EP1537926B1 - Process for manufacturing a high purity cast strand - Google Patents

Description

The invention relates to a method for producing a cast high-purity metal strand from a molten metal, preferably a molten steel, wherein the molten metal is supplied from a melt container controlled a distribution vessel and controlled by this distribution vessel is discharged into a continuous casting mold and wherein the molten metal at least in the distribution vessel during the stationary Casting operation is covered with a covering.

In particular, the invention relates to a starting method for a continuous casting plant for producing a cast high-purity metal strand from a molten metal, in particular a molten steel. The continuous casting plant can be equipped with a mold of any type. The cross-sectional format of the metal strand to be cast can also be arbitrary. However, especially in the production of thin metal strips with strip thicknesses below 6.0 mm and strip widths above 800 mm, special demands are made of the starting phase of the casting process in order to be able to produce a strip after the first few meters of cast strip which meets high quality standards. The invention particularly, but not exclusively, relates to strip casting with a two-roll caster according to the vertical two-roll casting process.

In the production of a cast high-purity metal strand with any continuous casting plant, the liquid metal is usually supplied from a ladle via at least one intermediate vessel or distribution vessel of a cooled mold in which the solidification process of the molten metal to a metal strand is at least initiated. The transfer of the molten metal from the ladle into the tundish takes place predominantly by means of shadow tubes and from the distributor into the mold through dip tubes, which dive into the melt pool of the respective downstream vessel in the stationary casting operation, thus ensuring as steady and uniform a flow and forwarding of the molten metal as far as possible allow the mold. Usually in the ladle, the molten metal accumulated in the tundish and, if appropriate, in the mold, is covered by a slag layer which protects the metal bath surface from oxidation. The basic arrangement of the melt receiving vessels in a multi-strand continuous casting plant for steel, for example, from US-A 5,887,647 known. The more intensive the metal bath movement takes place in the individual melting vessels, the more slag particles are introduced into the metal bath by the slag layer covering the molten metal, and the more particles of the refractory material from the lining of the melting vessels are likewise fed to the metal bath by erosion. At the same time, the deposition of impurity particles from the molten metal to the metal bath surface or into the slag layer is hindered by excessive metal bath movement. In the case of large-sized metal strands, there is still time in the mold for the separation of foreign substances from the bath surface. For small-sized strands and in particular for tapes of small thickness, the entry of foreign particles into the mold must be avoided as much as possible, since the possibilities for a separation of foreign particles are very limited in the mold.

In general, it is known that the quality of the cast strand is reduced when strong Badspiegelschwankungen occur, as they are unavoidable in the startup of the casting process in the first filling of the tundish, or as they occur during the performance of Pfannenwechsel the sequence casting, which usually with the in the distributor stock molten metal, the change time of the pan is bridged and is therefore poured with continuously decreasing distributor-Badspiegelhöhe. The stability of the melt flow in the distribution vessel is thereby greatly impaired and the molten metal is exposed to undesirable slag entry.

The object of the invention is therefore to avoid these disadvantages and difficulties of the prior art and to propose a method of the type described above, with which a metal strand of high purity can be poured with the beginning of the quasi-stationary casting phases, in which the start phase of Casting process can be kept as short as possible and at least mitigate the effects of non-stationary casting phases as quickly as possible.

This object is achieved in that the forming in a Erstfüllphase in the distribution vessel free bath surface of the molten metal after reaching a predetermined at least approximately stationary Angussbadspiegelhöhe is at least partially covered with a covering. All powdery or liquid agents which form a largely closed protective layer on the surface of the metal bath and thus hinder or completely prevent reoxidation on the surface of the metal bath can be used as covering agents.

The term "distribution vessel" here is not limited to the receiving vessel for molten metal, through which the transfer or distribution of molten metal is made possible in a mold, thus directly preceded by a mold, but may include all melt vessels between the ladle and the mold, in where the molten metal can be covered with a covering.

In order to achieve the at least approximately steady Angussbadspiegelhöhe as quickly as possible and thus the period expresses turbulent and uncontrolled Badbewegung in distribution vessel quickly, it is expedient that supplied until reaching the at least approximately stationary Angussbadspiegelhöhe molten metal in the distribution vessel, but no molten metal from the distribution vessel is dissipated.

Another expedient measure for rapidly achieving the at least approximately stationary runner bath level is that the supply of the molten metal into the distribution vessel until reaching a distributor bath level of 5% to 50%, preferably 10% to 30%, of the at least approximately stationary runner bath level takes place at approximately maximum filling rate and then the supply of molten metal in the distribution vessel, until reaching the at least approximately stationary Angussbadspiegelhöhe, with respect to the approximately maximum filling rate reduced filling rate. By "approximately maximum fill rate" is meant that the supply of the molten metal into the tundish at maximum or near maximum opening of the pan pusher. An approximately maximum filling rate in the context of the invention is given when 80% or more of a theoretically possible filling rate is achieved. This also prevents freezing of the slide slide opening in the Angießphase or a significant narrowing of the flow opening and thus reducing the flow rate.

Instead of the level of the bath level in the distribution vessel, the equivalent filling weight of the molten metal in the distribution vessel can also be used as the determining measured variable, for example for the supply amount of molten metal with maximum filling rate.

The reduced filling rate does not represent a constant value over the remaining filling time, but follows a continuous or stepwise decreasing time course, whereby the flow conditions in the distribution vessel continuously calm down.

It is also advantageous if the supply of the molten metal into the distribution vessel until immersion of the shadow tube in the introduced into the distribution vessel molten metal with approximately maximum filling rate and the supply of the molten metal in the distribution vessel then until reaching the at least approximately stationary Angussbadspiegelhöhe with a opposite the approximately maximum filling rate reduced filling rate takes place. The introduction of the molten metal below the bath level substantially reduces the bath movement on the metal bath surface.

To soothe the molten metal in the distribution vessel, it is expedient if the supply of molten metal into the distribution vessel is interrupted for a certain period of time upon reaching the at least approximately stationary runner bath level height. The closing of the pan slide after reaching the at least approximately stationary Angussbadspiegelhöhe has the advantage that existing foreign inclusions, especially non-metallic inclusions, float much faster at the bathroom mirror and in the slag, if a covering was already abandoned, can be deposited. The short-term interruption of the supply of melt is a good way to increase the quality of the molded product, while at the same time ensuring that the reopening of the pan slide is assured after this calming and deposition phase.

But it is also quite possible not to interrupt the supply of melt in the distribution vessel and immediately after reaching the at least approximately stationary Angussbadspiegelhöhe start with the filling of the continuous casting mold and thus initiate the stationary casting operation. This, however, provides a reduced time for the separation of foreign particles, which, however, can be compensated for by a different distribution of the filling rate.

Preferably, the period of interruption of the melt supply is between 8 seconds and 10 minutes, preferably between 60 and 270 seconds.

To avoid reoxidation on the metal bath surface, a covering agent is usually applied to the melt bath. Covering agents usually consist of a cover powder and form a slag layer.

A favorable time for the abandonment of a covering means is given if within a period of time which precedes the time of reaching the at least approximately stationary sprue level, the lowest possible filling rate is set and kept constant and, within this period, in particular in the second half thereof Covering agent is applied to the melt bath.

A further expedient possibility for the task of a covering means is given when the initial covering of the free bath surface with a covering means begins within the period of interruption of the supply of melt. Thus, the covering is applied only at a time at which already a substantial calming of the bath mirror has occurred. In contrast, an application of the covering at maximum filling rate would lead to a significant entry of foreign particles in the melt and an inhomogeneous distribution of the covering on the melt bath, since the velocities of the liquid phase at the bath level are quite 5 to 10 times higher than the largely stationary operation. The intensity of the surface turbulence increases with the square of the surface velocity. In addition, in this transient phase also pronounced Badspiegelwelligkeiten be expected, which favors the flooding of the covering in the metal bath.

Appropriately begins the first time covering the free bath surface with a covering in a period of time from 30 seconds, preferably from 8 seconds, before the resumption of the supply of melt after the interruption of the supply of melt.

It is also useful if the covering of the free bath surface with a covering at the earliest in a period of time before the beginning of the derivation of Molten metal from the distribution vessel begins, which preferably corresponds to at most half of the period of interruption of the supply of molten metal.

A further advantageous embodiment results when the covering of the free bath surface with a covering agent begins only after casting of the continuous casting mold.

So that the covering means is not drawn into the molten metal along the outer wall of the shadow tube in the vicinity of the shadow tube immersed in the molten metal, it is expedient if the region of the free bath surface surrounding the shadow tube in the distribution vessel is shielded by a covering with a covering means becomes. This is preferably done by shielding, which are formed by wall elements that either dip from above into the melt bath or protrude from below from the melt bath and surround the shadow tube at a distance. This purposefully creates a "hot spot" around the shadow tube and it is expedient if the wall elements form a closed chamber in which the shadow tube is integrated and the atmosphere enclosed in the chamber is rendered inert.

After resuming the supply of molten metal into the tundish, this supply of molten metal into the tundish is quantitatively regulated in response to the discharge of the molten metal from the tundish. The transfer of the molten metal from the distribution vessel into the downstream mold begins in time with the resumption of the supply of molten metal into the distribution vessel. Thus, the quasi-stationary Angussbadspiegelhöhe is largely maintained at a constant level.

The amount of molten metal supplied to the tundish and the amount of molten metal discharged from the tundish are substantially stationary when a steel strip is cast, with a casting thickness of 1.0 - 5.0 mm and a casting width of 1.0 m to 2.0 m Casting operation between 0.5 t / min and 4.0 t / min, preferably between 0.8 t / min and 2.0 t / min. This information refers to the use of a two-roll casting machine with the desired cast product and appropriate design.

Preferably, the capping agent is applied to the bath surface of the molten metal in the tundish in a surface area of low surface flow rate, waviness of the bath surface, and turbulence intensity.

A case by case manual task of the cover means requires adequate accessibility of the distribution vessel for the operator and also brings with the disadvantage of additional slag inclusions by the sudden local task of a larger amount of the covering with it. The covering agent is therefore applied in fine-grained form or in powder form, preferably with a semi-automatic or fully automatic feeding device.

The interior of the distribution vessel is shielded by a distributor cover against the free atmosphere, wherein it is expedient if during or before the Erstfüllphase an inerting of the distribution vessel takes place in order to largely eliminate the reactive oxygen inside the distribution vessel.

The setting and monitoring of the operating casting level height is preferably carried out via a distributor weight measurement or with an equivalent measuring method for level measurement. The runner level or the at least approximately steady runner level may also be determined by other direct or indirect measuring methods, such as, e.g. with floats, optical observation of the bath mirror surface, sound level measurement, eddy current measurement and similar measuring methods.

When the casting plant is restarted, the distribution vessel is free of molten metal at the beginning of the first filling phase and expediently also free of covering agents or slags, as well as refractory residues.

Fig. 1

eine schematische Darstellung einer Zweiwalzengießanlage mit einem Schmelzenbehälter und einem Verteilergefäß zur Durchführung des erfindungsgemäßen Verfahrens,

Fig. 2

den Verlauf einer Anfahrkurve für das Füllen des Verteilergefäßes (Füllrate) nach dem erfindungsgemäßen Verfahren in einer ersten Ausführungsform,

Fig. 3

den Verlauf einer Anfahrkurve für das Füllen des Verteilergefäßes (Füllrate) nach dem erfindungsgemäßen Verfahren in einer zweiten Ausführungsform,

Fig. 4

den zeitlichen Verlauf des Verteilergewichtes während des Füllens des Verteilergefäßes,

Fig. 5

den Verlauf von Anfahrkurven für das Füllen des Verteilergefäßes und der Stranggießkokille nach einer dritten Ausführungsform,

Fig. 6

ein Schattenrohr mit einer mechanischen Abschirmung gegen Kontakt mit Schlacke.

Further advantages and features of the present invention will become apparent from the following description of non-limiting embodiments, reference being made to the attached figures, which show:

Fig. 1

a schematic representation of a Zweiwalzengießanlage with a melt container and a distribution vessel for carrying out the method according to the invention,

Fig. 2

the course of a starting curve for the filling of the distribution vessel (filling rate) according to the inventive method in a first embodiment,

Fig. 3

the course of a starting curve for the filling of the distribution vessel (filling rate) according to the method according to the invention in a second embodiment,

Fig. 4

the time course of the distributor weight during the filling of the distributor vessel,

Fig. 5

the course of start-up curves for the filling of the distribution vessel and the continuous casting mold according to a third embodiment,

Fig. 6

a shade tube with a mechanical shield against contact with slag.

Fig. 1 shows a schematic representation of a Zweiwalzengießmaschine as a way to carry out the method according to the invention with the essential structural components for feeding the molten metal in the two oppositely rotating casting rolls 1, 2 and the end faces of the casting rollers can be pressed side plates 3 continuous casting mold 4. The molten metal is from a melt container 5, which is usually formed by a replaceable supported on fork arms 6 of a ladle turret ladle, passed through a shadow tube 7 in a distribution vessel 8. The shade tube 7 is associated with a slide closure 9 as a control element for the flow rate. From the distribution vessel 8, the molten metal flows through a submersible pouring tube 10 into the mold cavity 11 of the continuous casting mold 4. The submersible pouring tube 10 is also associated with a slide closure 12 for controlling the quantity of melt to be supplied to the continuous casting mold 4. The closure members can also be formed by plugs, which, projecting from above through the melt bath, controllably close the outflow opening of the respective melt container.

The amount of intermediate metal melt stored in the distribution vessel 8 is kept as constant as possible during the stationary casting process. This is achieved by setting a predetermined pouring height h of the molten metal in the distributor vessel and by keeping this level of pouring means largely intact by means of an inflow quantity control. A largely constant Gießspiegelhöhe ensures a uniform melt transfer into the continuous casting mold. 4

On the cooled cylinder jacket surfaces of the casting rolls 1, 2 are formed in the melt pool strand shells, which are rolled in the narrowest cross section between the casting rolls to a metal strand 13 of predetermined thickness and width, which is further discharged continuously from the casting plant.

Before the casting process is initiated with the melt supply, the continuous casting mold is prepared for the casting start, wherein the casting gap between the casting rolls is closed by a starter strand or appropriate provisions are made for a casting start without startup strand. Such a start-up method without using a Anfahrstranges is for example in the previously unpublished Austrian patent application A 1367/2002 described.

A melt container filled with molten metal is introduced into its pouring position above the distributor vessel. The filling process of the distribution vessel takes place in a possible embodiment according to the in Fig. 2 illustrated filling curve course. In the first filling phase (time interval t ₀ -t ₁ ), the molten metal is passed into the distributor vessel at the greatest possible opening of the slide closure, ie the molten metal enters the distribution vessel at approximately maximum filling rate ṁ _{fill, max} , the slide closure being kept closed on the outlet side of the distributor vessel becomes. From reaching a Badspiegelhöhe h _pool , which corresponds to approximately 40% of an at least approximately stationary Angussbadspiegelhöhe h _{pool, op} at time t ₁ , the filling rate is substantially continuously withdrawn until the at least approximately stationary Angussbadspiegelhöhe h _{pool, op is} reached.

Fig. 3 shows a further embodiment of a possible Füllkurvenverlaufes, wherein the fill rate ṁ _fill after reaching about 40% of at least approximately stationary Angussbadspiegelhöhe at time t ₁ gradually decreased in several stages is, wherein the reduction of the filling rate in the individual times t ₁ to t ₅ takes place so that a degressive approximation of the Badspiegelhöhe h _pool to the Angussbadspiegelhöhe h _{pool, op} is done.

Fig. 4 shows the increase of the distributor weight m _v over the filling time, starting from the empty weight m _{0 of} the distributor vessel to the distributor weight m ₅ , which is achieved upon reaching the at least approximately stationary Angussbadspiegelhöhe h _{pool, op} .

These illustrated filling curve courses already favor during the continuous filling process a subsidence of violent bath movement in the distribution vessel and in particular calm the metal bath surface. This settling phase in the distribution vessel is enhanced by interrupting the supply of melt for preferably a few minutes after reaching the at least approximately stationary Angussbadspiegelhöhe. Within this period, the task of a covering agent on the metal bath surface is started, which is carried out with a semi- or fully automatic feeding device 15 whose outlet opening opens above the bath level in a region of the distribution vessel with little surface turbulence ( Fig. 1 ). The fine-grained to dust-like covering agent is applied to the molten metal in a continuous trickle. This process is continued until complete coverage of the metal bath in the distribution vessel and if necessary repeated at any time during the casting process.

In addition, the distribution vessel 8 is covered with a distributor cover 16, with which the interior of the distribution vessel is shielded from the atmosphere ( Fig.1 ). This also gives the possibility to carry out an inerting of the interior even before the supply of molten metal.

Substantially at the same time as the re-introduction of the supply of melt into the distributor vessel, the introduction of the molten metal into the continuous casting mold, or its filling and the initiation of the continuous casting operation, begins. In this case, the amount of metal melt supplied to the distributor vessel is set as a function of the amount of melt introduced into the continuous casting mold. An operating bath level for the stationary casting operation may well deviate from the runner bath level and will be adjusted in the first phase of the stationary casting operation or as needed.

Deviations of the bath level from the desired at least approximately stationary sprue level or operating bath level are detected via a manifold weight measurement. As a result, a measured variable characteristic of the level of the bath level is continuously determined and used in a supply control loop for controlling the inflowing molten metal quantity as a control variable. For this purpose, the distribution vessel 8 is supported via measuring cells 17 on a support frame 18, for example a movable distribution carriage.

Fig. 5 shows in analogy to Fig. 2 the filling process of the distribution vessel on the basis of the filling rate ṁ _v and the Badspiegelhöhe h _pool in the time-dependent course. The molten metal is introduced in a filling phase t ₀ to t ₁ at the maximum opening of the slide valve closure on the melt vessel in the manifold and the filling process then continued with decaying filling rate in the period t ₁ to t ₄ . In a period of time before reaching the at least approximately stationary level of the bath level h _{pool, op} , which extends in the time interval t ₄ to t ₅ , the supply of melt takes place at a filling rate which is substantially reduced compared with the maximum filling rate but largely kept constant. The above-described interruption of the supply of melt is omitted here. At about midway between t ₄ and t ₅ , the task of covering means P on the melt bath in the distribution vessel begins. From the time t _{5 of} the casting operation begins with the filling of the continuous casting mold, wherein the filling rate of the mold ṁ _{m has} a time course, such as in Fig. 5 shown in the bottom diagram. At the same time, the fill rate in the distribution vessel ṁ _{v is} adjusted to an operating _{bath level} h _{pool, op} .

In Fig. 6 a possibility is shown which is intended to largely exclude the entry of masking agent applied to the melt bath into the interior of the melt bath in the region of the outer wall of the shadow tube 7. To the accumulated in the distribution vessel 8 molten metal flows through the vertically immersed in the melt shadow tube 7 more molten metal from the melt container 5 to continuously. The inflowing molten metal creates a suction action along the shadow tube 7 and, if necessary, draws slag / covering agent collected in this area downward into the molten metal. With a cover 21, which is cup-shaped, which surrounds the shadow tube with a radial distance to this and projects from above into the molten metal, the formed slag layer 20 is kept away from the critical area near the shadow tube. The interior of this upwardly closed cover 21 can over the Inert gas line 22 are rendered inert if necessary. The shadow tube 7 is located in the outflow direction of the molten metal only schematically illustrated flow-damping element 23 (turbo stop) firmly anchored in the distribution vessel, whereby the inflowing metal beam is strongly braked and redirected targeted.

The starting method described has been found to be particularly successful in connection with a distribution vessel, which in the WO 03/051560 is described and has a geometry that promotes the deposition of extraneous particles especially.

Claims (20)

1. Process for producing a high-purity cast metal strand from a metal melt, preferably a steel melt, in which the metal melt is fed in a controlled manner from a melt vessel (5) to a tundish (8) and is discharged in a controlled manner from this tundish into a continuous-casting mould (4), and the metal melt being covered by a covering agent at least in the tundish during steady-state casting operation, characterized in that the free bath surface of the metal melt which forms in the tundish during an initial filling phase is at least partially covered with a covering agent after a predetermined, at least approximately steady-state casting pool level (h_pool,op) has been reached.
2. Process according to Claim 1, characterized in that until the at least approximately steady-state casting pool level is reached, metal melt is fed into the tundish but no metal melt is discharged from the tundish.
3. Process according to one of the preceding claims, characterized in that the supply of metal melt into the tundish takes place at approximately the maximum filling rate (ṁ _fill,max) until a pool level of from 5% to 50%, preferably from 10% to 30%, of the at least approximately steady-state casting pool level is reached, and the supply of the metal melt into the tundish then takes place at a filling rate which is reduced compared to the approximately maximum filling rate until the at least approximately steady-state casting pool level is reached.
4. Process according to Claim 3, characterized in that the reduced filling rate follows a time curve which decreases continuously or in steps.
5. Process according to Claim 1 or 2, characterized in that the supply of metal melt into the tundish is carried out at the maximum filling rate until the shroud starts to be immersed into the metal melt introduced into the tundish, and the supply of metal melt into the tundish is then carried out at a filling rate which is reduced compared to the approximately maximum filling rate until the at least approximately steady-state casting pool level is reached.
6. Process according to one of Claims 3 to 5, characterized in that a filling rate which is as low as possible is set and kept substantially constant during a period of time which precedes the instant at which the at least approximately steady-state casting pool level is reached, and the covering agent is applied to the melt pool during this period of time, in particular the second half of this period of time.
7. Process according to one of Claims 1 to 5, characterized in that the supply of metal melt into the tundish is interrupted for a period of time when the at least approximately steady-state casting pool level is reached.
8. Process according to Claim 7, characterized in that the period of time for which the supply of melt is interrupted is between 8 sec and 10 min, preferably between 60 and 270 sec.
9. Process according to one of the preceding claims, characterized in that the initial covering of the free bath surface with a covering agent begins during the period of time when the supply of melt is interrupted.
10. Process according to one of Claims 1 to 7, characterized in that the initial covering of the free bath surface with a covering agent commences in a period of time starting from 30 sec, preferably from 8 sec, prior to resumption of the supply of melt after the supply of melt has been interrupted.
11. Process according to one of the preceding Claims 1 to 7, characterized in that the covering of the free bath surface with a covering agent commences at the earliest during a period of time prior to the commencement of the discharge of metal melt from the tundish, which period of time preferably corresponds to at most half the period of time for which the supply of metal melt is interrupted.
12. Process according to one of Claims 1 to 5, characterized in that the covering of the free bath surface with a covering agent only commences after initial casting into the continuous-casting mould.
13. Process according to one of the preceding claims, characterized in that that region of the free bath surface in the tundish which surrounds the shroud is kept free of coverage with a covering agent.
14. Process according to one of the preceding claims, characterized in that after the supply of metal melt into the tundish has resumed, this supply of metal melt into the tundish is quantitatively controlled as a function of the discharge of the metal melt from the tundish.
15. Process according to one of the preceding claims, characterized in that the quantity of metal melt fed to the tundish and the quantity of metal melt discharged from the tundish during casting of a steel strip on a two-roll casting installation in the substantially steady-state casting operation is between 0.5 t/min and 4.0 t/min, preferably between 0.8 t/min and 2.0 t/min.
16. Process according to one of the preceding claims, characterized in that the application of the covering agent to the bath surface of the metal melt takes place in a surface region with a low surface flow velocity, waviness of the bath surface and turbulence intensity.
17. Process according to one of the preceding claims, characterized in that the covering agent is applied in fine-grain or powder form, preferably using a semi-automatic or fully automatic application device.
18. Process according to one of the preceding claims, characterized in that the tundish is inerted during the initial filling phase.
19. Process according to one of the preceding claims, characterized in that the setting and monitoring of the operational casting level are effected by means of a tundish weight measurement or by using an equivalent measurement method for level measurement.
20. Process according to one of the preceding claims, characterized in that the tundish is free of metal melt when the initial filling phase commences.

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