Note: Descriptions are shown in the official language in which they were submitted.
CA 02653360 2008-11-25
Method and Device for Producing a Metal Strip by Continuous Casting
The invention relates to a method of producing a metal
strip using continuous casting, a strand, preferably a thin strand,
being initially cast in a casting machine and being diverted from
vertical downward travel to horizontal travel, and, in the travel
direction of the strand, the strand being subjected, downstream of
the casting machine, to a milling operation in a milling machine in
which at least one surface of the strand is milled off and
preferably two opposing surfaces are milled off.
In continuous casting of strands in a continuous-casting
system, surface flaws can occur, such as for instance oscillation
marks, casting powder errors, or surface cracks that run
longitudinally or transversely. These flaws can occur with
conventional and thin strand casting machines. The conventional
strands are sometimes descaled depending on the purpose of the
finished strip. Some strands are generally descaled at customer
request. The demands on surface quality for thin strand systems
are becoming increasingly stringent.
Descaling, grinding, or milling are options for surface
machining.
Descaling suffers from the disadvantage that the material
removed cannot be melted down again without further preparation due
to its high oxygen content. During grinding, metal splinters mix
with the grinding wheel dust so that the abraded material must be
disposed of. Both methods are difficult to adapt to the prevailing
transport speed.
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'Milling is therefore the primary type of surface
machining used. The hot milled cuttings are collected and can be
packetized and re-melted with no problem and without further
preparation, and can be re-introduced into the production process
in this manner. Moreover, it is easy to set the milling speed to
the transport speed (casting speed, finishing train speed,
advancing speed). The inventive method and the associated
apparatus therefore apply primarily to milling.
A method and a apparatus of the above-described type are
known that have a milling operation that takes place, or a milling
machine that is disposed, downstream of a continuous-casting
system. See CH 584 085 [US 4,047,468], DE 199 50 886, and also EP
0 053 274 and EP 0 881 017.
The publication of R. Borsi et al "Direct Thin Slab
Rolling at Algoma" in Iron and Steel Engineer, Association of Iron
and Steel Engineers, Pittsburgh, US, vol. 75, no. 5, May 1998,
pages 62 to 64, discloses continuous casting with his mass flow.
DE 71 11 221 discloses a similar solution. This document
depicts the machining of aluminum strips using the casting heat, in
which the machine is connected to the casting system.
In-line milling of the surface of a thin strand
(descaling, milling, etc.), on the upper and lower faces or even on
only one side, just upstream of a rolling mill has also already
been suggested; see EP 1 093 866.
DE 197 17 200 shows another embodiment of a surface
milling machine. It describes, inter alia, the variability of the
milling contour of the milling apparatus that is provided
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downstream of the continuous-casting system or upstream of a
rolling train.
EP 0 790 093 [US 4,436,9371, EP 1 213 076 [US 6,195,859],
and EP 1 213 077 (US 6,192,564] suggest another arrangement of an
in-line milling machine in a conventional hot-strip mill for
machining a rough strip and the embodiment of this arrangement.
In contrast, JP 1031 4908 describes descaling the
continuous cast strip downstream of the casting machine.
In DE 199 53 252 [US 6,436,205], the strand cast in a
casting machine is initially guided through a transverse separating
apparatus and then through various ovens before it is subjected to
a rolling operation.
During surface machining of the thin strands in a so-
called CSP system, approx. 0.1 B 2.5 mm are to be removed from the
hot strand surface, on one side or on both sides, in the machining
line ("in-line") as a function of detected surface flaws. A thin
strand that is as thick as possible is recommended (H = 60 B 120
mm) in order not to reduce output too much.
Surface machining and the apparatuses associated
therewith are not limited to thin strands, but rather can also be
used in-line downstream of a conventional thick strand casting
system and with strands that are cast with a thickness of more than
120 mm to up to 300 mm.
As a rule an in-line milling machine is not used for all
products in a rolling program, but rather only for those for which
stringent demands are made in terms of surface quality. This is
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advantageous for output reasons and reduces wear and tear on the
milling machine and therefore is reasonable.
There is a desire to employ the technology that is
already known even more efficiently and therefore with greater cost
efficiency. It should be possible to produce, although not
exclusively, high quality thin strands at high mass throughput.
The following should be noted regarding operational
parameters for a continuous-casting system:
The casting parameters for a few exemplary parameters
that can typically be attained for steels that are simple to cast
are shown in the following table:
Speed v Thickness d Speed x thickness v x d
[m/min] [nmm] [m/min x mml
7 50 350
6 65 390
3.7 100 370
1.7 210 357
These are speeds that as a rule are at the upper end of the
operational range. For high-strength materials where C > 0.3%,
silicon steel, and micro-alloyed steel, the speeds are typically
20% lower, i.e. 350 m/min x mm B 20% = 280 m/min x mm.
It has proven disadvantageous that strand surface quality
suffers at high mass flow or casting speed.
The underlying object of the invention is therefore to
improve a method and an apparatus of the above-described type such
that an improved production process or machining process can occur
with high efficiency. This should include in particular optimizing
with a focus on the required addition of heat into the casting
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strand and into the production process, and also and in particular
as concerns the rolling process that follows casting.
This object is attained by the invention using a method
characterized in that the strand is milled as a first mechanical
machining step after the strand has been diverted to horizontal
travel, the strand being cast with a thickness of at least 50 mm
and the strand being cast with a mass flow, as the product of
casting speed and strand thickness, of at least 350 m/min x mm, or
of at least 280 m/min x mm when casting high-temperature-resistant
materials with a carbon content of C > 0.,33, silicon steel, or
microalloyed steel when milling the strand immediately after
deflecting it to horizontal travel or after deflecting the strand
to horizontal travel and passing it through a thermal-treatment
unit and or an oven, the milling of the strip in the milling
machine entailing milling off the upper and lower strand faces at
one location along the travel direction, the relative amounts being
milled off the strand upper and lower faces being determined by
vertically setting drive rollers and/or guide plates upstream and
downstream of the milling cutter or the milling machine.
Upstream or downstream of the milling machine at least
one surface parameter of the strand can be measured and the
machining parameters during milling can be set as a function of the
one measured surface parameter. Milling depth preferably is
carried out as a function of the measured surface parameter.
Moreover, as a function of the measured surface parameter, at least
one milling cutter of the milling machine can be bent about a
horizontal axis that is perpendicular to its longitudinal axis.
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The strand can be cleaned prior to the measurement of the
surface parameter.
In accordance with one embodiment of the invention, the
strand is milled in the milling machine such that the strand upper
face and the strand lower face are milled off at the same location
in the travel direction. Alternatively, however, the strand is
milled in the milling machine such that the strand upper face and
the strand lower face are milled at two successive locations in the
travel direction.
The apparatus for producing a metal strip using
continuous casting, having a casting machine in which a strand,
preferably a thin strand, is cast, at least one milling machine
being provided downstream of the casting machine in the travel
direction of the strand, in which milling machine at least one
surface of the strand, preferably two opposing surfaces, can be
milled off, is inventively embodied such that in the travel
direction upstream and/or downstream of the milling machine means
are provided with which at least one surface parameter of the
strand can be measured, setting means being present with which at
least one milling cutter of the cutting machine can be displaced as
a function of the measured surface parameter.
These setting means can be embodied for adjusting the
milling depth of the milling cutter. It is also possible for the
setting means to be embodied for actuating the milling cutter.
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The means for measuring at least one surface parameter
can include a camera for determining the depth of cracks on the
strand surface. Furthermore, the means for measuring can permit
the geometric shape of the strand to be determined across its width
transverse to the travel direction.
The means for measuring at least one surface parameter
can be provided immediately downstream of the milling machine.
They can also be provided downstream of a finishing train that is
disposed downstream, in the travel direction, of the milling
machine. It has furthermore proven useful when the means for
measuring are provided downstream of a cooling section that is
disposed downstream, in the travel direction, of the milling
machine.
With the suggested solution it becomes possible to run at
a high casting speed and to operate the imanediately following
rolling process in an optimum manner. In particular acceptable
strip output temperatures out of the finishing train are attained
in this manner.
This leads to qualitatively improved production of
strands, in particular thin strands.
Specifically, by means of the invention it is possible to
increase the casting speed from the current level, at v x d > 350
m/min x mm, to approx. 480 B 650 m/min x mm, i.e. to increase it by
approx. 30% to 75%. Thus the following advantageously result:
the productivity of the system can be increased;
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sufficiently high production is made possible, even with
a continuous-casting system with low capital
expenditure costs; and,
particularly in continuous direct strand reduction, high
rolling temperatures are assured, especially when
surface milling, rather than descaling, takes place
before the rolling process.
Advantageously, high-quality strands result when the
milling machine, or where necessary, even a different surface
machining unit, is provided downstream of the casting system, in
that surface flaws are removed by milling.
Cooperation between a high-speed casting system and the
surface material removal, in particular milling, is critically
important for quality, especially the surface quality of the
product produced.
Illustrated embodiments of the invention are shown in the
drawings.
FIG. 1 is a schematic side view of an apparatus for
producing a metal strip using continuous casting in which a milling
machine, a roughing train, a heater, a finishing train, and a
cooling section are connected to a casting machine;
FIG. 2 shows an embodiment of the invention, alternative
to FIG. 1, in which the milling machine is provided downstream of
an oven and upstream of a finishing train and a cooling section;
FIG. 3 shows the upstream area of the apparatus in
accordance with FIGS. 1 and 2 in accordance with another
alternative embodiment of the invention;
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FIG. 4 shows a part of the apparatus in accordance with
FIGS. 1 and 2 in accordance with another alternative embodiment,
measuring means and setting means being provided that can be used
to influence the milling process;
FIG. 5 is a schematic view of the progression of the
casting errors over the casting speed;
FIG. 6 shows an example of the progression of the milling
depth during milling of the strand over strand length or over time;
and,
FIG. 7 is a front elevational view of a milling cutter
being subjected to a bending moment.
FIG. 1 shows an apparatus for producing a metal strip 1
using continuous casting. The corresponding strand 3 is
continuously cast in a casting machine 2 in a known manner. The
strand 3 is preferably a thin strand. in the strand segments 11,
the cast strand is diverted or bent in a known manner from its
vertical travel V to horizontal travel H. Immediately after
diversion to horizontal travel H, a profile measurement and surface
inspection can occur using means 8 for measuring. Thus the surface
quality of the strand and its geometric configuration can be
determined.
Connected to the means 8 in the travel direction F is a
milling machine 4 in which the strand 3 can be milled off on its
upper and lower faces.
It is essential that the strand 3 is milled as the first
mechanical machining step after the strand 3 is diverted to
horizontal travel H at high casting speed. It is specially
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provided here that the strand 3 is milled immediately after it is
diverted to horizontal travel H.
As will be seen, specifically adding the milling process
directly after casting results in technological advantages when
producing strands as high-speed thin strands. Specifically,
casting errors increase as casting speed increases such that
milling immediately after casting produces efficient preparation of
the strand for the subsequent process steps so that overall a very
economic process becomes possible.
Consequently it is desired that the strand 3 is cast with
a thickness of at least 50 mm. For mass flow (expressed as the
product of casting speed and strand thickness) a value of at least
350 m/min x mm has proven itself. The cooperation between these
process parameters and the milling of the strand that takes place
very far upstream results in great advantages in terms of
attainable strand quality and efficiency during finishing.
In the solution in accordance with FIG. 1, a roughing
train 12 is connected downstream of the milling machine 4. It is
followed by an oven 13, here an inductive heater. After a descaler
14, the strand travels into a finishing train 9. A cooling section
is provided downstream thereof in the travel direction F.
The system shown in FIG. 1 is particularly well suited
for continuous rolling of the strand 3. Integrating casting and
rolling results in a more economical process and more favorable
thermal efficiency in the system at high casting speed.
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The alternative system shown in FIG. 2 is constructed
similarly and is particularly well suited for combined continuous
or alternatively discontinuous rolling.
As in the solution of FIG. 1, after the cast strand has
been diverted to horizontal travel H, the unit 8 measures the
profile and inspects its surface. This is followed by a holding
oven or a closed roller conveyor unit 15. The oven 13, here an
inductive heater, is immediately downstream thereof.
Instead of the descaler 14 upstream of the finishing
train, a milling machine 4 is provided upstream of the finishing
train 9 for the purpose of optimizing temperature, it being
possible to provide inductive heaters 16 between the individual
roller units thereof. Finally, the cooling section 10 again
follows in the travel direction F.
The solution in accordance with FIG. 3 is distinguished
from those of FIGS. 1 and 2 in that the milling machine 4 is not
situated immediately after where the cast strand 3 has been bent
(apart from the measuring means 8, which are also provided in this
case), but rather in that the strand 3 is first guided through a
thermal equalization or temperature stabilizing unit 5 in the form
of a closed roller conveyor assembly. In this case, the two
milling cutters 6 of the milling machine 4 are provided one above
the other and machine the strand 3 on the upper and lower faces
simultaneously, driver rollers 21 and guide plates 22 upstream and
downstream of the milling cutters apportion the milling reduction
between the strand upper face and the strand lower face using
appropriate vertical adjustment of the two elements.
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The system shown in FIG. 3 is particularly suitable for
finishing thicker strands by means of high-speed casting, use for
thin strands by no means being precluded, however. The insulation
for the roller conveyor is provided as close as possible downstream
of the casting machine 2 and upstream of the milling machine 4.
As can be seen in FIG. 4, the milling process can occur
in the milling machine 4 in a closed control loop, depending on
milling parameters.
The strand 3 travels from an oven 13 into the milling
machine 4, the means 8 for profile measuring and/or surface
inspection being provided upstream of the milling machine.
In this case the strand 3 is again machined, i.e. milled,
on its upper and lower faces in the milling machine 4, machining
occurring however on the upper and on the lower faces at two
locations that are somewhat spaced from one another in the travel
direction F. The milling cutters 6 cooperate with support rollers
17. Measuring means 8 are again provided downstream of the milling
machine 4. After the surface machining, the high-temperature
strand 3 travels into a finishing train 9, measuring means 8 again
being provided downstream thereof.
The means 8 can have elements for optically determining
the strip shape (ski), which is indicated at reference 8= for the
means 8 farthest upstream in the travel direction. They can also
have strand profile and temperature measuring elements.
FIG. 4 further shows a controller 18 operating with or
without feedback and that receives the measurement values from the
measuring means 8 as input variables, in addition to the set points
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for the milling amounts for the upper face and strand lower face.
These means use stored algorithms to control the milling process
that is performed in the milling machine 4.
It is primarily the milling amount that is considered,
i.e. the depth of the roller-like milling cutters 6, that defines
the quantity of the material to be removed from the strand 3. This
can occur separately and differently for the upper face and the
bottom, as a function of the measured values.
The amount to be milled off derives from the surface
inspection of the strand, cracks and the geometric shape being
primary factors. A different reduction (depth) along the length of
the strand can result from this.
When the milling depth is being determined, the computed
milling wear is also taken into account in a cutting wear model
that determines the wear as a function of wear path, milling
volume, milling speed, material strength, etc.
A fixed milling amount can also be established using the
measured values.
Another option is to adapt the milling shape and bending
as a function of the measured profile (see also FIG. 7).
The surface result can be examined downstream of the
milling machine 4 and where necessary an adjustment can be made if
the measured values are not yet satisfactory.
FIG. 5 gives background information regarding the
suggested method. Here the progression of the casting errors E and
in particular their frequency is plotted against the casting speed
v. The casting speed range that extends to the broken line is the
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typical area for thin strands, the strand thickness being for
instance 60 mm. At the broken line, the product of casting speed
and casting thickness, also important, is v x d = 360 m/min x mm.
Casting errors increase sharply when the casting speed or
the product of casting thickness and speed increase further.
FIG. 6 schematically shows the milling reduction or
milling cutter depth s over time t or strand length. The solid
line is for the strand upper face, and the broken line is for the
strand lower face. The milling reduction, i.e. the depth s, is a
function of the detected errors. It can be seen that different
values can be given for the upper face and for the strand lower
face.
FIG. 7 illustrates how it is possible to influence the
milling result as a function of measured values in a particularly
advantageous manner in milling operations.
A roller-shaped milling cutter 6 is shown with
schematically indicated cutters 19. The milling contour, which is
created on the strand 3 using the milling process, can be
influenced in that a bending moment M is applied to the milling
cutter 6. The bending moment M is centered on a horizontal axis
that is perpendicular to a milling-cutter longitudinal axis 7.
The moment M can be produced by double forces FF that can
be applied to the shaft journals of the milling cutter 6. While
the line 7 marks the milling-cutter longitudinal axis when not
deformed, the bending curve 20 results when the forces FF are
applied. Then the milling cutter bends as shown. Since the
bending behavior of the milling cutter 6 as a function of the
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forces FF is known, it is possible to intentionally influence the
milling results if certain convexities are measured across the
strand width that can be intentionally influenced, i.e. eliminated,
by acting on the milling cutter 6 with the bending moment M.
Thus it is also possible to dynamically adapt the milling
process to the measured strand profile or to the measured strand
shape.
References 7 and 20 illustrate the neutral axes for the
milling cutter 6 for the two loads.
The milling reduction, i.e. the depth, can be adjusted
differently across the strand width or can be adapted to the
starting strand shape. The bending in the milling cutter can act
as the actuating element for the adjustment across this width.
This can be summarized as follows:
Since the output of a CSP system can be determined by the
casting machine, the invention suggests designing a casting machine
with a high casting speed. Given an extreme increase in casting
speed, instead of one CSP system with two strands with conventional
casting systems alternatively a one-strand CSP system with a high-
speed casting machine is preferred.
A high casting speed is also particularly necessary for
coupled casting and rolling (casting/rolling system) so that the
strip output temperature out of the finishing train is acceptable.
As casting speed increases, however, surface flaws (e.g.
scale, etc.) increase disproportionately (see FIG. 5). If a high
casting speed is selected, therefore, the increasingly poorer thin-
strand surface quality must be compensated for by a surface
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machining unit, to which end the invention provides the milling
process. That is, thin-strand high-speed casting becomes
reasonable when simultaneously using a thin-strand surface
machining unit so that high strip-surface quality or acceptable
strip-surface quality can be assured.
It is in particular suggested that thin strand surface
machining that is provided in the line downstream of the casting
system, within the oven, or upstream of the rolling mill be
performed for thin strands having a thickness greater than 50 mm
and/or having a mass flow (speed x thickness) greater than
350 m/min x mm. For example, the thin strand thickness to be
sought is approx. 60 B 110 mm at a casting speed of 6 B 9 m/min.
The typical mass flow is lower.
An increase in casting speed is reasonable not only for
thin strand systems. An advantageous application for thick strand
systems (H > 110 mm) is also conceivable. In this case, the
milling machine should be provided as close as possible downstream
of the continuous-casting system or the area between leaving the
casting system (last section roller) to the milling machine should
be closed by a roller conveyor housing so that the milling process
can occur at high casting speed at a high strand temperature to the
extent possible.
When needed, the milling process can be omitted at the
leading strand end and/or at the trailing strand end for the
purpose of protecting against milling damage. If a disadvantageous
surface shape (crossbow, ski, or other irregularity) is detected
optically, the milling amount, milling starting point, milling
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ending point, and milling profile setting are optionally made a
function thereof.
In order to minimize the milling material removal and to
adapt to the strand input profile, the milling cutter arrangement
forms a "milling crown" (analogous to the "roller crown") across
the width. The above-described milling roller journal bending in
accordance with FIG. 7 is provided for the purpose of dynamically
adapting to the strand shape.
During in-line milling of the surface, the strand speed
Vstrand is provided according to milling machine arrangement either
by the casting machine or the rolling mill. That is, travel speed
cannot be influenced by the milling machine. In order always to
set the optimum milling conditions, the milling cutter rotary speed
nmi11er is preferably adapted according to the formula
nmiller = K X Vatrand
where K is an empirically determined factor that depends on the
material.
The milling cutter rotary speed is controlled using the
milling model that is shown in FIG. 4 and that monitors the milling
result using the surface sensors.
The top and bottom of a milling roller can be seen in the
illustrated embodiments. At high required milling reductions per
side or given very hard materials it is conceivable to arrange two
milling cutter units, one after the other, on both the top and
bottom.
Instead of using roller milling cutters, it is also
possible to use other milling cutters, such as face cutters or even
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grinding tools or other surface removal tools (such as descaling
machines), at the provided locations.
The following in particular can be used as the cutting
material for the cutting plates of the milling cutters: HSS;
uncoated or preferably coated hard metals; ceramic; polycrystalline
cutting materials. As a rule conventional indexable inserts can be
used..
As explained, a surface inspection (camera, test for
cracks, roughness test) is recommended upstream and/or downstream
of the oven or upstream of the milling machine. The measured
signals are used for optimum employment of the milling. It is
possible to derive from them whether milling should be performed on
one or more sides or only in some longitudinal areas and what
extent of milling should be set. Preferably descaling or cleaning
of the strand is performed upstream of the inspection in order to
be able to do a precise and reliable surface analysis.
The usefulness of in-line strand inspection is also a
function of monitoring the effect of the casting system; monitoring
the effect of the electromagnetic brake; optimizing mold
oscillation curves; monitoring the surface at high speed; and
detecting cracks, casting powder errors, and other casting errors
in the early stages of the production process.
In addition, it is possible to examine the milling result
or the general surface condition by surface inspection immediately
downstream of the milling machine, downstream of the finishing
train, or downstream of the cooling section. The result is
monitored there and the amount milled off is optimized or minimized
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adaptively by means of a milling model (algorithm) and is thus
included in the overall system.
The milling cutter or the milling machine can be provided
at different locations. It can be downstream of the casting
system, within the oven, or upstream of the rolling mill.
Preferably it is used immediately upstream of the reshaping instead
of a descaler in order to maintain a high strip temperature in the
rolling mill, especially during continuous direct strand reduction,
which is particularly advantageous.
Preferably a milling model is used for controlling the
milling reduction, the beginning of milling, the end of milling,
and to adjust the milling cutter rotary speed. The milling model
takes the following into account when determining depth: set
points, actual values determined by the measuring means, computed
cutting wear, values found during previous milling (adaptation).
It is also possible to have an arrangement of a plurality
of milling cutters per side, one after the other, for greater
milling reduction.
Face cutters can also be used as an alternative to the
use of cylindrical cutters. However, basically other material-
removal methods can also be used, e.g. grinding tools or other
mechanical or melting material-removal tools (such as e.g.
descaling machines). Descaling is of particular interest for high-
speed continuous casting.
The first mechanical machining step addressed
inventively, which the milling is intended to represent, should be
understood such that in any case prior to the milling there is no
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mechanical machining that is typically used during continuous
casting. If for instance upstream of the milling there is minor
mechanical machining that is not typical for the method in terms of
its scale (e.g. minimal rolling with a reduction in thickness of a
few millimeters in a small frame or in a driver that is normally
present anyway), this shall not be construed as the first
mechanical machining in the sense of the invention.
List of reference figures: 8 Means for measurement
1 Metal strip 8' Means for measurement
2 Casting machine 9 Finishing train
3 Strand 10 Cooling section
4 Milling machine 11 Strand segments
Thermal equalization section 12 Roughing train
6 Milling cutter 13 Oven
7 Milling-cutter longitudinal 14 Descaling
axis
Holding oven/roller conveyor
housing
16 Inductive heater
17 Support roller
18 Controller
19 Cutter
Bending curve
21 Driver rollers
22 Guide plates
F Travel direction
V Vertical
H Horizontal
d Thickness of strand
v Casting speed
v x d Mass flow, expressed
as the product of speed and
thickness
M Bending moment
FF Force
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