U.S. patent application number 11/992856 was filed with the patent office on 2009-06-18 for method and device adjusting targeted combinations of properties of polyphase steel.
Invention is credited to Christian Bilgen, Wolfgang Henning, Ingo Schuster.
Application Number | 20090151821 11/992856 |
Document ID | / |
Family ID | 37908011 |
Filed Date | 2009-06-18 |
United States Patent
Application |
20090151821 |
Kind Code |
A1 |
Bilgen; Christian ; et
al. |
June 18, 2009 |
Method and Device Adjusting Targeted Combinations of Properties of
Polyphase Steel
Abstract
Compared to conventional steel products, polyphase steels have a
significantly improved combination of resistance and ductility and
are therefore becoming more and more important--especially for the
automobile industry. The currently most important steel groups for
the automobile industry are dual phase steels and TRIP steels. The
production of different polyphase steel resistance categories,
carried out directly on a hot strip, for meeting various
requirements, requires a highly extensive know-how and firstly a
corresponding adaptation of the alloy elements. According to the
invention, a heat treatment (30) with a variable heating
temperature and heating duration is carried out following the
actual production of polyphase steels with a standard analysis and
a standard process execution, whereby almost any combination of
different materials or combination of properties (height of yield
stress, level of tensile strength) can be adjusted.
Inventors: |
Bilgen; Christian;
(Dusseldorf, DE) ; Henning; Wolfgang; (Neuss,
DE) ; Schuster; Ingo; (Willich, DE) |
Correspondence
Address: |
FRIEDRICH KUEFFNER
317 MADISON AVENUE, SUITE 910
NEW YORK
NY
10017
US
|
Family ID: |
37908011 |
Appl. No.: |
11/992856 |
Filed: |
December 11, 2006 |
PCT Filed: |
December 11, 2006 |
PCT NO: |
PCT/EP2006/011909 |
371 Date: |
March 29, 2008 |
Current U.S.
Class: |
148/534 ;
148/579; 266/160 |
Current CPC
Class: |
C21D 7/13 20130101; C21D
8/0463 20130101; C21D 2211/002 20130101; C21D 8/0263 20130101; C21D
6/00 20130101; C21D 2211/008 20130101; C21D 1/26 20130101; C21D
1/25 20130101 |
Class at
Publication: |
148/534 ;
148/579; 266/160 |
International
Class: |
C21D 8/02 20060101
C21D008/02 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 10, 2006 |
DE |
10 2006 001 198.8 |
Claims
1. A method for adjusting targeted combinations of properties of
hot-rolled multiphase steels, whose multiphase structure includes
at least 30% ferrite and at most 50% 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, wherein following the cooling from the hot rolling (10)
or a later production step, for example, the production of
components, the desired combinations of strengths and elastic
limit-tensile strength ratios are adjusted in the multiphase steels
by a subsequent or intermediate annealing treatment (30, 35) with
variable annealing temperature and variable annealing time.
2. A method in accordance with claim 1, wherein the annealing
treatment (30, 35) is carried out in such a way that the resulting
microstructure consists of a ferritic base matrix and annealed
martensite or bainite with 10-50% of the area fraction, such that
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 such that the
level of tensile strength can be adjusted by the annealing
time.
3. A method in accordance with claim 1, wherein the annealing
treatment (30, 35) is carried out at an annealing temperature of
.ltoreq.600.degree. C. and an annealing time of .ltoreq.120 s.
4. A method in accordance with claim 1, wherein the annealing
treatment (30, 35) is carried out offline in a continuous annealing
installation.
5. A method in accordance with claim 1, wherein the annealing
treatment (30) is carried out online as part of a strip galvanizing
operation (40) in the heating stage of a galvanizing line before
entry into the zinc bath.
6. A method in accordance with claim 1, wherein the annealing
treatment (30, 35) is carried out on components that have already
been finish pressed.
7. A method in accordance with claim 1, wherein the annealing
treatment (35) is carried out as a targeted zonal treatment, i.e.,
in locally limited sections of a component.
8. A device for adjusting targeted combinations of properties of
hot-rolled multiphase steels, whose multiphase structure includes
at least 30% ferrite and at most 50% 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, especially for carrying out the method according to
claim 1, wherein a thermal installation is located in a freely
selectable place within the production plant or production line and
that an annealing treatment (30, 35) can be carried out in this
thermal installation at a variable annealing temperature of
.ltoreq.600.degree. C. and a variable annealing time of .ltoreq.120
s.
9. A device in accordance with claim 8, wherein the thermal
installation is a continuous furnace installed online in a
galvanizing line.
10. A device in accordance with claim 8, wherein the thermal
installation is a continuous annealing installation that is
operated offline.
11. A device in accordance with claim 8, wherein the thermal
installation is constructed in such a way that a zonal annealing
treatment (35) can be carried out on locally limited sections of a
component before or after its actual production as a finished
product.
Description
[0001] 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 50% 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.
[0002] 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.
[0003] In this regard, due to the significantly lower production
costs, the variant of production directly as hot strip offers
economic advantages and thus has very strong potential for the
future.
[0004] 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 are 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.
[0005] 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.
[0006] 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 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.
[0007] 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,
which makes them suitable especially for use in the automobile
industry.
[0008] EP 1 396 549 A1 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.degree. C. for an annealing time of 10-60 minutes in an
annealing furnace, starting from a run-in temperature of
850-1,050.degree. C. After descaling, the thin slabs are then
finish hot rolled in the range of 750-1,000.degree. C. and then
cooled to a coiling temperature of 300-530.degree. 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.
[0009] Finally, EP 1 394 279 B1 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.degree. C., resulting in a microstructure with an
ultrafine, crystalline, granular ferrite structure of 100-300 nm
with iron carbides deposited in the ferrite.
[0010] 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.
[0011] 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 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.
[0012] 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.
[0013] In accordance with the invention, the annealing treatment is
carried out at a variable annealing temperature of
.ltoreq.600.degree. C. and a likewise variable annealing time of
.ltoreq.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 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.
[0014] 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.
[0015] 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 components to values that otherwise would be
difficult to preset, since then the pressing force of the shaping
machines would not be sufficient.
[0016] 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.
[0017] 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
.ltoreq.600.degree. C. and an annealing time of .ltoreq.120 s. This
thermal installation can be a continuous annealing 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.
[0018] 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.degree. C. carried out on a
dual-phase steel produced as hot-rolled strip with a tensile
strength of 980-1,035 N/mm.sup.2, 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:
TABLE-US-00001 R.sub.p0.2 R.sub.m A Annealing Time (s) (MPa) (MPa)
R.sub.p0.2/R.sub.m (%) 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
[0019] These relationships presented for the example of the
dual-phase steel apply in the same way for TRIP steels as well.
[0020] 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.
[0021] FIG. 1 shows a flowchart of the annealing treatment of strip
material.
[0022] FIG. 2 shows a flowchart of the annealing treatment of wire
material.
[0023] FIG. 3 shows a flowchart of the annealing treatment of
components.
[0024] In FIGS. 1 to 3, the individual process steps which, in
accordance with the invention, are necessary for the annealing
treatment of strip material (FIG. 1), wire material (FIG. 2), and
components (FIG. 3) are shown in the form of 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.
[0025] FIG. 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.
[0026] In the process path 2 indicated in FIG. 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 further processing 80
into the finished product.
[0027] FIG. 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.
[0028] 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.
[0029] 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.
[0030] FIG. 3 shows possible process paths 1, 2, 3 for an annealing
treatment of components, such that for all three process paths,
after the controlled cooling 20, an additional process step
involving the production 60 of a blank is carried out first.
[0031] 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.
[0032] 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.
[0033] 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 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.
LIST OF REFERENCE NUMBERS
[0034] 1,2,3 process path [0035] 10 hot rolling [0036] 20
controlled cooling [0037] 30 annealing treatment of the entire
workpiece [0038] 35 zonal annealing treatment [0039] 40 strip
galvanizing [0040] 50 pressing of connecting elements [0041] 60
production of the blank [0042] 70 pressing of the components [0043]
80 further processing into the finished product
* * * * *