Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
TITLE OF INVENTION: AUSTENITIC STAINLESS STEEL
TECHNICAL FIELD
[0001]
The present invention relates to a stainless steel, more particularly to an
austenitic stainless steel.
BACKGROUND ART
[0002]
In facilities used under high temperature carburizing environment, such as
thermal power generation boilers and chemical plants, austenitic stainless
steels
containing increased contents of Cr and increased contents of Ni, or Ni-based
alloys
containing increased contents of Cr have been used as heat resistant steels.
These
heat resistant steels are austenitic stainless steels or Ni-based alloys each
containing
about 20 to 30% by mass of Cr and about 20 to 70% by mass of Ni.
[0003]
Pipes of the facilities such as thermal power generation boilers and chemical
plants are produced from steel material pipes. The steel material pipe is
produced
by melting and thereafter performing hot working on the above austenitic
stainless
steel or Ni-based alloy. Therefore, heat resistant steels are requested to
have high
hot workabilities. However, austenitic stainless steels typically have high
deformation resistances and low ductilities at high temperature. For that
reason,
there is a demand for austenitic stainless steels having excellent hot
workabilities.
[0004]
In what is called the shale gas revolution, inexpensive shale gas has been
produced in recent years. As compared with conventional raw materials such as
naphtha, use of shale gas as source gas in facilities such as chemical plants
is likely
to cause carburization, which is a corrosion phenomenon of a metallic tube
(e.g.,
reaction tube) used in the facilities such as chemical plants due to carbon
(C) derived
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from the source gas. Therefore, steels used in facilities such as chemical
plants are
requested to have excellent anti-carburizing properties.
[0005]
Stainless steels having increased anti-carburizing properties and anti-coking
properties are proposed in, for example, Japanese Patent Application
Publication No.
2005-48284 (Patent Literature 1).
[0006]
A stainless steel disclosed in Patent Literature 1 is made of a base material
including a chemical composition consisting of, in mass percent, C: 0.01 to
0.6%, Si:
0.1 to 5%, Mn: 0.1 to 10%, P: 0.08% or less, S: 0.05% or less, Cr: 20 to 55%,
Ni: 10
to 70%, N: 0.001 to 0.25%, 0 (oxygen): 0.02% or less, with the balance being
Fe and
unavoidable impurities. This stainless steel includes a Cr depleted zone in
its near-
surface portion, a Cr concentration in the Cr depleted zone is 10% or more and
less
than a Cr concentration in the base material, and a thickness of the Cr
depleted zone
is within 20 mm. Patent Literature 1 states that the anti-carburizing
properties and
the anti-coking properties are increased by forming a protection film mainly
made of
Cr203 coating film.
[0007]
However, in the stainless steel of Patent Literature 1, the protection film
mainly includes the Cr203 coating film. Therefore, the stainless steel suffers
from
an insufficient function of preventing oxygen and carbon from entering from an
external atmosphere, in particular, under a high temperature carburizing
environment.
As a result, internal oxidation and carburizing may occur in the material.
[0008]
Hence, International Application Publication No. W02010/113830 (Patent
Literature 2), International Application Publication No. W02004/067788 (Patent
Literature 3), and Japanese Patent Application Publication No. 10-140296
(Patent
Literature 4) disclose techniques relating to protection films that are
alternatives to
Cr203 coating films. Specifically, according to these literatures, a
protection film
mainly containing A1703, which is thermodynamically stable, is formed on a
surface
of heat resistant steel, as a protection film that is an alternative to the
Cr203 coating
films.
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[0009]
A cast product disclosed in Patent Literature 2 includes a casting made of a
heat resistant alloy that consists of, in mass percent, C: 0.05 to 0.7%, Si:
more than
0% to 2.5% or less, Mn: more than 0% to 3.0% or less, Cr: 15 to 50%, Ni: 18 to
70%,
Al: 2 to 4 %, and rare earth metals: 0.005 to 0.4 %, as well as W: 0.5 to 10%
and/or
Mo: 0.1 to 5%, with the balance being Fe and unavoidable impurities. The
casting
includes a barrier layer formed on its surface that is to be brought into
contact with a
high-temperature atmosphere, the barrier layer is an Al2O3 layer having a
thickness
of 0.5 p.m or more, 80% by area or more of an outermost surface of the barrier
layer
is A1203, and Cr-based particles disperse in an interface between the A1203
layer and
the casting, the Cr-based particles having a Cr concentration higher than that
of a
base of the alloy. Patent Literature 2 states that with added Al, a protection
film
mainly including an Al2O3 protection film is formed, and anti-carburizing
properties
are increased.
[0010]
A nickel-chromium casting alloy disclosed in Patent Literature 3 consists of,
up to 0.8 % of Carbon, up to 1% of silicon, up to 0.2% of manganese, 15% to
40% of
chromium, 0.5% to 13% of iron, 1.5% to 7% of aluminum, up to 2.5% of niobium,
up to 1.5% of titanium, 0.01% to 0.4% of zirconium, up to 0.06% of nitrogen,
up to
12% of cobalt, up to 5% of molybdenum, up to 6% of tungsten, and 0.019 % to
0.089% of yttrium, with the rest being nickel. Patent Literature 3 states that
with
added REM as well as Al, the nickel-chromium casting alloy including Al2O3,
which
serves as a protection film, with enhanced anti-peeling properties can be
provided.
[0011]
An austenitic stainless steel disclosed in Patent Literature 4 consists of, in
mass percent, C: 0.15% or less, Si: 0.9% or less, Mn: 0.2 to 2%, P: 0.04% or
less, S:
0.005% or less, S(%) and 0(%) at 0.015% or less in total, Cr: 12 to 30%, Ni:
10 to
35%, Al: 1.5 to 5.5%, B: 0.001 to 0.01%, N: 0.025% or less, Ca: 0 to 0.008%,
Cu: 0
to 2%, one or more elements of Ti, Nb, Zr, V, and Hf at 0 to 2% in total, one
or more
elements of W, Mo, Co, and Re at 0 to 3% in total, and one or more elements of
rare
earth metals at 0 to 0.05% in total, with the balance being Fe and unavoidable
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impurities. Patent Literature 4 states that with added Al, a protection film
mainly
including an Al2O3 protection film is formed, and an oxidation resistance is
increased.
CITATION LIST
PATENT LITERATURE
[0012]
Patent Literature 1: Japanese Patent Application Publication No. 2005-48284
Patent Literature 2: International Application Publication No.
W02010/113830
Patent Literature 3: International Application Publication No.
W02004/067788
Patent Literature 4: Japanese Patent Application Publication No. 10-140296
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0013]
However, in Patent Literature 2, the heat resistant alloy contains Cr at 50%
at
the maximum. Therefore, in a high temperature carburizing environment such as
a
hydrocarbon gas atmosphere, Cr may form its carbide on a steel surface. In
this
case, A1203, which serves as a protection film, is not formed uniformly. As a
result,
carburizing may occur.
[0014]
In addition, the casting item and the nickel-chromium casting alloy disclosed
in Patent Literatures 2 and 3 each have a high content of C, which
significantly
decreases their hot workabilities.
[0015]
Furthermore, in Patent Literature 3, a content of Ni is high, which
significantly increases a raw-material cost.
[0016]
In Patent Literature 4, anti-carburizing properties are not considered. As a
result, its anti-carburizing properties may be low.
[0017]
=
T=C.P1,1,P=I. ,.ASM
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An objective of the present invention is to provide an austenitic stainless
steel
that has excellent anti-carburizing properties even in a high temperature
carburizing
environment such as a hydrocarbon gas atmosphere, and provides an excellent
hot
workability in its production.
SOLUTION TO PROBLEM
[0018]
An austenitic stainless steel according to the present embodiment includes a
chemical composition consisting of, in mass percent, C: 0.03 to less than
0.25%, Si:
0.01 to 2.0%, Mn: 2.0% or less, P: 0.04% or less, S: 0.01% or less, Cr: 10 to
less than
22%, Ni: more than 30.0% to 40.0%, Al: more than 2.5% to less than 4.5%, Nb:
0.01
to 3.5%, N: 0.03% or less, Ca: 0.0005 to 0.05%, Mg: 0.0005 to 0.05%, Ti: 0 to
less
than 0.2%, Mo: 0 to 0.5%, W: 0 to 0.5%, Cu: 0 to 0.5%, V: 0 to 0.2%, and B: 0
to
0.01%, with the balance being Fe and impurities, and satisfying Formula (1).
0.40 (Ccr7CAI')/(Ccr/Cm) 0.80 (1)
Here, a Cr concentration (mass percent) in an outer layer of the austenitic
stainless steel is substituted for Car' in Formula (1). An Al concentration
(mass
percent) in the outer layer of the austenitic stainless steel is substituted
for CM'. A
Cr concentration (mass percent) in an other-than-outer-layer region of the
austenitic
stainless steel is substituted for Car. An Al concentration (mass percent) in
the
other-than-outer-layer region of the austenitic stainless steel is substituted
for CAI.
ADVANTAGEOUS EFFECTS OF INVENTION
[0019]
The austenitic stainless steel according to the present embodiment has
excellent anti-carburizing properties even in a high temperature carburizing
environment such as a hydrocarbon gas atmosphere, and provides an excellent
hot
workability in its production.
DESCRIPTION OF EMBODIMENTS
[0020]
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The present inventors conducted investigations and studies about anti-
carburizing properties of the austenitic stainless steel in a high temperature
carburizing environment and a hot workability in its production, and obtained
the
following findings. The high temperature carburizing environment refers to an
environment in a hydrocarbon gas atmosphere at 1000 C or more.
[0021]
(A) When an austenitic stainless steel or a Ni-based alloy is made to contain
Cr, Cr203 that is a protection film is formed on its steel surface, increasing
its anti-
carburizing properties. However, as described above, Cr203 is
thermodynamically
unstable. Hence, in the present invention, an A1203 coating film is formed on
a
surface of the steel. A1203 acts as a protection film. A1203 is
thermodynamically
more stable than Cr203 in the high temperature carburizing environment. That
is,
the A1203 coating film can increase the anti-carburizing properties of
austenitic
stainless steel even in an environment at 1000 C or more.
[0022]
(B) When Cr is excessively contained in Al-containing austenitic stainless
steel or Ni-based alloy, Cr binds with C derived from atmospheric gas in the
high
temperature carburizing environment. Cr binding with C forms a Cr carbide on
the
steel surface. The Cr carbide physically inhibits uniform formation of the
A1203
coating film on the steel surface. As a result, the anti-carburizing
properties of steel
are decreased. Therefore, the content of Cr needs to be limited to a certain
content.
[0023]
Meanwhile, Cr promotes uniform formation of the A1203 coating film.
Hereafter, this effect is called a Third Element Effect of Cr (referred to as
a TEE
effect below). A mechanism of the TEE effect is as follows. At the very
beginning of a heat treatment process to be described later, Cr is
preferentially
oxidized first in the steel surface, and Cr203 is formed. Therefore, an oxygen
partial pressure in the steel surface locally decreases. As a result, Al does
not
undergo the inside oxidation but forms a uniform A1203 coating film in
proximity to
the surface. Afterward, oxygen used in a form of Cr203 is incorporated into
A1203
Then, at the end of the heat treatment process, a protection film made only of
A1203
is formed. Likewise, Cr has the TEE effect even under the high temperature
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carburizing environment. That is, Cr promotes the uniform formation of the
A1203
coating film even under the high temperature carburizing environment.
Therefore,
to form a uniform Al2O3 coating film, Cr needs to be contained at a certain
content or
more.
[0024]
Accordingly, in order to promote inhibition of the production of a Cr carbide
and promote the formation of the Al2O3 coating film under the high temperature
carburizing environment, a content of Cr is set at 10 to less than 22% in the
present
invention.
[0025]
(C) For austenitic stainless steel, it is effective to make a ratio of a Cr
concentration in an outer layer to an Al concentration in the outer layer
moderately
lower than a ratio of a Cr concentration in an other-than-outer-layer region
to an Al
concentration in the other-than-outer-layer region. That is, when an
austenitic
stainless steel satisfies Formula (1), the anti-carburizing properties in the
high
temperature carburizing environment is increased.
0.40 5 (Car7CAI')/(Car/Cm) 5 0.80 (1)
Here, a Cr concentration (mass percent) in an outer layer of the austenitic
stainless
steel is substituted for Car' in Formula (1). An Al concentration (mass
percent) in
the outer layer of the austenitic stainless steel is substituted for CAI'. A
Cr
concentration (mass percent) in an other-than-outer-layer region of the
austenitic
stainless steel is substituted for Car. An Al concentration (mass percent) in
the
other-than-outer-layer region of the austenitic stainless steel is substituted
for CAI.
[0026]
Define Fl as Fl = (Car7CAI')/(Car/CA1). When Fl is 0.40 or more, the TEE
effect by Cr is sufficiently provided on the steel surface in the high
temperature
carburizing environment. In this case, the formation of the A1203 coating film
is
promoted. When Fl is 0.80 or less, the formation of the Cr carbide on the
steel
surface is inhibited in the high temperature carburizing environment.
Therefore, the
uniform Al2O3 coating film is formed. As a result, the anti-carburizing
properties
are increased.
[0027]
. ,
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(D) When a chemical composition of an austenitic stainless steel contains
0.0005% or more of calcium (Ca) and 0.0005% or more of magnesium (Mg), the hot
workability is increased. In contrast, when contents of these elements are
excessively high, a toughness and a ductility of an austenitic stainless steel
at high
temperature are decreased, resulting in a decrease in hot workability. For
this
reason, Ca: 0.0005 to 0.05%, and Mg: 0.0005 to 0.05% are contained.
[0028]
An austenitic stainless steel according to the present embodiment that is made
based on the above findings includes a chemical composition consisting of, in
mass
percent, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, P:
0.04% or
less, S: 0.01% or less, Cr: 10 to less than 22%, Ni: more than 30.0% to 40.0%,
Al:
more than 2.5% to less than 4.5%, Nb: 0.01 to 3.5%, N: 0.03% or less, Ca:
0.0005 to
0.05%, Mg: 0.0005 to 0.05%, Ti: 0 to less than 0.2%, Mo: 0 to 0.5%, W: 0 to
0.5%,
Cu: 0 to 0.5%, V: 0 to 0.2%, and B: 0 to 0.01%, with the balance being Fe and
impurities, and satisfying Formula (1).
0.40 (Ccr'/CAI')/(Ccr/CAI) 0.80 (1)
Here, a Cr concentration (mass percent) in an outer layer of the austenitic
stainless
steel is substituted for Ccr' in Formula (1). An Al concentration (mass
percent) in
the outer layer of the austenitic stainless steel is substituted for CAI'. A
Cr
concentration (mass percent) in an other-than-outer-layer region of the
austenitic
stainless steel is substituted for Ccr. An Al concentration (mass percent) in
the
other-than-outer-layer region of the austenitic stainless steel is substituted
for CAI.
[0029]
The above chemical composition may contain one or two or more types
selected from the group consisting of Ti: 0.005 to less than 0.2%, Mo: 0.01 to
0.5%,
W: 0.01 to 0.5%, Cu: 0.005 to 0.5%, V: 0.005 to 0.2%, and B: 0.0001 to 0.01.
[0030]
Hereafter, the austenitic stainless steel according to the present embodiment
will be described in detail. The sign "%" following each element means mass
percent unless otherwise noted.
[0031]
[Chemical Composition]
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A chemical composition of the austenitic stainless steel according to the
present embodiment contains the following elements.
[0032]
C: 0.03 to less than 0.25%
Carbon (C) binds mainly with Cr to form a Cr carbide in the steel, increasing
a creep strength in use in the high temperature carburizing environment. An
excessively low content of C results in failure to provide this effect. In
contrast, an
excessively high content of C causes a large number of coarse eutectic
carbides to be
formed in a solidification micro structure after the steel is cast, resulting
in a decrease
in a toughness of the steel. Consequently, a content of C is 0.03 to less than
0.25%.
A lower limit of the content of C is preferably 0.05%, more preferably 0.08%.
An
upper limit of the content of C is preferably 0.23%, more preferably 0.20%.
[0033]
Si: 0.01 to 2.0%
Silicon (Si) deoxidizes steel. If the deoxidation can be sufficiently
performed using another element, a content of Si may be reduced as much as
possible. In contrast, an excessively high content of Si results in a decrease
in the
hot workability. Consequently, the content of Si is 0.01 to 2.0%. A lower
limit of
the content of Si is preferably 0.02%, more preferably 0.03%. An upper limit
of the
content of Si is preferably 1.0%.
[0034]
Mn: 2.0% or less
Manganese (Mn) is unavoidably contained. Mn binds with S contained in
the steel to form MnS, increasing the hot workability of the steel. However,
an
excessively high content of Mn makes the steel too hard, resulting in
decreases in the
hot workability and weldability. Consequently, a content of Mn is 2.0% or
less. A
lower limit of the content of Mn is preferably 0.1%, more preferably 0.2%. An
upper limit of the content of Mn is preferably 1.2%.
[0035]
P: 0.04% or less
Phosphorus (P) is an impurity. P decreases the weldability and the hot
workability of the steel. Consequently, a content of P is 0.04% or less. An
upper
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limit of the content of P is preferably 0.03%. The content of P is preferably
as low
as possible. A lower limit of the content of P is, for example, 0.0005%.
[0036]
S: 0.01% or less
Sulfur (S) is an impurity. S decreases the weldability and the hot
workability of the steel. Consequently, a content of S is 0.01% or less. An
upper
limit of the content of S is preferably 0.008%. The content of S is preferably
as low
as possible. A lower limit of the content of S is, for example, 0.001%.
[0037]
Cr: 10 to less than 22%
Chromium (Cr) exhibits the above TEE effect to promote the formation of the
A1203 coating film in the heat treatment process and under the high
temperature
carburizing environment. In addition, Cr binds with C in the steel to form a
Cr
carbide, increasing the creep strength. An excessively low content of Cr
results in
failure to provide these effects. In contrast, an excessively high content of
Cr
causes Cr to bind with C derived from atmospheric gas (hydrocarbon gas) under
the
high temperature carburizing environment and form a Cr carbide on the steel
surface.
The formation of the Cr carbide on the steel surface causes local depletion of
Cr in
the steel surface. This lessens the TEE effect, resulting in failure to form
the
uniform A1203 coating film. An excessively high content of Cr further causes
the
Cr carbide on the steel surface to physically inhibit the formation of the
uniform
A1203 coating film. Consequently, a content of Cr is 10 to less than 22%. A
lower
limit of the content of Cr is preferably 11%, more preferably 12%. An upper
limit
of the content of Cr is preferably 21%, more preferably 20%. In the present
specification, the Cr carbide is divided into a Cr carbide formed in the steel
and a Cr
carbide formed on the steel surface. For the austenitic stainless steel
according to
the present embodiment, the Cr carbide in the steel is allowed to form, and
the Cr
carbide on the steel surface is inhibited.
[0038]
Ni: more than 30.0% to 40.0%
Nickel (Ni) stabilizes an austenite, increasing the creep strength. In
addition,
Ni increases the anti-carburizing properties of the steel. An excessively low
content
. -
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of Ni results in failure to provide these effects. In contrast, an excessively
high
content of Ni results not only in saturation of these effects but also in an
increase in
raw-material costs. Consequently, a content of Ni is more than 30.0% to 40.0%.
A lower limit of the content of Ni is preferably 31.0%, more preferably 32.0%.
An
upper limit of the content of Ni is preferably 39.0%, more preferably 38.0%.
[0039]
Al: more than 2.5% to less than 4.5%
Aluminum (Al) forms the A1203 coating film on the steel surface in the heat
treatment process and under the high temperature carburizing environment,
increasing the anti-carburizing properties of the steel. In particular, in the
high
temperature carburizing environment assumed in the present invention, the
A1203
coating film is thermodynamically stable as compared with Cr203 coating films
conventionally used. An excessively low content of Al results in failure to
provide
these effects. In contrast, an excessively high content of Al leads to a
decrease in
structural stability, resulting in a significant decrease in the creep
strength.
Consequently, a content of Al is more than 2.5% to less than 4.5%. A lower
limit
of the content of Al is preferably 2.55%, more preferably 2.6%. An upper limit
of
the content of Al is preferably 4.2%, more preferably 4.0%. In the austenitic
stainless steel according to the present invention, the content of Al means a
total
amount of Al contained in the steel material.
[0040]
Nb: 0.01 to 3.5%
Niobium (Nb) forms intermetallic compounds to be precipitation
strengthening phases (Laves phase and Ni3Nb phase) to cause precipitation
strengthening in crystal grain boundaries and in grains, increasing the creep
strength
of the steel. In contrast, an excessively high content of Nb causes the
intermetallic
compounds to be produced excessively, resulting in a decrease in the toughness
of
the steel. In addition, an excessively high content of Nb also results in a
decrease in
the toughness after long-time aging. Consequently, a content of Nb is 0.01 to
3.5%.
A lower limit of the content of Nb is preferably 0.05%, more preferably 0.1%.
An
upper limit of the content of Nb is preferably less than 3.2%, more preferably
3.0%.
[0041]
õ
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N: 0.03% or less,
Nitrogen (N) stabilizes austenite and is unavoidably contained. In contrast,
an excessively high content of N causes coarse nitride and/or carbo-nitride,
which
remains undissolved even after heat treatment, to be produced. The coarse
nitride
and/or carbo-nitride decreases the toughness of the steel. Consequently, a
content
of N is 0.03% or less. An upper limit of the content of N is preferably 0.01%.
A
lower limit of the content of N is, for example, 0.0005%.
[0042]
Ca: 0.0005 to 0.05%
Calcium (Ca) immobilizes S in a form of its sulfide, increasing the hot
workability. In contrast, an excessively high content of Ca results in a
decrease in
the toughness and the ductility. As a result, the hot workability decreases.
In
addition, an excessively high content of Ca results in a decrease in
cleanliness.
Consequently, a content of Ca is 0.0005 to 0.05%. A lower limit of the content
of
Ca is preferably 0.0006%, more preferably 0.0008%. An upper limit of the
content
of Ca is preferably 0.01%, more preferably 0.008%.
[0043]
Mg: 0.0005 to 0.05%
Magnesium (Mg) immobilizes S in a form of its sulfide, increasing the hot
workability of the steel. In contrast, an excessively high content of Mg
results in a
decrease in the toughness and the ductility. As a result, the hot workability
decreases. In addition, an excessively high content of Mg results in a
decrease in
cleanliness. Consequently, a content of Mg is 0.0005 to 0.05%. A lower limit
of
the content of Mg is preferably 0.0006%, more preferably 0.0008%. An upper
limit
of the content of Mg is preferably 0.01%, more preferably 0.008%.
[0044]
The balance of the chemical composition of the austenitic stainless steel
according to the present embodiment is Fe and impurities. Here, the impurities
mean elements that are mixed from ores and scraps used as raw material, a
producing
environment, or the like when the austenitic stainless steel is produced in an
industrial manner, and are allowed to be mixed within ranges in which the
impurities
have no adverse effect on the present invention.
--
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[0045]
[Optional Elements]
The above chemical composition of the austenitic stainless steel may further
contain Ti in lieu of a part of Fe.
[0046]
Ti: 0 to less than 0.2%
Titanium (Ti) is an optional element and need not be contained. If contained,
Ti forms intermetallic compounds to be precipitation strengthening phases
(Laves
phase and Ni3Ti phase) to cause precipitation strengthening, increasing the
creep
strength. In contrast, an excessively high content of Ti causes the
intermetallic
compounds to be produced excessively, resulting in a decrease in high-
temperature
ductility and the hot workability. In addition, an excessively high content of
Ti
results in a decrease in the toughness after long-time aging. Consequently, a
content of Ti is 0 to less than 0.2%. A lower limit of the content of Ti is
preferably
0.005%, more preferably 0.01%. An upper limit of the content of Ti is
preferably
0.15%, more preferably 0.1%.
[0047]
The above chemical composition of the austenitic stainless steel may further
contain, in lieu of a part of Fe, one or two elements selected from the group
consisting of Mo and W. All of these elements are optional elements and
increase
the creep strength of the steel.
[0048]
Mo: 0 to 0.5%
Molybdenum (Mo) is an optional element and need not be contained. If
contained, Mo is dissolved in the austenite, a parent phase. The dissolved Mo
causes solid-solution strengthening, increasing the creep strength. In
contrast, an
excessively high content of Mo results in a decrease in the hot workability.
Consequently, a content of Mo is 0 to 0.5%. A lower limit of the content of Mo
is
preferably 0.01%, more preferably 0.05%. An upper limit of the content of Mo
is
preferably 0.4%, more preferably 0.3%.
[0049]
W: 0 to 0.5%
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Tungsten (W) is an optional element and need not be contained. If contained,
W is dissolved in the austenite, the parent phase. The dissolved W causes
solid-
solution strengthening, increasing the creep strength. In contrast, an
excessively
high content of W results in a decrease in the hot workability. Consequently,
a
content of W is 0 to 0.5%. A lower limit of the content of W is preferably
0.01%,
more preferably 0.05%. An upper limit of the content of W is preferably 0.4%,
more preferably 0.3%.
[0050]
The above chemical composition of the austenitic stainless steel may further
contain Cu in lieu of a part of Fe.
[0051]
Cu: 0 to 0.5%
Copper (Cu) is an optional element and need not be contained. If contained,
Cu stabilizes the austenite. In addition, Cu causes precipitation
strengthening,
increasing a strength of the steel. In contrast, an excessively high content
of Cu
results in a decrease in the ductility and the hot workability of the steel.
Consequently, a content of Cu is 0 to 0.5%. A lower limit of the content of Cu
is
preferably 0.005%, more preferably 0.01%. An upper limit of the content of Cu
is
preferably 0.3%, more preferably 0.1%.
[0052]
The above chemical composition of the austenitic stainless steel may further
contain V in lieu of a part of Fe.
[0053]
V: 0 to 0.2%
Vanadium (V) is an optional element and need not be contained. If
contained, V forms intermetallic compounds, as with Ti, increasing the creep
strength of the steel. In contrast, an excessively high content of V makes a
volume
ratio of the intermetallic compounds in the steel excessively high, resulting
in a
decrease in the hot workability. Consequently, a content of V is 0 to 0.2%. A
lower limit of the content of V is preferably 0.005%, more preferably 0.01%.
An
upper limit of the content of V is preferably 0.15%, more preferably 0.1%.
[0054]
,==== y= =
CA 03028610 2018-12-19
- 15 -
The above chemical composition of the austenitic stainless steel may further
contain B in lieu of a part of Fe.
[0055]
B: 0 to 0.01%
Boron (B) is an optional element and need not be contained. If contained, B
segregates in grain boundaries, promoting precipitation of intermetallic
compounds
in the grain boundaries. This increases the creep strength of the steel. In
contrast,
an excessively high content of B results in decreases in the weldability and
the hot
workability of the steel. Consequently, the content of B is 0 to 0.01%. A
lower
limit of the content of B is preferably 0.0001%, more preferably 0.0005%. An
upper limit of the content of B is preferably 0.008%, more preferably 0.006%.
[0056]
[Formula (1)]
The austenitic stainless steel according to the present embodiment further
satisfies Formula (1).
0.40 (Ccr7CA11)/(Ccr/Cm) 0.80 (1)
Here, a Cr concentration (mass percent) in an outer layer of the austenitic
stainless
steel is substituted for Car' in Formula (1). An Al concentration (mass
percent) in
the outer layer of the austenitic stainless steel is substituted for CAI'. A
Cr
concentration (mass percent) in an other-than-outer-layer region of the
austenitic
stainless steel is substituted for Car. An Al concentration (mass percent) in
the
other-than-outer-layer region of the austenitic stainless steel is substituted
for CAI.
[0057]
In the present specification, the outer layer of the austenitic stainless
steel
means a range of 2 pm depth from the surface of the austenitic stainless
steel. The
2 pim depth from the surface means 2 jim depth from a surface of the base
metal.
When the austenitic stainless steel includes the A1203 coating film on its
surface, the
2 pm depth from the surface of the base metal means 2 1.tm depth from the
surface of
the base metal after the Al2O3 coating film is removed by descaling treatment.
That
is, the Cr concentration (mass percent) in the range of 21.1111 depth from the
surface of
the austenitic stainless steel (when the austenitic stainless steel includes
the A1203
coating film on its surface, it is the surface of the base metal after the
A1203 coating
CA 03028610 2018-12-19
- 16 -
film is removed by the descaling treatment) is substituted for Car' in Formula
(1).
The Al concentration (mass percent) in the range of 2 gm depth from the
surface of
the austenitic stainless steel (when the austenitic stainless steel includes
the A1203
coating film on its surface, it is the surface of the base metal after the
Al2O3 coating
film is removed by the descaling treatment) is substituted for CM' in Formula
(1).
The Cr concentration of the other-than-outer-layer region (mass percent) means
an
average Cr concentration (mass percent) in a region of the base material other
than
the outer layer. The Al concentration of the other-than-outer-layer region
(mass
percent) means an average Al concentration (mass percent) in the region of the
base
material other than the outer layer.
[0058]
As shown in Formula (1), in the austenitic stainless steel according to the
present embodiment, the ratio of the Cr concentration in the outer layer to
the Al
concentration in the outer layer is made moderately lower than the ratio of
the Cr
concentration of the base material to the Al concentration of the base
material. In
this case, the formation of the A1203 coating film is promoted as described
above.
As a result, the anti-carburizing properties are increased in the high
temperature
carburizing environment.
[0059]
Define Fl as Fl = (Cd/Cm')/(Car/CAL). Fl is an index of Cr behavior.
[0060]
When Fl is more than 0.80, the ratio of the Cr concentration of the outer
layer
to the Al concentration of the outer layer is excessively higher than the
ratio of the Cr
concentration of the base material to the Al concentration of the base
material. That
is, Car', the Cr concentration of the outer layer, is excessively high. In
this case, in
the high temperature carburizing environment, a Cr carbide is formed on the
steel
surface, physically inhibiting the formation of the uniform Al2O3 coating
film.
[0061]
When Fl is less than 0.40, the ratio of the Cr concentration of the outer
layer
to the Al concentration of the outer layer is excessively lower than the ratio
of the Cr
concentration of the base material to the Al concentration of the base
material. That
iS, Car' which is the Cr concentration of the outer layer, is excessively low.
In this
CA 03028610 2018-12-19
- 17 -
case, the TEE effect by Cr is not provided in the high temperature carburizing
environment. Therefore, the uniform A1203 coating film is not formed on the
steel
surface.
[0062]
Consequently, Fl is 0.40 to 0.80. A lower limit of Fl is preferably 0.42,
more preferably 0.44. An upper limit of Fl is preferably 0.79, more preferably
0.78.
[0063]
The Cr concentration Cr in the outer layer and the Al concentration CAI' in
the outer layer described above are determined by the following method. The
austenitic stainless steel is cut perpendicularly to its surface. In the range
of 2 1AM
depth from the surface of the cut austenitic stainless steel (when the
austenitic
stainless steel includes the Al2O3 coating film on its surface, it is the
surface of the
base metal after the Al2O3 coating film is removed by the descaling
treatment), any
five points (measurement points) are selected. The Cr concentrations and the
Al
concentrations at the measurement points are measured by EDX (Energy
Dispersive
X-ray Spectroscopy). Values determined by averaging the measured values are
defined as Car' (%) and CAI' (%).
[0064]
When the austenitic stainless steel includes the A1203 coating film on its
surface, the Cr concentration Car' in the outer layer and the Al concentration
CAI' in
the outer layer are measured after the descaling treatment is performed.
Conditions
for descaling the austenitic stainless steels conform to JIS Z 2290(2004).
[0065]
Analysis of the Cr concentration Car in the other-than-outer-layer region and
the Al concentration CAI in the other-than-outer-layer region described above
can be
conducted by a well-known component analysis method. Specifically, they are
determined by the following method. The austenitic stainless steel is cut
perpendicularly to its longitudinal direction (in a case of a steel pipe, it
is its axis
direction), and a measurement surface is prepared. A wall-thickness center
portion
of the measurement surface is pierced with a drill. By the piercing, machined
chips
are produced, and the machined chips are collected. The machined chips are
collected at four spots of the same measurement surface. When the austenitic
CA 03028610 2018-12-19
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stainless steel is a steel pipe, the machined chips are collected at four
spots provided
at 45 pitches. The collected machined chips are subjected to ICP-OES
(Inductively Coupled Plasma Optical Emission Spectrometry) to conduct an
elemental analysis of its chemical composition. A procedure of the analysis
according to the ICP-OES conforms to JIS G 1258(2007). Averages of the
measured values for the four spots are defined as the Cr concentration Cr in
the
other-than-outer-layer region (%) and the Al concentration CM in the other-
than-
outer-layer region (%).
[0066]
The austenitic stainless steel according to the present embodiment includes
the A1703 coating film on its surface after the heat treatment process to be
described
later. Therefore, the austenitic stainless steel of the present embodiment may
include the A1703 coating film on its surface. However, the A1203 coating film
can
be removed by a well-known method such as pickling treatment and shotpeening
performed after the heat treatment process. Therefore, in the austenitic
stainless
steel of the present embodiment, the Al2O3 coating film may be removed from
its
surface.
[0067]
[Grain Size]
The austenitic stainless steel according to the present embodiment preferably
has a grain size of 30 to 80 pi.m. When the grain size is 30 pim or more, the
creep
strength of the steel further increases. When the grain size is 801..tm or
less, grain
boundary diffusion of Al is promoted, which further promotes the formation of
the
A1203 coating film. The grain size is determined by the microscopic test
method for
a grain size specified in JIS G 0551(2013).
[0068]
A shape of the austenitic stainless steel according to the present embodiment
is not limited to a particular shape. The austenitic stainless steel is, for
example, a
steel pipe. An austenitic stainless steel pipe is used as a reaction tube for
a chemical
plant. The austenitic stainless steel may be a plate material, a bar material,
a wire
rod, or the like.
[0069]
CA 03028610 2018-12-19
- 19 -
[Producing Method]
As an example of a method for producing the austenitic stainless steel of the
present embodiment, description will be made about a method for producing a
steel
pipe.
[0070]
[Preparation Process]
A molten steel having the chemical composition described above is produced.
The molten steel is subjected to a well-known degassing treatment as
necessary.
The molten steel is cast to produce a starting material. The starting material
may be
an ingot made by an ingot-making process, or a cast piece such as a slab,
bloom, and
billet made by a continuous casting process. Alternatively, a tube-shaped
casting
may be produced by a centrifugal casting process.
[0071]
[Hot Forging Process]
Hot forging may be performed on the produced starting material to produce a
cylindrical starting material. By performing the hot forging, an interior
structure of
the molten steel produced in the preparation process can be modified from a
solidification micro structure to a regulated-grain-sized structure, which is
formed by
homogeneous grains. A temperature of the hot forging is, for example, 900 to
1200 C.
[0072]
[Hot Working Process]
Hot working is performed on the starting material produced through the
preparation process or the starting material produced by the hot forging
(cylindrical
starting material) to produce a steel material pipe. For example, a through
hole is
formed at a center of the cylindrical starting material by machining. The
cylindrical
starting material with the through hole formed is subjected to hot extrusion
to
produce the steel material pipe. A machining temperature of the hot extrusion
is,
for example, 900 to 1200 C. The steel material pipe may be produced by
performing piercing-rolling (the Mannesmann process etc.) on the cylindrical
starting
material.
[0073]
- ,
CA 03028610 2018-12-19
- 20 -
[Cold Working Process]
Cold working is performed on the steel material pipe subjected to the hot
working to produce an intermediate material. The cold working is, for example,
cold drawing or the like. In the cold working process, giving strain to the
steel
surface allows elements such as Al and Cr to move to the steel surface easily.
In
this case, the TEE effect is provided sufficiently. It is thereby possible to
obtain an
austenitic stainless steel in which Cr is moderately depleted in an outer
layer of the
steel and that satisfies Formula (1). This effect cannot be provided when a
working
ratio of the cold working is excessively low. An upper limit of the working
ratio of
the cold working is not particularly specified, but cold working with an
excessively
high working ratio is practically difficult to perform. Consequently, the
working
ratio of the cold working is 10 to 90%.
[0074]
[Heat Treatment Process]
Heat treatment is performed on the produced intermediate material in an air
atmosphere. By performing the heat treatment in the air atmosphere, the
uniform
A1203 coating film is formed on the steel surface. At that time, Cr in the
outer layer
of the steel is moderately depleted by the TEE effect. As a result, it is
possible to
obtain the austenitic stainless steel satisfying Formula (1).
[0075]
A temperature of the heat treatment is 900 to less than 1100 C, and a duration
of the heat treatment is 3.0 to 30.0 minutes.
[0076]
If the temperature of the heat treatment is less than 900 C, or the duration
of
the heat treatment is less than 3.0 minutes, the TEE effect cannot be provided
sufficiently. In this case, the Cr concentration Car' in the outer layer of
the steel
becomes excessively high, failing to satisfy Formula (1). Accordingly, a Cr
carbide
is formed on the steel surface under the high temperature carburizing
environment,
and the uniform A1203 coating film is not formed sufficiently. As a result,
the anti-
carburizing properties are decreased. Consequently, the temperature of the
heat
treatment is 900 C or more, and the duration of the heat treatment is 3.0
minutes or
more. In addition, when the temperature of the heat treatment is 900 C or
more,
CA 03028610 2018-12-19
- 21 -
and the duration of the heat treatment is 3.0 minutes or more, a grain size
becomes
30 inn or more.
[0077]
In contrast, if the temperature of the heat treatment is 1100 C or more,
scales
mainly made of Cr203 are formed excessively on the steel surface. Therefore,
Cr in
the outer layer of the steel is excessively depleted. In this case, the Cr
concentration
Ccr' in the outer layer of the steel becomes excessively low, failing to
satisfy Formula
(1). Accordingly, the TEE effect by Cr under the high temperature carburizing
environment is lessened, and the uniform A1203 coating film is not formed
sufficiently. As a result, the anti-carburizing properties are decreased. If
the
duration of the heat treatment duration is more than 30.0 minutes, scales
mainly
made of A1203 are formed excessively on the steel surface. Therefore, Al in
the
outer layer of the steel is excessively depleted. In this case, the Al
concentration
CAI' in the outer layer of the steel becomes excessively low, failing to
satisfy Formula
(1). Accordingly, the uniform A1203 coating film is not formed sufficiently
under
the high temperature carburizing environment. As a result, the anti-
carburizing
properties are decreased. Consequently, the temperature of the heat treatment
is
less than 1100 C, and the duration of the heat treatment is 30.0 minutes or
less. In
addition, when the temperature of the heat treatment is less than 1100 C, and
the
duration of the heat treatment is 30.0 minutes or less, a grain size becomes
80 1.1m or
less.
[0078]
When the temperature of the heat treatment is 900 to less than 1100 C, and
the duration of the heat treatment is 3.0 to 30.0 minutes, the TEE effect is
provided
sufficiently and appropriately, and the steel having a chemical composition
satisfying
Formula (1) is obtained. As a result, the anti-carburizing properties under
the high
temperature carburizing environment are increased.
[0079]
For the purpose of removing the scales formed on the surface, pickling
treatment may be performed on the intermediate material subjected to the heat
treatment. For the pickling, for example, a mixed acid solution of nitric acid
and
CA 03028610 2018-12-19
- 22 -
hydrochloric acid is used. A duration of the pickling is, for example, 30
minutes to
60 minutes.
[0080]
In addition, for the purpose of removing the scales on the steel surface and
giving strain to the steel surface of the intermediate material subjected to
the pickling
treatment, shot peening may be performed on the steel surface. In the shot
peening,
a starting material and a shape of shot media, and treatment conditions are
not
specified, but the starting material and the shape, and the treatment
conditions are set
to be sufficient for peeling the scales on the steel surface and giving the
strain to the
steel surface. The scales refer to, for example, A1203 By well-known methods
of
the pickling treatment, shot peening, and the like, the Al2O3 coating film can
be
removed.
[0081]
By the above producing method, the austenitic stainless steel according to the
present embodiment is produced. The above description is made about the method
for producing a steel pipe. However, a plate material, bar material, wire rod,
or the
like may be produced by a similar producing method (preparation process, hot
forging process, hot working process, cold working process, heat treatment
process).
It is particularly preferable to apply the austenitic stainless steel
according to the
present embodiment to steel pipes. Hence, the austenitic stainless steel
according to
the present embodiment is preferably an austenitic stainless steel pipe.
EXAMPLES
[0082]
[Producing Method]
Molten steels having chemical compositions shown in Table I were produced
using a vacuum furnace.
[0083]
[Table 1]
CA 03028610 2018-12-19
- 23 -
TABLE 1
CHEMICAL COMPOSITION (mass%, BALANCE: Fe AND IMPURITIES)
STEEL
ESSENTIAL ELEMENT OPTIONAL ELEMENT
TYPE
C Si Mn P S Cr Ni Al Nb N , Ca Mg Ti Mo W Cu V B
A 0.112 1.20 1.85 0.038 0.004 14.6 38.5 2.54 2.29 0.0044 0.0114 0.0263
B 0.089 1.28 1.59 0.024 0.002 11.3 31.8 2.67 0.65 0.0270 0.0448 0.0381
C 0.136 2.00 1.44 0.001 0.001 11.8 34.9 3.44 _0.11 0.0070 0.0323 0.0161
D 0.033 0.82 0.44 0.002 0.003 16.1 32.7 2.85 3.25 0.0185 0.0334 0.0098 ,
E 0.205 0.15 1.43 0.002 0.008 20.3 35.3 2.76 0.28 0.0077 0.0481 0.0160
F 0.183 0.24 1.17 0.028 0.006 20.2 33.7 2.89 2.13 0.0002 0.0320 0.0410
G 0.210 1.51 0.84 0.027 0.002 12.1 30.4 4.22 0.36 0.0186 0.0323 0.0015 0.15
- - - - -
H 0.224 1.13 2.00 0.027 0.009 10.1 34.0 2.81 1.69 0.0118 0.0384 0.0185 -
0.09 - - - -
I 0.091 1.30 1.61 0.024 0.002 11.4 31.9 2.69 0.68 0.0052 0.0453 0.0023 -
- 0.50 - - -
J 0.030 0.97 0.08 0.025 0.006 12.9 30.3 3.34 0.82 0.0156 0.0032 0.0164 -
- - 0.15 - -
K 0.221 0.83 1.57 0.016 0.009 12.4 36.4 2.90 0.54 0.0098 0.0021 0.0196 -
- - - 0.14 -
L 0.085 0.54 1.36 0.028 0.009 14.1 34.7 3.59 2.75 0.0271 , 0.0209 0.0269
0.0035
M 0.128 0.33 0.05 0.007 0.007 5.9 34.8 3.08 _1.27 0.0221 0.0211 0.0131
N 0.166 1.34 1.02 0.040 0.003 32.8 32.5 3.47 1.86 0.0169 0.0463 0.0240
0 0.119 0.58 1.28 0.023 0.001 15.3 33.7 1.55 3.33 0.0157 0.0092 0.0123
P 0.137 0.62 1.15 0.031 0.004 15.2 19.2 4.36 1.59 0.0119 0.0441 0.0066
Q 0.039 0.81 1.57 0.006 0.001 20.1 36.8 3.75 2.23 0.0124 0.0305 0.0001
R 0.185 0.28 0.22 0.026 0.003 17.6 35.4 3.38 3.28 0.0162 0.0224 0.1485
[0084]
Using the above molten steels, column-shaped ingots having an outer
diameter of 120 mm (30 kg) were produced. The ingots were subjected to the hot
forging and the hot rolling. After the hot rolling, the cold rolling was
performed in
conditions shown in Table 2 to produce intermediate materials having a
thickness of
15 mm. From each of the intermediate materials of respective steel types, two
8
mm x 20 mm x 30 mm plate materials were produced by machining. The heat
treatment was performed on the plate materials at temperatures and for
durations
shown in Table 2. After the heat treatment, the plate materials were water-
cooled to
produce test steel plates.
- 24 -
[0085]
[Table 2]
TABLE 2
-
Cr _ Al
COLD HEAT HEAT N GRAI CONCENTRATION
CONCENTRATION Cr CONCENTRATION AI CONCENTRATION ENTERING
TEST STEEL ROLLING TREATMENT TREATMENT Cc,' IN OUTER
Cd IN OUTER Co, Cc, IN OTHER-THAN- Ca IN OTHER-THAN- GC, C REDUCTION
IZ
Fl
SE
NUMBER TYPE WORKING TEMPERATURE DURATION LAYER LAYER /CAr OUTER-
LAYER OUTER-LAYER (CA QUANTITY OF AREA
RATIO (9(:,) ( C) (min) (PM)
AFTER HEAT AFTER HEAT REGION (%)
REGION (%) (%)
,
TREATMENT (%) TREATMENT (%) ,
_
1 A 36 1000 10 54 8.24 2.11 3.91 , 14.60 2.54
5.75 0.68 0.29 0 _
2 B 52 900 5 37 6.99 2.20 3.18 11.30 2.67
4.23 0.75 _ 0.23 0 .
3 C 40 900 20 47 , 5.60 2.91 1.92 11.80 3.44
3.43 0.56 0.09 0 _
4 D 41 1050 10 60 10.01 2.28 4.39 16.10 2.85
5.65 0.78 _ 0.18 0 P
- _
E 62 1000 10 42 11.94 2.23 5.35 20.30 2.76
7.36 0.73 0.22 0 ,D
.
,D
6 F 39 1000 5 54 13.22 2.54 5.20 20.20 2.89
6.99 0.74 0.14 0
.
03" ,
7 G 62 1050 10 45 7.29 3.42 2.13 12.10 4.22
2.87 0.74 0.11 0 1-
_
8 H 55 950 10 71 5.07 2.17 _ 2.34 10.10 2.81
3.59 0.65 _ 0.22 0 0" -
9 I 33 900 20 46 6.35 3.67 . 1.73 11.40 2.69
4.24 0.41 _ 0.28 0 1-
00
1
J - 39 950 20 49 8.44 3.11 . 2.71 12.90
3.34 3.86 0.70 , 0.12 0
1 11 K 48 1000 , 10 40 7.78 3.65 2.13 12.40
2.90 4.28 _ 0.50 _ 0.17 0 1-
_ _
12 L 49 _ 1000 10 44 10.02 3.46 2.90 14.10
3.59 3.93 _ 0.74 _ 0.07 0 . 13 A 7 1050 5 76
4.25 2.11 2.01 14.60 2.54 5.75 0.35 _ 0.51 0 =
14 B 32 700 5 21 9.36 2.21 _ 4.24 11,30 2.67
4.23 1.00 0.65 0 -
C - _
_
57 1300 20 131 4.21 3.18 1.32 11.80 3.44 3.43
0.39 0.58 0
1- _ .
16 D 61 1000 0.5 22 14.29 2.38 6.00 16.10 2.85
5.65 1.06 0.69 0
- -
,
_ L _
17 E 50 1050 90 95 12.68 1.82 6.97 20.30 2.76
7.36 0.95 0.54 0 .
_ _ ,
18 M 45 1000 10 58 1.92 2.35 0.82 5.90 3.08
1.92 0.43 0.75 0
- -
_ - _ _ _ _
19 N 47 900 20 43 15.89 3.06 5.19 32.80 3.47
9.45 0.55 0.60 0
_ _
0 64 900 5 35 9.34 1.23 7.59 15.30 1.55 9.87
0.77 0.83 0
1
_ 21 P 34 1050 5 58 10.15 3.88 2.62 15.20
1
4,36 T 1 ,
3.49 0.75
0.52 0 1
_
_ 22 0 30 1050 20 67 12.78 3.31 3.86 20.10
- 3.75 5.36 0.72 0.14 x
_
23 R 31 1000 5 59 11.49 2.97 3.87 17.60 3.38
5.21 0.74 0.21 x
I
.;.'
1
i
i
i
1
.,
,
CA 03028610 2018-12-19
- 25 -
[0086]
[Measurement of Austenite Grain size]
For each of the steel plates of the respective test numbers, from a center
portion of its cross section in a direction perpendicular to its rolling
direction, a test
specimen for microscopic observation was fabricated. Of the surfaces of the
test
specimen, a surface corresponding to the above cross section (referred to as
an
observation surface) was used, and the microscopic test method specified in
ASTM E
112 was performed, and the grain size was measured. Specifically, the
observation
surface was subjected to mechanical polishing, and thereafter etched using
etching
reagent, and crystal grain boundaries in the observation surface were exposed.
An
average grain size of ten visual fields on the etched surface was determined.
The
area of each visual field was about 0.75 mm2.
[0087]
[Measurement of Cr Concentration Ccr' in Outer Layer and Al Concentration CAI'
in
Outer Layer]
The steel plates of the respective test numbers were subjected to the
descaling
treatment under conditions conforming to JIS Z 2290(2004). Each of the steel
plates subjected to the descaling treatment was cut perpendicularly to its
rolling
direction, and a sample including a surface of the steel plate was taken. Each
of the
samples was embedded in resin, and its observation surface including a cross
section
of the vicinity to the surface of the steel plate was polished. On the
polished
observation surface, the above method was used to determine Ccr', the Cr
concentration and Cm', the Al concentration in the outer layer (range of 2 pun
depth
from the surface of the steel plate).
[0088]
[Measurement of Cr Concentration Ccr in Other-than-outer-layer region and Al
Concentration CAI in Other-than-outer-layer region]
The above method was used to determine the Cr concentration Ccr in the
other-than-outer-layer region and the Al concentration CAI in the other-than-
outer-
layer region.
[0089]
[Carburizing Test]
CA 03028610 2018-12-19
- 26 -
The steel plates of the respective test numbers were retained in H2-C1-14-0O2
atmosphere at 1100 C x 96 hours. After the carburizing, scales and the like
were
removed from surfaces of the steel plates by performing manual dry polishing
on the
surfaces using #600 abrasive paper. From the surfaces of the steel plates,
machined
chips for analysis of four layers were taken at 0.5 mm pitches. On the taken
machined chips for analysis, the C concentrations were measured by a high
frequency combustion infrared absorption method. Values obtained by
subtracting
the C concentration originally contained in the steel from results of the
measurement
were determined as C concentration increase quantities. An average of C
concentration increase quantities of the four layers was determined as an
entering C
quantity.
[0090]
[High-Temperature Tensile Test]
For each of the produced ingots, from its wall-thickness center portion, a
column-shaped tensile test specimen having a diameter of 10 mm and a length of
130
mm was cut out. Each tensile test specimen was subjected to a tensile test at
a
tensile speed (strain rate) of 10/s, and its hot workability was evaluated. In
the
present invention, when a reduction of area of a test specimen after the
tensile test at
900 C was 60% or more, the test specimen was determined as good (0), and when
the reduction of area was less than 60%, the test specimen was determined as
no
good (x).
[0091]
[Test Results]
Results of the tests are shown in Table 2.
[0092]
Referring to Table 2, as to a test number 1 to a test number 12, their
chemical
compositions were appropriate, their producing conditions were also
appropriate, and
Fl satisfied Formula (1). As a result, their entering C quantities were 0.4%
or less,
and they exhibited excellent anti-carburizing properties. In addition, their
values of
reduction of area in the high-temperature tensile test were 60 % or more, and
they
exhibited excellent hot workabilities.
[0093]
CA 03028610 2018-12-19
- 27 -
In contrast, as to a test number 13, its working ratio of the cold rolling was
excessively low. Accordingly, Fl was 0.35, failing to satisfy Formula (1). As
a
result, its entering C quantity was 0.51%, exhibiting poor anti-carburizing
properties.
[0094]
As to a test number 14, its temperature of the heat treatment was excessively
low. Accordingly, Fl was 1.00, failing to satisfy Formula (1). As a result,
its
entering C quantity was 0.65%, exhibiting poor anti-carburizing properties. In
addition, as to the test number 14, its grain size was 21 lam.
[0095]
As to a test number 15, its temperature of the heat treatment was excessively
high. Accordingly, Fl was 0.39, failing to satisfy Formula (1). As a result,
its
entering C quantity was 0.58%, exhibiting poor anti-carburizing properties. In
addition, as to the test number 15, its grain size was 131 jam.
[0096]
As to a test number 16, its duration of the heat treatment was excessively
short. Accordingly, Fl was 1.06, failing to satisfy Formula (1). As a result,
its
entering C quantity was 0.69%, exhibiting poor anti-carburizing properties. In
addition, as to the test number 16, its grain size was 22 ptm.
[0097]
As to a test number 17, its duration of the heat treatment was excessively
long.
Accordingly, Fl was 0.95, failing to satisfy Formula (1). As a result, its
entering C
quantity was 0.54%, exhibiting poor anti-carburizing properties. In addition,
as to
the test number 17, its grain size was 95 i_tm.
[0098]
As to Test Number 18, its content of Cr was excessively low. Accordingly,
the TEE effect by Cr was lessened. As a result, its entering C quantity was
0.75%,
exhibiting poor anti-carburizing properties.
[0099]
As to Test Number 19, its content of Cr was excessively high. Accordingly,
its Cr carbide inhibited the formation of the A1203 coating film. As a result,
its
entering C quantity was 0.60%, exhibiting poor anti-carburizing properties.
[0100]
CA 03028610 2018-12-19
- 28 -
As to Test Number 20, its content of Al was excessively low. Accordingly,
its A1203 coating film was not formed sufficiently. As a result, its entering
C
quantity was 0.83%, exhibiting poor anti-carburizing properties.
[0101]
As to Test Number 21, its content of Ni was excessively low. As a result, its
entering C quantity was 0.52%, exhibiting poor anti-carburizing properties.
[0102]
As to Test Number 22, its content of Mg was excessively low. As a result,
its value of reduction of area was less than 60%, exhibiting a low hot
workability.
[0103]
As to Test Number 23, its content of Mg was excessively high. As a result,
its value of reduction of area was less than 60%, exhibiting a low hot
workability.
[0104]
The embodiment according to the present invention has been described above.
However, the aforementioned embodiment is merely an example for practicing the
present invention. Therefore, the present invention is not limited to the
aforementioned embodiment, and the aforementioned embodiment can be modified
and implemented as appropriate without departing from the scope of the present
invention.
INDUSTRIAL APPLICABILITY
[0105]
The austenitic stainless steel according to the present invention is available
even in the high temperature carburizing environment such as a hydrocarbon gas
atmosphere, in which there are concerns about carburizing and coking. In
particular,
the austenitic stainless steel is suitable for application to steel for
reaction tube in
chemical industry plants such as ethylene producing plants, and the like.
