Description
Title of Invention:
AUSTENITIC STAINLESS STEEL AND METHOD FOR PRODUCING
AUSTENITIC STAINLESS STEEL MATERIAL
Technical Field
[00011
The present invention relates to austenitic
stainless steel and a method for producing an austenitic
stainless steel material, and more particularly to
austenitic stainless steel used in a corrosive
environment of a chemical plant or the like, and a method
for producing an austenitic stainless steel material.
Background Art
[00021
A steel material used in a chemical plant is
required to have excellent corrosion resistance as well
as strength. In particular, in a urea plant that is one
of chemical plants, high temperature strength and nitric
acid corrosion resistance are required. In a urea plant,
urea is generally produced by the following method. A
gaseous mixture containing ammonia and carbon dioxide is
condensed through a high pressure of 130 kg/cm2 or higher
in a high temperature range of 160 to 230°C. At this
time, urea is produced by synthesis reaction. Since urea
is produced under a high temperature and high pressure as
described above, the steel materials used in urea plants
are required to have excellent high temperature strength.
[00031
In the production process of urea described above,
an intermediate product called ammonium carbamate is
further produced. Corrosiveness of ammonium carbamate is
very strong. It is generally known that corrosion by
ammonium carbamate is correlated with corrosion by nitric
acid. Accordingly, steel materials for urea plants are
required to have not only high temperature strength but
also excellent nitric acid corrosion resistance.
[00041
Austenitic stainless steel typified by SUS316,
SUS317 and the like in JIS Standard has excellent
corrosion resistance. Therefore, these types of
austenitic stainless steel are used as steel materials
for plants.
[00051
With the objective of further improving the strength
and corrosion resistance of the austenitic stainless
steel as above, the following arts are proposed.
[00061
JP10-88289A (Patent Document 1) proposes Cr-Mn
austenitic steel excellent in strength and corrosion
resistance. In Patent Document 1, the crystal grains of
Cr-Mn austenitic steel are ultra-refined, and the average
grain size is 1 pm or less. Patent Document 1 indicates
that thereby, Cr-Mn austenitic steel having high strength
and excellent corrosion resistance is obtained.
[00071
JP6-256911A (Patent Document 2) proposes austenitic
stainless steel having excellent nitric acid corrosion
resistance even after cold working. In Patent Document 2,
Ni, Mn, C, N, Si and Cr contents in the steel are
controlled. Patent Document 2 indicates that thereby,
martensite production by strain induced transformation
after cold working is suppressed, and excellent nitric
acid corrosion resistance is obtained.
[0008]
JP2005-509751A (Patent Document 3) proposes ultra
austenitic stainless steel having excellent corrosion
resistance. In Patent Document 3, Cu is contained as
well as Cr, Ni, Mo and Mn. Patent Document 3 indicates
that by containing right amounts of these elements,
excellent corrosion resistance is obtained.
[00091
However, the austenitic stainless steel disclosed in
each of Patent Documents 1 to 3 sometimes cannot provide
sufficient high temperature strength while maintaining
nitric acid corrosion resistance.
Disclosure of the Invention
[OOlO]
An objective of the present invention is to provide
austenitic stainless steel having high temperature
strength and excellent nitric acid corrosion resistance.
[OOllI
Austenitic stainless steel according to the present
invention comprising, in mass percent, C: at most 0.050%,
Si: 0.01 to 1.00%, Mn: 1.75 to 2.50%, P: at most 0.050%,
S: at most 0.0100%, Ni: 20.00 to 24.00%, Cr: 23.00 to
27.00%, Mo: 1.80 to 3.20%, and N: 0.110 to 0.180%, the
balance being Fe and impurities, wherein a grain size
number of crystal grains based on JIS GO551 (2005) is at
least 6.0, and an area fraction of a o phase in the steel
is at most 0.1%.
[ 0 0 12 I
The austenitic stainless steel according to the
present invention has higher temperature strength and
excellent nitric acid corrosion resistance.
[00131
The austenitic stainless steel according to the
present invention may further comprising, in place of
some of the Fe, one or two types selected from a group
consisting of Ca: at most 0.0100%, Mg: at most 0.0100%,
and rare earth metal (REM) : at most 0.200%.
[00141
The method for producing an austenitic stainless
steel material according to the present invention
includes a step of preparing a starting material
comprising, in mass percent, C: at most 0.050%, Si: 0.01
to 1.00%, Mn: 1.75 to 2.50%, P: at most 0.050%, S: at
most 0.0100%, Ni: 20.00 to 24.00%, Cr: 23.00 to 27.00%,
Mo: 1.80 to 3.20%, and N: 0.110 to 0.180%, the balance
being Fe and impurities, a step of subjecting the
starting material to hot working to produce a steel
material, and a step of carrying out solution treatment
at a solution temperature of 1050 to llOO°C, for the
steel material.
[00151
The austenitic stainless steel material produced by
the production method according to the present invention
has higher temperature strength, and excellent nitric
acid corrosion resistance.
Best Mode for Carrying Out the Invention
[0016]
Hereinafter, an embodiment of the present invention
will be described in detail. In the following
description, " % " of contents of elements means mass
percent.
[00171
The present inventor made a study concerning high
temperature strength and nitric acid corrosion resistance
of austenitic stainless steel. As a result, the present
inventor obtained the following finding.
[0018]
(A) In order to obtain higher temperature strength,
1.75% or more of Mn is contained. Mn is dissolved in
steel, and enhances the high temperature strength of the
steel. Further, even if Mn is contained, the nitric acid
corrosion resistance of the steel is less likely to be
reduced. Accordingly, in order to obtain higher
temperature strength and excellent nitric acid corrosion
resistance, Mn is effective.
[00191
(B) If crystal grains are refined, the high
temperature strength and the nitric acid corrosion
resistance of austenitic stainless steel are enhanced.
More specifically, if the grain size number of crystal
grains based on JIS GO551 (2005) is 6.0 or larger,
excellent high temperature strength and nitric acid
corrosion resistance are obtained. Note that in the
present description, a revision year is written in the
parentheses written at the end of JIS Standard.
[00201
(C) A sigma phase (hereinafter, called a o phase)
reduces nitric acid corrosion resistance. Accordingly,
in order to obtain excellent nitric acid corrosion
resistance, production of o phases has to be suppressed.
Cr and Mo are dissolved in steel to enhance the high
temperature strength of the steel similarly to Mn.
However, Cr and Mo promote production of o phases.
Accordingly, in the present invention, a Cr content and
an Mo content are suppressed. More specifically, an
upper limit of the Cr content is set to be 27.00%, and an
upper limit of the Mo content is set to be 3.20%.
[00211
(Dl In order to suppress production of o phases, and
to obtain higher temperature strength, a solution
temperature in solution treatment is set to be 1050 to
1100°C. If the solution temperature is lower than 1050°C,
o phases are produced. More specifically, an area
fraction of the o phases in the steel exceeds 0.1%. As a
result, the nitric acid corrosion resistance is reduced.
On the other hand, if the solution temperature exceeds
llOO°C, the high temperature strength is reduced. If a
chemical composition is adjusted based on the above
described (A) and (C), and the solution temperature is
set to be 1050 to llOO°C, the high temperature strength
and the nitric acid corrosion resistance of the produced
austenitic stainless steel are enhanced. More
specifically, yield strength at 230°C becomes 220 MPa or
more, and a corrosion rate in a 65% nitric acid corrosion
test in conformity to JIS GO573 (1999) becomes 0.085
g/m2/h or less.
[00221
Based on the above finding, the present inventor
completed the present invention. Hereinafter, austenitic
stainless steel according to the present invention will
be described.
[00231
[Chemical composition]
The austenitic stainless steel according to the
present invention has the following chemical composition.
[00241
C: at most 0.050%
Carbon (C) combines with Cr to form a Cr carbide.
Cr carbides are precipitated on grain boundaries, and
enhance high temperature strength of steel. Meanwhile,
if C is excessively contained, a Cr depleted zone is
formed in the vicinity of the grain boundaries. The Cr
depleted zone reduces nitric acid corrosion resistance of
steel. Accordingly, a C content is at most 0.050%. A
lower limit of the C content is not especially set, and
if the C content is 0.002% or more, the above described
effect is remarkably obtained. An upper limit of the C
content is preferably less than 0.050%, and more
preferably is 0.030%. A far more preferable lower limit
of the C content is 0.010%.
[00251
Si: 0.01 to 1.00%
Silicon (Si) deoxidizes steel. Si further enhances
oxidation resistance of steel. Meanwhile, if Si is
excessively contained, Si segregates on grain boundaries.
The segregated Si reacts with a combusted slug containing
chlorides, and thereby, intergranular corrosion occurs.
If Si is excessively contained, the mechanical properties
such as ductility of the steel are further reduced.
Accordingly, an Si content is 0.01 to 1.00%. A lower
limit of the Si content is preferably higher than 0.01%,
more preferably is 0.10%, and far more preferably is
0.20%. An upper limit of the Si content is preferably
less than 1.00%, is more preferably 0.40%, and is far
more preferably 0.30%.
[0026]
Mn: 1.75 to 2.50%
Manganese (Mn) is dissolved in steel, and enhances
high temperature strength of the steel. Further, even if
Mn is contained, the nitric acid corrosion resistance of
the steel is less likely to be reduced. Accordingly, Mn
is effective in enhancing high temperature strength while
maintaining the nitric acid corrosion resistance of the
steel. Mn further deoxidizes steel. Further, Mn is an
austenite forming element, and stabilizes austenite
phases in a matrix. Mn further combines with S in steel
to form MnS and enhances hot workability of the steel.
Meanwhile, if Mn is excessively contained, workability
and weldability of the steel are reduced. Accordingly,
an Mn content is 1.75 to 2.50%. A lower limit of the Mn
content is preferably higher than 1.75%, is more
preferably 1.85%, and is far more preferably 1.90%. An
upper limit of the Mn content is preferably less than
2.50%, is more preferably 2.30%, and is far more
preferably 2.00%.
[00271
P: at most 0.050%
Phosphorus (P) is an impurity. P reduces
weldability and workability of steel. Accordingly, the
smaller the P content, the better. The P content is at
most 0.050%. An upper limit of the P content is
preferably less than 0.050%, is more preferably at most
0.020%, and is far more preferably at most 0.015%.
[0028]
S: at most 0.0100%
(Sulfur) S is an impurity. S reduces weldability
and workability of steel. Accordingly, the smaller the S
content, the better. The S content is at most 0.0100%.
An upper limit of the S content is preferably lower than
0.0100%, is more preferably 0.0020%, and is far more
preferably 0.0012%.
[00291
Ni: 20.00 to 24.00%
Nickel (Ni) is an austenite forming element, and
stabilizes austenite phases in a matrix. Ni further
enhances high temperature strength and nitric acid
corrosion resistance of steel. Meanwhile, if Ni is
, excessively contained, a dissolution limit of N decreases
to reduce the nitric acid corrosion resistance of the
steel on the contrary due to reduction in strength and
precipitation of nitrides. Accordingly, an Ni content is
20.00 to 24.00%. A lower limit of the Ni content is
preferably higher than 20.00%, is more preferably 21.00%,
and is far more preferably 22.00%. An upper limit of the
Ni content is preferably less than 24.00%, is more
preferably 23.00%, and is far more preferably 22.75%.
[00301
Cr: 23.00 to 27.00%
Chrome (Cr) enhances nitric acid corrosion
resistance of steel. Further, Cr is dissolved in steel
to enhance high temperature strength of the steel.
Meanwhile, if Cr is excessively contained, o phases are
precipitated in the steel, and the nitric acid corrosion
resistance of the steel is reduced. The o phase further
reduces weldability and workability of the steel.
Accordingly, a Cr content is 23.00 to 27.00%. A lower
limit of the Cr content is preferably higher than 23.00%,
is more preferably 24.00%, and is far more preferably
24.50%. An upper limit of the Cr content is preferably
less than 27.00%, is more preferably 26.00%, and is far
more preferably 25.50%.
[00311
Mo: 1.80 to 3.20%
Molybdenum (Mo) enhances nitric acid corrosion
resistance of steel. Further, Mo is dissolved in steel
to enhance high temperature strength of the steel.
Meanwhile, if Mo is excessively contained, o phases are
precipitated in the steel, and nitric acid corrosion
resistance of the steel is reduced. The o phase further
reduces weldability and workability of the steel.
Accordingly, an Mo content is 1.80 to 3.20%. A lower
limit of the Mo content is preferably higher than 1.80%,
is more preferably 1.90%, and is far more preferably
2.00%. An upper limit of the Mo content is preferably
less than 3.20%, is more preferably 2.80%, and is far
more preferably 2.50%.
[00321
N: 0.110 to 0.180%
Nitrogen (N) is an austenite forming element, and
stabilizes austenite phases in a matrix. Nitrogen
further forms fine nitrides to refine crystal grains, and
enhances high temperature strength of steel. Further,
nitrogen also has an effect of stabilizing a surface film,
and enhances nitric acid corrosion resistance. Meanwhile,
if N is excessively contained, nitrides are excessively
produced, whereby hot workability of steel is reduced,
and nitric acid corrosion resistance is further reduced.
Accordingly, an N content is 0.110 to 0.180%. A lower
limit of the N content is preferably higher than 0.110%,
is more preferably 0.120%, and is far more preferably
0.130%. An upper limit of the N content is preferably
less than 0.180%, is more preferably 0.170%, and is far
more preferably 0.160%.
[00331
The balance of the austenitic stainless steel
according to the present invention is Fe and impurities.
Impurities refer to elements that enter from ores and
scraps that are used as raw materials of the steel, the
environment of a production process, or the like.
[00341
[Grain size]
In the austenitic stainless steel according to the
present invention, the grain size number of the crystal
grains as measured by being corroded with use of about
20% of a nitric acid aqueous solution based on JIS GO551
(2005) is 6.0 or larger. If the grain size number is 6.0
or larger, the austenitic stainless steel has excellent
high temperature strength while maintaining nitric acid
corrosion resistance.
[00351
[Sigma phase area fraction]
In the austenitic stainless steel according to the
present invention, an area fraction of a sigma phase
(hereinafter, called a o phase) in the steel is at most
0.1%. Here, the area fraction of the o phase is
calculated by the following method.
[00361
A sample for microscopic observation is extracted
from an arbitrary spot of an austenitic stainless steel
material. A surface of the extracted sample is
mechanically polished, and etched. In the etched sample
surface, arbitrary six visual fields are observed with
use of a 400-power lens including 20 by 20, 400 lattices
.in total with an optical microscope. An observation
region of each of the visual fields is 225 p2. The
number of o phases existing on the lattice points in each
of the visual fields is counted, and a value obtained by
dividing the number of o phases existing on the lattice
points in the visual fields by a total number of lattice
points of the six visual fields (2400 points) is defined
as an area fraction of the o phase (in % ) .
[00371
In the present invention, the area fraction of the o
phase in the steel is at most 0.1%. Therefore, the
austenitic stainless steel according to the present
invention has excellent nitric acid corrosion resistance.
When the steel having the aforementioned chemical
composition is produced by a production method that will
be described later, the area fraction of the o phase
becomes at most 0.1%. An area fraction of the o phase is
preferably less than 0.05%, and is more preferably at
most 0.01%.
[0038]
The austenitic stainless steel of the present
invention having the above composition has excellent high
temperature strength and nitric acid corrosion resistance.
More specifically, the high temperature strength at 230°C
of the austenitic stainless steel according to the
present invention is 220 MPa or more. Yield strength
mentioned here is defined as 0.2% yield stress. Further,
the corrosion rate that is obtained by the 65% nitric
acid corrosion test (Huey test) in conformity with JIS
GO573 (1999) is at most 0.085 g/m2/h.
[ 0 0 3 9 I
A total content of C and N is preferably 0.145% or
more in the aforementioned chemical composition. In this
case, high temperature strength of the austenitic
stainless steel is further enhanced.
[0040]
[Selective element]
The austenitic stainless steel according to the
present invention further contains one or more types
selected from a group consisting of Ca, Mg and rare earth
metal (REM) . All of these elements enhance hot
workability of steel.
[00411
Ca at most 0.0100%
Calcium (Ca) is a selective element. Ca enhances
hot workablity of steel. Meanwhile, if Ca is excessively
contained, cleanliness of steel is reduced. Therefore,
nitric acid corrosion resistance and toughness of the
steel are reduced, and mechanical properties of the steel
are reduced. Accordingly, a Ca content is at most
0.0100%. If the Ca content is 0.0005% or more, the above
described effect is remarkably obtained. An upper limit
of the Ca content is preferably less than 0.0100%, and is
more preferably 0.0050%.
[00421
Mg: at most 0.0100%
Magnesium (Mg) is a selective element. Mg enhances
hot workability of steel. Meanwhile, if Mg is
excessively contained, cleanliness of the steel is
reduced. Therefore, nitric acid corrosion resistance and
toughness of the steel are reduced, and mechanical
properties of the steel are reduced. Accordingly, an Mg
content is at most 0.0100%. If the Mg content is 0.0005%
or more, the above described effect is remarkably
obtained. An upper limit of the Mg content is preferably
less than 0.0100%, and is more preferably 0.0050%.
LOO431
Rare earth metal (REM) : at most 0.200%
Rare earth metal (REM) is a selective element. REM
has a high affinity for S. Therefore, REM enhances hot
workability of steel. However, if REM is excessively
contained, cleanliness of the steel is reduced.
Therefore, nitric acid corrosion resistance and toughness
of the steel are reduced, and mechanical properties of
the steel are reduced. Accordingly, an REM content is at
most 0.200%. If the REM content is 0.001% or more, the
above described effect is remarkably obtained. An upper
limit of the REM content is preferably less than 0.150%,
and is more preferably 0.100%.
[00441
REM is a generic name of 17 elements that are
lanthanum (La) of atomic number 57 to lutetium (Lu) of
atomic number 71 in the periodic table, to which yttrium
(Y) and scandium (Sc) are added. The content of REM
means a total content of one or more types of these
elements.
[00451
When two types or more of Ca, Mg and REM are
contained, the total content of Ca, Mg and REM is
preferably at most 0.0150%. In this case, excellent hot
workability is obtained while nitric acid corrosion
resistance of steel is maintained.
[0046]
[Product ion method]
An example of a method for producing an austenitic
stainless steel material according to the present
invention will be described.
[00471
Molten steel having the aforementioned chemical
composition is produced by blast furnace or electric
furnace melting. Well-known degassing treatment is
applied to the produced molten steel as necessary.
[00481
Next, a starting material is produced from the
molten steel. More specifically, the molten steel is
formed into casting materials by a continuous casting
process. Casting materials are, for example, slabs,
blooms and billets. Alternatively, the molten steel is
formed into ingots by an ingot-making process. The
starting material mentioned in the present description is,
for example, the aforementioned casting material or ingot.
Next, the produced starting material (the casting
material or ingot) is subjected to hot working by a wellknown
method, and formed into an austenitic stainless
steel material. Examples of the austenitic stainless
steel material include steel pipes (seamless pipes or
welded steel pipes), steel plates, steel bars, wire rods,
forged steel and the like. Hot working is, for example,
piercing-roll, hot rolling, hot forging or the like. For
the austenitic stainless steel material after hot working,
cold working such as cold rolling and cold draw may be
carried out.
E00491
Solution treatment is carried out for the produced
austenitic stainless steel material. The temperature of
the solution treatment (solution temperature) is 1050 to
llOO°C. If the solution temperature is less than 1050°C,
o phases are produced, and the area fraction of the a
phase in the steel exceeds 0.1%. Meanwhile, if the
solution temperature exceeds llOO°C, the crystal grains
are coarsened, and the grain size number becomes smaller
than 6.0. If the solution temperature is 1050 to llOO°C,
the grain size number of the crystal grains is 6.0 or
larger, and the area fraction of the o phase becomes at
most 0.1%.
[OOSO]
A preferable holding (soaking) time period at the
solution temperature is one minute to ten minutes. An
upper limit of the soaking time period is preferably five
minutes. In the solution treatment, the steel is held at
the solution temperature for a predetermined time period,
and thereafter, is rapidly cooled.
[0051]
In the above process, the austenitic stainless steel
according to the present invention is produced.
Example
[00521
A plurality of types of austenitic stainless steel
materials were produced, and the high temperature
strength and the nitric acid corrosion resistance of each
of the steel materials were examined.
[00531
[Examination method]
The austenitic stainless steel of each of mark 1 to
mark 12 having the chemical composition shown in Table 1
was melted in a high-frequency heating vacuum furnace to
produce ingots.
[00541
[Table 11
Mark
Chemical composition (in mass%, bal
C I Si I M n l P I S I N i
ince being Fe and impurities) Solution
temperature --
Cr I Mo I N I Ca I R E M ( N ~ ~ ("C)
In each of the columns of the symbols of the
respective elements (C, Si, Mn, P, S, Ni, Cr, Mo, N, Ca,
REM) in Table 1, the content (mass%) of the corresponding
element in the steel of each of the marks is written.
The balance except for the elements described in Table 1
of the chemical composition of each of the marks is Fe
and impurities. In Table 1, indicates that the
corresponding element content is at an impurity level.
[0056]
The chemical compositions of marks 1 to 3, 6, 7 and
12 were within the range of the present invention.
Meanwhile, the Mn contents of marks 4 and 5 were less
than the lower limit of the Mn content of the present
invention. The Ni content of mark 8 was less than the
lower limit of the Ni content of the present invention,
and the Ni content of mark 9 exceeded the upper limit of
the Ni content of the present invention. The lower limit
of the N content of mark 10 was less than the lower limit
of the N content of the present invention, and the N
content of mark 11 exceeded the upper limit of the N
content of the present invention.
[00571
The respective produced ingots were subjected to hot
forging, and hot-rolling to produce an intermediate
material. Further, the intermediate material was
subjected to cold rolling to produce austenitic stainless
steel plates of a thickness of 30 mm.
[0058]
For the produced steel plates, solution treatment
was carried out at the solution temperatures shown in
Table 1. The holding time periods at the solution
temperatures were three minutes in all the marks. After
a lapse of the holding time period, the steel plates were
rapidly cooled (water-cooled).
[00591
[o phase area f ractionl
From arbitrary spots of the produced steel plates of
the respective marks, samples for microscopic test
observation were extracted. The surfaces of the
extracted samples were mechanically polished, and etched.
In the etched sample surfaces, arbitrary six visual
fields were observed with use of a 400-power lens
including 20 by 20, 400 lattices in total, with an
optical microscope. The area of each of the visual
fields was 225 pm2. The number of o phases existing on
the lattice points in each of the visual fields was
counted. The value obtained by dividing the total number
of counts of o phases by the total number of lattice
points (2400 points) of the six visual fields was
determined as the area fraction of the o phase (in % ) .
[ 0 0 6 0 I
[Microscope test of grain size]
Specimens were extracted from the produced steel
plates of the respective marks. With use of the
specimens, a microscope test of the grain size in
conformity with JIS GO551 (2005) was carried out, and the
grain size numbers of the austenitic crystal grains of
the respective marks were found.
[00611
[High temperature strength testl
From the produced steel plates of the respective
marks, round bar specimens each with the outside diameter
of the parallel part of 6 mm were extracted. With use of
the extracted round bar specimens, a high temperature
tension test in conformity with JIS GO567 (1998) was
carried out to find yield strength (MPa) of each of the
marks. The test temperature was 230°C. Further, 0.2%
yield stress was defined as the yield strength.
[0062]
[65% nitric acid corrosion testl
A 65% nitric acid corrosion test (Huey test) in
conformity with JIS GO573 (1999) was carried out, and the
nitric acid corrosion resistance of the steel plate of
each of the marks was examined. More specifically, from
the steel plate of each of the marks, a specimen of 40 mm
x 10 mm x 2 mm was extracted. The surface area of the
specimen was 1000 mm2. Further, a test solution with the
concentration of nitric acid being 65 mass% was prepared.
The specimens were immersed in the boiled test solution
for 48 hours (the first immersion test). After the test
ended, a new test solution was prepared, and the second
immersion test was carried out. More specifically, the
specimens were taken out from the test solution that was
used in the first immersion test, and the specimens were
immersed in the test solution for the second immersion
test for 48 hours. The immersion tests as above were
repeatedly performed five times (the first test to the
fifth test) .
[0063]
Before and after the respective immersion tests (the
first test to the fifth test), the masses of the
specimens were measured, and the differences (mass
losses) were found. Based on the mass losses, for each
of the immersion tests, the mass losses per unit area and
unit time of the specimens (hereinafter, called unit mass
losses, in g/m2/h) were found. The average value of the
unit mass losses of the five tests (the first test to the
fifth test) that were found was defined as a corrosion
rate (g/m2/h) .
[00641
[Test result]
The test result is shown in Table 2.
[00651
[Table 21
TABLE 2
1 I I
[00661
Referring to Table 2, the chemical compositions of
marks 1 to 3 were within the range of the chemical
composition of the present invention, and the solution
temperatures were within the range of 1050 to llOO°C.
Accordingly, the o phase area fractions of the austenitic
stainless steel plates of marks 1 to 3 were at most 0.1%,
and the grain size numbers were 6.0 or larger. Therefore,
the high temperature strengths of marks 1 to 3 were 220
MPa or more, and the corrosion rates thereof were at most
0.085 g/m2/h.
[0067]
Meanwhile, the Mn content of mark 4 was less than
the lower limit of the Mn content of the present
invention, and the solution temperature exceeded llOO°C.
Therefore, the grain size number of mark 4 was less than
6.0, and the high temperature strength thereof was less
than 220 Mpa.
[0068]
The Mn content of mark 5 was less than the lower
limit of the Mn content of the present invention.
Therefore, the high temperature strength of mark 5 was
less than 220 MPa.
[0069]
The chemical composition of mark 6 was within the
range of the chemical composition of the present
invention, but the solution temperature exceeded llOO°C.
Therefore, the grain size number of mark 6 was less than
6.0, and the high temperature strength thereof was less
than 220 MPa.
[00701
The chemical compositions of marks 7 and 12 were
within the range of the chemical composition of the
present invention, but the solution temperatures were
less than 1050°C. Therefore, the o phase area fractions
exceeded 0.1%. As a result, the corrosion rates exceeded
0.085 g/m2/h.
[00711
The Ni content of mark 8 was less than the lower
limit of the Ni content of the present invention.
Therefore, the high temperature strength was less than
220 MPa, and the corrosion rate exceeded 0.085 g/m2/h.
[00721
The Ni content of mark 9 exceeded the upper limit of
the Ni content of the present invention. Therefore, the
corrosion rate exceeded 0.085 g/m2/h.
[00731
The N content of mark 10 was less than the lower
limit of the N content of the present invention.
Therefore, the grain size number was smaller than 6.0.
Accordingly, the high temperature strength was less than
220 MPa, and the corrosion rate exceeded 0.085 g/m2/h.
[0074]
The N content of mark 11 exceeded the upper limit of
the N content of the present invention. Therefore, the
corrosion rate exceeded 0.085 g/m2/h.
[00751
Note that referring to marks 1 to 3, 7 and 12, the o
phase area fractions significantly declined as the
solution temperature increased. When the solution
temperatures were 1050°C or higher, the o phase area
fractions were at most 0.1%.
[00761
The embodiment of the present invention is described
above, and the aforementioned embodiment is only an
illustration for carrying out the present invention.
Accordingly, the present invention is not limited to the
aforementioned embodiment, and the aforementioned
embodiment can be carried out by being properly modified
within the range without departing from the gist of the
present invention.
Industrial Applicability
[00771
The present invention can be widely applied to the
steel materials that are required to have high
temperature strength and nitric acid corrosion resistance,
and can be applied to, for example, steel materials for
chemical plants. The present invention is especially
preferable for the steel materials for urea plants.

, L "-+
GXF1*%~
We Claim:
[Claim 11
Austenitic stainless steel comprising: in mass
percent,
C: at most 0.050%;
Si: 0.01 to 1.00%;
Mn: 1.75 to 2.50%;
P: at most 0.050%;
S: at most 0.0100%;
Ni: 20.00 to 24.00%;
Cr: 23.00 to 27.00%;
Mo: 1.80 to 3.20%; and
the balance being Fe and impurities,
wherein a grain size number of crystal grains based
on JIS GO551 (2005) is at least 6.0, and
an area fraction of a o phase is at most 0.1%.
[Claim 21
The austenitic stainless steel according to claim 1,
further comprising:
in place of some of the Fe,
at least one type selected from a group consisting
of
Ca: at most 0.0100%;
Mg: at most 0.0100%; and
rare earth metal (REM) : at most 0.200%.
[Claim 31
Q IGINALO '~36
The austenitic stainless steel according to claim 1
or claim 2,
wherein yield strength at 230°C is at least 220 MPa,
and a corrosion rate in a 65% nitric acid corrosion test
in conformity with JIS GO573 (1999) is at most 0.085
g/m2/h
[Claim 41
A method for producing an austenitic stainless steel
material, comprising:
a step of preparing a starting material comprising,
in mass percent, C: at most 0.050%, Si: 0.01 to 1.00%,
Mn: 1.75 to 2.50%, P: at most 0.050%, S: at most 0.01'00%,
Ni: 20.00 to 24.00%, Cr: 23.00 to 27.00%, Mo: 1.80 to
3.20%, and N: 0,110 to 0.180%, the balance being Fe and
impurities;
a step of subjecting the starting material to hot
working to produce a steel material; and
a step of carrying out solution treatment at a
solution temperature of 1050 to llOO°C, for the steel
material.
[Claim 51
The method for producing an austenitic stainless
steel material according to claim 4,
wherein the starting material further comprising, in
place of some of the Fe, at least one type selected from
a group consisting of Ca: at most 0.0100%, Mg: at most
0.0100%, and rare earth metal (REM): at most 0.200%.
Dated this 3rd day of January, 2014.
Ni on tee1 & Sumitomo Metal Corporation
(oNf itin 9, Amarcha'1n dm &an Mi) angaldas &
Suresh A. Shroff & Co.
Attorneys for the Applicant