Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to a cooled continuous casting mold for casting metal,
in particular steel, in slab format and having, in particular, a thickness
between 40
and 400mm and a width from 200mm to 3,S00mm, with mold walls formed of
plates and with cooling medium channels for cooling with the cross-section of
the
cooling medium channel changing along the mold height.
An abstract of JP-A-59 133 940 discloses a continuous casting mold in
which the width of the cooling channels increases from the mold inlet to the
mold
outlet.
With the help of Fig. I, the known interrelationships during continuous
casting of metal will be described. The continuous casting of metal, in
particular
steel, with oscillating molds l, but also with displacement or travelling
molds
formed, e.g., as twin-roller with a stationary roller core and a
circumferential mold
tubular shell, leads to a thermal flow J(2) along a potential drop U(3) from
the
mold or strand center 4 through the formed strand shell S, a usually available
slag
film 6 on the mold plate 7.I with a predetermined copper plate thickness 8 and
up
to the mold cooling water 9. Here, the numeral 8 designates the thickness of
the
copper plate between the slag and the mold coating water course or between hot
and cold faces. The mold cooling water 9 flows, with a controlled velocity
(10),
measured; e. g., in m/s, a predetermined pressure ( 11 ) which is measured in
bar at
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the mold cooling water inlet, and a controlled mold cooling water inlet
temperature, T=O(12) which is measured at the mold cooling water inlet,
parallel
to the mold height 13 in or against a direction l4 of the continuous casting
which is
measured in m/min, in order to absorb and remove the existing thermal flow J
(2).
The total thermal flow J (2), which is removed by the mold cooling water 9,
is determined by a total resistance R-total (15) which is determined by
separate
media 16 with their discrete resistances Ri (17) between the strand middle 4
and
the mold cooling water 9. The separate or discrete resistances 17 are
determined
by their length I (18), their specific thermal conductivity ~. (19), and their
conducting cross-section F (20), and determine, together with the potential
drop U
(3) and the thermal flow J (2), the mass flow equation (20.1). In this
equation, the
discrete resistances of separate media enter, between the mold center or
middle 4
and the course of the mold cooling water, as the resistance of the liquid
steel, the
strand shell, the refractory lining, and the mold plate which is formed, in
particular, of copper.
The incoming thermal flow at the face boundary 21 between the copper
plate 7 and the course of the cooling water 9 (called "cold face") must
overcome the interface resistance 22 between the copper of the mold plate and
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the cooling water, with the copper plate 7 defining a hot face temperature or
a
temperature gradient 25 between the face boundaries 21 and 21.1 which
designates a face boundary between the copper plate 7 and the slag film 6 or
the
strand shell 5. The temperature gradient depends on the strength of the
thermal
flow over the mold height (13) and on the interface resistance at the boundary
face copper/water (21 ). It is also known that the thermal flow diminishes
from
the liquid metal level 30 to the mold outlet 13.2 in accordance with a profile
2.1
known as a "thermal beam:"
The interface resistance 22 is determined by the size of the cooling
channels 26 extending parallel to each other over the mold height 13, here in
form of cooling slots, and having a width (2b. l ), depth (26.2), which define
a
flow cross-section Q (26.3), and a length (26.4) corresponding somewhat to the
.
mold height (13) minus the boundary layer (Nernst layer) of the cooling water,
which represents a function of the flow velocity (Fig. 3e). The resistance 17
is
determined from the percentage water overlay (27.2) over the mold width
determined as a difference between a maximal cooled mold width and non-
directly cooled mold width divided by the cooled mold width, or also in a
first
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approximation, defined as a distance cooling channel/cooling channel 27 minus
the web width 2.1 divided by the distance cooling channel/cooling channel (see
Fig. 3e). This relative water overlay corresponds to the conducting cross-
section F(20) expressed through a mass flow equation U=E Ri " J. Further, the
resistance 17 depends on the thickness (8) of the copper plate and on the
specific thermal conductivity ~. (19) and further on the water velocity (10)
which is a function of the water pressure (26.6) at the water inlet and the
flow
resistance (26.5} or of the pressure loss in the mold. The relative water
overlay
(27.2) can also be looked at as the conducting cross-section F (20) in the
sense
of the mass flow equation U=E Ri " J which, in the known molds, is constant
over the mold height (13), i.e., with cooling channels extending parallel to
each
other.
In the known molds, the interface resistance 22 is constant over the mold
height 13. The cooling channel can be formed either as cooling bores 28 (not
shown) with a constant diameter and with or without displacement bars 28.1 or
as cooling slots 26 with water guiding plates 26.7 (Figs. 3d and 3e) and with
a
constant cross-section (26.3}.
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In summary, it can be said that according to the state of the art, in any
mold format (in slab, bloom, billet, profile and strip plants, etc.), the
percentage
water overlay (27.2) over the mold width, whether the cooling bore 28 or the
cooling slots 26 are used, and over the mold height 13 geometrically and,
therefore, in its operational cooling action remains uniform.
These ISO-design or uniform design of the mold cooling over the mold
height leads, as a result of a strand shell being located closely adjacent to
the
s
wall immediately beneath the liquid metal level 30 and a following shrinkage
of
the strand shell 5 over the mold height 13, to an increased thermal flow and
thereby simultaneously to a high hot face temperature of the copper plate 23 .
This high temperature of the copper plate 23 leads to a danger of the
recrystallization temperature T-Cu-Re (31 ) of the rolled copper (see Fig. 3
c)
being exceeded.
This danger of exceeding the recrystallization temperature (T-Cu-Re) of
the mold plate increases with increased casting velocities. Fig. 2 shows an
overview of constructional and operational characteristics of thin slab and
standard slab molds, in form of a table.
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This tabular representation of the characteristic mold data shows that an
increased thermal loading of the mold represented by a load of 2.2/3.2 MVJ/m2,
which characterizes the thermal flow (2) or the thermal loading of the mold,
in
case of a thin slab (32) in comparison with a standard slab (33), leads to a
larger
percentage water overlay (27.2) of 60-40%, an increased water velocity (10) of
12-8 m/s, a smaller copper plate thickness (18.1) of 25-l5mm, and a higher
mold cooling water pressure (26.6) of 12-8 bar. This increased thermal loading
or the increased thermal flow in the mold leads, in case of a thin slab (32),
to a ~,
reduced slag film thickness (18.2) of .4-.2mm, an increased casting velocity
{14) of the thin slab (32), and a smaller slab thickness (34/32) or (34.1).
Simultaneously, one can see that the mold hot face temperature at the,
adjacent
to the steel, side (23), dependent on the casting velocity, lies between
300°C
and 400° and closer to the recrystallization temperature (31 ) of the
cold rolled
copper than for the standard slab. The recrystallization temperature of the
cold
rolled copper lies, dependent on the copper quality, between 350°C (Cu-
Ag)
and 700°C (Cu-CuZr) or 500°C (softening temperature).
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A further reduction of the Cu-plate thickness ( I 8.1 ) because of high
water pressure (at the mold cooling water inlet (26.6)) in the bores (28) or
cooling slots (26) and a resulting possible mechanical bulging of the adjacent
to
the steel, copper plate surface, hot face, would be di~cult to achieve.
Fig. 3 shows a known arrangement of water cooling for slab or thin slab
molds with cooling channels 26 and water guiding plates 26.7. Fig. 3a shows a
half of a broad side 7 of a slab mold with a narrow side 7.1, an immersion
inlet
35, steel flow 36, and a strand 37, with a strand shell 5 at the mold outlet.
This
figure also shows cooling channels 26 extending parallel to each other over
the
mold height 13, and the position of the liquid metal level 30.
Fig. 3b shows a cross-sectional view through the mold broad side 7 with
a water box 38 with a water inlet 38.1 and a water return or the water box
inlet
38.2. 38.1.1 or 38.2.1 designate transitions of the mold cooling water from
the
water box (38.1) into the cooling channels (26) or into the cooling bores (28
not
shown).
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Fig. 3b further shows a multi-part mold with locking bolts 39 which
connect the copper plate 40 with cooling slots with the water box 38 or the
copper plate 40.1 without the cooling slots with the water box 38 (see also
Fig.
3d). In case the copper plate 40.1 without cooling slots is used, an
intermediate
plate 41 with cooling slots 26.3 is provided. Alternatively, the intermediate
plate 41 can form directly the wall of the water box 41.1 (Fig. 4).
Fig. 3c shows the profiles of the mold hot face temperature 23, the
thermal flow J (2), and the recrystallization temperature T-Cu-Re (31 ), over
the -
mold height 13 according to the state of the art.
Fig. 3c shows that both profiles (23:1) (hot face temperature profile) and
(2.1) (thermal flow profile) are functionally similar, and that the thermal
loading (23) is close to the recrystallization temperature 31 of copper, in
particular, at high casting speeds which results in a short service life of
the
copper plate in the region of the liquid metal level 30.
Fig. 3d shows a horizontal cross-section through the mold and shows the
arrangement of the parallel cooling slots 26 with water guiding plate 27 and
the
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transitions (38.1.1/38.2.1) of the cooling water 9 from the water box outlet
38.1
into the cooling channels and from the cooling channels through the mold water
transition 38.2.1 to the water return 38.2.
Fig. 3e shows a horizontal cross-section of parallel cooling slots 26. The
figure shows the slot width 26.1, the percentage water overlay 27.2 which is
determined by the ratio of the cooling channel width to the distance cooling
channel/cooling channel 27, the cooling channel cross-section 26.3, the water
guiding plates 26.7, the distance cooling channel/cooling channel 27, and the
copper plate thickness 8. The constructive features over the old height are
shown in cross-sectional views A-A'-A~l and B-B'-B11, with a constant
conducting cross-section F (20) and a constant interface resistance (22) over
the
mold height being set based on the uniform thermal flow of the mold cooling
water 9 with a constant Nernst phase region (flow velocity=0) which is smaller
at an increase flow velocity (I0).
Fig. 4 shows possible know designs of a mold broad side 7 formed of a
copperplate and a water box 38. The mold can be formed of a copper plate 40
with cooling slots and a water box 38 {Fig. 4a), or of a copper plate 40.1
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without cooling slots 40.1 and with an intermediate plate 41 with cooling
slots
(sandwich) and the water box (Fig. 4b), or of a copper plate 40 without
cooling
slots 40.1, which are provided in an intermediate plate 41.1 that forms a wall
of
the water box (Fig. 4c). Fig 4d shows the profiles of the thermal flow J
(2.1),
the thermal loading over the mold height, and the recrystallization
temperature
(31 ) of the cold rolled copper plate.
The object of the invention is to provide a continuous casting mold in
which the thermal loading over the mold height, i.e., the thermal profile over
the mold height is uniform, whereby the mold hot face temperature at the
liquid
metal level can be reduced.
This object is achieved with a continuous casting mold having the
features of claim 1. Advantageous embodiments are set forth in the subclaims.
It is proposed, according to the invention, to improve the continuous
casting mold by reducing the width of the cooling medium channel in the
casting direction from the mold inlet to the mold outlet in accordance with
the
thermal flow profile over the mold height.
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The width designates the length of extension of the channel wall which
(substantially) extends along the hot plate inner wall. Here, the cooling
channels have preferably a rectangular cross-section. However, use of
elliptical
shapes can also be contemplated.
According to the invention, the face boundary surface between the mold
plate wall and the cooling water diminishes from the mold inlet to the mold
outlet.
According to a first embodiment, the width of the cooling medium
channels diminish approximately functionally to the thermal flow profile over
the mold height between the mold inlet and the mold outlet in the casting
direction, whereby the boundary lines or surfaces of a cooling medium channel
or adjacent cooling medium channels do not extend parallel to each other.
According to a second embodiment, the width of the cooling medium
channels diminishes approximately linear in the casting direction, whereby the
boundary lines or surfaces of a cooling medium channel or adjacent cooling
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medium channels extend not parallel to each other, but at an. acute angle to
each
other.
This means that the width of a cooling channel diminishes linearly over
the mold height, whereby the boundary surfaces of adjacent channels, which
have a rectangular cross-section, extend to each other at a defined angle, or
the
lines of adjacent channels, which have an elliptical cross-section, viewed in
a
plane extending through common center points of the channels and parallel to
the cooled plate surface, form with each other a defined angle.
According to a particular advantageous embodiment, the cooling
channels are so formed that the depth of the channels over the mold height
increases from the mold inlet to the mold outlet in the casting direction.
The depth is a measurement of the cooling channels which, together with
the width, is taken into account when calculating the surface area.
According to a particular advantageous embodiment, dependent on a
reduction of the width, an increase of the depth over the mold height so
changes
that a size of a respective cross-sectional surface of a cooling channel
remains
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constant from the mold inlet to the mold outlet, so that flow velocity of a
cooling medium, in the cooling medium channels, remains constant from the
mold inlet to the mold outlet. Because of a constant resistance of a cooling
channel between the mold cooling water inlet and the mold cooling water
outlet, the flow velocity of the cooling water does not change.
Preferably, the water boxes are used for supplying the cooling channels
formed in the mold wall plates. Here, the water box outlet is provided at the
,.
height of the mold inlet, and water box inlet is provided at the height of the
mold outlet. Advantageously, the water delivery takes place above the liquid
metal level at the mold inlet, and the water return takes place at the mold
outlet,
thereby in the region of the liquid metal level beneath which largest thermal
loading occurs, a cold, thermally non-loaded water with the greatest cooling
capacity or most remote from the vaporization point of water at a pressure
between 1 and 25 bar acts.
Other advantageous features are disclosed in claims 7 through 12.
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As cooling channels, cooling slots or cooling boxes can be used. The
cooling channels are provided in a side of the plate remote from the mold side
or in a separate intermediate plate. To obtain desired cross-sectional
surfaces,
the cooling slots are closed, over the mold height, with correspondingly
formed
water guiding plates the width of which is adapted, over the mold height from
the mold inlet to the mold outlet, to the changes of the width of the cooling
channel course, i.e., is reduced, and their thickness advantageously
correspondingly diminishes over the mold height from the mold inlet to the
mold outlet, so that they flash adjoin the remote side of the plate.
Figs. I through 4 show the prior act, and Figs. 5 and 6 show the
invention. The prior act has akeady been described in detail. The invention
will only be described by way of example in comparison with the prior act
based on Figs. 5 and 6. The elements the same as in Figs. 1 - 4 are designated
with the same reference numerals.
Fig. 5a show an invention in which adjacent cooling channels 29 or their
border lines do not extend parallel but rather have their width reduced from
the
mold inlet 13.1 or the liquid metal level 30 and to the mold outlet 13.2, so
that
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the channel cross-section or the conducting surface F(20) functionally
corresponds to the thermal flow density or to the thermal flow profile 2.1.
simultaneously, by a corresponding increase of the cooling channel depth 26.2
(fig. 5b), the flow cross-section Q(26.3) for the cooling water and thereby
the
flow velocity 26.5 of the water in the mold 1 is retained substantially
constant.
The border surfaces of the cooling channels in form of slots 29 do not extend
any more parallel to each other but form an acute angle 29.2 with each other.
The percentage water overlay 27.2 or the conducting cross-section 20 is, e.g.,
at
,.
the liquid metal Level 30, maximum 100% in case of casting a thin slab, and at
the mold outlet minimum 30%.
Fig.Sc shows a comparative thermal loading 23.3 of the mold plate over
the mold height in comparison with the thermal flow profile 2.1 and the
recrystallization temperature 31. The drawing figure shows that the hot fall
temperatures 23.2 of the cooper plate 7 is smaller, more uniformly
distributed,
and simultaneously, increases the service life of the copper plate.
Figs. 5d shows cross-sections A-A'-A" and B-B'-B" through the broad
sides 7 at the mold inlet 13.1 and the mold outlet 13.2 for both mold plate
(40)
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with non-parallel cooling slots and the sandwich solution, i.e., a mold plate
with
an intermediate plate 41 in which the non-parallel cooling channel 29
according
to the present invention are formed.
This drawing figure also makes clear, e.g. that the flow velocity, despite a
large water overlay in the region of the liquid metal level 30, remains
constant
because the flow cross-section Q (26.3) remains constant as a result of the
corresponding increase of the cooling channel depth 26.2 over the mold height
from the mold inlet to the mold outlet.
Fig. Se shows the cooling channels 29 at the mold inlet 13.1 and the mold
outlet 13.2 with their guiding plates 29.1 the width and thickness of which
changes.
Fig. 6 shows a comparison of the solution according to the invention
(Fig. 6b) with the prior act (Fig. 6a). Basically, the proposed solution with
regard to the cooling channels 29 with guiding plates 29.1 is transferable to
molds with cooling bores (not shown), with the bore cross-sections being
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changes, along the mold length, by using conical displacement bars (not
shown).
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List of Reference Numerals
1. Oscillating mold
2. Thermal flow
2.1 Profile of the thermal flow over the mold height, ("thermal
beam")
3. Drop of Potential, U
4. Mold or strand center
5. Strand shell
6. Slag film
7. Mold plate-broad side
7.1. Mold plate-marrow side
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8. Cooper plat thickness between slag and water or between hot and
cold face
9. Mold cooling water
10. Velocity of the mold cooling water, in m/s
11. Pressure_of the mold cooling water at the mold cooling water
inlet in bas:
12. Temperature of the mold cooling water at the cooling water inlet,
T-O in °C
13. Mold height parallel to the casting velocity in a withdrawal
direction or along the mold length.
13.1 Mold inlet
13.2 Mold outlet
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14. Strand casting direction with the casting velocity in m/min (max.
15 m/min)
15. Total resistance, R-total.
16. Individual media between the mold center (4) and mold water (9)
such as e.g., liquid steel, refractory material, strand shell, slag,
mold plate, e.g., of copper
17. Discrete resistance, RI
18. Resistance length R in m
18.1 Thickness of the copper plate 1-Cu, hotlcold face, in mm
19. Specific thermal conductivity, ~., in W/K x m
20. Conducting cross=section, F.
20.1 Mass flow equation U=ERi x J ; ERi=( 1 /7.. x F)~
21. Face boundary copper plate (7)/mold cooling water (9}, cold face
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,. ~ .
21.I Face boundary copper plate (7), slag film (6) or strand shell (5),
hot face
22. Interface resistance copper/water, Nernst boundary layer
23. Hot face temperature copper/case shell (hot face) of parallel
channels (26)
23.1 Profile of hot face temperature over the mold height.
23.2 Hot face-temperature of non-parallel cooling channels.
23.2.1 Thermoprofile of non-parallel cooling channels (29).
24. High temperature copper/water (cold face)
25. Profile of the temperatures cooper/water (cold face)
26. Cooling channels formed as cooling slot extending parallel to
each other over the mold height.
26.1 Cooling channel width
22
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26.2 Cooling channel depth.
26.3 Cooling channel cross-section or flow cross-section, Q
26.4 Cooling channel length corresponding to the mold height (13)
26.5 Flow resistance
26.6 Water pressure at the mold inlet
26.7 Water guilding plates
27. Distance, cooling channel/cooling channel
27.1 Web Width
27.2 Percentage water overlay over the mold width defined as
difference between a maximum cooled mold width less non-
directly cooled width divided by a cooled mold width or also in 1
approximately as a distance cooling channel/cooling channel less
the web width divided by the distance cooling channel/cooling
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channel, corresponding to conduct cross-section, F(20) in a sense
of mass flow equation (20)
28. Cooling bores
28.1 Displacement phase, displacement body
29. Cooling channels; displacement bars, extending non-parallel over
the mold height (3)
30. Region of the liquid metal level, liquid metal level
31. Recrystallization temperature of a cold rolled mold copper plat T-
Cu-Re.
32. Thin slabs, thin slab thickness 40-150mm
33. Standard slabs, slab thickness 400-150mm.
34. Slab thickness, strand thickness
34.1 Thin slabs from 150 to 40mm
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34.2 Standard slabs from 400 to 150mm
35. Immersion inlet, SEN
35.1 Cast compound
3 5 .2 Cast slag
36. Steel flow
a
37. Strand
38. Water box
38.1 Water inlet, water outlet
38.1.1. Transition for the mold cooling water from the water box (38.1)
in the cooling channels (26) or (29)
38.2. Water return flow, water box inlet
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38.2.1. Transition of the mold cooling water from the cooling channels
(26) or (29) in the water box (38.2)
39. Locking bolts water box%cooper plate
40. Copper plate with cooling channels
40.1. Copper plat without cooling channels and with an intermediate
plate (4 1)
41. Intermediate plate, with cooling channels (sandwich)
41.1. Intermediate plate (41 ) with cooling channels that forms directly
the water box wall.
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