RU2266798C2 - Method for metal continuous casting to mold and apparatus for performing the same - Google Patents

Abstract

FIELD: metallurgy, namely metal casting processes and equipment.

SUBSTANCE: apparatus includes mold, unit for feeding metal and electromagnetic device for agitating having two coils. First coil is excited by alternating current. Second coil is arranged upstairs first coil and it is made with possibility of changing AC excitation to DC excitation mode and vice versa. At first mode second coil is excited by alternating current. Rotation sign of magnetic field of second coil coincides with rotation sign of magnetic field created by means of first coil. In second mode second coil is excited by the same way but rotation sign of magnetic field is opposite relative to rotation sign of magnetic field created by means of first coil. In third mode second coil is excited by means of direct current for creating horizontally oriented magnetic field opposite relative to liquid flows direction in cross and in longitudinal planes of mold.

EFFECT: flexible control of agitation rate and melt metal flow.

13 cl, 4 dwg

Description

Technical field

The present invention relates to a method and apparatus for continuous or semi-continuous casting of metals and alloys, for example steel, into a mold that is open at both ends in the casting direction.

Description of the prior art

In operations of continuous casting of steel, the practice of mixing liquid steel in the mold of a continuous casting steel using an externally applied low-frequency electromagnetic field of alternating current has been established. Electromagnetic stirring, commonly referred to as EMF, is widely used in continuous casting of steel to improve the quality of the cast product and the productivity of the process. It was found that in order to satisfy the conditions of various casting methods, it is necessary to control the mixing motion in the area closely adjacent to the free surface of the melt, usually called the meniscus. Therefore, in the meniscus region, mixing motion with a certain intensity is required to control surface and subcortical porosity, subcrustal inclusions and other defects of round and square billets, obtained mainly from deoxidized Si-Mn steel. In other casting technologies, known as submerged crystallizer powder, the meniscus must be stable, and therefore restrictions are placed on the stirring movement on the meniscus. In addition to these two opposite requirements arising from different casting methods, a direct correlation was established between certain groups of creek defects and the intensity of mixing both in the meniscus and in the bulk in the mold. The need to control the mixing motion in these areas of the mold necessitated the creation of a number of techniques based on the use of electromagnetic fields of direct or alternating current. In US patent No. 4933005 it was proposed to control the mixing motion in the meniscus in the mold using a horizontal magnetic field of direct current. According to this patent, an external DC magnetic field applied externally interacts with the mixing flow in the meniscus region created by the main mixing device. This interaction generates an electromagnetic force that counteracts the movement of the liquid metal and thereby reduces the speed of this movement. This method of controlling the mixing speed has disadvantages. In conventional equipment designs used for continuous casting of round and square billets, the dimensions of the braking coil are limited, therefore, the strength of the DC magnetic field generated by the braking coil is sufficient only to reduce the mixing speed on the meniscus to 50-60 percent of the initial speed value.

Another known method for controlling the stirring movement in the meniscus is an alternating current electromagnetic coil double coil system described in US Pat. No. 5,699,850. According to this patent, an induction coil located in the upper part of the mold in the meniscus is excited by a current source independent of the current source of the main mixing device located in the lower part of the mold. Thus, the rotating magnetic field of the alternating current generated by the upper induction coil is controlled independently of the magnetic field of the main mixing device. When the direction of rotation of the magnetic fields created by the upper and main devices for mixing coincides, the mixing speed in the meniscus increases. This increase in speed can be controlled by introducing current into the upper coil. If the directions of rotation are opposite to each other, the upper mixing device turns into a magnetic brake with respect to the mixing flow in the meniscus region. By adjusting the current of this brake, it is possible to control the mixing speed in the meniscus in the interval from its initial value when no inhibitory effect is applied to effective zero when the magnetic moment of the brake is in equilibrium with the kinetic moment of the mixing flow in the meniscus.

The disadvantage of this method is that the inhibitory effect only affects the azimuthal component of the fluid flows caused by mixing or by the action of a jet poured into the mold. The longitudinal component of these flows remains unaffected by the alternating current magnetic field generated by the upper induction coil. These longitudinal fluid flows, depending on their intensity, create significant turbulence in the melt on the meniscus and in the area near the meniscus, which affects the working conditions of the casting method and product quality.

SUMMARY OF THE INVENTION

The present invention is based on the task of providing more flexible control of the mixing speed and melt flow, i.e. a molten metal duct in the melt meniscus in the mold of continuous casting devices used for manufacturing, for example, round and square billets.

The invention is solved by a device, the essential features of which are described in paragraph 1 of the claims, a method, the essential features of which are described in paragraph 7 of the claims, and a method, the essential features of which are described in paragraph 11 of the claims.

According to the present invention, the upper induction coil, in this context referred to as the "second induction coil", of the two-coil mixing system is excited by either direct or alternating current, depending on the desired effect on the mixing motion of the melt in the region adjacent to the upper free surface of the melt, and the main induction a coil, referred to as a “first induction coil”, always acts as a stirring device driven by alternating current, i.e. creating an alternating current magnetic field.

The second induction coil is preferably driven by alternating current from an independent source relative to the main mixing device, i.e. relative to the first induction coil. When this mixing system is used for casting using a measuring nozzle and there is a need to increase the mixing movement in the meniscus, then the upper induction coil operates in the mode of assistance to the main mixing device. Alternating current is also used to excite the upper induction coil when a complete or almost complete decrease in the mixing speed on the meniscus is required in the casting method with submerged metal casting.

A partial decrease in the stirring speed on the meniscus can be achieved by applying a horizontal direct current magnetic field. Such a partially inhibitory effect is required when casting using either a measuring nozzle or a submerged inlet nozzle, and it is necessary to adjust the mixing speed on the meniscus up to 50-60% of its initial value. In this case, direct current is used to drive the upper induction coil. Experience has shown that such braking intensity is sufficient in many cases using casting technology with submerged casting and when casting through a measuring nozzle. At high levels of mixing intensity, an additional decrease in the mixing speed is achieved by applying an alternating current magnetic field. Switching from alternating current to direct and vice versa is preferably carried out using electronic and programming (programmable) means, which are part of the power supply system. Fluid flows arising in the meniscus region as a result of mixing caused by the main device for mixing, the release of a jet of liquid metal and / or the movement of the mold will interact with the horizontal direct current magnetic field generated by the upper induction coil. As a result of the interaction between the horizontal direct current magnetic field and the fluid flows crossing this magnetic field at any angle not equal to zero, magnetic forces will arise that impede the movement of these flows. The maximum interaction is achieved when the angle between the magnetic field and the fluid flow is 90 degrees. As a result, the speed of mixing and longitudinal flows, including the jet being discharged straight down straight, will decrease. At the same time, meniscus turbulence will decrease and meniscus stability will increase, working conditions and the quality of cast products will improve.

Thus, due to the interchangeability of the magnetic fields of alternating current and direct current provided by one mixing system and created by the same induction coil located in the meniscus in the molds, the present invention can significantly increase the flexibility of controlling the mixing speed and turbulence on the meniscus, which provides more high productivity of the metallurgical process and the effectiveness of the mixing system.

The invention is a further improvement in the method and apparatus of a double coil mixing system. It can be widely applied to all electrically conductive materials, i.e. metals and alloys that can be mixed electromagnetically, and where it is required to control the mixing motion in some area or areas with minimal impact or in the complete absence of impact on the mixing motion in other areas of the liquid metal column. The invention can be applied to various special orientations of the mold. The mold can be mounted vertically, horizontally or obliquely.

Brief Description of the Drawings

Hereinafter the present invention will be described in more detail with reference to the embodiments given only as examples, and to the accompanying drawings, in which:

figure 1 depicts in schematic form a two-coil mixing system relative to the mold according to one variant embodiment of the invention,

figure 2 depicts a single-line diagram of possible electrical connections for the induction coils of a device according to one embodiment of the invention,

3 graphically represents the relationship between the current of the DC magnetic brake and the mixing speed on the meniscus and in the middle plane of the electromagnetic device for mixing in a mercury column, and

figure 4 graphically depicts the axial profiles of the measured mixing speed in a mercury bath with a square cross-section for a two-coil EMF system that works both with and without a brake based on the magnetic field of alternating current and direct current.

Description of preferred embodiments of the invention

Figure 1 shows a device for continuous or semi-continuous casting of metals according to one embodiment of the invention. The device comprises a mold 1, which is open at both ends in the casting direction, and means 2 for supplying hot melt 7 to the mold. This device is equipped with a two-coil electromagnetic stirring system (EMF) comprising a first induction coil 4 and a second induction coil 3. The second induction coil 3 is located on the upper end of the mold upstream than the first induction coil 4. Therefore, the first induction coil 4 is located lower in flow than the second induction coil 3. The first induction coil 4 acts as a mixing device and is excited by alternating current, which creates an alternating current magnetic field. The first induction coil 4 constitutes an alternating current electromagnetic mixer, capable of inducing (causing) the rotational movement of molten metal 7 in the mold 1 around the longitudinal axis of the mold 1 when it is excited. In FIG. 1, the melt is fed into the mold through a sprue tube 2 open under the upper melt surface, i.e. meniscus 5. Of course, you can also use other types of means for feeding the melt into the mold 1.

According to the invention, the second induction coil 3 is interchangeably excited either by direct current or by alternating current, depending on the desired effect on the stirring movement of the melt in the region adjacent to the upper free surface 5 of the melt. To control the type of current supplied to the second induction coil 3, the device is preferably provided with means 12, schematically shown in FIG. 2, for switching the current supplied to the second induction coil 3 from alternating current to direct and vice versa. Switching the current from alternating current to direct and vice versa is preferably carried out by electronic and programming means 12, which are part of the power supply of the system.

The second induction coil 3 is preferably driven by alternating current from an independent source with respect to the first induction coil 4. According to a preferred embodiment of the invention, a first power supply 10 is provided for supplying alternating current to the first induction coil 4, and a second power supply 11 is provided for interchangeably supplying alternating current and direct current into the second induction coil 3. The first and second power sources are schematically shown in figure 2. Means for switching AC to DC and vice versa are schematically shown at 12 in FIG. 2. Therefore, to power the second coil 3, you can choose either alternating current or direct current. This scheme allows you to independently control the mixing action of either the first or second induction coils, regardless of the pattern of mixing directions created by the first induction coil 4.

According to a preferred embodiment of the invention, the first induction coil 4 comprises a series of coils 8 located around the periphery of the mold 1. These coils 8 preferably have a multiphase and multipolar design. It is also preferred that the second induction coil 3 comprises a series of coils 9 located around the periphery of the mold 1. The coils 9 preferably also have a multiphase and multipolar design.

According to one aspect of the invention, the second induction coil 3 is configured to provide at least three different modes of operation, namely:

- the first mode, in which the second induction coil 3 is excited by alternating current, and the direction of rotation of the magnetic field generated by the second induction coil 3 coincides with the direction of rotation of the magnetic field generated by the first induction coil 4, due to which the magnetic field generated by the second induction coil 3 , increases the speed of the mixing motion induced by the first induction coil 4 in the melt region adjacent to the upper free surface 5 of the melt, while the mixing speed ra the alloy in this area is controlled by adjusting the magnitude of the alternating current supplied to the second induction coil 3;

- the second mode, in which the second induction coil 3 is excited by alternating current, and the direction of rotation of the magnetic field generated by the second induction coil 3 is opposite to the direction of rotation of the magnetic field created by the first induction coil 4, due to which the magnetic field generated by the second induction coil 3, reduces the speed of the mixing motion induced by the first induction coil 4 in the melt region adjacent to the upper free surface 5 of the melt, while the mixing speed melt in this region is controlled by adjusting the variable amount of current supplied to the second induction coil 3, and

- the third mode, in which the second induction coil 3 is excited by direct current in order to create a horizontally directed magnetic field of direct current, which induces electromagnetic forces in the melt 7, opposite to the direction of fluid flows in both transverse and longitudinal spatial planes of the mold 1 in the region melt adjacent to the upper free surface 5 of the melt, due to which the magnetic field generated by the second induction coil 3 reduces the speed of the mixing motion, ind -skilled first induction coil 4 in the region of the melt adjacent to the upper free surface 5 of the melt, the velocity of longitudinal flows produced in the melt 7, the stirring action of the first induction coil 4 as well as longitudinal flows produced by continuously release the melt into the mold 1.

The required operating mode is selected from the above modes depending on the casting process used. The required effect of the second induction coil 3 on the mixing motion of the melt in the area adjacent to the meniscus 5 varies depending on the type of casting process used.

According to the invention, the second induction coil 3 is excited either by direct current or by alternating current in order to provide a braking effect in the area of the mold meniscus to improve control of the stirring motion. The metallurgical efficiency of the EMF system is also achieved. The braking action of an alternating current magnetic field can control the mixing speed near the meniscus over a wide range, including the effective zero speed. The negative effect exerted by braking on the mixing motion in the mold volume is such that the mixing speed in this region can be reduced up to 20%. Providing an inhibitory effect with a horizontal direct current magnetic field can control the mixing speed in the meniscus in the range up to 50% of the initial speed value, without affecting the mixing motion in the mold volume. This is sufficient for most requirements arising in the continuous casting processes of steel with submerged casting.

The inhibitory effect resulting from the interaction between the horizontal direct current magnetic field generated by the second induction coil 3 and the rotating alteration flow in the meniscus region is mainly enclosed within the boundaries between the meniscus and the lower end of the magnetic brake. The stirring movement in the mold volume created by the main mixing device, i.e. the first induction coil 4, is practically not affected by the inhibitory effect in the meniscus created by the horizontal direct current magnetic field.

The intensity of the rotational flow in the melt 7 is characterized by its peripheral (angular) velocity U, which, in turn, depends on the parameters of the magnetic moment and its spatial distribution in the melt, as well as the size and geometry of the crystallizer cross section. For a system with a relatively small axisymmetric geometry, for example, with a cylindrical or square cross-section, the magnetic moment can be determined in accordance with the following expression:

T = 0.5πfσB ² R ⁴ L,

Where

T is the magnetic moment created by a 2-phase or 3-phase alternating current magnetic field;

f is the current frequency;

σ is the electrical conductivity of the liquid metal;

B - magnetic induction;

R is the radius of the mixing bath;

L is the length of the iron frame (yoke) of the device for mixing.

Independent control of the stirring movement near the meniscus 5, provided by the interchangeable use of alternating current and direct current to excite the second induction coil 3, allows for higher flexibility and accuracy in controlling the mixing process, as well as controlling the turbulence in the meniscus region caused by the longitudinal fluid flows created by the first induction coil 4, filled with a jet, shown at 18 in FIG. 1, and the oscillatory motion of the mold 1.

The decrease in the movement of the fluid flow in both the transverse and longitudinal planes caused by the DC brake, i.e. the second induction coil 3, when it is excited by direct current, occurs due to the occurrence of electromagnetic force, i.e. Lorentz forces resulting from the interaction between a direct current magnetic field and a moving flow of an electrically conductive liquid in accordance with the following expressions:

F = B × J

J = σ (E + UB),

Where

J is the density of the induced current in the melt,

U is the melt flow rate,

E is the electric potential.

Since the electromagnetic force F depends on both the magnitude of the magnetic induction B and the velocity U of the fluid flow, it is clear that a significant increase in current is required to reduce the fluid flow velocity to a value close to zero. In many practical situations of continuous casting, such a reduction in speed is not required. As shown in FIG. 3, the mixing speed near the meniscus of the mercury bath decreased from the initial speed of 7.3 rad / s to 2.7 rad / s when applying a direct current of 250 A. A linear extrapolation of the decrease in speed suggests that to reduce the mixing speed to effective zero level requires a current of 335 A.

A decrease in the stirring speed in the meniscus using the DC brake also has an inhibitory effect on the stirring speed in the middle plane of the main EMF, i.e. the first induction coil 4, similar to the effect created by the AC brake. Figures 3 and 4 show that the mixing speed in the middle plane of the EMF was approximately 11.7 rad / s or 86% of the initial value of 13.6 rad / s, when the mixing speed on the meniscus was reduced to 2.7 rad / with the help of a DC brake.

The present invention provides an improved method for controlling the movement of molten metal in both horizontal and longitudinal directions in the area of the meniscus of the mold. The longitudinal component of the motion of the molten metal induced by the main EMF and other means, such as the inflowing jet of molten metal discharged into the mold, is minimized using induction coils in the form of a modifier of the mixing device, i.e. a second induction coil located around the meniscus of the melt and excited by direct electric current, and more complete control of the mixing speed, i.e. its azimuthal component is provided using an alternating current magnetic field generated by a modifier of the mixing device.

The expression "induction coil" used in this description and the attached claims also encompasses an induction coil consisting of several separate coils, as shown in FIG.

The present invention, of course, is not limited to the preferred embodiments described above, and many ordinary modifications will be apparent to those of ordinary skill in the art without departing from the basic idea of the invention described in the appended claims.

Claims (13)

1. Device for continuous or semi-continuous casting of metals, containing a mold (1), open at both ends in the casting direction, means (2) for feeding the melt into the mold (1), the first electromagnetic induction coil (4), excited by alternating current and made with the possibility of inducing mixing motion in the melt (7) in the mold (1), and a second electromagnetic induction coil (3) located upstream than the first induction coil (4) and configured to control by the stirring movement of the melt in the region adjacent to the upper free surface (5) of the melt, characterized in that the second induction coil (3) is made interchangeably excited by either direct current or alternating current. 2. The device according to claim 1, characterized in that it contains means (12) for switching the current in the second induction coil (3) from alternating current to direct current and vice versa. 3. The device according to claim 1 or 2, characterized in that a first power source (10) is provided for supplying alternating current to the first induction coil (4), and a second power source (11) is provided for interchangeably supplying alternating and direct current to second induction coil (3). 4. The device according to claim 3, characterized in that the second power supply source (11) is equipped with electronic and programming means (12) for converting it from an alternating current source to a direct current source and vice versa. 5. A device according to any one of claims 1 to 4, characterized in that the second induction coil (3) comprises coils of a multiphase and multipolar design spaced around the periphery around the mold (1). 6. A device according to any one of claims 1 to 5, characterized in that the first induction coil (4) comprises coils of a multiphase and multipolar design spaced around the periphery around the mold (1). 7. A method of controlling the stirring movement in the mold (1) for continuous or semi-continuous casting of metals, according to which the mold (1) is open at both ends in the casting direction, the melt is fed to the mold (1), stirring motion in the melt (7) is induced in the mold (1) using the first electromagnetic induction coil (4) excited by alternating current, while the stirring movement of the melt in the region adjacent to the upper free surface (5) of the melt is controlled by a second electron omagnitnoy induction coil (3) disposed upstream than the first induction coil (4), characterized in that the second induction coil (3) interchangeably excite either direct current or alternating current. 8. The method according to claim 7, characterized in that the current in the second induction coil (3) is switched from alternating current to direct current and vice versa using switching means (12). 9. The method according to claim 7 or 8, characterized in that the first induction coil (4) is supplied with alternating current from the first power source (10), and the second induction coil (3) is interchangeably supplied with alternating and direct current from the second source ( 11) power supply. 10. The method according to claim 9, characterized in that the second power source (11) is converted from an alternating current source to a direct current source and vice versa using electronic and programming tools (12). 11. The method of controlling the stirring movement in the mold (1) for continuous or semi-continuous casting of metals, according to which the mold (1) is open at both ends in the casting direction, the melt is fed into the mold (1), stirring motion in the melt (7) is induced in the mold (1) using the first electromagnetic induction coil (4) excited by alternating current, while the stirring movement of the melt in the region adjacent to the upper free surface (5) of the melt is controlled by a second electron magnetic induction coil (3) located upstream than the first induction coil (4), characterized in that the second induction coil (3) is configured to provide three different modes of operation, namely: the first mode, in which the second induction coil (3) excite with alternating current and the direction of rotation of the magnetic field generated by the second induction coil (3), coincides with the direction of rotation of the magnetic field created by the first induction coil (4), due to which the magnetic field is created e the second induction coil (3), increases the speed of the stirring motion induced by the first induction coil (4) in the melt region adjacent to the upper free surface (5) of the melt, while the melt mixing speed in the said region is controlled by adjusting the magnitude of the alternating current supplied into the second induction coil (3), the second mode, in which the second induction coil (3) is excited by alternating current and the direction of rotation of the magnetic field created by the second induction coil (3), is opposite to the direction of rotation of the magnetic field generated by the first induction coil (4), due to which the magnetic field generated by the second induction coil (3) reduces the speed of the stirring motion induced by the first induction coil (4) in the melt region adjacent to the upper free surface ( 5) the melt, while the melt mixing rate in the mentioned region is controlled by adjusting the magnitude of the alternating current supplied to the second induction coil (3), and the third mode, in which the second the induction coil (3) is excited with direct current in order to create a horizontally directed magnetic field of direct current, which induces electromagnetic forces in the melt (7), opposite to the direction of fluid flows in both transverse and longitudinal spatial planes of the mold (1) in the melt region adjacent to the upper free surface (5) of the melt, due to which the magnetic field generated by the second induction coil (3) reduces the speed of the mixing motion induced by the first induction with a coil (4) in the melt region adjacent to the upper free surface (5) of the melt, the speed of the longitudinal flows created in the melt (7) by the mixing action of the first induction coil (4), as well as the longitudinal flows created by the continuous release of the melt into the crystallizer ( 1), while the operating mode is selected depending on the casting process used. 12. The method according to claim 11, characterized in that the first induction coil (4) is supplied with alternating current from the first power supply source (10), and the second and the induction coil (3) are supplied interchangeably with alternating and direct current from the second source (11) power supply. 13. The method according to p. 12, characterized in that the second power source (11) is converted from an alternating current source to a direct current source and vice versa using electronic and programming tools (12).

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