JP6547599B2 - Austenitic heat resistant steel - Google Patents

Description

  The present invention relates to an austenitic heat resistant steel.

  Hitherto, 18-8 austenitic heat resistant steels such as SUS304H, SUS316H, SUS321H, and SUS347H have been used as materials for equipment of boilers and chemical plants used under high temperature environment. However, in recent years, the operating conditions of the device under such high temperature environment are extremely severe, and the performance requirements for the used materials are becoming strict accordingly. Therefore, in the case of the 18-8 austenitic heat resistant steel, the high temperature strength, in particular, the creep rupture strength, the steam oxidation resistance, and the high temperature corrosion resistance are extremely insufficient.

  Therefore, an austenitic heat-resistant steel has been developed in which the creep rupture strength is improved by optimizing the content of various elements of the austenitic heat-resistant steel containing about 20% or more of Cr.

  Japanese Patent No. 3838216 discloses an austenitic stainless steel containing more than 20% and less than 28% Cr. The document describes that creep rupture strength, creep rupture ductility, and hot workability are improved by adding Cu, Nb and N in combination and controlling P and O according to the Cu content. It is done.

  According to Japanese Patent No. 4258678, the content of P, S, Sn, Sb, Pb, Zn and As in an austenitic stainless steel containing 15 to 30% of Cr is limited in the welding heat affected zone. It is disclosed that the resistance to brittleness cracking has been improved.

  Japanese Patent No. 4946758 discloses an austenitic stainless steel containing more than 22% and less than 30% of Cr by limiting the content of P, S, Sn, Sb, Pb, Zn and As. It is disclosed that the processability of is improved.

  Japanese Patent No. 5000805 discloses an austenitic stainless steel that contains 20 to 27% of Cr and is excellent in creep rupture strength, water vapor oxidation resistance, and the like.

  Patent No. 5670103 discloses that high creep rupture strength can be obtained by adding Mo, W and N in an austenitic stainless steel containing 18.0 to 26.0% of Cr. ing.

  Japanese Patent No. 5661001 discloses that, in an austenitic heat-resistant steel containing 18 to 23% of Cr, excellent high temperature strength and toughness after aging can be obtained by defining the austenite balance.

  In JP-A 2013-67843, excellent creep rupture strength can be obtained by optimizing the addition amounts of Mo, W and Nb in an austenitic heat-resistant steel containing 18 to 23% Cr. Is disclosed.

Patent No. 3838216 Patent No. 4258678 gazette Patent No. 4946758 Patent No. 5000805 Patent No. 5670103 Patent No. 5661001 JP, 2013-67843, A

  Recently, in the field of boilers for thermal power generation, for example, plans for raising the steam temperature to 650 to 700 ° C. or more are being promoted at home and abroad. If the steam temperature is raised to 650-700 ° C. or higher, the temperature of the components used will far exceed 700 ° C. In such an environment, the creep rupture strength and the corrosion resistance may be insufficient even with the austenitic heat-resistant steel described in the above-mentioned document.

  In addition, in the austenitic heat-resistant steel in which emphasis is placed on the improvement of the creep rupture strength, it has been clarified by the investigation of the present inventors that the creep rupture ductility tends to be low when held for a long time at high temperature.

  An object of the present invention is to provide an austenitic heat-resistant steel excellent in long-term structural stability and having high creep rupture strength and creep rupture ductility.

In the austenitic heat resistant steel according to one embodiment of the present invention, the chemical composition is, by mass%, C: 0.02 to 0.12%, Si: 0.1 to 2%, Mn: 0.1 to 3%, P: 0.04% or less, S: 0.02% or less, Cr: 20 to 26%, Ni: 26% or more and 35% or less, W: 1 to 5.5%, V: 0.01 to 1% Nb: 0.01 to 1%, B: 0.0005 to 0.008%, Mo: 0.3% or less, Al: 0.001 to 0.3%, Cu: 0.3% or less, Ti: 0.01% or less, N: more than 0.13% and 0.35% or less, REM: 0.003 to 0.10%, Mg: 0 to 0.05%, Ca: 0 to 0.05%, Zr : 0 to 0.1%, Hf: 0 to 0.5%, Ta: 0 to 1%, Re: 0 to 5%, Co: 0 to 10%, balance: Fe and impurities, the chemical composition is , The following formula ( ) And satisfying the (2).
0.15 [N residue] ≦ [REM] ≦ 0.7 [N residue] (1)
6.2 [N] [[W] 1 17.1 [N] (2)
Here, the content of REM, N, and W is substituted by mass% into [REM], [N], and [W], and [N residue] is a nitride when heated at 750 ° C. for 3000 hours. The amount of nitrogen deposited is substituted by mass%.

  According to the present invention, an austenitic heat resistant steel excellent in long-term structural stability and having high creep rupture strength and creep rupture ductility can be obtained.

The present inventors clarified that it is necessary to satisfy the following a) to e) in order to develop high creep rupture strength and creep rupture ductility.
a) Cr: 20 to 26%, Ni: more than 26% and 35% or less b) W: 1 to 5.5%, Mo: 0.3% or less c) V: 0.01 to 1%, Nb: 0 .01 to 1%, N: more than 0.13% and 0.35% or less d) 6.2 [N] [[W] 1 17.1 [N]
e) 0.15 [N residue] ≦ [REM] ≦ 0.7 [N residue]
Here, the content of REM, N, and W is substituted by mass% into [REM], [N], and [W], and [N residue] is a nitride when heated at 750 ° C. for 3000 hours. The amount of nitrogen deposited is substituted by mass%.

  In order to achieve both corrosion resistance and high-temperature long-term structural stability, it is necessary to set each content of Cr and Ni in the range of the above (a).

  In such an austenitic heat resistant steel containing Cr and Ni, in order to realize high creep rupture strength, the content of each of W and Mo is made the range of the above (b), and each of V, Nb, and N is It is effective to make the content in the range of the above (c).

  By containing W, in addition to the solid solution strengthening, the creep rupture strength is improved by the precipitation strengthening by the Laves phase or the like. On the other hand, Mo, which is considered to have the same strengthening action as W, precipitates the sigma phase in an austenitic heat resistant steel containing 20% or more of Cr when held at high temperature for a long time. Therefore, the Mo content needs to be strictly reduced to 0.3% or less.

  Furthermore, when V, Nb, and N are contained in appropriate amounts, nitrides such as (V, Nb) N and Z phases are precipitated, and creep rupture strength is significantly improved.

  However, W also promotes the precipitation of relatively coarse nitride π phase. In order to secure the precipitation of the solid solution W and Laves phase and the fine (V, Nb) N and Z phases, to obtain high creep rupture strength, to suppress the precipitation of excess π phase, and to obtain high creep rupture ductility The W content and the N content need to satisfy the relationship shown in the above (d). That is, in order to secure the precipitation amount of solid solution W and Laves phase, it is necessary to make W content more than 6.2 [N]. On the other hand, the W content is 17.1 [N] or less from the viewpoint of creep rupture strength improvement by securing fine nitrides, creep rupture ductility by precipitation of excessive π phase, and toughness reduction after high temperature long time heating. You need to

  In order to ensure a sufficient creep rupture ductility even when held at high temperature for a long time, the REM content and the amount of nitrogen deposited as a nitride when heated at 750 ° C. for 3000 hours are indicated by the above (e) It is necessary to satisfy the relationship. That is, when the REM content is too low relative to the amount of nitride, the intergranular strength is insufficient relative to the intragranular strength, and the creep rupture ductility is reduced. Therefore, the REM content needs to be 0.15 [N residue] or more. On the other hand, when the REM content is too high relative to the amount of nitride, creep rupture ductility also decreases due to excess inclusions and a decrease in the local melting point of grain boundaries. Therefore, REM content needs to be 0.7 [N residue] or more.

  The present invention has been completed based on the above findings. Hereinafter, the austenitic heat-resistant steel according to one embodiment of the present invention will be described in detail.

[Chemical composition]
The austenitic heat resistant steel according to the present embodiment has the chemical composition described below. In the following description, “%” of the content of the element means mass%.

C: 0.02 to 0.12%
Carbon (C) is an essential element to obtain high temperature tensile strength and creep rupture strength necessary for austenitic heat resistant steel. In order to obtain this effect, the C content needs to be 0.02% or more. On the other hand, when the C content exceeds 0.12%, undissolved carbides are formed or Cr carbides are increased, thereby deteriorating the mechanical properties such as ductility and toughness and weldability. Therefore, the C content is 0.02 to 0.12%. The lower limit of the C content is preferably 0.05%. The upper limit of the C content is preferably 0.10%.

Si: 0.1 to 2%
Silicon (Si) is contained as a deoxidizing element. Si is also an element effective for enhancing oxidation resistance, water vapor oxidation resistance and the like. In order to acquire this effect, it is necessary to make Si content 0.1% or more. On the other hand, if the Si content exceeds 2%, precipitation of intermetallic compound phases such as the σ phase is promoted, and toughness and ductility are lowered due to deterioration of the structural stability at high temperatures. In addition, the weldability also decreases. Therefore, the Si content is 0.1 to 2%. The lower limit of the Si content is preferably 0.2%. The upper limit of the Si content is preferably 1%.

Mn: 0.1 to 3%
Manganese (Mn), like Si, has a deoxidizing action on molten steel. Mn also fixes S, which is inevitably contained in steel, as sulfide and improves ductility at high temperatures. In order to acquire these effects, it is necessary to make Mn content 0.1% or more. On the other hand, when the Mn content exceeds 3%, precipitation of intermetallic compound phase such as σ phase is promoted, and the structure stability, the high temperature strength and the mechanical property are deteriorated. Therefore, the Mn content is 0.1 to 3%. The lower limit of the Mn content is preferably 0.2%. The upper limit of the Mn content is preferably 2%, more preferably 1.5%.

P: 0.04% or less Phosphorus (P) is contained in steel as an impurity, and significantly reduces weldability and ductility at high temperatures. Therefore, the P content is 0.04% or less. The P content is preferably as low as possible, preferably 0.03% or less, and more preferably 0.02% or less.

S: 0.02% or less Sulfur (S) is contained as an impurity in steel, as with P, and significantly reduces weldability and ductility at high temperatures. Therefore, the S content is 0.02% or less. The S content is preferably 0.01% or less, more preferably 0.003% or less, in the case where hot workability is important.

Cr: 20 to 26%
Chromium (Cr) plays an important role in improving corrosion resistance, such as oxidation resistance, water vapor oxidation resistance, high temperature corrosion resistance, etc., and it is also important to contribute to the improvement of creep rupture strength by forming Cr carbonitride. Element. In the present embodiment, the Cr content needs to be 20% or more in order to place importance on steam oxidation resistance and high temperature corrosion resistance under severe high temperature environments. On the other hand, when the Cr content exceeds 26%, the structure becomes unstable due to the precipitation of the σ phase and the like, and the weldability also deteriorates. Therefore, the Cr content is 20 to 26%. The lower limit of the Cr content is preferably 21%. The upper limit of the Cr content is preferably 25%.

Ni: more than 26% and 35% or less Nickel (Ni) is an element that stabilizes the austenite structure, and is also an important element to ensure corrosion resistance. In particular, in the austenitic heat resistant steel containing 20% or more of Cr as in the present embodiment, when the Ni content is insufficient, the sigma phase precipitates when held at high temperature for a long time, and creep rupture strength and creep rupture ductility And the toughness is significantly reduced. From the above balance with the Cr content, the Ni content needs to be higher than 26%. On the other hand, excessive Ni impairs weldability and economy, so the upper limit of the Ni content is set to 35%. Therefore, the Ni content is more than 26% and 35% or less. The lower limit of the Ni content is preferably 26.5%. The upper limit of the Ni content is preferably 33%. The Ni content should be adjusted in consideration of the content of the ferrite stabilizing element such as W.

W: 1 to 5.5%
Tungsten (W) contributes to the improvement of the creep rupture strength as a solid solution strengthening element, and precipitates the Fe ₂ W type Laves phase and the Fe ₇ W ₆ type μ phase, and significantly strengthens the creep rupture strength by precipitation strengthening. It is an important element to improve. These effects can not be obtained when the W content is less than 1%. On the other hand, when the W content exceeds 5.5%, while the effect of improving the strength is saturated, the structure stability is lowered, and the creep rupture ductility and the toughness are deteriorated by the excessive precipitates. Therefore, the W content is 1 to 5.5%. The lower limit of the W content is preferably 1.2%. The upper limit of the W content is preferably 5%, more preferably 4%. Furthermore, W also promotes the precipitation of relatively coarse nitride π phase, and therefore, it is necessary to control appropriately according to the N content as shown in the equation (d).

V: 0.01 to 1%
Vanadium (V) is an important element which forms fine carbonitrides with Nb and greatly contributes to the improvement of creep rupture strength. If the V content is less than 0.01%, this effect can not be obtained. On the other hand, when the V content exceeds 1%, the creep rupture strength and the toughness decrease, and the high temperature corrosion resistance also deteriorates. Therefore, the V content is 0.01 to 1%. The lower limit of the V content is preferably 0.05%, and more preferably 0.08%. The upper limit of the V content is preferably 0.8%, more preferably 0.7%.

Nb: 0.01 to 1%
Niobium (Nb) is an important element which forms fine carbonitrides together with V and greatly contributes to the improvement of creep rupture strength. Nb further suppresses precipitation of Cr carbonitrides at grain boundaries, and also contributes to the improvement of stress corrosion cracking resistance. If the Nb content is less than 0.01%, this effect can not be obtained. On the other hand, when the Nb content exceeds 1%, the creep rupture ductility and toughness decrease, and the hot workability also deteriorates. Therefore, the Nb content is 0.01 to 1%. The lower limit of the Nb content is preferably 0.05%, more preferably 0.08%. The upper limit of the Nb content is preferably 0.8%, more preferably 0.7%.

B: 0.0005 to 0.008%
Boron (B) is an important element which exists in carbides or in a matrix, and refines precipitated carbides, Laves phase, etc., or strengthens grain boundaries to improve creep rupture strength. In order to exert this effect, the B content needs to be 0.0005% or more. On the other hand, when the B content exceeds 0.008%, the ductility at high temperatures is lowered and the melting point is also lowered. Therefore, the B content is 0.0005 to 0.008%. The lower limit of the B content is preferably 0.001%. The upper limit of the B content is preferably 0.007%, more preferably 0.005%.

Mo: 0.3% or less Molybdenum (Mo) is an impurity mixed from a melting material and the like. Conventionally, Mo is considered to have an action equivalent to W as an element which is solid-solved in the matrix and contributes to the improvement of the creep rupture strength. However, when Mo is contained in the chemical composition of the present embodiment, the sigma phase precipitates when held at high temperature for a long time, and the creep rupture strength, the ductility, and the toughness decrease. In particular, in the present embodiment, since the content of Cr is 20% or more, the Mo content needs to be as low as possible. Therefore, the Mo content is 0.3% or less. The Mo content is preferably 0.2% or less.

Al: 0.001 to 0.3%
Aluminum (Al) is an element to be contained as a deoxidizer for molten steel. In order to exhibit this effect, it is necessary to make Al content 0.001% or more. However, if the Al content exceeds 0.3%, a large amount of nonmetallic inclusions precipitate, and the ductility, toughness, processability, etc. deteriorate. Therefore, the Al content is 0.001 to 0.3%. The lower limit of the Al content is preferably 0.005%, and more preferably 0.010%. The upper limit of the Al content is preferably 0.25%, and more preferably 0.2%.

Cu: 0.3% or less Copper (Cu) is an impurity mixed from a melting material and the like. Cu precipitates as a fine Cu phase in a normal austenitic heat resistant steel to improve creep rupture strength. However, Cu inhibits the ductility improvement effect by REM. After securing a sufficiently high creep rupture strength up to a high temperature of 700 ° C. or more with nitrides of V and Nb, Laves phase, etc., the Cu content needs to be as low as possible in this embodiment, which places importance on creep rupture ductility and toughness. . Therefore, the Cu content is 0.3% or less. The Cu content is preferably 0.2% or less.

Ti: 0.01% or less Titanium (Ti) is an impurity mixed from a melting material or the like. Since Ti forms coarse Ti nitrides that do not contribute to the improvement of creep rupture strength and consumes N, the improvement effect of the creep rupture strength due to the N content is reduced. Therefore, the Ti content is 0.01% or less. The Ti content is preferably 0.008% or less.

N: more than 0.13% and 0.35% or less Nitrogen (N) is an important element which forms a nitride together with V, Nb and the like to improve creep rupture strength. N also improves tensile strength by solid solution strengthening. N further has the effect of stabilizing austenite. In order to obtain these effects sufficiently, the N content needs to be higher than 0.13%. On the other hand, if the N content exceeds 0.35%, precipitation of excess nitride reduces ductility and toughness, and blow hole defects are formed in the steel. Therefore, the N content is more than 0.13% and 0.35% or less. The preferable lower limit of the N content is 0.15%. The preferred upper limit of the N content is 0.30%.

REM: 0.003 to 0.10%
The rare earth elements (REM) fix S in the grain boundaries as sulfides and improve creep rupture ductility particularly when held at high temperature for a long time. REM further improves the adhesion of the Cr ₂ O ₃ protective coating on the steel surface, in particular improving the oxidation resistance during repeated oxidation. These effects can not be obtained when the REM content is less than 0.003%. On the other hand, when the REM content exceeds 0.10%, inclusions such as oxides increase and the workability and weldability are impaired. Therefore, the REM content is 0.003 to 0.10%. The lower limit of the REM content is preferably 0.005%. The upper limit of the REM content is preferably 0.07%, and more preferably 0.05%.

  In addition, REM is a general term for 17 elements in total of Sc, Y and lanthanoid, and the content of REM indicates the total content of one or more elements of REM.

  The balance of the chemical composition of the austenitic heat resistant steel according to the present embodiment is Fe and impurities. The term "impurity" as used herein means an element mixed in from ore and scrap used as a raw material of steel, or an element mixed in from the environment of the manufacturing process or the like.

  The chemical composition of the austenitic heat resistant steel according to the present embodiment is, instead of part of the above Fe, at least one element selected from one or more of the following first to fourth groups. You may contain. The following elements are all selective elements. That is, any of the following elements may not be contained in the austenitic heat resistant steel according to the present embodiment. Moreover, only a part may be contained.

  More specifically, for example, only one group may be selected from the groups from the first group to the fourth group, and one or more elements may be selected from the group. In this case, it is not necessary to select all the elements belonging to the selected group. Alternatively, a plurality of groups may be selected from the first group to the fourth group, and one or more elements may be selected from each group. Also in this case, it is not necessary to select all the elements belonging to the selected group.

First group Mg: 0 to 0.05%, Ca: 0 to 0.05%
The elements belonging to the first group are Mg and Ca. These elements improve the high temperature ductility of austenitic heat resistant steels.

Mg: 0 to 0.05%
Mg fixes S as sulfide which degrades ductility at high temperature, and improves high temperature ductility. This effect can be obtained if any Mg is contained. On the other hand, when the Mg content exceeds 0.05%, the steel quality is impaired, and the high temperature ductility is impaired. Therefore, the Mg content is 0 to 0.05%. The lower limit of the Mg content is preferably 0.0005%, more preferably 0.001%. The upper limit of the Mg content is preferably 0.02%, and more preferably 0.01%.

Ca: 0 to 0.05%
Ca fixes S as sulfide which degrades ductility at high temperatures, and improves high temperature ductility. This effect can be obtained if any amount of Ca is contained. On the other hand, when the Ca content exceeds 0.05%, the steel quality is impaired, and the high temperature ductility is impaired. Therefore, the Ca content is 0 to 0.05%. The lower limit of the Ca content is preferably 0.0005%, more preferably 0.001%. The upper limit of the Ca content is preferably 0.02%, and more preferably 0.01%.

Second group Zr: 0 to 0.1%, Hf: 0 to 0.5%
The elements belonging to the second group are Zr and Hf. These elements improve the creep rupture strength of the steel.

Zr: 0 to 0.1%
Zirconium (Zr) improves creep rupture strength by refining the carbonitrides and strengthening the grain boundaries. This effect can be obtained if any of Zr is contained. On the other hand, when the Zr content exceeds 0.1%, the ductility at high temperatures is reduced. Therefore, the Zr content is 0 to 0.1%. The lower limit of the Zr content is preferably 0.005%, and more preferably 0.01%. The upper limit of the Zr content is preferably 0.06%, more preferably 0.05%.

Hf: 0 to 0.5%
Hafnium (Hf) improves creep rupture strength by precipitation strengthening with carbonitrides. This effect can be obtained if Hf is contained at all. On the other hand, when the Hf content exceeds 0.5%, the workability and weldability of the steel are impaired. Therefore, the Hf content is 0 to 0.5%. The lower limit of the Hf content is preferably 0.005%, more preferably 0.01%, and still more preferably 0.02%. The upper limit of the Hf content is preferably 0.4%, more preferably 0.3%.

Third group Ta: 0 to 1%, Re: 0 to 5%
The elements belonging to the third group are Ta and Re. These elements improve the high temperature strength and creep rupture strength of the steel.

Ta: 0 to 1%
Tantalum (Ta) forms carbonitrides and improves high temperature strength and creep rupture strength as a solid solution strengthening element. If any Ta is contained, this effect is obtained. On the other hand, if the Ta content exceeds 1%, the workability and mechanical properties of the steel are impaired. Therefore, the Ta content is 0 to 1%. The lower limit of the Ta content is preferably 0.01%, more preferably 0.05%, and still more preferably 0.1%. The upper limit of the Ta content is preferably 0.7%, more preferably 0.6%.

Re: 0 to 5%
Rhenium (Re) mainly improves the high temperature strength and creep rupture strength as a solid solution strengthening element. This effect is obtained as long as Re is contained. On the other hand, when the Re content exceeds 5%, the workability and mechanical properties of the steel are impaired. Therefore, the Re content is 0 to 5%. The lower limit of the Re content is preferably 0.01%, more preferably 0.1%, and still more preferably 0.5%. The upper limit of the Re content is preferably 4%, more preferably 3%.

Fourth group Co: 0 to 10%
An element belonging to the fourth group is Co. Co stabilizes the austenitic structure of the steel.

Co: 0 to 10%
Cobalt (Co), like Ni, stabilizes the austenitic structure and also contributes to the improvement of creep rupture strength. This effect can be obtained as long as Co is contained. On the other hand, when the Co content exceeds 10%, the effect is saturated and the economic efficiency is lowered. Therefore, the Co content is 0 to 10%. The Co content is preferably 0.5% or more, and more preferably higher than 1% in the viewpoint of the lower limit. The Co content is preferably 8% or less, more preferably 7% or less, in the viewpoint of the upper limit.

[About formula (1)]
The chemical composition of the austenitic heat resistant steel according to the present embodiment satisfies the following formula (1) while the content of each element is in the above-mentioned range.
0.15 [N residue] ≦ [REM] ≦ 0.7 [N residue] (1)
Here, REM content is substituted by mass% for [REM], and the amount of nitrogen that precipitates as a nitride when heated at 750 ° C. for 3000 hours is substituted for mass% for [N residue].

  In the present embodiment, in order to improve creep rupture ductility when held at high temperature for a long time, the content of REM is controlled according to the amount of nitride to be deposited. If the REM content is too low relative to the amount of the precipitated nitride, the intergranular strength is insufficient with respect to the intergranular strength, and the creep rupture ductility when held at high temperatures for a long time is reduced. Therefore, the REM content needs to be 0.15 [N residue] or more. On the other hand, when the REM content is too high relative to the amount of the precipitated nitride, the inclusions become excessive or the local melting point of the grain boundaries decreases, so that the creep when held for a long time at high temperature Fracture ductility is reduced. Therefore, REM content needs to be 0.7 [N residue] or less.

  [N residue] is measured as follows. Specimens for residual determination are collected from austenitic heat resistant steel. The specimen is subjected to an aging heat treatment of heating at 750 ° C. for 3000 hours. Chips are collected from the aging-treated test pieces. The collected chips are extracted with Br ester (10% bromine-methyl acetate), and the residue obtained after filtration is acid-decomposed, and then precipitated as a nitride by ammonia distillation separated amide sulfuric acid titration method defined in JIS G1228. Find the mass of nitrogen that is The mass of nitrogen determined is divided by the mass of chips to obtain [N residue] (unit: mass%). A mixed solution of sulfuric acid, copper sulfate and potassium sulfate is used for acid decomposition of the residue.

  [N residue] can be adjusted by the N content and the respective contents of V, Nb, W and the like which form a nitride. Specifically, if the content of these elements is increased, the [N residue] is increased.

[About formula (2)]
The chemical composition of the austenitic heat resistant steel according to the present embodiment further satisfies the following formula (2).
6.2 [N] [[W] 1 17.1 [N] (2)
Here, the contents of N and W are substituted for [N] and [W] by mass%.

  In addition to making each content of N and W into a predetermined range, in this embodiment, it is necessary to control W content appropriately according to N content. Specifically, in order to secure the precipitation amount of the solid solution W and the Laves phase, the W content needs to be 6.2 [N] or more. On the other hand, the W content is 17.1 [N] or less from the viewpoint of creep rupture strength improvement by securing fine nitrides, creep rupture ductility by precipitation of excessive π phase, and toughness reduction after high temperature long time heating. You need to

[Production method]
An example of a method for producing an austenitic heat resistant steel according to an embodiment of the present invention will be described. The manufacturing method of the austenitic heat-resistant steel by this embodiment is not limited to this example.

  Prepare materials of the above-mentioned chemical composition. Specifically, for example, the steel of the above-described chemical composition is melted and refined.

  Hot work the material. Hot working is, for example, hot rolling or hot forging. After hot working, cold working may be performed if necessary.

  The hot-worked material or the cold-worked material after hot working is subjected to solution heat treatment. Specifically, after holding the material at a predetermined temperature, watering is performed. The holding temperature of the solution treatment is not particularly limited, but is, for example, 900 to 1300 ° C.

  The embodiments of the present invention have been described above. According to this embodiment, an austenitic heat-resistant steel excellent in long-term structural stability and having high creep rupture strength and creep rupture ductility can be obtained. The austenitic heat resistant steel according to the present embodiment preferably has a creep rupture strength of at least 125 MPa at 700 ° C. for 10000 hours, and a creep rupture elongation at 700 ° C. of 130 MPa of at least 15%.

  The embodiments described above are merely illustrative for practicing the present invention. Therefore, the present invention is not limited to the embodiment described above, and the embodiment described above can be appropriately modified and implemented without departing from the scope of the invention.

  Hereinafter, the present invention will be more specifically described by way of examples. The present invention is not limited to these examples.

  Steels 1 to 6 and steels A to H having chemical compositions shown in Table 1 were melted using a high frequency vacuum melting furnace to obtain ingots of 30 kg each. In Table 1, "-" indicates that the content of the corresponding element is at the impurity level.

  Each ingot was heated to 1200 ° C., and then hot forged so that the finishing temperature was 1000 ° C., to obtain a plate having a thickness of 15 mm. The plate was softened and heat treated at 1100 ° C., then cold rolled to 10 mm, further held at 1180 ° C. for 30 minutes, and then water cooled.

  The amount [N residue] of nitrogen deposited as a nitride when heated at 750 ° C. for 3000 hours was determined for each of the water-cooled plates by the method described in the above embodiment. The results are shown in Table 1 together.

  A round bar test piece having a diameter of 6 mm and a marking distance of 30 mm was produced by machining from the thickness direction center of each plate parallel to the longitudinal direction, and a creep rupture test was carried out in accordance with JIS Z2271. The creep rupture test was conducted in the atmosphere at 700 ° C., 750 ° C. and 800 ° C., and the creep rupture strength at 700 ° C. for 10000 hours was estimated using the Larson-Miller parameter method. Moreover, the creep rupture test of 700 degreeC and 130 Mpa was implemented, and breaking elongation was measured. The results are shown in Table 2.

  The content of each element of steels 1 to 8 was within the range defined by the present invention. The chemical compositions of the steels 1 to 8 further satisfied the equations (1) and (2). Steels 1 to 8 have creep rupture strengths of 700 ° C. and 10000 hours (hereinafter referred to simply as “creep rupture strength”) of 125 MPa or more, and creep rupture elongations at 700 ° C. and 130 MPa (hereinafter referred to simply as “creep elongation and elongation”) Is more than 15%.

  Steels A and B had low creep rupture strength. This is considered to be because Steel B did not contain V because Steel A did not contain Nb.

  Steel C had high creep rupture strength but low creep rupture elongation. This is considered to be because the Nb content and V content of steel C were too high.

  Steel D had low creep rupture strength and creep rupture elongation. This is considered to be because the Ni content of steel D was too low.

  Steels E and F had high creep rupture strength but low creep rupture elongation. This is considered to be because the REM content was too low relative to the amount of N residue.

  Steels G and H had high creep rupture strength but low creep rupture elongation. This is considered to be because the REM content was too high relative to the amount of N residue.

  Steels I and J had low creep rupture strength. This is considered to be because the W content of steel I and steel J was too low relative to the amount of nitrogen.

  Steel K had high creep rupture strength but low creep rupture elongation. This is considered to be because the W content of steel K was too high relative to the amount of nitrogen.

  Steel L had low creep rupture strength and creep rupture elongation. This is considered to be because the Mo content of the steel L was too high.

  Steel M had low creep rupture strength. This is considered to be because the Ti content of the steel M was too high.

  Steel N had a high creep rupture strength but a low creep rupture elongation. This is considered to be because the Cu content of steel N was too high.

  According to the present invention, an austenitic heat-resistant steel suitably used as a steel pipe, a steel plate of a heat-resistant pressure-resistant member, a valve or the like in a power generation boiler, a plant for chemical industry or the like can be obtained.

Claims (3)

The chemical composition is in mass%,
C: 0.02 to 0.12%,
Si: 0.1 to 2%,
Mn: 0.1 to 3%,
P: 0.04% or less,
S: 0.02% or less,
Cr: 20 to 26%,
Ni: more than 26% and 35% or less,
W: 1 to 5.5%,
V: 0.01 to 1%,
Nb: 0.01 to 1%
B: 0.0005 to 0.008%,
Mo: 0.3% or less,
Al: 0.001 to 0.3%,
Cu: 0.3% or less,
Ti: 0.01% or less,
N: more than 0.13% and 0.35% or less,
REM: 0.003 to 0.10%,
Mg: 0 to 0.05%,
Ca: 0 to 0.05%,
Zr: 0 to 0.1%,
Hf: 0 to 0.5%,
Ta: 0 to 1%,
Re: 0-5%,
Co: 0 to 10%
Remainder: Fe and impurities,
An austenitic heat resistant steel, wherein the chemical composition satisfies the following formulas (1) and (2).
0.15 [N residue] ≦ [REM] ≦ 0.7 [N residue] (1)
6.2 [N] [[W] 1 17.1 [N] (2)
Here, the content of REM, N, and W is substituted by mass% into [REM], [N], and [W], and [N residue] is a nitride when heated at 750 ° C. for 3000 hours. The amount of nitrogen deposited is substituted by mass%. The austenitic heat resistant steel according to claim 1, wherein
An austenitic heat resistant steel, wherein the chemical composition contains, by mass%, at least one selected from at least one of the following first to fourth groups.
First group Mg: 0.0005 to 0.05%, Ca: 0.0005 to 0.05%
Second group Zr: 0.005 to 0.1%, Hf: 0.005 to 1%
Third group Ta: 0.01 to 1%, Re: 0.01 to 5%
Fourth Group Co: 0.05 to 10% The austenitic heat resistant steel according to claim 1 or 2, wherein
An austenitic heat resistant steel having a creep rupture strength of at least 125 MPa at 700 ° C. for 10000 hours and a creep rupture elongation at 700 ° C. of 130 MPa of at least 15%.