METAL-COATED STEEL STRIP
The present invention relates to the production
of strip, typically steel strip, which has a corrosionresistant
metal alloy coating that contains aluminiumzinc-
silicon-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.
More particularly, the present invention is
concerned with minimising the amount of top dross in the
alloy coating bath. Top dross is undesirable from the
viewpoints of cost of production and coating quality, as
is discussed further below.
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:
Al: 40 to 60 %
Zn: 30 to 60 %
Si: 0.3 to 3%
Mg: 0.3 to 10 %
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:
Al: 45 to 60 %
Zn: 35 to 50 %
Si: 1.2 to 2.5%
Mg 1.0 to 3.0%
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
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.
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%A1-Zn coating composition that also
comprises 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 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 coating composition, Mg brings about certain
beneficial effects on product performance, such as
improved cut-edge protection.
The applicant has found that Mg-containing molten
55%A1-Zn coating metal is susceptible to increased levels
of top dross generation compared to molten 55%A1-Zn
coating metal that does not contain Mg.
The term "top dross" is herein understood to
include any one or more of the following components on or
near the surface of the molten bath:
(a) an oxide film on the surface of a molten bath,
(b) molten metal droplets covered by an oxide film,
(c) gas bubbles having an oxide film as the wall of
the bubbles ,
(d) intermetallic particles that are formed in the
coating bath, including particles covered by an oxide
film, and
(e) combinations of any two or more of gas, molten
metal , and intermetallic particles covered by an oxide
film.
Items (b) , (c) , (d) , and (e) can be described as
the result of entrainment of molten metal, gas, and
intermetallic particles in the oxide film on or near the
surface of the molten bath.
During a line trial to hot-dip metal coat a Mgcontaining
55%A1-Zn alloy onto a steel strip that has been
conducted by the applicant it was shown that the level of
top dross generated in the coating bath was 6 to 8 times
that of the top dross formed in a 55%A1-Zn alloy coating
bath without Mg addition. Whilst not wishing to be bound
by the following comment, the applicant attributes the
generation of excessive top dross in Mg-containing molten
coating alloys to the reactivity and rapid oxidation of Mg
in the alloys and the changes in the properties of the
liquid metal (for example, surface tension) that result
from the addition of Mg to 55%A1-Zn alloy baths. More
particularly, Mg has a higher affinity for oxygen compared
with Al and therefore Mg oxidises much more readily than
Al . This is evident from the standard free energy of
formation (AG°) of the oxides which shows that: the
thermodynamic driving force for oxide formation is much
greater for Mg than for Al
1015kJ/mol at a bath operating temperature of 600 °C) .
Moreover, turbulence in the melt surface enhances both the
oxidation of molten metal in the bath and the entrainment
of the oxide film in the coating bath. The entrainment of
the oxide film in the coating bath results in the
entrainment of molten metal, gas, and intermetallic
particles in the oxide film in the molten bath and the
consequential formation of the dross components described
in items (b) , (c) , (d) , and (e) above. This top dross has
high volume fractions of voids, oxide stringers and dross
intermetallic particles entrained in the top dross.
The amount of top dross generated has a
significant impact on the cost of production of Mgcontaining
55%A1-Zn alloy coated steel. Top dross must be
removed from the bath surface periodically to prevent
surface defects on the coated steel strip. The removal of
top dross represents a cost to the producer of coated
steel strip due to the cost of the removal process and the
cost of top dross disposal or recycling. Reducing top
dross generation provides an opportunity to significantly
reduce the cost of production.
In addition, reducing top dross also provides an
opportunity to lead to improved surface quality of the
coated strip by reducing entrainment of oxide stringers
and suspended dross particles.
The above discussion is not to be taken as an
admission of the common general knowledge in Australia and
elsewhere .
The applicant has been able to reduce the top
dross levels in molten Al-Zn-Si-Mg alloy baths by the
addition to molten baths of (a) Ca, (b) Sr and (c) Ca and
Sr and the reduction in top dross levels has lead to
benefits in terms of production costs and product quality.
The addition of these elements is hereinafter referred to
as the addition of "Ca and/or Sr" . It is noted that the
above reference to the addition of Ca and Sr is not
intended to indicate that Ca is added before Sr. The
present invention extends to situations in which Ca and Sr
are added at the same time or at different times to molten
baths .
The applicant found that this reduction in top
dross generation in molten Al-Zn-Si-Mg alloy baths by the
addition of Ca and/or Sr to the baths is due to changes in
the entrainment of gas , molten metal and intermetallic
particles in oxide films in top dross in the baths
resulting from (a) changes to the apparent surface tension
at the liquid metal/oxide interface as a result of the Ca
and/or Sr addition and (b) changes in the nature of the
oxide film as a result of the Ca and/or Sr addition. The
changes in the nature of the oxide film reduce the level
of oxide stringers formed, which in turn assists in an
overall reduction in liquid droplet entrainment.
According to the present invention there is
provided a method of forming an Al-Zn-Si-Mg alloy coating
on a strip that includes dipping strip into a bath of
molten Al-Zn-Si-Mg alloy and forming a coating of the
alloy on the strip, with the bath having a molten metal
layer and a top dross layer on the metal layer, and the
method including controlling the conditions in the molten
bath to minimise the top dross layer in the molten bath.
The method may include controlling the conditions
in the molten bath to minimise entrainment of any one or
more of molten metal, gas, and intermetallic particles in
oxide films in the top dross layer.
The conditions in the bath may include the
composition of the alloy in the bath.
Hence, the method may include controlling the
composition of the bath to minimise the top dross layer in
the molten bath, for example by minimising liquid droplet
entrainment in oxide films in the top dross layer in the
bath.
The method may include controlling the
composition of the bath to minimise the top dross layer in
the molten bath by including Ca in the composition of the
bath.
The composition of the bath may include more than
50 ppm Ca. It is noted that all references to ppm in the
specification are references to ppm by weight
It is noted that the reference to amounts of
elements such as Ca and Sr as part of the composition of a
molten bath are understood herein to be references to the
concentrations of the elements in the molten metal layer
of the bath as opposed to the top dross layer in the bath.
The reason for this is that it is the standard practice of
the applicant to measure bath concentrations in the molten
metal layers of molten baths .
It is also noted that the applicant found that Ca
and Sr tend to segregate to the top dross layer of molten
baths and, as a consequence the top dross layer becomes
enriched with respect to Ca and Sr when compared to the
metal layer. Specifically, if there is x" wt.% of Ca or
Sr in the molten metal layer of a molten bath, there will
be a higher concentration of the element in the top dross
layer of the bath. For example, the applicant found in
laboratory work that in a bath with a nominal bath
composition of 90 ppm Ca, the Ca content of the top dross
layer increased to 100 ppm Ca. Similarly, the applicant
found that in a bath with a nominal composition of 400 ppm
Ca, the top dross layer was enriched substantially to 600
ppm. Similar enrichments were also observed for Sr in
laboratory work. For example, in a bath with a nominal
composition of 500 ppm Sr, after 3 hrs of processing the
top dross layer was enriched in Sr to 700 ppm. And in a
bath with a nominal composition of 750 ppm Sr, after 3 hrs
of processing the top dross layer was enriched to 1100 ppm
Sr. In practice, this means that, if it is required that
there be "x" wt.% of Ca or Sr in the molten metal layer of
a molten bath, it will be necessary to add an amount of Ca
or Sr that is greater than x" wt.% in the total bath to
compensate for the higher concentration of Ca or Sr that
will segregate to the top dross layer.
The composition of the bath may include more than
150 ppm Ca.
The composition of the bath may include more than
200 ppm Ca.
The composition of the bath may include less than
1000 ppm Ca.
The composition of the bath may include less than
750 ppm Ca.
The composition of the bath may include less than
500 ppm Ca.
The Ca may be added to the bath as required. It
could be by way of specific additions of Ca compounds on a
continuous or a periodic basis. It could also be by way
of the inclusion of Ca in Al and/or Zn ingots that are
provided as feed materials for the bath.
The method may include controlling the
composition of the bath to minimise the top dross layer in
the molten bath by including Sr in the composition of the
bath.
The composition of the bath may include more than
100 ppm Sr.
The composition of the bath may include more
than 150 ppm Sr.
The composition of the bath may include more than
200 ppm Sr.
The composition of the bath may include less than
1250 ppm Sr.
The composition of the bath may include less than
1000 ppm Sr.
The Sr may be added to the bath as required. I
could be by way of specific additions of Sr compounds on
continuous or a periodic basis. It could also be by way
of the inclusion of Sr in Al and/or Zn ingots that are
provided as feed materials for the bath.
The method may include controlling the
composition of the bath to minimise the top dross layer in
the molten bath by including Ca and Sr in the composition
of the bath.
The amounts of Ca and Sr in the composition may
be as described above, with adjustments to the amounts of
each element to compensate for the effect of an addition
of the other element on the top dross layer.
The method may include controlling the
composition of the bath to minimise the top dross layer in
the molten bath by including rare earth elements such as
yttrium and a combination of rare earths and Ca and/or Sr
in the composition of the bath.
The method may include controlling the
composition of the bath to minimise the top dross layer in
the bath by periodically monitoring the concentration of
any one or more of Ca, Sr, and rare earth elements that
are in the bath, and adding Ca, Sr, and rare earth
elements as required to maintain the bath composition for
the element or elements .
In a situation in which the Ca, Sr, and rare
earth elements are part of ingots of other elements that
are in the composition in the bath, the method may include
selecting any one or more of the sizes of the ingots, the
timing of the addition of the ingots , and the sequence of
the addition of the ingots to maintain the concentration
of Ca, Sr, and rare earth elements substantially constant
or within a preferred range of + or - 10% for the
elements .
The Al-Zn-Si-Mg alloy may comprise more than 0.3
% by weight Mg.
The Al-Zn-Si-Mg alloy may comprise more than 1.0
% by weight Mg.
The Al-Zn-Si-Mg alloy may comprise more than 1.3
% by weight Mg.
The Al-Zn-Si-Mg alloy may comprise more than 1.5
% by weight Mg.
The Al-Zn-Si-Mg alloy may comprise less than 3 %
by weight Mg.
The Al-Zn-Si-Mg alloy may comprise more than 2.5
% by weight Mg.
The Al-Zn-Si-Mg alloy may comprise more than 1.2
% by weight Si .
The Al-Zn-Si-Mg alloy may comprise the following
ranges in % by weight of the elements Al, Zn, Si, and Mg:
Al: 40 to 60 %
Zn: 30 to 60 %
Si: 0.3 to 3%
Mg: 0.3 to 10 %
In particular, the Al-Zn-Si-Mg alloy may
comprise the following ranges in % by weight of the
elements Al, Zn, Si, and Mg:
Al: 45 to 60 %
Zn: 35 to 50 %
Si: 1.2 to 2.5%
Mg 1.0 to 3.0%
According to the present invention there is also
provided an Al-Zn-Si-Mg alloy coating on a strip produced
by the above-described method.
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 the mass of dross versus
time for molten Al-Zn-Si alloy baths with and without Mg
and with and without Ca in experiments on dross generation
carried out by the applicant;
Figure 3 is a graph of the mass of dross versus
time for molten Al-Zn-Si alloy baths with and without Mg
and with and without Sr in experiments on dross generation
carried out by the applicant;
Figure 4 presents selected results from the
experimental work summarised in Figures 2 and 3 that
highlights the impact of Ca and Sr on top dross
generation ;
Figure 5 is a graph of the mass of dross versus
Ca content in Al-Zn-Si-Mg alloy baths after process times
of 1 and 3 hours ; and
Figure 6 is a graph of the mass of dross
generated versus time during the course of a line trial
carried out by the applicant.
With reference to Figure 1 , in use, coils of cold
rolled 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 Al-Zn-Si-Mg alloy. The Al-Zn-Si-Mg
alloy is maintained molten in the coating pot 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. 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 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 is indicated above, the applicant has found
that Al-Zn-Si-Mg alloy coating baths generate
substantially greater amounts of top dross in the baths
than is the case with conventional 55%A1-Zn alloy baths in
the coating lines of the applicant.
As discussed above, the applicant has conducted a
number of laboratory experiments and line trials to
determine whether it is possible to reduce the amount of
dross generated in an Al-Zn-Si-Mg alloy bath. As
discussed above, the applicant found that it was possible
to significantly reduce the level of top dross by the
addition of Ca or Sr to Al-Zn-Si-Mg alloys in coating
baths .
The experimental results on the effect of Ca and
Sr additions to coating baths on the level of top dross
generation in Al-Zn-Si-Mg alloy coating baths are
summarized in Figures 2 to 5 .
The experimental work was carried out on the
following alloy compositions, in wt. % for (a) an Al-Zn
alloy (referred to as "AZ" in the Figures) and (b) an AlZn-
Mg alloy (referred to as "MAZ" in the Figures) and (c)
these A Z and MAZ alloys plus parts per million (ppm) Ca
and Sr additions to these compositions :
AZ: 55Al-43Zn-l.5Si-0.5Fe
MAZ: 53Al-43Zn-2Mg-l.5Si-0.5Fe
MAZ + 236 ppm Ca.
MAZ + 90 ppm Ca.
MAZ + 400 ppm Ca.
MAZ + 500 ppm Sr.
MAZ + 750 ppm Sr.
MAZ + 800 ppm Sr.
It is noted that the concentrations of Ca and Sr
are the concentrations of these elements in the metallic
parts of molten baths .
In the experimental work the top dross generation
was simulated using a laboratory melting furnace and an
overhead mechanical stirrer. The laboratory set-up
consisted of the following components:
A melting furnace with clay graphite crucible.
A variable speed overhead mechanical stirrer with a
support stand.
Dross collector cup machined from high density
sintered boron-nitride ceramic and having a series of
drainage holes in the bottom of the cup and a series
of upstanding handles to allow the cup to be
positioned and removed from the crucible.
Stainless steel impel lor shaft.
Impellor machined from high density sintered boron
nitride ceramic.
The dross collector cup and the impellor were
fabricated from a high temperature material that is nonwetting
to molten A Z and MAZ alloys . The sintered boron
nitride ceramic of these components provided excellent
non-wetting characteristics and high temperature stability
in the coating bath.
For each experiment, 15kg of the coating alloy of
a required composition was formed in the crucible and held
at the process temperature of 600°C. The dross collector
cup was then inserted into the molten bath and was
retained in the bath until the melt temperature reached
the process temperature. Then the shaft impellor assembly
was lowered into the bath until the impellor just touched
the surface of the melt. The stirrer motor was then
switched on and the stirring speed was adjusted to 60RPM.
This experimental set-up resulted in shearing of the
surface of the bath without creating a vortex so that at
each revolution of the impellor a fresh melt was
continuously exposed to air to generate dross. The dross
generated was pushed to the side of the crucible and
accumulated on the side of the crucible. At the end of
each experiment the accumulated dross was removed from the
crucible by lifting the dross collector cup from the
crucible and allowing excess entrained bath metal to drain
into the crucible via holes in the dross collector cup.
What was left in the dross collector cup comprised the
entrained bath metal and dross intermetallic particles
covered with oxide film. This retained material was the
top dross generated in each experiment.
The experiments were conducted for durations of
0.5, 1.2, and 3 hrs .
After each experiment the dross collected was
removed and weighed and the results are plotted as shown
in Figures 2 to 5 .
Figures 2 to 4 are graphs of the mass of dross
versus time for the molten alloy baths , with the Figure 2
results focusing on the results for the Ca alloys and the
Figure 3 results focusing on the results for the Sr alloys
and the Figure 4 results highlighting selected results for
Ca and Sr from Figures 2 and 3 .
Figure 5 is a graph of the mass of dross versus
Ca content in molten alloy baths after process times of 1
and 3 hours .
Figures 2 to 5 clearly show that the level of top
dross generated in an Al-Zn-Si-Mg alloy bath can be
significantly reduced by additions of Ca or Sr to MAZ
alloy coating baths. More particularly, Figures 2 to 5
show that:
(a) MAZ alloy coating baths generate significantly
higher amounts of top dross that AZ alloy coating baths,
and
(b) the amount of top dross decreases significantly
with increasing amounts of Ca and Sr in the MAZ alloys .
The results shown in Figures 2 to 5 were further
confirmed for Ca in a line trial conducted for
approximately 2 weeks. The line trial was carried out on
the above-mentioned AZ alloy to which Mg and Ca were added
at different points in time during the course of the line
trial. Figure 6 shows the dross collected during the line
trial and that the results are consistent with what was
observed in the laboratory work. In particular, Figure 6
shows that there was a substantial increase in the amount
of dross generated in the molten bath with the addition of
Mg to the bath and a substantial decrease in the amount of
dross as a consequence of the addition of Ca to the bath.
As is indicated above, the applicant attributes
the reduction in the dross level to reduction in the
entrainment of molten metal, gas, and intermetallic
particles in the oxide film in the molten bath (i.e. in
the top dross layer in the bath) that resulted from (a)
changes to the apparent surface tension at the liquid
metal/oxide interface as a result of the Ca and Sr
additions and (b) changes in the nature of the oxide film
as a result of the Ca and Sr additions . The changes in
the nature of the oxide film reduced the level of oxide
stringers formed, which in turn assists in an overall
reduction in liquid droplet entrainment. The changes in
the entrainment lead to reductions in the level of top
dross generation in molten Al-Zn-Si-Mg alloys.
Ca and Sr are examples of elements that can be
added to a molten bath of an Al-Zn-Si-Mg alloy to reduce
the entrainment of molten metal, gas, and intermetallic
particles in the oxide film in the bath and thereby reduce
the level of dross in the bath. Other bath additions
include, by way of example, rare earth elements such as
yttrium and combinations of rare earths and calcium and
strontium and calcium/ strontium.
In practice, the Ca and/or Sr may be added to the
bath as required. It could be by way of specific
additions of Ca and/or Sr compounds on a continuous or a
periodic basis. It could also be by way of the inclusion
of Ca and/or in Al and/or Zn ingots that are provided as
feed materials for the bath.
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 strip that includes dipping strip into a bath of
molten Al-Zn-Si-Mg alloy and forming a coating of the
alloy on the strip, with the bath having a molten metal
layer and a top dross layer on the metal layer, and the
method including controlling the conditions in the molten
bath to minimise the top dross layer in the molten bath.
2 . The method defined in claim 1 including
controlling the conditions in the molten bath to minimise
the top dross layer in the molten bath by controlling the
conditions in the molten bath to minimise entrainment of
any one or more of molten metal, gas, and intermetallic
particles in oxide films in the top dross layer.
3 . The method defined in claim 1 or claim 2
including controlling the composition of the bath to
minimise the top dross layer in the molten bath.
. The method defined in claim 3 including
controlling the composition of the bath to minimise the
top dross layer in the molten bath by including Ca in the
composition of the bath.
5 . The method defined in claim 4 including
controlling the composition of the bath to include more
than 50 ppm Ca.
6 . The method defined in claim including controlling
the composition of the bath to include more than 150 ppm
Ca.
7. The method defined in claim 4 including
controlling the composition of the bath to include more
than 200 ppm Ca.
8. The method defined in any one of claims 4 to 7
including controlling the composition of the bath to
include less than 1000 ppm Ca.
9 . The method defined in any one of claims 4 to 7
including controlling the composition of the bath to
include less than 750 ppm Ca.
10. The method defined in any one of claims 4 to 7
including controlling the composition of the bath to
include less than 500 ppm Ca.
11 . The method defined in claim 3 including
controlling the composition of the bath to minimise the
top dross layer in the molten bath by including Sr in the
composition of the bath.
12 . The method defined in claim including controlling
the composition of the bath to include more than 100 ppm
Sr.
13. The method defined in claim 11 including
controlling the composition of the bath to include more
than 150 ppm Sr.
14 . The method defined in claim 11 including
controlling the composition of the bath to include more
than 200 ppm Sr.
15. The method defined in any one of claims 11 to 14
including controlling the composition of the bath to
include less than 1250 ppm Sr.
16. The method defined in any one of claims 11 to 14
including controlling the composition of the bath to
include less than 1000 ppm Sr.
17. The method defined in any one of claims 3 to 16
including controlling the composition of the bath to
minimise the top dross layer in the molten bath by
including Ca and Sr in the composition of the bath.
18. The method defined in any one of claims 3 to 17
including controlling the composition of the bath to
minimise the top dross layer in the molten bath by
including rare earth elements such as yttrium and
combinations of rare earths and Ca and/or Sr in the
composition of the bath.
19. The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy comprises more than
0.3 % by weight Mg .
20 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy comprises more than
1.0 % by weight Mg.
21 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy comprises less than 3
% by weight Mg.
22 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy comprises more than
1.2 % by weight Si.
23 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy comprises the
following ranges in % by weight of the elements Al, Zn,
Si, and Mg:
Al: 40 to 60 %
Zn: 30 to 60 %
Si: 0.3 to 3%
Mg: 0.3 to 10 %
2 . The method defined in any one of the preceding
claims wherein the Al-Zn-Si-Mg alloy comprises the
following ranges in % by weight of the elements Al, n ,
Si, and Mg:
Al: 45 to 60 %
Zn: 35 to 50 %
Si: 1.2 to 2.5%
Mg 1.0 to 3.0%