US3459587A

US3459587A - Method of controlling coating thickness

Info

Publication number: US3459587A
Application number: US613474A
Authority: US
Inventors: Darrell L Hunter; James C Siple
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1967-02-02
Filing date: 1967-02-02
Publication date: 1969-08-05
Anticipated expiration: 1986-08-05
Also published as: FR1554130A; NL162966C; NL162966B; AT283844B; LU55387A1; DE1696619B2; BE709842A; NL6801161A; GB1196073A; DE1696619A1; JPS512055B1; ES349633A1

Description

5, 1969 D. L. HUNTER ETAL 3,459,587

METHOD OF CONTROLLING COATING THICKNESS Filed Feb. 2, 1967 m 2 Sheets-Sheet 1 40 x as l I 48 I l/ c S 44 F- mvewrans. 34 46 DARRELL L.HU/VTER and .uwss a. sun:

5 w ijnm AHarney Aug. 5, 1969 o. L. HUNTER ETAL 3,459,587

METHOD OF CONTROLLING COATING THICKNESS 2 Sheets-Sheet 2 Filed Feb. 2. 1967 Boundary trip A Mot/an A A Air Coating Metal A h Boundary I INVENTORS. DARRELL L. HUNTER and Strip JAMES c. SIPLE' a y (J My Air Boundary Air Boundary Coating Metal A I rorney United States Patent M US. Cl. 117-102 5 Claims ABSTRACT OF THE DISCLOSURE A method is described for controlling the weight and distribution of a molten metal coating on a moving or continuous strip substrate by the use of fluid streams directed against each side of the coated strip before the coating solidifies. The fluid streams must be positioned so that they overlap but are displaced from each other by an amount of from to A the impingement height.

This invention relates to a coating weight control method. More particularly, the invention relates to a method of controlling the weight of a coating applied by a process wherein material to be coated is immersed into and withdrawn from a bath of the coating material. The invention is particularly suited for use in hot-dip coating operations wherein a substrate such as steel strip is coated with a metal such as zinc, aluminum, tin or lead, and alloys thereof.

Presently, coating rolls are used to control the weight and volume of coating material applied to a substrate. Coating rolls limit line speeds materially in continuous operations. At moderate line speeds of up to 200 feet/ minute, a generally satisfactory product may be produced with coating control by rolls. Occasionally, coating defects such as groove marks, edge build up and non-uniform distribution are encountered even at these relatively low line speeds, but these defects are generally not serious problems. The defects do become serious, however, at the higher line speeds, above 200 feet/minute, which are normally used in continuous coating lines for light gauge product, i.e. less than 0.0l8-inch thick. Accordingly, line speeds in systems using coating-roll control have had to be reduced for light gauge material to as little as 100 meet/minutea serious decrease in the production rate.

It has long been proposed to employ fluid streams, such as air or other gases, to control coating weight and distribution. Such systems would have the inherent advantage of permitting more rapid line speeds with attendant increase in production rates. As early as 1883, a patent was issued for a system of removing surplus materials from articles coated with material. This patent, No. 287,076 to H. A. Young, is typical of the so-called air knife technique which has been the subject of a great amount of investigation and development.

A major problem or defect that has been encountered when using fluid streams to control coatin thickness is the formation of built-up ridges of coating metal at the edge of the coated strip. This condition becomes less tolerable as the thickness of the strip being coated decreases in gauge. For example, at substrate thicknesses below about 0.025-inch, the presence of coating metal build up at the edge prevents satisfactory coiling of the product. This defect is typically caused by changes in the direction and velocity of the fluid stream at the edges of the strip. Product produced using coating rolls also exhibits coating build up at the edges of the strip and extreme care 3,459,587 Patented Aug. 5, 1969 ICC must be exercised to minimize this condition so the strip can be coiled.

Examination of the mechanisms involved which cause edge build up has shown that the coating is built up at the edge by the force of the fluid striking the coating. The pattern of fluid flow is such that a vortex is formed at the edges on opposite sides of the substrate. If there is no opposing force, some coating from one side will be blown around the edge forming a heavy coating or ridge on the opposite surface at the edge. The present invention avoids this and other disadvantages of fluid-stream, coating control by proper positioning of opposing fluid nozzles such that the force from each of the fluid streams prevents the formation of edge ridges which occur by the flowing around of coating from one side to the other.

It would seem logical that the optimum position of the fluid streams, e.g. gas jets or gas streams, to avoid edge build up would be to have each stream exactly opposite to the other, or stated in another way, to provide a mirror image. However, it has been found that if the gas streams are positioned exactly opposite each other, there is an excess of metal blown to the extreme edge forming a metal bead there. This is also undesirable in that it produces an uneven, rough edge, often unacceptable in a. commercial product. The present invention contemplates the proper critical positionin of the gas streams to achieve superior coating control without undesirable edge build up or edge bead formation so that the coated strip may be coiled in the conventional manner with a satisfactory edge.

It has been discovered that opposing gas streams directed to opposite sides of a coated substrate must be critically offset, one higher than the other, so that the opposing force of the higher stream can eliminate the edge build up or edge bead of coating metal. However, this difference in impinging gas streams must not exceed that point at which the force from the higher gas stream can blow coating around from the opposite side and thereby form a ridge.

The advantages of the invention described above and others will be made more apparent by the following description taken in conjunction with the accompanying drawings.

In the drawings:

FIGURE 1 is a schematic elevation view of a hot-dip, coating line equipped with a gas nozzle system for controlling coating thickness in accordance with the invention;

FIGURE 2 is a cross sectional view taken along lines 11-11 of FIGURE 1 schematically showing the nozzles positioned with respect to the coated strip;

FIGURE 3 is an enlarged detail showing schematically how the nozzles are positioned to effect coating control in accordance with one embodiment of the invention and a vertical section of one of the nozzles therein;

FIGURE 4 is a vertical section of an alternative embodiment of a portion of the gas nozzle shown in FIG- URE 3; and

FIGURES 5, 6 and 7 are schematic diagrams illustrating different affects on the coated strip produced by varying adjustments in the gas nozzles shown in FIGURE 3.

The arrangement shown in FIGURE 1 describes a typical system for hot-dip-coating a substrate but one equipped with gas jets to control coating weight in accordance with the invention. The terms gas jets and gas streams as used herein refer to a fluid, e.g. gaseous medium, which is blown against the coated substrate while the coating is still molten and as the substrate is withdrawn from the coating medium to effect control of the quantity, volume, thickness and distribution of coating material on the substrate. Any suitable gas may be used but an advantage of the invention is that a simple air stream, even of ambient temperatures, may be used effectively. Steam, other fluids, gases, etc., may obviously also be used.

As is shown in FIGURE 1, a substrate such as a steel strip is uncoiled in zone 1 and then passed through a heating zone 2 of an annealing furnace and then through the cooling zone 3 of the furnace into a bath 4 of coating material. Rolls 5 disposed within the coating material and referred to as sink rolls guide the strip through the coating bath, after which the coated strip passes between oppositely disposed gas steams 6 over suitable rolls 7 to a cooling run-out and into recoiling equipment 8.

The length of the orifices 10 emitting the gas streams from nozzles 6 should be several inches longer than the widest strip whose coating is to be controlled by the fluid streams. The increased length of the orifice allows for movement of the strip in a variable pass line. The cross sectional view shown in FIGURE 2 illustrates the extension of the orifice slots beyond the length of the strip therebetween. This view taken along lines IIII of FIGURE 1 shows the positioning of the nozzles with respect to the strip.

An arrangement and positioning of the gas streams and nozzles for delivering same is described in FIGURE 3. As can be seen therein, nozzles 10a and 1% are positioned on opposite sides of a strip to project a gas stream against the coated surfaces as the strip is withdrawn from a coating bath and while the coating is still molten.

Each nozzle 10a and 10b is comprised of a header section 12, a convergent throat section 14 and an orifice section 16 having a depth M. The nozzles are equipped with a pressure gauge 18 to indicate fluid pressure, and some means are provided to vary the lips 22' and 22" of the nozzle in the orifice section. A screen 20 is provided to achieve the desired fluid distribution as hereinafter described.

Also as shown in FIGURE 3, the angle of divergence 6 of the emitted gas stream is independent of both the gas pressure behind the orifice slot and of the orifice height S when the flow exceeds a Reynolds number of about 2000. Since the angle 0 is relatively constant, the height of impact H, i.e. impingement height, of the wiping force of the gas stream is established according to the following equation:

The flow pattern emitted from the nozzle is independent of presure and as a result, the impingement height H is dependent only on the orifice height S, the divergent difiusion angle 9, the distance of the center of the orifice to the strip D and the horizontal angle 6 of the orifice to the strip.

It has been discovered that in order to achieve coating weight control by the use of gas streams without causing the formation of undesirable edge beads or edge build up, the impingement height H must be offset enough so that the opposing force of the higher height H can eliminate the edge bead. However, the difference, X, in the opposing height H must not exceed that point at which the resultant force from the heightest height H can blow coating around to the opposite side and form a ridge. The invention, therefore, provides a zone of operation in which the position of the fluid nozzles produces a coated strip having little or no edge beads or heavy coating ridges at the edges of the strip. The zone within which the coating control can be so accomplished is defined as follows:

Minimum range.To prevent edge heads, the impingement heights H of the two opposing fluid jets should be off-set from perfect vertical alignment (mirror image placement) by an amount, X, which is at least of H;

Maximum range.To assure elimination of heavy coating ridges, the two opposing impingement heights H should 4 be off-set from perfect mirror image alignment by not more than A of H.

As can be seen, the fluid streams must be overlapping but not coincident with each other and the amount of displacement, X, of each gas stream should be within the range of /3 to /4 H.

A number of design factors may be adjusted to obtain optimum nozzle operation. For example, it has been found that the header section of the nozzle is related to the area of the orifice slot in a manner which depends upon the gas ent1y into the header. If the system of dual feeding is used where air enters into both ends of the header (double entry feeding) the ratio of the cross sectional area of the header to the area of the orifice slot should be advantageously at least 4 to 1. If the air enters into only one end of the header (single entry feed) the ratio of the cross sectional area of the header to the area of the orifice slot should be at least 8 to 1. The aforementioned ratio of the cross sectional area of the header to the area of the orifice slot will aid uniformity of profile emitted from the slot. In a preferred arrangement, the aforementioned ratio would be 5 to 1 for double feed entry and 10 to l for single feed entry. Ratios larger than these do relatively little to improve the emitted profile. The shape of the header may be almost any configuration which is practical to construct or necessary for space limitations. A standard pipe has, however, proven to be satisfactory.

The internal portion of the nozzle should, desirably, have a uniform distributing resistance through which the air must pass. This resistance may be in the form of a perforated plate or a screen 20 shown in FIGURE 3. The resistance helps prevent non-uniformity of flow which may result from fluid changing direction from the header to the throat section. For satisfactory distribution, the resistance of a minimum of Z-inches water is desirable. One satisfactory arrangement is by the use of a ZOO-mesh screen (about 33% void opening) with a ratio of a total area of screen to the ratio of the slot of 16 to 1.

Referring to FIGURE 3, it has been found that a uniform air-profile can be achieved in the most optimum manner by meeting the following conditions.

(1) The inner surface of the slot between the lips 22' and 22" across which the air passes should be smooth and parallel. Waves and lines along the surface may leave defects on the coated strip upon which the air strikes.

(2) The ends of the slot from which the air exits should be vertically aligned so that the lips 22' and 22" terminate in a plane normal to the centerline of the lips. If one lip extends beyond the other, the air pattern will be disrupted and rough coating may result.

(3) It is desirable to avoid any protiusions extending into the path of the air between the stream and the slot. The taper of the convergent throat section should connect with the air entrance of the slot with little or no protrusion of the lips into the air stream.

(4) To allow for movement of the strip into an imperfect pass line the length should be several inches longer than the widest strip (see FIGURE 2 above).

Adjustment of the orifice height S is achieved in the nozzle described in FIGURE 3 by means of the nut and bolt connections 26a and 26]) shown in the drawing. Thus, the upper lip section 22' can be moved vertically to achieve any desired orifice height and then reaffixed to the nozzle in a stable manner such as by tightening the bolt shown.

An alternative construction of the convergent throat and orifice sections of the gas nozzle shown in FIGURE 3 is illustrated in FIGURE 4. This embodiment is another arrangement for adjusting the slot or orifice height S. However, it is apparent that the adjustment of the slot height can also be achieved by screws, cams or other devices all of which can be operated manually, electrically, pneumatically or hydraulically.

In the embodiment shown in FIGURE 4, threaded

bolts

32 and 34 secure machined

sections

36 and 38 to the

flanges

40 and 42 of the convergent throat section. The slot is formed by machined

sections

44 and 46 which are removably fastened to the machined

sections

36 and 38 by means of

duplicate machine screws

48 and 50. The slot height is varied by insertion of

other sections

44 and 46 of different thickness to provide any desired slot height.

It is well known that the weight of all hot-dip coatings produced with coating rolls increases with increasing strip speed which necessitates adjustments in the position, speed, pressure or grooving of the rolls to maintain the same coating weight. Even when no coating control system is used, the coating on the strip as it is withdrawn from the coating bath is also dependent on the speed of the strip and increased speeds result in heavier coatings. However, when fluid streams are used to control the coating weight, variations in strip speed are easily compensated for by adjusting the fluid flow from the nozzles to maintain a constant coating weight. Such a method is readily adaptable to an automatic coating weight control system whereby the fluid flow from the nozzles is regulated according to the speed of the coating line, i.e. strip speed, to maintain the desired coating weight. It is also apparent that the coating weight on each side of the strip can be controlled independently to produce differentially coated strip.

A coating control system with fluid streams as described herein may function at varied height of the nozzle above the coating bath. Excessive agitation and splashing of the coating, however, can be avoided if the nozzles are positioned at least about 8 inches from the bath.

It has also been determined that coatings of satisfactory appearance and weights can be produced with any combination of nozzle-to-strip angles ranging from about 20 above the horizontal to about 45 below the horizontal. It should be noted that the centerline of the gas streams need no be inclined at identical numerical angles. It is possible that each nozzle may be inclined at somewhat different angles and it is only necessary that the relationship impingement heights as described herein be maintained. An angle of +10 has been found to be particularly effective for some coatings (e.g. terne) and angles of 5 to 45 (with an optimum angle of l5) also have proved to be desirable for galvanize coatings. The orifice height is not critical and openings of from 0.02-inch to about 0.25-inch have proven to be satisfactory. However, somewhat higher flow rates are required with increasing slot height to maintain similar coating weights.

On the other hand for a given orifice height, there is a difference in coating appearance as the nozzle-to-strip distance is increased. Coatings produced with a nozzleto-strip distance of /2-inch are generally uniform in appearance. In comparison, coatings produced with a nozzleto-strip distance of 1-inch generally exhibit a faint transverse wave pattern. Coatings produced with a nozzle-tostrip distance of more than about 1 /2 inches exhibit a more pronounced wave pattern which might be unsatisfactory for some critical applications. The waves become slightly less pronounced as the strip speed is increased. Nozzle-to-strip distances of from about A to 1 /2 inches are preferred.

Coating distribution is effectively controlled while varying the fluid flow rate. That is, for each orifice height, increasing the fluid flow rate causes a reduction in the coating weight without impairing the appearance of the coating. One convenient method for determining the air flow rate, for example, from a given orifice is to measure the air pressure within the nozzle. A preferred minimum pressure is about 0.4 p.s.i.g. Relatively higher pressures are required to produce satisfactory coatings with smaller orifices. i.e. 0.020-inch, and lower pressures are sufficient with the larger orifices, i.e. 0.250-inch. Orifice heights of 0.06 to 0.15-inch are considered the optimum range for galvanizing operations when using air streams for coating control.

If all other operating variables are constant, the coating weight of the strip after passing through the gas streams can be considered directly proportional to the line speed. Thus, for example, if the coating weight is 0.5 oz./sq. ft. at a line speed of ft./minute, the coating weight would be 1.0 oz./sq. ft. at a line speed of 300 ft./minute.

As discussed previously, a problem of prior proposed fluid stream systems for effecting coating control has been the formation of small beads of coating metal and/or coating build up at the extreme edges of the coated strip. The schematic diagrams shown in FIGURES 5, 6 and 7 illustrate different results obtained by various gas stream adjustments which may result in undesirable edge conditions. The vertical view of FIGURE 5 and the cross section of FIGURE 6 show the edge bead 52 formed with the gas streams positioned as mirror images against the coated substrate. The edge beads are formed along the extreme edge of the strip and represent a highly undesirable effect and waste of coating metal. The schematic view shown in FIGURE 7 illustrates the effect of misalignment of impinging lengths H with the formation of a coating ridge 54. If the opposing gas streams are not positioned within the critical range of oH to %H the affect will be similar to that resulting when only one gas stream is used. That is, the coating will be blown around the edge and deposited on the opposite side forming a heavy build up of coating or ridges on the surface near the edge. It is apparent that the strip cannot be coiled effectively with such a coating ridge present.

It has been determined that the gas stream impingement pressure is proportional to the pressure that feeds the orifice. Therefore, it is possible to control the coating weight simply by increasing or decreasing the pressure to the orifice. As a first approximation, the coating weights may be taken as inversely proportional to the orifice pressure. However, increases in line speed also increase the coating weight when other variables are not changed. For a sufficiently good first approximation, the coating weight may be considered as directly proportional to strip speed.

The distance D (see FIGURE 3) of the orifice to the strip also affects the final coating weight. When the strip is not located in the center of the spacing between the two nozzles, equal pressure will result in a difierence in coating weight on the two surfaces of the strip because the distance is not linear. For a given gas pressure, the average coating weight, i.e. total weight will increase when the strip moves away from the center position.

As an example of the above, with the strip centered between the two nozzles, the coating weight will be 0.5 oz./sq. ft. or a total coating weight of 1.0 oz./sq. ft. If the strip moves off-center /2-inch, the strip side closest to the nozzle will have a coating weight of only 0.3 oz./sq. ft. while the side further away from the nozzle will have a coating weight of 0.8 oz./sq. ft. The total will be 1.1 oz./sq. ft. or an increase of 0.1 oz./sq. ft. because the strip moved off of center.

For a given nozzle-to-strip distance D and a given orifice pressure, there is a decrease in coating weight with increased orifice height S. This is due to the increase in the mass of fluid emitted from the increased orifice openmg.

With a given nozzle-to-strip distance D and a given orifice pressure, the effect of nozzle elevation above the coating bath depends upon line speed and coating drainage. Since the increased strip speed will result in more coating being carried upward and a heavier coating, gravity drains back some of the excess metal pulled out of the coating bath, and there is a level at which the gravity drainage is in equilibrium. Above this equilibrium point, no more drainage occurs and the coating weight will be constant. For example, to produce 1.25 oz. coating at 450 ft./minute, nozzle elevation will be at least 10-inches. Above this point, the nozzle will produce the same coating weight, 1.25 02., at 12-inches just as readily as at 18-inches.

The following is a specific example of the manner of using a gas stream coating control process to produce galvanized coating on a steel strip. A pair of air nozzles each on opposite sides of the strip and of the design shown in FIGURE 3 are placed in an adjustable rigging above the coating bath of the type shown in copending application S.N. 613,476 by Robert W. Patterson filed of even date and now U.S. Patent 3,406,656. The adjustable rigging is such that each air nozzle can be adjusted horizontally and vertically and rotated to adjust the impingement angle of the air striking the strip. The strip emerging vertically from the galvanizing bath at a speed of 300 ft./minute is subjected to the wiping action of the air streams at a distance of 16-inches above the surface of the bath. The nozzles are positioned so that the axis of each air stream issuing from /;inch high slot orifice impinge on the strip at an angle of i.e. 15 below a position normal to the strip. The point of impact of the axis of one jet was off-set Ax-inch vertically from the point of impact of the axis of the air stream striking at opposite sides of the strip. The distance along the axis of each jet from the orifice to the strip is 4-inch. The air pressure downstream from the screen inside each of the nozzle in 30-inches of water and the coating weight produced in this mannner is 1.175 oz./ sq. ft. of sheet. The resultant coating is uniform in thickness and appearance across the width of the strip and the extreme edges do not exhibit ridges or beads.

Changes in the aforementioned specific set of line conditions generally require, for optimum operations, changes in the adjustment of the air stream. For example, a reduction in line speed would be accompanied by a lowering of the gas nozzles to a position closer to the coating bath. Extremely slow speeds, i.e. 50 ft./minute, would cause the nozzle setting to be as close as practical to the bath, that is about 6-inches above the surface. Slower line speeds would also require a reduction in the pressure within the nozzles to maintain the same coating weight. For example, if the strip speed is reduced from 300 to 150 ft./ minute, the gas pressure should decrease from 30-inches of water to 15-inches of water. If lighter coating weights are to be produced at any given speed, the gas pressure within the nozzles ought to be increased at a rate inversely proportional to the desired change in coating weight. The gas pressure within the nozzles can be easily regulated by a valve located in the main air supply line.

We claim:

1. An improvement in the method of controlling the thickness of a hot dip coating of molten metal on a moving substrate of steel strip by the use of gas streams ap plied to the coated surfaces of the subtrate before the molten metal coatings solidifies comprising projecting gaseous streams of controlled impingement heights against opposite surfaces of said moving substrate from gas nozzles positioned on both sides thereof such that the impingement height of the respective gas streams are overlapping but offset from each other by an amount of from to A the impingement height, said gas streams extending beyond the width of the substrate at the edges thereof.

2. An improvement according to claim 1 wherein said gas projected against said coated substrate to control the thickness thereof is air at ambient temperature.

3. .An improvement according to claim 1 wherein said gas streams are directed to said substrate at an angle from their centerlines to a plane normal to the substrate of from about plus 20 to about minus 45.

4. An improvement according to claim 3 wherein said angle is within the range of from about plus 10 to about minus 15.

5. An improvement according to claim 1 wherein said nozzles are positioned from about fit-inch to l /z-inches from the substrate.

References Cited UNITED STATES PATENTS 602,532 4/1898 Wilder. 794,704 11/ 1905 Fellows et al. 811,854 2/1906 Lee. 1,672,526 6/1928 Hawkins. 2,160,864 6/ 1939 Hill et al. 2,243,979 6/ 1941 Reynolds. 2,370,495 2/ 1945 Sebell. 2,390,007 11/1945 Sherman 11863 X 2,950,991 8/ 1960 Seymour 117131 X 3,231,415 1/1966 Grenley et al.

ALFRED L. LEAVITT, Primary Examiner J. R. BATTEN, 1a., Assistant Examiner US. Cl. X.R. 17-114; l1863