Note: Descriptions are shown in the official language in which they were submitted.
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METHOD AND DEVICE FOR ADJUSTING TARGETED COMBINATIONS
OF PROPERTIES OF POLYPHASE STEEL
The invention concerns a method and a device for
adjusting targeted combinations of properties of hot-rolled
multiphase steels, whose multiphase structure includes at
least 30% ferrite and at most 5056- martensite, for example,
dual-phase steels and TRIP steels, which are produced with a
standard analysis and standard process management in a
conventional hot rolling mill train, a thin slab casting and
rolling installation, or suitable narrow and medium strip mill
trains, or a wire mill.
Compared to conventional steel grades, multiphase steels
have a significantly improved combination of strength and
ductility and therefore are becoming increasingly important,
especially for use in the automotive industry. The most
important groups of steel for automobile manufacturing at the
present time are dual-phase steels and TRIP steels.
In this regard, due to the significantly lower production
costs, the variant of production directly as hot strip offers
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economic advantages and thus has very strong potential for the
future.
A characteristic feature of dual-phase steels is a low
elastic limit-tensile strength ratio, which is generally 50-
70%. Compared to HSLA steels, i.e., high-strength low-alloy
structural steels, besides the lower yield point at the same
level of tensile strength, significantly better elongation
values a.re obtained. For some applications (for example,
tubes), it may be desired that the elastic limit-tensile
strength ratio must be adjusted to well-defined values, but
nevertheless the elongation after fracture is as great as
possible.
Since the production of different strength classes
directly in the hot-rolled strip requires very extensive
process know-how, it is a prior art practice to adjust either
the chemical analysis or the process management for each
individual material, with TRIP steels basically having a
somewhat higher elastic limit-tensile strength ratio than
dual-phase steels.
EP 1 108 072 B1 discloses a method for producing dual-
phase steels, in which after the finish rolling, a two-stage
cooling is used to obtain a dual-phase microstructure that
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consists of 70-90% ferrite and 30-10% martensite. The first
(slow) cooling is carried out in a cooling line in which the
hot-rolled strip is cooled in a well-defined way at a cooling
rate of 20-30 K/s by successive separated water cooling
stations. In this cooling stage, the cooling is adjusted in
such a way that the cooling curve enters the ferrite range at
a temperature that is still high enough to allow rapid ferrite
formation. This first cooling is continued until at least 70%
of the austenite has been converted to ferrite, and then the
further (rapid) cooling follows immediately without a pause.
The special effect of TRIP steels (transformation-induced
plasticity) with a microstructure that comprises, for example,
40-70% ferrite, 15-40% bainite, and 5-20% residual austenite
is the transformation of the metastable residual austenite to
martensite when an external plastic deformation occurs. This
transformation, which is accompanied by an increase in volume
and plasticization of the ferritic matrix and which is
supported not only by the austenite but also by the
surrounding microstructural components, results in greater
strain hardening and leads all together to higher plastic
elongations. Steels produced in this way have an
extraordinary combination of high strength and high ductility,
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which makes them suitable especially for use in the automobile
industry.
EP 1 396 549 Al discloses a method for producing
pearlite-free hot-rolled steel strip with TRIP properties, in
which a steel melt, which contains, in addition to iron and
unavoidable impurities, at least one of the elements Ti or Nb
as an essential component and optionally one or more of the
following elements in the amounts indicated: 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 then finish hot rolled in the range of 750-1,000 C and
then cooled to a coiling temperature of 300-530 C in two
stages at a controlled cooling rate of the first stage of at
least 150 K/s and a cooling interruption of 4-8 seconds.
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
residual austenite content and its stability.
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Finally, EP 1 394 279 Bl discloses a method for producing
a low-carbon steel of high strength and high ductility with a
tensile strength of greater than 800 MPa, uniform elongation
of greater than 5%, and elongation after fracture of greater
than 20%. Starting from a hardened or hardened and tempered
feedstock, a steel with 0.20% C, 1.60% Mn, and admixtures of
boron and a martensite phase component of greater than 90%,
and after a cold rolling that constitutes more than 20% of the
total rolling, an annealing treatment is carried out at a
temperature of 500-600 C, resulting in a microstructure with
an ultrafine, crystalline, granular ferrite structure of 100-
300 nm with iron carbides deposited in the ferrite.
Using this prior art as a point of departure, the
objective of the invention is to specify a method and a device
with which multiphase steels produced with a standard analysis
and standard process management can be transformed to steel
grades with almost any desired combinations of properties.
The objective of the invention with respect to a method
is achieved with the characterizing features of Claim 1 in
such a way that following the cooling from the hot rolling or
a later production step, for example, the production of
components, the desired combinations of strengths and elastic
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limit-tensile strength ratios are adjusted in the multiphase
steels by a subsequent or intermediate annealing treatment
with variable annealing temperature and variable annealing
time. A device for carrying out the method is characterized
by the features of Claim 8. Advantageous refinements of the
invention are described in the dependent claims.
The annealing treatment of multiphase steels with a
standard analysis and standard process management, which is to
be carried out simply and with adaptation in accordance with
the invention after the actual production process has been
completed, makes it possible to adjust almost any desired
combinations of different materials and combinations of
properties (magnitude of the yield point, level of tensile
strength). By contrast, the production of different
multiphase steel strength classes directly in the hot-rolled
strip requires very extensive process know-how and suitable
adjustment of the alloying elements in advance.
In accordance with the invention, the annealing treatment
is carried out at a variable annealing temperature of < 600 C
and a likewise variable annealing time of <- 120 s in such a
way that the resulting microstructure consists of a ferritic
base matrix and annealed martensite or bainite with 10-50% of
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the area fraction. In this regard, the annealing temperature
affects primarily the magnitude of the yield point by finely
distributed precipitation of carbides at the grain boundaries
of the martensite or bainite, and the level of tensile
strength can be adjusted by the annealing time.
In accordance with the invention, the annealing treatment
can be carried out offline in a continuous annealing
installation, independently of upstream or downstream process
steps and adapted to existing circumstances, or it can be
carried out online in an existing process line, for example,
as part of a strip galvanizing operation in the heating stage
of a galvanizing line before entry into the zinc bath.
In accordance with the invention, it is also possible to
carry out the annealing treatment on components that have
already been finish pressed (frame members, wheels, connecting
elements, etc.), which results in subsequent improvement of
the mechanical properties of these components. The advantage
of this procedure is that the deformation into the component
can be carried out on a nicely cold-workable material with a
low elastic limit-tensile strength ratio with good elongation,
and thus the tool wear is kept comparatively low. The
annealing treatment that follows increases the strength of the
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components to values that otherwise would be difficult to
preset, since then the pressing force of the shaping machines
would not be sufficient.
In addition to the complete annealing treatment of a
component, it is also possible, in accordance with the
invention, to use a targeted zonal annealing treatment in
locally limited sections of the component. The goal here is
the partial replacement of welded tailor blanks. Where tailor
blanks are concerned, steels of high strength are
systematically welded onto specific sections of components in
order to produce desired stiffness values of components.
However, this welding operation could be eliminated if a zonal
annealing treatment is carried out in the given sections
instead.
In accordance with the invention, a device for adjusting
targeted combinations of properties in hot-rolled multiphase
steels by an annealing treatment is characterized by a thermal
installation, which is located in a freely selectable place
within the production plant or production line and in which an
annealing treatment can be carried out at an annealing
temperature of S 600 C and an annealing time of < 120 s. This
thermal installation can be a continuous annealing
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installation, in which the annealing treatment, for example,
of components, is carried out offline, or it is arranged
online in an existing process line, for example, as part of a
strip galvanizing operation in the heating stage of a
galvanizing line before entry into the zinc bath.
The mode of operation of the annealing treatment of the
invention is made clear by the following example. Some dual-
phase steels have anisotropic toughness properties in the
direction of rolling and transversely to the direction of
rolling. In a brief annealing treatment for 60 s at 500 C
carried out on a dual-phase steel produced as hot-rolled strip
with a tensile strength of 980-1,035 N/mm2, this aniosotropy of
the properties can be evened out in the two different
directions (isotropic properties). As the following table
shows, the untreated hot-rolled strip (annealing time 0 s) has
a significantly different development of the elongations after
fracture in the longitudinal and transversal directions of
rolling. As a result of the brief annealing treatment
(annealing time 1 min), the tensile strength declines
somewhat, but the values for the elongation after fracture
rise overall to a higher level:
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Annealing Time (s) Rpo,2 Rm Rpo_Z / Rm A
(MPa) (MPa) (o)
0 longitudinal 473 1035 0.46 13.0
transversal 469 981 0.48 7.8
60 longitudinal 503 839 0.60 17.7
transversal 513 881 0.58 18.1
These relationships presented for the example of the
dual-phase steel apply in the same way for TRIP steels as
well.
Further details on the possible performance of the above-
described annealing treatment of the invention are explained
in greater detail below on the basis of the flowcharts shown
in the accompanying schematic drawings.
-- Figure 1 shows a flowchart of the annealing treatment
of strip material.
-- Figure 2 shows a flowchart of the annealing treatment
of wire material.
-- Figure 3 shows a flowchart of the annealing treatment
of components.
In Figures 1 to 3, the individual process steps which, in
accordance with the invention, are necessary for the annealing
treatment of strip material (Figure 1), wire material (Figure
2), and components (Figure 3) are shown in the form of
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flowcharts, with the respective process path labeled with
numbered directional arrows. A common feature of all of the
flowcharts presented here is that a hot rolling step is first
carried out as the starting point, which is followed by a
controlled cooling from the hot rolling operation to realize a
multiphase microstructure. The further possible process steps
and the time of the annealing treatment that is carried out
for the different materials are described below.
Figure 1 shows possible process paths 1, 2 for an
annealing treatment of strip material before further
processing. In process path 1, after the hot rolling 10 and
the controlled cooling 20, an annealing treatment 30 is
carried out, and then the strip material is sent for further
processing 80 into the finished product. The annealing
treatment 30 can be carried out online, and a suitable
continuous furnace is to be installed in the existing process
line for this purpose.
In the process path 2 indicated in Figure 1, for example,
strip galvanizing 40 of the hot-rolled strip is carried out,
so that a continuous annealing treatment 30 can be carried out
online before that in the heating stage of the galvanizing
line. The strip galvanizing operation 40 is then followed by
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further processing 80 into the finished product.
Figure 2 shows possible process paths 1, 2, 3 for an
annealing treatment of wire material. In the illustrated
process path 1, the hot rolling 10 and then the controlled
cooling 20 are followed by the annealing treatment 30, which,
as in the case of the strip material, can be carried out
online. The annealing treatment 30 is followed directly by
the step of further processing 80 into the finished product.
In the case of process path 2, the annealing treatment
30, which here too can be carried out online, is followed by
another processing step, namely, the pressing 50 of connecting
elements, before the wire material is sent for further
processing 80 into the finished product.
Alternatively, this pressing 50 of connecting elements
can be carried out before the annealing treatment 30, as
indicated by process path 3. This then results in the
following succession of process steps: hot rolling 10,
controlled cooling 20, pressing 50 of connecting elements,
annealing treatment 30 and finally the further processing 80
into the finished product.
Figure 3 shows possible process paths 1, 2, 3 for an
annealing treatment of components, such that for all three
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process paths, after the controlled cooling 20, an additional
process step involving the production 60 of a blank is carried
out first.
In process path 1, which involves the production of
components with adjusted mechanical properties, the production
60 of the blank is followed by the pressing 70 of the
components. The entire component is then subjected to an
annealing treatment 30 and then to the further processing 80
into the finished product.
In process path 2, which involves the production of
components with prior local annealing treatment of the blank,
the production 60 of the blank is followed by a zonal
annealing treatment 35, so that the pressing 70 of the
components must be carried out on a blank that has already
received a local heat treatment and thus on a blank that has
locally altered mechanical properties.
Alternatively to process path 2, in process path 3, the
components are produced with subsequent local alteration of
the mechanical properties by a zonal annealing treatment 35 of
the component after it has already been pressed, so that the
pressing 70 of the components can be advantageously carried
out on the still untreated blank. After this zonal annealing
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treatment 35, the component, which has thus undergone local
alteration of its mechanical strength, can then be sent for
further processing 80 into the finished product.
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List of Reference Numbers
1,2,3 process path
hot rolling
controlled cooling
annealing treatment of the entire workpiece
zonal annealing treatment
strip galvanizing
pressing of connecting elements
production of the blank
pressing of the components
further processing into the finished product
