Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
  • B22D11/10—Supplying or treating molten metal
  • B22D11/11—Treating the molten metal
  • B22D11/114—Treating the molten metal by using agitating or vibrating means
  • B22D11/115—Treating the molten metal by using agitating or vibrating means by using magnetic fields

Definitions

• 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 .
• 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 .
• 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.
• 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) .
• inventions 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) .
• US-A-4.522.249 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 lKHz .
• O-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.
• 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 electromagnetic 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.
• 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.
• Crystallisers of great length 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.
• 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
• 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.
• 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 .
• 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.
• 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 .
• 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.
• the plate elements are serially connected to each other, which allows the whole system to be fed with a single current.
• 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.
• 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.
• 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.
• 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 Al 2 0 3 or Br 2 C + Al 2 0 3 , 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 .
• 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.
• 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.
• 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 .
• 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 .
• 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.
• Fig. la shows a diagram of a crystalliser according to the invention
• Figs, lb and lc show two variants of Fig. la
• Fig. 2 shows a diagram of a transverse section of Fig. la
• 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. la;
• 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. la;
• 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, la-lc show in diagram form a first embodiment of a crystalliser 10 achieving the invention.
• 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.
• 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.
• 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 .
• 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
• the insulating layer may be made of AI 2 O 3 , or Br 2 C + AI 2 O 3 , 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 .
• 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) .
• the currents 13 may be of the pulsating type and may be supplied by a multi-channel feeder.
• each channel, associated with a relative plate element 11, of the multi ⁇ channel feeder delivers pulsations of current 13 with a maximum amplitude of 200 A, with a duration of between 50 and 500 ⁇ s and a frequency of between 2 and 150 Hz.
• 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.
• 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 at point H; 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 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.
• a preferential embodiment of the invention uses twisted conductors so as to reduce the inductance of the system.
• only one channel of the feeder is used to feed all four plate elements 11 of the crystalliser 10 which are serially connected.
• each plate element 11 is connected to the adjacent one 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.
• 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
• 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.
• Fig. 8 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.
• the currents 13 flow along the crystalliser 10 in a zig-zag path, remaining parallel however to the axis 12.
• a preferential embodiment of the invention uses cables of an equal length for each connection.
• each plate element 11 is electrically divided in a lengthwise direction into two parts, upper 11a and lower lib.
• 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 lib of one side of the crystalliser 10 is likewise associated electrically with the lower part lib of the adjacent side.
• the upper parts 11a are connected by means of conductors 17a to a first channel of a multichannel feeder and the lower parts lib are connected by conductors 17b to a second channel of the same multi-channel feeder.
• the upper parts 11a and the lower parts lib 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.
• 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.
• 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.
• the coil 20 is associated with moving means which allow the coil to be translated lengthwise along the sidewalls of the crystalliser 10.
• 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.
• 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.
• the current 23 is fed through one or more electrodes immersed in the crystalliser or through the nozzle 26.
• the current 23 flows in the metal 15 being cast and the circuit is closed by the containing foot rolls 27.
• the circuit wherein the current 23 circulates is closed by means to extract the cast product or by the appropriate means for that purpose.
• 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.
• 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 .
• 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.
• 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.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Continuous Casting (AREA)
• Water Treatment By Electricity Or Magnetism (AREA)
• Crushing And Grinding (AREA)
• Mixers Of The Rotary Stirring Type (AREA)
• Mechanical Treatment Of Semiconductor (AREA)
• Crystals, And After-Treatments Of Crystals (AREA)

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Citations (8)

• 1997
  • 1997-07-10 IT IT97UD000122A patent/IT1295164B1/it active IP Right Grant
• 1998
  • 1998-07-09 ES ES98929575T patent/ES2172156T3/es not_active Expired - Lifetime
  • 1998-07-09 WO PCT/IB1998/001055 patent/WO1999002286A1/en active IP Right Grant
  • 1998-07-09 AU AU79276/98A patent/AU7927698A/en not_active Abandoned
  • 1998-07-09 EP EP98929575A patent/EP1001862B1/en not_active Expired - Lifetime
  • 1998-07-09 AT AT98929575T patent/ATE212886T1/de not_active IP Right Cessation
  • 1998-07-09 DE DE69803775T patent/DE69803775T2/de not_active Expired - Fee Related

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