U.S. patent application number 10/344192 was filed with the patent office on 2003-08-07 for highly stable, steel and steel strips or steel sheets cold-formed, method for the production of steel strips and uses of said steel.
Invention is credited to Engl, Bernhard, Heller, Thomas, Hoffmann, Harald, Menne, Manfred, Zimmermann, Werner.
Application Number | 20030145911 10/344192 |
Document ID | / |
Family ID | 7688074 |
Filed Date | 2003-08-07 |
United States Patent
Application |
20030145911 |
Kind Code |
A1 |
Hoffmann, Harald ; et
al. |
August 7, 2003 |
Highly stable, steel and steel strips or steel sheets cold-formed,
method for the production of steel strips and uses of said
steel
Abstract
A steel strip or sheet steel having good cold formability and
high-strength is described, comprising a light steel having (in
weight-percent) C: .ltoreq.1.00%, Mn: 7.00-30.00%, Al: 1.00-10.00%,
Si: >2.50-8.00%, Al+Si: >3.50-12.00%, B: >0.00-<0.01%,
as well as alternately Ni: <8.00%, Cu: <3.00%, N: <0.60%,
Nb: <0.30%, Ti: <0.30%, V: <0.30%, P: <0.01%, with the
remainder iron and unavoidable impurities.
Inventors: |
Hoffmann, Harald; (Dortmund,
DE) ; Engl, Bernhard; (Dortmund, DE) ; Menne,
Manfred; (Bochum, DE) ; Heller, Thomas;
(Duisburg, DE) ; Zimmermann, Werner; (Voerde,
DE) |
Correspondence
Address: |
Charles Guttman
Proskauer Rose
1585 Broadway
New York
NY
10036
US
|
Family ID: |
7688074 |
Appl. No.: |
10/344192 |
Filed: |
February 6, 2003 |
PCT Filed: |
June 13, 2002 |
PCT NO: |
PCT/EP02/06480 |
Current U.S.
Class: |
148/329 ;
148/546; 148/620 |
Current CPC
Class: |
C22C 38/002 20130101;
C22C 38/04 20130101; C21D 8/0205 20130101; C21D 8/0226 20130101;
C22C 38/02 20130101; C21D 8/0273 20130101; C22C 38/06 20130101;
C21D 8/0236 20130101 |
Class at
Publication: |
148/329 ;
148/546; 148/620 |
International
Class: |
C22C 038/04; C22C
038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 13, 2001 |
DE |
10128544.2 |
Claims
1. A light steel, having good cold formability and high strength,
with the following composition (in weight-percent):
5 C: 23 1.00% Mn: 7.00-30.00% Al: 1.00-10.00% Si: >2.50-8.00% Al
+ Si: >3.50-12.00% B: >0.00-<0.01% as well as alternately
Ni: <8.00% Cu: <3.00% N: <0.60% Nb: <0.30% Ti:
<0.30% V: <0.30% P: <0.01%, with the remainder iron and
unavoid- able impurities.
2. The light steel according to claim 1, characterized in that the
carbon content is 0.10-1.00 weight-percent.
3. The light steel according to one of the preceding claims,
characterized in that the Si content is >2.70
weight-percent.
4. The light steel according to one of the preceding claims,
characterized in that the boron content is 0.002 weight-percent to
0.01 weight-percent, in particular 0.003 to 0.008
weight-percent.
5. A steel strip or sheet steel produced from a steel having a
composition according to one of claims 1 to 4.
6. The steel strip or sheet steel according to claim 5,
characterized in that its tensile strength is at least 680 MPa.
7. The steel strip or sheet steel according to claim 5 or 6,
characterized in that the product of its tensile strength and its
elongation is at least 41,000 MPa.
8. The steel strip or sheet steel according to one of claims 4 to
7, characterized in that its yield point is up to 520 MPa.
9. A method for producing a cold formable, high-strength steel
strip or sheet steel, wherein an input stock, such as slabs, thin
slabs, or strip, is cast from a steel having a composition
according to one of claims 1 to 4, the cast input stock is heated
to =1100.degree. C. or used directly at such a temperature, the
preheated input stock is hot rolled to hot strip at a hot rolling
final temperature of at least 800.degree. C., and the finish-rolled
hot strip is coiled at a coiler temperature of 450.degree. C. to
700.degree. C.
10. The method according to claim 9,characterized in that the hot
strip is cold rolled to cold strip after the coiling.
11. The method according to claim 10, characterized in that the
cold strip is subjected to annealing at an annealing temperature of
600.degree. C. to 1100.degree. C.
12. The method according to claim 11, characterized in that the
annealing is performed as hood annealing at an annealing
temperature from 600.degree. C. to 750.degree. C.
13. The method according to claim 11, characterized in that the
annealing is performed as continuous annealing at an annealing
temperature of 750.degree. C. to 1100.degree. C.
14. The method according to one of claims 10 to 13, characterized
in that the cold strip is dressed.
15. The method according to one of claims 9 to 14, characterized in
that the cold rolling is performed at a cold rolling reduction of
30% to 75%.
16. The method according to one of claims 9 to 15, characterized in
that blanks, which are subsequently cold formed into finished
components, are produced from the respective hot or cold strip
obtained.
17. The method according to claim 16, characterized in that the
cold forming is performed as flow forming.
18. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing supporting vehicle body
components.
19. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing externally visible parts of
vehicle bodies.
20. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing wheels for vehicles, in
particular motor vehicles.
21. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing non-magnetic components.
22. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing components used in
cryoengineering.
23. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing internally or externally
hydroformed components.
24. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 7 for producing tubes, which are particularly
intended for the production of high-strength engine parts, such as
camshafts or piston rods.
25. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing components, such as armor
plates, intended for protection against pulsed stresses, such as
bombardment.
26. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing protection components, such as
helmets and body armor, intended for protection of persons against
pulsed stresses, such as bombardment.
27. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing components through flow
forming.
28. A use of a steel or a steel strip or sheet steel according to
one of claims 1 to 8 for producing gear parts.
29. The use according to claim 28, characterized in that the gear
parts are provided with teeth.
30. The use according to claim 28 or 29, characterized in that the
gear parts are produced through flow forming.
Description
[0001] The present invention relates to an Fe--Mn--Al--Si light
steel and steel strip or sheet steel having good cold formability
and high strength. In addition, the present invention relates to a
method of producing strips from such a steel and particularly
suitable uses of such a steel.
[0002] A light steel used for producing car body parts and for
low-temperature use is known from German Patent 197 27 759 C2. It
contains, in addition to Fe, 10% to 30% Mn, 1% to 8% Al, and 1% to
6% Si, the sum of the contents of Al and Si not exceeding 12%. In
this known steel, the carbon content, if any, is in the impurity
range.
[0003] In the light structural steel known from German Patent
Application 199 00 199 A1, in contrast, carbon is provided as an
optional alloy element. The known light steel has >7% to 27% Mn,
>1% to 10% Al, >0.7% to 4% Si, <0.5% C, <10% Cr,
<10% Ni, and <0.3% Cu. Furthermore, the steel may contain N,
V, Nb, Ti and P, the sum of these elements not to exceed 2%.
[0004] The light steel known from European Patent Application 1 067
203 A1 also contains carbon, in a range from 0.001 to 1.6%. In
addition, this steel has, besides Fe, 6-30% Mn, .ltoreq.6% Al,
.ltoreq.2.5% Si, .ltoreq.10% Cr, .ltoreq.10% Ni, and .ltoreq.5% Cu.
In addition, the steel may contain V, Ti, Nb, B, Zr, and rare
earths, the sum of their contents not exceeding 3%. The known steel
may also contain P, Sn, Sb, and As, the sum of the contents of
these elements not to be greater than 0.2%.
[0005] It has been shown that steels having these types of
compositions may only be hot and cold rolled with difficulties, in
spite of the presence of carbon. Thus, instabilities or cracks are
frequently seen at the strip edges, which makes the large-scale
production of strips or sheets from such steels difficult in
practice. Furthermore, the steels have very strong isotropic
deformation behavior, which is expressed in a high .DELTA.r value.
The further processing of the sheet steels produced according to
the known method is also made more difficult due to the poor
formability.
[0006] Well-formable steels having higher strengths are also
necessary for the manufacture of components which are provided with
teeth or comparable shaped elements. These components are typically
gear parts provided with inner or outer teeth. These may be
produced cost effectively and with high dimensional accuracy
through flow forming.
[0007] A method for producing gear parts through flow forming is
known from German Patent 197 24 661 C2. According to this known
method, a blank is produced from a microalloyed, high-strength
structural steel, which has a lower yield point of at least 500
N/mm.sup.2, from a sheet. This blank is then cold formed into the
gear through flow forming. In the course of forming in the teeth,
the sheet material is reformed up to the limit of its forming
ability. Subsequently, a surface of the workpiece provided with the
teeth is hardened without heat deformation while essentially
maintaining the temperature.
[0008] The object of the present invention is, proceeding from the
related art described above, to provide a light steel and/or a
steel strip or sheet steel produced therefrom having good
formability and good strength which may also be easily produced at
an industrial scale. In addition, a method of producing a steel
strip or sheet steel and preferred uses for the steel are to be
indicated.
[0009] This object is achieved by a light steel which has the
following composition (in weight-percent):
1 C: .ltoreq.1.00% Mn: 7.00-30.00% Al: 1.00-10.00% Si:
>2.50-8.00% Al + Si: >3.50-12.00% B: >0.00-<0.01% as
well as alternately Ni: <8.00% Cu: <3.00% N: <0.60% Nb:
<0.30% Ti: <0.30% V: <0.30% P: <0.01%
[0010] The remainder is Fe and unavoidable impurities. The
impurities include sulfur and oxygen in this case.
[0011] Surprisingly, it has been shown that the targeted addition
of boron leads to significant improvement of the properties and
producibility in steels according to the present invention. Thus,
the content of boron contained in the steel according to the
present invention causes reduction of the yield point, through
which the formability is significantly improved. The favorable
influence of the alloy on the mechanical-technological properties
of steel according to the present invention may be reinforced even
further if the carbon content is 0.10-1.00 weight-percent, i.e., if
at least 0.10 weight-percent carbon is detectable in the steel
according to the present invention.
[0012] In this case, the presence of these elements results in a
particularly good combination of mechanical and technological
properties. Thus, steel according to the present invention and/or
steel strip or sheet steel produced therefrom has a significantly
lower .DELTA.r value than sheet steel known from the related art of
the species discussed here.
[0013] In addition, cold-rolled steel strips and steel sheets
having the composition according to the present invention are
distinguished by comparatively low yield points, improved stretch
formability at elevated hardening exponents (n value), elevated
deep drawing quality (r value), and lower planar anisotropy
(.DELTA.r value) as well as an elevated product of yield point and
elongation. Thus, the tensile strength of steel strips and steel
sheets according to the present invention is at least 680 MPa. The
product of tensile strength and elongation is at least 41,000 MPa.
The yield point of steel sheets and strips according to the present
invention does not exceed 520 MPa. Simultaneously, steels according
to the present invention, and/or sheets and strips produced
therefrom, have an extraordinarily high uniform elongation of 20%
up to more than 45%. n values of up to 0.7 are achieved.
[0014] As a result, in this way an especially good cold formable
light steel strip or sheet steel is obtained which, due to its
comparatively high strength and low density, is particularly
suitable for the production of components for automobile bodies.
The outstanding ratio of strength and weight also makes sheet steel
produced according to the present invention suitable for the
production of wheels for vehicles, in particular motor vehicles,
for the production of internally or externally hydroformed
components, for the production of high-strength engine parts, such
as camshafts or piston rods, for the production of components, such
as armor plates, intended for protecting against pulsed stresses,
such as bombardment, and for protective elements which are intended
in particular for protecting people against bombardment. In
particular in the case of the latter application, the comparatively
low weight of the sheet steel according to the present invention
and simultaneously its high strength has a positive effect.
[0015] Sheet steels according to the present invention are
additionally especially suitable for producing non-magnetic
components if they have a purely austenitic microstructure.
[0016] Furthermore, it has been shown that the steels according to
the present invention maintain their strength even at especially
low temperatures. As such, they are particularly suitable for
producing components used in cryoengineering, such as containers or
pipes for cryoengineering.
[0017] The positive effects of boron in the steel used according to
the present invention may be achieved especially reliably if the
boron content is 0.002 weight-percent to 0.01 weight-percent,
particularly 0.003 to 0.008 weight-percent.
[0018] The C content, which lies in the range from 0.1% to 1.0%,
also ensures improved producibility of sheet steel and steel strip
according to the present invention. In steels according to the
present invention, the formation of intermetallic phases is
suppressed due to the presence of carbon. Cracks and instabilities
in the strip edge region, which arise in steel strips produced from
the known steels, are thus significantly reduced, the instabilities
especially being reduced with increasing C content. A further
improvement of the strip edge quality is achieved by adding boron.
As a result, strip edge instabilities may be almost completely
avoided through the combined addition of C and B.
[0019] Boron acts as a substitute for alloy element Mn in its
effect on the mechanical-technological properties. Thus, it has
been established that a steel having 20% Mn and 0.003% boron has a
similar property profile to a steel which contains 25% Mn, but no
B. Therefore, light structural steels according to the present
invention may have relatively low Mn contents and still have
relatively high strengths. This leads to reduced alloying element
costs and makes production of a light steel used according to the
present invention in smelting metallurgy easier.
[0020] In addition, the contents of C and B provided according to
the present invention open up a wide spectrum of hot rolling
parameters. Thus, it has been established that the characteristics
of steels according to the present invention obtained if high hot
rolling final temperatures and coiler temperatures are selected are
essentially identical to those which are obtained at low hot
rolling final temperatures and coiler temperatures. This
insensitivity during hot strip production also favors easy
producibility of sheet steels according to the present
invention.
[0021] Due to their Si contents, which are restricted to above 2.50
weight-percent, preferably above 2.70 weight-percent, steel strips
and steel sheets according to the present invention have improved
cold formability in comparison to those light steel strips or
sheets which have lower Si contents. The high content of Si is
expressed in more uniform yield point and tensile strength values
and in higher fracture elongation and uniform elongation values. In
addition, Si in steels according to the present invention leads to
higher r and n values and to isotropic implementation of the
mechanical properties. The upper limit of the sum of the contents
of Al and Si is 12%, since a sum of the Al and Si contents
exceeding this limit would produce the danger of embrittlement.
[0022] The steel strips and sheets according to the present
invention are preferably produced through a method in which an
input stock, such as slabs, thin slabs, or strip, made of a steel
according to the present invention having the composition described
above, is cast, the cast input stock is heated to =1100.degree. C.
or used directly at such a temperature, the preheated input stock
is hot rolled to hot strip at a hot rolling final temperature of at
least 800.degree. C., and the finish-rolled hot strip is coiled at
a coiler temperature of 450.degree. C. to 700.degree. C.
[0023] Because the hot strip is hot rolled at hot rolling final
temperatures of at least 800.degree. C. and coiled at lower
temperatures according to the present invention, the positive
effect of the carbon and, in particular, of the boron described is
used to its full extent. Thus, boron and carbon produce higher
tensile strength and yield point values in strips hot rolled in
this range with fracture elongation values which are still
acceptable. As the hot rolling final temperature increases, tensile
strength and yield point are reduced, while the elongation values
increase. Through variation of the hot rolling final temperatures
within the scope provided by the present invention, the desired
properties of the steel strip may be influenced easily and in a
targeted way.
[0024] Material embrittlement is reliably avoided by restricting
the coiler temperature to values of at most 700.degree. C. It has
been shown that brittle phases form at higher coiler temperatures,
which may, for example, cause material flaking and thus make
further processing more difficult or even impossible.
[0025] Even hot strip produced according to the present invention
is distinguished by good usage properties. If thinner sheets or
strips are to be produced, the hot strip may be cold rolled to cold
strip after the coiling, the cold rolling advantageously being
performed at a reduction of 30% to 75%. The cold strip obtained is
subsequently preferably subjected to annealing, the annealing
temperatures to be between 600.degree. C. and 1100.degree. C. The
annealing may be performed in this case in the hood in the
temperature range from 600.degree. C. to 750.degree. C. or
continuously in the annealing furnace at temperatures from
750.degree. C. to 1100.degree. C. Finally, it is favorable in
regard to the cold formability and the surface formation to dress
the cold strip in a final step.
[0026] A further, particularly advantageous use of steel according
to the present invention, and/or steel strips and sheets produced
therefrom, is the production of cold-formed components through flow
forming. For this purpose, blanks are produced from the steel,
which are then finished by flow forming. Due to its special
property profile, steel according to the present invention, and/or
sheet blanks produced therefrom, are particularly suitable for this
purpose.
[0027] A microstructure which is purely austenitic or which
comprises a mixture of ferrite and austenite with components of
martensite may result in the steel according to the present
invention as a function of the composition. The steels according to
the present invention may therefore be formed significantly better.
They compact significantly more strongly in the course of cold
forming than the known high-strength microalloyed or multiphase
steels used for production through flow forming. Thus, depending on
the cold forming, component strengths in the range from 1400
N/mm.sup.2 to 2200 N/mm.sup.2 may be achieved. Additional hardening
of the components produced after the cold forming may therefore be
dispensed with. It also has a favorable effect in regard to the
intended purpose, in particular for the production of toothed gear
components, if the steels used according to the present invention
for their production have their density reduced due to the high
content of light elements, such as Si and Al.
[0028] If a steel composed and constituted according to the present
invention is used, then heat treatment or surface hardening of the
flow formed component may be dispensed with. The danger of
distortion and scaling caused in the related art by these
additional processing steps therefore no longer exists if a steel
according to the present invention is used to produce toothed
elements subjected to localized strong stress in use. Thus, the
steel according to the present invention allows cost-effective
production of light, heavy-duty, and dimensionally stable
components through cold forming, in particular flow forming.
[0029] In the following, the present invention is described in more
detail with reference to exemplary embodiments and comparative
examples.
[0030] The compositions of four steels A, B, C, D, E are indicated
in Table 1, steels A, B, and C corresponding to the alloy provided
according to the present invention, while steels D and E are the
comparative examples.
2 TABLE 1 Steel C Mn Al Si B Fe, impurities A 0.5 15 3 3 0.003
remainder B 0.5 20 3 3 0.003 remainder C -- 20 3 3 0.003 remainder
D -- 14 3 3 -- remainder E -- 19 3 3 -- remainder
[0031] Steels A to E of the compositions concerned are melted and
cast into slabs. Subsequently, the slabs are preheated to a
temperature of 1150.degree. C. The preheated slabs are then hot
rolled and subsequently coiled.
[0032] Respective hot rolling final temperatures ET and coiler
temperatures HT, and the respective properties of tensile strength
R.sub.m, yield point R.sub.e, A.sub.50 elongation, uniform
elongation A.sub.g1, and n value of the hot strips obtained are
indicated in Table 2.
3TABLE 2 ET HT R.sub.e R.sub.m Steel [.degree.C.] [.degree. ]
[N/mm.sup.2] [N/mm.sup.2] A.sub.50 [%] A.sub.gl [%] n A 960 500 486
792 42 38 0.31 B 930 500 509 825 46 42 0.32 C 920 500 496 818 31 27
D 820 500 610 920 26 -- -- E 840 500 430 700 30 -- --
[0033] Except for the strip produced from steel D, which is not
according to the present invention, and which could not be cold
rolled, the hot strips obtained were subsequently cold rolled at a
degree of deformation of approximately 65% and continuously
annealed at 950.degree. C. The mechanical properties of the cold
rolled steel sheets obtained in this way are listed in Table 3.
4TABLE 3 R.sub.e R.sub.m Steel [N/mm.sup.2] [N/mm.sup.2] A.sub.50
[%] A.sub.gl [%] n r .DELTA.r A 408 775 64 64 0.33 1.02 -0.1 B 411
785 61.1 61.1 0.33 1.0 -0.06 C 284 714 58 56.8 0.39 1.05 0.17 D not
cold rollable E 382 744 52.5 50.3 0.32 0.82 0.25
[0034] It has been shown that the steel strips produced according
to the present invention from steels A to C have outstanding cold
formability. In this case, at high strength and high fracture
elongation, they each have an distinct isotropic deformation
behavior (r.about.1, .DELTA.r.about.0). Even in the steel strips
produced from steel C according to the present invention, which is
carbon-free but contains boron, have lower yield points, elevated
fracture and uniform elongations, and isotropic forming
behavior.
[0035] Therefore, all variants of sheet steels according to the
present invention are especially suitable for the production of car
body components, especially for the outer panels of an automobile
body, of wheels for vehicles, in particular motor vehicles, of
non-magnetic components, of containers used in cryoengineering, of
internally or externally hydroformed components, of tubes which are
particularly intended for the production of high-strength engine
parts, such as camshafts or piston rods, of components intended for
protection against pulsed stresses, such as bombardment, or
protective elements, such as armor plates, or body armor for human
or animal bodies. Heavy-duty gear parts, which are distinguished by
low weight and good usage properties, without requiring additional
heat treatment for this purpose, may also be produced from steel
sheets according to the present invention.
* * * * *