JP6317566B2 - Precipitation hardening type martensitic stainless steel, turbine member using the stainless steel, and turbine using the turbine member - Google Patents

Description

  The present invention relates to a steel material having high mechanical properties and corrosion resistance, and particularly relates to a precipitation hardening martensitic stainless steel, a turbine member using the stainless steel, and a turbine using the turbine member.

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 a steam turbine is to increase the length of a steam turbine blade (for example, a long blade). By enlarging the turbine blades, it becomes possible to convert more steam energy into turbine rotational force. In addition, the increase in the length of the turbine blades can be expected to have the secondary effect of shortening the equipment construction period and cost reduction by reducing the number of cabins.

  Currently, martensitic stainless steel is mainly used for the steam turbine long blades of ultra-supercritical power generation (USC) plants. Here, when the length of the steam turbine blade is increased, the centrifugal force applied to the turbine blade increases significantly (generally, the centrifugal force is proportional to the product of the mass and the radius of rotation). Therefore, a material having higher mechanical strength is required as a long blade material for a steam turbine. It is also desirable to have excellent toughness to prevent sudden failure.

  As a structural material having good mechanical strength and toughness, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2005-194626) describes a weight ratio of 12.25 to 14.25% Cr, 7.5 to 8.5% Ni, 1.0 ~ 2.5% Mo, 0.05% or less C, 0.2% or less Si, 0.4% or less Mn, 0.03% or less P, 0.005% or less S, 0.008% or less N, 0.90 A precipitation hardening martensitic steel is disclosed, which is composed of ˜2.25% Al and the balance being substantially Fe, with the sum of Cr and Mo being 14.25 to 16.75%.

  In Patent Document 2 (Japanese Patent Laid-Open No. 2011-225913), by mass ratio, C: 0.05 to 0.10%, Cr: 12.0 to 13.0%, Ni: 6.0 to 7.0%, Mo: 1.0 to 2.0%, Si: 0.01 to 0.05 , Mn: 0.06 to 1.0%, Nb: 0.3 to 0.5%, V: 0.3 to 0.5%, Ti: 1.5 to 2.5%, and Al: 1.0 to 2.3%, with the balance being Fe and inevitable impurities A hardenable martensitic stainless steel is disclosed.

  Patent Document 3 (Japanese Patent Laid-Open No. 2012-102638) includes, as a composition, C of 0.10% by mass or less, Cr of 13.0% by mass to 15.0% by mass, Ni of 7.0% by mass to 10.0% by mass, 2.0% by mass. More than 3.0 mass% Mo, 0.5 mass% to 2.5 mass% Ti, 0.5 mass% to 2.5 mass% Al, 0.5 mass% or less Si, 0.1 mass% to 1.0 mass% Mn, A precipitation hardening martensitic stainless steel, the balance of which consists of Fe and inevitable impurities, is disclosed.

JP 2005-194626 A JP 2011-225913 A JP 2012-102638 A

  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). In order to further increase the length of the turbine blades with the aim of improving the efficiency of the steam turbine, it has higher mechanical strength and higher toughness than conventional martensitic stainless steel (for example, Patent Documents 1 to 3). Materials are needed. In addition, since the steam turbine long blades are in a severe corrosive environment because they are used in a wet and dry alternating area, it is also desirable to have high corrosion resistance (for example, resistance to stress corrosion cracking (SCC)).

  In general, mechanical strength and corrosion resistance are in a trade-off relationship. Ordinary martensitic stainless steel has a high mechanical strength, but has a weak point in terms of corrosion resistance. On the other hand, precipitation hardening martensitic stainless steel with a large amount of Cr component and a small amount of C component is excellent in corrosion resistance but has a slight weakness in mechanical strength.

  Accordingly, an object of the present invention is a precipitation hardening martensitic stainless steel in which mechanical strength and toughness are balanced at a higher level than before and corrosion resistance (for example, SCC resistance and pitting potential) is excellent, and the stainless steel It is an object to provide a turbine member using steel and a turbine using the turbine member.

(I) One aspect of the present invention is a martensitic stainless 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,
11 mass% or more and 13 mass% or less of Cr (chrome),
7.5 mass% or more and 11 mass% or less of Ni (nickel),
0.9 mass% or more and 1.7 mass% or less of Al (aluminum),
0.85 mass% or more and 1.35 mass% or less of Mo (molybdenum),
1.75 mass% or more and 2.75 mass% or less of W (tungsten)
The balance consists of Fe (iron) and inevitable impurities,
“[Mo component amount] +0.5 [W component amount]” is 1.9 mass% to 2.5 mass%,
There is provided a precipitation hardening martensitic stainless steel characterized in that “[Mo component amount] / [W component amount]” is 0.4 or more and 0.6 or less.

  (II) Another aspect of the present invention provides a turbine member using the precipitation hardening martensitic stainless steel described above in order to achieve the above object.

  (III) Another aspect of the present invention provides a turbine rotor characterized in that, in order to achieve the above object, the turbine member is a steam turbine long blade, and the steam turbine long blade 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.

  ADVANTAGE OF THE INVENTION According to this invention, mechanical strength and toughness are balanced at a higher level than before and precipitation hardened martensitic stainless steel having excellent corrosion resistance, a turbine member using the stainless steel, and the turbine member are used. The turbine that was present can be provided. In addition, a thermal power plant using the turbine can be provided.

It is a perspective schematic diagram which shows an example of the steam turbine long blade which concerns on this invention. It is a section schematic diagram showing an example of the turbine concerning the present invention. It is a systematic schematic diagram showing an example of a thermal power plant according to the present invention. 5 is a graph showing the relationship between the ratio of [Mo component amount] to [W component amount] and tensile strength. 5 is a graph showing the relationship between the ratio of [Mo component amount] to [W component amount] and shock absorption energy.

The present invention can be modified or changed as follows in the precipitation hardening martensitic stainless steel (I) according to the present invention described above.
(I) Further containing 0.4% by mass or less of Ti (titanium).
(Ii) A part of the Ni is replaced with 3% by mass or less of Co (cobalt).
(Iii) It further contains at least one of Nb (niobium) and V (vanadium) in a total amount of 0.5% by mass or less.
(Iv) It further contains at least one of Si (silicon) of 0.1% by mass or less and Mn (manganese) of 1% by mass or less.
(V) 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.
(Vi) The intermetallic compound is a β-NiAl phase.
(Vii) The precipitation hardening martensitic stainless steel is subjected to aging heat treatment at 450 to 650 ° C after solution heat treatment at 850 to 950 ° C.

  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 hardened martensitic stainless steel)
Hereinafter, each component of the precipitation hardening type martensitic stainless steel according to the present invention will be described.

C component:
The C component is a component that suppresses the formation of a δ ferrite phase that adversely affects mechanical properties and corrosion resistance. In addition, it is a component that generates carbides such as Cr and Ti and contributes to precipitation hardening. 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, a deterioration in corrosion resistance due to a decrease in Cr concentration near the grain boundary, and a decrease in martensite transformation temperature. 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 improving corrosion resistance by forming a passive film on the surface of stainless steel. When the Cr content is less than 11% by mass, the corrosion resistance is not sufficiently ensured. On the other hand, if the Cr component amount exceeds 13% by mass, a δ ferrite phase is likely to be formed, which causes deterioration of mechanical properties and corrosion resistance. Therefore, the Cr component amount is desirably 11 to 13% by mass. 11.5-12.5 mass% is more desirable, and 11.75-12.25 mass% is still more desirable.

Ni component:
The Ni component is a component that suppresses the formation of the δ ferrite phase and contributes to an improvement in tensile strength by dispersion precipitation hardening of an intermetallic compound (for example, a Ni—Al compound). Furthermore, there is an effect of improving hardenability and toughness. If the amount of Ni component is less than 7.5% by mass, those effects become insufficient. On the other hand, if the amount of Ni component exceeds 11% by mass, the austenite phase remains and precipitates, which causes a decrease in mechanical strength (for example, tensile strength). Therefore, the amount of Ni component is desirably 7.5 to 11% by mass. 8.5-10.5 mass% is more desirable, and 9-10 mass% is still more desirable.

Al component:
The Al component is also a component that contributes to precipitation hardening by forming a Ni-Al intermetallic compound. When the amount of Al component is less than 0.9% by mass, the effect becomes insufficient. On the other hand, when the amount of Al component exceeds 1.7% by mass, excessive precipitation of Ni—Al intermetallic compounds and formation of δ ferrite phase are likely to cause deterioration of characteristics. Therefore, the amount of Al component is desirably 0.9 to 1.7% by mass. 1.1-1.5 mass% is more desirable, and 1.25-1.4 mass% is still more desirable.

Mo component:
The Mo component is a component that improves corrosion resistance and contributes to improvement of mechanical strength (for example, solid solution strengthening). When the amount of Mo component is less than 0.85% by mass, the effect becomes insufficient. On the other hand, when the amount of Mo component exceeds 1.35% by mass, the formation of δ ferrite phase and the excessive formation of intermetallic compounds with Fe (for example, Laves phase) are promoted, which causes deterioration of mechanical properties and corrosion resistance. Therefore, the amount of Mo component is desirably 0.85 to 1.35% by mass. 1 to 1.3% by mass is more desirable, and 1.1 to 1.2% by mass is even more desirable.

W component:
Similar to the Mo component, the W component is a component that improves corrosion resistance and contributes to improvement of mechanical strength (for example, solid solution strengthening). When the amount of W component is less than 1.75% by mass, the effect becomes insufficient. On the other hand, when the amount of W component exceeds 2.75% by mass, the formation of δ ferrite phase and the excessive formation of intermetallic compounds with Fe (for example, Laves phase) are promoted, which causes deterioration of mechanical properties and corrosion resistance. Therefore, the amount of W component is desirably 1.75 to 2.75 mass%. 2 to 2.5% by mass is more desirable, and 2.2 to 2.5% by mass is even more desirable.

  The balance of the component amounts of Mo and W in the composition is the most characteristic feature of the present invention. The total amount of Mo component and W component is the sum of [Mo component amount] and half of [W component amount] [[Mo component amount] + 0.5 [W component amount]] 1.9% to 2.5% % Or less, more preferably 2% by mass or more and 2.4% by mass or less. Further, the ratio of [Mo component amount] to [W component amount] “[Mo component amount] / [W component amount]” is preferably 0.4 or more and 0.6 or less. By controlling in such a range, the mechanical properties of martensitic stainless steel can be balanced at a higher level than before. For example, high strength (tensile strength of 1550 MPa or more) and high toughness (impact absorption energy of 30 J or more) can be achieved simultaneously.

Ti component:
The Ti component is a component that contributes to precipitation hardening by generating carbide and generating an intermetallic compound (for example, a Ni-Ti-Al compound). 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 0.4% by mass, mechanical properties (for example, toughness) are reduced due to excessive precipitation of intermetallic compounds or formation of harmful phases (for example, σ ferrite phase). Therefore, the amount of Ti component is desirably 0.4% by mass or less. 0.35 mass% or less is more desirable, and 0.3 mass% or less is further desirable.

Co component:
The Co component is a component that has the effect of suppressing the formation of the δ ferrite phase and adjusting the martensite transformation temperature to improve the uniformity of the martensite structure. In the present invention, the Co component is not an essential component, but it is preferable to replace a part of the Ni component with the Co component because of its action and effect. That is, the total component amount of Ni and Co is desirably 7.5% by mass or more and 11% by mass or less. However, if the amount of Co component exceeds 3% by mass, the austenite phase is likely to remain, and the amount of precipitation of Ni-Al intermetallic compounds is reduced, leading to a decrease in mechanical strength (for example, tensile strength). . Therefore, the amount of Co component is desirably 3% by mass or less, and more desirably 2.8% by mass or less.

Nb component:
The Nb component is a component that precipitates as a carbide 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 Nb component amount exceeds 0.5 mass%, it becomes a factor for promoting the formation of δ ferrite phase. 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 hardening more remarkable.

Si component:
The Si component is a deoxidizer and is a component that functions when the stainless 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, if the amount of Si component exceeds 1% by mass, a δ ferrite 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 step of stainless steel, it is not necessary to positively add Si components (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 stainless steel is dissolved, and is effective even in a small amount. It also has the effect of suppressing the formation of δ ferrite phase. 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, when the amount of the Mn component exceeds 1% by mass, the austenite phase tends to remain. 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 melting process of stainless steel, 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. Examples of inevitable impurities include P, S, Sb, Sn, As, and N, and at least one of these is included in the precipitation hardening martensitic stainless steel of the present invention.

  Since reduction of P component and S component can improve toughness without impairing mechanical strength, it is desirable to reduce it as much as possible. From the viewpoint of 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.

  Similarly, toughness can be improved by reducing Sb, Sn and As components. 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 desirable.

  N component has strong affinity with Al component and Ti component, and forms nitrides (for example, AlN, TiN) to reduce toughness. Moreover, the amount of precipitation of the intermetallic compound (for example, Ni—Al compound, Ni—Ti—Al compound) in the precipitation strengthening phase is reduced by the amount of nitride produced, and the mechanical strength is lowered. 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 precipitation hardening martensitic stainless steel according to the present invention is not particularly limited except that there are desirable heat treatment conditions in the heat treatment step, and the 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 solution heat treatment that is rapidly cooled after heating and holding at 800 ° C. to 1000 ° C. (more desirably, 850 ° C. to 950 ° C.). The solution heat treatment in the present invention refers to a heat treatment in which components (for example, Ni, Al, Ti) involved in the formation of precipitates are dissolved in a matrix and then rapidly cooled to obtain a martensite structure.

  After the solution heat treatment, it is desirable to perform an aging heat treatment that is gradually cooled after heating and holding at 450 ° C. to 650 ° C. (more desirably, 500 ° C. to 600 ° C.). The aging heat treatment in the present invention refers to a heat treatment for generating and precipitating intermetallic compounds (for example, β-NiAl phase) and carbides. By these solution heat treatment and aging heat treatment, it is possible to obtain a precipitation hardening martensitic stainless steel having a uniform martensite structure and a desirable microstructure in which precipitates are finely dispersed.

  Moreover, in order to reduce the retained austenite after the heat treatment, it is preferable to perform sub-zero treatment. The sub-zero treatment in the present invention is a heat treatment performed to transform residual austenite into martensite, and refers to a heat treatment that is cooled to a temperature lower than room temperature and maintained at that temperature. For example, a heat treatment is performed by cooling to −70 ° C. or lower using a refrigerant such as dry ice or liquid nitrogen and an organic solvent such as isopentane and holding for 4 hours or more.

  The manufacture of the turbine member using the martensitic stainless steel of the present invention may be performed on the stainless steel material after completion of all the above heat treatments, but the stainless steel before the aging heat treatment after the solution heat treatment. It is preferable to use a steel material (a state in which no Ni-Al compound or the like is deposited) from the viewpoint of workability and workability. In that case, an aging heat treatment may be performed after the shape processing.

(Turbine member)
The precipitation hardening martensitic stainless steel according to the present invention has both good mechanical properties and good corrosion resistance. Therefore, a turbine member (for example, a steam turbine long blade having a length of 50 inches or more or a gas turbine compressor) It can be suitably used as a wing). FIG. 1 is a schematic perspective view showing an example of a steam turbine long blade according to the present invention. As shown in FIG. 1, the steam turbine long blade 10 is an axial entry type, and includes a blade profile portion 11 and a blade root portion 12 on which high-speed steam collides. A stub 14 and a shroud 15 for connecting adjacent long blades 10 of the steam turbine rotor are formed in the vicinity of the center and the tip of the blade profile portion 11, respectively. In addition, an erosion shield 13 for protecting the blade profile portion 11 from corrosion (erosion) caused by collision of high-speed condensed steam collides with the tip region of the blade profile portion 11. The erosion shield 13 may be used according to the degree of erosion. Since the steel of the present invention has erosion resistance, the erosion shield 13 need not be used when the degree of erosion is low.

  An example of the erosion shield 13 is a Co-based alloy plate (for example, Stellite (registered trademark) plate), which can be joined using a method such as TIG welding, electron beam welding, or brazing. After erosion shield 13 bonding, it is desirable to perform stress relief heat treatment (SR heat treatment) at 550 ° C or higher and 650 ° C or lower (more preferably 570 ° C or higher and 630 ° C or lower) in order to remove residual stress that causes cracking. . Further, as another means for protecting the blade profile portion 11 from erosion, there is surface quenching in which the tip region of the blade profile portion 11 is locally heated by a laser having a large heat input amount to harden the surface layer portion.

(Turbine)
FIG. 2 is a schematic cross-sectional view showing an example of a turbine according to the present invention. As shown in FIG. 2, the low-pressure stage steam turbine 20 is roughly composed of a turbine rotor 21 that rotates by circulation of a working fluid (steam), and a turbine casing 25 that accommodates the turbine rotor 21. The turbine rotor 21 includes a rotating shaft 22 having a plurality of disks 23 arranged at predetermined intervals in the axial direction, and a plurality of turbine long blades radially fixed to the outer periphery of each of the plurality of disks 23. And 10. The turbine long blade 10 is usually designed so that the blade length becomes longer toward the downstream side of the working fluid. The turbine casing 25 has a plurality of stationary blades 26 fixed to the inner surface thereof so as to be positioned between the turbine long blades 10 adjacent in the axial direction of the turbine rotor 21, and has a working fluid inlet 27 and a working fluid discharge port 28. It has.

(Thermal power plant)
FIG. 3 is a system schematic diagram showing an example of a thermal power plant according to the present invention. As shown in FIG. 3, in the thermal power plant 30, first, high-temperature and high-pressure steam (working fluid) generated in the boiler 31 is reheated in the boiler 31 after working in the high-pressure steam turbine 32. Next, the reheated steam works in the intermediate pressure stage steam turbine 33 and then in the low pressure stage steam turbine 20. The work generated in the steam turbine is converted into electric power by the generator 34. The steam leaving the low-pressure stage steam turbine 20 is led to the condenser 35 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 Stainless Steel 1-12 and Comparative Stainless Steel 1-17)
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 × thickness × length = 100 mm × 30 mm × 1000 mm. Next, this square was cut into width × thickness × length = 50 mm × 30 mm × 120 mm to obtain a stainless steel starting material.

  Next, each stainless steel starting material was subjected to various heat treatments using a box electric furnace. First, as a solution heat treatment, water quenching was performed by holding at 900 ° C. for 1 hour and then immersing in water at room temperature. Next, as an aging heat treatment, the mixture was kept at 538 ° C. for 2 hours and then air-cooled to be taken out into the atmosphere at room temperature. No sub-zero treatment was performed on any sample.

  Tables 1 to 5 show chemical composition analysis values of the obtained ingots. In addition, although description in a table | surface is abbreviate | omitted, the inevitable impurities (P, S, Sb, Sn, and As) other than N component each satisfy | filled the prescription | regulation range of this invention.

(Various characteristic evaluation)
For each sample obtained above (invention stainless steels 1-12 and comparative stainless steels 1-17), 0.02% proof stress and tensile strength at room temperature as a microstructural observation and mechanical strength index. Elongation and squeezing were conducted as indicators, impact absorption energy at room temperature was taken as an indicator of toughness, and pitting corrosion potential was evaluated as an indicator of corrosion resistance. The outline of each evaluation test will be described.

  Microstructure observation was performed using an optical microscope. The criterion was “accepted” when the martensite structure had a precipitation amount of δ ferrite phase of 1.0% or less and a precipitation amount of residual austenite phase of 10% or less. The others were “failed”. The measurement of the precipitation amount of the δ ferrite phase was based on the point calculation method described in JIS G 0555. The amount of precipitation of the retained austenite phase was measured by X-ray diffraction.

  For the measurement of 0.02% proof stress and tensile strength, test pieces (distance between grades 30 mm, outer diameter 6 mm) were prepared from each sample obtained above, and a tensile test was performed at room temperature according to JIS Z 2241. . The judgment criteria of 0.02% proof stress and tensile strength were “pass” for 1000 MPa or more and 1550 MPa or more, respectively, and “fail” for those values below. In addition, the criteria for determining elongation and drawing were 10% or more and 30% or more, respectively, “pass”, and less than those values were “fail”.

  For the measurement of the impact absorption energy, a test piece having a 2 mm V-notch was prepared from each sample obtained above, and a Charpy impact test was performed at room temperature in accordance with JIS Z 2242. The criterion for determining the impact absorption energy was 30 J or more as “pass” and less than that value as “fail”.

For the measurement of pitting potential, prepare flat plate test pieces (length 15 mm, width 15 mm, thickness 3 mm) from each sample obtained above, and leave the measurement surface (area 1.0 cm ² ) The part was insulated. Measurement conditions were a potential scan at a sweep rate of 20 mV / min using a 3.0% NaCl aqueous solution at 30 ° C. as the test solution. The criterion of the pitting corrosion potential was 150 mV or more as “pass”, and less than that value was “fail”.

  The results of each evaluation test are shown in Tables 6 to 10.

  As shown in Tables 6 to 7, all of the inventive stainless steels 1 to 12 had a metal structure with a martensite structure as a matrix and a small amount of precipitation of δ ferrite phase and residual austenite phase. Further, it was confirmed that β-NiAl phase precipitates having a particle size of 10 nm or less were uniformly and finely dispersed in each metal crystal grain. The mechanical properties of mechanical strength (0.02% proof stress, tensile strength), flexibility (elongation, drawing) and toughness (impact absorption energy) were also acceptable. Furthermore, good results were obtained with respect to corrosion resistance (pitting corrosion potential). From these results, it was demonstrated that the precipitation hardening martensitic stainless steel according to the present invention balances mechanical strength, flexibility, toughness, and corrosion resistance at a higher level than before.

  On the other hand, Comparative Stainless Steels 1 to 17 failed in any one or more of the microstructure, mechanical strength, suppleness, toughness, and corrosion resistance, and there was no sample satisfying all required characteristics. Specifically, the comparative stainless steel 1 has a C component amount that is not within the scope of the present invention, and the microstructure, mechanical strength, and corrosion resistance are unacceptable.

  Comparative stainless steel 2 had a Cr content less than that of the present invention, and the corrosion resistance was unacceptable. Comparative stainless steel 3 had a Cr content greater than that of the present invention, and had a fine structure and suppleness and toughness.

  Comparative stainless steel 4 had a Ni component amount less than that of the present invention, and the flexibility and toughness were unacceptable. The comparative stainless steel 5 had a Ni component amount larger than that of the present invention, and the microstructure and mechanical strength were unacceptable.

  Comparative stainless steel 6 had an Al component amount less than that of the present invention, and its mechanical strength was unacceptable. Comparative stainless steel 7 had an Al component amount larger than that of the present invention, and the flexibility and toughness were unacceptable.

  In comparative stainless steel 12, the amount of Co component replacing Ni was larger than that of the present invention, and the microstructure, mechanical strength, flexibility and toughness were unacceptable. In comparative stainless steel 13, the total amount of Nb and V was larger than that of the present invention, and the flexibility and toughness were unacceptable. Comparative stainless steel 14 had a Ti component amount higher than that of the present invention, and the microstructure and toughness were unacceptable. The comparative stainless steel 15 had an amount of Si component larger than that of the present invention, and the microstructure, mechanical strength, flexibility and toughness were unacceptable. The comparative stainless steel 16 had an Mn component amount larger than that of the present invention, and the microstructure and mechanical strength were unacceptable. The comparative stainless steel 17 had an N component amount larger than that of the present invention, and the microstructure and mechanical strength were unacceptable.

  Next, the component amount balance of Mo and W in the composition will be considered from the test evaluation results of Invention Stainless Steel 1 and Comparative Stainless Steels 8-11. FIG. 4 is a graph showing the relationship between the ratio of [Mo component amount] to [W component amount] and the tensile strength, and FIG. 5 is the ratio of [Mo component amount] to [W component amount] and impact absorption energy. It is a graph which shows the relationship.

As shown in FIG. 4, the ratio of [Mo component amount] to [W component amount] “[Mo component amount] / [W component amount]” becomes smaller (the higher the W component ratio), the higher the tensile strength. It turns out that becomes large. This is considered to be an effect of the formation of a Laves phase (Fe ₂ W phase) in addition to solid solution strengthening by the W component. In comparative stainless steel 11, formation and precipitation of a Laves phase was confirmed in the microstructure observation.

On the other hand, as shown in FIG. 5, it can be seen that the smaller the “[Mo component amount] / [W component amount]” (the greater the W component ratio), the smaller the impact absorption energy. This is also considered to be due to the effect of the formation of the Laves phase (Fe ₂ W phase), and it is considered that the toughness has greatly deteriorated due to the excessive formation of the Laves phase. That is, from the results of FIGS. 4 to 5, it is confirmed that “[Mo component amount] / [W component amount]” of 0.4 or more and 0.6 or less is desirable in order to balance high strength and high toughness at a high level. Is done.

(Steam turbine long blade)
A steam turbine long blade (see FIG. 1) having a blade length of 51 inches was manufactured using the inventive stainless steel 1. First, vacuum carbon deoxidation was performed in a high vacuum state of 5.0 × 10 ⁻³ Pa or less to deoxidize molten steel by a chemical reaction “C + O → CO”. Subsequently, the electrode rod was formed by forging. Next, this electrode rod was immersed in molten slag, self-dissolved by Joule heat by energization, and solidified in a water-cooled mold to re-melt electroslag to obtain a high-grade steel ingot.

  The obtained steel ingot was hot forged and then stamped forged with a 51 inch airfoil and molded. After forming, the solution was heat treated as a solution heat treatment at 900 ° C. for 2 hours, and then rapidly cooled by forced cooling with a blower. Finished in a predetermined shape by cutting. Next, it was kept at 538 ° C. for 4 hours as an aging heat treatment, and then air-cooled. As the final finishing process, 51-inch long blades were manufactured by bending and surface polishing.

  Test pieces were collected from the tip, center and blade root of the obtained steam turbine long blade so as to be in the blade longitudinal direction, and the same evaluation test as described above was performed. As a result, in all parts, all items of microstructure, mechanical strength (0.02% proof stress, tensile strength), suppleness (elongation, drawing), toughness (impact absorption energy) and corrosion resistance (pitting corrosion potential) Showed acceptable characteristics.

  As described above, the precipitation hardening type martensitic stainless steel according to the present invention is excellent in the uniformity of the fine structure (martensite structure + dispersed precipitation of intermetallic compound), mechanical strength, flexibility, toughness and corrosion resistance. Is balanced at a higher level than before, and can be preferably applied to the extension of the long blades of the steam turbine. Further, the present invention can be applied as a turbine rotor having the steam turbine long blades, a steam turbine using the turbine rotor, and a thermal power plant using the steam turbine. Furthermore, it is applicable not only to a steam turbine but also to a blade for a gas turbine compressor.

  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 ... steam turbine long blade, 11 ... blade profile, 12 ... blade root,
13 ... erosion shield, 14 ... stub, 15 ... shroud,
20 ... Low-pressure steam turbine, 21 ... turbine rotor, 22 ... rotary shaft, 23 ... disk,
25 ... Turbine casing, 26 ... Stator blade, 27 ... Working fluid inlet, 28 ... Working fluid outlet,
30 ... Thermal power plant, 31 ... Boiler, 32 ... High-pressure steam turbine,
33… Medium pressure steam turbine, 34… Generator, 35… Condenser.

Claims (10)

A martensitic stainless steel in which intermetallic compounds are dispersed and precipitated,
0.1 mass% or less of C,
11 mass% or more and 13 mass% or less of Cr,
7.5 mass% or more and 11 mass% or less of Ni,
0.9 mass% or more and 1.7 mass% or less of Al,
0.85 mass% or more and 1.35 mass% or less of Mo,
1.75 mass% or more and 2.75 mass% or less of W,
The balance consists of Fe and inevitable impurities,
“[Mo component amount] +0.5 [W component amount]” is 1.9 mass% to 2.5 mass%,
"[Mo component amount] / [W component amount]" is Ri der 0.4 to 0.6,
The intermetallic compound is a β-NiAl phase,
The precipitation amount of δ ferrite phase is 1.0% or less, and the precipitation amount of residual austenite phase has a martensite structure of 10% or less,
A precipitation hardening martensitic stainless steel that combines 0.02% yield strength of 1000 MPa or more, tensile strength of 1550 MPa or more, impact absorption energy of 30 J or more, and pitting corrosion potential of 150 mV or more . In the precipitation hardening martensitic stainless steel according to claim 1,
A precipitation hardening martensitic stainless steel further comprising 0.4% by mass or less of Ti. In the precipitation hardening type martensitic stainless steel according to claim 1 or claim 2,
A precipitation hardening martensitic stainless steel, wherein a part of the Ni is replaced with 3% by mass or less of Co. In the precipitation hardening martensitic stainless steel according to any one of claims 1 to 3,
A precipitation hardening martensitic stainless steel further comprising at least one of Nb and V in a total amount of 0.5% by mass or less. In the precipitation hardening type martensitic stainless steel according to any one of claims 1 to 4,
A precipitation hardening martensitic stainless steel, further comprising at least one of 0.1 mass% or less of Si and 1 mass% or less of Mn. In the precipitation hardening martensitic stainless steel according to any one of claims 1 to 5,
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 hardening type martensitic stainless steel. A turbine member using the precipitation hardening type martensitic stainless steel according to any one of claims 1 to 6 . The turbine member according to claim 7 is a steam turbine long blade,
A turbine rotor using the steam turbine long blades. A steam turbine using the turbine rotor according to claim 8 . A thermal power plant using the steam turbine according to claim 9 .

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