JP2017155264A - Austenitic heat resistant steel and turbine component - Google Patents

Abstract

An object is to provide an austenitic heat-resistant steel and a turbine component that can ensure excellent tensile strength and tensile ductility while having sufficient creep strength. An austenitic heat-resisting steel according to an embodiment is, by mass, Ni: 24 to 50%, Cr: 5 to 13%, Co: 0.1 to 12%, Nb: 0.1 to 5%, V: 0.1-0.5%, Ti: 1.90-2.35%, Al: 0.01-0.30%, B: 0.001-0.01%, C: 0.001-0. 1%, Zr: 0.1 to 2.0%, P: 0.01 to 0.2%, with the balance being Fe and inevitable impurities. [Selection] Figure 1

Description

  Embodiments of the present invention relate to an austenitic heat resistant steel and a turbine component.

  In recent years, high efficiency of power plants has been promoted from the viewpoint of reducing carbon dioxide emissions into the atmosphere. Therefore, high efficiency of the steam turbine and gas turbine provided in the thermal power plant is required.

  In order to increase the efficiency of each turbine described above, it is effective to increase the inlet temperature of the working fluid introduced into the turbine. For example, a steam turbine is expected to be operated in the future when the temperature of steam as a working fluid is 650 ° C. or higher, and further about 700 ° C. Also in the gas turbine, the inlet temperature of the introduced working fluid tends to increase.

  Conventionally, ferritic heat-resistant steel or the like is used for turbine parts exposed to a temperature of about 600 ° C. However, it is problematic from the viewpoint of heat resistance that the turbine parts exposed to the high-temperature working fluid as described above are made of ferritic heat resistant steel. Therefore, the turbine component exposed to such a high-temperature working fluid is composed of austenitic heat-resistant steel, Ni-base alloy, Co-base alloy, or the like. Among these, the austenitic heat resistant steel has a higher service temperature as much as about 50 ° C. than the ferritic heat resistant steel, and the material cost is about 1/3 that of the Ni-based alloy. Therefore, by using austenitic heat-resistant steel, manufacturing costs can be reduced and higher efficiency can be achieved.

  Until now, many austenitic heat resistant steels have been developed mainly for improving high temperature creep strength. However, when austenitic steel is used as a large forging or casting material, the crystal grain size becomes large due to manufacturing restrictions, and the creep strength is sufficient, but the tensile strength and ductility cannot meet the design requirements. Occurs. In addition, in the case of austenitic steel having a large crystal grain size, there is a problem that attenuation occurs in ultrasonic flaw detection of the material and the defect detection ability is lowered. Known austenitic heat-resistant steels such as Alloy 286 are improved in high-temperature creep strength by using an intermetallic compound as a precipitation strengthening phase. However, no heat-resisting steel has been proposed that has sufficient creep strength as described above and ensures excellent tensile strength and tensile ductility.

JP 2011-195880 A

  In designing a high temperature structural material, the creep strength, tensile strength and tensile ductility of the material are important factors. However, it has been difficult for conventional austenitic heat resistant steels to ensure excellent tensile strength and tensile ductility while having sufficient creep strength.

  The problem to be solved by the present invention is to provide an austenitic heat-resisting steel and a turbine component that can ensure excellent tensile strength and tensile ductility while having sufficient creep strength.

  The austenitic heat-resisting steel according to the embodiment is, by mass, Ni: 24 to 50%, Cr: 5 to 13%, Co: 0.1 to 12%, Nb: 0.1 to 5%, V: 0.00. 1-0.5%, Ti: 1.90-2.35%, Al: 0.01-0.30%, B: 0.001-0.01%, C: 0.001-0.1% , Zr: 0.1 to 2.0%, P: 0.01 to 0.2%, the balance being Fe and inevitable impurities.

  The embodiment of the present invention can provide an austenitic heat resistant steel and a turbine component that can ensure excellent tensile strength and tensile ductility while having sufficient creep strength.

It is sectional drawing which shows an example of the turbine with which the austenitic heat-resisting steel of embodiment is applied.

  Embodiments of the present invention will be described below.

  The inventors of the present invention can finely control the crystal grain size after solidification by adding Zr and P in a conventional austenitic heat resistant steel, and obtain excellent tensile strength and tensile ductility while having creep strength. I found.

  In order to improve tensile strength and ductility with respect to austenitic heat-resisting steel, refinement of crystal grains is one effective means in accordance with Hall Petch's law. However, when manufacturing large forgings and castings with conventional austenitic steels, the solidification rate during casting is slow and sufficient crystal grain growth occurs, indicating a coarse grain structure, resulting in tensile strength and ductility. Absent.

Therefore, the present inventors have found that the addition of Zr and P enables the refinement of crystal grains even when the solidification rate is low.

By adding Zr and P, Zr and P are combined with oxygen in the molten metal to form an oxide. This oxide solidifies before the molten metal and is finely and uniformly distributed in the molten metal. This oxide exhibits a pinning effect when the molten metal solidifies and crystal grains grow, and functions to suppress grain growth. The austenitic steel which obtained the fine crystal grain by this effect succeeded in ensuring the outstanding tensile strength and ductility, having sufficient creep strength.

  The embodiment will be specifically described below. In the following description, “%” representing a composition component is “% by mass” unless otherwise specified.

  The austenitic heat resistant steel of the embodiment is, by mass, Ni: 24-50%, Cr: 5-13%, Co: 0.1-12%, Nb: 0.1-5%, V: 0.1 0.5%, Ti: 1.90-2.35%, Al: 0.01-0.30%, B: 0.001-0.01%, C: 0.001-0.1%, Zr : 0.1 to 2.0%, P: 0.01 to 0.2%, with the balance being Fe and inevitable impurities.

  Here, N, Si, Mn, S, etc. are mentioned as an unavoidable impurity in the austenitic heat-resistant steel of embodiment, for example.

  The austenitic heat-resisting steel of the embodiment is suitable as a material constituting a turbine component having a temperature during operation of 650 ° C. or higher, and further about 700 ° C. Examples of the turbine component include a turbine casing, a moving blade, a stationary blade, a turbine rotor, a screwing member, piping, and a valve. Here, as a screwing member, a bolt, a nut, etc. which fix various components inside a turbine casing or a turbine can be illustrated, for example. Examples of the piping include piping installed in a power generation turbine plant and the like through which a high-temperature and high-pressure working fluid passes.

  You may comprise all the parts of the above-mentioned turbine parts with the austenitic heat resistant steel of the embodiment. For example, you may comprise the one part site | part of the turbine components in which temperature becomes 650 degreeC or more with the austenitic heat-resistant steel of embodiment.

  The austenitic heat resistant steel of the present embodiment described above has a creep strength equivalent to that of the conventional austenitic heat resistant steel, and has higher tensile strength and tensile ductility than the conventional austenitic heat resistant steel. Therefore, the turbine component manufactured using the austenitic heat resistant steel of the embodiment also has the same characteristics as the austenitic heat resistant steel of the embodiment and has high reliability.

The above-described austenitic heat-resistant steel and turbine parts according to the embodiments can be applied to power generation turbines such as steam turbines, gas turbines, and CO ₂ turbines, for example.

  FIG. 1 shows an example of a turbine.

  The turbine 10 includes, for example, a casing 11, a turbine rotor 12, a turbine disk 13, a moving blade 14, and a stationary blade 15. The turbine rotor 12 is provided inside the casing 11 so as to penetrate therethrough. The turbine rotor 12 and the turbine disk 13 are welded and joined by a welding portion 16. The turbine 10 may not have one of the turbine rotor 12 or the turbine disk 13 or may be a combination of a plurality of turbine rotors 12 and the turbine disk 13. A plurality of rotor blades 14 are implanted around each turbine rotor 12 and turbine disk 13. A stationary blade 15 is arranged in front of the moving blade 14. The stationary blade 15 is supported by the casing 11. The moving blade 14 and the stationary blade 15 constitute one turbine stage.

  Next, the reason for limitation of each component range in the austenitic heat-resistant steel of the above-described embodiment will be described.

  (1) Ni (nickel)

  Ni dissolves in the Fe matrix and brings about solid solution strengthening of the matrix. These effects are exhibited when the Ni content is 24% or more. Further, when the Ni content is 50% or less, an increase in material cost and a decrease in workability can be suppressed. Therefore, the Ni content is determined to be 24 to 50%. A more preferable Ni content is 34 to 45%, a further preferable Ni content is 38 to 45%, and a most preferable Ni content is 38 to 41%.

  (2) Cr (chromium)

  Cr dissolves in the Fe matrix and brings about solid solution strengthening of the matrix. Moreover, since Cr raises the solid solution temperature of the γ ′ phase, the precipitation of the γ ′ phase is promoted. These effects are exhibited when the Cr content is 5% or more. Further, when the Cr content is 13% or less, a stable austenite structure is obtained and precipitation of the σ phase is suppressed. Therefore, the Cr content is determined to be 5 to 13%. A more preferable Cr content is 6 to 10%, and a still more preferable Cr content is 6 to 8%.

  (3) Co (cobalt)

  Co dissolves in the Fe matrix and causes solid solution strengthening of the matrix. These effects are exhibited when the Co content is 0.1% or more. Further, when the Co content is 12% or less, an increase in material cost and a decrease in yield strength can be suppressed. Therefore, the Co content is determined to be 0.1 to 12%. A more preferable Co content is 0.1 to 6%, and a more preferable Co content is 0.1 to 4%.

  (4) Nb (niobium)

Nb forms a solid solution in the Fe matrix and causes solid solution strengthening of the matrix. Nb forms a γ ′ phase and stabilizes it. These effects are exhibited when the Nb content is 0.1% or more. Further, when the Nb content is 5% or less, an increase in material cost and precipitation of a δ (Ni ₃ (Nb, Ta)) phase (intermetallic compound) can be suppressed. Therefore, the Nb content is set to 0.1 to 5% or less. A more preferable Nb content is 0.1 to 3%, and a more preferable Nb content is 0.1 to 2%.

  (5) V (Vanadium)

  V is solid-solved in the Fe matrix and causes solid solution strengthening of the matrix. These effects are exhibited when the V content is 0.1% or more. Further, when the V content is 0.5% or less, a stable austenite structure is obtained, and precipitation of the σ phase is suppressed. Therefore, the V content is determined to be 0.1 to 0.5%. A more preferable V content is 0.1 to 0.4%, and a more preferable V content is 0.1 to 0.3%.

  (6) Ti (titanium)

  Ti forms a γ 'phase and increases the strength. When the Ti content is 1.90% or more, the precipitation of the γ ′ phase can be promoted. In addition, when the Ti content is 2.35% or less, a stable austenite structure can be obtained, an increase in the linear expansion coefficient is suppressed, and a pinning effect is exhibited by forming an appropriate amount of carbide and nitride, thereby achieving a fine grain Resulting in improved ductility. Therefore, the Ti content is determined to be 1.90 to 2.35%.

  (7) Al (aluminum)

  Al forms a γ 'phase to increase the strength. When the Al content is 0.30% or less, the γ 'phase is sufficiently precipitated and exhibits a pinning effect, thereby improving the strength and ductility associated with fine graining. Therefore, the Al content is determined to be 0.01 to 0.30%. A more preferable Al content is 0.01 to 0.20%, and a more preferable Al content is 0.01 to 0.10%.

  (8) B (boron)

  B dissolves in the Fe matrix, and particularly segregates at the grain boundaries, thereby strengthening the grain boundaries. Further, when B contains a large amount of Ti, it has an effect of suppressing precipitation of the η phase. These effects are exhibited when the B content is 0.001% or more. Further, when the B content is 0.01% or less, a decrease in the melting point of the parent phase is suppressed, and the strength at high temperature is improved. Therefore, the B content is determined to be 0.001 to 0.01%. A more preferable content of B is 0.004 to 0.006%.

  (8) C (carbon)

  C forms a carbide with Cr, brings about fine graining due to the pinning effect, and forms a solid solution in the parent phase, resulting in solid solution strengthening of the mother phase. When the C content is less than 0.001%, the above-described effects are not sufficiently exhibited. On the other hand, when the C content exceeds 0.1%, the austenite structure is destabilized and the carbide is excessively coarsened to lower the high temperature strength. Therefore, the C content is determined to be 0.001 to 0.1%. A more preferable C content is 0.01 to 0.08%, and a more preferable C content is 0.01 to 0.05%.

  (9) Zr (zirconium)

  Zr forms an oxide with O, brings about fine graining due to the pinning effect, dissolves in the parent phase, and strengthens the solid solution of the mother phase. When the Zr content is less than 0.1%, the above-described effects are not sufficiently exhibited. On the other hand, if the content of Zr exceeds 2%, the austenite structure is destabilized, and oxides are precipitated too much to lower the high temperature strength. Therefore, the Zr content is determined to be 0.1 to 2.0%. A more preferable Zr content is 0.5 to 2.0%, and a more preferable Zr content is 1.0 to 2.0%.

  (10) P (phosphorus)

P forms an oxide with O, brings about fine graining due to the pinning effect, and improves the tensile strength and ductility. When the P content is less than 0.01%, the above-described effects are not sufficiently exhibited. On the other hand, when the Zr content exceeds 0.2%, the austenite structure is destabilized and the oxide is excessively precipitated, resulting in material embrittlement. Therefore, the P content is determined to be 0.01 to 0.2%. A more preferable content of P is 0.05 to 0.2%, and a more preferable content of Zr is 0.1 to 0.2%.
(9) N (nitrogen), Si (silicon), Mn (manganese) and S (sulfur)

  N, Si, Mn and S are classified as inevitable impurities in the austenitic heat-resistant steel of the embodiment. These inevitable impurities are preferably made to have a residual content as close to 0% as possible.

  Next, an austenitic heat resistant steel according to an embodiment and a method for manufacturing a turbine component manufactured using the austenitic heat resistant steel will be described.

  The austenitic heat-resistant steel of the embodiment is manufactured as follows, for example. First, the composition components constituting the austenitic heat resistant steel are, for example, vacuum induction melted (VIM), and the molten metal is poured into a predetermined mold to form an ingot. Then, the ingot is subjected to a solution treatment (solution heat treatment) and an aging treatment to produce an austenitic heat resistant steel.

  A turbine casing that is a turbine component is manufactured, for example, as follows. First, the composition components constituting the austenitic heat-resisting steel are, for example, vacuum induction melted (VIM), the molten metal is poured into a mold for forming the shape of a turbine casing, and cast into the atmosphere to produce a structure. . And a solution casing process and an aging process are given to a structure, and a turbine casing is produced.

  In addition, the composition component which comprises an austenitic heat-resisting steel is good also as, for example, carrying out electric furnace melting | dissolving (EF), argon-oxygen decarburization (AOD), and making a molten metal.

  For example, the rotor blade, the stationary blade, the turbine rotor, the screwing member, and the valve, which are turbine parts, are manufactured as follows. First, for example, vacuum induction melting (VIM) and electroslag remelting (ESR) are performed on the composition components constituting the austenitic heat-resistant steel of the embodiment, and the ingot is cast into a predetermined mold in a reduced-pressure atmosphere. And this ingot is arrange | positioned to the type | mold corresponding to the shape of the said turbine components, and forge processes, such as rolling, are given. Then, a moving blade, a stationary blade, a turbine rotor, and a screwing member are produced by performing solution treatment, aging treatment, and the like.

  In addition, the composition component which comprises austenitic heat-resisting steel is good also as a molten metal, for example by carrying out vacuum induction melting (VIM) and vacuum arc remelting (VAR). Moreover, the composition component which comprises austenitic heat-resisting steel is good also as a molten metal by carrying out vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR), for example.

  Piping which is a turbine part is manufactured as follows, for example. First, the components constituting the austenitic heat-resisting steel are subjected to vacuum induction melting (VIM) to form a molten metal, or electric furnace melting (EF) to perform argon-oxygen decarburization (AOD) to obtain a molten metal, and a cylindrical mold The molten metal is poured while rotating at a high speed. Subsequently, the molten metal is pressurized using the centrifugal force of rotation to produce a pipe-shaped structure (centrifugal casting). And a solution treatment and an aging treatment are given to a structure, and piping is produced.

  In addition, the method for producing the turbine component is not limited to the above-described method.

  Next, solution treatment and aging treatment will be described.

  The solution treatment is performed for the purpose of removal of processing strain, grain size adjustment, and γ single phase formation. In the solution treatment, the treatment member is maintained at a temperature of 885 to 995 ° C. for a predetermined time, and then rapidly cooled to room temperature. The effect described above can be obtained at a temperature of 885 ° C. or higher. Further, when the temperature is 995 ° C. or lower, excessive coarsening of crystal grains is suppressed. The rapid cooling is performed by, for example, water cooling or forced air cooling.

  The aging treatment is performed for precipitating a γ 'phase in crystal grains and imparting high temperature strength. In the aging treatment, the treatment member is maintained at a temperature of 700 to 760 ° C. for a predetermined time, and then cooled to room temperature. When the temperature is 700 ° C. or higher, the γ ′ phase is sufficiently precipitated. Further, when the temperature is 760 ° C. or lower, the decrease in the precipitation density due to the early coarsening of the γ ′ phase is suppressed. The cooling is performed by, for example, natural cooling in the atmosphere.

  Here, it will be described that in the austenitic heat resistant steel of the embodiment, excellent tensile strength and ductility can be obtained while having the high temperature creep strength of the conventional austenitic heat resistant steel.

Table 1 shows the chemical compositions of Sample 1 to Sample 12 used for the evaluation. Sample 1 to sample 9 are austenitic heat resistant steels in the chemical composition range of the present embodiment, and samples 10 to 12 are austenitic heat resistant steels whose chemical compositions are not in the chemical composition range of the present embodiment. It is steel and is a comparative example.

  A creep rupture test and a tensile test were performed on the austenitic heat resistant steels of Sample 1 to Sample 12.

  The test piece used for each test was produced as follows.

  Raw materials necessary for obtaining the compositional components constituting the austenitic heat-resisting steels of Sample 1 to Sample 12 having the chemical compositions shown in Table 1 were melted in a vacuum induction melting furnace to produce 2 kg ingots. The obtained ingot was subjected to a solution treatment. In the solution treatment, heating was performed at a temperature of 940 ° C. for 30 minutes, and then rapidly cooled to room temperature by forced air cooling. Subsequently, an aging treatment was performed on the ingot. In the aging treatment, heating was performed at a temperature of 760 ° C. for 16 hours, and then cooled to room temperature by natural cooling in the atmosphere.

  The creep rupture test was performed based on JIS Z 2271 with respect to the test piece by each sample. Moreover, the tensile test was implemented based on JISZ2201 with respect to the test piece by each sample.

  The creep rupture strength was determined as 700 ° C./100,000 hours creep rupture strength. The creep rupture strength at 700 ° C./100,000 hours is calculated by the Larson-Miller method based on the test results of a test time of 700 to 800 ° C. and a test stress of 200 to 400 MPa with a rupture time of about 1000 hours. It was calculated by inserting.

Table 2 shows the results of the creep rupture test and the tensile test.

  As shown in Table 2, the creep rupture strength was the same for Sample 1 to Sample 12, and sufficient characteristics were obtained. However, the tensile strength of samples 10 to 12 which are conventional austenitic heat resistant steels is 700 MPa or less, and the elongation is 20% or less. On the other hand, in the samples 1 to 9, the tensile strength is 700 MPa or more. Further, the elongation of Sample 1 to Sample 9 is 20% or more.

  From the above results, in samples 1 to 9, the tensile strength and ductility are improved while maintaining the creep strength of the conventional austenitic heat resistant steel.

  According to the embodiment described above, it is possible to reduce the linear expansion coefficient while maintaining high temperature strength and to secure sufficient weldability.

  Although several embodiments of the present invention have been described, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

10 turbines, 11 casings, 12 turbine rotors, 13 turbine disks, 14 blades, 15 stator blades, 16 welds.

Claims (8)

  By mass, Ni: 24-50%, Cr: 5-13%, Co: 0.1-12%, Nb: 0.1-5%, V: 0.1-0.5%, Ti: 1. 90-2.35%, Al: 0.01-0.30%, B: 0.001-0.01%, C: 0.001-0.1%, Zr: 0.1-2.0% , P: Austenitic heat-resisting steel containing 0.01 to 0.2%, the balance being Fe and inevitable impurities.   The austenitic heat-resistant steel according to claim 1, containing 0.5 to 2.0% of Zr by mass.   The austenitic heat resistant steel according to claim 1, containing 1 to 2.0% of Zr by mass.   The austenitic heat-resistant steel according to claim 1, containing 0.05 to 0.2% of P by mass.   The austenitic heat-resistant steel according to claim 1, containing 0.1 to 0.2% of P by mass.   The austenitic heat resistant steel according to any one of claims 1 to 5, having a tensile strength and ductility at room temperature of 700 MPa or more and 20% or more.   A turbine component in which at least a predetermined portion is formed using the austenitic heat-resistant steel according to any one of claims 1 to 5.   A turbine component welded using the austenitic heat-resistant steel according to any one of claims 1 to 5.

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