Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
TITLE OF INVENTION
DUPLEX STAINLESS STEEL AND METHOD FOR PRODUCING DUPLEX
STAINLESS STEEL
TECHNICAL FIELD
[0001]
The present invention relates to a duplex stainless steel and a method for
producing the duplex stainless steel.
BACKGROUND ART
[0002]
A duplex stainless steel having a dual phase structure consisting of the
ferrite
phase and the austenite phase is known to have excellent corrosion resistance.
A
duplex stainless steel is particularly superior in corrosion resistance
against pitting
and/or crevice corrosion (hereinafter referred to as "pitting resistance"),
which is
taken as a problem in an aqueous solution containing chlorides. A duplex
stainless
steel is therefore widely used in a wet environment containing chlorides, such
as
seawater. In a wet environment containing chlorides, a duplex stainless steel
is
used, for example, in a flow line pipe, an umbilical tube, and a heat
exchanger.
[0003]
In recent years, the corrosion conditions in the environment in which a duplex
stainless steel is used have been increasingly severe. A duplex stainless
steel is
therefore required to have more excellent pitting resistance. To further
enhance the
pitting resistance of a duplex stainless steel, a variety of technologies have
been
proposed.
[0004]
International Application Publication No. 2013/191208 (Patent Literature 1)
discloses a duplex stainless steel containing, in mass%, Ni: 3 to 8%, Cr: 20
to 35%,
Mo: 0.01 to 4.0%, and N: 0.05 to 0.60% and further containing one or more
types of
element selected from Re: 2.0% or less, Ga: 2.0% or less, and Ge: 2.0% or
less. In
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Patent Literature 1, the fact that the duplex stainless steel contains Re, Ga,
or Ge
increases the critical potential at which pitting occurs (pitting potential)
to enhance
the pitting resistance and crevice corrosion resistance.
[0005]
International Application Publication No. 2010/082395 (Patent Literature 2)
discloses a method for producing a duplex stainless steel pipe by performing
hot
working or hot working and further solid solution heat treatment on a duplex
stainless steel material containing, in mass%, Cr: 20 to 35%, Ni: 3 to 10%,
Mo: 0 to
6%, W: 0 to 6%, Cu: 0 to 3%, and N: 0.15 to 0.60% to produce a steel pipe for
cold
working and then performing cold rolling on the steel pipe. The method for
producing a duplex stainless steel pipe in Patent Literature 2 is a method for
producing a duplex stainless steel pipe having a minimum yield strength
ranging
from 758.3 to 965.2 MPa by performing cold rolling that allows the working
ratio Rd
(=exp[IIn(MYS)-In(14.5xCr+48.3xMo+20.7xW+6.9xN)1/0.1951) at the area
reduction ratio in the final cold rolling step to fall within a range from 10
to 80%.
Patent Literature 2 describes that the method described above provides a
duplex
stainless steel pipe that can be used, for example, in an oil well and a gas
well, shows
excellent corrosion resistance also in a carbon dioxide gas corrosion
environment or
a stress corrosion environment, and has high strength.
[0006]
Japanese Patent Application Publication No. 2007-84837 (Patent Literature 3)
discloses a duplex stainless steel containing, in mass%, Cr: 20 to 30%, Ni: 1
to 11%,
Cu: 0.05 to 3.0%, Nd: 0.005 to 0.5%, and N: 0.1 to 0.5% and/or Mo: 0.5 to 6%
and
W: 1 to 10%. In Patent Literature 3, the hot workability of the duplex
stainless steel
is enhanced because the duplex stainless steel contains Nd.
[0007]
National Publication of International Patent Application No. 2005-520934
(Patent Literature 4) discloses a super duplex stainless steel containing, in
weight%,
Cr: 21.0% to 38.0%, Ni: 3.0% to 12.0%, Mo: 1.5% to 6.5%, W: 0 to 6.5%, N: 0.2%
to 0.7%, and Ba: 0.0001 to 0.6% and having a pitting resistance equivalent
index
PREW that satisfies 40A3REW67. Patent Literature 4 describes that the thus
configured super duplex stainless steel is superior in corrosion resistance,
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embrittlement resistance, castability, and hot workability with formation of
intermetal phases, such as the brittle sigma (a) phase and the chi (x) phase,
suppressed.
CITATION LIST
PATENT LITERATURE
[0008]
Patent Literature 1: International Application Publication No. 2013/191208
Patent Literature 2: International Application Publication No. 2010/082395
Patent Literature 3: Japanese Patent Application Publication No. 2007-84837
Patent Literature 4: National Publication of International Patent Application
No. 2005-520934
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0009]
As described above, a duplex stainless steel having more excellent pitting
resistance has been required in recent years. Technical means other than the
technologies described in Patent Literatures 1 to 4 may therefore provide a
duplex
stainless steel showing excellent pitting resistance.
[0010]
An objective of the present disclosure is to provide a duplex stainless steel
having excellent pitting resistance and a method for producing the duplex
stainless
steel.
SOLUTION TO PROBLEM
[0011]
A duplex stainless steel according to the present disclosure has a chemical
composition consisting of, in mass%, Cr: more than 27.00% to 29.00%, Mo: 2.50
to
3.50%, Ni: 5.00 to 8.00%, W: 4.00 to 6.00%, Cu: 0.01 to less than 0.10%, N:
more
than 0.400% to 0.600%, C: 0.030% or less, Si: 1.00% or less, Mn: 1.00% or
less,
sol.A1: 0.040% or less, V: 0.50% or less, 0: 0.010% or less, P: 0.030% or
less, 5:
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0.020% or less, Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, and B: 0 to 0.0040% with
the
balance being Fe and impurities and satisfying Formula (1), and a
microstructure
consisting of 35 to 65 volume% of ferrite phase with the balance being an
austenite
phase. In the duplex stainless steel according to the present disclosure, an
area
fraction of Cu precipitated in the ferrite phase is 0.5% or less.
Cr+4.0 xMo+2.0 x W+20 xN-5 x ln(Cu)65.2 (1)
where, a content in mass% of each of the elements is substituted into a
corresponding symbol of the element in Formula (1).
[0012]
A method for producing a duplex stainless steel according to the present
disclosure includes a preparation step, a hot working step, a cooling step,
and a
solution heat treatment step. In the preparation step, a starting material
having the
chemical composition described above is prepared. In the hot working step, the
starting material is subjected to hot working at 850 C or more. In the cooling
step,
the starting material subjected to the hot working is cooled at a rate of 5
C/sec or
more. In the solution heat treatment step, the cooled starting material is
subjected
to a solution heat treatment at 1070 C or more.
ADVANTAGEOUS EFFECTS OF INVENTION
[0013]
The duplex stainless steel according to the present disclosure has excellent
pitting resistance. The method for producing the duplex stainless steel
according to
the present disclosure allows production of the duplex stainless steel
described above.
DESCRIPTION OF EMBODIMENTS
[0014]
The present inventors have investigated and studied an approach for
enhancing the pitting resistance of a duplex stainless steel. As a result, the
following findings have been achieved.
[0015]
Cr, Mo, and Cu are known to be effective in improvement of the pitting
resistance of a duplex stainless steel. Among Cr, Mo, and Cu, Cr and Mo are
believed to have a mechanism that enhances the pitting resistance of a duplex
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stainless steel as follows: Cr serves as a primary component of a passive film
as an
oxide on the surface of a duplex stainless steel. The passive film prevents
contact
between corrosion factors and the surface of the duplex stainless steel. As a
result,
the duplex stainless steel on the surface of which the passive film has been
formed
has enhanced pitting resistance. Mo is contained in the passive film and
further
enhances the pitting resistance of the passive film.
[0016]
On the other hand, among Cr, Mo, and Cu, Cu is believed to have a
mechanism that enhances the pitting resistance of a duplex stainless steel as
follows:
It is believed that there are the following two steps that cause pitting to
occur. The
first step is occurrence of pitting (initial stage). The next step is
propagation of the
pitting (propagation stage). It has been believed that Cu is effective in
suppressing
the propagation of pitting. Particularly in an acidic solution, an active site
where
the duplex stainless steel melts at high speed is formed on the surface of the
duplex
stainless steel. Cu coats the active site to suppress the melting of the
duplex
stainless steel. It has been believed that the thus functioning Cu suppresses
the
propagation of the pitting that occurs on a duplex stainless steel.
[0017]
It has been believed based on the mechanism described above that Cr, Mo,
and Cu are elements effective in improvement in pitting resistance of a duplex
stainless steel. Cr, Mo, and Cu have therefore been actively contained in a
duplex
stainless steel to enhance the pitting resistance. However, the following
findings
that had not been known have been obtained as a result of the studies
conducted by
the present inventors. Specifically, the present inventors have found that
among Cr,
Mo, and Cu, Cu instead lowers the pitting resistance in some cases at the
occurrence
of pitting (initial stage).
[0018]
Table 1 is a table showing the chemical compositions of test specimens
labeled with test numbers 2 and 5 and the pitting potential, which is an index
of the
pitting resistance, of the test specimens in Examples described later. The
chemical
compositions listed in two rows in Table 1 are those of steels of B and E,
correspond
to the test numbers 2 and 5, and are extracted from Table 3, which will be
described
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later. The chemical compositions in Table 1 are expressed in mass%, and the
balance is Fe and impurities. The pitting potentials listed in Table 1 are
those
labeled with the corresponding test numbers and are extracted from Table 4,
which
will be described later.
[0019]
[Table 1]
TABLE 1
Test No. Steel Cr Mo Ni W Cu N C Si Mn
2 B 28.10 3.11 5.31 4.19 0.14 0421 0.016
049 0.97
E 27.53 2.61 6.97 4.31 0.04 0419 0.016 048 0.92
Test No. Steel sol.A1 V 0 P S Ca Mg B
2 B 0.013 0.10 0.004 0.018 0.001 0.0025 0.0001 0.0019
5 E 0.017 0.10 0.005 0.016 0.001 0.0010 0.0025 0.0013
Test No. Steel Pitting potential (mVvs.SCE)
2 B 71
5E 346
[0020]
Referring to Table 1, the test specimen labeled with the test number 2 has a
higher Cu content than the Cu content in the test specimen labeled with the
test
number 5. Further, the test specimen labeled with the test number 2 has higher
Cr
and Mo contents than the Cr and Mo contents in the test specimen labeled with
the
test number 5. It can therefore be expected based on the findings in the
related art
that the test specimen labeled with the test number 2, which has higher Cr,
Mo, and
Cu contents, has more excellent pitting resistance than the test specimen
labeled with
the test number 5. The pitting potential, which is an index of the pitting
resistance,
of the test specimen labeled with the test number 2 is, however, 71 mVvs.SCE,
which is smaller than the pitting potential of 346 mVvs.SCE of the test
specimen
labeled with the test number 5.
[0021]
That is, the pitting resistance of the test specimen labeled with the test
number
2, which is expected based on the findings in the related art to have more
excellent
pitting resistance than the test specimen labeled with the test number 5, is
instead
smaller than the pitting resistance of the test specimen labeled with the test
number 5.
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In view of the fact described above, the present inventors have focused on the
microstructures of the test specimens labeled with the test numbers 2 and 5
and have
investigated the microstructures in more detail. As a result, the
investigation clearly
showed that the test specimen labeled with the test number 2 has a greater
area
fraction of Cu precipitated in the ferrite phase (called Cu area fraction in
ferrite
phase) than the test specimen labeled with the test number 5.
[0022]
In view of the fact described above, the present inventors have investigated
and studied the effect of Cu precipitated in the ferrite phase on the pitting
resistance
of the duplex stainless steel. Table 2 is a table showing the chemical
compositions
of test specimens labeled with the test numbers 3 and 6, the Cu area fractions
thereof
in the ferrite phase, and the pitting potential thereof, which is an index of
the pitting
resistance, in Examples described later. The chemical compositions listed in
two
rows in Table 2 are those of steel of C, correspond to the test numbers 3 and
6, and
are extracted from Table 3, which will be described later. The chemical
compositions in Table 2 are expressed in mass%, and the balance is Fe and
impurities. The Cu area fractions thereof in the ferrite phase listed in Table
2 are
those labeled with the corresponding test numbers and are extracted from Table
4,
which will be described later. The pitting potentials listed in Table 2 are
those
labeled with the corresponding test numbers and are extracted from Table 4,
which
will be described later.
[0023]
[Table 2]
TABLE 2
Test No. Steel Cr Mo Ni W Cu N C Si Mn
3 C 28.24 2.96 5.76 4.25 0.08 0416 0.014
0.51 0.91
6 C 28.24 2.96 5.76 4.25 0.08 0416 0.014
0.51 0.91
Test No. Steel sol.A1 V 0 P 5 Ca Mg
3 C 0.012 0.10 0.004 0.019 0.001 0.0015 0.0002 0.0012
6 C 0.012 0.10 0.004 0.019 0.001 0.0015 0.0002 0.0012
Test No. Steel Cu area fraction in ferrite phase (%) Pitting potential
(mVvs.SCE)
3 C 0.7 -12
6 C 0 204
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[0024]
Referring to Table 2, the test specimen labeled with the test number 3 and the
test specimen labeled with the test number 6 had the same chemical
composition.
On the other hand, the test specimen labeled with the test number 6 had a
smaller Cu
area fraction in the ferrite phase than the Cu area fraction in the ferrite
phase of the
test specimen labeled with the test number 3. As a result, the pitting
potential of the
test specimen labeled with the test number 6 was 204 mVvs.SCE, which was
greater
than the pitting potential of -12 mVvs.SCE of the test specimen labeled with
the test
number 3. That is, the test specimen labeled with the test number 6 had more
excellent pitting resistance than the test specimen labeled with the test
number 3 as a
result of a decrease in the amount of precipitation of Cu in the ferrite phase
in the test
specimen labeled with the test number 6.
[0025]
It has been believed as described above that increasing the Cr, Mo, and Cu
contents increases the pitting resistance. The present inventors have,
however,
found for the first time that Cu among Cr, Mo, and Cu is instead likely to
lower the
pitting resistance. The present inventors have further found that reduction in
the
amount of Cu precipitating in the ferrite phase allows enhancement of the
pitting
resistance, which is a finding that has not been known at all.
[0026]
No detailed reason why Cu precipitated in the ferrite phase lowers the pitting
resistance of a duplex stainless steel has been clarified. The present
inventors,
however, consider the reason as follows: Cu precipitated in the ferrite phase
is likely
to prevent uniform formation of a passive film. Therefore, in a case where a
large
amount of Cu has precipitated in the ferrite phase, the large amount of Cu is
likely to
lower the passive film's effect of suppressing the contact between corrosion
factors
and the surface of the duplex stainless steel. The present inventors believe
that
pitting occurs on the surface of the duplex stainless steel as a result of the
assumption
described above.
[0027]
A duplex stainless steel according to the present embodiment attained based
on the findings described above has a chemical composition consisting of, in
mass%,
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Cr: more than 27.00% to 29.00%, Mo: 2.50 to 3.50%, Ni: 5.00 to 8.00%, W: 4.00
to
6.00%, Cu: 0.01 to less than 0.10%, N: more than 0.400% to 0.600%, C: 0.030%
or
less, Si: 1.00% or less, Mn: 1.00% or less, sol.A1: 0.040% or less, V: 0.50%
or less,
0: 0.010% or less, P: 0.030% or less, S: 0.020% or less, Ca: 0 to 0.0040%, Mg:
0 to
0.0040%, and B: 0 to 0.0040% with the balance being Fe and impurities and
satisfying Formula (1), and a microstructure consisting of 35 to 65 volume% of
ferrite phase with the balance being the austenite phase. In the duplex
stainless
steel according to the present embodiment, the area fraction of Cu
precipitated in the
ferrite phase is 0.5% or less.
Cr+4.0 xMo+2.0 x W+20 xN-5 x ln(Cu)65.2 (1)
where, the content in mass% of each of the elements is substituted into the
corresponding symbol of the element in Formula (1).
[0028]
The duplex stainless steel according to the present embodiment has the
chemical composition described above and the microstructure described above,
and
the area fraction of Cu in the ferrite phase is 0.5% or less. As a result, the
duplex
stainless steel according to the present embodiment has excellent pitting
resistance.
[0029]
The chemical composition described above preferably contains, in mass%,
one or more types of element selected from the group consisting of Ca: 0.0001
to
0.0040%, Mg: 0.0001 to 0.0040%, and B: 0.0001 to 0.0040%.
[0030]
In this case, the duplex stainless steel according to the present embodiment
has enhanced hot workability.
[0031]
A method for producing a duplex stainless steel according to the present
embodiment includes a preparation step, a hot working step, a cooling step,
and a
solution heat treatment step. In the preparation step, a starting material
having the
chemical composition described above is prepared. In the hot working step, the
starting material is subjected to hot working at 850 C or more. In the cooling
step,
the starting material subjected to the hot working is cooled at a rate of 5
C/sec or
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more. In the solution heat treatment step, the cooled starting material is
subjected
to a solution heat treatment at 1070 C or more.
[0032]
The duplex stainless steel produced by the production method according to
the present embodiment has the chemical composition described above and the
microstructure described above, and the area fraction of Cu in the ferrite
phase is
0.5% or less. As a result, the duplex stainless steel produced by the
production
method according to the present embodiment has excellent pitting resistance.
[0033]
The duplex stainless steel according to the present embodiment will be
described below in detail.
[0034]
[Chemical composition]
The chemical composition of the duplex stainless steel according to the
present embodiment contains the following elements. The symbol % associated
with an element means mass% unless otherwise specified.
[0035]
[Essential elements]
The chemical composition of the duplex stainless steel according to the
present embodiment essentially contains the following elements:
[0036]
Cr: more than 27.00% to 29.00%
Chromium (Cr) forms a passive film as an oxide on the surface of the duplex
stainless steel. The passive film prevents contact between corrosion factors
and the
surface of the duplex stainless steel. As a result, occurrence of pitting on
the duplex
stainless steel is suppressed. Further, Cr is an element necessary for
achievement of
the ferrite structure in the duplex stainless steel. Achievement of a
sufficient ferrite
structure provides stable pitting resistance. Too low a Cr content provides no
effects described above. On the other hand, too high a Cr content lowers the
hot
workability of the duplex stainless steel. The Cr content therefore ranges
from
more than 27.00% to 29.00%. The lower limit of the Cr content is preferably
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27.50%, more preferably 28.00%. The upper limit of the Cr content is
preferably
28.50%.
[0037]
Mo: 2.50 to 3.50%
Molybdenum (Mo) is contained in the passive film and further enhances the
corrosion resistance of the passive film. As a result, the pitting resistance
of the
duplex stainless steel is enhanced. Too low a Mo content provides no effect
described above. On the other hand, too high a Mo content lowers the
workability
of, for example, the assembly of a steel pipe made of the duplex stainless
steel. The
Mo content therefore ranges from 2.50 to 3.50%. The lower limit of the Mo
content
is preferably 2.80%, more preferably 3.00%. The upper limit of the Mo content
is
preferably 3.30%.
[0038]
Ni: 5.00 to 8.00%
Nickel (Ni) is an austenite stabilizing element and is an element necessary
for
achievement of the ferrite/austenite dual phase structure. Too low a Ni
content
provides no effect described above. On the other hand, too high a Ni content
causes
imbalance between the ferrite phase and the austenite phase. In this case, the
duplex stainless steel is not stably produced. The Ni content therefore ranges
from
5.00 to 8.00%. The lower limit of the Ni content is preferably 5.50%, more
preferably 6.00%. The upper limit of the Ni content is preferably 7.50%.
[0039]
W: 4.00 to 6.00%
Tungsten (W) is contained in the passive film and further enhances the
corrosion resistance of the passive film, as in the case of Mo. As a result,
occurrence of the pitting on the duplex stainless steel is suppressed. Too low
a W
content provides no effect described above. On the other hand, too high a W
content is likely to cause the cr phase to precipitate easily, resulting in a
decrease in
toughness. The W content therefore ranges from 4.00 to 6.00%. The lower limit
of the W content is preferably 4.50%. The upper limit of the W content is
preferably 5.50%.
[0040]
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Cu: 0.01 to less than 0.10%
Copper (Cu) is an element effective in suppressing the propagation of the
pitting (propagation stage). Too low a Cu content provides no effect described
above. On the other hand, among Cr, Mo, and Cu, Cu lowers the pitting
resistance
at the occurrence of pitting (initial stage). The duplex stainless steel
according to
the present embodiment therefore has a lowered Cu content as compared with the
Cu
content in a duplex stainless steel of the related art. As a result, the
precipitation of
Cu in the ferrite phase is suppressed, and occurrence of pitting on the duplex
stainless steel (initial stage) is suppressed. Too high a Cu content causes
too large
an area fraction of Cu in the ferrite phase. In this case, the pitting
resistance of the
duplex stainless steel lowers. The Cu content therefore ranges from 0.01 to
less
than 0.10%. The upper limit of the Cu content is preferably 0.07%, more
preferably
0.05%.
[0041]
N: more than 0.400% to 0.600%
Nitrogen (N) is an austenite stabilizing element and is an element necessary
for achievement of the ferrite/austenite dual phase structure. N further
enhances the
pitting resistance of the duplex stainless steel. Too low a N content provides
no
effects described above. On the other hand, too high a N content lowers the
toughness and the hot workability of the duplex stainless steel. The N content
therefore ranges from more than 0.400% to 0.600%. The lower limit of the N
content is preferably 0.420%. The upper limit of the N content is preferably
0.500%.
[0042]
C: 0.030% or less
Carbon (C) is inevitably contained. That is, the C content is more than 0%.
C forms a Cr carbide in the crystal grain boundary, and the Cr carbide
increases the
corrosion susceptibility in the grain boundary. The C content is therefore
0.030%
or less. The upper limit of the C content is preferably 0.025%, more
preferably
0.020%. The C content is preferably minimized. Extreme reduction in the C
content, however, greatly increases the production cost. The lower limit of
the C
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content is therefore preferably 0.001%, and more preferably 0.005% in
consideration
of industrial production.
[0043]
Si: 1.00% or less
Silicon (Si) deoxidizes steel. In a case where Si is used as a deoxidizer, the
Si content is more than 0%. On the other hand, too high a Si content lowers
the hot
workability of the duplex stainless steel. The Si content is therefore 1.00%
or less.
The upper limit of the Si content is preferably 0.80%, and more preferably
0.70%.
The lower limit of the Si content is not limited to a specific value and is,
for example,
0.20%.
[0044]
Mn: 1.00% or less
Manganese (Mn) deoxidizes steel. In a case where Mn is used as a
deoxidizer, the Mn content is more than 0%. On the other hand, too high a Mn
content lowers the hot workability of the duplex stainless steel. The Mn
content is
therefore 1.00% or less. The upper limit of the Mn content is preferably
0.80%, and
more preferably 0.70%. The lower limit of the Mn content is not limited to a
specific value and is, for example, 0.20%.
[0045]
Sol. Al: 0.040% or less
Aluminum (Al) deoxidizes steel. In a case where Al is used as a deoxidizer,
the Al content is more than 0%. On the other hand, too high an Al content
lowers
the hot workability of the duplex stainless steel. The Al content is therefore
0.040%
or less. The upper limit of the Al content is preferably 0.030%, and more
preferably 0.025%. The lower limit of the Al content is not limited to a
specific
value and is, for example, 0.005%. In the present embodiment, the Al content
refers to the acid-soluble Al (sol.A1) content.
[0046]
V: 0.50% or less
Vanadium (V) is inevitably contained. That is, the V content is more than
0%. Too high a V content excessively increases the amount of the ferrite
phase,
resulting in decreases in toughness and corrosion resistance of the duplex
stainless
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steel in some cases. The V content is therefore 0.50% or less. The upper limit
of
the V content is preferably 0.40%, and more preferably 0.30%. The lower limit
of
the V content is not limited to a specific value and is, for example, 0.05%.
[0047]
0: 0.010% or less
Oxygen (0) is an impurity. That is, the 0 content is more than 0%. 0
lowers the hot workability of the duplex stainless steel. The 0 content is
therefore
0.010% or less. The upper limit of the 0 content is preferably 0.007%, and
more
preferably 0.005%. The 0 content is preferably minimized. Extreme reduction in
the 0 content, however, greatly increases the production cost. The lower limit
of
the 0 content is therefore preferably 0.0001%, and more preferably 0.0005% in
consideration of industrial production.
[0048]
P: 0.030% or less
Phosphorus (P) is an impurity. That is, the P content is more than 0%. P
lowers the pitting resistance and toughness of the duplex stainless steel. The
P
content is therefore 0.030% or less. The upper limit of the P content is
preferably
0.025%, and more preferably 0.020%. The P content is preferably minimized.
Extreme reduction in the P content, however, greatly increases the production
cost.
The lower limit of the P content is therefore preferably 0.001%, and more
preferably
0.005% in consideration of industrial production.
[0049]
S: 0.020% or less
Sulfur (S) is an impurity. That is, the S content is more than 0%. S lowers
the hot workability of the duplex stainless steel. The S content is therefore
0.020%
or less. The upper limit of the S content is preferably 0.010%, more
preferably
0.005%, and still more preferably 0.003%. The S content is preferably
minimized.
Extreme reduction in the S content, however, greatly increases the production
cost.
The lower limit of the S content is therefore preferably 0.0001%, and more
preferably 0.0005% in consideration of industrial production.
[0050]
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The balance of the chemical composition of the duplex stainless steel
according to the present embodiment is Fe and impurities. The impurities in
the
chemical composition mean contaminants, for example, from ore as a raw
material,
scraps, or the production environment in industrial production of the duplex
stainless
steel that are acceptable to the extent that the contaminants do not adversely
affect
the duplex stainless steel according to the present embodiment.
[0051]
[Optional elements]
The chemical composition of the duplex stainless steel according to the
present embodiment may arbitrarily contain the following elements:
[0052]
Ca: 0 to 0.0040%
Calcium (Ca) is an optional element and may not be contained. That is, the
Ca content may be 0%. When contained, Ca enhances the hot workability of the
duplex stainless steel. When Ca is contained even by a trace amount, the
effect
described above is provided to some extent. On the other hand, too high a Ca
content produces a coarse oxide, which lowers the hot workability of the
duplex
stainless steel. The Ca content is therefore 0 to 0.0040%. The lower limit of
the
Ca content is preferably 0.0001%, more preferably 0.0005%, and still more
preferably 0.0010%. The upper limit of the Ca content is preferably 0.0030%.
[0053]
Mg: 0 to 0.0040%
Magnesium (Mg) is an optional element and may not be contained. That is,
the Mg content may be 0%. When contained, Mg enhances the hot workability of
the duplex stainless steel, as does Ca. When Mg is contained even by a trace
amount, the effect described above is provided to some extent. On the other
hand,
too high a Mg content produces a coarse oxide, which lowers the hot
workability of
the duplex stainless steel. The Mg content is therefore 0 to 0.0040%. The
lower
limit of the Mg content is preferably 0.0001%, more preferably 0.0005%, and
still
more preferably 0.0010%. The upper limit of the Ca content is preferably
0.0030%.
[0054]
B: 0 to 0.0040%
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Boron (B) is an optional element and may not be contained. That is, the B
content may be 0%. When contained, B enhances the hot workability of the
duplex
stainless steel, as do Ca and Mg. When B is contained even by a trace amount,
the
effect described above is provided to some extent. On the other hand, too high
a B
content lowers the toughness of the duplex stainless steel. The B content is
therefore 0 to 0.0040%. The lower limit of the B content is preferably
0.0001%,
more preferably 0.0005%, and still more preferably 0.0010%. The upper limit of
the Ca content is preferably 0.0030%.
[0055]
[Formula (1)1
The chemical composition of the duplex stainless steel according to the
present embodiment satisfies the contents of the elements described above and
further satisfies the following Formula (1):
Cr+4.0 xMo+2.0 x W+20 xN-5 x ln(Cu)65.2 (1)
where, content in mass% of each of the elements is substituted into the
corresponding symbol of the element in Formula (1).
[0056]
The following definition is made: F1=Cr+4.0 xMo+2.0xW+20xN-5x1n(Cu).
Fl is an index representing the pitting resistance. When Fl is less than 65.2,
the
pitting resistance of the duplex stainless steel lowers. The following formula
is
therefore satisfied: F165.2. The lower limit of Fl is preferably 68.0, more
preferably 69.0, and still more preferably 70Ø The upper limit of Fl is not
limited
to a specific value and is, for example, 90Ø
[0057]
[Microstructure]
The microstructure of the duplex stainless steel according to the present
embodiment consists of ferrite and austenite. Specifically, the microstructure
of the
duplex stainless steel according to the present embodiment consists of 35 to
65
volume% of ferrite phase with the balance being the austenite phase. When the
volume ratio of the ferrite phase (hereinafter also referred to as ferrite
fraction) is less
than 35%, stress corrosion cracking is more likely to occur depending on the
environment in which the duplex stainless steel is used. On the other hand,
when
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the volume ratio of the ferrite phase is more than 65%, the toughness of the
duplex
stainless steel is more likely to lower. Therefore, the microstructure of the
duplex
stainless steel according to the present embodiment consists of 35 to 65
volume% of
ferrite phase with the balance being the austenite phase.
[0058]
[Method for measuring ferrite fraction]
In the present embodiment, the ferrite fraction of the duplex stainless steel
can
be determined by the following method: A test specimen for microstructure
observation is collected from the duplex stainless steel. When the duplex
stainless
steel is used to form a steel plate, a cross section of the steel plate that
is the cross
section perpendicular to the plate width direction of the steel plate
(hereinafter
referred to as observation surface) is polished. When the duplex stainless
steel is
used to form a steel pipe, a cross section of the steel pipe that is the cross
section
(observation surface) containing the axial direction and the wall thickness
direction
of the steel pipe is polished. When the duplex stainless steel is used to form
a steel
bar or a wire rod, a cross section of the steel bar or the wire rod that is
the cross
section (observation surface) containing the axial direction of the steel bar
or the wire
rod is polished. The polished observation surface is then etched by using a
liquid
that is the mixture of aqua regia and glycerin.
[0059]
Ten visual fields of the etched observation surface are observed under an
optical microscope. The area of each of the visual fields is, for example,
2000 m2
(at magnification of 500). In each of the visual fields, the ferrite and the
other
phases can be distinguished from each other based on contrast. The ferrite is
therefore identified based on the contrast in each observation. The area
fraction of
the identified ferrite is measured by using a point counting method compliant
with
HS G0555 (2003). The measured area fraction is assumed to be equal to the
volume fraction, which is then defined as a ferrite fraction (volume%).
[0060]
[Cu area fraction in ferrite phase]
The area fraction of Cu precipitated in the ferrite phase of the duplex
stainless
steel according to the present embodiment is 0.5% or less. It is believed as
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described above that Cu contained in the duplex stainless steel suppresses the
propagation of the pitting on the duplex stainless steel. The duplex stainless
steel
according to the present embodiment therefore contains Cu by an amount ranging
from 0.01 to less than 0.10%. On the other hand, in the duplex stainless steel
containing Cu by the amount ranging from 0.01 to less than 0.10%, metal Cu
precipitates in the ferrite phase in some cases. It has clearly been shown as
described above that Cu precipitated in the ferrite phase lowers the passive
film's
effect of suppressing occurrence of pitting. That is, metal Cu precipitated in
the
ferrite phase lowers the pitting resistance of the duplex stainless steel.
[0061]
The duplex stainless steel according to the present embodiment has a reduced
Cu area fraction in the ferrite phase to 0.5% or less. The occurrence of
pitting on
the duplex stainless steel is thus suppressed. The Cu area fraction in the
ferrite
phase is preferably minimized. The upper limit of the Cu area fraction in the
ferrite
phase is preferably 0.3%, and more preferably 0.1%. The lower limit of the Cu
area
fraction in the ferrite phase is 0.0%.
[0062]
[Method for measuring Cu area fraction in ferrite phase]
In the present specification, the Cu area fraction in the ferrite phase means
the
area fraction of Cu precipitated in the ferrite phase out of the
microstructure of the
duplex stainless steel with respect to the ferrite phase. In the present
embodiment,
the Cu area fraction in the ferrite phase can be measured by the following
method: A
thin film specimen for observation under a transmission electron microscope
(TEM)
is prepared by an FIB-micro-sampling method. To prepare the thin film
specimen,
a focused ion beam processing apparatus (MI4050 manufactured by Hitachi High-
Tech Science Corporation) is used. A thin film specimen for TEM observation is
prepared from an arbitrary portion of the duplex stainless steel. To prepare
the thin
film specimen, a mesh made of Mo and a carbon deposit film as a surface
protection
film are used.
[0063]
A field emission transmission electron microscope (JEM-2100F manufactured
by JEOL Ltd.) is used for the TEM observation. The TEM observation is
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performed at an observation magnification of 10000. The ferrite phase and the
austenite phase in a visual field differ from each other in terms of contrast.
The
crystal grain boundary is then identified based on the contrast. The phase of
a
region surrounded by each crystal grain boundary is identified by X-ray
diffraction
(XRD). Among the regions surrounded by the crystal grain boundaries, the area
of
the region identified as the ferrite phase is determined by image analysis.
[0064]
Element analysis based on energy dispersive X-ray spectrometry (EDS) is
performed on the visual field under observation to generate an element map.
Further, a precipitate can be identified based on the contrast. Therefore,
whether a
precipitate identified based on the contrast in the ferrite phase identified
by XRD is
metal Cu can be identified by EDS.
[0065]
The area of Cu precipitated in the identified ferrite phase is determined by
image analysis. The sum of the areas of Cu precipitated in the ferrite phase
is
divided by the sum of areas of the ferrite phase. The Cu area fraction (%) in
the
ferrite phase is thus measured.
[0066]
The duplex stainless steel according to the present embodiment satisfies both
the chemical composition including Formula (1) and the microstructure
including the
in-ferrite-phase Cu area fraction described above. The duplex stainless steel
according to the present embodiment therefore has excellent pitting
resistance.
[0067]
[Yield strength]
The yield strength of the duplex stainless steel according to the present
embodiment is not limited to a specific value. When the yield strength is 750
MPa
or less, however, the cold working can be omitted in the production process.
In this
case, the production cost can be reduced. The yield strength is therefore
preferably
750 MPa or less. The yield strength is more preferably 720 MPa or less. The
lower limit of the yield strength is not limited to a specific value and is,
for example,
300 MPa.
[0068]
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[Method for measuring yield strength]
The yield strength in the present specification means 0.2% proof stress
determined by a method compliant with JIS Z2241 (2011).
[0069]
[Shape of duplex stainless steel]
The shape of the duplex stainless steel according to the present embodiment is
not limited to a specific shape. The duplex stainless steel may be used in a
form of,
for example, a steel pipe, a steel plate, a steel bar, or a wire rod.
[0070]
[Production method]
The duplex stainless steel according to the present embodiment can be
produced, for example, by the following method: The production method includes
a
preparation step, a hot working step, a cooling step, and a solution heat
treatment
step.
[0071]
[Preparation step]
In the preparation step, a starting material having the chemical composition
described above is prepared. The starting material may be a cast piece
produced by
a continuous casting process (including round continuous casting) or a slab
produced
from the cast piece. The starting material may be a slab produced by
performing
hot working on an ingot produced by an ingot-making process.
[0072]
[Hot working step]
The prepared starting material is placed in a heating furnace or a soaking pit
and heated at a temperature ranging, for example, from 1150 to 1300 C. The
heated starting material is subsequently subjected to hot working. The hot
working
may be hot forging, hot extrusion using, for example, the Ugine-Sejournet
process or
the Ehrhardt push bench process, or hot rolling. The hot working may be
performed once or multiple times.
[0073]
The heated starting material is subjected to hot working at 850 C or more.
More specifically, the surface temperature of the steel material at the end of
the hot
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working is 850 C or more. When the surface temperature of the steel material
at
the end of the hot working is less than 850 C, a large amount of Cu
precipitates in
the ferrite phase. As a result, even a solution treatment, which will be
described
later, cannot sufficiently reduce the Cu area fraction in the ferrite phase in
some
cases. In this case, the pitting resistance of the duplex stainless steel
lowers. The
surface temperature of the steel material at the end of the hot working is
therefore
850 C or more. In a case where the hot working is performed multiple times,
the
surface temperature of the steel material at the end of the last hot working
is 850 C
or more. Precipitation of Cu in the ferrite phase can thus be suppressed at
the end
of the hot working. The upper limit of the surface temperature of the steel
material
at the end of the hot working is not limited to a specific value and is, for
example,
1300 C. The end of the hot working is the point of time within three seconds
after
the hot working ends.
[0074]
[Cooling step]
The starting material after the hot working is subsequently cooled at a rate
of
C/sec or more. Cu starts precipitating in the ferrite phase at around 850 C.
Therefore, if the cooling rate after the hot working is too slow, a large
amount of Cu
precipitates in the ferrite phase. As a result, even a solution treatment,
which will
be described later, cannot sufficiently reduce the Cu area fraction in the
ferrite phase
in some cases. In this case, the pitting resistance of the duplex stainless
steel lowers.
The cooling rate after the hot working is therefore 5 C/sec or more. In the
case
where the hot working is performed multiple times, "after the hot working"
refers to
"after the last hot working." That is, in the present embodiment, the starting
material after the last hot working is cooled at the rate of 5 C/sec or more.
The
upper limit of the cooling rate is not limited to a specific value. The
cooling
method is, for example, air cooling, water cooling, or oil cooling.
[0075]
[Solution heat treatment step]
The cooled starting material is subsequently subjected to a solution heat
treatment at 1070 C or more. The solution heat treatment causes the Cu
precipitated in the ferrite phase to dissolve. Performing the solution heat
treatment
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at 1070 C or more on the starting material in which the precipitation of Cu in
the
ferrite phase at the end of the hot working and after the cooling is
sufficiently
suppressed allows the Cu area fraction in the ferrite phase to be 0.5% or
less. The
upper limit of the solution heat treatment temperature is not limited to a
specific
value and is, for example, 1150 C. The treatment period of the solution heat
treatment is not limited to a specific value. The treatment period of the
solution
heat treatment ranges, for example, from 1 to 30 minutes.
[0076]
The duplex stainless steel according to the present embodiment can be
produced by carrying out the steps described above. In the present embodiment,
it
is preferable to perform no cold working because cold working increases the
production cost.
EXAMPLES
[0077]
Alloys having the chemical compositions shown in Table 3 were melted in a
50 kg vacuum furnace, the obtained ingots were heated at 1200 C, and the
heated
ingots were subjected to hot forging and hot rolling into steel plates having
a
thickness of 10 mm. The temperatures at the end of rolling shown in Table 4
are
the surface temperatures of the steel plates at the end of the hot rolling.
The post-
rolling cooling rates shown in Table 4 are the cooling rates after the hot
rolling.
Further, the steel plates were subjected to a solution treatment at the
solution
temperatures ( C) shown in Table 4 into test specimens labeled with the test
numbers.
[0078]
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[Table 3]
TABLE 3
Chemical composition (unit is mass%, balance is Fe and impurities)
Steel Fl
Cr Mo Ni W Cu N C Si Mn sol.A1 V 0 P S Ca Mg B
A
27.14 3.21 6.21 4.10 0.50 0.406 0.015 0.50 0.98 0.017
0.10 0.003 0.019 0.001 0.0019 0.0021 0.0017 59.8
B
28.10 3.11 5.31 4.19 0.14 0.421 0.016 049 0.97 0.013
0.10 0.004 0.018 0.001 0.0025 0.0001 0.0019 67.2
C
28.24 2.96 5.76 4.25 0.08 0.416 0.014 0.51 0.91 0.012
0.10 0.004 0.019 0.001 0.0015 0.0002 0.0012 69.5
D
27.01 2.50 5.29 4.00 0.09 0.401 0.017 0.52 0.92 0.014
0.10 0.005 0.017 0.001 0.0027 0.0034 0.0015 65.1
E
27.53 2.61 6.97 4.31 0.04 0.419 0.016 0.48 0.92 0.017
0.10 0.005 0.016 0.001 0.0010 0.0025 0.0013 71.1
F 27.88 3.05 5.34 5.61 0.07 0.501 0.016 0.49 0.94 0.015 0.11 0.003 0.017 0.001
- - 74.6
G 28.71 3.45 7.21 4.37 0.08 0.457 0.014 0.48 0.97 0.016 0.11 0.005 0.017 0.001
- - - 73.0 0
0
H
27.30 2.86 6.48 3.61 0.08 0.401 0.018 0.54 0.91 0.014
0.10 0.004 0.021 0.001 0.0018 0.0019 0.0021 66.6 0
0
I
27.04 2.23 7.62 4.19 0.07 0.405 0.016 0.51 0.92 0.019
0.11 0.003 0.023 0.001 0.0025 0.0014 0.0011 65.7
J
26.10 3.01 5.67 4.27 0.09 0.408 0.019 0.47 0.96 0.017
0.09 0.003 0.018 0.001 0.0013 0.0034 0.0017 66.9 0
0
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[0079]
[Table 4]
TABLE 4
Production conditions Analysis results
Cu area
End of Pitting
Test Post-rolling Solution Ferrite fraction
Steel rolling . potential Yield
Nlo. cooling rate temperature fraction in ferrite
strength
temperature (VdioomVvs.SCE) ( c/sec) (
C) (volume%) phase (MPa)
( C)
(%)
1 A 980 30 1120 44 0.8 -60 712
2 B 970 10 1100 48 0.6 71 680
3 C 1010 30 1050 39 0.7 -12 620
4 D 930 10 1100 43 0.1 85 719
E 950 30 1100 50 0.0 346 637
6 C 1000 30 1090 41 0.0 204 675
7 F 1020 10 1070 40 0.0 410 617
8 G 1060 10 1090 47 0.0 384 701
9 H 1050 10 1100 51 0.0 70 721
I 1100 30 1090 48 0.1 76 679
11 J 1040 10 1070 45 0.2 81 665
12 C 840 10 1070 44 1.1 -150 663
13 C 1000 3 1090 51 1.6 -71 714
[0080]
[Ferrite fraction measurement test]
The ferrite fraction (volume%) of each of the test specimens labeled with the
test numbers was measured by using the method described above. Table 4 shows
the results of the measurement. The balance of the microstructure of each of
the
test specimens labeled with the test numbers was the austenite phase.
[0081]
[In-ferrite-phase Cu area fraction measurement test]
The in-ferrite-phase Cu area fraction (%) of each of the test specimens
labeled
with the test numbers was measured by using the method described above. Table
4
shows the results of the measurement.
[0082]
[Pitting potential measurement test]
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The pitting potential of each of the test specimens labeled with the test
numbers after the solution treatment was measured. The test specimens were
each
first machined into a test specimen having a diameter of 15 mm and a thickness
of 2
mm. The obtained test specimens were each used to measure the pitting
potential in
25% NaClaq. at 80 C. The conditions other than the test temperature and the
NaCl
concentration were compliant with HS G0577 (2014). Table 4 shows the results
of
the measurement of pitting potential Vc'ioo of the test specimens labeled with
the test
numbers.
[0083]
[Tensile test]
The 0.2% proof stress of the test specimens labeled with the respective test
numbers was determined by using a method compliant with MS Z2241 (2011).
Table 4 shows the results of the determination.
[0084]
[Evaluation results]
Referring to Tables 3 and 4, the test specimens labeled with test numbers 5 to
8 had appropriate chemical compositions and were produced under appropriate
conditions. The test specimens labeled with the test numbers 5 to 8 therefore
were
the duplex stainless steel having a ferrite fraction ranging from 35 to 65
volume%
with the balance being the austenite phase, and the Cu area fraction in the
ferrite
phase was 0.5% or less. As a result, the pitting potential (mVvs.SCE) of each
of the
steel plates labeled with the test numbers 5 to 8 was 100 or more, which
represented
excellent pitting resistance.
[0085]
On the other hand, the test specimen labeled with test number 1 has too high a
Cu content. Further, Fl of the test specimen labeled with the test number 1
was
59.8, which did not satisfy Formula (1). The Cu area fraction in the ferrite
phase of
the test specimen labeled with the test number 1 was therefore 0.8%. As a
result,
the pitting potential (mVvs.SCE) of the test specimen labeled with the test
number 1
was -60, which did not represent excellent pitting resistance.
[0086]
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The test specimen labeled with test number 2 has too high a Cu content. The
Cu area fraction in the ferrite phase of the test specimen labeled with the
test number
2 was therefore 0.6%. As a result, the pitting potential (mVvs.SCE) of the
test
specimen labeled with the test number 2 was 71, which did not represent
excellent
pitting resistance.
[0087]
The solution temperature of the test specimen labeled with test number 3 was
1050 C, which was too low. The Cu area fraction in the ferrite phase of the
test
specimen labeled with the test number 3 was therefore 0.7%. As a result, the
pitting
potential (mVvs.SCE) of the test specimen labeled with the test number 3 was -
12,
which did not represent excellent pitting resistance.
[0088]
The content of each element of the test specimen labeled with test number 4
was appropriate, but Fl was 65.1, which did not satisfy Formula (1). As a
result,
the pitting potential (mVvs.SCE) of the test specimen labeled with the test
number 4
was 85, which did not represent excellent pitting resistance.
[0089]
The test specimen labeled with test number 9 had too low a W content. As a
result, the pitting potential (mVvs.SCE) of the test specimen labeled with the
test
number 9 was 70, which did not represent excellent pitting resistance.
[0090]
The test specimen labeled with test number 10 had too low a Mo content.
As a result, the pitting potential (mVvs.SCE) of the test specimen labeled
with the
test number 10 was 76, which did not represent excellent pitting resistance.
[0091]
The test specimen labeled with test number 11 had too low a Cr content. As
a result, the pitting potential (mVvs.SCE) of the test specimen labeled with
the test
number 11 was 81, which did not represent excellent pitting resistance.
[0092]
The temperature of the test specimen labeled with test number 12 at the end of
the hot rolling was 840 C, which was too low. The Cu area fraction in the
ferrite
phase of the test specimen labeled with the test number 12 was therefore 1.1%.
As
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a result, the pitting potential (mVvs.SCE) of the test specimen labeled with
the test
number 12 was -150, which did not represent excellent pitting resistance.
[0093]
The cooling rate at which the test specimen labeled with test number 13 was
cooled at the end of the hot rolling was 3 C/sec, which was too slow. The Cu
area
fraction in the ferrite phase of the test specimen labeled with the test
number 13 was
therefore 1.6%. As a result, the pitting potential (mVvs.SCE) of the test
specimen
labeled with the test number 13 was -71, which did not represent excellent
pitting
resistance.
[0094]
The embodiment of the present invention has been described. The
embodiment described above is, however, only an example for implementing the
present invention. The present invention is therefore not limited to the
embodiment
described above, and the embodiment described above can be changed as
appropriate
to the extent that the change does not depart from the substance of the
present
invention.
Date Recue/Date Received 2020-04-28
