EP1697070A2

EP1697070A2 - Sequential casting method for the production of a high-purity cast metal billet

Info

Publication number: EP1697070A2
Application number: EP04819582A
Authority: EP
Inventors: Erfindernennung liegt noch nicht vor Die
Original assignee: Voest Alpine Industrienlagenbau GmbH; Siemens VAI Metals Technologies GmbH and Co
Current assignee: SIEMENS VAI METALS Technologies GmbH
Priority date: 2003-12-02
Filing date: 2004-11-10
Publication date: 2006-09-06
Anticipated expiration: 2024-11-10
Also published as: ES2327646T3; WO2005053877A3; DK1697070T3; AU2004295039A1; AT413084B; WO2005053877A2; KR20060121255A; US20080173423A1; ATA19272003A; DE502004009542D1; KR101165478B1; US7789123B2; EP1697070B1; ATE432135T1; AU2004295039B2

Abstract

The invention relates to a sequential casting method for the production of a high-purity cast metal billet from a metal melt, wherein the metal melt is supplied from a melt container in a controlled manner to a distributor vessel and is then supplied from said distributor vessel in a controlled manner into a continuous casting ingot mold without any interruption. According to the invention, in order to cast a qualitatively high-value metal billet even when the melt vessel is changed and to ensure that the restart phase is kept as short as possible, the flow rate into the distributor vessel is greater than the outflow rate from the distributor vessel during a period of time ranging from the resumption of the supply of the metal melt to the distributor vessel until a quasi-stationary operating bath level height is obtained in the distributor vessel, wherein during 70 % - 100 % of said time period, the inflow rate into the distributor vessel is less than or equal to twice the outflow rate from the distributor vessel.

Description

Sequence casting process for the production of a cast metal strand of high purity

The invention relates to a sequence casting method for the continuous production of a cast metal strand of high purity from a molten metal, preferably a molten steel, the molten metal being fed from a melt container in a controlled manner to a distributor vessel and being discharged from this distributor vessel into a continuous casting mold in a controlled manner, and wherein the feed of molten metal into the distributor vessel is interrupted during the change of the melt vessel, while the supply of the molten metal is continued in the continuous casting mold.

Sequence casting is to be understood as a casting process in which several metal batches, which are delivered to the casting system in several melting vessels, are continuously cast into a single metal strand without interrupting the casting process. In this case, it is necessary to replace the melt vessel with another filled melt vessel in the shortest possible time after it has been emptied. Inevitably there is an interruption in the melt flow into the distribution vessel and it is necessary to measure the remaining amount in the distribution vessel so that a sufficient amount of residual metal melt is available to bridge the required changeover time in the distribution vessel until metal melt comes in again from the further melting vessel brought into the casting position the tundish can flow in. In order to be able to maintain the continuous casting process over the changeover time, it is common to reduce the casting speed of the casting system during the changeover time. The changeover time can be kept very short with a ladle turret.

The continuous caster itself can be equipped with a mold of any type, such as one or more oscillating plate or tubular molds, with caterpillar molds, with molds with revolving belts, or molds that are formed by rotating casting rolls with side insulation walls. The cross-sectional format of the metal strand to be cast can also be as desired, but this is particularly the case in the production of thin metal strips with strip thicknesses below 6.0 mm and strip widths over 800 mm special requirements for the start phase or restart phase of the casting process after a ladle change, because in particular because of the relatively small melt pool and the practically unchangeable metallurgical length up to the kissing point in a two-roll casting system, as well as the rapid solidification of a thin metal strand, a substantial reduction in the Pouring speed is not possible. Furthermore, it must be taken into account that when the melt is returned to the distribution vessel, there is an increased bath movement in the residual metal melt already covered with masking agent, and an increase in the amount of masking agent entering the metal bath occurs due to the increased wave formation on the bath surface. Furthermore, when the pan slide is opened, filling sand is entered into the distribution vessel, which requires a certain amount of time and a calming metal bath to float to the surface of the bath. The invention relates in particular to the casting of a metal strip with a two-roll caster according to the vertical two-roll casting process.

When producing a cast metal strand of high purity with any continuous casting installation, the liquid metal is usually fed from a ladle via at least one intermediate vessel or distribution vessel to a cooled mold in which the solidification process of the molten metal to at least one metal strand is initiated. The transfer of the molten metal from the ladle to the distribution vessel and from there to the mold is mainly carried out by immersion pipes or shadow pipes, which immerse in the molten pool of the downstream vessel in the stationary casting operation, thus ensuring a smooth and even flow and forwarding of the molten metal up to enable the mold. The metal melt accumulated in the ladle, the tundish and possibly in the mold is usually covered by a layer of slag, by means of which the metal surface is protected from oxidation. The basic arrangement of the melt receptacles in a multi-strand continuous casting plant for steel is known, for example, from US Pat. No. 5,887,647. The more intensely the metal bath movement in the individual melt vessels takes place, the more slag particles from the slag layer covering the metal melt are introduced into the metal bath and the more particles of the refractory material from the lining of the melt vessels are also fed to the metal bath by erosion. At the same time, the separation of foreign matter particles from the molten metal onto the metal bath surface or into the slag layer is hindered by excessive movement of the metal bath. With large-sized metal strands, such as strands with slab cross-sections, there is still time in the mold to separate foreign substances from the bath surface. In the case of small-sized strands and in particular in the case of strips of small thickness, the entry of foreign particles into the mold must be avoided as far as possible, since the possibilities for separating foreign particles tend to be more restricted in the mold.

It is generally known that the quality of the cast strand is reduced if there are strong fluctuations in the bath level, as are unavoidable in the initial phase of the casting process when the tundish is filled for the first time, or as they occur during the ladle change during sequence casting, which is usually done with the the change time of the pan is bridged in the metal melt in the distributor and is therefore poured with a continuously decreasing bath level. This greatly affects the stability of the melt flow in the distributor vessel 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 known prior art and to propose a sequence casting method of the type described at the outset, with which an increased entry of foreign particles into the molten metal and thus an analogue or increased one in the continuous casting mold also during the melting vessel change The entry of foreign particles into the solidification product is minimized and, in connection with the resumption of the quasi-stationary casting phase, a high-purity metal strand can be cast, in which this bridging phase can be kept as short as possible in the continuous casting process and with at least the effects of non-stationary casting phases , such as the melting vessel change, subside as quickly as possible.

This object is achieved according to the invention in that during a period of time from the resumption of the supply of molten metal into the distributor vessel until a quasi-stationary operating bath level in the distributor vessel is reached, the inflow rate into the distributor vessel is greater than the outflow rate from the distributor vessel, and during 70% up to 100%, preferably during 70% to 99%, in particular during 70% to 95%, this period of time the inflow rate into the distributor vessel is less than or equal to twice, preferably less than or equal to 1.5 times, the outflow rate from the distributor vessel , The minimum inflow rate into the tundish within this period depends very much on the reduction in the casting speed on the continuous caster during the change of the melting vessel. The inflow rate into the distribution vessel should, however, correspond to at least 0.5 times the maximum inflow rate during stationary casting operation during this period.

The term “distributor vessel” is not only limited to the receptacle for molten metal, by means of which the transfer or distribution of molten metal into a mold is made possible, thus directly preceding a mold, but can include all melt vessels between the ladle and the mold.

A further improvement in the quality of the cast strand from the resumption of the casting process is achieved if the supply of molten metal within the last 5% to 30% of the period from the resumption of supply of molten metal to the distribution vessel until the quasi-stationary operating bath level is reached an inflow rate is reduced compared to the inflow rate in the upstream period.

A shortening of the resumption phase of the casting process and a maximally secure opening of the melt container is achieved without impairing the quality of the cast product if the supply of molten metal immediately with resumption of the melt supply into the distribution vessel for 0.1% to 30%, preferably for 3% to 15% %, the period of time until the quasi-stationary operating bath level is reached in the distribution vessel with an essentially maximum inflow rate and the supply of molten metal then takes place until the quasi-stationary operating bath level is reached with a reduced filling rate.

“Maximum fill rate” is to be understood to mean that the metal melt is fed into the distribution vessel when the ladle slide opens at the maximum, that is to say with the maximum possible fill rate. This also freezes the ladle slide opening in the pouring phase or markedly narrows the flow opening and thus an undesirable reduction in the flow rate is avoided. The reduced filling rate does not necessarily represent a constant value over the remaining filling time until the quasi-stationary operating bath level is reached, but rather follows a continuously or gradually decreasing course of time, which means that the flow conditions in the tundish already calm down during the filling time.

To calm the molten metal in the distributor vessel, it can be expedient if the supply of molten metal into the distributor vessel is even interrupted for a certain period of time when the quasi-stationary operating bath level is reached. Closing the ladle slide after reaching the quasi-stationary operating bath level has the advantage that existing foreign inclusions, in particular non-metallic inclusions, float quickly on the bath level and can be separated into the covering slag. The brief interruption of the melt supply is a good way to increase the quality of the cast product, if at the same time it is ensured that the opening of the ladle slide is guaranteed after this calming and separation phase. The time period for the interruption of the melt supply is between 1 sec and 2 min, preferably between 10 sec and 70 sec, since the bath level immediately begins to decrease again due to the metal flowing off into the continuous casting mold.

To avoid reoxidation on the metal bath surface, a covering agent is usually applied to the melt bath at the start of the first casting sequence. This covering agent is retained in all the casting sequences in the distribution vessel. So that the covering means in the vicinity of the shadow tube immersed in the molten metal is not - even partially - drawn into the molten metal along the outer wall of the shadow tube, it is expedient if a region of the free bath surface in the distribution vessel directly surrounding the shadow tube is covered by a covering means is kept free or shielded at least during quasi-stationary operation, preferably continuously. This is preferably done by shielding means which are formed by wall elements which either dip into the melt bath from above or protrude from the melt bath from below and surround the shadow tube at a distance. In this way, a “hot spot” is created around the shadow pipe and it is expedient if the wall elements form a closed chamber into which the shadow pipe is integrated and the atmosphere enclosed in the chamber is rendered inert. It is important that the shielding agents are immersed in the melt bath to such an extent that they still dip into the distribution vessel even when the pan is being changed at a minimum bath level just before the melt is resumed. In this way, the slag-free zone around the shadow pipe is maintained even in this operating phase and the supply of molten metal with low turbulence in the metal bath below the bath surface is ensured.

If the supply of molten metal is briefly interrupted again after reaching the quasi-stationary operating bath level in the distribution vessel in order to additionally calm the bath movement and increase the separation rate of foreign particles, after the resumption of supply of molten metal in the distribution vessel, this supply of molten metal is stopped the distribution vessel is regulated in terms of quantity depending on the removal of the molten metal from the distribution vessel. The transfer of the molten metal from the distribution vessel into the downstream mold begins at the time when the supply of molten metal is resumed in the distribution vessel. With the control, the quasi-stationary operating pool level or the corresponding distributor weight is largely kept at a constant level.

Unless the melt supply into the distribution vessel is interrupted after the quasi-stationary operating bath level has been reached, at least during 70% to 100%, preferably during 70% to 99%, in particular during 70% to 95% of the period from the resumption of the supply of Metal melt in the distribution vessel until a quasi-stationary operating bath level in the distribution vessel is reached and / or from reaching the quasi-stationary casting level, the quantity of metal melt in the distribution vessel is regulated in terms of quantity depending on the removal of the metal melt from the distribution vessel. This regulation is based on a measurement of the current bath level height or the current distributor weight.

The amount of molten metal fed to the distributor vessel and the amount of molten metal discharged from the distributor vessel is between 0 when casting a steel strip with a casting thickness of 1.0-0.5 mm and a casting width of 1.0 to 2.0 m , 5 t / min and 4.0 t / min, preferably between 0.8 t / min and 2.0 t / min. This information relates to the use of a two-roll casting machine with the desired cast product of the appropriate design. In exceptional cases, it may be necessary to add cover agents in the distribution vessel. The covering agent is preferably applied to the bath surface of the molten metal in the intermediate vessel in a surface area with a low surface flow velocity, waviness of the bath surface and turbulence intensity.

A case-by-case manual application of the covering means requires sufficient accessibility of the distribution vessel for the operating personnel and additionally entails the disadvantage of additional slag inclusions due to the sudden local application of a larger amount of the covering means. The covering agent is therefore applied in fine-grained form or in powder form, preferably with a semi-automatic or fully automatic application device.

The interior of the distributor vessel is shielded from the free atmosphere by a distributor lid, it being expedient for the distributor vessel to be rendered inert during or before the initial filling phase in order to largely reduce the reactive oxygen inside the distributor vessel.

The setting and monitoring of the operating mold level is preferably carried out via a distributor weight measurement or with an equivalent measuring method. The operating pool level can also be determined using other direct or indirect measurement methods, such as e.g. b. with floats, optical observation of the bath surface, ultrasonic distance measurement, eddy current measurement and similar measuring methods.

In the case of sequence casting, the bath level in the distributor vessel decreases continuously during the change of the melt vessel, whereby the minimum bath level must not be fallen below, which depends very much on the shape of the distributor vessel and is therefore not generally determinable. If the level of the bath drops too far, especially in the resumption phase of the melt supply, in particular at maximum filling rate, there is an increased introduction of foreign particles into the metal melt, which spreads into the entire distributor vessel. In order to suppress or at least substantially dampen this effect, it is expedient if at least in the time period Between the resumption of the supply of the molten metal into the distribution vessel and the reaching of the quasi-stationary operating bath level, the metal melt contained in the distribution vessel is divided into two subsets by a compartment plate, a molten metal portion being fed from the melt container and a second subset of molten metal being fed into the Continuous casting mold is derived and a transfer of molten metal from the first subset to the second subset takes place continuously, the inflow rate to the first subset in the distribution vessel being greater than the outflow rate from the second subset, during 70% to 100%, preferably during 70% to 99 %, in particular during 70% to 95%, of the time period from the resumption of the supply of molten metal into the distributor vessel until the quasi-stationary operating bath level of the second subset in the distributor vessel reached the inflow rate to the first subset is less than or equal to twice the discharge rate from the second subset. The spatial division of the distribution vessel accordingly creates two areas, namely a first area in which great turbulence can occasionally occur and also essentially subside there, and a second area that remains largely isolated from this.

The positive effects from the spatial separation in the distribution vessel are additionally increased if the supply of molten metal within the last 5% to 30% J of the period from the resumption of supply of molten metal into the distribution vessel until the quasi-stationary operating bath level of the second subset is reached in the distribution vessel with an inflow rate that is reduced compared to the inflow rate in the upstream period.

The filling time can be shortened until the quasi-stationary operating bath level is reached if the supply of molten metal immediately with the resumption of the melt supply into the distribution vessel during 1% to 30%, preferably during 3% to 15%, of the period until the quasi - Stationary operating bath level of the second partial amount in the distribution vessel takes place with essentially maximum inflow rate and then metal melt is supplied with a reduced filling rate until the operating bath level of the second partial amount is reached in the distribution vessel. The transfer of molten metal from the first subset to the second subset, and thus from one region of the distributor vessel to the other part of the distributor vessel, takes place through one or more openings in the compartment plate. The metal melt can preferably be transferred from the first subset to the second subset through a free space in the compartment plate and the bottom of the distributor vessel. In this case, the compartment plate is not brought to the bottom of the distributor vessel.

However, it is also possible to design the compartment plate as a firmly anchored component of the distributor vessel and to provide at least one permanent flow channel near the bottom of the distributor vessel, which is entirely beneath the bath surface of the metal melt in all operating phases.

The quasi-stationary casting process begins when the quasi-stationary operating bath level is reached with the second subset of the molten metal in the second area of the distributor vessel. When this quasi-stationary operating bath level is reached for the second subset of the molten metal in the distribution vessel, the quantity of molten metal fed into the distribution vessel is regulated in terms of quantity depending on the removal of the molten metal from the distribution vessel. This regulation is based on a measurement of the current bath level height or the current distributor weight.

Further advantages and features of the present invention result from the following description of non-restrictive exemplary embodiments, reference being made to the accompanying figures, which show the following:

1 is a schematic representation of a two-roll casting plant with a melt container and a distributor vessel for carrying out the method according to the invention,

2 shows the course of a start-up curve for the refilling of the distribution vessel (filling rate) according to the inventive method in a first embodiment,

3 shows the course of a start-up curve for the refilling of the distribution vessel (filling rate) according to the inventive method in a second embodiment, 4 shows the time course of the distributor weight during the refilling of the distributor vessel,

5a shows the course of relevant process parameters during the change of a melt vessel according to a third embodiment of the invention,

5b shows the course of relevant process parameters during the change of a melt vessel according to a fourth embodiment of the invention

6 shows a shadow pipe with a shield against contact with slag,

7a shows a distribution vessel with a compartment plate in a first extended operating position,

Fig. 7b a distribution vessel with a .Abteilplatte in a second retracted operating position.

1 shows a schematic representation of a two-roll casting machine as a possibility for carrying out the method according to the invention with the essential structural components for feeding the molten metal into the continuous casting mold 4 formed by two casting rolls 1, 2 rotating in opposite directions and side plates 3 which can be pressed against the end faces of the casting rolls. The molten metal is transferred from a melt container 5, which is mostly formed by an interchangeable ladle supported on fork arms 6 of a ladle turret, through a shadow pipe 7 into a distributor vessel 8. A slide closure 9 is assigned to the shadow tube 7 as a control element for the flow rate or filling rate. The metal melt flows from the distribution vessel 8 in a quantity-controlled manner through an immersion pouring tube 10 into the mold cavity 11 of the continuous casting mold 4. The immersion pouring tube 10 is also assigned a slide closure 12 for regulating the amount of melt to be supplied to the continuous casting mold 4. The closure members can also be formed by stoppers which, projecting through the melt bath from above, controllably close the outflow opening of the respective melt container. The amount of molten metal stored in the distributor vessel 8 is kept as constant as possible during the continuous casting process. This is achieved in that a predetermined casting level h of the molten metal is set in the distributor vessel and this casting level is kept as constant as possible by regulating the flow rate. A largely uniform mold level ensures a uniform melt transfer into the continuous casting mold 4.

On the cooled cylindrical jacket surfaces of the casting rolls 1, 2, strand shells (not shown) form in the melt pool, which are rolled in the narrowest cross section between the casting rolls to form a metal strand 13 of predetermined thickness and width.

After the melt vessel 5 has been emptied, and the slag covering the metal melt in the melt vessel should not drain off as far as possible, the empty melt vessel is removed from the casting installation and brought into the casting position in the casting installation by a provided, filled melt vessel with metal melt prepared for the casting. During the period of about 2 minutes required for this, the casting process in the continuous casting mold is continued with the amount of melt remaining in the tundish, the operating bath level falling to a minimum bath level hp ₀₀ ι, _m i _n , at which the shadow pipe, however, is still immersed in the melt bath. Thus, when the melt supply is resumed in the distribution vessel, a direct impact of the metal melt on the slag layer covering the metal bath and thus its intensive mixing is avoided.

The filling process of the distribution vessel takes place according to a possible embodiment variant according to the filling curve profile shown in FIG. 2. There is a quantity of residual steel in the tundish that corresponds to a bath level h _p00 ). i _n corresponds. In a first filling phase (time period t ₀ - ti), the metal melt is passed into the distribution vessel with the greatest possible opening of the slide closure, ie the metal melt occurs with a maximum filling rate m πn. _max into the distribution vessel. Once a bath level h _p00 ] has been reached at time ti, the filling rate is essentially reduced continuously until the quasi-stationary operating _{bath level} h _poθ ι, op is reached, with 70% to 95% of the time period from the resumption of the supply of the metal melt in the distribution vessel until it reaches the quasi-stationary Operating _pool level h _poθ ι.opt the inflow rate into the tundish is smaller than twice the outflow rate from the tundish. At time t ₅ , the stationary filling rate m _st , which is characteristic of the stationary casting operation, is reached.

3 shows a further embodiment variant of a possible filling curve profile, the metal melt in a first filling phase (time period t ₀ -ti) with a maximum filling rate m _f iii. _m ax or approximately maximum filling rate (more than 80% of the maximum filling rate) and, after reaching the point in time ti, is gradually withdrawn in several stages, the reduction in the filling rate in the individual points in time ti to 1 ₅ taking place in such a way that a degressive approximation of the Bath mirror height h _p00 | to the operating pool level h _p ooi, op. At time t ₅ , the stationary filling rate m _{st is} reached, which is characteristic of the stationary casting operation.

Fig. 4 shows the increase in the distributor weight m _v over the filling time, starting from a distributor weight m ₀ , which corresponds to the empty weight of the distributor vessel and the weight of the residual melt amount remaining in the distributor vessel, up to the distributor weight m ₅ , which reaches the quasi-stationary operating bath level h _P o _t ι, ₀ p is achieved.

These filling curve profiles shown in FIGS. 2 and 3 already promote a fading away of the violent bath movement in the distribution vessel during the continuous filling process and in particular soothe the metal bath surface.

This calming phase in the distribution vessel can be further strengthened by temporarily interrupting the melt supply after the quasi-stationary operating bath level has been reached. Within this period of time or at any later point in time, a supplementary application of a covering agent can be carried out on the metal bath surface, if necessary, using a semi-automatic or fully automatic application device 15 (FIG. 1), the outlet opening of which above the bath level in one or more areas of the Distributor with little surface turbulence opens. The fine-grained to dust-like covering agent is applied to the molten metal in a continuous trickling process and is intended to ensure complete coverage of the metal bath in the distribution vessel. In addition, the distributor vessel 8 is covered with a distributor lid 16 with which the interior of the distributor vessel is shielded from the atmosphere. This also makes it possible to carry out an inertization of the interior even before the molten metal is supplied, in particular when the distributor vessel is filled for the first time.

When the quasi-stationary operating pool level is reached, the continuous casting operation is reintroduced. Here, the amount of the molten metal supplied to the distribution vessel is adjusted or regulated depending on the amount of melt introduced into the continuous casting mold from the distribution vessel. Deviations in the bath level height from the desired quasi-stationary operating bath level height are recorded using a distributor weight measurement. As a result, a measurement variable characteristic of the bath level is continuously determined and used as an actuating or control variable in an inflow control loop to regulate the amount of molten metal flowing in. For this purpose, the distributor vessel 8 is supported via measuring cells 17 on a supporting structure 18, for example a movable distributor carriage (FIG. 1).

5a shows the sequence casting method according to the invention using the example of a steel strip casting installation, with the course of characteristic parameters, such as the distributor _weight w _{t and} i _sh , the filling rate in the distributor m ι _adte , and the filling rate in the mold m _m0 | over a time axis _{d is} illustrated with a lead time, beginning before the change of a melt vessel is carried out, and with a lead time after the steady-state casting operation has been restarted. Measures are taken before the start of the change of vessel to facilitate the bridging of the changeover time of about 2 minutes by increasing the amount of melt available in the distribution vessel. This is done by increasing the filling rate m _{\ 3d \ e} by opening the slide closure on the melt pan, which means that more molten metal flows into the distributor vessel than flows into the continuous casting mold at the same time. As a result, the distributor weight increases to about 1.1 times the distributor weight in stationary casting operation. During the immediately following pan change, the filling rate in the tundish is: m ι _ad | _β = 0. At the same time, the casting speed in the belt casting machine is reduced and, if necessary, the casting level in the mold is lowered, so that the casting process in the continuous casting mold _is maintained with a reduced filling rate m _m0U i _d . As soon as the change of the melt vessel has been completed, the quasi-steady-state operating state is reached over a period of approximately 10 minutes The distributor vessel is restored by introducing molten metal into the distributor vessel up to a point in time with a maximum or approximately maximum fill rate and then, following a degressive curve, approaching the quasi-stationary operating pool level. The casting level in the distribution container, which is indirectly determined by a weight measurement follows the curve _do dis _h and displays in front of the vessel change the desired increase in the sense of supply and the subsequent drop to a value of about 80% of the distribution weight or Betriebsbadspiegelhöhe to Completion of the pan change.

According to a further embodiment, which is illustrated in FIG. 5b, the melt supply is resumed in the distribution vessel with a substantially reduced filling rate m ladiβ.start _> that is 0.8 to 1.2 times the filling rate m ι _a die, o _P t for stationary casting operation. This reduced filling rate can expediently be within a range of 0.5 to twice the filling rate m ι _ad iβ, opt. The filling rate is kept approximately constant over a wide range of times for the refilling of the distribution vessel. The basic advantage of this variant is the significantly lower rate of flow of the molten metal into the tundish, which results in significantly lower surface turbulence at the metal bath. The flow rate remains low enough to ensure a good separation rate of the non-metallic inclusions in the slag layer and to avoid re-entry of the slag. However, on the other hand, the time span for refilling the distribution vessel increases to up to 25 min with a simultaneously reduced filling rate in the mold. Depending on the steel quality to be cast and product requirements, an appropriate filling rate curve can be selected, which lies between the embodiments shown in FIGS. 5a and 5b.

6 shows a possibility which is intended to largely preclude the entry of covering means applied to the melt bath into the interior of the melt bath along or in the vicinity of the outer wall of the shadow tube. Metal melt flows continuously from the melt container 5 through the shadow tube 7, which is vertically immersed in the melt, to the metal melt already accumulated in the distribution vessel 8. The incoming molten metal creates a suction effect along the raw shadow and any slag / covering agent collected in this area is drawn down into the molten metal. With a cover 21 that is pot-shaped, which surrounds the shadow pipe at a radial distance from it and protrudes into the molten metal from above, the slag layer 20 formed is kept away from the critical area near the shadow pipe. The inside of this cover can be rendered inert via the protective gas line 22 if necessary. This cover expediently extends so far into the molten bath that the shadow tube can be immersed even at a minimum bath level height h _min . In order to continuously maintain the function of the cover 21, it is essential that the current bath level does not fall below the value h _min during the change of the melt vessel, ie it is imperative that the lower edge of the cover 21 is always immersed in the melt bath.

A flow-damping element 23 (turbostop) is firmly anchored in the distribution vessel opposite the shadow tube 7 in the outflow direction of the molten metal, as a result of which the liquid metal jet flowing into the distribution vessel is braked strongly.

The sequence casting method described has proven to be particularly successful in connection with a distributor vessel which is described in WO 03/051560 and has a geometry which particularly promotes the separation of particles foreign to the melt.

7a and 7b, a vertically movable compartment plate 24 is shown in two operating positions in connection with the distributor vessel 8. This embodiment is intended to achieve a functional separation in the distribution vessel. 7a shows the operating state in the distributor vessel immediately before being restarted with a new melt vessel. The molten metal still in stock in the distributor vessel is covered with a covering agent and flows off at a speed corresponding to the reduced casting speed. The compartment plate is still in a raised position and is lowered into the tundish to divide it into two areas as shown in Fig. 7b. With the retracted compartment plate, adverse effects during the first filling phase, which takes place with a maximum or approximately maximum filling rate, on the entire amount of melt in the distribution vessel are prevented, but at least greatly reduced. The melt feed is assigned to a first area 25, the discharge of the melt into the continuous casting mold is assigned to a second area 26. In the first area 25, the melt bath is substantially calmed down and one is separated Most of the foreign melt particles on the slag layer in the first area. In the second region 26, residual stocks of foreign particles still contained in the metal melt are separated into the slag layer covering the metal bath.

Claims

claims:

1. Sequence casting process for the continuous production of a cast metal strand of high purity from a molten metal, preferably a molten steel, the molten metal being fed from a melt container (5) to a distributor vessel (8) in a controlled manner and being discharged from this distributor vessel into a continuous casting mold (4), and wherein the supply of molten metal into the distribution vessel is interrupted during the change of the melt vessel, while the supply of the molten metal continues into the continuous casting mold, characterized in that during a period of time from the resumption of the supply of molten metal into the distribution vessel until a quasi stationary operating bath level in the distribution vessel, the inflow rate into the distribution vessel is greater than the outflow residues from the distribution vessel and wherein this Z for 70% to 100%, preferably 70% to 99%, in particular 70% to 95% The inflow rate into the tundish is less than or equal to twice, preferably less than or equal to 1.5 times, the outflow rate from the tundish.

2. Sequence casting method according to claim 1, characterized in that the inflow rate into the tundish corresponds to at least 0.5 times the maximum inflow rate in stationary casting operation.

3. Sequence casting method according to claim 1 or 2, characterized in that the supply of molten metal within the last 5% to 30% of the time period from the resumption of the supply of molten metal into the distribution vessel until reaching the quasi-stationary operating bath level with a compared to the Inflow rate in the upstream period of reduced inflow rate.

4. Sequence casting method according to one of the preceding claims, characterized in that the supply of molten metal immediately with the resumption of the melt supply into the distribution vessel during 0.1% to 30%, preferably during 3% to 15%, of the time period until the quasi-stationary operating bath level in the distribution vessel takes place with essentially maximum inflow rate and then, until the quasi-stationary operating bath level is reached, the supply of molten metal takes place with a reduced filling rate.

5. Sequence casting method according to one of the preceding claims 3 and 4, characterized in that the reduced filling rate follows a continuously or step-wise decreasing time profile.

6. Sequence casting method according to one of the preceding claims, characterized in that the supply of molten metal in the distribution vessel is interrupted for a period of time when the quasi-stationary operating bath level is reached.

7. Sequence casting method according to claim 6, characterized in that the time period of the interruption of the melt supply is between 1 sec and 2 min, preferably between 10 sec and 70 sec.

8. Sequence casting method according to one of the preceding claims, characterized in that a region directly surrounding the shadow tube of the free bath surface in the distribution vessel is kept free from a cover with a covering means at least during quasi-stationary operation, preferably continuously.

9. Sequence casting method according to one of the preceding claims 6 and 7, characterized in that after the resumption of the supply of molten metal in the distributor vessel, this supply of molten metal in the distributor vessel is regulated in terms of quantity depending on the removal of the molten metal from the distributor vessel.

10. Sequence casting method according to one of claims 1 to 8, characterized in that at least during 70% to 100%, preferably during 70% to 99%, in particular during 70% to 95% of the time period from the resumption of the supply of molten metal into the distributor vessel by Reaching a quasi-stationary operating pool level in the distribution vessel and / or from reaching the quasi-stationary operating pool level, the supply of molten metal to the distribution vessel is regulated in terms of quantity depending on the removal of the molten metal from the distribution vessel.

11. Sequence casting method according to one of the preceding claims, characterized in that the amount of the metal melt fed to the distributor vessel and the amount of metal melt discharged from the distributor vessel when casting a steel strip on a two-roll casting machine between 0.5 t / min and 4.0 t / min , preferably between 0.8 t / min and 2.0 t / min.

12. Sequence casting method according to one of the preceding claims, characterized in that, if necessary, a covering agent is applied to the bath surface of the molten metal in the distribution vessel and this task of a covering agent is carried out on the bath surface of the molten metal in a surface region with a low surface flow rate, waviness of the bath surface or turbulence intensity ,

13. Sequence casting method according to claim 12, characterized in that the covering agent is applied in fine-grained form or in powder form, preferably with a semi-automatic or fully automatic application device.

14. Sequence casting method according to one of the preceding claims, characterized in that the setting and monitoring of the quasi-stationary operating bath level is carried out by means of a distributor weight measurement or by an equivalent measuring method.

15. Sequence casting method according to one of the preceding claims, characterized in that at least in the period between the resumption of the supply of the molten metal into the distributor vessel and the reaching of the quasi-steady-state operating bath level, the molten metal contained in the distributor vessel is divided into two subsets by a compartment plate, a first one Partial amount of molten metal is fed from the melt container and is derived from a second subset of molten metal into the continuous casting mold and metal melt is continuously transferred from the first subset to the second subset, the inflow rate to the first subset in the distributor vessel being greater than the outflow rate from the second subset, while 70% to 100% , preferably during 70% to 99%, in particular during 70% to 95%, of the time period from the resumption of the supply of molten metal into the distribution vessel until the quasi-stationary operating bath level of the second partial amount in the distribution vessel, the inflow rate to the first partial amount is less than or equal to is twice the discharge rate from the second subset.

16. Sequence casting method according to claim 15, characterized in that the supply of molten metal within the last 5% to 30% of the period from the resumption of the supply of molten metal in the distribution vessel to reaching the quasi-stationary operating bath level of the second subset in the distribution vessel with a compared to the inflow rate in the upstream period reduced inflow rate.

17. Sequence casting method according to claim 15 or 16, characterized in that the supply of molten metal immediately with the resumption of the melt supply into the distribution vessel during 1% to 30%, preferably during 3% to 15%, of the period until the quasi-stationary operating bath level is reached the second subset in the distributor vessel takes place with an essentially maximum inflow rate and then, until the operating bath level of the second subset in the distributor vessel is reached, the supply of molten metal takes place with a reduced fill rate.

18. Sequence casting method according to one of claims 15 to 17, characterized in that the transfer of molten metal from the first subset to the second subset takes place through one or more openings in the compartment plate.

19. Sequence casting method according to one of claims 15 to 17, characterized in that the transfer of molten metal from the first Partial amount to the second partial amount through a space between the compartment plate and the bottom of the distribution vessel.

20. Sequence casting method according to one of claims 15 to 19, characterized in that when the quasi-stationary operating bath level is reached for the second subset of the molten metal in the distributor vessel, the supply of molten metal into the distributor vessel is regulated in terms of quantity depending on the removal of the molten metal from the distributor vessel ,