U.S. patent application number 16/192157 was filed with the patent office on 2019-03-21 for metal-coated steel strip.
The applicant listed for this patent is Bluescope Steel Limited. Invention is credited to Robert Ian Scott, Ross McDowall Smith, Joe Williams.
Application Number | 20190085438 16/192157 |
Document ID | / |
Family ID | 40951749 |
Filed Date | 2019-03-21 |
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United States Patent
Application |
20190085438 |
Kind Code |
A1 |
Scott; Robert Ian ; et
al. |
March 21, 2019 |
METAL-COATED STEEL STRIP
Abstract
A coating of an Al--Zn--Si--Mg alloy on a steel strip that is
applied by a hot dip process and is subsequently heat treated to
improve the ductility of the coating.
Inventors: |
Scott; Robert Ian; (Redfern,
AU) ; Williams; Joe; (Woonona, AU) ; Smith;
Ross McDowall; (Cordeaux Heights, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bluescope Steel Limited |
Melbourne |
|
AU |
|
|
Family ID: |
40951749 |
Appl. No.: |
16/192157 |
Filed: |
November 15, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12811214 |
Aug 25, 2010 |
|
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|
PCT/AU2009/000145 |
Feb 6, 2009 |
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16192157 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 2/12 20130101; C23C
2/40 20130101; C23C 2/28 20130101 |
International
Class: |
C23C 2/12 20060101
C23C002/12; C23C 2/40 20060101 C23C002/40; C23C 2/28 20060101
C23C002/28 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 7, 2008 |
AU |
2008900574 |
Claims
1. A method of forming a coating of a corrosion-resistant
Al--Zn--Si--Mg alloy on a steel strip that comprises: (a) passing
the steel strip through a hot dip coating bath that contains Al,
Zn, Si, and Mg and optionally other elements and forming an
Al--Zn--Si--Mg alloy coating on the strip, wherein the
Al--Zn--Si--Mg alloy comprises the following ranges in % by weight
of the elements aluminium, zinc, silicon, and magnesium: Aluminium:
40 to 60% Zinc: 40 to 60% Silicon: 0.3 to 3% Magnesium: at least
1.5 and less than 2.5%; (b) cooling the coated strip; and (c) heat
treating the coated strip to improve the ductility of the coating,
with the heat treating step comprising heating the strip from a
lower temperature to a hold temperature of 150-300.degree. C.,
holding the coated strip at the hold temperature for a period of
time up to 45 minutes, and slow cooling the coated strip at a
cooling rate of 40.degree. C./hr or less from the hold temperature
to a temperature of 100.degree. C. or less.
2. The method defined in claim 1 wherein the hold temperature is
less than 275.degree. C.
3. The method defined in claim 1 wherein the Al--Zn--Si--Mg alloy
comprises 55% aluminum.
4. The method defined in claim 1 comprising slow cooling the coated
strip from the hold temperature to a temperature of 100.degree. C.
or less at a cooling rate of 30.degree. C./hr or less.
5. The method defined in claim 1 wherein the silicon concentration
is less than 3.0 wt. %.
6. The method defined in claim 1 wherein the aluminium
concentration is at least 45 wt. %.
7. The method defined in claim 1 wherein the Al--Zn--Si--Mg alloy
does not contain deliberate additions of chromium and/or manganese.
Description
BACKGROUND
[0001] The present invention relates to strip, typically steel
strip, which has a corrosion-resistant metal alloy coating.
[0002] The present invention relates particularly to a
corrosion-resistant metal alloy coating that contains
aluminium-zinc-silicon-magnesium as the main elements in the alloy,
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. Hence, the phrase "Al--Zn--Si--Mg alloy" is understood
to cover alloys that contain such other elements as deliberate
alloying additions or as unavoidable impurities. 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.
[0003] The present invention relates particularly but not
exclusively to a method of enhancing the ductility of an
Al--Zn--Si--Mg coating on steel strip.
[0004] 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 ductility of coatings, particularly in areas (e.g.
tension bends) that are directly subjected to cold forming, is an
important issue for such end-use products (painted and
un-painted).
[0005] Typically, the Al--Zn--Si--Mg alloy of the present invention
comprises the following ranges in % by weight of the elements
aluminium, zinc, silicon, and magnesium:
[0006] Aluminium: 40 to 60%
[0007] Zinc: 40 to 60%
[0008] Silicon: 0.3 to 3%
[0009] Magnesium 0.3 to 10%
[0010] Typically, the corrosion-resistant metal alloy coating of
the present invention is formed on steel strip by a hot-dip coating
method.
[0011] 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 heat treatment furnace that is adjacent a
coating pot has an outlet snout that extends downwardly to a
location close to an upper surface of the bath.
[0012] 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.
[0013] 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.
[0014] The metal alloy coated strip then passes through a cooling
section and is subjected to forced cooling.
[0015] 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.
[0016] 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.
[0017] 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% Al--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.
[0018] The addition of Mg to this known composition of 55%
Al--Zn--Si coating composition has been proposed in the patent
literature for a number of years, see for example U.S. Pat. No.
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.
[0019] It has been established that when Mg is included in a 55%
Al--Zn coating composition, Mg brings about certain beneficial
effects on product performance, such as improved cut-edge
protection.
[0020] The above discussion is not to be taken as an admission of
the common general knowledge in Australia and elsewhere.
[0021] Invention
[0022] It has also been established by the applicant that the
addition of Mg to a 55% Al--Zn coating composition has a
significant negative impact on the coating ductility. This is
caused by the formation of coarse intermetallic phases in the
coating microstructure and a hardening effect of Mg on Al-rich
dendrites and Zn-rich interdendritic regions in the coating
microstructure.
[0023] Specifically, in relation to the hardening effect, the
applicant is aware that following solidification of a 55%
Al--Zn-1.5% Si metallic coating, an age hardening reaction occurs
wherein excess Zn dissolved in the Al-rich phase in the coating
precipitates as a metastable phase. This causes an increase in
strength of the Al-rich phase, and consequently increases the
effectiveness of any potential crack initiation sites. This age
hardening reaction results in a significant increase in coating
hardness within 2-4 weeks of coating solidification, and if cold
forming (e.g. roll forming) of tight bends in the metal alloy
coated steel (including painted metal-coated steel) is not carried
out soon after coating solidification, increased bend cracking can
result. In some situations this can be a significant problem.
[0024] The applicant has found that this age hardening also occurs
in Al--Zn--Si coatings containing Mg.
[0025] The present invention is a coating of an Al--Zn--Si--Mg
alloy on a steel strip that is applied by a hot dip process and is
subsequently heat treated to improve the ductility of the
coating.
[0026] The applicant has found that the resultant coating can be
cold formed with a reduced level of cracking on tension bends
compared to coatings that are not heat treated. The applicant has
also found that the benefit obtained during the heat treatment can
be long lasting. Specifically, improved ductility can be retained
for a period of 12 months or more.
[0027] Accordingly, the present invention provides an
Al--Zn--Si--Mg alloy coated steel strip produced by hot dip coating
the steel strip with the alloy and then heat treating the coated
strip.
[0028] According to the present invention there is also provided a
method of forming a coating of a corrosion-resistant Al--Zn--Si--Mg
alloy on a steel strip that comprises:
(a) passing the steel strip through a hot dip coating bath that
contains Al, Zn, Si, and Mg and optionally other elements and
forming an alloy coating on the strip, and (b) heat treating the
coated strip to improve the ductility of the coating.
[0029] Preferably the method comprises heat treating the coated
strip at a hold temperature of at least 150.degree. C.
[0030] The term "hold temperature" is understood herein to mean a
maximum temperature to which a coated strip is heated to and held
at during the course of a heat treatment cycle.
[0031] More preferably the method comprises heat treating the
coated strip at a hold temperature of at least 200.degree. C.
[0032] Typically, the method comprises heat treating the coated
strip at a hold temperature of at least 225.degree. C.
[0033] Preferably the method comprises heat treating the coated
strip at a hold temperature of less than 300.degree. C.
[0034] More preferably the method comprises heat treating the
coated strip at a hold temperature of less than 275.degree. C.
[0035] Preferably the method comprises holding the coated strip at
the hold temperature for up to 45 minutes.
[0036] More preferably the method comprises holding the coated
strip at the hold temperature for up to 30 minutes.
[0037] Preferably the method comprises slow cooling the heat
treated coated strip from the hold temperature to a temperature of
100.degree. C. or less.
[0038] The applicant has found that the cooling rate of heat
treated coated strip affects the durability of the softening
effect, i.e. the improved ductility, obtained by the heat treatment
and that it is preferable that the cooling rate be a "slow" cooling
rate.
[0039] More preferably the method comprises slow cooling the heat
treated coated strip from the hold temperature to a temperature of
80.degree. C. or less.
[0040] Preferably the cooling rate is 40.degree. C./hr or less.
[0041] More preferably the cooling rate is 30.degree. C./hr or
less.
[0042] The heat treatment step of the method may be carried out on
a batch or a continuous basis.
[0043] Typically, the Al--Zn--Si--Mg alloy of the present invention
comprises the following ranges in % by weight of the elements
aluminium, zinc, silicon, and magnesium:
[0044] Aluminium: 40 to 60%
[0045] Zinc: 40 to 60%
[0046] Silicon: 0.3 to 3%
[0047] Magnesium 0.3 to 10%
[0048] Preferably the magnesium concentration is less than 8 wt.
%.
[0049] Preferably the magnesium concentration is less than 3 wt.
%.
[0050] Preferably the magnesium concentration is at least 0.5 wt.
%.
[0051] Preferably the magnesium concentration is between 1 wt. %
and 3 wt. %.
[0052] More preferably the magnesium concentration is between 1.5
wt. % and 2.5 wt. %.
[0053] Preferably the silicon concentration is less than 3.0 wt.
%.
[0054] Preferably the silicon concentration is less than 1.6 wt.
%.
[0055] Preferably the silicon concentration is less than 1.2 wt.
%.
[0056] Preferably the silicon concentration is less than 0.6 wt.
%.
[0057] Preferably the aluminium concentration is at least 45 wt.
%.
[0058] Typically, the aluminium concentration is at least 50 wt.
%.
[0059] The Al--Zn--Si--Mg alloy does not contain deliberate
additions, i.e. additions above concentration levels that would be
regarded as impurity levels, of chromium and/or manganese.
[0060] The Al--Zn--Si--Mg alloy may contain other elements as
impurities or as deliberate additions.
[0061] Preferably the coating on the strip is no more than 30
microns.
[0062] According to the present invention there is also provided a
metal coated steel strip formed by the above method.
[0063] Preferably the metal coated steel strip is cold formed into
an end-use product, such as building products (e.g. profiled wall
and roofing sheets).
[0064] According to the present invention there is also provided a
method of forming a painted, metal coated steel strip that
comprises:
[0065] (a) passing the steel strip through a hot dip coating bath
that contains Al, Zn, Si, and Mg and optionally other elements and
forming an alloy coating on the strip,
[0066] (b) heat treating the coated strip to improve the ductility
of the coating;
[0067] (c) slow cooling the heat treated coated strip from the hold
temperature to a temperature of 100.degree. C. or less; and
[0068] (d) forming a coating of a paint on the cooled heat treated
coated strip.
[0069] Preferably the Al--Zn--Si--Mg alloy and the heat treatment
step are as described above.
[0070] According to the present invention there is also provided a
painted, metal coated steel strip formed by the above method.
[0071] Preferably the metal coated steel strip is cold formed into
an end-use product, such as building products (e.g. profiled wall
and roofing sheets).
[0072] Experimental Work
[0073] The present invention is based on experimental work carried
out by the applicant.
[0074] Specifically, the experimental work was carried out to
determine the following:
[0075] (a) if any improvement in the ductility of a 55% Al--Zn-1.5%
Si-2% Mg coating could be achieved by an annealing heat
treatment,
[0076] (b) the optimum holding temperature, and
[0077] (c) the ageing behaviour of heat treated coatings, including
heat treated coatings that have undergone a subsequent paint bake
cycle (PBC) heat treatment simulation.
[0078] The experimental work was carried out on samples of steel
strip that were coated with a 55% Al--Zn-1.5% Si-2% Mg alloy with a
coating density of 150 g/m.sup.2 (i.e. 75 g/m.sup.2 of each surface
of the strip samples) and then heat treated by heating the samples
to a range of different hold temperatures and holding the samples
at the temperatures for a pre-determined period of 30 minutes and
then cooling the heat treated samples to ambient temperature.
[0079] The experimental work also included a paint bake cycle (PBC)
heat treatment simulation for some of the samples. The PBC
treatment comprised heating samples to a peak metal temperature of
230.degree. C. at .about.7.degree. C./s, followed by water
quenching.
[0080] FIG. 1 shows the critical bend strain (CBS), i.e. the strain
in a coating that is required to initiate cracking, for samples
having the 55% Al--Zn-1.5% Si-2% Mg (150 g/m.sup.2 coating density)
coating held at different temperatures for the above predetermined
time of 30 minutes and then cooled to 80.degree. C. at a rate of
0.5.degree. C./min.
[0081] FIG. 1 shows that the CBS increased from 5.3% for the
as-received coated sample (i.e. the sample point at ambient
temperature) to a maximum of 8.3% for a coated samples that were
heat treated at hold temperatures in the range of 225-250.degree.
C. This constitutes a 56% increase in coating ductility--a
significant improvement. The Figure also shows that the CBS started
to increase at a hold temperature of 150.degree. C.
[0082] A semi-quantitative measure of cracking severity was also
used to assess the coating ductility of samples.
[0083] Crack Severity Rating (CSR) is an arbitrary tension bend
crack rating system commonly used within the 55% Al--Zn coating
community as a measure of coating ductility. A 2T bend is produced
and viewed under a stereomicroscope at a magnification of
15.times.. The cracking on the bend is then compared with a set of
standards, and assigned a number between 0 and 10, with 0
indicating no cracking is visible, and 10 representing severe
cracking. Hence, a lower CSR rating is preferable to a higher
rating.
[0084] FIG. 2 shows the CSR for samples having heat-treated 55%
Al--Zn-1.5% Si-2% Mg (150 g/m.sup.2) coatings as a function of hold
temperature. It is evident from the Figure that 225.degree. C. is
the optimum hold temperature in this experiment. Also, it is
evident from the Figure that the CSR started to improve at a hold
temperature of 150.degree. C.
[0085] FIG. 3 shows the ageing behaviour of (a) samples having
coatings of 55% Al--Zn-1.5% Si-2% Mg alloy that were heat treated
at the above-established optimum hold temperature of 225.degree. C.
for the above predetermined time of 30 minutes that were aged for
up to three months, (b) samples as described in item (a) that were
then subjected to a paint bake cycle treatment, (c) samples having
as-received coatings of 55% Al--Zn-1.5% Si-2% Mg alloy, and (d)
samples having coatings of 55% Al--Zn-1.5% Si-2% Mg alloy that were
subjected to a paint bake cycle treatment only.
[0086] For heat treated coatings, no significant reversion to the
as-received ductility was observed in three months, even when the
annealed coatings had undergone a subsequent paint bake cycle heat
treatment. Extrapolating these results leads to a conclusion that
the heat treatment at the hold temperature of 225.degree. C. for
the predetermined time period of 30 minutes would be effective for
a period greater than 12 months.
[0087] The above-described experimental work shows that heat
treatment of coatings of 55% Al--Zn-1.5% Si-2% Mg alloy on strip
improved the ductility of the coatings.
[0088] Many modifications may be made to the present invention
described above without departing from the spirit and scope of the
invention.
[0089] By way of example, whilst the experimental work was carried
out on a 55% Al--Zn-1.5% Si-2% Mg coating, the present invention is
also applicable to Al--Zn--Si--Mg coatings generally.
* * * * *