Note: Descriptions are shown in the official language in which they were submitted.
2l613q3
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.
BACKGROUND OF THE INVENTION
The conventional process for hot dip galvanizing of
low carbon steels comprises pretreatment of said steels in
a 25 to 35~ by weight zinc-ammonium-chloride (ZnNH4Cl) pre-
flux, followed by immersion in molten zinc or zinc alloy
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, ie., 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~ silicon and
are known as 'reactive steels' or silicon steels.
Steels containing phosphorus are also reactive steels
having an accepted measure of reactivity relative to the
silicon content of Si + 2.5 P (the silicon content plus 2.5
times the phosphorus content). This is called the effective
silicon content.
Silicon steels that have high reactivity pose problems
to the galvanizing process, producing thick, brittle and
2161393
'
uneven coatings, poor adherence and/or a dull or marbled
appearance. These coatings are known as 'reactive' or 'R'
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 the "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
differences in localized silicon levels on the surface of
the steel.
It is known in the prior art to control reactivity by
reducing 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 times on
low silicon steels produces unacceptably thin 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
2161393
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 having up to about 0.2~ by weight silicon content.
For steels having silicon levels above approximately 0.2~,
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 0.5
aluminum, 0.03 to 2.0~ tin and 0.001 to 2.0~ magnesium.
U.S. Pat. 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.
2~61393
,
Thus, in most cases, new high-quality installations are
usually required.
U.S. Pat. No. 4,168,972 issued September 25, 1979 and
U.S. Pat. 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 containing 5~ aluminum. The
process requires a special flux and double dipping not
generally accepted by commercial galvanizers.
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.
2 5 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 0.003%, and one of an element selected from the group
consisting of vanadium in the amount of at least 0. 06~,
preferably at least 0. 08~, titanium in the amount of at
least 0.03~, preferably at least 0.5~, and both vanadium
2161393
and titanium together in the amount of at least 0.02~
vanadium and at least 0.02~ titanium with a total of at
least 0.04~, preferably at least 0.06~ vanadium and
titanium, the balance zinc. The alloy of the invention for
galvanizing steel comprises, by weight, aluminum in the
amount of at least 0.003~, and one of an element selected
from the group consisting of vanadium in the amount of at
least 0.06~, preferably at least 0.08~, titanium in the
amount of at least 0.03~, preferably at least 0.5~, and
both vanadium and titanium together in the amount of at
least 0.02~ vanadium and at least 0.02~ titanium with a
total of at least 0.04~, preferably at least 0.06~ vanadium
and titanium, the balance zinc.
In another aspect, the invention relates to the pre-
treatment of the steel to be galvanized by applying a fluxcomprising about 15 - 20~ ZnNH4Cl in an aqueous solution.
BRIEF DESCRIPTION OF THE 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
titanium with Prime Western (PW) zinc, Figure 2 being a
graph showing average coating thickness versus silicon
content in a galvanizing bath of vanadiam with PW zinc, and
Figure 3 being a graph showing average coating thickness
versus silicon content in a galvanizing bath of vanadium
and titanium together with PW zinc;
Figure 4 is a graph showing average coating weight
versus silicon content in steel coupons produced by a four-
mlnute immersion in a galvanizing bath of vanadium with PW
zlnc;
2161393
Figure 5 is a graph showing average coating weight
versus silicon content in steel coupons produced by an
eight-minute immersion in a galvanizing bath of vanadium
with PW zinc;
Figure 6 is a graph showing average coating weight
versus silicon content in steel coupons produced by a four-
minute immersion in a galvanizing bath of titanium with PW
zinc;
Figure 7 is a graph showing average coating weight
versus silicon content in steel coupons produced by an
eight-minute immersion in a galvanizing bath of titanium
with PW zinc;
Figure 8 is a graph showing average coating weight
versus silicon content in steel coupons produced by a four-
minute immersion in a galvanizing bath of vanadium plustitanium with PW zinc; and
Figure 9 is a graph showing average coating weight
versus silicon content in steel coupons produced by an
eight-minute immersion in a galvanizing bath of vanadium
plus titanium with PW zinc.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference first to Figures 1, 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
2161393
the well-known Sandelin curve. The composition of the
steels used is listed in Table I following.
Table I
;L COMPOSITIONS: 1995 TRIALS
Steel MTL Chemical Comro~ n (%) Si
Alloy# Heat #Si P C S Mn AlEquivalent~
95-18 0.021<0.006 0.11 .0071 0.5g 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
95-51 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
95-280.32 < 0.006 0.09 .0069 0.76 n.a. 0.32
95-420.36 0.12 .0067 0~83 0~032 0~36
~ 6 95-21c0.46 <0~006 0.10 .0030 0.73 0.037 0.46
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
95-22b0.51 0.10 .0032 0.68 0.042 0.51
8 95-22c0.99<0.006 0.09 .0031 0.710.022 0.99
95-22d0.98 0.09 .0031 0.710.022 0.98
9 95-23a.019 0.02 0.09 n.a. 0.66Ø0100.07
95-23b~018 0~02 0~09 0~650.010 0.07
95-39.031 0.050 0.10 .0071 0.800.036 0.16
95-40.023 0.055 0.09 .0072 0.710.047 0.16
* Si quivalent = Si ~ 2.SP
n.a. = not available
2161393
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.
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 is an effective alloying element for
reducing the reactivity of high silicon steels. Generally,
galvanizing baths containing 0.08 to 0.12~ vanadium
effectively control excessive reactivity. However,
galvanized steel from a galvanizing bath containing an
alloy of zinc with vanadium rapidly forms a coloured
vanadium oxide layer on the surface and has excessive
coating roughness and occasionally bare spots. The effect
of aluminum in a zinc/vanadium bath for avoiding
undesirable coloration has been found to be first evident
at 0.003~ and, when the aluminum level is increased to
0.004~, vanadium oxide formation is essentially reduced and
the bath surface retains a grey, metallic surface for an
indefinite time.
Tests have shown that in a galvanizing bath containing
0.08 wt~ vanadium, 0.005 wt~ aluminum and the remainder
zinc of commercial purity, reactivity is controlled in
steels having a silicon content up to about 0.25 wt~ by the
presence of at least 0.06 wt~ vanadium, as shown in Figure
1. 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
2161393
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.
The coating thickness for high silicon steels matches
those of non-reactive, low silicon steel subjected to
conventional galvanizing procedures. However, coatings on
the galvanized steels may have bare spots. These bare spots
are attributed to a reaction of the aluminum and vanadium
with normal commercial ZACLONTM pre-flux comprising 25 to
30~ by wt zinc-ammonium-chloride (ZnNH4Cl). The reaction
oxides subsequently deposit on the surface, preventing
wetting of said surface. We have found that a pre-flux
solution reduced to 15 to 20~ by weight ZnNH4Cl completely
eliminates bare spots.
In an alternative embodiment of the process of the
present invention, titanium is used in place of vanadium.
Tests have shown that a galvanizing bath containing 0.05
wt~ titanium, 0.005 wt~ aluminum and the balance zinc of
commercial purity, the presence of at least about 0.05 wt~
titanium effectively controls reactivity in varying degrees
in steels having silicon contents up to about 0.5~, as
shown by Zn-Ti curve 14 in Figure 2. However, galvanized
coatings on steels containing 0.3~ to 0.5~ silicon had a
~'mixed reactivity" growth, producing rough coatings that
would likely be commercially unacceptable.
The titanium addition to the bath modifies the Fe-Zn
intermetallic layers on the reactive steels to produce the
more compact and even delta and zeta layers found on the
non-reactive steels. This suggests that much like the
vanadium, titanium is a strong silicide former that ties up
the silicon released from the steel during galvanizing,
allowing the zeta layer to stabilize. However, unlike
vanadium, titanium forms a ternary Zn-Fe-Ti intermetallic
layer at or near the steel surface where there is iron-
enrichment. The intermetallic particles are trapped in the
2161393
eta layer (outer layer) and hinder zinc drainage, thus
producing a thicker coating.
The coating microstructures produced by Zn-Ti coating
alloys show clearly that the thicker coatings obtained with
the titanium alloys are due to Zn-6~Fe-3~Ti intermetallic
particles that are present in the eta layer. A beneficial
side effect of the thicker eta layer is that low-silicon
steels have coatings that meet ASTM thickness standards
such as the ASTM A-123 Standard.
These intermetallic particles however cause higher
levels of dross production and increased zinc consumption.
It has been found that optimum results are obtained
when vanadium and titanium are combined in the galvanizing
bath. In accordance with a preferred embodiment of the
present invention, the process for hot dip galvanizing
includes a galvanizing bath comprising aluminum and both
vanadium and titanium in an amount of at least 0.02 wt~ of
one of vanadium or titanium and sufficient of the other for
a total of at least 0.06 wt~, the balance zinc of
commercial purity. More specifically, tests show very good
results with an alloy comprising 0.04 wt~ vanadium, 0.05
wt~ titanium, 0.005 wt~ aluminum and the remainder zinc of
commercial purity, as illustrated in Figure 3, Zn~V-Ti
curve 16. In this case, the good coating thickness control
was retained with up to almost 1.0~ silicon in the steel.
The results indicate that the combination of vanadium and
titanium together outperforms the single element additions
of concentration levels higher than the sum of the two
elements.
The process of the invention preferably includes pre-
treatment of the steel surface in a reduced-strength pre-
flux aqueous composition of zinc-ammonium-chloride
(ZnNH4Cl), specifically 15 to 20~ by weight ZnNH4Cl. The
delta and zeta layers of this process embodiment of the
invention for a zinc bath containing vanadium and titanium
are compact, even and very thin. The eta layer usually has
a fine dispersion of intermetallic particles and the
2161393
-
thickness is about 10~ thicker than obtained from the
vanadium alloy. Coating thickness meets ASTM requirements
for all steels including low silicon steels. This
combination of vanadium and titanium controls reactivity on
steels with up to at least 0.5~ silicon content and
provides bright metallic coatings void of bare spots.
The alloy composition and process of the invention
will now be described with reference to the following non-
limitative examples.
EXAMPLE 1
Immersion qalvanizing of reactive steels in a zinc alloY
containinq vanadium, showinq effects of aluminum on colour
su~presslon .
Preliminary trials were conducted to establish a
standard control based on the zinc-vanadium alloy process.
The experimental melts weighed 25 kg and were prepared in
a ceramic crucible that was electrically heated with
external radiant tubes. The crucible provided a 150 mm
diameter surface for galvanizing.
A PW bath, saturated with iron, was prepared and an
addition was made of 0.002~ aluminum brightener. Small
steel coupons (77 mm x 39 mm x 3 mm) were dipped for 4
minutes at 450~C, to develop a dipping procedure and to
produce control samples. The steels used were selected
from the group listed in Table II below. Galvanized
coatings were of good quality, and were normal for the
particular silicon-level in the steel.
2161393
~able II
Si l ~;~;L COMPOSITIONS
% of FIPmPn~c Reactivity
SNeOd Si P Mn C (Si+2.5P)
0.01 0.014 0.27 0.03 0.045
2 0.05 0.009 0.40 0.06 0.073
3 0.12 0.012 0.43 0.09 0.15
4 0.16 0.009 0.46 0.09 0.183
0.22 0.01 N.A. N.A. 0.245
6 0.29 0.013 N.A. N.A. 0.323
N.A. = Not Available
2161393
An addition of 0.04~ vanadium was made to the bath
when baseline galvanizing conditions were established. The
vanadium was added as a master alloy containing 2.3~
vanadium. Immediately after the vanadium alloy was
introduced into the bath, the surface became covered with
a yellow oxide layer (as opposed to the matte-grey that
formed before the addition of vanadium). The surface was
skimmed, but the yellow oxide appeared again within
seconds. The oxide layer became thicker with time and
changed in colour from yellow to purple, to dark blue,
within a period of a couple of minutes. A few coupons were
dipped to assess galvanizability under these bath
conditions. The galvanized coupons had a yellow
appearance, presumably because of the very thick and rough
vanadium-oxide layer. The trial continued by making
additions of aluminum at 0.0005~ increments. As the
aluminum level in the bath increased from 0.002 to 0.003~,
the time for the surface to form the yellow oxide also
increased. At 0.004~ aluminum, the bath surface retained
a metallic grey sheen for about 5 minutes after which it
would gradually change to a light yellow colour. When the
aluminum level was increased to 0.005%, the bath surface
retained the grey/metallic surface for an indefinite period
of time.
The vanadium in the bath was increased to 0.12~ to
ensure that 0.005~ aluminum was sufficient to control
surface oxidation even at the higher vanadium bath level.
The grey surface was maintained and oxide formation was
controlled. A few coupons galvanized in this bath had a
grey metallic sheen.
EXAMPLE 2
Pre-flux treatment of reactive steels to be qalvanized with
a zinc-vanadium alloy.
Although galvanized coatings from vanadium-alloy baths
with 0.005~ aluminum had a normal (metallic-grey)
appearance, they also contained many small bare spots.
Close ex~m;n~tion of these defect areas showed that the
14
2161393
steel surface had not been wetted, and the surface was
covered with black residues. Scanning Electron Microscope
and Energy Dispersive Spectrometer examination of these
residues detected high levels of aluminum, vanadium and
chlorine. The flux had reacted with the aluminum and
vanadium in the bath, and the reaction products deposited
on the surface were preventing wetting of the steel.
In the ensuing dipping trials, a number of processing
parameters were systematically varied to determine their
effects on surface wetting and to establish a galvanizing
procedure that would produce defect-free test coupons. The
list of parameters examined includes steel type and size,
bath temperature, immersion and withdrawal rates, pickling
time, pre-flux strength and type, and pre-flux drying time
and temperature.
Varying the pickling times and immersion and
withdrawal rates did not improve wetting. The occurrence
of bare spots was lower on the high reactivity steels.
Increasing the bath temperature eliminated the problem,
since the thicker steel would heat-up faster and release
the pre-flux at, or near, the surface. The galvanizing
temperature was raised to 480~C and the thicker (3 mm)
coupons were galvanized. The coatings produced were
totally free of bare spots.
Trials were conducted with the following preflux
composition: 30~ by weight Zaclon'M F (normal commercial
composition), 10~ by weight Zaclon'M F, and 25~ of a low-
fuming flux. Pre-flux drying temperatures and times were
also varied from 90~ to 110~C and 5 to 10 minutes,
respectively.
To evaluate the effect of the different pre-flux
conditions, steel coupons were galvanized at a bath
temperature of 455~C and an immersion time of 4 minutes.
The results showed that the most influential factor in
preventing bare spot formation was the pre-flux
concentration. The normal galvanizing flux (30~ Zaclon)
produced the most bare spots. The low-fuming flux produced
2161393
fewer bare spots. The 10~ Zaclon'M flux produced a bright
shiny coating without any bare spots. Drying temperatures
and times were found to have a lesser effect and were set
at 100~C and 5 minutes, respectively.
In later trials, the Zaclon flux concentration was
increased to as much as 20~ before bare spots would start
to occur. A higher flux concentration (greater than 10~)
may be required in a galvanizing plant to ensure that poor
wetting due to inadequacies of the up-stream pre-cleaning
operations (such as pickling and rinsing) can be overcome
by the pre-flux.
EXAMPLE 3
Galvanizinq Trials
Eight bath alloys were prepared for the galvanizing
trials. The alloying additions were made to PW grade zinc.
Bath samples were taken before and after the day's
galvanizing trial and analyzed by atomic absorption
analysis to ensure that the aluminum and vanadium levels
were maintained close to the nominal composition and to
determine the losses from galvanizing. The analysis
results for the various experimental baths are listed in
Table III.
Table~I
BATH ALLOY COMPOSITIONS
X Elemenl (A.A. Assay Results)
TrialBath
No. r ib Al Fe V Ti
Before Aher Before ARer Before After Before After
PW 0.004 0.004 0.036 0.026 ~
2PW+0.04%V 0.004 0.004 0.026 0.028 0.037 0.038
3PW+0.08%V 0.005 0.004 0.033 0.034 b.o80 0.079
4PW+0.12%V 0.004 0.003 0.029 0.032 0.132 0.132
5PW+0.05%Ti 0.005 0.004 Ø029 0.020 -- -- 0.043 0.037
6PW+0.10%Ti 0.004 N.A. 0.026 N.A. -- --- 0.095 N.A.
7PW+0.04%V 0.004 N.A. 0.031 N.A. 0.047 N.A. 0.045 N.A.
+ 0.05%Ti
8PW+0.08%V 0.004 0.002 0.025 0.020 0.066 0.069 0.053 0.046
+ 0.05%TI
N.A. = Not A~ailable
2161393
The typical composition of PW is shown in Table IV.
Table IV
COMPOSITIONS OF PW ZINC
Element P W (%) Element P W (%)
Pb 0.80 Cd 0.0019
Fe 0.009 Ca 0.00005
Al 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 0.00005
V --- Ge 0.0003
Sn 0.0001 In
Sb 0.0004 Ti 0.0002
Bi 0.002 Zn bal.
Ag 0.0004
21613~3
The vanadium additions were made with a 2.3~ master
alloy. The appropriate amount was stirred into the bath
and dissolved readily at the galvanizing temperature of
455~C. The titanium was added directly to the bath at a
temperature of 550~C. The bath was maintained at that
temperature for about 3 hours until the titanium was
dissolved. The temperature was then reduced to 455~C
before galvanizing began. All experimental baths were
saturated with iron and a 5~ aluminum master alloy was
added to maintain a 0.005~ aluminum level in the bath.
A bench scale line was set-up to process the test
samples consistently. The following steps were used:
- Degreasing: 0.25 g/cc NaOH solution at 70~C with
agitation for 10 minutes.
- Rinse: Tepid flowing water.
- Pickling: 15~ HCl at room temperature, inhibited
with RodineTM 85 (14000), for 20
minutes.
- Pre-flux: 10~ Zaclon'U F (ZnNH4Cl) at 60~C, for 2-
minute immersion.
-Drying: Oven dried for 5 min. at 110~C.
Twenty-five kg melts were prepared in a SiC crucible
that provided a galvanizing surface of 150 mm diameter.
The crucible was heated in a radiant tube furnace.
The galvanizing temperature was 455 + 2~C. The melt
surface was skimmed prior to immersion and just before the
test coupons were withdrawn. The test coupons were dipped
for 4-minute and 8-minute immersions. The immersion rate
was 40 mm/sec., while the withdrawal rate was 160 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 six steel
compositions, with silicon levels ranging from 0.01~ to
0.29~, are listed in
Table II. This table includes the respective Si + 2.5P
level for the steels, which takes into account the weighted
18
2161393
-
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.
Coatinq A~pearance: The test coupons were
photographed and classified under one of the three
following categories: Normal (N), Reactive (R) and
Mixed (M). A description for each type 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 Weiqht and Thickness: Coating weights were
determined by the chemical weigh-strip-weigh method
according to ASTM A90 Standard. Only a portion of the
test coupons, measuring 25 mm x 25 mm, was used for
this test. Results are reported in gm/m2.
Corresponding thicknesses (in microns) were also
calculated from the coating weights.
Optical thickness measurements were taken from the
metallographic sections. Average, maximum and minimum
thicknesses from each section examined were recorded.
Metalloqraphy: 25-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.
19
21 6 1 393
~ .
The appearance of the test coupons is summarized in
Table V. The three general categories used to describe the
samples are Normal, Reactive, and Mixed.
Table V
COATING APPEARANCE OF TEST COUPONS
Steel Coupon No. (%Si)
Trial Bath
No. D~ 1 2 3 4 5 6
(0.01)(0.05) (0. 1~) (0. 16)(0.22) (0.29)
PW N N R N R M
2PW+0.04%V N N R M R M
3PW+0.08%V N N N N M N
4PW+0.12%V N N N N N N
5PW+0.05 %Ti N N N N M N
6 ~PW+0. 10%Ti N M M M R N
7PW+0.04%V N N N N M N
+ 0.05%Ti
8PW+0.08%V N N N N M N
+ 0.05%Ti
A~ aldnce C ~ ,~ics: N = Normal PW - Corninco Prime Western Zinc
R = Reactive
M = Mixed Reactivity
2161393
The coating weight thickness results are presented in
graph form in Figures 4 to 9. The graphs show the average
coating weights developed on the steel coupons in the
various galvanizing trials. Average coating thicknesses,
calculated from the weights, are also shown on the graphs.
Because the phosphorus levels of the steels used in these
trials were very low, the Si + 2.5 P values did not
significantly vary from the percentage of silicon values in
the steels. Thus, to simplify matters, the reactivity
curves in Figs. 4 to 9 were plotted using the silicon
content of the various steels.
Figures 4 and 5 show the results of the vanadium
trials with PW zinc for the 4- and 8- minute immersions of
test coupons at 455~C. Figures 6 and 7 show the titanium
trials with PW zinc for the two immersion times of test
coupons at 455~C; the 4-minute immersion of Figure 6 was
done with the 0.05% titanium only. Figures 8 and 9 show
the results of the vanadium plus titanium trials with PW
zinc for 4- and 8- minute immersion times of test coupons
at 455~C.
Galvanizing with PW zinc produced normal or non-
reactive type coatings on Steels No. 1 and 2 (silicon
levels of 0.01 and 0.05). Steels No. 3, 4, 5 and 6
(silicon levels of 0.12 to 0.29) developed reactive-type
coatings.
Galvanizing in baths containing 0.04% vanadium
produced normal coatings on Steels No. 1 and 2, mixed
reactivity coatings on Steels 3 and 6, and reactive
2161393
coatings on Steels 4 and 5. Galvanizing in baths
containing 0.12~ vanadium produced normal (non-reactive)
coatings on all six steels. Inconsistent results were
obtained with Steel No. 5 (0.22~ silicon), where in some
instances, mixed reactivity structures were obtained. This
was later determined to be due to abnormal roughness of the
steel surface.
A noticeable difference between the non-reactive
coatings produced in baths containing vanadium is in the
delta-to-zeta ratio. The PW coatings had a 1:3 delta-to-
zeta ratio while the vanadium coatings had a 1:1 ratio.
The 0.05~ titanium addition to the PW bath produced
non-reactive microstructures on all but the No. 5 Steel.
The 0.1~ titanium produced non-reactive microstructures on
all six steels. Although the intermetallic layers of the
0.1~ titanium coatings were significantly thinner than
those of the 0.05~ titanium coatings, overall the 0.1
titanium coatings appeared to be thicker than the 0.05~
titanium coatings. This was because the eta layer of the
0.1~ titanium coatings was thicker. The reason for the
thicker eta layer is the relatively large intermetallic
particles contained in the layer that affected drainage.
Intermetallic particles were also present in the eta
layer of the 0.05~ titanium coatings. However, their size
was much smaller than those in the 0.1~ titanium coatings
and did not interfere as much with the drainage of the eta
layer. EDX analysis of these intermetallic particles
determined their composition to be Zn-6~Fe-3~Ti.
2161393
Coating microstructures on the 0.08% vanadium plus
0.05% titanium and on the 0.04% vanadium plus 0.05%
titanium samples were very similar to those produced with
the 0.05% titanium (except Steel No. 22) and 0.1% titanium
baths. The reactive-steel coatings were modified by the
alloy additions to produce the compact alloy layer
structure of the non-reactive steels. As was seen in the
vanadium coatings, the zeta layer was thinner, giving a
delta-to-zeta ratio of about 1:1. The small Zn-Fe-Ti
intermetallic particles that were observed in the 0.05%
titanium coatings were also present in these coatings. EDS
analysis of the particles determined that their composition
was similar to that of the titanium coatings.
Referring now to the PW zinc reactivity curves in
Figures 4 and 5, it can be seen that reactivity was low
with the 0.01% and 0.05~ silicon steels. Reactivity
increased to a maximum with the 0.12% silicon steel,
followed by a drop with the 0.16% silicon steel.
Reactivity increased again with the 0.22~ silicon steel and
decreased with the 0.29% silicon steel. This decrease was
unusual for a steel of this silicon level, 0.3~ silicon
steels normally performing as shown in Figure 1.
Additions of 0.04% vanadium to PW zinc decreased
coating weight by an average of 10-30% on all experimental
steels, except for Steel No. 3 (0.12% silicon), which
showed a substantial 35% and 48% coating thickness increase
for the 4-minute and 8-minute immersions, respectively.
23
2161393
-
The 0.04% vanadium did not significantly modify the coating
microstructures of the reactive steels (Nos. 3, 4, 5 and
6).
Additions of 0.08~ vanadium and 0.12~ vanadium to PW
zinc reduced the coating thickness of all the steels
tested. The most dramatic reduction was for the reactive
steels (Nos. 3, 4, 5 and 6), whose coating thickness now
matched those of the non-reactive, low-silicon steels.
Coatings for the low-silicon steels (Nos. 1 and 2), at 4-
minute immersion, were marginally thinner than required by
ASTM A-123 Standard of 610 g/m2 or 86 microns.
Some inconsistency in controlling the reactivity of
Steel No. 5(0.22~ silicon) was experienced at both the
0.08~ vanadium and 0.12~ vanadium levels due to the
roughness of the steel and, therefore, the observations for
Steel No. 5 were disregarded. It is believed that
excessive roughness of the steel surface, the presence of
microcracks, or the stress effects of previous cold working
can result in inconsistency in the control of reactivity.
Ex~m;n~tion of the microstructures obtained from the
0.08~ and 0.12~ vanadium trials showed that the thick,
porous intermetallic layers normally produced on the
reactive steels (Nos. 3, 4, 5 and 6) were modified by the
vanadium addition to produce the compact and even alloy
layers that were produced on the non-reactive steels (Nos.
1 and 2). One noticeable difference between them was that
the delta-to-zeta alloy layer ratio of the vanadium-
modified coatings was higher than that of the normal non-
24
2~61393
reactive coating structure. EDS analysis showed that the
Fe-Zn intermetallic layers produced in the vanadium
coatings had the same composition as the conventional delta
and zeta layers of the non-reactive coating. Vanadium was
detected in discrete pockets at the zeta/eta interface and
on the outer surface of the coating or eta layer. Since
the solubility of vanadium in the zeta phase is very low,
it is rejected at the zeta and delta interfaces. No
vanadium was detected in the alloy layers.
The results of investigations into the causes of
silicon-induced reactivity in steels can provide a logical
explanation of how the vanadium affects the microstructure
and produces a non-reactive coating. Silicon released from
the steel during galvanizing is insoluble in the zeta
layer. This creates an instability in the zeta and
prevents the formation of a compact, pore-free layer.
Vanadium in the bath combines with the silicon to form
vanadium silicides, 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.
The growth of the zeta and delta layers are then diffusion
controlled. Since the availability of iron is higher at
the delta/zeta interface, the delta layer grows thicker
than in normal coatings.
With reference to Figures 6 and 7, the addition of
0.05~ titanium to the PW baths reduced coating thicknesses
on all but the No. 5 (0.22~ silicon) Steel. Additional
2161393
coupons of the silicon steel were dipped to ensure that the
results were consistent, since the performance of this
steel was found to be questionable in the vanadium trials.
The results showed that 0.0S~ titanium level could not
consistently control the reactivity of this steel.
Observations for Steel No. 5 accordingly were disregarded.
Later trials, reported above with reference to Figure 2,
confirmed that steels with the same silicon level (0.22~
silicon) were controllable with 0.05~ titanium. Although
0.1~ titanium reduced the thicknesses of the coatings on
all six steels, coating thicknesses were 7-32~ greater with
0.1~ titanium than with 0.05~ titanium (excluding No. 5
Steel). Compared to the coating thicknesses obtained with
the 0.08~ vanadium alloy, the 0.05~ titanium and 0.1
titanium coatings were on average about 20~ and 45~
thicker, respectively. The coating microstructures show
clearly that the thicker coatings obtained with the
titanium alloys were due to the Zn-6%Fe-3~Ti intermetallic
particles that were present in the eta layer.
The titanium addition to the bath modified the Fe-Zn
intermetallic layers on the reactive steels to produce the
more compact and even delta and zeta layers on the non-
reactive steels. This suggests that, much like the
vanadium, titanium is a strong silicide former that ties up
the silicon released from the steel during galvanizing,
allowing the zeta layer to stabilize. However, unlike
vanadium, titanium forms a ternary Zn-Fe-Ti intermetallic
in the bath at the coating/melt interface where there is
26
2161393
iron-enrichment. The intermetallic particles are trapped
in the eta layer and hinder zinc drainage. A beneficial
side effect of the thicker eta layers in the titanium
samples was that they ensured that the low-silicon steels
did not have coatings that were thinner than specified by
ASTM standards.
The larger intermetallic particles in the 0.1~
titanium coatings were very similar to those found in
coatings galvanized in the nickel-zinc alloy baths
containing about 0.1% nickel (Figure 2). These large
intermetallics produce higher-than-normal amounts of dross
and, hence, increase zinc consumption and operating costs.
The effectiveness of the 0.08~ vanadium plus 0.05%
titanium alloy in controlling steel reactivity as shown in
Figures 8 and 9 was not totally unexpected, since the 0.08~
vanadium alloy, on its own (Figures 4 and 5), was
effective. However, the 0.04~ vanadium (Figures 4 and 5)
or 0.05~ titanium (Figure 2) alloys alone could not fully
control reactivity over the full range (0-l~Si) of steels
tested. But together, they successfully controlled
reactivity of all steels tested, indicating a synergism in
the performance of the two elements together.
The microstructures of both the 0.08~ vanadium plus
0.05~ titanium and 0.04~ vanadium plus 0.05% titanium
appeared very similar to those of the 0.05~ titanium alloy.
The delta and zeta layers were compact, even and very thin.
The eta layer had the fine intermetallic compound seen in
the 0.05~ titanium coatings. Coating thicknesses from both
2161393
alloys were on average about 10% thicker than those
obtained from the 0.08% vanadium alloy.
The 0.04% vanadium plus 0.05% titanium alloy (Figure
9) has an advantage over an 0.08% vanadium or 0.12%
vanadium alloys (Figure 5) since it does not produce
coatings that are below specification with the low-silicon
steels. It also has an economic advantage since
substitution of some of the expensive vanadium with the
less expensive titanium reduces the cost of the alloy.
Referring now to Figures 1-3 representing later trials, it
was found that as little as 0.02% vanadium and 0.05%
titanium together were sufficient to effectively control
reactivity in up to the 0.05% silicon levels in steel.
Alloy containing 0.04% vanadium and 0.02% titanium was also
found to perform similarly to the 0.02% vanadium, 0.05%
titanium alloy. It was determined that vanadium with
titanium in the amount of at least 0.02% of each vanadium
and titanium and at least 0.04%, preferably at least about
0.06%, of a total of vanadium and titanium, will control
ractivity to at least 0.5% silicon in steel.
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 to at least 0.5%. The
coatings produced also have a bright metallic lustre. The
process can be easily adapted to conventional galvanizing
2~61393
production equipment using normal galvanizing temperatures
and immersion times.
It will be understood, of course, that modifications
can be made in the embodiment of the invention illustrated
and described herein without departing from the scope and
purview of the invention as defined by the appended claims.
