JP2014080656A - Precipitation hardening type martensitic stainless steel and steam turbine long blade using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a precipitation hardening type martensitic stainless steel having excellent mechanical properties and corrosion resistance, and to provide a steam turbine long blade using the same.SOLUTION: A precipitation hardening type martensitic stainless steel contains, by mass ratio, C:0.1% or less, N:0.1% or less, Cr:10 to 15%, Ni:10 to 15%, Mo:0.5 to 2.5%, Al:1.0 to 3.0%, Si:1.0% or less, Mn:1.0% or less and the balance Fe with inevitable impurities. A steam turbine long blade is constituted by the precipitation hardening type martensitic stainless steel.

Description

  The present invention relates to a precipitation hardening type martensitic stainless steel having excellent structural stability, mechanical properties, and corrosion resistance, and a steam turbine long blade using the same.

In recent years, from the viewpoint of energy saving (for example, fossil fuel saving) and prevention of global warming (for example, suppression of CO ₂ gas generation amount), improvement in the efficiency of thermal power plants (for example, improvement in efficiency in steam turbines) has been desired. . One effective means for improving the efficiency of the steam turbine is to lengthen the steam turbine blades. In addition, the increase in the length of the steam turbine blades can be expected to have the secondary effect of shortening the equipment construction period and thereby reducing costs by reducing the number of cabins.

  In order to improve the reliability of a steam turbine, a long blade material excellent in both mechanical properties and corrosion resistance is required. Precipitation hardening type martensitic stainless steel has a high Cr addition and a low C addition, so it has excellent corrosion resistance, but has a poor balance between strength and toughness (see, for example, Patent Document 1).

JP 2005-194626 A

  An object of the present invention is to provide a precipitation hardening martensitic stainless steel having excellent mechanical properties and corrosion resistance.

  Precipitation hardening type martensitic stainless steel is, by mass ratio, C: 0.1% or less, N: 0.1% or less, Cr: 10-15%, Ni: 10-15%, Mo: 0.5-2.5%, Al: 1.0 -3.0%, Si: 1.0% or less, Mn: 1.0% or less, with the balance being Fe and inevitable impurities.

  ADVANTAGE OF THE INVENTION According to this invention, the precipitation hardening type martensitic stainless steel provided with the stability of the structure | tissue, mechanical characteristics, and corrosion resistance 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 mimetic diagram showing an example of a low-pressure stage rotor concerning the present invention. It is a mimetic diagram showing an example of a low-pressure stage turbine concerning the present invention. It is a schematic diagram which shows an example of the power plant which concerns on this invention.

  Hereinafter, the role of the component elements contained in the precipitation hardening martensitic stainless steel according to the present invention and the regulation of the addition amount will be described.

  In the following description, the amount of component elements added is expressed in mass ratio (%).

  Carbon (C) forms chromium carbide, and there are problems such as a decrease in toughness due to excessive precipitation of carbide and a decrease in corrosion resistance due to a decrease in Cr concentration near the grain boundary. Further, C significantly lowers the martensitic transformation end temperature point. For this reason, the amount of C needs to be suppressed and is preferably 0.1% or less, more preferably 0.05% or less.

  Nitrogen (N) forms TiN and AlN to reduce fatigue strength and adversely affects toughness. For this reason, the amount of N must be suppressed, and is preferably 0.1% or less, more preferably 0.05% or less.

Chromium (Cr) is an element that contributes to improving corrosion resistance by forming a passive film on the surface.
By setting the lower limit of addition to 10.0%, sufficient corrosion resistance can be secured. On the other hand, when Cr is added excessively, a harmful phase is precipitated and the mechanical properties are remarkably deteriorated, so the upper limit was made 15.0%. From the above, the amount of Cr needs to be 10.0 to 15.0%. 11.0 to 14.0% is desirable, and 12.0 to 13.0% is particularly preferable.

  Nickel (Ni) is an element that suppresses the formation of δ ferrite and contributes to the improvement of strength by precipitation hardening of the Ni—Al compound. In addition, hardenability and toughness are improved. In order to obtain the above effect sufficiently, the lower limit of addition needs to be 10.0%. On the other hand, if the addition amount exceeds 15.0%, a harmful phase precipitates and the target mechanical properties cannot be obtained. From the above points, the amount of Ni needs to be 10.0 to 15.0%. 11.0 to 14.0% is more preferable, and 12.0 to 13.0% is more preferable.

  Molybdenum (Mo) is an element that improves corrosion resistance. In order to obtain the target corrosion resistance, addition of at least 0.5% is necessary. On the other hand, if the addition amount exceeds 2.5%, formation of a harmful phase is promoted and the characteristics are deteriorated. From the above points, the amount of Mo needs to be 0.5 to 2.5%. 1.0 to 2.0% is more preferable, and 1.25 to 1.75% is particularly preferable.

  Aluminum (Al) is an element that forms a Ni-Al compound and contributes to precipitation hardening. In order to fully develop precipitation hardening, it is necessary to add at least 1.0%. When the addition amount exceeds 3.0%, excessive mechanical precipitation of Ni-Al compounds and deterioration of mechanical properties due to formation of harmful phases are caused. From the above points, the amount of Al needs to be 1.0 to 3.0%. 1.5 to 2.5% is more preferable, and 1.75 to 2.25% is particularly preferable.

  Silicon (Si) is a deoxidizer and is preferably 1.0% or less. This is because if it exceeds 1.0%, precipitation of δ ferrite becomes a problem. 0.5% or less is more desirable, and 0.25% or less is particularly preferred. If the carbon vacuum deoxidation method and the electroslag melting method are applied, the addition of Si can be omitted. In that case, it is preferable not to add Si.

Manganese (Mn) is added as a deoxidizing agent and a desulfurizing agent, but if it exceeds 1.0%, a harmful phase is excessively generated and the required strength cannot be obtained. When melting by vacuum induction melting (VIM) or vacuum arc remelting (VAR), it is not necessary to add Mn, more preferably 0.5% or less, and particularly preferably 0.25% or less.
As another element, tungsten (W) has the effect of improving the corrosion resistance like Mo. W can further enhance this effect by the combined addition with Mo. In this case, the total amount of addition of Mo and W needs to be the same as the addition of Mo alone to prevent the precipitation of harmful phases.
Niobium (Nb) forms carbides and contributes to strength improvement, but deteriorates manufacturability.
For this reason, when Nb is added, the amount of Nb added must be 1.0% or less. Nb can be replaced with vanadium (V). When Nb and V are added together, the total amount of addition must be the same as the addition of Nb alone. Although addition of these elements is not essential, precipitation hardening becomes more remarkable.

  The inevitable impurities in the present invention refer to components that are originally included in the raw materials or are included in the present invention due to being mixed in the manufacturing process, and are not intentionally added. Inevitable impurities include P, S, Sb, Sn, and As, and at least one of them is included in the present invention.

  Further, it is preferable to reduce P and S as much as possible because the toughness can be improved without impairing the tensile properties. P: 0.5% or less and S: 0.5% or less are preferable from the viewpoint of improving toughness. In particular, P: 0.1% or less and S: 0.1% or less are preferable.

  Toughness can be improved by reducing As, Sb, and Sn. For this reason, it is desirable to reduce the above elements as much as possible, and As: 0.1% or less, Sb: 0.1% or less, and Sn: 0.1% or less are preferable. In particular, As: 0.05% or less, Sb: 0.05% or less, and Sn: 0.05% or less are preferable.

  Next, the heat treatment of the present invention will be described.

  In the present invention, it is necessary to carry out a solution treatment for rapid cooling after heating and holding at 800 to 1050 ° C., preferably 850 to 1000 ° C. The solution treatment in the present invention refers to a heat treatment for dissolving a component such as Al or Ti involved in the formation of precipitates into the structure and obtaining a martensite structure at the same time. The martensite structure is a kind of steel matrix and has an excellent balance between strength and toughness. Subsequent to the solution treatment, it is necessary to perform an aging treatment of gradually cooling after heating at 450 to 650 ° C. The aging treatment in the present invention refers to a heat treatment for obtaining excellent strength by finely precipitating a Ni—Al compound or the like in the structure after the solution treatment.

  Further, when it is desired to reduce retained austenite, sub-zero treatment may be performed. The sub-zero treatment needs to be heated to room temperature in the atmosphere using an organic solvent such as dry ice and isopentane, holding at -70 ° C or lower for at least 4 hours.

  The application of the present invention to a steam turbine long blade will be described. Forming and bending work can be performed after aging treatment, but if these work is performed immediately after solution treatment in which Ni-Al compound etc. is not precipitated, high work efficiency is expected due to good workability. it can.

  The steam turbine long blade to which the present invention is applied can join a Co-based alloy stellite to the blade tip by TIG welding. This is a means for protecting the steam turbine long blades from erosion in which the blades are damaged by collision of condensed high-speed steam. Other means for attaching stellite include silver brazing, plasma transfer arc, and overlay welding using a laser. As another means for protecting the steam turbine blades from erosion, surface modification can be performed by a titanium nitride coating or the like. In addition, the blade tip surface is heated to the Ac3 transformation point or higher and cooled to room temperature by air cooling multiple times repeatedly to make it finer than the crystal grain size 6, and then the entire blade surface is subjected to aging treatment to make only the blade tip surface hard and resistant to heat. It can also be equipped with erosion. Since the present invention has a certain degree of erosion resistance, the above-described erosion countermeasure may be omitted if the erosion is not severe.

  FIG. 1 shows a steam turbine long blade (reference numeral 10) to which the present invention is applied. The long blades are the blade profile (symbol 1) that receives steam, the blade root (symbol 2) that implants the blade into the rotor, the stub (symbol 4) that is integrated with the adjacent wing by twisting, the continuous cover (symbol 5). This steam turbine long blade is an axial entry type whose blade root is an inverted Christmas tree shape. Further, a stellite plate is joined as an example of the erosion shield (reference numeral 3). Other means for attaching stellite include silver brazing, plasma transfer arc, and overlay welding using a laser. Surface modification can also be performed by titanium nitride coating or the like. In addition, since the present invention has a certain degree of erosion resistance, the erosion countermeasure described above may be omitted if the erosion is not severe.

  FIG. 2 shows a low-pressure stage rotor (reference numeral 20) to which the long blades of the present invention are applied. This low-pressure stage rotor has a double-flow structure, and the long blades are installed in a plurality of stages in the long blade implantation part (reference numeral 21) symmetrically. The long wing described above is installed in the final stage.

  FIG. 3 shows a low-pressure stage steam turbine (reference numeral 30) to which the low-pressure stage rotor of the present invention is applied. The steam turbine long blade (reference numeral 31) rotates by receiving the steam guided by the nozzle (reference numeral 32). The rotor is supported by a bearing (reference numeral 33).

  FIG. 4 shows a power plant (reference numeral 40) to which the low-pressure steam turbine of the present invention is applied. The high-temperature and high-pressure steam generated in the boiler (reference numeral 41) is reheated in the boiler after working in the high-pressure turbine (reference numeral 42). The reheated steam works in the intermediate pressure stage turbine (reference numeral 43) and then in the low pressure stage turbine (reference numeral 44). The work generated by the steam turbine is converted into electric power by a generator (reference numeral 45). The steam that exits the low-pressure turbine is guided to a condenser (reference numeral 46).

  Examples will be described below.

[Example 1]
(Sample preparation)
Samples were prepared in order to evaluate the relationship between the chemical composition of the precipitation hardening martensitic stainless steel according to the present invention, tensile strength, 0.02% yield strength, Charpy impact absorption energy, pitting potential, and microstructure. Table 1 shows the chemical composition of each sample.

First, the raw materials were melted using a high-frequency vacuum melting furnace (5.0 × 10 ⁻³ Pa or lower, 1600 ° C. or higher) so as to have the composition shown in Table 1. The obtained ingot was hot forged using a press forging machine and a hammer forging machine, and formed into a square material having a 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. Inventive alloys 1 to 13 were subjected to water quenching by immersion in room temperature water after holding at 925 ° C. for 1 hour as a solution heat treatment. Subsequently, as an aging heat treatment, air cooling was performed by keeping the solution at an arbitrary temperature of 450 to 650 ° C. for 2 hours and then taking it out into the air at room temperature.

Each sample obtained above was subjected to evaluation tests of tensile strength, Charpy impact absorption energy, pitting potential, and microstructure observation. The outline of each evaluation test will be described.
(Test method)
The tensile test was performed at room temperature in accordance with JIS Z 2241 by preparing test pieces (distance between grades 30 mm, outer diameter 6 mm) from each sample obtained above. The criteria for the determination of tensile strength and 0.02% proof stress were 1500 MPa or more and 1000 MPa or more as “pass”, and less than those values were “fail”. In addition, as for elongation and drawing, 10% or more and 30% or more were regarded as “pass”, and less than those values were regarded as “fail”.

  For the measurement of Charpy impact absorption energy, a test piece having a 2 mmV 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 Charpy impact absorption energy was 20 J or higher as “pass” and less than that value as “fail”.

For the evaluation of the pitting corrosion potential, a plate-shaped test piece (length 15 mm, width 15 mm, thickness 3 mm) is prepared from each sample obtained above, and the area of the measurement surface becomes 1.0 cm ² So that it was coated with an insulator. The test solution was evaluated under the conditions of a 3.0% NaCl solution, a solution temperature of 30 ° C., and a sweep rate of 20 mV / min.
The criterion of the pitting corrosion potential was 150 mV or more as “pass”, and less than that value was “fail”.

The criterion for determining the microstructure was “pass” if the precipitation amount of δ ferrite and retained austenite had a martensite structure with an area ratio of 1.0% or less and 10% or less, respectively. The others were “failed”. The measurement of the amount of δ ferrite precipitation was based on the point calculation method described in JIS G 0555. The amount of residual austenite deposited was measured by X-ray diffraction.
(Test results)
Inventive alloys 1 to 13 according to the present invention also passed the mechanical properties of tensile strength, 0.02% proof stress, elongation, squeezing and impact absorption energy. Furthermore, good results were obtained for the pitting corrosion potential. In addition, it was confirmed that the δ ferrite phase and retained austenite in the metal structure are within the target range and have a martensite structure.

  None of the comparative alloys 1 to 12 satisfied all the targets of each characteristic. In Comparative Alloys 1-8, the influence of main components such as Cr, Ni, Mo and Al was examined. Among them, Comparative Alloy 5 was a sample with a high amount of Al added and had high tensile strength and 0.02% yield strength. However, the elongation, squeezing and shock absorption energy were significantly below the target. This is probably because the precipitation amount of the strengthening phase was excessive. On the other hand, in Comparative Alloy 6, the amount of Al added was low, the tensile strength and the 0.02% proof stress were below the target, and a large amount of residual austenite was precipitated in the structure. Further, Comparative Alloys 9 to 12 examined the effect of impurity elements, but Comparative Alloy 9 was a sample with a high amount of C added, and the tensile strength, 0.02% proof stress, elongation, and impact absorption energy were below the target. The pitting potential was also below the target. This is presumably because the Cr concentration near the grain boundary decreased due to the formation of Cr carbide, and the corrosion resistance deteriorated. Further, a large amount of retained austenite was precipitated in the structure. Comparative alloy 12 was a sample with a high amount of N added, and the elongation, squeezing and impact absorption energy were significantly below the targets, and a large amount of residual austenite was precipitated in the structure.

[Example 2]
A steam turbine long blade using the present invention will be described. In this embodiment, an axial entry type steam turbine long blade having a blade length of 48 inches was manufactured using the alloy 1 shown in Table 1 as an invention material. As a method for producing the long blade, 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 of C + O → CO. Subsequently, the electrode rod was formed by forging. This electrode rod was immersed in molten slag and self-dissolved by Joule heat generated when an electric current was passed, and solidified in a water-cooled mold to remelt the electroslag to obtain a high-grade steel ingot. Next, after hot forging, die forging was performed with a 48-inch airfoil. After this, as a solution treatment, the mixture was heated and held at 980 ° C. for 2.0 hours and then forcedly cooled with a blower. Next, it was processed into a predetermined shape through a cutting process, and subsequently air-cooled after being heated and maintained at 525 ° C. for 4.0 hours as an aging treatment. As the final finishing process, the 48-inch long blade was made by bending and polishing the surface.

  Test pieces were collected from the tip, center, and root of the steam turbine long blade obtained by the above steps, and the same evaluation test as in Example 1 was performed. The direction of the collected specimen is the wing length direction.

  The microstructure of each part was a uniform martensite structure, and retained austenite and δ ferrite were not observed. In addition, the tensile strength, 0.02% proof stress, impact absorption energy, and pitting potential satisfied all targets regardless of the sampling position.

  The precipitation hardening type martensitic stainless steel of the present invention has excellent mechanical properties and corrosion resistance, and therefore can be applied to steam turbine long blades as well as blades for gas turbine compressors.

  1 ... Blade profile part, 2 ... Blade root part, 3 ... Erosion shield, 4 ... Stub, 5 ... Continuous cover, 10 ... Steam turbine long blade, 20 ... Integrated low-pressure turbine rotor, 21 ... Steam turbine long blade implantation 30: Integrated low-pressure turbine, 31 ... Steam turbine long blade, 32 ... Nozzle, 33 ... Bearing, 40 ... Power plant, 41 ... Boiler, 42 ... High-pressure turbine, 43 ... Medium-pressure turbine, 44 ... Low pressure Stage turbine, 45 ... generator, 46 ... condenser.

Claims (9)

  By mass ratio, C: 0.1% or less, N: 0.1% or less, Cr: 10-15%, Ni: 10-15%, Mo: 0.5-2.5%, Al: 1.0-3.0%, Si: 1.0% or less, Mn: precipitation hardened martensitic stainless steel containing 1.0% or less and the balance being Fe and inevitable impurities. The precipitation hardening martensitic stainless steel according to claim 1, further comprising at least one of Nb and V by 1.0% or less by mass. 3. The precipitation hardening martensitic stainless steel according to claim 1 or 2, further comprising W, wherein the total amount of Mo and W is the same as the addition of Mo alone.   4. The precipitation hardening martensitic stainless steel according to claim 1, wherein the inevitable impurity is at least one selected from P, S, Sb, Sn, and As.   5. The precipitation hardening martensitic stainless steel according to claim 1, wherein the temperature range of the solution treatment is 800 to 1050 ° C., and the temperature range of the aging treatment is 450 to 650 ° C. 6.   A steam turbine long blade using the precipitation hardened martensitic stainless steel according to any one of claims 1 to 5.   The steam turbine long blade according to claim 6, wherein a Co-based alloy stellite is joined to the blade tip.   A turbine rotor comprising the steam turbine long blade according to claim 7.   A steam turbine comprising the turbine rotor according to claim 8.