U.S. patent application number 16/138574 was filed with the patent office on 2019-03-14 for metal-coated steel strip.
The applicant listed for this patent is Bluescope Steel Limited. Invention is credited to Qiyang Liu, Ross Smith, Joe Williams.
Application Number | 20190078181 16/138574 |
Document ID | / |
Family ID | 39135420 |
Filed Date | 2019-03-14 |
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United States Patent
Application |
20190078181 |
Kind Code |
A1 |
Liu; Qiyang ; et
al. |
March 14, 2019 |
METAL-COATED STEEL STRIP
Abstract
A steel strip having a coating of an aluminium-zinc-silicon
alloy on at least one surface of the strip is disclosed. The strip
is characterised in that the aluminium-zinc-silicon alloy contains
less than 1.2 wt. % silicon and also contains magnesium. A method
of forming a coating of an aluminium-zinc-silicon alloy on a steel
strip is also disclosed. The method includes moving steel strip
upwardly through a coating pot containing a bath of an
aluminium-zinc-silicon alloy and having an opening in a bottom wall
of the pot and forming a coating of the alloy on the strip. The
method is characterized by minimizing residence time of steel strip
in contact with the aluminium-zinc-silicon alloy bath in the
pot.
Inventors: |
Liu; Qiyang; (Mount Keira,
AU) ; Williams; Joe; (Woonona, AU) ; Smith;
Ross; (Cordeaux Heights, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bluescope Steel Limited |
Melbourne |
|
AU |
|
|
Family ID: |
39135420 |
Appl. No.: |
16/138574 |
Filed: |
September 21, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12439605 |
Mar 27, 2009 |
|
|
|
PCT/AU07/01265 |
Aug 30, 2007 |
|
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16138574 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 2/12 20130101; B32B
15/012 20130101; C23C 2/06 20130101; Y10T 428/12757 20150115; C23C
2/02 20130101; C22C 21/10 20130101; C23C 2/40 20130101 |
International
Class: |
C22C 21/10 20060101
C22C021/10; C23C 2/40 20060101 C23C002/40; C23C 2/12 20060101
C23C002/12; C23C 2/06 20060101 C23C002/06; B32B 15/01 20060101
B32B015/01; C23C 2/02 20060101 C23C002/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2006 |
AU |
2006904727 |
Claims
1. A steel strip having a coating of an aluminium-zinc-silicon
alloy on at least one surface of the strip which is characterised
in that the aluminium-zinc-silicon alloy contains less than 1.2 wt.
% silicon and also contains greater than 1 wt. % magnesium.
2. The steel strip defined in claim 1 wherein the silicon
concentration is 0.2-0.5 wt. %.
3. The steel strip defined in claim 1-wherein the silicon
concentration is at least 0.2 wt. % less than 1.2 wt. %.
4. The steel strip defined in claim 1 wherein the silicon
concentration is at least 0.2 wt. %.
5. The steel strip defined in claim 1 wherein the coating has small
spangles.
6. The steel strip defined in claim 1 wherein the magnesium
concentration is less than 8 wt. %.
7. The steel strip defined in claim 1 wherein the magnesium
concentration is less than 3 wt. %.
8. The steel strip defined in claim 1 wherein the magnesium
concentration is between 1 and 3 wt. %.
9. The steel strip defined in claim 1 wherein the magnesium
concentration is between 1.5 and 2.5 wt. %.
10. The steel strip defined in claim 1 wherein the
aluminium-zinc-silicon alloy is a titanium diboride-modified
alloy.
11. The steel strip defined in claim 1 wherein the
aluminium-zinc-silicon alloy contains strontium and/or calcium.
12. The steel strip defined in claim 11 wherein the concentration
of (i) strontium or (ii) calcium or (iii) strontium and calcium
together is at least 2 ppm.
13. The steel strip defined in claim 11 wherein the concentration
of (i) strontium or (ii) calcium or (iii) strontium and calcium
together is less than 0.2 wt. %.
14. The steel strip defined in claim 11 wherein the concentration
of (i) strontium or (ii) calcium or (iii) strontium and calcium
together is less than 100 ppm.
15. The steel strip defined in claim 11 wherein the concentration
of (i) strontium or (ii) calcium or (iii) strontium and calcium
together is no more than 50 ppm.
16. The steel strip defined in claim 11 wherein the
aluminium-zinc-silicon alloy does not contain vanadium and/or
chromium as deliberate alloy elements--as opposed to being present
in trace amounts for example as unavoidable impurities due to
contamination in the molten bath.
17. A method of forming a coating of an aluminium-zinc-silicon
alloy on a steel strip includes moving steel strip upwardly through
a coating pot containing a bath of an aluminium-zinc-silicon alloy
and having an opening in a bottom wall of the pot and forming a
coating of the alloy on the strip and is characterized by
minimizing residence time of steel strip in contact with the
aluminium-zinc-silicon alloy bath in the pot.
18. The method defined in claim 17 wherein the residence time is
less than 0.75 seconds.
19. The method defined in claim 18 wherein the residence time is
less than 0.5 seconds.
20. The method defined in claim 17 wherein the residence time is at
least 0.2 seconds.
21. The method defined in claim 18 wherein the
aluminium-zinc-silicon alloy contains less than 1.2 wt. % silicon
and optionally is a magnesium containing alloy.
22. The method defined in claim 17 wherein the
aluminium-zinc-silicon alloy contains magnesium.
23. The method defined in claim 17 also includes the steps of:
successively passing the steel strip through a heat treatment
furnace and the bath of molten aluminium-zinc-silicon alloy, and:
(a) heat treating the steel strip in the heat treatment furnace;
and (b) hot-dip coating the strip in the molten bath and forming
the coating of the alloy with small spangles on the steel
strip.
24. A steel strip having a coating of an aluminum-zinc-silicon
alloy on at least one surface of the strip which is characterized
in that the aluminum-zinc-silicon alloy contains less than 1.2 wt.
% silicon and also contains greater than 1 wt. % and less than 2.5
wt. % magnesium.
Description
SUMMARY
[0001] The present invention relates to steel strip that has a
corrosion-resistant metal coating that is formed on the strip by
coating the strip in a molten bath of coating metal.
[0002] The present invention relates particularly but not
exclusively to metal coated steel strip that can be cold formed
(e.g. by roll forming) into an end-use product, such as roofing
products.
[0003] The present invention relates more particularly but not
exclusively to metal coated steel strip of the type described in
the preceding paragraph that has a corrosion-resistant metal
coating with small spangles, a coating with an average spangle size
of the order of less than 0.5mm.
[0004] The present invention relates more particularly but not
exclusively to metal coated steel strip of the type described above
that has a corrosion-resistant metal coating with small spangles
and includes an aluminium-zinc-silicon alloy that has a relatively
low concentration of silicon and also contains magnesium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 shows micrographs of steel substrates coated with
aluminum alloys according to methods of the present disclosure
DETAILED DESCRIPTION
[0006] Conventional aluminium-zinc-silicon alloys used to coat
steel strip generally comprise the following ranges in % by weight
of the elements aluminium, zinc and silicon:
[0007] aluminium: 45.0-60.0;
[0008] zinc: 37.0-46.0; and
[0009] silicon: 1.2-2.3.
[0010] Conventional aluminium-zinc-silicon alloys may also contain
other elements, such as, by way of example, any one or more of
iron, vanadium, and chromium, often as impurities.
[0011] Conventionally, an aluminium-zinc-silicon alloy coating on
steel strip is formed using a hot-dip metal coating method.
[0012] In the conventional hot-dip metal coating method steel strip
passes through one or more heat treatment furnaces and thereafter
into and through a bath of molten aluminium-zinc-silicon alloy
contained in a coating pot. A coating of aluminium-zinc-silicon
alloy forms on the steel strip as the strip moves through the
bath.
[0013] In a widely used conventional method the strip moves
downwardly into the bath and around one or more sink rolls in the
bath and thereafter upwardly from the bath.
[0014] It has also been proposed to provide an opening in a bottom
wall of a coating pot and to move strip vertically upwardly through
the opening into the bath and thereafter from the bath. This method
relies on the use of an electromagnetic plugging means that
prevents molten aluminium-zinc-silicon alloy flowing downwardly
from the pot via the opening.
[0015] The applicant has carried out research and development work
to optimize the composition and microstructure of
aluminium-zinc-silicon alloys and coatings formed from these alloys
on steel strip for given end-use applications and to optimize
coating practices for forming such coatings on steel strip.
[0016] The present invention was made in the course of research and
development work that focused on the impact of silicon in
aluminium-zinc-silicon alloys and on coating practices for forming
such metallic coatings on steel strip, with a particular objective
of achieving low levels of silicon in the metallic coatings and a
secondary objective of forming coatings with small spangles.
[0017] The term "small spangles" is understood herein to mean metal
coated strip that has spangles that are less than 0.5 mm,
preferably less than 0.2 mm, measured using the average intercept
distance method as described in Australian Standard AS1733.
[0018] The applicant found in the course of the work that reducing
the silicon concentration below the conventional 1.2 wt % minimum
mentioned above had advantages in terms of improving corrosion
resistance and forming small spangles and disadvantages in terms of
growth of an intermetallic alloy layer of aluminium, zinc and iron
between the steel strip and the aluminium-zinc-silicon alloy
coating.
[0019] It is known that aluminium-zinc-silicon alloy coatings with
relatively high aluminium contents (as in the production of
GALVALUME.RTM. coated steel) depend on silicon additions to prevent
a strongly exothermic reaction during metallic coating in which the
entire coatings become an alloy of aluminium, zinc and iron. Such
coatings would be highly brittle and commercially useless.
[0020] It is also known that without silicon additions the
exothermic reaction is so spectacular as to heat steel substrates
such that it glows bright red, and on occasion the coating may
actually show combustion.
[0021] The basis of the present invention is that the applicant has
found that growth of the undesirable intermetallic alloy layer can
be suppressed by [0022] (a) the addition of magnesium to the
aluminium-zinc-silicon alloy composition; and/or [0023] (b)
minimizing the residence time of steel strip in contact with a
coating bath.
[0024] With regard to point (a), a coated steel strip in accordance
with the present invention includes a coating of an
aluminium-zinc-silicon alloy on at least one surface of the strip
which is characterised in that the aluminium-zinc-silicon alloy
contains less than 1.2 wt. % silicon and also contains
magnesium.
[0025] Preferably the silicon concentration is 0.2-0.5 wt. % and
the concentration of magnesium is 0.5-8 wt. %.
[0026] Preferably the silicon concentration is at least 0.2 wt. %
and less than 1.2 wt. % and the concentration of magnesium is 0.5-1
wt. %.
[0027] Preferably the silicon concentration is at least 0.2 wt.
%.
[0028] In addition to suppressing growth of an intermetallic alloy
layer, the magnesium addition to the aluminium-zinc-silicon alloy
improves the corrosion resistance of the coating.
[0029] Preferably the coating has small spangles, as described
herein, i.e. spangles that are less than 0.5 mm, preferably less
than 0.2 mm, measured using the average intercept distance method
as described in Australian Standard AS1733.
[0030] The small spangle size improves the ductility of the coating
and compensates for an adverse effect of magnesium on ductility of
the coating.
[0031] Preferably the magnesium concentration is less than 8 wt.
%.
[0032] Preferably the magnesium concentration is less than 3 wt.
%.
[0033] Preferably the magnesium concentration is at least 0.5 wt.
%.
[0034] Preferably the magnesium concentration is between 1 wt. %
and 3 wt. %.
[0035] More preferably the magnesium concentration is between 1.5
wt. % and 2.5 wt. %.
[0036] Preferably the aluminium-zinc-silicon alloy is a titanium
diboride-modified alloy such as described in International
application PCT/US00/23164 (WO 01/27343) in the name of Bethlehem
Steel Corporation and contains up to 0.5 wt. % boron as titanium
diboride. The International application discloses that titanium
diboride minimises the spangle size of aluminium-zinc-silicon
alloys. The disclosure in the specification of the International
application is incorporated herein by cross-reference.
[0037] The aluminium-zinc-silicon alloy may contain other
elements.
[0038] Preferably the aluminium-zinc-silicon alloy contains
strontium and/or calcium.
[0039] The strontium and/or calcium addition to the
aluminium-zinc-silicon alloy substantially reduces the number of
surface defects described by the applicant as "rough coating" and
"pinhole-uncoated" defects and compensates for the increased number
of such surface defects that appear to be caused by magnesium.
[0040] The strontium and the calcium may be added separately or in
combination.
[0041] Preferably the concentration of (i) strontium or (ii)
calcium or (iii) strontium and calcium together is at least 2
ppm.
[0042] Preferably the concentration of (i) strontium or (ii)
calcium or (iii) strontium and calcium together is less than 0.2
wt. %.
[0043] Preferably the concentration of (i) strontium or calcium or
(iii) strontium and calcium together is less than 100 ppm.
[0044] More preferably the concentration of (i) strontium or (ii)
calcium or (iii) strontium and calcium together is no more than 50
ppm.
[0045] Preferably the aluminium-zinc-silicon alloy does not contain
vanadium and/or chromium as deliberate alloy elements--as opposed
to being present in trace amounts for example as unavoidable
impurities due to contamination in the molten bath.
[0046] With regard to point (b) above, the applicant has found that
the above-described coating method of moving steel strip upwardly
through a coating pot containing an aluminium-zinc-silicon alloy
and having an opening in a bottom wall of the pot is an effective
option to minimize residence time of steel strip in contact with
the aluminium-zinc-silicon alloy bath in the pot.
[0047] Thus, a method of forming a coating of an
aluminium-zinc-silicon alloy on a steel strip in accordance with
the present invention includes moving steel strip upwardly through
a coating pot containing a bath of an aluminium-zinc-silicon alloy
and having an opening in a bottom wall of the pot and forming a
coating of the alloy on the strip and is characterized by
minimizing residence time of steel strip in contact with the
aluminium-zinc-silicon alloy bath in the pot.
[0048] Preferably the residence time is less than 0.75 seconds.
[0049] More preferably the residence time is less than 0.5
seconds.
[0050] Preferably the residence time is at least 0.2 seconds.
[0051] Preferably the aluminium-zinc-silicon alloy is the
above-described low silicon containing alloy and optionally is a
magnesium containing alloy.
[0052] The method of forming the aluminium-zinc-silicon alloy
coating on the steel strip in accordance with the present invention
may also include the steps of: successively passing the steel strip
through a heat treatment furnace and the bath of molten
aluminium-zinc-silicon alloy, and: [0053] (a) heat treating the
steel strip in the heat treatment furnace; and [0054] (b) hot-dip
coating the strip in the molten bath and forming the coating of the
alloy with small spangles on the steel strip.
[0055] Preferably the heat treatment furnace has an elongated
furnace exit chute or snout that extends into the bath.
[0056] According to the present invention there is also provided
cold formed products made from the above-described metal coated
steel strip.
[0057] As is indicated above, the present invention is based on
research and development work carried out by the applicant.
[0058] The work included a series of experiments designed to
evaluate the impact of silicon and magnesium in
aluminium-zinc-silicon alloys on the microstructure, and more
particularly spangle size and intermetallic alloy layer growth, of
coatings of the alloys on steel strip samples.
[0059] The experiments were carried out using
aluminium-zinc-silicon alloys containing, in wt. %:
[0060] (a) 0.5 Si, 0.0 Mg;
[0061] (b) 0.5 Si, 2.0 Mg;
[0062] (c) 0.28 Si, 0.0 Mg; and
[0063] (d) 0.28 Si, 2.0 Mg.
[0064] The above-described alloys were coated onto steel samples
and the samples were evaluated.
[0065] The results of the experiments are summarized in Table 1
below and in the micrographs of FIGS. 1.
[0066] The samples were found to have an aluminium-zinc-silicon
alloy coating and an intermetallic alloy layer between the coating
and the steel substrate.
TABLE-US-00001 TABLE 1 Intermetallic Intermetallic Spangle Spangle
Layer Layer Si Size Size Thickness Thickness Content (mm) - 0.0
(mm) - 2.0 (mm) - 0.0 (mm) - 2.0 (wt %) wt % Mg wt. % Mg wt % Mg wt
% Mg 0.5 0.3 0.29 5 .mu.m 2-3 .mu.m with 10 .mu.m with outbursts
outbursts up to 6 .mu.m 0.28 0.2 0.19 10 .mu.m 2-3 .mu.m with 15
.mu.m with outbursts outbursts up to 8 .mu.m
[0067] The micrographs show the steel substrate, the intermetallic
alloy layer on the substrate, and the coating (referred to as
"Al/Zn layer" in the micrographs) on the intermetallic layer of
each sample.
[0068] It is evident from Table 1 and the micrographs that: [0069]
(a) the thickness of the intermetallic alloy layer increased as the
Si concentration decreased--compare the samples containing 0.0 wt.
% Mg and the two different concentrations of Si; [0070] (b) the
samples containing 2.0 wt. % Mg had substantially less
intermetallic alloy layer growth than the samples containing 0.0
wt. % Mg; [0071] (c) Si concentration made very little difference
to the thickness of the intermetallic alloy layers in the samples
containing 2.0 wt. % Mg; and [0072] (d) smaller spangles were
formed with samples having the lower Si concentration of 0.28 wt.
%.
[0073] Many modifications may be made to the preferred embodiment
described above without departing from the spirit and scope of the
present invention.
* * * * *