U.S. patent application number 11/915338 was filed with the patent office on 2008-08-21 for austenitic lightweight steel and use thereof.
This patent application is currently assigned to Piotr R. Scheller. Invention is credited to Heiner Gutte, Piotr R. Scheller, Andreas Weiss.
Application Number | 20080199345 11/915338 |
Document ID | / |
Family ID | 36649769 |
Filed Date | 2008-08-21 |
United States Patent
Application |
20080199345 |
Kind Code |
A1 |
Scheller; Piotr R. ; et
al. |
August 21, 2008 |
Austenitic Lightweight Steel and Use Thereof
Abstract
Austenitic lightweight steel with a characteristic value of the
cold formability higher than 30,000 MPa %, with tensile strengths
between 600 and 800 MPa, and fracture strains over 50% has a
chromium content of >2.0%<18%, a silicon content of
>1.0%<4%, a manganese content of >2.0%<20%, an aluminum
content of >0.05%<4%, wherein the steel is within an alloying
range determined by (Cr.sub.equ=14; Ni.sub.equ=14.5);
(Cr.sub.equ=14; Ni.sub.equ=17.5) (Cr.sub.equ=20; Ni.sub.equ=10)
(Cr.sub.equ=20; Ni.sub.equ=13), wherein the chromium and nickel
equivalents are calculated based on the following relations (1) and
(2): Cr.sub.equ=% Cr+% Mo+1.5% Si+0.5% W+0.9% Nb+4% Al+4% Ti+1.5% V
(1) Ni.sub.equ=% Ni+30% C+18% N+0.5% Mn+0.3% Co+0.2% Cu-0.2% Al (2)
from the chemical composition of the steel, wherein the values
represent wt. % and the balance is largely iron and other steel
accompanying elements.
Inventors: |
Scheller; Piotr R.;
(Radebeul, DE) ; Weiss; Andreas; (Freiberg,
DE) ; Gutte; Heiner; (Freiberg, DE) |
Correspondence
Address: |
GUDRUN E. HUCKETT DRAUDT
SCHUBERTSTR. 15A
WUPPERTAL
42289
DE
|
Assignee: |
Scheller; Piotr R.
Radebeul
DE
|
Family ID: |
36649769 |
Appl. No.: |
11/915338 |
Filed: |
May 8, 2006 |
PCT Filed: |
May 8, 2006 |
PCT NO: |
PCT/DE2006/000797 |
371 Date: |
November 23, 2007 |
Current U.S.
Class: |
420/56 ;
420/8 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/06 20130101; C22C 38/38 20130101 |
Class at
Publication: |
420/56 ;
420/8 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 23, 2005 |
DE |
10 2005 024 029.1 |
Claims
1-11. (canceled)
12. Austenitic lightweight steel with good cold formability and a
characteristic value of the cold formability higher than 30,000 MPa
%, with tensile strengths between 600 and 800 MPa and fracture
strains over 50%, the steel comprising: a chromium content of more
than 2.0% but less than 18%; a silicon content of more than 1.0%
but less than 4%; a manganese content of more than 2.0% but less
than 20%; an aluminum content of more than 0.05% but less than 4%,
wherein the steel is within an alloying range determined by
coordinates of four points: (Cr.sub.equ=14; Ni.sub.equ=14.5)
(Cr.sub.equ=14; Ni.sub.equ=17.5) (Cr.sub.equ=20; Ni.sub.equ=10)
(Cr.sub.equ=20; Ni.sub.equ=13), wherein the chromium and nickel
equivalents are calculated based on the following relations (1) and
(2): Cr.sub.equ=% Cr+% Mo+1.5% Si+0.5% W+0.9% Nb+4% Al+4% Ti+1.5% V
(1) Ni.sub.equ=% Ni+30% C+18% N+0.5% Mn+0.3% Co+0.2% Cu-0.2% Al (2)
from the chemical composition of the steel, wherein the values
represent wt. % and the balance is largely iron and other steel
accompanying elements (O, P, S).
13. Lightweight steel according to claim 11, wherein the nickel
content is 0 to 10%, the niobium content is 0 to 1.2%, the carbon
content is 0.01 to 0.15%, the nitrogen content is 0 to 0.1%, the
copper content is 0 to 4%, the cobalt content is 0 to 1%, the
molybdenum content is 0 to 4%, the tungsten content is 0 to 3%, the
titanium content is 0 to 1%, and the vanadium content is 0 to
0.15%.
14. Lightweight steel according to claim 11, wherein the carbon
content is 0.04%, the chromium content is 13%, the silicon content
is 1.5%, the niobium content is 0.15%, the nickel content is 7.9%,
the manganese content is 8.1%, the aluminum content is 0.11%, and
the nitrogen content is 0.02%.
15. Lightweight steel according to claim 11, wherein the carbon
content is 0.03%, the chromium content is 15.82%, the silicon
content is 1.22%, the nickel content is 7.5%, the manganese content
is 5.8%, and the aluminium content is 0.11%.
16. Lightweight steel according to claim 12 as material for
hot-rolled sheet metal and strips.
17. Lightweight steel according to claim 12 as material for
cold-rolled sheet metal and strips.
18. Lightweight steel according to claim 12 as material for
crash-loaded components and strengthening structural
components.
19. Lightweight steel according to claim 12 for non-flat products
and fastening components.
20. Lightweight steel according to claim 12 wherein the steel is
heat-treated.
21. Lightweight steel according to claim 12 as material for
weather-resistant and slow-corroding parts.
22. Lightweight steel according to claim 12 as material for
stainless parts.
Description
[0001] The innovation relates to an austenitic lightweight steel,
and the use thereof Steels exhibiting tensile strengths of more
than 600 MPa are referred to as lightweight steels due to the
tensile strength per unit of weight being higher compared to
aluminum.
STATE OF THE ART
[0002] Stainless austenitic steels are distinguished by a high
corrosion resistance and, as a rule, good cold formability as well.
The cold formability and the energy absorptivity of stainless
austenitic steels can be increased by a TRIP effect (transformation
induced plasticity). Both tensile strengths and fracture strains
obtained are relatively high. The alloying range in which a TRIP
effect occurs in stainless, cold-formable CrNi steels and CrNiMn
steels has not been specified yet. Previously, stainless,
cold-formable austenitic steels exhibiting the TRIP effect have
only been characterized by some special properties. Said steels
exhibit tensile strengths of about 520 to 850 MPa while exhibiting
fracture strains of about 60 to 45% [1, 2]. A typical steel showing
the TRIP effect is a stainless steel with 17 to 18% chromium and 8
to 10% nickel such as the steel X5 CrNi 18 10 (1.4301).
[0003] In addition to the stainless austenitic steels,
high-manganese TRIP/TWIP-steels (twinning induced plasticity) and
the LIP-steels (light induced plasticity) are cold-formable steels.
Due to the increased tensile strength of the TRIP/TWIP- and the
LIP-steels they are also referred to as lightweight steels.
Austenitic TRIP/TWIP-steels exhibit tensile strengths of higher
than about 650 to 1100 MPa. The associated fracture strains range
from about 80 to 40% [1, 3, 4].
[0004] The chemical composition of the steels is disclosed in DE
197 27 759 A [3]. According to [3], these steels contain 10 to 30%
manganese, typically with silicon and aluminum added. They are not
alloyed with chromium. A steel with 20% manganese, 3% silicon and
3% aluminum is typical [3, 4, 5].
[0005] Austenitic LIP-steels have only been tested on a laboratory
scale. They are reported to reach tensile strengths of about 1000
to 1100 MPa and fracture strains ranging from about 60 to 50%.
According to [6], the chemical composition has not yet been
published.
[0006] Cold formability and energy absorptivity, tensile strength
and fracture strain of said steels are increased by a TRIP, TWIP,
or superimposed TRIP and TWIP effects.
[0007] When during mechanical straining the austenite transforms,
induced by deformation, to .epsilon.- and/or .alpha.'-martensite, a
TRIP effect is observed. As a result, the plastic deformability and
the tensile strength will rise. Twinning will further intensify
these property changes. High strain hardening capability will then
be observed. Relatively high tensile strengths are then reached
with relatively low 0.2% yield strengths so that, typically, low
yield strength ratios are recorded.
[0008] For assessing the cold formability of said steels, the
product of the tensile strength multiplied by the maximum
elongation can be used as characteristic value. The product of
tensile strength times maximum elongation is in the range of from
about 25,000 to 38,000 MPa % for the austenitic TRIP steels, over
38,000 to 57,000 MPa % for the TRIP/TWIP steels, and over 57,000
MPa % for the LIP steels [3-7]. The energy absorptivity of the TRIP
steels and TRIP/TWIP steels reaches values of 0.45 to 0.5
J/mm.sup.3. This means that on crash loading these steels exhibit a
large strain reserve [3, 4, 5]. For the LIP steels, corresponding
values have not been published.
[0009] In the austenitic TRIP and TRIP/TWIP steels, cold
formability and energy absorptivity are achieved by affecting the
austenitic structure as a result of mechanical straining during a
cold deformation process. The microstructure of the austenite,
particularly with respect to the formation of stacking faults and
twins, as well as the formation of deformation-induced
.epsilon.-martensite and .alpha.'-martensite are affected. The
different mechanisms can principally be influenced via the stacking
fault energy of the austenite, which depends on the chemical
composition of the austenite [5, 8]. Furthermore, niobium aids in
developing fine grain and thus has a good effect on the mechanical
properties. In addition, niobium causes setting of the carbon and
hence leads to improved corrosion properties.
[0010] Until now, silicon contents of more than 1% are alloyed to
austenitic steels in order to achieve heat resistance, or improved
scaling resistance, respectively, in connection with high chromium
contents. Silicon and aluminum show a high oxygen activity, which
can have an effect on the castability and the level of purity. For
this reason, as a rule, the contents of said elements are chosen to
be at a minimum unless they are alloyed with the aim of improving
special properties.
[0011] Silicon and aluminum are ferrite stabilizing elements.
Therefore, the contents of these elements are limited in austenitic
steels in order to prevent ferrite formation. With the exception of
the high manganese TWIP steels, aluminum has not been used as
alloying constituent in austenitic steels. Unlike other
accompanying and alloying elements, the influence of aluminum on
the chromium and/or nickel equivalent has not yet been
reported.
[0012] A chromium content of more than about 12% causes a passive
layer to form, making the stainless steels corrosion-resistant.
Austenitic steels with 12% chromium are, as a rule,
weather-resistant and slow-corroding. Resistance against rusting is
increased in these steels. High-manganese austenitic steels,
however, are not chromium-alloyed. Thus, they do not rank among the
stainless, slow-corroding weather-resistant steels.
In the usual austenitic steels, manganese is used as an
austenite-forming and nickel-substituting element. Therefore
manganese is added to austenitic steels mainly for cost
reasons.
[0013] It is a prerequisite of the creation of deformation-induced
.epsilon.-martensite that the structure consists of austenite. In
order to reach the TRIP effect, corresponding chromium and nickel
equivalents are required in the chemical composition. This means
that the chemical composition of the steels must be adjusted with
respect to the ferrite and austenite stabilizing elements.
[0014] The stainless manganese and nitrogen alloyed austenitic
steels 1.4371 (X2 CrMnNiN 17 7 5), 1.4372 (X12 CrMnNiN 17 7 5) and
1.4373 (X12 CrMnNiN 17 9 5), and the steels AISI 201 and 202, which
may be nitrogen-alloyed or do not contain nitrogen, are, as far as
their Cr, Ni and Mn contents is concerned, covered by the patent in
some partial ranges. Said steels are listed in the Stahlschlussel
[7]. However, they do not contain aluminum.
Literature Cited:
[0015] [1] Schroder, T.: Technische Rundschau 1/2 (2005), pp. 48-52
[0016] [2] DIN 17 440 and DIN 17 441 [0017] [3] Frommeyer, G.:
Unexamined laid-open patent application, DE 197 27 759 A1. [0018]
[4] Frommeyer, G.: Patent specification, DE 197 27 759 C2 [0019]
[5] Grassel, O., L. Kruger, G. Frommeyer and L. W. Meyer: Int. J.
Plasticity 16(2000), pp. 1391-1409 [0020] [6] Bode, R. et al.:
stahl und eisen 8(2004), pp. 19 to 26 [0021] [7] Stahischlussel
(Steel Index), Verlag Stahlschlussel Wegst GmbH [0022] [8]
Martinez, L. G. et al.: Steel research 63(1992)5, pp. 221-223
[0023] Therefore, the invention, which is disclosed in the
independent claims, aims at providing further austenitic
lightweight steels with good cold formability, a characteristic
value of the cold formability of higher than 30,000 MPa %, as well
as tensile strengths between 600 and 800 MPa and fracture strains
over 50%.
[0024] The invention solves the problem in that the austenitic
steel according to the invention is alloyed with silicon, aluminum
and chromium while containing manganese. An improvement of the
formability of said steel is achieved with the aid of alloying
measures, especially by adding silicon within the limits of higher
than 1.0% up to 4.0%, aluminum within the limits of 0.05% up to
4.0% while lowering the chromium content to less than 18%.
It has been found that aluminum affects both the chromium and the
nickel equivalents. The efficiency factor for aluminum takes this
into account when calculating the chromium equivalent according to
claim 1. Also in claim 1, an effect on the nickel equivalent is
taken into account by a coefficient. As a consequence, aluminum
lowers the nickel equivalent.
[0025] Preferably, aluminum leads to improved mechanical properties
and increased cold formability and energy absorption at
temperatures above room temperature, i.e. at temperatures where
most technological cold forming processes take place.
[0026] Based on these conditions, the required chemical composition
of the steel according to the invention can be determined as
mentioned above. As far as the Cr-, Ni- and Mn-contents are
concerned, the known stainless manganese- and nitrogen-alloyed
austenitic steels 1.4371 (X2 CrMnNiN 17 7 5), 1.4372 (X12 CrMnNiN
17 7 5) and 1.4373 (X12 CrMnNiN 17 9 5), and the steels AISI 201
and 202, which may be nitrogen-alloyed or do not contain nitrogen,
are covered by the claim in partial ranges. Said steels are listed
in the Stahischlussel [7]. However, they do not contain aluminum.
In addition, the steel according to the invention differs from
these steels in containing more silicon, and also partly in regard
to its use. Particularly, the solid-solution strengthening effect
of the nitrogen in said steels is used in order to obtain, unlike
with steels exhibiting good cold formability, relatively high 0.2%
yield strengths. The nitrogen-alloyed steels are then preferably
used as spring steels. The steels of the 201 and 202 grades that
are not nitrogen-alloyed are characterized by lower 0.2% yield
strengths compared with the nitrogen-alloyed steels of the same
grade. Therefore the cold formability of these steels is a little
better so that components made of these steels are used in
household articles, in apparatus construction, building industry,
etc.
[0027] The advantages achieved by the invention particularly
resides in that with the lightweight steels of the invention
improved mechanical properties and increased cold formability as
well as energy absorption are reached. It is thus possible to
manufacture cost-effective steels such as austenitic CrNiMn steels
with lowered Ni contents. Said steels exhibit better properties or
similar properties in comparison to the properties of, for example,
commercial stainless CrNi steels of the 18/8 or 18/10 grades.
Furthermore, weather-resistant or slow-corroding lightweight steels
with high levels of strength and toughness can successfully be
produced. The steels according to the invention cold-form very
well, similar to the chromium-free, high-manganese TWIP-steels.
[0028] The invention will be explained with the aid of the
following preferred embodiments.
[0029] The austenitic steels according to the invention comprise
two different steel grades. The first steel grade comprises
stainless austenitic steels containing about 12.0% to 18.0%
chromium. The second steel grade comprises austenitic steels
containing more than 2.0% and less than 12.0% chromium. Steels of
the second grade are not stainless, but exhibit a higher resistance
against rusting as a result of the chromium-, nickel- and silicon
content thereof, in this respect, they are thus different from the
previous austenitic TRIP/TWIP steels in spite of a similar
potential of properties. A multitude of said steels can therefore
be considered as weather-resistant or slow-corroding. Particularly
steels containing 10% to 12% chromium exhibit distinct slow
corrosion rates.
[0030] According to claim 2, a preferred composition is that the
nickel content is lower than 10% but also 0%, the niobium content
is lower than 1.2% but also 0%, the carbon content is between 0.01%
and 0.15%, the nitrogen content is lower than 0.1% but also 0%, the
copper content is lower than 4% but also 0%, the cobalt content is
lower than 1% but also 0%, the molybdenum content is lower than 4%
but also 0%, the tungsten content is lower than 3% but also 0%, the
titanium content is lower than 1% but also 0%, and the vanadium
content is lower than 0.15% but also 0%.
[0031] Preferably, such an austenitic steel with .epsilon.-TRIP
effect, good cold formability and increased rusting resistance
according to claim 3 has a carbon content of 0.04%, a chromium
content of 13%, a silicon content of 1.5%, a niobium content of
0.15%, a nickel content of 7.9%, a manganese content of 8.1%, a
nitrogen content of 0.02% and an aluminum content of 0.11%, balance
largely iron. The structure of the steel consists of metastable
austenite. The steel shows a marked .epsilon.-TRIP effect. A
relatively high hardening capability is achieved. The 0.2% yield
strength is 210 MPa, the tensile strength is 645 MPa. The steel
reaches a maximum elongation of 65%. This means that the
characteristic value calculated as the product of the fracture
strain times the tensile strength is determined to be 38,055 MPa %.
The energy absorption is about 0.5 J/mm.sup.3. The steel forms an
oxidation layer containing iron, chromium and silicon, said layer
under atmospheric conditions causing weather resistance, or slow
corrosion, respectively.
[0032] Particularly preferred is a stainless austenitic steel with
.epsilon.-TRIP effect and good cold formability according to claim
4 that has a carbon content of 0.03%, a chromium content of 15,82%,
a silicon content of 1.22%, a nickel content of 7.50%, a manganese
content of 5.80% and an aluminum content of 0.11%, balance largely
iron. The structure of the steel consists of metastable austenite.
The steel shows an austenitic basic structure with a marked
.epsilon.-TRIP effect at room temperature. A relatively low yield
strength ratio is observed as a result of high hardening
capability. The 0.2% yield strength is about 197 MPa, the tensile
strength is 620 MPa. The steel reaches a maximum elongation of 64%.
That means that the value that characterizes cold formability,
calculated as the product of the fracture strain times the tensile
strength, is determined to be 39,820 MPa %. The energy absorption
is about 0.5 J/mm.sup.3.
* * * * *