TECHNICAL FIELD
[0001]
The present invention relates to an austenitic heat resistant steel.
BACKGROUND ART
[0002]
In a thermal power plant, operating conditions of a power generation boiler have
been increasingly higher in terms of temperature and pressure so as to increase power
generation efficiency from a viewpoint of reduction of environmental loads. Thus,
materials of a superheater tube, a reheater tube, and the like used in the power generation
boiler are required to have properties such as corrosion resistance, as well as excellent
high temperature strength.
[0003]
Hence, austenitic heat resisting alloys that are made to contain a large amount of
Nb and N have been developed as materials having favorable high temperature strengths.
For example, Patent Documents 1 to 6 each disclose an austenitic steel that is made to
contain predetermined amounts of Nb and N to have an improved high temperature
strength.
LIST OF PRIOR ART DOCUMENTS
PATENT DOCUMENT
[0004]
Patent Document 1: JP62-133048A
Patent Document 2: JP2000-256803A
Patent Document 3: JP2003-268503A
Patent Document 4: WO 2009/044796
2
Patent Document 5: WO 2013/073055
Patent Document 6: JP2014-1436A
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[0005]
When used at high temperature, the austenitic heat resistant steels 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 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 austenitic heat resistant steel.
[0008]
(1) An austenitic heat resistant 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: less than 0.0050%,
Cu: 2.2 to 3.8%,
3
Ni: 8.0 to 11.0%,
Cr: 17.7 to 19.3%,
Mo: O.ol 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, wherein
a difference between a content ofNb and an amount ofNb analyzed as extraction
residues satisfies Formula (i) shown below;
0.170 ~ Nb - NbER ~ 0.480 (i)
where Nb in the formula means the content ofNb (mass%) contained in the steel,
and NhER means the amount ofNb (mass%) analyzed as extraction residues.
[0009]
(2) The austenitic heat resistant steel according to the above (1 ), wherein
Formula (ii) shown below is satisfied;
-2B + 0.185 ~ Nb- NbER ~ -4B + 0.480 (ii)
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.
4
[0010]
(3) The austenitic heat resistant steel according to the above (1) or (2), wherein
the chemical composition contains one or more elements selected from, in mass%:
[0011]
Co: 0.01 to 1.00%,
W: 0.01 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%.
(4) The austenitic heat resistant steel according to any one of the above (1) to (3),
wherein Formula (iii) shown below is satisfied;
0.08P- 2B + 0.200 ~ Nb- NhER ~ -0.4P- 4B + 0.450 (iii)
where symbols of elements in the formula mean the contents (mass%) of the
elements contained in the steel, and NhER means the amount ofNb (mass%) analyzed as
extraction residues.
[0012]
(5) The austenitic heat resistant steel according to any one of the above (1) to (4),
wherein the chemical composition contains, in mass%, P: 0.010 to 0.040%.
[0013]
(6) The austenitic heat resistant steel according to any one of the above (1) to (5),
wherein the chemical composition contains, in mass%, P: 0.020 to 0.038%.
ADVANTAGEOUS EFFECTS OF INVENTION
[0014]
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
5
resistance in its use at high temperature.
BRIEF DESCRIPTION OF DRAWING
[0015]
[Figure 1]
Figure 1 is a diagram illustrating a bevel shape in EXAMPLE.
DESCRIPTION OF EMBODIMENTS
[0016]
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).
[0017]
(a) In a steel that can exert a high creep strength, a precipitation amount of carbonitride
and nitride containing Nb was small before its use. 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]
Further, in a case where excellent creep strength was exerted more stably, carbide
containing Cr precipitated at grain boundaries fmely in the use of the steel, and a large
amount of B was dissolved in the carbide. It could be additionally confirmed that, in a
case where a large amount of P was contained, the creep strength of the steel tends to be
further increased.
[0019]
In contrast, a steel having a poor creep strength included a large precipitation
amount of carbo-nitride and nitride containing Nb before its use. Further, with use, the
carbo-nitride and nitride containing Nb that precipitated fmely in grains were small in
amount and coarsened at an early stage.
6
[0020]
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. In addition, it is preferable to adjust an amount of Nb
dissolved in a matrix (hereinafter, simply referred to as "dissolved Nb") depending on a
content of B, which has the same effect as Nb.
[0021]
(b) However, in a case where Nb and B were contained in a large amount, fine
cracks occurred in some cases at crystal grain boundaries in a weld heat affected zone.
This tendency was prominent in a case where P was contained in a large amount, and on
a crack fracture surface, traces of local melting of grain boundaries were recognized,
where concentration of Nb and/or P occurred. This was considered because these
elements were caused to segregate in grain boundaries through weld thermal cycles,
decreasing a melting point, by which the grain boundaries were locally melted.
[0022]
When the 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.
[0023]
(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 control the amount of dissolved Nb within its appropriate range so that
compatibility between stable, high creep strength and favorable weld crack resistance are
established. Likewise, since B and P have an influence on both creep strength and weld
crack resistance, the amount of dissolved Nb is desirably adjusted depending on a content
ofB, and more desirably adjusted depending on contents ofB and P.
[0024]
The present invention is made based on the fmdings described above.
7
Requirements of the present invention will be described below in detail.
[0025]
1. Chemical composition
Reasons for limiting a content of each element are as follows. In the following
description, the symbol "%" for contents means "mass%".
[0026]
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
Cis thus set to 0.04% or more. The content ofC is preferably set to 0.06% or more, and
more preferably set to 0.07% or more. However, ifC is contained excessively, the effect
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.
[0027]
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, more preferably set to 0.05% or more, and still more
preferably set to 0.10% 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, more preferably set to 0.25% or less, and still more preferably set to 0.20%
or less.
[0028]
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%
8
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
of Mn is preferably set to 1.30% or less, and more preferably set to 1.00% or less.
[0029]
P: 0.001 to 0.040%
P (phosphorus) is an element contained in the steel as an impurity but has an
effect of increasing creep strength. This would be because P has an influence on solidsolution
strengthening or a precipitation condition. A content ofP is thus set to 0.001%
or more. The content ofP is preferably set to 0.005% or more, more preferably set to
0.010% or more, and still more preferably set to 0.020% or more.
[0030]
However, if P is contained excessively, crack susceptibility of a weld heat
affected zone in welding is increased. The content of P is thus set to 0.040% or less.
The content ofP is preferably set to 0.038% or less, and more preferably set to 0.035%
or less.
[0031]
S: less than 0.0050%
As with P, S (sulfur) is contained in the steel as an impurity and increases crack
susceptibility of a weld heat affected zone in welding. A content of S is thus set to less
than 0.0050%. The content of S is preferably set to less than 0.0020%, more preferably
set to 0.0018% or less, and still more preferably set to 0.0015% or less. Note that the
content of S is preferably reduced as much as possible, but an extreme reduction of the
content of S leads to a rise in steel making costs. The content of S is thus preferably set
to 0.0001% or more, and preferably 0.0002% or more.
[0032]
Cu: 2.2 to 3.8%
Cu (copper) increases stability of an austenitic structure and finely precipitates
in use of the austenitic heat resistant steel, contributing to improvement of creep strength.
A content of Cu is thus set to 2.2% or more. The content of Cu is preferably set to 2.5%
9
or more, and more preferably set to 2. 7% or more. However, if Cu is contained
excessively, hot workability is decreased. The content of Cu is thus set to 3.8% or less.
The content of Cu is preferably set to 3.5% or less, and more preferably set to 3.3% or
less.
[0033]
Ni: 8.0 to 11.0%
Ni (nickel) stabilizes an austenitic structure, contributing to improvement of
creep strength. A content of Ni is thus set to 8.0% or more. The content of Ni is
preferably set to 8.2% or more, and more preferably set to 8.5% or more. However,
since Ni is an expensive element, a high content of Ni leads to a rise in costs and also
increases stability of austenite, decreasing weldability. The content ofNi is thus set to
11.0% or less. The content ofNi is preferably set to 10.8% or less, and more preferably
set to 10.5% orless.
[0034]
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,
ifCr is contained excessively, stability of an austenitic structure is impaired, which causes
production of sigma phases, decreasing creep strength. The content of Cr is thus set to
19.3% or less. The content ofCr is preferably set to 19.0% or less, and more preferably
set to 18.8% orless.
[0035]
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
10
in a decrease in creep strength. In addition, 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.
[0036]
Nb: 0.400 to 0.650%
Nb (niobium) precipitates in a form of its fme 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.
[0037]
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 steel according to
the present invention, an amount of dissolved Nb, namely, a difference between the
content ofNb and an amount ofNb 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 of Nb. Further, it is preferable to set the amount of dissolved Nb within a
predetermined range depending on the content ofB or depending on the content ofB and
P.
[0038]
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 0.00 I 0% 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
11
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.
[0039]
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, fme 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.
[0040]
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%
or less, and more preferably set to 0.020% or less. On the other hand, an extreme
reduction of AI 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.
[0041]
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.
[0042]
12
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.
[0043]
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, production
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.
[0044]
W: Oto 1.00%
W (tungsten) has an effect of improving creep strength at high temperature by
being dissolved in a matrix or forming its fine intermetallic compound phases. Thus, it
may be contained as necessary. However, even if W 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 ofW 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.
[0045]
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
13
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.
[0046]
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.
[0047]
Ta: 0 to 0.40%
As with Ti and V, Ta (tantalum) forms its fme carbide and fme carbo-nitride,
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.
[0048]
Sn: 0 to 0.0300%
Sn (tin) has an effect of increasing weldability 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
14
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.
[0049]
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.
[0050]
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
workability is rather decreased. A content of Mg is thus set to 0.0100% or less. The
content of Mg 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.
[0051]
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.
15
[0052]
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 ofmisch metal.
[0053]
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.
[0054]
2. Amount of dissolved Nb
OfNb contained in the austenitic heat resistant steel, Nb that is present in a form
of its precipitate before the use of the austenitic heat resistant steel contributes to
improvement of creep strength, but the effect provided by the contribution is minor. In
contrast, dissolved Nb precipitates finely and densely in grains in a form of its carbonitride
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.
[0055]
To provide a high creep strength stably, it is effective to keep an adequate amount
of dissolved Nb, that is, difference between an amount ofNb contained in the steel (the
content ofNb) and an amount ofNb present in a form of its precipitate before use of the
austenitic heat resistant steel (i.e., an amount ofNb analyzed as residues).
[0056]
In contrast, Nb contained in the steel segregates at crystal grain boundaries in a
weld heat affected zone through weld thermal cycles in welding. Nb lowers a solidus
temperature of steel, and thus the crystal grain boundaries at which Nb segregates are
locally melted, causing weld cracks. Compared with Nb present in the steel in a form
16
17
a result, the carbo-nitride and nitride containing Nb do not precipitate fmely in grains at
high-temperature use of the austenitic heat resistant steel. In addition, these precipitates
coarsen at an early stage. This results in a failure to improve creep strength. The
amount of dissolved Nb is therefore set to 0.170% or more. The amount of dissolved
Nb is preferably set to 0.180% or more, more preferably set to 0.185% or more, and still
more preferably set to 0.190% or more.
[0062]
However, if the amount of dissolved Nb is more than 0.480%, weld crack
susceptibility of a weld heat affected zone is further increased in welding. The amount
of dissolved Nb is therefore set to 0.480% or less. The amount of dissolved Nb is
preferably set to 0.460% or less, more preferably set to 0.440% or less, and still more
preferably set to 0.400% or less.
[0063]
Further, as described above, it is preferable to adjust the amount of dissolved Nb,
with consideration given to the effect ofB of increasing creep strength by being dissolved
in the Cr carbide and fmely precipitating at grain boundaries. Specifically, it is
preferable that the amount of dissolved Nb satisfy Formula (ii) shown below.
[0064]
-2B + 0.185 ~ Nb- NhER ~ -4B + 0.480 (ii)
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.
[0065]
This is because when the amount of dissolved Nb is not less than the left side
value of Formula (ii), after the amount of dissolved Nb is ensured, the Cr carbide in which
B is dissolved fmely precipitates at grain boundaries, further improving creep strength.
At the same time, this is because when the amount of dissolved Nb is not more than the
right side value of Formula (ii), weld crack resistance can be improved.
[0066]
In addition, as described above, while it is considered that P influences solid18
solution strengthening and a precipitation condition, improving creep strength, P lowers
a solidus temperature and segregates at grain boundaries in a weld heat affected zone
during welding, increasing weld crack susceptibility, as with Band Nb. For that reason,
it is still more preferable to adjust the amount of dissolved Nb depending on the content
ofB as well as the content ofP. Specifically, it is preferable that the amount of dissolved
Nb satisfy Formula (iii) shown below.
[0067]
0.08P- 2B + 0.200 :>; Nb- NhER :>; -0.4P- 4B + 0.450 (iii)
where symbols of elements in the formula mean the contents (mass%) of the
elements contained in the steel, and NhER means the amount ofNb (mass%) analyzed as
extraction residues.
[0068]
This is because when the amount of dissolved Nb is not less than the left side
value of Formula (iii), after the amount of dissolved Nb is ensured, the Cr carbide in
which B is dissolved finely precipitates at grain boundaries, and it is possible to favorably
control the solid-solution strengthening and the precipitation condition brought by P.
This is considered to result in further improvement of creep strength. At the same time,
this is because when the amount of dissolved Nb is not more than the right side value of
Formula (iii), more stable weld crack resistance can be ensured.
[0069]
Note that the amount ofNb analyzed as extraction residues in the formula above
can be measured by the following procedure. From the steel, a test specimen having a
predetermined size is taken. This test specimen is subjected to anodic dissolution by a
constant-current electrolysis with a current density of 20 mA/cm2 in which 10 vol.%
acetylacetone-1 mass% tetramethylammonium chloride methanol solution is used as its
electrolyte, by which carbo-nitrides and nitrides are extracted as residues. The extracted
residues are subjected to acid decomposition and then inductively coupled plasma (ICP)
optical emission spectrometry, by which a mass ofNb in the residues is measured. By
dividing the mass ofNb in the residues by a dissolved mass of the test material, an amount
ofNb present in a form of its carbo-nitride and nitride is determined. The determined
19
amount ofNb is the amount ofNb analyzed as the extraction residues.
[0070]
3. Production method
A preferable method for producing the steel according to the present invention
will be described. The steel according to the present invention exerts its advantageous
effects irrespective of its production method as long as the steel has the configuration
described above; nonetheless, the steel can be produced stably by a production method
described below, for example.
[0071]
A steel having the chemical composition described above is preferably machined
and formed into a frnal shape, that is, a product shape. 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 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.
[0072]
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.
[0073]
Subsequently, solution heat treatment is preferably performed. In the solution
heat treatment, it is preferable to perform the heat treatment with its heat treatment
temperature set within a temperature range of 1100 to 123 o•c and its soaking time set to
1 to 12 minutes.
[0074]
If the heat treatment temperature is less than 11 oo•c, precipitate containing Nb
20
formed before the forming step is not dissolved in a matrix sufficiently, failing to ensure
a sufficient amount of dissolved Nb. In addition, residual strain induced in the forming
step cannot be removed. It is considered that this consequently causes the carbo-nitride
or nitride containing Nb not to precipitate fmely and densely for a long time in a usage
environment where the austenitic heat resistant steel is exposed to high temperature,
failing to provide stable creep strength. The heat treatment temperature of the solution
heat treatment is therefore preferably set to 11 00°C or more. The heat treatment
temperature is more preferably set to 1120°C or more.
[0075]
On the other hand, if the heat treatment temperature is more than 1230°C,
although the amount of dissolved Nb is sufficient, weld crack susceptibility is increased
at a weld heat affected zone due to grain-boundary segregation of Nb as well as
coarsening of grains. The heat treatment temperature is therefore preferably set to
1230°C or less. The heat treatment temperature is more preferably set to 1200°C or less.
[0076]
Likewise, if the soaking time of the solution heat treatment is less than 1 minute,
precipitate containing Nb formed before the forming step is not dissolved in a matrix
sufficiently, failing to ensure a sufficient amount of dissolved Nb. In addition, residual
strain induced in the forming step cannot be removed. As a result, a desired creep
strength is less likely to be provided. The soaking time is therefore preferably set to 1
minute or more. The soaking time is more preferably set to 2 minutes or more.
[0077]
On the other hand, if the soaking time is more than 12 minutes, although the
amount of dissolved Nb is sufficient, weld crack susceptibility is increased at a weld heat
affected zone due to grain-boundary segregation ofNb as well as coarsening of grains.
The soaking time is therefore preferably set to 12 minutes or less. The soaking time is
more preferably set to 10 minutes or less. Note that the heat treatment is performed in
a heat treatment furnace, and an atmosphere for the heat treatment is only required to
conform to a conventional method. For example, an air atmosphere used in a normal
heat treatment or an atmosphere for bright heat treatment is conceivable.
21
[0078]
After heating is performed at the heat treatment temperature and for the soaking
time within their respective ranges, it is preferable to perform cooling. A method for the
cooling is not limited to a particular method; however, a cooling rate for the temperature
range of 1000 to 600°C is preferably set to 0.4 °C/s or more, and more preferably set to
1.0°C/s or more. A preferable method for the cooling is forced cooling in which the
cooling is forcibly performed by spraying coolant such as water and air on the steel.
Examples of the forced cooling include water cooling and forced air cooling. When the
forced cooling is performed, it is preferable to perform control in such a manner that a
difference between the heat treatment temperature and a temperature of the steel at a time
of starting the forced cooling (hereinafter, simply referred to as "cooling start temperature
difference") is 40°C or less.
[0079]
The cooling start temperature difference is preferably 0°C. However, it is
difficult for a normal equipment system, for example, in the process of production using
an actual machine, to bring the cooling start temperature difference to 0°C. The cooling
start temperature difference is therefore more preferably 1 oc or more, and still more
preferably is 2°C or more. Note that the heat treatment temperature means a temperature
of the steel at a time of performing the heat treatment, and the temperature of the steel
means a surface temperature of the steel.
[0080]
Further, the cooling is preferably performed until the temperature of the steel
decreases to 300°C or less. Satisfying these production conditions enables the amount
of dissolved Nb to be adjusted within its appropriate range.
[0081]
The present invention will be described below more specifically with reference
to examples, but the present invention is not limited to these examples.
EXAMPLE
22
[0082]
Steel types A to T having chemical compositions shown in Table I were melted
and cast into ingots, and the ingots were subjected to hot forging to have a thickness of
25 mm, subjected to hot rolling to have a thickness of 18 mm, and then subjected to cold
rolling to be formed to have a thickness of 12 mm.
[0083]
[Table 1]
23
Table 1
Steel Chemical composition (mass%, Balance: Fe and Impurities)
type c Si Mn p s Cu Ni Cr Mo Nb B N AI 0 Co w Ti v Ta Sn Ca 1\/g REM
A 0.07 0.25 0.75 0.028 0.0012 3.2 9.0 18.7 0.08 0.476 0.0030 0.095 0.011 0.007 - - - - - - - - -
B 0.07 0.22 0.80 0.030 0.0011 2.9 9.4 18.3 0.12 0.510 0.0035 0.110 0.009 0.008 0.05 - 0.03 - - - - - 0.0030
c 0.09 0.05 0.71 0.025 0.0005 3.3 8.8 18.5 0.23 0.628 0.0055 0.072 0.012 0.008 - - - - - - - - -
D 0.06 0.14 0.60 0.022 0.0015 3.5 8.5 18.2 0.53 0.424 0.0020 0.105 0.012 0.010 - 0.32 - - - 0.0020 0.0018 - -
E 0.06 0.20 0.98 0.026 0.0008 2.7 10.4 19.0 0.06 0.586 0.0049 0.090 0.008 0.009 - - - 0.02 0.03 - - 0.0012 -
F 0.10 0.17 0.70 0.035 0.0014 2.5 8.3 18.5 0.50 0.450 0.0015 0.120 0.009 0.011 - - - - - - - - -
G 0.08 0.08 0.95 0.020 0.0002 3.0 8.8 18.8 0.44 0.636 0.0059 0.061 0.010 0.008 - - - - - - - - -
H 0.08 0.12 1.03 O.D38 0.0015 2.4 9.3 18.3 0.32 0.408 0.0013 0.148 0.011 0.010 - - - - - - - - -
I 0.05 0.25 0.80 0.017 0.0011 2.3 8.4 19.1 0.05 0.420 0.0019 0.071 0.012 0.009 - - - - - - - - -
J 0.06 0.24 0.88 0.011 0.0012 2.2 8.6 18.9 0.03 0.418 0.0020 0.069 0.010 0.010 - - - - - - - - -
K 0.05 0.28 0.57 0.022 0.0006 2.4 8.3 19.2 0.49 0.388 0.0021 0.070 0.009 0.008 - - - - - - - - -
L 0.05 0.20 1.20 0.039 0.0019 3.6 10.7 18.3 0.08 0.662 0.0053 0.098 0.010 0.008 - - - - - 0.022 - - -
M 0.07 0.19 0.83 O.D38 0.0010 3.3 10.6 18.4 0.10 0.523 0.0062 0.112 0.009 0.010 - - - - - - - - -
N 0.09 0.23 1.01 0.014 0.0011 2.9 9.2 18.2 0.06 0.412 0.0036 0.113 0.014 0.010 - - - - - - - - -
0 0.08 0.20 0.95 0.032 0.0015 3.2 9.2 18.7 0.05 0.460 0.0055 0.110 0.012 0.008 - - - - - - - - -
p 0.05 0.27 0.90 0.0004 0.0011 2.4 8.3 18.1 0.04 0.412 0.0018 0.063 0.014 0.008 - - - - - - - - -
Q 0.09 0.19 0.78 0.045 0.0019 3.2 10.5 18.8 0.06 0.635 0.0058 0.095 0.011 0.010 - - - - - - - - -
R 0.08 0.12 0.66 O.D38 0.0015 3.2 18.2 19.0 0.15 0.420 0.0032 0.101 0.012 0.011 - - - - - 0.018 - - -
s 0.06 0.15 0.92 0.036 0.0018 3.0 10.8 22.0 0.22 0.445 0.0041 0.090 0.012 0.009 - - - - - - - - -
T 0.04 0.23 0.57 0.012 0.0010 2.2 8.3 18.2 - 0.410 0.0020 0.065 0.010 0.008 - - - - - - - - -
u 0.10 0.15 0.80 0.005 0.0010 3.4 8.4 18.8 0.33 0.552 0.0038 0.101 0.010 0.007
Underline value: indicating that the value fell out of the range in the chemical composition specified in the present invention.
24
[0084]
Thereafter, from the starting materials subjected to the cold rolling, sheet
materials that were 12 mm thick x 100 mm wide x 100 mm long were fabricated by
machining. The fabricated sheet materials were subjected to the solution heat treatment
under conditions shown in Tables 2 and 3. Note that, in the solution heat treatment,
water cooling was performed after heating so that the cooling start temperature difference
fell within ranges shown in Tables 2 and 3. The water cooling was performed until
temperatures of the steels reached 300°C or less, so that steels including an austenitic
structure were given, which were used as test materials. Note that examples where their
cooling start temperature differences are 0°C indicate that the cooling was performed
immediately after the solution heat treatment. Here, in every method for the cooling, its
cooling rate was 0.4°C/s or more in the temperature range of 1000 to 600°C.

CLAIMS
[Claim 1]
An austenitic heat resistant steel comprising
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: less than 0.0050%,
Cu: 2.2 to 3.8%,
Ni: 8.0 to 11.0%,
Cr: 17.7 to 19.3%,
Mo: O.oi 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
REM: 0 to 0.0800%,
with the balance: Fe and impurities, wherein
a difference between a content ofNb and an amount ofNb analyzed as extraction
33
residues satisfies Formula (i) shown below;
0.170 ~ Nb - NhER ~ 0.480 (i)
where Nb in the formula meaos the content ofNb (mass%) contained in the steel,
aod NhER meaos the amount ofNb (mass%) aoalyzed as extraction residues.
[Claim 2]
The austenitic heat resistaot steel according to claim 1, wherein Formula (ii)
shown below is satisfied;
-2B + 0.185 ~ Nb- NhER ~ -4B + 0.480 (ii)
where symbols of elements in the formula meao the contents (mass%) of the
elements contained in the steel, aod NhER means the amount ofNb (mass%) aoalyzed as
extraction residues.
[Claim 3]
The austenitic heat resistaot steel according to claim 1 or 2, 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%, aod
REM: 0.0005 to 0.0800%.
[Claim 4]
The austenitic heat resistaot steel according to aoy one of claims 1 to 3, wherein
Formula (iii) shown below is satisfied;
0.08P- 2B + 0.200 ~ Nb- NhER ~ -0.4P- 4B + 0.450 (iii)
where symbols of elements in the formula meao the contents (mass%) of the
elements contained in the steel, aod NhER means the amount ofNb (mass%) aoalyzed as
extraction residues.
34
[Claim 5]
The austenitic heat resistant steel according to any one of claims 1 to 4, wherein
the chemical composition contains, in mass%, P: 0.010 to 0.040%.
[Claim 6]
The austenitic heat resistant steel according to any one of claims 1 to 5, wherein
the chemical composition contains, in mass%, P: 0.020 to 0.038%.