GB2033804A - Mould for continuous vertical direct-chill casting of billets or blocks - Google Patents

Abstract

An annular graphite liner 2 provides a continuation of a lower, inner metal surface of a mould and, to reduce heat conduction at the top of the mould, the upper part is spaced from the metal of the mould as by a slot 3, Figure 3, or by extension, Figure 4, of the liner. There may be an insulation or lubricant in the slot 3 and a hot top 4 with an apertured plate 6. <IMAGE>

Description

SPECIFICATION Mould and mould assembly This invention relates to a mould assemblyforthe continuous or semi-continuous direct chill casting in the vertical manner of metals and more particularly of aluminium and magnesium and aluminium and magnesium base alloys.

In such casting it is necessary to have some means of controlling the metal level in the mould and this function is frequently performed by a float control.

Such a procedure is however inconvenient where large mumbers of strands, ground billet, are being cast simultaneously since each strand requires to be equipped with its own float control. Furthermore, the use of such float controls implies that the metal drops in level on entering the mould, whereas with a readily oxidisable metal such as aluminium, it is highly desirable to employ "level pouring" whereby the metal flows along a launder to the mould without change in the level of the free surface of the metal.

Various difficulties have been encountered in devising satisfactory methods of level pouring. For example, it has been found necessary to provide some form of "hot top" to prevent the metal from freezing prematurely in the upper part of the mould.

This need has led to inventions such as that of US Patent No 2,983,972.

In our British Patent No 1,389,784 there is disclosed a mould assembly for continuous orsemi- continuous direct chill casting of solid sections in the vertical manner comprising a mould, an insulating container for maintaining a sump of hot metal to be cast, launder means for level pouring metal into the container, plate of material of low thermal conductivity located between the mould and the container, the plate defining a plurality of spaced apertures providing a flow path from the container to the mould, the inner surface of the mould in use will contact liquid metal being made of graphite, and means for applying cooling water directly to the outerwall ofthe mould so that the cooled part ofthat wall extends downstream from a transverse plane not more than 1 inch downstream of the aperture plate.

Whilst this mould assembly gives improved results as regards the casting speeds that can be employed and the surface quality and internal structure of the castings qbtained, we have nevertheless found that certain kinds of surface inperfection remain on the cast surface. For most purposes these are of no practical consequence, butforcertain applications, such as billets for continuous extrusion presses, a higher standard of surface quality may be desired. One such surface defect is the presence of close form folding, and whilst the condition can be eliminated by very rapid casting this is not always practicable since central cracking may then occur.

We have investigated another such type of surface defect and have found it to originate through premature solidification of metal at the joint between the mould and the insulating container and in particular on any gasket used to provide a metaltight seal atthis point. Such gaskets are conveniently made of alumino silicate fibre paper or felt and if metal solidifies thereon the gasket may be plucked leafing to a recurrent surface defect on the emer gent casting.

British patentspecification 1,000,845 discloses a graphite mould surrounded by sleeve of heat in sulating material partially separated from a water cooled block by air gaps. The arrangement is primarily for casting iron alloys and would not be suitable for the continuous or semi-continuous direct chill casting in the vertical manner of alumi nium and magnesium and aluminium and magnesium base alloys.

It is accordingly an object of the present invention to provide an improved mould and an assembty including such mould.

According to one aspect of the present invention there is provided a mould forthe continuous or semi-continuous vertical direct chill casting of liquid metal comprising an open annular metal mould part having an annular graphite liner in the upper part thereof, in which the inner working surface of the liner is continuous with the inner surface of the metal mould part, the lower part of the outer surface of the liner being in intimate contact with the-metal mould part and the upper part ofthe outer surface of the liner being spaced therefrom. The uppersurface ofthe liner may extend above the upper surface of the metal mould part. Alternatively, an annular slot may be provided between the upper part of the liner and the upper part of the metal mould part, The upper surface of the liner may be substantially level with the upper surface of the mould.The slot may be at least 0.25 mm wide and have an axial length of at least 4.0 mm. Preferably, the slot is at least 0.5 mm wide and 10.0 to 20.0 mm long.

The invention also provides a mould assembly comprising a mould according to the preceding paragraph and an insulating container or hot top for maintaining a head of liquid metal above the mould.

Preferably launder means is provided for level pouring liquid metal into the hot top. The hot top may be of the closed head or open type and may be supported on the upper surface of the liner with the intermediary of a gasket. Preferably the base of the hot top overhangs the mould cavity. The horizontal overhang may be limited to approximately 3 mm where it is desired to achieve narrow subsurface segregation zones.

If desired the base of the hot top may be stepped to engage over the liner in such a manner that a gasket disposed therebetween extends vertically at a position in which it could come into contactwith liquid metal.

The above and other aspects of the present invention will not be described by way of example with reference to the accompanying drawing in which Figure 1 is a diagrammatic sectional view of one side of the upper part of a metal mould incorporating a liner; Figure 2 is a view similarto Figure 1 and showing part of a hottop; Figure 3 is a view similar to Figure 2 showing a different construction of metal mould and a closed head hot top; and Figure 4 is a view similar to Figure 1 showing a modified construction.

Referring to the drawing, Figure 1 shows a vertical section through a metal mould 1, provided with a graphite liner 2, and a slot 3, to restrict heat transfer between the upper part of the liner and the upper part of the mould. Figure 2 illustrates a similar mould fitted with a refractory hot top 4, so shaped that a sealing gasket 5 is disposed vertically at the point where it could come in contact with liquid metal.

Figure 3 illustrates a mould 1 of different shape fitted with a liner 2, and provided with a slot 3, the hot top 4, being of the closed head type having an aperture plate 6. A gasket 5 is provided between the graphite liner 2 and the hot top 4. The arrows at A-A define the horizontal overhang of the hot top which may be approximately 3 mm. Figure 4 shows another arrangement in which the graphite liner 2 projects upwardly above the upper surface 7 of a modified mould 1 without a slot 3.

Thus in all the arrangements described the lower part 2a of the outer surface of the liner is in intimate contact with the metal mould 1 while the upper part 2b of the outer surface of the liner is spaced therefrom.

In all the arrangements described the inner surface of the liner is continuous with the inner surface of the mould and where a slot is provided between the mould and the upper end of the liner this slot should be at least 0.25 mm wide and 4.0 mm long. It is preferable however that it should be at least 0.5 mm wide and 10 to 20 mm long. Good results have been obtained with a slot width of 1.0 mm. It will also be understood that launder means (not shown) are preferably associated with the mould assembly for level pouring liquid metal into the hot top.

Whilst it is generally preferred to leave the slot empty, so as to provide a simple air barrier to heat flow, the slot may be filled with insulating material of low thermal conductivity. It may also be filled with a lubricant which may be allowed to percolate through the graphite liner, if sufficiently porous, orto enter the mould via radial grooves in the upper surface of the graphite liner or to enter the mould through the wicking action of a gasket of fibrous material.

We have cast many tonnes of electrical purity billet of 178 mm dia in equipment as illustrated in Figure 2 using a graphite liner 2, 51 mm long and a slot 3, 14 mm long and 1 mm wide at a speed of approximately 180 mm/min and have obtained very satisfactory surface quality, suitable for use with continuous extrusion presses. To match this surface quality from a conventional mould required a casting speed of 300-350 mm/min, but at this speed trouble was encountered with centre cracking. We have also cast 2014 alloy through a 300 dia mould of the type shown in Figure 3 but without a taper on the aperture plate (ie, without a reduced overhang) at a speed of 60 mm/min and obtained very good surface quality with a peripheral zone width of only 3 mm and showing reduced intensity of segregation.

It is well known that there is a relationship between the length of the metal column exposed to the cooling action of the mould and the width of the sub-surface zone of the casting showing pronounced segregation. There are however practical operating difficulties in working with low metal depths in open head moulds and with very short moulds in hot top systems. An advantage of the present invention is that by lowering the rate of heat transfer in the region of the joint between the liner and the metal mould part the thickness of the segregated subsurface layer of the casting is also reduced without introducting operating difficulties.

Thus the present invention provides a mould and mould assembly incorporating a barrier to heat conduction through the upper part of the mould, said barrier taking the form either of a slot behind a graphite liner or the ambient atmosphere around the upper end of the liner, with the result that the temperature at the upper part of the liner is raised thereby reducing the risk of incipient solidification at the joint between the mould and the hot top and the rate of heat transfer in the vicinity of the lower end of the liner is reduced. Thus with the barrier the phenomenom of close form folding can then be overcome at much lower casting speeds than normal.

We have thus found the present invention to be expecially suitable for the achievement of very smooth cast surfaces in unalloyed aluminium, particularly aluminium of electrical purity, and for producing more highly alloyed compositions with very thin layers of sub-surface segregation and freedom from close form folding.

Although as described above reference has been made to casting billets it will be understood'that the invention is equally applicable to casting blocks.

Claims (16)

1. A mould for the continuous or semicontinuous vertical direct chill casting of liquid metal comprising an open annular metal mould part having an annular graphite liner in the upper part thereof in which the inner, working surface of the liner is continuous with the inner surface of the metal mould part, the lower part of the outer surface of the liner being in intimate contact with the metal mould part and the upper part of the outer surface of the liner being spaced therefrom.

2. A mould according to Claim 1 in which the upper surface of the liner extends above the upper surface of the metal mould part.

3. A mould according to Claim 1 in which an annular slot is provided between the upper part of the liner and the upper part of the metal mould part.

4. A mould according to Claim 1 or Claim 3 in which the upper surface of the liner is substantially level with the upper surface of the mould.

5. A mould according to any one of Claims 1,3 and 4 in which the slot is at least 0.25 mm wide and has an axial length of at least 4.0 mm.

6. A mould according to Claim 5 in which the slot is at least 0.5 mm wide and has an axial length of 10 to 20 mm.

7. A mould assembly comprising a mould according to any one of the preceding claims and an insulating container or hot top for maintaining a head of liquid metal above the mould.

8. An assembly according to Claim 7 comprising launder means for level pouring liquid metal into the hot top.

9. An assembly according to Claim 7 or Claim 8 in which the hot top is of the closed head type.

10. An assembly according to Claim 7 or Claim 8 in which the hot top is of the open type.

11. An assembly according to any one of Claims 7 to 10 in which the hot top is supported on the upper surface of the liner with the intermediary of a gasket.

12. An assembly according to Claim 11 in which the base of the hot top overhangs the mould cavity.

13. An assembly according to Claim 12 in which the overhang is approximately 3 mm.

14. An assembly according to any one of Claims 7 to 13 in which the base of the hot top is stepped to engage over the liner in such a manner that a gasket disposed therebetween extends vertically at a position in which it could come into contact with liquid metal.

15. A mould substantially as herein described with reference to Figure 1 or Figure 2 or Figure 3 or Figure 4 of the accompanying drawing.

16. A mould assembly substantially as herein described with reference to Figure 2 of Figure 3 of the accompanying drawing.