U.S. patent application number 16/483049 was filed with the patent office on 2020-07-23 for austenitic heat resistant alloy and method for producing same.
The applicant listed for this patent is Nippon Steel Corporation. Invention is credited to Hirokazu Okada, Toshihide Ono, Hiroyuki Semba, Mitsuru Yoshizawa.
Application Number | 20200232081 16/483049 |
Document ID | / |
Family ID | 63108183 |
Filed Date | 2020-07-23 |
United States Patent
Application |
20200232081 |
Kind Code |
A1 |
Semba; Hiroyuki ; et
al. |
July 23, 2020 |
Austenitic Heat Resistant Alloy and Method for Producing Same
Abstract
Provided is an austenitic heat resistant alloy having a chemical
composition consisting of, in mass %: C: 0.02 to 0.12%; Si: 2.0% or
less; Mn: 3.0% or less; P: 0.030% or less; S: 0.015% or less; Cr:
20.0% or more and less than 28.0%; Ni: more than 35.0% and 55.0% or
less; Co: 0 to 20.0%; W: 4.0 to 10.0%; Ti: 0.01 to 0.50%; Nb: 0.01
to 1.0%; Mo: less than 0.50%; Cu: less than 0.50%; Al: 0.30% or
less; N: less than 0.10%; Mg: 0 to 0.05%; Ca: 0 to 0.05%; REM: 0 to
0.50%; V: 0 to 1.5%; B: 0 to 0.01%; Zr: 0 to 0.10%; Hf: 0 to 1.0%;
Ta: 0 to 8.0%; Re: 0 to 8.0%; and the balance: Fe and impurities,
wherein a shortest distance from a center portion to an outer
surface portion of a cross section of the alloy is 40 mm or more,
the cross section being perpendicular to a longitudinal direction
of the alloy, an austenite grain size number at the outer surface
portion is -2.0 to 4.0, an amount of Cr which is present as a
precipitate satisfies [Cr.sub.PB/Cr.sub.PS.ltoreq.10.0], and
[YS.sub.S/YS.sub.B.ltoreq.1.5] and [TS.sub.S/TS.sub.B.ltoreq.1.2]
are satisfied at a normal temperature.
Inventors: |
Semba; Hiroyuki;
(Chiyoda-ku, Tokyo, JP) ; Okada; Hirokazu;
(Chiyoda-ku, Tokyo, JP) ; Yoshizawa; Mitsuru;
(Chiyoda-ku, Tokyo, JP) ; Ono; Toshihide;
(Chiyoda-ku, Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Nippon Steel Corporation |
Tokyo |
|
JP |
|
|
Family ID: |
63108183 |
Appl. No.: |
16/483049 |
Filed: |
February 9, 2017 |
PCT Filed: |
February 9, 2017 |
PCT NO: |
PCT/JP2017/004824 |
371 Date: |
August 2, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/005 20130101;
C22C 38/48 20130101; C22F 1/16 20130101; C22C 38/44 20130101; C22C
38/58 20130101; C22C 19/055 20130101; C22C 38/50 20130101; Y02P
10/212 20151101; C22C 30/02 20130101; C22F 1/10 20130101 |
International
Class: |
C22F 1/16 20060101
C22F001/16; C22C 30/02 20060101 C22C030/02 |
Claims
1. An austenitic heat resistant alloy having a chemical composition
consisting of, in mass %: C: 0.02 to 0.12%; Si: 2.0% or less; Mn:
3.0% or less; P: 0.030% or less; S: 0.015% or less; Cr: 20.0% or
more and less than 28.0%; Ni: more than 35.0% and 55.0% or less;
Co: 0 to 20.0%; W: 4.0 to 10.0%; Ti: 0.01 to 0.50%; Nb: 0.01 to
1.0%; Mo: less than 0.50%; Cu: less than 0.50%; Al: 0.30% or less;
N: less than 0.10%; Mg: 0 to 0.05%; Ca: 0 to 0.05%; REM: 0 to
0.50%; V: 0 to 1.5%; B: 0 to 0.01%; Zr: 0 to 0.10%; Hf: 0 to 1.0%;
Ta: 0 to 8.0%; Re: 0 to 8.0%; and the balance: Fe and impurities,
wherein a shortest distance from a center portion to an outer
surface portion of a cross section of the alloy is 40 mm or more,
the cross section being perpendicular to a longitudinal direction
of the alloy, an austenite grain size number at the outer surface
portion is -2.0 to 4.0, an amount of Cr which is present as a
precipitate obtained by an extraction residue analysis satisfies a
following formula (i), and mechanical properties at a normal
temperature satisfy following formula (ii) and formula
Cr.sub.PB/Cr.sub.PS.ltoreq.10.0 (i) YS.sub.S/YS.sub.B.ltoreq.1.5
(ii) TS.sub.S/TS.sub.B.ltoreq.1.2 (iii) where meaning of each
symbol in the formulas is as follows: Cr.sub.PB: amount of Cr which
is present at center portion as precipitate obtained by extraction
residue analysis Cr.sub.PS: amount of Cr which is present at outer
surface portion as precipitate obtained by extraction residue
analysis YS.sub.B: 0.2% proof stress at center portion YS.sub.S:
0.2% proof stress at outer surface portion TS.sub.B: tensile
strength at center portion TS.sub.S: tensile strength at outer
surface portion.
2. The austenitic heat resistant alloy according to claim 1,
wherein the chemical composition contains one or more elements
selected from a group consisting of, in mass %: Mg: 0.0005 to
0.05%; Ca: 0.0005 to 0.05%; REM: 0.0005 to 0.50%; V: 0.02 to 1.5%;
B: 0.0005 to 0.01%; Zr: 0.005 to 0.10%; Hf: 0.005 to 1.0%; Ta: 0.01
to 8.0%; and Re: 0.01 to 8.0%.
3. The austenitic heat resistant alloy according to claim 1,
wherein 10,000-hour creep rupture strength at 700.degree. C. in the
longitudinal direction at the center portion is 100 MPa or
more.
4. A method for producing an austenitic heat resistant alloy, the
method comprising the steps of: performing hot working on an ingot
or a cast piece having the chemical composition according to claim
1; and thereafter performing heat treatment where the ingot or the
cast piece is heated to a heat-treatment temperature T (.degree.
C.) ranging from 1100 to 1250.degree. C., is held for 1000 D/T to
1400 D/T (min), and is cooled with water, wherein symbol "D"
denotes a maximum value (mm) of a linear distance between an
arbitrary point on an outer edge of a cross section of the alloy
and another arbitrary point on the outer edge, the cross section
being perpendicular to a longitudinal direction of the alloy.
5. The method for producing an austenitic heat resistant alloy
according to claim 4, wherein in the step of performing the hot
working, the working is performed one or more times in a direction
substantially perpendicular to the longitudinal direction.
6. The austenitic heat resistant alloy according to claim 2,
wherein 10,000-hour creep rupture strength at 700.degree. C. in the
longitudinal direction at the center portion is 100 MPa or
more.
7. A method for producing an austenitic heat resistant alloy, the
method comprising the steps of: performing hot working on an ingot
or a cast piece having the chemical composition according to claim
2; and thereafter performing heat treatment where the ingot or the
cast piece is heated to a heat-treatment temperature T (.degree.
C.) ranging from 1100 to 1250.degree. C., is held for 1000 D/T to
1400 D/T (min), and is cooled with water, wherein symbol "D"
denotes a maximum value (mm) of a linear distance between an
arbitrary point on an outer edge of a cross section of the alloy
and another arbitrary point on the outer edge, the cross section
being perpendicular to a longitudinal direction of the alloy.
8. The method for producing an austenitic heat resistant alloy
according to claim 7, wherein in the step of performing the hot
working, the working is performed one or more times in a direction
substantially perpendicular to the longitudinal direction.
Description
TECHNICAL FIELD
[0001] The present invention relates to an austenitic heat
resistant alloy and a method for producing the same.
[0002] Conventionally, for thermal power generation boilers,
chemical plants and the like which are used in a high temperature
environment, 18-8 austenitic stainless steels, such as SUS304H,
SUS316H, SUS321H, and SUS347H, have been used as materials for
apparatuses.
[0003] In recent years, however, ultra super critical boilers,
where temperature and pressure of steam are increased to enhance
efficiency, have been newly installed worldwide. The use conditions
of apparatuses in such a high temperature environment have become
extremely severe, and therefore, properties which materials being
used are required to possess have become strict. Under such
circumstances, using 18-8 austenitic stainless steel, which is
conventionally used, has become extremely insufficient in terms of
not only corrosion resistance but also high temperature strength,
particularly creep rupture strength.
[0004] To overcome the above problems, various studies have been
made. For example, Patent Documents 1 to 4 disclose austenitic
steel excellent in high temperature strength and corrosion
resistance. Further, Patent Document 5 discloses austenitic
stainless steel excellent in high temperature strength and
corrosion resistance. According to Patent Documents 1 to 5, the
amount of Cr is increased to 20% or more, and W and/or Mo are
contained so as to enhance high temperature strength.
LIST OF PRIOR ART DOCUMENTS
Patent Document
[0005] Patent Document 1: JP61-179833A
[0006] Patent Document 2: JP61-179834A
[0007] Patent Document 3: JP61-179835A
[0008] Patent Document 4: JP61-179836A
[0009] Patent Document 5: JP2004-3000A
SUMMARY OF INVENTION
Technical Problem
[0010] Large-sized structural members made of a material for
apparatuses, such as thermal power generation boilers or chemical
plants, are hot rolled or hot forged and then subjected to final
heat treatment without cold rolling before putting into use.
Accordingly, the grain size is relatively large. For this reason,
usually, there is a problem that 0.2% proof stress and tensile
strength at a normal temperature, which are defined as the
specifications of materials, are lower than those of a material
obtained by performing final heat treatment after cold rolling.
[0011] In addition to the above, in a large-sized structural
member, a cooling speed at the time of performing heat treatment
varies largely from region to region and hence, there is a
variation from region to region in the amount of solid solution
elements which contribute to strengthening the member as
precipitates during use at a high temperature. There is also a
problem that creep rupture strength varies due to such variation.
Accordingly, it is difficult to adopt steel disclosed in Patent
Documents 1 to 5 to a large-sized structural member.
[0012] The present invention has been made to overcome the above
problems, and an objective of the present invention is to provide
an austenitic heat resistant alloy and a method for producing the
same which exhibits sufficient 0.2% proof stress and tensile
strength at a normal temperature, and sufficient creep rupture
strength at a high temperature in large-sized structural
members.
Solution to Problem
[0013] The present invention has been made to overcome the above
problems, and the gist of the present invention is the following
austenitic heat resistant alloy and method for producing the
same.
[0014] (1) An austenitic heat resistant alloy having a chemical
composition consisting of, in mass %:
[0015] C: 0.02 to 0.12%;
[0016] Si: 2.0% or less;
[0017] Mn: 3.0% or less;
[0018] P: 0.030% or less;
[0019] S: 0.015% or less;
[0020] Cr: 20.0% or more and less than 28.0%;
[0021] Ni: more than 35.0% and 55.0% or less;
[0022] Co: 0 to 20.0%;
[0023] W: 4.0 to 10.0%;
[0024] Ti: 0.01 to 0.50%;
[0025] Nb: 0.01 to 1.0%;
[0026] Mo: less than 0.50%;
[0027] Cu: less than 0.50%;
[0028] Al: 0.30% or less;
[0029] N: less than 0.10%;
[0030] Mg: 0 to 0.05%;
[0031] Ca: 0 to 0.05%;
[0032] REM: 0 to 0.50%;
[0033] V: 0 to 1.5%;
[0034] B: 0 to 0.01%;
[0035] Zr: 0 to 0.10%;
[0036] Hf: 0 to 1.0%;
[0037] Ta: 0 to 8.0%;
[0038] Re: 0 to 8.0%; and
[0039] the balance: Fe and impurities, wherein
[0040] a shortest distance from a center portion to an outer
surface portion of a cross section of the alloy is 40 mm or more,
the cross section being perpendicular to a longitudinal direction
of the alloy,
[0041] an austenite grain size number at the outer surface portion
is -2.0 to 4.0,
[0042] an amount of Cr which is present as a precipitate obtained
by an extraction residue analysis satisfies a following formula
(i), and
[0043] mechanical properties at a normal temperature satisfy
following formula (ii) and formula (iii):
Cr.sub.PB/Cr.sub.PS.ltoreq.10.0 (i)
YS.sub.S/YS.sub.B.ltoreq.1.5 (ii)
TS.sub.S/TS.sub.B.ltoreq.1.2 (iii)
[0044] where meaning of each symbol in the formulas is as
follows:
[0045] Cr.sub.PB: amount of Cr which is present at center portion
as precipitate obtained by extraction residue analysis
[0046] Cr.sub.PS: amount of Cr which is present at outer surface
portion as precipitate obtained by extraction residue analysis
[0047] YS.sub.B: 0.2% proof stress at center portion
[0048] YS.sub.S: 0.2% proof stress at outer surface portion
[0049] TS.sub.B: tensile strength at center portion
[0050] TS.sub.S: tensile strength at outer surface portion.
[0051] (2) The austenitic heat resistant alloy described in the
above (1), wherein the chemical composition contains one or more
elements selected from a group consisting of, in mass %:
[0052] Mg: 0.0005 to 0.05%;
[0053] Ca: 0.0005 to 0.05%;
[0054] REM: 0.0005 to 0.50%;
[0055] V: 0.02 to 1.5%;
[0056] B: 0.0005 to 0.01%;
[0057] Zr: 0.005 to 0.10%;
[0058] Hf: 0.005 to 1.0%;
[0059] Ta: 0.01 to 8.0%; and
[0060] Re: 0.01 to 8.0%.
[0061] (3) The austenitic heat resistant alloy described in the
above (1) or (2), wherein 10,000-hour creep rupture strength at
700.degree. C. in the longitudinal direction at the center portion
is 100 MPa or more.
[0062] (4) A method for producing an austenitic heat resistant
alloy, the method including the steps of:
[0063] performing hot working on an ingot or a cast piece having
the chemical composition described in the above (1) or (2); and
[0064] thereafter performing heat treatment where the ingot or the
cast piece is heated to a heat-treatment temperature T (.degree.
C.) ranging from 1100 to 1250.degree. C., is held for 1000 D/T to
1400 D/T (min), and is cooled with water,
[0065] wherein symbol "D" denotes a maximum value (mm) of a linear
distance between an arbitrary point on an outer edge of a cross
section of the alloy and another arbitrary point on the outer edge,
the cross section being perpendicular to a longitudinal direction
of the alloy.
[0066] (5) The method for producing an austenitic heat resistant
alloy described in the above (4), wherein
[0067] in the step of performing the hot working, the working is
performed one or more times in a direction substantially
perpendicular to the longitudinal direction.
Advantageous Effects of Invention
[0068] The austenitic heat resistant alloy of the present invention
has small variation in mechanical properties from region to region,
and is excellent in creep rupture strength at a high
temperature.
DESCRIPTION OF EMBODIMENTS
[0069] Hereinafter, the respective requirements of the present
invention are described in detail.
[0070] 1. Chemical Composition
[0071] The reasons for limiting respective elements are as follows.
In the description made hereinafter, symbol "%" for content refers
to "mass %".
[0072] C: 0.02 to 0.12%
[0073] C (carbon) forms carbides so that C is an indispensable
element for maintaining high temperature tensile strength and creep
rupture strength required for an austenitic heat resistant alloy.
Accordingly, it is necessary to set a content of C to 0.02% or
more. However, when the C content exceeds 0.12%, not only
undissolved carbides are formed, but also Cr carbides increase and
hence, mechanical properties, such as ductility and toughness, and
weldability deteriorate. Accordingly, the C content is set to a
value ranging from 0.02 to 0.12%. The C content is preferably 0.05%
or more and 0.10% or less.
[0074] Si: 2.0% or less
[0075] Si (silicon) is contained as a deoxidizing element. Further,
Si is an element effective in increasing oxidation resistance,
steam oxidation resistance and the like. Si is also an element
which facilitates the flow of a casting material. However, when a
content of Si exceeds 2.0%, the formation of intermetallic
compounds, such as a .sigma. phase, is promoted and hence,
stability of micro-structure at a high temperature deteriorates,
thus lowering toughness and ductility. When the Si content exceeds
2.0%, weldability is also lowered. Accordingly, the Si content is
set to 2.0% or less. When importance is placed on structural
stability, the Si content is preferably set to 1.0% or less. When a
deoxidizing action is sufficiently ensured by other elements, it is
not particularly necessary to set the lower limit of the Si
content. However, when importance is placed on a deoxidizing
action, oxidation resistance, steam oxidation resistance and the
like, the Si content is preferably set to 0.05% or more, and more
preferably set to 0.10% or more.
[0076] Mn: 3.0% or less
[0077] Mn (manganese) has a deoxidizing action in the same manner
as Si, and also has an action of fixing S, which is inevitably
contained in the alloy, as a sulfide, thus improving ductility at a
high temperature. However, when a content of Mn exceeds 3.0%, the
precipitation of intermetallic compounds, such as a .sigma. phase,
is promoted and hence, structural stability, and mechanical
properties, such as high temperature strength, deteriorate.
Accordingly, the Mn content is set to 3.0% or less. The Mn content
is preferably 2.0% or less, and more preferably 1.5% or less. It is
not necessary to set the lower limit of the Mn content. However,
when importance is placed on an action of improving ductility at a
high temperature, the Mn content is preferably set to 0.10% or
more, and more preferably set to 0.20% or more.
[0078] P: 0.030% or less
[0079] P (phosphorus) is inevitably mixed in the alloy as an
impurity, and remarkably lowers weldability and ductility at a high
temperature. Accordingly, a content of P is set to 0.030% or less.
It is preferable to reduce the P content to as much as possible.
The P content is preferably set to 0.020% or less, and more
preferably set to 0.015% or less.
[0080] S: 0.015% or less
[0081] S (sulfur) is inevitably mixed in the alloy as an impurity
in the same manner as P, and remarkably lowers weldability and
ductility at a high temperature. Accordingly, a content of S is set
to 0.015% or less. When importance is placed on hot workability,
the S content is preferably set to 0.010% or less, more preferably
set to 0.005% or less, and further preferably set to 0.003% or
less.
[0082] Cr: 20.0% or more and less than 28.0%
[0083] Cr (chromium) is an important element which excellently
exhibits an action of improving corrosion resistance, such as
oxidation resistance, steam oxidation resistance, and high
temperature corrosion resistance. However, when a content of Cr is
less than 20.0%, these advantageous effects cannot be obtained. On
the other hand, when the Cr content increases, particularly to
28.0% or more, the micro-structure is made unstable due to the
precipitation of a .sigma. phase or the like, and weldability also
deteriorates. Accordingly, the Cr content is set to a value ranging
of 20.0% or more and less than 28.0%. The Cr content is preferably
21.0% or more, and more preferably 22.0% or more. Further, the Cr
content is preferably 26.0% or less, and more preferably 25.0% or
less.
[0084] Ni: more than 35.0% and 55.0% or less
[0085] Ni (nickel) is an element which makes the austenitic
structure stable, and is also an element important to ensure
corrosion resistance. To maintain the balance with the Cr content,
it is necessary to set a content of Ni to more than 35.0%. On the
other hand, excessively high Ni content increases costs and hence,
the Ni content is set to 55.0% or less. The Ni content is
preferably 40.0% or more, and more preferably 42.0% or more.
Further, the Ni content is preferably 50.0% or less, and more
preferably 48.0% or less.
[0086] Co: 0 to 20.0%
[0087] It is not always necessary to contain Co (cobalt). However,
in the same manner as Ni, Co makes the austenitic structure stable,
and also contributes to enhancing creep rupture strength.
Accordingly, Co may be contained in lieu of a part of Ni. However,
when a content of Co exceeds 20.0%, the effect is saturated and
hence, economic efficiency is lowered. Accordingly, the Co content
is set to a value ranging from 0 to 20.0%. The Co content is
preferably 15.0% or less. When it is desired to obtain the
advantageous effects, the Co content is preferably set to 0.5% or
more.
[0088] W: 4.0 to 10.0%
[0089] W (tungsten) is dissolved in a matrix, thus not only
contributing to enhancing creep rupture strength as a
solid-solution strengthening element, but also precipitating as a
Fe.sub.2W Laves phase or a Fe.sub.7W.sub.6 .mu. phase so that creep
rupture strength is significantly enhanced. Accordingly, W is an
important element. However, when a content of W is less than 4.0%,
the advantageous effects cannot be obtained. On the other hand,
even if the W content is set to more than 10.0%, an effect of
enhancing strength is saturated, and structural stability and
ductility at a high temperature deteriorate. Accordingly, the W
content is set to a value ranging from 4.0 to 10.0%. The W content
is preferably 5.0% or more, and more preferably 5.5% or more.
Further, the W content is preferably 9.0% or less, and more
preferably 8.5% or less.
[0090] Ti: 0.01 to 0.50%
[0091] Ti (titanium) is an element which forms carbo-nitrides, thus
having an effect of enhancing creep rupture strength. However, when
a content of Ti is less than 0.01%, sufficient effects cannot be
obtained. On the other hand, when the Ti content exceeds 0.50%,
ductility at a high temperature is lowered. Accordingly, the Ti
content is set to a value ranging from 0.01 to 0.50%. The Ti
content is preferably set to 0.05% or more, and more preferably set
to 0.10% or more. Further, the Ti content is preferably set to
0.40% or less, and more preferably set to 0.35% or less.
[0092] Nb: 0.01 to 1.0%
[0093] Nb (niobium) has an action of forming carbo-nitrides, thus
enhancing creep rupture strength. However, when a content of Nb is
less than 0.01%, sufficient effects cannot be obtained. On the
other hand, when the Nb content exceeds 1.0%, ductility at a high
temperature is lowered. Accordingly, the Nb content is set to a
value ranging from 0.01 to 1.0%. The Nb content is preferably 0.10%
or more. Further, the Nb content is preferably 0.90% or less, and
more preferably 0.70% or less.
[0094] Mo: less than 0.50%
[0095] Mo (molybdenum) is an element which is dissolved in a
matrix, thus contributing to enhancing creep rupture strength as a
solid-solution strengthening element and hence, Mo has been
conventionally considered as an element having substantially the
same action as W. However, the inventors of the present invention
have made studies, and found the following. When Mo is contained in
combination in an alloy which contains the amounts of W and Cr, a
.sigma. phase may precipitate after long-term use and hence, creep
rupture strength, ductility and toughness may be lowered.
Accordingly, it is desirable to reduce a content of Mo as much as
possible, and the Mo content is set to less than 0.50%. It is
preferable to limit the Mo content to less than 0.20%.
[0096] Cu: less than 0.50%
[0097] In the present invention, Cu (copper) lowers a fusing point,
thus lowering hot workability and weldability. Accordingly, it is
desirable to reduce a content of Cu as much as possible, and the Cu
content is set to less than 0.50%. It is preferable to limit the Cu
content to less than 0.20%.
[0098] Al: 0.30% or less
[0099] Al (aluminum) is an element which is contained as a
deoxidizer for molten steel. However, when a content of Al exceeds
0.30%, ductility at a high temperature deteriorates. Accordingly,
the Al content is set to 0.30% or less. The Al content is
preferably 0.25% or less, and more preferably 0.20% or less. When
it is desired to obtain the advantageous effect, the Al content is
preferably set to 0.01% or more, and more preferably set to 0.02%
or more.
[0100] N: less than 0.10%
[0101] N (nitrogen) is an element having an action of making the
austenitic structure stable, and is an element inevitably contained
when an ordinary melting method is adopted. However, in the present
invention where Ti is contained as an indispensable element, it is
preferable to reduce a content of N as much as possible so as to
prevent Ti from being consumed by the formation of TiN. However, in
the case of atmospheric melting, it is difficult to extremely
reduce the N content. Accordingly, the N content is set to less
than 0.10%.
[0102] In the chemical composition of the austenitic heat resistant
alloy of the present invention, the balance consists of Fe and
impurities. It is preferable to set a content of Fe to 0.1 to
40.0%. In this embodiment, "impurity" means a component which is
mixed in industrially producing the alloy due to various causes,
such as raw materials including ores or scrap, or production steps,
and which is allowed to be mixed without adversely affecting the
present invention.
[0103] The austenitic heat resistant alloy of the present invention
may further contain one or more kinds selected from a group
consisting of Mg, Ca, REM, V, B, Zr, Hf, Ta, and Re.
[0104] Any of Mg, Ca or REM has an action of fixing S as sulfides
to enhance high temperature ductility. Accordingly, when it is
desired to obtain greater high temperature ductility, one or more
kinds of these elements may be positively contained within the
following range.
[0105] Mg: 0.05% or less
[0106] Mg (magnesium) has an action of fixing S, which inhibits
ductility at a high temperature, as sulfides, thus improving high
temperature ductility. Accordingly, Mg may be contained so as to
obtain this advantageous effect. However, when a content of Mg
exceeds 0.05%, cleanliness is lowered, and high temperature
ductility is impaired on the contrary. Accordingly, when Mg is
contained, the amount of Mg is set to 0.05% or less. The Mg content
is more preferably set to 0.02% or less, and further preferably set
to 0.01% or less. On the other hand, to obtain the advantageous
effect with certainty, the Mg content is preferably set to 0.0005%
or more, and more preferably set to 0.001% or more.
[0107] Ca: 0.05% or less
[0108] Ca (calcium) has an action of fixing S, which inhibits
ductility at a high temperature, as sulfides, thus improving high
temperature ductility. Accordingly, Ca may be contained so as to
obtain this advantageous effect. However, when a content of Ca
exceeds 0.05%, cleanliness is lowered, and high temperature
ductility is impaired on the contrary. Accordingly, when Ca is
contained, the amount of Ca is set to 0.05% or less. The Ca content
is more preferably set to 0.02% or less, and further preferably set
to 0.01% or less. On the other hand, to obtain the advantageous
effect with certainty, the Ca content is preferably set to 0.0005%
or more, and more preferably set to 0.001% or more.
[0109] REM: 0.50% or less
[0110] REM has an action of fixing S as sulfides, thus improving
high temperature ductility. REM also has an action of improving
adhesiveness of a Cr.sub.2O.sub.3 protection film on a steel
surface, thus improving oxidation resistance particularly when the
alloy is repeatedly oxidized. Further, REM contributes to
strengthening grain boundaries, thus having an action of enhancing
creep rupture strength and creep rupture ductility. However, when a
content of REM exceeds 0.50%, the amount of inclusions, such as an
oxide increases and hence, workability and weldability are
impaired. Accordingly, when REM is contained, the amount of REM is
set to 0.50% or less. The REM content is more preferably set to
0.30% or less, and further preferably set to 0.15% or less. On the
other hand, to obtain the advantageous effects with certainty, the
REM content is preferably set to 0.0005% or more, more preferably
set to 0.001% or more, and further preferably set to 0.002% or
more.
[0111] REM indicates 17 elements in total, including Sc, Y, and the
lanthanoids. The REM content means the total content of these
elements.
[0112] The total content of Mg, Ca and REM may be 0.6% or less.
However, the total content is more preferably 0.4% or less, and
further preferably 0.2% or less.
[0113] Any of V, B, Zr, or Hf has an action of enhancing high
temperature strength and creep rupture strength. Accordingly, when
it is desired to obtain greater high temperature strength and
greater creep rupture strength, the alloy may positively contain
one or more kinds of these elements within the following range.
[0114] V: 1.5% or less
[0115] V (vanadium) has an action of forming carbo-nitrides to
enhance high temperature strength and creep rupture strength.
Accordingly, V may be contained so as to obtain these advantageous
effects. However, when a content of V exceeds 1.5%, high
temperature corrosion resistance is lowered and, further, ductility
and toughness deteriorate due to the precipitation of a brittle
phase. Accordingly, when V is contained, the amount of V is set to
1.5% or less. The V content is more preferably set to 1.0% or less.
On the other hand, to obtain the advantageous effect with
certainty, the V content is preferably set to 0.02% or more, and
more preferably set to 0.04% or more.
[0116] B: 0.01% or less
[0117] B (boron) is present in carbide or in a matrix. B has not
only an action of promoting micronization of precipitated carbide,
but also an action of strengthening grain boundaries, thus
enhancing creep rupture strength. However, when a content of B
exceeds 0.01%, ductility at a high temperature is lowered, and a
fusing point is also lowered. Accordingly, when B is contained, the
amount of B is set to 0.01% or less. The B content is more
preferably 0.008% or less, and further preferably 0.006% or less.
On the other hand, to obtain the advantageous effects with
certainty, the B content is preferably set to 0.0005% or more, more
preferably set to 0.001% or more, and further preferably set to
0.0015% or more.
[0118] Zr: 0.10% or less
[0119] Zr (zirconium) is an element which promotes micronization of
carbo-nitrides, and which enhances creep rupture strength as a
grain boundary strengthening element. However, when a content of Zr
exceeds 0.10%, ductility at a high temperature is lowered.
Accordingly, when Zr is contained, the amount of Zr is set to 0.10%
or less. The Zr content is more preferably 0.06% or less, and
further preferably 0.05% or less. On the other hand, to obtain the
advantageous effects with certainty, the Zr content is preferably
set to 0.005% or more, and more preferably set to 0.01% or
more.
[0120] Hf: 1.0% or less
[0121] Hf (hafnium) has an action of contributing to strengthening
precipitation as carbo-nitrides, thus enhancing creep rupture
strength. Accordingly, Hf may be contained so as to obtain these
advantageous effects. However, when a content of Hf exceeds 1.0%,
workability and weldability are impaired. Accordingly, when Hf is
contained, the amount of Hf is set to 1.0% or less. The Hf content
is more preferably set to 0.8% or less, and further preferably set
to 0.5% or less. On the other hand, to obtain the advantageous
effects with certainty, the Hf content is preferably set to 0.005%
or more, more preferably set to 0.01% or more, and further
preferably set to 0.02% or more.
[0122] The total content of V, B, Zr, and Hf is preferably 2.6% or
less, and more preferably 1.8% or less.
[0123] Either one of Ta or Re dissolves in austenite forming a
matrix, thus having an action of solid-solution strengthening.
Accordingly, when it is desired to obtain greater high temperature
strength and creep rupture strength due to an action of
solid-solution strengthening, one or both of these elements may be
positively contained within the following range.
[0124] Ta: 8.0% or less
[0125] Ta (tantalum) has an action of forming carbo-nitrides, and
also has an action of enhancing high temperature strength and creep
rupture strength as a solid-solution strengthening element.
Accordingly, Ta may be contained so as to obtain these advantageous
effects. However, when a content of Ta exceeds 8.0%, workability
and mechanical properties are impaired. Accordingly, when Ta is
contained, the amount of Ta is set to 8.0% or less. The Ta content
is more preferably set to 7.0% or less, and further preferably set
to 6.0% or less. On the other hand, to obtain the advantageous
effects with certainty, the Ta content is preferably set to 0.01%
or more, more preferably set to 0.1% or more, and further
preferably set to 0.5% or more.
[0126] Re: 8.0% or less
[0127] Re (rhenium) has an action of enhancing high temperature
strength and creep rupture strength mainly as a solid-solution
strengthening element. Accordingly, Re may be contained so as to
obtain these advantageous effects. However, when a content of Re
exceeds 8.0%, workability and mechanical properties are impaired.
Accordingly, when Re is contained, the amount of Re is set to 8.0%
or less. The Re content is more preferably set to 7.0% or less, and
further preferably set to 6.0%. On the other hand, to obtain the
advantageous effects with certainty, the Re content is preferably
set to 0.01% or more, more preferably set to 0.1% or more, and
further preferably set to 0.5% or more.
[0128] The total content of Ta and Re is preferably 14.0% or less,
and more preferably 12.0% or less.
[0129] 2. Grain Size
[0130] Austenite grain size number at outer surface portion: -2.0
to 4.0
[0131] When an austenitic grain size at an outer surface portion is
extremely large, 0.2% proof stress and tensile strength at a normal
temperature are lowered. On the other hand, when an austenitic
grain size at an outer surface portion is extremely small, it
becomes impossible to maintain high creep rupture strength at a
high temperature. Accordingly, the austenite grain size number at
the outer surface portion is set to a value ranging from -2.0 to
4.0. In a production process for a Ni-based alloy, by properly
adjusting a heat-treatment temperature and holding time after hot
working and a cooling method, it is possible to set the grain size
number at the outer surface portion to a value which falls within
the range after final heat treatment.
[0132] 3. Size Shortest Distance From Center Portion to Outer
Surface Portion: 40 mm or More
[0133] As described above, in a large-sized structural member, in
addition to a problem that 0.2% proof stress and tensile strength
at a normal temperature are lowered, there is also a problem that
creep rupture strength varies from region to region. However, the
austenitic heat resistant alloy according to the present invention
exhibits sufficient 0.2% proof stress and tensile strength at a
normal temperature, and sufficient creep rupture strength at a high
temperature in large-sized structural members. That is, the present
invention can obtain remarkable advantageous effects in members
having a thick wall.
[0134] Accordingly, in the austenitic heat resistant alloy of the
present invention, the shortest distance from the center portion to
the outer surface portion of a cross section is set to 40 mm or
more, the cross section being perpendicular to a longitudinal
direction. To obtain more remarkable advantageous effects of the
present invention, the shortest distance from the center portion to
the outer surface portion is preferably 80 mm or more, and more
preferably 100 mm or more. In this embodiment, the shortest
distance from the center portion to the outer surface portion
refers to a radius (mm) of a cross section when an alloy has a
columnar shape, and the shortest distance refers to a half-length
(mm) of the short side of a cross section when an alloy has a
quadrangular prism shape, for example.
[0135] As described later, the heat resistant alloy according to
the present invention is obtained by performing hot working, such
as hot forging or hot rolling on an ingot, or a cast piece,
obtained by continuous casting or the like, for example. When an
ingot is used, the longitudinal direction of a heat resistant alloy
substantially refers to a direction along which a top portion and a
bottom portion of the ingot are connected. When a cast piece is
used, the longitudinal direction of a heat resistant alloy
substantially refers to the longitudinal direction of the cast
piece.
[0136] 4. Amount of Cr Which is Present as Precipitate Obtained by
Extraction Residue Analysis
Cr.sub.PB/Cr.sub.PS23 10.0 (i)
[0137] where meaning of each symbol in the formula (i) is as
follows:
[0138] Cr.sub.PB: amount of Cr which is present at center portion
as precipitate obtained by extraction residue analysis
[0139] Cr.sub.PS: amount of Cr which is present at outer surface
portion as precipitate obtained by extraction residue analysis
[0140] In a production process for an alloy, after heat treatment,
which is performed after the hot working, is performed, undissolved
Cr precipitations (mainly carbides) are generated at crystal grain
boundaries or within grains. Particularly at the center portion of
the alloy, a cooling speed is slower than that at the outer surface
portion of the alloy and hence, the amount of Cr precipitates tends
to increase. Accordingly, when a value of Cr.sub.PB/Cr.sub.PS
exceeds 10.0, it becomes impossible to maintain high creep rupture
strength at a high temperature. On the other hand, it is not
necessary to set the lower limit value of Cr.sub.PB/Cr.sub.PS.
However, there is a tendency that the amount of precipitates
increases more at the center portion than at the outer surface
portion and hence, Cr.sub.PB/Cr.sub.PS is preferably set to 1.0 or
more.
[0141] An extraction residue analysis is performed by the following
procedure. First, test coupons for measuring Cr precipitates are
obtained from the center portion and the outer surface portion of
the cross section of an alloy specimen, the cross section being
perpendicular to the longitudinal direction of the alloy specimen.
The surface area of each test coupon is obtained and, thereafter,
only the base metal of the alloy specimen is completely
electrolyzed in a 10% acetylacetone--1% tetramethyl ammonium
chloride--methanol solution under an electrolysis condition of 20
mA/cm.sup.2. Then, the solution after electrolysis is performed is
filtered through a 0.2 .mu.m filter to extract precipitates as a
residue. Thereafter, the extracted residue is decomposed with an
acid, and is analyzed using an inductively coupled plasma emission
spectrophotometer (ICP-AES) to measure a content (mass %) of Cr
contained as undissolved Cr precipitate, and a value of
Cr.sub.PB/Cr.sub.PS is obtained based on the measured value.
[0142] 5. Mechanical Properties
YS.sub.S/YS.sub.B.ltoreq.1.5 (ii)
TS.sub.S/TS.sub.B.ltoreq.1.2 (iii)
[0143] where meaning of each symbol in the formulas is as
follows:
[0144] YS.sub.B: 0.2% proof stress at center portion
[0145] YS.sub.S: 0.2% proof stress at outer surface portion
[0146] TS.sub.B: tensile strength at center portion
[0147] TS.sub.S: tensile strength at outer surface portion
[0148] In a large-sized structural member, a cooling speed at the
time of performing heat treatment varies from region to region and
hence, there is a tendency that great variations occur in
mechanical properties from region to region due to the difference
in the cooling speed. If there is a large difference in 0.2% proof
stress and tensile strength at a normal temperature between the
center portion and the outer surface portion of the large-sized
structural member, there arises a problem that some regions do not
satisfy the specifications.
[0149] Accordingly, with respect to the austenitic heat resistant
alloy according to the present invention, mechanical properties at
a normal temperature satisfy the formula (ii) and formula (iii). It
is not necessary to set the respective lower limit values of these
formulas. However, there is a tendency that mechanical
characteristics at the center portion are inferior to mechanical
characteristics at the outer surface portion and hence, either one
of formula (ii) or formula (iii) is preferably set to 1.0 or
more.
[0150] 0.2% proof stress and tensile strength are obtained in such
a way that round bar tensile test coupons, each having a parallel
portion with a length of 40 mm, are cut out by mechanical
processing from the center portion and the outer surface portion of
the alloy parallel to the longitudinal direction, and a tensile
test is performed on these test coupons at a room temperature. The
tensile test is performed in accordance with JIS Z 2241 (2011).
[0151] 6. Creep Rupture Strength
[0152] The austenitic heat resistant alloy of the present invention
is used in a high temperature environment, thus being required to
be excellent in high temperature strength, particularly, in creep
rupture strength. Accordingly, 10,000-hour creep rupture strength
at 700.degree. C. in the longitudinal direction is preferably 100
MPa or more at the center portion of the heat resistant alloy of
the present invention.
[0153] Creep rupture strength is obtained by the following method.
First, round bar creep rupture test coupons, described in JIS Z
2241 (2011), and having a diameter of 6 mm and a gage length of 30
mm, are cut out by mechanical processing from the center portions
of the alloys parallel to the longitudinal direction. Then, a creep
rupture test is performed in the atmosphere of 700.degree. C.,
750.degree. C., and 800.degree. C. to obtain 10,000-hour creep
rupture strength at 700.degree. C. by a Larson-Miller parameter
method. The creep rupture test is performed in accordance with JIS
Z 2271 (2010).
[0154] 7. Production Method
[0155] The austenitic heat resistant alloy of the present invention
can be produced by performing hot working on an ingot or a cast
piece having the above chemical composition. In the above step of
performing hot working, processing is performed such that the
longitudinal direction of the alloy in the final shape aligns with
the longitudinal direction of the ingot or the cast piece forming a
starting material. Hot working may be performed only in the
longitudinal direction. However, to obtain a more uniform
micro-structure at a higher working ratio, hot working may be
performed one or more times in a direction substantially
perpendicular to the longitudinal direction. After the hot working
is performed, hot working of another method, such as hot extrusion,
may be further performed when necessary.
[0156] In producing the austenitic heat resistant alloy of the
present invention, after the above step, final heat treatment
described below is performed so as to minimize variation in metal
micro-structure and mechanical properties from region to region,
thus maintaining high creep rupture strength.
[0157] First, the alloy on which hot working was performed is
heated to a heat-treatment temperature T (.degree. C.) ranging from
1100 to 1250.degree. C., and is held for 1000 D/T to 1400 D/T (min)
within such a range. In this embodiment, symbol "D" denotes the
diameter (mm) of the alloy when the alloy has a columnar shape, and
"D" denotes a diagonal distance (mm) when the alloy has a
quadrangular prism shape, for example. That is, symbol "D" denotes
the maximum value (mm) of a linear distance between an arbitrary
point on the outer edge of the cross section of the alloy and
another arbitrary point on the outer edge, the cross section being
perpendicular to a longitudinal direction of the alloy.
[0158] When the heat-treatment temperature is less than
1100.degree. C., the amount of undissolved chromium carbide or the
like increases, thus lowering creep rupture strength. On the other
hand, when the heat-treatment temperature exceeds 1250.degree. C.,
grain boundaries are dissolved or grains are remarkably coarsened
so that ductility is lowered. Accordingly, it is more desirable to
set the heat-treatment temperature to 1150.degree. C. or above and
1230.degree. C. or below. Further, when the holding time is less
than 1000 D/T (min), undissolved chromium carbide at the center
portion increases so that Cr.sub.PB/Cr.sub.PS falls outside a range
defined by the present invention. On the other hand, when the
holding time exceeds 1400 D/T (min), grain at the outer surface
portion is coarsened so that the austenite grain size number falls
outside the range defined by the present invention.
[0159] Immediately after the alloy is heated and held, the alloy is
cooled with water. This is because when a cooling speed becomes
lower, particularly at the center portion of the alloy, a large
amount of undissolved Cr precipitates is generated at crystal grain
boundaries or within grains so that there is a possibility that the
formula (i) is not satisfied.
[0160] Hereinafter, the present invention is described more
specifically with reference to examples. However, the present
invention is not limited to these examples.
EXAMPLE
[0161] Alloys having the chemical compositions shown in Table 1
were melted in a high-frequency vacuum furnace to prepare ingots
each having an outer diameter of 550 mm, and a weight of 3t.
TABLE-US-00001 TABLE 1 Chemical composition (in mass %, balance: Fe
and impurities) Alloy C Si Mn P S Cr Ni Co W Ti Nb 1 0.075 0.38
1.12 0.008 0.001 21.5 41.3 -- 4.6 0.41 0.73 2 0.043 0.42 0.95 0.006
0.002 25.3 44.2 7.3 8.4 0.22 0.45 3 0.090 0.40 1.07 0.010 0.001
26.8 48.5 -- 6.1 0.15 0.29 4 0.041 0.43 1.24 0.009 0.003 24.6 51.1
-- 5.2 0.17 0.24 5 0.030 0.51 1.06 0.011 0.002 27.5 53.7 -- 4.8
0.25 0.71 6 0.064 0.24 1.57 0.014 0.001 23.4 47.2 -- 6.4 0.47 0.60
7 0.102 0.78 0.59 0.008 0.001 25.6 50.6 -- 5.7 0.19 0.43 8 0.056
0.43 1.25 0.012 0.001 20.9 38.4 -- 4.9 0.20 0.39 9 0.048 0.69 1.68
0.015 0.002 24.7 52.1 -- 6.5 0.34 0.25 A 0.073 0.40 1.08 0.007
0.001 21.7 41.6 -- 4.5 0.45 0.73 B 0.076 0.42 1.10 0.008 0.001 21.4
41.0 -- 4.8 0.41 0.75 C 0.045 0.39 1.05 0.008 0.002 25.0 45.2 7.0
8.2 0.23 0.41 D 0.044 0.40 1.01 0.007 0.002 24.9 44.8 7.1 8.0 0.24
0.42 E 0.044 0.42 0.98 0.007 0.001 25.2 45.7 7.4 8.1 0.24 0.44
Chemical composition (in mass %, balance: Fe and impurities) Alloy
Mo Cu Al N Mg Ca REM V B Others 1 0.08 0.15 0.14 0.031 -- -- -- --
-- -- 2 0.05 0.07 0.03 0.015 -- -- -- -- 0.0051 -- 3 0.06 0.11 0.25
0.026 0.0012 0.002 0.01 0.6 -- Zr: 0.01, Ta: 1.4 4 0.13 0.08 0.09
0.019 -- -- 0.06 -- 0.0063 Hf: 0.3, Re: 1.2 5 0.34 0.21 0.16 0.024
-- -- 0.11 -- -- -- 6 0.07 0.13 0.20 0.018 -- -- -- -- 0.0017 -- 7
0.09 0.10 0.09 0.034 -- -- -- -- -- Zr: 0.05 8 0.11 0.15 0.12 0.072
-- -- -- 0.7 -- -- 9 0.14 0.08 0.17 0.044 -- 0.009 -- -- -- -- A
0.05 0.14 0.10 0.035 -- -- -- -- -- -- B 0.07 0.15 0.11 0.040 -- --
-- -- -- -- C 0.08 0.07 0.05 0.025 -- -- -- -- 0.0050 -- D 0.08
0.08 0.05 0.019 -- -- -- -- 0.0052 -- E 0.07 0.08 0.04 0.018 -- --
-- -- 0.0053 --
[0162] The obtained ingots were processed to have a columnar shape
with an outer diameter of 120 to 480 mm by hot forging, and final
heat treatment was performed under conditions shown in Table 2 to
obtain alloy member specimens. Alloys 1, 2 and 4 were subjected to
forging in a direction substantially perpendicular to the
longitudinal direction after hot forging in the longitudinal
direction and before final heat treatment and, thereafter, final
hot forging was further performed in the longitudinal
direction.
TABLE-US-00002 TABLE 2 Outer Heat-treatment Holding diameter D
temperature T 1000 1400 time Cooling Alloy (mm) (.degree. C.) D/T
D/T (min) method 1 450 1180 381 534 480 water cooling 2 350 1200
292 408 360 water cooling 3 200 1150 174 243 220 water cooling 4
480 1150 417 584 540 water cooling 5 250 1210 207 289 260 water
cooling 6 300 1200 250 350 310 water cooling 7 120 1180 102 142 130
water cooling 8 300 1180 254 356 295 water cooling 9 520 1200 433
607 570 water cooling A 450 1180 381 534 660 ** water cooling B 450
1180 381 534 200 ** water cooling C 350 1070 ** 327 458 340 water
cooling D 350 1270 ** 276 386 340 water cooling E 350 1200 292 408
360 air cooling ** ** indicates that production conditions do not
satisfy those defined by the present invention.
[0163] A test coupon for observing micro-structure was obtained
from the outer surface portion of each specimen, and the cross
section in the longitudinal direction was polished with emery paper
and a buff. Thereafter, the test coupon was etched with a mixed
acid, and optical microscopic observation was performed. The grain
size number on an observation surface was obtained in accordance
with a determination method defined by JIS G 0551 (2013) where the
grain size number is determined based on crossing line segments
(grain size).
[0164] Next, test coupons for measuring the amount of Cr
precipitates were obtained from the center portion and the outer
surface portion of the cross section of each specimen, the cross
section being perpendicular to the longitudinal direction of the
specimen. The surface area of each test coupon was obtained and,
thereafter, only the base metal of the alloy specimen was
completely electrolyzed in a 10% acetylacetone--1% tetramethyl
ammonium chloride--methanol solution under an electrolysis
condition of 20 mA/cm.sup.2. Then, the solution after electrolysis
was performed was filtered through a 0.2 .mu.m filter to extract
precipitates as a residue. Thereafter, extracted residue was
decomposed with an acid, and was subjected to ICP-AES measurement
to measure a content (mass %) of Cr contained as undissolved Cr
precipitate and, then, a value of Cr.sub.PB/Cr.sub.PS was obtained
based on the measured value.
[0165] Tensile test coupons, each having a parallel portion with a
length of 40 mm, were cut out by mechanical processing from the
center portion and the outer surface portion of each specimen
parallel to the longitudinal direction, and a tensile test was
performed on these test coupons at a room temperature so as to
obtain 0.2% proof stress and tensile strength. Further, creep
rupture test coupon, having a parallel portion with a length of 30
mm, was cut out by mechanical processing from the center portion of
each specimen parallel to the longitudinal direction. Then, a creep
rupture test was performed in the atmosphere of 700.degree. C.,
750.degree. C., and 800.degree. C. to obtain 10,000-hour creep
rupture strength at 700.degree. C. by a Larson-Miller parameter
method.
[0166] These results are collectively shown in Table 3.
TABLE-US-00003 TABLE 3 Grain size number at Creep outer surface
rupture Alloy portion Cr.sub.PB/Cr.sub.PS YS.sub.S/YS.sub.B
TS.sub.S/TS.sub.B strength.sup.# 1 -1.1 6.9 1.2 1.0 112 Inventive 2
0.2 3.4 1.3 1.1 128 example 3 2.2 5.8 1.0 1.0 115 4 0.6 7.9 1.2 1.0
118 5 -0.4 6.5 1.2 1.1 116 6 -0.7 5.7 1.3 1.1 119 7 1.2 2.8 1.1 1.0
118 8 1.0 4.4 1.2 1.0 114 9 -1.3 8.7 1.4 1.2 118 A -2.5 * 6.0 .sup.
1.6 * .sup. 1.3 * 110 Comparative B 3.5 4.6 1.1 1.1 92 example C
.sup. 5.7 * 12.6 * 1.2 1.1 93 D -2.8 * 2.4 .sup. 1.6 * .sup. 1.4 *
97 E 0.5 14.8 * 1.3 1.0 95 * indicates that conditions fall outside
the range of the present invention. .sup.#indicates 10,000-hour
creep rupture strengths at 700.degree. C.
[0167] The alloy A and the alloy B have substantially the same
chemical composition as the alloy 1, and are formed into a final
shape same as that of the alloy 1 by hot forging. However, a
holding time in heat treatment falls outside the production
conditions defined by the present invention. Due to such holding
time, the alloy A has the result that the grain size number at the
outer surface portion falls outside the range defined by the
present invention, and a value of YS.sub.S/YS.sub.B and a value of
TS.sub.S/TS.sub.B fall outside the range defined by the present
invention. Accordingly, the alloy A has a large variation in
mechanical characteristics from region to region. The alloy B falls
outside the range defined by the present invention with respect to
creep rupture strength and, as a result, creep rupture strength of
the alloy B is remarkably lower than that of the alloy 1.
[0168] Alloys C, D, and E have substantially the same chemical
composition as the alloy 2, and are formed into a final shape same
as that of the alloy 2 by hot forging. The alloy C is lower than
the range defined by the present invention with respect to the
heat-treatment temperature and hence, the grain size number at the
outer surface portion and a value of Cr.sub.PB/Cr.sub.PS fall
outside the ranges defined by the present invention. As a result,
creep rupture strength of the alloy C is remarkably lower than that
of the alloy 2.
[0169] The alloy D is higher than the range defined by the present
invention with respect to a heat-treatment temperature and hence,
the grain size number at the outer surface portion and a value of
YS.sub.S/YS.sub.B and a value of TS.sub.S/TS.sub.B fall outside the
range defined by the present invention. As a result, creep rupture
strength of the alloy D is remarkably lower than that of the alloy
2.
[0170] With regard to the alloy E, a cooling method in final heat
treatment was not water cooling but was air cooling and hence, a
cooling speed was remarkably low. Accordingly a value of
Cr.sub.PB/Cr.sub.PS falls outside the range defined by the present
invention and, as a result, creep rupture strength of the alloy E
is remarkably lower than that of the alloy 2. On the other hand,
the alloys 1 to 9 which satisfy all specifications of the present
invention have small variation in mechanical characteristics, and
favorable creep rupture strength.
INDUSTRIAL APPLICABILITY
[0171] The austenitic heat resistant alloy of the present invention
has small variation in mechanical properties from region to region,
and is excellent in creep rupture strength at a high temperature.
Accordingly, the austenitic heat resistant alloy of the present
invention is preferably applicable to a large-sized structural
member for a thermal power generation boiler, a chemical plant or
the like which is used in a high temperature environment.
* * * * *