Note: Descriptions are shown in the official language in which they were submitted.
~2~953~
Kobayashi et al
TITLE OF THE INVENTION
Organic Coa~ed Steel Strip Having Improved
Bake Hardenability and Method for Making
BACKGROUND OF THE INVENTION
This invention relates to corrosion resistant, organic
coated steel strips having improved bake hardenability and
drawability and finding application in automobiles.
As often encountered in the manufacture of
automobiles, strip steel is pressed formed to a desired
shape and then coated with a protective coating typically by
electrophoretic painting followed by baking at elevated
temperatures. The term bake hardenability designates that
the strip steel hardens during the baking of such a coating.
Usually, the bake hardenability of strip steel is evaluated
in terms of an increase of yield strength by baking a 2%
pre-stressed, steel stxip at 170C for 20 minutes and
measuring the yield strength.
In these years, there exists a greater demand for
further improving the corrosion resistance of automotive
strip steel. A number of rust-preventive steel strips have
been proposed to meet such a demand and many of them are
successfully used. These rust- or corrosion~preventive
steel strips are surface treated steel strips including ~inc
and ZillC alloy hot dl~pec1 steal, ZillC ~n~ zine alloy
electroplated, and zinc rlch painted, typically organic zinc
rich painted steel strips. In addition, composite coated
steel strip have also been developed wherein a plated steel
strip is covered with an organic coating. These composite
coated steel strips are known to be the currently rnost
improved corrosion-preventive steel strips.
For energy saving and drivability improvernent, an
increasing amo~nt of high tensile strip steel has been used
- 2 - ~5~3~
in the manufacture of automobiles. To compensate for the loss of
dent resistance resulting from thickness reduction, desired is a
steel strip which exhibits a low yield strength prior to press
forming and increases its yield stren~th during paint baking. Also
in common drawing steel strips, bake hardening after press forming
is a phenomenon favourable for increasing dent resistance
particularly when the strips are used as automobile outer plates.
Steel strips are thus desired to have both deep drawability and
bake hardenability.
10Thus, there is a need for corrosion-preventiv~ steel strips
capable of satisfying not only corrosion resistance, but also a
variety of re~uirements such as light weight, safety and rigidity.
One conventional commercially available steel meeting such
consideration is a class of zinc and zinc alloy plated steel strips
15having bake hardenability. An organic coating is applied to a zinc
or zinc alloy plated steel strip. The organic coating on the steel
must be baked at a temperature of higher than 150C in order to
convert it into a hardened one. Thus the organic coated steel
strip has been hardened prior to press forming and is thus not
20amenable to drawing.
More particularly, some of conventional automotive organic
coated or painted steel strips are known under the trademark of
~incrometal (Diamond Shamrock) as disclosed in Canadian Patents
Nos. 863,894, 918,516, 940,427, 9~3,442, 962,923, 970,668, and
25979,337, and some are disclosed in Canadian Patent No. 1,200,723,
U.S. Patent No. 4,659,39~, and Japanese Kokai (laid open
applications) Nos. 60-86281 and 60-105535. All these coated strips
suffer from the above-mentioned problem because they must be baked
at a temperature in excess oE 150C in order to convert the organic
30coating into a hardened one. The bake hardenabiLity o~' ~te~l
substrates themselve~ could not be b~neelciaLLy utilized.
SUMM~ OF TH~ INV~N~IQ~
An object of the present invention is to provide a novel and
improved organic coated steel strip which exhibits bake
_ 3 _ 12,S~3~
hardenability and good workabili~y even after baking of the organic
coating.
Another object of the present inven~ion is to provide a method
for making such an organic coated steel strip.
A further object of the present invention is to provide a
method for making an organic coated steel strip having improved
bake hardenability and capable of maintaining a high proportion of
chromium fixed while preventing chromium from being dissolved out
during alkaline degreasing and/or chemical conversion as used in
an automotive coating process.
According to a broad aspect of the present invention, there
is provided a method for making an organic coated steel strip
having improved bake hardenability of 3.5 to 6.1 kgf/mm2, comprising
the steps of: depositing a layer of a zinc base alloy on one
surface of an extra low carbon steel substrate containing 0.001 to
0.008 weight percent carbon and having bake hardenability in a
weight of 10 to 40 g/m2, subjecting said substrate to a chromate
treatment to form a chromate layer on the zinc base alloy layer in
a weight of at least 10 mg/m2 calculated as metallic chromium, and
applying an organic coating on the chromate layer and baking the
coating at a tempe:rature of up to 150C.
In a further broad aspect the present invention relates to a
method for making an organic coated steel strip having improved
bake hardenability of 3.5 to 6.1 kgf/mm2, comprising the steps of:
depositing a layer of a zinc base alloy on one surface of an extra
low carbon steel substrate containing 0.001 to 0.008 weight percent
carbon and having bake hardenability, subjecting said substrate to
a chromate treatment to form a chromate layer on the zi.nc base
alloy layer, and applying an organic coating on the chromate layer
and baking the coating at a temperature o~ up to 150C, wherein ~aid
chromate treatment uses ~n a~ueou~ cllromate ~olution containing a
chromate compound, a reducing agent, and at least one member
selected from acid residues, resins and silica.
~ ~25~3~
_RIEF DESCRIPTION OE' THE DRAWINGS
In order that those skilled in the art will readily understand
the practice of the present invention, the following description
is made with reference to the accompanying drawings, in which:
5FIG. l is a diagram showing a stable chromium fixing region
in relation to baking temperature and Cr6+/Cr3~;
FIG. 2 is a diagram showing a chromium fixing proportion as
a function of the amount of methanol added to a chromate solution;
FIG. 3 is a diagram showing a chromium fixing proportion as
10a function of the amount of phosphoric acid added to a chromate
solution;
FIG. 4 is a diagram showing a chromium fixing proportion as
a function of the amount of a resin added to a chromate solution;
FIG. 5 is a diagram showing a chromium fixing proportion as
15a function of the amount of silica added to a chromate solution;
and
FIG. 6 is a diagram showing the bake hardenability expressed
in BH value of steel strip as a function of the baking temperature
of organic coating.
~25~3~
DE ILED DESCRIPTION OF THE INVENTION
The present invention pro~ides a highly corrosion
resistant, organic coated steel strip capable of maintaining
improved bake hardenability after press forming, which is prepared
by preparing a drawing extra 'ow carbon steel substrate having bake
hardenability, depositing a layer of a zinc base alloy on one
surface of the substrate, subjecting said substrate to a chromate
treatment to form a chromate layer on the zinc base alloy layer,
and applying an organic coating on the chromate layer and baking
the coating at a temperature of up to 150C.
As previously described, automotive organic coated or
painted steel strips must be baked at a temperature in excess of
150C in order to convert the organic coating into hardened one.
The reason is that in case of ZincrometalR with which cold rolled
steel is coated, for example, baking must be effected at a
sufficiently high temperature to cause the powder zinc-containing
chromate to react with the steel substrate. The use of a high-
boiling solvent to dissolve a high molecular weight resin also
requires a baking treatment at a temperature of higher than 150C.
A si.milar requirement is imposed on plated steel strips,
for example, composite zinc rich/organic coated steel strips as
disclosed in Canadian Patent No. 1,200,723. That is, baking of the
organic coating must be at a high temperature as encountered for
the cold rolled steel mentioned above.
Also, steel strips having a thin film organic coating other
than the zinc rich paint are baked at a high temperature because
a binding agent capable of crosslinking at a high temperature, for
example, melamine resin must be added in the state o~` the a~t.
For all these convent:i.ona:l. o~ganic coatLng~, .Lt I.s not
contemplated to take a~vclntclge of the bake hardenability of
``~
~25~32
strip steel as done ln the present invention. Therefore,
the conventional coated strips have poor workability.
In order to take advantaye of the bake hardenability
of strip steel while maintaining the corrosion resistance as
available with ~onventional coatings, we have investigated
the organic coating which can exhibit sufficient corrosion
resistance even when baked at a temperature of 150C or
lower. We have discovered that when a cold rolled substrate
of extra low carbon steel having bake hardenability by
nature is provided with an organic corrosion-preventive
coating by a baking treatment at a temperature of up to
150~C so as not to lose the inherent bake hardenability,
there is available an optimum range of overall coating
consisting of plating plus pre-treatment plus organic
coating in which the organic corrosion-preventi~e coating
can provide corrosion resistance, workability and
weldability equal or superior to those of conventional
highly corrosion resistant organic coated or painted steel
strips.
Therefore, the present invention provides an organic
coated steel strip having improved bake hardenability,
comprising an extra low carbon steel substrate having bake
hardenability, a layer of a zinc base alloy deposited on one
surface of said substrate in a weight of 10 to 40 g/m2, a
chromate layer formed on said zinc base alloy layer in a
weight of at least 10 mg/m2 calculated as metallic chromium,
and an organic coating, optionally containlng silica,
attached to said chromate laye~ b~ baking at a tempe~ature
of up to 150C.
According to the present invention, the temperature at
which the organic coating is baked is limited to 150C or
lower in order to maintain the bake hardenability. The
reason will become clear from the description of a series of
experiments.
;;9~3;~:
The starting steel was an extra low carbon steel
consisting of, in percentage by weight, 0.003% C, 0.01~ Si~
0.16% Mn, 0.04~ Al, 0.070% P, 0.026% Nb and balance
essentially ~e. The strip steel was cold rolled at a draft
of 80% to a thickness of 0.7 mm, heat treated by soaking at
850C for 30 seconds and cooled to 650C at a rate of
~5C/sec. in a continuous annealing furnace, and then skin
pass rolled at a draft of 1.0%. The resulting extra low
carbon steel strip having bake hardenability was determined
for mechanical properties, exhibiting a yield strength (YS~
of 20 kgf/mm2, a tensile strength (TS~ of 35 kgf/mm2, an
elongation (El) of 45~, and a bake hardenability (BH) of 5
kgf/mm2. As previously defined, BH is equal to YS of baked
strip minus YS of initial strip.
An organic coating was applied to the steel strip and
baked at various temperatures for one minute. The bake
hardenability of the baked steel strip is shown in FIG. 6 as
a function of the baking temperature. As seen from the
curve in FIG. 6, the steel strip experiences a sudden drop
in BH value when the baking temperature exceeds 150C,
losing the inherent bake hardenability.
The preferred cold rolled steel strips having bake
hardenability with which the present invention starts are
bake hardenable, cold rolled steel strips comprising, in
percentage by weight, 0.001 to 0.008% of C, up to 0.5% of
Si, 0.05 to 1.2% of Mn, up to 0.1~ of P, 0~01 to o.oa~ 0f
Al, the aluminum b~ing at le~st ~3 tlmes the perce~t N, from
3 times the percant C to ~ tim0s th~ pe.rcent C plus 0.02~ of
Nb, up to 0.05~ of Ti, and balance essentially Fe, the
strips being continuously annealed to have a BE1 value of 3
to 6 kgf/mm2. The extra low carbon steel materials tlndergo
little hardening dl~ring mild baking at temperatures of 150C
or lower so that they maintain their own bake hardenability.
Although the reason is not exactly understood, it is
expected that bake hardenable steel strips of extra low
~9~3;2
carbon steel and those of low carbon steel have a different
distribution of C in solid solution form within grains even
though both have the same apparent sH value.
In the extra low carbon steels previously defined as
providing the preferred bake hardenable cold rolled steel
strips, the contents of the respective elements are limited
to certain ranges.
Carbon, C preferably ranges from 0.001 to 0.008 wt%.
Contents of less than 0.001 wt% lead to the loss of the
solid solution carbon contributing to bake hardening.
Steels having more than 0.008 wt% of carbon exhibit too high
yield strength and low ductility and r value (Lankford
value).
Silicon, Si preferably ranges up to 0.5 wt%. In
excess of 0.5 wt%, an oxide film will form to detract from
chemical conversion amenability.
Manganese, Mn preferably ranges from 0.05 to 1.2 wt%.
Red shortness diminishes at less than 0.05 wt% whereas r
value is reduced in excess of 1.2 wt%.
Phosphorus, ~ preferably ranges up to 0.1 wt%. Steel
becomes brittle with P contents in excess of 0.1 wt%.
Aluminum, Al preferably ranges from 0.01 to 0.08 wt%
and at least 8 times the percent N. At least 0.01 wt% of Al
is necessary to fix nitrogen. Al contents of more than 0.08
wt% undesirably generate many inclusions. The function of
Al to fix nitrogen provides the additional requirement that
its content be at least 8 timeC3 the perc0nt N~
Niobium, N~ preferabl~ ranyes ~rom 3 tlmes the percent
C to 8 times the percent C plus 0.02~. Below the lower
limit of 3 times the percent C, a too lar~er amount of solid
solution carbon is left, inhibiting the formation of an
aggregate structure contributing to drawability during cold
rolling recrystallization. Ductility is impaired in excess
of 8 times the percent C plus 0.02%.
~595~2
Titanium, Ti preferably ranges up to 0.05 wt~ because
the bake hardenability of steel is lost in excess of this
limit.
The steel strips are preferably controlled to a BH
value of 3 to 6 kgf/mm2. A value of less than 3 kgf/mm2 is
a substantial loss of bake hardenability. Steels having BH
values in excess of 6 kgf/mm2 undergo severe deterioration
upon aging and stretcher strain during working.
The extra low carbon steel strips are plated with zinc
based alloys by any known deposition techniques, typical
electrodeposition. Some non-limiting examples of the zinc
base alloy platings include Zn-Ni alloy platings preferably
having a nickel content of 5 to 13 wt~; Zn-Fe alloy plating
preferably having an iron content of 8 to 25 wt%; Zn-Co-
Al2O3-Cr2O3 alloy platings preferably having a cobalt
content of 1 to 5 wt%; Zn-Al alloy platings preferably
having an aluminum content of 1 to 15 wt%; Zn-Ni/Fe-P
double-layered alloy platings preferably having a phosphorus
content of 0.0003 to 5~ by weight based on the weight of Fe-
P; Zn-Fe/Fe-P double-layered alloy platings preferably
having a phosphorus content of 0~0003 to 5% by weight based
on the weight of Fe-P. These zinc base alloy platings which
have corrosion resistance several times higher than the
conventional pure zinc plating are effective in achieving
the objects of the present inventi~n. The amount of zinc
alloy plated, that is, plating weight should range from 10
to 40 gram per square meter (g/m2). Corrosion resistance is
insufficient with less than 10 g/m2 whereas plating weights
in excess of 40 g/m2 provide no additional benefit in
corrosion resistance improvement and are thus uneconomical.
In the above-listed zinc base alloys, the contents of
the respective elements are preferably limited to certain
ranges.
The Zn-Ni alloys preferably have a nickel content of 5
to 13 wt%. ~ess than 5 wt% of Ni provides insufficient
1259S32
corrosion resistance whereas a plating containing more than
13 wt% of Ni is too hard.
The Zn-Fe alloys preferably have an iron content of 8
to 25 wt%. Less than 8 wt% of Fe provides insufficient
corrosion resistance whereas red rust will often generate in
excess of 25 wt~ of Fe.
The Zn-Co-A12O3-Cr2O3 alloys preferably have a cobalt
content of 1 to 5 wt%. Less than 1 wt% of Co provides
insufficient corrosion resistance whereas more than 5 wt% of
Co is uneconomical.
The Zn-Al alloys preferably have an aluminum content
of 1 to 15 wt%. Less than 1 wt% of Al provides insufficient
corrosion resistance whereas sacrificial corrosion
prevention is lost in excess of 15 wt% of Al.
The Fe-P alloys for the double-layered Zn-Ni/Fe-P and
Zn-Fe/Fe-P platings preferably have a phosphorus content of
0.0003 to 5% by weight based on the weight of Fe-P.
Platings having less than 0.0003 wt% of P are less
susceptible to chemical conversion. More than 5 wt% of P is
uneconomical because of reduced current efficiency during
plating process.
The zinc base alloy platings are subjected to a
chromate treatment in order to improve their adherence to
subsequently applied organic coatings and hence, the
corrosion resistance of the overall structure. The chromate
treatment is carried out to produce a chromate film of at
least 10 mg/m2 of metallic chromium. Either coating or
electrolytic chromate treatment is advantageous in
controlling the amount of chromate film to such a level.
Chromate films of less than 10 mg/m2 of metallic chromium
have insufficient corrosion resistance and poor adherence to
the subsequently applied organic coatings.
On the chromate layer thus formed is applied an
organic coating which is baked at a temperature of up to
150C and assists in improving corrosion prevention. The
~59532
1 1
organic coating compositions used in the practice of the
present invention contain as a main ingredient, a resin
selected from the following three groups:
(1) water dispersible resins including acrylic,
polyethylene, epoxy, and alkyd resins and modified ones
thereof;
(2) solvent type resins including epoxy and polyester
resins and modified ones thereof; and
(3) UV- or electron radiation-curable resins such as
acrylic, epoxy, and polyurethane resins,
alone or a mixture thereof.
When resin (1) or (2) listed above is used, there are
contemplated some methods for assisting in fully hardening
the resin by a heat treatment at a low temperature of up to
150C, for example, the use of low temperature curing
agents, for example, metal salt catalysts such as cobalt
naphthenate, optionally in combination with amine curing
agents such as diethylene triamine. With these curing
agents added, the curing process can proceed at room
temperature. An organic coating may be more readily formed
by using a method adequate for the particular type of
organic resin used. Examples of the curing agents which
exert their function upon heating include urea resins,
melamine resins, benzoguanamine resins, block isocyanate
resins, and phenol resins.
Known examples of UV- or electron radiation-curable
resins (3) are acrylic resin coatings utilizing a benzoin
ether as a photopolymerization initiator and epoxy resin
coatings utilizing an aromatic diazonium salt as a
photopolymerization initiator. Exemplary of the resins
having an electron radiation-curable functional group there
may be given acrylic and epoxy resins utilizing epoxy-acid
addition reaction and polyurethane resins utilizing
isocyanate-hydrogen group addition reaction. These resins
(3) can advantageously maintain the bake hardenability of
1259S~2
12
the steel substrate substantially unchanged because they can
be baked at temperatures of several ten degree in
Centigrade.
The resinous coating composition of any of resins (1)
to (3~ is applied to the chromate layer to a thickness of
0.5 to 3 ~m. Organic coatings of less than 0.5 ~m thick
provide insufficient corrosion resistance whereas more than
3 ~m thickness adversely affects weldability.
The resinous coating composition may further contain
up to 60% by weight of silica sol for the purpose of
improving corrosion resistance. Coating compositions
containing more than ~0% by weight of silica sol are too
viscous and tend to gel.
The composite coated steel strips mentioned above are
improved rust-preventive steel strips. As previously
mentioned, a chromate treatment is often employed as a
preliminary treatment for the organic coating in order to
enhance corrosion resistance. A usual automotive part
coating process proceeds according to the scheme of blank
(organic coated steel) - assembly - alkaline degreasing -
chemical conversion - electrophoretic deposition -
intermediate coating - top coating. Since the temperature
at which the chromate and resin films are baked is
controlled relatively low to maintain bake hardenability
according to the present invention, there is the likelihood
that when a conventional chromate solution is used in the
preliminary treatment, chromium be dissolved out during the
alkaline degreasing and chemical conversion, imposing a
problem to spent liquid disposal.
We have found that in alkaline degreasing of an
automotive steel strip comprising a zinc alloy-plated steel
substrate which has been subjected to a chromate treatment
and an organic coating treatment, the chromium can be
1'~5!:~S32
dissolved out so that the degreased steel strip has only a
markedly reduced amount of chromate attached thereto.
To overcome this drawback, we have studied the
addition of reducing agents, acids, resins, and silica to
the chromate solution, and arrived at the method of the
present invention.
Therefore, the present invention according to the
other aspect provides a method for making an organic coated
steel strip having improved bake hardenability, comprising
the steps of:
preparing an extra low carbon steel substrate having
bake hardenability,
depositing a layer of a zinc base alloy on one surface
of the substratej preferably in a weight of 10 to 40 g/m2,
subjecting said substrate to a chromate treatment to
form a chromate layer on the zinc base alloy layer,
preferably in a weight of at least 10 mg/m2 calculated as
metallic chromium, the chromate treatment using an aqueous
chromate solution containing a chromate compound, a reducing
agent, and at least one member selected from acid residues,
resins and silica, and
applying an organic coating on the chromate layer and
baking the coating at a temperature of up to 150~C.
The organic coated steel strips produced by the method
of the present invention experience controlled dissolving
out of chromium during alkaline d~greasing and/or chemical
conversion in the automotive coating process without a loss
of corrosion resistance.
The chromate treatment will be described in more
detail. We have made an experiment to examine the
proportion of chromium fixed during alkaline degreasing
and/or chemical conversion in the automotive coating
process. The starting steel strip is an extra low carbon
steel consisting of, in percentage by weight, 0.003~ C,
0.01% Si, 0.16% Mn, 0.04% Al, 0.070% P, O.G26% Nb, and
1~59532
balance essentially Fe. A zinc base alloy, typically Zn-Ni
alloy was plated on the strip in a plating weight of 10 to
40 g/m2, a chromate solution having a ratio of hexavalent to
trivalent chromium (Cr6+/Cr3+) of from 80/20 to 20/80 was
applied and baked to the zinc base plating, and then a resin
in water or solvent, typically epoxy resin was applied and
baked to the chromate film. It is to be noted that the
ratio of Cr6+/Cr3+ was measured by the redox titration
technique. The maximum temperature to which the strip was
heated was from room temperature to 150C for both the
chromate and resin film baking steps.
When a conventional chromate solution, that is, free
of any additives as defined in the present invention, is
used, higher temperatures at which the applied chromate and
resin films are baked cause more hexavalent chromium to be
reduced to trivalent chromium, resulting in an increased
chromium fixing proportion. For example, when the films
were baked at temperatures of higher than 150C, the percent
of chromium remaining fixed after alkaline degreasing was at
least 80%, which level is acceptable in the automotive
coating process. With attention paid to the steel
substrate, however, yield strains are induced therein during
the process. There arise some problems including removal of
such yield strain as well as increased yield stress and
deteriorated press formability.
Therefore, it is intended in the present invention to
improve the percent chromium fixed, provided that the
maximum baking temperature is limited to the range between
room temperature and 150C.
According to one embodiment of the present invention,
chromate solutions having added thereto methanol as a
reducing agent and phosphoric acid as an acid residue was
applied and baked to plated steel strips at temperatures of
from room temperature to 150C and then a resin was applied
and baked to the chromate film at a temperature of from room
~259S32
temperature to 150C. The chromium fixing proportion, that
is, percent chromium fixed of the strips was plotted in FIG.
1 as a function of the Cr6+/Cr3+ ratio of the chromate
solution and the maximum baking temperature. The range of
Cr6+/Cr3+ ratio within which the percent chromium fixed is
80~ or higher is depicted hatched as a favorable region in
FIG. 1. Although the reducing agent used is methanol and
the additive used is phosphoric acid, similar results are
obtained when other reducing agents are used and/or other
additives such as acids other than phosphoric acid, resins
and silica are used.
To achieve a favorable percent chromium fixed of 80%
or higher with the maximum baking temperature ranging from
room temperature to 150~C as shown in FIG. 1, we have found
that favorable results are obtained by adding a reducing
agent and at least one additive selected from acid residues,
resins, and silica to the chromate solution.
Examples of the reducing agents added to the chromate
solution include methanol, aqueous hydrogen peroxide,
ethylene glycol, succinic acid, succinimide, but are not
limited thereto. The reducing agent is added in an amount
sufficient to provide a RAH of 0.2 to 1.9 per gram molecule
of CxO3. The term RAH is the gram atoms of hydrogen
contained in the reducing agent. With RAH of less than 0.2,
the percent chromium fixed is reduced to an unacceptable
level even when the additive as defined below is
additionally used. The chromate solution becomes gel if RAH
is above 1.9.
The additive which is used in the chromate solution in
combination with the reducing agent are selected from acid
residues, resins, and silica. They are described in more
detail.
(1) Acid residues
Preferred acid residuex are provided by such acids as
phosphoric acid and boric acid. They are added in an amount
. ~
~5953~
16
to give a A X/CrO3 ratio of from 0.05 to 0.3 by weight
wherein A x represents an acid residue. Ratios of less than
0.05 will result in a percent chromium fixed of less than
80% whereas ratios of more than 0.3 will result in poor
corrosion resistance.
(2) Resins
Preferred examples of the resins added to the chromate
solution include acrylic resins having an acid value of at
least 250 and acrylic resins having acrylic acid monomer
1G and/or methacrylic acid monomer added to stabilize them.
They are added in an amount to give a resin/CrO3 ratio
of from 0.1 to 20 by weight. Ratios of less than 0.1 will
result in a percent chromium fixed of less than 80% whereas
ratios of more than 20 will deteriorate the adherence of the
chromate film to the underlying substrate.
~3) Silica
Silica added to the chromate solution is preferably
colloidal silica. Silica is added in an amount to give a
SiO2/CrO3 ratio of from 0.3 to 3.0 by weight. Ratios of
less than 0.3 will result in a percent chromium fixed of
less than 80% whereas ratios of more than 3.0 will
deteriorate the adherence of the chromate film to the
underlying substrate.
When a steel strip carries a chromate film resulting
from the chromate solution having added the reducing agent
and the additive as defined above, the steel can maintain a
significantly high percent chromium fixed at the end of
alkaline degreasing and chemical conversion in the
automotive coating process. To demonstrate the maintenance
of high percent chromium fixed, an experiment was made us-ng
an immersion type alkaline degreasing solution commonly used
in the automotive coating process.
FIG. 2 graphically shows the percent chromium fixed as
a function of the amount of methanol added as the reducing
agent. It is seen that only the addition of methanol, that
~Z59532
17
is, reducing agent mostly results in a percent chromium
fixed of less than 80~. However, the RAH/CrO3 ratio range
from 0.2 to 1.9 gives a ratio of Cr6+/Cr3+ in the range of
from 80/20 to 20/80. Then, the percent chromium fixed can
be 80% or higher as seen from FIG. 1 by adding at least one
additive selected from (1) acid residues, (2) resins, and
(3) silica to the chromate solution while keeping the
maximum baking temperature within the range of from room
temperature to 150C.
FIG. 3 graphically shows the percent chromium fixed as
a function of the amount of phosphoric acid added as
producing an acid residue to the chromate solution having
methanol added as the reducing agent (RAH/CrO3=1.0). It is
seen that when the amount of phosphoric acid added is at
least 0.1 calculated as PO43 /CrO3, a percent chromium fixed
of 100% is advantageously achieved even at a ratio of
Cr6+/Cr3+ of 80/20. It is also seen that when the ratio of
Cr6+/Cr3+ is 20/80, a percent chromium fixed of 100% is
advantageously achieved even at a ratio of PO43 /CrO3 of
0.01. When phosphoric acid is added in an amount to give a
PO43 /CrO3 ratio of 0.3 or higher, the phosphoric acid due
to its non-volatile nature adversely affects the
subsequently applied and baked resin and hence, the
corrosion resistance of the product.
FIG. 4 graphically shows the percent chromium fixed as
a function of the amount of a resin added to the chromate
solution having methanol added as the reducing agent
(RAH/CrO3=1.0~. The resin used is an acidic acrylic resin.
The amount of resin added is expressed as a weight
ratio of resin solids/CrO3. It is seen that a resin
solids/CrO3 ratio in the range beween 0.1 and 20.0 is
effective in improving the percent chromium fixed. Such a
ratio of more than 20.0 adversely affects the adherence of
chromate film to the underlying substrate, and hence, the
workability and weldability of the product.
'
~25953~
18
FIG. 5 graphically shows the percent chromium fixed as
a function of the amount of silica added to the chromate
solution having methanol added as the reducing agent
(~AH/CrO3=1.0). The silica used is ultrafine particulate
silica anhydride.
The amount of silica added is expressed as a weight
ratio of SiO2/CrO3. It is seen that a SiO2/CrO3 ratio of at
least 0.3 is effective in achieving a percent chromium fixed
of 80~ or higher. Such a ratio of more than 3.0 adversely
affects the adherence of chromate film to the underlying
substrate, and hence, the weldability of the product.
The foregoing experimental results indicate that the
addition of a reducing agent in combination with (1) an acid
residue, (2) a resin or (3) silica to the chromate solution
is effective in increasing the percent chromium fixed at the
end of alkaline degreasing in the automotive coating
process. The additive effects of these agents are estimated
as follows.
The reducing agent such as methanol, aqueous hydrogen
peroxide and ethylene glycol is added to the chromate
solution. The reducing agent reduces chromic acid to lower
the Cr6+/Cr3+ ratio. The percent chromium fixed is then
increased because the proportion of hexavalent chromium
which is more liable to dissolve away is decreased.
The additives, (1) acid residue, (2) resin, and (3)
silica added to the chromate solution have the following
functions.
(1) Addition of acid residue to chromate solution
Two sets of samples were prepared by applying a
chromate solution having phosphoric acid residue added and
an acid residue-free chromate solution followed by baking.
Analysis of the samples from the acid residue-containing
chromate solution indicates peaks probably attributable to
the hydrate or hydroxide of trivalent chromium in addition
to the peaks of trivalent and hexavalent chromium. No
1~59~
1 9
substantial difference is observed in the proportion of
he~avalent chromium between the acid residue-introduced and
free solutions. When the chromate films are evaluated as
applied, the percent chromium fixed does not depend on the
introduction of acid residue. However, when a resin is
subsequently applied and baked to the chromate films, the
hydrate or hydroxide of trivalent chromium is firmly
attached to the resin, eventually preventing hexavalent
chromium from dissolving out. It has also been observed
that hexavalent chromium itself reacts with the resin to
reduce its quantity. These two mechanisms decrease the
dissolving out of hexavalent chromium.
(2) Addition of resin to chromate solution
When the chromate solution contains an acrylic resin
or a similar resin which is of acidic type and stable in the
chromate solution, the Cr6+/Cr3+ ratio of the solution
remains unchanged, but a bond is created between hexavelent
chromium and the resin at the end of baking to prevent the
hexavalent chromium from dissolving out.
(3) Addition of silica to chromate solution
When a chromate film is used alone without subsequent
resin coating, the percent chromium fixed decreases with the
increasing amount of silica added to the chromate solution.
Analysis of silica-containing chromate films indicates
more OH groups than in silica-free chromate films. The
distribution of silicon in the chromate film is also
determined to find that Si is concentrated in a surface
layer.
These indicate that silica forms a rigid film at the
surface layer of the chromate film in which the chromate
itself is present as the hydrate and hydroxide of trivalent
chromium. When a resin is subsequently applied and baked to
such a chromate film, the resin is firmly attached to the O~
group of the chromate film so that the resulting resin-
~2sss3~
coated steel strip is characte~ized by the controlled
dissolving-out of chromium.
EXAMPLES
Examples of the present invention are given below by
way of illustration and not by way of limitation.
Example 1
The composition of cold rolled steel strips having
bake hardenability used in a series of runs is shown in
Table 1 together with their mechanical properties.
Using bake hardenable steel strips, blank Nos. 1-6 as
shown in Table 1, test specimens were prepared by depositing
a zinc base alloy plating on the blank, subjecting the
plating to a chromate treatment, and then applying an
organic coating followed by baking. The treating procedure
is shown in Table 2.
The test specimens were determined for corrosion
resistance, weldability, workability, and bake
hardenability, with the results shown in Table 3.
The tests are conducted by the following procedures.
1. Corrosion Resistance
1-a) Salt spray test (SST)
A salt spray test was carried out by crosshatching the
organic coating on each specimen, and spraying a 5% NaCl
solution at 35C to the coating. The time was observed
until red rust generated.
1-b) Cycle corrosion test (CCT)
A cycle corrosion test was carried out by subjecting
each specimen to corrosion cycles each consistiny of
spraying of 5% NaCl at 35C for 4 hours, drying at 60C for
2 hours, and allowing to stand in wet conditions at 50C, RH
95% for 2 hours. The number of cycles was counted until red
rust generated.
2. Weldability
125953~
21
Two pieces of each specimen were partially overlapped
and subjected to continuous spot we]ding using an R type
electrode (40R) each under a compresslon force of 170 kg for
a welding period of 10 cycles (50Hz, 1/5 sec). The number
of permissible continuous welding spots was determined.
3. Workability
Each specimen in a disk form having a diameter of 90
mm was subjected to a cupping test by drawing to a cup shape
having a diameter of 50 mm and a depth of 25 mm (Blank Hold
1Q Force 1 Ton). An adhesive tape was applied to and removed
from the worked area to determine the removal of coating as
expressed in mg/circumference.
4. Bake Hardenability
Each specimen was pre-stressed 2% and then subjected
to a baking treatment at 170C for 20 minutes. The yield
strength (YS) of the specimen was measured to determine an
increase of YS in kgf¦mm2.
The data in Table 3 demonstrate that the organic
coated steel strips obtained according to the present
invention have high corrosion resistance, good weldability
and workability while maintaining satisfactory bake
hardenability.
~25~3~3'~
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Table 3
Corrosion
resistance Weld- Workability BH
Blank SST CCT ability (mg/circum- (kgf2
N _ (hour) (cycle) (spots) ference) /mm )
Invention A-l ~ >2500 >200 >1000 <1 4.5
A-2 ¦ >2500 >200 >1000 <1 4.5
A-3 ¦ >2500 >200 >1000 <1 4.5
A-4 ~ 1 >2500 >200 >1000 <1 4.5
A-5 ¦ >2500 >200 >1000 <1 4.5
A-6 ¦ >2500 >200 >1000 <1 4.5
A-7 1 >2500 >200 >1000 <1 4.5
A-8 >2000 >150 >1000 <1 4.5
A-9 >2000 >150 >1000 <1 4.5
A-10 >2000 >150 >1000 <1 4.5
A-ll 1 >2000 >150 >1000 <1 4.5
A-12 >2000 >150 >1000 <1 4.5
A-13 >2000 >150 >1000 <1 4.5
A-14 >2000 >150 >1000 <1 4.5
B-l ~ >2500 >200 >1000 <1 3.5
B-2 ~ 2 >2500 >200 >1000 <1 3.5
B-3 J >2500 >200 >1000 <1 3.5
C-l ~ >2500 >200 >1000 <1 4.6
C-2 ~3 >2500 >200 >1000 <1 4.6
C-3 J >2500 >200 >1000 <1 4.6
D-l ~ >2500 >200 >1000 <1 5.2
D-2 l >2500 >200 >1000 <1 5.2
D-3 ~ 4 >2500 >200 >1000 <1 5.2
D-4 J >2500 >200 >1000 <1 5.2
E-1 1 >2500 >200 >1000 <1 6.1
E-2 5 >2500 >200 >1000 <1 6.1
E-3 >2500 >200 >1000 <1 6.1
125953~
26
Table 3 (cont'd)
Corrosion
resistance Weld- Workability BH
Blank SST CCT ability ~mg/circum- (kYf2
No. (hour) (cycle) (spots) ference) /mm )
Invention F-l ~ >2500 >200 >1000 <1 5.0
F-2 6>2500>200 >1000 <1 5.0
F-3 >2500 >200 >1000 <1 5.0
Comparative G-l 1000 100 >1000 <1 4.5
runs G-2 1500 100 >1000 <1 4.5
G-3 1>2000>150 700 1-2 4.5
G-4 >2000 >150 500 1-2 4.5
G-5 >2000 >150 300 1-2 4.5
H 1>2000>150 500 60 0
I 1>2000>150 >1000 10 0
J 1>2000>150 >1000 5 0
~259~:i3':'
Example 2
The cold rolled steel strips having bake hardenability
used were the same as used in Example 1. Their compositions
are shown in Table 1 together with their mechanical
properties.
Using bake hardenable steel strips, blank ~os. 1-6 as
shown in Table 1, test specimens were prepared by depositing
a zinc base alloy plating on the blank, subjecting the
plating to a chromate treatment, and then applying an
organic coating followed by baking. The treating procedure
is shown in Table 4.
The test specimens were determined for corrosion
resistance, weldability, workability, bake hardenability,
and percent chromium fixed, with the results shown in Table
5.
The procedures for measuring the former four
properties are the same as in Example 1.
Percent chromium fixed was determined by degreasing a
specimen with a commonly used immersing alkaline degreasing
solution. Using fluorescent X-ray analysis, the number of
chromium counts was determined before and after the alkaline
degreasing.
The data in Table 5 demonstrate that the organic
coated steel strips produced according to the method of the
G5 present invention have high corrosion resistance, good
weldability and workability while maintaining satisfactory
bake hardenability and high percent chromium fixed.
~ ~J'353~
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1~59S32
31
Table 5
Corrosion
resistance Weld- Workability BH %
Blank SST CCT ability (mg/circum- (kgf2 Cr
No. (hour) (cycle) (spots) ference) /mm ) fixed
Invention A-l >2500 >200 >lO00 <l 4.6 >90
A-2 >2500 >200 >lO00 <l 4.7 >90
A-3 >2500 >200 >lO00 <l 4.3 >90
A-4 >2500 >200 >lO00 <l 4.4 >90
A-5 l >2500 >200 >lO00 <l 4.4 >90
A-6 >2500 >200 >lO00 <l 4.4 >90
A-7 >2500 >200 >lO00 <l 4.4 >90
A-8 >2500 >200 >lO00 <l 4.4 >90
A-9 , >2500 >200 >lO00 <l 4.4 >90
B-l ~ >2500 >200 >lO00 <l 4.7 >90
B-2 ¦ >2500 >200 >lO00 <l 4.5 >90
B-3 ~2 >2500 >200 >lO00 <l 4.7 >90
B-4 J >2500 >200 >lO00 <l 4.5 >90
C-l ~ I >2500 >200 >lO00 <l 4.5 >90
C-2 >2500 >200 >lO00 <l 4.5 >90
C-3 >2500 >200 >lO00 <l 4.5 >90
C-4 3 >2500 >200 >lO00 <l 4.3 >90
C-5 1 >2500 >200 >lO00 <l 4.5 >90
C-6 ~ >2500 >200 >lO00 <l 4.5 >90
D-l >2500 >200 >lO00 <l 4.5 >90
D-2 4 >2500 >200 >lO00 <l 4.5 >90
D-3 >2500 >200 >lO00 <l 4.5 >90
E-l 1 >2500 >200 >lO00 <l 4.7 >90
E-2 >2500 >200 >lO00 <l 4.7 >90
E-3 5 >2500 >200 >lO00 <l 4.7 >90
E-4 >2500 >200 >lO00 <l 4.5 >90
F-l 1 >2500 >200 >lO00 <l 4.5 >90
F-2 >2500 >200 >lO00 <l 4.5 >90
F-3 6 >2500 >200 >lO00 <l 4.7 >90
F-4 ~ >2500 >200 >lO00 <l 4.7 >90
~ ~5~'3S3~
32
Table 5 (cont'd)
Corrosion
resistance Weld- Workability BH %
Blank SST CCT ability (mg/circum- (kgf2 Cr
N _ (hour) (cycle) (spots) ference) /mm ) fixed
Comparative G-l~ >2500 >200 >1000 <1 4.7 50
runs G-2 1 >2500>200 >1000 <1 4.7 60
G-3 1>2500>200 >1000 <1 4.8 50
G-4 >2500>200 >1000 <1 4.7 50
H-l~ 1500 150 >1000 <1 4.5 >90
H-2 ¦ 1500 100 >1000 <1 4.3 >90
H-3 t2 >2500>200 400 2.0 4.5 >90
H-4 J >2500>200 400 1.5 4.0 >90
I-l~ ~
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I-3 3 - - gelled
I-4
