JP5932622B2 - Austenitic heat resistant steel and turbine parts - Google Patents

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. Here, the CO ₂ turbine drives the turbine using CO ₂ generated by combustion of natural gas and oxygen as a working fluid. In the CO ₂ turbine, a system in which most of the generated CO ₂ is circulated to the combustor is adopted, and CO ₂ emission is reduced. Therefore, the CO ₂ turbine is attracting attention from the viewpoint of protecting the global environment.

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, 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.

  Known austenitic heat resistant steels include SUS316 and Alloy286. Recently, a high-strength austenitic heat-resistant steel using an intermetallic compound or the like as a new precipitation strengthening phase has also been proposed.

Japanese Patent Laid-Open No. 11-195880

  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. It is also known that conventional austenitic heat resistant steels have lower yield strength and tensile strength than ferritic heat resistant steels.

  The problem to be solved by the present invention is to provide an austenitic heat-resisting steel and a turbine component capable of reducing the linear expansion coefficient and improving the tensile properties.

  The austenitic heat resistant steel of the embodiment is Ni: 24-27% by mass, Cr: 16-25% by mass, V: 0.1-0.5% by mass, Ti: 1.9-2.35% by mass, Al : 0.35 to 2% by mass, B: 0.001 to 0.01% by mass, with the balance being Fe and inevitable impurities.

  Embodiments of the present invention will be described below.

In an austenitic heat-resistant steel, if Cr, Mo, W is added excessively, σ ((Ni, Cr) (Mo, Co)) phase (intermetallic compound), η (Ni ₃ Ti) phase (intermetallic compound), etc. Precipitates and the creep strength and ductility decrease. However, the present inventors have found that by adding an appropriate amount of these elements, these elements are dissolved in the austenite matrix. As a result, it has been clarified that the solid phase strengthening improves the strength of the matrix and reduces the linear expansion coefficient.

Al and Ti are elements that form a γ ′ (Ni ₃ (Al, Ti)) phase. Since this γ ′ phase is coherently precipitated with respect to the parent phase, precipitation strengthening can be achieved by addition of Al and Ti. Further, the linear expansion coefficient in the γ ′ phase is small. Therefore, the present inventors have found that the linear expansion coefficient of the austenitic heat resistant steel can be reduced by increasing the amount of precipitation of the γ ′ phase.

  From the above viewpoints, the present inventors have found a chemical composition of an austenitic heat-resistant steel that can reduce the linear expansion coefficient and improve tensile properties.

  Hereinafter, embodiments of the present invention will be specifically described. In the following description, “%” representing a composition component is “% by mass” unless otherwise specified.

  The austenitic heat resistant steel (M1) of the embodiment is Ni: 24-27%, Cr: 16-25%, V: 0.1-0.5%, Ti: 1.9-2.35%, Al : 0.35 to 2%, B: 0.001 to 0.01%, with the balance being Fe and inevitable impurities.

  The austenitic heat resistant steel (M1) of the above-described embodiment may further contain 10% or less (including the case where either Mo or W is 0) in total of Mo and W.

  The austenitic heat resistant steel (M1) of the above-described embodiment may further contain 5% or less (including the case where either Ta or Nb is 0) in total of Ta and Nb.

  In the austenitic heat-resisting steel (M1) of the above-described embodiment, Mo and W are summed up to 10% or less (including the case where either Mo or W is 0), and Ta and Nb are summed up. It may further contain 5% or less (including the case where either Ta or Nb is 0).

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

Moreover, it is preferable that the austenitic heat resistant steel of the present invention 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 ) / L ₀ (T−T ₀ ) (1)

The average linear expansion coefficient at the temperature from room temperature to 700 ° C. is calculated by using the length (L ₀ ) at room temperature (T ₀ ) and the length (L) at temperature (T = 700 ° C.), 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 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 is destroyed 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 present invention is suitable as a material constituting a turbine component having a temperature during operation of 650 ° C. or higher, and 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.

  Here, you may comprise all the site | parts of an above-described turbine component with the austenitic heat-resisting steel of this 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 this Embodiment.

  The austenitic heat resistant steel of the present embodiment described above has a lower coefficient of linear expansion and excellent tensile properties than conventional austenitic heat resistant steel. Therefore, a turbine component manufactured using the austenitic heat resistant steel of the present embodiment also has the same characteristics as the austenitic heat resistant steel and has high reliability.

The austenitic heat-resistant steel and turbine parts described above 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. When the Ni content is less than 24%, the above-described effects are not sufficiently exhibited. On the other hand, if the Ni content exceeds 27%, the material cost increases and the workability decreases. Therefore, the Ni content is determined to be 24-27%. A more preferable Ni content is 25 to 27%.

(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. When the Cr content is less than 16%, the above-described effects are not sufficiently exhibited. On the other hand, if the Cr content exceeds 25%, the austenite structure becomes unstable, and the σ phase tends to precipitate. Therefore, the Cr content is determined to be 16 to 25%. A more preferable Cr content is 20 to 25%.

(3) 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. When the V content is less than 0.1%, the above-described effects are not sufficiently exhibited. On the other hand, if the V content exceeds 0.5%, the austenite structure is destabilized and the σ phase tends to precipitate. Therefore, the V content is determined to be 0.1 to 0.5%. A more preferable V content is 0.2 to 0.4%.

(4) 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 less than 1.9%, solid solution strengthening of the mother phase and precipitation of the γ ′ phase cannot be promoted. On the other hand, if the Ti content exceeds 2.35%, the austenite structure becomes unstable and the η phase tends to precipitate. Therefore, the Ti content is determined to be 1.9 to 2.35%. A more preferable Ti content is 2.0 to 2.2%.

(5) 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. If the Al content is less than 0.35%, solid solution strengthening of the mother phase and precipitation of the γ ′ phase cannot be promoted. On the other hand, if the Al content exceeds 2%, the austenite structure becomes unstable, and workability and weldability deteriorate due to excessive precipitation of the γ 'phase. Therefore, the Al content is determined to be 0.35 to 2%. A more preferable Al content is 0.5 to 1.5%.

(6) 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. When the B content is less than 0.001%, the above-described effects are not sufficiently exhibited. On the other hand, if the B content exceeds 0.01%, the melting point of the parent phase is lowered and the hot workability is lowered. 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%.

(7) Mo (molybdenum) and W (tungsten)
Mo and W are solid-dissolved in the Fe parent phase, resulting in solid solution strengthening of the parent phase and a decrease in linear expansion coefficient. When the total content of Mo and W (Mo + W) exceeds 10%, the material cost increases and the hot workability decreases. Therefore, the content ratio of (Mo + W) is set to 10% or less. Moreover, in order to fully exhibit the above-described effect, the lower limit of the content ratio of (Mo + W) is preferably 1.5% or more. A more preferable content ratio of (Mo + W) is 6 to 8%. Note that the content of either Mo or W may be zero.

(8) Ta (tantalum) and Nb (niobium)
Ta and Nb are dissolved in the Fe matrix, resulting in solid solution strengthening and a decrease in the linear expansion coefficient of the matrix. Ta and Nb form a γ ′ phase and stabilize it. When the total content of Ta and Nb (Ta + Nb) exceeds 5%, the material cost increases and the δ (Ni ₃ (Nb, Ta)) phase (intermetallic compound) is likely to precipitate. Therefore, the content ratio of (Ta + Nb) is set to 5% or less. Moreover, in order to fully demonstrate the above-mentioned effect, it is preferable that the lower limit of the content rate of (Ta + Nb) is 0.5% or more. A more preferable content ratio of (Ta + Nb) is 2 to 4%. Note that the content of either Ta or Nb may be zero.

(9) 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.

  Here, the austenitic heat resistant steel of the 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. The ingot is subjected to a solution heat treatment (solution 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. . Then, the structure is subjected to a solution heat treatment and an aging treatment to produce a turbine casing.

  In addition, for example, it is good also as a molten metal the composition component which comprises an electric furnace melt | dissolution (EF), argon-oxygen decarburization (AOD), and comprises an austenitic heat-resistant steel.

  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 a solution heat treatment, an aging treatment, and the like.

  In addition, for example, the composition component constituting the austenitic heat-resistant steel may be formed as a molten metal by vacuum induction melting (VIM) and vacuum arc remelting (VAR). Further, for example, the composition component constituting the austenitic heat-resisting steel may be formed as a molten metal by vacuum induction melting (VIM), electroslag remelting (ESR), and vacuum arc remelting (VAR).

  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). Then, the structure is subjected to a solution heat treatment and an aging treatment to produce a pipe.

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

  Next, the solution heat treatment and the aging treatment will be described.

  The solution heat treatment is performed for the purpose of removal of processing strain, grain size adjustment, and γ single phase formation. In the solution heat treatment, the processing member is maintained at a temperature of 885 to 995 ° C. for a predetermined time, and then rapidly cooled to room temperature. When the temperature is lower than 885 ° C., the above effects cannot be obtained sufficiently. On the other hand, when the temperature exceeds 995 ° C., excessive coarsening of crystal grains occurs. The rapid cooling is performed by, for example, water cooling or forced air cooling.

  The aging treatment is performed to precipitate a γ 'phase in the crystal grains, impart high temperature strength, and lower the linear expansion coefficient. 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 lower than 700 ° C., the γ ′ phase is not sufficiently precipitated. On the other hand, in the case of a temperature exceeding 760 ° C., the γ ′ phase is coarsened early and the precipitation density is reduced. The cooling is performed by, for example, natural cooling in the atmosphere.

(Evaluation of linear expansion coefficient and tensile properties)
Here, it will be described that the austenitic heat-resistant steel of the embodiment has a low coefficient of linear expansion and excellent tensile properties.

  Table 1 shows the chemical compositions of Sample 1 to Sample 17 used for the evaluation. Samples 1 to 12 are austenitic heat resistant steels in the chemical composition range of the present embodiment, and Samples 13 to 17 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.

  Samples 1 to 17 were subjected to a coefficient of linear expansion, a tensile test, and a creep rupture test for the austenitic heat resistant steels.

  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 17 having the chemical compositions shown in Table 1 were melted in a vacuum induction melting furnace to produce 2 kg ingots. This ingot was melted by an arc melting furnace and formed into a plate member by hot rolling. The obtained plate-like member was subjected to a solution heat treatment. In the solution heat 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, the test piece for the tensile test, 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 linear expansion coefficient was measured in accordance with JIS Z 2285 for the test piece of each sample. In the tensile test, 0.2% proof stress was measured based on JIS Z 2201 under the conditions of room temperature for the test pieces 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 linear expansion coefficient, 0.2% proof stress, and 700 ° C / 100,000 hours creep rupture strength.

  Moreover, in order to investigate the presence or absence of the intermetallic compound in the austenitic heat resistant steel, the structure of the test piece after the creep rupture test was observed. In the tissue observation, an optical microscope and a scanning electron microscope were used.

As shown in Table 2, Samples 1 to 12 have a linear expansion coefficient of 18 × 10 ⁻⁶ / K or less, and high values are obtained for both 0.2% proof stress and creep rupture strength. That is, it can be seen that Samples 1 to 12 have a low coefficient of linear expansion and excellent tensile properties.

On the other hand, some of Samples 13 to 17 have a linear expansion coefficient of 18 × 10 ⁻⁶ / K or less, but both 0.2% proof stress and creep rupture strength are lower than those of Sample 1 to Sample 12. . That is, Sample 13 to Sample 17 had a low coefficient of linear expansion and excellent results in tensile properties.

  As a result of the structure observation, in Samples 1 to 12, no intermetallic compounds such as σ phase, η phase, and δ phase were observed, and a healthy structure was maintained. On the other hand, in Samples 13 to 17, in those containing excessive amounts of W, Mo, etc. in Comparative Examples 1 to 7, precipitation of σ phase, η phase, δ phase, etc. was observed, and the structure was unstable. I found out.

  According to the embodiment described above, it is possible to reduce the linear expansion coefficient and improve the tensile characteristics.

  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 (6)

  Ni: 24-27 mass%, Cr: 16-25 mass%, V: 0.1-0.5 mass%, Ti: 1.9-2.35 mass%, Al: 0.35-2 mass%, B: An austenitic heat-resisting steel containing 0.001 to 0.01% by mass, the balance being Fe and inevitable impurities.   The austenitic heat-resisting steel according to claim 1, containing 20 to 25% by mass of Cr.   The austenitic heat-resistant steel according to claim 1 or 2, further comprising 10% by mass or less in total of Mo and W.   The austenitic heat resistant steel according to any one of claims 1 to 3, further comprising 5% by mass or less of Ta and Nb in total. The austenitic heat-resistant steel according to any one of claims 1 to 4, 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 part is produced using the austenitic heat-resistant steel according to any one of claims 1 to 5.