Flow Speed Control In Continuous Casting

TECHNICAL FIELD

The present disclosure relates to the field of continuous casting of metals and in particular proposes an arrangement for controlling flow speed in a thin slab caster.

BACKGROUND

Stability control is essential in the process of high-speed continuous thin slab casting. Modern, high-productivity thin slab casters can have throughputs of 8 tons per minute and above. In this scenario, the inlet flow speeds of the molten steel when leaving the submerged entry nozzle (SEN) into the mold are high, leading to strong turbulent effects with the risk of unstable, fluctuating, time-varying flow patterns in the upper part of the strand. Reducing these effects is crucial in order to obtain homogeneous and constant thermal and flow conditions for the fluid steel to solidify evenly in the mold.

In continuous casters of today, slab production is often diversified for different grades and dimensions. To accommodate for these different caster outputs, operation of the thin slab caster varies dynamically with width, casting speed, types of SENs, SEN immersion, superheat, mold funnel types, etc. A challenging aspect of the process is to provide an equivalent solidification environment independent of the caster parameters, with conditions favoring homogeneous solidification. Particularly present in high-speed thin slab casters is the risk of excessive meniscus flow speeds, fluctuations, turbulence and biased flows, which may cause mold powder entrainment or variations in the initial shell solidification.

An electromagnetic brake (EMBR) offers a great alternative to counteract these potential quality-reducing phenomena in a dynamic manner for thin slab casters, as it does not only brake the molten steel flow in the mold, but is also able to adjust this braking force by control of the electric current to the brake, to a suitable level according to the incoming flow speed of the steel.

Traditional deterministic (or open-loop) EMBR control applies different electric currents to the EMBR for different casting conditions. Appropriate current settings are typically found by trials evaluating steel quality and process stability, and by numerical and physical modelling. Apart from being cumbersome, time-consuming and expensive, these methods work on a large scale and lack the sharpness of handling local and specialized events.

EP2633928B1 is an attempted refinement of EMBR control, namely, by arranging a plurality of independently controllable magnetic brakes in different zones of the casting mold. This allows the operator a certain freedom to counteract left/right asymmetries or depth gradients in the flow of molten metal. By the configuration of the magnetic poles, however, the magnetic brakes can only apply magnetic fields of which the direction of at least one local magnetic field in the mold is opposite to the direction of other local magnetic field in the mold. In other words, a braking arrangement according to EP2633928B1 with one left and one right braking zone can be operated in modes such as a (+, −) mode, a (−, +) mode, but is unable to function in, for example, a (+, +) or (+, 0) mode.

SUMMARY

One objective of the present disclosure is to propose a flow speed control arrangement allowing more versatile, flexible and/or more adaptable flow speed control in a mold for continuous casting of metal. The objective is achieved by the invention as defined by the independent claims.

In a first aspect, there is provided an arrangement for controlling flow speed in a mold for continuous casting of metal, comprising: at least two first front cores with associated first magnetic coils arranged on one side of the mold; at least two second front cores with associated second magnetic coils arranged on an opposite side of the mold in substantial alignment with the first front cores; and an external magnetic loop connecting the second front cores to the first front cores, to allow a one-directional magnetic flux to pass through the mold from the first front cores to the second front cores or vice versa. According to an embodiment, the flow speed control arrangement further comprises a control interface enabling independent control of two subsets of the first magnetic coils.

Because of the combination of the external magnetic loop and the control interface enabling some of the first magnetic coils to be controlled independently of other ones of the first magnetic coils, the flow speed control arrangement is able to provide a one-directional magnetic flux with different intensity in different areas of the mold. A one-directional magnetic flux is one which is everywhere directed from the mold side proximate to the first front cores to the mold side proximate to the second front cores, unless it is locally zero, or a magnetic flux which is everywhere directed from the mold side proximate to the second front cores to the mold side proximate to the first front cores. While the presence of the external magnetic loop allows the generation of a one-directional magnetic flux, it is also possible to apply a flux of the (+, −) or (−, +) type, wherein the net flux may be zero (e.g., if the left/right magnitudes are equal) or non-zero (e.g., if the left/right magnitudes are different). The presence of the external magnetic loop lifts the restriction as stated in EP2633928B1 that the direction of at least one local magnetic field in the mold is opposite to the direction of other local magnetic field in the mold.

In an embodiment, the control interface enables independent control of two or more subsets of the second magnetic coils. This is in addition to the independent control, which the control interface allows, of two or more subsets of the first magnetic coils. An effect of the controllability of the second magnetic coils is that the geometry and/or local strength of the magnetic flux may be controlled more precisely.

The subsets of the first magnetic coils and/or the subsets of the second magnetic coils may be differently positioned with respect to a lateral direction of the mold. For example, in an embodiment where the flow speed control arrangement comprises one left and one right first front core and one left and one right second front core, the associated two left magnetic coils may be controllable independently from the associated two right magnetic coils. This may allow more precise tuning of the applied magnetic flux with respect to the lateral direction, and thereby more precise control of the flow speed, including the flow geometry.

In a variation, the flow speed control arrangement may comprise two left and two right first front cores and two left and two right second front cores, wherein the two left first front cores may be arranged at different heights, to provide good coverage of the vertical direction of the mold. Similarly, each of the right first, left second and right second front cores may be arranged at different heights. According to this variation, the magnetic coils associated with the two left first front cores are controllable independently from the magnetic coils associated with the two right first front cores. There is furthermore an optional—not mandatory—control independence (i) between the magnetic coils associated with the upper and lower left first front core, (ii) between the magnetic coils associated with the upper and lower left second front core, (iii) between the magnetic coils associated with the upper and lower right first front core, (iv) between the magnetic coils associated with the upper and lower right second front core, and/or (v) between the magnetic coils associated with the two left second front cores and the magnetic coils associated with the two right second front cores.

In the embodiments discussed above, the independent control may be achieved by the fact that the control interface comprises electric terminals for energizing the magnetic coils of each subset. In other words, an electrically separate terminal (or terminal pair) is provided for each subset. Alternatively, if the control interface comprises a processor and is at least partially implemented in software, the control independence may be achieved by software instructions to this effect.

In an embodiment, the control interface is adapted for coordinated control of magnetic coils associated with pairs of aligned front cores. For example, the magnetic coil associated with an (upper) left first front core and the magnetic coil associated with an (upper) left second front core are controlled in a coordinated manner. These cores may be aligned in the sense that their symmetry axes, which are generally parallel with the transversal direction of the mold, substantially coincide. Coordinated control is to be understood in the sense that substantially equal or proportional control signals or energizing currents are applied to both magnetic coils, so that the resulting magnetic fluxes through both coils are comparable or substantially equal. This may be achieved by providing the control interface with a common electric terminal (or pair of terminals) for energizing the magnetic coils of those magnetic coils that are to be controlled in a coordinated manner. Similarly, for a control interface comprising a processor, the coordinated control may be achieved by providing corresponding software instructions.

In an embodiment, the magnetic coils are controlled on the basis of sensor data relating to the temperature distribution or temperature gradient in the mold or relating to characteristics of the meniscus. The sensor data may have a spatial resolution with respect to a lateral direction of the mold. This is to say, the sensor data may comprise at least one left-side and one right-side value. In embodiments where the spatial resolution is even finer, there may be three or more different sensor data values corresponding to an equal number of points or areas distributed along the lateral direction of the mold.

In an embodiment, the first and/or second front cores are provided with flux-shaping elements. This may cause a spatially non-uniform magnetic flux to pass through the mold. The flux-shaping elements may be reconfigurable.

In an embodiment, the external magnetic loop comprises a first and a second level core, which may be retractable away from the mold to allow mold exchange or maintenance, and an external yoke. This provides a magnetic circuit susceptible of channeling the magnetic field in a substantially closed loop, that is, from the second front cores, through the second level core, the external yoke and the first level core, up to the first front cores, from where the magnetic flux passes transversally through the mold and reaches the second front cores.

In an embodiment, the flow speed control arrangement is supported in such manner that it can move independently of the mold. Typically, to allow the casting to proceed smoother, the mold is mounted on an oscillation table. The flow speed control arrangement, which does not benefit from being subjected to oscillations, should be mounted on a different support structure than the oscillation table. Since the oscillation table thereby has to support less weight, it may have a simpler design, be more economical to operate, and suffer less wear and fatigue.

In a second aspect, there is provided a system for continuous casting of metal, which comprises a mold, a supply of molten metal and the flow speed control arrangement with the above characteristics. Preferably, the system is a thin slab caster.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. The terms flow speed control arrangement, electromagnetic braking arrangement, electromagnetic brake (EMBR) and arrangement, for short, may be used interchangeably in this disclosure. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects and embodiments are now described, byway of example, with reference to the accompanying drawings, on which:

FIGS. 1 and 2 are a partial cutaway perspective views of a thin slab caster with a single magnetic coil on each side of the casting mold;

FIG. 3 is a schematic top view of a thin slab caster comprising an external yoke, in which the magnetic coils on either side of the mold are not independently controllable;

FIG. 4 is a schematic top view of a thin slab caster comprising independently controllable left and right magnetic coils on each side of the mold and two internal yokes arranged so that the directions of the magnetic field in the left and right are opposite to each other;

FIG. 5 is a schematic top view of a thin slab caster comprising independently controllable left and right magnetic coils on each side of the mold and an external yoke, according to an embodiment of the invention;

FIG. 6 is a schematic front view of a configuration of flux-shaping elements arranged on a front core of the thin slab caster;

FIG. 7a is a perspective view of a front core of the thin slab caster comprising a configuration of flux-shaping elements;

FIG. 7b is a schematic front view of the configuration of flux-shaping elements shown in FIG. 7a;

FIG. 8 is a schematic top view of a thin slab caster comprising independently controllable left and right magnetic coils on each side of the mold and an external yoke, according to an embodiment of the invention, in which a processor, control interfaces and sensors have been indicated;

FIG. 9 is a perspective view of a casting mold in the wall of which there is a plurality of horizontally arranged optical fibers for sensing a temperature distribution in the mold; and

FIG. 10 includes a lateral section (lower part) of a SEN for a thin slab caster, in which a velocity distribution v(x) with respect to the lateral direction x and a meniscus height h have been indicated, and a transversal section (upper part) through the line B-B.

DETAILED DESCRIPTION

The aspects of the present invention will now be described more fully with reference to the accompanying drawings, on which certain embodiments of the invention are shown.

These aspects may, however, be embodied in many different forms and should not be construed as limiting; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and to fully convey the scope of all aspects of invention to those skilled in the art. Like reference symbols refer to like elements throughout the description, as summarized in the below Table of Symbols.

FIGS. 1 and 2, which are cutaway views differing by the amount of opaque objects that have been removed, show a thin slab caster system with an electromagnetic braking arrangement of a generic type. In operation, a SEN 13 releases molten metal, such as steel or a ferrous or non-ferrous alloy, into a mold 2. By the downward force of its own gravity and the weight of subsequently added metal higher up in the mold 2, the metal moves vertically to reach cooler regions of the mold 2, where it solidifies (crystallizes) gradually, eventually to leave the mold 2 as a continuous slab. The mold 2 may for example be made of copper, optionally with a lubricated or coated inner surface to regulate friction, may have a cross section measuring about 100 mm by 1400 mm and be adapted for a casting speed of 5.5 m/min.

The electromagnetic braking arrangement shown in FIGS. 1 and 2 comprises a first front core 3 (visible in FIG. 2 only) on the near side of the mold 2 and a second front core 5 (visible in FIG. 2 only) on the far side of the mold 2. One magnetic coil 4, 6 surrounds each front core 3, 5 and is thereby associated with said front core 3, 5. Electric terminals 10 for energizing at least the magnetic coil 4 on the near side are shown. Subdivided proximate portions 3.1, 5.1 of the first and second front cores 3, 5 extend up to the surface of the mold 2 through corresponding passages of cooling medium channels 11.1, 12.1, which are adapted to remove excess heat. The electromagnetic braking arrangement further comprises a first and a second level core 8, 9, which interface with the front cores 3, 5 and further interface with a double-sided magnetic yoke 7 serving to close the magnetic circuit. The interfaces in the magnetic circuit may be solid or include air gaps. The magnetic yoke 7 and level cores 8, 9 may be of a ferromagnetic material, such as iron.

The hollow arrows illustrate the direction of the local magnetic flux while the magnetic coils 4, 6 are energized. Under the action of the energized magnetic coils 4, 6, the metal flow in the mold 2 underneath the SEN 13 is exposed to a static magnetic field B, substantially perpendicular to the flow velocity v. The metal therefore experiences a braking eddy current force

F=σ(E+v×B)×B,

which is substantially opposite to v, where E is the local electric field and σ the conductivity in suitable units. The electromagnetic braking arrangement shown in FIGS. 1 and 2 may not allow independent control of the flow at different lateral positions of the mold 2.

FIG. 3 is a schematic top view of a thin slab caster with an electromagnetic braking arrangement that comprises an external yoke 7. The present thin slab caster has similar characteristics as the one which was described with reference to FIGS. 1 and 2. The electromagnetic braking arrangement, with which the thin slab caster is equipped, comprises a single magnetic coil 4, 6 on each side of the mold 2. The magnetic coils are energized in accordance with signals generated by a connected control interface 14, to provide a magnetic flux similar to the one suggested by the solid arrows. While the control interface 14 may allow independent control over the two magnetic cores 4, 6, it is not possible to give the transversal magnetic field different intensities in a left and right side of the mold 2.

FIG. 4 is a top view of a still further prior art casting system having an electromagnetic braking arrangement with a left and right first front core 3a, 3b, which are associated with a left and right first magnetic coil 4a, 4b, as well as a left and right second front core 5a, 5b, which are associated with a left and right second magnetic coil 6a, 6b. The magnetic flux is allowed to circulate by virtue of a first internal magnetic yoke 15 interfacing with the left and right first front cores 3a, 3b and a second internal magnetic yoke 16 interfacing with the left and right second front cores 5a, 5b. Because the magnetic flux suggested by the solid arrows is recirculated through the mold 2, like in EP2633928B1, the depicted electromagnetic braking arrangement can only generate such magnetic fields that the directions of the magnetic field in the left and right side are opposite to each other. This applies regardless of the controllability of the magnetic coils 4a, 4b, 6a, 6b.

The present invention proposes solutions for improving the controllability of the magnetic braking field. FIG. 5 is a schematic top view of a thin slab caster with a flow speed control arrangement 1 comprising independently controllable left and right magnetic coils 4, 6 on each side of the mold as well as an external yoke 7, according to an embodiment of the invention. A left-side control interface 14a controls the energization of the magnetic coils 4a, 6a corresponding to the left first and second front cores 3a, 5a. Preferably, the two coils are controlled in a coordinated manner, in the sense described above. A right-side control interface 14b is arranged to energize the corresponding coils at the right side of the mold 2. This flow speed control arrangement 1 allows independent control of the magnetic field passing through different lateral positions of the mold 2. The magnetic flux may circulate through the external loop including the external yoke 7, rather than through the mold 2. Regarding generic properties of the flow speed control arrangement 1 according to the invention, reference is made to the above description of the electromagnetic braking arrangements shown in FIGS. 1-4.

In a variation of the embodiment shown in FIG. 5, also the first and second level cores 8, 9 may be divided into a left first, a right first, a left second and a right second level core. Then, the first left level core will interface with the first left front core and so forth.

Not explicitly shown in FIG. 5 is the support structure for the flow speed control arrangement 1. The flow speed control arrangement 1 is preferably supported in such manner as to be able to move independently of the mold 2. While the mold 2 may be mounted on an oscillation table, the flow speed control arrangement 1 is preferably mounted on a different support structure than the oscillation table. Since in this manner the oscillation table may carry a lighter load, its design may be simplified.

FIG. 6 is a schematic view of an arrangement of flux-shaping elements arranged on a proximate portion 5.1, 6.1 of a front core 5, 6 of the flow speed control arrangement 1 in the thin slab caster. The filled squares correspond to portions extending relatively closer to the mold 2, while the empty squares end relatively further from the mold 2. Since the front cores 5, 6, which may be of mild steel, iron or another ferromagnetic material, have much higher magnetic permeability than air, the magnetic flux will prefer the shorter air gap and concentrate there. Therefore, the local magnetic field will be relatively stronger at the shorter air gap than at the longer air gap, so that the magnetic flux passing through the mold 2 will be distributed with higher flexibility. This magnetic flux distribution effect may be even more pronounced if the front core on the opposite side of the mold 2 has symmetric flux-shaping elements.

The configuration of the flux-shaping elements can be adapted to the expected flow pattern, in view of the inner geometry of the mold 2, the properties of the SEN 13, the casting speed etc., so that a suitable braking action is achieved. In some embodiment, the flux-shaping elements may be reconfigured after deployment, so as to become useful in a different casting process or to incorporate later insights about a given casting process. The reconfigurability is ensured if the flux-shaping elements are provided as a plurality of freely positionable magnetic protrusions 17, of the type shown in FIG. 7a. The protrusions 17 may be bars of iron or another ferromagnetic material which are releasably fitted into recesses of each front core. A reconfiguration of the flux-shaping elements is preferably carried out between successive continuous casting batches.

In the example shown in FIG. 6, the flux-shaping elements will cause the magnetic flux to be relatively stronger in the lower portion, except for the central portion. In the further example shown in FIG. 7b, the flux-shaping elements are arranged in an approximately bowl-like shape corresponding to the positions on the mold 2 where more intense braking is expected to be necessary. The width of each of FIGS. 6 and 7b corresponds approximately to the full width of the mold 2. The height may correspond to an upper portion of the mold 2, as suggested by the perspective drawings in FIGS. 1 and 2.

It is recalled that, according to an embodiment of the invention, the left and right sides of each configuration shown preferably belong to a respective left and right first (or second) front core with an associated magnetic coil. This achieves a dynamic left/right controllability in addition to the option of reconfiguring the flux-shaping elements between casting batches. In other embodiments with a larger number of magnetic coils, the lateral resolution of the controllability may be even finer. While the patterns shown in FIGS. 6 and 7b are mirror symmetric with respect to the left/right direction, it is also possible to use asymmetric patterns. Asymmetric braking force distribution resulting from such patterns may be beneficial to stabilize an asymmetric casting jet from the SEN 13.

FIG. 8 is a schematic top view of a thin slab caster comprising independently controllable left and right magnetic coils 4a, 4b, 6a, 6b on each side of the mold 2 and an external yoke 7, according to an embodiment of the invention. Further provided are a processor 18, left and right control interfaces 14a, 14b for energizing the magnetic coils, as well as left and right sensors 19a, 19b for detecting various flow parameters. The flow parameters may include a temperature distribution or temperature gradient in the mold 2, a meniscus height profile, meniscus speed, meniscus height fluctuations and/or another meniscus characteristic. The control interfaces 14a, 14b may be connected to—or may be implemented as—thyristor power converters, such as the converters in the applicant's DCS family.

The local temperature may be sensed using an arrangement of optical fibers using the methods and devices disclosed in WO2017032488A1; see especially FIGS. 1a, 1b, 1c, 1d and 2 therein. This disclosure's FIG. 9 is a perspective view of a casting mold 2 and an upper portion of a SEN 13. In the wall of the mold 2, there is a sensor array comprising a plurality of optical fibers (dashed lines) extending horizontally up to lateral apertures, which fibers allow sensing of a temperature distribution or temperature gradient with high spatial resolution. The resulting sensor data can unveil solidification anomalies and can also in detail capture the meniscus shape and predict the meniscus flow velocity. As an alternative to FIG. 9, vertically arranged optical fibers may be used. A fully distributed measurement system readily captures flow speeds and fluctuations on both left and right sides of the SEN 13 and can easily be connected to a left/right independent flow speed control arrangement 1 of the kind described above, to manage flow asymmetries. The high-resolution measurement of temperatures in domains close to the meniscus provides sufficient information to control left and right flow speeds independently. A possible alternative control method is based on a split set of mold level sensors with separate information on levels and fluctuations from the left and right sides.

FIG. 10 shows a SEN 13 and a resulting inlet speed distribution v(x). The lower portion of the figure is a lateral section of the SEN 13, which is embodied as a two-channel fishtail-shaped nozzle. In the upper portion of FIG. 10 is a transversal section along B-B, where it can be seen that the SEN 13 has a flat cross section which is substantially aligned with the lateral direction of the mold 2. Mold-level sensors may allow the velocity distribution v(x) and the meniscus height h to be tracked, so that the flow speed control arrangement 1 may be controlled to apply a suitable braking magnetic field for stabilizing the flow. The magnetic field may be adapted to have a shape suitable to stabilize the flow of molten metal and to help guide momentum toward the meniscus in a double-roll flow pattern, while at the same time minimizing meniscus fluctuations and regulating meniscus local flow speeds. In one example, for a 100×1400 mm mold and a skew inlet velocity condition with a ±50% speed variation, the left and right magnitudes of the applied magnetic field must differ by approximately 23% in order for the flow speeds at ±440 mm from the lateral center of the mold 2 to be equalized. With this magnetic field applied, the meniscus flow asymmetry is substantially leveled out and the flow speed peaks dampened.

Returning to the description of FIG. 8, the inventors have realized that an automatic meniscus flow speed and asymmetry control can be set up using a left/right independent flow speed control arrangement 1 in combination with an online flow measurement sensor such as the mold-level sensors discussed above. A closed control loop may be implemented in the processor 18 provided as an indus-trially suitable computer environment for robust, continuous operation. The control loop may for example execute a PID algorithm. As processor 18, one may select an ABB Ability™ Optimold Monitor marketed by the applicant.

When the flow speeds of the meniscus in the mold are being accurately predicted, the closed-loop control system controls the control interfaces 14a, 14b of the flow speed control arrangement 1 to apply varying braking magnetic or electromagnetic fields to counter-act meniscus speeds that are too low or high. It is understood that the left-side control interface 14a controls the energization of both the left first magnetic coil 4a and the left second magnetic coil 6a; and that the right-side control interface 14b controls the energization of both the right first magnetic coil 4b and the right second magnetic coil 6b. In the same way, the control loop cooperates with the flow speed control arrangement 1 to mitigate flow pattern asymmetries. For instance, a greater flow speed in one lateral half of the mold 2 may be suppressed by a locally strengthened DC (i.e., non-oscillating) magnetic field. Control can also be carried out with respect to data from an electromagnetic level sensor, where high frequency feedback can be utilized to obtain detailed level and fluctuation information in the probing locations. This enables meniscus speed control and stability control of the meniscus level and in the upper part of the mold 2.

In an embodiment, the control loop includes two parts, a first one being the decision of the EMBR current based on the process inputs, such as casting speed, SEN geometry, SEN depth, steel grade, mold dimension and similar process characteristics. The decision may rely, in part, on magnetohydrodynamic simulations and/or recorded empirical data. The second part is the dynamic control of the EMBR. The meniscus level sensor 19 which is located on the left and right side of the mold 2 measures the meniscus level and meniscus fluctuations, wherein the transient values may be taken as input to realize the dynamic control of EMBR left/right currents. The dynamic control may include recurring positive and negative adjustments of the EMBR current value which was obtained initially based on the process inputs.

In a further embodiment, the processor 18 connected to the control interface 14 is configured to control the magnetic coils on the basis of numerical simulations of the transient flow dynamics in the mold.

The aspects of the present invention have mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, which is defined by the appended claims.

Flow Speed Control In Continuous Casting

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