Note: Descriptions are shown in the official language in which they were submitted.
CA 02529837 2009-03-11
METHOD AND INSTALLATION FOR THE PRODUCTION OF HOT-ROLLED
STRIP WITH A DUAL-PHASE MICROSTRUCTURE
The invention concerns a method for producing hot-rolled
strip with a dual-phase microstructure consisting of ferrite and
martensite, wherein at least 70?, of the austenite is transformed
to ferrite from the hot-rolled state by a controlled two-stage
cooling operation after the finish rolling to a strip
temperature below the martensite start temperature in a cooling
line that consists of successive, spaced water cooling units.
Systematic microstructural transformation by controlled
cooling is steels is well known, and to produce dual-phase
steels, this controlled cooling is carried out after the working
of the hot strip is complete. The adjustment of the attainable
dual-phase microstructure depends essentially on the cooling
rates that are technically possible in the installation and on
the chemical composition of the steel. In this regard, it is
important in any case to achieve sufficient ferrite formation of
at least 70: in the first cooling stage. During this first
1
CA 02529837 2005-12-19
cooling stage, transformation of the austenite in the pearlite
stage should be avoided.
The cooling capacity of the second cooling stage following
the first cooling stage must be sufficiently high that coiling
temperatures below the martensite start temperature are reached.
Only then is the formation of a dual-phase microstructure with
ferritic and martensitic constituents ensured.
The previously known production of dual-phase steels is
unproblematic at low strip speeds or with sufficiently long
cooling lines. However, at very high strip speeds, the
beginning of the second cooling stage can. be shifted so far in
the present cooling line that the subsequent martensite
formation remains incomplete or does not occur at all. This
results in a mixed microstructure consisting of ferrite, bainite
and some martensite, so that the desired mechanical properties
of a pure dual-phase microstructure are not obtained.
EP 0 747 495 Bl describes a method for producing high-
strength steel plate with a microstructure consisting of 75%
ferrite, at least loo martensite and possibly bainite and
retained austenite. Accordingly, this is not a microstructure
of pure dual-phase steels. A steel microalloyed with niobium is
used as the alloy. It is produced by systematically cooling the
2
CA 02529837 2005-12-19
hot-rolled steel plate, wherein a rapid cooling follows a slow
cooling or, alternatively, a rapid cooling precedes the slow
cooling. A cooling rate of 2-15 C/s within a cooling time of 8-
40 s is given for the first cooling stage to a final temperature
between the AR1 point and 730 C. In the second cooling stage,
the steel is cooled to a temperature of 300 C at a cooling rate
of 20-150 C/s. In the alternative method, in which the rapid
cooling stage precedes the slow cooling stage, rapid cooling is
carried out to a temperature below the Ar.3 point at a cooling
rate of 20-150 C/s.
EP 1 108 072 Bl describes a method for producing dual-phase
steels, in which a dual-phase microstructure consisting of 70-
90% ferrite and 30-10% martensite is achieved with a two-stage
cooling operation (first slow, then rapid) carried out after the
finish rolling. The first (slow) cooling is carried out in a
cooling line in which the hot-rolled strip is cooled in a well-
defined way by successive, spaced water cooling zones at a
cooling rate of 20-30 K/s. In this connection, the cooling is
adjusted in such a way that the cooling curve enters the ferrite
range at a temperature that is still so high that ferrite
formation can occur rapidly. The first cooling is continued
until at least 70% of the austenite has transformed to ferrite.
3
CA 02529837 2011-02-11
This cooling stage is immediately followed by the other (rapid)
cooling stage without any holding time.
Proceeding from the aforementioned prior art with the
various possible means that have been described for producing
dual-phase microstructure, the objective of the invention is to
specify a method by which and an installation in which the
production of hot-rolled strip with dual-phase microstructure
can be carried out in a conventional continuous casting and
rolling installation with the local limitations that exist there
and thus with the given time constraints. The cooling line of
an installation of this.type is characterized by the fact that
the total length generally does not exceed 50 m and that compact
cooling is not provided.
The objective with respect to the method is achieved with
the method steps described herein. The method is
characterized by the fact that, to obtain a hot-rolled strip
with a dual-phase microstructure consisting of 70-95% ferrite
and 30-5% martensite with high mechanical strength and high
formability (tensile strength greater than 600 MPa, elongation
after fracture at least 25%) in the cooling line of a continuous
casting and rolling installation, starting from a steel with the
following chemical composition: 0.01-0.08% C, 0.9% Si, 0.5-1.6%
4
CA 02529837 2011-02-11
Mn, 1.2% Al, 0.3-1.2o Cr, with the remainder consisting of Fe
and customary trace elements, the two-stage controlled cooling
is carried out from a finish rolling strip temperature Tfinietõ
such that A3 - 100 K < Tfinish < A3 - 50 K, to a coiling strip
temperature Tcc,1iiõuu < 300 C (< martensite start temperature) ,
wherein the cooling rate V1,2 in both cooling stages is V = 30-
150 K/s, and preferably V = 50-90 K/s, the first cooling stage
is carried out until the cooling curve enters the ferrite range,
and then the heat of transformation liberated by the
transformation of the austenite to ferrite is used for
isothermally holding the strip temperature thereby reached for a
holding time of 5 s until the beginning of the second cooling
stage.
Due to the short length of conventional cooling lines in
existing continuous casting and rolling installation, the
production of hot-rolled strip with a dual-phase microstructure
is possible only with a special cooling strategy. To allow a
special cooling strategy of this type to be carried out, it is
absolutely necessary to maintain certain limits of chemical
composition, such as those listed above, so that the
desired degree of transformation can be achieved with the short
total cooling time that is available.
CA 02529837 2005-12-19
The cooling strategy involves two cooling stages that have
selectively variable cooling rates and are interrupted by an
isothermal holding time of a maximum of 5 s. The beginning of
the holding time, which corresponds to the end of the first
cooling stage, is determined by the entrance of the cooling
curve into the ferrite range, i.e., the point at which the
austenite starts to transform to ferrite. The entire desired
transformation of the austenite to at least 700 ferrite occurs
in the short isothermal cooling interruption of a maximum of 5
s, during which, in accordance with the invention, the liberated
heat of transformation holds the temperature at a constant value
by compensating unavoidable air cooling. This holding time is
then immediately followed by the second cooling stage, during
which the hot-rolled strip is cooled to a temperature below
300 C. Since this temperature is below the martensite start
temperature, the desired level of martensite, which is the
second constituent of the dual-phase microstructure, is thus
obtained.
In addition to the use of a short holding time, the cooling
strategy is defined by an exactly defined, predetermined cooling
rate for both cooling stages. This cooling rate is V = 30-150
K/s, and preferably V = 50-90 K/s. It depends on the geometry
6
CA 02529837 2005-12-19
of the hot-rolled strip and on the chemical composition of the
grade of steel that is used. In regard to these cooling rates,
it should be noted that a cooling rate of less than 30 K/s is
not possible due to the small amount of time available in the
conventional cooling line of a continuous casting and rolling
installation, and cooling rates greater than 150 K/s also cannot
be attained in conventional cooling lines.
Compared to prior-art methods for producing dual-phase hot-
rolled strip, the method of the invention is characterized not
only by the fact that the initial steel has a different chemical
composition but also by the fact that
(a) the finish rolling temperature is well below the A3
temperature,
(b) cooling is carried out to a temperature below 300 C in
the second cooling stage,
(c) the cooling rates are below 150 K/s and above 30 K/s,
(d) there is a very short holding time of a maximum of 5
seconds, during which no cooling occurs, between the two cooling
stages, and
(e) the transformation to ferrite occurs isothermally.
A continuous casting and rolling installation for carrying
out the method of the invention is characterized by a
7
CA 02529837 2005-12-19
conventional cooling line that is' installed after the last
finishing stand and has several successive, spaced water cooling
units, which can be automatically controlled. The spray bars
present in each cooling unit are arranged in such a way that a
specific amount of water is uniformly sprayed onto the upper and
lower surfaces of the hot-rolled strip. The total amount of
water can be automatically controlled by turning individual
spray bars on or off during rolling. The number and arrangement
of the water spray bars that are turned on can be variably
adjusted in advance to obtain an optimum adjustment of the
entire cooling line to the cooling conditions that are to be
established.
Further details, features and characteristic of the
invention are explained in greater detail below with reference
to the specific embodiment of the invention that is illustrated
in the schematic drawings.
-- Figure 1 shows a time-temperature cooling curve of a
hot-rolled strip.
-- Figure 2 shows a layout of a cooling line in a
continuous casting and rolling installation with a 6-stand
finishing train.
-- Figure 3 shows a layout of a cooling line in a
8
CA 02529837 2005-12-19
continuous casting and rolling installation with a 7-stand
finishing train.
-- Figure 1 shows an example of a time-temperature cooling
curve of a hot-rolled strip that was cooled by the method of the
invention on the-runout roller table in a cooling line 1. The
hot-rolled strip, which had the following composition: 0.060 C,
0.10 Si, 1.2% Mn, 0.0150 P, 0.06% S, 0.036% Al, 0.150 Cu, 0.0540
Ni, 0.71% Cr, the remainder consisting of Fe and customary trace
elements, was cooled in a first cooling stage at a cooling rate
V1 of 54 K/s from an adjusted finish rolling temperature Tflnisn of
800 C to a hot-rolled strip temperature of 670 C, at which the
cooling curve entered the ferrite range. During a holding time
of about 4 s, the temperature of the hot-rolled strip remained
at this holding temperature and then the final cooling
was carried out in a second cooling stage, in which the strip
was cooled to a temperature below 300 C (about 250 C coiling
temperature) at a cooling rate V2 of 84 K/s. Tests on the hot-
rolled strip produced by this method, which had a dual-phase
microstructure in the desired range of at least 700 ferrite and
less than 200 martensite, yielded a tensile strength of 620 MPa
combined with a ratio of yield stress to tensile strength of
0.52.
9
CA 02529837 2005-12-19
Figure 2 shows an example of a layout of a cooling line 1
of a conventional continuous casting and rolling installation.
The cooling line 1, through which the hot-rolled strip passes in
direction of conveyance 8, is located between the last finishing
stand 2 and the coiler 5. A temperature-measuring point 6 for
monitoring the temperature of the hot-rolled strip 10 entering
the cooling line 1 is located between the last finishing stand 2
and the first water cooling unit 31. The cooling line 1 shown in
Figure 2 comprises a total of eight cooling units 31-7 and 4.
The latter is often realized as a trimming zone 4. More
generally, a conventional cooling line comprises six to nine
cooling units, depending on the particular continuous casting
and rolling installation.
The example illustrated in Figure 2 is the typical layout
of a cooling line for a 6-stand continuous casting and rolling
installation, as is apparent from the gap between cooling units
3-7 and 4. Subsequent conversion to a 7-stand finishing train
often requires that, for example, the first cooling unit
(cooling zone) 31 be moved to the rear into the structural gap
between the cooling units 37 and 4. In this case, the cooling
line 1' has a layout of the type shown in Figure 3, which
differs from the layout of the cooling line 1 in Figure 2 only
CA 02529837 2005-12-19
by the elimination of this sturctural gap between the cooling
units 37 and 4. Therefore, the reference numbers of the
individual structural parts and assemblies of Figure 3 are the
same as the reference numbers of Figure 2. An exception to this
is the first cooling unit 31', whose upper spray bar, in contrast
to the spray bar of cooling unit 31 in Figure 2, is designed with
the standard length of the cooling units 32 to 37.
In most cases, each cooling unit has four spray bars on
both the upper side and the lower side. Each spray bar in turn
consists of two rows of small water pipes for cooling the upper
surface of the strip 10' and the lower surface of the strip 10".
As a special feature, the cooling unit 31 in Figure 2 is
shortened by one spray bar on the upper side due to limited
space.
In contrast to the upstream cooling units 31-7, which have
one switchable valve 7 per spray bar, the trimming zone 4 has
two valves 7 for each spray bar. This means that in the
trimming zone, each row of small cooling pipes can be
individually controlled, and thus the amount of water can be
more finely controlled.
The delivery speed of the strip from the finishing train
varies with the rolled thickness of the finished strip.
11
CA 02529837 2005-12-19
Accordingly, the mode of operation of the cooling line must be
adjusted to be able to adjust the time-temperature control
necessary for the adjustment of the strip properties. For a
strip thickness of 3 mm, for example, the first required cooling
level is attained with the cooling units 31 and 32, while the
second cooling level is realized with cooling units 3,,, 36, 37,
and 4. Due to the altered boundary conditions for a finished
strip with a thickness of 2.0 mm, only cooling units 3h, 37, and
4 need to be used for the second cooling stage.
12
CA 02529837 2005-12-19
List of Reference Numbers
1 cooling line
2 last finishing stand
31-7 water cooling units
4 water cooling unit (trimming zone)
coiler
6 temperature-measuring point
7 switchable valve
8 direction of conveyance
hot-rolled strip
10' upper surface of the strip
10" lower surface of the strip
V1 cooling rate of the first cooling stage
VZ cooling rate of the second cooling stage
Til,l::i, strip temperature after the last finishing stand
T,.),,at. strip temperature after the holding time
T,7,,111õg strip temperature at the end of cooling (coil
temperature)
13
