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 the continuous casting of metals and alloys, for example, steel.
• Electromagnetic stirring of liquid steel within the mold is broadly employed in continuous casting mainly to improve quality of the strand surface/sub-surface and solidification structure (i.e., structure refinement, soundness and chemical homogeneity) .
• the two most common practices of continuous steel casting impose entirely opposite requirements to the stirring conditions within the region of molten metal near its free surface at the mold top, i.e. the meniscus region. Accordingly, casting mainly Al-killed steel grades via a submerged entry nozzle under mold powder requires meniscus stability in order to prevent disruption of mold lubrication and powder entrapment into the cast body.
• a rotary stirring motion at the meniscus causes meniscus depression in the centre, waves, and excessive erosion of the casting nozzle when stirring intensity exceeds a certain level.
• casting of Si-Mn deoxidized steel with an open stream is often accompanied by the defects of cast product surface. Pinholes, blowholes, surface slag entrapment and subsurface inclusions are examples of those surface defects.
• an intensive stirring motion of the molten metal is required in the meniscus region. The same requirement applies for casting low deoxidized, or so-called rimming substitute steel.
• overly intensive stirring motion of the meniscus may cause undesirable deterioration of the surface by producing deep oscillation marks and lappings.
• the main drawback of this method is that the induction coil adjacent to the meniscus region provides only a deceleration of the stirring velocity produced by EMS. In the case when an intensive stirring action within the meniscus region is required, this method would need to relinquish its decelerating action by de- energizing the coil. The coil is not intended to enhance stirring action if the need of such enhancement arises, for the reasons discussed above.
• W is the angular stirring velocity
• R is the radius of the stirred pool g is the acceleration due to gravity
• the depth of meniscus depression h approaches zero when the angular stirring velocity at the meniscus caused by EMS is equalized by counter-stirring angular velocity produced by a braking induction coil.
• Another possible way of alleviating the problem of meniscus instability and decreasing stirring motion at the surface is an application of a strong horizontal D.C. magnetic field to the meniscus region.
• a strong horizontal D.C. magnetic field produces an electromagnetic (Lorentz) force directed opposite to the liquid metal motion and thereby reduces that motion velocity, providing a quiescent surface.
• An application of this concept is described in the U.S. Patent No. 4,933,005 of June 12, 1990, assigned to the assignee hereof.
• An electromagnetic volume force will be produced in either of two situations, firstly, when an A.C. rotating magnetic field interacts with liquid metal which is in the state of complete rest, the metal will be set into a motion with a velocity lower than that of the A.C. field; and, secondly, when a stationary, i.e. D.C. magnetic field, interacts with liquid metal already in motion.
• the volumetric magnetic force is proportional to velocity slip, i.e., the difference between the velocities of magnetic field and liquid metal, in accord
• ⁇ is the electrical conductivity of liquid metal
• B is the magnetic flux density
• W f is the angular velocity of magnetic field
• W m is the angular velocity of liquid metal
• R is the radius of liquid metal pool
• an electromagnetic A.C. coil similar to but smaller than that of a main electromagnetic stirrer installed downstream is arranged around the mold in the meniscus area.
• This device is in essence another induction stirrer, similar to the main stirrer which is arranged axially symmetrical around the mold and farther down from the meniscus.
• the coil in the upper part of the mold is intended to counterbalance and equalize, or enhance, depending on specific objectives, the stirring motion in the adjacent volume of liquid metal, the metal motion which is originated by the main stirrer. Therefore, the working function of this stirrer is to modify the pattern and/or intensity of the stirring induced by the main stirrer and henceforth the device performing that function will be called A.C. magnetic stirring modifier or A.C. MSM.
• the action of the A.C. MSM is typically contained within the upper portion of molten metal pool, comprising approximately 10 to 15 percent of its volume confined by the mold.
• both the A.C. MSM and M-EMS operate at a common frequency determined by the parameters of the mold.
• the current supplied to both sets of the coils can be of the same variable value or it can be controlled separately.
• the invention is broadly applicable to all electroconductive materials, i.e. metals and alloys, which can be electromagnetically stirred and where control of stirring intensity is required within some region or regions without interference with stirring within other regions of the liquid pool.
• the invention is applicable to a wide variety of spacial orientation of a vessel containing the molten method.
• a casting mold may be arranged vertically, inclined or horizontally.
• Figure 1 is a schematic of an arrangement of an A.C. magnetic stirring modifier and an electromagnetic stirrer (EMS) , with respect to a casting mold in accordance with one embodiment of the invention
• Figure 2 is a schematic representation of the magnetic flux density axial profiles for the A.C. magnetic stirring modifier and the EMS of Figure 1 and the axial profile of rotational stirring velocity produced thereby;
• Figure 3 is a graphical representation of the relationship of meniscus depression without and with an A.C. magnetic stirring modifier at varying current of an EMS
• Figure 4 is a single-line diagram of possible electrical connections for the induction coils of the A.C. magnetic stirrer modifier and the EMS of Figure 1
• Figure 5 is an elevational sectional view of the mechanical arrangement of the A.C. MSM and the EMS within the mold housing and corresponding to the schematic arrangement of Figure 1.
• Figure 1 is a schematic depiction of an arrangement of an A.C. MSM and an EMS within a mold housing assembly of a continuous casting machine 10 in accordance with one embodiment of the present invention.
• Figure 5 is a more detailed depiction of the mechanical elements of the mold assembly.
• a series of induction coils 12, is arranged equally spaced around the periphery of a vertical casting mold 14, at its lower portion to comprise an A.C. electromagnetic stirrer (EMS).
• a casting ceramic tube 18 is axially located with respect to the strand of molten metal 16, when casting with a submerged entrance nozzle is being performed.
• A.C. MSM induction coils 20, are equally spaced around the vertical mold 14, adjacent to a free upper surface or meniscus 22 of the strand of molten metal 16.
• the EMS coils 12 are designed to induce a strong rotational flow of molten metal in the strand of molten metal 16 within the mold 14.
• the intensity of this rotational flow is characterized by its rotational velocity U R which, in turn, depends on the parameters comprising the expression: ,
• a maximum value of rotational velocity is attained within and about the region of molten metal defined by a characteristic length of stirrer L which corresponds to a magnetic flux density B distribution along stirring axis.
• a typical magnetic flux density distribution for the two sets of induction coils 12 and 20 are shown in Figure 2.
• the value of the maximum stirring velocity within and about the active stirring zone L and the rate of its axial attenuation within the metal 16 determine the stirring velocity at the meniscus area 22 in the absence of other effects.
• the stirring velocity value and its lengthwise axial range depend on the stirrer length L, the radius of the stirred pool R, and the roughness of the solidification interface with liquid metal. Accordingly, it is difficult to quantitatively and accurately predict the stirring velocity at the meniscus, based upon the design and operating parameters of the EMS coils 12 and the distance from EMS neutral axis to the meniscus.
• the stirring velocity at the meniscus generally is about 0.5 to 0.7 (about 50 to 70 percent) of maximum stirring velocity value while the EMS coils 12 are located at a lowest position with respect to the meniscus. Therefore, a substantial stirring action can be expected at the meniscus area produced by the EMS coils even if the latter is located at the farthest possible distance from the meniscus. Meniscus depression and, more generally, turbulence at this location manifest themselves as a result of this stirring action.
• the meniscus depression depth is strongly correlated to the angular stirring velocity at the meniscus.
• the meniscus stirring velocity and depression are proportional to the current supplied to the EMS coils 12, as shown schematically in Figure 3.
• the meniscus depression for industrial systems can range from approximately 6 to 27 mm, for example.
• the induction coils 20 of A.C. MSM are energized, to induce a stirring action within the liquid metal at the meniscus opposite to that caused by the EMS coils 12. All the previous considerations with respect to a rotary movement of liquid metal are applicable to the stirring produced by the A.C. MSM coils 20.
• the A.C. MSM coils 20 are substantially smaller and require less power for their operation than the EMS coils 12 due to a much less stirring velocity expected for them to produce to counteract the rotational motion at the meniscus induced by the EMS coils 12.
• the A.C. MSM coils 20 are energized from a power supply common with the EMS coils 12, as shown by single line diagrams in Figure 4.
• Schemes I and II appearing in Figure 4 show the A.C. MSM and EMS coils 20 and 12 respectively connected in series and, therefore, operating at the time same current and frequency supplied from a common power source.
• the coil connections presented in Scheme I provide for unidirectional rotating magnetic fields produced by both the EMS and A.C. MSM coils. This mode of operation is employed for enhancing the stirring motion at the meniscus area by the A.C. MSM coils 20.
• the coil connections presented in Scheme II provide for counter-rotating magnetic fields and cause counter-rotating liquid metal motions in the areas corresponding to the EMS and A.C. MSM coils.
• the current level supplied to the A.C. MSM coils 20 may have an independent control from that of the EMS coils 12, as shown by Scheme III in Figure 4.
• This arrangement allows for independent control of stirring actions of either of the EMS or the A.C. MSM coils regardless of the directional pattern of stirring, namely unirotational or counter-rotational.
• the independent control of stirring motion at the meniscus provided by the use of the A.C. MSM coils 20 enables a greater flexibility of the stirring process control with a possibility of achieving equalization of the opposite stirring motions at the meniscus, and minimization of its depression, as illustrated in Figure 3.
• the line OA corresponds to the meniscus depression caused by the stirring induced by EMS coils 12 without being opposed or added by A.C. MSM stirring.
• the line OD represents meniscus depression associated with isolated stirring action induced by the A.C. MSM coils 20.
• the meniscus depression In order to equalize the stirring velocities caused by the EMS and A.C. MSM coils, the meniscus depression must be of the same value in either of the situations. For example, if the meniscus depression caused by EMS stirring corresponds to the level A, then counter-rotational stirring provided by A.C. MSM stirring should have corresponding meniscus depression, i.e. level D.
• the line OC is the resultant of two opposite stirring actions produced respectively by the EMS and AC MSM coils and equalized at the meniscus.
• the line AB represents the resultant of two unidirectional stirring actions.
• the range of stirring enhancement expressed through the meniscus depression can be adjusted in accordance with the casting practice requirements, so that the stirring intensity of EMS is fully utilized.
• the present invention provides an improved method of controlling disturbance of the free surface of molten steel or other metal or alloy being cast through a mold and caused by electromagnetic stirring applied to the liquid metal, to minimize such disturbance or achieve an enhanced, within a single casting unit stirring motion at the meniscus, by employing an induction modifier in the form of an electromagnetic stirrer adjacent to the location of the meniscus. Modifications are possible within the scope of this invention.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Continuous Casting (AREA)
• Furnace Details (AREA)
• Mixers With Rotating Receptacles And Mixers With Vibration Mechanisms (AREA)

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• 1994
  • 1994-01-14 AT AT94904542T patent/ATE154767T1/de not_active IP Right Cessation
  • 1994-01-14 DE DE69403950T patent/DE69403950T3/de not_active Expired - Lifetime
  • 1994-01-14 ES ES94904542T patent/ES2106501T5/es not_active Expired - Lifetime
  • 1994-01-14 CA CA002153995A patent/CA2153995C/en not_active Expired - Fee Related
  • 1994-01-14 WO PCT/CA1994/000018 patent/WO1994015739A1/en active IP Right Grant
  • 1994-01-14 EP EP94904542A patent/EP0679115B2/de not_active Expired - Lifetime

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