US 2992941 A
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July 18, 1961 P. J. WHITLEY ET AL 2,992,941
EXIT MACHINE FOR COATING APPARATUS AND METHOD OF CONTROLLING COATING THICKNESS Filed May 7, 1958 United States Patent 2 992 941 EXIT MACHINE FOR coAT1No APPARATUS AND METHOD OF CONTROLLING COATING THICK- NESS Paul J. Whitley, John E. Kemplin, and Henry L. Jones,
Middletown, Ohio, assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Filed May 7, 1958, Ser. No. 733,708 13 Claims. (Cl. 117-102) This invention relates to an exit machine for metal coating apparatus and to a method of controlling the thickness or weight of metallic coatings.
In the coating of a metallic strand by the hot dip method, it is conventional to have the strand issue from the pot between a pair of exit rolls which may, in part at least, be submerged in the hot coating bath. It will be understood that the term strand as herein used is inclusive of strip or sheet material and wire.
When a strand issues from the coating pot between the exit rolls, the pumping action of the rolls and dragging action of the strip produce the result that a quantity of coating metal is pulled up out of the pot between the strand and the exit rolls and forms a pool or meniscus between the strand and the nip of the rolls. The faster the strip is passed through the apparatus, the greater the pumping and dragging action and the deeper the meniscus thus formed.
It has been found that the meniscus has a great effect on the thickness of coating which is produced. The larger the meniscus, the heavier the coating will be and the smaller the meniscus, the lighter the coating.
Where it is desired to produce lighter weight coatings at a speed, or to maintain a given weight of coating at higher speeds, it is clear that something has to be done to lower or reduce the meniscus as required.
It is common practice in the art to control the size or depth of the meniscus by an adjustment upward and downward of the exit rolls, or conversely, by adjusting the level of metal in the pot. If, at a given speed, the exit rolls are adjusted upwardly out of the bath, the
meniscus tends to be reduced, and if the exit rolls are lowered into the bath the depth of the meniscus is in- .creased. However, if the exit rolls are raised too high,
the metal seal is broken and then the coating will become heavy and non-uniform.
It has also been attempted, with some success, to control the weight of coating by braking the exit rolls to slow them down or even stop them entirely, thereby lowering the meniscus and thereby the weight of coating. Moreover, this is not fully effective-pumping action of the rolls is decreased or stopped, but the dragging of the strip is not afiected. Also, this practice tends to cause stripes or lines in the finished strip, which of course detract from the appearance of the finished product.
It is therefore the principal object of the invention to provide a new method for controlling coating thickness which may be practiced instead of, or in addition to, adjustment of the exit rolls as above outlined, and to provide an apparatus for carrying out such a method, whereby desired weights of coating may be applied at speeds higher than was heretofore believed possible.
Reference is made to the drawing forming a part hereof and in which the single figure is a diagrammatic represent-ation in cross-section of the exit rolls and associated apparatus according to the present invention.
Briefly, in the practice of the invention, the exit rolls are rotatably mounted so that they rotate partially submerged in the molten coating metal. The size of the meniscus is controlled, in part at least, by directing a blast at the meniscus formed between the strand and the nip of the rolls, and controlling the force of such blast to provide a back pressure to the pumping and dragging actions of the rolls and strand, so that by increasing the force of the blast these actions are impeded and the meniscus is reduced. Other known practices may be used in addition to the blast.
Referring more particularly to the drawing, a molten metal coating bath is indicated generally at 10 and a strand 11 is shown as passing upwardly out of the bath 10, between the exit rolls 12 and 13. The pumping and dragging actions of the rolls and strand produce a meniscus on each side of the strand between the strand and the nip of the rolls as indicated at 14 and 15. The coating on the strand is indicated at 16 and 17.
18 and 19 indicate pipes extending parallel to the axis of the exit rolls and provided with nozzles 20 and 21 through which the blasts 22 and 23 issue. The pipes 18 and 19 are supplied by air under pressure through the lines 24 and 25 respectively and these lines are provided with pressure control valves 26 and 27. Air under pressure is supplied to the valves 26 and 27 through a line 28 provided with a shut-off valve 29 and supplied from a source of air under pressure 30.
By the provision of the separate control valves 26 and 27, it will be clear that the force of the blast issuing from the nozzles 20 and 21 need not be the same. Thus, the blast 22 may be made stronger than the blast 23 or vice versa, so that different weights of coating may be applied to the two sides of the strip. It will be understood that in the case of strip, the pipes 18 and 19' extend over the full width of the strip and are provided with sufiicient nozzles 20 and 21 to provide a substantially continuous blast along the entire meniscus. It will be understood that additional blasts may be directed at the edges of the strip to prevent or reduce edge build-up.
If the strand is being coated with aluminum and other metals having similar characteristics in the molten state, it will be sufiicient to provide the air blast as above described. However, if the coating metal is zinc or other metal having characteristics similar to zinc when molten, it has been found that if air alone is used for the blast,
. the exit rolls become roughened and the coating quality will be poor. The exact reason for this is not at present cured by the provision of heat exchange means to preheat the air because as the air issues from the nozzles 29 and 21 it expands, as is well known, and is cooled in its expansion so that by the time it strikes the metal surface it is too cool to produce the desired result.
For this reason, there are provided the additional pipes 31 and 32 having the nozzles 33 and 34 respectively and these pipes are supplied respectively through lines 35 and 36 with a combustible gas. They are supplied further through the lines 37 and 38 with air for combustion of the gas and the gas and air flowing through the lines 35, 36, 37 and 38 may be controlled by suitable valves 39, 40, 41 and 42.
Thus there will issue from the nozzles 33 and 34 flames which are directed at the blast so as to heat the blast so that it will be hot when it strikes the meniscus.
By adjusting the pressure by means of the valves 26 and 27, the level of the meniscus on either or both sides of the strand may be raised and lowered without the necessity of changing the position of the exit rolls.
It will be understood that by experimentation a gas may be found which will burn at a suitable temperature, having regard to the type of coating metal, that air will not be required for its combustion; and in such cases the lines 37 and 38 may be omitted. With a gas requiring air for combustion, it may be possible to achieve the required blast at the proper temperature without a separate air blast; and in the latter case the air lines 24, 25 and 28 may be omitted. With some coating metals such as aluminum the blast may be a steam blast, and it higher temperature blasts are required, the steam may be superheated.
In the production of zinc coatings on steel particularly it is undesirable for many applications to have a large amount of alloying between the coating metal and the metal of the strand. The heat supplied to the blast by the present method and apparatus is only sufficient to prevent the blast from causing the coating metal to freeze and roughen the surface of the exit rolls, but is not sufficient to provide additional heat on the coated metal surface which would promote an alloying action. It is therefore important that the blast be directed primarily at the meniscus and that it not play to any appreciable extent on the coating 16 and 17, nor on the exit rolls 12 and 13.
By following the process disclosed herein and by using the apparatus described, weights of coating on the order of one-half of perviously obtainable weights have been produced. Also, desired weights of coating have been maintained at speeds which previously produced impracticably heavy and non-uniform coatings.
It will be clear that various modifications may be made without departing from the spirit of the invention and no limitation is therefore intended other than such limitations as are set forth in the claims which follow.
What is claimed is:
1. The method of controlling coating thickness in the hot dip coating of a strand of metal with a molten coating metal, wherein the strand issues from the molten coating metal between a pair of exit rolls, and a meniscus of the coating metal is formed between the strand and the nip of the exit rolls by the pumping action of the exit rolls and strand; which includes the steps of directing a blast primarily at said meniscus, said blast not playing to any appreciable extent on the coating nor on the exit rolls, to create a back pressure to said pumping action, and controlling the force of said blast to maintain said meniscus at a level to produce the desired weight of coatmg.
2. The method of claim 1, wherein the blast is a blast of air.
3. The method of claim 2, wherein said blast of air is heated to prevent too rapid a chilling of the coating metal.
4. The method of claim 3, which includes the step of playing a flame on the blast of air to heat the same.
5. The method of claim 1, wherein the blast is a blast of steam.
6. The method of claim 1, wherein the blast is a blast of a gas which is combustible at approximately the temperature of the molten coating metal.
7. An exit machine for use in the hot dip coating of a metallic strand with a molten metal, comprising a pair of exit rolls rotatable on fixed axes, and between which said strand passes in issuing from the molten coating metal, said rolls being partly immersed in said molten coating metal, means for directing a blast primarily at the meniscus of molten coating metal formed by the pumping action of the strand and exit rolls between the strand and the nip of the rolls, and not playing to any appreciab-le extent on the coating nor on the exit rolls, and means for controlling the force of said blast.
8. Apparatus according to claim 7, wherein said strand is a strip and wherein a blast is directed at the meniscus on each side of said strip and wherein separate means are provided for controlling the force of the blast on each side of the strip.
9. Apparatus according to claim 7, wherein the blast 1s air.
10. Apparatus according to claim 7, wherein the blast is air and means are provided to heat said blast.
11. Apparatus according to claim 7, wherein said blast is air and means are provided for directing a flame at the blast.
12. Apparatus according to claim 7, wherein the blast is a blast of steam.
13. Apparatus according to claim 7, wherein the blast is a blast of gas which is combustible at approximately the temperature of the molten coating metal.
References Cited in the file of this patent UNITED STATES PATENTS 811,854 Lee Feb. 6, 1906 1,672,526 Hawkins June 5, 1928 2,034,348 Lytle Mar. 17, 1936 2,135,652 Whitfield et al. Nov. 8, 1938 2,708,171 Inglefield May 10, 1955 FOREIGN PATENTS 352,911 Great Britain Jan. 9, 1930