Process for producing thin-hot rolled strip

The invention relates to a process for producing thin hot-rolled steel strip with a final thickness of <1 mm from strip-cast feedstock in sequential steps, including surface descaling a cast feedstock, rolling the feedstock in six passes in a tandem Steckel roll line, coiling and uncoiling the feedstock after its first and second passage through the tandem Steckel roll line in furnaces arranged on the entry and exit sides of the tandem Steckel roll line. After a third passage through the Steckel roll line, the feedstock is coiled up in an exit-side coiling device having two furnaces one atop the other, which serves as a storage device for the roughed strip. Simultaneously with the coiling of the first strip, a second strip is uncoiled from the storage device. The cropped front end of the second strip is welded to the cropped rear end of the already uncoiled roughed strip. After passing through a second roughed strip storage device, the roughed strip that has been welded into an endless strip is fed to the finishing train, where it is reduced to the desired finished strip thickness.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a process for producing thin hot-rolled steel 
strip with a final thickness of &lt;1 mm from strip-cast feedstock in 
sequential steps, wherein the solidified feedstock leaving the casting 
machine is divided into roughed strip lengths, heated in a straight-flow 
furnace to roll temperature, and then rolled in a reverse-operated tandem 
Steckel roll line and in an attached finishing train. 
2. Description of the Related Art 
In prior art processes, the rolling of ferritic and steel strips to 
thicknesses of approximately 0.7 mm is performed by a continuous operation 
using rolling machines with a large number of stands and a correspondingly 
high investment cost. In the prior art, welding machines used to attach 
the roughed strips to each other to produce an endless strip move with the 
roughed strip at the entry speed of the roughed strip into the rolling 
mill so that the end of one roughed strip is attached to the beginning of 
another roughed strip without stopping production. The use of 
synchronously movable welding machines also leads to high expenses. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a rolling mill for 
producing extremely thin finished strip, preferably having a thickness of 
approximately 0.5 mm, which achieves high capacity with few stands, 
thereby resulting in a low investment cost, and is used without capacity 
limitations for strips thicker than 1.2 mm. 
To attain this object, the present invention comprises the following 
process steps: 
a) after surface descaling of a strip-cast 90 mm thick feedstock, rolling 
the feedstock through six roll passes in a tandem Steckel roll line having 
two Steckel roll stands, wherein the feedstock completes two roll passes 
for each passage through the tandem Steckel roll line; 
b) coiling and uncoiling a first feedstock strip in coiling furnaces 
arranged on the entry and exit sides of the tandem Steckel roll line after 
each of a first and a second passage of the first feedstock strip through 
the tandem Steckel roll line; 
c) coiling the first feedstock strip after its third passage through the 
tandem Steckel roll line in one of two furnaces in an exit-side coiling 
device, wherein the two furnaces are arranged one atop the other and the 
exit-side coiling device is a storage device for the roughed strips; 
d) uncoiling a second feedstock strip from the other of the two furnaces in 
the exit side coiling device simultaneously with the coiling of the first 
feedstock strip in the exit-side coiling device; 
e) welding a cropped front end of the second feedstock strip to a cropped 
rear end of an already uncoiled roughed strip to create an endless strip; 
f) feeding the endless strip to a finishing train for reducing the endless 
strip to a desired finished strip thickness. 
Thus, according to the present invention, a slab of feedstock that is 
approximately 90 mm thick, or less, is produced from a casting machine and 
is divided at the exit of the casting machine by shears or other dividing 
devices into finite roughed strip lengths. The divided roughed strip 
lengths of the feedstock are evenly tempered by being passed through a 
furnace such, for example, as a rotary hearth furnace and are then fed, 
evenly tempered, to a roughing train. The roughing train includes a 
descaling device, a tandem Steckel roll line having two Steckel roll 
stands and with bilaterally arranged coiling furnaces or with a coiling 
device having two furnaces one atop the other, known as a Cremona box, 
attached directly to the exit-side of the tandem Steckel roll line. The 
Cremona box has upper and lower coiling devices, which may be alternately 
used. 
The roughed strip length that exits the rotary hearth furnace is first 
descaled, and then reduced in thickness through the two Steckel roll 
stands of the tandem Steckel roll line to such an extent that it can be 
coiled in coiling furnaces arranged on the exit side. The next pass is 
carried out on the tandem Steckel roll line in reversed operation, whereby 
the roughed strip is fed to an entry-side coiling furnace. Before the 
second entry of the roughed strip onto the roll line, the surface may 
again be descaled. To prevent scale from being rolled into the material, 
descaling may also be performed before the final (third) reversing pass. 
In the third reversing pass, the strip again passes through the two 
Steckel roll stands of the tandem Steckel roll line toward a finishing 
train. The roughed strip has thus undergone a total of six reducing 
passes. After this, the exiting roughed strip is coiled up in one of the 
furnaces of the aforementioned "Cremona box." 
It has proved advantageous to use one of the upper and lower coiling 
devices of the Cremona box as the exit-side coiling furnace after the 
first passage of the roughed strip through the tandem Steckel roll line 
instead of using a separate exit-side coiling furnace between the Steckel 
roll stands and the Cremona box. This allows the separate exit-side 
coiling furnace between the Steckel stands and the "Cremona box" to be 
dispensed with. 
After the roughed strip has been coiled in either the upper or lower 
coiling device of the Cremona box, the furnace body is swivelled, and the 
roughed strip end, which now becomes the roughed strip head, is fed to a 
shears to have the head scrap cut off. The roughed strip is then passed 
through a welding machine (which welds the roughed strip head to a 
previously uncoiled roughed strip. The roughed strip is then supplied to a 
roughed strip storage device, at whose end is located a further descaling 
device to remove scale that has formed in the meantime. 
Located directly behind the descaling device is a finishing mill train, 
which reduces the strip to a thickness that ensures the safe transport of 
the strip via the exit roller table to the coiler. In front of the coiler 
is a rapid cutting shears that, in the case of continuous rolling, later 
redivides the strips to attain the desired strip sizes and weights. 
According to the invention, welding of the cropped roughed strips occurs at 
a stationary location. For this purpose, after the roughed strip has been 
taken by the coiler, the length of roughed strip needed for the welding of 
the second roughed strip to the first roughed strip during standstill is 
stored in the roughed strip storage device, such for example, as a loop 
tower, in front of the finishing train. Thus, while the first strip is 
rolled in the finishing train, the second roughed strip is simultaneously 
rolled in the roughing train and coiled up in the other of the lower and 
upper furnace of the Cremona box. 
After the head scrap removal, the second roughed strip is welded by means 
of the welding machine to the end of the first roughed strip. During the 
welding procedure, the contents of the roughed strip storage device in 
front of the finishing train are fed to the finishing train, without the 
rolling process in the finishing train having to be interrupted. The upper 
furnace of the Cremona box, now empty, swivels back into the pick-up 
position to be ready to receive the third roughed strip, which has been 
produced simultaneously with the rolling process of the second strip into 
the finishing train. 
As soon as the finished strip is clamped between the last active finishing 
stand and the coiler, the finished strip thickness can be reduced by 
adjustments of the finishing train, and the roll speed can be 
correspondingly increased. In this way, the goal of rolling 0.5 mm-thick 
hot strip at an austenitic temperature level is achieved. 
The rapid cutting shears in front of the coiler enable the redivision of 
the endless strip to produce the desired coil size. During the passage of 
the second roughed strip through the finishing train, the storage device 
in front of the finishing train is again filled, to ensure the time needed 
to weld the next roughed strip to the roughed strip just rolled. This 
process occurs repeatedly with the same rhythm. 
Preferably, the roughed strip storage device arranged directly in front of 
the finishing train is heated and protected by a protective gas. The 
capacity of the roughed strip storage device set in accordance with the 
exit speed of the finished strip and the time needed to weld the roughed 
strip ends. 
If the process is terminated for any of various reasons, the finished strip 
thickness should again be brought to a size that permits easy tailing from 
the finishing train and secure transport via the exit roller table. 
The invention creates, for the first time, a machine for producing 
extremely thin finished strip that has a high capacity and uses a low 
number of stands, thereby incurring a low capital investment. Moreover, 
when the welding machine and the storage device in front of the finishing 
train are not used, thicker strip sizes (thicker than 1.2 mm) may be 
produced without the capacity limitations associated with the welding 
machine and storage device. Therefore, according to another feature of the 
invention, it is proposed that the roughed strip be rolled in the tandem 
Steckel roll line in six passes to a thickness of, for example, 3 mm. 
The casting machine capacity and the rolling capacities of the roughing and 
finishing trains are designed so that continuous operation is guaranteed 
and the desired thermal and geometric demands on the finished strip are 
met. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of the disclosure. For a better understanding of the invention, its 
operating advantages, and specific objects attained by its use, reference 
should be had to the drawing and descriptive matter in which there are 
illustrated and described preferred embodiments of the invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS 
The single FIG. 1 shows an entire roll train for rolling and finishing 
strip material comprising two ladle turning towers 1 and 1' arranged in 
front of two continuous casting machines 2 and 2' so that the ladle 
turning towers 1 and 1' respectively supply the continuous casting machine 
2 and 2' with cast melt. The continuous casting machines 2 and 2', which 
are bow-type continuous casting machines in the preferred embodiment with 
a horizontal run-out, have flame cutting machines or shears 3 and 3' at 
their exit-side ends. Although bow-type continuous casting machines 2 and 
2' are shown, other types of casting machines may also be used. The shears 
3 and 3' divide slabs of the cast material into roughed strips. These 
roughed strips are fed into a rotary hearth furnace 4 and 4', where they 
are brought to an even temperature, i.e. tempered, over their 
cross-section. The roughed strips are transported by a ferry 5 to the 
beginning of a rolling mill line. The two casting machines 2 and 2' are 
thus connected to the rolling mill in this fashion. After passing through 
the rotary hearth furnace 4 and 4', the roughed strip is descaled in a 
scale washer 6 and fed to a tandem Steckel roll line 7 having two Steckel 
roll stands. Since the tandem Steckel roll line 7 has two roll stands, the 
roughed strip undergoes two roll passes for each passage through the 
tandem Steckel roll line 7. After its first passage through the tandem 
Steckel roll line 7, the roughed strip is coiled up in an exit-side 
coiling furnace 8. Subsequently, the roughed strip is further reduced in 
by a second passage through the tandem Steckel roll line 7 back toward the 
entrance of the roll line. After the second passage, the material is 
coiled up in an entry-side coiling furnace 9. The third passage through 
the roll line 7 reduces the material to a desired roughed strip thickness. 
This roughed strip is received and stored in a coiling device 10, which is 
a Cremona furnace comprising two furnaces arranged one atop the other, 
each of which is equipped with a coiling mandrel to hold the roughed 
strip. The housings of these furnaces can be swivelled to bring the end of 
the roughed strip into the correct position for the subsequent work steps. 
The furnaces 8 and 9 serve as storage devices to compensate for short 
process irregularities in the following machine sections and are also 
used, as needed, as "Steckel furnaces" after the first and second 
passages. In an optional embodiment, the exit-side coiling furnace 8 is 
omitted and one of the two furnaces of the coil storage device 10 is used 
instead of the exit-side coiling device 8. This embodiment simplifies the 
roughed strip rolling train. 
The roughed strip is fed from alternating ones of the two furnaces of the 
coil storage device 10 to a shears 11, which crops the head of the roughed 
strip. The cropped roughed strip passes through a welding machine 12 where 
it is welded to the cropped end of a previously uncoiled roughed strip. 
The welded roughed strip is then fed through a looping tower 13 and a 
scale washer 14, and rolled in a multi-stand finishing train 15 into the 
desired finished strip. During the welding of the cropped rough strip, the 
looping tower 13 feeds stored roughed strip to the scale washer 14 and 
multistand finishing train 15 so that the finishing train is run without 
interruption. Once the weld is complete, the looping tower is re-filled 
for continuous feeding of the finishing train 15 during the next welding 
procedure. 
After passing through the finishing train 15, the roughed strip runs 
through a rapid cutting shears 18 via an exit roller table 16 with an 
integrated strip cooling device 17 to a coiling device 19 where the 
roughed strip is coiled up. Coil removal carts 20 and other handling 
devices, such, for example, as a strapping machine 21, scales 22 and 
marking devices 23, follow the coiling device 19. The finished coil is 
transported from these handling devices by a coil transport device 24 to a 
coil storage area 25 and stored. 
The invention is not limited by the embodiments described above which are 
presented as examples only but can be modified in various ways within the 
scope of protection defined by the appended patent claims.