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.
[0003] In other areas of the world, including Europe or China, highly efficient USC boilers
have been constructed one after another from the viewpoint of CO
2 reduction as a global environmental countermeasure.
[0004] 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.
[0005] 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.
[0006] Patent Literature 2 discloses an austenitic stainless steel superior in high temperature
corrosion thermal fatigue cracking resistance.
[0007] Patent Literature 3 discloses a heat-resistant austenitic stainless steel superior
in high temperature strength and cyclic oxidation resistance.
[0008] 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.
[0009] 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.
[0010] 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
for utilizing solid solution strengthening and nitride precipitation strengthening
so as to make up low strength of a low carbon stainless steel.
Patent Literature 7 relates to an austenitic stainless steel consisting of, in percent
by mass, C: 0.05-0.15 %, Si: not more than 2 %, Mn: 0.1-3%, P: 0.05-0.30 %, S: not
more than 0.03 %, Cr: 15-28 %, Ni: 8-55 %, Cu: 0-3.0 %, Ti: 0.05-0.6 %, REM: 0.001-0.5
%, sol. Al: 0.001-0.1%, N: not more than 0.03 %, and the balance being Fe and incidental
impurities.
[0011]
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
Patent Literature 7: EP 1 867 743 A1
SUMMARY OF INVENTION
Technical Problem
[0012] 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.
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] There is however a drawback in any of the methods.
[0018] 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.
[0019] 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.
[0020] In the case of a method of adding a large amount of Cr, since a high content of brittle
phase such as σ-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.
[0021] 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.
[0022] 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.
[0023] 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
[0024] 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,
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.
[0025] 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
[0026] The means for achieving the object includes the following aspects.
- <1> 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 O,
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%:
wherein in Formula (1), each element symbol represents a content (mass%) of each element.
wherein in Formula (1), each element symbol represents the content of each element
(mass%)).
- <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, as measured according to ASTM E112, 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 700°C and 10,000 hours is 140 MPa or more.
- <6> The austenitic stainless steel according to any one of <1> to <5>, wherein the
effective M content Meff is 0.002% to 0.250%.
Advantageous Effects of Invention
[0027] 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.
DESCRIPTION OF EMBODIMENTS
[0028] Embodiments of the invention will be described below.
[0029] 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.
[0030] The content of an element expressed by "%" and an effective M content Meff expressed
by "%" both mean herein "mass%".
[0031] Further, the content of C (carbon) may be herein occasionally expressed as "C content".
The content of another element may be expressed similarly.
[0032] 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 O, 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%.
wherein, in Formula (1), each element symbol represents the content (mass%) of each
element.
[0033] The chemical composition of the steel of the embodiment includes 17.00 to 19.00%
of Cr.
[0034] In other words, the steel of the embodiment belongs to the 18 Cr - based austenitic
stainless steel.
[0035] 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 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.
[0036] 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.
[0037] 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.
[0038] 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 effective M content Meff in the above range.
[0039] 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.
[0040] 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.
[0041] 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.
[0042] 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).
[0043] This finding is a novel finding contrary to heretofore common sense.
[0044] 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 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.
[0045] 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%).
[0046] 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.
[0047] 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.
[0048] 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.
C: 0.05 to 0.13%
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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 cracking
to reduce toughness. Therefore, the C content is 0.13% or less, and is preferably
0.12% or less.
Si: 0.10 to 1.00%
[0053] 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.
[0054] Meanwhile, when the Si content exceeds 1.00%, the workability declines, and a brittle
phase such as a σ-phase precipitates at a high temperature. Therefore, the Si content
is 1.00% or less, and is preferably 0.80% or less.
Mn: 0.10 to 3.00%
[0055] 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.
[0056] 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.
P: 0.040% or less
[0057] P is an impurity element, which disturbs workability and weldability.
[0058] 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.
[0059] Preferably the P content is as low as possible, and may be even 0%.
[0060] 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.
S: 0.020% or less
[0061] S is an impurity element, which disturbs workability, weldability, and stress corrosion
cracking resistance.
[0062] 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.
[0063] Even in a case in which S is added for improvement of molten metal flow in welding,
the S content is added at 0.020% or less, and is preferably added at 0.010% or less.
[0064] Preferably the S content is as low as possible, and may be even 0%.
[0065] 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 production cost
greatly. Therefore, the S content may be 0.001% or more from the viewpoint of production
cost.
Cr: 17.00 to 19.00%
[0066] 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.
[0067] 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.
[0068] 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.
Ni: 12.00 to 15.00%
[0069] 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.
[0070] When the Ni content is less than 12.00%, the addition effect is not obtained adequately,
and formation of a brittle phase (σ-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.
[0071] 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.
Cu: 2.00 to 4.00%
[0072] 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.
[0073] 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.
[0074] 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.
Mo: 0.01 to 2.00%
[0075] 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.
[0076] 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.
[0077] 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.
W: 2.00 to 5.00%
[0078] 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, and therefore
it is an element to contribute to stable maintenance of the strength at a high temperature
for a long period of time.
[0079] 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.
[0080] 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.
2Mo+W: 2.50 to 5.00%
[0081] 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.
[0082] 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.
V: 0.01 to 0.40%
[0083] 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.
[0084] 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.
Ti: 0.05 to 0.50%
[0085] 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.
[0086] 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.
[0087] 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.
[0088] 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.
[0089] 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.
Nb: 0.15 to 0.70%
[0090] 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.
[0091] Further, Nb is, similar to Ti, an element contributing to improvement of the high
temperature strength due to precipitation of a fine Laves phase.
[0092] 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.
[0093] 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.
Al: 0.001 to 0.040%
[0094] Al is an element which functions as a deoxidizing element in steel making to purify
a steel.
[0095] 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.
[0096] Meanwhile, when the Al content exceeds 0.040%, a large amount of nonmetallic inclusion
is formed 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.
B: 0.0010 to 0.0100%
[0097] 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.
[0098] 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.
[0099] 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.
[0100] Meanwhile, when the B content exceeds 0.0100%, a boron compound is formed 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.
N: 0.0010 to 0.0100%
[0101] 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 the steel of the embodiment, a nitride disturbs stress corrosion cracking resistance,
and therefore N is not added actively.
[0102] 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 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.
[0103] 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.
[0104] 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% or more, and is preferably 0.0020%
or more, and more preferably 0.0030% or more.
[0105] Meanwhile, when the N content exceeds 0.0100%, a nitride is formed to 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.
Nd: 0.001 to 0.20%
[0106] 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.
[0107] 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.
[0108] However, since the bonding strength of Nd with N, O, 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, O content, and S content
to the extent possible.
[0109] When the Nd content is less than 0.001 %, even if the N content, O content, and S
content are reduced, the addition effect of Nd cannot be obtained adequately. Therefore
the Nd content is 0.001% or more, and is preferably 0.002% or more, and more preferably
0.005% or more.
[0110] 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.
[0111] 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%.
[0112] With respect to the steel of the embodiment, Zr, Bi, Sn, Sb, Pb, As, and O 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.
[0113] 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.
[0114] 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 O (oxygen) remains inevitably in a stainless steel.
[0115] 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 O are required to be reduced to the extent possible so as to prepare
a high purity steel.
Zr: 0.002% or less
[0116] 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.
[0117] 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.
[0118] As described above, Zr is a harmful element in terms of high temperature strength
and stress corrosion cracking resistance. Therefore in a relational expression (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.
[0119] 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.
[0120] 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.
Bi: 0.001% or less
[0121] 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.
[0122] 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.
[0123] 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.
Sn: 0.010% or less
Sb: 0.010% or less
Pb: 0.001% or less
As: 0.001% or less
[0124] 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.
[0125] However, the contents of the elements are required to be reduced to the extent as
possible.
[0126] Considering source materials composition and refining limits, the upper limits of
the Sn content and the Sb content are set at 0.010% respectively. The Sn content and
the Sb content are preferably 0.005% or less respectively.
[0127] 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.
[0128] Any of the Sn content, the Sb content, the Pb content, and the As content may be
0%.
[0129] However, 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.
Zr+Bi+Sn+Sb+Pb+As: 0.020% or less
[0130] 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.
[0131] The total content of the 6 impurity elements in the steel of the embodiment is 0.020%
or less.
[0132] The total content of the 6 impurity elements is preferably 0.015% or less, and more
preferably 0.010% or less.
[0133] 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%.
O: 0.0090% or less
[0134] O (oxygen) remaining inevitably after refining a molten steel is an element used
as an index of the content of a nonmetallic inclusion.
[0135] When O 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 O 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.
[0136] The O content may be 0%. However, O may occasionally remain after refining inevitably
at 0.0001% or so. Therefore, from the viewpoint of production cost, the O content
may be 0.0001% or even more.
[0137] 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.
[0138] Any of the elements is an optional element, and therefore the contents thereof may
be respectively 0%.
Co: 0.80% or less
[0139] 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.
[0140] 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.
[0141] When the steel of the embodiment contains Co, the Co content is preferably 0.01%
or more, and more preferably 0.03% or more.
Ca: 0.20% or less
[0142] Ca is an optional element, and the Ca content may be 0%.
[0143] 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.
[0144] 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.
Mg: 0.20% or less
[0145] Mg is an optional element, and the Mg content may be 0%.
[0146] 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.
[0147] 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.
Total of one or more of Y, Sc, Ta, Hf, Re or lanthanoid elements other than Nd: 0.20%
or less
[0148] 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
content of the elements may be 0%.
[0149] 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
more.
[0150] 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.
[0151] A remainder excluding the aforementioned elements from the chemical composition of
the steel of the embodiment is Fe and impurities.
[0152] 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 preferably
to 0.001% or less.
[0153] 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%.
[0154] The effective M content Meff will be described below.
wherein in Formula (1), each element symbol represents the content (mass%) of each
element.
[0155] 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.
[0156] 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.
[0157] 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.
[0158] 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.
[0159] 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.
[0160] In Formula (1) defining an effective M content Meff, the moiety of "(B-11·N/14)"
represents the content of B that effectively functions (namely, the content of B that
is not bound with N, among the B that has been added).
[0161] 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 (≈11).
[0162] In Formula (1), "13·(B-11·N/14)" obtained as above is added to the Nd content ("Nd
+13·(B-11·N/14)"). Nd is an element that functions effectively similarly as B for
securing superior high temperature strength and superior stress corrosion cracking
resistance.
[0163] 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.
[0164] The impurity element Zr, by forming a nitride and an oxide, functions to reduce a
synergistic effect of combined addition of Nd and B.
[0165] In Formula (1) the reduction effect of Zr is weighted by multiplying the Zr content
by 1.6 (≈144/91), which is the ratio of the atomic weight of Nd (≈144) to the atomic
weight of Zr (≈91), to "1.6·Zr".
[0166] In Formula (1) the "1.6·Zr" is subtracted from the "Nd+13·(B-11·N/14)".
[0167] As described above, the addition amounts of Nd and B necessary for obtaining superior
high temperature strength and superior stress corrosion cracking resistance, and 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).
[0168] 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
[0169] In this regard, when the N content or the Zr content is high, the effective M content
Meff may take a negative value.
[0170] 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.
[0171] There is no particular restriction on the metallic structure of the steel of the
embodiment.
[0172] 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).
[0173] Specifically, with respect to the steel of the embodiment, the ASTM grain size number
of the metallic structure thereof is preferably 7 or less.
[0174] 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 σ phase can be conceivably
obtained.
[0175] 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.
[0176] 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.
[0177] 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).
[0178] 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.
[0179] 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.
[0180] A steel of the embodiment is superior in high temperature strength (especially, creep
rupture strength) as described above.
[0181] 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.
[0182] In this regard, 700°C is a temperature higher than an actual usage temperature.
[0183] 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.
[0184] 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).
[0185] 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.
[0186] 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.
[0187] 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.
[0188] A steel of the embodiment may be a heat-treated steel plate or a heat-treated steel
tube or pipe.
[0189] 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.
[0190] 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.
[0191] 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.
[0192] 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.
[0193] 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 performing 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.
[0194] Hereinabove the steel of the embodiment has been described.
[0195] 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 resistance.
[0196] 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.
[0197] 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 from
20 to 100 mm).
EXAMPLES
[0198] Next, Examples of the invention will be described, but conditions in the Examples
are just examples of conditions adopted for confirming the feasibility and 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 they are within
the scope of the claims.
[0199] In the Examples, 30 kinds of steels, whose chemical compositions are shown in Table
1 and Table 2 (Continuation of Table 1), were produced by melting.
[0200] 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).
[0201] 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.
[0202] 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 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.
[0203] 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 O content, the N
content and the like were limited, and the Nd content and the B content were regulated
within an appropriate range.
[0204] 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.
[0205] In melt-producing Comparative Steels 21, 23, 24, 27, and 29 at least one of the 6
impurity elements and O (oxygen) was added intentionally.
[0206] In melt-producing Comparative Steels 21, 24, and 26, N (nitrogen) was added intentionally.
[0207] In melt-producing Comparative Steels 21 to 23, 25, 27, and 28, at least one of B
or Nd was not added.
[0208] In melt-producing Comparative Steel 21, Cu was added at an insufficient content,
and Mo, W, V, and Ti were not added.
[0209] In melt-producing Comparative Steel 30, W was added at an insufficient content.
[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 |
|
5 |
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 |
|
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 |
|
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 |
|
15 |
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 |
|
20 |
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 |
Comparative Steel |
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 |
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 |
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 |
30 |
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 |
[Table 2]
(Continuation of Table 1) |
Class |
Steel |
B |
N |
Nd |
Meff |
Zr |
Bi |
Sn |
Sb |
Pb |
As |
Sub-total (X) |
O |
Others |
|
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 |
|
5 |
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 |
Inventive Steel |
10 |
0.0036 |
0.0074 |
0.11 |
0.080 |
0.001 |
<0.001 |
0.005 |
0.001 |
<0.001 |
<0.001 |
0.007 |
0.0063 |
|
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 |
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 |
|
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 |
|
|
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 |
|
15 |
0.0025 |
0.0050 |
0.09 |
0.071 |
<0.001 |
<0.001 |
0.008 |
<0.001 |
<0.001 |
<0.001 |
0.008 |
0.0068 |
Mg:0.0012, Co:0.20 Hf:0.002 |
|
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 |
|
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 |
|
Comparative Steel |
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 |
-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 |
|
25 |
0.0100 |
0.0098 |
0 |
0.027 |
0.002 |
<0.001 |
<0.001 |
0.002 |
0.001 |
0.001 |
0.006 |
0.0087 |
|
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 |
|
30 |
0.0037 |
0.0078 |
0.11 |
0.077 |
0.001 |
<0.001 |
0.002 |
0.005 |
0.001 |
<0.001 |
0.009 |
0.0075 |
|
- Explanation of Table 1 and Table 2 -
[0210] A numerical value represents the content of each element (mass%).
[0211] An underlined numerical value is a value outside the range of the chemical composition
of the embodiment.
[0212] A remainder of each steel excluding the elements listed in Table 1 and Table 2 is
Fe and impurities.
[0213] 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%.
[0214] 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%.
<Production and Heat Treatment (1200°C) of Test Material>
[0215] 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.
[0216] By hot forging the obtained steel ingot, a 15 mm-thick steel plate was obtained.
[0217] By cutting a surface of the obtained 15 mm-thick steel plate, an approx. 12 mm-thick
steel plate was obtained.
[0218] 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.
[0219] 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.
<Measurement of ASTM Grain Size>
[0220] 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.
[0221] The results are shown in Table 3.
<Measurement of High Temperature Strength>
[0222] A creep rupture test piece with a size of 6 mmφ and a length of the parallel portion
of 30 mm was cut out from the heat-treated test material, whose longitudinal 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.
[0223] The results are shown in Table 3.
<Stress Corrosion Cracking Test on Base Material>
[0224] A corrosion test piece with a width of 10 mm × a thickness of 4 mm × a length of
40 mm was sliced out from the heat-treated test material. The sliced out corrosion
test piece is hereinafter called a "base material".
[0225] The base material was subjected to a thermal aging treatment at 650°C for 10 hours.
[0226] 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 µm or more was examined.
[0227] The results of the above are shown in Table 3.
<Stress Corrosion Cracking Test on Weld HAZ (Heat Affected Zone) Equivalent Material
>
[0228] A corrosion test piece with a width of 10 mm × a thickness of 4 mm × a length of
40 mm was sliced out from the heat-treated test material.
[0229] 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.
[0230] A thermal aging treatment and a Strauss test were conducted on the obtained weld
HAZ equivalent material similarly as the stress corrosion cracking test on the base
material, and presence or absence of a crack with a depth of 100 µm or more was examined.
[0231] The results are shown in Table 3.
[Table 3]
Class |
Steel |
ASTM grain size number |
High temperature strength (700°C, 10000 hours creep rupture strength) (MPa) |
Stress corrosion cracking test result (Existence of crack with depth of 100 µm or
more) |
Base material |
Weld HAZ 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 |
|
5 |
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 |
|
15 |
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 |
|
20 |
4.5 |
149 |
No crack |
No crack |
Comparative Steel |
21 |
6.0 |
95 |
Cracked |
Cracked |
22 |
4.5 |
125 |
Cracked |
Cracked |
23 |
3.8 |
137 |
Cracked |
Cracked |
24 |
4.5 |
110 |
Cracked |
Cracked |
25 |
2.3 |
107 |
Cracked |
Cracked |
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 |
30 |
5.6 |
125 |
No crack |
No crack |
[0232] As shown in Table 3, all of the metallic structures of Inventive Steels 1 to 20,
and Comparative Steels 21 to 30 were coarse grain structures with an ASTM grain size
number of 7 or less.
[0233] 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
347H).
[0234] 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.
[0235] 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 µm or more was not observed. From the results, it was demonstrated that Inventive
Steels 1 to 20 had superior stress cracking resistance.
[0236] Meanwhile, with respect to Comparative Steels 21 to 28, a crack with a depth of 100
µm or more was observed.
[0237] 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.
[0238] 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.
[0239] Further, from the results of Comparative Steel 24, in which the Meff was within a
range from 0.0001 to 0.25%, and the O content was beyond 0.0090%, and the N content
was beyond 0.0100%, it was demonstrated that a combination of the O 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.
[0240] 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.
[0241] 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.
[0242] 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.
[0243] 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.
<Relationship between Crystal Grain Size and Stress Corrosion Cracking>
[0244] 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.
[0245] 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.
[0246] The results are shown in Table 4.
[0247] 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.
[0248] 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.
[0249] The results are shown in Table 4.
[Table 4]
Class |
Steel |
Heat treatment temperature (°C) |
ASTM grain size number |
Stress corrosion cracking test result (Measurement result of crack depth) |
Base material |
Weld HAZ equivalent material |
Inventive Steel |
1 |
1200 |
4.3 |
<10 µm |
<10 µm |
10 |
6.2 |
<10 µm |
<10 µm |
17 |
6.8 |
<10 µm |
<10 µm |
1 |
|
8.1 |
Microcrack of approx. 20 µm |
Microcrack of approx. 20 µm |
10 |
1125 |
9.2 |
Microcrack of approx. 20 µm |
Microcrack of approx. 20 µm |
17 |
|
9.6 |
Microcrack of approx. 20 µm |
Microcrack of approx. 20 µm |
Comparative Steel |
21 |
1200 |
6.0 |
3 mm |
3 mm or more, many |
23 |
3.8 |
2 mm |
3 mm or more, many |
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 |
[0250] 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.
[0251] 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.
[0252] 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.
[0253] 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.
[0254] As described above, stress corrosion cracking was reduced remarkably in Inventive
Steels 1, 10, and 17 compared to Comparative Steels 21 and 23.