet having a zinc
electroplating layer, with a plating weight of 60 g/m2
per surface of the steel sheet, formed on one surface of
the steel sheet and a zinc-cobalt alloy electroplating
layer, with a plating weight of 5 g/m2 per surface of the
steel sheet, formed on the æinc electroplating layer, the
relationship between a frictional coefficient of the
electrogalvanized steel sheet and a cobalt content in
the zinc-cobalt alloy electroplating layer was investigated.
In Fig. 2, the ordinate represents the frictional
coefficient of the electrogalvanized steel sheet, and
the abscissa represents the cobalt content in the zinc-
cobalt alloy electroplating layer.
As is clear from Fig. 2, when the cobalt content
in the zinc-cobalt alloy electroplating layer is under
3 wt.~ relative to the zinc-cobalt allov electroplating
layer, the electrogalvanized steel sheet has a relatively
large frictional coefficient of at least 0.2, so that
the electrogalvanized steel sheet having such a
relatively large frictional coefficient is poor in
antifriction.
As described above, the frictional coefficient
-- 1~ --
2i~ ~3 ~
of the electrogalvanized steel sheet increases when the
cobalt content in the zinc-cobalt alloy electroplating
layer is under 3 wt.~ relative to the zinc-cobalt alloy
electroplating layer~ The reason of this is estimated
to be as follows: It is impossible to sufficiently
increase hardness of the zinc-cobalt alloy electroplating
layer with such a low cobalt content.
The content of manganese, nickel, iron or chromium
in the zinc alloy electroplating layer also exerts an
important effect on the frictional coefficient of the
electrogalvanized steel sheet as in the above-mentioned
cobalt content in the zinc alloy electroplating layer.
The zinc alloy electroplating layer should
therefore contain at least one element selected from the
group consisting of cobalt, manganese, nickel, iron and
chromium in an amount of at least 3 wt.~ relative to the
zinc alloy electroplating layer.
When the content of the above-mentioned at least
one element selected from that group in the zinc alloy
electroplating layer is over 99 wt.~ relative to the
zinc alloy electroplating layer, on the other hand,
phosphate crystals of the phosphate film formed on the
zinc alloy electroplating layer prior to the electro-
painting, become coarser, thus resulting in a lower
chipping resistance of the electrogalvanized steel sheet.
- 18 -
63~ 3
Therefore, the content of the above-mentioned at least
one element selected from that group should be limited
within a range of from 3 to 99 wt.% relative to the zinc
alloy electroplating layer.
The plating weight of the zinc alloy electro-
plating layer exerts an important effect on antifriction
of the electrogalvanized steel sheet and the manufacturing
cost thereof. When the plating weight of the zinc alloy
electropla-ting layer is under 1 g/m2 per surface of the
steel sheet, the zinc alloy electroplating layer has
only a low covering ratio over the entire surface of the
zinc electroplating layer, so that most part of the surface
of the zinc electroplating layer is exposed, thus making
it impossible to impart an excellent antifriction to the
lS electrogalvanized steel sheet. Even when the plating
weight of the zinc alloy electroplating layer is over
20 g/m2, on the other hand, the effect of imparting an
excellent antifriction to the electrogalvanized steel
sheet cannot further be improved, and in addition, the
manufacturing cost of the electrogalvanized steel sheet
becomes uneconomically higher. The plating weight of the
zinc alloy electroplating layer should therefore be
limited within a range of from 1 to 20 g/m2 per surface
of the steel sheet.
Now, the electrogalvanized steel sheet of the
-- 19 --
~ ~ J~ 3~J
present invention, having two electroplating layers and
excellent in antifriction, corrosion resistance and
painting finish, is described ~urther in detail by means
of examples while comparing with e~amples for comparison.
EXAMPLES
~ach of cold-rolled steel sheets having a thick-
ness of 0.7 mm and a center-line mean rouahness (Ra) within
a range of from 0.8 to 1.0 ~um was subjected to a conven-
tional degreasing treatment and a conventional pickling
treatment to remove rust from the both surfaces thereof.
Then, the steel sheet from the both surfaces of which rust
was thus removed, was subjected to an electroplating under
the conditions shown in Table 1 to form a zinc electro-
plating layer on each of the both surfaces of the steel
sheet.
Then, the steel sheet having -the zinc electro-
plating layer formed on each of the both suraces thereof
was subjected to another electroplating under other
conditions also shown in Table 1 to form a zinc alloy
electroplating layer on the zinc electroplating layer.
Thus, samples of the electrogalvanized steel sheet
within the scope of the present invention (hereinafter
referred to as the "samples of the invention") Nos.
1 to 66 were prepared.
- 20 -
2 ~
For each of the samples of the invention Nos. 1
to 66, the plating weight per surface of the steel sheet
and the center-line mean roughness (Ra) of the zinc
electropainting layer, as well as elements other than
zinc and the contents thereof in the zinc alloy electro-
plating layer, and the plating weight per surface of
the steel sheet of the zinc alloy electroplating layer
are also shown in Table 1.
~J~30.
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and content thereof
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_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Cr (BF4 ) 3 ~
o cl~ c~ o a~ o~ a~ ~ o _ _ _ _ _ _ o o N a 2 S 0 4 ~ , o
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ ~ NaCH3COO ' _
O _ O O O O O O O O O O O O O O O Flow velbcth ~m/sec. ) ~
~ O g ~ O O ~ O O ~ ~ ~ ~ _ ~ ~ ~ Bath temperature (C) ~.
_ _ _ _ _ _ _ _ _ O O O O O O _ _ pH value
O O 0 ~ O O O ~ O O ~ ~ O~ O O O O Electr c current ~
~ O ~0 O _ _ O O _ O O O _ _ ~ _ _ Electroplating time(se ~. _
O O O O O O O O O O O O O O O O O Plating weight (g/m2) ~ ~
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Surface roughness (ym) ~ w
Z 2 ~ 2 Z Z 2 Z o F F O O ~ O O and content thereof
~ _ _ ~0 _ _ ~ ~ _ ~ ~ _ O _ ~ ~ _ Plating weight (g/m2)
O ~ O . _ O O O O O _ ~ O O O _ O Frictional coefficient ~ ho
Corrosion resistance ~ ~Z
_ ~ ~ ~ ~ ~ ~ Painting finish
- ~z -
r Sample of the inventiOn
cn cn _ co~ ~ cn ~ co ro ~ o co co _ ~o cn o
_ O ZnS04 71120 ~ n~ N~
~ O O ~ _ ~ ~ ~ ~ O O ~ ~ ~ ~ O ~ Na2S04 ~Qo ~c
ro ro n~ ro n~ ro r~ n~ r.~ r~ 1~ n~ r.~ n~ r.~ r~ ro Flow velocity of
O O O O O O O O O O O O c~ O O O O plating bath (m/sec.) ~c
O O O O O O O O O ~O ~O O O O O g O Bath temperature (C)
1~ _ ~ _ ~ r.~ r.~ _ r~ _ ro r~ r~ r~ _ _ ~ H value `Qc
O O O O O O O O O O O O O O O O O P ~
O ~ ~ O _ ~ ~ ~ ~ ~ ~ ~ _ ~ ~ ~ ~ Electric current ~C
co ~ ~ ~ ~ r~ r~ ro rn rn rn rn ~n r~ r~ rn rn Electroplating time(sec.)
_ _ cn o o o cn co oo o rO ~ -- ~ r~ cn cn ZnS0 4 ~ 7 H 2 _
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ coS04 71120 ~1
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ r~,
l l l l l l l l l l l l l 3 3 O ~ Ni504~6~12o ~Q
_ ,_ o o o o o o r~ o r ro r~ _ _ _ _ FeS04~71120 ,~ 8
l l l l l l l l l _ __ _ _ _ l _ _ Cr2~S04)3 ~ o~
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Mn(BF~) 2 o o
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ , zn(BF4) 2 o o 3
r.~ r~ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ H3B03 ~r ~ (D
O _ _ _ _ _ _ _ _ _ _ _ , _ _ _ _ llO(cH2cH2o)n~ ~ ~ w
l l O ~O oo ~O O ~O oo O ~O ~O ~O O~ ~O ~O ~O Na2So4 ~ ~
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~ ~D
l l ~ rO `~ r~ rO rO r~ ~ ~ ~ ~ NaCH3C00 Q
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ ~,r
r~ 1~ ro _ _ _ _ r~ r~ r~> n~ r~ r.~ r.~ r.~ r~ Flow velocity of d
o o o or co cn cn cn o cn o o o o o o o platinq bath (m/sec.)
c o cn o cn cn o cn cn cn cn cn o cn cn cn cn Bath temperature (C)
o r.~ oo oo oo oo oo oo oo or or co oo _ r.~ r~ ro pH value
cn cn cn cn ~n cn cn cn 0 cn cn cn cn cn cn cn cn EleCtriC Current(A/dm2) ¦
_ co co co ~o cn cn ~ ~ ~ ~ ~ ~ ~ ~ ~ c~ Electroplating time(sec.
rv cn o o o __ o cn cn cn co co co o o o o _ _
O c O O c~ O ~ ~ c~ O ~ O ~ ~ O ~ O Platinq weight (g~m2)
_ _ _ _ ~ _ _ _ _ _ _ _ _ _ _ _ SurEace roughness (jum)
r~ ~ r.~ r~ ~ ~ ~ ~ ~ ~ ~ r~ ~ r~ ra P~ ~
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
~ ~ ~ ~ ~ ~ ~ ~ ~ Z ,~ 2 z Element other than Zn ~ r~
n ~ ~ ~ r~ ~ ~ _. _ _. _. and content thereof
co co o cn cn co cn o cn n ro cn _ oo o cn cn (wt.~) ~Q~
_ ~n _ o co cn cn cn cn cn _ _ _ co co cn cn Plating weight (g/m ) ~0
O ,O O ,O ,O O O O ,O _ O _ O O _ _ _ Frictional coefficient ~rnO
_ r.~ _ _ n~ _ _ _ r~ ro r~ _ r I _ _ _ ro I_ ~
? ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Corrosion resistance ~ ~
_ _ _ _ _ _ _ _ _ _ > _ _ _ _ _ _ Painting finish ~ rr
- sz -
= Sam >le of the i~ ~venti )n _ = =
c~ Cn C cO ~ cn o cO cn ~n cO cn ~n cn cO o
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ :5~ _
O O O O _ O _ O _ O _ _ O ZnS04 7H20 ~o
_ _ _ _ _ __ _ _ _ _ _ _ _ Q D)U~, Il~ ~i
_ g g g o _ _ _ _ _ _ _ _~ _ _ Na~504 ~Q O (D~a
~o ~o ~o ro r~ ~o ro ro to ro ro ro to ro t~ Flow velocity of ~ ~
O O O O ~ O O O O O O O O O O plating bath (m/sec.) ~ g
o cn ~ cn o cn cn ~n ~n cn ~n cn ~n ~n o Bath temperature (C) ~
ro r ro r ro ro ro ro ro ro r ro ro ro ro pll value ~o
_ _ _, _ I _ ~ _ _ _ _ _ _ _ ~ ~ Electric current ~o
O O O O O O O O O O O O O O O density (A/dm2)
~ r~ rn ~ c ~ c ~o ~ cn ro rn ~ ~O ~`S Electroplating time(sec.) 3
CO CO CO CO CO CO CO oo CO CO CO CO CO oo oo
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _
o ro _ _ _ ~n o ro co o o _ _ _ _ ZnS04 7H2
O O l l l ~O l O l ~O l l l l l cos04 7H2
_ _ _ __ _ _ _ _ _ _ _ _ _ ~
O O _ _ __ _ O O _ _ O __ _ _ _ NiS04 6il2o 8
__ _ _ _ _ _ r r ro _ _ _ _ FeS04 ' 7H20 ~) ~
~n Ul _ _ _ un U~ _ n _ ~ _ _ _ _ Cr2(S04)3 . o
_ __ ro ro r ~ _ _ _ _ ~ _ _ _ _ Mn(BF4)2 o o
un cn ~n __ _ _ _ _ _ Un o o o Zn(BF4)2 o
ro ro ro l l ~ ~ ~ l ~ rO H3B3 ~ a
_ _ _ _ _ _ _ _ _ _ _ _ _ :1 ~
r~n ,ro ,ro ~ ~ ~ ~ ~ ~ ro ro r~n ~n H0(CH2cH20)nH ~Q N I_
_ _ _ _ _ _ _ _ _ _ __ _ _
o o o l l l l l ~n rO g gCr(BF4)3 ~ ~
_ _ _ _ _ _ _ _ _ _ _ _ _ o
Q Q _ _ _ ~o Q O O Q Q _ _ _ _ Na2So4 ~ (D
l l l _ _ ro _ ro ro _ _ _ _ _ _ NaCH 3COO _ (D
ro ro ro ro ~o ro ro ro ~ t ro ~o co to ro Flo~ velocity of
O O O O O O O O O O O O O O O platinc bath (m/sec.) ~
cn o o o o cn o o o o o ~ o o ~ Bath temperature (C) g
~ _ ro ,ro ,ro ~ _ ~ ~ _ _ to P to ,to pH value
to to o o o to to t I to cn _ o o o o ~
cl~ o _ cn cn oo oo 0 oo ~n ~n un ~n ~n cn densitv (A/dm2) (D
cn cn oo o o ~n _ o o r 9 o0 O un cn Electroplating time(sec _ _
O g O O g O O 8 2 O O O O O O Plating ~eiaht (~/m2) ~ ~
_ _ _ _ _ _ _ _ _ _ _ _ _ ~ r~''
to to ro ro to t~ ~ to ro tO ~O t~ ro t~ to Surface roughness (,um) ~
g 2 ~o~2 ~F _ _ ~ 2 2 ~ ~ lo _ _ _ ~~~0~
co o _ o g o o ~n ~n ~n ~ ~ ~ ~ Element other than Zn ~
_ 7 CO _ _ ~.0 CO g _ cn cn o9o O O O (wt.%) 3 1--
~n cn o o cn o _ _ o _ _ o ~ cn co Plating weight (g/m2)
o o r~ o o ro r~ ro ro ro ro .o O O _ Frictional coefficient
_ _ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ Corrosion resistance
_~ Painting finish
2 ~ 6~
For comparison purposes, each of steel sheets
identical with those in the samples of the invention Nos.
1 to 66 was subjected to a conventional degreasing
treatment and a conventional pickling treatment to remove
S rust from the both surfaces thereof. Then, the steel
sheet from the both surfaces of which rust was thus
removed, was subjected to an electroplating under the
conditions shown in Table 2 to form a zinc electroplating
layer on each of the both surfaces of the steel sheet.
Then, the steel sheet having the zinc electro-
plating layer formed on each of the both surfaces thereof
was subjected to another electroplating under other
conditions also shown in Table 2 to form a zinc alloy
electroplating layer on the zinc electroplating layer.
Thus, samples of the electrogalvanized steel sheet
outside the scope of the present invention (hereinafter
referred to as the "samples for comparison") Nos. 1 to
10 were prepared. Each of the samples for comparison
Nos. 1 and 2 had only a zinc electroplating layer as a
single layer.
The plating weight per surface of the steel
sheet and the center-line mean roughness tRa) of the
zinc electroplating layer for each of the samples for
comparison Nos. 1 to 10, as well as elements o~her
than zinc and the contents thereof in the zinc alloy
- 26 -
V .~
electroplating layer, and the plating weight per surface
of the steel sheet of the zinc alloy electroplating
layer for each of the samples for comparison Nos. 3 to
10 are also shown in Table 2~
- 27 -
- 8Z ~
¦ Sample of the invention
~ _ _ _ _ _ _ _ _ ~ o ~ O N ~
ns04-7H2o ~ ~q
O _ ~ _, ~ O _ ~ _ ~ Na2S04 ~ o O o
__ O O O O O O __ O O Flow velocity of ~c.) ~ R
O ~ ~ ~ ~ ~ O ~ ~ ~ Bath temperature (C)
_ _ _ _ _ _ _ _ _ _ q
O O O O O O O O O O pH value ~c
_ ~ _ ~ ~ ~ ~ ~ ~ . Electric current 2 ~o
Electroplatinq time(s~ c ` _
_ O ~ ~ ~ O _ _ ZnSO4 7H2o
~ ~ ~ ~ ~ ~ ~ ~ _ _ CoS04 7H2
O~ O O oo . , , , , NiSO4-6H20 ~q
_ _ _ _ _ _ _ _ _ _ FeS04-7~120 ~ 8
, ' ' ' ' ' ' ' ' ' cr2(S04)3 ~ g
_ _ _ _ _ _ _ _ _ _ ~ :~
_ _ _ _ _ _ _ _ _ _ Mn(BF~)2 0 o
_ _ _ _ _ _ _ _ _ _ Zn(BF4)2 ~ o
_ _ _ _ _ _ _ _ _ _ H3B03 . ~ 3
, , , . . . . , , , H0(CH2CH20)nH ~ N
_ _ _ _ _ _ _ _ _ _ ~ t) 1~
_ _ _ _ _ _ _ _ _ _ Cr(BF4)3 ~ v
c~ c~ O~ ~ O ~O co ~O _ _ Na 2 SO 4
NaC~3C00 ~
_ _ _ _ _ _ _ --rt
_ O O O O O O O _ _ FloW VelbCthY (m/SeC- ) .
O O O ~ ~ O ~ ~ _ _ Bath temperature (C) ~
_ _ _ _ _ _ _ _ l l pH value ~a
~ ~ ~ r~ r~ ra ro ~ _ _ PJ
_ _ _ density (A/dm )
~ _ ~ _ ~ _ ~ ~O . . Electroplatingtime(sec.
ul o o o o ~ ~n ~ _ _ _ _
_ O ~ O _ O O O Plating weight (g/m2) ~ ~
_ ~ _ _ _ ~ _ _ _ O Surface roughness (ym) ~q
_ _ _ _ _ _ _ _ _ _ , ~1
2 Z. Z Z ~ o~ ~ g Element other than Zn
.. .. _ _ O _ O _ . l and content thereof
_ ~ ~ _ _ _ _ _ _ _ (wt.O ~
~ -o ~ ~O ~ O ~ ~O . . Plating weight (g/m2) ~
o o o ~ o ~o o ~n
~ O O O O O _ O ~ O Frictionalcoefficienl
_ _ _ _ _ _ _ _ _ _ I ~- Y
~ Q ~ ~ ~ ~ ~ ~ ~ ~ Corrosion resistance
_ _ _ _ _ _ _ ~ _ _ Paintin~ f~nish
Then, for each of the thus prepared samples of
the invention Nos. 1 to 66 and the samples for comparison
Nos. 1 to 10, antifriction, corrosion resistance and
painting finish were investigated by means of performance
tests as described below. The results of these tests are
also shown in Tables 1 and 2.
(1) Antifriction test:
A mineral oil type anticorrosive oil for a steel
sheet (product name: NOX RUST 530F40) made by Parker
Industries, Inc. was applied onto one surface of each
sample. The frictional coefficient of the sample onto
the one surface of which the anticorrosive oil was applied,
was measured with the use of an apparatus as shown in
Fig. 3, thereby evaluating antifriction of the sample on
the basis of the thus measured frictional coefficient.
The apparatus for measuring the frictional co-
efficient of the sample comprised, as shown in Fig. 3, a
rack 2; a supporting stand 5, provided vertically movably
on the rack 2 along a plurality of guide rods 12 and 13
attached vertically to the rack 2, and having a plurality
of rollers 6 on the upper end thereof; a supporting stand
driving mechanism (not shown) for vertically moving the
supporting stand Si a first load cell 3, provided between
the supporting stand 5 and the rack 2, for measuring
the force applied to the supporting stand 5; a pressing
3 ~
block 4 fitted to a frame 3 fixed to the rack 2 so as to
project toward the supporting stand 5; a horizontally
movable sliding table 7 mounted on the rollers 6 of
the supporting stand 5 between the supporting stand 5 and
the pressing block 4; a sliding table driving mechanism
(not shown), provided on another rack 11, for horizontally
moving the sliding table 7; and a second load cell 9,
provided between an oper~ting rod 10 connected to the
sliding table driving mechanism and one end of the
slidina table 7, for measuring the force applied to the
sliding table 7.
By operating the supporting stand driving mech-
anism, the supporting stand 5 was moved upward to lift up
the sliding table 7 on the upper surface of which a sample
lS 1 was placed. Thus, the upper surface of the sample 1
was pressed against the lower end of the pressing block
4, and the force N in the arrow A direction was measured
by means of the first load cell 8. Then, by operating
the sliding table driving mechanism, the sliding table
7 was horizontally moved in the arrow B direction, together
with the sample 1 placed on the upper surface thereof,
and the force F applied to the sliding -table 7 was
measured by means of the second load cell 9 at the moment
when the sliding table 7 reached the moving speed of 1 m/
~5 minute. The ratio of the force F to the force N, i.e.,
~ ~n -
the ratio F/N was determined, and the thus determined
value was used as the frictional coefficient.
(2) Corrosion resistance test:
Each of the samples having a width o 70 mm and a
length of 150 mm was subjected to a dipping type phosphat-
ing for a steel sheet for automobile in a phosphating
solution (product name: PBL3080) made by Nihon perkerizing
Co., Ltd., to form a phosphate film on the surface of the
sample. Then, the sample was subjected to a cation type
electropainting with the use of a paint (product name:
ELECRON 9400) made by Kansai Paint Co., Ltd., to form a
painting film having a thickness of 20 ~um on the phosphate
film. Then, a notch was provided on the thus formed
painting film. A salt spray test was carriea out on the
sample having the thus notched painting film. More
specifically, the sample was exposed to the open air for
a period of one year, during which salt water having a
sodium chloride content of 5 wt.% was sprayed over the
sample at a rate of twice a week. Then, the maximum
blister width of the painting film was measured on
one side of the notch on the sample after the salt
spray test, and corrosion resistance was evaluated by
means of the thus measured maximum blister width of the
painting film. The criteria for evaluation were as
follows:
2~3~3~
A: a maximum blister width of under 1 mm;
B: a maximum blister width within a range of from
1 mm to under 2 mm;
C: a maximum blister width within a range of from
2 mm to under 2.5 mm; and
D: a maximum blister width of at least 2.5 mm.
t3) Painting finish test:
First, 100 sheets of each sample having a width
of 70 mm and a length of 150 mm were prepared. As in the
case of the corrosion resistance test, each of the thus
prepared samples was subjected to a dipping type phosphating
for a steel sheet for automobile in a phosphating solution
(product name: PB 3080) made by Nihon Perkerizing Co., Ltd.,
to form a phosphate film on the surface of the sample.
Then, the sample was subjected to a cation type electro-
painting with the use of a paint (product name: ELECRON
9400) made by Kansai Paint Co., Ltd., to form a painting
film having a thickness of 20 jum on the phosphat~ film.
Then, the number of samples having a defect in the thus
formed painting film caused by bubbles was counted to
determine the ratio of the number of such defective
samples to the 100 samples, and painting finish was
evaluated on the basis of the thus determined value,
i.e., the defect occurrence ratio~ The criteria for
evaluation were as follows:
- 32 -
2 ~ 3 ~
A: a defect occurrence ratio of 0~;
B: a defect occurrence ratio within a range of from
1 to 5%; and
C: a defect occurrence ratio of over 5~.
S As is clear from Table 1, all the samples of the
invention Nos. 1 to 66 ha~ a frictional coefficient of
up to 0.17, and were therefore excellent in antifriction.
The samples of the invention Nos. 16 and 17 showed
a maximum blister width within a range of from 1 mm to
under 2 mm in the corrosion resistance test, and were
therefore excellent in corrosion resistance. All the
samples of the invention Nos. 1 to 15 and 18 to 66, except
for the samples of the invention Nos. 16 and 17, had a
maximum blister width of under 1 mm in the corrosion
resistance test, and were therefore particularly excellent
in corrosion resistance. Each of the samples of the
invention Nos. 16 and 17 was slightly inferior in
corrosion resistance to each of ~he samples of the
invention Nos. 1 to 15 and 18 to 66 because the plating
weight of the zinc electroplating layer of each of the
samples of the invention Nos. 16 and 17 was smaller than
the plating weight of the zinc electroplating layer of
each of the samples of the invention Nos. 1 to 15 and
18 ~o 66.
In addition, all the samples of the invention
2 ~ 3 2
Nos. 1 to 66 showed a defect occurrence ratio of 0~ in
the painting finish test, thus having an excellent
painting finish.
As is evident from the above description, all
the samples of the invention Nos. 1 to 66 were excellent
in antifriction, corrosion resistance and paintin~ finish.
As is clear from Table 2, in contrast, none of
the samples for comparison Nos. 1 to 10 satisfied
simultaneously the following three favorable merits
possessed by each of the samples of the invention Nos. 1
to 66:
(i~ A frictional coefficient of up to 0.17 in the
antifriction test;
(ii) a maximum blister width of under 2 mm in the
corrosion resistance test; and
(iii) a defect occurrence ratio of 0% in the painting
~inish test.
Among others, the samples for comparison Nos. 1
to 3, 5 to 7 and 9 to 10, except for the samples for
comparison Nos. 4 and 8 had a large frictional coefficient
of at least 0.3.
Furthermore, a high-viscosity lubricant oil
(product name: FERROCOTE 61-MAL-HCL-l) made by Nippon
Quaker Chemical Co., Ltd. was applied onto the zinc
3 ~
electroplating layer as the single layer of the sample for
comparison No. 2 having a large frictional coefficient,
and an antifriction test as described above was effected
on the sample for comparison No. 2 applied with the high-
viscosity lubricant oil on the zinc electroplating layer
thereof. The above-mentioned sample for comparison No. 2
had a frictional coefficient of 0.11. This revealed
that the samples of the invention Nos. 1 to 66 applied
with the easily removable anticorrosive oil had sub-
stantially the same antifriction as that of the sample
for comparison No. 2 applied with the high-viscosity
lubricant oil which is very difficult to remove.
According to the present invention, as described
above in detail, it is possible to provide an electro-
lS galvanized steel sheet having two electroplating layers
and excellent in antifriction, corrosion resistance and
painting finish, thus providing industrially useful
effects.
