U.S. patent application number 17/597745 was filed with the patent office on 2022-05-12 for duplex stainless steel material.
The applicant listed for this patent is NIPPON STEEL CORPORATION. Invention is credited to Toshio MOCHIZUKI, Mikiko NOGUCHI, Yusaku TOMIO.
Application Number | 20220145438 17/597745 |
Document ID | / |
Family ID | 1000006140017 |
Filed Date | 2022-05-12 |
United States Patent
Application |
20220145438 |
Kind Code |
A1 |
NOGUCHI; Mikiko ; et
al. |
May 12, 2022 |
DUPLEX STAINLESS STEEL MATERIAL
Abstract
The duplex stainless steel material according to the present
disclosure has a chemical composition consisting of, in mass %, C:
0.030% or less, Si: 0.20 to 1.00%, Mn: 0.50 to 7.00%, P: 0.040% or
less, S: 0.0100% or less, Al: 0.100% or less, Ni: 4.20 to 9.00%,
Cr: 20.00 to 28.00%, Mo: 0.50 to 2.00%, Cu: 1.90 to 4.00%, N: 0.150
to 0.350%, V: 0.01 to 1.50%, and one or more types of element
selected from the group consisting of Ca: 0.0001 to 0.0200% and Mg:
0.0001 to 0.0200%, with the balance being Fe and impurities, and
satisfying Formulae (1) and (2) described in the description, a
microstructure consisting of 35.0 to less than 50.0% of ferrite in
volume ratio and austenite as the balance, and a yield strength of
550 MPa or more.
Inventors: |
NOGUCHI; Mikiko;
(Chiyoda-ku, Tokyo, JP) ; TOMIO; Yusaku;
(Chiyoda-ku, Tokyo, JP) ; MOCHIZUKI; Toshio;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL CORPORATION |
Tokyo |
|
JP |
|
|
Family ID: |
1000006140017 |
Appl. No.: |
17/597745 |
Filed: |
August 18, 2020 |
PCT Filed: |
August 18, 2020 |
PCT NO: |
PCT/JP2020/031050 |
371 Date: |
January 21, 2022 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/001 20130101;
C22C 38/06 20130101; C22C 38/005 20130101; C22C 38/54 20130101;
C22C 38/002 20130101; C22C 38/48 20130101; C22C 38/02 20130101;
C22C 38/58 20130101; C22C 38/46 20130101; C22C 38/44 20130101; C22C
38/42 20130101; C22C 38/50 20130101 |
International
Class: |
C22C 38/58 20060101
C22C038/58; C22C 38/54 20060101 C22C038/54; C22C 38/50 20060101
C22C038/50; C22C 38/48 20060101 C22C038/48; C22C 38/46 20060101
C22C038/46; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/06 20060101 C22C038/06; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 19, 2019 |
JP |
2019-149844 |
Claims
1. A duplex stainless steel material comprising: a chemical
composition consisting of, in mass %, C: 0.030% or less, Si: 0.20
to 1.00%, Mn: 0.50 to 7.00%, P: 0.040% or less, S: 0.0100% or less,
Al: 0.100% or less, Ni: 4.20 to 9.00%, Cr: 20.00 to 28.00%, Mo:
0.50 to 2.00%, Cu: 1.90 to 4.00%, 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%, B: 0 to 0.0200%, and rare earth metal: 0
to 0.200%, and one or more types of elements selected from the
group consisting of: Ca: 0.0001 to 0.0200%, and Mg: 0.0001 to
0.0200%, with the balance being Fe and impurities, satisfying
Formulae (1) and (2); a microstructure consisting of 35.0 to less
than 50.0% of ferrite in volume ratio and austenite as the balance;
and a yield strength of 550 MPa or more: 4 . 5 .times. 0 .ltoreq.
Mn + Cu .ltoreq. 9.50 ( 1 ) 13 .times. Cr - 19 .times. Ni + 21
.times. Mo - 17 .times. Cu + 63 .times. Mn + 8 .times. Si + 984
.times. N .gtoreq. 580 ( 2 ) ##EQU00007## where, a content in mass
% of a corresponding element is substituted for each symbol of an
element in Formulae (1) and (2).
2. The duplex stainless steel material according to claim 1,
wherein the chemical composition contains one or more types of
elements selected from the 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%, and Hf: 0.001 to 0.100%.
3. The duplex stainless steel material according to claim 1,
wherein the chemical composition contains one or more types of
elements selected from the group consisting of: B: 0.0005 to
0.0200%, and rare earth metal: 0.001 to 0.200%.
4. The duplex stainless steel material according to claim 2,
wherein the chemical composition contains one or more types of
elements selected from the group consisting of: B: 0.0005 to
0.0200%, and rare earth metal: 0.001 to 0.200%.
Description
TECHNICAL FIELD
[0001] The present disclosure relates to a duplex stainless steel
material.
BACKGROUND ART
[0002] There are cases in which oil wells or gas wells
(hereinafter, oil wells and gas wells are collectively referred to
simply as "oil wells") become a corrosive environment containing a
corrosive gas. Here, the 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] So far, as a method for improving the corrosion resistance
of the steel material, there is known a method of increasing the
content of chromium (Cr) and forming a passivation film mainly
composed of Cr oxide on the surface of the steel material.
Therefore, in an environment where excellent corrosion resistance
is required, a duplex stainless steel material having an increased
Cr content is used in some cases. On the other hand, a duplex
stainless steel material having a duplex microstructure consisting
of a ferrite phase and an austenite phase is excellent in corrosion
resistance with respect to pitting and/or crevice corrosion
(hereinafter, referred to as "pitting resistance") which is a
problem in an aqueous solution containing chlorides.
[0004] In recent years, furthermore, deep wells below sea level are
being actively developed. Therefore, there is a need to enhance the
strength of duplex stainless steel materials. That is, there is a
growing demand for a duplex stainless steel material that achieves
both high strength and excellent pitting resistance in a compatible
manner.
[0005] Japanese Patent Application Publication No. 05-132741
(Patent Literature 1), Japanese Patent Application Publication No.
09-195003 (Patent Literature 2), Japanese Patent Application
Publication No. 2014-043616 (Patent Literature 3), and Japanese
Patent Application Publication No. 2016-003377 (Patent Literature
4) each propose a duplex stainless steel that has high strength and
excellent corrosion resistance.
[0006] The duplex stainless steel disclosed in Patent Literature 1
has a chemical composition consisting of, in weight %, C: 0.03% or
less, Si: 1.0% or less, Mn: 1.5% or less, P: 0.040% or less, S:
0.008% or less, Sol. Al: 0.040% or less, Ni: 5.0 to 9.0%, Cr: 23.0
to 27.0%, Mo: 2.0 to 4.0%, W: more than 1.5 to 5.0%, and N: 0.24 to
0.32%, with the balance being Fe and unavoidable impurities, in
which PREW (=Cr+3.3(Mo+0.5W)+16N) is 40 or more. Patent Literature
1 discloses that this duplex stainless steel exhibits excellent
corrosion resistance and high strength.
[0007] The duplex stainless steel disclosed in Patent Literature 2
consists of, in weight %, C: 0.12% or less, Si: 1% or less, Mn: 2%
or less, Ni: 3 to 12%, Cr: 20 to 35%, Mo: 0.5 to 10%, W: more than
3 to 8%, Co: 0.01 to 2%, Cu: 0.1 to 5%, and N: 0.05 to 0.5%, with
the balance being Fe and unavoidable impurities. Patent Literature
2 discloses that this duplex stainless steel has more excellent
corrosion resistance, without lowering the strength.
[0008] The duplex stainless steel disclosed in Patent Literature 3
has a chemical composition consisting of, in mass %, C: 0.03% or
less, Si: 0.3% or less, Mn: 3.0% or less, P: 0.040% or less, S:
0.008% or less, Cu: 0.2 to 2.0%, Ni: 5.0 to 6.5%, Cr: 23.0 to
27.0%, Mo: 2.5 to 3.5%, W: 1.5 to 4.0%, N: 0.24 to 0.40%, and Al:
0.03% or less, with the balance being Fe and impurities, in which a
a phase susceptibility index X (=2.2Si+0.5Cu+2.0Ni+Cr+4.2Mo+0.2W)
is 52.0 or less, a strength index Y (=Cr+1.5Mo+10N+3.5W) is 40.5 or
more, and a pitting resistance equivalent PREW
(=Cr+3.3(Mo+0.5W)+16N) is 40 or more. In the micro-structure of the
steel, in a cross section in a thickness direction that is parallel
to a rolling direction, when a straight line is drawn to be
parallel to the thickness direction from the outer layer to a depth
of 1 mm, the number of boundaries between a ferrite phase and an
austenite phase which intersect with the straight line is 160 or
more. Patent Literature 3 discloses that the strength of this
duplex stainless steel can be enhanced without loss of corrosion
resistance, and by combining the use of cold working with a high
reduction rate, this duplex stainless steel exhibits excellent
hydrogen embrittlement resistance characteristics.
[0009] The duplex stainless steel disclosed in Patent Literature 4
has a chemical composition consisting of, in 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, Sol. 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 a metal microstructure
composed of a duplex microstructure of a ferrite phase and an
austenite phase, in which there is no precipitation of a sigma
phase, a proportion of the ferrite phase in the metal
microstructure is 50% or less in area ratio, and the number of
oxides having a particle size of 30 pm or more existing in a visual
field of 300 mm.sup.2 is 15 or less. Patent Literature 4 discloses
that this duplex stainless steel is excellent in strength, pitting
resistance, and low-temperature toughness.
CITATION LIST
Patent Literature
[0010] Patent Literature 1: Japanese Patent Application Publication
No. 05-132741 [0011] Patent Literature 2: Japanese Patent
Application Publication No. 09-195003 [0012] Patent Literature 3:
Japanese Patent Application Publication No. 2014-043616 [0013]
Patent Literature 4: Japanese Patent Application Publication No.
2016-003377
SUMMARY OF INVENTION
Technical Problem
[0014] As described above, in recent years, there is a growing
demand for a duplex stainless steel material that has higher
strength than before and that exhibits excellent pitting
resistance. Specifically, there is a growing demand for a duplex
stainless steel material which has a yield strength of 550 MPa or
more and exhibits excellent pitting resistance. Therefore, a duplex
stainless seamless steel material which has a yield strength of 550
MPa or more and excellent pitting resistance may be obtained by a
technique other than those disclosed in the aforementioned Patent
Literatures 1 to 4.
[0015] A duplex stainless steel material is also sometimes
subjected to hot rolling or hot working, such as hot extrusion,
during production. Therefore, in addition to high strength and
excellent pitting resistance, a duplex stainless steel material
also needs to be excellent in hot workability. However, in the
aforementioned Patent Literatures 1 to 4, hot workability has not
been examined.
[0016] An objective of the present disclosure is to provide a
duplex stainless steel material having a yield strength of 550 MPa
or more, excellent pitting resistance and excellent hot
workability.
Solution to Problem
[0017] A duplex stainless steel material according to the present
disclosure has: a chemical composition consisting of, in mass
%,
[0018] C: 0.030% or less,
[0019] Si: 0.20 to 1.00%,
[0020] Mn: 0.50 to 7.00%,
[0021] P: 0.040% or less,
[0022] S: 0.0100% or less,
[0023] Al: 0.100% or less,
[0024] Ni: 4.20 to 9.00%,
[0025] Cr: 20.00 to 28.00%,
[0026] Mo: 0.50 to 2.00%.
[0027] Cu: 1.90 to 4.00%,
[0028] N: 0.150 to 0.350%,
[0029] V: 0.01 to 1.50%.
[0030] Nb: 0 to 0.100%,
[0031] Ta: 0 to 0.100%,
[0032] Ti: 0 to 0.100%,
[0033] Zr: 0 to 0.100%.
[0034] Hf: 0 to 0.100%,
[0035] B: 0 to 0.0200%, and
[0036] rare earth metal: 0 to 0.200%, and
[0037] one or more types of elements selected from the group
consisting of
[0038] Ca: 0.0001 to 0.0200%, and
[0039] Mg: 0.0001 to 0.0200%,
[0040] with the balance being Fe and impurities, and
[0041] satisfying Formulae (1) and (2);
[0042] a microstructure consisting of 35.0 to less than 50.0% of
ferrite in volume ratio and austenite as the balance; and
[0043] a yield strength of 550 MPa or more:
4 . 5 .times. 0 .ltoreq. Mn + Cu .ltoreq. 9.50 ( 1 ) 13 .times. Cr
- 19 .times. Ni + 21 .times. Mo - 17 .times. Cu + 63 .times. Mn + 8
.times. Si + 984 .times. N .gtoreq. 580 ( 2 ) ##EQU00001##
[0044] where, a content in mass % of a corresponding element is
substituted for each symbol of an element in Formulae (1) and
(2).
Advantageous Effects of Invention
[0045] A duplex stainless steel material according to the present
disclosure has a yield strength of 550 MPa or more, excellent
pitting resistance and excellent hot workability.
BRIEF DESCRIPTION OF DRAWING
[0046] FIG. 1 is a view illustrating a relation between a value of
Fn2 and a yield strength (MPa) of a steel material in the present
Example.
DESCRIPTION OF EMBODIMENT
[0047] The present inventors conducted investigations and studies
regarding a duplex stainless steel material having a yield strength
of 550 MPa or more, excellent pitting resistance and excellent hot
workability. As a result, the present inventors obtained the
following findings.
[0048] First, the present inventors have considered that if a
duplex stainless steel material has a chemical composition
consisting of, in mass %. C: 0.030% or less, Si: 0.20 to 1.00%, Mn:
0.50 to 7.00%, P: 0.040% or less, S: 0.0100% or less, Al: 0.100% or
less, Ni: 4.20 to 9.00%, Cr: 20.00 to 28.00%, Mo: 0.50 to 2.00%,
Cu: 1.90 to 4.00%, 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%, B: 0 to 0.0200%, and rare earth metal: 0 to 0.200%, and one
or more types of elements selected from the uroup consisting of Ca:
0.0001 to 0.0200% and Mg: 0.0001 to 0.0200%, with the balance being
Fe and impurities, there is a possibility that a duplex stainless
steel material having a yield strength of 550 MPa or more,
excellent pitting resistance and excellent hot workability will be
obtained.
[0049] As described above, a duplex stainless steel material has a
characteristic of being excellent in pitting resistance. Here, the
microstructure of the duplex stainless steel material having the
aforementioned chemical composition consists of ferrite and
austenite. Note that, as used herein, "consists of ferrite and
austenite" means that the amount of any phase other than ferrite
and austenite is negligibly small.
[0050] First, the present inventors found that, with respect to a
duplex stainless steel material having the aforementioned chemical
composition in which the microstructure consists of ferrite and
austenite, the pitting resistance is increased by appropriately
controlling the volume ratios of ferrite and austenite.
Specifically, the present inventors found that the pitting
resistance of the duplex stainless steel material is increased by
making the volume ratio of ferrite 35.0 to less than 50.0%.
Therefore, the microstructure of the duplex stainless steel
material according to the present embodiment is a microstructure
consisting of 35.0 to less than 50.0% of ferrite in volume ratio
and austenite as the balance.
[0051] Thus, in the duplex stainless steel material according to
the present embodiment having the chemical composition and
microstructure described above, the volume ratio of austenite is
equal to or more than the volume ratio of ferrite. On the other
hand, the strength of austenite is lower than the strength of
ferrite. That is, in the duplex stainless steel material having the
aforementioned chemical composition and microstructure, because
austenite, which has lower strength, is contained in a greater
amount than ferrite, which has higher strength, the strength as a
steel material overall is liable to be low. Therefore, the present
inventors investigated various approaches for increasing the
strength in a duplex stainless steel material having the
aforementioned chemical composition and microstructure. As a
result, the present inventors obtained the following findings.
[0052] As a chemical composition for increasing the yield strength
of a duplex stainless steel material, first, the present inventors
focused on manganese (Mn) and copper (Cu). Mn dissolves in a steel
material and increases the yield strength of the steel material.
Further, Cu precipitates as fine Cu precipitates in a steel
material, and increases the yield strength of the steel material.
That is, the present inventors considered that if the Mn content
and Cu content are increased, the yield strength of the steel
material will increase.
[0053] Here, it is defined that Fn1=Mn+Cu. When Fn1 is increased,
the yield strength of the steel material increases. However, it was
revealed that in a duplex stainless steel material having the
aforementioned chemical composition and microstructure, if Fn1 is
too high, although the yield strength of the steel material will
increase, the hot workability of the steel material will decrease.
Therefore, the duplex stainless steel material according to the
present embodiment satisfies the following Formula (1). As a
result, on the condition that the other requirements according to
the present embodiment are satisfied, the duplex stainless steel
material of the present embodiment can achieve both high yield
strength and excellent hot workability in a compatible manner.
4.50 .ltoreq. Mn + Cu .ltoreq. 9.50 ( 1 ) ##EQU00002##
[0054] Where, a content in mass % of a corresponding element is
substituted for each symbol of an element in Formula (1).
[0055] On the other hand, even in the case of a duplex stainless
steel material that has the aforementioned chemical composition and
microstructure and that satisfies Formula (1), there were some
cases in which a yield strength of 550 MPa or more could not be
consistently obtained. Therefore, next, the present inventors
conducted detailed investigations and studies with regard to
increasing the yield strength of a duplex stainless steel material
having the aforementioned chemical composition and microstructure
by a method other than a method that adjusts the aforementioned
Fn1. As a result, the present inventors obtained the following
findings.
[0056] As described above, in the duplex stainless steel material
according to the present embodiment that contains a large amount of
austenite, the yield strength of the steel material overall is
liable to be low due to the characteristics of austenite. That is,
if the strength of the austenite can be increased, the yield
strength of the duplex stainless steel material can be increased.
Specifically, as an approach for increasing the strength of
austenite, the present inventors focused on the dissolved nitrogen
(N) amount. N dissolves in the steel material and increases the
strength of the steel material. That is, if N can be caused to
selectively dissolve in austenite, the strength of austenite can be
selectively increased, and as a result, there is a possibility of
increasing the yield strength of the duplex stainless steel
material.
[0057] Taking into consideration the above findings, the present
inventors discovered that, with respect to a duplex stainless steel
material having the aforementioned chemical composition and
microstructure and satisfying Formula (1), if the chemical
composition is further caused to satisfy the following Formula (2),
the yield strength of the duplex stainless steel material is
increased.
13 .times. Cr - 19 .times. Ni + 21 .times. Mo - 17 .times. Cu + 63
.times. Mn + 8 .times. Si + 984 .times. N .gtoreq. 580 ( 2 )
##EQU00003##
[0058] Where, a content in mass % of a corresponding element is
substituted for each symbol of an element in Formula (2).
[0059] It is defined that Fn2
=13.times.Cr-19.times.Ni+21.times.Mo-17.times.Cu+63.times.Mn+8.times.Si+9-
84.times.N. FIG. 1 is a view illustrating a relation between a
value of Fn2 and a yield strength (MPa) of steel materials in the
present Example. FIG. 1 was created using the value of Fn2 and the
yield strength (Tv Pa) with respect to, among Examples that are
described later, Examples having the aforementioned chemical
composition and microstructure and satisfying Formula (1). Note
that the yield strength was determined by a method to be described
later.
[0060] Referring to FIG. 1, in the relation between Fn2 and the
yield strength, an inflection point exists in the vicinity of
Fn2=580. Further, it can be confirmed that when Fn2 is 580 or more,
the yield strength markedly increases. Accordingly, the duplex
stainless steel material according to the present embodiment has
the aforementioned chemical composition, and a microstructure
consisting of 35.0 to less than 50.0% of ferrite in volume ratio
and austenite as the balance, in which Fn1 is 4.50 to 9.50, and
furthermore Fn2 is 580 or more. As a result, the duplex stainless
steel material according to the present embodiment had a high yield
strength of 550 MPa or more, excellent pitting resistance, and
excellent hot workability.
[0061] 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.
[1]
[0062] A duplex stainless steel material having:
[0063] a chemical composition consisting of, in mass %,
[0064] C: 0.030% or less,
[0065] Si: 0.20 to 1.00%,
[0066] Mn: 0.50 to 7.00%,
[0067] P: 0.040% or less,
[0068] S: 0.0100% or less,
[0069] Al: 0.100% or less,
[0070] Ni: 4.20 to 9.00%,
[0071] Cr: 20.00 to 28.00%,
[0072] Mo: 0.50 to 2.00%,
[0073] Cu: 1.90 to 4.00%,
[0074] N: 0.150 to 0.350%,
[0075] V: 0.01 to 1.50%,
[0076] Nb: 0 to 0.100%,
[0077] Ta: 0 to 0.100%,
[0078] Ti: 0 to 0.100%,
[0079] Zr: 0 to 0.100%,
[0080] Hf: 0 to 0.100%,
[0081] B: 0 to 0.0200%, and
[0082] rare earth metal: 0 to 0.200%, and
[0083] one or more types of elements selected from the group
consisting of
[0084] Ca: 0.0001 to 0.0200%, and
[0085] Mg: 0.0001 to 0.0200%,
[0086] with the balance being Fe and impurities, and
[0087] satisfying Formulae (1) and (2);
[0088] a microstructure consisting of 35.0 to less than 50.0% of
ferrite in volume ratio and austenite as the balance; and
[0089] a yield strength of 550 MPa or more:
4 . 5 .times. 0 .ltoreq. Mn + Cu .ltoreq. 9.50 ( 1 ) 13 .times. Cr
- 19 .times. Ni + 21 .times. Mo - 17 .times. Cu + 63 .times. Mn + 8
.times. Si + 94 .times. N .gtoreq. 580 ( 2 ) ##EQU00004##
[0090] where, a content in mass % of a corresponding element is
substituted for each symbol of an element in Formulae (1) and
(2).
[2]
[0091] The duplex stainless steel material according to [1],
wherein
[0092] the chemical composition contains one or more types of
elements selected from the group consisting of:
[0093] Nb: 0.001 to 0.100%,
[0094] Ta: 0.001 to 0.100%,
[0095] Ti: 0.001 to 0.100%,
[0096] Zr: 0.001 to 0.100%. and
[0097] Hf: 0.001 to 0.100%.
[3]
[0098] The duplex stainless steel material according to [1] or [2],
wherein
[0099] the chemical composition contains one or more types of
elements selected from the group consisting of:
[0100] B: 0.0005 to 0.0200%, and
[0101] rare earth metal: 0.001 to 0.200%.
[0102] Hereinafter, the duplex stainless steel material according
to the present embodiment will be described in detail. Note that
"%" concerning an element means mass % unless otherwise
specified.
[0103] [Chemical Composition]
[0104] The chemical composition of the duplex stainless steel
material according to the present embodiment contains the following
elements.
[0105] C: 0.030% or less
[0106] Carbon (C) is unavoidably contained. That is, the lower
limit of the C content is more than 0%. If the C content is too
high, C will form Cr carbides at crystal grain boundaries and
increase corrosion susceptibility at the grain boundaries even if
the contents of other elements are within the range of the present
embodiment. As a result, the pitting resistance of the steel
material will deteriorate. Therefore, the C content is 0.030% or
less. An upper limit of the C content is preferably 0.028%, and
more preferably 0.025%. The C content is preferably as low as
possible. However, an extreme reduction of the C content will
significantly increase the production cost. Therefore, when
industrial manufacturing is taken into consideration, a lower limit
of the C content is preferably 0.001%, and more preferably
0.005%.
[0107] Si: 0.20 to 1.00%
[0108] Silicon (Si) deoxidizes steel. If the Si content is too low,
the aforementioned effect cannot be sufficiently obtained even if
the contents of other elements is within the range of the present
embodiment. On the other hand, if the Si content is too high, the
low-temperature toughness and hot workability of the steel material
will deteriorate even if the contents of other elements are within
the range of the present embodiment. Therefore. the Si content is
0.20 to 1.00%. A lower limit of the Si content is preferably 0.25%,
and more preferably 0.30%. An upper limit of the Si content is
preferably 0.80%, and more preferably 0.60%.
[0109] Mn: 0.50 to 7.00%
[0110] Manganese (Mn) deoxidizes steel and desulfurizes steel. Mn
also dissolves in the steel material and increases the strength of
the steel material. Furthermore, Mn enhances the hot workability of
the steel material. If the Mn content is too low, the
aforementioned effects cannot be sufficiently obtained even if the
contents of other elements are within the range of the present
embodiment. On the other hand, if the Mn content is too high, Mn
will segregate at grain boundaries together with impurities such as
P and S, and the corrosion resistance of the steel material in a
high-temperature environment will deteriorate even if the contents
of other elements are within the range of the present embodiment.
Therefore, the Mn content is 0.50 to 7.00%. A lower limit of the Mn
content is preferably 0.75%, more preferably 0.90%, further
preferably 1.75%, further preferably 2.00%, and further preferably
2.20%. An upper limit of the Mn content is preferably 6.50%, and
more preferably 6.20%.
[0111] P; 0.040% or less
[0112] Phosphorus (P) is an impurity. That is, the lower limit of
the P content is more than 0%. If the P content is too high, P will
segregate at grain boundaries and the low-temperature toughness of
the steel material will deteriorate even if the contents of other
elements are within the range of the present embodiment. Therefore,
the P content is 0.040% or less. An upper limit of the P content is
preferably 0.035%, and more preferably 0.030%. The P content is
preferably as low as possible. However, an extreme reduction of the
P content will significantly increase the production cost.
Therefore, when industrial manufacturing is taken into
consideration, a lower limit of the P content is preferably 0.001%,
and more preferably 0.003%.
[0113] S: 0.0100% or less
[0114] Sulfur (S) is an impurity. That is, the lower limit of the S
content is more than 0%. If the S content is too high, S will
segregate at grain boundaries and the low-temperature toughness and
hot workability of the steel material will deteriorate even if the
contents of other elements are within the range of the present
embodiment. Therefore, the S content is 0.0100% or less. An upper
limit of the S content is preferably 0.0085%, and more preferably
0.0030%. The S content is preferably as low as possible. However,
an extreme reduction of the S content will significantly increase
the refining cost in the steel making process. Therefore, when
industrial manufacturing is taken into consideration, a lower limit
of the S content is preferably 0.0001%, and more preferably
0.0002%.
[0115] Al: 0.100% or less
[0116] Aluminum (Al) is unavoidably contained. That is, a lower
limit of the Al content is more than 0%. Al deoxidizes the steel.
On the other hand, if the Al content is too high, coarse
oxide-based inclusions are formed and low-temperature toughness of
the steel material deteriorates even if the contents of other
elements are within the range of the present embodiment. Therefore,
the Al content is 0.100% or less. A lower limit of the Al content
is preferably 0.001%, more preferably 0.005%, and further
preferably 0.010%. An upper limit of the Al content is preferably
0.090%, and more preferably 0.085%. Note that the Al content
referred to in the present description means the content of
"acid-soluble Al," that is, sol. Al.
[0117] Ni: 4.20 to 9.00%
[0118] Nickel (Ni) is an element that stabilizes the austenitic
structure of a steel material. That is, Ni is an element necessary
for obtaining a stable microstructure consisting of ferrite and
austenite. Ni also enhances the corrosion resistance of the steel
material in a high-temperature environment. If the Ni content is
too low, the aforementioned effect cannot be sufficiently obtained
even if the contents of other elements are within the range of the
present embodiment. On the other hand, if the Ni content is too
high, the volume ratio of austenite becomes too high and the
strength of the steel material decreases even if the content of
other elements is within the range of the present embodiment.
Therefore, the Ni content is 4.20 to 9.00%. A lower limit of the Ni
content is preferably 4.30%, more preferably 4.35%, further
preferably 4.40%, further preferably 4.50%, and further preferably
4.60%. An upper limit of the Ni content is preferably 8.50%, more
preferably 8.00%, further preferably 7.50%, further preferably
7.00%, and further preferably 6.75%.
[0119] Cr: 20.00 to 28.00%
[0120] Chromium (Cr) enhances the corrosion resistance of the steel
material in a high-temperature environment. Specifically, Cr forms
a passivation film as an oxide on the surface of the steel
material, and thereby increases the corrosion resistance of the
steel material. Cr also increases the volume ratio of the ferritic
structure of the steel material. By obtaining a sufficient ferritic
structure, the corrosion resistance of the steel material is
stabilized. If the Cr content is too low, the aforementioned
effects cannot be sufficiently obtained even if the contents of
other elements are within the range of the present embodiment. On
the other hand, if the Cr content is too high, the hot workability
of the steel material deteriorates even if the contents of other
elements are within the range of the present embodiment. Therefore,
the Cr content is 20.00 to 28.00%. A lower limit of the Cr content
is preferably 20.50%, more preferably 21.00%, and further
preferably 21.50%. An upper limit of the Cr content is preferably
27.50%, more preferably 27.00%, and further preferably 26.50%.
[0121] Mo: 0.50 to 2.00%
[0122] Molybdenum (Mo) enhances the corrosion resistance of the
steel material in a high-temperature environment. If the Mo content
is too low, the aforementioned effect cannot be sufficiently
obtained even if the contents of other elements are within the
range of the present embodiment. On the other hand, if the Mo
content is too high, hot workability of the steel material
deteriorates even if the contents of other elements are within the
range of the present embodiment. Therefore, the Mo content is 0.50
to 2.00%. A lower limit of the Mo content is preferably 0.60%, more
preferably 0.70%, and further preferably 0.80%. An upper limit of
the Mo content is preferably less than 2.00%, more preferably
1.85%, and further preferably 1.50%.
[0123] Cu: 1.90 to 4.00%
[0124] Copper (Cu) increases the strength of the steel material by
precipitation strengthening. Cu further enhances the corrosion
resistance of the steel material in a high-temperature environment.
If the Cu content is too low, the aforementioned effect cannot be
sufficiently obtained even if the contents of other elements are
within the range of the present embodiment. On the other hand, if
the Cu content is too high, hot workability of the steel material
deteriorates even if the contents of other elements are within the
range of the present embodiment. Therefore, the Cu content is 1.90
to 4.00%. A lower limit of the Cu content is preferably 2.00%, more
preferably more than 2.00%, further preferably 2.10%, further
preferably 2.20%, and further preferably 2.50%. An upper limit of
the Cu content is preferably 3.90%, more preferably 3.75%, and
further preferably 3.50%.
[0125] N: 0.150 to 0.350%
[0126] Nitrogen (N) is an element that stabilizes the austenitic
structure of a steel material. That is, N is an element necessary
for obtaining a stable microstructure consisting of ferrite and
austenite. N further enhances the corrosion resistance of the steel
material. If the N content is too low, the aforementioned effect
cannot be sufficiently obtained even if the contents of other
elements are within the range of the present embodiment. On the
other hand, if the N content is too high, toughness and hot
workability of the steel material will deteriorate even if the
contents of other elements are within the range of the present
embodiment. Therefore, the N content is 0.150 to 0.350%. A lower
limit of the N content is preferably 0.170%, more preferably
0.180%, and further preferably 0.200%. An upper limit of the N
content is preferably 0.340%, and more preferably 0.330%.
[0127] V: 0.01 to 1.50%
[0128] Vanadium (V) forms a carbonitride and increases the strength
of the steel material. If the V content is too low, the
aforementioned effect cannot be sufficiently obtained even if the
contents of other elements are within the range of the present
embodiment. On the other hand, if the V content is too high, the
strength of the steel material will be too high and the toughness
of the steel material will deteriorate even if the contents of
other elements are within the range of the present embodiment.
Therefore, the V content is 0.01 to 1.50%. A lower limit of the V
content is preferably 0.02%, more preferably 0.03%, and further
preferably 0.05%. An upper limit of the V content is preferably
1.20%, and more preferably 1.00%.
[0129] The chemical composition of the duplex stainless steel
material according to the present embodiment contains one or more
types of elements selected from the group consisting of Ca and Mg.
That is, the chemical composition of the duplex stainless steel
material according to the present embodiment may contain at least
one of Ca and Mg, or may contain both Ca and Mg. In other words,
either one of Ca and Mg need not be contained. In short, the
content of either one of Ca and Mg may be 0%. Each of these
elements improves the hot workability of the steel material.
[0130] Ca: 0.0001 to 0.0200%
[0131] Calcium (Ca) immobilizes S in the steel material as sulfide
to make it harmless, and thereby improves the hot workability of
the steel material. On the other hand, if the Ca content is too
high, even if the contents of other elements are within the range
of the present embodiment, oxides in the steel material coarsen and
the toughness of the steel material deteriorates. Therefore, when
contained, the Ca content is 0.0001 to 0.0200%. A preferable lower
limit of the Ca content for more effectively obtaining the
aforementioned effect is 0.0003%, more preferably 0.0005%, further
preferably 0.0008%, and further preferably 0.0010%. An upper limit
of the Ca content is preferably 0.0180%, and more preferably
0.0150%.
[0132] Mg: 0.0001 to 0.0200%
[0133] Magnesium (Mg) immobilizes S in the steel material as
sulfide to make it harmless, and thereby improves the hot
workability of the steel material. On the other hand, if the Mg
content is too high, even if the contents of other elements are
within the range of the present embodiment, oxides in the steel
material coarsen and the toughness of the steel material
deteriorates. Therefore, when contained, the Mg content is 0.0001
to 0.0200%. A preferable lower limit of the Mg content for more
effectively obtaining the aforementioned effect is 0.0003%, more
preferably 0.0005%, further preferably 0.0008%, and further
preferably 0.0010%. An upper limit of the Mg content is preferably
0.0180%, and more preferably 0.0150%.
[0134] The balance of the chemical composition of the duplex
stainless steel material according to the present embodiment is Fe
and impurities. Here, impurities in a chemical composition means
those which are mixed from ores and scraps as the raw material or
from the production environment when industrially producing the
duplex stainless steel material, and which are permitted within a
range not adversely affecting the duplex stainless steel material
of the present embodiment.
[0135] Note that various elements may be mentioned as examples of
impurities. The impurities may be one type of element only, or may
be two or more types of elements. The impurities are, for example,
Co, W, Sb and Sn. There may be a case where these elements are
contained, for example, as impurities having the following
contents:
[0136] Co: 0.30% or less, W: 0.30% or less, Sb: 0.30% or less, and
Sn: 0.30% or less.
[0137] [Optional Elements]
[0138] The chemical composition of the duplex stainless steel
material described above may further contain one or more types of
elements selected from the group consisting of Nb, Ta, Ti, Zr, and
Hf in lieu of a part of Fe. All of these elements are optional
elements and increase the strength of the steel material.
[0139] Nb: 0 to 0.100%
[0140] Niobium (Nb) is an optional element and does not have to be
contained. That is, the Nb content may be 0%. When contained, Nb
forms a carbonitride and increases the strength of the steel
material. If even a small amount of Nb is contained, the
aforementioned effect can be obtained to some extent. However, if
the Nb content is too high, the strength of the steel material
becomes too high and the toughness of the steel material
deteriorates even if the contents of other elements are within the
range of the present embodiment. Therefore, the Nb content is 0 to
0.100%. A lower limit of the Nb content is preferably more than 0%,
more preferably 0.001%, further preferably 0.002%. An upper limit
of the Nb content is preferably 0.080%, and more preferably
0.070%.
[0141] Ta: 0 to 0.100%
[0142] Tantalum (Ta) is an optional element and does not have to be
contained. That is, the Ta content may be 0%. When contained, Ta
forms a carbonitride and increases the strength of the steel
material. If even a small amount of Ta is contained, the
aforementioned effect can be obtained to some extent. However, if
the Ta content is too high, the strength of the steel material
becomes too high and the toughness of the steel material
deteriorates even if the contents of other elements are within the
range of the present embodiment. Therefore, the Ta content is 0 to
0.100.degree. 4). A lower limit of the Ta content is preferably
more than 0%, more preferably 0.001%, further preferably 0.002%,
and further preferably 0.003%. An upper limit of the Ta content is
preferably 0.080%, and more preferably 0.070%.
[0143] Ti: 0 to 0.100%
[0144] Titanium (Ti) is an optional element and does not have to be
contained. That is, the Ti content may be 0%. When contained, Ti
forms a carbonitride and increases the strength of the steel
material. If even a small amount of Ti is contained, the
aforementioned effect can be obtained to some extent. However, if
the Ti content is too high, the strength of the steel material
becomes too high and the toughness of the steel material
deteriorates even if the contents of other elements are within the
range of the present embodiment. Therefore, the Ti content is 0 to
0.100%. A lower limit of the Ti content is preferably more than 0%,
more preferably 0.001%, further preferably 0.002%. An upper limit
of the Ti content is preferably 0.080%, and more preferably
0.070%.
[0145] Zr: 0 to 0.100%
[0146] Zirconium (Zr) is an optional element and does not have to
be contained. That is, the Zr content may be 0%. When contained, Zr
forms a carbonitride and increases the strength of the steel
material. If even a small amount of Zr is contained, the
aforementioned effect can be obtained to some extent. However, if
the Zr content is too high, the strength of the steel material
becomes too high and the toughness of the steel material
deteriorates even if the contents of other elements are within the
range of the present embodiment. Therefore, the Zr content is 0 to
0.100%. A lower limit of the Zr content is preferably more than 0%,
more preferably 0.001%, further preferably 0.002%, and further
preferably 0.003%. An upper limit of the Zr content is preferably
0.080%, and more preferably 0.070%.
[0147] Hf: 0 to 0.100%
[0148] Hafnium (Hf) is an optional element and does not have to be
contained. That is, the Hf content may be 0%. When contained, Hf
forms a carbonitride and increases the strength of the steel
material. If even a small amount of Hf is contained, the
aforementioned effect can be obtained to some extent. However, if
the Hf content is too high, the strength of the steel material
becomes too high and the toughness of the steel material
deteriorates even if the contents of other elements are within the
range of the present embodiment. Therefore, the Hf content is 0 to
0.100%. A lower limit of the Hf content is preferably more than 0%,
more preferably 0.001%, further preferably 0.002%. An upper limit
of the Hf content is preferably 0.080%, and more preferably
0.070%.
[0149] The chemical composition of the duplex stainless steel
material described above may further contain one or more types of
elements selected from the group consisting of B and rare earth
metal, in place of part of Fe. All of these elements are optional
elements and enhance the hot workability of the steel material.
[0150] B: 0 to 0.0200%
[0151] Boron (B) is an optional element and does not have to be
contained. That is, the B content may be 0%. When contained, B
suppresses segregation of S at grain boundaries in the steel
material and enhances the hot-workability of the steel material. If
even a small amount of B is contained, the aforementioned effect
can be obtained to some extent. However, if the B content is too
high, boron nitride (BN) is produced, thereby deteriorating the
low-temperature toughness of the steel material even if the
contents of other elements are within the range of the present
embodiment. Therefore, the B content is 0 to 0.0200%. A lower limit
of the B content is preferably more than 0%, more preferably
0.0005%, further preferably 0.0010%, further preferably 0.0015%,
and further preferably 0.0020%. An upper limit of the B content is
preferably 0.0180%, more preferably 0.0150%, and further preferably
0.0100%.
[0152] Rare earth metal: 0 to 0.200%
[0153] Rare earth metal (REM) is an optional element and does not
have to be contained. That is, the REM content may be 0%. When
contained, REM immobilizes S in the steel material as sulfide to
make it harmless, and thus improves the hot-workability of the
steel material. If even a small amount of REM is contained, the
aforementioned effect can be obtained to some extent. However, if
the REM content is too high, the oxide in the steel material
becomes coarse, thereby deteriorating the toughness of the steel
material even if the contents of other elements are within the
range of the present embodiment. Therefore, the REM content is 0 to
0.200%. A lower limit of the REM content is preferably more than
0%, more preferably 0.005%, and further preferably 0.010%. An upper
limit of the REM content is preferably 0.180%, more preferably
0.150%, further preferably 0.120%. and further preferably
0.100%.
[0154] Note that REM in this description is Scandium (Sc) of atomic
number 21, Yttrium (Y) of atomic number 39, and one or more types
of elements selected from the group consisting of lanthanum (La) of
atomic number 57 to lutetium (Lu) of atomic number 71, which are
called lanthanoids. Moreover, the REM content in the present
description is the total content of these elements.
[0155] [Regarding Formula (1)]
[0156] The chemical composition of the duplex stainless steel
material according to the present embodiment also satisfies the
following Formula (1).
4.50 .ltoreq. Mn + Cu .ltoreq. 9.50 ( 1 ) ##EQU00005##
[0157] Where, a content in mass % of a corresponding element is
substituted for each symbol of an element in Formula (1).
[0158] Fn1 (=Mn+Cu) is an index relating to the strength and the
hot workability of the duplex stainless steel material. If Fn1 is
too low, even if the other components are within the range of the
present embodiment, a yield strength of 550 MPa or more cannot be
obtained. On the other hand, if Fn1 is too high, even if the other
components are within the range of the present embodiment, the hot
workability of the duplex stainless steel material will
deteriorate. Therefore, in the chemical composition of the duplex
stainless steel material according to the present embodiment, Fn1
is 4.50 to 9.50. As a result, on the condition that the other
requirements of the present embodiment are satisfied, the duplex
stainless steel material can achieve both yield strength and hot
workability in a compatible manner.
[0159] A lower limit of Fn1 is preferably 4.55, more preferably
4.60, further preferably 4.70, and further preferably 5.00. An
upper limit of Fn1 is preferably 9.20, more preferably 9.00,
further preferably 8.70, and further preferably 8.50.
[0160] [Regarding Formula (2)]
[0161] The chemical composition of the duplex stainless steel
material according to the present embodiment also satisfies the
following Formula (2).
13 .times. Cr - 19 .times. Ni + 21 .times. Mo - 17 .times. Cu + 63
.times. Mn + 8 .times. Si + 984 .times. N .gtoreq. 580 ( 2 )
##EQU00006##
[0162] Where, a content in mass % of a corresponding element is
substituted for each symbol of an element in Formula (2).
[0163] Fn2
(=13.times.Cr-19.times.Ni+21.times.Mo-17.times.Cu+63.times.Mn.+-
-.8.times.Si+984.times.N) is an index indicating a distribution
state of N in the ferrite and the austenite. If Fn2 is too low,
even if the other components are within the range of the present
embodiment, a large proportion of N will be distributed in the
ferrite, and the amount of dissolved N in the austenite will
decrease. Consequently, the yield strength of the duplex stainless
steel material will decrease. Therefore, in the chemical
composition of the duplex stainless steel material according to the
present embodiment, Fn2 is 580 or more. As a result, on the
condition that the other requirements of the present embodiment are
satisfied, the amount of dissolved N in the austenite increases,
and the yield strength of the duplex stainless steel material can
be increased to 550 MPa or more.
[0164] A lower limit of Fn2 is preferably 590, more preferably 600,
and further preferably 610. An upper limit of Fn2 is not
particularly limited. However, in the range of the chemical
composition described above, the upper limit of Fn2 is practically
1087.
[0165] [Microstructure]
[0166] The microstructure of the duplex stainless steel material
according to the present embodiment consists of, in volume ratio,
35.0 to less than 50.0% of ferrite, and austenite as the balance.
As used herein, "consists of ferrite, and austenite as the balance"
means that the amount of any phase other than ferrite and the
austenite is negligibly small. For example, in the chemical
composition of the duplex stainless steel material according to the
present embodiment, volume ratios of precipitates and inclusions
are negligibly small as compared with volume ratios of ferrite and
austenite. That is, the microstructure of the duplex stainless
according to the present embodiment may contain minute amounts of
precipitates, inclusions, etc., in addition to ferrite and
austenite.
[0167] Further, in the microstructure of the duplex stainless steel
material according to the present embodiment, the volume ratio of
ferrite is 35.0 to less than 50.0%. If the volume ratio of ferrite
is too low, the strength and/or corrosion resistance of the steel
material may deteriorate. On the other hand, if the volume ratio of
ferrite is too high, the corrosion resistance of the steel material
deteriorates. Further, if the volume ratio of ferrite is too high,
the low-temperature toughness and/or the hot workability of the
steel material may deteriorate. Therefore, in the microstructure of
the duplex stainless steel material according to the present
embodiment, the volume ratio of ferrite is 35.0 to less than
50.0%.
[0168] A lower limit of the volume ratio of ferrite is preferably
35.5%, and more preferably 36.5%. An upper limit of the volume
ratio of ferrite is preferably 48.0%, more preferably 47.0%, and
further preferably 45.0%.
[0169] In the present embodiment, the volume ratio of ferrite in
the duplex stainless steel material can be determined by a method
conforming to ASTM E562 (2011). A test specimen for microstructure
observation is prepared from an arbitrary location in the duplex
stainless steel material according to the present embodiment. Here,
the arbitrary location from which the test specimen is prepared is
not particularly limited. For example, the test specimen is
prepared from a center portion in a thickness direction of the
steel material. An observation surface at which to carry out the
microstructure observation is not particularly limited. For
example, a cross section perpendicular to a rolling direction of
the duplex stainless steel material is adopted as the observation
surface. Note that the size of the test specimen is not
particularly limited, and it suffices that an observation surface
of 5 mm.times.5 mm can be obtained.
[0170] The observation surface of the test specimen taken is
mirror-polished. The minor-polished observation surface is
electrolytically etched in a 7% potassium hydroxide etching
solution to reveal the microstructure. The observation surface on
which the microstructure has been revealed is observed in 10 fields
of view using an optical microscope. The visual field area is not
particularly limited, and, for example, is 1.00 mm.sup.2 (at a
magnification of 100 times). In each field of view, ferrite is
identified from contrast. The area ratios of the identified ferrite
are measured by a point counting method conforming to ASTM E562
(2011). In the present embodiment, an arithmetic average value of
the area ratios of ferrite obtained in the 10 fields of view is
defined as the volume ratio (%) of ferrite.
[0171] [Yield Strength of Duplex Stainless Steel Material]
[0172] The yield strength of the duplex stainless steel material
according to the present embodiment is 550 MIPa or more. By having
the chemical composition and microstructure described above, the
duplex stainless steel material according to the present embodiment
exhibits excellent pitting resistance and excellent hot
workability, even when the yield strength is 550 MPa or more.
[0173] A lower limit of the yield strength of the duplex stainless
steel material according to the present embodiment is preferably
560 MPa, and more preferably 570 MPa. Note that an upper limit of
the yield strength of the duplex stainless steel material according
to the present embodiment is not particularly limited. The upper
limit of the yield strength of the duplex stainless steel material
according to the present embodiment is, for example, 700 MPa. The
upper limit of the yield strength may be 690 MPa, may be 680 MPa,
or may be 670 MPa.
[0174] 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 conforming to ASTM E8/E811.4 (2013). A round bar test
specimen is prepared from the steel material according to the
present embodiment. If the steel material is a steel plate, the
round bar test specimen is prepared from a center portion of the
thickness. If the steel material is a steel pipe, the round bar
test specimen is prepared from a center portion of the wall
thickness. The size of the round bar test specimen is, for example,
as follows: a parallel portion diameter is 6 mm and a parallel
portion length is 30 mm. Note that an axial direction of the round
bar test specimen is parallel with the rolling direction of the
steel material. The tensile test is carried out in the atmosphere
at room temperature (25.degree. C.) using the prepared round bar
test specimen, and the obtained 0.2% offset proof stress is defined
as the yield strength (MPa).
[0175] [Pitting Resistance of Duplex Stainless Steel Material]
[0176] By having the aforementioned chemical composition and the
aforementioned microstructure, the duplex stainless steel material
according to the present embodiment exhibits excellent pitting
resistance. In the present embodiment, excellent pitting resistance
is defined as follows.
[0177] Specifically, the duplex stainless steel material according
to the present embodiment is subjected to a corrosion test in
accordance with "Method E" specified in ASTM G48 (2015). A test
specimen for the corrosion test is prepared from the steel material
according to the present embodiment. If the steel material is a
steel plate, the test specimen is prepared from a center portion of
the thickness. If the steel material is a steel pipe, the test
specimen is prepared from a center portion of the wall thickness.
The size of the test specimen is, for example, as follows: a
thickness of 3 mm, a width of 25 mm, and a length of 50 nm. A
longitudinal direction of the test specimen is parallel with the
rolling direction of the steel material.
[0178] A solution of 6%FeCl.sub.3 1%HCl is adopted as the test
solution. The test specimen is immersed in the test solution so
that the solution volume to specimen area ratio is 5 mL/cm.sup.2 or
more. The temperature at the start of the test is set to 20.degree.
C., and the temperature of the test solution is increased by
5.degree. C. every 24 hours. The temperature when pitting occurs on
the test specimen is defined as the critical pitting temperature
(CPT). If the obtained CPT is 25.degree. C. or more, it is
determined that the duplex stainless steel material exhibits
excellent pitting resistance.
[0179] [Hot Workability of Duplex Stainless Steel Material]
[0180] By having the aforementioned chemical composition and the
aforementioned microstructure, the duplex stainless steel material
according to the present embodiment exhibits excellent hot
workability. In the present embodiment, the excellent hot
workability is defined as follows.
[0181] Specifically, the duplex stainless steel material according
to the present embodiment is subjected to a hot workability test
(Gleeble test). A test specimen for the Gleeble test is prepared
from the steel material according to the present embodiment. If the
steel material is a steel plate, the test specimen is prepared from
a center portion of the thickness. If the steel material is a steel
pipe, the test specimen is prepared from a center portion of the
wall thickness. The test specimen is, for example, a round bar test
specimen having a diameter of 10 mm and a length of 130 mm. A
longitudinal direction of the test specimen is parallel with the
rolling direction of the steel material.
[0182] The test specimen heated to 1000.degree. C. is subjected to
a tensile test at a strain rate of 10s.sup.-1 to cause the test
specimen to break. A reduction value (%) is determined based on the
broken test specimen. If the obtained reduction value is 40% or
more, it is determined that the duplex stainless steel material
exhibits the excellent hot workability.
[0183] [Shape of Duplex Stainless Steel Material]
[0184] The shape of the duplex stainless steel material according
to the present embodiment is not particularly limited. The duplex
stainless steel material, for example, may be a steel pipe, may be
a steel plate, may be a steel bar, or may be a wire rod. Preferably
the 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 550 MPa
or more, excellent pitting resistance, and excellent hot
workability.
[0185] [Method for Producing Duplex Stainless Steel Material]
[0186] A method for producing a steel pipe will be described as one
example of a method for producing the duplex stainless steel
material according to the present embodiment which has the
above-described configuration. Note that the method for producing
the duplex stainless steel material according to the present
embodiment is not limited to the production method described
below.
[0187] An example of the method for producing the duplex stainless
steel material according to the present embodiment includes a
starting material preparation step, a hot working step, and a
solution treatment step. Hereinafter, each production step will be
described in detail.
[0188] [Starting Material Preparation Step]
[0189] In the starting material preparation step, a starting
material having the above-described chemical composition is
prepared. The starting material may be prepared by producing it, or
may be prepared by purchasing it from a third party. That is, the
method for preparing the starting material is not particularly
limited.
[0190] When the starting material is produced, the production is
performed by, for example, the following method. A molten steel
having the above-described chemical composition is produced. By
using the molten steel, a cast piece (a slab, a bloom, or a billet)
is produced by a continuous casting method. A steel ingot may be
produced by an ingot-making method by using the molten steel. If
desired, a slab, a bloom or an ingot may be subjected to blooming
to produce a billet. The starting material is produced by the step
described above.
[0191] [Hot Working Step]
[0192] In the hot working step, the starting material prepared by
the above-described preparation step is subjected to hot working to
produce a steel material. The hot working may be hot forging, may
be hot extrusion, or may be hot rolling. A method for performing
hot working is not particularly limited, and a well-known method
may be used.
[0193] If the steel material is a steel pipe, for example, the
steel material may be subjected to hot working by the
Ugine-Sejournet process or the Ehrhardt push bench process (that
is, hot extrusion). If the steel material is a steel pipe, for
example, the 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.
[0194] [Solution Treatment Step]
[0195] In the solution treatment step, the steel material produced
by the aforementioned hot working step is subjected to a solution
treatment. A method for performing the solution treatment is not
particularly limited, and a well-known method may be used. For
example, the steel material is charged into a heat treatment
furnace, and after being held at a predetermined temperature, is
quenched. Note that, in the case of performing a solution treatment
by charging the steel material into a heat treatment furnace,
holding the steel material at a predetermined temperature, and
thereafter quenching the steel material, the temperature at which
the solution treatment is performed (solution treatment
temperature) means the temperature (.degree. C.) of the heat
treatment furnace for performing the solution treatment. Similarly,
the holding time at the solution treatment temperature (solution
treatment time) means a time from when the starting material is
charged into the inside of the heat treatment furnace for
performing the solution treatment until the starting material is
taken out from the heat treatment furnace.
[0196] Preferably, the solution treatment temperature in the
solution treatment step of the present embodiment is set to 900 to
1200.degree. C. If the solution treatment temperature is too low,
precipitates (for example, a a phase that is an intermetallic
compound or the like) may remain in the steel material after the
solution treatment. In this case, the pitting resistance of the
steel material deteriorates. Furthermore, if the solution treatment
temperature is too low, in some cases the volume ratio of ferrite
in the steel material after the solution treatment will be less
than 35.0%, and the strength and/or corrosion resistance of the
steel material may deteriorate. On the other hand, if the solution
treatment temperature is too high, in some cases the volume ratio
of ferrite in the steel material after the solution treatment will
be 50.0% or more, and the pitting resistance of the steel material
may deteriorate. Furthermore, in this case, the low-temperature
toughness and hot workability of the steel material may
deteriorate.
[0197] Therefore, when performing the solution treatment by
charging the steel material into a heat treatment furnace, holding
the steel material at a predetermined temperature, and thereafter
performing quenching, the solution treatment temperature is
preferably set within the range of 900 to 1200.degree. C. A lower
limit of the solution treatment temperature is more preferably
920.degree. C., and further preferably 940.degree. C. An upper
limit of the solution treatment temperature is more preferably
1180.degree. C., and further preferably 1160.degree. C.
[0198] When performing the solution treatment by charging the steel
material into a heat treatment furnace, holding the steel material
at a predetermined temperature, and thereafter performing
quenching, the solution treatment time is not particularly limited,
and may be in accordance with a well-known condition. The solution
treatment time is, for example, 5 to 180 minutes. The quenching
method is, for example, water cooling.
[0199] Note that, as necessary, the steel material on which the
solution treatment was performed may be subjected to a pickling
treatment. In this case, the pickling treatment may be performed by
a well-known method and is not particularly limited. Further, if
the steel material on which the solution treatment was performed is
subjected to cold working, the strength of the steel material will
be too high and the toughness of the steel material will
deteriorate. Therefore, it is preferable not to subject the duplex
stainless steel material according to the present embodiment to
cold working.
[0200] Through the steps described above, the duplex stainless
steel material according to the present embodiment can be produced.
Note that the method for producing the duplex stainless steel
material described above is one example, and the duplex stainless
steel material may be produced by another method. Hereunder, the
present invention is described in more detail by way of
Examples.
EXAMPLES
[0201] Molten steels having the chemical compositions shown in
Table 1 were melted using a 50 kg vacuum furnace, and ingots were
produced by an ingot casting method. Note that the symbol "-" in
Table 1 means that the content of the corresponding element was at
an impurity level. Further, Fn1 and Fn2 that were determined based
on the chemical composition described in Table 1 and the
definitions described above are shown in Table 2.
TABLE-US-00001 TABLE 1 Chemical composition (in mass %, the balance
being Fe and impurities) Steel C Si Mn P S Al Ni Cr Mo Cu N A 0.015
0.52 3.10 0.020 0.0005 0.080 4.81 25.20 0.99 3.20 0.216 B 0.013
0.56 4.45 0.019 0.0005 0.075 5.29 26.21 1.12 3.62 0.218 C 0.015
0.52 5.06 0.019 0.0005 0.081 4.82 25.17 0.99 3.19 0.228 D 0.014
0.54 4.32 0.019 0.0003 0.075 5.00 22.25 0.98 3.20 0.199 E 0.015
0.53 5.58 0.020 0.0002 0.070 4.91 25.01 1.02 3.20 0.242 F 0.016
0.56 3.09 0.020 0.0002 0.081 4.82 25.11 0.99 2.99 0.203 G 0.015
0.51 5.05 0.019 0.0005 0.077 4.80 25.13 0.99 2.50 0.233 H 0.015
0.52 3.10 0.020 0.0005 0.080 4.81 25.20 0.99 3.20 0.216 I 0.015
0.52 5.06 0.019 0.0005 0.081 4.82 25.17 1.48 3.19 0.228 J 0.015
0.51 5.05 0.019 0.0005 0.077 4.80 25.13 0.99 2.21 0.233 K 0.015
0.52 3.10 0.020 0.0005 0.080 4.81 25.20 0.99 3.20 0.216 L 0.015
0.52 1.61 0.016 0.0003 0.029 4.59 27.88 1.57 2.78 0.231 M 0.014
0.49 6.90 0.017 0.0005 0.040 5.73 27.64 1.13 3.80 0.194 N 0.014
0.51 2.78 0.019 0.0004 0.075 5.98 25.00 0.98 2.52 0.187 O 0.014
0.50 2.59 0.023 0.0002 0.012 4.83 23.25 0.99 3.40 0.159 P 0.014
0.52 0.65 0.021 0.0002 0.014 5.03 25.00 1.10 3.97 0.187 Q 0.014
0.50 2.05 0.023 0.0002 0.012 4.83 23.25 0.99 3.89 0.159 R 0.014
0.50 1.01 0.023 0.0002 0.012 4.83 23.25 0.99 2.29 0.159 S 0.014
0.52 0.97 0.021 0.0002 0.014 5.03 25.00 1.10 2.44 0.187 T 0.015
0.44 2.53 0.016 0.0005 0.023 3.04 22.62 0.89 2.13 0.205 U 0.014
0.54 3.64 0.021 0.0004 0.031 5.38 18.47 1.14 2.46 0.238 V 0.015
0.50 2.22 0.016 0.0002 0.033 5.57 24.56 0.98 2.44 0.053 Steel V Ca
Mg Nb Ta Ti Zr Hf B REM A 0.10 0.0022 -- -- -- -- -- -- -- -- B
0.11 -- 0.0019 -- -- -- -- -- -- -- C 0.09 0.0027 0.0023 -- -- --
-- -- -- -- D 0.10 0.0022 -- -- -- -- -- -- -- -- E 0.11 0.0020 --
-- 0.003 -- 0.001 -- -- -- F 0.11 -- 0.0023 -- -- 0.001 -- 0.001 --
-- G 0.10 0.0020 -- -- -- -- -- -- 0.0020 -- H 0.10 0.0022 -- -- --
-- -- -- 0.0019 -- I 0.09 0.0027 0.0020 -- -- -- -- -- 0.0020 0.012
J 0.10 0.0020 -- 0.002 -- 0.002 -- -- 0.0020 -- K 0.10 0.0022 -- --
0.003 -- 0.003 0.002 0.0019 -- L 0.10 0.0020 -- -- -- -- -- -- --
-- M 0.09 0.0023 -- 0.002 -- -- -- -- 0.0021 0.010 N 0.11 0.0027
0.0023 0.002 -- -- -- -- 0.0021 -- O 0.05 0.0020 0.0022 -- -- -- --
-- -- -- P 0.05 0.0023 -- -- -- -- -- -- -- -- Q 0.05 0.0020 0.0023
-- 0.001 -- 0.001 -- -- -- R 0.05 0.0020 -- -- -- -- -- -- -- -- S
0.05 0.0023 -- 0.002 -- -- -- -- 0.0023 -- T 0.11 0.0017 -- -- --
-- -- -- 0.0021 -- U 0.12 0.0022 0.0022 -- -- -- -- -- -- -- V 0.10
0.0021 -- -- -- 0.003 -- -- 0.0020 --
TABLE-US-00002 TABLE 2 Solution treatment Ferrite Hot temper-
volume work- Test ature ratio YS CPT ability Number Steel Fn1 Fn2
(.degree. C.) (%) (MPa) (.degree. C.) test 1 A 6.30 615 980 46.4
622 30 E 2 B 8.07 702 1000 45.7 607 35 E 3 C 8.25 749 1050 48.5 625
35 E 4 D 7.52 633 1050 44.8 589 30 E 5 E 8.78 793 950 48.3 650 30 E
6 F 6.08 604 1000 45.0 603 35 E 7 G 7.55 765 980 46.1 620 30 E 8 H
6.30 615 980 46.4 618 30 E 9 I 8.25 760 1000 47.6 615 35 E 10 J
7.26 770 1050 43.8 585 35 E 11 K 6.30 615 1050 46.7 612 35 E 12 L
4.39 594 980 48.9 531 30 E 13 M 10.70 839 1050 43.5 642 35 NA 14 N
5.30 552 1000 36.8 537 30 E 15 O 5.99 497 1050 38.8 538 35 E 16 P
4.62 414 1000 39.1 522 30 E 17 Q 5.94 455 1000 38.4 530 35 E 18 R
3.30 416 1050 39.9 524 40 E 19 S 3.41 460 1070 36.0 513 40 E 20 T
4.66 584 1050 58.0 594 20 E 21 U 6.10 587 1050 38.9 588 20 E 22 V
4.66 389 1000 38.6 518 30 E 23 G 7.55 765 750 33.0 640 20 E
[0202] The ingot of each Test Number was heated to 1200.degree. C.,
and subjected to hot forging and hot working to produce a steel
plate having a thickness of 10 mm. The steel plate of each Test
Number was subjected to a solution treatment in which the steel
plate was held for 15 minutes at the solution treatment temperature
described in Table 2. The steel plates of the respective Test
Numbers that had been subjected to the solution treatment were
water-cooled.
[0203] [Evaluation Tests]
[0204] The steel plates of the respective Test Numbers that had
been subjected to the aforementioned solution treatment were
subjected to microstructure observation, a tensile test, a
corrosion test, and a hot workability test which are described
below.
[0205] [Microstructure Observation]
[0206] The steel plate of each Test Number was subjected to
microstructure observation by the above-described method conforming
to ASTM E562 (2011) to determine the ferrite volume ratio (%). Note
that, in the present Example, a test specimen for microstructure
observation was prepared from the center portion of the thickness
of the steel plate of each Test Number, and a cross section
perpendicular to the rolling direction was adopted as the
observation surface. Further, the microstructure of the steel plate
of each Test Number was a microstructure consisting of ferrite and
austenite. Table 2 shows the ferrite volume ratios (%) determined
for the steel plate of each Test Number.
[0207] [Tensile Test]
[0208] A tensile test was carried out on the steel plate of each
Test Number by the above-described method conforming to ASTM E8/E8M
(2013) to determine yield strength (MPa). In the present Example,
the round bar test specimen for the tensile test was prepared from
the center portion of the thickness of the steel plate of each Test
Number, and the axial direction of the round bar test specimen was
parallel to the rolling direction. The 0.2% offset proof stress
obtained in the tensile test was defined as the yield strength
(MPa). Table 2 shows the yield strength determined for the steel
plate of each Test Number as "YS (MPa)".
[0209] [Corrosion Test]
[0210] A corrosion test was carried out on the steel plate of each
Test Number by the above-described method conforming to ASTM G48
(2015) Method E to evaluate pitting resistance. In the present
Example, the test specimen for the corrosion test was prepared from
the center portion of the thickness of the steel plate of each Test
Number. The size of the test specimen was as follows: a thickness
of 3 mm, a width of 25 mm, and a length of 50 mm, and the
longitudinal direction of the test specimen was parallel with the
rolling direction.
[0211] The test specimen of each Test Number was immersed in a test
solution (6%FeCl.sub.3+1%HCl) at 20.degree. C. so that the solution
volume to specimen area ratio was 5 mL/cm.sup.2 or more. Every 24
hours from the time at which the test specimen was immersed in the
test solution, the temperature of the test solution was increased
by 5.degree. C., and whether or not pitting had occurred was
confirmed with the naked eyes. The temperature when pitting
occurred was defined as the CPT (.degree. C.). The CPT (.degree.
C.) obtained in the corrosion test for the steel plate of each Test
Number is shown in Table 2.
[0212] [Hot Workability Test]
[0213] A hot workability test (Gleeble test) was carried out on the
steel plate of each Test Number to evaluate hot workability.
Specifically, a round bar test specimen having a diameter of 10 mm
and a length of 130 mm was prepared from the steel plate of each
Test Number. The round bar test specimen was prepared from the
center portion of the thickness of the steel plate of each Test
Number. Note that the longitudinal direction of the round bar test
specimen was parallel with the rolling direction.
[0214] After heating the round bar test specimen of each Test
Number to 1000.degree. C., a tensile test was carried out at a
strain rate of 105.sup.1 to cause the round bar test specimen of
each Test Number to break. A reduction value (%) was determined
based on the broken round bar test specimen of each Test Number. If
the obtained reduction value was 40% or more, it was determined
that the steel plate of the relevant Test Number exhibited
excellent hot workability ("E" (Excellent) in Table 2). On the
other hand, if the obtained reduction value was less than 40%, it
was determined that the steel plate of the relevant Test Number did
not exhibit excellent hot workability ("NA" (Not Acceptable) in
Table 2). The evaluation result of the hot workability test for the
steel plate of each Test Number is shown in Table 2.
[0215] [Evaluation Results]
[0216] Referring to Table 1 and Table 2, in the steel plates of
Test Numbers 1 to 11, the chemical composition was appropriate, Fn1
was in the range of 4.50 to 9.50, and Fn2 was 580 or more. In
addition, the production method performed with respect to the steel
plates of Test Numbers 1 to 11 was a preferable production method
described in the present description. As a result, the steel plates
of Test Numbers 1 to 11 had a microstructure in which the volume
ratio of ferrite was 35.0 to less than 50.0%, with austenite as the
balance. In addition, the yield strength of the steel plates of
Test Numbers 1 to 11 was 550 MPa or more. Further, for the steel
plates of Test Numbers 1 to 11, the CPT was 25.degree. C. or more
and excellent pitting resistance was exhibited. Furthermore, the
steel plates of Test Numbers 1 to 11 exhibited excellent hot
workability in the hot workability test.
[0217] On the other hand, in the steel plate of Test Number 12, Fn1
was less than 4.50. As a result, the yield strength of the steel
plate of Test Number 12 was less than 550 MPa and the desired yield
strength was not obtained.
[0218] In the steel plate of Test Number 13, Fill was more than
9.50. As a result, the steel plate of Test Number 13 did not
exhibit excellent hot workability in the hot workability test.
[0219] In the steel plates of Test Numbers 14 to 17, Fn2 was less
than 580. As a result, the yield strength of the steel plates of
Test Numbers 14 to 17 was less than 550 MPa and the desired yield
strength was not obtained.
[0220] In the steel plates of Test Numbers 18 and 19, Fn1 was 4.50
or less. Further, in the steel plates of Test Numbers 18 and 19,
Fn2 was less than 580. As a result, the yield strength of the steel
plates of Test Numbers 18 and 19 was less than 550 MPa and the
desired yield strength was not obtained.
[0221] In the steel plate of Test Number 20, the Ni content was too
low. Consequently, the volume ratio of ferrite in the steel plate
of Test Number 20 was 50.0% or more. As a result, for the steel
plate of Test Number 20, the CPT was less than 25.degree. C. and
excellent pitting resistance was not exhibited.
[0222] In the steel plate of Test Number 21, the Cr content was too
low. As a result, for the steel plate of Test Number 21, the CPT
was less than 25.degree. C. and excellent pitting resistance was
not exhibited.
[0223] In the steel plate of Test Number 22, the N content was too
low. Furthermore, in the steel plate of Test Number 22, Fn2 was
less than 580. As a result, the yield strength of the steel plate
of Test Number 22 was less than 550 MPa and the desired yield
strength was not obtained.
[0224] In the steel plate of Test Number 23, the solution treatment
temperature in the production process was too low. Consequently,
the volume ratio of ferrite in the steel plate of Test Number 23
was less than 35.0%. As a result, for the steel plate of Test
Number 23, the CPT was less than 25.degree. C. and excellent
pitting resistance was not exhibited.
[0225] The embodiment of the present disclosure has been described
so far. 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 not departing from the
spirit thereof.
* * * * *