TECHNICAL FIELD
[0001]
The present disclosure relates to a steel material, and more specifically relates
to an austenitic stainless steel material.
BACKGROUND ART
[0002]
A steel material that is used in chemical plant facilities such as oil refinery
plants and petrochemical plants is required to have high temperature strength. An
austenitic stainless steel material is used as a steel material for use in these chemical
plant facilities.
[0003]
A chemical plant facility includes a plurality of apparatuses. The respective
apparatuses included in a chemical plant facility are, for example, a vacuum
distillation unit, a desulfurization unit, and a catalytic reforming unit. These
apparatuses include a heating furnace pipe, a reactor, a tank, a heat exchanger, piping
and the like. The average temperatures during operation of the respective
apparatuses differ from each other. Hereinafter, the average temperature during
operation is referred to as an "average operation temperature". The operation
temperature varies significantly depending on the raw material and products
processed at the chemical plant facility. Further, the apparatuses in a chemical plant
facility also include multiple apparatuses that operate at an average operation
temperature of more than 600 to 750C.
[0004]
An apparatus that operates at an average operation temperature of more than
600 to 750C needs to have high creep strength.
[0005]
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International Application Publication No. WO2018/043565 (Patent Literature
1) discloses improving the creep strength of an austenitic stainless steel material to
be used in a high-temperature region. An austenitic stainless steel disclosed in
Patent Literature 1 has a chemical composition which consists of, in mass%, C:
0.030% or less, Si: 0.10 to 1.00%, Mn: 0.20 to 2.00%, P: 0.040% or less, S: 0.010%
or less, Cr: 16.0 to 25.0%, Ni: 10.0 to 30.0%, Mo: 0.1 to 5.0%, Nb: 0.20 to 1.00%, N:
0.050 to 0.300%, sol. Al: 0.0005 to 0.100%, B: 0.0010 to 0.0080%, Cu: 0 to 5.0%,
W: 0 to 5.0%, Co: 0 to 1.0%, V: 0 to 1.00%, Ta: 0 to 0.2%, Hf: 0 to 0.20%, Ca: 0 to
0.010%, Mg: 0 to 0.010%, and rare earth metal: 0 to 0.10%, with the balance being
Fe and impurities, and which satisfies Formula (1). Here, Formula (1) is as follows:
B + 0.004 - 0.9C + 0.017Mo2  0.
CITATION LIST
PATENT LITERATURE
[0006]
Patent Literature 1: International Application Publication No. WO2018/043565
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0007]
In this connection, when newly constructing a chemical plant facility or
repairing a chemical plant facility, the steel material used for the apparatuses in the
chemical plant facility is welded at the site where the chemical plant is located. In
recent welding work, in order to reduce the number of welding passes, there are
many cases in which welding with higher heat input for which the heat input is made
large is adopted.
[0008]
As described above, a steel material to be used at an average operation
temperature of more than 600C is required to be excellent in high temperature
strength. Consequently, the steel material tends to be thick-walled and/or larger in
size. When such kind of steel material is welded, a large amount of residual stress
is generated in a heat affected zone (hereinafter, also referred to as a "HAZ").
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When such kind of steel material is used at an average operation temperature of more
than 600C, a stress relaxation process occurs in which the residual stress in the heat
affected zone relaxes. In the stress relaxation process, carbides form within grains
during the course of the recovery of residual stress in the heat affected zones, and
secondary induced precipitation hardening occurs. A difference between the
hardness within grains and the hardness at grain boundaries increases due to the
secondary induced precipitation hardening. As a result, in some cases stress
relaxation cracking occurs at the grain boundaries. Therefore, in a steel material to
be used for a long time period at an average operation temperature of more than 600
to 750C, it is desirable not only that the creep strength is high, but also that stress
relaxation cracking can be suppressed, that is, that the stress relaxation cracking
resistance is high.
[0009]
The austenitic stainless steel proposed in Patent Literature 1 exhibits excellent
creep strength. However, in Patent Literature 1, stress relaxation cracking
resistance is not investigated.
[0010]
An objective of the present disclosure is to provide an austenitic stainless steel
material that has high creep strength even when used at an average operation
temperature of more than 600 to 750C after welding with higher heat input, and
furthermore, has excellent stress relaxation cracking resistance even after use for a
long time period at an average operation temperature of more than 600 to 750C after
welding with higher heat input.
SOLUTION TO PROBLEM
[0011]
An austenitic stainless steel material having:
a chemical composition consisting of, in mass%,
C: 0.030% or less,
Si: 1.50% or less,
Mn: 2.00% or less,
P: 0.045% or less,
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S: 0.0300% or less,
Cr: 15.00 to 25.00%,
Ni: 8.00 to 20.00%,
N: 0.050 to 0.250%,
Nb: 0.10 to 1.00%,
Mo: 0.05 to 5.00%,
B: 0.0005 to 0.0100%,
Ti: 0 to 0.50%,
Ta: 0 to 0.50%,
V: 0 to 1.00%,
Zr: 0 to 0.10%,
Hf: 0 to 0.10%,
Cu: 0 to 4.00%,
W: 0 to 5.00%,
Co: 0 to 1.00%,
sol. Al: 0 to 0.100%,
Ca: 0 to 0.0200%,
Mg: 0 to 0.0200%,
rare earth metal: 0 to 0.100%,
Sn: 0 to 0.010%,
As: 0 to 0.010%,
Zn: 0 to 0.010%,
Pb: 0 to 0.010%, and
Sb: 0 to 0.010%,
with the balance being Fe and impurities,
wherein
a ratio of a dissolved N amount (mass%) in the austenitic stainless steel
material with respect to a content of N (mass%) in the austenitic stainless steel
material is 0.40 to 0.90.
ADVANTAGEOUS EFFECTS OF INVENTION
[0012]
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The austenitic stainless steel material of the present disclosure has high creep
strength even when used at an average operation temperature of more than 600 to
750C after welding with higher heat input, and furthermore, has excellent stress
relaxation cracking resistance even after use for a long time period at an average
operation temperature of more than 600 to 750C after welding with higher heat
input.
DESCRIPTION OF EMBODIMENTS
[0013]
The present inventors conducted studies regarding an austenitic stainless steel
material that has high creep strength even when used at an average operation
temperature of more than 600 to 750C after welding with higher heat input, and
furthermore, has excellent stress relaxation cracking resistance even after use for a
long time period at an average operation temperature of more than 600 to 750C after
welding with higher heat input. Hereinafter, an environment in which an average
operation temperature ranges from more than 600 to 750C is also referred to as a
"high temperature environment".
[0014]
The present inventors initially conducted studies relating to stress relaxation
cracking resistance. It is considered that stress relaxation cracking occurs as a
result of the following mechanism. In a high temperature environment, Cr carbides
form at grain boundaries in the steel material. Consequently, Cr depleted zones
(decarburized zones) are formed along grain boundaries. Cr depleted zones are
soft. Therefore, a strength difference between the inside of grains that underwent
secondary induced precipitation hardening and the Cr depleted zones along grain
boundaries increases. As a result, stress relaxation cracking occurs.
[0015]
Therefore, in order to increase stress relaxation cracking resistance, it is
effective to suppress formation of Cr depleted zones along grain boundaries. In
order to suppress formation of Cr depleted zones, it is necessary to suppress
formation of Cr carbides in the steel material. To suppress formation of Cr
carbides, it is effective to reduce the content of C, and furthermore, to contain Nb in
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the steel material to cause C in the steel material to combine with Nb as NbC in order
to thereby inhibit C in the steel material from combining with Cr.
[0016]
The present inventors conducted studies regarding chemical compositions for
a steel material while taking into account the matters described above. As a result,
the present inventors considered that if an austenitic stainless steel material has a
chemical composition consisting of, in mass%, C: 0.030% or less, Si: 1.50% or less,
Mn: 2.00% or less, P: 0.045% or less, S: 0.0300% or less, Cr: 15.00 to 25.00%, Ni:
8.00 to 20.00%, N: 0.050 to 0.250%, Nb: 0.10 to 1.00%, Mo: 0.05 to 5.00%, B:
0.0005 to 0.0100%, Ti: 0 to 0.50%, Ta: 0 to 0.50%, V: 0 to 1.00%, Zr: 0 to 0.10%,
Hf: 0 to 0.10%, Cu: 0 to 4.00%, W: 0 to 5.00%, Co: 0 to 1.00%, sol. Al: 0 to 0.100%,
Ca: 0 to 0.0200%, Mg: 0 to 0.0200%, rare earth metal: 0 to 0.100%, Sn: 0 to 0.010%,
As: 0 to 0.010%, Zn: 0 to 0.010%, Pb: 0 to 0.010%, and Sb: 0 to 0.010%, with the
balance being Fe and impurities, stress relaxation cracking resistance can be
increased while also increasing creep strength.
[0017]
With the aforementioned chemical composition, formation of Cr depleted
zones can be suppressed. However, even with the aforementioned chemical
composition, because C and Cr are contained, formation of Cr depleted zones will
still occur. Therefore, the present inventors investigated suppressing stress
relaxation cracking by means based on a different concept to the conventional means.
The present inventors conducted studies regarding a method which keeps the content
of C to 0.030% or less to thereby suppress the occurrence of Cr depleted zones to a
minimum and, in addition, even if Cr depleted zones occur, strengthens the Cr
depleted zones.
[0018]
Because a Cr depleted zone is a decarburized zone, precipitation
strengthening by carbides cannot be used in a Cr depleted zone. Therefore, the
present inventors considered causing nitrides to precipitate in the steel material when
used in a high temperature environment. Because C is not used in the formation of
nitrides, Cr depleted zones (decarburized zones) do not become larger. During use
in a high temperature environment, if nitrides precipitate in Cr depleted zones formed
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near grain boundaries, softening near grain boundaries can be suppressed by
precipitation strengthening. Therefore, a strength difference between the inside of
the grains that underwent secondary induced precipitation hardening and the Cr
depleted zones formed along the grain boundaries can be reduced, and stress
relaxation cracking resistance can be increased. In addition, by strengthening the
Cr depleted zones, creep strength also increases.
[0019]
In addition, in order to exhibit the aforementioned stress relaxation cracking
suppression effect and high creep strength together, during use in a high temperature
environment, it is important that, in addition to securing a dissolved N amount for
forming nitrides which act to perform precipitation strengthening of Cr depleted
zones and the inside of grains, nitrides are caused to precipitate in advance in the
steel material prior to use. By forming nitrides in the steel material prior to use, a
pinning effect of the nitrides occurs and the grains can be refined. If the grains can
be refined, the amount (coverage) of Cr carbides precipitating at the grain boundaries
will be low, and in addition, the amount of grain-boundary segregation of
phosphorus (P) and sulfur (S) will be small. In such a case, a decrease in the
hardness of grain boundaries and near grain boundaries can be suppressed, and a
strength difference between the inside of the grains and the grain boundaries and Cr
depleted zones can be reduced. Therefore, the stress relaxation cracking resistance
of the steel material will increase.
[0020]
As described above, the present inventors considered that by forming nitrides
and refining grains by the pinning effect in a steel material prior to use in a high
temperature environment, and also forming nitrides in the steel material to strengthen
Cr depleted zones during use in a high temperature environment, stress relaxation
cracking resistance can be increased. Further, as the result of taking into
consideration the compatibility between the creep strength and the stress relaxation
cracking resistance, the present inventors discovered that if a steel material has the
aforementioned chemical composition and, furthermore, the ratio of the dissolved N
amount in the steel material to the content of N in the steel material is 0.40 to 0.90, it
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is possible to achieve compatibility between the creep strength and the stress
relaxation cracking resistance.
[0021]
The austenitic stainless steel material of the present embodiment that has been
completed based on the above findings is as follows.
[0022]
[1]
An austenitic stainless steel material, having:
a chemical composition consisting of, in mass%,
C: 0.030% or less,
Si: 1.50% or less,
Mn: 2.00% or less,
P: 0.045% or less,
S: 0.0300% or less,
Cr: 15.00 to 25.00%,
Ni: 8.00 to 20.00%,
N: 0.050 to 0.250%,
Nb: 0.10 to 1.00%,
Mo: 0.05 to 5.00%,
B: 0.0005 to 0.0100%,
Ti: 0 to 0.50%,
Ta: 0 to 0.50%,
V: 0 to 1.00%,
Zr: 0 to 0.10%,
Hf: 0 to 0.10%,
Cu: 0 to 4.00%,
W: 0 to 5.00%,
Co: 0 to 1.00%,
sol. Al: 0 to 0.100%,
Ca: 0 to 0.0200%,
Mg: 0 to 0.0200%,
rare earth metal: 0 to 0.100%,
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Sn: 0 to 0.010%,
As: 0 to 0.010%,
Zn: 0 to 0.010%,
Pb: 0 to 0.010%, and
Sb: 0 to 0.010%,
with the balance being Fe and impurities,
wherein
a ratio of a dissolved N amount (mass%) in the austenitic stainless steel
material with respect to a content of N (mass%) in the austenitic stainless steel
material is 0.40 to 0.90.
[0023]
[2]
The austenitic stainless steel material according to [1], wherein
the chemical composition contains at least one or more elements belonging to
any group among a first group to a fourth group:
first group:
Ti: 0.01 to 0.50%,
Ta: 0.01 to 0.50%,
V: 0.01 to 1.00%,
Zr: 0.01 to 0.10%, and
Hf: 0.01 to 0.10%;
second group:
Cu: 0.01 to 4.00%,
W: 0.01 to 5.00%, and
Co: 0.01 to 1.00%;
third group:
sol. Al: 0.001 to 0.100%;
fourth group:
Ca: 0.0001 to 0.0200%,
Mg: 0.0001 to 0.0200%, and
rare earth metal: 0.001 to 0.100%.
[0024]
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Hereunder, the austenitic stainless steel material of the present embodiment
are described in detail. The symbol "%" in relation to an element means "mass%"
unless specifically stated otherwise.
[0025]
[Regarding chemical composition]
The chemical composition of the austenitic stainless steel material of the
present embodiment contains the following elements.
[0026]
C: 0.030% or less
Carbon (C) is unavoidably contained. In other words, the content of C is
more than 0%. C forms M23C6-type carbides at grain boundaries. In this case, Cr
depleted zones form at grain boundaries, and the stress relaxation cracking resistance
of the steel material decreases. If the content of C is more than 0.030%, even if the
contents of other elements are within the range of the present embodiment, the stress
relaxation cracking resistance of the steel material will markedly decrease.
Therefore, the content of C is 0.030% or less. A preferable upper limit of the
content of C is 0.026%, more preferably is 0.024%, further preferably is 0.022%,
further preferably is 0.020%, and further preferably is 0.018%. The content of C is
preferably as low as possible. However, if the content of C is excessively reduced,
the production cost will increase. Therefore, for industrial production, a preferable
lower limit of the content of C is 0.001%, and further preferably is 0.002%.
[0027]
Si: 1.50% or less
Silicone (Si) is unavoidably contained. In other words, the content of Si is
more than 0%. Si deoxidizes the steel in the steel making process. In addition,
when the steel material is used in a high temperature environment (an average
operation temperature of more than 600 to 750C), Si increases the oxidation
resistance and steam oxidation resistance of the steel material. If even a small
amount of Si is contained, the aforementioned effects are obtained to a certain extent.
However, if the content of Si is more than 1.50%, even if the contents of other
elements are within the range of the present embodiment, weld cracking
susceptibility will markedly increase. In addition, sigma phase ( phase) will form
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in the steel material due to use for a long time period in a high temperature
environment. The  phase will reduce the toughness of the steel material.
Therefore, the content of Si is 1.50% or less. A preferable lower limit of the
content of Si is 0.01%, more preferably is 0.05%, further preferably is 0.10%, further
preferably is 0.15%, and further preferably is 0.18%. A preferable upper limit of
the content of Si is 1.40%, more preferably is 1.20%, further preferably is 1.00%,
further preferably is 0.80%, further preferably is 0.70%, further preferably is 0.60%,
and further preferably is 0.50%.
[0028]
Mn: 2.00% or less
Manganese (Mn) is unavoidably contained. In other words, the content of
Mn is more than 0%. Mn combines with S in the steel material to form MnS, and
thereby increases the hot workability of the steel material. Mn also deoxidizes the
weld zone of the steel material during welding. If even a small amount of Mn is
contained, the aforementioned effects are obtained to a certain extent. However, if
the content of Mn is more than 2.00%, even if the contents of other elements are
within the range of the present embodiment, sigma phase ( phase) will easily form
during use in a high temperature environment. The  phase will reduce the
toughness of the steel material during use in a high temperature environment.
Therefore, the content of Mn is 2.00% or less. A preferable lower limit of the
content of Mn is 0.01%, more preferably is 0.10%, further preferably is 0.40%,
further preferably is 0.50%, and further preferably is 0.60%. A preferable upper
limit of the content of Mn is 1.80%, more preferably is 1.60%, further preferably is
1.50%, further preferably is 1.30%, further preferably is 1.10%, and further
preferably is 0.95%.
[0029]
P: 0.045% or less
Phosphorus (P) is unavoidably contained. In other words, the content of P is
more than 0%. During welding with higher heat input, P segregates at grain
boundaries of the steel material. As a result, the stress relaxation cracking
resistance decreases. If the content of P is more than 0.045%, even if the contents
of other elements are within the range of the present embodiment, the stress
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relaxation cracking resistance will decrease. Therefore, the content of P is 0.045%
or less. A preferable upper limit of the content of P is 0.035%, and more preferably
is 0.030%. The content of P is preferably as low as possible. However,
excessively reducing the content of P will raise the production cost of the steel
material. Therefore, when taking into consideration normal industrial production, a
preferable lower limit of the content of P is 0.001%, and more preferably is 0.002%.
[0030]
S: 0.0300% or less
Sulfur (S) is unavoidably contained. In other words, the content of S is more
than 0%. During welding with higher heat input, S segregates at grain boundaries
of the steel material. As a result, the stress relaxation cracking resistance decreases.
If the content of S is more than 0.0300%, even if the contents of other elements are
within the range of the present embodiment, the stress relaxation cracking resistance
will decrease. Therefore, the content of S is 0.0300% or less. A preferable upper
limit of the content of S is 0.0150%, more preferably is 0.0100%, further preferably
is 0.0050%, and further preferably is 0.0030%. The content of S is preferably as
low as possible. However, excessively reducing the content of S will raise the
production cost of the steel material. Therefore, when taking into consideration
normal industrial production, a preferable lower limit of the content of S is 0.0001%,
and more preferably is 0.0002%.
[0031]
Cr: 15.00 to 25.00%
Chromium (Cr) increases the oxidation resistance and corrosion resistance of
the steel material during use of the steel material in a high temperature environment.
If the content of Cr is less than 15.00%, even if the contents of other elements are
within the range of the present embodiment, the aforementioned effect will not be
sufficiently obtained. On the other hand, if the content of Cr is more than 25.00%,
even if the contents of other elements are within the range of the present
embodiment, the stability of austenite in the steel material in a high temperature
environment will decrease. In this case, the creep strength of the steel material will
decrease. Therefore, the content of Cr is 15.00 to 25.00%. A preferable lower
limit of the content of Cr is 16.00%, more preferably is 16.20%, and further
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preferably is 16.40%. A preferable upper limit of the content of Cr is 24.00%, more
preferably is 23.00%, further preferably is 22.00%, further preferably is 21.00%,
further preferably is 20.00%, and further preferably is 19.00%.
[0032]
Ni: 8.00 to 20.00%
Nickel (Ni) stabilizes austenite and increases the creep strength of the steel
material in a high temperature environment. If the content of Ni is less than 8.00%,
even if the contents of other elements are within the range of the present
embodiment, the aforementioned effect will not be sufficiently obtained. On the
other hand, if the content of Ni is more than 20.00%, the aforementioned effect will
be saturated and, furthermore, the production cost will increase. Therefore, the
content of Ni is 8.00 to 20.00%. A preferable lower limit of the content of Ni is
8.50%, more preferably is 9.00%, further preferably is 9.20%, and further preferably
is 9.40%. A preferable upper limit of the content of Ni is 18.00%, more preferably
is 16.00%, further preferably is 15.00%, and further preferably is 14.00%.
[0033]
N: 0.050 to 0.250%
Nitrogen (N) dissolves in the matrix (parent phase) and stabilizes austenite.
In addition, the dissolved N forms fine nitrides in the steel material during use in a
high temperature environment. Because the fine nitrides strengthen Cr depleted
zones, the stress relaxation cracking resistance of the steel material increases. The
fine nitrides formed during use in a high temperature environment also increase the
creep strength by precipitation strengthening. If the content of N is less than
0.050%, even if the contents of other elements are within the range of the present
embodiment, the aforementioned effects will not be sufficiently obtained. On the
other hand, if the content of N is more than 0.250%, even if the contents of other
elements are within the range of the present embodiment, Cr nitrides (Cr2N) will
form at grain boundaries. In such a case, the amount of nitrides precipitated near
grain boundaries will decrease. Consequently, the strength near grain boundaries
will decrease. As a result, a difference between the strength within the grains and
the strength at the grain boundaries will increase, and the stress relaxation cracking
resistance will decrease. Therefore, the content of N is 0.050 to 0.250%. A
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preferable lower limit of the content of N is 0.052%, more preferably is 0.055%, and
further preferably is 0.060%. A preferable upper limit of the content of N is
0.200%, more preferably is 0.150%, and further preferably is 0.120%.
[0034]
Nb: 0.10 to 1.00%
Niobium (Nb), together with N, forms fine nitrides in the steel material during
use in a high temperature environment. Because the fine nitrides strengthen Cr
depleted zones, the stress relaxation cracking resistance of the steel material
increases. The fine nitrides formed during use in a high temperature environment
also increase the creep strength by precipitation strengthening. Nb also combines
with C to form MX-type Nb carbides. When Nb carbides form and fix C, the
amount of dissolved C in the steel material decreases. By this means, during use of
the steel material in a high temperature environment, formation of Cr carbides at
grain boundaries is suppressed and the stress relaxation cracking resistance of the
steel material increases. If the content of Nb is less than 0.10%, even if the contents
of other elements are within the range of the present embodiment, the
aforementioned effects will not be sufficiently obtained. On the other hand, if the
content of Nb is more than 1.00%, even if the contents of other elements are within
the range of the present embodiment, nitrides and carbides will excessively form.
In this case, the strength within the grains will become excessively high, and a
strength difference between the inside of the grains and the grain boundaries will be
large. Consequently, stress concentration will occur at grain boundary interfaces,
and the stress relaxation cracking resistance will decrease. Therefore, the content of
Nb is 0.10 to 1.00%. A preferable lower limit of the content of Nb is 0.20%, more
preferably is 0.23%, further preferably is 0.25%, further preferably is 0.30%, and
further preferably is 0.35%. A preferable upper limit of the content of Nb is 0.80%,
more preferably is 0.60%, and further preferably is 0.50%.
[0035]
Mo: 0.05 to 5.00%
Molybdenum (Mo) suppresses formation and growth of M23C6-type Cr
carbides at grain boundaries during use of the steel material in a high temperature
environment. As a result, the stress relaxation cracking resistance of the steel
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material increases. In addition, as a solid-solution strengthening element, Mo
increases the creep strength of the steel material in a high temperature environment.
If the content of Mo is less than 0.05%, even if the contents of other elements are
within the range of the present embodiment, the aforementioned effects will not be
sufficiently obtained. On the other hand, if the content of Mo is more than
5.00%, even if the contents of other elements are within the range of the present
embodiment, formation of intermetallic compounds such as Laves phases within
grains will be markedly promoted. In such a case, the strength within the grains
will be excessively high, and the strength difference between the inside of the grains
and the grain boundaries will be large. Consequently, stress concentration will
occur at grain boundary interfaces, and the stress relaxation cracking resistance will
decrease. Therefore, the content of Mo is 0.05 to 5.00%. A preferable lower limit
of the content of Mo is 0.06%, more preferably is 0.10%, further preferably is 0.15%,
further preferably is 0.20%, further preferably is 0.24%, further preferably is 0.28%,
and further preferably is 0.32%. A preferable upper limit of the content of Mo is
4.00%, more preferably is 3.00%, further preferably is 2.00%, further preferably is
1.50%, and further preferably is 1.00%.
[0036]
B: 0.0005 to 0.0100%
Boron (B) segregates at grain boundaries during use of the steel material in a
high temperature environment, and thereby increases the grain boundary strength.
Therefore, B increases the stress relaxation cracking resistance of the steel material.
If the content of B is less than 0.0005%, even if the contents of other elements are
within the range of the present embodiment, the aforementioned effect will not be
sufficiently obtained. On the other hand, if the content of B is more than 0.0100%,
even if the contents of other elements are within the range of the present
embodiment, formation of Cr carbides at the grain boundaries will be promoted. In
such a case, the stress relaxation cracking resistance of the steel material will
decrease. Therefore, the content of B is 0.0005 to 0.0100%. A preferable lower
limit of the content of B is 0.0012%, more preferably is 0.0014%, further preferably
is 0.0016%, further preferably is 0.0018%, and further preferably is 0.0020%. A
preferable upper limit of the content of B is 0.0080%, more preferably is 0.0060%,
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further preferably is 0.0050%, further preferably is 0.0040%, further preferably is
0.0035%, and further preferably is 0.0030%.
[0037]
The balance in the chemical composition of the austenitic stainless steel
material according to the present embodiment is Fe and impurities. Here, the term
"impurities" refers to elements which, during industrial production of the austenitic
stainless steel material, are mixed in from ore or scrap that is used as a raw material,
or from the production environment or the like, and which are allowed within a range
that does not adversely affect the austenitic stainless steel material of the present
embodiment.
[0038]
Among the impurities, the respective contents of Sn, As, Zn, Pb and Sb are as
follows.
Sn: 0 to 0.010%
As: 0 to 0.010%
Zn: 0 to 0.010%
Pb: 0 to 0.010%
Sb: 0 to 0.010%
Tin (Sn), arsenic (As), zinc (Zn), lead (Pb) and antimony (Sb) are each an
impurity. The content of Sn may be 0%. Similarly, the content of As may be 0%.
The content of Zn may be 0%. The content of Pb may be 0%. The content of Sb
may be 0%. If contained, each of these elements segregates at grain boundaries and
lowers the fusing point of the grain boundaries, and reduces the bonding strength of
the grain boundaries. If the content of Sn is more than 0.010%, even if the contents
of other elements are within the range of the present embodiment, the hot workability
and weldability of the steel material will decrease. Similarly, if the content of As is
more than 0.010%, even if the contents of other elements are within the range of the
present embodiment, the hot workability and weldability of the steel material will
decrease. If the content of Zn is more than 0.010%, even if the contents of other
elements are within the range of the present embodiment, the hot workability and
weldability of the steel material will decrease. If the content of Pb is more than
0.010%, even if the contents of other elements are within the range of the present
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embodiment, the hot workability and weldability of the steel material will decrease.
If the content of Sb is more than 0.010%, even if the contents of other elements are
within the range of the present embodiment, the hot workability and weldability of
the steel material will decrease. Therefore, the content of Sn is 0 to 0.010%. The
content of As is 0 to 0.010%. The content of Zn is 0 to 0.010%. The content of
Pb is 0 to 0.010%. The content of Sb is 0 to 0.010%.
[0039]
[Regarding optional elements]
[First group of optional elements]
The chemical composition of the austenitic stainless steel material according
to the present embodiment may also contain one or more elements selected from the
group consisting of Ti, Ta, V, Zr and Hf in lieu of a part of Fe. These elements
each combine with C to form a carbide and thereby reduce the amount of dissolved
C, and in this way these elements further increase the stress relaxation cracking
resistance of the steel material.
[0040]
Ti: 0 to 0.50%
Titanium (Ti) is an optional element, and need not be contained. In other
words, the content of Ti may be 0%. When contained, Ti combines with C in the
steel material to form carbides. By this means, the formation of Cr carbides is
suppressed, and the stress relaxation cracking resistance of the steel material further
increases. If even a small amount of Ti is contained, the aforementioned effect is
obtained to a certain extent. However, if the content of Ti is more than 0.50%, even
if the contents of other elements are within the range of the present embodiment,
carbides will excessively precipitate in the grains. In such a case, the strength
within the grains will become excessively high, and the strength difference between
the inside of the grains and the grain boundaries will be large. Consequently, stress
concentration will occur at grain boundary interfaces, and the stress relaxation
cracking resistance will, on the contrary, decrease. Therefore, the content of Ti is 0
to 0.50%. A preferable lower limit of the content of Ti is more than 0%, more
preferably is 0.01%, further preferably is 0.02%, and further preferably is 0.03%. A
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preferable upper limit of the content of Ti is 0.45%, more preferably is 0.40%,
further preferably is 0.35%, and further preferably is 0.30%.
[0041]
Ta: 0 to 0.50%
Tantalum (Ta) is an optional element, and need not be contained. In other
words, the content of Ta may be 0%. When contained, Ta combines with C to form
carbides. By this means, the formation of Cr carbides is suppressed, and the stress
relaxation cracking resistance of the steel material further increases. If even a small
amount of Ta is contained, the aforementioned effect is obtained to a certain extent.
However, if the content of Ta is more than 0.50%, even if the contents of other
elements are within the range of the present embodiment, carbides will excessively
precipitate in the grains. In such a case, the strength within the grains will become
excessively high, and the strength difference between the inside of the grains and the
grain boundaries will be large. Consequently, stress concentration will occur at
grain boundary interfaces, and the stress relaxation cracking resistance will, on the
contrary, decrease. Therefore, the content of Ta is 0 to 0.50%. A preferable lower
limit of the content of Ta is more than 0%, more preferably is 0.01%, further
preferably is 0.02%, further preferably is 0.03%, and further preferably is 0.05%. A
preferable upper limit of the content of Ta is 0.45%, more preferably is 0.40%,
further preferably is 0.35%, and further preferably is 0.30%.
[0042]
V: 0 to 1.00%
Vanadium (V) is an optional element, and need not be contained. In other
words, the content of V may be 0%. When contained, V combines with C to form
carbides. By this means, the formation of Cr carbides is suppressed, and the stress
relaxation cracking resistance of the steel material further increases. If even a small
amount of V is contained, the aforementioned effect is obtained to a certain extent.
However, if the content of V is more than 1.00%, even if the contents of other
elements are within the range of the present embodiment, carbides will excessively
precipitate in the grains. In such a case, the strength within the grains will become
excessively high, and the strength difference between the inside of the grains and the
grain boundaries will be large. Consequently, stress concentration will occur at
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grain boundary interfaces, and the stress relaxation cracking resistance will, on the
contrary, decrease. Therefore, the content of V is 0 to 1.00%. A preferable lower
limit of the content of V is more than 0%, more preferably is 0.01%, further
preferably is 0.02%, further preferably is 0.04%, and further preferably is 0.06%. A
preferable upper limit of the content of V is 0.50%, more preferably is 0.40%, further
preferably is 0.35%, and further preferably is 0.30%.
[0043]
Zr: 0 to 0.10%
Zirconium (Zr) is an optional element, and need not be contained. In other
words, the content of Zr may be 0%. When contained, Zr combines with C to form
carbides. By this means, the formation of Cr carbides is suppressed, and the stress
relaxation cracking resistance of the steel material further increases. If even a small
amount of Zr is contained, the aforementioned effect is obtained to a certain extent.
However, if the content of Zr is more than 0.10%, even if the contents of other
elements are within the range of the present embodiment, carbides will excessively
precipitate in the grains. In such a case, the strength within the grains will become
excessively high, and the strength difference between the inside of the grains and the
grain boundaries will be large. Consequently, stress concentration will occur at
grain boundary interfaces, and the stress relaxation cracking resistance will, on the
contrary, decrease. Therefore, the content of Zr is 0 to 0.10%. A preferable lower
limit of the content of Zr is more than 0%, more preferably is 0.01%, and further
preferably is 0.02%. A preferable upper limit of the content of Zr is 0.09%, more
preferably is 0.08%, further preferably is 0.07%, and further preferably is 0.06.
[0044]
Hf: 0 to 0.10%
Hafnium (Hf) is an optional element, and need not be contained. In other
words, the content of Hf may be 0%. When contained, Hf combines with C to form
carbides. By this means, the formation of Cr carbides is suppressed, and the stress
relaxation cracking resistance of the steel material further increases. If even a small
amount of Hf is contained, the aforementioned effect is obtained to a certain extent.
However, if the content of Hf is more than 0.10%, even if the contents of other
elements are within the range of the present embodiment, carbides will excessively
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precipitate in the grains. In such a case, the strength within the grains will become
excessively high, and the strength difference between the inside of the grains and the
grain boundaries will be large. Consequently, stress concentration will occur at
grain boundary interfaces, and the stress relaxation cracking resistance will, on the
contrary, decrease. Therefore, the content of Hf is 0 to 0.10%. A preferable lower
limit of the content of Hf is more than 0%, more preferably is 0.01%, and further
preferably is 0.02%. A preferable upper limit of the content of Hf is 0.09%, more
preferably is 0.08%, further preferably is 0.07%, and further preferably is 0.06%.
[0045]
[Second group of optional elements]
The chemical composition of the austenitic stainless steel material according
to the present embodiment may also contain one or more elements selected from the
group consisting of Cu, W and Co in lieu of a part of Fe. Each of these elements
further increases the creep strength of the steel material at an average operation
temperature of more than 600 to 750C.
[0046]
Cu: 0 to 4.00%
Copper (Cu) is an optional element, and need not be contained. In other
words, the content of Cu may be 0%. When contained, during use of the steel
material in a high temperature environment, Cu precipitates as a Cu phase in the
grains, and further increases the creep strength of the steel material by precipitation
strengthening. If even a small amount of Cu is contained, the aforementioned effect
is obtained to a certain extent. However, if the content of Cu is more than 4.00%,
during use in a high temperature environment, in some cases the precipitation
amount of Cu phase will increase and the creep ductility will decrease. Therefore,
the content of Cu is 0 to 4.00%. A preferable lower limit of the content of Cu is
more than 0%, more preferably is 0.01%, further preferably is 0.05%, further
preferably is 0.10%, further preferably is 0.20%, and further preferably is 0.30%. A
preferable upper limit of the content of Cu is 3.50%, more preferably is 3.00%,
further preferably is 2.50%, and further preferably is 2.00%.
[0047]
W: 0 to 5.00%
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Tungsten (W) is an optional element, and need not be contained. In other
words, the content of W may be 0%. When contained, during use of the steel
material in a high temperature environment, W further increases the creep strength of
the steel material by solid-solution strengthening. If even a small amount of W is
contained, the aforementioned effect is obtained to a certain extent. However, if the
content of W is more than 5.00%, even if the contents of other elements are within
the range of the present embodiment, the stability of austenite will decrease and the
toughness will decrease. Therefore, the content of W is 0 to 5.00%. A preferable
lower limit of the content of W is more than 0%, more preferably is 0.01%, further
preferably is 0.10%, further preferably is 0.20%, further preferably is 0.25%, and
further preferably is 0.30%. A preferable upper limit of the content of W is 4.00%,
more preferably is 3.00%, further preferably is 2.50%, further preferably is 2.00%,
and further preferably is 1.50%.
[0048]
Co: 0 to 1.00%
Cobalt (Co) is an optional element, and need not be contained. In other
words, the content of Co may be 0%. When contained, Co stabilizes austenite and
further increases the creep strength of the steel material at an average operation
temperature of more than 600 to 750C. If even a small amount of Co is contained,
the aforementioned effects are obtained to a certain extent. However, if the content
of Co is more than 1.00%, even if the contents of other elements are within the range
of the present embodiment, the raw material cost will increase. Therefore, the
content of Co is 0 to 1.00%. A preferable lower limit of the content of Co is more
than 0%, more preferably is 0.01%, further preferably is 0.04%, and further
preferably is 0.10%. A preferable upper limit of the content of Co is 0.90%, more
preferably is 0.80%, further preferably is 0.70%, and further preferably is 0.60%.
[0049]
[Third group of optional elements]
The chemical composition of the austenitic stainless steel material according
to the present embodiment may, in addition, contain Al in lieu of a part of Fe. Al
deoxidizes the steel in the steel making process.

We claim:
1. An austenitic stainless steel material, comprising:
a chemical composition consisting of, in mass%,
C: 0.030% or less,
Si: 1.50% or less,
Mn: 2.00% or less,
P: 0.045% or less,
S: 0.0300% or less,
Cr: 15.00 to 25.00%,
Ni: 8.00 to 20.00%,
N: 0.050 to 0.250%,
Nb: 0.10 to 1.00%,
Mo: 0.05 to 5.00%,
B: 0.0005 to 0.0100%,
Ti: 0 to 0.50%,
Ta: 0 to 0.50%,
V: 0 to 1.00%,
Zr: 0 to 0.10%,
Hf: 0 to 0.10%,
Cu: 0 to 4.00%,
W: 0 to 5.00%,
Co: 0 to 1.00%,
sol. Al: 0 to 0.100%,
Ca: 0 to 0.0200%,
Mg: 0 to 0.0200%,
rare earth metal: 0 to 0.100%,
Sn: 0 to 0.010%,
As: 0 to 0.010%,
Zn: 0 to 0.010%,
Pb: 0 to 0.010%, and
Sb: 0 to 0.010%,
with the balance being Fe and impurities,
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wherein
a ratio of a dissolved N amount (mass%) in the austenitic stainless steel
material with respect to a content of N (mass%) in the austenitic stainless steel
material is 0.40 to 0.90.
2. The austenitic stainless steel material according to claim 1, wherein
the chemical composition contains at least one or more elements belonging to
any group among a first group to a fourth group:
first group:
Ti: 0.01 to 0.50%,
Ta: 0.01 to 0.50%,
V: 0.01 to 1.00%,
Zr: 0.01 to 0.10%, and
Hf: 0.01 to 0.10%;
second group:
Cu: 0.01 to 4.00%,
W: 0.01 to 5.00%, and
Co: 0.01 to 1.00%;
third group:
sol. Al: 0.001 to 0.100%;
fourth group:
Ca: 0.0001 to 0.0200%,
Mg: 0.0001 to 0.0200%, and
rare earth metal: 0.001 to 0.100%.