METHOD FOR MODELING THE BEHAVIOUR OF A ROUND ROLLING MILL

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of modeling forging processes, and in particular the modeling of circular rolling processes. STATE OF THE ART

[0002] Modeling forging processes is a major industrial challenge today because it allows for the design of new forging ranges or the optimization of existing ranges. A forging range is the set of shaping operations used, using specific tools, to develop a billet until a defect-free raw part of the desired shape is obtained. Reliable and realistic modeling of all these operations, and in particular the behavior of the tools and the billet, allows for the reduction of the design time for a new forging range, since it also reduces the number of test pieces to be produced to validate the new range.

[0003] Forging processes using a hydraulic press or, more generally, vertical forging machines are generally easy to model because the principle of controlling the press is simple. Indeed, there is only a translational movement of the press during forging. However, these forging processes are not very suitable for the production of crowns or seamless rings, for example. In order to allow optimal use of the material, a ring rolling process is used to produce such parts.

[0004] In the case of ring rolling, modeling the behavior of the tools is complicated to implement, because it is necessary to take into account in a synchronized manner the simultaneous translational and rotational movements of different tools. Similarly, the movements of the different tools lead to several simultaneous changes in the parameters of the billet, which must be controlled. As a reminder, the principle of a ring rolling process is generally to reduce the section and height of the billet to increase its diameter, in a controlled manner.

[0005] To model this type of process, it is necessary to control the movement of all the tools that make up the circular rolling mill. Generally, the tool commands are given as input to a rolling mill control system by an operator, and depend on the part to be rolled. When the operator wishes to roll a new part, he must first determine the input data, which are not only the final dimensions of the part to be obtained, but also the joint evolution of these dimensions.

[0006] The rolling process can be modeled numerically by finite element calculations that do not take into account the adaptive operation of the rolling mill tools managed by the control system. In this case, it is generally necessary to machine at least one part to control the quality of the forging range and recover acquisition data, in particular tool movement data and force data exerted by the tools during the rolling of the part, to integrate them into the modeling of the rolling process.

[0007] However, this practice has several limitations. On the one hand, it is necessary to produce a part to carry out a modeling, which results in a loss of time and significant cost. On the other hand, each model produced is only relevant to a particular part shape and a particular tool, and it is therefore impossible to predict the behavior of the different tools if the rough shape of the cylindrical part to be produced is different. Finally, such models are complicated to put into data, because there is a need to rework the acquisitions.

[0008] Finite element calculation models exist that take into account the adaptive operation of the rolling mill tools managed by the control system.

[0009] To date, there are machine control models integrated into calculation codes such as Simufact. However, the models used do not take into account the mechanical aspects of the circular rolling mill 1. The model predictions are therefore not very accurate, and it is necessary to forge real parts to validate new forging ranges. Typically, existing models can significantly underestimate the duration of the rolling process, for example by 10%. Existing models can also simulate radial and axial forces F cone that are not representative of reality.

[0010] Document KR 20110090423 A shows a method for modeling the behavior of a circular rolling mill. In this method, the deformation of the part is calculated by finite elements and the translation speeds of the mandrel and the tapered rollers are adapted to this deformation in order to minimize the load on the mandrel and the rollers. PRESENTATION OF THE INVENTION

[0011] An aim of the invention is to remedy at least in part the aforementioned drawbacks by proposing a modeling method taking into account the behavior of all the moving tools of a circular rolling mill, and their interactions, making it possible to determine forging ranges reliably and quickly.

[0012] This aim is achieved by the present invention by means of a method for modeling the behavior of a circular rolling mill intended to roll a cylindrical part from a setpoint, the circular rolling mill comprising at least one conical roller, configured to have a translational movement in a first direction, and a mandrel, configured to have a translational movement in a second direction, the setpoint comprising a setpoint in speed of increase of an external diameter of said cylindrical part as a function of said external diameter, and a setpoint in height of the cylindrical part in the first direction as a function of a thickness of the cylindrical part in the second direction,said method comprising the steps of: E1- obtaining a first set of parameters characteristic of the behavior of the circular rolling mill by means of a control formula linking a translation speed of the mandrel in the second translation direction to the speed of increase of the external diameter and depending on the setpoint; E2- Calculation by finite elements of a value of force exerted on the conical roller from the first set of parameters; E3- Comparison of the value of force exerted on the conical roller calculated with at least one threshold value of force admissible by the circular rolling mill, and if the value of force exerted on the conical roller is greater than the threshold value of admissible force, obtaining a second set of parameters, so that the setpoint in increase speed is not respected,correction of the second set of parameters in order to obtain a third set of parameters; if the force value exerted on the calculated tapered roller is less than the admissible force threshold value, correction of the first set of parameters by taking into account a stiffness of the mandrel in order to obtain a third set of parameters; said third set of parameters being characteristic of the behavior of the circular rolling mill for the given instruction.,

[0013] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which: the first set of parameters obtained by the control formula includes a displacement speed of the conical roller ḣ and a translation speed of the mandrel ṡ ; the steering formula is given by: with s the position of the mandrel, ṡthe translation speed of the mandrel, Ḋ the rate of increase of the external diameter D of the part to be rolled; during the comparison step E3, the at least one threshold value of admissible force depends on the external diameter of the cylindrical part; during the correction step, a deformation of a mandrel cage, modeled as a spring of stiffness constant k is taken into account, so that the third set of parameters is obtained by an offset of at least one parameter characteristic of the behavior of the mandrel of the circular rolling mill. DESCRIPTION OF FIGURES

[0014] Other characteristics, aims and advantages of the invention will emerge from the following description, which is purely illustrative and non-limiting, and which must be read in conjunction with the appended drawings in which: There figure 1 schematically illustrates a circular rolling mill allowing the rolling of a cylindrical part. The Figures 2a And2b schematically illustrate steps of a method for modeling the behavior of a circular rolling mill according to the invention. The figure 3 schematically illustrates a cylindrical part that can be obtained by a circular rolling process. The figure 4 illustrates examples of input instructions for a circular rolling mill. The Figure 5 schematically illustrates a system for controlling the movements of mobile tools in the circular rolling mill of the figure 1 . There figure 6 illustrates different levels defined by threshold force values ​​admissible by a conical roller of the rolling mill. The figure 7 is a graph representing the evolution of the axial force at the level of a conical roller, calculated by a model obtained by a method according to the invention, by a model of the prior art, and measured experimentally. The figure 8is a graph representing the evolution of the radial force at the level of a mandrel, calculated by a model obtained by a method according to the invention, by a model of the prior art, and measured experimentally. The figure 9 is a graph representing the evolution of the external diameter of a part during rolling, calculated by a model obtained by a method according to the invention, by a model of the prior art, and measured experimentally. The figure 10 is a graph representing the evolution of the rate of increase of the external diameter of a part during rolling, calculated by a model obtained by a method according to the invention, by a model of the prior art, and measured experimentally.

[0015] Only the elements necessary for understanding the invention have been shown. To facilitate reading of the drawings, similar elements bear identical references throughout the figures. DETAILED DESCRIPTION OF THE INVENTION

[0016] There figure 1 schematically illustrates moving tools of a system allowing the production of a cylindrical part called crown 5 during a circular rolling process illustrated on the Figures 2a And 2b . The circular rolling mill 1 comprises at least one conical roller 3, in translation in a first direction Y, and in rotation in a roller direction X'. In the illustrated embodiment, the circular rolling mill 1 comprises an upper conical roller 3 and a lower conical roller 3'.

[0017] The circular rolling mill comprises a motor cylinder 4 rotating around an axis tangent to the first direction Y, substantially vertical. The motor cylinder 4 is controlled in rotation speed by a control unit 10, illustrated schematically in the Figure 5 .

[0018] The rolling mill 1 comprises another cylindrical tool called mandrel 2, also rotating around an axis in the first direction Y. The mandrel 2 can translate in a second direction X, substantially orthogonal to the first direction Y. The translational movement and the rotational movement of the mandrel 2 are controlled by the control unit 10.

[0019] There figure 3illustrates an example of a cylindrical crown 5 which can be obtained by rolling a billet with the circular rolling mill 1. In order to form a crown 5 having a height a in the first direction Y and a thickness e and an external diameter D in the second direction X, an operator can place the crown 5 under the at least one conical roller 3 and between the mandrel 2 and the motor cylinder 4.

[0020] A rotational movement of the conical roller 3 simultaneous with a translational movement of the conical roller 3 makes it possible to change the height a of the crown 5. In a linked manner, rotational movements of the motor cylinder 4 and the mandrel 2, simultaneous with a translational movement of the mandrel 2, make it possible to change the thickness e of the crown.

[0021] All the movements of the tools simultaneously modify the external or outside diameter D of the crown 5.

[0022] In order to achieve the desired dimensions of the crown 5, input instructions must be given as input to the control unit 10 (also called the control system 10) of the circular rolling mill 1. The input instructions include at least the desired dimensions of the cylindrical crown 5 to be rolled. Preferably, the input instructions also include the laws of evolution of the dimensions of the crown 5.

[0023] In an exemplary embodiment, the instruction includes an increase speed instruction Ḋ ( D ) of the external diameter of the cylindrical crown 5 as a function of the external diameter D.

[0024] The instruction may include a height instruction a(e) of the cylindrical crown 5 in the first direction Y as a function of the thickness e of the crown 5 in the second direction X. An example of such input instructions is illustrated in the figure 4 .

[0025] From the instructions given at the input of the control unit 10, the control unit 10 can control the translational and rotational movements of the tools 2, 3, 4 in motion of the rolling mill 1, as illustrated schematically in the Figure 5 Each forging range is associated with a specific instruction. In particular, the circular rolling process stops once the desired external diameter Dcible is reached.

[0026] A first step E1 of the method for modeling the behavior of the circular rolling mill 1 is to determine a control formula, making it possible to link the input setpoint to at least one movement of a mobile tool 2,3 of the rolling mill 1. The control formula makes it possible to obtain a first set of parameters characteristic of the behavior of the circular rolling mill 1.

[0027] Preferably, it is a question of determining a control formula making it possible to link the input instruction to all the movements of the mobile tools 2, 3 of the rolling mill 1.

[0028] In an exemplary embodiment, it is a question of connecting the speed of increase Ḋ of the external diameter D of the crown 5 to be rolled, associated with a particular forging range, at the speed of increase e of the thickness s of the crown 5 according to the second direction X. The rate of increase e of the thickness e is directly linked to the translational movement of the mandrel 2 in the second direction X and therefore to the translational speed of the mandrel ṡ

[0029] Preferably, the relationship between the rate of increase Ḋ of the external diameter D and the rate of increase ethe thickness e also depends on the other parameters of the crown 5, i.e. its height a, its external diameter D and its thickness e.

[0030] In order to determine the control formula describing the different movements of rolling mill 1, it is possible to study the principle of the control loop of rolling mill 1 which makes it possible to manage all the movements of the mobile tools 2, 3, 4 from the instructions entered by the operator.

[0031] In an exemplary embodiment, the control formula which has been identified to reproduce part of the behavior of rolling mill 1 as well as the main instructions entered by the operator in the control unit 10 of rolling mill 1 is given by: s ˙ = s D − 2 . s + h ˙ s ˙ D − s . s h D ˙ with s the position of the mandrel directly linked to the thickness e of the crown, ṡ the translation speed of the mandrel directly linked to the growth speed eof the thickness e of the crown, h the position of the conical roller, ḣ tapered roller travel speed ḣ.

[0032] This non-linear control formula makes it possible to obtain a first translation speed of the mandrel ṡ of the theoretical thickness s, respecting the entry instructions, as illustrated on the figure 4 .

[0033] It is also possible to relate the rate of increase Ḋ of the external diameter D of the crown 5 to be rolled, at the rate of increase ȧ of the height a of the crown 5 according to the first direction Y.

[0034] Preferably, the relationship between the rate of increase Ḋ of the external diameter D and the rate of increase ḣ of the position h also depends on the other parameters of the crown 5, that is to say its height a, its external diameter D and its thickness e.

[0035] The control formula for obtaining the first set of parameters characteristic of the behavior of circular rolling mill 1 can be integrated into a finite element calculation code. In an exemplary embodiment, it can be integrated into the Forge calculation code by the Transvalor calculation code editor.

[0036] The finite element calculation code makes it possible to model the rolling process and in particular to calculate a force F cone exerted by the conical roller 3 on the crown 5 during a rolling process as a function of the first set of parameters.

[0037] In order to improve the control model obtained previously, the mechanical characteristics of the circular rolling mill 1 can be taken into account. In particular, it is possible to integrate into the modeling a first mechanical model translating a limitation in force on the conical roller 3, and a second mechanical model translating the elasticity of the mandrel 2.

[0038] It is also necessary to manage the interaction of these two mechanical models with the model of the circular rolling control loop. Finally, it was necessary to make assumptions consistent with the circular rolling process to make the models interact with each other. This makes it possible to obtain a final control model representative of reality. Force limitation integrated into the conical roller 3

[0039] In order not to damage the rolling mill 1, it is necessary that the force F cone exerted on the conical roller 3 does not exceed a threshold value F threshold . In an exemplary embodiment, the threshold value F threshold depends on the external diameter D of the crown 5 during rolling. For example, the threshold value F threshold (D) may depend on the position in the second direction X of a point on the external edge of diameter D of the crown 5, in contact with a surface of the conical roller 3.

[0040] There figure 6illustrates an exemplary embodiment in which four force threshold values ​​are defined F threshold (D) = {N1,N2,N3,N4}, defining four force limitation levels associated with three different external diameter thresholds D. The force threshold values ​​depend on the type of rolling mill 1. For example, we can have N1=50 tonnes, N2=100 tonnes, N3=150 tonnes and N4=200 tonnes. Dmax corresponds to the maximum external diameter that can be rolled. Dmax is preferably less than the axial dimension of the tapered roller 3 in the second direction X.

[0041] In order to manage the interaction of this first mechanical model with the model of the control loop of the circular rolling mill 1 control formula, two different situations can be defined.

[0042] Preferably, the first mechanical model intervenes when the radial force F cone of the conical roller 3 exceeds the threshold value F threshold .

[0043] The modeling method comprises a step E3 of comparing the force value F cone exerted on the calculated conical roller 3 with the force threshold value F threshold admissible by the rolling mill 1.

[0044] If the force value F cone exerted on the conical roller is greater than the threshold value F admissible force threshold, a second set of parameters is obtained, corresponding to the first set of corrected parameters.

[0045] During the force limitation, the control formula, for example one of the control formulas presented previously, is no longer applied. A constant force is applied by the conical roller 3 on the crown 5. The first mechanical model will thus modify the displacement of the conical roller 3, in order to ensure the maximum admissible force. The applied force of the first mechanical model being reduced compared to the theoretical force calculated by the finite element calculation code, this will imply a slowdown in the rate of increase Ḋ of the external diameter D.

[0046] In an exemplary embodiment, the second set of parameters is calculated by a control formula following only the height input instruction h(s) of the crown 5 in the first direction Y as a function of the thickness s of the crown 5 in the second direction X, so that the increase speed instruction Ḋ is not respected.

[0047] The modeling presented makes it possible to more reliably account for the behavior of rolling mill 1, in the transition zones where the force applied F cone by the conical roller 3 of the model would be too high to be tolerable by the tool.

[0048] There figure 7 illustrates a comparison between the axial force F cone calculated by the proposed modeling method, in comparison with a prior art modeling not taking into account this mechanical model, and with experimental force measurements. It can be seen that the proposed modeling method makes it possible to more reliably account for the axial force on the conical roller 3.

[0049] During the rolling process, the calculated diameter D of the crown 5 will increase to a diameter value that changes the force level. If the calculated force value exerted on the tapered roller 3 is lower than the new permissible force threshold value, the parameter set is calculated according to the control formula in normal operation.

[0050] During step E3 of the proposed modeling method, the first set of parameters or the second set of parameters, if any, is corrected in order to obtain a third set of corrected parameters, making it possible to translate the kinematics of all the moving tools of the circular rolling mill 1, and in particular of the mandrel 2. The third set of parameters is thus characteristic of the behavior of the circular rolling mill 1 for the given setpoint. Taking into account the stiffness of the mandrel 2

[0051] In order to improve the modeling process, it is planned to take into account the stiffness of mandrel 2 when calculating the third set of parameters. The control formula as determined during the first step E1 uses a theoretical position of mandrel 2.

[0052] In an exemplary embodiment, the mandrel 2 may be contained in a cage which deforms elastically during the rolling process, which has the consequence of disturbing the position of the mandrel 2. It has in fact been empirically observed that the actual thickness e of the crown 5 was generally greater than the theoretical thickness e. It was identified that the control unit 10 of the circular rolling mill 1 did not take into account the deformation of the cage of the mandrel 2 in the control during the rolling process.

[0053] The elastic deformation of the mandrel cage 2 can be modeled simply as the deformation of a spring fixed between the mandrel 2 and the jack 21 allowing the translational movement in the second direction X of the mandrel 2.

[0054] We can consider that the cage of the mandrel 2 behaves like a spring of stiffness k, radially exerting a force on the crown 5 depending on the position of the mandrel 2 along the axis of the second direction X. In this exemplary embodiment, we obtain the values ​​of the third set of parameters by adding an offset, positive or negative (in English "offset").

[0055] The offset applied as correction may depend on the radial force calculated by the finite element calculation code.

[0056] In particular, in the example where the parameter set includes the rate of increase ṡof the thickness s of the crown 5, which depends directly on the translational movement of the mandrel 2, the addition of this second mechanical modeling is relevant because taking into account the stiffness of the mandrel 2 makes it possible to change the theoretical stroke of the latter, which is notably influenced by the virtual deformation of the cage.

[0057] Preferably, this second mechanical model can be fully integrated by Transvalor into the Forge calculation code.

[0058] There figure 8 illustrates a comparison between the radial force calculated by the proposed modeling method, in comparison with a prior art modeling not taking into account this second mechanical model, and with experimental force measurements. It can be seen that the proposed modeling method makes it possible to more reliably account for the radial force on the mandrel 2.

[0059] Thus, the method of modeling the behavior of the circular rolling mill 1 as presented makes it possible to obtain a complete model predicting the behavior of the circular rolling installation.

[0060] To best explain the contribution of what has been developed, we can compare the results of the modeling using a prior art rolling mill servo model and using the new model presented taking into account the two mechanical models translating the mechanical characteristics of the mandrel 2 and the conical roller 3.

[0061] THE figures 9 And 10 illustrate the comparison between the evolution of the external diameter D of the crown 5 and the growth rate Ḋ of the external diameter of the crown 5, calculated by a model of the prior art, by the model presented and obtained by experimental measurements.

[0062] It can be seen that the modeling process described makes it possible to obtain a new model that more realistically follows the actual changes in the parameters. Thus, for a setpoint corresponding to a new forging range, it is possible to predict all the parameters characterizing the evolution of the part to be rolled and the behavior of rolling mill 1.

[0063] In particular, this allows the optimization of the rolling operation without producing actual parts. It is thus possible to reduce the design costs of forging ranges.

[0064] Finally, the proposed modeling process makes it possible to predict the duration of the rolling process, which is not known a priori and depends on the part to be rolled. In particular, this makes it possible to determine whether there is a risk of cold forging the part. This control model also makes it possible to limit the risk of part rejection during production.

Claims

1. A method for modelling the behavior of a circular rolling mill (1), intended to roll a cylindrical part based on a setpoint, the circular rolling mill comprising at least one tapered roller (3), configured to have a movement of translation along a first direction (Y), and a mandrel (2), configured to have a movement of translation along a second direction (X), the setpoint comprising a setpoint of the speed of increase of an outer diameter of said cylindrical part as a function of said outer diameter, and a setpoint for the height of the cylindrical part along the first direction as a function of a thickness of the cylindrical part along the second direction, comprising the steps of: E1- obtaining a first set of parameters characteristic of the behavior of the circular rolling mill (1) by means of a control formula connecting a translation speed of the mandrel (2) along the second direction of translation to the speed of increase of the outer diameter and a function of the setpoint; E2- Finite-element computation of a value of the force exerted on the tapered roller (3) based on the first set of parameters; E3- Comparing the computed value of the force exerted on the tapered roller (3) with at least one threshold force value permissible by the circular rolling mill (1), and if the value of the force exerted on the tapered roller is greater than the permissible force threshold value, obtaining a second set of parameters, such that the speed-of-increase setpoint is not observed, correcting the second set of parameters by taking into account the stiffness of the mandrel to obtain a third set of parameters; if the computed value of the force exerted on the tapered roller is less than the permissible force threshold value, correcting the first set of parameters by taking into account the stiffness of the mandrel to obtain a third set of parameters; said third set of parameters being characteristic of the behavior of the circular rolling mill for the given setpoint.

2. The modelling method as claimed in claim 1, wherein the first set of parameters obtained by the control formula comprises a speed h of displacement of the tapered roller and a speed ṡ. of translation of the mandrel.

3. The modelling method as claimed in any of the preceding claims, wherein the control formula is given by: s ˙ = s D − 2 . s + h ˙ s ˙ D − s . s h D ˙ with s the position of the mandrel, ṡ the speed of translation of the mandrel, Ḋ the speed of increase of the outer diameter D of the part to be rolled (5).

4. The modelling method as claimed in any of the preceding claims, wherein, during the comparing step E3, the at least one permissible force threshold value depends on the outer diameter of the cylindrical part.

5. The modelling method as claimed in any of the preceding claims, wherein, during the correcting step, the deformation of a cage of the mandrel, modelled as a spring of stiffness constant k is taken into account, such that the third set of parameters is obtained by an offset of at least one characteristic parameter of the behavior of the mandrel of the circular rolling mill.