U.S. patent application number 16/759798 was filed with the patent office on 2020-10-22 for duplex stainless steel and method for producing duplex stainless steel.
The applicant listed for this patent is NIPPON STEEL CORPORATION. Invention is credited to Takahiro OSUKI, Masayuki SAGARA, Yusaku TOMIO, Yusuke UGAWA.
Application Number | 20200332378 16/759798 |
Document ID | / |
Family ID | 1000004945685 |
Filed Date | 2020-10-22 |
United States Patent
Application |
20200332378 |
Kind Code |
A1 |
SAGARA; Masayuki ; et
al. |
October 22, 2020 |
DUPLEX STAINLESS STEEL AND METHOD FOR PRODUCING DUPLEX STAINLESS
STEEL
Abstract
A duplex stainless steel with occurrence of pitting suppressed
is provided. 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.Al: 0.040% or less, V: 0.50% or less, O: 0.010% or less, P:
0.030% or less, and S: 0.020% or less with the balance being Fe and
impurities and satisfying Formula (1), a microstructure consisting
of 35 to 65 volume % of ferrite phase with the balance being the
austenite phase, and the area fraction of Cu precipitated in the
ferrite phase is 0.5% or less.
Cr+4.0.times.Mo+2.0.times.W+20.times.N-5.times.ln(Cu).gtoreq.65.2
(1)
Inventors: |
SAGARA; Masayuki;
(Chiyoda-ku, Tokyo, JP) ; TOMIO; Yusaku;
(Chiyoda-ku, Tokyo, JP) ; OSUKI; Takahiro;
(Chiyoda-ku, Tokyo, JP) ; UGAWA; Yusuke;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
1000004945685 |
Appl. No.: |
16/759798 |
Filed: |
November 14, 2018 |
PCT Filed: |
November 14, 2018 |
PCT NO: |
PCT/JP2018/042114 |
371 Date: |
April 28, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/02 20130101;
C22C 38/002 20130101; C21D 6/005 20130101; C22C 38/44 20130101;
C22C 38/42 20130101; C22C 38/46 20130101; C21D 6/004 20130101; C21D
8/005 20130101; C22C 38/001 20130101; C22C 38/06 20130101; C21D
6/008 20130101; C21D 2211/005 20130101; C22C 38/54 20130101; C22C
38/04 20130101; C21D 2211/001 20130101 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/54 20060101 C22C038/54; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/04 20060101 C22C038/04; C22C 38/06 20060101
C22C038/06; C22C 38/02 20060101 C22C038/02; C22C 38/00 20060101
C22C038/00; C21D 6/00 20060101 C21D006/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 15, 2017 |
JP |
2017-220172 |
Claims
1. A duplex stainless steel comprising: 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.Al: 0.040% or less, V: 0.50%
or less, O: 0.010% or less, P: 0.030% or less, S: 0.020% or less,
Ca: 0 to 0.0040%, Mg: 0 to 0.0040%, B: 0 to 0.0040%, and with the
balance being Fe and impurities, and satisfying Formula (1), a
microstructure consisting of 35 to 65 volume % of ferrite phase
with the balance being an austenite phase, wherein an area fraction
of Cu precipitates in the ferrite phase is 0.5% or less:
Cr+4.0.times.Mo+2.0.times.W+20.times.N-5.times.ln(Cu).gtoreq.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).
2. The duplex stainless steel according to claim 1, wherein the
chemical composition 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%.
3. A method for producing a duplex stainless steel, the method
comprising the steps of: preparing a starting material having 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.Al:
0.040% or less, V: 0.50% or less, O: 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): subjecting the starting material to hot
working at 850.degree. C. or more; cooling the starting material
subjected to the hot working at a rate of 5.degree. C./sec or more;
and subjecting the cooled starting material to a solution heat
treatment at 1070.degree. C. or more:
Cr+4.0.times.Mo+2.0.times.W+20.times.N-5.times.ln(Cu).gtoreq.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).
Description
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 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[{In(MYS)-In(14.5.times.Cr+48.3.times.Mo+20.7.times.W+6.9.times.N)}/-
0.195]) 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 40.ltoreq.PREWs.ltoreq.67. Patent Literature 4
describes that the thus configured super duplex stainless steel is
superior in corrosion resistance, embrittlement resistance,
castability, and hot workability with formation of intermetal
phases, such as the brittle sigma (.sigma.) phase and the chi
(.chi.) phase, suppressed.
CITATION LIST
Patent Literature
[0008] Patent Literature 1: International Application Publication
No. 2013/191208
[0009] Patent Literature 2: International Application Publication
No. 2010/082395
[0010] Patent Literature 3: Japanese Patent Application Publication
No. 2007-84837
[0011] Patent Literature 4: National Publication of International
Patent Application No. 2005-520934
SUMMARY OF INVENTION
Technical Problem
[0012] 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 to 4 may therefore provide a duplex stainless steel
showing excellent pitting resistance.
[0013] 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
[0014] 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.Al:
0.040% or less, V: 0.50% or less. O: 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 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.times.Mo+2.0.times.W+20.times.N-5.times.ln(Cu).gtoreq.65.2
(1)
[0015] where, a content in mass % of each of the elements is
substituted into a corresponding symbol of the element in Formula
(1).
[0016] 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.degree. C.
or more. In the cooling step, the starting material subjected to
the hot working is cooled at a rate of 5.degree. C./sec or more. In
the solution heat treatment step, the cooled starting material is
subjected to a solution heat treatment at 1070.degree. C. or
more.
Advantageous Effects of Invention
[0017] 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
[0018] 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.
[0019] 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 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.
[0020] 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.
[0021] 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).
[0022] 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 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.
TABLE-US-00001 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 0.421 0.016 0.49 0.97 5 E 27.53 2.61 6.97
4.31 0.04 0.419 0.016 0.48 0.92 Test No. Steel sol. Al V O 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 Pitting
potential No. Steel (mVvs SCE) 2 B 71 5 E 346
[0023] 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.
[0024] 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. 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 faction in ferrite phase) than the test
specimen labeled with the test number 5.
[0025] 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.
TABLE-US-00002 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 0.416 0.014 0.51 0.91 6 C 28.24 2.96 5.76
4.25 0.08 0.416 0.014 0.51 0.91 Test No. Steel sol. Al V O P S Ca
Mg B 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 Cu area
fraction Pitting potential No. Steel in ferrite phase (%) (mVvs
SCE) 3 C 0.7 -12 6 C 0 204
[0026] 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.
[0027] 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.
[0028] 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.
[0029] A duplex stainless steel according to the present embodiment
attained based on the findings described above 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.Al: 0.040%, or
less. V: 0.50% or less, O: 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.times.Mo+2.0.times.W+20.times.N-5.times.ln(Cu).gtoreq.65.2
(1)
[0030] where, the content in mass % of each of the elements is
substituted into the corresponding symbol of the element in Formula
(1).
[0031] 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.
[0032] 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%.
[0033] In this case, the duplex stainless steel according to the
present embodiment has enhanced hot workability.
[0034] 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.degree. C.
or more. In the cooling step, the starting material subjected to
the hot working is cooled at a rate of 5.degree. C./sec or more. In
the solution heat treatment step, the cooled starting material is
subjected to a solution heat treatment at 1070.degree. C. or
more.
[0035] 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.
[0036] The duplex stainless steel according to the present
embodiment will be described below in detail.
[0037] [Chemical Composition]
[0038] 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.
[0039] [Essential Elements]
[0040] The chemical composition of the duplex stainless steel
according to the present embodiment essentially contains the
following elements:
[0041] Cr: more than 27.00% to 29.00%
[0042] 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 27.50%, more preferably 28.00%. The
upper limit of the Cr content is preferably 28.50%.
[0043] Mo: 2.50 to 3.50%
[0044] 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%.
[0045] Ni: 5.00 to 8.00%
[0046] 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%.
[0047] W: 4.00 to 6.00%
[0048] Tungsten (V) 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 a 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%.
[0049] Cu: 0.01 to less than 0.10%
[0050] 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%.
[0051] N: more than 0.400% to 0.600%
[0052] 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%.
[0053] C: 0.030% or less
[0054] 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 content is therefore
preferably 0.001%, and more preferably 0.005% in consideration of
industrial production.
[0055] Si: 1.00% or less
[0056] 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%.
[0057] Mn: 1.00% or less
[0058] 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%.
[0059] Sol. Al: 0.040% or less
[0060] 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.Al)
content.
[0061] V: 0.50% or less
[0062] 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 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%.
[0063] O: 0.010% or less
[0064] Oxygen (O) is an impurity. That is, the O content is more
than 0%. O lowers the hot workability of the duplex stainless
steel. The O content is therefore 0.010% or less. The upper limit
of the O content is preferably 0.007%, and more preferably 0.005%.
The O content is preferably minimized. Extreme reduction in the O
content, however, greatly increases the production cost. The lower
limit of the O content is therefore preferably 0.0001%, and more
preferably 0.0005% in consideration of industrial production.
[0065] P: 0.030% or less
[0066] 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.
[0067] S: 0.020% or less
[0068] 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.
[0069] 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.
[0070] [Optional Elements]
[0071] The chemical composition of the duplex stainless steel
according to the present embodiment may arbitrarily contain the
following elements:
[0072] Ca: 0 to 0.0040%
[0073] 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%.
[0074] Mg: 0 to 0.0040%
[0075] 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%.
[0076] B: 0 to 0.0040%
[0077] 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%.
[0078] [Formula (1)]
[0079] 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.times.Mo+2.0.times.W+20.times.N-5.times.ln(Cu).gtoreq.65.2
(1)
[0080] where, content in mass % of each of the elements is
substituted into the corresponding symbol of the element in Formula
(1).
[0081] The following definition is made:
F1=Cr+4.0.times.Mo+2.0.times.W+20.times.N-5.times.ln(Cu). F1 is an
index representing the pitting resistance. When F1 is less than
65.2, the pitting resistance of the duplex stainless steel lowers.
The following formula is therefore satisfied: F1.gtoreq.65.2. The
lower limit of F1 is preferably 68.0, more preferably 69.0, and
still more preferably 70.0. The upper limit of F1 is not limited to
a specific value and is, for example, 90.0.
[0082] [Microstructure]
[0083] 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 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.
[0084] [Method for Measuring Ferrite Fraction]
[0085] 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.
[0086] 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 .mu.m.sup.2 (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 JIS G0555
(2003). The measured area fraction is assumed to be equal to the
volume fraction, which is then defined as a ferrite fraction
(volume %).
[0087] [Cu Area Fraction in Ferrite Phase]
[0088] 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 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.
[0089] 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%.
[0090] [Method for Measuring Cu Area Fraction in Ferrite Phase]
[0091] 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.
[0092] A field emission transmission electron microscope (JEM-2100F
manufactured by JEOL Ltd.) is used for the TEM observation. The TEM
observation is 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.
[0093] 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.
[0094] 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.
[0095] 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.
[0096] [Yield Strength]
[0097] 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.
[0098] [Method for Measuring Yield Strength]
[0099] The yield strength in the present specification means 0.2%
proof stress determined by a method compliant with JIS Z2241
(2011).
[0100] [Shape of Duplex Stainless Steel]
[0101] 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.
[0102] [Production Method]
[0103] 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.
[0104] [Preparation Step]
[0105] 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.
[0106] [Hot Working Step]
[0107] 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.degree. 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.
[0108] The heated starting material is subjected to hot working at
850.degree. C. or more. More specifically, the surface temperature
of the steel material at the end of the hot working is 850.degree.
C. or more. When the surface temperature of the steel material at
the end of the hot working is less than 850.degree. 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.degree. 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.degree. 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.degree. C. The end of the hot
working is the point of time within three seconds after the hot
working ends.
[0109] [Cooling Step]
[0110] The starting material after the hot working is subsequently
cooled at a rate of 5.degree. C./sec or more. Cu starts
precipitating in the ferrite phase at around 850.degree. 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.degree. 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.degree. 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.
[0111] [Solution Heat Treatment Step]
[0112] The cooled starting material is subsequently subjected to a
solution heat treatment at 1070.degree. C. or more. The solution
heat treatment causes the Cu precipitated in the ferrite phase to
dissolve. Performing the solution heat treatment at 1070.degree. 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.degree. 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.
[0113] 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
[0114] Alloys having the chemical compositions shown in Table 3
were melted in a 50 kg vacuum furnace, the obtained ingots were
heated at 1200.degree. 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
(.degree. C.) shown in Table 4 into test specimens labeled with the
test numbers.
TABLE-US-00003 TABLE 3 Chemical composition (unit is mass %,
balance is Fe and impurities) Steel Cr Mo Ni W Cu N C Si Mn sol. Al
V O P S Ca Mg B F1 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 0.49 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 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 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
TABLE-US-00004 TABLE 4 Analysis results Production conditions Cu
area End of fraction Pitting rolling Post-rolling Solution Ferrite
in ferrite potential Yield Test temperature cooling rate
temperature fraction phase Vc'.sub.100 strength No. Steel (.degree.
C.) (.degree. c./sec) (.degree. C.) (volume %) (%) (mVvs SCE) (MPa)
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 5 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 10 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
[0115] [Ferrite Fraction Measurement Test]
[0116] 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.
[0117] [In-Ferrite-Phase Cu Area Fraction Measurement Test]
[0118] 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.
[0119] [Pitting Potential Measurement Test]
[0120] 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.degree. C. The conditions other than the test
temperature and the NaCl concentration were compliant with JIS
G0577 (2014). Table 4 shows the results of the measurement of
pitting potential Vc'.sub.100 of the test specimens labeled with
the test numbers.
[0121] [Tensile Test]
[0122] The 0.2% proof stress of the test specimens labeled with the
respective test numbers was determined by using a method compliant
with JIS Z2241 (2011). Table 4 shows the results of the
determination.
[0123] [Evaluation Results]
[0124] 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.
[0125] On the other hand, the test specimen labeled with test
number 1 has too high a Cu content. Further, F1 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.
[0126] 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.
[0127] The solution temperature of the test specimen labeled with
test number 3 was 1050.degree. 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.
[0128] The content of each element of the test specimen labeled
with test number 4 was appropriate, but F1 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.
[0129] 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.
[0130] 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.
[0131] 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.
[0132] The temperature of the test specimen labeled with test
number 12 at the end of the hot rolling was 840.degree. 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 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.
[0133] The cooling rate at which the test specimen labeled with
test number 13 was cooled at the end of the hot rolling was
3.degree. 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.
[0134] 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.
* * * * *