Note: Descriptions are shown in the official language in which they were submitted.
CA 02338743 2001-01-23
Method and Installation for Producing Dual-Phase Steel
The invention relates to a method and a device for producing dual-
phase steel with a two-phase microstructure of 70 to 90 % ferrite
and 30 to 10 % martensite from the hot-rolled state by a controlled
temperature guiding and defined cooling strategy during the coolirg
of the steels, inter alia by means of water cooling after their
finish rolling, wherein in a first cooling stage the cooling curve
enters the ferrite region and, after reaching the required ferrite
contents, further cooling to temperatures below the martensite
starting temperature is carried out in a second cooling stage.
The targeted structural transformation by a corresponding cooling
of the steels is known. For example, in DE 44 16 752 Al a method
for generating hot wide strip is described in which, before the
first transformation, between the continuous casting device and a
compensation furnace, the surface temperature of the slab is
reduced to a sufficient depth (at least 2 mm) so that a structural
transformation from austenite to ferrite/pearlite is achieved. In
this context, the cooling time is selected such that at least 70 %
austenite is transformed into ferrite/pearlite. A renewed
transformation into austenite with new orientation of the austenite
grain boundaries is carried out subsequently in the compensation
furnace. In this way, it is to be achieved that even scrap metal
of second quality, in particular, scrap metal with copper contents,
can be used as a raw material without undesirable accumulations of
copper on the grain boundaries of the primary austenite.
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When manufacturing dual-phase steels, one takes also advantage of
an occurring structural transformation by means of a targeted
cooling, but now temporally after the transformation has occurred.
The adjustment
of a dual-phase microstructure depends in this connection
significantly on the cooling speeds made possible by the device
techriology arid on the SLeel composition. Important for the
manufacturc of dual-phasc steels is a sufficient ferrite formation
in the first cooling stage.
With respect to device technology, a sufficient ferrite forrriaLiuli
is achieved, for example, by cooling with water to a-~emperature of
approximately 620 - 650 C with subsequent air cooling. The
duration of air cooling (approximately 8 seconds) is selected such
that at least 70 % of the austenite is transtormed into ferrite
before Llie second cooling stage begins. A transformation into the
pearlitc stagc should be avoided during the first cooling stage as
well as during air cooling.
In the second cooling stage there must still be so much cooling
capacity present that hasp temperatures below the martensi.te
starting temperature are achieved. Only then the formation of a
dual-phase microstructure with ferrite and martensite components is
ensured. This known manufacture presents no problem For small
strip speeds because sufficierit cuoling capacities for the
martensite transformation are available at the end of the first
cooling stage.
For very high strip speeds, however, the beginning of the second
cooling stage can be displaced within the current cooling stretch
to such an extent that the s~~hgequent martensite formation occurs
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only incompletely or not at all because thcn the cooling capacity
for adjustment of the required low-temperature (< 2.20 C) is tlc
longer sufficient. A mixed microstructure of ferrite, bainite and
proportions of martensite will result that cannot fulfill the
desired mechanical properties of a pure dual-phase microstructure.
From EP-A-0 747 495 a method for manufacture of hot-rolled steel
sheet is known whose structure comprises at least '/5 % territe and
at least 10 % martensite. For its manufacture, the steel is cooled
in a targeted fashion after hot-roll-ing, in particular, in a first
cooling stage with a cooling rate of 2 to 15 (7/s within a time
period of 8 to 40 seconds to a temperature between Ar, point and
730 C and thereafter in a second cooling stage with a cooling rate
of 20 to 150 per secoiid to a temperature of. 300 C. As an
alternativc, a quick cooling with a cooling rate of 20 to 150 C/s
is used before the f i rst cooling stage that leads to a temperature
below the Ar; point.
Frum the printed publication Patent Abstracts of Japan vol. 006,
No. 191(C-127), 30 September 1982, and JP 57 104650 A (Kobe Steel
Ltd.), 29 June 1982, a method for manufacturing a hot-rolled steel
sheet comprised of ferrite and a proportion of 1 to 30 % martensite
is known which is also generated by a two-stage cooling. According
to this method, cooling i's carried out slowly to a temperature
between the Ar: point and 550 C at a cooling rate of 5 to
30 C/second and, subsequently, cooling is carried out with a fast
cooling rate of > 30 C/s to a temperature in Lhe range of 350 to
500 C in a second cooling stage.
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Based on this known prior art, it is an object of the invention to
provide a method and a device for producing dual-phase steel
wherein a fast and quantitatively sufficient structural
transformation of the austenite into ferrite is possible even at
high strip speeds.
The above object is solved according to the invention in that
during the first cooling stage the cooling curve of the steels is
adjusted with such a low cooling speed of 20 K/s to 30 K/s that the
cooling curve enters the ferrite region with a temperature still
so high that the ferrite formation can take place quickly and that
already at least 70 % of the austenite has been transformed into
ferrite before the beginning of the second cooling stage.
With the inventively slower cooling with a cooling speed lower than
in known methods, the cooling curve enters the ferrite region
temporally later but at a higher temperature than in the known
methods, i.e., the transformation of the austenite into ferrite
begins slightly delayed but at a higher temperature than in the
known methods and it occurs also faster as a result of the higher
temperature. It is especially beneficial when the ferrite region
is reached as quickly as possible while at the same time the
transformation temperature is high.
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In comparison to the known methods, a degree of transformation of
at least 70 ~ is reached so early that there is sufficient cooling
capacity in a given cooling stretch for the subsequent martensite
formation. This means that at the end of the first cooling stage a
sufficiently large quantity of austenite has been transformed into
ferrite so that the conventionally performed air cooling can be
eliminated and the second cooling stage can follow immediately
after the first cooling stage.
In order to perform the cooling with the desired low cooling speed,
the principle of a dispersed cooling is applied according to the
invention. This is a water cooling process in which water is
applied to the goods to be cooled by water cooling stages arranged
successively at a spacing. By adjusting the number of the water
cooling stages, their spacing from one another, as well as the
effective length of the water cooling stages, the cooling speed as
well as the applied water quantity can be optimally adjusted to the
goods to be cooled (the mass of the goods to be cooled and/or the
surface of the goods to be cooled) . The cooling can also be
realized by a cooling medium quantity that can be adjusted
continuously.
As a result of the adjustment to the goods to be cooled, the
dispersed cooling can be temporally expanded until the desired
degree of transformation has been reached without there being the
risk that, as in the known methods of fast cooling, the cooling
curve leaves the ferrite region already beforehand as a result of
cooling that is too intensive.
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In comparison to cooling according to the prior art, by means of a
dispersed cooling or cooling with a continuously adjustable cooling
medium quantity, less water is applied until the transformation
temperature is reached. This differential water quantity can now
be applied during the transformation in order to force the carbon
separation from the ferrite into the residual austenite and to
thereby accelerate the ferrite formation. The residual austenite
regions are enriched with carbon to such an extent that they
transform into martensite already at cooling speeds of 20 - 30 K/s.
Since a defined holding period for the cooling in air is no longer
needed in order to ensure a sufficient ferrite formation, the
production of dual-phase steels can be realized on a portion of the
cooling stretch. In this connection, the employed portion of the
cooling stretch is very much shorter than in known methods with air
cooling.
When the required microstructure components for dual-phase steels
can be adjusted without air cooling, this results in significant
advantages for the operator. Fewer device components are required
for the production of dual-phase steels. At the same time, the
production spectrum can be broadened in comparison to the prior art
with changed process and strip parameters (for example, higher
strip speed).
A device for performing the method of the invention is
characterized by a cooling stretch arranged behind the last
finishing roll stand and comprised of several water cooling stages
positioned successively at a spacing or cooling systems with a
continuously adjustable cooling medium quantity. The number of
CA 02338743 2007-08-07
water cooling stages, their effective length and their spacing from one
another are changeable according to the invention so that this
cooling stretch can be adapted in a simple way to changing
geometries of the goods to be cooled as well as to different strip
speeds.
In one aspect, the present invention resides in a method for
producing dual-phase steels from the hot-rolled state with a two-
phase microstructure of 70 to 90 % ferrite and 30 to 10 %
martensite by a controlled temperature guiding and defined cooling
strategy during the cooling of the steels, inter alia by means of
water cooling after their finish rolling, wherein in a first
cooling stage at a cooling rate of < 30 K/s the cooling curve
enters the ferrite region and, after reaching the required ferrite
contents, further cooling is carried out in a second cooling stage
at a cooling rate of > 30 K/s to temperatures below the martensite
starting temperature, characterized in that a) the first cooling
stage (14) is carried out in a cooling stretch of water cooling
stages (7), arranged successively at a spacing, or in a cooling
system with continuously changeable cooling medium quantity with a
cooling rate of 30 K/s adjusted such b) that the cooling curve
(10) enters the ferrite region a temperature still so high that
the ferrite formation can take place quickly; and c) before begin
of the second cooling stage (16), which follows without
intermediate air cooling and holding time directly after the first
cooling stage (14), already at least 70 % of the austenite is
transformed to ferrite.
In another aspect, the present invention resides in a method for
the manufacture of dual-phase steels from the hot-rolled state with a
two-phase microstructure of 70 to 90 % ferrite and 30 % to 10 %
martensite by controlled temperature application and defined cooling
strategy during the cooling of the steels, inter alia by means of water
cooling after finish rolling whereby, in a first cooling stage with slow
cooling rate, the cooling curve runs into the ferrite sector, and in a
second cooling stage at a higher cooling rate is further cooled to
temperatures below the martensite start temperature, wherein the first
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cooling stage (14) is carried out in an adjusted manner in a cooling
stretch consisting of water cooling stages (7) arranged at intervals
behind one another, with a cooling rate of 20-30 K/s, such that a) the
cooling curve (10) runs into the ferrite sector still at such a high
temperature that the ferrite formation can take place rapidly; and b)
before the start of the second cooling stage (16), which follows
immediately from the first cooling stage (14), without intermediate air
cooling and dwell time, already at least 70 % of the austenite is
converted into ferrite, and, during the conversion of the austenite into
ferrite, the cooling of the first cooling stage is continued until the
desired ferrite content of at least 70% is attained.
Further advantages, details, and features of the invention result
from the following description of an embodiment schematically
illustrated in the drawings.
It is shown in:
Fig. 1 a schematic illustration of the fast cooling and the
dispersed cooling as well as their arrangement in a mill
train;
Fig. 2 a time-temperature-transformation curve;
Fig. 3 the degree of austenite transformation for fast
transformation;
Fig. 4 the degree of austenite transformation for dispersed
cooling.
In Fig. 1 the end of a mill train is schematically illustrated. It
is comprised of the last finish roll stand (1), the rolling stock
or goods to be cooled (2), and a hasp (3) with deflection rolls or
drivers (4) . Above this part of a mill train two different cooling
stretches are shown. With the cooling stretch (5) according to the
prior art an early, fast cooling of the goods to be cooled (2) is
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realized by a continuous water supply. In the cooling stretch (6)
according to the invention water cooling stages (7) are arranged
successively at a spacing so that the cooling is "dispersed".
The different transformation results caused by the differenr
cooling methods (5, 6) are represented in an exemplary fashion in
the following schematic illustrations.
In Fig. 2, a time-temperature-transformation curve of the course of
the cooling curve (9) for cooling according to known methods a:.d
the cooling curve (10) for a dispersed cooling are illustrated,
wherein on the abscissa the time (Z) in seconds and on the ordinate
the temperature (T) in C are indicated.
The cooling curve (9) shows the cooling course for the strategy
conventionally employed nowadays (early, fast cooling to a certain
holding temperature with subsequent air cooling, followed by
further cooling to lower temperatures below the martensite starting
temperature). The first cooling stage (11) of the cooling curve
reaches relatively early the transformation region for the ferrite
formation (F = ferrite region) at the point (8) and also remains in
this region (F) for a relatively long time as a result of the
holding time (12) with air cooling before a further cooling to a
temperature below the martensite starting temperature (M =
martensite, B = bainite, P = pearlite) takes place by means of the
second cooling stage (13) starting at the point (17).
In contrast, with the dispersed cooling the cooling curve (10) with
its first cooling stage (14) reaches the ferrite region (F) at the
point (15) later in comparison to the cooling curve (9) Since
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after reaching the ferrite region (F) the dispersed cooling is
initially maintained, no time-consuming waiting period with air
cooling is required, and the cooling curve (10) leaves the ferrite
region (F) earlier.
The dispersed cooling is maintained within the ferrite region (F)
until the desired degree of transformation has been reached. The
further cooling by means of the second cooling stage (16) is
carried out directly thereafter.
The austenite transformation rates which can be achieved with the
described different cooling strategies, i.e., the known fast
cooling and the dispersed cooling, can be seen in the two next
illustrations of Figs. 3 and 4. The cooling time (Z) in seconds and
the degree of transformation (U) of the austenite transformation
into ferrite are illustrated on the abscissa and on the ordinate,
respectively.
In the fast cooling (Fig. 3), during the first cooling stage (11)
of the cooling curve (9) first a strong ferrite formation up to
approximately 53 % takes place which then increases during the
following air cooling (12) to approximately 62 %. However, this is
not sufficient for the production of dual-phase steels.
In contrast thereto, with the dispersed cooling (Fig. 4) according
to cooling curve (10) a considerably higher ferrite contents has
already been formed in the first cooling stage (14) in the same
time period and approximately 82 % austenite has already been
transformed into ferrite before the second cooling stage (16)
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occurs (the dual-phase steels produced nowadays have generally a
contents of > 80 % ferrite).
The invention is not limited to the exemplary cooling curves
described in the illustrations; other cooling curves as, for
example, in cooling systems with a continuously changing cooling
medium quantity are possible which, in keeping with the invention,
result in higher transformation temperatures. Also, the invention
is not limited to water cooling; other cooiing systems can also be
employed which lead to an earlier reaching of the ferrite region at
high temperatures.
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