U.S. patent application number 10/589945 was filed with the patent office on 2008-09-04 for cr-al-steel for high-temperature application.
Invention is credited to Kenneth Goransson, Andreas Rosberg, Eva Witt.
Application Number | 20080210348 10/589945 |
Document ID | / |
Family ID | 31989618 |
Filed Date | 2008-09-04 |
United States Patent
Application |
20080210348 |
Kind Code |
A1 |
Goransson; Kenneth ; et
al. |
September 4, 2008 |
Cr-Al-Steel for High-Temperature Application
Abstract
The present invention relates to a product of ferritic stainless
steel manufactured according to the process of this invention,
which product has increased resistance to cyclic and continuous
thermal load and oxidation at elevated temperatures and which has
improved mechanical properties at said temperatures as well as use
thereof in the form of wire, strip, foil and/or tube in
high-temperature applications such as in catalytic converter
applications, in heating and furnace applications and which has the
following composition (in % by weight): less than 1% of Ni, 15-25%
of Cr, 4.5-12% of Al, 0.5-4% of Mo, 0.01-1.2% of Nb, 0-0.5% of Ti,
0-0.5% of Y, Sc, Zr and/or Hf, 0-0.2% of one or more rare earth
metals (REM) such as, for instance, Ce or La, 0-0.2% of C, 0-0.2%
of N, with the balance iron and normally occurring impurities.
Inventors: |
Goransson; Kenneth; (Gavle,
SE) ; Rosberg; Andreas; (Sandviken, SE) ;
Witt; Eva; (Sandviken, SE) |
Correspondence
Address: |
DRINKER BIDDLE & REATH (DC)
1500 K STREET, N.W., SUITE 1100
WASHINGTON
DC
20005-1209
US
|
Family ID: |
31989618 |
Appl. No.: |
10/589945 |
Filed: |
February 21, 2005 |
PCT Filed: |
February 21, 2005 |
PCT NO: |
PCT/SE2005/000249 |
371 Date: |
April 22, 2008 |
Current U.S.
Class: |
148/537 ;
420/40 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/26 20130101; C22C 38/04 20130101; C22C 38/02 20130101; C22C
38/22 20130101 |
Class at
Publication: |
148/537 ;
420/40 |
International
Class: |
C22C 38/18 20060101
C22C038/18; C22C 38/06 20060101 C22C038/06 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 23, 2004 |
SE |
0400452-9 |
Claims
1. Ferritic steel alloy comprising the following composition (in %
by weight): less than 1% of Ni, 15-25% of Cr, 4.5-12% of Al, 0.5-4%
of Mo, 0.01-1.2% of Nb, 0-0.5% of Ti, 0-0.5% of Y, Sc, Zr and/or
Hf, 0-0.2% of one or more rare earth metals (REM), 0-0.2% of C,
0-0.2% of N, with the balance iron and normally occurring
impurities.
2. Ferritic steel alloy according to claim 1 wherein Mo entirely or
partly is replaced by W.
3. Ferritic steel alloy according to claim 1 wherein it contains
one or more rare earth metals (REM).
4. Ferritic steel alloy according to claim 1 wherein it contains at
least 0.1% in total of Ti, Nb, Zr and/or Hf.
5. Method of producing a ferritic steel alloy according to claim 1
comprising coating a substrate alloy with Al or an alloy of Al, the
substrate alloy having the following composition (in % by weight):
less than 1% of Ni, 15-27% of Cr, 0-5% of Al, 0.5-5% of Mo, 0.01-2%
of Nb, 0-0.5% of Ti, 0-0.5% of Y, Sc, Zr and/or Hf, 0-0.2% of one
or more rare earth metals (REM), 0-0.2% of C, 0-0.2% of N, with the
balance iron and normally occurring impurities.
6. Product in the form of wire, strip, foil and/or tube for use in
high-temperature applications wherein it is produced from a
ferritic steel alloy according to claim 1.
7. Use of a ferritic steel alloy according to claim 1 as a
supporting material in catalytic converter applications.
8. Use of a ferritic steel alloy according to claim 1 in heating
and furnace applications.
9. Ferritic steel alloy according to claim 1, wherein the one or
more rare earth metals (REM) is Ce or La.
10. Method according to claim 5, wherein the one or more rare earth
metals (REM) is Ce or La.
Description
[0001] The present invention relates to a product of ferritic
stainless steel manufactured according to the process of this
invention, which product has increased resistance to cyclic and
continuous thermal load and oxidation at increased temperatures and
which has improved mechanical properties at said temperatures as
well as the use thereof in the form of wire, strip, foil and/or
tube in high-temperature applications such as in catalytic
converter applications, in heating and furnace applications.
BACKGROUND
[0002] Fe--Cr--Al-alloys have extensive use in the temperature
range above 900.degree. C. Thanks to the protective oxide on the
surface, they resist cyclic and continuous thermal load and
oxidation until the material is depleted of the oxide former, e.g.,
Al. The limiting factors for the manufacture and the service life
of the entire device are the total content of Al and the mechanical
strength.
DESCRIPTION OF PRIOR ART
[0003] Metallic high-temperature materials in, for instance,
catalytic converters or for applications for resistive heating are
today normally based on thin strips or wire of ferritic
Fe--Cr--Al-alloys having at least 4.5% of Al and small amounts of
reactive elements added. The high ductility of the metal gives a
good resistance to mechanical and thermal fatigue. Aluminum in
contents above approx. 4.5% by weight, together with the reactive
elements, imparts the material the possibility of forming a thin,
protective aluminum oxide upon heating. Furthermore, the reactive
elements cause that the oxide gets a considerably reduced tendency
of peeling or flaking, i.e., to come loose from the metal upon
cooling or mechanical deformation. Conventional Fe--Cr--Al-alloys
have, however, a great disadvantage: they are mechanically very
weak at high temperature, and tend, therefore, to be considerably
deformed also at small stresses by virtue of, e.g., acceleration,
changes of pressure, mechanical impacts or changes of temperature.
The alloy disclosed in EP-B-290 719, which is intended for use in
the manufacture of heating elements for resistive heating of
furnaces etc., as well as construction parts in catalytic
converters, solves the problem of decreasing the elongation of the
substrate material in relation to the protective the oxide layer as
a consequence of the combined effect of addition of Ti and Zr to
the alloy.
[0004] Ferritic steel materials having low content of carbon are
also embrittled by grain growth upon use in temperatures above
800.degree. C. The low content of carbon is required in order to
obtain an optimal oxidation resistance of the alloy and enable
plastic cold working since contents of carbon above approx. 0.02%
by weight have an embrittling effect by increasing the brittle
transition temperature of the material. Elements that are used for
solid solution hardening of high-temperature materials, such as Mo
and/or W, are regarded to have a considerable negative impact on
the oxidation properties, and therefore the desirable content of
these elements may be limited to at most 1% such as in U.S. Pat.
No. 4,859,649 or at most 0.10% as in EP 0667400.
SUMMARY
[0005] Therefore, it is an object of the present invention to
provide an alloy of a ferritic stainless steel having elevated
resistance to cyclic and continuous thermal load and oxidation at
elevated temperatures.
[0006] It is an additional object of the present invention to
provide a ferritic stainless steel that has improved mechanical
properties for the use in applications with cyclic and continuous
thermal load and oxidation at elevated temperatures such as, e.g.,
supporting material in converter applications, such as
catalysts.
[0007] It is an additional object of the present invention to
provide a ferritic stainless steel for the use in heating
applications and in furnace applications.
[0008] It is an additional object of the present invention to
provide a ferritic stainless steel in the form of wire, strip, foil
and/or tube.
[0009] It is an additional object of the present invention to
provide a process for the manufacture of a product of said
alloy.
DESCRIPTION OF THE FIGURES
[0010] FIG. 1 shows results of the oxidation testing at
1000.degree. C. as a function of the change of mass versus time for
examples D and E as well as comparative examples 1 and 3.
[0011] FIG. 2 shows results of the oxidation testing at
1100.degree. C. as a function of the change of mass versus time for
examples C, E and G as well as comparative example 1.
DESCRIPTION OF THE INVENTION
[0012] These objects are met by means of a ferritic stainless steel
having the following composition (in % by weight): [0013] less than
1% of Ni, [0014] 15-25% of Cr, [0015] 4.5-12% of Al, [0016] 0.5-4%
of Mo, [0017] 0.01-1.2% of Nb, [0018] 0-0.5% of Ti, [0019] 0-0.5%
of Y, Sc, Zr and/or Hf, [0020] 0-0.2% of one or more rare earth
metals (REM) such as, for instance, Ce or La, [0021] 0-0.2% of C,
[0022] 0-0.2% of N, with the balance iron and normally occurring
impurities.
[0023] The final product may be manufactured in the form of wire,
strip, foil and/or tube.
[0024] The final product according to the present invention is
manufactured as a homogeneous material or a laminate or a material
having a concentration gradient of Al, where the content of Al
increases toward said surface of the product. Thus, the manufacture
may be effected by coating a substrate material and a substrate
alloy, respectively, with Al or an alloy of Al, especially by
coating strips of a substrate alloy of a thickness below 1 mm with
an alloy of Al.
[0025] By this two-step process, the mechanical properties and
oxidation resistance of the alloy can be improved and optimized
independently of each other. This process also enables a
simplification of the production process when manufacture via
conventional pyrometallurgy of materials having average contents of
Al above the average above 4.5% is associated with great yield
losses by virtue of brittleness. An additional advantage of this
process is that a final material may be manufactured having a
gradient of Al, such that the content of Al increases toward the
surface, which entails improved oxidation resistance since the
formation of fast growing oxides such as chromium and iron oxides
is prevented and the mechanical properties of the final material
are improved.
[0026] The substrate alloy may be manufactured by conventional
pyrometallurgy or, for instance, powder metallurgy with the
intended composition, and then the alloy is hot- and cold-rolled to
final desired dimension. In production by a coating process, before
the coating the substrate material has the following composition
(in % by weight): [0027] less than 1% of Ni, [0028] 15-27% of Cr,
[0029] 0-5% of Al, [0030] 0.5-5% of Mo, [0031] 0.01-2% of Nb,
[0032] 0-0.5% of Ti, [0033] 0-0.5% of Y, Sc, Zr and/or Hf, [0034]
0-0.2% of one or more rare earth metals (REM) such as, for
instance, Ce or La, [0035] 0-0.2% of C, [0036] 0-0.2% of N, [0037]
with the balance iron and normally occurring impurities.
[0038] The most suitable composition of the substrate material is
the following (in % by weight): [0039] less than 1% of Ni, [0040]
16-25% of Cr, [0041] 0.5-4% of Al, [0042] 0.7-4% of Mo, [0043]
0.25-1.0% of Nb, [0044] 0-0.5% of Y, Sc, Zr and/or Hf, [0045]
0-0.5% of Ti, [0046] 0-0.1% of one or more rare earth metals (REM)
such as, for instance, Ce or La, [0047] 0.02-0.2% of C, [0048]
0-0.05% of N, [0049] with the balance iron and normally occurring
impurities.
[0050] The material may be used in the as coated condition or after
a diffusion-annealing. The most favourable compositions of the
substrate material before coating are obtained if it contains 2-4%
of Al. This aluminium content imparts the final product an
increased oxidation resistance and results in a simplified
production process, i.e., the risk of production disturbances in
comparison with the manufacture of a material having a aluminum
content above 4% is considerably decreased. After coating with
alloy of Al, the material should in total contain a content of Al
that is greater than 4.5% by weight.
[0051] Mechanical stability and resistance to grain growth are
provided by the presence of precipitations of carbides and/or
nitrides of one or some of the elements Ti, Nb, Zr, Hf. Increased
strength at high temperatures, i.e., temperatures above approx.
800.degree. C. is also provided by the presence of Mo and/or W in
solid solution. In the alloy according to the present invention, Mo
may entirely or partly be replaced by W with a maintained effect on
the alloy.
[0052] Addition of Zr and/or Hf and REM and/or Y and/or Sc provides
an increased resistance to peeling and flaking of the formed oxide.
The contents of the final product of the same elements may be
supplied by adding these in the substrate alloy and/or in the alloy
of Al that is used in the coating. The alloy according to the
present invention should totally contain at least 0.1% by weight of
Ti+Nb+Zr+Hf.
[0053] Most compositions of the alloy according to the invention
can be manufactured by conventional metallurgy. However, by the
two-step process according to the present invention, a material is
obtained the microstructure of which is controlled, the oxidation
properties of which are improved, the mechanical properties of
which are optimised and improved, and the maximum aluminum content
of which is not limited by the embrittling effect that contents of
Al above approx. 5% by weight normally may give, both upon cold and
hot working. Furthermore, the process to coat a substrate material
with an alloy of Al provides a finished product the contents of
which of, e.g., Mo, Nb and C can be considerably higher than in a
conventionally manufactured material without the presence of these
elements resulting in any noticeable deterioration of the oxidation
properties.
[0054] Coating of the substrate alloy with alloy of Al may be
effected by previously known processes such as, for instance,
dipping in melt, electrolytic coating, rolling together of strips
of the substrate alloy and the aluminum alloy, deposition of solid
alloy of Al from a gas phase by so-called CVD or PVD technique. The
coating with alloy of Al may be effected after the substrate alloy
having been rolled down to desired final thickness of the product,
or in larger thickness. In the latter case, a diffusion-annealing
may be carried out in order to achieve a homogenization of the
material, and then rolling in one or more steps is carried out in
order to provide the finished product. Rolling may also be effected
directly on a coated product according to the present invention
having greater thickness than the desired final thickness. In this
case, the rolling may be followed by annealing.
[0055] The thickness of the coated layer of Al may be varied
depending on the thickness of the substrate material, the desired
aluminum content in the final product and the aluminum content in
the substrate material. However, the total content of Al in the
finished product has to, as has been mentioned above, always be at
least 4.5% by weight. The product may be used in the form of an
annealed, homogeneous material or a laminate or a material having a
concentration gradient of Al where the content of Al is higher at
the surface than in the centre of the material. For a material
having a concentration gradient, a lower total content and average
content down to 4.0% by weight, respectively, can be allowed if the
aluminum content at a distance of at most 5 .mu.m from the surface
is more than 6.0% by weight.
[0056] Examples of useful aluminum alloys are pure Al, Al alloyed
with 0.5-25% by weight of Si, Al alloyed with 0-2% by weight of one
or more of the elements Ce, La, Y, Zr, Hf. Depending on the coating
process used, different compositions of the alloy of Al are more
suitable than others. Thus, it is, upon coating from melt,
desirable that the melting point is low and that a homogeneous
material or a eutectic mixture is deposited. Upon coating by
rolling-on, it is required that the material is ductile and has
similar mechanical properties as the substrate so that coating and
substrate are deformed in the similar way.
EXAMPLE 1
[0057] Table 1 shows compositions of examined alloys. Example C and
comparative example 1 were prepared in the conventional way by
pyrometallurgy and hot working. From comparative example 1, 50
.mu.m thick strips were also prepared via hot rolling and cold
rolling. Comparative example 1 is an alloy that today is used as
supporting material in catalytic converters. This material has
sufficient oxidation resistance for this use. However, the
mechanical strength thereof is low and is regarded to be the
limiting factor of the service life of the entire device.
[0058] The very low ductility at room temperature (2% elongation at
fracture) of the alloy according to example C entails that this
alloy hardly can be manufactured in the form of thin strips.
However, the same alloy has, as is seen in table 1, a very good
high temperature strength, thus at 700.degree. and 900.degree. C.
the ultimate strength, for instance, is approx. 100% higher than
for comparative example 1. The oxidation resistance of example C
and comparative example 1 at 1100.degree. C. is shown in FIG. 2.
The oxidation rate of example C is 5% higher than of comparative
example 1, which means that the materials can be considered as
equivalents as regards oxidation resistance.
EXAMPLE 2
[0059] Table 1 shows compositions of examined alloys. Examples A
and B and comparative examples 1 and 2 were prepared in the
conventional way by pyrometallurgy and hot working. Then 50 .mu.m
thick strips of all alloys were also prepared via hot rolling and
cold rolling. The alloys according to examples A and B are all
sufficiently ductile at room temperature in order to be able to be
cold-rolled to very thin strips of good productivity.
[0060] Examples D and E and comparative example 3 correspond to
cold-rolled strips of alloy according to examples B and C and
comparative example 2, respectively, which was coated by
vaporization or sputtering with Al on both sides in such a quantity
that the total content of Al corresponded to 5.5-6% (see table
3).
TABLE-US-00001 TABLE 3 Thick-ness Coated Desired Measured Sub-
before thickness of total con- coating strate coating coated Al
tent of Al thickness Example alloy [.mu.m] alloy [.mu.m] [%]
[.mu.m] D A 50 5 6 E B 50 4 6 4.1 Comparative Cf. 50 5 6 4.7
example 3 example 2
[0061] The obtained thickness of Al was measured by means of GDOES
(glow discharge optical emission spectroscopy), a method that
enables accurate measuring of compositions and thicknesses of thin
surface layers. The analyses showed that a total content of Al of
5-6% had been attained. These samples were oxidized in air at
1000.degree. C. for up to 620 h, which is shown in FIG. 1. The
alloys according to examples D and E are superior to the alloy
according to comparative example 3, while the conventionally
manufactured alloy of Fe--Cr--Al in comparative example 1 has a
significantly better oxidation resistance than examples D and E of
the alloy according to the invention.
EXAMPLE 3
[0062] Examples F and G and comparative example 4 have the same
composition as the alloys according to examples D and E and
comparative example 3 having been annealed at 1050.degree. C. for
10 min with the purpose of providing an equalising of the content
of Al in the material. The ductility of the material was determined
by a bending test where the smallest bending radius that the
material could be bent to without fracture was determined, see
table 4.
TABLE-US-00002 TABLE 4 Smallest Results of Diffusion an- bending
tensile testing Ex- nealing in H.sub.2 radius without at
900.degree. C. ample Composition [min/1050.degree. C.] fracture
[mm] [Rm/MPa] F The same as 10 0.5 46 example D G The same as 10
0.38 81 example E Com- The same as 10 2.5 could not be parative
comparative measured due example example 3 to brittleness 4
[0063] The smallest radius that the material was tested at was 0.38
mm. The alloys according to the invention have a ductility being
superior to comparative example 4. The alloy according to
comparative example 4 proved to be so brittle that this alloy has
to be regarded as less suitable for the use in catalytic
converters. The alloy according to example G has an ultimate
strength at 900.degree. C. that is equally good as the
conventionally manufactured material according to the invention,
example C, and twice as high as the conventionally manufactured
alloy of Fe--Cr--Al in comparative example 1. This means that, upon
the assumption that the oxidation resistance is sufficient, this
alloy can be used in a thickness that is half of the thickness of a
conventional material, and thereby enable an increase in efficiency
and a reduction of the material cost for the manufacture of
catalytic converters.
[0064] The alloy according to example G was oxidation tested at
1100.degree. C. together with the alloy according to examples C and
E as well as comparative example 1, which is shown in FIG. 2. An
improved oxidation resistance is obtained with the alloy according
to example G, both by comparison with the same material without
diffusion-annealing (example E) and with conventionally
manufactured alloys. The comparison between example G and example C
is especially interesting, since these correspond to alloys having
very similar composition but different ways of production: the
alloy according to example G is prepared by cold rolling to desired
thickness, followed by Al coating and annealing while example C has
been prepared with desired content of Al in the alloy from the
beginning. Apart from the improved production properties of a
material that has been prepared in the way of example G, in
addition this alloy has a better oxidation resistance than example
C. The relatively seen lower oxidation resistance that example C
has in comparison with comparative example 1 may be explained by a
negative effect on the oxidation resistance by virtue of the
presence of Mo and Nb in the alloy according to example C. It is
known that these elements may deteriorate the oxidation resistance
of an alloy. In example G, these negative effects are absent, which
may be interpreted as a positive result of example G having been
prepared by Al-coating. Thus, this method of manufacture is
favourable as regards the oxidation resistance of the alloy.
[0065] To sum up, it may be observed that by the combined effect of
high contents of Mo and Nb, a considerable improvement in the
strength is provided in comparison with the material that is used
today as well as that by using the described process, this material
may be imparted the oxidation resistance required upon use at high
temperatures of materials in weak dimensions and the above
mentioned product forms.
[0066] The product of ferritic stainless steel manufactured
according to the process of this invention has increased resistance
to cyclic and continuous thermal load and oxidation at elevated
temperatures and has improved mechanical properties at said
temperatures, which makes it suitable for use in high-temperature
applications such as in catalytic converter applications and in
heating and furnace applications in the form of wire, strip, foil
and/or tube.
TABLE-US-00003 TABLE 1 Example C N Ni Al Cr Nb Mo Zr REM Ti Si Mn A
0.007 0.074 0.10 <0.01 22.23 0.73 1.05 0.065 0.036 0.10 0.13 B
0.097 0.032 0.08 0.83 22.11 0.77 2.02 0.002 0.009 0.12 0.11 0.11 C
0.099 0.043 0.08 4.9 21.87 0.79 2.00 0.002 0.11 0.12 0.11 0.12
Comparative 0.01 0.01 0.3 5.3 20 0.01 0.01 0.01 0.03 0.01 0.3 0.3
example 1 Comparative 0.100 0.034 3.12 <0.01 22.28 0.74 2.04
<0.01 0.049 0.12 0.12 example 2
TABLE-US-00004 TABLE 2 Room temperature 700.degree. C. 900.degree.
C. Example Rp0.2 Rm A5 Rp0.2 Rm A5 Rp0.2 Rm A5 A 359 470 26 150 165
29 40 59 67 B 442 561 22 145 172 58 51 73 80 C 514 579 2 234 267 46
48 84 110 Comparative example 1 480 670 25 50 140 90 20 40 150
Comparative example 2 555 614 9 148 168 56 42 67 91
* * * * *