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001P4033

Patent Document 4: WO 2013/073055

Patent Document 5: JP2014-1436A Patent Document 6: JP2003-268503A

SUMMARY OF INVENTION TECHNICAL PROBLEM [0005]

When used at high temperature, the austenitic heat resistant steel s disclosed in Patent Documents
1 to 6 may prone to variation in time taken to be ruptured at a certain level of stress, and there
is room for improvement in terms of providing stable creep strength. Even when the stable creep
strength is provided, containing of Nb makes cracking likely to occur in welding; thus the
problem is that it is difficult to establish compatibility between stable creep strength and weld
crack resistance.
[0006]

An objective of the present invention is to solve the problem described above and to provide a
method for producing an austenitic heat resistant steel having a stable, favorable creep strength
and an excellent weld crack resistance in its use at high temperature.

SOLUTION TO PROBLEM [0007]

The present invention is made to solve the problem described above, and the gist of the present
invention is the following method for producing an austenitic heat resistant steel.
[0008]

(1) A method for producing an austenitic heat resistant steel in which a difference between a
content ofNb and an amount ofNb analyzed as extraction residues satisfies Formula (i) shown below,
the method including:
a forming step of machining and forming a steel into a product shape, the steel

including a chemical composition consisting of, in mass%:

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C: 0.04 to 0.12%, Si: 0.01 to 0.30%, Mn: 0.50 to 1.50%, P: 0.001 to 0.040%, S: 0.0050% or less, Cu:
2.2 to 3.8%,
Ni: 8.0 to 11.0%, Cr: 17.7 to 19.3%, Mo: O.Ql to 0.55%, Nb: 0.400 to 0.650%,
B: 0.0010 to 0.0060%, N: 0.050 to 0.160%, Al: 0.025% or less,
0: 0.020% or less, Co: 0 to 1.00%,
W: 0 to 1.00%, Ti: 0 to 0.40%, V: 0 to 0.40%, Ta: 0 to 0.40%,
Sn: 0 to 0.0300%, Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%, and

REM: 0 to 0.0800%,

with the balance: Fe and impurities;

a solution heat treatment step of performing, after the forming step, heat treatment
under conditions including a heat treatment temperature satisfying Formula (ii) shown below and a
soaking time satisfying Formula (iii) shown below; and
a cooling step of performing cooling after the solution heat treatment step:

0.170 :o; Nb - NbER :o; 0.480 (i)

-250Nb + 1200 :o; T :o; -lOONb + 1290 (ii)

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405 - 0.3Tt2475 - 1.5T (iii)

where a symbol of an element in the formulas indicates a content (mass%) of the element contained
in the steel, and symbols in the formulas are defined as follows:
NhER (mass%): amount ofNb analyzed as extraction residues

T (°C): heat treatment temperature t (sec): soaking time.

[0009]

(2) The method for producing an austenitic heat resistant steel according to the

above (1), wherein in the cooling step, the cooling is performed in a form of forced cooling,
and
a difference between the heat treatment temperature and a temperature of the steel at a time of
starting the forced cooling satisfies Formula (iv) shown below:
0L\T lOONb- 5 (iv)

where a symbol of an element in the formula indicates a content (mass%) of the element contained in
the steel, and a symbol in the formula is defined as follows:
LlT (°C): difference between the heat treatment temperature of the solution heat treatment and the
temperature of the steel at the time of starting the forced cooling. [0010]

(3) The method for producing an austenitic heat resistant steel according to the above (1) or (2),
wherein a difference between a content ofNb and an amount ofNb analyzed as extraction residues
satisfies Formula (v) shown below:
-2B + 0.185Nb - NhER-4B + 0.480 (v)

where symbols of elements in the formula means the contents (mass%) of the elements contained in
the steel, and NhER means the amount ofNb (mass%) analyzed as extraction residues.
[0011]

(4) The method for producing an austenitic heat resistant steel according to any one of the above
(1) to (3), wherein the chemical composition contains one or more elements selected from, in
mass%:
Co: 0.01 to 1.00%,

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[0012]

W: O.Ql to 1.00%, Ti: 0.01 to 0.40%, V: 0.01 to 0.40%, Ta: 0.01 to 0.40%,
Sn: 0.0002 to 0.0300%, Ca: 0.0002 to 0.0100%,
Mg: 0.0002 to 0.0100%, and

REM: 0.0005 to 0.0800%.

(5) The method for producing an austenitic heat resistant steel according to any

one of the above (1) to (4), wherein the chemical composition contains, in mass%, P: 0.020 to
0.040%.

ADVANTAGEOUS EFFECTS OF INVENTION [0013]

According to the present invention, it is possible to provide an austenitic heat resistant steel
having a stable, favorable creep strength and an excellent weld crack resistance in its use at high
temperature.

BRIEF DESCRIPTION OF DRAWING [0014]
[Figure 1]

Figure 1 is a diagram illustrating a bevel shape in EXAMPLE.

DESCRIPTION OF EMBODIMENTS [0015]

The present inventors conducted various studies for improving a high temperature strength,
specifically a creep strength, and a weld crack resistance of an austenitic heat resistant steel
containing Nb and N, and obtained the following fmdings (a) to (c).

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[0016]

(a) It was found that a creep strength of steel is strongly influenced by solution

heat treatment performed after a final forming step, and even the same chemical
composition results in different creep strengths under different conditions of the solution heat
treatment. To be specific, stable provision of a high creep strength was associated with an
increase in the heat treatment temperature or the soaking time.
[0017]

When a high creep strength was stably provided as described above, a precipitation
amount of carbo-nitride and nitride containing Nb was small in a state before use, that is, after
the solution heat treatment and cooling. In addition, in the use of the steel where the steel
is exposed to high temperature, the carbo-nitride and nitride containing Nb precipitated
in grains finely and densely and were present stably for a long time.
[0018]

In contrast, in a case of a poor creep strength, the precipitation amount of the carbo-nitride and
nitride containing Nb observed before use was large. Inaddition, with use, the carbo-nitride
and nitride containing Nb that precipitated finely in grains were small in amount and coarsened
at an early stage. Further, a steel having a small content ofNb is required to be subjected to
the solution heat treatment at a higher temperature so as to provide a high creep strength stably.
For those reasons, in order to provide stable creep strength, it is desirable to dissolve Nb in a
matrix adequately in advance and to cause precipitates containing Nb to precipitate at a usage
stage.
[0019]

(b) As for the weld crack resistance, it was observed that cracking susceptibility tended to be
increased with an increase in the heat treatment temperature or the soaking time. Additionally,
weld cracks occur at crystal grain boundaries in a weld heat affected zone, and on a crack fracture
surface, traces of local melting of grain boundaries were recognized, where concentration of Nb
occurred. Further, a steel containing a large amount ofNb caused weld cracks in some cases
even when the steel was subjected to the solution heat treatment at a lower temperature.

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[0020]

When Nb dissolved in a matrix (hereinafter, simply referred to as "dissolved

Nb") is compared with Nb present in a form of its precipitate, the dissolved Nb has a greater
influence on weld crack susceptibility. The reason for this is that the dissolved Nb needs no
time for being dissolved in a matrix through weld thermal cycles, and the Nb is likely to be
concentrated at grain boundaries. As a result, weld cracks are likely to occur.
[0021]

(c) In view of the above, it is found that the amount of dissolved Nb has a major influence on both
properties of creep strength and weld crack resistance. It is therefore necessary to
appropriately control the amount of dissolved Nb and conditions of the solution heat treatment
that influences the amount of dissolved Nb so that compatibility between stable, high creep
strength and favorable weld crack properties are established. The amount of dissolved Nb is also
influenced by a content of B, and it is therefore preferable to control their ranges appropriately.
In controlling the amount of dissolved Nb, it is naturally desirable to adjust a content of Nb
within its appropriate range, and it is additionally desirable to manage a heat treatment
temperature and a soaking time in the solution heat treatment appropriately based on the content
ofNb.
[0022]

The present invention is made based on the fmdings described above and is a method for producing an
austenitic heat resistant steel in which a difference between a content ofNb and an amount ofNb
analyzed as extraction residues satisfies Formula (i) described later. Requirements of the
present invention will be described below.
[0023]

1. Forming step

1-1. Forming method

In the production method according to the present invention, a steel having a chemical
composition described below is machined and formed into a final shape, that is, a product shape in
a forming step. A method for the machining and formation is not limited to a particular
method; the method may be casting for the formation using a mold

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or may be plastic working. In a case where the plastic working is employed for the formation, hot
rolling, hot forging, cold rolling, cold forging, cold drawing or the like is conceivable for
example, and a working temperature may be within any temperature range such as a hot
temperature range, a cold temperature range, and a warm temperature range. Note that heat
treatment and pickling may be performed in a forming step as necessary.
[0024]

The product shape provided by the machining and formation is not limited to a particular shape,
either. Examples of a conceivable product shape include a plate shape, a tubular shape, a bar
shape, a wire shape, an H shape, and an I shape, and in addition, a peculiar shape provided by
using a mold.
[0025]

1-2. Chemical composition of steel

In the production method according to the present invention, it is also important to control a
chemical composition of a steel to be machined and formed. The chemical composition of the steel
being a starting material to be subjected to plastic forming is controlled by, for example, melting
and refining a starting material having predetermined chemical components. The starting material
is typically produced into ingots or blooms by casting. The ingots or blooms are then subjected
to plastic working such as hot rolling and cold rolling.
[0026]

A chemical composition of a steel to be a starting material for working will be described below.
In the description, the symbol"%" for contents means "mass%" unless otherwise noted.
[0027]

C: 0.04 to 0.12%

C (carbon) stabilizes an austenitic structure and forms fme carbides, improving creep strength in
high-temperature use of the austenitic heat resistant steel. A content of C is thus set to 0.04%
or more. The content of C is preferably set to 0.06% or more, and more preferably set to 0.07%
or more. However, if C is contained excessively, the effect

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of C is saturated, and carbides precipitate in a large quantity, decreasing creep ductility. The
content of C is thus set to 0.12% or less. The content of C is preferably set to 0.10% or less,
and more preferably set to 0.09% or less.
[0028]

Si: 0.01 to 0.30%

Si (silicon) has a deoxidation effect in the process of production. Further, Si is an element
useful in improving corrosion resistance and oxidation resistance at high temperature. A
content of Si is thus set to 0.01% or more. The content of Si is preferably set to 0.03%
or more, and more preferably set to 0.05% or more. However, if Si is contained excessively,
stability of an austenitic structure is decreased, leading to a decrease in toughness and creep
strength. The content of Si is thus set to 0.30% or less. The content of Si is preferably set to
0.28% or less, and more preferably set to 0.25% or less.
[0029]

Mn: 0.50 to 1.50%

As with Si, Mn (manganese) has a deoxidation effect. In addition, Mn stabilizes an
austenitic structure, contributing to improvement of creep strength. A content ofMn is
thus set to 0.50% or more. The content ofMn is preferably set to 0.60% or more, and more
preferably set to 0.70% or more. However, if Mn is contained excessively, the excessively
contained Mn leads to embrittlement, further resulting in a decrease in creep ductility. The
content ofMn is thus set to 1.50% or less. The content ofMn is preferably set to 1.30% or less,
and more preferably set to 1.00% or less.
[0030]

P: 0.001 to 0.040%

P (phosphorus) is an element contained in the steel as an impurity, and if P is reduced
excessively, production costs are increased; therefore, a content of P is thus set to 0.001% or
more. The content ofP is preferably set to 0.005% or more, and more preferably set to 0.010%
or more. Further, P has an effect of increasing creep strength. To exert the effect, the content
of P is preferably set to 0.020% or more.
[0031]

9

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[0035]

Cr: 17.7 to 19.3%

Cr (chromium) contributes to improvement of oxidation resistance and corrosion

resistance at high temperature. Cr also forms its fine carbide, contributing to ensuring of
creep strength. A content of Cr is thus set to 17.7% or more. The content of Cr is preferably
set to 18.0% or more, and more preferably set to 18.2% or more. However, if Cr is contained
excessively, stability of an austenitic structure is impaired, decreasing creep strength. The
content of Cr is thus set to 19.3% or less. The content of Cr is preferably set to 19.0% or
less, and more preferably setto 18.8% or less.
[0036]

Mo: 0.01 to 0.55%

Mo (molybdenum) is dissolved in a matrix, contributing to improvement of creep strength and
tensile strength at high temperature. A content of Mo is thus set to
0.01% or more. The content of Mo is preferably set to 0.03% or more, and more preferably
set to 0.05% or more. However, ifMo is contained excessively, the effect of Mo is saturated.
Further, stability of an austenitic structure is impaired, rather resulting in a decrease in creep
strength. Further, since Mo is an expensive element, excessive containing ofMo leads to a rise
in costs. The content ofMo is thus set to 0.55% or less. The content ofMo is preferably set to
0.53% or less, more preferably set to 0.50% or less, and still more preferably set to 0.40% or
less.
[0037]

Nb: 0.400 to 0.650%

Nb (niobium) precipitates in a form of its fine carbo-nitride and fme nitride, contributing to
improvement of creep strength. A content ofNb is thus set to 0.400% or more. The content ofNb
is preferably set to 0.420% or more, and more preferably set to 0.450% or more. However, ifNb is
contained excessively, the excessively contained Nb leads to occurrence of weld cracks at a weld
heat affected zone in welding. In addition, carbo-nitride and nitride of Nb precipitate
in a large quantity, decreasing ductility of the material. The content ofNb is thus set to
0.650% or less. The content ofNb is preferably set to 0.630% or less, and more preferably set to
0.600% or less.

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[0038]

Note that the content ofNb means the amount ofNb contained in the austenitic

heat resistant steel. That is, the content ofNb means a total of an amount of dissolved Nb and
an amount ofNb present in a form of its precipitate. For the production method according to the
present invention, an amount of dissolved Nb, namely, a difference between the content of Nb
and an amount of Nb that is analyzed as extraction residues (an amount of Nb present in a form of
its precipitate) is controlled as a predetermined range of the content ofNb.
[0039]

B: 0.0010 to 0.0060%

B (boron) has an effect of fmely dispersing grain boundary carbides, improving creep strength. A
content of B is thus set to 0.0010% or more. The content of B is preferably set to 0.0020%
or more, and more preferably set to 0.0030% or more. However, if B is contained
excessively, crack susceptibility of a weld heat affected zone in welding is increased. The
content ofB is thus set to 0.0060% or less. The content ofB is preferably set to 0.0055% or
less, and more preferably set to 0.0050% or less. [0040]

N: 0.050 to 0.160%

N (nitrogen) stabilizes an austenitic structure and is dissolved or precipitates in a form of
nitrides, contributing to improvement of creep strength. A content ofN is thus set to 0.050% or
more. The content ofN is preferably set to 0.070% or more, and more preferably set to 0.090% or
more. However, ifN is contained excessively, fine nitrides precipitate in a large quantity,
leading to a decrease in creep ductility and toughness. The content of N is thus set to 0.160%
or less. The content of N is preferably set to
0.140% or less, and more preferably set to 0.120% or less. [0041]

AI: 0.025% or less

AI (aluminum) has a deoxidation effect. However, if AI is contained excessively,
cleanliness of the steel deteriorates, and hot workability is decreased. A content of AI is
thus set to 0.025% or less. The content of AI is preferably set to 0.023%

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or less, more preferably set to 0.020% or less, and still more preferably set to 0.0017% or less.
On the other hand, an extreme reduction of A! leads to a rise in steel making costs and in
addition fails to provide the effect. The content of AI is thus preferably set to
0.001% or more, and more preferably set to 0.002% or more. [0042]

0: 0.020% or less

0 (oxygen) is contained in the steel as an impurity, and if 0 is contained excessively,
hot workability is decreased. In addition, toughness and ductility are impaired. A
content of 0 is thus set to 0.020% or less. The content of 0 is preferably set to 0.018% or
less, and more preferably set to 0.015% or less. Note that no particular lower limit will be
imposed on the content of 0, but an extreme reduction of the content of 0 results in a rise in
production costs. The content of 0 is thus preferably set to
0.001% or more, and more preferably set to 0.002% or more. [0043]

In the chemical composition, in addition to the elements described above, one or more elements
selected from Co, W, Ti, V, Ta, Sn, Ca, Mg, and REM may be contained within their respective ranges
described below. Reasons for limiting a content of each element will be described.
[0044]

Co: 0 to 1.00%

As with Ni, Co (cobalt) has an effect of stabilizing an austenitic structure, contributing to
improvement of creep strength. Thus, it may be contained as necessary. However, Co is a very
expensive element, and if Co is contained excessively, costs rise. A content of Co is thus set to
1.00% or less. The content of Co is preferably set to 0.90% or less, and more preferably set to
0.80% or less. On the other hand, to exert the effect, the content of Co is preferably set to
0.01% or more, and more preferably set to 0.03% or more.
[0045]

W: Oto 1.00%

W (tungsten) has an effect of improving creep strength at high temperature by

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being dissolved in a matrix or forming its fme intermetallic compound phases. Thus, it may be
contained as necessary. However, even ifW is contained excessively, the effect is saturated,
and stability of an austenitic structure is impaired, rather resulting in a decrease in
creep strength. Further, since W is an expensive element, excessive containing ofW
results in a rise in production costs. A content ofW is thus set to 1.00% or less. The
content of W is preferably set to 0.90% or less, and more preferably set to
0.80% or less. On the other hand, to exert the effect, the content of W is preferably set to
0.01% or more, and more preferably set to 0.03% or more.
[0046]

Ti: 0 to 0.40%

Ti (titanium) combines with carbon and nitrogen to form its fine carbide and fme carbo-nitride,
having an effect of improving creep strength at high temperature. Thus, it may be contained as
necessary. However, ifTi is contained excessively, its precipitate precipitates in a large
quantity, leading to a decrease in creep ductility and toughness. A content ofTi is thus set to
0.40% or less. The content ofTi is preferably set to 0.35% or less, and more preferably set to
0.30% or less. On the other hand, to exert the effect, the content of Ti is preferably set to
0.01% or more, and more preferably set to 0.02% or more.
[0047]

V: Oto 0.40%

As with Ti, V (vanadium) forms its fme carbide and fine carbo-nitride, having an effect of
improving creep strength at high temperature. Thus, it may be contained as necessary.
However, if V is contained excessively, its precipitate precipitates in a large quantity, leading
to a decrease in creep ductility and toughness. A content of V is thus set to 0.40% or less.
The content of V is preferably set to 0.35% or less, and more preferably set to 0.30% or less.
On the other hand, to exert the effect, the content of V is preferably set to 0.01% or more, and
more preferably set to 0.02% or more.
[0048]

Ta: 0 to 0.40%

As with Ti and V, Ta (tantalum) forms its fine carbide and fme carbo-nitride,

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having an effect of improving creep strength at high temperature. Thus, it may be contained
as necessary. However, if Ta is contained excessively, its precipitate precipitates
in a large quantity, leading to a decrease in creep ductility and toughness. A content ofTa is
thus set to 0.40% or less. The content ofTa is preferably set to 0.35% or less, and more
preferably set to 0.30% or less. On the other hand, to exert the effect, the content ofTa is
preferably set to 0.01% or more, and more preferably set to 0.02% or more.
[0049]

Sn: 0 to 0.0300%

Sn (tin) has an effect of increasing weldabillity considerably. Thus, it may be contained as
necessary. However, if Sn is contained excessively, crack susceptibility of a weld heat affected
zone in welding is increased, and hot workability in the process of production is impaired. A
content of Sn is thus set to 0.0300% or less. The content of Sn is preferably set to 0.0250% or
less, and more preferably set to 0.0200% or less. On the other hand, to exert the effects, the
content of Sn is preferably set to 0.0002% or more, and more preferably set to 0.0005% or more.
[0050]

Ca: 0 to 0.0100%

Ca (calcium) has an effect of improving hot workability. Thus, it may be contained as
necessary. However, if Ca is contained excessively, Ca combines with oxygen, which decreases
cleanliness significantly, rather impairing hot workability. A content of Ca is thus set to
0.0100% or less. The content of Ca is preferably set to
0.0080% or less, and more preferably set to 0.0060% or less. On the other hand, to exert

the effects, the content of Ca is preferably set to 0.0002% or more, and more preferably set to
0.0005% or more.
[0051]

Mg: 0 to 0.0100%

As with Ca, Mg (magnesium) has an effect of improving hot workability. Thus, it may be
contained as necessary. However, if Mg is contained excessively, Mg combines with
oxygen, which decreases cleanliness significantly. As a result, hot

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workability is rather decreased. A content ofMg is thus set to 0.0100% or less. The content
ofMg is preferably set to 0.0080% or less, and more preferably set to 0.0060% or less. On the
other hand, to exert the effects, the content of Mg is preferably set to
0.0002% or more, and more preferably set to 0.0005% or more. [0052]

REM: 0 to 0.0800%

As with Ca and Mg, REM has an effect of improving hot workability in the process of production.
Thus, it may be contained as necessary. However, if REM is contained excessively, REM combines
with oxygen, which decreases cleanliness significantly. As a result, hot workability is rather
decreased. A content of REM is thus set to 0.0800% or less. The content of REM is preferably
set to 0.0600% or less, and more preferably set to 0.0500% or less. On the other hand, to exert
the effect, the content of REM is preferably set to 0.0005% or more, and more preferably set to
0.0010% or more.
[0053]

REM refers to Sc (scandium), Y (yttrium), and lanthanoids, 17 elements in total, and the content of
REM means a total content of these elements. Industrially, REM is often added in a form of misch
metal.
[0054]

In the chemical composition according to the present invention, the balance is Fe and impurities.
The term "impurities" herein means components that are mixed in steel in producing the steel
industrially from raw materials such as ores and scraps and due to various factors in the producing
process, and are allowed to be mixed in the steel within their respective ranges in which the
impurities have no adverse effect on the present invention.
[0055]

2. Solution heat treatment step

OfNb contained in the steel, Nb that is present in a form of its precipitate before the use of the
austenitic heat resistant steel contributes to creep strength, but the effect provided by the
contribution is minor. In contrast, dissolved Nb precipitates fmely and

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densely in grains in a form of its carbo-nitride or its nitride in use of the austenitic heat
resistant steel at high temperature for a long time, greatly contributing to improvement of creep
strength and stabilization of the improved creep strength.
[0056]

In addition, even when the dissolved Nb is adequate, if a heat treatment temperature in the
solution heat treatment is too low, or a heat treatment temperature in the solution heat treatment
is too short, residual strain induced in prior steps is not released sufficiently, and the
carbo-nitride or the nitride coarsens in an early stage in use of the austenitic heat resistant
steel at high temperature, and the precipitation strengthening effect of the dissolved Nb
disappears in an early stage. As a result, creep strength is less likely to be stable.

CLAIMS
[Claim 1]
A method for producing an austenitic heat resistant steel in which a difference
between a content ofNb and an amount ofNb analyzed as extraction residues satisfies
Formula (i) shown below, the method comprising:
a forming step of machining and forming a steel into a product shape, the steel
including a chemical composition consisting of, in mass%:
C: 0.04 to 0.12%,
Si: 0.01 to 0.30%,
Mn: 0.50 to 1.50%,
P: 0.001 to 0.040%,
S: 0.0050% or less,
Cu: 2.2 to 3.8%,
Ni: 8.0 to 11.0%,
Cr: 17.7 to 19.3%,
Mo: O.Ql to 0.55%,
Nb: 0.400 to 0.650%,
B: 0.0010 to 0.0060%,
N: 0.050 to 0.160%,
AI: 0.025% or less,
0: 0.020% or less,
Co: 0 to 1.00%,
W: 0 to 1.00%,
Ti: 0 to 0.40%,
V: 0 to 0.40%,
Ta: 0 to 0.40%,
Sn: 0 to 0.0300%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%, and
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REM: 0 to 0.0800%,
with the balance: Fe and impurities;
a solution heat treatment step of performing, after the forming step, heat
treatment under conditions including a heat treatment temperature satisfying Formula (ii)
shown below and a soaking time satisfying Formula (iii) shown below; and
a cooling step of performing cooling after the solution heat treatment step:
0.170 ~ Nb - NbER ~ 0.480 (i)
-250Nb + 1200 ~ T ~ -lOONb + 1290 (ii)
405 - 0.3T ~ t ~ 2475 - l.ST (iii)
where a symbol of an element in the formulas indicates a content (mass%) of the
element contained in the steel, and symbols in the formulas are defined as follows:
NhER (mass%): amount ofNb analyzed as extraction residues
T (°C): heat treatment temperature
t (sec): soaking time
[Claim 2]
The method for producing an austenitic heat resistant steel according to claim 1,
wherein
in the cooling step, the cooling is performed in a form of forced cooling, and
a difference between the heat treatment temperature and a temperature of the
steel at a time of starting the forced cooling satisfies Formula (iv) shown below:
O~L\T~lOONb-5 (iv)
where a symbol of an element in the formula indicates a content (mass%) of the
element contained in the steel, and a symbol in the formula is defined as follows:
Ll T (°C): difference between the heat treatment temperature of the solution heat
treatment and the temperature of the steel at the time of starting the forced cooling.
[Claim 3]
The method for producing an austenitic heat resistant steel according to claim 1
or 2, wherein a difference between a content of Nb and an amount of Nb analyzed as
extraction residues satisfies Formula (v) shown below:
-2B + 0.185 ~ Nb- NhER ~ -4B + 0.480 (v)
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001P4033
where symbols of elements in the formula mean the contents (mass%) of the
elements contained in the steel, and NbER means the amount ofNb (mass%) analyzed as
extraction residues.
[Claim 4]
The method for producing an austenitic heat resistant steel according to any one
of claims 1 to 3, wherein the chemical composition contains one or more elements
selected from, in mass%:
Co: 0.01 to 1.00%,
W: O.Ql to 1.00%,
Ti: 0.01 to 0.40%,
V: 0.01 to 0.40%,
Ta: 0.01 to 0.40%,
Sn: 0.0002 to 0.0300%,
Ca: 0.0002 to 0.0100%,
Mg: 0.0002 to 0.0100%, and
REM: 0.0005 to 0.0800%.
[Claim 5]
The method for producing an austenitic heat resistant steel according to any one
of claims 1 to 4, wherein the chemical composition contains, in mass%,
P: 0.020 to 0.040%.