Continuous casting mold assembly

Disclosed is a mold assembly characterized by efficient and controlled removal of heat from molten metal during continuous casting, providing high casting speeds in conjunction with superior surface finish on the cast product. The mold assembly includes a refractory mold body having a longitudinal solidification chamber therein and a plurality of longitudinal cooling bores spaced around the solidification chamber. The cooling bores extend only partially through the mold body in the direction of the solidification chamber to define an insulating section adjacent the inlet end thereof to minimize heat removal from the molten metal source and a cooling section adjacent the outlet end when cooling probes containing a circulating coolant are inserted into the cooling bores. The cooling probes are adjustable along the length of the cooling bores to accurately control the position of the solidification front in the solidification chamber and provide optimum heat transfer from the molten metal for rapid solidification with superior surface finish. In a preferred embodiment, the mold assembly includes a longitudinal bore defining a plurality of solidification chambers of increasing diameter in the direction of the outlet end for producing two or more diameters of cast product by positioning the solidification front first in one chamber and then another by suitable adjustment of the cooling probes.

FIELD OF THE INVENTION 
The present invention relates to the continuous casting of metals and, more 
particularly, to molds for use in such processes. 
DESCRIPTION OF THE PRIOR ART 
Continuous casting of both ferrous and non-ferrous metals and alloys is a 
well known technique in the metallurgical art, for example, as represented 
by the Rossi et al patent, U.S. Pat. No. 3,399,716 issued Sept. 3, 1968, 
among many others. Of course, in such a dynamic process which transforms 
hot molten metal into a solid metal shape, the mold in which 
solidification takes place is extremely important. In the continuous 
casting of ferrous alloys, water-cooled copper molds have been 
successfully utilized. On the other hand, for non-ferrous metals and 
alloys, such as copper, copper base alloys, aluminum, aluminum base alloys 
and the like, water-cooled graphite molds have met with widespread use, 
for example, as represented by the Kolle patent, U.S. Pat. No. 3,459,255 
issued Aug. 5, 1969 and the Adamec et al patent, U.S. Pat. No. 3,592,259 
issued Dec. 10, 1971. As further illustrated in the Woodburn patent, U.S. 
Pat. No. 3,590,904 issued July 6, 1971, water-cooled graphite molds have 
also been utilized in casting slabs or ingots of metals or alloys in a 
non-continuous manner. 
In the die casting art, it is known to provide a metallic mold with cooling 
bores and to insert a cooling probe sealably within each bore for 
injecting coolant such as liquid carbon dioxide therein, for example, see 
the Carlson patent, U.S. Pat. No. 3,667,248 issued June 6, 1972. The 
injection carbon dioxide of course is transformed from the liquid to the 
gaseous phase by absorption of heat from the mold and the gas is directed 
by a suitable conduit through the probe to compressor and condensor means 
for recycling into the cooling bores. 
SUMMARY OF THE INVENTION 
The present invention provides an improved mold assembly for the continuous 
casting of metals and alloys characterized by controlled and efficient 
heat removal from the solidifying metal. As a result, higher casting 
speeds can be realized with the improved mold assembly in conjunction with 
improved surface quality of the cast product. In addition, mold life is 
also appreciably enhanced. 
With the aid of a preferred mold assembly of the invention, two or more 
sizes of cast product can be produced from one mold assembly without 
interrupting the casting process while still retaining the advantageous 
features enumerated above. 
In a typical embodiment of the present invention, the improved mold 
assembly includes a refractory mold body having a longitudinal 
solidification chamber therethrough with an inlet end to receive molten 
metal from a crucible or other source and an outlet end through which the 
solidified product exits. An important feature of the improved mold 
assembly is the provision in the mold body of a plurality of longitudinal 
cooling bores spaced peripherally around the central solidification 
chamber, the cooling bores having an open end at the outlet end of the 
mold body and extending only partially into the mold body in the direction 
of the inlet end so that an insulating section is defined adjacent the 
inlet end and a cooling section is adjacent the outlet end of the 
solidification chamber. Another important feature of the improved mold 
assembly is the provision of a plurality of elongated cooling probes, each 
typically comprising an inner feed tube and concentric outer return tube, 
inside of which a coolant such as water circulates. The cooling probes are 
adapted for insertion into the cooling bores spaced around the periphery 
of the solidification chamber to a preselected distance in the direction 
of the inlet end to accurately control the solidification front within the 
molten metal at the desired location along the length of the 
solidification chamber. By providing a peripheral insulating section 
adjacent the inlet end of the mold body and a peripheral cooling section 
adjacent the outlet end of the mold body, heat removal from the crucible 
supplying the molten metal is minimized while heat removal from the molten 
metal in the cooling section of solidification chamber is maximized, 
thereby significantly enhancing the heat removal efficiency of the mold 
assembly. Such increased heat efficiency results in significantly higher 
casting speeds. In addition, by changing the position of the 
solidification front relative to the speed of casting by simply moving the 
cooling probes into or out of the peripheral cooling bores, the surface 
quality or finish of the cast product can be optimized. Also, periodic 
changing of the position of the solidification front along the length of 
the solidification chamber reduces wear of the chamber wall, extending the 
useful life of the mold body substantially. 
These advantages as well as the capability to produce two or more sizes of 
cast product from a single mold without interrupting the casting process 
are obtainable with a particular preferred mold assembly which includes as 
an important feature a longitudinal bore which defines two or more 
solidification chambers of increasing cross-section, e.g. increasing 
diameter, toward the outlet end of the mold body. By suitably adjusting 
the depth of the cooling probes in the peripheral cooling bores, the 
position of the solidification front within the molten metal can be 
located in a particular solidification chamber of a first diameter and 
then in another chamber of a second diameter to produce the desired sizes 
of cast product. Thus, interruption of the casting process to exchange 
molds is totally unnecessary.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIGS. 1 and 2, a typical mold body 2 useful in the 
invention is illustrated. Although graphite is a preferred material for 
the mold body, other refractory materials will of course be usable and can 
be selected as desired depending upon the type of metal or alloy to be 
cast among other factors. A graphite mold body 2 has proved especially 
satisfactory in continuously casting leaded brass (60 w/o Cu, 40 w/o Zn, 2 
w/o Pb) having a solidification temperature of about 
870.degree.-880.degree. C. The mold body 2 includes a central cylindrical 
bore therethrough which defines a cylindrical solidification chamber 4 for 
producing a cast bar product, the bore including enlarged ends one of 
which defines inlet end 6 through which molten metal enters the chamber 
and outlet end 8 through which the solidified product exits. Inlet end 6 
is connected to the discharge nozzle of a conventional crucible (not 
shown) or other vessel containing the molten metal metal to be 
continuously cast. The mold body typically is oriented in the horizontal 
plane although vertical or other orientations are of course possible and 
well known in the art. Spaced around the circumference or periphery of 
solidification chamber 4 are a plurality of cylindrical cooling bores 10 
which have an open end at the outlet end of the mold body and extend 
partially into the mold body in the direction of inlet end 6 to provide a 
peripheral insulating section 12 and peripheral cooling section 14 along 
the length of the mold body when the cooling probes are inserted therein. 
As shown, the longitudinal axes of the cooling bores are substantially 
parallel with the longitudinal axis of the chamber 4. Insulating section 
12 is adjacent the inlet end 6 and functions more or less as insulating 
means between the peripheral cooling section 14 and the crucible 
containing the hot molten metal to minimize heat removal from the crucible 
itself and molten metal until it reaches the vicinity of cooling section 
14. Cooling section 14 adjacent the outlet end 8 provides highly efficient 
and concentrated heat removal from the molten and solidifying metal 
passing therethrough when the cooling probes are inserted in cooling bores 
10. 
A typical cooling probe 13 is shown in cross-section in FIG. 3 as 
comprising essentially an inner feed tube 15 and concentric outer return 
tube 16 inside of which coolant, such as water, circulates as indicated by 
the arrows. As can be seen, the outer return tube 16 includes a closed end 
16a to seal one end of the cooling probe. At the other end, the tubes 
penetrate and are sealed within a manifold 20. Feed tube 15 includes an 
extension 15a passing outside the manifold for connection to a coolant 
supply whereas outer return tube 16 has an open end inside the manifold 
for discharging the returning coolant therein. Discharge tube 22 conveys 
the returning coolant from the manifold for cooling and recycling or for 
disposal. Preferably, feed and return tubes 15 and 16 are made of highly 
heat conductive metal such as copper although other materials may be 
utilized. 
FIG. 4 illustrates a plurality of such cooling probes 13 inserted into 
cooling bores 10 of the mold body to provide the mold assembly of the 
present invention. The cooling probes are shown inserted at different 
distances only for purposes of clarity; generally during casting, all the 
cooling probes are inserted into the cooling bores to the same distance or 
depth to assure a uniform solidification front in the molten metal. By 
adjusting the speed of casting, i.e., speed with which solidified bar is 
withdrawn from the outlet end 8, and the position of the cooling probes 
within the cooling bores 10, the location of the solidification front of 
the molten metal within the solidification chamber, in particular cooling 
section 14, can be readily adjusted to provide an optimum surface finish 
on the cast bar product. Of course, the parameters of casting speed and 
cooling probe insertion distance for production of an optimum surface 
finish will vary with the chemistry of molten metal or alloy being 
solidified, the size of the cast product to be produced, the initial 
temperature of the molten metal and other factors. However, these 
parameters are readily determinable by simple and well known continuous 
casting procedures. Generally, for a constant casting speed, the 
solidification front can be translated toward the inlet end or outlet end 
by simply increasing or decreasing, respectively, the distance the cooling 
probes are inserted into cooling bores 10. By using the mold assembly of 
the invention, very efficient transfer cooling is achieved and the bulk of 
the cooling is from the liquid/solid bar in a transverse direction with a 
minimum amount of heat extracted longitudinally. Thus, the metal only in 
solidification chamber 4 is cooled and heat is not extracted and lost from 
metal contained in the crucible. As a result, control over the cooling 
process is considerably improved in conjunction with much improved 
efficiency. The net effect is that higher casting speeds can be realized 
while producing a cast product with superior surface finish. 
Of course, to optimize heat transfer from the mold body to the cooling 
probes, the dimensions of the cooling bores and probes must be properly 
correlated. Cooling bores 10 mm in diameter and cooling probes having a 
nominal outer diameter (copper return tube outer diameter) of 10 mm have 
proved satisfactory in this regard. Great care is used in reaming out the 
cooling bores in the mold body and the outer surface of the cooling probe 
is coated with colloidal graphite to provide good contact between the 
cooling probe and cooling bore wall. Of course, these dimensions can be 
varied as desired depending upon the size of the mold body employed. The 
aforementioned dimensions have been employed with a cylindrical mold body 
having a length of 292 mm and a diameter of 90 mm, the solidification 
chamber therein having a diameter of 21.26 mm. 
FIGS. 5-8 illustrate somewhat schematically actual casting results obtained 
with the mold body and cooling probes described in detail hereinabove. In 
FIGS. 5 and 6, a leaded brass described more fully under International 
Copper Research Specification Cu Zn 39Pb2 was cast from melt temperatures 
of about 962.degree. C. and 1025.degree. C., respectively. This alloy has 
a solidification temperature of about 870.degree.-880.degree. C. The 
casting speed in each figure was about 14 cm/min with cooling probes 
inserted so that the probe tips P were 155 mm from the outlet end in FIG. 
5 and 60 mm from the outlet end in FIG. 6. The water flow rate in each 
probe of FIG. 5 was 3.9 liter/min whereas that in each probe of FIG. 6 was 
7.15 liter/min. As shown, the solidification front A in FIG. 5 was found 
to be 216 mm from the outlet end and that in FIG. 6 was only 105 mm from 
the outlet end. In FIG. 7, the casting speed was 36 cm/min, the probe tips 
P were inserted 60 mm from the outlet end and the water flow rate was 16.6 
liter/min. Under these conditions, the solidification front was 205 mm 
from the outlet end. In FIG. 8, casting speed was increased to 61 cm/min 
and the probe tips were inserted farther so that they were 140 mm from the 
outlet. The water flow rate in the probes was the same as in FIG. 7. The 
solidification front was determined to be 185 mm from the outlet end in 
this instance. It should be noted that in all of these casting trials, the 
surface finish of the resulting solidified bar was excellent, being 
characterized by a fine-grained surface skin and remelted smooth surface 
at the pulse interface, and would not require further surface treatment 
prior to hot stamping or forging. It is believed that the improved heat 
transfer characteristics of the mold assembly are primarily responsible 
for the excellent surface finish obtained on the cast product. It is also 
apparent from the figures that by adjusting the position of the cooling 
probes within the cooling bores and the casting speed, the location of the 
solidification front can be varied at will. Variation of the position of 
the solidification front is extremely useful as a means to reduce wear of 
the walls of the solidification chamber and thus to considerably increase 
the life of the mold body. 
FIG. 9 illustrates a modified mold body 2' for use in a preferred mold 
assembly of the invention for producing two or more diameters of cast bar 
product. The notable difference between the modified mold body 2' of FIG. 
9 and that shown in FIG. 1 is that the former includes as an important 
feature a longitudinal bore defining two or more solidification chambers 
4', 4" of increasing diameter D.sub.1, D.sub.2 toward the outlet end 8' of 
the mold body. Cooling bores 10' identical to those of FIG. 1 are spaced 
about the periphery of the central bore and receive cooling probes (not 
shown) identical to those already described. By adjusting the position of 
the cooling probes in cooling bores 10' in relation to the casting speed, 
as described hereinbefore, the solidification front can be located in 
solidification chamber 4' to produce the smallest diameter cast bar and 
then can be brought forward by further adjustment toward the outlet end 
into solidification chamber 4" to produce larger diameter bars as desired. 
There is a tapered transition chamber between solidification chambers 4' 
and 4" to allow the solidification front to be judiciously repositioned 
along the length of the mold body during the changeover from casting bar 
of diameter D.sub.1 to bar of diameter D.sub.2. It should be noted that 
bar of smaller diameter D.sub.1 can be produced after the larger bars by 
simply inserting the cooling probes a greater distance into the cooling 
bores. Thus with the multiple solidification chambers, it is possible to 
continuously cast one diameter in required tonnage and then others in 
required tonnages without changing molds or interrupting the flow of 
molten metal. For purposes of illustration, typical values of D.sub.1 and 
D.sub.2 might be 21.26 mm and 26.16 mm, respectively. 
Another notable modification to mold body 2' of FIG. 9 is the provision for 
injection of a gaseous and possibly liquid coolant into the exit chamber 
4'" and through the static air gap formed between the solidified bar and 
chamber wall as a result of solidification shrinkage upon casting most 
metals. The coolant then flows out radial discharge passages 5'. This flow 
of coolant, preferably an inert gas such as nitrogen, is advantageous 
since it considerably increases the rate of heat transfer from the solid 
hot bar. A pressurized gas cylinder of nitrogen provides a useful means 
for introducing the inert gas coolant into chamber 4'", although other 
injection means may be employed. In the embodiment thus far described, 
exit chamber 4'" typically would have a diameter D.sub.3 of 28.16 mm. By 
making diameter D.sub.3 greater than D.sub.2, the so-called static air gap 
is effectively enlarged and thereby facilitates flow of the coolant 
therethrough. Although the mold assembly of the invention has been 
described in detail as it relates to the production of a cast bar product 
of circular cross-section, it is apparent that other product shapes can be 
produced with the mold assembly by suitable modification to the shape of 
the solidification chamber. For example, FIG. 10 illustrates one type of 
mold body useful for producing a cast product of rectangular 
cross-section. Other cooling probe constructions may be utilized so long 
as they are adapted to be moved in and out of the cooling bores and to 
provide sealed, circulating coolant therein. Of course, other 
modifications will occur to those skilled in the art and it is desired to 
cover in the appended claims all such modifications as fall within the 
true spirit and scope of the invention.