EP0036342A1 - Process for controlling the cooling of an ingot in a continuous-casting plant - Google Patents

Abstract

The cooling of a cast steel slab in a continuous casting installation is controlled by dividing the cast product into successive fictitious elements and periodically calculating the water flow values of the cooling water delivered to the successive cooling sections in the secondary cooling zone of the installation as a function of the age of the elements in these sections. The quantity of heat extracted in the mold is taken into account by periodically determining the water flow values in the different zones by means of a computer on the basis of a first curve giving the variations of the quantity of heat extracted from a unitary mass of the cast product as a function of the time while the cast product passes from the point of emergence from the mold to at least the zone of solidification, and a second curve giving the variations of the surface temperature of the cast product during this passage as a function of time. Before each calculation of the water flow values, the first curve is corrected in direct dependence on the quantity of heat extracted from the product in the mold.

Description

In a continuous casting installation, the molten metal begins to solidify in the ingot mold, where a relatively thin skin is formed, then solidification continues in the secondary cooling zone equipped with nozzles or spraying booms or water atomization.

The function of this secondary cooling by nozzles or ramps is to ensure regular growth of the skin formed in the mold to achieve complete solidification of the product cast in the form of a bar after a predetermined time. Sufficient water flows must be projected onto the bar to maintain the temperature of the skin of the cast product at a value low enough for it to have adequate mechanical strength. On the other hand, if too large water flow rates are used, the temperature of the product poured into the straightening zone of the bar from the curved state to the straight state will be too low; there will follow a decrease in the ductility of the surface of the cast metal such that the deformations due to the straightening of the bar will be greater than the limit deformations acceptable by the metal in this zone. In general, uncontrolled cooling of the cast product can be the cause of significant metallurgical defects, in particular of internal and surface cracks.

To optimize the production of the installation and the quality of the cast products, it is therefore essential to control the cooling of the cast product. Different systems, more or less advanced, have been proposed for this purpose.

This is how we realized installations where the total cooling water flow was kept proportional to the speed of extraction of the poured product, repairing it. tition of the flow between the different zones being predetermined. There are also installations where the cooling water flow is adjusted according to the pouring speed, so as to maintain the watering rate (water flow / metal flow) proportional to this speed.

In other installations, the total water flow used in the cooling zone or the partial flows of one or more sections of this zone are periodically adjusted according to the age of the fictitious slices of poured product formed during time intervals equal to the setting period, located in the corresponding cooling zone or section. For this, one or more variation curves of water flow rates as a function of the age of the product are used which are pre-established from test results and calculations.

All these systems which are content to maintain the cooling water flow rates at variable set values according to different laws but always predetermined, ignore the progress of the cooling process and do not take into account the variations of certain parameters, in particular the amount of heat extracted in the mold and the thermal profile of the casting bar.

The present invention provides a method and a system for taking into account the actual behavior and the thermal history of the cast product.

The process which is the subject of the invention is characterized in that the set values of the cooling water flow rates of the various sections of the secondary cooling zone are periodically determined by means of a computer from a first curve representing the variations, as a function of the residence time in the casting machine, of the quantity of heat extracted from a unit mass of the cast product, during its journey from the free surface of the metal in the ingot mold to the zoned solidification or beyond, and a second curve associated with the previous one due to the laws of thermal behavior and representing the variations, as a function of the residence time in the casting machine, of the surface temperature of the cast product. These curves were defined beforehand using a mathematical model for simulating the thermal behavior of the cast product calibrated by experimental results.

By dividing the product poured into fictitious elementary slices and periodically determining the age of each slice, we calculate, from the curves, the amount of heat to be extracted from each slice and its surface temperature, then the heat exchange coefficient for each slice. Using a curve giving the variations in the specific cooling water flow rates as a function of the heat exchange coefficient, the water flow rates to be projected on each unit are determined, then the flow rate setpoints are calculated. of water from each section of the cooling zone by integrating the water flows for all the units located at the instant considered in each section and the supply water flows of the plants are maintained by regulators different sections equal to the respective setpoints.

The equations from which the curves of the quantity of heat extracted and the surface temperature are established include parameters whose value can vary from one casting to another: nature of the metal cast, format of the product cast. It is therefore necessary to have a set of curves for each steel grade and each format that is planned to be cast.

The plotting of these curves also depends on the amount of heat extracted in the mold. The method of the invention allows this parameter to be easily taken into account. Calculations have indeed shown that variations in efficiency thermal city of the ingot mold only shift in time the curve of the quantity of heat extracted as a function of time in the secondary cooling zone, that is to say that the different curves of variation of the quantity of heat extracted in this zone can be deduced from each other by a simple translation along the time axis.

In accordance with the invention, the quantity of heat extracted in an ingot mold is determined, the curve of the quantity of heat extracted is corrected by shifting it parallel to the time axis so that it passes through the point whose coordinates are, on the one hand, the residence time of the product poured into the mold and, on the other hand, the quantity of heat extracted in the mold, and the set values of the water flow rates are calculated from the corrected curve.

To determine the quantity of heat extracted in the mold, one can measure the flow and the heating of the cooling water of the mold or the flows and the heating of the cooling water of the four faces of the mold.

It is also possible to determine the quantity of heat extracted in an ingot mold from data stored in the memory of the computer and established by predictive simulation calculations and / or by tests.

When the actual surface temperature of the product poured at the outlet of the ingot mold is not equal to the temperature given by the surface temperature curve, this curve is corrected by connecting by a straight line or a 2nd or 3rd degree curve the point whose the coordinates are, on the one hand, the residence time in the mold and, on the other hand, the surface temperature of the product at the outlet of the mold at a point on the curve corresponding to an upper section of the cooling zone smoothing, and the set values of the water flow rates are calculated from the corrected curve.

The surface temperature of the product poured at the outlet of the ingot mold is measured by means of an optical pyrometer or calculated from the amount of heat extracted in the ingot mold by means of a curve established using forecast simulation calculations.

Normally, the surface temperature profile which is imposed in the secondary cooling zone makes it possible to reach the desired temperature at the exit from the zone, in particular at the straightening point of the bar in a curved casting installation. However, the efficiency of the cooling device may vary accidentally, for example by fouling or wear of the nozzles.

In accordance with another characteristic of the invention, the surface temperature of the bar measured just at the outlet of the cooling zone or in the vicinity of the straightening point in the case of a curved machine is periodically compared with the desired temperature and, if the difference between these two values is greater than a predetermined value, the reference values of the water flow rates of the last sections of the secondary cooling zone are corrected. First of all, the reference value of the water flow rate of the last section is corrected; the importance of this correction is a function of the temperature difference. If at the end of a predetermined time this difference is still too great, it is possible to correct the set value of the water flow rates of one or more previous sections.

Furthermore, in order to respect the metallurgical constraints, a family of curves of quantity of heat extracted and of surface temperature are used which correspond to different extraction speeds of the cast product. In accordance with another characteristic of the invention, several classes of extraction speed are defined and a set of curves of the quantity of heat extracted and of the surface temperature are associated with each class. For a given extraction speed, the set values of the water flow rates are calculated from the set of curves corresponding to the class into which this speed enters.

When the speed is modified for a short time (of the order of 2 to 4 minutes) and then returns to its initial value, the set values for the water flow rates for the duration of the transient regime are calculated from the curves corresponding to the initial speed.

When the extraction speed takes a new value and keeps it for a period of time whose duration is greater than a predetermined value (approximately 5 minutes), it is established after the expiration of this period of predetermined duration and from the two sets of curves corresponding to the initial and final speeds a law of variation of the set values of the water flow rates which limits the speed of the variations of the surface temperatures to a predetermined value (from 10 ° C / minute to 200 ° C / minute depending on the case) and the target values are calculated by applying this law until they correspond to the new extraction speed.

We can also divide the product poured into fictitious elementary slices, periodically determine the age of each slice, calculate the average extraction speed for the slice considered, calculate the specific water flow to be projected on this slice from the set of curves corresponding to this average speed and calculate the set value of the water flow rate for each section of the cooling zone by integrating the water flow rates calculated for all the units located in the section considered. The average extraction speed of a slice is defined as the quotient of the distance traveled by this slice in the machine by the age of this slice.

Since the average speed of the slices in a given section of the cooling zone varies gradually over time from the initial speed to the final speed, the water flows of the different sections, whose setpoints are calculated periodically as indicated above, will evolve gradually from the values they had at the initial speed to those which correspond to the new speed.

This division of the product cast into fictitious elementary slices is moreover generally used in the method of the invention, in the same way as in certain known methods, to determine at a given instant the age of the different slices, information from which the quantities of heat to be extracted, the surface temperatures, the exchange coefficients, the specific water flows and the set values for the water flows of the different sections of the cooling zone are calculated.

• La figure 1 est le schéma d'une machine de coulée continue courbe et du système de contrôle du refroidissement de la barre coulée conforme à l'invention ;
• la figure 2 est une courbe représentant les variations en fonction du temps de la température superficielle de la barre pendant son déplacement dans la machine de coulée ;
• la figure 3 est une courbe représentant les variations en fonction du temps de la quantité de chaleur extraite d'une masse unitaire du produit coulé pendant son déplacement dans la machine de coulée depuis la surface libre du métal dans la lingotière ;
• la figure 4 montre plusieurs courbes de variation de la température superficielle en fonction du temps pour différentes vitesses d'extraction de la barre ;
• la figure- 5 montre plusieurs courbes de variation de la quantité de chaleur extraite en fonction du temps pour différentes vitesses d'extraction de la barre ; et
• la figure 6 est une courbe formée de plusieurs segments valables dans les différentes sections de la zone de refroidissement et représentant les variations du coefficient d'échange thermique superficiel en fonction du débit d'eau spécifique.

The description which follows refers to the accompanying drawings which illustrate the process of the invention and in which:

• FIG. 1 is the diagram of a curved continuous casting machine and of the system for controlling the cooling of the casting bar according to the invention;
• FIG. 2 is a curve representing the variations as a function of time of the surface temperature of the bar during its movement in the casting machine;
• FIG. 3 is a curve representing the variations as a function of time of the quantity of heat extracted from a unit mass of the product cast during its movement in the casting machine from the free surface of the metal in the ingot mold;
• Figure 4 shows several variation curves of the surface temperature as a function of time for different extraction speeds of the bar;
• FIG. 5 shows several curves of variation of the quantity of heat extracted as a function of time for different extraction speeds of the bar; and
• FIG. 6 is a curve formed by several valid segments in the different sections of the cooling zone and representing the variations of the surface heat exchange coefficient as a function of the specific water flow rate.

The machine for the continuous casting of steel shown diagrammatically in FIG. 1 essentially comprises an ingot mold 10, a corset of guide rollers 12, straightening rollers 14 and a cooling device comprising nozzles or spray or atomization grouped by sections, all the nozzles or booms of the same section being connected in parallel to a supply pipe provided with a valve 16 whose opening is controlled by a regulator 18 to maintain the supply flow equal at a set flow rate set by a computer 20. The nozzles or booms are distributed all around the casting bar or, in the case of a bar with rectangular section, only on its large faces. Means are provided for manually adjusting the distribution between the different nozzles or booms of a section, according to their position, of the total flow of water supplying this section.

The machine is equipped with various measuring devices, the information of which is transmitted to the computer 20: thermometric rod 22 for measuring the temperature of the molten metal in the distributor 24, thermometric probes 26 for measuring the temperature of the water of cooling of the ingot mold, at its inlet and outlet, flow meter 28 for measuring the flow rate of the cooling water of the ingot mold, pulse generator 30 for measuring the extraction speed of the bar and the calculation of the age of the elements of the bar, pyro meter 32 for measuring the surface temperature of the bar in the vicinity of the straightening point, etc.

From this information and data stored in memory in the computer, the latter determines at regular intervals the set values of the water supply flow rates of the various sections of the cooling device. This regular time interval is for example between 1 and 50 seconds.

The principle of cooling control according to the invention is to maintain over time the evolution of the solidification of the bar regardless of the operating regime of the casting machine. For this, a law of variation of the quantity of heat C extracted per kilo of steel is imposed as a function of the residence time in the machine t (FIG. 3) with which is associated a law of variation of the surface temperature T of the bar as a function of the residence time in the machine t (FIG. 2). These laws essentially depend on the grade of steel, the size of the bar and the speed of extraction. In practice, for a given bar format, steel grades and extraction speeds will be grouped into different classes. For steel grades, the number of classes will depend on the order book of the steelworks. The extraction speeds can for example be grouped into three classes: high, medium and low. We must therefore have three sets of curves C = f (t) and T = g (t) for each bar format and each steel grade class.

Figures 4 and 5 show the families of curves T = g (t) and C = f (t) respectively, corresponding to the different speed classes V ₁ , V ₂ and V ₃ for a given bar format and a shade class of steel given with V ₁ <V ₂ <3

The temperature plotted on the curves T = g (t) can be the point temperature on the midline of a face of the bar, or the average temperature over the center lines of the four faces of the bar, or the average temperature over the width of one face or over the entire periphery of the bar; it can also be the average temperature on the middle part of one face of the bar or the average of the average temperatures of the middle parts of the four faces of the bar.

All these curves are defined by parametric equations or by point values introduced into memory in the computer. Data on the steel grade and the format of the bar are entered into the computer, before each casting, to allow it to select the set of corresponding curves. The extraction speed is continuously measured by means of the pulse generator 30 and the computer chooses at each instant the set of curves corresponding to the average speed deduced from these measurements.

From the selected set of curves, the computer can, at any time, calculate the surface heat exchange coefficient K for each element of the bar from C and T and deduce the specific water flow q to be projected on the unit of area of the element considered using a curve K = h (q) stored in the memory of the computer and shown in FIG. 6; this curve can be unique for the entire cooling zone or be formed by several distinct curve segments valid in the different sections of the zone. This curve K = h (q) can be relative to the entire periphery of the bar; in this case we consider the global phenomenon. It can also relate to the middle parts of the four faces of the bar; in this case we consider the local phenomena on the periphery of the bar.

This calculation is carried out periodically, for example every 10 s, and the bar is divided into elements the length of which is that of the slice cast during the interval of time between two successive calculations. The serial number assigned to each section from its production therefore makes it possible at any time to know its age and its position in the machine. Assuming that, for a fictitious elementary slice of the bar, the curves C = f (t) and T = g (t) give the values C ₁ and T ₁ for a residence time t ₁ and C ₂ and T ₂ for their residence time t ₂ = t ₁ + Δt, the quantity of heat extracted from a unit mass of this slice, during the time period Δ t will be Δ C = C ₂ - C ₁ .

If L is the length of the slice, 1 the width of the bar, e its thickness and f the specific mass of the metal, the heat flux extracted from this slice during the period at t will be:

et le coefficient d'échange thermique sur la périphérie de la tranche sera :

T = Tl + T2 étant la température superficielle moyenne de 2 la tranche.The heat flux density extracted by the lateral surface S of the periphery of the wafer will be:

and the heat exchange coefficient on the periphery of the wafer will be:

T = Tl + T2 being the average surface temperature of 2 the slice.

The curve K = h (q) gives, from the calculated value of K, the specific water flow q for this elementary section which makes it possible to calculate the water flow Q = qx S to be projected onto the lateral surface of the slice.

This method is slightly modified when the faces of the bar are not cooled by spraying water over their entire width. We then only consider the parts medians of the faces of the wafer, that is to say that S represents only the total surface of these median parts and T is the average surface temperature on this surface.

After calculating the water flow rates to be projected on each bar slice located at a given time in the cooling zone, the set values of the supply water flow rates of the different sections of the cooling zone are calculated by integration. cooling and the calculated values are transmitted to the respective controllers 18.

In the case where nozzles or atomizing booms are used in which the water jets are divided into very fine droplets by means of compressed air, the total water flow rate of the cooling zone is calculated and deduced therefrom the total air flow to be used, using an equation or a curve establishing a relationship between these two flows. Means are provided for manually adjusting the distribution between the different sections of the total air flow supplying the secondary cooling zone.

To determine the amount of heat to be extracted from each elementary slice of the bar in the secondary cooling zone, it is necessary to take into account the amount of heat actually extracted in the mold. For this we use a basic curve C = f (t) (in solid lines in Figure 3) corresponding to the operating conditions (steel grade, bar format, extraction speed) which we shift parallel to the time axis to pass it through point A, the coordinates of which are equal, respectively, to the residence time of the product poured into the ingot mold t ₁ and to the quantity of heat c ₁ actually extracted in the ingot mold; this new curve of general formula C = f (ta) is shown in broken lines in FIG. 3.

At each calculation step, the computer 20 determines, on the basis of the values of the flow rate and of the temperatures at the inlet and at the outlet of the cooling water of the ingot mold, measured continuously by the probes 26 and the flow meter 28, or from a file of values established using forecast simulation calculations, the quantity of heat extracted in the mold and deduces the time lag which must be taken into account for the determination of the quantities of heat to be extracted in the secondary cooling zone.

The surface temperature of the bar at the outlet of the ingot mold is measured by means of an optical pyrometer or is calculated from a curve established using forecast simulation calculations and giving the evolution of this temperature as a function the amount of heat extracted in the mold; this curve is stored in the computer memory. If this temperature T'l is different from the theoretical temperature Tl provided by the curve T = g (t) (in solid lines in FIG. 2) corresponding to the operating conditions, the computer will correct the start of this curve by admitting, by example, a linear variation of the temperature from the outlet of the ingot mold to a predetermined point in the upper part of the secondary cooling zone (dashed line), so as to find the theoretical temperature at this point. It is this corrected curve that the computer will use to determine the surface temperature for the calculation of the set values for the water flows.

The measurement of the surface temperature of the bar in the vicinity of the straightening point is continuously transmitted to the computer 20 by the pyrometer 32. If its value deviates too much from the desired value (for example if the difference is greater than 50 ° C), the computer modifies the setpoint values calculated for the last section (s) of the secondary cooling zone accordingly. Calcu The reader first corrects the setpoint for the water flow from the last cooling section; the extent of the correction depends on the difference between the measured temperature and the desired temperature. Then after a certain time, which depends on the position of the last cooling section relative to the rectification point, the computer corrects the set values for the water flow rates of the last two cooling sections if the difference between the temperatures is still too important. At the end of a period which depends on the position of the penultimate cooling section relative to the righting point, the computer maintains or not, depending on the temperature difference, the correction of the setpoints of the water flow rates of the two last sections. Optionally, this could gradually correct the water flow setpoints for the last three or four sections of the cooling zone.

When the extraction speed varies, two cases should be considered: if the speed is modified for a short time, of the order of 2 to 4 minutes, and then returns to its initial value, the thermal profiles are imposed during this transient regime C = f (t) and T = g (t) corresponding to the initial speed, that is to say that the computer continues to use a set of curves corresponding to the class of speeds into which the initial speed enters; this is what happens, for example, when changing a pocket or dispatcher. If, on the contrary, the extraction speed takes a new value and keeps it for a time greater than a predetermined time, for example 5 minutes, after the expiration of this time, the thermal profiles corresponding to the new speed are imposed. extraction. In fact, we gradually pass from the thermal profiles corresponding to the initial speed to those corresponding to the new speed by following a law determined by the computer and which limits the speed of surface heating or cooling to a maximum value between 10 ° C / minute and 200 ° C / minute.

The gradual transition from the old thermal profiles to the new ones can also be carried out as follows: at each calculation step, the computer assigns to each elementary section of the bar an average speed depending on its position in the machine and its age and calculates the water flow to be projected on the section considered using the set of curves C = f (t) and T = g (t) corresponding to the speed class into which this average speed enters. The set values of the feed water flow rates of each section of the cooling zone are calculated by integrating the water flow rates calculated for each elementary unit located in the section considered. As this average speed varies gradually to finally become equal to the new speed, if it is stable, the set values of the water flows for each section of the cooling zone will gradually change from the values they had at the initial speed up to the values corresponding to the new speed.

In addition to the set values for the water flow rate for the different sections of the secondary cooling zone and, if applicable, for the total air flow rate used for water atomization, the computer can supply other information: proposal of an optimal extraction speed in steady state which depends on the nature of the metal, the format of the product and the temperature of the metal in the distributor, alarms if the temperature of the steel in the distributor exceeds the limits imposed, if the calculated water flows are greater than predetermined maximum values, if the difference between the measured and calculated water flows is greater than 10%, if the actual extraction speed is greater than the optimal speed, if the surface temperature at the righting point is too low, etc.

The computer can also be advantageously used to check the condition of the cooling device secondary between two flows. For this, the different sections of the cooling device will be supplied; after setting flow rate setpoints using the calculator, the actual flow rates and actual pressures will be measured and the measured values compared to the calculated values. If the device is in good condition (no wear, no fouling, no leakage), there should be no significant differences between these values; in particular for a given flow rate, the pressure measured must comply with the pressure calculated. The calculated pressures are determined by the computer using the pressure-flow curves which are stored in the computer memory and which are pre-established on the basis of test and calculation results.

• - La zone de refroidissement secondaire doit avoir une longueur importante pour permettre un meilleur contrôle de la solidification de la barre ; en particulier, l'extrémité aval de cette zone doit se trouver le plus près possible du point de redressement afin d'obtenir en ce point une température superficielle de la barre la plus proche'possible de la température imposée par les contraintes métallurgiques.
• - La zone de refroidissement doit être divisée en un nombre aussi grand que possible de sections alimentées séparément de manière à réaliser un contrôle précis du refroidissement et suivre au plus près les lois d'échange thermique imposées.
• - Les dispositifs d'arrosage utilisés dans la zone de refroidissement secondaire doivent avoir une large plage de réglage des débits d'eau. Les profils thermiques imposés dépendent en effet du format de la barre, de la nuance de l'acier et de la vitesse d'extraction et doivent être obtenus par des réglages différents des dispositifs d'arrosage pour couvrir une large gamme de nuances et de vitesses, en régime permanent et en régime transitoire, il faut donc disposer pour chaque section de la zone de refroidissement d'une large plage de réglage du refroidissement, donc d'une large plage de réglage des débits d'eau.

In order for the method of automatic control of the secondary cooling object of the invention to have maximum efficiency, the following conditions must be fulfilled

• - The secondary cooling zone must have a significant length to allow better control of the solidification of the bar; in particular, the downstream end of this zone must be as close as possible to the straightening point in order to obtain at this point a surface temperature of the bar as close as possible to the temperature imposed by metallurgical constraints.
• - The cooling zone must be divided into as large a number of sections supplied separately as possible in order to achieve precise control of the cooling and to follow the imposed heat exchange laws as closely as possible.
• - The sprinkler systems used in the secondary cooling zone must have a wide range of adjustment of the water flow rates. The imposed thermal profiles depend in fact on the format of the bar, the grade of the steel and the extraction speed and must be obtained bare by different settings of the sprinkler devices to cover a wide range of grades and speeds, in steady state and in transient state, it is therefore necessary to have for each section of the cooling zone a wide range of cooling settings , therefore a wide range of adjustment of water flow rates.

For the realization of this last condition, it will be advantageous to use the atomization devices which are the subject of patents and certificate of addition 74/00227, 74/09449, 75/38986 and 76/32685, in the name of the plaintiff. These devices allow variations in the water flow rates in a ratio of 1 to 6 or even 1 to 10, while conventional spraying devices allow variations in the flow rates in a ratio of only 1 to 3.

Claims (13)

1. Method for controlling the cooling of the product poured in a continuous casting installation according to which the product poured is divided into fictitious elementary slices and the set values of the water flow rates of the various sections of the secondary cooling zone are periodically calculated. as a function of the age of the sections located in said sections, characterized in that the set values for the water flow rates of the various sections of the secondary cooling zone are determined periodically, by means of a computer, from '' a first curve giving the variations as a function of time of the quantity of heat extracted from a unit mass of the cast product, during its journey from the free surface of the metal in the ingot mold to the complete solidification zone or at beyond, and of a second curve giving the variations as a function of time, during the said path, of the surface temperature of the cast product, and in c e that, before each calculation, from the setpoint values of the flow rates, the first curve is corrected by: as a function of the quantity of heat extracted in the ingot mold. 2. Method for controlling the cooling of the product poured in a continuous casting installation according to which the product poured is divided into fictitious elementary slices and the set values of the water flow rates of the various sections of the secondary cooling zone are periodically calculated. depending on the age of the sections located in said sections, characterized in that the quantity of heat to be extracted from each section is periodically determined by means of a calculator, using a first curve giving variations in the quantity of heat extracted from a unit mass of the product cast as a function of the residence time of the wafer in the casting machine, this first curve being corrected, before each calculation, as a function of the quantity of heat extracted in ingot mold, the temperature is determined superficial of the section considered using a second curve giving the variations of the surface temperature as a function of the residence time of the section in the casting machine, the heat exchange coefficient is calculated from the values thus determined for the section considered, the specific water flow rate for the section considered is determined using another curve giving the variations in the specific water flows as a function of the heat exchange coefficient, the flow rate of water to be sprayed on the section considered, the water flows for all the sections located at the instant considered in each section of the cooling zone are integrated to determine the set values for the water flows of the different sections, and maintaining, by means of regulators, the water supply flow rates of the different sections equal to the respective set values. 3. Method according to claim 1 or 2, characterized in that the first curve is corrected by shifting it parallel to the time axis so as to pass it through the point whose coordinates are, on the one hand, the residence time of the product cast in the mold and, on the other hand, the amount of heat extracted in the mold. 4. Method according to claim 1, 2 or 3, characterized in that one determines from measurements or results of forecast calculations the surface temperature of the product poured at the outlet of the mold and if this temperature differs from the corresponding temperature given by the second curve, this curve is corrected before each calculation of the flow rate setpoints, by connecting by a straight line or a 2nd or 3rd degree curve the point whose coordinates are, on the one hand, the residence time in ingot mold and, on the other hand, said surface temperature determined from measurements or from results of forecast calculations at a point on this second curve corresponding to an upper section of the secondary cooling zone. 5. Method according to any one of the preceding claims, characterized in that the surface temperature of the cast product measured at the outlet of the cooling zone is periodically compared with the desired temperature and, if the difference between these two values is greater than a predetermined value, the set values of the water flow rates of the last section or sections of the cooling zone are corrected. 6. Method according to any one of the preceding claims, characterized in that, for a given bar format and steel grade, the extraction speeds of the cast product are grouped into several classes, a set of first is established and second curves for each speed class, the extraction speed is measured and the flow rate setpoints are calculated using the set of curves corresponding to the speed class into which the measured speed enters. 7. Method according to claim 6, characterized in that when the speed of extraction of the cast product varies, the substitution of the set of curves corresponding to the new speed for the set of curves corresponding to the initial speed is only carried out the expiration of a predetermined period, if the extraction speed has not returned to its initial value. 8. Method according to claim 7, characterized in that the substitution of one set of curves for another is carried out gradually so that the speed of the variations in the surface temperature of the cast product is between 10 ° C and 200 ° C per minute. 9. Method according to claim 6, characterized in that periodically determines the age of each elementary slice, the average extraction speed of each is calculated section, we calculate the specific water flow rate for the section considered using the set of curves corresponding to this average extraction speed, we calculate the water flow rate to be projected on the section considered and we integrate the flow rates d of all the slices in each section of the cooling zone to determine the set values for the water flow rates of the different sections. 10. Method for controlling the cooling of the product poured in a continuous casting installation equipped with nozzles or spray bars, characterized in that the total air flow supplying the nozzles or pipes of the cooling zone is calculated at l Using an equation establishing a relationship between the air and water flow rates in the cooling zone. 11. Control method according to any one of the preceding claims, characterized in that the second curve gives the variations, as a function of the residence time, of the surface temperature on the median line of one face of the cast product or the average surface temperatures on the midlines of the four faces. 12. Control method according to any one of claims 1 to 10, characterized in that the second curve gives the variations, as a function of the residence time, of the average surface temperature over at least a median part of a face of the cast product. 13. Control method according to any one of claims 1 to 10, characterized in that the second curve gives the variations, as a function of the residence time, of the average temperature over the entire width of one or more faces of the product. sunk.

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