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 coating of a corrosion-resistant metal alloy that contains aluminium-zincsilicon- 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 a coating of an Al-Zn-Si-Mg
alloy on a strip that includes dipping uncoated strip into a
bath of molten Al-Zn-Si-Mg alloy and forming a coating of the
alloy on the strip.
Typically, the Al-Zn-Si-Mg alloy of the present invention
comprises the following ranges in % by weight of the elements
Al , Zn , Si , and Mg :
Zn: 30 to 60 %
Si: 0.3 to 3%
Mg: 0.3 to 10 %
Balance Al and unavoidable impurities.
More typically, the Al-Zn-Si-Mg alloy of the present
invention comprises the following ranges in % by weight of the
elements Al , Zn, Si, and Mg:
Zn: 35 to 50 %
Si: 1.2 to 2.5%
Mg 1.0 to 3.0%
Balance Al and unavoidable impurities.
The Al-Zn-Si-Mg alloy may contain other elements that are
present in the 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 .
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 partixdla dissolution of the
metal strip into the coating during the coating process.
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, 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 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 .
It has been established that when Mg is included in a
55%A1-Zn alloy coating, 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 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 there was greater dissolution of Al-Zn-Si-Mg alloy
coatings into quench water than was the case with conventional
Al-Zn coatings and the dissolution resulted in precipitates in
quench water that caused a rapid deterioration of cooling
water circuit heat exchangers and caused undesirable coatings
to form on cooling water storage tank surfaces in the quench
water circuits in the metal coating lines. The precipitation
problem is a potentially serious maintenance issue.
After identifying the precipitate problem and carrying
out further research and development work, the applicant found
that pH control of cooling water and to a lesser extent
cooling water temperature control made it possible to reduce
the extent of precipitate formation and allowed the cooling
water heat exchangers to perform in a practical manner. More
particularly, the applicant found that the precipitate problem
could be addressed by suppressing the alkalinity of cooling
water via pH control of cooling water and to a lesser extent
cooling water temperature control (operating at low
temperatures) to thereby reduce the corrosiveness of the
cooling water towards Al-Zn-Si-Mg alloy coatings.
According to the present invention there is provided a
method of forming a coating of an Al-Zn-Si-Mg alloy on a steel
strip to form an Al-Zn-Mg-Si coated steel strip, the method
including the steps of dipping steel strip into a bath of
molten Al-Zn-Si-Mg alloy and forming a coating of the alloy on
exposed surfaces of the steel strip and cooling the coated
strip with cooling water, with the cooling step including
controlling the pH of cooling water to be in a range of pH 5-
9 .
The cooling step may include controlling the pH of
cooling water to be less than 8 .
The cooling step may include controlling the pH of
cooling water to be less than 7 .
The cooling step may include controlling the pH of
cooling water to be less than 7.5.
The cooling step may include controlling the pH of
cooling water to be greater than 5.5.
The cooling step may include controlling the pH of
cooling water to be greater than 6 .
The cooling step may include controlling the temperature
of cooling water to be in a range of 25-80°C.
The cooling step may include controlling the temperature
of cooling water to be less than 70°C.
The cooling step may include controlling cooling water
temperature to be less than 60°C.
The cooling step may include controlling cooling water
temperature to be less than 55°C.
The cooling step may include controlling cooling water
temperature to be less than 50°C.
The cooling step may include controlling cooling water
temperature to be less than 45°C.
The cooling step may include controlling cooling water
temperature to be greater than 30°C.
The cooling step may include controlling cooling water
temperature to be greater than 35°C.
The cooling step may include controlling cooling water
temperature to be greater than 40°C.
The cooling step may include controlling the pH by adding
acid to the cooling water.
The 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 cooling step may be a water quench step.
The 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 cooling step may be an open loop in which cooling
water is not recycled in the cooling step.
The cooling step may include controlling the operating
conditions to cool the coated strip to a temperature range of
28-55°C.
The 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 steps of pre-treating strip to clean the strip
before the hot dip coating step, controlling the thickness of
the coated strip immediately after the coating step, rolling
the coated strip, treating the coated strip with a passivation
solution, and coiling the coated strip.
The Al-Zn-Si-Mg alloy may include more than 0.3 % by
weight Mg.
The Al-Zn-Si-Mg alloy may include more than 1.0 % by
weight Mg.
The Al-Zn-Si-Mg alloy may include 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 include less than 3 % by weight
Mg.
The Al-Zn-Si-Mg alloy may include more than 2.5 % by
weight Mg.
The Al-Zn-Si-Mg alloy may include more than 1.2 % by
weight Si .
The Al-Zn-Si-Mg alloy may include less than 2.5 % by
weight Si .
The Al-Zn-Si-Mg alloy may include the following ranges in
% by weight of the elements Al , Zn, Si, and Mg:
Zn: 30 to 60 %
Si: 0.3 to 3%
Mg: 0.3 to 10 %
Balance Al and unavoidable impurities.
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 coating 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.
The steel may be a low carbon steel .
The present invention also provides an Al-Zn-Mg-Si 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 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 one embodiment 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;
Figure 2 is a graph of the Al and Ca concentrations in
cooling water used during the course of a plant trial carried
out by the applicant; and
Figure 3 is a graph of the Mg and Zn concentrations in
cooling water used during the course of the plant trial
carried out by the applicant.
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 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 A l and unavoidable
impurities. 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. 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 5-8, more typically in a range of 5.5-
7.5 and (b) the temperature of the cooling water supplied to
the spray system is controlled to be in a relatively low
temperature range of 30-50°C. Both control steps (a) and (b)
minimise dissolution of 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.
As discussed above, the applicant has conducted extensive
research and development work in relation to Al-Zn-Si-Mg alloy
coatings on steel strip.
The research and development work included plant trials
on metal coating line MCLl at the Springhill operations of the
applicant. The plant trials found that white precipitates
formed in the quench system of the line when the line was
operating with Al-Zn-Si-Mg alloy as a coating alloy for steel
strip. Significantly, it was found that these white
precipitates eventually blocked the quench system heat
exchanger. The Springhill metal coating lines are similar in
general terms to the line shown in Figure 1 and include a
closed loop quench step on each of the three lines (MCLl,
MCL2 , and MCL3) . Each closed loop processes a relatively
small volume (approx 5000L) of water. The cooling water is
cooled by dedicated heat exchangers on each line. The white
precipitates formed on cooling system equipment surfaces and
covered an initial layer of grey material. The grey layer was
found to contain Al(OH) 3 and A120 3 .3H20 from previous line
operations using conventional Al-Zn alloys. The white
precipitates were found to contain Mg4Al2 (OH) 14 .3H20 and
A120 3 .3H20 . These magnesium/aluminium oxy/hydroxides also
contained magnesium carbonate compounds.
The plant trials carried out by the applicant comprised
initial plant trials on Al-Zn-Si-Mg alloys in two groups of
alloy compositions that identified the precipitate problem in
the first instance and later more extensive plant trials that
confirmed the precipitate problem and evaluated several
options to minimise the problem. Group (a) alloys include the
following ranges in % by weight of the elements Al , n , Si,
and Mg: Al : 2 to 19 % , Si: 0.01 to 2 % , Mg: 1 to 10 % , and
balance Zn and unavoidable impurities. Group (b) alloys
include the following ranges in % by weight of the elements
Al, Zn, Si, and Mg: Al : 30 to 60 % , Si: 0.3 to 3 % , Mg: 0.3 to
10 % , and balance Zn and unavoidable impurities.
The following description focuses on the later plant
trials .
The later plant trials on the MCL1 line were carried out
by hot dip coating steel strip with the following alloys in
coating baths: (a) a known Al-Zn alloy (hereinafter referred
to as "AZ") and (b) an Al-Zn-Si-Mg alloy (hereinafter referred
to as "AM") having the following compositions, in wt. % :
• AZ: 55Al-43Zn-l .5Si-0 .45Fe-incidental impurities.
• AM: 53Al-43Zn-2Mg-l .5Si-0 .45Fe-incidental impurities (a
group (b) alloy) .
The later plant trials on the MCL1 line are summarised
below.
QUENCH SYSTEM - NO CONTROL
The first week of the plant trials on the MLC1 line was
run with the A Z (Al-Zn) alloy and produced standard Zincalume
(Registered Trade Mark) coated strip. The line was run in
accordance with established operating conditions. In terms of
the water cooling step on the line, the quench water was at a
temperature of 50-60°C upstream of the water sprays. There was
no pH control of the quench water. Under these conditions the
quench water became saturated with aluminium and the pH
increased to around 8.5 (at 60°C) .
A s soon as Mg (and a small amount of Ca for dross
control) was added to the metal coating pot to adjust the A Z
alloy composition to the AM (Al-Zn-Si-Mg) alloy coating
composition the pH started to rise and eventually reached
10.0. The quench water became milky white and the inlet
screens to the quench pumps became blocked with milky white
precipitates and had to be removed. The quench water was
analysed and the results of the analysis are presented in
Table 1 .
Table 1 - Quench Tank White Precipitate - AM alloy
A typical Al-Zn scale is almost all aluminium.
Consequently, the Table 1 data indicates that a surface layer
rich in Mg and Ca was dissolving in the quench water. The
proportion of Mg and Ca relative to Al in the quench deposits
was much higher than in the metal pot. The presence and
quantity of carbon in Table 1 also indicated that both Ca and
Mg were forming carbonates in the quench water. The white
precipitates were found to contain Mg4Al2 (OH) 4 .3H20 and
A12q 3 .3H20 . These magnesium/aluminium oxy/hydroxides also
contained magnesium carbonate compounds .
The presence of the white precipitate in the quench water
caused the quench heat exchanger to become blocked quickly.
When operating with conventional Al-Zn alloy compositions, the
quench heat exchanger on the line would typically last 9
months. The presence of magnesium and calcium made a
significant change to the surface characteristics of the
coated strip and increased the dissolution of the oxide layer
during the water cooling step.
The applicant considered a range of options to prevent or
minimise the dissolution for the AM Al-Zn-Si-Mg alloy coating.
The applicant settled on a strategy of suppressing the
alkalinity of cooling water via pH control of cooling water
and to a lesser extent cooling water temperature control to
thereby reduce the corrosiveness of the cooling water towards
Al-Zn-Si-Mg alloy coatings. The plant trials tested two
options, namely pH control and cooling water temperature
control, as discussed below.
QUENCH SYSTEM - pH CONTROL
A trial to control quench tank pH using phosphoric aci
ran for 4 days. The control system was set to allow a pre
determined [OH~] ion value of 1.0 x 10 6 mol/L.
Table 2 provides the values of the pH set point for
different water quench tank temperatures to maintain a set
Table 2 Quench Tank pH requirements for constant [OH ]
concentration
The pH and the concentration of the dosing acid were 1 .6
and 53.6 g/L H3P04 respectively. During the trial the dosing
acid consumption was quite low, approximately 17L/day, or less
than about lL/day of concentrated phosphoric acid (85 wt%) .
Quench tank dosing proved effective at controlling white
precipitate formation and preventing quench heat exchanger
blockage. Another outcome of pH dosing was that the pH probe
did not foul .
QUENCH SYSTEM - LOW TEMPERATURE CONTROL
A t the end of the above-described pH control trial
period, the set point temperature for the quench tank sprays
was lowered 50 °C to 35 °C , and pH dosing was discontinued. The
quench tank was flushed with water to remove residual salts
from the pH control trial. This change caused wet strip
conditions further downstream but it also showed that
temperature is an important variable for quench tank control.
During the period of the low temperature operation (24 hours)
there was no increase in differential pressure across the
quench tank heat exchanger . The quench tank temperature was
typically 15 °C higher than the spray temperature. During the
low temperature trial the quench tank temperature was 48-50 °C
rather than the 65-70 °C typical of normal MCL1 quenching
conditions .
After 24 hours the set point was increased to 50 °C to
determine whether temperature is a critical variable. The
quench heat exchanger differential pressure started to
increase immediately - indicating the formation of
precipitates in the heat exchanger.
After 10 hours the set point was lowered to 40 °C but this
seemed to have little impact. When the quench heat exchanger
differential pressure reached 110 kPa the set point was
returned to 50 °C and the quench tank was dosed with acid to
bring the pH down and pH control was reactivated. Dosing was
left on during the run down of the pot in the final days of
the trial. The quench water became clear and the quench heat
exchanger differential pressure stabilised during this time.
QUENCH WATER ANALYSIS
Samples of quench water were collected and analysed
during the trials. The results are shown in Figures 2 and 3 .
In Figures 2 and 3 the periods 1-4 represent pH control
(1) , low temperature control (35 °C ) (2) , quench tank set point
at 50 °C (3), and quench tank set point at 40 °C , respectively.
With reference to the Figures, both aluminium and calcium
seem to follow the same trend (Figure 2 ) . Lower quench tank
temperature and pH dosing lowered the level of these ions in
the quench water, with the calcium levels dropping
substantially. Without control the level of Al in the quench
water is considerably higher for Al-Zn-Si-Mg alloy coatings
than Al-Zn alloy coatings (typical Al-Zn concentrations in
quench water are 4-20 mg/L) . The impact of pH control on
magnesium concentration is shown in Figure 3 . It increased
considerably during the 4 day test period. Increased
magnesium levels are also evident for cooler quench tank
conditions. Zinc levels also increased during pH control and
for the coldest quench tank trial (35 °C ) but was still at low
levels overall.
CONCLUSIONS
The above trials and other research and development work
of the applicant established that Al-Zn-Si-Mg alloy coated
strip is far more reactive in cooling water than Al-Zn alloy
coated strip and lead to rapid deterioration of the quench
heat exchangers and coatings of the quench tank surfaces, with
the higher reactivity being due in large part to magnesium and
calcium. Lower quench tank temperatures and pH control
reduced the impact of magnesium and calcium dissolution in the
quench water and allowed the quench heat exchangers to perform
in a practical manner.
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 or immersion tanks.

CLAIMS :
1 . A method of forming an Al-Zn-Si-Mg alloy coating on a
steel strip to form a coating of an Al-Zn-Mg-Si on a steel
strip, the method including the steps of dipping steel strip
into a bath of molten Al-Zn-Si-Mg alloy and forming a coating
of the alloy on exposed surfaces of the steel strip and
cooling the coated strip with cooling water, with the cooling
step including controlling the pH of cooling water to be in a
range of pH 5-9.
2 . The method defined in claim 1 wherein the cooling step
includes controlling the pH of cooling water to be less than
8 .
3 . The method defined in claim 1 or claim 2 wherein the
cooling step includes controlling the pH of cooling water to
be less than 7 .
4 . The method defined in any one of the preceding claims
wherein the cooling step includes controlling the pH of
cooling water to be greater than 6 .
5 . The method defined in any one of the preceding claims
wherein the cooling step includes controlling the temperature
of cooling water to be in a range of 25-80°C.
6 . The method defined in any one of the preceding claims
wherein the cooling step includes controlling the temperature
of cooling water to be less than 70°C.
7 . The method defined in any one of the preceding claims
wherein the cooling step includes controlling cooling water
temperature to be less than 60°C.
8 . The method defined in any one of the preceding claims
wherein the cooling step includes controlling cooling water
temperature to be less than 55°C.
9 . The method defined in any one of the preceding claims
wherein the cooling step includes controlling cooling water
temperature to be less than 50°C.
10. The method defined in any one of the preceding claims
wherein the cooling step includes controlling cooling water
temperature to be less than 45°C.
11. The method defined in any one of the preceding claims
wherein the cooling step includes controlling cooling water
temperature to be greater than 30°C.
12. The method defined in any one of the preceding claims
wherein the cooling step includes controlling cooling water
temperature to be greater than 40°C.
13. The method defined in any one of the preceding claims
wherein the cooling step includes controlling the pH by adding
acid to the cooling water.
1 . The method defined in any one of the preceding claims
wherein the strip cooling step includes controlling the
chemistry of the cooling water.
15. The method defined in any one of the preceding claims
wherein the strip cooling step includes controlling the
chemistry and the pH by adding acid to the cooling water.
16. The method defined in any one of the preceding claims
wherein the cooling step includes a water quench step.
17. The method defined in any one of the preceding claims
wherein the cooling step includes 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.
18. The method defined in any one of claims 1-16 wherein the
cooling step includes an open loop in which cooling water is
supplied from a cooling tower to the coated strip and
collected and recirculated through the cooling tower.
19. The method defined in any one of the preceding claims
wherein the cooling step includes controlling the operating
conditions to cool the coated strip to a temperature range of
30-55°C.
20. The method defined in any one of the preceding claims
includes other steps including any one or more of the steps of
pre-treating strip to clean the strip before the hot dip
coating step, rolling the coated strip, treating the coated
strip with a passivation solution, and coiling the coated
strip.
21. The method defined in any one of the preceding claims
wherein the Al-Zn-Si-Mg alloy includes more than 0.3 % by
weight Mg.
22. The method defined in any one of the preceding claims
wherein the Al-Zn-Si-Mg alloy includes more than 1.0 % by
weight Mg.
23. The method defined in any one of the preceding claims
wherein the Al-Zn-Si-Mg alloy includes more than 1.3 % by
weight Mg.
2 . The method defined in any one of the preceding claims
wherein the Al-Zn-Si-Mg alloy includes more than 1.5 % by
weight Mg.
25. The method defined in any one of the preceding claims
wherein the Al-Zn-Si-Mg alloy includes less than 3 % by weight
Mg.
26. The method defined in any one of the preceding cl
wherein the Al-Zn-Si-Mg alloy includes more than 2.5 %
weight Mg.
27. The method defined in any one of the preceding cl
wherein the Al-Zn-Si-Mg alloy includes more than 1.2 %
weight Si .
28. The method defined in any one of the preceding claims
wherein the Al-Zn-Si-Mg alloy includes less than 2.5 % by
weight Si .
29. The method defined in any one of the preceding claims
wherein the Al-Zn-Si-Mg alloy includes the following ranges
% 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.
30. The method defined in any one of the preceding claims
wherein the Al-Zn-Si-Mg alloy includes the following ranges
% 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
31. An Al-Zn-Mg-Si alloy coated steel strip produced by
the method defined in any one of the preceding claims.
32. The Al-Zn-Mg-Si alloy coated steel strip defined in cliam
31 wherein the the Al-Zn-Si-Mg alloy used to form the 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.