Stirring of cast billets or blooms using an oscillating stirrer and control program

Abstract

Liquid steel (1) is poured from above into a continuous mold (2). A steel strand (3) with an already solidified strand shell (4) and a still liquid core (5) is drawn from the bottom of the continuous mold (2). The steel strand (3) is fed from above into a holding device (6) without changing direction, where it remains during a solidification period. During this period, the steel strand (3) gradually solidifies into a billet or ingot. During solidification, the liquid core (5) is stirred by a strand agitator (9). Furthermore, during solidification, the strand agitator (9) is moved vertically along the steel strand (3) between an upper and a lower reversal point (P1, P2) with an oscillation stroke (h).

Description

field of technology

[0001] The present invention is based on a manufacturing method for a billet, wherein liquid steel is poured from above into a continuous mold, a steel strand with an already solidified strand shell and a still liquid core is drawn off from the bottom of the continuous mold and the steel strand with the already solidified strand shell and the still liquid core is introduced from above into a holding device without changing direction, in which the steel strand remains during a solidification time in which the steel strand gradually solidifies, wherein the liquid core is stirred by means of a strand stirrer during the solidification time.

[0002] The present invention is further based on a control program having machine code that can be processed by a control device of a continuous casting machine for producing a bloom, wherein the processing of the machine code by the control device causes the control device to control a strand stirrer of a holding device, in which a steel strand having an already solidified strand shell and a still liquid core gradually solidifies into the bloom during a solidification time, in such a way that the liquid core is stirred by means of the strand stirrer during the solidification time.

[0003] The present invention further relates to a control device of a continuous casting machine for producing a bloom, wherein the control device is programmed with such a computer program so that during operation it controls a strand stirrer of a holding device in which a steel strand having an already solidified strand shell and a still liquid core gradually solidifies into the bloom during a solidification time, such that during the solidification time the liquid core is stirred by means of the strand stirrer.

[0004] The present invention further relates to a continuous casting machine for producing a bloom, wherein the continuous casting machine has a continuous mold into which liquid steel is poured from above and from which a steel strand with an already solidified strand shell and a still liquid core is drawn off at the bottom, wherein the continuous casting machine has a holding device below the continuous mold into which the steel strand with the already solidified strand shell and the still liquid core is introduced from above without a change of direction and in which the steel strand remains during a solidification time in which the steel strand gradually solidifies to form the bloom, wherein the holding device has a strand stirrer which can be moved vertically along the steel strand and by means of which the liquid core is stirred during the solidification time, wherein the continuous casting machine has a control device by which at least the strand stirrer is controlled. State of the art

[0005] Such manufacturing processes are generally known. Purely by way of example, reference can be made to WO 2015 / 079 071 A2, WO 2018 192 903 A1, and EP 3 251 773 B1. These documents explain manufacturing processes of the above-mentioned type in detail.

[0006] Continuous casting machines with strand agitators are well known. These agitators improve the internal quality of the cast strand. In particular, center segregation can be reduced.

[0007] Continuously operating continuous casting plants usually have several strand stirrers. For example, a mold stirrer is often present, which stirs the still-liquid melt in the continuous mold. Stirring often also occurs at one or more points after it leaves the continuous mold. Viewed in the casting direction, the last strand stirrer is usually located near the location on the continuous casting machine where the bottom tip of the cast steel strand forms. The strand stirrers are usually stationary, meaning they cannot be moved or can only be moved slightly in the casting direction.

[0008] Even semi-continuous continuous casting plants, such as those typically used for casting blooms, often feature strand agitators. In contrast to continuous casting plants, however, designs are known that feature only a single strand agitator that can be moved in the casting direction. This is also explained in the patent documents mentioned above.

[0009] US 2017 / 0 216 908 A1 discloses a manufacturing method for a bloom, in which liquid steel is poured into a continuous mold from above, a steel strand with an already solidified strand shell and a still liquid core is withdrawn from the bottom of the continuous mold, and the steel strand with the already solidified strand shell and the still liquid core is introduced from above into a holding device without changing direction, in which the steel strand remains during a solidification period in which the steel strand gradually solidifies into the bloom. A strand stirrer is arranged in the area of the continuous mold, by means of which the liquid strand is stirred: The partially solidified strand can be stirred in a tertiary cooling system by means of a stirring coil, wherein the stirring coil can be displaced in the withdrawal direction.

[0010] US Pat. No. 4,375,830 A discloses an operating method for a continuous casting mold in which liquid steel is poured into a continuous mold from above, and a steel strand with an already solidified strand shell and a still-liquid core is drawn from the bottom of the continuous mold. The steel strand is deflected into a horizontal position via an arc-shaped secondary cooling system. A strand stirrer is arranged in one area of the secondary guide, by means of which the liquid core is stirred. The strand stirrer is moved with an oscillating stroke between an upper and a lower reversal point.

[0011] US Pat. No. 3,987,841 A discloses an operating method for a continuous casting mold in which liquid steel is poured into a continuous mold from above, and a steel strand with an already solidified strand shell and a still-liquid core is drawn from the bottom of the continuous mold. The steel strand is deflected into a horizontal position via an arc-shaped secondary cooling system. A strand stirrer is arranged in one area of the secondary guide, by means of which the liquid core is stirred. The strand stirrer is not moved along the casting axis of the metal strand. This is also not possible due to the design of the strand stirrer.

[0012] US Pat. No. 5,762,127 A discloses an operating method for a continuous casting mold in which liquid steel is poured into a continuous mold from above, and a steel strand with an already solidified strand shell and a still-liquid core is drawn from the bottom of the continuous casting mold. A strand stirrer is arranged in each of the upper and middle sections of the continuous casting mold, by means of which the liquid core is stirred. The strand stirrers are not moved along the casting axis of the metal strand.

[0013] WO 2013 / 174 512 A2 discloses an operating method for a continuous casting mold in which liquid steel is poured into a continuous casting mold from above, and a steel strand with an already solidified strand shell and a still liquid core is drawn from the continuous casting mold from below. A strand stirrer is arranged in a secondary guide area to stir the liquid core. The strand stirrer cannot be moved along the casting axis of the metal strand. Summary of the invention

[0014] Despite the use of strand stirrers even in semi-continuous continuous casting plants, center segregations still occur in the blooms cast with these.

[0015] The object of the present invention is to provide possibilities by means of which the center segregations can be reduced in a simple and reliable manner to a greater extent than in the prior art.

[0016] The object is achieved by a manufacturing method having the features of claim 1. Advantageous embodiments of the manufacturing method are the subject of dependent claims 2 to 11.

[0017] According to the invention, a production method of the type mentioned at the outset is designed in that the strand stirrer is moved during the solidification time with an oscillation stroke in an oscillating manner between an upper and a lower reversal point.

[0018] Through this oscillation, a single strand stirrer can efficiently prevent center segregation along the entire length of the cast steel strand. The strand stirrer can simultaneously perform the functions of both a typical strand stirrer in the narrower sense (i.e., a stirrer that stirs the steel strand in a region where the strand shell is still relatively thin) and a final stirrer (i.e., a stirrer that stirs the steel strand in the region of the bottom tip). A single stirrer thus performs functions that are usually performed with two stirrers.

[0019] In a simple embodiment, the upper reversal point is fixed. In particular, the upper reversal point can be at a predetermined distance from the position of an upper edge of the fully cast steel strand, for example, it can coincide with the upper edge of the fully cast steel strand or be up to approximately 30 cm below the upper edge or slightly above the upper edge.

[0020] Preferably, however, the upper turning point is variable. In this case, the position of the upper turning point can be adapted, in particular, to the solidification behavior of the steel strand. For example, for this purpose, the upper turning point can be determined as a function of an upper location or as a function of a lower location. In this case, the upper location is the location along the cast steel strand in the holding device at which the liquidus temperature prevails. Analogously, the lower location is the location along the cast steel strand in the holding device at which the solidus temperature prevails. With this procedure, the upper turning point initially moves downwards during the solidification time – namely when the steel strand is introduced into the holding device – and then gradually upwards.The upper turning point may, for example, be determined such that it is located a predetermined distance above the upper location or a predetermined distance above the lower location.

[0021] In a completely analogous manner, in a simple embodiment, the lower reversal point is fixed. In particular, the lower reversal point can be at a predetermined distance from the position of a lower edge of the fully cast steel strand, for example, it can coincide with the lower edge of the fully cast steel strand, or it can be up to approximately 30 cm above the lower edge, or it can be slightly below the lower edge.

[0022] Likewise, in a completely analogous manner, the lower reversal point is preferably also variable. In this case, the position of the lower reversal point can also be adapted, in particular, to the solidification behavior of the steel strand. For example, for this purpose, the lower reversal point can be determined as a function of the upper location or as a function of the lower location. With this procedure - again completely analogous to the upper reversal point - the lower reversal point initially moves downwards and then gradually upwards during the solidification time. The lower reversal point can, for example, be determined such that it is located a predetermined distance above or below the upper location or a predetermined distance above or below the lower location.

[0023] The position of the upper and lower reversal points can in particular be coordinated in such a way that, on the one hand, the upper reversal point is at or below the upper edge of the fully cast steel strand and, on the other hand, as long as the distance of the lower reversal point from the upper edge of the fully cast steel strand is at least as great as a fixed nominal stroke, the upper reversal point is determined as a function of the lower reversal point such that the oscillation stroke is equal to the fixed nominal stroke. If and as long as the lower reversal point is spaced from the upper edge by less than the fixed nominal stroke, the upper reversal point is set equal to the upper edge of the steel strand. The latter case can occur both during the insertion of the steel strand into the holding device and when the steel strand is fully inserted into the holding device.By adjusting the upper and lower reversal points accordingly, stirring efficiency is optimized.

[0024] The upper location and / or the lower location can be determined as needed. For example, the upper location and / or the lower location can be determined as functions of time from the start of casting or from the end of casting. This approach is relatively simple to implement. However, it is preferred that the upper location and / or the lower location be determined using a thermodynamic model of the steel strand. This provides an improved determination of the upper location and / or the lower location.

[0025] Typically, the strand stirrer is designed as an electromagnetic stirring coil supplied with at least one alternating current. Preferably, the at least one alternating current is varied during the oscillation of the strand stirrer depending on its current position. Current intensities, frequencies, phase angles, and other variables characterizing the at least one alternating current can be varied as needed.

[0026] The object is further achieved by a control program having the features of claim 12. According to the invention, the processing of the machine code by the control device causes the control device to control the strand stirrer in such a way that, during the solidification time, the strand stirrer is additionally moved with an oscillation stroke between an upper and a lower reversal point.

[0027] Preferably, the processing of the machine code by the control device even causes the control device to implement at least one of the advantageous operating modes of the strand stirrer explained above in connection with the manufacturing method.

[0028] The object is further achieved by a control device having the features of claim 14. According to the invention, the control device is programmed with a control program according to the invention, so that the control device controls the strand stirrer as explained above.

[0029] The object is further achieved by a continuous casting machine having the features of claim 15. According to the invention, in a continuous casting machine of the type mentioned at the outset, the control device is designed as a control device according to the invention, so that the strand stirrer is moved in an oscillating manner between an upper and a lower reversal point during the solidification time with an oscillation stroke. Short description of the drawings

[0030] The above-described properties, features, and advantages of this invention, as well as the manner in which they are achieved, will become clearer and more readily understood in connection with the following description of the embodiments, which are explained in more detail in conjunction with the drawings. Herein, in schematic representation: FIGS 1 to 5 show various states during the casting of a steel strand, FIG 6 show a time diagram, FIGS 7 to 9 show longitudinal sections through various sections of a cast steel strand, FIG 10 show a block diagram and FIGS 11 to 14 show time diagrams. Description of the embodiments

[0031] To produce a bloom, the following steps are taken: FIG 1 bis 5 Liquid steel 1 is poured from above into a continuous casting mold 2 of a continuous casting machine and a steel strand 3 is drawn from the bottom of the continuous casting mold 2. The steel strand 3 often has a large cross-section, for example, in the case of a circular cross-section, a diameter of 600 mm and more, sometimes up to over 1000 mm. Due to the relatively low solidification rate, the steel strand 3 - see in particular FIG 7 - an already solidified strand shell 4 and a still liquid core 5. In this state—i.e., with the already solidified strand shell 4 and the still liquid core 5—the steel strand 3 is introduced from above into a holding device 6 of the continuous casting machine. The holding device 6 is often referred to in the prior art as tertiary cooling. The holding device 6 is arranged below the continuous mold 2. The introduction of the steel strand 3 into the holding device 6 thus takes place without a change in direction, unlike its withdrawal from the continuous mold 2.

[0032] The FIG 1 bis 5 show different phases of the casting process.

[0033] In the presentation of FIG 1 Casting has just begun. A strand head 7 of a dummy strand is still in the continuous mold 2 and seals it at its bottom. In the illustration of FIG 2 The strand head 7 has just passed a secondary cooling zone of the continuous casting machine. In the secondary cooling zone, the steel strand 3 is withdrawn from the continuous casting mold 2 by means of strand guide rollers and supported. Furthermore, the steel strand 3 is cooled in the secondary cooling zone by means of cooling nozzles. The strand head 7 is shown in the illustration of FIG 2 just entered the holding device 6.

[0034] In the presentation of FIG 3 The feeding of the liquid steel 1 has just been completed. At this point, the strand head 7 is located in the holding device 6, but has not yet reached its bottom. The steel strand 3 itself, however, has now reached its full length l. This is usually between 10 m and 20 m.

[0035] In the presentation of FIG 4 the steel strand 3 is further drawn out of the continuous mold 2. In the illustration of FIG 5 the strand head 7 has reached the bottom of the holding device 6. An upper edge 8 of the steel strand 3 is generally flush or almost flush with an upper edge of the holding device 6.

[0036] For further details on the casting of the steel strand 3, reference is made to the prior art, for example to the aforementioned EP 3 251 773 B1.

[0037] FIG 6 shows in a solid line the location of the swamp tip as a function of time t. The swamp tip can be, for example, an upper location O1 (see FIG 10 ) at which the liquidus temperature TL prevails in the center of the steel strand 3. Alternatively, a lower location O2 (see also FIG 10 ) at which the solidus temperature TS prevails in the center of the steel strand 3.

[0038] At a time t1 (compare FIG 1 ) the casting of the steel strand 3 is started. A base point of the steel strand 3 is therefore still above the holding device 6. The sump tip is just forming. As the casting process continues, the base point of the steel strand 3 moves downwards in accordance with the withdrawal of the steel strand 3 from the continuous mold 2. The sump tip is located slightly above the base point of the steel strand 3. At a time t2 (compare FIG 3 ), the feeding of the liquid steel 1 to the continuous mold 2 is terminated. At time t2, the base of the steel strand 3 moves further downwards, thus has not yet reached its lowest point. The sump tip has moved slightly upwards relative to the base. At time t3 (compare FIG 5 ) the strand head 7 and thus also the base point of the steel strand 3 has reached its lowest point. In the meantime, the sump tip has moved slightly upwards again relative to the base point. However, it is located deep in the holding device 6. As the process continues, the sump tip gradually moves upwards until it reaches the upper edge 8 of the steel strand 3 at a time t4. Only at time t4 has the steel strand 3 solidified into the bloom. The steel strand 3 remains in the holding device 6 at least until time t4 (usually even beyond). However, the thickness of the strand shell 4 gradually increases over the entire length of the steel strand 3, so that the sump tip moves upwards.

[0039] The setting time can be defined as the period between times t1 and t4 or between times t3 and t4, as required. Regardless of whether one or the other specification is made, the setting time is generally in the range of several hours, for example, between 5 and 15 hours, usually 10 hours or more. The casting time can be defined as the period between times t1 and t2 or between times t1 and t3, as required. Regardless of whether one or the other specification is made, the casting time is considerably shorter than the setting time. The casting time is typically a maximum of 3 hours, often in the range of approximately 1 hour.

[0040] Even while the steel strand 3 is in the holding device 6, the steel strand 3 still has the strand shell 4 and the liquid core 5. The FIG 7 bis 9 each show a cross-section through the steel strand 3 immediately after complete insertion into the holding device 6. FIG 7 shows a cross-section in the upper part of the steel strand 3, FIG 8 a cross-section in the middle area of the steel strand 3. FIG 9 shows a cross-section in the area of the sump tip of steel strand 3.

[0041] During the solidification period, the liquid core 5 is stirred by means of a strand stirrer 9 of the holding device 6. The strand stirrer 9 is only FIG 8 The strand stirrer 9 is, as shown in FIG 8 The steel strand 3 can be moved vertically along the steel strand 3 by means of a drive 10, as indicated by a double arrow. The drive 10 can be designed as a hydraulic cylinder. However, other designs are also possible, in particular as an electric drive.

[0042] The strand stirrer 9 is operated according to FIG 10 controlled by a control device 11 of the continuous casting machine. If necessary, other components of the continuous casting machine can also be controlled by the control device 11, for example the feeding of the liquid steel 1 into the continuous mold 2, the withdrawal of the steel strand 3 from the continuous mold 2 and the associated introduction into the holding device 6, as well as various cooling processes, for example of the continuous mold 2 and of the steel strand 3 in the secondary cooling zone and the holding device 6. The control device 11 is programmed with a control program 12. The control program 12 comprises machine code 13, which can be processed by the control device 11. The processing of the machine code 13 by the control device 11 causes the control device 11 to control the strand stirrer 9 in a manner that is explained in more detail below.

[0043] Firstly, the liquid core 5 is stirred by means of the strand stirrer 9 during the solidification period. In particular, in the case of the conventional design of the strand stirrer 9, in which the strand stirrer 9 is designed as an electromagnetic stirring coil, the stirring coil is FIG 9 fed with at least one alternating current I. The further configuration can be as required. Advantageous configurations of a stirring coil, by means of which the liquid core 5 can be stirred in various ways, are explained in more detail, for example, in WO 2017 / 162 418 A1. The precise manner in which the liquid core 5 is stirred is of secondary importance within the scope of the present invention. The only decisive factor is that the precise effect on the liquid core 5 is determined by the alternating current I (for example, its current intensity, its frequency, and, in the case of multiple phases of the alternating current I, the phase relationships between the phases).

[0044] On the other hand - and this is the core subject of the present invention and is used in conjunction with the FIG 11 bis 14 As explained above, the strand stirrer 9 is moved during the solidification period with an oscillation stroke h, oscillating between an upper reversal point P1 and a lower reversal point P2. For this purpose, a position value p is specified for the drive 10. The position value p is time-dependent according to the desired oscillation.

[0045] In connection with the FIG 11 bis 14 Possible oscillations of the strand stirrer 9 are explained in more detail below. FIG 11 bis 14 emanate from FIG 6 .

[0046] In the representation according to FIG 11 both the upper reversal point P1 and the lower reversal point P2 are fixed. Consequently, an oscillation occurs with a uniform, time-constant oscillation stroke h. The frequency of the oscillation can also be constant. However, it can also be varied over time. Furthermore, the oscillation can be sinusoidal or non-sinusoidal as required. A non-sinusoidal movement can occur, for example, if the oscillation stroke h and / or the oscillation frequency is relatively large (in particular, the product exceeds a limit value), so that with a sinusoidal movement, the strand stirrer 9 would have to be moved in the middle area between the upper and lower reversal points P1, P2 at a speed that can no longer be achieved by the drive 10.For the same reason, it may also be possible that the acceleration required in the region of the upper and / or lower reversal point P1, P2 during a sinusoidal oscillation can no longer be achieved. However, other reasons for a non-sinusoidal movement of the strand agitator 9 are also possible. If desired, the at least one alternating current I can also be varied during the oscillation of the strand agitator 9 depending on the current position p of the strand agitator 9.

[0047] The upper reversal point P1 can, in particular, have a predetermined distance a1 from the position of the upper edge 8 of the fully cast steel strand 3. The distance a1 is typically less than 100 cm. The distance a1 can be positive or negative as required. Analogously, the lower reversal point P2 can, in particular, have a predetermined distance a2 from the position of a lower edge of the fully cast steel strand 3. The distance a2 is typically also less than 100 cm. The distance a2 can also be positive or negative as required. The position of the lower edge of the fully cast steel strand 3 corresponds to the location of the strand head 7.

[0048] In the representation according to FIG 12 the upper reversal point P1 is fixed. Regarding the exact position of the upper reversal point P1, the explanations for FIG 11 The lower reversal point P2, however, is variable. In particular, the lower reversal point P2 is FIG 12 determined depending on the upper location O1. For example, the lower reversal point P2 can be determined such that it coincides with the upper location O1 or is a predetermined distance above or below the upper location O1. Alternatively, the lower reversal point P2 can be determined depending on the lower location O2. In this case, the lower reversal point P2 is usually determined such that it lies above the lower location O2 and is a predetermined distance from the lower location O2. Due to the fact that the upper reversal point P1 is fixed, but the lower reversal point P2 is variable, an oscillation occurs with a time-varying oscillation stroke h. The explanations regarding FIG 11 on the frequency of the oscillation and the shape of the oscillation as well as the possible variation of the alternating current I also apply to FIG 12 .

[0049] To determine the upper location O1 and / or the lower location O2, the control device 11 implements according to FIG 10 preferably a thermodynamic model 14 of the steel strand 3. Using the thermodynamic model 14, the temperature and solidification behavior of the steel strand 3 is modeled online by the control device 11 based on a heat conduction equation and a phase transformation equation. At least the heat conduction equation is a differential equation.

[0050] This often also applies to the phase transformation equation. The heat conduction equation and the phase transformation equation are coupled and solved iteratively. Model 14 is two-dimensional or three-dimensional. Corresponding models are well known to experts.

[0051] In the representation according to FIG 13 Both the upper reversal point P1 and the lower reversal point P2 are variable. In particular, the lower reversal point P2 is FIG 13 - as well as FIG 12 - determined depending on the upper location O1. Alternatively, the lower reversal point P2 can be determined depending on the lower location O2. The corresponding statements on FIG 12 are applicable in an analogous manner. A completely analogous procedure can be used for the upper reversal point P1. The determination must simply be made in such a way that the upper reversal point P1 lies above the lower reversal point P2. The explanations for FIG 11 on the frequency of the oscillation and the shape of the oscillation as well as the possible variation of the alternating current I also apply to FIG 13 .

[0052] The in connection with FIG 13 The embodiment explained, in which both the upper reversal point P1 and the lower reversal point P2 are variable, is particularly preferred. Within the scope of this embodiment, it is particularly possible that the lower reversal point P2 is determined first and then (if possible) the upper reversal point P1 is determined as a function of the lower reversal point P2 such that the oscillation stroke h is equal to a fixed nominal stroke hN, i.e. is constant. This determination is possible as long as the distance of the lower reversal point P2 from the upper edge 8 of the completely cast steel strand 3 is at least as great as the nominal stroke hN. If, on the other hand, the lower reversal point P2 is spaced from the upper edge 8 by less than the fixed nominal stroke hN, the upper reversal point P1 is set equal to the upper edge 8 of the steel strand 3. This state occurs briefly between times t1 and t2, i.e. while the steel strand 3 is being introduced into the holding device 6.Second, this condition occurs shortly before time t4. During these two periods, the oscillation stroke h is therefore smaller than the nominal stroke hN.

[0053] FIG 14 shows a further embodiment in which both the upper reversal point P1 and the lower reversal point P2 are variable. The lower reversal point P2 as such and also the upper reversal point P1 as such are in FIG 14 not shown. Instead, the position p of the strand stirrer 9 is shown as a function of time t.

[0054] Within the framework of the design according to FIG 14 the lower reversal point P2 remains at an upper level N1 until the steel strand 3 has been introduced to a significant extent into the holding device 6. Thereafter, the lower reversal point P2 is gradually lowered until it reaches a lower level N2. During the lowering, it can in particular follow the introduction of the steel strand 3 into the holding device 6. Thereafter, the lower reversal point P2 remains at the lower level N2 for some time. During a short period of time, the lower reversal point P2 is lowered below the lower level N2 and then gradually raised to the upper level N1. The upper level N1 is preferably reached at time t4. Within the scope of the embodiment according to FIG 14 the strand stirrer 9 preferably always oscillates with the nominal stroke nH.

[0055] The present invention has many advantages. In particular, the prior art usually involves multiple strand stirrers 9, each of which covers only a portion of the cast steel strand 3. Within the scope of the present invention, however, one and the same strand stirrer 9 can stir the liquid core 5 over the entire length l, or at least over almost the entire length l, of the steel strand 3. Nevertheless, a very high quality of the steel strand 3 can be achieved, particularly in its interior.

[0056] It should be noted that the embodiments of the present disclosure are illustrative and not limiting. The scope of the present invention is defined in the appended claims. List of reference symbols

[0057] 1Liquid steel 2Continuous mold 3Steel strand 4Strand shell 5Liquid core 6Holding device 7Strand head 8Top edge of the steel strand 9Strand agitator 10Drive 11Control device 12Control program 13Machine code 14Model a1, a2Distances hOscillation stroke hNNominal stroke lLength of the steel strand IAlternating current N1Upper level N2Lower level pPosition value O1Upper location O2Lower location P1Upper reversal point P2Lower reversal point tTime TLLiquidus temperature TSSolidus temperature t1 to t4Time points

Claims

1. Production method for a billet or a bloom, - wherein liquid steel (1) is poured from above into an open-ended mould (2), at the bottom a steel strand (3) with an already solidified strand shell (4) and a still liquid core (5) is removed from the open-ended mould (2) and the steel strand (3) with the already solidified strand shell (4) and the still liquid core (5) is introduced from above without changing direction into a holding device (6) in which the steel strand (3) remains during a solidification time in which the steel strand (3) gradually solidifies to form the billet or the bloom, - wherein the liquid core (5) of the stationary steel strand (3) is stirred during the solidification time by means of a strand stirrer (9), characterized in that, during the solidification time, the strand stirrer (9) is moved with an oscillation stroke (h) vertically in an oscillating manner along the steel strand (3) between an upper and a lower reversal point (P1, P2).

2. Production method according to Claim 1, characterized in that the upper reversal point (P1) is fixed, in particular has a predetermined distance (a1) from the position of an upper edge (8) of the completely cast steel strand (3).

3. Production method according to Claim 1, characterized in that the upper reversal point (P1) is variable.

4. Production method according to Claim 3, characterized in that the upper reversal point (P1) is determined in dependence on an upper location (01) at which the liquidus temperature (TL) prevails, or in dependence on a lower location (O2) at which the solidus temperature (TS) prevails.

5. Production method according to one of Claims 1 to 4, characterized in that the lower reversal point (P2) is fixed, in particular has a predetermined distance (a2) from the position of a lower edge of the completely cast steel strand (3).

6. Production method according to one of Claims 1 to 4, characterized in that the lower reversal point (P2) is variable.

7. Production method according to Claim 6, characterized in that the lower reversal point (P2) is determined in dependence on an upper location (01) at which the liquidus temperature (TL) prevails, or in dependence on a lower location (O2) at which the solidus temperature (TS) prevails.

8. Production method according to Claim 1, characterized - in that the lower reversal point (P2) and the upper reversal point (P1) are variable, - in that the upper reversal point (P1) is at or lies below an upper edge (8) of the completely cast steel strand (3), - in that, as long as the distance of the lower reversal point (P2) from the upper edge (8) of the completely cast steel strand (3) is at least as large as a fixed nominal stroke (hN), the upper reversal point (P1) is determined in dependence on the lower reversal point (P2) in such a way that the oscillation stroke (h) is equal to the fixed nominal stroke (hN), and - in that, if and as long as the lower reversal point (P2) is spaced apart from the upper edge (8) by less than the fixed nominal stroke (hN), the upper reversal point (P1) is equated to the upper edge (8) of the steel strand (3).

9. Production method according to Claim 8, characterized in that the lower reversal point (P2) is determined in dependence on an upper location (01) at which the liquidus temperature (TL) prevails, or in dependence on a lower location (O2) at which the solidus temperature (TS) prevails.

10. Production method according to Claim 4, 7 or 9, characterized in that the upper location (01) and / or the lower location (O2) are ascertained by means of a thermodynamic model (14) of the steel strand (3).

11. Production method according to one of the preceding claims, characterized in that the strand stirrer (9) is in the form of an electromagnetic stirring coil which is fed with at least one alternating current (I), and in that the at least one alternating current (I) is varied during the oscillation of the strand stirrer (9) in dependence on the current position of the strand stirrer (9).

12. Control program, which comprises machine code (13) which can be executed by a control device (11) of a continuous casting machine for producing a billet or a bloom, wherein the execution of the machine code (13) by the control device (11) causes the control device (11) to actuate a strand stirrer (9) of a holding device (6), in which a stationary steel strand (3) comprising an already solidified strand shell (4) and a still liquid core (5) gradually solidifies during a solidification time to form the billet or the bloom, in such a way that, during the solidification time, the liquid core (5) is stirred by means of the strand stirrer (9) and the strand stirrer (9) is moved with an oscillation stroke (h) vertically in an oscillating manner along the steel strand (3) between an upper and a lower reversal point (P1, P2).

13. Control program according to Claim 12, characterized in that the execution of the machine code (13) by the control device (11) causes the control device (11) to carry out the additional method steps of at least one of Claims 2 to 11.

14. Control device of a continuous casting machine for producing a billet or a bloom, wherein the control device is programmed with a computer program (12) according to Claim 12 or 13, such that, during operation, it actuates a strand stirrer (9) of a holding device (6), in which a stationary steel strand (3) comprising an already solidified strand shell (4) and a still liquid core (5) gradually solidifies during a solidification time to form the billet or the bloom, at least in such a way that, during the solidification time, the liquid core (5) is stirred by means of the strand stirrer (9) and the strand stirrer (9) is moved with an oscillation stroke (h) in an oscillating manner between an upper and a lower reversal point (P1, P2).

15. Continuous casting machine for producing a billet or a bloom, - wherein the continuous casting machine comprises an open-ended mould (2) into which liquid steel (1) is poured from above and from which at the bottom a steel strand (3) with an already solidified strand shell (4) and a still liquid core (5) is removed, - wherein the continuous casting machine comprises, below the open-ended mould (2), a holding device (6) into which the steel strand (3) with the already solidified strand shell (4) and the still liquid core (5) is introduced from above without changing direction and in which the steel strand (3) remains during a solidification time in which the steel strand (3) gradually solidifies to form the billet or the bloom, - wherein the holding device (6) comprises a strand stirrer (9) which is movable vertically along the steel strand (3) and by means of which the liquid core (5) of the stationary steel strand (3) is stirred during the solidification time, - wherein the continuous casting machine comprises a control device (11), by which at least the strand stirrer (9) is controlled, characterized in that the control device (11) is designed according to Claim 14, such that the strand stirrer (9) is moved during the solidification time with an oscillation stroke (h) in an oscillating manner between an upper and a lower reversal point (P1, P2).

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