GB2077643A - Wiping cooling water from a continuous casting - Google Patents

Abstract

Cooling water is removed from the surface of the strand (25) at a predetermined level below the base of the mould (22) by a wiper consisting of segments (26) which are brought into their operative (i.e. wiping) position after the ingot butt (28) has passed. The segments may be shaped as appropriate to wiping various ingot shapes. <IMAGE>

Description

SPECIFICATION Expanding wiper This invention relates to the continuous or semi-continuous direct chill casting of metals and in particular of aluminium and aluminium base alloys.

In the casting of certain alloy compositions, and especially those with high alloy content, problems can arise with cracking of the casting. It is well known that in the casting of round sections this cracking problem can be alleviated by reducing or delaying the chilling action of the water as it impinges on the emerging ingot. Thus it has been proposed to remove the water from the billet surface at a point below that of initial impingement by means of air knives or a wiper ring, the latter comprising a sheet of rubber or similar pliable material with a circular hole of sufficient size to fit the billet being cast. It has also been proposed to apply the cooling water at a prescribed distance below the mould so that the onset of the quench is thereby delayed.

Use of wipers as hitherto practised has given rise to operating inconvenience and is attended by increasing safety hazard, since the risk of metal run-out at the start of the cast is increased.

Moreover, since the butt of a billet is normally longer in diameter than the billet itself, the butt will not readily pass through the wiper ring, leading to hang-up of the billet in the mould, thus further increasing the casting hazard. Alternatively, part only of the butt may pass through the wiper aperture, leading to tilting of the billet and metal spillage. If the aperture of the wiper is large enough to allow the butt to pass through without difficulty, then the wiping action on the bilet surface is not efficient. Use of a delayed quench similarly increases the risk of metal runout and makes greater demands on operator skill at the start of the cast than does the normal casting normal casting operation.

With rectangular ingots cracking problems can be more severe than with rounds, but in this case the use of wiper rings is impractible because of the more extensive butt swelling which occurs with rectangular shapes. Furthermore, the thickness and section shape of rectangular ingots cast through a given rectangular mould vary with casting speed, so that even if blocks without butt swelling could be cast it would still be necessary to change the wiper size and shape for significant changes in casting speed if the wiping is to remain effective. It will be understood that by rectangular moulds is meant moulds which in shape are essentially rectangular; such moulds may have rounded corners, D-shaped ends and major faces comprised of several straight sections at small inclination to each other.

An object of the present invention is to provide a simple and convenient means whereby difficult alloys with high cracking tendency can be cast easily in rectangular as well as round shapes. This is preferably achieved by means of an expanding wiper device, comprised of segments, which can be opened before the start of casting and closed when the butt of the ingot has descended below the level of the wiper device. Casting can thus be started in an easy and safe manner, and the technical advantages of casting with wipers, hitherto unrealized for rectangular ingots, can thereafter be enjoyed, irrespective of ingot shape and without the difficulties and hazards previously associated with the use of wipers on round ingots.

According to one aspect of the present invention there is provided a method of direct chill casting an ingot of metal in which, after a predetermined length of ingot has been cast, the cooling water is thereafter substantially removed from the ingot surface at a predetermined level below the base of the mould.

According to another aspect of the present invention there is provided a method of direct chill casting an ingot of metal in which, after a predetermined length of ingot has been cast, a plurality of co-operating water-deflecting surfaces, each shaped to fit a portion of the periphery of the cast ingot, are moved into positions immediately adjacent to the cast ingot and at a predetermined level below the base of the mould so as to form a substantially continuous collar around the ingot whereby the cooling water is substantially removed from the surface of the cast ingot.

The length of ingot cast before moving the deflecting surfaces into position will be sufficient to bring the ingot butt below the predetermined level at which the deflecting surfaces operate.

The latter level will be from 50 to 400 mm and generally from 75 to 225 mm below the base of the mould. A preferred level is about 50 mm above the bottom of the liquid metal sump, the depth of the latter being a readily determined parameter.

The water-deflecting surfaces, conveniently termed wiper segments, may usefully be of sandwich or double sandwich construction being preferably composed of one or more layers of flexible material such as synthetic rubber placed between plates of rigid material, for example of aluminium, the flexible material projecting beyond the metal backing. The flexible material is preferably brought firmly into contact with the ingot surface. The wiper segments will generally be disposed substantially in a horizontal manner, but may be tilted or curved in such a way as to assist in shedding the water into the casting pit, well clear of the wiped surfaces of the ingot.

Surface temperature measurements taken on the ingot at the end of the cast will generally be in the range 175-400"C instead of under 50"C as in normal DC casting practice, thereby greatly reducing the levels of residual stress developed in the cast ingot.

The wiper segments may be moved into position by any known means, for example manually with toggle arms actuated by a lever conveniently accessible to the casting operator. Alternatively, the desired movement may be effected electrically or pneumatically. The wiper segments may be hinged or provided with guides to facilitate the desired movement.

Some degree of lateral overlap between the flexible blades of the wiper segments may be employed if desired, some of the segments being applied at slightly different levels for this purpose.

The present invention is particularly suitable for the casting or crack-sensitive aluminium base alloys of Al-Zn-Mg-Cu, Al-Cu-Mg and other types. Flexible materials which may be used comprise natural and synthetic rubbers, including silicone rubbers, felts of refractory fibres, and abrasion resistant polymers.

The scope of the present invention and the manner in which it may be performed are further illustrated by the following figures and examples.

Figure 1 is a view of one wiper segment for use in casting rectangular ingot.

Figure 2 is a schematic view of the casting of a rectangular ingot employing two wiper segments of the kind shown in Fig. 1.

Figures 3 and 4 show in plan form two arrangements of wiper segments for use in the casting of rectangular ingot.

Figure 5 shows in plan form one arrangement of wiper segments for use in casting round ingot.

Figure 6 illustrates a hinged joint between two wiper segments.

Figure 7 is a vertical section through a casting system for rectangular ingot showing two hinged wiper segments.

Figure 8 shows in vertical section a form of wiper segment for use in casting rectangular ingot in which the ingot wall is contacted by a fold of flexible material.

Figure 9 shows in vertical section a form of wiper segment for use in casting rectangular ingot in which the ingot wall is contacted by a roller composed of or coated with flexible material.

In Fig. 1 a wiping blade of synthetic rubver 11, shaped to conform with one half of the periphery of a rectangular ingot, is sandwiched between an upper metal plate 1 2 and a lower metal plate 13, the two plates being bolted together so as to retain the wiping blade firmly in position. The plates may be curved as shown to assist in shedding the water which is removed from the ingot surface by the action of the wiping blade. The lower plate 1 3 may have a lateral extension 14 so as to assist in shedding water removed from the minor faces of the block.

In Fig. 2, liquid metal 21 enters a rectangular water-cooled mould 22 by means of a downspout and floating distributor system 23. Cooling water flows from the mould in a row of jets 24 which impinge on to the ingot surfaces 25. The water is then wiped off the ingot surface by two wiper segments 28 of the type illustrated in Fig. 1, here shown in the closed position, the blades of which co-operate to form a continuous collar 27 enclosing the ingot. The swollen butt of the ingot 28 has passed below the level of the wiper segments 27. Water removed from the ingot surface is shed in a curtain 29 well clear of the ingot.

Figs. 3 and 4 indicate in plan form two arrangements of wiper segments 31 and 41 for casting rectangular ingot. The wiper segments are shown in each case in the open position.

When the ingot butt has descended below the level of the wiper segments the latter are moved together so as to form a continuous collar round the ingot as shown in Fig. 2.

Fig. 5 indicates an arrangement of three wiper segments 51, shown in the open position, for the casting of round ingot. Alternatively, two wiper segments only may be employed in which case it may be convenient for them to be hinged. Fig. 6 shows parts of two such wiper segments 61 connected by a hinge 62, the segments being in the open position.

Fig. 7 shows two wiper segments 71 of the kind illustrated in Fig. 1. The segments are connected by hinge 72 to a support bracket 73 attached to the mould 74 or the mould support table (not shown). A wiper segment in the open position is shown at A in which case the cooling water from the mould 74 runs down the surface of the ingot in the usual manner 75. With the wiper segment in the closed position, as at B, the cooling water is removed from the ingot surface by the wiper blades 76 and falls 77 into the casting pit (not shown) well clear of the rest of the ingot 78. The wiper segments at B are applied to the ingot surface at approximately 50 mm above the base of the liquid metal sump 79.

In Fig. 8 the wiper segment 81 comprises a fold of flexible material 82 in place of one or more blades and in use the fold of flexible material makes contact with the wall of the ingot 83.

In Fig. 9 the wiper segment takes the form of a roller 91 composed of or coated with flexible material. The wiper segment is hinged at 92, and in use is moved from the open position A to the closed position B. In position B the flexible material is in contact with the surface of the ingot 93.

The utility of the expanding wiper device of the present invention will be seen from the following Example in which various attempts were made to cast 7075, 7050 or 7010 alloy in rectangular form. The alloys were in most cases made up from super purity aluminium with a view to achieving enhanced fracture toughness, but casting difficulties were encountered with block cracking to a greater degree than in casting the same compositions when made from the grade of aluminium normally used. As is well known, cracking tendency increases with ingot size.

EXAMPLE Melts were prepared of the crack sensitive alloys 7075, 7050 and 7010 using super purity aluminium, and attempts made to cast crack-free rolling blocks using a range of casting conditions suitable for the casting of these composition when made with the grade of aluminium normally. Rolling blocks were also cast under the same conditions but using an expanded wiper device in accordance with Figs. 1 and 2, the wiper segments being moved into position by means of toggle arms activated by a lever, as soon as the ingot butt had descended below the level of the wiper device. The wiper device was applied at a point approximately 50 mm above the base of the liquid metal sump. At the end of the cast, surface temperatures of the blocks cast without wipers were less than 50"C.

In this series of experiments attempts were made to use a static, one-piece wiper, as can be used for round ingots, but as anticipated great difficulties were experienced with the ingot butt and this approach was abandoned as impracticable.

It will be seen that in casting a crack-sensitive composition such as 7075 and 7050 made from super purity aluminium the expanding wiper device is of great practical benefit.

TABLE I Whether Expanding ingot Size No of No of Cracked ingots Wiper Used Alloy Base Purity mm No of Casts Soudn longots or Abandoned Cests Remarks 7075 HP 380 x 125 2 -- 2 SP 380 x 125 42 111/2* 30+ *Includes 1 half block No +1 block lost through casting machine malfunction 7050 SP 380 x 125 6 1 5 7075 SP 500 x 175 9 7 2+ +1 block lost through casting machine malfunction Yes 7050 SP 500 x 175 3 3 0 HP 960 x 300 1 1 0 7010 HP 960 x 300 1 1 0 HP high purity aluminium SP super purity aluminium

Claims (14)

1. A method of direct chill casting an ingot of metal in which after a predetermined length of ingot has been cast the cooling water is thereafter substantially removed from the ingot surface at a predetermined level below the base of the mould.

2. A method of direct chill casting an ingot of metal in which after a predetermined length of ingot has been cast a plurality of co-operating water-deflecting surfaces, each shaped to fit a portion of the periphery of the cast ingot, are moved into positions immediately adjacent to the cast ingot and at a predetermined level below the base of the mould so as to form a substantially continuous collar around the ingot whereby the cooling water is substantially removed from the surfaces of the cast ingot.

3. A method as claimed in claim 2 in which the water-deflecting surfaces comprise flexible material which makes contact with the ingot surface.

4. A method as claimed in any of claims 1-3 in which the water-deflecting surfaces are applied at a level of 50-400 mm below the base of the mould.

5. A method as claimed in claim 4 in which the water-deflecting surfaces are applied at a level of 75-225 below the base of the mould.

6. A method as claimed in claim 4 in which the water-deflecting surfaces are applied at a level of about 50 mm above the base of the liquid metal sump.

7. A method as claimed in any one of claims 1-6 in which the surface temperature of the ingot on removal from the casting pit is in the range 175-400"C.

8. A method as claimed in any of claims 1-7 in which the ingot is essentially rectangular in shape.

9. A wiper device for use in direct chill casting comprising a plurality of co-operating waterdeflecting surfaces, each shaped to fit a portion of the periphery of a cast ingot, and together combining to form a substantially continuous collar around said ingot, together with means for moving the deflecting surfaces into positions immediately adjacent to the ingot and for removing them therefrom.

10. A wiper device as claimed in claim 9 in which the water-deflecting surfaces comprise blades of flexible material.

11. A wiper device as claimed in claim 9 or claim 10 for use with a cast ingot of essentially rectangular shape.

1 2. A wiper device as claimed in claim 9 for a rectangular ingot in which the waterdeflecting surfaces comprise folds of flexible material.

1 3. A wiper device as claimed in claim 9 for a rectangular ingot in which the waterdeflecting surfaces comprise rollers.

14. A wiper device as claimed in any of claims 8-11 situated from 50-400 mm below the base of a direct chill casting mould and disposed co-axially therewith.