Process For Separating Non-metallic Inclusions From Hot Liquid Metal

Abstract

The invention relates to a process for separating non-metallic inclusions from hot liquid metal and to a casting tube for carrying out such process. The method is employed in continuous casting, in which the metal is permitted to flow into a water-cooled mould, if desired via a tundish, whereupon the bar is drawn out of the mould and further cooled. In this process the metal, when flowing through the tundish and/or when entering the mould, is deflected under formation of at least one metal stream directed in upward direction. The length a of this metal stream amounts to from 3 to 30 cm and its speed v is adjusted to increase proportionally thereto in a range of at least 3.5 to 31 cm/sec up to maximally 17.5 to 45 cm/sec so that at the surface of the metal or of the slag or casting powder layer a wave is formed. When a and v are maintained within the mentioned range, a sufficient separation is guaranteed and the formation of turbulences tending to rupture the slag layer is avoided.

Description

The invention relates to a process for separating nonmetallic inclusions from hot liquid metal, in particular Al-killed, soft steel, in a continuous casting plant, in which the metal, if desired after passing through a tundish in which a metal sump is formed covered by a slag layer, is permitted to flow into a water-cooled mould, whose casting level is covered with a slag layer or with a casting powder, whereupon the bar is drawn out of the mould and further cooled, and to a casting tube for carrying out such process.

It is known that non-metallic inclusions decrease the workability of metals, lead to surface flaws in the rolled product and impair the mechanical properties of the end product. In soft, Al-killed steel, which is further processed to cold-rolled sheets which are used as autobody sheets in the car industry, it is necessary to keep the content of non-metallic inclusions as low as possible because otherwise the high demands placed in the shapeability and surface quality cannot be met. Non-metallic inclusions in Al-killed steels are substantially Al oxides occurring in the desoxidation of the steel. It is also possible that the aluminum is oxidized and thus aluminum oxides are formed when the aluminum containing steel gets into contact with air, e.g. when the steel stream runs into a mould or in a tundish. Although these non-metallic inclusions have a lower specific weight than the liquid steel, the nonmetallic particles do not separate by themselves by floating up; rather, a considerable amount of these particles remains in the steel and thus impairs the quality of the sheets made of such steel.

In order to avoid these difficulties, it has already been proposed in a process described in the German provisional publication No. 1,959,570 to use in continuous casting casting tubes with closed bottom and lateral outflow openings from which the steel flows out below the steel level in the continuous casting mould obliquely in upward direction so that non-metallic particles are guided to the surface. Though it is possible in this manner to reduce the amount of non-metallic inclusions in the marginal zone of the steel bar, the overall content of non-metallic inclusions still remains relatively high. In a continuous casting mould, there are very unfavourable conditions for the purification of the steel by separating non-metallic particles. In the known process, turbulences may occur and, as a result non-metallic particles already guided to the surface are again drawn into the steel and remain in the solidifying bar. Another disadvantage resides in that for an adaptation of the current to various cross sections of the continuous casting mould different casting tubes have to be employed.

The invention is aimed at avoiding the described difficulties and disadvantages and, in a process of the kind defined in the introduction, resides in that the metal, when flowing through the tundish and/or when entering the mould is deflected under formation of at least one metal stream directed in upward direction, the length of this metal stream directed in upward direction amounting to from 3 to 30 cm and its speed being adjusted to increase proportionally thereto in a range of at least 3.5 to 31 cm/sec up to maximally 17.5 to 45 cm/sec (field A in FIG. 8), so that at the surface of the metal or of the slag or casting powder layer a wave or vault is formed.

The invention utilizes in a singular manner the properties of the low wettability and of the small specific weight of the non-metallic particles as compared to liquid steel for the separation: the current of the metal directed from below in upward direction is to guide to the surface each metal particle and each slag particle, but the current must not be so strong that the slag layer would be torn up or that a turbulence might arise at the surface whereby particles might be drawn downwardly. By the invention, an optimal separation effect is achieved.

The process according to the invention is advantageously carried out by employing a casting tube which immerses below the surface of the metal level in the water-cooled continuous casting mould, the casting tube head having a closed bottom and at least one laterally extending canal for deflecting the metal, with the measure that the total metal is permitted to flow through at least one canal of refractory material which joins the laterally extending canal of the casting tube head and extends vertically upwardly, the length of the canal extending in upward direction being such that the "directed" path length of the metal stream between the deflection and the mouth of the canal amounts to at least 4 cm and that the "free" path length of the metal stream between the mouth of the canal and the metal surface amounts to from 3 to 30 cm.

It is also possible to carry out a twofold separation under formation of metal streams directed in upward direction, both when the tundish is flown through and when the metal flows into the mould.

In the production of broad steel slabs, in particular of slabs of more than 1.5 m width, from steels comprising up to 0.20 % C, 0.25 to 1.60 % Mn, 0.02 to 0.1 % Al and if desired up to 0.30 % Si, balance iron and customary impurities, which are destined for the production of cold rolled sheets with high surface quality, in particular of sheets which are subjected to cold shaping, it is advantageous to permit the metal to flow from two or more casting tubes from the tundish into the mould under formation of deflection streams directed in upward direction, all deflection streams and the horizontal longitudinal axis of the mould lying preferably in a vertical plane.

A casting tube suitable for carrying out the process according to the invention is provided with a casting tube head with a closed bottom and at least one canal extending from the inner bore of the casting tube in lateral direction for deflecting the metal, and is characterized in that the laterally extending canal is joined by at least one canal of refractory material extending vertically in upward direction and having a length of at least 4 cm.

The cross section of the canal extending in upward direction or of the canals extending in upward direction in the casting tube head is dimensioned such that for a specific casting output the chosen metal stream speed is obtained. This canal cross section may be calculated from the relation saying that the casting output is equal to the product of canal cross section and desired metal stream speed.

According to a preferred embodiment, the casting tube according to the invention is shaped in a manner that in the casting tube head two canals extending vertically upwardly are arranged diametrically opposed to each other relative to the central inner bore of the casting tube.

By means of casting tubes according to the invention and by maintaining the defined stream length and the appurtenant speeds, it is possible to produce continuous casting products having a minimum of non-metallic inclusions, i.e. products that are highly pure. The invention has proven particularly satisfactory for the following steel types: reference steel- steel- steel- steel- data: type 1 type 2 type 3 type 4 ______________________________________ % C 0.05 0.05 0.15 0.18 % Si 0.00 0.10 0.20 0.20 % Mn 0.25 0.30 0.50 1.40 % Al 0.05 0.03 0.03 0.03 ______________________________________

In order that the invention may be more fully understood it shall now be described with reference to the accompanying drawings.

FIG. 1 is a schematical vertical sectional view of a casting ladle, a tundish and the upper part of a continuous casting plant.

FIGS. 2 and 3 show details of a casting tube head with two canals extending vertically upwardly, FIG. 2 being a vertical sectional view, and FIG. 3 a horizontal sectional view along the line III--III of FIG. 2. FIG. 4 is a similar representation as FIG. 1 for an embodiment of the process according to the invention in which a primary separation of non-metallic particles takes place in the tundish and a secondary separation takes place in the continuous casting mould.

FIG. 5 shows an embodiment in which a casting tube according to the invention is directly joined to a casting ladle.

FIG. 6 is a horizontal sectional view along the line VI-VI of FIG. 5.

FIG. 7 is a vertical sectional view of a casting tube with a single canal extending vertically upwardly.

FIG. 8 is a diagram illustrating the connection between the current speed of the metal stream and its "free" path length a.

In FIG. 1, numeral 1 denotes the lower part of a casting ladle from which e.g. aluminum killed steel runs in a stream 2 into a tundish 3. The steel stored in the tundish 3 is covered by a casting powder or slag layer 4 in order to prevent heat losses or the access of air, respectively. Into the bottom 5 of the tundish 3 two casting tubes 6,7 are inserted side by side, whose casting tube heads 8,9 are illustrated in detail in FIGS. 2 and 3. The inner spaces of the casting tubes 6,7 are connected by means of horizontal bores or canals 10 with canals 12,13 extending vertically upwardly, so that in the casting tube head a deflection of the metal streams by 180.degree. takes place. The four metal streams flowing out in upward direction through the canals 12,13 produce waves or vaults 14,15,16,17 on the metal level surface of the continuous casting mould without rupturing the casting powder layer 18 on the metal level. According to the invention, between the level 20, defined by the outflow openings of the canals 12, 13 directed in a vertically upward direction, and the metal level 19, there is to be a distance a of 3 to 30 cm. When the distance a is smaller than 3 cm, there is the danger that the metal will sink below the level 20, which would cause a disadvantageous oxidation of the metal. When the distance a is greater than 30 cm, there is the danger that not all non-metallic particles will get the surface. The distance a is adjusted by lifting or lowering, respectively, the tundish, as indicated in FIG. 1 by a double arrow. The non-metallic particles are absorbed and held by the slag or casting powder layer 18 as soon as they get to the surface and get in contact with said layer.

In order to guarantee that the metal streams deflected in the casting tube head by 180.degree. flow out vertically upwardly, the vertical guiding path b in the vertical canals 12,13 of the casting tube head is not to be shorter than 4 cm; b thus may be referred to as "directed", and a as "free" path length of the metal streams.

The metal stream speed v at the exit of the metal from the canals 12, 13 is defined by the casting output or throughflow amount/ time unit and the canal cross section, i.e. the product of canal cross section and metal stream speed v corresponds to the casting output. This metal stream speed may be adjusted for a predetermined casting output by adequate dimensioning of the canal cross section. v is obviously also dependent on the ferrostatic pressure, i.e. on the height difference between the casting level 19 and the surface of the metal sump in the tundish 3. An adjustment of the metal stream speed is necessary in order to obtain the desired separation effect. It has to be adjusted to the distance a as explained above. The connection between a and v will be explained in greater detail with reference to FIG. 8.

Numeral 21 denotes a water-cooled copper mould from which a bar having a liquid core 22 and an already solidified shell 23 is drawn out and further cooled. Numeral 24 denotes supporting and guiding rolls. For regulating the casting speed or the lowering speed, respectively, of the cast bar, stoppers (not shown) to be actuated by rods may be provided with which the casting tubes 6,7 may be closed in a known manner. Also in the casting ladle 1, a stopper may be provided for regulating the stream 2. The embodiment according to FIG. 1 is preferably used for the production of steel slabs having a width of more than 1.5 m from steels comprising up to 0.20 % C, 0.25 to 1.60 % Mn, 0.02 to 0.1 % Al and, if desired, up to 0.30 % Si, balance iron and customary impurities, destined for the production of cold rolled sheets with highest surface quality, in particular of sheets to be subjected to cold shaping.

FIG. 4 shows a further embodiment. The tundish 25 is provided with a cover 26 having an opening 27 through which the steel 2 runs into the tundish 25 from a casting ladle (not shown). For the primary separation of non-metallic particles, a tubular body 28 of refractory material is provided in the tundish 25 which surrounds the metal stream 2 at a distance. This body has close to its bottom 29 a lateral opening 30, and the liquid steel is deflected by 180.degree. in upward direction by means of a canal 32 extending vertically upwardly and formed by a shoulder 31. The metal thus deflected is permitted to flow out towards the surface of the metal with such a speed that a vault 33 is formed on the metal. The level 20 defined by the mouth of the canal 32 is--as already described in connection with FIG. 1--at a distance a of from 3 to 30 cm below the metal level 19 in the tundish 25. The "guided" path length of the metal stream in the canal 32 is denoted with b and amounts to at least 4 cm. By dimensioning the cross section of the canal 32 in an appropriate manner, the speed v is adjusted in dependence on the distance a as has already been explained in connection with FIG. 1--so that the slag or casting powder layer 4 is not ruptured. Part of the non-metallic particles contained in the steel is flushed into the slag or casting powder layer 4 and separated from the steel. This phase may be referred to as primary separation. The remainder of the non-metallic particles or inclusions is separated by employing a casting tube 35 according to the invention which extends into the mould 21. This phase may be referred to as secondary separation. The casting speed may be regulated by means of a stopper 34. When steel flows out of the casting tube head 36 which has bevelled outflow openings as indicated at 37, vaults 38,39 covering relatively big areas are formed into which the non-metallic particles are flushed whereupon they are absorbed by the casting powder layer 18. Numeral 20 again denotes the level defined by the outflow openings of the casting tube head 36, which level is at the distance a below the casting level 19; a, the "free" path length of the metal streams, lies in the range of from 3 to 30 cm. The length b of the vertical canals 12,13 in the casting tube head 36, corresponding to the "directed" path length of the metal streams, amounts to at least 4 cm.

In the embodiment according to FIG. 5, the casting tube 35 is directly connected with the bottom of a steel casting ladle 1 which is provided with a liftable and lowerable stopper 34 for regulating the steel supply into the continuous casting mould 21. The casting tube 35 may also be designed in two parts, the connecting line being suitably in the area of the ladle bottom. In this case, a new casting tube may be connected with the ladle by means of suitable connecting means, e.g. by a bayonet catch. a, the distance between the casting level 19 and the level of the casting tube outflow holes, again amounts to from 3 to 30 cm, and b to at least 4 cm.

FIG. 6 shows the preferred arrangement of the casting tube head 36 relative to the horizontal longitudinal axis 40 of the slab mould 21: the longitudinal axis 41 of the casting tube 35 and the axes of the vertical canals 12,13 intersect the longitudinal axis 40, i.e. all axes lie in a common vertical plane. In FIGS. 5 and 6, the casting tube 35 and the casting tube head 36 are illustrated at an enlarged scale as compared to the continuous casting mould 21 and the ladle 1, respectively, so as to render the details better visible.

The embodiment according to FIG. 5 has the great advantage that no tundish need be used; thus, it becomes possible e.g. to adjust the temperature of the liquid metal in the casting ladle lower than when a tundish is used which causes heat losses. If a tundish is used, it is also possible to arrange above the tundish a casting ladle 1 with its casting tube 35, as shown in FIG. 5, in a manner that the steel is deflected in the tundish below the metal surface and is permitted to flow in upward direction. In this manner, the tubular hollow body 28 illustrated in FIG. 4 with the lateral exit opening 30, the shoulder 31 and the canal 32 extending vertically upwardly formed thus, may be replaced by the casting tube 35.

Lastly it is also posible to use, both in the continuous casting mould and in the tundish, casting tubes according to FIG. 7 with a single laterally extending canal 42 and a single upwardly extending canal 43 with a length b, from which canal a metal stream 44 containing non-metallic particles is directed vertically upwardly towards the metal surface; the stream 44 has a "free" path length a so that these non-metallic particles are separated into thhe casting powder or slag layer 18 in the area of a wave or vault 45.

In all cases, it is essential to the invention that each individual metal stream directed in upward direction obtains a specific speed v lying in a certain range in dependence on its free path length a. In the diagram according to FIG. 8, this connection is illustrated. On the abscissa the "free" path length of the metal stream or of the metal streams is plotted in cm, which length corresponds to the distance a, and on the ordinate, the speed v in cm/sec. Within the range of 3 to 30 cm for the distance a, the appurtenant speed v is to lie according to the invention in the shaded field A. When a has the smallest value, the speed thus may amount to from 3.5 to 17.5 cm/sec and increase proportionally up to maximally 31.0 to 45 cm/sec when a reaches its greatest value. When the lower limit 46 is transgressed, the separation would be insufficient (field C), whereas when the upper limit 47 is transgressed, the speed would be too great, and turbulence currents would occur at the metal surface so that the slag particles would be drawn downwardly. In this case too, the separation would be likewise insufficient (field B). Slight variations of a and v may occur during casting; this, however, is no disadvantage, provided that a and v are maintained in the field "A" of FIG. 4, which is possible in normal casting operation without any difficulties.

It is very simple to carry out the process according to the invention in practice. Prior to casting a specific distance a and an appurtenant steel stream speed v are chosen which are to be maintained during casting. b, the "directed" path length of the metal streams, corresponds to the length of the vertical canals 12,13; 13; 32; 43. The distance a in the mould is adjusted by lifting or lowering the tundish 3, 25 or the casting ladle 1 relative to the casting level 19. When the stationary tubular hollow body 28 with its shoulder 32 is used, as described in connection with FIG. 4, the distance a is adjusted prior to casting by filling the tundish to the level 19, and changed during casting by changing the supplied steel amount relative to the amount of metal flowing out into the mould 21.

In order to obtain the desired speed v during casting, the necessary cross section of the vertically upwardly extending canals 12,13 in the casting head 36 is calculated with the aid of the casting output and consideration of the ferrostatic pressure, which in the embodiment according to FIG. 4 corresponds to the level difference between the metal level 19 in the tundish and the casting level 19 in the continuous casting mould 21, and then the canal cross sections are dimensioned. The same applies to the dimensioning of the canals of all other casting tube heads illustrated in the embodiments.

The canal cross sections may have any desired shape; they may be oval, round or angular. This also applies to the canal 32 of the tubular body 28 in FIG. 4. In this case, within the tubular body 28 a metal level is adjusted of about the same height as in the remaining part of the tundish.

Claims

We claim:

1. In a process for separating non-metallic inclusions from hot liquid metal in a continuous casting plant, in which the metal flows into a water-cooled mould containing a quantity of liquid metal having a surface covered with a layer of slag and in which a bar of metal is drawn out of the mould and further cooled, the improvement comprising deflecting the liquid metal, when it enters the mould, into at least one stream of metal, the stream originating at a predetermined distance of from 3 to 30 cm below the surface of the liquid metal in the mould and being directed in an upward direction to impinge against the layer of slag and form a wave in the layer of slag without previously contacting any lateral portion of a solidifying shell of the bar of metal, the speed of the stream of metal being adjusted to remain within a range of from 3.5 to 31 cm/sec when the predetermined distance is 3 cm and linearly increasing with distance to a range of from 17.5 to 45 cm/sec when the predetermined distance is 30 cm.

2. The process set forth in claim 1, wherein the non-metallic inclusions are separated from Al-killed soft steel.

3. The process set forth in claim 1, for producing broad steel slabs from steels comprising up to 0.20 % C, 0.25 to 1.60 % Mn, 0.02 to 0.1 % Al, 0 to 0.30 % Si, balance iron and customary impurities, which slabs are destined for the production of cold rolled sheets with high surface quality, wherein the metal is permitted to flow from at least two casting tubes from the tundish into the mould under formation of deflection streams directed in upward direction.

4. The process set forth in claim 3, wherein broad slabs having a width of more than 1.5 m are produced destined to be subjected to cold shaping.

5. The process set forth in claim 3, wherein all deflection streams and the horizontal longitudinal axis of the mould lie in a vertical plane.

6. The process set forth in claim 1, wherein the metal flows into the water-cooled mould through a casting tube immersed below the surface of the liquid metal in the mould, the casting tube having at least one longitudinally extending inner bore through which the metal flows and a casting tube head at the lower end of the tube, the casting tube head having (a) a closed bottom, (b) at least one canal for deflecting the liquid metal extending laterally from the inner bore, and (c) at least one canal of refractory material joining the laterally extending canal and extending vertically upwardly, the length of the upwardly extending canal being at least 4 cm between the top of the laterally extending canal and the mouth of the upwardly extending canal, the length of the stream of metal between the mouth of the upwardly extending canal and the surface of the metal in the mould being from 3 to 30 cm.

7. In a process for separating non-metallic inclusions from hot liquid metal in a continuous casting plant, in which the liquid metal, after passing through a tundish in which a metal sump is formed covered with a layer of slag, is permitted to flow into a water-cooled mould containing a quantity of liquid metal having a surface covered by a layer preventing access of air and in which a bar of metal is drawn out of the mould and further cooled, the improvement comprising deflecting the metal, after it flows through the tundish and when it enters the mould, into at least one stream of metal, said stream originating at a predetermined distance of from 3 to 30 cm below the surface of the liquid metal in the mould and being directed in an upward direction to impinge against the access preventing layer and form a wave in the access preventing layer without previously contacting any lateral portion of a solidifying shell of the bar of metal, the speed of the stream being adjusted to remain within a range of from 3.5 to 31 cm/sec when the predetermined distance is 3 cm and linearly increasing with distance to a range of from 17.5 to 45 cm/sec when the predetermined distance is 30 cm.

8. The process set forth in claim 7, wherein the layer preventing the access of air is a layer of casting powder.

9. The process set forth in claim 7, wherein the liquid metal is deflected, when it enters the tundish, into at least one stream of metal, the stream of metal in the tundish originating below the layer of slag on the metal sump and being directed in an upward direction to impinge against the layer of slag and form a wave in the layer of slag, so that a primary separation of inclusions occurs in the tundish and a secondary separation of inclusions occurs in the mould.