Plant For Continuous Casting Without Deep Casting Stream Penetration

Abstract

The invention proposes a plant for continuously casting liquid metals comprising a casting mould, a tundish arranged above the casting mould, adapted to be gastightly sealed and evacuated, and provided with a draining tube extending below the metal level in the mould, a supply ladle for liquid metal, and pneumatic means for conveying the metal from the supply ladle into the tundish, a chamber which is connected with a pneumatic pressure conduit and arranged below the tundish, into which chamber the supply ladle charged with liquid metal is inserted, an ascending pipe line starting from the bottom of the ladle, penetrating the lid of the chamber and leading into the tundish, the length of said ascending pipe line being dimensioned that the level difference between the metal in the tundish and the metal in the supply ladle exceeds 1.4 m, i. e. the height of the ferrostatic column at atmospheric pressure, and the length of the draining tube being dimensioned that the level difference between the metal in the tundish and the metal in the mould does not exceed 1.5 m. This plant is of great advantage because by releasing the atmospheric ferrostatic pressure the metal flows into the casting mould practically without exerting any pressure so that the solidifying casting shell is prevented from being damaged. The invention permits also of subjecting the steel to a highly effective vacuum treatment.

Description

The invention relates to a plant for continuous casting of hot liquid metals and to a continuous casting method in which this plant is used.

In known continuous casting plants the casting stream falling vertically into the casting mould penetrates deeply into the yet liquid core of the bar, which may damage the solidifying casting shell. When leaving the mould a damaged casting shell cannot resist the pressure of the metal melt contained in the interior of the bar and the liquid metal may break through so that the plant is damaged and the cast product is wasted. The casting stream penetrating into the liquid core of the bar does not permit casting of bars having flat cross sections of any desired small thickness, because the danger of damage to the casting shell increases greatly when the thickness of the bar is reduced. It has been attempted to reduce the kinetic energy of the metal falling into the mould, by using draining pipes with a closed bottom and laterally arranged mouths. However, such outlet tubes are difficult to manufacture, they are rapidly worn and have not shown the desired effect in practice. The invention is aimed at avoiding the deep penetration of the casting stream.

Further it is a task of the invention to provide for a casting plant, which may also be built into existing steel works in which casting cranes are used having a relatively small lifting height. Known casting plants are provided either with a straight mould, to which a longer vertical bar guide is joined, or with a circularly curved mould, to which a bar guide having the form of a quarter circle is joined. In any case, particularly in vertical plants, the construction height of the plant is considerable and the casting ladle has to be liftable from the mill floor level to the area of the mould. In steel works not designed to have a casting plant built in the lifting height of the casting cranes frequently is insufficient.

A further aim of the invention consists in obtaining an effective vacuum treatment of the steel in a plant for continuous casting; such a vacuum treatment may bring about apart from lowering the contents of hydrogen and non-metallic inclusions, also a reduction of the oxygen content in the steel. This, as well as an even and appropriate casting temperature is a prerequisite for the production of excellent bars when rimming steels are cast according to the continuous casting method. In a known plant liquid steel is sucked by vacuum from a ladle into a tundish of relatively large dimensions in which a bath of rather great depth is maintained. Despite of the considerable expenditure of equipment, the degassing effect is insufficient and the temperature of the metal in the tundish falls rapidly. A further disadvantage rests in the small level distance between the metal in the ladle and in the tundish, which level difference at a vacuum of e.g. 0.5 Torr amounts to maximally 1.4 m (ferrostatic pressure column). Therefore, ladles having a capacity of 50 - 300 mt of steel and a vertical extension of about 2.5 to 4 m and which are arranged at a lower level than the tundish, cannot be degassed continuously.

When alloy steels are cast with a low casting temperature in known continuous casting plants a metallurgical disadvantage resides in that the metals or alloys added into the ladle or during tapping are dissolved rather slowly so that the chemical composition of the bar varies greatly over its entire length.

The invention meets the mentioned tasks by avoiding the disadvantages referred to above; it starts from a plant for continuously casting liquid metals comprising a casting mould, a tundish arranged above the casting mould and adapted to be gastightly sealed and evacuated, which tundish is provided with a draining tube extending below the metal level in the mould, a supply vessel for liquid metal, and pneumatic conveying devices serving to convey the metal from the supply vessel into the tundish; and is, according to its main feature, characterized in that in a manner known per se a pressure chamber connected with a pneumatic conduit is provided below the tundish, into which pressure chamber a ladle charged with liquid metal may be inserted and that an ascending pipe line starting from the bottom of the ladle and penetrating the lid of the pressure chamber leads into the tundish, the length of said ascending pipe line being such that the level difference between the metal in the tundish and the metal in the ladle is always greater than 1.4 m, i.e. the height of the ferrostatic column at atmospheric pressure, and that the length of the draining tube is such that the level difference between the metal in the tundish and the metal in the mould is maximally 1.5 m so that when a vacuum is created the ferrostatic pressure is compensated and the metal flows into the mould practically without exerting any pressure. Obviously, the level difference between the metal in the tundish and the metal in the mould must not be smaller than 1.4 m when the tundish is completely evacuated, because otherwise the metal column in the draining tube would break. With a completely evacuated tundish the level difference thus lies between the mentioned limits of 1.5 and 1.4 m. When the tundish is not completely evacuated the level difference should correspond to the vacuum employed.

In the plant according to the invention the liquid metal does not get into contact with atmospheric oxygen so that no undesired oxidation may occur; thus the content of non-metallic inclusions is reduced and flaws caused by such inclusions are thus avoided in sheets produced from the cast bars.

A casting plant in which a ladle may be inserted into the pressure chamber which is connected by means of an ascending pipe line with a tundish, as is provided by the present invention, is known per se. In the known plant, however, the tundish cannot be evacuated so that neither the metal may be supplied into the mould without exertion of pressure nor is it possible to provide for vacuum treatment.

The lower part of the draining tube reaching into the casting mould may broaden in downward direction to be funnel-shaped. By providing for a continuous transition from a smaller to a larger cross section within the funnel-shaped part of the draining tube the speed with which the metal leaves the tundish is further reduced. When slabs are cast the cross section of the funnel-shaped part may also be oval.

Suitably the lower part of the draining tube and the casting mould are gastightly enclosed by a hood which is connectable with a conduit for the supply of a protective gas, e.g. argon, so that above the metal level in the casting mould a gas space subjected to slight overpressure can be produced. The drain suitably comprises two parts which are releasably connected with each other by means of connecting elements such as flanges and brackets. The lower part of the drain may be made of metal-ceramic material containing primarily molybdenum and zircon oxide.

The plant according to the invention may also include the features that the ladle is provided with a slanting bottom and the ascending pipe line starts from slightly above the lowest place of the ladle bottom; further, that the ascending pipe line is arranged eccentrically of the center axis of the ladle. Suitably, the pressure chamber and the ladle are arranged on a traveling vehicle.

The ascending pipe line may comprise an ascending tube which is connected with the lid of the pressure chamber and a supply tube which is inserted in the floor of the tundish, the upper end of the ascending tube and the lower end of the supply tube being releasably connected by means of a slide valve connection. In order to have sufficient space available when the ascending tube and the supply tube of the tundish are connected, the tundish is designed to be liftable and lowerable. Furthermore, the tundish may also be rotatable so as to provide for room whenever servicing work has to be carried out.

According to a preferred embodiment the tundish is provided with one or several draining tubes, each reaching into one casting mould, the lower ends of said draining tubes being situated somewhat below the connection level of the ascending tube and the supply tube. Thus, when the ascending tube and the supply tube are connected or disconnected, it is not necessary to lift the end of the draining tube out of the metal sump in the mould, so that any impurification of the steel in the mould owing to access of oxygen is avoided.

Preferably the tundish is provided with a slanting floor or is arranged to be slanting, the upper edge of the supply tube being situated on a higher level than the upper edge of the draining tube. With such an arrangement owing to the influence of the vacuum in the tundish the steel running over the upper edge of the supply tube is degassed in an explosion-like process in which the steel is distributed into a plurality of droplets and owing to the thus obtained large surface is subjected to a highly effective vacuum treatment.

In the tundish, above the mouth of the supply tube a highly refractory hollow body with a nozzle-like opening may be provided for changing the direction of the metal stream which is conveyed in upward direction through the pressure conduit.

The cross section of the ascending and supply tubes may further be greater than the cross section of the draining tube. Thus it becomes possible to quickly fill and drain the tundish.

The continuous casting plant according to the invention is particularly suitable for a method of continuous casting in which a plurality of ladles are cast successively without interruption. According to the present invention it is proposed for such methods to compensate temporarily the pneumatic pressure acting upon the liquid metal in the ladle or to reduce said pressure to a degree that metal flows back from the tundish into the supply ladle so as to equalize differences in concentration and temperature of the liquid metal. By employing this measure the bar obtains a highly uniform chemical composition over its entire length because an effective circulation of the stock of metal is achieved in the ladle on account of considerable portions of the liquid metal temporarily flowing back from the tundish into the ladle and the eccentric arrangement of the ascending tube. Metallic additions, e.g. manganese, introduced into the ladle prior to its being filled, dissolve under the influence of the circulatory movement and are evenly distributed. The temperature is qualized. Thereby, a non-oxidizing atmosphere may be maintained in the tundish.

When employing the plant of the invention a highly effective method for degassing the metal to be cast resides in that the tundish is evacuated while the metal is conveyed from the ladle into the tundish. The metal enters the evacuated tundish under the action of the pneumatic conveying pressure and is distributed into a plurality of droplets. Hence, the degassing effect is excellent.

Also when it is not intended to subject the liquid metal to a vacuum treatment the tundish may be kept under reduced pressure during casting so as to reduce the kinetic energy of the metal stream running from the tundish into the continuous casting mould and deep penetration of the metal stream into the liquid sump is avoided.

A non-oxidizing gas may be used for generating the pneumatic pressure acting upon the surface of the metal in the ladle.

In order that these and further features of the plant according to the invention and of the continuous casting method in which this plant is used may be more fully understood, they will now be carefully described with reference to the accompanying drawings, in which FIG. 1 is a vertical sectional view of the entire plant, FIG. 2 is a top view of a tundish with four draining tubes, FIG. 3 is a vertical sectional view of a tundish with two draining tubes, and FIG. 4 is a vertical sectional view of a tundish with one draining tube. FIGS. 5, 6, 7 are vertical sectional views showing preferred embodiments of tundishes. FIG. 8 shows a plant in which a draining tube broadening to be funnel-shaped is used, equally in a vertical sectional view.

In FIG. 1 numeral 1 denotes a ladle having a slanting bottom 2 and being filled with liquid steel 3 upon which a thin slag layer 4 is floating. This supply ladle 1 is inserted in a pressure chamber 5 comprising the bottom part 6 and the lid part 7 which is provided with a refractory insulation 8. The bottom part 6 and the lid part 7 are connected with each other by means of a flange connection 9 which may easily be opened. A duct 10 leads into the pressure chamber 5 through which compressed air or preferably a non-oxidizing gas, such as argon, may be introduced under pressure. A refractory ascending tube 12 is built into the lid 7 in a manner that its lower end comes to stand slightly above the lowest point of the ladle floor 2 when the lid 7 is placed onto the bottom part 6. The pressure chamber 5 is arranged on a vehicle 11 so that it may travel together with the casting ladle 1 on the floor level 36, beneath the pouring platform 35, to the continuous casting plant.

The tundish 13 which is designed in two parts, is arranged on the pouring platform 35. Preferably the tundish has a circular cross section; its dimensions are designed as small as possible in order to reduce the temperature loss of the steel. The tundish 13 comprises a closed sheet casing 14, 15. The bottom part is lined with highly refractory material 17, while the upper part may be made of refractory insulating material 16. The tundish 13 is arranged at an acute angle to the pouring platform 35. At the lowest point a supply tube 18 with a conical end piece 19 is vertically inserted into the bottom of the tundish. The supply tube 18 has the same diameter as the ascending tube 12. The tundish 13 is liftable and lowerable by means of a hydraulically operated cylinder 29. It may be lifted in vertical direction approximately up to the position 13' shown in dotted lines, which the tundish occupies prior to operation. The cylinder 29 is arranged on the pouring platform 35 to be rotatable. This revolving bearing is denoted by numeral 30 and serves to enable swinging of the tundish 13 so that re-lining is facilitated and the continuous casting mould 31 can be removed. The supply tube 18 of the tundish 13 is connected to a slide valve which may be designed as a rotary slide valve 25. The slide valve 25 is provided with a guide 26 into which the supply tube 18 engages from above and the ascending pipe 12 engages from below so that a form locking connection is achieved. The guide 26 is water cooled so as to prevent any breakthrough of the liquid steel in the area of the connection in case of leakages. Water supply and draining tubes are indicated with numerals 27 and 28, respectively.

The tundish 13 may be provided with a refractory stopper 21, which is operated by a pneumatic cylinder 22. This stopper serves to close the draining tube 20 or to regulate the outflowing amount of liquid steel. At the site where the stopper rod passes through the upper wall of the tundish it is surrounded by a casing 23 which provides a gastight sealing.

In working position the draining tube 20 immerses below the metal level 32 in the continuous casting mould 31. The cast bar 33 is withdrawn from the mould vertically downwardly and is gradually deflected. The guide rollers for the bar 33 are indicated by numeral 34.

Before casting is started, a non-oxidizing gas, preferably argon, is blown into the tundish 13 through the opening 24 of the tundish 13, and its interior, the ascending and supply tubes 12, 18 and the draining tube 20 are flushed and filled with the protective gas respectively so that the liquid steel cannot be oxidized when it enters and flows through the tundish. It is also possible to connect the opening 24 to a vacuum pump not shown in the drawing.

When operation is started, excess pressure is generated in the pressure chamber 5 by introducing compressed air or nitrogen via the conduit 10; this excess pressure acts upon the slag 4, and the steel 3 is thus conveyed into the ascending pipe 12 and into the tundish 13, at first to the level 37. Then, by releasing the pressure in the pressure chamber 5 the steel may be caused to flow back into the casting ladle 1 so that the steel is effectively mixed in the ladle and differences in concentration and temperature of the steel are eliminated. This process may be carried out prior or during casting and, if desired, repeatedly. During normal operation the liquid steel gains approximately the level as indicated at 38 in the tundish and flows continuously through the draining tube 20 into the casting mould 31. When the casting ladle 1 is drained completely except for a small remainder, the slide valve 25 is closed, after the steel in the tundish 13 has been permitted to gain the level 39. In order to be able to move the ladle away and to replace it by another one the pressure in the pressure chamber 5 is released and the tundish 13 is lifted some centimeters by means of the hydraulic cylinder 29 so that the supply tube 18 which is closed by means of the slide valve gets out of engagement with the ascending tube 12, while the bottom edge of the outflow 20 is still below the metal surface level 32. The vehicle 11 with the pressure chamber 5 and the emptied ladle 1 is removed and a second vehicle, carrying a second pressure chamber with a filled ladle is brought to replace it. The tundish 13 is lowered again and the connection with the ascending pipe line is reestablished. After pressure gas has been introduced into the pressure chamber, the slide valve 25 may be opened again so that the steel may flow into the tundish 13, in which the metal level has meanwhile fallen from level 39 to level 38. The amount of liquid metal stored in the tundish 13 has to be calculated on the period during which the ladles are exchanged. In this manner it is possible to cast liquid steel continuously and to keep the construction height of the entire plant very small as may be derived from FIG. 1. If the ladle 1 is arranged on a vehicle 11 it is possible to use a ladle without a stopper which greatly increases operation safety.

FIG. 2 shows a tundish 40 with four drains 20', 20", 20'", 20"" situated above the moulds for four bars 33', 33", 33'", 33""; FIG. 3 shows a tundish 41 with two outflows 20', 20" and the respective stoppers 21', 21". The mouth of the supply tube 18' is arranged to be lower than the upper openings of the draining tubes. In the embodiment according to FIG. 4 a recess 45 is provided in a tundish 42 which comprises an upper part 43 and a bottom part 44; this tank serves for the formation of a sump for the liquid metal which has been lifted to the level 37' through the supply tube 18'. In operation the metal reaches level 38' and, prior to the exchange of the ladles, level 39'. The capacity difference between level 39' and 38' is based on the period necessary for the exchange of vessels. The outflow 20' may also be provided with a stopper. Neither the vacuum pump nor the conduit for joining this pump to the tundish are shown in FIGS. 2 to 4.

In FIGS. 5, 6 and 7 preferred embodiments of tundishes 46, 58, 60 are shown which are used when the steel is to be degassed. In these embodiments the upper edge of the supply tube 18' is situated at the same height as the level 51 of the liquid metal in the tundish (FIG. 5) or above this level (FIG. 6). The tundish 46, 58, 60 may be connected to a vacuum conduit 49 which is arranged off the axis of the supply tube 18'.

Before the tundish 46 is evacuated the outflow 20' is closed with a pot-like cover 53 which is pressed onto an element 52 by means of fixing brackets 54 (FIG. 5). An appropriate vacuum sealing 55 provides for a gastight closure. The cover 53 may be made of steel e.g. As soon as a certain amount of steel is present in the tundish 46, the floor 56 of the cover 53 will melt through and the metal stream 57 may flow into the casting mould 31 which instance signifies the beginning of casting.

When the steel enters into the evacuated tundish 46, 58, 60 the steel is atomized in an explosion-like process into a plurality of droplets; thus its surface is extremely widened and the steel is rapidly and effectively degassed and there is no necessity of providing for a large surface 51 of the metal sump. In the drawing the metal droplets are diagrammatically shown and indicated by numeral 50. The lining is indicated with numeral 48; a perforated brick 47 serves for inserting a supply tube 18' having a conical shoulder 19' so that it may be exchanged from the outside (FIG. 5). The effect of the vacuum on the liquid steel can be improved if a highly refractory hollow body 61 with a nozzle-like bore is built in above the inlet tube 18' (FIG. 7). The hollow body 61 is arranged in a manner that the axis of its bore is slanted in relation to the axis of the supply tube 18' so that the metal droplets 50 cover as long a distance as possible in the tundish 60 before they impinge upon the metal sump.

The tundishes 42, 58, 60 may, of course, be also provided with stoppers which may be built in and operated similarly as described in relation with FIG. 1. The amount of metal flowing into the casting mould may be regulated by changing the metal level in the tundish or by actuating the stopper.

According to a preferred embodiment the tundishes are evacuated also during casting in order to reduce or compensate the kinetic energy of the metal stream entering the mould 31. When the height difference h (FIG. 7) between the melt level 32 in the mould 31 and the melt level 51 in the tundish 60 amounts to about 1.4 m and when a vacuum of about 0.5 Torr is employed the steel flows into the mould 31 practically without pressure because the ferrostatic pressure has been compensated. Thus a deep penetration of the steel into the liquid core 62 and damaging of the already solidified marginal layer 63 of the bar 33 is prevented. It is possible to use casting tubes 20' without bottoms, the manufacture of which is cheap and simple.

Also when tundishes according to FIGS. 5, 6 and 7 are used a portion of the liquid metal may be permitted to flow back from the tundish 46, 58, 60 into the ladle 1, in order to achieve an equalization of concentration or temperature. However, it is a precondition that the level 51 of the liquid metal be sufficiently high above the opening of the draining tube 20' so as to avoid an interruption of casting. The metal present in the ascending tube 12 and in the supply tube 18' flows back when the pressure chamber 5 is relieved of pressure and the tundish 46, 58, 60 of the vacuum.

In order to guarantee that portions of metal will quickly flow back into the ladle 1 and that the tundishes 13, 40, 41, 46, 58, 60 are filled quickly, the cross section of the ascending tubes 12 and supply tubes 18, 18' are designed considerably larger than the cross section of the draining tube or tubes 20, 20'.

FIG. 8 shows a plant in which a draining tube 20 comprising two parts 20 a and 20 b is used. The end of this tube is broadening to be funnel-shaped. The bottom part 20 a of the drain and the mould 31 are gastightly enclosed by a hood 65 which is connected to a conduit 66 for the supply of argon. Thus a gas space 67 may be produced above the level 32 of the metal in the mould, which gas space is subjected to a slight over-pressure. The lower part 20 a of the draining tube 20 is made of a metal-ceramic material containing primarily molybdenum and zircon oxide and is releasably connected with the upper part 20 b by means of flanges 68, 69 and brackets 70.

The invention may advantageously also be used in vertical continuous casting plants, whose pouring platform is situated e.g. 20 m above mill floor; in such vertical plants it is not only difficult but also very dangerous to lift the filled, heavy ladles to the necessary height. However, when employing the invention, the ladle may be arranged at any distance below the pouring platform even in vertical plants, so that the platform need not carry too heavy loads and may be built as a lighter construction. As a rule compressed air of 4 to 5 at. gauge will always be available in a steel mill so that the liquid steel may be conveyed over a height difference of about 5.6 to 7 m. If, in addition to compressed air, also a vacuum is used in the tundish, a height of 7 to 8.4 m may be overcome. Thus all types of plants may benefit from the advantage that the heavy casting ladle may be arranged at any low position desired, and only in individual cases it will be necessary to increase the pressure of the compressed air in the pressure chamber, which, however, may easily be carried out.

Claims

What I claim is:

1. A plant for the continuous casting of metals comprising

a supply vessel for containment of a portion of molten metal,

a pressure chamber enclosing the supply vessel and adapted to be gas-tightly sealed,

an ascending pipe in communication with the tundish and the supply vessel and adapted to convey metal from the supply vessel into the tundish,

a casting mold,

a draining tube connected to the tundish and adapted to convey molten metal from the tundish into the casting mold,

means for supporting said supply vessel, and tundish and said casting mold at respective heights such that during operation of said plant, the level of molten metal in said tundish may be maintained more than 1.4 m above the level of molten metal in said supply vessel and above the level of molten metal in said mold by a distance enabling gravity flow of metal from said tundish to said mold and not exceeding 1.5 m, and

pressure means for conveying the metal from the supply vessel into the tundish and then conveying the metal from the tundish into the casting mold with a minimum of force on the metal in the casting mold,

said pressure means comprising means for increasing the pressure in the supply vessel above atmospheric pressure and means for lowering the pressure in the tundish below atmospheric pressure.

2. The plant set forth in claim 1, wherein the lower end of the draining tube is broadened in downward direction to be funnel-shaped.

3. The plant set forth in claim 1, wherein the lower end of the draining tube and the mold are gas-tightly enclosed by a hood which is connectable with a conduit for the supply of protective gas selected from the group including argon, so that above the metal surface in the mold a gas space subjected to slight overpressure can be produced.

4. The plant set forth in claim 1, wherein the draining tube comprises two parts which are releasably connected with each other.

5. The plant set forth in claim 4, wherein the lower part of the draining tube is made of a metal-ceramic material comprising molybdenum and zircon oxide.

6. The plant set forth in claim 1, wherein the supply vessel is provided with a slanting bottom and the top of the ascending pipe is positioned slightly above the bottom of the supply vessel.

7. The plant set forth in claim 1, wherein the ascending pipe is arranged eccentrically of the longitudinal axis of the supply vessel.

8. The plant set forth in claim 1, wherein the pressure chamber and the supply vessel are adapted to be movable.

9. The plant set forth in claim 1, wherein the ascending pipe comprises an ascending tube which is connected to the top of the pressure chamber and a supply tube which is connected to the bottom of the tundish, the upper end of the ascending tube and the lower end of the supply tube being releasably connected by means of a slide valve connection.

10. The plant set forth in claim 1, wherein the tundish is designed to be liftable, lowerable and rotatable.

11. The plant set forth in claim 9, wherein the tundish is provided with a plurality of draining tubes, each of which is leading into one casting mould, the bottom ends of each of said draining tubes being situated somewhat below the connection level of the ascending tube and the supply tube.

12. The plant set forth in claim 1, wherein the tundish is provided with a slanting bottom, the top of the ascending pipe being positioned above the top of the draining tube.

13. The plant set forth in claim 1, wherein the tundish is positioned at an angle from the horizontal, and the top of the ascending pipe is situated above the top of the draining tube.

14. The plant set forth in claim 1, wherein a refractory body having a nozzle-like opening is provided in the tundish at the top of the ascending pipe, said body being positioned to change the direction of the metal which is upwardly conveyed through the ascending pipe.

15. The plant set forth in claim 1, wherein the cross section of the ascending pipe is greater in size than the cross section of the draining tube.