Method and device for thin-slab strand casting

Abstract

The invention relates to a method for thin-slab strand casting, comprising the following steps: feeding a metal melt into a mold, molding a partially solidified thin-slab strand from the metal melt in the mold, reducing the flow velocity of the metal melt in the partially solidified thin-slab strand by means of an electromagnetic brake (EMBR) arranged in the region of the mold, and leading away the partially solidified thin-slab strand from the mold by means of a strand-guiding system, wherein unsolidified parts of the partially solidified thin-slab strand are stirred by means of an electromagnetic stirrer arranged below the mold downstream in the strand draw-off direction of the thin-slab strand, wherein a traveling electromagnetic field is produced by means of the electromagnetic stirrer in a region of the thin-slab strand that lies at a distance between 20 and 7,000 millimeters from the mold in the strand draw-off direction.

Description

State of the art

[0001] The present invention is based on a method for continuously casting thin slabs according to the preamble of claim 1.

[0002] It is generally known from the prior art to produce thin slabs using the continuous casting process. This process involves creating a metallic melt which is transferred into a tundish by means of a steel pouring ladle. From the tundish, the metallic melt flows through a pouring tube into a mold, which is cooled and moved in an oscillating manner. In the mold, the metallic melt forms a strand with a solidified shell and a largely unsolidified cross-section within the solidified shell. Upon leaving the mold, the strand is picked up by a transport system with a plurality of strand guide rollers, between which the strand is guided through the so-called casting arch and cooled until it has completely solidified.It is also known to slow the flow velocity of the molten metal inside the partially solidified strand within the mold using an electromagnetic brake (EMBR). The goal here is to reduce the flow velocity of the molten steel at the bath surface and even out the bath surface profile in order to improve lubrication between the strand and the mold and reduce strand surface defects that can arise from the entrapment of casting slag.

[0003] For the production of thin slabs with thicknesses between 40 and 120 millimeters, the mold typically has a funnel-shaped, widened cross-section in the upper part and a rectangular cross-section in the lower part. Due to these thin thicknesses, the solidification times in continuous thin-slab casting are comparatively short, and the proportion of liquid melt inside the partially solidified strand is low. This inevitably results in a coarse, strictly oriented, columnar-crystalline microstructure in the continuous casting of thin slabs. However, such a microstructure can have a detrimental effect on the surface and internal properties of the products produced from the thin slabs.For example, depending on the steel grade and casting conditions, products made from thin slab material may exhibit longitudinal streaks on the product surface, inhomogeneous mechanical properties, microstructural banding, core segregation, reduced HIC resistance (hydrogen induced cracking) and susceptibility to internal cracking.

[0004] It is known from conventional thick-slab continuous casting that longitudinal streaking in dynamo steels can be avoided by casting with very low superheat. However, thick-slab continuous casting has a comparatively long solidification time, so that superheating of the steel melt in the tundish below approximately 12 Kelvin is sufficient to achieve sufficient microstructure refinement. Microstructure refinement can be considered sufficient if the expansion of the globulitic core zone in the thickness direction is more than 30%. To achieve the same effect with thin slabs, the shorter solidification times would require such low superheat that casting problems would arise in the form of clogging of the immersion tubes in the mold (so-called "clogging"), which can result in strand surface defects or even strand breakouts.

[0005] It is also known from the literature (e.g., "Improved quality and productivity in slab casting by electromagnetic breaking and stirring," C. Crister et al., 41st Steelmaking Seminar International, Resende, Brazil, May 23-26, 2010, pages 1-15) that electromagnetic stirrers are used in some thick slab continuous casters to refine the solidification microstructure. The stirrers are installed either in the mold area or several meters below the mold bath level.

[0006] From the document DE 698 24 749 T2, a device for casting metal is also known, which comprises a mold for forming a cast strand and means for supplying a primary flow of hot molten metal to the mold. The device comprises a magnetic device which applies a static or periodic magnetic field to the flow of metal in the non-solidified parts of the cast strand in order to act on the molten metal in the mold during casting. This is intended to slow and divide the flow of the hot metal in order to achieve a secondary flow pattern in the mold. Furthermore, it is also known from this document to provide a further device in the form of an electromagnetic stirrer to act on the melt in the mold or on the melt downstream of the mold.However, this document does not disclose in which area the electromagnetic stirrer should be arranged with respect to the mold.

[0007] The use of an electromagnetic brake and / or an electromagnetic stirrer in the continuous casting of steel for thick slab formats is also known from the publications DE 21 2009 000 056 U1 and DE 10 2009 056 000 A1.

[0008] DE 100 20 703 A1 discloses a method and apparatus for the continuous casting of thin slabs, which comprise ultrasonic transducers mounted in the mold area and an electromagnetic brake. The ultrasonic transducers cause vibration of the mold wall to reduce thermal stress, frictional forces, adhesives, and longitudinal depressions.

[0009] JPH-08 323454 A discloses a process for the continuous casting of thin slabs, in which superheating of less than 0 °C is to be set in the mold or in which the unsolidified melt is to be stirred below the mold. During the discharge of the strand, complete solidification of the strand is to be prevented within the bending area of ​​the strand below the mold, in which a plurality of spinning rolls are arranged, in order to allow the incompletely solidified strand to be compressed by the spinning rolls. The aim of the process is to produce thin slabs with low center segregation.

[0010] From DE 195 42 211 A1 an electromagnetic stirring device for continuous slab casting molds is known, by means of which the flow velocity of the melt in the mold can be slowed down.

[0011] From EP 0 092 126 A1 a method for stirring liquid metal in a mold is known, in which the areas in the mold which are not stirred by the pouring stream penetrating into the melt are stirred by the interaction of a static magnetic field and the currents induced when the pouring stream is braked.

[0012] From JP 2003-326339 A a method for the continuous casting of thin slabs is known in which the flows in the mold are influenced by applying an almost U-shaped electromagnetic field around the immersion tube.

[0013] The particular difficulty in thin-slab continuous casting is achieving significant microstructure refinement given the short solidification times and the small volume of liquid in the interior of the strand compared to thick-slab continuous casting. The present invention solves this problem.

[0014] It is an object of the present invention to provide a method and apparatus for producing thin slabs by continuous casting, which, despite short solidification times and comparatively small volumes of liquid in the interior of the strand, enable the creation of a large core zone with a fine-grained, globulitic structure in the thin slab strand, in order to prevent the disadvantages caused by a coarse, strictly oriented, columnar crystalline structure in the thin slab strand in the prior art. Furthermore, the risk of dip tube clogging due to excessively low superheating is to be avoided. This object is achieved by a method for thin slab continuous casting according to claim 1, comprising the method steps: feeding a metallic melt into a mold, forming a partially solidified thin slab strand from the metallic melt in the mold,Reducing the flow velocity of the metallic melt in the partially solidified thin slab strand by means of an electromagnetic brake (EMBR) arranged in the region of the mold and removing the partially solidified thin slab strand from the mold by means of a strand guide system, wherein non-solidified parts of the partially solidified thin slab strand are stirred by means of an electromagnetic stirrer arranged downstream below the mold along the strand withdrawal direction of the thin slab strand, wherein an electromagnetic traveling field is generated by means of the electromagnetic stirrer in a region of the thin slab strand located between 0.9 and 3.8 m from the bath level of the mold along the strand withdrawal direction.

[0015] The device according to the invention has the advantage over the prior art that, through a concept specifically designed for thin-slab continuous casting, a refinement of the solidification structure inside the thin-slab strand is achieved. The simultaneous use of an electromagnetic brake prevents the stirrer-induced increase in the flow velocity of the molten steel in the mold area from leading to excessively strong local bath level fluctuations, i.e., bath level fluctuations of more than 15 mm, for example. High turbulence at the bath level can lead to strand breakouts or strand surface defects due to casting slag trapped at the bath level of the mold. Both strand breakouts and strand surface defects should be avoided.Surprisingly, it has been shown that electromagnetic stirring at a distance of 0.9 to 3.8 m below the bath level of the mold, and in particular from the underside of the mold, causes accelerated and uniform overheating reduction, which advantageously leads to the formation of a sufficiently large core zone with a fine-grained, globulitic structure inside the thin slab strand, i.e. in particular at least 30% in the thickness direction, while coarse, columnar crystalline structures are limited by the stirring. Despite the short solidification times and small-volume liquid fractions inside the thin slab strand that are typical in the continuous casting of thin slabs, this fine-grained, globulitic core zone forms in the solidification structure, thereby greatly reducing the formation of columnar crystals between the edge zone and the central region of the strand.The expansion of the globulitic core zone in the thickness direction is then at least 30%. In the manufactured product, longitudinal striations, microstructural banding, core segregation, and susceptibility to internal cracking can thus be reduced, and HIC resistance and the homogeneity of the mechanical and magnetic properties can be increased. Furthermore, higher and non-critical superheating can advantageously be maintained, thus eliminating the risk of casting defects in the form of dip tube clogging and resulting strand surface defects or strand breakouts. It is conceivable that, for example, the present process uses superheating of the molten steel in the tundish between 10 and 50 Kelvin, preferably around 20 Kelvin.By means of the electromagnetic stirrer, an electromagnetic traveling field is generated in a region of the thin slab strand located between 0.9 and 3.8 m from the bath level of the mold along the strand withdrawal direction. For the purposes of the present invention, a region of the thin slab strand located between 0.9 and 3.8 m from the bath level of the mold is understood to be the region of the thin slab strand that is between 0.9 and 3.8 m from the bath level in the mold, which is typically around 100 millimeters below the top of the mold. The electromagnetic stirrer is preferably arranged such that the traveling field acts directly below the mold on the parts of the strand that have not yet solidified, since a positive influence on the grain structure by the traveling field is no longer possible in parts of the strand that have already solidified.The electromagnetic traveling field is preferably generated in an area between 1.5 and 2.5 m away from the bath level of the mold along the strand withdrawal direction. According to the invention, the position of the electromagnetic stirrer or of the electromagnetic alternating field along the strand withdrawal direction is thus defined by the distance to the bath level in the mold: the distance to the bath level along the strand withdrawal direction is between 0.9 and 3.8 meters and preferably between 1.5 and 2.5 meters. For the purposes of the present invention, in particular either a single electromagnetic stirrer is arranged on one side of the thin slab strand, either on the fixed side or the loose side, or a separate electromagnetic stirrer is arranged on each side, i.e. on both the fixed side and the loose side.The fixed side refers in particular to the broad side of the strand guide segments whose position always remains unchanged and serves as a so-called reference line. Adjustments to the strand thickness formats are then always made via the opposite loose side. The method according to the invention is used in particular for the production of thin slabs by continuous casting and the hot-rolled or cold-rolled strip produced therefrom. The hot-rolled or cold-rolled strip is used in particular for the production of electrical sheets (non-grain-oriented or grain-oriented) or sheets of high-strength steels with yield strength values ​​greater than 400 megapascals (e.g., quenched and tempered steel). A thin slab within the meaning of the present invention comprises in particular a slab with a thickness between 40 and 120 millimeters.For a precise description of the geometric relationships, two transverse directions are mentioned below in addition to the strand withdrawal direction: a first transverse direction and a second transverse direction. The first transverse direction always runs perpendicular to the strand withdrawal direction and parallel to the strand surface normal of the slab's broadside, while the second transverse direction always runs perpendicular to the strand withdrawal direction and parallel to the strand surface on the slab's broadside. The slab's broadside refers to the side of the rectangular cross-section of the thin slab strand that has the largest dimensions. The first and second transverse directions are therefore both perpendicular to the strand withdrawal direction and perpendicular to each other.

[0016] Advantageous embodiments and further developments of the invention can be found in the dependent claims and the description with reference to the drawings.

[0017] According to a preferred embodiment of the present invention, the unsolidified parts are stirred within the mold and / or during the removal of the partially solidified thin slab strand from the mold through the strand guide system by means of the electromagnetic stirrer positioned beneath the mold. This advantageously ensures that during stirring, the proportion of not yet solidified metallic melt inside the thin slab strand is still sufficiently large, i.e., at least 50% of the strand thickness, in order to obtain a core zone with the largest possible cross-section and a fine-grained, globulitic structure, i.e., in order to obtain a globulitic core zone with an extension in the thickness direction of the slab of at least 30%.

[0018] According to a further preferred embodiment of the present invention, the electromagnetic stirrer is adjusted such that the electromagnetic traveling field runs along a second transverse direction, which runs perpendicular to the strand withdrawal direction and parallel to a strand surface on a broad side of the thin slab strand, from a first edge region of the thin slab strand to a second edge region of the thin slab strand opposite the first edge region. In this way, the not yet solidified metallic melt in the thin slab strand is stirred, so that fine, globulitic grains form in the solidification structure during solidification.Preferably, the electromagnetic traveling field is reversed after a period of 1 to 60 seconds, particularly preferably between 1 and 10 seconds, so that the electromagnetic traveling field subsequently runs along the second transverse direction from a second edge region of the thin slab strand to the first edge region of the thin slab strand. After the period of 1 to 60 seconds, preferably again 1 to 10 seconds, has elapsed again, the electromagnetic traveling field is reversed again, and the cycle begins again.

[0019] According to an alternative preferred embodiment of the present invention, a bidirectional, symmetrical electromagnetic traveling field is generated across the width of the thin slab strand by means of the electromagnetic stirrer, wherein the electromagnetic stirrer is adjusted such that a first subfield of the electromagnetic traveling field runs from the center of the thin slab strand to a first edge region of the thin slab strand, and such that a second subfield of the electromagnetic traveling field runs from the center to a second edge region of the thin slab strand opposite the first edge region. This electromagnetic traveling field is preferably maintained for 1 to 60 seconds, particularly preferably between 1 and 10 seconds. Thereafter, the electromagnetic traveling field generated by the electromagnetic stirrer, and thus the direction of the two subfields, are reversed.This reversed electromagnetic traveling field is also preferably maintained for between 1 and 60 seconds, and particularly preferably between 1 and 10 seconds. Afterward, the electromagnetic traveling field is reversed again, and the cycle begins again. This preferred embodiment ensures symmetrical stirring of the not yet solidified metallic melt within the already solidified edge zone of the thin slab strand, resulting in a symmetrical solidification structure with fine, globulitic grains.

[0020] According to a further alternative preferred embodiment of the present invention, it is provided that a bidirectional, symmetrical electromagnetic traveling field is generated across the width of the thin slab strand by means of the electromagnetic stirrer, wherein the electromagnetic stirrer is adjusted such that a first subfield of the electromagnetic traveling field runs from a first edge region of the thin slab strand to the center of the thin slab strand and that a second subfield of the electromagnetic traveling field runs from a second edge region of the thin slab strand opposite the first edge region to the center of the thin slab strand. This electromagnetic traveling field is preferably maintained for 1 to 60 seconds, in particular between 1 and 10 seconds. Thereafter, the electromagnetic traveling field generated by the electromagnetic stirrer and thus the direction of the two subfields are reversed.This reversed electromagnetic traveling field is also maintained for between 1 and 60 seconds, in particular between 1 and 10 seconds. Afterward, the electromagnetic traveling field is reversed again, and the cycle begins again. This preferred embodiment also ensures symmetrical stirring of the not yet solidified metallic melt within the already solidified edge zone of the thin slab strand, resulting in a symmetrical solidification structure with fine, globulitic grains.

[0021] According to an embodiment not according to the invention, it is provided that an electromagnetic traveling field is generated across the width of the thin slab strand by means of the electromagnetic stirrer, the magnetic flux density of which is on average preferably 0.1 to 0.6 Tesla, particularly preferably 0.3 to 0.5 Tesla, and very particularly preferably substantially 0.4 Tesla. It has been shown that an alternating field with amplitudes in the range of preferably 0.1 to 0.6 Tesla, particularly preferably 0.3 to 0.5 Tesla, and very particularly preferably substantially 0.4 Tesla, is sufficient to achieve accelerated and uniform superheat reduction in the metallic melt.This effect is advantageously achieved by an electromagnetic stirrer adjusted so that the flow velocity of the unsolidified particles in the partially solidified thin slab strand is a maximum of 0.7 meters per second or at least 0.2 meters per second, and preferably between 0.2 and 0.7 meters per second. The associated circulation of the unsolidified particles in the thin slab strand ensures accelerated and uniform reduction of superheat and thus the desired microstructure refinement, without the need to select a lower superheat from the outset, which would drastically increase the risk of immersion tube clogging.

[0022] According to a further preferred embodiment of the present invention, the electromagnetic stirrer is set such that the stirring frequency is at least 0.1 Hz or at most 10 Hertz, and preferably between 1 and 10 Hz. This stirring frequency range has been shown to be particularly advantageous. At a stirring frequency of less than 0.1 Hz, there is no electromagnetic traveling field, so no stirring effect occurs. If the stirring frequency is greater than 10 Hz, the penetration depth of the electromagnetic traveling field into the strand interior is too small, and no microstructure refinement is achieved.

[0023] According to a further preferred embodiment of the present invention, an electromagnetic field is generated within the mold by means of the electromagnetic brake, the magnetic flux density of which is preferably 0.1 to 0.3 Tesla, particularly preferably 0.15 to 0.25 Tesla, and very particularly preferably substantially 0.2 Tesla. This advantageously slows down the flow velocity of the metallic melt between the partially solidified edge regions of the strand, thus preventing mold level fluctuations, as well as surface defects (so-called shell defects) and internal defects (e.g., casting slag inclusions) resulting from mold level fluctuations.

[0024] According to a further preferred embodiment of the present invention, the magnetic field strengths of the electromagnetic traveling field generated by the electromagnetic stirrer and the field generated by the electromagnetic brake are coordinated with one another. It has been shown that matching the magnetic field strengths of the electromagnetic traveling field generated by the electromagnetic stirrer and the field generated by the electromagnetic brake is advantageous. The matching is preferably achieved by increasing the magnetic field strength of the field of the electromagnetic brake by 20 to 80% of its base value to values ​​between 0.1 and 0.3 Tesla when the electromagnetic stirrer is switched on. In this context, the base value is understood to be the magnetic field strength of the field of the electromagnetic brake as it is typically used without the additional use of an electromagnetic stirrer.Typical basic settings for an electromagnetic brake without the use of an electromagnetic stirrer are fields with magnetic field strengths between 0.08 and 0.2 Tesla.

[0025] A further object of the present invention to achieve the object mentioned above is a device for the continuous casting of thin slabs, in particular using the method according to the invention, which device comprises a feed means for feeding a metallic melt, a mold for forming a partially solidified thin slab strand from the metallic melt, an electromagnetic brake arranged in the region of the mold for reducing the flow velocity of the metallic melt inside the partially solidified strand within the mold, and a strand guide system for removing the partially solidified thin slab strand from the mold, wherein the device further comprises an electromagnetic stirrer arranged downstream below the mold along the strand withdrawal direction of the thin slab strand for stirring non-solidified parts of the partially solidified thin slab strand,the electromagnetic stirrer is spaced between 0.9 and 3.8 m from the bath level of the mold along the strand withdrawal direction.

[0026] The device according to the invention has the advantage over the prior art that the metallic melt is stirred by the electromagnetic stirrer during continuous casting, thereby refining the solidification structure inside the thin slab strand. Stirring the metallic melt ensures accelerated and uniform superheat reduction, which advantageously leads to the formation of a core zone with a fine-grained, globulitic structure inside the thin slab strand, while coarse columnar crystalline structures are broken up by the stirring. Despite the short solidification times and small-volume liquid fractions inside the thin slab strand typical of continuous casting of thin slabs, this fine-grained, globulitic core zone forms in the solidification structure, thereby avoiding or at least suppressing the formation of columnar crystals between the edge zone and the center region of the strand.The products manufactured from the thin slabs thus exhibit significantly reduced longitudinal streaks, microstructural lineation, and susceptibility to internal cracking, as well as increased HIC resistance and homogeneity of the mechanical and magnetic properties. The electromagnetic stirrer generates, in particular, a spatially and / or temporally variable magnetic field in the region of the thin slab strand. The electromagnetic stirrer preferably comprises a linear field stirrer arranged on one of the two broad sides of the thin slab strand. However, it would also be conceivable for a linear field stirrer to be arranged on each of the two opposite broad sides of the thin slab strand. Alternatively, the electromagnetic stirrer comprises a rotating field stirrer or a helical stirrer.

[0027] The electromagnetic stirrer is positioned along the strand withdrawal direction of the thin slab strand below the electromagnetic brake. This advantageously achieves rapid and uniform reduction of superheat in the not yet solidified parts of the thin slab strand before solidification progresses into the interior of the thin slab strand, thus achieving a refinement of the solidification microstructure. In general, the proportion of the globulitic core zone in the thin slab increases the closer the electromagnetic stirrer is positioned to the meniscus of the thin slab strand or to the bath level.At the same time, however, it must be ensured that the electromagnetic stirrer is also effective in the lower region of the mold so that early and rapid reduction of overheating in the interior of the strand is achieved, and that the flows in the molten metal generated by the electromagnetic stirrer do not lead to increased bath level fluctuations or increased local bath level elevations in the mold. The distance between the electromagnetic stirrer and the bath level is between 0.9 and 3.8 meters and preferably between 1.5 and 2.5 meters. Furthermore, it is particularly provided that the electromagnetic stirrer is spaced 20 to 1000 millimeters, preferably 20 to 200 millimeters and particularly preferably 20 to 40 millimeters from a surface of the thin slab strand along the first transverse direction.

[0028] The device according to the invention is used in particular for the production of thin slabs using the continuous casting process and the hot-rolled or cold-rolled strip produced therefrom. The hot-rolled or cold-rolled strip is used in particular for the production of electrical steel sheets (non-grain-oriented or grain-oriented) or sheets of high-strength steels with yield strengths greater than 400 megapascals (e.g., quenched and tempered steel). A thin slab within the meaning of the present invention particularly comprises a slab with a thickness between 40 and 120 millimeters.

[0029] According to a further preferred embodiment of the present invention, the electromagnetic stirrer comprises a linear field stirrer for generating an electromagnetic traveling field in the region of the thin slab strand, wherein the direction of travel of the electromagnetic traveling field is aligned parallel to the second transverse direction. The electromagnetic stirrer is in particular configured such that a first subfield of the electromagnetic traveling field runs from the center of the thin slab strand to a first edge region of the thin slab strand, and a second subfield of the electromagnetic traveling field runs from the center to a second edge region of the thin slab strand opposite the first edge region. This electromagnetic traveling field is maintained for between 1 and 60 seconds, preferably between 1 and 10 seconds.The process is then reversed, so that the first sub-field runs from the first edge region of the thin slab strand, and the second sub-field runs from the second edge region of the thin slab strand, opposite the first edge region, to the center of the thin slab strand. This field is also held for between 1 and 60 seconds, preferably between 1 and 10 seconds. After that, the cycle starts again. This advantageously achieves a uniform and symmetrical flow inside the strand and thus also a uniform dissipation of superheat. On the one hand, this should bring about a homogeneous microstructure refinement inside the strand, and on the other hand, uniform strand shell growth across the strand width. This prevents the formation of strand breakouts or longitudinal surface cracks.

[0030] According to an embodiment not according to the invention, the electromagnetic stirrer is set such that the flow velocity of the metallic melt generated by the stirrer is at least 0.2 meters per second or at most 0.7 meters per second and in particular lies between 0.2 and 0.7 meters per second. In this way, it is ensured that, on the one hand, the strand shell growth on the narrow side of the strand is not weakened too much (reduction in the risk of strand breakage) and, on the other hand, severe element depletions (so-called white bands, i.e. depletion of C, Mn, Si, P, S, etc.) at the solidification front in the effective range of the stirrer are avoided. It has been shown that the flow velocity should not be less than 0.2 meters per second, because otherwise sufficient microstructure refinement cannot be achieved.For example, a globulitic core zone whose thickness extension is less than 30% may be considered insufficient. Furthermore, the flow velocity should not exceed 0.7 meters per second to avoid depletion of the melt in alloying elements near the solidification front. Depletion of alloying elements in the melt near the solidification front is measurable in the solidified material. This phenomenon is referred to as "white bands" or "white streaks." White bands lead to inhomogeneous properties of the final product.

[0031] According to a further preferred embodiment of the present invention, the electromagnetic brake in the upper half of the mold is spaced 20 to 150 millimeters, preferably 25 to 100 millimeters, and particularly preferably substantially 75 millimeters from a surface of the thin slab strand along the first transverse direction. For the purposes of the present invention, the aforementioned distance is understood to mean, in particular, the smallest distance between the electromagnetic brake and the strand surface.

[0032] Further details, features, and advantages of the invention will become apparent from the drawings and the following description of preferred embodiments with reference to the drawings. The drawings merely illustrate exemplary embodiments of the invention, which do not limit the essential inventive concept. Short description of the drawings

[0033] Figure 1 shows a schematic sectional view of an apparatus for continuous casting thin slabs according to an exemplary embodiment of the present invention. Figures 2a and 2b show schematic detailed views of the apparatus for continuous casting thin slabs according to the exemplary embodiment of the present invention in the region of the mold and below the mold. Embodiments of the invention

[0034] In the various figures, identical parts are always provided with the same reference symbols and are therefore usually named or mentioned only once.

[0035] In Figure 1 is a schematic sectional view of an apparatus 1 for producing thin slabs by continuous casting according to an exemplary embodiment of the present invention.

[0036] In the present example, metallic melt 2 is transferred from a steel ladle 6 into a distributor 3 and poured from the distributor 3 via a pouring tube 4 (feeding means) into a mold 5 of the device 1. The flow through the pouring tube is controlled by a plug 8 or a slide valve depending on the meniscus level 7 in the mold 5. The mold 5 comprises a mold with a downwardly open passage opening of rectangular cross-section. The broad sides 28 of the mold are spaced between 40 and 120 millimeters apart so that the mold 5 is suitable for casting thin slabs. The mold consists of water-cooled copper plates, which cause the supplied metallic melt to solidify in the edge region of the mold 5.In the mold 5, a thin slab strand 9 with a solidified shell 10 and a largely unsolidified cross-section 11 within the solidified shell 10 is thus formed from the continuously fed metallic melt 2. The mold 5 optionally oscillates to prevent the strand surface from adhering to the mold 5. The thin slab strand 9 passes through the mold 5 along a vertical strand withdrawal direction 15. Upon leaving the downwardly open mold 5, the thin slab strand 9 is picked up by a transport system 12 (also referred to as a strand guide system) with a plurality of strand guide rollers 13 and guided through a so-called casting arch 14. The thin slab strand 9 is cooled until it has completely solidified.

[0037] In addition to the strand withdrawal direction 15, a first transverse direction 18 and a second transverse direction 30 are Figure 1The first transverse direction 18 runs perpendicular to the strand withdrawal direction 15 and parallel to a strand surface normal of the slab broadside 28 (the slab broadside 28 protrudes Figure 1 into the plane of the drawing), while the second transverse direction 30 runs perpendicular to the strand withdrawal direction 15 and parallel to the strand surface on the slab broad side 28, ie perpendicular to the first transverse direction 18.

[0038] An electromagnetic brake 16 (EMBR: Electromagnetic Brake) is arranged in the upper region of the mold 5, which slows the flow velocity of the metallic melt 2 inside the already partially solidified thin slab strand 9 and thus reduces bath level fluctuations in the mold 5. In the present example, the electromagnetic brake 16 comprises two coils arranged on either side of the thin slab strand 9. The electromagnetic brake 16 generates an electromagnetic field within the mold 5, the magnetic flux density of which is preferably 0.1 to 0.3 Tesla and particularly preferably substantially 0.2 Tesla. By slowing the flow velocity of the metallic melt 2 between the partially solidified edge regions 10 of the thin slab strand 9, casting level fluctuations, as well as surface defects (so-called shell defects) and internal defects (e.g., casting slag inclusions) resulting from casting level fluctuations can be prevented.

[0039] Below the mold 5, the device 1 according to the invention has an electromagnetic stirrer 17 for stirring non-solidified parts of the partially solidified thin slab strand 9. In the present example, the electromagnetic stirrer 17 comprises a linear field stirrer, which extends along one of the two broad sides 28 of the strand. The linear field stirrer generates an electromagnetic traveling field 19 across the width of the thin slab strand 9 (see Figures 2a and 2b), which runs cyclically back and forth between a first edge region 20 of the thin slab strand 9 and an opposite second edge region 21 of the thin slab strand 9 along a second transverse direction 30 perpendicular to the strand withdrawal direction 15 and parallel to the broad side 28 of the strand surface. The electromagnetic traveling field 19 is generated in a region located between 0.9 and 3.8 m, preferably between 1.5 and 2.5 m, from the bath level of the mold 5 along the strand withdrawal direction 15 and has an average magnetic flux density of between 0.1 and 0.6 Tesla and preferably essentially 0.4 Tesla. The electromagnetic traveling field stirs the metallic melt, thereby bringing about accelerated and uniform overheating reduction in the metallic melt.This advantageously leads to the formation of a larger core zone with a fine-grained, globulitic structure inside the thin slab strand 9, while coarse columnar crystalline structures are restricted by the electromagnetic stirring. It is not according to the invention that this effect is achieved by an electromagnetic stirrer 17 adjusted such that the flow velocity of the non-solidified parts in the partially solidified thin slab strand is less than 0.7 meters per second and preferably between 0.2 and 0.7 meters per second. Despite the short solidification times and small-volume liquid components inside the thin slab strand 9 typical of continuous casting of thin slabs, the fine-grained, globulitic core zone then forms in the solidification structure, thereby suppressing the formation of columnar crystals between the edge zone and the central region of the thin slab strand 9.In a final product made from continuously cast thin slabs, longitudinal streaks, microstructural banding, core segregation, and susceptibility to internal cracking can be reduced, and HIC resistance and the homogeneity of the mechanical and magnetic properties can be increased. In this case, for example, casting is carried out with superheating—i.e., with a temperature difference between the melt and the actual temperature minus the liquidus temperature—between 10 and 50 Kelvin, preferably around 30 Kelvin. Thus, a higher and non-critical superheat can be maintained, thus eliminating the risk of casting defects in the form of immersion tube clogging and resulting strand surface defects or strand breakouts.

[0040] The device and method described above are used to produce thin slabs, particularly for hot-rolled or cold-rolled strip. The hot-rolled or cold-rolled strip is used primarily for the production of electrical steel sheets (non-grain-oriented or grain-oriented) or sheets of high-strength steels with yield strengths greater than 400 megapascals (e.g., quenched and tempered steel).

[0041] The upper figure shows that the feed means comprises the pouring tube 4, which is immersed in the metallic melt 2 located in the mold 5, and pouring holes 22 formed below the casting level 7 on the pouring tube 4 in the lower part of the pouring tube 4. The metallic melt 2 is introduced via the pouring holes 22 at an angle to the strand withdrawal direction 15 of the thin slab strand 9 (see flow arrows 23). The electromagnetic traveling field 19, induced by the electromagnetic stirrer 17 (not shown), is arranged below the mold 5. The electromagnetic stirrer 17, which is arranged below the mold 5, generates the electromagnetic traveling field 19 below the mold 5, which in turn causes flows that can reach into the mold 5 - under certain circumstances even to the bath level. In the embodiment according to Figure 2aThe electromagnetic stirrer 17 is configured such that the electromagnetic traveling field 19 comprises two subfields, a first subfield 24 and a second subfield 25. The first subfield 24 of the electromagnetic traveling field 19 moves cyclically back and forth between a center 26 of the thin slab strand 9 and the first edge region 20 of the thin slab strand 9, while the second subfield 25 of the electromagnetic traveling field 19 moves cyclically back and forth between the center 26 and the second edge region 21 of the thin slab strand 9. The movement of the electromagnetic traveling field 19 is schematically represented by the movement arrows 27. The division of the electromagnetic traveling field 19 into two bidirectional, symmetrical subfields leads to a uniform and symmetrical flow inside the thin slab strand 9 and thus also to a rapid and uniform dissipation of the superheat.On the one hand, this is intended to bring about a homogeneous microstructure refinement in the interior of the strand and, on the other hand, uniform strand shell growth across the strand width. In this way, the electromagnetic stirring is prevented from giving rise to the potential risk of strand breakage or longitudinal surface cracks. The electromagnetic stirrer 17 is further not adjusted according to the invention such that the flow velocity of the metallic melt at the solidification front generated by the stirrer is between 0.2 and 0.7 meters per second. In this way, it is ensured that, on the one hand, strand shell growth on the narrow side of the strand is not excessively weakened (reduction in the risk of strand breakage) and, on the other hand, severe element depletions (so-called white bands, i.e. depletion of C, Mn, Si, P, S, etc.) at the solidification front in the effective range of the electromagnetic stirrer 17 are avoided.In addition, the electromagnetic stirrer 17 must be adjusted such that the flows generated by the electromagnetic stirrer 17 in the metallic melt 2 do not lead to increased bath level fluctuations or to increased local bath level elevations in the mold 5. The magnetic field strengths of the electromagnetic stirrer 17 and the electromagnetic brake 16 should be coordinated with one another. The coordination is achieved, for example, by increasing the magnetic field strength of the electromagnetic brake 16 by 20 to 80% of its base value to values ​​between 0.1 and 0.3 Tesla when the electromagnetic stirrer 17 is switched on. In this context, the base value is understood to be the magnetic field strength of the electromagnetic brake 16 as it is typically used without the additional use of an electromagnetic stirrer 17.Typical basic settings for an electromagnetic brake 16 without the use of an electromagnetic stirrer 17 are 0.08 to 0.2 Tesla.

[0042] In the lower illustration of Figure 2a The rectangular cross-section of the through-hole of the mold 5 can be seen schematically. The electromagnetic traveling field 19 or the two subfields 24, 25 travel along the broad sides 28 through the thin slab strand 9.

[0043] Alternatively, the electromagnetic traveling field 19 is not divided into two subfields 24, 25, but runs cyclically back and forth along the second transverse direction 30 between the first edge region 20 of the thin slab strand 9 and the opposite second edge region 21 of the thin slab strand 9. This embodiment is shown by way of example in Figure 2b illustrated.

[0044] The following examples were carried out using a device according to Figures 1 and 2a carried out: Example 1:

[0045] A measure of the success of the refinement of the solidification structure within the thin slab strand is the proportion of the globulitic core zone (GKZ). The extent of the globulitic core zone in percent is defined as GKZ (%) = D GKZ (mm) / D (mm) ▪ 100, where D GKZ = thickness of the globulitic core zone and D = slab thickness.

[0046] A test was therefore conducted using steel grade S420MC, a casting speed of 5 meters per minute, a tundish superheat of 30 Kelvin, a strand thickness of 65 millimeters, a strand width of 1550 millimeters, and a mold height of 1100 millimeters. The electromagnetic brake (EMBR) was located in the upper half of the mold and the electromagnetic stirrer (EMS) was located below the mold behind non-magnetic rollers of the transport system. The electromagnetic stirrer, or rather the electromagnetic alternating field of the electromagnetic stirrer, was positioned at a distance of 2960 millimeters from the mold level. The results shown in the following table were obtained: e.g. Magnetic field strength (T) Distance from the strand surface (mm) GKZ (%) only EMBR GKZ (%) EMBR + EMS 1 EMBR 0,1 75 0 - 10 40 - 50 EMS 0,1 310 2 EMBR 0,2 75 0 - 10 50 - 60 EMS 0,4 310

[0047] The test series demonstrate that by connecting an electromagnetic stirrer arranged below the mold, the proportion of the globulitic core zone (GKZ) increases from 0 to 10 percent to a proportion of 40 to 60 percent. Example 2:

[0048] A relationship between overheating of the steel melt in the tundish and the proportion of the globulitic core zone on the one hand and the resulting longitudinal streakiness on the finished strip in dynamo steels and the center segregation was experimentally determined on dynamo steels with 2.4% silicon: Superheat (K) GKZ in thickness direction (%) Longitudinal streaks on cold-rolled finished strip Center segregation 37 0 strong medium 24 3 strong medium 11 6 medium - strong medium 6 30 easy - medium easy - medium 3 50-70 no no

[0049] It follows that, to avoid longitudinal streaking and reduce central segregation, the proportion of the globulitic core zone (GKZ) should be at least 30 percent and preferably greater than 50 percent. However, overheating below 20 K should be avoided, as otherwise problems such as clogging of the immersion tubes in the mold would occur, which could result in strand surface defects or even strand breakouts.

[0050] The following example shows, using the dynamo steel with 2.4% silicon and thin slabs with a thickness of 63 millimeters, a tundish superheat of 30 Kelvin, a strand width of 1550 millimeters, and a mold height of 1100 millimeters, the casting level was 1000 millimeters above the mold bottom, the stirring frequency was 6 Hz, and the flow velocity at the solidification front was 0.4 m / s, that by appropriately selecting the distance between the casting level and the electromagnetic stirrer (EMS), the required proportion of the globulitic core zone (GKZ) of at least 30 percent and preferably at least 50 percent can be achieved at different casting speeds VG: GKZ (%) Strand shell thickness "S" on the respective broad side (mm) Distance of the EMS from the bath level (m) VG = 4.0 m / min VG = 5.0 m / min VG = 6.0 m / min 30 22 4,8 6,1 7,3 40 19 3,6 4,5 5,4 50 16 2,6 3,2 3,8 60 13 1,7 2,1 2,5

[0051] The above series of measurements show that, at the casting speeds (VG) typical for thin-slab continuous casters between 4 and 6 m / min, the electromagnetic stirrer must be positioned between 2.6 and 3.8 meters below the mold's bath level for a 50 percent portion of the globulitic core zone, and between 1.7 and 2.5 meters for a 60 percent portion. However, satisfactory results are also achieved with a distance of between 3.6 and 7.3 meters between the electromagnetic stirrer and the bath level.

[0052] The distance between the bath level of the mold and the electromagnetic stirrer is therefore between 0.9 and 3.8 m and preferably between 1.5 and 2.5 m. List of reference symbols

[0053] 1Device 2Metallic melt 3Distributor 4Pouring tube 5Mould 6Steel pouring ladle 7Mould level 8Plug 9Thin slab strand 10Solidified strand shell 11Unsolidified cross-section 12Transport system 13Strand guide roller 14Casting bend 15Strand withdrawal direction 16Electromagnetic brake 17Electromagnetic stirrer 18First transverse direction (runs perpendicular to the strand withdrawal direction and parallel to the strand surface normal of the slab broadside) 19Electromagnetic traveling field 20First edge area 21Second edge area 22Pour-out holes in the lower part of the pouring tube 23Flow arrow 24First subfield 25Second subfield 26Center 27Movement arrow 28Broadsides 29Mould bottom 30Second transverse direction (runs perpendicular to Strand withdrawal direction and parallel to the strand surface on the slab broadside or runs perpendicular to the strand withdrawal direction and perpendicular to the first transverse direction) 31Mould top

Claims

1. Method for the continuous casting of thin slabs comprising the method steps of: - feeding a molten metal (2) into a mold (5), - molding a partially solidified thin-slab strand (9) from the molten metal (2) in the mold (5), - reducing the flow rate of the molten metal (2) in the partially solidified thin-slab strand (9) by means of an electromagnetic brake (16) arranged in the region of the mold (5) and - removing the partially solidified thin-slab strand (9) from the mold (5) by means of a strand guiding system (12), characterized in that - unsolidified parts of the partially solidified thin-slab strand (9) are stirred by means of an electromagnetic stirrer (17) arranged underneath the mold (5) downstream along the strand takeoff direction (15) of the thin-slab strand (9), - a traveling electromagnetic field (19) being produced by means of the electromagnetic stirrer (17) in a region of the thin-slab strand (9) that lies at a distance from the bath level of the mold (5) of between 0,9 and 3,8 m along the strand takeoff direction (15).

2. Method according to Claim 1, wherein the traveling electromagnetic field (19) is generated in a region of the thin-slab strand (9) that is at a distance from the bath level of the mold (5) of between 1,5 and 2,5 m along the strand takeoff direction (15).

3. Method according to one of the preceding claims, wherein an electromagnetic field is generated within the mold (5) by means of the electromagnetic brake (16), wherein, in the upper half of the mold, the electromagnetic brake (16) is at a distance from a surface of the thin-slab strand of preferably between 20 and 150 millimeters along a first transverse direction (18), which runs perpendicularly to the strand takeoff direction (15) and parallel to a strand surface normal on a broad side (28) of the thin-slab strand (9).

4. Method according to one of the preceding claims, wherein the electromagnetic stirrer (17) is set in such a way that, along a second transverse direction (30), which runs perpendicularly to the strand takeoff direction (15) and perpendicularly to the first transverse direction (18), the traveling electromagnetic field (19) runs from a first outer region (20) of the thin-slab strand (9) to a second outer region (21) of the thin-slab strand (9) that is opposite from the first outer region (20).

5. Method according to one of Claims 1 to 3, wherein a bidirectional, symmetrical traveling electromagnetic field (19) is generated over the width of the thin-slab strand (9) by means of the electromagnetic stirrer (17), the electromagnetic stirrer (17) being set in such a way that a first subfield (24) of the traveling electromagnetic field (19) runs from a center (26) of the thin-slab strand (9) to a first outer region (20) of the thin-slab strand (9) and that a second subfield (25) of the traveling electromagnetic field (19) runs from the center (26) of the thin-slab strand (9) to a second outer region (21) of the thin-slab strand (9) that is opposite from the first outer region (20).

6. Method according to one of Claims 1 to 3, wherein a bidirectional, symmetrical traveling electromagnetic field (19) is generated over the width of the thin-slab strand (9) by means of the electromagnetic stirrer (17), the electromagnetic stirrer (17) being set in such a way that a first subfield (24) of the traveling electromagnetic field (19) runs from a first outer region (20) of the thin-slab strand (9) to a center (26) of the thin-slab strand (9) and that a second subfield (25) of the traveling electromagnetic field (19) runs from a second outer region (21) of the thin-slab strand (9) that is opposite from the first outer region (20) to the center (26) of the thin-slab strand (9).

7. Method according to one of the preceding claims, wherein a traveling electromagnetic field of which the magnetic flux density is on average preferably 0.1 to 0.6 tesla, particularly preferably 0.3 to 0.5 tesla and most particularly preferably substantially 0.4 tesla, is generated in the region of the thin-slab strand (9) by means of the electromagnetic stirrer (17).

8. Method according to one of the preceding claims, wherein the electromagnetic stirrer (17) is set in such a way that the stirring frequency is at least 0.1 Hz or at most 10 Hertz and preferably between 0.1 and 10 Hz.

9. Method according to one of the preceding claims, wherein an electromagnetic field of which the magnetic flux density is preferably 0.1 to 0.3 tesla, particularly preferably 0.15 to 0.25 tesla and most particularly preferably 0.2 tesla, is generated within the mold (5) by means of the electromagnetic brake (16).

10. Method according to one of the preceding claims, wherein the method is used for producing thin slabs for the production of hot strip or cold strip, in particular for producing electric sheets or sheets of higher-strength steels preferably with yield strength values greater than 400 megapascals.

11. Device (1) for the continuous casting of thin slabs, in particular by means of a method according to one of the preceding claims, comprising - a feeding means for supplying a molten metal (2), - a mold (5) for molding a partially solidified thin-slab strand (9) from the supplied molten metal (2), - an electromagnetic brake (16), arranged in the region of the mold (5), for reducing the flow rate of the molten metal (2) inside the partially solidified thin-slab strand (9) and - a strand guiding system (12) for removing the partially solidified thin-slab strand (9) from the mold (2), characterized in that - the device (1) comprises an electromagnetic stirrer (17), arranged underneath the mold (5) downstream along the strand takeoff direction (15) of the thin-slab strand (9), for stirring unsolidified parts of the partially solidified thin-slab strand (9), that is at a distance from the bath level of the mold (5) of between 0,9 and 3,8 m along the strand takeoff direction (15).

12. Device (1) according to Claim 11, wherein the electromagnetic stirrer (17) is at a distance from the bath level of the mold (5) of between 1,5 and 2,5 m along the strand takeoff direction (15).

13. Device (1) according to either of Claims 11 and 12, wherein the electromagnetic stirrer (17) comprises a linear field stirrer for generating a traveling electromagnetic field (19) in the region of the thin-slab strand (9), the running direction of the traveling electromagnetic field (19) being aligned perpendicularly to the strand takeoff direction (15) and parallel to a second transverse direction (30), which runs perpendicularly to the strand takeoff direction (15) and parallel to a strand surface on a broad side (28) of the thin-slab strand (9), and the running direction of the traveling electromagnetic field (19) being reversible.

14. Device (1) according to one of Claims 11 to 13, wherein the electromagnetic stirrer (17) is at a distance from a surface of the thin-slab strand (9) of 20 to 1000 millimeters, preferably 20 to 200 millimeters and particularly preferably 20 to 40 millimeters, along a first transverse direction (18), which runs perpendicularly to the strand takeoff direction (15) and perpendicularly to the second transverse direction (30).

15. Device (1) according to one of Claims 11 to 14, wherein the electromagnetic stirrer (17) is configured in such a way that the stirring frequency is between 0.1 and 10 Hz.

16. Device (1) according to one of Claims 11 to 14, wherein, in the upper half of the mold, the electromagnetic brake (16) is at a distance from a surface of the thin-slab strand of preferably between 20 and 150 millimeters along the first transverse direction (18).

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