pocument Name] Description
[Title of Invention] HIGH-STRENGTH AUSTENITIC STAINLESS STEEL FOR
HIGH-PRESSURE HYDROGEN GAS
[Technical Field]
[OOO 11
The present invention relates to a high-strength stainless steel for high-pressure
hydrogen gas, which has a tensile strength of 800 MPa or higher, and has excellent
mechanical properties in a high-pressure hydrogen gas environment.
[Background Art]
[00021
In recent years, the development of fuel-cell vehicles that run using hydrogen as the
fuel and researches on practical hydrogen stations for supplying hydrogen to %el-cell
vehicles have been advanced. A stainless steel is one of candidate materials used for
these applications; still, in a high-pressure hydrogen gas environment, the stainless steel
may be susceptible to embrittlement caused by hydrogen gas (hydrogen environment
embrittlement). In accordance with the Exemplified Standards of Compressed Hydrogen
Vehicle Container stipulated in the High Pressure Gas Safety Act, the use of austenitic
SUS3 16L is approved as a stainless steel that is not susceptible to hydrogen embrittlement.
[0003]
In consideration of the necessity for reduced weight of fuel-cell vehicle and for
high-pressure operation of hydrogen station, however, for a stainless steel used for a
container and a pipe, there has been a need for stainless steel that has a strength higher than
that of the existing SUS3 16L, especially has a tensile strength of 800 MPa or higher and is
0 not susceptible to hydrogen environment embrittlement in a hydrogen gas environment.
That is, assuming the use of high-pressure hydrogen of about 70 MPa, it is estimated that
the SUS316L requires a pipe and container to have a wall thickness of 20 mm or larger,
which leads to a significant increase in empty vehicle weight, so that higher strength of
steel is indispensable.
[0004]
As a method for enhancing the strength of steel, cold rolling can be cited as a
typical method. Patent Document 1 gives a description concerning the cold rolling and
the hydrogen environment embrittlement property of austenitic stainless steel.
[0005]
As means for strengthening the austenitic stainless steel and improving the
hydrogen embrittlement property of the austenitic stainless steel without relying on
strengthening by cold rolling, Patent Documents 2 and 3 propose high-strength stainless
steels for high-pressure hydrogen gas, in which precipitation strengthening by means of
I fine nitrides is utilized.
I
[0006]
Patent Document 2 proposes a high-strength austenitic stainless steel in which 7 to
30% of Mn, 15 to 22% of Cr, and 5 to 20% of Ni are contained as principal components,
and Patent Document 3 proposes a high-strength austenitic stainless steel in which 3 to
30% of Mn, more than 22% to 30% or less of Cr, and 17 to 20% of Ni are contained as
principal components. These Documents indicate that a tensile strength of 800 MPa or
higher can be realized in a state of solid solution heat treatment.
[Citation List]
[Patent Document]
[0007]
[Patent Document 11 WO 200411 11285
[Patent Document 21 WO 2004/083477
[Patent Document 31 WO 2004/083476
[Summary of Invention]
[Technical Problem]
[0008]
In Patent Document 1, the influence of cold rolling on the hydrogen environment
embrittlement has also been studied for SUS316L, in which it is verified that the cold
rolling in the reduction of area of 30% or less does not have a great influence on the
hydrogen environment embrittlement property, indicating the possibility that a tensile
strength of about 800 MPa can be realized by cold rolling in the reduction of area of 20 to
30%. However, the high-strength austenitic stainless steel has a problem of the decrease
in elongation and in hydrogen environment embrittlement property by cold rolling. The
invention described in Patent Document 1 discloses, as measures against this problem, a
technique in which cold rolling is performed at two or more stages, and by performing cold
rolling in different rolling directions, the decrease in hydrogen environment embrittlement
property and the decrease in elongation are restrained; however, the application of this
invention inevitably requires considerably complicated cold rolling.
[0009]
Further, in the case where a cold-rolled material is welded, local softening may be
caused by welding heat affect. Therefore, it is difficult to join the materials by a welded
joint, and the joint of the materials is restricted to a mechanical joint. To reduce the
weight of fuel-cell vehicle or to streamline the piping system for the hydrogen station,
there has been a strong need for a stainless steel that has a high strength and has no
problem even if being welded. In this case, such means for achieving strengthening by
cold rolling is difficult to apply in some respects.
[OO lo]
The austenitic stainless steels described in Patent Documents 2 and 3 realize a high
0 strength of 800 MPa or higher in a state after solid solution heat treatment. However, in
Patent Document 2, when the Mn content is less than 7%, a sufficient hydrogen
environment embrittlement property cannot be obtained, and a sufficient strength cannot be
realized in a state of solid solution heat treatment. Also, in the steel relating to Patent
Document 3, both of Cr concentration and Ni concentration are considerably high, so that
this steel has a disadvantage of considerably high alloy cost.
[OOl 11
The austenitic stainless steel described in Patent Document 2 can be produced at a
somewhat low alloy cost as compared with the steel described in Patent Document 3.
Therefore, if the stainless steel can be used in high-pressure hydrogen applications even if
the stainless steel has low content of Mn of less than 7% as compared with Patent
Document 2, an advantage is brought about in industrial production, since the steels of this
Mn content range have been used conventionally in applications such as the nuclear field,
and a common ingot can be used.
[0012]
The present invention has been made in view of the present situation, and
accordingly an objective thereof is to provide an austenitic stainless steel that has a high
strength such that the tensile strength is 800 MPa or higher and is excellent in hydrogen
environment embrittlement property in the composition range of less than 7% of Mn,
which austenitic stainless steel has not been realized in Patent Document 2.
[Solution to Problem]
[0013]
The present inventors conducted various studies to solve the problem, and
resultantly obtained the findings described in items (a) to (d) below.
[OO 1 41
(a) By utilizing nitrogen as a solute element, the strength of stainless steel can be
enhanced. However, the addition of a large amount of nitrogen decreases the stacking
fault energy, and therefore has an adverse influence such that the distortion at the
deformation time is localized, and the durability against hydrogen environment
embrittlement is decreased.
[00 1 51
(b) By making grains fine, the resistance to hydrogen environment embrittlement of
high-nitrogen steel can be enhanced. As a method for making grains fine, there is a
method in which by precipitating fine alloy carbo-nitrides at the time of final solid solution
heat treatment, the growth of grains is restrained by the pinning effect. In order to
produce fine carbo-nitrides and to make the grains of high-nitrogen steel fine, it is most
effective to add V or Nb. However, in the conventional method, although V and Nb
precipitate as nitrides, V and Nb agglomerate and coarsen because of a small amount of
precipitate nucleus, so that the pinning effect cannot be achieved sufficiently.
[OO 161
(c) As a method for solving this problem, a production process involving solid
solution heat treatment, cold rolling, and secondary heat treatment is effective. In the
initial solid solution heat treatment, the alloying elements are dissolved sufficiently. In
the next cold rolling step, distortion is given, whereby the amount of precipitate nucleus of
carbo-nitrides precipitating at the time of the next secondary heat treatment is increased,
the carbo-nitrides are precipitated finely, and the grains are made fine.
[00 171
(d) That is, in an alloy system having a Mn content lower than that of Patent
Document 2, by performing cold rolling at an intermediate stage of two heat treatments,
the precipitation of carbo-nitrides is stimulated, and by the resultant refinement effect of
austenite grains and the precipitation strengthening action due to the precipitation itself of
carbo-nitrides, a high strength can be attained, and also the resistance to hydrogen
environment embrittlement can be enhanced.
[0018]
The present invention has been completed based on the findings, and the gists
thereof are austenitic stainless steels for high-pressure hydrogen gas described in items (1)
to (3) below.
[00 191
(1) An austenitic stainless steel for high-pressure hydrogen gas consisting, by mass
percent, of C: 0.10% or less, Si: 1.0% or less, Mu: 3% or more to less than 7%, Cr: 15 to
30%, Ni: 10% or more to less than 17%, Al: 0.10% or less, N: 0.10 to 0.50%, and at least
one kind of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%, the balance being Fe and impurities,
wherein in the impurities, the P content is 0.050% or less and the S content is 0.050% or
less, the tensile strength is 800 MPa or higher, the grain size number (ASTM El 12) is No.
8 or higher, and alloy carbo-nitrides having a maximum diameter of 50 to 1000 nm are
contained in the number of 0.4lW2 or larger in cross section observation.
[0020]
(2) An austenitic stainless steel for high-pressure hydrogen gas consisting, by mass
percent, of C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more to less than 7%, Cr: 15 to
30%, Ni: 10% or more to less than 17%, Al: 0.10% or less, N: 0.10 to 0.50%, and at least
one kind of V: 0.010 to 1.0% and Nb: 0.01 to 0.50%, further containing one or more kinds
of elements of at least one group selected from element groups of a first group to a fourth
group described below, the balance being Fe and impurities, wherein in the impurities, the
P content is 0.050% or less and the S content is 0.050% or less, the tensile strength is 800
MPa or higher, the grain size number (ASTM El 12) is No. 8 or higher, and alloy carbonitrides
having a maximum diameter of 50 to 1000 nm are contained in the number of
0.4lPm2 or larger in cross section observation.
First group elements ... Mo: 0.3 to 3.0% and W: 0.3 to 6.0%
Second group elements ... Ti: 0.001 to 0.5%, Zr: 0.001 to 0.5%, Hfi 0.001 to 0.3%, and Ta:
0.001 to 0.6%
Third group elements ... B: 0.0001 to 0.020%, Cu: 0.3 to 5.0%, and Co: 0.3 to 10.0%
Fourth group elements ... Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, La: 0.0001 to
0.20%, Ce: 0.0001 to 0.20%, Y: 0.0001 to 0.40%, Sm: 0.0001 to 0.40%, Pr: 0.0001 to
0.40%, and Nd: 0.0001 to 0.50%
[002 11
(3) The austenitic stainless steel for high-pressure hydrogen gas described in item
(1) or (2), wherein the austenitic stainless steel is subjected to solid solution heat treatment
at a temperature of 1100 to 1200°C, next being subjected to cold rolling in which the
reduction of area is 20% or more, and thereafter is again subjected to heat treatment in the
temperature range of 900°C or higher and lower than the solution treatment temperature.
[Advantageous Effect of Invention]
[0022]
According to the present invention, there can be provided a high-strength austenitic
stainless steel that has a tensile strength of 800 MPa or higher and is excellent in hydrogen
environment embrittlement property in the composition region of less than 7% of Mn.
EDescription of Embodiment]
[0023]
The reasons for restricting the chemical composition and metal micro-structure of a
steel plate in the present invention are as follows:
[0024]
(A) Chemical composition of steel
The operational advantages of each component of steel and the preferable content of
each component are described below. The symbol "%" concerning the content of each
element means "mass percent",
[0025]
C: 0.10% or less
In the present invention, C (carbon) is not an element that is added positively. If
the C content is more than 0.10%, carbides precipitate at the grain boundaries, and exert an
adverse influence on toughness and the like. Therefore, the C content is restrained to
0.10% or less. The C content is preferably 0.04% or less, further preferably 0.02% or less.
The C content should be as low as possible. However, the extreme reduction in C content
leads to an increase in refrning cost, so that it is desirable to make the C content 0.001% or
more in practical application.
[0026]
Si: 1 .O% or less
If Si (silicon) is contained in large amounts, Si forms an intermetallic compound
with Ni, Cr, or the like, or promotes the formation of an intermetallic compound such as
sigma phase, so that, in some cases, the hot workability is decreased remarkably.
Therefore, the Si content is 1.0% or less. Preferably, the Si content is 0.5% or less. The
Si content should be as low as possible. However, considering the refining cost, it is
desirable to make the Si content 0.01% or more.
[0027]
Mn: 3% or more to less than 7%
Mn (manganese) is an inexpensive austenite stabilizing element. In the steel of the
present invention, due to a proper combination with Cr, Ni, N, and the like, Mn contributes
to the enhancement of strength and the improvement in ductility and toughness. The
present invention also has an aim of finely precipitating carbo-nitrides and making the
grains fine. In the case where the amount of dissolved N is small, even if the steel
undergoes the later-described process consisting of solid solution heat treatment, cold
rolling, and secondary heat treatment, carbo-nitrides having a sufficient number density
cannot be precipitated, and it becomes difficult to enhance the strength due to finer
austenite grains. Therefore, 3% or more of Mn must be contained. If the Mn content is
7% or more, the technique described in Patent Document 2 can be applied. Therefore, in
the present invention, the upper limit of the Mn content is less than 7%. For these reasons,
the Mn content is specified so as to be 3% or more to less than 7%. The preferable lower
limit of the Mn content is 4%. Also, the Mn content is effective wheh being 6.5% or less,
especially effective when being 6.2% or less.
[0028]
Cr: 15 to 30%
Cr (chromium) is an essential component because it is an element for ensuring
corrosion resistance as a stainless steel. The Cr content must be 15% or more. However,
if the Cr content is excessively high, coarse carbides such as M23C6, which decrease the
ductility and toughness, are easily formed in large amounts. Therefore, the proper Cr
content is 15 to 30%. The Cr content is preferably 18 to 24%, further preferably 20 to
23.5%.
[0029]
Ni: 10% or more to less than 17%
Ni (nickel) is added as an austenite stabilizing element. In the steel of the present
invention, due to a proper combination with Cr, Mn, N, and the like, Ni contributes to the
enhancement of strength and the improvement in ductility and toughness. Therefore, the
Ni content is 10% or more. However, if the Ni content is 17% or more, the effect
saturates, and the material cost increases. For these reasons, the proper Ni content is 10%
or more to less than 17%. The Ni content is preferably 11 to 15%, fbrther preferably 11.5
to 13.5%.
[0030]
Al: 0.10% or less
A1 (aluminum) is an important element as a deoxidizer. However, if the Al
content is more than 0.10% and Al remains in large amounts, the formation of an
intermetallic compound such as sigma phase is promoted. Therefore, to attain both of the
strength and toughness intended by the present invention, the A1 content must be restricted
to 0.10% or less. In order to reliably achieve the deoxidizing effect, the A1 content is
desirably 0.00 1 % or more. The A1 content is preferably 0.05% or less, further preferably
C 0.03% or less. In this description, A1 means so-called "sol. A1 (acid soluble Al)".
[003 11
N: 0.10 to 0.50%
N (nitrogen) is the most important solid-solution strengthening element, and at the
same time, in the present invention, makes the grains fine due to the formation of fine alloy
carbo-nitrides, contributing to the enhancement of strength. To utilize N for the
enhancement of strength, 0.10% or more of N must be contained. However, if the N
content is more than 0.50%, coarse nitrides are formed, and therefore the mechanical
properties such as toughness decrease. Therefore, the N content is 0.10 to 0.50%. The
lower limit of the N content is preferably 0.20%, further preferably 0.30%.
[0032]
V: 0.01 to 1.0% and/or Nb: 0.01 to 0.50%
V (vanadium) and Nb (niobium) are important elements in the steel of the present
invention. To promote the formation of alloy carbo-nitrides and to contribute to finer
grains, either one or both of V and Nb must be contained. For these purposes, 0.01% or
more of V andlor Nb must be contained. On the other hand, even if more than 1 .O% of V
andlor more than 0.50% of Nb are contained, the effect saturates, and the material cost
increases, so that the upper limits of the V content and the Nb content are 1 .O% and 0.50%,
respectively. The V content is preferably 0.10 to 0.30%, and the Nb content is preferably
0.15 to 0.28%. The containing of both of V and Nb is more effective.
[0033]
P: 0.050% or less
P (phosphorus), which is an impurity, is an element that exerts an adverse influence
on the toughness and the like of steel. The P content is 0.050% or less, and is preferably
as low as possible. The P content is preferably 0.025% or less, further preferably 0.018%
or less.
[0034]
S: 0.050% or less
S (sulk), which is an impurity, is an element that, like P, exerts an adverse
influence on the toughness and the like of steel. The S content is 0.050% or less, and is
preferably as low as possible. The S content is preferably 0.010% or less, W e r
preferably 0.005% or less.
[0035]
The steel in accordance with the present invention has the above-described
chemical composition, and in the steel, the balance consists of Fe and impurities. The
"impurities" in the "Fe and impurities" mean components that mixed in on account of
various factors in the production process, including raw materials such as ore or scrap,
when a steel is produced on an industrial scale, the components being allowed to exist in
the range such that they do not an adverse influence on the present invention.
LO03 61
The steel in accordance with the present invention can contain, as necessary, one or
more kinds of components selected from at least one group of the first group to the fourth
group described below. Hereunder, the components belonging to these groups are
described.
[0037]
The elements belonging to the first group are Mo and W. These elements have a
common operational advantage of stimulating the formation and stabilization of carbonitrides
and contributing to solid-solution strengthening. The reasons for restricting the
contents of these elements are as described below.
[0038]
Mo: 0.3 to 3.0%, W: 0.3 to 6.0%
Mo (molybdenum) and W (tungsten) have an effect of forming carbo-nitrides and
thereby making the grains fine, and also contribute to solid-solution strengthening. Either
of these elements achieves the effect when the content of each of these elements is 0.3% or
more, so that these elements can be contained as necessary. However, even if these
elements are contained excessively, the effect saturates. Therefore, if these elements are a contained, the contents thereof should be as follows: Mo: 0.3 to 3.0%, and W: 0.3 to 6.0%.
[0039]
The elements belonging to the second group are Ti, Zr, Hf, and Ta. These
elements have a common operational advantage of stimulating the formation of carbonitrides.
[0040]
Ti: 0.001 to 0.5%, Zr: 0.001 to 0.5%, Hf: 0.001 to 0.3%, Ta: 0.001 to 0.6%
Ti (titanium), Zr (zirconium), Hf (hafhium), and Ta (tantalum), which, like V and
Nb, have an effect of forming alloy carbo-nitrides and thereby making the grains fine, can
be contained as necessary. This effect can be achieved by containing 0.001% or more of
each of these elements. However, even if these elements are contained excessively, the
effect saturates. Therefore, the upper limits of the contents of these elements are
respectively as follows: Ti: 0.5%, Zr: 0.5%, Hf: 0.3%, and Ta: 0.6%. The upper limits of
I
contents of Ti and Zr are preferably 0.1%, M e r preferably 0.03%. The upper limit of
the Hf content is preferably 0.08%, further preferably 0.02%. The upper limit of the Ta
content is preferably 0.4%, further preferably 0.3%.
[004 11
The elements belonging to the third group are By Cu, and Co. These elements
contribute to the enhancement of strength. The reasons for restricting the contents of
these elements are as described below.
[0042]
B: 0.0001 to 0.020%
B (boron), which makes the precipitates fine, and decrease the austenite grain
diameter, whereby increasing the strength, can be contained as necessary. The effect
thereof is achieved when the B content is 0.0001% or higher. On the other hand, if the B
content is excessive, a compound of low melting point is formed, and the hot workability
may be decreased. Therefore, the upper limit of the B content is 0.020%.
100431 * Cu: 0.3 to 5.0%, Co: 0.3 to 10.0%
Cu (copper) and Co (cobalt) are austenite stabilizing elements, and contribute to the
enhancement of strength due to solid-solution strengthening. Therefore, 0.3% or more of
either one or both of these elements can be contained as necessary. However, because of
the balance between effect and material cost, the upper limits of the contents of Cu and Co
are 5.0% and 10.0%, respectively.
[0044]
The elements belonging to the fourth group are Mg, Ca, La, Ce, Y, Sm, Pr, and Nd.
These elements have a common action for preventing solidification cracking at the time of
casting.
[0045]
Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, La: 0.0001 to 0.20%, Ce: 0.0001 to
0.20%, Y: 0.0001 to 0.40%, Sm: 0.0001 to 0.40%, Pr: 0.0001 to 0.40%, and Nd: 0.0001 to
0.50%
Mg (magnesium) and Ca (calcium), and La (lanthanum), Ce (cerium), Y (yttrium),
Sm (samarium), Pr (praseodymium), and Nd (neodymium) among transition metals have
an action for preventing solidification cracking at the time of casting. Therefore, one or
more kinds of these elements may be contained as necessary. The effect can be achieved
by containing 0.0001% or more of each of these elements. On the other hand, if these
elements are contained excessively, the hot workability decreases. Therefore, the upper
limits of the contents of these elements are as follows: Mg and Ca: 0.0050%, La and Ce:
0.20%, Y, Sm and Pr: 0.40%, and Nd: 0.50%.
[0046]
(B) Micro-structure of the steel
The nitrogen used in the present invention is effective in performing solid-solution
strengthening, but has an action such that the distortion at the time of deformation is
localized by decreasing the stacking fault energy, and the durability against hydrogen
environment embrittlement is decreased. However, by decreasing the grain diameter,
@ both of the enhancement of strength to 800 MPa or higher and the prevention of hydrogen
environment embrittlement are enabled. In order to prevent hydrogen environment
embrittlement, the grain size number (ASTM El 12) is No. 8 or higher, preferably No. 9 or
higher, and further preferably No. 10 or higher.
[0047]
In order to make the grains fine, the pinning utilizing alloy carbo-nitrides is
effective. To achieve this effect, alloy carbo-nitrides having a size of 50 to 1000 nm must
be contained in the number of 0 . 4 1o~r l~arg er in cross section observation. These alloy
carbo-nitrides are those that contain Cr, V, Nb, Mo, W, Ta, and the like as principal
components, and have a crystalline structure of Z phase, that is, Cr(Nb, V)(C, N) or of MX
type (M: Cr, V, Nb, Mo, W, Ta, and the like, X: C, N). The alloy carbo-nitrides in the
present invention are carbo-nitrides scarcely containing Fe. Even if Fe is contained, the
amount of Fe is 1 atom% or less. Also, the carbo-nitrides in the present invention include
those in which the content of C (carbon) is extremely low, that is, those consisting of
nitrides.
[0048]
(C) Production method
In order to make grains fine as described in (B) and to precipitate fine alloy carbonitrides
having a desired number density, the ordinary method cannot be used. However,
the steel of the present invention can be produced by successively performing the solid
solution heat treatment, cold rolling, and secondary heat treatment described below.
[0049]
The first solid solution heat treatment must be performed at a temperature of
1000°C or higher, preferably 1100°C or higher, to dissolve alloying elements sufficiently.
However, if the solid solution heat treatment temperature is higher than 1 200°C, the grains
are coarsened extremely. Therefore, the upper limit of the solid solution heat treatment
temperature is 1200°C. Hereunder, for convenience, the heat treatment temperature in the
solid solution heat treatment is referred to as a "T1 temperature".
[OOSO]
In the solid solution heat treatment in accordance with the present invention,
solution treatment of a degree necessary for precipitating carbo-nitrides in the later
secondary heat treatment has only to be performed, and all of the carbo-nitride forming
elements need not necessarily be dissolved. The steel material having been subjected to
solid solution heat treatment is preferably cooled rapidly from the solid solution heat
treatment temperature. In this case, water cooling (shower water cooling or dipping) is
preferable.
[005 11
Also, concerning the solid solution heat treatment, an independent solid solution
heat treatment step need not necessarily be provided. By performing rapid cooling after a
process of hot working such as hot extrusion, the equivalent effect can be achieved. For
example, rapid cooling has only to be performed after hot extrusion at about 1 150°C.
[0052]
Next, in order to increase the amount of precipitate nucleus of carbo-nitrides, cold
rolling is performed at a cold rolling ratio such that the reduction of area is 20% or more.
The upper limit of cold rolling ratio is not restricted especially. However, considering the
working ratio at the time when an ordinary member is subjected to cold rolling, 90% or
less of cold rolling ratio is preferable. Finally, in order to remove distortion caused by
cold rolling and to making the grains fine by precipitating fine carbo-nitrides, secondary
heat treatment is performed at a temperature lower than the T1 temperature. Hereunder,
for convenience, the heat treatment temperature in the secondary heat treatment is referred
to as a "T2 temperature".
[0053]
The T2 temperature is less than the T1 temperature. In order to make the grains
finer, the upper limit of the T2 temperature is preferably made [Tl treatment temperature -
20°C], and further preferably made [Tl treatment temperature - 50°C]. Specifically, the
upper limit of the T2 temperature is preferably made 1 150°C, and M e r preferably made
1080°C. On the other hand, the lower limit of the T2 temperature is 900°C because if the
T2 temperature is lower than 900°C, coarse Cr carbides are formed, and therefore the
micro-structure becomes non-uniform.
[Examples]
[0054]
In the following, the effects of the present invention are explained based on
examples.
[0055]
Fifty kilograms of each of stainless steels having the chemical compositions given
in Table 1 was vacuum melted and hot-forged to form a block having a thickness of 40 to
60 mrn.
[0056]
[Table 11
-
-Stee
A
A
C
D
E
F
3
Jl-
I
J
K
L
2
p
Q
J
-s-
J-
1
1
4
y
2
1
2
3
4
5
6
7
' h a
Table 1
0.36 1 4.95 1 0.014 1 e0.001 1 12.96 1 22.01 1 0.22
cope of the invention steel.
Thereafter, the block was hot-rolled to a predetermined thickness, and was
subjected to one-hour solid solution heat treatment, cold rolling, and one-hour secondary
heat treatment, whereby an 8-rnm thick plate material was formed. In Table 2, the solid
solution heat treatment temperature (TI temperature) of each test No. is expressed by
Tl("C), and the secondary heat treatment temperature (T2 temperature) thereof is
expressed by T2("C). The cold rolling ratio of each test No. is also shown in Table 2.
[Table 21
Table 2
* shows out of scope of the invention steel.
** shows out of scope of the invention method.
[0059]
A specimen was sampled and embedded with a resin so that the cross section
perpendicular to the rolling direction of the plate material can be observed, and after
Ted
No.
1
2
3
4
5
6
7
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
Number of
carbo-nitrides
[XI 0/25pm2)
35
42
49
55
33
41
28
29
27
20
23
82
12
15
65
31
25
29
27
28
28
25
24
27
34
31
58
65
52
25
24
31
33
25
24
38
0.3'
0.2'
0.5*
31
0.3'
0.1 *
22
28
0.2'
steel
A
A
A
A
A
A
A
B
C
D
E
F
G
H
1
J
K
L
M
N
0
P
Q
R
S
T
U
V
W
X
Y
Z
1
2
3
A
A
A
A
A
A
4
5
6
7
TS
[MPa)
826
814
822
828
819
82 1
81 5
808
805
812
809
865
805
812
854
812
81 1
808
809
814
830
815
806
803
830
809
825
842
81 5
805
808
83 1
812
813
822
802
666'
654*
704'
805
688"
581'
813
802
560'
T I
[OC)
1100
1100
1100
1150
1150
1100
1200
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1100
1000
1000
1250"'
950"
1100
1100
1100
1100
1100
1100
1100
1100
Relative
rupture
elongation (%)
96
98
103
101
92
93
82
96
98
92
93
92
85
86
89
100
101
98
99
100
99
104
99
99
98
96
95
95
96
105
101
98
99
101
1 02
65
63
53
58
75
73
74
55
45
73
Cold
rolling
ratio
25
40
50
50
25
40
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
40
40
0*'
15"
25
40
25
25
25
25
c
.0- C s
al .zal
Ical
.> C
eo E" 0 o
T2
[OC)
900
1000
950
1050
1050
1050
1150
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
1000
950
950
1000
1000
1000
1000
1100-
850
1000
1000
1000
1000
Grain size
No.
10.2
10.6
11.0
10.5
9.1
9.6
8.2
10.3
10.5
10.3
10.4
10.3
10.1
10.5
10.3
10.2
10.3
10.3
10.2
10.2
10.3
10.3
10.2
10.4
10.3
10.2
10.2
10.2
10.3
10.4
10.4
10.3
10.3
10.9
10.9
7.5'
7.2*
6.7:
7.2'
7.8'
7.5'
7.3'
10.5
10.6
7.7'
electrolytic etching, the grain size number (in conformity to ASTM El 12) was measured.
Also, similarly, by using a resin embedding material in the cross section direction, the
number of precipitates was measured by the observation under an electron microscope
using the extraction replica method. A region of 25 pm2 was observed at ~10,000
magnification in ten visual fields, and precipitates having a size of 50 to 1000 nrn were
measured. The precipitates measured in examples were carbo-nitrides of Z phase of
rhombic structure containing Cr, V, Nb, C, N, and the like, or of MX type of tetragonal
structure containing Cr, Nb, V, C, N, and the like.
[0060]
A round-bar tensile test specimen having a diameter of 3 mm in its parallel part was
sampled in the longitudinal direction of the plate material, and a tensile test was conducted
at a strain rate of 3 x 10'1s in the atmosphere at normal temperature or in high-pressure
hydrogen gas of 85 MPa at normal temperature to measure tensile strength (TS) and
rupture elongation. Since hydrogen has a remarkable influence on the decrease in
ductility, the ratio of rupture elongation in hydrogen to rupture elongation in the
atmosphere was made a relative rupture elongation, and it was interpreted that if the
relative rupture elongation is 80% or more, preferably 90% or more, the decrease in
ductility caused by hydrogen is slight, and the resistance to hydrogen environment
embrittlement is excellent.
[0061]
The strain rate of 3 x 10-~/isn the above-described tensile test is considerably lower
than the strain rate of 10~1s in the tensile test in the high-pressure hydrogen gas
environment, which has been used in the conventional documents. The reason for this is
that in the recent evaluation standards in durability evaluation against hydrogen
environment embrittlement, the evaluation test at a very low strain rate, in which the
hydrogen environment embrittlement susceptibility of austenitic stainless steel becomes
higher, is recommended.
[0062]
@ Table 2 summarized the grain size number, the number of carbo-nitrides, tensile
strength (TS), and relative rupture elongation of steel being tested. Test Nos. 1 to 35 are
example embodiments of the present invention, in which the grain size number was No. 8
or higher, a sufficient number of carbo-nitrides were precipitated, the TS was 800 MPa or
higher, and the relative rupture elongation was also 80% or more, a sufficient resistance to
hydrogen environment embrittlement being attained.
[0063]
Test Nos. 36 to 41 are comparative examples. In test No. 36, the solid solution
heat treatment temperature T1 was too high, the grains were coarsened, and the resistance
to hydrogen environment embrittlement was poor. In test No. 37, the solid solution heat
treatment temperature T1 was too low, the number density of carbo-nitrides was low, the
grains were coarsened, and the resistance to hydrogen environment embrittlement was poor.
In test Nos. 38 and 39, the cold rolling ratio was low, the precipitation number of carbonitrides
was insufficient, the grains were coarsened, and the resistance to hydrogen
environment embrittlement was poor. In test No. 40, the secondary heat treatment
temperature T2 was too high, the grains were coarsened, and the resistance to hydrogen
environment embrittlement was poor. In test No. 41, the final solid solution heat
treatment temperature T2 was too low, the number density of carbo-nitrides was low, the
grains were coarsened, and the resistance to hydrogen environment embrittlement was poor.
[0064]
Test Nos. 42 to 45 are comparative examples, in which the chemical composition of
steel material was out of the range of the present invention. In test No. 42, the Mn
content was too low, and resultantly N (nitrogen) could not be contained suficiently, the
grains were coarsened, the strength was low, and the resistance to hydrogen environment
embrittlement was poor. In test No. 43, the Ni content was low, 6 ferrite was formed, and
the resistance to hydrogen environment embrittlement was poor. In test No. 44, the Cr
content was high, coarse Cr carbides were formed, and the resistance to hydrogen
environment embrittlement was poor. In test No. 45, the N (nitrogen) content was low,
the grains were coarsened, the strength was low, and the resistance to hydrogen
environment embrittlement was poor.
[Industrial Applicability]
[0065]
As described above, according to the present invention, even an austenitic stainless
steel containing less than 7% of Mn can be made a high-strength steel excellent in
hydrogen environment embrittlement property by interposing a cold rolling step between
two heat treatments, and therefore can be used for pipes and containers for high-pressure
hydrogen gas.

We claim: ;
[Claim 11
An austenitic stainless steel for high-pressure hydrogen gas consisting, by mass
percent, of C: 0.10% or less, Si: 1.0% or less, Mn: 3% or more to less than 7%, Cr: 15 to
30%, Ni: 10% or more to less than 17%, Al: 0.10% or less, N: 0.10 to 0.50%, and at least
one kind of V: 0.01 to 1.0% and Nb: 0.01 to 0.50%, the balance being Fe and impurities,
wherein in the impurities, the P content is 0.050% or less and the S content is 0.050% or
less, the tensile strength is 800 MPa or higher, the grain size number (ASTM El 12) is No.
8 or higher, and alloy carbo-nitrides having a maximum diameter of 50 to 1000 nm are
contained in the number of 0.4lPm2 or larger in cross section observation.
[Claim 21
An austenitic stainless steel for high-pressure hydrogen gas consisting, by mass
percent, of C: 0.10% or less, Si: 1 .O% or less, Mn: 3% or more to less than 7%, Cr: 15 to
30%, Ni: 10% or more to less than 17%, Al: 0.10% or less, N: 0.10 to 0.50%, ahd at least
one kind of V: 0.0 10 to 1 .O% and Nb: 0.01 to 0.50%, further containing one or more kinds
of elements of at least one group selected fiom element groups of a first group to a fourth
group described below, the balance being Fe and impurities, wherein in the impurities, the
P content is 0.050% or less and the S content is 0.050% or less, the tensile strength is 800
MPa or higher, the grain size number (ASTM El 12) is No. 8 or higher, and alloy carbonitrides
having a maximum diameter of 50 to 1000 nm are contained in the number of
0.4lPm2 or larger in cross section observation.
First group elements ... Mo: 0.3 to 3.0% and W: 0.3 to 6.0%
Second group elements ... Ti: 0.001 to 0.5%, Zr: 0.001 to 0.5%, Hf: 0.001 to 0.3%, and Ta:
0.001 to 0.6%
Third group elements ... B: 0.0001 to 0.020%, Cu: 0.3 to 5.0%, and Co: 0.3 to 10.0%
Fourth group elements ... Mg: 0.0001 to 0.0050%, Ca: 0.0001 to 0.0050%, La: 0.0001 to
0.20%, Ce: 0.0001 to 0.20%, Y: 0.0001 to 0.40%, Sm: 0.0001 to 0.40%, Pr: 0.0001 to
0.40%, and Nd: 0.0001 to 0.50%
The austenitic stainless steel for high-pressure hydrogen gas according to claim 1 or
2, wherein the austenitic stainless steel is subjected to solid solution heat treatment at a
I temperature'of 1000 to 1200°C, next being subjected to cold rolling in which the reduction
of area is 20% or more, and thereafter is again subjected to heat treatment in the
temperature range of 900°C or higher and lower than the solution treatment temperature.
Dated this 2nd day of July, 2013.
Nippon Steel & Sumitomo Metal Corporation
L 4 . h A w A
(Varun Sharma)
1 of Amarchand & Mangaldas &
I Suresh A. Shroff & Co.
I Attorneys for the Applicant