EP3134220B2 - Method and device for thin-slab strand casting - Google Patents

Description

State of the art

The present invention is based on a method for thin slab continuous casting according to the preamble of claim 1.

It is generally known from the prior art to produce thin slabs using the continuous casting process. This involves producing a metallic melt which is transferred to a distributor using a steel pouring ladle. From the distributor, the metallic melt flows through a pouring pipe 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. When it leaves the mold, the strand is picked up by a transport system with a large number 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 down the flow rate of the metallic melt inside the already partially solidified strand within the mold using an electromagnetic brake (EMBR: Electromagnetic Brake). The aim here is to reduce the flow velocity of the molten steel at the bath surface and to even out the bath surface profile in order to improve the lubrication between the strand and the mold and to reduce strand surface defects that can arise from the capture of casting slag.

To produce thin slabs with thicknesses between 40 and 120 millimeters, the mold typically has a funnel-shaped, expanded cross-section in the upper part and a rectangular cross-section in the lower part. Due to these small thicknesses, the solidification times in thin slab continuous 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 structure when thin slabs are continuously cast. However, such a structure can have a detrimental effect on the surface and internal properties of the products made from the thin slabs. For example, depending on the type of steel and casting conditions, products made from the thin slab material may exhibit longitudinal streaks on the product surface, inhomogeneous mechanical properties, microstructure lines, core segregation, reduced HIC resistance (hydrogen induced cracking) and susceptibility to internal cracking.

It is known from conventional thick slab continuous casting that longitudinal streaks in dynamo steels can be avoided by casting with very low superheating. In thick slab continuous casting, however, there is a comparatively long solidification time, so that superheating of the steel melt in the tundish below approx. 12 Kelvin is sufficient to achieve sufficient microstructure refinement. The microstructure refinement can be described as sufficient if the expansion of the globulitic core zone in the thickness direction is more than 30%. In order to achieve the same effect with thin slabs, such low superheating would have to be selected due to the shorter solidification times 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 breakages.

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

From the publication DE 698 24 749 T2 Furthermore, a device for casting metal is known which comprises a mold for forming a cast strand and means for supplying a primary flow of hot metallic melt to the mold. The device has 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 down and divide the flow of the hot metal in order to achieve a secondary flow pattern in the mold. In addition, it is also known from this document to provide a further device in the form of an electromagnetic stirrer in order 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 is to be arranged in relation to the mold.

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

Out of DE 100 20 703 A1 A method and a device for the continuous casting of thin slabs are known, which comprise ultrasonic transmitters mounted in the mold area and an electromagnetic brake. The ultrasonic transmitters cause the mold wall to vibrate in order to reduce the temperature load, friction forces, adhesives and longitudinal depressions.

From JPH-08 323454 A a process for the continuous casting of thin slabs is known, in which a superheat of less than 0 °C is to be set in the mold or in which the non-solidified melt is to be stirred below the mold. During the discharge of the strand, the complete solidification of the strand is to be prevented below the mold within the bending area of the strand, in which a large number of spinning rollers are arranged, in order to enable the not completely solidified strand to be compressed by the spinning rollers. The aim of the process is to produce thin slabs with low center segregation.

Out of 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.

Out of 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 the melt are stirred by the interaction of a static magnetic field and the currents induced when the pouring stream is braked.

Out of JP 2003-326339 A A process 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.

The particular difficulty in thin slab continuous casting is to achieve a 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.

It is an object of the present invention to provide a method and a device for producing thin slabs using the continuous casting process, 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 directed, columnar crystalline structure in the thin slab strand in the prior art. Furthermore, the risk of immersion tube clogging due to excessive overheating should be avoided. This object is achieved with 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 rate 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 by means of the electromagnetic stirrer an electromagnetic traveling field is generated in an area of the thin slab strand is produced.

The device according to the invention has the advantage over the prior art that a refinement of the solidification structure inside the thin slab strand is achieved by a concept for electromagnetic stirring specifically designed for thin slab continuous casting, and the simultaneous use of an electromagnetic brake prevents the increase in the flow rate of the molten steel in the mold area induced by the stirrer from leading to unacceptably strong local bath level fluctuations, ie 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 caught on 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 an accelerated and uniform reduction in overheating, 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. at least 30% in the direction of thickness, while coarse, columnar crystalline structures are limited by the stirring. Despite the short solidification times and small volume liquid components inside the thin slab strand that are typical for the continuous casting of thin slabs, this fine-grained, globulitic core zone forms in the solidification structure, which greatly reduces the formation of columnar crystals between the edge zone and the center area of the strand. The extent of the globulitic core zone in the direction of thickness is then at least 30% in particular. In the manufactured product, longitudinal streaks, microstructure, core segregation and susceptibility to internal cracking can thus be reduced and the HIC resistance and the homogeneity of the mechanical and magnetic properties can be increased. It is also advantageous to maintain a higher and non-critical overheating, so that the risk of casting problems in the form of immersion tube blockages and resulting strand surface defects or strand breakages is eliminated. It is conceivable that in the present process, for example, overheating of the steel melt in the tundish between 10 and 50 Kelvin, preferably around 20 Kelvin, is used. Using the electromagnetic stirrer, an electromagnetic traveling field is generated in an area of the thin slab strand that is between 0.9 and 3.8 m away from the bath level of the mold along the strand withdrawal direction. For the purposes of the present invention, an area of the thin slab strand that is between 0.9 and 3.8 m away from the bath level of the mold is understood to mean the area of the thin slab strand that is between 0.9 and 3.8 m away 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 in such a way that the traveling field acts on the parts of the strand that have not yet solidified immediately below the mold, since the traveling field can no longer have a positive influence on the grain structure in parts of the strand that have already solidified. The electromagnetic traveling field is preferably generated in an area that is 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 the electromagnetic alternating field along the strand withdrawal direction is 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. In the sense 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 is in particular the broad side of the strand guide segments, the position of which 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 producing thin slabs in the continuous casting process and hot strip or cold strip made from them. The hot strip or cold 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. tempering steel). A thin slab in the sense of the present invention comprises in particular a slab with a thickness of between 40 and 120 millimeters. For a precise description of the geometric relationships, in addition to the strand withdrawal direction, two transverse directions, a first transverse direction and a second transverse direction, are mentioned below. The first transverse direction always runs perpendicular to the strand withdrawal direction and parallel to the strand surface normal of the slab broadside, while the second transverse direction always runs perpendicular to the strand withdrawal direction and parallel to the strand surface on the slab broadside. The slab broadside is understood to be the side of the rectangular cross-section of the thin slab strand that has the greater extent. The first and second transverse directions therefore both run perpendicular to the strand withdrawal direction and perpendicular to each other.

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

According to a preferred embodiment of the present invention, the non-solidified 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, which is positioned below 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%.

According to a further preferred embodiment of the present invention, the electromagnetic stirrer is set in such a way 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. The electromagnetic traveling field is preferably reversed after a period of 1 to 60 seconds, particularly preferably between 1 and 10 seconds, so that the electromagnetic traveling field then 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 time period of 1 to 60 seconds has elapsed again, preferably again 1 to 10 seconds, the electromagnetic traveling field is reversed again and the cycle begins again.

According to an 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 the center of the thin slab strand to a first edge region of the thin slab strand and 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. After that, 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 to 60 seconds and particularly preferably between 1 and 10 seconds. After that, 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, so that a symmetrical solidification structure with fine, globulitic grains is created.

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 set 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 held for 1 to 60 seconds, in particular between 1 and 10 seconds. After that, 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 held for between 1 to 60 seconds, in particular between 1 and 10 seconds. After that, the electromagnetic traveling field is reversed again and the cycle starts again. This preferred embodiment also ensures a symmetrical stirring of the not yet solidified metallic melt within the already solidified edge zone of the thin slab strand, so that a symmetrical solidification structure with fine, globulitic grains is created.

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 essentially 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 essentially 0.4 Tesla is sufficient to achieve accelerated and uniform overheating reduction in the metallic melt. This effect is advantageously achieved by an electromagnetic stirrer set in such a way that the flow velocity of the non-solidified parts 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 is preferably between 0.2 and 0.7 meters per second. The associated circulation of the non-solidified parts in the thin slab strand ensures the accelerated and uniform reduction of the superheat and thus the desired microstructure refinement, without having to select a lower superheat from the outset, which would drastically increase the risk of immersion tube clogging.

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 a maximum of 10 Hertz and is preferably between 1 and 10 Hz. It has been shown that this stirring frequency range is particularly advantageous. At a stirring frequency of less than 0.1 Hz, there is no electromagnetic traveling field, so that no stirring effect occurs. If the stirring frequency is greater than 10 Hz, the penetration depth of the electromagnetic traveling field into the interior of the strand is too low and no refinement of the structure is achieved.

According to a further preferred embodiment of the present invention, it is provided that 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 essentially 0.2 Tesla. This advantageously slows down the flow rate of the metallic melt between the partially solidified edge regions of the strand and thus prevents fluctuations in the casting level, as well as surface defects (so-called shell defects) and internal defects (e.g. casting slag inclusions) resulting from fluctuations in the casting level.

According to a further preferred embodiment of the present invention, it is provided that the magnetic field strengths of the electromagnetic traveling field caused by the electromagnetic stirrer and the field caused by the electromagnetic brake are coordinated with one another. It has been shown that a coordination of the magnetic field strengths of the electromagnetic traveling field caused by the electromagnetic stirrer and the field caused by the electromagnetic brake is advantageous. The coordination is preferably carried out by increasing the magnetic field strength of the field of the electromagnetic brake by 20 to 80% of its basic value to values between 0.1 and 0.3 Tesla when the electromagnetic stirrer is switched on. The basic value in this context is understood to be the magnetic field strength of the field of the electromagnetic brake, as it is typically achieved without the additional use of an electromagnetic stirrer. is used. 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.

A further object of the present invention to solve the problem mentioned at the outset is a device for thin slab continuous casting, in particular using the method according to the invention, which has 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 rate 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 has 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, wherein the electromagnetic stirrer is spaced between 0.9 and 3.8 m from the bath level of the mold along the strand withdrawal direction.

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 achieving a refinement of the solidification structure inside the thin slab strand. Stirring the metallic melt ensures accelerated and uniform overheating 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 stirring. Despite the short solidification times and small-volume liquid components 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 avoiding or at least suppressing the formation of columnar crystals between the edge zone and the center region of the strand. The products made from the thin slabs therefore have significantly reduced longitudinal streaks, microstructures 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 area of the thin slab strand. The electromagnetic stirrer preferably comprises a linear field stirrer, which is 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 helicoidal stirrer.

The electromagnetic stirrer is arranged along the direction of the thin slab strand's withdrawal below the electromagnetic brake. This advantageously achieves rapid and even reduction of overheating in the parts of the thin slab strand that have not yet solidified before solidification progresses into the interior of the thin slab strand, so that the solidification structure is refined. In principle, the proportion of the globulitic core zone in the thin slab is greater the closer the electromagnetic stirrer is arranged 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 area of the mold so that early and rapid reduction of overheating in the interior of the strand is achieved, and that the currents in the metallic melt 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.

The device according to the invention is used in particular for producing thin slabs using the continuous casting process and hot strip or cold strip made from them. The hot strip or cold strip is used in particular for producing electrical sheets (non-grain-oriented or grain-oriented) or sheets of high-strength steels with yield strength values greater than 400 megapascals (e.g. tempering steel). A thin slab in the sense of the present invention includes in particular a slab with a thickness of between 40 and 120 millimeters.

According to a further preferred embodiment of the present invention, it is provided that 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. It is then reversed so that the first subfield runs from the first edge region of the thin slab strand and the second Subfield from the second edge area of the thin slab strand, opposite the first edge area, 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 from the beginning. This advantageously achieves a uniform and symmetrical flow inside the strand and thus also a uniform removal of the overheating. On the one hand, this should bring about a homogeneous microstructure refinement inside the strand and, on the other hand, a uniform strand shell growth across the strand width. In this way, strand breakouts or longitudinal surface cracks are prevented from occurring.

According to an embodiment not according to the invention, the electromagnetic stirrer is set such that the flow rate of the metallic melt generated by the stirrer is at least 0.2 meters per second or a maximum of 0.7 meters per second and in particular is 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 (reducing the risk of strand breakage) and, on the other hand, strong element depletions (so-called white bands, i.e. depletion of C, Mn, Si, P, S, etc.) on the solidification front in the area of action of the stirrer are avoided. It has been shown that the flow rate 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 extent in the thickness direction is less than 30% can be considered insufficient. The flow velocity should also not be greater than 0.7 meters per second to avoid a depletion of the melt in alloying elements in the area of the solidification front. The depletion of the melt in alloying elements in the area of the solidification front is measurable in the solidified material. This phenomenon is referred to as "white bands" or "white stripes". White bands lead to inhomogeneous properties of the end product.

According to a further preferred embodiment of the present invention, it is provided that 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. In the sense of the present invention, the aforementioned distance is to be understood in particular as the smallest distance between the electromagnetic brake and the strand surface.

Further details, features and advantages of the invention emerge from the drawings and from the following description of preferred embodiments with reference to the drawings. The drawings merely illustrate exemplary embodiments of the invention which do not restrict the essential inventive concept.

Short description of the drawings

Figure 1

shows a schematic sectional view of an apparatus for thin slab continuous casting according to an exemplary embodiment of the present invention.

Figures 2a and 2b

show schematic detailed views of the apparatus for thin slab continuous casting according to the exemplary embodiment of the present invention in the region of the mold and below the mold.

embodiments of the invention

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

In Figure 1 is a schematic sectional view of a device 1 for producing thin slabs in the continuous casting process according to an exemplary embodiment of the present invention.

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 pipe 4 (feeding means) into a mold 5 of the device 1. The flow through the pouring pipe is controlled by a stopper 8 or a slide valve depending on the pouring level 7 in the mold 5. The mold 5 comprises a mold with a downwardly open passage opening with a 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 area of the mold 5. In the mold 5, a thin slab strand 9 is formed from the continuously supplied metallic melt 2 with a solidified shell 10 and a largely unsolidified cross-section 11 within the solidified shell 10. Optionally, the mold 5 oscillates to prevent the strand surface from sticking to the mold 5. The thin slab strand 9 passes through the Mould 5 along a vertical strand withdrawal direction 15. When leaving the downwardly open mould 5, the thin slab strand 9 is picked up by a transport system 12 (also referred to as 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 is completely solidified.

In addition to the strand withdrawal direction 15, a first transverse direction 18 and a second transverse direction 30 are Figure 1 The 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 drawing plane), while the second transverse direction 30 runs perpendicular to the strand withdrawal direction 15 and parallel to the strand surface on the slab broadside 28, ie perpendicular to the first transverse direction 18.

In the upper area of the mold 5, an electromagnetic brake 16 (EMBR: Electromagnetic Brake) is arranged, which slows down the flow rate 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 both sides 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 essentially 0.2 Tesla. By slowing down the flow rate of the metallic melt 2 between the partially solidified edge areas 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.

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 that is 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 along the strand withdrawal direction 15 between 0.9 and 3.8 m, preferably between 1.5 and 2.5 m, away from the bath level of the mold 5 and comprises on average a magnetic flux density between 0.1 and 0.6 Tesla and preferably essentially 0.4 Tesla. The electromagnetic traveling field leads to stirring of the metallic melt, which causes an accelerated and uniform reduction in overheating 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 set in such a way that the flow speed 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 that are typical in the continuous casting of thin slabs, the fine-grained, globulitic core zone then forms in the solidification structure, whereby the formation of columnar crystals between the edge zone and the center region of the thin slab strand 9 is suppressed. In a final product made from continuously cast thin slabs, longitudinal streaks, microstructure, core segregation and susceptibility to internal cracking can be reduced and the HIC resistance and the homogeneity of the mechanical and magnetic properties can be increased. In this case, for example, casting is carried out with overheating, i.e. with a temperature difference between the actual melt temperature minus the liquidus temperature, between 10 and 50 Kelvin, preferably around 30 Kelvin. A higher and non-critical overheating can therefore be maintained, so that the risk of casting problems in the form of immersion tube blockages and resulting strand surface defects or strand breakages is eliminated.

The device or method described above is used to produce thin slabs, particularly for hot strip or cold strip. The hot strip or cold strip is used in particular to produce electrical sheets (non-grain-oriented or grain-oriented) or sheets of high-strength steels with yield strength values greater than 400 megapascals (e.g. tempering steel).

The upper figure shows that the feed means comprises the pouring pipe 4, which is immersed in the metallic melt 2 in the mold 5, and pouring holes 22 formed on the pouring pipe 4 below the pouring level 7 in the lower part of the pouring pipe 4. The metallic melt 2 is introduced by means of 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 2a the 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 shown schematically 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 overheating. 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 width of the strand. In this way, the electromagnetic stirring prevents the potential risk of strand breakage or longitudinal surface cracks from occurring. The electromagnetic stirrer 17 is also not adjusted according to the invention such that the flow rate of the metallic melt generated by the stirrer at the solidification front is 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, strong 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 in such a way that the currents generated by the electromagnetic stirrer 17 in the metallic melt 2 do not lead to increased bath level fluctuations and do not lead 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 takes place, 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. The base value in this context 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.

In the lower illustration of Figure 2a the rectangular cross-section of the through-opening 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.

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.

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

Example 1:

A measure of the success of the refinement of the solidification structure inside 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 with D _GKZ = thickness of the globulitic core zone and D = slab thickness.

A test was therefore carried out with the steel grade S420MC, a casting speed of 5 meters per minute, a superheat in the tundish of 30 Kelvin, a strand thickness of 65 millimeters, a strand width of 1550 millimeters and a mold height of 1100 millimeters, in which the electromagnetic brake (EMBR) was arranged in the upper half of the mold and the electromagnetic stirrer (EMS) was arranged below the mold behind non-magnetic rollers of the transport system. The electromagnetic stirrer or the electromagnetic alternating field of the electromagnetic stirrer was arranged at a distance of 2960 millimeters from the casting level. The results shown in the following table were achieved: example 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

The test series show that by switching on 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:

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 determined experimentally on dynamo steels with 2.4% silicon: overheating (K) GKZ in thickness direction (%) Longitudinal streaking 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

It follows from this that in order to avoid longitudinal streaking and to reduce central segregation, the proportion of the globulitic core zone (GKZ) should be at least 30 percent and preferably greater than 50 percent. Overheating of less than 20 K should, however, be avoided, as otherwise problems would arise in the form of clogging of the immersion tubes in the mold (so-called "clogging"), which could result in strand surface defects or even strand breakages.

The following example shows, using the dynamo steel with 2.4% silicon and thin slabs with a thickness of 63 millimeters, a superheat in the tundish of 30 Kelvin, a strand width of 1550 millimeters and a mold height of 1100 millimeters, the casting level was 1000 millimeters above the bottom of the mold, the stirring frequency was 6 Hz, the flow velocity at the solidification front was 0.4 m/s, that by choosing the distance between the casting level and the electromagnetic stirrer (EMS) accordingly, 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 V _G : GKZ (%) Strand shell thickness "S" on the respective broad side (mm) Distance of the EMS from the bath surface (m) V _G = 4.0 m/min V _G = 5.0 m/min V _G = 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

The above series of measurements show that the electromagnetic stirrer must be positioned between 2.6 and 3.8 meters below the bath level of the mold for a portion of 50 percent of the globulitic core zone and between 1.7 and 2.5 meters for a portion of 60 percent of the globulitic core zone at the casting speeds ( _{V G} ) usual for thin slab continuous casting plants of between 4 and 6 m/min. 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.

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

1

device

2

metallic melt

3

distributor

4

pouring pipe

5

mold

6

steel ladle

7

pouring surface

8

Plug

9

thin slab strand

10

Solidified strand shell

11

Non-solidified cross-section

12

transport system

13

strand guide roller

14

pouring arch

15

strand withdrawal direction

16

Electromagnetic brake

17

Electromagnetic stirrer

18

First transverse direction (runs perpendicular to the strand withdrawal direction and parallel to the strand surface normal of the slab broadside)

19

electromagnetic traveling field

20

first edge area

21

Second marginal area

22

pouring holes in the lower part of the pouring tube

23

flow arrow

24

First subfield

25

Second subfield

26

center

27

movement arrow

28

broadsides

29

bottom of the mold

30

Second transverse direction (runs perpendicular to the 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)

31

mold top

Claims (16)

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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