METHOD OF PRODUCING METAL-COATED STEEL STRIP
TECHNICAL FIELD
The present invention relates to the production of metal
strip, typically steel strip, which has a corrosion-resistant
metal alloy coating.
The present invention relates particularly, although by
no means exclusively, to the production of strip, typically
steel strip, which has a coating of a corrosion-resistant
metal alloy that contains magnesium as one of the main
elements in the alloy.
The present invention relates particularly, although by
no means exclusively, to the production of strip, typically
steel strip, which has coatings of corrosion-resistant metal
alloys such as Zn-Mg based alloys, Al-Mg based alloys and Al-
Zn-Mg based alloys.
In particular, the present invention relates to a hot-dip
metal coating method of forming a coating of a metal alloy
that contains magnesium as one of the main elements in the
alloy on a strip that includes dipping uncoated strip into a
bath of molten alloy and forming a coating of the alloy on the
strip .
Depending on the end-use application, the metal -coated
strip may be painted, for example with a polymeric paint, on
one or both surfaces of the strip. In this regard, the metalcoated
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.
BACKGROUND ART
One corrosion resistant metal alloy coating that is used
widely in Australia and elsewhere for building products,
particularly profiled wall and roofing sheets, is an Al-Zn
alloy coating composition, more particularly a coating formed
from a 55%A1-Zn alloy that also comprises Si in the alloy.
The profiled sheets are usually manufactured by cold forming
painted, metal alloy coated strip. Typically, the profiled
sheets are manufactured by roll -forming the painted strip.
The addition of Mg to this known composition of 55%A1-Zn
alloy 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.
Another Al-Zn-Si-Mg alloy coating that is described in
the patent literature although not commercially available in
Australia is formed from Al-Zn-Si-Mg alloys that contain in %
by weight: Al : 2 to 19 % , Si: 0.01 to 2%, Mg: 1 to 10 % ,
balance Zn and unavoidable impurities. The alloy coating is
described and claimed in Australian patent 758643 entitled
"Plated steel product, plated steel sheet and precoated steel
sheet having excellent resistance to corrosion" in the name of
Nippon Steel Corporation.
It has been established that when Mg is included in Al-Zn
alloy coating compositions, 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. The present invention is the
result of part of this research and development work.
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 research and development work that is relevant to the
present invention included a series of plant trials on metal
coating lines of the applicant to investigate the viability of
forming particular metal alloy coatings, namely Al-Zn-Si-Mg
alloy coatings, on steel strip on these metal coating lines.
The plant trials found that Al-Zn-Si-Mg alloy coatings are far
more reactive with quench water used to cool metal alloy
coatings on strip after coated strip leaves molten alloy baths
in the metal coating lines than conventional Al-Zn coatings.
More particularly, the applicant found that:
(a) there was greater dissolution of Al-Zn-Si-Mg alloy
coatings into quench water than was the case with conventional
Al-Zn coatings;
(b) the greater dissolution of the metal alloy coatings could
result in removal of a corrosion resistant native oxide layer,
as described herein, from exposed surfaces of the coatings;
(c) the removal of the native oxide layers from surfaces of
the Al-Zn-Si-Mg alloy coatings exposed the Al-Zn-Si-Mg alloy
coatings to corrosion that caused defects such as crevices,
pits, black spots, voids, channels, and speckles in surfaces
of the coated strip; and
(d) the surface defects had a negative impact on the
effectiveness of subsequent passivation of the coated strip
with a passivation solution.
The term "native oxide" is understood herein to mean the
first oxide to form on the surface of the metal alloy coating,
with its chemical make-up being intrinsically dependent on the
composition of the metal alloy coating.
More particularly, the applicant has found that the
native oxide layer is important in terms of preventing
corrosion of an underlying metal alloy coating layer as the
coated strip is processed downstream of the metal coating
bath. In particular, the applicant has found that it is
important to maintain the native oxide layer at least
substantially intact in order to maintain a metal alloy
coating layer that has a suitable surface quality for
passivation with a passivation solution. More particularly,
the applicant has found that complete removal of the native
oxide layer can lead to corrosion of the metal alloy coating
before a downstream passivation step, with the corrosion
including any one of the following surface defects of
crevices, pits, black spots, voids, channels, and speckles.
The applicant has realised that the above-described
problem is not confined to coatings of Al-Zn-Si-Mg alloy and
extends generally to metal alloy coatings of alloys that
contain Mg and, as a consequence are more reactive in
downstream processing operations, such as water quenching, on
coated strip.
According to the present invention there is provided a
method of forming a coating of a metal alloy on a steel strip
to form a metal alloy coated steel strip, the method including
a hot dip coating step of dipping steel strip into a bath of
molten metal alloy and forming a metal alloy coating on
exposed surfaces of the steel strip, with a native oxide layer
as defined herein forming on the metal alloy coating of the
metal alloy coated strip as it emerges from the metal coating
bath, and the method including controlling the method
downstream of the hot dip coating step and/or selecting the
metal coating composition to maintain the native oxide layer
at least substantially intact on the metal alloy coating.
The control step may be any suitable step.
The control step may be particular operating conditions
in one or more than one downstream method step.
The control step may be a selection of the composition of
the metal alloy coating composition to minimise removal of the
native oxide layer in method steps downstream of the metal
coating bath.
The control step may be a combination of particular
operating conditions in one or more than one downstream method
step and the selection of the composition of the metal alloy
coating composition.
The method may include a step of treating the metal alloy
coated strip with a passivation solution, and the method may
include controlling the method between the hot dip coating
step and the passivation step to maintain the native oxide
layer at least substantially intact on the metal alloy
coating .
The method may include a step of cooling the metal alloy
coated strip with cooling water, and the step of controlling
the conditions downstream of the hot dip coating step may
include controlling the water cooling step to maintain the
native oxide layer at least substantially intact on the metal
alloy coating. The applicant has found that one of more than
one of pH control, temperature control, and specific chemical
composition of cooling water can minimise removal of the
native oxide layer on the metal alloy coated strip.
The strip cooling step may include controlling the pH of
cooling water to be in a range of pH 5-9.
The strip cooling step may include controlling the pH of
cooling water to be less than 8 .
The strip cooling step may include controlling the pH of
cooling water to be less than 7 .
The strip cooling step may include controlling the pH of
cooling water to be greater than 6 .
The strip cooling step may include controlling the
temperature of cooling water to be in a range of 25-80°C.
The strip cooling step may include controlling the
temperature of cooling water to be less than 70°C.
The strip cooling step may include controlling cooling
water temperature to be less than 60°C.
The strip cooling step may include controlling cooling
water temperature to be less than 55°C.
The strip cooling step may include controlling cooling
water temperature to be less than 50°C.
The strip cooling step may include controlling cooling
water temperature to be less than 45 °C
The strip cooling step may include controlling cooling
water temperature to be greater than 30°C.
The strip cooling step may include controlling cooling
water temperature to be greater than 35°C.
The strip cooling step may include controlling cooling
water temperature to be greater than 40°C.
The strip cooling step may include controlling the pH by
adding acid to the cooling water.
The strip cooling step may include controlling the pH by
adding acid and other salts, buffers, wetting agents,
surfactants, coupling agents, etc.
The acid may be any suitable acid such as phosphoric acid
and nitric acid by way of example.
The strip cooling step may be a water quench step.
The strip cooling step may be a closed loop in which
water is circulated through a circuit that supplies water to
the coated strip and collects and cools water and returns the
cooled water for cooling the coated strip.
The closed loop may include a water storage tank, a spray
system for supplying water to the coated strip from the tank,
and a heat exchanger for cooling water after it has been
sprayed onto the strip.
The strip cooling step may be an open loop in which
cooling water is not recycled in the cooling step.
The strip cooling step may include controlling the
operating conditions to cool the coated strip to a temperature
range of 28-55°C.
The strip cooling step may include controlling the
operating conditions to cool the coated strip to a temperature
range of 30-50°C.
The method may include other steps including any one or
more of the following steps in addition to the above-described
hot dip coating step, water cooling step, and passivation
step :
(a) pre-treating strip to clean the strip before the hot dip
coating step,
(b) controlling the thickness of the coated strip immediately
after the coating step,
(c) rolling the coated strip, and
(d) coiling the coated strip.
The metal alloy coating may be formed from Zn-Mg based
alloys, Al-Mg based alloys, and Al-Zn-Mg based alloys, with
each of these alloys including other elements such as Si, and
with the additional elements being the result of deliberate
alloying additions or as unavoidable impurities.
One type of metal alloy of particular interest to the
applicant for coatings is Al-Zn-Si-Mg alloys.
The Al-Zn-Si-Mg alloy may include the following ranges in
% by weight:
Al: 2 to 19 %
Si: 0.01 to 2 %
Mg: 1 to 10 %
Balance Zn and unavoidable impurities.
The Al-Zn-Si-Mg alloy is not confined to the composition
ranges of the elements Al , Zn, Si, and Mg described in the
preceding paragraph and extends to Al-Zn-Si-Mg alloy
compositions generally.
By way of example, 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.
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 Al-Zn-Si-Mg alloy may contain other elements that are
present as deliberate alloying additions or as unavoidable
impurities. The other elements may include by way of example
any one or more of Fe, Sr, Cr, and V .
By way of particular example, the other elements may
include Ca for dross control in molten coating baths.
It is noted that the composition of the as-solidified
coating of the Al-Zn-Si-Mg alloy may be different to an extent
to the composition of the Al-Zn-Si-Mg alloy used to form the
coating due to factors such as partial dissolution of the
metal strip into the coating during the coating process.
The steel may be a low carbon steel .
The present invention also provides a metal alloy coated
steel strip produced by the above-described method.
The Al-Zn-Si-Mg alloy used to form the coating of the Al-
Zn-Mg-Si alloy coated steel strip may include the following
ranges in % by weight:
Al: 2 to 19 %
Si: 0.01 to 2 %
Mg: 1 to 10 %
Balance Zn and unavoidable impurities
The Al-Zn-Si-Mg alloy used to form the coating of the Al-
Zn-Mg-Si alloy coated steel strip 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
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention is described further by way of
example with reference to the accompanying drawings of which:
Figure 1 is a schematic drawing of a continuous metal
coating line for forming an Al-Zn-Si-Mg alloy coating on steel
strip in accordance with the method of the present invention;
and
Figures 2 (a) to 2 (d) are graphs illustrating the results
of x-ray photoelectron spectroscopy analysis of the metal
alloy coating surfaces of metal alloy coated steel strip
samples .
DESCRIPTION OF EMBODIMENTS
With reference to Figure 1 , 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
preheat 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 held in a coating pot 6 and is coated with the Al-
Zn-Si-Mg alloy. Typically, the Al-Zn-Si-Mg alloy in the
coating pot 6 comprises in % by weight: Zn: 30 to 60 % , Si:
0.3 to 3 % , Mg: 0.3 to 10 % , and balance Al and unavoidable
impurities. It is noted that the Al-Zn-Si-Mg alloy may
contain other ranges of these elements. It is also noted that
the Al-Zn-Si-Mg alloy may contain other elements as deliberate
additions or as impurities. For example, the coating pot 6
may also contain Ca for dross control in the molten bath. The
Al-Zn-Si-Mg alloy is maintained molten in the coating pot at a
selected temperature 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. Both surfaces of the strip are coated with the Al-Zn-
Si-Mg alloy as it passes through the bath.
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 exposed surfaces of the Al-Zn-Si-Mg alloy coating
oxidise as the coated strip moves through the gas wiping
station and a native oxide layer forms on the exposed surfaces
of the coating. A s indicated above, the native oxide is the
first oxide to form on the surface of the metal alloy coating,
with its chemical make-up being intrinsically dependent on the
composition of the metal alloy coating, including Mg oxide, Al
oxide, and a small amount of oxides of other elements of the
Al-Zn-Si-Mg alloy coating.
The coated strip is then passed through a cooling section
7 and is subjected to forced cooling by means of a water
quench step. The forced cooling may include a forced air
cooling step (not shown) before the water quench step. The
water quench step is, by way of example, a closed loop in
which water sprayed onto coated strip is collected and then
cooled for re-use to cool coated strip. The cooling section 7
includes a coated strip cooling chamber 7a, a spray system 7b
that sprays water onto the surface of the coated strip as it
moves through the cooling chamber 7a, a water quench tank 7c
for storing water that is collected from the cooling chamber
7b, and a heat exchanger 7d for cooling water from the water
quench tank 7c before transferring the water to the spray
system 7b. In accordance with one embodiment of the present
invention (a) the pH of the cooling water supplied to the
spray system 7b is controlled to be in a range of pH 5-9,
typically in a range of 6-8, and (b) the temperature of the
cooling water supplied to the spray system is controlled to be
in range of 30-50°C. The applicant has found that both
control steps (a) and (b) minimise removal of the native oxide
layer on the Al-Zn-Si-Mg alloy coating on the coated strip.
The pH and temperature control may be achieved, by way of
example, by using a pH probe and a temperature sensor in an
overflow tank of the water quench tank 7c and supplying data
from the probe/sensor to a PLC and calculating required acid
additions to maintain the pH at predetermined set points for
pH and the water temperature, with any acid additions and
temperature adjustments being made so that the water in the
water quench tank 7c is controlled to the set points for pH
and temperature. This is not the only possible option for
achieving pH and temperature control .
The pH, temperature, and chemical control may also be
achieved, by way of example, by using a once through water
cooling system where the quench water is not recirculated and
the input water has pH and temperature properties as described
above .
The cooled, coated strip is then passed through a rolling
section 8 that conditions the surface of the coated strip.
This section may include one or more of skin pass and tension
leveling operations.
The conditioned strip is then passed through a
passivation section 10 and coated with a passivation solution
to provide the strip with a degree of resistance to wet
storage and early dulling.
The coated strip is thereafter coiled at a coiling
station 11.
A s discussed above, the applicant has conducted extensive
research and development work in relation to Al-Zn-Si-Mg alloy
coatings on steel strip.
A s discussed above, the applicant has found in the
research and development work that the native oxide layer that
forms as the metal alloy coated strip moves through the gas
wiping station is important in terms of minimising corrosion
of the underlying metal alloy coating as the coated strip is
processed downstream of the bath.
In particular, the applicant has found that it is
important to maintain the native oxide layer at least
substantially intact in order to maintain a metal alloy
coating that has a suitable surface quality for passivation
with a passivation solution.
More particularly, the applicant has found that total
removal of the native oxide layer can lead to corrosion of the
metal alloy coating before a downstream passivation step, with
the corrosion including any one of the following surface
defects of crevices, pits, black spots, voids, channels, and
speckles .
The research and development work relevant to the native
oxide issue included x-ray photoelectron spectroscopy (XPS)
depth profiling analysis to assess the conditions of the
surfaces of a series of metal alloy coatings.
The graphs of Figures 2 (a) to 2 (d) are the results of XPS
analysis of various materials, representing a set of possible
metal coating surface conditions.
The graph of Figure 2 (a) is an XPS depth profile of the
surface of an Al-Zn-Si-Mg alloy coated steel panel produced on
the Hot Dip Process Simulator (HDPS) at the research
facilities of the applicant. The HDPS is a state-of-the-art
unit purpose-built to the specifications of the applicant by
Iwatani International Corp (Europe) GmbH. The HDPS unit
comprises a molten metal pot furnace, an infrared heating
furnace, gas wiping nozzles, de-drossing mechanisms, gas
mixing and dewpoint management functions, and computerized
automatic control systems. The HDPS unit is capable of
simulating a typical hot dip cycle on a conventional metal
coating line. The horizontal axis of Figure 2(a) marked
"Etchine time" refers to the etching time in the analysis and
indicates the depth of the coating from the surface of the
coating. Each of the lines in the Figure indicates a different
atomic component in the coating. Figure 2(a) indicates that a
thin oxide layer of approximately 9nm thickness was detected
on the Al-Zn-Si-Mg alloy coated steel panel. The oxide layer
consisted primarily of aluminium and magnesium oxides. The
HDPS has gas cooling but no water quench, and thus the oxide
layer is representative of oxides forming on the surface of
the molten coating at elevated temperatures. One
characteristic of the oxide layer is the presence of a small
portion of calcium oxide (~2at%Ca) resulted from a low level
of Ca addition in the molten coating bath for dross control.
The oxide is described as a "native oxide" by the applicant,
as it is the first oxide to form on the surface of the metal
coating and its chemical make-up is intrinsically dependant on
the composition of the metal coating.
The graph of Figure 2 (b) is an XPS depth profile of the
surface of an Al-Zn-Si-Mg alloy coated steel panel produced on
one of the applicant' s metal coating lines where there is a
water quench step in the production loop and the pH and
temperature of the quench water is controlled. The pH was
controlled with nitric acid addition to be pH 5-8 and the
temperature was controlled to be 35-55°C. Figure 2(b) shows
that a small portion only of the native oxide was removed due
to the water quench. However, the presence of Ca indicates
that the native oxide was not totally removed. Moreover,
there was no corrosion of the underlying Al-Zn-Si-Mg alloy
coating. Significantly, Figure 2(b) also indicates that under
the particular water quench conditions, it was possible to
maintain a partial native oxide layer.
The graph of Figure 2 (c) is an XPS depth profile of the
surface of an Al-Zn-Si-Mg alloy coated steel panel produced on
another metal coating line of the applicant, where there is
also a water quench step in the production loop. The pH was
controlled to be pH 5-8 and the temperature was controlled to
be 35-55°C. Figure 2(c) shows that the conditions of the
water quench were such that there was partial removal of the
native oxide layer and Ca was detected at levels lower than
that in Figures 2 (a) or 2 (b) . Some new oxide formed on the
surface of the Al-Zn-Si-Mg alloy coating, presumably during or
following the quench process. Nevertheless, there was no
corrosion attack on the underlying structure of the Al-Zn-Si-
Mg alloy coating.
The graph of Figure 2 (d) is an XPS depth profile of the
surface of an Al-Zn-Si-Mg alloy coated steel panel produced on
yet another metal coating line of the applicant, where there
is also a water quench step in the production loop. The pH
was controlled to be greater than pH 9 and the temperature was
controlled to be greater than 50°C. Figure 2(d) shows that
the water quench conditions resulted in complete removal of
the native oxide layer and obvious corrosion attack on the
underlying structure of the Al-Zn-Si-Mg alloy coating. The
new oxide layer that formed on the surface of the metal
coating was characterized by a substantial presence of zinc
oxide (corrosion product) in the layer and a much greater
layer thickness. This resulted in an unsatisfactory
passivation outcome.
The research and development work described with
reference to Figures 2 (a) to 2 (d) indicates that water quench
conditions that maintain the integrity of the underlying
structure of a metal alloy coating allow the metal alloy
coating to achieve a satisfactory passivation outcome, whereas
water quench conditions that cause any corrosion attack to the
underlying structure of the metal coating impair the ability
of the metal alloy coating to be properly passivated.
Many modifications may be made to the present invention
described above without departing from the spirit and scope of
the invention.
By way of example, whilst the embodiment of the metal
coating line shown in Figure 1 includes a coated strip cooling
section 7 that includes water sprays, the present invention is
not so limited and extends to any suitable water cooling
system, such as dunk tanks.
By way of further example, whilst the description of the
invention in relation to the Figures focuses on control of a
water cooling step in a metal coating line, the invention is
not so limited and the control may be otherwise achieved and
may, for example, include selection of metal alloy coating compositions that form more resistant native oxide layers.

CLAIMS :
1. A method of forming a coating of a metal alloy on a steel
strip to form a metal alloy coated steel strip, the method
including a hot dip coating step of dipping steel strip into a
bath of molten metal alloy and forming a metal alloy coating
on exposed surfaces of the steel strip, with a native oxide
layer forming on the metal alloy coating of the metal alloy
coated strip after it emerges from the metal coating bath, and
the method including controlling the method downstream of the
hot dip coating step and/or selecting the metal coating
composition to maintain the native oxide layer intact on the
metal alloy coating during the downstream steps.
2 . The method defined in claim 1 including a step of
treating the metal alloy coated strip with a passivation
solution, and the method including controlling the method
between the hot dip coating step and the passivation step to
maintain the native oxide layer at least substantially intact
on the metal alloy coating.
3 . The method defined in claim 1 or claim 2 including a step
of cooling the metal alloy coated strip with cooling water,
and the step of controlling the conditions downstream of the
hot dip coating step including controlling the strip cooling
step to maintain the native oxide layer at least substantially
intact on the metal alloy coating.
. The method defined in claim 3 wherein the strip cooling
step includes controlling the pH of cooling water to be in a
range of pH 5-9.
5 . The method defined in claim 3 wherein the strip cooling
step includes controlling the pH of cooling water to be less
than 8 .
6 . The method defined in claim 3 wherein the strip cooling
step includes controlling the pH of cooling water to be less
than 7 .
7 . The method defined in claim 3 wherein the strip cooling
step includes controlling the pH of cooling water to be
greater than 6 .
8 . The method defined in any one of claims 3 to 7 wherein
the strip cooling step includes controlling the temperature of
cooling water to be in a range of 25-80°C.
9 . The method defined in any one of claims 3 to 7 wherein
the strip cooling step includes controlling the temperature of
cooling water to be less than 70°C.
10. The method defined in any one of claims 3 to 7 wherein
the strip cooling step includes controlling cooling water
temperature to be less than 60°C.
11. The method defined in any one of claims 3 to 7 wherein
the strip cooling step includes controlling cooling water
temperature to be less than 55°C.
12. The method defined in any one of claims 3 to 7 wherein
the strip cooling step includes controlling cooling water
temperature to be less than 50°C.
13. The method defined in any one of claims 3 to 7 wherein
the strip cooling step includes controlling cooling water
temperature to be greater than 40°C.
1 . The method defined in any one of claims 3 to 13 wherein
the strip cooling step includes controlling the pH by adding
acid to the cooling water.
15. The method defined in any one of claims 3 to 13 wherein
the strip cooling step includes controlling the chemistry of
the cooling water.
16. The method defined in any one of claims 3 to 13 wherein
the strip cooling step includes controlling the chemistry and
the pH by adding acid to the cooling water.
17. The method defined in any one of claims 3 to 16 wherein
the strip cooling step includes controlling the operating
conditions to cool the coated strip to a temperature range of
30-50°C.
18. The method defined in any one of the preceding claims
wherein the metal alloy coating is formed from Zn-Mg based
alloys, Al-Mg based alloys, and Al-Zn-Mg based alloys, with
each of these alloys including other elements such as Si, with
the additional elements being the result of deliberate
alloying additions or as unavoidable impurities.
19. The method defined in any one of the preceding claims
wherein the metal alloy coating is a coating formed from an
Al-Zn-Si-Mg alloy.
20. The method defined in claim 19 wherein the Al-Zn-Si-Mg
alloy includes the following ranges in % by weight:
Al: 2 to 19 %
Si: 0.01 to 2 %
Mg: 1 to 10 %
Balance Zn and unavoidable impurities
21. The method defined in claim 19 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: 0.3 to 10 %
Balance Al and unavoidable impurities.
22 . A metal alloy coated steel strip produced by the method
defined in any one of the preceding claims.