Note: Descriptions are shown in the official language in which they were submitted.
CA 02413521 2002-12-04
COMPOSITION FOR CONTROLLINIs SPANGLE SIZE,
A COATED STEEL PRODUCT, AND
A COATING METHOD
Field of the Invention
The present invention is directed to a coating composition, a coated steel
product, and a
method of making, and in particular, to an aluminum-zinc coating composition
employing
effective amounts of a particulate compound constituent to enhance tension
bend rust stain
performance and the appearance of the sheet when painted and reduce spangle
facet size.
Background Art
The coating of steel components with aluminum-based coating alloys, commonly
referred
to as hot dip coating, is well knov~m in the prior art. One particular type of
coating is
trademarked as Ga.lvalume~, which is owned by B:(EC International, Inc., and
is
representative of an aluminum-zinc coating alloy.
These materials are advantageous as building materials, particularly wall and
roof
construction due to their corrosion resistance, durability, heat reflection,
and paintability.
Typically, these materials are manufactured by passing a steel product such as
a sheet or plate
through a bath of a melted alloy coating composition comprising aluminum,
zinc, and silicon.
The amount of coating applied to the steel products is controlled by wiping,
and then the
products are cooled. One characteristic of the coating applied to the steel
product is its grain
size or spangle facet size.
U.S. Patent Nos. 3,343,930 to Borzillo et al., 5,049,202 to Willis et al. and
5,789,089 to Maki
et al. disclose methods and techniques for the manufacture of steel sheets
coated with these
aluminum-zinc alloys.
CA 02413521 2002-12-04
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European Patent Application No. 0 905270 A2 to Komatsu et al. discloses
another coating
process utilizing zinc, aluminum, and magnesium. This application is directed
at solving the
corrosion problems associated with baths containing magnesium as an alloying
element.
Further, it is disclosed that the undesirable stripe pattern occurring in
magnesium-containing
baths does not occur in baths without magnesium.
United States PatentNo. 5,571,566 to Cho discloses another method of
manufacturing coated
steel sheet using an aluminum-zinc-silicon alloy. The object of the Cho patent
is to provide
a more effcient production method for manufacturing coated steel sheet. Cho
meets this
object by uniformly minimizing the size of spangles by introducing a large
number of
spangle particles into the coating, which limits subsequent growth of the
spangles because
these particles interfere with their respective growth resulting in a smaller
spangle facet size.
The seed effect is achieved by using titanium as part of the molten coating
composition.
A similar disclosure with respect to the use of titanium in coating baths to
minimize spangle
facet size is disclosed in an article entitled "Minimization of Galvalume
Spangle facet size By
Titanium Addition To Coating Bath", by Cho, presented for the INTERZAC 94
Conference
in Canada in 1994. In this article, the author indicates that elements such as
titanium, boron,
and chromium produce finer spangles in a Galvalume coating, such a disclosure
consisted
with the disclosure of the Cho patent.
Notwithstanding the improvements suggested by Cho, presently used coated steel
products
still have disadvantages. One disadvantage is that, when the coated steel
product is to be
painted, a temper rolling is required to flatten the product in preparation
for painting. Another
problem is cracking when the product is a sheet and is bent. When this sheet
product is bent,
the coating can crack, the crack exposing the steel to the environment and
premature
corrosion. With presently available coated steel sheets, large cracks can
form, thereby
compromising the corrosion resistance of the sheet product.
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In light of the deficiencies in the prior art, a need has developed to provide
an aluminum-zinc
coated steel product with improved bending performance, reduced spangle facet
size, and
improved painted surface appearance. The present invention solves this need by
providing
a method of coating a steel product, a coating composition and a coated steel
article which,
when experiencing surface cracking during bending, is still corrosion
resistant and does not
require temper rolling when the coated steel product is painted. The coating
composition is
modified with one or more particulate compound constituents such as titanium
boride,
aluminum boride and the like.
Additionally, discussions with customers have led to the realization that
there is a long felt
need to develop a less expensive product that provides the metallic look of
high cost
aluminum, stainless steel, and other like metal materials used in
architectural applications, for
example, interior functional/decorative applications such as door kick plates,
counter back
splashes, cabinets, furniture, and the like. The present invention is capable
of solving this
need by providing a brushed 55% A1 and Zn alloy coated steel article that
provides the
appearance of a brushed stainless steel article or brushed aluminum article.
Prior to the
present invention, such a brushed metallic look was not available in a 55% Al
and Zn alloy
coated steel sheet product.
Summary of the Invention
Accordingly, it is a first object of the present invention to provide an
improved hot dip
coating composition for steel products.
Another object of the present invention is a method of coating a steel product
using a
modified aluminum-zinc coating alloy.
Still further objects of the present invention are to provide a coated steel
product with
enhanced tension bend rust stain performance and painted appearance.
CA 02413521 2002-12-04
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One other object of the present invention is a coated steel article employing
a modified
coating alloy composition.
Yet another object of the invention is a method of coating and then painting a
steel product,
whereby the coated steel product does not require temper rolling before
painting.
A still further object of the present invention is to provide a brushed Al-Zn
alloy coating
having an improved visual appearance as compared to brushed conventional
Galvalume
coatings.
A still further object of the present invention is to provide a brushed Al-Zn
alloy coated
product having an improved visual appearance capable of providing an
alternative materials
application for more expensive brushed aluminum or 'brushed stainless steel
material
applications.
Other objects and advantages of the present invention will become apparent as
a description
thereof proceeds.
In satisfaction of the foregoing objects and advantages, the present invention
is an
improvement in the art of hot dip coating of steel products using an aluminum-
zinc coating
alloy. The composition of the aluminum-zinc alloy is modified by adding an
effective amount
of one or more of a particulate compound constituent selected from the group
consisting of
boride compounds having one of titanium and aluminum, aluminide compounds
containing
titanium and iron, and carbide compounds containing titanium, vanadium,
tungsten, and iron.
Preferably, the constituent is one of TiC, TiB2, AIBz, A1B12, and TiAl3.
The constituent can be prepared in various ways as part of the modification
step, e.g., as part
of aprecursor or master alloy ingot or bath containing principally aluminum,
the master alloy
then added to an aluminum-zinc bath in the necessary proportions to arrive at
a final bath
CA 02413521 2002-12-04
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composition suitable for coating and providing the benefrts of the invention
as a result of the
modifier constituent. The constituent can be added to the master alloy as
particulate
compounds or can be formed in-situ in the master alloy to add to the actual
coating bath.
More particularly, the composition of the coating bath can be modified by: ( 1
) directly adding
the particles (as a powder) to the coating bath or a pre-melt pot which feeds
the coating bath;
(2) adding an ingot that contains the required particles; the ingot may be
aluminum with
particles, zinc with particles, a zinc-aluminum alloy with particles, etc.;
the ingot may be
added to a main coating pot or a pre-melt pot; (3 ) adding molten bath
containing the required
particles, wherein the liquid may be aluminum with particles, zinc with
particles, a zinc-
aluminum alloy with particles, etc.; (4) in-situ reaction in the main pot or
pre-melt pot, for
example by the reaction of elemental species, such as titanium and boron in an
aluminum
feed melt, or the reaction of salts on the feed melt pot to produce particles.
The particle size of the constituent in the coating bath can vary but
preferably ranges from
about 0.01 and 25 microns. When practicing the invention, a spangle facet size
of a coated
product can range as low as 0.05 mm and up to 2.0 mm.
The effective amount of the constituent is considered to be that amount which
reduces the
spangle facet size of the coated product, causes an increase in the number of
cracks while
maintaining a smaller crack size than conventional aluminum-zinc coated
products, and does
not require temper rolling when painting. An overall weight percentage range
of the
constituent, boride, carbide, or aluminide, based on the alloy bath is
believed to be between
about 0.0005 and ~.5%. When the constituent is a boride, a preferred weight
percentage of
the constituent as part of the coating bath can range between about 0.041 and
0.5%. When
the constituent is a carbide, a preferred weight percentage can range between
about 0.0005
and 0.01 %.
CA 02413521 2002-12-04
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The invention also provides a coated steel article employing a coating
containing the
particulate compound constituent as well as the coating composition as applied
to the steel
product. The product is preferably a steel sheet or plate for construction
purposes.
S Brief Descri,~tion of the Drawings
Reference is now made to the drawings of the invention wherein:
Figure 1 is a graph comparing the use of titanium boride a>nd titanium as melt
additives for
hot dip coating in terms of spangle facet size and titanium. content.
Figure 2 is a graph comparing the use of titanium boride and aluminum boride
as melt
additives for hot dip coating in terms of spangle facet size and boron
content.
Figure 3 is a graph comparing the use of titanium carbide as a melt additive
for hot dip
coating in terms of spangle facet size and carbon content.
Figure 4 is a graph showing bend test result comparisons for coating
compositions modified
with titanium and titanium boride.
Figure 5 is a graph comparing crack area and number of cracks for a coating
composition
containing titanium boride and a conventional coated steel product.
Figure 6a-6c are photomicrographs showing spangle facet size for a
conventionally coated
product and a TiB2-modified product.
Figure 7a-7c are photomicrographs showing spangle facet size for a
conventionally coated
product with and without titanium.
CA 02413521 2002-12-04
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Figure 8a-8c are photomicrographs showing spangle facet size for a
conventionally coated
product and a TiC-modified product.
Figure 9a-9c are photomicrographs showing spangle facet size for a
conventionally coated
product and an Al2-Al, ~ modified product.
Description of the Preferred Embodiments
The present invention advances the art of hot dipping or coating steel
products, particularly
plate and sheet products, using an aluminum-zinc molten alloy bath, e.g., a
Galvalume bath.
According to the invention, the coating bath is modified with particulate
compound
constituents to reduce the spangle facet size of the coated steel product.
With the addition
of the particulate constituents, improvements may also be realized in the
performance of the
coated steel product in terms of tension bend rust staining. Tension bend rust
staining is a
1 ~ discrete pattern of cosmetic red rust running along the rib of a
prepainted, roll formed,
building panel caused by cracking of the metallic coating and paint.
The surface of the coated steel product also yields a painted appearance that
is superior to
conventional Galvalume product. This is believed to allow for the production
of smooth
coated steel sheet product without the need for temper rolling. Eliminating
the extra
processing step of temper rolling also reduces energy consumption, eliminates
possible waste
streams associated with temper rolling, and simplifies the x>roduction
process.
In its broadest embodiments, the invention entails a novel composition for a
coating of steel
product, a method of making such a coating, and the article made from such
method.
When coating steel products with an aluminum-zinc coating bath, the processing
steps of
forming the bath to the desired composition and passing the steel product to
be coated
through the bath are well known. As a result, a further description of the
prior art methods
CA 02413521 2002-12-04
- g
and apparatus to accomplish this conventional coating is not deemed necessary
for
understanding of the invention.
The composition of the prior art aluminum-zinc alloy baths is well known as
discussed in the
Borzillo et al. and Cho patents, and the Cho publication noted above.
Generally, this bath
comprises about 55% aluminum, a level of silicon, generally about 1.6% by
weight, and the
balance zinc. Other variations in the composition are within the scope of the
invention as
would be conventionally known to those of ordinary skill in the art.
According to the invention, the aluminum-zinc molten bath is modified with a
particulate
compound constituent to achieve improvements in terms of reduced spangle facet
size,
improved surface finish, reduction in crack size, and potential improvements
in tension bend
rust staining. The particulate compound constituent can be a boride, carbide,
or aluminide.
Preferably, the boride compounds include titanium boride (TiB2), and aluminum
boride (A1B?
and AlBlz). The particulate compound constituent as a carbide can be titanium
carbide,
vanadium carbide, tungsten carbide, and iron carbide, and as an aluminide,
titanium
aluminide (TiAI;) and iron aluminide. The level of the particulate compound
constituent is
set as an amount to effectively reduce the spangle facet size over that of
conventional
coatings, with or without elemental titanium. While the effective amount may
vary depending
on which compound is selected, it is anticipated that the amount would range
from about
0.0005% to about 3.5% by weight of the carbon, boron, or aluminide of the
composition of
the coating bath. For carbon, a more preferred range is between about 0.005%
and 0.10% by
weight of the bath. In terms of titanium concentration, a titanium boride
containing coating
melt bath could have a titanium concentration between about 0.001 % and 0.1 %
by weight
of the bath. For the boride compound, the boron weight percentage in the bath
can range
from 0.001% to 0.5% by weight.
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Table 1 shows broad claimed ranges for the particle additions if only a single
type of particle
is added:
TABLE 1
Coating Wt.% Particle
Bath in the Melt
Composition
(Wt.%)
Nominally
55%Al-1.6%Si-bal.
Zn
Ti B C
TiB, 0.002-1.0 0.001-0.5 -- 0.007-3
.5
A1B, -- 0.001-0.5 -- 0.010-5.0
AIB,, -- 0.001-0.5 -- 0.005-2.5
TiC 0.0019-1.9 -- 0.0005-0.5 0.0025-2.5
For example, for 100g of melt, the amount of TiB2 particle addition should be
0.007-3.5
grams.
The values in Table I assume stoichiometric additions. Excess Ti (in the case
of TiC or TiB2)
is permissible, but not necessary.
Table 2 shows preferred ranges or optimal ranges for the particle additions:
TABLE 2
ParticleCoating Bath Wt.% Particles
Type Composition in the Melt
(Wt.%)
Nominally 55%Al-1.6%Si-bal.
Zn
Ti B C
TiB2 0.01-0.05 0.002-0.1 -- 0.014-0.7
AIB~ -- 0.02-0.05 -- 0.2-0.5
AlB 12 0.02-0.05 -- 0.2-0.5
TiC 0.011-0.38 -- 0.003-0.1 0.015-0.5
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The particle size of the particulate constituent should range between about
0.01 and about
25 microns. By coating a steel product using the inventive method, spangle
facet sizes are
produced which range from as low as 0.05 up to 2.0 mm.
The molten bath used to coat this steel product containing the modified
aluminum-zinc alloy
composition can be prepared in a number of ways. In one method, a master alloy
of
aluminum is prepared and is modified with the particulate compound
constituent. This bath
is then added to an aluminum-zinc coating bath, the proportions of the two
baths calculated
to arrive at a target bath composition containing the effective amount of the
particulate
compound constituent. The modified alloy bath would still track the
conventional weight
percentages of the aluminum, zinc and silicon for these types of coating
baths, e.g., about
55% aluminum, 1-2% silicon, the balance zinc, since the effective amount of
the particular
compound constituent is a relatively low weight percentage of the overall bath
amount.
Methods formaking master alloys are taught in United States Patent Nos.
5,415,708 to Young
et al. and 3,785,807.
Secondly, the master alloy containing the particles could be added to the
coating bath in the
form of a solid ingot. The ingot may be primarily Al, primarily Zn, or an
alloy containing Zn,
Al, and/or Si along with the spangle refining particles.
Alternatively, the particulate compound constituents could be added directly
to the
aluminum-zinc bath prior to coating a steel product.
When using aluminum boride as a bath modifier, boron particles can be added to
an
aluminum master alloy to facilitate incorporation of the particles into the
melt and improve
even distribution of the particles throughout the melt. Alternatively,
aluminum boride
particles can be added to the aluminum-zinc bath in the appropriate amounts.
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When producing an aluminum master alloy with the particulate compound
constituents such
as titanium boride, some excess titanium may exist in the bath. This excess
may range from
0.01 % to 10% relative to the total mass of boron added. In terms of the
stoichiometry,
titanium additions in excess of one mole of titanium for 2 moles of boron may
range from
0.002 to 4.5 excess moles. It is not believed that the excess titanium,
whether present through
the use of titanium boride or another titanium-containing compound such as
titanium carbide
or the like, is necessary to obtain the spangle refinement associated with the
invention.
In preparing the alloy bath for coating, the particulate compound constituent
can be
l 0 introduced as a powder or formed in the bath itself. For example, titanium
boride powders
could be added to an aluminum bath in the appropriate weight percentages.
Alternatively,
elemental titanium and boron could be added to an aluminum melt and heated at
sufficiently
high temperatures to form titanium boride particles therein. It is preferred
that the compound
particles be added to the master alloy since this processing is much more
effective in terms
of energy consumption. Similar processing techniques can be employed for the
caxbides and
aluminides.
It is believed that the presence of titanium and boron in a coating bath alone
will not produce
the grain refining benefits demonstrated above as compared to adding a
compound
particulate such as titanium boride. It has been reported that in aluminum
casting, the
separate addition of titanium and boron to an aluminum melt did not produce
titanium boride
particles when added at temperatures below 1000°C (1832°F).
Instead, the titanium reacted
with the aluminum to form TiAI; particles. Since the coating process is
generally conducted
at much lower temperatures, i.e., 593°C (1100°F), adding
titanium and boron in elemental
form to an Al-Zn coating bath would produce similar behavior. In addition, the
kinetics of
titanium and boron dissolution will be very slow at the low temperatures
associated with the
coating method. Thus, when forming the titanium boride in the bath itself, it
is necessary to
go beyond conventional melting parameters to achieve t;he necessary
particulate for use in
the invention.
CA 02413521 2002-12-04
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The inventive coating method produces a coated article, wherein the coating
has a coating
composition including the added particulate compound constituent described
above. The
coated product can then be painted as is known in the art without the need for
temper rolling
or skin passing.
While titanium and aluminum borides, and titanium aluminide have been
exemplified as
spangle refiners, other carbides, such as vanadium carbide, tungsten carbide,
iron carbide,
and aluminum compounds such as iron aluminide, are also believed to be within
the scope
of the invention.
In order to demonstrate the unexpected benefits associated with the invention,
studies were
done comparing coated steel products using an aluminum titanium master alloy
and an
aluminum titanium boride master alloy. These master alloys were added to the
aluminum-
zinc coating alloys to form a coating bath for the steel to be tested. Figure
1 compares two
curves based on the master alloys noted above, the curves relating spangle
facet size and the
titanium content of the melt in weight percent. As is evident from Figure 1,
the use of a
master alloy with titanium boride significantly refines the: spangle facet
size, particularly at
much lower additional levels of titanium. For example, at a titanium content
of 0.02% by
weight, the reported spangle facet size is about 0.3 mm as compared to a
spangle facet size
of 1.4 mm when only titanium is used. Thus, not only does the boride modifier
reduce
spangle facet size, it also reduces cost by lowering the amount of titanium
needed.
Figure 2 shows a similar comparison between a master alloy containing titanium
boride and
a master alloy of aluminum and boron. Figure 2 shows that the titanium boride
refiner
achieves a smaller spangle facet size for boron levels up to about 0.03% by
weight, when
compared to a master alloy of just aluminum and boron. However, when comparing
Figures
l and 2, the use of an aluminum boride particulate compound constituent to
reduce spangle
facet size is more effective than just titanium.
CA 02413521 2002-12-04
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Figure 3 shows a graph exhibiting behavior for a coating composition modified
with titanium
carbide that is similar to the TiB2-modified coating of Figure 1.
Besides minimizing the spangle facet size, the use of the particulate compound
constituent
according to the invention also allows the coated steel product to tolerate
more severe
bending without cracking. Referring now to Figure 4, a comparison is made
between
products coated with a coating bath alloy composition. employing just titanium
and one
employing 0.05% weight titanium boride. The spangle facet size is decreased
from 1.5 mm
to 0.1 mm when titanium boride is used. When the coated products are subjected
to conical
bend tests, the coating thickness of the product was plotted against the
radius at which no
crack occurred. Conical bend tests are tests that generally follow ASTM D522-
93a. The
product employing titanium boride as a particulate compound constituent in the
coating bath
decreased the no-crack radius by 23%.
Another unexpected result associated with the invention is the formation of
more numerous
but small cracks during bending as compared to conventional aluminum-zinc
alloy coatings
of sheet product. Referring to Figure 5, it can be seen that the titanium
boride-modified
aluminum zinc coated steel product has a significantly higher number of cracks
than
conventional aluminum zinc. However, the conventional product has a
significantly increased
crack area as compared to the titanium boride modified product. The smaller
but more
uniformly distributed cracks of the invention promote crack bridging by paint
films. This
bridging then facilitates choking off of corrosion products quicker than the
larger cracks
associated with conventional aluminum zinc coatings would. Thus, the titanium
boride-
coated product would exhibit improved corrosion resistance over prior art
products.
The graph of Figure 5 was based on bending a coated sample on a 1 /16"
cylindrical bend. The
size of the cracks were measured after bending and a 19.71 square millimeter
surface portion
was examined for the number of cracks and their size. The maximum crack size
in the
inventive product is less than half (41 %) of the size of the maximum crack
size in the
CA 02413521 2002-12-04
-14-
conventional product. This behavior is beneficial in preventing or reducing
tension bend rust
staining, where it is thought that the size of the worst cracks are what
control the tension
bend rust staining behavior of a coating.
Another equally important attribute of the invention is the surface quality of
the inventive
coated steel product and its improved suitability for painting. Table 3 shows
profilometry
results for a number of conventionally aluminum-zinc coated products and
products coated
with the titanium boride modified aluminum zinc alloy. The conventional
product is noted
as a Galvalume coating in Table 3. This table shows that the surface waviness
(WCA) of the
coated product of the invention is substantially lower than the as-coated and
temper rolled
conventional Galvalume product. The average waviness ofthe as-coated and
titanium boride-
modified sheet is 67% better than the as-coated regular Galvalume product
produced under
identical conditions. The minimal spangle Galvalurne waviness with the product
of the
invention is 50% better than the larger spangle mill produced temper rolled
Galvalume. The
titanium boride-modified minimum spangle Galvalume does not require temper
rolling to
reduce waviness, and is ideal for high speed coil coating applications. The
appearance of the
painted product is superior to large spangled as-coated and skin-passed
Galvalume.
TABLE 3
Profilometry Results For A Number Of Conventional Galvalume Coatings And TiB2,
Modified Minimum Spangle Ga:lvalume
Coating Surface ID/
Process/Line Condition Ra(win) Rt(~in)W~a(~,in)PC(ppi)
Galvalume w/TiB, As-coated 24.3 273.4 15.9 167
Master Alloy
Pilot Line ConventionalAs-coated 16.7 196.1 48.4 58.0
Galvalume
Average Mill ProducedAs-coated 21.6 271.2 61.3 97.5
Galvalume
Temper Rolled47.3 354.9 39.6 153.5
CA 02413521 2002-12-04
~15-
Figures 6A-9C compare the invention to the prior art and demonstrate the
reduction in
spangle facet size. Figures CA-6C show the effect of TiB2 added in the form of
an Al-5%Ti-1
%B master alloy, wherein a significant refinement of spangle facet size is
achieved as
compared to conventional Galvalume coatings. Similar reductions in spangle
facet size are
shown in Figures 8A-8C and 9A-9C when titanium carbide and aluminum borides
are used
as modifiers. Most importantly, when comparing Figures 6A-6C and Figure 7A-7C,
particularly, Figures 6C and 7C, the addition of titanium alone does not
produce the same
spangle facet size reduction. In fact, the presence of titanium alone as
compared to TiB2 only
marginally decreases spangle facet size.
If boride additions fall below a specific concentration range, the appearance
of the spangle
size in the hot-dip coating becomes non-uniform and inconsistent within the
same coil as
well as from coil to coil. On the other hand, when bo:ride additions are
greater than the
specific range, spangle size is no longer visible to the naked eye.
Additionally, at the lower
boride concentration levels, below the specific range, the small additions to
the hot-dip bath
are difficult to measure and control, adding to the problem of inconsistency
in spangle size.
In certain instances, visual spangle size is desirable in Galvalume like hot-
dip coated
products. Such visibly spangled products are widely used. in large
construction applications,
for example, roofing and siding in large industrial and agricultural type
structures. However,
customers view inconsistent spangle size as a coating quality problem as well
as an aesthetic
problem. Variation in spangle size manifests itself as a non-uniform
appearance from panel
to panel on the roof or sides of a building, which in turn is objectionable to
the building
owner.
A more uniform, consistent spangle size may be produced by adding a small
amount of TiB2
grain refiner to the hot-dip coating bath. By making bath additions of between
about 0.0008-
0.0012% by weight boron in the form of boride particles to the bath we are
able to produce
a consistent spangle facet size of between about 400 to S00 microns (measured
using the
CA 02413521 2002-12-04
-16-
mean intercept length method described in ASTM E 112). Producers and customers
consider
such controlled spangle size products superior in visual appearance as
compared to a
conventional spangle aluminum-zinc coated products where boride additions fall
outside the
specified range.
As such, an invention has been disclosed in terms of preferred embodiments
thereof which
fulfills each and every one of the objects of the present invention as set
forth above and
provides new and improved coated steel product, a method of making and a
coating
composition therefore.
Of course, various changes, modifications, and alterations from the teachings
of the present
invention may be contemplated by those skilled in the art without departing
from the
intended spirit and scope thereof. It is intended that the present invention
only be limited by
the terms of the appended claims.
20
