Four-high rolling mill, in particular for cold finishing a strip of metal material

Abstract

The present invention relates to a four-high rolling mill (1), in particular for finishing a strip (N) of metal material, wherein said rolling mill (1) comprises a housing (2) comprising: - a pair of work rolls (11, 12) comprising an upper work roll (11) and a lower work roll (12), between which a strip (N) is fed; - a pair of back-up rolls (21, 22) comprising an upper back-up roll (21), resting on the upper work roll (11) in operating conditions, and a lower back-up roll (22), whereon the lower work roll (12) rests in said operating conditions, wherein said rolling mill (1) comprises a control system (3) for controlling the flatness of the strip (N). The main feature of the present invention lies in the fact that said control system (3) comprises at least one heating system (30) associated with at least one back-up roll (21, 22) for thermally increasing the diameter of said at least one back-up roll (21, 22), wherein the increased diameter of said at least one back-up roll (21, 22) results in load being applied to the associated work roll (11, 12), causing it to undergo an inflection that corrects a deformation of said work roll (11, 12).

Description

[0001] FOUR-HIGH ROLLING MILL, IN PARTICULAR FOR COLD FINISHING A STRIP OF METAL MATERIAL

[0002] DESCRIPTION

[0003] The present invention relates to a four-high rolling mill, in particular for cold-finishing a metal strip, according to the preamble of claim 1.

[0004] The invention finds application especially in the field of cold-finishing rolling mills, wherein cold-rolling is typically used in order to reduce the thickness of a product to be rolled (usually a strip made of metal, e.g. steel, copper or alloys thereof, aluminium or alloys thereof).

[0005] It is known in the art that a rolling mill generally comprises a rolling structure or mill housing comprising an upper work roll and a lower work roll stacked over each other and having their axes substantially horizontal and parallel to each other; between the two stacked work rolls a strip or product to be rolled is fed in order to reduce its thickness. Usually, the two work rolls are motorized and turn in opposite directions (i.e. the directions of rotation of the two work rolls are opposite).

[0006] “Four-high” rolling mills are known in the art, which also comprise two back-up rolls, i.e.:

[0007] - an upper back-up roll, positioned on top of the upper work roll, and

[0008] - a lower back-up roll, positioned under the lower work roll.

[0009] In this context, also the back-up rolls have their axes substantially horizontal and parallel to each other; the function of the back-up rolls is to counter the inflection of the work rolls under the rolling load, in order to ensure proper flatness of the rolled strip.

[0010] As is known in the art, work rolls and back-up rolls are laterally connected to the rolling structure or cage by means of respective chocks, which are in turn associated to both a first side of the rolling mill (wherein said first side can also be defined as “operator side”) and a second side of the rolling mill (wherein said second side can also be defined as “drive side”); in fact, the rolling mills known in the art comprise motorized means for moving the work rolls and / or the back-up rolls, said motorized means being associated with the rolls on said drive side of the rolling mill.

[0011] In a four-high rolling mill for cold-finishing a metal strip (wherein said rolling mill can be defined as “finisher rolling mill”), in particular for an aluminium or copper strip, the minimum thickness of the metal strip can be reduced down to 12 micron (double-rolled metal strip); therefore, it is apparent that any minimal deformation of the surface of the work rolls from the theoretical geometry can heavily affect the flatness of the rolled metal strip.

[0012] It is known in the art that work rolls are normally grinded with a “crown” at the centre of table that constitutes the outer surface of the work roll, wherein said crown helps to counter the following phenomena arising during the rolling process and affecting the profile, and hence the flatness, of the rolled strip:

[0013] - rolling load acting upon the rolls’ surface and causing it to bend;

[0014] - separating and bending forces applied to the areas of the work rolls proximal to the chocks;

[0015] - rolling energy causing the work rolls to overheat, resulting in thermal expansion of the profile of said work rolls.

[0016] In this context, the rolling mills currently known in the art are designed to include a system for automatically controlling the flatness of the work rolls, wherein said control system controls the evolution of the thermal profile of the work rolls and their deformation, in particular in the areas adjacent to the chocks, in an attempt to keep the strip properly flat during the whole rolling process.

[0017] Usually the problems relating to the flatness of the strip in the central area of the work rolls are handled by said control system by monitoring the cooling of the rolls via continuous modulation of a rolling fluid (typically consisting of rolling oil) that is sprayed onto the surface of the work rolls by at least one cooling bar, so as to control the transfer of the heat generated during the rolling process, and hence the thermal deformation of the work rolls.

[0018] As to the problems relating to the flatness of the strip in the areas adjacent to the chocks, they are handled by controlling the separating and bending force of the work rolls which, by causing a positive / negative inflection of the work rolls, counters those deformations of the latter which are caused by heating resulting from rolling energy; however, the deformation imposed on the work rolls by separating and bending forces has no appreciable influence on the central area of the work rolls (and also, as a consequence, on the central area of the strip).

[0019] The strategy of controlling the deformation of the work rolls by modulating the rolling fluid (or rolling oil) sprayed onto the various sectors of the surface of the work rolls is effective when there is a temperature difference between the surface of the work roll and the rolling fluid.

[0020] However, the temperature of the rolling fluid cannot fall below certain values, because it dictates the viscosity of the fluid itself, and hence the rolling speed (which is also dependent on the roughness of the work rolls and on the thickness reduction to be obtained during the rolling pass under way).

[0021] As a consequence, whenever the work rolls and the strip to be laminated are cold, and therefore their temperature is almost equal to that of the rolling fluid (for example: at the beginning of the rolling process and / or during rolling passes in which a very low thickness reduction is produced and / or when a very hard material is processed, e.g. a very hard aluminium alloy), the rolled strip will have very poor flatness, in that the central band of such strip will be less tightened than the strip edges; in technical jargon, this phenomenon is known as “loose centre and tightened edges”.

[0022] In an attempt to overcome this problem, some solutions are known in the art which employ at least one back-up roll (typically an upper one) with a geometry that is mechanically or hydraulically variable, namely by means of mechanical and / or hydraulic elements fitted within at least one back-up roll and configured to vary the external geometry of said back-up roll. In particular, such a solution increases, whether mechanically or hydraulically, the diameter of the back-up roll (in particular, of the central part of the surface of said back-up roll), so that the increased diameter results in load being applied to the associated work roll (in particular, a load to the central surface of the work roll), thereby causing the latter to undergo an inflection that corrects the deformation currently suffered by the work roll in the absence of any deformation contribution given by the thermal profile of the roll.

[0023] While it is effective, such a solution has nonetheless the drawback that it is rather expensive and complex from a technical point of view, and therefore also from a maintenance point of view.

[0024] In this frame, it is therefore the main object of the present invention to provide a four- high rolling mill, in particular for cold-finishing a metal strip, so conceived as to overcome the drawbacks of prior-art solutions.

[0025] In particular, it is one object of the present invention to provide a four-high rolling mill, in particular for cold-finishing a metal strip, wherein said rolling mill is so conceived as to ensure optimal flatness of the rolled strip, even when the thickness of the strip is very low. In this context, it is one object of the present invention to provide a four-high rolling mill, in particular for cold-finishing a metal strip, so conceived as to adequately counter those phenomena which may arise during the rolling process and which adversely affect the profile of the work rolls, and hence the flatness of the rolled strip, in particular said phenomena being the following: rolling load exerted on the surface of the work rolls, which causes inflection thereof; separating and bending forces applied in the areas of the work rolls proximal to the chocks; rolling energy causing the work rolls to overheat, resulting in thermal expansion of their profile.

[0026] It is another object of the present invention to provide a four-high rolling mill, in particular for cold-finishing a metal strip, so constructed as to adequately correct any strip flatness errors both in the areas adjacent to the chocks and in the central area of the work rolls, during the entire rolling process.

[0027] It is a further object of the present invention to provide a four-high rolling mill, in particular for cold-finishing a metal strip, so constructed as to adequately correct any strip flatness errors whenever the work rolls and the material being rolled are cold, e.g. at the beginning of the rolling process and / or during rolling passes where flatness is poor due to a very low thickness reduction and / or when a very hard material, e.g. a very hard aluminium alloy, is processed.

[0028] It is yet another object of the present invention to provide a four-high rolling mill, in particular for cold-finishing a metal strip, that represents an effective solution for controlling the flatness of the strip to be rolled, while at the same time not being excessively expensive and technically complex and, as a consequence, difficult to maintain.

[0029] Further objects, features and advantages of the present invention will become apparent in light of the following detailed description and of the annexed drawings, which are provided herein merely by way of non-limiting explanatory example, wherein:

[0030] - Fig. 1 shows a schematic cross-sectional view of a four-high rolling mill, in particular for finishing a metal strip, according to the present invention, wherein some components of said rolling mill are shown in a first operating condition;

[0031] - Fig. 2 shows the rolling mill of Fig. 1, wherein some components of said rolling mill are shown in a second operating condition;

[0032] - Figures 3a to 3d show different cross-sectional views of some components of the rolling mill according to the present invention; - Fig. 4 is a perspective view of some components of the rolling mill according to the present invention.

[0033] Referring now to the annexed drawings, reference numeral 1 designates as a whole a four- high rolling mill, in particular for finishing a metal strip, according to the present invention.

[0034] As shown in Fig. 1, the rolling mill 1 comprises a rolling structure or housing (generally identified by reference numeral 2) comprising an upper work roll 11 and a lower work roll 12, between which a strip N to be rolled is fed; as is known, the typical purpose of rolling is to reduce the thickness of said strip N.

[0035] Said upper work roll 11 and lower work roll 12 are stacked over each other and have a first longitudinal axis Al and a second longitudinal axis A2, respectively, which are substantially horizontal and substantially parallel to each other; also, the work rolls 11, 12 are connected to the rolling housing 2 of the rolling mill 1 by means of respective chocks (not shown).

[0036] As far as said strip N is concerned, it is made of metal, e.g. aluminium or alloys thereof, copper or alloys thereof, steel, etc.

[0037] The rolling mill 1 according to the present invention further comprises a pair of back-up rolls 21, 22, wherein said pair of back-up rolls 21, 22 include:

[0038] - an upper back-up roll 21, resting on the upper work roll 11 in operating conditions;

[0039] - a lower back-up roll 22, whereon the lower work roll 12 rests in said operating conditions.

[0040] The back-up rolls 21, 22 are also preferably connected to the rolling housing 2 of the rolling mill 1 by means of respective chocks (not shown), and have a third longitudinal axis A3 and a fourth longitudinal axis A4, respectively, which are substantially horizontal and substantially parallel to each other; as a result, the third longitudinal axis A3 of the upper back-up roll 21 and the fourth longitudinal axis A4 of the lower back-up roll 22 are substantially horizontal and substantially parallel to the first longitudinal axis Al of the upper work roll 11 and to the second longitudinal axis A2 of the lower work roll 12.

[0041] In this context, the chocks of the work rolls 11, 12 and of the back-up rolls 21, 22 develop substantially horizontal in said rolling mill 1, so as to connect the work rolls 11, 12 and the back-up rolls 21, 22 laterally to the rolling housing 2; said chocks are associated with both a first side of the rolling mill 1 (wherein said first side can also be defined as “operator side”) and a second side of the rolling mill 1 (wherein said second side can also be defined as “drive side”). Also, the rolling mill 1 comprises motorized means (not shown in the drawings) for moving the rolls 11, 12, 21, 22, said motorized means being preferably associated with the rolls 11, 12, 21, 22 (in particular, to the work rolls 11, 12, the rotation of which also drives the back-up rolls 21, 22) on said second side, or “drive side”.

[0042] Therefore, the present invention is ideally applicable to a four-high rolling mill 1, in that the rolling housing 2 comprises two work rolls 11, 12 and two back-up rolls 21, 22; it is nevertheless clear that the provisions of the present invention may also be used in a rolling mill 1 having a different configuration.

[0043] The rolling mill 1 according to the present invention further comprises a control system (designated as a whole by reference numeral 3 in Figures 1 and 2) for controlling the flatness of the strip N passing between said upper work roll 11 and lower work roll 12.

[0044] In accordance with the present invention, said control system 3 comprises at least one heating system (designated as a whole by reference numeral 30 in the annexed drawings) associated with at least one back-up roll 21, 22 for thermally increasing the diameter of said at least one back-up roll 21, 22, wherein the increased diameter of said at least one back-up roll 21, 22 results in load being applied to the associated work roll 11, 12, causing it to undergo an inflection that corrects a deformation of said work roll 11, 12.

[0045] Figures 3a to 4 show a preferred embodiment of said heating system 30, wherein said heating system 30 is associated with the upper back-up roll 21, in particular by means of a support crosspiece 30T fastened to frame 2S (shown in Fig. 4) of the housing 2, wherein said support crosspiece 30T develops substantially parallel to the axes Al, A2, A3, A4 of the rolls 11, 12, 13, 14. It is however clear that the heating system 30 according to the present invention may be associated with the lower back-up roll 22, or that at least two heating systems 30 may be provided, wherein a first heating system 30 is associated with the upper back-up roll 21 and a second heating system 30 is associated with the lower back-up roll 22.

[0046] The heating system 30 according to the present invention comprises a plurality of heating elements 31, in particular heads 31 of the induction type, arrayed on a support frame 32 (see Fig. 3a) which is substantially parallel to the longitudinal axis A3, A4 of said at least one back-up roll 21, 22. In this regard, it should be noted that, in the embodiment shown in the accompanying drawings (wherein the heating system 30 is only associated with the upper back-up roll 21), the support frame 32 is substantially parallel to the third longitudinal axis A3 of the upper back-up roll 21. It should also be noted that the heating system 30 may comprise a device (not shown in the annexed drawings) configured to monitor the temperature (in particular, the temperature increase resulting from the activation of the heating elements 31) of at least one back-up roll 21, 22, in particular wherein said device may be mounted to the support frame 32.

[0047] Said heating elements 31 (or heads 31) are preferably arrayed in at least two rows extending longitudinally on the support frame 32, wherein the heating elements 31 of a first longitudinal row are longitudinally offset or stepped relative to the heating elements 31 of a second longitudinal row; this ensures to obtain a better longitudinal coverage (and, hence, a more uniform heating) of the back-up roll 21, 22 with which the heating system 30 is associated.

[0048] In particular, each heating element 31 is about 75 mm wide, and therefore covers a band approximately 75 mm wide of the back-up roll 21, 22.

[0049] In accordance with a preferred embodiment, the heating system 30 comprises moving means 33, 34 connected to the housing 2 by means of the support crosspiece 30T and configured to move the heating elements 31 towards said at least one back-up roll 21, 22 and to move the heating elements 31 away from said at least one back-up roll 21, 22. In substance, said moving means 33, 34 are configured to move the heating elements 31 in such a way as to switch them from a first operating condition (which may also be defined as “idle” condition, shown in Fig. 1) to a second operating condition (which may also be defined as “working” condition, shown in Fig. 2), and vice versa (i.e. to switch them from the “working ” condition to the “idle” condition).

[0050] Said moving means 33, 34 comprise at least one main actuator 33, in particular comprising a pneumatic cylinder 33A equipped with a pneumatic brake 33B (see Fig. 3d), wherein said main actuator 33 is configured to connect the support crosspiece 30T (in turn fastened to the frame 2S of the housing 2) to the support frame 32, and wherein said at least one main actuator 33 permits moving the heating elements 31 towards / away from said at least one back-up roll 21, 22.

[0051] In this context, the support frame 32 preferably comprises at least one end-of-stroke element 32A (shown in Fig. 4 and consisting of, for example, a spacer extending perpendicularly from said support frame) configured to prevent any contact between the heating elements 31 and said at least one back-up roll 21, 22; preferably, said end-of- stroke element 32A is connected to a pressure switch (not shown in the annexed drawings, and set to approximately 2.5 bar), the signal of which is monitored by a control unit (identified in Figures 1 and 2 by reference UC) of the rolling mill 1 to trigger the activation of the pneumatic brake 33B and stop the rod of the pneumatic cylinder 33A when said signal indicates a value above a predefined threshold (i.e. approximately 2.5 bar), so that said rod can move no further until the pneumatic brake 33B is released.

[0052] Preferably, said moving means 33, 34 comprise at least one secondary actuator 34 connected to the support crosspiece 30T and to the support frame 32, and configured to provide a fine positioning of the heating elements 31 relative to said at least one back-up roll 21, 22.

[0053] In accordance with the present invention, said at least one secondary actuator 34 comprises a first cylinder 34A and a second cylinder 34B positioned on opposite sides relative to said primary actuator 33 (as shown in Fig. 4), in particular said cylinders 34A, 34B being of the pneumatic type and of the short-stroke type (i.e. preferably having a stroke of approximately 10 mm).

[0054] Preferably, said first cylinder 34A and second cylinder 34B comprise each an adjustment mechanism 34C for adjusting their return stroke, in particular mechanically; for example, the adjustment mechanism 34C of said cylinders 34A, 34B may comprise an adjustment screw or a technically equivalent element.

[0055] In accordance with the present invention, the heating system 30 comprises translation means (designated as a whole by reference numeral 35 in Figures 3c and 4) configured to keep the support frame 32 (and, hence, the heating elements 31) parallel to the longitudinal axis A3, A4 of said at least one back-up roll 21, 22, in particular during the operation of the main actuator 33 and / or of the secondary actuators 34.

[0056] Preferably, said translation means 35 comprise a pair of gearings 35A with straight-cut rack and pinion, mutually timed by a synchronization bar 35B that connects said gear mechanisms 35A via toothed joints 35C.

[0057] In accordance with the present invention, the heating system 30 further comprises rotation means (designated as a whole by reference numeral 36 in Figures 3a and 4) associated with the support frame 32 and configured to dispose the surfaces of the heating elements 31 facing the back-up roll 21, 22 in a substantially tangent way with respect to said at least one back-up roll 21, 22, regardless of its position (which, in turn, depends on the diameter of the back-up roll 21, 22 and on the diameter of the work rolls 11, 12).

[0058] In this context, said rotation means 36 comprise: - two pairs of additional cylinders 36A, in particular of the pneumatic type, wherein each cylinder 36A comprises a rod secured to the support frame 32 and a jacket,

[0059] - two brackets 32B, wherein each bracket 32B is integral with the jacket of a pair of said additional cylinders 36A and with the rods of the first cylinder 34A and second cylinder 34B, wherein said cylinders 36A are configured in a manner such that their actuation determines the support frame 32 to pivot or rotate relative to the surface of the back-up roll 21, 22 in order to keep the heating elements 31 (i.e. the surface of said heating elements 31 that faces towards the back-up roll 21, 22) substantially tangent to said backup roll 21, 22.

[0060] As shown in Figures 1 and 2, the control system 3 for controlling the flatness of the strip N comprises:

[0061] - sensor means (generally identified by reference numeral 4 in Figures 1 and 2) configured to detect a flatness defect of the strip N passing between said upper work roll 11 and lower work roll 12,

[0062] - a control unit UC (which may be a control unit controlling the operation of the whole rolling mill 1), wherein said sensor means 4 are associated with the control unit UC, and wherein said control unit UC is configured to activate the heating system 30 as a function of a detection of said flatness defect of the strip N made by said sensor means 4. For example, said sensor means 4 may comprise at least one roll for measuring the flatness of the strip (also known as “flatness roll”).

[0063] In this context, the control unit UC can be configured to control the heating elements 31 selectively, i.e. to only activate those heating elements 31 which are located over that section of the back-up roll 21, 22 which has generated a flatness defect on the strip N, such defect having been detected by the sensor means 4.

[0064] For example, if, after the rolling mill 1 has been cold-started, there is a need for thermally increasing the diameter (i.e. creating a convex portion) of the central band of said at least one back-up roll 21, 22 in order to correct the deformation of the work rolls 11, 12 in a central band thereof, then the control unit UC will only supply power to those heating elements 31 which are located over the central band of said at least one back-up roll 21, 22. Thermal simulations and calculations have shown that an increase in the temperature of the surface of a back-up roll 21, 22 from 42°C to 65°C results in an increase in the diameter of the latter amounting to approximately 0.10 / 0.12 mm on the radius. This increase produces an inflection of the work roll 11, 12 along its central band (about 200 mm centred in the middle), which, for a strip N having a width of 1200 mm, will produce a strip flatness variation of up to approximately -30 I-units. The width of the rolled strip being equal, the traditional system fitted to prior-art rolling mills, which generates separating and bending forces, can only produce a strip flatness variation as small as -15 / - 18 I-units over the whole 600mm central band (centred in the middle); therefore, there is a substantial difference in the effect on strip flatness between the heating system 30 according to the present invention and a prior-art system applying separating and bending forces to the work rolls.

[0065] As a further example, if, while rolling a strip N, zones with local flatness defects (e.g. near the edges of the strip N) are formed, it will be possible, according to the present invention, to activate the corresponding heating elements 31 in order to produce a local increase in the diameter of the back-up rolls 21, 22, resulting in a local mechanical action being exerted on the work roll 11, 12 which will affect the flatness of the strip N.

[0066] In accordance with the present invention, the control system 3 for controlling the flatness of the strip N may also comprise cooling means 40 associated with at least one work roll 11, 12, so configured as to correct a flatness defect of the strip N, in particular said defect having been detected by the sensor means 4.

[0067] In one embodiment, said cooling means 40 may comprise at least one cooling bar configured to distribute or spray a rolling fluid (e.g. consisting of rolling oil) onto at least one work roll 11, 12 (Figures 1 and 2 show two cooling bars, wherein each cooling bar is associated with a respective work roll 11, 12 to spray rolling fluid onto it).

[0068] In this context, the heating system 30 can be activated by the control system 3 for controlling the flatness of the strip N (in particular, by the control unit UC) whenever the work rolls 11, 12 and / or the strip N to be rolled are cold, i.e. when they have a temperature almost equal to that of the rolling fluid (e.g. at the beginning of the rolling process and / or during rolling passes where flatness is poor due to a very low thickness reduction and / or a very hard material, e.g. a very hard aluminium alloy). As the rolling process goes on, and rolling energy produces heat that increases the temperature of the work rolls 11, 12, the control system 3 for controlling the flatness of the strip N (in particular, the control unit UC) may turn off the heating system 30 and turn on the cooling means 40 to distribute rolling fluid onto the work rolls 11, 12. It is however clear that the heating system 30 and the cooling means 40 may also be turned on / off simultaneously by the control system 3 for controlling the flatness of the strip N (in particular, by the control unit UC).

[0069] In accordance with the present invention, the control system 3 for controlling the flatness of the strip N may also comprise a management system (designated as a whole by reference numeral 50 in Figures 1 and 2) for controlling the separating and / or bending forces applied to the work rolls 11, 12, in particular said management system 50 being configured to control the separating and / or bending forces of said work rolls 11, 12 in order to counter the deformations undergone by the latter because of the heating produced by the rolling energy.

[0070] The management system 50 can also be activated by the control system 3 for controlling the flatness of the strip N either alternatively to, or simultaneously with, the heating system 30 and / or the cooling means 40.

[0071] From the above description it emerges that the operation of the rolling mill 1 of the present invention is preferably automatic, in particular said rolling mill 1 comprising a control system 3 equipped with a control unit UC connected to the various components of the rolling mill 1 and configured to manage its operation, in particular the operation of the heating system 30 for heating at least one back-up roll 21, 22 and / or of the cooling means 40 for cooling at least one work roll 11, 12 and / or of the management system 50 for controlling the separating and / or bending forces of the work rolls 11, 12. It is however clear that the present invention is also applicable to the case wherein the operation of the heating system 30 and / or of the cooling means 40 and / or of the management system 50 is controlled directly by a user of the rolling mill 1.

[0072] The following will describe a method for controlling the flatness of a strip N by means of a rolling mill 1 conceived in accordance with the teachings of the present invention, wherein said rolling mill 1 is equipped with a rolling structure or housing 2 comprising:

[0073] - an upper work roll 11 and a lower work roll 12, between which said strip N is fed;

[0074] - an upper back-up roll 21, resting on the upper work roll 11 in operating conditions;

[0075] - a lower back-up roll 22, whereon the lower work roll 12 rests in said operating conditions, and wherein said rolling mill 1 comprises a control system 3 for controlling the flatness of the strip N.

[0076] The method according to the present invention comprises the following phases: a) associating a heating system 30 with at least one back-up roll 21, 22; b) activating (i.e. electrically power) at least some of a plurality of heating elements 31 of said heating system 30 in order to thermally increase the diameter of said at least one back-up roll 21, 22, wherein the increased diameter of said at least one back-up roll 21, 22 results in load being applied to the associated work roll 11, 12, causing it to undergo an inflection that corrects a deformation of said work roll 11, 12 (wherein said inflection makes it possible to obtain the correct flatness of the strip N).

[0077] The method according to the present invention further comprises a phase c) of detecting a flatness defect of the strip N, in particular the detection of said flatness defect being made by sensor means 4 associated with a control unit UC of said control system 3.

[0078] As concerns the above-described phases a) to c), it should be noted that they must not necessarily be executed in the indicated order; for example, said phase c) may be executed before or after the other phases, or between phase a) and phase b).

[0079] In this context, said phase a) is carried out by activating moving means 33, 34 configured to move said heating elements 31 towards said at least one back-up roll 21, 22.

[0080] In particular, said phase a) comprises the following steps: al) activating at least one main actuator 33 and at least one secondary actuator 34 (included in said moving means 33, 34) in order to bring an end-of-stroke element 32A in contact with said at least one back-up roll 21, 22; a2) activating rotation means 36 of the heating system 30 in order to dispose the surfaces of the heating elements 31 facing the back-up roll 21, 22 in a substantially tangent way with respect to said at least one back-up roll 21, 22; a3) activating said at least one secondary actuator 34 in order to obtain a backward movement of the heating elements 31 from said at least one back-up roll 21, 22, wherein the extent of said backward movement is such as to leave a gap between the heating elements 31 and said at least one back-up roll 21, 22.

[0081] In particular, said backward movement substantially corresponds to a return stroke of at least one cylinder 34A, 34B (preferably, a stroke of approximately 10 mm) minus the adjustment (approximately 7 mm) made by an adjustment mechanism 34C, in particular of a mechanical type; as a result, said backward movement is such as to leave a gap of approximately 3 mm between the heating elements 31 and said at least one back-up roll 21, 22.

[0082] As far as step al) is concerned, it is preferably carried out by means of a main actuator 33 comprising a pneumatic cylinder 33A provided with a pneumatic brake 33B, and comprises the following sub-steps: ala) detecting a contact between said end-of-stroke element 32A and said at least one back-up roll 21, 22, in particular by means of a pressure switch (preferably set to approximately 2.5 bar); a2b) actuating said pneumatic brake 33B to block the rod of the pneumatic cylinder 33A. It should be noted that the method of the present invention can be implemented during the operation of the rolling mill 1 (i.e. when the rolling mill 1 is already in operation to reduce the thickness of said strip N); in this case, the method includes a phase f-az) of actuating the work rolls 11, 12 and the back-up rolls 21, 22, wherein said phase f-az) precedes phase c) of detecting a flatness defect of the strip N.

[0083] As an alternative, the method of the present invention may also be implemented at the beginning of the rolling process; in this case, said phase f-az) of actuating the work rolls 11, 12 and the back-up rolls 21, 22 can be executed after phase a) of associating the heating system 30 with at least one back-up roll 21, 22.

[0084] The method of the present invention may additionally comprise one or more of the following phases: d) activating cooling means 40, in particular comprising at least one cooling bar configured to distribute or spray a rolling fluid (e.g. consisting of rolling oil) onto at least one work roll 11, 12; e) activating a management system 50 for controlling separating and / or bending forces of the work rolls 11, 12 in order to control the separating and / or bending force of said work rolls 11, 12 and counter the deformations of the same; f) monitoring the temperature (in particular, the temperature increase following the activation of the heating elements 31) of at least one back-up roll 21, 22 by means of a device (not shown in the accompanying drawings) configured to measure said temperature, in particular wherein said device may be mounted to the support frame 32.

[0085] As regards the method according to the present invention, it should be noted that one or more phases of said method are preferably controlled (in a substantially automatic manner) by a control unit UC.

[0086] The advantages of the present invention are apparent from the above description.

[0087] Indeed, the peculiar provisions of the present invention provide a four-high rolling mill 1, in particular for cold-finishing a metal strip, so conceived as to overcome the drawbacks of prior-art solutions.

[0088] In particular, the teachings of the present invention make it possible to manufacture a rolling mill 1 that ensures optimal flatness of the rolled strip N, even when the thickness of the strip N is very low.

[0089] A further advantage of the rolling mill 1 according to the present invention lies in the fact that it has been conceived to adequately counter the phenomena occurring during the rolling process and adversely affecting the profile of the work rolls 11, 12, and hence the flatness of the strip N. In particular, said phenomena are the following: rolling load applied to the surface of the work rolls 11, 12, which causes them to undergo an inflection, separating and bending forces applied to the areas of the work rolls 11, 12 proximal to the chocks, rolling energy overheating the work rolls 11, 12 and causing a thermal expansion of their profiles.

[0090] In this context, the rolling mill 1 according to the present invention is so constructed as to be able to adequately handle any problems concerning the flatness of the strip N both in the areas adjacent to the chocks and in the central area of the work rolls 11, 12, during the entire rolling process.

[0091] Another advantage of the present invention lies in the fact that the rolling mill 1 is so constructed as to be able to adequately handle said problems concerning the flatness of the strip N even when the work rolls 11, 12 and / or the strip N to be rolled are cold, e.g. at the beginning of the rolling process and / or during rolling passes where the thickness reduction is very low and / or when a very hard material, e.g. a very hard aluminium alloy, is processed.

[0092] A further advantage of the present invention lies in the fact that the rolling mill 1 is such as to provide an effective solution for controlling the flatness of the strip N to be rolled, while at the same time being such as to provide a solution which is not excessively expensive and technically complex, thus being also easy to maintain.

[0093] The rolling mill 1 described herein by way of example may be subject to many possible variations without departing from the novelty spirit of the inventive idea; it is also clear that in the practical implementation of the invention the illustrated details may have different shapes or be replaced with other technically equivalent elements.

[0094] It can therefore be easily understood that the present invention is not limited to the abovedescribed rolling mill 1, but may be subject to many modifications, improvements or replacements of equivalent parts and elements without departing from the inventive idea, as clearly specified in the following claims.

Claims

CLAIMS1. A four-high rolling mill (1), in particular for finishing a strip (N) of metal material, wherein said rolling mill (1) comprises a housing (2) comprising:- a pair of work rolls (11, 12) comprising an upper work roll (11) and a lower work roll (12), between which a strip (N) is fed;- a pair of back-up rolls (21, 22) comprising an upper back-up roll (21), resting on the upper work roll (11) in operating conditions, and a lower back-up roll (22), whereon the lower work roll (12) rests in said operating conditions, wherein said rolling mill (1) comprises a control system (3) for controlling the flatness of the strip (N), said rolling mill (1) being characterized in that said control system (3) comprises at least one heating system (30) associated with at least one back-up roll (21, 22) for thermally increasing the diameter of said at least one backup roll (21, 22), wherein the increased diameter of said at least one back-up roll (21, 22) results in load being applied to the associated work roll (11, 12), causing it to undergo an inflection that corrects a deformation of said work roll (11, 12).

2. The rolling mill (1) according to claim 1, characterized in that said at least one heating system (30) comprises a plurality of heating elements (31), in particular heads (31) of the induction type, arrayed on a support frame (32) which is substantially parallel to a longitudinal axis (A3, A4) of said at least one back-up roll (21, 22).

3. The rolling mill (1) according to claim 2, characterized in that said heating elements (31) are arrayed in at least two rows extending longitudinally on the support frame (32), wherein the heating elements (31) of a first longitudinal row are longitudinally offset or stepped relative to the heating elements (31) of a second longitudinal row.

4. The rolling mill (1) according to one or more of claims 2 and 3, characterized in that the heating system (30) comprises moving means (33, 34) connected to the housing (2) by means of a support crosspiece (30T) and configured to move the heating elements (31) towards said at least one back-up roll (21, 22) and to move the heating elements (31) away from said at least one back-up roll (21, 22).

5. The rolling mill (1) according to claim 4, characterized in that said moving means (33, 34) comprise at least one main actuator (33), in particular comprising a pneumatic cylinder (33A) equipped with a pneumatic brake (33B), wherein said main actuator (33)is configured to connect the support crosspiece (30T) to the support frame (32), and wherein said at least one main actuator (33) permits moving the heating elements (31) towards / away from said at least one back-up roll (21, 22).

6. The rolling mill (1) according to one or more of claims 4 and 5, characterized in that said moving means (33, 34) comprise at least one secondary actuator (34) connected to the support crosspiece (30T) and to the support frame (32) and configured to provide a fine positioning of the heating elements (31) relative to said at least one back-up roll (21, 22).

7. The rolling mill (1) according to claim 6, characterized in that said at least one secondary actuator (34) comprises a first cylinder (34A) and a second cylinder (34B) positioned on opposite sides relative to said primary actuator (33), in particular said first and second cylinders (34A, 34B) being of the pneumatic type and of the short-stroke type.

8. The rolling mill (1) according to claim 7, characterized in that said first cylinder (34A) and second cylinder (34B) comprise each an adjustment mechanism (34C) for adjusting their return stroke, in particular mechanically.

9. The rolling mill (1) according to one or more of the preceding claims, characterized in that the support frame (32) comprises at least one end-of-stroke element (32A) configured to prevent contact between the heating elements (31) and said at least one back-up roll (21, 22).

10. The rolling mill (1) according to one or more of the preceding claims, characterized in that the heating system (30) comprises translation means (35) configured to keep the support frame (32) parallel to a longitudinal axis (A3, A4) of said at least one back-up roll (21, 22), in particular during the operation of the main actuator (33) and / or of the secondary actuators (34).

11. The rolling mill (1) according to one or more of the preceding claims, characterized in that the heating system (30) comprises rotation means (36) associated with the support frame (32) and configured to dispose the surfaces of the heating elements (31) facing the back-up roll (21, 22) in a substantially tangent way with respect to said at least one backup roll (21, 22).

12. The rolling mill (1) according to one or more of the preceding claims, characterized in that said control system (3) comprises:- sensor means (4) configured to detect a flatness defect of the strip (N) passing between said upper work roll (11) and lower work roll (12),- a control unit (UC), wherein said sensor means (4) are associated with the control unit (UC), and wherein said control unit (UC) is configured to activate the heating system (30) as a function of a detection of said flatness defect of the strip (N) made by said sensor means (4).

13. A method for controlling the flatness of a strip (N) passing between an upper work roll (11) and a lower work roll (12) of a rolling mill (1) according to one or more of the preceding claims, wherein said rolling mill (1) is provided with a housing (2) comprising:- an upper back-up roll (21) resting on the upper work roll (11) in operating conditions;- a lower back-up roll (22) whereon the lower work roll (12) rests in said operating condition, and wherein said rolling mill (1) comprises a control system (3) for controlling the flatness of the strip (N), said method being characterized in that it comprises the following phases: a) associating a heating system (30) of said control system (3) with at least one back-up roll (21, 22); b) activating, or supplying electrical power to, at least some of a plurality of heating elements (31) of said heating system (30) in order to thermally increase the diameter of said at least one back-up roll (21, 22), wherein the increased diameter of said at least one back-up roll (21, 22) results in load being applied to the associated work roll (11, 12), causing it to undergo an inflection that corrects a deformation of said work roll (11, 12), wherein said inflection makes it possible to obtain the correct flatness of the strip (N).

14. The method according to claim 13, characterized in that it comprises a phase c) of detecting a flatness defect of the strip (N), in particular the detection of said flatness defect being made by sensor means (4) associated with a control unit (UC).

15. The method according to one or more of claims 13 and 14, characterized in that said phase a) is carried out by activating moving means (33, 34) configured to move said heating elements (31) towards said at least one back-up roll (21, 22), wherein said phase a) comprises the following steps: al) activating at least one main actuator (33) and at least one secondary actuator (34) included in said moving means (33, 34) in order to bring an end-of-stroke element (32A) in contact with said at least one back-up roll (21, 22); a2) activating rotation means (36) of the heating system (30) in order to dispose thesurfaces of the heating elements (31) facing the back-up roll (21, 22) in a substantially tangent way with respect to said at least one back-up roll (21, 22); a3) activating said at least one secondary actuator (34) in order to obtain a backward movement of the heating elements (31) from said at least one back-up roll (21, 22), wherein the extent of said backward movement is such as to leave a gap between the heating elements (31) and said at least one back-up roll (21, 22).

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