The present invention relates to the production
of metal strip, typically steel strip, which has a
corrosion-resistant metal alloy coating that contains
aluminium, zinc, silicon, and magnesium as the main
elements in the alloy, and is hereinafter referred to as
an "Al-Zn-Si-Mg alloy" on this basis.
In particular, the present invention relates to a
hot-dip metal coating method of forming an Al-Zn-Si-Mg
alloy coating on a strip that includes dipping uncoated
strip into a bath of molten Al-Zn-Si-Mg alloy and forming
a coating of the alloy on the strip.
Typically, the Al-Zn-Si-Mg alloy of the present
invention comprises the following ranges in % by weight of
the elements Al, Zn, Si, and Mg:
Zn: 30 to 60%
Si : 0 .3 to 3%
Mg: 0.3 to 10%
Balance : Al and unavoidable impurities .
More typically, the Al-Zn-Si-Mg alloy of the
present invention comprises the following ranges in % by
weight of the elements Al, Zn, Si, and Mg:
Zn: 35 to 50%
Si: 1.2 to 2.5%
Mg: 1.0 to 3.0%
Balance : Al and unavoidable impurities .
The Al-Zn-Si-Mg alloy coating may contain other
elements that are present as deliberate alloying additions
or as unavoidable impurities. Hence, the phrase "Al-Zn-
Si-Mg alloy" is understood herein to cover alloys that
contain such other elements as deliberate alloying
additions or as unavoidable impurities. The other
elements may include by way of example any one or more of
Ca, Ti, Fe, Sr, Cr, and V .
Depending on the end-use application, the metalcoated
strip may be painted, for example with a polymeric
paint, on one or both surfaces of the strip. In this
regard, the metal-coated strip may be sold as an end
product itself or may have a paint coating applied to one
or both surfaces and be sold as a painted end product.
The present invention relates particularly but
not exclusively to steel strip that is coated with the
above -described Al-Zn-Si-Mg alloy and is optionally coated
with a paint and thereafter is cold formed (e.g. by roll
forming) into an end-use product, such as building
products (e.g. profiled wall and roofing sheets).
BACKGROUND TO THE INVENTION
One corrosion resistant metal coating composition
that is used widely in Australia and elsewhere for
building products, particularly profiled wall and roofing
sheets, is a 55% by weight Al-Zn coating composition that
also comprises Si. It is noted that, unless otherwise
stated, all references to percentages are references to
percentages by weight.
The profiled sheets are usually manufactured by
cold forming painted, metal alloy coated strip.
Typically, the profiled sheets are manufactured by rollforming
the painted strip.
The microstructure of coatings of the coating
composition on profiled sheets typically comprises Al-rich
dendrites and Zn-rich interdendritic channels .
The addition of Mg to this known composition of
55%Al-Zn-Si coating composition has been proposed in the
patent literature for a number of years , see for example
US patent 6,635,359 in the name of Nippon Steel
Corporation, but Al-Zn-Si-Mg coatings on steel strip are
not commercially available in Australia.
It has been established that when Mg is included
in a 55%A1-Zn-Si coating composition, Mg brings about
certain beneficial effects on product performance, such as
improved cut-edge protection.
The applicant has carried out extensive research
and development work in relation to Al-Zn-Si-Mg alloy
coatings on strip such as steel strip which has included
plant trials . The present invention is the result of part
of this research and development work.
During the course of plant trials, the applicant
noticed a defect on the surface of Al-Zn-Si-Mg alloy
coated steel strip. The plant trials were carried out
with an Al-Zn-Si-Mg alloy having the following
composition, in wt. % : 53Al-43Zn-2Mg-1.5Si-0.45Fe and
incidental impurities . The applicant was surprised that
the defect occurred. The applicant had not observed the
defect in extensive laboratory work on Al-Zn-Si-Mg alloy
coatings. Moreover, since noticing the defect in plant
trials , the applicant has not been able to reproduce the
defect in the laboratory. The applicant has not observed
the defect on standard 55%A1-Zn alloy coated steel strip
that has been available commercially in Australia and
elsewhere for many years .
The applicant has found that the defect has a
number of different forms, including streaks, patches, and
a wood grain pattern. The defect is described internally
by the applicant as an "ash" mark.
A severe example of the defect is shown in Figure
1 , which is a photograph of a part of the surface of an
Al-Zn-Si-Mg alloy coated steel strip from the plant trials
captured under outdoor viewing conditions - low angle in
direct sunlight. In Figure 1 the defect manifests itself
as darker areas taking a number of shapes . In this example
the ash mark defect appears as (a) a patch (a well-defined
area that is uniformly darker than the surrounding area) ,
(b) a streak (a narrow area extending along the length of
the strip which is darker than the surrounding area) and
(c) a wood grain pattern (an area extending along the
length of the strip, with clear darker lines and lighter
lines between the darker lines . i.e. similar to a wood
grain) , on the coated steel strip surface when viewed at
low viewing angles under "optimum" lighting. The
applicant has found that as the viewing angle increases
towards the perpendicular, the visual distinction of the
defect rapidly decreases until it can no longer be seen,
with no obvious coating artefacts present at the surface,
e.g. metal spots, dross or spangle variation.
The applicant has found that the defect is not
confined to the morphologies shown in Figure 1 and can be
other configurations of darker areas .
The defect is a concern to the applicant from the
viewpoint of the aesthetic appearance of coated strip.
This is a very important issue commercially.
The above discussion is not to be taken as an
admission of the common general knowledge in Australia and
elsewhere.
SUMMARY OF THE INVENTION
The applicant has found that the above-described
ash mark defect is due to variations in the Al/Zn ratio on
the surface of Al-Zn-Si-Mg alloy coatings, specifically, a
decrease in the surface Al/Zn ratio within the defect
area, owing to an increased average width of Zn-rich
interdendritic channels on the surface of the coatings .
The applicant has observed that the variations in
Al/Zn ratio that are relevant to the defect are in, but
not necessarily limited to the outermost 1-2mm of the
coating cross section.
The applicant has also found that the defect is
most easily detected by elemental mapping of the defect
boundary with an electron probe microanalyser
According to the present invention there is
provided a method of forming a coating of an Al-Zn-Si-Mgbased
alloy on a substrate , such as although not limited
to a steel strip, that is characterised by controlling
conditions in (a) a bath containing the Al-Zn-Si-Mg-based
alloy for coating the substrate and (b) downstream of the
molten coating bath so that there is a uniform Al/Zn ratio
across the surface of the coating formed on the substrate.
The term "uniform" in the context of the Al/Zn
ratio is understood herein to mean a variation of
typically less than 0.1 in the Al/Zn ratio between any two
or more independent 1 mm x 1 mm areas as measured by
Energy Dispersive X-Ray Spectroscopy (EDS) .
Notwithstanding the aforementioned Al/Zn ratio variation
limit, the suitability of the coating for commercial use
and hence the meaning of the word "uniform" is defined by
the visual surface appearance under optimum lighting
conditions .
According to the present invention there is
provided a method of forming an Al-Zn-Si-Mg alloy coating
on a steel strip to form the above-described Al-Zn-Mg-Si
coated steel strip, the method including dipping steel
strip into a bath of molten Al-Zn-Si-Mg alloy and forming
a coating of the alloy on exposed surfaces of the steel
strip, and the method including controlling conditions in
the molten coating bath and downstream of the coating bath
so that there is a uniform Al/Zn ratio across the surface
of the coating formed on the steel strip.
Whilst not wishing to be bound to the following
comment, the applicant believes that the defect may be due
to a non-uniform surface/sub-surface distribution of Mg2Si
in the microstructure of the coatings. The applicant has
observed an increased nucleation rate of Mg2Si in the lower
half of the coating cross section within the defect
region .
The method may include controlling any suitable
conditions in the molten coating bath and downstream of
the coating bath.
By way of example, the method may include
controlling any one or more of the composition of the
molten coating bath, and the rate of cooling the coated
steel strip after the coated steel strip leaves the molten
coating bath.
Typically, the method includes controlling the Ca
concentration of the molten coating bath.
Typically, the Ca concentration of the molten
coating bath is determined by a generally standard
practice in the industry of taking coating bath samples
and analysing the samples by any one of a number of known
analysis options such as XRF and ICP, with measurement
tolerances typically of plus/minus 10 ppm.
The method may include controlling the Ca
concentration to be at least 100 ppm.
The method may include controlling the Ca
concentration to be at least 120 ppm.
The method may include controlling the Ca
concentration to be less than 200 ppm.
The method may include controlling the Ca
concentration to be less than 180 ppm.
The Ca concentration may be any other suitable
concentration range .
Typically, the method includes controlling the Mg
concentration of the molten coating bath.
Typically, the Mg concentration of the molten
coating bath is determined by a generally standard
practice in the industry of taking coating bath samples
and analysing the samples by any one of a number of known
analysis options such as XRF and ICP, with measurement
tolerances typically of plus/minus 10 ppm.
The method may include controlling the Mg
concentration to be at least 0.3%.
The method may include controlling the Mg
concentration to be at least 1.8%.
The method may include controlling the Mg
concentration to be at least 1.9%.
The method may include controlling the Mg
concentration to be at least 2%.
The method may include controlling the Mg
concentration to be at least 2.1%.
The Mg concentration may be any other suitable
concentration range .
The method may include controlling the postcoating
bath cooling rate to be less than 40°C/s while the
coated strip temperature is in the temperature range 400 °C
to 510°C.
The applicant has found that, for the coating
alloy compositions tested, the coating temperature range
of 400°C to 510°C is significant and that cooling quickly
in this range is undesirable due to accentuating
variations in the Al/Zn ratio to the extent that the
differences become visually apparent as the ash mark
defect. The selection of the cooling rate to be less than
40°C/s within this temperature range is based on
minimising accentuating variations in the Al/Zn ratio.
The applicant has also found that coating
temperatures below 400°C have no significant impact on the
Al/Zn ratio at the surface of a coating.
The applicant has also found that temperatures
above 510°C have no significant impact on the uniformity
of Al/Zn ratio.
It is emphasised that, in any given situation,
the limits of the significant temperature range will be
dependent on the coating alloy composition and the
invention is not necessarily confined to the coating
temperature range of 400°C to 510°C.
The method may include controlling the postcoating
bath cooling rate to be less than 35°C/s while the
coated strip temperature is in the temperature range 400 °C
to 510°C.
The method may include controlling the postcoating
bath cooling rate to be greater than 10°C/s in the
temperature range 400°C to 510°C.
The method may include controlling the postcoating
bath cooling rate to be greater than 15°C/s in the
temperature range 400°C to 510°C.
Typically, the cooling rate of coated strip is
controlled via a computerised model .
The applicant believes that the selection of any
one or more than one of Ca concentration, Mg concentration
and post-coating bath cooling rate is independent of
coating mass .
In general terms , the invention appears to be
independent of coating mass .
Typically, the coating mass is 50-200 g/m2 .
The Al-Zn-Si-Mg alloy may comprise more than 1.8%
by weight Mg.
The Al-Zn-Si-Mg alloy may comprise more than
1.9% Mg.
The Al-Zn-Si-Mg alloy may comprise more than
2% Mg.
The Al-Zn-Si-Mg alloy may comprise more than
2.1% Mg.
The Al-Zn-Si-Mg alloy may include less than
3% Mg.
The Al-Zn-Si-Mg alloy may include less than
2.5% Mg.
The Al-Zn-Si-Mg alloy may include more than
1.2% Si.
The Al-Zn-Si-Mg alloy may include less than
2.5% Si.
The Al-Zn-Si-Mg alloy may include the following
ranges in % by weight of the elements Al, Zn, Si, and Mg:
Zn: 30 to 60%
Si: 0.3 to 3%
Mg: 0.3 to 10%
Balance : Al and unavoidable impurities .
In particular, the Al-Zn-Si-Mg alloy may include
the following ranges in % by weight of the elements Al ,
Zn, Si, and Mg:
Zn: 35 to 50%
Si: 1.2 to 2.5%
Mg: 1.0 to 3.0%
Balance : Al and unavoidable impurities .
The steel may be a low carbon steel.
According to the present invention there is also
provided an Al-Zn-Mg-Si coated steel strip produced by the
above-described method.
According to the present invention there is also
provided an Al-Zn-Mg-Si coated steel strip that includes a
uniform Al/Zn ratio on the surface of the Al-Zn-Si-Mg
alloy coating.
According to the present invention there is also
provided an Al-Zn-Mg-Si coated steel strip that includes a
uniform Al/Zn ratio on the surface or the outermost 1-2 mm
of the Al-Zn-Si-Mg alloy coating.
According to the present invention there is also
provided a profiled wall and roofing sheet that has been
roll formed or press formed or otherwise formed from the
above-described Al-Zn-Mg-Si coated steel strip.
DESCRIPTION OF DRAWINGS
The present invention is described further by
way of example with reference to the accompanying drawings
of which:
Figure 1 is the above-described photograph of
part of the surface of the Al-Zn-Si-Mg alloy coated steel
strip from the plant trials captured under ideal viewing
conditions; and
Figure 2 is a schematic drawing of one embodiment
of a continuous production line for producing steel strip
coated with an Al-Zn-Si-Mg alloy in accordance with the
method of the present invention.
DESCRIPTION OF EMBODIMENT OF THE INVENTION
With reference to Figure 2 , in use, coils of
cold-rolled low carbon steel strip are uncoiled at an
uncoiling station 1 and successive uncoiled lengths of
strip are welded end to end by a welder 2 and form a
continuous length of strip.
The strip is then passed successively through an
accumulator 3 , a strip cleaning section 4 and a furnace
assembly 5 . The furnace assembly 5 includes a preheater,
a pre-heat reducing furnace, and a reducing furnace.
The strip is heat treated in the furnace assembly
5 by careful control of process variables including: (i)
the temperature profile in the furnaces, (ii) the reducing
gas concentration in the furnaces, (iii) the gas flow rate
through the furnaces, and (iv) strip residence time in the
furnaces (i.e. line speed).
The process variables in the furnace assembly 5
are controlled so that there is removal of iron oxide
residues from the surface of the strip and removal of
residual oils and iron fines from the surface of the
strip.
The heat treated strip is then passed via an
outlet snout downwardly into and through a molten bath
containing an Al-Zn-Si-Mg alloy having a Ca concentration
in a range of 100-200 ppm in a coating pot 6 and is coated
with Al-Zn-Si-Mg alloy. The Al-Zn-Si-Mg alloy is
maintained molten in the coating pot at a selected
temperature in a range of 595-610°C by use of heating
inductors (not shown) . Within the bath the strip passes
around a sink roll and is taken upwardly out of the bath.
The line speed is selected to provide a selected immersion
time of strip in the coating bath to produce a coating
having a coating mass of 50-200 g/m 2 on both surfaces of
the strip.
After leaving the coating bath 6 the strip passes
vertically through a gas wiping station (not shown) at
which its coated surfaces are subjected to jets of wiping
gas to control the thickness of the coating.
The coated strip is then passed through a cooling
section 7 and subjected to forced cooling at a selected
cooling rate greater than 10°C/s but less than 40°C/s
while the coated strip temperature is between 400 °C and
510°C. The cooling rate may be any suitable cooling rate
at coated strip temperatures less than 400 °C or greater
than 510°C.
The cooled, coated strip is then passed through a
rolling section 8 that conditions the surface of the
coated strip.
The coated strip is thereafter coiled at a
coiling station 10.
As discussed above, the applicant has conducted
extensive research and development work in relation to Al-
Zn-Si-Mg alloy coatings on steel strip which includes
plant trials and the applicant noticed a defect on the
surface of Al-Zn-Si-Mg alloy coated steel strip during
plant trials . The plant trials were carried out with an
Al-Zn-Si-Mg alloy having the following composition, in wt.
% : 53Al-43Zn-2Mg-1.5Si-0.45Fe and incidental impurities.
The applicant was surprised that the defect occurred. The
applicant had not observed the defect in extensive
laboratory work on Al-Zn-Si-Mg alloy coatings. Moreover,
since noticing the defect in plant trials, the applicant
has not been able to reproduce the defect in the
laboratory. The applicant has not observed the defect on
standard 55%Al-Zn alloy coated steel strip that has been
available commercially in Australia and elsewhere for many
years. Moreover, as discussed above, the applicant has
found that the defect has a number of different forms,
including streaks, patches, and a wood grain pattern, and
severe examples of each of these forms of the defect are
shown in Figure 1 .
As is discussed above, the applicant has found
that the above-described defect is due to variations in
the Al/Zn ratio on the surface of Al-Zn-Si-Mg alloy
coatings and may be due to a non-uniform distribution of
Mg2Si in the microstructure of the of coatings and the
invention includes controlling conditions in the molten
coating bath and downstream of the coating bath so that
there is a uniform Al/Zn ratio across the surface of the
coating formed on the steel strip.
The method of the invention includes controlling
any suitable conditions in the molten coating bath and
downstream of the coating bath so that there is a uniform
Al/Zn ratio (in accordance with the definition on page 5 )
across the surface of the coating, i.e. on or within the
outermost 1-2 mm of the coating cross section, formed on
the steel strip.
By way of example, the embodiment of the method
of the invention described in relation to Figure 2
includes controlling (a) the Ca concentration in the
molten coating bath, (b) the Mg concentration of the
molten coating bath, and (c) the rate of cooling the
coated steel strip after the coated steel strip leaves the
molten coating bath, as described above in the description
of Figure 2 .
It is noted that the invention is not confined to
controlling this combination of conditions.
Many modifications may be made to the present
invention described above without departing from the
spirit and scope of the invention.

CLAIMS :
1 . A method of forming an Al-Zn-Si-Mg alloy coating
on a steel strip to form the above-described Al-Zn-Mg-Si
coated steel strip, the method including dipping steel
strip into a bath of molten Al-Zn-Si-Mg alloy and forming
a coating of the alloy on exposed surfaces of the steel
strip, and the method including controlling conditions in
the molten coating bath and downstream of the coating bath
so that there is a uniform Al/Zn ratio across the surface
of the coating formed on the steel strip.
2 . The method defined in claim 1 includes
controlling any one or more of the composition of the
molten coating bath, and the rate of cooling the coated
steel strip after the coated steel strip leaves the molten
coating bath.
3 . The method defined in claim 1 or claim 2 includes
controlling the Ca concentration of the molten coating
bath.
4 . The method defined in any one of the preceding
claims includes controlling the Ca concentration of the
molten coating bath to be at least 100 ppm.
5 . The method defined in any one of the preceding
claims includes controlling the Ca concentration of the
molten coating bath to be less than 200 ppm.
6 . The method defined in any one of the preceding
claims includes controlling the Mg concentration of the
molten coating bath to be at least 1.8%.
7 . The method defined in any one of the preceding
claims includes controlling the post-coating bath cooling
rate to be less than 40°C/s while the coated strip
temperature is between 400 °C and 510 °C.
8 . The method defined in any one of the preceding
claims includes controlling the post-coating bath cooling
rate to be greater than 10°C/s while the coated strip
temperature is between 400 °C and 510 °C.
9 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy includes more than
1.8% by weight Mg.
10. The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy includes less than 3%
by weight Mg.
11 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy includes less than
2.5% by weight Mg.
12 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy includes more than
1.2% by weight Si.
13. The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy includes less than
2.5% by weight Si.
14 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy includes the
following ranges in % by weight of the elements Al , Zn,
Si , and Mg :
Zn: 30 to 60%
Si: 0.3 to 3%
Mg: 1.8 to 10%
Balance: Al and unavoidable impurities.
15. The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy includes the
following ranges in % by weight of the elements Al, Zn,
Si, and Mg:
Zn: 35 to 50%
Si: 1.2 to 2.5%
Mg: 1.8 to 3.0%
Balance : Al and unavoidable impurities .
16. An Al-Zn-Mg-Si coated steel strip produced by the
method defined in any one of the preceding claims.
17 . An Al-Zn-Mg-Si coated steel strip that includes a
uniform Al/Zn ratio on the surface or the outermost 1-2mm
of the Al-Zn-Si-Mg alloy coating.
18 . A profiled wall and roofing sheet that has been
roll formed or press formed or otherwise formed from the
Al-Zn-Mg-Si coated steel strip defined in claim 16 or
claim 17 .