U.S. patent application number 15/505388 was filed with the patent office on 2017-09-21 for austenitic stainless steel.
This patent application is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Tomoaki HAMAGUCHI, Hiroyuki HIRATA, Atsuro ISEDA, Kana JOTOKU, Hirokazu OKADA, Toshihide ONO, Hiroyuki SEMBA, Katsuki TANAKA.
Application Number | 20170268085 15/505388 |
Document ID | / |
Family ID | 57440490 |
Filed Date | 2017-09-21 |
United States Patent
Application |
20170268085 |
Kind Code |
A1 |
ISEDA; Atsuro ; et
al. |
September 21, 2017 |
AUSTENITIC STAINLESS STEEL
Abstract
An austenitic stainless steel with a chemical composition
including in terms of mass %: 0.05 to 0.13% of C, 0.10 to 1.00% of
Si, 0.10 to 3.00% of Mn, 0.040% or less of P, 0.020% or less of S,
17.00 to 19.00% of Cr, 12.00 to 15.00% of Ni, 2.00 to 4.00% of Cu,
0.01 to 2.00% of Mo, 2.00 to 5.00% of W, 2.50 to 5.00% of 2Mo+W,
0.01 to 0.40% of V, 0.05 to 0.50% of Ti, 0.15 to 0.70% of Nb, 0.001
to 0.040% of Al, 0.0010 to 0.0100% of B, 0.0010 to 0.0100% of N,
0.001 to 0.20% of Nd, 0.002% or less of Zr, 0.001% or less of Bi,
0.010% or less of Sn, 0.010% or less of Sb, 0.001% or less of Pb,
0.001% or less of As, 0.020% or less of Zr+Bi+Sn+Sb+Pb+As, 0.0090%
or less of O, and a remainder including Fe and impurities.
Inventors: |
ISEDA; Atsuro; (Tokyo,
JP) ; OKADA; Hirokazu; (Tokyo, JP) ; SEMBA;
Hiroyuki; (Tokyo, JP) ; HIRATA; Hiroyuki;
(Tokyo, JP) ; HAMAGUCHI; Tomoaki; (Tokyo, JP)
; JOTOKU; Kana; (Tokyo, JP) ; ONO; Toshihide;
(Tokyo, JP) ; TANAKA; Katsuki; (Tokyo,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION
Tokyo
JP
|
Family ID: |
57440490 |
Appl. No.: |
15/505388 |
Filed: |
June 3, 2016 |
PCT Filed: |
June 3, 2016 |
PCT NO: |
PCT/JP2016/066695 |
371 Date: |
February 21, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C 38/06 20130101;
C22C 38/42 20130101; C22C 38/46 20130101; C22C 38/54 20130101; C22C
38/60 20130101; C22C 38/44 20130101; C22C 38/58 20130101; C22C
38/005 20130101; C22C 38/008 20130101; C21D 6/004 20130101; C22C
38/02 20130101; C22C 38/48 20130101; C22C 38/00 20130101; C22C
38/04 20130101; C22C 38/50 20130101; C22C 38/002 20130101; C22C
38/001 20130101 |
International
Class: |
C22C 38/60 20060101
C22C038/60; C22C 38/50 20060101 C22C038/50; C22C 38/48 20060101
C22C038/48; C22C 38/46 20060101 C22C038/46; C22C 38/00 20060101
C22C038/00; C22C 38/42 20060101 C22C038/42; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/54 20060101 C22C038/54; C22C 38/44 20060101
C22C038/44 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 5, 2015 |
JP |
2015-114665 |
Claims
1. An austenitic stainless steel with a chemical composition
consisting of in terms of mass %: 0.05 to 0.13% of C, 0.10 to 1.00%
of Si, 0.10 to 3.00% of Mn, 0.040% or less of P, 0.020% or less of
S, 17.00 to 19.00% of Cr, 12.00 to 15.00% of Ni, 2.00 to 4.00% of
Cu, 0.01 to 2.00% of Mo, 2.00 to 5.00% of W, 2.50 to 5.00% of
2Mo+W, 0.01 to 0.40% of V, 0.05 to 0.50% of Ti, 0.15 to 0.70% of
Nb, 0.001 to 0.040% of Al, 0.0010 to 0.0100% of B, 0.0010 to
0.0100% of N, 0.001 to 0.20% of Nd, 0.002% or less of Zr, 0.001% or
less of Bi, 0.010% or less of Sn, 0.010% or less of Sb, 0.001% or
less of Pb, 0.001% or less of As, 0.020% or less of
Zr+Bi+Sn+Sb+Pb+As, 0.0090% or less of O, 0.80% or less of Co, 0.20%
or less of Ca, 0.20% or less of Mg, 0.20% or less in total of one
or more of Y, Sc, Ta, Hf, Re or lanthanoid elements other than Nd,
and a remainder consisting of Fe and impurities; wherein an
effective M content Meff defined by the following Formula (1) is
0.0001 to 0.250%: Effective M content Meff=Nd+13(B-11N/14)-1.6Zr
Formula (1) wherein in Formula (1), each element symbol represents
a content (mass %) of each element.
2. The austenitic stainless steel according to claim 1, wherein the
chemical composition comprises, in terms of mass %, one or more of:
0.01 to 0.80% of Co, 0.0001 to 0.20% of Ca, or 0.0005 to 0.20% of
Mg.
3. The austenitic stainless steel according to claim 1, wherein the
chemical composition comprises, in terms of mass %, 0.001 to 0.20%
in total of one or more of Y, Sc, Ta, Hf, Re or lanthanoid elements
other than Nd.
4. The austenitic stainless steel according to claim 1, wherein an
ASTM grain size number of a metallic structure thereof is 7 or
less.
5. The austenitic stainless steel according to claim 1, wherein a
creep rupture strength at 700.degree. C. and 10,000 hours is 140
MPa or more.
6. The austenitic stainless steel according to claim 1, wherein the
effective M content Meff is 0.002 to 0.250%.
7. The austenitic stainless steel according to claim 2, wherein the
chemical composition comprises, in terms of mass %, 0.001 to 0.20%
in total of one or more of Y, Sc, Ta, Hf, Re or lanthanoid elements
other than Nd.
8. The austenitic stainless steel according to claim 2, wherein an
ASTM grain size number of a metallic structure thereof is 7 or
less.
9. The austenitic stainless steel according to claim 2, wherein a
creep rupture strength at 700.degree. C. and 10,000 hours is 140
MPa or more.
10. The austenitic stainless steel according to claim 2, wherein
the effective M content Meff is 0.002 to 0.250%.
11. The austenitic stainless steel according to claim 3, wherein an
ASTM grain size number of a metallic structure thereof is 7 or
less.
12. The austenitic stainless steel according to claim 3, wherein a
creep rupture strength at 700.degree. C. and 10,000 hours is 140
MPa or more.
13. The austenitic stainless steel according to claim 3, wherein
the effective M content Meff is 0.002 to 0.250%.
14. An austenitic stainless steel with a chemical composition
comprising in terms of mass %: 0.05 to 0.13% of C, 0.10 to 1.00% of
Si, 0.10 to 3.00% of Mn, 0.040% or less of P, 0.020% or less of S,
17.00 to 19.00% of Cr, 12.00 to 15.00% of Ni, 2.00 to 4.00% of Cu,
0.01 to 2.00% of Mo, 2.00 to 5.00% of W, 2.50 to 5.00% of 2Mo+W,
0.01 to 0.40% of V, 0.05 to 0.50% of Ti, 0.15 to 0.70% of Nb, 0.001
to 0.040% of Al, 0.0010 to 0.0100% of B, 0.0010 to 0.0100% of N,
0.001 to 0.20% of Nd, 0.002% or less of Zr, 0.001% or less of Bi,
0.010% or less of Sn, 0.010% or less of Sb, 0.001% or less of Pb,
0.001% or less of As, 0.020% or less of Zr+Bi+Sn+Sb+Pb+As, 0.0090%
or less of O, 0.80% or less of Co, 0.20% or less of Ca, 0.20% or
less of Mg, 0.20% or less in total of one or more of Y, Sc, Ta, Hf,
Re or lanthanoid elements other than Nd, and a remainder comprising
Fe and impurities; wherein an effective M content Meff defined by
the following Formula (1) is 0.0001 to 0.250%: Effective M content
Meff=Nd+13(B-11N/14)-1.6Zr Formula (1) wherein in Formula (1), each
element symbol represents a content (mass %) of each element.
15. The austenitic stainless steel according to claim 14, wherein
the chemical composition comprises, in terms of mass %, one or more
of: 0.01 to 0.80% of Co, 0.0001 to 0.20% of Ca, or 0.0005 to 0.20%
of Mg.
16. The austenitic stainless steel according to claim 14, wherein
the chemical composition comprises, in terms of mass %, 0.001 to
0.20% in total of one or more of Y, Sc, Ta, Hf, Re or lanthanoid
elements other than Nd.
17. The austenitic stainless steel according to claim 14, wherein
an ASTM grain size number of a metallic structure thereof is 7 or
less.
18. The austenitic stainless steel according to claim 14, wherein a
creep rupture strength at 700.degree. C. and 10,000 hours is 140
MPa or more.
19. The austenitic stainless steel according to claim 14, wherein
the effective M content Meff is 0.002 to 0.250%.
Description
TECHNICAL FIELD
[0001] The present invention relates to austenitic stainless
steel.
BACKGROUND ART
[0002] There has been an advancing tendency since 1990s in Japan
with respect to a boiler toward high temperature and high pressure,
and the current mainstream is an Ultra Super Critical power (USC)
boiler for a steam temperature beyond 600.degree. C.
[0003] In other areas of the world, including Europe or China,
highly efficient USC boilers have been constructed one after
another from the viewpoint of CO.sub.2 reduction as a global
environmental countermeasure.
[0004] As a source material steel to be used for a heat exchanger
tube to generate high temperature high pressure steam in a boiler,
and for a pipe of a boiler, a steel material with superior high
temperature strength has been demanded and various steel materials
have been developed recently.
[0005] For example, Patent Literature 1 discloses an 18 Cr-based
austenitic stainless steel superior in high temperature strength as
well as superior in steam oxidation resistance.
[0006] Patent Literature 2 discloses an austenitic stainless steel
superior in high temperature corrosion thermal fatigue cracking
resistance.
[0007] Patent Literature 3 discloses a heat-resistant austenitic
stainless steel superior in high temperature strength and cyclic
oxidation resistance.
[0008] Patent Literature 4 discloses an austenitic stainless steel
exhibiting superior toughness even after exposure to a high
temperature environment for a prolonged period of time.
[0009] Patent Literature 5 discloses a high strength austenitic
stainless steel with a creep rupture strength at 800.degree. C. for
600 hours of 100 MPa or more.
[0010] Patent Literature 6 discloses a method for securing a high
temperature strength (a method of adding a large amount of N) by
which a large amount of nitrogen (N) is added for utilizing solid
solution strengthening and nitride precipitation strengthening so
as to make up low strength of a low carbon stainless steel.
Patent Literature 1: Japanese Patent No. 3632672
Patent Literature 2: Japanese Patent No. 5029788
Patent Literature 3: Japanese Patent No. 5143960
Patent Literature 4: Japanese Patent No. 5547789
Patent Literature 5: Japanese Patent No. 5670103
Patent Literature 6: Japanese Patent No. 3388998
SUMMARY OF INVENTION
Technical Problem
[0011] Generally, in designing the chemical composition of a
material steel to be used for a heat exchanger tube used in a high
temperature range and a pipe of a boiler used in a high temperature
range, importance is placed on high temperature strength (for
example, creep strength), high temperature corrosion resistance,
steam oxidation resistance, thermal fatigue resistance, etc.,
however corrosion resistance in a temperature range from normal
temperature to approx. 350.degree. C. (for example, stress
corrosion cracking resistance in water) is less valued. This is
because the corrosion resistance in a temperature range from room
temperature to approx. 350.degree. C. has been heretofore addressed
by fabrication technique or operation control technique.
[0012] However, there arises recently a big problem that stress
corrosion cracking occurs in water in a range of room temperature
to low temperature (approx. 350.degree. C. or less) due to a
inhomogeneous metallic structure or an heterogeneous carbide
precipitation at a heating processed portion, such as a welded
portion or a bending portion.
[0013] For example, during a hydrostatic pressure test of a boiler,
or a shut-down of a boiler, since water is stored for an extended
period of time inside heat exchanger tubes, where stress corrosion
cracking may occur remarkably.
[0014] Stress corrosion cracking of stainless steel may occur
because a crystal grain boundary becomes susceptible to selective
corrosion due to precipitation of a Cr-based carbide or generation
of a zone with a low Cr concentration (Cr depleted zone) in the
vicinity of a crystal grain boundary.
[0015] As a method for preventing stress corrosion cracking of an
18 Cr-based austenitic stainless steel, heretofore:
[0016] a method for suppressing formation of a grain boundary Cr
carbide by reduction of a C amount (a low carbon addition
method),
[0017] a method for suppressing formation of a grain boundary Cr
carbide by addition of Nb and Ti, which have higher capability of
forming a carbide than Cr, to form a MC carbide to fix C (a
stabilizing heat treatment method),
[0018] a method for suppressing formation of a Cr depleted zone by
addition of Cr at 22% or more to suppress selective corrosion at a
grain boundary (a method of adding a large amount of Cr), or the
like is known.
[0019] There is however a drawback in any of the methods.
[0020] In the case of a low carbon addition method, there is a
tendency that a carbide effective for high temperature strength is
not formed and the high temperature strength declines.
[0021] In the case of a stabilizing heat treatment method, since a
stabilizing heat treatment is done at a temperature as low as
approx. 950.degree. C., a high temperature strength, especially
creep strength tends to be impaired.
[0022] In the case of a method of adding a large amount of Cr,
since a high content of brittle phase such as .sigma.-phase is to
be formed, it is required to add a large amount of expensive Ni for
stabilization of a metallic structure and maintenance of high
temperature strength, so that the cost of source materials tends to
increase greatly.
[0023] The method described in Patent Literature 6 (a method of
adding a large amount of N) is a method devised for replacing the
aforementioned conventional methods.
[0024] The method of adding a large amount of N is a method by
which a large amount of N is added for utilizing solid solution
strengthening and nitride precipitation strengthening so as to make
up low strength of a low carbon stainless steel.
[0025] However, it was found there is a problem that according to
the method of Patent Literature 6 (the method of adding a large
amount of N), a large amount of nitride is formed against
expectation to cause stress corrosion cracking, or sufficient high
temperature strength cannot be obtained in a high temperature range
of 700.degree. C. or higher
[0026] Under such circumstances, it has been demanded to achieve
superior high temperature strength and superior stress corrosion
cracking resistance with respect to 18 Cr-based austenitic
stainless steel without depending on the low carbon addition
method, the stabilizing heat treatment method, the method of adding
a large amount of Cr, and the method of adding a large amount of N,
which are conventional methods.
[0027] An object of the invention is to provide an austenitic
stainless steel, which is an 18 Cr-based austenitic stainless steel
securing superior high temperature strength and superior stress
corrosion cracking resistance.
Solution to Problem
[0028] The means for achieving the object includes the following
aspects.
[0029] <1> An austenitic stainless steel with a chemical
composition consisting of in terms of mass %:
0.05 to 0.13% of C,
0.10 to 1.00% of Si,
0.10 to 3.00% of Mn,
[0030] 0.040% or less of P, 0.020% or less of S,
17.00 to 19.00% of Cr,
12.00 to 15.00% of Ni,
2.00 to 4.00% of Cu,
0.01 to 2.00% of Mo,
2.00 to 5.00% of W,
2.50 to 5.00% of 2Mo+W,
0.01 to 0.40% of V,
0.05 to 0.50% of Ti,
0.15 to 0.70% of Nb,
0.001 to 0.040% of Al,
0.0010 to 0.0100% of B,
0.0010 to 0.0100% of N,
0.001 to 0.20% of Nd,
[0031] 0.002% or less of Zr, 0.001% or less of Bi, 0.010% or less
of Sn, 0.010% or less of Sb, 0.001% or less of Pb, 0.001% or less
of As, 0.020% or less of Zr+Bi+Sn+Sb+Pb+As, 0.0090% or less of O,
0.80% or less of Co, 0.20% or less of Ca, 0.20% or less of Mg,
0.20% or less in total of one or more of Y, Sc, Ta, Hf, Re or
lanthanoid elements other than Nd, and a remainder consisting of Fe
and impurities;
[0032] wherein an effective M content Meff defined by the following
Formula (1) is 0.0001 to 0.250%:
Effective M content Meff=Nd+13(B-11N/14)-1.6Zr Formula (1)
[0033] wherein in Formula (1), each element symbol represents a
content (mass %) of each element.
Effective M content Meff=Nd+13(B-11N/14)-1.6Zr Formula (1)
[0034] wherein in Formula (1), each element symbol represents the
content of each element (mass %)).
<2> The austenitic stainless steel according to <1>,
wherein the chemical composition comprises, in terms of mass %, one
or more of: 0.01 to 0.80% of Co, 0.0001 to 0.20% of Ca, or 0.0005
to 0.20% of Mg. <3> The austenitic stainless steel according
to <1> or <2>, wherein the chemical composition
comprises, in terms of mass %, 0.001 to 0.20% in total of one or
more of Y, Sc, Ta, Hf, Re or lanthanoid elements other than Nd.
<4> The austenitic stainless steel according to any one of
<1> to <3>, wherein an ASTM grain size number of a
metallic structure thereof is 7 or less. <5> The austenitic
stainless steel according to any one of claim <1> to
<4>, wherein a creep rupture strength at 700.degree. C. and
10,000 hours is 140 MPa or more.
Advantageous Effects of Invention
[0035] According to the invention, an austenitic stainless steel,
which is an 18 Cr-based austenitic stainless steel securing
superior high temperature strength and superior stress corrosion
cracking resistance, is provided.
DESCRIPTION OF EMBODIMENTS
[0036] Embodiments of the invention will be described below. [0037]
A numerical range expressed by "x to y" herein includes the values
of x and y in the range as the lower and upper limit values,
respectively.
[0038] The content of an element expressed by "%" and an effective
M content Meff expressed by "%" both mean herein "mass %".
[0039] Further, the content of C (carbon) may be herein
occasionally expressed as "C content". The content of another
element may be expressed similarly.
[0040] An austenitic stainless steel of the embodiment (hereinafter
also referred to as "the steel of the embodiment") is an austenitic
stainless steel with a chemical composition consisting of in terms
of mass %: 0.05 to 0.13% of C, 0.10 to 1.00% of Si, 0.10 to 3.00%
of Mn, 0.040% or less of P, 0.020% or less of S, 17.00 to 19.00% of
Cr, 12.00 to 15.00% of Ni, 2.00 to 4.00% of Cu, 0.01 to 2.00% of
Mo, 2.00 to 5.00% of W, 2.50 to 5.00% of 2Mo+W, 0.01 to 0.40% of V,
0.05 to 0.50% of Ti, 0.15 to 0.70% of Nb, 0.001 to 0.040% of Al,
0.0010 to 0.0100% of B, 0.0010 to 0.0100% of N, 0.001 to 0.20% of
Nd, 0.002% or less of Zr, 0.001% or less of Bi, 0.010% or less of
Sn, 0.010% or less of Sb, 0.001% or less of Pb, 0.001% or less of
As, 0.020% or less of Zr+Bi+Sn+Sb+Pb+As, 0.0090% or less of O,
0.80% or less of Co, 0.20% or less of Ca, 0.20% or less of Mg,
0.20% or less in total of one or more of Y, Sc, Ta, Hf, Re or
lanthanoid elements other than Nd, and a remainder consisting of Fe
and impurities; wherein an effective M content Meff defined by the
following Formula (1) is 0.0001 to 0.250%.
Effective M content Meff=Nd+13(B-11N/14)-1.6Zr Formula (1)
[0041] wherein, in Formula (1), each element symbol represents the
content (mass %) of each element.
[0042] The chemical composition of the steel of the embodiment
includes 17.00 to 19.00% of Cr.
[0043] In other words, the steel of the embodiment belongs to the
18 Cr-based austenitic stainless steel.
[0044] As described above, it is demanded that superior high
temperature strength and superior stress corrosion cracking
resistance is achieved for 18 Cr-based austenitic stainless steel
without depending on the low carbon addition method, the
stabilizing heat treatment method, the method of adding a large
amount of Cr, and the method of adding a large amount of N, which
are conventional methods.
[0045] According to the steel of the embodiment, superior high
temperature strength and superior stress corrosion cracking
resistance may be secured without depending on the low carbon
addition method, the stabilizing heat treatment method, the method
of adding a large amount of Cr, and the method of adding a large
amount of N, which are conventional methods.
[0046] The reason of such an effect to be obtained with the steel
of the embodiment is presumed as follows, provided that the
invention be not restricted in any way by the following
presumption.
[0047] In the case of the steel of the embodiment, grain boundary
purification and strength improvement may be achieved by adding Nd
and B combinedly at the above respective contents, and further by
adjusting the effective M content Meff in the above range.
[0048] Further, in the case of the steel of the embodiment, purity
refinement is achieved by limiting the contents of Zr, Bi, Sn, Sb,
Pb, and As, which are impurities (hereinafter also collectively
referred to as "6 impurity elements"), in the above ranges.
[0049] It is conceivable that superior high temperature strength
and superior stress corrosion cracking resistance may be secured by
the grain boundary purification, the strength improvement, and the
purity refinement without depending on any of the low carbon
addition method, the stabilizing heat treatment method, and the
method of adding a large amount of Cr.
[0050] Further, in the case of the steel of the embodiment,
conceivably precipitation strengthening through precipitation of a
fine carbide and precipitation of a fine and stable Laves phase
becomes possible by reducing N (nitrogen) to the extent possible
(specifically to 0.0100% or less) and adding W at the above
content.
[0051] As the result, in 18 Cr-based austenitic stainless steel,
superior high temperature strength may be presumably secured
without depending on the method of adding a large amount of N (see,
for example Patent Literature 6).
[0052] This finding is a novel finding contrary to heretofore
common sense.
[0053] Ordinarily, a carbide and a Laves phase precipitate
preferentially around a nitride and on a nitride at a crystal grain
boundary to impair the high temperature strength and corrosion
resistance. In other word, when a nitride is present, both
precipitation of a fine carbide, and precipitation of a fine and
stable Laves phase become difficult, and the high temperature
strength is not improved. Especially, when a coarse Zr nitride is
present, precipitation of a fine carbide, and precipitation of a
fine and stable Laves phase become more difficult, and therefore N
and Zr are reduced to the extent as possible.
[0054] However, a trace amount of N forms a precipitation nucleus
made of fine carbide, which contributes to improvement of high
temperature strength. Therefore, in the steel of the embodiment, N
is not an impurity element but a useful element, and controlled in
a very low content range (specifically, from 0.0010 to
0.0100%).
[0055] By regulating the N content in the steel of the embodiment
from 0.0010 to 0.0100%, both of precipitation strengthening with a
fine carbide and precipitation strengthening with a fine and stable
Laves phase may be achieved effectively. As the result, the high
temperature strength can be secured and the metallic structure can
be stabilized in a temperature range of 700.degree. C. or
higher.
[0056] In other words, in the steel of the embodiment, enhancement
of the strength can be achieved without depending on precipitation
strengthening with a nitride, and stabilization of the metallic
structure can be achieved without forming a brittle phase, etc. The
technique has not been known conventionally.
[0057] Firstly, the chemical composition and its preferable
embodiment of the steel of the embodiment will be described below,
and then an effective M content Meff (Formula (1)), etc. will be
described.
[0058] C: 0.05 to 0.13%
[0059] C is an essential element for formation of a carbide, and
stabilization of an austenitic structure, as well as improvement of
high temperature strength and stabilization of a metallic structure
at a high temperature.
[0060] With respect to the steel of the embodiment, stress
corrosion cracking can be prevented without utilizing strengthening
by addition of N, or without reducing C.
[0061] Provided that the C content is 0.05% or more, which is
because, when the C content is less than 0.05%, improvement of high
temperature creep strength, and stabilization of a metallic
structure at a high temperature becomes difficult. The C content is
preferably 0.06% or more.
[0062] Meanwhile, when the C content exceeds 0.13%, a coarse Cr
carbide precipitates at a crystal grain boundary, which may cause
stress corrosion cracking or welding cracking to reduce toughness.
Therefore, the C content is 0.13% or less, and is preferably 0.12%
or less.
[0063] Si: 0.10 to 1.00%
[0064] Si is an element which functions as a deoxidizing agent
during steel making, and prevents steam oxidation at a high
temperature. However, when the Si content is less than 0.10%, the
addition effect is not obtained adequately. Therefore, the Si
content is 0.10% or more, and is preferably 0.20% or more.
[0065] Meanwhile, when the Si content exceeds 1.00%, the
workability declines, and a brittle phase such as a .sigma.-phase
precipitates at a high temperature. Therefore, the Si content is
1.00% or less, and is preferably 0.80% or less.
[0066] Mn: 0.10 to 3.00%
[0067] Mn is an element which makes S harmless by forming MnS with
S as an impurity element to contribute to improvement of a hot
workability, as well as to stabilization of a metallic structure at
a high temperature. However, when the Mn content is less than
0.10%, the addition effect is not obtained adequately. Therefore,
the Mn content is 0.10% or more, and is preferably 0.20% or
more.
[0068] Meanwhile, when the Mn content exceeds 3.00%, the
workability and weldability decrease. Therefore, the Mn content is
3.00% or less, and is preferably 2.60% or less.
[0069] P: 0.040% or less
[0070] P is an impurity element, which disturbs workability and
weldability.
[0071] When the P content exceeds 0.040%, the workability and
weldability decrease remarkably. Therefore, the P content is 0.040%
or less, and is preferably 0.030% or less, and more preferably
0.020% or less.
[0072] Preferably the P content is as low as possible, and may be
even 0%.
[0073] However, P may inevitably get mixed in from steel raw
materials (raw material ore, scrap, etc.), and reduction of the P
content to below 0.001% will increase the production cost greatly.
Therefore, the P content may be 0.001% or more from the viewpoint
of production cost.
[0074] S: 0.020% or less
[0075] S is an impurity element, which disturbs workability,
weldability, and stress corrosion cracking resistance.
[0076] When the S content exceeds 0.020%, the workability,
weldability, and stress corrosion cracking resistance decrease
remarkably. Therefore the S content is 0.020% or less.
[0077] Even in a case in which S is added for improvement of molten
metal flow in welding, the S content is added at 0.020% or less,
and is preferably added at 0.010% or less.
[0078] Preferably the S content is as low as possible, and may be
even 0%.
[0079] However, S may inevitably get mixed in from steel source
materials (raw material ore, scrap, etc.) and reduction of the S
content to below 0.001% will increase the production cost greatly.
Therefore, the S content may be 0.001% or more from the viewpoint
of production cost.
[0080] Cr: 17.00 to 19.00%
[0081] Cr is a major element of an 18 Cr-based austenitic stainless
steel, which contributes to improvement of oxidation resistance,
steam oxidation resistance, and stress corrosion cracking
resistance, as well as to stabilization of the strength or metallic
structure with a Cr carbide.
[0082] When the Cr content is less than 17.00%, the addition effect
may be not obtained adequately. Therefore, the Cr content is 17.00%
or more. The Cr content is preferably 17.30% or more, and more
preferably 17.50% or more.
[0083] Meanwhile, when the Cr content exceeds 19.00%, a large
amount of Ni becomes necessary for maintaining the stability of an
austenitic structure, and further a brittle phase is formed to
decrease high temperature strength or toughness. Therefore, the Cr
content is 19.00% or less. The Cr content is preferably 18.80% or
less, and more preferably 18.60% or less.
[0084] Ni: 12.00 to 15.00%
[0085] Ni is an element to form austenite, and as a major element
of an 18 Cr-based austenitic stainless steel contributes to
improvement of high temperature strength and workability as well as
to stabilization of a metallic structure at a high temperature.
[0086] When the Ni content is less than 12.00%, the addition effect
is not obtained adequately, and formation of a brittle phase
(.sigma.-phase, etc.) is promoted at a high temperature due to
imbalance with the content of a ferrite forming element, such as
Cr, W, and Mo. Therefore, the Ni content is 12.00% or more. The Ni
content is preferably 12.50% or more.
[0087] Meanwhile, when the Ni content exceeds 15.00%, the high
temperature strength and the economic efficiency decrease.
Therefore, the Ni content is 15.00% or less, and is preferably
14.90% or less, more preferably 14.80% or less, and further
preferably 14.50% or less.
[0088] Cu: 2.00 to 4.00%
[0089] Cu is an element, which precipitates as a fine Cu phase that
is stable at a high temperature, to contribute to improvement of
high temperature strength.
[0090] When the Cu content is less than 2.00%, the addition effect
is not obtained adequately. Therefore, the Cu content is 2.00% or
more, and is preferably 2.20% or more, and more preferably 2.50% or
more.
[0091] Meanwhile, when the Cu content exceeds 4.00%, the
workability, creep ductility, and strength decrease. Therefore, the
Cu content is 4.00% or less, and is preferably 3.90% or less, more
preferably 3.80% or less, and further preferably 3.50% or less.
[0092] Mo: 0.01 to 2.00%
[0093] Mo is an essential element for improvement of the corrosion
resistance, high temperature strength, and stress corrosion
cracking resistance. Further, Mo is an element that contributes to
formation of a Laves phase that is stable at a high temperature for
a long time period of time and a carbide, through a synergistic
effect with W to be added combinedly.
[0094] When the Mo content is less than 0.01%, the addition effect
is not obtained adequately. Therefore, the Mo content is 0.01% or
more, and is preferably 0.02% or more.
[0095] Meanwhile, when the Mo content exceeds 2.00%, a large amount
of brittle phase is formed to decrease the workability, high
temperature strength, and toughness. Therefore, the Mo content is
2.00% or less, and is preferably 1.80% or less, more preferably
1.50% or less, and further preferably 1.30% or less.
[0096] W: 2.00 to 5.00%
[0097] W is an essential element for improvement of the corrosion
resistance, high temperature strength, and stress corrosion
cracking resistance. Further, it is an element to contribute to
precipitation of a Laves phase stable at a high temperature for a
long time period of time and a carbide, through a synergistic
effect with Mo to be added combinedly. Further, W is slower in
terms of diffusion at a high temperature than Mo, and therefore it
is an element to contribute to stable maintenance of the strength
at a high temperature for a long period of time.
[0098] When the W content is less than 2.00%, the addition effect
is not obtained adequately. Therefore, the W content is 2.00% or
more, and is preferably 2.10% or more.
[0099] Meanwhile, when the W content exceeds 5.00%, a large amount
of brittle phase is formed to decrease the workability, and
strength. Therefore, the W content is 5.00% or less, and is
preferably 4.90% or less, more preferably 4.80% or less, and
further preferably 4.70% or less.
[0100] 2Mo+W: 2.50 to 5.00%
[0101] Combined addition of Mo and W contributes to improvement of
the high temperature strength, stress corrosion cracking
resistance, and high temperature corrosion resistance. When 2Mo+W
(Wherein Mo represents a Mo content, and W represents a W content.
The same holds hereinbelow.) is less than 2.50%, the synergistic
effect of the combined addition cannot be obtained adequately.
Therefore, 2Mo+W is 2.50% or more, and is preferably 2.60% or more,
more preferably 2.80% or more, and further preferably 3.00% or
more.
[0102] Meanwhile, when 2Mo+W exceeds 5.00%, the strength or
toughness decreases, and the stability of a metallic structure is
also decreased at a high temperature. Therefore 2Mo+W is 5.00% or
less, and is preferably 4.90% or less.
[0103] V: 0.01 to 0.40%
[0104] V is an element contributing to improvement of high
temperature strength by forming a fine carbide together with Ti and
Nb. When the V content is less than 0.01%, the addition effect is
not obtained adequately. Therefore, the V content is 0.01% or more,
and is preferably 0.02% or more.
[0105] Meanwhile, when the V content exceeds 0.40%, the strength or
stress corrosion cracking resistance decreases. Therefore, the V
content is 0.40% or less, and is preferably 0.38% or less.
[0106] Ti: 0.05 to 0.50%
[0107] Ti is an element contributing to improvement of high
temperature strength by forming a fine carbide together with V and
Nb, and contributing also to improvement of stress corrosion
cracking resistance through suppression of precipitation of a Cr
carbide at a crystal grain boundary by fixing C.
[0108] In a conventional N-adding austenitic stainless steel, not
only the effect of N addition is not obtained effectively due to
precipitation of a nitride in clumps, but also the stress corrosion
cracking resistance is decreased due to precipitation of a coarse
Cr carbide at a grain boundary.
[0109] The inventors have found, with respect to an 18 Cr-based
austenitic stainless steel, that an advantageous action effect of a
fine Ti carbide can be obtained by controlling the N content at a
very low level, and that, specifically, a fine Laves phase
precipitates using a fine Ti carbide as a nucleus, as a result of
which the high temperature strength of the steel is enhanced
remarkably.
[0110] When the Ti content is less than 0.05%, the addition effect
is not obtained adequately, and therefore, the Ti content is 0.05%
or more. Combined addition of Nb and V is preferable, and the Ti
content is preferably 0.10% or more.
[0111] Meanwhile, when the Ti content exceeds 0.50%, a clumpy
precipitate is precipitated to decrease the strength, toughness,
and stress corrosion cracking resistance. Therefore, the Ti content
is 0.50% or less, and is preferably 0.45% or less.
[0112] Nb: 0.15 to 0.70%
[0113] Nb is an element contributing to improvement of high
temperature strength by forming a fine carbide together with V and
Ti, and contributing also to improvement of stress corrosion
cracking resistance through suppression of precipitation of a Cr
carbide at a crystal grain boundary by fixing C.
[0114] Further, Nb is, similar to Ti, an element contributing to
improvement of the high temperature strength due to precipitation
of a fine Laves phase.
[0115] When the Nb content is less than 0.15%, the addition effect
is not obtained adequately. Therefore, the Nb content is 0.15% or
more, and is preferably 0.20% or more.
[0116] Meanwhile, when the Nb content exceeds 0.70%, a clumpy
precipitate is precipitated to decrease the strength, toughness,
and stress corrosion cracking resistance. Therefore, the Nb content
is 0.70% or less, and is preferably 0.60% or less.
[0117] Al: 0.001 to 0.040%
[0118] Al is an element which functions as a deoxidizing element in
steel making to purify a steel.
[0119] When the Al content is less than 0.001%, purification of a
steel cannot be achieved adequately. Therefore, the Al content is
0.001% or more, and is preferably 0.002% or more.
[0120] Meanwhile, when the Al content exceeds 0.040%, a large
amount of nonmetallic inclusion is formed to decrease the stress
corrosion cracking, high temperature strength, workability,
toughness, and stability of a metallic structure at a high
temperature. Therefore, the Al content is 0.040% or less, and is
preferably 0.034% or less.
[0121] B: 0.0010 to 0.0100%
[0122] B is an element for achieving securance of superior high
temperature strength and superior stress corrosion cracking
resistance by combined addition with Nd, which is an important
element in the steel of the embodiment. Therefore, B is an
essential element. B is an element not only to contribute to
improvement of the high temperature strength through segregation at
a crystal grain boundary, but also to contribute to formation of a
carbide, micronization of a Laves phase, and stabilization of a
metallic structure, which are effective for improvement of the high
temperature strength.
[0123] Further, B is an element, which makes N (present in the
steel of the embodiment at 0.0010 to 0.0100%) harmless as BN, and
contributes to improvement of the high temperature strength and
stress corrosion resistance.
[0124] When the B content is less than 0.0010%, residual B, which
has not been consumed as a nitride, namely B able to contribute to
improvement of the high temperature strength and stress corrosion
resistance cannot be secured. As the result, when the B content is
less than 0.0010%, a synergistic effect (to be described below) of
combined addition with Nd (and securance of effective M content) is
not obtained, so that the high temperature strength and stress
corrosion cracking resistance are not improved. Therefore the B
content is 0.0010% or more, and is preferably 0.0015% or more.
[0125] Meanwhile, when the B content exceeds 0.0100%, a boron
compound is formed to decrease the workability, weldability, and
high temperature strength. Therefore the B content is 0.0100% or
less, and is preferably 0.0080% or less, and more preferably
0.0060% or less.
[0126] N: 0.0010 to 0.0100%
[0127] N (nitrogen) is a useful element with respect to a general
18 Cr-based austenitic stainless steel for improvement of the high
temperature strength through solid solution strengthening and
precipitation strengthening with a nitride. However with respect to
the steel of the embodiment, a nitride disturbs stress corrosion
cracking resistance, and therefore N is not added actively.
[0128] However, since a small amount of N forms a precipitation
nucleus for a fine precipitate effective for improvement of high
temperature strength, such small amount of N is allowed in the
steel of the embodiment, as is used for forming a precipitation
nucleus for a fine precipitate effective for improvement of high
temperature strength.
[0129] Namely according to the fundamental thought with respect to
the steel of the embodiment, N is not added actively, but is
allowed only in a small content range, which is different from the
prior art.
[0130] When the N content is less than 0.0010%, formation of a
precipitation nucleus for a fine precipitate, which is effective
for improvement of high temperature strength, is difficult.
Therefore the N content is 0.0010% or more, and is preferably
0.0020% or more, and more preferably 0.0030% or more.
[0131] Meanwhile, when the N content exceeds 0.0100%, a nitride is
formed to decrease the high temperature strength and stress
corrosion cracking resistance. Therefore the N content is 0.0100%
or less, and is preferably 0.0090% or less, more preferably 0.0080%
or less, and further preferably 0.0070% or less.
[0132] Nd: 0.001 to 0.20%
[0133] Nd is an element to improve remarkably the high temperature
strength and stress corrosion cracking resistance through a
synergistic effect (described below) of combined addition with
B.
[0134] With respect to the steel of the embodiment, as described
above, the stress corrosion cracking resistance is improved by
micronizing a carbide and a Laves phase effective for improvement
of the high temperature strength, by securing the long term
stability, and further by strengthening a crystal grain boundary
through combined addition of Nd and B.
[0135] However, since the bonding strength of Nd with N, O, or S is
extremely strong, even when it is added as metal Nd, it is consumed
to precipitate as a harmful precipitate, and the addition effect is
hardly exhibited adequately. Therefore, for obtaining fully the
addition effect, it is necessary to reduce the N content, O
content, and S content to the extent possible.
[0136] When the Nd content is less than 0.001%, even if the N
content, O content, and S content are reduced, the addition effect
of Nd cannot be obtained adequately. Therefore the Nd content is
0.001% or more, and is preferably 0.002% or more, and more
preferably 0.005% or more.
[0137] Meanwhile, when the Nd content exceeds 0.20%, the addition
effect is saturated, and an oxide-based inclusion is formed, so
that the strength, workability, and economy are decreased.
Therefore the Nd content is 0.20% or less, and is preferably 0.18%
or less, more preferably 0.15% or less, and further preferably
0.10% or less.
[0138] For the sake of easier securance of the aforementioned
effective M content Meff, the Nd content is preferably in a range
of from 0.002 to 0.15%, and more preferably from 0.005 to
0.10%.
[0139] With respect to the steel of the embodiment, Zr, Bi, Sn, Sb,
Pb, As, and O are treated as impurity elements for the sake of
securance of superior characteristics of the steel of the
embodiment, and the contents of the elements are limited.
[0140] Ordinarily, as a source material for a stainless steel,
scraps such as alloy steel are used mainly. The scraps contain,
although at low contents, Zr, Bi, Sn, Sb, Pb, and As (6 impurity
elements), which get mixed in a stainless steel (product)
inevitably.
[0141] Further, when a facility for melting, etc. in a production
process of a stainless steel is contaminated by production of
another alloy, the 6 impurity elements may get mixed in a stainless
steel (product) from the facility for melting, etc., and O (oxygen)
remains inevitably in a stainless steel.
[0142] With respect to the steel of the embodiment, for the sake of
securance of superior high temperature strength and superior stress
corrosion cracking resistance, Zr, Bi, Sn, Sb, Pb, As, and O are
required to be reduced to the extent possible so as to prepare a
high purity steel.
[0143] Zr: 0.002% or less
[0144] Zr is ordinarily not contained. However Zr may get mixed
from scraps, etc. and/or a facility for melting, etc. contaminated
by production of another alloy to form an oxide and a nitride. The
nitride functions as a nucleus for precipitation of a precipitate
such as a Laves phase.
[0145] However, in a case in which a clumpy precipitate is
precipitated with a nucleus of a nitride, the high temperature
strength and stress corrosion cracking resistance are
disturbed.
[0146] As described above, Zr is a harmful element in terms of high
temperature strength and stress corrosion cracking resistance.
Therefore in a relational expression (Formula (1)) introduced for
the sake of securance of superior high temperature strength and
superior stress corrosion cracking resistance, a term of "-1.6Zr"
expressing a negative action effect has been added.
[0147] Since the amount of Zr is preferably as low as possible, the
upper limit of the Zr content is set at 0.002% which is close to
the analytical limit (0.001%). The Zr content is preferably 0.001%
or less.
[0148] The Zr content may be 0%. However, Zr may occasionally get
mixed inevitably at 0.0001% or so. Therefore, from the viewpoint of
production cost, the Zr content may be 0.0001% or even more.
[0149] Bi: 0.001% or less
[0150] Bi is an element which is ordinarily not contained. However,
Bi may get mixed from scraps, etc. and/or a facility for melting,
etc. contaminated by production of another alloy, and disturbs high
temperature strength and stress corrosion cracking resistance.
[0151] Since the Bi content is required to be reduced to the extent
possible, the upper limit of the Bi content is set at 0.001% which
is the analytical limit.
[0152] The Bi content may be 0%. However, Bi may occasionally get
mixed inevitably at 0.0001% or so. Therefore, from the viewpoint of
production cost, the Bi content may be 0.0001% or even more.
[0153] Sn: 0.010% or less
[0154] Sb: 0.010% or less
[0155] Pb: 0.001% or less
[0156] As: 0.001% or less
[0157] Sn, Sb, Pb, and As are elements, which easily get mixed from
scraps, etc. and/or a facility for melting, etc. contaminated by
production of another alloy, and are hardly removed in a refining
process.
[0158] However, the contents of the elements are required to be
reduced to the extent as possible.
[0159] Considering source materials composition and refining
limits, the upper limits of the Sn content and the Sb content are
set at 0.010% respectively. The Sn content and the Sb content are
preferably 0.005% or less respectively.
[0160] Further, the upper limits of the Pb content and the As
content are set at 0.001% respectively. Pb and As are preferably
0.0005% or less respectively.
[0161] Any of the Sn content, the Sb content, the Pb content, and
the As content may be 0%.
[0162] However, the elements may inevitably get mixed at 0.0001% or
so. Therefore from the viewpoint of production cost, the content of
any of the elements may be 0.0001% or even more.
[0163] Zr+Bi+Sn+Sb+Pb+As: 0.020% or less
[0164] In a case in which the invention steel contains inevitably
Zr, Bi, Sn, Sb, Pb, and As (6 impurity elements), for the sake of
securance of superior high temperature strength and superior stress
corrosion cracking resistance through a synergistic effect of
combined addition of Nd and B, not only the individual contents of
the 6 impurity elements is required to be limited but also the
total of the contents of the 6 impurity elements
(Zr+Bi+Sn+Sb+Pb+As; wherein each element symbol represents the
content of each element) is required to be limited to 0.020% or
less for achieving higher purity.
[0165] The total content of the 6 impurity elements in the steel of
the embodiment is 0.020% or less.
[0166] The total content of the 6 impurity elements is preferably
0.015% or less, and more preferably 0.010% or less.
[0167] Meanwhile, for the sake of securance of superior high
temperature strength and superior stress corrosion cracking
resistance, the total content of the 6 impurity elements is
preferably as low as possible. Therefore, the lower limit of the
total content of the 6 impurity elements is 0%.
[0168] O: 0.0090% or less
[0169] O (oxygen) remaining inevitably after refining a molten
steel is an element used as an index of the content of a
nonmetallic inclusion.
[0170] When O exceeds 0.0090%, an Nd oxide is formed to consume Nd
and form a fine carbide or Laves phase, so that the improvement
effect on high temperature strength and stress corrosion cracking
resistance cannot be obtained. Therefore, the O content is 0.0090%
or less, and is preferably 0.0080% or less, more preferably 0.0070%
or less, and further preferably 0.0050% or less.
[0171] The O content may be 0%. However, O may occasionally remain
after refining inevitably at 0.0001% or so. Therefore, from the
viewpoint of production cost, the O content may be 0.0001% or even
more.
[0172] The chemical composition of the steel of the embodiment may
include one or more of Co, Ca, or Mg, and/or one or more of
lanthanoid elements except Nd, Y, Sc, Ta, Hf, or Re.
[0173] Any of the elements is an optional element, and therefore
the contents thereof may be respectively 0%.
[0174] Co: 0.80% or less
[0175] Co may become a contaminant source in producing another
steel. Therefore, the Co content is 0.80% or less, and is
preferably 0.60% or less.
[0176] A steel of the embodiment is not required to contain Co
(namely, the Co content may be 0%), however from the viewpoint of
further stabilization of a metallic structure and improvement of
high temperature strength, Co may be contained.
[0177] When the steel of the embodiment contains Co, the Co content
is preferably 0.01% or more, and more preferably 0.03% or more.
[0178] Ca: 0.20% or less
[0179] Ca is an optional element, and the Ca content may be 0%.
[0180] Ca may be added as a finishing element for deoxidation.
Since the steel of the embodiment contains Nd, it is preferable
that the same is deoxidized by Ca in a refining process. When the
steel of the embodiment contains Ca, from the viewpoint of
obtaining more effectively a deoxidation effect, the Ca content is
preferably 0.0001% or more, and more preferably 0.0010% or
more.
[0181] Meanwhile, when the Ca content exceeds 0.20%, the amount of
a nonmetallic inclusion increases to lower the high temperature
strength, stress corrosion cracking resistance, and toughness.
Therefore the Ca content is 0.20% or less, and is preferably 0.15%
or less.
[0182] Mg: 0.20% or less
[0183] Mg is an optional element, and the Mg content may be 0%.
[0184] Mg is an element, which contributes to improvement of high
temperature strength or corrosion resistance by addition of a small
amount thereof. When the steel of the embodiment contains Mg, from
the viewpoint of obtaining more effectively the effect, the Mg
content is preferably 0.0005% or more, and more preferably 0.0010%
or more.
[0185] Meanwhile, when the Mg content exceeds 0.20%, the strength,
toughness, corrosion resistance, and weldability are lowered.
Therefore the Mg content is 0.20% or less, and is preferably 0.15%
or less.
[0186] Total of one or more of Y, Sc, Ta, Hf, Re or lanthanoid
elements other than Nd: 0.20% or less
[0187] Any of Y, Sc, Ta, Hf, Re and lanthanoid elements other than
Nd (namely, La, Ce, Pr, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and
Lu) is an optional element, and the total content of the elements
may be 0%.
[0188] Although Y, Sc, Ta, Hf, Re and lanthanoid elements other
than Nd are expensive, they are elements acting to enhance a
synergistic effect of combined addition of Nd and B. When the steel
of the embodiment contains one or more of the elements, the total
content of the elements is preferably 0.001% or more, and more
preferably 0.005% or more.
[0189] Meanwhile, when the total content of Y, Sc, Ta, Hf, Re and
lanthanoid elements other than Nd exceeds 0.20%, the amount of a
nonmetallic inclusion increases to lower the strength, toughness,
corrosion resistance, and weldability. Therefore the total content
is 0.20% or less, and is preferably 0.15% or less.
[0190] A remainder excluding other than the aforementioned elements
from the chemical composition of the steel of the embodiment is Fe
and impurities.
[0191] The impurities referred to herein mean one or more of
elements other than the aforementioned elements. The contents of
the elements (impurities) other than the aforementioned elements
are preferably limited to 0.010% or less respectively, and more
preferably to 0.001% or less.
[0192] With respect to the chemical composition of the steel of the
embodiment, an effective M content Meff defined by the following
Formula (1) is from 0.0001 to 0.250%.
[0193] The effective M content Meff will be described below.
Effective M content Meff=Nd+13(B-11N/14)-1.6Zr Formula (1)
[0194] wherein in Formula (1), each element symbol represents the
content (mass %) of each element.
[0195] The effective M content Meff is an index defining a
quantitative relationship between Nd and B, which are essential for
improvement of high temperature strength and stress corrosion
cracking resistance.
[0196] Formula (1) defining an effective M content Meff is a
relational expression discovered by the inventors from the
viewpoint of securance of superior high temperature strength and
superior stress corrosion cracking resistance.
[0197] Formula (1) is basically a relational expression, in which
to the content of Nd to function effectively for securance of
superior high temperature strength and superior stress corrosion
cracking resistance, the content of B also to function effectively
is added, and the content of Zr to function harmfully against
securance of superior high temperature strength and superior stress
corrosion cracking resistance is subtracted.
[0198] With respect to the steel of the embodiment, N is reduced to
the extent as possible so as to suppress formation of a nitride in
order to secure superior high temperature strength and superior
stress corrosion cracking resistance.
[0199] However, when a steel is produced industrially, some amount
of N inevitably gets mixed in a steel. If the N mixed in a steel
forms BN, the function of B cannot be obtained. Therefore, it is
necessary to secure B not bound with N.
[0200] In Formula (1) defining an effective M content Meff, the
moiety of "(B-11N/14)" represents the content of B that effectively
functions (namely, the content of B that is not bound with N, among
the B that has been added).
[0201] In Formula (1), "(B-11N/14)" (the content of B not bound
with N) is multiplied by 13 to "13(B-11N/14)" for weighting the
content of B which functions effectively. In this regard, 13 is a
ratio of the atomic weight of Nd (.apprxeq.144) to the atomic
weight of B (.apprxeq.11).
[0202] In Formula (1), "13(B-11N/14)" obtained as above is added to
the Nd content ("Nd+13(B-11N/14)"). Nd is an element that functions
effectively similarly as B for securing superior high temperature
strength and superior stress corrosion cracking resistance.
[0203] In Formula (1) in addition to "Nd+13(B-11N/14)", there is a
term "-1.6Zr" for subtracting the content of Zr that is harmful
against securance of superior high temperature strength and
superior stress corrosion cracking resistance.
[0204] The impurity element Zr, by forming a nitride and an oxide,
functions to reduce a synergistic effect of combined addition of Nd
and B.
[0205] In Formula (1) the reduction effect of Zr is weighted by
multiplying the Zr content by 1.6 (.apprxeq.144/91), which is the
ratio of the atomic weight of Nd (.apprxeq.144) to the atomic
weight of Zr (.apprxeq.91), to "1.6Zr".
[0206] In Formula (1) the "1.6Zr" is subtracted from the
"Nd+13(B-11N/14)".
[0207] As described above, the addition amounts of Nd and B
necessary for obtaining superior high temperature strength and
superior stress corrosion cracking resistance, and the limited
amount of Zr being harmful to securance of superior high
temperature strength and superior stress corrosion cracking
resistance can be quantified by an effective M content Meff defined
by Formula (1) (specific examples will be described in Examples in
detail).
[0208] When the effective M content Meff is less than 0.0001%, it
is difficult to achieve superior high temperature strength and
superior stress corrosion cracking resistance. Therefore the
effective M content Meff is 0.0001% or more, and is preferably
0.001% or more, more preferably 0.002% or more, and further
preferably 0.010% or more
[0209] In this regard, when the N content or the Zr content is
high, the effective M content Meff may take a negative value.
[0210] Meanwhile, when the effective M content Meff exceeds 0.250%,
the improvement effect on high temperature strength and stress
corrosion cracking resistance according to the effective M content
Meff is saturated, and the economy declines, and moreover the
strength, toughness, workability, and weldability decrease.
Therefore, the effective M content Meff is 0.250% or less, and is
preferably 0.200% or less, and more preferably 0.150%.
[0211] There is no particular restriction on the metallic structure
of the steel of the embodiment.
[0212] The metallic structure of the steel of the embodiment is
preferably a coarse grain metallic structure from the viewpoint of
improvement of high temperature strength (for example, high
temperature creep strength between 700.degree. C. and 750.degree.
C.).
[0213] Specifically, with respect to the steel of the embodiment,
the ASTM grain size number of the metallic structure thereof is
preferably 7 or less.
[0214] When the metallic structure of the steel of the embodiment
is a coarse grain structure with an ASTM grain size number of 7 or
less, a suppression effect on grain boundary sliding in creep,
change in a metallic structure by element diffusion through a
crystal grain boundary, and formation of precipitation site for an
.sigma. phase can be conceivably obtained.
[0215] Therefore, from the viewpoint of improvement of the high
temperature strength, it is preferable that the metallic structure
of the steel of the embodiment is a coarse grain structure with an
ASTM grain size number of 7 or less.
[0216] Meanwhile, in the case of a conventional steel, when the
metallic structure of a steel is a coarse grain metallic structure,
stress corrosion cracking is apt to occur due to segregation of an
impurity element at a crystal grain boundary.
[0217] However, in the case of the steel of the embodiment,
segregation of an impurity element at a crystal grain boundary is
reduced owing to higher purification. Therefore, with respect to
the steel of the embodiment, even with a coarse grain metallic
structure (for example, a metallic structure with an ASTM grain
size number of 7 or less), the stress corrosion cracking is
suppressed (namely, superior stress corrosion cracking resistance
may be maintained).
[0218] From the above viewpoints, the ASTM grain size number of the
metallic structure of the steel of the embodiment is preferably 7
or less, and more preferably 6 or less.
[0219] There is no particular restriction on the lower limit of the
ASTM grain size number of a metallic structure. From the viewpoint
of suppression of decreasing in creep ductility and welding
cracking, the lower limit of the ASTM grain size number of a
metallic structure is preferably 3.
[0220] A steel of the embodiment is superior in high temperature
strength (especially, creep rupture strength) as described
above.
[0221] There is no particular restriction on the specific range of
the high temperature strength of the steel of the embodiment. The
creep rupture strength at 700.degree. C. and 10,000 hours of the
steel of the embodiment is preferably 140 MPa or more.
[0222] In this regard, 700.degree. C. is a temperature higher than
an actual usage temperature.
[0223] Therefore, the creep rupture strength at 700.degree. C. and
10,000 hours of 140 MPa or more means that the high temperature
characteristic is remarkably superior.
[0224] Specifically, a high temperature strength at which the creep
rupture strength is 140 MPa or more at 700.degree. C. and 10,000
hours is a high temperature strength that is remarkably superior to
a 347H steel (18 Cr-12Ni-Nb), which is used widely in the world as
a conventional 18 Cr-based austenitic stainless steel (see, for
example, Inventive Steels 1 to 20, and Comparative steel 21 in
Table 3 below).
[0225] A creep rupture strength less than 140 MPa may be easily
achievable by extension of the conventional art, however it is
difficult to achieve a creep rupture strength of 140 MPa or more by
mere extension of the prior art.
[0226] In contrast, in the case of the steel of the embodiment, a
creep rupture strength of 140 MPa or more at 700.degree. C., which
is higher than an actual service temperature, and 10,000 hours
(superior high temperature strength) can be attained by fine
precipitation of a carbide and a Laves phase, the Laves phase
precipitates during creep, by means of optimization of the chemical
composition, optimization of the effective M content Meff by the Nd
content and the B content, higher degree of purification by
limiting the amount of impurity elements, etc.
[0227] There is no particular restriction on a method for producing
the steel of the embodiment, and a publicly known method for
producing an austenitic stainless steel may be appropriately
adopted.
[0228] A steel of the embodiment may be a heat-treated steel plate
or a heat-treated steel tube or pipe.
[0229] From the viewpoint of easy formation of a coarse grain
structure and easy improvement of the high temperature strength
(for example, creep rupture strength), the heating temperature of
the heat treatment is preferably from 1050 to 1250.degree. C., more
preferably from 1150.degree. C. to 1250.degree. C.
[0230] Although there is no particular restriction on the mode of
cooling after the heating during the heat treatment, and either of
quenching (for example, water cooling) and air cooling is
acceptable, quenching is preferable, and water cooling is more
preferable.
[0231] The heat-treated steel plate or the heat-treated steel tube
or pipe is obtained for example by preparing a steel plate or a
steel tube or pipe having an chemical composition of the
aforementioned steel of the embodiment, then heating the prepared
steel plate or the prepared steel tube or pipe at, for example,
from 1050 to 1250.degree. C. (preferably from 1150.degree. C. to
1250.degree. C.), and thereafter cooling the same.
[0232] The steel plate or the steel tube or pipe having the
chemical composition (a steel plate or a steel tube or pipe before
a heat treatment) may be prepared according to an ordinary
method.
[0233] A steel tube or pipe having the chemical composition may be
prepared, for example, by casting a molten steel having the
chemical composition to form a steel ingot or a steel billet, and
performing at least one kind of a processing selected from the
group consisting of hot extrusion, hot rolling, hot forging, cold
drawing, cold rolling, cold forging, and cutting, on the obtained
steel ingot or steel billet.
[0234] Hereinabove the steel of the embodiment has been
described.
[0235] There is no particular restriction on an application of the
steel of the embodiment, and the steel of the embodiment may be
applied to any application demanding securance of high temperature
strength and stress corrosion cracking resistance.
[0236] The steel of the embodiment is a material steel suitable
for, for example, a heat-resistant and pressure-resistant heat
exchanger tube or a pipe for a boiler, a chemical plant, or the
like; a heat-resistant forged product; a heat-resistant steel bar;
or a heat-resistant steel plate.
[0237] The steel of the embodiment is a material steel especially
suitable for a heat-resistant and pressure-resistant heat exchanger
tube to be placed inside a boiler (for example, a heat-resistant
and pressure-resistant heat exchanger tube with an outer diameter
of from 30 to 70 mm, and a thickness of from 2 to 15 mm), or a pipe
of boiler (for example, a pipe with an outer diameter of from 125
to 850 mm, and a thickness of from 20 to 100 mm).
EXAMPLES
[0238] Next, Examples of the invention will be described, but
conditions in the Examples are just examples of conditions adopted
for confirming the feasibility and effectiveness of the invention,
and the invention is not limited to such condition examples.
Indeed, many alternative conditions may be adopted for the
invention, insofar as the object of the invention is achieved
without departing from the spirit and scope of the invention.
[0239] In the Examples, 30 kinds of steels, whose chemical
compositions are shown in Table 1 and Table 2 (Continuation of
Table 1), were produced by melting.
[0240] In Table 1 and Table 2, Steels 1 to 20 are Inventive Steels
which are examples of the invention (hereinafter also referred to
as "Inventive Steels 1 to 20" respectively), and Steels 21 to 30
are Comparative Steels which are comparative examples (hereinafter
also referred to as "Comparative Steels 21 to 30"
respectively).
[0241] Comparative Steel 21 is a general-purpose steel 347H
(18Cr-12Ni-Nb) and is a standard material for comparison between
the prior art and Inventive Steels 1 to 20.
[0242] In melt-producing Inventive Steels 1 to 20, as a Fe source,
high purity Fe obtained by smelting in a blast furnace and a
converter and secondary refining by a vacuum oxygen degassing
process was used, and as an alloy element, a high purity alloy
element analyzed in advance was used. Further, before
melt-producing any of Inventive Steels 1 to 20, the furnace for
melt-producing Inventive Steels 1 to 20 was washed adequately, and
special care was taken so as to prevent contamination with
impurities.
[0243] Under the above special control, in producing Inventive
Steels 1 to 20, the 6 impurity elements (specifically, Zr, Bi, Sn,
Sb, Pb, and As) content, the O content, the N content and the like
were limited, and the Nd content and the B content were regulated
within an appropriate range.
[0244] In melt-producing Comparative Steels 23 to 30, the high
purity Fe source was used also. Further, in melt-producing
Comparative Steels 23 to 30, the chemical compositions were
adjusted as follows.
[0245] In melt-producing Comparative Steels 21, 23, 24, 27, and 29
at least one of the 6 impurity elements and O (oxygen) was added
intentionally.
[0246] In melt-producing Comparative Steels 21, 24, and 26, N
(nitrogen) was added intentionally.
[0247] In melt-producing Comparative Steels 21 to 23, 25, 27, and
28, at least one of B or Nd was not added.
[0248] In melt-producing Comparative Steel 21, Cu was added at an
insufficient content, and Mo, W, V, and Ti were not added.
[0249] In melt-producing Comparative Steel 30, W was added at an
insufficient content.
TABLE-US-00001 TABLE 1 Class Steel C Si Mn P S Cr Ni Cu Mo W 2Mo +
W V Ti Nb Al Inven- 1 0.09 0.20 0.80 0.015 0.001 18.10 14.20 3.01
0.10 4.02 4.22 0.03 0.20 0.21 0.008 tive 2 0.08 0.35 1.50 0.025
0.002 18.52 14.85 3.52 0.78 2.57 4.13 0.02 0.35 0.52 0.015 Steel 3
0.06 0.12 1.25 0.019 0.001 17.58 12.12 2.42 0.05 3.21 3.31 0.08
0.06 0.42 0.005 4 0.12 0.22 0.56 0.008 0.003 18.02 13.85 2.88 0.02
3.11 3.15 0.15 0.22 0.69 0.002 5 0.07 0.38 0.21 0.020 0.001 18.03
14.00 3.02 0.32 2.05 2.69 0.05 0.30 0.25 0.022 6 0.11 0.15 2.45
0.006 0.001 18.41 13.92 3.45 0.02 3.21 3.25 0.38 0.06 0.66 0.013 7
0.10 0.41 0.86 0.029 0.005 17.99 12.79 2.89 0.04 3.89 3.97 0.02
0.25 0.34 0.007 8 0.08 0.20 1.52 0.012 0.010 18.07 13.24 3.14 1.22
2.01 4.45 0.04 0.33 0.44 0.015 9 0.06 0.56 1.68 0.020 0.003 17.65
13.71 3.25 0.30 3.00 3.60 0.03 0.45 0.31 0.024 10 0.12 0.39 0.98
0.017 0.001 18.61 14.68 3.06 0.68 3.01 4.37 0.10 0.21 0.55 0.005 11
0.06 0.50 1.00 0.022 0.018 18.00 14.22 2.90 1.23 2.01 4.47 0.28
0.38 0.63 0.038 12 0.08 0.11 0.73 0.025 0.010 17.42 13.87 3.37 0.02
3.25 3.29 0.33 0.08 0.55 0.017 13 0.06 0.20 0.32 0.029 0.003 17.69
12.88 2.87 0.08 4.72 4.88 0.19 0.11 0.35 0.009 14 0.11 0.35 0.21
0.010 0.007 18.21 14.53 2.99 0.50 3.21 4.21 0.26 0.28 0.41 0.010 15
0.09 0.45 1.05 0.023 0.001 18.10 14.01 3.10 0.31 3.79 4.41 0.17
0.37 0.42 0.031 16 0.07 0.30 1.22 0.011 0.002 17.93 13.70 2.69 0.08
3.52 3.68 0.16 0.10 0.39 0.019 17 0.12 0.26 0.69 0.028 0.001 17.88
12.55 3.82 0.05 4.11 4.21 0.20 0.28 0.60 0.025 18 0.06 0.46 1.40
0.027 0.004 18.09 14.74 2.99 1.21 2.13 4.55 0.14 0.33 0.28 0.033 19
0.09 0.35 0.28 0.008 0.001 18.01 14.12 3.11 0.55 3.33 4.43 0.05
0.17 0.37 0.009 20 0.08 0.17 0.72 0.005 0.001 17.87 13.73 2.74 0.15
2.97 3.27 0.02 0.34 0.42 0.020 Compar- 21 0.09 0.45 1.53 0.026
0.001 18.52 12.06 0.01 0 0 0 0 0 0.65 0.001 ative 22 0.08 0.35 1.23
0.028 0.002 17.95 12.01 2.45 0.01 4.03 4.05 0.01 0.06 0.45 0.015
Steel 23 0.06 0.45 0.58 0.025 0.005 17.56 13.04 3.10 0.01 3.52 3.54
0.02 0.07 0.32 0.036 24 0.07 0.37 0.23 0.015 0.001 17.06 12.14 2.02
0.33 2.03 2.69 0.02 0.35 0.24 0.001 25 0.13 0.69 1.23 0.028 0.015
17.53 12.23 2.10 0.03 2.51 2.57 0.01 0.06 0.15 0.006 26 0.11 0.36
0.14 0.028 0.009 17.23 12.03 2.04 0.52 2.23 3.27 0.01 0.05 0.16
0.004 27 0.08 0.25 0.36 0.017 0.001 18.20 12.01 2.53 0.20 2.22 2.62
0.02 0.06 0.20 0.012 28 0.07 0.89 0.15 0.032 0.005 18.02 13.01 2.03
1.12 2.03 4.27 0.05 0.06 0.23 0.035 29 0.12 0.15 0.32 0.028 0.001
18.30 12.80 3.21 0.05 2.13 2.23 0.10 0.11 0.17 0.021 30 0.10 0.92
0.40 0.029 0.001 17.52 12.63 2.78 0.48 1.81 2.77 0.05 0.13 0.20
0.022
TABLE-US-00002 TABLE 2 (Continuation of Table 1) Sub-total Class
Steel B N Nd Meff Zr Bi Sn Sb Pb As (X) O Others Inven- 1 0.0040
0.0080 0.18 0.149 0.001 <0.001 0.005 <0.001 <0.001
<0.001 0.006 0.0021 tive 2 0.0015 0.0025 0.01 0.004 <0.001
<0.001 <0.001 <0.001 <0.001 <0.001 0 0.0030 Co: 0.40
Steel 3 0.0052 0.0098 0.15 0.118 <0.001 <0.001 0.005
<0.001 <0.001 <0.001 0.005 0.0056 4 0.0033 0.0053 0.02
0.007 0.001 <0.001 <0.001 0.003 <0.001 <0.001 0.004
0.0086 La: 0.01 5 0.0055 0.0015 0.18 0.235 0.001 <0.001 0.005
0.002 <0.001 <0.001 0.008 0.0050 Ce: 0.18 6 0.0018 0.0085
0.08 0.015 0.001 <0.001 0.009 <0.001 <0.001 <0.001
0.010 0.0045 7 0.0023 0.0056 0.07 0.043 <0.001 <0.001 0.001
0.001 <0.001 <0.001 0.002 0.0078 Mg: 0.0015 8 0.0047 0.0088
0.05 0.021 <0.001 <0.001 0.009 <0.001 <0.001 <0.001
0.009 0.0088 9 0.0023 0.0065 0.04 0.004 <0.001 <0.001 0.008
0.005 <0.001 <0.001 0.013 0.0078 Ta: 0.15, Y: 0.003 10 0.0036
0.0074 0.11 0.080 0.001 <0.001 0.005 0.001 <0.001 <0.001
0.007 0.0063 11 0.0010 0.0090 0.09 0.011 <0.001 <0.001
<0.001 0.001 <0.001 <0.001 0.001 0.0060 Pr: 0.002, Ca:
0.002 12 0.0035 0.0075 0.06 0.029 <0.001 <0.001 0.007 0.001
<0.001 <0.001 0.008 0.0038 Ca: 0.0005 13 0.0044 0.0042 0.02
0.033 0.001 <0.001 0.005 0.001 <0.001 <0.001 0.007 0.0047
14 0.0036 0.0035 0.07 0.079 0.001 <0.001 0.005 0.002 <0.001
<0.001 0.008 0.0055 Re: 0.010 15 0.0025 0.0050 0.09 0.071
<0.001 <0.001 0.008 <0.001 <0.001 <0.001 0.008
0.0068 16 0.0017 0.0063 0.10 0.058 <0.001 <0.001 0.005 0.001
<0.001 <0.001 0.006 0.0089 Mg: 0.0012, Co: 0.20 Hf: 0.002 17
0.0029 0.0075 0.08 0.039 0.001 <0.001 0.005 <0.001 <0.001
<0.001 0.006 0.0064 18 0.0038 0.0081 0.05 0.017 <0.001
<0.001 0.008 <0.001 <0.001 <0.001 0.008 0.0041 19
0.0017 0.0087 0.07 0.002 0.001 <0.001 <0.001 0.002 <0.001
<0.001 0.003 0.0077 Sc: 0.002 20 0.0026 0.0077 0.08 0.035
<0.001 <0.001 <0.001 <0.001 <0.001 <0.001 0
0.0084 Compar- 21 0 0.0110 0 -0.114 0.001 <0.001 0.002 0.003
<0.001 <0.001 0.006 0.0102 ative 22 0.0023 0.0063 0 -0.036
0.001 <0.001 0.002 0.001 <0.001 0.001 0.005 0.0089 Steel 23
0.0010 0.0073 0 -0.070 0.005 <0.001 0.005 0.001 0.001 0.001
0.013 0.0088 24 0.0017 0.0105 0.10 0.013 0.001 <0.001 0.001
0.001 0.001 <0.001 0.004 0.0170 25 0.0100 0.0098 0 0.027 0.002
<0.001 <0.001 0.002 0.001 0.001 0.006 0.0087 26 0.0068 0.0530
0.02 -0.433 <0.001 <0.001 0.008 0.001 <0.001 <0.001
0.009 0.0085 27 0.0047 0.0055 0 -0.009 0.010 0.010 0.013 0.002
0.003 <0.001 0.038 0.0089 28 0 0.0070 0.15 0.075 0.002 <0.001
0.005 0.001 <0.001 <0.001 0.008 0.0085 29 0.0028 0.0089 0.10
0.034 0.007 <0.001 0.004 0.008 <0.001 <0.001 0.019 0.0079
30 0.0037 0.0078 0.11 0.077 0.001 <0.001 0.002 0.005 0.001
<0.001 0.009 0.0075
[0250] --Explanation of Table 1 and Table 2--
[0251] A numerical value represents the content of each element
(mass %).
[0252] An underlined numerical value is a value outside the range
of the chemical composition of the embodiment.
[0253] A remainder of each steel excluding the elements listed in
Table 1 and Table 2 is Fe and impurities.
[0254] An Meff was calculated according to Formula (1). In this
regard, for a steel in which the Zr content is less than 0.001%
(written as "<0.001" in Table 2), the Meff was calculated by
regarding the Zr content as 0%.
[0255] Sub-total (X) shows the total content (mass %) of the 6
impurity elements (specifically, Zr, Bi, Sn, Sb, Pb, and As). In
this regard, for an element with a content of less than 0.001%
(written as "<0.001" in Table 2), the sub-total (X) was
calculated by regarding the content of the element as 0%.
[0256] <Production and Heat Treatment (1200.degree. C.) of Test
Material>
[0257] A steel having an chemical composition shown in Table 1 and
Table 2 was melted by vacuum melting and cast to obtain a 50
kg-steel ingot.
[0258] By hot forging the obtained steel ingot, a 15 mm-thick steel
plate was obtained.
[0259] By cutting a surface of the obtained 15 mm-thick steel
plate, an approx. 12 mm-thick steel plate was obtained.
[0260] By performing cold rolling on the obtained approx. 12
mm-thick steel plate at a cross-section reduction rate of approx.
30% an approx. 8 mm-thick platy test material was obtained.
[0261] A heat treatment at 1200.degree. C. was performed on the
test material by heating the test material to 1200.degree. C., then
keeping test material at 1200.degree. C. for 15 min, and thereafter
cooling the test material with water.
[0262] <Measurement of ASTM Grain Size>
[0263] The ASTM grain size of the test material after the heat
treatment was measured according to ASTM E112. A measurement
position of an ASTM grain size was near the central part in the
thickness direction of a longitudinal cross-section of the test
material.
[0264] The results are shown in Table 3.
[0265] <Measurement of High Temperature Strength>
[0266] A creep rupture test piece with a size of 6 mm.phi. and a
length of the parallel portion of 30 mm was cut out from the
heat-treated test material, whose longitudinal direction was the
longitudinal direction of the test piece. The creep rupture test
piece was subjected to a long term creep rupture test at
700.degree. C. for 10,000 hours or longer, and a creep rupture
strength (MPa) at 700.degree. C. and 10,000 hours was measured as a
high temperature strength.
[0267] The results are shown in Table 3.
[0268] <Stress Corrosion Cracking Test on Base Material>
[0269] A corrosion test piece with a width of 10 mm.times.a
thickness of 4 mm.times.a length of 40 mm was sliced out from the
heat-treated test material. The sliced out corrosion test piece is
hereinafter called a "base material".
[0270] The base material was subjected to a thermal aging treatment
at 650.degree. C. for 10 hours.
[0271] A Strauss test (ASTM A262, Practice E: Sensitization
evaluation) was performed on the base material after the thermal
aging treatment, and presence or absence of a crack with a depth of
100 .mu.m or more was examined.
[0272] The results of the above are shown in Table 3.
[0273] <Stress Corrosion Cracking Test on Weld HAZ (Heat
Affected Zone) Equivalent Material>
[0274] A corrosion test piece with a width of 10 mm.times.a
thickness of 4 mm.times.a length of 40 mm was sliced out from the
heat-treated test material.
[0275] The sliced-out test piece was heated at 950.degree. C. for
25 sec using a Greeble tester (Joule heating in vacuum). A weld HAZ
equivalent material (i.e. a weld heat affected zone equivalent
material) was obtained by blowing He for cooling after the
heating.
[0276] A thermal aging treatment and a Strauss test were conducted
on the obtained weld HAZ equivalent material similarly as the
stress corrosion cracking test on the base material, and presence
or absence of a crack with a depth of 100 .mu.m or more was
examined.
[0277] The results are shown in Table 3.
TABLE-US-00003 TABLE 3 High temperature Stress corrosion strength
cracking test result (700.degree. C., (Existence of crack with ASTM
10000 hours depth of 100 .mu.m or more) grain creep rupture Weld
HAZ size strength) Base equivalent Class Steel number (MPa)
material material Inven- 1 4.3 165 No crack No crack tive 2 5.2 152
No crack No crack Steel 3 3.1 148 No crack No crack 4 5.1 170 No
crack No crack 5 3.8 163 No crack No crack 6 6.5 160 No crack No
crack 7 6.8 150 No crack No crack 8 4.2 172 No crack No crack 9 5.2
161 No crack No crack 10 6.2 178 No crack No crack 11 4.5 163 No
crack No crack 12 3.7 155 No crack No crack 13 5.1 156 No crack No
crack 14 6.1 162 No crack No crack 15 4.8 147 No crack No crack 16
4.0 152 No crack No crack 17 6.8 164 No crack No crack 18 3.1 157
No crack No crack 19 5.2 161 No crack No crack 20 4.5 149 No crack
No crack Compar- 21 6.0 95 Cracked Cracked ative 22 4.5 125 Cracked
Cracked Steel 23 3.8 137 Cracked Cracked 24 4.5 110 Cracked Cracked
25 2.3 107 Cracked Cracked 26 3.1 123 Cracked Cracked 27 5.3 85
Cracked Cracked 28 5.1 73 Cracked Cracked 29 4.5 81 No crack No
crack 30 5.6 125 No crack No crack
[0278] As shown in Table 3, all of the metallic structures of
Inventive Steels 1 to 20, and Comparative Steels 21 to 30 were
coarse grain structures with an ASTM grain size number of 7 or
less.
[0279] As shown in Table 3, the high temperature strengths of
Inventive Steels 1 to 20 were superior strengths of 147 MPa or
more, which were approx. 1.5 times or more higher than the high
temperature strength of Comparative Steel 21 (general-purpose steel
347H).
[0280] Meanwhile, the high temperature strengths of Comparative
Steels 21 to 30 were as low as 137 MPa or less, which were inferior
to the high temperature strengths of Inventive Steels 1 to 20.
[0281] As shown in Table 3, with respect to Inventive Steels 1 to
20 in both a base material and a weld HAZ equivalent material of an
Inventive Steel, a crack with a depth of 100 .mu.m or more was not
observed. From the results, it was demonstrated that Inventive
Steels 1 to 20 had superior stress cracking resistance.
[0282] Meanwhile, with respect to Comparative Steels 21 to 28, a
crack with a depth of 100 .mu.m or more was observed.
[0283] More particularly, from the results of Comparative Steel 21,
in which neither B nor N was added, and Comparative Steels 22, 23,
25, and 27, in which B but not Nd was added, it was demonstrated
that addition of Nd is effective for improvement of high
temperature strength and stress corrosion cracking resistance.
[0284] Further, from the results of Comparative Steel 26, in which,
although Nd and B were added combinedly, the N content was
excessive and the Meff was less than 0.0001 mass %, it was
demonstrated that a combination of the N content of 0.0100% or less
and the Meff of 0.0001 to 0.250% was effective for improvement of
high temperature strength and stress corrosion cracking
resistance.
[0285] Further, from the results of Comparative Steel 24, in which
the Meff was within a range from 0.0001 to 0.25%, and the O content
was beyond 0.0090%, and the N content was beyond 0.0100%, it was
demonstrated that a combination of the O content of 0.0090% or less
and the N content of 0.0100% or less was effective for improvement
of high temperature strength and stress corrosion cracking
resistance.
[0286] The reason behind the low high temperature strength of
Comparative Steel 24 is presumed that Nd and B were consumed as an
oxide or a nitride respectively and fine precipitation
strengthening did not develop.
[0287] From the results of Comparative Steel 28, it was
demonstrated that the B content of 0.0010% or more was effective
for improvement of high temperature strength and stress corrosion
cracking resistance.
[0288] Further, from the results of Comparative Steel 29, it was
demonstrated that the Zr content of 0.002% or less was effective
for improvement of high temperature strength.
[0289] Further, from the results of Comparative Steel 30, it was
demonstrated that the W content of 2.00% or more was effective for
improvement of high temperature strength.
[0290] <Relationship Between Crystal Grain Size and Stress
Corrosion Cracking>
[0291] The following tests were conducted to examine the
relationship between the crystal grain size and the stress
corrosion cracking of a steel with respect to Inventive Steels 1,
10, and 17, as well as Comparative Steels 21 and 23.
[0292] Firstly, an ASTM grain size measurement, a stress corrosion
cracking test on a base material, and a stress corrosion cracking
test on a weld HAZ equivalent material were conducted according to
the aforementioned methods with respect to the test material that
had been subjected to the aforementioned heat treatment at
1200.degree. C. In this regard, the depth of a crack was measured
and the cracking conditions were observed precisely in the stress
corrosion cracking tests on a base material and a weld HAZ
equivalent material.
[0293] The results are shown in Table 4.
[0294] Next, the test material that had not been subjected to the
aforementioned heat treatment at 1200.degree. C. was subjected to a
heat treatment at 1125.degree. C. by heating the test material to
1125.degree. C., then keeping test material at 1125.degree. C. for
15 min, and thereafter cooling the test material with water.
[0295] With respect to the test material having received the heat
treatment at 1125.degree. C., an ASTM grain size measurement, a
stress corrosion cracking test on a base material, and a stress
corrosion cracking test on a weld HAZ equivalent material were
conducted similarly as the test material having received the heat
treatment at 1200.degree. C.
[0296] The results are shown in Table 4.
TABLE-US-00004 TABLE 4 Stress corrosion cracking test result
(Measurement result Heat ASTM of crack depth) treatment grain Weld
HAZ temperature size Base equivalent Class Steel (.degree. C.)
number material material Inven- 1 1200 4.3 <10 .mu.m <10
.mu.m tive 10 6.2 <10 .mu.m <10 .mu.m Steel 17 6.8 <10
.mu.m <10 .mu.m 1 1125 8.1 Microcrack Microcrack of approx. of
approx. 20 .mu.m 20 .mu.m 10 9.2 Microcrack Microcrack of approx.
of approx. 20 .mu.m 20 .mu.m 17 9.6 Microcrack Microcrack of
approx. of approx. 20 .mu.m 20 .mu.m Compar- 21 1200 6.0 3 mm 3 mm
or ative more, many Steel 23 3.8 2 mm 3 mm or more, many 21 1125
9.3 3 to 4 mm 3 mm or more, many 23 8.0 2 to 3 mm 3 mm or more,
many
[0297] As shown in Table 4 and the aforementioned Table 3, the
metallic structures of test materials having received the heat
treatment at 1200.degree. C. with respect to Inventive Steels 1,
10, and 17, and Comparative Steels 21 and 23 were coarse grain
structures with an ASTM grain size number of 7 or less.
[0298] Meanwhile, as shown in Table 4, the metallic structures of
test materials having received the heat treatment at 1125.degree.
C. with respect to Inventive Steels 1, 10, and 17, and Comparative
Steels 21 and 23 became fine grain structures with an ASTM grain
size number of 8 or more.
[0299] Further, as shown in Table 4, with respect to Inventive
Steels 1, 10, and 17, in both the cases of fine grain structures
(ASTM grain size number 8 or more) and coarse grain structures
(ASTM grain size number 7 or less), the stress corrosion cracking
resistance was adequately reduced compared to Comparative Steels 21
and 23.
[0300] In contrast to the Inventive Steels, with respect to
Comparative Steels 21 and 23 in both the cases of fine grain
structures (ASTM grain size number 8 or more) and coarse grain
structures (ASTM grain size number 7 or less), the crack depth in a
stress corrosion cracking test was 2 mm or more and remarkable
stress corrosion cracking occurred. Especially, in a weld HAZ
equivalent material a large number of cracks with a depth of 3 mm
or more appeared.
[0301] As described above, stress corrosion cracking was reduced
remarkably in Inventive Steels 1, 10, and 17 compared to
Comparative Steels 21 and 23.
[0302] The entire contents of the disclosures by Japanese Patent
Application No. 2015-114665 are incorporated herein by
reference.
[0303] All documents, patent applications, and technical standards
described in this specification are herein incorporated by
reference to the same extent as if each individual document, patent
application, or technical standard was specifically and
individually indicated to be incorporated by reference.
* * * * *