The invention is concerned with the descaling of metal slabs and ingots prior to rolling.

In general, plate mills have a descaling system for removing scale from the slabs before they are rolled and in most modern mills high pressure water jets are used for this descaling.

A cross section of a typical modern descaler is illustrated in Figure 1. The descaler contains two top headers - 1 and 2 - and two bottom headers - 3 and 4. For descaling, the slabs are transported by the rollers 5 in the direction indicated by the arrow 6 so that the slabs pass through the descaler. While many descalers have two sets of headers, as illustrated in Figure 1 , normally only one set (i.e. one top header and one bottom header) is used at a time and the other set is reserved as a standby set. It is quite common for descaling nozzles to become blocked and if this happens then the mill switches to the standby set of headers until such a time as it is convenient to carry out maintenance on the blocked nozzles.

Figure 2 illustrates one of the top headers 1 , 2. The high pressure descaling water, typically at around 180 Bar pressure or more, enters the header 1 through - in this design - pipes at both ends 9. The descaling jets 8 spray this high pressure water onto the slab.

In order to descale the surface of the slab properly with high pressure water it is important that the impact pressure of the water on the slab surface is high enough. In order to achieve this most descaling systems use so-called 'flat fan' type nozzles as illustrated in Figure 3. These produce a fan shaped jet which is very narrow in one plane but has a fan shape - angle α in Figure 3 - in the other plane. In order to achieve sufficient impact pressure for good descaling the parameters that the designer can change include: -

• Supply pressure • Size of nozzle / Nozzle angle α / Flow Rate. For a given nozzle size the flow rate is proportional to the square root of the supply pressure - so for a given nozzle size and supply pressure the flow rate is effectively set as well.

• The 'standoff' distance - h2 in Figure 3. The shorter the standoff distance the higher the impact pressure but the more nozzles are required to cover the full width of the slab.

Other important parameters include the angle of inclination of the nozzle from the vertical - β in Figure 3 - and the offset angle γ_ The angle of inclination ensures that the high pressure water and scale bouncing back from the surface of the slab does not interfere with the direct jet whilst the offset angle ensures that neighbouring jets do not interfere with each other. Generally, for a given 'standoff distance h2, the spacing between the nozzles E is chosen to give a small overlap D between adjacent nozzles. This small overlap ensures that the full surface of the slab is descaled even if the setting of the header height is slightly wrong for the thickness of the slab. The total flow for the header is obviously the number of nozzles times the flow for each nozzle.

It will be obvious from Figure 3 that during operation of the descaler the actual standoff distance h2 must be close to the design figure for h2. If the actual standoff distance is greater than the design figure then the impact pressure of the jets will be reduced and descaling will not be as effective. If the actual standoff distance is significantly less than the design figure then the jets will no longer overlap and the slab would have stripes of scale left along it. Most plate mills use a variety of slab thicknesses and therefore the top headers can usually be adjusted for height using screwjacks, hydraulic cylinders or other actuators. The control system sets the correct header height for a particular slab before the slab enters the descaler so that the standoff h2 is approximately the same whatever the slab thickness. Obviously the bottom headers do not need to be moved because the bottom surface of the slab is always in the same place - on top of the rollers 5.

In recent years there has been a trend towards producing very thick plates from ingots instead of from continuously cast slabs.

The problem with existing designs of descaler comes when they are used for ingots. Ingots 10 have a trapezoidal shape as illustrated in Figure 4. On a large ingot the difference in thickness between the thinner end and the thicker end of the ingot can be up to around 80 mm or more. Since the standoff distance h2 on a typical descaler is around 150 mm then it is not possible to descale the full length of the ingot properly with the header of a conventional descaler set at a fixed height. If the header height is set correctly for the thick end of the ingot then the impact pressure at the thin end would be too low for good descaling. If the header height was set for the thin end of the ingot then the jets will not overlap properly at the thick end and stripes of scale would remain.

One solution to this problem is to adjust the header height along the length of the ingot as it passes through the descaler. However, in practise, this is quite difficult to do. First of all there is the problem of tracking exactly where the ingot is within the descaler in order to synchronize the movement of the headers. Then there is the difficulty of moving the headers fast enough. Descaling normally takes place at around 1 m/sec speed so the whole process only takes a few seconds. To move the header 80 mm in this short time would require very fast movement. Another problem is that on some descaler designs it is not possible to move the headers when the descaling flow is switched on because they use telescopic feed units which effectively lock up when the pressure is applied.

Another solution to this problem is to allow the ingot itself to push the top header up to the correct height. Existing designs often have protection plates 1 1 which prevent the slabs or ingots from hitting the nozzles if the header height is not set correctly. It would be possible to design these protection plates in such a way that the protection plate and the top header were pushed up by the ingot as soon as a minimum standoff distance was reached. The problem with this solution is that the protection plates would wear very rapidly because they would be in contact with all the ingots and, possible, many of the slabs as well.

The objective of the invention is to descale ingots along their full length without the problems discussed in the prior art.

According to a first aspect of the invention, metal descaling apparatus comprises the features set out in claim 1 attached hereto. Although there is, in principle, a large number of possible arrangements for the nozzles, they may conveniently be arranged in linear arrays. More conveniently, the arrays are arranged orthogonal to the datum parallel to the direction of movement of the metal and in a particularly preferred embodiment, the nozzles are equally spaced along the array.

According to a second aspect of the invention, a method of descaling metal comprises the features set out in claim 5 attached hereto.

In one preferred method according to the invention, a metal slab of substantially constant thickness is passed below the arrays with at least one array being employed for descaling. The one array is arranged at a sufficient distance above the metal to allow for partial overlap of the descaling fluid sprays on the metal surface.

In another preferred embodiment, according to the invention, a metal slab of substantially constant thickness is passed below the arrays with both arrays being employed for descaling. The arrays are arranged to ensure coverage of the metal surface at every distance from the datum, by the spray from at least one nozzle.

In another preferred embodiment, descaling fluid is directed through the nozzles in both arrays while passing a metal slab of varying thickness therebelow. The arrays are arranged within a range of distances above the metal whereby partial overlap of the descaling fluid sprays from one array is ensured on the metal surface in the thinnest region of the metal, and coverage of the metal surface at every distance from the datum, by the spray from at least one nozzle, is ensured in the thickest region of the metal. The invention will now be described, by non-limiting example, with reference to the following figures in which:

figure 1 illustrates a cross section of a typical modern descaler;

figure 2 illustrates one of the top headers of figure 1 ;

figure 3 illustrates the 'flat fan' type nozzles typically used for descaling metal in modern plate mills;

figure 4 illustrates the typical tapered, trapezoidal shape of metal ingots used in modern plate mills;

figure 5 shows the spray pattern for one of the headers according to the invention at the normal standoff distance of 150 mm;

figure 6 shows the spray pattern for the same header at a reduced standoff distance of 70 mm and

figure 7 shows the spray pattern obtained from the combined headers according to the invention.

Referring to figure 8, in a particular preferred embodiment, the invention is realised by arranging two arrays of nozzles 17 (which are similar in terms of size, number and spacing of nozzles) on headers 1 , 2 and offsetting the headers relative to each other. Datum 7 is parallel to the direction 6 in which the metal passes below the nozzles and the effect of this arrangement is that each of the nozzles 17 on header 2 are each located at a different distance from the datum than any of the nozzles on header 1 . Thus, the sprays produced by header 2 are centered on different longitudinal lines than any of the sprays produced by header 1. At a 'normal' standoff distance, the arrays are arranged such that the spray patterns from one set of nozzles overlap and one or both arrays descale the full width of the metal. (As previously noted, this 'normal' distance depends on a number of parameters but typically may be in the region of 150mm. At a 'minimum' standoff distance, for example when the arrays are above the thicker end of an ingot, the two arrays descale different, but overlapping, regions of the metal. Typically, the minimum standoff distance may be in the region of 70mm.

The spray patterns of the two headers interlace with each other and both headers are used at the same time when descaling metal of varying thickness such as ingots.

Referring to figure 5, it can be seen that at the normal standoff distance of 150 mm, there is a small overlap between each jet and the impact pressure is satisfactory for descaling.

Figure 6 shows that, at a reduced standoff distance of 70 mm, there are obvious gaps between each of the jets. This is the standoff distance that would be achieved on the thick end of an ingot if the header height was set for the thin end of the ingot and the ingot was 80 mm thicker at the thick end.

However, when descaling ingots, the second descaling header is also used. The second header is arranged so that its jets interlace with the jets of the first header so that the whole surface of the ingot gets descaled as illustrated in Figure 7. In Figure 7, spray pattern 12 is the spray pattern that the first header makes on the thin end of the ingot at 150 mm standoff. The whole width of the ingot is descaled because each jet overlaps slightly with its neighbour. Spray pattern 13 is the spray pattern that the second header makes on the thin end of the ingot at 150 mm standoff. Again the whole width is descaled. The thin end of the ingot therefore gets descaled twice.

Spray pattern 14 is the spray pattern which the first header makes on the thick end of the ingot at 70 mm standoff. Due to the reduced standoff distance there are gaps between the jets 16 and the whole width is not descaled. Spray pattern 15 is the spray pattern which the second header makes on the thick end of the ingot at 70 mm standoff. However the jets of the second header are deliberately offset relative to the first header so that spray pattern 15 fills in the gaps 16 in spray pattern 14 and hence the whole width of the ingot is descaled.

Clearly at points between the minimum thickness and the maximum thickness the gaps between the jets 16 will be smaller and there will be more overlap of patterns 14 and 15. However, providing that the impact pressure at the maximum standoff is sufficient for good descaling then the overlap of patterns 14 and 15 is acceptable.

Another advantage of the invention is that for ordinary slabs which are difficult to descale (e.g. because of their particular metallurgy) the operator could decide to use both headers together and to set the standoff distance to (e.g.) 70 mm instead of 150 mm. This would give very much higher impact pressure and hence better descaling and the offset of the second header relative to the first would ensure that the whole width would still get descaled at the shorter standoff distance.