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DESCRIPTION
AUSTENITIC STAINLESS STEEL
Technical Field
[0001] The present invention relates to austenitic stainless steel.
Background Art
[0002] There has been an advancing tendency since 1990s in Japan with respect
to a
boiler toward high temperature and high pressure, and the current mainstream
is an Ultra
Super Critical power (USC) boiler for a steam temperature beyond 600 C.
In other areas of the world, including Europe or China, highly efficient USC
boilers have been constructed one after another from the viewpoint of CO2
reduction as a
global environmental countermeasure.
As a source material steel to be used for a heat exchanger tube to generate
high
temperature high pressure steam in a boiler, and for a pipe of a boiler, a
steel material
with superior high temperature strength has been demanded and various steel
materials
have been developed recently.
[0003] For example, Patent Literature 1 discloses an 18 Cr - based austenitic
stainless
steel superior in high temperature strength as well as superior in steam
oxidation
resistance.
Patent Literature 2 discloses an austenitic stainless steel superior in high
temperature corrosion thermal fatigue cracking resistance.
Patent Literature 3 discloses a heat-resistant austenitic stainless steel
superior in
high temperature strength and cyclic oxidation resistance.
[0004] Patent Literature 4 discloses an austenitic stainless steel exhibiting
superior
toughness even after exposure to a high temperature environment for a
prolonged period
of time.
Patent Literature 5 discloses a high strength austenitic stainless steel with
a creep
rupture strength at 800 C for 600 hours of 100 MPa or more.
[0005] Patent Literature 6 discloses a method for securing a high temperature
strength (a
method of adding a large amount of N) by which a large amount of nitrogen (N)
is added
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for utilizing solid solution strengthening and nitride precipitation
strengthening so as to
make up low strength of a low carbon stainless steel.
[0006]
Patent Literature 1: Japanese Patent No. 3632672
Patent Literature 2: Japanese Patent No. 5029788
Patent Literature 3: Japanese Patent No. 5143960
Patent Literature 4: Japanese Patent No. 5547789
Patent Literature 5: Japanese Patent No. 5670103
Patent Literature 6: Japanese Patent No. 3388998
SUMMARY OF INVENTION
Technical Problem
[0007] Generally, in designing the chemical composition of a material steel to
be used
for a heat exchanger tube used in a high temperature range and a pipe of a
boiler used in a
high temperature range, importance is placed on high temperature strength (for
example,
creep strength), high temperature corrosion resistance, steam oxidation
resistance, thermal
fatigue resistance, etc., however corrosion resistance in a temperature range
from normal
temperature to approx. 350 C (for example, stress corrosion cracking
resistance in water)
is less valued. This is because the corrosion resistance in a temperature
range from
room temperature to approx. 350 C has been heretofore addressed by fabrication
technique or operation control technique.
[0008] However, there arises recently a big problem that stress corrosion
cracking
occurs in water in a range of room temperature to low temperature (approx. 350
C or less)
due to a inhomogeneous metallic structure or an heterogeneous carbide
precipitation at a
heating processed portion, such as a welded portion or a bending portion.
For example, during a hydrostatic pressure test of a boiler, or a shut-down of
a
boiler, since water is stored for an extended period of time inside heat
exchanger tubes,
where stress corrosion cracking may occur remarkably.
[0009] Stress corrosion cracking of stainless steel may occur because a
crystal grain
boundary becomes susceptible to selective corrosion due to precipitation of a
Cr - based
carbide or generation of a zone with a low Cr concentration (Cr depleted zone)
in the
vicinity of a crystal grain boundary.
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[0010] As a method for preventing stress corrosion cracking of an 18 Cr -
based
austenitic stainless steel, heretofore:
a method for suppressing formation of a grain boundary Cr carbide by reduction
of a C amount (a low carbon addition method),
a method for suppressing formation of a grain boundary Cr carbide by addition
of Nb and Ti, which have higher capability of forming a carbide than Cr, to
form a MC
carbide to fix C (a stabilizing heat treatment method),
a method for suppressing formation of a Cr depleted zone by addition of Cr at
22% or more to suppress selective corrosion at a grain boundary (a method of
adding a
large amount of Cr), or the like is known.
[0011] There is however a drawback in any of the methods.
In the case of a low carbon addition method, there is a tendency that a
carbide
effective for high temperature strength is not formed and the high temperature
strength
declines.
In the case of a stabilizing heat treatment method, since a stabilizing heat
treatment is done at a temperature as low as approx. 950 C, a high temperature
strength,
especially creep strength tends to be impaired.
In the case of a method of adding a large amount of Cr, since a high content
of
brittle phase such as a-phase is to be formed, it is required to add a large
amount of
expensive Ni for stabilization of a metallic structure and maintenance of high
temperature
strength, so that the cost of source materials tends to increase greatly.
[0012] The method described in Patent Literature 6 (a method of adding a large
amount
of N) is a method devised for replacing the aforementioned conventional
methods.
The method of adding a large amount of N is a method by which a large amount
of N is added for utilizing solid solution strengthening and nitride
precipitation
strengthening so as to make up low strength of a low carbon stainless steel.
[0013] However, it was found there is a problem that according to the method
of Patent
Literature 6 (the method of adding a large amount of N), a large amount of
nitride is
formed against expectation to cause stress corrosion cracking, or sufficient
high
temperature strength cannot be obtained in a high temperature range of 700 C
or higher
[0014] Under such circumstances, it has been demanded to achieve superior high
temperature strength and superior stress corrosion cracking resistance with
respect to 18
Cr - based austenitic stainless steel without depending on the low carbon
addition method,
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the stabilizing heat treatment method, the method of adding a large amount of
Cr, and the
method of adding a large amount of N, which are conventional methods.
An object of the invention is to provide an austenitic stainless steel, which
is an
18 Cr - based austenitic stainless steel securing superior high temperature
strength and
superior stress corrosion cracking resistance.
Solution to Problem
[0015] The means for achieving the object includes the following aspects.
[0016] <1> An austenitic stainless steel with a chemical composition
consisting of
in telins of mass%:
0.05 to 0.13% of C,
0.10 to 1.00% of Si,
0.10 to 3.00% of Mn,
0.040% or less of P,
0.020% or less of S,
17.00 to 19.00% of Cr,
12.00 to 15.00% of Ni,
2.00 to 4.00% of Cu,
0.01 to 2.00% of Mo,
2.00 to 5.00% of W,
2.50 to 5.00% of 2Mo+W,
0.01 to 0.40% of V,
0.05 to 0.50% of Ti,
0.15 to 0.70% of Nb,
0.001 to 0.040% of Al,
0.0010 to 0.0100% of B,
0.0010 to 0.0100% of N,
0.001 to 0.20% of Nd,
0.002% or less of Zr,
0.001% or less of Bi,
0.010% or less of Sn,
0.010% or less of Sb,
0.001% or less of Pb,
5
0.001% or less of As,
0.020% or less of Zr+Bi+Sn+Sb+Pb+As,
0.0090% or less of 0,
0.80% or less of Co,
0.20% or less of Ca,
0.20% or less of Mg,
0.20% or less in total of one or more of Y, Sc, Ta, Hf, Re or lanthanoid
elements other
than Nd, and
a remainder consisting of Fe and impurities;
wherein an effective M content Meff defined by the following Formula (1) is
0.0001 to 0.250%;
Effective M content Meff = Nd +13.(B -11=N/14) Formula (1)
wherein in Formula (1), each element symbol represents a content (mass%) of
each element.
[0017] Effective M content Meff Nd +13.(B -11=N/14) -1.6=Zr Formula (1)
wherein in Formula (1), each element symbol represents the content of each
element (mass%)).
[0018] <2> The austenitic stainless steel according to <1>, wherein the
chemical
composition comprises, in terms of mass%, one or more of: 0.01 to 0.80% of Co,
0.0001
.. to 0.20% of Ca, or 0.0005 to 0.20% of Mg.
<3> The austenitic stainless steel according to <1> or <2>, wherein the
chemical
composition comprises, in terms of mass%, 0.001 to 0.20% in total of one or
more of Y,
Sc, Ta, Hf, Re or lanthanoid elements other than Nd.
<4> The austenitic stainless steel according to any one of <1> to <3>,
wherein an
ASTM grain size number of a metallic structure thereof is 7 or less.
<5> The austenitic stainless steel according to any one of <1> to <4>,
wherein a
creep rupture strength at 700T and 10.000 hours is 140 MPa or more.
Advantageous Effects of Invention
[0019] According to the invention, an austenitic stainless steel, which is an
18 Cr -
based austenitic stainless steel securing superior high temperature strength
and superior
stress corrosion cracking resistance, is provided.
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DESCRIPTION OF EMBODIMENTS
[0020] Embodiments of the invention will be described below.
A numerical range expressed by "x to y" herein includes the values of x
and y in the range as the lower and upper limit values, respectively.
The content of an element expressed by "%" and an effective M content Meff
expressed by "%" both mean herein "mass%".
Further, the content of C (carbon) may be herein occasionally expressed as "C
content". The content of another element may be expressed similarly.
[0021] An austenitic stainless steel of the embodiment (hereinafter also
referred to as
"the steel of the embodiment") is an austenitic stainless steel with a
chemical composition
consisting of in terms of mass%: 0.05 to 0.13% of C, 0.10 to 1.00% of Si, 0.10
to 3.00%
of Mn, 0.040% or less of P, 0.020% or less of S, 17.00 to 19.00% of Cr, 12.00
to 15.00%
of Ni, 2.00 to 4.00% of Cu, 0.01 to 2.00% of Mo, 2.00 to 5.00% of W, 2.50 to
5.00% of
2Mo+W, 0.01 to 0.40% of V, 0.05 to 0.50% of Ti, 0.15 to 0.70% of Nb, 0.001 to
0.040%
of Al, 0.0010 to 0.0100% of B, 0.0010 to 0.0100% of N, 0.001 to 0.20% of Nd,
0.002%
or less of Zr, 0.001% or less of Bi, 0.010% or less of Sn, 0.010% or less of
Sb, 0.001% or
less of Pb, 0.001% or less of As, 0.020% or less of Zr+Bi+Sn+Sb+Pb+As, 0.0090%
or
less of 0, 0.80% or less of Co, 0.20% or less of Ca, 0.20% or less of Mg,
0.20% or less in
total of one or more of Y, Sc, Ta, Hf, Re or lanthanoid elements other than
Nd, and a
remainder consisting of Fe and impurities; wherein an effective M content Meff
defined
by the following Formula (1) is 0.0001 to 0.250%.
[0022] Effective M content Meff = Nd+13 = (B-11.N/14)-1.6=Zr Formula (1)
wherein, in Formula (1), each element symbol represents the content (mass%) of
each element.
[0023] The chemical composition of the steel cf the embodiment includes 17.00
to
19.00% of Cr.
In other words, the steel of the embodiment belongs to the 18 Cr - based
austenitic stainless steel.
As described above, it is demanded that superior high temperature strength and
superior stress corrosion cracking resistance is achieved for 18 Cr - based
austenitic
stainless steel without depending on the low carbon addition method, the
stabilizing heat
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treatment method, the method of adding a large amount of Cr, and the method of
adding a
large amount of N, which are conventional methods.
[0024] According to the steel of the embodiment, superior high temperature
strength and
superior stress corrosion cracking resistance may be secured without depending
on the
low carbon addition method, the stabilizing heat treatment method, the method
of adding
a large amount of Cr, and the method of adding a large amount of N, which are
conventional methods.
The reason of such an effect to be obtained with the steel of the embodiment
is
presumed as follows, provided that the invention be not restricted in any way
by the
following presumption.
[0025] In the case of the steel of the embodiment, grain boundary purification
and
strength improvement may be achieved by adding Nd and B combinedly at the
above
respective contents, and further by adjusting the ?,ffective M content Meff in
the above
range.
Further, in the case of the steel of the embodiment, purity refinement is
achieved
by limiting the contents of Zr, Bi, Sn, Sb, Pb, and As, which are impurities
(hereinafter
also collectively referred to as "6 impurity elements"), in the above ranges.
It is conceivable that superior high temperature strength and superior stress
corrosion cracking resistance may be secured by the grain boundary
purification, the
strength improvement, and the purity refinement without depending on any of
the low
carbon addition method, the stabilizing heat treatment method, and the method
of adding
a large amount of Cr.
[0026] Further, in the case of the steel of the embodiment, conceivably
precipitation
strengthening through precipitation of a fine carbide and precipitation of a
fine and stable
Laves phase becomes possible by reducing N (nitrogen) to the extent possible
(specifically to 0.0100% or less) and adding W at the above content.
As the result, in 18 Cr - based austenitic stainless steel, superior high
temperature
strength may be presumably secured without depending on the method of adding a
large
amount of N (see, for example Patent Literature 6).
This finding is a novel finding contrary to heretofore common sense.
[0027] Ordinarily, a carbide and a Laves phase precipitate preferentially
around a nitride
and on a nitride at a crystal grain boundary to impair the high temperature
strength and
corrosion resistance. In other word, when a nitride is present, both
precipitation of a fine
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carbide, and precipitation of a fine and stable Laves phase become difficult,
and the high
temperature strength is not improved. Especially, when a coarse Zr nitride is
present,
precipitation of a fine carbide, and precipitation of a fine and stable Laves
phase become
more difficult, and therefore N and Zr are reduced to the extent as possible.
However, a trace amount of N forms a precipitation nucleus made of fine
carbide, which contributes to improvement of high temperature strength.
Therefore, in
the steel of the embodiment, N is not an impurity element but a useful
element, and
controlled in a very low content range (specifically, from 0.0010 to 0.0100%).
By regulating the N content in the steel of the embodiment from 0.0010 to
0.0100%, both of precipitation strengthening with a fine carbide and
precipitation
strengthening with a fine and stable Laves phase may be achieved effectively.
As the
result, the high temperature strength can be secured and the metallic
structure can be
stabilized in a temperature range of 700 C or higher.
In other words, in the steel of the embodiment, enhancement of the strength
can
be achieved without depending on precipitation strengthening with a nitride,
and
stabilization of the metallic structure can be achieved without forming a
brittle phase, etc.
The technique has not been known conventionally.
[0028] Firstly, the chemical composition and its preferable embodiment of the
steel of
the embodiment will be described below, and then an effective M content Meff
(Formula
(1)), etc. will be described.
[0029] C: 0.05 to 0.13%
C is an essential element for formation of a carbide, and stabilization of an
austenitic structure, as well as improvement of high temperature strength and
stabilization
of a metallic structure at a high temperature.
With respect to the steel of the embodiment, stress corrosion cracking can be
prevented without utilizing strengthening by addition of N, or without
reducing C.
Provided that the C content is 0.05% or more, which is because, when the C
content is less than 0.05%, improvement of high temperature creep strength,
and
stabilization of a metallic structure at a high temperature becomes difficult.
The C
content is preferably 0.06% or more.
[0030] Meanwhile, when the C content exceeds 0.13%, a coarse Cr carbide
precipitates
at a crystal grain boundary, which may cause stress corrosion cracking or
welding
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cracking to reduce toughness. Therefore, the C content is 0.13% or less, and
is
preferably 0.12% or less.
[0031] Si: 0.10 to 1.00%
Si is an element which functions as a deoxidizing agent during steel making,
and
prevents steam oxidation at a high temperature. However, when the Si content
is less
than 0.10%, the addition effect is not obtained adequately. Therefore, the Si
content is
0.10% or more, and is preferably 0.20% or more.
[0032] Meanwhile, when the Si content exceeds 1.00%, the workability declines,
and a
brittle phase such as a s-phase precipitates at a high temperature. Therefore,
the Si
content is 1.00% or less, and is preferably 0.80% or less.
[0033] Mn: 0.10 to 3.00%
Mn is an element which makes S harmless by forming MnS with S as an
impurity element to contribute to improvement of a hot workability, as well as
to
stabilization of a metallic structure at a high temperature. However, when the
Mn
content is less than 0.10%, the addition effect is not obtained adequately.
Therefore, the
Mn content is 0.10% or more, and is preferably 0.20% or more.
Meanwhile, when the Mn content exceeds 3.00%, the workability and
weldability decrease. Therefore, the Mn content is 3.00% or less, and is
preferably
2.60% or less.
[0034] P: 0.040% or less
P is an impurity element, which disturbs workability and weldability.
When the P content exceeds 0.040%, the workability and weldability decrease
remarkably. Therefore, the P content is 0.040% or less, and is preferably
0.030% or less,
and more preferably 0.020% or less.
.. [0035] Preferably the P content is as low as possible, and may be even 0%.
However, P may inevitably get mixed in from steel raw materials (raw material
ore, scrap, etc.), and reduction of the P content to below 0.001% will
increase the
production cost greatly. Therefore, the P content may be 0.001% or more from
the
viewpoint of production cost.
[0036] S: 0.020% or less
S is an impurity element, which disturbs workability, weldability, and stress
corrosion cracking resistance.
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When the S content exceeds 0.020%, the workability, weldability, and stress
corrosion cracking resistance decrease remarkably.'Therefore the S content is
0.020% or
less.
Even in a case in which S is added for improvement of molten metal flow in
5 welding, the S content is added at 0.020% or less, and is preferably
added at 0.010% or
less.
[0037] Preferably the S content is as low as possible, and may be even 0%.
However, S may inevitably get mixed in from steel source materials (raw
material ore, scrap, etc.) and reduction of the S content to below 0.001% will
increase the
10 production cost greatly. Therefore, the S content may be 0.001% or more
from the
viewpoint of production cost.
[0038] Cr: 17.00 to 19.00%
Cr is a major element of an 18 Cr - based austenitic stainless steel, which
contributes to improvement of oxidation resistance, steam oxidation
resistance, and stress
corrosion cracking resistance, as well as to stabilization of the strength or
metallic
structure with a Cr carbide.
When the Cr content is less than 17.00%, the addition effect may be not
obtained
adequately. Therefore, the Cr content is 17.00% or more. The Cr content is
preferably
17.30% or more, and more preferably 17.50% or more.
[0039] Meanwhile, when the Cr content exceeds 19.00%, a large amount of Ni
becomes
necessary for maintaining the stability of an austenitic structure, and
further a brittle
phase is formed to decrease high temperature strength or toughness. Therefore,
the Cr
content is 19.00% or less. The Cr content is preferably 18.80% or less, and
more
preferably 18.60% or less.
[0040] Ni: 12.00 to 15.00%
Ni is an element to form austenite, and as a major element of an 18 Cr - based
austenitic stainless steel contributes to improvement of high temperature
strength and
workability as well as to stabilization of a metallic structure at a high
temperature.
[0041] When the Ni content is less than 12.00%, the addition effect is not
obtained
adequately, and formation of a brittle phase (u-phase, etc.) is promoted at a
high
temperature due to imbalance with the content of a ferrite forming element,
such as Cr,
W, and Mo. Therefore, the Ni content is 12.00% or more. The Ni content is
preferably
12.50% or more.
=
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[0042] Meanwhile, when the Ni content exceeds 15.00%, the high temperature
strength
and the economic efficiency decrease. Therefore, the Ni content is 15.00% or
less, and
is preferably 14.90% or less, more preferably 14.80% or less, and further
preferably
14.50% or less.
[0043] Cu: 2.00 to 4.00%
Cu is an element, which precipitates as a fine Cu phase that is stable at a
high
temperature, to contribute to improvement of high temperature strength.
When the Cu content is less than 2.00%, the addition effect is not obtained
adequately. Therefore, the Cu content is 2.00% or more, and is preferably
2.20% or
more, and more preferably 2.50% or more.
[0044] Meanwhile, when the Cu content exceeds 4.00%, the workability, creep
ductility,
and strength decrease. Therefore, the Cu content is 4.00% or less, and is
preferably
3.90% or less, more preferably 3.80% or less, and further preferably 3.50% or
less.
[0045] Mo: 0.01 to 2.00%
Mo is an essential element for improvement of the corrosion resistance, high
temperature strength, and stress corrosion cracking resistance. Further, Mo is
an
element that contributes to formation of a Laves phase that is stable at a
high temperature
for a long time period of time and a carbide, through a synergistic effect
with W to be
added combinedly.
[0046] When the Mo content is less than 0.01%, the addition effect is not
obtained
adequately. Therefore, the Mo content is 0.01% or more, and is preferably
0.02% or
more.
[0047] Meanwhile, when the Mo content exceeds 2.00%, a large amount of brittle
phase
is formed to decrease the workability, high temperature strength, and
toughness.
Therefore, the Mo content is 2.00% or less, and is preferably 1.80% or less,
more
preferably 1.50% or less, and further preferably 1.30% or less.
[0048] W: 2.00 to 5.00%
W is an essential element for improvement of the corrosion resistance, high
temperature strength, and stress corrosion cracking resistance. Further, it is
an element
.. to contribute to precipitation of a Laves phase stable at a high
temperature for a long time
period of time and a carbide, through a synergistic effect with Mo to be added
combinedly. Further, W is slower in terms of diffusion at a high temperature
than Mo,
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and therefore it is an element to contribute to stable maintenance of the
strength at a high
temperature for a long period of time.
[0049] When the W content is less than 2.00%, the addition effect is not
obtained
adequately. Therefore, the W content is 2.00% or more, and is preferably 2.10%
or
more.
[0050] Meanwhile, when the W content exceeds 5.00%, a large amount of brittle
phase
is formed to decrease the workability, and strength. Therefore, the W content
is 5.00%
or less, and is preferably 4.90% or less, more preferably 4.80% or less, and
further
preferably 4.70% or less.
[0051] 2Mo+W: 2.50 to 5.00%
Combined addition of Mo and W contributes to improvement of the high
temperature strength, stress corrosion cracking resistance, and high
temperature corrosion
resistance. When 2Mo+W (Wherein Mo represents a Mo content, and W represents a
W
content. The same holds hereinbelow.) is less than 2.50%, the synergistic
effect of the
combined addition cannot be obtained adequately. Therefore, 2Mo+W is 2.50% or
more, and is preferably 2.60% or more, more preferably 2.80% or more, and
further
preferably 3.00% or more.
[0052] Meanwhile, when 2Mo+W exceeds 5.00%, the strength or toughness
decreases,
and the stability of a metallic structure is also decreased at a high
temperature.
Therefore 2Mo+W is 5.00% or less, and is preferably 4.90% or less.
[0053] V: 0.01 to 0.40%
V is an element contributing to improvement of high temperature strength by
forming a fine carbide together with Ti and Nb. When the V content is less
than 0.01%,
the addition effect is not obtained adequately. Therefore, the V content is
0.01% or
more, and is preferably 0.02% or more.
[0054] Meanwhile, when the V content exceeds 0.40%, the strength or stress
corrosion
cracking resistance decreases. Therefore, the V content is 0.40% or less, and
is
preferably 0.38% or less.
[0055] Ti: 0.05 to 0.50%
Ti is an element contributing to improvement of high temperature strength by
forming a fine carbide together with V and Nb, and contributing also to
improvement of
stress corrosion cracking resistance through suppression of precipitation of a
Cr carbide at
a crystal grain boundary by fixing C.
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[0056] In a conventional N-adding austenitic stainless steel, not only the
effect of N
addition is not obtained effectively due to precipitation of a nitride in
clumps, but also the
stress corrosion cracking resistance is decreased due to precipitation of a
coarse Cr
carbide at a grain boundary.
The inventors have found, with respect to an 18 Cr - based austenitic
stainless
steel, that an advantageous action effect of a fine Ti carbide can be obtained
by
controlling the N content at a very low level, and that, specifically, a fine
Laves phase
precipitates using a fine Ti carbide as a nucleus, as a result of which the
high temperature
strength of the steel is enhanced remarkably.
[0057] When the Ti content is less than 0.05%, the addition effect is not
obtained
adequately, and therefore, the Ti content is 0.05% or more. Combined addition
of Nb
and V is preferable, and the Ti content is preferably 0.10% or more.
[0058] Meanwhile, when the Ti content exceeds 0.50%, a clumpy precipitate is
precipitated to decrease the strength, toughness, and stress corrosion
cracking resistance.
Therefore, the Ti content is 0.50% or less, and is preferably 0.45% or less.
[0059] Nb: 0.15 to 0.70%
Nb is an element contributing to improvement of high temperature strength by
forming a fine carbide together with V and Ti, and contributing also to
improvement of
stress corrosion cracking resistance through suppression of precipitation of a
Cr carbide at
a crystal grain boundary by fixing C.
Further, Nb is, similar to Ti, an element contributing to improvement of the
high
temperature strength due to precipitation of a fine Laves phase.
[0060] When the Nb content is less than 0.15%, the addition effect is not
obtained
adequately. Therefore, the Nb content is 0.15% or more, and is preferably
0.20% or
more.
Meanwhile, when the Nb content exceeds 0.70%, a clumpy precipitate is
precipitated to decrease the strength, toughness, and stress corrosion
cracking resistance.
Therefore, the Nb content is 0.70% or less, and is preferably 0.60% or less.
[0061] Al: 0.001 to 0.040%
Al is an element which functions as a deoxidizing element in steel making to
purify a steel.
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When the Al content is less than 0.001%, purification of a steel cannot be
achieved adequately. Therefore, the Al content is 0.001% or more, and is
preferably
0.002% or more.
[0062] Meanwhile, when the Al content exceeds 0.040%, a large amount of
nonmetallic
inclusion is foinied to decrease the stress corrosion cracking resistance,
high temperature
strength, workability, toughness, and stability of a metallic structure at a
high
temperature. Therefore, the Al content is 0.040% or less, and is preferably
0.034% or
less.
[0063] B: 0.0010 to 0.0100%
B is an element for achieving securance of superior high temperature strength
and superior stress corrosion cracking resistance by combined addition with
Nd, which is
an important element in the steel of the embodiment. Therefore, B is an
essential
element. B is an element not only to contribute to improvement of the high
temperature
strength through segregation at a crystal grain boundary, but also to
contribute to
formation of a carbide, micronization of a Laves phase, and stabilization of a
metallic
structure, which are effective for improvement of the high temperature
strength.
[0064] Further, B is an element, which makes N (present in the steel of the
embodiment
at 0.0010 to 0.0100%) harmless as BN, and contributes to improvement of the
high
temperature strength and stress corrosion resistance.
[0065] When the B content is less than 0.0010%, residual B, which has not been
consumed as a nitride, namely B able to contribute to improvement of the high
temperature strength and stress corrosion resistance cannot be secured. As the
result,
when the B content is less than 0.0010%, a synergistic effect (to be described
below) of
combined addition with Nd (and securance of effective M content) is not
obtained, so that
the high temperature strength and stress corrosion cracking resistance are not
improved.
Therefore the B content is 0.0010% or more, and is preferably 0.0015% or more.
[0066] Meanwhile, when the B content exceeds 0.0100%, a boron compound is
foimed
to decrease the workability, weldability, and high temperature strength.
Therefore the B
content is 0.0100% or less, and is preferably 0.0080% or less, and more
preferably
0.0060% or less.
[0067] N: 0.0010 to 0.0100%
N (nitrogen) is a useful element with respect to a general 18 Cr - based
austenitic
stainless steel for improvement of the high temperature strength through solid
solution
strengthening and precipitation strengthening with a nitride. However with
respect to
CA 02954755 2017-01-10
the steel of the embodiment, a nitride disturbs stress corrosion cracking
resistance, and
therefore N is not added actively.
However, since a small amount of N forms a precipitation nucleus for a fine
precipitate effective for improvement of high temperature strength, such small
amount of
5 N is allowed in the steel of the embodiment, as is used for forming a
precipitation nucleus
for a fine precipitate effective for improvement of high temperature strength.
[0068] Namely according to the fundamental thought with respect to the steel
of the
embodiment, N is not added actively, but is allowed only in a small content
range, which
is different from the prior art.
10 [0069] When the N content is less than 0.0010%, formation of a
precipitation nucleus
for a fine precipitate, which is effective for improvement of high temperature
strength, is
difficult. Therefore the N content is 0.0010% cr more, and is preferably
0.0020% or
more, and more preferably 0.0030% or more.
[0070] Meanwhile, when the N content exceeds 0.0100%, a nitride is formed to
15 decrease the high temperature strength and stress corrosion cracking
resistance.
Therefore the N content is 0.0100% or less, and is preferably 0.0090% or less,
more
preferably 0.0080% or less, and further preferably 0.0070% or less.
[0071] Nd: 0.001 to 0.20%
Nd is an element to improve remarkably the high temperature strength and
stress
corrosion cracking resistance through a synergistic effect (described below)
of combined
addition with B.
With respect to the steel of the embodiment, as described above, the stress
corrosion cracking resistance is improved by miCronizing a carbide and a Laves
phase
effective for improvement of the high temperature strength, by securing the
long term
stability, and further by strengthening a crystal grain boundary through
combined addition
of Nd and B.
[0072] however, since the bonding strength of Nd with N, 0, or S is extremely
strong,
even when it is added as metal Nd, it is consumed to precipitate as a harmful
precipitate,
and the addition effect is hardly exhibited adequately. Therefore, for
obtaining fully the
addition effect, it is necessary to reduce the N content, 0 content, and S
content to the
extent possible.
[0073] When the Nd content is less than 0.001%, even if the N content, 0
content, and S
content are reduced, the addition effect of Nd cannot be obtained adequately.
Therefore
CA 02954755 2017-01-10
16
the Nd content is 0.001% or more, and is preferably 0.002% or more, and more
preferably
0.005% or more.
[0074] Meanwhile, when the Nd content exceeds 0.20%, the addition effect is
saturated,
and an oxide-based inclusion is formed, so that the strength, workability, and
economy
are decreased. Therefore the Nd content is 0.20% or less, and is preferably
0.18% or
less, more preferably 0.15% or less, and further preferably 0.10% or less.
[0075] For the sake of easier securance of the aforementioned effective M
content Meff,
the Nd content is preferably in a range of from 0.002 to 0.15%, and more
preferably from
0.005 to 0.10%.
[0076] With respect to the steel of the embodiment, Zr, Bi, Sn, Sb, Pb, As,
and 0 are
treated as impurity elements for the sake of securance of superior
characteristics of the
steel of the embodiment, and the contents of the elements are limited.
[0077] Ordinarily, as a source material for a stainless steel, scraps such as
alloy steel are
used mainly. The scraps contain, although at low contents, Zr, Bi, Sn, Sb, Pb,
and As (6
impurity elements), which get mixed in a stainless steel (product) inevitably.
[0078] Further, when a facility for melting, etc. in a production process of a
stainless
steel is contaminated by production of another alloy, the 6 impurity elements
may get
mixed in a stainless steel (product) from the facility for melting, etc., and
0 (oxygen)
remains inevitably in a stainless steel.
[0079] With respect to the steel of the embodiment, for the sake of securance
of superior
high temperature strength and superior stress corrosion cracking resistance,
Zr, Bi, Sn, Sb,
Pb, As, and 0 are required to be reduced to the extent possible so as to
prepare a high
purity steel.
[0080] Zr: 0.002% or less
Zr is ordinarily not contained. However Zr may get mixed from scraps, etc.
and/or a facility for melting, etc. contaminated by production of another
alloy to form an
oxide and a nitride. The nitride functions as a nucleus for precipitation of a
precipitate
such as a Laves phase.
However, in a case in which a clumpy precipitate is precipitated with a
nucleus
of a nitride, the high temperature strength and stress corrosion cracking
resistance are
disturbed.
[0081] As described above, Zr is a harmful element in terms of high
temperature
strength and stress corrosion cracking resistance. Therefore in a relational
expression
CA 02954755 2017-01-10
17
(Formula (1)) introduced for the sake of securance of superior high
temperature strength
and superior stress corrosion cracking resistance, a term of "-1.6=Zr"
expressing a
negative action effect has been added.
[0082] Since the amount of Zr is preferably as low as possible, the upper
limit of the Zr
content is set at 0.002% which is close to the analytical limit (0.001%). The
Zr content
is preferably 0.001% or less.
The Zr content may be 0%. However, Zr may occasionally get mixed
inevitably at 0.0001% or so. Therefore, from the viewpoint of production cost
, the Zr
content may be 0.0001% or even more.
[0083] Bi: 0.001% or less
Bi is an element which is ordinarily not contained. However, Bi may get mixed
from scraps, etc. and/or a facility for melting, etc. contaminated by
production of another
alloy, and disturbs high temperature strength and stress corrosion cracking
resistance.
Since the Bi content is required to be reduced to the extent possible, the
upper
limit of the Bi content is set at 0.001% which is the analytical limit.
The Bi content may be 0%. However, Bi may occasionally get mixed
inevitably at 0.0001% or so. Therefore, from the viewpoint of production cost
, the Bi
content may be 0.0001% or even more.
[0084] Sn: 0.010% or less
Sb: 0.010% or less
Pb: 0.001% or less
As: 0.001% or less
Sn, Sb, Pb, and As are elements, which easily get mixed from scraps, etc.
and/or
a facility for melting, etc. contaminated by production of another alloy, and
are hardly
removed in a refining process.
However, the contents of the elements are required to be reduced to the extent
as
possible.
Considering source materials composition and refining limits, the upper limits
of
the Sn content and the Sb content are set at 0.019% respectively. The Sn
content and the
Sb content are preferably 0.005% or less respectively.
Further, the upper limits of the Pb content and the As content are set at
0.001%
respectively. Pb and As are preferably 0.0005% or less respectively.
CA 02954755 2017-01-10
18
[0085] Any of the Sn content, the Sb content, the Pb content, and the As
content may be
0%.
ITowever, the elements may inevitably get mixed at 0.0001% or so. Therefore
from the viewpoint of production cost , the content of any of the elements may
be
0.0001% or even more.
[0086] Zr+Bi+Sn+Sb+Pb+As: 0.020% or less
In a case in which the invention steel contains inevitably Zr, Bi, Sn, Sb, Pb,
and
As (6 impurity elements), for the sake of securance of superior high
temperature strength
and superior stress corrosion cracking resistance through a synergistic effect
of combined
addition of Nd and B, not only the individual contents of the 6 impurity
elements is
required to be limited but also the total of the contents of the 6 impurity
elements
(Zr+Bi+Sn+Sb+Pb+As; wherein each element symbol represents the content of each
element) is required to be limited to 0.020% or less for achieving higher
purity.
The total content of the 6 impurity elements in the steel of the embodiment is
0.020% or less.
The total content of the 6 impurity elements is preferably 0.015% or less, and
more preferably 0.010% or less.
Meanwhile, for the sake of securance of superior high temperature strength and
superior stress corrosion cracking resistance, the total content of the 6
impurity elements
is preferably as low as possible. Therefore, the lower limit of the total
content of the 6
impurity elements is 0%.
[0087] 0: 0.0090% or less
0 (oxygen) remaining inevitably after refining a molten steel is an element
used
as an index of the content of a nonmetallic inclusion.
When 0 exceeds 0.0090%, an Nd oxide is formed to consume Nd and form a
fine carbide or Laves phase, so that the improvement effect on high
temperature strength
and stress corrosion cracking resistance cannot be obtained. Therefore, the 0
content is
0.0090% or less, and is preferably 0.0080% or less, more preferably 0.0070% or
less, and
further preferably 0.0050% or less.
[0088] The 0 content may be 0%. However, 0 may occasionally remain after
refining
inevitably at 0.0001% or so. Therefore, from the viewpoint of production cost
, the 0
content may be 0.0001% or even more.
CA 02954755 2017-01-10
19
[0089] The chemical composition of the steel of the embodiment may include one
or
more of Co, Ca, or Mg, and/or one or more of lanthanoid elements except Nd, Y,
Sc, Ta,
Hf, or Re.
Any of the elements is an optional element, and therefore the contents thereof
may be respectively 0%.
[0090] Co: 0.80% or less
Co may become a contaminant source in producing another steel. Therefore,
the Co content is 0.80% or less, and is preferably 0.60% or less.
A steel of the embodiment is not required to contain Co (namely, the Co
content
may be 0%), however from the viewpoint of further stabilization of a metallic
structure
and improvement of high temperature strength, Co may be contained.
When the steel of the embodiment contains Co, the Co content is preferably
0.01% or more, and more preferably 0.03% or more.
[0091] Ca: 0.20% or less
Ca is an optional element, and the Ca content may be 0%.
Ca may be added as a finishing element for deoxidation. Since the steel of the
embodiment contains Nd, it is preferable that the same is deoxidized by Ca in
a refining
process. When the steel of the embodiment contains Ca, from the viewpoint of
obtaining more effectively a deoxidation effect, the Ca content is preferably
0.0001% or
more, and more preferably 0.0010% or more.
[0092] Meanwhile, when the Ca content exceeds 0.20%, the amount of a
nonmetallic
inclusion increases to lower the high temperature strength, stress corrosion
cracking
resistance, and toughness. Therefore the Ca content is 0.20% or less, and is
preferably
0.15% or less.
[0093] Mg: 0.20% or less
Mg is an optional element, and the Mg content may be 0%.
Mg is an element, which contributes to improvement of high temperature
strength or corrosion resistance by addition of a small amount thereof. When
the steel of
the embodiment contains Mg, from the viewpoint of obtaining more effectively
the effect,
the Mg content is preferably 0.0005% or more, and more preferably 0.0010% or
more.
[0094] Meanwhile, when the Mg content exceeds 0.20%, the strength, toughness,
corrosion resistance, and weldability are lowered. Therefore the Mg content is
0.20% or
less, and is preferably 0.15% or less.
CA 02954755 2017-01-10
[0095] Total of one or more of Y, Sc, Ta, Hf, Re or lanthanoid elements other
than Nd:
0.20% or less
Any of Y, Sc, Ta, Hf, Re and lanthanoid elements other than Nd (namely, La,
Ce,
Pr, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu) is an optional element,
and the total
5 content of the elements may be 0%.
Although Y, Sc, Ta, Hf, Re and lanthanoid elements other than Nd are
expensive,
they are elements acting to enhance a synergistic effect of combined addition
of Nd and
B. When the steel of the embodiment contains one or more of the elements,
the total
content of the elements is preferably 0.001% or more, and more preferably
0.005% or
10 .. more.
[0096] Meanwhile, when the total content of Y, Sc, Ta, Hf, Re and lanthanoid
elements
other than Nd exceeds 0.20%, the amount of a nonmetallic inclusion increases
to lower
the strength, toughness, corrosion resistance, and weldability. Therefore the
total
content is 0.20% or less, and is preferably 0.15% or less.
15 [0097] A remainder excluding the aforementioned elements from the
chemical
composition of the steel of the embodiment is Fe and impurities.
The impurities referred to herein mean one or more of elements other than the
aforementioned elements. The contents of the elements (impurities) other than
the
aforementioned elements are preferably limited to 0.010% or less respectively,
and more
20 preferably to 0.001% or less.
[0098] With respect to the chemical composition of the steel of the
embodiment, an
effective M content Meff defined by the following Formula (1) is from 0.0001
to 0.250%.
The effective M content Meff will be described below.
[0099] Effective M content Meff = Nd +13-(B -11.N/14) -1.6-Zr Formula (1)
wherein in Formula (1), each element symbol represents the content (mass%) of
each element.
[0100] The effective M content Meff is an index defining a quantitative
relationship
between Nd and B, which are essential for improvement of high temperature
strength and
stress corrosion cracking resistance.
[0101] Formula (1) defining an effective M content Meff is a relational
expression
discovered by the inventors from the viewpoint of securance of superior high
temperature
strength and superior stress corrosion cracking resistance.
CA 02954755 2017-01-10
21
Formula (1) is basically a relational expression, in which to the content of
Nd to
function effectively for securance of superior high temperature strength and
superior
stress corrosion cracking resistance, the content of B also to function
effectively is added,
and the content of Zr to function harmfully against securance of superior high
temperature strength and superior stress corrosion cracking resistance is
subtracted.
[0102] With respect to the steel of the embodiment, N is reduced to the extent
as
possible so as to suppress formation of a nitride in order to secure superior
high
temperature strength and superior stress corrosion cracking resistance.
However, when a steel is produced industrially, some amount of N inevitably
gets mixed in a steel. If the N mixed in a steel forms BN, the function of B
cannot be
obtained. Therefore, it is necessary to secure B not bound with N.
[0103] In Formula (1) defining an effective M content Meff, the moiety of "(B
-11-N/14)" represents the content of B that effec'ively functions (namely, the
content of B
that is not bound with N, among the B that has been added).
In Formula (1), "(B -11.N/14)" (the content of B not bound with N) is
multiplied
by 13 to "13.(B -11-N/14)" for weighting the content of B which functions
effectively.
In this regard, 13 is a ratio of the atomic weight of Nd (----144) to the
atomic weight of B
In Formula (1), "13.(B -11.N/14)" obtained as above is added to the Nd content
("Nd +13.(B -11.1\1/14)"). Nd is an element that functions effectively
similarly as B for
securing superior high temperature strength and superior stress corrosion
cracking
resistance.
[0104] In Formula (1) in addition to "Nd +13-(B -11-N/14)", there is a term "-
1.6-Zr" for
subtracting the content of Zr that is harmful against securance of superior
high
temperature strength and superior stress corrosion cracking resistance.
[0105] The impurity element Zr, by forming a nitride and an oxide, functions
to reduce a
synergistic effect of combined addition of Nd and B.
In Formula (1) the reduction effect of Zr is weighted by multiplying the Zr
content by 1.6 (z144/91), which is the ratio of the atomic weight of Nd (-144)
to the
atomic weight of Zr (z91), to "1.6-Zr".
In Formula (1) the "1.6-Zr" is subtracted from the "Nd +13.(B -11=N/14)".
[0106] As described above, the addition amounts of Nd and B necessary for
obtaining
superior high temperature strength and superior stress corrosion cracking
resistance, and
CA 02954755 2017-01-10
22
the limited amount of Zr being harmful to securance of superior high
temperature strength
and superior stress corrosion cracking resistance can be quantified by an
effective M
content Meff defined by Formula (1) (specific examples will be described in
Examples in
detail).
[0107] When the effective M content Meff is less than 0.0001%, it is difficult
to achieve
superior high temperature strength and superior stress corrosion cracking
resistance.
Therefore the effective M content Meff is 0.0001% or more, and is preferably
0.001% or
more, more preferably 0.002% or more, and further preferably 0.010% or more
In this regard, when the N content or the Zr content is high, the effective M
content Meff may take a negative value.
[0108] Meanwhile, when the effective M content Meff exceeds 0.250%, the
improvement effect on high temperature strength and stress corrosion cracking
resistance
according to the effective M content Meff is saturated, and the economy
declines, and
moreover the strength, toughness, workability, and weldability decrease.
Therefore, the
effective M content Meff is 0.250% or less, and is preferably 0.200% or less,
and more
preferably 0.150% or less.
[0109] There is no particular restriction on the metallic structure of the
steel of the
embodiment.
The metallic structure of the steel of the embodiment is preferably a coarse
grain
metallic structure from the viewpoint of improvement of high temperature
strength (for
example, high temperature creep strength between 700 C and 750 C).
Specifically, with respect to the steel of the embodiment, the ASTM grain size
number of the metallic structure thereof is preferably 7 or less.
When the metallic structure of the steel of the embodiment is a coarse grain
structure with an ASTM grain size number of 7 or less, a suppression effect on
grain
boundary sliding in creep, change in a metallic structure by element diffusion
through a
crystal grain boundary, and formation of precipitation site for an a phase can
be
conceivably obtained.
Therefore, from the viewpoint of improvement of the high temperature strength,
it is preferable that the metallic structure of the steel of the embodiment is
a coarse grain
structure with an ASTM grain size number of 7 or less.
CA 02954755 2017-01-10
23
[0110] Meanwhile, in the case of a conventional steel, when the metallic
structure of a
steel is a coarse grain metallic structure, stress corrosion cracking is apt
to occur due to
segregation of an impurity element at a crystal grain boundary.
However, in the case of the steel of the embodiment, segregation of an
impurity
element at a crystal grain boundary is reduced owing to higher purification.
Therefore,
with respect to the steel of the embodiment, even with a coarse grain metallic
structure
(for example, a metallic structure with an ASTM grain size number of 7 or
less), the stress
corrosion cracking is suppressed (namely, superior stress corrosion cracking
resistance
may be maintained).
[0111] From the above viewpoints, the ASTM grain size number of the metallic
structure of the steel of the embodiment is preferably 7 or less, and more
preferably 6 or
less.
There is no particular restriction on the lower limit of the ASTM grain size
number of a metallic structure. From the viewpoint of suppression of
decreasing in
creep ductility and welding cracking, the lower limit of the ASTM grain size
number of a
metallic structure is preferably 3.
[0112] A steel of the embodiment is superior in high temperature strength
(especially,
creep rupture strength) as described above.
There is no particular restriction on the specific range of the high
temperature
strength of the steel of the embodiment. The creep rupture strength at 700 C
and 10,000
hours of the steel of the embodiment is preferably 140 MPa or more.
[0113] In this regard, 700 C is a temperature higher than an actual usage
temperature.
Therefore, the creep rupture strength at 700 C and 10,000 hours of 140 MPa or
more means that the high temperature characteristic is remarkably superior.
Specifically, a high temperature strength at which the creep rupture strength
is
140 MPa or more at 700 C and 10,000 hours is a high temperature strength that
is
remarkably superior to a 347H steel (18 Cr-12Ni-Nb), which is used widely in
the world
as a conventional 18 Cr - based austenitic stainless steel (see, for example,
Inventive
Steels 1 to 20, and Comparative steel 21 in Table 3 below).
[0114] A creep rupture strength less than 140 MPa may be easily achievable by
extension of the conventional art, however it is difficult to achieve a creep
rupture
strength of 140 MPa or more by mere extension of the prior art.
CA 02954755 2017-01-10
24
In contrast, in the case of the steel of the embodiment, a creep rupture
strength of
140 MPa or more at 700 C, which is higher than an actual service temperature,
and
10,000 hours (superior high temperature strength) can be attained by fine
precipitation of
a carbide and a Laves phase, the Laves phase precipitates during creep, by
means of
optimization of the chemical composition, optimization of the effective M
content Meff
by the Nd content and the B content, higher degree of purification by limiting
the amount
of impurity elements, etc.
[0115] There is no particular restriction on a method for producing the steel
of the
embodiment, and a publicly known method for producing an austenitic stainless
steel may
be appropriately adopted.
A steel of the embodiment may be a heat-treated steel plate or a heat-treated
steel
tube or pipe.
From the viewpoint of easy formation of a coarse grain structure and easy
improvement of the high temperature strength (for example, creep rupture
strength), the
heating temperature of the heat treatment is preferably from 1050 to 1250 C,
more
preferably from 1150 C to 1250 C.
Although there is no particular restriction on the mode of cooling after the
heating during the heat treatment, and either of quenching (for example, water
cooling)
and air cooling is acceptable, quenching is preferable, and water cooling is
more
preferable.
[0116] The heat-treated steel plate or the heat-treated steel tube or pipe is
obtained for
example by preparing a steel plate or a steel tube or pipe having an chemical
composition
of the aforementioned steel of the embodiment, then heating the prepared steel
plate or
the prepared steel tube or pipe at, for example, from 1050 to 1250 C
(preferably from
1150 C to 1250 C), and thereafter cooling the same.
The steel plate or the steel tube or pipe having the chemical composition (a
steel
plate or a steel tube or pipe before a heat treatment) may be prepared
according to an
ordinary method.
A steel tube or pipe having the chemical composition may be prepared, for
example, by casting a molten steel having the chemical composition to form a
steel ingot
or a steel billet, and perfouning at least one kind of a processing selected
from the group
consisting of hot extrusion, hot rolling, hot forging, cold drawing, cold
rolling, cold
forging, and cutting, on the obtained steel ingot or steel billet.
CA 02954755 2017-01-10
[0117] Hereinabove the steel of the embodiment has been described.
There is no particular restriction on an application of the steel of the
embodiment, and the steel of the embodiment may be applied to any application
demanding securance of high temperature strength and stress corrosion cracking
5 resistance.
The steel of the embodiment is a material steel suitable for, for example, a
heat-resistant and pressure-resistant heat exchanger tube or a pipe for a
boiler, a chemical
plant, or the like; a heat-resistant forged product; a heat-resistant steel
bar; or a
heat-resistant steel plate.
10 The steel of the embodiment is a material steel especially suitable for
a
heat-resistant and pressure-resistant heat exchanger tube to be placed inside
a boiler (for
example, a heat-resistant and pressure-resistant heat exchanger tube with an
outer
diameter of from 30 to 70 mm, and a thickness of from 2 to 15 mm), or a pipe
of boiler
(for example, a pipe with an outer diameter of from 125 to 850 mm, and a
thickness of
15 from 20 to 100 mm).
EXAMPLES
[0118] Next, Examples of the invention will be described, but conditions in
the
Examples are just examples of conditions adopted for confirming the
feasibility and
20 effectiveness of the invention, and the invention is not limited to such
condition
examples. Indeed, many alternative conditions may be adopted for the
invention,
insofar as the object of the invention is achieved without departing from the
spirit and
scope of the invention.
[0119] In the Examples, 30 kinds of steels, wh'pse chemical compositions are
shown in
25 Table 1 and Table 2 (Continuation of Table 1), were produced by melting.
In Table 1 and Table 2, Steels 1 to 20 are Inventive Steels which are examples
of
the invention (hereinafter also referred to as "Inventive Steels 1 to 20"
respectively), and
Steels 21 to 30 are Comparative Steels which are comparative examples
(hereinafter also
referred to as "Comparative Steels 21 to 30" respectively).
[0120] Comparative Steel 21 is a general-purpose steel 347H (18Cr-12Ni-Nb) and
is a
standard material for comparison between the prior art and Inventive Steels 1
to 20.
[0121] In melt-producing Inventive Steels 1 to 20, as a Fe source, high purity
Fe
obtained by smelting in a blast furnace and a converter and secondary refining
by a
CA 02954755 2017-01-10
26
vacuum oxygen degassing process was used, and as an alloy element, a high
purity alloy
element analyzed in advance was used. Further, before melt-producing any of
Inventive
Steels 1 to 20, the furnace for melt-producing Inventive Steels 1 to 20 was
washed
adequately, and special care was taken so as to prevent contamination with
impurities.
Under the above special control, in producing Inventive Steels 1 to 20, the 6
impurity elements (specifically, Zr, Bi, Sn, Sb, Pb, and As) content, the 0
content, the N
content and the like were limited, and the Nd content and the B content were
regulated
within an appropriate range.
[0122] In melt-producing Comparative Steels 23 to 30, the high purity Fe
source was
used also. Further, in melt-producing Comparative Steels 23 to 30, the
chemical
compositions were adjusted as follows.
In melt-producing Comparative Steels 21, 23, 24, 27, and 29 at least one of
the 6
impurity elements and 0 (oxygen) was added intentionally.
In melt-producing Comparative Steels 21, 24, and 26, N (nitrogen) was added
intentionally.
In melt-producing Comparative Steels 21 to 23, 25, 27, and 28, at least one of
B
or Nd was not added.
In melt-producing Comparative Steel 21, Cu was added at an insufficient
content, and Mo, W, V, and Ti were not added.
In melt-producing Comparative Steel 30, W was added at an insufficient
content.
[0123]
[Table 1]
Class Steel C Si Mn P S Cr Ni Cu Mo W
2Mo+W V Ti Nb Al
1
0.09 0.20 0.80 0.015 0.001 18.10 14.20 3.01 0.10 4.02 4.22
0.03 0.20 0.21 0.008
2
0.08 0.35 1.50 0.025 0.002 18.52 14.85 3.52 0.78 2.57 4.13
0.02 0.35 0.52 0.015
3 0.06 0.12 1.25 0.019 0.001 17.58 12.12 2.42 0.05 3.21 3.31
0.08 0.06 0.42 0.005
4 0.12 0.22 0.56 0.008 0.003 18.02 13.85 2.88 0.02 3.11 3.15
0.15 0.22 0.69 0.002
0.07 0.38 0.21 0.020 0.001 18.03 14.00 3.02 0.32 2.05 2.69
0.05 0.30 0.25 0.022
6 0.11 0.15 2.45 0.006 0.001 18.41 13.92 3.45 0.02 3.21 3.25
0.38 0.06 0.66 0.013
7 0.10 0.41 0.86 0.029 0.005 17.99 12.79 2.89 0.04 3.89 3.97
0.02 0.25 0.34 0.007
8 0.08 0.20 1.52 0.012 0.010 18.07 13.24 3.14 1.22 2.01 4.45
0.04 0.33 0.44 0.015
9 0.06 0.56 1.68 0.020 0.003 17.65 13.71 3.25 0.30 3.00 3.60
0.03 0.45 0.31 0.024
Inventive 10 0.12 0.39 0.98 0.017 0.001 18.61 14.68 3.06 0.68 3.01 4.37
0.10 0.21 0.55 0.005
Steel 11
0.06 0.50 1.00 0.022 0.018 18.00 14.22 2.90 1.23 2.01 4.47
0.28 0.38 0.63 0.038 9
12
0.08 0.11 0.73 0.025 0.010 17.42 13.87 3.37 0.02 3.25 3.29
0.33 0.08 0.55 0.017 .
13 0.06 0.20 0.32 0.029 0.003 17.69 12.88 2.87 0.08 4.72 4.88
0.19 0.11 0.35 0.009 o,
14 0.11 0.35 0.21 0.010 0.007 18.21 14.53 2.99 0.50 3.21 4.21
0.26 0.28 0.41 0.010
0.09 0.45 1.05 0.023 0.001 18.10 14.01 3.10 0.31 3.79 4.41
0.17 0.37 0.42 0.031 .
,
'
16
0.07 0.30 1.22 0.011 0.002 17.93 13.70 2.69 0.08 3.52 3.68
0.16 0.10 0.39 0.019 .
,
17 0.12 0.26 0.69 0.028 0.001 17.88 12.55 3.82 0.05 4.11 4.21
0.20 0.28 0.60 0.025 ,
18 0.06 0.46 1.40 0.027 0.004 18.09 14.74 2.99 1.21 2.13 4.55
0.14 0.33 0.28 0.033
19 0.09 0.35 0.28 0.008 0.001 18.01 14.12 3.11 0.55 3.33 4.43
0.05 0.17 0.37 0.009
0.08 0.17 0.72 0.005 0.001 17.87 13.73 2.74 0.15 2.97 3.27 0.02 0.34 0.42
0.020
21 0.09 0.45 1.53 0.026 0.001 18.52 12.06 0.01 0 0 0
0 0 0.65 0.001
22 0.08 0.35 1.23 0.028 0.002 17.95 12.01 2.45 0.01 4.03 4.05
0.01 0.06 0.45 0.015
23
0.06 0.45 0.58 0.025 0.005 17.56 13.04 3.10 0.01 3.52 3.54
0.02 0.07 0.32 0.036
24 0.07 0.37 0.23 0.015 0.001 17.06 12.14 2.02 0.33 2.03 2.69
0.02 0.35 0.24 0.001
Comparative 25 0.13 0.69 1.23 0.028 0.015 17.53 12.23 2.10 0.03 2.51 2.57
0.01 0.06 0.15 0.006
Steel 26 0.11 0.36 0.14 0.028 0.009 17.23 12.03 2.04 0.52 2.23 3.27
0.01 0.05 0.16 0.004
27 0.08 0.25 0.36 0.017 0.001 18.20 12.01 2.53 0.20 2.22 2.62
0.02 0.06 0.20 0.012
28 0.07 0.89 0.15 0.032 0.005 18.02 13.01 2.03 1.12 2.03 4.27
0.05 0.06 0.23 0.035
29 0.12 0.15 0.32 0.028 0.001 18.30 12.80 3.21 0.05 2.13 2.23
0.10 0.11 0.17 0.021
0.10 0.92 0.40 0.029 0.001 17.52 12.63 2.78 0.48 1.81 2.77
0.05 0.13 0.20 0.022
[0124]
[Table 2] (Continuation of Table 1)
Class Steel B N Nd Meff Zr Bi Sn Sb Pb As Sub-total0 Others
(X)
1 0.0040 0.0080 0.18 0.149 0.001 <0.001 0.005 <0.001 <0.001
<0.001 0.006 0.0021
2 0.0015 0.0025 0.01 0.004 <0.001 <0.001 <0.001 <0.001 <0.001
<0.001 0 0.0030 Co:0.40
3 0.0052 0.0098 0.15 0.118 <0.001 <0.001 0.005 <0.001 <0.001
<0.001 0.005 0.0056
4 0.0033 0.0053 0.02 0.007 0.001 <0.001 <0.001 0.003 <0.001
<0.001 0.004 0.0086 La:0.01
0.0055 0.0015 0.18 0.235 0.001 <0.001 0.005 0.002 <0.001 <0.001 0.008
0.0050 Ce:0.18
6 0.0018 0.0085 0.08 0.015 0.001 <0.001 0.009 <0.001 <0.001
<0.001 0.010 0.0045
7 0.0023 0.0056 0.07 0.043 <0.001 <0.001 0.001 0.001 <0.001
<0.001 0.002 0.0078 Mg:0.0015
8 0.0047 0.0088 0.05 0.021 <0.001 <0.001 0.009 <0.001 <0.001
<0.001 0.009 0.0088
9 0.0023 0.0065 0.04 0.004 <0.001 <0.001 0.008
0.005 <0.001 <0.001 0.013 0.0078 Ta:0.15, Y:0.003
0.0036 0.0074 0.11 0.080 0.001 <0.001 0.005 0.001 <0.001 <0.001 0.007
0.0063
Inventive
11 0.0010 0.0090 0.09 0.011
<0.001 <0.001 <0.001 0.001 <0.001 <0.001 0.001 0.0060 Pr:0.002,
Ca:0.002 9
Steel
.
12 0.0035 0.0075 0.06 0.029 <0.001 <0.001 0.007 0.001 <0.001
<0.001 0.008 0.0038 Ca:0.0005
o,
13 0.0044 0.0042 0.02 0.033 0.001 <0.001 0.005 0.001 <0.001
<0.001 0.007 0.0047
co
:31
14 0.0036 0.0035 0.07 0.079 0.001 <0.001 0.005 0.002 <0.001
<0.001 0.008 0.0055 Re:0.010
0.0025 0.0050 0.09 0.071 <0.001 <0.001 0.008 <0.001 <0.001 <0.001 0.008
0.0068
0
.,
Co:0 0012,
. 1
16 0.0017 0.0063 0.10 0.058 <0.001 <0.001 0.005 0.001 <0.001
<0.001 0.006 0.0089 Mg:0. .
,
Hf:0.002
,
17 0.0029 0.0075 0.08 0.039 0.001 <0.001 0.005 <0.001 <0.001
<0.001 0.006 0.0064 .
18 0.0038 0.0081 0.05 0.017 <0.001 <0.001 0.008 <0.001 <0.001
<0.001 0.008 0.0041
19 0.0017 0.0087 0.07 0.002 0.001 <0.001 <0.001 0.002 <0.001
<0.001 0.003 0.0077 Sc:0.002
20 0.0026 0.0077 0.08 0.035 <0.001 <0.001 <0.001 <0.001 <0.001
<0.001 0 0.0084
21 0 0.0110 0 -
0.114 0.001 <0.001 0.002 0.003 <0.001 <0.001 0.006 0.0102
22 0.0023 0.0063 () -
0.036 0.001 <0.001 0.002 0.001 <0.001 0.001 0.005 0.0089
23 0.0010 0.0073 0 -
0.070 0.005 <0.001 0.005 0.001 0.001 0.001 0.013 0.0088
24 0.0017 0.0105 0.10 0.013 0.001 <0.001 0.001 0.001 0.001 <0.001
0.004 0.0170
Comparative 25 0.0100 0.0098 0
0.027 0.002 <0.001 <0.001 0.002 0.001 0.001 0.006 0.0087
Steel 26 0.0068 0.0530 0.02 -0.433 <0.001 <0.001 0.008 0.001 <0.001
<0.001 0.009 0.0085
27 0.0047 0.0055 0 -
0.009 0.010 0.010 0.013 0.002 0.003 <0_001 0.038 0.0089
28 0
0.0070 0.15 0.075 0.002 <0.001 0.005 0.001 <0.001 <0.001 0.008 0.0085
29 0.0028 0.0089 0.10 0.034 0.007 <0.001 0.004 0.008 <0.001
<0.001 0.019 0.0079
0.0037 0.0078 0.11 0.077 0.001 <0.001 0.002 0.005 0.001 <0.001 0.009 0.0075
CA 02954755 2017-01-10
29
[0125] - Explanation of Table 1 and Table 2 -
A numerical value represents the content of each element (mass%).
An underlined numerical value is a value outside the range of the chemical
composition of the embodiment.
A remainder of each steel excluding the elements listed in Table 1 and Table 2
is
Fe and impurities.
An Meff was calculated according to Formula (1). In this regard, for a steel
in
which the Zr content is less than 0.001% (written as "<0.001" in Table 2), the
Meff was
calculated by regarding the Zr content as 0%.
Sub-total (X) shows the total content (mass%) of the 6 impurity elements
(specifically, Zr, Bi, Sn, Sb, Pb, and As). In this regard, for an element
with a content of
less than 0.001% (written as "<0.001" in Table 2), the sub-total (X) was
calculated by
regarding the content of the element as 0%.
[0126] <Production and Heat Treatment (1200 C) of Test Material>
A steel having an chemical composition shown in Table 1 and Table 2 was
melted by vacuum melting and cast to obtain a 50 kg- steel ingot.
By hot forging the obtained steel ingot, a 15 mm-thick steel plate was
obtained.
By cutting a surface of the obtained 15 mm-thick steel plate, an approx. 12
mm-thick steel plate was obtained.
By performing cold rolling on the obtained approx. 12 mm-thick steel plate at
a
cross-section reduction rate of approx. 30% an approx. 8 mm-thick platy test
material was
obtained.
A heat treatment at 1200 C was performed on the test material by heating the
test
material to 1200 C, then keeping test material at 1200 C for 15 min, and
thereafter
cooling the test material with water.
[0127] <Measurement of ASTM Grain Size>
The ASTM grain size of the test material after the heat treatment was measured
according to ASTM E112. A measurement position of an ASTM grain size was near
the
central part in the thickness direction of a longitudinal cross-section of the
test material.
The results are shown in Table 3.
[0128] <Measurement of High Temperature St:ength>
A creep rupture test piece with a size of 6 mm4) and a length of the parallel
portion of 30 mm was cut out from the heat-treated test material, whose
longitudinal
CA 02954755 2017-01-10
direction was the longitudinal direction of the test piece. The creep rupture
test piece
was subjected to a long term creep rupture test at 700 C for 10,000 hours or
longer, and a
creep rupture strength (MPa) at 700 C and 10,000 hours was measured as a high
temperature strength.
5 The results are shown in Table 3.
[0129] <Stress Corrosion Cracking Test on Base Material>
A corrosion test piece with a width of 10 mm x a thickness of 4 mm x a length
of
mm was sliced out from the heat-treated test material. The sliced out
corrosion test
piece is hereinafter called a "base material".
10 The base material was subjected to a thet mal aging treatment at 650
C for 10
hours.
A Strauss test (ASTM A262, Practice E. Sensitization evaluation) was performed
on the base material after the thermal aging treatment, and presence or
absence of a crack
with a depth of 100 1.tm or more was examined.
15 The results of the above are shown in Table 3,
[0130] <Stress Corrosion Cracking Test on Weld HAZ (Heat Affected Zone)
Equivalent
Material >
A corrosion test piece with a width of 10 mm x a thickness of 4 mm x a length
of
40 mm was sliced out from the heat-treated test material.
20 The sliced-out test piece was heated at 950 C for 25 sec using a Greeble
tester
(Joule heating in vacuum). A weld HAZ equivalent material (i.e. a weld heat
affected
zone equivalent material) was obtained by blowing He for cooling after the
heating.
A thermal aging treatment and a Strauss test were conducted on the obtained
weld HAZ equivalent material similarly as the sress corrosion cracking test on
the base
25 material, and presence or absence of a crack with a depth of 100 p.m or
more was
examined.
The results are shown in Table 3.
CA 02954755 2017-01-10
31
,
[0131]
[Table 3]
Stress corrosion cracking test
High temperature result
ASTM strength (Existence of crack with
depth
Class Steel grain size (700 C, 10000 hours of 100 1.tm or
more)
number creep rupture strength) Weld HAZ
(MPa) Base material equivalent
material
1 4.3 165 No crack No crack
2 5.2 152 No crack No crack
3 3.1 148 No crack No crack
4 5.1 170 No crack No crack
3.8 163 No crack No crack
6 6.5 160 No crack No crack
7 6.8 150 No crack No crack
8 4.2 172 No crack No crack
9 5.2 161 No crack No crack
Inventive 10 6.2 178 No crack No crack
Steel 11 4.5 163 No crack No crack
12 3.7 155 No crack No crack
13 5.1 156 No crack No crack
14 6.1 162 No crack No crack
4.8 147 No crack No crack
16 4.0 152 No crack No crack
17 6.8 164 No crack No crack
18 3.1 157 No crack No crack
19 5.2 161 No crack No crack
4.5 149 No crack No crack
21 6.0 95 Cracked Cracked
22 4.5 125 Cracked Cracked
23 3.8 137 Cracked Cracked
24 4.5 110 Cracked Cracked
Comparative 25 2.3 107 Cracked Cracked
Steel 26 3.1 123 Cracked Cracked
27 5.3 85 Cracked Cracked
28 5.1 73 Cracked Cracked
29 4.5 81 No crack No crack
5.6 125 No crack No crack
[0132] As shown in Table 3, all of the metallic structures of Inventive Steels
1 to 20, and
5 Comparative Steels 21 to 30 were coarse grain structures with an ASTM
grain size
number of 7 or less.
[0133] As shown in Table 3, the high temperature strengths of Inventive Steels
1 to 20
were superior strengths of 147 MPa or more, which were approx. 1.5 times or
more
higher than the high temperature strength of Comparative Steel 21 (general-
purpose steel
10 34711).
CA 02954755 2017-01-10
32
Meanwhile, the high temperature strengths of Comparative Steels 21 to 30 were
as low as 137 MPa or less, which were inferior to the high temperature
strengths of
Inventive Steels 1 to 20.
[0134] As shown in Table 3, with respect to Inventive Steels 1 to 20 in both a
base
material and a weld HAZ equivalent material of an Inventive Steel, a crack
with a depth
of 100 gm or more was not observed. From the results, it was demonstrated that
Inventive Steels 1 to 20 had superior stress cracking resistance.
Meanwhile, with respect to Comparative Steels 21 to 28, a crack with a depth
of
100 um or more was observed.
[0135] More particularly, from the results of Comparative Steel 21, in which
neither B
nor N was added, and Comparative Steels 22, 23, 25, and 27, in which B but not
Nd was
added, it was demonstrated that addition of Nd is effective for improvement of
high
temperature strength and stress corrosion cracking resistance.
[0136] Further, from the results of Comparative Steel 26, in which, although
Nd and B
were added combinedly, the N content was excessive and the Meff was less than
0.0001
mass%, it was demonstrated that a combination of the N content of 0.0100% or
less and
the Meff of 0.0001 to 0.250% was effective for improvement of high temperature
strength
and stress corrosion cracking resistance.
[0137] Further, from the results of Comparative Steel 24, in which the Meff
was within
a range from 0.0001 to 0.25%, and the 0 content was beyond 0.0090%, and the N
content
was beyond 0.0100%, it was demonstrated that a combination of the 0 content of
0.0090% or less and the N content of 0.0100% or less was effective for
improvement of
high temperature strength and stress corrosion cracking resistance.
The reason behind the low high temperature strength of Comparative Steel 24 is
presumed that Nd and B were consumed as an oxide or a nitride respectively and
fine
precipitation strengthening did not develop.
[0138] From the results of Comparative Steel 28, it was demonstrated that the
B content
of 0.0010% or more was effective for improvement of high temperature strength
and
stress corrosion cracking resistance.
Further, from the results of Comparative Steel 29, it was demonstrated that
the Zr
content of 0.002% or less was effective for improvement of high temperature
strength.
Further, from the results of Comparative Steel 30, it was demonstrated that
the W
content of 2.00% or more was effective for improvement of high temperature
strength.
CA 02954755 2017-01-10
33
[0139] <Relationship between Crystal Grain Size and Stress Corrosion Cracking>
The following tests were conducted to examine the relationship between the
crystal grain size and the stress corrosion cracking of a steel with respect
to Inventive
Steels 1, 10, and 17, as well as Comparative Steels 21 and 23.
[0140] Firstly, an ASTM grain size measurement, a stress corrosion cracking
test on a
base material, and a stress corrosion cracking test on a weld HAZ equivalent
material
were conducted according to the aforementioned methods with respect to the
test material
that had been subjected to the aforementioned heat treatment at 1200 C. In
this regard,
the depth of a crack was measured and the cracking conditions were observed
precisely in
the stress corrosion cracking tests on a base material and a weld HAZ
equivalent material.
The results are shown in Table 4.
[0141] Next, the test material that had not been subjected to the
aforementioned heat
treatment at 1200 C was subjected to a heat treatment at 1125 C by heating the
test
material to 1125 C, then keeping test material at 1125 C for 15 min, and
thereafter
cooling the test material with water.
With respect to the test material having received the heat treatment at 1125
C, an
ASTM grain size measurement, a stress corrosion cracking test on a base
material, and a
stress corrosion cracking test on a weld HAZ equivalent material were
conducted
similarly as the test material having received the heat treatment at 1200 C.
The results are shown in Table 4.
[0142]
[Table 4]
Heat ASTM Stress corrosion cracking test result
treatment (Measurement result of crack depth)
Class Steel grain size
temperature
number Weld HAZ equivalent
( C) Base material
material
1 4.3 <10 pm <10 gm
10 1200 6.2 <10 pm <10 p.m
17 6.8 <10 pm <10 gm
Microcrack of Microcrack of approx.
1
Inventive 1 8. approx. 20 pm 20 pm
Steel
Microcrack of Microcrack of approx.
1125
10 9.2 approx. 20 pm 20 pm
17 9.6 Microcrack of Microcrack of approx.
approx. 20 tim 20 um
I.
34
Heat ASTM Stress corrosion cracking test
result
treatment (Measurement result of crack
depth)
Class Steel grain size
temperature
number Weld HAZ
equivalent
( C) Base material
material
21 1200 6.0 3 mm 3 mm or more,
many
Comparative 23 3.8 2 mm 3 mm or more,
many
Steel 21 1125 9.3 3 to 4 mm 3 mm or more,
many
23 8.0 2 to 3 mm 3 mm or more,
many
[0143] As shown in Table 4 and the aforementioned Table 3, the metallic
structures of
test materials having received the heat treatment at 1200 C with respect to
Inventive
Steels 1, 10, and 17, and Comparative Steels 21 and 23 were coarse grain
structures with
an ASTM grain size number of 7 or less.
Meanwhile, as shown in Table 4, the metallic structures of test materials
having
received the heat treatment at 1125 C with respect to Inventive Steels 1, 10,
and 17, and
Comparative Steels 21 and 23 became fine grain structures with an ASTM grain
size
number of 8 or more.
[0144] Further, as shown in Table 4, with respect to Inventive Steels 1, 10,
and 17, in
both the cases of fine grain structures (ASTM grain size number 8 or more) and
coarse
grain structures (ASTM grain size number 7 or less), the stress corrosion
cracking was
adequately reduced compared to Comparative Steels 21 and 23.
In contrast to the Inventive Steels, with respect to Comparative Steels 21 and
23
in both the cases of fine grain structures (ASTM grain size number 8 or more)
and coarse
grain structures (ASTM grain size number 7 or less), the crack depth in a
stress corrosion
cracking test was 2 mm or more and remarkable stress corrosion cracking
occurred.
Especially, in a weld HAZ equivalent material a large number of cracks with a
depth of 3
mm or more appeared.
[0145] As described above, stress corrosion cracking was reduced remarkably in
Inventive Steels 1, 10, and 17 compared to Comparative Steels 21 and 23.
CA 2954755 2018-05-10
