Strip casting installation with rotary hearth furnace and method for producing hot-rolled steel strip therefrom

An invention is directed to a method and an installation for producing a hot-rolled steel strip from continuously cast primary material in consecutive work steps, wherein the primary material is brought to hot rolling temperature after solidification and is inserted into a rolling mill for rolling out into a finished strip, wherein the primary material is coiled after reaching the hot rolling temperature, is stored intermediately in a rotary hearth furnace, and is recalled, as needed, uncoiled and rolled out to form a finished strip, the intermediate storage being effected immediately prior to entering the finishing train, the coils being preferably stored intermediately in a horizontal position and with a horizontal coiling axis, a subsequent heating being effectable during the intermediate storage.

BACKGROUND OF THE INVENTION 
The invention relates to a method and an installation for producing a 
hot-rolled steel strip from continuously cast primary material in 
consecutive work steps, wherein the primary material is brought to hot 
roll temperature after solidification and is inserted into a rolling mill 
for rolling out into a finished strip. 
A method which is known from German Offenlegungsschrift Pat. No. 32 41 745 
provides that a strip-shaped cast strand be wound into a coil immediately 
after leaving the casting machine and passing the transverse cutting 
device and then, after heating to rolling temperature, uncoiled again and 
fed to a rolling mill for rolling out into final cross sections. A 
disadvantage of this known installation is that it is difficult to heat 
wound up coils and a very long time at a great cost in energy and 
operation costs is required for this. 
Known strip casting installations usually use tunnel furnaces in order to 
bring the primary material to the desired hot rolling temperature, 
wherein, however, the material strands are not coiled. A substantially 
lower expenditure of energy is required compared to strip coiling. In 
multiple-strand installations, holding furnaces must be used in any event 
because of the resulting long transportation distances. 
Bogie hearth continuous furnaces, hoisting hearth or car-bottom furnaces 
are primarily taken into consideration as holding furnaces; however, all 
of these have various disadvantages. With the use of a bogie hearth 
continuous furnace, it is not possible to charge and discharge coils at 
different times, since when discharging a bogie hearth without another 
bogie hearth moving up behind it, a gap occurs which is not protected from 
heat radiation. The charging and discharging must therefore always be 
effected simultaneously. In addition, the empty bogie hearths must be 
brought back from the discharge side to the charge side. A subsequent 
heating of the car, or, at any event, of the ceramic benches, is required 
in order to avoid black spots (so-called skid marks). During disturbances 
in the rolling mill train, the coils present at the coil winding station 
cannot be received by the furnace. There must be an additional storage 
furnace for storing the coils. Both doors must be opened simultaneously 
during the charging and discharging of a bogie hearth, which leads to 
higher heat losses. 
A hoisting hearth furnace requires a plurality of separately driven 
hoisting hearth beam systems (walking-beam type systems) in order to 
enable a separate charging and discharging of coils. In the event of a 
disturbance in the rolling mill, no coils can be received by the furnace 
for storage. A second furnace installation is also required in this case 
if coils are to be stored. 
A car-bottom furnace can receive one or more coils depending on the design. 
If the storage capacity must be the same as the ladle contents, a 
plurality of car-bottom furnaces are required. The disadvantage in a 
plurality of furnaces is that the traveling distances to the uncoilers 
vary and, because of this, there is no uniformity of temperature of the 
coils. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a method for 
producing hot-rolled steel strips and a continuous casting installation 
with hot-strip rolling mill arranged downstream, so that the 
aforementioned disadvantages can be avoided and the difficulties can be 
eliminated and so that work may be carried out while adapting to different 
production quantities of the individual strands in multiple-strand 
installations, as well as during operating disturbances, in an economical 
manner, i.e. with high work loads, and in which, in particular, relatively 
low investment costs are required. 
Pursuant to this object, and others which will become apparent hereafter, 
one aspect of the present invention resides in the primary material being 
wound into coils after reaching the hot rolling temperature, 
intermediately stored in a rotary hearth furnace and recalled, as needed, 
uncoiled and rolled out to form finished strips. 
In an advantageous construction of the invention, the intermediate storage 
is effected immediately prior to the entrance into the finishing train. 
The temperature distribution is accordingly made uniform even during only 
very short holding times, which has advantageous results for the rolling 
process. 
According to another embodiment of the invention, the coils are stored 
intermediately in a horizontal position or with a horizontal coiling axis, 
so that a particularly simple charging is provided, accompanied by the use 
of known and proven manipulators. On the other hand, it is also possible 
to store the coils intermediately with preferably vertical coiling axis or 
in any other coil position. 
It is advisable that heat be supplied during the intermediate storage. In 
this way, heat losses at the surface can be compensated for by means of 
subsequent heating and ideal rolling conditions are accordingly provided. 
The horizontal position of the coils is particularly effective in this 
instance. 
For the strip casting installation with continuous rolling mill arranged 
downstream, the object is met in that a coiler and a rotary hearth furnace 
as a second furnace, as well as an uncoiler, are arranged between the 
first furnace and the finishing train. In a particularly advantageous 
manner, a plurality of coils which are formed from parallel primary 
material strands can be stored intermediately in only one rotary hearth 
furnace. Accordingly, considerable investment costs can be saved. 
According to further embodiment of the invention, the rotary hearth furnace 
is integrated in the production line and comprises a charging and 
discharging location in each instance. Space is saved with this step and 
an emergency storage during operating disturbances is simultaneously 
provided. 
In addition, however, the rotary hearth furnace can be arranged adjacent to 
the production line and be provided with only one charging and discharging 
location, so that the heat losses are prevented to great advantage. The 
charging device is arranged, optionally, in the inner circle of the 
furnace with the advantage of an additionally reduced space requirement. 
In an additional embodiment, which is very suitable for many possibilities 
of use, the rotary hearth furnace has an outer diameter of approximately 
13.5 m and is designed for receiving 10 coils. Other design data are given 
as a function of the ladle contents of the strip casting installation. The 
strip casting installation is generally designed as a double-strand 
installation/two single-strand strip casting installations and is equipped 
with two coilers arranged downstream of the furnaces. 
In another embodiment of the invention, the first furnace is constructed as 
a shortened roller hearth furnace, without a soaking zone, comprising 
induction heatino arranged upstream. In this way, the entire overall 
length of the installation can be greatly reduced, since the roller hearth 
furnace only has a length of approximately 80 m with a heating zone and a 
short buffer zone. The induction heating requires little space and 
introduces the absent heat quantities into the strip within the briefest 
time in a very advantageous manner. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, a strip casting installation with two strands 1', 1" is 
designated by 1, a transverse cutting device 2, e.g. a flame-cutting 
machine or shears, being arranged downstream of the strip casting 
installation 1 for the purpose of dividing the cast strips 1', 1" leaving 
the casting installation 1 at a speed of approximately 6-7 m/min, at most 
8 m/min, into partial pieces of the same length of e.g. 65 m. The 
individual partial lengths of the strips are then stored intermediately 
one after the other in two roller hearth furnaces 6 of approximately 150 m 
length and brought to a homogenous hot rolling temperature of 
approximately 1050 to 1100 degrees Celsius. The roller hearth furnaces 6 
comprise, e.g. on the input side, a 3 m long unheated foldable hood 
adjoining a 70 m long heating zone, a 65 m long soaking zone, and a buffer 
zone of 12 m. The partial lengths leaving the furnaces 6 are wound up into 
coils 11 in a coiler or two coilers 10. The coiling speed is approximately 
2-3 m/s. The coils 11 have a coil weight of between 22 and 27 tons 
according to the strip width between 1100 and 1350 mm at a thickness of 40 
mm and a strip length of 65 m. The coils 11 are fed to a charging device 
18 from coil transferring stations 12 via a transporting system 14, which 
charging device 18 brings the coils 11 into a rotary hearth furnace 20, 
according to the invention. 
The rotary hearth furnace 20 is advantageously arranged in the line between 
a two-strand strip casting machine (or a plurality of single-strand strip 
casting installations) and the hot-rolled wide strip rolling mill 40. It 
is substantially designed as a storage furnace or soaking furnace and 
comprises e.g. approximately 10 to 25 annularly distributed storage places 
21. The annular furnace 20 comprises a fire-proof insulation on all sides, 
wherein closable charging openings 24, 25, in the present instance a 
charging 24 and discharging opening 25, are preferably provided in the 
outer and/or inner wall 22, 23. The storage places 21 within the furnace 
20 are movable on a circular path between the stationary outer and inner 
wall 22, 23 at desired speed and in a desired direction. A coil 11 is 
removed via discharging device 28 in each instance and fed to an uncoiler 
30 by means of a suitable coil transferring device 29. The transfer can be 
effected as desired, e.g. also by means of a discharging device 28 
constructed so as to be swivelable between the discharging position and 
the uncoiler 30. The uncoiled strip is then conveyed to the finishing 
rolling mill train 40 in a known manner via shears 32 and a scale-breaking 
installation 34, wherein the uncoiled strip is rolled out in the finishing 
rolling mill train 40 to the final rolled thickness to form the finished 
strip 41. After leaving the last rolling mill stand of the finishing train 
40 with a delivery temperature of approximately 860 degrees Celsius, the 
finished strip 41 passes through a cold rolling train in a known manner to 
be coiled up subsequently by an underground coiler (not shown) at a 
temperature of approximately 560 degrees Celsius. 
The charging and discharging of coils 11 can be effected independently from 
one another with respect to time. During trouble-free operation of the 
rolling installation 40, the coils 11 only pass through a partial area of 
the furnace 20, corresponding to the required holding time between the 
charging and discharging stations 18, 28. If disturbances occur in the 
rolling mill train 40, additional coils 11 can be stored intermediately. 
When putting the rolling mill train 40 back into operation again, the 
coils 11 can be removed from the furnace 20 in desired sequence. The 
transporting of the coils 11 through the furnace 20 is effected on a 
hearth which is prepared for this purpose. This hearth has a furnace room 
temperature and prevents black spots, i.e. so-called skid marks. During 
the holding time, a temperature compensation takes place in the coil. The 
furnace 20 can be designed in an advantageous manner for a scale-resistant 
heating. 
The furnace combination according to FIG. 2 has the advantage that the 
total length of the furnaces 6, 20 in the production line is shorter than 
a roller hearth furnace 6 according to FIG. 1, since the soaking zone can 
be dispensed with. The length of the roller hearth furnaces 6 in this 
instance is approximately 80 m with a strip length of 65 m, consisting of 
a heating zone of 68 m and a buffer zone of 12 m, which is required for 
the subsequent method steps. The strips 1', 1" run through the furnaces 6 
at casting speed during the subsequent heating. When the strip end reaches 
the roller hearth furnace 6, it is conveyed out of the respective furnace 
6 at coiling speed, i.e. at approximately 2-3 m/s, and wound into coils 11 
in the coilers 10. The holding time of a strip in the roller hearth 
furnace 6 decreases continuously from the tip of the strip to the end of 
the strip. The front portion of the strip remains in the furnace 6 during 
the entire casting time and therefore has a higher temperature. As the 
holding time decreases, a partial length of the strip may possibly not be 
brought to the temperature level required for the subsequent rolling 
process. Foldable hoods 3 are provided for removing residual strip pieces 
and so as to reduce the heat losses. 
An induction heater 4 is provided to introduce the absent heat quantities 
into the strip. The heating power of the induction heater 4 is raised 
corresponding to the absent amount of heat, wherein the switch-on time is 
calculated. The data for this are the measured strip temperature, the 
casting speed and the furnace room temperature. The individual inductors 4 
are switched off when the end of the strip has passed them. A temperature 
compensation in the coil 11 to the required temperature profile can be 
effected in the rotary hearth furnace 20 arranged downstream. The 
arrangement and manner of operation of the rotary hearth furnace 20 and of 
the units arranged prior to and subsequent to the latter correspond to the 
description of FIG. 1. However, as an alternative for a sufficient 
temperature compensation, the roller hearth furnaces 6 can also be 
lengthened, wherein the individual controlled zones move alternately to 
heating and soaking temperature, respectively. 
The rotary hearth furnace 20 can also be arranged adjacent to the 
production line of the strip casting installation according to FIG. 3 and, 
in so doing, serves as a pure coil storage furnace. The charging or 
discharging of the coils 11 is effected at the same location via the 
charging opening 24. The possibility of keeping the distances of the 
transporting system 14 short is provided by means of this. As can also be 
seen particularly from FIG. 3a, closable openings having a door 24, 26 in 
each instance are provided for the charging device 18 in the outer and 
inner wall 22, 23 of the rotary hearth furnace 20, the charging device 18 
being arranged in the inner circle of the furnace. In addition, two slides 
27, which close the rest of the furnace space when charging, are arranged 
in the furnace 20. This measure keeps heat losses and atmospheric exchange 
low. The transporting of the coils 11 to and from the rotary hearth 
furnace 20 is effected by means of a coil transport system 14. The coils 
11 can be recalled from the furnace 20 in desired sequence. Alternatively, 
the rotary hearth furnace 20 can be designed for an externally arranged 
charging device 18. 
The transporting system 14 according to FIG. 4 is particularly advisable 
for installations in which a greater distance must be bridged between the 
strip casting machines 1 and the rolling mill 40. The main area of use is 
therefore the conversion of conventional rolling mills into strip casting 
installations with an adjoining continuous rolling mill train. Hoisting 
stations 13 for lifting the coils 11 to the level of the transporting 
system 14, i.e. to the height of transporting cars 15, are provided behind 
the coilers 10 and the coil transfer devices 12. The cars 15 are equipped 
with removable heat insulating hoods which prevent high heat losses during 
the traveling of the coils 11 to the rotary hearth furnace 20. Each car 15 
has its own drive and can be moved independently. The rail guidance 16 is 
dependent on the local peculiarities and project tasks. The heat 
insulating hoods of the cars 15 are equipped with burners, pilot burners, 
etc. and a temperature measurement, and can be heated or re-heated at 
provided places. The rotary hearth furnace 20 is arranged so as to adjoin 
the rail line 16 in the transporting direction, its charging openings 24, 
25 lying opposite one another. The manner of operation of the device and 
the successive units substantially corresponds to the description relating 
to FIG. 1. 
The rotary hearth furnace 20 can also be arranged adjacent to the 
production line of the strip casting installation 1 according to FIG. 5 
similar to the installation shown in FIG. 3 and in this case again serves 
as a pure coil storage furnace. The charging or discharging of the coils 
11 is effected at the same location via the charging opening 24. 
Accordingly, the possibility is again provided of keeping the distances of 
the transporting system 14 short. The coils 11 can be recalled from the 
furnace 20 in desired sequence. 
The steps, according to the invention, are not restricted to the embodiment 
examples shown in the drawings. Thus, for example, one or more rotary 
hearth furnaces can be provided in desired arrangement, e.g. can also be 
used in multiple-strand installations, without departing from the 
framework of the invention. Moreover, it can also be advisable to arrange 
the rotary hearth furnace directly behind the casting machine or the 
shears and to replace the previously used tunnel furnaces entirely. The 
respective constructional design for adapting to the specific utilization 
of the installation is left to the discretion of the person skilled in the 
art. 
While the invention has been illustrated and described as embodied in a 
strip casting installation with a rotary hearth furnace and a method for 
producing hot-rolled steel strip therefrom, it is not intended to be 
limited to the details shown, since various modifications and structural 
changes may be made without departing in any way from the spirit of the 
present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fairly constitute essential 
characteristics of the generic or specific aspects of this invention.