METHOD FOR PREHEATING A WORKING ROLL FOR ROLLING

Abstract

The invention relates to a method for preheating at least one working roll for rolling, involving phases of: determining a target thermal expansion profile, determining an effective mean temperature profile of longitudinal segments of the working roll and for determining a target mean temperature profile, activating a plurality of inductors distributed along the working roll in order to reduce a difference between the effective mean temperature and the target mean temperature.

Claims

1. A process for preheating at least one work roll of a rolling mill intended to roll a metallic strip so that the work roll has a target thermal expansion profile (?d.sub.(i).sup.c).sub.1?i?Ns determined along Ns longitudinal segments of the work roll, the rolling mill including a thermal control device including Ni induction coils distributed along the longitudinal axis of the work roll facing the Ns longitudinal segments, the process comprising the following phases: a. determining the target thermal expansion profile (?d.sub.(i).sup.c).sub.1?i?Ns, at a calculation time t.sub.k, based on predefined values of input parameters Pe representative of the dimensions and mechanical and thermal properties of the metallic strip to be rolled, and a first predefined physical model M1 expressing a relationship between the input parameters Pe and the target thermal expansion profile (?d.sub.(i).sup.c).sub.1?i?Ns; b. determining an effective mean temperature profile (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?Ns along Ns longitudinal segments of the work roll, based on an effective thermal power profile (P.sub.Q(i).sup.eff(t.sub.k+1)).sub.1?i?Ni generated by the Ni induction coils and measured beforehand, and a second predefined physical model M2 expressing a relationship between the effective thermal power profile (P.sub.Q(i).sup.eff(t.sub.k)).sub.1?i?Ni and the effective mean temperature profile (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?Ns; c. determining (30) a target mean temperature profile (T.sub.(i).sup.c(t.sub.k)).sub.1?i?Ns along the Ns longitudinal segments of the work roll, based on the target thermal expansion profile (?d.sub.(i).sup.c (t.sub.k)).sub.1?i?Ns determined and the effective mean temperature profile (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?Ns determined; d. determining a deviation ?T(t.sub.k) between the target mean temperature profile (T.sub.(i).sup.c(t.sub.k)).sub.1?i?Ns and the effective mean temperature profile (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?Ns; e. determining a convergence criterion based on the deviation ?T(t.sub.k) determined, and stopping the preheating when the convergence criterion is satisfied, and continuing the phases of the preheating process when the convergence criterion is not satisfied; f. activating of the induction coils including the following: determining a target thermal power profile (P.sub.Q(i).sup.c (t.sub.k)).sub.1?i?Ni to be supplied by the Ni induction coils based on the deviation ?T(t.sub.k) determined; activating the induction coils such that they supply the target thermal power profile (P.sub.Q(i).sup.c (t.sub.k)).sub.1?i?Ni determined; measuring an effective thermal power profile (P.sub.Q(i).sup.eff(t.sub.k)).sub.1?i?Ni actually supplied by the induction coils; g. repeating b/ to f/ until the convergence criterion is satisfied, by incrementing the calculation time t.sub.k.

2. The preheating process according to claim 1, wherein the target thermal expansion (?d.sub.(i).sup.c).sub.1?i?Ns, effective mean temperature (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?Ns and target mean temperature (T.sub.(i).sup.c(t.sub.k)).sub.1?i?Ns profiles are determined for the longitudinal segments intended to be in contact with the metallic strip to be rolled.

3. The preheating process according to claim 1, wherein the determining the target thermal power profile (P.sub.Q(i).sup.c(t.sub.k)).sub.1?i?Ni includes the following: identifying the longitudinal segment, of index jmax, for which the deviation ?T.sub.(jmax) (t.sub.k) is maximum, and defining the target thermal power P.sub.Q(jmax).sup.c (t.sub.k) at a maximum value; determining the target thermal power of the other longitudinal segments such that P.sub.Q(j).sup.c (t.sub.k)=P.sub.Q(jmax).sup.c(t.sub.k)?(?T.sub.(j)(t.sub.k)/?T.sub.(jmax)(t.sub.k)).

4. The preheating process according to claim 3, wherein an induction coil of index j is only activated when the ratio ?T.sub.(j)(t.sub.k)/?T.sub.(jmax)(t.sub.k) is greater than or equal to a predefined threshold value R.sub.T, otherwise the coil remains inactive.

5. The preheating process according to claim 1, wherein the thermal control device includes coolers distributed along the longitudinal axis of the work roll facing the Ns longitudinal segments, and including activating the coolers based on the deviation ?T(t.sub.k) between the target mean temperature profile (T.sub.(i).sup.c(t.sub.k)).sub.1?i?Ns and the effective mean temperature profile (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?Ns.

6. The preheating process according to claim 1, wherein the phase of determining the effective mean temperature profile (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?Ns is performed by digital simulation, the work roll being discretized according to a 2D axisymmetric mesh.

7. The preheating process according to claim 1, wherein the metallic strip is produced from an aluminum alloy.

8. A rolling process comprising the following: a. preheating at least one work roll, optionally two work rolls, of a mill intended to roll a metallic strip according to the process of claim 1, b. rolling the metallic strip with the thus preheated at least one work roll, optionally two work rolls.

9. The rolling process according to claim 8, wherein the metallic strip comprises an aluminum alloy, optionally the aluminum alloy is an alloy chosen from AA2014, AA2017, AA2024, AA2027, AA2046, AA2050, AA2056, AA2060, AA2074, AA2098, AA2139, AA2195, AA2198, AA2214, AA2219, AA2519, AA2524, AA2618, AA2654, AA3003, AA3004, AA3005, AA3103, AA3104, AA3105, AA5005, AA5049, AA5050, AA5052, AA5083, AA5086, AA5088, AA5150, AA5154, AA5182, AA5186, AA5200, AA5251, AA5252, AA5254, AA5383, AA5454, AA5456, AA5657, AA5754, AA6016, AA6056, AA6060, AA6061, AA6063, AA6082, AA6156, AA6182, AA6909, AA7010, AA7011, AA7017, AA7019, AA7020, AA7021, AA7022, AA7039, AA7040, AA7049, AA7050, AA7056, AA7072, AA7075, AA7079, AA7099, AA7122, AA7150, AA7175, AA7178, AA7449, AA7450 or AA7475 alloy.

10. The rolling process according to claim 9, wherein the aluminum alloy is cladded on at least one side, optionally two sides with a 1000 Series alloy according to the Aluminum Association, optionally AA1050 alloy or with AA7072 alloy.

11. The rolling process according to claim 8, wherein the rolling of the metallic strip is a hot rolling.

12. The rolling process according to claim 11, wherein the temperature of the aluminum alloy, optionally cladded, prior to hot rolling thereof is at least 350? C. and at most 510? C.

13. The rolling process according to claim 11, wherein the surface temperature of the preheated work roll is at least 200? C. and at most 320? C.

14. The rolling process according to claim 8, wherein in that the rolling of the metallic strip is a cold rolling.

15. The rolling process according to claim 14, wherein the surface temperature of the preheated work roll is at least 100? C. and at most 200? C.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0031] Further aspects, aims, advantages and features of the invention will become more apparent on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, with reference to the appended drawings wherein:

[0032] FIG. 1A is a schematic and partial view of a mill stand of a mill according to an embodiment, in a cross-section along the longitudinal axis of the rolling mill, illustrating the thermal control device;

[0033] FIG. 1B is a schematic and partial view of a work roll of the mill stand of FIG. 1A, in a cross-section along the longitudinal axis of the work roll, illustrating the segmentation of the work roll into Ns longitudinal segments and the longitudinal distribution of the Ni induction coils;

[0034] FIG. 2A is a schematic and partial view of a work roll, in a cross-section along its longitudinal axis, showing an example of a thermal expansion profile;

[0035] FIG. 2B illustrates an example of a target thermal expansion profile of the work roll, as well as the parameters A, B, xx and u for characterizing it;

[0036] FIG. 3 is a flow chart of a preheating process implemented by the rolling mill in FIG. 1A according to an embodiment.

DETAILED DISCLOSURE OF PARTICULAR EMBODIMENTS

[0037] In the figures and hereinafter in the description, the same references represent identical or similar elements. Furthermore, the different elements are not represented to scale so as to favor the clarity of the figures. Moreover, the different embodiments and alternative embodiments are not mutually exclusive and can be combined with each other. Unless specified otherwise, the terms substantially, approximately, of the order of mean within 10%, and preferably within 5%. Moreover, the terms between . . . and . . . and equivalents mean that the bounds are included, unless stated otherwise.

[0038] The invention relates to a process for preheating at least one work roll of a mill stand, making it possible to thermally expand the work roll locally, according to a predetermined target profile, before the metallic strip to be rolled is introduced into the roll gap. Hereinafter in the description, the metallic strip is produced based on aluminum, without the invention however being limited to this type of material.

[0039] The target thermal expansion profile of the preheating accounts for the features of the metallic strip to be rolled, and corresponds substantially to that generated during the rolling operation per se, by adding heat essentially obtained from the deformation of the metallic strip in the roll gap. In addition, following the preheating operation, the work roll then has a thermal stability similar or substantially identical to that during the rolling operation.

[0040] The target thermal expansion profile is predefined such that, during the rolling operation, the rolled metallic strip has, at the exit of the roll gap, the desired thickness profile and flatness. To this end, the flatness of a rolled metallic strip is defined based on an index IP corresponding to the ratio of the elongation of a material fiber relative to the mean length L of the fibers, such that: IP=?L/L?10.sup.5.

[0041] Hereinafter in the description, the profile of a physical quantity associated with the work roll is the variation (or the distribution) of this physical quantity along the longitudinal axis of the roll. On the other hand, the profile of the metallic strip is the variation (or the distribution) of thickness in a cross-section along a transversal axis (widthwise) of the metallic strip.

[0042] FIG. 1A is a schematic and partial view of a rolling mill 1 including several successive mill stands 10, in a cross-section along the longitudinal axis of the rolling mill. Here only one mill stand 10 is shown. FIG. 1B is a schematic and partial view of the mill stand 10, in a cross-section along the longitudinal axis of the work roll.

[0043] Here and for the rest of the description, an orthogonal three-dimensional direct reference XYZ is defined, where the axis X is oriented along the rolling direction and corresponds to the longitudinal axis of the rolling mill 1 and the metallic strip 2 during rolling, the axis Y corresponds to the longitudinal axis of the rolls, and the axis Z is oriented along the height of the mill stand 10. The terms upstream and downstream are defined with reference to the longitudinal axis of the rolling mill 1, i.e., here to the axis X.

[0044] In this example, the rolling mill 1 can include several successive mill stands 10 to roll a single metallic strip 2. It also includes a thermal control device 20 adapted to control the thermal expansion profile of at least one of the work rolls 11 by means of a plurality of induction coils 21 and optionally coolers 22.

[0045] Each mill stand 10 is here of quarto type and includes here a pair of work rolls 11 (lower and upper rolls), and a pair of back up rolls 12 (lower and upper). Obviously, other configurations are possible, such as sexto or Sendzimir type stands, inter alia. Each work roll 11 of the mill stands 10 can be equipped with induction coils 21 and optionally coolers 22 of the thermal control device 20. The rolling mill 1 can however include, upstream from the mill stands 10, at least one reversible stand not equipped with induction coils of the thermal control device.

[0046] The work rolls 11 are equipped here with induction coils 21 and coolers 22. They each have a diameter d.sub.ref at ambient temperature, outside operation, which is here constant along the longitudinal axis Y: d.sub.ref(y)=d.sub.0. This diameter can, alternatively, not be constant and can have a grinding profile: d.sub.ref(y)=d.sub.0+?d.sub.rec(y) mentioned above.

[0047] The thermal control device 20 includes a plurality of induction coils 21 and optionally a plurality of coolers 22, connected to a processing unit 23. It makes it possible to generate, within the scope of the preheating of the work roll 11 in question, and therefore before the rolling operation, a thermal expansion profile at the work roll 11 in question which is substantially equal to a predetermined target profile.

[0048] For this, the work roll 11 is considered to be discretized along its entire length, along the longitudinal axis Y, into Ns successive longitudinal segments 11s, preferably of the same dimension. For example the work roll 11 can be discretized into several longitudinal segments 11s of width equal to approximately 20 mm along the axis Y.

[0049] The thermal control device 20 includes Ni induction coils 21, where here Ni?Ns. They are distributed along the longitudinal axis Y facing the Ns successive longitudinal segments, at a rate here of 1 induction coil for several successive longitudinal segments 11s. Not all the longitudinal segments 11s necessarily include induction coils 21, in particular the longitudinal segments 11s located at the edge of the work roll 11 and which are not intended to form the roll gap (no contact with the metallic strip 2). The induction coils 21 can be placed upstream and/or downstream from the work roll 11. In this example, Ni induction coils are located upstream and Ni induction coils are located downstream from the work roll 11.

[0050] The induction coils 21 are adapted to transmit a thermal energy in the longitudinal segments 11s of the work roll 11. It consists here of electromagnetic induction heating, in that each induction coil 21 generates a magnetic field which induces an alternating electric current in the longitudinal segment(s) 11s facing which it is disposed. The electromagnetic power received by the longitudinal segments 11s is converted by Joule effect into heat power, which thus results in an increase in the mean temperature of the longitudinal segments 11s in question.

[0051] The induction coils 21 are activated and supply a thermal power in response to a control signal from the control unit 23 which defines a target value of the thermal power. However, it appears that the induction coils 21 may not supply the target value of the thermal power effectively. In addition, they each include a sensor (not shown) adapted to supply the processing unit 23 with a measurement of the thermal power actually supplied.

[0052] The thermal control device 10 can also include coolers 22, distributed along the work roll 11. Each cooler 22 can be a nozzle for spraying a cooling liquid. These coolers 22 thus make it possible to reduce the mean temperature of the longitudinal segments 11s of the work roll. They can be more or less numerous than the induction coils 21. Furthermore, the longitudinal arrangement of the coolers 22 may not coincide with that of the induction coils 21.

[0053] The processing unit 23 is adapted to perform calculations at different successive calculation times t.sub.k, and to control the induction coils 21 and where applicable the coolers 22 such that the effective mean temperature profile of the longitudinal segments 11s (and therefore the effective thermal expansion profile) is substantially equal to the target mean temperature profile (and therefore to a target thermal expansion profile).

[0054] The processing unit 23 includes a programmable processor capable of executing instructions recorded in a data recording medium. It furthermore includes a memory containing the instructions required for the implementation of the preheating process. It is also adapted to store the data calculated at each calculation time t.sub.k. It implements moreover two predefined physical models M1 and M2.

[0055] The first predefined physical model M1 expresses a relationship between, on one hand, input parameters Pe representative of dimensions and the mechanical and thermal properties of the metallic strip 2 to be rolled, and, on the other, a target thermal expansion profile of the work roll defined at the Ns longitudinal segments 11s.

[0056] The predefined physical model M1 can be a database (scales) obtained beforehand for example experimentally and/or digitally. Thus, to obtain the desired properties of the metallic strip 2 (thickness profiles, flatness, etc.) at the exit of the roll gap, the predefined physical model M1 establishes a relationship between the target thermal expansion profile required to obtain these properties of the metallic strip 2, and the input parameters Pe.

[0057] The input parameters Pe relate in particular to the mechanical features of the metallic strip 2 to be rolled such as the type of aluminum alloy, the thermal features such as the temperature of the metallic strip 2 at the entry of the mill stand 10 and the desired coiling temperature, the dimensions of the metallic strip 2 to be rolled such as its width W, the initial thickness H and the output thickness h. Further features can be taken into account. These input parameters Pe make it possible to estimate the rolling force and therefore the heat produced in the roll gap when rolling the metallic strip 2, as well the deflection of the work rolls under the mechanical force, these thermal and mechanical expansions being intended to be compensated by the preheating process according to the invention.

[0058] The target thermal expansion profile corresponds to the distribution along the longitudinal axis Y of the local variation ?d.sub.th(y) in diameter of the work roll 11 on account of a temperature variation ?T between two successive calculation times t.sub.k. It therefore consists of a variation in diameter relative to the reference profile d.sub.ref(y), whether the latter includes the grinding component ?d.sub.rec(y) or not. The target thermal expansion profile is independent of the calculation time t.sub.k, and is determined at the start of the preheating process (it can however be adjusted according to the thermal state of the preceding mills (e.g., roughing stand). It is noted ?d.sub.th.sup.c(y) when the profile is defined along the continuous y-axis y along the longitudinal axis Y, and is noted (?d.sub.th(i).sup.c).sub.1?i?Ns or more simply ?d.sub.th.sup.c (vector of Ns values) when it is defined along the Ns longitudinal segments 11s.

[0059] To this end, FIG. 2A illustrates a schematic and partial view, in a cross-section along the longitudinal axis Y, of a work roll 11 having a thermal expansion profile ?d.sub.th(y). The thermal expansion profile ?d.sub.th.sup.c(y) corresponds to the deviation with the reference profile d.sub.ref(y). In this example, the reference profile d.sub.ref(y) is constant (no grinding component). Obviously, the thermal expansion profile ?d.sub.th.sup.c(y) is not to scale to favor the clarity of the figure.

[0060] FIG. 2B illustrates an example of a target thermal expansion profile ?d.sub.th(y) of a work roll 11 determined by the predefined physical model M1, showing the parameters for characterizing such a profile, the values of these parameters being stored in the predefined physical model M1. These parameters are noted here A, B, u and xx. In this example, A is equal to 0.2 mm, B is equal to 0.18 mm, xx to 500 mm, u to 400 mm, for a set of predefined input parameters Pe, of which a width W of the metallic strip 2 to be rolled equal to 2000 mm.

[0061] The target thermal expansion profile ?d.sub.th(y) is here a parabola over a distance W/2-xx from the center of the work roll 11 along the longitudinal axis Y (more specifically from the center of the table of the roll 11), with an amplitude A at the center, and an amplitude B at the y-axis xx. Then, between the position xx and the end of the table of the roll, the profile has a decline given by a function erf. The parameter u makes it possible to calculate the y-axis W/2-xx+u from the center of the table of the roll for which the profile equals B/2. Obviously, these parameters are given by way of illustration and other parameters can be used to characterize the target thermal expansion profile.

[0062] The predefined physical model M1 therefore includes the values of the parameters A, B, xx and u, which are dependent on the input parameters Pe. These values can furthermore correspond to a standardized profile for a reference ratio W to H (width of the metallic strip 2 over the input thickness H), for example here for W=1800 mm and H=18 mm. This standardization for a reference ratio W/H being useful when the input thickness is dependent in particular on the limits of the mill stand. These values can be given for the mill stand 10 in question, in particular for the first mill stand 10 as well as for the subsequent mill stands 10 (by means for example of a homothetic adaptation). Indeed, the preheating process can consist of heating the different mill stands 10 of the rolling mill before the rolling operation per se.

[0063] Moreover, when the rolling mill 1 includes a reversible stand upstream from the mill stands 10, the predefined physical model M1 can in particular provide an update of the values of the parameters A, B, xx and u according to the amplitude of the thermal expansion profile of the work rolls 11 of the reversible stand. Thus, for example, the value of this amplitude is known and is subtracted from the value of the parameter A. The reversible stand is an upstream stand which is not thermally controlled by the thermal control device 20, in that it does not include induction coils 21. On the other hand, it includes here coolers 22.

[0064] Finally, as stated hereinafter, when the target thermal expansion profile has been determined, the processing unit 23 performs its discretization along the Ns longitudinal segments 11s.

[0065] The predefined physical model M2 expresses, for each calculation time t.sub.k, a relationship between a measured effective thermal power profile (P.sub.Q(i).sup.eff(t.sub.k)).sub.1?i?Ni and an effective mean temperature profile (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?N of the work roll 11. Thereafter, the following vector notation is used for these profiles: P.sub.Q.sup.eff (t.sub.k) and T.sup.eff(t.sub.k), where the vectors respectively include Ni and Ns values.

[0066] The effective thermal power profile P.sub.Q.sup.eff(t.sub.k) corresponds to the measurements made by the sensors of the Ni induction coils 21 at the calculation time t.sub.k and sent to the processing unit 23. Based on these measurements, the predefined physical model M2 determines the mean thermal energy received by the longitudinal segments 11s in question between the two consecutive calculation times. The predefined physical model M2 can be a database (scales) obtained beforehand for example experimentally and/or digitally.

[0067] The effective mean temperature profile T.sup.eff(t.sub.k) corresponds to the mean temperature of the Ns longitudinal segments 11s at the calculation time t.sub.k, and is dependent in particular on the mean thermal energy received from the induction coils 21 (and where applicable the mean thermal energy lost on account of the coolers 22), and therefore the measured effective thermal power profile P.sub.Q.sup.eff(t.sub.k). It consists of a mean temperature of the longitudinal segments 11s of the work roll 11, and not only the mean temperature of the surface of the roll 11 in the longitudinal segments 11s. The mean temperature is therefore constant at all points of the volume of each longitudinal segment 11s in question.

[0068] The effective mean temperature profile T.sup.eff(t.sub.k) is here determined by means of digital simulation software by resolution of the predefined physical model M2 where the work roll 11 is discretized into an axisymmetric mesh, where the meshes are formed along the longitudinal axis Y of the Ns longitudinal segments 11s. It can consist of a 1D axisymmetric mesh where each mesh corresponds to a longitudinal segment, or a 2D axisymmetric mesh, i.e., along the longitudinal axis Y (longitudinal segments) and along a radial axis. The predefined physical model M2 is a physical model which assesses the incoming and outgoing thermal flows, and accounts for the thermal diffusion along the longitudinal axis Y. Thus, according to the mean thermal energy received by each longitudinal segment 11s (induction coils 21) and the mean thermal energy lost (coolers), the input parameters which make it possible to estimate a mechanical force and therefore a thermal energy produced in the roll gap during the rolling operation, the predefined physical model M2 (e.g., 2D axisymmetric model) is solved by digital simulation, for example in finite differences, and makes it possible to determine the effective mean temperature profile T.sup.eff(t.sub.k) of the Ns longitudinal segments.

[0069] FIG. 3 illustrates a flow chart of a preheating process of a work roll 11 of a rolling mill 1 according to an embodiment. Obviously, the process can relate to the two work rolls 11 of the mill stand in question, as well as all of the mill stands of the rolling mill 1. The process is implemented before the rolling operation per se of the metallic strip 2, and stops when the rolling operation starts. The preheating can however be activated once again when the convergence criterion mentioned below is no longer satisfied.

[0070] As a general rule, the preheating process includes a preliminary phase 10 of determining the target thermal expansion profile ?d.sup.c of the work roll, subsequently followed by several phases performed at each calculation time t.sub.k, namely a phase 20 of determining the effective mean temperature profile T.sup.eff(t.sub.k) of the work roll, a phase 30 of determining the target mean temperature profile T.sup.c(t.sub.k) of the work roll, then, based on a deviation between the two mean temperature profiles T.sup.eff(t.sub.k) and T.sup.c(t.sub.k) determined, a phase of activating or not the induction coils based on a target thermal power profile P.sub.Q.sup.eff(t.sub.k) that has been determined.

[0071] As stated above, the work roll 11 is discretized into Ns longitudinal segments 11s, and the thermal control device 20 includes Ni induction coils 21 distributed along the longitudinal axis Y, where here Ns?Ni. In this example, for clarity purposes, the coolers 22 that may be included in the thermal control device 20 are disregarded.

[0072] Phase 10: determining a target thermal expansion profile (?d.sub.th(i).sup.c).sub.1?i?Ns of the work roll 11. The vector notation ?d.sub.th is used hereinafter.

[0073] During a step 11, input parameters Pe representative of the mechanical features of the metallic strip 2 to be rolled such as the type of aluminum alloy, the thermal features such as the temperature of the metallic strip at the entry of the mill stand and the desired coiling temperature, the dimensions of the metallic strip 2 to be rolled such as its width W, the initial thickness H and the output thickness h are defined.

[0074] During a step 12, the processing unit 23 then determines the target thermal expansion profile ?d.sub.th based on the input parameters Pe defined and by means of the predefined physical model M1 implemented in the memory of the processing unit 23.

[0075] During a step 13, the target thermal expansion profile ?d.sub.th.sup.c(y) is discretized on the Ns longitudinal segments 11s, to thus obtain the profile (?d.sub.th(i).sup.c).sub.1?i?Ns (noted ?d.sub.th.sup.c.

[0076] Note that the thermal expansion profile ?d.sub.th, as well as the target T.sup.c(t.sub.k) and effective T.sup.eff(t.sub.k) mean temperature profile may only be defined for the longitudinal segments 11s intended to be contact with the metallic strip 2, i.e., here for the longitudinal segments 11s of index ranging from iwi to iwf.

[0077] The subsequent phases 20 to 60 are performed iteratively at different successive times, the time being discretized at a predefined calculation frequency, for example every 45 seconds. Thus, a calculation time t.sub.k also referred to as current time is associated with each iteration of rank k.

[0078] Phase 20: determining an effective mean temperature profile (T.sub.(i).sup.eff(t.sub.k)).sub.1?i?Ns (noted T.sup.eff(t.sub.k)) of the Ns longitudinal segments 11s of the work roll 11.

[0079] During a step 21, the effective mean temperature profile T.sup.eff(t.sub.k) of the work roll 11 is determined. As stated above, it consists of the mean temperature of each longitudinal segment 11s of the work roll 11 (constant temperature on the surface and in the volume of the longitudinal segment), and not only the surface temperature. This mean temperature is responsible for the (mean) thermal expansion of the longitudinal segment 11s in question.

[0080] This profile is determined using an effective thermal power profile (P.sub.Q(i).sup.eff(t.sub.k+1)).sub.1?i?Ni (and noted vectorially P.sub.Q.sup.eff(t.sub.k)) supplied by the induction coils 21 and measured beforehand by the sensors of the induction coils 21 at the calculation time t.sub.k+1, and by means of the second predefined physical model M2 expressing a relationship between the effective thermal power profile P.sub.Q.sup.eff (t.sub.k) and the effective mean temperature profile T.sup.eff(t). During the first calculation time t.sub.k=1, the induction coils 21 having not yet supplied the thermal power, the effective temperature profile T.sup.eff(t.sub.k=1) can be equal to ambient temperature, or can be equal to a predefined temperature (uniform or not) corresponding to the thermal state of the work rolls following prior rolling operations for example.

[0081] Phase 30: determining a target mean temperature profile (T.sub.(i).sup.c)(t.sub.k)).sub.1?i?Ns (noted T.sup.c(t.sub.k)) of the Ns longitudinal segments 11s of the work roll 11.

[0082] During a step 31, the target mean temperature profile T.sup.c(t.sub.k) of the Ns longitudinal segments 11s of the work roll 11 is defined, based on the effective mean temperature profile T.sup.eff(t.sub.k) and the target thermal expansion profile ?d.sub.th.sup.c. For this, a longitudinal segment 11, here of index iwi, where one of the lateral edges of the metallic strip 2 is located, is taken as a reference. Then, the target mean temperature is calculated based on the following equation: ?j=iwi, iwf T.sub.(j).sup.c(t.sub.k)=T.sub.(iwi).sup.eff (t.sub.k)+(?d.sub.(j).sup.c??d.sub.(iwi).sup.c)/(??d.sub.ref(j))), where d.sub.ref(j) is the reference diameter at the index j, and where a is the mean thermal expansion coefficient of the work roll 11. Obviously, other calculations, are possible.

[0083] Phase 40: determining a deviation ?T(t.sub.k) between the target mean temperature profile T.sup.c(t.sub.k) and the effective mean temperature profile T.sup.eff(t.sub.k).

[0084] During a step 41, a maximum deviation ?T.sub.(jmax)(t.sub.k) is determined here between the target mean temperature profile T.sup.c(t.sub.k) and the effective mean temperature profile T.sup.eff(t.sub.k). For this, the longitudinal segment 11s of index jmax located between iwi and iwf for which the temperature deviation is maximum: ?T.sub.(jmax)(t.sub.k)=.sub.j=iwi,iwf.sup.max (T.sub.(j).sup.c(t.sub.k))?T.sub.(j).sup.eff (t.sub.k) The aim is here that of identifying the induction coil 21 closest to this longitudinal segment of index jmax of which the target thermal power will be brought to a maximum value.

[0085] Phase 50: convergence criterion

[0086] During a step 50, a convergence criterion is determined wherein a comparison is made to a predefined threshold value E of a deviation Ec(t.sub.k) representative of the deviation ?T(t.sub.k) between the target mean temperature profile T.sup.c(t.sub.k) and the effective mean temperature profile T.sup.eff(t.sub.k). This deviation Ec(t.sub.k) is therefore also representative of the deviation between the target thermal expansion profile ?d.sup.c(t.sub.k) and the effective thermal expansion profile ?d.sup.eff(t.sub.k).

[0087] The deviation Ec(t.sub.k) can be defined in different ways. It can consist of the local maximum value ?T.sub.(jmax)(t.sub.k) between the target mean temperature profile T.sup.c(t.sub.k) and the effective mean temperature profile T.sup.eff(t.sub.k). It can also consist of a point-by-point comparison between the target mean temperature profile T.sup.c(t.sub.k) and the effective mean temperature profile T.sup.eff(t.sub.k), for example a mean or a sum, optionally weighted, of the difference in absolute value between these two profiles. It can also consist of an activation time of the induction coil 21 associated with the longitudinal segment of index jmax, i.e., that for which the temperature deviation ?T.sub.(jmax)(t.sub.k) is maximum.

[0088] The convergence criterion is considered as satisfied when the deviation Ec(t.sub.k) is less than or equal to the threshold value E, in which case the preheating of the work roll(s) is considered as completed (step 70). An item of information can then be given to the user of the rolling mill 1, for example the deviation Ec(t.sub.k) in question, or an item of remaining heating time information (ratio between the temperature deviation and the injected thermal power). However, the convergence criterion is considered as not satisfied when the deviation Ec(t.sub.k) is greater than the threshold value E, in which case the preheating process continues with phase 60. Note that phase 50 is performed here between phases 40 and 60, but it can obviously be performed at other times of the process, for example after phase 60. In the case where the convergence criterion is not satisfied, the process continues with phase 60.

[0089] Phase 60: determining a target thermal power profile (P.sub.Q(i).sup.c (t.sub.k).sub.1?i?Ni (noted P.sub.Q.sup.c(t.sub.k)) and activating the induction coils accordingly.

[0090] During a step 61, the target thermal power profile P.sub.Q.sup.c (t.sub.k) to be supplied by the induction coils 21 is determined. It is possible to this end to envisage only activating the induction coils 21 intended to be facing the metallic strip during the rolling operation, i.e., located facing the longitudinal segments 11s of indexes between iwi and iwf. For this, the target thermal power of the induction coil 21 of index jmax is defined at 100% of the maximum thermal power P.sub.Q,max. The target thermal power P.sub.Q(j).sup.eff (t.sub.k) to be supplied by the other induction coils 21 of index j is then defined as being equal to the maximum thermal power P.sub.Q,max modulated by the ratio ?T.sub.(j)(t.sub.k)?T.sub.(jmax)(t.sub.k). To account for the lateral thermal diffusion, an activation threshold can be taken into account: thus, when the ratio ?T.sub.(jmax)(t.sub.k)/?T.sub.(jmax)(t.sub.k) is less than a predefined threshold R.sub.T, the induction coil 21 of index j in question is not activated. The target thermal power profile P.sub.Q.sup.c (t.sub.k) of the induction coils 21 is thus obtained.

[0091] During a step 62, a control signal is sent by the processing unit 23 to the induction coils 21 so that they supply a target thermal power P.sub.Q.sup.c (t.sub.k). According to the value of the target thermal power P.sub.Q(j).sup.c (t.sub.k), the induction coils 21 are activated or not and supply (or attempt to supply) the determined target thermal power. In an equivalent way, the control signal can be sent to the coolers 22 when the local effective mean temperature is greater than the local target mean temperature, so as to reduce the corresponding deviation.

[0092] During a step 63, each sensor of the induction coils 21 measures the thermal power P.sub.Q(j).sup.eff (t.sub.k) actually supplied, here simultaneously with their operation, and sends the measured value thereof to the processing unit 23. These values thus form an effective thermal power profile P.sub.Q.sup.eff(t.sub.k).

[0093] When the convergence criterion has not been satisfied, phases 20 to 50 are then repeated, and the calculation time t.sub.k is incremented.

[0094] On the other hand, if it has been satisfied, this phase 60 may not have been performed, and the information is given to the operator of the rolling mill 1 that the effective mean temperature profile T.sup.eff(t.sub.k) has converged toward the target mean temperature profile T.sup.c(t.sub.k), and therefore the effective thermal expansion profile ?d.sub.th.sup.eff (t.sub.k) has converged toward the target thermal expansion profile ?d.sub.th.sup.c(t.sub.k). The rolling operation of the metallic strip 2 can therefore start and the induction coils 21 can be deactivated, immediately (or not), insofar as the heat produced by rolling the metallic strip 2 in the roll gap will trigger a thermal expansion of the work roll 21 corresponding to the target thermal expansion profile ?d.sub.th.sup.c.

[0095] It appears that the preheating process according to the invention makes it possible to preheat simply and effectively the work roll(s) 11 before the implementation of the rolling of the metallic strip 2 per se. Indeed, the use of induction coils 21 and a predefined physical model M2 receiving the measurements of the effective thermal power of the induction coils 21 make it possible to modify quickly and precisely the effective mean temperature profile so that it tends toward the target mean temperature profile.

[0096] Indeed, the induction coils 21 modify the mean surface and volume temperature of the longitudinal segments 11s, and not only the surface temperature such as the nozzles for spraying a heating liquid, which makes it possible to use a simplified predefined physical model M2, for example a 2D axisymmetric type model, which directly determines the mean temperature of the longitudinal segments of the work roll without involving the measurement of the surface temperature. On the other hand, in the prior art where nozzles for spraying a heating liquid are provided, the physical model requires more complexity and must determine the mean temperature based on the measurement of the surface temperature (hence the use of specific sensors). Furthermore, the injected thermal power is sent directly into the longitudinal segments of the work roll, with no exchange coefficient, since there is overheating by Joule effect of the eddy currents induced. On the other hand, with a heating liquid (e.g., water), the energy efficiency is impacted by the exchange coefficient, and the maximum overheating is limited by the boiling point of the liquid.

[0097] Particular embodiments have just been described. Various variants and modifications are possible within the scope of the invention. Thus, as mentioned above, the thermal control device can include coolers distributed along the longitudinal axis Y of the work roll, and the processing unit can send a control signal to the coolers based on the deviation ?T(t.sub.k) between the target mean temperature profile T.sup.c(t.sub.k) and the effective mean temperature profile T.sup.eff(t.sub.k).

[0098] In a preferred embodiment, the metallic strip comprises an aluminum alloy, preferably the aluminum alloy is an alloy chosen, according to the aluminum association designation, from AA2014, AA2017, AA2024, AA2027, AA2046, AA2050, AA2056, AA2060, AA2074, AA2098, AA2139, AA2195, AA2198, AA2214, AA2219, AA2519, AA2524, AA2618, AA2654, AA3003, AA3004, AA3005, AA3103, AA3104, AA3105, AA5005, AA5049, AA5050, AA5052, AA5083, AA5086, AA5088, AA5150, AA5154, AA5182, AA5186, AA5200, AA5251, AA5252, AA5254, AA5383, AA5454, AA5456, AA5657, AA5754, AA6016, AA6056, AA6060, AA6061, AA6063, AA6082, AA6156, AA6182, AA6909, AA7010, AA7011, AA7017, AA7019, AA7020, AA7021, AA7022, AA7039, AA7040, AA7049, AA7050, AA7056, AA7072, AA7075, AA7079, AA7099, AA7122, AA7150, AA7175, AA7178, AA7449, AA7450 or AA7475 alloy.

[0099] In an embodiment, the metallic strip is a cladded aluminum alloy. In an embodiment, the aluminum alloy is cladded on at least one side, preferably two sides with a 1000 Series alloy according to the Aluminum Association, preferably AA1050 alloy or with AA7072 alloy. In a preferred embodiment, the central part of the cladded aluminum is AA2024 alloy or AA2524 alloy and the cladding is a 1000 Series alloy, preferably AA1050. In another preferred embodiment, the central part of the cladded aluminum is AA7075, AA7175 or AA7475 alloy and the cladding is AA7072 alloy. Cladded aluminum alloys are known as cladded product in the standard NF EN 12258-1.

[0100] In a preferred embodiment, the rolling of the metallic strip is a hot rolling. Preferably, the hot rolling is carried out with a rolling mill which is part of a plurality of hot mills operating in tandem, preferably preceded by a hot reversing mill.

[0101] In an embodiment, the temperature of the aluminum alloy, optionally cladded, prior to its hot rolling is at least 350? C. and at most 510? C. or 490? C. or 470? C. or 450? C. or 430? C. or 410? C. or 390? C. or 370? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, prior to its hot rolling is at least 370? C. and at most 510? C. or 490? C. or 470? C. or 450? C. or 430? C. or 410? C. or 390? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, prior to its hot rolling is at least 390? C. and at most 510? C. or 490? C. or 470? C. or 450? C. or 430? C. or 410? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, prior to its hot rolling is at least 410? C. and at most 510? C. or 490? C. or 470? C. or 450? C. or 430? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, prior to its hot rolling is at least 430? C. and at most 510? C. or 490? C. or 470? C. or 450? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, prior to its hot rolling is at least 450? C. and at most 510? C. or 490? C. or 470? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, prior to its hot rolling is at least 470? C. and at most 510? C. or 490? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, prior to its hot rolling is at least 490? C. and at most 510? C.

[0102] In an embodiment, the temperature of the aluminum alloy, optionally cladded, after its hot rolling is at least 230? C. and at most 370? C. or 350? C. or 330? C. or 310? C. or 290? C. or 270? C. or 250? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, after its hot rolling is at least 250? C. and at most 370? C. or 350? C. or 330? C. or 310? C. or 290? C. or 270? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, after its hot rolling is at least 270? C. and at most 370? C. or 350? C. or 330? C. or 310? C. or 290? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, after its hot rolling is at least 290? C. and at most 370? C. or 350? C. or 330? C. or 310? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, after its hot rolling is at least 310? C. and at most 370? C. or 350? C. or 330? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, after its hot rolling is at least 330? C. and at most 370? C. or 350? C. In another embodiment, the temperature of the aluminum alloy, optionally cladded, after its hot rolling is at least 350? C. and at most 370? C.

[0103] In an embodiment, the surface temperature of the preheated work roll is at least 200? C. and at most 320? C. or 300? C. or 280? C. or 260? C. or 240? C. or 220? C. In another embodiment, the surface roll temperature during hot rolling is at least 220? C. and at most 320? C. or 300? C. or 280? C. or 260? C. or 240? C. In another embodiment, the surface roll temperature during hot rolling is at least 240? C. and at most 320? C. or 300? C. or 280? C. or 260? C. In another embodiment, the surface roll temperature during hot rolling is at least 260? C. and at most 320? C. or 300? C. or 280? C. In another embodiment, the surface roll temperature during hot rolling is at least 280? C. and at most 320? C. or 300? C. In another embodiment, the surface roll temperature during hot rolling is at least 300? C. and at most 320? C.

[0104] In another preferred embodiment, the rolling of the metallic strip is a cold rolling. Preferably, the cold rolling is carried out with a rolling mill which is part of a plurality of cold mills operating in tandem.

[0105] In an embodiment, the surface temperature of the preheated work roll is at least 100? C. and at most 200? C. or 180? C. or 160? C. or 140? C. or 120? C. In another embodiment, the surface roll temperature during cold rolling is at least 120? C. and at most 200? C. or 180? C. or 160? C. or 140? C. In another embodiment, the surface roll temperature during cold rolling is at least 140? C. and at most 200? C. or 180? C. or 160? C. In another embodiment, the surface roll temperature during cold rolling is at least 160? C. and at most 200? C. or 180? C. In another embodiment, the surface roll temperature during cold rolling is at least 180? C. and at most 200? C.

[0106] For example, the two work rolls, of diameter 700 mm, of a hot mill were each equipped with 33 induction coils of 80 mm along the length of said rolls. The work rolls were discretized into 20 mm long segments for the implementation of the preheating process according to the invention. The use of the process for preheating the rolls made it possible to do away with the use of starting metallic strips to stabilize the thermal profiles required to hot roll aluminum alloys. This improvement relates in particular to AA5083, AA5086, AA5088, AA5182, AA5052, AA5754, AA2098, AA2198, AA2195, AA2024 and AA2524 alloys. This improvement also relates to cladded aluminum alloys, the central part being an AA2024 or AA2524 aluminum alloy and the cladding being AA1050. This improvement also relates to cladded aluminum alloys, of which the central part is an AA7075 or AA7175 aluminum alloy and of which the cladding is AA7072.