EP3178958B1

EP3178958B1 - Austenitic stainless steel

Info

Publication number: EP3178958B1
Application number: EP16803537.6A
Authority: EP
Inventors: Atsuro Iseda; Hirokazu Okada; Hiroyuki Semba; Hiroyuki Hirata; Tomoaki Hamaguchi; Kana JOTOKU; Toshihide Ono; Katsuki Tanaka
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2015-06-05
Filing date: 2016-06-03
Publication date: 2019-05-08
Anticipated expiration: 2036-06-03
Also published as: US20170268085A1; WO2016195106A1; KR20170020918A; CN106795610A; JPWO2016195106A1; EP3178958A1; JP6112270B1; CA2954755C; CN106795610B; JP2017089013A; CA2954755A1; ES2734051T3; JP6350686B2; KR101934219B1; EP3178958A4

Description

Technical Field

The present invention relates to austenitic stainless steel.

Background Art

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.
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.
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.
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.

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

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.
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.
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.
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.

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 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.
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.
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
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.
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

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%: $Effective M content Meff = Nd + 13 \cdot (B - 11 \cdot N / 14) - 1.6 \cdot Zr$
  - wherein in Formula (1), each element symbol represents a content (mass%) of each element.
$Effective M content Meff = Nd + 13 \cdot (B - 11 \cdot N / 14) - 1.6 \cdot Zr$
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

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

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.
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%. $Effective M content Meff = Nd + 13 \cdot (B - 11 \cdot N / 14) - 1.6 \cdot Zr$
wherein, in Formula (1), each element symbol represents the content (mass%) of each element.
The chemical composition of the steel of 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 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.
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.
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.
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.
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.
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.
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.
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%

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.
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%

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.
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%

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.

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.
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.

S: 0.020% or less

S is an impurity element, which disturbs workability, weldability, and stress corrosion cracking resistance.
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 welding, the S content is added at 0.020% or less, and is preferably added at 0.010% or less.
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 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%

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.
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%

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.
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.
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%

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.
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%

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.
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.
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%

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.
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.
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%

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.
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%

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.
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%

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.
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.
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.
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%

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.
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.

Al: 0.001 to 0.040%

Al is an element which functions as a deoxidizing element in steel making to purify a steel.
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.
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%

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.
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.
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.
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%

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.
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.
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.
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.
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%

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.
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.
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.
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.
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%.
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.
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.
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.
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

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.
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.
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.

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.

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 0.010% 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.
Any of the Sn content, the Sb content, the Pb content, and the As content may be 0%.
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

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%.

O: 0.0090% or less

O (oxygen) remaining inevitably after refining a molten steel is an element used as an index of the content of a nonmetallic inclusion.
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.
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.
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%.

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.

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.
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

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.
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

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%.
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.
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.
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 preferably to 0.001% or less.
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. $Effective M content Meff = Nd + 13 \cdot (B - 11 \cdot N / 14) - 1.6 \cdot Zr$
wherein in Formula (1), each element symbol represents the content (mass%) of each element.
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.
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.
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.
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.
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).
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).
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.
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.
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 (≈144/91), which is the ratio of the atomic weight of Nd (≈144) to the atomic weight of Zr (≈91), to "1.6·Zr".
In Formula (1) the "1.6·Zr" is subtracted from the "Nd+13·(B-11·N/14)".
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).
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.
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.
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 σ 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.
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).
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.
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.
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).
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.
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.
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.
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 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.
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 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.
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

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.
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.
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).
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.
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.
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.
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 O (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.

[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	Mg:0.0012, Co:0.20 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
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 -

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%.

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.

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.

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.
The results are shown in Table 3.

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".
The base material was subjected to a thermal 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 µm or more was examined.
The results of the above are shown in Table 3.

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 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 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.

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)
Class	Steel	ASTM grain size number		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

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.
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).
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.
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.
Meanwhile, with respect to Comparative Steels 21 to 28, a crack with a depth of 100 µm or more was observed.
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.
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.
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.
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.
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.

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.
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.
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.

[Table 4]

Class	Steel	Heat treatment temperature (°C)	ASTM grain size number	Stress corrosion cracking test result (Measurement result of crack depth)
Class	Steel	Heat treatment temperature (°C)	ASTM grain size number	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	1200	3.8	2 mm	3 mm or more, many
	21	1125	9.3	3 to 4 mm	3 mm or more, many
	23	1125	8.0	2 to 3 mm	3 mm or more, many

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.
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.
As described above, stress corrosion cracking was reduced remarkably in Inventive Steels 1, 10, and 17 compared to Comparative Steels 21 and 23.

Claims

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%: $Effective M content Meff = Nd + 13 \cdot (B - 11 \cdot N / 14) - 1.6 \cdot Zr$
wherein in Formula (1), each element symbol represents a content (mass%) of each element.
The austenitic stainless steel according to claim 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.
The austenitic stainless steel according to claim 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.
The austenitic stainless steel according to any one of claims 1 to 3, wherein an ASTM grain size number, as measured according to ASTM E112, of a metallic structure thereof is 7 or less.
The austenitic stainless steel according to any one of claims 1 to 4, wherein a creep rupture strength at 700°C and 10,000 hours is 140 MPa or more.
The austenitic stainless steel according to any one of claims 1 to 5, wherein the effective M content Meff is 0.002% to 0.250%.