Note: Descriptions are shown in the official language in which they were submitted.
CA 02686377 2009-12-04
METHOD FOR HOT ROLLING AND FOR HEAT TREATMENT OF A STRIP OF
STEEL
The invention relates to a method for hot rolling and for
heat treatment of a strip of steel.
The hardening and subsequent tempering of steel components
is common practice. This has the result that a desired
combination of strength and toughness of the material can
be specifically adjusted. This technology is also used in
principle in the production of high-strength steel sheet in
sheet plants. It is described in EP 1 764 423 Al. In this
case, after heating the slab and rolling down to the final
thickness on the heavy plate stand in a plurality of
reversing passes, the sheet is cooled to room temperature
at high speed, for example, i.e. the hardening process is
carried out. This is followed by the tempering process,
i.e. re-heating of the strip to, for example, 600 C
followed by renewed cooling. Thus, sheet having different
properties can be produced flexibly in small batch sizes in
a sheet stand.
Similar methods are known from JP 04 358022 A, from JP 04
358023 A and from JP 58 009919 A.
As in the sheet production sector, the demand for types of
steel having very high strength is also increasing
continuously in strip production, i.e. the demand for so-
called high-strength and ultrahigh strength steels. These
materials are used, inter alia, in motor vehicles, cranes,
containers and in pipes.
It is thus the object of the present invention to provide a
method whereby high-strength and ultrahigh-strength strip
having sufficient toughness can be produced more
economically in a strip plant. In particular, it should
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advantageously be possible to produce QT steels by this
method.
The solution of this object by the invention is
characterised in that the method comprises the steps:
a) heating the slab to be rolled;
b) rolling the slab to the desired strip thickness;
c) cooling the strip, wherein after cooling the strip has
a temperature above the ambient temperature;
d) coiling the strip into a coil;
e) uncoiling the strip from the coil;
f) heating the strip;
g) cooling the strip and
h) removing the strip,
wherein before heating according to step f), the strip has
a temperature above the ambient temperature and wherein
when carrying out step d) the coil is located at a coiling
station and when carrying out step e) the coil is located
at an uncoiling station spatially remote from the coiling
station, wherein the coil is transported from the coiling
station to the uncoiling station between steps d) and e).
Step e) can directly follow step d).
During cooling or after cooling according to step c) and/or
according to step g), the strip can be subjected to a
straightening process. It can also be subjected to a
straightening process between the uncoiling according to
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step e) and the heating according to step f) . It can also
be subjected to a straightening process between the heating
according to step f) and the removal according to step h).
Said straightening process can be effected by deflecting
the strip around base, deflecting, driving or other
rollers.
The straightening process is usually carried out by means
of a roller straightening machine or screwed-down strip
deflecting rollers or, according to a special embodiment of
the invention, on a so-called skin-pass frame.
The strip can also be subjected to a straightening process
during the heating according to above step f).
The cooling of the strip according to step c) can comprise
a laminar cooling and downstream intensive cooling. The
cooling of the strip according to step g) can also comprise
a laminar cooling or alternatively or additively air
cooling.
At least parts of the cooling device can be configured as
zone cooling which acts in zones over the width of the
strip.
The cooling of the strip can also be carried out by means
of a high-pressure bar so that cleaning and/or descaling of
the strip is possible at the same time.
The heating of the strip according to step f) can comprise
inductive heating. Alternatively, direct flame impingement
of the strip can be effected. In the latter case, it is
preferably provided that the direct flame impingement of
the strip is effected by a gas jet comprising at least 750
oxygen, preferably comprising almost pure oxygen, in which
a gaseous or liquid fuel is mixed.
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A further development provides that the inductive heating
of the strip takes place in inert gas (protective gas).
The removal of the strip according to step h) can comprise
coiling the strip. The removal of the strip according to
step h) according to claim 1 can also comprise pushing off
plate-like cut portions of the strip.
Before the cooling according to step c) the strip
preferably has a temperature of at least 750 C.
After the cooling according to step c) and before coiling
according to step d), the strip preferably has a
temperature of at least 25 C and at most 400 C, preferably
between 100 C and 300 C.
A further development furthermore provides that after the
heating according to step f), the strip preferably has a
temperature of at least 400 C, preferably between 400 C and
700 C. Meanwhile, after the cooling according to step g)
and before the removal according to step h), the strip can
preferably have a temperature of at most 200 C, preferably
between 2 5 C and 2000C.
The heating of the strip can take place at different
intensity over the strip width.
Finally, it can be provided that the steps e) to g) are
carried out in reversing mode for which a coiling station
located after the cooling according to step g) is used.
It can furthermore be provided that the flatness of the
strip and/or the temperature of the strip (the latter
preferably by means of a temperature scanner) is measured
at least at two locations in the strip treatment
installation for monitoring the quality of the strip.
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The running speed of the strip through the strip treatment
installation, the, in particular, zone-related strip
heating, the adjustment of the straightening rolls and/or
the in particular zone-related cooling can be controlled or
regulated by a process model.
During passage through the strip treatment installation,
the strip can be held under a defined strip tension at
least in sections by means of drivers. This applies
particularly in the area of the intensive cooling section.
In order to ensure that the strip runs centrally in the
driver, in the roller straightening unit or in the
intensive cooling section, a strip lateral guide is
preferably located in front thereof.
An alternative embodiment of the method for hot rolling and
for heat treatment of a strip of steel comprises the steps:
a) heating the slab to be rolled;
b) rolling the slab to the desired strip thickness;
c) cooling the strip, wherein after cooling the strip has a
temperature above the ambient temperature;
d) coiling the strip on a first coiler;
e) reversing the strip between the first coiler and a
second coiler wherein the strip is subject to heating
between the coilers,
wherein before the heating according to step e), the strip
has a temperature above the ambient temperature.
This method can also be combined with the aforesaid
embodiments.
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In the case of materials for which no tempering is
required, i.e. for which the strength and toughness
properties already meet the requirements after step d),
process steps a) to d) can be used by themselves alone.
The following further developments have proved successful:
A strip tension can be built up by means of drivers before
and after the cooling of the strip.
The strip can be guided transversely to its longitudinal
axis by means of a lateral guidance. The lateral guidance
can preferably take place in the area of the cooling of the
strip, in particular in the area of the laminar cooling of
the strip.
The lateral guidance of the strip can furthermore take
place before the driver and can open after passing the
strip head and close again at the strip end for the purpose
of guidance.
Measurement of the strip temperature can be made by means
of a low-temperature radiation pyrometer. The measurement
of the strip temperature can preferably be made before,
inside and/or after the cooling and/or heating devices.
The production spectrum of a hot wide strip mill differs
appreciably from that of a heavy plate mill. A plurality of
high-strength and ultrahigh strength types of steel newly
developed over the last few decades now exist, whose
properties can be adjusted by specific rolling and/or
cooling strategies. A suitable method for this is quenching
of the strip at a high cooling rate after rolling, followed
by re-heating to temperatures above the phase
transformation temperature.
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The classical QT steels (Q: quenched; T: tempered) which
can be produced in this way are already produced on heavy
plate stands. However, they can be produced substantially
more economically in hot wide strip mills.
Moreover, thinner, ultrahigh-strength strips having lower
temperature and thickness tolerance as well as strip
flatness can be produced more reliably on hot strip mills.
It is therefore appropriate and advantageous to shift parts
of production from heavy plate stands to strip mills.
In addition, there are many new types of steel which cannot
be produced on heavy plate stands. The method presented
here is particularly suitable for the group of multi-phase
steels. By means of a significantly enlarged spectrum of
temperature-time profiles and in particular, by means of
the possibility of interrupting the cooling and temporarily
increasing the temperature again, it is possible to produce
structures having almost any combinations of phase
constituents which cannot be envisaged at the present time.
In addition, it is possible to make precipitation processes
take place and thus specifically introduce second phases
which are a characteristic of modern types of steels.
In addition, properties required for the higher alloy
contents in conventional production can be adjusted by the
method presented.
Advantages of the separate arrangement of the rolling and
cooling process on the one hand and the tempering process
on the other hand are the flexibility of the method (no
mixed rolling is necessary), the flexible adjustment of the
temperature time profile of the strip and that one's own
coils or coils from other installations can be processed.
Coils or plates can also be cut depending on the intended
use of the strip or the coilability. The plates are
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preferably cut at higher temperature, i.e. at the tempering
temperature.
Advantages of the coupled arrangement of the rolling and
cooling process and the tempering process are the
particularly large energy saving and in the case of coils
which are difficult to coil and bind, the use of a special
coiler with direct transfer to avoid the so-called watch-
spring problem. Furthermore, rapid further processing or
delivery of the strip in the case of direct further
processing is achieved. Finally, mention should be made of
the greater possibility for influencing the microstructure
of the strip in the arrangements mentioned.
Exemplary embodiment of the invention are shown in the
drawings. In the figures:
Fig. 1 shows schematically a hot strip mill for producing a
steel strip according to a first embodiment of the
invention,
Fig. 2 shows an alternative embodiment of the hot strip
mill to Fig. 1,
Fig. 3 shows an exemplary temperature profile of the strip
over the conveying direction of the hot strip mill,
Fig. 4 shows the fundamental structure of a straightening
machine with integrated intensive cooling as a section from
the hot strip mill according to Figs. 1 or 2,
Fig. 5 shows the fundamental structure of a straightening
machine with integrated heating as a section from the hot
strip mill according to Figs. 1 or 2,
Fig. 6 shows schematically a hot strip mill with an
alternative embodiment of a first process step.
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Figure 1 shows a hot strip mill in which a strip 1 is
initially processed in a first process stage (given by I)
and then in a second process stage (given by II).
In the first process stage, i . e . in a rolling and cooling
process, a slab is first rolled in a multi-stand rolling
mill. Of the rolling mill, only the last three finishing
stands 7 are shown in which the strip 6 having an
intermediate thickness has been rolled. The temperature
distribution in the strip or the flatness can then be
measured. The strip 1 then passes in the conveying
direction F into a strip cooling system 8 which is divided
here into an intensive laminar cooling system 9 with so-
called edge masking and a laminar strip cooling system 10.
The conveying speed is, for example, 6 m/s. The cooled
strip 1 then enters into an intensive cooling system 11 in
which, according to a preferred embodiment of the
invention, a straightening machine and driver are
integrated (details in Fig. 4) . Drivers can be provided
before and after the intensive cooling system 11.
The intensive cooling system 11 can be followed by another
measurement of the temperature distribution and the
flatness of the strip. A low-temperature radiation
pyrometer is preferably used at these low temperatures. A
temperature measurement is also feasible inside the
intensive cooling system between two squeeze or driver
rolls for the purpose of temperature-coolant regulation.
The strip 1 is then coiled in a coiling station 3 by a
coiler 12 or 13.
The coil 2 then enters the second process stage, i.e. the
tempering process.
Here the coil 2 is initially uncoiled in an uncoiling
station 4 and then fed to a straightening machine 14 (this
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can be located before and/or after the following furnace)
After a temperature equalisation has taken place over the
length and width of the strip in a zone 15, the strip 1
enters into a furnace 16. It is possible and advantageous
to integrate a straightening machine in the furnace 16
similarly to the cooling (details in Fig. 5) . Here the
strip 1 can be heated in continuous or in reversing mode.
An oxyfuel furnace or an induction furnace are preferably
used, the heating time being between 10 and 600 seconds.
This is followed by trimming shears 17 or shears 18. The
strip 1 then enters into a laminar strip cooling system or
alternatively into an air cooling system 19. This can be
followed by a straightening machine 20. A plate pushing
unit 21 or a coiler 22 in a coiling station 5 are then
furthermore indicated in Fig. 1.
A skin-pass stand can also be arranged here instead of a
straightening machine 14 or 20.
Coils from other hot strip mills can also be introduced
instead of the uncoiling station 4.
In contrast, a direct connection of the two process stages
I and II can be seen in Fig. 2 (the installation is not
shown fully fitted). The last stands of a hot wide strip
mill (finishing mill 7), the strip cooling system 8, and
the coilers 12 and 13 of the first process stages are shown
similarly here. The last coiler 23 is provided for winding
the higher-strength strips. In this case, this can
advantageously comprise a special coiler for simple winding
of the high-strength steels. In this case, the coiler 23 is
a so-called transfer coiler. The coil does not need to be
bound there. Pivotable pinch rolls hold the strip under
tension during turning into the unwinding position. The
winding is therefore directly followed by further
processing in the tempering line (second process stage).
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The further transport takes place similarly to the solution
according to Fig. 1.
Particular advantages here are the energy saving for strips
of higher winding temperature and the rapid further
transport of the coils from the first to the second process
stage. It is thus provided that the strip 1 already has a
temperature above the ambient temperature To before the
heating in the furnace 16.
In addition, for special strip it is also possible to
provide reversing of the strip between the two coilers 23
and 22 in to be able to carry desired temperature profiles
or treatments of the strip.
Preferably in the case of shorter strip and/or sufficiently
dimensioned component spacings, direct further transport of
the strip 1 from the first process stage to the second
process stage is provided without intermediate coiling of
the strip 1 and/or subsequent reversing from coiler 22 to
coiler 23. In this case, therefore the coiler 23 is not
used but after the strip end runs out from the roll mill
the tempering process is carried out directly at low or
initially high and then lower speed.
Alternatively, this operating mode is applied to strip
independently of the thickness and the speed. Then the
coiler 23 is initially not used and the furnace is also not
operating. The strip is wound on coiler 22. The tempering
process is then carried out reversingly between coiler 22
and 23.
A preferred temperature profile for the strip 1 along the
strip mill is shown in Fig. 3 in correspondence with Fig.
2. The cooling at the end of the line is preferably water
or air cooling.
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However, cooling can also be effected by means of a high-
pressure beam. Cleaning or descaling of the strip surface
is thereby carried out at the same time.
The production quantity of the rolling plant is usually
higher than in the tempering process since the rolling
speed of the strip is greater than the tempering speed. A
so-called mixing rolling operation is therefore possible to
optimally utilise the rolling mill. This means that a
number of strips is wound on coilers 12 and 13 whilst the
further processing of the higher-strength strip takes place
in the tempering line.
The production of the strip is therefore divided according
to the invention substantially into two process stages
which will be specified subsequently as an example with
further optional steps.
First process stage:
- Heating of slabs (thick or thin slabs) and subsequent
rolling in a multi-stand hot wide strip mill;
- Intensive cooling of the strip on the delivery roller
table;
- Passing through a straightening machine;
- Winding the strip into a coil.
In order to improve the flatness of the high-strength
strip, strip edge heating before a conventional finishing
train, edge masking in the first cooling section units and
a straightening machine are advantageous.
At higher winding temperatures, fast coil transport to the
subsequent second process stage is advantageous to save
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heating energy during tempering. The coil can then be
transported under a heat insulating hood tc reduce the
temperature loss and ensure more uniform material
properties.
Second process stage:
- Unwinding the coil,
- Optionally straightening the strip in a straightening
machine if lack of flatness is present;
- Optionally equalising the strip temperature by zonal
cooling or heating before the actual tempering treatment
for making the strip temperature uniform over the strip
length and width;
- Tempering the strip, i.e. continuous re-heating by means
of induction heating or energetically advantageously in a
gas-heated continuous furnace (e.g. oxyfuel furnace using
so-called DFI method);
- Trimming the strip;
- Subsequent cooling of the strip;
- Renewed straightening of the strip;
- Renewed winding of the strip into a coil.
Alternatively, the strips can be cut into plates before the
furnace, after the furnace and/or directly before the plate
pushing unit. The cutting of plates is particularly
advantageous in the case of strip which is difficult to
wind. Cutting at the tempering temperature is advantageous
since the strip has a lower strength there.
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In the case of thicker strip and/or high-strength steels
which can no longer be cut, a flame cutting machine, a
laser cutting machine or a thermal cutting machine can be
provided for cutting.
The said oxyfuel furnace in which the so-called DFI oxyfuel
method (direct flame impingement) is carried out for
tempering comprises a special furnace in which (almost)
pure oxygen instead of air and gaseous or liquid fuel are
mixed and the resulting flame is directed directly onto the
strip. This not only optimises the combustion process but
also reduces the nitrogen oxide emissions. The scale
properties are also favourable or the scale growth is very
small (operated with air undershooting). The high flow rate
of the gases even has a cleaning effect on the strip
surface. This type of heating is particularly advantageous
with regard to strip surface quality. High heat densities
with as good efficiency as in inductive heating can be
achieved with this method.
Instead of a successively arranged cooling section and
inline straightening machine in the first or second process
stage, the straightening machine and the strip cooling can
also be accommodated combined in one unit. The
straightening rollers are then at the same time used as
water squeeze rollers and thus ensure a cooling effect
which is as uniform as possible over the width of the strip
since any strip transverse curvature and lack of flatness
is eliminated directly it forms. The straightening rollers
are adjusted individually depending on the strip
temperature and the material quality with the assistance of
a straightening machine model so that overstretching of the
strip surface is avoided. Drivers before and after the
cooling section unit ensure strip tension for as long as
possible even when the stand or the coiler tension is not
built up. Part of the strip cooling can be carried out in
the form of strip zone cooling in order to be able to
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actively influence the temperature distribution. The
cooling-straightening unit is indicated in Figures 1 and 2.
Details on this are deduced from Fig. 4. Possible arbitrary
combinations for straightening, cooling and squeezing can
be seen in this figure. For the secure threading-in process
of the strip head particularly in the case of thinner
strip, the cooling-straightening unit is executed as
raisable and pivotable as indicated in Fig. 4 (see double
arrow). The straightening rolls are individually
adjustable.
A temperature scanner for the strip can be provided before
and/or after the joint arrangement of straightening machine
and cooling which can be seen in Fig. 4. A strip head form
detector (for detecting a ski or waves) can be positioned
in front of the installation shown.
Drivers 24, a pure cooler unit 25, straightening rolls 26
and combined squeeze rolls/drivers 27 can be identified in
detail in Fig. 4. Furthermore, nozzles of intensive cooling
system can be seen.
In this case, an alternating arrangement of cooling,
straightening and drive roller units is possible. The
amount of straightening is adjusted individually depending
on the material of the strip and the temperature. The
straightening-cooling unit is raisable and pivotable.
As can be seen in Fig. 5, the straightening and heating
process 14, 16 of the second process stage can also be
combined with the installation shown. Similarly, the amount
of straightening can be adapted to the present strip
temperature and the strip material. In this case, the skin
effect (higher surface temperature) of the induction
heating (or a direct flame impingement in the DFI oxyfuel
method) has a positive effect. At the same time, the
straightening rolls hold the strip in position and avoid
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lack of flatness so that (inductive) heating which is as
efficient as possible is possible in the long fillet
portion of the strip. Drivers 29 before and after the
heating- straightening unit hold the strip under tensile
stress 30. For secure threading in of the strip head, the
induction coils 32 as well as the straightening and
transfer rollers 31 are designed as vertically adjustable.
The use of the cooling- straightening unit (Fig. 4) or the
heating- straightening unit (Fig. 5) is not restricted to a
strip installation but can also be provided in a heavy
plate installation.
A temperature scanner for the strip can be provided before
and/or after the joint arrangement of straightening machine
and heating which can be seen in Fig. 5.
In order to be able to influence the temperature
distribution over the strip width in the second process
stage during the inductive heating, transverse field
inductors are used, inter alia, which can be displaced
transversely to the strip running direction or conveying
direction F. By this means, if necessary, the strip edges
for example can be heated more strongly or heated less
intensively.
Equalisation of the strip temperature over the length and
the width of the strip by specific cooling (zone cooling)
or heating at warm or cold strip sections can optionally
take place before heating the strip to the tempering
temperature. This should be provided in particular when
coils not completely cooled to ambient temperature are to
be handled. By this means the passage of the coil through
the coil store can be shortened. A coil tracking system
(model) as well as the measured temperature distributions
during unwinding of the coil are used for optimum control
of the heating or cooling systems.
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Welded-to-order, high wear-resistant roller materials are
used for the straightening rolls in order to ensure a long
life and good strip quality.
Temperature scanners and flatness meters within the line
indirectly monitor the quality of the strip and serve as a
signal for adjusting and regulating members such as, for
example, for the throughput speed, the heating power, the
adjustment of the straightening rolls and the cooling which
are controlled by a process model.
Figure 6 shows the first process stage in a somewhat
modified embodiment. By analogy with Fig. 1, Fig. 6 shows
the rear part of the finishing train 7, laminar strip
cooling units 9, 10 as well as an intensive cooling system
11 and the coiling stations 3. In this embodiment the
intensive cooling system 11 and a strip straightening unit
36.1, 36.2 are located at various positions. Drivers 34 and
35 are located before and after the intensive cooling
system 11. A strip tension can hereby be maintained within
the intensive cooling system 11 for almost the entire strip
length without the strip being clamped in the stand or
coiler system. Thus, any strip waves which may occur are
pulled out and a cooling effect which is as uniform as
possible is achieved.
In order to ensure that the strip runs centrally in the
drivers 34, 35 and/or in the intensive cooling system 11, a
strip lateral guide 33.1 is particularly advantageously
located in front thereof. After the strip head has passed
the driver 33.1 and the intensive cooling system 11, the
lateral guide 33.1 is opened again so that the water flow
in the laminar strip cooling system 10 is not hindered. The
guide 33.2 then takes over the guiding task for the
remainder of the strip. Similarly for the strip end the
guide 33.1 is briefly adjusted again after the end has left
the finishing train to counteract any straying of the strip
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end. In order to minimise the cooling section length, the
lateral guide 33.1 is therefore preferably located inside
the strip cooling unit 10.
The straightening rolls 36.1, 36.2 before the respective
coiling stations 3 are dipped into the strip plane after
building up the strip tension and provide a strip
straightening effect by looping around the base, deflecting
or drive rollers. A similar operating mode is practised
when deflecting rollers 26 (see Fig. 4) are located inside
the intensive cooling section 11.
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REFERENCE LIST
1 Strip (after the finishing train with final thickness)
2 Coil
3 Coiling station
4 Uncoiling station
Coiling station
6 Strip (inside the finishing train with intermediate
thickness)
7 Finishing train
8 Strip cooling
9 Intensive laminar strip cooling
Laminar strip cooling
11 Intensive cooling
12 Coiler
13 Coiler
14 Straightening machine
Zone
16 Furnace
17 Trimming shears
18 Shears
19 Air cooling or laminar strip cooling
Straightening machine
21 Plate pushing unit
22 Coiler
23 Coiler
24 Driver
Pure cooling unit
26 Straightening roll
27 Squeeze roll/driver
28 Nozzles of intensive cooling system
29 Driver
Tensile stress
31 Transfer roller
32 Induction coil
33.1 Lateral guide before the first driver/before the
intensive cooling
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33.2 Lateral guide before the coiler driver
34 Driver before the intensive cooling
35 Driver after the intensive cooling
36.1 Straightening roll before the first winding station
36.2 Straightening roll before the second winding station
I. First process stage
II. Second process stage
F Conveying direction
To Ambient temperature
