U.S. patent number 3,808,033 [Application Number 05/235,766] was granted by the patent office on 1974-04-30 for continuous metallic strip hot-dip metal coating apparatus.
This patent grant is currently assigned to National Steel Corporation. Invention is credited to John T. Mayhew.
United States Patent |
3,808,033 |
Mayhew |
April 30, 1974 |
CONTINUOUS METALLIC STRIP HOT-DIP METAL COATING APPARATUS
Abstract
Apparatus and method in which streams of gas under pressure are
impinged against opposite faces of a continuous metallic strip
emerging from a molten coating metal bath to thereby control the
final coating weight on the strip, there being close control of the
temperature of the molten coating metal in the bath by means of
cooling fluid in heat exchange relation with a portion of the
molten coating metal in the bath.
Inventors: |
Mayhew; John T. (Toronto,
OH) |
Assignee: |
National Steel Corporation
(Pittsburgh, PA)
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Family
ID: |
26675231 |
Appl.
No.: |
05/235,766 |
Filed: |
March 17, 1972 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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6137 |
Jan 27, 1970 |
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46825 |
Jun 16, 1970 |
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704193 |
Nov 7, 1967 |
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6137 |
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598380 |
Dec 1, 1966 |
3499418 |
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409053 |
Nov 5, 1964 |
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46825 |
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757522 |
Aug 27, 1968 |
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375264 |
Jun 15, 1964 |
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704193 |
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374953 |
Jun 15, 1964 |
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Current U.S.
Class: |
427/349; 118/63;
118/429; 427/398.4; 427/431; 118/419; 427/10; 427/398.5;
427/433 |
Current CPC
Class: |
B05C
11/06 (20130101); C23C 2/20 (20130101) |
Current International
Class: |
C23C
2/14 (20060101); C23C 2/20 (20060101); B05C
11/06 (20060101); B05C 11/02 (20060101); B05c
011/10 (); B05c 003/02 () |
Field of
Search: |
;117/12M,114R,114A,114B,114C,64R
;118/4,5,7,8,9,63,419,423,424,429 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Kendall; Ralph S.
Attorney, Agent or Firm: Shanley and O'Neil
Parent Case Text
CROSS REFERENCES TO RELATED APPLICATIONS
This application is a division of copending application Ser. No.
6,137, filed Jan. 27, 1970, which in turn was a continuation of
application Ser. No. 598,380, filed Dec. 1, 1966, now U.S. Pat. No.
3,499,418, which in turn was a continuation-in-part of application
Ser. No. 409,053 filed Nov. 5, 1964, now abandoned. The present
application is also a continuation-in-part of copending application
Ser. No. 46,825 filed June 16, 1970 which is a continuation of Ser.
No. 757,522 filed Aug. 27, 1968, now abandoned, as a continuation
of patent application Ser. No. 375,264 filed June 15, 1964 and is
also a continuation-in-part of copending application Ser. No.
704,193 filed Nov. 7, 1967 as a continuation of patent application
Ser. No. 374,953 filed June 15, 1964, application Ser. No. 375,264
and application Ser. No. 374,953 being now abandoned.
Claims
1. Continuous metallic strip hot-dip metal coating apparatus
comprising
a. molten coating metal bath means having the upper surface of the
molten coating metal of the bath means exposed to a gaseous
atmosphere,
b. roll means defining a travel path for continuous metallic strip
through a portion of the molten coating metal of the bath means,
thence through a coating weight control zone and upwardly to a
point where the molten coating metal applied to the strip in the
bath has solidified, the coating weight control zone extending
upwardly from the exposed surface of the molten coating metal
through the portion of the travel path in which the weight of the
still molten coating metal applied to the strip in the bath can be
controlled, the roll means comprising a first roll submerged in the
molten coating metal and a second roll positioned above the bath
means at a point where the molten coating metal applied to the
strip in the bath means has solidified,
c. nozzle means having two linearly extended, narrow gas outlet
means, each gas outlet means having a length greater than about 23
inches and at least equal to the width of the strip, each gas
outlet means being shaped to deliver a concentrated stream of gas
under pressure of shape and mass along the length of the gas outlet
means to give desired coating weight across the width of the
continuous metallic strip,
d. means for supplying gas under pressure to the nozzle means,
e. support means for positioning the nozzle means in the coating
weight control zone with each linearly extended gas outlet means
facing and symmetrically disposed in spaced relation to an opposite
planar surface of the strip and spaced above the upper surface of
the bath, with each gas outlet means positioned to direct a stream
of gas under pressure against the strip and across the width
thereof with the major component of motion of the stream of gas
perpendicular to the opposed planar surface of the strip,
f. means for continuously controlling the mass of gas supplied to
the nozzle means,
g. heat exchange means having portions immersed in the bath for
control of the temperature of the molten coating metal carried out
of the bath on the strip,
h. means for introducing a stream of cooling fluid into the heat
exchange means, and
i. means associated with means (h) for controlling the rate of
introduction
2. The combination of claim 1 further comprising
j. means for measuring the temperature of the bath, and
k. means actuated by means (j) acting on means (i) to control the
rate of introduction of cooling fluid to maintain the temperature
of the bath at a
3. The combination of claim 1 in which
j. means (g) is disposed in the bath in the neighborhood of the
zone where
4. Continuous metallic strip hot-dip metal coating process
comprising the steps of
a. applying molten coating metal in excess of desired coating
weight to continuous metallic strip by passing the strip
longitudinally through a bath of molten coating metal having an
upper surface which is exposed to a gaseous atmosphere,
b. passing the continuous strip from the molten coating metal bath
through a coating weight control zone located immediately above the
upper surface of the molten coating metal bath in which zone the
molten coating metal applied to the strip in the bath remains
molten,
c. forming linearly extended thin streams of gas, each of length at
least as great as the width of the strip,
d. impinging a said stream of gas against each of the opposite
planar sides of the coated strip in the coating weight control zone
across the full width of the strip in a line space above the upper
surface Of the bath with a major component of motion of the stream
of gas perpendicular to the strip,
e. each stream of gas being shaped along its length in step (c) to
have mass flow across the width of the strip such as to result in
desired coating weight across the width of the strip,
f. controlling the mass of gas impinging against each planar side
of the coated strip in the coating weight control zone to maintain
uniform along the length of the strip the weight of coating metal
remaining on the strip leaving the coating control zone, and
g. subjecting the molten coating metal in the bath to heat exchange
with a heat transfer medium to control the temperature of the
molten coating
5. The process of claim 4 including
h. measuring the temperature of the bath, and
i. controlling the amount of heat transfer medium to maintain
the
6. The process of claim 4 in which the molten coating metal in the
bath in the neighborhood of the zone where the strip leaves the
path is subjected to heat exchange in step (g).
Description
BACKGROUND OF THE INVENTION
This invention is concerned with molten metal coating of metal
sheet material and in particular relates to a novel method and
apparatus which provides faster and more efficient production,
better product control and improved product.
Although there have been many highly productive improvements in
metal coating operations over the last twenty-five years, molten
metal coating itself, especially control of coating weight, has
remained essentially the same. Prior to this invention, molten
metal coating operations, have relied on mechanical contact with
strip at the exit side of a coating bath. This has been a slow
cumbersome process, making coating weight control one of the
biggest drawbacks and bottlenecks, especially in continuous strip
practice.
Since hot-dip zinc and zinc alloy coating, herein termed
galvanizing, is the commonest form of molten metal coating
operations, the invention will be described in this
environment.
The invention makes a radical departure from prior art practice by
providing a coating control method and apparatus which accurately
determine coating weight in continuous strip galvanizing
operations. The coating control method and apparatus of the
invention leaves the strip free from the marks and damage
occasioned by coating rolls, and the like, eliminates changing and
cleaning of such mechanical contact devices, and provides numerous
unexpected advantages such as increased line speeds, better
operational control, a choice of manual or automatic coating weight
control, smoother finish, more uniform coatings, and better
corrosion protection with less consumption of coating metal.
There are pronounced contrasts between the teachings of the
invention and past theories on wiping coating advanced for hot-dip
tinplating. For example, the U.S. Pats. to Steele, No. 850,548,
Sebell, No. 2,370,495, Sherman, No. 2,390,007, and the British Pat.
specification No. 588,281 disclose use of a liquid or equate use of
a liquid and compressed gas in tinplating. There are similar
contrasts between the teachings of the invention and the wiping
action of a high velocity stream of steam passing between a coated
surface and internal surfaces of a throat to blow excess metal from
the surfaces of a material as disclosed in the U.S. Pats. to
Underwood, No. 2,080,518, No. 2,095,537 and No. Re. 19,758. Such
theories have no application in the galvanizing industry and in
fact none of these prior art theoretical disclosures is known to
have found practical application in hot-dip metal coating of any
kind. In practice, coating rolls, despite their many shortcomings
and difficulties, remain in use throughout the strip steel
galvanizing industry.
The present invention overcomes these problems by controlling
coating with what is herein termed a gaseous barrier. Coating
control by gaseous barrier leaves the strip free from the marks and
damage occasioned by coating rolls, eliminates changing of rolls
and other mechanical problems and provides numerous unexpected
advantages such as increased line speeds, better operational
control, smoother finish, and so forth, which will be discussed
below.
In making a departure from conventional mechanical contact coating
control processes difficulties were encountered in obtaining a
smooth finish and in maintaining a uniform coating over sustained
periods. These difficulties were encountered even though recognized
variables in the process were maintained within normal ranges. In
solving these problems a significant discovery was made in
uncovering a critical relation of certain bath properties to
effective control of the coating. With this discovery, uniformly
coated product can be readily produced over sustained periods, an
objective difficult to obtain under some conditions.
Sustained, high-yield, hot-dip galvanizing operations are made
possible in the present invention by critical control of a number
of factors. The strip is guided from the bath and passes into a
coating control zone with its travel path determined and with strip
travel limited to longitudinal movement. The strip is shaped to a
desired cross-sectional configuration and coating metal on the
strip is in excess of desired final coating weight. A linearly
extended, thin stream of compressed and heated gas is impinged
substantially perpendicularly against the moving strip. The
compressed gas is at a minimum desired pressure which will
establish a gaseous barrier to passage of a quantity of molten
coating metal greater than desired final coating thickness. With
this apparatus excess coating metal is returned without turbulence
to the coating bath and the final coating on the strip is uniform,
smooth, and free from surface imperfections.
Just before leaving the bath and coming into a coating control
zone, the strip is led through a portion of the bath in which the
metal is held at a controlled temperature so that the temperature
of coating metal carried from the bath on exiting strip is
substantially uniform regardless of changes in strip width or gage
and line speed. Applicant has discovered that this uniformity in
coating metal temperature eliminates difficulties encountered in
maintaining desired coating weight and quality which stem from
variations in coating metal viscosity and the compressibility
factor of a gaseous barrier.
In further description of the invention, reference will be had to
the accompanying drawings wherein like numbers are used to denote
like parts wherever possible;
FIG. 1 is a schematic drawing of apparatus embodying the invention
and for carrying out the method;
FIG. 2 is a sectional view of typical coating apparatus of the
invention;
FIG. 3 is a schematic front elevational view of apparatus embodying
the invention with some of the parts shown in section;
FIG. 4 is an enlarged sectional view of nozzle structure of the
invention;
FIG. 5 is a reduced plan view of the structure of FIG. 4.
In carrying out the invention steel strip is prepared for hot-dip
coating with cleaning and/or annealing apparatus 12 shown
diagrammatically in FIG. 1. After preparation, the strip is
delivered through controlled atmosphere chute 14 into coating bath
16 where molten coating in excess of desired final coating weight
adheres to the strip. The temperature of strip 18 is ordinarily
elevated several hundred degrees above atmospheric temperature and
heat is added to coating bath 16 by strip 18 or in another practice
the coating bath adds heat to the strip. In either situation, with
different gages and widths of strip, with changing temperature of
the strip itself due to differing heat treatment requirements, and
with different strip speeds, which are all part of everyday
galvanizing conditions, bath temperature tends to fluctuate widely,
with the changing masses of strip entering the bath. Compensation
for these differences has been difficult and ordinarily required
manipulation of strip temperature or speed to the detriment of
efficient operation of a line.
Close control of the coating metal temperature at the strip exit
zone of the bath is critical for sustained uniformity when
controlling coating weight with the gaseous barrier process taught
by the invention. This significant factor was not recognized in the
practice described in parent application Ser. No. 282,474, now U.S.
Pat. No. 3,499,418. The temperature of the coating metal at the
strip exit zone of the bath influences both coating weight control
and the behavior of the bath surface below the coating control
apparatus. Therefore, in accordance with one embodiment of the
invention, a portion 20 of the coating bath is maintained at a
substantially constant temperature by submerged temperature
regulator tubes 21, regardless of changes in sensible heat effects
due to changes in strip steel width, gage, temperature and/or line
speed. Strip 18, after passage around sink roll 22, passes upwardly
through portion 20 of the bath 16 toward top roll 23. Thus the
temperature of the coating metal on strip 18 as the strip exits
from the bath is determined by controlled temperature portion 20
and is held substantially constant at a desired level.
The effect of regulating coating metal temperature in the present
invention on uniformity of coating and reduction of bath turbulence
becomes more clear when the interrelation of temperature and
fluidity is considered. As the temperature of the molten metal in
the coating pot rises the fluidity of the coating metal increases
and less coating metal is dragged from the bath, more coating metal
is held back by the gaseous barrier and the bath surface is more
easily disturbed.
The effect of substantially constant coating metal temperature and
resulting constant viscosity at the strip exit area is especially
important in the gaseous barrier taught by the invention. Unlike
the rigid nature of the prior art finishing rolls the gaseous
barrier of the present invention constitutes a barrier to the
passage of undesired coating metal which is of a yielding nature.
Final coating weight is determined by the amount of coating metal
encountering the gas barrier and its resistance to flow, or
viscosity. Therefore, with other factors constant, a uniform
viscosity which affects both drag out of coating metal and action
of the gas barrier is required if coating weight remaining on the
strip is to remain constant. Stabilizing coating metal temperature
in the strip exit zone of the bath stabilizes the drag out of metal
and the viscosity of the molten metal meeting the barrier, other
factors being constant. In practice such temperature stabilization
is preferably carried out by selective cooling of a portion of the
bath as shown in FIG. 1. However, certain cold strip type lines
with borderline bath heating capacity may require selective heating
of that portion of the bath in order to maintain a desired
temperature level for coating metal.
Uniformity of temperature of the coating metal on the exiting strip
is critical where sustained, uniform coating weight production is
desired. Other reasons which have not been fully explained at this
time may exist for this criticality in applicant's pneumatic
process. Additionally, applicant has discovered that more uniform,
smoother finish coating results when there is a substantial excess
of coating metal to be held back by the gaseous barrier. Of course
if less than desired final coating weight were to be dragged out
because of excessive fluidity, the need for regulating coating
metal temperature to obtain proper drag out of coating metal would
be clear. However, the smoother finish referred to is achieved by
virtue of acting on a substantial excess of coating metal over and
above the minimal required merely to meet coating weight
specifications by holding back some coating metal.
Another important teaching of the invention provides for
positioning the moving strip as it passes through the coating
control zone. The strip is constrained to purely longitudinal
movement by eliminating any lateral vibrating or transverse swaying
of the strip during its upward travel. The strip should be shaped
to minimize buckles and wavy edges in the strip as much as
possible. Also the strip is positioned so that each surface of the
strip is uniformly spaced across its width from its respective
adjacent nozzle.
After exit from the coating bath the strip passes through a coating
control zone where a gaseous barrier established by superheated
steam jets from nozzles 24 and 26 determines the final coating
weight. The steam or other heated gas is made to impinge uniformly
across the full width of the strip and is confined to a thin stream
(five- to fifteen-thousandths inch) in the direction of strip
travel.
FOr proper coating metal removal nozzles 24 and 26 must be
positioned to impinge the steam against the strip while the coating
metal is molten and at proper temperature. These and other
considerations can require the nozzles to be positioned in close
proximity, about 4 to 5 inches, above the coating bath.
To obtain desired conformation, movement, and spacing in the
coating control zone, roll guides are specially positioned as close
as possible to the coating control zone. Referring to FIG. 1, guide
roll 28 contacts the strip below but adjacent the bath surface and
applies a force to the strip which may cooperate with an upper
guide roll 30 to control positioning of the strip.
Guide roll 28 is preferably freely rotatable and not driven. Both
guide rolls 28 and 30 make contact with the strip as close as
possible to the coating control zone without interfering with the
coating operation and the coated surface, respectively. In the
latter case, with roll 30, heat is removed from the strip after
exit from the coating control zone. Cooling air or wet steam from
spout 32 is used to solidify the coating before contact with guide
roll 30. An important discovery of applicant, reasons for which are
not fully known, is that upper roll 30 can be positioned closer to
the bath with the jet process of the present invention than is
possible with coating control rolls. For example when wet steam is
discharged from spout 32 to minimize spangle formation by rapidly
solidifying the coating the spout can be positioned about 5 to 6
feet above the bath rather than the customary 8 to 12 feet above
the bath used with coating control rolls. Guide roll 30, which is
positioned a short distance (one to three feet) above spout 32, can
then be correspondingly closer to the bath. Because of this, roll
30 is more effective in obtaining the desired strip placement in
the coating control zone.
The shaft of guide roll 28 is positioned as close to nozzles 24 and
26 as possible without disturbing coating weight control; for
example, 7 or 8 inches below nozzles 24 and 26, with the top of the
roll submerged about two to three inches beneath the coating bath
surface has been found to be optimum in a coil galvanizing lines of
present design.
A coating control machine in accordance with the invention is shown
in more detail in FIG. 2. The strip 18 passes upwardly from sink
roll 22 in contact with guide roll 28 and between nozzles 24 and
26. The nozzles are supported in slides 34, 36, 38 and 40, which
permit the nozzles to move toward and away from the strip.
Adjustment gearing 42 and 44 which may be operable by motors 42',
44' (see FIG. 3) is connected to each nozzle for selection of
spacing between each nozzle and its adjacent surface of strip 18.
The adjustment means are mounted on both longitudinal ends of the
nozzles, are calibrated and are adjustable from the same side of
the machine.
The nozzles and slides are supported by frame members 46 and 48
which are separable to permit installation of the machine without
cutting the continuous strip. Frame member 46 also supports arm 49
which holds the bearing for guide roll 28. The lateral displacement
of strip 18 between sink roll 22 and guide roll 28 is exaggerated
in the FIG. 2 showing. In practice, a shaping and placement force
is applied to strip 18 by a lateral displacement of strip 18 of
around three inches between sink roll 22 and guide roll 28. An
oppositely directed force may be applied at roll 30. Strip 18 moves
upwardly along a substantially vertical path since any lateral
offsetting is minor compared to the overall length of the
longitudinal path between the sink roll 22 and top roll 23, usually
forty to sixty feet, or more.
Guide roll 28 is spaced seven and one-half inches from nozzles 24
and 26 in the machine shown. This spacing can be 15 inches or more
above roll 28 dependent on a number of conditions. However, the
object is to position nozzles 24 and 26 as close to guide roll 28
as possible in order to take advantage of the planar configuration
of the strip imposed by roll 28. In positioning the nozzles
however, turbulence of the bath and return of coating metal to the
bath must be considered.
The coating metal on the strip should preferably be above
800.degree.F. at the time of contact with the coating control jet.
Composition of the galvanizing spelter may affect this;
conventional galvanizing spelter includes aluminum additions and
impurity level percentages of lead, antimony, cadmium, etc., and
has a melting temperature in the neighborhood of 790.degree.F. The
temperature of zone 20 is maintained in a range of roughly
800.degree. to 860.degree.F. dependent on product and stabilized to
avoid changes greater than roughly 10.degree. to 20.degree.; e.g.
around 825.degree. to 840.degree.F. is preferred for most of
today's flat rolled steel galvanized products. In stabilizing the
temperature of zone 20, a temperature differential up to 30.degree.
or higher may exist between zone 20 and the remainder of the
bath.
FIG. 3 shows a front elevation, partially in section, of coating
apparatus in which a fluid coolant tube 50 is submerged in coating
bath 16. In practice a plurality of such tubes can be used to
define a zone of temperature regulated cooled spelter. For safety
purposes tube 50 may be surrounded by conduit 52 containing a heat
conductive material 54, such as molten lead. Flow of fluid in tube
50 is controlled by valve 56 and may be responsive to temperature
control apparatus 58 which receives signals from temperature
measuring device 60. Water is a preferred coolant.
Superheater 62 controls the temperature of a heated gas, such as
superheated steam or air. Temperature of the heated gas is
preferably held substantially constant with varying flow demands.
In practice a temperature around 850.degree.F. is prefered but
satisfactory operation can be obtained within a range of, roughly
500.degree. to 1,500.degree.F. Temperature measurements at
indicator 64 can be used to automatically control valve 63 which
controls fuel flow to superheater 62 to maintain gas temperature at
the desired constant value.
The pressure at nozzle 24 or 26 cannot be conveniently measured
without disturbing the coating. Pressure measurements read at
control meter 66, or a similar location, give satisfactory results
once relative values for a given installation are established.
Valve 68 controls the pressure of the heated gas delivered to
nozzle 24 and is responsive to selected pressures at control meter
66. A similar control is provided for nozzle 26 and substantially
equal pressures are used on both surfaces of the strip when equal
coating weights on each surface are desired.
For automatic coating weight control, a noncontact coating
thickness measurement device, such as beta ray back scattering gate
67 is positioned on each side of strip 18. Thickness measurements
from the beta ray gages are delivered to coating weight control
apparatus 69 and coordinated with the selected coating weight to
vary the pressure delivered to each coating control nozzle or
control the spacing between each nozzle and its respective side of
the strip 18. Control signals are delivered over the dotted lines
shown to a pressure control meter, such as 66, for each surface of
the strip and to motors 42', 44' for actuation of the spacing
controls 42 and 44.
Details of linearly extended nozzle structure are shown in FIGS. 4
and 5. Nozzle structure 70 includes two die members 72 and 74 which
mate to form a linearly extended gas manifold 76. Die members 72
and 74 are joined by a series of bolts 78. The separation between
members 72 and 74 determines the nozzle opening or passageway 84
and is set by use of shim stock 80. Spacing means 80 can vary in
thickness between 0.005 and 0.015 inches. Passageway 84 has an
inlet opening into manifold 76 and an outlet facing the strip. Gas
is supplied through a plurality of apertures 82, in order to obtain
substantially uniform gas pressures across the full longitudinal
length of manifold 76. The gas exits through linearly extended
passageway 84. It is to be noted that the angle of entry of the gas
with respect to the plane of the exit is shown at 90.degree. but
the invention is not limited to 90.degree.; however, a substantial
angular relationship is desired in order to obtain uniform gas
dispersal and exit velocity across the linearly extended nozzle
opening 84. Typical dimensions for nozzle structure used in
obtaining data for the examples presented are:
A 573/4 " E 1/2" B 287/8" F 1/2" C 18" G 11/2" D 54" H .015"
One of the primary objects of nozzle structure used in gas barrier
coating control of strip is a linearly extended gaseous stream of
uniform gas pressure across the strip. The thickness of the gaseous
barrier in a direction parallel to the strip motion is dependent on
the nozzle opening which will give proper flow. Larger nozzle
openings give greater gas mass and permit a greater mass of molten
coating to be held back. Larger openings also avoid clogging by
foreign matter; openings of .015 inches have been found
satisfactory for mill use in this latter regard.
In the gas barrier principle, as taught by the present invention,
the mass of the gas impinging against the molten coating is a
dominant factor. The effect of mass can be seen from a culvert
stock example where approximately 1,090 pounds of steam per hour at
a line speed of 110 feet per minute produced 21/2 ounce per square
foot coating while at a line speed of 130 feet per minute
approximately 2,200 pounds per hour produced light commercial
coating near 0.6 ounce per square foot.
From an operational point of view, pressure change can be used for
changing the mass of the superheated steam or other gas used. For
example, with increasing line speeds the mass of coating to be held
back to maintain constant coating weight increases. This increase
in mass can be achieved by increasing gas pressure. Alternatively,
the mass of the gas can be increased by increasing the area of the
nozzle slot without increasing the gas pressure.
Speed of the line is an important factor; it has been observed that
on a continuous galvanizing line, under similar operating
conditions of nozzle location and superheated steam pressure, a
speed of 100 feet per minute produced a "light commercial coat"
while 200 feet per minute produced a commercial ounce and a quarter
coating. Line speed could be used to control coating weight but, in
practice, an operator would run a line at a maximum speed for a
particular gage material as determined by other factors. The gas
barrier could be set at the optimum height above the bath, the
optimum gas opening, the gas pressure or nozzle spacing or both
being then varied to control the coating weight.
Final coating weight at a given line speed can be controlled by
either gas pressure or proximity of the linearly extended nozzles
to the strip, or both. As described above the temperature of the
coating metal applied to the strip is held substantially constant.
Briefly, higher strip speeds, lower impinging gas pressure, and
greater distances between nozzle and strip produce heavier coating
weights; lower strip speeds, high impinging gas pressure, and lower
distances between the nozzle and the strip produce lighter coating
weights. Generally the strip speed is selected based on other
limiting factors, e.g., the annealing capacity of the line, and the
line is ordinarily run at the maximum speed available considering
such limitation factors. It is desirable to maintain a minimum
steam pressure regardless of other related coating control factors
although to meet variations in required coating weight either steam
pressure or nozzle spacing can be changed. In practice changing of
nozzle spacing is preferred because of the desire to maintain a
minimum gas pressure and to avoid over or under correcting when gas
pressure controls are employed. With automatic controls either
spacing or pressures can be changed readily to meet coating
requirements within a selected low pressure range. Typical
production examples are included below.
TABLE I
Continuous-strip galvanizing with nozzles 41/2 to 51/2 inches above
bath level, coating metal at exit side of bath held at or near
825.degree.F., nozzle opening of 0.015 inch, substantially
perpendicular impingement, superheated steam temperature about
840.degree.F., spacing between each nozzle and its adjacent strip
surface about one-half inch.
__________________________________________________________________________
RUN 1 RUN 2 RUN 3 RUN 4
__________________________________________________________________________
Strip thickness (inches) .020 .0183 .018 .0172 Strip width (inches)
36 9/16 28 241/2 27 13/16 Topside pressure (lb./in..sup.2) 35 30 37
32 Bottomside pressure (lb./in..sup.2) 34 30 38 31 Speed of line
(fpm) 170 220 230 230 Coating weight (oz./ft..sup.2) .59 .82 .59
.99 (Total of both surfaces)
__________________________________________________________________________
TABLE II
Continuous-strip galvanizing with nozzles 41/2 to 51/2 inches above
bath level, coating metal temperature at exit side of bath held at
or near 825.degree.F., nozzle opening 0.015 inch, substantially
perpendicular impingement, superheated steam temperature about
840.degree.F., spacing between each nozzle and its adjacent strip
surface about three-fourths inch.
______________________________________ RUN 5 RUN 6 RUN 7 RUN 8
______________________________________ Strip Thickness (inches)
.018 .0217 .021 .0157 Strip Width (inches) 36 36 231/4 24 Topside
Pressure (lb./in..sup.2) 38 29 42 42 BOttomside Pressure
(lb./in..sup.2) 40 29 41 41 Speed of Line (fpm) 200 200 210 230
Coating Weight (oz./ft..sup.2) .54 .76 .53 .56
______________________________________
TABLE III
The following differing products were run over a 3 hour period on
the same line with the nozzles 41/2 to 51/2 inches above bath
level, nozzle opening 0.015 inch, substantially perpendicular
impingement, and spacing between each nozzle and its adjacent strip
surface about three-fourths inch. The coating metal temperature at
the exit side of the bath was held at or near 825.degree.F. despite
wide changes in strip heat added to pot by changes in steel mass
introduced and varying line speeds.
______________________________________ RUN 9 RUN 10 RUN 11
______________________________________ Strip Thickness (inches)
.018 .0157 .0187 Strip Width (inches) 30 30 35 Topside Pressure
(lb./in..sup.2) 32 26 28 Bottomside Pressure (lb./in..sup.2) 32 26
28 Speed of Line (fpm) 230 130 180 Coating Weight (oz./ft..sup.2)
.94 .94 .97 Product Form Coils Sheets Coils Use Pipes Warehouse
Roofing Stock ______________________________________
Differential-coat is readily produced by controlling gas pressure
on each surface. From observation or production of
differential-coat on a continuous galvanizing line, the light side
coating is controlled more effectively by the gas barrier apparatus
than with any known method. Imperfections in the strip on the light
side of the strip are not a problem with the gas barrier apparatus
and a smoother light side coating results. Differential galvanized
product having less than 0.1 ounce per square foot on the light
side and more than 0.3 ounce per square foot on the heavy side was
produced using 55 pounds per square inch steam pressure on the
bottom side manifold (light side of the differential coat) and 45
pounds per square inch steam pressure on the top side manifold.
In producing product with equal coating weight on both surfaces,
the strip is ordinarily passed midway between the coating control
nozzles and the steam pressure on each nozzle is about the same. In
order to make differential coat product, the nozzle on the light
coating side of the strip can be moved closer to the strip or the
steam pressure can be increased or both. In practice, changing the
spacing of the nozzles is preferred as shown in the following
table.
TABLE IV
With perpendicular disposition of the nozzles, drawing quality
stock, 0.0503 inch gage, 373/8 inches width, was produced at a line
speed of 80 feet per minute with the following settings:
Nozzle Spacing Pressure Coating Weight (lb./in..sup.2)
(lb./ft..sup.2) ______________________________________ Light Side
1/4" 25 .19 Heavy Side 11/4" 22 .48
______________________________________
Adjustment in spacing between nozzles to change coating weight is
an advantage of the substantially perpendicular impingement concept
which is not readily available with angled impingement. Spacing of
angled nozzles cannot be changed without changing the point of
impact of the gas with the strip. Therefore the point of impact for
one nozzle may be readily offset from the other with an angled
disposition. One result can be an edge buildup of coating metal.
With the nozzles disposed substantially perpendicularly to the
strip this problem does not exist and a new means of adjusting
coating weight, by adjusting nozzle spacing is available to the
operator.
With the present invention, high strip speed is not a limiting
factor whereas, with mechanical contact methods, coating control
was one of the major speed limiting factors. Other operations such
as annealing or coiling, etc., may place some limit on a particular
line but, with the present invention, the coating operation itself
will not limit line speeds with present day molten metal coating
lines of any type. In fact, it has been found that the gas barrier
principle of this invention produces smoother finishes at higher
speeds.
Some of the advantages of the gas barrier principle of coating
control include increased production, improved quality and more
economic production. Increased production results from the faster
line speeds available with this invention over those with the prior
art practice; also, less down time for a line since there is no
necessity to change coating rolls, etc. Improved quality results
from the avoidance of coating roll marks and the smoother finish
produced by the gas barrier method. Improved economy results from
the increased production referred to above, increased percentage
yields, and elimination of a number of post coating treatments to
improve coating surface.
* * * * *