JP5657964B2 - High-strength Ni-base forged superalloy and manufacturing method thereof - Google Patents
High-strength Ni-base forged superalloy and manufacturing method thereof Download PDFInfo
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- JP5657964B2 JP5657964B2 JP2010205741A JP2010205741A JP5657964B2 JP 5657964 B2 JP5657964 B2 JP 5657964B2 JP 2010205741 A JP2010205741 A JP 2010205741A JP 2010205741 A JP2010205741 A JP 2010205741A JP 5657964 B2 JP5657964 B2 JP 5657964B2
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C19/00—Alloys based on nickel or cobalt
- C22C19/03—Alloys based on nickel or cobalt based on nickel
- C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
- C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
- C22C19/056—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 10% but less than 20%
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C19/00—Alloys based on nickel or cobalt
- C22C19/03—Alloys based on nickel or cobalt based on nickel
- C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
- C22C19/051—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W
- C22C19/055—Alloys based on nickel or cobalt based on nickel with chromium and Mo or W with the maximum Cr content being at least 20% but less than 30%
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
- C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
- C22F1/10—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of nickel or cobalt or alloys based thereon
Description
本発明は、Ｎｉ基鍛造超合金に関し、特に高いクリープ強度を有し、主に蒸気タービンの高温部材のように高温で用いられるNi基鍛造超合金とその製造方法に関する。 The present invention relates to a Ni-based forged superalloy, and particularly to a Ni-based forged superalloy having a high creep strength and mainly used at a high temperature like a high-temperature member of a steam turbine and a method for producing the same.
昨今、石炭火力発電プラントの高効率化を目指して、蒸気温度が700℃以上である火力発電プラントの開発が進められている。これまでの蒸気タービンの高温部材には、鉄系の材料である12Cr系フェライト鋼が用いられている。しかしながら、12Cr系フェライト鋼は、使用環境として蒸気温度で650℃が限界であると言われている。そこで、それに代わって、析出強化型合金であるNi基超合金が700℃級の蒸気タービン高温部材として検討されている。 In recent years, with the aim of increasing the efficiency of coal-fired power plants, development of thermal power plants with a steam temperature of 700 ° C. or higher is underway. Conventionally, 12Cr ferritic steel, which is an iron-based material, is used for high-temperature members of steam turbines. However, the 12Cr ferritic steel is said to have a limit of 650 ° C in terms of steam temperature as a use environment. Therefore, instead of this, a Ni-based superalloy, which is a precipitation-strengthened alloy, has been studied as a steam turbine high-temperature member of 700 ° C class.
蒸気タービン高温部材は大型のものが多いため、析出強化型Ni基超合金には良好な大型鋼塊製造性や熱間鍛造性が求められる。また、熱膨張係数の比較的小さいフェライト鋼と組み合わせて使用されることも考えられるため、析出強化型Ni基超合金には低熱膨張性も求められる。例えば、Ni基超合金のFENIX-700（株式会社日立製作所製）は、非常に優れた組織安定性と強度とを有し、大型鋼塊製造性も優れている。Ni基超合金のM252はフェライト鋼に近い線膨張係数を有する合金であり、フェライト鋼と組み合わせて使用ができる。また、特許文献１や特許文献２では、低熱膨張係数と優れた高温強度とを有するNi基超耐熱合金が提案されている。
Since many steam turbine high-temperature members are large, precipitation-strengthening Ni-base superalloys are required to have good large steel ingot manufacturability and hot forgeability. Further, since it may be used in combination with a ferritic steel having a relatively low thermal expansion coefficient, the precipitation strengthened Ni-base superalloy is also required to have low thermal expansion. For example, the Ni-base superalloy FENIX-700 (manufactured by Hitachi, Ltd.) has very excellent structure stability and strength, and is excellent in large steel ingot manufacturability. Ni-base superalloy M252 is an alloy having a linear expansion coefficient close to that of ferritic steel, and can be used in combination with ferritic steel.
材料の高温クリープ特性は多様な因子によって決まるが、材料組織もその因子の一つである。例えば、結晶粒界はクリープ破断の起点となることが知られている。そのため、結晶粒界を減らすことでクリープ変形を抑制することができる。その例として、最新鋭のガスタービンの動翼では、クリープ強度を高めるために、破断起点となる結晶粒界をなくした単結晶材から製造されているものがある。 The high temperature creep characteristics of materials are determined by various factors, and the material structure is one of the factors. For example, it is known that a crystal grain boundary is a starting point of creep rupture. Therefore, creep deformation can be suppressed by reducing the crystal grain boundaries. As an example, there is a state-of-the-art gas turbine rotor blade that is manufactured from a single crystal material that eliminates the grain boundary that is the starting point of fracture in order to increase the creep strength.
結晶粒界を減らす他の手段としては、材料の結晶粒径を大きくすることが考えられる。しかしながら、本発明者等が検討したところ、結晶粒径を単純に粗大化する手段は、Ni基鍛造超合金のクリープ強度を高めることができるが、それだけではクリープ延性が低下してしまうという新たな問題が見出された。 As another means for reducing the crystal grain boundary, it is conceivable to increase the crystal grain size of the material. However, as a result of the study by the present inventors, the means for simply coarsening the crystal grain size can increase the creep strength of the Ni-based forged superalloy, but that alone will decrease the creep ductility. A problem was found.
したがって、本発明の目的は、多結晶体の各結晶粒径を大きくしてクリープの高強度化を図り、同時に多結晶体の延性の低下を抑制した高強度Ni基鍛造合金およびその製造方法を提供することにある。 Accordingly, an object of the present invention is to provide a high-strength Ni-based forged alloy that increases the crystal grain size of the polycrystalline body to increase the strength of creep and at the same time suppresses the decrease in ductility of the polycrystalline body, and a method for producing the same. It is to provide.
本発明の１つの態様は、上記目的を達成するため、次のような特徴を有する。
本発明のNi基鍛造超合金は、組成として、0.005質量％以上0.2質量％以下のC（炭素）、0質量％以上1質量％以下のSi（ケイ素）、0質量％以上1質量％以下のMn（マンガン）、10質量％以上24質量％以下のCr（クロム）、およびMo（モリブデン）とW（タングステン）の少なくとも１種を「[Mo濃度]＋0.5[W濃度]」で規定した時に5質量％以上17質量％以下、1質量％以上2質量％以下のAl（アルミニウム）、0.5質量％以上3.5質量％以下のTi（チタン）、0質量％以上10質量％以下のFe（鉄）、および：0.002質量％以上0.02質量％以下のB（ホウ素）と0.01質量％以上0.2質量％以下のZr（ジルコニウム）の少なくとも１種を含有し、残部が48質量％以上80質量％以下のNi（ニッケル）と不可避不純物でなるNi基鍛造超合金であって、前記Ni基鍛造超合金は多結晶体であり、熱処理後の前記結晶の平均粒径が72μm以上289μm以下であり、前記Ni基鍛造超合金は前記熱処理後に結晶粒界に沿って複数の粒状析出物が析出しており、前記多結晶体の任意断面における前記粒状析出物の平均長さ（結晶粒界に沿った長さ、前記粒状析出物１個あたりの前記結晶粒界の平均被覆長さ）が0.5μm以上2.5μm以下であることを特徴とする。なお上述したように、本発明においてNi基鍛造超合金とは、多結晶体の状態にあるものを意味する。
One aspect of the present invention has the following features in order to achieve the above object.
The Ni-based forged superalloy of the present invention has a composition of 0.005 mass% to 0.2 mass% C (carbon), 0 mass% to 1 mass% Si (silicon), 0 mass% to 1 mass% At least one of Mn (manganese), Cr (chromium) of 10 mass% to 24 mass%, and Mo (molybdenum) and W (tungsten) is defined as “[Mo concentration] +0.5 [W concentration]”. Sometimes 5 mass% to 17 mass%, 1 mass% to 2 mass% Al (aluminum), 0.5 mass% to 3.5 mass% Ti (titanium), 0 mass% to 10 mass% Fe (iron) ), And: containing at least one of 0.002 mass% or more 0.02 mass% or less of B (boron) and 0.01 mass% or more and 0.2 mass% or less of Zr (zirconium), with the balance being 48 mass% or more and 80 mass% or less A Ni-based forged superalloy composed of Ni (nickel) and inevitable impurities, the Ni-based forged superalloy being a polycrystalline body, The average grain size of the subsequent crystal is 72 μm or more and 289 μm or less, and the Ni-based forged superalloy has a plurality of granular precipitates precipitated along the grain boundary after the heat treatment, and the arbitrary cross section of the polycrystalline body The average length of the granular precipitate in (the length along the crystal grain boundary, the average covering length of the crystal grain boundary per one granular precipitate) is 0.5 μm or more and 2.5 μm or less. To do. As described above, in the present invention, the Ni-based forged superalloy means one in a polycrystalline state.
また、本発明の他の態様は、上記目的を達成するため、次のような特徴を有する。
本発明のNi基鍛造超合金の製造方法は、組成として、0.005質量％以上0.2質量％以下のC、0質量％以上1質量％以下のSi、0質量％以上1質量％以下のMn、10質量％以上24質量％以下のCr、MoとWの少なくとも１種を「[Mo濃度]＋0.5[W濃度]」で規定した時に5質量％以上17質量％以下、1質量％以上2質量％以下のAl、0.5質量％以上3.5質量％以下のTi、0質量％以上10質量％以下のFe、および0.002質量％以上0.02質量％以下のBと0.01質量％以上0.2質量％以下のZrの少なくとも１種を含有し、残部が48質量％以上80質量％以下Niと不可避不純物でなるNi基鍛造超合金に対して、溶体化熱処理を施す工程を有し、前記溶体化熱処理が、1100℃以上1160℃以下の第１溶体化熱処理と、980℃以上1080℃以下の第２溶体化熱処理とからなる２段階溶体化熱処理であることを特徴とする。
Another aspect of the present invention has the following characteristics in order to achieve the above object.
The production method of the Ni-based forged superalloy according to the present invention includes, as a composition, 0.005 mass% to 0.2 mass% C, 0 mass% to 1 mass% Si, 0 mass% to 1 mass% Mn, 10 5% by mass to 17% by mass, 1% by mass to 2% by mass when at least one of Cr, Mo and W is specified by “[Mo concentration] + 0.5 [W concentration]”. % Al, 0.5 mass% to 3.5 mass% Ti, 0 mass% to 10 mass% Fe, 0.002 mass% to 0.02 mass% B, and 0.01 mass% to 0.2 mass% Zr. A solution heat treatment is performed on a Ni-based forged superalloy containing at least one kind and the balance being 48 mass% or more and 80 mass% or less of Ni and inevitable impurities, and the solution heat treatment is performed at 1100 ° C. It is a two-stage solution heat treatment comprising the first solution heat treatment at 1160 ° C. or lower and the second solution heat treatment at 980 ° C. or higher and 1080 ° C. or lower.
また、本発明の更に他の態様は、上記目的を達成するため、次のような特徴を有する。
本発明のNi基鍛造超合金の製造方法は、組成として、0.005質量％以上0.2質量％以下のC、0質量％以上1質量％以下のSi、0質量％以上1質量％以下のMn、10質量％以上24質量％以下のCr、MoとWの少なくとも１種を「[Mo濃度]＋0.5[W濃度]」で規定した時に5質量％以上17質量％以下、1質量％以上2質量％以下のAl、0.5質量％以上3.5質量％以下のTi、0質量％以上10質量％以下のFe、および0.002質量％以上0.02質量％以下のBと0.01質量％以上0.2質量％以下のZrの少なくとも１種を含有し、残部が48質量％以上80質量％以下Niと不可避不純物でなるNi基鍛造超合金に対して、溶体化熱処理を施す工程を有し、前記溶体化熱処理が、980℃以上1080℃以下で24時間以上保持する１段溶体化−長時間熱処理であることを特徴とする。
Still another aspect of the present invention has the following characteristics in order to achieve the above object.
The production method of the Ni-based forged superalloy according to the present invention includes, as a composition, 0.005 mass% to 0.2 mass% C, 0 mass% to 1 mass% Si, 0 mass% to 1 mass% Mn, 10 5% by mass to 17% by mass, 1% by mass to 2% by mass when at least one of Cr, Mo and W is specified by “[Mo concentration] + 0.5 [W concentration]”. % Al, 0.5 mass% to 3.5 mass% Ti, 0 mass% to 10 mass% Fe, 0.002 mass% to 0.02 mass% B, and 0.01 mass% to 0.2 mass% Zr. A solution heat treatment is performed on a Ni-based forged superalloy containing at least one kind and the balance being 48 mass% or more and 80 mass% or less of Ni and inevitable impurities, and the solution heat treatment is performed at 980 ° C. It is characterized in that it is a one-stage solution-long-time heat treatment that is maintained at 1080 ° C. or lower for 24 hours or longer.
本発明によると、超合金多結晶体の各結晶粒径を大きくしてクリープの高強度化を図り、同時に超合金の結晶粒界に沿って適切な大きさと量の粒状析出物を析出させることで多結晶体の延性の低下を抑制した高強度Ni基鍛造超合金を提供することができる。また、そのような微細構造を形成するNi基鍛造超合金の製造方法を提供することができる。 According to the present invention, the crystal grain size of the superalloy polycrystal is increased to increase the creep strength, and at the same time, the appropriate size and amount of granular precipitates are precipitated along the superalloy grain boundaries. Thus, it is possible to provide a high-strength Ni-based forged superalloy that suppresses the decrease in ductility of the polycrystalline body. Moreover, the manufacturing method of the Ni base forge superalloy which forms such a microstructure can be provided.
（本発明の基本思想）
本発明者らは、前記課題を解決するために、Ni基鍛造超合金の望ましい微細構造と微細構造を制御できる熱処理方法を鋭意検討した。その結果、Ni基鍛造超合金の熱処理において、２段階溶体化熱処理という新規な熱処理方法を開発した。第１溶体化熱処理では、γ’相の溶体化熱処理における温度を従来よりも高く（具体的には、炭化物が固溶する温度域に）設定して超合金の結晶粒を粗大化させる。引き続いて、第２溶体化熱処理としてγ’相の固溶温度以上で炭化物の固溶温度以下の中間温度域で溶体化熱処理を施し、超合金の各結晶粒界上に粒状炭化物を析出させるとともに結晶粒内にも粒状炭化物を析出させる。その結果、超合金の結晶粒界に沿って粒状炭化物が鎖状に連なったような微細構造が得られる。
(Basic idea of the present invention)
In order to solve the above-mentioned problems, the present inventors have intensively studied a desirable microstructure and a heat treatment method capable of controlling the microstructure of the Ni-based forged superalloy. As a result, a new heat treatment method called two-step solution heat treatment was developed for heat treatment of Ni-base forged superalloys. In the first solution heat treatment, the temperature in the solution heat treatment of the γ ′ phase is set higher than that in the conventional case (specifically, in a temperature range in which carbides are dissolved), and the crystal grains of the superalloy are coarsened. Subsequently, as a second solution heat treatment, a solution heat treatment is performed at an intermediate temperature range not lower than the solid solution temperature of the γ 'phase and not higher than the solid solution temperature of the carbide to precipitate granular carbide on each grain boundary of the superalloy. Particulate carbide is also precipitated in the crystal grains. As a result, a fine structure in which granular carbides are connected in a chain form along the crystal grain boundary of the superalloy is obtained.
さらに、本発明者らは、別の熱処理方法として、γ’相の固溶温度以上で炭化物の固溶温度以下の中間温度域で長時間保持する（例えば、24時間以上、より好ましくは48時間以上保持する）というもう１つの新規な熱処理方法を開発した。この熱処理（１段溶体化−長時間熱処理）を施すことによっても、超合金の結晶粒界に沿って粒状炭化物が鎖状に連なったような微細構造が得られる。なお、２段階溶体化熱処理または１段溶体化−長時間熱処理の後に、Ni基鍛造超合金の用途に応じて適当な時効熱処理を実施することは好ましい。 Furthermore, as another heat treatment method, the present inventors hold for a long time in an intermediate temperature range not lower than the solid solution temperature of the γ ′ phase and not higher than the solid solution temperature of the carbide (for example, 24 hours or more, more preferably 48 hours). Another new heat treatment method has been developed. By performing this heat treatment (one-step solution-long-time heat treatment), a fine structure in which granular carbides are chained along the crystal grain boundary of the superalloy can be obtained. In addition, it is preferable to perform an appropriate aging heat treatment according to the use of the Ni-base forged superalloy after the two-stage solution heat treatment or the first-stage solution heat treatment-long-time heat treatment.
図１は、本発明に係るNi基鍛造超合金の溶体化熱処理後の微細構造の１例を示した断面模式図である。図中には、比較として本発明の溶体化熱処理を実施しなかったNi基鍛造超合金の微細構造例（比較のNi基鍛造超合金Ａ，Ｂ）も併せて示した。図１に示したように、本発明に係るNi基鍛造超合金の多結晶体は、粗大化した結晶粒1の粒界2に沿って粒状炭化物3が鎖状に連なるように析出した微細構造を有している。また、粒状炭化物3は粗大化した結晶粒1の内部にも析出している。
FIG. 1 is a schematic cross-sectional view showing an example of a microstructure after solution heat treatment of a Ni-based forged superalloy according to the present invention. In the drawing, as a comparison, an example of a microstructure of a Ni-based forged superalloy not subjected to the solution heat treatment of the present invention (comparative Ni-based forged superalloys A and B) is also shown. As shown in FIG. 1, the polycrystalline Ni of the Ni-based forged superalloy according to the present invention has a microstructure in which
一方、本発明の溶体化熱処理を実施しなかったNi基鍛造超合金多結晶体の１つは（比較のNi基鍛造超合金Ａ）、粒状炭化物3が比較的小さい結晶粒1’の粒界2に沿って析出するとともに、該比較的小さい結晶粒1’の内部に析出した微細構造を有していた。また、本発明の溶体化熱処理を実施しなかったNi基鍛造超合金多結晶体の他の１つは（比較のNi基鍛造超合金Ｂ）、粗大化した結晶粒1の粒界2に沿って炭化物微粒子3’が連続的に連なって膜状に結晶粒1を覆うように析出した微細構造を有していた。
On the other hand, one of the Ni-base forged superalloy polycrystals not subjected to the solution heat treatment of the present invention (comparative Ni-based forged superalloy A) is a grain boundary of crystal grains 1 'in which the
種々の微細構造を有するNi基鍛造超合金多結晶体に対してクリープ試験を行ったところ、比較のNi基鍛造超合金Ａは、良好なクリープ延性を示したが、クリープ強度において更なる向上が望まれる結果であった。比較のNi基鍛造超合金Ｂは、結晶粒の粗大化に伴ってクリープ強度が向上したが、クリープ延性が著しく低下する結果であった。これらに対し、本発明に係るNi基鍛造超合金は、結晶粒の粗大化に伴って良好なクリープ強度を示すとともに、良好なクリープ延性を示すことが見出された。本発明はこれらの知見を基にして完成された。 When a creep test was performed on a Ni-based forged superalloy polycrystal having various microstructures, the comparative Ni-based forged superalloy A showed good creep ductility, but the creep strength was further improved. The desired result. The comparative Ni-based forged superalloy B improved the creep strength as the crystal grains became coarser, but resulted in a significant decrease in creep ductility. On the other hand, it has been found that the Ni-based forged superalloy according to the present invention exhibits a good creep strength as well as a good creep ductility as the crystal grains become coarser. The present invention has been completed based on these findings.
以下、本発明に係る実施形態について、より詳細に説明する。ただし、本発明はここで取り上げた実施形態に限定されることはなく、要旨を変更しない範囲で適宜組み合わせや改良が可能である。 Hereinafter, embodiments according to the present invention will be described in more detail. However, the present invention is not limited to the embodiments taken up here, and can be appropriately combined and improved without departing from the scope of the invention.
前述したように、本発明に係るNi基鍛造超合金は、組成として、0.005質量％以上0.2質量％以下のC、0質量％以上1質量％以下のSi、0質量％以上1質量％以下のMn、10質量％以上24質量％以下のCr、およびMoとWの少なくとも１種を「[Mo濃度]＋0.5[W濃度]」で規定した時に5質量％以上17質量％以下、1質量％以上2質量％以下のAl、0.5質量％以上3.5質量％以下のTi、0質量％以上10質量％以下のFe、および：0.002質量％以上0.02質量％以下のBと0.01質量％以上0.2質量％以下のZrの少なくとも１種を含有し、残部が48質量％以上80質量％以下のNiと不可避不純物でなるNi基鍛造超合金であって、前記Ni基鍛造超合金は多結晶体であり、熱処理後の前記結晶の平均粒径が72μm以上289μm以下であり、前記Ni基鍛造超合金は前記熱処理後に結晶粒界に沿って複数の粒状析出物が析出しており、前記多結晶体の任意断面における前記粒状析出物の平均長さ（結晶粒界に沿った平均長さ、前記粒状析出物１個あたりの前記結晶粒界の平均被覆長さ）が0.5μm以上2.5μm以下であることを特徴とする。 As described above, the Ni-based forged superalloy according to the present invention has a composition of 0.005 mass% to 0.2 mass% C, 0 mass% to 1 mass% Si, 0 mass% to 1 mass%. When Mn, Cr of 10% to 24% by mass, and at least one of Mo and W are defined by “[Mo concentration] + 0.5 [W concentration]”, 5% to 17% by mass, 1% % To 2% by mass of Al, 0.5% to 3.5% by mass of Ti, 0% to 10% by mass of Fe, and B: 0.002% to 0.02% by mass of B and 0.01% to 0.2% by mass of B % Ni-based forged superalloy containing at least one kind of Zr with a balance of 48 mass% to 80 mass% and inevitable impurities, the Ni-based forged superalloy being polycrystalline. The average grain size of the crystal after the heat treatment is 72 μm or more and 289 μm or less, and the Ni-based forged superalloy precipitates a plurality of granular precipitates along the crystal grain boundary after the heat treatment. And the average length of the granular precipitates in an arbitrary cross section of the polycrystal (average length along the crystal grain boundaries, average covering length of the crystal grain boundaries per one granular precipitate) It is 0.5 μm or more and 2.5 μm or less.
また、本発明は、上記の発明に係るNi基鍛造超合金において、以下のような改良や変更を加えることができる。
（１）前記多結晶体の任意断面における前記結晶粒界の合計長さに対する前記粒状析出物の合計長さの比率（被覆率）が、50％以上である。
（２）前記多結晶体の任意断面における前記粒状析出物の個数が、前記結晶粒界10μmあたり3個以上である。
（３）前記粒状析出物は、Cr炭化物と、Mo炭化物および/またはW炭化物から主に構成される。
（４）前記熱処理は、1100℃以上1160℃以下で施される第１溶体化熱処理と、それに引き続いて980℃以上1080℃以下で施される第２溶体化熱処理とを含む。
（５）前記熱処理は、980℃以上1080℃以下で24時間以上保持する溶体化熱処理を含む。
（６）組成として、0質量％以上20質量％以下のCo、0質量％以上1質量％以下のNbを更に含有する。
（７）組成として、「[Al濃度]/([Al濃度]＋0.56[Ti濃度])」で表わされる値が0.45以上0.70以下である。
（８）上記のNi基鍛造超合金からなる石炭火力発電プラントのボイラーに用いるボイラー管である。
（９）上記のNi基鍛造超合金からなる蒸気タービンのブレードである。
（１０）上記のNi基鍛造超合金からなる蒸気タービンのケーシングボルトである。
In addition, the present invention can add the following improvements and changes to the Ni-based forged superalloy according to the present invention.
(1) The ratio (coverage) of the total length of the granular precipitates to the total length of the crystal grain boundaries in an arbitrary cross section of the polycrystal is 50% or more.
(2) The number of the granular precipitates in an arbitrary cross section of the polycrystalline body is 3 or more per 10 μm of the crystal grain boundary.
(3) The granular precipitate is mainly composed of Cr carbide, Mo carbide and / or W carbide.
(4) The heat treatment includes a first solution heat treatment performed at 1100 ° C. to 1160 ° C. and a second solution heat treatment performed at 980 ° C. to 1080 ° C. subsequently.
(5) The heat treatment includes a solution heat treatment for holding at 980 ° C. or higher and 1080 ° C. or lower for 24 hours or longer.
(6) The composition further contains 0% by mass or more and 20% by mass or less Co, and 0% by mass or more and 1% by mass or less Nb.
(7) As a composition, a value represented by “[Al concentration] / ([Al concentration] +0.56 [Ti concentration])” is 0.45 or more and 0.70 or less.
(8) A boiler tube used for a boiler of a coal-fired power plant made of the above-described Ni-based forged superalloy.
(9) A steam turbine blade made of the above Ni-based forged superalloy.
(10) A casing bolt for a steam turbine made of the above-described Ni-based forged superalloy.
（Ni基鍛造超合金の組成）
次に、本発明に係るNi基鍛造超合金の組成について説明する。
C成分は、炭化物を形成することにより超合金結晶粒の過度の粗大化を防止する効果を有する。ただし、過剰の添加は、炭化物がストリンガー状に析出しやすくなり、加工方向に対する直角方向の延性を低下させる。更にTiと結合して炭化物を形成した場合、Ni基鍛造超合金の析出強化相となるγ’相を形成するためのTi量が減少して強度が低下する。よって、C成分量は、0.005質量％以上0.2質量％以下が好ましい。0.005質量％以上0.15質量％以下がより好ましく、0.005質量％以上0.08質量％以下が更に好ましく、0.005質量％以上0.05質量％以下が最も好ましい。
(Composition of Ni-based forged superalloy)
Next, the composition of the Ni-based forged superalloy according to the present invention will be described.
The C component has an effect of preventing excessive coarsening of superalloy crystal grains by forming carbides. However, excessive addition makes it easy for the carbide to precipitate in a stringer shape, and reduces ductility in the direction perpendicular to the processing direction. Further, when carbide is formed by combining with Ti, the amount of Ti for forming the γ ′ phase that becomes the precipitation strengthening phase of the Ni-based forged superalloy decreases, and the strength decreases. Therefore, the amount of component C is preferably 0.005% by mass or more and 0.2% by mass or less. 0.005% by mass or more and 0.15% by mass or less is more preferable, 0.005% by mass or more and 0.08% by mass or less is more preferable, and 0.005% by mass or more and 0.05% by mass or less is most preferable.
Si成分とMn成分は、ともに超合金溶製時に脱酸剤として用いられ、少量添加でも効果がある。ただし、過剰の添加は、熱間加工性の低下や使用時の靭性を損なう。よって、Si成分量とMn成分量は、それぞれ0質量％以上1質量％以下が好ましい。それぞれ0質量％以上0.5質量％以下がより好ましく、0質量％以上0.1質量％以下が更に好ましく、0質量％以上0.01質量％以下が最も好ましい。 Both the Si component and the Mn component are used as deoxidizers when melting the superalloy, and even when added in a small amount, it is effective. However, excessive addition impairs hot workability and toughness during use. Accordingly, the Si component amount and the Mn component amount are each preferably 0% by mass or more and 1% by mass or less. Each is preferably 0% by mass or more and 0.5% by mass or less, more preferably 0% by mass or more and 0.1% by mass or less, and most preferably 0% by mass or more and 0.01% by mass or less.
Cr成分は、マトリックスに固溶して、超合金の耐酸化性を向上させる効果を有する。特に700℃を超える高温では、10質量％以上の添加が必要である。ただし、過剰の添加は、超合金の塑性加工が困難となる。よって、Cr成分量は10質量％以上24質量％以下が好ましい。15質量％以上24質量％以下がより好ましく、18質量％以上22質量％以下が更に好ましく、19質量％以上21質量％以下が最も好ましい。 The Cr component has the effect of improving the oxidation resistance of the superalloy by dissolving in the matrix. In particular, at a high temperature exceeding 700 ° C., addition of 10% by mass or more is necessary. However, excessive addition makes it difficult to plastically process the superalloy. Therefore, the Cr component amount is preferably 10% by mass or more and 24% by mass or less. 15 mass% or more and 24 mass% or less are more preferable, 18 mass% or more and 22 mass% or less are still more preferable, and 19 mass% or more and 21 mass% or less are the most preferable.
Mo成分とW成分は、ともに超合金の熱膨張係数を下げる効果がある重要な元素であり、少なくとも１種を必須添加する。Mo成分とW成分の合計添加量は、「[Mo濃度]＋0.5[W濃度]」で規定した時に5質量％以上17質量％以下が好ましい。合計添加量が5質量％未満では上記効果が得られず、17質量％を超えると超合金の塑性加工が困難となる。「[Mo濃度]＋0.5[W濃度]」は、5質量％以上15質量％以下がより好ましく、5質量％以上12質量％以下が更に好ましい。一方、W成分の比率が高いとLAVES相が形成されやすく延性や熱間加工性が低下するため、Mo成分の単独添加は好ましい。その場合、8質量％以上12質量％以下が好ましく、9質量％以上11質量％以下がより好ましい。 Both the Mo component and the W component are important elements having an effect of lowering the thermal expansion coefficient of the superalloy, and at least one kind is essentially added. The total addition amount of the Mo component and the W component is preferably 5% by mass or more and 17% by mass or less as defined by “[Mo concentration] +0.5 [W concentration]”. If the total amount added is less than 5% by mass, the above effect cannot be obtained, and if it exceeds 17% by mass, it becomes difficult to perform plastic working of the superalloy. “[Mo concentration] +0.5 [W concentration]” is more preferably 5% by mass or more and 15% by mass or less, and further preferably 5% by mass or more and 12% by mass or less. On the other hand, when the ratio of the W component is high, a LAVES phase is easily formed, and ductility and hot workability are lowered. Therefore, it is preferable to add the Mo component alone. In that case, 8 mass% or more and 12 mass% or less are preferable, and 9 mass% or more and 11 mass% or less are more preferable.
Al成分は、時効熱処理によりγ’相と呼ばれる金属間化合物（Ni3Al）を形成し、超合金の高温強度を高める効果を有する。時効析出強化を得るためには1質量％以上が必要であるが、2質量％を越えると熱間加工が困難となる。よって、Al成分量は、1質量％以上2質量％以下が好ましく、1質量％以上1.8質量％以下がより好ましい。 The Al component has an effect of increasing the high temperature strength of the superalloy by forming an intermetallic compound (Ni 3 Al) called γ ′ phase by aging heat treatment. In order to obtain aging precipitation strengthening, 1% by mass or more is required. However, if it exceeds 2% by mass, hot working becomes difficult. Therefore, the amount of Al component is preferably 1% by mass or more and 2% by mass or less, and more preferably 1% by mass or more and 1.8% by mass or less.
Ti成分は、Al成分と共に析出強化相となるγ’相（Ni3(Al,Ti)）を形成する。Ni3Alからなるγ’相よりもNi3(Al,Ti)からなるγ’相の方が、更に高い高温強度が得られる。時効析出強化を得るためには0.5質量％以上が必要である。一方、3.5質量％を越えるとγ’相が不安定になり、高温においてγ’相からη相への変態が起こり易くなって高温強度が低下するとともに、熱間加工性が低下する。よって、Ti成分量は、0.5質量％以上3.5質量％以下が好ましい。1質量％以上3質量％以下がより好ましく、1.2質量％以上2.5質量％以下が更に好ましく、1.2質量％以上1.8質量％以下が最も好ましい。 The Ti component forms a γ ′ phase (Ni 3 (Al, Ti)) that becomes a precipitation strengthening phase together with the Al component. Ni 3 comprising gamma of Al toward the phase 'than phase Ni 3 (Al, Ti) gamma consisting' is higher high-temperature strength. In order to obtain aging precipitation strengthening, 0.5 mass% or more is necessary. On the other hand, if it exceeds 3.5% by mass, the γ ′ phase becomes unstable, and the transformation from the γ ′ phase to the η phase is likely to occur at a high temperature, resulting in a decrease in high-temperature strength and a decrease in hot workability. Therefore, the amount of Ti component is preferably 0.5% by mass or more and 3.5% by mass or less. 1 mass% or more and 3 mass% or less are more preferable, 1.2 mass% or more and 2.5 mass% or less are more preferable, and 1.2 mass% or more and 1.8 mass% or less are the most preferable.
前述のように、本超合金においてAl成分とTi成分とのバランスは重要である。γ’相中のAl成分の割合が高くなるほど延性は向上するが、逆に強度は低下する。本発明においては、十分な延性を確保することが重要であることから、γ’相中のAl成分の割合を規定するため「[Al濃度]/([Al濃度]＋0.56[Ti濃度])」で表わされる値を設定する。この値が0.45より低いと十分な延性が得られない。逆に0.7を越えると強度が不足する。よって、この値は0.45以上0.70以下が好ましく、0.45以上0.60以下がより好ましい。 As described above, the balance between the Al component and the Ti component is important in this superalloy. As the proportion of the Al component in the γ 'phase increases, the ductility improves, but conversely the strength decreases. In the present invention, since it is important to ensure sufficient ductility, in order to define the ratio of the Al component in the γ ′ phase, “[Al concentration] / ([Al concentration] +0.56 [Ti concentration] ) "Is set. When this value is lower than 0.45, sufficient ductility cannot be obtained. Conversely, if it exceeds 0.7, the strength will be insufficient. Therefore, this value is preferably 0.45 or more and 0.70 or less, and more preferably 0.45 or more and 0.60 or less.
Fe成分は、必ずしも添加する必要はないが、超合金の熱間加工性を改善する作用があるため、必要に応じて添加することができる。ただし、10質量％を越えると、超合金の熱膨張係数が大きくなるとともに、耐酸化性が劣化する。よって、Fe成分量は、0質量％以上10質量％以下が好ましく、0質量％以上5質量％以下がより好ましく、0質量％以上2質量％以下が更に好ましい。 The Fe component does not necessarily need to be added, but can be added as necessary because it has an effect of improving the hot workability of the superalloy. However, if it exceeds 10% by mass, the thermal expansion coefficient of the superalloy increases and the oxidation resistance deteriorates. Therefore, the amount of Fe component is preferably 0% by mass or more and 10% by mass or less, more preferably 0% by mass or more and 5% by mass or less, and further preferably 0% by mass or more and 2% by mass or less.
B成分およびZr成分は、ともに結晶粒界を強化し、超合金の高温における延性を高める効果があるため、少なくとも1種を添加する。ただし、それぞれ過剰に添加すると熱間加工性を劣化させる。よって、B成分量は0.002質量％以上0.02質量％以下が好ましく、Zr成分量は0.01質量％以上0.2質量％以下が好ましい。 Since both the B component and the Zr component have the effect of strengthening the grain boundaries and increasing the ductility of the superalloy at a high temperature, at least one kind is added. However, when each is added excessively, hot workability is deteriorated. Therefore, the B component amount is preferably 0.002% by mass or more and 0.02% by mass or less, and the Zr component amount is preferably 0.01% by mass or more and 0.2% by mass or less.
本超合金組成の残部はNi成分と不可避的不純物である。Ni成分量が48質量％未満だと高温強度が不足し、80質量％を超えると延性が低下する。よって、Ni成分量は48質量％以上80質量％以下が好ましく、50質量％以上75質量％以下がより好ましく、54質量％以上72質量％以下が更に好ましい。 The balance of the superalloy composition is Ni components and inevitable impurities. If the Ni content is less than 48% by mass, the high temperature strength is insufficient, and if it exceeds 80% by mass, the ductility decreases. Therefore, the amount of Ni component is preferably 48% by mass to 80% by mass, more preferably 50% by mass to 75% by mass, and further preferably 54% by mass to 72% by mass.
なお、上記以外の成分元素に関して、少量であれば本発明のNi基鍛造超合金の特性に特段の悪影響を与えない元素を該超合金に含有させてもよい。具体的には、0.05質量％以下のP（リン）、0.01質量％以下のS（硫黄）、1質量％以下のNb（ニオブ）、20質量％以下のCo（コバルト）、5質量％以下のCu（銅）、0.01質量％以下のMg（マグネシウム）、0.01質量％以下のCa（カルシウム）、0.02質量％以下のO（酸素）、0.05質量％以下のN（窒素）、0.1質量％以下のREM（希土類金属）である。Nb成分量は0.8質量％以下がより好ましく、Co成分量は5質量％以下がより好ましい。 In addition, regarding the component elements other than those described above, an element that does not particularly adversely affect the characteristics of the Ni-based forged superalloy according to the present invention may be contained in the superalloy if the amount is small. Specifically, 0.05 mass% or less P (phosphorus), 0.01 mass% or less S (sulfur), 1 mass% or less Nb (niobium), 20 mass% or less Co (cobalt), 5 mass% or less Cu (copper), 0.01 mass% or less Mg (magnesium), 0.01 mass% or less Ca (calcium), 0.02 mass% or less O (oxygen), 0.05 mass% or less N (nitrogen), 0.1 mass% or less REM (rare earth metal). The Nb component amount is more preferably 0.8% by mass or less, and the Co component amount is more preferably 5% by mass or less.
（平均結晶粒径）
本発明に係るNi基鍛造超合金は、多結晶体であって、72μm以上289μm以下の平均結晶粒径を有している。また、本発明のNi基鍛造超合金は、できるだけ結晶粒径が揃った均等な組織であることが望ましい。結晶粒の大きさは、日本工業標準規格（JIS）における「鋼のオーステナイト結晶粒度試験方法」（JIS G 0551）で、結晶粒度番号（GS No.）0.99〜5.0の範囲が適当である。すなわち、下限である平均結晶粒径72μmは粒度番号5.0である。平均結晶粒径が72μmより小さいと、従来のNi基鍛造超合金に比べて、クリープ強度の高強度化が十分図れない。一方、上限である289μm（結晶粒度番号＝0.99）より大きな平均結晶粒径では、本発明の結晶粒界組織としても延性の低下が著しくなる。また、289μmより大きな平均結晶粒径のNi基鍛造超合金は、超音波透過性も低下することから大型部材を作製した際の超音波探傷試験による欠陥検出性が悪くなる。平均結晶粒径は、141μm以上282μm以下（結晶粒度番号＝1.0〜3.0）がより好ましい。なお、結晶粒径は、JISで定めるところの混粒（粒度番号が3以上異なる結晶粒が共存すること）でないものとする。
(Average crystal grain size)
The Ni-based forged superalloy according to the present invention is a polycrystalline body and has an average crystal grain size of 72 μm or more and 289 μm or less. In addition, the Ni-based forged superalloy of the present invention desirably has a uniform structure with as much crystal grain size as possible. The size of the crystal grain is suitably in the range of the grain size number (GS No.) 0.99 to 5.0 in the “Austenite grain size test method for steel” (JIS G 0551) in the Japanese Industrial Standards (JIS). That is, the lower limit average crystal grain size 72 μm is the grain size number 5.0. If the average grain size is smaller than 72 μm, the creep strength cannot be sufficiently increased as compared with the conventional Ni-based forged superalloy. On the other hand, when the average crystal grain size is larger than the upper limit of 289 μm (grain size number = 0.99), the ductility is significantly lowered even in the grain boundary structure of the present invention. In addition, Ni-base forged superalloys having an average crystal grain size larger than 289 μm also have reduced ultrasonic transmission, so that the defect detectability by an ultrasonic flaw detection test when a large-sized member is produced is deteriorated. The average crystal grain size is more preferably 141 μm or more and 282 μm or less (crystal grain size number = 1.0 to 3.0). Note that the crystal grain size is not a mixed grain as defined by JIS (a crystal grain having a grain size number of 3 or more coexists).
（結晶粒界の析出物及び析出形態）
本発明に係るNi基鍛造超合金は、多結晶体の結晶粒界に沿って複数の粒状析出物が析出しており、該粒状析出物の平均長さが0.5μm以上2.5μm以下であることが好ましく、0.5μm以上1.5μm以下がより好ましい。ここで、粒状析出物の平均長さとは、多結晶体の任意断面における結晶粒界に沿った平均長さであり、言い換えると、１個の粒状析出物によって覆われている結晶粒界の平均長さ（粒状析出物１個あたりの結晶粒界の平均被覆長さ）と定義する。また、結晶粒界における粒状析出物は、Cr炭化物と、Mo炭化物および/またはW炭化物で主に構成され、Ti炭化物を含む場合もある。粒状析出物の平均長さが0.5μmより小さいと、粒界の強化（粒界結合性の向上）に寄与することが難しい。一方、この平均長さが大き過ぎる（すなわち粒状析出物１個あたりが覆う粒界の平均長さが大きくなる）と、逆に粒界結合性の低下を招きやすくなる。実験的に確認したところ、粒状析出物の平均長さが2.5μm以下であれば延性低下が抑制され、1.5μm以下であればより好ましい。
(Precipitates and precipitation forms at grain boundaries)
In the Ni-based forged superalloy according to the present invention, a plurality of granular precipitates are precipitated along the crystal grain boundaries of the polycrystalline body, and the average length of the granular precipitates is 0.5 μm or more and 2.5 μm or less. Is preferably 0.5 μm or more and 1.5 μm or less. Here, the average length of the granular precipitate is an average length along the crystal grain boundary in an arbitrary cross section of the polycrystal, in other words, the average of the crystal grain boundary covered by one granular precipitate. The length is defined as the average coating length of the grain boundaries per granular precipitate. Further, the granular precipitates at the grain boundaries are mainly composed of Cr carbide, Mo carbide and / or W carbide, and may include Ti carbide. If the average length of the granular precipitates is less than 0.5 μm, it is difficult to contribute to strengthening of grain boundaries (improving grain boundary bonding). On the other hand, if this average length is too large (that is, the average length of the grain boundary covered by one granular precipitate is increased), conversely, the grain boundary connectivity tends to be lowered. When experimentally confirmed, the average length of the granular precipitates is suppressed to 2.5 μm or less, and a decrease in ductility is suppressed, and 1.5 μm or less is more preferable.
上述した粒状析出物の大きさ（平均長さ）に加えて、粒状析出物の析出量（すなわち粒状析出物の数）も十分な粒界強度（粒界結合性）を得るために重要である。本発明に係るNi基鍛造超合金は、多結晶体の任意断面における結晶粒界の合計長さに対する粒状析出物の合計長さの比率（被覆率）が、50％以上であることが好ましい。具体的には、上述した平均長さを有する粒状析出物が、多結晶体の任意断面における結晶粒界10μmあたり、3個以上析出していることが好ましい。4個以上がより好ましく、5個以上が更に好ましい。 In addition to the size (average length) of the granular precipitates described above, the amount of granular precipitates (that is, the number of granular precipitates) is also important for obtaining sufficient grain boundary strength (intergranular bondability). In the Ni-based forged superalloy according to the present invention, the ratio (coverage) of the total length of the granular precipitates to the total length of the grain boundaries in an arbitrary cross section of the polycrystal is preferably 50% or more. Specifically, it is preferable that three or more granular precipitates having the above average length are precipitated per 10 μm of grain boundaries in an arbitrary cross section of the polycrystalline body. 4 or more is more preferable, and 5 or more is more preferable.
（Ni基鍛造超合金の製造方法）
本発明に係るNi基鍛造超合金の製造方法は、熱処理工程（特に溶体化熱処理工程）に最大の特徴を有する。他の工程に特段の限定はなく、従前の方法を利用することができる。以下、該熱処理について詳細に説明する。
(Ni-base forged superalloy manufacturing method)
The method for producing a Ni-based forged superalloy according to the present invention has the greatest feature in a heat treatment step (particularly a solution heat treatment step). There is no special limitation in other processes, and a conventional method can be used. Hereinafter, the heat treatment will be described in detail.
（２段階溶体化熱処理）
前述したように、本発明者等は２段階溶体化熱処理という新規な熱処理方法を開発した。第１段溶体化熱処理は、1100℃以上1160℃以下の温度で行われる。第１段溶体化熱処理を実施することで、短時間で超合金結晶粒を粗大化することが可能となる。ただし、1160℃より高い温度で溶体化熱処理すると、粒成長速度が大きくなり過ぎるため、平均結晶粒径を289μm以下に制御することが困難になる。第１段溶体化熱処理の段階で平均結晶粒径が289μmより大きくなると、その後の熱処理（第２段溶体化熱処理を含む）を実施しても十分な延性を得ることが困難になる。また、前述したように、そのような超合金から大型部品を製造した場合、超音波探傷試験による欠陥検出性が悪くなる問題がある。一方、1100℃以上ではほとんどの炭化物がマトリックス中に固溶する（溶体化する）ため、粒界移動が容易になり結晶粒が粗大化しやすくなる。さらに粗大化を促進するためには、第１段溶体化熱処理は1125℃以上がより好ましい。
(Two-stage solution heat treatment)
As described above, the present inventors have developed a novel heat treatment method called a two-step solution heat treatment. The first stage solution heat treatment is performed at a temperature of 1100 ° C. or higher and 1160 ° C. or lower. By performing the first stage solution heat treatment, the superalloy crystal grains can be coarsened in a short time. However, when solution heat treatment is performed at a temperature higher than 1160 ° C., the grain growth rate becomes too high, and it becomes difficult to control the average crystal grain size to 289 μm or less. When the average crystal grain size becomes larger than 289 μm in the first stage solution heat treatment stage, it becomes difficult to obtain sufficient ductility even if the subsequent heat treatment (including the second stage solution heat treatment) is performed. Further, as described above, when a large part is manufactured from such a superalloy, there is a problem that the defect detectability by the ultrasonic flaw detection test is deteriorated. On the other hand, most of the carbides dissolve in the matrix (solution) at 1100 ° C. or higher, which facilitates grain boundary movement and facilitates coarsening of crystal grains. In order to further promote the coarsening, the first-stage solution heat treatment is more preferably 1125 ° C. or higher.
本発明の２段階溶体化熱処理において、第２段溶体化熱処理は、980℃以上1080℃以下の温度で行われる。熱力学計算の結果、本発明の超合金系では、γ’相の固溶温度が最高でおよそ980℃である。そのため、γ’相を溶体化状態とするには、最低でも980℃以上とする必要がある。一方、第２段溶体化熱処理温度が1080℃より高くなると炭化物が固溶しはじめる。この第２段溶体化熱処理は、γ’相は固溶状態の温度域であるが、熱力学的に炭化物は析出可能である。ただし、この熱処理は炭化物析出のための過冷度が小さいことから（すなわち核生成頻度が小さいことから）、少ない数の炭化物が成長しながら結晶粒界上に析出する。その結果、結晶粒界に沿って粒状炭化物が鎖状に連なるように析出した微細構造が得られ、クリープ延性を高めることができる。また、一部の粒状炭化物は結晶粒内にも析出する。 In the two-stage solution heat treatment of the present invention, the second-stage solution heat treatment is performed at a temperature of 980 ° C. or higher and 1080 ° C. or lower. As a result of thermodynamic calculation, in the superalloy system of the present invention, the solid solution temperature of the γ ′ phase is about 980 ° C. at the maximum. For this reason, in order to bring the γ ′ phase into a solution state, it is necessary that the temperature be at least 980 ° C. or higher. On the other hand, when the second-stage solution heat treatment temperature is higher than 1080 ° C., the carbide starts to dissolve. In this second-stage solution heat treatment, the γ 'phase is in a temperature range where the solution is in a solid solution state, but carbide can be precipitated thermodynamically. However, in this heat treatment, since the degree of supercooling for carbide precipitation is small (that is, because the nucleation frequency is low), a small number of carbides are deposited on the grain boundaries while growing. As a result, it is possible to obtain a microstructure in which the granular carbides are precipitated so as to be chain-like along the crystal grain boundary, and the creep ductility can be improved. Some granular carbides are also precipitated in the crystal grains.
前述の第１段溶体化熱処理の後に、この温度域の熱処理（第２段溶体化熱処理）を実施せずに通常の時効熱処理を施すと、大きな化学ポテンシャル差（過冷度、過飽和度）を駆動力として、結晶粒界に沿って炭化物の核生成が一気に起こり、多数の炭化物微粒子が連結して結晶粒を膜状に覆うように析出した微細構造となる。その結果、粒界強度（粒界結合性）が低下し、十分なクリープ延性が得られない。第２段溶体化熱処理の実施は、炭化物析出が一気に起こることを抑制する効果があり、時効熱処理時に結晶粒界で炭化物微粒子の膜状析出を抑制することが可能となる。 If the normal aging heat treatment is performed without performing the heat treatment in this temperature range (second-stage solution heat treatment) after the first-stage solution heat treatment described above, a large chemical potential difference (supercooling degree, supersaturation degree) is obtained. As a driving force, nucleation of carbides occurs at once along the crystal grain boundary, and a fine structure is formed in which a large number of carbide fine particles are connected and deposited so as to cover the crystal grains in a film shape. As a result, the grain boundary strength (grain boundary bondability) is lowered, and sufficient creep ductility cannot be obtained. The implementation of the second-stage solution heat treatment has an effect of suppressing the occurrence of carbide precipitation all at once, and it is possible to suppress film-like precipitation of carbide fine particles at the crystal grain boundary during the aging heat treatment.
（１段溶体化−長時間熱処理）
本発明者等は、別の溶体化熱処理方法として、980℃以上1080℃以下の温度域（γ’相の固溶温度以上で炭化物の固溶温度以下の中間温度域）で長時間保持する（具体的には24時間以上、より好ましくは48時間以上）熱処理を開発した。この１段溶体化−長時間熱処理よっても、本発明に係るNi基鍛造超合金の結晶粒を粗大化させ、結晶粒界に沿って粒状炭化物が鎖状に連なるように析出した微細構造を形成することが可能である。その結果、Ni基鍛造超合金のクリープ延性を高めることができる。１段溶体化−長時間熱処理は、前述の２段階溶体化熱処理と比べて長時間を要するものの、炭化物が固溶しない温度域かつ過飽和度が小さい温度域であるため、炭化物微粒子の大量生成を防止することができる。さらに、温度を変化させる必要がないことから被熱処理体内部に温度分布が生じることを防ぐことができるので、より均等な大きさの結晶粒を形成するのに好適である。
(One-step solution-long-time heat treatment)
As another solution heat treatment method, the present inventors hold for a long time in a temperature range from 980 ° C. to 1080 ° C. (intermediate temperature range from the solid solution temperature of the γ ′ phase to the solid solution temperature of the carbide) ( Specifically, heat treatment was developed for 24 hours or more, more preferably 48 hours or more). Even with this one-step solution-long-time heat treatment, the crystal grains of the Ni-base forged superalloy according to the present invention are coarsened to form a fine structure in which granular carbides are precipitated in a chain along the grain boundaries. Is possible. As a result, the creep ductility of the Ni-based forged superalloy can be increased. Although the first-stage solution-long-time heat treatment requires a longer time than the above-mentioned two-stage solution heat-treatment, it is a temperature range in which carbides do not form a solid solution and the degree of supersaturation is small. Can be prevented. Furthermore, since it is not necessary to change the temperature, it is possible to prevent the temperature distribution from occurring inside the body to be heat-treated, which is suitable for forming crystal grains having a more uniform size.
（時効熱処理）
溶体化熱処理の後、時効熱処理が行われる。本発明において、時効熱処理に特段の限定はなく、従前の時効熱処理を実施することができる。１例として、クリープ強度と延性の観点から検討を行った結果、820℃以上880℃以下の温度で第１時効熱処理を行い、その後、600℃以上800℃以下の温度で第２時効熱処理を行うことは好ましい。該２段階の時効熱処理を行うことで、良好なクリープ強度とクリープ延性とを両立した特性が得られる。
(Aging heat treatment)
An aging heat treatment is performed after the solution heat treatment. In the present invention, the aging heat treatment is not particularly limited, and the conventional aging heat treatment can be performed. As an example, as a result of investigation from the viewpoint of creep strength and ductility, the first aging heat treatment is performed at a temperature of 820 ° C to 880 ° C, and then the second aging heat treatment is performed at a temperature of 600 ° C to 800 ° C. It is preferable. By performing the two-stage aging heat treatment, it is possible to obtain characteristics satisfying both good creep strength and creep ductility.
（石炭火力発電プラント用高温部材）
以上説明してきたように、本発明に係るNi基鍛造超合金は、良好な高温機械的特性を有することから、石炭火力発電プラントの高温部材として好適に利用することができる。図２は、700℃級石炭火力発電プラントとそれに使用される高温部材の外観例を示す模式図である。図２に示したように、700℃級石炭火力発電プラントは、ボイラー10で加熱された高温蒸気（例えば700〜750℃）が蒸気タービン20の高圧タービン21、中圧タービン22、低圧タービン23に順次供給され、蒸気タービンシャフトに連結された発電機30を回転させて発電するシステムである。本発明に係るNi基鍛造超合金は、高温蒸気に直接曝されかつ大きな機械的応力が掛かるボイラー管11や高圧タービンブレード24やケーシングボルト25などに好適に利用することができる。
(High temperature components for coal-fired power plants)
As described above, the Ni-based forged superalloy according to the present invention has good high-temperature mechanical properties, and can be suitably used as a high-temperature member for a coal-fired power plant. FIG. 2 is a schematic diagram showing an external appearance example of a 700 ° C. class coal-fired power plant and a high-temperature member used therefor. As shown in FIG. 2, in the 700 ° C. class coal-fired power plant, high-temperature steam (for example, 700 to 750 ° C.) heated by the
ボイラー管11は、本発明の溶体化熱処理を実施後で時効熱処理を未実施の状態のものを利用することが好ましい。この理由は、ボイラー10を組み立てる際、ボイラー管11を溶接によって接続することが多く、該溶接施工によって割れが生じないようにボイラー管11を軟化状態にしておく必要があるためである。また、溶接施工したボイラー管11を700℃級石炭火力発電プラントで使用した場合、使用中に高温蒸気によって実質的な時効熱処理が施され、マトリックス中にγ’相が析出して良好な高温機械的特性が得られる。一方、高圧タービンブレード24やケーシングボルト25は、本発明の溶体化熱処理と前述の時効熱処理を実施した状態のものを利用することが好ましい。
It is preferable to use the
以下、本発明を実施例に基づいて更に詳しく説明するが、本発明はこれらに限定されるものではない。 EXAMPLES Hereinafter, although this invention is demonstrated in more detail based on an Example, this invention is not limited to these.
はじめに、表１に示す組成となるように、真空誘導溶解（VIM）及びエレクトロスラグ再溶解（ESR）のダブルメルトプロセスによって溶造し、得られた鋳塊に対して熱間鍛造を施して超合金出発材とした。 First, the composition shown in Table 1 is melted by a double melt process of vacuum induction melting (VIM) and electroslag remelting (ESR). The alloy starting material was used.
上記の超合金出発材に対して、種々の熱処理を実施してNi基鍛造超合金（実施例１〜１８および比較例１〜７）を用意した。得られた各Ni基鍛造超合金（実施例１〜１８および比較例１〜７）に対して、断面の微細構造観察とクリープ試験を行った。溶体化熱処理と時効熱処理を施した場合の実施例１〜６の結果を表２に示し、実施例７〜１６の結果を表３に示し、比較例１〜８の結果を表４に示す。なお、表２〜４のクリープ試験条件は700℃で45 kgf/mm2とした。 Various heat treatments were performed on the above superalloy starting materials to prepare Ni-based forged superalloys (Examples 1 to 18 and Comparative Examples 1 to 7). Each Ni-based forged superalloy obtained (Examples 1 to 18 and Comparative Examples 1 to 7) was subjected to cross-sectional microstructure observation and a creep test. The results of Examples 1 to 6 when solution heat treatment and aging heat treatment are performed are shown in Table 2, the results of Examples 7 to 16 are shown in Table 3, and the results of Comparative Examples 1 to 8 are shown in Table 4. The creep test conditions in Tables 2 to 4 were 45 kgf / mm 2 at 700 ° C.
表２〜４では、使用した合金No、実施した熱処理条件、超合金結晶の平均粒径（μm）と結晶粒度番号（JISに定めるもの、G）、析出物の平均長さ（μm）、粒界10μmあたりの平均析出数、被覆率（％）、及び、クリープ試験結果（クリープ破断時間、クリープ伸び、絞り）を示した。なお、析出物が炭化物微粒子による膜状析出物の場合は（図１中の比較のNi基鍛造超合金Ｂ参照）、便宜上、粒界に沿った膜状被覆の１区切りを１つの析出物と見なして、長さと析出物の数を計数した。 In Tables 2 to 4, the alloy No. used, the heat treatment conditions carried out, the average grain size (μm) and grain size number of the superalloy crystal (G, JIS), the average length of the precipitate (μm), and the grain size The average number of precipitates per 10 μm boundary, coverage (%), and creep test results (creep rupture time, creep elongation, drawing) were shown. When the precipitate is a film-like precipitate due to carbide fine particles (see comparative Ni-based forged superalloy B in FIG. 1), for convenience, one break of the film-like coating along the grain boundary is referred to as one precipitate. The length and the number of precipitates were counted.
比較例１は従来の一般的な熱処理によるものである。比較例１は、良好なクリープ延性を示したが、クリープ強度において更なる向上が望まれる結果であった。そこで、溶体化熱処理温度を高めて超合金結晶粒を粗大化したところ、クリープ強度が向上したが、クリープ延性が著しく低下する結果であった（比較例２〜４参照）。また、比較例２〜４は、粗大化した超合金結晶粒の粒界に沿って炭化物微粒子が連続的に連なって膜状に結晶粒を覆うように析出した微細構造を有していた。比較例５は、粗大化した結晶粒の粒界に沿って粒状炭化物が鎖状に連なるように析出した微細構造を有していたが、超合金結晶粒が大きくなり過ぎてクリープ延性が低下する結果であった。比較例６は、比較例５と同様に粗大化した結晶粒の粒界に沿って粒状炭化物が鎖状に連なるように析出した微細構造を有していたが、粒状析出物による粒界に対する被覆率が小さ過ぎてクリープ破断時間（すなわち、クリープ強度）が不十分な結果であった。 Comparative Example 1 is based on a conventional general heat treatment. Although Comparative Example 1 showed good creep ductility, it was a result that further improvement in creep strength was desired. Therefore, when the solution heat treatment temperature was raised to coarsen the superalloy crystal grains, the creep strength was improved, but the creep ductility was significantly reduced (see Comparative Examples 2 to 4). Further, Comparative Examples 2 to 4 had a microstructure in which carbide fine particles were continuously connected along the grain boundaries of coarsened superalloy crystal grains and deposited so as to cover the crystal grains in a film shape. Comparative Example 5 had a microstructure in which granular carbides were precipitated so as to be chained along the grain boundaries of the coarsened crystal grains, but the superalloy crystal grains became too large and the creep ductility was lowered. It was a result. Comparative Example 6 had a microstructure in which granular carbides were precipitated so as to be chain-like along the grain boundaries of the coarsened crystal grains as in Comparative Example 5, but the grain boundaries were covered with the granular precipitates. The rate was too small and the creep rupture time (ie, creep strength) was insufficient.
これらに対し、本発明に係るNi基鍛造超合金（実施例１〜１６）は、結晶粒の粗大化に伴って良好なクリープ強度を示すとともに、必要十分なクリープ延性を示した。具体的には、従来のNi基鍛造超合金（例えば比較例１）に比して、1.3〜1.7倍のクリープ破断時間を示すとともに、0.5〜1.1倍のクリープ延性を示した。 On the other hand, the Ni-based forged superalloys according to the present invention (Examples 1 to 16) exhibited good creep strength along with the coarsening of crystal grains, and exhibited necessary and sufficient creep ductility. Specifically, the creep rupture time was 1.3 to 1.7 times that of a conventional Ni-based forged superalloy (for example, Comparative Example 1), and the creep ductility was 0.5 to 1.1 times.
次に、700℃級石炭火力発電プラントのボイラー管を想定して、溶体化熱処理を実施後で時効熱処理を未実施の状態の試料（実施例１７〜１８および比較例７）に対するクリープ試験（750℃×19.6 kgf/mm2）を実施した。微細組織とクリープ試験結果を表５に示す。表５に示したように、本発明に係るNi基鍛造超合金（実施例１７〜１８）は、従来のNi基鍛造超合金（例えば比較例７）に比して、約1.5倍のクリープ破断時間を示すとともに、同等のクリープ延性を有することが実証された。 Next, assuming a boiler tube of a 700 ° C. class coal-fired power plant, a creep test (750 to 750) was performed on samples (Examples 17 to 18 and Comparative Example 7) in which aging heat treatment was not performed after solution heat treatment was performed. ℃ × 19.6 kgf / mm 2 ). Table 5 shows the microstructure and creep test results. As shown in Table 5, the Ni-based forged superalloys according to the present invention (Examples 17 to 18) are about 1.5 times creep ruptured as compared with the conventional Ni-based forged superalloy (for example, Comparative Example 7). It has been demonstrated that it exhibits time and has equivalent creep ductility.
1…粗大化した結晶粒、1’…比較的小さい結晶粒、2…粒界、
3…粒状炭化物、3’…炭化物微粒子、
10…ボイラー、11…ボイラー管、
20…蒸気タービン、21…高圧タービン、22…中圧タービン、23…低圧タービン、
24…高圧タービンブレード、25…ケーシングボルト。
1 ... coarsened grains, 1 '... relatively small grains, 2 ... grain boundaries,
3 ... granular carbide, 3 '... carbide fine particles,
10 ... Boiler, 11 ... Boiler tube,
20 ... steam turbine, 21 ... high pressure turbine, 22 ... medium pressure turbine, 23 ... low pressure turbine,
24 ... high pressure turbine blade, 25 ... casing bolt.
Claims (14)
前記Ni基鍛造超合金は所定の熱処理が施された多結晶体であり、前記結晶の平均粒径が72μm以上289μm以下であり、
前記Ni基鍛造超合金は結晶粒界に沿って複数の粒状析出物が析出しており、前記多結晶体の任意断面における前記粒状析出物の平均長さが0.5μm以上2.5μm以下であることを特徴とするNi基鍛造超合金。 As composition, the additive component is 0.005 mass% to 0.2 mass% C, 0 mass% to 1 mass% Si, 0 mass% to 1 mass% Mn, 10 mass% to 24 mass% Cr, When at least one of Mo and W is defined as “[Mo concentration] + 0.5 [W concentration]”, 5% by mass to 17% by mass, 1% by mass to 2% by mass Al, 0.5% by mass to 3.5% at least one result, the balance being 48 mass% or less by mass of Ti, 0 wt% to 10 wt% of Fe, and 0.002 wt% 0.02 wt% or less of B and 0.01 wt% to 0.2 wt% Zr Ni-based forged superalloy composed of Ni and unavoidable impurities of not less than 80% and not more than 80% by mass,
The Ni-based wrought superalloy is polycrystal predetermined heat treatment is performed, the average particle diameter before Symbol crystal is less 289μm least 72 .mu.m,
The Ni-based wrought superalloy has a plurality of particulate precipitates along the crystal grain boundary precipitates, the average length of the granular precipitates at any cross section is 0.5μm or 2.5μm below the polycrystal Ni-base forged superalloy characterized by that.
前記多結晶体の任意断面における前記結晶粒界の合計長さに対する前記粒状析出物の合計長さの比率（被覆率）が、50％以上であることを特徴とするNi基鍛造超合金。 In the Ni-based forged superalloy according to claim 1,
A Ni-based forged superalloy characterized in that the ratio (coverage) of the total length of the granular precipitates to the total length of the crystal grain boundaries in an arbitrary cross section of the polycrystal is 50% or more.
前記多結晶体の任意断面における前記粒状析出物の個数が、前記結晶粒界10μmあたり3個以上であることを特徴とするNi基鍛造超合金。 In the Ni-based forged superalloy according to claim 1 or 2,
The Ni-based forged superalloy characterized in that the number of the granular precipitates in an arbitrary cross section of the polycrystalline body is 3 or more per 10 μm of the crystal grain boundary.
前記粒状析出物が、Cr炭化物と、Mo炭化物および／またはW炭化物から構成されることを特徴とするNi基鍛造超合金。 In the Ni-based forged superalloy according to any one of claims 1 to 3,
Wherein the particulate precipitate, Cr carbide and, Mo carbide and / or W carbide or al structure Ni-based wrought superalloy, characterized in that it is made.
組成として、0質量％超20質量％以下のCoおよび／または0質量％超1質量％以下のNbを更に含有することを特徴とするNi基鍛造超合金。 In the Ni-based forged superalloy according to any one of claims 1 to 4 ,
A Ni-based forged superalloy further comprising, as a composition, more than 0% by mass and 20% by mass of Co and / or more than 0% by mass and 1% by mass of Nb.
組成として、「[Al濃度]/([Al濃度]＋0.56[Ti濃度])」で表わされる値が0.45以上0.70以下であることを特徴とするNi基鍛造超合金。 In the Ni-based forged superalloy according to any one of claims 1 to 5 ,
A Ni-based forged superalloy having a composition represented by “[Al concentration] / ([Al concentration] +0.56 [Ti concentration])” of 0.45 or more and 0.70 or less.
前記所定の熱処理は、1100℃以上1160℃以下で施される第１溶体化熱処理と、それに引き続いて980℃以上1080℃以下で施される第２溶体化熱処理とを含むことを特徴とするNi基鍛造超合金。 In the Ni-based forged superalloy according to any one of claims 1 to 6 ,
The predetermined heat treatment includes a first solution heat treatment performed at 1100 ° C. to 1160 ° C. and a second solution heat treatment performed at 980 ° C. to 1080 ° C. Base forging superalloy.
前記所定の熱処理は、980℃以上1080℃以下で24時間以上保持する溶体化熱処理を含むことを特徴とするNi基鍛造超合金。 In the Ni-based forged superalloy according to any one of claims 1 to 6 ,
Wherein the predetermined heat treatment, Ni-based wrought superalloy which comprises a solution heat treatment for holding 980 ° C. or higher 1080 ° C. or less for 24 hours or more.
前記所定の熱処理は溶体化熱処理を施す工程を有し、
前記溶体化熱処理が、1100℃以上1160℃以下の第１溶体化熱処理と、それに引き続いて行われる980℃以上1080℃以下の第２溶体化熱処理とからなる２段階溶体化熱処理であり、
前記第１溶体化熱処理における温度と保持時間とを制御することによって前記平均粒径を前記の範囲内に制御し、
前記第２溶体化熱処理における温度と保持時間とを制御することによって前記粒状析出物の析出を前記の性状に制御することを特徴とするNi基鍛造超合金の製造方法。 A method for producing a Ni-based forged superalloy according to any one of claims 1 to 6 ,
The predetermined heat treatment includes a step of performing a solution heat treatment,
The solution heat treatment, and 1100 ° C. or higher 1160 ° C. or less of the first solution heat treatment, are two-step solution heat treatment der consisting in a 980 ° C. or higher 1080 ° C. or less of the second solution heat treatment performed subsequently thereto,
By controlling the temperature and holding time in the first solution heat treatment, the average particle size is controlled within the range,
A method for producing a Ni-based forged superalloy characterized by controlling the precipitation of the granular precipitates to have the above-mentioned properties by controlling the temperature and holding time in the second solution heat treatment .
前記所定の熱処理は溶体化熱処理を施す工程を有し、
前記溶体化熱処理が、980℃以上1080℃以下で24時間以上保持する１段溶体化−長時間熱処理であり、
前記１段溶体化−長時間熱処理における温度と保持時間とを制御することによって前記平均粒径を前記の範囲内に制御し、かつ前記粒状析出物の析出を前記の性状に制御することを特徴とするNi基鍛造超合金の製造方法。 A method for producing a Ni-based forged superalloy according to any one of claims 1 to 6 ,
The predetermined heat treatment includes a step of performing a solution heat treatment,
The solution heat treatment, 1 step solution holding 980 ° C. or higher 1080 ° C. or less for 24 hours or more - Ri long heat treatment der,
The average particle size is controlled within the above range by controlling the temperature and holding time in the first-stage solution-long-time heat treatment, and the precipitation of the granular precipitates is controlled to the above properties. A method for producing a Ni-based forged superalloy.
前記所定の熱処理は、前記溶体化熱処理を施す工程の後に、更に時効熱処理を施す工程を有し、
前記時効熱処理が、820℃以上880℃以下の第１時効熱処理と、600℃以上800℃以下の第２時効熱処理とからなる２段階時効熱処理であることを特徴とするNi基鍛造超合金の製造方法。 The method for producing a Ni-based forged superalloy according to claim 12 or claim 13,
The predetermined heat treatment has a step of further performing an aging heat treatment after the step of performing the solution heat treatment,
Production of a Ni-based forged superalloy characterized in that the aging heat treatment is a two-stage aging heat treatment comprising a first aging heat treatment of 820 ° C or higher and 880 ° C or lower and a second aging heat treatment of 600 ° C or higher and 800 ° C or lower. Method.
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| JP5921401B2 (en) * | 2012-02-10 | 2016-05-24 | 株式会社東芝 | Ni-based alloy, method for producing the same, and turbine component |
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| RU2542194C1 (en) * | 2014-02-19 | 2015-02-20 | Открытое акционерное общество Научно-производственное объединение "Центральный научно-исследовательский институт технологии машиностроения" ОАО НПО "ЦНИИТМАШ" | Refractory nickel-based alloy for casting gas turbine working blades |
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