EP0736350B1 - Method and device for controlling crown adjustment on rolls in a metal strip casting machine - Google Patents

Abstract

Casting device for a metal strip (9) where the solidification is effected by passing the molten metal between two counter rotating rolls (1,1') on horizontal axes. The rolls (1,1') are cooled by the internal circulation of a cooling fluid and the roll gap defines the casting space (2) for the molten metal. The outer surfaces (3,3') of the rolls have a specified roughness. The casting space (2) is rendered inert by blowing a given quantity of a gas or gas mixture across a cover (10) covering the casting space (2). The camber of the rolls (1,1') is regulated by changing the quantity of injected gas and/or the nature of the gas or the composition of the gas mixture, at least around the surface of each roll upstream of the contact zone with the molten metal. Also claimed is device (14,15,16) for blowing the gas and a system (7,18,19,20,21,22,22',23,23',24,24') for modulating the quantity of gas injected and/or the nature of the gas or the composition of the gas mixture. The gas mixture used may be a mixture of nitrogen and argon.

Description

The invention relates to the casting of thin metallurgical products obtained directly from liquid metal. More specifically, it relates to installations for casting thin strips, in particular steel, by solidifying the liquid metal against two close cylinders with horizontal axes, rotated in opposite directions and internally cooled.

Such installations are described, in particular, in documents WO-A-9402269 and EP-A-0409645. This latter document forms the basis of the preamble to the independent claims. It shows a casting between cylinders of thin strips of steel, provided with a cover covering the casting space and means for modulating the composition and / or the flow rate of the gas inerting the casting space. This modulation is used to control the surface quality of the tape.

In installations for casting thin steel strips between two counter-rotating cylinders, the thickness profile of the strip depends closely on the shape of the external surfaces of the cylinders in the casting space. Ideally, this profile of the strip should be rectangular or slightly convex to ensure the smooth running of the cold rolling step and a satisfactory regularity of the thickness of the final product. For this purpose, the generatrices of each cylinder should remain rectilinear or slightly concave, in particular at the level of the neck, ie of the region of the casting space where the cylinders are closest to each other. other. In practice, this is not the case, due to the intense thermal stresses to which the cylinders are subjected. Thus a cylinder which, when cold, would have a perfectly rectilinear generator, would see, under the effect of expansion, its outer surface become convex. The thickness profile of the solidified strip being the faithful reproduction of the section of the casting space at the neck, we would obtain a strip whose thickness would increase significantly and gradually from the center to the edges. This would be detrimental to the smooth running of the cold rolling of the strip and to the quality of the products which would result therefrom.

This is why this expansion is usually anticipated by giving the exterior surface of each cylinder a slightly concave profile, having a "domed" in the center of the cylinder, that is to say a difference in radius relative to the edges. The optimum value of this cold convex varies according to the dimensions of the cylinder, and can be, for example, around 0.5 mm. In this way, during the expansion of the cylinder, there is a reduction in this convexity, and the profile of the cylinder in the casting space tends to approach a rectilinear profile. The value of this convex during casting depends on the materials of the cylinders and the cooling system of the cooled ferrule which constitutes the periphery of the cylinder, of the geometry of this ferrule, and also of its method of attachment to the core of the cylinder, which can allow greater or lesser expansion of the ferrule. But it also depends on operating conditions which can vary from one casting to another, or even also during the same casting, such as the height of liquid metal present in the casting space and the intensity of the heat flow extracted from the metal. by the cylinder cooling means.

It would be important to have means giving the operator in charge of the operation of the casting machine the possibility of acting to a certain extent on the crown of the cylinders, so as to permanently obtain an optimal crown regardless of the casting conditions. and their variations. In addition, this would avoid having to use separate pairs of cylinders, each having a different initial curvature, for each grade that one wishes to pour under optimal conditions.

One way of regulating this bulge could consist in modulating the flow of heat extracted from the metal by acting on the flow rate of the cooling water which circulates inside the shell of each cylinder. In fact, the variations in the bulge that could be obtained by this means alone would be minimal, of the order of a few 1/100 of mm. The reason is that this water flow only tolerates being modified in small proportions compared to the maximum admissible flow, under penalty of deteriorating too appreciably the conditions in which the heat transfers take place between the shell and the 'water. It would then no longer be possible to satisfactorily control the conditions for solidification of the metal.

Document JP-A-58-23549 shows an installation for casting a thin strip by depositing liquid metal on a single cylinder. The cylinder is cooled by sending on its surface, upstream of the casting zone, a fluid whose flow can be modulated along the width of the cylinder, so as to control the convexity of the cylinder. Data collected by thickness gauges measuring the thickness of the strip cast at different points are used for this purpose.

The object of the invention is to provide operators with a means enabling them to adjust the convexity of the cylinders during casting with sufficient latitude.

To this end, the subject of the invention is a process for casting a metal strip, in particular steel, according to which the said strip is solidified by adding liquid metal between two counter-rotating cylinders with horizontal axes cooled by internal circulation. of a cooling fluid, defining between them a pouring space, and the external surfaces of which have a roughness, and an inerting of said pouring space is carried out by insufflation of a given quantity of a gas or of a mixture of gases through a cover covering said pouring space,
characterized in that the convexity of said cylinders is adjusted by modulating the quantity supplied and / or the nature of said gas or the composition of said gas mixture, at least in the vicinity of the surface of each cylinder upstream from its contact zone with liquid metal.

The invention also relates to an installation for casting a metal strip, in particular steel, of the type comprising two counter-rotating cylinders with horizontal axes, cooled by an internal circulation of a cooling fluid, defining between them a casting space intended receiving the liquid metal, and the external surfaces of which have a roughness, a device for blowing a gas or a mixture of gases through a cover covering said pouring space, and means for modulating the quantity blown in and / or the nature of said gas or the composition of said gas mixture at least in the vicinity of the surface of each cylinder upstream from its zone of contact with the liquid metal, characterized in that it comprises means for measuring or calculating the bulge cylinders in said casting space, or a quantity representative of said convexity of the cylinders - in said casting space, and means - for automatically controlling said modulation of the quantity supplied and / or of the nature of said gas as a function of the data collected by said means for measuring or calculating the crown of the cylinders in the casting space, or of a quantity representative of said crown.

As will be understood, the invention consists in modulating the quantity and / or the composition of the gas present in the immediate vicinity of the surface of each cylinder, just before the latter comes into contact with the meniscus of liquid metal. , or these two parameters, in order to adjust the convexity of the cylinders. In fact, when the casting rolls are not smooth but have roughness on their surface, the quantity and composition of the gas present in the hollow parts of the surface of the cylinder have a direct influence on the coefficient of heat transfer between the metal and the cylinder. It is through this that we will vary the heat flow extracted from the metal on which the expansion of the cylinder, and therefore its crown, depends. This variation of the crown of the cylinders can be carried out during casting, depending on the particular conditions at the time.

The invention will be better understood on reading the description which follows, given with reference to the single appended figure. This schematically shows, in cross section, an installation for casting metal strips between two cylinders allowing the implementation of the invention.

As has been said, the expansion of the cylinders is in particular governed by the heat flow which they extract from the metal present in the casting space. Thus, experience has shown the inventors that the instantaneous heat flux Φ _i extracted by a cylinder of a given portion of metal with which it is in contact, expressed in MW / m ² , can be written: $${\text{Φ}}_{\text{i}}{\text{= A. t}}_{\text{i}}{}^{\text{-0.35}}$$ t _i is the time that has elapsed since said portion of metal was brought into contact with the cylinder at the meniscus, that is to say the area where the cylinder meets the free surface of the liquid metal present in the space of casting. The fact that Φ _i decreases when t _i increases reflects the deterioration in the quality of heat transfers as the temperature of the metal decreases. A is a heat transfer coefficient, expressed in MW / m ² .s ^0.35 , the value of which depends on the conditions prevailing at the metal-cylinder interface.

From this expression of the instantaneous heat flux, we can calculate the average heat flux Φ _m extracted from any portion of the skin during solidification and cooling which is in contact with the cylinder. This is achieved by integrating Φ _i over all of this skin, the various portions of which are distinguished by the time since which they have been in contact with the cylinder. This time is between 0 for a portion of the skin located at the meniscus, and t _c for a portion of the skin which leaves the cylinder at the neck. t _c can be calculated in function of the length of the arc of contact between the metal and the cylinder and the speed of rotation of the cylinders. Φ _m can therefore be written:

Furthermore, Φ _m can be measured by means of the flow rate Q of cooling water passing through the cylinder, the temperature variation ΔT of this water between its entry and exit from the cylinder and the contact surface S between the metal and cylinder, depending on: $${\text{Φ}}_{\text{m}}\text{= Q.ΔT / S}$$

Knowing t _c we can deduce A by the calculation according to: $${\text{A = 0.65 Φ}}_{\text{m}}{\text{/ t}}_{\text{vs}}{}^{\text{-0.35}}{\text{= 0.65 Q ΔT / S t}}_{\text{vs}}{}^{\text{-0.35}}$$

It has been said that the value of A depends on the conditions at the metal-cylinder interface. One of the most important features of this interface is the roughness of the surface of the cooled cylinder shell. It has been found that a perfectly smooth ferrule surface and having a uniform thermal conductivity could cause the appearance of defects on the cast strip. The reason is that the effect of contraction of the skin of the strip during its cooling opposes the adhesion forces of this same skin on the shell. This competition is a source of tension inside the skin, which can lead to the appearance of superficial micro-cracks. To remedy these problems, it is commonly accepted that it is preferable to use cylinders whose ferrule has a certain roughness, that is to say an alternation of smooth areas (or raised areas) and recessed areas compared to the previous ones, distributed regularly or randomly. On smooth and raised areas, the metallic skin normally adheres to the shell and can cool quickly. The width of the recessed areas is, on the other hand, calculated so that the metal being solidified only partially fills them, and so that, under the effect of the surface tension forces, it does not reach the bottom of these hollow. In line with at least the central parts of these recesses, the metal is therefore not in direct contact with a cooled surface. There is thus created on the skin, in line with these hollows, a series of zones having a slight relief, and whose solidification and cooling are less advanced than on the rest of the skin. They constitute, in a way, a reserve of metal which has a certain elasticity, and can absorb without cracking the superficial stresses linked to the contraction of the skin. To obtain a satisfactory surface condition of the cast strip, it has been imagined to provide different types of engravings on the ferrules of the cylinders, such as intersecting grooves in v-section. More recently, it has been proposed to provide dimples on the ferrule of substantially circular or oval shape, not touching, and having a diameter of 0.1 to 1.2 mm and a depth of 5 to 100 μm (see the document EP 0309247).

Before coming into contact with the liquid metal, the recessed areas are filled with the gas which constitutes the boundary layer of the atmosphere overhanging the rotating cylinder, and which the latter carries with it. When they come into contact with the meniscus and are then covered by the metal skin during solidification, the gas which filled them is trapped there. It is by means of this gas that the cooled walls of the hollows which are not in contact with the skin will nevertheless participate in the extraction of the heat flux from the metal. The calculated value of the coefficient A takes into account the effect of the roughness of the shell on the overall heat transfer between the metal and the cylinder.

Very generally, the surface of the liquid steel is not exposed to the ambient air, otherwise there would be pollution of the metal by the formation of oxidized inclusions. This formation would also lead to consumption of the most easily oxidizable elements present in the steel. To isolate the surface of the air, the pouring space is most often capped with a device forming a cover. Under this cover, a gas totally inert towards the liquid metal (for example argon) is blown in the direction of the surface of the liquid steel, or a gas which it is tolerable for it to partially dissolve. in liquid metal (for example nitrogen in the case where a stainless steel is poured into which a low nitrogen content is not particularly sought after), or a mixture of such gases. To avoid wear problems, both of the cylinders and of the cover, the latter does not generally bear on the cylinders, but is kept at a very short distance from their surface (a few mm). The disadvantage of such an arrangement is that the cylinders carry with them, in particular in the hollows of their surface, a boundary layer of air whose oxidizing power is unfavorable to the quality of the metal which comes into contact with it at the meniscus. and below. In certain cases, this problem is remedied by carrying out, in addition to the insufflation directed towards the surface of the liquid steel, an insufflation of argon and / or nitrogen in the immediate vicinity of the surface of the cylinders, where it is overhung by the cover. It is carried out with an adjustable flow rate, which must be sufficient to cause a dilution of the air boundary layer, so as to make it lose most of its oxidizing power. It is this solution which is used, in particular, in French application FR 94 14571.

Due to the differences between their physical and chemical properties, not all gases and gas mixtures usable for the protection of liquid metal have the same influence on heat transfers between the metal and the cylinder. It is thus observed that these transfers take place more efficiently when nitrogen is used as the inerting gas rather than argon. A probable explanation of this phenomenon is that, argon being practically insoluble in steel, it remains entirely inside the recessed ranges. It therefore forms in permanently a gas mattress between the bottom of the recessed areas and the metal skin, which helps to prevent significant penetration of the metal into the recesses. On the other hand, the nitrogen trapped in the recesses is more or less (depending on the shade cast) absorbed by the metal when the latter is not yet completely solidified. In general, the quantity of gas present in the recesses is also a function of the flow of blown gas, in particular in the immediate vicinity of the cylinders. For an equal blown gas flow, the quantity of gas remaining present in each recessed range is therefore lower in the case of the use of nitrogen than in the case of the use of argon. Thus, nitrogen cannot hinder the penetration of the metal into the hollows as much as argon, and we find ourselves in solidification conditions closer to those of a smooth cylinder. In other words, if it is argon which essentially constitutes the gaseous boundary layer entrained by the cylinders up to the meniscus, the heat transfer coefficient A between the cylinder and the metal skin during solidification is lower than in the case where the boundary layer is constituted by nitrogen. And in the case where a mixture of these two gases is used, a decrease in A is observed when the percentage of argon in the mixture blown increases in the vicinity of the surface of the cylinders, upstream of the meniscus, from the value A ₀ that A takes in the case of pure nitrogen: $${\text{A = A}}_{\text{0}}\text{- K (\% Ar)}$$

Experience shows that, for different austenitic stainless steels and a given roughness of the cylinders, A ₀ can for example vary between 4.2 and 4.8 and K is of the order of 0.025 in the range of lower argon contents or equal to 30%. Beyond this limit, there is a marked decrease in the influence of the argon content on the value of A. In the case of ferritic stainless steels, the influence of the argon content on A is less marked, and it becomes relatively weak in the case of carbon steels. These observations are related to the differences in nitrogen solubility in these different types of grades: the more the nitrogen is soluble in the steel, the more its partial or total replacement in the inerting gas by an insoluble gas modifies the conditions at the gas / metal interface. This means that the variant of the process according to the invention in which the crowning of the cylinders is adjusted by varying the nature of the inerting gas or the composition of the inerting gas mixture finds its preferred application in the casting of stainless steels, in particular austenitics. The variant according to which the setting of the convex is obtained only by modulating the gas flow blown is intended more particularly for carbon steels. It is understood that it is also possible to play on both parameters, bit rate and composition.

The operator can experimentally determine the value of the thermal flux passing through the cylinder, and deduce therefrom by calculation the value of A, knowing the casting speed. From this value of A, thanks to previous experiences or modeling techniques, for each type of roughness of the cylinders and for each grade category, it deduces the curvature of the cylinder which would be to be expected if the cylinder had a perfectly straight generator when cold. The operator finally deduces the shape correction which it is preferable to apply construction to the cylinder so that, in at least most of the real experimental conditions, it is possible to obtain a cylinder whose generatrices would take, hot , the rectilinear or slightly concave shape desired, just by playing on the composition and / or the flow rate of the inerting gas, in accordance with the invention.

To modify the nature of the inerting gas, the operator has the possibility of using either pure nitrogen or pure argon in order to be able, for a given gas flow rate and casting conditions, to have the choice between two rounded cylinders. But of course, it is preferable to give the possibility of using a mixture of these two gases (or of any other suitable gases) in respective proportions varying at will according to the needs of the setting of the convex, so as to carry out this setting as precisely as possible.

A nonlimiting example of a device allowing the implementation of the invention is shown diagrammatically in the single figure. Conventionally, the casting installation comprises two cylinders 1, 1 'brought together, energized internally cooled and rotated in opposite directions around their horizontal axes by means not shown, and means for supplying liquid steel 2 into the 'pouring space defined by the external surfaces 3, 3' of the cylinders 1, 1 'and closed laterally by two refractory plates, one of which 4 is visible in FIG. 1. These supply means comprise a nozzle 5 connected to a distributor not shown, and whose lower end plunges under the surface 6 of the liquid steel 2 which contains the casting space. The liquid steel 2 begins its solidification on the external surfaces 3, 3 'of the cylinders 1, 1' by forming skins 7, 7 ', the junction of which at the neck 8, that is to say of the zone where the difference between the cylinders 1, 1 ′ is the smallest, gives rise to a solidified strip 9 a few mm thick, which is continuously extracted from the casting installation. The inerting of the pouring space is provided by a cover 10 crossed by the nozzle 5, and which is supported on two blocks 11, 11 'extending over the entire width of the cylinders 1, 1'. The lower faces 12, 12 'of these blocks 11, 11' are shaped so as to match the curvatures of the external surfaces 3, 3 'of the cylinders 1, 1' and to be defined with them, when the inerting device is in service , a space 13, 13 ′ of width "e" equal to a few mm. The inerting gas is first supplied by a pipe 14 passing through the cover 10 and opening above the surface 6 of the liquid steel 2 present in the casting space. This pipe 14 is connected to a gas tank 15, containing for example nitrogen or argon, and the flow rate and the blowing pressure of which are controlled by a valve 16.

On the other hand, for the implementation of the method according to the invention, a blowing of gas with controlled flow rates and composition is carried out through the blocks 11, 11 ′. A nitrogen tank 17 provided with a valve 18 and an argon tank 19 provided with a valve 20 are connected to a mixing chamber 21. It is from this mixing chamber 21 that the gas or, more generally, the gaseous mixture which, according to the invention, will constitute the boundary layer entrained by the external surfaces of the cylinders 1, 1 ′ up to their zones of contact with the surface 6 of the liquid metal contained in the pouring space which constitute the meniscus. To this end, a pipe 22 provided with a valve 23 leaves the mixing chamber 21 and brings a portion of the gas mixture which is there in the block 11, where a slot 24 (or a plurality of close holes, or a porous element) distributes it as uniformly as possible in the space 13 defined by the lower face 12 of the block 11 and the external face 3 of the cylinder 1. The valve 23 makes it possible to adjust the flow rate and the pressure of the mixture gaseous. A symmetrical device, comprising a line 22 'provided with a valve 23' also brings the gas mixture into the block 11 'then, through a slot 24', into the space 13 'separating the block 11' and the cylinder 1 ' .

As a variant, it is also possible to provide, for each block 11, 11 ′, gas supply devices which are completely independent of one another, so that the compositions of the gas mixtures present in the spaces 13, 13 ′ can be adjusted separately. , and therefore the crown of each of the cylinders 1, 1 '. One can thus take into account a possible difference in the cooling conditions of each of the cylinders 1, 1 '. On the other hand, one can also choose to also take the gas blown under the cover 10 in the mixing chamber 21, and thus give it the same composition as the gas mixture which should form the boundary layer on the surface of the cylinders 1, 1 '.

Another variant of the device according to the invention consists, as in the French application 94 14571 already cited, in providing inside each block 11, 11 'a second slot (or another functionally equivalent member) similar to the slot 24 , 24 ', and opening upstream of the latter in the space 13, 13' relative to the progression of the surface 3, 3 'of the cylinder 1, 1'. This second slot directs the gas which comes from it towards the outside of the space 13, 13 ', while the slot 24, 24' directs the gas which leaves there towards the casting space, therefore in the direction of progression. from the surface 3, 3 'of the cylinder 1, 1'. There is thus obtained a better seal of the space 13, 13 'vis-à-vis the external environment, where a finer control of the composition of the boundary layer The regulation of the convexity of the cylinders 1, 1' finds it easier.

Likewise, the gas or the gaseous mixture brought into the spaces 13, 13 ′ separating the blocks 11, 11 ′ and the cylinders 1, 1 ′ can be found not only in the state gaseous, as has been implicitly assumed so far, but also in the liquid state. One can also plan to reheat it by regulating its temperature.

It should be understood that the inerting device which has just been described constitutes only one example of implementation of the invention, and that any other device making it possible to control the composition of the gas present above the casting space, and in particular of the gaseous boundary layer entrained by the external surface of each cylinder up to the meniscus could also be suitable.

In order to control the convexity of the cylinders during casting according to the method of the invention, the operator (or the automations) which is responsible for the operation of the casting installation must have a certain amount of information for s 'ensure that the composition and the flow rate of the inerting gas adopted effectively lead to the desired convexity, and therefore to an adequate quality for the product. To this end, one possibility consists in continuously collecting the data (flow rate of cooling water, variation of its temperature between inlet and outlet of the cylinder) making it possible to calculate the heat flux passing through the cylinder, to calculate it at close intervals and to deduce the curvature such as mathematical models and / or prior calibrations make it possible to predict it. Another way of proceeding is to continuously measure the volume of the cylinders in an area as close as possible to the casting space, to deduce therefrom the value of the volume in these contact zones and to adjust the composition of the gas. inerting accordingly. This curved measurement can be carried out using, for example, a set of non-contact shape measurement sensors, such as capacitive sensors or laser sensors, distributed along at least one generator of one of the cylinders. , or better, of two sets of such sensors each installed on a different cylinder. The single figure shows schematically such sensors 25, 25 ', which are connected to a calculation unit 26. This also receives the information cited above which allows it to calculate the heat fluxes passing through the cylinders 1, 1', and it consequently controls the respective openings of the valves 18, 20, in order to regulate the flow rate and the composition of the gas mixture to the values which provide a convex considered optimal to the cylinders 1, 1 ′. The measurement of the thermal profile of the strip along its width, carried out at the outlet of the cylinders, can also give at least qualitative indications on the bulge imposed by the cylinders, since the temperature difference between the center of the strip and the areas closer to the banks is an index of variations in the thickness of the strip. Finally, it is possible to install downstream of the cylinders a device for direct measurement of the thickness of the strip and of its variations according to its width, such as X-ray gauges, by means of which one can directly observe the effects of the bulging of the cylinders on the strip, and, if necessary, correct the crown by the method according to the invention.

It is also conceivable to combine the method according to the invention with a control of the convexity by the flow of water for cooling the cylinders. It has been said previously that it was difficult to obtain only by this latter method variations of large amplitude of the bulge. However, it can be used to finely complete a coarser adjustment of the convex effect previously effected by action on the flow rate and / or the composition of the inerting gas.

The invention is, of course, not limited to the casting of steel strips, and can be applied to the casting of other metallic materials.

Claims (10)

1. Process for casting a metal strip, especially a steel strip, in which the solidification of the said strip is brought about by supplying liquid metal between two counter-rotating rolls having horizontal axes, cooled by the internal circulation of a coolant, defining a casting space between them, and the external surfaces of which rolls have a certain roughness, and the said casting space is inerted by injecting a given amount of a gas or of a gas mixture through a cover sitting over the said casting space, characterized in that the crown of the said rolls is controlled by modulating the injected amount and/or the nature of the said gas or the composition of the said gas mixture, at least near the surface of each roll upstream of its region of contact with the liquid metal.
2. Process according to Claim 1, characterized in that the said control of the crown is completed by varying the flow rate of the said coolant.
3. Plant for casting a metal strip (9), especially a steel strip, of the type comprising two counter-rotating rolls (1, 1') having horizontal axes, cooled by the internal circulation of a coolant, defining between them a casting space intended to receive the liquid metal (2), and the external surfaces (3, 3') of which rolls have a certain roughness, a device ( 14, 15, 16) for injecting a gas or a gas mixture through a cover (10) sitting over the said casting space, and means (17, 18, 19, 20, 21, 22, 22', 23, 23', 24, 24') for modulating the injected amount and/or the nature of the said gas or the composition of the said gas mixture at least near the surface (3, 3') of each roll (1, 1') upstream of its region of contact with the liquid metal (2), characterized in that it includes means (25, 25', 26) for measuring or calculating the crown of the rolls (1, 1') in the said casting space, or a quantity representative of the said crown of the rolls (1, 1') in the said casting space, and means (26) for automatically controlling the said modulation of the injected amount and/or the nature of the said gas according to the data collected by the said means (25, 25', 26) for measuring or calculating the crown of the rolls (1, 1') in the casting space, or a quantity representative of the said crown.
4. Plant according to Claim 3, characterized in that the said cover (10) includes two blocks (11, 11'), the lower face (12, 12') of each block defining, with the external surface (3, 3') of one of the said rolls (1, 1'), a space, the said blocks (11, 11') extending over the entire width of the said rolls (1, 1'), and means (24, 24') for injecting the said gas or the said gas mixture, modulated in terms of amount and/or nature or composition, into the said space.
5. Plant according to Claim 3 or 4, characterized in that the said gas mixture is a nitrogen/argon mixture.
6. Plant according to one of Claims 3 to 5, characterized in that the means for measuring the crown of the rolls (1, 1') include at least one set of shape-measuring sensors (25, 25') placed along a generatrix of one of the rolls (1, 1').
7. Plant according to one of Claims 3 to 6, characterized in that the said means (26) for calculating the crown of the rolls (1, 1') include means for measuring the heat flux passing through the rolls (1, 1').
8. Plant according to one of Claims 3 to 7, characterized in that the said quantity representative of the crown of the rolls (1, 1') is the thickness profile of the strip (9) over its width.
9. Plant according to Claim 8, characterized in that it includes means for measuring the variations in the temperature of the said strip (9) over its width.
10. Plant according to Claim 9, characterized in that it includes means for directly measuring the thickness profile of the said strip (9) over its width.