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 present invention relates to a method, during casting of a cast strand in a mould which is open m both ends of the casting direction, of controlling and distributing a rotating magnetic field which is applied to act on and stir molten parts of the cast strand in the mould.
• the magnetic field is applied by means of a two-phase or polyphase stirrer arranged adjacent to the mould.
• the invention also relates to a two- phase or polyphase mould stirrer for carrying out the invented method.
• a melt is supplied to the mould by means of a free tapping j et , open casting, or through a casting tube, closed casting.
• a cast strand is formed by the melt being cooled.
• at least one solidified self-supporting surface layer on the casting strand has been formed.
• the flow of molten metal in the non-solidifled portions of a cart strand can be controlled by means of a two-phase or poly- pnase mould stirrer to which is applied a rotating magnetic field to act on and stir the melt m these parts of the cast strand, electric currents then being induced m the melt.
• a rotating magnetic field By cooperation between these induced electric currents and the applied magnetic field, forces arise which act on the melt and give rise to a rotation, a stirring, of the melt.
• the movements of the melt must be con ⁇ trolled and distributed. This makes demands on the properties and field-strength distribution of the magnetic field applied to the melt, as well as on current intensity and current den ⁇ sity and the direction of the induced electric currents.
• the demands vary between different casting processes, such as open and closed casting.
• closed casting that is, when melt is supplied to the mould through a casting tube which opens out into the melt below the meniscus, a sufficiently strong stirring in the non- solidified parts of the cast strand is desired to ensure the desired metallurgical result regarding casting structure, etc.; however, stirring and flow rate at the meniscus should be avoided.
• closed casting is referred to as casting with mould power since the upper surface of the melt is often covered with a so-called casting powder.
• a casting powder has a plurality of functions such as to thermally insulate the upper surface of the melt, protect it against oxidation and other reactions, and to prevent the adhesion of the solidified surface layer to the mould wall, as well as to take up particles separated from the melt.
• a sufficient displacement cannot normally be achieved by the variation in the position of the meniscus in the casting direction which is possible m a mould of the type referred to here, or by displacement in the casting direction of a stirrer designed and arranged in accordance with the prior art.
• the current intensity of the current which is fed through the phase windings can be varied.
• this entails increased capital costs since the phase windings in such a case must be dimensioned based on the greatest possible current intensity.
• the risk of powder being drawn down during powder casting increases.
• One object of the invention is to suggest a method of con ⁇ trolling and distributing a rotating magnetic field which is apclied to a cast strand present m a mould by means of two- pnase or polyphase stirrers for stirring the non-solidifled portions of a cast strand, whereby a sufficiently strong stirring to obtain the desired metallurgical result is ensured a certain distance down in the cast strand whereas the stirr ⁇ ing near the meniscus can be controlled and checked such that 5 optimum conditions can be offered with one and the same mould stirrer both during open and closed casting, such that suffi ⁇ cient stirring of the melt near the meniscus is obtained during open casting whereas this stirring is limited, inter alia to minimize the risks of powder being drawn down, during 0 closed casting.
• Another object is to provide a two-phase or polyphase mould stirrer for carrying out the invented method.
• molten metal or metal alloy such as steel
• a mould which is open m both ends of the casting direction.
• the molten metal is cooled, and during the passage through the mould a cast strand is formed.
• the cast strand exhibits at least one self-suppor ⁇ ting surface layer and residual melt contained therein.
• the flow or the stirring of liquid metal in non-solidified por- 5 tions of the formed cast strand is achieved and maintained by applying at least one rotating magnetic field to act on molten parts of the cast strand during the solidification of the metal
• This rotating magnetic field is applied by means of a tv/o-phase or polyphase mould stirrer arranged adjacent to the ⁇ mould
• the mould stirrer comprises two or more sub-stirrers.
• Each one of these sub-stirrers comprises at least one magnetic core and an ac-fed phase winding, arranged around the core, which are arranged in such a way that the magnetic field which is generated inside the mould, on a level with the cores, ⁇ essentially comprises a magnetic field-strength component, B ⁇ , ciiented across the casting direction.
• the phase windings included m the mould stirrer are arranged sufficiently near the inner wall of the mould and are designed m such a way that the magnetic field which is generated inside the mould on a level with the phase windings mentioned essentially comprises a magnetic field- strength component, B z , oriented parallel to the casting direction.
• the relative position in the casting direction is adjusted between the cores included m the mould stirrer and the upper surface of the melt, the meniscus.
• the differences m the orientation of the magnetic field strength between the levels at the phase windings and the cores, respectively, which are obtained when the cores are arranged and designed according to the invention, are utilized.
• the magnetic field-strength component across the casting direction By, dominates on a level with the cores while at the same time the magnetic field-strength component parallel to the casting direction, B z , dominates on a level with the phase windings.
• the cores included in the mould stirrer are arranged with their upper edge at a sufficiently large distance under the meniscus, preferably more than 25 mm under the meniscus, while
• the front edge of the phase winding is arranged sufficiently close to the inner surface of the mould wall, preferably more less than 150 mm from the inner surface of the mould wall, whereby the flow of the melt at and near the meniscus is braked by the generation and the application to the melt of braking moments on a level with the phase windings by the action of the force which arises when the component of the magnetic field in the casting direction, B z , and the electric current which are induced in the melt by the magnetic field cooperate.
• the cores included in the mould stirrer are suitably arranged with their upper edge 25 mm to 200 mm under the meniscus, while at the same time phase windings m the mould stirrer are arranged with their front edge 10 mm to 150 mm from the inner surface of the mould wall. This results in optimum conditions for closed casting, powder casting.
• the flow and turbulence of the melt at the meniscus are limited and controlled by the strong braking moments arising ust the meniscus, while at the same time a powerful stirring -.s maintained somewhat down m the cast strand.
• the invention When the invention is applied to open casting, wherein melt is supplied to the mould by means of a free tapping et which hits the upper surface of the melt, the meniscus, the cores included in the mould stirrer are arranged with their upper 0 edge sufficiently near the meniscus, preferably less than 100 mm from the meniscus.
• a sufficiently strong flow is achieved and maintained at and near the meniscus by means of the forces which are generated by cooperation between the component of the magnetic field across the casting direction, ⁇ By, and the electric currents induced in the melt by the magnetic field while at the same time the component B z of the magnetic field, on a level with the above-mentioned phase windings, is applied near the surface of the meniscus without achieving any force directed opposite to the flow.
• this case 5 substantially comprises a magnetic field-strength component, B v , oriented across the casting direction, whereas the magnetic field-strength component, B z , which is generated on a level with the phase windings and oriented parallel to the casting direction, does not act on the molten metal but is situated above or upstream of the meniscus.
• the cores are arranged with their upper edge 50 to 100 mm below the menis ⁇ cus, while at the same time the phase windings included in the mould stirrer are arranged with their front edge 50 to 100 mm frorr the inner surface of the mould stirrer and, during open casting, the cores included m the mould stirrer are arranged with their upper edge less than 100 mm below tne meniscus.
• a two-phase or polyphase mould stirrer for carrying out the invented method which is arranged in the form of two or more sub-stirrers, each one at least comprising one magnetic core and one ac-fed phase winding arranged around the core, in a preferred embodiment the phase windings included in the mould stirrer are arranged m the form of foil-wound coils, where the foil-wound coils are arranged with their front edge suffi ⁇ ciently close to the mould wall for the magnetic field which is generated inside the mould on a level with the foil wind ⁇ ings to essentially comprise a magnetic field-strength com ⁇ ponent, B z , oriented parallel to the casting direction, and for the foil windings to receive sufficiently indirect cooling from the cooling means arranged for cooling the mould.
• Figures 1 and 2 show the field-strength distribution and the moment which are obtained during open and closed casting, respectively, with a stirrer arranged according to the prior art, optimally placed relative to the meniscus.
• Figures 3 and 4 show a mould with a stirrer arranged according to the invention.
• Figure 5 shows the distribution in a plane in the casting direction for the field strength and moment of the magnetic field as well as the magnetic field, the electric currents which are induced in the mould during stirring, and the forces which arise by coopera ⁇ tion between the magnetic field and induced currents when the cores and phase windings included in the mould are arranged and designed according to the invention during open casting
• Figure 6 shows the same during closed casting where the stirrer is located at essentially the same level in the mould whereas its location relative to the meniscus has been achieved by means of a possible displacement of the meniscus position for the mould.
• the situation regarding the desired stirring of the melt at the meniscus 17 is the reverse, and the applied rotating magnetic field is arranged such that its field strength on a level with the meniscus is limited by applying the field-strength maximum Bmax of the magnetic field at a level essentially under the meniscus 17, whereby the magnetic field strength B on a level with the meniscus 17 is essentially lower than the maximum value Bmax as 1S shown m Figure 2.
• the mould stirrer is usually oversized so that it can be fed with a higher current intensity and apply a magnetic field to the melt which, at the meniscus, exhibits a sufficient magnetic field strength to achieve the required flow at the meniscus 17.
• the mould stirrer is usually oversized so that it can be fed with a higher current intensity and apply a magnetic field to the melt which, at the meniscus, exhibits a sufficient magnetic field strength to achieve the required flow at the meniscus 17.
• mould stirrers During casting of metal m a mould, which is open in both ends in the casting direction, with cooled wall plates 12a, 12b, 12c, 12d, the metal melt is cooled and formed into a cast strand 11.
• One or more mould stirrers is/are arranged to apply a rotating magnetic field to the non-solidifled portions of the cast strand 13.
• the mould stirrers according to one embo ⁇ diment of the invention which is shown in Figures 3 and 4, comprise four sub-stirrers which are arranged adjacent to the wall plates 12a, 12b, 12c, 12d of the mould.
• the sub-stirrers comprise at least iron cores 14a, 14b, 14c, 14d and phase windings 15a, 15b, 15c, 15d arranged therearound and are surrounded by a yoke 16 of a magnetically conducting material, whereby the magnetic circuits are closed.
• Figure 5 shows the distribution in a plane in the casting direction for the field strength and moments of the magnetic iield as well as the magnetic field, the electric currents which are induced in the mould during stirring, and the forces ⁇ 'hicn arise by cooperation between the magnetic field and the induced currents when the cores and the phase windings inclu ⁇ ded in the mould are arranged and designed according to the invention during open casting
• Figure 6 shows the same during closed casting where the stirrer is located at essen ⁇ tially the same level in the mould whereas its location rela- tive to the meniscus is achieved by means of a possible dis ⁇ placement of the meniscus position of the mould.
• the mould stirrer Since the cores 14a, 14b, 14c, 14d and the phase windings 15a, 15b, 15c, 15d included m the mould stirrer are arranged and designed according to the invention, it is sufficient with that dis- placement of the position of the meniscus 17 relative to these
• the cores 14a, 14b, 14c, 14d and the phase windings 15a, 15b, 15c, 15d included in the mould stirrers are arranged and designed according to that embodiment of the invention which is shown in Figures 3 and 4 where each one of the phase windings is designed m the form of a foil-wound coil.
• the foil-wound phase windings 15a, 15b, 15c, 15d are very compact and therefore need no separate cooling but are given sufficient indirect cooling through the cooling circuit of the mould.
• mould stirrers are then arranged at moulds where more poles than four can be arranged to control and distribute a magnetic field applied to the melt m a mould according to the invention.
• a magne- 5 tic feedback 16 a magnetic yoke
• the magnetic yoke 16 is arranged to surround the mould and the sub-stirrers.
• 25 windings 15a, 15b, 15c, 15d included in the sub-stirrers is arranged with its front edge at a distance away from the melt which is smaller than 150 mm. Since the mould wall normally has a thickness which usually exceeds 8 mm, the phase winding is preferably arranged at a distance away from the inner ° surface of the mould wall of between 10 and 150 mm.
• the iron cores included m the sub-stirrers are arranged more than 25 mm below the meniscus, preferably 25 to 200 mm belo. the meniscus During closed casting of /.eak rectangular blanks, so-called billets, and to a certain extent also during
• the phase windings included m the sub-stirrers are preferably arranged with their front edge at a distance from the inner surface of the mould wall, that is, from the melt, which lies within an interval of 50 to 100 mm, and the iron cores 14a, 14b, 14c, 14d included in the sub-stirrers are arranged with their upper edge 50 to 100 below the meniscus.
• the cores 14a, 14b, 14c, 14d and the phase windings 15a, 15b, 15c, 15d inclu ⁇ ded are arranged such that the propagation and the magnetic field strength B of the rotating magnetic field applied to and acting on the non-solidified portions 13 of the cast strand are controlled and distributed in such a way that the flow of the melt at and adjacent the meniscus essentially comprises a magnetic field-strength component, By, oriented across the casting direction, which gives rise to a sufficiently strong stirring at the meniscus.
• each one of the iron cores 14a, 14b, 14c, 14d included in the sub-stirrers is arranged with its upper edge less than 100 mm from the meniscus.
• the iron cores 14a, 14b, 14c, 14d included in the sub-stirrers are preferably arranged with their upper edge less than 100 mm below the meniscus.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Continuous Casting (AREA)
• Casting Or Compression Moulding Of Plastics Or The Like (AREA)
• Confectionery (AREA)

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• 1994
  • 1994-12-15 SE SE9404370A patent/SE513627C2/sv not_active IP Right Cessation
• 1995
  • 1995-12-14 AT AT95941323T patent/ATE193986T1/de active
  • 1995-12-14 WO PCT/SE1995/001506 patent/WO1996018469A1/en active IP Right Grant
  • 1995-12-14 EP EP95941323A patent/EP0797487B1/de not_active Expired - Lifetime
  • 1995-12-14 ES ES95941323T patent/ES2150593T3/es not_active Expired - Lifetime
  • 1995-12-14 CN CN95197585A patent/CN1083308C/zh not_active Expired - Fee Related
  • 1995-12-14 DE DE69517599T patent/DE69517599T2/de not_active Expired - Fee Related

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