The present invention relates to a method for the cold rolling of metallic rolled stock, which for plastic deformation enters a roll gap formed between oppositely rotating rolls on a run-in side and leaves the roll gap on a run-out side, deformation heat that is produced being removed by supplying an industrial gas which is at a lower temperature than the rolled stock.

In cold rolling, a strand - for example in the form of a strip, profile or sheet of steel, nonferrous metals, aluminium or other metals - is continuously fed to a rolling stand and cold-formed in it. By contrast with hot forming, the rolled stock is not heated before the plastic deformation.

The change in shape below the respective recrystallization temperature of the metals brings with it advantageous changes in properties, for example an increase in strength and hardness. However, the deformation energy that is introduced into the rolled stock causes internal friction, and consequently heat, which on the one hand changes the material properties of the rolled stock and on the other hand leads to gradual heating up of the rolling stand, and consequently to changes in the properties of the product, such as the surface and flatness of the rolled strip and oxidation.

In order to avoid this, it is proposed, for example in EP 0 054 172 Al, to separate the working surfaces of the rolls and the rolled stock hydrodynamically by the use of cooling lubricants and at the same time to cool them. To influence the friction coefficient between the surface of the rolled stock and the surfaces of the rolls, in addition to the rolling oil emulsion sprayed into the roll gap, a rolling base oil is applied to the surface of the rolled strip before it runs into the rolling stand, in dependence on the strip tension values measured downstream of the rolling stand. The additional application of a base oil to the rolled strip allows the flatness of the cold rolled strip to be influenced.

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However, it has been found that, in particular, metal fines from the forming process are given off to the lubricants, and that as a result their properties and lubricity are impaired. A further disadvantage of the cooling lubricants is that they partly remain behind on the surface and must be labouriously removed before the further processing of the rolled strip.

Proposed as a solution to this problem in DE 199 53 230 is a method of the generic type mentioned at the beginning, with inert gas in gaseous or liquid form, which is at a lower temperature than the temperature of the rolled stock in the roll gap, being blown into the region of the roll gap from the run-in side and from the run-out side. As a result, the rolled stock passing through the roll gap is locally bathed in inert gas, so that a protective atmosphere is locally formed there, preventing corrosion of the surfaces of the rolled stock and the surfaces of the rolls in the region of the roll gap and at the same time bringing about a reduction in the friction in the roll gap by what is referred to there as "gas cooling lubrication". Nitrogen, noble gases or carbon dioxide are proposed as the inert gas.

By contrast with conventional, liquid cooling lubricants, inert gas for "gas cooling lubrication" offers particularly good protection from oxidation by completely displacing the ambient air and avoids remains of lubricant on the surface of the rolled stock. However, the proposed cooling of the surface of the rolled stock may cause moisture to condense and cause corrosion when it leaves the inert environment, and on the other hand the oxygen content of the ambient air is reduced by the sup- ply of large amounts of inert gas, which can lead to health problems for the personnel working in the vicinity of the rolling stand.

In view of this, the present invention is based on the object of providing a cold rolling method which avoids the aforementioned problems.

On the basis of a method of the generic type mentioned at the beginning, this object is achieved according to the invention by a measured value for the surface

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temperature of at least one of the rolls being determined, and by the supply of industrial gas being set on the basis of the measured value.

The development according to the invention of the known method for "gas cooling lubrication" essentially consists in that the industrial gas is supplied in a controlled manner on the basis of the surface temperature of at least one roll. As a result, with minimal use of industrial gas, the roll temperature is kept constant, that is to say counteracting both gradual heating up of the rolls and gradual cooling down.

Since only as much industrial gas is supplied as is required for removing the de- formation heat in the roll gap and factional heat at the bearings, of the rolls and for setting the temperature of the surfaces of the rolls, the composition of the ambient air is changed as little as possible.

At the same time, the problems that accompany heating up, with regard to corro- sion and changes of the surface of the rolled stock, are avoided in just the same way as condensation on the surfaces of the rolls or rolled stock due to excessive cooling.

The surface temperature is determined at one of the rolls - or at a number of rolls with subsequent averaging. The known temperature measuring methods, in particular optical measuring methods, are suitable for the determination.

Industrial gases that are suitable for "gas cooling lubrication" are inert gases such as nitrogen, argon, carbon dioxide and mixtures thereof - but also air, as explained in more detail further below.

It has proven to be favourable if the measured value for the surface temperature of the roll is determined on the run-in side.

Due to absorption of the heat produced as a result of the plastic deformation of the rolled stock and the friction between the roll and the rolled stock, the surface temperature of the roll increases as it rotates about its axis of rotation from the run-in side to the run-out side. In addition, the roll is heated by the frictional heat pro-

duced at the bearings. In the region of the run-out side, the surface temperature of the roll is therefore at its highest and may have irregular temperature peaks, which in the region of the run-in side are levelled out and not falsified by previously applied industrial gas. Therefore, the surface temperature of the roll in the region of the run-in side is particularly suitable as a measure for determining the necessary amount of industrial gas.

It has also proven successful in this connection if the industrial gas is directed onto the outer cylindrical surface of at least one of the rolls.

By contrast with the known methods explained above, in the case of this modification of the method the industrial gas is not supplied to the surface of the rolled stock or directly to the roll gap, but is applied directly to the surface of the roll. In the case of the method according to the invention, particular attention is given to the setting of the surface temperature of the rolls to a predetermined temperature or within a predetermined temperature range. The direct cooling of the roll ensures that the deformation heat in the roll gap and frictional heat at the bearings of the rolls is completely or largely removed.

With a view to this, it proves to be particularly advantageous to direct the industrial gas onto the outer cylindrical surface of the roll on the run-out side.

As already explained further above, the surface temperature of the roll is at its highest in the region of the run-out side. The industrial gas is therefore used most effectively on the run-out side of the roll, since the temperature difference between the surface of the rolls and the industrial gas that is necessary for heat transfer is at its greatest and, as a result, the required amount of industrial gas can be minimized.

The high-impact application of the industrial gas to the surface of the rolls further improves the effectiveness of the use of the industrial gas, so that, in a preferred variant of the method, the industrial gas is sprayed onto the outer cylindrical surface of the roll.

High-impact spraying of the industrial gas allows the amount of gas required to be further reduced.

A further improvement with regard to the efficiency of the use of gas is achieved if the industrial gas is directed onto the roll in a stream of coolant, the main direction of flow of which is essentially perpendicular to the outer cylindrical surface of the roll.

This measure also makes it easier in particular to set a high impact pulse when spraying the inert gas onto the surface of the rolls.

The industrial gas is supplied in a cryogenic gaseous form, but with preference in a liquid form. In the case of a particularly preferred variant of the method according to the invention, a gas containing carbon dioxide is used as the industrial gas.

Carbon dioxide is inexpensive and can be easily handled.

Particularly easy handling is obtained if the carbon dioxide is kept liquid at ambient temperature in a supply line and is directed onto the outer cylindrical surface on the run-out side of at least one of the rolls as a cryogenic carbon dioxide gas or in the form of a mixture of dry ice and carbon dioxide gas.

The carbon dioxide is kept available in a liquid state in the supply line without complex cooling measures. This storage of the liquid carbon dioxide in the supply line that. is preferred according to the invention requires only moderate pressures even at ambient temperature. When the liquid carbon dioxide expands at ambient pressure, a transformation of the liquid carbon dioxide into solid and gaseous carbon dioxide takes place. The heat of evaporation and sublimation that is necessary for this brings about a cooling down of the carbon dioxide and leads to particularly effective heat removal from the surface of the rolls.

The fact that the carbon dioxide is directed onto the outer cylindrical surface on the run-out side of at least one of the rolls avoids impairments of the surface of the

rolled stock that may occur if dry ice, which forms when liquid carbon dioxide expands at ambient pressure, is drawn into the gap. Small amounts of unused ice sublimate without leaving any residue on the rolled stock on the run-out side of the rolls.

The liquid carbon dioxide supplied via the supply line of the roll is preferably sprayed with high impact onto the surface of the roll that is to be cooled in a gaseous and solid (ice) form via at least one nozzle.

In a further preferred variant of the method according to the invention, an industrial gas which contains liquefied or gaseous cryogenic air is used.

The industrial gas is supplied to the surface to be cooled in a gaseous form, but with preference in liquid form. The use of air for cooling is unproblematical with regard to contamination of the air in the working area, so that even relatively high use of industrial gas can be safely maintained when a particularly low surface temperature of the roll is desired or particularly great amounts of heat are to be removed. It has been found that the proportion of oxygen in the air is not critical with respect to oxidation of the rolls and of the rolled stock when setting a low temperature level.

The control of the supply of industrial gas on the basis of the average surface temperature of the rolls takes place manually or automatically. It is preferred, however, for the surface temperature of the rolls to be kept at a setpoint value by means of an automatic control, the throughflow of the industrial gas or a parameter which can be correlated with it being used as the manipulated variable of the control.

The industrial gas demand required for maintaining the surface temperature is set by means of the automatic control by changes of the mass throughflow or volume throughflow of the industrial gas. The accuracy that can be achieved on the basis of the automatic temperature control allows the use of industrial gas to be minimized further.

It has proven successful if the setpoint value of the surface temperature of the rolls lies in the range between +/- 20% of the maximum temperature of the rolled stock on the run-in side.

The surface temerature of the roll niay be controlled to a constant setpoint temperature value within a tolerance range, which lies between +/- 20% of the maximum temperature of the rolled stock on the run-in side, or it is just ensured that the surface temperature keeps within this tolerance range.

The industrial gas is preferably directed onto the roll along a cooling section, which extends parallel to the longitudinal axis of the rolls.

The cooling section extends over the entire length of the roll or a large part of the entire length and so allows particularly uniform cooling of the surface of the roll and the rolled stock, so that stresses and changes in shape caused by local differences in temperature are reduced. For applying the industrial gas to the roll that is to be cooled along the cooling section, a spraying device is preferably used, arranged along the longitudinal axis of the rolls and on the run-out side of the rolled stock.

The invention is described in more detail below on the basis of exemplary embodiments and a patent drawing in which, as the single figure,

Figure 1 shows in a schematic representation a side view of a rolling stand with coolant nozzles arranged on the run-out side of the rolls for the automatically controlled supply of coolant according to the method as provided by the invention.

Figure 1 schematically shows a cold rolling stand with two rolls 1, 2 arranged one vertically above the other, which form a roll gap 3, through which rolled stock in the form of a metal strip 4 is drawn in the direction of the arrow 9.

Provided on the strip delivery side "B" of the rolling stand are nozzles 5 for the supply of a coolant in liquid form to the surface of the rolls. The nozzles 5 are formed as tubular nozzles which extend along the entire length of both rolls 1, 2. The liquid jet 6 emerging from the nozzles 5 is directed at the surface of the rolls I ₅ 2, its main direction of propagation, which is identified by the dashed line 5a, extending perpendicularly in relation to the outer cylindrical surface 20 of the respective roll 1, 2.

The surface temperature of the rolls 1 and 2 is recorded at a measuring point 11 on the strip entry side "A" by means of the pyrometers 7 and 8. The pyrometers 7 and 8 are each connected to a temperature controller 9 and 10, by means of which the measured value of the surface temperature is recorded and evaluated for controlling the supply of coolant.

For this purpose, pneumatically activated throughflow control valves 14 and 15, which are respectively connected to one of the temperature controllers 9, 10, are fitted in the supply lines for the coolant 12, 13.

For the operation of the cold rolling stand, the rolls 1, 2 are driven in a rotating manner, whereby the rolled stock 4 is drawn through the roll gap 3 while undergoing plastic deformation. The surface temperature of the rolls 1 and 2 is recorded by means of the pyrometers 7, 8 and the measured value is passed on to the temperature controllers 9, 10. The surface temperature of the rolls 1, 2 is controlled by means of the temperature controllers 9, 10 to a predetermined setpoint value, which may lie in the temperature range between minus 20°C and plus 6O ⁰ C. In the exemplary embodiment, the setpoint value is set to 20°C. The manipulated variable for this is the mass flow or volume flow of the coolant sprayed onto the rolls 1, 2, which is supplied to the nozzles 5 via the control valves 14, 15.

Liquid carbon dioxide is used as the coolant, fed into the supply lines 12, 13 from a liquefied gas tank 18 via a feed line 16 which can be closed by means of a ball- cock 17, the carbon dioxide being located in the supply lines 12, 13 in a liquid state of aggregation in the range from 252 to 304 K at a pressure of 19 to 73 bar.

Preferably, the carbon dioxide is supplied in a liquid form at about 292 K and 56 bar. When it leaves the nozzles 5, gaseous carbon dioxide and dry ice form, impinging with high impact on the surface of the rolls. Evaporation and sublimation of the carbon dioxide has the result that the cooling of the rolls 1, 2 takes place very effectively.