Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD AND APPARATUS FOR REMOVAL OF COOLING WATER
FROM INGOTS BY MEANS OF WATER JETS
TECHNICAL FIELD
This invention relates to casting of metal ingots. More particularly,
the invention relates to the cooling of such ingots as they emerge from a
casting apparatus by the application and removal of cooling water to the
outer surfaces of the ingots.
BACKGROUND ART
There are various techniques for casting metal ingots, such as
direct chill (DC) casting (a technique that includes electromagnetic casting
(EMC)), hot top technologies for the production of rolling slab ingots,
forging ingots, extrusion ingots (billets), etc. These various casting
techniques may involve the application of cooling media to the external
surface of the ingots as they emerge from the mold to ensure ingot surface
solidification and to reduce the likelihood of molten metal bleedout from
the interior of the ingot before the ingot becomes fully solid. Frequently,
the ingots are cast vertically, but horizontal casting is also practiced as,
for
example, in horizontal direct chill casting (HDC). In the case of vertical
direct chill casting, in particular, cooling water is directed onto the outer
surface of the ingot around the bottom of the mold and the cooling water
flows down the sides of the ingot.
For some purposes, it is desirable to remove the cooling water from
the surface of the ingot at a certain distance from the mold exit. This
reduces the rate of cooling of the ingot from that point on because the
surface becomes air cooled rather than water cooled. As shown, for
example, in U.S. patent 4,237,961 to Zinniger on December 9, 1980, the
cooling water may be removed by means of physical wipers or squeegee-
like devices that contact the metal surface, but the surface of the ingot is
still hot and wiper devices may quickly become degraded, especially if
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there is an instance of molten metal bleed-out that brings molten metal into
contact with the elastomeric material of the wiper or the metal of the
supporting structure. It may also be difficult to employ mechanical wipers
of this kind at an early stage in the casting process. The geometry of the
butt (bottom) of the ingot makes mechanical wiping schemes difficult,
especially in the case of thin ingots. For example, in DC casting, during
the initial fill, start down, primary and secondary curl, metal sometimes
dribbles or bleeds out of the mold and the molten metal may collect on the
wiper and burn the elastomeric contact material prior to its being able to
wipe the ingot. Therefore, the wiper is not usually deployed until after the
incidence of butt-curl, i.e. only after the ingot has emerged by 10 to 14
inches. Wipers which mechanically engage the ingot cannot be engaged
prior to final curl, so again the first 10 to 14 inches of the ingot is
substantially cooled prior to any water being removed. After wiper
engagement, the dissimilar temperatures between the butt portion and run
portion generates varied metallurgical structures and stresses which can
result in further processing problems or the formation of scrap while
casting, preheating and rolling.
It is known to remove cooling water by means of jets of gas, such
as compressed air, that blow the cooling water from the cast metal, for
example as disclosed in US patent 2,705,353 to Zeigler which issued on
April 5, 1955. However, compressed air wipers are costly to install and
use because of inefficiencies involved in pressurizing compressible gases.
US patent 5,685,359 to Wagstaff et al. shows coolant spray holes
with overlapping spray patterns for use in direct secondary cooling, but the
spray holes are not used for coolant water removal.
US patent 5,431,214 to Ohatake et al. mentions cooling water jets,
but again such jets are not used for coolant water removal.
There is a need for improved ways of removing surface cooling
water from such ingots.
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DISCLOSURE OF THE INVENTION
An exemplary embodiment of the invention provides a method of
removing cooling water from a surface of a metal ingot, wherein the
cooling water streams over the surface in a casting direction. The method
involves directing one or more water sprays onto the surface of the ingot at
an angle and rate of flow effective to cause the cooling water streaming
over the surface to separate from the surface as the cooling water
encounters the sprays. Preferably, enough of the cooling water is
removed to allow natural film boiling to occur, thereby removing all of the
cooling water within a short distance of the water sprays.
Another exemplary embodiment provides an apparatus for
removing cooling water from a surface of a metal ingot, wherein the
cooling water streams over the surface in a casting direction. The
apparatus includes one or more nozzles adapted to direct water sprays
onto the surface, the nozzles being positioned and angled such that the
water sprays are effective in use to cause the cooling water streaming
over the surface to separate from the surface as the cooling water
encounters the sprays. The apparatus also includes one or more conduits
for supplying water to the nozzles, and pressurizing apparatus for
pressurizing water supplied to the nozzles.
According to these exemplary embodiments, water jets or sprays
are used to remove cooling water from the surface of an ingot as it is being
cast. The apparatus for producing the water jets is economical to provide
and operate given that the removal medium is water (which may be taken
from the same source as the water used for cooling the ingot). The
apparatus and method may be used early during casting operations and
close to the outlet of the casting mold as the jets are not affected by
molten metal bleed out and they follow any variations in the profile of the
ingot as it is being produced.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention is described in more detail in the following with
reference to the accompanying drawings, in which:
Fig. 1 is a vertical cross-section of a known direct chill casting mold
provided with a mechanical wiper for removal of cooling water;
Fig. 2 is a horizontal cross-section of an ingot being cast by DC
casting showing an exemplary embodiment of apparatus for removing
cooling water;
Fig. 3 is an enlargement of part of the apparatus of Fig. 2, showing
water jets in action;
Fig. 4 is a vertical section of part of the apparatus of Fig. 2 prior to
activation of the water jets;
Fig. 5 is the same view as Fig. 4 but showing the apparatus
following activation of the water jets;
Fig. 6 is a vertical cross-section similar to that of Fig. 5 but showing
an exemplary embodiment that makes use of a scupper to remove cooling
water;
Fig. 7 is a horizontal cross-section of an alternative exemplary
embodiment that makes use of a corrugated shield wall to form channels
for cooling water stripped from the ingot surface.
Figs 8 to 10 illustrate alternative embodiments making use of
narrow cylindrical water jets to remove cooling water from an ingot; and
Fig. 11 is a cross-section illustrating an exemplary embodiment
applied to horizontal DC casting.
BEST MODES FOR CARRYING OUT THE INVENTION
The exemplary embodiments of the present invention may be used
with apparatus of many kinds that employ streams of water to cool a
newly-formed metal ingot, e.g. an ingot of a non-ferrous or light metal,
such as an ingot of an aluminum, magnesium or copper alloy. However,
the exemplary embodiments are especially suitable for use with DC
casting apparatus and one form of such apparatus is shown in Fig. 1 and
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is briefly described below so that the preferred and exemplary
embodiments may be better understood, although it is to be noted that the
present invention is not limited to equipment of this kind.
Fig. 1 is a vertical cross-section of a direct chill casting mold
5 producing a metal ingot and showing a known arrangement for removing
cooling water from the outer surface of the ingot. This apparatus is
disclosed in U.S. patent publication no. 2007/0102136 to Wagstaff et al.,
published on May 10, 2007 (the disclosure of which is specifically
incorporated herein by this reference). The mold is indicated generally at
10 and it is provided with an open upper entrance 11 and an open lower
exit 12. Molten metal is introduced into the entrance of the mold as
indicated by arrow 13. The mold includes a primary cooling channel 14
filled with recirculating cooling water 15 that cools the inner wall of the
mold. The molten metal cools adjacent to the mold wall and forms an
embryonic ingot 16 that emerges from the mold. The embryonic ingot has
a molten metal sump 17 surrounded by a solid metal shell 18 which
increases in thickness as the ingot descends until full solidification occurs
at a point remote from the exit 12 of the mold to form a fully solid ingot 19.
Streams or jets of cooling water 20 are poured onto the surface of the
ingot from the channel 14 adjacent to the lower exit 12 of the mold to help
to form and maintain the solid outer shell 18 around the molten metal
sump. The water streams down along the sides of the embryonic ingot,
but is removed by a mechanical wiper 21 positioned at a distance X from
the exit of the mold. The cooling water 20 removed in this way forms
streams 22 spaced from the ingot 19 that have no further cooling effect.
The wiper is in the form of an annulus made of a soft flexible or
elastomeric material that physically contacts the outer surface of the ingot
to wipe away the cooling water. The wiper is held in a rigid holder (not
shown) made of metal or the like. In the apparatus of Fig. 1, distance X is
made such as to allow the ingot to "self homogenize". Of course, there are
other reasons why the cooling water may be removed at a predetermined
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distance from the mold, so the exemplary embodiments are not limited to
this one purpose.
In preferred exemplary embodiments of the present invention, a
mechanical wiper of the kind shown at 21 may be replaced by a series of
water jets that remove the cooling water from the surface of the ingot.
This is shown by way of example in Figs. 2 to 11 of the accompanying
drawings. Fig. 2 shows a horizontal cross-section of an ingot at a distance
below a direct chill casting mold where cooling water is to be removed.
The ingot 19 (or embryonic ingot 16) having a downwardly streaming
surface layer of cooling water 20 is completely surrounded at a narrow
horizontal spacing by a short solid vertical wall 25 (made, for example, of a
metal such as aluminum or stainless steel) that extends downwardly from
a bottom wall 26 of a direct chill casting mold 10 (see Figs. 4 and 5). The
wall 25 is not essential, but acts as a shield to prevent water from spraying
onto any other ingots that may be cast at the same time in adjacent areas.
The wall 25 is penetrated by a number of holes or slots 27 all positioned at
the same vertical height in the illustrated embodiment. An elongated
nozzle 28 extends through each slot from outside the wall and terminates
a short distance from the outer surface 29 of the ingot. As best seen in
Fig. 2, the nozzles 28 on each side of the ingot 19 are connected to a
manifold 30 that supplies water under pressure to the nozzles, and the
manifolds are connected together in series by high pressure flexible hoses
31, 32 and 33. The first manifold in the series is connected by a flexible
high pressure hose 34 to an apparatus 35, e.g. a pump, for supplying
water under pressure. When supplied with water under pressure in this
way, the nozzles each spray a jet 36 (Fig. 3) of water towards the surface
29 of the ingot. It will be noted that each of the nozzles forms a jet 36
having the shape of a flat fan of water. Thus, the jets 36 are generally flat
in vertical side view but expand outwardly in plan view, so that they extend
vertically by a much smaller distance than they extend horizontally. The
fan shaped jets 36 are preferably partially overlapping, as shown. The
angle at the apex of the water jets (as shown in the plan view of Fig. 3) is
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preferably at least 65 , and may be 72 , or more. The nozzles are
preferably spaced from each other (and/or from the ingot) by distances
effective to provide an overlap of the water jets of 1 - 2 inches at the ingot
surface 29. While this arrangement is particularly preferred, it will be
noted from later embodiments that nozzles producing water jets of other
shapes may alternatively be employed, e.g. cylindrical jets, and that
overlap of the jets may not always be necessary.
The manifolds 30 may be of any size and shape, but are preferably
square in cross-section (e.g. of 11/4 inches per side) and the nozzles 28 are
preferably arranged at intervals of up to about 5 inches from each other,
although this may be varied to suit particular molds and spacing
arrangements. For standard DC casting equipment, the manifolds 30 may
be, for example, 1720 mm long (long side of ingot) and 560 mm long
(short side of ingot). The pressure of the water supplied to the nozzles 28
should be adequate for the removal of most or all of the coolant water from
the surface of the ingot and is preferably at least 80 psi up to about 150
psi, and more preferably is in the range of 100 -120 psi, to give a rate of
flow at each nozzle of at least 0.4 gallons per minute per linear inch of
distance around the mold circumference (gpm/in) up to about 1.5 gpm/in,
(ideally in the range of 0.6 - 1.0 gpm/in). The mold discharge flow rate
(flow rate relating to the overall water discharge from the mold in advance
of the wipers) is preferably at least 0.6 gpm/in up to about 1.5 gpm/in, and
is preferably in the range of 0.7 - 1.0 gpm/in. The high pressure hoses 31,
32, 33 and 34 are preferably attached to the manifolds by quick release
fittings so that they may be easily disconnected and re-connected to allow
the replacement of one or more of the manifolds if they become blocked or
otherwise require attention. Moreover, the manifolds 30 are preferably
supported on equipment (not shown) that allows them to be moved closer
to or further away from the ingot 19, and/or closer to or further away from
the casting mold. Also, it is desirable to make the nozzles rotatable about
a horizontal axis to make it possible to adjust the angle of spray relative to
the ingot surface, as circumstances dictate.
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The action of the jets is best shown in Figs. 4 and 5, which are
detailed vertical sectional views in the region of the bottom wall 26 of the
casting mold 10. The manifolds 30 have been omitted from these
drawings for the sake of simplicity but are positioned immediately outside
the walls 25. Fig. 4 shows the situation before the jets are started.
Nozzles 28 extend through the vertical wall 25 and face the surface 29 of
the ingot 19 emerging from an exit 12 of the casting mold. Cooling water
20 is streamed onto the surface 29 from apertures in the bottom of
channel 14 of the mold and the water streams in a continuous layer
downwardly along the outer surface of the ingot (as represented by arrow
A). Without operating the water jets, the cooling water streams down the
ingot in this way until it reaches the bottom of the ingot or a water
collection pool. As shown in Fig. 5, in order to remove the cooling water at
distance X from the bottom of the mold, the nozzles 28 are supplied with
water under pressure to create flat fan-shaped jets 36 of water that contact
the surface 29 of the ingot. When the jets have sufficient momentum
(volume of water and rate of flow), and a suitable angle a relative to the
surface 29 (preferably in the range of 65 to 75 , and more preferably 68 to
72 ) with a component of movement countercurrent to the direction of flow
of the cooling water 20, they strip the cooling water 20 from the ingot
surface and force it to adopt an upward flow 40 (as indicated by arrow B)
after leaving the ingot surface 29. This means that the nozzles 28 are
preferably angled upwardly from the horizontal (when the streams 20 flow
downwardly) at an angle of 15 to 25 , and more preferably 19 to 22 ,
although the most effective angle may be determined in particular
situations by trial and experimentation. The overlap of the jets further
helps to remove the streaming water from the ingot because the
momentum created by the water in the overlap region helps to cause the
streaming water to spray away from the ingot with an "interactive fountain"
effect. Ideally, sufficient cooling water is removed in this way to leave just
a thin residual film that quickly dries off due to the high temperature of the
ingot.
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Preferably, the upward flow 40 of cooling water is caused to bounce
off the bottom wall 26 of the casting mold without impacting the junction
between the ingot and the mold and entering the mold cavity, and is then
caused to run down the inside surface 42 of the vertical wall 25 (as
indicated by arrow C) so that there is no further contact between the
cooling water and the surface 29 of the ingot beyond distance X. The
cooling water is thus stripped from the surface without any direct contact
from mechanical parts of the apparatus.
It should be noted that sufficient cooling water should be stripped
from the surface 29 to achieve a desired reduction of cooling of the ingot
beyond distance X. Ideally, all or substantially all of the cooling water is
removed in this way, but this is not always essential (or perhaps possible)
because small amounts of cooling water remain beyond distance X.
However, these residual amounts normally disappear quickly or even
instantly due to evaporation caused by the heat of the ingot. Also,
according to the cooling effect desired in any particular case, a small
amount of residual cooling water may be acceptable, even if it does not
disperse immediately by evaporation. Preferably, at least 90% of the
volume of the cooling water above point X, more preferably at least 95%,
and even more preferably at least 99%, is removed by the water jets
themselves to leave just a sub-film that is quickly or even substantially
instantly removed by evaporation.
The spacing of the nozzles from the ingot is preferably optimized
according to the following considerations. The closer the nozzles are
positioned to the ingot, the higher will be the momentum of the water in the
jets as they contact the ingot surface, but the more at risk the nozzles will
be from damage if molten metal bleeds out of the mold or ingot during the
casting operation. Also, the closer the nozzles are positioned to the ingot,
the greater the number of nozzles will be required in order to provide a
constant line of impacting water around the entire periphery of the ingot.
Therefore, the spacing of the nozzles from the ingot should be made as far
as possible without causing the momentum of the water in the jets to
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diminish to a point below their effectiveness for stripping cooling water
from the ingot.
The distance X at which the water jets are applied to the ingot
surface depends on the reason for the desired water stripping operation.
5 As noted above, the water stripping may be required for "in-situ
homogenization", in which case the distance X is one that allows the
temperature of the ingot to rise to the homogenization range following
water stripping. Cooling water removal may alternatively be carried out for
stress relief within the ingot. In the case of more conventional wiping used
10 with hard alloys, a greater distance X is employed and a flash boiling
effect
of any residual cooling water may not be so important.
It should also be noted that the distance X may, in some cases, be
chosen to differ on different sides of the ingot. The short sides of the ingot
(ingot ends) may have a jet contact point that is higher (closer to the mold)
than that required for the long faces of the ingot (rolling faces). Also,
thinner ingots may have water contact points that are higher than those
required for thicker ingots. However, the rate of flow and pressure of the
water jets would normally be the same on all sides of the ingot, unless the
streaming water is acted upon by a different force on different sides of the
ingot (e.g. gravity in the case of horizontal direct chill casting). In such a
case, the flow rate and/or pressure would be varied on different sides of
the ingot to achieve the desired degree of water stripping from each ingot
face.
The ideal angle of the nozzles to produce the cooling water
stripping effect can be determined by manually adjusting the angle of the
jets (e.g. by rotating the manifolds 30) and observing the results. This may
be done in a preliminary run of the casting apparatus and then maintained
at the same angle for all subsequent casting runs of the same
characteristics.
It should be noted that the exemplary embodiments of the present
invention may be especially effective when used with the means of cooling
water application disclosed in U.S. patent 5,685,359 to Wagstaff
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mentioned above. This means of cooling employs a split jet/dual jet
arrangement for ingot cooling purposes at the exit of the casting mold.
For reasons of safety, performance and maintenance, the hoses
and manifolds through which the water passes will need filters, shut off
valves and other conventional equipment. For example, a 50 mesh filter
may be provided to protect the nozzles from blockage. Such a filter may
be provided on the supply side of the apparatus 35 for supplying the water
under pressure in order to minimize loss of performance of the apparatus.
The apparatus 35 may be a pump capable of generating for example 150
psi or more of water pressure and a rate of water flow of 115 gallons per
minute or more. Suitable pumps may be obtained, for example, from
Pioneer Pump Inc., of 310 South Sequoia Parkway, Canby, OR 97013,
U.S.A. (e.g. model SC32C1 0). The same water that is used for cooling
may be employed for the nozzles, or it may be supplied from a different
source. The water may be substantially pure, but may contain various
additives, such as ethylene glycol. When the water contains such
additives, it must of course be supplied from a source different from the
cooling water. The water may also contain unintentional additives,
particularly if recycled cooling stream water is used. The water is
generally at ambient temperature when fed to the nozzles.
The nozzles 28 are preferably capable of delivering about 0.8 to 1.0
(or even 1.5 or more) gallons of water per minute over an arc of at least
65 (preferably 72 ) at a pressure of 120 psi. Such nozzles may be
obtained, for example, from Spraying Systems Co. of P.O. Box 7900,
Wheaton, Illinois 60189-7900, U.S.A. The nozzles are preferably used
with extenders to allow them to project sufficiently through the shield wall
25 to avoid interruption by contact with the reverse flow of cooling water
streaming along the inner surface of the wall.
An alternative embodiment is shown in Fig. 6. In this embodiment,
the underside 26 of the mold 10 is provided with a scupper 50 to collect
the cooling water 20 stripped from the ingot 19 before it descends down
wall 25 to the level of the nozzles 28. This avoids the possibility that the
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stripped cooling water may interrupt or adversely affect the operation of
the nozzles 28 or the shape or power of the water jets 36. Water collected
in the scupper 50 flows to the ends of the mold and is allowed to pour
away from the ingot or is removed through suitable channels (not shown).
Another alternative arrangement is shown in Fig. 7 which makes
use of a shield wall 25 having a corrugated shape in plan view. The
nozzles 28 project through the wall 25 at positions where the wall is
closest to the surface 29 of the ingot 19. After curling back away from the
ingot in the manner shown in Fig. 5, cooling water 20 stripped from the
mold by the jets 36 tends to stream into the vertical channels 52 formed
between the points of the wall 25 closest to the ingot. This directs the
cooling water away from the nozzles 28 and water jets 36, thereby
minimizing any likelihood of interference with the jets.
Figs. 8 to 10 show embodiments where narrow cylindrical water jets
are used instead of the fan shaped jets of the above embodiments. In Fig.
8, the jets 36 (which are upwardly angled as in previous embodiments)
penetrate the layer of cooling water 20 to the surface 29 of the ingot 19
and then spread out to separate the cooling water from the ingot surface.
In the case of Fig. 9, after contacting the ingot 19, the jets 36 spread
sufficiently to contact each other and form a combined "interactive
fountain" 54 between the positions of the nozzles. This effect is created by
adjusting the pressure and flow rates of the nozzles sufficiently. The
cooling water layer becomes completely separated from the ingot.
In the case of Fig. 10, the effect shown in Fig. 9 is accentuated by
angling the nozzles towards each other to maximize the separation of the
cooling water from the ingot surface.
Fig. 11 shows an exemplary embodiment of the invention applied to
horizontal DC casting. In horizontal direct chill casting apparatus, the
positions of the nozzles may have to be adjusted to allow the water wiping
jets to contact the top surface of the ingot at a different distance from the
casting mold relative to the bottom surface of the ingot. In addition, in the
illustrated embodiment, a scupper 50 is used at the upper side of the ingot
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to collect and remove cooling water 20 stripped from the ingot. Without
such a means of collecting and removing the stripped cooling water, it
would fall back on the ingot and adversely affect the cooling characteristics
of the ingot. At the lower side of the ingot, cooling water 20 may fall
naturally from the ingot 19 as shown, or alternatively a series of water jets
may also be applied to remove the cooling water at a distinct distance from
the mold. However, a scupper such as the one 50 used at the upper side
of the mold, will not be needed at the lower side of the mold because
cooling water stripped from the ingot will anyway stream away from the
ingot under the action of gravity. As in the embodiment of Fig. 6, the
scupper 50 removes the collected cooling water to the ends of the mold
and disposes of it without allowing it to come into contact with the ingot or
the nozzles.
While the embodiments described above are preferred, various
modifications and alternatives are possible. As already noted, the
exemplary embodiments may be employed with various kinds of casting
apparatus, not just the DC casting apparatus of Fig. 1. Moreover, the
invention is suitable for use with metals of various kinds, particularly
alloys
of aluminum, magnesium and copper. Use with the casting of aluminum
alloys is particularly preferred.
