METHOD AND APPARATUS FOR RHEOCASTING

METHODE ET APPAREIL DE RHEOFORMAGE

English Abstract

Abstract
A method and apparatus for rheocasting (slurry
casting) molten metal employ a stirring chamber located
upstream of a casting mold. Electromagnetic stirring is
employed in both the stirring chamber and the casting
mold. Structure is provided for minimizing secondary
recirculating flows in the molten metal as it flows
downstream through the apparatus, for preventing hangers
and for eliminating the columnar dendritic zone at the
periphery of the casting. The efficiency of utilization
of the magnetic field is optimized as is the agitation
required for producing a desired fine, spheroidal,
degenerate dendritic grain structure in the solidified
casting.

Claims

- 22 -
THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. An apparatus for use in the continuous casting
of molten metal, said apparatus comprising:
a casting mold having an inlet and an outlet;
a chamber located upstream of said mold and
having an inlet for receiving molten metal and an outlet
vertically aligned with said inlet of the mold;
magnetic stirring means disposed around said
chamber, for inducing, in molten metal contained in said
chamber, a primary circulatory flow in a first
rotational sense:
said magnetic stirring means having an upstream
end and a downstream end and having a linear dimension
between its upstream and downstream ends substantially
less than the linear distance between said inlet of the
chamber and said outlet of the casting mold;
and means for substantially reducing secondary
recirculating flows caused in said molten metal by said
electromagnetic stirring means.
2. An apparatus as recited in claim 1 wherein said
means for substantially reducing said secondary
recirculating flows comprises:
an upstream constriction in said chamber at a
location not substantially further upstream than the
upstream end of the magnetic stirring means;
and a downstream constriction in said chamber
at a location not substantially further downstream than
the downstream end of the magnetic stirring means;
each of said constrictions defining an opening
for molten metal passage, each of said openings having a
cross-sectional area substantially less than the cross-
sectional area of the chamber between said upstream and
downstream constrictions.
3. An apparatus as recited in claim 2 wherein:

- 23 -
said downstream constriction is at said outlet
of the chamber.
4. An apparatus as recited in claim 2 or 3
wherein:
said upstream constriction is at said inlet of
the chamber.
5. An apparatus as recited in claim 2 wherein:
said cross-sectional area of the opening in
said downstream constriction is between about one-fourth
and about one-half of said cross-sectional area of said
chamber.
6. An apparatus as recited in claim 5 wherein:
said cross-sectional area of the opening in
said upstream constriction is substantially the same as
the cross-sectional area of the opening in said
downstream constriction.
7. An apparatus as recited in claim 1 wherein said
means for substantially reducing said secondary
recirculating flows comprises:
at least one additional magnetic means aligned
with and spaced from said first-recited magnetic
stirring means and comprising either (a) magnetic brake
means or (b) another magnetic stirring means for
inducing in molten metal a primary circulatory flow in a
rotational sense opposite that induced by said first-
recited magnetic stirring means.
8. An apparatus as recited in claim 7 wheren said
additional magnetic means comprises:
at least one additional magnetic means located
upstream of said first-recited magnetic stirring means;
and at least one additional magnetic means
located downstream of said first-recited magnetic
stirring means.
9. An apparatus as recited in claim 1 and
comprising:

- 24 -
a linear conduit composed of refractory
material and extending between said outlet of the
chamber and said inlet of the mold, for confining molten
metal flowing from said chamber into said mold;
said mold comprising means for solidifying a
solid peripheral skin around the molten metal in said
mold;
and means for preventing said solid peripheral
skin in the mold from extending upstream beyond a
predetermined location.
10. An apparatus as recited in claim 1 and
comprising:
magnetic stirring means disposed around the
outside of said mold;
said mold being cylindrical and having a
diameter no more than about 150 mm (6 in.) and a wall
thickness between about 1.6 and 4.8 mm (1/16 - 3/16
in.);
said mold being composed of a metallic material
having a conductivity no greater than about 0.29 x 108
ohm m) 1.
11. An apparatus for use in the casting of molten
metal, said apparatus comprising:
a casting mold having an inlet and an outlet;
a chamber located upstream of said mold and
having an inlet for receiving molten metal and an outlet
linearly aligned with said inlet of the mold;
magnetic stirring means disposed around said
chamber;
a conduit composed of refractory material and
extending between said outlet of the chamber and said
inlet of the mold, for confining molten metal flowing
from said chamber into said mold;
said mold comprising means for solidifying a
solid peripheral skin around the molten metal in said
mold;

- 25 -
and first preventing means for preventing said
solid peripheral skin in the mold from extending
upstream beyond a predetermined location.
12. An apparatus as recited in claim 11 wherein:
said conduit has a downstream end;
said mold has an upstream end;
and said first preventing means comprises a
ceramic break ring sandwiched between the downstream end
of said conduit and the upstream end of said mold.
13. An apparatus as recited in claim 11 wherein:
said conduit includes a portion extending
downstream into said mold and having an outer surface;
said mold includes an upstream portion having
an inner surface;
said outer surface on the downstream portion of
the conduit and said inner surface on the upstream
portion of the mold define a substantially annular space
therebetween;
and said first preventing means comprises means
for introducing a pressurized inert gas into said space
to prevent molten metal in said mold from entering said
space.
14. An apparatus as recited in claim 13 and
comprising:
magnetic stirring means disposed around said
mold and which creates turbulence in the molten metal in
said mold;
and second preventing means for preventing said
turbulence from splashing molten metal upstream into
said space adjacent said interior surface of the mold's
upstream portion.
15. An apparatus as recited in claim 14 wherein
said second preventing means comprises:
lip means composed of refractory material and
extending inwardly from said interior surface, adjacent
the downstream end of said space.

- 26 -
16. An apparatus as recited in claim 11 wherein:
said chamber comprises heat-extracting means
capable of forming a solid peripheral skin around the
molten metal in said chamber;
and said apparatus comprises third preventing
means for preventing any solid peripheral skin which
forms in said chamber from growing downstream into said
conduit.
17. An apparatus as recited in claim 16 wherein
said third preventing means comprises:
a constriction at said outlet of the chamber.
18. An apparatus for use in the casting of molten
metal, said apparatus comprising:
means for confining a volume of molten metal
flowing downstream;
magnetic stirring means, disposed around said
confining means, for inducing, in said downstream
flowing volume of molten metal, primary circulatory flow
in a first rotational sense;
and means for substantially reducing secondary
recirculating flows caused in said volume of molten
metal by said magnetic stirring means.
19. An apparatus as recited in claim 18 wherein
said means for substantially reducing said secondary
recirculating flows comprises:
at least one additional magnetic means aligned
with and spaced from said first-recited magnetic
stirring means and comprising either (a) magnetic brake
means or (b) another magnetic stirring means for
inducing, in said downstream-flowing volume of molten
metal, primary circulatory flow in a rotational sense
opposite that induced by said first-recited magnetic
strirring means.
20. An apparatus as recited in claim 19 wherein
said additional magnetic means comprises:

- 27 -
at least one additional magnetic means located
upstream of said first-recited magnetic stirring means;
and at least one additional magnetic means
located downstream of said first-recited magnetic
stirring means.
21. An apparatus as recited in claim 20 wherein
said confining means is a casting mold.
22. An apparatus for use in the continuous casting
of molten metal, said apparatus comprising:
a cylindrical casting mold having an inlet and
an outlet;
magnetic stirring means disposed around the
outside of said mold;
said mold having a diameter no more than about
150 mm (6 in.) and a wall thickness between about 1.6
and 4.8 mm (1/16 - 3/16 in.);
and said mold being composed of a metallic
material having a conductivity no greater than about
0.29 x 108 (ohm m)-1.
23. An apparatus as recited in claim 22 wherein:
said mold allows at least 50% of the magnetic
field developed by said magnetic stirring means to
penetrate to the interior of said mold when employing an
electromagnetic frequency of about 60 Hertz.
24. An apparatus as recited in claim 23 and which
provides a skin depth for the magnetic field in the mold
of at least 12.7 mm (1/2 in.) when employing an
electromagnetic frequency of about 60 Hertz.
25. An apparatus as recited in claim 22 wherein:
said mold is composed of a metallic material
consisting essentially of, in wt. %:
Be 0.55
Co 2.4
Zr 0.25
Cu balance.

- 28 -
26. A method for use in the casting of molten
metal, said method comprising the steps of:
providing a column of molten metal flowing
downstream, said column having upstream and downstream
ends;
magnetically stirring said column, at a first
location between its ends, to induce in the molten metal
at said first location a primary circulatory flow in a
first rotational sense;
and substantially reducing secondary
recirculating flow caused by said magnetic stirring.
27. A method as recited in claim 26 wherein said
step of substantially reducing secondary recirculating
flows comprises performing at least one of the following
procedures:
(a) magnetically stirring the molten metal at a
location spaced from said first location to induce in
said molten metal at a second location, at which said
secondary recirculating flow occurs, primary circulatory
flow in a rotational sense opposite that of said primary
circulatory flow at said first location; and
(b) magnetically braking said first-recited
secondary recirculating flow at said second location.
28. A method as recited in claim 27 wherein:
one of said procedures is performed at a
location upstream of said first location;
and one of said procedure is performed at a
location downstream of said first location.
29. A method as recited in claim 26 wherein said
primary circulatory flow has an upstream terminus in
said column and a downstream teminus in said column, and
said step of substantially reducing secondary
recirculating flows comprises:
constricting said column at a first location
not substantially further upstream than said upstream
terminus;

- 29 -
and constricting said column at a second
location not substantially further downstream than said
downstream terminus.
30. A method as recited in claim 29 wherein:
said column has a cross-sectional area at each
of said constricting locations substantially less than
the cross-sectional area of said column between said
first and second contricting locations.
31. A method as recited in claim 30 wherein:
said cross-sectional area of the column at each
of said constricting locations is between about one-
fourth and about one-half of said cross-sectional area
of said column between said constricting locations.
32. A method for rheocasting molten metal to
produce a degenerate, dendritic microstructure
comprising substantially spheroidal grains, said method
comprising:
providing a column of molten metal flowing
downstream;
confining said column to a substantially
circular cross-section;
subjecting said column of molten metal to
electromagnetic agitation in a stirring zone;
allowing said molten metal to cool as it flows
downstream through said stirring zone;
said method being conducted in accordance with
the following equation -
<IMG> >1,000
where: B is the magnetic field strength, in Tesla
R is the radius, in meters, of the molten
metal column undergoing stirring in the
stirring zone
.alpha. is the electrical conductivity of the

- 30 -
molten metal, in (ohm meters)-1
.omega. is the angular frequency of the stirring
in the stirring zone, in radians/second
? is the density of the molten metal, in
kg/m3
L is the latent heat of fusion of the molten
metal, in Joules/m3
Q is the rate of heat extraction from the
molten metal in the stirring zone,
in Watts/m2.
33. A method as recited in claim 32 wherein:
<IMG> < 2500 S
34. A method as recited in claim 33 wherein:
RL/Q < 100 S.
35. In an apparatus for casting molten metal
wherein said apparatus comprises a casting mold having
mold walls, an inlet and an outlet end, a device for
preventing the formation of columnar dendrites extending
into the interior of said mold from the inside surface
of said mold walls, said device comprising:
a chamber located upstream of said mold and
having an inlet for receiving molten metal and an outlet
communicating with the inlet end of said mold;
means for cooling the molten metal in said
chamber;
magnetic stirring means associated with said
chamber for agitating molten metal in said chamber and
for cooperating with said cooling means to deliver to
said mold an agitated volume of cooled metal consisting
essentially of primarily molten metal with 0-30 wt.%
solid metal which, when present, is in the form of
particles which form a slurry with said molten metal.

- 31 -
36. In a method for casting molten metal in a
casting mold having mold walls and inlet and outlet
ends, a procedure for preventing the formation of
columnar dendrites extending into the interior of said
mold from the inside surface of said mold wall, said
procedure comprising the steps of:
providing a stirring zone upstream of said
mold;
flowing molten metal downstream through said
stirring zone and into said mold;
cooling the molten metal in said stirring zone;
magnetically stirring said molten metal as it
flows downstream through said stirring zone to agitate
said molten metal in said stirring zone;
said cooling step and said stirring step
cooperating to deliver to said mold an agitated volume
of cooled metal consisting essentially of primarily
molten metal with 0-30 wt.% solid metal which, when
present, is in the form of particles which form a slurry
with said molten metal.
37. In a method as recited in claim 36 wherein the
metal undergoing casting is a ferrous alloy.
38. In a method as recited in claim 36 wherein
said volume of cooled metal delivered to said mold has a
temperature below the liquidus temperature of the metal
when it enters the mold.

Description

METHOD AND APPARATUS FOR RHEOCASTING
Back~round Of The Invention
The present invention relates generally to methods
and apparatuses for solidifying molten metal and more
particuluarly to methods and apparatuses for doing so
employing rheocasting. Rheocasting, also known as
slurry casting, is a procedure in which molten ~etal is
subjected to vigorous agitation as it undergoes
solidification. Absent such agitation, dendrites would
form as the metal solidifies. A dendrite is a
solidified particle shaped like an elongated stem having
transverse branches extending therefrom.
Vigorous agitation converts the normally dendritic
microstructure of the solidifying metal into a non-
dendritic form comprising discreter degenerate dendritesin a liquid matrix. The agitatiorl, which may be either
mechanical or electromagnetic, shears the tips of the
solidifying dendrites, and this produces a metal slurry
composed of relatively fine, spheroidal, nondendritic
particles or grains in a liquid matrix.
The rheocast material is typically fully
solidified, then reheated to a semi-solid state
temperature, and then subjected to forming under
pressure, e.g. die forming. When the material is in a
semi-solid ~tate, it has a microstructure composed of
solid particles in a liquid matrix.
It is desirable that there be a relatively fine
grain size when metallic material is formed under
pressure while in a semi-solid state. Fine grains or
particles flow more readily than do coarse grains during
forming under pressure in a semi-solid state. For
example, one desirable steel microstructure for semi-
solid forming has an aim austenitic grain size, when in
a solid state, of no greater than about 150 microns.
A procedure in which molten metallic material is
solidified by rheocasting and then reheated to a semi-

- 2 - 2~
solid state followed by forming under pressure is
disclosed in Young U.S. Patent No. 4,565,241. This
patent discloses maintaining, within a specified range,
the ratio between (a) the shear rate of the metal
undergoing agitation and (b) the solidlfication rate of
that metal. Doing so produces certain desired results
from the standpoint of microstructure and forming
costs. Either mechanical or electromagnetic agitation
are contemplated.
The shear rate obtained with mechanical agitation
may be ascertained with reasonable accuracy. However,
that is not the case when electromagnetic agitation is
employed; in such a case, complex mathematical models
are required to calculate the shear rate. These moclels
lS require one to estimate the viscosity of the metal
undergoing rheocasting, and that viscosity depends
lar~ely upon the proportion of solid phase in the metal
undergoing rheocasting. The proportion of solid phase
can vary from 0 to 80%, and over that range of solid
phase, the viscosity can vary over several orders of
magnitude. As a result, the calculated value of the
shear rate can vary over several orders of magnitude
depending upon the estimated viscosity of the metal
undergoing rheocasting.
Another consideration involved in the
electromagnetic stirring of molten metal undergoing
rheocasting is the efficiency with which the
electromagnetic field is employed. Rheocasting
typically employs a casting mold having open upstream
and downstream ends, and rheocasting can be a continuous
type of casting. Copper alloys having hi~h thermal
conductivities are the only materials that have been
found suitable for constructing molds employed in the
rheocasting of metals such as steel. The lower
conductivities of other materials cause excessive
thermal distortion. However, the electrical

2 ~
conductivities of copper alloys are almost directly
proportional to their thermal conductivities. As a
result, when a rheocasting mold is made from materials
conventionally employed for that purpose, there i9
produced a very effective shield to electromaglletic
stirring fields.
To overcome this shielding ef~ect, it has been
conventional to use electromagnetic stirring fields with
a frequency of lO Hertz or less when stirring steel in a
rheocasting mold. However, with such low
electromagnetic stirring frequencies, the angular
velocity of the molten metal wlthin the mold is
relatively low, e.g. no greater than about lO
revolutions per second. ~n rheocasting, it would be
desirable to use an electromagnetic stirring field
having frequencies of 30 to 60 Hertz, preferably at t:he
upper end of that range.
In the rheocasting Oe steel, the molten steel can
form a continuous column of liquid many meters long.
Generally, an electromagnetic stirrer will extend over
only a small portion of the liquid column. ~he stirring
effect of such a device will extend up to 15 diameters
upstream and downstream of the stirring device due to
secondary recirculating flows. Primary circulatory flow
occurs in planes transverse to the axis of the column of
molten metal, while secondary recirculating flows occur
in planes transverse to the planes in which primary
circulatory flow occcurs. The secondary flows will
absorb about half of the stirring energy introduced into
the metal column and thus reduce the maximum rotational
or angular velocity that can be imparted to the material
undergoing agitation. Reducing the secondary
recirculating flows is a desirable aim.
Another problem which can arise in a rheocàsting
process is the occurrence of hangers. A hanger is a
solidified peripheral skin which hangs up on the walls

~ 7~3
of the casting mold or confinement chamber in which
solidification begins, rather than moving downstream at
the same rate as the rest of the metal undergoing
solidification. This can result in a breakout at the
outlet of the casting mold, i.e. molten metal leaking
through the skin of the partially soliclified metal.
A third problem which can arise in a rheocasting
process is the occurrence of a columnar, dendritic zone
at the periphery of the casting. This peripheral,
columnar, dendritic zone has a structure that is
unsuitable for forming in the semi-solid state and thus
reduces the yield of rheocast feedstock obtained from
the rheocasting process.
Summary Of ~he Invent~on
The present invention employs processing conditions
and apparatus features which eliminate or minimize the
problems discussed above.
In one embodiment, an apparatus in accordance with
the present invention comprises a casting mold having an
inlet and an outlet. A stirring chamber, which may have
interior walls composed of refractory material, is
located upstream of the continuous casting mold. The
stirring chamber has an inlet for receiving molten
metal, e.g. from a tundish, and an outlet aligned with
the inlet of the mold. A magnetic stirring element is
disposed around the chamber, and there is at least one
other magnetic stirring element disposed around the
mold. A linear conduit composed of refractory material
extends between the outlet of the stirring chamber and
the inlet of the mold, for confining molten metal
flowing from the stirring chamber into the mold.
The apparatus produces a degenerate dendritic
microstructure comprising substantially spheroidal
grains having a relatively fine grain size. This
desirable microstructure is provided utilizing

- 5 - 2~ 7~
electromagnetic agitation and a combination of
processing conditions which are controlled in accordance
with an equation which does not require the use of
complex mathematical models to calculate the shear rate
produced by the electromagnetic agitation. All of the
parameters entering into the equation (to be described
subsequently in detail in the detailed description) can
be readily determined with reasonable accuracy.
The casting mold employed in the apparatus allows
one to use a magnetic frequency, for stirring purposes,
up to about 60 Hertz, while permitting at least 5~ of
the magnetic field developed by the electromagnetic
stirring element to penetrate to the interior of the
mold. These advantages are a result of the particular
dimensions of the mold and of the particular metallic
material of which the mold is composed. In operation,
the mold provides a desirable combination of thermal
conductivity, ~or heat extraction purposes, and magnetic
fleld efficiency for agitation purposes.
In the mold, a solid, peripheral skin is solidified
around the molten metal in the mold. Structure is
provided for preventing the solid peripheral skin in the
mold from extending upstream beyond a predetermined
level, an occurrence which could cause undesirable
hangers to form. In one embodiment, there is a ceramic
break ring sandwiched between (a) the downstream end of
the conduit which communicates with the mold and (b) the
upstream end of the mold. The break ring is used
together with a procedure in which the downstream flow
of the metallic material through the casting mold is
stopped, reversed, and then reinitiated. This procedure
breaks up any hangers which may have a tendency to form
at the location of the ceramic break ring.
In another embodiment of mold which prevents the
3~ formation of hangers, the upstream end portion of the
mold includes structure which defines an annular space

'3
-- 6 --
surrounding the column of metallic material flowing
through the upstream end portion of the mold. An inert
gas is introduced into this annular space to prevent
molten metal from entering that space. This in turn
prevents hangers from forming at the upstream end
portion of the mold.
Electromagnetic agitation of the molten metal
within the mold creates turbulence which can cause the
molten metal to splash upstream into the annular space
described in the preceding paragraph. Additional
structure is provided to minimize that splashing.
The stirring chamber includes heat-extracting
structure capable of forming a solid peripheral skin
around the molten metal in that chamber. In a further
embodiment of the present invention, structure is
provided which prevents any solid peripheral skin which
forms in the stirring chamber from growing downstream
into the con~uit which communicates the stirring chamber
with the mold. This structure is typically in the form
of a constriction at the downstream end of the stirring
chamber.
As noted above, at least one electromagnetic
stirring element is disposed around each of the stirring
chamber and the continuous casting mold. Each magnetic
stirring element induces, in the adiacent downstream-
flowing vo}ume of molten metal, a primary circulating
flow in a first rotational sense. Associated with each
of these primary electromagnetic stirring elements is
structure for substantially reducing secondary
recirculating flows caused in the volume of molten metal
by the primary electromagnetic stirring element.
One embodiment of such structure employs at least
one additional electromagnetic element linearly aligned
with and spaced from the primary magnetic stirring
element. This additional electromagnetic element may be
either (a) an electromagnetic brake or (b) another

- 7 - 2~
electromagnetic stirring element for inducing, in the
downstream-flowing volume of molten metal, primary
circulatory flow in a rotational sense opposite that
induced by the primary electromagnetic stirring
element. One such additional electromagnetic element
may be located upstream of the primary electromagnetic
stirring element, and another additional electromagnetic
element may be located downstream of the primary
electromagnetic stirring element.
Another embodiment of structure for preventing
secondary recirculating flow, in the stirring chamber,
comprises ~a) an upstream constriction in the stirring
chamber at a location not substantially further upstream
than the upstream end of the electromagnetic stirring
element and ~b) a downstream constriction in the
stirring chamber at a location not substantially further
downstream than the downstream end of the
electromagnetic stirring element. Each of the
constrictions defines an opening Eor molten metal
passage, and each of those openings has a cross-
sectional area substantially less than the cross-
sectional area of th~ stirring chamber between the
upstream and downstream constrictions.
Other features and advantages are inherent in the
apparatus and methods claimed and disclosed or will
become apparent to those skilled in the art from the
following detailed description in conjunction with the
accompanying diagrammatic drawings.
Brief Description of the Drawings
Fig. 1 is an elevational view, partially in
section, illustrating an embodiment of apparatus in
accordance with the present invention;
Fig. 2 is an enlarged, fragmentary, vertical
sectional view of part of the apparatus illustrated in
Fig. l;
`
,~ ~

- 8 ~ 7 ~
Fig. 3 is an enlarged, vertical sectional view of
another part of an apparatus in accordance with an
embodiment of the present invention;
Fig. 4 is a fragmentary, vertical sectional view of
part of an apparatus in accordance with another
embodiment of the present inventiorl;
Fig. 5 is a fragmentary, vertical sectional view of
part o an apparatus in accordance with a further
embodiment of the present invention;
Fig. 6 is a fragmentary, vertical sectional view of
part of an apparatus in accordance with still a further
embodiment of the present invention;
Fig. 7 is a fragmentary, vertical sectional view of
part of an apparatus in accordance with yet another
embodiment of the present invention;
Fi~. 8 is a vertical sectional view illustrating an
embodiment of a casting mold in accordance with the
present invention; and
Fig. 9 i9 a sactional view taken along line 9-9 in
Fig. 8.
Detailed Description
Referring initially to Fig. 1, indicated generally
at 20 is a rheocasting apparatus constructed in
accordance with an embodiment of the present
invention. Apparatus 20 is associated with a ladle 21,
mounted on a ladle car 22, for pouring molten metal,
such as molten steel, into a tundi~h 23, from which the
molten metal exits through a tundish outlet 24 lnto
rheocasting apparatus 20.
In the embodiment illustrated herein, rheocasting
apparatus 20 comprises a vertically disposed, casting
mold 34 having an upper inlet 35 and a lower outlet
36. Located above mold 34 is a vertically disposed
stirring chamber 26 having interior walls composed of
refractory material, stainless steel or other suitable
.

g
material. Although not shown in Fig. 1, stirring
chamber 26 is typically enclosed within a water cooled,
stainless steel shell or jacket. Stirring chamber 26
has an upper inlet 27 for receiving molten metal from
tundish 23 and a lower outlet 28 vertically aligned with
inlet 35 of mold 34. A magnetic stirring element 30 is
disposed around the stirring chamber, and there is at
least one other magnetic stirring element 38 disposed
around mold 34. The linear dimension between the
upstream and downstream ends of magnetic stirring
element 30 is substantially less than the linear
distance between the stirring chamber's upper inlet 27
and the casting mold's lower outlet 36. A vertically
disposed conduit 32 composed of refractory material
extends between outlet 28 of stirring chamber 26 and
inlet 35 of mold 34, for confining molten metal
descending from the stirring chamber into the mold.
Conduit 32 may be enclosed within a metal shell 33. The
stirring chamber and the casting mold, as well as the
conduit extending between the two, are typically
cylindrical in cross-section.
Extending upwardly through outlet 36 of mold 34 is
a dummy element 42 connected by a rod 43 to a withdrawal
mechanism 44 which can be hydraulic or electrically
powered.
When a casting operation begins, molten metal is
poured from ladle 21 into tundish 23 from which the
molten metal descends, in sequence, through stirring
chamber 26 and conduit 32 into mold 34. A solid casting
bottom forms at dummy element 42 which is withdrawn from
casting mold outlet 36 by withdrawal mechanism 44 to
allow a partially solidified casting, typically having a
solid peripheral skin and a metal slurry interior, to
exit from mold 34. Solidification of the casting's
interior proceeds to completion either inside or outside
mold 34, and in the latter case, solidification may be

- 10 ~
assisted by employing an external cooling medium, such
as water sprays, in a conventional manner.
Typically, a slurry of solid particles in molten
metal is contained within the solidified peripheral skin
of the casting in mold 34, and that slurry is stirred by
electromagnetic stirring element 38. ~tirring chamber
26 contains molten metal or a slurry o~ solid particles
in molten metal, and the molten metal or slurry
undergoes stirring in chamber 26 by electromagnetic
stirring element 30. Both electromagnetic stirring
elements 30 and 38 have two poles and are of
conventional construction.
~ solidified casting produced by apparatus 20 has a
degenerate dendritic microstructure comprising
substantially spheroidal grains having a relatively fine
grain size. This microstructure is the result, at :Least
in part, of the electromagnetic agitation the metal
undergoes in stirring chamber 26 and mold 34. Other
features of the present invention which contribute to
the desirable microstructure described two sentences
above will be described subsequently.
There is a descending vertical column of metal
which is totally or partially molten extending all the
way from stirring chamber inlet ~7 to mold outlet 36.
: 25 The agitation caused by the stirring chamber's
electromagnetic stirring element 30 creates primary
circulatory flow within stirring chamber 26, and the
mechanical agitation caused in the molten metal within
mold 34 by electromagnetic stirring element 38 creates
primary circulatory flow within mold 34. Primary
circulatory flow occurs in planes transverse to the
vertical axis of the column of molten metal.
Primary circulatory flow is not the only stirring
effect caused by each of the electromagnetic stirring
elPments. In addition, there are secondary
recirculating flows which extend above and below the
.

location of primary circulatory flow, in planes
transverse to the planes in which primary circulatory
flow occurs. The stirring effects may have a total
vertical extent of up to 15 diameters, extending both
above and below the location of the electromagnetic
stirring element which produces the primary circulatory
flow. Secondary recirculating flows are undesirable
because they will absorb a substantial part (e.g. about
one-half) of the stirring energy introduced into the
ln material undergoing stirring and thus reduce the maximum
rotational or anyular velocity that can be imparted to
that material. ~igs. 2 and 3 illustrate structure for
reducing secondary recirculating flows.
With refe~ence to Fig. 2, there is an upper
constriction 46 in stirring chamber 26 at a vertical
level not substantially higher than the vertical level
of the upper end of magnetic stirring element 30. There
is also a lower constriction 47 in chamber 26 at a
vertical level not substantially lower than the vertical
level of the lower end of electromagnetic stirring
element 30. Upper constriction 46 has an openi~g 48l
and lower constriction 47 has an opening 49. Each of
openings 48, 49 have a cross-sectional area
substantially less than the cross-sectional area of
chamber 26 between upper and lower constrictions 4~, 47
respectively. Constrictions 46 and 47 reduce secondary
recirculating flows caused by electromagnetic stirring
element 30.
The stirring chamber's lower constriction may be at
the lower outlet of the stirring chamberl and the
stirring chamber's upper constriction may be at the
upper inlet of the stirring chamber. The cross-
sectional area of the opening in the lower constriction
is between about 1/4 and about l/2 of the cross-
sectional area of the chamber. The cross-sectional area
of the opening in the upper constriction is

- 12 - 2~rl~ 7
substantially the same as the cross-sectional area of
the opening in the lower constriction, in preferred
embodiments.
The expedients described in connection with Fig. 2
for reducing secondary recirculating flows are
applicable only to a stirring chamber but not to a mold.
Fig. 3 illustrates an expedient which may be
applicable to either a stirring chamber or a mold, to
reduce the secondary recirculating flows. In Fig. 3,
there is shown a confining chamber 126 having an upper
inlet 127 and a lower outlet 128, without constrictions
anywhere in the chamber. Disposed around chamber 126 is
a first electromagnetic stirring element 138. Also
associated with chamber 126 is at least one addltional
electromagnetic element vertically aligned with and
spaced from electromagnetic stirring element 138. The
additional electromagnetic stirring elements are at 139
and 140 in ~ig. 3.
An electromagnetic element such as 13~ or 140 may
be in the form of a magnetic brake, or it may be in the
form of another electromagnetic stirring element for
;~ inducing in the molten metal within chamber 126 a
primary circulatory flow which (a) rotates in a sense
opposite that induced by electromagnetic stirring
element 138 and tb) is located at the level of element
139 or 140. If the additional electromagnetic element
is in the form of a magnetic brake, no primary
circulatory flow is induced into the molten metal by
that particular electromagnetic element, but the brake
does substantially reduce the secondary recirculating
flows at the level of the brake. An electromagnetic
brake employs a DC field which stops or reduces the
secondary recirculating flows but cannot create a
primary circulatory flow itself.
The employment of electromagnetic elements, such as
139 and 140 above and~or below the principal

- 13 - 2~ 7~
electromagnetic stirring element, to prevent secondary
recirculating flows, is useful not only with a stirring
chamber but also with a casting mold such as mold 34.
In such a case, the multiplicity of electromagnetic
elements 138-140 would replace the single
electromagnetic stirring element 38 illustrated in E~ig.
1.
Solidification of the molten steel occurs primarily
in mold 34, although some solidification can occur in
the stirring chamber. It is desirable to form a
peripheral skin in mold 34. It is undesirable to form a
peripheral skin in the stirring chamber, or anywhere
upstream of mold 34. Solidification in the stirring
chamber can be anywhere from 0 to 50~, for example,
depending upon the thickness of the refractory walls of
which the interior of the stirring chamber is composed,
and upon the temperature of the cooling fluid which is
circulated through the stainless steel water ja~ket
which typically encloses the stirring chamber. To the
extent that solidiication occurs in the stirring
chamber, it is desirable to confine such solidification,
as much as possible, to solid particles which form part
of a slurry, composed of molten metal and solidified
metal particles, and which undergoes agitation in the
stirring chamber and exits the stirring chamber in that
form.
A variation of the procedure described above can be
employed to prevent the formation of columnar dendrites
extending into the interior of mold 34 from the inside
surface of the mold walls. The formation of columnar
dendrites is particularly a problem when the metal is
undergoing rheocasting. Preventing the formation of
such dendrites is accomplished by cooling the metal
undergoing agitation in the separate stirring chamber
upstream of the casting mold so as to deliver to mold 34
an agitated volume of cooled metal consisting

- 14 - 2~ 3
essentially of primarily molten metal with 0-30 wt.%
solid metal which, when present, is in the form of
particles which form a slurry with the molten metal, as
described above. Preferably the metal is at a
temperature below the liquidus temperature when it
enters the casting mold. The procedure described in
this paragraph i9 applicable to ferrous alloys~ for
example.
Care must be taken to avoid hangers in the casting
mold and upstream of the casting mold, and various
embodiments of structure for doing so will now be
described in connection with Figs. 4-7.
Referring initially to Fig. 4, a stirring chamber
226 has a constriction 47 at the outlet of the stirring
chamber t and the constriction has an opening 49
communicating with a conduit 232 in turn communlcating
with casting mold 3~. A ceramic break ring 40 is
~ocated between upper inlet 35 of casting mold 3~l and
the lower end of conduit 232.
Stirring chamber 226 is composed oE a refractory
substrate, or some other suitable substrate, and the
substrate is typically surrounded by a water cooled,
stainless steel cooling jacket (not shown in Fig. 4).
Conduit 232 is composed of refractory material of
sufficient thickness to prevent any substantial
solidification from occurring in the conduit.
Stirring chamber 226 has vertically disposed
sidewalls 227 extending upwardly from constriction 47.
Depending upon the extent of cooling which takes place
in stirring chamber 226, it is possible for a peripheral
skin to solidify within chamber 226 at sidewalls 227.
Constriction 47 prevents any solid peripheral skin which
may form at wall 227 from growing downwardly into
conduit 232, and this assists in preventing the
formation o~ hangers upstream of mold 34.

- 15 ~ 7 3
The descending column of metal undergoes virtually
no cooling as it descends through refractory conduit 232
which is heavily insulated. Mold 34, however, is
composed of a highly thermally conductive material, such
as copper or copper alloy, and the moLd is cooled by
cooling coils (not shown in Fig. 4). Therefore, as the
hot metal enters mold 34, there is (1) a substantial
chilling effect on the metal at upper lnlet 35 of mold
34 and (2) the danger of hanger formatlon at the upper
end portion of mold 34. A number of expedients are
utilized to prevent the formation of such hangers.
In one instance, as shown in Fig. 4, a ceramic
break ring 40 is employed, either alone or together with
a procedure in which the descent of metallic material
through the mold is stopped, reversed, and then re-
initiated, employing the withdrawal mechanism 44 and
associated structure 42, 43 shown in Fig. 1. The
procedure described in the preceding sentence breaks up
any hangers wllich may have a tendency to form at the
location of the ceramic break ring. This procedure can
be repeated periodically throughout the casting
operation to minimize the formation of hangers at the
upper end portion of mold 34. The ceramic break ring
also prevents any solid peripheral skin which forms at
the top of mold 34 from extending upwardly beyond ring
40, an occurrence which would be undesirable.
Another expedient for preventing the formation of
hangers at the upper end portion of mold 34 is
illustrated in Fig. 5. A stirring chamber 326 has a
lower constriction 347 with an opening 349 communicating
with a conduit 332 extending downwardly through the
upper inlet 3S of mold 34. At least a portion of
conduit 332 extends downwardly into the upper portion of
mold 34, or the entire conduit may do so, as shown in
Fig. 5. Mold 34 includes an upper portion having an
inner surface 355. There is an outer surface 356 on

- 16 - 2~
that portion of conduit 332 which extends into the upper
portion of mold 34. Outer conduit surface 356 and inner
mold surface 355 define between them a substantially
annular space 350. Communicating with annular space 350
is an inlet conduit 351 which extends through a bottom
wall 345 of stirring chamber 326. Outlet conduit 352
communicates with a pressur~ relief valve 353. Inlet
conduit 351 communicates with a source of pressurized
gas (not shown).
Pressuriæed gas is introduced through inlet 351
into annular space 350, and the resulting pressure in
annular space 350 is sufficient to prevent molten metal
in mold 34 from rising into annular space 350, thereby
preventing the formation of a pexipheral skin therein,
and minimizing the danger of hangers forming at the
upper end portion of mold 34. Any peripheral skin which
does solidify within mold 34 is located for the most
part below annular apace 350 t as shown at 346 in Fig.
5. The pressurized gas can be withdrawn from annular
space 350 by opening valve 353 on outlet conduit 352.
Molten metal within mold 34 is subjected to
agitation by electromagnetic stirring element 38. Such
agitation creates turbulence in the molten metal within
mold 34, and this may cause the molten metal to splash
upwardly into annular space 350. Structure to minimize
that splashing is shown in Fig. 6.
More particularly, extending inwardly from interior
surface 355 of the mold's upper portion, adjacent the
lower end of peripheral space 350, is a lip 358 composed
of refractory material. Lip 358 is in the form of a
ring having an interior opening through which extends
conduit 332. Lip 358 provides a barrier which prevents
molten metal below the lip from splashing upwardly into
annular space 350.
Referring now to Fig. 7, in the embodiment
illustrated therein, stirring chamber 226 has a lower

2 ~
- 17 -
constriction 47, as in the embodiment illustrat~d in
Fig. 4, but outlet opening 49 at the bottom of
constriction 47 does not communicate with a conduit
such as 232 in the embodiment of Fig. 4. Instead,
outlet opening 49 communicates directly with upper inlet
35 of mold 34, there being no ceramic break ring at the
top of mold 34, as there is at 40 in the embodiment of
Fig. 4. In the embodiment of Fig. 7, hangers are
prevented from occurring at the top of mold 34 by
employing the procedure, described above, in which the
descent of metallic material through the mold is
stopped, reversed, and then reinitiated. Moreover, as
was described in connection with stirring chamber 226
illustrated in Fig. 4, any peripheral skin which may
solidify on stirring chamber wall 227 in Fig. 7 is
prevented from descending further downwardly by
constriction 47.
Stirring chamber 226 in Fig. 7 i5 illustrated as
having a re~ractory substrate with a stainless steel
water jacket 228 containing passages 229 for circulating
cooling water.
Referring now to Figs. 8 and 9, there is
illustrated an embodiment of a mold 334 constructed in
accordance with the present invention. Mold 334 has an
upper inlet 335, a lower outlet 336 and vertically
disposed peripheral ribs 337 containing passages (not
shown) for circulating a cooling fluid, such as water.
Mold 334 is cylindrical and has a diameter (D) no more
than about 152 mm (6 in.). The wall thickness (t) of
the cylinder, excluding ribs 337, is between about 1.6
and 4.8 mm (1/16-3/16 in.).
Mold 334 is composed of a metallic material having
a conductivity no greater than about 0.29 x 1o8
(ohm m) 1. A preferred example of such a metallic
material consists essentially of, in wt. %:

7 7 ~
~ 18 -
Be 0.55
Co 2.4
Zr 0.25
Cu balance.
I.ike the casting molds illustrated in Figs. 1 and
4-7, mold 334 is surrounded by at least one
electromagnetic stirring element, such as 38 in Figs. 1
and 4-7. Although mold 334 is composed of a copper
alloy having a high thermal conductivity, as well as a
high electrical conductivity, mold 334 does not have the
problem of low magnetic field efficiency associated with
other molds composed of copper base alloy. As a result,
the electromagnetic stirring elements associated with
mold 334 may be operated at an electromagnetic stlrring
field Erequ~ncy of 30 to 60 Hertz, preferably at the
upper end of that range. When doing 50, the mold allows
at least 50% of the magnetic field developed by the
electromagnetic stirring element to penetrate to the
interior of the mold, even when employing a frequency of
about 60 Hertz. This is due to the comblnation of mold
dimensions and metallic composition described above.
Such a mold will provide a magnetic skin depth of at
least about 1/2 in. (12.7 mm) when one employs a
magnetic frequency, at the electromagnetic stirring
element, of about 60 Hertz. To ensure that at least 50%
of the magnetic field penetrates the mold, the parameter
D2t2/4(d)4 must be less than 3, where D and t are the
mold diameter and thickness, respectively, and d is the
skin depth of the magnetic field in the mold.
The skin depth of the magnetic field is inversely
proportional to the square root of the multiplication
product of (a) the electrical conductivity of the mold
material times (b) the angular frequency of the stirring
field. The angular frequency, in radians per second, is
equal to the magnetic frequency in Hertz times 2 ~. One
revolution equals 2 ~ radians. The skin depth should be

2 ~
-- 19 --
at least about 12.7 mm (1/2 in.) when employing an
electromagnetic frequency of about 60 Hertz, and this
requires that the mold be composed of a metallic
material having the conductivity noted above.
In summary, a magnetic field efficiency of at least
50% is obtained when the mold diameter is no greater
than about 6 inches (152 mm), the thickness of the mold
wall is in the range 1/16-3/16 in. tl.6 to 4.8 mm), and
the skin depth of the magnetic field in the mold is
greater than about 1/2 in. (12.7 mm). The skin depth
will meet the requirements noted in the preceding
sentence when (a) the mold is composed of a metallic
material having a conductivity no greater than about
0.~9 x 108 (ohm m)~l and (b) the angular frequency of
the stirring field corresponds to an electromagnetic
frequency of 60 Hertz, i.e. 120 ~ radians per second~,
Within an electromagnetic frequency range of 30-60
Hertz, the higher the electromagnetic frequency, the
higher the angular stirring frequency within the mold.
Above a frequency of 60 Hertz, there is too large a loss
in efficiency of utilization of the magnetic field.
Below about 30 Hertz, there is too large a drop in the
agitation or stirring produced within the mold. An
electromagnetic frequency within the range 30-60 Hertz
provides a desired efficiency of utilization of the
magnetic field together with a desired amount of
agitation, provided all of the other parameters noted
below are employed. The electromagnetic field intensity
in both the stirrin~ chamber and the mold should be in
the range 400 to 3,000 Gauss for a 60 Hertz stirring
field.
The various embodiments of apparatus described
above produce a degenerate, dendritic microstructure
comprising substantially spheroidal grains having a
relatively fine grain size. This desirable
microstructure is provided by utilizing electromagnetic

2 ~
- 20 -
agitation and a combination of processing conditions
which are controlled in accordance with an equation
which does not require the use of complex mathematical
models to calculate the shear rate produced by the
S electromagnetic agitation. All of the parameters
entering into the equation, described immediately below,
can be readily determined with reasonable accuracy
LBR ~_ 2 >4,000
where: B is the magnetic field strength, in Tesla
(1 Tesla = 400 Gauss)
R is the radius, in meters, of the molten
metal column undergoing stirring in the
stirring ~one
lS o is the electrical conductivity of t:he
molten metal, in (ohm meters)~l
is the angular frequency of the stirring
in the stirring zone, in radians/second
~ is the density of the molten metal, in
kg/m3
L is the latent heat of fusion of the
molten metal, in Joules/m3
Q is the rate of heat extraction from the
molten metal in the stirring zone,
in Watts/m2.
When stirring is performed in accordance with the
foregoing equation, the resulting microstructure has
grains which are satisfactorily rounded or spheroidal.
In addition, to obtain microstructures with sufficiently
small grain sizes, the following additional conditions
should be satisfied:
RL < 2500 S (préEerably < 100 S)
Q
As noted above, one desirable steel microstructure
for semi-solid forming has an aim austenitic grain size,

- 21 -
when in a solid state, of no greater than about 150
microns.
The apparatus and methods described above produce a
solidiied metallic material, e.g. steel, having a
S cylindrical shape with a diameter of about 3 to 6 inches
(76 to 152 mm). The cylindrical metallic material is
subsequently cut into blanks, and the blanks can be
readily formed under pressure ~e.g. die forming~ when
heated to a semi-solid state.
The diameter of the mold is primarily determined by
the diameter desired for the casting exiting from the
mold, subject to lower and upper diameter limits of 3
and 6 in. (76 and 152 mm) for optimizing the efficiency
of utilization of the magnetic field generated by the
electromagnetic stirring element. The cross-sectional
area or diameter of the stirring chamber is related to
the diameter of the mold. The larger the mold diameter,
the larger the diameter required for the stirring
chamber. A typical stirring chamber has a diameter in
the range 3-20 in. (76-508 mm), preferably 3-3 in. (76-
229 mm).
The foregoing detailed description has been given
for clearness of understanding only, and no unnecessary
limitations should be understood therefrom, as
modifications will be obvious to those skilled in the
art. For example, the methods and apparatuses of the
present invention are described above in the context of
a vertical disposition, but they can be readily adapted,
to a large extent, for employment in a horizontal
disposition. In addition, many of the features and
advanta~es described above in the context of an
apparatus having a circular cross-section would be
applicable to other types of cross-sections.

Representative Drawing