WO2017119415A1

WO2017119415A1 - Austenitic heat-resistant alloy and method for manufacturing same

Info

Publication number: WO2017119415A1
Application number: PCT/JP2017/000056
Authority: WO
Inventors: 礼文河内; 真木　純; 西山　佳孝
Original assignee: 新日鐵住金株式会社
Priority date: 2016-01-05
Filing date: 2017-01-04
Publication date: 2017-07-13
Also published as: JP6493566B2; KR102090201B1; JPWO2017119415A1; SG11201805206PA; EP3401415A4; KR20180095640A; US20190010565A1; CN108474072A; EP3401415A1; CA3009770A1

Abstract

Provided is an austenitic heat-resistant alloy having high creep strength and high toughness even in a high-temperature environment. This austenitic heat-resistant alloy has a chemical composition containing, in terms of mass%, 0.03 to less than 0.25% C, 0.01-2.0% Si, 2.0% or less of Mn, 10 to less than 30% Cr, more than 25% to 45% Ni, more than 2.5% to less than 4.5% Al, 0.2-3.5% Nb, and 0.025% or less of N, the remainder comprising Fe and unavoidable impurities, and P and S being 0.04% or less and 0.01% or less, respectively, of the impurities. The gross moment of precipitates 6 µm or larger in the structure of the austenitic heat-resistant alloy is 5% or less.

Description

Austenitic heat-resistant alloy and manufacturing method thereof

The present invention relates to a heat resistant alloy and a method for producing the same, and more particularly to an austenitic heat resistant alloy and a method for producing the same.

Conventionally, 18-8 stainless steel is used as heat resistant steel in equipment such as boilers and chemical plants used in high temperature environments. 18-8 stainless steel is an austenitic stainless steel containing about 18% Cr and about 8% Ni, and examples thereof include SUS304H, SUS316H, SUS321H, and SUS347H in the JIS standard.

In recent years, the use conditions of equipment in a high temperature environment have become extremely severe, and higher creep strength is required than 18-8 stainless steel. Recently, an advanced ultra-supercritical power generation program has been promoted to raise the steam temperature of about 600 ° C. to 700 ° C. or higher in a boiler for thermal power generation. Also in the chemical plant, in order to increase the operation efficiency, an increase in the operation temperature is planned. Steel materials used in these high-temperature environments are required to have excellent creep resistance as well as high creep strength.

Heat resistant materials with improved corrosion resistance have been proposed in, for example, Japanese Patent Application Laid-Open No. 02-115348 (Patent Document 1) and Japanese Patent Application Laid-Open No. 07-316751 (Patent Document 2). Since these Al alloys have a high Al content, an Al ₂ O ₃ film is formed on the surface at high temperatures during use. This coating provides high corrosion resistance.

However, the heat resistant alloys disclosed in Patent Documents 1 and 2 described above may have a low creep strength in a high temperature environment of 700 ° C. or higher.

As a heat-resistant material having high creep strength in a high temperature environment of 700 ° C. or higher, a heat-resistant alloy containing Ni and Co and a γ ′ phase (Ni ₃ Al) as a strengthening phase has been developed. Such heat-resistant alloys are, for example, Ni-base alloys such as Alloys 617, 263, and 740. However, the alloy raw materials for these heat-resistant alloys are expensive. Furthermore, since the processability is low, the manufacturing cost is also increased.

Therefore, a heat-resistant alloy that is less expensive than the Ni-based alloy and has excellent creep strength is proposed in Japanese Patent Laid-Open No. 2014-43621 (Patent Document 3) and Japanese Patent Laid-Open No. 2013-227644 (Patent Document 4). ing.

The austenitic heat-resistant alloy disclosed in Patent Document 3 is in mass%, C: less than 0.02%, Si: 2% or less, Mn: 2% or less, Cr: 15-26%, Ni: 20-35% Al: 0.3% or less, P: 0.04% or less, S: 0.01% or less and N: 0.05% or less, Ti: 3.0% or less (including 0%), Including one or more selected from V: 3.0% or less (including 0%), Nb: less than 2.3% (including 0%), and Ta: 2.0% or less (including 0%) F1 represented by f1 = 2Ti + 2V + Nb + (1/2) Ta satisfies 1.5 to 6.0, and the remainder has a chemical composition composed of Fe and impurities. Patent Document 3 describes that the austenitic heat-resistant alloy has excellent high-temperature strength and toughness due to precipitation strengthening of Laves phase and γ ′ phase.

The austenitic heat-resistant alloy disclosed in Patent Document 4 is, in mass%, C: less than 0.02%, Si: 0.01-2%, Mn: 2% or less, Cr: 20% or more and less than 28%, Ni : More than 35% to 50% or less, W: 2.0 to 7.0%, Mo: less than 2.5% (including 0%), Nb: less than 2.5% (including 0%), Ti : Less than 3.0% (including 0%), Al: 0.3% or less, P: 0.04% or less, S: 0.01% or less and N: 0.05% or less, the balance being Fe1 and impurities, and f1 represented by f1 = 1 / 2W + Mo is 1.0 to 5.0, f2 represented by f2 = 1 / 2W + Mo + Nb + 2Ti is 2.0 to 8.0, and f3 = Nb + 2Ti. The f3 represented has a chemical composition of 0.5 to 5.0. Patent Document 4 describes that the austenitic heat-resistant alloy has excellent high-temperature strength and toughness due to precipitation strengthening of Laves phase and γ ′ phase.

Japanese Patent Laid-Open No. 02-115348 Japanese Patent Laid-Open No. 07-316751 JP 2014-43621 A JP 2013-227644 A

However, in the case of an alloy that uses a strengthening mechanism based on the Laves phase and the γ 'phase, such as the heat resistant alloys of Patent Documents 3 and 4, the creep strength and toughness after aging for a long time may decrease.

An object of the present invention is to provide an austenitic heat resistant alloy having high creep strength and high toughness even in a high temperature environment.

The austenitic heat-resistant alloy according to the present embodiment is, in mass%, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0% or less, Cr: 10 to 30 %, Ni: more than 25 to 45%, Al: more than 2.5 to less than 4.5%, Nb: 0.2 to 3.5%, N: 0.025% or less, Ti: 0 to 0.2 %: W: 0-6%, Mo: 0-4%, Zr: 0-0.1%, B: 0-0.01%, Cu: 0-5%, Rare earth elements: 0-0.1 %, Ca: 0 to 0.05%, and Mg: 0 to 0.05%, the balance is made of Fe and impurities, and P and S in the impurities are each P: 0.04% or less, And S: having a chemical composition of 0.01% or less. In the structure, the total volume ratio of precipitates having an equivalent circle diameter of 6 μm or more is 5% or less. Here, the precipitate is, for example, carbide, nitride, NiAl, and α-Cr.

The austenitic heat-resistant alloy according to the present embodiment has long-term high-temperature strength and excellent toughness even in a high-temperature environment.

The present inventors investigated and examined the creep strength and toughness of the austenitic heat resistant alloy in a high temperature environment of 700 ° C. or higher (hereinafter simply referred to as a high temperature environment), and obtained the following knowledge.

As described above, a heat-resistant alloy containing a Laves phase or a γ ′ phase such as Ni ₃ Al has a high creep strength in a high-temperature environment. However, since these precipitated phases become coarse when used for a long time in a high temperature environment, the creep strength and toughness of the heat-resistant alloy are lowered.

On the other hand, if a precipitate such as carbide, nitride, NiAl, α-Cr, etc. can be finely dispersed while using a heat-resistant alloy in a high-temperature environment, high creep strength and high toughness can be maintained even after long-term use. These precipitates increase the grain boundary strength by covering the crystal grain boundaries. Furthermore, if these precipitates precipitate in the grains, the deformation resistance of the heat-resistant alloy increases and the creep strength increases.

In order to increase the creep strength and toughness by the fine precipitates described above, the structure of the heat-resistant alloy before use is controlled as follows.

[Limit of amount of precipitates with equivalent circle diameter of 6 μm or more]
In the solidified structure after casting the heat-resistant alloy, there are precipitates of carbide, nitride, NiAl, α-Cr, etc. (hereinafter simply referred to as precipitates). These precipitates are generated in a liquid phase in which solute elements existing between dendrites are concentrated. These precipitates usually have a coarse shape and are unevenly distributed in the tissue. Therefore, the toughness of the heat resistant alloy is reduced.

Furthermore, even when the solution treatment is performed, these precipitates are not easily dissolved and remain in a coarse state. If these precipitates remain coarsely in the heat-resistant alloy, it is difficult to form fine precipitates during use in a high temperature environment. Therefore, the total volume ratio of coarse precipitates in the heat-resistant alloy is preferably as low as possible.

If the total volume fraction of precipitates having an equivalent circle diameter of 6 μm or more (hereinafter referred to as coarse precipitates) is 5% or less in the structure of the heat-resistant alloy, a sufficient amount can be obtained while using the heat-resistant alloy in a high-temperature environment. Fine precipitates can be deposited, and high creep strength and toughness can be obtained.

In order to make the total volume ratio of coarse precipitates in the structure 5% or less, the C content in the heat-resistant alloy is set to less than 0.25%. Furthermore, the cross-section reduction rate during hot forging is set to 30% or more. In this case, coarse precipitates are uniformly dispersed by hot forging. Therefore, the precipitate can be dissolved in the solution treatment in the subsequent step, and the total volume ratio of the coarse precipitate becomes 5% or less.

The austenitic heat-resistant alloy according to the present embodiment completed based on the above knowledge is mass%, C: 0.03 to less than 0.25%, Si: 0.01 to 2.0%, Mn: 2.0. %: Cr: 10 to less than 30%, Ni: more than 25 to 45%, Al: more than 2.5 to less than 4.5%, Nb: 0.2 to 3.5%, N: 0.025% or less Ti: 0 to less than 0.2%, W: 0 to 6%, Mo: 0 to 4%, Zr: 0 to 0.1%, B: 0 to 0.01%, Cu: 0 to 5%, Rare earth elements: 0 to 0.1%, Ca: 0 to 0.05%, and Mg: 0 to 0.05%, the balance is composed of Fe and impurities, and P and S in the impurities are respectively It has a chemical composition of P: 0.04% or less and S: 0.01% or less. In the structure, the total volume ratio of precipitates having an equivalent circle diameter of 6 μm or more is 5% or less.

The chemical composition is, by mass, Ti: 0.005 to less than 0.2%, W: 0.005 to 6%, Mo: 0.005 to 4%, Zr: 0.0005 to 0.1%, And B: One or more selected from the group consisting of 0.0005 to 0.01% may be contained.

The chemical composition may contain one or more selected from the group consisting of Cu: 0.05 to 5% and rare earth elements: 0.0005 to 0.1% by mass.

The chemical composition may contain at least one selected from the group consisting of Ca: 0.0005 to 0.05% and Mg: 0.0005 to 0.05% by mass.

The method for producing the austenitic heat-resistant alloy includes a step of performing hot forging at a cross-section reduction rate of 30% or more on a cast material having the above-described chemical composition, and heating the material after hot forging. A step of producing an intermediate material by performing inter-processing, and a step of performing a solution treatment on the intermediate material at 1100 to 1250 ° C.

Hereinafter, the austenitic heat-resistant alloy of this embodiment will be described in detail. “%” Regarding an element means mass% unless otherwise specified.

[Chemical composition]
The austenitic heat-resistant alloy according to the present embodiment is, for example, an alloy tube. The chemical composition of the austenitic heat-resistant alloy contains the following elements.

C: 0.03 to less than 0.25% Carbon (C) forms a carbide and increases the creep strength. Specifically, during use in a high temperature environment, C combines with an alloy element within a grain boundary and within the grains to form fine carbides. Fine carbides increase deformation resistance and increase creep strength. If the C content is too low, this effect cannot be obtained. On the other hand, if the C content is too high, many coarse eutectic carbides are formed in the solidified structure after casting of the heat-resistant alloy. Since the eutectic carbide remains coarse in the structure even after the solution treatment, the toughness of the heat-resistant alloy is lowered. Furthermore, if coarse eutectic carbides remain, fine carbides are less likely to precipitate during use in a high temperature environment, and the creep strength decreases. Therefore, the C content is 0.03 to less than 0.25%. The minimum with preferable C content is 0.05%, More preferably, it is 0.08%. The upper limit with preferable C content is 0.23%, More preferably, it is 0.20%.

Si: 0.01 to 2.0%
Silicon (Si) deoxidizes the heat-resistant alloy. Si further enhances the corrosion resistance (oxidation resistance and steam oxidation resistance) of the heat-resistant alloy. Si is an element inevitably contained, but the content of Si may be as small as possible when deoxidation can be sufficiently performed with other elements. On the other hand, if the Si content is too high, the hot workability decreases. Therefore, the Si content is 0.01 to 2.0%. The minimum with preferable Si content is 0.02%, More preferably, it is 0.03%. The upper limit with preferable Si content is 1.0%.

Mn: 2.0% or less Manganese (Mn) is unavoidably contained. Mn combines with S contained in the heat-resistant alloy to form MnS and enhances the hot workability of the heat-resistant alloy. However, if the Mn content is too high, the heat-resistant alloy becomes too hard and the hot workability and weldability deteriorate. Therefore, the Mn content is 2.0% or less. The minimum with preferable Mn content is 0.1%, More preferably, it is 0.2%. The upper limit with preferable Mn content is 1.2%.

Cr: 10 to less than 30% Chromium (Cr) improves the corrosion resistance (oxidation resistance, steam oxidation resistance, etc.) of the heat-resistant alloy in a high-temperature environment. Further, Cr is finely precipitated as α-Cr during use in a high temperature environment, thereby increasing the creep strength. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, the stability of the structure decreases and the creep strength decreases. Therefore, the Cr content is 10 to less than 30%. The minimum with preferable Cr content is 11%, More preferably, it is 12%. The upper limit with preferable Cr content is 28%, More preferably, it is 26%.

Ni: Over 25-45%
Nickel (Ni) stabilizes austenite. Ni further enhances the corrosion resistance of the heat-resistant alloy. If the Ni content is too low, these effects cannot be obtained. On the other hand, if the Ni content is too high, these effects are not only saturated, but hot workability is reduced. If the Ni content is too high, the raw material cost further increases. Therefore, the Ni content is more than 25 to 45%. The minimum with preferable Ni content is 26%, More preferably, it is 28%. The upper limit with preferable Ni content is 44%, More preferably, it is 42%.

Al: more than 2.5 to less than 4.5% Aluminum (Al) is combined with Ni to form fine NiAl during use in a high temperature environment, and increases the creep strength. Further, Al enhances corrosion resistance in a high temperature environment of 1000 ° C. or higher. If the Al content is too low, these effects cannot be obtained. On the other hand, if the Al content is too high, the structural stability is lowered and the strength is lowered. Therefore, the Al content is more than 2.5 to less than 4.5%. The minimum with preferable Al content is 2.55%, More preferably, it is 2.6%. The upper limit with preferable Al content is 4.4%, More preferably, it is 4.2%. In the austenitic heat-resistant alloy according to the present invention, the Al content means the total amount of Al contained in the steel material.

Nb: 0.2-3.5%
Niobium (Nb) forms a Laves phase and a Ni ₃ Nb phase as precipitation strengthening phases, and precipitates and strengthens the crystal grain boundaries and crystal grains, thereby increasing the creep strength of the heat-resistant alloy. If the Nb content is too low, the above effect cannot be obtained. On the other hand, if the Nb content is too high, the Laves phase and the Ni ₃ Nb phase are excessively generated, and the toughness and hot workability of the alloy are lowered. If the Nb content is too high, the toughness after aging for a long time further decreases. Therefore, the Nb content is 0.2 to 3.5%. The minimum with preferable Nb content is 0.35%, More preferably, it is 0.5%. The upper limit with preferable Nb content is less than 3.2%, More preferably, it is 3.0%.

N: 0.025% or less Nitrogen (N) stabilizes austenite and is inevitably contained in a normal dissolution method. In addition, during use in a high temperature environment, N combines with the alloy element in the grain boundaries and grains to form fine nitrides. Fine nitride increases deformation resistance and increases creep strength. However, if the N content is too high, coarse nitrides that remain undissolved even after the solution treatment are formed and the toughness of the alloy is lowered. Therefore, the N content is 0.025% or less. The upper limit of the preferable N content is 0.02%, more preferably 0.01%.

P: 0.04% or less Phosphorus (P) is an impurity. P decreases the weldability and hot workability of the heat-resistant alloy. Therefore, the P content is 0.04% or less. The upper limit with preferable P content is 0.03%. The P content is preferably as low as possible.

S: 0.01% or less Sulfur (S) is an impurity. S decreases the weldability and hot workability of the heat-resistant alloy. Therefore, the S content is 0.01% or less. The upper limit with preferable S content is 0.008%. The S content is preferably as low as possible.

The balance of the chemical composition of the austenitic heat-resistant alloy of this embodiment is composed of Fe and impurities. Here, the impurities are those mixed from the ore, scrap, or production environment as raw materials when industrially producing austenitic heat-resistant alloys, and are allowed within a range that does not adversely affect the present invention. Means what will be done.

[Arbitrary elements]
The chemical composition of the austenitic heat-resistant alloy described above may further contain one or more selected from the group consisting of Ti, W, Mo, Zr and B, instead of a part of Fe. All of these elements are optional elements and increase the creep strength.

Ti: 0 to less than 0.2% Titanium (Ti) is an optional element and may not be contained. When it is contained, a Laves phase and a Ni ₃ Ti phase that are precipitation strengthening phases are formed, and the creep strength is increased by precipitation strengthening. However, if the Ti content is too high, the Laves phase and the Ni ₃ Ti phase are excessively generated, and the hot ductility and hot workability are reduced. If the Ti content is too high, the toughness after aging for a long time further decreases. Therefore, the Ti content is 0 to less than 0.2%. The minimum with preferable Ti content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable Ti content is 0.15%, More preferably, it is 0.1%.

W: 0-6%
Tungsten (W) is an optional element and may not be contained. When contained, it dissolves in the austenite of the matrix (matrix), and increases the creep strength by solid solution strengthening. Further, W forms a Laves phase in the crystal grain boundaries and in the crystal grains, and increases the creep strength by precipitation strengthening. However, if there is too much W content, a Laves phase will be generated excessively and hot ductility, hot workability, and toughness will fall. Accordingly, the W content is 0 to 6%. The minimum with preferable W content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable content of W is 5.5%, More preferably, it is 5%.

Mo: 0-4%
Molybdenum (Mo) is an optional element and may not be contained. When contained, it dissolves in the austenite of the parent phase and increases the creep strength by solid solution strengthening. Mo further forms a Laves phase in the grain boundaries and in the crystal grains, and increases the creep strength by precipitation strengthening. However, if the Mo content is too high, the Laves phase is excessively generated and the hot ductility, hot workability, and toughness are reduced. Therefore, the Mo content is 0 to 4%. The minimum with preferable Mo content is 0.005%, More preferably, it is 0.01%. The upper limit with preferable Mo content is 3.5%, More preferably, it is 3%.

Zr: 0 to 0.1%
Zirconium (Zr) is an optional element and may not be contained. When contained, Zr increases creep strength by grain boundary strengthening. However, if the Zr content is too high, the weldability and hot workability of the heat-resistant alloy are lowered. Therefore, the Zr content is 0 to 0.1%. The minimum with preferable Zr is 0.0005%, More preferably, it is 0.001%. The upper limit with preferable Zr content is 0.06%.

B: 0 to 0.01%
Boron (B) is an optional element and may not be contained. When contained, the creep strength is increased by grain boundary strengthening. However, if the B content is too high, weldability decreases. Therefore, the B content is 0 to 0.01%. A preferable lower limit of B is 0.0005%, and more preferably 0.001%. The upper limit with preferable B content is 0.005%.

The chemical composition of the austenitic heat-resistant alloy described above may further contain one or more selected from the group consisting of Cu and rare earth elements instead of a part of Fe. Any of these elements is an arbitrary element and improves the corrosion resistance of the heat-resistant alloy.

Cu: 0 to 5%
Copper (Cu) is an optional element and may not be contained. When contained, it promotes the formation of an Al ₂ O ₃ film in the vicinity of the surface and enhances the corrosion resistance of the heat-resistant alloy. However, if the Cu content is too high, not only the effect is saturated, but also the high temperature ductility is lowered. Therefore, the Cu content is 0 to 5%. The minimum with preferable Cu content is 0.05%, More preferably, it is 0.1%. The upper limit with preferable Cu content is 4.8%, More preferably, it is 4.5%.

Rare earth elements: 0 to 0.1%
The rare earth element (REM) is an optional element and may not be contained. When it is contained, S is fixed as a sulfide to enhance hot workability. REM further forms oxides to increase corrosion resistance, creep strength, and creep ductility. However, if the REM content is too high, inclusions such as oxides increase, thereby reducing hot workability and weldability and increasing manufacturing costs. Therefore, the REM content is 0 to 0.1%. The minimum with preferable REM content is 0.0005%, More preferably, it is 0.001%. The upper limit with preferable REM content is 0.09%, More preferably, it is 0.08%.

In this specification, REM is a general term for a total of 17 elements of Sc, Y, and a lanthanoid. The REM content means the content of an element when the REM contained in the heat-resistant alloy is one of these elements. When two or more types of REM are contained in the heat-resistant alloy, the REM content means the total content of these elements. REM is generally contained in misch metal. For this reason, for example, it may be added in the form of misch metal so that the REM content falls within the above range.

The chemical composition of the austenitic heat-resistant alloy described above may further include one or more selected from the group consisting of Ca and Mg instead of a part of Fe. Any of these elements is an arbitrary element and improves the hot workability of the heat-resistant alloy.

Ca: 0 to 0.05%
Calcium (Ca) is an optional element and may not be contained. When it is contained, S is fixed as a sulfide to enhance hot workability. On the other hand, if the Ca content is too high, toughness, ductility and cleanliness will be reduced. Therefore, the Ca content is 0 to 0.05%. A preferable lower limit of Ca is 0.0005%. The upper limit with preferable Ca content is 0.01%.

Mg: 0 to 0.05%
Magnesium (Mg) is an optional element and may not be contained. When contained, it fixes S as a sulfide and improves the hot workability of the heat-resistant alloy. On the other hand, if the Ca content is too high, toughness, ductility and cleanliness will be reduced. Therefore, the Ca content is 0 to 0.05%. A preferable lower limit of Ca is 0.0005%. The upper limit with preferable Ca content is 0.01%.

[Total volume ratio of precipitates with coarse equivalent diameter of 6 μm or more (coarse precipitates): 5% or less]
As described above, the austenitic heat-resistant alloy of the present embodiment precipitates fine precipitates during use in a high temperature environment, increases creep strength, and maintains toughness. Examples of the precipitate include carbide, nitride, NiAl, and α-Cr. If the precipitate is coarse, creep strength and toughness are lowered. Therefore, in the heat-resistant alloy before use, it is preferable that there are few coarse precipitates. If the total volume fraction of precipitates (coarse precipitates) with an equivalent circle diameter of 6 μm or more in the microstructure of the heat-resistant alloy is 5% or less, fine precipitates precipitate during use in a high-temperature environment and creep. Strength and toughness are increased. The upper limit with the preferable total volume ratio of a coarse precipitate is 4%, More preferably, it is 3%. Here, the equivalent circle diameter means the diameter (μm) when the area of the precipitate is converted into the area of the circle.

[Method for measuring the total volume fraction of coarse precipitates in the structure]
The total volume ratio of coarse precipitates in the structure of the austenitic heat-resistant alloy of this embodiment can be measured by the following method.

Take a specimen with a vertical cross section from the surface of the heat-resistant alloy material. For example, when the austenitic heat-resistant alloy material is an alloy pipe, a test piece is sampled from the central thickness portion of the cross section perpendicular to the axial direction.

After polishing the cross section (observation surface) of the collected specimen, the observation surface is etched with a mixed acid solution of hydrochloric acid and nitric acid. A scanning electron microscope (SEM) is used to photograph 10 fields of view on the observation surface to create an SEM image (reflection electron image). Each field of view is 100 μm × 100 μm.

In the SEM image, the precipitates and the matrix have different contrasts. The area of the precipitate specified by the difference in contrast is obtained, and the equivalent circle diameter of each precipitate is calculated. After the calculation, a precipitate (coarse precipitate) having an equivalent circle diameter of 6 μm or more is specified.

求める Calculate the total area of the specified coarse precipitates. The ratio (%) of the total area of coarse precipitates to the visual field area is obtained. Since the area ratio of the precipitate corresponds to the volume ratio, the obtained ratio of the coarse precipitate is defined as the total volume ratio (%) of the coarse precipitate.

The shape of the austenitic heat-resistant alloy according to this embodiment is not particularly limited. An austenitic heat-resistant alloy is, for example, an alloy tube. Austenitic heat-resistant alloy pipes are used as boiler pipes and chemical plant reaction pipes. The austenitic heat-resistant alloy may be a plate material, a rod material, or a wire material.

[Production method]
As an example of a method for producing the austenitic heat-resistant alloy of the present embodiment, a method for producing an alloy tube will be described. The manufacturing method of the present embodiment includes a step of preparing a material having the above-described chemical composition (preparation step), a step of hot forging the prepared material (hot forging step), and a hot forged material. And a step of producing an intermediate material by performing hot working (hot working step) and a step of performing solution heat treatment on the intermediate material (solution heat treatment step). Hereinafter, each step will be described.

[Preparation process]
A molten steel having the above chemical composition is produced. A well-known degassing process is implemented with respect to molten steel as needed. A raw material is manufactured by casting using molten steel. The material may be an ingot obtained by an ingot-making method or a slab such as a slab, bloom or billet obtained by a continuous casting method.

[Hot forging process]
A cylindrical material is manufactured by hot forging the manufactured material. In hot forging, the cross-section reduction rate defined by the formula (1) is set to 30% or more.
Cross-sectional reduction rate = 100- (cross-sectional area of material after hot working / cross-sectional area of material before hot forging) × 100 (%) (1)

As described above, precipitates such as eutectic carbides are present in the structure of the material produced by casting. These precipitates are coarse, and there are many that have an equivalent circle diameter of 6 μm or more. Such coarse precipitates are difficult to dissolve in a solution treatment in a later step.

If the cross-section reduction rate in the hot forging process is 30% or more, coarse precipitates are destroyed during hot forging and the size is reduced. For this reason, the precipitate is easily dissolved in the solution heat treatment in the subsequent step. As a result, the volume ratio of precipitates having an equivalent circle diameter of 6 μm or more is 5% or less.

The preferable cross-sectional reduction rate is 35% or more, and more preferably 40% or more. Although the upper limit of the cross-section reduction rate is not particularly limited, it is 90% in consideration of productivity.

[Hot working process]
Hot working is performed on the hot-forged material (cylindrical material) to manufacture an alloy base tube that is an intermediate material. For example, a through hole is formed in the center of a cylindrical material by machining. Hot extrusion is performed on the cylindrical material in which the through holes are formed, and an alloy base tube is manufactured. A cylindrical raw material (intermediate material) may be manufactured by piercing and rolling a cylindrical material. Cold working may be performed on the intermediate material after hot working. The cold working is, for example, cold drawing or the like. An intermediate material is manufactured by the above process.

[Solution heat treatment process]
Solution heat treatment is performed on the manufactured intermediate material. The precipitate in the intermediate material is dissolved by solution heat treatment.

The heat treatment temperature in the solution heat treatment is 1100 to 1250 ° C. If the heat treatment temperature is less than 1100 ° C., the precipitate is not sufficiently dissolved, and as a result, the volume fraction of the coarse precipitate exceeds 5%. On the other hand, if the heat treatment temperature is too high, the austenite grains are coarsened and the productivity is lowered.

When the heat treatment temperature is 1100 to 1250 ° C., the precipitate is sufficiently dissolved, and the total volume ratio of the coarse precipitate is 5% or less.

The solution heat treatment time is not particularly limited. The solution heat treatment time is, for example, 1 minute to 1 hour.

For the intermediate material after the solution heat treatment, pickling treatment may be performed for the purpose of removing scale formed on the surface. For pickling, for example, a mixed acid solution of nitric acid and hydrochloric acid is used. The pickling time is, for example, 30 to 60 minutes.

Furthermore, a blasting process using a projection material may be performed on the intermediate material after the pickling process. For example, blasting is performed on the inner surface of the alloy tube. In this case, a processed layer is formed on the surface, and corrosion resistance (oxidation resistance and the like) is increased.

The austenitic heat-resistant alloy of this embodiment is manufactured by the above manufacturing method. In addition, the manufacturing method of the alloy pipe was demonstrated above. However, you may manufacture a board | plate material, a bar, a wire, etc. with the same manufacturing method (a preparation process, a hot forging process, a hot working process, a solution heat treatment process).

[Production method]
Molten steel having the chemical composition shown in Table 1 was produced using a vacuum melting furnace.

A cylindrical ingot (30 kg) having an outer diameter of 120 mm was manufactured using the molten steel. The ingot was hot forged at a cross-sectional reduction rate shown in Table 2 to produce a rectangular material. The rectangular material was hot-rolled and cold-rolled to produce a plate-like intermediate material having a thickness of 1.5 mm. The intermediate material was subjected to a solution treatment for 10 minutes at the heat treatment temperature shown in Table 2. After holding for 10 minutes, the intermediate material was water-cooled to produce an alloy sheet.

[Creep rupture test]
A test piece was prepared from the manufactured alloy sheet. The test piece was sampled in parallel to the longitudinal direction (rolling direction) from the thickness center of the alloy sheet. The test piece was a round bar test piece, the diameter of the parallel part was 6 mm, and the distance between the gauge points was 30 mm. A creep rupture test was performed using the test piece. The creep rupture test was conducted in an air atmosphere at 700 to 800 ° C. Based on the obtained breaking strength, the creep strength (MPa) at 700 ° C. at 1.0 × 10 ⁴ hours was determined by the Larson-Miller parameter method.

[Charpy impact test]
The manufactured alloy sheet was subjected to aging treatment at 700 ° C. for 8000 hours, followed by water cooling. A V-notch Charpy impact test piece defined in JIS Z2242 (2005) was collected from the thickness direction center of the plate after aging treatment. The notch was made parallel to the longitudinal direction of the alloy sheet. The test piece had a width of 5 mm, a height of 10 mm, a length of 55 mm, and a notch depth of 2 mm. At 0 ° C., a Charpy impact test in accordance with JIS Z2242 (2005) was performed to determine an impact value (J / cm ² ).

[Test results]
The test results are shown in Table 2.

Referring to Table 2, the chemical compositions of Test No. 1 to Test No. 11 were appropriate, and the volume fraction of coarse precipitates was 5% or less. As a result, the creep strength was 140 MPa or more, indicating an excellent creep strength. Furthermore, the Charpy impact value was 40 J / cm ² or more, and excellent toughness was exhibited even after long-term aging treatment.

On the other hand, in test number 12, the C content was too high. Therefore, the volume ratio of the coarse precipitate exceeded 5%. As a result, the creep strength was less than 140 MPa, and the Charpy impact value was less than 40 J / cm ² .

In test number 13, the Al content was too low. Therefore, the creep strength was less than 140 MPa. It is considered that the amount of NiAl deposited was small.

In test number 14, the Al content was too high. Therefore, the creep strength was less than 140 MPa. It is considered that the structure was not stable and the creep strength was low because the Al content was too high.

In test number 15, the Cr content was too low. Therefore, the creep strength was less than 140 MPa. This is probably because the amount of α-Cr deposited was small.

In test number 16, the Cr content was too high. Therefore, the creep strength was less than 140 MPa. It is considered that the structure was not stable and the creep strength was low because the Cr content was too high.

In test number 17, the cross-sectional reduction rate during hot forging was less than 30%. Therefore, the total volume ratio of coarse precipitates exceeded 5%. As a result, the creep strength was less than 140 MPa, and the Charpy impact value was less than 40 J / cm ² .

In test number 18, the solution heat treatment temperature was less than 1100 ° C. Therefore, the total volume ratio of coarse precipitates exceeded 5%. As a result, the creep rupture strength was less than 140 MPa, and the Charpy impact value was less than 40 J / cm ² .

In test number 19, the Nb content was too high. Therefore, the Charpy impact value was less than 40 J / cm ² .

In test number 20, the Nb content was too low. Therefore, the creep strength was less than 140 MPa.

The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be implemented by appropriately changing the above-described embodiment without departing from the spirit thereof.

The austenitic heat-resistant alloy of the present invention can be widely used in a high temperature environment of 700 ° C. or higher. In particular, it is particularly suitable for use as an alloy pipe in a power generation boiler, a chemical industry plant or the like exposed to a high temperature environment of 700 ° C. or higher.

Claims

% By mass
C: 0.03 to less than 0.25%,
Si: 0.01 to 2.0%,
Mn: 2.0% or less,
Cr: 10 to less than 30%,
Ni: more than 25 to 45%,
Al: more than 2.5 to less than 4.5%,
Nb: 0.2 to 3.5%
N: 0.025% or less,
Ti: 0 to less than 0.2%,
W: 0-6%
Mo: 0-4%,
Zr: 0 to 0.1%,
B: 0 to 0.01%
Cu: 0 to 5%,
Rare earth elements: 0-0.1%,
Ca: 0 to 0.05% and
Mg: 0 to 0.05% contained,
The balance consists of Fe and impurities,
P and S in the impurity are each
P: has a chemical composition of 0.04% or less, and S: 0.01% or less,
An austenitic heat-resistant alloy, wherein the total volume fraction of precipitates having an equivalent circle diameter of 6 μm or more in the structure is 5% or less.
The austenitic heat-resistant alloy according to claim 1,
The chemical composition is
Ti: 0.005 to less than 0.2%,
W: 0.005 to 6%,
Mo: 0.005 to 4%,
Zr: 0.0005 to 0.1%, and
B: An austenitic heat-resistant alloy containing one or more selected from the group consisting of 0.0005 to 0.01%.
The austenitic heat-resistant alloy according to claim 1 or 2,
The chemical composition is
Cu: 0.05 to 5%, and
Rare earth element: An austenitic heat-resistant alloy characterized by containing at least one selected from the group consisting of 0.0005 to 0.1%.
The austenitic heat-resistant alloy according to any one of claims 1 to 3,
The chemical composition is
Ca: 0.0005 to 0.05%, and
Mg: One or more selected from the group consisting of 0.0005 to 0.05%, an austenitic heat resistant alloy.
A step of hot forging the material having the chemical composition according to any one of claims 1 to 4 at a cross-section reduction rate of 30% or more;
A process of producing an intermediate material by performing hot working on the hot forged material;
And a step of performing a solution treatment on the intermediate material at 1100 to 1250 ° C., and a method for producing an austenitic heat-resistant alloy.