U.S. patent application number 15/519845 was filed with the patent office on 2017-11-16 for duplex stainless steel and method for producing the same.
The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Junko IMAMURA, Hideya KAMINAKA, Hiroshi KAMIO, Kouichi TAKEUCHI.
Application Number | 20170327915 15/519845 |
Document ID | / |
Family ID | 55761004 |
Filed Date | 2017-11-16 |
United States Patent
Application |
20170327915 |
Kind Code |
A1 |
KAMIO; Hiroshi ; et
al. |
November 16, 2017 |
DUPLEX STAINLESS STEEL AND METHOD FOR PRODUCING THE SAME
Abstract
A duplex stainless steel is provided that has a chemical
composition comprising, by mass %, C: 0.03% or less, Si: 1.0% or
less, Mn: 1.0% or less, P: 0.04% or less, S: 0.01% or less, Cu: 0.1
to 1.0%, Ni: 5.0 to 7.5%, Cr: 22.0 to 26.0%, W: 6.0 to 12.0%, N:
0.20 to 0.32%, Mo: 0.01% or less, and a balance: Fe and impurities,
in which a metal micro-structure contains, by area ratio, 0.40 to
0.60 of an .alpha.-phase, with a balance being a .gamma.-phase and
0.01 or less of other phases.
Inventors: |
KAMIO; Hiroshi; (Tokyo,
JP) ; KAMINAKA; Hideya; (Tokyo, JP) ; IMAMURA;
Junko; (Tokyo, JP) ; TAKEUCHI; Kouichi;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
55761004 |
Appl. No.: |
15/519845 |
Filed: |
October 23, 2015 |
PCT Filed: |
October 23, 2015 |
PCT NO: |
PCT/JP2015/079962 |
371 Date: |
April 18, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/04 20130101;
C21D 6/005 20130101; C22C 38/42 20130101; C21D 8/005 20130101; C21D
2211/005 20130101; C21D 8/0247 20130101; C22C 38/001 20130101; C22C
38/00 20130101; C21D 6/008 20130101; C21D 8/0226 20130101; C22C
38/02 20130101; C22C 38/44 20130101; C22C 30/02 20130101; C22C
38/002 20130101; C21D 2211/001 20130101; C21D 2211/004 20130101;
C21D 8/0263 20130101; C21D 8/0205 20130101; C21D 6/004
20130101 |
International
Class: |
C21D 8/00 20060101
C21D008/00; C22C 38/42 20060101 C22C038/42; C22C 38/04 20060101
C22C038/04; C22C 38/02 20060101 C22C038/02; C21D 6/00 20060101
C21D006/00; C21D 6/00 20060101 C21D006/00; C22C 30/02 20060101
C22C030/02; C22C 38/00 20060101 C22C038/00; C21D 6/00 20060101
C21D006/00; C22C 38/44 20060101 C22C038/44; C22C 38/00 20060101
C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 24, 2014 |
JP |
2014-217632 |
Claims
1. A duplex stainless steel having a chemical composition
comprising, by mass %, C: 0.03% or less, Si: 1.0% or less, Mn: 1.0%
or less, P: 0.04% or less, S: 0.01% or less, Cu: 0.1 to 1.0%, Ni:
5.0 to 7.5%, Cr: 22.0 to 26.0%, W: 6.0 to 12.0%, N: 0.20 to 0.32%,
Mo: 0.01% or less, and balance: Fe and impurities, wherein, a metal
micro-structure contains, by area ratio, 0.40 to 0.60 of an
.alpha.-phase, with the balance of a .gamma.-phase and 0.01 or less
of other phases.
2. The duplex stainless steel according to claim 1, wherein a
pitting potential corresponding to 100 .mu.A/cm.sup.2 when immersed
in a 250 g/L NaCl aqueous solution that is held at 90.degree. C. is
600 mV (vs. SCE) or more.
3. The duplex stainless steel according to claim 1, wherein a
chemical composition of an outermost surface of a passivation film
after immersion for 24 hours in a testing liquid having a pH of 1
satisfies formula (i) below: W/(Fe+Cr).gtoreq.0.09 (i) where, each
symbol of an element in the formula represents a content (at %) of
each element in the outermost surface of the passivation film.
4. A method for producing a duplex stainless steel, including, with
respect to a steel having a chemical composition according to claim
1, performing a heat treatment of heating to a temperature range of
1150 to 1300.degree. C., and after holding the steel in the
temperature range, cooling at a cooling rate that is equal to or
higher than a cooling rate of water cooling.
Description
TECHNICAL FIELD
[0001] The present invention relates to a duplex stainless steel
and a method for producing the same.
BACKGROUND ART
[0002] There is a need for a stainless steel that has excellent
corrosion resistance for use in applications in which corrosion in
a high-temperature and high-concentration chloride environment is a
problem, such as in the chemical industry field.
[0003] A duplex stainless steel containing a large amount of Cr
(first-generation duplex stainless steel: SUS 329J4L or the like)
exhibits excellent corrosion resistance in comparison to
conventional stainless steel as typified by SUS 304 or SUS 316L.
However, in recent years, the environments in which stainless steel
is used have become more severe, and it is no longer possible to
exhibit satisfactory corrosion resistance using the traditional
duplex stainless steels.
[0004] Patent Document 1, Patent Document 2 and Patent Document 3
disclose duplex stainless steels (second-generation duplex
stainless steels) of which, as the increasing severity of usage
environments increases, corrosion resistance is accordingly
improved by utilizing Mo and N in accordance with pitting
resistance equivalent (PRE, PREW) values represented by the
following formula (1) and formula (2) that are known as indices
that indicate the corrosion resistance of duplex stainless steel.
However, even in the case of these second-generation duplex
stainless steels, corrosion resistance is insufficient in a
seawater environment.
PRE=Cr+3.3Mo+16N (1)
PREW=Cr+3.3(Mo+0.5W)+16N (2)
[0005] Patent Document 4, Patent Document 5, Patent Document 6,
Non-Patent Document 1 and Non-Patent Document 2 disclose duplex
stainless steels containing W (third-generation duplex stainless
steels). The third-generation duplex stainless steels have
excellent corrosion resistance to the traditional second-generation
duplex stainless steels, and are widely used in seawater
environments.
LIST OF PRIOR ART DOCUMENTS
Patent Document
[0006] Patent Document 1: JP62-180043A [0007] Patent Document 2:
JP2-258956A [0008] Patent Document 3: JP5-132741A [0009] Patent
Document 4: JP62-56556A [0010] Patent Document 5: JP5-132741A
[0011] Patent Document 6: JP8-170153A
Non Patent Document
[0011] [0012] Non-Patent Document 1: Anthony Comer, Lisa Looney,
Corrosion and fatigue characteristics of positively polarised Zeron
100 base & weld metal in synthetic seawater, International
Journal of Fatigue, Vol 28, 826-834. [0013] Non-Patent Document 2:
Corrosion Center News, No. 059 (2012)
SUMMARY OF INVENTION
Technical Problem
[0014] Even in the case of third-generation duplex stainless
steels, corrosion resistance is insufficient in a hot concentrated
chloride environment that is more severe than seawater, such as in
the chemical industry field.
[0015] An objective of the present invention is to provide a duplex
stainless steel that, by improving the corrosion resistance of
third-generation duplex stainless steel, can solve a problem of
corrosion under a hot concentrated chloride environment such as in
the chemical industry field, as well as a method for producing the
duplex stainless steel.
Solution to Problem
[0016] Heretofore the influence of W on corrosion resistance and
the action mechanism thereof have been considered to be the same as
the mechanism of Mo. However, as a result of detailed studies
conducted to examine the action mechanisms which contribute to
corrosion resistance of Mo and W, the present inventors found that
there is a mistake in the conventional findings with respect to the
corrosion resistance under severe environments.
[0017] FIG. 1 illustrates polarization curves of pure W and pure Mo
under a corrosive environment. As shown in FIG. 1, even in a region
in which Mo is eluted, almost no W is eluted. Thus, it is expected
that the influences of Mo and W on improving corrosion resistance
are significantly different.
[0018] Therefore, detailed studies were performed on the corrosion
resistance of duplex stainless steel for which the chemical
composition of a third-generation duplex stainless steel was
adopted as a basis and which contained a large amount of W but did
not contain Mo. As a result, the following findings were
obtained.
[0019] (a) By appropriately adjusting the chemical composition and
production method to obtain an .alpha.+.gamma. duplex
micro-structure in which there is no precipitation of an
.sigma.-phase or a .chi.-phase, duplex stainless steel having
excellent corrosion resistance under an environment in which hot
concentrated chloride is present is obtained. The corrosion
resistance at such time exceeds a corrosion resistance that is
predicted from the relational expression of PREW.
[0020] (b) By appropriately adjusting the chemical composition and
production method, a passivation film that is formed under an
environment in which hot concentrated chloride having a low pH is
present can be made a passivation film that is rich in W. A
passivation film that is rich in W dramatically improves corrosion
resistance under the aforementioned environment.
[0021] The present invention has been made based on the above
findings, and the gist of the present invention is a duplex
stainless steel and a production method therefor which are
described hereunder.
[0022] (1) A duplex stainless steel having a chemical composition
comprising, by mass %,
[0023] C: 0.03% or less,
[0024] Si: 1.0% or less,
[0025] Mn: 1.0% or less,
[0026] P: 0.04% or less,
[0027] S: 0.01% or less,
[0028] Cu: 0.1 to 1.0%,
[0029] Ni: 5.0 to 7.5%,
[0030] Cr: 22.0 to 26.0%,
[0031] W: 6.0 to 12.0%,
[0032] N: 0.20 to 0.32%,
[0033] Mo: 0.01% or less, and
[0034] balance: Fe and impurities, wherein,
[0035] a metal micro-structure contains, by area ratio, 0.40 to
0.60 of an .alpha.-phase, with the balance of a .gamma.-phase and
0.01 or less of other phases.
[0036] (2) The duplex stainless steel according to the above (1),
wherein a pitting potential corresponding to 100 .mu.A/cm.sup.2
when immersed in a 250 g/L NaCl aqueous solution that is held at
90.degree. C. is 600 mV (vs. SCE) or more.
[0037] (3) The duplex stainless steel according to the above (1) or
(2), wherein a chemical composition of an outermost surface of a
passivation film after immersion for 24 hours in a testing liquid
having a pH of 1 satisfies formula (i) below:
W/(Fe+Cr).gtoreq.0.09 (i)
[0038] where, each symbol of an element in the above formula
represents a content (at %) of each element in the outermost
surface of the passivation film.
[0039] (4) A method for producing a duplex stainless steel,
including, with respect to a steel having a chemical composition
according to the above (1), performing a heat treatment of heating
to a temperature range of 1150 to 1300.degree. C., and after
holding the steel in the temperature range, cooling at a cooling
rate that is equal to or higher than a cooling rate of water
cooling.
[0040] Note that, in the present invention, the term
".alpha.-phase" refers to a ferritic phase and the term
".gamma.-phase" refers to an austenite phase.
Advantageous Effects of Invention
[0041] According to the present invention, a duplex stainless steel
that has excellent corrosion resistance is obtained. The duplex
stainless steel is suited for use in the chemical industry field
and the like in which corrosion under a hot concentrated chloride
environment is a problem.
BRIEF DESCRIPTION OF DRAWINGS
[0042] FIG. 1 is a view illustrating polarization curves of pure W
and pure Mo under a corrosive environment.
[0043] FIG. 2 is a view illustrating the relation between a value
for W/(Fe+Cr) at an outermost surface of a passivation film and
pitting potential according to an example.
DESCRIPTION OF EMBODIMENTS
[0044] An embodiment of the present invention is described
hereunder. Hereinafter, the symbol "%" as used with respect to the
content of respective elements refers to "mass %."
[0045] 1. Chemical Composition of Base Metal
[0046] C: 0.03% or Less
[0047] C is an austenite former and is effective for stabilizing an
austenite phase. However, in stainless steel with a high Cr content
such as in the present invention, if the C content exceeds 0.03%,
there is a risk that Cr carbides will precipitate and corrosion
resistance will deteriorate. Therefore, the C content is made 0.03%
or less. Preferably, the C content is 0.01% or less. The above
effect will be achieved if even a trace amount of C is contained,
and hence the lower limit thereof is not particularly defined.
However, to adequately achieve the above effect, a C content of
0.003% or more is preferable.
[0048] Si: 1.0% or Less
[0049] Si is effective as a deoxidizing component of steel.
However, if the Si content is excessive, there is a concern that
the Si will promote precipitation of an .sigma.-phase and a
.chi.-phase. Therefore, the Si content is made 1.0% or less.
Preferably, the Si content is 0.5% or less. Although the Si content
may be substantially zero if deoxidation is to be performed with
another element, it is preferable to contain 0.2% or more of Si to
adequately achieve the above effect.
[0050] Mn: 1.0% or Less
[0051] Mn is an austenite former and contributes to stabilization
of an austenite. However, if the Mn content is excessive, there is
a concern that MnS that acts as a corrosion starting point will
crystallize or precipitate. Therefore, the Mn content is made 1.0%
or less. Preferably the Mn content is 0.5% or less. The above
effect will be achieved if even a trace amount of Mn is contained,
and hence the lower limit thereof is not particularly defined.
However, to adequately achieve the above effect, an Mn content of
0.1% or more is preferable.
[0052] P: 0.04% or Less
[0053] P is an impurity element that is unavoidable during the
production process, and if the content thereof is excessive there
is a risk that workability will be reduced. Therefore, the P
content is made 0.04% or less. Preferably, the P content is 0.01%
or less.
[0054] S: 0.01% or Less
[0055] S is an impurity element that is unavoidable during the
production process, and if the content thereof is excessive there
is a risk that workability will be reduced. There is also a concern
that MnS which acts as a corrosion starting point will crystallize
or precipitate. Therefore, the S content is made 0.01% or less.
Preferably, the S content is 0.004% or less.
[0056] Cu: 0.1 to 1.0%
[0057] Cu is an austenite former, and is effective for improving
resistance to sulfuric acid. Cu is also effective for assisting the
formation of a passivation film that contains a large amount of W.
Specifically, Cu has an effect of promoting a cathode reaction, and
accelerating the formation of a passivation film containing a large
amount of W. Therefore, the content of Cu is made 0.1% or more.
However, if the content of Cu is excessive, there is a concern that
the excessive content may deteriorate formability. Accordingly, the
Cu content is set in a range of 0.1 to 1.0%. A preferable lower
limit is 0.4%, and a preferable upper limit is 0.6%.
[0058] Ni: 5.0 to 7.5%
[0059] Ni is an austenite former. In order to obtain an
.alpha.+.gamma. duplex micro-structure with a desirable balance
with respect to the relation with ferrite forming elements such as
Cr and W, an Ni content in the range of 5.0 to 7.5% is necessary. A
preferable lower limit is 6.0%, and a preferable upper limit is
6.8%.
[0060] Cr: 22.0 to 26.0%
[0061] Cr is a ferrite forming element and is also a basic element
that is effective for improving corrosion resistance. If the Cr
content is insufficient or excessive, a temperature range in which
an .alpha.+.gamma. duplex micro-structure can be stably obtained
narrows. Therefore, the Cr content is set in a range of 22.0 to
26.0%. A preferable lower limit is 23.0%, and a preferable upper
limit is 25.5%.
[0062] W: 6.0 to 12.0%
[0063] W is a ferrite forming element and is also an important
element for developing excellent corrosion resistance. If the W
content is insufficient or is excessive, an .alpha.+.gamma. duplex
micro-structure cannot be stably obtained. Therefore, the W content
is set as a value within a range of 6.0 to 12.0%. A preferable
lower limit is 8.0%, and a preferable upper limit is 11.0%.
[0064] N: 0.20 to 0.32%
[0065] N is an austenite former, and is an effective element for
improving thermal stability and corrosion resistance of a duplex
stainless steel. To obtain an .alpha.+.gamma. duplex
micro-structure with a desirable balance with respect to a relation
with ferrite forming elements such as Cr and W, an N content of
0.20% or more is necessary. However, if the N content exceeds
0.32%, there is a risk that the toughness and corrosion resistance
of the steel will noticeably deteriorate due to the production of
nitride. Therefore, the N content is set in a range of 0.20 to
0.32%. A preferable lower limit is 0.24%, and a preferable upper
limit is 0.28%.
[0066] Mo: 0.01% or Less
[0067] Mo is a ferrite forming element, similarly to Cr and W.
However, when Mo is contained in the chemical composition, the Mo
decreases the solubility of W, and hence it is necessary to make
the Mo content as low as possible. Therefore, the Mo content is
made 0.01% or less, and preferably is 0.008% or less.
[0068] The chemical composition of the duplex stainless steel of
the present invention contains each of the aforementioned elements
in the respectively defined ranges, with the balance of Fe and
impurities. The term "impurities" refers to components that are
contained in raw materials such as ore or scrap or that are mixed
in due to other causes when industrially producing the steel
material.
[0069] 2. Metal Micro-Structure of Base Metal
[0070] The base metal has an .alpha.+.gamma. duplex micro-structure
in which the area ratio of the .alpha.-phase is from 0.40 to 0.60
and the balance is the .gamma.-phase as well as other phases for
which the area ratio is 0.01 or less. In phases other than the
.alpha.-phase and .gamma.-phase, particularly in an .sigma.-phase
and .chi.-phase, a Cr depleted zone is formed around the phase, and
consequently corrosion resistance is degraded. Therefore, although
it is preferable that the total area ratio of those phases is zero,
a total area ratio of 0.01 or less is permissible. Note that, since
the corrosion resistance may be degraded if the proportion of the
.gamma.-phase is large, preferably the area ratio of the
.gamma.-phase is made 0.58 or less.
[0071] 3. passivation Film
[0072] If the duplex stainless steel having the aforementioned
chemical composition and metal micro-structure is produced under
appropriate conditions, a passivation film formed under an
environment in which hot concentrated chloride having a low pH is
present can be made a passivation film that is rich in W. Although
Fe and Cr in a passivation film undergo corrosion in a low pH
environment, a passivation film that contains a large amount of W
that effectively contributes to corrosion resistance is excellent
in corrosion resistance.
[0073] Further, in a case where the chemical composition of an
outermost surface of the passivation film after immersion for 24
hours in a testing liquid having a pH of 1 satisfies the following
formula (i), it is possible to dramatically improve the corrosion
resistance of the duplex stainless steel. The left-hand value in
the following formula (i) is more preferably set to 0.10% or
more.
W/(Fe+Cr).gtoreq.0.09 (i)
[0074] Where, each symbol of an element in the above formula
represents a content (at %) of each element in the outermost
surface of the passivation film.
[0075] 4. Method for Producing Duplex Stainless Steel
[0076] The duplex stainless steel of the present invention is made
into a product by melting under production conditions that are
generally adopted, performing necessary processes such as hot
working and cold working, and finally performing a heat treatment
including heating in a temperature range of 1150 to 1300.degree.
C., and after holding the steel in this temperature range, cooling
at a cooling rate that is equal to or higher than a cooling rate of
water cooling.
[0077] This is because, if the aforementioned heat treatment
temperature is less than 1150.degree. C., precipitation of the
.sigma.-phase or .chi.-phase is inevitable, while on the other
hand, if the aforementioned heat treatment temperature exceeds
1300.degree. C., there is a risk that an .alpha.+.gamma. duplex
micro-structure in which the area ratio of the .alpha.-phase is
from 0.4 to 0.6 and the balance is substantially a .gamma.-phase
cannot be obtained. Therefore, the heat treatment is performed in a
temperature range from 1150 to 1300.degree. C. Although the holding
time will vary depending on the thickness of the duplex stainless
steel, the holding time may be appropriately selected within a
range of 1 to 120 min.
[0078] If the cooling rate after the steel is held in the
aforementioned temperature range is excessively slow there is a
risk that an .sigma.-phase or a .chi.-phase will precipitate during
the cooling process, and therefore the steel is cooled at a cooling
rate that is equal to or higher than the cooling rate of water
cooling. More specifically, it is sufficient to perform cooling at
a cooling rate of 40.degree. C./s or more.
[0079] Hereunder, the present invention is described specifically
by way of an example, although the present invention is not limited
to the following example.
Example 1
[0080] Ingots having the chemical compositions shown in Table 1
were melted in a 17-kg vacuum furnace, and then subjected to hot
rolling to a thickness of 4 to 8 mm. Each of the steels was
adjusted so that a pitting resistance equivalent PREW value defined
by the following formula was around 43 to 44,
PREW=Cr+3.3(Mo+0.5W)+16N
[0081] Where, each symbol of an element in the above formula
represents a content (mass %) of the element in the steel.
[0082] Thereafter, after heating and holding at the temperatures
shown in Table 1, the respective steels were subjected to water
cooling and specimens were obtained. Commercially available
stainless steels having the chemical compositions shown in Table 2
were also prepared as specimens. With respect to these specimens,
observation of the metal micro-structure of the base metal,
measurement of corrosion resistance, and component analysis of a
passivation film were performed.
[Table 1]
TABLE-US-00001 [0083] TABLE 2 Heat treatment Cooling Steel Chemical
composition (by mass %, balance: Fe and impurities) temperature
rate No. C Si Mn P S Cu Ni Cr N W Mo PREW (.degree. C.) (.degree.
C/s) 30 0.019 0.40 0.75 0.025 0.001 <0.01 * 6.39 24.530 0.17
<0.01 * 3.22 37.9 1100 # 41 31 0.011 0.41 0.77 0.021 0.001
<0.01 * 6.34 24.550 0.17 <0.01 * 3.24 38.0 42 32 0.013 0.39
0.71 0.024 0.001 <0.01 * 6.45 24.810 0.19 <0.01 * 3.23 38.5
39 * indicates that conditions do not satisfy those defined by the
present invention. # indicates that production conditions do not
satisfy the preferrable conditions described in the present
invention.
TABLE-US-00002 TABLE 1 Heat treatment Cooling Steel Chemical
composition (by mass %, balance; Fe and impurities) temperature
rate No. C Si Mn P S Cu Ni Cr N W Mo PREW (.degree. C.) (.degree.
C./s) 1 0.003 0.26 0.48 <0.001 0.001 0.49 6.96 25.04 0.26 8.44
<0.01 43.0 1200 45 2 0.004 0.26 0.49 <0.001 0.001 0.49 6.96
25.07 0.26 8.46 <0.01 43.1 43 3 0.003 0.27 0.49 <0.001 0.001
0.49 6.97 25.02 0.25 8.46 <0.01 43.2 50 4 0.003 0.26 0.48
<0.001 0.001 0.49 6.96 25.04 0.26 8.44 <0.01 43.9 1150 49 5
0.004 0.26 0.49 <0.001 0.001 0.49 6.96 25.07 0.26 8.46 <0.01
43.1 42 6 0.003 0.27 0.49 <0.001 0.001 0.49 6.97 25.02 0.25 8.46
<0.01 43.2 44 7 0.003 0.26 0.48 <0.001 0.001 0.49 6.96 25.04
0.26 8.44 <0.01 43.0 1050 # 40 8 0.004 0.26 0.49 <0.001 0.001
0.99 6.96 25.07 0.26 8.46 <0.01 43.1 42 9 0.003 0.27 0.49
<0.001 0.001 0.49 6.97 25.02 0.25 8.46 <0.01 43.2 39 10 0.003
0.26 0.48 <0.001 0.001 0.49 6.96 25.04 0.26 8.44 <0.01 43.0
950 # 37 11 0.004 0.26 0.49 <0.001 0.001 0.49 6.96 25.07 0.26
8.46 <0.01 43.1 41 12 0.003 0.27 0.49 <0.001 0.001 0.49 6.97
25.02 0.25 8.46 <0.01 43.2 38 13 0.003 0.26 0.48 <0.001 0.001
0.49 6.96 25.04 0.26 8.44 <0.01 43.0 850 # 33 14 0.004 0.26 0.49
<0.001 0.003 0.49 6.96 25.07 0.26 8.46 <0.01 43.1 31 15 0.003
0.27 0.49 <0.001 0.001 0.49 6.97 25.02 0.25 8.46 <0.01 43.1
34 16 0.012 0.29 0.49 0.027 0.001 0.50 6.97 25.53 0.30 1.95 * 3.21
* 44.1 1100 # 40 17 0.013 0.29 0.47 0.025 0.001 0.49 6.97 25.50
0.30 1.94 * 3.23 * 44.1 38 18 0.010 0.30 0.48 0.024 0.001 0.49 6.95
25.55 0.30 2.02 * 3.22 * 44.2 39 19 0.019 0.51 0.46 0.024 <0.001
0.44 6.69 25.15 0.26 2.07 * 3.09 * 43.0 39 20 0.015 0.49 0.49 0.025
<0.001 0.46 6.67 25.31 0.26 2.09 * 3.08 * 43.1 39 21 0.016 0.47
0.48 0.024 <0.001 0.50 6.70 25.30 0.26 2.06 * 3.11 * 43.2 39 22
0.003 0.26 0.48 <0.001 0.001 0.49 6.96 25.04 0.26 8.44 <0.01
43.0 38 23 0.004 0.26 0.49 <0.001 0.001 0.49 6.96 25.07 0.26
8.46 <0.01 43.1 39 24 0.003 0.27 0.49 <0.001 0.001 0.49 6.97
25.02 0.25 8.46 <0.01 43.2 38 25 0.003 0.26 0.48 <0.001 0.001
<0.01 * 6.96 25.42 0.27 8.52 <0.01 43.8 1200 50 26 0.007 0.30
0.48 <0.001 <0.001 <0.01 * 6.89 25.13 0.26 8.58 <0.01
43.5 45 27 0.008 0.29 0.49 <0.001 0.001 <0.01 * 7.12 25.10
0.27 8.49 <0.01 43.4 45 28 0.003 0.26 0.48 <0.001 0.001 0.49
6.96 25.04 0.26 8.44 <0.01 43.0 1150 0.05 # 29 0.003 0.26 0.48
<0.001 0.001 <0.01 * 6.96 25.42 0.27 8.52 <0.01 43.8 0.1 #
* indicates that conditions do not satisfy those defined by the
present invention. # indicates that production conditions do not
satisfy the preferrable conditions described in the present
invention.
[0084] <Observation of Metal Micro-Structure of Base
Metal>
[0085] A cross-section of each specimen was observed under an
optical microscope at a magnification of 500, and the area ratios
of the .alpha.-phase and the .gamma.-phase were measured. In
addition, the presence/absence of an .alpha.-phase and a
.chi.-phase was verified, and steel in which there was no
.sigma.-phase or .chi.-phase precipitation was marked with
".largecircle.", while steel in which precipitation of at least one
of .sigma.-phase and .chi.-phase was observed was marked with "x",
and the total area ratios of these phases were measured.
[0086] <Measurement of Corrosion Resistance>
[0087] A disk-like test specimen having a diameter of 15 mm and a
plate thickness of 2 mm was cut out from each specimen, and the
surface was finished by #600 wet polishing. Testing was performed
in accordance with JIS G 0577 (2014), and a pitting potential
V'C100 corresponding to 100 .mu.A/cm.sup.2 was measured. Note that,
since an environment in which hot concentrated chloride is present
was assumed, a 250 g/L NaCl aqueous solution that was kept at
90.degree. C. was used as the aqueous solution.
[0088] <Component Analysis of Passivation Film>
[0089] Some of the specimens were immersed for 24 hours in a
testing liquid having a pH of 1, and thereafter measurement of each
main metal element in the passivation film was performed by X-ray
photoelectron spectroscopy and a value of W/(Fe+Cr) in the
outermost surface of the passivation film was calculated.
[0090] The results of the above measurements are summarized in
Table 3.
TABLE-US-00003 TABLE 3 Metal micro-structure Total Presence/ ratio
Left-hand Pitting Test Steel Ratio of Ratio of absence of of
.sigma., value in potential No. No. .alpha.-phase .gamma.-phase
.sigma., .chi.-phase .chi.-phase formula (i).sup..dagger. (mV vs.
SCE) 1 1 0.55 0.45 .smallcircle. -- 0.10 663 Inventive 2 2 0.55
0.45 .smallcircle. -- 0.10 661 example 3 3 0.52 0.48 .smallcircle.
-- 0.10 701 4 4 0.57 0.43 .smallcircle. -- 0.09 630 5 5 0.46 0.54
.smallcircle. -- 0.10 685 6 6 0.44 0.56 .smallcircle. -- 0.11 722 7
7 0.38 * 0.60 x 0.02 * 0.03 188 Comparative 8 8 0.43 0.55 x 0.02 *
0.02 152 example 9 9 0.35 * 0.63 x 0.02 * 0.03 218 10 10 0.34 *
0.62 x 0.04 * 0.03 220 11 11 0.29 * 0.66 x 0.05 * 0.03 238 12 12
0.42 0.54 x 0.04 * 0.04 308 13 13 0.38 * 0.55 x 0.07 * 0.01 -4 14
14 0.37 * 0.56 x 0.07 * 0.01 12 15 15 0.43 0.51 x 0.06 * 0.01 44 16
16 * 0.48 0.52 .smallcircle. -- 0.07 416 17 17 * 0.51 0.49
.smallcircle. -- 0.06 404 18 18 * 0.48 0.52 .smallcircle. -- 0.06
433 19 19 * 0.55 0.45 .smallcircle. -- 0.06 378 20 20 * 0.52 0.48
.smallcircle. -- 0.06 404 21 21 * 0.51 0.49 .smallcircle. -- 0.06
411 22 22 0.34 * 0.66 .smallcircle. -- 0.08 574 23 23 0.31 * 0.69
.smallcircle. -- 0.07 509 24 24 0.33 * 0.65 x 0.02 * 0.03 294 25 25
* 0.55 0.45 .smallcircle. -- 0.05 523 26 26 * 0.50 0.50
.smallcircle. -- 0.05 526 27 27 * 0.53 0.47 .smallcircle. -- 0.05
514 28 28 0.56 0.33 x 0.11 * 0.01 -44 29 29 * 0.57 0.30 x 0.13 *
0.01 -68 30 30 * 0.68* 0.32 .smallcircle. -- -- 219 31 31 * 0.66 *
0.34 .smallcircle. -- -- 230 32 32 * 0.66 * 0.34 .smallcircle. --
-- 248 * indicates that conditions do not satisfy those defined by
the present invention .sup..dagger.W/(Fe + Cr) .gtoreq. 0.09 . . .
(i)
[0091] As shown in Table 3, in test Nos. 1 to 6 in which the
chemical composition and metal micro-structure satisfied the
specification of the present invention, the pitting potential was
600 mV or more and favorable corrosion resistance was
exhibited.
[0092] In contrast, the results showed that the corrosion
resistance was inferior in test Nos. 16 to 21, 25 to 27 and 29 to
32 in which at least the chemical composition deviated from the
range specified by the present invention and in test Nos. 7 to 15,
22 to 24 and 28 in which at least the metal micro-structure
deviated from the range specified by the present invention.
[0093] As shown in FIG. 2, there is a constant correlation between
the value of W/(Fe+Cr) in the outermost surface of the passivation
film and the pitting potential, and it is possible to make the
pitting potential 600 mV or more when the value for W/(Fe+Cr) is
0.09 or more.
INDUSTRIAL APPLICABILITY
[0094] According to the present invention, a duplex stainless steel
having excellent corrosion resistance is obtained. The duplex
stainless steel is suitable for use in the chemical industry field
and the like in which corrosion under a hot concentrated chloride
environment is a problem.
* * * * *