METAL COATED STEEL STRIP
The present invention relates to strip, typically
steel strip, which has a corrosion-resistant metal alloy
coating of an alloy that contains aluminium, zinc, and
silicon and is hereinafter referred to as an "Al-Zn-Si
alloy" on this basis .
The present invention relates particularly but not
exclusively to a corrosion-resistant metal alloy coating
that contains aluminium, zinc, silicon, and magnesium as
the main elements in the alloy coating and is hereinafter
referred to as an "Al-Zn-Si—Mg alloy" on this basis. The
alloy coating may contain other elements that are present
as deliberate alloying additions or as unavoidable
impurities .
The present invention relates particularly but not
exclusively to steel strip that is coated with the above -
described Al-Zn-Si-Mg alloy and can be cold formed (e.g.
by roll forming) into an end-use product, such as roofing
products .
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%.
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) .
The present invention relates particularly but not
exclusively to a cold formed (e.g. roll formed) end-use
product (e.g. profiled wall and roofing sheet) comprising
steel strip that is coated with the above-described Al-Zn-
Si-Mg alloy and is optionally coated with a paint.
Typically, the corrosion-resistant metal alloy
coating is formed on steel strip by a hot dip coating
method.
In the conventional hot-dip metal coating method,
steel strip generally passes through one or more heat
treatment furnaces and thereafter into and through a bath
of molten metal alloy held in a coating pot.
The metal alloy is usually maintained molten in the
coating pot by the use of heating inductors. The strip
usually exits the heat treatment furnaces via an outlet
end section in the form of an elongated furnace exit chute
or snout that dips into the bath. Within the bath the
strip passes around one or more sink rolls and is taken
upwardly out of the bath and is coated with the metal
alloy as it passes through the bath.
After leaving the coating bath the metal alloy coated
strip passes through a coating thickness control station,
such as a gas knife or gas wiping station, at which its
coated surfaces are subjected to jets of wiping gas to
control the thickness of the coating.
The metal alloy coated strip then passes through a
cooling section and is subjected to forced cooling.
The cooled metal alloy coated strip may thereafter be
optionally conditioned by passing the coated strip
successively through a skin pass rolling section (also
known as a temper rolling section) and a tension levelling
section. The conditioned strip is coiled at a coiling
station.
The aluminium and zinc are provided in an Al-Zn-Si
alloy coating on a steel strip for corrosion resistance.
The aluminium, zinc, and magnesium are provided in an
Al-Zn-Si alloy coating on a steel strip for corrosion
resistance .
The silicon is provided in both alloy types to
prevent excessive alloying between a steel strip and the
molten coating in the hot-dip coating method. A portion
of the silicon takes part in a quaternary alloy layer
formation but the majority of the silicon precipitates as
needle-like, pure silicon particles during solidification.
These needle-like silicon particles are also present in
the inter-dendritic regions of the coating.
One corrosion resistant metal coating composition
that has been used widely in Australia and elsewhere for
building products , particularly profiled wall and roofing
sheets, for a considerable number of years is an Al-Zn-Si
alloy composition comprising 55%A1 . 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. However, Al-Zn-Si-Mg alloy coatings on steel
strip are not commercially available in Australia.
The above description is not to be taken as an
admission of the common general knowledge in Australia or
elsewhere.
It has been found by the applicant that magnesium and
vanadium enhance specific aspects of corrosion performance
of 55%A1-Zn-Si alloy metallic coated steel strip.
In particular, it has been found by the applicant
that when Mg is included in a 55%A1-Zn-Si coating
composition, it brings about certain beneficial effects on
product performance, such as improved cut-edge protection,
by changing the nature of corrosion products formed in
both marine and acid rain environments . This improvement
in corrosion performance has been demonstrated by research
work carried out by the applicant including comprehensive
accelerated corrosion testing and outdoor exposure testing
carried out by the applicant. For magnesium additions,
the improvement in the level of edge undercutting for
metallic coated steel with a paint coating is more
pronounced than the improvement in bare surface corrosion
of the metallic coating in marine environments .
It has also been found by the applicant that when V
is included in Al-Zn-Si alloy coating compositions, the V
brings about certain beneficial effects on product
performance . The applicant has found that the level of
mass loss from bare (unpainted) metallic coated steel
strip surfaces tested on outdoor exposure is reduced by an
average of 33% for a range of environments. As distinct
from magnesium, the improvement in coating loss from bare
(unpainted) surfaces is much greater than improvements in
the level of edge undercutting for metallic coated steel
strip with a paint coating.
The present invention is a metal, typically steel,
strip that has a coating of an Al-Zn-Si alloy that
contains 0.3-10 wt.% Mg and 0.005-0.2 wt.% V in order to
take advantage of the above-mentioned complementary
aspects of corrosion performance of the coating.
More particularly, the addition of the Mg and the V
improves both the bare mass loss of the strip and the edge
undercutting of painted, metallic coated strip to a level
that is greater than could be obtained by larger separate
additions of each respective element alone.
The coating alloy may be an Al-Zn-Si-Mg alloy that
comprises the following ranges in % by weight of the
elements Al, n , Si, and Mg:
Al: 40 to 60 %
Zn: 30 to 60 %
Si: 0.3 to 3%
Mg: 0.3 to 10 %
The coating alloy may be an Al-Zn-Si-Mg alloy that
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 coating alloy may contain less than 0.15 wt.% V .
The coating alloy may contain less than 0.1 wt.% V .
The coating alloy may contain at least 0.01 wt.% V .
The coating alloy may contain at least 0.03 wt.% V .
The coating alloy may contain other elements .
The other elements may be present as unavoidable
impurities and/or as deliberate alloy additions.
By way of example, the coating alloy may contain any
one or more of Fe, Cr, Mn, Sr, and Ca.
The coating may be a single layer as opposed to
multiple layers .
The coating may be a coating that does not include a
non-equilibrium phase.
The coating may be a coating that does not include an
amorphous phase.
The coated metal strip may have a paint coating on an
outer surface of the alloy coating.
The present invention is also a cold formed (e.g.
roll formed) end-use product (e.g. profiled wall and
roofing sheet) comprising steel strip that is coated with
the above-described coating alloy and is optionally coated
with a paint.
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; and
Figure 2 is an Anodic Tafel plot showing a comparison
of coating alloys , including an embodiment of an alloy
coating in accordance with the present invention.
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 present invention is based
on research work carried out by the applicant on the known
55%A1-Zn-Si alloy coating on steel strip which found that
magnesium and vanadium enhance specific aspects of
corrosion performance of the coated steel strip.
The research work included accelerated corrosion
testing and outdoor exposure testing in acidic and marine
environments for extended time periods .
The Anodic Tafel plot in Figure 2 illustrates the
results of a part of the research work. The plot shows
the logarithm of the current density ("J" - in A/cm2)
against the electrode potential (in Volts) for 3 alloy
compositions . The plot shows the results of research work
on coatings of (a) the known 55%A1-Zn-Si alloy ("AZ") , (b)
an Al-Zn-Si-Zn alloy containing Ca ("AM(Ca)"), and (c) an
Al-Zn-Si-Zn alloy containing V in accordance with one
embodiment of the present invention ("AM(V)").
The plot of Figure 2 compares the corrosion
performance of the alloy coatings (a) , (b) , and (c) . The
plot and other results obtained by the applicant indicate
that:
(a) the AM(V) alloy coating of the present
invention had a lower corrosion current at a given
corrosion potential than the other alloy coatings (1.5-2
times improvement of AM(V) over AM(Ca));
(b) the AM(V) alloy coating of the present
invention had more noble corrosion potential compared to
AM (Ca) (+0.03 V and +0.11 V respectively);
(c) the AM(V) alloy coating of the present
invention had more noble pitting potential compared to
AM (Ca) (+0.04 V and +0.18 V respectively); and
(d) the AM(V) alloy coating of the present
invention had significantly lower oxidative current under
anodic polarisation - compared to AM(Ca) , at -0.25 V , the
oxidative current is about 20000 times less for AM(V) .
These improvements in the resistance for anodic
dissolution of the alloy layer imply that upon exposure of
the alloy coating of the present invention to corrodants
(salt, acid, and dissolved oxygen) the metallurgical phase
will corrode at a slow rate and the mode of corrosion will
be generalised and less prone to localised and pitting
corrosion mode. These properties will impart a longer
life in an end-use product, as it will be rendered less
likely to red rust staining, metal coating blistering and
substrate perforation.
Many modifications may be made to the present
invention as described above without departing from the
spirit and scope of the invention.

CLAIMS
1 . A metal strip that has a coating of an Al-Zn-Si alloy
that contains 0.3-10 wt.% Mg and 0.005-0.2 wt.% V .
2 . The metal strip defined in claim 1 wherein the
coating alloy is an Al-Zn-Si-Mg alloy that 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 %
3 . The metal strip defined in claim 1 or claim 2 where
the coating alloy is an Al-Zn-Si-Mg alloy that 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%.
4 . The metal strip defined in any one of the preceding
claims wherein the alloy coating contains less than
0.15 wt.% V .
5 . The metal strip defined in any one of the preceding
claims wherein the alloy coating contains less than
0.1 wt.% V .
6 . The metal strip defined in any one of the preceding
claims wherein the alloy coating contains at least
0.01 wt.% V .
7 . The metal strip defined in any one of the preceding
claims wherein the alloy coating contains at least
0.03 wt.% V .
8 . The metal strip defined in any one of the preceding
claims wherein the alloy coating contains other elements
present as unavoidable