EP0841112A1 - Process for casting between cylinders - Google Patents

Abstract

A detection method of continuous casting between rolls of steel products comprises measuring continuously the roll separation strain (RSF) and a signal representative of the variations in this strain (RSF) generated as a function of time. The signal is used to act upon the roll separation to compensate for any untrue roundness of the rolls. During these measurements the signal is separated into different harmonic components and the resulting comparison of the harmonic components thus obtained with some reference harmonics is representative of the state of defects in the rolls. This state of defects in the rolls allows the definition of different rules for conducting the casting process as a function of the gravity of the roll defects measured.

Description

The present invention relates to continuous casting between cylinders of thin metallic products, in particular steel.

According to this known technique, the manufactured product, for example a thin strip of steel a few millimeters thick, is obtained by pouring the molten metal into a casting space defined between two cylinders with parallel axes, cooled and driven in rotation in the opposite direction. On contact with the cold walls of the cylinders, called ferrules, the metal solidifies and the solidified metal skins, driven by the rotation of the cylinders, meet at the neck between the cylinders, to form the said strip, extracted downwards.

The operation of the casting process between rolls is subject to various constraints relating both to the product cast and to the implementation of the casting installation.

The cast strip must in particular have a section corresponding, in shape and dimensions, to the desired section, the actual section of the strip being a direct function of the space, called the air gap, between the rolls at the level of the neck.

For this, there is known a method for regulating the continuous casting between rolls, described in patent application FR-A-2728817, according to which the roll spacing force (RSF) is measured and the action is acted on accordingly. relative position of said cylinders. This process makes it possible to act on the relative position of the cylinders to move them apart in the event of excessive force or to bring them closer in the event of excessively weak force, in particular in order to avoid breakthroughs of liquid metal or even a rupture of the tape, and also to avoid damage to the cylinders in the event of over-solidification cast metal.

Furthermore, it is known that a runout of the cylinders cannot be completely avoided, on the one hand for mechanical reasons and on the other hand due to the thermal deformations undergone by the ferrule, during the first contact of the molten metal at when starting the casting, and also later during the rotation of the cylinders. It is already known to compensate for this runout, which will be called hereinafter "normal runout" (or even "mechanical runout" although being partly of thermal origin), by automatically acting on the position of the bearings d 'at least one of the cylinders as a function of the angular position of the cylinders, so as to keep the air gap as constant as possible. Given the practical impossibility of directly measuring the air gap, it has already been proposed to use as a parameter representative of the runout a signal supplied by the means for measuring the force of spacing of the cylinders, the compensation system out of round being then combined with a regulation system such as that described in the document FR-A-2728817 already cited.

The implementation of these methods does not however make it possible to detect in real time certain faults liable to disturb the casting or even lead to its stopping, or to permanently damage the cylinders.

There are already known methods of detecting defects, visual or otherwise, making it possible to detect defects linked to the casting process, to the thermo-hydraulic of the molten metal, or even those known under the name of "shiny bands". This latter type of defect corresponds to a localized decrease in the surface roughness of the rolls, which causes variations in the cooling of the strip which can be detected by temperature measurements carried out on the cast strip. But the observation of these defects can only be done a posteriori, on the strip already formed, and therefore belatedly after their appearance. However, these defects can damage the surface condition of the cylinders, and this all the more so since they are perceived late, this damage then being able to become irreparable.

Certain faults could a priori be detected on the basis of direct observation of the signal representing the spacing force of the cylinders. However, the variations in this signal represent both variations in force due to the normal run-out and variations due to other parameters or events which may occur during casting. A direct observation of the effort signal therefore does not make it possible to determine the share of each of these causes in the variations of the signal.

The present invention aims to solve the problems mentioned above and aims to allow, from the measurement of the cylinder separation force (RSF), the real-time detection of faults, before an amplification of these faults causes irreparable damage, particularly to the cylinders. The invention also aims to allow monitoring of the evolution of these defects, in order to be able to offer the operator corrective actions or interruption of the casting according to the severity of said defects.

With these objectives in view, the subject of the invention is a process for continuous casting between cylinders of thin metallic product, in particular steel, according to which the force of spacing of the cylinders is continuously measured during casting, and generating a signal representative of the variations in the spacing force (RSF) as a function of time, and acting, in particular as a function of said signal, on the spacing of the cylinders to compensate for the runout of the cylinders, this process being characterized in that, in order to detect faults other than the runout of the cylinders, said signal is broken down into different harmonic components, and these said components are compared harmonics with reference harmonics of corresponding rank, the results of said comparison being representative of a fault state of the casting process, and rules of conduct of the casting process are defined as a function of the results of said comparison.

The inventors have in fact been able to establish, following numerous tests carried out on an industrial scale, that there is a certain relationship between the variations in the signals representative of the spacing force and the appearance of defects during casting. For example, the appearance on a cylinder of the defect called the shiny strip is characterized by the presence of a disturbance on the signal of the measured spacing force. This disturbance is cyclical and manifests itself at each revolution of the cylinder. It reflects an over-solidification of the product when it passes to the neck and results in variations in the force which are much faster than those which can be generated for example by variations in the thickness of the solidified product.

The inventors then imagined making a decomposition into harmonics of said signals so as to differentiate in these signals the part which can be attributed to the normal false round and that coming from other causes. They thus verified, by comparison of the harmonic components noted during various flows, that, although the signals representative of the spacing force vary in particular as a function of the runout, even when the latter is compensated by a system of compensation, variations of certain harmonic components corresponded to the appearance of defects during casting. It therefore appeared that an analysis, carried out continuously during the flows, of these harmonic components could make it possible, by comparison with a reference obtained experimentally during flows considered without defects, to detect almost real-time deviations revealing such casting defects, much faster than by known methods.

An explanatory hypothesis of the relation existing between the variations of the harmonic components and the presence of casting defects is that the normal runout causes variations of the signal representative of the force of spacing of the cylinders (RSF) which are mainly slow and soft , in other words that said signal is released, due to said normal runout, essentially a harmonic component of low rank, of frequency equal to the frequency of rotation of the cylinders. On the other hand, real defects, such as the bright bands mentioned above, mainly generate sudden variations of said signal and therefore higher order harmonics. Typically, the spectrum of the signal representative of the force of spacing of the cylinders and resulting from the only normal runout is characterized by a significant harmonic component of rank 0 (for example 70% of the total amplitude of the signal) and harmonics of higher rank in rapid decay (20% for the harmonic of rank 1, 10% for the harmonic of rank 2). We rarely notice the presence of higher order harmonics. On the other hand, in the case of the presence of bright bands, the distribution of the harmonics is different from the above case, the presence of an over-solidification front at the level of the brilliant band generating more high harmonics.

It is specified that here and thereafter, the term harmonic of rank i denotes the component of the signal at a frequency F _i = 2 ⁱ F ₀ , F ₀ being the fundamental frequency corresponding to the speed of rotation of the cylinders. Similarly, we will denote by h _i the amplitude of the harmonic components of rank i, and by H _i a value representative of the harmonics of rank i considered over a predetermined number of revolutions of the cylinders.

According to a particular provision of the invention when a system for regulating the air gap, as described above, is used, it is possible to use as a signal representative of the variations in the force of spacing of the cylinders (RSF), originating of the measurement of said force, an associated signal used as a setpoint for displacement of the bearings of at least one cylinder. That is to say that the signal which is then broken down into different harmonic components is directly linked to said displacement setpoint produced by a runout compensation module, and therefore reflecting the variations in the spacing force.

To carry out the decomposition of the signal into its various harmonic components, it is possible in particular to use a fast Fourier transform applied to the signal representative of the force of spacing of the cylinders (RSF), this signal therefore being either directly the measurement signal of the 'spreading force, that is to say a corresponding signal produced by said module for compensation for runout.

In a preferred arrangement of the invention, the value H _i representative of each harmonic of rank i is calculated as being an average value of the amplitudes h _i of each harmonic, determined over a given number of revolutions of the cylinders. The value H _i representative of each harmonic being calculated as being an average over a given number of turns of the amplitudes measured, this makes it possible to attenuate the effect of random and localized defects in time and space, non-repetitive over several turns cylinder. Thus, if the defect is caused by a lasting problem on a cylinder, the system will completely integrate this data at the end of said number of turns, while the effect of harmonics only appearing on a low number of turns, significantly lower than the said number of turns given, will be considerably reduced.

The comparison of the measured signal with a signal of a flow deemed good can be done in different ways. One can simply compare term by term the values H _i representative of each harmonic of the signal measured with reference values H _ir coming from measurements carried out during flows considered good, and verify that the sum of the differences of the values H _i representative of each harmonic with the reference H _ir values is not too high. We can also compare the proportion of each harmonic compared to a proportional reference distribution. However, preferably, the comparison will be made on the basis of a barycenter of the harmonics, this barycenter being calculated by weighting each harmonic by a predetermined coefficient, so as to give the different harmonics relative importance by unevenly weighting the latter. This method of calculation is justified by experimental observations: during a casting considered good, the first harmonic is the most important, the importance of the different harmonics being decreasing according to the increasing rank of the harmonics considered. By weighting the harmonics of higher rank by an adapted coefficient, the variations of these harmonics of higher rank will be somehow amplified, making their appearance or increase more easily perceptible in the result of the barycenter calculation.

   et normer ce barycentre par la fréquence fondamentale pour obtenir un rapport R = B_f/F₀ qui pourra être comparé à une valeur de référence R₀ prédéterminée, de manière à s'affranchir d'éventuelles différences de fréquence fondamentale, et donc de vitesse effective des cylindres, entre la coulée considérée et la référence.We can for example calculate a frequency barycenter B _f by assigning to each frequency of harmonics a coefficient representing the amplitude of the harmonic considered: $${\text{B}}_{\text{f}}{\text{(Hz) = Σ H}}_{\text{i}}{\text{∗ F}}_{\text{i}}{\text{/ Σ H}}_{\text{i}}$$

and normalize this barycenter by the fundamental frequency to obtain a ratio R = B _f / F ₀ which can be compared with a predetermined reference value R ₀ , so as to overcome any differences in fundamental frequency, and therefore in effective cylinder speed, between the casting considered and the reference.

We can moreover calculate the derivative dR / dt and also compare the result with a second predetermined threshold, thus making it possible to follow the evolution of the ratio R over time, a rapid evolution of R being in a way the sign of an aggravation fast of a fault.

• A représentant l'amplitude totale des variations : A = Σ H_i
• R représentatif de la part ou de l'importance des défauts dans le signal,
• et E = dR/dt
     on peut établir un tableau de décision, comme on le verra par la suite, qui pourra être utilisé pour proposer en temps réel à l'opérateur des actions correctives sur certains paramètres de coulée, dans le but visé de remédier à des défauts le plus rapidement possible après leur apparition.

With the values of the different parameters:

• A representing the total amplitude of the variations: A = Σ H _i
• R representative of the share or the importance of the defects in the signal,
• and E = dR / dt
  we can establish a decision table, as we will see later, which can be used to propose in real time to the operator corrective actions on certain casting parameters, with the aim of remedying faults as quickly as possible possible after their appearance.

• la figure 1 est une vue schématique d'un dispositif de coulée entre cylindres avec un système de régulation de type connu en soi, mais utilisant une décomposition en harmonique du signal de compensation de faux rond,
• la figure 2 représente un tableau de décision permettant de définir la démarche à suivre au cours de la coulée en fonction des valeurs des différents paramètres fournis par le procédé selon l'invention,
• les figures 3a, 3b, 3c et 3d présentent, sous forme de tracés représentant les variations des différents paramètres mesurés ou calculés, les résultats obtenus lors d'une coulée jugée bonne avec procédé de compensation de faux rond,
• les figures 4a, 4b, 4c et 4d présentent les tracés correspondants obtenus lors d'une coulée jugée mauvaise.

Other advantages and particularities will appear on reading the detailed description which follows of examples of embodiment of the invention, given by way of indication and in no way limitative, to be read in conjunction with the appended drawings among which:

• FIG. 1 is a schematic view of a device for casting between cylinders with a regulation system of the type known per se, but using a harmonic decomposition of the runout compensation signal,
• FIG. 2 represents a decision table making it possible to define the approach to be followed during casting as a function of the values of the various parameters provided by the method according to the invention,
• FIGS. 3a, 3b, 3c and 3d show, in the form of plots representing the variations of the various parameters measured or calculated, the results obtained during a casting judged to be good with a method of compensation for runout,
• Figures 4a, 4b, 4c and 4d show the corresponding plots obtained during a casting deemed bad.

The casting installation, shown only partially in Figure 1, conventionally comprises, and known per se, two cylinders 1, 2, with parallel axes, spaced from each other by a distance called the air gap. It corresponds to the desired thickness of the cast strip, minus the crushing due to the RSF. The two cylinders 1, 2 are rotated in opposite directions, at the same speed. They are carried by bearings 3, 4, diagrammatically represented, of two supports 5, 6 mounted on a chassis 7. The support 5, and therefore the axis of the corresponding cylinder 1, is fixed relative to the chassis 7. The other support 6 is movable in translation on the chassis 7. Its position is adjustable and determined by thrust jacks 9 acting so as to bring the supports 5, 6 closer or further apart, from one another. Means for measuring the force between the cylinders (RSF), such as load cells 8, are arranged between the fixed support 5 and the chassis 7. Sensors 10 make it possible to measure the position of the mobile support 6, and therefore variations in position with respect to a predetermined setpoint position as a function of the desired thickness of the strip.

During casting, the molten metal is poured between the cylinders, and begins to solidify on contact with their cooled walls, forming solidified skins which are driven by the rotation of the cylinders and meet substantially at the neck 11 between the cylinders to form the solidified strip extracted downwards. In doing so, the metal exerts on the cylinders a separation force (RSF), measured by the load cells 8, this force being variable in particular according to the degree of solidification of the metal.

To regulate this effort, and guarantee the continuity of the casting, the casting installation includes a regulation system. In this regulation system, the difference between the effort setpoint signal and the effort signal measured by the effort sensor 8 is calculated by a first comparator 12. The signal for this difference is introduced into a regulator force 13 which determines a position setpoint signal introduced into a second comparator 14. The force signal measured by the force sensor 8 is also introduced into a runout compensation system 15 which performs a decomposition into harmonics of the force signal and generates signals H ₁ , H ₂ , H ₃ for compensation of each of said harmonics. These signals H ₁ , H ₂ and H ₃ are added to an adder 16 which generates a position correction setpoint signal which is transmitted to the second comparator 14. The output signal from the second comparator 14 is introduced into a third comparator 17 as well as a position signal from the position sensor 10. The output signal from the third comparator 17 is introduced into the position regulator 18 which controls the jacks 9.

The rotation of the cylinders 1 and 2 is ensured respectively by motors 19 and 20 controlled by a speed regulator 21. This speed regulator 21 receives a signal from a thickness regulator 22 which itself receives a setpoint signal from thickness, the force signal emitted by the force sensor 8 and the position signal emitted by the position sensor 10.

An action on the cylinders 9 is carried out automatically by this regulation system, which allows for example to act on the cylinders 9 in the direction leading to a spacing of the cylinders to reduce the spreading force (RSF), or vice versa in the direction of bringing the cylinders closer together to increase the force. Similarly, this system makes it possible to carry out at least partial compensation for the normal runout, that is to say to compensate for any misalignment existing between the axis of the ferrule and its axis of rotation as well as the irregularities of shape d '' a cylinder, whether these are of mechanical or thermal origin. The regulation system then takes these shape and coaxial defects into account in order to give a displacement instruction to the thrust jacks 9 controlling the air gap of the cylinders in order to maintain this air gap as constant as possible during the rotation of the cylinders.

We will now describe a preferred method for determining the various parameters A, R and E which will be used to warn the operator of the presence of faults and the seriousness of these.

In this method, a harmonic decomposition of the signal representative of the cylinder spacing force is used, this decomposition being carried out in the runout compensation module 15 using a Fourier transform. We could just as easily carry out the same operation not using a Fourier transform but with a Laplace transform or any other mathematical or signal processing operation such as for example the use of filters making it possible to obtain the same result, namely a decomposition of a signal into different harmonic components.

The values H _{i are} then calculated as indicated above, that is to say by performing an average of the amplitudes h _i over a predetermined number of revolutions of the cylinders, for example over the last ten revolutions. It will be noted that the preceding method for calculating the coefficients H _i is given only by way of example and is not at all limiting. Representative H _i values of each harmonic of rank i can also be calculated as being the effective value of the amplitude h _i of the harmonics or any other calculated value characterizing these said harmonics, this calculation being able to be made according to an arithmetic mean, according to the method of least squares or by any other method.

Whatever the calculation method, the values H _i are representative of the relative amplitude of each harmonic of rank i and of frequency F _i .

The criterion B _{f is} then calculated as being a frequency barycenter of the different harmonics. That is to say that the barycenter of the frequencies of the harmonics considered is calculated, each value F _i being assigned a weight constituted by the corresponding value H _i ,
is : $${\text{B}}_{\text{f}}{\text{= Σ H}}_{\text{i}}{\text{x F}}_{\text{i}}{\text{/ Σ H}}_{\text{i}}\text{.}$$

We will generally use only harmonics of rank 0, 1 and 2. However, it would obviously be possible to take into account more harmonics.

In order to be able to make valid comparisons at different rotation speeds of the cylinders, the ratio R _f = B _F / F ₀ , F ₀ corresponding to the rotation frequency of the cylinders is then calculated.

• amplitude globale des variation du signal : A=H₁+H₂+H₃,
• barycentre normé : $${\text{R}}_{\text{f}}{\text{= (<figure-callout id="1" label="F" filenames="imgf0001.png,imgf0002.png" state="{{state}}">F</figure-callout>}}_{\text{1}}{\text{<figure-callout id="1" label="xH" filenames="imgf0001.png,imgf0002.png" state="{{state}}">xH</figure-callout>}}_{\text{1}}{\text{+<figure-callout id="2" label="F" filenames="imgf0001.png" state="{{state}}">F</figure-callout>}}_{\text{2}}{\text{<figure-callout id="2" label="xH" filenames="imgf0001.png" state="{{state}}">xH</figure-callout>}}_{\text{2}}{\text{+<figure-callout id="3" label="F" filenames="imgf0001.png" state="{{state}}">F</figure-callout>}}_{\text{3}}{\text{xH}}_{\text{3}}{\text{)/((<figure-callout id="1" label="H" filenames="imgf0001.png,imgf0002.png" state="{{state}}">H</figure-callout>}}_{\text{1}}{\text{+<figure-callout id="2" label="H" filenames="imgf0001.png" state="{{state}}">H</figure-callout>}}_{\text{2}}{\text{+H}}_{\text{3}}{\text{)xF}}_{\text{0}}\text{)}$$
  
• évolution de R_f dans le temps : E=dR_f/dt.

In the case taken as an example where only the first three harmonics are taken into account, the following three criteria are obtained:

• overall amplitude of the signal variation: A = H ₁ + H ₂ + H ₃ ,
• standardized barycenter: $${\text{R}}_{\text{f}}{\text{= (<figure-callout id="1" label="F" filenames="imgf0001.png,imgf0002.png" state="{{state}}">F</figure-callout>}}_{\text{1}}{\text{<figure-callout id="1" label="xH" filenames="imgf0001.png,imgf0002.png" state="{{state}}">xH</figure-callout>}}_{\text{1}}{\text{+ <figure-callout id="2" label="F" filenames="imgf0001.png" state="{{state}}">F</figure-callout>}}_{\text{2}}{\text{<figure-callout id="2" label="xH" filenames="imgf0001.png" state="{{state}}">xH</figure-callout>}}_{\text{2}}{\text{+ <figure-callout id="3" label="F" filenames="imgf0001.png" state="{{state}}">F</figure-callout>}}_{\text{3}}{\text{xH}}_{\text{3}}{\text{) / ((<figure-callout id="1" label="H" filenames="imgf0001.png,imgf0002.png" state="{{state}}">H</figure-callout>}}_{\text{1}}{\text{+ <figure-callout id="2" label="H" filenames="imgf0001.png" state="{{state}}">H</figure-callout>}}_{\text{2}}{\text{+ H}}_{\text{3}}{\text{) xF}}_{\text{0}}\text{)}$$
  
• evolution of R _f over time: E = dR _f / dt.

A comparison of these different criteria calculated during casting with a predetermined threshold then makes it possible to detect for the casting in progress if such a defect appears.

• H₀ = 700 µm, H₁ = 200 µm, H₂ = 100 µm, avec
• F₀ = 0,2 Hz, F₁ = 0,4 Hz et F₂ = 0,8 Hz ,
• alors B_f = 0,3 Hz et R_f = 1,5 .

For example, in a case where the signal representative of the force between the cylinders is the signal from the runout compensation module, that is to say expressed in displacement value of the mobile cylinder, and in the presence of the only normal runout, one can have:

• H ₀ = 700 µm, H ₁ = 200 µm, H ₂ = 100 µm, with
• F ₀ = 0.2 Hz, F ₁ = 0.4 Hz and F ₂ = 0.8 Hz,
• then B _f = 0.3 Hz and R _f = 1.5.

If a bright band appears, these values become respectively 350 µm, 350 µm and 300 µm for H ₀ , H ₁ , H ₂ , and then R _f = 2.25.

It is thus seen that by simply fixing an adequate threshold on R _f , for example R _fseuil = 1.6, the passage of R _f above this threshold can activate an alarm signaling a fault.

A better appreciation of the severity of the defects can be obtained by simultaneously taking into account the three criteria mentioned above.

For this, one could for example use a decision table as shown in FIG. 2 which indicates directly to the operator the defective state of the casting, that is to say gives him an indication of the presence, the importance, and the development of faults and signals the need to take corrective actions, such as modifications to certain casting parameters to try to remedy the faults that have arisen, or at worst the need to stop the casting to avoid irreparable damage to the casting facility.

• A "petit" est le signe de faibles variations de l'effort d'écartement des cylindres, la coulée se déroule dans de bonnes conditions,
• quand A est "moyen",
• si R et E sont "petit", ce qui signifie pas ou peu de défauts,, la coulée se déroule encore dans de bonnes conditions,
• si R est "petit" et E "grand", cela peut signifier que, bien qu'il n'y ait pas de présence réelle de défauts, le point de fonctionnement de l'installation est instable, pour des raisons liées essentiellement au faux rond "normal", et une alarme du procédé de coulée est déclenchée pour avertir l'opérateur d'un besoin de modifier par exemple les conditions thermiques de la virole (température ou débit de l'eau de refroidissement),
• si R est "grand" et E "petit", ce qui signale la présence de défauts, sans tendance notable à une éventuelle aggravation de ceux-ci, une alarme du procédé de coulée est déclenchée,
• si R et E sont "grand", signalant la présence de défauts et l'aggravation de ceux-ci, un arrêt de la coulée est demandé,
• quand A est "grand",
• si R et E sont "petit", aucun défaut latent n'est signalé, le faux rond normal est correctement compensé, mais l'amplitude des déplacements du cylindre mobile pour réaliser cette compensation est importante, ce qui n'est pas grave pour la coulée elle-même, mais peut révéler des problèmes de géométrie des cylindres,
• si R est "grand" et E "petit", ce qui signale en plus la présence de défauts, mais sans aggravation notable, une alarme du procédé de coulée est déclenchée,
• si E est "grand", quelque soit la valeur de R, une forte aggravation des défauts est signalée et un arrêt rapide de la coulée est demandé.

This table presents for example the procedure to be followed according to the relative values of the coefficients A, R _f and E:

• A "small" is the sign of small variations in the spacing force of the cylinders, the casting takes place in good conditions,
• when A is "average",
• if R and E are "small", which means no or few defects, the casting still takes place in good conditions,
• if R is "small" and E "large", this can mean that, although there is no real presence of faults, the operating point of the installation is unstable, for reasons essentially related to the false round "normal", and an alarm of the casting process is triggered to warn the operator of a need to modify for example the thermal conditions of the shell (temperature or flow of cooling water),
• if R is "large" and E "small", which signals the presence of faults, without any noticeable tendency for them to worsen, an alarm of the casting process is triggered,
• if R and E are "large", signaling the presence of faults and their worsening, a stop of the casting is requested,
• when A is "big",
• if R and E are "small", no latent fault is reported, the normal runout is correctly compensated, but the amplitude of the displacements of the movable cylinder to achieve this compensation is important, which is not serious for the casting itself, but can reveal cylinder geometry problems,
• if R is "large" and E "small", which additionally signals the presence of faults, but without noticeable deterioration, an alarm of the casting process is triggered,
• if E is "large", whatever the value of R, a strong aggravation of the defects is signaled and a rapid stopping of the casting is requested.

It will be noted that the characters "small", "medium" and "large" of the different criteria are assessed by comparison with experimental data acquired during previous flows.

By way of illustration of the possibilities of detecting faults by the method according to the invention, we can refer to Figures 3a, 3b, 3c and 3d which show the variations of the different parameters measured and calculated during a casting with a runout compensation process deemed good, and to Figures 4a, 4b, 4c and 4d which show comparatively the curves obtained during a casting having shiny strip defects.

Figures 3a and 4a show the variations in the spacing force of the cylinders expressed as a percentage of the admissible RSF, measured for 40 minutes from the start of casting.

FIGS. 3b and 4b show the evolution during this time of the parameter A, that is to say the average amplitude over 10 turns, in μm, of the displacement of the bearings of the mobile cylinder controlled by the module for compensating the runout.

Figures 3c and 4c show in time correspondence the evolution of the parameter R.

Figures 3d and 4d show in correspondence on the same graph the changes in the values H ₀ , H ₁ and H ₂ , representative of the amplitudes of the harmonics of rank 0, 1 and 2, the first (H ₀ ) being represented at the bottom of the diagram. , the second (H ₁ ) in the middle and the third (H ₂ ) at the top.

It is noted that, in the case of the casting judged good, the growth of A for approximately the first 20 minutes corresponds to a similar growth of H ₀ and essentially reflects the evolution of the compensation for runout, until a stability is obtained. of A in the vicinity of 50 µm, signaling an almost perfect out-of-round compensation. There is also a stability of the parameter R after ten minutes, after an excursion of R towards higher values, corresponding to a relatively large amplitude of H ₂ during the same period of start of casting.

By comparison, the plots of Figures 4b, 4c and 4d, relating to a flow whose course has been highly disturbed, show large amplitudes of H ₁ and H ₂ for approximately 40 minutes, with a high value of a during the same period, and above all a high value of R.

It will be readily understood from these readings that a comparison, carried out in real time during casting, of the values of A and especially of R with predetermined thresholds would have made it possible to quickly detect the defects corresponding to the large amplitudes of the harmonics H ₁ and H ₂ , and to act immediately on the casting parameters to prevent them from getting worse.

The invention is not limited to the methods of calculating the various parameters indicated above solely by way of example.

In particular, by always using the same values H _i representative of the amplitude of each harmonic, it will be possible to calculate another barycenter B of the harmonic spectrum of the value representative of the force of spacing of the cylinders, for example by then assigning to each value H _i a judiciously chosen weighting coefficient making it possible to accentuate in the calculated value of this barycenter the influence of the harmonics of higher rank, which are significant of defects. Whatever the type of barycenter calculation used, we will use values representative of the different harmonics and weighting coefficients relating to each harmonic such that we can easily follow the evolution of the value of the barycenter and compare it with experimental values with a view to defining a defective level in real time by comparison with the defective state (trouble-free casting, disturbed casting, bad casting having led to a stop or damage to the cylinders, etc.) of the previous castings.

To compare the harmonics, we may also define a reference distribution of the amplitudes of the harmonics, as a percentage of each harmonic relative to the total signal, for example by assuming a priori that the first harmonic represents 66% of this signal, the second 17% and the third also 17% . We can then follow the evolution of this distribution during each casting and, by comparison with the reference distribution, easily assess any deviations. This comparison could for example be made by calculating a sum R _d of the differences between the proportion H _i / A of each harmonic component in the measured signal representative of the spacing force and the reference proportion α _i : R _d = pos (α ₀ -H ₀ / A) + pos (H ₁ / A-α ₁ ) + ... + pos (H _i / A-α _i ), (i.e., each element of this sum n ' is counted only if it is positive). In this way, if the proportion of the harmonic of rank 0 is greater than the reference proportion or if the proportion of a harmonic of rank greater than or equal to 1 is less than the reference proportion, the difference relative to the harmonic considered is not taken into account. For example, if the first harmonic represents for example 98% of A, the second 2% and the third 0%, which would correspond to an almost total absence of harmonics of rank greater than 0 and therefore to an absence of faults, R _d = 0.

In the case where the continuous casting installation between cylinders would not include a system for regulating the air gap as a function of the runout, it would obviously be possible to apply the method according to the invention described above by taking directly as a signal subjected to a harmonic decomposition the direct measurement of the variations in the force of spacing of the cylinders (RSF), the use of the values H _i coming from the module of out-of-roundness remaining however particularly practical when such a module of compensation already exists in the and installs already, within the framework of its usual operation, the required decomposition into harmonics.

Claims (10)

Continuous casting process between cylinders of thin metallic products, in particular steel, according to which the roll spacing force (RSF) is continuously measured during casting, and a signal representative of the variations in l is generated. spacing force (RSF) as a function of time, and action is taken, in particular as a function of said signal, on the spacing of the cylinders to compensate for the runout of the cylinders, characterized in that, with the aim of detecting faults other than the runout of the cylinders, said signal is broken down into different harmonic components, and these said harmonic components are compared with reference harmonics of corresponding rank, the results of said comparison being representative of a fault state of the casting process , and we define, based on the results of said comparison, rules of conduct of the casting process. Method according to Claim 1, characterized in that the said representative signal, resulting from the measurement of the variations in the spacing force of the cylinders (RSF), is an associated signal used as a setpoint for displacement of the bearings of a cylinder in a loop regulating the spacing between said cylinders. Method according to any one of the preceding claims, characterized in that a Fourier transform is used in order to decompose said signal representative of the force between the cylinders (RSF) into different harmonic components. Method according to any one of the preceding claims, characterized in that, to carry out the comparison, a value H _i corresponding to the average of the amplitudes h _i of the harmonics of this rank measured over a number of turns given. Method according to any one of the preceding claims, characterized in that a comparison is made of a barycenter of harmonics, this barycenter being calculated by weighting a value representative of each harmonic by a predetermined coefficient. Method according to claim 5, characterized in that a frequency barycenter (B _f ) = (Σ (H _i × F _i )) / (ΣH _i ) is calculated in which the representative value of each harmonic is its frequency F _i and the weighting coefficient H _i represents the amplitude of the harmonic considered. Method according to claim 6, characterized in that the comparison is carried out on the basis of a ratio R _f = B _f / F ₀ , where F ₀ is the frequency corresponding to the speed of rotation of the cylinders. Method according to claim 1, characterized in that the comparison is carried out using as comparison criterion the proportion H _i / A of each harmonic component with respect to the signal representative of the spacing force, H _i representing the amplitude of the harmonic of rank i and A = ΣH _i . Method according to claim 8, characterized in that the result of the comparison is represented by the sum R _d = pos (α ₀ -H ₀ / A) + pos (H ₁ / A-α ₁ ) + ... + pos (H _i / A-α _i ). Method according to one of claims 7 or 9, characterized in that a decision table is used to determine the procedure to be followed for casting, according to the values of the criteria: - A = ΣH _i , - R (R _f or R _d ), - E = dR / dt.

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