META -COATED STEEL STRIP
The present invention relates to the production
of metal strip, typically steel strip, which has a
corrosion-resistant metallic coating that contains
aluminium-zinc-silicon-magnesium as the main elements ,
although not necessarily the only elements, in the coating
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 coating method of forming a metallic coating on a
strip that includes dipping uncoated strip into a bath of
molten Al-Zn-Si-Mg alloy and forming a coating o the
alloy on the strip.
Typically, the composition of the molten Al-Zn-
Si-Mg alloy comprises the following ranges in % by weight
of the elements Al Zn , S , and Mg :
Zn: 30 to 60%
Si: 0.3 to 3%
Mg: 0.3 to 10%
Balance Al and unavoidable impurities .
More typically, the composition of the molten
Al-Zn-Si-Mg alloy 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 composition o the molten Al-Zn-Si-Mg alloy
may contain other elements that are present in the molten
alloy 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 Fe, Sr, Cr, and V .
Depending on the end-use application, the
metallic coated strip may be painted, for example with a
polymeric paint, on one or both surfaces of the strip. In
this regard, the metallic 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 has a metallic coating
formed from the above-described molten Al-Zn-Si-Mg alloy
composition and is also 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) .
One corrosion resistant metal coating bath
composition that is used widely in Australia and elsewhere
for building products , particularly profiled wall and
roofing sheets, is a 55%A1-Zn alloy coating composition
that also contains Si . 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-Si alloy 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 alloy coating composition, Mg brings
ab ou t 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 .
The present invention is based on a finding of
the applicant during the cours of the research and
development work that forming an Al-Zn-Si-Mg alloy coating
on a steel strip so that there is an intermediate alloy
layer having a selected composition and preferably a
selected crystal structure between an Al-Zn-Si-Mg alloy
coating overlay layer and the steel strip can improve the
corrosion performance of the coated strip. The research
and development work also found that the selected
composition and preferred crystal structure of the
intermediate alloy layer that can improve corrosion
performance of the coated strip is not an inevitable
outcome of the selection of the Al-Zn-Si-Mg alloy
composition for use in a hot dip coating bath, and a
number of factors such as but not limited to molten Al-Zn-
Si-Mg alloy bath composition and hot dip process
conditions, typically strip immersion time and coating pot
temperature, are relevant factors to forming the
intermediate alloy layer having the required composition
and the preferred crystal structure.
According to the present invention there is
provided a metallic coated steel strip that includes a
steel strip and a metallic coating on at least one side of
the strip, with the metallic coating including an Al-Zn-
Si-Mg overlay alloy layer and an intermediate alloy layer
between the steel strip and the overlay alloy layer, and
wherein the intermediate alloy layer has a composition of,
by weight, 4 .0-12 .0%Zn, 6.0-17.0%Si, 20 .0-40 .0%Fe , 0.02-
0.50%Mg, and balance Al and unavoidable impurities.
The intermediate alloy layer may be formed as an
intermetallic phase of elements in the compositions of the
molten Al-Zn-Mg-Si alloy and the steel strip.
Alternatively, the intermediate alloy layer and
the Al-Zn-Mg-Si overlay alloy layer may be formed as
separate layers .
The intermediate alloy layer may include, by
weight, 5.0-10.0%Zn, 7.0-14.0%Si (typically .5-1 .0%Si) ,
25. 0-37. 0%Fe, 0.03-0. 25%Mg, balance Al and unavoidable
impurities .
The intermediate alloy layer may include, by
weight, 6.0-9.0%Zn, 8.0-12.0%Si, 28 .0-35 .0%Fe , 0.05-
0.15%Mg, balance Al and unavoidable impurities.
The intermediate alloy layer may include, by
weight, 0.01-0. 2%Ca.
The intermediate alloy laye may include, by
weight , 0.1-3.0%Cr.
The intermediate alloy layer may include, by
weight , 0.1-13.0%Mn.
The intermediate alloy layer may include , by
weight, 0.1-2.0%V.
The intermediate alloy layer may have a thickness
of 0.1-5.0um as measured on a cross-section through the
thickness of the coating.
The intermediate alloy layer may have a thickness
of 0.3-2.0um as measured on a cross-section through the
thickness of the coating.
The intermediate alloy layer may have a thickness
of 0.5-1.0um as measured on a cross-section through the
thickness of the coating.
The intermediate alloy layer may include
substantially columnar crystals measuring 50-1000nm in a
short diameter as measured on a cross section through the
thickness of the coating.
The intermediate alloy layer may include
substantially equiaxial crystals measuring 50-4000nm in a
long diameter as measured on a cross section through the
thickness of the coating.
The intermediate alloy layer may include a
mixture of columnar crystals and equiaxial crystals.
The intermediate alloy layer may include body
centred cubic crystals.
The Al, Zn, Si and Fe concentrations of the
intermediate alloy layer may satisfy the formula
FeioAl32Si Zn3 .
The Al, Zn, Si and Fe concentrations of the
intermediate alloy layer may satisfy the formula
e i0Al34Si4 n 2.
v
- 6 -
The strip may be a passivated strip, for example
using a Cr-containing or Cr-free passivation system.
The strip may include a resin coating on an
exposed surface of the Al-Zn-Mg-Si alloy coating.
A molten Al-Zn-Si-Mg alloy for forming the
metallic coating may include more than 0.3% by weight Mg.
The molten Al-Zn-Si-Mg alloy for forming the
metallic coating may include more than 1.0% by weight Mg.
The molten Al-Zn-Si-Mg alloy for forming the
metallic coating may include more than 1.3% by weight Mg.
The molten Al-Zn-Si-Mg alloy for forming the
metallic coating may include more than 1.5% by weight Mg
The molten Al-Zn-Si-Mg alloy for forming the
metallic coating may include less than 3% by weight Mg.
The molten Al-Zn-Si-Mg alloy for forming the
metallic coating may include less than 2.5% by weight Mg.
The molten Al-Zn-Si-Mg alloy for forming the
metallic coating may include more than 1.2% by weight Si.
The molten Al-Zn-Si-Mg alloy for forming the
metallic coating may include less than 2.5% by weight Si.
The molten Al-Zn-Si-Mg alloy for forming the
metallic coating 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 molten Al-Zn-Si-Mg alloy for
forming the metallic coating 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 a method of forming a metallic coating on a steel
strip to form the above-described metallic coated steel
strip, the method including dipping steel strip into a
bath of a molten Al-Zn-Si-Mg alloy and forming a metallic
coating of the alloy on exposed surfaces of the steel
strip, and the method including controlling any one or
more of the composition of the molten alloy bath, the
temperature of the molten alloy bath, and the immersion
time of the steel strip in the molten alloy bath to form
the intermediate alloy layer between the steel strip and
the Al-Zn-Mg-Si overlay alloy layer.
The molten Al-Zn-Si-Mg alloy may have the
composition described above. For example, the molten 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 present invention is described further by
way of example with reference to the accompanying drawings
of which:
Figure 1 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;
Figure 2 is a graph of Q-Fog life (time in hours
to 5% surface red rust) for samples of a known Al-Zn-Si
alloy coating on steel strip and an Al-Zn-Si-Mg alloy
coating on steel strip in accordance with the invention;
Figure 3 presents the results of further
experimental work on samples of a known Al-Zn-Si alloy
coating on steel strip and an Al-Zn-Si-Mg alloy coating on
steel strip in accordance with the invention;
Figure 4 presents the results of further
experimental work on samples of Al-Zn-Si-Mg alloy coatings
on steel strip in accordance with the invention;
Figure 5 presents the results of further
experimental work on samples of Al-Zn-Si-Mg and Al-Zn-Si-
Mg-Cr alloy coatings on steel strip in accordance with the
invention ;
Figure 6 is a graph showing the effect of pot
temperature on the compositions of the intermediate alloy
layers and, in turn, on the Q-Fog mass losses of samples
coated with the same Al-Zn-Si-Mg alloy;
Figure 7 is a graph of experimental results
showing the effect of Mg and Si in coating bath
compositions of Al-Zn-Si-Mg alloys on the mass of
intermediate alloy layers of samples of metallic coated
strip in accordance with the invention;
Figure 8 is a graph of the thicknesses of
intermediate alloy layers of samples of metallic coated
strip in accordance with the invention and other metallic
coated steel strip versus immersion time in coating baths
used to form the coatings on the samples ; and
Figure 9 is a graph of the thicknesses of
intermediate alloy layers of samples of metallic coated
strip in accordance with the invention versus thicknesses
of the overlay alloy layers on the samples .
With reference to the continuous production line
for coating steel strip shown diagrammatically in 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 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 molten Al-Zn-Si-Mg alloy. The Al-Zn-
Si-Mg alloy is maintained molten in the coating pot at a
selected temperature by use of heating inductors (not
shown) or other suitable heating options . Within the bath
the strip passes around a sink roll (not shown) positioned
in the bath 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 molten 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 coated strip is then passed through a cooling
section 7 and subjected to forced cooling.
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 and found that
forming a metallic coating that includes an overlay alloy
layer and an intermediate alloy layer having a selected
composition and preferably a selected crystal structure
between the overlay alloy layer and the steel strip can
improve the corrosion performance of the metallic coated
strip.
The research and development work included work
carried out by hot dip coating steel strip samples with
the following molten alloy compositions: (a) a known Al-
Zn-Si alloy (hereinafter referred to as "AZ") , (b) an Al-
Zn-Si-Mg alloy (hereinafter referred to as "MAZ") in
accordance with the invention and (c) a MAZ alloy plus 0.1
wt .% Cr in accordance with the invention , having the
following molten alloy compositions, in wt.%:
· AZ: 55Al-43Zn-l .5Si-0 . 5Fe-incidental impurities.
• MAZ: 53Al-43Zn-2Mg-l .5Si-0 .45Fe-incidental
impurities .
· MAZ + 0.1 wt.% Cr-incidental impurities.
The molten alloys were coated onto exposed
surfaces of steel strip samples with a double-sided
coating mass of 125 g/m2 and 150 g/m2 . One group of
samples was produced on a metal coating line ("MCL") at
the Wollongong operations of the applicant and another
group of samples was produced on a Hot Dip Process
Simulator ("HDPS") at research facilities of the applicant
in Wollongong. Experimental work was primarily conducted
on the HDPS. The HDPS is a state-of-the-art unit purposebuilt
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 .
Coated samples were tested for corrosion
resistance (Q-Fog cyclic corrosion test performance) and
were subjected to micro structural analysis using a
scanning electron microscope and other analytical
equipment.
Figure 2 is a graph of -Fog life (time in hours
to 5% surface red rust) for the following samples:
• MCL AZ150 - AZ alloy coating with a double-sided
coating mass of 150 g/m2 produced on the metal
coating line .
• MCL MAZ125 - MAZ alloy coating with a double-sided
coating mass of 125 g/m2 produced on the metal
coating lin .
• HDPS MAZ125 - MAZ alloy coating with a double-sided
coating mass of 125 g/m2 produced on the Hot Dip
Proces s Simulator .
• HDPS MAZ125+0 .l%Cr - MAZ+0.1%Cr alloy coating with a
double-sided coating mass of 125 g/m produced on the
Hot Dip Process Simulator.
It is evident from Figure 2 that the MAZ alloy
coating samples had significantly longer Q-Fog lives and
therefore significantly better corrosion resistance than
the AZ alloy coating sample, with the MAZ+0.1%Cr sample
having the best performance of all of the samples.
Figure 2 illustrates the improvement in corrosion
resistance as a consequence of the addition of Mg to an AZ
to form the MAZ alloy. Figure also illustrates that a
small addition of 0.1% Cr to an MAZ alloy produced a
further significant improvement in corrosion resistance.
Figure 3 illustrates further the contribution of
Mg to the improvement in corrosion resistance of MAZ alloy
coatings when compared to AZ alloy coatings . The results
shown in Figure 3 are the result of experimental work on
the following samples:
• MCL AZ150 - AZ alloy coating with a double-sided
coating mass of 150 g/m2 produced on the metal
coating line .
• MCL MAZ125 - MAZ alloy coating with a double-rsided
coating mass of 125 g/m2 produced on the metal
coating line.
The left hand side of Figure 3 is two SEM back
scattered electron images of sections through the
thickness of both samples . The right side of Figure 3 is
a graph of -Fog life (time in hours to 5% surface red
rust) or the samples . Both samples were produced on the
same metal coating line. The SEM images show that the
samples have different coating microstructures due to the
presence of Mg in the MAZ alloy. The SEM images also show
that the coating of both samples includes an overlay alloy
layer 11 and an intermediate alloy layer 12 (referred to
as an "Alloy layer" in this and other Figures) between the
steel strip 13 (referred to as "Base steel" in the
Figures) and the overlay layers 11. The intermediate
alloy layer is an intermetallic layer formed from elements
in the molten alloy bath and the steel strip. The graph
shows that the MAZ alloy coating sample had a
significantly longer Q-Fog life and therefore
significantly better corrosion resistance than the AZ
alloy coating sample, possibly attributed to the presence
of Al/Zn/MgZn 2 eutectic and Mg2Si phases in the
microstructure of the MAZ alloy coating overlay, although
the intermediate layer may have also contributed to the
difference in corrosion performance.
Figure 4 presents the results of further
experimental work on a MAZ alloy coating that focused on
the contribution of the intermediate alloy layer 12
between the steel strip 13 and the overlay alloy layers 11
of the following samples :
MCL MAZ125 - MAZ alloy coating with a double-sided
coating mass of 125 g/m2 produced on the metal coating
line.
• HDPS MAZ125 - MAZ alloy coating with a double-sided
coating mass of 125 g/m2 produced on the Hot Dip
Process Simulator.
The le t hand side of Figure 4 is two SEM back
scattered electron images of sections through the
thickness of both samples . The right side of Figure 4 is
a graph of Q-Fog life (time in hours to 5% surface red
rust) or the samples . Both samples were coated with the
same molten alloy composition - MAZ alloy. One sample was
produced on the metal coating line and the other sample
was produced on the Hot Dip Process Simulator. Both
samples had substantially the same coating thickness -
approximately 18 microns. The graph shows that the HDPS
MAZ125 alloy coating sample had a significantly longer -
Fog life and therefore significantly better corrosion
resistance than the MCL MAZ125 alloy coating sample. The
SEM images show that the HDPS MAZ125 coating sample had a
thicker intermediate alloy layer than the MCL MAZ125
coating sample due to a longer immersion time (2 .5 seconds
on the HDPS versus 1 .0 second on the MCL) Figure 4 is an
indication that the intermediate alloy layer 12
contributed to the better corrosion resistance of the HDPS
MAZ125 coating sample, i.e. a thicker intermediate alloy
layer 12 produced a longer Q-Fog life.
Figure 5 presents the results of further
experimental work that focused on the contribution of Cr
on the corrosion performance of the following samples :
• HDPS MAZ125+0 .l%Cr - AZ+0 .l%Cr alloy coating with a
double- sided coating mass of 125 g/m2 produced on the
Hot Dip Process Simulator.
• HDPS MAZ125 - MAZ alloy coating with a double-sided
coating mass of 125 g/m2 produced on the Hot Dip
Process Simulator.
The left h and side of Figure 5 is two SEM back
scattered electron images and two SEM-EDS elemental maps
of sections through the thickness of both samples showing
the micros true ure of the sections and the distribution of
Cr through the sections . The right side of Figure 5 is a
graph of -Fog life (time in hours to 5% surface red rust)
for the samples . Both samples were produced on the Hot
Dip Process Simulator. Both samples had substantially the
same coating thickness and substantially the same
intermediate alloy layer thickness. In effect, the only
difference between the samples is the 0.1% Cr 'in one of
the samples . It is evident f om the graph that the Cr
resulted in the HDPS MAZ125+0 .l%Cr alloy coating sample
having a significantly longer Q-Fog life and therefore
significantly better corrosion resistance than the HDPS
MAZ125 alloy coating sample. It is also evident from the
SEM-EDS maps that there was a higher concentration of Cr
in the intermediate alloy layer of the HDPS MAZ125+0 .l%Cr
alloy coating sample. It follows that the Cr in the
intermediate alloy layer of the HDPS MAZ125+0 .l%Cr coating
sample contributed to the improved corrosion resistance of
this sample .
The research and development work included
extensive work to establish the contribution of the
intermediate alloy layer to the corrosion resistance of
Al-Zn-Si-Mg alloy coating either by virtue of the
composition of the intermediate alloy layer or by virtue
of the crystal structure of the intermediate alloy layer.
This work identified the following composition
range of the intermediate alloy layer, by weight, that
offers the optimum Al-Zn-Si-Mg alloy coating corrosion
performance:
Zn 4.0-12.0%,
Si 6.0-17.0%,
Fe 20.0-40.0%,
Mg 0.02-0.50%,
Balance Ά1 and unavoidable impurities .
The corrosion performance of the Al-Zn-Si-Mg
alloy coating is inferior outside the above composition
range of the intermediate alloy layer of the coating.
The above composition range of the intermediate
alloy layer was determined by extensive testing (including
but not limited to, coating corrosion through Q-Fog test
and outdoor exposure, coating ductility through T bend
tests etc) of Al-Zn-Si-Mg alloy coatings on steel samples
with molten alloy bath compositions across the ranges of
AZ + 0-5.0%Si, 0-5.0%Mg, 0-0.1%Cr, 0-0.4%Mn, 0-0. 1%V, and
0-0. l%Ca at strip immersion times o 0.3-20 seconds and
pot temperatures of 595- 640 °C to identify samples that
delivered desirable performances . A wide range of
analytical techniques were employed to (a) study the
chemical compositions, thicknesses and crystal structures
of the intermediate alloy layers and (b) develop an
understanding of critical properties of the intermediate
alloy layer that contribute to the performances of the
final coated product. Figures 2-9 are a sample of the
results of this research and development work.
The research and development work also found that
the above composition range of the intermediate alloy
layer is not an inevitable outcome of the selection of the
molten Al-Zn-Si-Mg alloy bath composition and that factors
such as but not limited to molten Al-Zn-Si-Mg alloy
composition and hot dip process conditions , typically
strip immersion time and coating pot temperature, are
relevant factors to rming an intermediate alloy layer
having a required composition. In particular, although it
may not necessarily be obvious to a person skilled in the
art, the chemical compositions, thicknesses and crystal
structures of the intermediate alloy layers illustrated in
the Figures are interrelated and contribute to the
performance of the coated strip as a whole.
Figures 6 is a graph showing the effect of
coating pot temperature on the compositions of the
intermediate alloy layers and, in turn, on the Q-Fog mass
losses, of two Al-Zn-Si-Mg alloy coatings. Samples were
prepared at two different pot temperatures, 600° C and
620 °C respectively, using the same molten Al-Zn-Si-Mg
alloy in the coating bath and the same immersion time (1
second) . The coated samples were analysed to determine
the compositions of the intermediate alloy layers. The
intermediate alloy compositions are set out in the table
below the bar graph in Figure 6 . The thicknesses of the
intermediate alloy layers are also presented in the table.
The samples were subjected to the same Q-Fog corrosion
test procedure. Figure 6 shows that the 610 °C and 620 °C
pot temperatures produced different intermediate alloy
layer compositions. At 620 °C pot temperature the
intermediate alloy layer composition was outside the
invention composition range (Si<6% in particular) . As a
result, the corrosion performance of the Al-Zn-Si-Mg alloy
coating su f rs , despite a greater intermediate alloy
layer thickness, which would otherwise have been
advantageous if the composition of the intermediate alloy
layer was the same as that at 600 °C pot temperature (or
within the invention range) . The compositions of the
intermediate alloy layers were analysed using an
inductively- coupled plasma spectrometry (ICP) technique.
In accordance with this technique, firstly the Al-Zn-Si-Mg
alloy coating overlay was removed by submerging the sample
in a 1 : aqueous HC1 solution inhibited by sodium arsenite
(9g per litre) . The intermediate alloy layer was then
dissolved using a RODINE ® inhibited HC1 solution and the
resultant solution was analysed by ICP.
Figure 7 is a graph of the mass of intermediate
alloy layers of samples of metallic coated steel strip
obtained under the same hot dip process conditions (1
second immersion time at 600 °C pot temperature) in
accordance with the invention versus concentrations of Mg
and Si in coating baths of an Al-Zn-Si-Mg alloy used to
form the coatings on the samples . Figure 7 shows that the
mass of the intermediate alloy layer decreased with
increasing Mg and Si concentrations in coating baths .
Figure 8 is a graph of the thicknesses of
intermediate alloy layers of samples of metal coated strip
in accordance with the invention versus immersion time in
coating baths of coating alloys used to form the coatings
on the samples. Figure 8 presents the results of work on
3 different molten alloy bath compositions. One molten
alloy is a known Al-Zn-Si alloy (the "AZ" alloy in the
Figure) . Another molten alloy is an Al-Zn-Si-Mg alloy
that also includes Ca in accordance with the invention
(the "AMCa alloy in the Figure") . The 3rd molten alloy is
a known Al-Zn-Si-Mg alloy having 5.0% Mg and 4.0% Si (the
"5 .0%Mg .0%Si" alloy in the Figure). Figure 8 shows that
the molten alloy bath composition and the immersion time
in the molten alloy bath have an impact on the thickness
of the intermediate alloy layer of coated steel strip.
Figure 9 is a graph of the thicknesses of
intermediate alloy layers of coatings of metallic coated
strip samples in accordance with the invention versus the
thicknesses of the overlay alloy layers of the coatings on
the samples . Figure 9 shows that the intermediate alloy
layer thickness increased with the overlay alloy layer
thickness . It follows from Figure 9 that it is therefore
desirable to minimize any coating mass variation across
the surface of the entire coated strip to maintain uniform
corrosion performance.
Apart from the direct contribution of the
intermediate alloy layer to the corrosion performance of
the Al-Zn-Si-Mg alloy coating by virtue of the composition
and/or thickness of the intermediate alloy layer, the
applicant has also found that the crystal structure of the
intermediate alloy layer can have an indirect impact on
the corrosion performance of the overall MAZ alloy coating
by way of cracking. The applicant has found that the
intermediate alloy layer is one significant source of
crack initiation when the Al-Zn-Mg-Si alloy coated strip
is subjected to high strain operations such as roll
forming. Coarse intermediate alloy layer crystal
structures result in. wider and more numerous cracks
penetrating through the overlay alloy layer of the coating
and the corrosion performance of the Al-Zn-Mg-Si alloy
coated strip will suffer. Although the intermediate alloy
layer can include eguiaxial , columnar or a mixture of
eguiaxial and columnar crystals, to minimise cracking, it
is desirable to control the size of columnar crystals to
b no more than lOOOnm in a short diameter as measured on
a cross section through the thickness of the coating,
and/or the size of eguiaxial crystals to be no more than
4000nm in a long diameter as measured on a cross section
through the thickness of the coating.
Although, from the corrosion performance point of
view, it is desirable to have the presence of a
substantial intermediate alloy layer (or no thinner than
0. um) , it is disadvantageous if the intermediate alloy
layer is too thick (or thicker than 5 um) , as this induce
cracking and impair the roll formability of the coated
stri .
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 research and
development work described above in relation to Figures 2
9 focused on coatings formed from coating baths of
particular Al-Zn- S -Mg alloys, the present invention is
not confined to these particular alloys.

CLAIMS :
1 . A metallic coated steel strip that includes a
steel strip and a metallic coating on at least one side of
the strip, with the metallic coating including an Al-Zn-
Mg-Si overlay alloy layer and an intermediate alloy layer
between the steel strip and the overlay alloy layer, and
wherein the intermediate alloy layer has a composition of,
by weight, 4.0-12. 0%Zn, 6.0-17.0%Si, 20 .0-40 .0%Fe , 0.02-
0.50%Mg, and balance Al and unavoidable impurities.
2 . The strip de ned in claim 1 wherein the
intermediate alloy layer includes, by weight, 0.01-0.2%Ca.
3 . The strip defined in claim 1 or claim 2 wherein
the intermediate alloy layer includes, by weight, 0.1-
3.0%Cr.
. The strip defined in any one of the preceding
claims wherein the intermediate alloy layer includes, by
weight, 0.1-13.0%Mn.
5 . The strip defined in any one of the preceding
claims wherein the intermediate alloy layer includes , by
weight, 0.1-2.0%V.
6 . The strip defined in any one of the preceding
claims wherein the intermediate alloy layer has a
thickness of 0 .1-5 .Oum as measured on a cross-section
through the thickness of the coating.
. The strip defined in claim 6 wherein the
intermediate alloy layer has a thickness of 0.3-2. Oum as
measured on the cross-section through the thickness of the
coating.
8 . The strip defined in claim 7 wherein the
intermediate alloy layer has a thickness of 0.5-1.0um as
measured on the cross-section through the thickness of the
coating.
9 . The strip defined in any one of the preceding
claims wherein the intermediate alloy layer includes
substantially columnar crystals measuring 50-1000nm in a
short diameter as measured on a cross section through the
thickness of the coating.
10 . The strip defined in any one of claims 1 to 8
wherein the intermediate alloy layer includes
substantially equiaxial crystals measuring 50-4000nm in a
long diameter as measured on a cross section through the
thickness of the coating.
11 . The strip defined in any one of claims 1 to 8
wherein the intermediate alloy layer includes a mixture of
columnar crystals and equiaxial crystals.
12 . The strip defined in any one of claims 1 to 8
wherein the intermediate alloy layer includes body centred
cubic crystals .
13 . The strip defined in any one of the preceding
claims wherein Al, Zn, Si and Fe concentrations of the
intermediate alloy layer satisfy the formula FexoAl32Si n3.
1 . The strip defined in any one of the preceding
claims wherein Al , Zn , Si and Fe concentrations of the
intermediate alloy layer satisfy the formula 10Al3 S n2 .
15 . The strip defined in any one of the preceding
claims is a passivated strip using a Cr-containing or Crfree
passivation system.
16. The strip defined in any one of the preceding
claims includes a resin coating on an exposed surface of
the metallic coating.
17. The strip defined in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating includes more than 0.3 % by weight Mg.
18. The strip defined in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating includes more than 1.0 % by weight Mg .
19. The strip defined in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating includes more than 1.3 % by weight Mg.
20. The strip defined in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating includes more than 1.5 % by weight Mg .
21 . The strip defined in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating includes less than 3 % by weight Mg.
22. The strip defined in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating includes less than 2.5 % by weight Mg.
23 . The strip de ned in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating includes more than 1.2 % by weight Si .
2 . The strip defined in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating 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
25. The strip defined in any one of the preceding
claims wherein a molten Al-Zn-Si-Mg alloy for forming the
metallic coating 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 1.0 to 3.0%
balance Al and unavoidable impurities
26. A method of forming a metallic coating on a steel
strip to form the metallic coated steel strip defined in
any one of the preceding claims , the method including
dipping steel strip into a bath of a molten Al-Zn-Si-Mg
alloy and forming a metallic coating of the alloy on
exposed surfaces of the steel strip, and the method
including controlling any one or more of the composition
of the molten alloy bath, the temperature of the molten
alloy bath, and the immersion time of the steel strip in
the molten alloy bath to form the intermediate alloy layer
between the steel strip and the Al-Zn-Mg-Si overlay alloy
layer .