Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
1. Field of the Invention. '
The present invention relates to plate for a metal mold for continuous
casting of metal and having a funnel--shaped pouring region narrowing in
casting direction to a size of a cast billet, and a rear wall provided with
slots in
which a cooling medium flows.
The present invention also relates to a method of cooling of a metal mold
plate.
2. Description of tb~e Pxxor Art
The object of a metal mold is to form a billet and to absorb the heat
released by the melt upon shell growth. At the metal mold outlet, the shell
must
be so thick that it can withstand thermal and mechanical loads and, at the
same
tizxxe, there would not be teared up of the billet.
In a steel mold, the heat is absorbed by the cooling water which flows in
drilled channels or slots which are milled in the rear wall of the plate and
which
form, together with an outer surface of a water box, rectangular channels.
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Conventionally, ire accordance with the existent technology, the cooling
channels or slots extend vertically, i.e., in the casting direction to achieve
as
uniform as possible heat removal over the plate width. The arrangement of the
bores and slots and the selection of their dimensions are effected in such a
way
that as uniform as possible removal of heat over the entire surface of the
plate is
insured.
In addition, upon designnz~g of a mold plate, the art'an,gement of the
tbxeads or of the thread inserts for receiving the screws, with which the
plate is
attached to the water box, should be taken into consideration. The number of
the attachment poixtts should be sufficiently large to prevent the deformation
of
the plate above a tolerable amount as result of an application of a thermal
load
during a casting process.
For casting slabs, the vertically extending slots extend over the entire
plate.
The drilled plate are relatively expensive in manufacture and, therefore,
are preferably used where only a small deformation of the metal mold cavity is
acceptable as, e.g., during a continuous casting of blooms.
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Further, despite large thermal loads during contact with a steel melt, a
correct design of and a selection of a material for a mold plate exclude
formation of cracks in the working surface of the metal mold plate.
Nontheless,
the use of the plate Leads, with time, to the damage of the working surface
such
as mechanical wear during the casting process, the appearance of scratches as
a
result of lead-in and draw-out of the dummy bar and of the adjustment of the
narrow sides during the casting process, and to local deformations which
result
in the formation of a clearance with the narrow sides.
The service life of a metal mold plate depends substantially on the
frequency, position, and depth of such damages and the number of possible
millings of the working surface at which respective layers of the surface are
mechanically removed.
The idea to provide a funnel-shaped pouring region can be traced back.to
the tendency to cast as thin a billet as possible which billet, after exiting
the
casting machine, is separated into slabs and is fed, through a furnace,
directly to
a rolling train.
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The dimensions of the pouring region are essentially determined by the
cross-section of a cast billet, the dimensions of the submerged entry nozzle
and
its submergence into the melt.
During operation of several continuous casting plants with drilled and
slotted metal mold plates formed of different copper alloys and having funnel-
shaped pouring regions having different dimensions, it was found out that
after
a comparatively small number of several hundred castings, cracks were formed
in the copper mold plate at the height of the bath level. After, in average,
three-
four millings, therefore, in comparison with a metal mold plate having an even
working surface, the treated plate had a service life of about 1000 melts
which
nurx~ber is in several times smaller that the usual number of melts for the
plate
with the even working surface.
The fissures are the result of the fatigue of the mold material resulting
from alternating plastic strains. The measurement with thermo elements
provided on the metal mold plate and a static evaluation of the occurrence of
the scratches have shown that the scratches are always located in the
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transitional region of the funnel-shaped part and the plain lateral parts of
the
mold and~are not caused by an excessive local thermal load.
The distribution of the stresses arid strains over the width of a metal mold
plate having a flat or arched working surface is determined, by, in addition
to
the cross-section, the number of attachment points, their positions, and
spring
constant of the attachment screws. The smaller is the number of the attachment
points, the more uneven is the distribution of stresses and strains over the
metal
mold plate. They concentx-ate, dependent on the ax~r~gement of the attachment
points, increasingly in the region of one of the outer Vertical roves of the
attachment points on each side. 'therefore, a prerequisite for a uniform
distribution of stresses and elongations over the width of a metal mold plate,
with regard to the prevention of cracks, is provision of a sufficicntly large
amount of the attachment points.
Further investigations and calculation of the stresses and strains have
shown that additional strains in the transitional region between funnel and
lateral parallel portions favorably influences the formation of the cracks in
the
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metal mold plate which are traced back to the existence of a camber that
serves
for formation of the funnel-shaped pouring region.
'1 he mechanism of the appearance of the additional strains can be
explained with reference to Fig. 1 which shows a portion of a cross-sectional
view of an upper region of a drihed metal mold platel. The plate is secured to
the water box 2 with screws S 1, S2, S3, S4, S5, S6. The billet width is
determined by the position of the narrow side plate 3. The location of the
cooling bores is determined by working thickness d~, i.e., by the distaxxce of
the
bores :Crorn. the working surface, and by a distance d2 from the rear wall.
The heating of the metal mold plate I by a heat flow q, e.g., during a
casting phase or as result of the displacement of the bath level, leads, as a
result
o~ the expansion of the plate material, to a length change in the direction
toward
the narrow side which, in case of an even working surface, is not prevented or
obstructed. Therefore, no additional stresses or strains occur in the plate.
In a
metal mold with a funnel-shaped pouring region having a width B, the working
surface of the plate additionally buckles because this region cannot freely
expand with screw S 1, S2, S3, S4, SS, S6 at points defined by positions X1,
X2,
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X3, 7C4, X5, X6. Upon cooling of the plate, e.g., after the end of the casting
process, because of tlae contraction, a length change in the opposite
direction
takes place, and the previously buckled region of the working surface will
addition~,ally be stretched.
The distribution of this additional alternating strains over the plate width
is predetermined by the profile of the arched or bulged region which, in this
case, is determined by radii R1 and R2 and by the funnel opening T, by the
positions X1, X2, X3, X4, X5, X6 of the attachment screws S1, S2, S3, S4, S5,
S6 with xespect to the cowrse of this pxo'hle and by the elasticity of the
attachment screws S 1, S2, S3, S4, S 5, Sf .
'The intensity of this additional alternating strains are determined,
together with the thermal load applied to the metal mold plate by the heat
flow
q and by the characteristics of the material of the plate, the following
parameter:
the working thickness d~, between the working surface and the cooling bores,
the plate thickness d2 between the cooling bores and the rear wall, and the
width
B/2 of the arched or buckled regzon.
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The calculations conducted for a drilled and slotted metal mold plate with
a funnel-shaped pouring region, which is used in casting, have shown that the
additional aItexnanz~g strains, together with strains caused by the general
thermal Load, reach a value that results in a very rapid fatigue of the plate
material due to the plastic variable deformation.
if the load acting on the arched metal mold plate is to be reduced by
constructive measures to an uncritical amount with regard to susceptibility to
formation of cxaclcs, it is necessary to incxease the sriffness of the plate
cross-
section which, nn the simplest case, is achieved by increasing the thickness
between the cooling bores or slots and the rear wall of the plate from dZ to a
necessary value.
Because no temperature gradient exists behind the cooling channels, the
increase of the plate thickness by increasing the thickness of the plate
between
the rear wall and the cooling channel leads, at the one and sarne thermal
load, to
a more uniform distribution of stresses and elongations over the plate width.
The expansion of the metal mold plate in a horizontal directions which
favors the formation of the cracks in the working surface decreases because
the
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plate behind the bores is not heated strong enough in comparison with the
region between the working surface and the cooling bores, and the average
temperature over the mold cross-section becomes generally smaller_
Therefore, drilled plate with an adequately large thickness d2 between
the cooling bores and the rear wall and which is attached to the water box
with
a suffciently large number of screws S 1, S2, S3, S4, S5, S6, insures that the
working surface, up to a predetermined thermal load q max rcrnains free of
fissures, and that, upon exceeding this thermal load, the cracks axe uniformly
distributed over the width of the plate.
The drawback of such a drilled plate consists inn that the manufacture of
such drilled plate is very expensive in comparison with a slotted plate.
Accordingly, an object of the present invention is to provide a metal
mold plate with a funnel-shaped pouring region and with water-cooled slots on
the rear wall and the working surface of which remains crack-free up to a
predetermined thermal load q max and which, upon the thermal load exceeding
the predetermined thermal load, has the cracks uniformly distributed over the
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plate width, so that the plate has the samc service life as the plate with
llat~
walls.
SUMMARY OF THE INVEhT'I'rUN
This and other objects of the present invention,~which will become
apparent hereinafter, are achieved by forming the slots in the rear wall of
the
plate in such a manner that at least in one height section o~ the arched
region of
the plate, the course of the slots deviates from a horizontal by no more then
30°.
The present invention is based on a premise that in order to prevent the
formation of cracks on the working surface at a predetermined working
thickness, the plate cross-section should be as large as possible because the
increased stiffriess and a smaller horizontal thermal expansion, which are
associated with an increased cross-section, lead to a uniform distribution of
the
stz-esses and strains over the width of the plate.
The novel features of the present invention, which are considercd as
characteristic for the invention, are set forth in particular in the appended
claims. The invention itself, however, both as to its construction and its
mode
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of operation, together with additional advantages and objects thereof, will be
best understood from the following detailed description of preferred
embodiments, when read with reference to the accompanying drawings.
BRIEF D)CSCRIPTION OF THE DRAWINGS:
The drawings show:
Pig. 1. a portion of a cross-section view of an upper region of a
conventional funnel-shaped metal mold plate with a plurality of
cooling bores;
Fig_ 2a_ a rear view o~ a metal mold plate with a plurality of slots and
with a funnel-shaped pouring region according to the prevent
invention;
Fig. 2b. a rear view of the metal mold plate shown in Fig.2a.; and
Fig. 3. a cross-sectional view along line C-C of the metal mold
plate shown in Fig. 2b and provided with inserts and bores for
locking screws.
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DETAILED DESCRIPTION OF PREFERRED EMBODIMENT
Fig. 2 shows a metal mold plate 4 of a continuous casting metal mold
according to the present invention. The metal mold plate 4 has, as shown in
Fig. 2, a funnel--shaped arcuate pouring region 5 which begins at a mold upper
edge 6 and diminishes toward the narrow sides 3 and in the pouring direction
to
the size of a cast billet.
The rear side of the metal mold plate 4, at least in its upper region, has a
plurality of slots 7 in which a cooling water flows. The course of the slots 7
deviates from a horizontal plane by no more than 30°. However, in a
preferred
embodiment of the present invention, the slots extend over the width of the
plate 4 substantially horizontally, as shown in Fig. 2.
The depth and/or the width of the slots 7 and the distance between them
can remain constant or vary over the height and/or width o~ the mold plate 4.
According to a further preferred embodiment of the present invention, it
is contemplated to insert inserts 9 into the slots 7 in order to increase the
flow
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velocity and to reduce the consumption of the cooling water. The inserts 9 can
be made of any suitable material.
Along the slots 7, the inserts 9 are connected, in any suitable manner with
webs limiting the slots 7. In this way, the inserts 9 additionally increase
the
stiffizess ox rigidity of the plate cross-section. The inserts 9 permit to
reduce the
thiclrness of the plate 4 necessary for the required stiffness of the metal
mold
cross-section. The reduction of the thickness of the plate 4 permits, in turn,
to
reduce the dimension of the metal mold. ,A, further advantage of using the
inserts 9 consists in the reduction of costs of the materials. The reduction
of the
dimensions of the metal mold perzxoits to reduce the amount of the material of
which the metal mold is made, usually a copper alloy which is comparatively
more expensive than steel of which the inserts 9 are made.
,A.nd important advantage of the present invention consists in the
possibility of separate delivery of cooling medium for different height
regions.
By a controlled influence of the heat transfer in the metal mold at different
height sections, the quality of the cast billet can be improved.
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Accordingly to the invention, the cooling medium can be delivered at one
side of the mold plate 4 and removal at the other side. Advantageously,
however, the fluid medium is delivered ai both sides and is removed at the
middle or is delivered at the middle and is removed at both sides. Because in
both cases, the increase of the temperature of the cooling medium dozing its
flow through the cooling slots is reduced in half, the unevenness of the heat
transfer over the width of the metal mold plate 4 is also reduced.
The cooling medium can flow in different slots 7 in opposite directions.
This additionally reduces the uxzevenness of the heat transfer over the width
of
the metal mold plate 4. This is because upon flo~uv of the cooling nZedaum in
opposite directions, the increase of the temperature of the cooling medium is
transmitted back to the stream of the cooling medium with a smaller .
temperature_
Though the present invention was shown and described with references
to the preferred embodiments, such are merely illustrative of the present
invention and axe not to be constzucted as a limitation thereof and various
modifications of the present invention will be apparent to those skilled in
the
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art. It is therefore not intended that the present invention be limited to the
disclosed embodiments or details thereof, and the present invention includes
all
variations and/or alternative embodiments within the spirit and scope of the
present invention as defined by the appended claims.
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