Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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DESCRIPTION
TITLE OF INVENTION
DUPLEX STAINLESS STEEL MATERIAL
TECHNICAL FIELD
[0001]
The present disclosure relates to a duplex stainless steel material.
BACKGROUND ART
[0002]
Oil wells and gas wells (hereinafter, oil wells and gas wells are collectively
referred to simply as "oil wells") may be in a corrosive environment
containing a
corrosive gas. Here, the term "corrosive gas" means carbon dioxide gas and/or
hydrogen sulfide gas. That is, steel materials for use in oil wells are
required to
have excellent corrosion resistance in a corrosive environment.
[0003]
To date, as a method for enhancing the corrosion resistance of a steel
material,
there has been a known method that increases the content of chromium (Cr) and
forms a passive film mainly composed of Cr oxides on the surface of the steel
material. Therefore, a duplex stainless steel material in which the content of
Cr has
been made high may in some cases be used in an environment where excellent
corrosion resistance is required. It is known that, in particular, a duplex
stainless
steel material exhibits excellent corrosion resistance in seawater.
[0004]
In recent years, deep wells below sea level are being actively developed. On
the other hand, steel materials used for deep wells below sea level are
required to not
only have excellent corrosion resistance, but also to have high strength and
excellent
low-temperature toughness. Therefore, there is a need for duplex stainless
steel
materials that have high strength and excellent low-temperature toughness.
[0005]
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Japanese Patent Application Publication No. 10-60597 (Patent Literature 1),
International Application Publication No. W02012/111536 (Patent Literature 2),
and
Japanese Patent Application Publication No. 2016-3377 (Patent Literature 3)
each
propose a technique for increasing the strength and low-temperature toughness
of a
duplex stainless steel material.
[0006]
Patent Literature 1 discloses a duplex stainless steel material which contains
ferrite in an amount of 60 to 90% in area fraction, in which a Ni balance
value (= Ni
+ 0.5Mn + 30(C +N) - 1.1(Cr + 1.5Si + Mo + 0.5Nb) + 8.2) is -15 to -10, and
which
satisfies the formula (content of Al x content of N 0.0023 x Ni balance value
+
0.357). It is described in Patent Literature 1 that this duplex stainless
steel material
has high strength and excellent toughness.
[0007]
Patent Literature 2 discloses a duplex stainless steel material that has a
chemical composition consisting of, by mass%, C: 0.030% or less, Si: 0.20 to
1.00%,
Mn: 8.00% or less, P: 0.040% or less, S: 0.0100% or less, Cu: more than 2.00
to
4.00% or less, Ni: 4.00 to 8.00%, Cr: 20.0 to 30.0%, Mo: 0.50 to less than
2.00%, N:
0.100 to 0.350%, and Al: 0.040% or less, with the balance being Fe and
impurities,
and that has a microstructure in which a ferrite ratio is 30 to 70%, and the
hardness
of ferrite is 300 HVlOgf or more. It is described in Patent Literature 2 that
this
duplex stainless steel material has high strength and high toughness.
[0008]
Patent Literature 3 discloses a duplex stainless steel material that is a
duplex
stainless steel tube which has a chemical composition consisting of, by mass%,
C:
0.03% or less, Si: 0.2 to 1%, Mn: 0.5 to 2.0%, P: 0.040% or less, S: 0.010% or
less,
Al: 0.040% or less, Ni: 4 to less than 6%, Cr: 20 to less than 25%, Mo: 2.0 to
4.0%,
N: 0.1 to 0.35%, 0: 0.003% or less, V: 0.05 to 1.5%, Ca: 0.0005 to 0.02%, and
B:
0.0005 to 0.02%, with the balance being Fe and impurities, and has a
microstructure
composed of a duplex microstructure of a ferrite phase and an austenite phase
in
which there is no precipitation of sigma phase, and in which a proportion
occupied
by the ferrite phase in the steel microstructure is 50% or less in area
fraction, and the
number of oxides having a particle size of 30 lam or more present in a visual
field of
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300 mm2 is 15 or less. It is described in Patent Literature 3 that this duplex
stainless steel material is excellent in strength, pitting resistance, and low-
temperature toughness.
CITATION LIST
PATENT LITERATURE
[0009]
Patent Literature 1: Japanese Patent Application Publication No. 10-60597
Patent Literature 2: International Application Publication No. W02012/111536
Patent Literature 3: Japanese Patent Application Publication No. 2016-3377
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0010]
As described above, the aforementioned Patent Literatures 1 to 3 disclose
duplex stainless steel materials that have high strength and excellent low-
temperature
toughness. However, a duplex stainless steel material that has high strength
and
excellent low-temperature toughness may also be obtained by a technique other
than
the techniques disclosed in the aforementioned Patent Literatures 1 to 3.
[0011]
An objective of the present disclosure is to provide a duplex stainless steel
material that has high strength and excellent low-temperature toughness.
SOLUTION TO PROBLEM
[0012]
A duplex stainless steel material according to the present disclosure consists
of, by mass%,
C: 0.030% or less,
Si: 0.20 to 1.00%,
Mn: 0.50 to 7.00%,
P: 0.040% or less,
S: 0.0200% or less,
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Al: 0.100% or less,
Ni: 4.20 to 9.00%,
Cr: 20.00 to 30.00%,
Mo: 0.50 to 2.00%,
Cu: 0.50 to 3.00%,
N: 0.150 to 0.350%,
V: 0.01 to 1.50%,
Nb: 0 to 0.100%,
Ta: 0 to 0.100%,
Ti: 0 to 0.100%,
Zr: 0 to 0.100%,
Hf: 0 to 0.100%,
W: 0 to 0.200%,
Co: 0 to 0.500%,
Sn: 0 to 0.100%,
Sb: 0 to 0.100%,
Ca: 0 to 0.020%,
Mg: 0 to 0.020%,
B: 0 to 0.020%, and
rare earth metal: 0 to 0.200%,
with the balance being Fe and impurities,
wherein:
a yield strength is 552 MPa or more; and
when, in a microstructure, an austenite grain with a minor axis of 20 lam or
more is defined as primary austenite, and the balance of austenite is defined
as
secondary austenite,
the microstructure is composed of, in volume ratio, ferrite in an amount of 35
to 55%, the primary austenite in an amount of 40 to 55%, and the secondary
austenite
in an amount of 5 to 20%.
ADVANTAGEOUS EFFECT OF INVENTION
[0013]
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The duplex stainless steel material according to the present disclosure has
high strength and excellent low-temperature toughness.
BRIEF DESCRIPTION OF DRAWINGS
[0014]
[FIG. 1] FIG. 1 is a schematic diagram illustrating the appearance of
microstructure
during microstructure observation at a cross section at a central portion of
the wall
thickness of a duplex stainless seamless steel pipe that is one example of the
duplex
stainless steel material according to the present embodiment, the cross
section being
perpendicular to the pipe axis direction of the duplex stainless steel
seamless pipe.
[FIG. 2] FIG. 2 is a view illustrating the relation between a volume ratio (%)
of
secondary austenite and a lowest temperature ( C) at which an absorbed energy
is 30
J/cm2 or more in duplex stainless steel materials which satisfy the chemical
composition described above among the present examples.
DESCRIPTION OF EMBODIMENTS
[0015]
First, the present inventors conducted studies regarding obtaining a duplex
stainless steel material having a yield strength of 80 ksi (552 MPa) or more
as a high
strength. That is, the present inventors conducted investigations and studies
regarding a method for obtaining a duplex stainless steel material that
achieves both
a yield strength of 80 ksi or more and excellent low-temperature toughness. As
a
result, the present inventors obtained the following findings.
[0016]
First, the present inventors conducted studies from the viewpoint of the
chemical composition with respect to a duplex stainless steel material that
achieves
both a yield strength of 80 ksi or more and excellent low-temperature
toughness.
As a result, the present inventors considered that if a duplex stainless steel
material
has a chemical composition consisting of, by mass%, C: 0.030% or less, Si:
0.20 to
1.00%, Mn: 0.50 to 7.00%, P: 0.040% or less, S: 0.0200% or less, Al: 0.100% or
less,
Ni: 4.20 to 9.00%, Cr: 20.00 to 30.00%, Mo: 0.50 to 2.00%, Cu: 0.50 to 3.00%,
N:
0.150 to 0.350%, V: 0.01 to 1.50%, Nb: 0 to 0.100%, Ta: 0 to 0.100%, Ti: 0 to
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0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, W: 0 to 0.200%, Co: 0 to 0.500%, Sn:
0
to 0.100%, Sb: 0 to 0.100%, Ca: 0 to 0.020%, Mg: 0 to 0.020%, B: 0 to 0.020%,
and
rare earth metal: 0 to 0.200%, with the balance being Fe and impurities, there
is a
possibility that a yield strength of 80 ksi or more and excellent low-
temperature
toughness can be obtained.
[0017]
Here, the microstructure of a duplex stainless steel material having the
chemical composition described above is composed of ferrite and austenite.
Specifically, the microstructure of a duplex stainless steel material having
the
chemical composition described above is composed of, in volume ratio, ferrite
in an
amount of 35 to 55% with the balance being austenite. Note that, in the
present
description, the phrase "composed of ferrite and austenite" means that the
amount of
any phase other than ferrite and austenite is negligibly small.
[0018]
Next, with respect to a duplex stainless steel material having the chemical
composition described above in which the volume ratio of ferrite is 35 to 55%,
the
present inventors conducted detailed studies regarding a method for obtaining
a yield
strength of 80 ksi or more and excellent low-temperature toughness. Here, in a
duplex stainless steel material having the chemical composition described
above,
ferrite is harder than austenite. Therefore, especially in a low-temperature
environment, minute cracks generated in the duplex stainless steel material
tend to
easily propagate through the ferrite.
[0019]
Therefore, with respect to a duplex stainless steel material having the
chemical composition described above in which the volume ratio of ferrite is
35 to
55%, the present inventors conducted studies regarding causing fine austenite
to
disperse in the ferrite so as to increase the low-temperature toughness of the
ferrite.
If austenite is finely dispersed in ferrite, there is a possibility that the
low-
temperature toughness of the ferrite can be selectively increased. In such
case,
there is a possibility that the low-temperature toughness of a duplex
stainless steel
material having the chemical composition described above can be increased.
[0020]
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On the other hand, as the result of detailed studies conducted by the present
inventors it was revealed that in a duplex stainless steel material having the
chemical
composition described above, there is a possibility that the yield strength
will be
reduced by causing fine austenite to disperse in ferrite. In other words, in a
duplex
stainless steel material having the chemical composition described above,
there is a
possibility that if the austenite is merely refined, even if the low-
temperature
toughness is increased, a yield strength of 80 ksi or more will not be
obtained.
[0021]
Therefore, the present inventors considered causing ferrite having a volume
ratio of 35 to 55%, coarse austenite, and fine austenite to be intermixed in
the
microstructure of a duplex stainless steel material. In this case, there is a
possibility
that not only will the low-temperature toughness of the duplex stainless steel
material
be increased by the fine austenite dispersed in the ferrite, but also that the
yield
strength can be maintained by the coarse austenite.
[0022]
Specifically the present inventors classified austenite in the microstructure
of
a duplex stainless steel material into coarse austenite grains with a minor
axis of 20
pm or more, and fine austenite grains which is the remaining austenite other
than the
coarse austenite grains. More specifically, in the austenite of the
microstructure of
a duplex stainless steel material, the present inventors defined an austenite
grain with
a minor axis of 20 pm or more as "primary austenite", and defined the balance
of the
austenite as "secondary austenite". This point will be described more
specifically
using the drawings.
[0023]
FIG. 1 is a schematic diagram illustrating the appearance of microstructure
during microstructure observation at a cross section at a central portion of
the wall
thickness of a duplex stainless steel seamless pipe that is one example of the
duplex
stainless steel material according to the present embodiment, the cross
section being
perpendicular to the pipe axis direction of the duplex stainless steel
seamless pipe.
The vertical direction of an observation visual field region 10 in FIG. 1
corresponds
to the pipe radial direction of the duplex stainless steel seamless pipe. The
horizontal direction of the observation visual field region 10 in FIG. 1
corresponds to
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the pipe circumferential direction of the duplex stainless steel seamless
pipe. That
is, the observation visual field region 10 in FIG. 1 corresponds to a plane
perpendicular to the pipe axis direction. Note that, the observation visual
field
region 10 in FIG. 1 has a length of 200 pm in the vertical direction and a
length of
200 pm in the horizontal direction.
[0024]
Referring to FIG. 1, regions shown in black represent ferrite 20, and regions
shown in white represent austenite 30. Among the austenite 30, an austenite
grain
with a minor axis of 20 pm or more is defined as primary austenite 31, and an
austenite grain with a minor axis of less than 20 pm is defined as secondary
austenite
32. Note that, in the observation visual field region 10, the
ferrite 20, the primary
austenite 31, and the secondary austenite 32 can be identified by a method
described
later.
[0025]
Next, the present inventors used method described later to evaluate the yield
strength and the low-temperature toughness of duplex stainless steel materials
which
had the chemical composition described above and in which the volume ratio of
ferrite was 35 to 55%. As a result, it was revealed that in a duplex stainless
steel
material which has the chemical composition described above and in which the
volume ratio of ferrite is 35 to 55%, if the volume ratio of primary austenite
is 40 to
55% and, in addition, the volume ratio of secondary austenite is 5 to 20%,
both a
yield strength of 80 ksi or more and excellent low-temperature toughness can
be
achieved. This point will now be described specifically using the drawings.
[0026]
FIG. 2 is a view illustrating the relation between a volume ratio (%) of
secondary austenite and a lowest temperature ( C) at which an absorbed energy
is 30
J/cm2 or more in duplex stainless steel materials which satisfy the chemical
composition described above among examples that are described later. FIG. 2
was
created using a volume ratio of secondary austenite (%) and a lowest
temperature
( C), that is an index of low-temperature toughness, at which an absorbed
energy
was 30 J/cm2 or more in, among examples described later, duplex stainless
steel
materials which satisfied the chemical composition described above and which
had a
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microstructure including ferrite in an amount of 35 to 55% in volume ratio,
and
primary austenite in an amount of 40 to 55% in volume ratio.
[0027]
Note that, the volume ratio of secondary austenite and the lowest temperature
at which an absorbed energy is 30 J/cm2 or more were determined using methods
described later. Further, a white circle (0) in FIG. 2 means a steel material
whose
yield strength was 552 MPa or more. A black circle (*) in FIG. 2 means a steel
material whose yield strength was less than 552 MPa.
[0028]
Referring to FIG. 2, it can be confirmed that in a duplex stainless steel
material having the chemical composition and microstructure described above,
when
the volume ratio of secondary austenite is 5% or more, the lowest temperature
at
which an absorbed energy is 30 J/cm2 or more is -20 C or less, and thus
excellent
low-temperature toughness is exhibited. Referring further to FIG. 2, it can be
confirmed that in a duplex stainless steel material having the chemical
composition
and microstructure described above, when the volume ratio of secondary
austenite is
more than 20%, although excellent low-temperature toughness is exhibited, a
yield
strength of 552 MPa or more is not obtained. That is, referring to FIG. 2, it
can be
confirmed that in a duplex stainless steel material having the chemical
composition
and microstructure described above, if the volume ratio of secondary austenite
is 5 to
20%, both a yield strength of 552 MPa or more and excellent low-temperature
toughness can be achieved.
[0029]
Therefore, the duplex stainless steel material according to the present
embodiment has the chemical composition described above, and is composed of,
in
volume ratio, ferrite in an amount of 35 to 55%, primary austenite in an
amount of 40
to 55%, and secondary austenite in an amount of 5 to 20%. As a result, the
duplex
stainless steel material according to the present embodiment has a yield
strength of
80 ksi (552 MPa) or more and has excellent low-temperature toughness.
[0030]
The gist of the duplex stainless steel material according to the present
embodiment, which has been completed based on the above findings, is as
follows.
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[0031]
[1]
A duplex stainless steel material consisting of, by mass%,
C: 0.030% or less,
Si: 0.20 to 1.00%,
Mn: 0.50 to 7.00%,
P: 0.040% or less,
S: 0.0200% or less,
Al: 0.100% or less,
Ni: 4.20 to 9.00%,
Cr: 20.00 to 30.00%,
Mo: 0.50 to 2.00%,
Cu: 0.50 to 3.00%,
N: 0.150 to 0.350%,
V: 0.01 to 1.50%,
Nb: 0 to 0.100%,
Ta: 0 to 0.100%,
Ti: 0 to 0.100%,
Zr: 0 to 0.100%,
Hf: 0 to 0.100%,
W: 0 to 0.200%,
Co: 0 to 0.500%,
Sn: 0 to 0.100%,
Sb: 0 to 0.100%,
Ca: 0 to 0.020%,
Mg: 0 to 0.020%,
B: 0 to 0.020%, and
rare earth metal: 0 to 0.200%,
with the balance being Fe and impurities,
wherein:
a yield strength is 552 MPa or more; and
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when, in a microstructure, an austenite grain with a minor axis of 20 lam or
more is defined as primary austenite, and the balance of austenite is defined
as
secondary austenite,
the microstructure is composed of, in volume ratio, ferrite in an amount of 35
to 55%, the primary austenite in an amount of 40 to 55%, and the secondary
austenite
in an amount of 5 to 20%.
[0032]
[2]
The duplex stainless steel material according to [1], containing one or more
elements selected from a group consisting of:
Nb: 0.001 to 0.100%,
Ta: 0.001 to 0.100%,
Ti: 0.001 to 0.100%,
Zr: 0.001 to 0.100%,
Hf: 0.001 to 0.100%,
W: 0.001 to 0.200%,
Co: 0.001 to 0.500%,
Sn: 0.001 to 0.100%,
Sb: 0.001 to 0.100%,
Ca: 0.001 to 0.020%,
Mg: 0.001 to 0.020%,
B: 0.001 to 0.020%, and
rare earth metal: 0.001 to 0.200%.
[0033]
Note that, the shape of the duplex stainless steel material according to the
present embodiment is not particularly limited. The duplex stainless steel
material
according to the present embodiment may be a steel pipe, may be a round steel
bar (a
solid material), or may be a steel plate. Note that, the term "round steel
bar" means
a steel bar in which a cross section perpendicular to the axial direction is a
circular
shape. Further, the steel pipe may be a seamless steel pipe or may be a welded
steel
pipe.
[0034]
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Hereunder, the duplex stainless steel material according to the present
embodiment is described in detail.
[0035]
[Chemical composition]
The chemical composition of the duplex stainless steel material according to
the present embodiment contains the following elements. The symbol "%"
relating
to an element means "mass%" unless otherwise noted.
[0036]
C: 0.030% or less
Carbon (C) is unavoidably contained. That is, the lower limit of the content
of C is more than 0%. C forms Cr carbides at grain boundaries and increases
corrosion susceptibility at the grain boundaries. Therefore, if the content of
C is too
high, corrosion resistance of the steel material will decrease even if the
contents of
other elements are within the range of the present embodiment. Therefore, the
content of C is to be 0.030% or less. A preferable upper limit of the content
of C is
0.028%, and more preferably is 0.025%. The content of C is preferably as low
as
possible. However, extremely reducing the content of C will significantly
increase
the production cost. Therefore, when industrial manufacturing is taken into
consideration, a preferable lower limit of the content of C is 0.001%, more
preferably
is 0.003%, and further preferably is 0.005%.
[0037]
Si: 0.20 to 1.00%
Silicon (Si) deoxidizes the steel. If the content of Si is too low, the
aforementioned advantageous effect will not be sufficiently obtained even if
the
contents of other elements are within the range of the present embodiment. On
the
other hand, if the content of Si is too high, the low-temperature toughness of
the steel
material will decrease even if the contents of other elements are within the
range of
the present embodiment. Therefore, the content of Si is to be 0.20 to 1.00%. A
preferable lower limit of the content of Si is 0.25%, and more preferably is
0.30%.
A preferable upper limit of the content of Si is 0.80%, more preferably is
0.70%, and
further preferably is 0.60%.
[0038]
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Mn: 0.50 to 7.00%
Manganese (Mn) deoxidizes the steel, and desulfurizes the steel. Mn also
increases hot workability of the steel material. If the content of Mn is too
low, the
aforementioned advantageous effects will not be sufficiently obtained even if
the
contents of other elements are within the range of the present embodiment. On
the
other hand, Mn segregates to grain boundaries together with impurities such as
P and
S. Therefore, if the content of Mn is too high, corrosion
resistance of the steel
material will decrease even if the contents of other elements are within the
range of
the present embodiment. Therefore, the content of Mn is to be 0.50 to 7.00%. A
preferable lower limit of the content of Mn is 0.75%, and more preferably is
1.00%.
A preferable upper limit of the content of Mn is 6.50%, and more preferably is
6.20%.
[0039]
P: 0.040% or less
Phosphorus (P) is unavoidably contained. That is, the lower limit of the
content of P is more than 0%. P segregates to grain boundaries. Therefore, if
the
content of P is too high, the low-temperature toughness and corrosion
resistance of
the steel material will decrease even if the contents of other elements are
within the
range of the present embodiment. Therefore, the content of P is to be 0.040%
or
less. A preferable upper limit of the content of P is 0.035%, and more
preferably is
0.030%. The content of P is preferably as low as possible. However, extremely
reducing the content of P will significantly increase the production cost.
Therefore,
when industrial manufacturing is taken into consideration, a preferable lower
limit of
the content of P is 0.001%, and more preferably is 0.003%.
[0040]
S: 0.0200% or less
Sulfur (S) is unavoidably contained. That is, the lower limit of the content
of S is more than 0%. S segregates to grain boundaries. Therefore, if the
content
of S is too high, the low-temperature toughness and corrosion resistance of
the steel
material will decrease even if the contents of other elements are within the
range of
the present embodiment. Therefore, the content of S is to be 0.0200% or less.
A
preferable upper limit of the content of S is 0.0180%, and more preferably is
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0.0160%. The content of S is preferably as low as possible. However, extremely
reducing the content of S will significantly increase the production cost.
Therefore,
when industrial manufacturing is taken into consideration, a preferable lower
limit of
the content of S is 0.0005%, and more preferably is 0.0010%.
[0041]
Al: 0.100% or less
Aluminum (Al) is unavoidably contained. That is, the lower limit of the
content of Al is more than 0%. Al deoxidizes the steel. On the other hand, if
the
content of Al is too high, coarse oxide-based inclusions will form and the low-
temperature toughness of the steel material will decrease even if the contents
of other
elements are within the range of the present embodiment. Therefore, the
content of
Al is to be 0.100% or less. A preferable lower limit of the content of Al is
0.001%,
more preferably is 0.005%, and further preferably is 0.010%. A preferable
upper
limit of the content of Al is 0.090%, and more preferably is 0.085%. Note
that, as
used in the present description, the term "content of Al" means the content of
"acid-
soluble Al," that is, the content of sol. Al.
[0042]
Ni: 4.20 to 9.00%
Nickel (Ni) stabilizes the austenitic microstructure of the steel material.
That is, Ni is an element necessary for obtaining a stable duplex
microstructure
composed of ferrite and austenite. Ni also enhances corrosion resistance of
the steel
material. If the content of Ni is too low, the aforementioned advantageous
effects
will not be sufficiently obtained even if the contents of other elements are
within the
range of the present embodiment. On the other hand, if the content of Ni is
too high,
even if the contents of other elements are within the range of the present
embodiment,
the volume ratio of austenite will be too high and the yield strength of the
steel
material will decrease. Therefore, the content of Ni is to be 4.20 to 9.00%. A
preferable lower limit of the content of Ni is 4.25%, more preferably is
4.30%,
further preferably is 4.35%, further preferably is 4.40%, and further
preferably is
4.50%. A preferable upper limit of the content of Ni is 8.75%, more preferably
is
8.50%, further preferably is 8.25%, further preferably is 8.00%, and further
preferably is 7.75%.
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[0043]
Cr: 20.00 to 30.00%
Chromium (Cr) forms a passive film as an oxide on the surface of the steel
material and thereby enhances corrosion resistance of the steel material. Cr
also
increases the volume ratio of the ferritic microstructure of the steel
material. By
obtaining a sufficient ferritic microstructure, corrosion resistance of the
steel material
is stabilized. If the content of Cr is too low, the aforementioned
advantageous
effects will not be sufficiently obtained even if the contents of other
elements are
within the range of the present embodiment. On the other hand, if the content
of Cr
is too high, hot workability of the steel material will decrease even if the
contents of
other elements are within the range of the present embodiment. Therefore, the
content of Cr is to be 20.00 to 30.00%. A preferable lower limit of the
content of
Cr is 20.50%, more preferably is 21.00%, further preferably is 21.50%, and
further
preferably is 22.00%. A preferable upper limit of the content of Cr is 29.50%,
more
preferably is 29.00%, and further preferably is 28.00%.
[0044]
Mo: 0.50 to 2.00%
Molybdenum (Mo) enhances corrosion resistance of the steel material. Mo
also dissolves in the steel and increases the yield strength of the steel
material. In
addition, Mo forms fine carbides in the steel and increases the yield strength
of the
steel material. If the content of Mo is too low, the aforementioned
advantageous
effects will not be sufficiently obtained even if the contents of other
elements are
within the range of the present embodiment. On the other hand, if the content
of
Mo is too high, hot workability of the steel material will decrease even if
the contents
of other elements are within the range of the present embodiment. Therefore,
the
content of Mo is to be 0.50 to 2.00%. A preferable lower limit of the content
of Mo
is 0.55%, more preferably is 0.60%, and further preferably is 0.70%. A
preferable
upper limit of the content of Mo is less than 2.00%, more preferably is 1.85%,
and
further preferably is 1.50%.
[0045]
Cu: 0.50 to 3.00%
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Copper (Cu) increases the yield strength of the steel material. If the content
of Cu is too low, the aforementioned advantageous effect will not be
sufficiently
obtained even if the contents of other elements are within the range of the
present
embodiment. On the other hand, if the content of Cu is too high, the hot
workability of the steel material will decrease even if the contents of other
elements
are within the range of the present embodiment. Therefore, the content of Cu
is to
be 0.50 to 3.00%. A preferable lower limit of the content of Cu is 0.60%, more
preferably is 0.80%, further preferably is 0.90%, further preferably is 1.00%,
and
further preferably is 1.50%. A preferable upper limit of the content of Cu is
2.90%,
more preferably is 2.75%, and further preferably is 2.50%.
[0046]
N: 0.150 to 0.350%
Nitrogen (N) stabilizes the austenitic microstructure of the steel material.
That is, N is an element necessary for obtaining a stable duplex
microstructure
composed of ferrite and austenite. N also enhances corrosion resistance of the
steel
material. If the content of N is too low, the aforementioned advantageous
effects
will not be sufficiently obtained even if the contents of other elements are
within the
range of the present embodiment. On the other hand, if the content of N is too
high,
the low-temperature toughness and hot workability of the steel material will
decrease
even if the contents of other elements are within the range of the present
embodiment.
Therefore, the content of N is to be 0.150 to 0.350%. A preferable lower limit
of
the content of N is 0.170%, more preferably is 0.180%, and further preferably
is
0.190%. A preferable upper limit of the content of N is 0.340%, and more
preferably is 0.330%.
[0047]
V: 0.01 to 1.50%
Vanadium (V) increases the yield strength of the steel material. If the
content of V is too low, the aforementioned advantageous effect will not be
sufficiently obtained even if the contents of other elements are within the
range of
the present embodiment. On the other hand, if the content of V is too high,
even if
the contents of other elements are within the range of the present embodiment,
strength of the steel material will be too high, and the low-temperature
toughness and
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hot workability of the steel material will decrease. Therefore, the content of
V is to
be 0.01 to 1.50%. A preferable lower limit of the content of V is 0.02%, more
preferably is 0.03%, further preferably is 0.05%, and further preferably is
0.10%. A
preferable upper limit of the content of V is 1.20%, and more preferably is
1.00%.
[0048]
The balance of the chemical composition of the duplex stainless steel material
according to the present embodiment is Fe and impurities. Here, the term
"impurities" in the chemical composition refers to substances which are mixed
in
from ore and scrap as the raw material or from the production environment or
the
like when industrially producing the duplex stainless steel material, and
which are
permitted within a range that does not adversely affect the duplex stainless
steel
material according to the present embodiment.
[0049]
[Optional elements]
The chemical composition of the duplex stainless steel material according to
the present embodiment may further contain one or more elements selected from
the
group consisting of Nb, Ta, Ti, Zr, Hf, and W in lieu of a part of Fe. Each of
these
elements is an optional element, and each of these elements increases strength
of the
steel material.
[0050]
Nb: 0 to 0.100%
Niobium (Nb) is an optional element, and does not have to be contained.
That is, the content of Nb may be 0%. When contained, Nb forms carbo-nitrides
and increases strength of the steel material. If even a small amount of Nb is
contained, the aforementioned advantageous effect will be obtained to a
certain
extent. However, if the content of Nb is too high, even if the contents of
other
elements are within the range of the present embodiment, strength of the steel
material will be too high and the low-temperature toughness of the steel
material will
decrease. Therefore, the content of Nb is to be 0 to 0.100%. A preferable
lower
limit of the content of Nb is more than 0%, more preferably is 0.001%, further
preferably is 0.002%, further preferably is 0.003%, and further preferably is
0.005%.
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A preferable upper limit of the content of Nb is 0.080%, and more preferably
is
0.070%.
[0051]
Ta: 0 to 0.100%
Tantalum (Ta) is an optional element, and does not have to be contained.
That is, the content of Ta may be 0%. When contained, Ta forms carbo-nitrides
and
increases strength of the steel material. If even a small amount of Ta is
contained,
the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Ta is too high, even if the contents of other
elements are
within the range of the present embodiment, strength of the steel material
will be too
high and the low-temperature toughness of the steel material will decrease.
Therefore, the content of Ta is to be 0 to 0.100%. A preferable lower limit of
the
content of Ta is more than 0%, more preferably is 0.001%, further preferably
is
0.002%, further preferably is 0.003%, and further preferably is 0.005%. A
preferable upper limit of the content of Ta is 0.080%, more preferably is
0.070%, and
further preferably is 0.050%.
[0052]
Ti: 0 to 0.100%
Titanium (Ti) is an optional element, and does not have to be contained.
That is, the content of Ti may be 0%. When contained, Ti forms carbo-nitrides
and
increases strength of the steel material. If even a small amount of Ti is
contained,
the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Ti is too high, even if the contents of other
elements are
within the range of the present embodiment, strength of the steel material
will be too
high and the low-temperature toughness of the steel material will decrease.
Therefore, the content of Ti is to be 0 to 0.100%. A preferable lower limit of
the
content of Ti is more than 0%, more preferably is 0.001%, further preferably
is
0.002%, further preferably is 0.003%, and further preferably is 0.005%. A
preferable upper limit of the content of Ti is 0.080%, and more preferably is
0.070%.
[0053]
Zr: 0 to 0.100%
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Zirconium (Zr) is an optional element, and does not have to be contained.
That is, the content of Zr may be 0%. When contained, Zr forms carbo-nitrides
and
increases strength of the steel material. If even a small amount of Zr is
contained,
the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Zr is too high, even if the contents of other
elements are
within the range of the present embodiment, strength of the steel material
will be too
high and the low-temperature toughness of the steel material will decrease.
Therefore, the content of Zr is to be 0 to 0.100%. A preferable lower limit of
the
content of Zr is more than 0%, more preferably is 0.001%, further preferably
is
0.002%, further preferably is 0.003%, and further preferably is 0.005%. A
preferable upper limit of the content of Zr is 0.080%, and more preferably is
0.070%.
[0054]
Hf: 0 to 0.100%
Hafnium (Hf) is an optional element, and does not have to be contained.
That is, the content of Hf may be 0%. When contained, Hf forms carbo-nitrides
and
increases strength of the steel material. If even a small amount of Hf is
contained,
the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of Hf is too high, even if the contents of other
elements are
within the range of the present embodiment, strength of the steel material
will be too
high and the low-temperature toughness of the steel material will decrease.
Therefore, the content of Hf is to be 0 to 0.100%. A preferable lower limit of
the
content of Hf is more than 0%, more preferably is 0.001%, further preferably
is
0.002%, further preferably is 0.003%, and further preferably is 0.005%. A
preferable upper limit of the content of Hf is 0.080%, and more preferably is
0.070%.
[0055]
W: 0 to 0.200%
Tungsten (W) is an optional element, and does not have to be contained.
That is, the content of W may be 0%. When contained, W forms carbo-nitrides
and
increases strength of the steel material. If even a small amount of W is
contained,
the aforementioned advantageous effect will be obtained to a certain extent.
However, if the content of W is too high, even if the contents of other
elements are
within the range of the present embodiment, strength of the steel material
will be too
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high and the low-temperature toughness of the steel material will decrease.
Therefore, the content of W is to be 0 to 0.200%. A preferable lower limit of
the
content of W is more than 0%, more preferably is 0.001%, further preferably is
0.002%, further preferably is 0.003%, and further preferably is 0.005%. A
preferable upper limit of the content of W is 0.180%, and more preferably is
0.150%.
[0056]
The chemical composition of the duplex stainless steel material according to
the present embodiment may further contain one or more elements selected from
the
group consisting of Co, Sn, and Sb in lieu of a part of Fe. Each of these
elements is
an optional element, and each of these elements enhances corrosion resistance
of the
steel material.
[0057]
Co: 0 to 0.500%
Cobalt (Co) is an optional element, and does not have to be contained. That
is, the content of Co may be 0%. When contained, Co forms a coating on the
surface of the steel material, and thereby enhances corrosion resistance of
the steel
material. Co also increases hardenability of the steel material and stabilizes
strength of the steel material. If even a small amount of Co is contained, the
aforementioned advantageous effects will be obtained to a certain extent.
However,
if the content of Co is too high, the production cost will increase extremely,
even if
the contents of other elements are within the range of the present embodiment.
Therefore, the content of Co is to be 0 to 0.500%. A preferable lower limit of
the
content of Co is more than 0%, more preferably is 0.001%, further preferably
is
0.010%, and further preferably is 0.020%. A preferable upper limit of the
content
of Co is 0.480%, more preferably is 0.460%, and further preferably is 0.450%.
[0058]
Sn: 0 to 0.100%
Tin (Sn) is an optional element, and does not have to be contained. That is,
the content of Sn may be 0%. When contained, Sn enhances corrosion resistance
of
the steel material. If even a small amount of Sn is contained, the
aforementioned
advantageous effect will be obtained to a certain extent. However, if the
content of
Sn is too high, even if the contents of other elements are within the range of
the
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present embodiment, liquation cracking will occur at grain boundaries, which
will
cause hot workability of the steel material to decrease. Therefore, the
content of Sn
is to be 0 to 0.100%. A preferable lower limit of the content of Sn is more
than 0%,
more preferably is 0.001%, further preferably is 0.002%, and further
preferably is
0.003%. A preferable upper limit of the content of Sn is 0.080%, and more
preferably is 0.070%.
[0059]
Sb: 0 to 0.100%
Antimony (Sb) is an optional element, and does not have to be contained.
That is, the content of Sb may be 0%. When contained, Sb enhances corrosion
resistance of the steel material. If even a small amount of Sb is contained,
the
aforementioned advantageous effect will be obtained to a certain extent.
However,
if the content of Sb is too high, even if the contents of other elements are
within the
range of the present embodiment, high-temperature ductility of the steel
material will
decrease, and hot workability of the steel material will decrease. Therefore,
the
content of Sb is to be 0 to 0.100%. A preferable lower limit of the content of
Sb is
more than 0%, more preferably is 0.001%, further preferably is 0.002%, and
further
preferably is 0.003%. A preferable upper limit of the content of Sb is 0.080%,
and
more preferably is 0.070%.
[0060]
The chemical composition of the duplex stainless steel material according to
the present embodiment may further contain one or more elements selected from
the
group consisting of Ca, Mg, B, and rare earth metal in lieu of a part of Fe.
Each of
these elements is an optional element, and each of these elements increases
hot
workability of the steel material.
[0061]
Ca: 0 to 0.020%
Calcium (Ca) is an optional element, and does not have to be contained.
That is, the content of Ca may be 0%. When contained, Ca fixes S in the steel
material as a sulfide to make it harmless, and thereby increases hot
workability of the
steel material. If even a small amount of Ca is contained, the aforementioned
advantageous effect will be obtained to a certain extent. However, if the
content of
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Ca is too high, even if the contents of other elements are within the range of
the
present embodiment, oxides in the steel material will coarsen and the low-
temperature toughness of the steel material will decrease. Therefore, the
content of
Ca is to be 0 to 0.020%. A preferable lower limit of the content of Ca is more
than
0%, more preferably is 0.001%, further preferably is 0.002%, further
preferably is
0.003%, and further preferably is 0.005%. A preferable upper limit of the
content
of Ca is 0.018%, and more preferably is 0.015%.
[0062]
Mg: 0 to 0.020%
Magnesium (Mg) is an optional element, and does not have to be contained.
That is, the content of Mg may be 0%. When contained, Mg fixes S in the steel
material as a sulfide to make it harmless, and thereby increases hot
workability of the
steel material. If even a small amount of Mg is contained, the aforementioned
advantageous effect will be obtained to a certain extent. However, if the
content of
Mg is too high, even if the contents of other elements are within the range of
the
present embodiment, oxides in the steel material will coarsen and the low-
temperature toughness of the steel material will decrease. Therefore, the
content of
Mg is to be 0 to 0.020%. A preferable lower limit of the content of Mg is more
than
0%, more preferably is 0.001%, further preferably is 0.002%, further
preferably is
0.003%, and further preferably is 0.005%. A preferable upper limit of the
content
of Mg is 0.018%, and more preferably is 0.015%.
[0063]
B: 0 to 0.020%
Boron (B) is an optional element, and does not have to be contained. That is,
the content of B may be 0%. When contained, B suppresses segregation of S in
the
steel material to grain boundaries, and thereby increases hot workability of
the steel
material. If even a small amount of B is contained, the aforementioned
advantageous effect will be obtained to a certain extent. However, if the
content of
B is too high, even if the contents of other elements are within the range of
the
present embodiment, boron nitride (BN) will be formed and will cause the low-
temperature toughness of the steel material to decrease. Therefore, the
content of B
is to be 0 to 0.020%. A preferable lower limit of the content of B is more
than 0%,
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more preferably is 0.001%, further preferably is 0.002%, further preferably is
0.003%, and further preferably is 0.005%. A preferable upper limit of the
content
of B is 0.018%, and more preferably is 0.015%.
[0064]
Rare earth metal: 0 to 0.200%
Rare earth metal (REM) is an optional element, and does not have to be
contained. That is, the content of REM may be 0%. When contained, REM fixes
S in the steel material as a sulfide to make it harmless, and thereby
increases hot
workability of the steel material. If even a small amount of REM is contained,
the
aforementioned advantageous effect will be obtained to a certain extent.
However,
if the content of REM is too high, even if the contents of other elements are
within
the range of the present embodiment, oxides in the steel material will coarsen
and the
low-temperature toughness of the steel material will decrease. Therefore, the
content of REM is to be 0 to 0.200%. A preferable lower limit of the content
of
REM is more than 0%, more preferably is 0.001%, further preferably is 0.005%,
further preferably is 0.010%, and further preferably is 0.020%. A preferable
upper
limit of the content of REM is 0.180%, and more preferably is 0.160%.
[0065]
Note that, in the present description the term "REM" means one or more
elements selected from the group consisting of scandium (Sc) which is the
element
with atomic number 21, yttrium (Y) which is the element with atomic number 39,
and the elements from lanthanum (La) with atomic number 57 to lutetium (Lu)
with
atomic number 71 that are lanthanoids. Further, in the present description the
term
"content of REM" refers to the total content of these elements.
[0066]
[Yield strength]
The yield strength of the duplex stainless steel material according to the
present embodiment is 552 MPa or more. The duplex stainless steel material
according to the present embodiment has the chemical composition described
above,
and is composed of, in volume ratio, ferrite in an amount of 35 to 55%,
primary
austenite in an amount of 40 to 55%, and secondary austenite in an amount of 5
to
20%. As a result, the duplex stainless steel material according to the present
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embodiment has excellent low-temperature toughness even when the yield
strength is
80 ksi (552 MPa) or more.
[0067]
A preferable lower limit of the yield strength of the duplex stainless steel
material according to the present embodiment is 560 MPa, more preferably is
570
MPa, and further preferably is 580 MPa. Although not particularly limited, the
upper limit of the yield strength of the duplex stainless steel material
according to the
present embodiment is, for example, 724 MPa.
[0068]
The yield strength of the duplex stainless steel material according to the
present embodiment can be determined by the following method. Specifically, a
tensile test is performed by a method in accordance with ASTM E8/E8M (2021). A
test specimen is prepared from the steel material according to the present
embodiment. If the steel material is a steel plate, a round bar test specimen
is
prepared from a center portion of the thickness. If the steel material is a
steel pipe,
an arc-shaped test specimen having a thickness which is the same as the wall
thickness of the steel pipe and having a width of 25.4 mm and a gage length of
50.8
mm is prepared. If the steel material is a round steel bar, a round bar test
specimen
is prepared from an R/2 position in a cross section perpendicular to the axial
direction of the round steel bar. Note that, as used in the present
description, the
term "R/2 position" means the center position of a radius R in a cross section
perpendicular to the axial direction of the round steel bar. In the case of
preparing a
round bar test specimen, the size of the round bar test specimen is, for
example, as
follows: the parallel portion diameter is 6 mm and the gage length is 24 mm.
Note
that, the longitudinal direction of the round bar test specimen and the arc-
shaped test
specimen is to be parallel to the rolling direction of the steel material. A
tensile test
is carried out at normal temperature (25 C) in air using the test specimen,
and the
obtained 0.2% offset proof stress is defined as the yield strength (MPa).
[0069]
[Microstructure]
The microstructure of the duplex stainless steel material according to the
present embodiment is composed of ferrite and austenite. In the present
description,
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the phrase "composed of ferrite and austenite" means that the amount of any
phase
other than ferrite and austenite is negligibly small. For example, in the
microstructure of the duplex stainless steel material according to the present
embodiment, the volume ratios of precipitates and inclusions are negligibly
small as
compared with the volume ratios of ferrite and austenite. That is, the
microstructure
of the duplex stainless steel material according to the present embodiment may
contain minute amounts of precipitates, inclusions and the like, in addition
to ferrite
and austenite.
[0070]
In the microstructure of the duplex stainless steel material according to the
present embodiment, the volume ratio of ferrite is 35 to 55%. If the volume
ratio of
ferrite is too low, the yield strength will decrease. On the other hand, if
the volume
ratio of ferrite is too high, the low-temperature toughness of the steel
material will
decrease. However, in the microstructure of a duplex stainless steel material
which
has the chemical composition described above and which is produced by a
preferable
production method to be described later, the volume ratio of ferrite is 35 to
55%.
Therefore, in the microstructure of the duplex stainless steel material
according to the
present embodiment, the volume ratio of ferrite is 35 to 55%.
[0071]
In the microstructure of the duplex stainless steel material according to the
present embodiment, the volume ratio of primary austenite is 40 to 55% and the
volume ratio of secondary austenite is 5 to 20%. In the present description,
in the
austenite of the microstructure of the duplex stainless steel material, an
austenite
grain with a minor axis of 20 lam or more is defined as primary austenite, and
the
balance of the austenite is defined as secondary austenite.
[0072]
As described above, the fine secondary austenite disperses in ferrite and
increases the low-temperature toughness of the ferrite. As a result, the low-
temperature toughness of the duplex stainless steel material increases. If the
volume ratio of the secondary austenite is too low, the aforementioned
advantageous
effect will not be sufficiently obtained. On the other hand, if the volume
ratio of the
secondary austenite is too high, the yield strength of the duplex stainless
steel
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material will decrease. As a result, a duplex stainless steel material having
a yield
strength of 80 ksi or more will not be obtained. Therefore, in the present
embodiment, the volume ratio of secondary austenite is to be 5 to 20%. A
preferable lower limit of the volume ratio of secondary austenite is 6%, more
preferably is 7%, and further preferably is 10%. A preferable upper limit of
the
volume ratio of secondary austenite is 19%, more preferably is 18%, and
further
preferably is 15%.
[0073]
In addition, as described above, because the coarse primary austenite is
present, the yield strength of the duplex stainless steel material according
to the
present embodiment increases. If the volume ratio of the primary austenite is
too
low, the aforementioned advantageous effect will not be sufficiently obtained.
On
the other hand, if the volume ratio of the primary austenite is too high, the
volume
ratio of secondary austenite and/or ferrite will decrease, and in some cases
the low-
temperature toughness of the produced duplex stainless steel material will
decrease.
Therefore, in the present embodiment, the volume ratio of primary austenite is
to be
40 to 55%. A preferable lower limit of the volume ratio of primary austenite
is 41%,
more preferably is 42%, and further preferably is 45%. A preferable upper
limit of
the volume ratio of primary austenite is 54%, more preferably is 53%, and
further
preferably is 50%.
[0074]
As described above, the duplex stainless steel material according to the
present embodiment has the chemical composition described above, and is
composed
of, in volume ratio, ferrite in an amount of 35 to 55%, primary austenite in
an amount
of 40 to 55%, and secondary austenite in an amount of 5 to 20%. As a result,
the
duplex stainless steel material according to the present embodiment has a
yield
strength of 80 ksi or more and has excellent low-temperature toughness. Note
that,
in the present embodiment, as described later, microstructure observation is
performed at a cross section perpendicular to the rolling direction of the
duplex
stainless steel material. That is, in the duplex stainless steel material
according to
the present embodiment, at a cross section perpendicular to the rolling
direction of
the duplex stainless steel material, the microstructure is composed of, in
volume ratio,
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ferrite in an amount of 35 to 55%, primary austenite in an amount of 40 to
55%, and
secondary austenite in an amount of 5 to 20%.
[0075]
The volume ratios of ferrite, primary austenite, and secondary austenite of
the
duplex stainless steel material according to the present embodiment can be
determined by the following method. First, a test specimen for microstructure
observation is prepared from the duplex stainless steel material according to
the
present embodiment. If the steel material is a steel plate, a test specimen
having an
observation surface with dimensions of 5 mm in the width direction and 5 mm in
the
thickness direction is prepared from a center portion of the thickness. If the
steel
material is a steel pipe, a test specimen having an observation surface with
dimensions of 5 mm in the pipe radial direction and 5 mm in the pipe
circumferential
direction is prepared from a central portion of the wall thickness. If the
steel
material is a round steel bar, a test specimen having an observation surface
(5 mm x
mm) perpendicular to the axial direction of the round steel bar is prepared
from an
R/2 position in a cross section perpendicular to the axial direction of the
round steel
bar. Note that, the size of the test specimen is not particularly limited as
long as the
aforementioned observation surface can be obtained.
[0076]
The observation surface of the prepared test specimen is mirror-polished.
The mirror-polished observation surface is electrolytically etched in a 7%
potassium
hydroxide etching solution to reveal the microstructure. The observation
surface
where the microstructure has been revealed is observed in 10 visual fields
using an
optical microscope. The area of each visual field is, for example, 40000ium2
(200
pm x 200 pm). In each visual field, ferrite and austenite are identified based
on
contrast. The area fraction of the identified ferrite is determined. The
method for
determining the area fraction of ferrite is not limited, and it suffices to
use a well-
known method. For example, the area fraction of ferrite can be determined
using
image analysis software.
[0077]
The austenite grain with a minor axis of 20 pm or more is identified among
the austenite of each visual field identified based on contrast. Note that, in
the
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present description, the minor axis of austenite is defined as follows. First,
any
austenite is identified in each visual field. Note that, in the present
description, the
phrase "any austenite is identified" means one austenite grain whose outer
circumference is surrounded by ferrite is identified. A line segment with the
longest length among line segments linking an arbitrary two points of the
outer
circumference of the relevant austenite is defined as the major axis of the
relevant
austenite. A rectangle that circumscribes the relevant austenite is then drawn
in a
manner so that the major axis of the relevant austenite is the long side. At
this time,
the short side of the drawn rectangle is defined as the minor axis of the
relevant
austenite.
[0078]
In each visual field, the area fraction of the identified austenite with a
minor
axis of 20 lam or more (the primary austenite) is determined. The method for
determining the area fraction of the primary austenite is not limited, and it
suffices to
use a well-known method. For example, the area fraction can be determined
using
image analysis software. Further, the area fraction (%) of secondary austenite
can
be determined using the area fraction (%) of ferrite and the area fraction (%)
of
primary austenite as determined by the above methods, and the following
Formula
(A).
(Area fraction (%) of secondary austenite) = 100 - {(area fraction (%) of
ferrite) + (area fraction (%) of primary austenite)} (A)
[0079]
In the present embodiment, the arithmetic average value of the area fractions
(%) of ferrite obtained in the 10 visual fields by the above method is defined
as the
volume ratio (%) of ferrite. In addition, in the present embodiment, the
arithmetic
average value of the area fractions (%) of primary austenite obtained in the
10 visual
fields by the above method is defined as the volume ratio (%) of primary
austenite.
Further, in the present embodiment, the arithmetic average value of the area
fractions
(%) of secondary austenite obtained in the 10 visual fields by the above
method is
defined as the volume ratio (%) of secondary austenite.
[0080]
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Note that, as mentioned above, in the duplex stainless steel material
according
to the present embodiment, in some cases the microstructure includes
precipitates or
inclusions or the like in addition to ferrite, primary austenite, and
secondary austenite.
However, as described above, the volume ratios of precipitates or inclusions
or the
like are negligibly small as compared with the volume ratios of ferrite,
primary
austenite, and secondary austenite. Therefore, in the present description,
when
calculating the volume ratios of ferrite, primary austenite, and secondary
austenite by
the above method, the volume ratios of precipitates, inclusions and the like
are
ignored.
[0081]
[Low-temperature toughness]
The duplex stainless steel material according to the present embodiment has
the chemical composition described above, and is composed of, in volume ratio,
ferrite in an amount of 35 to 55%, primary austenite in an amount of 40 to
55%, and
secondary austenite in an amount of 5 to 20%. As a result, the duplex
stainless steel
material according to the present embodiment has excellent low-temperature
toughness even when the yield strength thereof is 80 ksi (552 MPa) or more. In
the
present embodiment, excellent low-temperature toughness is defined as follows.
[0082]
The low-temperature toughness of the duplex stainless steel material
according to the present embodiment is evaluated by a Charpy impact test in
accordance with ASTM E23 (2018). A V-notch test specimen in accordance with
ASTM E23 (2018) is prepared from the steel material according to the present
embodiment. Specifically, if the steel material is a steel plate, a V-notch
test
specimen having a notched surface parallel to the thickness direction and the
rolling
direction is to be prepared from a center portion of the thickness. If the
steel
material is a steel pipe, a V-notch test specimen having a notched surface
parallel to
the wall thickness direction and the pipe axis direction is to be prepared
from a
central portion of the wall thickness. If the steel material is a round steel
bar, a V-
notch test specimen having a notched surface parallel to the radial direction
of the
cross section and to the rolling direction is to be prepared from an R/2
position in a
cross section perpendicular to the axial direction. Note that, the
longitudinal
CA 03241527 2024- 6- 18
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direction of the V-notch test specimen is to be parallel to the rolling
direction of the
steel material.
[0083]
The prepared V-notch test specimen is subjected to a Charpy impact test in
accordance with ASTM E23 (2018). The Charpy impact test is performed under
eight conditions set in increments of 10 C in the range of 0 to -70 C, and the
absorbed energy (J) at each temperature is determined. The determined absorbed
energy (J) is divided by the cross-sectional area (cm2) of the V-notch test
specimen
to determine the absorbed energy (J/cm2) per unit area at each temperature.
Note
that, the term "cross-sectional area of the V-notch test specimen" means the
area of a
cross section perpendicular to the longitudinal direction of the V-notch test
specimen
at a position at the bottom of the V-notch. Specifically, when using a full-
size 2-
mm V-notch test specimen, the absorbed energy (J/cm2) per unit area can be
determined by dividing the determined absorbed energy (J) by the cross-
sectional
area 0.8 cm2 (width of 0.8 cm x thickness of 1.0 cm) of the V-notch test
specimen.
[0084]
From among the absorbed energies per unit area at each temperature that are
determined, the lowest temperature ( C) at which an absorbed energy is 30
J/cm2 or
more is determined. Specifically, for example, in a case where the absorbed
energy
per unit area at 0 C, -10 C, -20 C, and -30 C is 30 J/cm2 or more, and the
absorbed
energy per unit area at -40 C, -50 C, -60 C, and -70 C is less than 30 J/cm2,
the
lowest temperature is -30 C. In the present embodiment, if the lowest
temperature
at which an absorbed energy per unit area is 30 J/cm2 or more is -20 C or
less, it is
determined that the duplex stainless steel material has excellent low-
temperature
toughness. Note that, in the present description the absorbed energy per unit
area is
also referred to simply as "absorbed energy".
[0085]
[Shape of duplex stainless steel material]
As described above, the shape of the duplex stainless steel material according
to the present embodiment is not particularly limited. The duplex stainless
steel
material according to the present embodiment, for example, may be a steel
pipe, may
be a steel plate, may be a round steel bar, or may be a wire rod. Preferably,
the
CA 03241527 2024- 6- 18
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duplex stainless steel material according to the present embodiment is a
seamless
steel pipe. In a case where the duplex stainless steel material according to
the
present embodiment is a seamless steel pipe, even if the wall thickness is 5
mm or
more, the duplex stainless steel material has a yield strength of 552 MPa or
more and
excellent low-temperature toughness.
[0086]
[Production method]
One example of a method for producing the duplex stainless steel material
according to the present embodiment composed as described above will now be
described. Note that, a method for producing the duplex stainless steel
material
according to the present embodiment is not limited to the production method
described hereunder. One example of a method for producing the duplex
stainless
steel material of the present embodiment includes a starting material
preparation
process, a hot working process, a secondary austenite precipitation treatment
process,
and a solution treatment process. Hereunder, each production process is
described
in detail.
[0087]
[Starting material preparation process]
In the starting material preparation process according to the present
embodiment, a starting material having the chemical composition described
above is
prepared. The starting material may be prepared by producing the starting
material,
or may be prepared by purchasing the starting material from a third party.
That is,
the method for preparing the starting material is not particularly limited.
[0088]
In the case of producing the starting material, for example, the starting
material is produced by the following method. A molten steel having the
chemical
composition described above is produced. A cast piece (a slab, a bloom, or a
billet)
is produced by a continuous casting process using the molten steel. An ingot
may
also be produced by an ingot-making process using the molten steel. As
required, a
slab, a bloom, or an ingot may be subjected to blooming to produce a billet.
The
starting material is produced by the above process.
[0089]
CA 03241527 2024- 6- 18
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[Hot working process]
In the hot working process according to the present embodiment, the starting
material prepared in the aforementioned preparation process is subjected to
hot
working to produce an intermediate steel material. In the present description,
the
term "intermediate steel material" refers to a plate-shaped steel material in
a case
where the end product is a steel plate, refers to a hollow shell in a case
where the end
product is a steel pipe, refers to a steel material having a circular cross-
sectional
shape in a case where the end product is a round steel bar, and refers to a
wire-
shaped steel material in a case where the end product is a wire rod. The hot
working may be hot forging, may be hot extrusion, or may be hot rolling. The
hot
working method is not particularly limited, and it suffices to use a well-
known
method.
[0090]
If the intermediate steel material is a hollow shell (seamless steel pipe), in
the
hot working process, for example, the Ugine-Sejournet process or the Ehrhardt
push
bench process (that is, hot extrusion) may be performed, or the intermediate
steel
material may be subjected to piercing-rolling (that is, hot rolling) according
to the
Mannesmann process. Note that, hot working may be performed only one time or
may be performed multiple times. For example, after performing the
aforementioned piercing-rolling on the starting material, the aforementioned
hot
extrusion may be performed. For example, in addition, after performing the
aforementioned piercing-rolling on the starting material, tube drawing may be
performed. That is, in the hot working process, hot working is performed by a
well-
known method to produce an intermediate steel material having the desired
shape.
[0091]
Note that, if the steel material is a round steel bar or a steel plate, the
intermediate steel material may be produced as follows. If the steel material
is a
round steel bar, first, the starting material is heated in a heating furnace.
Although
not particularly limited, the heating temperature is, for example, 1100 to
1300 C.
After being extracted from the heating furnace, the starting material is
subjected to
hot working to produce an intermediate steel material in which a cross section
perpendicular to the axial direction is a circular shape. The hot working is,
for
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example, blooming performed using a blooming mill or hot rolling performed
using a
continuous mill. In a continuous mill, a horizontal stand having a pair of
grooved
rolls arranged one on the other in the vertical direction, and a vertical
stand having a
pair of grooved rolls arranged side by side in the horizontal direction are
alternately
arranged.
[0092]
If the steel material is a steel plate, first, the starting material is heated
in a
heating furnace. Although not particularly limited, the heating temperature
is, for
example, 1100 to 1300 C. After being extracted from the heating furnace, the
starting material is subjected to hot rolling using a blooming mill and a
continuous
mill to produce an intermediate steel material in the shape of a steel plate.
[0093]
[Secondary austenite precipitation treatment process]
In the secondary austenite precipitation treatment process according to the
present embodiment, the intermediate steel material produced by the
aforementioned
hot working process is subjected to a heat treatment to cause secondary
austenite to
precipitate in the intermediate steel material. Specifically, in the present
embodiment, in the secondary austenite precipitation treatment process,
preferably
the intermediate steel material is heated, and is held for three minutes or
more within
the range of 900 to 960 C.
[0094]
Preferably, the heating rate from 400 to 800 C when heating the intermediate
steel material is set to 0.35 C/sec or more. In the case of an intermediate
steel
material having the chemical composition described above, if the heating rate
from
400 to 800 C is too slow, in some cases flaws or cracks may occur in the
intermediate steel material due to precipitates temporarily formed when the
temperature is increasing. Therefore, in the present embodiment, preferably
the
heating rate from 400 to 800 C when heating the intermediate steel material is
set to
0.35 C/sec or more. Although not particularly limited, the upper limit of the
heating rate from 400 to 800 C when heating the intermediate steel material
is, for
example, 0.60 C/sec. Note that, a method for heating the intermediate steel
material is not particularly limited, and a well-known method can be used. For
CA 03241527 2024- 6- 18
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example, the intermediate steel material may be heated using a holding furnace
or a
high-frequency heating furnace.
[0095]
In the present embodiment, preferably the intermediate steel material heated
at the aforementioned heating rate is held for three minutes or more at a
temperature
within the range of 900 to 960 C. In the secondary austenite precipitation
treatment
process, if the temperature at which the intermediate steel material is held
(holding
temperature) is too high, secondary austenite will not precipitate
sufficiently. In
such case, a sufficient volume ratio of secondary austenite will not be
obtained in the
produced duplex stainless steel material. As a result, sufficient low-
temperature
toughness will not be obtained in the produced duplex stainless steel
material. On
the other hand, if the holding temperature is too low, too large an amount of
secondary austenite will precipitate. In such case, the volume ratio of
primary
austenite in the produced duplex stainless steel material will decrease. As a
result,
in some cases the yield strength of the steel material will be less than 552
MPa.
[0096]
Therefore, in the secondary austenite precipitation treatment process
according to the present embodiment, preferably the holding temperature is set
within the range of 900 to 960 C. A more preferable lower limit of the holding
temperature is 905 C, and further preferably is 910 C. A more preferable upper
limit of the holding temperature is 955 C, and further preferably is 950 C.
[0097]
In the secondary austenite precipitation treatment process, if the time
(holding
time) for which the intermediate steel material is held within the range of
900 to
960 C is too short, secondary austenite will not precipitate sufficiently. In
such
case, a sufficient volume ratio of secondary austenite will not be obtained in
the
produced duplex stainless steel material. As a result, sufficient low-
temperature
toughness will not be obtained in the produced duplex stainless steel
material.
Therefore, in the secondary austenite precipitation treatment process
according to the
present embodiment, preferably the holding time is set to three minutes or
more. A
more preferable lower limit of the holding time is four minutes, and further
preferably is five minutes.
CA 03241527 2024- 6- 18
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[0098]
Note that, the upper limit of the holding time is not particularly limited.
However, even if the holding time is very long, the effect thereof will be
saturated.
Therefore, in the secondary austenite precipitation treatment process
according to the
present embodiment, the upper limit of the holding time is preferably set to
20
minutes. A more preferable upper limit of the holding time is 19 minutes, and
further preferably is 18 minutes.
[0099]
[Solution treatment process]
In the solution treatment process, a solution treatment is performed on the
intermediate steel material subjected to the aforementioned secondary
austenite
precipitation treatment process. A method for performing the solution
treatment is
not particularly limited, and it suffices to perform a well-known method. For
example, the intermediate steel material is loaded into a heat treatment
furnace, and
after being held at a desired temperature, is rapidly cooled. Note that, in
the case of
performing a solution treatment by loading a hollow shell into a heat
treatment
furnace, holding the hollow shell at a desired temperature, and thereafter
performing
rapidly cooling, the term "solution treatment temperature" means the
temperature
( C) of the heat treatment furnace for performing the solution treatment. In
this
case, in addition, the term "solution treatment time" means the time for which
the
intermediate steel material is held at the solution treatment temperature.
[0100]
Preferably, the solution treatment temperature in the solution treatment
process of the present embodiment is set within the range of 980 to 1110 C. If
the
solution treatment temperature is too low, precipitates (for example, a phase
that is
an intermetallic compound or the like) may remain in the intermediate steel
material
after the solution treatment. In such case, the corrosion resistance of the
produced
duplex stainless steel material will decrease. Furthermore, if the solution
treatment
temperature is too low, too much secondary austenite will precipitate. In such
case,
in the produced duplex stainless steel material, in some cases the yield
strength of the
steel material may be less than 552 MPa. On the other hand, if the solution
treatment temperature is too high, in some cases the precipitated secondary
austenite
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will dissolve and the volume ratio of secondary austenite in the produced
duplex
stainless steel material will decrease. In such case, the low-temperature
toughness
of the steel material will decrease.
[0101]
When performing a solution treatment by loading an intermediate steel
material into a heat treatment furnace, holding the intermediate steel
material at a
desired temperature, and thereafter performing rapid cooling, the solution
treatment
time is not particularly limited, and it suffices that the solution treatment
time is in
accordance with a well-known condition. The solution treatment time is, for
example, 5 to 180 minutes. The rapid cooling method is, for example, water
cooling.
[0102]
[Other processes]
Note that, as necessary, the duplex stainless steel material on which the
solution treatment was performed may be subjected to a pickling treatment. In
this
case, the pickling treatment is not particularly limited and it suffices that
the pickling
treatment is performed by a well-known method. Further, the duplex stainless
steel
material on which the solution treatment was performed may be subjected to
cold
rolling. Even in a case where cold rolling is performed, as long as the
aforementioned requirement regarding the volume ratio of ferrite, the volume
ratio of
primary austenite, and the volume ratio of secondary austenite is satisfied, a
yield
strength of 80 ksi (552 MPa) or more and excellent low-temperature toughness
can
both be achieved.
[0103]
The duplex stainless steel material according to the present embodiment can
be produced by performing the processes described above. Note that the method
for producing the duplex stainless steel material described above is one
example, and
the duplex stainless steel material may also be produced by the other methods.
Hereunder, the present invention is described in more detail by way of
examples.
EXAMPLES
[0104]
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Molten steels having the chemical compositions shown in Table 1A and Table
1B were melted using a 50 kg vacuum furnace, and ingots were produced by an
ingot-making process. Note that, the symbol "2 in Table 1B means that the
content
of the corresponding element was at an impurity level. For example, it means
that
the content of Nb, the content of Ta, the content of Ti, the content of Zr,
the content
of Hf, the content of W, the content of Co, the content of Sn, the content of
Sb, the
content of Ca, the content of Mg, the content of B, and the content of REM of
Test
No. 1 were each 0% when rounded off to third decimal places.
[0105]
[Table 1A]
TABLE lA
Steel Chemical Composition (unit is mass%; balance is
Fe and impurities)
Type C Si Mn P S Al Ni Cr Mo Cu N V
A 0.006 0.46 5.70 0.003 0.0033 0.058 4.80 23.40 1.40 2.20 0.190 0.96
B 0.022 0.50 1.70 0.018 0.0122 0.052 6.80 24.80 1.00 2.00 0.220 0.96
C 0.015 0.40 2.10 0.027 0.0001 0.046 5.10 24.60 1.30 2.50 0.190 0.78
D 0.021 0.35 5.70 0.030 0.0002 0.041 6.00 22.10 0.70 1.30 0.240 0.13
E 0.023 0.58 5.80 0.004 0.0116 0.064 7.10 26.20 1.10 2.00 0.180 0.83
F 0.006 0.45 4.90 0.010 0.0155 0.016 7.50 29.00 1.20 1.40 0.300 0.19
G 0.024 0.40 5.90 0.005 0.0133 0.090 6.20 25.30 1.00 2.10 0.170 0.14
H 0.010 0.52 3.00 0.028 0.0078 0.062 6.40 26.10 1.00 1.10 0.290 0.56
I 0.024 0.38 3.80 0.004 0.0005 0.051 4.30 27.70 1.30 1.10 0.200 0.46
J 0.023 0.42 5.80 0.025 0.0048 0.038 6.90 22.30 0.70 1.80 0.210 0.92
K 0.021 0.38 5.00 0.011 0.0033 0.074 7.50 26.40 1.00 1.50 0.240 0.85
L 0.017 0.34 6.00 0.029 0.0030 0.031 7.30 27.50 1.00 2.20 0.250 0.62
M 0.013 0.32 2.50 0.005 0.0088 0.021 5.50 26.20 1.20 1.90 0.240 0.32
N 0.019 0.37 3.80 0.017 0.0090 0.080 6.70 28.00 1.40 1.90 0.220 0.70
O 0.008 0.45 4.30 0.016 0.0059 0.068 6.50 26.70 1.10 1.80 0.230 0.29
P 0.012 0.38 2.30 0.013 0.0061 0.025 7.20 26.40 0.70 2.10 0.220 0.24
Q 0.025 0.41 2.50 0.018 0.0137 0.067 4.70 26.40 0.90 2.10 0.220 0.79
R 0.020 0.44 4.60 0.020 0.0117 0.030 6.90 27.30 0.80 1.50 0.250 0.62
S 0.022 0.52 1.30 0.018 0.0019 0.040 5.90 26.00 1.00 2.60 0.340 0.50
T 0.020 0.59 3.00 0.008 0.0122 0.029 4.90 26.60 0.70 2.20 0.320 0.79
U 0.005 0.51 0.70 0.008 0.0148 0.018 5.50 27.70 0.90 1.60 0.160 0.94
/ 0.018 0.88 6.60 0.024 0.0055 0.074 8.80 29.50 1.90 0.60 0.155 1.35
W 0.015 0.55 0.75 0.025 0.0012 0.033 6.00 21.30 1.80 2.80 0.220 0.05
X 0.023 0.37 4.80 0.024 0.0010 0.013 4.50 27.70 1.50 2.60 0.380 0.47
Y 0.021 0.41 1.90 0.029 0.0144 0.073 4.60 25.30 1.20 2.60 0.120 0.80
Z 0.008 0.31 1.40 0.019 0.0152 0.060 2.30 25.50 1.40 1.50 0.290 0.91
CA 03241527 2024- 6- 18
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[0106]
[Table 1B]
TABLE 1B
Steel Chemical Composition (unit is mass%; balance is
Fe and impurities)
Type Nb Ta Ti Zr Hf W Co Sn Sb Ca Mg B REM
A- - - - - - -
B 0.015 - - - - - - - - - -
- -
C - 0.034 - - - - - - - - -
- -
D - - 0.053 - - - - - - -
- -
E - - 0.067 - - - - - - -
i -
F - - - - 0.060 - - - - - -
- -
G - - - - - 0.050 - - - - -
- -
H - - - - - 0.078 - - - -
- -
I - - - - - - - 0.042 - - -
- -
J - - - - - - - 0.050 - -
- -
K - - - - - - - - - 0.010 -
- -
L - - - - - - - - - - 0.010
- -
M - - - - - - - - - -
0.014 -
N - - - - - - - - - - - -
0.028
O 0.035 0.014 - - - - 0.193
- - - - -
P - - 0.013 - - - - - 0.007 -
- -
Q - - - - - - 0.022 - -
0.009 - -
R - - - 0.029 - - - 0.057
- - 0.008 0.094
S- - - - - - - -
- -
T- - - - - - - - - - -
- -
U- - - - - - - - -
- - -
/ 0.071 - - - - 0.180 0.450 -
- - - -
W - 0.070 - - - - - -
- 0.150
X- - - - - - - - - -
-
Y_ _ _ _ _ _ _ _ _ _ _
_ _
Z- - - - - - - - - - - -
-
[0107]
The ingot of each steel type was heated to the heating temperature ( C) for
hot
working described in Table 2, and thereafter subjected to hot rolling to
produce a
hollow shell (seamless steel pipe) having an outer diameter of 177.8 mm and a
wall
thickness of 12.65 mm. The hollow shell of each test number on which the hot
working had been performed was subjected to a secondary austenite
precipitation
treatment in which the hollow shell was heated at a heating rate ( C/sec) from
400 to
800 C described in Table 2, and held for only a holding time (mins) at a
holding
CA 03241527 2024- 6- 18
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temperature ( C) which are described in Table 2. In addition, a solution
treatment
was performed at a solution treatment temperature ( C) for a solution
treatment time
(mins) which are described in Table 2.
[0108]
[Table 2]
TABLE 2
Hot Working Secondary Austenite Precipitation Treatment Solution
Treatment
Heating Rate Solution
Solution
Test Steel Heating from 400 to
Holding Holding Treatment Treatment
Number Type Temperature
800 C Temperature Time Temperature Time
( C) ( C) (mins)
( C/sec) ( C)
(mins)
1 A 1270 0.43 930 17 985
23
2 B 1250 0.52 940 10 1020
28
3 C 1270 0.42 960 5 1075
36
4 D 1290 0.43 950 4 1085
71
E 1300 0.49 940 6 1080 51
6 F 1290 0.35 950 3 1025
89
7 G 1270 0.55 950 15 1065
29
8 H 1300 0.54 960 5 1095
21
9 I 1260 0.47 920 9 1020
57
J 1280 0.53 900 3 1015 51
11 K 1260 0.46 940 17 985
63
12 L 1270 0.47 930 13 1070
49
13 M 1290 0.48 940 20 1105
67
14 N 1300 0.54 930 6 1035
54
0 1290 0.54 905 8 1040 45
16 P 1290 0.50 900 10 1015
61
17 Q 1300 0.39 915 12 1035
76
18 R 1300 0.49 910 3 1030
88
19 S 1260 0.43 960 3 1030
35
T 1260 0.49 960 5 1040 40
21 U 1260 0.40 900 20 1055
33
22 V 1270 0.41 950 5 1070
20
23 W 1270 0.42 950 5 1070
20
24 A 1250 0.48 955 1 1075
47
0 1290 0.53 905 1 1040 40
26 A 1290 0.36 850 3 1000
66
27 0 1290 0.37 850 5 1030
35
28 R 1290 0.52 1000 8 1070
33
29 S 1270 0.38 1000 5 1030
35
F 1250 0.46 - - 1000 30
31 S 1270 0.55 - - 1030
35
32 G 1270 0.44 920 3 940
22
33 S 1270 0.35 920 10 930
20
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- 40 -
34 Q 1270 0.15 915 12 1035
35
35 T 1260 0.13 960 5 1040
30
36 X 1270 0.40 960 5 1075
40
37 Y 1250 0.48 910 15 1020
25
38 Z 1270 0.36 930 15 1080
27
[0109]
A seamless steel pipe of each test number was obtained by the above process.
The obtained seamless steel pipe of each test number was subjected to a
tensile test, a
microstructure observation test, and a Charpy impact test. Note that, cracking
was
confirmed in the obtained seamless steel pipes of Test Nos. 34 and 35.
Therefore,
these seamless steel pipes were not subjected to evaluation tests.
[0110]
[Tensile test]
The seamless steel pipe of each test number excluding Test Nos. 34 and 35
was subjected to a tensile test in accordance with ASTM E8/E8M (2021).
Specifically, an arc-shaped test specimen for a tensile test was prepared from
a
central portion of the wall thickness of the seamless steel pipe of each test
number.
The thickness of the arc-shaped test specimen was made the same as the wall
thickness of the steel pipe, the width was made 25.4 mm, and the gage length
was
made 50.8 mm. The arc-shaped test specimen of each test number was used to
carry out a tensile test at normal temperature (25 C) in air, and the 0.2%
offset proof
stress (MPa) was determined. The determined 0.2% offset proof stress was
defined
as the yield strength (MPa). The obtained yield strength of each test number
is
shown in the column "YS (MPa)" in Table 3.
[0111]
[Table 3]
TABLE 3
Microstructure Low-temperature
Test YS
Toughness
Number (MPa) Ferrite
Primary Austenite Secondary Austenite
Lowest
Volume Ratio Volume Ratio Temperature
Volume Ratio (%) (%) (%) (
C)
1 655 39 42 19 -30
2 572 45 46 9 -50
3 614 51 41 8 -30
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4 627 36 48 16 -
70
669 38 52 10 -50
6 703 40 50 10 -
20
7 614 43 50 7 -
20
8 683 46 46 8 -
40
9 634 47 46 7 -
20
717 36 50 14 -40
11 600 40 52 8 -
40
12 696 42 52 6 -
30
13 676 46 44 10 -
40
14 621 39 48 13 -
70
558 37 46 17 -40
16 676 36 48 16 -
60
17 565 44 44 12 -
60
18 600 36 50 14 -
40
19 689 53 42 5 -
20
676 43 51 6 -20
21 552 37 43 20 -
50
22 593 50 43 7 -
40
23 599 48 44 8 -
40
24 607 47 50 3 -
10
567 47 51 2 -10
26 517 36 42 22 -
50
27 523 36 41 23 -
50
28 641 54 45 1 0
29 566 56 42 2 -
10
710 44 55 1 0
31 572 55 44 1 0
32 503 38 23 39 -
20
33 520 40 22 38 -
30
34
_
36 621 46 50 4 -
10
37 510 45 27 28 -
50
38 707 69 28 3
10
[0112]
[Microstructure observation test]
The seamless steel pipe of each test number excluding Test Nos. 34 and 35
was subjected to microstructure observation, and the volume ratios of ferrite,
primary
austenite, and secondary austenite were determined. Specifically, a test
specimen
for microstructure observation having an observation surface with dimensions
of 5
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mm in the pipe radial direction x 5 mm in the pipe circumferential direction
was
prepared from a central portion of the wall thickness of the seamless steel
pipe of
each test number. The observation surface of the test specimen of each test
number
was polished to obtain a mirror surface, and then electrolytically etched in a
7%
potassium hydroxide etching solution. The observation surface on which the
microstructure had been revealed by the electrolytic etching was observed in
10
visual fields using an optical microscope. The area of each visual field was
40000
tim2 (200 lam x 200 lam).
[0113]
In each visual field of each test number, phases other than ferrite and
austenite
in the microstructure were negligibly small. That is, the seamless steel pipe
of each
test number had a microstructure composed of ferrite, primary austenite, and
secondary austenite. In each visual field of each test number, ferrite and
austenite
were each identified based on contrast. In addition, austenite with a minor
axis of
20 lam or more (primary austenite) was identified by the method described
above.
The area fractions (%) of the identified ferrite and primary austenite were
determined
by image analysis. In addition, based on the area fractions (%) of ferrite and
primary austenite, the area fraction (%) of secondary austenite was determined
by
Formula (A) which is described above. The arithmetic average value of the area
fractions of ferrite in the 10 visual fields was defined as the ferrite volume
ratio (%).
The arithmetic average value of the area fractions of primary austenite in the
10
visual fields was defined as the primary austenite volume ratio (%). The
arithmetic
average value of the area fractions of secondary austenite in the 10 visual
fields was
defined as the secondary austenite volume ratio (%). The determined ferrite
volume
ratio (%), primary austenite volume ratio (%), and secondary austenite volume
ratio
(%) of each test number are shown in Table 3.
[0114]
[Charpy impact test]
The seamless steel pipe of each test number excluding Test Nos. 34 and 35
was subjected to a Charpy impact test in accordance with ASTM E23 (2018), and
the
low-temperature toughness was evaluated. First, a V-notch test specimen for
the
Charpy impact test was prepared from the seamless steel pipe of each test
number in
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accordance with ASTM E23 (2018). Specifically, a V-notch test specimen having
a
notched surface parallel to the wall thickness direction and the pipe axis
direction
was prepared from a central portion of the wall thickness of the seamless
steel pipe
of each test number. Note that, the longitudinal direction of the V-notch test
specimen was parallel to the rolling direction of the steel material. Further,
the V-
notch test specimen was a full-size V-notch test specimen (having a width of
10 mm,
a thickness of 10 mm, and a length of 55 mm), and the V-notch depth was made 2
mm. Here, the term "width" of the V-notch test specimen means
the distance
between the surface in which the V-notch was formed and the surface on the
opposite side thereto in the V-notch test specimen.
[0115]
For each test number, three of the prepared V-notch test specimens were
cooled under each of the eight conditions, i.e. a temperature of 0 C, -10 C, -
20 C, -
30 C, -40 C, -50 C, -60 C, and -70 C, respectively. Each of the cooled test
specimens was subjected to the Charpy impact test in accordance with ASTM E23
(2018), and the absorbed energy (J) was determined. Note that, the arithmetic
average value for the three test specimens at each temperature was defined as
the
absorbed energy (J). The determined absorbed energy (J) was divided by the
cross-
sectional area (cm2) of the V-notch test specimen to determine the absorbed
energy
per unit area (J/cm2) at each temperature. Note that, the cross-sectional area
of the
V-notch test specimen (cm2) was taken as 0.8 cm2 (width of 0.8 cm x thickness
of
1.0 cm) as defined by the aforementioned method.
[0116]
Among the determined absorbed energies per unit area at the respective
temperatures, the lowest temperature ( C) at which the absorbed energy per
unit area
is 30 J/cm2 or more was determined. The determined lowest temperature of each
test number is shown in Table 3.
[0117]
[Evaluation results]
Referring to Table lA to Table 3, in the seamless steel pipes of Test Nos. 1
to
23, the chemical composition was appropriate. In addition, the production
method
was also the preferable production method described in the present
description. As
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a result, the yield strength was 552 MPa or more. Further, the volume ratio of
ferrite was 35 to 55%, the volume ratio of primary austenite was 40 to 55%,
and the
volume ratio of secondary austenite was 5 to 20%. As a result, the lowest
temperature at which the absorbed energy per unit area was 30 J/cm2 or more
was -
20 C or less. That is, the seamless steel pipes of Test Nos. 1 to 23 had a
yield
strength of 80 ksi or more and excellent low-temperature toughness.
[0118]
On the other hand, for the seamless steel pipes of Test Nos. 24 and 25, the
holding time in the secondary austenite precipitation treatment process was
too short.
As a result, the volume ratio of secondary austenite was less than 5%.
Consequently, the lowest temperature at which the absorbed energy per unit
area was
30 J/cm2 or more was more than -20 C. That is, these seamless steel pipes did
not
have excellent low-temperature toughness.
[0119]
For the seamless steel pipes of Test Nos. 26 and 27, the holding temperature
in the secondary austenite precipitation treatment process was too low. As a
result,
the volume ratio of secondary austenite was more than 20%. Consequently, the
yield strength was less than 552 MPa. That is, these seamless steel pipes did
not
have a yield strength of 80 ksi or more.
[0120]
For the seamless steel pipes of Test Nos. 28 and 29, the holding temperature
in the secondary austenite precipitation treatment process was too high. As a
result,
the volume ratio of secondary austenite was less than 5%. Consequently, the
lowest
temperature at which the absorbed energy per unit area was 30 J/cm2 or more
was
more than -20 C. That is, these seamless steel pipes did not have excellent
low-
temperature toughness.
[0121]
The seamless steel pipes of Test Nos. 30 and 31 were not subjected to the
secondary austenite precipitation treatment process. As a result, the volume
ratio of
secondary austenite was less than 5%. Consequently, the lowest temperature at
which the absorbed energy per unit area was 30 J/cm2 or more was more than -20
C.
That is, these seamless steel pipes did not have excellent low-temperature
toughness.
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[0122]
For the seamless steel pipes of Test No. 32 and 33, the solution treatment
temperature in the solution treatment process was too low. As a result, the
volume
ratio of primary austenite was less than 40%, and in addition, the volume
ratio of
secondary austenite was more than 20%. Consequently, the yield strength was
less
than 552 MPa. That is, these seamless steel pipes did not have a yield
strength of
80 ksi or more.
[0123]
For the seamless steel pipes of Test Nos. 34 and 35, in the secondary
austenite
precipitation treatment process, the heating rate from 400 to 800 C was too
slow.
As a result, cracking was confirmed in these seamless steel pipes. Therefore,
the
yield strength and the low-temperature toughness of these seamless steel pipes
were
not evaluated.
[0124]
In the seamless steel pipe of Test No. 36, the content of N was too high. As
a result, the volume ratio of secondary austenite was less than 5%.
Consequently,
the lowest temperature at which the absorbed energy per unit area was 30 J/cm2
or
more was more than -20 C. That is, this seamless steel pipe did not have
excellent
low-temperature toughness.
[0125]
In the seamless steel pipe of Test No. 37, the content of N was too low. As a
result, the volume ratio of primary austenite was less than 40%, and in
addition, the
volume ratio of secondary austenite was more than 20%. Consequently, the yield
strength was less than 552 MPa. That is, this seamless steel pipe did not have
a
yield strength of 80 ksi or more.
[0126]
In the seamless steel pipe of Test No. 38, the content of Ni was too low. As
a result, the volume ratio of ferrite was more than 55%, the volume ratio of
primary
austenite was less than 40%, and furthermore, the volume ratio of secondary
austenite was less than 5%. Consequently, the lowest temperature at which the
absorbed energy per unit area was 30 J/cm2 or more was more than -20 C. That
is,
this seamless steel pipe did not have excellent low-temperature toughness.
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[0127]
An embodiment of the present disclosure has been described above.
However, the embodiment described above is merely an example for carrying out
the
present disclosure. Therefore, the present disclosure is not limited to the
above-
described embodiment, and can be implemented by appropriately modifying the
above-described embodiment within a range that does not depart from the gist
of the
present disclosure.
REFERENCE SIGNS LIST
[0128]
Observation Visual Field Region
Ferrite
Austenite
31 Primary Austenite
32 Secondary Austenite
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