Note: Descriptions are shown in the official language in which they were submitted.
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GALVANIZING OF REACTIVE ST FLS
~ACKGRO 1 D OF THE INVENTION
(i) Field of the Invention
This invention relates to a galvanizing alloy and process and, more
particularly, relates to a
galvanizing alloy and an immersion galvanization process adapted to control
the undesirable
effects associated with galvanizing reactive steels.
(ii) Description of the Related Art
The conventional process for hot dip galvanizing of low carbon steels
comprises pretreatment
of said steels in a 20% to 30%, by weight, zinc-ammonium-chloride (ZnNH4Cl)
pre-flux,
followed by immersion in molten zinc or zinc allow baths. The 'normal' or 'N'
coating structure
produced on low reactivity steel by conventional hot dip galvanizing processes
has well defined,
compact alloy (intermetallic) layers. The predominant growth mode in this type
of coating is by
solid-state diffusion of iron and zinc, and thus well established
intermetallic (delta and zeta)
layers control the rate of the galvanizing reaction. The diffusion reaction
rate decreases as the
coating thickness increases, thus permitting predictable, consistent coverage.
The normal coating
has a bright metallic lustre.
Recent developments in the manufacture of low-alloy high-strength steels
include continuous
casting. In the continuous casting process, it is necessary to add elements
that 'kill' or deoxidize
the steel i.e. prevent gaseous products which produce porosity. Silicon is
commonly employed
for this purpose. These steels, as a result, generally contain between 0.01%
to 0.3%, by weight,
silicon but may include up to or more than about 0.5 wt% silicon and are known
as 'reactive
steels' or silicon steels.
Phosphorus in the steel also affects reactivity having an accepted measure of
reactivity that is
approximately 2.5 times that of silicon. Thus, the silicon content plus 2.5
times the phosphorus
content is known as the effective silicon content of the steel.
SUBSTITUTE SHEET (RULE 26)
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Silicon steels that have high reactivity pose problems to the galvanizing
process,
producing thick, brittle and uneven coatings, poor adherence and/or a dull or
marbled
appearance. These coatings are known as 'reactive' coatings. The high
reactivity of
the silicon steels also causes excessive zinc consumption and excessive dross
formation.
Silicon released from the steel during galvanizing is insoluble in the zeta
layer. This
creates an instability in the zeta layer and produces thick, porous
intermetallic layers.
The microstructure is characterized by a very thin and uneven delta layer
overlaid by
a very thick and porous zeta layer. The porous intermetallic; layer allows
liquid bath
metal to react near the steel interface during the entire immersion period.
The result is
a linear growth mode with immersion time that allows the formation of
excessively
thick coatings. These coatings are generally very rough, undesirably thick,
brittle and
dull in appearance.
Steels with silicon levels between 0.05 to 0.15 (i.e. around tl~e "Sandelin
Peak" area),
may also develop a 'mixed' reactivity or 'M' coating. This coating is
characterized
by a combination of reactive and non-reactive areas on the same steel which is
believed to be due to differences in localized silicon levels on the surface
of the steel.
It is known in the prior art to control reactivity by producing bath
temperature and
immersion time at a rate inversely proportional to the silicon content of the
steel.
Lower bath temperatures, in the order of 430°C, and reduced immersion
times, tend to
control reactivity on high silicon steels. However, using low bath
temperatures and
reduced times on low silicon steels produces unacceptably l;hin coating
thicknesses.
Thus, the galvanizer must know the silicon content of the steel beforehand and
adjust
the hot dip parameters accordingly. This approach cannot be implemented if
steel
reactivity is not known or if components to be galvanized comprise parts of
different
reactivities welded together. With low-temperature galvanizing, productivity
can be
poor because of the need to increase immersion times.
It is also known to control steel reactivity by adding alloy elements to the
zinc galvanizing bath. One such addition is nickel in a process known as the
TechnigalvaTM (or Nickel-Zinc) process. A nickel content of 0.05 to 0.10% by
weight in the zinc bath effectively controls reactive steels
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having up to about 0.2% by weight silicon content. For steels having silicon
levels above
approximately 0.2 wt%, this nickel-zinc process is not effective and thus it
is only a partial
solution to the reactive steel galvanizing problem. Low reactivity (normal)
steels, when
galvanized by the nickel-zinc process, pose the same difficulty as seen in low
temperature
galvanizing in that coating thickness may be unacceptably thin. With this
process, it is thus
preferred that the galvanizer know the reactivity of the steel beforehand and
adjust galvanizing
conditions accordingly, both of which are difficult to accomplish in practice.
Under some
conditions, this process also produces dross that tends to float in the bath
and be drawn out on
the workpiece, producing unacceptable coatings.
Another alloy used to control reactivity is that disclosed in French Patent
No. 2,366,376, granted
October 27, 1980, for galvanizing reactive steels, known as the PolygalvaTM
process. The alloy
comprises zinc of commercial purity containing by weight 0.1 to 1.5% lead,
0.01 to 0.05%
aluminum, 0.03 to 2.0% tin, and 0.001 to 2.0% magnesium.
US Patent No. 4,439,397, granted March 27, 1984, discusses the accelerated
rate at which the
magnesium and aluminum are consumed or lost in this PolygalvaTM process for
galvanizing steel.
Procedures are presented to overcome the inherent difficulty in replenishing
deficient aluminum
or magnesium in the zinc alloy galvanizing bath. The process has serious
limitations in that the
steel has to be meticulously degreased, pickled, pre-fluxed and oven-dried to
obtain good quality
product free of bare spots. Thus, in most cases, new high-quality
installations are usually
required.
US Patent No. 4,168,972, issued September 25, 1979, and US Patent No.
4,238,532, issued
December 9, 1980, also disclose alloys for galvanizing reactive steels. The
alloys presented
include variations of the PolygalvaTM alloy components of lead, aluminum,
magnesium and tin
in zinc.
It is known in the prior art that aluminum included in the galvanizing bath
reduces the reactivity
of the high silicon steels. A process known as the SupergalvaTM process
includes an alloy of zinc
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containing 5 wt% aluminum. The process requires a special flux and double
dipping not
generally accepted by commercial galvanizers.
Co-Pending US Patent Application No. 08/667,830 filed June 20, 1996, describes
a new alloy
and process for controlling reactivity in steels with silicon content up to 1
wt%. The alloy
comprises zinc of commercial purity containing, by weight, one or both of
vanadium in the
amounts of at least 0.02% to 0.04% and titanium in the amounts of at least
0.02% to 0.05%.
It is a principal object of the present invention to provide a process and
alloy to effectively
control reactivity on a full range of steels including low and high silicon
steels. The process
should also produce coatings of acceptable and uniform thickness over the full
range of steels.
Another object of the invention is to provide an alloy and process which uses
standard
galvanizing equipment operated under normal conditions for galvanizing steels
of mixed
reactivity without the need to adjust for variations in steel chemistry.
SUMMARY OF THE INVENTION
The disadvantages of the prior art thus may be substantially overcome by
providing a new
galvanizing alloy and process which can be readily adapted to standard hot-dip
galvanizing
equipment.
In its broad aspect, the process of the invention for galvanizing steel,
including reactive steels,
by immersion comprises immersing said steel in a molten bath of a zinc alloy
comprising, by
weight, aluminum in the amount of at least 0.001% to 0.007%, preferably 0.002%
to 0.004%,
tin in the amount of at least 0.5% to a maximum of 2%, preferably at least
0.8%, and one of an
element selected from the group consisting of vanadium in the amount of at
least 0.02%,
preferably 0.05% to 0.12%, titanium in the amount of at least 0.03%,
preferably 0.06% to 0.10%,
and both vanadium and titanium together in the amount of at least 0.02%
vanadium and at least
0.01% titanium for a total of at least 0.03%, preferably 0.05 wt% to 0.15%, of
vanadium and
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titanium, the balance zinc containing up to 1.3 wt% lead. The alloy of the
invention for
galvanizing steel comprises, by weight, aluminum in the amount of at least
0.001 % to 0.007%,
preferably 0.002 to 0.004%, tin in the amount of at least 0.5% to a maximum of
2%, preferably
at least 0.8%, and one of an element selected from the group consisting of
vanadium in the
amount of at least 0.02%, preferably 0.05% to 0.12%, titanium in the amount of
at least 0.03%,
preferably 0.06% to 0.10%, and both vanadium and titanium together in the
amount of at least
0.02% vanadium and at least 0.01% titanium for a total of at least 0.03%,
preferably 0.05% to
0.15%, of vanadium and titanium, the balance zinc containing up to 1.3 wt%
lead. In an
embodiment of the invention for use in zinc-nickel alloy baths, the alloy may
comprise, by
weight, aluminum in the amount of at least 0.001%, tin in the amount of 0.5%
to 2%, and
vanadium with nickel in the amount of at least 0.02% vanadium and at least
0.02% nickel to a
maximum of 0.15% vanadium and nickel collectively. Titanium may be added in an
amount of
at Least 0.01 % titanium to a maximum of 0.2% vanadium, nickel and titanium.
In a further
embodiment, for use in a zinc alloy bath, the alloy is comprised of aluminum
in the amount of
at least 0.001%, tin in the amount of about 0.5% to about 2%, vanadium in the
amount of 0.02
to 0.12%, bismuth in the amount of 0.05% to 0.1%, and the balance zinc.
BRIEF DESCRIPTION OF TH DRAWINGS
The process of the invention and the alloy produced thereby will now be
described with reference
to the following drawings, in which:
Figure 1 to 3 are graphs illustrating galvanized coating thickness of a
variety of
galvanizing coatings on steel surfaces having a silicon content ranging from 0
to
1.0 wt% under conditions of eight-minute immersion at 450°C, Figure 1
being a
graph showing average coating thickness versus silicon content in a
galvanizing
bath of Prime Western (PW) zinc with tin and vanadium, Figure 2 being a graph
showing average coating thickness versus silicon content in a galvanizing bath
of PW zinc with tin and titanium, and Figure 3 being a graph showing average
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coating thickness versus silicon content in a galvanizing bath of PW zinc with
tin
and both vanadium and titanium together; and
Figure 4 is a graph illustrating kettle material weight losses for a variety
of
galvanizing alloys.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference to Figures I, 2 and 3 of the drawings, curve 10 typifies the
variation of thickness
in microns of a coating of zinc of commercial purity, such as conventional
Prime Western (PW),
on a steel surface as a function of the silicon content of the steel. The term
"commercial purity"
used herein will be understood to include Prime Western, High Grade and
Special High Grade
zinc. Under these conditions of bath temperature (450°C) and immersion
time (8 minutes), the
thickness of zinc coating peaks at a thickness of about 260 microns at a
silicon content of about
0.15 wt%, decreases to a thickness of about 175 microns at a silicon content
of about 0.2 wt%,
and then increases to a maximum thickness of about 375 microns at a silicon
content of about
0.5 wt%, decreasing in thickness slightly to a silicon content of 1.0 wt%.
This curve 10 will be
recognized as being very similar to the well-known Sandelin Curve. The
composition of the
steels used is listed in Table I below.
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TABLE I
STEEL COMPOSITION : 1995 TRIALS
SteelMTL Chemical Si
Composition
(%)
wt.
AlloyHeat
~
Equivalent*
Si P C S Mn AI
1 95-18 0.021 <0.006 0.11 .0071 0.59 0.019 0.021
95-20 0.019 0.11 .0051 0.76 0.035 0.019
2 95-4a 0.15 <0.006 0.10 .0037 0.71 0.015 0.15
95-4b 0.15 0.10 .0026 0.70 0.016 0.15
3 95-4c 0.21 <0.006 0.10 .0029 0.73 0.007 0.21
95-4d 0.21 0.11 .0038 0.73 0.005 0.21
95-SI 0.19 0.13 .0073 0.77 0.046 0.19
4 95-21a0.29 <0.006 0.10 .0030 0.70 0.035 0.29
95-21b0.30 0.10 .0028 0.71 0.046 0.30
95-28 0.32 <0.006 0.09 .0069 0.76 n.a. 0.32
95-42 0.36 0.12 .0067 0.83 0.032 0.36
6 95-21 0.46 <0.006 0.10 .0030 0.73 0.037 0.46
c
95-21d0.46 0.10 .0029 0.73 0.036 0.46
7 95-22a0.51 <0.006 0.09 .0036 0.68 0.040 0.51
95-2260.51 0.10 .0032 0.68 0.042 0.51
8 95-22c0.99 <0.006 0.09 .0031 0.71 0.022 0.99
95-22d0.98 0.09 .0031 0.71 0.022 0.98
9 95-23a.019 0.02 0.09 n.a. 0.6G 0.010 0.07
95-236.018 0.02 0.09 0.65 0.010 0.07
95-39 .031 0.050 0.10 .0071 0.80 0.036 0.16
95-40 .023 0.055 0.09 .0072 0.71 0.047 0.16
Si quivalent= Si + 2.SP
n.a. = Not available
In accordance with ASTM Standards e.g. the ASTM A-123 Standard (610 g/m2 or 86
microns
for 3.2 to 6.4 mm thick steel plate), a uniform coating thickness of about 100
microns is desired
in order to meet minimum thickness requirements while avoiding the expense and
waste of thick
coatings. Also, excessive thickness of zinc coatings on reactive steels and
steels of mixed
reactivity due to high or variable silicon contents, usually produce rough,
porous, brittle and
generally unsightly coatings which can have poor adherence to the underlying
steel surface.
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It is generally accepted that the addition to the galvanizing bath of strong
silicide formers may
neutralize the influence of silicon in reactive steels. It has been found that
vanadium alone is an
effective alloying element for reducing the reactivity of silicon steels with
up to 0.25 wt% Si.
Vanadium in the bath is believed to combine with the silicon to form vanadium
silicides as inert
particles that become dispersed in the zeta layer. The silicon-free iron can
then react with zinc
to form a very compact and smooth layer that prevents liquid bath metal from
reaching the delta
layer. In essence, the vanadium effectively suppresses reactivity by
stabilizing the growth of the
zeta layer in the coating, which controls the growth rate by a diffusion
process.
It has been found that tin is also an effective element for reducing the
reactivity of steels. Tests
have shown that a galvanizing bath containing 2.5 wt% to 5 wt% tin can control
reactivity in
steels with up to 1 % silicon content. However, tests have also shown that tin
in amounts greater
than 2 wt% react rapidly with the galvanizing kettle wall steel at galvanizing
temperatures.
When the tin level in the galvanizing bath is below 2%, the reaction with the
kettle steel proceeds
at a slow rate, which is comparable to that of the commercial grade zinc.
However, when the
level of tin in a galvanizing bath is 2%, the presences of tin controls
reactivity in steels with only
up to 0.3% silicon.
The presence of at least 0.02 wt% vanadium, preferably 0.05 wt% to 0.12 wt%,
the solubility
limit of vanadium, in combination with 0.5 wt% to 2 wt% tin, controls
reactivity in steels having
up to 1 wt% silicon. Tests have shown that in galvanizing baths containing 1
to 1.2 wt% tin,
0.002 wt% aluminium, and the balance zinc of commercial purity containing 0.8
wt% lead, the
presence of 0.05 wt% and 0.08 wt% vanadium effectively controls reactivity to
varying degrees
in steels having silicon contents up to 1 %, as shown by the Sn-V curves 11
and 12 in Figure 1.
Zinc of commercial purity, such as conventional Prime Western, contains up to
1.3 wt% lead,
typically about 0.8% lead. However, other grades of zinc available such as
High Grade and
Special High Grade have lower contents of lead. There is a growing tendency to
reduce and
eliminate the presence of Lead in galvanizing because of environmental, health
and safety
concerns. It has been observed that bare spots in galvanized coatings could be
produced from
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galvanizing baths without lead or with reduced lead contents at lower levels
of tin at about 1 wt%
tin with 0.05 wt% vanadium and 0.002 wt% aluminium on steels having lower
silicon contents.
It was found that by the addition of 0.05% to 0.5 wt% of bismuth, preferably
0.05% to 0.1 wt%
bismuth, to Zn-Sn-V alloys containing 0.5 wt% to 2 wt% tin, 0.05 wt% to 0.12
wt% vanadium,
0.001 wt% to 0.007 wt% aluminium, the balance zinc, uniformly thick bright
galvanized coatings
having spangling and free of bare spots were produced. The presence of bismuth
was particularly
beneficial for tin contents in the range of 1 - 1.5 wt% tin.
In an alternative embodiment of the process of the present invention, titanium
is used in place
of vanadium. The presence of at least 0.03 wt% titanium, preferably 0.06 wt%
to 0.1 wt%, in
combination with 0.5 wt% to 2.0 wt% tin, controls reactivity in steels having
up to about 0.5 wt%
silicon. Tests have shown in a galvanizing bath containing 1.8 wt% tin, 0.002
wt% aluminum
and the balance zinc of commercial purity, the presence of 0.06 wt% and 0.10
wt% titanium
effectively controls reactivity to varying degrees in steels having silicon
contents up to about 0.5
wt %, as shown by Sn-Ti curve 13 in Figure 2. Increasing the titanium content
in the galvanizing
bath to 0.1 wt% did not increase the maximum silicon level controlled as seen
by Sn-Ti curve
14 in Figure 2.
However, the titanium addition to the bath forms a ternary Zn-Fe-Ti
intermetallic which
increases the amount of dross and ash during galvanizing and contributes to
high rates of
titanium consumption or depletion in the bath. It also adversely affects the
appearance of the
galvanized coating by eliminating the distinctive large spangle formed with
the tin-vanadium
alloy which most galvanizing customers favour.
Small amounts of titanium added to the tin-vanadium alloy as a substitute for
a portion of the
vanadium can be used to lower the level of vanadium in the alloy, without the
adverse effects of
the high titanium-tin alloy. The presence of at least 0.02 wt% vanadium and at
least 0.01 wt%
titanium, preferably 0.05 wt% to 0.1 wt% vanadium and titanium collectively,
controls reactivity
in steels having up to 1 wt% silicon. In a galvanizing bath containing 1 wt%
tin, 0.002 wt%
aluminum, and the balance zinc of commercial purity, the presence of 0.06 wt%
vanadium and
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0.02 wt% titanium effectively controls reactivity in steels having silicon
contents up to 1 wt%,
as shown by Sn-V-Ti curve 16 in Figure 3. Reducing the vanadium content in the
alloy may be
desirable in some cases to offset the high cost of vanadium as compared to
titanium.
An other embodiment of the alloy composition of the invention has utility in
zinc-nickel alloy
baths containing a typical nickel content of 0.05 wt% to 0.08 wt% nickel, and
up to 0.1 wt%
nickel, and comprises aluminum in the amount of at least 0.001 wt%, tin in the
amount of about
0.5 wt% to about 2 wt%, and vanadium with nickel in the amount of at least
0.02 wt% vanadium
and at least 0.02 wt% nickel to a maximum of 0.15 wt% vanadium and nickel
collectively. The
alloy compositions and the process of the invention will now be described with
reference to the
following non-limitative examples.
EXAMPLE 1
Long term immersion experiments of kettle steel in zinc all~y baths to
determine rate of attack
on the steel and maximum allowable limit for tin in the galvanizing alloy.
Four alloys were prepared and samples from kettle steel were immersed in each
alloy for a period
of about 11 days at a temperature of 480°C. This immersion temperature
was about 30°C higher
than the normal galvanizing bath temperature to accelerate the reaction of the
alloys with the
kettle steel samples. All the baths were saturated with iron at the start of
the experiments and an
addition of 0.004 wt% aluminium was made. The baths were analyzed during the
11-day trial
period and additions were made as needed to maintain the nominal bath
compositions. The four
alloy compositions are listed in Table II below.
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TABLE II
Alloy Alloy Composition
% wt
No. DesignationSn V Ti Ni
1 PW - - - -
2 Sn-Ni 2.5 - - 0.05
3 V-Ti - 0.04 0.05 -
4 Sn-V 1.0 0.05 - -
The composition of alloy No. 2 (Sn-Ni) is a high tin alloy. The composition of
alloy No. 3
(V-Ti) is included in US Patent Application No. 08/667,830. The composition of
alloy No. 4
(Sn-V) is an embodiment of alloy of the subject Patent Application.
Fifty kg melts were prepared in a SiC crucible that was heated in a radiant
tube furnace. Four
steel samples measuring 32 x 51 x 25 mm were immersed in each alloy bath.
Analysis of the
kettle steel showed its composition to contain, by weight, 0.09 wt% carbon,
0.02 wt% silicon,
0.006 wt% phosphorus and 0.27 wt% manganese. The samples were machined (to
remove
surface scale), degreased with acetone, pickled in hydrochloric acid, weighed,
measured, and
pre-fluxed in ZnNH4Cl, prior to immersion in the alloy baths.
The samples were removed after approximately 2, 4, 7 and 11 days immersion.
The coatings on
the samples were removed by immersion in hot sodium hydroxide solution,
followed by cold
hydrochloric solution, and re-weighed.
The differences in weight loss were divided by the initial surface areas of
the samples to
determine weight loss in gms per unit area in mm2. The results are shown in
the graph of Figure
4, as weight loss in g/mm2 versus the immersion period in hours.
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The curves in Figure 4 show that the weight losses for alloy baths No. 3 (V-Ti
curve) and No.
4 (Sn-V curve) are comparable to No. 1 (PW curve). The weight loss from alloy
bath No. 2
(Sn-Ni curve) after 150 hours is about six times as great as the others (Nos.
l, 3 and 4). More
importantly, the slope of the No. 2 alloy curve is very steep, indicating that
the reaction with the
steel follows a rapid linear growth with immersion time that results in the
formation of
excessively thick coatings.
An additional PW melt was prepared and additions of tin were made at 0.2 wt%
increments, from
0.5 wt% to 2.5 wt% tin. Kettle steel samples were immersed at 480°C and
inspected after 24
hours and 48 hours. If no evidence of excessive coating growth was observed
after 48 hours, the
tin content in the bath was increased by 0.2 wt%. When evidence of excessive
growth was first
observed, the tin content in the bath was reduced by 0.2 wt% and steel samples
were immersed
for a period of about two weeks to ensure that the coating growth rate was
normal. From these
experiments, it was determined that when the tin content in the bath exceeded
2 wt%, the
abnormal or excessive growth rate began to occur.
~~AMPLE 2
Galvanizin Tg rials
Ten alloys were prepared for laboratory-scale galvanizing trials. The alloying
additions were
made to PW grade zinc. The typical composition of PW is shown in Table III
below.
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TABLE III
COMPOSITIONS OF PW ZINC
Element PW (%) Element PW (%}
Pb 0.80 Cd 0.0019
Fe 0.009 Ca 0.00005
A1 0.004 Zr -
Si 0.0004 Cu 0.0032
Mn 0.007 Mg 0.00002
Ni 0.0005 As -
Cr 0.0001 B -
Ti 0.0002 Ga o.oooos
V - Ge 0.0003
Sn 0.0001 In -
Sb 0.0004 Ti 0.0002
Bi 0.002 Zn bal.
Ag 0.0004 - -
The various experimental baths are listed in Table IV. All experimental baths
were saturated
with iron and appropriate amounts of a 5 wt% aluminum master alloy were added
to maintain
a 0.002 wt% (brightener) aluminum level in the bath. The tin additions were
made with high
purity tin ingot. The vanadium additions were made with a Zn-2.3 wt% V master
alloy, and the
titanium additions were made with a Zn-4 wt% Ti master alloy.
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TABLE IV
BATH ALLOY COMPOSITIONS
Bath % Element
Trial No. Designation Sn V Ti
1 PW - - -
2 PW+Sn 1.8 - -
3 PW+Sn+V 1.8 0.04 -
4 PW+Sn+V 0.4 0.12 -
PW+Sn+V 1.0 0.05 -
6 PW+Sn+V 1.2 0.08 -
7 PW+Sn+Ti 1.8 - 0.06
8 PW+Sn+Ti 1.8 - 0.10
9 PW+Sn+V+Ti 1.0 0.06 0.02
PW+Sn+V+Ti 1.0 0.03 0.02
Note: All baths saturated in iron and contain 0.002 wt% aluminum brightener.
A bench scale line was set up to process the test samples consistently. The
following steps were
taken:
1. Degreasing: 0.25 g/cc NaOH solution at 70°C with agitation for ten
minutes
2. Rinse: Tepid flowing water
3. Pickling: 15 wt% Hcl at room temperature, inhibited with RodineTM 85
(1:4000),
for 20 minutes
4. Pre-flux: 20 wt% ZaclonTM K (ZnNH4Cl) at 60°C, for two minute
immersion.
S. Drying: Oven-dried for five minutes at 110°C.
Twenty-five kg melts were prepared in a SiC crucible that provided a
galvanizing surface of 150
mm in diameter. The crucible was heated in a radiant tube furnace.
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The galvanizing temperature was 4S0 f 2°C. The melt surface was skimmed
prior to immersion
and just before the test coupons were withdrawn. The test coupons were dipped
for eight-minute
immersions. The immersion rate was 40 mm/sec while the withdrawal rate was 60
mm/sec. The
samples were air-cooled at room temperature (no quenching).
Hot-rolled low-carbon silicon-killed steel coupons, measuring 77 mm x 39 mm x
3 mm, were
used. The ten steel compositions, with silicon levels ranging from about 0.02
wt% to 1 wt%, are
listed in Table 1. This table includes the respective Si-equivalent or Si +
2.SP level for the steels,
which takes into account the weighted effect of phosphorus as it relates to
the reactivity
behaviour of the steel.
The galvanized coatings produced in the experiments were evaluated by the
following methods:
Coating~~earance
The test coupons were photographed and classified under one of the three
following categories:
Normal, Reactive or Mixed. A description for each category of coating
appearance is as follows:
Normal: The typical coating of a low-reactivity steel, usually bright and
relatively
smooth with visible spangle.
Reactive: The typical coating of a reactive steel, usually matte-grey with no
visible
spangle.
Mixed: The typical coating of a steel that has both reactive and non-reactive
areas. The
coating is usually very rough and varies from thin in low-reactivity areas to
thick in the reactive areas.
Coating Thickness
Coating thickness measurements were made using an electromagnetic thickness
gauge. The
coating thickness results are presented in graph form in Figures 1 to 3 and
constitute the steel
reactivity curves.
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Metallo~~ra,~nhv~
Twenty-five mm long pieces were cut from representative areas of the test
coupons and prepared
by conventional metallographic techniques for microscopic examination. All
test samples were
examined by optical microscopy. Selected samples were examined with a scanning
electron
microscope (SEM) and energy dispersive x-ray micro-analysis (EDS) was
performed on selected
samples as required.
From these galvanizing trials, the maximum effective steel silicon levels
controlled by the
various bath alloys were determined and they are presented in Table V. As a
reference, results
of single element additions of tin, vanadium, titanium and nickel, obtained
from past trials, are
included in Table V.
TABLE V
MAXIMUM EFFECTIVE SILICON (ESI) LEVEL IN STEEL CONTROLLED BY ALLOY ADDITION
Bath Alloy Maximum
Addition
(%)
PW Alloy Sn V Ti Ni Esi
Single 1.8* - - 0.09 0.20
Element - 0.12 - - 0.25
Addition - - 0.10 - 0.3 0
Sn+V 1.8' 0.04 - - 0.50
Combination0.42 0.12 - - 0.50
1.03 0.05 - - 0.50
1.24 0.08 - - 1.0
Sn+Ti 1.8 - 0.06 - O.SM
1.8 - 0.10 - O.SM
Sn+V+Ti 1.0 0.06 0.02 - 1.0
1.0 0.03 0.02 - 0.5
Notes:
1. High Sn - Low V
2. Low Sn - High V
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3. Preferred composition for 0.5% Esi
4. Preferred composition for 1.0% Esi
M Marginal control with various amounts of mixed reactivity and heavier
coatings than
when fully controlled.
The results show that, as a single element addition, the maximum effective
silicon level
controlled is about 0.3 wt%. When tin and vanadium are combined, 0.5 wt%
effective silicon
can be controlled with a minimum level of 0.04 wt% vanadium and a tin level of
I .8 wt% (which
is near the maximum allowable level), and with a minimum level of 0.4 wt% tin
and a 0.12 wt%
vanadium level. A preferred composition for controlling the 0.5 wt% Si level
is 1.0 wt% tin with
0.05 wt% vanadium. The 1.0 wt% effective silicon can be controlled with a
preferred
composition of 1.2 wt% tin and 0.08 wt% vanadium.
When tin is combined with titanium, the maximum effective silicon level that
was controlled was
0.5 wt%, even when the maximum allowable amount of 1.8 wt% tin and an amount
of 0.1 wt%
titanium were added to the galvanizing bath.
When vanadium and titanium are added together, it is possible to control the
0.5 wt% effective
silicon with additions of 1.0 wt% tin, 0.03 wt% vanadium, and 0.02 wt%
titanium and the I wt%
effective silicon level with additions of I .0 wt% tin, 0.06 wt% vanadium, and
0.02 wt% titanium.
The addition of titanium to the tin and vanadium alloy allows for a reduction
in the amount of
vanadium needed to control at the 0.5 wt% and 1.0 wt% effective silicon
levels.
EXAMPLE 3
Addition of Bismuth
Trials were conducted on 77 mm x 39 mm x 3 mm low silicon steel coupons which
were
pretreated by an acetone rinse and scrubbing, pickling in 15% HCL solution for
10 - 15 minutes,
preflux of ZACLON KTM (20° Be) for 2 minutes at 70 ° C and oven-
dried at 100 °C for 5 minutes.
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The coupons were galvanized by immersion for 4 minutes in zinc alloy baths of
Special High
Grade 25 kg melt saturated with iron and containing 0.004 wt% aluminum, 1 wt%
tin, 0.05 wt%
vanadium and varying amounts of bismuth at a temperature of 450° C.
The test results are shown in Table VI.
TABLE VI
BATH ALLOY COMPOSITIONS -SHG + Sn + V + Bi
Trial No. % Element Observations
Sn V Bi
I 1.0 0.05 Severe bare spots and small
spangling
2 1.0 0.05 0.05 Substantially complete elimination
of
bare spots, and larger spangling
3 1.0 0.05 0.1 Free of bare spots and larger
spangles
4 1.0 0.05 0.2 Free of bare spots and very
large
spangles
1.0 0.05 0.5 Free of bare spots and very
large
spangles
Note: all baths contain 0.004 wt% aluminum brightener.
The presence at least 0.05 wt% bismuth was found to be effective in obviating
bare spots and in
enhancing spangling of the galvanized coating. An upper limit of bismuth of
0.1 wt% bismuth
was found economically viable, amounts in excess of 0.1% up to 0.5% did not
improve the
quality of coating.
The invention provides a number of important advantages. Galvanized coatings
produced in
accordance with the invention are complete and uniform and of desired
thickness on low and
high silicon steels including steel having silicon content from 0.01 wt% to at
least 0.5 wt%. The
coatings produced also have a bright metallic lustre. The process can be
easily adapted to
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conventional galvanizing production equipment using normal galvanizing
temperatures and
immersion times.
AMENDED SHEET
IPE.g/EP
