Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
AUSTENITIC HEAT RESISTANT ALLOY AND METHOD FOR PRODUCING THE
SAME
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, SUS31611, 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.
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LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[0005]
Patent Document 1: JP61-179833A
Patent Document 2: JP61-179834A
Patent Document 3: JP61-179835A
Patent Document 4: 1P61-179836A
Patent Document 5: JP2004-3000A
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0006]
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.
[0007]
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.
[0008]
The present invention has been made to overcome the above problems, and an
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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
[0009]
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.
[0010]
(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%;
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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 (iii):
CrpB/Crps10.0 (i)
YSs/YSB...c,1.5 (ii)
TSs/TSB.1.2 (iii)
where meaning of each symbol in the formulas is as follows:
CrpB: amount of Cr which is present at center portion as precipitate obtained
by
extraction residue analysis
Crps: amount of Cr which is present at outer surface portion as precipitate
obtained by extraction residue analysis
YSB: 0.2% proof stress at center portion
YSs: 0.2% proof stress at outer surface portion
TSB: tensile strength at center portion
TSs: tensile strength at outer surface portion.
[0011]
(2) The austenitic heat resistant alloy described in the above (1), wherein
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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%.
[0012]
(3) The austenitic heat resistant alloy described in the above (1) or (2),
wherein
10,000-hour creep rupture strength at 700 C in the longitudinal direction at
the
center portion is 100 MPa or more.
[0013]
(4) A method for producing an austenitic heat resistant alloy, the method
including the steps of:
performing hot working on an ingot or a cast piece having the chemical
composition described in the above (1) or (2); and
thereafter performing heat treatment where the ingot or the cast piece is
heated
to a heat-treatment temperature T ( C) ranging from 1100 to 1250 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.
[0014]
(5) The method for producing an austenitic heat resistant alloy described in
the
above (4), wherein
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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
[0015]
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
[0016]
Hereinafter, the respective requirements of the present invention are
described
in detail.
[0017]
1. Chemical composition
The reasons for limiting respective elements are as follows. In the
description
made hereinafter, symbol "%" for content refers to "mass%".
[0018]
C: 0.02 to 0.12%
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.
[0019]
Si: 2.0% or less
Si (silicon) is contained as a deoxidizing element. Further, Si is an element
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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 a 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.
[0020]
Mn: 3.0% or less
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 a 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.
[0021]
P: 0.030% or less
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.
[0022]
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S: 0.015% or less
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.
[0023]
Cr: 20.0% or more and less than 28.0%
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 a 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.
[0024]
Ni: more than 35.0% and 55.0% or less
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.
[0025]
Co: 0 to 20.0%
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,
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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.
[0026]
W: 4.0 to 10.0%
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 Fe2W Laves phase or a Fe7W6 t 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.
[0027]
Ti: 0.01 to 0.50%
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.
[0028]
Nb: 0.01 to 1.0%
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
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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.
[0029]
Mo: less than 0.50%
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 a 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%.
[0030]
Cu: less than 0.50%
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%.
[0031]
Al: 0.30% or less
Al (aluminum) is an element which is contained as a deoxidizer for molten
steel.
However, when a content ofAl 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.
[0032]
N: less than 0.10%
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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%.
[0033]
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.
[0034]
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.
[0035]
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.
[0036]
Mg: 0.05% or less
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
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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.
[0037]
Ca: 0.05% or less
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.
[0038]
REM: 0.50% or less
REM has an action of fixing S as sulfides, thus improving high temperature
ductility. REM also has an action of improving adhesiveness of a Cr203
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.
[0039]
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REM indicates 17 elements in total, including Sc, Y, and the lanthanoids. The
REM content means the total content of these elements.
[0040]
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.
[0041]
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.
[0042]
V: 1.5% or less
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.
[0043]
B: 0.01% or less
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
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preferably set to 0.0015% or more.
[0044]
Zr: 0.10% or less
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.
[0045]
Hf: 1.0% or less
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.
[0046]
The total content of V, B, Zr, and Hf is preferably 2.6% or less, and more
preferably 1.8% or less.
[0047]
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.
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[0048]
Ta: 8.0% or less
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.
[0049]
Re: 8.0% or less
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.
[0050]
The total content of Ta and Re is preferably 14.0% or less, and more
preferably
12.0% or less.
[0051]
2. Grain size
Austenite grain size number at outer surface portion: -2.0 to 4.0
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
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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 4Ø 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.
[0052]
3. Size
Shortest distance from center portion to outer surface portion: 40 mm or more
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.
[0053]
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.
[0054]
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
16
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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.
[0055]
4. Amount of Cr which is present as precipitate obtained by extraction residue
analysis
CrpB/Crps 10.0 (i)
where meaning of each symbol in the formula (i) is as follows:
Crpa: amount of Cr which is present at center portion as precipitate obtained
by
extraction residue analysis
Crps: amount of Cr which is present at outer surface portion as precipitate
obtained by extraction residue analysis
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 CrpB/Crps 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 CrpB/Crps. However, there is a tendency that the amount of precipitates
increases
more at the center portion than at the outer surface portion and hence,
CrpB/Crps is
preferably set to 1.0 or more.
[0056]
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
17
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completely electrolyzed in a 10% acetylacetone - I% tetramethyl ammonium
chloride -
methanol solution under an electrolysis condition of 20 mAJcm2. Then, the
solution
after electrolysis is performed is filtered through a 0.2 tm 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 CrpB/Crps
is obtained based on the measured value.
[0057]
5. Mechanical properties
YSs/YSB1.5 (ii)
TSs/TSB1.2 (iii)
where meaning of each symbol in the formulas is as follows:
YSB: 0.2% proof stress at center portion
YSs: 0.2% proof stress at outer surface portion
TSB: tensile strength at center portion
TSs: tensile strength at outer surface portion
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.
[0058]
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.
18
CA 03052547 2019-08-02
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[0059]
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).
[0060]
6. Creep rupture strength
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 C in the longitudinal direction is preferably 100 MPa or more at the
center portion
of the heat resistant alloy of the present invention.
[0061]
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 C, 750 C, and 800 C to obtain 10,000-hour
creep
rupture strength at 700 C by a Larson-Miller parameter method. The creep
rupture test
is performed in accordance with JIS Z 2271 (2010).
[0062]
7. Production method
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
19
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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.
[0063]
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.
[0064]
First, the alloy on which hot working was performed is heated to a heat-
treatment
temperature T ( C) ranging from 1100 to 1250 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.
[0065]
When the heat-treatment temperature is less than 1100 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 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 C or above
and 1230 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 CrpB/Crps
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.
[0066]
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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.
[0067]
Hereinafter, the present invention is described more specifically with
reference
to examples. However, the present invention is not limited to these examples.
EXAMPLE
[0068]
Alloys having the chemical compositions shown in Table I were melted in a
high-frequency vacuum furnace to prepare ingots each having an outer diameter
of 550
mm, and a weight of 3t.
[0069]
[Table 1]
21
Table 1
Chemical composition (in mass%, balance: Fe and impurities)
Alloy
-
C Si Mn P S Cr Ni Co W Ti Nb Mo Cu Al N Mg Ca REM V B Others
, I 0.075 0.38 1.12 0.008 0.001 21.5 41.3 - 4.6 0.41
0.73 0.08 0.15 0.14 0.031 - - _ - - -
-
_
2 0.043 0.42 0.95 0.006 0.002 25.3 44.2 7.3 _ 8.4
0.22 0.45 0.05 0.07 0.03 0.015 - - .. - 0.0051
_
_ _ .
3 0.090 0.40 1.07 0.010 0.001 26.8 48.5 -
6.1 0.15 0.29 0.06 0.11 0.25 0.026 0.0012 0.002 0.01 0.6 -
Zr:0.01,Ta:1.4
. . _
4 0.041 0.43 1.24 0.009 0.003 24.6 51.1 - 5.2
0.17 0.24 0.13 0.08 0.09 0.019 - - 0.06 - 0.0063 Hf0.3,Re:1.2
_ _
0.030 0.51 1.06 0.011 0.002 27.5 53.7 - 4.8 0.25 0.71
0.34 0.21 0.16 0.024 - - 0.11 - -
_
_
6 0.064 0.24 1.57 0.014 0.001 23.4 47.2 - 6.4 0.47 0.60
0.07 0.13 0.20 0.018 - - _ - 0.0017 -
_ _
7 0.102 0.78 0.59 0.008 0.001 25.6 50.6 - 5.7 0.19 0.43
0.09 0.10 0.09 0.034 - .. - .. _ .. - .. - .. Zr:0.05
_ .. _
8 0.056 0.43 125 0.012 0 001 20.9 38.4 - 4.9
0.20 0.39 0.11 0.15 0.12 0.072 - - - 0.7
- - P
_ _
9 0.048 0.69 1.68 0.015 0.002 24.7 52.1 -
6.5 0.34 0.25 0.14 0.08 0.17 0.044 - 0.009 - - - - 0
,.,
0
_
_
0,
A 0.073 0.40 1.08 0.007 0.001 21.7 41.6 - 4.5 0.45 0.73
0.05 0.14 0.10 0.035 - - - - - - "
0,
0.
B 0.076 0.42 1.10 0.008 0.001 21.4 41.0 - 4.8 0.41 0.75
0.07 0.15 0.11 0.040 - - - - -
- _ .
0
C 0.045 0.39 1.05 0.008 0.002 25.0 45_2 7.0 8.2 0.23 0.41 0.08 0.07 0.05 0.025
- - - - 0.0050 - 1-
0
. _
1
0
D 0.044 0.40 1.01 0.007 0.002 24.9 44.8 7.1 8.0 0.24 0.42 0.08 0.08 0.05 0.019
- - - 0.0052 - 00
1
_ _ _ _
0
E 0.044 0.42 0.98 0.007 0.001 25.2 45.7 7.4 8.1 0.24 0.44 0.07 0.08 0.04 0.018
- - - - 0.0053 -
o
cp
co
41-
c.rµ
1-.
CA 03052547 2019-08-02
001P3451
[0070]
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.
[0071]
[Table 2]
Table 2
Outer diameter Heat-treatment Holding time
Alloy 1000D/T 1400D/T Cooling method
D (mm) temperature T ( C) (min)
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
. .
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.
[0072]
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
23
CA 03052547 2019-08-02
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JIS G 0551 (2013) where the grain size number is determined based on crossing
line
segments (grain size).
[0073]
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/cm2.
Then, the solution after electrolysis was performed was filtered through a 0.2
pm 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 CrpB/Crps was
obtained based
on the measured value.
[0074]
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 C, 750 C, and 800 C to obtain 10,000-hour creep rupture
strength at
700 C by a Larson-Miller parameter method.
[0075]
These results are collectively shown in Table 3.
[0076]
[Table 3]
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Table 3
Grain size number at Creep rupture
Alloy CrpB/Crps YSs/YSB TSsiTSB
outer surface portion strength
1 -1.1 6.9 1.2 1.0 112
2 0.2 3.4 1.3 1.1 128
-
3 2.2 5.8 1.0 1.0 115
4 0.6 7.9 1.2 1.0 118
Inventive
-0.4 6.5 1.2 1.1 116
example
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 1.6 * 1.3 * 110
B 3.5 4.6 1.1 , 1.1 92
' Comparative
C 57* 12.6* 1.2 1.1 93
example
D -2.8 * 2.4 1.6 * 1.4 * 97
E 0.5 14.8 * _ 1.3 1.0 95
* indicates that conditions fall outside the range of the present invention.
# indicates 10,000-hour creep rupture strengths at 700 C.
[0077]
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 YSs/YSB and a value of TSs/TSB 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.
[0078]
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
CA 03052547 2019-08-02
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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 CrpB/Crps 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.
[0079]
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 YSs/YSa and a value of TSs/TSB 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.
[0080]
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 Crpa/Crps 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
[0081]
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.
26
