Note: Descriptions are shown in the official language in which they were submitted.
The invention relates to a finishing method and
apparatus for conventional continuous hot-dip coating of
a ferrous base metal strip with a molten coating metal,
and more particularly to a method and apparatus whereby
the coated ferrous base metal strip, upon exiting the
coating bath, is maintained in an essentially oxygen-free
atmosphere until jet finished with a non-oxidizing or
inert gas.
The method and the apparatus of the present in-
vention are applicable to the hot-dip coating of a ferrous
base metal strip with zinc, zinc alloys, aluminum, aluminum
alloys, terne, lead and those coating metals or coating
metal alloys which have oxide forming characteristics such
that acceptable finishing cannot be accomplished by con-
ventional jet practice or by conventional exit rolls.While not intended to be so limite~, for purposes of an
exemplary showing the method of the present invention will
be described as applied to galvanizing. The method can be
practiced on various types of galvanizing lines. For
example, the method of the present invention is applicable
to fluxless, hot-dip metallic coa~ing of ferrous base metal
strip wherein it is necessary to subject the strip surfaces
to a preliminary treatment which provides the strip with
oxide free surfaces and preferably brings the strip to a
temperature approximating that of the molten zinc or zinc
alloy coating bath at the time the strip is caused to pass
beneath the surface thereof. One of the principal types of
anneal-in-line~ fluxless, preliminary treatments is the
p~
so-called Sendzimir process or oxidation-reduction prac-
tice disclosed in ~.S. Patents 2,110~893 and 2,197,622.
Another anneal-in-line, fluxless, preliminary treatment
in common use is the so-called Selas process or hiyh
intensity direct fired furnace practice disclosed in
.S. Patent 3 r 320,085.
In the Sendzimir process a ferrous base metal strip
is heated in an oxidizing furnace (which may be a direct
fired furnace) to a temperature of about 370C to about
~85C without atmosphere control, withdrawn into air to
form a controlled surface oxide layer varying in
appearance from light yellow to purple or even blue,
introduced into a reduction furnace containing a hydrogen
and nitrogen atmosphere wherein the stock is heated to
about 735C to about 925C and the controlled oxide
layer is completely reduced. The stock is then passed
into a cooling section containing a protective reducing
atmosphere, such as a hydrogen nitrogen mixture, brought
approximately to a temperature of the molten coating
metal bath, and then is led beneath the bath surface
while still surrounded by the protective atmosphere.
In the Selas process the ferrous base metal strip is
passed through a direct fired preheat furnace section.
The strip is heated by direct combustion of fuel and air
producing gaseous products of combustion containing at
least about 3% combustibles in the form of carbon monoxide
and hydrogen. The strip reaches a temperature of about
535C to about 760C while maintaining bright surfaces
completely free of oxidation. The strip is then passed
into a reducing section which is in sealed relation to
the preheat section and which contains a hydrogen and
nitrogen atmosphere, wherein it may be further heated
by radiant tubes to about 650C to about 925~C and
tl~ereafter cooled approximately to the ~olten cGatinc3
~etal bath temperature. The strlp is then led beneath
ti-e batll surface while surrounded by the protective
atmosphere.
Other related pretreatment techniques are taught
in U.S. Patents Re 29,726 - 3,837,790 - 4,123,291 -
4,123,~92 and 4,140,552. The above mentioned prior art
patents constitute non-limiting examples of fluxless,
continuous galvanizing processes to which the method
of the present invention is applicable. When such
conventional strip preparation techniques as are taught
in the above mentioned prior art are used, it is necessary
that the base metal strip be maintained in a protective
atmosphere at least until it passes beneath the surface
of the bath of molten zinc or zinc alloy.
Such a protective atmosphere is not a requirement
when flux or chemical strip preparation techniques
of the type taught in United States Patents 2,824,020
and 2,824,021 are employed. B:riefly, when such chemical
strip preparation techniques axe used, the ferrous base
metal strip is caused to pass through a flux bath and
through means to assure the proper thickness of the flux
coating on the strip. The ferrous base metal strip is
then conducted through a heating chamber wherein
the strip is heated to evaporate the water in the flux
solution. Thereafter, the fer~ous base metal stxip is
; further heated to raise it t~ that temperature approaching
tne maximum temperature of stability of flux coating
on the strip. The strip is then caused to pass
beneath the surface of the bath of molten zinc or zinc
alloy so as to be coated. The method of the present
invention is equally applicable to galvanizing lines
utilizing such flux or chemical pretreatment systems.
From the above it will be evident that the method
of thè present invention is not limited to the use of
an~ particular pretreatment of ti-le ferrous hase ~etal
strip in the galvanizing line and the terms "pretreatment"
or "pretreated" (as used herein and in the claims with
reference to tlle ferrous base metal strip) are to be
interpreted broadly to include any of the conventional
pretreatment systems exemplified by the above noted
prior art. In general, these terms refer to ar.y
appropriate pretreatment technique, the result of which
is such that, during the actual coating step wherein the
ferrous ~ase metal strip passes through the molten bath
of zinc or zinc alloy, it will be at or will achieve
the ?roper coating temperature and its surfaces will be
oxide-free.
In conventional continuous hot dip salvanizing, it
is usual to cause the two-side coated strip LO exit the
molten coating metal bath into the ambient atmosphere.
The most widely used finishing and coating weight control
technique is to direct the coated strip between jet
};nives or nozzles which cause a blast of air or steam to
impinge upon both sides of the coated strip, xeturning
excess coating metal `LO the bath. This finishing
technique however, has a number of definite drawbaGks.
One drawbac~ is the formation of dross at the surface of
tne molten coating metal bath. ~he formation of such dross
represents considerable loss of zinc values and some
of tne surface dross i5 often dra~;n up by the coated strip
passing through the jet blast, and forming visible
surface dross lump defects in the coated product.
~ nother common coating defect or non-uniformity
related to conventional jet finishing is frequently
referred to as "coating ripples" or "ocean waves". Coating
r~ .f-~
ripples can essential1y be described as wave-like non-
~miformities in coating thickness in the longitudinal
(rolling) dlrection of the coated strip. Coating
ripples can vary in severity from essentially none to
very heavy ripples which are often called "coating sags".
It is very difficult to completely eliminate co~ting
ripples in con~entional jet finishing practice ~nd nearly
impossible at speeds below about 45.4 meters per minute~
High speed, close positionirlg of the ~et nozzles to the
strip, high aluminum content in the zinc bath and
minimum coating weight are means employed to reduce
coating ripple severity in conventional practice
Yet another hot-dip zinc coating non-uniformity is
commonly referred to as "spangle rellef". Spangle relief
has two aspects. One is the variation of sur~ace profile
(zinc thickness) across the zinc crystal from one boundary
to the opposite boundary. The other is a depressed
spangle boundary which surrounds each spangle or crystal.
Both aspects are related to the dendritic solidifcation
characteristics of zinc coa~ings. Spangle relief can
be reduced by such methods as purposely causing part of
the zinc coating to alloy with the ferrous base metal a
by decreasing the lead content of the zinc bath, or by
antimony additions to the zinc bath. However, none of
these methods is entirely satisfactory. As a result,
many methods have been developed to suppress spangle
formation. That is, to minimize final spangle size to
such an extent that the spangles are hardly visible to
the naked eye. For example, United States Patents
3,379,557 and 3,756,844 teach spangle minimizing methods.
Most methods involve spraying water or water solutions
against the molten coating to quench the coating and
create many nucleation sites. While spangle minimizing
7~
s ~
methods are effective in minimi~ir.g spanyle relief, uch
methods suffer operating and maintenance shor-tco~in~s
when considered on a routine basis and the results
achieved by ther,l are not alway consistent. S~angle
minim1zing methods do notlling to overcor,e jet finishing
ripples and coatiny dross.
lhe various hot dip coating non-uniformities present
in conventionally jet finished ~inc coatinas can be
~asked by temper (skin-pass) rolling. Iiowever, temper
rolling causes the non-uniformities to ~ecome imprinted
in the base me~al. As a result of this non-uniform
cold working of the base metal, the defects may
reap?ear when critical surface items sùch as automotive
body parts are stamped or formed.
~.nother major proble~ area encountered with con-
ventional jet finishing is that of coating control at
the edges of the strip. One edge proble~ is that of zinc
coating thickness over a narrow band immediately adjacent
each edge of the coated strip. l~he coating thickness
of these bands is greater than the coating thickness over
the rest of the strip width. If this coating thickness
differential is great enough, edge ~uild-up or spooling
will occur when the continuous strip is coiled under
tension.
O~her troublesome problems include edge berries
(small balls of oxide) which are attached to the strip
edge and are pulled though the jet blast. Furthermore,
an edge defect co~only ~nown as "feathered oxide" occurs
~uring low speed jet finishing. Feathered oxide is char-
acterized ~y discontinuous patches of heavy coating
metal oxide which pull through the jet blast. They
appear much like feathers which extend inwardly fro~ the strip
edges with the tips thereof pointing toward the center
of the strip.
~ any methods have been used by prior art workers
to reduce build-up and oxide control problems at the
strip edges. Tapered jet nozzle slot openings are
commonly used where the slot opening of the jet finishing
nozzle continuously increases in width from the center of
the jet nozzles to its ends. Such a contoured jet
finishing no7zle is taught in United States Patent
4,1~7,347~
Other methods to control edge coating include curving
the jet nozzles so that the nozzle is closer to the strip
at the strip edges than at the strip center. Also, vanes
or nozzle extensions have been used at the strip edges
to bring the nozzle closer to the edges than to the center
of the strip. Still other methods include the use of
shutters and auxiliary jets, both internal and external
to the main jet nozzles, to alter the jet wiping force
at the strip edges as compared to the jet wiping force
at the strip center.
All prior art methods fall short of producing optimum
edge control with a minimum of operator attention, maximum
coating metal economy, sufficient edge control for low
speed operation and proper edge control over a wide range
of strip widths.
Yet another problém area encountered with convention-
al jet finishing invol~es coating weights and line speeds.
The viscous interaction between the coating metal and the
strip is proportional to strip speed. At slow speeds,
the prior art was faced with the problem of ripple form-
ation. To combat this, it was found that reducing the jet
finishing flow rate will break up the oxide and more
evenly distribute it. However, low jet finishing pressure
and close positioning of the jet finishing nozzles at the
time creates an edge build-up problem. Prior art workers
therefore have had to adjust the parameters to control
edge build-up and ripples and this has necessitated higher
line speeds. As an examp]e, it has been common practice
to use conventional jet finishing only with strip speeds
above 30 meters per minute to produce commercial class
coating weight (ASTM A525, G-90). Edge build-up problems
on G-90 coating (coating weight of 275 g/m2) commonly occur
at speeds below about 45.5 meters per minute. Minimum
operating speed for heavier coatings, such as 564 g/m2
(G-185), are even more restrictive and edge-to-edge coating
uniformity deteriorates with increasing coating weight.
Another essential practice in most prior art
jet finishing operations is to position the jet nozzles
virtually directly opposed such that the jet streams are
in direct interference beyond the edges of the strip. This
interference results in extremely high and objectionable
noise levels. If the jet nozzles are operated while
verticall~ offset with respect to each other, a wrap-
around effect can result whereby the last jet to operateon the strip causes a bead of heavy coating metal to form
along the edge on the opposite side of the strip. In
addition to the noise problem and the need for precise
adjustment of the jet nozzles by the operator, opposed
operation can result in coating metal splatter being blown
off of the strip edge by one nozzle and into the noæzle
opening of the opposed nozzle.
Prior art workers have hitherto jet finished
hot-dipped, two-side coated ga~vanized and aluminized strip
30 wi~h ni~rogen. Such jet finishing, however, has been per-
formed in an ambient atmosphere. In jet finishing, less
nitrogen is required than air. However, the results
achieved b~ such finishing are more nearly like those
achieved in jet finishing with air in an ambient at~osphere
than like the results achieved by the present methcd.
United States Patents 4,107,357 and 4,114,563 and German Patent
2,656,535 are exemplary of patents teaching methods for coating one side only
of a ferrous base metal strip. In the practice of these processes, the ccated
strip after contact with the coatin~ k~ath is ~intained in a protective, non-
oxidiziny atmosphere and is jet finished with nitrogen or a non-oxidizing gas.
However, the primary purpose of these steps is to prevent oxidation of that
side of the ferrous base metal strip not coated or, if the uncoated side has
an oxide film thereon, to prevent adherence of the coating metal to the oxide
film.
I'he present invention is kased upon the discovery that if, in a con-
ventional, continuous, hot-dip, two-side galvanizing process, the coated metal
as it exits the coating bath is surrounded by an enclosure in which a sub-
stantially oxygen-free atmosphere is maintained and if within the enclosure
the coated strip is jet finished with a non-oxidizing or inert gas, the finish-
;ng prablems encountered with conventional finishing methods are markedly re-
duced or eliminated. m e finishing method of the present invention does not
eliminate the need for minimizing spangle, but for the first time makes
spangle m~nimizing techniques effective and consistent~ ~ross is greatly
reduced together with dross-related problems and jet finishing ripples are
eliminated, even at slcw operating speeds. With the marked reduction in
dross fonmation, loss of zinc values to top skimmings is greatly reduced.
One of the most significant aspects of the present invention is the
discovery that all coating control prablems at the strip edges are campletely
eliminated with the exclusion of oxygen fram the finishing process. Minimum
operating speeds are no longer limited ~y edge build-up problems, but rather
only by the desired coating weight relative to the amount of coating metal
naturally pulled
A~ X 3
up by the strip to the finishing jet nozzles. It has
been found, for example, that excellent quality coatings
of 275 g/m2 can be produced without difficulty at speeds
as low as 9.1 meters per minute. Edge wrap around does
not occur, so that the jet nozzles can be vertically
offset, eliminating the need for precise positioning,
greatly reducing noise, and eliminating the zinc
splattering hazard. Jet nozzle design may be simplified
to utilize a nozzle having a ~lot-like nozzle opening
of uniform width throughout its length, doing away with
the multitude of special jet nozzle designs, methods
and accessories which have been used ~or controlling
edge build-up. Superior uniform coating results edge-
to-edge for all coating weights because the center
profile need no longer be distorted to compensate for
heavy edges.
Heretofore, in the practice of conventional
two side jet finishing, neither the mechanism of ripple
formation nor that causing edge build-up problems was
completely understood. In the process of conventional
jet finishing, a pneumatic dam effect is created whereby
the desired amount of coating metal is metered through
the jet barrier to form the fin.;shed coating. At this
metering point the excess coating metal pulled up with
the strip, beyond that required forthe finished coating,
is returned to the coating bath. This process is des-
cribed in detail in United States Patent 4,078,103.
While applicants do not wish to be bound by
theory, it appears as a result of the present invention
that the coating ripples and heavy edge coating in con-
ventional jetfinishing are caused entirely by coating metaloxide. At some point in the jet interaction region,probably
just above the point of zero surface velocity, fresh
(unoxidized) coating metal is being exposed and, as it is
~,.J. ~ f~
exposed~ it immediately forms a very light oxide skin.
The continuity of flow or distribution of this very
light oxide skin onto the finished coating determines
the occurrence of coating ripples. In conventional
S practice, the jet periodlcally restrains the oxide film.
The film builds Up until the jet no longer can restrain
it. At this time a segment of relatively heavy oxide
break~ off and passes with the finished coating. The
segment as it passes carries with it coating beneath
which is heavier than that which is metered on when
the oxide film is restrained. This process is repeated
many times each second as ripples are formed.
A similar mechanism is believed to be operable in
- creating heavy coating metal along the strip edges. How-
ever, at the edges, geometry becomes an additional
important factor in that there is no wiping force directed
against the edge surfaces. Relatively heavy oxide is
permitted to pass through the je~ interaction area more
or less continuously carrying with it heavy coating
beneath. This oxide envelope around each strip edge
surface is the "container" which permits æinc wrap-
around to occur when the jet nozzles are vertically
offset.
These coating nonuniformities are caused by oxide
on the molten coating metal and they are eliminated in
the present invention by avoiding oxidation~
The zinc coated product produced by the method of
the present invention has such excellent surface qualities
- after temper rolling that it is suitable for use in
exposed automotive body panels, appliance applications
and the like. The practice of the present invention lends
itself well to short immersion, shallow coating pot
practice utili~ing a partially submerged pot roll.
,.~
f~
; A ~
Accordina to the invention there is provided a
finishing ~rocess for continuous hot-d:ip, two-side coatin~
of a ferrous base metal strip with a molten coating metal
of the ty?e wherein said ferrous base metal strip is caused
to enter a bath of said molten coatin~ metal contained
in a coating ~ot, said ferrous base metal stri~ having
been treated to bring it to a coating temperature suf-
ficiently high to prevent casting of said coating metal
thereon and low enough to ~revent excess coating metal-
base metal alloying and to render the surfaces of said
strip clean and free of oxide as it passes through said
molten coating metal bath, characterized by the steps of
providing an enclosure in sealed relationship with said
bath for said two-side coated ferrous base metal strip
as it exits said bath and an exit in said enclosure for
said coated strip, maintaining a non-oxidizing atmosphere
within said enclosure, locating a jet finishing nozzle to
either si-le of said coated strip within said enclosure,
jet finishing said coated strip with a non-oxidizing gas
and maintaining said jet finishing gas and said atmos-
phere within said enclosure at an oxygen level of less than
about 200 ppm, wherebv to render said two-side coated strip
free of oxide-caused coating nonuniformities.
Finishing apparatus in accordance with the
present invention adapted to ~ractice of the above ~rocess
is characterized by an enclbsure for ferrous base metal
strip as it exits a molten coating metal bath, the en-
closure having an open bottom and èxtending into the
molten coatina metal bath, the enclosure having an exit
slot formed therein for said strip, a pair of jet finishing
nozzles located within the enclosure, above the molten
coating metal bath and to each side of the strip, means
to maintain a non-oxidizing gas within the enclosure having
an oxygen level of less than about 200 ppm, and means to
supply the jet finishing nozzles ~lith a non-oxidizing gas
having an oxygen level of less than about 200 ppm.
f~ L~ ;~J
BRI~F D~SCRIPTION OF Tll~ Dr~wI~lGs
~ igure 1 is a fragmentary, semi-diagra~matic, cross
sectional elevational view of an exemplary continuous
hot-dip galvanizing line equipped to practice the method
of the present invention.
Figure 2 is an enlarged, fragmentary, semi-diagra;,~
matic, cross section view of the coating end of the
galvanizing line of Fiyure 1.
Figures 3, 4 and 5 are enlarged, fragmentary, semi-
1 diagrammatic cross sectional views similar to Figure 2,
but illustrating various pot roll arrangements.
Figure 6 is a fragmentary, semi-diagrammatic cross
sectional view illustrating the enclosure of the present
invention as constituting an integral part of the snout
through which the strip enters the bath of molten coating
metal.
~ igure 7 is a fragmentary, semi-diagrammatic cross
sectional view, similar to Figure G, but showing a
partially submerged pot roll and the use of a pump for
the molten coatlng metal.
DETAILED DESCRIPTION OF THE I~VENTION
While not intended to be so limited, for purposes of
an exemplary showing the method of the present invention
will be described as applied to a Selas-type galvanizing
line. Turning to Figure 1, the coating line is generally
indicated at 1. The strip preparation furnace of the
coating line comprises a direct fired furnace 2, a
controlled atmosphere heating furnace 3, a first cooling
section 4, a second cooling section 5 and a snout 6. It
will be noted that the snout 6 is configured to extend
below the upper surface of a bath 7 of molten coating
: zinc or zinc alloy, located in a coating pot 8.
14
'I~he ferrous base metal strip g to be prepared en~crs
the direct fired furnace 2 over rolls 10 and 11 and
throuyh sealing rolls 12 and 13, so located as to minimi~e
the escape of products of combustion through the entrance
opening 14 of preheat furnace 2. The direct fired furnace
2 operates at a temperature on the order of 1260C. The
function of the direct fired furnace is to quickly
burn away oil and the like from the surfaces of the
ferrous base metal strip 9, while providing partial heating
for annealing the strip. The direct fired furnace, at the
temperature indicated, will be sufficient to heat the
entering strip to a temperature of from about 535C to
about 760C by the time it passes from the direct fired
furnace to the controlled atmosphere heating furnace 3.
The ferrous base metal strip 9 passes about turn-
around rolls 15 and 16 and be~ins an upward travel through
contolled atmosphere heating furnace 3. Thereafter, the
strip passes about turn-arouncl roll 17 and continues
downwardly again through furnace 3. The controlled
atmosphere heating furnace may be of the radiant tube
type and will further raise the temperature of the ferrous
base metal strip 9 to from abc)ut 650C to about 925C,
depending upon the nature of the ferrous base metal strip
and the desired final characteristics of the base metal
strip~
The strip preparation furnace of the coating line 1
may have one or more cooling chambers. For purposes of
this exemplary showing, the strip preparation furnace is
illustrated as having the two cooling chambers ~ and 5.
From the controlled atmosphere heating furnace 3 the strip
9 passes about turn-around rolls 18 and 19 and enters
cooling chamber 4. Chamber 4 may be of the tube cooling
type well known in the art. In the exemplary illustration,
the ferrous strip 9 makes th~ee vertical flights through
cooling chamber 9, passing about turn-around rolls 20
and 21. Thereafter, the ferrous base metal strip 9
passes about turn-around rolls 22 and 23 to enter the
second cooling chamber 5 which may be of the jet cooling
type, again well known in the art.
The temperature to which the ferrous base metal
strip 9 is cooled will depend upon a number of factors.
Sln^e the molten coating metal 7 in coating pot 8 is
zinc or zinc alloy, the ferrous base metal strip will
preferably be cooled to approximately 450C. In some
instances, however, the strip itself may be used as an
additional means to introduce heat into the molten coating
metal bath 7. ~nder these circumstances the ferrous base
metal strip 9 may be introduced into the bath 7 at a
temperature somewhat higher than the melting point of the
zinc or zinc alloy therein. Where the strip is not relied
upon as one of the heat sources for the bath 7, the strip
may he introduced into the bath at a temperature slightly
below that of the bath. In any event, the strip
temperature should be sufficiently high as to prevent
casting of the molten coating metal thereon. By the
same token, the strip temperature must not be so high
as to bring about excess coating metal-base metal alloying.
From the cooling chamber 5 the ferrous base metal
strip 9 passes about turn-around roll 24 and enters the
snout 6. It will be noted that the free end of snout
6 extends downwardly below the surface of the zinc or
zinc alloy bath 7. The ferrous base metal strip passes
about turn-down roll 25 and is directed downwardly into
; bath 7. Within the bath, the strip is guided by one or
more coating pot rolls so as to exit in a substantially
vertical flight. In the embodiment shown, a single coating
pot roll 26 is illustrated. The two-side coated ferrous
base metal stxip 9a exits the molten coating metal bath 7
f~
1~,
and enters an enclosure 27, the lower end of which e~tends
into the molten coating metal bath 7 to form a seal
therewith. Within enclosure 27, the two-sided coated
ferrous base metal strip 9a is caused to pass between a
pair of jet finishing nozzles 28 and ~9.
It will be noted from Figure 1 that the upper end
of the direct fired furnace 2 is connected by a conduit
30 to an exhaust fan 31. The outlet 32 of exhaust fan
31 may be connected directly to a stack or to waste gas
heat reclamation means (not shown). The strip preparation
furnace of coating line 1 can be operated above
atmospheric pressure ~to prevent the introduction therein
of oxygen from the ambient atmosphere) by controlling the
discharge rate of the products of combustion from the
directed fired furnace 2. ~o this end, a damper 33 may
be located in conduit 30. The parameters under which the
~ strip preparation furnace of coating line 1 is run
; do not constltute a limitation OII the present invention.
Reference is now made to Figure 2 wherein the snout 6,
coating pot 8 and enclosure 27 of Figure 1 are shown
enlarged. Like parts have been given like index numerals.
In the embodiment of Figures 1 and 2, snout 6 and the
enclosure 27 are illustrated as constituting wholly
separate structures. It will be understood by one skilled
in the art that the enclosure 27 could constitute an
integral part of snout 6. When a chemical and flux
pretreatment system is used, the snout 6 can be eliminated~
; In accordance with the method of the present inven-
tion, a non-oxidizing atmosphere is maintained within
enclosure ~7 having an oxygen content of less than
about ~00 ppm, and preferably less than about 100 ppm.
Any appropriate non-oxidizing or inert atmosphere may be
used. A nitrogen atmosphere is preferred as being the
'~. " i.6f fi ~ f~
most economical. The je~ nozzles 28 and 29 can serve as
the source of the atmosphere within enclosure 27, although
additional atmospllere inlets such as the inlet 34 may ~e
provided, if required.
A portion of the nitrogen atmosphere within enclosure
27 may ~e withdrawn and recirculated through the jet
finishing nozzles 28 and 29. This is diagrammatically
illustrated in Figure 2. The enclosure 27 is provided
with an outlet 35. The outlet 35 is preferably connected
to a hi.gh temperature baghouse 35a for collecting zinc
oxide particles. From the baghouse 35a, the atmosphere
withdrawn from enclosure 27 passes to a heat exchanger 36.
The heat exchanger 3G is connected as at 37 to the input
38 of a blower 39. The purpose of the heat exchanser is
to cool the nitrogen from enclosure 27 ahead of blower
39 to prevent overheating of bearings and seals in the
blower. The baghouse 35a cou].d be located between heat
exchanger 36 and blower 39, although it is preferred that
it be ahead of excilallger 36 to prevent clogging of the
heat exchanger fins with zinc dust. tlle output 40 of
blower 39 is connected by conduits 41 and q2 to jet
finishing nozzles 28 and 29. The conduits or lines 41 and
: 42 may contain valves 43 and 44, respectively, so that
the plenum pressure of the jet finishing nozzles 28 and
29 can be adjusted. It has been found that through the
use of such a baghouse-heat exchanger-blower-sealed
conduit system, more than 50% of the high purity nitrogen
requirement can be recirculated ~rom enclosure 27 through
jet finishing nozzles 28 and 29, thus reducing the nitrogen
consumption. Make-up nitrogen may be introduced into the
sys~em via conduit 45 connected to the line 37 between
heat exchanger 36 and the intake 38 of blower 39. The
atmosphere recirculation rate is so adjusted as to avoid
infiltration of air through the slot 46 through which the
coated strip 9a exits enclosure 27.
~h~,
1~
The ~e, no7~1es 23 an~i 29 are located to either
side of the two side coated base ~etal strip 9a and
directly opposite each other, as sho~n in Fiqure 1.
However, since the above noted edqe ~roble~s includinc
~ra~-around have been eliminated ~y the ~resent ~et~od,
it is preferre~ that the jet no?.zles 28 a~d 29 he
staggered vertically ~ith respect to each other as sho-~n
in Figure 2. This ~revents clogaing of the no~zles d~e
to zinc s~latter and blow-off from one nozzle to the
lQ other, as exolained above. ~ither jet knife can be
located above the other. The higher of the two jet
finishing no~zles (in this instance jet finishing nozzle
23) can be located up to about 0.~ m or more above the
bath. The ~et finishing no~zles 28 and 29 may be ver i-
cally offset with respect to each other by any amountdesired. Generally, they are of~set from 5 to 15.25
centimetexs. Usually the noz-les will be within ahout
3.8 centimeters of the stri~ 1hen the jet finishing
noz-les are offset, the noise level of the finishing ste~
caused hy the no7zles is greatly reduced. The jet noz~.les
2~ and 29 can be of si~le construction, having a si~,Ple
rectangular jet o~ening and heing free of cu~ved li~s,
shutters, vanes, or other devices. Excellent results
have been achieved using jet finishing noæ~les having a
simple rectan~ular o~ening with a uniform width of from
; 1.25 to 2.05 ~m throuahout its length.
The enclosure 27 is ~rovided ~ith an exit oPening
or slot 46 for the ~wo-side coated ferrous base ~etal
stri~ 9a. Care ~ust be ta]:en to assure that ambient air
is not as~irated through the slot 46 due to high ~as
~elocities and turhulent effects o~erating within the
enclosure near the slot 46. A~hient air as~irated through
the slot 46 would cause excessive oxyaen to be present in
enclosure 270 The use of baffles or additional nitroqen
19
purging around the strip exit 46 may assist in pre~entinc~
such air aspiration. Iiowever, excellent results have
been achieved hy sim~ly providiny a short chimney 47
and locating exit slot 46 atop chimney 47.
The enclosed finishing method of the present invention
permits a short immersion, shallow coating pot practice
utili~ing a partially submerged coating pot roll. ~his
is true because the method of the present invention
minimizes the formation of oxide on the surface of the
bath and on the partially submerged pot roll. Such a
practice has a number of advantages. First of all, it
utilizes a smaller coating bath. Furthermore, the
amount of the ferrous base metal strip immersed and the
duration of immersion are ~reatly reduced, thereby
reducing the arnount of iron values going into solution
in the coating metal from the ferrous base metal strip.
Figure 3 illustrates such a short immersion,
shallow coating pot practice. In Figure 3 a coating
pot is shown at 48. The coating pot 48 is similar to
coating pot 8 of Figure 2 with the exception that it is
shallower. The coating pot 48 contains a bath of
molten coating metal 49 which is of considerably less
volume than the bath 7 of Figure 2.
A snout 50, equivalent to snout 6 of Figure 2,
is shown with its lowermost end located beneath the
surface of bath 49 so as to be sealed thereby. The
snout 50 contains a turn-down roll 51 equivalent to
turn-down roll 25 of Figure 2. A pot roll is illustrated
at 52. The pot roll 52 differs from pot roll 26 of
Figure 2 in that it is only partially submerged in the
molten coating metal bath 49. The apparatus of Figure
3 includes an enclosure 53 which is equivalent in every
way to enclosure 27 of Figure 2 with the exception that
2()
its lower rear edge 53a is bent sli~htly downwardly ar~d
inwardly so as to make a seal with the molten coatiny
bath 49 while at the same time providing clearance fGr
the flight of the uncoated ferrous base metal strip 5
between turn-down roll 51 and the pot roll 52. The
coated ferrous base metal strip 54a is shown passing
between a pair of jet finishing nozzles 55 and 5~ and
upwardly through a chimney 57 and an exit slot 58
equivalent to chimney 47 and exit slot 46 of Figure 2.
The operation of the coating and finishing apparatus
shown in F~gure 3 is substantially identical to that
described with respect to Figure 2. ~gain, jet finishing
nozzles 55 and 56 may be connected to a recirculating
system (not shown) of the type illustrated in Figure 2.
The primary difference between the operation illustrated
in Figure 3 and that illustrated in Figure 2 lies in the
fact that pot roll 52 is only partially submerged which
provides the above noted advantages.
The amount by which pot roll 52 is submerged in
bath ~9 can be varied. In Figure 3 pot roll 52 is
shown more than half submerged. With appropriate
configuration of snout 50 and the portion 53a of
enclosure 53, the pot roll 52 could be less than half
submerged, particularly in those instances where it is
desirable to maintain the roll bearings ~not shown)
above the~bath surface.
The pool of molten metal S9 between pot roll 52
and the ferrous base metal strip 54 engaging the pot
roll must be of sufficient size to assure adequate
coating of the back side or roll side of the ferrous
base metal strip. It will be understood that the size
of the pool 59 will decrease as the amount by which
pot roll 52 is submerged decreases. It is within
the scope o~ the present invention to augment this
situation through the use of a grooved r,~ot rGll 52 c~r
means to pump additional ~,olten coa~inc; ~etal into t~.e
pool 59 (as will be described hereinafter).
Fic~ure 4 illustrates anothcr arrangement to ass~re
adequate coating o~ the backside or roll side of the
ferrous base metal strip in shallow pot practice. In
Figure 4 an enclosure is illustrated at 60 which may
be identical to the enclosure 27 of Figure 2, having a
pair of jet finishing nozzles 61 and 62, an exit
chimney 63 and an inlet 64 for the inert or non-oxidizing
atmosphere. It will be understood that the enclosure
60 may be provided with the atmosphere recirculation
system described with respect to Figure 2. The lower
end of enclosure 60 is submerged in a bath 65 of molten
coating metal located in a shallow pot 66. Figure ~ also
illustrates a conventional snout 67, similar to snout 6
of Figure 2. Again it will be noted that the lowermost
end of snout 67 extends below the surface of the molten
coating metal bath 65.
The ferrous base metal strip to be coated is shown
at 68. The strip passes about turn-down roll 69 in
snout 67 and enters the bath as it pass about a ~irst
pot roll 70. From pot roll 70 it extends to a second
pot roll 71 which directs the coated strip 68a upwardly
through enclosure 60.
The amount by which pot rolls 70 and 71 extend into
tlle molten coating b?th 65 can be varied. For purposes
of this exemplary showin~, pot roll 70 and 71 are
illustrated as extending into the molten coating metal
22
bath 65 b~. all amount less than 1/2 their diameters. r~'ile
eY~istence of the submerged strip 68b between pot rolls
70 and 71 assuresadequate coating of the backside or
roll side of the ferrous base metal strip. It has bc-er,
determined that the shallo~! pot practice of the type
just described with respect to Figures 3 and 4 does
not significantly change the enclosed nitrogen finishir.g
characteristics or advantages described with respect to
Figures 1 and 2.
As has already been made evident, the apparatus of
the present invention may utilize various pot roll
arrangements. Another arrangement is illustrated in
Figure 5 in this Figure, a conventional coating pot is
shown at 72 containing a molten coating metal bath 73.
lS A snout 74, equivalent to snout 6 of Figure 2 has its
lower end submerged in the molten coating metal bath
73 and is provided with a turn-down roll 75, equivalent
to turn down roll 25 of Figure 2. An enclosure 76 has
its lower end submerged in the molten coating metal bath
73. The enclosure 76 may be identical to enclosure
27 of Figure 2, having an exit chimney 77 and an
atmosphere inlet 78, if required. The enclosure contains
a pair of jet finishing nozzles 79 and 80 equivalent to
jet finishing nozzles 28 and 29 of Figure 2. Again, the
enclosure 76 may be provided with the atmosphere
recirculating system (not shown) of Figure 2.
In this embodiment, the ferrous base metal strip
81 to be coated enters the molten coating metal bath 73
and passes about a series of three pot rolls 82, 83 and
84. Rolls 83 and 84 are stabilizer rolls and provide
strip shape control, assuring flatness of the coated
~ ~ ~r~
strip 81a as it passes betweerl jct finis~ing noz~lcs
7~ and 80.
In all of the embodiments thus far described, the
enclosure and the snout have been illustrated as
se~arate structures. It is also within the scope
of the present invention, however, to provide a snout
and an enclosure which constitute an inteyral, one-
piece structure. This is illustrated in Figure ~.
In Figure 6 a conventional coating pot 25 is show..
containing a bath 86 of molten coating metal. The snout-
enclosure structure is generally indicated at 87, havin~a snout ~ortion 87a and an enclosure portion 87b. The
snout portion 87a is similar to snout 6 of F'igure 2 and
has a turn-down roll 88 located therein. Turn-down roll
88 is equivalent to turn-down roll 25 of Figure 2. The
enclosure portion 87b is similar to enclosure 27 and has
an exit chimney 89. The enclosure portion 87b may be
provided with an atmosphere inlet 90 equivalent to inlet
34 of Figure 2. Jet knives 91 and 92 are located within
the enclosure portion 87b and are in every way equivalent
to jet knives 28 and 29 of Figure 2. It will further be
understood that the enclosure portion 87b may be provided
with an atmosphere recirculating system (not shown)
equivalent to that described with respect to Figure 2.
In the embodiment of Figure 6, a submerged pot roll is
shown at 93.
~ nder normal circumstances, the snout portion 87a
and the enclosure portion 87b will contain different
atmospheres and therefore some sort of seal means should
be provided therebetween. The seal means may take any
appropriate form. For purposes of an exemplary 5howing,
the seal means i5 illustrated as being made up of two
,L1:~L,,~
24
pairs of sealing rolls 94-95 and 96-97.
It is within the scope of the invention to provide
an inlet 98 for an appropriate non-oxidizing gas bet~een
sealing rolls 94-95 and sealing rolls 96-97. It is
preferable that the non-oxidizing atmosphere between
sealing rolls 94-95 and sealing rolls 96-97 be at a
pressure slightly higher than the pressure of the
atmosphere in snout portion 87a and enclosure portion
87b. This assures that either the enclosure portion 87b
or the strip preparation furnace associated with snout
87a can be shut down without contaminating the other. It
will also prevent contamination of the atmosphere within
hood portion 87b from sources at the entry end of the
conventional strip preparation apparatus.
The strip 99 to be coated passes about turn-down
roll 8~ and between sealing roll pairs 94-95 and 96~97.
The strip 99 enters the bath and passes about pot
roll 93. Thereafter, the coated strip 99a passes upwardly
between jet finishing nozzles 91 and 92, exiting through
exit chimney 89. Thus, the operation of the apparatus
and the advantages achieved thereby are essentially
the same as has been described with respect to Figures
1 and 2.
The unitary snout-enclosure of Figure 6 can also
be applied to shallow pot practice. This is illustrated
in Figure 7. In Figure 7, the snout-enclosure apparatus
is identical to that of Figure 6 and like parts have been
given like index numerals. In the embodiment of Figure 7,
a shallow pot is shown at 100, containing a shallow bath
102 of molten coating metal. In this instance, a pot
roll 103 is shown being partially submerged in the molten
~etal coating bath 102. For purposes of this exemplary
sho~ing, the pot roll 103 is illustrated as bcing subrnerged
by an amo~lnt less than half its diameter. The pot roll
103 could, of course, be submerged by an amount more than
half its diameter, as is shown with respect to pot roll
52 of Figure 3. It would even be possible to provide the
apparatus of Figure 7 with a pair of pot rolls of the type
described with respect to Figure 4.
In Figure 7, however, for purposes of an exemplary
showing the apparatus is illustrated as being provided
with a pump for the molten coating metal of bath 102,
the outlet of the pump being shown at 104. The pump
outlet 104 creates a pool 105 of molten coating metal
between the ferrous base metal strip 99 and pot roll 103,
which pool assures adequate coating of the back or roll
side of the ferrous base metal strip. Such a pump for
the molten coating metal could be provided for the
embodiment of Figure 3, if the pool 59 of Figure 3 were
inadequate. In all embodiments of the present invention,
where the pot roll is only partially submerged, it would
be within the scope of the invention to use a grooved
pot roll. The grooves carry molten coating metal to the
roll side of the ferrous base metal strip.
Since the method of the present invention eliminates
the oxide problems with respect to the strip and strip
edges, it has been found that relatively heavy coatings
can be achieved at lower line speeds, such coatings
having excellent surface characteristics. For example,
with minimum controlled wipe by the jet finishing nozzles,
coating weights of up to about 543g/m2 (about 1.78 ounces
per square foot) have been achieved at a line speed of 12
~eters per minute i~ the lab~ratory.
~6
A laboratory galvanizing line utiliziny a 10.16
cm strip was provided with an enclosure similar to en-
closure 27 of Figure 2. The chimney 47 was 15.25 cm high
and provided with an exit slot 46 having a width of 31.75
~m and a length of 12.7 cm. The enclosure was provided
with a pair of jet finishing nozzles (equivalent to jet
nozzles 28 and 29 of Figure 2), each having a slot-like
opening of 1.27 mm width throughout its length. The
lower of the two jet finishing nozzles was maintained at
a distance of four inches from the bath surface. The other
jet nozzle was offset vertically and upwardly therefrom
by 12.7 mm. The jet nozzles were maintained at a distance
from the strip of about 6.35 mm. The enclosure was pro-
vided with a recirculating system of the type shown in
Figure 2. Make up nitrogen was added at the rate of 85 m
per hour and the nitrogen atmosphere within the enclosure
was maintained at a pressure of 12.7 mm of water.
The ferrous base metal strip was 0.381 mm cold
rolled steel with relatively smooth surfaces at 1.27 ~ m
and 3.546 Kp/m. During this run, a coating of 183 g/m2
was being produced and a line speed of 21.4 m per minute
was used. The influence of oxygen contamination in the
enclosure containing high purity nitrogen was evaluated
by metering compressed air into the recirculating system
in increasing amounts until defects were observed in the
molten coating. With oxygen below tO ppm the molten
coating was ylossy~ smooth, free of visible oxide and
without sign of edge problems. The solidified coating
showed a dead flat spangle without spangle boundary
relief. As the oxygen was purposely increased, no ripples
occurred at an oxygen level of 140 ppm. Detrimental
finishing effects were first observed at an oxygen level
within the enclosure of about 20n ppm in the form of edge
~, . 2 L~ J
oxide berries, ripples, a ridge of heavy edge metal, and some
spangle relief. These conditions become steadily more pronounced
as the oxygen level was increased to 600 ppm. Surface oxide bands
developed when the oxygen level reached a~out 700 ppm. These
oxide bands extended inwardly from the strip edge and increased
to gross feathers of oxicle when the oxygen level reached 850 ppm.
This run showed that the enclosed nitrogen finishing method
of the present invention produces a smoo-th, uniform hok~dip zinc
coating finish without the common ripple, dross, oxide curtain
and edge build-up defects associated with conventional edge
finishing. Simplified jet finishing nozzles with uniform slot
openings can be used and can be vertically offset without zinc
splatter and without heavy edge coating or coating wrap-around.
The noise level of the finishing step was drastically reduced not
only by virtue of the fact that the nozzles can be offset with
respect to each other. The relationship between oxygen contamin-
ation of the finishing gas and the coating surface quality was
clearly demonstrated. The oxygen level within the enclosure
should be maintained at less than about 200 ppm and preferably
less than about 100 ppm. In other similar runs nitrogen at the
rate of 85 m per hour was circulated through the jet finishing
nozzles using abou~ 42.5 m3 per hour or less make-up nitrogen,
confirm;ng the ability to recirculate more than
~i~
2~
5G~ of the high purity fini.shing gas reouirement.
In ail of the embodiments illustratecl in the Figures,
the enclosure is shown in semi-diagrammatic fashion. I
wi.ll ke understood by one skilled in the art that the
enclosure will be provided with suitable support means
and the like. Furthermore, the enclosure may be remGvable
in whole or in part for maintenance or if reyular air
finishing is to be practiced.
; Modifications may be made in the invention without
departing from the spirit of it.
