Abstract
A method for semi-continuous casting of a strand (1) of steel in a strand casting machine and a strand casting machine for such casting. The strand has little segregation of the center and porosity. Yet it is castable rapidly. The method steps are: at a casting start of the strand casting machine, pouring liquid steel into an open-ended mold (2). The mold is closed by a cold strand (6). The liquid steel forms, together with the cold strand, a completely solidified strand start (1a) and subsequently forms a semi-solidified strand (1b). Then extracting the semi-solidified strand (1b) from the open-ended mold (2). Supporting and guiding the semi-solidified strand (1b) in a strand guide (3). Cooling the semi-solidified strand (1b) by secondary cooling (4) at the casting end of the strand casting machine, ending the pouring of liquid steel into the open-ended mold (2) and forming a strand end (1c). Extracting the strand end (1c) from the open-ended mold (2). Ending the extraction such that the strand end (1c) lies outside the open-ended mold (2). Ending the secondary cooling (4). Controlling or regulating cooling of the semi-solidified strand (1b) until complete solidification of the strand (1) in a tertiary cooling zone (5) of the strand casting machine. The cooling effect is stronger at the strand start (1a) and decreases towards the strand end (1c). Discharging the strand (1) from the strand casting machine.
Claims
1. A method for semi-continuous casting of a strand made of steel in a strand casting machine, wherein the strand casting machine comprises: a cooled open-ended mold configured for primary cooling of the strand, followed by a strand guide for supporting and guiding movement of the strand, and having secondary cooling located and configured for cooling the strand, followed by tertiary cooling for cooling the strand, the method comprising steps of: starting casting in the strand casting machine, comprising pouring liquid steel into the open-ended mold while closing off the mold by a dummy bar and so that the liquid steel forms with the dummy bar a fully solidified strand start area which is then followed by a partially solidified strand; extracting the partially solidified strand from the open-ended mold; supporting and guiding the partially solidified strand in the strand guide, where the partially solidified strand is cooled by the secondary cooling; at the end of the casting in the strand casting machine, ending the pouring of liquid steel into the open-ended mold and forming a strand end; extracting the strand end from the open-ended mold; ending the extraction of the strand end, such that the strand end is then located outside the open-ended mold; then ending the secondary cooling; providing controlled or regulated cooling of the partially solidified strand until full solidification of the strand in the tertiary cooling zone of the strand casting machine, wherein the cooling takes place more strongly at the strand start and in a decreasing manner toward the strand end; and discharging the strand from the strand casting machine.
2. The method as claimed in claim 1, further comprising setting the cooling of the partially solidified strand in the tertiary cooling zone by influencing at least one from the group consisting of: thermally insulating the strand; heating the strand; and surface cooling the strand.
3. The method as claimed in claim 2, further comprising heating the partially solidified strand in the tertiary cooling zone by a heating device for setting the cooling of the strand.
4. The method as claimed in claim 3, wherein the heating device is displaceable in an extraction direction of the strand casting machine and displacing the heating device in the extraction direction.
5. The method as claimed in claim 2, further comprising protecting the partially solidified strand from cooling too rapidly in the tertiary cooling zone by thermal insulation.
6. The method as claimed in claim 5, further comprising setting the insulating effect of the thermal insulation.
7. The method as claimed in claim 2, further comprising heating the strand end by head heating.
8. The method as claimed in claim 2, further comprising cooling the surface of the partially solidified strand in the tertiary cooling zone by a cooling device.
9. The method as claimed in claim 2, further comprising stirring the partially solidified strand in the tertiary cooling zone by a stirring coil that is stationary or displaceable in the extraction direction, or rotating the partially solidified strand about its own axis alternately in the clockwise direction and the counterclockwise direction in the tertiary cooling zone.
10. A strand casting machine for carrying out the method as claimed in claim 1 comprising: an open ended mold configured for receiving liquid steel and for cooling the liquid steel to form a strand of the steel in the mold; a device for extracting the strand from the open-ended mold and a device for discharging the strand from the strand casting machine; the open-ended mold is cooled for providing primary cooling of the strand in the mold; a strand guide following the mold and configured for supporting and guiding the strand out of the mold, in an extraction direction, the strand guide having a secondary cooling zone for cooling the strand; a tertiary cooling zone following the strand guide for cooling the strand further, the tertiary cooling zone has a heating device which is displaceable in the extraction direction of the strand casting machine, for the controlled or regulated further cooling of the partially solidified strand.
11. The strand casting machine as claimed in claim 10, further comprising the tertiary cooling zone has thermal insulation that is statically settable or settable in a controlled or regulated manner.
12. The strand casting machine as claimed in claim 10, further comprising a plurality of the tertiary cooling zones offset transversely to the extraction direction of the strand casting machine; and a machine head of the strand casting machine, comprising the open-ended mold and the secondary cooling zone, is connectable to and separable from the tertiary cooling zone.
13. The strand casting machine as claimed in claim 12, further comprising the plurality of the tertiary cooling zones are arranged one after another in an arcuate, circular, or linear arrangement for receiving the strand from the machine head.
14. The strand casting machine as claimed in claim 11, further comprising the adjustable thermal insulation comprises at least one insulation panel which is displaceable in the extraction direction (A) or is pivotable with respect to the extraction direction.
15. The strand casting machine as claimed in claim 11, further comprising the strand casting machine comprises a strand extraction carriage configured for extracting the strand, from the secondary cooling zone, wherein the strand extraction carriage is displaceable in the extraction direction.
16. The strand casting machine as claimed in claim 12, further comprising the strand extraction carriage is connected to the machine head and both the extraction carriage and the machine head are displaceable transversely to the extraction direction.
17. The strand casting machine as claimed in claim 12, further comprising the machine head is stationary and the strand is displaceable transversely to the extraction direction.
18. The strand casting machine as claimed in claim 10, further comprising a plurality of the tertiary cooling zones offset transversely to the extraction direction of the strand casting machine; and a machine head of the strand casting machine, comprising the open-ended mold.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0068] Further advantages and features of the present invention can be gathered from the following description of nonlimiting exemplary embodiments, wherein, in the figures:
[0069] FIG. 1 schematically shows, in sub-FIGS. 1A-1F, the method steps in the semi-continuous casting of an ingot made of steel.
[0070] FIGS. 2A and 2B show two alternative embodiments of tertiary cooling for the semi-continuous casting of an ingot made of steel.
[0071] FIGS. 3A and 3B show the chronological sequence of a heating unit for heating an ingot in tertiary cooling.
[0072] FIG. 4 shows the temperatures at various zones during the cooling of the strand 1 in the tertiary cooling zone 5. It refers to the zones in FIGS. 4A, B and C.
[0073] FIGS. 4A, 4B and 4C show temperature changes in the molded and cooled strand after progressively longer periods of time of cooling.
[0074] FIG. 5 shows the temperature profiles over time with respect to FIGS. 4, 4A, 4B and 4C.
[0075] FIG. 6A shows an elevational, partially, cross-sectional side view and FIG. 6B shows a front view of a strand casting machine according to the invention.
[0076] FIGS. 7A and 7B show a machine head of a strand casting machine according to the invention respectively in FIG. 7A in an elevational view in the direction of FIG. 6A and a top view in FIG. 7B.
[0077] FIGS. 8A and 8B schematically respectively show an upright strand before discharge from the tertiary cooling zone and then discharge of a fully solidified strand from the tertiary cooling zone.
[0078] FIG. 8C is a top view of FIG. 8A
DESCRIPTION OF THE EMBODIMENTS
[0079] FIGS. 1A-1F show the method steps in the semi-continuous casting of a strand 1 in a strand casting machine.
[0080] In FIG. 1A, liquid steel is poured from a ladle tundish (not shown separately) via a submerged entry nozzle into a cooled open-ended mold 2, wherein, at the start of casting in the strand casting machine, the open-ended mold 2 is closed off in a fluid-tight manner by the dummy bar 6 such that a meniscus M is set in the mold. As a result of the liquid steel joining to the head of the dummy bar 6, a fully solidified strand start 1a (see FIG. 1C) is formed. As a result of the primary cooling of the cooled open-ended mold 2, the partially solidified strand 1b in FIG. 1B following the fully solidified strand start 1a counter to the extraction direction A is not fully solidified but has merely a thin strand shell and a liquid core. In order to keep the meniscus M in the mold 2 more or less constant in spite of the liquid steel flowing in via the submerged entry nozzle, the strand 1 is extracted from the mold 2. To this end, in FIG. 1A, the strand casting machine has a strand extraction carriage 11 which comprises the dummy bar 6 itself, a threaded spindle 12, a threaded nut 13 and a motor 14 for displacing the strand extraction carriage 11 in the extraction direction A. The motor 14 is connected to the threaded nut 13 via a transmission and the threaded spindle 12 and has a drive shaft for the threaded spindle 12.
[0081] In FIG. 1B, the strand 1 has already been extracted further from the open-ended mold 2, wherein the strand 1 is supported in the strand guide 3 following the mold 2 by a plurality of strand guide rollers 3a, guided and cooled by a plurality of cooling nozzles 4a in the secondary cooling 4. In the process, the strand 1 forms a viable strand shell which can withstand the ferrostatic pressure. In this way, breaching of the strand 1 is prevented.
[0082] In FIG. 1C, the strand start 1a has already passed through the secondary cooling 3 of the strand casting machine and has passed into the tertiary cooling zone 5. In the tertiary cooling zone 5, the strand 1 is cooled further slowly in a controlled or regulated manner, such that full solidification takes place in the center of the partially solidified strand 1b with an upwardly oriented direction. As a result, either a globular or at least a dendritic microstructure avoiding thread porosity forms. In order to prevent the partially solidified strand 1b from cooling too rapidly, the tertiary cooling zone 5 has thermal insulation 9 and a heating device 7, illustrated in FIG. 1F. FIG. 2a shows an example of thermal insulation 9 for tertiary cooling, wherein the atmosphere between the strand 1 and the thermal insulation hood 9 is evacuated by a vacuum pump (in this case a jet pump 15). To this end, a pressure port of the jet pump 15 is connected to a compressed air system and the suction port of the jet pump 15 is connected to the space inside the thermal insulation 9. By way of this measure, oxidation, i.e. scaling, of the strand 1 is additionally prevented. Moreover, as a result of the vacuum treatment, the not yet fully solidified melt in the strand is degassed. The thermal insulation 9 has a plurality of insulation panels 9a (FIGS. 2A and 2B, which can be closed (opening angle 0°), opened (opening angle 90°) or partially opened (90°>opening angle>0°) independently of one another.
[0083] In FIG. 1D, casting in the strand casting machine has been ended, and so a strand end 1c forms. As a result of the extraction of the strand end 1c from the mold 2, the meniscus M of FIG. 1D is below the mold 2, illustrated by a dashed line, according to method steps in FIGS. 1A-1C.
[0084] FIG. 1E shows the situation after the strand end 1c of the strand 1 has passed through the secondary cooling zone 3, the secondary cooling has been ended and the strand end 1c terminates flush with the upper end of the tertiary cooling zone 5. In the tertiary cooling zone 5, the slow, controlled or regulated cooling of the partially solidified strand 1b is ensured by the thermal insulation 9 and by the heating of the strand by the heating device 7 that is displaceable in the extraction direction A (see FIG. 1F). After the separation and lifting of the machine head, comprising the open-ended mold 2, the strand guide 3 and the secondary cooling 4, away from the tertiary cooling 5, the strand end 1c is heated by inductive head heating 10, so as to prevent the strand end 1c from cooling too rapidly.
[0085] According to FIGS. 1A-1F, a, for example round steel strand 1 having a diameter of 1200 mm and a length of 10 m was produced. The extraction rate of the strand 1 from the open-ended mold 2 is 0.25 m/min. As a result of the thermal insulation 9 and the reheating of the strand 1 by the displaceable heating device 7, the complete solidification of the strand 1 is achieved only after 13 h. The casting of the strand, without the slow cooling of the strand in the tertiary cooling zone 5, had already been ended after 46 min, however. Since casting ended rapidly compared with the slow full solidification, it is advantageous, in order to increase the throughput of the semi-continuous casting method, for the machine head (no longer illustrated in FIG. 1F) to be separated from the tertiary cooling zone 5 and displaced transversely to the extraction direction A to a further tertiary cooling zone 5. There, a new strand can be cast, while the strand 1 illustrated in FIG. 1F continues to be cooled slowly. After the slow cooling of the strand 1 until its complete solidification in FIG. 8A, the strand is tipped down from its upright orientation of FIG. 8A to its position on the roller bed 37, as described below discharged from the strand casting machine, for example by a device according to FIGS. 8A and 8B.
[0086] FIG. 2A illustrates a first alternative embodiment of the tertiary cooling zone 5 from FIG. 1. In this case, the space between the strand 1 and the thermal insulation 9 is evacuated by a jet pump 15, with the result that good thermal insulation and slow cooling are achieved. Furthermore, the surface of the strand 1 is protected against scaling and the residual melt is degassed. The jet pump is simple and wear-free. The pressure port thereof is connected to a compressed air port P and the suction port thereof is connected to the space within the tertiary cooling zone that is to be evacuated. Blowing off can take place against ambient pressure U. The inductive head heating 10 is advantageous compared with plasma heating, since the magnetic field also acts through the thermal insulation of the strand end 1c.
[0087] FIG. 2B shows a second alternative of the tertiary cooling zone 5 from FIG. 1. In this case, the insulation slats 9a of the thermal insulation 9 are pivotable with respect to the extraction direction, such that the air exchange between the ambient air and the strand 1 in the interior of the tertiary cooling zone 9 is settable. Merely to illustrate the function of the insulation slats 9a, the insulation slats 9a have been illustrated closed on the right-hand side of the strand 1 and opened through 10° with respect to the extraction direction A on the left-hand side. The adjustment of the slats 9a can take place either manually or by way of actuators.
[0088] FIG. 3A schematically shows the chronological sequence of the displacement path s of the inductive heating device 7 for reheating the lateral surface of the strand 1. In this case, the heating device 7 is illustrated by solid lines in the upper region of the strand 1 and by dashed lines in the lower region. FIG. 3B shows a reciprocating path of movement of the heating device 7. Since the solidification front moves from the bottom upward during cooling (i.e. from the strand start 1a to the strand end 1c), the displacement path s of the heating device 7 also shortens over time. The displacement downward toward 1a in FIG. 3A is greater at the start of tertiary cooling and smaller toward the end of the cooling, since the solidification is faster or earlier from bottom to top in FIGS. 3A and 3B. As an alternative to a displaceable heating device 7, a plurality of heating devices (e.g. burners) arranged in a manner distributed along the length of the tertiary cooling zone 5 in the extraction direction A could also be used.
[0089] FIG. 4 shows the temperatures in ° C. of the strand 1 produced according to FIG. 1 in the sectional illustration, 3 h after the start of casting FIG. 4A, 8.3 h after the start of casting FIG. 4B and at full solidification of the strand 1, approx. 13 h after the start of casting FIG. 4C. The numbered lines in FIG. 4 provide information in the following way: The numbers 1-13 represent the respective temperatures in ° C. at the zones defined by the lines in FIGS. 4A, 4B and 4C. The first line, “1.495e+03” means 1.495×10.sup.3, which is 1495. This also guides a reader to understand the other temperatures in FIG. 4. The chronological sequence of the temperatures of the strand 1 at different positions on the surface and in the center of the strand are illustrated in FIG. 5. It is apparent therefrom that the casting of the strand and thus also the primary and secondary cooling is ended 46 min after the start of casting and subsequently the strand 1 is cooled in a controlled manner only by the tertiary cooling 5.
[0090] FIGS. 6a and 6b illustrate a vertical strand casting machine according to the invention in two views. The liquid steel is poured from a ladle 30 via a shroud tube into the tundish 31, and subsequently the melt flows via a submerged entry nozzle (SEN, not illustrated) into the open-ended mold 2. As a result of the primary cooling in the mold 2, a partially solidified strand 1 having a viable strand shell forms. In the mold 2, the melt is influenced yet further by an optional stirring device 32. The strand 1 is supported in the strand guide 3, guided and cooled further in the secondary cooling zone 4. At least the open-ended mold 2, the stirring coil 32, the strand guide 3 having the secondary cooling zone 4, and optionally also the tertiary cooling zone 5, are displaceable on a casting car 33 on the casting platform G. The strand 1, together with the dummy bar 6, is extracted from the open-ended mold 2 via the strand extraction carriage 11. To this end, the strand extraction carriage 11 is driven via four threaded spindles 12 and guided by additional guide rails 34, wherein a motor is connected to the threaded nut 13 via a transmission and the threaded spindle 12. After the casting operation has been ended and the strand 1 has been set down on the anvil 40, the casting car 33 can be displaced transversely to the extraction direction A to a further casting station, since the casting of the partially solidified strand, i.e. without the tertiary cooling of the strand 1, requires much less time than the tertiary cooling of the strand 1 until the full solidification thereof. In the tertiary cooling zone 5, the strand 1 is cooled slowly by the thermal insulation 9 and optionally by a heating device (not illustrated here), such that the solidification in the center of the strand takes place with an upwardly oriented solidification front.
[0091] A more detailed illustration of the machine head of the strand casting machine from FIGS. 6A and 6B is illustrated in FIGS. 7A and 7B.
[0092] FIG. 7A shows a strand guide 3 between the open ended mold 2 which provides primary cooling of the strand. The strand guide rollers 3a on the guide direct the strand out of the guide and past water stripper 36.
[0093] FIGS. 8A and 8B schematically show an embodiment of the discharging of the fully solidified strand 1 from the tertiary cooling zone. The strand 1 is supported laterally by two brackets 38, such that even very different diameters (see outline in FIG. 8C) can be cast in the strand casting machine. In FIG. 8C, the strand 1 has already been pivoted out with respect to the vertical and is resting on the brackets 38. In FIG. 8B, the strand 1 is deposited via the pivot drive 39 on a roller bed 37 where it can be removed in the direction of the arrow.
[0094] Although the invention has been described and illustrated in more detail by the preferred exemplary embodiments, the invention is not limited by the disclosed examples and other variants can be derived therefrom by a person skilled in the art without departing from the scope of protection.
LIST OF REFERENCE SIGNS
[0095] 1 Strand [0096] 1a Strand start [0097] 1b Partially solidified strand [0098] 1c Strand end [0099] 2 Open-ended mold, primary cooling [0100] 3 Strand guide [0101] 3a Strand guide rollers [0102] 4 Secondary cooling, secondary cooling zone [0103] 4a Cooling nozzle [0104] 5 Tertiary cooling, tertiary cooling zone [0105] 6 Dummy bar [0106] 7 Heating device [0107] 9 Thermal insulation [0108] 9a Insulation panel [0109] 10 Head heating [0110] 11 Strand extraction carriage [0111] 12 Threaded spindle [0112] 13 Threaded nut [0113] 14 Motor [0114] 15 Jet pump [0115] 30, 30′ Ladle [0116] 31 Tundish [0117] 32 Stirring coil [0118] 33 Casting car [0119] 34 Guide rail [0120] 35 Oscillating device [0121] 36 Water stripper [0122] 37 Roller bed [0123] 38 Bracket [0124] 39 Pivot drive [0125] 40 Anvil [0126] A Extraction direction [0127] G Casting platform [0128] M Meniscus [0129] P Pressure in a compressed air system [0130] s Displacement path [0131] U Ambient pressure
