GB2028875A - Production of Minimum Spangle in Galvanised Sheet - Google Patents

Abstract

A matte finish with minimum spangle is formed on hot dip metal coated metal strip by spraying the molten metal coating with a stream of gas containing water particles, the water particles being injected into the gas stream between the gas discharge. nozzle and the coated strip. It is particularly effective in galvanising steel strip.

Description

SPECIFICATION Production of Minimum Spangle in Galvanised Sheet The present invention relates to the surface treatment of a coated metallic strip to provide a matte finish and is particularly concerned with the production of a galvanised steel strip with a coating exhibiting minimum spangle.

It is well known that galvanised coatings invariably have a large crystalline structure the individual crystals being known as "spangles".

This phenomenon is undesirable in many applications. It is undesirable particularly when the galvanised strip is to be coated with another substance, such as paint. Frequently the paint will not spread but will ball up on the crystals and even when the paint will spread the coating so formed is not smooth because of the unevenness of the large spangles.

One common practice to overcome the problem of spangling by producing a smooth galvanised coating involves temper rolling with shot-blasted rolls. A smooth finish is produced by such rolling to which paint will adhere. However, in order to achieve this desired type of finish, the strip must be rolled several times. This is obviously undesirable as it adds further expense to the manufacturing process.

Another common practice is to direct a spray of wet steam at the galvanised surface or surfaces at a point above the galvanising bath from which the strip is moving upwardly and while the coating is still molten so as to freeze the coating instantaneously before a recrystallisation pattern has started to appear. Apart from the high expense involved in following the above practice due to the large energy consumption, the finish is not always of good quality.

It is therefore an object of the present invention to overcome the above disadvantages.

According to one aspect of the present invention there is provided a method for the surface treatment of a molten metallic coating on a continuous metallic strip to produce a matte finish, the method comprising discharging gas from a source for incidence upon the surface of the strip, and, after discharge of the gas from the source, but before it reaches the strip, injecting into the gas a stream of water particles for dispersion within the gas and deposition upon the molten metal.

Preferably the gas comprises two streams arranged to intersect each other as, or just before, they reach the strip. Suitably the water particle stream is arranged to meet the gas streams at their point of intersection. Conveniently the gas water stream extends at least across the width of the strip.

Preferably gas curtains are provided at or near the outer edges of the streams for flow towards the strip and parallel to strip movement so that the gas streams are directionally confined.

Suitably the gas is air and conveniently the water particles have a size ranging between 90 and 150fly Preferably the water particles are arranged to deposit upon the strip at a stage immediately preceding the point where the coating metal solidifies.

Suitably the coating metal is applied to the strip by moving the strip into and out of a bath containing the molten coating metal.

Conveniently the metal coating is a galvanising metal.

According to another aspect of the present invention, there is provided apparatus for the surface treatment of a molten metallic coating on moving mettalic strip to produce a matte finish, the apparatus comprising means for discharging gas for incidence upon the strip and, located externally of the gas discharge means, means for injecting water particles into the gas after discharge so that the particles disperse within the gas before subsequent deposition upon the molten metal.

Preferably the gas discharge means is adapted to provide two gas streams intersecting each other as or just before they reach the strip.

Suitably the gas discharge means comprises a chamber for receiving a flow of the gas, the chamber terminating in two slots effective to discharge the gas as the two intersecting streams.

Conveniently the means for injecting water particles is associated with the chamber and is located at a position intermediate the slots, the injecting means being arranged to inject the particles at the point where the gas streams intersect. The means for injecting water particles may comprise a series of atomising nozzles linearly arranged along the chamber.

Preferably means are provided to enable gas curtains to be formed at or near the outer edges of the streams for flow towards and parallel to the movement of the strip so that the gas streams are directionally confined. Suitably the means for forming gas curtains comprise gaps in the chamber wall at right angles to the slots through which air flowing within the chamber can escape.

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a side view in section of an apparatus for the production of a matte finished galvanised coating in accordance with the present invention and, Figure 2 is a part front view of the apparatus shown in Figure 1.

Referring to the drawings, Figure 1 shows a metal strip 1, in this case steel strip, travelling upwardly from a coating bath (not shown) from which the strip has just emerged. The strip 1 is coated on both sides with a molten layer of a coatant metal, in this case zinc. Just at the point at which the molten zinc is about to solidify, two convergent streams of air, shown by arrows 2 and 3 and emanating from the apparatus 4, impinge on the strip 1. In addition, streams of water particles, shown by the arrows 5, are injected for incorporation into the air streams 2 and 3 preferring at their point of intersection. The particles are thereby dispersed within the air streams and are deposited onto the molten zinc so as to provide a matte finish.While not shown, an apparatus similar to apparatus 4 is located opposite that shown so as to direct streams of air and water particles onto the other side of the strip and thus produce a matte finish on that surface.

The apparatus 4 comprises an elongate chamber 6, which is sealed at one end (not shown) and communicates at its other end with a source of compressed air (not shown). The chamber 6 terminates in two slots 7 and 8 located one above the other. The slots 7 and 8 are respectively defined by pairs of upper lips 9, 10 and lower lips 11, 12. The lips 9 and 12 are integral with the chamber 6, while the lips 10, 11 are welded to either side of an inner plate 13. The ends of lips 10, 11 are chamfered so as to angle the slots 7 and 8 in the appropriate direction whereby the streams of air issuing from the slots 7, 8 intersect before they impinge upon the strip.

Extending linearly along the outside of the chamber 6 and located at spaced intervals are a series of nozzles 14 for atomising water into particles (see Figure 2). The nozzles 14 are made by Delavon Watson under the number C 1580.

Each nozzle 14 projects through a circular aperture 1 5 in an outer plate 1 6 which also extends along the length of the chamber 6. The plate 1 6 is secured at several positions to the chamber 6 by screws 17. As shown in Figure 1, the screw bodies are located in lugs 18, 1 9 extending along the length of the lips 1 0 and 11 to which the lugs 18, 19 are welded.

Referring to Figure 1, each nozzle 14 communicates with a circular manifold 20 by way of an inlet pipe 19, the manifold 20 being sealed at one end (not shown) and communicating at the other end with a supply of water under pressure at the same end of the chamber 6 as the air source. The manifold 20 which is located behind the plate 1 6 is located in several circular clamps 21 (only one shown) at intervals along the manifold 20. The clamps 21 are themselves welded to abutments 22 which are welded to the plate 16.

The nozzles 14 eject water as a flat V-shaped stream of atomised water particles in the direction shown by the arrows 5. The nozzles 14 are arranged so that the streams of particles are injected into the convergent air streams at the point where the air streams intersect. The water particles have a size of about 90y.

While not shown, the lips 9, 10, 11 and 12 are provided with subsidiary slots at or near their outer edges extending at right angles back from their respective main slots 7 and 8 to the wall of the chamber 6. These subsidiary slots enable the air to escape as streams arranged at right angles to the main slots 7 and 8 and to form air curtains confining the flow of the main air streams to that area bounded by the air curtains.

Claims (22)

Claims

1. A method for treating the surface of a molten mettalic coating upon continuous mettalic strip to produce a matte finish comprising discharging gas from a source for incidence upon the surface of the strip, and, after discharge of the gas from the source, but before it reaches the strip, injecting into the gas a stream of water particles for dispersion within the gas and deposition upon the molten metal.

2. A method as claimed in claim 1, wherein two gas streams are discharged to intersect substantially at incidence upon the coating.

3. A method as claimed in claim 1, wherein two gas streams are discharged to intersect prior to incidence upon the coating.

4. A method as claimed in claim 2 or claim 3, wherein the stream of water particles is injected to meet the gas streams substantially at their intersection.

5. A method as claimed in any preceding claim wherein the gas/water streams are incident upon the coating substantially across the width of the strip.

6. A method as claimed in any preceding claim wherein the gas/water streams are incident upon the coating substantially immediately before the coating solidifies.

7. A method as claimed in any preceding claim, wherein the water particles are produced by water atomising nozzles.

8. A method as claimed in any preceding claim wherein the water particle size lies within the range 2Qi-1 50.

9. A method as claimed in any preceding claim wherein the gas is discharged from slots which form the outlet of a gas supply system.

10. A method as claimed in any preceding claim, wherein the gas is air.

11. A method as claimed in any preceding claim wherein the coating metal is zinc and the strip is steel.

12. A method as claimed in claim 11, wherein the steel strip is coated by passing through a bath of molten zinc.

13. Apparatus for treating the surface of a molten mettalic coating upon moving mettalic strip to produce a matte finish, the apparatus comprising means for discharging gas for incidence upon the strip and, located externally of the gas discharge means, means for injecting water particles into the gas after discharge so that the particles disperse within the gas before subsequent deposition upon the molten metal.

14. Apparatus as claimed in claim 13, wherein the gas discharge means comprise two slots forming the outlet of a chamber for receiving gas, the slots being arranged to discharge gas as two intersecting streams.

1 5. Apparatus as claimed in claim 14 wherein the chamber wall incorporates gaps inclined in the slots to permit gas flow through the gaps to provide a gas curtain effective to directionally constrain the gas streams from the nozzles.

16. Apparatus as claimed in any of claims 13 to 1 5 wherein water particles are produced by water atomising nozzles.

17. Apparatus as claimed in any of claims 13 to 1 6 wherein water atomising nozzles are arranged along the width of the strip.

18. Apparatus as claimed in claim 17 wherein an array of nozzles are disposed between two slots effective to discharge gas.

1 9. Apparatus substantially as hereinbefore described with reference to the accompanying drawings.

20. Apparatus substantially as shown in and adapted to operate substantially as herein described with reference to the accompanying drawings.

21. A method for surface treatment substantially as hereinbefore described with reference to the accompanying drawings.

22. Apparatus for performing the method of claim 21.