KR101148594B1 - Process for producing a high-purity cast metal strand - Google Patents

Abstract

The present invention relates to a method for producing a high purity cast metal strand from a metal melt, wherein the metal melt is fed from the melt vessel to the tundish in a controlled manner and discharged from the tundish into the continuous-casting mold in a controlled manner. The metal melt is covered by a covering agent at least in a tundish during steady state casting operations. In this way a free bath of the metal melt formed in the tundish during the initial filling step, in order to ensure that the high quality metal strands are cast and at least as short as possible immediately after the start of the steady state casting step. It is proposed that the surface be at least partially covered with a coating after the surface has reached a predefined at least approximate steady state casting pool level.

Description

High purity cast metal strand manufacturing method {PROCESS FOR PRODUCING A HIGH-PURITY CAST METAL STRAND}

1 is a side view of a two-roll casting plant with a melt vessel and a tundish for carrying out the method according to the invention.

2 is a graph showing a starting curve for filling (filling speed) a tundish using the first embodiment of the method according to the invention.

3 is a graph showing a starting curve for filling (filling speed) a tundish using a second embodiment of the method according to the invention.

4 is a graph showing a curve of time of tundish weight during filling of tundish.

5 is a graph showing a starting curve for the filling of a continuous-casting mold and a tundish according to a third embodiment of the present invention.

6 shows a lid in which contact with slag is mechanically prevented.

The present invention relates to a method for producing a high purity cast metal strand from a metal melt, preferably a steel melt, wherein the metal melt is fed from the melt vessel to the tundish in a controlled manner and in a controlled manner. The melt is discharged from the tundish into a continuous-casting mold, wherein the metal melt is covered by a covering agent at least in the tundish during a steady-state casting operation.

In particular, the invention relates to a method of starting a continuous-casting facility for producing high purity cast metal strands from metal melts, in particular steel melts. The continuous-casting facility will have a permanent mold of any intended design. In addition, any cross-sectional format of the metal strand to be cast may be used. However, in the beginning of the casting process, it is possible to produce strips that meet high quality requirements after the first few meters of the casting strip, particularly when trying to manufacture thin metal strips having a thickness of less than 6.0 mm and a width of 800 mm. Special requirements apply. In particular, the present invention relates to strip casting using a two-roll casting plant, which is illustratively based on a vertical two-roll casting method.

During the manufacture of high purity cast metal strands using any intended continuous-casting equipment, liquid metal is generally fed from the casting ladle via at least one tundish into a cooled mold and solidifies the metal melt to solidify the metal melt. The process for forming the is initiated at least in a cooled mold. The transfer of the metal melt from the casting ladle to the tundish is greatly influenced by the shroud, and the transfer of the metal melt from the tundish into the mold is submerged in the melt pool of the downstream vessel during the steady state casting operation. It is greatly influenced by the submerged pipe, which allows the metal melt to be immersed and flow and delivered as calmly and uniformly as possible into the mold. In the case of metal melts collected in casting ladles, it is common for the tundish (and possibly mold) to be covered by a slag layer which protects the surface of the metal bath from oxidation. The basic construction of a melt-holding vessel for a multi-strand continuous casting installation of steel is known, for example US Pat. No. 5,887,647. The stronger the movement of the metal bath in the individual melt vessel, the more slag particles enter the metal bath from the slag layer covering the metal melt, and the more refractory particles enter the metal bath from the lining of the melt vessel as a result of erosion. . At the same time, the separation of foreign matter particles from the metal bath surface or into the slag layer is hindered by excessively powerful movement of the metal bath. In the case of large-format metal strands, there is still time to separate foreign matter from the bath surface in the mold. In the case of small-format strands, especially in the case of strips of thin thickness, the introduction of foreign matter into the mold should be avoided as far as possible, since the possibility of separating and removing foreign matter particles in the mold is extremely limited.

It is generally known that the quality of address strands deteriorates when significant full level fluctuations occur, for example when ladles are exchanged at the start of the casting process during initial filling of the tundish or in a series of casting processes. In the case of such ladle exchange, casting is generally performed by means of a metal melt held in the tundish during the ladle exchange time and thus casting is carried out using a full level which is continuously reduced in the tundish. Thereby, the stability of the metal flow in the tundish is very negatively affected and the metal melt is exposed to undesirable slag inflow.

It is therefore an object of the present invention to obviate these disadvantages and difficulties of the known art, and to provide a method of the type described in the introduction, in which a high quality metal strand is to be cast immediately after the start of the steady state casting step. It is possible to make this starting step last as short as possible and to at least remove any effects resulting from the non-steady casting step as soon as possible.

According to the present invention, this object is that the free bath surface of the metal melt formed in the tundish during the initial filling step is at least partly covered with the coating after reaching a predefined at least approximately steady state casting pool level. Is achieved by the fact. Suitable coatings include all powdered or liquid formulations that form a substantially continuous protective layer that floats on the metal bath surface and thus completely prevent or prevent reoxidation on the metal bath surface.

As used herein, the term "tundish" is not limited to a holding vessel for a metal melt that is disposed upstream of the mold to allow the metal melt to be delivered or supplied into a permanent mold, and the metal melt is to be covered with a coating. It may include all melt vessels between the permanent mold and the casting ladle.

Until at least the near steady casting pool level is reached as quickly as possible and thus quickly passes through the severe turbulent, uncontrolled bathing movement in the tundish. It is desirable to allow the metal melt to be fed into the tundish while not discharging the metal melt from such tundish.

Further preferred means aimed at quickly reaching at least near steady state casting full level are at least 5% to 50% of near steady state casting full level, preferably at 10% to 30% tundish pull level. Feeding the metal melt into the tundish at nearly maximum fill rate until it is reached, and into the tundish at a small fill rate compared to the nearly maximum fill rate until reaching a near steady state casting full level. It includes supplying. The term “near maximum fill speed” may be understood to mean feeding the metal melt into the tundish with the ladle slide in the maximum or near maximum open position. In the sense of the present invention, the almost maximum filling speed means reaching 80% or more of the theoretically possible filling speeds. In addition, this prevents the ladle slide from freezing up in the initial casting step or prevents the flow opening from narrowing significantly, thereby preventing the quantitative flow from being reduced.

As an alternative to the full level in the tundish, for example for the quantitative supply of the metal melt at the maximum filling rate, the filling weight of the metal melt in the equivalent tundish may be used as a measurement parameter for the determination.

The reduced filling speed over the remaining filling time does not represent a constant value, but by following a time curve that is continuously or gradually reduced, the flow conditions in the tundish continue to stabilize.

The metal melt feed into the tundish is carried out at a near maximum fill rate until the lid begins to submerge into the metal melt introduced into the tundish, and then into the tundish until at least a nearly steady casting full level is reached. It is also preferred that the supply of the metal melt is carried out at a filling rate less than said almost maximum filling rate. The introduction of the metal melt below the bath level significantly reduces bath motion on the metal bath surface.

In order to stabilize the metal melt in the tundish, it is desirable to stop feeding the metal melt into the tundish for a period of time when at least the nearly steady state casting pool level is reached. Closing the ladle slides after reaching a near steady state casting pool level has the advantage that existing foreign matter inclusions, in particular nonmetallic inclusions, rise more quickly to the bath level and can then be separated into slag if the coating has already been added. . If at the same time it is reliably ensured that the reopening of the ladle slide follows this settling and separation step, a short interruption of the melt feed offers a great possibility of improving the quality of the cast product.

However, the filling of the continuous-casting mold may be initiated without interrupting the supply of the melt into the tundish and the steady state casting operation may be carried out at least immediately after reaching the nearly steady state casting full level. This means that the time required for the separation of foreign matter particles is shortened, but this may be compensated for by different distributions of filling rate.                         

Preferably, the time for stopping the supply of the melt is from 8 seconds to 10 minutes, preferably from 60 seconds to 270 seconds.

In order to avoid reoxidation on the metal bath surface, it is common to apply a coating to the melt bath. The coating generally comprises coating powder and forms a slag layer.

Desirable coating application moments are obtained when the slowest possible filling speed is set and kept constant for a period prior to the moment of reaching a near steady state casting full level, the coating during this time period, in particular It is applied to the melt pool during the second half of that period.

A more preferred option with regard to the application of the coating is provided when the application of the free bath surface with the coating begins during the time when the supply of the melt is stopped. The result is that the coating is only applied when the bath level has already stabilized significantly. In contrast, applying the coating when the fill rate is at its maximum can introduce significant foreign particles into the melt pool and cause non-uniform coating distribution on the melt pool, where the velocity of the liquid phase at the bath level is substantially This is because the speed is generally 5 to 10 times faster than during normal operation. The intensity of surface turbulence is increased by a factor that is the square of the surface velocity. In addition, significant bath level waviness in this temporary step also facilitates the inflow of the coating into the metal bath.

Preferably, the initial coating that begins to coat the free bath surface with the coating begins 30 seconds, preferably 8 seconds before the supply of the melt is resumed after the supply of the melt is stopped.

It is also desirable to begin applying the free bath surface with the coating at the earliest of the period before discharging the metal melt from the tundish, wherein the period before discharging the metal melt preferably stops supplying the metal melt. It is equivalent to less than half of the term.

Additionally, it is also desirable to cover the free bath surface with a coating only after the initial casting into the continuous-casting mold.

The area of the free bath surface in the tundish surrounding the sheath is covered with the sheath to prevent the coating (at least partially) from entering the metal melt along the outer wall of the sheath partially submerged in the metal melt in the region adjacent the sheath. It is desirable to shield what is. This is preferably achieved by shielding means formed by wall elements which penetrate into the melt pool from above or protrude out of the melt pool from below and enclose the sheath at a certain distance. This intentionally creates a hot spot around the lid, with the wall element preferably forming a closed chamber in which the lid is integrated and the atmosphere in the chamber is inert.

After the supply of the metal melt into the tundish is resumed, the supply of this metal melt into the tundish is quantitatively controlled as a function of the metal melt discharge from the tundish. Transferring the metal melt from the tundish to the permanent mold downstream begins with respect to time when the supply of the metal melt into the tundish is resumed. This maintains a quasi-steady state casting full level at a substantially constant level.

When the casting thickness is 1.0-5.0mm and the casting width is 1.0m to 2.0m, when the almost steady state casting operation is made, the amount of metal melt supplied into the tundish and the amount of metal melt discharged from the tundish are 0.5 t / min to 4.0t / min, preferably 0.8t / min to 2.0t / min. This enables proper design and makes it possible to produce the intended cast product using a two-roll casting device.

Preferably, the application of the coating to the bath surface of the metal melt takes place at surface areas with low surface flow rates, bath surface waves, and low turbulence strength.

Manually applying the coating has the disadvantage that the operator must have considerable access to the tundish, and the additional slag inclusion caused by the relatively large amount of coating applied locally. Thus, the coating is preferably applied in the form of fine powder, using a semi-automatic or fully automatic application device.

The interior of the tundish is shielded from the open atmosphere by a tundish cover, in which case it is preferred that the tundish be inerted during or before the initial filling step to substantially remove the reactive oxygen inside the tundish. .

Preferably, the setting and monitoring of the working casting level is made by tundish gravimetric measurement or by using an equivalent measuring method for level measurement. The working casting level or at least nearly steady state casting pool level may also be used, for example, using floats, visual observation of the full level surface, acoustic level measurement, eddy current measurement, and similar measurement methods. Determination can be made using direct or indirect measurement methods.

When the casting plant is restarted, the tundish at the beginning of the initial filling step preferably does not contain a metal melt and also does not contain residues of coatings, slags or refractory materials.

Additional advantages and features of the present invention will become apparent from the following description of exemplary embodiments with reference to the accompanying drawings.

1 shows a two-roll casting apparatus capable of carrying out the process according to the invention, the casting apparatus comprising main structural parts for supplying a metal melt into the continuous-casting mold 4, which mold It is formed by two casting rollers 1, 2 rotating opposite to each other and a side plate 3 which can be pressed against the end side of the casting roller. The metal melt is transferred into the tundish 8 through the lid 7 from the melt vessel 5 formed by the exchangeable casting ladle supported on the fork arm 6 of the ladle turntable. The lid 7 comprises a slide closure 9 as part for controlling the quantitative flow. Under quantitative control, the metal melt flows from the tundish 8 through the submersion pipe 10 into the mold cavity 11 of the continuous-casting mold 4. Similarly, the submersion pipe 10 includes a slide closure 12 for controlling the amount of melt fed to the continuous-casting mold 4. The closure members may also be formed by stoppers that protrude through the melt pool from above and close the outlet flow opening of each melt vessel.

During the steady state casting operation, the amount of metal melt temporarily held in the tundish 8 is kept as constant as possible. This is achieved by the predetermined height h of the metal melt set in the tundish, which is maintained substantially by quantitative control of the inflow flow. The substantially constant casting level ensures uniform delivery of the melt into the continuous-casting mold 4.

A strand shell (not shown) which is rolled in the narrowest cross-sectional area between the casting rolls to form a metal strand 13 having a predetermined thickness and width is continuously conveyed and discharged from the casting facility, and the casting roller 1, On the cooled cylindrical sides of 2) and in the melt pool.

Prior to starting the casting operation with melt feeding, a continuous-casting mold is prepared for casting initiation, where the casting nip between the casting rollers is closed by the initial strand or to start casting without the initial strand. Appropriate precautions are taken. A start-up method of the type that does not use an initial strand is described, for example, in Austrian patent application 1367/2002 (currently unpublished).

)로 턴디쉬내로 유입된다. t₁ 의 순간에서 적어도 거의 정상상태의 주조 풀 높이(h_{pool, op})의 약 40%에 상당하는 풀 레벨(h_pool)에 도달하면, 충진 속도는 적어도 거의 정상상태의 주조 풀 레벨(h_{pool, op})에 도달할 때까지 실질적으로 연속적으로 감소된다. The melt vessel filled with the metal melt is moved to a casting position above the tundish. In one possible variant embodiment, the filling operation of the tundish is carried out according to the filling curve shown in FIG. 2. During the first filling step (t ₀ -t ₁ period), with the slide closure on the discharge side of the tundish closed, the metal melt flows into the tundish with the slide closure open to the maximum possible extent, ie the metal The melt has an approximate maximum fill rate (

Into the tundish. At a moment of t ₁ , when the pool level h _pool is at least equivalent to about 40% of the nearly steady-state casting pool height h _{pool, op} , the filling rate is at least nearly steady-state casting pool level h _{pool. , op} ) is reduced substantially continuously until it is reached.

)는 t₁ 의 순간에서 적어도 거의 정상상태의 주조 풀 높이의 약 40%에 도달한 후에는 다수의 스테이지(stages)에서 단계적으로 감소되며, 충진 속도의 감소는 풀 레벨(h_pool)이 주조 풀 높이(h_{pool, op})에 체감적으로(degressively) 접근하도록 각각의 순간들(t₁ 내지 t₅)에서 실시된다. 3 shows another variant of a possible fill curve, wherein the fill speed (

) Decreases in stages in a number of stages after reaching at least about 40% of the nearly steady-state casting pool height at the moment of t ₁ , and the decrease in filling rate is indicated by the pool level h _pool . It is carried out at each of the instants t ₁ to t ₅ to degressively approach the height h _{pool, op} .

4 shows the tundish weight (m _v ) over the fill time when the tundish weight (m _v ) has reached at least a nearly steady casting pool height (h _{pool, op} ) from the tundish bin weight (m ₀ ). Increase to ₅ ).

Even during continuous filling operations, this depicted filling curve reduces the strong movement of the pool in the tundish and in particular stabilizes the metal pool surface. This steady state in the tundish is facilitated by intermediate feeding of the melt, preferably for several minutes after reaching at least the nearly steady state of the casting pool level. During this time, the coating begins to be applied to the metal pool surface by the semi-automatic or fully automatic application device 15, the discharge opening of the application device being opened above the pool height in the tundish region with low surface turbulence. (See Figure 1). The finely divided coating in the form of dust continuously falls onto the metal melt. This operation continues until the metal pool in the tundish is completely covered and is repeated when necessary during the casting operation.

The tundish 8 is also covered by a tundish cover 16 that shields the interior of the tundish from the atmosphere (see FIG. 1). It also makes it possible to make the internal space inert before the metal melt is fed.

Feeding the metal melt into the continuous-casting mold, or filling the continuous-casting mold and initiating the continuous casting operation begins substantially simultaneously with resuming feeding the melt into the tundish. The amount of metal melt fed into the tundish is determined as a function of the amount of melt introduced into the continuous-casting mold. The working pool level during the steady state casting operation can be very easily released from the casting pool level and can be set during the start phase of the steady state casting operation or as needed.

Deviations from the intended at least near steady state casting pool level or working pool level are recorded by tundish gravimetric measurements. As a result, the measurement variable which is an indicator of the full level is continuously measured and used as a control variable in the inflow control circuit for controlling the flow amount of the metal melt. For this purpose, the tundish 8 is supported via a measuring cell 17 on the support frame 18, for example on a movable tundish car.

) 및 풀 레벨(h_pool)을 기초로 턴디쉬의 충진 작업을 시간에 따른 곡선 형태로 도시한다. 충진 단계(t₀ 내지 t₁)중에, 금속 용융체는 용융체 용기의 슬라이드 폐쇄부가 최대 범위로 개방된 상태에서 턴디쉬내로 도입되며, 이어서 시간 구간 t₁ 내지 t₄에서는 감소되는 충진속도로 충진 작업이 계속된다. 시간 구간 t₄ 내지 t₅ 에 걸쳐진 적어도 거의 정상상태의 주조 풀 레벨(h_{pool, op})에 도달하기 전의 기간 동안, 용융체는 실질적으로 일정하게 유지되는 최대 충진 속도에 비해 상당히 감소된 충진 속도로 공급된다. 이러한 경우에, 전술한 용융체 공급의 중단은 발생하지 않는다. 시간 간격 t4 내지 t5 의 중간 정도에서, 피복제(P)는 턴디쉬내의 용융체 풀에 도포되기 시작한다. 시간 t5 로부터 연속-주조 몰드의 충진과 함께 주조 작업이 시작되며, 연속-주조 몰드의 충진 속도(

)는 예를 들어 도 5 의 아래쪽에 도시된 시간 곡선을 따른다. 동시에, 턴디쉬의 충진 속도(

)는 작업 풀 레벨(h_{pool, op})에 셋팅된다. Similar to FIG. 2, FIG. 5 shows the filling speed (

The filling operation of the tundish is shown in the form of a curve over time based on the level and h _pool . During the filling step t ₀ to t ₁ , the metal melt is introduced into the tundish with the slide closure of the melt vessel open to the maximum range, followed by filling operation at a reduced filling rate in the time intervals t ₁ to t _4. Continues. During the period before reaching the at least nearly steady state casting pool level (h _{pool, op} ) over the time intervals t ₄ to t ₅ , the melt is fed at a significantly reduced filling rate compared to the maximum filling rate which remains substantially constant. do. In this case, the interruption of the melt supply described above does not occur. In the middle of the time interval t4 to t5, the coating P begins to be applied to the melt pool in the tundish. The casting operation starts with filling of the continuous-casting mold from time t5, and the filling speed of the continuous-casting mold (

) Follow the time curve shown at the bottom of FIG. 5, for example. At the same time, the filling speed of the tundish (

) Is set at the job pool level (h _{pool, op} ).

6 shows exemplary options for substantially preventing the coating applied to the metal melt from entering the interior of the melt pool in the outer wall region of the lid 7. An additional metal melt flows continuously from the melt vessel 5 into the metal melt already accumulated in the tundish 8 through the lid 7 partially submerged in the melt of the tundish. The incoming metal melt creates a sucking action along the lid 7 and draws the slag / covering agent collected in the area into the metal melt. The formed slag layer 20 is held away from the critical area adjacent the cover by a cover 21 in the form of a pot, which covers the cover at a radial distance from the cover and into the metal melt from above. Extrude If necessary, a shielding gas line 22 may be inserted into the interior of this cover 21 with the upper end closed. Along the direction in which the metal melt flows out, a flow-damping element 23 (turbostop), shown only in contour form, is fixed in the tundish against the lid 7, and as a result The incoming metal jets are significantly slowed down and intentionally diverted.

The start-up method described above has proved particularly successful when used in combination with the tundish described in WO 03/051560, which has a geometry which particularly facilitates the separation of foreign matter particles into the melt.

The present invention allows high quality metal strands to be cast even immediately after the start of the steady state casting step, such that the starting step can last as short as possible and at least as quickly as possible any effect resulting from the non-steady state casting step. To be removed.

Claims (21)

The metal melt is fed from the melt vessel 5 to the tundish 8 in a controlled manner and discharged from the tundish into the continuous-casting mold 4 in a controlled manner, the metal melt being subjected to steady state casting operation. A method for producing a high purity cast metal strand from a metal melt, at least covered by a coating in a tundish, The free bath surface of the metal melt formed in the tundish during the initial filling step is at least partly covered with the coating after reaching the predefined steady state casting pool level (h _{pool, op} ) and the steady state casting pool. Supplying the molten metal into the tundish for a predetermined time when the level is reached. The method of claim 1, wherein the molten metal is not discharged from the tundish while the molten metal is supplied into the tundish until the steady state full cast level is reached. The method of claim 1 or 2, wherein the maximum filling speed until reaching a full level of 5% to 50% of the steady state casting full level (

Feeding the molten metal into the tundish at a reduced filling rate relative to the maximum filling rate until reaching a steady state casting full level. High purity casting metal strand manufacturing method. 4. The method of claim 3, wherein the reduced fill speed follows a time curve that decreases continuously or stepwise. The method of claim 1 or 2, wherein the metal melt supply into the tundish is carried out at the maximum filling rate until the lid begins to submerge into the metal melt introduced into the tundish and reaches a steady state casting pool level. Supplying the molten metal into the tundish until the filling rate is reduced rather than the maximum filling rate. 4. The method of claim 3, wherein a filling rate as slow as possible is set and kept constant for a period prior to the moment the steady state casting pool level is reached, wherein the coating is applied to the melt pool during the second half of this time period. A high purity cast metal strand manufacturing method characterized by the above-mentioned. delete The method of claim 1, wherein the supply of the melt is stopped for 8 seconds to 10 minutes. The method of claim 1 or 2, wherein the initial coating that begins to coat the free bath surface with the coating begins during a period in which the supply of the melt is interrupted. 3. The initial coating according to claim 1 or 2, wherein the initial coating which begins to coat the free bath surface with the coating starts in a period of 30 seconds before the supply of the melt is resumed after the supply of the melt is stopped. Method of manufacturing high purity cast metal strands. 3. The method according to claim 1 or 2, wherein the application of the free bath surface with the coating begins at the earliest of the periods prior to the discharge of the metal melt from the tundish, wherein the period before the discharge of the metal melt is reduced. A method for producing a high purity cast metal strand, characterized by less than half of the period of discontinuance. The method of claim 1 or 2, wherein the covering of the free bath surface with the coating is carried out only after the initial casting into the continuous-casting mold. The method of claim 1 or 2, wherein the area of the free bath surface in the tundish surrounding the sheath is kept uncovered with a coating. The method according to claim 1 or 2, wherein after the supply of the metal melt into the tundish is resumed, the supply of this metal melt into the tundish is quantitatively controlled as a function of the discharge of the metal melt from the tundish. A high purity cast metal strand manufacturing method characterized by the above-mentioned. 3. The method according to claim 1 or 2, wherein the amount of metal melt supplied into the tundish and the amount of metal melt discharged from the tundish during the casting of the steel strip in a two-roll casting plant in which the casting operation is performed in a steady state is 0.5. t / min to 4.0t / min. 3. The method for producing a high purity cast metal strand according to claim 1 or 2, wherein the coating of the coating on the bath surface of the metal melt is performed in a surface region having a low surface flow rate, a wave of the bath surface, and a turbulent strength. . 3. The method of claim 1 or 2, wherein the coating is applied in the form of micro-particles or powders. 3. The method of claim 1 or 2, wherein the tundish is inactivated during the initial filling step. Method according to claim 1 or 2, characterized in that the setting and monitoring of the working casting level is made by tundish gravimetric measurement or using an equal measuring method for level measurement. 3. The method of claim 1 or 2, wherein the tundish is free of metal melt when the initial filling step begins. The method of claim 17, Method for producing a high purity cast metal strand, characterized in that the coating is applied using a semi-automatic or fully automatic application device.

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