METHOD AND DEVICE FOR THIN-SLAB STRAND CASTING

Abstract

A method for continuous casting of thin slabs may involve feeding a molten metal into a mold, molding a partially solidified thin-slab strand from the molten metal in the mold, reducing a flow rate of the molten metal in the partially solidified thin-slab strand by way of an electromagnetic brake positioned in a region of the mold, and removing the partially solidified thin-slab strand from the mold by way of a strand guiding system. Unsolidified parts of the partially solidified thin-slab strand may be stirred by an electromagnetic stirrer arranged underneath the mold downstream along a strand takeoff direction of the thin-slab strand. Further, a traveling electromagnetic field may be produced by the electromagnetic stirrer in a region of the thin-slab strand that is at a distance from the mold of between 20 and 7000 millimeters along the strand takeoff direction.

Claims

1.-24. (canceled)

25. A method for continuous casting of thin slabs, the method comprising: feeding a molten metal into a mold; molding a partially solidified thin-slab strand from the molten metal in the mold; reducing a flow rate of the molten metal in the partially solidified thin-slab strand by using an electromagnetic brake disposed in a region of the mold; removing the partially solidified thin-slab strand from the mold by a strand guiding system; and stirring unsolidified parts of the partially solidified thin-slab strand using an electromagnetic stirrer disposed beneath the mold downstream along a strand takeoff direction of the thin-slab strand, wherein the electromagnetic stirrer is disposed a distance from the mold of between 20-7000 millimeters along the strand takeoff direction and produces a traveling electromagnetic field in a region of the thin-slab strand.

26. The method of claim 25 wherein the electromagnetic field is generated in a region of the thin-slab strand that is at a distance from the mold of between 50-3000 millimeters along the strand takeoff direction.

27. The method of claim 25 wherein the electromagnetic brake generates an electromagnetic field within the mold, wherein in an upper half of the mold the electromagnetic brake is at a distance from a surface of the thin-slab strand of between 20-150 millimeters along a first transverse direction that runs perpendicular to the strand takeoff direction and parallel to a strand surface normal on a broad side of the thin-slab strand.

28. The method of claim 27 wherein the electromagnetic stirrer is configured such that along a second transverse direction that runs perpendicular to the strand takeoff direction and perpendicular to the first transverse direction the traveling electromagnetic field runs from a first outer region of the thin-slab strand to a second outer region of the thin-slab strand that is opposite the first outer region.

29. The method of claim 28 further comprising reversing the traveling electromagnetic field after 1 to 60 seconds in such a way that the traveling electromagnetic field runs along the second transverse direction from the second outer region of the thin-slab strand to the first outer region of the thin-slab strand.

30. The method of claim 25 wherein the traveling electromagnetic field is a bidirectional, symmetrical traveling electromagnetic field that extends over a width of the thin-slab strand, wherein a first subfield of the traveling electromagnetic field runs from a center of the thin-slab strand to a first outer region of the thin-slab strand and a second subfield of the traveling electromagnetic field runs from the center of the thin-slab strand to a second outer region of the thin-slab strand that is opposite the first outer region.

31. The method of claim 30 further comprising reversing the traveling electromagnetic field after 1 to 60 seconds such that the first subfield runs from the first outer region of the thin-slab strand to the center of the thin-slab strand and the second subfield runs from the second outer region of the thin-slab strand to the center of the thin-slab strand.

32. The method of claim 25 wherein the traveling electromagnetic field is a bidirectional, symmetrical traveling electromagnetic field that extends over a width of the thin-slab strand, wherein a first subfield of the traveling electromagnetic field runs from a first outer region of the thin-slab strand to a center of the thin-slab strand and a second subfield of the traveling electromagnetic field runs from a second outer region of the thin-slab strand that is opposite the first outer region to the center of the thin-slab strand.

33. The method of claim 32 further comprising reversing the traveling electromagnetic field after 1 to 60 seconds such that the first subfield runs from the center of the thin-slab strand to the first outer region and the second subfield runs from the center of the thin-slab strand to the second outer region.

34. The method of claim 25 wherein the traveling electromagnetic field generated in the region of the thin-slab strand has a magnetic flux density of on average 0.1 to 0.6 tesla.

35. The method of claim 25 wherein the electromagnetic stirrer is configured such that a flow rate of the unsolidified parts of the partially solidified thin-slab strand is between 0.2 and 0.7 meters per second.

36. The method of claim 25 wherein the electromagnetic stirrer is configured such that a stirring frequency is between 0.1 and 10 Hz.

37. The method of claim 25 wherein an electromagnetic field generated within the mold by the electromagnetic brake has a magnetic flux density of 0.1 to 0.3 tesla.

38. The method of claim 25 further comprising producing thin slabs with a thickness of 40 to 120 millimeters.

39. The method of claim 25 further comprising producing thin slabs for production of hot strip or cold strip for producing electric sheets or sheets of high-strength steel having a yield strength value of more than 400 megapascals.

40. A device for continuous casting of thin slabs comprising: a feeding means for supplying a molten metal; a mold for molding a partially solidified thin-slab strand from the molten metal; an electromagnetic brake disposed in a region of the mold, the electromagnetic brake for reducing a flow rate of the molten metal inside the partially solidified thin-slab strand; a strand guiding system for removing the partially solidified thin-slab strand from the mold; and an electromagnetic stirrer disposed under the mold downstream along a strand takeoff direction of the thin-slab strand, the electromagnetic stirrer for stirring unsolidified parts of the partially solidified thin-slab strand, wherein the electromagnetic stirrer is disposed at a distance from the mold of between 20 and 7,000 millimeters along the strand takeoff direction.

41. The device of claim 40 wherein the electromagnetic stirrer is disposed at a distance from the mold of between 50 and 3,000 millimeters along the strand takeoff direction.

42. The device of claim 40 wherein the electromagnetic stirrer comprises a linear field stirrer for generating a traveling electromagnetic field in a region of the thin-slab strand, wherein a running direction of the traveling electromagnetic field is aligned perpendicular to the strand takeoff direction and parallel to a second transverse direction, the second transverse direction running perpendicular to the strand takeoff direction and parallel to a strand surface on a broad side of the thin-slab strand, wherein the running direction of the traveling electromagnetic field is reversible.

43. The device of claim 42 wherein the electromagnetic stirrer is configured such that a first subfield of the traveling electromagnetic field runs from a center of the thin-slab strand to a first outer region of the thin-slab strand and a second subfield of the traveling electromagnetic field runs from the center of the thin-slab strand to a second outer region of the thin-slab strand that is opposite the first outer region.

44. The device of claim 42 wherein the electromagnetic stirrer is configured such that the traveling electromagnetic field runs along the second transverse direction from a first outer region of the thin-slab strand to a second outer region of the thin-slab strand that is opposite the first outer region.

45. The device of claim 42 wherein the electromagnetic stirrer is disposed at a distance from a surface of the thin-slab strand of between 20 and 1,000 millimeters along a first transverse direction that runs perpendicular to the strand takeoff direction and perpendicular to the second transverse direction.

46. The device of claim 45 wherein in an upper half of the mold, the electromagnetic brake is at a distance from a surface of the thin-slab strand of between 20 to 150 millimeters along the first transverse direction.

47. The device of claim 40 wherein the electromagnetic stirrer is configured such that at least one of a flow rate of the unsolidified parts in the partially solidified thin-slab strand is between 0.2 and 0.7 meters per second, or a stirring frequency is between 0.1 and 10 Hz.

48. The device of claim 40 configured to produce thin slabs for production of hot strip or cold strip for electric sheets or sheets of high-strength steel having a yield strength value of more than 400 megapascals.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1 shows a schematic view of a sectional image of a device for the continuous casting of thin slabs according to an exemplary embodiment of the present invention.

[0030] FIGS. 2a and 2b show schematic views of details of the device for the continuous casting of thin slabs according to the exemplary embodiment of the present invention in the region of the mold and underneath the mold.

EMBODIMENTS OF THE INVENTION

[0031] In the various figures, the same parts are always provided with the same designations, and are therefore in each case also generally only referred to or mentioned once.

[0032] In FIG. 1, a schematic view of a sectional image of a device 1 for producing thin slabs by the continuous casting method according to an exemplary embodiment of the present invention is represented.

[0033] In the present example, molten metal 2 from a steel casting ladle 6 is transferred into a tundish 3 and cast from the distributor 3 by way of a casting tube 4 (feeding means) into a mold 5 of the device 1. The flow through the casting tube is controlled in dependence on the casting level 7 in the mold 5 by a plug 8 or a slide. The mold 5 comprises a mold with a downwardly open through-opening of a rectangular cross section. The broad sides 28 of the mold are spaced apart by between 40 and 120 millimeters, in order that the mold 5 is suitable for the casting of thin slabs. The mold consists of water-cooled copper plates, which have the effect of solidifying the supplied molten metal in the outer region of the mold 5. Consequently, a thin-slab strand 9 with a solidified shell 10 and a mostly not yet solidified cross section 11 within the solidified shell 10 forms in the mold 5 from the continuously supplied molten metal 2. Optionally, the mold 5 oscillates, in order that the surface of the strand is prevented from becoming attached to the mold 5. The thin-slab strand 9 runs through the mold 5 along a vertical strand takeoff direction 15. When it leaves the downwardly open mold 5, the thin-slab strand 9 is taken up by a transporting system 12 (also referred to as the strand guiding system) with a multiplicity of strand guiding rollers 13 and is passed through a so-called casting bow 14. The thin-slab strand 9 is thereby cooled down until it has solidified through completely.

[0034] Apart from the strand takeoff direction 15, a first transverse direction 18 and a second transverse direction 30 are sketched in FIG. 1. The first transverse direction 18 in this case runs perpendicularly to the strand takeoff direction 15 and parallel to a strand surface normal of the slab broad side 28 (in FIG. 1, the slab broad side 28 extends into the plane of the drawing), while the second transverse direction 30 runs perpendicularly to the strand takeoff direction 15 and parallel to the strand surface on the slab broad side 28, i.e. therefore perpendicularly to the first transverse direction 18.

[0035] Arranged in the upper region of the mold 5 is an electromagnetic brake 16 (EMBR), which slows down the flow rate of the molten metal 2 inside the already partially solidified thin-slab strand 9 and thereby reduces fluctuations in the bath level 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. An electromagnetic field of which the magnetic flux density is preferably 0.1 to 0.3 tesla, and particularly preferably substantially 0.2 tesla, is generated within the mold 5 by the electromagnetic brake 16. The braking of the flow rate of the molten metal 2 between the partially solidified outer regions 10 of the thin-slab strand 9 has the effect that fluctuations in the casting level, and also surface defects resulting from fluctuations in the casting level (so-called shell defects) and internal defects (for example casting slag inclusions), can be prevented.

[0036] Underneath the mold 5, the device 1 according to the invention comprises an electromagnetic stirrer 17 for stirring unsolidified 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 over the width of the thin-slab strand 9 a traveling electromagnetic field 19 (see FIGS. 2a and 2b), which cyclically runs back and forth between a first outer region 20 of the thin-slab strand 9 and an opposite second outer region 21 of the thin-slab strand 9 along a second transverse direction 30, which is perpendicular to the strand takeoff direction 15 and parallel to the broad side 28 of the strand surface. The traveling electromagnetic field 19 is generated in a region that is at a distance from the mold 5 or from the underside 29 of the mold of between 20 and 7000 millimeters, preferably between 50 and 3000 millimeters, along the strand takeoff direction 15 and comprises on average a magnetic flux density of between 0.1 and 0.6 tesla and preferably of substantially 0.4 tesla. The traveling electromagnetic field leads to a stirring of the molten metal, whereby an accelerated and uniform reduction of the overheating in the molten metal is brought about. This advantageously leads to the formation of a larger core zone with a fine-grained, globular microstructure inside the thin-slab strand 9, while coarse columnar crystalline structures are restricted by the electromagnetic stirring. This effect is advantageously achieved by an electromagnetic stirrer 17 that is set in such a way that the flow rate of the unsolidified 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. In spite of the short solidifying-through times that are typical in the case of continuous casting of thin slabs and small-volume proportions of liquid inside the thin-slab strand 9, the fine-grained, globular core zone then forms in the solidification structure, whereby the occurrence of columnar crystals between the outer zone and the central region of the thin-slab strand 9 is suppressed. Consequently, in an end product produced from the continuously cast thin slabs, longitudinal striations, microstructural stringers, core segregations and internal crack susceptibilities can be reduced and the HIC resistance and the homogeneity of the mechanical and magnetic properties can be increased. In the present case, casting is performed for example with an overheating, i.e. a temperature difference of the actual temperature of the molten material minus the liquidus temperature, of between 10 and 50 kelvins, preferably around 30 kelvins. Therefore, a higher, uncritical overheating can be retained, so that the risk of casting problems in the form of immersion tube clogging and resultant strand surface defects or strand ruptures is eliminated.

[0037] With the device described above and the method described above, thin slabs are produced, in particular for hot strip or cold strip. Hot strip or cold strip is used in particular for producing electric sheets (not grain-oriented or grain-oriented) or sheets of higher-strength steels with yield strength values greater than 400 megapascals (for example heat-treatable steel).

[0038] In FIGS. 2a and 2b, schematic views of details of the device 1 for the continuous casting of thin slabs in the region of the mold and underneath the mold according to the exemplary embodiment of the present invention explained above on the basis of FIG. 1 are represented. In the upper region of FIGS. 2a and 2b there is respectively illustrated a view of a sectional image along a sectional image plane parallel to the strand takeoff direction 15 and a plane parallel to the second transverse direction 30. In the lower region of FIGS. 2a and 2b there is respectively illustrated a view of a sectional image along a sectional image plane perpendicular to the strand takeoff direction 15, i.e. to the first transverse direction 18 and to the second transverse direction 30, in the region of the electromagnetic stirrer 17, which corresponds to the cross section of the strand 9.

[0039] It can be seen in each case from the upper figure that the feeding means comprises the casting tube 4, which is immersed in the molten metal 2 located in the mold 5, and, underneath the casting level 7, discharge holes 22 formed on the casting tube 4 in the lower part of the casting tube 4. The molten metal 2 is introduced by means of the discharge holes 22 at an angle to the strand takeoff direction 15 of the thin-slab strand 9 (see flow arrows 23). Arranged underneath the mold 5 is the traveling electromagnetic field 19, induced by the electromagnetic stirrer 17 that is not represented. The electromagnetic stirrer 17, which is arranged underneath the mold 5, generates underneath the mold 5 the traveling electromagnetic field 19, which in turn brings about flows that can extend into the mold 5under some circumstances even up to the bath level. In the case of the exemplary embodiment according to FIG. 2a, the electromagnetic stirrer 17 is configured in such a way that the traveling electromagnetic field 19 comprises two subfields, a first subfield 24 and a second subfield 25. The first subfield 24 of the traveling electromagnetic field 19 cyclically migrates back and forth between a center 26 of the thin-slab strand 9 and the first outer region 20 of the thin-slab strand 9, while the second subfield 25 of the traveling electromagnetic field 19 cyclically migrates back and forth between the center 26 and the second outer region 21 of the thin-slab strand 9. The movement of the traveling electromagnetic field 19 is schematically represented by the movement arrows 27. The dividing of the traveling electromagnetic field 19 into two bidirectional, symmetrical subfields leads to a uniform and symmetrical flow inside the thin-slab strand 9, and consequently also to a rapid and uniform removal of the overheating. On the one hand, a homogeneous microstructural refinement inside the strand and on the other hand a uniform growth of the strand shell over the width of the strand are intended to be brought about as a result. In this way the potential risk of a strand rupture or longitudinal surface cracks occurring is prevented due to the electromagnetic stirring. The electromagnetic stirrer 17 is preferably also set in such a way that the flow rate of the molten metal produced 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 growth of the strand shell on the strand narrow side is not weakened too much (reduction of the risk of strand rupture) 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 area of action of the electromagnetic stirrer 17 are avoided. Moreover, the electromagnetic stirrer 17 must be set in such a way that the flows in the molten metal 2 that are produced by the electromagnetic stirrer 17 do not lead to increased fluctuations in the bath level and to increased local excessive bath levels in the mold 5. In this case, the magnetic field strengths of the electromagnetic stirrer 17 and of the electromagnetic brake 16 should be made to match one another. The matching takes place for example by the magnetic field strength of the electromagnetic brake 16 being raised by 20 to 80% of its base value to values of between 0.1 and 0.3 tesla when the electromagnetic stirrer 17 is included. Understood as the base value in this connection is 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.

[0040] In the lower representation of FIG. 2a, the rectangular cross section of the through-opening of the mold 5 can be schematically seen. The traveling electromagnetic field 19 or the two subfields 24, 25 migrate through the thin-slab strand 9 along the broad sides 28.

[0041] Alternatively, the traveling electromagnetic field 19 is not divided into two subfields 24, 25, but cyclically runs along the second transverse direction 30 back and forth between the first outer region 20 of the thin-slab strand 9 and the opposite second outer region 21 of the thin-slab strand 9. This exemplary embodiment is illustrated by way of example in FIG. 2b.

[0042] The following exemplary embodiments were carried out with a device according to FIGS. 1 and 2a.

Exemplary Embodiment 1

[0043] A measure of the success of the refinement of the solidification structure inside the thin-slab strand is the proportion of the globular core zone (GCZ). The extent of the globular core zone in percent is defined as GCZ (%)=D.sub.GCZ (mm) ID (mm).Math.100 where D.sub.GCZ=the thickness of the globular core zone and D=the slab thickness.

[0044] A test was therefore carried out with the steel grade S420MC, a casting rate of 5 meters per minute, an overheating in the tundish of 30 kelvins, 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 underneath the mold, downstream of a magnetic rollers of the transporting system. The electromagnetic stirrer or the alternating electromagnetic field of the electromagnetic stirrer was arranged at a distance of 2960 millimeters from the casting level. The results presented in the following table were thereby achieved:

TABLE-US-00001 Magnetic Distance from GCZ (%) GCZ (%) field the strand EMBR EMBR + Ex. strength (T) surface (mm) only 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

[0045] The series of tests demonstrate that the inclusion of an electromagnetic stirrer arranged underneath the mold has the effect that the proportion of the globular core zone (GCZ) of 0 to 10 percent increases to a proportion of 40 to 60 percent.

Exemplary Embodiment 2

[0046] A connection between overheating of the molten steel in the tundish and the proportion of the globular core zone on the one hand and the resultant longitudinal striations on the finished strip in the case of dynamo steels and the core segregation was experimentally determined on dynamo steels with 2.4% silicon:

TABLE-US-00002 GCZ in Longitudinal thickness striations on Overheating direction the finished Core (K) (%) cold-rolled strip segregation 37 0 severe moderate 24 3 severe moderate 11 6 moderate-severe moderate 6 30 mild-moderate mild-moderate 3 50-70 none none

[0047] It follows from this that, to avoid longitudinal striations and to reduce the core segregation, the proportion of the globular core zone (GCZ) should be at least 30 percent and preferably greater than 50 percent. An overheating of less than 20 K should be avoided however, since otherwise problems in the form of clogging of the immersion tubes in the mold would occur, which may result in strand surface defects or even strand ruptures.

[0048] It is shown below by the example of the dynamo steel with 2.4% silicon and thin slabs with a thickness of 63 millimeters, an overheating in the tundish of 30 kelvins, a strand width of 1550 millimeters and a mold height of 1100 millimeters; the casting level lay 1000 millimeters above the underside of the mold, the stirring frequency was 6 Hz, the flow rate at the solidification front was 0.4 m/s; that, by appropriate choice of the distance between the casting level and the electromagnetic stirrer (EMS), the required proportion of the globular core zone (GCZ) of at least 30 percent and preferably at least 50 percent can be achieved with different casting rates V.sub.G:

TABLE-US-00003 Strand shell thickness Distance of the EMS from S on the bath level (m) GCZ the respective V.sub.G = 4.0 V.sub.G = 5.0 V.sub.G = 6.0 (%) broad side (mm) m/min m/min 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

[0049] The above series of measurements show that, with the casting rates (V.sub.G) customary for thin-slab continuous casting installations of between 4 and 6 m/min, for a proportion of the globular core zone of 50 percent the electromagnetic stirrer must be arranged between 2.6 and 3.8 meters underneath the bath level of the mold, for a proportion of 60 percent between 1.7 and 2.5 meters. Satisfactory results are however also already achieved with a distance of the electromagnetic stirrer from the bath level of between 3.6 and 7.3 meters.

[0050] The distance between the mold or the underside of the mold and the electromagnetic stirrer consequently is advantageously between 20 and 7000 millimeters and preferably between 50 and 3000 millimeters. Alternatively, it is also evident that a distance between 100 and 7000 millimeters, between 500 and 6500 millimeters, between 700 and 6300 millimeters, between 700 and 4400 millimeters or between 700 and 2800 millimeters is particularly advantageous.

LIST OF DESIGNATIONS

[0051] 1 Device [0052] 2 Molten metal [0053] 3 Tundish [0054] 4 Casting tube [0055] 5 Mold [0056] 6 Steel casting ladle [0057] 7 Casting level [0058] 8 Plug [0059] 9 Thin-slab strand [0060] 10 Solidified strand shell [0061] 11 Unsolidified cross section [0062] 12 Transporting system [0063] 13 Strand guiding roller [0064] 14 Casting bow [0065] 15 Strand takeoff direction [0066] 16 Electromagnetic brake [0067] 17 Electromagnetic stirrer [0068] 18 First transverse direction (runs perpendicularly to the strand takeoff direction and parallel to the strand surface normal of the slab broad side) [0069] 19 Traveling electromagnetic field [0070] 20 First outer region [0071] 21 Second outer region [0072] 22 Discharge holes in the lower part of the casting tube [0073] 23 Flow arrow [0074] 24 First subfield [0075] 25 Second subfield [0076] 26 Center [0077] 27 Movement arrow [0078] 28 Broad sides [0079] 29 Mold underside [0080] 30 Second transverse direction (runs perpendicularly to the strand takeoff direction and parallel to the strand surface on the slab broad side and runs perpendicularly to the strand takeoff direction and perpendicularly to the first transverse direction) [0081] 31 Mold upper side