CN1449313A - Method and device for continuous casting of metals in a mold - Google Patents

Abstract

A device for continuous or semi-continuous casting of metals, comprising a casting mold (1) which is open in both ends in the casting direction, means (2) for supplying melt to the mold (1), a first electromagnetic induction coil (4) energized by A.C. current and adapted to induce a stirring motion to the melt (7) in the mold (1), and a second electromagnetic induction coil (3) arranged upstream of the first induction coil (4) and adapted to control the stirring motion of the melt in the region adjacent to the upper free surface (5) of the melt. The second induction coil (3) is arranged to be interchangeably energized by either D.C. or A.C. current. The invention also relates to methods for control of stirring motion in a casting mold (1).

Description

Method and apparatus for continuous casting of metal in a mold

technical field

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

Background technique

It is a long-established practice in continuous casting steel operations to agitate molten steel in continuous casting molds by means of externally applied low frequency AC electromagnetic fields. Electromagnetic stirring, commonly known as EMS, is widely used in continuous steel casting to improve the quality of as-cast products and process productivity. It has been demonstrated that, in order to meet the conditions of different casting practices, it is necessary to control the stirring motion in the region immediately adjacent to the free surface of the melt, commonly referred to as the meniscus. A certain intensity of agitation motion is therefore required in the meniscus region in order to control surface and subsurface porosity, surface inclusions and other defects in billets and ingots produced primarily from Si-Mn deoxidized steels. Another casting practice, commonly known as submersion pouring below the mold powder, requires a stable meniscus, so it imposes a limit on the stirring motion at the meniscus. In addition to these two opposing demands emanating from casting practice, there is also a direct correlation between certain sets of strand defects and agitation strength in the meniscus region and in the mold body. The need to control the agitating motion in those areas of the mold thus necessitates techniques based on the application of DC or AC electromagnetic fields. Thus in US Patent No. 4,933,005 the control of the stirring movement in the region of the meniscus of the casting mold by means of a horizontal DC magnetic field is described. According to this patent, an externally applied DC magnetic field interacts with the agitation flow in the meniscus region formed by the main agitator. An electromagnetic force is generated from this interaction, and this force opposes the liquid metal motion, thereby reducing the speed of that motion. This method of agitation rate control has limitations. In the usual outlines of equipment for continuous casting of billets and ingots, the size of the brake coil is limited, as a result of which the DC magnetic field strength generated by the brake is only sufficient to reduce the stirring speed at the meniscus to the initial value of the speed fifty to sixty percent of that.

Another prior art known method aimed at controlling the agitation motion of the meniscus region is a dual coil EMS system operated with alternating current, which is described in US Patent No. 5,699,850. According to this patent, the induction coils arranged in the upper meniscus region of the casting mold are powered by a power supply which is independent of the main stirrer power supply arranged in the lower part of the casting mould. The rotating AC magnetic field generated by the upper induction coil is thus controlled independently of the main stirrer magnetic field. When the magnetic fields generated by the upper and main agitators coincide, the agitation velocity within the meniscus increases. This speed increase can be controlled by the current input to the upper coil. With the directions of rotation opposite to each other, the upper agitator becomes a magnetic brake with respect to the agitated flow in the region of the meniscus. By adjusting the brake current, the agitation velocity in the meniscus region can be controlled within a range from its initial value when no braking action is applied to the braking torque and the angular momentum of the agitated flow in the meniscus region Actual zero value at equilibrium.

As a disadvantage of this method, the braking effect only has an effect on the horizontal component of the fluid flow produced by agitation or by impingement of the pouring stream discharged into the mould. The longitudinal components of these flows remain unaffected by the AC magnetic field generated by the upper induction coil. These longitudinal fluid flows, according to their intensity, cause significant turbulence in the melt at the meniscus and in the region adjacent to the meniscus, thus affecting the performance of the pouring practice and the quality of the product.

Contents of the invention

It is an object of the present invention to provide a more flexible agitation rate and therefore melt flow control of the liquid metal flow in the region of the meniscus of the melt of a continuous casting mold used to produce eg billets and ingots.

The object of the invention is achieved by means of a device with the features of claim 1 , a method with the features of claim 7 and a method with the features of claim 11 .

According to the invention, the upper induction coil of the dual-coil agitation system, referred to herein as the "second induction coil", is powered by direct or alternating current depending on the desired effect on the agitation motion of the melt in the region adjacent to the upper free surface of the melt. The main induction coil, referred to as "the first induction coil" here, is always working as a stirrer powered by alternating current, that is, generating an alternating magnetic field.

The second induction coil is preferably powered by alternating current from a power source independent from the main stirrer, ie from the first induction coil. When the agitation system uses pouring with sizing nozzles and it is desired to enhance the agitation motion in the meniscus region, the upper induction coil operates in mode assisting the main agitator. Alternating current is also used to energize the upper induction coil when a completely or nearly completely reduced agitation velocity is required at the meniscus for submerged pour casting practices.

A local reduction in the stirring speed at the meniscus can be achieved by applying a horizontal DC magnetic field. This local braking action is required in the following cases: Casting with sizing nozzles or submerged inlet nozzles, and the agitation velocity at the meniscus needs to be controlled within at most sixty or fifty percent of its initial value . In this case a direct current is used to excite the upper induction coil. Experience has shown that such a braking strength is sufficient in many submerged pouring casting practices and for pouring through sizing nozzles. At high levels of agitation intensity, a further reduction in agitation speed was achieved by means of the application of an alternating magnetic field. Converting the current from AC to DC or vice versa is best done by electronic and programming means which form part of the system's power supply. Fluid flow in the meniscus region resulting from agitation by the main agitator, discharge flow of liquid metal and/or movement of the mold interacts with the horizontal DC magnetic field generated by the upper induction coil. As a result of the interaction between the DC magnetic field and any fluid flows traversing the magnetic field at angles other than zero degrees, magnetic forces develop and impede the movement of these flows. The maximum degree of interaction occurs at an angle of 90° between the magnetic field and the fluid flow. As a result, the speed of the stirring motion and the longitudinal flow including discharge of the pouring stream straight down will be reduced. Meniscus turbulence is also reduced as a result, resulting in improved meniscus stability, process behavior and cast product quality.

The present invention therefore brings flexibility to control agitation speed and meniscus turbulence in view of the interchangeability of the AC and DC magnetic fields provided by a single agitation system and generated by the same induction coil placed in the meniscus region of the mold. Significant improvement and results in increased metallurgical performance effectiveness and agitation system efficiency.

The present invention is a further improvement of the double-coil agitation system method and equipment. The present invention can be widely applied to all conductive materials that can be electromagnetically stirred, that is, metals and alloys, and where it is necessary to control the stirring motion in one or more areas, even if the stirring motion of said area and other areas of the fluid metal column has interference, which is also minimal. The invention is applicable to a wide variety of specific mold orientations. Casting molds can be placed vertically, horizontally or obliquely.

Description of drawings

The invention is now explained in more detail, by way of example only, with reference to different embodiments and figures.

Figure 1 schematically discloses a dual coil agitation system associated with a casting mold according to an embodiment of the present invention;

Figure 2 is a single-line diagram of a possible electrical connection of an induction coil of a device according to an embodiment of the present invention;

Figure 3 is a graphical representation of the relationship between DC magnetic brake current in a mercury column and the stirring velocity at the meniscus and in the mid-plane of the electromagnetic stirrer; and

Figure 4 is a diagrammatic representation of multiple axial sections for measuring agitation velocities within a square section mercury bath for a dual coil EMS system operating with and without AC and DC magnetic field brakes.

Detailed ways

Figure 1 discloses an apparatus for continuous or semi-continuous casting of metal according to an embodiment of the present invention. The apparatus comprises a casting mold 1 open at both ends in the pouring direction, and means 2 for supplying the casting mold with hot melt. The device is equipped with a dual-coil electromagnetic stirring (EMS) system comprising a first induction coil 4 and a second induction coil 3 . The second induction coil 3 is arranged at the top of the casting mold and upstream of the first induction coil 4 . Thus, the first induction coil 4 is arranged downstream of the second induction coil 3 . The first induction coil 4 works as a stirrer and is powered by an alternating current generating an alternating magnetic field. The first induction coil 4 constitutes an AC electromagnetic stirrer, and is designed to cause the molten metal 7 in the casting mold 1 to generate a rotational movement around the longitudinal axis of the casting mold 1 when power is supplied. In FIG. 1 the melt is supplied into the mold by means of a sprue 2 which opens below the upper surface of the melt (meniscus 5 ). Of course other types of devices can also be utilized for supplying the mold 1 with melt.

According to the invention, the second induction coil 3 is interchangeably powered by direct or alternating current depending on the effect of the desired melt agitation motion in the region adjacent to the upper free surface 5 of the melt. In order to control the type of current supplied to the second induction coil 3, the device is preferably equipped with means 12 schematically shown in FIG. 2 for converting the current reaching the second induction coil 3 from AC to DC or vice versa. The conversion of this current from AC to DC or vice versa is preferably carried out by the electronics and programming unit 12 which forms part of the system power supply.

The second induction coil 3 is preferably powered by an alternating current from a power source independent from the first induction coil 4 . According to a preferred embodiment of the present invention, a first power source 10 is provided to supply AC current to the first induction coil 4 , and a second power source 11 is provided to supply AC and DC current to the second induction coil 3 interchangeably. The first and second power sources are schematically represented in FIG. 2 . The means for converting the current from alternating current to direct current and vice versa are also schematically indicated at 12 in FIG. 2 . Therefore, either AC or DC current can be selected to power the second coil 3 . This arrangement allows independent control of the agitation action of the first or second induction coil 4 irrespective of the agitation direction pattern produced by the first induction coil 4 .

According to a preferred embodiment of the invention, the first induction coil comprises a series of coils 8 arranged around the periphery of the casting mold 1 . These coils 8 preferably have a multiphase and multipole arrangement. Preferably the second induction coil 3 also comprises a series of coils 9 arranged around the periphery of the mold 1 . These coils 9 preferably also have a multiphase and multipole arrangement.

According to one aspect of the present invention, the second induction coil 3 can provide at least three different working modes, namely:

- a first mode in which the second induction coil 3 is powered by an alternating current and the magnetic field generated by the second induction coil 3 rotates in the same direction as the magnetic field generated by the first induction coil 4 and thus generated by the second induction coil 3 The magnetic field strengthens the agitation motion velocity produced by the first induction coil 4 in the region of the melt adjacent to the free surface 5 of the melt, and the agitation velocity of the melt in said region is controlled by adjusting the alternating current supplied to the second induction coil 3 value to control,

- a second mode in which the second induction coil 3 is powered by an alternating current and the magnetic field generated by the second induction coil 3 rotates in the opposite direction to the magnetic field generated by the first induction coil 4 and is therefore generated by the second induction coil 3 The magnetic field reduces the stirring motion speed generated by the first induction coil 4 in the area of the melt adjacent to the free surface 5 of the melt, and the stirring speed of the melt in said area is controlled by adjusting the alternating current supplied to the second induction coil 3 values are controlled, and

- a third mode, in which the second induction coil 3 is powered by a direct current, so that a horizontally oriented direct current magnetic field is generated, which in the region of the melt adjacent to the upper free surface 5 of the melt in the transverse as well as in the longitudinal spatial plane of the casting mold 1 7 generates an electromagnetic force opposite to the flow direction of the fluid, whereby the magnetic field generated by the second induction coil 3 reduces the speed of the stirring motion generated by the first induction coil 4 in the region of the melt adjacent to the free upper surface 5 of the melt, As well as the longitudinal flow produced in the melt 7 by the stirring action of the first induction coil 4 and the velocity of the longitudinal flow produced by the continuous discharge of the melt into the mold 1 .

The desired working mode is selected from the above-mentioned various modes according to the pouring method adopted. The required effect of the second induction coil 3 on the stirring movement of the melt in the region adjacent to the meniscus 5 varies with the type of casting method used.

According to the invention the second induction coil 3 is powered by either direct or alternating current in order to produce a braking action in the region of the meniscus of the mold to improve the control of the stirring movement. In addition, the metallurgical effectiveness of the EMS system is also realized. Braking with an alternating magnetic field can control the velocity of the agitation at the meniscus over a wide range including zero actual velocity. The reverse impact produced by braking the stirring movement within the mold body allows the stirring speed to be reduced by up to twenty percent in this region. The braking action provided by the horizontal DC magnetic field enables the control of the stirring velocity in the meniscus region within fifty percent of the original velocity value and does not affect the stirring movement in the main body of the mold. This is sufficient for most continuous steel casting practices in submerged pouring situations.

The braking action resulting from the interaction between the horizontal DC magnetic field generated from the second induction coil 3 and the rotating agitated flow in the region of the meniscus is mostly confined within the boundaries between the meniscus and the bottom end of the magnetic brake. The stirring movement inside the mold body produced by the main stirrer, ie the first induction coil 4 , remains virtually unaffected by the braking action in the meniscus region produced by the horizontal DC magnetic field.

The intensity of the rotational flow inside the melt 7 is characterized by its rotational (angular) velocity U which in turn depends on the parameters of the magnetic torque and its spatial distribution within the melt, as well as on the dimensions and geometry of the mold cross-section. For relatively small axisymmetric geometries, i.e. cylindrical or square cross-section systems, the magnetic torque can be determined according to the following formula:

T＝0.5πfσB ² R ⁴ L

here:

T is the magnetic torque generated by two-phase or three-phase AC magnetic field,

f is the current frequency,

σ is the conductivity of the liquid metal,

B is the magnetic flux density,

R is the radius of the stirring tank, and

L is the length of the yoke iron of the agitator.

The independently controlled agitation motion at the meniscus 5 provided by the interchangeable use of alternating or direct current for energizing the second induction coil 3 allows more flexible and accurate control of the agitation process and turbulence in the meniscus region , the turbulent flow is caused by the first induction coil 4, the pouring flow shown by the reference number 18 in FIG. 1 and the longitudinal fluid flow introduced by the vibration of the mold 1.

The reduction of the fluid flow in the transverse and longitudinal planes by the DC brake, i.e. the second induction coil 4, when supplied with DC current, occurs as a result of the interaction between the DC magnetic field and the moving conductive fluid flow according to the following formula: Lorentz forces what happened:

F=B×J

J＝σ(E+UB)

here:

J is the induced current density inside the melt,

U is the velocity of the melt stream, and

E is the electric potential.

The electromagnetic force F depends on the magnitude of the magnetic flux density B and the fluid flow velocity U. Obviously, it is necessary to significantly increase the current in order to reduce the fluid flow velocity to a level close to zero. This speed reduction is unnecessary in many continuous casting practices. As shown in Figure 3, the agitation speed at the meniscus of the mercury bath decreases from the initial 7.3 rad/s to 2.7 rad/s when the current input value of the DC current is 250A. Linear extrapolation of the velocity reduction indicates that 335A is required to reduce the agitation velocity to a practical zero level.

Reducing the agitation velocity in the meniscus region by means of a DC brake also exerts a braking effect on the agitation velocity of the main EMS, ie the midplane of the first induction coil 4, similar to the effect produced by an AC brake. Figures 3 and 4 show that the stirring velocity at the EMS midplane is about 11.7 rad/s or 86% of the initial value of 13.6 rad/s when the DC braking at the meniscus reduces the stirring velocity to 2.7 rad/s.

The present invention provides an improved method of controlling the movement of liquid metal in the meniscus region of a casting mold both horizontally and vertically. The longitudinal component of the liquid metal motion produced by the main EMS and by other means such as the liquid metal pouring stream discharged into the mould, is minimized by employing an induction coil in the form of an agitation regulator, the second induction coil, and placing it in the Around the meniscus region of the melt and powered by DC current, the more complex agitation speed, that is, the horizontal component control, is achieved by using the AC magnetic field generated by the agitation regulator.

The expression "induction coil" used in this description and in the appended claims also includes an induction coil comprising several individual coils, as shown in FIG. 2 .

Of course, the invention is not restricted in any way to the preferred embodiments described above, and it is obvious that a person skilled in the art can make many variations thereon without going beyond the basic idea of the invention defined in the appended claims.

Claims (13)

1. one kind is used for continuously or the equipment of semi-continuous casting metal, it comprises a mold (1), one device (2), one first electromagnetic induction coil (4) and one second electromagnetic induction coil (3), mold (1) is along cast direction both ends open, device (2) is used for fused mass is fed to mold (1), first electromagnetic induction coil (4) is by the alternating current power supply and be adapted such that the fused mass (7) in the mold (1) produces agitation movement, and second electromagnetic induction coil (3) is arranged in first induction coil (4) upstream and be suitable for the fused mass agitation movement in Free Surface (5) zone on contiguous this fused mass of control, it is characterized in that second induction coil (3) is arranged to be powered interchangeably by direct current or alternating current.

2. equipment as claimed in claim 1 is characterized in that, this equipment comprises device (12), and the electric current that it will arrive second induction coil (3) converts direct current to by interchange and can reverse.

3. equipment as claimed in claim 1 or 2, it is characterized in that, one first power supply (10) is provided with and is used for to first induction coil (4) supply alternating current, and a second source (11) setting to be used for be that second induction coil (3) is supplied the AC and DC electric current convertibly.

4. equipment as claimed in claim 3 is characterized in that, second source (11) is being equipped electronics and programmer (12), and it is used for changing it into dc source from AC power, and can reverse.

5. as each described equipment in the above-mentioned claim, it is characterized in that second induction coil (3) is included in mold (1) coil of the peripheral heterogeneous and multipole layout that separates in edge on every side.

6. as each described equipment in the above-mentioned claim, it is characterized in that first induction coil (4) is included in mold (1) coil of the peripheral heterogeneous and multipole layout that separates in edge on every side.

7. one kind is used to control metal continuously and the method for the interior agitation movement of semi-continuous casting mold (1), mold (1) is along cast direction both ends open, fused mass is fed in the mold (1), make the fused mass (7) in the mold (1) produce agitation movement by means of first induction coil (4) by the alternating current power supply, by means of second electromagnetic induction coil (3) that is arranged in first electromagnetic induction coil (4) upstream the fused mass agitation movement in Free Surface (5) zone on the contiguous fused mass is controlled, it is characterized in that second induction coil (3) is powered convertibly by direct current or alternating current.

8. method as claimed in claim 7 is characterized in that, the electric current of supplying with second induction coil (3) is converted to direct current and can reverse from interchange by conversion equipment (12).

9. as claim 7 or 8 described methods, it is characterized in that first induction coil (4) is from one first power supply (10) supply alternating current, and second induction coil (3) is supplied the AC and DC electric current convertibly from a second source (11).

10. method as claimed in claim 9 is characterized in that, second source (11) is transformed into dc source and can reverses from AC power by electronics and programmer (12).

11. one kind is used to control metal continuously and the method for the interior agitation movement of semi-continuous casting mold (1), mold (1) is along cast direction both ends open, fused mass is fed in the mold (1), make the fused mass (7) in the mold (1) produce an agitation movement by means of first induction coil (4) by the alternating current power supply, by means of second electromagnetic induction coil (3) that is arranged in first electromagnetic induction coil (4) upstream the fused mass agitation movement in Free Surface (5) zone on the contiguous fused mass is controlled, it is characterized in that, second induction coil (3) can provide three kinds of different mode of operations, promptly

-the first pattern, second induction coil wherein, (3) power by alternating current, and by second induction coil, (3) the magnetic field direction of rotation of Chan Shenging with by first induction coil, (4) the magnetic field direction of rotation unanimity of Chan Shenging, therefore by second induction coil, (3) magnetic field that produces is strengthened by first induction coil, (4) Free Surface on contiguous this fused mass, (5) the agitation movement speed that produces in the fused mass zone, fused mass agitation speed in this zone is fed to second induction coil by adjusting, (3) alternating current flow valuve is controlled

-the second pattern, wherein second induction coil (3) is powered by alternating current, and the magnetic field direction of rotation that is produced by second induction coil (3) is opposite with the magnetic field direction of rotation that is produced by first induction coil (4), therefore the agitation movement speed by generation in the fused mass zone of first induction coil (4) Free Surface (5) on contiguous this fused mass is lowered in the magnetic field that is produced by second induction coil (3), fused mass agitation speed in this zone is controlled by the alternating current flow valuve that adjusting is fed to second induction coil (3), and

-Di three-mode, wherein second induction coil (3) is powered by DC current, so that produce the D.C. magnetic field of a horizontal alignment, produce the electromagnetic force opposite in the horizontal and inherent fused mass in longitudinal space plane (7) of the fused mass inherent mold in zone (1) of its Free Surface (5) on contiguous this fused mass with fluid flow direction, the speed by the agitation movement of generation in the fused mass zone of first induction coil (4) Free Surface (5) on contiguous this fused mass is lowered in the magnetic field that is produced by second induction coil (3) whereby, and by vertically flowing that first induction coil (4) and agitaion produce in fused mass (7) by means of fused mass being entered continuously the speed that vertically flows that mold (1) produces

This mode of operation is selected according to the casting method that is adopted.

12. method as claimed in claim 11 is characterized in that, first induction coil (4) is by one first power supply (10) supply alternating current, and second induction coil (3) is supplied the AC and DC electric current convertibly by a second source (11).

13. method as claimed in claim 12 is characterized in that, second source (11) is transformed into dc source and can reverses from AC power by electronics and programmer (12).

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