US3465811A - Plants for the continuous casting of steel - Google Patents

Description

l 1969 GAETAN DE COYE DE CASTELET 3,465,311

PLANTS FOR THE CONTINUOUS CASTING OF STEEL Filed May 26, 1966 United States Patent M 3,465,811 PLANTS FOR THE CONTINUOUS CASTING OF STEEL Gaia'tan de Coye de Castelet, Billaneourt, France, assignor to Societe des Aciers Fins de lEst, Billancourt, Hauts de Seine, France Filed May 26, 1966, Ser. No. 553,121 Claims priority, applicgation France, Nov. 15, 1965,

Int. Cl. B22d 11/12 11/14; C04b 35/14 US. Cl. 164-481 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to the continuous casting of steel performed with the assistance of a steelwork ladle poured into an intermediate or distributor vessel feeding one or a plurality of tubular cooled ingot moulds.

It is known that in conventional plants for the continuous casting of steel the jet of liquid steel flowing from the ladle into the distributor vessel, and also from this vessel into the ingot mould, is subjected to an oxidation detrimental both to the process and to the quality of the final products. When the steel contains aluminium, alumina is deposited On the nozzles of the ladle and vessel, thus clogging these passages. In the case of ordinary grade steels, it is noted that these nozzles are attacked by the iron oxide FeO and moistened by the steel, thus leading to the formation of more or less oxidized metal candles during the solidification. This effect interferes with a proper orientation of the teeming jet and in the long run produces a clogging necessitating the cleaning, by means of an oxygen blow-pipe, of the lower portion of the nozzle. Holdups and resumptions of casting operations subsequent to these cleaning steps are attended both by a surface alteration and by an axial shrinkage pipe or hole which are subsequently eliminated by separating the defective portions of the ingot. The loss of material resulting therefrom averages 2%.

In the conventional continuous-casting process refractory fragments from the nozzle lining or the plug are entrained and constitute a foreign material which become encrusted in the ingot skin during the solidification phase, thus altering the surface condition of the ingots which must subsequently be freed of these incrustations by using a chisel or any other suitable means. The loss thus incurred averages 6% All the drawbacks set forth hereinabove are completely removed by the present invention. This invention is applicable to continuous steel casting plants comprising a group of two vessels, namely a steelwork ladle adapted to be teemed through a refractory pipe or duct for feeding an intermediate vessel adapted to be discharged through at least one refractory nozzle or hole formed in the bottom of the vessel and having a refractory, substantially vertical pipe extension for feeding at least one tubular cooled ingot mould. A controlled device for starting and stopping the casting is associated with the ladle and vessel, the control device of the intermediate vessel comprising a plug or like stop- Patented Sept. 9, 1969 per rod adapted to engage the inlet orifice of the discharge nozzle or hole.

According to the present invention, the length of said pip-es is sufficient to enable their free end to extend through the slag layer overlying the liquid steel, in which their outlet orifice is immersed. Advantageously, sintered porous vitreous silica will be used for making these pipes. Moreover, particularly satisfactory results can be ob tained by meeting various requirements such as the existence of a reducing atmosphere in the intermediate vessel, the protection of the metal surface in the ingot mould by means of a thin layer of synthetic slag, and the covering of the slag layers by means of a pulverulent refractory substance having heat-insulating properties, such as graphite.

This invention is also concerned with the methods of operating the plant broadly described hereinabove and with certain constructional details as will appear from the following description, in which various forms of embodiment are described by way of example with reference to the attached diagrammatic drawings, in which:

FIGURE 1 is a diagrammatic sectional view of a plant according to this invention;

FIGURES 2 to 5 show in section the manners in which the pipes according to this invention are secured to the bottom of a vessel;

FIGURE 6 illustrates diagrammatically and in sectional view a modified form of embodiment of a plant according to this invention.

FIGURE 1 shows only the essential elements of a plant according to the invention. Therefore, the handling apparatus or means for positioning these elements, the supports therefor, the oxy-acetylenic ingot cutting follower devices, the members and devices constituting the process line from the setting ingots to the final discharge of the steel cut sections, and most of the control, supervision and regulation apparatus, inter alia, are not shown in this figure.

A steelwork ladle 1 containing the liquid steel is equipped with a plug and a movable stopper-rod 2 adapted to obturate the inlet orifice of a nozzle 3 provided at the bottom of the ladle. The device for starting and stopping the teeming operation is provided with manual, servo-assisted manual, or automatic control means.

This nozzle 3 has an extension in the form of a refractory pipe 5 extending through an orifice40 formed in the vault 6 of an intermediate vessel 7 underlying the ladle 1. The length of the refractory pipe 5 is such that its lower end extends through the slag layer 8 covering the liquid steel 9 and its outlet orifice 10 is immersed in the liquid metal.

The intermediate vessel 7 comprises a number of outlet nozzles corresponding to the number of ingot moulds to be fed from this vessel. In the case illustrated, the vessel 7 comprises only one discharge nozzle 11 adapted to be closed by a member such as a stopper-rod or plug 22 having control means 23, also of the manual, servoassisted manual, or automatic control type.

A pipe 12 leads into the cover or vault 6 to permit the introduction, into the space left between the slag 8 and the cover or vault 6, of a non-oxidizing and preferably reducing gas such as, inter alia, propane, coke-oven gas, etc.

Like the nozzle 3 of ladle 1, the nozzle 11 of vessel 7 has a depending tubular refractory extension pipe 13 of which the lower end projects to a certain extent into a vertical tubular ingot mould 14 of conventional type, for example having a copper double-wall with cooling water circulation means. The pipe 13 has a length sufiicient to cause its outlet orifice 15, in the operative position, to be immersed in the liquid steel 16 covered by a slag 17,

the steel level being maintained at a substantially constant value by exerting on the ingot position a gravity-responsive action transmitted through driving cylinders or rollers 18.

Of course, the pipes 5 and 13 partially immersed in the steel baths must have a high resistance to heat gradients. To this end, they are made from materials well known for this specific property, such as sintered porous vitreous silica. The discovery of the possibility of utilizing this material is inasmuch surprising and novel as this product is considerably less refractory than many products such as zircon currently used for making nozzles and plugs for continuous casting processes. This is also inasmuch surprising as high-alumina refractory products, considerably less reactive with iron oxide and steelwork slags, are frequently resorted to. Experience teaches that by using strongly acid synthetic slag ingot moulds the outer surface of the pipe according to this invention is not attacked, whereas on the other hand the inner wall is also not attacked. Only a fine, clear, vitreous, nonoxidized film is formed.

These elements of sintered silica, a commercially available material, are prepared by applying known methods not within the scope of the present invention. The pipes may be obtained by preparing vitreous silica, grinding same to a fine pulverulent state, forming a suspension thereof in pure water with an emulsifier to obtain a kind of slip subsequently poured into a porous plaster mould in which the powder is regularly deposited to form a firm crust on the mould walls, then discharging the excess slip, stripping the plaster mould by opening same, drying the form thus obtained and finally sintering the mass according to any process well known to ceramists.

The pipes may be attached to the ladle or intermediate vessel in any suitable manner. FIGURES 2 to 5 of the drawings illustrate a few examples of pipe fastening procedures. The nozzle and the pipe may form an integral unit, respectively 19 and 20 (FIGURES 2 and 3) or two separate members (FIGURES 4 and 5) interconnected by screw-threads 21 (FIGURE 4) or a flange 22 (FIG- URE 5), or a bayonet fitting. The two members may be either abutting (FIGURE 4) or separated by elements either of the ladle or of the vessel (FIGURE 5). The nozzle diameter may be the same as the pipe diameter (FIG- URE 2); in this case the pipe is filled completely by the jet of liquid metal which contacts and moistens the entire inner wall. The pipe may also have a greater diameter than the nozzle (FIGURES 3, 4 and 5) so that the vertical steel jet be separated from the pipe wall by a confined atmosphere from which the oxygen is quickly removed.

If in most instances the depending and immersed pipes are cylindrical with a circular cross-section, in certain specific cases they may have a variable thickness or a different cross-sectional configuration. Thus, in the casting of small square sections such as billets having a side length of, say, 90 to 120 mm., it may be advantageous to use a square cross-sectional shape.

Since the steel teeming is no more visible to the operators, a device may be provided for warning them when upon completion of the casting the steel has drained off from the intermediate vessel, so that the operators may plug the casting nozzles with the stopper-rods and thus prevent the slag from flowing from this intermediate vessel into the ingot moulds.

To this end, the vessel 7 is equipped with a measuring device comprising one or a plurality of sensors so adjusted as to detect any reduction in the level of the liquid steel bath below or in close proximity to a value at which the slag might be entrained into the pipe or pipes. These sensors are adapted to control through a relay or amplifying means a tell-tale visual or sound signal to Warn the operators that the casting is completed.

Thus, the measuring device may comprise an emitter of gamma rays directed vertically through the liquid metal towards the aforesaid sensor. In this case, the rays are absorbed as a function of the thickness of the metal layer through which they passed.

The sensors may also consist of spring-balances 21 (FIGURE 1) consisting of resilient metal parts on which distortion gauges of the electrical resistance type are cemented, these spring-bances being disposed under the vessel 7. By using these resistance gauge spring-balances inserted in a Wheatstone bridge it is possible to simultaneously measure the weight of the metal and the weight of the slag contained in the vessel for actuating the signal for warning the operators that the casting operation is completed.

Of course, the continuous casting plant according to this invention may also be equipped with automatic regulating means. Thus, for example, the steel bath in the vessel 7 may be kept to a constant level by utilizing the sensors of this vessel for controlling the maneuvering members 4 of ladle 1. This control acttion may be obtained for example by using a relay or amplifying means 24 and an electromagnet actuator 25.

Finally, water sprinkling sprays are disposed in the known manner at the outlet end of the ingot mould 14, the water spray serving the purpose of accelerating the cooling of the ingot 26 during its solidification.

With this invention it is also possible use a steelwork rocking ladle, as shown in FIGURE 6. In this case, a pipe 27 of ladle 28 is bent, its inlet orifice 29 being located near the ladle bottom and its outlet end extending in such a manner through a slot 30 formed in the cover 31 of vessel 7 that its outlet orifice 32 is completely immersed in the liquid steel bath therein. The ladle is carried by a handling apparatus so disposed that it can tip the ladle about its horizontal axis 34 located substantially in the plane of the outlet orifice 32. Of course, the inlet orifice 29 must be positioned as close as possible to the level of this axis 34 yet near the bottom of the ladle 28. This ladle further comprises a barrier-forming partition 35 for preventing the slag and the liquid steel from overflowing when the ladle is inclined. As shown, this pipe 27 acts somewhat like a teapot spout, the steel flowing into the vessel and, as in the first form of embodiment, the steel contacts only the vitreous silica of the pipes 13 and 27.

This invention is also concerned with the manner in which the plant is operated, in connection more particularly not only with the protection layers convering the liquid steel surfaces but also with the length of the ingotmoulds and the level at which the application of sprinkling water begins, the choice of these layers, length and level being subordinate to the actual design of the plant according to this invention.

In fact, the depth at which the pipe 13 is immersed must be carefully adjusted in the ingot mould 14 (FIG- URE 1). Between the free steel level and the lower outlet orifice of the pipe, there is an annular, more or less stagnating zone of liquid steel between the wall of the cooled ingot mould and the pipe, so that, with due consideraton for the heat-insulating influence exerted by the pipe, the steel contained in this zone will cool at a higher rate whereby, given an equal distance from the free surface of the liquid bath, the crust will be thicker. As a result, the distance in relation to the free bath surface Whereat the setting ingot is detached from the ingot mould is shorter. Therefore, according to this invention, it is well to reduce the length of the ingot mould 14 and to apply the secondary cooling action by water sprinkling at a higher level, by means of sprays 36, directly to the wall of ingot 26. All these steps are advantageous in that they reduce the risk of perforation and somewhat accelerate the solidfication. However, there is a limit in this direction, for the depth at which the pipe 13 is immersed should not be such that the free steel surface might tend to solidify within the ingot mould. It is relatively easy to find the best setting by the make-and-try method.

As contrasted with the conventional technique concerning the use of slags in continuous-casting ingot moulds, the steel teeming from the vessel 7 into the ingot mould 14 according to this invention will not pass through this slag which, therefore, remains perfectly still and prevented from being emulsified or entrained by the steel mass. According to this invention, it is therefore possible to cover the horizontal steel bath surface in the ingot mould with a fine layer of fusible synthetic slag 17. The latter will advantageously be covered with a refractory heat-insulating powder 38 such as graphite, whereby the slag cooling may be reduced accordingly.

It may be noted that the liquid steel in vessel 7 and ladle 1 or 28 is also covered with a protective slag 39, 8 and 41, respectively.

The method of this invention constitutes a considerable improvement in the continuous casting technique. The improvement relates essentially to the regular operation, the quality of the products and the use of the continuous casting technique in applications which were considered as impracticable up to now.

According to this invention, there is no direct contact whatsoever between the moving liquid steel mass on the one hand and the atmospheric air, on the other hand, from the steelwork teeming ladle to the ingot solidifying in the mould. Thus, in the case of aluminium alloys, the clogging of nozzles by alumina deposits is safely avoided. In the ease of ordinary grade steels, the nozzles are no more attacked by iron oxide FeO, thus avoiding the moistening of said nozzles by the steel and the formation of more or less oxidized metal candles obturating the nozzles in the long run. Under these conditions, the periodic cleaning of these nozzles becomes unnecessary, as well as the consequent holdup and the resumption of the casting procedure which are extremely detrimental from the dual point of view of quality and output.

During comparative tests carried out with a four-jet continuous casting plant, without providing any pipe under the teeming ladle, but providing two immersed pipes in the two ingot moulds respectively and casting without using pipes on the other two lines, a reduction of over 50% was observed in the incrustations of refractory materials in the skin of the castings produced in the two lines equipped with pipes between the teeming vessel and the ingot moulds, in comparison with the conventional casting lines showed no change. Therefore, the average losses caused by cleaning operations dropped from 6% to 3% in the two lines equipped with pipes according to this invention. If a similar gain is reckoned with by using the pipe between the teeming ladle and the intermediate vessel, the total loss of materials may be taken as being inferior to 1%, as contrasted with 8% in the conventional processes, for the whole of the surface cleaning and the scraps resulting from holdups and the resumption of a normal teeming operation.

It was also observed that by using these pipes (which prevent the oxidation of the metal jet) no reaction takes place between the carbon and the iron oxide, thus eliminating skin flaws, at the beginning of the ingot mould solidification. Such skin flaws have disappeared completely.

Similarly, in the casting of resulfurized steels giving a reverse liquation of the sulfurous products between the dendrites of the basalt layers, the latter disappeared completely with the use of pipes according to this invention.

All this leads to a considerable reduction in the socalled line defects in rolling-mill products.

It is thus possible to use nozzles of a slightly greater diameter without any risk of producing the defects resulting from a more or less eccentric teeming jet. The use of this pipe prevents the steel jet, even if the latter is broken, from oxidising; besides, any risk of projecting cold steel drops onto the walls of the ingot mould, as well as the incrustation of cold droplets into the ingot skin, are definitely precluded.

The use, according to this invention, of immersed pipes and the covering of the free steel surface in the ingot mould with a slag layer permit dispensing completely with the use of ingot oil, thus eliminating the risks of hydrogen aborption, flaws or flakes inherent to this use.

In a conventional casing process the aluminium content was kept within very narrow limits; in fact, the oxidation of the aluminium content modified the flow capacity of steel. As contracted thereto, by using the immersed pipe of this invention it is possible to increase the aluminium content of steel without altering either the fluidity, or the flowing capacity of steel, while eliminating any risks of clogging the nozzle.

As the steels are better deoxidized and calmed, skin flaws causing lines in rolling-mill products are eliminated.

The steels obtained by applying the method and means of this invention have a poor inclusion rate and their stress limit in the transverse direction is improved. It is even possible to continuously cast extra-mild aluminiumcalrned steel having less than 15% C, for deep-stamping metal sheets.

It is also possible to arrange at the bottom of the vessel a plurality of nozzles each aligned with an underlying pipe extending into a common ingot mould for casting fiat ingots or slabs for metal-sheet rolling-mills.

Another advantageous feature of the method as taught by practical experience lies in the complete removal of the risk of perforation or break-through due to the better formation of the solidified ingot skin as the ingot emerges from the ingot mould.

What is claimed is:

1. In a continuous steel-casting plant wherein the liquid steel throughout its path from a teeming ladle to at least one tubular, cooled ingot mold contacts only the refractory materials of the plant and non-oxidizing fluids, comprising a ladle having a nozzle of refractory material depending therefrom and plunging through a protective slag layer into said liquid steel contained in an intermediate vessel having a reducing atmosphere therein and adapted to be fed through said refractory nozzle from said ladle, at least one refractory nozzle disposed in the bottom of the vessel and formed with substantially vertical refractory pipe extensions for feeding liquid steel to said molds, the free surface of the liquid steel in said ingot mold being protected by a fine layer of synthetic slag covered with a pulverulent refractory substance having heat insulating properties, a control device for starting and stopping the teeming from said ladl and said vessel, the control device of said vessel comprising a stopper rod adapted to engage the inlet orifice of the relevant nozzle, said pipe extensions consisting of sintered porous vitreous silica and their length being sufiicient to cause the free end to extend through the slag layer covering the liquid steel and to cause their outlet orifice to be immersed in the liquid steel contained in said mold during a casting operation.

2. A plant according to claim 1, wherein said ladle nozzle is disposed in the bottom of said ladle and is substantially vertical, straight and aligned with a substantially vertical refractory pipe the lower end of which plunges through said protective slag layer into said liquid steel contained in said vessel, the control device of said ladle comprising a stopper rod adapted to engage the nozzle inlet orifice of said ladle.

3. A plant according to claim 2, wherein said refractory pipe is made of porous sintered vitreous silica.

4. A plant according to claim 1, wherein said ladle is a tilting ladle having fixed to it a siphon-like bent refractory pipe made of porous sintered vitreous silica and adapted to pour the liquid steel from said ladle to said vessel, both ends of said pipe being immersed in liquid metal through the protective slag layer, the one end in said ladle and the other end in said vessel, the

tilting axis of the ladle being situated near the plunging 3,349,838 10/1967 Baier 164281 end of said refractory pipe in said vessel. 3,354,940 11/1967 Walther et a1. 164281 References Cited 1 30 O8 HQREIGN PATENTS ,2 9 3/19 0 France. 2 973 278 i PATENTS 106 69 5 1395648 3/1965 France a 1sc 3,301,635 1/1967 Bergna et a1. 10669 X OTHER REFERENCES 3,349,838 10/1967 Baier 164281 Modern Refractory Practice, 4th edition, Harbison- 3,125,440 3/ 1964 Hornak et a1. 164-66 X Walker Refractories Company, 1961, pp. 89-94, 38 and 3,315,323 4/1967 Sperth et a1. 16482 10 586. 2,825,104 3/1958 J 164-155 X 3 066,364 12/1962 164F281 X J. SPENCER OVERHOLSER, Primary Exrnamer 3,201,837 3/1965 Sylvester 164-337 X R. SPENCER ANNEAR, Assistant Examiner 3,245,126 5/1966 Phillips et a1. 16466 3,287,773 11/1966 Woodburn 164-82 15 U.S.C1.X.R.

3,318,363 5/1967 Goss 164281 X 23182;10669

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