Title of Invention

A METHOD AND A ROLLING INSTALLATION FOR COLD ROLLING OF A STRIP

Abstract

(57) Abstract: In a process for the cold rolling of a metal strip (b) between the rolls of roll stands (c1,c2,c3) arranged one after the other, a modification to the clamping setting of the rolls of at least one (c2, c3) of the said stands (c1,c2,c3) is made in real time in order to compensats for the out-of-roundness defacts of the rolls of the said stand (c2,c3) and the said modification is evaluated by analysing, in the frequenoy domain, a measurement signal corresponding to the tension in the strip (B) immediacaly upstream of the said stand (c2,c3) and by extracting from the tension measurement signal the periodic variations in the signal, the frequencias of which correspond to the speeds of rotation of the said rolls, so as to obtain a compensation signal proportional to the pariodic variations, The response time of the compensation regulation and the thickness uniformity of the strip (B) leaving the rolling mill are substantially improved. The compensation device (p,2 p.3) is connected to the means (a2,a3) for olamping the stand (c2,c3) and to a tension sensor (t2,t3) located immediately upstream of the stand (c2,c3).

Full Text

The invention relates to a process and a plant for rolling thin sheet metal, especially steel sheets. For the production of a sheet-metal strip by-rolling, a great deal of attention is paid to thickness uniformity, especially along the strip or in the rolling direction. This thickness uniformity is even more critical when thin metal sheets are produced, especially metal sheets for packaging and/or for drinks cans. The strips are generally rolled in plants comprising a succession of rolling stands, making them run in the gap after each stand, delimited by the rolling rolls. Conventionally rolling is carried out by controlling each stand, especially on the basis of settings of the tension between the stands and/or the clamping of the strip to be rolled between two working rolls of a stand. Thus, referring to Figure 1, a rolling plant comprising three rolling stands CI, C2, C3 may be controlled by a control device S, especially based on measurement of the inter-stand tension in the strip B by sensors T2, T3 and/or by acting on the successive clamping means of each stand Al, A2, A3. The strict circularity of the rolling rolls is one of the conditions for obtaining a constant thickness over the length of the strip. Nevertheless, the methods of machining the rolling rolls do not enable a perfectly circular shape to be attained and the rolls often have a slight ovalization. Furthermore, even when grinding does enable a circular shape to be attained, a slight ovalization may still occur or be accentuated on the rolls in service due to the effect, for example, of thermal stresses. This slight ovalization of the rolling rolls, also called "out-of-roundness" or "eccentricity", is manifested, for example, by variations of a few tens of micrometres in the gap between two working rolls of a rolling stand, during rolling. These variations in the thickness of the gap are periodic, have a frequency proportional to the speed of rotation of the rolls and cause periodic variations in thickness along the sheet-metal strip leaving each rolling stand. These variations in thickness of the strip are called out-of-roundness defects or eccentricity defects. These thickness variations are typically about 0.5 ^un for each stand of a plant, which would represent a variation of 0.2% on a 0.25 mm thick strip. Such a thickness variation in a metal strip is unacceptable for most uses, especially in the field of packaging and drinks cans. Processes are known for compensating for this ovalization or "out-of-roundness" of the rolls during rolling and for obtaining a more uniform thickness along the strip. According to the first process, in a first step prior to rolling a strip, the roll stands of the plant are rotated "empty", applying a clamping setting for the working rolls of each stand and the "out-of-roundness" defect is logged by measuring, in terms of amplitude and phase, the variations in force between the said rolls and then, in a second step during rolling of a strip, an out-of -roundness compensation signal having the same amplitude as the previously measured defect, but in phase opposition, is applied to the clamping setting for the rolls of each stand. Such an out-of-roundness compensation process is an off-line compensation process which is not effective when the out-of-roundness defect is not constant, especially when it varies during rolling, for example due to the effect of distortions of the rolls resulting from thermal stresses. A second, real-time, out-of-roundness compen¬sation process is known in which, during rolling, the thickness of the strip leaving each stand is measured, the periodic variations in the thickness which correspond to the rotation frequencies of the rolls and which have a constant phase shift with it are logged and the ampli¬tude and phase of the signal to be applied to the clamping setting in order to compensate for the out-of-roundness is calculated therefrom. For this purpose, referring to Figure 2, an out-of-roundness compensation device is attached to the plant described previously and comprises thickness sensors El, E2 arranged downstream of the said stands, sensors VI, V2 for detecting the angular position of the rolls, and compensators PI, P2. From the signals supplied by the sensors El, VI for PI and E2, V2 for P2, the compensators PI, P2 evaluate the clamping setting compensation signal to be applied to the means Al, A2 for clamping the stands CI, C2 in order to eliminate the out-of-roundness defects of the rolls. Such a real-time out-of-roundness compensation process therefore requires the installation of many measurement sensors on each stand. The thickness measurement sensor is always offset by a few metres, for example 2.5m, after the gap exit of a stand, which causes a delay in logging the out-of-roundness defects of the rolls of the said stand with respect to the creation of the defect itself within the gap of the stand. Thus, the delay in detecting the defect may be about 0.5 s for a stand/sensor distance of 2.5 m and a running speed of the strip in the stand of 300 m/min. Moreover, it was observed that there was considerable noise on the thickness measurement signal supplied by the sensors El, E2 and that it was often difficult to discriminate between the periodic thickness variations in. the sheet relating to the out-of-roundness defects of the rolls compared to the other thickness variations of other origins. In order to evaluate the compensation signal to be applied to the clamping means Al, A2 by the compensators PI, P2, the thickness measurement signal is analyzed in the frequency domain, especially by a Fourier transform, the signals which correspond to the frequency of rotation of the rolls of the stands CI, C2 are extracted from the spectrum obtained and a compensation signal is generated from these extracted signals. In order thus to produce a reliable and accurate compensation signal, so that the signals extracted from the thickness measurement really do represent out-of-roundness defects and not other phenomena, it is therefore necessary to extend the Fourier transform integration time, which imroves the frequency resolution of the analysis of the measurement signal, in order to make it easier to extract the out-of-roundness signals from the measurement signal, which avoids or limits random or inaccurate defect detections. However, extending the integration time further increases the delay in detecting the out-of-roundness defect with respect to the instant at which the defect itself was created. This long integration time is necessary in order to obtain a stable regulation of the operation of the out-of-roundness compensation device. Thus, there results an overall response time, between the creation of an out-of-roiindness defect on a stand and the complete compensation for the said defect, for the regulation of the compensation device which is much too long, during which time the thickness defects of a running strip are not properly corrected. Thus, by way of example, the response times of the components of the compensation device may be: typical response time of a thickness measurement sensor: 50 ms; delay due to the stand/sensor distance: 500 ms; response time of the clamping means: 50 to 70 ms. With such a device, the out-of-roundness compensation control loop will have a response time of about several tenths of a second. If the running speed is high and since the strip obviously passes through several roll stands, at the end of rolling the portions of strip along which the out-of-roundness defects persist represent a major part of the strip (up to one half). This no longer makes it possible to provide the commercial guarantee required, for example the guarantee of a thickness variation of less than 2.5% or 3%. Thus, the real-time out-of-roundness condensation process does not yet have the required performance to provide a guarantee that the thickness is sufficiently uniform over an entire rolled strip, especially in the case of the rolling of thin metal sheets. Moreover, this compensation process requires expensive equipment, associated especially with the number of sensors to be installed. The object of the invention is to provide a rolling process which enables a metal strip to be rolled with a very high thickness uniformity along the entire strip. The object of the invention is also to provide a reliable and economic device for implementing this process. The subject of the invention is a process for the cold rolling of a strip between the rolls of rolling stands arranged one after the other, in which process a modification is made, in real time, to the roll clamping setting of at least one of the said stands in order to compensate for the out-of-roundness defects of the rolls of the said stand and in which the said modification is evaluated by analysing in the frequency domain a signal corresponding to the measurement of a rolling pareuneter specific to the said stand and by extracting from the measurement signal the periodic variations in the signal, the frequencies of which correspond to the speeds of rotation of the said rolls, so as to obtain a compensation signal proportional to the said period variations, characterized in that the said measurement is a measurement of the tension in the strip immediately upstream of the said stand. The subject of the invention is also a rolling plant comprising a succession of rolling stands inter-stand tension measurement sensors, clamping means for each stand, a control device, especially for controlling the said clamping means, and at least one means for compensating for out-of-roundness defects of the rolling roils of a stand of the succession of stands which is connected to the means for clamping the said stand in order to deliver an out-of-roundness defect compensation signal to them, characterized in that the compensation measurement sensor located immediately upstream of the said stand in order to receive the measurement signal from the tension measurement sensor {T2, T3) and evaluate the said compensation signal. Accordingly the present invention provides a method of cold rolling a strip (B) between the cylinders of rolling housings (C1 C2 C3) disposed one after the other in which out of round defects of at least one of the rolling housings are corrected by the following steps: - measuring in continuous the tension in the strip (B) immediately upstream of the said rolling housing for obtaining a tension measurement signal, - analysing the tension measurement signal as a function of frequency, - extracting from the terrain measurement signal, at least the periodic variations whose frequencies correspond to the instantaneous speed of rotation of the cylinders of the rolling housing, - elaborating a compensation signal proportional to the extracted periodic variations, - modifying a tightening set-point value of the rolling housing in real time through the compensation signal Accordingly the present invention also provides a rolling installation for cold rolling of a strip by the method claimed in claim 1 comprising a succession of rolling housings (Ci C2 C3), sensors (T2 T3), for measuring the inter-housing tension, each rolling housing having tightening means (Aj, A2, A3) for tightening the cylinders of the rolling housing and at least one rolling housing having means of compensation of out-of-round defects, comprising: - a sensor for measuring the tension of the strip immediately upstream of the rolling housing, - an out-of-round defect compensation device (P'2, P'3) connected to the sensor for measuring the tension and to the tightening means of the rolling housing comprising frequency analysing means of the tension measurement signal for extracting from the signal, at least the periodic variations whose frequencies correspond to an instantaneous speed of rotation of the cylinders of the rolling housing and for elaborating a compensation signal proportional to the extracted periodic variations provided in real time to the tightening means of the rolling housing for modifying a tightening set-point value. The invention will be more clearly understood on reading the description which will follow, given by way of example, and with reference to the appended figures in which: - Figure 1 is a partial diagram of a conventional cold-rolling plant with its control device; - Figure 2 represents the same rolling plant as in Figure 1, provided with a device for compensating tor the out-of-roundness of the rolls according to the prior art; - Figure 3 represents the same rolling plant as in Figure I, but provided with a device for compensating for the out-of-roundness of the rolls according to the invention. Referring to Figure 3, the rolling plant comprises three rolling stands CI, C2, C3, a control device S, inter-stand tension measurement sensors T2, T3, means AI, A2, A3 for clamping the stands CI, 02, C3 and compensation devices P'2, P'3. Measurement of the inter-stand tension corresponds to the tension in the strip between each stand and the sensors T2, T3 provided for this purpose are, for example, deflection-type tensiometers. The clamping means Al, A2, A3 are mainly controlled by a setting delivered by the control device S. The function of the compensation devices P'2, P' 3, as previously for the compensators P2, P3, is to evaluate the compensatiozi signal intended for correcting, in real time, the clamping setting of the means A2, A3 for clazaplng the stands C2, C3 for the purpose of eliminating the out-of-roundness defect of the rolls of the said stands. In order to evaluate the compensation signal, according to the invention, the compensation devices P2 and P3 are connected to the tension sensors T2 and T3. Conventionally, the rolling rolls of the stands C2, C3 are ground but do have ovalization or out-of-roundness defects which, during rolling without compensation, would cause variations in the gap between the working rolls. In a manner known per se, a strip B is rolled by controlling the operation of the stands CI, C2, C3 of the plant with the aid of the control device S, especially on the basis of the measurement of the inter-stand tension in the strip B by the sensors T2, T3 and by acting on the successive means Al, A2, A3 for clamping each stand. Furthermore, for the purpose of rolling the metal strip B while maintaining the thickness variations along the strip within a range which is appreciably less than the ovalization defects of the rolls of the stands C2 and C3, a signal for compensating for the out-of-roundness of the said rolls is applied to the clamping setting of the clamping means A2, A3. According to the invention, the compensators P'2, P'3 deliver the said compensation signal to the clamping means A2, A3 by evaluating the signal on the basis of the measurement signal delivered by the sensors T2, T3 upstream of the stands C2, C3. The invention is therefore applicable after the second stand of the rolling plant as far as the final stand without installing additional sensors, since tension measurement sensors T2, T3 are already installed between each stand in order to allow the device S to control the rolling plant conventionally. Thus, for example, the compenaatlon device P'2 is designed, in a manner known per se, to extract from the signal from the sensor T2 the periodic variations in the tension in the strip B upstream of the stand C2 which have a frequency equal to the speed of rotation of the rolls of the stand C2 and to generate a compensation signal proportional to the said extracted variations. According to one alternative form of the invention, the compensation device P'2 also takes into account the harmonics in the out-of-roundness defects, that 1B to say the periodic variations in the tension in the strip B which have a frequency which is a multiple of the speed of rotation of the rolls of the stand C2. In order to extract these periodic variations, a Fourier transform is generally carried out. According to the Invention, it was observed that the tension measurement signal could be integrated over a much shorter time than in the processes of the prior art while at the same time detecting the out-of-roundness defects of the rolls of the stand C2 reliably and accurately. Although, for example, the two working rolls of the stand C2 may have slightly different speeds of rotation, especially because of slight differences in diameter, it Is not necessary to choose a sufficiently long Integration time to enable the out-of-roundness defect of one of the rolls to be discriminated from that of the other and it proves to be sufficient to evaluate the compensation signal on the basis of the average amplitude of the defect measured. Surprisingly, it was observed that there is much less noise on the out-of-roundness defect detection signal when it comes from the tension measurement of a sensor T2 upstream of the stand C2 than when it comes especially from the thickness measurement of a sensor E2 downstream of the stand C2, as in the process of the prior art shown in Figure 2. Thus, in order to log the out-of-roundness defects of the rolls of one stand, by analysing the frequencies of the measurement signal delivered by a tension sensor upstream of the said stand, a very fine frequency resolution is no longer necessary for reliable and accurate detection of the said out-of-roundness defect, which enables the Fourier transform integration times to be substantially decreased. Thus, according to the invention, since the out-of-roundness defect sensor is a tension sensor, there is no delay between the appearance of a defect and its detection, contrary to the process of the prior art. Still according to the invention, the compensation devices are programmed in order to analyse the measurement signals, in the frequency domain, with integration times much shorter than in Che prior art. Overall therefore, the response time of the out-of-round compensation device according to the invention is considerably shortened compared to the devices of the prior art. Thus, for rolls rotating at 60 revolutions per minute, i.e. 1 Hz, a tension measurement sensor having a response time of 16 ms, clamping means having a response time to application of a setting of 70 ms, the overall response time of the out-of-roundness compensation control according to the process of the invention is only approximately 2.5 revolutions of a roll, i.e. approxi¬mately 2.5 seconds. The response time of the clamping means and the maximum rate of clamping of these means may be limiting and critical factors for implementing the process according to the invention; preferably, the response time of the clamping means must remain less than the time for one revolution of the rolls. Since the speed of the strip, and therefore the speed of rotation of the rolls, increases from the upstream end to the downstream end of a rolling plant, these conditions may be fulfilled only for the first stands of the plant and it is not always possible to equip rolling plants fully with the compensation device according to the invention. The compensation device and its operation which are described for the stand 2 are also applicable to the stand 3, or to any other downstream stands. According to the invention, and when all the rolling stands of a plant are equipped with the device according to the invention, metal strips are obtained at the end of rolling which have longitudinal thickness variations of less than 5 fixa, that is to say less than 0.7% for an average thickness of 0.26 mm. WE CLAIM: 1. A method ofcold rolling a strip (B) between the cylinders of rolling housings (C1 C2 C3) disposed one after the other in which out-of-round defects of at least one of the rolling housings are corrected by the following steps: - measuring in continuous the tension in the strip (B) immediately upstream of the said rolling housing for obtaining a tension measurement signal, - analysing the tension measurement signal as a function of frequency, - extracting from the tension measurement signal, at least the periodic variations whose frequencies correspond to the instantaneous speed of rotation of the cylinders of the rolling housing, - elaborating a compensation signal proportional to the extracted periodic variations, - modifying a tightening set-point vaiue of the rolling housing in real time through the compensation signal. 2. A rolling installation for cold rolling of a strip by the method claimed in claim 1 comprising a succession of rolling housings (Ci C2 C3), sensors (T2 T3), for measuring the inter-housing tension, each rolling housing having tightening means (A1, A2, A3) for tightening the cylinders of the rolling housing and at least one rolling housing having means of compensation of out-of-round defects, comprising: - a sensor for measuring the tension of the strip immediately upstream of the rolling housing, - an out-of-round defect compensation device (P'2, P'3) connected to the sensor for measuring the tension and to the tightening means of the rolling housing comprising frequency analysing means of the tension measurement signal for extracting from the signal, at least the periodic variations whose frequencies correspond to an instantaneous speed of rotation of the cylinders of the rolling housing and for elaborating a compensation signal proportional to the extracted periodic variations provided in real time to the tightening means of the rolling housing for modifying a tightening set-point value. 3. The rolling installation according to claim 2, wherein the sensor for measuring the tension of the strip is a deflector tensiometer. 4. A method of cold rolling a strip substantially as herein described with reference to figures 2 & 3 of the accompanying drawings. 5. A rolling installation for cold rolling of a strip substantially as herein described with reference to the accompanying drawings.

Documents:

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999-mas-96 abstract.pdf

999-mas-96 claims.pdf

999-mas-96 correspondence others.pdf

999-mas-96 correspondence po.pdf

999-mas-96 description (complete).pdf

999-mas-96 drawings.pdf

999-mas-96 form-2.pdf

999-mas-96 form-26.pdf

999-mas-96 form-4.pdf

999-mas-96 form-6.pdf

999-mas-96 petition.pdf