WO2002040199A2

WO2002040199A2 - Process of and apparatus for ingot cooling during direct casting of metals

Info

Publication number: WO2002040199A2
Application number: PCT/CA2001/001628
Authority: WO
Inventors: Joseph Langlais; Luc Belley
Original assignee: Alcan International Limited
Priority date: 2000-11-15
Filing date: 2001-11-15
Publication date: 2002-05-23
Also published as: US20020174971A1; AU2002220397A1; WO2002040199A3

Abstract

A process and apparatus for producing a cast and cooled metal ingot. The process comprises: casting a molten metal by a direct chill casting operation to form a metal ingot emerging from a mould (10), and directing one or more streams of liquid coolant onto an outer surface of the metal ingot adjacent to the mould at positions spaced around the periphery of the ingot to achieve a rate of heat extraction from the ingot. The one or more streams of liquid coolant are orientated at an angle relative to the outer surface of the ingot, and the angle is varied during the casting operation to change the rate of heat extraction from the ingot to minimize cooling-related defects in the cast and cooled ingot.

Description

PROCESS OF AND APPARATUS FOR INGOT COOLING DURING DIRECT CASTING OF METALS

TECHNICAL FIELD

This invention relates to the cooling of ingots as they are formed during metal casting procedures. More particularly, the invention relates to cooling processes and apparatus that allows the cooling effect to be varied at different times during the casting process.

BACKGROUND ART

Direct chill (DC) casting of metals is a well known procedure that is frequently used for the formation of ingots (i.e. elongated bodies, sometimes also called billets or slabs) of non-ferrous metals, such as alloys of aluminum, zinc, magnesium, etc. The DC casting procedure can be done in the vertical or horizontal direction. In the vertical procedure, molten metal is poured into the top of an annular mould and quickly caused to solidify (at least at the periphery) before the metal exits the bottom ofthe mould as an ingot. The procedure is commenced by positioning a bottom block in the lower opening ofthe mould during the initial pouring step, and then lowering the bottom block at a suitable rate of descent to allow the ingot shell to form and solidify before it exits the mould. Due to the vertical nature ofthe such a casting procedure, the emerging ingot normally descends into a pit positioned beneath the casting apparatus, and ingots having a length of 10 to 12 m are normally produced before the casting procedure is repeated. Horizontal casting procedures are similar in that a starter- block is used in the opening of a horizontally oriented annular mould until the initial mould fill occurs at which time the starter-block is displaced horizontally. In horizontal DC casting an essentially continuous ingot is produced, which is then sawn to length as required. The annular mould has a mould body normally defining a rectangular casting cavity for producing ingots of rectangular cross-section. The mould body may also be circular, square or any other suitable shape. The mould body usually has a hollow interior through which a liquid coolant (e.g. water) may be passed to provide primary cooling for the metal. The mould body has a mould surface that contacts and shapes the outer periphery ofthe ingot as it is being formed, and the liquid coolant passing through the mould body at the rear ofthe mould surface ensures that metal solidification takes place at the required rate. In addition, casting apparatus of this kind is provided with a means of secondary cooling (often referred to as direct cooling) ofthe metal. For example, jets of water or other liquid coolant are directed onto the outer surfaces ofthe metal ingot as the ingot emerges from the mould. This provides the bulk ofthe ingot cooling and has a major effect on the ingot microstructure.

However, different rates of cooling ofthe ingot are required at different stages of ingot formation. The start ofthe casting operation is referred to as the start-up phase (often referred to as the butt-forming stage) when the bottom or starter block is positioned at the mould opening and is initially displaced. After this initial stage, steady state casting may commence and continue until the ingot is fully formed. During the start-up phase, heat extraction from the ingot must be lower than in the steady state casting phase in order to prevent various problems and defects, e.g. excessive ingot butt curl, hot/cold fissures, tearing, cold-shut, run out, bleeding, etc. As steady-state casting commences, the rate of heat extraction can be increased. However, even during steady-state casting, the cooling requirements may change due to changes in the casting rate or surface characteristics ofthe ingot microstructure.

There are several ways by which the rate of heat extraction can be varied. For example, the amount of cooling liquid may be varied during different production phases, or jets of water may be pulsed (rapidly turned on and off) at different rates at different phases to achieve different rates of cooling. Alternatively or additionally, air or other gases may be entrained or dissolved in the liquid jets in different amounts to modify the effective cooling co-efficients of the coolant at different times. However, such methods usually do not produce even cooling effects and can therefore give unsatisfactory results. They are difficult to control and produce variable results.

An alternative arrangement is disclosed in U.S. Patent No. 5,582,230, which issued on December 10, 1996 to Robert B. Wagstaff, et al. and was assigned to Wagstaff, Inc. In this apparatus, the mould body is provided with two series of channels staggered relative to each other around the periphery ofthe lower end ofthe mould body so that cooling water can be directed as individual streams onto the emerging ingot surface. The channels ofthe first series are all oriented to produce water streams that impinge against the ingot surface at angles of 45 degrees, and the channels ofthe second series are all orientated to produce water streams that impinge against the ingot surface at angles of 22.5 degrees. Because ofthe high angle of incidence ofthe 45 degree streams, substantial portions ofthe coolant rebounds from the ingot surface and form a region of spray directly in the path ofthe 22.5 degree streams. The effect is to widen the bands of turbulence in the coolant layers in contact with the ingot and to minimize or eliminate regions of laminar flow ofthe coolant. By combining streams of 45 degrees and 22.5 degrees in this way during the steady-state phase of casting greater heat extraction can be achieved than in the initial phase, when only the 22.5 degree streams are employed. However, the degree of control ofthe rate of heat extraction is still rather unsatisfactory, and the transition from the 22.5 degree to 45 degree jet may cause disturbances to the ingot, since there is an abrupt change in the heat transfer coefficient as soon as the 45 degree jet is started.

U.S. Patent No. 4,351,384, which issued on September 28, 1982 to

David G. Goodrich, and was assigned to Kaiser Aluminum & Chemical Corporation, also makes use of streams of water arranged at different angles to the ingot surface to achieve direct cooling. This patent is particularly concerned with electromagnetic DC casting in which an electromagnetic effect is used to hold the solidifying metal a slight distance away from the casting surface ofthe annular mould. The invention is concerned in particular with defects that are characteristic of electromagnetic DC casting. The coolant streams at different angles are directed to the ingot so as to intersect a short distance away from the metal surface. By controlling the velocity and/or volume of one or both ofthe coolant streams, the point of impact of the coolant with the metal surface can be brought closer to the discharge point ofthe mould, which is desirable to prevent defects during the start of casting in the electromagnetic process. By causing the two streams issuing from the mould to intersect before they strike the ingot surface, the increased turbulence increases the tendency for the coolant to remain on the ingot surface.

Despite these approaches to cooling modification during DC casting, there is still a need for an improved way of varying direct cooling effects to allow the formation of cast ingots of high quality and achieve a better control ofthe cooling, especially in the transition from the startup to the steady state.

DISCLOSURE OF THE INVENTION

An object ofthe invention is to facilitate variation of cooling of an ingot during DC casting at different times during the casting procedure.

Another object ofthe invention is to provide an improved means to control the cooling of an ingot during the casting procedure.

According to one aspect ofthe invention, there is provided a process of producing a cast and cooled metal ingot, comprising: casting a molten metal by a direct chill casting operation to form a metal ingot emerging from a mould, and directing one or more streams of liquid coolant onto an outer surface ofthe metal ingot adjacent to the mould at positions spaced around the periphery ofthe ingot to achieve a rate of heat extraction from the ingot. The one or more streams of liquid coolant are orientated at an angle relative to the outer surface ofthe ingot, and the angle is varied during said casting operation to change the rate of heat extraction from said ingot. The angle ofthe liquid coolant streams is preferably varied continuously during said casting operation and preferably in between predetermined limits in response to at least one measured parameter ofthe casting system. The liquid coolant streams preferably exit the mould along a single line, which may be a straight line, or a simple curve. Preferably, each ofthe one or more streams is formed by the combination of two or more streams internally within the mould to form a single stream exiting the mould.

According to another aspect ofthe invention, there is provided an apparatus for producing a cast and cooled metal ingot, comprising: a direct chill casting mould having an annular body defining a casting cavity, for casting molten metal into a metal ingot having a periphery, and a mould outlet from which said metal ingot emerges as casting proceeds during a casting operation. One or more openings are provided in the annular body adjacent to the mould outlet for directing one or more streams of liquid coolant onto an outer surface of the metal ingot at positions spaced around the periphery ofthe ingot to achieve a rate of heat extraction from the ingot. An orientating arrangement is provided within the annular body for orientating the one or more streams emerging from the openings at an angle relative to the surface ofthe ingot, and for enabling variation ofthe angle as the casting operation proceeds.

In the above apparatus, the orienting arrangement preferably causes the angle ofthe one or more streams to vary continuously. The one or more openings preferably lie along a single line, which may be a straight line or a simple curve.

Preferably, the orienting arrangement comprises, for each ofthe one or more openings, two or more internal channels that meet internally within the mould body to form a single channel before exiting the mould. This orienting means is controlled so as to cause the angle of impingement on the one or more coolant streams to vary between predetermined limits in response to one or more measured casting parameters. The orienting arrangement is preferably selected from the group consisting of a hydraulic means, a mechanical means, or a pneumatic means or a combination of such means. The one or more streams of liquid coolant may comprise a plurality of streams of coolant or only a single stream of coolant from a continuous slot outlet running around the mould periphery. The single slot outlet may be connected to a series of internal coolant channels or to a pair of internal slot-like coolant channels.

According to another aspect ofthe invention, there is provided a direct chill casting mould having an annular body defining a casting cavity, for casting molten metal into a metal ingot having a periphery, and a mould outlet from which the metal ingot emerges as casting proceeds during a casting operation, and one or more openings in the annular body adjacent to the mould outlet for directing one or more streams of liquid coolant onto an outer surface ofthe metal ingot at positions spaced around the periphery ofthe ingot to achieve a rate of heat extraction from the ingot. The mould also comprises an orientating arrangement within the annular body for orientating the one or more streams emerging from the openings at an angle relative to the surface ofthe ingot, and for enabling variation ofthe angle as the casting operation proceeds to minimize cooling-related defects in the cast and cooled ingot.

According to yet another aspect ofthe invention, there is provided a mould having a mould body and one or more coolant chambers within the mould body. The mould is made up of a single monolithic piece that forms an inner surface of the mould, a top wall, a bottom wall and dividing walls dividing the mould body internally into the coolant chambers, and that forms three sides of each coolant chamber, and a separate closing plate providing a exterior wall ofthe mould and forming a fourth side of each the coolant chamber. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a simplified sketch showing a casting and cooling operation using apparatus according to one preferred form ofthe present invention;

Fig. 2 is a cross-section ofthe annular body ofthe mould at one side ofthe apparatus as shown in Fig. 1 ;

Fig. 3 is a cross-section ofthe annular body ofthe mould at one side ofthe apparatus as shown in Fig. 1 illustrating a further embodiment ofthe present invention;

Fig. 4 is a view similar to Fig. 1 showing a position where coolant liquid is supplied to the upper chamber ofthe annular body;

Fig. 5 is an underneath plan view ofthe annular body of Fig. 1;

Fig. 6 is an additional schematic view showing how the coolant exit holes in the annular body of Fig. 1 may be placed;

Fig. 7 is a partial view on an enlarged scale of part ofthe annular body of Fig. 5;

Fig. 8 is a partial cross-section ofthe part ofthe annular body of Fig. 6 rotated through 90 degrees, and also showing part of an adjacent cast ingot;

Fig. 9 is a cross-section ofthe annular body ofthe mould at one side ofthe apparatus showing a further embodiment ofthe present invention;

Fig. 10 is a cross-section ofthe annular body ofthe mould showing yet a further embodiment ofthe present invention; and

Figs. 11, 12 and 13 are cross-sections of further embodiments ofthe invention. BEST MODES FOR CARRYING OUT THE INVENTION

Figure 1 ofthe accompanying drawings shows a simplified representation of a casting and cooling operation according to the present invention employing apparatus according to one preferred embodiment. The illustrated embodiment is particularly suited for casting and cooling aluminum and aluminum alloys, but could be employed with other metals capable of being DC cast.

This apparatus (which in this embodiment is arranged for so-called vertical casting, i.e. for casting operations wherein the ingot descends vertically from the mould as it is cast) includes an axially vertical annular mould 10 (open at its upper and lower ends 11 and 12, respectively) to which molten metal 14 is supplied through a dip tube 15 for casting an ingot 20. The mould 10 is in the form of an annular body 18, which is made from a material having good thermal conductivity. The annular body 18 has a vertical inner wall 19 providing a casting surface 21 of desired horizontal cross-section (in this case, rectangular). A parting layer (lubricant) may be applied to the casting surface 21 to reduce sticking. The casting surface 21 defines a casting cavity 22 for the molten metal. It will be understood that the inner wall configuration determines the cross-sectional shape ofthe resulting ingot.

The lower end ofthe mould 10 is provided with a plurality of outlets 70 through which a cooling liquid 28 is projected as individual streams onto the outer periphery 29 ofthe ingot 20 immediately adjacent to the mould 10 for extracting heat from the ingot emerging from the mould to cool and solidify the metal. This cooling arrangement is described in more detail later.

At the start of a casting operation, the lower end ofthe casting cavity is closed by a bottom block 30, which is supported by a hydraulic ram 31. As molten metal poured into the casting cavity 22 solidifies at the lower end ofthe cavity, the bottom block 30 is drawn vertically downwards by operation ofthe ram 31. The solidifying base ofthe ingot 20 being cast, resting on the bottom block 30, then begins to emerge from the lower end ofthe casting cavity 22. Molten metal is continuously supplied to the upper end ofthe casting cavity 22 through dip tube 15 that opens downwardly into an upper portion ofthe casting cavity 22, so as to maintain the pool of molten metal 14 in the casting cavity at a substantially constant level as the solidifying ingot is progressively withdrawn from the mould, i.e. as the bottom block 30 is moved downwardly.

During the DC casting operation thus described, molten metal 14 within the casting cavity 22 solidifies around the periphery ofthe casting surface 21 and is cooled in part by heat transfer to the externally chilled mould inner wall 19 but mainly by impingement of coolant 28 directly on the initially formed solid shell. This solidification progresses sufficiently far inward towards the centre ofthe mould that the ingot emerging from the lower end ofthe mould has an externally solid and self-sustaining shell 33 even though the central portion or core 34 ofthe emerging ingot is still molten. With an effectively continuous supply of molten metal to the mould, and correspondingly continuous downward advance ofthe cast ingot from the mould, the molten central core 34 ofthe ingot emerging from the mould extends downwardly as a molten metal sump (constituting the lower end ofthe molten metal pool in the mould) of progressively decreasing cross- section in the downward direction, until the ingot becomes entirely solid.

The mould 18 contains two internal chambers 54, 55 used to provide cooling to the mould body and to deliver coolant 26 via channels 60, 61 to impinge on the ingot surface. The arrangement of internal chambers and channels is described in detail in the following.

A source of coolant is provided to each chamber 54, 55 via pipes 83, 84 and control valves 81, 82.

Fig. 2 is an enlarged cross-sectional view ofthe annular mould body 18 as shown at the left-hand side ofthe apparatus of Fig. 1. The mould body 18 is made up of three main pieces: a central portion 37, and top and bottom plates 38 and 39, respectively. These pieces are held together by upper and lower bolts, 40 and 41, respectively, that pass through sleeves 42 and 43, respectively, and are secured at their free ends 44 and 45, respectively, in opposite ends of a threaded hole 46 located in a central divider 47 ofthe central portion 37. The enlarged heads 48 and 49, respectively, ofthe bolts 40 and 41 are recessed as shown in short bores 51 and 52, respectively, in the top and bottom plates 38 and 39. Elastomeric O-ring seals 56, 57, 58, 59 and 59A are provided between the central portion 37 and the top and bottom plates 38 and 39 to prevent leakage of coolant.

The central portion 37, in cooperation with the top and bottom plates 38 and 39, forms upper and lower chambers, 54 and 55, respectively, within the annular body 18. These chambers are separated from each other by the central horizontal divider 47. The chambers each continuously encircle the central casting cavity 22 ofthe mould, but are separate from each other. At regularly spaced positions around the annular mould 18, coolant channels 60 and 61 are provided within the central portion 37 ofthe mould. At their respective inner ends 62 and 63, coolant channels 60 communicate with the upper chamber 54, and coolant channels 61 communicate with lower chamber 55 via narrow cross channels 64 and 65, respectively. Larger encircling grooves 66 and 67, provided for ease of manufacture, are sealed by O-rings 68 and 69, respectively. The channel 60 from the upper chamber 54 is orientated downwardly and inwardly at an angle of 22 degrees relative to the vertical casting surface 21 (i.e. relative to the adjacent surface 29 ofthe ingot 20). The channel 61 from the lower chamber 55 is orientated downwardly and inwardly at an angle of 45 degrees to the vertical casting surface 21.

In all cases, the outer ends 68 and 69 ofthe channels 60 and 61 overlap or coincide to form a single combined outlet 70 for both channels. The single outlet 70 is located in an inwardly and downwardly sloping undercut wall 72 at the lower end ofthe casting surface 21. The horizontal distance between the inner and outer ends ofthe undercut wall (as shown by arrows A) is typically about 6 mm. The inner end surface ofthe bottom plate 39 slopes slightly outwardly and downwardly at 73 (e.g. at an angle of typically 12 to 15 degrees to the vertical) beneath the undercut wall 72. The horizontal distance and angle is chosen to avoid any tendency for the coolant streams to "attach" to the mould wall, particularly when they are directed downwards at the smallest angles from the vertical. The channels 60 and 61 may be in the form of circular holes and join to form a single outlet 70. However, adjacent single outlets may be joined together to form a continuous slot, such a continuous slot being fed from several pairs of channels 60 and 61. Alternatively, channels 60 and 61 may be elongated slot like channels joining to form a single elongated slot outlet 70.

Fig. 3 is an enlarged cross-sectional view of an annular mould body 18 according to a further preferred embodiment ofthe invention. The embodiment is similar to that illustrated in Figure 2 except for the following details. Baffle plates 100 and 101 are placed within the upper and lower chambers 54, 55 mounted at one end within grooves machined in the top and bottom faces of central portion 37 and sealed with elastomeric seals 102, 103 against the top and bottom plates 38 and 39. The baffle plates divide each ofthe chambers 54, 55 into an outer section 54a, 55a and an inner section 54b, 55b. Each baffle plate is provided with a series of uniformly spaced, uniformly sized holes 104, 105 to provide fluid communication between the outer and inner sections. The coolant channels 60 and 61 as previously described terminate in the inner section ofthe upper and lower chambers. Coolant is delivered to the outer sections ofthe upper and lower chambers, and flows through the holes 104, 105 in the baffle plate to the inner sections and from there via coolant channels 60, 61 to the exterior ofthe mould.

Fig. 4, which illustrates another embodiment, shows how cooling liquid (represented by arrow 26) can be supplied to the upper chamber 54 from below. A tubular element 75 passes through a hole 76 in bottom plate 39, completely through the lower chamber 55 and the central divider 47, and communicates with the upper chamber 54 at the free end 77 thereof. Elastomeric O-rings 78 and 79 seal the tubular element 75 to prevent coolant leakage from the lower and upper chambers 55 and 54. The tubular element 75 is attached at its outer end 80 to a coolant supply pipe (not shown) so that coolant can be supplied under pressure to the upper chamber 54. Coolant is supplied to the lower chamber 55 from below in a similar manner via a tubular element (not shown) that extends through the bottom plate 39 into the lower chamber 55.

Fig. 5 is an underneath plan view ofthe annular body 18 ofthe previous drawings showing the outlet (lower) side ofthe mould. As shown, the annular body 18 and the casting cavity 22 are rectangular so that an ingot (not shown) of rectangular cross-section is produced. The vertical casting surface 21 terminates at the sloping undercut wall 72 in which the combined outlets 70 ofthe channels 60 and 61 (not visible in Fig. 4) are located. As shown, these outlets 70 are spaced at regular intervals around the periphery ofthe casting cavity 22. The tapering inner end wall 73 ofthe bottom plate 39 are also visible in this view. The outlets also all lie in a single line and are not staggered or paired. In the case of circular moulds, the single line is most often a straight line. In Fig. 5, the outlets 70 are all shown to lie on a single straight line along each side ofthe rectangular. However, in such moulds, or in square, T-shaped and similar moulds, the single line may be most advantageously in the form of a simple curve particularly along the long faces ofthe ingot.

Figs. 6A-6D show the kinds of single lines of outlets 70 that may be employed. Fig. 6A shows a straight line (as used in Fig. 5). Figs. 6B through 6D show various curve lines that may be employed along a side (generally the long side) of a rectangular mould. Figs. 6B and 6C illustrate forms of such lines have a single inflection point along a side ofthe mould, and Figure 6C shows a form have three inflection points, which is the maximum number of inflection points that would be used along each side of a mould.

The secondary or direct cooling ofthe ingot is effected by streams 28 of liquid coolant (see Fig. 2) exiting outlets 70 and contacting the periphery 29 ofthe ingot 20 emerging from the annular body 18. Although each of these streams contains coolant from both channels 60 and 61, only as single stream 28 of coolant liquid emerges from each outlet 70 and projects onto the ingot 20. The angle at which each stream is projected against the ingot 20 is determined by the relative rate of flow of coolant liquid in the channels 60 and 61. When the rate of flow through the upper channel 60 is much greater than the flow through the lower channel 61, the stream 28 emerges from the outlet 70 at an angle similar to the angle ofthe upper channel 60, i.e. 22 degrees. On the other hand, when the rate of flow of coolant through the lower channel 61 is much greater than the rate of flow through the upper channel 60, the stream emerges from the outlet 70 at an angle that approximates the angle ofthe lower channel, i.e. 45 degrees. Relative flow rates intermediate these two conditions produce streams that emerge at particular angles within the range of 22 to 45 degrees.

The relative rates of flow of coolant through the channels 60 and 61 is controlled by valves 81 and 82 (see Fig. 1) and in the coolant supply lines 83 and 84 to the chambers 54 and 55 within the annular body 18. These lines are fed with coolant from a pump 85 fed with cooling liquid (represented by arrow 26), after filtration, from a sump (not shown) where coolant collects after use. Fresh coolant may be added to the sump to replace coolant lost to evaporation.

Two flow valves 81 and 82 may be used for control as described above. However, it is possible as well to leave the valve 81 (controlling coolant flow to the upper chamber and to the channels 60 at 22 degrees) fully open at all times (or even eliminated altogether), and control the angle of coolant stream exiting the mould solely by adjusting valve 82 (controlling the flow through the channels 61 angled at 45 degrees).

Since the angle of contact ofthe streams 28 with the periphery 29 ofthe ingot can be varied at will within the range of angles mentioned above, and since the rate of heat extraction ofthe ingot is dependent on the angle ofthe streams 28, the rate of cooling ofthe ingot can be varied during the casting operation. As noted above, it is generally necessary in DC casting to reduce the rate of cooling during the initial start-up procedure (when the bottom block 30 may be moving slowly) as the ingot butt emerges from the mould, and then to increase the rate of cooling during the steady state casting operation. It may sometimes be desirable to vary the rate of cooling during the steady state casting operation, e.g. if the surface ofthe ingot becomes unusually hot or cool, or if undesirable surface effects appear.

It may be desirable to link the control ofthe angle ofthe streams 28 with apparatus for measuring casting parameters such as ingot surface temperature, metal sump temperature, casting rate, starter block position, or intrinsic coolant properties. An intrinsic coolant property may include coolant temperature, coolant chemical composition, including gas content, or coolant quenchability coefficient. In Fig. 1 apparatus for measuring surface temperature and for measuring coolant quenchability coefficient is shown. Thus, a temperature sensor 90 may be provided in permanent or temporary contact with the surface ofthe ingot 20 at a suitable distance from the lower end 12 ofthe mould 10. Signals from the temperature sensor may fed via line 91 to a controller 92 (e.g. a computer) that adjusts the relative flow rates of coolant in the channels 60 and 61 by actuating the flow control valves 81 and 82 via lines 93 and 94. The rate of adjustment required for any sensed temperature may be programmed into the controller 92 for fully automatic operation. An apparatus for measuring ingot surface temperature is described for example in U.S. Patent No. 6,056,041 assigned to Alcan International Limited. The preferred location for measuring the ingot surface temperature is at a predetermined location with respect to the point at which the coolant stream 28 impinges on the ingot outer surface once the steady state casting has been achieved (sometimes referred to as the "normal" secondary coolant impingement point).

Similarly the quenchability coefficient ofthe coolant stream can be measured by extracting a portion 95 ofthe coolant stream 26 and passing it through an apparatus 96 for measuring this coefficient. An apparatus for measuring coolant quenchability coefficients is described for example in U.S. Patent No. 5,918,473 assigned to Alcan International Limited. The output signal from the coolant quenchability apparatus may be similarly fed via line 97 to the controller 92. The surface temperature and coolant quenchability coefficient may be used alone or in combination with each other or with other measured parameters to continuously control the angle ofthe coolant discharging on the ingot surface and thereby continuously and smoothly control the rate of heat removal.

Although, in the above embodiment, the upper channel 60 is set at an angle of 22 degrees to the ingot surface 29 and the lower channel 61 is set at an angle of 45 degrees, the angles of these channels may be varied, if required. For example, the angle ofthe lower channel 61 may be chosen from the range of greater than the angle ofthe upper channel 60 up to about 90 degrees but preferably up to about 60 degrees. The upper channel 60 may be chosen from a range of less than the angle ofthe lower channel 61 to a minimum of about 15 degrees (more preferably, a minimum of about 18 degrees). The angles ofthe upper and lower channels should, of course, be sufficiently different that a large variation in the angle ofthe emerging coolant stream may be obtained.

As shown more clearly in Figs. 7 and 8, the outer ends 68 and 69 of upper and lower channels 60 and 61 coincide at a common outlet 70. The ends of each channel 60 and 61 may be perfectly concentric at the common outlet 70, but there may be some variation as long as there is a significant overlap. Most preferably, the centre X ofthe end of one channel does not extend beyond the periphery Y of the end ofthe other channel. When this occurs, the outlet 70 may take on the shape of a "figure 8", i.e. with a substantially narrow waist portion positioned between two enlarged ends. The two channels 60 and 61 actually become one single channel at a distance B from the outlet 70 within the annular body 18. This distance B depends in practice on geometrical factors, e.g. the diameters ofthe channels and the angle of their convergence.

The stream 28 preferably has a diameter (or approximate diameter as the stream may not be quite cylindrical) in the range of 3.2 to 12.7 mm (1/8 to 1/2 inch), and preferably about 4.8 mm (3/16 inch). The number of outlets 70 provided around the casting cavity 22 ofthe mould may be the same as in a conventional mould, e.g. evenly spaced with a distance of 4.8 to 7.9 mm (3/16 or 5/16 inch) centre to centre.

The outlets 70 are preferably formed in undercut sloping surfaces 72 so that the streams 28 emerge from the annular body at a steep angle to the adjacent surface 72 and at a distance C from the adjacent surface 29 ofthe ingot. This distance allows the coolant to emerge as the required single streams and to impinge upon the metal surface at a distance D from the lower end ofthe casting surface 21 ofthe mould.

The sloping wall 73 immediately below the outlets 70 preferably slopes rearwardly and downwardly away from the outlets 70 to prevent coolant streams from "attaching" to this surface and therefore emerging at an incorrect angle of approach.

It would be possible to combine more than two channels, e.g. three channels, for even finer control over the angle ofthe emerging single combined stream 28, in which case the annular body would be provided with three coolant chambers. However, it is normally suitable to combine just two streams, as in the illustrated embodiment.

As noted above, the relative rates of flow of coolant liquid through the respective channels 60 and 61 is adjusted to manipulate the angle ofthe resulting single streams emerging from the outlets 70. It is desirable always to maintain at least a minimum rate of flow in each channel 60 and 61, i.e. the flow to one ofthe channels 60 or 61 should preferably not be entirely shut off or a venturi effect may be created, drawing liquid or air from the closed channel, and causing an uneven liquid flow from the outlet 70.

The above description represents the preferred embodiment ofthe coolant stream orienting arrangement of this invention. Other orienting arrangements may also be used. In Fig. 9, the annular mould body 110 is provided with a single internal chamber 111. The chamber communicates with the mould exterior via a tapered channel 112 which terminates in a series of holes 113 or a continuous slot for delivering coolant to the surface ofthe ingot. The upper wall ofthe chamber 112 is set at an angle of 22 degrees from the vertical and the lower wall at an angle of 45 degrees from the vertical. A deflecting plate 114 is mounted within the tapered channel on a pivot 115 located near the end adjacent the holes 113. An internal baffle 117, containing uniform, spaced holes 118, in mounted within the chamber 111 and divides it into an inner section I l ia and an outer section 11 lb. Coolant is delivered to the outer section ofthe chamber from a single source 116. In use, the deflector plate is rotated about the pivot 115 so that the amount of coolant flowing through the upper portion ofthe tapered channel can be varied with respect to that flowing through the lower portion ofthe tapered channel. The streams are joined prior to exiting the holes 113 so as to form a single stream having a direction dependent on the relative flows through the top and bottom channel sections.

In Fig. 10, the annular mould body 110 is provided with a single internal chamber 111. The chamber is divided into an inner I l ia and outer 111b portion by means of a baffle plate 117 containing a series of equally spaced uniform holes 118. The inner chamber communicates with the exterior of the mould by means of two channels 120, 121, which terminate in a single hole 122 for delivery of coolant to the ingot surface. The upper channel 120 is oriented downwardly and inwardly at an angle of 22 degrees relative to the vertical casting surface and the lower channel 121 at an angle of 45 relative to the same surface. Two sliding valves 123, 124 are mounted between the inside surface 125 ofthe inner portion ofthe chamber and the baffle plate, and these valves are moved in the vertical direction by shafts 126, 127 passing through the upper and lower sides ofthe mould body, and sealed to the mould body by means of elastomeric seals 128, 129. The valves extend along the entire inside face ofthe chamber so that the upper valve 123 can, in its lowest position, cover all the upper channels 120 along the length ofthe mould, and the lower valve 124 can, in its highest position, cover all the lower channels 121 along the length ofthe mould. In operation, the vertical positions ofthe valves is varied to control the relative flow through the upper and lower channel and thereby to change the direction ofthe coolant stream exiting the mould. The sliding valves 123, 124 can in another embodiment, be replaced by pneumatically activated elastomeric bladders, that are controlled by air/vacuum valves external to the mould so as to expand and contract and thereby alternatively cover or uncover the openings to the channels.

The embodiments of Figs. 9 and 10 both use internal mechanical or pneumatic orienting arrangements to change the angle ofthe coolant stream exiting the mould. As a result they are less preferred embodiments ofthe invention than that depicted in Figs. 2, 3 and 4 which provide a continuously adjustable fluidic orienting arrangement with no internal moving parts.

Fig. 11 is a cross-section of a further embodiment ofthe invention, using a similar baffle arrangement to that of Fig. 3. However, in this case, the bottom flange ofthe mould, the interior face, the horizontal plate dividing the two coolant chambers and the top flange ofthe mould are formed from a monolithic piece 200 into which the coolant delivery channels 201, 202 are drilled. Baffle plates 203, 204 are mounted adjacent to one interior face ofthe coolant chambers, spaced from the faces by machined ledges 205 into which sealing gaskets 206 are installed, and held in place by bolts 207. The coolant chambers are closed at the outside surface ofthe mould by a single closing plate 208, bolted to the upper and lower mould flanges and the horizontal dividing plate, by means of bolts 209 and sealing gaskets 201 are provided in the upper and lower mould flanges. The outer surface 211 ofthe horizontal dividing plate and the inner surface 212 ofthe closing plate are in mechanical contact and provide sufficient seal between the two coolant chambers.

Fig. 12 is a cross-section of a further embodiment ofthe invention, similar to Fig. 11, except that the closing plate 220 is mounted by means of bolts 221 through the upper and lower mould flanges. Gaskets 222 are provided in the upper and lower mould flanges, and also in the end 223 ofthe horizontal dividing plate. A mounting bracket 224 for mounting the mould within a casting table (not shown) is provided, attached to the closing plate 220 by means of bolts 225 which are welded at one end 226 to the closing plate. The mounting bracket may also be welded to the closing plate. A similar mounting plate may be used in the embodiment of Fig. 11.

Fig. 13 is a cross-section of a further embodiment ofthe invention, similar to Fig. 12, except that the mould is mounted on a portion ofthe casting table 230 by means of bolts 231 welded into the bottom flange by means of a weld on the inside surface. Such a mounting system is also applicable to the embodiment of

The closing plates of Figs. 11 to 13 represent a general means of forming a mould containing multiple coolant chambers, in which a monolithic piece is used to form the inner face ofthe mould, the top surface, bottom surface and any interior dividing surfaces, and the coolant chambers are closed by a single closing plate on the exterior surface ofthe mould. The closing plate may be held in place by bolts passing into the monolithic piece and sealing gaskets between the face of the closing plate and the monolithic piece.

Without wishing to be bound by any theory, it can be stated that, when secondary or direct cooling is employed during DC casting, the cooling achieved by the liquid coolant passes through at least four stages. First, at high temperature, the coolant is vaporized on contact with the hot metal surface and the vapour may insulate the metal from further contact with the coolant, so that the overall cooling rate is low. As the metal cools somewhat there is a transition phase leading to a nucleate boiling phase in which bubbles form in the coolant film present on the metal surface. In this phase, the rate of heat extraction may be quite high, but is somewhat uneven. Finally, the coolant forms a continuous film on the metal surface that cools by convection, whereby even cooling can be achieved. The use of a high angle of approach ofthe cooling streams onto the metal surface helps the cooling to transition quickly from the vapour phase, through the nucleate boiling phase and to the convection phase. During the start- up it is desirable to provide a continuous, smooth and reproducible increase ofthe angle of approach ofthe coolant streams to ensure that a controllable cast startup can be achieved for a wide variety of alloys and that cast failures are minimized. The control over the approach angle ofthe streams according to the present invention makes this rapid transition possible.

The invention may, if desired, be used in combination with conventional procedures for varying the rate of cooling of an ingot during DC casting, e.g. by pulsing the streams rapidly on and off during one phase ofthe casting procedure (e.g. during start-up), or by introducing a gas into the coolant liquid at various stages ofthe casting procedure to vary the cooling coefficient ofthe combined cooling medium.

It has been found that the invention works with several alloys that are difficult to cast with conventional means. The invention is therefore suitable for use with most other DC castable metals and alloys.

While the invention has been described in connection with a vertical DC casting apparatus, it could also be used equally effectively with the so-called horizontal DC casting apparatus that is capable of casting longer ingot lengths.

Claims

WHAT WE CLAIM IS:

1. A process of producing a cast and cooled metal ingot, comprising: casting a molten metal by a direct chill casting operation to form a metal ingot emerging from a mould, and directing one or more streams of liquid coolant onto an outer surface of said metal ingot adjacent to the mould at positions spaced around the periphery ofthe ingot to achieve a rate of heat extraction from the ingot, characterized in that said one or more streams of liquid coolant are orientated at an angle relative to the outer surface ofthe ingot, and said angle is varied during said casting operation to change said rate of heat extraction from said ingot to minimize cooling-related defects in said cast and cooled ingot.

2. A process according to Claim 1, characterized in that said angle may be varied continuously.

3. A process according to Claim 1, characterized in that each of said one or more streams is formed by the combination of two or more streams internally within the mould to form a single stream exiting the mould.

4. A process according to Claim 1, Claim 2 or Claim 3, characterized in that said angle is varied continuously between predetermined limits in response to at least one measured parameter ofthe casting system.

5. A process according to Claim 4, characterized in that said at least one measured parameter is selected from the group including ingot surface temperature measurement, metal sump temperature, stool cap position, casting speed and intrinsic coolant property.

6. A process according to Claim 5, characterized in that said intrinsic coolant property is a coolant quenchability factor.

7. A process according to Claim 5, characterized in that said ingot surface temperature measurement is measured at a predetermined distance from a point at which the said streams of coolant impinge on the outer surface of the ingot after the cast has reached a steady state.

8. A process according to any one of Claims 1 to 7, characterized in that said one or more streams of liquid coolant exit the mould along a single line.

9. A process according to Claim 8, characterized in that said single line is selected from the group consisting of a straight line, and a curve having no more than three inflection points on one side ofthe mould.

10. A process according to any one of Claims 1 to 9, characterized in that each individual stream of said one or more streams is produced by passing liquid coolant under pressure through at least two channels, each channel being orientated at a different angle with respect to said surface of said ingot, and combining said coolant from said channels at a common outlet of said channels to produce said individual stream.

11. A process according to Claim 10, characterized in that said angle of said one or more streams relative to the outer surface ofthe ingot is varied by changing the relative flow ofthe coolant in said at least two channels provided for each individual stream.

12. Apparatus for producing a cast and cooled metal ingot, comprising: a direct chill casting mould having an annular body defining a casting cavity, for casting molten metal into a metal ingot having a periphery, and a mould outlet from which said metal ingot emerges as casting proceeds during a casting operation, and one or more openings in said annular body adjacent to said mould outlet for directing one or more streams of liquid coolant onto an outer surface of said metal ingot at positions spaced around the periphery ofthe ingot to achieve a rate of heat extraction from the ingot, characterized in that said apparatus comprises an orientating arrangement within said annular body for orientating said one or more streams emerging from said openings at an angle relative to the surface of the ingot, and for enabling variation of said angle as said casting operation proceeds to minimize cooling-related defects in the cast and cooled ingot.

13. Apparatus according to Claim 12, characterized in that said orienting arrangement causes the said angle ofthe said one or more streams to be continuously variable.

14. Apparatus according to Claim 12 or Claim 13, characterized in that said one or more openings lie along a single line.

15. Apparatus according to Claim 14, characterized in that said single line is selected from the group consisting of a straight line and a curve have three or less inflection points along a side ofthe said mould.

16. Apparatus according to Claim 12, Claim 13, Claim 14 or Claim 15, characterized in that said orienting arrangement comprises, for each ofthe said one or more openings, two or more internal channels that meet

-internally within the mould body to form a single channel before exiting the mould.

17. Apparatus according to any one of Claims 12 to 16, characterized in that said orienting arrangement is selected from the group consisting of a hydraulic means, a mechanical means, or a pneumatic means.

18. Apparatus according to any one of Claims 12 to 17, characterized in that said orienting means is controlled so as to cause the angle of impingement on the said one or more coolant sfreams to vary between predetermined limits in response to one or more measured casting parameters.

19. Apparatus according to Claim 18, characterized in that said at least one measured parameter is selected from the group including ingot surface temperature measurement, metal sump temperature, stool cap position, casting speed and intrinsic coolant property.

20. Apparatus according to Claim 19, characterized in that said intrinsic coolant property is the coolant quenchability factor.

21. Apparatus according to Claim 19, characterized in that said ingot surface temperature measurement is measured at a predetermined distance from the point at which the said streams of coolant impinge on the outer surface ofthe ingot after the cast has reached a steady state.

22. A direct chill casting mould having an annular body defining a casting cavity, for casting molten metal into a metal ingot having a periphery, and a mould outlet from which said metal ingot emerges as casting proceeds during a casting operation, and one or more openings in said annular body adjacent to said mould outlet for directing one or more sfreams of liquid coolant onto an outer surface of said metal ingot at positions spaced around the periphery ofthe ingot to achieve a rate of heat extraction from the ingot, characterized in that said mould comprises an orientating arrangement within said annular body for orientating said one or more streams emerging from said openings at an angle relative to the surface of the ingot, and for enabling variation of said angle as said casting operation proceeds to minimize cooling-related defects in the cast and cooled ingot.

23. A mould having a mould body and one or more coolant chambers within the mould body, characterized in that a single monolithic piece forms an inner surface ofthe mould, a top wall, a bottom wall and dividing walls dividing said mould body internally into said coolant chambers, and forming three sides of each coolant chamber, and a separate closing plate providing a exterior wall ofthe mould and forming a fourth side of each said coolant chamber.