Description
Title of Invention: DUPLEX STAINLESS STEEL
Technical Field
[00011
The present invention relates to stainless steel,
more specifically, to duplex stainless steel.
Background Art
[00021
Oil and natural gas produced from oil fields and gas
fields contain associated gas. The associated gas
contains corrosive gas, such as carbon dioxide gas (COz)
and/or hydrogen sulfide (HzS). Line pipes transport oil
and natural gas containing the above corrosive gas.
Consequently, in line pipes, stress corrosion cracking
(SCC) , sulfide stress cracking (SSC) , and general
corrosion cracking account for reduction in wall
thickness may cause problems in some cases.
[00031
SCC and SSC cause rapid propagation of the cracking.
Hence, SCC and SSC penetrate line pipes in a short time
since they occur. In addition, SCC and SSC occur locally.
For theses reasons, corrosion resistance, particularly,
SCC resistance and SSC resistance are required in steel
material for use in line pipes.
- 2 -
[ 0 0 04 I
Duplex stainless steel has high corrosion resistance.
Hence, duplex stainless steel is used as steel for line
pipes.
[00051
High strengthening of steel pipes attains reduction
in wall thickness of the steel pipes for the line pipes,
resulting in reduction in production cost. In this sense,
high strengthening is required in the duplex stainless
steel for use in the line pipes. JP 2003-171743A (Patent
Literature 1) and JP 5-132741A (Patent Literature 2)
suggest duplex stainless steel having high strength.
[0006]
Patent Literature 1 discloses the following: the
duplex stainless steel of Patent Literature 1 contains Mo
of at least 2.00% as well as W. Solid-solution
strengthening of Mo and W enhances strength of the duplex
stainless steel. The duplex stainless steel of Patent
Literature 1 contains Cr of 22.00 to 28.00%, and Ni of
3.00 to 5.00%. This configuration enhances corrosion
resistance of the duplex stainless steel.
[00071
Patent Literature 2 discloses the following: the
duplex stainless steel of Patent Literature 2 contains Mo
of at least 2.00% as well as W. In the duplex stainless
steel, PREW = Cr + 3.3 (Mo + 0.5W) + 16N is at least 40.
The contents of Mo and W enhance strength of the duplex
stainless steel. PREW of at least 40 enhances corrosion
resistance of the duplex stainless, as well.
Disclosure of the Invention
[0008]
Unfortunately, each duplex stainless steel disclosed
in Patent Literature 1 and Patent Literature 2 has a high
content of Mo. If the Mo content is high, a sigma phase
(o phase) is likely to be generated. The o phase
precipitates during producing and welding the steel. The
o phase is hard and brittle, which reduces toughness and
corrosion resistance of the duplex stainless steel.
Particularly, steel pipes for used in line pipes are
welded on the site where the line pipes are installed.
Hence, it is preferable to suppress precipitation of the
o phase particularly in the duplex stainless steel for
use in line pipes.
[00091
As described above, high SCC resistance and high SSC
resistance are required in an environment having
accompanied gas containing carbon dioxide gas and/or
hydrogen sulfide (referred to as a I1chloride
environment,I1 hereinafter). Oil fields and gas fields
that have been recently developed are located at a deep
level. Oil fields and gas fields located at a deep level
have a chloride environment whose temperature is 80°C to
150°C. Consequently, in the duplex stainless steel for
use in line pipes, SCC resistance and SSC resistance
excellent even in such a high-temperature chloride
environment are required.
[OOlOI
An object of the present invention is to provide
duplex stainless steel having high strength, SCC
resistance and SSC resistance excellent in a hightemperature
chloride environment, and capable of
suppressing precipitation of the o phase.
[OOllI
Duplex stainless steel according to the present
invention comprises, in mass%, C: at most 0.03%; Si: 0.2
to 1%; Mn: more than 5.0% to at most 10%; P: at most
0.040%; S: at most 0.010%; Ni: 4.5 to 8%; sol. Al: at
most 0.040%; N: more than 0.2% to at most 0.4%; Cr: 24 to
29%; Mo: 0.5 to less than 1.5%; Cu: 1.5 to 3.5%; W: 0.05
to 0.2%; the balance being Fe and impurities, and
satisfies Formula (1): Cr + 8Ni + Cu + Mo + W/2 2
65 ... (I), where a symbol of each element in Formula (1)
represents a content of the element (in mass%).
[00121
The duplex stainless steel according to the present
invention has high strength, and SCC resistance and SSC
resistance excellent in a high-temperature chloride
environment. In addition, precipitation of the o phase
is suppressed.
[00131
The aforementioned duplex stainless steel may
further comprise V: at most 1.5% instead of part of Fe.
[00141
The aforementioned duplex stainless steel may
further comprise one or more types selected from a group
of Ca: at most 0.02%, Mg: at most 0.02%, and B: at most
0.02% instead of part of Fe.
Brief Description of Drawings
[00151
[Figure 11 Figure 1 is a drawing showing a relation among
a Mn content, yield strength and precipitation of a o
phase in duplex stainless steel.
[Figure 21 Figure 2 is a drawing showing a relation among
a Mo content, the yield strength and precipitation of the
o phase in the duplex stainless steel.
[Figure 31 Figure 3 is a drawing showing a relation among
the Mn content, F1 = Cr + 8Ni + Cu + Mo + W/2, and SCC
resistance.
[Figure 4A] Figure 4A is a plan view of a plate material
produced in Example.
[Figure 4Bl Figure 4B is a front view of the plate
material shown in Figure 4A.
[Figure 5A] Figure 5A is a plan view of a welded joint
produced in Example.
[Figure 5B] Figure 5B is a front view of the welded joint
shown in Figure 5A.
Best Mode for Carrying Out the Invention
[00161
Hereinafter, an embodiment of the present invention
will be described in detail with reference to drawings.
Same or equivalent components in the drawings are denoted
with the same reference numerals, and repeated
explanation thereof is omitted. A symbol 11%" for a
content of each element means mass% unless otherwise
mentioned.
[00171
The present inventors have conducted investigations
and studies on strength, SCC resistance and SSC
resistance in a high-temperature chloride environment,
and suppression of a o phase precipitation of duplex
stainless steel. As a result, the present inventors have
obtained the following findings.
[0018]
(A) Mo enhances strength of steel, but encourages
precipitation of the o phase. Hence, it is preferable to
suppress the Mo content to be as small as possible. W is
expensive, and thus it is also preferable to suppress the
W content to be as small as possible.
[0019]
(B) As the Mo content and the W content are more
reduced, the strength of the duplex stainless steel
becomes more reduced. Hence, instead of increasing the
- 7 -
Mo content and the W content, the Mn content is increased
so as to enhance the strength of the duplex stainless
steel.
[00201
Figure 1 is a drawing showing a relation among the
Mn content, the yield strength, and the o phase
precipitation. Figure 2 is a drawing showing a relation
among the Mo content, the yield strength, and the o phase
precipitation. Figure 1 and Figure 2 are obtained based
on a tensile test and a o-phase area ratio measurement
test in Example 1 and in Example 3, as described later.
In Figure 1 and Figure 2, open marks l1Ol1 indicate that
no o phase was observed in the o-phase area ratio
measurement test, and solid marks I1@l1 indicate that the
o phase was observed.
[00211
With reference to Figure 1 and Figure 2, as the Mo
content becomes higher, the yield strength becomes
greater, and similarly, as the Mn content becomes higher,
the yield strength becomes greater in the duplex
stainless steel. If the Mn content is more than 5.0%,
the yield strength of the duplex stainless steel becomes
at least 550 MPa, resulting in high strength.
[0022]
If the Mo content is high, the o phase is observed
in the duplex stainless steel; to the contrary, no o
phase is observed in the duplex stainless steel even if
the Mn content is high. Hence, the Mn content of more
than 5.0% enhances strength of the duplex stainless, and
also suppresses generation of the o phase instead of
using Mo and W.
[00231
(C) If the Mn content is more than 5.0%, a corrosion
film formed on a surface of the duplex stainless steel
becomes unstable in the high-temperature chloride
environment. If the corrosion film becomes unstable, the
SCC resistance becomes deteriorated in the hightemperature
chloride environment.
[00241
In order to enhance the SCC resistance of the duplex
stainless steel having the Mn content of more than 5.0%,
the Ni content is defined to be at least 4.5%. Ni is
effective for stabilizing the corrosion film in the
duplex stainless steel having the Mn content of more than
5.0%. The Ni content of at least 4.5% enhances the SCC
resistance of the duplex stainless steel having the Mn
content of more than 5.0%.
[0025]
(D) In order to enhance the SCC resistance of the
duplex stainless steel having the Mn content of more than
5.0%, the duplex stainless steel preferably satisfies the
following Formula (1) in addition to the above (C) .
Cr + 8Ni + Cu + Mo + W/2 2 65 ... (I),
where, a symbol of each element in Formula (1) represents
mass% of the element.
[00261
All of Cr, Ni, Mo, and W stabilize the corrosion
film. F1 is defined to be F1 = Cr + 8Ni + Cu + Mo + ~/2.
If F1 satisfies Formula (I), a stable corrosion film can
be formed even if the Mn content is more than 5.0%.
Hence, the SCC resistance of the duplex stainless steel
becomes high.
[0027]
Figure 3 is a drawing showing a relation among the
Mn content, F1, and the SCC resistance. Figure 3 was
obtained based on the result of the SCC test in Example 3
described later. In Figure 3, open marks 1 1 0 1 1 indicate
that no SCC was observed, and solid marks ll@" indicate
that SCC was observed.
[OOZ~I
With reference to Figure 3, in the duplex stainless
steel having the Mn content of more than 5.0%, if F1 is
at least 65, excellent SCC resistance can be attained
without relying on the content of Mn. On the other hand,
if F1 value is less than 65, SCC occurs in the duplex
stainless steel having the Mn content of at least 5.0%.
Hence, in the case of the duplex stainless steel having
the Mn content of at least 5.0%, excellent SCC resistance
can be attained by satisfying Formula (1).
[0029]
Based on the above findings, the present inventors
have completed the duplex stainless steel according to
the present embodiment. Hereinafter, the duplex
stainless steel according to the present embodiment will
be described in detail.
[00301
[Chemical Composition]
The duplex stainless steel according to the present
invention includes the following chemical composition.
[00311
C: at most 0.03%
Carbon (C) stabilizes an austenite phase in the
steel, as similar to Nitrogen (N). On the other hand, if
the C content is excessively high, coarse carbide is
likely to precipitate, and the corrosion resistance of
the steel, particularly, the SCC resistance thereof
becomes deteriorated. Accordingly, the C content is
defined to be at most 0.03%. The upper limit of the C
content is preferably less than 0.03%, more preferably
0.02%, and further more preferably less than 0.02%.
[00321
Si: 0.2 to 1%
Silicon (Si) secures flowability of welding metal at
the time of welding the duplex stainless steel to each
other. Hence, generation of weld defects is suppressed.
On the other hand, an excessively high Si content
generates intermetallic compound represented by the a
- 11 -
phase. Accordingly, the Si content is defined to be 0.2
to 1%. The lower limit of the Si content is preferably
more than 0.2%, more preferably 0.35%, and further more
preferably 0.40%. The upper limit of the Si content is
preferably less than I%, more preferably 0.80%, and more
preferably 0.65%.
[00331
Mn: more than 5.0% to at most 10%.
Manganese (Mn) enhances solubility of N in the steel.
Hence, Mn suppresses precipitation of the o phase as well
as enhances strength of the steel. On the other hand, if
the Mn content is excessively high, the corrosion
resistance (SSC resistance and SCC resistance) of the
steel becomes deteriorated. Hence, the Mn content is
defined to be more than 5.0% to at most 10%. The lower
limit of the Mn content is preferably 5.5%, and more
preferably more than 6.0%. The preferable upper limit of
the Mn content is less than 10%.
[00341
P: at most 0.040%
Phosphorus (P) is an impurity. P deteriorates the
corrosion resistance and toughness of the steel. Hence,
the P content is preferably as small as possible. The P
content is defined to be at most 0.040%. The P content
is preferably less than 0.040%, more preferably at most
0.030%, and further more preferably at most 0.020%.
[0035]
S: at most 0.010%
Sulfur (S) is an impurity. S deteriorates hot
workability of the steel. S generates sulfide, which
initiates pitting. Accordingly, the S content is
preferably as small as possible. The S content is
defined to be at most 0.010%. The S content is
preferably less than 0.010%, more preferably at most
0.007%, and further more preferably at most 0.002%.
[0036]
Ni: 4.5 to 8%
Nickel (Ni) stabilizes the austenite phase in the
steel. Ni enhances the corrosion resistance of the steel,
as well. In the case of the Mn content of more than 5.0%
as similar to the present embodiment, Ni stabilizes the
corrosion film of the steel in the high-temperature
chloride environment. On the other hand, the excessively
high Ni content reduces the ratio of the ferrite phase in
the duplex stainless steel. The intermetallic compound
represented by the o phase significantly precipitates, as
well. Accordingly, the Ni content is defined to be 4.5%
to 8%. The lower limit of the Ni content is preferably
more than 4.5%, and more preferably more than 5%. The
upper limit of the Ni content is preferably less than 8%,
more preferably 7%, and further more preferably 6.5%.
[00371
Sol. Al: at most 0.040%
Aluminum (Al) deoxidizes the steel. On the other
hand, if the A1 content is excessively high, A1 combines
with N in the steel to generate AlN, which deteriorates
the corrosion resistance and the toughness of the steel.
Accordingly, the A1 content is defined to be at most
0.040%. The preferable lower limit of the A1 content is
0.005%. The upper limit of the A1 content is preferably
less than 0.040%, more preferably 0.030%, and further
more preferably 0.020%. In the present embodiment, the
A1 content denotes a content of acid-soluble A1 (Sol. Al).
[0038]
N: more than 0.2% to at most 0.4%
Nitrogen (N) is a strong austenite former, and N
enhances thermal stability, strength, and corrosion
resistance (particularly pitting resistance) of the
duplex stainless steel. On the other hand, an
excessively high N content is likely to cause blow holes
that are welding defects. In addition, coarse nitride is
generated due to thermal influence at the time of welding,
which deteriorates the toughness and the corrosion
resistance of the steel. Accordingly, the N content is
defined to be more than 0.2% to at most 0.4%. The upper
limit of the N content is preferably less than 0.4%, more
preferably 0.35%, and further more preferably 0.30%.
[00391
Cr: 24 to 29%
Chrome (Cr) enhances the corrosion resistance of the
steel, and particularly enhances the SCC resistance
thereof in the chloride environment. On the other hand,
if the Cr content is excessively high, intermetallic
compound represented by the o phase significantly
precipitates, which deteriorates hot workability and
weldability of the steel. Accordingly, the Cr content is
defined to be 24 to 29%. The lower limit of the Cr
content is preferably more than 24%, more preferably
24.5%, and further more preferably 25%. The preferable
upper limit of the Cr content is less than 29%.
[0040]
Mo: 0.5 to less than 1.5%
Molybdenum (Mo) enhances the SSC resistance and the
SCC resistance of the steel, and particularly enhances
the SSC resistance thereof. On the other hand, if the Mo
content is excessively high, intermetallic compound
represented by the o phase significantly precipitates.
Accordingly, the Mo content is defined to be 0.5 to less
than 1.5%. The lower limit of the Mo content is
preferably more than 0.5%, more preferably 0.7%, and
further more preferably 0.8%. The upper limit of the Mo
content is preferably 1.4%, and more preferably 1.2%.
[0041]
Cu: 1.5 to 3.5%
Copper (Cu) strengthens a passivation film in the
high-temperature chloride environment, and enhances the
SCC resistance of the steel. Cu also suppresses
generation of the o phase at a boundary between a ferrite
phase and an austenite phase. Specifically, extremely
refined Cu precipitates in matrixes at the time of high
heat input welding. Precipitating Cu becomes a site for
nucleation of the o phase. The precipitating Cu competes
with the boundary between the ferrite phase and the
austenite phase that is the original nucleation site of
the o phase. Consequently, the precipitation of the o
phase is suppressed at the boundary between the ferrite
phase and the austenite phase. Cu enhances the strength
of the steel. On the other hand, an excessively high Cu
content rather deteriorates the hot workability of the
steel. Accordingly, the Cu content is defined to be 1.5
to 3.5%. The lower limit of the Cu content is preferably
more than 1.5%, and more preferably 2.0%. The upper
limit of the Cu content is preferably less than 3.5%, and
more preferably 3.0%.
roo421
W: 0.05 to 0.2%
Tungsten (W) enhances the SSC resistance and the SCC
resistance of the steel. On the other hand, an
excessively high W content rather saturates this effect,
resulting in increase in production cost. Accordingly,
the W content is defined to be 0.05% to 0.2%. The lower
limit of the W content is preferably more than 0.05%.
The upper limit of the W content is preferably less than
0.2%, and more preferably 0.15%.
[00431
The balance of the duplex stainless steel according
to the present embodiment consists of iron (Fe) and
impurities. The impurities herein denotes elements mixed
from minerals or scraps used as row materials of the
steel, or through an environment of the manufacturing
process, and the like.
[00441
The duplex stainless steel according to the present
embodiment may further comprise V instead of part of Fe.
[00451
V: at most 1.5%
Vanadium (V) is an selective element. V enhances
the corrosion resistance of the steel, and particularly
enhances the corrosion resistance of the steel in an
acidic environment. Even a slight content of V can
attain this effect. On the other hand, an excessively
high V content extremely increases the ratio of the
ferrite phase in the steel, resulting in deterioration of
the toughness and the corrosion resistance. Accordingly,
the V content is defined to be at most 1.5%. The
preferable lower limit of the V content is 0.05%.
[00461
The duplex stainless steel of the present embodiment
further comprises one or more types of elements selected
- 17 -
from a group of Ca, Mg, and B instead of part of Fe. Ca,
Mg, and B enhance the hot workability of the steel.
[00471
Ca: at most 0.02%
Mg: at most 0.02%
B: at most 0.02%
Calcium (Ca) , magnesium (Mg), and boron (B) are all
selective elements. All of Ca, Mg, and B enhance the hot
workability of the steel. For example, at the time of
producing a seamless steel pipe through the skew rolling
process, high hot workability is required. In such a
case, if one or more of Ca, Mg, and B are contained, the
hot workability of the steel is enhanced. Even a slight
content of any of these elements can attain this effect.
On the other hand, if one or more of these elements has
an excessively high content, oxide, sulfide, and
intermetallic compound in the steel become increased.
Oxide, sulfide, and intermetallic compound initiate
pitting, which deteriorates the corrosion resistance of
the steel. Accordingly, the Ca content is defined to be
at most 0.02%, the Mg content is defined to be at most
0.02%, and the B content is defined to be at most 0.02%.
[00481
Each preferable lower limit of the Ca content, the
Mg content, and the B content is 0.0001%. Each upper
limit of the Ca content, the Mg content, and the B
content is preferably less than 0.02%, more preferably
0.010%, and further more preferably 0.0050%.
[00491
[Formula (1) 1
The chemical composition of the duplex stainless
steel according to the present embodiment further
satisfies Formula (1) .
Cr + 8Ni + Cu + Mo + W/2 2 65 ... (I),
where a symbol of each element in Formula (1) represents
a content of the element (in mass%).
[00501
All of Cr, Ni, Cu, Mo, and W stabilize the corrosion
film of the duplex stainless steel having the Mn content
of more than 5.0% in the high-temperature chloride
environment. Ni stabilizes the corrosion film the most
among these elements. Accordingly, the Ni content is
multiplied by a coefficient of I18l1. Meanwhile, W has a
small contribution ratio of stabilizing the corrosion
film. Hence, the W content is multiplied by a
coefficient of n1/21.1
[00511
As shown in Figure 3, if F1 = Cr + 8Ni + Cu + Mo +
W/2 is at least 65, the SCC resistance is enhanced in the
duplex stainless steel having the Mn content of more than
5.0%. On the other hand, if F1 is less than 65, the SCC
resistance is reduced in the duplex stainless steel
having the Mn content of more than 5.0% in the hightemperature
chloride environment.
[0052]
[Yield Strength]
The yield strength of the duplex stainless steel
according to the present invention is at least 550 MPa.
The yield strength is defined by a 0.2% proof stress. In
the duplex stainless steel according to the present
invention, while the contents of Mo and W that are
elements for enhancing the strength are reduced, Mn that
is also an element for enhancing the strength is
contained at a content of more than 5.0%. Accordingly,
it is possible to attain high strength of at least 550
MPa .
[00531
[Producing Method]
A producing method of the duplex stainless steel
according to the present invention will be described,
hereinafter. Duplex stainless steel is melted, which has
the aforementioned chemical composition and satisfies
Formula (1). The duplex stainless steel may be melted
using an electric furnace, or using an Ar-O2 gaseousmixture
bottom blowing decarburization furnace (AOD
furnace). The duplex stainless steel may be melted using
a vacuum oxygen decarburization furnace (VOD furnace).
The melted duplex stainless steel may be produced into an
ingot through the ingot-making process, or may be
produced into a cast piece (slab, bloom, or billet)
through the continuous casting process.
[0054]
A duplex stainless steel material is produced using
the produced ingot or cast piece. The duplex stainless
steel material is a duplex stainless steel plate or a
duplex stainless steel pipe, for example.
[0055]
The duplex stainless steel plate may be produced in
the following manner, for example. The produced ingot or
slab is subjected to hot working so as to produce a
duplex stainless steel plate. The hot working is hot
forging or hot rolling, for example.
[00561
The duplex stainless steel pipe may be produced in
the following manner, for example. Each produced ingot,
slab, or bloom is subjected to hot working to produce a
billet. The produced billet is subjected to hot working
to produce a duplex stainless steel pipe. The hot
working is piercing rolling with the Mannesmann process,
for example. As the hot working, hot extrusion or hot
forging may be carried out, instead. The produced duplex
stainless steel pipe may be a seamless steel pipe or a
welded steel pipe.
[00571
If the duplex stainless steel pipe is a welded steel
pipe, the above duplex stainless steel plate may be bent
into an open pipe, for example. Both the longitudinal
ends of the open pipe are welded using a well-known
method, such as a submerged arc welding or the like,
thereby producing a welded steel pipe.
[00581
The produced duplex stainless steel material is
subjected to solid solution heat treatment. Specifically,
the duplex stainless steel material is charged in a heat
treatment furnace, and is soaked at a well-known solid
solution heat treatment temperature (900 to 1200°C).
After the soaking, the duplex stainless steel material is
rapidly cooled by water cooling or the like.
[00591
In the above manner, the duplex stainless steel
material is produced. The produced duplex stainless
steel material has a yield strength of at least 550 Mpa.
The duplex stainless steel material according to the
present embodiment is an as-solid-solution heat-treated
material.
Example 1
[0060]
Duplex stainless steel plates including multiple
kinds of chemical compositions were produced, and
evaluations of the yield strength and the a phase
susceptibility were conducted on each produced duplex
stainless steel plate.
[00611
[Test Method]
Each molten steel of the marks A to K having each
chemical composition shown in Table 1 was produced using
the vacuum furnace. An ingot was produced from each
produced motel steel. The weight of each ingot was 150
kg -
[0062]
[Table 11

[0063]
F1 values (left side of Formula (1)) are recorded in
the column "Flu of Table 1.
[0064]
Each ingot was heated at 1250°C. The heated ingot
was hot-forged into a steel plate having a thickness of
40 mm. Each steel plate was heated at 1250°C. The heated
steel plate was hot-rolled into a steel plate having a
thickness of 15 mm.
[0065]
Each produced steel plate was subjected to solid
solution heat treatment so as to produce a specimen steel
plate. Specifically, each steel plate was soaked at a
temperature of 1025 to 1070°C for 30 minutes, and
thereafter, the soaked steel plate was cooled with water.
Each specimen steel plate was produced in the above
manner.
[0066]
[Tensile Test]
A round tensile specimen was collected from the
specimen steel plate of each mark. Each round tensile
specimen had a diameter of 4 mm in its straight portion,
and a length of 20 mm. The longitudinal direction of the
round tensile specimen was vertical to the rolling
direction of the specimen steel plate. Each round
tensile specimen was subjected to a tensile test at a
normal temperature (25OC) so as to measure the yield
strength (MPa). The 0.2% proof stress was defined as the
yield strength.
[0067]
[o-phase Area Ratio Measurement Test]
Generally, it is said that the o phase precipitates
at a temperature of 850 to 900°C. Accordingly , the o
phase susceptibility was evaluated for the specimen steel
plate of each mark in the following manner. Each
specimen steel plate was soaked at a temperature of 900°C
for ten minutes. A specimen having a surface vertical to
the rolling direction of the specimen steel plate
(ref erred to as a llobservations urfa ce1I,h ereinafter) was
collected from each soaked specimen steel plate. The
observation surface of each collected specimen was
mirror-polished as well as etched.
[ 0 0 6 8 I
Using an optical microscope with 500x magnification,
any four fields were selected in the etched cross section,
and image analysis was made on each field. An area of
each filed used in the image analysis was approximately
4000 pm2. The area ratio (%) of the o phase in each
field was found through the image analysis. An average
area ratio (%) obtained in the four fields was defined as
the area ratio (%) of the o phase in the specimen steel
plate of each mark. If the area ratio of the o phase was
at least 1%, it was determined that the o phase
precipitated. If the area ratio of the o phase was less
than 1%, it was determined that no o phase precipitated.
[Test Result]
Table 2 shows the test result.
[Table 21
- -
[00711
In Table 2, the column "YS (MPa)" shows the yield
strength (MPa) of the specimen steel plate of each mark.
The column I1o-phases u~ceptibilitys~h~o ws the result of
the o-phase area ratio measurement test of the specimen
steel plate of each mark. I1NFn indicates that it was
determined that no o phase precipitated. IVF" indicates
that it was determined that the o phase precipitated.
- 27 -
[0072]
With reference to Table 2, each chemical composition
of the marks A to F was within the range of the chemical
composition of the present invention, and also each F1
value satisfied Formula (1). Hence, the yield strength
of each specimen material of the marks A to F was at
least 550 MPa, and no o phase precipitated.
[00731
To the contrary, each Mn content of the marks G and
H was less than the lower limit of the Mn content of the
present invention. Hence, each yield strength of the
marks G and H was less than 550 MPa.
[00741
Each Mn content of the marks I to K was less than
the lower limit of the Mn content of the present
invention. In addition, each Mo content of the marks I
to K was more than the upper limit of the Mo content of
the present invention. Hence, although each yield
strength of the marks I to K was at least 550 MPa, the o
phase precipitated in all the specimen steel plates of
the marks I to K.
Example 2
[00751
A welded joint was produced using each specimen
steel plate of the marks C and D, and the marks I and J,
and the o phase susceptibility was evaluated for each
welded joint .
[0076]
[Test Method]
Four plate materials 10 shown in Figure 4A and
Figure 4B were produced from each specimen steel plate of
the marks C, Dl I, and J. Figure 4A is a plan view of
each plate material 10, and Figure 4B is a front view of
each plate material 10. In Figure 4A and Figure 4B, each
numerical value to which llmmui s attached denotes a
dimension (unit : mm) .
[00771
As shown in Figure 4A, and Figure 4B, each plate
material 10 had a thickness of 12mm, a width of 100 mm,
and a length of 200 mm. The plate material 10 had a Vtype
groove face 11 whose groove angle was .30° at the
longer side. Each plate material 10 was produced through
machining.
[0078]
Two of the produced plate materials 10 were disposed
such that the V-type groove surface 11 of one plate
material 10 opposed that of the other plate material 10.
The two plate materials 10 were welded through the TIG
welding, and two welded joints 20 shown in Figure 5A and
Figure 5B were produced for each mark. Figure 5A is a
plan view of the welded joint 20, and Figure 5B is a
front view of the welded joint 20. Each welded joint 20
included a front face 21, and a back face 21, and also
included a welded portion 30 at its central portion. The
welded portion 30 was formed from the front face 21
through the multi-layer welding so as to extend in the
longitudinal direction of the plate material 10. The
welded portion 30 of each mark had each chemical
composition shown in Table 3, and was formed using a
welding material having an outer diameter of 2 mm.
[00791
[Table 31
[ 0 0 8 0 I
Of the two welded joints 20 of each mark, one welded
joint 20 had heat input of 15 kJ/cm in the TIG welding.
The other welded joint 20 had heat input of 35 kJ/cm in
the TIG welding.
[0081]
[o-phase Area Ratio Measurement Test]
The welded joint 20 of each test number was cut in
the longitudinal direction of the welded portion 30, and
also in the vertical direction to the front face 21.
After the cutting, the cross section of the welded joint
20 was mirror-polished, and etched. After the etching,
using the optical microscope with 500x magnification,
- 30 -
Chemical Composition (Unit: Mass%, Balance: Fe and Impurities)
B
0.0013
Cu
0.5
W
2.02
sol.Al
0.003
Ni
9.3
C
0.02
Cr
25.3
Mn
0.52
Si
0.31
Mo
2.95
P
0.007
S
0.002
four fields were selected in a welding heat affected zone
(HAZ) in the vicinity of the welded portion included in
the etched cross section, and image analysis was
conducted on each field. The area of each filed used in
the image analysis was approximately 40000pm2. The area
ratio (%) of the o phase in each field (HAZ) was found
through the image analysis. The average area ratio (%)
in these four fields was defined as the area ratio (%) of
the o phase within the HAZ of the test number of interest.
If the area ratio of the o phase was at least I%, it was
determined that the o phase precipitated. If the area
ratio of the o phase was less than I%, it was determined
that no o phase precipitated.
[00821
[Test ~esult]
Table 4 shows the test result.
[0083]
[Table 41
TABLE4
- -
Comparative
Example Steel
Category
Inventive
Example Steel
Mark
C
D
Heat Input
I
J
15kJ/cm
NF
NF
35kJ/cm
NF
NF
F
F
F
F
In Table 4, the column "15 kJ/cml1 in the column
"Heat InputH shows the test result of each mark whose
heat input of the TIG welding was 15 k~/cm. The column
"35 kJ/cmH in the column "Heat Inputn shows the test
result of each mark whose heat input of the TIG welding
was 35 kJ/cm. "NFH in each column indicates that the
area ratio of the o phase was less than I%, and no o
phase precipitated. "FU in each column indicates that
the area ratio of the o phase was at least I%, and the o
phase precipitated.
[0085]
With reference to Table 4, the chemical compositions
of the mark C and the mark D were within the range of the
chemical composition of the present invention, and the F1
value satisfied Formula (1). Hence, no o phase
precipitated in the HAZ at the both heat inputs of the
TIG welding (15 kJ/cm and 35kJ/cm).
[0086]
To the contrary, each Mo content of the mark I and
the mark J was more than the upper limit of the Mo
content of the present invention. Hence, the o phase
precipitated in the HAZ at each heat input of the TIG
welding (15 kJ/cm, and 35k~/cm).
Example 3
[00871
As similar to Example 1, multiple duplex stainless
steel plates having multiple types of chemical
compositions were produced. The yield strength, the
existence of the o phase, the SSC resistance, and the SCC
resistance were evaluated for each of the produced duplex
stainless steel plates.
[0088]
[Test Method]
Each molten steel of the marks A to L, the marks M
to Z, and the marks AA to AC having each chemical
composition shown in Table 5 was produced using a vacuum
furnace. An ingot was produced from each molten steel.
The mass of each ingot was 150 kg.
[0089]
[Table 51
[0090]
A specimen steel plate of each mark was produced
under the same producing condition as that of Example 1.
- 34 -
g m a I-
- LL
-
E
9
-c .
O O O N a D
z q0 q0 q0 q0 q0
> rz rz rz rz O
a D a D W r c D r m g g g g g g g b 0 0 0 0 0 0 0
~ & o o ? ~ o ' a 9 k 9 9 ~ o 6 o q o ? o
m w m m m w a a a a a m b m w w m $ N N N N N N N N N N N N N N N N N
o o o ~ ~ a ~ m a ~ a ~ m a ~
8 8 8 8 8 , 8 ,
-
.-
-
.0-
i . - a a m r r n c c b c b c o
2
b
3
8 t F 5 0 0
~ O r m r O m O r m O r
f
r 5
S
" ? 9 ' f ? Y k 9 9 9 9 9 k b Y
c n a o c n c n c n ~ o o o o r c n m ~
CDCDCDCDCDbbbbbbCDaCO
a0D
8
0
- = 8 % ' $ % S 8 8 8 S 8 8 7 8 O r N r
~ ° C 4 ~ C 4 ~ ~ ~ ~ ~ G z m w m m * * m m m m m m r +
; a s z q q ~ g ~ ~ a a s s
8
< m O O Y u J Z Z O L O E ( D + 3 > ~ X ~ N $ $ $
soia5l ij c E c
$-! 3 G
, ,
,
0 8 F 8 S Z Z t 3 F F F G ? F o
q q q . - q q q q q q q ' % =
E c n o c n o ~ c n c n c n c n c n c n o c n o c n
0
fj 9 -
al
z : ; ,Ed
0
,
,
0
&
8
aDaDwabmmmmmmaDmmmam
0 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
aD
3
~ 1 1 1 1 1 ~ 1 ° 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
m
0 0
~ ~ 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
; 9 z z z z z z ~ z z z z 3 ~ $ z z z ~ ~ z ~ z z z
, o , o o
0 0 0 0 0 0 0 0 0 0
o ~ ~ o ~ q ' f ? w k m 8
g ? S S Z S S S Z S S E Z S S 8 Z 8 8 8 8 S Z 6 6 z a U 0 ' 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
~ m ~ z z z ~ 8 8 g g 8 g ~ g ~ ~ ~ 0 r r O O O O 0
P 6 6 6 6 6 5 6 6 6 6 6 6 6 6 6 5 6 6 6 6 6 6 6 6
7
, S 86 ,
a ~ m
r
8 % '
o ~ ~ o c n m ~ m m m c n m r o m m o o o a a a o m
m m m m r - * N r m w m r o c n m m b b m m w w m
c n o o ~ ~ b b b b b b o b m o a ~ m o o a ~ c n m b a
o o o o o o o o o o o o o o o o 8 ~ 8 8 8 8 8 8
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 6 0 0 0 0
m m m b w N m a a D a m m m w m m m a a a m m m ( V
0 0 0 0 0 0 0 0 0 0 0 0 0 ~ ~ ~ 0 ~ ~ 0 0 0 0 0
, ~ , b c n c n b ~ ~ , ~ ~ ~ ! ~ ~ ~ , ~ ~ m a b ( w m * m w w w w w * w n w m w ' $ 8 E E 8 8 G % %
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
$ ? N $ S
q q 8 e 1
0 0 0 0
m a o m
, 80 80 38 38 ,
r r r r r r
N N O ~ O ~
K i w 4 % 2 i 4 3 i
, a 0
, N
0 - 0
r O O O O O O O O r r O O r r r O O ~ ~ r q
N
N N O O O
3
,
13 3
N r a 8 N a 6 q g q
! ! o o o o o
8 s ; % i Z % 3
d i + i i i
a a ~ m a ~ m
6 6 6 Z 6
The yield strength (MPa) of the specimen steel plate of
each mark was found in the same manner as that in Example
1. The 0-phase area ratio measurement test was conducted
on the specimen steel plate of each mark in the same
manner as that in Example 1.
[00911
The following SCC and SSC tests were conducted on
the specimen steel plate of each mark, and the SCC
resistance and the SSC resistance of the specimen steel
plate of each mark were evaluated.
[00921
[SCC Test]
A 4-point bending test specimen (referred to simply
as a llspecimen"h,e reinafter) was collected from the
specimen steel plate of each mark. Each specimen had a
length of 75 mm, a width of 10 mm, and a thickness of 2
mm. The longitudinal direction of the specimen was
vertical to the rolling direction of the specimen steel
plate. Each specimen was bent by 4-point bending. In
compliance with ASTM G39, deflection for each specimen
was determined in such a manner that the stress applied
to each specimen become equal to the 0.2% proof stress of
this specimen.
[00931
An autoclave having a temperature of 150°C where co2
at 3 MPa was pressurized and enclosed was prepared. Each
specimen to which bend was applied was immersed in an
- 35 -
NaCl solution of 25% in mass% for 720 hours in this
autoclave. After 720 hours had passed, it was evaluated
whether or not cracking was generated in each specimen.
Specifically, the cross section of each specimen at a
portion where tensile stress was applied was observed
using the optical microscope with 100x magnification so
as to visually determine whether or not there is any
cracking.
[00941
[SSC Test]
A 4-point bending test specimen was collected from
the specimen steel plate of each mark in the same manner
as that of the SCC test. Each specimen was bent by 4-
point bending in the same manner as that of the SCC test.
[00951
An autoclave having a temperature of 90°C where C02
at 3 MPa and H2S at 0.003 MPa were pressurized and
enclosed was prepared. Each specimen to which the bend
was applied was immersed in the autoclave in an NaCl
solution of 5% in mass% for 720 hours. After 720 hours
had passed, it was evaluated whether or not cracking was
generated in each specimen in the same manner as that of
the SCC test.
[0096]
[Test Result]
Table 6 shows the test result.
[00971
[Table 61
In Table 6, the column I1SCC Resistanceu shows the
evaluation result of the SCC test. The column "SSC
Resistance" shows the evaluation result of the SSC test.
In each column, "NFV indicates that no cracking was
observed. "FU indicates that cracking was observed.
[00991
With reference to Table 6, each chemical composition
of the marks A to F and the marks L to R was within the
range of the chemical composition of the present
invention, and the F1 value also satisfied Formula (1).
Hence, the yield strength was at least 550 MPa, and no o
phase precipitated. As a result, no SCC and no SSC were
observed in these specimen steel plates.
[OlOO]
To the contrary, the Mn content of the mark S was
less than the lower limit of the Mn content of the
present invention. Hence, the yield strength was less
than 550 MPa. The N content of the mark S was also less
than the lower limit of the N content of the present
invention. Hence, pitting occurred in the SCC test, and
SCC was observed in the SCC test. In addition, The Mo
content of the mark S was also less than the lower limit
of the Mo content of the present invention. Hence, SSC
was observed in the SSC test.
[01011
Each Ni content of the marks T to V was less than
the lower limit of the Ni content of the present
invention, and the F1 value did not satisfy Formula (1) .
Hence, SCC was observed in the SCC test.
[0102]
The Cu content of the mark W was less than the lower
limit of the Cu content of the present invention. Hence,
the yield strength of the mark W was less than 550 MPa.
In addition, the Mo content of the mark W was less than
the lower limit of the Mo content of the present
invention. Hence, SSC was observed in the SSC test. In
the mark W, the Ni and Cr contents were less than the Ni
and Cr contents of the present invention, and the F1
value did not satisfy Formula (1). The C content was
more than the C content of the present invention. Hence,
in the mark W, SCC was observed in the SCC test. It can
be considered that the Ni content and the Cr content were
excessively low, and excessive C generated Cr carbide in
the mark W, and thus the corrosion film became unstable,
and SCC occurred.
[0103]
The Cr content of the mark X was less than the Cr
content of the present invention, and the F1 value did
not satisfy Formula (1) . In the mark XI the C content
was more than the C content of the present invention.
Hence, SCC was observed in the SCC test in the mark X.
In the mark X, it can be considered that the Cr content
was excessively low, and excessive C generated Cr carbide,
and thus the corrosion film became unstable, and SCC
occurred.
[01041
Each N content of the mark Y and the mark Z was less
than the lower limit of the N content of the present
invention, and the F1 value did not satisfy Formula (1).
Hence, pitting was generated, and SCC was observed in the
SCC test.
[01051
Each chemical composition of the mark A24 to the mark
AC was within the range of the chemical composition of
the present invention. The F1 value of each mark did not
satisfy Formula (1) , though. Hence, in the marks A24 to
the mark AC, SCC was observed in the SCC test. It can be
considered that Formula (1) was not satisfied in these
marks AA to AC, and thus the corrosion film became
unstable, resulting in generation of SCC.
[0106]
The embodiment of the present invention has been
described above, but the aforementioned embodiment was
merely exemplified for embodying the present invention.
Accordingly, the present invention is not limited to the
aforementioned embodiment, and the aforementioned
embodiment may be appropriately modified to be carried
out without departing from the spirit and scope of the
present invention.
We Claim:
[Claim 11
Duplex stainless steel comprising, in mass%,
C: at most 0.03%;
Si: 0.2 to 1%;
Mn: more than 5.0% to at most 10%;
P: at most 0.040%;
S: at most 0.010%;
Ni: 4.5 to 8%;
sol. Al: at most 0.040%;
N: more than 0.2% to at most 0.4%;
Cr: 24 to 29%;
Mo: 0.5 to less than 1.5%;
Cu: 1.5 to 3.5%;
W: 0.05 to 0.2%;
the balance being Fe and impurities,
wherein
the duplex stainless steel satisfies Formula (1) :
Cr + 8Ni + Cu + Mo + W/2 2 65 . . . (I),
where a symbol of each element in Formula (1) represents
a content of the element (in mass%).
[Claim 21
The duplex stainless steel according to claim 1,
further comprising V: at most 1.5% instead of part of Fe.
[Claim 31
The duplex stainless steel according to claim 1 or
claim 2, further comprising one or more types selected
from a group of Ca: at most 0.02%, Mg: at most 0.02%, and
B: at most 0.02% instead of part of Fe.