Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a casting machine
having a pair o~ travelling block mold assembly capable
of continuously casting a thin plate product at high
speed.
The so-called block casting method of carrying
out continuous casting while travelling in the casting
dlrection an assembly which comprises block mold members
endlessly connected to one another has appeared these
days. This block casting method is intended to cast
10 thin products continuously and at high speed and it
attracts attention as a continuous casting method which
will become prevailing in the future.
In the case of the conventional continuous casting
method which uses the stationary mold, the product in the ~--
15 process of solidification, is forcedly pulled out of the
stationary mold. Therefore, they are broken because of
their shells bonded by walls of the stationary mold.
This is the so-called "sticker type breakout". In the
case of the block casting method, however, the sticker
20 type breakout can be effectively avoided because molding
blocks can be drived in response to the speed at which
the product in the mold is solidified. According to the
block casting method, therefore, casting speed can be
made ten times or higher than that of the conventional
25 continuous casting machine having the stationary mold.
A block type casting machine for casting thin
products each having a thickness of several tens
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rnillimeters, for example, is provided with a pair
of travelling block units (block mold assembly). Each
of these paired travelling block units comprises a
plurality of block mold members each having L-shaped
section and endlessly connected to one another, and both
of these units are travelled synchronously with each
other. Casting cavity is defined by a plurality of the
block mold members.
As shown in Fig. 1, however, the cooling
deficiency of molten steel 4 takes place at seam 3,
along which block mold members 2 are connected to
each other, to thereby form solidification-delayed
portion 6 along seam 3, when molten steel 4 comes
into contact with casting walls of the block mold
members 2 and shell 5 starts to develop at a certain
speed. Contraction stress thus concentrates to
solidification-delayed portion 6 along seam 3 of the
block members and crack forms on the surface oE the cast
product along the dendrite axis of the product. This is
the so-called "transversal crack". This transversal
crack also forms in the case of the common stationary
mold. It forms in the wide face, extending from one end
to the other end of the width of the product in worst
case.
When a flaw parallel to the casting direction
exists on the inner wall of the block mold member 2,
local cooling deficiency is caused at this flaw to form
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the solidification-delayed portion, which cooperates
with the distortion of the product in the width direc-
tion thereof to result the so-called "longitudinal
crack" on the surface of the product.
These transversal and longitud:inal cracks can be
avoided to some extent by controlling cooling speed to
the product at the secondary cooling zone which ~ollows
the casting process. Because the casting speed is high,
however, it is difficult to control the cooling speed to
the product and it is impossible to completely eliminate
these surface flaws from the product. This makes it
necessary to apply a treatment such as -the scarfing to
the surface of the product before the following rolling -~
process, thereby reduces the productivity and causes to
a higher cost.
The object of the present invention is to
provide a casting machine capable of preventing flaws
such as the transversal and longitudinal cracks at the
surfaces of the cast products. T t is particularly to
provide a casting machine capable of preventing from
the surface flaws on the products in the case where the
products for thin plates are continuously cast at high
speed.
According to an aspect of the present invention,
a casting machine comprising; at least a pair of tra-
velling mold assemblies, each having a plurality of
block mold members endlessly connected to one another
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to define a cavity and a means for driving the block
mold members such that the block mold members o~ one
travelling block mold assembly move in synchronism with
those of the other in a casting direction, wherein
S grooves are formed in the shape of a latticed on at
least inner walls of the block mold members.
When solidification delays of molten steel exist
intentionally caused at the lattice of grooves on the
surface of the cast product, they can be scattered
over the whole surface oE the product, including those
portions thereof which correspond to seams of the block
mold members connected and vertical flaws on the casting
walls of the block mold members. This can reduce the
concentration of contraction stress.
The cavity defined by the block mold members may
have any sectional shape such as the slender rectangle,
slender ellipse, triangle, hexagon, and octagon.
Whatever sectional shape the cavity may have, however,
it is preferable to form the lattice of grooves at the
wide face of the mold wall.
It is preferable that each oE the grooves formed on
the mold wall is small in depth and width. It is more
preferable that each of the grooves has a depth D of
0.5 mm and a width W of 0.5 - 1.0 mm.
It is preferable that the pitch interval L of the
latticed grooves is 5 - 10 mm. When the groove pitch L
is smaller than 5 mm, contact between the molten steel
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and the casting walls of the block mold members becomes
incomplete. As the result, the molten steel is not
cooled enough to thereby increase the frequency of so
called breakout. When the groove pitch L is larger than
lO mm, however, the merit of preventing the transversal
and longitudinal cracks cannot be found. These are the
reasons why the groove pitch L is kept 5 - lO mm.
This invention can be more fully understood from
the Eollowing detailed description when taken in
conjunction with the accompanying drawings, in which:
Fig. l is a vertically-sectioned partial view
showing conventional block mold members which is under
casting;
Fig. 2 is a schematic view of a block casting
system in which an embodiment of casting machine
according to the present invention is employed;
Fig. 3 is a transversally sectioned view showing
block mold members;
Fig. 4 is a perspective view showing a part of the
block mold members arranged to one another;
Fig. 5 is an enlarged perspective view showing
grooves formed on the casting wall of a block mold
member;
Figs. 6 through 8 are sectional views showing
variations of the groove formed on the casting walls of
the block mold members; and
Fig. 9 is a graph, on a horizontal axis of which
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groove pitches L are plotted and on a vertical axis of
which the frequency of cracks caused in products and the
index of breakout occurrence are plotted to explain how
the quality of products cast is influenced by groove ,
pitches L.
Some embodiments of the present invention will be
described in detail with reEerence to the accompanying
drawings.
As shown in Fig. 2, tundish 10 is located on the
upstream side of block type continuous casting machine 20
and nozzle 14 of -tundish 10 through which molten steel
; is fed is inserted into the upstream side of a cavity 27
oE casting machine 20.
As shown in Fig. 3, cavity 27 is defined by block
mold assembly 22 of paired upper and lower casting units
21.
Plural spray nozzles 32 and pinch rollers (not
shown) are located on the downstream side of casting
machine 20. Casting machine 20 is tilted by a certain
angle.
Casting machine 20 includes the paired casting
units 21 combined to face each other. Each of block
mold assembly 22 comprises a plurality of block mold
members 23, substantially the same in shape, endlessly
connected to one another between a pair of gear wheels
28 and 29 and these members 23 of each of assembly 22
are endlessly driven from upstream to downstream, or
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from downstream to upstream by a drive mechanism (not
shown). Speeds "a" and "b" at which upper and lower
block mold assemblies 22 are driven and speed c at which
product 3l is pulled out of cavity 27 are computer-
controlled in response to the volume of the molten steelfed and the change of the molten surface in the cavity
27.
As shown in Fig. 4, each of block mold members 23
has an L-shaped section in a direction perpendicular to
the casting direction and it is provided with a lattice
of grooves 25 formed all over its wider inner wall 24a.
In other words, grooves 25 extend along seam 3 of adja-
cent block mold members 23 connected and from one end to
the other end of wider inner wall 24a. They also extend
in a direction perpendicular to the wider inner wall 24a
of member 23.
Grooves 25 may be formed on a narrower inner
wall 24b of block mold member 23 in addition to
wider inner wall 24a thereofO Or they may be formed
on a part of wider inner wall 24a, that is, only that
portion of wall 24a which is adjacen-t to seam 3 of mem- -
bers 23 connected.
As shown in Fig. 5, each of latticed grooves
25 has a V-shaped section whose depth ~ is smaller
than its width W. For example, groove width W is
0.5 - 1.0 mm, groove depth smaller than 0.5 mm, and
groove pitch (distance between adjacent grooves)
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L 5 - lO mm.
As shown in Figs. 6 through ~, the sectional shape
of the ~roove may be changed to saw-tooth-shaped groove
34, U~shaped groove 35 or rectangular shaped groove 36.
There will be described a case where low-carbon
aluminum killed steel in which carbon of about 0.05
weight percents is contained is continuously cast
through above-described machine 20.
Molten steel 12 adjusted to have certain components
and a certain temperature is transferred from a ladle
(not shown) to tundish 10. rlolten steel 12 thus trans-
ferred into tundish 10 is fed into cavity 27, into which
a dummy bar (not shown) is inserted, through nozzle 14.
The cavity 27 is previously under the atmosphere of
inert gas and casting is carried out, Eeeding the molten
steel 12 into the cavity 27 while pulling the dummy bar
out of the cavity 27 and keeping the front end of nozzle
14 separated from the molten surface in the cavity 27.
This so-called "open casting method" may be carried out
or the so-called "closed casting method" which is
carried out keeping the front end of nozzle 14 immersed
in the molten steel 12 in the cavity 27 may be employed.
Further, the feeding volume of the molten steel is pre-
cisely adjusted by a sensor and a controller (not shown)
located on the side of the tundish lO to best control
both of the speed "a" and "b" at which each of casting
units 21 is endlessly driven and the speed "c" at which
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the product 3l is pulled out of the cavity 27.
When rnolten steel 12 is fed into cavity 27 through
nozzle 1~, it contacts the castlng walls of water-cooled
block members 23 which define cavity 27 and becomes
shell 26, which is further cooled by spraying water
through spray nozzles 32 and solidiied completely.
Casting speed is more than 20 meters per minute, for
example.
- Cooling deficiencies are caused at seams 3 and
grooves 25 during solidifcation of molten steel in
the mold~ Solidification delays are thus caused
and tensile stress (contraction stress) is created
at these specific areas when the molten steel is
solidified and shrunk. Because the contraction
stress is scattered on all of the surface of the
shell 26, however, -the concentration of the contraction
stress can be reduced to some extent.
Even if cracks should be caused, they can be
stopped by grooves 25 not to develop to larger ones
because some grooves 25 will be formed perpendicular
to them.
Shell 26 is developed to have a sufficient
thickness and a certain strength at the outlet of the
mold. The solidification of product 31 is controlled by
water jetted through spray nozzles 32, during pulling
shell 26 out of the mold. A thin slab 50 mm thick and
1000 mm wide, is produced in this manner.
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The quality o~ slabs producting by the block
members which have various kinds of grooves thereon
will be described referrirlg to Fig. 9.
Fig. 9 is a graph, on the horiæontal axis of which
groove pitch L of the V-shaped grooves is plotted and on
the vertical axis o~ which the Erequency of cracks
caused in products and the index of breakout occurrence
are plotted to explain the quality of slab 31 changing
the groove pitch L variously but keeping other con-
ditions substantially the same. Tn the Fig. 9, blackspots represent transversal cracks, whlte spots repre-
sent longitudinal cracks, and white triangles represent
breakout occurrence. The index of breakout occurrence
means in this case that frequencies of breakout caused
in products are replaced by indexes. As apparent from
Fig. 9, the frequency of transversal and longitudinal
cracks caused in products is quickly increased when
groove pitch L becomes longer than 10 mm. When groove
pitch L is shorter than 5 mm, the index of breakout
occurrence is increased because contact between the
molten steel in the cavity and the inner walls of the
block mold members becomes insufficient and the molten
steel is not cooled enough, accordingly. When low-
carbon aluminum killed steel is to be cast, therefore,
it is preferable that groove pitch L is in a range of
5 - 10 mm.
Although each of the latticed grooves formed on the
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inner wall,s oE the block mold mem~ers in the cavity has
the V-shaped section in the above-described embodiment,
it may be like saw-tooth-shaped groove 34, U-shaped
groove 35 or rectangular-shaped groove 36, as shown in
;~ 5 Figs. 6 through ~.
Although the latticed grooves have been formed
only on the wider inner wall 24a oE each oE the block
mold members 23 in the cavity in the case of the above-
described embodiment, they may be formed on the narrower
inner wall 24b of each of the block mold members 23, or
on a part of the wider inner wall 24a, or adjacent to
the seams along which the block mold members 23 are
connected to one another.
According to the present invention, the occurrence
of transversal and longitudinal cracks on surface of
slab 31 can be effectively prevented. After~treatments -~
(e.g. surface treatment etc.) which will be applied to
surfaces of slab 31 at the following process can be thus
reduced to a great e~tent. The productivity of products
can be therefore enhanced and their manufacturing cost
can be made lower.
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