JP2015092010A - Precipitation strengthening type ferritic heat resistant steel, turbine high temeperature member using heat resistant steel, and turbin using turbine high temeperature member - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide precipitation strengthening type ferritic heat resistant steel having balance of mechanical strength and oxidation resistance at high temperature at a higher level than conventional one, a turbine high temperature member using the heat resistant steel and a turbine using the turbine high temperature member.SOLUTION: The precipitation strengthening type ferritic heat resistant steel contains 0.1 mass% or less of C, 12 mass% to 20 mass% of Cr, 2 mass% to 6 mass% of Ni, 2 mass% to 8 mass% of Al, 2.5 mass% to 10 mass% of W, 0.5 mass% to 2.5 mass% of Mo, 2 mass% to 6 mass% of Co, 0.001 mass% to 0.01 mass% of B and the balance Fe with inevitable impurities.

Description

  The present invention relates to a heat resistant steel having excellent high temperature strength and oxidation resistance, and more particularly to a precipitation strengthened ferritic heat resistant steel, a turbine high temperature member using the heat resistant steel, and a turbine using the turbine high temperature member. is there.

In recent years, from the viewpoint of energy saving (for example, saving of fossil fuel) and protection of the global environment (for example, suppression of CO ₂ gas generation amount), improvement in efficiency of a thermal power plant (for example, improvement in efficiency in a steam turbine) has been desired. One effective means for improving the efficiency of the steam turbine is to increase the main steam temperature. For example, a significant improvement in thermal efficiency can be expected by increasing the main steam temperature from 600 ° C to 650 ° C. Further, even when the steam temperature does not increase, it is possible to expect an improvement in thermal efficiency by reducing a part of the amount of steam used for cooling the steam turbine member.

  Currently, various heat-resistant steels (austenitic, martensitic, and ferritic) are used for steam turbine members (for example, moving blades and stationary blades) of ultra-supercritical power generation (USC) plants. Here, in order to raise the main steam temperature to 650 ° C., a material having higher mechanical strength is required as a material of the steam turbine member. It is also desired to have excellent oxidation resistance from the viewpoint of long-term stability / reliability. As the mechanical strength, the creep rupture strength that greatly affects the high temperature characteristics is the most important.

  As a structural material having good mechanical strength, for example, in Patent Document 1 (Japanese Patent Publication No. 8-30251), by weight, C: 0.05 to 0.20%, Mn: 0.05 to 1.5%, Ni: 0.05 to 1.0% , Cr: 9.0 to 13.0%, Mo: 0.05 to 0.50% (not including 0.50%), W: 2.0 to 3.5%, V: 0.05 to 0.30%, Nb: 0.01 to 0.20%, Co: 2.1 to 10.0%, A ferritic heat resistant steel containing N: 0.01 to 0.1%, the balance being substantially made of Fe and inevitable impurities, and particularly limiting Si to 0.15% or less is disclosed.

  In Patent Document 2 (Japanese Patent Laid-Open No. 9-296258), by weight, C: 0.05 to 0.20%, Si: 0.15% or less, Mn: 1.5% or less, Ni: 1.0% or less, Cr: 8.5 to 13.0%, Mo: 3.50% or less, W: 3.5% or less, V: 0.05-0.30%, Nb: 0.01-0.20%, Co: 5.0% or less, B: 0.001-0.020%, N: 0.005-0.040%, O: 0.010% or less, H: A heat-resistant steel containing 0.00020% or less and having a total tempered martensite structure is disclosed.

  In Patent Document 3 (Japanese Patent Publication No. 8-30249), by weight, carbon 0.05 to 0.2%, silicon 0.1% or less, manganese 0.05 to 1.5%, chromium 8.0% to less than 13.0%, nickel 1.5% or less, vanadium 0.1 -0.3%, niobium 0.01-0.1%, nitrogen 0.01-0.1%, aluminum 0.02% or less, molybdenum less than 0.50%, tungsten 0.9-3.0%, and molybdenum and tungsten content [Mo], [W] A heat-resisting steel that is an iron-base alloy that satisfies each of the predetermined relational expressions, and basically includes a martensite matrix that contains almost no δ-ferrite phase and huge grain boundary carbides in the metal structure. It is disclosed.

  In Patent Document 4 (Japanese Patent Laid-Open No. 8-13102), by weight, C: 0.05 to 0.15%, Si: 0.5% or less, Mn: 0.05 to 0.50%, Cr: 17 to 25%, Ni: 7 to 20%, Cu: 2.0-4.5%, Nb: 0.10-0.80%, B: 0.001-0.010%, N: 0.05-0.25%, sol. Al: 0.003-0.030% and Mg: 0-0.015%, Furthermore, an austenitic heat-resistant steel containing either or both of Mo: 0.3 to 2.0% and W: 0.5 to 4.0%, the balance being Fe and inevitable impurities is disclosed.

Japanese Patent Publication No.8-30251 JP-A-9-296258 Japanese Patent Publication No.8-30249 JP-A-8-13102

  While demand for protecting the global environment is increasing worldwide, energy demand continues to increase. In order to meet these conflicting demands, further improvement in efficiency is strongly demanded for thermal power plants (especially steam turbines). As described above, increasing the main steam temperature is very effective for improving the efficiency of the steam turbine.

  A main steam temperature of 650 ° C has been a long-term goal for steam turbines, and many research and developments of heat-resistant steel have been made for practical application. Unfortunately, however, a 650 ° C. class steam turbine has not been put into practical use even today. In other words, the conventional heat-resistant steel (for example, Patent Documents 1 to 4) means that the required characteristics are not sufficiently satisfied.

  In general, austenitic heat-resistant steels have the advantage of being excellent in high-temperature strength and oxidation resistance, but have a weak point in thermal fatigue caused by temperature changes due to a relatively large thermal expansion coefficient (that is, long-term It has weak points from the viewpoint of stability / reliability). Martensitic heat resistant steel has the advantage of high dislocation density due to martensite structure and very high mechanical strength due to precipitation hardening of carbonitride, but dislocation recovery and precipitate aggregation in high temperature and long time environment. It is easy to cause coarsening and has weak points from the viewpoint of long-term stability / reliability. Ferritic heat-resistant steel, on the other hand, has the advantage of excellent long-term stability / reliability, since it has a low dislocation density in the matrix crystal grains and has little microstructural change even in high-temperature and long-time environments. It has a weak point of being relatively low.

  Therefore, an object of the present invention is to provide a precipitation strengthened ferritic heat resistant steel in which mechanical strength and oxidation resistance at high temperatures are balanced at a higher level than before, a turbine high temperature member using the heat resistant steel, and the turbine high temperature It is providing the turbine using a member.

(I) One aspect of the present invention is a ferritic heat resistant steel in which an intermetallic compound is dispersed and precipitated in order to achieve the above object,
C (carbon) of 0.1% by mass or less,
12 mass% or more and 20 mass% or less of Cr (chrome),
Ni (nickel) of 2 mass% or more and 6 mass% or less,
Al (aluminum) of 2 mass% or more and 8 mass% or less,
W (tungsten) of 2.5 mass% or more and 10 mass% or less,
0.5 mass% or more and 2.5 mass% or less of Mo (molybdenum),
2% to 6% by weight of Co (cobalt),
0.001 mass% or more and 0.01 mass% or less B (boron),
There is provided a precipitation strengthened ferritic heat resistant steel characterized in that the balance is Fe (iron) and inevitable impurities.

  (II) Another aspect of the present invention provides a high-temperature turbine member using the precipitation-strengthened ferritic heat resistant steel described above in order to achieve the above object.

  (III) In order to achieve the above object, another aspect of the present invention provides a turbine rotor characterized in that the turbine high temperature member is a turbine rotor shaft, and the turbine rotor shaft is used.

  (IV) According to another aspect of the present invention, there is provided a steam turbine using the above-described turbine rotor in order to achieve the above object.

  (V) Another aspect of the present invention provides a thermal power plant using the steam turbine described above in order to achieve the above object.

  According to the present invention, precipitation-strengthened ferritic heat resistant steel in which mechanical strength and oxidation resistance at high temperatures are balanced at a higher level than before, a turbine high temperature member using the heat resistant steel, and the turbine high temperature member The used turbine can be provided. In addition, a thermal power plant using the turbine can be provided.

It is a cross-sectional schematic diagram which shows an example of the steam turbine using the member for steam turbines concerning this invention. It is a systematic schematic diagram showing an example of a thermal power plant according to the present invention.

The present invention can be modified or changed as follows in the precipitation strengthened ferritic heat resistant steel (I) according to the present invention described above.
(I) Further containing 5% by mass or less of Ti (titanium).
(Ii) It further contains at least one of Nb (niobium) and V (vanadium) in a total amount of 0.5% by mass or less.
(Iii) It further contains at least one of 1 mass% or less of Si (silicon) and 1 mass% or less of Mn (manganese).
(Iv) The inevitable impurity is at least one of P (phosphorus), S (sulfur), Sb (antimony), Sn (tin), As (arsenic), and N (nitrogen), Is 0.5 mass% or less, S is 0.5 mass% or less, Sb is 0.1 mass% or less, Sn is 0.1 mass% or less, As is 0.1 mass% or less, and N is 0.1 mass% or less.
(V) The intermetallic compound is a Laves phase and a β-NiAl phase.
(Vi) The precipitation strengthened ferritic heat resistant steel is subjected to a normalizing treatment at 1000 ° C. or higher and 1200 ° C. or lower and then a tempering treatment at 600 ° C. or higher and 800 ° C. or lower.

  Hereinafter, embodiments according to the present invention will be described with reference to the drawings. However, the present invention is not limited to the embodiments described here, and can be appropriately combined and improved without departing from the technical idea of the present invention.

(Composition of precipitation strengthened ferritic heat resistant steel)
Hereinafter, each component of the precipitation strengthening ferritic heat resistant steel according to the present invention will be described.

C component:
The C component is a component that generates carbides such as Cr and Ti and contributes to precipitation strengthening. However, if the amount of the C component exceeds 0.1% by mass, it causes a decrease in toughness due to excessive precipitation of carbides and a deterioration in corrosion resistance due to a decrease in Cr concentration near the grain boundary. Further, the excessive C component significantly promotes the austenite phase of the matrix. Therefore, the amount of component C is preferably 0.1% by mass or less. 0.05 mass% or less is more desirable, and 0.025 mass% or less is further desirable.

Cr component:
The Cr component is a component that contributes to the improvement of corrosion resistance by forming a passive film on the surface of the heat resistant steel, and is a component that contributes to the stabilization and single phase of the ferrite phase. When the Cr component amount is less than 12% by mass, their effects are insufficient. On the other hand, when the Cr component amount exceeds 20% by mass, a harmful phase (for example, σ phase: Fe—Cr intermetallic compound) is likely to be generated, which causes deterioration of mechanical properties. Therefore, the Cr content is preferably 12 to 20% by mass. 14-18 mass% is more desirable, and 15-17 mass% is still more desirable.

Ni component:
The Ni component is a component that generates a Ni—Al-based intermetallic compound (for example, β-NiAl phase) and contributes to improvement of high-temperature strength (for example, creep rupture strength) by dispersion precipitation hardening of the phase. Ni-Al intermetallic compounds are mainly dispersed and precipitated in the matrix crystal grains. It also has the effect of improving toughness. When the amount of Ni component is less than 2% by mass, the effects thereof are insufficient. On the other hand, when the amount of Ni component exceeds 6 mass%, the austenite phase is precipitated and becomes a factor that inhibits the ferrite phase from becoming a single phase. Therefore, the Ni component amount is desirably 2 to 6% by mass. 3-5 mass% is more desirable, and 3.5-4.5 mass% is still more desirable.

Al component:
The Al component is also a component that generates a Ni—Al-based intermetallic compound (for example, a β-NiAl phase) and contributes to precipitation hardening and improvement of high-temperature strength. If the amount of Al component is less than 2% by mass, their effects are insufficient. On the other hand, when the amount of Al component exceeds 8% by mass, Ni—Al-based intermetallic compounds are excessively precipitated, which causes deterioration of toughness. Therefore, the amount of Al component is desirably 2 to 8% by mass. 3-7 mass% is more desirable, and 4-6 mass% is still more desirable.

W component:
The W component is a component that contributes to improvement in mechanical strength (solid solution strengthening) by dissolving in the matrix. In addition, it is a component that contributes to the improvement of high-temperature strength by forming a Laves phase (for example, Fe ₂ W phase) by combining with iron. The Laves phase precipitates mainly along the grain boundaries of the matrix crystal. When the amount of the W component is less than 2.5% by mass, the effects thereof are insufficient. On the other hand, when the amount of W component exceeds 10% by mass, excessive formation of a Laves phase is promoted and mechanical characteristics (for example, toughness) are deteriorated. Therefore, the amount of W component is preferably 2.5 to 10% by mass. 3-9 mass% is more desirable, and 4-8 mass% is still more desirable.

Mo component:
Similar to the W component, the Mo component is a component that contributes to improvement in mechanical strength (solid solution strengthening) by solid solution in the matrix. In addition, it is a component that contributes to the improvement of high-temperature strength by combining with iron to form a Laves phase. By combined addition with the above W component, the effect of improving the high temperature strength can be further enhanced. The Laves phase in this case is considered to be an Fe ₂ (W, Mo) phase. When the amount of Mo component is less than 0.5% by mass, these effects are insufficient. On the other hand, if the amount of Mo component exceeds 2.5% by mass, excessive formation of Laves phase is promoted and mechanical characteristics (for example, toughness) are deteriorated. Therefore, the amount of Mo component is desirably 0.5 to 2.5% by mass. It is more preferably 1 to 2% by mass, and further preferably 1.25 to 1.75% by mass.

Co component:
The Co component is a component that contributes to improvement of high-temperature strength (for example, creep rupture strength) by promoting the refinement of the precipitated Ni—Al-based intermetallic compound. When the amount of Co component is less than 2% by mass, the effect is insufficient. On the other hand, when the amount of Co component exceeds 6% by mass, the austenite phase is precipitated, which becomes a factor for inhibiting the ferrite phase from becoming a single phase. Therefore, the amount of Co component is desirably 2 to 6% by mass or less. 2-5 mass% is more desirable, and 2-4 mass% is still more desirable.

B component:
The B component is a component that contributes to the improvement of the high-temperature strength by suppressing the aggregation and coarsening of the precipitated Ni—Al intermetallic compound (promoting fine dispersion). When the amount of component B is less than 0.001% by mass, the effect is insufficient. On the other hand, when the amount of component B exceeds 0.01% by mass, it becomes a factor of deteriorating mechanical properties (for example, toughness) and weldability. Therefore, the amount of component B is preferably 0.001 to 0.01% by mass. 0.002 to 0.009 mass% is more desirable, and 0.003 to 0.007 mass% is still more desirable.

The precipitation strengthened ferritic heat resistant steel of the present invention has a Ni—Al intermetallic compound (for example, β-NiAl phase) and a Laves phase (for example, Fe ₂ W phase, Fe ₂ (W, Mo)) as a precipitation strengthening phase. The greatest feature is that the high temperature strength (for example, creep rupture strength) is improved by complex precipitation of the phase) and the oxidation resistance is ensured by adjusting the amount of Cr component. As described above, the Ni—Al intermetallic compound is finely dispersed and precipitated mainly in the matrix crystal grains. The Laves phase precipitates mainly along the grain boundaries of the matrix crystal.

Ti component:
The Ti component is a component that contributes to precipitation strengthening by generating carbide and generating an intermetallic compound (Ni-Ti-based compound, for example, Ni ₃ Ti phase). In addition, Ti carbide is generated in preference to Cr carbide, and as a result, the formation of Cr carbide is suppressed and the corrosion resistance is improved. In the present invention, the Ti component is not an essential component, but it is preferable to add it because of its effect. However, when the amount of Ti component exceeds 5% by mass, excessive precipitation of intermetallic compounds causes a decrease in mechanical properties (for example, toughness). Therefore, the amount of Ti component is desirably 5% by mass or less. 4 mass% or less is more desirable, and 3 mass% or less is more desirable.

Nb component:
The Nb component is a component that precipitates as a carbide or carbonitride and contributes to an improvement in mechanical strength. In the present invention, the Nb component is not an essential component, but it is preferable to add it because of its action and effect. However, when the amount of Nb component exceeds 0.5 mass%, it becomes a factor which deteriorates manufacturing workability. Therefore, the Nb component amount is desirably 0.5% by mass or less, and more desirably 0.45% by mass or less.

V component:
The V component can be added in place of the Nb component. In that case, the total addition amount is desirably the same as that in the case of adding Nb alone. That is, it is desirable to add at least one of Nb and V in a total amount of 0.5% by mass or less, and more preferably 0.45% by mass or less. In the present invention, the V component is not an essential component, but by adding it in combination with the Nb component, there is an effect of making precipitation strengthening more remarkable.

Si component:
The Si component is a deoxidizer and is a component that functions when the heat-resistant steel is dissolved, and is effective even in a small amount. In the present invention, the Si component is not an essential component, but it is preferable to add it because of its effect. However, when the amount of Si component exceeds 1% by mass, an austenite phase is easily generated, which causes deterioration of characteristics. Therefore, the amount of Si component is desirably 1% by mass or less. 0.5 mass% or less is more desirable, and 0.25 mass% or less is further desirable. In addition, when performing a carbon vacuum deoxidation method, an electroslag remelting method, or the like in the melting process of heat-resistant steel, it is not necessary to positively add Si component (Si may not be added).

Mn component:
The Mn component is a deoxidizing agent and a desulfurizing agent, and is a component that functions when the heat-resistant steel is dissolved, and is effective even in a small amount. In the present invention, the Mn component is not an essential component, but it is preferable to add it because of its action and effect. However, if the amount of the Mn component exceeds 1% by mass, an austenite phase is easily generated, which causes deterioration of characteristics. Therefore, the amount of Mn component is desirably 1% by mass. 0.5% by mass is more desirable, and 0.25% by mass is even more desirable. In addition, when the vacuum induction melting method (VIM), the vacuum arc remelting method (VAR), or the like is performed in the heat-resistant steel melting step, it is not necessary to positively add the Mn component (Mn may not be added).

Inevitable impurities:
In the present invention, inevitable impurities refer to components that are not intentionally added. In other words, it refers to a component that was originally included in the raw material, or a component that is inevitably mixed in the manufacturing process. Inevitable impurities include, for example, P, S, Sb, Sn, As and N, and at least one of these is included in the precipitation strengthened ferritic heat resistant steel of the present invention.

  The P component and the S component are components that adversely affect hot workability and mechanical properties (for example, toughness), and are desirably reduced as much as possible. From the viewpoint of hot workability and toughness, it is desirable that the P component amount be 0.5% by mass or less and the S component amount be 0.5% by mass or less. P of 0.1% by mass or less and S of 0.1% by mass or less are more desirable.

  The Sb component, Sn component, and As component are components that adversely affect high-temperature strength (for example, creep rupture strength). Therefore, it is desirable to reduce these components as much as possible, and 0.1% by mass or less of Sb, 0.1% by mass or less of Sn, and 0.1% by mass or less of As are desirable. 0.05% by mass or less of Sb, 0.05% by mass or less of Sn, and 0.05% by mass or less of As are more preferable.

  N component has strong affinity with Al component, Ti component, and B component, and forms nitrides (for example, AlN, TiN, BN) to reduce fatigue strength and toughness. In addition, the amount of precipitation of intermetallic compounds (for example, Ni—Al compounds, Ni—Ti compounds) in the precipitation strengthening phase is reduced by the amount of nitrides generated, thereby reducing the mechanical strength. For this reason, it is desirable to reduce the N component as much as possible, and it is preferably 0.1% by mass or less. N of 0.05% by mass or less is more desirable.

(Production method)
The method for producing a precipitation strengthened ferritic heat resistant steel according to the present invention is not particularly limited except that there are desirable heat treatment conditions in the heat treatment step, and a conventional method can be used. Hereinafter, the heat treatment of the present invention will be described.

  In the present invention, it is desirable to perform a normalizing process in which cooling is performed at a speed equal to or higher than air cooling after heating and holding at 1000 ° C. to 1200 ° C. (more desirably, 1050 ° C. to 1150 ° C.). The normalizing treatment in the present invention is a heat treatment for sufficiently dissolving the components related to the formation of precipitates (for example, Ni, Al, W, Mo, Ti) in the matrix and obtaining a uniform ferrite phase structure. Point to.

  After the normalizing treatment, it is desirable to perform a tempering treatment in which the sample is gradually cooled after being heated at 600 ° C. to 800 ° C. (more desirably 650 ° C. to 750 ° C.). The tempering treatment in the present invention refers to a heat treatment for generating and precipitating intermetallic compounds (for example, β-NiAl phase, Laves phase). By these normalizing treatment and tempering treatment, it is possible to obtain a precipitation strengthened ferritic heat resistant steel having a uniform ferrite phase structure and a desirable fine structure in which precipitates are dispersed and precipitated.

  The manufacture of the high temperature turbine member using the ferritic heat resistant steel of the present invention may be performed on the heat resistant steel material after all the heat treatments are completed, but after the normalizing process and before the tempering process. From the viewpoint of workability and workability, it is preferable to use a heat resistant steel material (a state in which a strengthening phase of an intermetallic compound is not precipitated). In that case, an aging heat treatment may be performed after the shape processing.

(Turbine high temperature member and turbine)
The precipitation strengthened ferritic heat resistant steel according to the present invention has good mechanical properties and good corrosion resistance. Therefore, the high temperature turbine member (for example, a high pressure steam turbine or medium pressure steam turbine having a main steam temperature of 600 ° C. or higher). And a turbine rotor shaft of a high-medium pressure integrated steam turbine.

  FIG. 1 is a schematic cross-sectional view showing an example of a steam turbine using a steam turbine member according to the present invention. The steam turbine shown in FIG. 1 is a high-medium pressure integrated steam turbine 10 in which a high-pressure stage steam turbine and an intermediate-pressure stage steam turbine are integrated. In the high-pressure stage steam turbine (left half in the figure), a high-pressure internal casing 11 and a high-pressure external casing 12 outside thereof are formed, and high- and medium-pressure axles (with high-pressure turbine blades 13 implanted in these casings) A high and medium pressure integrated turbine rotor shaft 14) is provided. High-temperature and high-pressure steam is obtained by a boiler (not shown), passes through a main steam pipe (not shown), a flange elbow 15 constituting a main steam inlet, a main steam inlet 16, and a nozzle box 17. Guided to high-pressure first stage turbine blade 13 '. Steam enters from the center side of the high-medium pressure integrated turbine rotor shaft 14 and flows in the direction of the rotor bearing portion 14 ′ and the bearing 18 on the high-pressure steam turbine side. The main steam temperature in this steam turbine is assumed to be 650 ° C.

  The steam discharged from the high-pressure stage steam turbine is reheated by a reheater (not shown), and then guided to an intermediate-pressure stage steam turbine (the right half in the figure). The medium pressure steam turbine rotates a generator (not shown) with the high pressure steam turbine. Like the high-pressure stage steam turbine, the intermediate-pressure stage steam turbine has an intermediate-pressure inner casing 21 and an intermediate-pressure outer casing 22, and the intermediate-pressure turbine blades 23 are planted on the high- and intermediate-pressure integrated turbine rotor shaft 14. It is installed. The reheated steam enters from the center side of the high- and medium-pressure integrated turbine rotor shaft 14 and is guided to the intermediate-pressure first stage turbine blade 23 ′ and is directed to the rotor bearing portion 14 ″ and bearing 18 ′ on the intermediate-pressure stage steam turbine side. Flowing into.

  It is particularly preferable that the turbine rotor shaft, which is exposed to high-temperature steam and is subjected to great stress and is a large high-temperature member, is composed of the precipitation-strengthened ferritic heat resistant steel according to the present invention. Similarly, members that are exposed to high-temperature steam such as high-temperature bolts (not shown) for tightening the casing, seals 24 and piping (for example, nozzle box 17, boiler tube (not shown)) are also strengthened by precipitation according to the present invention. Preferably, it is made of type ferritic heat resistant steel.

(Thermal power plant)
FIG. 2 is a system schematic diagram showing an example of a thermal power plant according to the present invention. FIG. 2 shows an example in which the high-pressure stage steam turbine and the intermediate-pressure stage steam turbine are separate and are tandemly connected via the turbine rotor shaft. As shown in FIG. 2, in the thermal power plant 30, first, the high-temperature and high-pressure steam generated in the boiler 31 is reheated in the boiler 31 after working in the high-pressure stage steam turbine 32. Next, the reheated steam works in the low pressure stage steam turbine 34 after working in the intermediate pressure stage steam turbine 33. The work generated in the steam turbine is converted into electric power by the generator 35. The steam exiting the low-pressure stage steam turbine 34 is led to the condenser 36 to become water, and then returned to the boiler 31.

  EXAMPLES Hereinafter, although this invention is demonstrated in more detail based on an Example, this invention is not limited to these.

(Production of invention heat resistant steels 1 to 10 and comparative heat resistant steels 1 to 2)
First, the raw material was melted using a high-frequency vacuum melting furnace (5.0 × 10 ⁻³ Pa or less, 1600 ° C. or more). The obtained ingot was hot forged using a 1000 ton forging machine and a 250 kgf hammer forging machine, and formed into square bars of width × length × thickness = 100 mm × 1000 mm × 30 mm. Next, this square was cut into width × length × thickness = 50 mm × 120 mm × 30 mm to obtain a heat resistant steel starting material.

  Next, each heat-resistant steel starting material was subjected to various heat treatments using a box electric furnace. First, as a normalizing treatment, water quenching was performed by holding at 1100 ° C. for 1 hour and then immersing in room temperature water. Next, as a tempering treatment, the substrate was kept at 700 ° C. for 2 hours and then air-cooled to be taken out into the atmosphere at room temperature.

  Tables 1 and 2 show chemical composition analysis values of the obtained ingots. The comparative heat resistant steels 1 and 2 are heat resistant steels (commercially available) currently used as materials for high temperature members of 600 ° C. class steam turbines. Although not shown in the table, inevitable impurities (P, S, Sb, Sn, As, and N) each satisfied the specified range of the present invention.

(Various characteristic evaluation)
For each sample obtained above (invention heat resistant steels 1 to 10 and comparative heat resistant steels 1 to 2), microstructure observation, creep rupture strength as an index of high temperature strength, and oxide scale thickness as an index of oxidation resistance Each evaluation test was conducted. The outline of each evaluation test will be described next.

  In the microstructure observation, the structure of the matrix and the state of the precipitate were observed using an optical microscope or a scanning electron microscope.

As an evaluation of the high temperature strength, a creep test is performed up to 10 ³ hours at a temperature range of 600 to 700 ° C. with a predetermined stress, and the obtained test results are used for 10 ³ hours using Larson Miller parameters. The creep rupture strength at 10 ⁴ hours and 10 ⁵ hours was determined. For practical applications of 650 ° C. class steam turbine 10 for ³ hours at 190 MPa or more, 10 ⁴ hours at 120 MPa or more and 10 ⁵ hours creep rupture strength of at least 100 MPa it is considered at least necessary. Therefore, they were used as pass / fail criteria.

As an evaluation of oxidation resistance, a steam oxidation test was conducted. After maintaining 10 ³ hours at 600 to 700 ° C. at a steam atmosphere, to measure the average thickness of the oxide scale layer formed on the surface of the sample. Criteria for oxidation resistance, 50 [mu] m or less is 600 ° C. × 10 ³ hours average thickness of the oxide scale layer, 60 [mu] m at 650 ° C. × 10 ³ hours or less, a 70μm or less at 700 ° C. × 10 ³ hours as "pass" The value exceeding the value was regarded as “fail”.

  The results of high temperature strength are shown in Tables 3 to 4, and the results of oxidation resistance are shown in Tables 5 to 6.

  In the observation of the microstructure, the comparative heat resistant steels 1 and 2 had a matrix of martensite structure, and carbide and / or carbonitride precipitates (particle size: several μm) were dispersed. On the other hand, the invention heat resistant steels 1 to 10 all have a ferrite phase as a matrix, and β-NiAl phase precipitates (particle size: 10 nm or less) are uniformly finely dispersed in each of the matrix crystal grains. It was confirmed that a Laves phase precipitate was deposited along the boundary.

In the high temperature strength (creep rupture strength at 650 ° C.), as shown in Tables 3 to 4, all the heat resistant steels (invention heat resistant steels 1 to 10 and comparative heat resistant steels 1 to 2) are 190 MPa in 10 ³ hours. The above creep rupture strength was shown. However, the comparative heat resistant steel 2 did not satisfy 120 MPa or more in 10 ⁴ hours, and the comparative heat resistant steels 1 and 2 did not satisfy 100 MPa or more in 10 ⁵ hours. In other words, it was confirmed that the invention heat resistant steels 1 to 10 have little decrease in creep rupture strength on the long-time side and satisfy the high temperature strength requirement standard for putting a 650 ° C. class steam turbine to practical use.

In oxidation resistance (average thickness of oxide scale layer at 10 ³ hours), as shown in Tables 5-6, all heat resistant steels (invention heat resistant steels 1 to 10 and comparison) at 600 ° C. × 10 ³ hours Heat resistant steels 1-2) showed an average thickness of the oxide scale layer of 50 μm or less. However, the comparative heat resistant steels 1 and 2 could not satisfy 60 μm or less and 70 μm or less at 650 ° C. × 10 ³ hours and 700 ° C. × 10 ³ hours, respectively. In other words, it was confirmed that the invention heat-resistant steels 1 to 10 have little formation of an oxide scale layer even on the high temperature side, and satisfy the requirements for oxidation resistance for putting a 650 ° C. class steam turbine into practical use.

  As described above, the precipitation-strengthened ferritic heat-resistant steel according to the present invention balances mechanical strength and oxidation resistance at high temperatures at a higher level than before. Can be preferably applied to a steam turbine high-temperature member (for example, a turbine rotor shaft) corresponding to the above. The turbine high-temperature member according to the present invention is not limited to the turbine rotor shaft, and can be applied as a high-temperature high-pressure member such as a boiler tube or a casing bolt. Further, the present invention can be applied as a turbine rotor using the turbine rotor shaft, a steam turbine using the turbine rotor, and a thermal power plant using the steam turbine.

  Note that the above-described embodiments have been specifically described in order to help understanding of the present invention, and the present invention is not limited to having all the configurations described. For example, a part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. Further, a part of the configuration of each embodiment can be deleted, replaced with another configuration, or added with another configuration.

10 ... High and medium pressure integrated steam turbine, 11 ... High pressure internal casing, 12 ... High pressure external casing,
13 ... High-pressure turbine blade, 13 '... High-pressure first stage turbine blade,
14 ... High and medium pressure integrated turbine rotor shaft, 14 ', 14 "... Rotor bearing
15 ... Flange / Elbow, 16 ... Main steam inlet, 17 ... Nozzle box, 18, 18 '... Bearing,
21 ... Medium pressure internal compartment, 22 ... Medium pressure external compartment,
23 ... Medium pressure turbine blade, 23 '... Medium pressure first stage turbine blade, 24 ... Seal,
30 ... Thermal power plant, 31 ... Boiler, 32 ... High-pressure steam turbine,
33 ... Medium pressure stage steam turbine, 34 ... Low pressure stage steam turbine, 35 ... Generator, 36 ... Condenser.

Claims (11)

A ferritic heat resistant steel in which intermetallic compounds are dispersed and precipitated,
0.1 mass% or less of C,
12 to 20% by mass of Cr,
Ni of 2 mass% or more and 6 mass% or less,
Al of 2 mass% or more and 8 mass% or less,
2.5 wt% to 10 wt% W,
0.5% to 2.5% by mass of Mo,
2% to 6% by weight of Co,
0.001% by mass or more and 0.01% by mass or less B and
A precipitation strengthened ferritic heat resistant steel characterized in that the balance consists of Fe and inevitable impurities. In the precipitation strengthened ferritic heat resistant steel according to claim 1,
A precipitation-strengthened ferritic heat resistant steel further comprising 5% by mass or less of Ti. In the precipitation strengthened ferritic heat resistant steel according to claim 1 or claim 2,
A precipitation strengthened ferritic heat resistant steel further comprising at least one of Nb and V in a total amount of 0.5% by mass or less. In the precipitation strengthened ferritic heat resistant steel according to any one of claims 1 to 3,
A precipitation-strengthened ferritic heat resistant steel, further comprising at least one of 1 mass% or less of Si and 1 mass% or less of Mn. In the precipitation strengthened ferritic heat resistant steel according to any one of claims 1 to 4,
The inevitable impurity is at least one of P, S, Sb, Sn, As and N;
The P is 0.5 mass% or less, the S is 0.5 mass% or less, the Sb is 0.1 mass% or less, the Sn is 0.1 mass% or less, the As is 0.1 mass% or less, and the N is 0.1 mass% or less. Precipitation strengthened ferritic heat resistant steel characterized by In the precipitation strengthened ferritic heat resistant steel according to any one of claims 1 to 5,
A precipitation strengthened ferritic heat resistant steel, wherein the intermetallic compound is a Laves phase and a β-NiAl phase. In the precipitation strengthened ferritic heat resistant steel according to any one of claims 1 to 6,
The precipitation strengthened ferritic heat resistant steel is subjected to a normalizing treatment at 1000 to 1200 ° C. and then subjected to a tempering treatment at 600 to 800 ° C. .   A high temperature turbine member using the precipitation strengthened ferritic heat resistant steel according to any one of claims 1 to 7. The turbine high temperature member according to claim 8 is a turbine rotor shaft,
A turbine rotor using the turbine rotor shaft.   A steam turbine using the turbine rotor according to claim 9.   A thermal power plant using the steam turbine according to claim 10.

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