Note: Descriptions are shown in the official language in which they were submitted.
CA 02770378 2012-02-07
[Document Name] Description
[Title of Invention] DUPLEX STAINLESS STEEL
[Technical Field]
[0001]
The present invention relates to a ferrite-austenite duplex stainless steel
excellent in
stress corrosion cracking resistance, in particular to a duplex stainless
steel suitable as a
steel material for line pipes transporting petroleum, natural gas or the like.
[Background Art]
[0002]
In petroleum and natural gas produced from oil fields and gas fields, such
corrosive
gasses as carbon dioxide gas (CO2) and hydrogen sulfide (H2S) are present as
associated
gases. In line pipes transporting such highly corrosive petroleum and natural
gas, stress
corrosion cracking (SCC), sulfide stress cracking (SSC), and general corrosion
functioning
as a factor for wall thickness reduction and the like offer problems. In
particular, stress
corrosion cracking (SCC) and sulfide stress cracking (SSC) are fast in rate of
progression,
hence the time in which the crack penetrates line pipes is short, and such
penetration
occurs locally to offer more serious problems. Accordingly, the steel
materials for such
line pipes as aforementioned are required to have excellent corrosion
resistance.
[0003]
As steel materials excellent in corrosion resistance, what is called duplex
stainless
steels composed of ferrite-austenite phases have hitherto been used. For
example, Patent
Document 1 describes a duplex stainless steel containing Cu in a content of 1
to 3% and
improved in corrosion resistance in chloride and sulfide environments. Patent
Document
2 describes a duplex stainless steel in which the strength, toughness and
seawater
- 1 -
CA 02770378 2012-02-07
resistance are improved by appropriately regulating the contents of Cr, Ni,
Cu, Mo, N and
W and by controlling the area fraction of the ferrite phase to 40% through
70%.
[Citation List]
[Patent Document 1] WO 96/18751
[Patent Document 2] JP2003-171743A
[Summary of Invention]
[Technical Problem]
[0005]
In the duplex stainless steel described in Patent Document 1, the degradation
of the
corrosion resistance of the weld zone tends to occur during large heat input
welding. In
the duplex stainless steel described in Patent Document 2, intermetallic
compounds
precipitate in the weld zone during large heat input welding, and hence
embrittlement and
degradation of the corrosion resistance tend to occur in the weld zone, and
additionally, on
the assumption of the transportation of petroleum or natural gas, insufficient
is the stress
corrosion cracking resistance in a chloride environment containing corrosive
associated
gases such as carbon dioxide gas and hydrogen sulfide.
[0006]
The present invention has been performed for the purpose of solving the
aforementioned problems, and an object of the present invention is to provide
a duplex
stainless steel excellent in the weldability during large heat input welding
and excellent in
the stress corrosion cracking resistance in the chloride environment
containing corrosive
associated gases.
[Solution to Problem]
[0007]
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The present inventors performed a series of various experiments and detailed
studies for the purpose of actualizing in a duplex stainless steel the
improvement of the
weldability during large heat input welding and the improvement of the stress
corrosion
cracking resistance in the chloride environment. Consequently, the present
inventors
have obtained the following findings (a) to (f).
[0008]
(a) The stress corrosion cracking resistance of a duplex stainless steel can
be
improved by strengthening with Mo the passivation film mainly composed of Cr.
On the
other hand, for the purpose of preventing the precipitation of intermetallic
compounds
during large heat input welding, it is necessary to regulate the contents of
Cr and Mo.
However, in a high-temperature chloride environment containing carbon dioxide
gas and
hydrogen sulfide, when the contents of Cr and Mo are reduced, it is impossible
to obtain
any excellent stress corrosion cracking resistance in the vicinity of a weld
zone.
[0009]
(b) For the purpose of improving the stress corrosion cracking resistance
while the
contents of Cr and Mo are being regulated, it is only required that the
passivation film
mainly composed of Cr be able to be strengthened with an element other than
Mo. In this
connection, Cu is an element having a function to reduce the corrosion rate of
a steel
material in an acidic environment. Accordingly, the inclusion of Cu in an
appropriate
content in addition to Cr and Mo enables to stabilize the passivation film and
to strengthen
the passivation film.
[0010]
Figure 4 is a graph in which for duplex stainless steels having various
chemical
compositions, used in Examples described later, the content (mass%) of "Cr" is
plotted on
the X-axis and the content (mass%) of "7Mo + 3Cu" is plotted on the Y-axis.
With the
straight line of "7Mo + 3Cu = -2.2Cr + 66" as a boundary, the graph can be
divided into
the upper right section of the "determination (0) of non-occurrence of stress
corrosion
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cracking" and the lower left section of the "determination (x) of occurrence
of stress
corrosion cracking."
[0011]
Accordingly, it can be derived that by containing Cr, Mo and Cu so as to
satisfy the
relation of the following formula (1), the passivation film can be
strengthened:
2.2Cr + 7Mo + 3Cu > 66 (1)
wherein the symbols of elements in formula (1) respectively represent the
contents (unit:
mass%) of the elements in the steel.
[0012]
When the content of Cu is 2% or less by mass%, no sufficient corrosion
resistance
is obtained. Accordingly, Cu is required to be contained in a content
exceeding 2%.
[0013]
(c) When a duplex stainless steel is welded, the micro-structure in the
vicinity of the
weld zone is heated in a short time and then cooled in a short time. For the
purpose of
preventing the precipitation of the intermetallic compound (the sigma phase)
in such a
micro-structure in which heating and cooling are conducted in a short time, it
is important
to suppress the nucleation and the nuclear growth of the sigma phase.
[0014]
(d) The driving force for the nucleation of the sigma phase is increased with
the
increase of the content of Ni. Accordingly, when only the suppression of the
production
of the sigma phase is considered, Ni has only not to be contained. However,
when Ni is
not contained, the ratio between the ferrite phase and the austenite phase
largely deviates
from 1:1, and the toughness and the corrosion resistance are degraded.
Accordingly, for
the purpose of suppressing the production of the sigma phase while the
degradation of the
toughness and the degradation of the corrosion resistance are being prevented,
Ni is
required to be contained in an appropriate content depending on the contents
of Cu and N.
Specifically, by containing Ni so as to satisfy the relation of the following
formula (2), the
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production of the sigma phase can be suppressed without degrading the
toughness and the
corrosion resistance:
Cr + 11Mo + 10Ni < 12(Cu + 30N) (2)
wherein the symbols of elements in formula (2) respectively represent the
contents (unit:
mass%) of the elements in the steel.
[0015]
The left hand side of formula (2) represents the driving force for the
precipitation of
the sigma phase; among the components constituting the duplex stainless steel,
Cr, Mo and
Ni are the elements to increase the driving force for the nucleation of the
precipitation of
the sigma phase; on the basis of various tests, it has been found that the
degrees of
contribution of Mo and Ni are 11 times and 10 times the degree of contribution
of Cr,
respectively.
[0016]
On the other hand, the right hand side of formula (2) conversely represents
the
deterrent force against the precipitation of the sigma phase, and on the basis
of various
tests, it has been found that the degree of contribution of N is 30 times the
degree of
contribution of Cu, and the deterrent force of Cu is 12 times the driving
force of Cr.
[0017]
The manifestation mechanism of the deterrent force against the precipitation
of the
sigma phase due to Cu and N is as follows. The presence of a Cu atom or an N
atom in
the vicinity of each of the Ni atoms present in the crystal lattice suppresses
the decrease of
the interface energy in the ferrite/austenite phase interface, which is the
site of the
nucleation of the sigma phase; thus, the decrease amount of the free energy at
the time of
the precipitation reaction of the sigma phase is made small, and hence the
driving force for
the crystal nucleation can be made small to be associated with the
aforementioned
manifestation mechanism. Additionally, Cu precipitates in the matrix as a
Cu
concentrated phase in an ultrafine manner, hence a large number of nucleation
sites of the
sigma phase are dispersed so as to compete against the fen-ite/austenite phase
interface
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which is the proper nucleation site, and consequently, there occurs an effect
to retard the
sigma phase production, otherwise fast in growth, in the ferrite/austenite
phase boundary.
[0018]
(e) By containing an appropriate amount of Ni to satisfy the relation of the
aforementioned formula (2), Cu atoms and N atoms can be located in the
vicinities of the
Ni atoms present in the crystal lattice. In this case, it is possible to
suppress the decrease
amount of the interface energy in the ferrite phase/austenite phase interface,
which is the
nucleation site of the sigma phase. Accordingly, it is possible to reduce the
decrease
amount of the free energy at the time of the precipitation reaction of the
sigma phase, and it
is possible to reduce the driving force for the nucleation of the sigma phase.
Consequently, it is possible to suppress the production of the sigma phase.
[0019]
(f) The nuclear growth of the sigma phase can be suppressed by containing an
appropriate amount of Cu. Specifically, the inclusion of an appropriate amount
of Cu
enables the precipitation of an ultrafine Cu concentrated phase in the matrix
during large
heat input welding. The Cu concentrated phase serves as the nucleation site of
the sigma
phase, and hence by precipitating a large number of Cu concentrated phases in
a dispersed
manner, the Cu concentrated phases can be made to compete against the ferrite
phase/austenite phase interface, which is the proper nucleation site.
Consequently, the
growth of the sigma phase in the ferrite phase/austenite phase interface can
be retarded.
[0020]
The present invention has been perfected on the basis of the aforementioned
findings, and the gist of the present invention resides in the following items
(1) to (4)
regarding duplex stainless steel.
[0021]
(1) A duplex stainless steel that has a chemical composition consistingõ by
mass%,
of C: 0.03% or less, Si: 0.2 to 1%, Mn: 5.0% or less, P: 0.040% or less, S:
0.010% or less,
sol. Al: 0.040% or less, Ni: 4 to 8%, Cr: 20 to 28%, Mo: 0.5 to 2.0%, Cu: more
than 2.0%
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and 4.0% or less and N: 0.1 to 0.35%, with the balance being Fe and
impurities; wherein
the duplex stainless steel satisfies the relations of the following formulas
(1) and (2):
2.2Cr + 7Mo + 3Cu > 66 (1)
Cr + 11Mo + 10Ni < 12(Cu + 30N) (2)
wherein the symbols of elements in formulas (1) and (2) respectively represent
the contents
(unit: mass%) of the elements in the steel.
[0022]
(2) The duplex stainless steel according to the item (1) above, which further
contains, by mass%, V: 1.5% or less, in place of part of Fe.
[0023]
(3) The duplex stainless steel according to the item (1) or (2) above, which
further
contains, by mass%, one or more selected from among Ca: 0.02% or less, Mg:
0.02% or
less and B: 0.02% or less, in place of part of Fe.
[0024]
(4) The duplex stainless steel according to any one of the items (1) to (3)
above,
which further contains, by mass%, rare earth metal(s): 0.2% or less, in place
of part of Fe.
[Advantageous Effects of Invention]
[0025]
The duplex stainless steel according to the present invention is excellent in
the
weldability during large heat input welding and excellent in the stress
corrosion cracking
resistance in a chloride environment.
[Brief Description of Drawings]
[0026]
[Figure 1] Figure 1 is a view illustrating a plate material prepared by
mechanical working,
(a) being a plane view and (b) being a front view.
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[Figure 21 Figure 2 is a view illustrating a weld joint, (a) being a plane
view and (b) being
a front view.
[Figure 3] Figure 3 is an oblique perspective view illustrating a specimen.
[Figure 4] Figure 4 is a graph showing the relations between the chemical
compositions of
the duplex stainless steels according to Examples, wherein the mark 0
represents the
"determination of non-occurrence of stress corrosion cracking" and the mark x
represents
the "determination of occurrence of stress corrosion cracking."
[Description of Embodiments]
[0027]
Hereinafter, the functional effect of the chemical composition of the duplex
stainless steel according to the present invention is described, together with
the reasons for
limiting the contents in the chemical composition. In this connection, "%"
related to the
contents means "mass%."
[0028]
C: 0.03% or less
C is an element effective in stabilizing the austenite phase. However, when
the
content of C exceeds 0.03%, carbides tend to precipitate, and the corrosion
resistance is
degraded. Accordingly, the content of C is set at 0.03% or less.
[0029]
Si: 0.2 to 1%
Si is able to ensure the fluidity of the molten metal during welding, and
hence is an
element effective in preventing weld defects. For the purpose of obtaining
this effect. Si
is required to be contained in a content of 0.2% or more. On the other hand,
when the
content of Si exceeds 1%, intermetallic compounds (such as the sigma phase)
tend to be
produced. Accordingly, the content of Si is set at 0.2 to 1%. The content of
Si is
preferably 0.2 to 0.5%.
[0030]
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Mn: 5.0% or less
Mn is a component effective in improving the hot workability through the
desulfurization and deoxidation effects during melting of the duplex stainless
steel. Mn
also has a function to increase the solubility of N. However, when the content
of Mn
exceeds 5.0%, the corrosion resistance is degraded. Accordingly, the content
of Mn is set
at 5.0% or less.
[0031]
P: 0.040% or less
P is mixed in the steel as an impurity, and degrades the corrosion resistance
and the
toughness of the steel. Accordingly, the content of P is set at 0.040% or
less.
[0032]
S: 0.010% or less
S is mixed in the steel as an impurity, and degrades the hot workability of
the steel.
Sulfides offer the origins of the occurrence of pitting and degrade the
pitting resistance of
the steel. For the purpose of avoiding these adverse effects, the content of S
is set at
0.010% or less. The content of S is preferably 0.007% or less.
[0033]
sol. Al: 0.040% or less
Al is a component effective as a deoxidizer of the steel. On the other hand,
when
the content of N in the steel is large, Al precipitates as AIN (aluminum
nitride), and
degrades the toughness and the corrosion resistance of the steel. Accordingly,
the content
of Al is set at 0.040% or less. The content of Al as referred to in the
present invention
means the content of acid-soluble Al (what is called sol. Al). Al is used as a
deoxidizer in
the duplex stainless steel according to the present invention, because the
content of Si as a
component effective deoxidizer is suppressed , and hence. However, when the
duplex
stainless steel is produced by vacuum melting, it is not necessary to contain
Al.
[0034]
Ni: 4 to 8%
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Ni is a component effective in stabilizing austenite. When the content of Ni
exceeds 8%, the resultant decrease of the amount of ferrite makes it difficult
to ensure the
fundamental properties of the duplex stainless steel and also facilitates the
production of
intermetallic compounds (such as the sigma phase). On the other hand, when the
content
of Ni is less than 4%, the amount of ferrite comes to be too large and thus
the features of
the duplex stainless steel are lost. The solubility of N in ferrite is small,
and hence due to
the amount of ferrite becoming too large, nitrides precipitate and the
corrosion resistance is
degraded. Accordingly, the content of Ni is set at 4 to 8%.
[0035]
Cr: 20 to 28%
Cr is a component effective in maintaining the corrosion resistance. For the
purpose of obtaining the SCC resistance in a chloride environment, Cr is
required to be
contained in a content of 20% or more. On the other hand, when the content of
Cr
exceeds 28%, the precipitation of intermetallic compounds (such as the sigma
phase)
comes to be remarkable, and the degradation of the hot workability and the
degradation of
the weldability are caused. Accordingly, the content of Cr is set at 20 to
28%.
[0036]
Mo: 0.5 to 2.0%
Mo is an element extremely effective in improving the SCC resistance. For the
purpose of obtaining this effect, Mo is required to be contained in a content
of 0.5% or
more. On the other hand, when the content of Mo exceeds 2.0%, the
precipitation of
intermetallic compounds is remarkably accelerated during large heat input
welding, and the
degradation of the hot workability and the degradation of the weldability are
caused.
Accordingly, the content of Mo is set at 0.5 to 2.0%. The content of Mo is
preferably 0.7
to 1.8% and more preferably 0.8 to 1.5%.
[0037]
Cu: more than 2.0% and 4.0% or less
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Cu is a component effective in strengthening the passivation film mainly
composed
of Cr in a chloride environment containing corrosive acidic gasses (such as
carbon dioxide
gas and hydrogen sulfide gas). Additionally, Cu precipitates in the matrix in
an ultrafine
manner during large heat input welding to become nucleation sites of
intermetallic
compounds (the sigma phase) so as to compete against the ferrite/austenite
phase interface
which is the proper nucleation site. Consequently, there occurs retardation of
the sigma
phase production, otherwise fast in growth, in the ferrite/austenite phase
interface. For
the purpose of obtaining these effects, Cu is required to be contained in a
content
exceeding 2.0%. On the other hand, when Cu is contained in a content exceeding
4.0%,
the hot workability of the steel is impaired. Accordingly, the content of Cu
is set to be
more than 2.0% and 4.0% or less.
[0038]
N: 0.1 to 0.35%
N is a powerful austenite former, and is effective in improving the thermal
stability
and the corrosion resistance of the duplex stainless steel. The duplex
stainless steel
according to the present invention contains Cr and Mo, which are ferrite
formers, in large
amounts, and hence N is required to be contained in a content of 0.1% or more
for the
purpose of establishing an appropriate balance between ferrite and austenite.
On the other
hand, when the content of N exceeds 0.35%, the toughness and the corrosion
resistance of
the steel are degraded due to the occurrence of blow holes as weld defects,
the nitride
production caused by the thermal effects during welding or the like.
Accordingly, the
content of N is set at 0.1 to 0.35%.
[0039]
In addition to the aforementioned chemical composition, Cr, Mo, Ni, Cu and N
are
required to satisfy the following formulas (1) and (2):
2.2Cr + 7Mo + 3Cu > 66 (1)
Cr + 11Mo + 10Ni < 12(Cu + 30N) (2)
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wherein the symbols of elements in formulas (1) and (2) respectively represent
the contents
(unit: mass%) of the elements in the steel.
[0040]
In the duplex stainless steel according to the present invention, the contents
of Cr
and Mo are regulated for the purpose of suppressing the precipitation of the
intermetallic
compounds. Accordingly, for the purpose of strengthening the passivation film
mainly
composed of Cr, Cu is required to be contained in an appropriate amount in
addition to Mo.
In this connection, when the value of "2.2Cr + 7Mo + 3Cu" is 66 or less, a
sufficient
resistance against the stress corrosion cracking (SCC) in a chloride
environment cannot be
ensured as the case may be. Accordingly, the requirement of the above
presented formula
(1) is specified.
[0041]
When the value of "Cr + 11Mo + 10Ni" is equal to or larger than the value of
"12(Cu + 30N)," the production of the intermetallic compounds in the
ferrite/austenite
phase boundary during large heat input welding cannot be sufficiently
suppressed as the
case maybe. In consideration of this point, the requirement of the above
presented
formula (2) is specified.
[0042]
The duplex stainless steel according to the present invention has the
aforementioned
chemical composition, and the balance is composed of Fe and impurities. The
impurities
as referred to herein mean the components which are mixed due to various
factors in the
production process including raw materials such as ores and scraps when the
duplex
stainless steel is industrially produced, and are tolerated within the range
not adversely
affecting the present invention.
[0043]
The duplex stainless steel according to the present invention may contain, in
addition to the aforementioned elements, one or more of the elements selected
from at
least one group of the following first to third groups.
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[0044]
First group: V: 1.5% or less
Second group: Ca, Mg, B: 0.02% or less
Third group: rare earth metal (REM): 0.2% or less
Hereinafter, these optional elements are described in detail.
[0045]
First group: V: 1.5% or less
V may be contained if necessary. V is effective in improving the corrosion
resistance (in particular, the corrosion resistance in an acidic environment)
of the duplex
stainless steel. More specifically, by containing V in combination with Mo and
Cu, the
crevice corrosion resistance can be improved. However, when the content of V
exceeds
1.5%, there is an adverse possibility that the amount of ferrite is
excessively increased, and
the toughness and the corrosion resistance are degraded; accordingly, the
content of V is
set at 1.5% or less. For the purpose of stably displaying the improvement
effect due to V
of the corrosion resistance of the duplex stainless steel, it is preferable to
contain V in a
content of 0.05% or more.
[0046]
Second group: One or more selected from among Ca: 0.02% or less, Mg: 0.02% or
less and B: 0.02% or less
One or more selected from among Ca, Mg and B may be contained if necessary.
Each of Ca, Mg and B has an effect to fix S (sulfur) and 0 (oxygen) to improve
the hot
workability. In the duplex stainless steel according to the present invention,
the content
of S is regulated so as to be low, and hence the hot workability can be
satisfactory even
when Ca, Mg or B is not contained. However, in the case where further hot
workability is
demanded under severe working conditions such as the case of the production of
seamless
pipes based on a skew rolling method, it is possible to further improve the
hot workability
of the duplex stainless steel by containing one or more selected from among
Ca, Mg and B.
On the other hand, when the content of each of these elements exceeds 0.02%,
there is an
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adverse possibility that the amount of nonmetallic inclusions (such as the
oxides and
sulfides of Ca, Mg or B) is increased and such inclusions offer the origins of
pitting and the
degradation of the corrosion resistance occurs. Accordingly, when these
elements are
contained, the content of each of these elements is set at 0.02% or less. When
two
selected from among Ca, Mg and B are contained, the upper limit of the total
content is
0.04%; and when three of Ca, Mg and B are contained, the upper limit of the
total content
is 0.06%. For the purpose of stably displaying the improvement effect of the
hot
workability due to Ca, Mg or B, it is preferable to contain Ca, Mg and B each
alone or in
total, in a content of "S(mass%) + (1/2).0(mass%)" or more.
[0047]
Third group: rare earth metal (REM): 0.2% or less
REM may be contained if necessary. Similarly to Ca, Mg and B, a rare earth
metal also has an effect to fix S or 0 to enable further improvement of the
hot workability
of the duplex stainless steel. On the other hand, when the content of the rare
earth metal
exceeds 0.2%, there is an adverse possibility that the amount of nonmetallic
inclusions
(such as the oxides and sulfides of the rare earth metal) is increased and
such inclusions
offer the origins of pitting and the degradation of the corrosion resistance
occurs.
Accordingly, when the rare earth metal is contained, the content of the rare
earth metal is
set at 0.2% or less. For the purpose of stably displaying the improvement
effect of the hot
workability due to REM, it is preferable to contain REM in a content of
"S(mass%) +
(1/2).0(mass%)" or more.
[0048]
REM as referred to herein is a generic name of the 17 elements consisting of
the 15
lanthanoid elements and Y and Sc, and one or more of these elements may be
contained.
The content of REM means the total content of such elements.
[0049]
The duplex stainless steel according to the present invention can be produced
by the
production equipment and the production method used for the usual commercial
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production. For example, for the melting of the duplex stainless steel, there
can be used
an electric furnace, an Ar-02 mixed gas bottom blowing decarburization furnace
(AOD
furnace), a vacuum decarburization furnace (VOD furnace) or the like. The
molten steel
obtained by melting may be cast into ingots, or may be cast into rod-like
billets or the like
by a continuous casting method.
[Examples]
[0050]
The duplex stainless steels (Present Inventions: Test Nos. 1 to 11, the
Comparative:
Test Nos. 12 to 25) having the chemical compositions shown in below-presented
Table 1
were melted by using a vacuum furnace of 150 kg in capacity, and cast into
ingots. Next,
each of the ingots was heated to 1250 C, and forged into a 40-mm thick plate
material.
Subsequently, each of the plate materials was again heated to 1250 C, and
rolled so as to
have a thickness of 15 mm by hot rolling (the working temperature: 1050 C or
higher);
then each of the plate materials after rolling was subjected to a solid
solution heat
treatment (a treatment of water cooling after being maintained in a soaked
manner at
1070 C for 30 minutes) to prepare a test steel plate.
[0051]
[Table 1]
- 15-
Table 1
Chemical composition
(mass%, the balance: Fe and impurities)
Test No. C , Si Mn P S Ni sol-Al N
Cr Mo Cu V Ca Mg B REM
...
1 0.015 0.50 1.51 0.010 0.0008 4.21 0.020 0.152 20.3 1.98 3.41 - -
- - -
2 0.015 0.50 1.50 0.015 0.0010 5.50 0.020 0.211 22.1 1.95 2.92 0.15 -
- - -
3 0.015 0.50 1.48 0.014 0.0007 4.51 0.020 0.181 23.2 1.97 2.08 0.07 -
- - -
w) 4 0.015 0.50 1.55 0.014 0.0008 5.09 0.020 0.156 22.9 1.05
3.15 - - - - -
=
5 0.015 0.50 1.52 0.016 0.0011 4.08 0.020 0.192 23.9 1.96 2.20 0.06 0.0015
- - -
o
..
+4 6 0.021 0.42 1.53 0.017 0.0005 5.19 0.022 0.210 24.1 1.55
2.12 - - - - -
,1)
> 7 0.017 0.51 1.52 0.012 0.0004 7.82 0.013 0.305 25.2 1.02
2.51 - - 0.0200 -
=
,-,
8 0.017 0.51 1.03 0.011 _ 0.0008 5.19 0.013 0.215 24.9 1.02 3.24 -
- - 0.0005 -
9 0.015 0.50 1.03 0.014 0.0006 5.22 0.014 0.228 26.0 0.51 2.07 - -
- - 0.0012 n
10 0.016 0.50 1.03 0.015 0.0009 5.22 0.014 0.202 27.1 0.50 2.15 0.08 -
- 0.0008 - 0
I.)
11 0.016 0.50 1.02 0.013 0.0007 5.18 0.012 0.223 27.0 0.52 3.20 0.01 -
- - 0.0010
-.3
0
12 0.016 0.49 1.52 0.011
0.0008 5.21 0.012 0.232 18.1* 1.94 _. 3.22 - - - - - u.)
-.3
13 0.016 0.50 1.55 0.015 0.0005 5.22 0.008 0.085" 20.2 1.99 2.05 - -
- - - co
i
I.)
14 a 0.015 0.49 4.90 0.014 0.0005 4.04 0.019 0.224 20.1 1.03
3.10 - - - - - _ H
15 0.016 0.46 7.11* 0.014 0.0008 2.01* 0.023 0.208 22.2 0.95 2.89 - -
- - - "
1
16 0.015 0.48 5.08* 0.015 0.0009 3.52* 0.023 0.262 23.2 0.52 3.11 - -
- - 0
I.)
c.) _
1
-' 17 0.036* 0.68 4.94 0.012 0.0004 1.49* 0.027 0.238 24.0
0.96 2.10 - - - 0
-.3
5` 18 0.015 0.48 1.02 0.011 0.0001 5.08 0.028 0.231 24.2 0.52
1.90* - - - -
2. 19 0.015 0.50 1.03 0.011 0.0005 5.02 0.032 0.302 25.1 1.05
1.15* - - -
=
.6 20 0.015 0.43 0.98 0.011 0.0003 5.06 0.019 0.148 25.1 0.51
2.10 - - - - -
21 0.015 0.49 1.03 0.016 0.0006 5.08 0.020 0.185 24.8 2.11* 1.21* - -
- - -
22 0.016 0.50 1.01 0.013 0.0005 5.56 0.019 0.182 25.1 0.11* 2.10 - -
- -
23 0.015 0.50 1.02 0.012 0.0008 6.10 0.015 0.182 26.2 0.02* 2.12 - -
- -
24 0.011 0.48 1.54 0.012 0.0009 5.12 0.020 0.155 26.7 1.04 1.55* - -
- -
25 0.014 0.49 1.56 0.015 0.0008 4.98 0.015 0.164 26.8 0.02* 2.10 - -
- -
* shows out of scope of the invention.
CA 02770378 2012-02-07
[0052]
For the purpose of evaluating the weldability of each of these test steel
plates, first
prepared were plate materials of 12 mm in thickness, 100 mm in width and 200
mm in
length, each having on a long side a V-type groove of a groove angle of 30
degrees.
Figure 1 shows a plate material 10 which is prepared by mechanical working. In
Figure 1,
(a) is a plan view and (b) is a front view.
[0053]
Next, as shown in Figure 2, for each of the test steels, two pieces of the
plate
material 10 having a shape shown in Figure 1 were prepared and arranged so as
for the
groove faces to butt each other; then, a weld joint 20 was prepared by
performing multi-
layer welding based on tungsten inert gas (TG) welding from the one side of
each of the
plate materials. Figure 2(a) is a plan view and Figure 2(b) is a front view of
the weld
joint 20. As the welding material 30 of each of the weld joints 20, a welding
material of 2
mm in outer diameter prepared from the Test No. 1 in Table 1 was used commonly
for
all the test steels. The welding was performed under the condition of the heat
input
amount of 30 kJ/cm, which was particularly highly efficient for a common
welding
working of stainless steel.
[0054]
Next, a specimen was sampled from the back side (the first layer side of the
weld
bead) of each of the weld joints 20 obtained as described above. Specifically,
under the
conditions that the penetration bead and the scales during welding were
allowed to remain,
a specimen of 2 mm in thickness, 10 mm in width and 75 mm in length was
sampled.
Figure 2 shows the region to be sampled as a specimen with a dotted line.
[0055]
Figure 3 shows an oblique perspective view of a sampled specimen 40. In the
specimen 40 shown in Figure 3, the upper surface is the rolled surface (the
lower surface of
the weld joint in Figurer 2). As shown in Figure 3, the longitudinal direction
of the
specimen 40 is a direction perpendicular to the weld line. Each of the
specimens 40 was
- 17-
CA 02770378 2012-02-07
sampled in such a way that one of the two boundary lines between the welding
material 30
and the plate material 10, on the surface (the rolled surface) of the
concerned specimen 40,
was to be located in the center of the surface of the concerned specimen 40.
[0056]
By using each of the obtained specimens, a four-point bending test was
performed.
In the four-point bending test, a stress corresponding to the yield stress of
the specimen
was applied to the specimen in a NaC1 aqueous solution (150 C) having a
concentration of
25 mass% into which CO2 at 3 MPa had been injected under pressure. The test
time of
the four-point bending test was 720 hours.
[0057]
After the four-point bending test, for each of the specimens, the
occurrence/nonoccurrence of the stress corrosion cracking was examined by
visually
observing the exterior appearance and also by the observation (magnification
of field of
vision: 500 times) with an optical microscope in the cross-sectional direction
(the direction
perpendicular to the upper surface of the specimen in Figure 3). The results
of the
observation are shown in Table 2. In Table 2, the cases where no stress
corrosion
cracking occurred are marked with "0" and the cases where the stress corrosion
cracking
occurred are marked with "x."
[0058]
[Table 2]
- 18-
Table 2
Test No. 2.2Cr+7Mod-3Cu Stress corrosion cracking (Cr+111V1o+10Ni)-12(Cu+30N)
Precipitation of the sigma phase
. .
1 68.75 0 -11.46
0
2 71.03 0 -12.45
0
3 71.07 0 -0.15
0
4 67.18 0 -8.61
0
m
=
0 5 72.9 0 -9.26
0
.-
-5 6 70.23 0 -7.99
0
Q
0
7 70.11 0 -25.3
=
.
8 71.64 0 -28.26
0
9 66.98 0 -23.11
0
. 69.57 0 -13.72
0 (-)
11 72.64 0 -34.16
0 0
12 63.06* x -30.62
0 I.)
-.1
-.1
13 64.52* x 39.09*
x 0
u.)
14 60.73* x -46.01
0
CO
i
15 64.16* x -56.81
0 I.)
0
i * 16 64.01 x -
67.52 0 H
0.)
N
I
-,-; 17 65.82* x -
61.42 0 0
E 18 62.58* x -
25.24 0 I.)
1
0
2-, 19 66.02 x -
35.67 0
E
L5 20 65.09* x 2.83*
x
21 72.96 x 17.69*
x
22 62.29* x -8.81
0
23 64.14* x -3.54
0
24 70.67 x 14.94*
x
25 65.4* x -7.42
0
* shows out of scope of the invention.
CA 02770378 2012-02-07
[0059]
In each of the weld joints (see Figure 2), the cross-section perpendicular to
the weld
line and the rolled surface was mirror polished and etched, and then an image
analysis of
the cross-section was performed by using an optical microscope with a
magnification of
field of vision of 500 times. Thus, the area fraction of a trace amount of the
sigma phase
in HAZ (weld heat affected zone) was measured, and the case where the area
fraction of
the sigma phase is 1% or more was determined that the precipitation of the
sigma phase
occurred. The determination results are shown in Table 2. In Table 2, the
cases
determined that no precipitation of the sigma phase occurred are marked with
"0" and the
cases determined that the precipitation of the sigma phase occurred are marked
with "x."
[0060]
Figure 4 is a graph showing the relation between " 7Mo (mass%) + 3Cu (mass%)"
and "Cr (mass%)" for the duplex stainless steels of Test Nos. 1, 4, 6, 13 and
20. In this
connection, as shown in Table 2, no stress corrosion cracking occurred in the
specimens
prepared from the duplex stainless steels of Test Nos. 1, 4 and 6, whereas the
stress
corrosion cracking occurred in the specimens prepared from the duplex
stainless steels of
Test Nos. 13 and 20. Accordingly, as shown in Figure 4, when a border line is
drawn
between the " 7Mo (mass%) + 3Cu (mass%)" values of the duplex stainless steels
of Test
Nos. 1, 4 and 6 and the " 7Mo (mass%) + 3Cu (mass%)" values of the duplex
stainless
steels of Test Nos. 13 and 20, the border line is represented by the following
formula (3):
7Mo (mass%)+ 3Cu (mass%) = -2.2Cr (mass%) + 66 (3)
[0061]
From the relation shown in Figure 4, it can be seen that in the case where the
" 7Mo
+ 3Cu" value is larger than the "-2.2Cr + 66" value, namely, the case where
the duplex
stainless steel satisfies the relation of the aforementioned formula (1), it
is possible to
prevent the occurrence of the stress corrosion cracking. In other words, as
shown in
Tables 1 and 2, no stress corrosion cracking occurred in the specimens
prepared from the
duplex stainless steels of Test Nos. 1 to 11 in which the requirements for the
chemical
- 20 -
CA 02770378 2012-02-07
composition specified in the present invention and the aforementioned relation
of formula
(1) were satisfied. On the other hand, the stress corrosion cracking occurred
in the
specimens prepared from the duplex stainless steels of Test Nos. 12 to 18, 20,
22, 23 and
25. The
stress corrosion cracking occurred in these duplex stainless steels of Test
Nos. 19,
21 and 24 probably because the duplex stainless steels of Test Nos. 19, 21 and
24 satisfied
the relation of formula (1), but the contents of Cu (see Table 1) in these
duplex stainless
steels did not satisfy the requirement of the present invention.
[0062]
Also, as shown in Table 2, no trace amount of the sigma phase precipitated in
the
HAZ in the weld joints prepared from the duplex stainless steels of Test Nos.
1 to 12, 14 to
19, 22, 23 and 25 satisfying the aforementioned relation of formula (2). On
the other
hand, a trace amount of the sigma phase precipitated in each of the weld
joints prepared
from the duplex stainless steels of Test Nos. 13, 20, 21 and 24 not satisfying
the relation of
formula (2).
[0063]
As can be seen clearly from the above-described results, the duplex stainless
steels
satisfying the requirements of the present invention can suppress the
precipitation of the
intermetallic compounds during large heat input welding, and each have an
excellent stress
corrosion cracking resistance in chloride environments.
[Industrial Applicability]
[0064]
The duplex stainless steels according to the present invention are excellent
in
weldability during large heat input welding and excellent in the stress
corrosion cracking
resistance in chloride environments.
[Reference Signs List]
[0065]
-21 -
CA 02770378 2012-02-07
10: Plate material
20: Weld joint
30: Welding material
40: Specimen
- 22 -
