U.S. patent application number 11/297418 was filed with the patent office on 2006-08-31 for austenitic stainless steel for hydrogen gas and method for its manufacture.
Invention is credited to Masaaki Igarashi, Mitsuo Miyahara, Kazuhiko Ogawa, Tomohiko Omura, Hiroyuki Semba.
Application Number | 20060193743 11/297418 |
Document ID | / |
Family ID | 33549224 |
Filed Date | 2006-08-31 |
United States Patent
Application |
20060193743 |
Kind Code |
A1 |
Semba; Hiroyuki ; et
al. |
August 31, 2006 |
Austenitic stainless steel for hydrogen gas and method for its
manufacture
Abstract
An austenitic stainless steel for use in a hydrogen gas
atmosphere comprises, in mass %, C: 0.10% or less, Si: 1.0% or
less, Mn: 0.01 to 30%, P: 0.040% or less, S: 0.01% or less, Cr: 15
to 30%, Ni: 5.0 to 30%, Al: 0.10% or less, N: 0.001 to 0.30 % with
the balance Fe and inevitable impurities. An X-ray (111)
integration intensity of a cross section along the direction
rectangular to the working direction is five times that in a random
direction or less, and the X-ray integration intensity ratio of a
cross section along the working direction satisfies
I(220)/I(111).ltoreq.10. The high strength steel can also contain
one or more of the groups of Mo and W; V, Nb, Ta, Ti, Zr and Hf; B;
Cu and Co; Mg, Ca, La, Ce, Y, Sm, Pr and Nd.
Inventors: |
Semba; Hiroyuki; (Sanda-shi,
JP) ; Igarashi; Masaaki; (Sanda-shi, JP) ;
Omura; Tomohiko; (Osaka, JP) ; Miyahara; Mitsuo;
(Kobe-shi, JP) ; Ogawa; Kazuhiko;
(Nishinomiya-shi, JP) |
Correspondence
Address: |
CLARK & BRODY
1090 VERMONT AVENUE, NW
SUITE 250
WASHINGTON
DC
20005
US
|
Family ID: |
33549224 |
Appl. No.: |
11/297418 |
Filed: |
December 9, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/JP04/08380 |
Jun 9, 2004 |
|
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|
11297418 |
Dec 9, 2005 |
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Current U.S.
Class: |
420/53 |
Current CPC
Class: |
C22C 38/44 20130101;
C22C 38/02 20130101; C22C 38/58 20130101; C22C 38/001 20130101 |
Class at
Publication: |
420/053 |
International
Class: |
C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 10, 2003 |
JP |
2003-165670 |
Claims
1. An austenitic stainless steel for hydrogen gas characterized by
having a chemical composition comprising, in mass percent, C: at
most 0.10%, Si: at most 1.0%, Mn: 0.01-30%, P: at most 0.040%, S:
at most 0.01%, Cr: 15-30%, Ni: 5.0-30%, sol. Al: at most 0.10%, N:
0.001-0.30%, and a remainder of Fe and impurities, and by having a
structure such that the x-ray integrated intensity I(111) of a
cross section in a direction perpendicular to the direction of
working is at most 5 times that of a random direction and such that
the x-ray integrated intensity ratio I(220)/I(111) of a cross
section in the direction of working is at most 10.
2. An austenitic stainless steel as set forth in claim 1 wherein
the chemical composition further contains, in mass percent, at
least one of Mo: 0.3-3.0% and W: 0.3-6.0%.
3. An austenitic stainless steel as set forth in claim 1 wherein
the chemical composition further contains, in mass percent, at
least one of V: 0.001-1.0%, Nb: 0.001-1.0%, Ta: 0.001-1.0%, Ti:
0.001-1.0%, Zr: 0.001-1.0%, and Hf: 0.001-1.0%.
4. An austenitic stainless steel as set forth in claim 2 wherein
the chemical composition further contains, in mass percent, at
least one of V: 0.001-1.0%, Nb: 0.001-1.0%, Ta: 0.001-1.0%, Ti:
0.001-1.0%, Zr: 0.001-1.0%, and Hf: 0.001-1.0%.
5. An austenitic stainless steel as set forth in claim 3 wherein
the chemical composition further contains, in mass percent, at
least one of Nb: greater than 0.20% not greater than 1.0%, Ta:
greater than 0.40% not greater than 1.0%, and Ti: greater than
0.10% not greater than 1.0%, and wherein C+N is .ltoreq.0.05%.
6. An austenitic stainless steel as set forth in claim 4 wherein
the chemical composition further contains, in mass percent, at
least one of Nb: greater than 0.20% not greater than 1.0%, Ta:
greater than 0.40% not greater than 1.0%, and Ti: greater than
0.10% not greater than 1.0%, and wherein C+N is .ltoreq.0.05%.
7. An austenitic stainless steel as set forth in claim 1 wherein
the chemical composition further contains, in mass percent,
0.0001-0.020% of B.
8. An austenitic stainless steel as set forth in claim 2 wherein
the chemical composition further contains, in mass percent,
0.0001-0.020% of B.
9. An austenitic stainless steel as set forth in claim 3 wherein
the chemical composition further contains, in mass percent,
0.0001-0.020% of B.
10. An austenitic stainless steel as set forth in claim 4 wherein
the chemical composition further contains, in mass percent,
0.0001-0.020% of B.
11. An austenitic stainless steel as set forth in claim 5 wherein
the chemical composition further contains, in mass percent,
0.0001-0.020% of B.
12. An austenitic stainless steel as set forth in claim 6 wherein
the chemical composition further contains, in mass percent,
0.0001-0.020% of B.
13. An austenitic stainless steel as set forth in claim 1 wherein
the chemical composition further contains, in mass percent, at
least one of Cu: 0.3-2.0% and Co: 0.3-5.0%.
14. An austenitic stainless steel as set forth in claim 1 wherein
the chemical composition further contains, in mass percent, at
least one of Mg: 0.0001-0.0050%, Ca: 0.0001-0.0050%, La:
0.0001-0.20%, Ce: 0.0001-0.20%, Y: 0.0001-0.40%, Sm: 0.0001-0.40%,
Pr: 0.0001-0.40%, and Nd: 0.0001-0.50%.
15. An austenitic stainless steel as set forth in claim 1
characterized in that the austenite average grain diameter is at
most 20 .mu.m.
16. An austenitic stainless steel as set forth in claim 14
characterized in that the austenite average grain diameter is at
most 20 .mu.m.
17. A method of manufacturing an austenitic stainless steel for
hydrogen gas characterized by carrying out plastic working with a
decrease in cross section of 10-50% in a temperature range of room
temperature to 200.degree. C. on a steel having a chemical
composition as set forth in claim 1, and then performing final
plastic working with a reduction in cross section of at least 5% in
a direction different from the direction of working of the
above-described plastic working.
Description
TECHNICAL FIELD
[0001] This invention relates to a stainless steel for use in a
hydrogen gas environment which has excellent mechanical properties
(strength and ductility) and corrosion resistance and to a method
for its manufacture. In addition, the present invention relates to
equipment used in a hydrogen gas environment such as piping, gas
cylinders, and valves for hydrogen gas made from such a stainless
steel.
[0002] A stainless steel according to the present invention is
particularly suitable as a steel for structural equipment which is
exposed to a high pressure hydrogen gas environment in fuel cell
automobiles and hydrogen gas stations, and particularly for piping,
gas cylinders, and valves.
BACKGROUND ART
[0003] As is well known, fuel cell automobiles obtain electric
power using hydrogen and oxygen as fuels. They have attracted
attention as the next generation of clean automobiles which do not
discharge carbon dioxide (CO.sub.2) or harmful substances such as
nitrogen oxides (NO.sub.x) or sulfur oxides (SO.sub.x) as do
conventional gasoline-powered or diesel-powered automobiles. In
Japan, under the guidance of the Ministry of Economy, Trade, and
Industry, it is planned to introduce 5 million vehicles by the year
2020.
[0004] At present, the biggest problem with respect to fuel cell
automobiles is how to realize the practical generation and storage
of hydrogen as a fuel, and various types of research and
development are being pursued.
[0005] Conventional methods thereof include a method in which a
hydrogen gas cylinder is directly mounted on a vehicle, a method in
which methanol or gasoline is reformed to obtain hydrogen by a
reformer mounted on a vehicle, and a method in which a
hydrogen-storing alloy which can absorb hydrogen is mounted on a
vehicle.
[0006] Each of these methods has strengths and weaknesses, but in
Japan, in December of 2002, the first fuel cell automobile in the
world having a hydrogen gas cylinder mounted thereon was sold, and
a number of the vehicles are already being used as government
vehicles by the Ministry of Land, Infrastructure, and Transport,
for example.
[0007] However, although present fuel cell automobiles have a
maximum speed of approximately 150 km per hour and an output of
approximately 100, horsepower which is close to the performance of
a gasoline-powered automobile used as a private vehicle, due to
limitations on the size of gas cylinders, the distance for which
they can continuously run is at most only 300 km, and this is an
impediment towards their general use.
[0008] At present, an increasing amount of research and development
is being carried out for promoting the spread of fuel cell
automobiles as the next generation of clean automobile by improving
and lowering the cost of fuel cell automobiles having a high
pressure hydrogen gas cylinder mounted thereon. To achieve these
goals, it is necessary to overcome the following problems.
[0009] Namely, there are problems such as lengthening the
continuous running distance, providing infrastructure such as gas
stations, and developing safer technology for hydrogen.
[0010] It is calculated that in order to increase the running
distance to 500 km, for example, it is necessary to increase the
pressure of hydrogen in a vehicle-mounted gas cylinder from the
present value of 35 MPa to 70 MPa. In addition, instead of existing
gasoline stations, hydrogen gas stations will be necessary. As a
result, it will be necessary to provide for the generation,
transport, and storage of high pressure hydrogen gas and rapid
filling thereof (supply to vehicles).
[0011] Since hydrogen gas is flammable, it is necessary to exercise
special care when handling it. However, there are many unknown
matters concerning the interaction of ultrahigh pressure hydrogen
gas exceeding 50 MPa and the structure of components of equipment,
and there is a strong desire for establishment of technology for
its safe utilization.
[0012] The fuel cell automobiles which were sold last year used
already existing SUS316 austenitic stainless steel, the soundness
of which is already widely recognized. This is because its
susceptibility to hydrogen gas embrittlement in a hydrogen gas
environment of up to about 35 MPa is better compared to other
structural steels (such as STS480 (JIS G 3455) low carbon steel or
SUS304 stainless steel) and because techniques for working and
welding it are already, established.
[0013] However, in order to use SUS316 steel under a hydrogen gas
pressure increased from 35 MPa to 70 MPa, piping which
conventionally had an outer diameter of 26.2 mm and an inner
diameter of 20 mm (a pipe wall thickness of 3.1 mm) must be changed
to piping having an outer diameter of 34.7 mm and an inner diameter
of 20 mm (pipe wall thickness of 7.35 mm), since the conventional
material does not have sufficient strength unless its wall
thickness is at least doubled and its weight is at least 3 times as
large. Therefore, a large increase in the weight mounted on a
vehicle and an increase in the size of hydrogen gas stations are
unavoidable. These are great impediments to its practical
application.
[0014] As disclosed in Japanese Published Unexamined Patent
Applications Hei 5-98391, Hei 5-65601, Hei 7-216453, and Hei
7-26350, for example, it is generally known that the strength of
usual austenitic stainless steel can be improved by cold working,
and that it is possible to increase the strength and reduce the
wall thickness of a pipe by drawing or by rolling.
[0015] FIG. 1 is a graph showing a typical relationship between the
degree of cold working (percent reduction in cross section) and
tensile strength. It can be seen therefrom that a high strength can
be realized by increasing the degree of cold working.
DISCLOSURE OF THE INVENTION
[0016] However, when strengthening is carried out by cold working
in this manner, although a high strength is obtained, ductility and
toughness are markedly decreased. FIG. 2 is a graph showing the
relationship between the elongation in the direction perpendicular
to the direction of working in cold working and the degree of cold
working (hereunder referred to as "the percent reduction in cross
section"). It can be seen that elongation greatly decreases as the
degree of cold working increases. In actual practice, elongation of
at least about 30% is desirable, but when the degree of cold
working is large, a decrease in elongation becomes a problem.
[0017] Accordingly, in light of safety in handling high pressure
hydrogen gas, it has been thought that cold working could not be
employed to obtain a high strength.
[0018] The object of the present invention is to provide an
austenitic stainless steel which has excellent mechanical
properties and corrosion resistance and which can be used in a
hydrogen gas environment such as one containing high pressure
hydrogen gas at 70 MPa or above and to provide a method for its
manufacture.
[0019] The present inventors studied the causes of a degradation in
mechanical properties accompanying working of various types of
austenitic stainless steels in detail. As a result of detailed
study of the effects of the chemical composition and the metallic
structure (the microstructure) of a material on a deterioration in
mechanical properties, including hydrogen embrittlement in a high
pressure hydrogen gas environment such as at least 70 MPa, and on
corrosion resistance, they obtained the following new
knowledge.
[0020] (1) When an austenitic stainless steel is subjected to cold
working, its strength increases, but when cold working is performed
in one direction, a strong anisotropy of mechanical properties
develops. In particular, the ductility and toughness and torsional
properties markedly decrease in the direction perpendicular to the
direction of working.
[0021] (2) When an excessive amount of dislocations are introduced
into austenitic stainless steel, it is thought that penetration of
hydrogen into the steel is difficult compared to conventional
ferritic steel. However, in a high pressure hydrogen gas
environment, penetration of hydrogen is easily enabled, and
susceptibility to hydrogen embrittlement increases. In addition, it
was found that if the susceptibility to hydrogen embrittlement in
the direction of working is compared to that in the direction
perpendicular to the direction of working, the susceptibility to
hydrogen embrittlement is markedly increased in the direction
perpendicular to the direction of working.
[0022] FIG. 3 is a graph showing the relationship between the
degree of cold working and hydrogen embrittlement in the direction
of working and the direction perpendicular thereto. The
above-described tendency is clear therefrom.
[0023] (3) With an austenitic stainless steel, if strength is
increased by cold working, a texture structure is achieved as the
degree of cold working increases. In usual cold rolling, a rolled
texture structure is formed such that {112} is parallel to the
rolling surface and <11{overscore (1)}> is parallel to the
direction of working (rolling), or such that {110} is parallel to
the rolling surface and <001> is parallel to the direction of
working. With pipes or wires or forged materials, a fiber texture
structure is formed in which <11{overscore (1)}> or
<001> is parallel to the direction of working
(elongation).
[0024] Namely, if it is attempted to give an austenitic stainless
steel a high strength by conventional cold working methods, in
every situation, the texture structure in which <11{overscore
(1)}> or <001> is parallel to the direction of working
(elongation) is inevitably formed.
[0025] Formation of such a texture structure can be measured by
measurement of the x-ray integrated intensity I(hkl) (h, k, and l
are Miller indices) obtained by x-ray diffraction of the rolling
surface. The degree of formation of the above-described texture
structure can be obtained by measurement of the x-ray integrated
intensity I(111) or I(002) for a cross section perpendicular to the
direction of working.
[0026] (4) The susceptibility to hydrogen embrittlement in the
direction of working increases as the degree of formation of the
x-ray integrated intensity I(111) of a cross section perpendicular
to the direction of working increases. When the degree of the
formation thereof exceeds 5, the susceptibility to hydrogen
embrittlement indicated by elongation (hydrogen)/elongation (air)
becomes .ltoreq.0.75. In other words, if the degree of formation of
a cross section perpendicular to the direction of working is made 5
or less, susceptibility to hydrogen embrittlement in the direction
of working can be decreased. Here, elongation (hydrogen) means the
elongation in a tensile test in a hydrogen gas environment, and
elongation (air) means the elongation in a tensile test in air.
[0027] FIG. 4 is a graph showing the relationship between the x-ray
integrated intensity I(111) of a cross section perpendicular to the
direction of working and resistance to hydrogen embrittlement for
the direction of working and the direction perpendicular thereto.
From FIG. 4, it can be seen that the resistance to hydrogen
embrittlement in the direction of working has a strong correlation
to the x-ray integrated intensity I(111).
[0028] (5) Although susceptibility to hydrogen embrittlement in the
direction perpendicular to the direction of working has a
correlation to the x-ray integrated intensity I(111) of a cross
section perpendicular to the direction of working, it has an
extremely strong correlation to the x-ray integrated intensity
I(220) and the x-ray integrated intensity I(111) of a cross section
in the direction of working. When the ratio I(220)/I(111) exceeds
10, susceptibility to hydrogen embrittlement enormously increases
(susceptibility to hydrogen embrittlement as expressed by
elongation (hydrogen)/elongation (air).ltoreq.0.75). In other
words, if the x-ray integrated intensity ratio I(220)/I(111) of a
plane in the direction of working is made 10 or less,
susceptibility to hydrogen embrittlement perpendicular to the
direction of working can be decreased.
[0029] FIG. 5 is a graph showing the relationship between the x-ray
integrated intensity ratio I(220)/I(111) of a cross section in the
direction of working and resistance to hydrogen embrittlement for
the direction of working and the direction perpendicular thereto.
From FIG. 5, it can be seen that hydrogen embrittlement in the
direction perpendicular to the direction of working has a strong
correlation to the x-ray integrated intensity ratio
I(220)/I(111).
[0030] (6) By working a material in a series of steps of
deformation which are not employed by conventional working methods,
a specific texture structure is developed, and as a result, it is
possible to manufacture an austenitic stainless steel having an
extremely low susceptibility to hydrogen embrittlement in a high
pressure hydrogen gas environment.
[0031] Namely, by carrying out plastic working with a reduction in
cross section of 10-50% in a temperature range from room
temperature to 200.degree. C. and then performing plastic working
of at least 5% in a direction different from the direction of
working of the above-described plastic working on an austenitic
stainless steel having a specific composition to be described
below, the x-ray integrated intensity I(111) of a cross section
perpendicular to the direction of working can be suppressed to at
most 5 times that of a random direction, and the x-ray integrated
intensity ratio I(220)/I(111) of a cross section in the direction
of working can be suppressed to at most 10. As a result, it is
possible to markedly decrease susceptibility to hydrogen
embrittlement.
[0032] Here, "the direction of working" does not mean "the
direction of plastic working itself" but means "the direction of
plastic deformation of the material being worked".
[0033] (7) Summarizing the above, when applying cold working to an
austenitic stainless steel having a composition described below, to
limit the x-ray integrated intensity I(111) of a cross section
perpendicular to the direction of working to at most 5 times that
of a random direction and limiting the x-ray integrated intensity
ratio I(220)/I(111) of a cross section in the direction of working
to at most 10, it is possible to obtain an austenitic stainless
steel which in spite of having a high strength has excellent
toughness and reduced susceptibility to hydrogen is embrittlement
and low anisotropy and which can be used in a hydrogen gas
environment at a high pressure such as 70 MPa or above.
[0034] Thus, the present invention is an austenitic stainless steel
for hydrogen gas having a chemical composition comprising, in mass
percent, C: at most 0.10%, Si: at most 1.0%, Mn: 0.01-30%, P: at
most 0.040%, S: at most 0.01%, Cr: 15-30%, Ni: 5.0-30%, Al: at most
0.10%, N: 0.001-0.30%, and a remainder of Fe and impurities and
having a structure such that the x-ray integrated intensity I(111)
of a cross section perpendicular to the direction of working is at
most 5 times that of a random direction and such that the x-ray
integrated intensity ratio I(220)/I(111) of a cross section in the
direction of working is at most 10.
[0035] The direction of working herein means the direction of
plastic deformation of the material being worked.
[0036] The above-described chemical composition according to the
present invention may further include at least one element selected
from the following groups.
[0037] (1) At least one of Mo: 0.3-3.0% and W: 0.3-6.0%.
[0038] (2) At least one of V: 0.001-1.0%, Nb: 0.001-1.0%, Ta:
0.001-1.0%, Ti: 0.001-1.0%, Zr: 0.001-1.0%, and Hf: 0.001-1.0%.
[0039] (3) B: 0.0001-0.020%
[0040] (4) At least one of Cu: 0.3-2.0% and Co: 0.3-5.0%.
[0041] (5) At least one of Mg: 0.0001-0.0050%, Ca: 0.0001-0.0050%,
La: 0.0001-0.20%, Ce: 0.0001-0.20%, Y: 0.0001-0.40%, Sm:
0.0001-0.40%, Pr: 0.0001-0.40%, and Nd: 0.0001-0.50%.
[0042] In a preferred mode of the present invention, the average
austenite grain diameter is at most 20 .mu.m.
[0043] In order to manufacture an austenitic stainless steel for
hydrogen gas according to the present invention, an austenitic
stainless steel having the above-described chemical composition is
subjected to plastic working with a reduction in cross section of
10-50% in a temperature range from room temperature to 200.degree.
C., and then plastic working of at least 5% is carried out in a
direction different from the direction of working of the
above-described plastic working.
[0044] In this manner, according to the present invention, it is
possible to obatin a high strength austenitic stainless steel which
does not undergo hydrogen embrittlement even in a high pressure
hydrogen gas environment such as 70 MPa or above and which does not
have anisotropy of mechanical properties. It exhibits particularly
excellent properties in vessels, piping, valves, and the like which
are used in a hydrogen gas station or a fuel cell automobile and
are exposed to a high pressure hydrogen gas environment.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] FIG. 1 is a view showing the relationship between the degree
of cold working and the tensile strength of a conventional
steel.
[0046] FIG. 2 is a view showing the relationship between the degree
of cold working and the elongation of a conventional steel.
[0047] FIG. 3 is a view showing that the resistance to hydrogen
embrittlement in the direction of working greatly differs from that
in the direction perpendicular to the direction of working.
[0048] FIG. 4 is a view showing that the resistance to hydrogen
embrittlement in the direction of working has a strong correlation
to the x-ray integrated intensity I(111) of a cross section in a
direction perpendicular to the direction of working.
[0049] FIG. 5 is a view showing that the resistance to hydrogen
embrittlement in the direction perpendicular to the direction of
working has a strong correlation to the x-ray integrated intensity
I(220)/I(111) of a cross section in the direction of working.
[0050] FIG. 6 is a view showing the relationship between grain
diameter and resistance to hydrogen embrittlement in examples.
BEST FORM FOR CARRYING OUT THE INVENTION
[0051] The chemical composition of an austenitic stainless steel
according to the present invention and the reasons for limits
thereon will be explained below in detail. In this specification,
percent with respect to the chemical composition of steel means
mass percent unless otherwise specified.
[0052] The content of C is made at most 0.10%. In an austenitic
stainless steel, there are often cases in which corrosion
resistance is increased by precipitation of M.sub.23C.sub.6-type
carbides (M is Cr, Mo, Fe, or the like) or MC-type carbides (M is
Ti, Nb, Ta, or the like), but in a steel according to the present
invention, precipitation of carbides is not mandatory. Rather,
there are cases in which precipitation at grain boundaries has an
adverse effect on toughness and the like after cold working. Thus,
C is limited to at most 0.10%. The lower the content of C the
better, and preferably it is at most 0.04%. Taking into
consideration refining costs, it is not necessary to make the C
content zero, and preferably it is at least 0.001%.
[0053] When at least one of below-described Nb, Ta, and Ti is
included in the amounts of Nb: greater than 0.20% and at most 1.0%,
Ta: greater than 0.40% and at most 1.0%, and Ti: greater than 0.10%
and at most 1.0% in order to obtain a higher strength, C+N is
limited to at most 0.05%.
[0054] Si is known as an element which is effective for improving
corrosion resistance in various environments, but if a large amount
thereof is added, it forms intermetallic compounds with Ni, Cr, and
the like, it promotes the formation of sigma phase and other
intermetallic compounds, and there are cases in which it markedly
lowers hot workability. Therefore, the content of Si is made at
most 1.0% and preferably at most 0.5%. In the same manner as for C,
taking into consideration the refining costs of Si, it is not
necessary to make the content of Si zero, and preferably it is at
least 0.001%.
[0055] Mn is not only effective in minute amounts as a deoxidation
and desulfurization agent, but in addition, there are cases in
which it is added in large amounts as an inexpensive austenite
stabilizing element. In the steel of the present invention, by
appropriate combinations thereof with Cr, Ni, N, and the like, it
contributes to high strength and increased ductility and toughness.
Therefore, Mn is added in an amount of at least 0.01%. However, if
it exceeds 30%, there are cases in which hot workability and
weathering resistance decrease, so it is made 0.01-30%. Preferably
it is 0.1-20%.
[0056] Cr is essential as an element for improving corrosion
resistance in the above-described environment of use, so it is
included in an amount of at least 15%. However, if a large amount
is added, a large amount of nitrides such as CrN and Cr.sub.2N or
M.sub.23C.sub.6-type carbides are formed, so the content of Cr is
made 15-30%. Preferably it is 15-27%.
[0057] Ni is added as an austenite stabilizing element, but in the
steel of the present invention, when it is suitably combined with
Cr, Mn, N, or the like, it contributes to high strength and
improvements in ductility and toughness in the direction
perpendicular to the direction of working. Therefore, Ni is added
in an amount of at least 5.0%. From the standpoint of costs, it is
undesirable to add it in excess of 30%, so it is made 5.0-30%.
Preferably it is 6-23%.
[0058] The content of Al is made at most 0.10%. Al is an important
element as a deoxidizing agent. However, if a large amount remains
in excess of 0.10%, it promotes the formation of sigma phase and
other intermetallic compounds, which is undesirable from the
standpoint of achieving the strength and toughness which are the
objects of the present invention.
[0059] N is an important solid solution strengthening element. When
it is included in a suitable range in conjunction with Mn, Cr, Ni,
C, and the like, it suppresses the formation of sigma phase and
other intermetallic compounds, and it contributes to an increase in
toughness, particularly in the direction perpendicular to the
direction of working. For this purpose at least 0.001% is added.
However, if it is added in excess of 0.30%, cold workability
decreases, so it is made 0.001-0.30%. When at least one of
below-described Nb, Ta, and Ti is added in the range of Nb: greater
than 0.20% and at most 1.0%, Ta: greater than 0.40% and at most
1.0%, and Ti: greater than 0.10% and at most 1.0% with the object
of obtaining a higher strength, C+N is restricted to a range of at
most 0.05%.
[0060] Mo and W contribute to obtaining a high strength as solid
solution strengthening elements, so at least one thereof may be
added as necessary. However, if a large amount thereof is added,
they destabilize austenite, so when adding Mo or W, the amounts
thereof are made Mo: 0.3-3.0% and W: 0.3-6.0%.
[0061] V, Nb, Ta, Ti, Zr, and Hf form cubic carbonitrides and
contribute to an increase in strength, and if necessary at least
one of these may be added. However, if a large amount of
carbonitrides thereof precipitate, ductility and toughness in the
direction perpendicular to the direction of working decrease, and
the contents thereof in the steel according to the present
invention are each made 0.001-1.0%.
[0062] When it is desired to obtain a higher strength, at least one
of Nb, Ta, and Ti is contained in the ranges of Nb: greater than
0.20% and at most 1.0%, Ta: greater than 0.40% and at most 1.0%,
and Ti: greater than 0.10% and at most 1.0%, and C+N is preferably
restricted to the range of at most 0.05%.
[0063] B contributes to refinement of precipitates and refinement
of austenite crystal grain, and if necessary at least 0.0001% of B
may be added. However, if a large amount of B is added, it forms
low melting point compounds, and there are cases in which it
decreases hot workability. Thus, its upper limit is made
0.020%.
[0064] Cu and Co are austenite stabilizing elements. In a steel of
the present invention, when they are suitably combined with Mn, Ni,
C, or Cr, they contribute to an increase in strength, and
optionally, at least one thereof may be added in an amount of at
least 0.3%. However, from the standpoint of costs, it is
unnecessary to add a large amount thereof, and the content thereof
is defined as Cu: 0.3-2.0% and Co: 0.3-5.0%.
[0065] Mg, Ca, and, of the transition elements, La, Ce, Y, Sm, Pr,
and Nd act to prevent occurrence of cracks during solidification at
the time of casting when present in the ranges set forth for the
steel of the present invention, and they have the effect of
reducing a decrease in ductility resulting from hydrogen
embrittlement after long periods of use. If necessary, therefore,
at least one of any of Mg: 0.0001-0.0050%, Ca: 0.0001-0.0050%, La:
0.0001-0.20%, Ce: 0.0001-0.20%, Y: 0.0001-0.40%, Sm: 0.0001-0.40%,
Pr: 0.0001-0.40%, and Nd: 0.0001-0.50% may be added.
[0066] In the steel of the present invention, there is no marked
deterioration in the general properties of the steel even if at
most 0.040% of P and at most 0.01% of S are present as impurities.
Each of these is generally known as an element which by nature has
a harmful effect on ductility, workability, and the like. However,
in the steel of the present invention, an impurity content of this
level does not cause any problems.
[0067] An austenitic stainless steel according to the present
invention has a tensile strength on the level of at least 800 MPa
and preferably at least 900 MPa, and it has an elongation of at
least 30%. The steel can be used in the form of plates, pipes,
rods, shaped members, and wire, for example. If necessary, it can
further undergo surface treatment such as plating.
[0068] In order to obtain a texture structure having a reduced
anisotropy which is characteristic of the present invention, first
cold working (first plastic working) having a degree of cold
working of 10-50% expressed as percent reduction in cross section,
and second cold working (second plastic working) in a direction of
working different from that of the first cold working and having a
degree of cold working of at least 5% are carried out. Some
examples of methods of plastic working when preparing tubes, for
example, are pipe forming by cold drawing or pipe forming by cold
rolling combined with pipe expansion with a plug or pipe expansion
by spinning. A combination of the above-described pipe forming and
swaging in the axial direction is also effective.
[0069] There are no particular restrictions on the direction of
working of the first plastic working and the direction of working
of the second plastic working as long as anisotropy of the
structure of a cold worked product in the present invention can be
eliminated. However, considering making of steel pipes and the
like, the directions of working are preferably perpendicular to
each other. As already stated, the direction of working refers to
the direction of plastic deformation of the material being worked.
For example, when carrying out drawing of a steel pipe, the
direction of working is the lengthwise direction of the steel pipe,
and when carrying out swaging to compress a steel pipe, it is the
radial direction of the steel pipe.
[0070] The order in which the first and second cold working are
performed is usually such that the working with the larger degree
of working is carried out first and then the working with the
smaller degree of working is carried out. However, in the present
invention, there is no particular restriction as long as prescribed
shaping can be carried out and anisotropy of the structure can be
eliminated.
[0071] When one or both of the first and second working is carried
out in multiple steps, the steps of the first and second cold
working can be suitably combined with each other. For example, at
first the first cold working can be carried out a number of times,
and then the direction of working can be changed and the second
cold working can be carried one or more times, and then the
direction can again be changed and the first cold working can be
carried out, or the order may be the opposite of the above.
[0072] In the present invention, the "direction of working" when
measuring the x-ray integrated intensity which evaluates the
anisotropy of the metal structure can be the direction of working
of either the first cold working or the second cold working as long
as the requirements of the present invention are satisfied.
However, for convenience, in the examples of this specification,
the direction of working is the direction in which the largest cold
working is carried out. Specifically, when carrying out cold
working of a steel pipe, it is the lengthwise direction.
[0073] In the present invention, such a texture structure is
obtained in order to improve resistance to hydrogen embrittlement,
and it is sufficient to form such a texture structure at least in a
surface layer portion where contact with a hydrogen gas atmosphere
takes place. Accordingly, after pipe forming, it is permissible to
eliminate anisotropy of the texture structure only in the surface
layer portion (the inner surface or the outer surface of the pipe)
by shot peening.
[0074] The effects of carrying out the present invention will be
described in greater detail by examples.
EXAMPLES
[0075] Table 1 shows examples of the chemical composition (mass
percent) of an austenitic stainless steel according to the present
invention and of a comparative steel. TABLE-US-00001 manufacturing
Steel C Si Mn P S Cr Ni sol-Al N Mo W others process Present
invention 1 0.005 0.39 1.78 0.018 0.0007 18.2 9.5 0.009 0.045 A 2
0.016 0.36 1.03 0.001 0.0008 25.6 20.7 0.011 0.089 B 3 0.018 0.40
2.36 0.027 0.0007 27.8 28.7 0.007 0.152 A 4 0.014 0.43 10.51 0.010
0.0008 18.9 10.3 0.006 0.281 A 5 0.015 0.47 15.23 0.019 0.0007 24.1
21.1 0.010 0.279 A 6 0.020 0.38 1.78 0.029 0.0008 16.4 12.7 0.009
0.090 2.14 B 7 0.046 0.46 9.12 0.004 0.0010 16.8 13.0 0.006 0.275
2.46 A 8 0.004 0.39 1.73 0.025 0.0008 16.5 12.2 0.005 0.090 4.25 B
9 0.012 0.09 1.78 0.015 0.0009 17.1 12.6 0.009 0.090 2.63 0.34V B
10 0.016 0.41 1.80 0.018 0.0070 17.5 13.4 0.004 0.094 1.96 0.55Nb A
11 0.024 0.38 1.77 0.025 0.0008 15.6 11.6 0.003 0.087 2.87 0.81Ta B
12 0.015 0.37 1.10 0.018 0.0006 16.8 11.7 0.003 0.089 0.27Ti B 13
0.015 0.38 1.22 0.019 0.0005 15.6 12.6 0.009 0.085 2.67 0.12Zr A 14
0.016 0.31 1.27 0.006 0.0010 16.7 12.8 0.004 0.052 1.97 0.36Hf B 15
0.017 0.36 1.19 0.023 0.0010 17.1 11.8 0.003 0.053 2.01 0.44V +
0.29Nb B 16 0.013 0.82 0.89 0.015 0.0010 16.6 12.0 0.005 0.056 1.92
0.39Ti + 0.52Zr A 17 0.018 0.40 0.95 0.018 0.0007 16.7 12.9 0.002
0.048 2.58 0.005B B 18 0.016 0.36 1.79 0.018 0.0005 17.0 12.1 0.008
0.052 2.57 1.21Cu A 19 0.013 0.40 0.96 0.028 0.0005 16.2 11.7 0.009
0.054 2.38 3.46Co B 20 0.014 0.38 0.91 0.027 0.0008 16.9 12.5 0.011
0.057 0.0032Mg A 21 0.014 0.28 0.90 0.016 0.0008 17.2 13.3 0.011
0.092 2.33 0.0044Ca B 22 0.015 0.30 1.09 0.016 0.0016 17.0 12.0
0.015 0.089 2.07 0.081La B 23 0.006 0.33 0.97 0.027 0.0009 16.7
13.0 0.008 0.094 1.98 0.059Ce A 24 0.092 0.25 1.00 0.035 0.0010
15.9 12.1 0.007 0.088 2.04 0.118Y B 25 0.055 0.27 1.01 0.018 0.0007
15.7 13.0 0.005 0.093 2.89 0.025Sm A 26 0.015 0.27 0.98 0.017
0.0008 16.9 12.0 0.011 0.084 0.033Pr B 27 0.014 0.31 1.49 0.018
0.0010 15.6 12.1 0.008 0.090 2.21 0.38Nd B 28 0.011 0.29 1.30 0.027
0.0008 16.1 12.9 0.008 0.087 2.86 0.0024Ca + 0.022Y A 29 0.011 0.36
1.22 0.027 0.0007 16.7 11.9 0.006 0.060 2.06 0.030Ce + 0.081Nd B 30
0.014 0.38 1.39 0.025 0.0006 17.5 12.0 0.012 0.029 2.63 0.57Nb A 31
0.017 0.40 1.78 0.010 0.0008 15.8 11.7 0.008 0.024 2.99 0.80Ta A 32
0.015 0.39 1.80 0.020 0.0007 17.0 12.5 0.005 0.023 1.83 0.45Ti B
comparative A 0.020 0.26 1.85 0.027 0.0007 17.1 13.1 0.012 0.054
2.28 No cold rolling B .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw.
.dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. XX (10%) C .dwnarw.
.dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw.
.dwnarw. .dwnarw. XX (20%) D .dwnarw. .dwnarw. .dwnarw. .dwnarw.
.dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. XX (30%) E
.dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw.
.dwnarw. .dwnarw. .dwnarw. XX (40%) F .dwnarw. .dwnarw. .dwnarw.
.dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. XX
(50%) G .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw. .dwnarw.
.dwnarw. .dwnarw. .dwnarw. .dwnarw. XX (65%) H 0.159* 0.28 1.76
0.028 0.0006 16.2 13.3 0.008 0.045 2.94 B I 0.021 0.25 1.84 0.029
0.0006 12.4* 13.1 0.007 0.045 2.46 B J 0.024 0.34 1.80 0.028 0.0007
33.5* 12.2 0.008 0.091 1.89 B K 0.019 0.32 11.56 0.028 0.0004 16.4
11.7 0.005 0.342* 2.72 B L 0.023 0.38 1.82 0.029 0.0009 18.0 9.8
0.008 0.047 XX (40%) M 0.014 0.43 14.89 0.027 0.0008 24.6 21.4
0.009 0.272 XX (40%) N 0.021 0.35 1.80 0.026 0.0009 16.1 12.3 0.010
0.089 2.20 XX (50%) O 0.054 0.11 1.85 0.024 0.0009 16.9 12.8 0.010
0.092 2.63 0.37V XX (50%) *Outside the range of the present
invention XX: Conventional, % in parentheses means the degree of
working
[0076] 150 kg of each of the steels having the compositions shown
in Table 1 were melted using a vacuum induction melting furnace and
cast into ingots. Soaking was then performed for 4 hours at
1200.degree. C., after which hot forging was carried out at
1000.degree. C. or above to obtain a plate with a thickness of 35
mm and a width of 100 mm. Solid solution treatment was then carried
out by holding for 20 minutes at 1000.degree. C. followed by water
cooling.
[0077] For the steels according to the present invention, the
water-cooled plates were subjected to cold rolling of 30%, after
which cold rolling of 10% was carried out in the direction
perpendicular to the previous direction of working (manufacturing
method A), or the water-cooled plates were subjected to cold
rolling of 40% and then to cold rolling of 10% in the direction
perpendicular to the previous direction of working (manufacturing
method B) to obtain test materials.
[0078] For comparative steel A, a test material was obtained by the
above-described hot forging and solid solution treatment without
further processing.
[0079] For comparative steels B-G, after the above-described hot
forging and solid solution treatment, cold rolling of 10-65% in a
single direction was carried out to obtain test materials.
[0080] For comparative steels H-K, test materials were obtained by
the above-described manufacturing method B.
[0081] For comparative steels L and M, after hot forging and solid
solution treatment, cold rolling of 40% was carried out in a single
direction to obtain test materials.
[0082] For comparative steels N and O, after hot forging and solid
solution treatment, cold rolling of 50% was carried out in a single
direction to obtain test materials.
[0083] From each of the test materials obtained in this manner,
tensile test pieces with a diameter of 4 mm and GL of 20 mm,
hydrogen gas environment tensile test pieces with a diameter of
2.54 mm and GL of 30 mm, and Charpy impact test pieces measuring 10
mm.times.10 mm.times.55 mm with a 2-mm V-notch were cut in the
direction of final cold rolling (namely, in the direction of the
final plastic deformation of the test materials). A tensile test
was carried out at room temperature in air, and a tensile test
under a hydrogen gas atmosphere was carried out at room temperature
in high pressure hydrogen gas at 70 MPa. Both were carried out at a
strain rate of 10.sup.-6/second, and the results of steels
according to the present invention and comparative steels were
compared.
[0084] The results are shown in Table 2. TABLE-US-00002 TABLE 2
X-ray Integrated Elongation X-ray Intensity (111) Intensity Ratio
TS YS Elongation (hydrogen)/ Steel to Random Direction (220)/(111)
(MPa) (MPa) (%) Elongation(air) Present Invention 1 2.4 4.1 848 448
45.7 0.94 2 3.8 7.3 1072 487 38.9 0.83 3 3.5 6.6 1042 512 42.2 0.89
4 4.6 9.5 1328 572 31.8 0.79 5 4.6 9.2 1248 604 34.9 0.79 6 3.5 6.8
1086 479 40.0 0.85 7 4.4 9.1 1327 562 33.0 0.78 8 3.7 7.2 1075 447
42.0 0.81 9 3.9 7.6 1142 478 37.6 0.80 10 2.8 5.2 948 455 46.1 0.87
11 4.2 8.3 1159 470 37.2 0.81 12 3.9 7.6 1122 488 37.0 0.80 13 2.8
5.2 946 448 42.3 0.90 14 3.5 6.7 1062 438 40.4 0.88 15 3.1 5.9 1000
448 40.6 0.89 16 2.5 4.5 858 457 46.9 0.92 17 3.2 6.1 1025 461 39.8
0.83 18 2.7 5.0 914 459 45.1 0.93 19 3.6 7.0 1082 462 38.2 0.81 20
2.9 5.3 926 446 46.0 0.87 21 3.9 7.6 1131 454 39.7 0.82 22 4.3 8.4
1172 479 35.7 0.84 23 2.8 5.2 931 450 46.7 0.88 24 4.3 8.4 1159 475
38.9 0.83 25 3.1 5.7 977 491 45.0 0.91 26 3.8 7.4 1130 463 37.7
0.86 27 4.0 7.8 1149 468 37.9 0.81 28 3.0 5.7 981 441 44.0 0.86 29
4.0 7.7 1099 438 40.9 0.83 30 4.3 8.6 1224 592 35.6 0.82 31 4.7 9.4
1257 586 34.2 0.79 32 4.8 9.6 1372 606 31.6 0.76 Comparative A 1.1
1.2 561 285 48.3 0.99 B 2.3 2.8 703 362 36.8 0.89 C 3.3 4.9 806 409
27.4 0.82 D 4.2 8.7 884 428 20.3 0.76 E 5.4 16.5 948 448 13.7 0.42
F 6.5 18.7 1030 495 10.9 0.38 G 7.6 19.8 1105 487 7.4 0.31 H 3.5
6.7 1024 425 15.3 0.90 I 3.2 6.0 986 445 44.4 0.62 J 3.7 7.1 1075
420 21.6 0.83 K 4.1 8.1 1148 490 16.7 0.85 L 6.5 12.0 948 451 18.2
0.45 M 6.0 11.6 854 443 19.5 0.49 N 7.1 16.3 1072 467 13.6 0.38 O
7.3 17.0 1168 476 11.2 0.34
[0085] As can be seen from the results of comparative steels A-G of
Table 2, when strength was increased by cold working, ductility
greatly decreased. When a necessary degree of cold working to
achieve the room temperature tensile strength (TS) of at least 800
Mpa was applied, the x-ray integrated intensity I(111) of a cross
section perpendicular to the direction of working was over 5 times
that of a random direction, and the x-ray integrated intensity
ratio I(220)/I(111) of a cross section in the direction of working
exceeded 10. In such a case, resistance to hydrogen embrittlement
greatly decreases, and this becomes a major problem in actual
use.
[0086] In contrast, steels of the present invention of Nos. 1-32 of
Table 2 each had an x-ray integrated intensity I(111) of a cross
section in the direction perpendicular to the direction of working
(a cross section perpendicular to the direction of working) of at
most 5 times that in a random direction, and the x-ray integrated
intensity ratio I(220)/I(111) of a cross section in the direction
of working (a working direction cross section) of at most 10. The
strength TS at room temperature was at least 800 MPa, YS was at
least 400 MPa, and elongation was at least 30%. In addition,
susceptibility to hydrogen embrittlement which was evaluated by the
ratio of the ductility in a tensile test in a hydrogen gas
environment to that in a tensile test in air was extremely low.
[0087] In comparative steels H-O their chemical compositions were
outside the range for the steels of the present invention, or the
degree of formation of the texture structure was such that the
x-ray integrated intensity I(111) of a cross section perpendicular
to the direction of working was greater than 5 times that of a
random direction, or the x-ray integrated intensity ratio
I(220)/I(111) of a cross section in the direction of working was
greater than 10. The susceptibility to hydrogen embrittlement as
evaluated by the ratio of the ductility in a tensile test in a
hydrogen gas atmosphere to the ductility in a tensile test in air
was extremely high.
[0088] In this example, cold rolling was carried out twice on a
plate in different directions of working, but the same effects as
in this example can be obtained when carrying out cold rolling two
times in different directions of working on a steel pipe (such as
pipe forming by cold drawing and pipe expansion with a plug).
[0089] FIG. 6 shows the results of evaluation of susceptibility to
hydrogen embrittlement when solid solution heat treatment was
carried out at various temperatures in the range of
950-1150.degree. C. (30 minutes of holding followed by water
cooling) after hot forging of inventive steel No. 6 of Table 1,
after which above-described manufacturing method A was performed
and test materials having various grain sizes were prepared.
Susceptibility to hydrogen embrittlement was evaluated in the same
manner as with the above-described evaluation based on the ratio of
the ductility in a tensile test in a hydrogen gas atmosphere to the
ductility in a tensile test in air. As can be seen from the results
shown in FIG. 6, by making the austenite average grain diameter at
most 20 .mu.m, susceptibility to hydrogen embrittlement becomes
extremely low.
INDUSTRIAL APPLICABILITY
[0090] According to the present invention, an austenitic stainless
steel can be provided which has excellent mechanical properties
(strength and ductility) and corrosion resistance for use as a
component of structural equipment which is exposed to high pressure
hydrogen gas, specifically which is used in a hydrogen environment
such as in a fuel cell automobile or a hydrogen gas station.
* * * * *