EP1001862B1

EP1001862B1 - Electromagnetic stirring method for crystallisers and relative crystalliser

Info

Publication number: EP1001862B1
Application number: EP98929575A
Authority: EP
Inventors: Milorad Pavlicevic; Anatoly Fedorovich Kolesnichenko; Alfredo Poloni; Andrea Codutti
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 1997-07-10
Filing date: 1998-07-09
Publication date: 2002-02-06
Anticipated expiration: 2018-07-09
Also published as: EP1001862A1; ES2172156T3; DE69803775T2; IT1295164B1; ITUD970122A1; WO1999002286A1; AU7927698A; ATE212886T1; DE69803775D1

Abstract

Electromagnetic stirring method for crystallisers, of the type including plates or based thereon, employed in the continuous casting of billets, blooms, slabs, round pieces, the method employing means to generate a magnetic field interacting with the metal cast inside the crystalliser, the method including the circulation of electric currents (13) directly along the sidewalls of the crystalliser (10), the principal component of the currents (13) having a direction parallel to the casting axis (12) of the crystalliser (10) and a desired direction and intensity, the currents (13) generating electromagnetic forces with a direction perpendicular to the casting axis (12) with the function of obtaining a separation of the skin of the solidifying metal from the sidewall of the crystalliser and a consequent reduction in the friction between the skin and the sidewall. Crystalliser to achieve the method as above whereby each plate element (11), or each group of plate elements (11), is electrically insulated lengthwise from the adjacent plate elements (11), or the adjacent group of plate elements (11), and is associated with its own electric power supply means generating an electric current (13) whose principal component has a direction parallel to the casting axis (12) of the crystalliser (10) and the desired intensity and direction.

Description

This invention concerns an electromagnetic stirring method for crystallisers and the relative crystalliser as set forth in the respective main claims.

The invention is applied to machines performing continuous casting of billets, blooms and slabs and, in particular, thin slabs in the field of the production of iron and steel.

BACKGROUND OF THE INVENTION

The state of the art of the continuous casting field covers the use of electromagnetic stirring devices associated externally with the sidewalls of a crystalliser in order to generate an electromagnetic field interacting with the molten metal being cast.

In the state of the art this electromagnetic field mainly has the purpose of improving the surface quality of the product by intervening on the liquid metal and improving its solidification characteristics.

To be more exact, such electromagnetic stirring devices anticipate and control the separation of the solidified skin from the crystalliser and thus make it possible to increase the casting speed and avoid risks that the skin itself might break.

The electromagnetic devices of the state of the art normally comprise one or more coils or inductors positioned in cooperation with the outside of the sidewall of the crystalliser and generally close to the zone of the beginning of solidification of the metal.

Embodiments have been disclosed in which the coil or inductor generates a stationary alternating magnetic field (see the article "Improvement of Surface Quality of Steel by Electromagnetic Mold" taken from the documents of the International Symposium on the "Electromagnetic Processing of Materials" - Nagoya 1994) or else generates an alternating magnetic field modulated in amplitude (see the article "Study of Meniscus Behavior and Surface Properties During Casting in a High-Frequencies Magnetic Field" taken from "Metallurgical and Materials Transaction" - Vol.26B, April 1995).

Other embodiments disclosed provide for the magnetic field generated to be periodically pulsating with waves defined by successions of pulses of a substantially constant amplitude (US-A-4,522,249) or else for the magnetic field to be generated by electromagnetic waves of a development which is attenuated until it is eliminated within a half-period (SU-A-1021070 and SU-A-1185731). To be more exact, US-A-4.522.249, on which is based the preamble of claims 1 and 16, teaches that a helical coil wound around the crystalliser for the whole of its length is fed by means of a pulsating direct current with a duration of from 10 to 100 milliseconds, with an amplitude of between 5 and 20 kA and with a frequency of repetition of around 1KHz. This current generates radial forces which act on the crystalliser and make it vibrate, thus improving the surface quality of the product.

The action of vibration on the crystalliser, however, can cause breakage through fatigue, and in fact often does.

The embodiment proposed by US-A-4.522.249, moreover, does not generate on the metal produced any actions of the travelling field variety, nor any multimodal excitations, which are those which make it possible to obtain a really useful result.

WO-A-80/01999 and FR-A-2.632.549 include electromagnetic devices consisting of poles, radial to the crystalliser, on which respective coils are wound, the devices being arranged on different levels and are made to work in a staggered manner.

The coils are fed with alternating current, low frequency single phase or multi-phase, and generate forces which are mainly oriented in an azimuth direction and only by reflection in a lengthwise direction along the axis of the crystalliser.

The function of these electromagnetic devices is to mix the liquid steel in the crystalliser in an azimuth direction so as to produce a helical motion upwards or downwards.

US-A-4.933.005 includes permanent coils or magnets operating both in correspondence with the meniscus and also in a desired zone of the crystalliser.

The coils arranged along the crystalliser, and far from the meniscus, generate forces which are prevalently of the azimuth type (azimuth stirring) or helical (helical stirring) or longitudinal (longitudinal stirring); the coils arranged in correspondence with the meniscus generate forces which oppose the movement of the liquid part of the product.

The coils which are far from the meniscus serve to move the liquid part of the product so as to obtain the known metallurgical results deriving from electromagnetic stirring; the coils which cooperate with the meniscus act as an electromagnetic brake so as to diminish the consequent distorsions of the meniscus caused by the electromagnetic stirring action generated by the other coils and so as to reduce the turbulence caused by the introduction of the material into the crystalliser.

WO-A-94/15739 includes two classical coils for electromagnetic stirring of which one is positioned on the meniscus.

Both coils are fed by means of low frequency multi-phase alternating current, possibly with different intensities of current; the direction of the magnetic field travelling on the pole extensions may also be different.

The forces generated are applied to the liquid part of the product in an azimuth direction.

The function of the underlying coil is to provide a maximum intensity azimuth stirring; the function of the coil on the meniscus is to contrast the distorsion on the meniscus produced by the stirring of the first coil or, alternatively, to increase the effect on the meniscus according to the particular type of process or kind of casting (type of steel).

EP-A-0.511.465 provides a coil for electromagnetic stirring which can be displaced along the axis of the crystalliser so that it is possible to adapt the electromagretic stirring effect in the liquid metal according to different metallurgical requirements.

EP-A-0.489.202 provides coils cooperating with the crystalliser and fed by direct current, which generate a constant magnetic field with the appropriate direction; these coils act as a brake on the liquid steel leaving the submerged nozzle and prevent it from affecting the already solidified skin; at the same time, they reduce the entrapment of slag.

US-A-4.867.786 and JP-A-56-126.048 provide coils which produce azimuth flows in order to mix the liquid part with a stirring effect in an azimuth direction in order to obtain the desired stirring effect.

US-A-4523628 provides to generate simultaneously a static magnetic field and a magnetic field which is variable in a sinusoidal direction; the magnetic fields are generated by coils wound in an azimuth direction around the axis of the crystalliser.

JP-A-5212512 provides to generate electromagnetic forces on the melted metal at the level of the meniscus so as to encourage the introduction and control the flow of lubricating powders; the electromagnetic forces are generated by coils arranged in an azimuth direction around the crystalliser and fed with high frequency alternating current.

JP-A-832350 provides to use plates applied to the walls of the crystalliser and made of particular materials in order to concentrate and encourage the flow of currents in particular zones of the crystalliser; these currents however flow orthogonal to the crystalliser and therefore to the movement of the melted metal.

JP-A-52134817 provides to apply a pulsed current flowing in an azimuth direction with respect to the crystalliser and being applied to the high zone of the crystalliser, around the meniscus, in order to reduce the risk of breakout and to improve the surface quality of the skin.

JP-A-58212843 provides to use electrodes introduced into the bath of molten metal inside the crystalliser so as to generate in the metal currents which melt the protecting and lubricating powders; however it does not provide that the currents cooperate with an external magnetic field produced by currents which flow along the crystalliser.

"Metallurgie des Stranggiessens" also provides currents induced in the molten metal from the tundish as far as the foot rolls, but in this case too there is no cooperation between the currents and a pulsed magnetic field generated by currents which flow on the walls of the crystalliser in a lengthwise direction thereto.

Experimental tests have shown that such configurations of the electromagnetic field acting in the crystalliser are not suitable to achieve the desired results in view of the different conditions which take place within the solidifying metal.

These different conditions, which are due to the different physical state and different temperature of the solidifying metal, cause an interaction between the magnetic field and the metal, this interaction being different from one zone to another of the crystalliser and therefore not being the best along the whole length of the crystalliser.

Moreover, the problems found in connecting the external inductors and the crystalliser are known to the state of the art; these problems include the dispersion and attenuation of the electromagnetic field generated, which causes a reduction in the intensity of the forces acting on the molten metal.

The problem of mechanical deformation to which the inductors may be subjected during use is also known.

It should also be noted that, at present, the tendency to use crystallisers of great length, in order to achieve high casting speeds, is becoming more and more marked.

Crystallisers of great length, in fact, make it possible to remove a great quantity of heat from the liquid metal, thus encouraging the formation of a skin of solidified metal of a thickness suitable to prevent the skin from breaking when the product leaves the crystalliser even when the casting speed is very high and therefore when there is less time for the skin to solidify.

With the electromagnetic configurations such as are known to the state of the art, wherein the current is fed in a direction orthogonal to the axis of the crystalliser, the flow lines of the magnetic field induced on the product wind round the crystalliser along its length.

Due to the great length of the crystalliser, the flow lines are dispersed, with a consequent loss of efficiency, so that the maximum forces which can be exerted on the cast product will never be the maximum obtainable for a given intensity of current applied.

In conclusion therefore, devices known to the state of the art do not make it possible to exploit in the best possible way the currents fed to the electromagnetic devices, which makes it necessary to use high intensities of current and causes dispersion and performances which are not of the best.

The present applicants have designed, tested and embodied this invention to overcome these shortcomings and to achieve further advantages.

SUMMARY OF THE INVENTION

This invention is set forth and characterised in the respective main claims, while the dependent claims describe variants of the idea of the main embodiment.

The purpose of the invention is to provide an electromagnetic stirring method for crystallisers in continuous casting machines for billets, blooms, slabs or round pieces, and the relative crystalliser, which will be able to carry out at least the following functions in an optimum manner and with an improved efficiency:

to apply to the solidified skin of the cast product pulsating electromagnetic forces which will encourage the separation of the skin from the sidewalls of the crystalliser; this facilitates the flow of the product along the crystalliser and eliminates the problem of sticking, particularly in the area where the skin first solidifies, thus improving the surface quality of the product;
to reduce friction between the skin of the cast product and the sidewall of the crystalliser by making the solid skin vibrate, and also the liquid part where necessary, in order to increase the casting speed;
not to use traditional systems of mechanical oscillation of the ingot mould and therefore of the crystalliser, with a resulting improvement of the surface quality of the product and preventing the formation of oscillation marks;
to control the effect on the meniscus according to the necessities of the process in order to improve not only lubrication but also the surface quality and the inner quality of the product;
to exploit the resonance capability of the solidified skin and the skin-liquid system, by exciting the specific resonance modes of the cast product zone by zone in a differentiated manner, in order to improve heat exchange in the musky zone so as to facilitate a growth of the product along symmetrical axes with a resulting improvement in the inner quality of the continuously cast product;
to use the travelling field configuration to induce in the liquid part an azimuth stirring so as to obtain an optimum result in the cast product;
to improve the heat exchange in the lower part of the crystalliser where the skin becomes separated from the crystalliser and thus increase the total quantity of heat removed from the crystalliser, making higher casting speeds possible and at the same time improving the quality of the product.

The invention includes the generation of a magnetic field, which acts on the metal cast inside the crystalliser, directly connecting the sidewalls of the crystalliser to electric supply means.

The crystalliser may have any section whatsoever, polygonal or circular, defined by at least two plate elements which are electrically insulated, completely or only partly, from each other.

According to a variant, the crystalliser is structured as a single element and includes electric insulation means which divide it into several parts which are electrically insulated from each other.

According to the invention, each plate or part of the crystalliser is fed with a respective current which has a direction parallel to the axis of the crystalliser and a desired sense.

According to a variant, the current is of the pulsating type.

Each current, whether pulsating or not, which flows along the respective plate element, or group of plates, generates an electromagnetic field whose flow lines close transversely on the plate element.

Therefore, the flow lines close on a space which is limited to the width alone of the relative plate element; this space is much less than the length of the crystalliser.

Therefore, with the same current applied, the invention makes it possible to generate electromagnetic forces of an intensity greater than those generated by electromagnetic systems such as are known to the state of the art, without any loss of efficiency due to dispersion.

According to a variant, the plate elements are serially connected to each other, which allows the whole system to be fed with a single current.

According to a first embodiment, all the plate elements of the crystalliser are fed by a current with the same direction; according to a variant, each plate element is fed with a current in the opposite direction to that circulating in the adjacent plate elements.

According to another variant, each longitudinal plate, or group of longitudinal plates, is fed with desired direction, duration, frequency and intensity of current so as to obtain desired effects on the metal being cast.

According to the invention, moreover, by feeding the current in the appropriate manner to the different plate elements, or groups of plates, it is possible to correlate the individual longitudinal zones of the crystalliser to different parameters, such as intensity, wave form, duration, etc.

Thus it is possible to generate electromagnetic forces with differentiated intensity, direction and frequency zone by zone so as to obtain the desired and variable effects on the metal being cast, for example by exciting the individual modes of resonance of the product according to the greater or lesser degree of solidification; it is also possible to adapt the electromagnetic forces to the conformation of the crystalliser.

According to a variant, the inner sidewalls of the crystalliser are covered with a layer of electrical insulation which has good thermal conductivity; it is thus possible to avoid direct electrical contact between the metal being cast and the sidewalls of the crystalliser, but does not limit the heat exchange necessary for the process of solidification of the liquid metal.

The insulating layer can be made of Al₂O₃ or Br₂C + Al₂O₃, or AlN or also amorphous diamond carbon or otherwise.

The crystalliser according to the invention may include a single cooling system or each plate element may be cooled autonomously.

According to a variant, there is a secondary effect superimposed over the principal effect of the forces produced by currents having a direction parallel to the axis of the crystalliser and circulating along the plates of the crystalliser; this secondary effect is provided by electromagnetic forces produced by currents having a direction azimuth to the axis of the crystalliser and fed to appropriate electromagnetic devices arranged outside the plates themselves.

This is to obtain more significant variations in the electromagnetic forces applied to the metal being cast in correspondence with the various transverse sections of the crystalliser.

In a preferred embodiment of the invention the electromagnetic devices consist of coils arranged in a position outside the crystalliser, coaxial to the axis of the crystalliser and positioned in correspondence with one or more transverse sections of the crystalliser where it is desired to obtain a particular effect on the cast metal.

In this embodiment, the azimuth currents fed to the outer coils may be of a very reduced intensity since their effect is used only in addition to the currents already circulating on the plates of the crystalliser and generated with a direction parallel to the axis thereof.

With these additional coils located at several transverse sections of the crystalliser, it is possible, for example, to make the cast product vibrate by exciting it locally.

For example, it is possible to excite the molten metal with frequencies of excitation corresponding or near to the frequencies of resonance, which are different at different points on the crystalliser according to the specific physical state, for example the degree of solidification, and the temperature of the metal being cast.

This condition of resonance achieved in a variable manner along the longitudinal extent of the crystalliser generates a better separation of the skin from the sidewalls of the crystalliser and therefore an easier and faster downward sliding of the metal, with an improvement also in the surface quality of the product.

By varying the forces exerted on the solid skin of the metal being cast it is also possible to manage the heat exchange with the crystalliser, which makes it possible, for example, to adapt the heat exchange to the type of steel and the type of process.

Moreover, by using outer coils, it is also possible to generate volumetric waves on the surface of the meniscus which make it possible to form a gap between the sidewall of the crystalliser and the skin of the metal which has just solidified; the gap facilitates the introduction of lubricants, either liquid or in powder form.

According to a variant of the invention, at least some of the coils outside the crystalliser are movable with respect to the casting axis of the metal, so that they can assume the optimum position from time to time according to the different casting conditions and the different effects which are to be obtained on the metal.

According to a further variant, the pulsed currents which flow parallel to the casting axis are not fed directly to the plates of the crystalliser but are produced by magnetic induction by means of outer inductors which are electrically insulated from the plates.

BRIEF DESCRIPTION OF THE DRAWINGS

The attached Figures are given as a non-restrictive example, and show some preferred embodiments of the invention as follows:

Fig. 1a: shows a diagram of a crystalliser according to the invention;
Figs. 1b and 1c: show two variants of Fig. 1a,
Fig. 2: shows a diagram of a transverse section of Fig. 1a;
Fig. 3: shows an embodiment of a crystalliser according to the invention;
Fig. 4: shows another embodiment of a crystalliser according to the invention;
Fig. 5: shows a variant of Fig. 1a;
Fig. 6: shows a transverse section of a variant of Fig. 5;
Fig. 7: shows a transverse section of another variant of Fig. 5;
Fig. 8: shows a further embodiment of a crystalliser according to the invention;
Fig. 9: shows a variant of Fig. 8;
Fig. 10: shows a variant of Fig. 1a;
Fig. 11: shows a transverse section of a possible application of the system according to the invention;
Fig. 12: shows a longitudinal section of an enlarged detail of a crystalliser according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Figs. 1a-1c show in diagram form a first embodiment of a crystalliser 10 achieving the invention.

In this first embodiment, the crystalliser 10 is rectangular in section defined by four plate elements 11 made of electrically conductive material, insulated from each other by a longitudinal insulation element 30.

According to the invention, each plate element 11 is fed directly with a current 13 which has a direction parallel to the axis 12 of the crystalliser 10 and an ascending or descending sense.

In the case shown in Fig. 1a, the four plate elements 11 are fed with respective currents 13 with an ascending sense; according to a variant which is not shown here, the currents 13 all have a descending sense.

In the variant shown in Fig. 1b, two adjacent plate elements 11 are fed with ascending currents 13 and the other two plate elements 11 are fed with descending currents 13.

Furthermore, in the variant shown in Fig. 1c, each plate element 11 is fed with a current 13 in the opposite direction to that fed to the other two adjacent plate elements 11.

The senses of the currents can be inverted as the casting process continues, to obtain the desired effects on the metal.

According to the invention, so as to prevent direct electric contact between the metal 15 being cast and the side walls of the crystalliser, the latter are lined on the inside by an insulating layer 28.

The insulating layer 28 has good characteristics of heat conductivity, which encourages heat exchange with the metal 15 being cast and therefore does not create any problems with the removal of heat which causes the progressive solidification of the liquid metal.

The insulating layer may be made of Al₂O₃, or Br₂C + Al₂O₃, or AlN or amorphous diamond carbon, or other.

The crystalliser 10 according to the invention may have a single cooling system or each plate element 11 may be cooled autonomously..

According to the invention, the current 13 circulating on the respective plate elements 11 generate a magnetic field whose lines of force 14 close substantially transversely on the plate element 11 itself, that is to say, in the direction of the width of the plate element 11 (Fig. 2).

With the system according to the invention, therefore, it is possible to use electric currents with a lower intensity than those used in systems known to the state of the art, because the force lines 14 of the magnetic field induced close on a much more limited space.

The currents 13 may be of the pulsating type and may be supplied by a multi-channel feeder.

In one embodiment of the invention, each channel, associated with a relative plate element 11, of the multi-channel feeder delivers pulsations of current 13 with a maximum amplitude of 200kA, with a duration of between 50 and 500µs and a frequency of between 2 and 150 Hz.

The variant shown in Fig. 5 shows a crystalliser 10 wherein each sidewall is defined by a plurality of plate elements 11, in this case three, which are electrically insulated from each other and fed with respective currents 13 which have a different sense than the currents 13 circulating in the adjacent plate elements 11.

In this embodiment it is possible to differentiate the currents fed to the individual plate element 11 of the crystalliser 10 in intensity, frequency and sense, so as to obtain the desired variable effects zone by zone on the metal 15 being cast.

In the embodiment shown in Fig. 6, it can be seen how it is possible to intensify or limit the electromagnetic forces acting on the metal 15 being cast according to the geometry of the section of the crystalliser 10, in this case with a H-shape; for example, it is possible to limit the electromagnetic forces in correspondence with the edges so as not to risk breaking the skin, and to intensify them in correspondence with the straight sides or obtuse angles.

The same considerations apply for more complex sections which are not shown here, or also for circular sections as shown in Fig. 7.

In the embodiment shown in Fig. 3, the currents 13 are applied to the respective plate elements 11 by means of bracket elements 16, each of which is connected to a respective channel of a multi-channel feeder, which is not shown here by means of conductors 17, either rigid, for example bars, or flexible, for example cables.

When flexible conductors 17 are used, a preferential embodiment of the invention uses twisted conductors so as to reduce the inductance of the system.

In the variant shown in Fig. 4, only one channel of the feeder is used to feed all four plate elements 11 of the crystalliser 10 which are serially connected.

In this case, each plate element 11 is connected to the adjacent ore by means of conductor bridges 18 which electrically associate the diametrically opposite peaks of each plate element 11 with the corresponding peaks of the adjacent plate element 11; each plate element 11, with the exception of two which are connected directly to a channel of the feeder by means of conductors 17, is electrically connected in series with the adjacent plate element 11.

In this way, the current 13 flows substantially diagonally along each individual plate element 11 so as to follow a substantially zig-zag path, along the serially connected plate elements 11 or along the sidewalls of the crystalliser 10.

Given that the plate elements 11 are very long and narrow, the principal component of each current 13 has a direction parallel to the axis 12 of the crystalliser 10, while there is also a secondary component with an azimuth direction with respect to the same axis 12; in this way the effect of the secondary component of the current is superimposed over the effect of the principal component.

The embodiment shown in Fig. 8 is similar to that of Fig. 4, only that the adjacent plate elements 11 are electrically connected in series, not in correspondence with the peaks, but in correspondence with the longitudinal terminal ends by means of conductors 19, either rigid or flexible.

In this embodiment too, the currents 13 flow along the crystalliser 10 in a zig-zag path, remaining parallel however to the axis 12.

When the conductors 19 consist of flexible cables, a preferential embodiment of the invention uses cables of an equal length for each connection.

In the variant shown in Fig. 9, each plate element 11 is electrically divided in a lengthwise direction into two parts, upper 11a and lower 11b.

This makes it possible to vary the electromagnetic forces acting on the metal 15 being cast in a lengthwise direction, thus making the system according to the invention more flexible and adapting the electromagnetic forces induced to the different state of solidification of the metal.

In this case, each upper part 11a of one side of the crystalliser 10 is electrically associated, by means of cables 19, to the upper part 11a of the adjacent side and each lower part 11b of one side of the crystalliser 10 is likewise associated electrically with the lower part 11b of the adjacent side.

In the embodiment shown, the upper parts 11a are connected by means of conductors 17a to a first channel of a multi-channel feeder and the lower parts 11b are connected by conductors 17b to a second channel of the same multi-channel feeder.

According to a variant which is not shown here, the upper parts 11a and the lower parts 11b of each side of the crystalliser are individually fed by a respective channel of the multi-channel feeder.

The variant shown in Fig. 10 makes it possible to regulate in a differentiated manner the electromagnetic forces applied to the metal 15 being cast on several transverse sections, in this case one, of the crystalliser 10.

For this purpose, there is at least a coil 20 outside the crystalliser 10, coaxial with the axis 12 of the latter and fed with a current 21 in an azimuth direction to the axis 12.

The current 21 generates electromagnetic forces whose effect is superimposed over the principal electromagnetic forces generated by the currents 13 parallel to the axis 12.

In correspondence with the coil 20 therefore, a desired variation in the electromagnetic forces acting on the metal 15 being cast is obtained.

In this embodiment, the current 21 necessary to feed the coil 20 may be of a very limited intensity, since the forces produced by this current 21 are superimposed over the principal forces produced by the currents 13.

According to a variant, the coil 20 is associated with moving means which allow the coil to be translated lengthwise along the sidewalls of the crystalliser 10.

According to another variant, there are several coils 20, individually fed, at various heights of the crystalliser 10.

In the embodiment shown in Fig. 11, in order to increase the electromagnetic forces acting on the metal 15 being cast, and in particular on the solidified skin 115, a current 23 of the pulsating type is applied directly to the metal 15 being cast.

The current 23 according to the invention may reach a maximum intensity of 5000A and have the same sense, duration and frequency as those of the currents 13, or different therefrom.

In this case, the current 23 is fed to the metal 15 being cast by means of two electrodes 24 immersed in the liquid metal 15 contained in the tundish 25.

According to other variants which are not shown here, the current 23 is fed through one or more electrodes immersed in the crystalliser or through the nozzle 26.

In the embodiment shown, the current 23 flows in the metal 15 being cast and the circuit is closed by the containing foot rolls 27.

According to other variants which are not shown here, the circuit wherein the current 23 circulates is closed by means to extract the cast product or by the appropriate means for that purpose.

As shown in Fig. 11, the current 23 flows principally on the outer face of the solidified skin 115 which is electrically insulated from the sidewalls of the crystalliser 10 by means of an insulating layer 28.

The electromagnetic field produced by the currents 13 circulating in the plate elements 11 of the crystalliser 10 interacts with the currents 23 and generates forces which facilitate the separation of the solidified skin 115 from the sidewalls of the crystalliser 10.

In this embodiment, moreover, the currents 23 leaving the nozzle 26 spread out: towards the outside of the crystalliser 10, thus creating pulsating forces inside the liquid metal 15 which produce a stirring effect which improves the inner quality of the metal 15 being cast.

Fig. 12 shows a variant which can be used in all the embodiments described, in order to obtain the maximum possible pulsating forces on the skin 115 of the metal 15 being cast.

According to this embodiment, the currents 13 circulating in the plate elements 11 flow in correspondence with the zone of the sidewall of the crystalliser 10 as near as possible to the skin 115 of the metal 15 being cast.

For this purpose, the conductor 19, whether it be of the rigid or flexible type, associated with one channel of the feeder, is electrically associated only in correspondence with the inner edge of the terminal end of the plate elements 11; on the remaining surface of the terminal end there is an insert 29 made of insulating material.

In order to prevent dispersion of the current 13 towards the outside, on the outer surface of the plate elements 11 there is a plurality of notches 22, in this case transverse, with the function of increasing the impedence of the outer surface.

Claims

Electromagnetic stirring method for crystallisers, of the type including plates or based thereon, employed in the continuous casting of billets, blooms, slabs, round pieces, the method employing means to generate a magnetic field interacting with the metal cast inside the crystalliser, the method being characterised in that it includes the circulation of electric currents (13) directly in and along the sidewalls of the crystalliser (10), the principal component of the currents (13) having a direction parallel to the casting axis (12) of the crystalliser (10) and having a desired direction and intensity, the currents (13) generating electromagnetic forces with a direction perpendicular to the casting axis (12) with the function of obtaining a separation of the skin of the solidifying metal from the sidewall of the crystalliser and a consequent reduction in the friction between the skin and the sidewall.
Method as in Claim 1, characterised in that the currents (13) are of the pulsating type and generate pulsating electromagnetic forces which interact with the cast product.
Method as in Claim 1 or 2, characterised in that the individual plate elements (11) of the crystalliser (10), electrically insulated from each other lengthwise, are fed with respective currents (13) with their own specific parameters of intensity, frequency, direction and duration.
Method as in Claim 3, characterised in that the individual plate elements (11) of the crystalliser (10), electrically insulated from each other lengthwise, are sub-divided transversely into several parts (11a,11b) electrically insulated from each other and fed with respective currents (13) with their own specific parameters of intensity, frequency, direction and duration.
Method as in any claim hereinbefore, characterised in that it includes the circulation of currents (21) with a substantially azimuth direction to the casting axis (12) in cooperation with at least a desired transverse section of the crystalliser (10).
Method as in Claim 5, characterised in that the effect of the azimuth current (21) is superimposed over the effect of the principal current (13) parallel to the casting axis (12) with the function of exciting the modes of resonance of the cast product inside the crystalliser (10) for the relative transverse zone of the crystalliser (10).
Method as in any claim hereinbefore, characterised in that it includes the circulation of a current (23) directly in the metal (15) being cast.
Method as in Claim 7, characterised in that the current (23) made to circulate directly in the metal (15) has the opposite direction from the current (13) circulating in the plate elements (11) of the crystalliser (10).
Method as in Claim 7, characterised in that the current (23) circulating directly in the metal (15) is induced by electrodes (24) associated with the tundish (25).
Method as in Claim 7, characterised in that the current (23) circulating directly in the metal (15) is fed directly to the nozzle (26).
Method as in Claim 7, characterised in that the current (23) circulating directly in the metal (15) is induced by electrodes immersed in the crystalliser (10).
Method as in any claim from 7 to 11 inclusive, characterised in that the current (23) circulating directly in the metal (15) closes on the foot rolls (27) and/or the means to extract the product.
Method as in any claim hereinbefore, characterised in that the currents (13) with a direction parallel to the casting axis (12) are made to circulate in the sidewalls of the crystalliser (10) as near as possible to the skin (115) of the metal (15) being cast.
Method as in any claim hereinbefore, characterised in that the pulsating currents (13) with a direction parallel to the casting axis (12) have a maximum amplitude of 200kA, a duration of between 50 and 500µs and a frequency of between 2 and 150Hz.
Method as in any claim hereinbefore, characterised in that the pulsed currents (13) with a direction parallel to the casting axis (12) are produced by means of magnetic induction by outer inductors which are electrically insulated from the relative plates (11) of the crystalliser (10).
Crystalliser for the continuous casting of billets, blooms, slabs, round pieces, the crystalliser being of the type with plates or based thereon and cooperating with means suitable to generate an electromagnetic field interacting with the metal cast inside, the crystalliser being characterised in that each plate element (11), or each group of plate elements (11), is electrically insulated lengthwise from the adjacent plate elements (11), or the adjacent group of plate elements (11), and is associated with its own electric power supply means generating an electric current (13) whose principal component has a direction parallel to the casting axis (12) of the crystalliser (10) and the desired intensity and sense.
Crystalliser as in Claim 16, characterised in that at least some of the plate elements (11), or groups of plate elements (11), are electrically connected in series and associated with a common means to supply the electric current (13).
Crystalliser as in Claim 17, characterised in that it includes flexible conductor elements (19) which electrically connect two adjacent plate elements (11), or two adjacent groups of plate elements (11).
Crystalliser as in Claim 18, characterised in that the flexible conductors (19) are twisted cables.
Crystalliser as in Claim 17, characterised in that it includes bridge conductors (18) in correspondence with the edges, electrically connecting two adjacent plate elements (11), or two adjacent groups of plate elements (11).
Crystalliser as in any claim from 16 to 20 inclusive, characterised in that, in cooperation with at least one defined trarsverse zone, it is associated with at least an electromagnetic device (20) outside the crystalliser (10) and coaxial with the casting axis (12), electric currents (21) flowing through the electromagnetic device (20) and having an azimuth direction with respect to the casting axis (12).
Crystalliser as in Claim 21, characterised in that at least one electromagnetic device (20) is movable at least lengthwise along the crystalliser (10).
Crystalliser as in any claim from 16 to 22 inclusive, characterised in that each plate element (11) is sub-divided into a plurality of longitudinal segments (11a, 11b), electrically insulated from each other and fed with respective currents (13).
Crystalliser as in any claim from 16 to 23 inclusive, characterised in that the plate elements (11) include notches (22) on the outer part to increase the impedence of the outer face of the crystalliser (10) and to concentrate the currents (13) circulating in the plate elements (11) in the forward zone of the plate element (11) nearest the cast metal (15).
Crystalliser as in any claim from 16 to 24 inclusive, characterised in that it includes means to generate a current (23) directly in the cast metal (15).
Crystalliser as in Claim 25, characterised in that these means comprise electrodes (24) immersed in the liquid metal contained in the tundish (25).
Crystalliser as in Claim 25, characterised in that these means consist of the nozzle (26).
Crystalliser as in Claim 25, characterised in that these means comprise electrodes (24) immersed in the liquid metal (15) inside the crystalliser itself.
Crystalliser as in any claim from 16 to 28 inclusive, characterised in that it comprises means to close the circuit of the electric current (13) circulating directly in the liquid metal (15).
Crystalliser as in Claim 29, characterised in that these means comprise the means to extract the cast product and/or the foot rolls (27).