Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING A MOLD TUBE
The present invention relates to a mpthod for producing a mold
tube from copper or a copper alloy for a continuous casting
mold in accordance with the features set forth in the
definition bf the species in Claim 1.
Tubular chill molds made of copper or copper alloys for
casting profiles made of steel or other metals having a high
melting point have often been described in the related art.
Mold tubes typically have a uniform wall thickness in a
horizontal cross-sectional plane that increases in the
direction of the strand due to the inner conicity of the mold,
tube. The inner conicity is adapted to the solidification
behavior of the strand and the continuous casting parameters.
The heat flux has a predominantly two-dimensional
characteristic and leads to widely differing cooling rates of
the steel strand. A particularly strong shell growth and
shrinkage behavior of the strand is evident in the corners of
the mold tube, since it is here that substantial amounts of
heat are dissipated due to the unequal casting surface to
cooling surface ratio. The rate of cooling is lower in the
lateral surfaces of a mold tube than in the corner regions due
to the virtually equal casting surface to cooling surface
ratio. At the same time, a greater heat flux is imposed
thereon. The leads to a reduced shell growth relative to the
corner regions.
Due to the different cooling ratios within a mold tube, a
different strand shell growth rate results over the horizontal
cross section. Consequently, tensile and compressive stresses
occur in the strand shell. Since the strength of the strand
shell is relatively low immediately following solidification,
such stresses readily lead to internal and external defects in
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the form of cracks in the billets. There is also an increased
risk of the strand shell rupturing underneath the mold.
Therefore, efforts are currently underway to optimize the mold
tube by providing a homogeneous dissipation of heat over the
horizontal cross section, as well as a maximum dissipation of
heat over the entire dwell time of the strand in the mold.
This is all the more important since, the thermal loading of
the mold tubes increases in response to rising casting rates.
It is, therefore, necessary to seek a most optimal possible
cooling to prevent damage to the mold tube, the aim being to
prolong the service life of the mold tubes.
A higher rate of heat dissipation can be achieved by providing
additional cooling grooves, as is described, for example, in
the European Patent Application EP 1 792 676 Al. The cooling
grooves are adapted in the depth and configuration thereof to
the amount of heat to be removed, the corner regions of the
mold tube being excluded. The grooves are produced by
cutting-type machining since they are formed as depressions in
the surface.
The German Patent Application DE 36 15 079 Al describes a
method for producing open-ended molds for continuous casting
for continuous casting machines, where a tube is calibrated by
an inner mandrel and, on the outside, is pulled through a die
that imparts the outer contour to the mold. Molds having
curvilinear shapes can also be manufactured in this manner.
It is, therefore, an object of the present invention to
provide a method for producing a mold tube that will make
possible the inexpensive manufacture of mold tubes featuring
optimized heat dissipation. This objective is achieved by a
method having the features of Claim 1.
Advantageous refinements of the present invention constitute
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the subject matter of the dependent claims.
The method according to the present invention for producing a
mold tube from copper or a copper alloy provides for a tube
blank to be formed on a mandrel, which determines the inner
shape of the mold, by externally applied force, and, following
the shaping process, for the mandrel to be removed from the
mold tube. In the process, the tube blank is passed through a
die that has a shaping structure for a cold formed profile on
the outer surface of the mold tube, so that the cold formed
profile is produced by the die upon shaping of the tube blank
on the mandrel.
The method according to the present invention, therefore,
provides for a noncutting production of the cold formed
profile that is achieved by a special shaping structure in the
die. By applying the method according to the present
invention, the cold formed profile is able to be produced much
faster and more economically than if cutting-type machining
were used.
In the context of the present inventj_on, it is self-evident
that it is not ruled out for a cutting-type machining to be
additionally carried out in order to make local adaptations,
for example, to mill in grooves used for fastening the mold
tube to the interior of a water jacket. However, the basic
principle is based on the approach of producing the cold
formed profile in a continuous casting process using
noncutting shaping.
This enables the cold formed profile to extend from the top to
the bottom end of the mold tube. With regard to the exact
embodiment of the cold formed profiling, consideration must be
given to the heat dissipated from the strand during the dwell
time in the mold tube.
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The heat dissipation is derived, inter alia, from the outer
surface of the mold tube that is in contact with cooling
water. This is represented by the mathematical formulation:
Q --axAxLT, Qdescribing the heat flux, athe
heat-transfer coefficient of the outer mold surface into the
cooling water, and LT the increase in the temperature of the
cooling water during the cooling phase along the mold tube. In
this context, the amount of heat to be dissipated is in
proportion to the heat-transfer surface. Increasing the size
,of the outer surface by the cold profiling allows more heat to
be released to the ambient environment, i.e., to the cooling
water. Thus, the cold profiling results an enlarged surface
area, this increase in the surface area being determined by
the structure of the die.
The present invention provides for the cold formed profile to
be produced preferably in response to the pulling of the tube
blank through the die. The corner regions of the mold tube may
be thereby excluded in order not to additionally increase the
size of the heat-transfer surface in this region. The cold
formed profile may itself be configured as a grooved profile
having an undulated structure or as a zigzag profile. An
undulated profile or also a zigzag profile may be more readily
realized by the process of pulling through the die than are
individual, mutually spaced apart grooves having a rectangular
cross section, for example.
In the context of the present invention, it is considered to
be especially advantageous for the cold formed profile to be
produced with an amplitude height range of from 0.5 to 5 mm;
in the case of a zigzag profile, an opening angle being
produced between two adjacent zags within a range of from 15
to 90 , and, in the case of an undulated profile, the distance
between two adjacent grooves being 1 to 14 mm.
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*
In one advantageous embodiment, the amplitude height
is 0.5 to 1.5 mm. The opening angle is preferably within a
range of from 45 to 60 .
In principle, the method according to the present invention is
suitable for all known forms of mold tubes, whether the cross
section be circular, rectangular or square. In the same way,
double-, U- 9,r L-shaped cross-sectional profiles may be
produced using the method according to the present invention.
The mandrel used in the method according to the present
invention may have a conical shape. It may have a one-part or
a multipart design. The mandrel itself may also be
curvilinear, thereby making it possible for the method
according to the present invention to be used for producing
the mold tubes for circular-arc continuous casting machines.
The present invention is described in greater detail in the
following on the basis of the exemplary embodiments
illustrated in the drawing, whose figures show:
FIG. 1 a sectional representation through the wall region
of a mold tube having a zigzag-shaped cold formed
profile;
FIG. 2 a sectional representation through the wall region '
of a mold tube having an undulated, cold formed
profile; and
FIG. 3 a perspective view of the corner region of a mold
tube.
FIG. 1 shows a detail of a mold tube 1. Specifically, it is a
question of one fourth of a mold tube which, if fully
represented, would define a rectangular interior space.
Therefore, mold tube 1 has a-corner region 2, as well as side
walls 3, 4; in the top portion of the image plane, side wall 3
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being longer than side wall 4, which is to the right in the
image plane.
Illustrated mold tube 1 is made of copper or of a copper alloy
and is produced by the pulling of a tube blank (not shown in
detail) through a die. In this context, the mold blank had
been formed on a mandrel (likewise not shown in detail). The
inner contour of mold tube 1 is formed by the external force
applied by the die. The geometry of the die determines the
outer geometry of mold tube 1. It is the outer geometry of the
mold tube that is relevant to the inventive method. FIG. 1
shows that, in some regions, outer surface 5 features a cold
formed profile 6, 7 while, in other regions, it does not.
Specifically, in this exemplary embodiment, corner region 2 is
smooth, i.e., configured without any cold formed profile. Cold
formed profiles are only located in the region of side
walls 3, 4. While cold formed profile 6 of upper side wall 3
in the top portion of the image plane extends directly to the
beginning of corner region 2, i.e., ends where the curvature
of corner region 2 begins, cold formed profile 7 of shorter
side wall 4, which is to the right in the image plane, is
located at a somewhat greater distance from corner region 2.
This means that corner region 2 initially merges
transitionally into a region 8 in which outer surface 5 of
side wall 4 is unrounded and smooth. Only then does cold
formed profile 7 begin.
Cold formed profiles 6, 7 are identical in design. It is a
question of zigzag profiles. Overall, therefore, the grooves
of the zigzag profile are identical in form. They have a
uniform amplitude height H, which, in this exemplary
embodiment, has dimensions on the order of 0.5 to 1.5 mm and,
in particular, 1 mm. Angle W, which is measured between
mutually adjacent flanks of two zags, is within the range
of 15 to 90 . In this exemplary embodiment, it is 60 .
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The specific embodiment of FIG. 2 differs from that of FIG. 1
merely in the shape of cold formed profiles 6a, 7a. Cold
formed profiles 6a, 7a are not shaped as zigzag profiles, but
rather as undulated profiles. The amplitude height is within
the range of 0.5 to 5 mm and, here, may also be preferably
within a range of from 0.5 to 1.5 mm, in particular may be
1 mm. Overall, therefore, it is discernible that both
profiles 6a, 7a are uniform. Mutually adjacent grooves 9
between two crests 10 are all spaced, apart at the same
distance. The distance is 1 to 14 mm. Between the flanks,
illustrated angle Wl, in turn, is 60 .
In a perspective view, FIG. 3 shows corner region 2 of mold
tube 1 illustrated in FIG. 1. On outer surface 5 thereof,
corner region 2 is smooth, while a cold formed profile 6 is
produced on side wall 3 which is to the left in the image
plane. Situated in the image plane above cold formed profile 6
as a recess milled into side wall 3, is a transverse
groove 11. Another transverse groove 12 is located in other
side wall 4. Transverse grooves 11, 12 extend into corner
region 2. Mold tube 1 may be fixed in position via transverse
grooves 11, 12. No further cold formed profile is configured
in the image plane above transverse grooves 11, 12. Cold
formed profile may be removed by a cutting-type machining, for
example, in order to produce a smooth surface for sealing mold
tube 2 in a water cooling box.
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List of Reference Numerals
1 - mold tube
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la - mold tube
2 - corner region
3 - side.wall
3a - side wall
4 - side wall
4a - side wall
- outer surface
6 - cold formed profile
6a - cold formed profile
7 - cold formed profile
7a - cold formed profile
8 - region
9 - groove
- crest
11 - transverse groove
12 - transverse groove
W - angle
W1 - angle
H - amplitude height
=
H1 - amplitude height
8
