Note: Descriptions are shown in the official language in which they were submitted.
'~.3q~
SP~CIFICATION
excellent coating perfor~ance
Technical field
.
This invention relates to a plated steel shee-t
excellent in the appearance of a coating ~hen applied in
such a use that cationic electrodeposition coating is
carried out as in the case of components for automobile
bodies.
_ac~ground art
In North America, Canada, North Europe and
elsewhere, rock sa t is sprayed in winter to prevent
roads from free~ing, and therefore steel sheets used for
the co~ponents of automobile bodies are required to have
excellent corrosion-resisting performance. For thi~
reason, in such a use, there have been recently applied
pure zinc-plated steel sheets or zinc alloy-plated steel
sheets (for example, Zn-Fe alloy-plated steel sheets, Zn-
Ni alloy-plated steel sheets, and so forth) having
excellent corrosion resistance.
However, there has been a problem that, in the case
of a single-layer coating, these plated steel sheets may
bear craterings generated on a coating film when the
cationic electrodeposition coating is carried out after
pho~phating, to give poor appearance of the coating.
Now, as a steel sheet that has solved the problem of
the coating appearance, a double-layer plated steel sheet
has been p~oposed, wherein an Fe coating that can achieve
a good electrodeposition coating performance is further
applied on a pure Zn or Zn alloy coating. Conventionally
kno~n stee1 sheets of this type may include those wherein
an upper layer comprises an Fe-Zn alloy coating having Fe
content of 60 to 9o wt.~, and -those wherein an upper layer
comprises an Fe coating, and it is true that the
application of the cationic electrodeposition coating on
these double-layer plated steel shee-ts may result in
generation of a decreased number of craterings on a
coating film and can improve the coating appearance.
~ owever, in order to lessen the generation of
craterings on a coating film by providing the Fe-Zn alloy
coating having Fe content of 60 to 90 wt.%, the coating
weight must be made not less than 5 g/m2 (per one sidel,
necessarily resulting in higher production cost.
Moreover, this Fe alloy coating is so hard and brittle
that an infinite number of crack may be formed when a
plated steel sheet is worked into a component, with the
result that the lower layer i9 exposed at the cracked
portion. ~herefore, when the electrodeposition coating is
carried out, it ollows that the electrodeposition coating
is directly applied on the lower layer, and also that
craterings are liable to be generated on the coating film.
On the other hand, in the case of the Fe coating,
which is softer than the Fe-Zn alloy coa-ting, no crack i5
generated even when the plated steel sheet is worked into
a co~ponent, and only a little cratering is generated on
the coating film. However, if the coating weight is less
than 3 g/m ~per one side), there is seen variation in the
quantity of the generation of craterings. Although the
variation factor has not been made clear, -this i5
presumably because the covering rate in the upper layer
coating relative to the lower layer coating is so poor, or
the purity of the upper layer coating is so high, that
large crystals of phosphate may tend to be for~ed during
phosphating which is a pre-treatm~nt for the
electrodeposition coating, and, as a re~ult, the rate of
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covering by the phosphate crystals on the surface of a
coating may be lowered and also the variation in the
coveriny rate may be caused to bring about a difference in
the electrolytic conduction for electrodepos.ition coating,
between the phosphate deposited portion and non-deposited
portion. Therefore, in order to lessen the generation of
craterings on a coa~ing film by providing the Fe coating,
the coatin~ wei~ht must be 3 g~m (per one side), also
necessarily resulting in higher production cost.
Taking account of the fact that the
electrodeposition coating performance has not been perfect
even in the double-layer plated steel sheet obtained by
applying the Fe coating on the pure Zn coating or Zn alloy
coating as mentioned above, this invention aims at
providing a plated steel sheet that has been improved the
electrodeposition coating performance and yet can achieve
lower production C05t.
Disclosure o
This invention provides a plated steel sheet which
is comprised of a plated steel sheet comprising a steel sh~et;
a lower layer coating of pure Zn or a Zn alloy, provided on
the steel sheet, and an upper layer coating of an Fe-Zn alloy
having an Fe percentage of 50 wt.% or more, provided on said
lower layer coating, said upper layer coating containing from
about O.O01 to 3 wt.~ of boron, thereby making it possible to
prevent craterings being generated on a coating film and
lessen the upper layer coating weight.
The reason why the plated steel sheet of this
invention can have an excellent electrodeposition coating
- performance i~ presumed to be that the presence of boron
added to a bath at the time of the plating ~or the upper
layer can improve the uniformitr of the electrodeposition
to give a uniform coating, and also that the boron
contained in the upper layer can serve as a nucleus at the
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1 3~,~J6~ ~_! r~
time of the deposition of phosphate when a phosphating is
applied as a pre-treatment for the electrodeposition
coating, to form a fine and dense phosphate coating, and,
as a result, the electrolytic conduction on the entire
surface of a steel sheet becomes uniform, whereby a
coating material can be uniformly electrodeposited at the
time the electrodepositior~ coating is carried out, and
thus the generation of craterings can be stably
restrained.
Best mode for workin~ the invention
The plated steel sheet of this invention can
restrain the generation of craterings in a coating film
even when the coating weight is not more than 3 g/m2 per
one side in the case the upper layer comprises the boron-
containing Fe coating, or even when the coating weight is
not more than 5 g/m2 per one side in the case the upper
layer comprises the above element-cor.taining Fe alloy
coating, This is presumably because, even if the coating
weight is small, the upper layer can cover the lower layer
so excellently that the rate of covering of the lower
layer by the upper layer can be improved.
In the case the upper layer comprises the Fe-Zn
alloy coatlng, it has been conventionally i~possible to
lessen the generation of craterings in a coating film
unless Fe content is controlled to less than 60 to 100
wt.%. However, the presence of boron contained makes it
possible to lessen the generation of craterings in a
coating film even if the Fe content is decreased to 50 to
100 wt.%. This is presumably because of the above
mentioned effect of improving the phosphating performance,
and once the Fe content can be decreased like this, the
difference in the corrosion potential between the upper
layer and the lower layer becomes smalll whereby the
corrosion resistance of the coa-tings as a whole for a long
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period can be improved.
The amount of boron contained in the upper layer may
preferably be controlled to 0.001 to 3 wt.%. This is
because the boron amount of less -than 0.001 wt.~ may
result in no difference ~rom an upper layer containing no
boron in respect of the generation of craterings and
variation thereof in the electrodeposition coating, and
the boron amount more than 3 wt.% may result in saturation
of the effect so that it is meaningless to make the amount
larger than that.
The covering weight on the upper layer (per one
side) may be preferably controlled to 0.5 to 10 gtm2 in
the case of the boron-containing Fe coating, and 0.2 to ~
g/m2 in the case of the Fe alloy coating. This is because
the weight of less than 0.5 g/m2 or 0.2 g/m2 may make it
impossible to achieve perfect covering of the lower layer
to bring about the generation of craterings in a coating
film to be caused by the exposure of the lower layer at
the time of the electrodeposition coating, and the weight
more than 10 g/m2 or 3 g/m2 may result in saturation of
the ef~ect of restraining the generation of craterin~s in
a coating film so that it is unnecessary to make the
coating weight larger than that. Meanwhile, in the case
of the conventional upper layer coating comprising the Fe-
Zn alloy containing no boron, the effect of restraining
the generation of craterings in a coating film has been
saturated when the weight is 10 g 10/m2 g. The effect by
virtue of the addition of boron can be seen also in this
- point, in respect of the decrease in the coating weight.
In order to codeposit boron into the upper layer in
an amount of 0.001 to 3 wt.% according to electroplating,
th~ plat.ing may be carried out by adding one or more of
boron compound(s) such as boric acid, metaboric acid,
water soluble metaborate, water soluble tetraborate and
tetrafluorsborate to an ordinary Fe plating bath or a
plating bath of an Fe alloy such as an Fe-Zn alloy and an
, ~., ~
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~;
Fe-Ni alloy, and adjusting the pH of the bath to 1 to 3.
The steel sheet according to this in~ention can
improve the performance in the electrocleposition coating
of a pure Zn-plated or Zn alloy-plated steel sheet.
However, in the case the lower layer comprises an Zn alloy
coating of a Zn-Ni or Zn-Fe alloy, the generation of
craterings can be restrained even with imclusion of a
trace amount of o~e or more of elements su~h as Ni ~in
respect of the Zn-Fe alloy), Fe ~in respect of the Zn-Ni
alloy), Co, Cr, Mn, Mo and Ti. In the case the lower
layer comprises a pure Zn coating, it can be also
restrained even with respect to coatings obtained by
carrying out electroplating, vacuum deposition or hot
dipping. Further, in the case the upper layer comprises
the Zn alloy coating, it can be also restrained even with
respect, in addition to the coatings formed by the above
methods, to those which are alloyed into a Zn-Fe alloy by
thermal diffusion after carrying out hot dip ~inc coating
or vacuum zinc deposition as in the case of an alloyed
zinc-plated steel sheet.
This invention will be described below more
specifically by ~xamples.
Example 1
On a cold rolled steel sheet of 0.8 mm thick, usual
treatments of degreasing and acid pickling were applied to
make its surface clean. Therea~ter, a lower layer coating
comprising pure Zn, an Zn-Fe alloy or Zn-Ni alloy was
first provided according to electroplating under the
conditions as shown in Table 1, and next an upper layer
coating comprising Fe or Fe-B was provided on it according
to the same plating method.
Subsequently, from the thus plated steel sheet,
samples were collected by 10 sheets per each plating
condition, which were treated with a commercially
available phbsphating solution (Bondelite #3030*; produced by
Nippon Parker Co.), followed by carrying out cationic
*Trade mark
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electrodeposition coating to examine the number of
craterings generated on the coatings after drying by
baking. In the electrodeposition, ~lcon* #9000 (produced
by Kansai Paint Co., Ltd.) was used as a coating material,
which was electrodeposited according to an instantaneous
voltage-increasing ~ethod (the so-called "dokan" method)
under 300 V to have a coating film thickness of 25 um, and
the baking was carried out at 1~0C for 20 minutes. The
number of craterings generated in the coating film when
provided with an Fe-B coating as the upper layer is shown
in Table 2.
As will be clear ~rom Table 2, the steel sheets of
this invention, provided with an Fe-B coating as the upper
layer, show less generation o~ craterings and variation
thereof than those in conventional double-layer plated
steel sheets provided with an upper layer Zn coating and
having a good electrodepo~ition coating perfor~ance, even
with a lower layer comprising a pure Zn coating or a Zn
alloy coating.
*Trade mark
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Table 1
Conditions for lower layer coating
_ _ ... .
Pure Zn coating Zn-Fe alloy coating
Plating bath composition:
Zinc sulfate 240 g/lit Ferrous sulfate ,'80 g/lit
Sodium sulfate 75 g/lit Zinc sulfate 75 g/lit
Sodium sulfate 85 g/lit
Plating conditions:
pH 1.5 pH 1.6
Bath temperature 50C Bath temperature 50C
Current density 20 A/dm2 Current density 40 A/dm
Others:
Fe content 15 to 20 wt.%
- Coating weight 20 g/m2
_ (~E~r one side)
Conditions for lower layer coating
(Cont'd)
Zn-Ni alloy coating
Plating bath composition:
Nickel sulfate 260 g/lit
Zinc sulfate 150 g/lit
Sodium sulfate 70 g/lit
Plating conditions:
pH 2.0
Bath temperature 55C
Current densit-y 40 A/dm
Others:
Ni content 11 to 12 wt.%
Coating weight 20 g/m2
(~er one side)
. ~ .. . .
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Table 1 (cont'd)
Condit.ions for upper layer coating
_ Fe-B coating _
Plating bath composition:
Ferrous sulfate 250 g/lit
Sodium sulfate 70 g/lit
Tartaric acid 3 g/lit
Sodium metaborate 10 to 50 g/lit
Plating conditions:
Current density 20 to 30 A/dm2
pH 1.0 to 3.0
Bath temperature ~0 C
Others:
Boron content was controlled depending on the
combination of the concentration of sodium
metaborate and the pH. Plating of boron-free
: - Fe was carried out without addition of sodium
metaborate. :
Table 2
Type of Upper layer coatin~ (Fe-B)
Sam- lower layer B content Coating Number of
ple ~ __ weight
(wt.%) (g/m
Present invention:
l Zn 0.050 4 0 to 5
2 ~I 0.21 20 to 5
3 Zn-Ni 0.002 8 0 to 6
4 " 0.00~ 5 0 to 5
Zn-Fe 0.032 11 to 5
6 " 0.75 4 0 to 5
~ " 2.6 3 1 to 5
_______________________ __________________________________
Comparati~e Example:
1 Zn 0 0 10 to 18
2 " 0 2 4 to 25
3 Zn-Ni O 3 5 t o 2 3
4 Zn-Fe 0 1 15 to 28
Zn-Ni 0.0004 3 2 to 22
6 Zn-Fe 0.003 0.163 to 205
. _ _ _ _Note 1~ Comparative Examples 1 to 4 i9 provided with an
upper layer comprising a boron-free Fe coating.Note Z) The coating weight in the upper layer refers to
that per one side. (Ditto in Table 4, Examples 2
and 3 shown below)Note 3) The number of craterings refers to the minimum
to maximum number in 10 sheets (per coating area
of 5 cm x 5 cm).
Exa~ple ?
~ cold rolled steel sheet of 0.8 ~m thick was
treated in the same manner as in Example 1 to make its
surface clean. Thereafter, a lower layer coating
comprising a Zn-Fe alloy or a Zn-Ni alloy was first
provided under the conditions as shown in Table 3, and
next an upper layer coating comprising al boron containing
Fe-Zn alloy or a boron-free Fe-Zn alloy was provided on
it.
Subsequently, samples were collected fro~ this
plated steel sheet, and subjected to phosphating and
electrodeposition coating in the same manner as in ~xample
1 to produce coated steel sheets having a coating film
thickness of 23 um. The number of craterings generated in
the coating film when provided with a boron-containing
high Fe-Zn alloy coating as the upper layer is shown in
Table 4.
Table 3
Conditions for lower layer coating
.
Zn-Fe alloy coating _ Zn-Ni alloy coating
Plating bath composition:
Ferrous sulfate 280 g/lit Nickel sulfate 260 gtlit
Zinc sulfate 75 g/lit Zinc sulfate 150 g/lit
Sodium sulfate ~5 g/lit Sodium sulfate 70 g/lit
Plating conditions:
Current density 40 A/dm Current density 40 A/dm
Bath temp. 50 C Bath temperature 55C
pH 1.6 pH 2.0
Others:
Fe content 15 to 20 wt.% Ni content 11 to 12 wt.%
Coating weight 20 g/m Coating weight 20 g~m
( er one side~ (per one side)
P
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Table 3 (Cont'd)
Conditions for upper layer coating
_
_oro containin~ hi
Plating bath composition:
: Ferrous sulfate Z50 g/lit
Zinc sul~ate2B ~lit
Sodium sul~ate 75 g/lit
Sodium ~etaborate 10 to 50 g/lit
Plating conditions.
Current density 40 to ~O A/dm2
Bath temp. 50 C
p~l 1.5 to 3.0
Others:
Fe content was controlled based on the
combination of the current density and the pH,
and ~oron content was controlled based on the
combination of the concentration of sodium
metaborate and the pH.
.
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Table 4
. ~
Type of Upper__ayer coatin~
__
Sa~- lower layer B con- Fe con- Coating Number o~
ple coating tent tent wei~ht
(wt.%) (wt.%3 ~g/m2,
Present invention: one side)
1 Zn-Ni 0.003 82 5 2
2 " 0.018 gS ~ 1
3 " 2.21 55 3
4 " 0.08 65 1 2
" O.g2 72 0.7 2
6 Zn-Fe 0.20 90 0.4 3
~ " 1.62 85 1 0
8 " 2.61 ~3 7 0
9 " 0.05 60 1.5 0
_________ ________________________________________________
Comparative Example:
1 Zn-Ni 0 55 5 25
2 " 0 68 8 3
3 Zn-Fe 0 70 1 19
: 4 " 0 69 2 12
" 0 85 6 5
. ~ . _ _ _
(Note) Comparative Examples are provided with an upper
layer comprising a boron-free Fe-Zn alloy
: coating.
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Example 3
A steel strip of 0.6 mm thick and 300 mm wide was
r-educed in a pre-treatment oven of a gas reduction ~yste~,
and thereafter passed through a pressuri~ing chamber for
preventing inflow of gas or air and a seal roll chamber in
which pressure is stepwise reduced by means of a numher of
seal rolls, and then introduced into a first vacuum
deposition cha~ber equipped with a vacuum deposition Zn
bath of an electrical resistance heating system at a lower
side of the position of the steel strip, to apply a
deposition coating of pure Zn on one side of the steel
strip. Subsequently, the steel strip was guided to a
second vacuu~ deposition chamber disposed at a lower side
of the above first vacuum deposition chamber and having
the same construction as the first vacuum deposition
chamber to apply a deposition coating of pure 2n on the
opposite side of the steel strip, and thereafter passed
through a seal roll chamber and a pre~surizing cha~ber to
produce a deposited steel strip having a coating weight of
50 g~ (per one side). The plating was carried out under
the conditions of a steel strip moving speed of 15 ~/min
a~d a vacuum degree of 0.01 Torr in both the first and
second vacuum deposition chambers.
Next, part of the pure Zn-plated steel strip thus
produced was introduced in an oven having an atmosphere of
a mixed gas ~dew point: -25C) comprising 3 % of H2 and 97
% of N2, and heated to 280C to for~ the coating into a Zn-
Fe alloy, thereby producing a steel strip provided with a
coating of a Zn-Fe alloy ha~ing a~ Fe percentage of 10
wt.%.
Thereafter, this Zn-Fe alloy-plated steel strip and
the pure Zn-plated steel strip were subjected to
electroplating to respectively provide a Fe coating or a
Fe-B coating under the same conditions as those for the
upper layer coating show~ in Tahle 1, and a boron-
containing high Fe-Zn coating under the same conditions as
those for the upper layer coating shown in Table 3,
followed by carrying out elec~rodeposition coatiny under
the same conditions as in E~ample 1. The states of
generation of craterings in the coating film are shown in
Table 5 and Table 6.
Table 5
Type of Upper layer_coatin~ __
Sam- lower layer B content Coating Number of
ple coating wei~ht craterin~s
,.
(wt.%) (g/m~, one
Present invention: side)
1 Zn 0.041 3 0 to 5
2 " 0.23 2 0 to 5
3 " 0.002 9 0 to 5
4 Zn-Fe 0.003 5 1 to 5
" 0.06 4 0 to 5
6 " 2.3 2 1 to 5
Comparative Example:
: 1 Zn 0 0 tO to 20
2 " 0 2 5 to 25
3 Zn-Fe 0 4 4 to 25
4 " 0 2 15 to 30
Zn 0.0004 5 3 to 30
6 " 0.02 0.15 to S0
7 Zn-Fe 0.0003 6 5 to 30
8 " 0.005 0.14 to 47
(Note) Comparative E~amples 1 to ~ are provided with an
upper layer comprising a boron-free Fe coating.
~3~
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I'able 6
.
Type of Up~ layer coating _
Sam- lower layer B con- Fe con- Coating Number of
coatin~ tent tent weight_ rater n~_
(wt.%) (wt.%) (g/m ,
one side)
Present i nvent i on:
1 2n-Ni 0.003 a 1 5
2 " ~.02 93 2
3 " 0.12 60
4 " 2.1 55 0.6 2
Zn-Fe 0.006 58 0.5 2
6 " 0.6 72 1 0
7 " 0.4 83 ~ 0
8 " 2.6 90 1.5 0
_____________________________________________ ___ ________
Comparative Example:
1 Zn 0 56 5 26
2 " 0 ~0 B 10
3 Zn-Fe 0 61 2 23
4 " 0 86 6 13
. . . _ _ _ . , ,
~Note) Comparative Examples are provided with an upper
layer comprising a boron-free Fe-Zn alloy
coating.
Possibil-ity of industrial utilization
The plated steel sheet according to -this invention
can achieve a good coating appearance when used not only
in automobile body components but also in other components
such as electrical equipl~ent co~ponents for do~estic use
and construc-tion components on which the electrodeposition
coating is carried out. Since also having an excellent
coating performance for coating materials other than the
coating materials for the electrodeposition, the pr0sent
steel sheet can be also applied in such a use for general
coating.
