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, 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:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    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.  
           [0003]    2. Background Art  
           [0004]    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 of the mould as an ingot. The procedure is commenced by positioning a bottom block in the lower opening of the 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 of the 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.  
           [0005]    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 of the ingot as it is being formed. In addition, casting apparatus of this kind is provided with a means of secondary cooling (often referred to as direct cooling) of the metal. For example, jets of water or other liquid coolant are directed onto the outer surfaces of the metal ingot as the ingot emerges from the mould. This provides the bulk of the ingot cooling and has a major effect on the ingot microstructure.  
           [0006]    However, different rates of cooling of the ingot are required at different stages of ingot formation. The start of the 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 of the ingot microstructure.  
           [0007]    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 heat transfer 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.  
           [0008]    An alternative arrangement is disclosed in U.S. Pat. No. 5,582,230, which issued on Dec. 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 of the lower end of the mould body so that cooling water can be directed as individual streams onto the emerging ingot surface. The channels of the first series are all oriented to produce water streams that impinge against the ingot surface at angles of 45 degrees, and the channels of the second series are all orientated to produce water streams that impinge against the ingot surface at angles of 22.5 degrees. Because of the high angle of incidence of the 45 degree streams, substantial portions of the coolant rebounds from the ingot surface and form a region of spray directly in the path of the 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 of the 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 of the 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.  
           [0009]    U.S. Pat. No. 4,351,384, which issued on Sep. 28, 1982 to David G. Goodrich, and was assigned to Kaiser Aluminum &amp; 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 of the 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 of the coolant streams, the point of impact of the coolant with the metal surface can be brought closer to the discharge point of the 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.  
           [0010]    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 of the cooling, especially in the transition from the startup to the steady state.  
         SUMMARY OF THE INVENTION  
         [0011]    An object of the invention is to facilitate variation of cooling of an ingot during DC casting at different times during the casting procedure.  
           [0012]    Another object of the invention is to provide an improved means to control the cooling of an ingot during the casting procedure.  
           [0013]    According to one aspect of the 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 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 said casting operation to change the rate of heat extraction from said ingot.  
           [0014]    The angle of the liquid coolant streams is preferably varied continuously during said casting operation. It is preferably in between predetermined limits and is preferably varied in response to at least one measured parameter of the 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 of the 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.  
           [0015]    According to a preferred embodiment, this provides 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 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 said 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 said casting operation to change said rate of heat extraction from said ingot to minimize cooling-related defects in said cast and cooled ingot. Each of the 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.  
           [0016]    It is further preferred that the one or more streams of liquid coolant exit the mould along a single line.  
           [0017]    According to another aspect of the 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 of the 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 of the ingot, and for enabling variation of the angle as the casting operation proceeds.  
           [0018]    In the above apparatus, the orienting arrangement preferably causes the angle of the 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.  
           [0019]    Preferably, the orienting arrangement comprises, for each of the 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 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.  
           [0020]    Thus, a further preferred embodiment comprises 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 the metal ingot emerges as casting proceeds during a casting operation. One or more openings are provided in the annular body lying in a single line adjacent to said 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 of the 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 of the ingot, and for enabling variation of the angle as the casting operation proceeds to minimize cooling-related defects in the cast at an angle relative to the surface of the ingot, and for enabling variation of the angle as said casting operation proceeds to minimize cooling-related defects in the cast and cooled ingot.  
           [0021]    According to a further preferred embodiment, the above orienting arrangement comprises, for each of the 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.  
           [0022]    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. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is a simplified sketch showing a casting and cooling operation using apparatus according to one preferred form of the present invention;  
         [0024]    [0024]FIG. 2 is a cross-section of the annular body of the mould at one side of the apparatus as shown in FIG. 1;  
         [0025]    [0025]FIG. 3 is a cross-section of the annular body of the mould at one side of the apparatus as shown in FIG. 1 illustrating a further embodiment of the present invention;  
         [0026]    [0026]FIG. 4 is a view similar to FIG. 1 showing a position where coolant liquid is supplied to the upper chamber of the annular body;  
         [0027]    [0027]FIG. 5 is an underneath plan view of the annular body of FIG. 1;  
         [0028]    [0028]FIG. 6 is an additional schematic view showing how the coolant exit holes in the annular body of FIG. 1 may be placed;  
         [0029]    [0029]FIG. 7 is a partial view on an enlarged scale of part of the annular body of FIG. 5;  
         [0030]    [0030]FIG. 8 is a partial cross-section of the part of the annular body of FIG. 6 rotated through 90 degrees, and also showing part of an adjacent cast ingot;  
         [0031]    [0031]FIG. 9 is a cross-section of the annular body of the mould at one side of the apparatus showing further embodiment of the present invention; and  
         [0032]    [0032]FIG. 10 is a cross-section of the annular body of the mould showing yet a further embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    [0033]FIG. 1 of the 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.  
         [0034]    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 . 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 of the resulting ingot.  
         [0035]    The lower end of the mould  10  is provided with a plurality of outlets through which a cooling liquid  28  is projected as individual streams onto the outer periphery  29  of the 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.  
         [0036]    At the start of a casting operation, the lower end of the 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 of the cavity, the bottom block  30  is drawn vertically downwards by operation of the ram  31 . The solidifying base of the ingot  20  being cast, then resting on the bottom block  30 , then begins to emerge from the lower end of the casting cavity  22 .  
         [0037]    Molten metal is continuously supplied to the upper end of the casting cavity  22  through dip tube  15  that opens downwardly into an upper portion of the 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.  
         [0038]    During the DC casting operation thus described, molten metal  14  within the casting cavity  22  solidifies around the periphery of the 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 of the mould that the ingot emerging from the lower end of the mould has an externally solid and self-sustaining shell  33  even though the central portion or core  34  of the emerging ingot is still molten. With an effectively continuous supply of molten metal to the mould, and correspondingly continuous downward advance of the cast ingot from the mould, the molten central core  34  of the ingot emerging from the mould extends downwardly as a molten metal sump (constituting the lower end of the molten metal pool in the mould) of progressively decreasing cross-section in the downward direction, until the ingot becomes entirely solid.  
         [0039]    The mould  18  contains two internal chambers  54 ,  55  used to provide cooling to the mould body and to deliver coolant  28  via channels  60 ,  61  to impinge on the ingot surface. The arrangement of internal chambers and channels is described in detail in the following.  
         [0040]    A source of coolant is provided to each chamber via pipes  83 ,  84  and control valves  81 ,  82 .  
         [0041]    [0041]FIG. 2 is an enlarged cross-sectional view of the annular mould body  18  as shown at the left-hand side of the 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  of the central portion  37 . The enlarged heads  48  and  49 , respectively, of the 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  53 ,  54  and  55  are provided between the central portion  37  and the top and bottom plates  38  and  39  to prevent leakage of coolant.  
         [0042]    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  of the 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  of the mould. At their respective inner ends, 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  of the 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 .  
         [0043]    In all cases, the outer ends  68  and  69  of the 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 of the casting surface  21 . The horizontal distance between the inner and outer ends of the undercut wall (as shown by arrows A) is typically about 6 mm. The inner end surface of the 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 .  
         [0044]    [0044]FIG. 3 is an enlarged cross-sectional view of the annular mould body  18  as shown at the left-hand side of the apparatus of FIG. 1, but shows a further preferred embodiment of the invention. The embodiment is similar to that illustrated in FIG. 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 of the chambers  54 ,  55  into an outer section  54   a ,  55   a  and an inner section  54   b ,  55   b . 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 of the upper and lower chambers. Coolant is delivered to the outer sections of the upper and lower chambers, and flows through the holes in the baffle plate to the inner sections and from there via to coolant channels to the exterior of the mould.  
         [0045]    [0045]FIG. 4 shows how cooling liquid  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 .  
         [0046]    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 .  
         [0047]    [0047]FIG. 5 is an underneath plan view of the annular body  18  of the previous drawings showing the outlet (lower) side of the 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  of the channels  60  and  61  (not visible in FIG. 4) are located. As shown, these outlets  70  are spaced at regular intervals around the periphery of the casting cavity  22 . The tapering inner end wall  73  of the bottom plate  39  are also visible in this view. The outlets also all lie in a single line, i.e. do not lie vertically one above the other. 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 of the 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 of the ingot.  
         [0048]    Although the outlets lie on a single line, that line can be in the form of any convenient curve. FIGS.  6 A- 6 D show in exaggerated form preferred embodiments of the kinds of single lines that may be employed. FIG. 6A shows a straight line (as used in FIG.  5 ). FIGS. 6B through 6D show various curved 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 maximum or minimum along a side of the mould, and FIG. 6C shows a form having three maxima or minima, which is the maximum number of such points that would be used along each side of a mould.  
         [0049]    The secondary or direct cooling of the ingot is effected by streams  28  of liquid coolant (see FIG. 1) exiting outlets  70  and contacting the periphery  29  of the ingot  20  emerging from the annular body  18 . Although each of these streams contains coolant from both channels  60  and  61 , only a 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 of the 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 of the 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.  
         [0050]    The relative rates of flow of coolant through the channels  60  and  61  is controlled by valves  81  and  82  and in the coolant supply lines  83  and  84  to the chambers  54  and  55  within the annular body  18  (see FIG. 1). These lines are fed with coolant from a pump  85  fed with cooling liquid  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. The relative rates of flow may be adjusted either to maintain a constant total flow or allow variations in total flow as well as the relative flow.  
         [0051]    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 channel 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 channel at 45 degrees).  
         [0052]    Since the angle of contact of the streams  28  with the periphery  29  of the ingot can be varied at will within the range of angles mentioned above, and since the rate of heat extraction of the ingot is dependent on the angle of the streams  28 , the rate of cooling of the 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 of the ingot becomes unusually hot or cool, or if undesirable surface effects appear.  
         [0053]    As shown in FIG. 1, it may be desirable to link the control of the angle of the streams  28  with apparatus for measuring casting parameters such as ingot surface temperature, metal sump temperature, casting rate, starter block position, or coolant properties. A 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 of the ingot  20  at a suitable distance from the lower end  12  of the 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 filly automatic operation. An apparatus for measuring ingot surface temperature is described for example in U.S. Pat. No. 6,056,041 assigned to Alcan International Limited and incorporated herein by reference. The preferred location for measuring the ingot surface temperature is at a predetermined location with respect to the point at which the coolant stream  26  impinges on the ingot outer surface once the steady state casting has been achieved (sometimes referred to as the “normal” secondary coolant impingement point).  
         [0054]    Similarly the quenchability coefficient of the coolant stream can be measured by extracting a portion  95  of the 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. Pat. No. 5,918,473 assigned to Alcan International Limited and incorporated herein by reference. The output signal from the coolant quenchability apparatus may be similarly fed via line  97  to the controller  92 .  
         [0055]    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 of the coolant discharging on the ingot surface and thereby continuously and smoothly control the rate of heat removal.  
         [0056]    Although, in the above embodiment, the upper channel is set at an angle of 22 degrees to the ingot surface  29  and the lower channel is set at an angle of 45 degrees, the angles of these channels may be varied, if required. For example, the angle of the lower channel  61  may be chosen from the range of greater than the angle of the 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 of the lower channel  61  to a minimum of about 15 degrees (more preferably, a minimum of about 18 degrees). The angles of the upper and lower channels should, of course, be sufficiently different that a large variation in the angle of the emerging coolant stream may be obtained.  
         [0057]    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 of the end of one channel does not extend beyond the periphery Y of the end of the other channel. When this occurs, the outlet  70  may take on the shape of a “figure of 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 of the channels and the angle of their convergence.  
         [0058]    The stream  28  preferably has a diameter (or approximate diameter as the stream may not be quite cylindrical) in the range of about 3-13 mm, and preferably about 5 mm. The number of outlets  70  provided around the casting cavity  22  of the mould may be the same as in a conventional mould, e.g. evenly spaced with a distance of 4 to 12 mm centre to centre.  
         [0059]    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  of the 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 of the casting surface  21  of the mould.  
         [0060]    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.  
         [0061]    It would be possible to combine more than two channels, e.g. three channels, for even finer control over the angle of the 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.  
         [0062]    As noted above, the relative rates of flow of coolant liquid through the respective channels  60  and  61  is adjusted to manipulate the angle of the resulting single streams emerging from the outlets  70 . It is preferable always to maintain at least a minimum rate of flow in each channel  60  and  61 , i.e. the flow to one of the 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 .  
         [0063]    The above description represents the preferred embodiment of the coolant stream orienting arrangement of this invention. Other orienting arrangements may also be used.  
         [0064]    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 of the ingot. The upper wall of the 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  111   a  and an outer section  111   b . Coolant is delivered to the outer section of the 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 of the tapered channel can be varied with respect to that flowing through the lower portion of the 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.  
         [0065]    In FIG. 10, the annular mould body  110  is provided with a single internal chamber  111 . The chamber is divided into an inner  111   a  and outer  111   b  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  of the inner portion of the 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 of the mould body, and sealed to the mould body by means of elastomeric seals  128 ,  129 . The valves extend along the entire inside face of the chamber so that the upper valve  123  can, in its lowest position, cover all the upper channels  120  along the length of the mould, and the lower valve  124  can, in its highest position, cover all the lower channels  121  along the length of the mould. In operation, the vertical positions of the valves is varied to control the relative flow through the upper and lower channel and thereby to change the direction of the 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.  
         [0066]    The embodiments of FIGS. 9 and 10 both use internal mechanical or pneumatic orienting arrangements to change the angle of the coolant stream exiting the mould. As a result they are less preferred embodiments of the invention than that depicted in FIGS. 2, 3 and  4  which provide a continuously adjustable fluidic orienting arrangement with no internal moving parts.  
         [0067]    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 of the 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 of the angle of approach of the 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 of the streams according to the present invention makes this rapid transition possible.  
         [0068]    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 of the casting procedure (e.g. during start-up), or by introducing a gas into the coolant liquid at various stages of the casting procedure to vary the cooling coefficient of the combined cooling medium.  
         [0069]    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.  
         [0070]    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.