Note: Descriptions are shown in the official language in which they were submitted.
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Description
Continuous Steel Casting Machine
And Method
Technical Field:
5This invention relates to high temperature metal
continuous casting machines~ and more particularly, to
systems for cooling the mold with sprayed coolant fluid
and for controlling the extent of expansion and contrac-
tion of the mold during use.
Background Art:
In conventiona} continuous metal casting machines,
molten metal is passed through a mold for solidif.ication
~nto a desired shape. As the molten metal passes through
the mold, an outer shell forms and hardens. In many
machines, the mold ~s vertically oriented and as the metal
strand continues to solidify, it is bent through an angle
of 90 so that it moves horizontally,.and it i5 subsequently
cut into individual segments. In the case of horizontal
continuous casting machines, the strand is not bent
through the 90 angle but exits the mold horizontally.
In both vertical and horizontal machines, the strands
are then cut into segments or predetermined lengths.
The temperature of molten steel is typically
2850 F, although with certain grades the temperature
may be as low as 2600 F. In general, although most of
the references herein are to steel casting, the inven-
tivn contemplates the casting of any metal or metal alloy
: whose liquid temperature exceeds 260~ F.
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The mold which forms the steel strand contains
the liquid steel and provides for its initial solidifi-
cation, that is, hardening of the outer shell. The
solidifying strand is extracted continuously from the
bottom of the mold at a rate equal to that of the in-
coming liquid steel at the top, the production rate being
determined by the time required for the outer shell to
harden sufficiently so as to contain the inner core of
liquid steel by the time the strand exits the mold.
The liquid steel is cooled in almost all present day
casting machines by providing a water system which cir-
culates a stream of cooling water around the mold. The
water enters at the bottom of a pressure-tight vessel
which surrounds the mold and travels upwardly in a direction
opposite to that of the moving liquid steel. The "counter
current" water flow has been found to be adequate for
heat transfer in continuous steel casting machines. Such
cooling systems embody a baffle jacket closely surround-
ing the mold, defining an annular space through which
2~ the cooling water flows.
It has been found in practice that when ~he mold
heats and expands due to direct contact with the molten
metal being cast, it has the effect of reducing the
annular space available for cooling water circulation
~5 between the mold and the baffle jacket. This reduces
the rate of heat extraction, permitting further heating
and expansion of the mold, thereby further reducing the
annular space and the amount of cooling water circulated
for cooling. This condition: continues, and in extreme
cases can ultimately result in contact between the mold
wall and the inside surface of the bafle jacket. When such contact
occurs, ~otal mel~own of the m~ld can result,but in any event,
cooling is drastically reduced and the strand quality
deteriorates.
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To attempt to control and maintain the relative
positions of the mol~ wall and the baEfle jacket,
continuous casting machine designers sometimes weld
small spacer bars or pins onto the interior surface of
the baffle jacket. This method is relatively unsuccess-
ful, however, since the mold expands in the area between
the spacers, thus further increasing the deformation of
the mold, Moreover, such mold dèformation will actually
cause rhomboidal bulging and other s~rand shape defor-
mations~ If mold expansion exceeds 0.0755 inch, the
stresses generated in the mold will exceed the yield
strength of the mold copper and result in permanent
deformation of the mold, further exacerbating the de-
terioration of strand quality and destroying the mold.
In applicant's prior U.S. patent, number 4,494,5~4,
~ spray cooling system is disclosed which alleviates
many of the problems found in the systems described
above. The spray cooling system described in this patent
i~ much more efficient in cooling the mold and controlling
its expansion than are the prior art systems described
above. As a result, higher quality cast steel strands
are achieved when the spray cooling system disclosed
in patent 4,494,594 is used~
: ~lowever, neither the baffle jacket design nor the
prior spray cooling system disclosed by applicant in its
prior U~S. patent 4,494,594 considers the differential
expansion zones to be found in a continuous casting
: machine (meniscus, midsection, lower section), and the
prior devices lack any means for constantly monitoring
mold expansion in these zones and adjusting the cooling
rate to maintain a predetermined maximum expansion.
: This inability to control mold exp~nsion with conventional
mold cooling systems forces continuous casting machine
operators or researchers to accept the adverse effect~
; 35 of uncontrolled mold expansion on the cast structure,
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and the associated adverse effects on s~bsequent proces-
sing operations and finished product q~ality that can
result.
Disclosure of the Invention:
In accordance with the present invention, a
continuous metal casting machine is provided in which
one or more parameters of the spray cooling system are
defined and established to main~ain the desired mold
configuration and in which mold distortion is detected
and corrected.
Applicant has found that by utilizing certain
conditions and by modifying other operating conditions
as disclosed in prior V.S. patent number 4,494,S94
it can accurately control mold expansion and consequently
control the quality of the continuously cast strand
within the mold. More specifically, applicant has
found that by carefully regulating the coolant directed
against a zone in an area extendin~ from approximately
2~ inches above the meniscus level to approximately 2~
inches below the meniscus level, it can most effectively
control the distortion of the moldO In most continuous
casting mac~ine for billets, slabs and other shapes,
this zone is at a point roughly two to 14 inches below
the top of the mold assembly. It is in this meniscus
zone that approximately 70~ of the heat of the molten
metal must be extracted in order to initiate rapid and
uniform solidification of the molten metal. Applicant
has also determined that the maximum allowable mold
expansion during casting with respect to surface and
subsurface quality of the cast strand is about 0.0755
inch in directions transverse to the longitudinal axis
of the m~ld, with 0, 0550 inch being the preferred limit
at the meniscus zoneO Expansion of up to 0.0755 inch
can be tolerated at the mid and lower zones without
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de~rimental effect to the cast strand. If these ma~im~ms
axe exceeded, the cast strand can have: surface cracks,
both transverse and longitudinal; subsurface cracks,
from 0.025 inch to 1.250 inch below the surface; rhomboidal
S bulging and other shape deformations; and excessive oscil-
lation mark depths (from 0.007 inch to 0.100 inch~O In
addition, excessive mold wear can occur.
To detect and correct these problems in accordance
with the invention, applicant contacts a distortion
measuring gauge or gauges to the outside (cooled) surfaces
of the mold and monitors the expansion of each face during
the casting operation. Expansion of the mold on any or
all desired faces is then controlled within selected
limits by selectively adjusting the rate of heat extraction
at that face.
In achieving this accurate control of mold expan-
sion during the casting operation, the following parameters
are maintained in the spray nozzles:
~a) spray angles of the cooling water no7zles at
the meniscus zone are selected so as not to exceed
100;
(b) spray nozzle pressure at the meniscus zone is
maintained at or above 15 psig;
(c) spray pa~tern overlap at the meniscus zone is
designed so that ~he coverage pattern between
adjacent nozzles is uniform per unit area of
sprayed surface; and
~d) water droplet size in the meniscus zone is
maintained in the range of from about 475 microns
to about 1450 microns.
A~ the other zones, expansion control is attained
following the conditions set forth in applicant's prior
U.S. patent 4,494,594.
Since so much heat must be extracted from the molten
metal at the meniscus zone, the steam barrier generated
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by the evaporating spray water must be penetrated by
the water droplets in order to effectuate rapid and uni-
form cooling. When the spray angle exceeds about 100
degrees, the force vector o the water droplets perpen-
dicular to the surface of the mold to be cooled at theedges o the spray pattern is not sufficient to penetrate
the steam barrier. Likewise, the water droplets will not
penetrate the steam barrier when the water pressure at
the nozzle is less than 15 psig. Moreover, since water
droplet size influences the ability of the water spray
to pene~rate the steam barrier and also the rapidity
with which heat extraction occurs, best operating con-
ditions are obtained when droplet sizes are confined
to the range set forth above.
Most shape deformations of the cast product are
initiated in the mold of the casting machine due to non-
un$form expansion of the mold which results in non-uni-
form contact between the metal being cast and the cooled
mold wall. By ope~ating within the above listed para-
meters and by monitoring the extent of distortion of the
mold wall during casting, it is possible to ef~ect what-
ever mold expansion conditions are desired by operators
or researchers by increasinq or decreasing the amount o
cooling water sprayed onto the mold. The condition and
quality of the cast product is also consequently controlled~
For example, if desired, one area of the mold can be
permitted to expand while another area is simultaneously
drawn in, or all mold areas can be controlled to the
same degree of expansion/contraction.
With the present invention, the operator o~ the
casting machine can monitor expansion conditions throuqh-
out the casting process and react accordinqly to conditions
as they may occur within the mold, making minor or ma~or
adjustments to the cooling system as required, thereby
influencing the quality of the solidifyin~ strand4 It
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is therefore possible to effect complete control of
the solidification process on all continuous casting
machines, producing billets, bloom, slabs, rounds or
other shapes. Moreover, since the operator can monitor
mold expansion, and hence strand and mold contact, he
now has the ability to control cast strand and mold
contact, mold stresses and elevated surface temperatures
of the copper mold and thereby directly and beneficially
influence mold wear and/or deterioration. Since mold
deterioration due to high temperatures of the cast
metal occurs most rapidly at the meniscus zone, the
ability to continuously monitor and control mold expansion
and temperature will allow the operat~r to directly and
beneficially influence mold wear patterns.
In slab casting, the mold consists of plates of
copper held together at the corners to form a large cavity
~etween the plates. Since in most cases, a slab has a
considerably larger cross-sectional area, greater quantities
of heat must be removed and greater ferrostatic pressures
must be contained by the copper mold. In conventional
mold systems for slab casting, the above facts require
further support systems on the back (cooled) surface of
the copper plate. The mechanisms that make this support
neces~ary are substantially the same as encountered in
casting smaller ~billet and/or bloom) cross sections.
With the present invention, it is possible for the designer
and operator to significantly reduce the back up support
systems and in most cases eliminate secondary support
systems in the ~old altogether.
Brief Description of the Drawings:
Further objects, features and advantages of the
invention will become apparent upon consideration of the
following detailed description in conjunction with the
drawings, in which like reference characters designate
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like parts throughout the several views, and wherein:
Fig. 1 is a ragmentary view in longitudinal
vertical section of the mold and bafle jacket of a
prior ar~ system;
Fig. 2 is a t~ansverse sectional view of the
apparatus of figure l;
Fig. 3 is a v;ew similar to figure 1, showing
diagrammatically the effects of mold expansion relative
to the coolant fluid space;
Fig. 4 is a view similar to figure 2, showing
the mold expansion depicted in ~igure 3;
Fig. 5 is a view similar to figure 2 of a further
form of prior art system in which spacers are used to
attempt to control mold copper expansion and thus to
alleviate the reduction of the coolant fluid space
upon expansion of the mold;
Fig. 6 ~s a view similar to figure 4 of the
system of figure 5, showing the increased distortion
of the mold when spacers are used;
Fig. 7 is a longitudinal sectional view of a
spray cooled mold section of a continuous casting
machine incorporating the invention therein;
Fig. 8 is a transverse sectional view of the
system of figure 7~
~ig. 9 is a somewhat enlarged, schematic view
of a portion of a mold and a pair of adjacent spray
no~zles, showing the overlap of the adjacent sprays
to achieve substantially uniform spray coverage per
unit area of the surface being sprayed;
Fig. 10 is a diagrammatic:view in transverse
section of a mold in which the expansion of the
various areas of the mold is controlled in accordance
with the invention;
F~g. 11 is a transverse sectional view of a
portion of a slab mold, showing the manner in which
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the copper pla~es are secured together to form the mold;
FigO 12 is a view similar to figure ll, showin~
a back-up support system for the mold of figure 11; and
Fig. 13 is a Yiew similar to figure ll, schemati-
cally illustrating the distortion of the slab mold platescaused by temperature and ferrostatic pressures.
Best Mode for Carrying Out the Invention:
Referrin~ first to figures l through 6 and ll
through 12, prior art systems are depicted generally
at lO and lO', respectively, In figures l through 6,
a mold ll extends concentrically ~ithin a baffle jacket
l~, definlng an annular space 13 therebetween for cir-
culation of coolant water as indicated by arrows A.
As shown in figures 3 and 4, when the mold is heated
by flow of molten metal therethrough, it expands
and thereby decre~ses the size of the annular space 13,
thereby altering the quantity and velocity of the cooling
water sweeping over the mold surface and influencing
heat transfer.
2Q In figures 5 and 6, an arrangement is shown
at 14 which is an attempt to overcome the problem
illustrated by figures 3 and 4. In this prior art
arrangementl pins or spacers 15 are placed against
the outer surface of the mold. However, as shown in
figure 6, the ~old deformation is not prevented by
this system and, in fact, the spacers act to further
increase the deformation of the mold.
The slab mold lO' shown in figures ll ~hrough
13 comprises a plurality of plates 16 bolted together
to form the slab mold cavity 17. A back-up suppor~
system 18 is bolted onto the copper plates on both
sides of the mold, as shown in figure 12. In figure
13, the back-up-support system is eliminated and sprays
of water are used to control expansion of the platesO
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For instance, a greater flow of cooling water could
be appl~ed in the vicinity of the ~iddle of the mold
and a lesser flow applied toward the edges.
The present invention is represented qenerally
at 20 in figure 7~ and comprises a frame 21 in which a
copper mo]d 22 with an open inlet encl and an open out-
let end is mounted at the top in re~istry with a central
opening 23 through top wall 24 of the frame. In a
typical construction, the frame 21 may be made of A-36
steel, and the mold tube may be made of DHP-grade
copper. A stream of molten steel 25 is poured into
the mold, at a rate relative to the rate of solidifi-
cation and strand withdrawal, to position the menisGus
25a in the upper region of the mold, i.e. within a
range of from about two inches to about 14 inches ~elow
the top of the mold. The mold is not connected at its
bottom end to the bottom 26 of the frame, but instead
simply hangs from its upper end - remaining free of
connection with the bottom wall 26. The lower end of
the mold is in alignment with an opening 27 in the
bottom wall 26, through which the strand is withdrawn.
Rather than the baffle jacket and annular space
for cooling fluid as depicted in figures 1 through 6,
the invention utilizes a plurality of s~ray pipes 28
spaced around the mold. Each pipe 28 carries a plurality
of spray nozzles 29 for forming a spray of water as
depicted at 30. Water is supplied to the pipes and
nozzles by a supply pipe 31. The spacing of the nozzles
relative to the mold and to each other is essentially
the same as set forth in applicant's earlier U.S. patent
4,494,594. Further, the selection of nozzle sizes,
water flow rate and water tnozzle) pressure are all
essentially as set forth in said patent. However, as
noted previously herein, the spray angle is se~ so that
it does not go over about 100, the spray overlap is
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selected so that the spray coverage is uniform through-
out the area being sprayed, the droplet size of the
spray particles is maintained in the range of from
about 475 microns to about 1450 micrc~ns, the mold
expansion is limited to a maximum of 0.0755 inch per
face t0.0550 inch being preferred at the meniscus
zone), and the water pressure is not permitted ~o go
below about 15 psig.
With respect to spray overlap! reference is
made to figure 9, which depicts the spray overlap in
accordance with the invention. Applicant has observed
that approximately 80% of the total water flow produced
by the spray nozzles occurs over about 50% of the cover-
age area - shown here as occurring over a central area
C located concentrically within the overall spray
pattern O produced by the nozzle '-?9. Thus, the nozzles
29 are adjusted relative to one another and to the mold,
taking into consideration the maximum spray angle and
the water pressure, so that the central areas C of
adjacent sprays just touch one another.
In accordance wi~h the invention, one or more
distortion measuring instruments 32 are associated
with the mold to measure the extent of expansion and
contraction of the mold during the casting operation.
As shown in figures 7, 8 and 9, these measuring instru-
ments comprise an elongate arm 33 extending into proximity
with the mold surface and having a probe or finger 34
projecting from the end thereof and into contact with
the outer surface of the mold. An indicator 35 is
connected with the probe to indicate displacements of
the probe and hence the mold surface contacted by the
probe, The indicator is preferablY mounted so as to
be readily visible to the operator of the casting machine
or linked to a computer for automatic control and
adjustment, Other types of deflection measuring instru-
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ments may be used, if desired. By observin~ the
indicator, the operator can ascertain the extent of
distortion or expansion of the moldO As noted pre-
viously, it is importan~ to maintain the distortion
below abou~ 0.0755 inch, and preferably below about
0.0550 inch at the meniscus zone. In order to main~
tain the expansion of the mold within this li~it, the
flow of coolant fluid directed against the mold is
controlled. This may be accomplished in a number of
ways, including ~ flow controller 36 which may be man-
ually operated or automatically operated in response
to a sensed reading by the indicator tshown at 37 in
dot-and-dash lines in figure 7). Further, the spray
pipes 28 may be constructed in sections 28a, 28b, etc.,
each supplying a number of nozzles 29 and supplied by
its own supply pipe 31a, 31b, etc., controlled by a
controller 36a, 36b, etc. (shown in dot-and-dash lines
in figure 7).
Nozzles may be positioned to direct coolant fluid
directly agains~ the face of the mold, as shown in dot-
and-dash lines at 38 in figure 8.
By maintaining the operating conditions within
the parameters described herein and by measuring and
controlling the expansiun and c~ntraction of the mold
during a casting operation, the extent of distortion
of respective faces A, B, C and D can be accurately
controlled as shown in dot and-dash lines in figure
10. Consequently, the quality of the cast strand can
be controlled and need not be accepted with defects
therein as noted earlier herein, such defects being
associated with scrap losses incurred during subse-
quent processing of the cast strand and with the cost
of metallurgical claims arising from the shipment of
sub-standard finished products to end users.
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Although the invention has been described with
reference ~o particular embodiments, it is to be under-
stood that these embodiments are merely illustrative
of the application of the principles of the invention.
Numerous modifica~ions may be made therein and other
arrangements may be devised without departing from
the spirit and scope of the invention.
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