Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to the manufacture of ingot molds.
Ingot molds used in the production of steel ingots usually consist of
upright cast iron, box-like shells open at one or both ends, and weigh from
0.8 to 1.5 times as much as the ingot cast therein. To close the bottom for
casting steel in those molds open at both ends, i.e., either big-end-down open-
top or big-end-down bottle-top molds, the mold is placed upright, big end
down on a thick cast iron stool, which serves as the bottom closure for the
mold cavity. A reasonably close fit between the mold and stool should be
assured to prevent leakage of molten steel therebetween. Those molds
which are open at one end only, i.e. big-end-up closed bottom molds, are
already closed and hence need no mold stool in combination therewith. It is
common practice, however, to place a hot-top over the open end of this type
mold when steel is cast therein. Here again it is necessary that a
reasonably close fit be maintained between the mold and hot-top to prevent
leakage of molten steel therebetween.
Ingot molds are usually manufactured in accordance with long
established foundry techniques wherein a suitable cavity is formed within a
sand mold and cast iron poured into the cavity and allowed to solidiy in
the shape thereof. Although the as-sand-cast surface of the resulting mold is
suitable for most surfaces, it has been necessary to machine the as sand cast
bottom surface of a newly cast big-end-down type mold to provide a smooth,
flat surface. This is because the bottom surface of the mold must rest
securely against the mold stool during teeming as noted above for big-end-down
molds. Even the as-sand-cast upper surface of big-end-up closed bottom molds
must also be machined to provide a smooth flat surface to form a good seal with
the hot top placed thereon. To avoid confusion hereinafter, these surfaces
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which must be flat and smooth will be referred to as "big-end surfaces" rather
than "bottom surfaces" in the case of big-end-down open and bottle-top
molds and "top surfaces" in the case of big-end-up closed bottom molds.
Since such machining operations are of course costly and time-
consuming, alternate methods of producing ingot molds having smooth, flat
big-end surfaces have been developed. Specifically, chill casting techniques
are widely used wherein an iron plate is incorporated into the mold for
making ingot molds to form the big-end surface of the ingot mold cast there-
against. Since the chill-plate is smoother than the sand mold surface and
more resistant to iron erosion, the resulting big-end surface on the ingot
mold is sufficiently smooth and flat that no machining is required. This of
course works equally well with closed bottom molds as they are usually cast
upside down so that the big-end of the opening can be formed on such a chill-
plate.
However, it has been found that ingot molds cast on chill-casting
stools have a considerably shorter useful life than do molds cast on sand
stools. One study has shown that ingot molds cast on sand stools have a 22%
longer life than ingot molds cast on chill casting stools. This difference
in mold life is primarily due to the differences in macrostructure and micro-
structure between sand-cast surfaces and chill-cast surfaces. The sand-cast
surfaces cool more slowly resulting in a more uniform, macrostructure and
microstructure, which is less crack-sensitive.
According to the present invention, there is provided a method of
manufacturing a cast iron ingot mold having a smooth, flat as-cast big-end
surface, comprising casting the ingot mold in a sand mold having a smooth,
flat, rigid, thermally insulating board incorporated into the sand mold and
providing the surface against which the big-end surface of the resulting ingot
mold is formed.
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The invention is further described, by way of example,
with reference to the accompanying drawings, in which:-
Figure 1 is a schematic sectional view of a mold as setup to cast an ingot mold according to one embodiment of this
invention,
Figure 2 is a close-up sectional view of the lower left-
hand portion of the cavity shown in Figure 2, and
Figure 3 is a prospective view of a rigid board as used
in the Figure 1 embodiment.
As already noted, the crux of this invention resides in
the use of a rigid insulation board to form the big-end surface
of the mold cavity. To this end, the rigidized insulation board
can be incorporated into either conventional sand-casting tech-
niques and equipment or conventional chill-casting techniques
and equipment. Simply stated then the process of this invention
involves the casting of an ingot mold in a conventional sand mold
having neither sand nor a chill~plate forming the big-end surface
of the mold cavity, but rather having a rigidized insulation board.
Obviously, there are numerous forms and techniques by which such an
insulation board could be incorporated into a mold to form the big-
end surface of the mold cavity. Perhaps the simplest method is to
produce a conventional sand mold wherein a large flat insulation
board is provided as the stool, and a sand mold placed thereover.
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One method we have preferred to use is illustrated in Figures l and
2 and utilizes conventional chill-casting equipment with a separable core
barrel and stool. Equally good results can be achieved with an integral
stool and core barrel. This first mentioned equipment comprises a chill-cast-
ing stool 10 having a shallow cavity 12 in the center thereof. Cavity 12 has
two sloped walls 14 and 16. A hollow core barrel 18 having a flange 22 is
designed to set into cavity 12 so that sloped wall 24 on flange 22 mates
reasonably with the lower sloped wall 16 in cavity 12. This arrangement is
provided to assure that core barrel 18 is positioned at the center of chill-
casting stool 10 when the mold is assembled for casting. An outer flask
30 having a sand retaining ring 32 is adapted to be set on the periphery of
the chill casting stool 10 encircling core barrel 18 at a sufficient distance
to form the desired cavity.
To prepare the above equipment for casting an ingot mold, it is first
necessary to compact molding sand 36 against the inside surface of flask 30 to
shape the outer walls of the intended ingot mold, and to compact molding sand
38 against the outer walls of core barrel 18 to shape the inside wall of the
intended ingot mold. When the molding sand 38 is in place, core barrel 18
is placed into the cavity 12 in chill-casting stool 10. According to prior art
practice, molding sand 46 is then compacted into the annular space provided
between core barrel 18 and surface 14 on stool 10 and then the flask 30 is
positioned in place. Contrary thereto, the practice of this invention then
requires that a rigidized insulation board 40 be placed over chill-casting
stool 10 to form the bottom surface of cavity 42. Rigidized insulation board
40 has an annular configuration so as to completely encircle core barrel 18.
In order to seal board 40 against chill-casting stool 10 to prevent molten
metal from getting therebetween, the inside edge of board 40 can be angled
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downward to form a lip 44 which mates with wall 14 of cavity 12. Molding sand
46 is then compacted between lip 44 and flange 20 or core barrel 18 to provide
a smooth extension from the upper surface of board 40 to core sand 38. Rigid-
ized insulation board 40 is of sufficient width so that the outer edge thereof
will extend under sand retaining ring 30 to seal the outer edge thereof. Lastly,
a flask-stool sealant 48 is placed around the edge of chill-casting stool 10,
encircling board 40, and then flask 30, to which ring 32 and sand 36 are
attached, is placed thereover to form cavity 42. It should be noted that
sealant 48 is not always necessary.
The moderately complicated inside sealing arrangement, i.e. the
interplay between lip 44 and molding sand 46, is somewhat necessitated by the
specific design of the chill-casting equipment used. Other equipment designs
which utilize flat chill-casting stools can employ a more simplified board
design. For example, the inside edge thereof may be flat and extend under the
core barrel and sealed in much the same manner as the outside edge is sealed
under flask 30, as shown.
In addition to the basic mold construction as shown in the drawings
as described above, it is of course necessary to provide a basin, runner and
gate ~not shown) according to conventional practice through which the mold is
cast.
The rigidized insulation board 40 can be made from any insulative
material which can be formed into a smooth board which will not deteriorate
or erode when contacted by the molten iron during or after casting. For this
material we have preferred to use a mixture of aluminosilicate fibers, such as
K~OWOOL* or FIBERFRAX* and a colloidal silica blnder. The fibers are first
felted from a chopped fiber and binder slurry and compressed to form a board
* Trade Mark
:
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shape by conventional vacuum forming techniques. Thereafter, the form is
dried at 220F to remove and provide increased rigidity and strength.
Increased strength may be necessary to support loads caused by the ferro-
static head when the ingot mold is cast, as well as provide adequate erosion
resistance. This can be accomplished by reimpregnating the dried board with
colloidal silica, or by adding inorganic fillets, such as hollow glass spheres
or powders, which may also be substituted for fibers to lower material costs.
In addition, mineral wool or calcium silicate fibers may be substituted. We
have found that suitable boards are characterized by the following properties:
thickness, 1/4 to 1/2 inch; density, 15 - 30 lb/cu.ft.; thermal conductivity,
1 - 2 Btu-in./sq.ft./F/hr. In addition, the above described materials are
quite suitable in that they will not stick to the cast metal, and can be
applied to hot as well as cold chill-casting stools.
The above detailed embodiment is of course tailored to be used with
the specific chill-casting equipment utilized. It would be obvious therefore
that the details would vary somewhat with other types of equipment. For
example, when chill casting equipment is not available, the insulative board
40 could consist of a simple flat disk shaped board embedded into the upper
surface of a sand stool.
During the investigations attempting to solve the problems discussed
above, several different approaches were tested such as the use of insulative
sheets and spray coatings. All such tests were considered unsatisfactory.
The use of an insulative board in the production at the laboratory of a 7" x
7" cast iron ingot did result in the production of a smooth surface as well as
a macrostructure and microstructure resembling that deslred. As a result of
the successful laboratory trial, an initial plant trial was conducted in which
three ingot molds ranging in weight from 35,800 lb. to 51,200 lb. were produced
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on insulating boards glued to the chill-casting stool. All three molds had
smooth flat bottoms that did not require machining. One of the molds had 59
pours before condemnation which was considerably better than the consumption
rate for the mold size. (The other two molds were lost.) As a result of the
successful laboratory trial and initial plant trial, eight commercial sized
ingot molds (46,440 lb) were produced using a procedure substantially as
described above. The boards used were 3/8-inch thick and made of alumino-
silicate fibers bonded and rigidized with colloidal silica. Several individual
pieces were used and taped together with fiberglass tape but were not attached
to the stool. The table below shows the results of that test.
Total No. of Pours
Mold No. Bottom Machined Before Scrapping
38 Yes 45
39 No 72
41 No 72
44 No 18
No 54
48 No Mold Lost
49 No 59
Yes 33
In the above test, all of the as-cast bottoms produced were quite
smooth as compared to sand-cast surfaces. Mold No. 38 could have been used
without machining but was nevertheless machined to remove a very slight ridge
at the core seal. Mold No. 50 required machining because of difficulties
encountered which were considered unusual. After casting, the boards were
easily separated from the casting. In a few places where portions of the
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boards did stick to the casting, such portions were easily scraped off with a
putty knife. As shown in the table, all molds had good life, except for
Mold No. 44. It is believed, however, that this mold may well have failed due
to causes other than macrostructural and microstructural defects in the bottom
surface. Mold 48 was lost in the mill and could not be traced. At the time
these test molds were made, other conventional ingot molds made at the same
time with chill-casting techniques were noted and their histories followed.
These other ingot molds were scrapped after a total number of pours ranging
from 31 to 54. The over-all life improvement can readily be seen.
