US3428023A

US3428023A - Quenching zinc metal coatings with atomised water spray

Info

Publication number: US3428023A
Application number: US379977A
Authority: US
Inventors: Arthur Cox; Carl V Gladysz; Arthur G Linke
Original assignee: Dominion Foundries and Steel Ltd
Current assignee: ArcelorMittal Dofasco Inc
Priority date: 1964-07-02
Filing date: 1964-07-02
Publication date: 1969-02-18
Anticipated expiration: 1986-02-18

Description

3,428,023 QUENCHING ZINC METAL COATINGS WITH ATOMISED WATER SPRAY Filed July 2, 1964 A- CQX ET AL Feb. 18, 1969 I of 2 Sheet 4% 5 M h m m 9 m 2 2 5 2 a 2 .r r 2 o m 2 '1 H I H /M l H 0 y 2 m o 2 w. a a m m FIG. 2.

PAT E N T AGEN TS,

Feb. 18, 1969 A. cox ET AL 3,428,023

' Qi ;EEIIi"JHING ZINC METAL COATINGS WITH ATOMISED WATER SPRAY Filed July 2, 1964 Sheet 2 0f 2 Inventors:

ARTHUR COX CARL V. G'LADYSZ ARTHUR G. LINKE F|G.4..

mow zy PATENT AGENTS United States Patent 8 Claims Int. Cl. Bc 9/12; C21d 1/62 ABSTRACT OF THE DISCLOSURE Apparatus for controlling the grain or crystal size of a zinc metal coating on a metal strip quenches the coating with a spray of water atomised and directed by gas under pressure. Uniformity of quenching across the width of the strip is obtained by arranging that the impingement patterns overlap on the strip without being coincident thereon as it moves past the nozzles.

This invention is concerned with a method of, and apparatus for, controlling the grain or crystal size of a metal coating applied to a base, and especially'to such a method and apparatus for controlling the grain or crystal size of a zinc coating applied to a continuous ferrous strip.

In the well-known continuous zinc galvanising processes the protective coating is applied to a ferrous strip by passing the heated strip through a bath of the molten zinc. In one such process the strip issuing from the bath is passed between opposed wiping rolls, which wipe most of the zinc from the surfaces, leaving thereon very thin coatings of the order of 4 inch thick. Such coatings comprise essentially an iron-zinc alloy, and do not provide full protection against corrosion, but the resultant wiped coated surface provides an excellent base for the reception of a coat of paint. If the strip is not wiped the coatings obtained are of the order of 7 inch thick, and the strip is known as fully coated. The solidified coatings present a grain or crystal structure (commonly called spangle and generally so called hereinafter) which can vary from a size which is readily visible to the naked eye to one requiring the use of a low power microscope to be seen. In commercial practice it is desirable to be able to adjust the size of spangle obtained to suit the end-use of the product. For example, with many products intended for the domestic market a large spangle is required, so that the galvanising is seen by the purchaser. With other products, and especially when they are to be painted, it is desirable to have a spangle so small as not to be seen with the naked eye, since large spangles stand out in relief due to variations in the grain or crystal thickness, and this efiect is still evident when the surface has been painted.

It is not usually necessary to provide a spangle greater than that obtained when the strip issuing from the zinc bath cools normally in air. A method commonly used hitherto to obtain a smaller spangle comprises quenching the coatings with jets of steam, or steam and water, after the strip has travelled a predetermined distance from the bath exit. It is essential that the spangle size be relatively uniform across the width of the strip, since otherwise it presents a streaked appearance that renders it inacceptable for commercial use. Such uniformity has been difficult to achieve with the methods and apparatus used hitherto, and relatively large quantities of the expensive end product must be scrapped or designated as seconds.

It is an object of the invention to provide a new apparatus for use in combination with continuous zinc galvanizing equipment for controlling the grain or crystal size of the zinc coatings applied to the ferrous strip.

In accordance with the present invention there is provided apparatus for applying zinc metal coatings to the surfaces of a continuous strip of metal comprising, means for containing a bath of molten zinc, and means for moving the said strip of metal into the bath and vertically out of the bath to coat the surfaces thereof with respective layers of molten zinc, in combination with means for quenching the zinc coatings to control the grain or crystal size thereof, the quenching means for each surface and its zinc coating comprising a respective single row of a plurality of spray nozzles, the said row extending generally horizontally transversely of the direction of movement of the strip, each spray nozzle producing a spray of Water atomised and directed along a respective projection axis by gas under pressure to impinge upon the associated coating. In one such arrangement alternate nozzles of the said row have their projection axes at least approximately parallel to one another, and immediately successive nozzles have their axes inclined at a diiference angle to one another of up to 45 degrees, the arrangement being such that the impingement patterns of the nozzles overlap on the said strip in its movement past the nozzles without being coincident thereon. In another such arrangement there are provided means mounting the said nozzles for rotation about respective axes coincident with their projection axes, and means for rotating the said nozzles to adjust their impinged patterns on the said strip, so that they overlap on said strip in its movement past the nozzles without being coincident thereon.

Specific embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings wherein:

FIGURE 1 is a side elevation of apparatus in accordance with the invention with a part of the structure 3 broken away for clarity of illustration,

FIGURE 2 is a plan view of the apparatus of FIG- URE 1,

FIGURE 3 is an end elevation as seen from the right in FIGURE 1, again with part of the structure 3 broken away, and

FIGURE 4 is a plane end elevation as seen from the left in FIGURE 1.

In the process to be described a continuous strip 1 of thin ferrous sheet material is passed through a bath of molten zinc, from the exit 2 of which bath it is pulled vertically (by means not shown) and passes to a cooling tunnel (not shown) having its entrance disposed vertically above the bath exit. The conditions are arranged to be such that the emerging strip is coated with a layer of liquid zinc of predetermined thickness, usually of the order of 7 inch thick. If maximum spangle is to be obtained then the coatings are allowed to cool by radiation and by their contact with the ambient air, and in a typical installation the spangle Will be apparent by the time that the strip moving at, say, 200-250 feet per minute has travelled about 30 feet. The spangle produced under such conditions typically is about /2 to inch wide, with a variation in thickness of about i% inch.

Apparatus in accordance with the invention comprises a rigid vertically-extending frame 3 providing guide posts 4 and mounting a vertically-extending screw-threaded rod 5 for rotation about its longitudinal axis. The rod 5 can be rotated at will he means of an electric motor 6 connected thereto by a gear 7, rotation of the rod producing vertical movement of a nut 8 guided for such movement by the posts 4. A rigid U-shaped frame 9 is fastened to the nut 8 for movement therewith, and in operation is disposed with its spaced arms 10 extending on opposite sides of the strip 1 and parallel to the plane thereof. Each arm 10 has mounted thereon a respective nozzle carrier frame 11, which is carried by brackets 12 and rollers 13 for movement towards and away from the strip 1. Movement of the frames 11 to adjust their spacing from the strip 1 is produced by manual operation of a chain 14 passing around a pulley 15. The pulley is fast with a shaft 16 having screw-threaded end portions of opposite hand engaged in nuts 17 carried by the respective frame 11.

The front edge of each frame 11 carries two upstanding brackets 18 mounting a rod 19 for rotation about its longitudinal axis. Each rod has attached thereto by respective clamp brackets a single horizontal row comprising a plurality (eleven in this particular embodiment) of airoperated, water-spray nozzles 20. It will be seen that the angle at which each nozzle directs its water spray along a respective projection axis towards the strip 1, relative to a horizontal plane perpendicular to the strip, can be adjusted by rotation of the respective rod 19. Such rotation is produced by manual operation of a respective chain 21 passing around a pulley 22, the resultant rotation of the pulley causing rotation of the rod via a speed-reducing gear 23 that permits fine adjustment of the rods angular position. The gear is mounted on a bracket 24 which is fixed to the frame 11 and which also mounts a dial 25 cooperating with a pointer 26 fixed to the rod 19 to indicate to the operator the said angle of inclination of the projection axes of the nozzles. This angle can be adjusted between approximately i45", and in this specific embodiment is adjustable between :15". It will also be noted that while alternate nozzles along the row are all inclined at the same angle, immediately successive nozzles are inclined at a slightly different angle, so that the corresponding impingement patterns of the nozzles overlap on the said strip without being coincident thereon, thereby producing a completely uniform spray pattern. The difference angle between the projection axes of the two sets of nozzles can have any value from to 45, preferably is between and 25, and in this embodiment is All of the spray nozzles are supplied with operating air via a respective pipe, such as 27, and a respective shut-off cock, such as 28, from a common header pipe 29. Similarly, they are all supplied with control air via a respec tive pipe, such as 30, and a respective cock, such as 31, from a common header 32. The nozzles are supplied with water via respective hoses, such as 33, each of which is connected to a water supply via items which are not shown, comprising a respective shut-off cock, pressure regulator and pressure gauge.

In operation, when the size of spangle required in the finished end-product has been determined, the operator makes a preliminary adjustment of the vertical distance of the nozzles from the zinc bath exit, their horizontal distance from the moving strip, and their inclination to the said horizontal plane. Only the number of nozzles required to adequately cover the full width of the strip are operated, and this number can be preset by operation of the corresponding hand cocks 28 and 31, or it can be changed remotely at any time by cutting off the supply of control air to the nozzles to be put out of action. Water and air are then supplied to the nozzles, and subsequent adjustments made while the apparatus is in operation.

It is at present believed that the problem of a streaked coating arises mainly because the liquid zinc coating moves easily transversely of the strip under comparatively small transverse forces, causing local variations in the coating thickness. A very obvious streak is produced if one of the nozzles is blocked, but they can also be produced if there is an appreciable overlap or underlap between adjacent nozzles, or if there is appreciable variation in the evenness of the spray pattern produced by each nozzle, and they will be produced if two patterns coincide on the strip.

The cooling and crystallising process begins almost immediately after the strip issues from the bath, and coarse control of the spangle is achieved by adjusting the length of cooling time before quenching of the coatings with the water spray, i.e., by adjusting the distance between the bath exit and the line of application of the water sprays.

This distance cannot be too small, or only the strip surface will be cooled and the residual heat of the strip will remelt the zinc, so that substantially maximum spangle will be obtained. Fine control is obtained by adjustment of the nozzle angle, and it is believed that this is due to precooling of the coatings by downward moving spray before they are impinged by the main water spray. The distance of the nozzles from the strip must be adjusted carefully to ensure that the spray patterns from adjacent nozzles do not coincide in their impingement on the strip, although they may overlap so that a part of the moving strip passes successively through the two overlapping patterns. This adjustment also controls the pressure of the spray impingement on the coatings; closer spacing giving higher pressure resulting in faster cooling and smaller spangle. A limit is set to the closeness of the nozzle spacing in that too high pressures cause the production of channels in the liquid coating with resultant streaks.

The spray pattern of the nozzles 20 is generally fanshaped as seen from above or below, giving a horizontallyextending rectangular-shaped impingement area on the moving strip. Adjustment of the nozzles to control the pattern overlap without coincidence can also be done by rotating them about their projection axis, and, for example, the nozzles can be coupled together by suitable gearing so that they are rotated simultaneously by a single control.

Preferably, the following conditions of operations obtain:

The spray nozzles are of the type intended primarily for the spraying of paint, in which external atomisation of the water is effected by the air, the nozzle being specifically designed to produce as uniform a spray as possible over the full width of the spray pattern. Such a nozzle is supplied with operating air at between 10 and pounds per square inch. (p.s.i.), preferably 40 to 60 p.s.i., and at a rate of 10 to 25 cubic feet per minute. The water is supplied at a pressure of 1-40 p.s.i., preferably 20-30 p.s.i. The operating air header 29 is of generous size to ensure uniform pressure is applied to all the nozzles, and this is also the reason for the use of individual hoses 33 to supply the water to the nozzles. In this specific embodiment the nozzles are spaced at 4 /2 inches centres. Although in the embodiments described air is referred to as the atomising and directing medium the component gases of air or other suitable gases can also be used.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for applying zinc metal coatings to the surfaces of a continuous strip of metal comprising means for containing a bath of molten zinc, and means for moving the said strip of metal into the bath and vertically out of the bath to coat the surfaces thereof with respective layers of molten zinc, in combination with means for quenching the zinc coatings to control the grain or crystal size thereof, the quenching means for each surface and its zinc coating comprising a respective single row of a plurality of spray nozzles, the said row extending generally horizontally transversely of the direction of movement of the strip, each spray nozzle producing a spray of water atomised and directed along a respective projection axis by gas under pressure to impinge upon the associated coating, alternate nozzles of the said row having their projection axes at least approximately parallel to one another, and immediately successive nozzles having their axes inclined at a difference angle to one another of up to 45 degrees, the arrangement being such that the impingement patterns of the nozzles overlap on the said strip in its movement past the nozzles without being coincident thereon.

2. Apparatus as claimed in claim 1, wherein the said difference angle is between 5 and 25 degrees.

3. Apparatus as claimed in claim 1, and comprising means mounting the said nozzles for rotation about respective axes coincident with their projection axes, and means for rotating the said nozzles for adjustment of the overlap of their impingement patterns on the said strip,

4. Apparatus as claimed in claim 1, and comprising means mounting the said spray nozzles for movement parallel to the direction of movement of the strip, means mounting the nozzles for movement in directions perpendicular to the plane of the said strip, and means mounting the nozzles for adjustment of the inclination of their projection axes to a horizontal plane which is perpendicular to the plane of the said strip, from an angle degrees above the said horizontal plane to an angle 15 degrees below the horizontal plane.

5. Apparatus as claimed in claim 1, wherein the nozzles are supplied with air at from 40 to 60 pounds per square inch, and at a rate of from 10 to 25 cubic feet per minute, and the water is supplied by individual duct means to each nozzle at from to 30 pounds per square inch. A

6. Apparatus for applying zinc metal coatings to the surfaces of a continuous strip of metal comprising means for containing a bath of molten zinc, and means for moving the said strip of metal into the bath and vertically out of the bath to coat the surfaces thereof with respective layers of molten zinc, in combination with means for quenching the zinc coatings to control the grain or crystal size thereof, the quenching means for each surface and its zinc coating comprising a respective single row of a plurality of spray nozzles, the said row extending generally horizontally transversely of the direction of movement of the strip, each spray nozzle producing a spray of Water atomised and directed along a respective projection axis by gas under pressure to impinge upon the associated coating, means mounting the said nozzles for rotation about respective axes coincident with their projection axes, and means for rotating the said nozzles to adjust their impinged patterns on the said strip so that they overlap on said strip in its movement past the nozzles without being coincident thereon.

7. Apparatus as claimed in claim 6, and comprising means mounting the said spray nozzles for movement parallel to the direction of movement of the strip, means mounting the nozzles for movement in directions perpendicular to the plane of the said strip, and means mounting the nozzles for adjustment of the inclination of their projection axes to a horizontal plane which is perpendicular to the plane of the said strip, from an angle 15 degrees above the said horizontal plane to an angle 15 degrees below the horizontal plane.

8. Apparatus as claimed in claim 6, wherein the nozzles are supplied with air at from to pounds per square inch, and at a rate of from 10 to 25 cubic feet per minute, and the water is supplied by individual duct means to each nozzle at from 20 to 30 pounds per square inch.

References Cited UNITED STATES PATENTS 1,277,986 9/1918 Merrill 118-325 XR 3,189,490 6/1965 Scott 134 3,191,918 6/1965 Kamm 266-3 3,300,198 1/1967 Clumpner et al. 266-6 XR 3,310,295 3/1967 Haak 2666 3,148,080 9/1964 Mayhew 118-69 X FOREIGN PATENTS 1,125,322 3/1962 Germany.

258,633 9/ 1926 Great Britain.