EP0116496B1 - Regulating process for the secondary cooling of a continuous-casting machine - Google Patents

Abstract

In regulating the flow of cooling water sprayed upon a metallurgical product such as a steel slab in a machine for the continuous casting of such products which then undergo a straightening operation, the present, past and future speeds of the product are taken into account so as to compensate a projected change in the temperature of the product in the straightening stage, due to a planned or expected modification of the cooling conditions. These conditions are established by a regulating system responsive to changes in the speed of the product; the anticipated temperature change is compensated by substituting for the true speed, as a controlling parameter, a fictitious speed lying between the true speed and an advance image of an anticipated speed change whose effects upon the temperature are to be neutralized.

Description

The invention relates to a method of adjusting the secondary cooling of a machine for continuously casting a metallurgical product, of the type according to which the current speed and the past speed of the metallurgical product are taken into account, as known per se.

• - d'assurer la solidification complète du produit avant un certain niveau dans la machine (redres- sage/oxycoupage),
• - d'assurer une bonne tenue mécanique de la peau solidifiée le long de la machine et, en particulier, d'éviter les problèmes de gonflement dus à une température de surface trop élevée et générateurs de criques internes et de ségrégation centrales marquées,
• - d'assurer une certaine régularité dans le refroidissement du produit et d'éviter par suite les brusques réchauffements ou refroidissements susceptibles de créer des criques au front de solidification (criques médianes),
• - de maintenir la température de surface au décintrage dans la zone de bonne forgeabilité du métal et d'éviter ainsi la formation de criques transversales sur l'intrados.

The importance of secondary cooling, both on the quality of the cast products and on the productivity of the casting machine is well established. A good adjustment of the secondary cooling allows in particular:

• - ensure complete solidification of the product before a certain level in the machine (straightening / flame cutting),
• - to ensure good mechanical strength of the solidified skin along the machine and, in particular, to avoid the problems of swelling due to too high a surface temperature and generators of internal cracks and marked central segregation,
• - ensure a certain regularity in the cooling of the product and consequently avoid sudden heating or cooling likely to create cracks at the solidification front (middle cracks),
• - to maintain the surface temperature during decinking in the zone of good forgeability of the metal and thus to avoid the formation of transverse cracks on the lower surface.

In steady state, finding the optimal secondary cooling setting amounts to finding a distribution of water along the product poured into the different cooling zones of the machine which ensures these conditions by optimizing another parameter directly related to the productivity of the machine: the casting speed. This optimization is currently well under control.

In a variable regime where the evolution of the pouring speed is imposed, optimizing means finding a method of managing the water flows allowing the above conditions to be satisfied at all times.

Different models of secondary cooling management have already been proposed. They all use the speed of casting as active parameter for the calculation of the water flows, but can be divided into different groups according to the adopted method: one finds in particular a first group of models according to which one determines the water flows in the different watering zones depending only on the instantaneous pouring speed (models taking only the present into account). These models are generally ill-suited to the casting of slabs for heavy plate, for example they generally do not make it possible, during abrupt slowing down of the casting, to maintain the surface temperature of the slab outside the zone of poor forgeability of the casting shade.

In a second group of models, one determines these water flows as a function of an average speed defined from past history and present of the casting speed (models taking into account the past and the present).

These models are therefore based on:

• 1. The definition of a parameter characterizing in each watering zone the past and present history of the cast product. In most cases, this will be the average age of the product elements present at all times in a given area; the age of a product element is defined as the time spent by this element in the machine since its creation in an ingot mold (the product being, for the needs of the model, virtually considered as a series of elementary sections or "elements") .
• 2. The choice of a watering curve based on metallurgical criteria and indicating for each zone the flow of water to be sprayed as a function of the value of the parameter previously described.

They are characterized by a non-constant distribution of water between the different zones, and the need to use a computer due to the numerous calculations to be carried out to determine, at regular time intervals, the average age of the elements in the different zones. .

The different models in this group differ from each other by the choice of watering curves and the cooling criteria to which they obey, and the method of calculating the average age.

Reference will be made in particular to French patent application no. 80/05592 (FCB) or to Belgian patent BE-A-827 040, serving as a basis for the preparation of the preamble of independent claim 1, or to the model described by J False in a publication of the Revue de Métallurgie - June 1978, p. 404-415, and whose organization chart for the writing of the computer program is given in the last three pages of this memo.

The watering curves are chosen to best achieve the objectives of cooling, in particular to maintain the surface temperature in the decintration zone above the pocket of poor forgeability of the cast product (i.e. above 900 ° C. approximately as a rule) to avoid the formation of transverse cracks on the lower surface.

Even the most sophisticated secondary cooling management systems still have difficulties in achieving this objective, due to the important transient regimes specific to continuous casting, such as changing the grade of steel, replacing the distributor or more simply the start and end of casting.

This is all the more true since the last cooling zones are very limited in the cooling adjustment range. In general, moreover, the last zone, just upstream of the decining point, is often devoid of cooling means. Also, in the event of significant variations in speed, there is little that can be done to correct the thermal profile of the part of the product located in these terminal areas. This is not very serious, if the transition consists of an increase in speed, because, in this case, the temperature increases. But it does not go too far in this direction, because of the risks of swelling or cutting of the product on a liquid core.

On the other hand, gravity can be systematic in the event of slowing down (or stopping) of the product, because, in this case, the temperature drops irresistibly and can be found in the region of bad forgeability and this even if one stops cooling, by simple game of radiation loss.

The object of the invention is to propose a method for adjusting the cooling which is free from the drawbacks mentioned.

This object is achieved, within the framework of a process of the aforementioned type at the head of this thesis, because one takes into account not only the present and past speeds of the product but also its future speed so as to compensate by anticipation a change in the temperature of the product in the decoupling zone due to a planned change in its speed.

More specifically, the secondary cooling being managed by a regulation system parameterized on the actual rate of pouring of the product, the temperature change at the declinering point is compensated by anticipation by temporarily introducing into the regulation system instead of the real speed a fictitious speed between the current speed and the future speed whose effects on temperature are to be compensated for.

In other words, a “decoy” is introduced into the regulatory system.

The invention is based in part on the analysis of the situations encountered in the continuous casting process, this analysis showing that approximately 90% of the events are foreseeable: it is thus possible to provide for a change of the distributor basket, or a feed delay with , for example, half an hour in advance. It is therefore possible to intervene in advance and compensate for the subsequent cooling of the product in the final zone following the slowdown, by prior overheating (with respect to the normal regime) by means of an anticipated reduction in the cooling regime.

• - la figure 1 est un graphique des profils de vitesse de coulée réelle ou factice,
• - la figure 2 est un graphique de l'évolution de température en fonction de la progression d'un élément donné du produit, dans trois cas respectivement idéal, modifié par un événement, et corrigé par anticipation selon l'invention,
• - la figure 3 est un graphique de l'évolution de température du produit au niveau du décintrage pour les éléments successifs qui y parviennent.

The invention will be better understood thanks to the description which will be made of it and with reference to the appended drawings in which:

• - Figure 1 is a graph of real or dummy casting speed profiles,
• FIG. 2 is a graph of the temperature evolution as a function of the progression of a given element of the product, in three respectively ideal cases, modified by an event, and corrected by anticipation according to the invention,
• - Figure 3 is a graph of the temperature evolution of the product at the level of decinking for the successive elements which reach it.

In Figure 1 is shown the evolution of the casting speed V as a function of time t. The solid curve represents the actual casting speed: the speed remains constant for a certain time (part a), then, an event, for example a change of distributor, imposes a modification of speed, according to a profile b, going up to l 'possible stopping of the casting.

Whereas in the known regulation system this actual speed is taken as a regulation parameter, according to the invention, an anticipated dummy setpoint, such as c, d or e, is introduced into the regulating system according to the degree of anticipation chosen or possible. Events are not all predictable with the same advance. In addition, even if they are predictable with a large advance, we do not necessarily anticipate their arrival with the total advance: as a general rule (but not critical) we adopt the maximum anticipation compatible with all metallurgical constraints of casting. This avoids too much anticipation which can cause a temperature beyond the acceptable threshold on the surface of the product at the level of decintring or along the product.

The anticipated speed profile is not necessarily identical to the actual speed profile at the time of the event, especially since, if the event is in itself predictable, the exact speed profile is not necessarily known with accuracy in advance, and this is all the more so since one generally has poor control over speed drops in practice.

FIG. 2 represents, as a function of the position L of an element of the product poured over the metallurgical height, the evolution of the temperature T of said element. The discontinuous curve f represents the ideal temperature profile where the temperature is seen to decrease from the maximum temperature at the outlet of the ingot mold to the temperature corresponding to the forgeability threshold M, generally around 900 ° C. at the level of the Declining zone N.

Curve g represents the temperature profile during an event characterized by a drop in casting speed. This event disturbs the regulation and causes the surface temperature to drop below the forgeability threshold, in particular at the level of the drop zone. This problem arises in particular from the fact that the heat regulation operates fairly well for liquid steel, but is more difficult to control for solid steel, therefore, essentially at the level of the last elements, below the liquid well of the product. .

The curve h represents the temperature curve obtained thanks to the invention, where, having introduced into the regulation management system, an anticipated dummy speed profile, it was possible to maintain the temperature profile above the threshold of forgeability M.

FIG. 3 represents the curve for the evolution of the surface temperature at the level of the decinking zone, for elements taken out of the ingot mold at times S. Four curves have been represented: the curve w corresponds to the evolution constant in a conventional regulation management system configured on the real speed curve ab; the curves x, y, z correspond to the evolution observed in the context of the invention by anticipating the event according to the respective speed profiles c, d, e of FIG. 1.

If, for example, the threshold of bad forgingability is set at 1000 ° C. (represented by the line k), it can be seen that the change of distributor distributes each curve w, x, y, z below said threshold, from the point A. But whereas for a conventional system, the curve w only goes back above the threshold at point B, for the regulation according to the invention according to the curve z, we are above the threshold from the point C, located approximately halfway between A and B. In other words, thanks to the z regulation, the length of the cast product reaching decinking is substantially divided by two at a temperature below the threshold of good forgeability.

The realization of the computer program for the implementation of the invention will not pose any major difficulty from the teaching contained in the publication mentioned at the beginning and from the organization chart given on the following pages and that it will suffice to complete by inserting an anticipation procedure defining a fictitious rate of pouring which makes it possible to intervene on the cooling of the product before the disturbing event actually occurs.

Claims (2)

1. A method for regulating the secondary cooling of a machine for continuously casting a metallurgical product subjected to a straightening operation, according to which the secondary cooling is controlled by a regulation system based on the parameter of the casting speed of the product, said method being of the type which takes the current and past speeds of the product into consideration as being known, characterized in that a temperature change of the product in the straightening zone due to a predicted modification of its speed is compensated in advance by introducing into the regulation system in place of the real speed a fictitious speed ranging between the current speed and the future speed whose effects on temperature are to be compensated.

2. A method according to claim 1, characterized in that a decrease in the casting speed is compensated by an anticipated reduction of cooling.

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