This invention relates to a method and device for cooling of rolling mill work rolls, in particular for cryogenic cooling in aluminium rolling.

In order to provide flatness control across the width of the rolled strip it is necessary to be able to control the cooling across the width of the roll.

Examples of methods of varying heating and/or cooling across the width of the work rolls are described in GB2012198 where an array of adjustable liquid jets are provided. The majority of existing systems use kerosene or water based coolants, but GB2466458 describes a system which uses cryogenic coolant.

Whilst it is possible to use conventional valves, similar to those used for kerosene or water based coolants, with cryogenic coolants in practise there are many difficulties involved.

The objective of the invention is to provide for a simpler and more reliable method of controlling the cooling when using cryogenic coolants.

In accordance with a first aspect of the present invention a method of controlling a profile of a surface of one or more work rolls of a rolling mill comprises supplying a cryogenic coolant to a nozzle at a substantially constant flow rate; and controlling a gas supply to supply gas to the cryogenic coolant, whereby if the cooling power provided by the coolant at the surface of the work roll is greater than the cooling power required, controlling the gas supply to the coolant, such that liquid coolant is atomised by the gas before being sprayed from the nozzle onto the surface of the work roll; and if the cooling power provided by the coolant at the surface of the work roll is less than the cooling power required, controlling the gas supply, such that the coolant is supplied from the nozzle as a liquid to the surface of the work rolls.

If the cooling power is correct, no changes are made. Otherwise, when substantial cooling is required, an un-atomised liquid jet is sprayed directly from the fluid outlet onto the work roll, but when little or no cooling is required to maintain the work roll within a desired temperature range, the gas supply is switched on and atomises the liquid at the fluid outlet, so that an atomised spray is sprayed onto the work roll. The liquid flow to each of the nozzles in a zone remains substantially the same, but the cooling effect of an atomised spray from a nozzle is significantly less than that of an un-atomised liquid jet. Preferably, the method further comprises comparing data relating to the work roll profile with reference data; and adjusting the gas supply according to the result of the comparison.

The effect of operating conditions on the work roll profile may be controlled in comparison with reference data to allow the cooling power to be adjusted as necessary.

Preferably, the gas supply is adjusted, only when the result of the comparison differs by more than a predetermined range.

Preferably, the data comprises rolled strip flatness measurements.

Preferably, the method comprises deriving data relating to the work roll profile using feedback from a strip flatness sensor and flatness control system in a controller; and thereafter controlling the gas supply to the coolant.

In one embodiment, initially 100% of coolant supplied to the surface of the one or more work rolls is atomised.

Preferably, the method further comprises controlling a mark-space ratio of a control mechanism of the gas supply to adjust the cooling according to the cooling requirement.

In one embodiment, the coolant supply is divided into zones, each zone having multiple nozzles and a common gas supply, whereby all nozzles in each zone provide coolant to the workroll in the same manner.

Using a common gas supply to a plurality of nozzles in a zone ensures that the coolant supplied from all the nozzles in that zone is in the form of an atomised spray, or remains as liquid coolant, rather than some nozzles in a zone supplying liquid and other nozzles in the same zone supplying an atomised spray. This gives better profile control.

Alternatively, the coolant supply is divided into zones, each zone having multiple nozzles and two or more gas supplies, whereby, within a zone coolant from at least one nozzle is atomized.

Using multiple gas supplies in a single zone gives the option of atomising coolant from some, but not all of the nozzles. The advantage of this is that it allows fine tuning if a very small region of the workroll within that zone needs a reduced amount of cooling to improve the profile.

Preferably, the cryogenic coolant comprises nitrogen. In accordance with a second aspect of the present invention, a cooling device for one or more work rolls of a rolling mill comprises a nozzle to receive a cryogenic coolant; a fluid outlet from the nozzle for supplying the cryogenic coolant to a surface of the work roll; a gas supply; and a controller for controlling supply of gas from the gas supply to the cryogenic coolant, whereby the device is adapted to supply a liquid directly to the surface of the work roll, or to spray an atomised liquid at the surface of the work roll.

Preferably, the device further comprises at least one sensor for providing feedback to the controller of the effect of the cooling power at the surface of the work roll.

Preferably, the sensor comprises a strip flatness measuring device.

Preferably, the cryogenic coolant comprises liquid nitrogen.

An example of a method and apparatus according to the present invention will now be described with reference to the accompanying drawings in which:

Figure 1 illustrates a conventional spray nozzle to supply coolant to the surface of a work roll;

Figure 2 illustrates an arrangement in which a multiple nozzle header may be used with a cryogenic valve and source of cryogenic liquid;

Figure 3 illustrates a first example of a cooling device according to the present invention;

Figure 4 illustrates a second example of a cooling device according to the present invention;

Figure 5 shows how the temperature of a rotating steel work roll changes with time using the cooling device of Fig.3; and,

Figure 6 illustrates the cooling device of Fig.3 and 4 in situ with work rolls of a rolling mill.

Fig.1 illustrates a conventional spray system for cooling a work roll in an aluminium rolling mill, e.g. using either kerosene or cryogenic liquid as the coolant. A steel work roll 1 is mounted on an axis 2 for rotation in anti-clock wise direction 3 or clockwise direction 4. A flow of liquid from a supply header 5 which receives the liquid via pipes and valves (not shown), passes through a valve 10 and into a spray nozzle header 9 and then through nozzles 6 which produce jets 7 which spray the liquid onto a surface 8 of the work roll. The supply header 5 usually supplies all of the zones across the width of the roll and there is at least one valve 10, zone header 9 and nozzle 6 for each cooling zone. The jets 7 may be flat jet type, or cone type, or column jets.

The valve 10 may be either a proportional type valve such as the valve described in US4081141 which adjusts the flow over a range of values, or the valve could be an on-off type valve.

One problem with proportional type flow control valves is that as the flow is reduced the pressure on the outlet side of the valve is also reduced. The change in pressure alters the spray pattern 7 and this causes problems with controlling the cooling particularly at low flows where the spray pattern 7 may not contact the roll surface 8 properly. An additional problem with cryogenic coolants is that the pressure drop across a proportional valve tends to create a lot of gas which changes the cooling effect. For this reason most systems use on-off valves because the spray pattern when the spray is switched on is more consistent. A common method of controlling the cooling using on-off valves is to pulse the sprays and to control the mark-space ratio as described in GB2156255. An alternative method is to have several valves within each zone where each valve supplies a different sized nozzle as described in GB2012198. Different combinations of nozzles can therefore be selected to adjust the total cooling flow.

However, for flatness control purposes when using cryogenic fluids, a significant issue is the large number of cycles of spraying that is required by these valves to provide a continuous operation and flatness control. Valves which are used in the water or kerosene based cooling systems are unsuitable for use with cryogenic liquids because the valves need to be able to operate with cryogenic fluids at very low temperatures and achieve a high number of cycles. The fatigue life of materials at very low temperatures results in the valves failing in a relatively short space of time.

US20080048047 describes a method of controlling the flow rate of a cryogenic liquid through a cryogenic nozzle in which a throttling gas at a pressure greater than or equal to the pressure of the cryogenic liquid is used to control the cryogenic liquid flow rate. An increase in gas pressure reduces the mass flow rate of the liquid. In a spray tube with multiple nozzles, the liquid flow rate from nozzles near the centre of the tube may be greater than the liquid flow rate from nozzles at the ends.

However, the inventors have found that even with a valve which is capable of operating at low temperatures and for high numbers of cycles, part of the problem is caused by the fact that when using a nozzle header with a plurality of outlets in an array, the plane of which is substantially perpendicular to an axis of a pipe supplying the liquid cryogen, there is inconsistency in the spray pattern. An example of this is shown in Fig.2. A source of liquid cryogen, such as nitrogen, is supplied from supply header 5 through a valve 10 and pipe 12 to a spray nozzle header 9. In this example, there is a single pipe 12 running from the source of the cryogenic liquid to the back of the header 9. When the valve 10, which is positioned between the supply header 5 and the spray nozzle header 9, is open liquid flows through the valve and the pipe 12 and out of the header through a plurality of nozzles 6a, 6b evenly to give a desired spray pattern on the surface 8 of the work roll 1. However, when the valve is switched off, there is still some cryogenic liquid 14 remaining in the spray nozzle header 9. As the cryogenic liquid in the spray nozzle header 9 warms up it vaporises and creates pressure in the spray nozzle header, even though the valve 10 is closed. The gas 13 collects in the upper part of the header, whilst the boiling liquid 14 remains in the lower part of the header, so gas emerges through the upper nozzles 6a and liquid from the lower nozzles 6b. The consequence of this is that the total cooling effect of the sprays 7a, 7b is both unpredictable and slow to respond to the operation of the valve 10.

Thus, a new design of cooling device is proposed in which it is possible to switch between levels of cooling of the surface of the work roll, without needing to use a valve which suffers from fatigue at cryogenic temperatures, or which causes the nozzle header to provide an uneven spray pattern when the coolant is switched off.

A first embodiment of the present invention is illustrated in Fig.3 in which a coolant comprising cryogenic liquid 20 from a cryogenic header 21 passes through an orifice 22 in an atomising nozzle 23. The preferred option is to use nitrogen as the cryogenic liquid. The coolant is supplied to the surface 8 of a work roll 1, either as a liquid jet, or else the liquid is atomised before reaching the surface of the work roll, i.e. the liquid jet is broken up into droplets. The size of the droplets so formed by atomising depends upon the velocity of the gas supplied. When the gas supply 24 is switched off the orifice 22 produces a conventional liquid spray pattern on the roll surface 8, such as a column type jet or a cone type jet. When it is desired to reduce the cooling power at the work roll surface produced by the spray in a particular zone, a gas supply is switched on and gas passes through inlets 29 to form a gas spray 25 which comes into contact 26 with the cryogenic liquid 20 as it emerges from the orifice 22. The gas supply and the valves which switch the gas supply on or off are not at cryogenic temperatures. The effect of this is to atomise the liquid and an atomised spray 27 is sprayed onto the surface 8 of the work roll. This atomised spray has a lesser cooling effect than the direct contact of liquid with the surface which is produced by the conventional liquid spray pattern when the gas supply 24 is off. The Leidenfrost effect causes the atomised liquid droplets coming into contact with a much hotter surface to form a layer of vaporised gas between the surface and the droplets, reducing the cooling effect of the liquid droplets. By contrast a jet of cryogenic liquid is sufficiently powerful to pass through the vaporised gas layer. By maintaining the liquid flow rate through the header 21 and orifices 22 substantially constant at all times and changing the form in which the coolant reaches the roll from liquid to atomised spray, there is no risk of getting unpredictable and variable cooling because the output of the nozzles in the zone is either all liquid, or all atomised spray, rather than the variable and partial result as described with respect to Fig.2.

A second embodiment of the present invention is illustrated in Fig.4. As before, the coolant comprising cryogenic liquid 20 from a cryogenic header 21 passes through an orifice 22 in an atomising nozzle 23. When the gas supply 24 is not switched on, the orifice 22 produces a conventional liquid spray pattern on the work roll surface 8, such as a column type jet or a cone type jet. When it is desired to reduce the cooling effect of the spray in a particular zone, the gas supply is switched on and gas 24 passes through inlets 29 to exits 30 leading into the orifice 22, so that the gas comes into contact with the liquid 20 which has passed through a first part of the orifice 22 to reach that point 30. In a second part of the orifice 22 after that point, an atomised spray 27 is sprayed through and out of the nozzle 23 onto the surface 8 of the work roll. This atomised spray has a lesser cooling effect than the direct contact of liquid with the surface which is produced by the conventional liquid spray pattern when the gas supply 24 is off, as explained above.

In both embodiments, when the gas is switched off for nozzles in a zone, then the cryogenic liquid is supplied directly onto the roll and this produces a high heat transfer. However, when the gas is switched on, the liquid is atomised and the spray of atomised liquid has a significantly reduced cooling power. In some cases, the cooling effect of the atomised spray may be close to zero. Thus, turning the gas on and off has the effect of switching the cooling power between high (directly liquid jet) and low (atomised spray), but the liquid flow through the nozzles in the zone remains substantially the same, with no need for separate control of the liquid flow in each zone. Thus, the cooling power may still be controlled by zone to get the desired shape control of the work roll, by switching the gas supply on or off in a particular zone, according to whether more or less cooling is required there, but there is no change in liquid flow, it is consistent across all zones, whether higher or lower cooling effect is required. Switching the control gas is straightforward and may be implemented with conventional valves capable of operating with a high number of cycles such as a diaphragm type valve,. Each zone has a separate control valve, or valves, to allow the appropriate control of cooling effectiveness in each zone. Using a non-cryogenic gas to atomise the cryogenic liquid, conventional valves can be used to switch the gas supply on and off, so the cost and reliability issues of using valves rated for cryogenic conditions is avoided. The inlets 29 are typically angled, so that the gas enters the orifice 22 or the exit of the nozzle (depending upon the embodiment) and mixes with the liquid flow at an angle of less than 90 degrees, more typically at between 40 and 50 degrees to the longitudinal axis of the orifice 22, to atomise the liquid. The controller may store predetermined mark space ratios for the gas supply valves to produce specified amounts of cooling power.

The mixing of the gas and liquid in the atomising nozzles may take place internally, or externally as illustrated in Figs.3 and 4. Usually, the gas is supplied at a higher pressure than the liquid. By changing the relative values of pressure, flow and temperature of the gas and liquid the effectiveness of the cooling is defined.

Fig.5 illustrates the effect on roll temperature of switching the control gas supply on and off, thereby changing the cooling efficiency of the cryogenic liquid applied to a rotating steel work roll, similar to the ones used in the rolling of aluminium. The temperature is the temperature of a rotating roll in Celsius, with samples taken at regular time intervals. Ideally the gas pressure is much higher than the liquid pressure, but this need not always be the case. According to the choice of liquid, the gas pressure may be anything from 4 to 14 times the liquid pressure. In the invention, the gas supply may be only 0.7 to 0.9 times that of the liquid. Higher gas pressure relative to the liquid pressure allows the rolls to warm up more quickly. For test purposes, the roll was heated internally to a representative temperature and a cryogenic liquid was sprayed onto the surface using an atomising nozzle, but with the gas supply 24 switched off. Initially the temperature is constant as there is a steady state. For this example, the temperature ranges between 72. OC and 72.2C. Once the gas supply is switched on at the point labelled 41, then the temperature rises to 73 C because there is less cooling effect on the surface of the work roll. At the point labelled 42 the gas supply is switched off. The temperature drops as the cooling efficiency of the liquid has increased. At the point labelled 43 the gas supply 24 is turned back on, the cooling efficiency reduces and the temperature rises again.

Fig.6 illustrates use of atomising nozzles of the present invention, such as the embodiments of Figs.3 and 4, in a rolling mill. A metal strip 50 to be rolled, passes through the nip of work rolls 8, which have corresponding back-up rolls 52. The rolled strip then passes over guide rolls 53. A cooling system comprising atomising nozzles 23 is supplied with cryogenic liquid from a store 54 controlled by a valve 56 set on a constant setting, via cryogenic header 21 and pipes 22. The cooling may be provide both above and below the strip 50. A gas store 57 supplies gas 24 to the nozzles 23 through individual control valves 58 under control of controller 59 via control line 63. Each header has multiple nozzles 23, a control valve 58 per nozzle and a single gas supply 57 per header 21. Each nozzle sprays only a single zone of the work roll 8 The controller may receive data from one or both of an operator via input 60, or feedback via line 61 from a shapemeter 62 and uses this to adjust the gas flow and hence the cooling effect of the sprays from the nozzles 23 onto the work roll.

The cooling device of the present invention not only controls whether liquid, or atomised spray, or even only control gas at sufficiently high pressure, reaches the work roll, but also allows liquid droplet size to be controlled, so that finer cooling control is possible than in prior art systems. By adjusting the pressure of the gas supply which interacts with the cryogenic liquid, the size of droplets formed during atomising may be varied. The smaller the droplets so formed, the less the cooling power at the work roll surface.