GB2352731A

GB2352731A - Strip cooling apparatus

Info

Publication number: GB2352731A
Application number: GB9917758A
Authority: GB
Inventors: Daryl Wyn Lewis; Ralph Parker; John Watson
Original assignee: Corus UK Ltd; British Steel PLC
Current assignee: Corus UK Ltd; British Steel PLC
Priority date: 1999-07-29
Filing date: 1999-07-29
Publication date: 2001-02-07
Also published as: AU6172000A; WO2001009397A3; WO2001009397A2; GB9917758D0

Abstract

Apparatus for cooling steel strip 10 in an annealing apparatus comprises a plurality of gas jets 22, 24 and 26 arranged in an array extending transverse to the strip 10. At least some of the jets 24 and 26 are directed towards an edge of the strip 10 at an angle offset from perpendicular to the mean position of the strip 10 of either 2-15 degrees or between 25-75 % of the angle of divergence of the jet. The jets 22 in the central portion of the array may be aimed perpendicularly to the mean web position. The jets may be angled alternately in opposite directions (not shown).

Description

1 2352731 CONTROL OF WEBS The present application relates to control of

webs. It is particularly, but not exclusively, concerned with the stability of elongate metallic webs such as in the continuous annealing of steel strip.

In the continuous annealing of steel strip, the moving strip is cooled at very high rates, typically above 50'c per second. This process is performed by passing the strip between many cold gas jets which impinge on both sides of the strip and remove thermal energy via forced heat transfer. Strip is often in a vertical plane, but this iG not essential although it will be assumed in this application.

The cooling rate can be controlled by the gas flow rates, the thermal capacity of the gas, and the number of jets in operation. The jets issue from nozzles mounted on "blow boxes" which are typically arranged in pairs, one on either side of the strip. Nozzles can simply be straight pipes, or can be more complex designs. Each blow box is typically subdivided into a number of sections, and the flow to each pair and the sections therewithin can be controlled allowing the distribution and rate of cooling to be adjusted to meet the cooling rate required of any particular type of strip.

2 The gap between the strip and nozzles is typically adjustable between 50 and 200 mm, and is usually set somewhere between 50 and 100 mm.

A limiting factor in the cooling rate is that as the volume flow rate of the gas is increased, the strip tends to become unstable in torsion in the area between the blow boxes. This is commonly referred to as "'flutter" and is a torsional vibration about the vertical axis at the centre of the strip. In some cases this can result in displacements at the edges of the strip which are large enough to cause contact between this strip and the ends of the nozzles. This then causes damage to both the strip and the nozzles.

The strip is stabilised by pairs of rolls between each pair of blow boxes. This means that the length of unsupported strip between the rolls is large. For other reasons, the tension in the strip must relatively low. As a result there is little mechanical restraint to limit the amplitude of flutter.

It is desired to increase the potential cooling rate so that the speed of the strip through the blow boxes can be increased, thereby increasing the productivity of tha entire annealing line. At present, the onset of flutter is a limiting factor in doing so.

The present invention therefore comprises apparatus for cooling a web comprising a plurality of gas jets directed towards the surface thereof, the jets being arranged in an array extending transverse to the web, at least a proportion of the jets being aimed towards an edge of the web.

There can be one or more portions within the array, for example at the centre, in which the jets are aimed perpendicularly to the mean web position. In that case, 3 jets in other portions of the array (such as at either edge) are deflected. However, a wide variety of configurations in which the jets are suitably deflected will achieve better control of the strip, for example arrangements in which the jets are all angled in the same direction, all jets are angled alternately in opposite directions, and arrangements in which some or all nozzles are angled in various combinations of directions whilst the remainder are left normal to the strip. Equally, the nozzles can be retained in their normal position, and the strip rotated relative to the nozzles about its vertical axis.

A suitable offset is between 20 and 15'. However, greater offsets are possible although there does tend to be a reduction in cooling power. A particularly preferred range is between 60 and 100.

The jets tend to have an inherent angle of divergence in the gas in which they emit, which depends on the design of the jets. An alternative approach to the angle of deflection is therefore to include an offset between 25% and 75% of that angle 3f divergence, more preferably between 40 and 60% of that diversion. Typically. a figure which is about half that angle is ideal.

An embodiment will now be described, by way of example, with reference to the accompanying figures, in which:

Figure 1 is an illustration of the known arrangement; Figure 2 illustrates an embodiment of the present invention.

4 Referring to figure 1, the steel strip 10 is supported between arrays 12, 14 of gas jets such as that illustrated at 16. Each jet issues a plume of gas 18 which diverges during its travel to strip 10 and then impinges upon a surface of the strip. Heat is then removed from the strip through forced convective transfer.

As illustrated in figure 1 where the strip is normal to the jets, the situation is a case of unstable equilibrium, i.e. due to the symmetry of the configuration there is no tendency for the strip to rotate. However, if the strip is rotated through a small angle, the interaction between the jets and the surface of the strip result in a torque which tends to increase the rotation of the strip. As the angle increases the torque initially increases, reaches a maximum and then decreases, probably eventually reversing in direction and thus tending to restore the angle of the strip towards the normal. From the experimental data available, the angle at which the torque falls to zero prior to reversal is small, of the order of 6 degrees.

This non linear relation between the torque and the angle between the jets and the strip, the inertia of the strip and further torques resulting form the tension and/or stiffness of the strip form a classical case in which self induced oscillations develop with a limiting amplitude related to the actual angle-torque relationship and the magnitude of the maximum torque. The maximum torque is a function of the gas density and the jet velocities. At low velocities, there is insufficient torque at small angles to overcome the natural stiffness of the strip so there is no oscillation, but in most practical applications high gas velocities are required to achieve the required rates of heat transfer and oscillations develop to unacceptable amplitudes.

In the present invention the initial deflection of the jets from the normal is of the same order of magnitude as (or greater than) the angle at which the torque on the strip reverse and becomes a restoring torque and with a proportion of the jets deflected in each direction a stable system is achieved.

Figure 2 illustrates an embodiment of the invention. Within array 14 the central jets 22 remain perpendicular to the strip, but he outermost portion 24, 26 on either side are deflected outwardly by an angle of about 6'. This is repeated within the array 12 on the opposite side of the strip 10. Thus, when the strip is positioned centrally, the outermost sets of nozzles 24,26 will each set up four opposed torques within the strip. A small deflection of the strip will not be sufficient to change the sign of any of these torques, and the closer approach of the strip to some nozzles may compensate at least partly for the change in angle. As a result there will be a range of angles for the strip over which it will remain stable.

It should be emphasised that this explanation as to how instability arises is simply the opinion of the applicant and not susceptible to direct verification. However, the strip 10 has been found to be significantly more stable up to a significantly higher gas velocity if the jets are deflected in the manner described herein..

Another arrangement of nozzles which is effective in reducing flutter is to deflect successive nozzles along the array in alternate directions. A f urther arrangement is for all nozzles in the array to be deflected in the same direction. It would then be advisable to deflect nozzles of a vertically adjacent row in the opposite direction.

It will be appreciated that many variations could be made to the embodiment of Figure 2. For example, all nozzles could be deflected, or fewer nozzles could be deflected.

Claims

1 Apparatus for cooling a web, comprising a plurality of gas jets directed towards the surface thereof, the jets being arranged in an array extending transverse to the web, at least a proportion of the jets being aimed towards an edge of the web.

2. Apparatus according to claim 1 comprising one or more portions within the array in which the jets are aimed perpendicularly to the mean web position.

3. Apparatus according to claim 2 in which the one or more portions are in a central portion of the array.

4. Apparatus according to claim 1 in which jets are angled alternately in opposite directions.

5. Apparatus according to any preceding claim in which those jets which are offset are offset between 20 and 150.

6. Apparatus according to claim 5 in which the offset is between 60 and 10'.

7. Apparatus according to any one of claims 1 to 4 in which the offset is between 25% and 75% of the angle of divergence of gas emitted by the jet.

8. Apparatus according to claim 7 in which the offset is between 40% and 60%.

9. Apparatus according to any preceding claim in which the web is steel sheet.

10. Apparatus substantially as herein described with reference to and/or as illustrated in the accompanying drawings.