Liquid metal processing

A method of removing heat from molten metal in which the metal is caused to flow freely within a gaseous media or vacuum over a body disposed within the metal stream such that any tendency for the metal to solidify during its passage is effective on the surface of said body against which the solidifying metal contracts into intimate contact therewith. The body may be forcibly cooled, e.g. by water. Accordingly, since the liquid metal is not enveloped by a channel or pipe, the solidifying shell tends to shrink on to the body, no gap is created, and this much higher heat transfer coefficients between the shell, the liquid metal, and the cooling body are achieved.

This invention relates to a method of, and apparatus for, processing liquid 
metal. 
For many metallurgical casting processes being developed in recent years it 
is necessary or advantageous to deliver the liquid metal to a metal 
forming medium at temperatures close to or below its liquidus. Such a 
forming medium might be for example a casting mould, continuous casting 
mould or a rolling mill used for "roll casting." The 
advantages--operational, metallurgical and economic--of casting or forming 
from a low superheat stock material are well known and indeed are 
identified in UK patent no. 2117687 for example. 
Equipment for cooling `superheated` metals ready for forming processes is 
described in this UK patent, and in thi equipment the superheat, and 
perhaps some of the latent heat, of the liquid metal is removed during its 
passage through a hollow carrier through the walls of the carrier. A solid 
shell usually forms as the liquid metal flows across the cooled surfaces 
of this carrier. This shell will also in most cases form a continuous 
lining of the inside perimeter of the carrier which then shrinks away from 
the inner surface of the carrier forming a gap between it and the inner 
surface. The carrier itself may also expand due to an increase in its 
temperature further increasing the size of the air gap. For a water cooled 
copper carrier through which molten steel is passing the contraction of 
the solidified shell and expansion of the copper carrier might typically 
contribute to a total difference in dimensions of 0.8%. The resultant gap 
is a barrier to heat transfer from the processed metal to the carrier. 
It is one object of this invention to mitigate this problem. 
From one aspect, the present invention provides a method of removing heat 
from molten metal in which the metal is caused to flow freely within a 
gaseous media over a body disposed within the metal stream such that any 
tendency for the metal to solidify during its passage is effective on the 
surface of said body against which the solidifying metal contracts into 
intimate contact therewith. 
Preferably an `impact` pad is provided at the upstream end of the body and 
a parting spacer is provided at the downstream end to cause the reforming 
of a coherent stream and to facilitate ready removal of the solidified 
shell after each processing run; the impact pad also prevents erosion of 
the leading edge of the body. 
The body may be shaped such that it extends in the longitudinal direction 
e.g. cylindrical or pear shaped in section, it may be asymmetrical or it 
may be symmetrical e.g. like a sphere, and the flow over the cooling body 
may be contained within a restricted zone e.g. by dams affixed to the 
body. Several of such bodies may be `cascaded` in the direction of flow of 
the molten metal stream. 
In accordance with this aspect of the invention since the liquid metal 
envelopes the cooling body the solidifying shell tends to shrink into it, 
no gap is created as hitherto and accordingly much higher heat transfer 
coefficients between the shell, and thus the liquid metal, and the cooling 
body are achieved. For this geometry the higher heat transfer between the 
solidified shell and the cooled body leads to the formation of a thicker 
skull which in turn leads to a greater area for heat transfer so 
that--assuming a constant heat transfer coefficient between the liquid and 
solid steel--the power output increment does not diminish with increasing 
heat transfer as for an `internal` steel flow through a carrier described 
above. 
It is however, not essential for the liquid metal to envelope the cooler, 
gravitational forces alone will enchance the close contact between the 
solidifying metal and the cooler and to this end plate-type coolers may be 
adopted, particularly to give larger heat transfer areas. 
The body may be interposed between a melt storage vessel and a forming 
mechanism such as a mould or the nip of a rolling mill used for roll 
casting, and a distribution box may be sited between the vessel and the 
said body. 
Preferably the body is forcibly cooled, e.g. by water. 
Another object of the invention is to distribute one or more liquid metal 
streams into a casting mould and to do this in such a manner that the 
momentum with which the steel arrives in the liquid pool is minimised. 
This is particularly a problem when feeding liquid steel from single 
nozzles to roll casting equipment where irregularities, imperfections, and 
breakouts can occur in the cast product. For this objective the parting 
spacer described above is also of benefit to promote the formation of a 
coherent stream leaving the unit. In this role the body over which the 
steam flows is either cooled or non-cooled such that although little heat 
is removed the liquid steel is still delivered in a distributed low 
momentum manner. In this instance the body might be located above the 
liquid metal pool or alternatively partially immersed in it. The liquid 
feed stream or streams may be circular or of other section, e.g. 
rectangular. 
Again, when the body is in cylindrical form it may be designed to rotate, 
imparting a horizontal component of velocity to the emergent stream, eg to 
feed a single roll caster or a horizontal casting machine, or to induce a 
greater shear rate at the skull/liquid interface.

Referring now to the drawings, FIG. 1 shows the effect of gap size on heat 
transfer for a water cooled copper carrier of circular section when a 
solidified steel shell has formed inside the carrier. From this it can be 
seen that minimising such a gap below a certain limit will have a dramatic 
effect on heat transfer. This is difficult to achieve for a metal passing 
through a hollow carrier since the metal is encased by the cooling medium 
and will tend to shrink away from it. If however, the liquid metal encases 
the cooling medium then any solidified shell formed will tend to shrink on 
to the cooling medium. 
The method of removing heat at high rates from a liquid metal can be best 
understood by means of the apparatus shown in FIG. 2 which incorporates 
the principles of achieving intimate contact between the metal and the 
cooling medium and minimising the liquid volume to cooled surface ratio. 
Referring now to FIG. 2 superheated molten metal contained in vessel 1 
flows through an elongated nozzle 2 on to an impact pad 3 and thence 
around a water-cooled copper cooler 4, the metal flowing across a 
shell-parting stream collecting device 5 at the downstream end of the 
cooler into the nip of a roll casting machine 6. The cooler is cooled by 
water flowing through channels 7 conveniently supplied through the ends of 
the body. The nozzle outlet 2 is shaped to maintain an integral consistent 
stream of metal against the cooler. 
The stream collector device 5 serves to gather the cooled melt into a 
single coherent stream for supply to e.g. casting moulds or other forming 
mechanism. The device 5 can be designed to allow easier removal of the 
shell after use if desired, the shell breaking into two about the impact 
pad. The impact pad 3, which prevents erosion of the cooler surface, is 
shaped so as to ensure a smooth flow of metal on to the cooler; it may be 
integral with the nozzle 2 or with the cooler. 
The cooler may also be tapered along an axis perpendicular to the melt flow 
to aid easier skull removal and/or obviate the requirement for a shell 
parting device. 
In FIG. 3, liquid metal passes from the thermally insulated vessel 1 to a 
distribution box 9 which serves to spread the flow along the length of the 
water cooled copper cooler 10. From this box the liquid metal flows over a 
ceramic weir 11 and falls a short distance on to the top of the cooler. 
Shaped refractory weirs 12 attached to the cooler ensure that the liquid 
streams land in a metal pool 13 and that the flow 14 which passes over the 
weirs and down the sides of the cooler is ordered and spread evenly over 
its length. Thereafter the metal flows over a refractory collecting device 
15 and falls as a gathered stream into a caster 6 or other forming device 
as before. 
It is not essential for the molten metal to envelope the cooler and in FIG. 
4, the delivery system is the same as in FIG. 3 but the cylindrical water 
cooled copper cooler 17 is offset so that the liquid metal falls into a 
pool 18 formed between a refractory dam 19 and the top of the cooler. In 
this case the outflow 20 is over the top of the cooler down one side only; 
a reduction in potential heat removal as compared with the previous 
embodiment is balanced by a more ordered flow off the cooler. 
In FIG. 5 there is shown a modification of FIG. 4 in which the cylindrical 
water cooled copper cooler is replaced by a plate type 21. This design 
allows control of the thickness/velocity of the metal flow 22 over the 
cooler by variation of its inclination and also gives the possibility of 
significant increases in cooled area. The importance of this latter factor 
is illustrated by FIG. 6 which shows the dependence of heat removal on the 
area of contact between liquid metal and the cooled copper cooler surface. 
The size and shape of the apparatus can readily be chosen to remove heat at 
specific rates for a given teeming rate, and to produce streams of 
desirable profile, and a multiple array of such coolers may be placed in 
the path of the liquid metal flow; the whole system may be enclosed in a 
chamber with a positive pressure of inert or inactive gas to protect the 
exposed surfaces of the flowing metal. 
Devices may also be used to modify the flow of liquid metal over the cooler 
surface to perhaps make the metal flow more turbulent to improve heat 
transfer or to contain the metal flow in a desired path. Magnetic fields 
and/or electric fields may be used for this purpose. Appendages to the 
cooler may also serve to modify the metal flow characteristics, e.g. ribs 
or protrusions which might serve to further enhance heat transfer by 
promoting turbulent flow. 
Another aspect of this invention is that the cooler can be used singly or 
in combination with other identical, similar or compatible coolers to slow 
down and distribute the flow of a liquid metal providing a uniform low 
velocity feed of liquid metal to the casting mould or forming process. In 
this embodiment the cooler may be replaced by a non-cooling body so that 
little or no heat is removed from the liquid but it is delivered in a 
diffused but coherent stream. 
In this regard reference is now made to FIG. 7a. Superheated metal is 
contained in vessel 23 and flows through discharge nozzles 24 on to an 
impact pad 25 and thence around the body 26. As the metal stream flows 
over this body it is distributed longtudinally and leaves as a coherent 
film. The stream collector device 27 serves to promote the formation of 
the coherent stream leaving the unit and dams 28 serve to contain the 
liquid stream. 
FIG. 7b shows a typical horizontally--disposed twin roll casting mill. The 
two rolls 29 are shown and within the nip of the rolling mill there is a 
pool of metal 30; the cast product 31 is shown emerging from the mill. To 
simplify the drawing the devices used to contain the liquid pool at the 
ends of the rolls are not shown in this instance. 
In practice the FIG. 7a apparatus would be disposed just above the metal 
pool 30 in the nip or alternatively partially immersed in it. As shown the 
device is uncooled its role being just to distribute the metal feed. 
However it may readily be used with a cooled body such as to achieve the 
dual benefits of distribution and a metal feed close to or below its 
liquidus.