Note: Descriptions are shown in the official language in which they were submitted.
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METHOD FOR PRODUCING HOT STRIP WITH A
MULTIPHASE STRUCTURE
The invention concerns a method for producing hot-rolled
strip that consists of TRIP (transformation-induced
plasticity) steel with a multiphase microstructure and with
both high strength values and outstanding deformation
properties, where the TRIP steel strip is produced from the
hot-rolled state by controlled cooling after the last rolling
stand.
The adjustment of the microstructure is a complex matter
in TRIP steels, since, besides ferrite and bainite, a third
phase is present in the form of retained austenite or, after a
subsequent deformation, in the form of martensite. TRIP
steels are now usually produced in a two-stage heat cycle.
The starting material is hot-rolled or cold-rolled strip, in
which an approximately 50% a- 50% y initial microstructure is
adjusted. Due to the higher solubility of carbon in
austenite, austenite has a higher carbon concentration. After
the annealing treatment, rapid cooling is carried out past the
ferrite and pearlite range into the bainite range, in which
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isothermal conditions are maintained for some time. The
austenite is partially transformed to bainite, and at the same
time the remainder of the austenite becomes further enriched
with carbon. In this way, the martensite start temperature Ms
is reduced to values below ambient temperature, and
consequently the retained austenite also continues to exist at
ambient temperature. The final microstructure consists of 40-
70% ferrite, 15-40% bainite, and 5-20% retained austenite.
The special effect of TRIP steels is the transformation
of the metastable retained austenite to martensite when
external plastic deformation occurs. The transformation of
the austenite to martensite is accompanied by an increase in
volume, which is supported not just by the austenite alone but
rather by the surrounding microstructural components as well.
The ferritic matrix is plasticized, which in turn results in
greater strain hardening and leads overall to higher plastic
elongations. Steels produced in this way have an
extraordinary combination of high strength and high ductility,
which makes them suitable especially for use in the automobile
industry.
The process management described above which is presently
used mostly for the industrial production of TRIP steels, is
complicated and expensive due to the additional annealing and
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cooling treatment after the rolling operation, which is the
reason that attempts have been made to produce these TRIP
steels directly as hot-rolled strip in industrial production
plants for hot strip production. For example, EP 1 396 549 Al
discloses a method for producing pearlite-free hot-rolled
steel strip with TRIP properties in a continuously running
operational process, in which a steel melt, which contains, in
addition to iron and unavoidable impurities, 0.06-0.3% C; 0.1-
3.0% Si; 0.3-1.1% Mn (with the total amount of Si and Mn being
1.5-3.5%); 0.005-0.15% of at least one of the elements Ti or
Nb as an essential component; and optionally one or more of
the following elements: max. 0.8% Cr; max. 0.8% Cu; and max.
1.0% Ni, is cast into thin slabs, which are annealed at 1,000-
1,200 C for an annealing time of 10-60 minutes in an annealing
furnace, starting from a run-in temperature of 850-1,050 C.
After descaling, the thin slabs are finish hot rolled in the
range of 750-1,000 C and then cooled to a coiling temperature
of 300-530 C. The controlled cooling is carried out in two
stages at a cooling rate of the first stage of at least 150
K/s with a cooling interruption of 4-8 seconds.
Alternatively, it is proposed that the controlled cooling be
carried out continuously at a cooling rate of 10-70 K/s
without a holding interruption. Finally, a third possibility
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is proposed, in which the cooling is controlled in such a way
that the hot rolled strip is cooled in a first phase to a
temperature of about 80 C above coiling temperature within 1-7
seconds and is then cooled to coiling temperature by air
cooling. Besides the prescribed process management, the
presence of Ti and/or Nb is important, since these elements
remain in solution until the start of the hot rolling and,
upon their subsequent precipitation, improve, among other
properties, the grain fineness of the hot-rolled strip and
increase the retained austenite content and its stability.
Using this prior art as a point of departure, the
objective of the invention is to specify a method which allows
simpler and more economical production of TRIP steels in
existing plants and in which an annealing treatment and the
addition of alloying elements that are otherwise not
absolutely necessary can be eliminated.
This objective is achieved by the characterizing features
of Claim 1, according to which the production of the hot-
rolled strip in a thin-slab continuous casting and rolling
plant (CSP plant) with a predetermined chemical composition of
the steel grade that is used within the following limits:
0.12-0.25% C; 0.05-1.8% Si; 1.0-2.0% Mn; the remainder Fe and
customary accompanying elements is carried out with a combined
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rolling and cooling strategy in such a way that a
microstructure is obtained which consists of 40-70% ferrite,
15-45% bainite, and 5-20% retained austenite, such that
= the finish rolling of the hot-rolled strip for
adjusting a very fine austenite grain (d < 8 pm) during the
last deformation is carried out at temperatures of 770-830 C,
just above Ar3 in the range of metastable austenite, and
= immediately after the last rolling stand, a controlled
two-stage cooling of the hot-rolled strip to a strip
temperature in the range of bainite formation of 320-480 C is
carried out with a holding time at about 650-730 C, whose
start is determined by the entry of the cooling curve into the
ferrite range and whose duration is determined by the
transformation of the austenite to at least 40% ferrite.
In contrast to the usual procedure that was described
earlier, in accordance with the invention, in an
austenitically finish rolled hot strip, the typical
microstructure for a TRIP steel is adjusted immediately after
the last rolling stand by a two-stage cooling operation in the
cooling line. In this connection, the adjustment of the
appropriate microstructure requires extensive process know-how
as well as very exact maintenance of the necessary process
parameters. Due to the narrow tolerance range for the
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production of TRIP steels on hot wide strip mills, since the
introduction of thin-slab continuous casting and rolling
technology, a plant configuration has been available which
provides much better conditions for the direct production of
TRIP steels than hot-rolled strip, compared to conventional
hot-rolled strip mills. Due to the high degree of uniformity
of temperature over the thickness, width, and length of the
strip, TRIP steels with constant mechanical properties can be
reproducibly produced in this way. Due to the short length of
the conventional cooling lines used in this process in
existing continuous casting and rolling mills, the production
of hot-rolled strip with TRIP microstructure is possible only
with a special rolling and cooling strategy.
The rolling strategy of the invention is used for
adjusting a very fine austenite grain (d < 8 pm) during the
last deformation, which has an accelerating effect on the
ferrite transformation in the subsequent cooling line.
Therefore, the finish rolling of the strip takes place at
temperatures of 770-830 C, just above Ar3 in the range of
metastable austenite.
The successful implementation of the cooling strategy
makes it absolutely necessary to maintain certain limits of
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chemical composition in order to realize the desired degree of
transformation within the short total cooling time that is
available. Therefore, the chemical analysis proposed for the
production of TRIP steels varies within the following limits:
0.12-0.25% C, 0.05-1.8% Si, 1.0-2.0% Mn, the remainder Fe and
customary accompanying elements.
The cooling strategy involves two-stage cooling with the
option of using different cooling rates in each stage. The
start of the holding time at temperatures of 650-730 C is
determined by the entry of the cooling curve into the ferrite
range. The desired transformation of the austenite to at
least 40% ferrite then takes place during the following brief
cooling interruption. The holding time is then immediately
followed by the second cooling stage, in which the hot-rolled
strip is cooled to a temperature of 320-480 C. The
transformation of austenite to at least 15% bainite takes
place at this temperature.
In addition to the use of a short holding time, the
cooling strategy is determined by an exactly defined,
predetermined cooling rate for the two cooling stages. This
cooling rate is V = 30-150 K/s and preferably V = 50-90 K/s,
depending on the geometry of the hot-rolled strip and the
chemical composition of the steel grade that is used. In
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regard to these cooling rates, it should be noted that a
cooling rate less than 30 K/s is not possible due to the small
amount of time that is available in the conventional cooling
line of a continuous casting and rolling plant, and that
cooling rates greater than 150 K/s likewise cannot be realized
in cooling lines of this type, which consist of a succession
of water cooling zones spaced a certain distance apart.
The hot-rolled strip produced with the method of the
invention with TRIP steel properties for different strength
levels with an elastic limit tensile strength ratio Rpo.2 / Rm
in the range of 0.45-0.75 has the following combinations of
tensile strength Rm and percentage elongation after fracture A:
Rm = 600-700 MPa => A > 25%
Rm = 700-800 MPa => A > 23%
Rm = 800-900 MPa ~ A > 21%
Rn, = 900-1,000 MPa ~ A > 18%
Rm > 1,000 MPa ~ A > 15%
Further details and advantages of the invention are
explained in greater detail below with reference to the
specific embodiment of the invention illustrated in the
accompanying drawings.
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-- Figure 1 shows a CSP plant.
- Figure 2 shows a modified cooling line of the CSP
plant.
-- Figure 3 shows cooling curves for a dual-phase steel
and a TRIP steel in a TTT diagram.
Figure 1 shows the layout of a conventional CSP plant 1
schematically. In the illustrated example, it comprises the
following main components in the direction of conveyance (from
left to right in the drawing): the casting installation with
two strands 2, the strand guides 3, the soaking furnaces 4
with a furnace transverse conveyor 5, a multiple-stand rolling
mill 6, the cooling line 10, and coilers 8.
Figure 2 shows a modified cooling line 10 of a CSP plant
1, which is necessary for carrying out cooling in accordance
with the invention and is already known from EP 1 108 072 Bl,
which describes a method for producing dual-phase steel. This
modified cooling line 10 of the CSP plant 1 is installed
downstream of the last finish rolling stand 61. The cooling
line 10 has several successive water cooling zones 111_7, 12
that are spaced a certain distance apart and can be
automatically controlled. The water cooling zones 111_7, 12
are equipped with water spray heads 13, which evenly spray the
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upper and lower surfaces of the hot-rolled strip 7 with a
specific amount of water. The positioning of the water
cooling zones 111_7, 12 within the cooling line 10, their
number, their spacing, and the number of water spray heads 13
per water cooling zone 111_7, 12 are chosen in such a way that
the desired cooling rate of the two cooling stages can be
variably adjusted in advance in order to achieve optimum
adaptation of the water cooling zones 111_7, 12 to the cooling
conditions that are to be adjusted. Automatic control of the
amount of water sprayed thus makes it possible, even during
the cooling operation, to make any necessary change in the
cooling rate.
An additional water cooling zone 12 is installed a
greater distance from the last water cooling zone 117 of the
first cooling stage than the distance between the individual
zones of water cooling zones 111_7. The second cooling stage
is carried out in this additional water cooling zone 12. In
this water cooling zone 12, in contrast to the water cooling
zones 111_7 of the first cooling stage, there is a
significantly larger number of water spray heads 13 in order
to carry out forced intensive cooling over a shorter distance.
The distance between the last water cooling zone 117 of the
first cooling stage and the water cooling zone 12 of the
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second cooling stage is chosen sufficiently large to obtain
the holding time necessary to achieve transformation of the
austenite to at least 40% ferrite, as prescribed by the
invention, at the predetermined strip speed.
Figure 3 shows a TTT diagram with the transformation
lines for ferrite, pearlite, and bainite and with the
temperature lines (20, 21, 22, 24) for Ac3, Acl, and Ms.
Horizontal shift arrows 27 for the transformation lines and
vertical shift arrows 28 for the temperature lines show the
effect of existing or added alloying elements on the position
of these transformation and temperature lines in the TTT
diagram. The cooling curve 25 for the production of a dual-
phase steel and the cooling curve 26 for the production of a
TRIP steel in accordance with the invention are plotted in
this TTT diagram as examples. At approximately the same start
temperature (above Ac3) at the start of cooling and
approximately the same holding time temperature (above Acl), a
significantly different microstructural composition is
obtained due to the different courses of the cooling and the
different compositions of the initial steels. According to
the plotted cooling curve 25 for the dual-phase steel, the
cooling curve 25 passes only into the ferrite range and ends
below the martensite start temperature line 22, which is well
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above room temperature 23, so that, as desired, a dual
microstructure that consists only of ferrite and martensite is
obtained. On the other hand, the cooling curve 26 for the
production of a TRIP steel in accordance with the invention
passes first through the ferrite range and then through the
bainite range and ends above the martensite start temperature
line 24, which is now below room temperature 23, so that
transformation to martensite during cooling does not take
place, and, in accordance with the invention, a microstructure
is obtained that consists of ferrite, bainite, and some
retained austenite.
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LIST OF REFERENCE NUMBERS
1 CSP plant
2 casting installation with two strands
3 strand guide
4 soaking furnace
furnace transverse conveyor
6 multiple-stand rolling mill 6
6' last rolling stand
7 hot-rolled strip
8 coiler
9 temperature measurement
cooling line
111_7 water cooling zones
12 water cooling zone
13 water spray heads
Ac3 temperature line
21 Acl temperature line
22 martensite start temperature line for a dual-phase steel
23 room temperature line
24 martensite start temperature line for a TRIP steel
cooling curve for a dual-phase steel
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26 cooling curve for a TRIP steel
27 horizontal shift arrows of the transformation lines
28 vertical shift of the temperature lines
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