U.S. patent application number 16/067751 was filed with the patent office on 2019-01-10 for austenitic heat resistant alloy and method for producing the same.
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 Norifumi Kochi, Jun Maki, Yoshitaka Nishiyama.
Application Number | 20190010565 16/067751 |
Document ID | / |
Family ID | 59274572 |
Filed Date | 2019-01-10 |
United States Patent
Application |
20190010565 |
Kind Code |
A1 |
Kochi; Norifumi ; et
al. |
January 10, 2019 |
Austenitic Heat Resistant Alloy and Method for Producing the
Same
Abstract
Provided is an austenitic heat resistant alloy having high creep
strength and high toughness even in a high temperature environment.
This austenitic heat resistant alloy has a chemical composition
consisting of: in mass %, C: 0.03 to less than 0.25%, Si: 0.01 to
2.0%, Mn: not more than 2.0%, Cr: 10 to less than 30%, Ni: more
than 25 to 45%, Al: more than 2.5 to less than 4.5%, Nb: 0.2 to
3.5%, N: not more than 0.025%, with the balance being Fe and
impurities, wherein P and S in the impurities are respectively, P:
not more than 0.04% and S: not more than 0.01%. In the structure, a
total volume ratio of precipitates having a circle equivalent
diameter of not less than 6 .mu.m is not more than 5%.
Inventors: |
Kochi; Norifumi;
(Chiyoda-ku, Tokyo, JP) ; Maki; Jun; (Chiyoda-ku,
Tokyo, JP) ; Nishiyama; Yoshitaka; (Chiyoda-ku,
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: |
59274572 |
Appl. No.: |
16/067751 |
Filed: |
January 4, 2017 |
PCT Filed: |
January 4, 2017 |
PCT NO: |
PCT/JP2017/000056 |
371 Date: |
July 2, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 6/004 20130101;
C22C 19/05 20130101; C22C 38/50 20130101; C22C 38/005 20130101;
C22C 38/002 20130101; C21D 8/00 20130101; C22C 38/42 20130101; C22C
30/00 20130101; C22C 38/02 20130101; C22C 38/54 20130101; C22C
38/48 20130101; C22C 38/44 20130101; C22F 1/10 20130101; C21D
2211/001 20130101; C21D 7/13 20130101; C22F 1/00 20130101; C22C
38/04 20130101; C21D 6/02 20130101; C22C 19/058 20130101; C22C
19/055 20130101; C21D 8/10 20130101; C22C 38/06 20130101; C22C
38/001 20130101 |
International
Class: |
C21D 6/00 20060101
C21D006/00; C22C 38/54 20060101 C22C038/54; C22C 38/50 20060101
C22C038/50; C22C 38/44 20060101 C22C038/44; C22C 38/42 20060101
C22C038/42; C22C 38/48 20060101 C22C038/48; C22C 38/06 20060101
C22C038/06; C22C 38/04 20060101 C22C038/04; C22C 38/02 20060101
C22C038/02; C22C 38/00 20060101 C22C038/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 5, 2016 |
JP |
2016-000375 |
Claims
1. An austenitic heat resistant alloy comprising a chemical
composition, consisting of: in mass %, C: 0.03 to less than 0.25%,
Si: 0.01 to 2.0%, Mn: not more than 2.0%, Cr: 10 to less than 30%,
Ni: more than 25 to 45%, Al: more than 2.5 to less than 4.5%, Nb:
0.2 to 3.5%, N: not more than 0.025%, Ti: 0 to less than 0.2%, W: 0
to 6%, Mo: 0 to 4%, Zr: 0 to 0.1%, B: 0 to 0.01%, Cu: 0 to 5%, rare
earth metals: 0 to 0.1%, Ca: 0 to 0.05%, and Mg: 0 to 0.05%, with
the balance being Fe and impurities, wherein P and S in the
impurities are respectively, P: not more than 0.04% and S: not more
than 0.01%, and wherein in a structure, a total volume ratio of
precipitates having a circle equivalent diameter of not less than 6
.mu.m is not more than 5%.
2. The austenitic heat resistant alloy according to claim 1,
wherein the chemical composition contains one or more kinds
selected from the group consisting of, in mass %, Ti: 0.005 to less
than 0.2%, W: 0.005 to 6%, Mo: 0.005 to 4%, Zr: 0.0005 to 0.1%, and
B: 0.0005 to 0.01%.
3. The austenitic heat resistant alloy according to claim 1,
wherein the chemical composition contains one or more kinds
selected from the group consisting of, in mass %, Cu: 0.05 to 5%,
and rare earth metals: 0.0005 to 0.1%.
4.-5. (canceled)
6. The austenitic heat resistant alloy according to claim 2,
wherein the chemical composition contains one or more kinds
selected from the group consisting of, in mass %, Cu: 0.05 to 5%,
and rare earth metals: 0.0005 to 0.1%.
7. The austenitic heat resistant alloy according to claim 1,
wherein the chemical composition contains one or more kinds
selected from the group consisting of, in mass %, Ca: 0.0005 to
0.05%, and Mg: 0.0005 to 0.05%.
8. The austenitic heat resistant alloy according to claim 2,
wherein the chemical composition contains one or more kinds
selected from the group consisting of, in mass %, Ca: 0.0005 to
0.05%, and Mg: 0.0005 to 0.05%.
9. The austenitic heat resistant alloy according to claim 3,
wherein the chemical composition contains one or more kinds
selected from the group consisting of, in mass %, Ca: 0.0005 to
0.05%, and Mg: 0.0005 to 0.05%.
10. The austenitic heat resistant alloy according to claim 6,
wherein the chemical composition contains one or more kinds
selected from the group consisting of, in mass %, Ca: 0.0005 to
0.05%, and Mg: 0.0005 to 0.05%.
11. A method for producing an austenitic heat resistant alloy
comprising steps of: performing hot forging at a reduction of area
of not less than 30% on a starting material having the chemical
composition according to claim 1; producing an intermediate
material by performing hot working on the starting material after
hot forging; and performing solution treatment at 1100 to
1250.degree. C. on the intermediate material.
Description
TECHNICAL FIELD
[0001] The present invention relates to a heat resistant alloy and
a method for producing the same, and more specifically to an
austenitic heat resistant alloy and a method for producing the
same.
BACKGROUND ART
[0002] Conventionally, 18-8 stainless steel has been used for a
heat resistant steel in facilities such as boilers and chemical
plants, which are used in high temperature environments. The 18-8
stainless steel is an austenitic stainless steel containing about
18% of Cr and about 8% of Ni, and is, for example, SUS304H,
SUS316H, SUS321H, and SUS347H in the JIS standard.
[0003] In recent years, use conditions of facilities in high
temperature environments have become significantly harsh, and there
is a need for high creep strength even higher than that of the 18-8
stainless steel. Further recently, the Advanced-Ultra Super
Critical pressure power generation plan has been promoted in which
a conventional steam temperature of about 600.degree. C. is
increased to not less than 700.degree. C. in a boiler for thermal
power generation. Moreover, to improve operation efficiency,
increasing the operation temperature is also planned in chemical
plants. For steel materials to be used in these high temperature
environments, high creep strength as well as excellent corrosion
resistance is required.
[0004] Heat resistant materials with enhanced corrosion resistance
have been proposed in Japanese Patent Application Publication No.
02-115348 (Patent Literature 1) and Japanese Patent Application
Publication No. 07-316751 (Patent Literature 2). An Al.sub.2O.sub.3
film is to be formed on surfaces of these heat resistant alloys in
a high temperature range during use since these alloys have a high
Al content. The film provides excellent corrosion resistance.
[0005] However, the above-described heat resistant alloys disclosed
in Patent Literatures 1 and 2 may exhibit insufficient creep
strength in a high temperature environment of not less than
700.degree. C.
[0006] As a heat resistant material having a high creep strength in
a high temperature environment of not less than 700.degree. C., a
heat resistant alloy containing Ni and Co, and also containing a
.gamma.' phase (Ni.sub.3Al) as a strengthening phase has been
developed. Examples of such heat resistant alloy include Ni-based
Alloys 617, 263, and 740. However, alloying raw materials for those
heat resistant alloys are expensive. Further, since these alloys
have low workability, production cost thereof tends to be high.
[0007] Accordingly, heat resistant alloys which are inexpensive
than the above-described Ni-base alloys and are excellent in creep
strength have been proposed in Japanese Patent Application
Publication No. 2014-43621 (Patent Literature 3) and Japanese
Patent Application Publication No. 2013-227644 (Patent Literature
4).
[0008] The austenitic heat resistant alloy disclosed in Patent
Literature 3 has a chemical composition containing, in mass %, C:
less than 0.02%, Si: not more than 2%, Mn: not more than 2%, Cr: 15
to 26%, Ni: 20 to 35%, Al: not more than 0.3%, P: not more than
0.04%, S: not more than 0.01%, and N: not more than 0.05%, and
further containing one or more kinds selected from Ti: not more
than 3.0% (including 0%), V: not more than 3.0% (including 0%), Nb:
less than 2.3% (including 0%), and Ta: not more than 2.0%
(including 0%), wherein a condition of f1: 1.5 to 6.0 where
f1=2Ti+2V+Nb+(1/2)Ta is satisfied, with the balance being Fe and
impurities. Patent Literature 3 states that the above-described
austenitic heat resistant alloy has excellent high temperature
strength and toughness due to precipitation strengthening by a
Laves phase and a .gamma.' phase.
[0009] The austenitic heat resistant alloy disclosed in Patent
Literature 4 has a chemical composition consisting of, in mass %,
C: less than 0.02%, Si: 0.01 to 2%, Mn: not more than 2%, Cr: not
less than 20% and less than 28%, Ni: more than 35% and not more
than 50%, W: 2.0 to 7.0%, Mo: less than 2.5% (including 0%), Nb:
less than 2.5% (including 0%), Ti: less than 3.0% (including 0%),
Al: not more than 0.3%; P: not more than 0.04%, S: not more than
0.01%, and N: not more than 0.05%, with the balance being Fe and
impurities, wherein the chemical composition further satisfies f1:
1.0 to 5.0, where f1=1/2W+Mo; f2: 2.0 to 8.0, where
f2=1/2W+Mo+Nb+2Ti; and f3: 0.5 to 5.0, where f3=Nb+2Ti. Patent
Literature 4 states that the above-described austenitic heat
resistant alloy has excellent high temperature strength and
toughness due to precipitation strengthening by a Laves phase and a
.gamma.' phase.
CITATION LIST
Patent Literature
[0010] Patent Literature 1: Japanese Patent Application Publication
No. 02-115348 [0011] Patent Literature 2: Japanese Patent
Application Publication No. 07-316751 [0012] Patent Literature 3:
Japanese Patent Application Publication No. 2014-43621 [0013]
Patent Literature 4: Japanese Patent Application Publication No.
2013-227644
SUMMARY OF INVENTION
Technical Problem
[0014] However, as in the heat resistant alloys of Patent
Literatures 3 and 4, in a case of an alloy which takes advantage of
strengthening mechanism by a Laves phase and a .gamma.' phase,
creep strength and toughness may deteriorate after long-hours
aging.
[0015] An objective of the present invention is to provide an
austenitic heat resistant alloy having high creep strength and high
toughness even in a high temperature environment.
Solution to Problem
[0016] An austenitic heat resistant alloy according to the present
embodiment has a chemical composition consisting of: in mass %, C:
0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: not more than 2.0%,
Cr: 10 to less than 30%, Ni: more than 25 to 45%, Al: more than 2.5
to less than 4.5%, Nb: 0.2 to 3.5%, N: not more than 0.025%, Ti: 0
to less than 0.2%, W: 0 to 6%, Mo: 0 to 4%, Zr: 0 to 0.1%, B: 0 to
0.01%, Cu: 0 to 5%, rare earth metals: 0 to 0.1%, Ca: 0 to 0.05%,
and Mg: 0 to 0.05%, with the balance being Fe and impurities,
wherein P and S in the impurities are respectively P: not more than
0.04% and S: not more than 0.01%. In the structure, a total volume
ratio of precipitates having a circle equivalent diameter of not
less than 6 .mu.m is not more than 5%. Where, the precipitates are,
for example, carbides, nitrides, NiAl, and .alpha.-Cr.
Advantageous Effects of Invention
[0017] The austenitic heat resistant alloy according to the present
embodiment has a high temperature strength for long hours and
excellent toughness even in a high temperature environment.
DESCRIPTION OF EMBODIMENTS
[0018] The present inventors have conducted investigation and
research on creep strength and toughness of austenitic heat
resistant alloys in a high temperature environment of not less than
700.degree. C. (hereinafter, simply referred to as a "high
temperature environment"), and have obtained the following
findings.
[0019] As described so far, a heat resistant alloy containing a
Laves phase and .gamma.' phase such as Ni.sub.3Al has a high creep
strength in a high temperature environment. However, since these
precipitation phases are coarsened when used for long hours in a
high temperature environment, creep strength and toughness of the
heat resistant alloy deteriorate.
[0020] On the other hand, provided that precipitates such as
carbides, nitrides, NiAl, .alpha.-Cr, and the like can be caused to
precipitate in a finely dispersed manner during use of the heat
resistant alloy in a high temperature environment, it is possible
to maintain a high creep strength and high toughness even in
long-hours use. These precipitates cover grain boundaries, thereby
increasing grain boundary strength. Further, when these
precipitates precipitate inside grains, deformation resistance of
the heat resistant alloy is increased, thereby increasing the creep
strength.
[0021] To increase the creep strength and toughness by the
above-described fine precipitates, the structure of the heat
resistant alloy before use is controlled as follows.
[0022] [Limitation of Amount of Precipitates Having a Circle
Equivalent Diameter of not Less than 6 .mu.m]
[0023] In a solidified structure after casing of a heat resistant
alloy, precipitates such as carbides, nitrides, NiAl, .alpha.-Cr,
and the like (hereinafter, simply referred to as "precipitates")
are present. These precipitates are generated in a liquid phase in
which solution elements that are present between dendrites are
condensed. These precipitates have typically coarse shapes, and are
non-uniformly dispersed in the structure. Therefore, the toughness
of the heat resistant alloy deteriorates.
[0024] Further, even when subjected to solution treatment, these
precipitates are not likely to dissolve, remaining in a coarse
state. When these precipitates remain in a coarse state in a heat
resistant alloy, it is not likely that fine precipitates are formed
during use in a high temperature environment. Therefore, a total
volume ratio of coarse precipitates in a heat resistant alloy is
preferably as low as possible.
[0025] Provided that the total volume ratio of precipitates having
a circle equivalent diameter of not less than 6 .mu.m (hereinafter,
referred to as "coarse precipitates") is not more than 5% in the
structure of a heat resistant alloy, an enough amount of fine
precipitates can be caused to precipitate during use of heat
resistant alloy in a high temperature environment, and thus high
creep strength and high toughness can be obtained.
[0026] To make the total volume ratio of coarse precipitates in the
structure not more than 5%, the C content in the heat resistant
alloy is made to be less than 0.25%. Further, the reduction of area
during hot forging is made not less than 30%. In this case, coarse
precipitates are uniformly dispersed by hot forging. Because of
that, in a solution treatment in a later step, precipitates can be
dissolved, and thus the total volume ratio of coarse precipitates
will be not more than 5%.
[0027] The austenitic heat resistant alloy according to the present
embodiment, which has been completed based on the above findings,
has a chemical composition consisting of, in mass %, C: 0.03 to
less than 0.25%, Si: 0.01 to 2.0%, Mn: not more than 2.0%, Cr: 10
to less than 30%, Ni: more than 25 to 45%, Al: more than 2.5 to
less than 4.5%, Nb: 0.2 to 3.5%, N: not more than 0.025%, Ti: 0 to
less than 0.2%, W: 0 to 6%, Mo: 0 to 4%, Zr: 0 to 0.1%, B: 0 to
0.01%, Cu: 0 to 5%, rare earth metals: 0 to 0.1%, Ca: 0 to 0.05%,
and Mg: 0 to 0.05%, with the balance being Fe and impurities,
wherein P and S in the impurities are respectively, P: not more
than 0.04% and S: not more than 0.01%. In the structure, the total
volume ratio of precipitates having a circle equivalent diameter of
not less than 6 .mu.m is not more than 5%.
[0028] The above-described chemical composition may contain, in
mass %, one or more kinds selected from the group consisting of,
Ti: 0.005 to less than 0.2%, W: 0.005 to 6%, Mo: 0.005 to 4%, Zr:
0.0005 to 0.1%, and B: 0.0005 to 0.01%.
[0029] The above-described chemical composition may contain, in
mass %, one or more kinds selected from the group consisting of,
Cu: 0.05 to 5%, and rare earth metals: 0.0005 to 0.1%.
[0030] The above-described chemical composition may contain, in
mass %, one or more kinds selected from the group consisting of,
Ca: 0.0005 to 0.05% and Mg: 0.0005 to 0.05%.
[0031] A method for producing the above-described austenitic heat
resistant alloy includes steps of: performing hot forging at a
reduction of area of not less than 30% on a starting material
having the above-described chemical composition; producing an
intermediate material by performing hot working on the starting
material after hot forging; and performing solution treatment at
1100 to 1250.degree. C. on the intermediate material.
[0032] Hereinafter, an austenitic heat resistant alloy of the
present embodiment will be described in detail. The symbol "%"
relating to elements means, unless otherwise stated, mass %.
[Chemical Composition]
[0033] The austenitic heat resistant alloy of the present
embodiment is, for example, an alloy pipe. The chemical composition
of the austenitic heat resistant alloy contains the following
elements.
[0034] C: 0.03 to Less than 0.25%
[0035] Carbon (C) forms carbides and increases creep strength.
Specifically, C combines with an alloy element to form fine
carbides at crystal grain boundaries and in crystal grains during
use in a high temperature environment. The fine carbides increase
deformation resistance and increase creep strength. When the C
content is too low, this effect cannot be obtained. On the other
hand, when the C content is too high, a large number of coarse
eutectic carbides are formed in a solidified structure after
casting of the heat resistant alloy. Since the eutectic carbides
remain coarse in the structure even after solution treatment, they
deteriorate toughness of the heat resistant alloy. Further, if the
coarse eutectic carbides remain, fine carbides are not likely to
precipitate during use in a high temperature environment, and thus
creep strength decreases. Thus, the C content is 0.03 to less than
0.25%. A lower limit of the C content is preferably 0.05%, and more
preferably 0.08%. An upper limit of C content is preferably 0.23%,
and more preferably 0.20%.
[0036] Si: 0.01 to 2.0%
[0037] Silicon (Si) deoxidizes a heat resistant alloy. Si further
improves corrosion resistance (oxidation resistance and steam
oxidation resistance) of a heat resistant alloy. While Si is an
element that is inevitably contained, the Si content may be as
small as possible when deoxidization is sufficiently carried out by
other elements. On the other hand, when the Si content is too high,
hot workability deteriorates. Therefore, the Si content is 0.01 to
2.0%. A lower limit of the Si content is preferably 0.02%, and more
preferably 0.03%. An upper limit of the Si content is preferably
1.0%.
[0038] Mn: Not More than 2.0%
[0039] Manganese (Mn) is inevitably contained. Mn combines with S
contained in a heat resistant alloy to form MnS, thereby improving
hot workability of the heat resistant alloy. However, when the Mn
content is too high, the heat resistant alloy becomes too hard, and
hot workability and weldability deteriorate. Therefore, the Mn
content is not more than 2.0%. A lower limit of the Mn content is
preferably 0.1%, and more preferably 0.2%. An upper limit of the Mn
content is preferably 1.2%.
[0040] Cr: 10 to Less than 30%
[0041] Chromium (Cr) improves corrosion resistance (oxidation
resistance, steam oxidation resistance, etc.) of a heat resistant
alloy in a high temperature environment. Cr further finely
precipitates as .alpha.-Cr during use in a high temperature
environment to increase creep strength. When the Cr content is too
low, these effects cannot be obtained. On the other hand, when the
Cr content is too high, the stability of the structure
deteriorates, and the creep strength decreases. Therefore, the Cr
content is 10 to less than 30%. A lower limit of the Cr content is
preferably 11%, and more preferably 12%. An upper limit of the Cr
content is preferably 28%, and more preferably 26%.
[0042] Ni: More than 25 to 45%
[0043] Nickel (Ni) stabilizes austenite. Ni further improves the
corrosion resistance of a heat resistant alloy. When the Ni content
is too low, such effect cannot be obtained. On the other hand, when
the Ni content is too high, not only such effect is saturated, but
also hot workability deteriorates. Further, when the Ni content is
too high, the raw material cost increases. Therefore, the Ni
content is more than 25 to 45%. A lower limit of the Ni content is
preferably 26%, and more preferably 28%. An upper limit of the Ni
content is preferably 44%, and more preferably 42%.
[0044] Al: More than 2.5 to Less than 4.5%
[0045] Aluminum (Al) combines with Ni to form fine NiAl during use
in a high temperature environment, thereby increasing creep
strength. Al further improves corrosion resistance in a high
temperature environment of not less than 1000.degree. C. When the
Al content is too low, these effects cannot be obtained. On the
other hand, when the Al content is too high, structural stability
deteriorates, and strength decreases. Therefore, the Al content is
more than 2.5 to less than 4.5%. A lower limit of the Al content is
preferably 2.55%, and more preferably 2.6%. An upper limit of the
Al content is preferably 4.4%, and more preferably 4.2%. In the
austenitic heat resistant alloy according to the present invention,
the Al content means the total Al amount contained in the steel
material.
[0046] Nb: 0.2 to 3.5%
[0047] Niobium (Nb) forms a Laves phase and a Ni.sub.3Nb phase
which work as precipitation strengthening phases, and
precipitation-strengthens crystal grain boundaries and crystal
grains, thereby increasing creep strength of a heat resistant
alloy. When the Nb content is too low, the above effect cannot be
obtained. On the other hand, when the Nb content is too high, the
Laves phase and the Ni.sub.3Nb phase are excessively generated,
thereby deteriorating toughness and hot workability of the alloy.
When the Nb content is too high, toughness after long-hours aging
will also deteriorate. Therefore, the Nb content is 0.2 to 3.5%. A
lower limit of the Nb content is preferably 0.35%, and more
preferably 0.5%. An upper limit of the Nb content is preferably
less than 3.2%, and more preferably 3.0%.
[0048] N: Not More than 0.025%
[0049] Nitrogen (N) stabilizes austenite and is inevitably
contained in a usual melting method. In addition, N combines with
an alloy element to form fine nitrides at crystal grain boundaries
and in crystal grains during use in a high temperature environment.
Fine nitrides increase deformation resistance, thereby increasing
creep strength. However, when the N content is too high, it forms
coarse nitrides which remain undissolved even after solution
treatment, thus decreasing toughness of the alloy. Therefore, the N
content is not more than 0.025%. An upper limit of the N content is
preferably 0.02%, and more preferably 0.01%.
[0050] P: Not More than 0.04%
[0051] Phosphorus (P) is an impurity. P deteriorates weldability
and hot workability of a heat resistant alloy. Therefore, the P
content is not more than 0.04%. An upper limit of P content is
preferably 0.03%. The P content is preferably as low as
possible.
[0052] S: Not More than 0.01%
[0053] Sulfur (S) is an impurity. S deteriorates weldability and
hot workability of a heat resistant alloy. Therefore, the S content
is not more than 0.01%. An upper limit of the S content is
preferably 0.008%. The S content is preferably as low as
possible.
[0054] The balance of the chemical composition of the austenitic
heat resistant alloy of the present embodiment consists of Fe and
impurities. Herein, the term impurity means what are introduced
from ores and scraps as raw materials, or production environments
when industrially producing an austenitic heat resistant alloy, and
what are permitted within a range not adversely affecting the
present invention.
[Optional Elements]
[0055] The chemical composition of the above-described austenitic
heat resistant alloy may contain, in lieu of part of Fe, one or
more kinds selected from the group consisting of Ti, W, Mo, Zr, and
B. All of these elements are optional elements, and increase creep
strength.
[0056] Ti: 0 to Less than 0.2%
[0057] Titanium (Ti) is an optional element and may not be
contained. When contained, Ti forms a Laves phase and a Ni.sub.3Ti
phase, which each act as a precipitation strengthening phase, and
creep strength is increased by the precipitation strengthening.
However, when the Ti content is too high, the Laves phase and the
Ni.sub.3Ti phase are excessively generated, thereby deteriorating
high temperature ductility and hot workability. Further, when the
Ti content is too high, toughness after long-hours aging
deteriorates. Therefore, the Ti content is 0 to less than 0.2%. A
lower limit of Ti content is preferably 0.005%, and more preferably
0.01%. An upper limit of Ti content is 0.15%, and more preferably
0.1%.
[0058] W: 0 to 6%
[0059] Tungsten (W) is an optional element and may not be
contained. When contained, W dissolves into austenite which is the
mother phase (matrix), thereby increasing creep strength by solid
solution strengthening. Further, W forms a Laves phase at crystal
grain boundaries and in crystal grains, thereby increasing creep
strength by precipitation strengthening. However, when the W
content is too high, the Laves phase is excessively generated,
thereby deteriorating high-temperature ductility, hot workability,
and toughness. Therefore, the W content is 0 to 6%. A lower limit
of the W content is preferably 0.005%, and more preferably 0.01%.
An upper limit of W content is preferably 5.5%, and more preferably
5%.
[0060] Mo: 0 to 4%
[0061] Molybdenum (Mo) is an optional element and may not be
contained. When contained, Mo dissolves into austenite of the
mother phase, thereby increasing creep strength by solid solution
strengthening. Mo further forms a Laves phase at crystal grain
boundaries and in crystal grains, thereby increasing creep strength
by precipitation strengthening. However, when the Mo content is too
high, the Laves phase is excessively generated, thereby
deteriorating high temperature ductility, hot workability and
toughness. Therefore, the Mo content is 0 to 4%. A lower limit of
the Mo content is 0.005%, and more preferably 0.01%. An upper limit
of the Mo content is preferably 3.5%, and more preferably 3%.
[0062] Zr: 0 to 0.1%
[0063] Zirconium (Zr) is an optional element and may not be
contained. When contained, Zr increases creep strength by grain
boundary strengthening. However, when the Zr content is too high,
weldability and hot workability of a heat resistant alloy
deteriorate. Therefore, the Zr content is 0 to 0.1%. A lower limit
of the Zr content is preferably 0.0005%, more preferably 0.001%. An
upper limit of the Zr content is preferably 0.06%.
[0064] B: 0 to 0.01%
[0065] Boron (B) is an optional element and may not be contained.
When contained, B increases creep strength by grain boundary
strengthening. However, when the B content is too high, weldability
deteriorates. Therefore, the B content is 0 to 0.01%. A lower limit
of B is preferably 0.0005%, and more preferably 0.001%. An upper
limit of the B content is preferably 0.005%.
[0066] The chemical composition of the above-described austenitic
heat resistant alloy may contain, in lieu of part of Fe, one or
more kinds selected from the group consisting of Cu and rare earth
metals. All of these elements are optional elements, and increase
corrosion resistance of a heat resistant alloy.
[0067] Cu: 0 to 5%
[0068] Copper (Cu) is an optional element and may not be contained.
When contained, Cu facilitates formation of an Al.sub.2O.sub.3 film
in the vicinity of the surface, thereby enhancing corrosion
resistance of a heat resistant alloy. However, when the Cu content
is too high, not only such effect is saturated, but also the high
temperature ductility deteriorates. Therefore, the Cu content is 0
to 5%. A lower limit of the Cu content is preferably 0.05%, and
more preferably 0.1%. An upper limit of the Cu content is
preferably 4.8%, and more preferably 4.5%.
[0069] Rare Earth Metals: 0 to 0.1%
[0070] Rare earth metals (REM) are optional elements and may not be
contained. When contained, REM each immobilize S as a sulfide,
thereby improving hot workability. REM further form oxides to
improve corrosion resistance, creep strength, and creep ductility.
However, when the REM content is too high, inclusions such as
oxides increase, thereby deteriorating hot workability and
weldability, and increasing production cost. Therefore, the REM
content is 0 to 0.1%. A lower limit of the REM content is
preferably 0.0005%, and more preferably 0.001%. An upper limit of
the REM content is preferably 0.09%, and more preferably 0.08%.
[0071] The term REM as used herein is a general term for a total of
17 elements including Sc, Y and lanthanoide series. When the REM
contained in a heat resistant alloy is one kind of these elements,
a REM content means the content of that element. When the REM
contained in the heat resistant alloy is not less than two kinds,
the REM content means the total content of those elements. REM are
generally contained in Mischmetal. Therefore, REM may be added in
the form of Mischmetal such that the REM content is within the
above-described range.
[0072] The chemical composition of the above-described austenitic
heat resistant alloy may contain, in lieu of part of Fe, one or
more kinds selected from the group consisting of Ca and Mg. All of
these elements are optional elements, and improve hot workability
of a heat resistant alloy.
[0073] Ca: 0 to 0.05%
[0074] Calcium (Ca) is an optional element and may not be
contained. When contained, Ca immobilizes S as a sulfide, thereby
improving hot workability. On the other hand, when the Ca content
is too high, toughness, ductility and cleanliness deteriorate.
Therefore, the Ca content is 0 to 0.05%. A lower limit of Ca is
preferably 0.0005%. An upper limit of the Ca content is preferably
0.01%.
[0075] Mg: 0 to 0.05%
[0076] Magnesium (Mg) is an optional element and may not be
contained. When contained, Mg immobilizes S as a sulfide, thereby
improving hot workability of a heat resistant alloy. On the other
hand, when the Ca content is too high, toughness, ductility and
cleanliness deteriorate. Therefore, the Ca content is 0 to 0.05%. A
lower limit of Ca is preferably 0.0005%. An upper limit of the Ca
content is preferably 0.01%.
[Total Volume Ratio of Precipitates (Coarse Precipitates) Having a
Circle Equivalent Diameter of not Less than 6 .mu.m: Not More than
5%]
[0077] As described so far, in the austenitic heat resistant alloy
of the present embodiment, fine precipitates are caused to
precipitate during use in a high temperature environment, and thus
creep strength and maintaining toughness are increased. Examples of
the precipitate include carbides, nitrides, NiAl, and .alpha.-Cr.
When the precipitate is coarse, creep strength and toughness
deteriorate. Therefore, coarse precipitates are preferably as small
in amount as possible in a heat resistant alloy before use.
Provided that the total volume ratio of precipitates having a
circle equivalent diameter of not less than 6 .mu.m (coarse
precipitates) is not more than 5% in the structure of the heat
resistant alloy, fine precipitates are caused to precipitate during
use in a high temperature environment, and thus creep strength and
toughness are increased. An upper limit of the total volume ratio
of coarse precipitates is preferably 4%, and more preferably 3%.
Here, the circle equivalent diameter of a precipitate means a
diameter (.mu.m) of a circle which has the same area as that of the
precipitate.
[Measurement Method of Total Volume Ratio of Coarse Precipitates in
Structure]
[0078] A total volume ratio of coarse precipitates in the structure
of an austenitic heat resistant alloy of the present embodiment can
be measured by the following method.
[0079] A test specimen of a vertical section to the surface is
sampled from a heat resistant alloy material. For example, when the
austenitic heat resistant alloy material is an alloy pipe, a test
specimen is sampled from a middle portion of wall thickness of a
section normal to the axial direction.
[0080] After a section (observation surface) of the sampled test
specimen is polished, the observation surface is etched by a mixed
acid solution of hydrochloric acid and nitric acid. Arbitrary 10
visual fields in the observation surface are imaged by using a
scanning electron microscope (SEM) to create SEM images
(backscattered electron images). Each visual field is 100
.mu.m.times.100 .mu.m.
[0081] In a SEM image, a precipitate and the matrix have different
contrast, respectively. By determining area of a precipitate which
is identified from difference in contrast, a circle equivalent
diameter of each precipitate is calculated. After calculation,
precipitates having a circle equivalent diameter of not less than 6
.mu.m (coarse precipitates) are identified.
[0082] A total area of the identified coarse precipitates is
determined. Also a proportion (%) of the total area of coarse
precipitates to the area of the visual field is determined. Since
an area ratio of precipitate corresponds to a volume ratio thereof,
the determined proportion of coarse precipitates is defined as a
total volume ratio (%) of coarse precipitates.
[0083] The shape of the austenitic heat resistant alloy of the
present embodiment is not particularly limited. The austenitic heat
resistant alloy is, for example, an alloy pipe. An austenitic heat
resistant alloy pipe is used for piping for boilers and a reaction
pipe for chemical plants. The austenitic heat resistant alloy may
be a plate, a bar, or a wire.
[Production Method]
[0084] A production method of an alloy pipe will be described as an
example of the method for producing an austenitic heat resistant
alloy of the present embodiment. The production method of the
present embodiment includes: a step of preparing a starting
material having the above-described chemical composition
(preparation step); a step of hot forging the prepared starting
material (hot forging step); a step of producing an intermediate
material by performing hot working on the hot forged starting
material (hot working step); and a step of performing solution heat
treatment on the intermediate material (solution heat treatment
step). Hereinafter, each step will be described.
[Preparation Step]
[0085] Molten steels having the above-described chemical
compositions are produced. The molten steels are subjected as
needed to a well-known degassing treatment. Using a molten steel, a
starting material is produced by casting. The starting material may
be an ingot by an ingot-making process, a slab by a continuous
casting process, or a cast piece such as a bloom, and a billet.
[Hot Forging Step]
[0086] The produced starting material is subjected to hot forging
to produce a columnar starting material. In the hot forging, the
reduction of area defined by Formula (1) is made not less than
30%.
Reduction of area=100-(sectional area of starting material after
hot forging/sectional area of starting material before hot
forging).times.100(%) (1)
[0087] As described above, precipitates such as eutectic carbides
are present in the structure of the starting material produced by
casting. These precipitates are coarse, and a large number of them
have a circle equivalent diameter of not less than 6 .mu.m. Such
coarse precipitates are not likely to dissolve even in a solution
treatment in a later step.
[0088] Provided that the reduction of area is not less than 30% in
the hot forging step, the coarse precipitates are broken off during
hot forging, thereby decreasing in size. Therefore, the
precipitates are more likely to dissolve in the solution heat
treatment in a later step. As a result of this, the volume ratio of
precipitates having a circle equivalent diameter of not less than 6
.mu.m will become not more than 5%.
[0089] The reduction of area is preferably not less than 35%, and
more preferably not less than 40%. Although the upper limit of the
reduction of area is not particularly limited, it will be 90% when
considering productivity.
[Hot Working Step]
[0090] The hot forged starting material (columnar starting
material) is subjected to hot working, to produce an alloy raw pipe
which is the intermediate material. For example, a through hole is
formed at a center of the columnar starting material by machining.
The columnar starting material formed with a through hole is
subjected to hot extrusion, to produce an alloy raw pipe. The alloy
raw pipe (intermediate material) may be produced by
piercing-rolling of the columnar starting material. The
intermediate material after hot working may be subjected to cold
working. The cold working is, for example, cold drawing, etc. The
intermediate material is produced through the above described
steps.
[Solution Heat Treatment Step]
[0091] The produced intermediate material is subjected to solution
heat treatment. By the solution heat treatment, precipitates in the
intermediate material are dissolved.
[0092] The heat treatment temperature in the solution heat
treatment is 1100 to 1250.degree. C. When the heat treatment
temperature is less than 1100.degree. C., the precipitates will not
sufficiently dissolve and, as a result, the volume ratio of coarse
precipitates will be more than 5%. On the other hand, when the heat
treatment temperature is too high, austenite grains are coarsened,
thus deteriorating productivity.
[0093] When the heat treatment temperature is 1100 to 1250.degree.
C., the precipitates sufficiently dissolve, and the total volume
ratio of coarse precipitates will be not more than 5%.
[0094] The solution heat treatment time is not particularly
limited. The solution heat treatment time is, for example, one
minute to one hour.
[0095] The intermediate material after the solution heat treatment
may be subjected to pickling treatment for the purpose of removing
scales formed on the surface. For the pickling, for example, a
mixed acid solution of nitric acid and hydrochloric acid is used.
The pickling time is, for example, 30 to 60 minutes.
[0096] Further, the intermediate material after pickling treatment
may be subjected to blasting treatment by use of blast media. For
example, the blasting treatment is performed on the inner surface
of the alloy pipe. In this case, a worked layer is formed on the
surface, thereby improving corrosion resistance (oxidation
resistance, etc.).
[0097] By the production method described so far, the austenitic
heat resistant alloy of the present embodiment is produced. It is
noted that a production method of an alloy pipe has been described
in the above. However, a plate, a bar, a wire, or the like may be
produced by a similar production method (the preparation step, hot
forging step, hot working step, and solution heat treatment
step).
Examples
[Production Method]
[0098] Molten steels having chemical compositions shown in Table 1
were produced by using a vacuum melting furnace.
TABLE-US-00001 TABLE 1 Chemical composition (mass %, the balance
being Fe and impurities) Test No. C Si Mn Cr Ni Al Nb N P S Others
1 0.120 0.14 1.13 20.31 35.69 3.14 0.74 0.0021 0.011 0.004 -- 2
0.150 0.18 1.31 25.14 40.66 3.56 0.94 0.0037 0.008 0.006 -- 3 0.060
0.13 0.96 28.67 30.69 3.22 1.85 0.0012 0.013 0.006 Mg: 0.0025 4
0.210 0.20 0.64 18.97 41.45 3.87 1.72 0.0021 0.012 0.008 Ca: 0.0021
5 0.120 0.18 1.25 24.36 28.21 4.42 2.45 0.0021 0.008 0.006 B: 0.003
6 0.140 0.14 1.41 21.64 35.64 2.84 0.65 0.0018 0.003 0.009 Ti: 0.14
7 0.080 0.16 1.09 22.65 32.67 3.65 2.46 0.0021 0.011 0.004 W: 4.57
8 0.160 0.20 1.25 19.04 33.67 3.14 2.28 0.0023 0.012 0.006 Zr: 0.03
9 0.110 0.19 1.02 21.36 31.69 3.24 1.17 0.0011 0.010 0.007 Mo: 2.14
10 0.065 0.89 1.18 15.09 28.09 3.94 1.94 0.0227 0.025 0.008 REM:
0.032 11 0.079 0.04 1.26 24.03 40.95 3.52 3.01 0.0112 0.021 0.007
Cu: 3.56 12 0.815 0.21 0.98 23.14 31.64 3.55 1.05 0.0025 0.013
0.006 -- 13 0.140 0.11 1.04 20.64 30.27 1.56 1.49 0.0029 0.012
0.006 -- 14 0.110 0.14 0.97 22.64 33.94 5.47 0.99 0.0017 0.011
0.003 -- 15 0.151 0.12 0.85 7.69 40.36 2.97 2.81 0.0022 0.007 0.005
-- 16 0.140 0.20 1.07 35.68 32.82 3.85 1.07 0.0025 0.006 0.006 --
17 0.220 0.14 1.34 25.66 30.41 3.24 0.49 0.0015 0.002 0.008 -- 18
0.140 0.15 0.75 28.64 34.90 3.84 2.50 0.0018 0.008 0.007 -- 19
0.090 0.46 0.91 19.89 38.14 2.88 4.11 0.0041 0.004 0.004 -- 20
0.157 1.91 1.11 17.38 42.26 3.44 0.07 0.0124 0.001 0.004 --
[0099] The above-described molten steels were used to each produce
a columnar ingot (30 kg) having an outer diameter of 120 mm. Each
ingot was subjected to hot forging at a reduction of area shown in
Table 2 to produce a rectangular starting material. The rectangular
starting material was subjected to hot rolling and cold rolling to
produce a planar intermediate material having a thickness of 1.5
mm. The intermediate material was subjected to a solution treatment
in which the intermediate material was held at a heat treatment
temperature shown in Table 2 for 10 minutes. After being held for
10 minutes, the intermediate material was water cooled to produce
an alloy plate.
TABLE-US-00002 TABLE 2 Solution heat Reduction of treatment Creep
area (%) Total volume ratio (%) temperature strength Charpy impact
Test No. during forging of coarse precipitates (.degree. C.) (MPa)
value (J/cm.sup.2) 1 52 1.2 1215 152.3 50.3 2 65 1.6 1220 154.6
48.3 3 58 0.8 1185 149.6 55.6 4 75 2.1 1195 156.7 51.3 5 81 1.3
1200 154.3 50.1 6 68 1.4 1225 151.2 51.4 7 74 1.2 1205 155.9 52.3 8
77 2.3 1210 157.6 53.6 9 59 1.1 1200 154.6 52.1 10 70 0.7 1205
150.9 53.6 11 68 0.4 1190 151.2 50.5 12 45 10.4 1225 116.8 24.6 13
56 1.4 1230 120.3 70.2 14 65 1.9 1235 108.2 67.5 15 72 1.5 1215
124.3 70.6 16 68 1.1 1225 114.4 57.8 17 8.7 8.5 1195 121.1 28.7 18
65 7.7 1040 126.4 25.9 19 43 0.6 1175 164.7 21.9 20 52 1.8 1220
109.6 48.8
[Creep Rupture Test]
[0100] A test specimen was made from the produced alloy plate. The
test specimen was sampled from a central portion of the thickness
of the alloy plate in parallel with the longitudinal direction
(rolling direction). The test specimen was a round bar specimen, of
which diameter of the parallel portion was 6 mm and gauge length
was 30 mm. A creep rupture test was conducted by using the test
specimen. The creep rupture test was performed in an atmosphere of
700 to 800.degree. C. Based on obtained rupture strength, a creep
strength (MPa) at 1.0.times.10.sup.4 hours at 700.degree. C. was
determined by the Larson-Miller parameter method.
[Charpy Impact Test]
[0101] The produced alloy plate was subjected to aging treatment in
which it is held for 8000 hours at 700.degree. C., and thereafter
was water cooled. A V-notch Charpy impact test specimen specified
in JIS Z2242 (2005) was sampled from a middle portion in the
thickness direction of the plate stock after aging treatment. The
notch was formed in parallel with the longitudinal direction of the
alloy plate. The test specimen had a width of 5 mm, a height of 10
mm, a length of 55 mm, and a notch depth of 2 mm. At 0.degree. C.,
a Charpy impact test in accordance with JIS Z2242 (2005) was
performed to determine an impact value (J/cm.sup.2).
[Test Results]
[0102] Test results are shown in Table 2.
[0103] Referring to Table 2, the chemical compositions of Test No.
1 to Test No. 11 were appropriate, and the volume ratios of coarse
precipitates were not more than 5%. As a result, the creep strength
was not less than 140 MPa, showing excellent creep strength.
Further, the Charpy impact values were not less than 40 J/cm.sup.2,
thus exhibiting excellent toughness even after long-hours aging
treatment.
[0104] On the other hand, in Test No. 12, the C content was too
high. Because of that, the volume ratio of coarse precipitates was
more than 5%. As a result, the creep strength was less than 140
MPa, and the Charpy impact value was less than 40 J/cm.sup.2.
[0105] In Test No. 13, the Al content was too low. Because of that,
the creep strength was less than 140 MPa. This may be because the
precipitation amount of NiAl was small.
[0106] In Test No. 14, the Al content was too high. Because of
that, the creep strength was less than 140 MPa. Because the Al
content was too high, conceivably, the structure was not
stabilized, resulting in low creep strength.
[0107] In Test No. 15, the Cr content was too low. Because of that,
the creep strength was less than 140 MPa. This may be because the
precipitation amount of .alpha.-Cr was small.
[0108] In Test No. 16, the Cr content was too high. Because of
that, the creep strength was less than 140 MPa. Because the Cr
content was too high, conceivably, the structure was not
stabilized, resulting in low creep strength.
[0109] In Test No. 17, the reduction of area during hot forging was
less than 30%. Because of that, the total volume ratio of coarse
precipitates was more than 5%. As a result, the creep strength was
less than 140 MPa, and the Charpy impact value was less than 40
J/cm.sup.2.
[0110] In Test No. 18, the solution heat treatment temperature was
less than 1100.degree. C. Because of that, the total volume ratio
of coarse precipitates was more than 5%. As a result, the creep
rupture strength was less than 140 MPa, and the Charpy impact value
was less than 40 J/cm.sup.2.
[0111] In Test No. 19, the Nb content was too high. Because of
that, the Charpy impact value was less than 40 J/cm.sup.2.
[0112] In Test No. 20, the Nb content was too low. Because of that,
the creep strength was less than 140 MPa.
[0113] So far embodiments of the present invention have been
described. However, the above described embodiments are merely
examples for carrying out the present invention. Therefore, the
present invention will not be limited to the above described
embodiments, and can be carried out by appropriately modifying the
above described embodiments within the range not departing from the
spirit thereof.
INDUSTRIAL APPLICABILITY
[0114] The austenitic heat resistant alloy of the present invention
can be widely used in a high temperature environment of not less
than 700.degree. C. It is particularly suitable as an alloy pipe to
be used such as in boilers for power generation and in plants for
chemical industry, which are exposed to a high temperature
environment of not less than 700.degree. C.
* * * * *