NZ621776B2 - Metal-coated steel strip - Google Patents
Metal-coated steel strip Download PDFInfo
- Publication number
- NZ621776B2 NZ621776B2 NZ621776A NZ62177612A NZ621776B2 NZ 621776 B2 NZ621776 B2 NZ 621776B2 NZ 621776 A NZ621776 A NZ 621776A NZ 62177612 A NZ62177612 A NZ 62177612A NZ 621776 B2 NZ621776 B2 NZ 621776B2
- Authority
- NZ
- New Zealand
- Prior art keywords
- bath
- alloy
- method defined
- weight
- composition
- Prior art date
Links
- 229910052751 metal Inorganic materials 0.000 title claims abstract description 24
- 239000002184 metal Substances 0.000 title claims abstract description 24
- 229910000831 Steel Inorganic materials 0.000 title description 15
- 239000010959 steel Substances 0.000 title description 15
- 229910045601 alloy Inorganic materials 0.000 claims abstract description 86
- 239000000956 alloy Substances 0.000 claims abstract description 86
- REDXJYDRNCIFBQ-UHFFFAOYSA-N aluminium(3+) Chemical class [Al+3] REDXJYDRNCIFBQ-UHFFFAOYSA-N 0.000 claims abstract description 82
- 229910007981 Si-Mg Inorganic materials 0.000 claims abstract description 60
- 229910008316 Si—Mg Inorganic materials 0.000 claims abstract description 60
- 238000000576 coating method Methods 0.000 claims abstract description 39
- 239000011248 coating agent Substances 0.000 claims abstract description 38
- 239000000203 mixture Substances 0.000 claims abstract description 33
- 229910052749 magnesium Inorganic materials 0.000 claims abstract description 8
- 229910052725 zinc Inorganic materials 0.000 claims abstract description 7
- 238000007598 dipping method Methods 0.000 claims abstract description 4
- 239000012535 impurity Substances 0.000 claims description 6
- 238000005275 alloying Methods 0.000 claims description 5
- 229910052742 iron Inorganic materials 0.000 claims description 5
- 239000000463 material Substances 0.000 claims description 5
- 229910052712 strontium Inorganic materials 0.000 claims description 4
- 229910052790 beryllium Inorganic materials 0.000 claims description 3
- 229910052804 chromium Inorganic materials 0.000 claims description 3
- 229910052803 cobalt Inorganic materials 0.000 claims description 3
- 150000001875 compounds Chemical class 0.000 claims description 3
- 229910052802 copper Inorganic materials 0.000 claims description 3
- 229910052748 manganese Inorganic materials 0.000 claims description 3
- 229910052759 nickel Inorganic materials 0.000 claims description 3
- 230000000737 periodic Effects 0.000 claims description 3
- 229910052719 titanium Inorganic materials 0.000 claims description 3
- 229910052720 vanadium Inorganic materials 0.000 claims description 3
- 229910052791 calcium Inorganic materials 0.000 abstract description 6
- 239000011575 calcium Substances 0.000 abstract 4
- OYPRJOBELJOOCE-UHFFFAOYSA-N calcium Chemical compound [Ca] OYPRJOBELJOOCE-UHFFFAOYSA-N 0.000 abstract 2
- 239000011777 magnesium Substances 0.000 description 24
- 229910018137 Al-Zn Inorganic materials 0.000 description 6
- 229910018573 Al—Zn Inorganic materials 0.000 description 6
- 238000004519 manufacturing process Methods 0.000 description 6
- 238000002474 experimental method Methods 0.000 description 5
- XEEYBQQBJWHFJM-UHFFFAOYSA-N iron Substances [Fe] XEEYBQQBJWHFJM-UHFFFAOYSA-N 0.000 description 5
- 239000002245 particle Substances 0.000 description 5
- 238000000034 method Methods 0.000 description 4
- 229910052582 BN Inorganic materials 0.000 description 3
- PZNSFCLAULLKQX-UHFFFAOYSA-N N#B Chemical compound N#B PZNSFCLAULLKQX-UHFFFAOYSA-N 0.000 description 3
- 239000000919 ceramic Substances 0.000 description 3
- 239000008199 coating composition Substances 0.000 description 3
- -1 aluminium- zinc-silicon-magnesium Chemical compound 0.000 description 2
- 238000001816 cooling Methods 0.000 description 2
- 238000005260 corrosion Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 239000000155 melt Substances 0.000 description 2
- 238000002844 melting Methods 0.000 description 2
- 230000000717 retained Effects 0.000 description 2
- 238000009736 wetting Methods 0.000 description 2
- 230000003466 anti-cipated Effects 0.000 description 1
- OKTJSMMVPCPJKN-UHFFFAOYSA-N carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
- 239000004927 clay Substances 0.000 description 1
- 229910052570 clay Inorganic materials 0.000 description 1
- 238000004140 cleaning Methods 0.000 description 1
- 239000010960 cold rolled steel Substances 0.000 description 1
- 238000010924 continuous production Methods 0.000 description 1
- 238000003618 dip coating Methods 0.000 description 1
- 229910002804 graphite Inorganic materials 0.000 description 1
- 239000010439 graphite Substances 0.000 description 1
- 238000010438 heat treatment Methods 0.000 description 1
- VBMVTYDPPZVILR-UHFFFAOYSA-N iron(2+);oxygen(2-) Chemical group [O-2].[Fe+2] VBMVTYDPPZVILR-UHFFFAOYSA-N 0.000 description 1
- 229910001092 metal group alloy Inorganic materials 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000006011 modification reaction Methods 0.000 description 1
- 239000003921 oil Substances 0.000 description 1
- 238000005096 rolling process Methods 0.000 description 1
- 238000010008 shearing Methods 0.000 description 1
- 210000004894 snout Anatomy 0.000 description 1
- 239000010935 stainless steel Substances 0.000 description 1
- 229910001220 stainless steel Inorganic materials 0.000 description 1
- 238000003756 stirring Methods 0.000 description 1
Classifications
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C18/00—Alloys based on zinc
- C22C18/04—Alloys based on zinc with aluminium as the next major constituent
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C2204/00—End product comprising different layers, coatings or parts of cermet
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
- C23C2/06—Zinc or cadmium or alloys based thereon
-
- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
- C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
- C23C2/36—Elongated material
- C23C2/40—Plates; Strips
Abstract
Disclosed is 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 Al-Zn-Si-Mg alloy containing in % by weight: Al: 2 to 19 %, Si: 0.1 to 2%, Mg: 1 to 10%, and Zn: 80 to 97 %, and with the bath having a molten metal layer and a top dross layer on the metal layer, and the method including providing calcium (Ca) in the composition of the bath to minimise the top dross layer in the molten bath. o 97 %, and with the bath having a molten metal layer and a top dross layer on the metal layer, and the method including providing calcium (Ca) in the composition of the bath to minimise the top dross layer in the molten bath.
Description
METAL-COATED STEEL STRIP
The present invention relates to the production
of strip, typically steel strip, which has a corrosion-
resistant metal alloy coating that contains aluminium-
zinc-silicon-magnesium as the main elements in the alloy
in the following ranges in % by weight:
Al: 2 to 19 %
Si: 0.1 to 2 %
Mg: 1 to 10 %
Zn: 80 to 97 %
The above alloy is described and claimed in
Australian patent 758643 entitled “Plated steel product,
plated steel sheet and precoated steel sheet having
excellent resistance to corrosion” in the name of Nippon
Steel Corporation.
The above alloy is hereinafter referred to as an
“Al-Zn-Si-Mg alloy”.
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, Ti, Cu, Ni, Co, Ca, Mn, Be, Sr, Ca, Cr, and V.
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.
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.
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.
International application
entitled “Metal-Coated Steel Strip” in the name of the
applicant is concerned with minimising the amount of top
dross in an alloy coating bath of an alloy that contains
aluminium-zinc-silicon-magnesium as the main elements in
the alloy. The invention described and claimed in the
International application is based on laboratory work and
line trials on coating bath alloy compositions containing,
53% Al, 43% Zn, 2% Mg, 1.5% Si, and 0.5% Fe, with the
percentages being percentages by weight, and different
amounts of Ca and Sr in the baths carried out by the
applicant. The coating alloys were coated onto steel
strip. The invention was made during the course of a
research and development project that investigated the
addition of Mg to a known corrosion resistant metal
coating composition, namely 55%Al-Zn-Si, that is used
widely in Australia and elsewhere for building products,
particularly profiled wall and roofing sheets. At the
time the invention of the International application was
made, the addition of Mg to this known composition of
55%Al-Zn-Si coating composition had 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 were
not commercially available in Australia. More
particularly, it had 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. However, the
applicant found in the research and development project
that Mg-containing molten 55%Al-Zn coating metal is
susceptible to increased levels of top dross generation
compared to molten 55%Al-Zn coating metal that does not
contain Mg. During a line trial involving hot-dip metal
coating a Mg-containing 55%Al-Zn alloy onto a steel strip
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%Al-Zn alloy coating
bath without Mg addition. This amount of top dross
generated has a significant impact on the cost of
production of Mg-containing 55%Al-Zn alloy coated steel
and product quality. The applicant found in the research
and development project that the amount of top dross could
be greatly reduced by the addition of Ca and/or Sr to a
coating bath.
The applicant has now found that there is
substantial top dross generated in hot dip coating steel
strip in a molten bath containing the above-described Al-
Zn-Si-Mg alloy containing in % by weight: Al: 2 to 19 %,
Si: 0.1 to 2%, Mg: 1 to 10 %, and Zn: 80 to 97 % and that
the top dross is undesirable in terms of cost of
production and product quality. The applicant had not
anticipated that top dross would have been as significant
an issue with the above-described Al-Zn-Si-Mg alloy.
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 containing
in % by weight: Al: 2 to 19 %, Si: 0.1 to 2%, Mg: 1 to 10
%, and Zn: 80 to 97 % by the addition to molten baths of
Ca, and the reduction in top dross levels has lead to
benefits in terms of production costs and product quality.
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 Al-Zn-Si-Mg alloy containing
in % by weight: Al: 2 to 19 %, Si: 0.1 to 2%, Mg: 1 to 10
%, and Zn: 80 to 97 %, and with the bath having a molten
metal layer and a top dross layer on the metal layer, and
the method including providing Ca in the composition of
the bath to minimise the top dross layer in the molten
bath.
The Al-Zn-Si-Mg alloy 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, Ti, Cu, Ni, Co, Ca,
Mn, Be, Sr, Cr, and V.
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.
The composition of the bath may include more than
100 ppm Ca.
The composition of the bath may include more than
200 ppm Ca.
The composition of the bath may include more than
250 ppm Ca.
The composition of the bath may include more than
300 ppm Ca.
The composition of the bath may include less than
2000 ppm Ca.
The composition of the bath may include less than
1500 ppm Ca.
The composition of the bath may include less than
1000 ppm Ca.
It is noted that the references to amounts of
elements such as Ca 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
tends to segregate to the top dross layer of molten baths
and, as a consequence the top dross layer becomes enriched
with respect to Ca when compared to the metal layer.
Specifically, if there is “x” wt.% of Ca 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. In practice,
this means that, if it is required that there be “x” wt.%
of Ca in the molten metal layer of a molten bath, it will
be necessary to add an amount of Ca that is greater than
“x” wt.% in the total bath to compensate for the higher
concentration of Ca that will segregate to the top dross
layer.
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
concentration of Ca in the bath to minimise the top dross
layer in the molten 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
Ca that is in the bath, and adding Ca as required to
maintain the bath composition for the element.
In a situation in which the Ca is 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 substantially constant or
within a preferred range of + or – 10% for the elements.
The Al-Zn-Si-Mg alloy may include more than 8 %
by weight Al.
The Al-Zn-Si-Mg alloy may include more than 10 %
by weight Al.
The Al-Zn-Si-Mg alloy may include less than 15 %
by weight Al.
The Al-Zn-Si-Mg alloy may include less than 12 %
by weight Al.
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 %
by weight Mg.
The Al-Zn-Si-Mg alloy may include more than 2 %
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 3 %
by weight Mg.
The Al-Zn-Si-Mg alloy may include less than 5 %
by weight Mg.
The Al-Zn-Si-Mg alloy may include less than 4 %
by weight Mg.
The Al-Zn-Si-Mg alloy may include more than 0.15
% by weight Si.
The Al-Zn-Si-Mg alloy may include less than 1.2 %
by weight Si.
The Al-Zn-Si-Mg alloy may include less than 1 %
by weight Si.
The Al-Zn-Si-Mg alloy may include less than 0.25
% by weight Si.
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; and
Figure 2 is a graph of the mass of dross versus
Ca concentration for molten Al-Zn-Si-Mg alloy baths with
and without Ca in experiments on dross generation 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
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 containing in % by
weight: Al: 2 to 19 %, Si: 0.1 to 2%, Mg: 1 to 10 %, and
Zn: 80 to 97 % generate substantial amounts of top dross
in the baths that is undesirable in terms of production
costs and product quality.
As discussed above, the applicant conducted a
number of laboratory experiments to determine whether it
is possible to reduce the amount of dross generated in Al-
Zn-Si-Mg alloy baths having compositions, in % by weight,
of: Al: 2 to 19 %, Si: 0.1 to 2%, Mg: 1 to 10 %, and Zn:
80 to 97 %.
As discussed above, the applicant found that it
was possible to significantly reduce the level of top
dross by the addition of Ca to such Al-Zn-Si-Mg alloys in
coating baths.
The experimental results for experiments of 3
hours duration on the effect of Ca additions to coating
baths on the level of top dross generation in Al-Zn-Si-Mg
alloy coating baths is summarized in Figure 2.
The experimental work was carried out on the
following alloy compositions, in wt. % for (a) an Al-Zn-
Si-Mg alloy and (b) this alloy plus parts per million
(ppm) Ca additions to the composition:
• Alloy: Al: 11.2% Al; Mg: 3%; Si: 0.19%; Zn: balance;
and unavoidable impurities
• Alloy + 500 ppm (0.05 wt.%) Ca.
• Alloy + 750 ppm (0.075 wt.%) Ca.
• Alloy + 1500 ppm (0.15 wt.%) Ca.
It is noted that the concentrations of Ca 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 impellor 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 non-
wetting to the coating alloy tested in the experimental
work. 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 460 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 for the 3
hour experiments as shown in Figure 2.
Figure 2 is a graph of the mass of dross
generated versus Ca concentration for the molten alloy
baths.
Figure 2 clearly shows that the level of top
dross generated in an Al-Zn-Si-Mg alloy bath can be
significantly reduced by additions of Ca to coating baths.
More particularly, Figure 2 shows that the amount of top
dross decreases significantly with increasing amounts of
Ca in the coating baths.
In practice, the Ca may be added to a coating
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
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 (22)
1. A method of forming an Al-Zn-Si-Mg alloy coating on a strip that includes dipping strip into a bath of 5 molten Al-Zn-Si-Mg alloy and forming a coating of the alloy on the strip, with the Al-Zn-Si-Mg alloy containing in % by weight: Al: 2 to 19 %, Si: 0.1 to 2%, Mg: 1 to 10 %, and Zn: 80 to 97 %, and with the bath having a molten metal layer and a top dross layer on the metal layer, and 10 the method including providing Ca in the composition of the bath to minimise the top dross layer in the molten bath, and the method further including controlling the composition of the bath to minimise the top dross layer in the bath by periodically monitoring the concentration of 15 Ca that is in the bath and adding Ca as required to maintain the bath composition for the element.
2. The method defined in claim 1 wherein the Al-Zn- Si-Mg alloy contains other elements that are present as 20 deliberate alloying additions or as unavoidable impurities.
3. The method defined in claim 2 wherein the other elements include any one or more of Fe, Ti, Cu, Ni, Co, 25 Cr, Mn, Be, Sr, and V.
4. The method defined in any one of the preceding claims wherein the composition of the bath includes more than 50 ppm Ca.
5. The method defined in any one of the preceding claims wherein the composition of the bath includes more than 100 ppm Ca. 35
6. The method defined in any one of the preceding claims wherein the composition of the bath includes more 7415740_1 (GHMatters) P88480.NZ than 200 ppm Ca.
7. The method defined in any one of the preceding claims wherein the composition of the bath includes more 5 than 250 ppm Ca.
8. The method defined in any one of the preceding claims wherein the composition of the bath includes less than 2000 ppm Ca.
9. The method defined in any one of the preceding claims wherein the composition of the bath includes less than 1500 ppm Ca. 15
10. The method defined in any one of the preceding claims includes adding Ca by way of specific additions of Ca compounds on a continuous or a periodic basis.
11. The method defined in any one of the preceding 20 claims includes adding Ca in Al and/or Zn ingots that are provided as feed materials for the bath.
12. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes more than 8 25 % by weight Al.
13. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes more than 10 % by weight Al.
14. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes less than 15 % by weight Al. 7415740_1 (GHMatters) P88480.NZ
15. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes less than 12 % by weight Al. 5
16. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes more than 1 % by weight Mg.
17. The method defined in any one of the preceding 10 claims wherein the Al-Zn-Si-Mg alloy includes more than 2 % by weight Mg.
18. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes more than 15 2.5 % by weight Mg.
19. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes less than 5 % by weight Mg.
20. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes less than 4 % by weight Mg. 25 21. The method defined in any one of the preceding claims wherein the Al-Zn-Si-Mg alloy includes more than
0.15 % by weight Si.
22. An Al-Zn-Si-Mg alloy coating on a strip produced 30 by the method defined in any one of the preceding claims. 7415740_1 (GHMatters) P88480.NZ
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
AU2011904293A AU2011904293A0 (en) | 2011-10-18 | Metal-Coated Steel Strip | |
AU2011904293 | 2011-10-18 | ||
PCT/AU2012/001262 WO2013056305A1 (en) | 2011-10-18 | 2012-10-18 | Metal-coated steel strip |
Publications (2)
Publication Number | Publication Date |
---|---|
NZ621776A NZ621776A (en) | 2016-03-31 |
NZ621776B2 true NZ621776B2 (en) | 2016-07-01 |
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