JP2015086432A - Austenitic heat resistant steel and turbine component - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an austenitic heat resistant steel and a turbine component which can reduce coefficients of linear expansion while maintaining high temperature strength.SOLUTION: An austenitic heat resistant steel comprises, by mass %, Ni: 24-40%, Cr: 5-13%, Co: 0.1-12%, Nb: 0.1-5%, V: 1-14%, Ti: 1.9-2.35%, Al: 0.35-2%, B: 0.001-0.01%, and C: 0.001-0.1%, with the balance being Fe and inevitable impurities.

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. High efficiency is also required for a CO ₂ turbine that can be installed in a thermal power plant.

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 and the CO ₂ 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, austenitic heat-resistant steel has a higher service temperature of about 50 ° C. than ferritic heat-resistant steel, and has a material cost of about 1/3 that of Ni-based alloys. Therefore, by using austenitic heat-resistant steel, manufacturing costs can be reduced and higher efficiency can be achieved.

  On the other hand, austenitic heat-resisting steel has the property of having a high linear expansion coefficient. This property is a disadvantage in application to high-temperature equipment. In known austenitic heat resistant steels such as Alloy 286, the yield strength is improved by using an intermetallic compound as a precipitation strengthening phase. However, no heat-resisting steel has been proposed in which the linear expansion coefficient is reduced while maintaining high-temperature strength.

JP 2011-195880 A

  In designing a high-temperature structural material, the thermal expansion property of the material is an important factor. However, since the linear expansion coefficient of the conventional austenitic heat resistant steel is about 1.5 times the linear expansion coefficient of the ferritic heat resistant steel, a problem of thermal expansion difference occurs.

  The problem to be solved by the present invention is to provide an austenitic heat-resistant steel and a turbine component that can reduce the linear expansion coefficient while maintaining high-temperature strength.

  The austenitic heat resistant steel of the embodiment is, by mass, Ni: 24 to 40%, Cr: 5 to 13%, Co: 0.1 to 12%, Nb: 0.1 to 5%, V: 1 to 14% Ti: 1.9-2.35%, Al: 0.35-2%, B: 0.001-0.01%, C: 0.001-0.1%, the balance being Fe and Consists of inevitable impurities.

  Embodiments of the present invention will be described below.

  The present inventors have found that when V is dissolved in the matrix of an austenitic heat resistant steel, there is an effect of reducing the linear expansion coefficient. However, even if V is added alone, it does not dissolve in the matrix phase, but precipitates as a Cr-V brittle phase, and it has been found that the high temperature strength of the material is lowered. Therefore, the present inventors added an appropriate amount of C together with an appropriate amount of V to precipitate a Cr—C-based carbide that does not affect the high temperature strength and sufficiently dissolve V in the matrix, thereby increasing the linear expansion coefficient. It was found that an austenitic heat-resistant steel having a high temperature strength equivalent to that of the conventional steel can be obtained.

  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 Ni: 24-40%, Cr: 5-13%, Co: 0.1-12%, Nb: 0.1-5%, V: 1-14%, Ti Contains: 1.9-2.35%, Al: 0.35-2%, B: 0.001-0.01%, C: 0.001-0.1%, the balance being Fe and inevitable Consists of impurities.

  Here, examples of inevitable impurities in the austenitic heat-resistant steel according to the embodiment include Si, Mn, P, and S.

The austenitic heat-resistant steel of the embodiment preferably has an average linear expansion coefficient of 18 × 10 ⁻⁶ / K or less at a temperature from room temperature to 700 ° C. Here, the average linear expansion coefficient is the following equation (1) using the length (L ₀ ) at room temperature (T ₀ ) and the length (L) at a predetermined temperature (T) in the same test piece. Sought by.
Average linear expansion coefficient = (L−L ₀ ) / (T−T ₀ ) / L ₀ Formula (1)

The above-mentioned average linear expansion coefficient at a temperature from room temperature to 700 ° C. is calculated using the length (L ₀ ) at room temperature (T ₀ ) and the length (L) at temperature (T = 700 ° C.). It is obtained by (1).

Here, when an austenitic heat-resistant steel having a high linear expansion coefficient is used, for example, in a power plant, the life and performance of the power plant are hindered. Specifically, for example, when such austenitic heat-resistant steel is used for a turbine component, thermal fatigue due to expansion and contraction at the time of start-up of the power plant occurs excessively, and the turbine component breaks early. . Therefore, in order to avoid such a problem, it is preferable that the average linear expansion coefficient of the austenitic heat-resistant steel at a temperature from room temperature to 700 ° C. is 18 × 10 ⁻⁶ / K or less.

  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, and piping. 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 lower coefficient of linear expansion than the conventional austenitic heat resistant steel and has a high temperature strength equivalent to that of 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.

  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 causes solid solution strengthening and a decrease in the linear expansion coefficient of the matrix. Ni also stabilizes the austenite structure. These effects are exhibited when the Ni content is 24% or more. Further, when the Ni content is 40% 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 40%. A more preferable content of Ni is 28 to 38%, and a more preferable content of Ni is 32 to 36%.

(2) Cr (chromium)
Cr dissolves in the Fe matrix and causes solid solution strengthening and a reduction in linear expansion coefficient 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%.

(3) Co (cobalt)
Co dissolves in the Fe matrix and causes solid solution strengthening and a reduction in linear expansion coefficient 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 2 to 6%.

(4) Nb (niobium)
Nb forms a solid solution in the Fe matrix and causes solid solution strengthening and a reduction in the linear expansion coefficient 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%.

(5) V (Vanadium)
V forms a solid solution in the Fe matrix and causes solid solution strengthening and a reduction in the linear expansion coefficient of the matrix. These effects are exhibited when the V content is 1% or more. Further, when the V content is 14% or less, a stable austenite structure is obtained, and precipitation of the σ phase is suppressed. Therefore, the V content is determined to be 1 to 14%. A more preferable V content is 2 to 10%, and a more preferable V content is 2 to 6%.

(6) Ti (titanium)
Ti is an element that forms a γ 'phase while solid-solution strengthening the mother phase by dissolving in the Fe mother phase. When the Ti content is 1.9% or more, solid solution strengthening of the parent phase and precipitation of the γ ′ phase can be promoted. Further, when the Ti content is 2.35% or less, a stable austenite structure is obtained and precipitation of the η phase is suppressed. Therefore, the Ti content is determined to be 1.9 to 2.35%. A more preferable Ti content is 2.0 to 2.2%.

(7) Al (aluminum)
Al is an element that forms a γ ′ phase while solid-solution strengthening the mother phase by dissolving in the Fe mother phase. When the Al content is 0.35% or more, solid solution strengthening of the parent phase and precipitation of the γ ′ phase can be promoted. In addition, when the Al content is 2% or less, a stable austenite structure is obtained, and excessive precipitation of the γ ′ phase is suppressed, and deterioration of workability and weldability is suppressed. Therefore, the Al content is determined to be 0.35 to 2%. A more preferable Al content is 0.5 to 1.5%.

(8) C (carbon)
C forms carbides with Cr or forms a solid solution in the matrix, and causes solid solution strengthening of the matrix. This effect is exhibited when the C content is 0.001% or more. In addition, when the C content is 0.1% or less, a stable austenite structure is obtained, and the coarsening of the carbide is suppressed to suppress a decrease in high-temperature strength. Therefore, the C content is determined to be 0.001 to 0.1%. The preferable C content is 0.01 to 0.1%, the more preferable C content is 0.04 to 0.08%, and the still more preferable C content is 0.06 to 0.08%. It is.

(9) 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 a decrease in hot workability is suppressed. 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%.

(10) Si (silicon), Mn (manganese), P (phosphorus) and S (sulfur)
Si, Mn, P 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, and the screwing member, 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 to electric furnace melting (EF) to perform argon-oxygen decarburization (AOD) to form a molten metal. 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.

(Evaluation of linear expansion coefficient and high temperature strength)
Here, in the austenitic heat resistant steel of the embodiment, it will be explained that the linear expansion coefficient is low while maintaining the high temperature strength of the conventional austenitic heat resistant steel.

  Table 1 shows the chemical compositions of Sample 1 to Sample 14 used for the evaluation. Samples 1 to 10 are austenitic heat resistant steels in the chemical composition range of the present embodiment, and samples 11 to 14 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.

  About the austenitic heat-resisting steel of sample 1 to sample 14, the measurement of the average linear expansion coefficient and the creep rupture test were performed.

  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 14 having the chemical compositions shown in Table 1 were melted in a vacuum induction melting furnace to produce 2 kg ingots. This ingot was formed into a plate-like member by hot rolling. The obtained plate-like member 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, the plate member was subjected to an aging treatment. 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 test piece for measuring the linear expansion coefficient and the test piece for the creep rupture test were taken from the plate member so that the stress axis was parallel to the forging direction.

  The average linear expansion coefficient was measured in accordance with JIS Z 2285 for the test piece of each sample. The creep rupture test was performed based on JIS Z 2271 with respect to the test piece by each sample. The creep rupture strength at 700 ° C./100,000 hours is extrapolated by the Larson-Miller method based on the test result of about 1000 hours at a test temperature of 700 to 800 ° C. and a test stress of 200 to 400 MPa. It was sought after. Table 2 shows the test results of average linear expansion coefficient and 700 ° C./100,000 hours creep rupture strength.

As shown in Table 2, the average linear expansion coefficient in Samples 11 to 14 which are conventional austenitic heat resistant steels is 18 × 10 ⁻⁶ / K or more. On the other hand, in sample 1 to sample 10, the creep rupture strength is comparable to that of sample 11 to sample 14, and the average linear expansion coefficient is 18 × 10 ⁻⁶ / K or less. That is, in the samples 1 to 10, the linear expansion coefficient is reduced while maintaining the high temperature 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.

  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.

Claims (5)

  By mass, Ni: 24-40%, Cr: 5-13%, Co: 0.1-12%, Nb: 0.1-5%, V: 1-14%, Ti: 1.9-2. 35%, Al: 0.35 to 2%, B: 0.001 to 0.01%, C: 0.001 to 0.1%, the balance being Fe and inevitable impurities Austenitic heat-resistant steel.   2. The austenitic heat-resistant steel according to claim 1, which contains 2 to 10% of V by mass.   The austenitic heat-resistant steel according to claim 1, which contains 0.01 to 0.1% of C by mass. The austenitic heat-resistant steel according to any one of claims 1 to 3, wherein an average linear expansion coefficient at a temperature from room temperature to 700 ° C is 18 × 10 ^-6 / K or less.   A turbine part, wherein at least a predetermined portion is produced using the austenitic heat-resistant steel according to any one of claims 1 to 4.