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Translated Text of WO 03/092931 A1 (PCT/FP03/02384)
with Amended Claims Incorporated Therein
ADJUSTMENT OF HEAT TRANSFER TN CONTINUOUS CASTING
MOLDS, ESPECTALLY IN THE MENISCUS REGION
The invention concerns a mold for the continuous casting of
molten metals, especially steel, with cooling channels, such as
cooling grooves, cooling slits, or cooling bores, in the ;side of
the mold that faces away from the melt contact surface.
.A continuous casting mold, especially a CSP (compact strip
production) mold of conventional design in the form of a ?late
mold, for the continuous casting of steel blooms or slabs is
usually constructed with sidewalls, each of which consists of a
support wall arid an inner plate that is mounted on the support
wall and comes into contact with the molten metal. Cool~.nt
channels that are parallel to one another are preferably
provided on the side of the inner plate that faces the s~.pport
wall and can be formed as slots that are open towards the'
support wall.
In CSP molds of current design, the heat transfer
conditions along the height of the mold can be varied wi~:hin
lim~rs, especially in a region above and below the level of the
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molten metal. For example, the wall temperature of the mc:l.d can
hP reduced above the level of the molten metal. When the heat
transfer is reduced in the area of the molten metal level and/or
above the molten metal level, however, the temperature of the
mold increases. This has the following advantages:
-- casting flux melts faster due to the hotter mold ~n the
region of the molten metal level;
- the faster melting of the casting flux improves tie
lubricating effect between the strand and the mold, which has
the effect of improving the surface of the strand;
-w better lubrication leads to a lower mold surface below
the molten metal level, which results in reduced thermal
stresses and reduced cracking tendency and thus a higher service
life of the mold; and
-- hotter regions of the mold above the molten metal level
reduce the compressive stresses in the regions below the molten
metal level; this also reduces cracking and prolongs the service
life of the mold.
It is known from measurements on continuous casting molds
that the distribution of the heat flux densities has a m;.~ximum
20-80 mm below the molten metal level and then decreases in the
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manner of a bell curve both in the casting direction and in the
opposite direction. The region of increased heat flux dei~siry
is about 120 mm long.
An associated graph of the temperature distribution c.af the
melt in the mold corresponds to the curvature of a horizoaztal
parabola with tm"~ in the region of the increased heat flu3:
density.
The document DE 38 40 448 C2 describes a continuous ~astirzg
mold, especially a plate mold, each of whose sidewalls is formed
by a support wall and an inner plate, which is mounted on the
support wall and is in cml.act with the molten metal, and
wherein parallel coolant channels are provided on the side of
the inner plane facing the support wall and are formed as slits
that are opcn towards the support wx7.l. The width of these
slits is smaller and their depth greater than the width c.f the
ribs between the slits.
RP 4 551 311 B1 describes a liquid-cooled, adjustab'.e-width
plate mold for the continuous casting of steel strands im slab
format, especially in a thickness of less than 100 mm. °.n this
mold, the transverse dimensions of the broad-side plates and
narrow-side plates are designed for the purpose of incre.:xsing
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Che cross section of the strand, the narrow-side plates bEing
arranged essentially parallel to each other over the heigr.t of
the mold, whereas the broad-side plates are concave at lc<<at in
the region where the slab is at minimal width, in such a gray
that, in cross section, the height of the highest point o;. the
curved mold wall reaches a maximum of 12 mm per 1,000 mrn ~of slab
width relative to an inscribed rectangle on the inlet side of
the mold, and where the shape of the broad-side plates at the
strand exit end of the mold corresponds to the shape of tae
strand to be produced. The broad-side plates are flat in the
area where the narrow-side plates can be adjusted, and slit-like
channels are arranged in the side that faces away from tr.e
shaping side.
EP 0 968 779 Al pertains to the formation of the bread side
of a slab mold with a casting plate with an inner surface.. and an
outer surface opposite the inner surface, such that the k>road
Eide has an upper and a lower region, and such that at le:~ast the
upper region has a middle region and two lateral regions
arranged on either side of it. The cited document propo;:~es that
the inner surface of the ca8ting plate be provided with ~.indercut
grooves to form r_ooling channels, and that the grooves b.
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covered in a positive-lacking way by fillers, which are ii.serted
into the undercuts.
US Patent 5,207,266 pertains to a water-cooled coppe:° mold,
which comprises a copper plate with a back frame fastened to it
to form cooling channels, where the widths of the main ch..innels
are greater in the bolt-fastening areas than they are in .he
other areas. The design of the mold calls for larger cha:inels
between the channels on the right and between those on th~~ left
of the mounting bolts, excluding the screwed joints. Hra.zch
channels between the main channels and the enlarged channels are
provided, where at least the branch channels and the branch
areas of the main channels have larger water surface areas than
the main channels arid the enlarged channels do.
Intensive cooling, i.e., heat dissipation, from the region
between the meniscus and the outlet of the mold is essential for
the rapid and reliable formation and especially for the t.niform
formation of a crack-free strand shell. The following
possibilities exist for this in previously known molds:
-- sparing the cooling water flow rate to a relativc.~ly high
value;
-- reducing the temperature of the cooling water; a:zd
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-- enlarging the heat-exchange surfaces in the coolix,g
channels by the use of cooling fins.
The aforesaid variants are already being used frequently in
practice in the design of molds for continuous casting plants.
The contact plate of the mold, which generally consi:~ts of
a copper alloy, is in " direct contact~~ Wlth the molten ~.nd
solidified metal. The contact plate, which is also refereed to
as the copper plate, is a part that is subject to wear an3 is
mounted on a support, which is usually made of steel. This
reusable support is called a water box.
The mold itself acts as a crystallizes; i.e., so much
energy is removed from the introduced molten steel that a load-
bearing strand shell forms, which can then be continuously
withdrawn from the mold. Under these conditions, a first strand
shell forms at the height of the filling level in the mold,
i.e., at the so-called meniscus. The term "meniscus~~ is
applied to the region where the strand shell first forms, this
is the region where the contact surface of the mold, the solid
and molten casting aids, the molten steel, and the stran~~ shell
meet. Casting fluxes and oils are used as casting aids. They
separate the metal and the copper from each other by lub:ication
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and control the local heat transfer (Figure 8).
The first volume element of the strand shell formed mt the
meniscus migrates through the mold dL the take-off apccd. On
the basis of the given temperature gradient between the molten
steel and the coolanC, a local energy flux develops in th~:
direction of the Cooling channels. Its energy content is
removed through the cooling channels, through which the c;~olant,
usually water, flows. The thickness of the strand shell
increases accordingly.
The cooling channels formed in the mold structure can be
either completely within the copper plate or completely within
the water box component. Combination designs are also )cr.own.
In addition, there are standard variants in which filler pieces
are installed between the water box and the copper plate to form
suitable cooling channels.
For reasons related to manufacturing technology, codling
channels with rectangular or circular cross sections are very
common. Corner regions can be rounded. However, U-shapca, L-
shapes, and T-shapes of any desired orientation relative to the
contact surface Can also be produced by suitable filler ~~ieces.
The cooling channels, arranged either individually or in groups,
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typically proceed in the casting direction, i.e., from toFj to
bottom, and they are usually equidistant from the contact
,urface. The goal of these efforts is to achieve a aoolimg
effect through the contact surface of the mold that is as
homogeneous as possible, which is often successful to onl~r a
limited extent. in the area of the fastening points. Cooling
channels with different cross sections and/or geometric shapes
are often placed side by side to optimize further the unirormity
of the cooling effect over the casting width (Figure 10).
AlI of these designs have in common the property that the
geometry of an individual cooling slit remains the same with
respect t.o shape and cross-sectional area along its entire
length. As a result of this design, the cooling channel surface
area that can be utilized fox cooling remains unchanged Tong
the length of the cooling channel. It can also be deduced from
the quantitative balance along an imagined flow path that the
flow rate remains constant along the entire length of ths:
cooling channel.
There is only one special design for cooling channe:_ bores
which attempts to deal with this problem. Tn this desig.-i,
central displacement pins can be inserted from above or '.allow.
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--..
Since the length of the displacement pin is usually shortE:r than
the bore itself, a cross-sectional constriction occurs in the
cooling channel, which leads to an acceleration of the cor,lant
in this transition zone. Tn the narrowed cross-sectional
region, the coolant then flows faster, which intensifies :he
cooling effect correspondingly. However, the effective c.~oling
surface for the cooling channel remains unaffected by this
measure.
The cooling channel designs that have been customary until
now are aimed at a cooling effect that it, as homogeneous as
possible, and in these designs, no consideration is giver. to the
inhomogeneous thermal load distribution actually present on the
mold plate. On the basis of the analysis, which is nece.~sarily
multidimensional, two in homogeneities in the thermal lo~.d
distribution are to be distinguished:
-- the inhomogeneity parallel to the casting direct_.on; and
-- the inhomogeneity perpendicular to the casting
direction,
In the casting direction, the heat transfer from the: molten
steel to the coolant in the cooling channel can be analy.~ed in a
simplified way as one-dimensional heat conduction throug'..i
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i-.~ r.
several layers. The following terms must be considered ii.. the
energy balance equation:
1. heat transfer from the molten steel to Lhe strand shell
that has formed,
2, heat conduction by the strand shell,
3. heat conduction by the lubricant layer,
4. heat conduction by the copper plate, and
5. heat transfer to the coolant.
Expansion terms do not have to be considered in the ateady-
state case.
One cause of the nonuniform thermal load distribut:icn over
the length of the mold is contained in the term of heat
conduction by the strand shell, since a strand shell foz-ct.s first
of all in the meniscus and continues to grow in the casting
direction. The heat transfer thus decreases with increa~:ing
thickness of the strand shell, Therefore, if all the otf:er
parameters remain constant, it is to be expected that the' heat
flux has its highest value at the meniscus and then decreases
continuously in the cac~ting direction. A mean heat flux can be
derived by integration wer the entire length of the coo:~ing
channel. Due to the multidimensionality of the heat conduction,
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i.e., no heat input occurs above the meniscus, the theoretically
sharp curve of the heat flux density will be smoothed, anc.l the
position of the maximum will shift iir l.he casting directi«n
(Figure 9).
Operating measurements of local heat flux densities
establish that the local values in the meniscus region ca::n be
1.5 to 3 times higher than the mean heat flux, while the ~.ralues
at the mold base can be 0.3 to 0.6 times lower. The maxi~num is
located 20-70 mm below the actual position of the meniscus,
depending on the plant and the process parameters. The ai~solute
values of the mean heat flux densities depend on the casting
flux and especially on the casting rate. For example, mean heat
flux densities that have been published are around 1.0 MH/m2 at a
casting speed of 0.9 m/min, 2_0 MW/m= at 3.0 m/min, and 3.0 MW/ma
at 5.5 m/min. The local heat flux densities tv be expected can
at least be estimated by using these factors.
The nonuniform distribution of the heat flux denait~~ in the
casting direction causes the primary thermal wear of the mold
plate to occur almost exclusively in the meniscus region. This
manifests itself in scoring, cracking, deformation, and ~:ven
flaking of layers that may have been applied earlier.
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The load ~n the mold plate is also highly variable ir. the
lateral direction. Inhomogeneities usually result from tl~e
molten steel flow field Chat develops in the mold. The
processes are closely linked to the geometric design of the
steel submerged nozzle which introduces the steel, to the
contact surface geometry, and to other process variables.
Steady-state and non-steady-state processes at the menisc~.~s
cause a usually plant-specific inhomogeneous development of the
meniscus. The inhomogeneous development of the meniscus is also
associated with an inhomogeneous heat distribution, so that the
primary damage does not develop uniformly over the width of the
mold but rather becomes concentrated in certain places.
Proceeding from the prior art specified above, the
objective of the invention is to adjust the heat transfex, which
is the determining factor for the cooling effect of the cooling
channels, to the local heat flux density of the contact f:urface
of the mold that is in contact with the molten metal by r.reans of
a special geometric design of the heat-transfer surface a-.reas of
a cooling channel or a group of cooling channels.
This objective is achieved by the invention in acco::dance
with the features of Claim 1.
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Further means of influencing the heat transfer in
accordance with the invention are specified in the dependE~nt
claims. In this regard, it is possible, for example, to
influence the local cooling effect of a channel by local
variation of the channel's shape, cross-sectional area,
circumference, boundary surface properties, orientation, ind
arrangement relative to the contact surface.
In addition, it is possible, fc~r example, to increase or
decrease the effective heat-exchange surfaces at the base of the
channel or on the sidewalls.
For example, the surface area of the base or lateral
surfaces of the cooling channels is significantly increased,
nearly doubled in fact, by the formation of scores in the base
or lateral surfaces, which results in a higher heat flux density
with a considerably more intensive cooling effect at the same
flow rate of the coolant. This has the important advantage that
the temperatures of the mold are considerably reduced, s« that
it is possible to reduce not only the stress on the mold
material but also the water pressures for the cooling wai:er.
For example, comparative temperature calculations y.;.elded
the following values:
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-- smooth surface of the heat-exchange surface at thEa base
of cooling grooves (°G1:
507° temperature raciy 173° temperature facing the.a
the strand ; water;
-- increased surface in accordance with the invention:
462° temperature facing 131° temperature facing th.3
the strand ; water;
-45° difference -42° difference.
Thp numbers clearly demonstrate the positive effect :~f the
measure of the invention. Artificial enlargement of the vooling
channel surfaces can also be realized in drilled CSP molds,
preferably in the meniscus region, with the use of a broaching
tool.
Other refinements of the invention are specified in other
dependent claims. zn this regard, the artificial enlargE.ment of
the r_ooling channel surface is not undertaken above the 7eve1 of
thp molten metal, because in this region of the mold, it would
be preferable to reduce the heat transfer to promote the melting
o.f the casting flux.
A reduction of the heat transfer above the molten m~:etal
level is achieved by:
-- insertion of sleeves in cooling bores above the molten
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metal level;
-- coating the bores above the molten metal level; a»d
-- introducing inserts made ct a material of lower thermal
conductivity above the molten metal level.
At the same time, a hotter region of the mold above ':he
molten metal level reduces stresses in the mold and thus -educes
cracking of the strand and simultaneously increases the
availability of the mold.
zn this regard, it was found to be especially advant:~geous
to use the measure of adapting the amount of heat being
dissipated via the heat-transfer surface areas of the cocling
channels to the heat flux density distribution in the channels
in a manner which varies along the height of the mold.
This evens out the temperature gradients in the mole; along
the height of the mold even more, avoids relatively largE
material stresses in the strand shell that is starting tc~ form,
and prevents the formation of cracks in the strand shell
The invention is explained in greater detail below raith
reference to specific embodiments:
-- Figure 1 shows an enlarged cross section of part of a
mold wall perpendicular to its main dimension;
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-- Figtu~e 2 shows another cross section of part of tr:e mold
wall according to Figure l;
-- Figure 3 shows Cooling c:hdilnel boles with acorcc can
their inner surfaces;
-- Figures ~ and 5 show corresponding parts of heat-
exchange surfaces without and with an enlarged base surfa~.e;
-- Figure 6 shows the behavior of the heat flux density q
as a funr..tion of the height H of the mold below the molte.z metal
level;
-- Figure 7 shows a graph of the depth of the furrows R as
a function of the height of the mold with the associated
behavior of a temperature curve T, likewise below the molten
metal level with T",a,% above and below the meniscus region;
-- Figure 8 shows a cross section of part of a mold wall
with roofing channels and the associated heat flux;
-- Figure 9 shows two graphs side by side for compax.~ison
with the mean or overall heat flux density and temperatu~~e;
-- b'igure 10 shown parts of coolant channels with tl~e
formation of comparable heat-exchanger bases;
-- Figure 11 shows additional designs of heat-exchanger
bases; and
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-- Figure 12 shows a distribution, adapted along the height
of the mold, of the heat flux density distribution With qrt~
below the molten metal level.
Figure 1 shows an enlarged view of a part 10 of a side 2 of
a mold wall that faces away from the melt with a slit-like:
cooling groove 1 formed in it. The cooling groove has a ~.~idth B
and a depth T. In accordance with the invention, the bas:
region of the cooling groove 1 is formed with a profile t.aat has
scoring 3, which approximately doubles its surface compared to a
planar design, e.g., as shown in Figure 4.
The heat dissipation of the heat-transfer surface arias of
the cooling grooves, slits, or bares can be adapted to the heat
flux density distribution of the mold in a manner which varies
over the height of the mold, as is shown, for example, ir. Figure
6.
Far this purpose, it is provided that the scoring 3 has a
variable depth 4, of, for example, s-4 mm and a dihedral angle
of 30-60° between adjacent scores fox the purpose of var3-ing the
intensity of the heat transfer, as shown purely by way of
example in bigure 7. The scores 3 can be formed with a c.~ihedral
angle of up to about 60°, with a height of up to about 4 mm, and
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with a spacing "A «, thus resembling the profile of a scraw
thread. Naturally, scoring with other shapes can be prov:.ded to
enlarge the cooling surface, e.g., wave-shaped, trapc.:oida:~l,
dentiform, or the like.
Figure 2 shows a part 10 of a mold wall, which compr:~ses a
section of a support wall 5 and a section of an. inner pla:e 6,
which are tightly joined together, preferably screwed tog;ther.
The inner plate 6 is penetrated by cooling channels 7, which are
formed as slits that are open towards the support wall 5 end are
covered by the support wall 5. Tn accordance with the
invention, the bases of the slits are provided with heat-
exchange surfaces 3, in which scores are provided to produce an
artificially increased heat flux density.
Figure 3 shows an arbitrary section 10 of a mold wall with
cooling channel bores 8 arranged therein, which have innE.r walls
9 provided With grooves or scores 3.
On the basis of the schematically indicated parts oi.
coolant channels 7, 7' with heat-exchanger bases i1 and ::_2,
which are to be compared with each other, I~'igures 4 and !> show a
smooth configuration 11 and a configuration consisting o:.: scores
1.7 and the corresponding temperature values. The drawin~;~s show
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a clear reduction of the temperatures far the design with the
scored base 12, the conditions under which the process
parameters to be compared were determined being strictly
identical_
Figure ~ shows a heat flux density distribution adapted
along the height of the mold in accordance with the inven~::ion
with q""X f.nr a limited region below the molten metal leve:.
(bath). Correspondingly, the temperature curve T in Figure 7
shows a temperature maximum T",~ between points 14 and 15 ~nith R"~x
within the region Z3 to 17 of variable depth R of the hea~-
exchange grooves. The heat-exchange grooves 3 begin at 13 at
the height of the molten metal level. The maximum groove depth
4 is reached at point 14. This maximum groove depth continues
as far as point 15, and then the groove depth is reduced to the
original level as point 16 is approached.
Figure 8 shows a cross section of a broad-sidewall c.f a
mold, which comprises a support plate 20 with a contact elate 18
mounted on it, a layer of casting aid, and a schematical7.y
suggested coolant channel 7, a strand shell 19 developing in the
rasting direction, and the associated heat flux.
Supplementing Figures 6 and 7, Figure 9 shows graph; of the
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/.~ r1
behavior of the local heat flux deusity/temperature compa~~ed to
the heat-transfer cooling channel surface as a function oi~ the
position of the meniscus.
Figures z0 and 11 show different possible designs fo:- the
cooling slits, especially for their base region.
Figure 12 provides a tabular listing of:
-- the channel cross-sectional areas;
-- the effective cooling charuiel wall areas;
-- their distance from the contact surface; and
-- the resulting effective cooling effect
of the r_orresponding design modifications in Figures 10 and 11,
wherein all values are relative values and are to be considered
only examples.
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. ,
Figures 6 and 7:
KEY:
Bad = bath (molten metal level)
Figure 8:
KEY:
Kuhlmittel = coolant
Kokille = mold
Giei3hi7.fsmittel = casting aid
Meniskus = meniscus
Metal7.schmelze = molten metal
warmest..rnm = heat flux
Gief~rir..htung = casting direction
Figure 9:
KEY:
lokale Warmestromdichte/Temperatur - local heat flux
density/temperature
Meniskus = meniscus
hzw . Tmpx = Amax or T",ax
Giei~riehtung = casting direction
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mittlere bzw. globale Warmestromdichte bzw. Temperatur = r:ean or
overall heat flux density or temperature
lokale warmeubertragende Kuhlkanalflache = local heat-transfer
cooling channel surface
variabel fiber Anzahl, Form, Tiefe von Kiihlkanalnuten = variable
by the number, shape, arid depth of cooling channel grooves
Figure 12:
KEY:
Kokillenfui3 = mold base
Meniskusbereich = meniscus region
Kokillenkopf = mold head
Kontaktflache zum Stahl = contact surface with steel
Kanalquerschnittsflache = channel cross-sectional area
Wirks. Kuhlkanalwandflache = effective cooling channel we~.ll area
Abstand zur Kontaktflache = distance to the contact surf-ce
Kuhlwirkung = cooling effect
Werte Bind Relativewerte and nur exemplarisch = values awe
relative values and are given only as examples
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r. ~
List ~ Reference Numbers
1. cooling grooves
Z . side facing away
3. scores
4, depth
5. support wall
6. inner plate
7. coolant channel
8. coolant bore
9. wall part
10. section
11. beginning of the heat-exchange scores at the height of the
molten metal level
12. maximum groove depth
13. end of the maximum groove depth
14, end of the depth reduction of the grooves
15.-17. constant groove depth reached
18. contact plate, contact surface
1.9. strand shell
20. support plate
Z1
