U.S. patent application number 11/718498 was filed with the patent office on 2009-01-08 for method for producing high strength steel strips or sheets with twip properties, method for producing a component and high-strength steel strip or sheet.
This patent application is currently assigned to THYSSENKRUPP STEEL AG. Invention is credited to Jens-Ulrik Becker, Harald Hofmann, Manfred Menne, Jochen Wans.
Application Number | 20090010793 11/718498 |
Document ID | / |
Family ID | 34959180 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090010793 |
Kind Code |
A1 |
Becker; Jens-Ulrik ; et
al. |
January 8, 2009 |
Method For Producing High Strength Steel Strips or Sheets With Twip
Properties, Method For Producing a Component and High-Strength
Steel Strip or Sheet
Abstract
A method for producing cold-formable, high-strength steel strips
or sheets with TWIP properties, wherein in successive working steps
are carried out without interruption, uses a molten material of the
following composition (mass %): C: 0.003-1.50%, Mn: 18.00-30.00%,
Ni: .ltoreq.10.00%, Si: .ltoreq.8.00%, Al: .ltoreq.10.00%, Cr:
.ltoreq.10.00%, N: .ltoreq.0.60%, Cu: .ltoreq.3.00%, P:
.ltoreq.0.40%, S: .ltoreq.0.15%, selectively one or more components
from the Se, Te, V, Ti, Nb, B, REM, Mo, W, Co, Ca and Mg group
provided that the total content of Se, Te is .ltoreq.0.25%, the
total content of V, Ti, Nb, B, REM is .ltoreq.4.00%, the total
content of Mo, W, Co is .ltoreq.1.50% and the total content of Ca,
Mg is .ltoreq.0.50%, the rest being iron and melting conditioned
impurities, wherein the content of Sn, Sb, Zr, Ta and As, whose
total content is equal to or less than 0.30% is included in said
impurities.
Inventors: |
Becker; Jens-Ulrik;
(Duisburg, DE) ; Hofmann; Harald; (Dortmund,
DE) ; Menne; Manfred; (Bochum, DE) ; Wans;
Jochen; (Meerbusch, DE) |
Correspondence
Address: |
PROSKAUER ROSE LLP
ONE INTERNATIONAL PLACE
BOSTON
MA
02110
US
|
Assignee: |
THYSSENKRUPP STEEL AG
Duisburg
DE
|
Family ID: |
34959180 |
Appl. No.: |
11/718498 |
Filed: |
November 3, 2004 |
PCT Filed: |
November 3, 2004 |
PCT NO: |
PCT/EP2004/012407 |
371 Date: |
February 19, 2008 |
Current U.S.
Class: |
420/38 ; 148/541;
29/527.7; 420/36; 420/41; 420/42; 420/44; 420/45; 420/56; 420/57;
420/58; 420/72; 420/73; 420/74; 420/75; 420/76 |
Current CPC
Class: |
C21D 6/005 20130101;
C21D 8/0205 20130101; Y10T 29/49991 20150115; C21D 8/0215 20130101;
C22C 38/04 20130101 |
Class at
Publication: |
420/38 ;
29/527.7; 148/541; 420/36; 420/41; 420/42; 420/45; 420/44; 420/56;
420/57; 420/58; 420/72; 420/73; 420/74; 420/75; 420/76 |
International
Class: |
C22C 38/44 20060101
C22C038/44; B23P 17/00 20060101 B23P017/00; B22D 11/045 20060101
B22D011/045; C22C 38/52 20060101 C22C038/52; C22C 38/58 20060101
C22C038/58; C22C 38/50 20060101 C22C038/50; C21D 8/02 20060101
C21D008/02; C22C 38/42 20060101 C22C038/42 |
Claims
1. A method for producing cold-formable, high-strength steel strips
or sheets with TWIP properties, wherein in successive working steps
carried out without interruption a molten material of the following
composition (mass %): C: 0.003-1.50%, Mn: 18.00-30.00%, Ni:
.ltoreq.10.00%, Si: .ltoreq.8.00%, Al: .ltoreq.10.00%, Cr:
.ltoreq.10.00%, N: .ltoreq.0.60%, Cu: .ltoreq.3.00%, P:
.ltoreq.0.40%, S: .ltoreq.0.15%, selectively one or more components
from the Se, Te, V, Ti, Nb, B, REM, Mo, W, Co, Ca and Mg group
provided that the total content of Se, Te is .ltoreq.0.25%, the
total content of V, Ti, Nb, B, REM is .ltoreq.4.00%, the total
content of Mo, W, Co is .ltoreq.1.50% and the total content of Ca,
Mg is .ltoreq.0.50%, the rest being iron and melting conditioned
impurities, wherein the content of Sn, Sb, Zr, Ta and As, whose
total content is equal to or less than 0.30% is included in said
impurities, is applied to a conveyor and is cooled thereon, until
it is solidified into a pre-strip, the pre-strip is removed from
the conveyor belt, the removed pre-strip is exposed if required to
heat treatment, the pre-strip is heat-rolled at a hot-rolling
temperature of at least 700.degree. C. into a hot strip with a
completely re-crystallized structure, and the hot strip is wound at
a winding temperature of up to 750.degree. C.
2. The method according to claim 1, wherein the C content of the
molten material is 0.2-0.8 mass %.
3. The method according to claim 1, wherein the Mn content of the
molten material is at least 20 mass %.
4. The method according to claim 1, wherein the total Se and Te
contents of the molten material are at least 0.01 mass %.
5. The method according to claim 1, wherein the total V, Ti, Nb and
REM contents of the molten material are at least 0.01 mass %.
6. The method according to claim 1, wherein the B content of the
molten material is at least 0.001 mass %.
7. The method according to claim 1, wherein the total Mo, W and Co
contents are at least 0.01 mass %.
8. The method according to claim 1, wherein the total Ca and Mg
contents are at least 0.001 mass %.
9. The method according to claim 1, wherein the pre-strip is cooled
down during the heat treatment carried out if required.
10. The method according to claim 1, wherein the pre-strip is
heated up during the heat treatment, carried out if required, to a
hot-rolling start temperature.
11. The method according to claim 1, wherein the thickness of the
hot strip obtained is .ltoreq.3 mm.
12. The method according to claim 1, wherein the winding
temperature is at least 450.degree. C.
13. The method according to claim 1, wherein the hot strip is
cold-rolled after winding.
14. The method according to claim 13, wherein the thickness of the
cold strip obtained is .ltoreq.0.8 mm.
15. The method according to claim 13, wherein the cold strip is
subjected to annealing at an annealing temperature of 60020
C.-1,100.degree. C.
16. The method for producing a component, wherein by using the
method in accordance with claim 1, a hot or cold strip is produced
wherein a pre-product is produced from the hot or cold strip
obtained and wherein the pre-product is afterwards finally
cold-formed into the component.
17. The method according to claim 16, wherein the cold-forming of
the blank is carried out by flow-turning.
18. A steel strip or sheet with TWIP properties, produced by the
method in accordance with claim 1 with a brittle/ductile transition
temperature T.sub.ue of .ltoreq.-40.degree. C.
19. The steel strip or sheet according to claim 18, wherein its
average r-value r.sub.m is 1.0+/-0.15 and its .DELTA.r value is
-0.20 to +0.20.
Description
[0001] The invention relates to a method for producing
high-strength, cold-formable steel strip or sheet comprising TWIP
properties from an Fe--C--Mn lightweight structural steel, a method
for manufacturing components as well as a high-strength steel strip
or sheet, which comprises TWIP properties.
[0002] So-called "Hadfield steels", which apart from iron contain,
as the main alloying elements, 11-14 mass % Mn and 1.1-1.4 mass %
C, have already been known for a long time. Steels with such a high
manganese content are marked by very high tensile strength and
work-hardening due to the effect of repeated impact or
friction.
[0003] In addition, austenitic steels with even higher Mn-contents
are known, which possess so-called "TWIP" properties
("TWIP"="Twinning Induced Plasticity"). The steels concerned
together with low weight and good strength possess high ductility
when mechanically loaded in consequence of a twinning formation of
the grains of the structure arising in the course of mechanical
loading. This twinning formation directly facilitates the
deformation of the steel. The twinning also contributes thereto
because it limits the mobility of dislocations, in order to
increase the flow stress of the steel in the event of mechanical
loading. The ductility of TWIP steel is possibly additionally
assisted by a martensitic .gamma./.alpha. transformation generally
accompanying the twinning formation.
[0004] A method for producing steel strips from Fe--C--Mn alloys of
the type described above is known from EP 1 067 203 B1. In
accordance with the known method a molten material, which contains
0.001-1.6 mass % C, 6-30 mass % Mn, up to 10 mass % Ni, wherein the
total content of Mn and Ni is 16 mass % up to 30 mass %, up to 2.5
mass % Si, up to 6 mass % Al, up to 10 mass % Cr, as well as P, Sn,
Sb and As, provided that the total content of these elements is
maximum 0.2 mass %, S, Se and Te provided that the total of these
elements is maximum 0.5 mass %, V, Ti, Nb, Zr and rare earth metals
(REM) provided that the total of these elements is maximum 3 mass
%, Mo and W provided that the total of these elements is limited to
maximum 0.5 mass %, the rest being iron and melting conditioned
unavoidable impurities, is cast in a conventional twin-roll strip
casting machine into a thin strip of 1.5 mm to 10 mm in thickness.
The thin strip obtained in this way is then directly, or possibly
after intermediate hot-rolling with subsequent winding, cold-rolled
to 10%-90% reduction in one or more stages into cold strip and
afterwards subjected to re-crystallization annealing.
[0005] Apart from the use of twin-roll casting machines, in
technical parlance also called "Double Roller" or "Twin Roller",
cast strip can also be produced by the so-called "Direct Strip
Casting" process, for which the abbreviation "DSC process" is
normally used. With this method the molten material to be cast is
poured from the foundry ladle, into a dispensing vessel, by which
it is applied to a continuously revolving conveyor belt. Within the
area of the conveyor belt the molten material is cooled
intensively, so that it is solidified into a hard pre-strip on
reaching the end of the conveyor belt. Subsequently, the pre-strip
normally passes through a secondary cooling stage before it is
heat-rolled likewise without interruption immediately after this
cooling stage. Heat-rolling can take place in one or more rolling
stands. After heat-rolling further controlled cooling takes place,
before the finished hot strip is wound into a coil.
[0006] A possibility of producing steel strips from Fe--Mn--Al--Si
alloys using the DSC process is described in the essay "DEFORMATION
AND MECHANICAL PROPERTIES OF HIGH MANGANESE TRIP ALLOYS" by Renata
Vi{hacek over (s)}{hacek over (c)}orova et al. published in
Proceedings at IDDRG International Deep Drawing Research Group 2004
Conference, 24-26 May 2004, Sindelfingen, Verlag Stahleisen GmbH,
2004, ISBN 3-514 00708-X, pages 261-269. Apart from a general
reference to the possibility of producing TWIP steel using the DSC
process, in this publication as a specific example of an
Fe--Mn--Al--Si-alloy cast in this way there is a steel possessing
TRIP properties, which apart from iron and melting conditioned
impurities comprises (in mass %) 16.2% Mn, 2.36% Al, 2.47% Si,
0.084% C, 0.007% S and 0.0093% N.
[0007] Dependent on its composition TRIP steels
("TRIP"="Transformation Induced Plasticity") have particularly high
strength with a degree of elongation comparable to conventional
two-phase steels or a high stretch capability with a strength
comparable to the conventional two-phase steels. In contrast TWIP
steel has a more balanced combination of properties with optimum
transformation behaviour during the shaping of the component and in
the event of sudden mechanical stress.
[0008] But all variants of known metal sheet produced from
lightweight structural steel of this type, although they possess
high strength, have specific characteristic disadvantages. Thus,
for example, wide ranging of the brittle-ductile transition
temperature, heavy dependence of the properties on temperature or
more anisotropic deformation behaviour occur.
[0009] In addition, steels with a high Mn-content can only be hot
and cold-rolled with difficulty due to their intrinsic high
strength. This is shown to be particularly critical in the case of
the high-strength TWIP steels of the type discussed here. Thus with
such steels instabilities or tears frequently appear at the edges
of the strip, which in practice make large-scale production and
processing of strip or sheet from such steels difficult. Also due
to the extreme hardness, which steel with Mn-contents of 18 mass %
and more possesses even in the just cast condition before
heat-rolling, large capital investment in production plant is
necessary, in order to produce thin hot strip from such steels,
from which cold strip of narrow thickness can then be produced at
reasonable cost. However, especially in the field of motor vehicle
body construction there is increased demand for such thin
cold-rolled metal sheets, which have low weight with high strength
and good deformation and hardening behaviour in the event of an
accident.
[0010] The object of the invention consisted in creating, on the
basis of the prior art described above, a method for producing
steel strips and sheets having TWIP properties with high manganese
content, which enables products with optimum combination of
properties and equally optimum utility value to be made available
at reduced cost. Furthermore, a method for producing high-strength
components from a steel of the type initially described was to be
indicated. Finally, a steel strip or sheet was also to be created,
which possesses particularly good deformation behaviour.
[0011] With respect to the method for producing cold-formable,
high-strength steel strips or sheets with TWIP properties, this
object was achieved in that the following successive working steps
are carried out according to the invention without interruption:
[0012] a molten material of the following composition (mass %):
[0013] C: 0.003-1.50%, [0014] Mn: 18.00-30.00%, [0015] Ni:
.ltoreq.10.00%, [0016] Si: .ltoreq.8.00%, [0017] Al:
.ltoreq.10.00%, [0018] Cr: .ltoreq.10.00%, [0019] N: .ltoreq.0.60%,
[0020] Cu: .ltoreq.3.00%, [0021] P: .ltoreq.0.40%, [0022] S:
.ltoreq.0.15%, selectively one or more elements from the Se, Te, V,
Ti, Nb, B, REM, Mo, W, Co, Ca. and Mg group, provided that the
total content of Se, Te is .ltoreq.0.25%, the total content of V,
Ti, Nb, B, REM is .ltoreq.4.00% the total content of Mo, W, Co is
.ltoreq.1.50% and the total content of Ca, Mg is .ltoreq.0.50%, the
rest being iron and melting conditioned impurities, wherein the
content of Sn, Sb, Zr, Ta and As, whose total content is equal to
or less than 0.30%, is included in said impurities, is applied to a
conveyor belt and is cooled thereon, until it is solidified into a
pre-strip, [0023] this pre-strip is removed from the conveyor belt,
[0024] the removed pre-strip is exposed if required to heat
treatment, [0025] the pre-strip is heat-rolled at a final
hot-rolling temperature of at least 700.degree. C. into a hot strip
with a completely re-crystallized structure, and [0026] the hot
strip is wound at a winding temperature of up to 750.degree. C.
[0027] With regard to the method for producing a high-strength
component the invention achieves the object specified above in
that, by using the method according to the invention, hot or cold
strip is produced, from which a pre-product is then possibly
produced, which afterwards is finally cold-formed into the
component.
[0028] Due to the special way in which it is produced steel strip
or sheet produced by means of the method according to the invention
comprises a unique optimum combination of properties right down to
temperatures which lie far below 0.degree. C. Accordingly, steel
strip or sheet produced according to the invention is characterized
in that its brittle/ductility transition temperature T.sub.ue lies
under -40.degree. C. The transition temperature T.sub.ue concerned
is normally determined with the cupping test or notched bar impact
test.
[0029] Thus, it can be ensured when using steel strips or sheets
according to the invention, for example when producing motor
vehicle body panels or comparable applications that the superior
deformation capacity of these steel strips and sheets is constant
over the entire temperature range in which such applications are
normally used.
[0030] The invention is based on the realization that steels with
an Mn-content of 18 mass % and above can be processed using the
presently known DSC process in a particularly advantageous way, if
at the same time the final hot-rolling temperature and winding
temperature are adjusted in a way according to the invention. Due
to the fact that the hot-rolling temperature is at least
700.degree. C., typically at least 850.degree. C., a completely
re-crystallized hot strip is obtained after hot-rolling, which is
extremely suitable for subsequent cold-forming. Because the winding
temperature of maximum 750.degree. C., typically maximum
550.degree. C. is also selected, so that grain boundary oxidation
of the finished hot strip is avoided as far as possible, surface
defects only appear to a minimum extent on the hot strip obtained
after winding. Therefore, hot strip produced according to the
invention or cold strip made therefrom can be protected
particularly satisfactorily with metal coatings, in order to
improve its corrosion resistance for example.
[0031] A particular advantage of the method according to the
invention is that during the hot phase of the production process
used according to the invention, the strip does not need to be
diverted from a vertical to a horizontal direction. Instead the
pre-strip cast from the molten material according to the invention,
both during its solidification on the conveyor belt and during
subsequent hot-rolling, as well as heat treatment preceding
hot-rolling if required, runs exclusively in a horizontally-aligned
direction with the consequence that any critical bending of the
strip can be avoided in the hot phase of the production process.
This makes it possible to produce steel strip from particularly
heat resistant steel materials without problems occurring due to
the still poor transformation capacity of these materials. In
contrast to casting strip with the known strip casting machines,
therefore, the risk of having to abort the casting operation, for
example, due to the breaking of only insufficiently ductile cast
strip does not exist when using the DSC process according to the
invention.
[0032] A further advantage of the method according to the invention
consists in that pre-strip can be cast in a thickness, which is far
greater than that attainable with conventional strip casting. Thus,
pre-strip, whose thickness is typically more than 10 mm, in
particular more than 12 mm, can be produced without difficulty with
the method according to the invention. Pre-strip of this kind of
more than 15 mm or more than 20 mm in thickness, for example, is
formed during subsequent hot-rolling using high strain degrees into
a thin hot strip, which is typically less than 3 mm, in particular
less than 2 mm in thickness.
[0033] The heavy deformation during hot-rolling leads to the fact
that, in contrast to conventional strip casting by means of a
twin-roll casting machine, the original casting structure of the
pre-strip is, as far as possible, completely eliminated and a hot
strip structure is produced which, due to its particularly
homogeneous, completely re-crystallized structure and due to the
most extensive elimination of cavities, is marked by particularly
good ductility. Accordingly, hot-forming of the cast pre-strip is
preferably carried out using the method according to the invention
so that high degrees of deformation of preferably more than 60%, in
particular up to 95%, are attained. In this way for example hot
strips of 1 mm in thickness, which at low cost can afterwards be
cold-rolled into cold strips directly suitable for use in motor
vehicle body construction, can be produced from pre-strip of large
thickness, despite the fact that the steel alloys processed
according to the invention as standard possess high heat
resistance.
[0034] A further substantial advantage of the method according to
the invention lies in the fact that it is substantially more
tolerant in the processed molten material in relation to the
presence of alloying elements, which are problematic in the
conventional process. Thus, such molten materials, which apart from
considerable contents of phosphorus, sulphur and copper can have
impurities in the form of relatively high contents of Sn, Sb, Zr,
TA and As in total of up to 0.30 mass %, can also be cast with high
success. This enables higher contents in accompanying elements to
be tolerated without the possibility of producing a correspondingly
alloyed steel strip according to the invention being impaired as a
result.
[0035] The invention thus allows economical production of molten
material using the electric-arc furnace route employing cheaper
inferior scrap iron. It is therefore possible to move way from
using blast furnaces responsible for high CO.sub.2-emissions.
[0036] Processing, possible through the invention, of molten
materials whose composition can be varied to high tolerances,
renders the possibility of using non-optimum alloying materials
with corresponding impurities and thus additionally reduces the
costs for alloying materials. The high cost of blast furnace coke
can be avoided.
[0037] The segregation profile, problematic with conventional
vertical continuous casting, is substantially reduced when
processing, according to the invention, steel of the type under
discussion. Also, irregular casting structures, which arise with
conventional continuous casting is homogenized using a method
according to the invention.
[0038] The strength and ductility of the finished steel strip or
sheet are higher with the production method according to the
invention than in cases, where a comparable alloy is processed by
conventional continuous casting.
[0039] Finally, the method according to the invention can be used
on production lines, which require a substantially lower capital
investment than conventional continuous casting plant. Accordingly,
capital outlay is less than for a conventional continuous casting
wide hot strip plant. Also, the method according to the invention
enables the width to be adjusted coil by coil. The output
attainable with a production line operating according to the
invention is comparable with conventional continuous casting
plants. The C-content of the alloy processed according to the
invention can be 0.003 mass % to 1.6 mass %. Preferably, this lies
in the range of 0.2 mass % to 0.8 mass %. If the C-content is at
least 0.2 mass % the risk of carbon depletion in the molten
material is minimized. Carbon content of more than 0.8 mass % can
make it more difficult to optimise the content of other alloying
elements with regard to achieving advantageous mechanical
properties.
[0040] The preferably selected carbon content of 0.2-0.8% ensures
the improved possibility of producing steel sheet and strip
according to the invention. Tears and instabilities in the strip
edge region are substantially reduced, the instabilities in
particular becoming less with increasing carbon content.
[0041] Additionally, the carbon content proposed according to the
invention opens up a wide spectrum of hot-rolling parameters. Thus,
it has been found that the characteristic values of steels
according to the invention obtained when selecting high final
hot-rolling temperatures and winding temperatures are substantially
the same as those which are obtained at low final hot-rolling
temperatures and winding temperatures. Also, this insensitivity
favours the simple and sure feasibility of the method according to
the invention.
[0042] The manganese content of the alloy processed according to
the invention is at least 18 mass %, in particular at least 20 mass
%. Steels possessing such high Mn-content of the type processed
according to the invention reliably have TWIP properties.
[0043] Since the total content of Mn and Ni in the case of the
steel under discussion should not exceed 30 mass %, the nickel
content is limited up to 10 mass %.
[0044] The silicon content of a molten material processed according
to the invention can be up to 8 mass %, this element being added if
especially lightweight steel is required. Furthermore, a higher Si
content can be used, in order to substitute correspondingly reduced
C and Mn contents while still maintaining the TWIP properties.
[0045] For the same purpose aluminium in amounts of up to 10 mass %
can be optionally added to the molten material processed according
to the invention.
[0046] Chrome can be added to the steel processed according to the
invention in order to improve corrosion resistance. A limitation of
the Cr content to maximum 10 mass % is expedient with regard to
cost criteria, since above this limit only small characteristic
improvements are to be observed.
[0047] Surprisingly, it has been shown that the presence of
selenium and tellurium results in improvement of the wetting
behaviour when the composition is applied to the conveyor belt, on
which the molten material is afterwards solidified into the
pre-strip. An advantageous embodiment of the invention accordingly
proposes that the total Te and Se contents in the molten material
are at least 0.01 mass %.
[0048] V, Ti, Nb and REM amounts can be included, in order to
benefit from the positive effect, known per se, of these
micro-alloying elements with regard to the mechanical properties of
steels of the type processed according to the invention. In
accordance with a further embodiment of the invention it is
therefore proposed that the molten material cast into the pre-strip
contains a total of at least 0.01 mass % of V, Ti, Nb and/or REM.
The property-improving effect (isotropy) of B however already
occurs, if B is present in an amount of at least 0.001 mass %.
[0049] The total content of molybdenum, tungsten and cobalt can be
up to 1.5 mass %, in order to benefit from the known
property-improving effects of these elements. Also, Ca and Mg
amounts in a total of 0.5 mass % can be proposed, if the effects,
likewise known per se, of these elements are to be exploited in the
case of steels of the type processed according to the
invention.
[0050] Nitrogen amounts of up to 0.6 mass % can be added, in order
to exploit the strength-increasing and anti-corrosive effect of
nitrogen in steels of the type under discussion.
[0051] As a result, when using the method according to the
invention and exploiting the possibilities of the alloying concept
according to the invention a particularly well cold-formable
lightweight structural steel strip or sheet is obtained, which is
suitable, in particular due to its comparatively high strength, for
producing motor vehicle body panels. Likewise, steel sheet produced
according to the invention is suitable for producing wheels for
vehicles, in particular motor vehicles, for producing internal high
pressure or external high pressure formed components, for producing
high-strength engine parts, such as cam shafts or piston rods, for
producing components designed to protect against pulse-type
striking pressures, i.e. bombardment, such as armour plate as well
as protective elements, which are intended to protect humans, in
particular against bombardment.
[0052] Steel sheets according to the invention with purely
austenitic structure are also especially suitable for producing
non-magnetic components.
[0053] Moreover, it has been shown that the steel strips or sheets
produced according to the invention maintain their tensile strength
even at particularly low temperatures. So it can be guaranteed, as
mentioned, that transition from the ductile to the brittle
behaviour in the case of steel strip or sheet produced according to
the invention only takes place at a transition temperature of below
-40.degree. C. Accordingly, steel products produced according to
the invention are particularly suitable for fabricating components
used in cryogenic technology such as vessels or pipes for
refrigeration purposes.
[0054] The isotropic deformation behaviour of steel strips and
sheets produced according to the invention is particularly
remarkable. Thus, steel strips and sheets, whose average r-value
r.sub.m is 1.0+/-0.15 and whose .DELTA.r value is -0.2 to 0.2, can
be easily made available by means of the invention.
[0055] Because the hot strip is hot-rolled according to the
invention at a final hot-rolling temperature of at least
700.degree. C., apart from avoiding grain boundary oxidation,
already mentioned, the positive effect of carbon is exploited to
the full. Thus, in the case of strip hot-rolled in this range,
carbon brings about higher tensile strength and yield point values
with still acceptable degrees of elongation. As the final
hot-rolling temperature increases, the tensile strength and yield
strength decrease, while the degrees of elongation rise. As a
result of varying the final hot-rolling temperatures within the
limits specified by the invention, the desired properties of the
yielded steel strip can therefore be influenced in a controlled and
simple manner.
[0056] The heat treatment possibly carried out between the
solidification of the pre-strip on the conveyor belt and
hot-rolling is intended to bring the temperature of the pre-strip
to a level on the basis of which optimum hot-rolling results are
achieved. Accordingly, the heat treatment in the way known per se
may comprise additional controlled cooling, wherein the pre-strip
is brought to a hot-rolling start temperature, which is optimum for
hot-rolling. However, it is just as conceivable to carry out heat
treatment by heating up the pre-strip, whenever the structure of
the pre-strip should be influenced by such heat treatment or a rise
in the temperature of the pre-strip to the optimum hot-rolling
start temperature is necessary.
[0057] Already hot strip produced according to the invention is
marked by good usage properties. If thinner sheets or strips are to
be produced, then the hot strip can be cold-rolled into cold strip
after winding, wherein cold-rolling is advantageously carried out
with a cold-rolling strain degree of 10% to 90%, preferably 30% to
75%.
[0058] Due to the possibility provided by the method according to
the invention of producing thin hot strip whose structure is
completely recrystallized from relatively thick pre-strip using a
high strain degrees, it is easily possible when cold-rolling to
produce cold strip in a thickness of 0.8 mm or less, for example
0.6 mm. Such thickness of metal sheet is demanded especially for
motor vehicle body construction.
[0059] In order to avoid impairment of the surface quality through
scale adhering to the hot strip during cold-rolling, the hot strip
can be pickled before cold-rolling.
[0060] Preferably, the cold strip obtained after one stage or
multi-stage cold-rolling can be subjected to annealing, wherein the
annealing temperatures should lie between 600.degree. C. and
1,100.degree. C. Annealing can take place in a stationary furnace
within the temperature range of 600.degree. C. to 750.degree. C. or
on the run at temperatures of 700.degree. C. to 1,100.degree.
C.
[0061] If scale forms during annealing, then in order to improve
the surface quality of the final cold strip it may be expedient to
also subject the annealed hot strip to acidic pickling. This
applies in particular if the cold strip unfinished in order to
achieve optimum surface quality and dimensional precision as well
as optimum mechanical properties.
[0062] A first advantageous use of steel strips or sheets produced
according to the invention lies in producing cold-formed components
by flow-turning pressing. To this end blanks are made from the
steel, which are then formed by flow-turning. Due to its special
characteristic profile steel strip or sheet produced according to
the invention or sheet metal blanks made therefrom are especially
suitable for this purpose.
[0063] Good ductile steel with higher strengths of the type
produced according to the invention can be used for manufacturing
components, which are equipped with toothing or comparable shaped
elements. These components are typically transmission parts
equipped with internal or external toothing. These can be produced
economically and with high dimensional precision by flow-turning. A
method for manufacturing transmission parts by flow-turning is
known from DE 197 24 661. In accordance with this known method a
blank is formed from a metal sheet made of a micro-alloyed
high-strength structural steel, which possesses a lower yield point
of at least 500 N/mm.sup.2. This blank is then cold-formed into
gearing by flow-turning. While the toothing is being produced, the
metal sheet is formed to the limit of its transforming capacity.
Finally, a surface of the work-piece equipped with toothing is
hardened substantially while maintaining the temperature and
causing no thermal warping.
[0064] Depending on the composition a purely austenitic or a
structure consisting of a mixture of ferrite and austenite with
percentages of martensite can be obtained in steel strip or sheet
produced according to the invention. The steels according to the
invention can therefore be transformed substantially better. In the
course of cold-forming they solidify substantially more strongly
than high-strength micro-alloyed or multi-phase steels used, as is
known, for producing components by flow-turning. Thus, component
strengths in the range of 1,400 N/mm.sup.2 to 2,200 N/mm.sup.2 can
be obtained in every case after cold-forming. Additional hardening
of the components being produced can be dispensed with therefore
after the cold-forming.
[0065] When using steel composed and produced according to the
invention heat treatment or surface hardening of the component by
flow-turning is therefore no longer necessary. The risk of warping
and scale-formation, caused by these additional process stages in
the case of the prior art, does not exist with production according
to the invention. This is positively noticeable, particularly in
the production of toothed components subjected to locally heavy
stress in service. Thus, the steel according to the invention
facilitates the economic production of light-weight high stressable
and dimensionally-precise components by cold-forming, in particular
flow-turning.
[0066] As a result the method according to the invention
facilitates the economic production of light-weight, highly
stressable steel strips and sheets, which form the base product for
the possible production, requiring low capital investment, of
dimensionally-precise components by cold-forming.
[0067] Also, all variants of steel sheet according to the invention
are especially suitable for producing vehicle body components,
particularly the external panels of a motor vehicle body or
load-bearing components for vehicle bodies, wheels for vehicles, in
particular motor vehicles, non-magnetic components, vessels, used
in cryogenic technology, internal high pressure or external high
pressure-formed components, tubes which are designed particularly
for producing high-strength engine parts, such as cam shafts or
piston rods, components designed for protecting against pulse-type
striking pressures, such as bombardment, or protective elements,
such as armour plate, or body armour for the human or animal
body.
[0068] Likewise highly stressable gear components, which are
characterized by minimum weight and good performance properties,
can be made of steel sheet according to the invention without
additional heat treatment needed for this purpose.
[0069] The invention is described in detail below on the basis of
exemplary embodiments.
[0070] Table 1 shows the composition of steels A, B, C, D, E and
V1, of which steels A-E belong to the steels processed in a way
according to the invention, while steel V1 is indicated for
comparison purposes only.
TABLE-US-00001 TABLE 1 C Mn Al Si B Steel [mass %] [mass %] [mass
%] [mass %] [mass %] A 0.5 20 3 3 0.003 B 0.6 20 -- -- -- C 0.4 30
8 -- -- D 0.05 20 3 3 -- E 0.05 20 3 3 0.003 V1 0.8 15 -- -- -- The
rest being iron and steel production impurities
[0071] The steels are molten in each case and cast into pre-strip
using the DSC process. In this case the molten material was poured
by means of a dispensing spout onto a revolving, heavily cooled
conveyor belt, on which it has been intensively cooled down
additionally by liquid cooling working from above. The molten
material being solidified in such a way on the conveyor belt into
the pre-strip was then removed from the conveyor belt and again
subjected to secondary cooling in the directly adjoining stage.
[0072] The steel strips emerging from the secondary cooling, still
possessing a sufficiently high temperature, were then again
hot-rolled directly afterwards while exploiting the heat retained
therein to a thickness of 2 mm, wherein the hot-rolling temperature
was 900.degree. C.
[0073] The hot strip obtained in this way was then wound at a
winding temperature of 500.degree. C. into a coil.
[0074] Winding was followed by cold-rolling, wherein the hot strip
was formed with a strain degree of approx. 62.5% into cold strip,
which was 0.75 mm in thickness.
[0075] The cold strips were then annealed while running to
recrystallization at temperatures of 950.degree. C.
[0076] The mechanical properties: yield point Re, tensile strength
Rm, extension A80, uniform elongation Ag, n-, r and .DELTA.r values
of the cold strips KA-KE produced in this way from the steels A-E
and the strip KV1 produced from the comparison steel V1 are
indicated in Table 2.
TABLE-US-00002 TABLE 2 Cold Re Rm A80 Ag strip [N/mm.sup.2]
[N/mm.sup.2] [%] [%] n r .DELTA.r Property KA 492 864 59.3 58.0
0.301 0.90 -0.007 TWIP KB 444 1050 64.3 60.1 0.445 0.96 -0.03 TWIP
KC 576 891 32.8 36.4 0.24 0.63 -0.15 TWIP weak KD 384 708 63.4 63.0
0.329 0.96 -0.14 TWIP KE 342 792 65.6 64.8 0.354 0.95 -0.17 TWIP
KV1 512 1107 46.3 42.6 0.441 0.86 0.22 TWIP
[0077] It is shown that the steel strips A-E produced from the
steels A-E in a way according to the invention possess outstanding
cold ductility at the same time with high strengths and high
elongation at rupture. At the same time in each case they comprise
a pronounced isotropic behaviour. As such they are especially
suitable for producing cold-formed components, which are exposed to
high stress in service. The characteristic profile of KC indicated
in Table 2 is worse than that of KV1, which is due to the only weak
TWIP effect. The advantage of KC relative to KV1 lies in the high
density reduction as a result of the high Al content.
[0078] On the contrary the comparison steel V1 comprising TRIP
properties possesses high strengths with comparatively low
characteristic values A80 and AG, which represent a substantially
worse transformation capacity. This substantially worse deformation
behaviour is also evident from the substantially worse r and
.DELTA.r values relative to the steels A-E.
* * * * *