JP3962743B2 - Precipitation hardening type martensitic steel, method for producing the same, turbine rotor blade and steam turbine using the same - Google Patents

Description

  The present invention relates to a precipitation hardening martensitic steel having high strength, high toughness, and good delayed cracking resistance, a method for producing the same, and a turbine rotor blade and a steam turbine using the same.

  In order to improve the thermal efficiency of the steam turbine, it is advantageous to increase the blade length of the low-pressure final stage blade. In order to lengthen the turbine blade, a blade material with high specific strength is required. However, in the 3600 rpm steam turbine, at present, the steel blade has a limit of 40 inches, and titanium alloy is used for the 45 inches.

  As a high-strength steel material for turbine blades, Japanese Patent Application Laid-Open No. 2001-98349 discloses that 0.13 to 0.40% C, 0.5% or less Si, and 1.5% or less by weight. Mn, 2 to 3.5% Ni, 8 to 13% Cr, 1.5 to 4% Mo, 0.02 to 0.3% Nb and Ta in total, and 0.05 A martensitic steel having a composition consisting of ~ 0.35% V, 0.04 ~ 0.15% N and the balance Fe is described.

  A number of techniques have been published as high-strength, high-toughness, high-corrosion-resistant precipitation-hardening stainless steels. Among them, the martensite single-phase one has a weight ratio of 0 in Table 1 of Japanese Patent No. 3251648. 0.08% or less of C, 0.7 to 2.5% of Si, 3.0% or less of Mn, 6.0 to 7.2% of Ni, and 10.0 to 17.0% of Cr, 0.5-2.0% Cu, 0.5-3.0% Mo, 0.15-0.45% Ti, 0.015% or less N, A precipitation hardening martensitic steel having a composition comprising S of 003% or less and the balance Fe is described.

JP 2001-98349 A Japanese Patent No. 3251648 (Table 1)

  As a high strength steel material for turbine blades, a blade length of 45 inches for a 3600 rpm steam turbine has a tensile strength of 1350 MPa or more, and as a blade material of a blade length of 50 inches, a tensile strength of 1500 MPa or more. It is required to satisfy strength, room temperature Charpy absorption energy of 20 J or more, high toughness, and good delayed crack resistance (SCC) characteristics. However, in the tempered martensitic steel whose strength is adjusted by quenching and tempering as described in JP-A-2001-98349, delayed cracking may occur when the tensile strength is increased to 1350 MPa or more as described later. There is. On the other hand, although precipitation hardening type martensitic steel has high strength, high toughness, and high corrosion resistance, the conventional Cu, Nb and Ti precipitates alone are insufficient in strength for the required value of tensile strength of 1500 MPa or more. . The technology described in Japanese Patent No. 3251648 achieves high strength and high toughness by refining the crystal grain size, but it is difficult to obtain a fine grain structure in a thick part such as a turbine blade root, There is a problem that strength and toughness are insufficient.

  In view of the above problems, the present invention is a precipitation hardening martensite having a high tensile strength of 1350 MPa or more, a toughness of Charpy absorbed energy at room temperature of 20 J or more, and satisfying good delayed cracking resistance. It is an object of the present invention to provide a site steel, a manufacturing method thereof, and a turbine rotor blade and a steam turbine using the same.

  In order to achieve the above object, the precipitation hardening type martensitic steel according to the present invention has a weight ratio of 12.25 to 14.25% Cr, 7.5 to 8.5% Ni, 1 0.0-2.5% Mo, 0.05% or less C, 0.2% or less Si, 0.4% or less Mn, 0.03% or less P, 0.005% % Of S, 0.008% or less of N, 0.90 to 2.25% of Al, the balance being substantially Fe, and the sum of Cr and Mo is 14.25 to 16.75. %.

The precipitation hardening martensitic steel according to the present invention is
Cr equivalent = [Cr] +2 [Si] +1.5 [Mo] +5.5 [Al] +1.75 [Nb] +1.5 [Ti]
Ni equivalent = [Ni] +30 [C] +0.5 [Mn] +25 [N] +0.3 [Cu]
Preferably, the Cr equivalent is less than 28.0 and the Ni equivalent is less than 10.5. In the above formula, the unit in parentheses is% by weight. By adjusting the Cr equivalent and the Ni equivalent in this way, precipitation of the δ ferrite phase and the residual austenite phase in the alloy structure can be reliably prevented, and high toughness and good hot forgeability can be ensured.

  The total of Cr and Mo is more preferably 15.5 to 16.75%. By limiting the total amount of Cr and Mo in this way, the tensile strength is 1500 MPa or higher, the strength is high, the room temperature Charpy absorbed energy is 20 J or higher, and the toughness is satisfied, and satisfactory delayed cracking resistance is satisfied. Precipitation hardening type martensitic steel can be provided.

  Further, the Al content is more preferably more than 1.35% and not more than 2.25%. In this way, by limiting the amount of Al in a high range, the tensile strength is high strength of 1500 MPa or more, the Charpy absorbed energy at room temperature is 20 J or more, high toughness, and satisfactory delayed cracking characteristics are satisfied. Hardened martensitic steel can be provided. Further, by limiting the amount of Al in a high range as described above, a tensile strength of 1500 MPa or more can be achieved even at a high aging temperature of 550 ° C. or higher, and since the aging temperature is high, the total amount of Cr and Mo is 15.5. Good delayed cracking characteristics can be satisfied even in the low range of% or less.

  Another aspect of the present invention is a method for producing a precipitation hardening type martensitic steel, wherein the weight ratio is 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, 005% or less S, 0.008% or less N, 0.90 to 1.35% Al, and the balance substantially consists of Fe, and the total of Cr and Mo is 14.25 to 16. A 75% steel slab is subjected to solution treatment at 910 ° C to 940 ° C, and then subjected to aging treatment at 510 ° C to 550 ° C. In this case, the total of Cr and Mo is preferably 15.5 to 16.75%.

  Moreover, the manufacturing method of the precipitation hardening type martensitic steel which concerns on this invention, as another form, by weight ratio, 12.25-14.25% Cr, 7.5-8.5% 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, S of 005% or less, N of 0.008% or less, Al of more than 1.35% and 2.25% or less, and the balance substantially consist of Fe, and the total of Cr and Mo is 14. A steel piece of 25 to 16.75% is subjected to a solution treatment at 910 ° C. to 940 ° C. and then subjected to an aging treatment at 550 ° C. to 600 ° C. In any of the above forms, the method for producing precipitation hardening martensitic steel according to the present invention preferably has the Cr equivalent of less than 28.0 and the Ni equivalent of less than 10.5.

  Another aspect of the present invention is a turbine rotor blade using the martensitic steel described above. As a result, the blade length of 45 inch class long blades (for 3600 rpm steam turbine), which conventionally used a titanium alloy, can be made of steel, and the cost can be reduced.

  The present invention, as yet another aspect, is a steam turbine comprising the turbine rotor blade using the martensite steel and a rotor using 9-12Cr steel at least for the long blade planting portion. Features. By using 9-12Cr steel for the long blade planting portion in this way, the SCC strength of the blade groove can be improved, and a low-cost and highly reliable steam turbine can be provided.

  Thus, according to the present invention, precipitation-hardened martensitic steel having a high tensile strength of 1350 MPa or more, a high toughness of Charpy absorbed energy at room temperature of 20 J or more, and satisfying good delayed cracking resistance properties. And a manufacturing method thereof, and a turbine rotor blade and a steam turbine using the same.

  Below, the component contained in the precipitation hardening type martensitic steel which concerns on this invention, and its content are demonstrated. In the following description, unless otherwise specified,% indicating the content is a weight ratio.

  Chromium (Cr) needs to contain at least 12.25% Cr in order to obtain excellent corrosion resistance and excellent delayed cracking resistance (SCC) characteristics. On the other hand, if the Cr content exceeds 14.5%, a large amount of δ ferrite phase precipitates, which causes deterioration of mechanical properties such as tensile strength and toughness. Therefore, the upper limit of the Cr content is preferably 14.25% for safety. For these reasons, the Cr content is set in the range of 12.25 to 14.25%.

  Nickel (Ni) is an indispensable element that suppresses precipitation of the δ ferrite phase and contributes to precipitation hardening by forming an intermetallic compound with aluminum (Al). In the present invention, in order to obtain high strength and high toughness characteristics, it is necessary to contain at least 7.5% Ni. On the other hand, if the Ni content exceeds 8.5%, a residual austenite phase is generated, and the required strength cannot be obtained. Therefore, the Ni content is set in the range of 7.5 to 8.5%.

  Molybdenum (Mo) is an alloy element effective in improving corrosion resistance and delayed cracking resistance (SCC) characteristics together with chromium (Cr), and in order to obtain the effect, at least 1.0% of Mo is contained. There is a need. On the other hand, if the Mo content exceeds 2.5%, precipitation of the δ ferrite phase is promoted, which causes a decrease in toughness. Therefore, the Mo content is set in the range of 1.0 to 2.5%.

  Further, it was found that the sum of Cr and Mo has a good correlation with the limit tensile strength (delayed crack initiation limit strength) at which cracks are generated by the delayed crack test. Therefore, in order to develop excellent delayed cracking resistance (SCC) characteristics with a tensile strength of 1350 MPa or more, the total amount of Cr and Mo was set in the range of 14.25 to 16.25%. Furthermore, the total amount of Cr and Mo is more preferably limited to a range of 15.5 to 16.25% in order to develop excellent SCC resistance even at a tensile strength of 1500 MPa or more. As will be described later, when the Al content is high and the tensile strength is 1500 MPa or more even when the aging temperature is 550 ° C. or higher, the total amount of Cr and Mo may be as low as less than 15.5%. Excellent delayed cracking characteristics can be satisfied.

In view of high strength, high toughness, and excellent mechanical properties, the precipitation of the δ ferrite phase is preferably within 1% by volume fraction. Precipitation of the δ ferrite phase can be avoided by setting the Cr equivalent to less than 28.0. Even if precipitation of the δ ferrite phase is avoided, the desired strength cannot be obtained if the retained austenite phase is precipitated. Precipitation of the retained austenite phase can be avoided by making the Ni equivalent less than 10.5. That is, by setting the Cr equivalent to less than 28.0 and the Ni equivalent to less than 10.5, precipitation of both the δ ferrite phase and the retained austenite phase can be avoided. In addition, Cr equivalent and Ni equivalent are represented by the following formula | equation.
Cr equivalent = [Cr] +2 [Si] +1.5 [Mo] +5.5 [Al] +1.75 [Nb] +1.5 [Ti]
Ni equivalent = [Ni] +30 [C] +0.5 [Mn] +25 [N] +0.3 [Cu]

  Carbon (C) is an element effective in suppressing the δ ferrite phase, but when the C content increases, a residual austenite phase is generated, and a martensite single phase structure is not formed by cooling after solution treatment. A sufficient strength cannot be obtained. Moreover, the precipitation of carbides adversely affects the corrosion resistance. Therefore, the upper limit of the C content is set to 0.05%. More preferably, it is 0.01 to 0.05%.

  Aluminum (Al) contributes to precipitation hardening by forming an intermetallic compound with nickel (Ni), and is an indispensable important element in the present invention. In order to exhibit effective precipitation hardening ability, it is necessary to contain at least 0.90% Al. On the other hand, if the Al content exceeds 2.25%, excessive precipitation or the formation of δ ferrite phase causes a significant decrease in toughness and hot forgeability. Therefore, the Al content is set in the range of 0.90 to 2.25%.

  It was also found that the tensile strength was improved as the Al content was increased. In particular, by limiting the Al content to a high range of higher than 1.35% and not higher than 2.25%, even at a high aging temperature condition of 550 ° C. or higher, which is sufficiently over-aged, a tensile strength of 1500 MPa or more, Charpy High strength and high toughness with an absorption energy of 20 J or more can be achieved. In addition, by limiting the Al content to the above high range and limiting the aging temperature to a high range of 550 ° C. or higher, the total amount of Cr and Mo is limited to a low range of less than 15.5%. The occurrence of delayed cracking can be suppressed. Furthermore, by limiting the contents of Cr and Mo to the above low range, precipitation of the δ ferrite phase can be suppressed, and the phase stability of the large steel ingot can be improved.

  Manganese (Mn) is an element effective in suppressing the formation of the δ ferrite phase. However, when the Mn content is increased, a retained austenite phase is generated, and sufficient strength cannot be obtained. Therefore, the upper limit of the Mn content is set to 0.4% as the limit content that can be manufactured by the atmospheric dissolution method and can satisfy the targets of strength and toughness. According to the vacuum induction melting method, the vacuum arc remelting method, the electroslag remelting method, etc., it is not always necessary to add Mn, and the Mn content is 0.1% or less, more preferably 0.05% or less. can do.

  Silicon (Si) is an element effective as a deoxidizer for molten steel. However, when the Si content increases, the formation of the δ ferrite phase is promoted, and the strength and toughness are reduced. Therefore, the upper limit of the Si content is set to 0.2% as the limit content that can be manufactured by the atmospheric dissolution method and can satisfy the targets of strength and toughness. According to the vacuum induction melting method, the vacuum arc remelting method, the electroslag remelting method, etc., it is not always necessary to add Si, and the Si content is 0.1% or less, more preferably 0.05% or less. can do.

  Phosphorus (P) does not contribute to the improvement of strength, but adversely affects toughness. Therefore, from the viewpoint of securing toughness, it is desirable to reduce the P content as much as possible, and the upper limit is set to 0.03% as the limit content that can be manufactured by the atmospheric dissolution method and can satisfy the goals of strength and toughness. It was. A more preferable range is 0.005% or less. In this case, it is preferable that Si is 0.1% or less and Mn is 0.1% or less.

  Sulfur (S) is present in steel as non-metallic inclusions such as MnS, and adversely affects fatigue strength, toughness, corrosion resistance, and the like. Therefore, it is desirable to reduce the S content as much as possible, and the upper limit is made 0.005%.

  Nitrogen (N) is an element effective in suppressing the δ ferrite phase, but when the N content increases, the strength becomes insufficient due to the formation of residual austenite phase. Furthermore, the increase in N content has an adverse effect on toughness, similar to phosphorus (P). Therefore, the upper limit of the N content is set to 0.008%.

  Niobium (Nb) and tantalum (Ta) can also be added as other elements. Nb and Ta form carbides and are effective in improving the strength, but deteriorate the toughness and hot forgeability. Therefore, when adding these, it is good to make the upper limit of the total content of Nb and Ta 0.01%. Further, the balance of the steel of the present invention is basically Fe, but includes impurities inevitably mixed.

  Next, the heat treatment of the precipitation hardening type martensitic steel according to the present invention will be described. Precipitation hardening type martensitic steel having the chemical composition defined above is first melted and forged into a predetermined shape, preferably heated to a temperature of 910 to 940 ° C., and then cooled with water or forced air. Perform solution treatment. By setting the solution temperature to 910 ° C. or higher, precipitates are solidified, and undissolved precipitates are reduced to ensure toughness. Moreover, by setting the solution temperature to 940 ° C. or less, a fine structure can be obtained by suppressing coarsening of crystal grains, and good toughness can be obtained. The heating time is not particularly limited but is preferably 0.5 to 3 hours.

  After the solution treatment, when the Al content is up to 1.35%, the solution is preferably heated to a temperature of 510 to 550 ° C. and then air-cooled to perform an aging treatment. By setting the heating temperature of the aging treatment to 510 ° C. or higher, the Charpy absorbed energy at room temperature of the obtained steel can be set to 20 J or higher, and high toughness can be obtained. Moreover, by setting the temperature of the aging treatment to 550 ° C. or less, the tensile strength at room temperature of the obtained steel can be set to 1350 MPa or more, and high strength can be obtained. In particular, by setting the temperature of the aging treatment to 530 ° C. or less, the tensile strength can be set to 1500 MPa or more, and higher strength can be obtained. The heating time for the aging treatment is not particularly limited, but is preferably 3 to 5 hours.

  Moreover, when Al content exceeds 1.35%, after solution treatment, after heating to the temperature of preferably 550-600 degreeC, it cools by air and an aging treatment is performed. By setting the heating temperature of the aging treatment to 550 to 600 ° C., the obtained steel can have a tensile strength at room temperature of 1350 MPa or more and a Charpy absorbed energy of 20 J or more, and can achieve high strength and high toughness. it can. In particular, by limiting the temperature of the aging treatment to 550 to 580 ° C., the tensile strength can be increased to 1500 MPa or more, and higher strength can be obtained. The heating time for the aging treatment is not particularly limited, but is preferably 3 to 5 hours.

  Next, a turbine blade using the precipitation hardening type martensitic steel according to the present invention will be described. FIG. 1 shows an example of a 45-inch blade for a 3600 rpm steam turbine. As shown in FIG. 1, the long blade 1 has a serrated blade root 4, and the blade root 4 is implanted in a rotor (not shown) in a side entry manner. A plurality of long blades 1 are radially planted on the outer periphery of the rotor, and the long blades 1 adjacent to each other on the left and right are combined via the shroud 2 and the stub 3 to form an annular turbine blade row.

  The erosion shield 5 is for preventing erosion caused by water droplets, and usually a Co-based alloy stellite plate is brazed. Moreover, the hardened layer by the surface hardening using a laser or a high frequency can also be used. Since the precipitation hardening type martensitic steel according to the present invention has a hardness of about 450 Hv, the erosion shield 5 can be omitted in a mild environment. Although FIG. 1 shows an integral shroud rotor blade in which the shroud 2 is integrally formed with the blade, the present invention is not limited to this and can be applied to a conventional blade.

  As described above, by employing the precipitation hardening type martensitic steel according to the present invention for the turbine rotor blade, the conventional 45 inch class titanium blade can be replaced with the steel blade, and a significant cost reduction can be achieved. The turbine rotor blade according to the present invention is not limited to a blade length of 45 inch class long blade (for 3600 rpm steam turbine). As described above, by changing the chemical composition and heat treatment conditions, for example, Al When the content is up to 1.35%, the total amount of Cr and Mo is 15.5 to 16.75%, the aging conditions are set to 510 to 530 ° C., and the Al content exceeds 1.35% By setting the aging condition to 550 to 580 ° C., a tensile strength of 1500 MPa or more can be obtained, so it is also possible to make a blade length of 50 inch class long blade (for 3600 rpm steam turbine). Moreover, it can respond | correspond to various long blades, for example, can respond to the 54-inch class and 60-inch class long blades for 3000 rpm steam turbines, and can also respond to the long blades for 1500/1800 rpm steam turbines.

  Next, the steam turbine provided with the turbine rotor blade according to the present invention will be described. As described above, the precipitation hardened martensitic steel according to the present invention is used for turbine blades, so that it is possible to cope with 45 inch and 50 inch class long blades for 3600 rpm steam turbines. However, since the centrifugal stress of the rotor blade groove increases as the blade weight increases from the titanium alloy blade, the conventional low alloy steel rotor material has insufficient SCC strength of the blade groove. Therefore, by adopting high strength 9-12Cr steel having higher SCC strength than low alloy steel as a rotor material and combining it with the long blades using the martensitic steel of the present invention, the steel length of 45 inches and 50 inches is made. A steam turbine with blades can be provided.

As the high-strength 9-12Cr steel, for example, 9-12Cr steel described in JP-A-11-131190 and JP2948324 can be employed. In particular, 0.05 to 0.2% C, 2.5% or less Ni, 8 to 11% Cr, 0.3 to 2% Mo, 0.1 to 0.3% V,. It contains 01 to 0.08% N and 0.02 to 0.15% Nb, and the balance consists of Fe and inevitable impurities, Si among the inevitable impurities is 0.1% or less, and Mn is 0. .3% or less, P is 0.015% or less, S is 0.008% or less, high strength heat resistant steel, 0.08 to 0.25% C, 0.10% or less Si, 0.10 % Mn, 0.05 to 1.0% Ni, 10.0 to 12.5% Cr, 0.6 to 1.9% Mo, 1.0 to 1.95% W, 0 .10 to 0.35% V, 0.02 to 0.10% Nb, 0.01 to 0.08% N, 0.001 to 0.01% B, 2.0 to 8.0 % Co with the balance being substantially F And Cr equivalent (Cr equivalent = Cr + 6Si + 4Mo + 1.5W + 11V + 5Nb-40C-2Mn-4Ni-2Co-30N) determined by the following formula of a heat-resistant steel consisting of a tempered martensite base is 7.5% or less, and (B + 0 .5N) is equivalent to 0.030% or less, Nb equivalent expressed as (Nb + 0.4C) is 0.12% or less, and Mo equivalent expressed as (Mo + 0.5W) is 1.40 to 2.45%, and among impurity elements, S is suppressed to 0.01% or less and P is suppressed to 0.03% or less, and mainly M ₂₃ C ₆ type carbides and intermetallic compounds are used. Precipitates at grain boundaries and martensitic lath boundaries, and MX type carbonitrides are precipitated inside martensitic laths, and the total amount of these deposited precipitates is 1.8 to 4.5%. High strength and high toughness heat-resistant steel, which is formed from heat steel is preferred.

  FIG. 2 shows an example of a steam turbine according to the present invention. FIG. 2 is a cross-sectional view of an integrated low pressure turbine rotor constructed from a single rotor material. As shown in FIG. 2, since the integrated rotor 20 is composed of all 9-12Cr rotor material 23 including the long blade planting portion 21, the SCC strength of the blade groove is made of steel of the present invention 45. It is possible to improve the strength to withstand inch and 50 inch class long blades. However, since the 9 to 12 Cr rotor material is more expensive than the low alloy steel rotor material, there is a possibility that the cost merit of using the steel long blade instead of the titanium long blade may be reduced. Therefore, 9-12Cr steel is used for the blade groove of only the final stage of the turbine in which 45-inch and 50-inch blades for a 3600 rpm turbine and 54-inch and 60-inch blades for a 3000 rpm turbine are implanted, and the other parts are made of conventional low-power. By using alloy steel, cost can be reduced. An example is shown in FIGS.

  FIG. 3 is a cross-sectional view of a welded low-pressure turbine rotor in which a portion including the long blade planting portion and other portions are welded. As shown in FIG. 3, the welded rotor 30 includes a rotor both ends including a long blade planting portion 31 configured by a 9-12Cr steel rotor material 33 and a rotor central portion configured by a low alloy steel rotor material 75. These are welded and joined at the welded portion 37. By adopting such a configuration, the 9-12Cr steel rotor material 33 is used only for about half of the entire welded rotor 30, so that the cost can be reduced.

  FIG. 4 is a cross-sectional view of a shrink-fit low-pressure turbine rotor in which a portion including the long blade planting portion and other portions are shrink-fit. As shown in FIG. 4, the shrink-fit rotor 40 is obtained by shrink-fitting a 9 to 12 Cr steel disk 45 in which the long blade planting portion 41 is integrally formed with a rotor body made of a low alloy steel rotor material 43. Are joined. With such a configuration, 9-12Cr steel is used for only a part of the entire shrink-fit type rotor 40, so that the cost can be reduced more remarkably.

  Hereinafter, the present invention will be described based on examples. Table 1 shows the chemical composition (% by weight) of the high-strength steel according to the steam turbine long blade material, and the balance consists of Fe and inevitable impurities. Each sample was vacuum-melted by 50 kg and then hot forged into square bars or round bars and subjected to the following heat treatment.

  Samples 1 and 2 are 12Cr steels with high strength and high toughness, which were oil-quenched after heating at 1100 ° C. for 2 hours, tempered by heating at any temperature in the range of 400 to 650 ° C. for 3.5 hours and air-cooling. . Sample 3 is 17-4PH steel, which is a long blade material for current use, and it was air-cooled and quenched after heating at 1038 ° C. × 1 h, and was subjected to aging treatment by heating at an arbitrary temperature in the range of 450 to 650 ° C. × 3 h. . Sample 4 was a commercial steel called maraging steel, which was 820 ° C. × 2 h heated and air cooled and quenched, then heated at an arbitrary temperature in the range of 410 to 550 ° C. × 5 h and then air cooled to perform aging treatment. Samples 5 to 11 were steels of the present invention, which were 925 ° C. × 1 h heated and air cooled and quenched, and then heated at an arbitrary temperature in the range of 450 to 620 ° C. × 4 h and then air cooled to perform aging treatment. Samples 12 and 13 were comparative steels of the present invention, which were 925 ° C. × 1 h heated, air cooled and quenched, 550 ° C. × 4 h heated, air cooled and aged.

  About these samples 1-13, the tension test and the Charpy impact test were done at room temperature (20 degreeC). The results are shown in FIGS. FIG. 5 is a graph showing changes in tensile strength with respect to the aging / tempering temperature of each sample. FIG. 6 is a graph showing the change in Charpy absorbed energy with respect to the aging / tempering temperature of each sample. As shown in FIGS. 5 and 6, samples 5 and 6 of the steels of the present invention have tensile strength, which is a characteristic required for blades having a blade length of 45 inches (for 3600 rpm steam turbines) due to aging at about 550 ° C. 1350 MPa or more and Charpy absorbed energy of 20 J or more were satisfied. Moreover, due to aging at about 510 ° C., a tensile strength of 1500 MPa or more and a Charpy absorbed energy of 20 J or more, which are characteristics required for a blade material of a blade length of 50 inches (for 3600 rpm steam turbine), were satisfied.

  Moreover, as shown in FIGS. 5 and 6, Sample 7 of the steel of the present invention with an Al content of 1.36% satisfied a tensile strength of 1500 MPa or more and a Charpy absorbed energy of 20 J or more by aging at about 550 ° C. . As shown in FIGS. 5 and 6, Sample 10 of the steel of the present invention with an Al content of 2.13% satisfied a tensile strength of 1500 MPa or more and a Charpy absorbed energy of 20 J or more by aging at about 580 ° C. In order to compare the results when the amount of Al is increased, graphs summarizing Sample 5 and Samples 7 to 11 are shown in FIGS.

  As shown in FIG. 7, at an aging temperature of 550 ° C., the tensile strength improved as the amount of Al increased. From the viewpoint of delayed cracking, it is desirable that the aging temperature is as high as possible and the overaging condition is desirable. When the aging temperature is sufficiently overaged at 550 ° C. and the Al content is higher than 1.35%, it is 1500 MPa or more. It was found that tensile strength can be obtained. Further, as shown in FIG. 8, the Charpy absorbed energy tends to decrease as the tensile strength is improved, but the Charpy absorbed energy of 20 J or more is satisfied even at the tensile strengths of 1350 MPa and 1500 MPa. It was found that a Charpy absorbed energy of 20 J or more can be obtained if the tensile strength is 1580 MPa or less.

  Further, as shown in FIGS. 5 and 6, the sample 11 in which the Mn amount is 0.39%, the Si amount is 0.19%, and the P amount is 0.024%, each amount of Mn, Si, and P is The same excellent tensile strength and Charpy absorbed energy as those of the reduced inventive steel samples could be obtained.

  On the other hand, Sample 12, which is a comparative steel having a Cr equivalent of more than 28.0, had a very low tensile strength of about 800 MPa because a large amount of δ ferrite phase was precipitated, and did not satisfy the required strength characteristics. Further, in the sample 13 with a Ni equivalent of more than 10.5, which is a comparative steel, a retained austenite phase was generated, and therefore, Charpy absorbed energy was lower than 20 J under aging conditions of 550 ° C. at which the tensile strength was 1350 MPa, and necessary toughness was obtained I couldn't.

  Sample 1 of high strength 12Cr steel, which is a steel type different from the steel of the present invention, has a target value of 1350 MPa class with a tensile strength of 1350 MPa or more and Charpy absorbed energy of 20 J or more by tempering at about 500 ° C. or less. Although satisfied, it did not reach the tensile strength of the 1500 MPa class. Sample 2 had a tensile strength of 1350 MPa or less when tempered above about 500 ° C., and Charpy absorbed energy was 20 J or less when tempered at about 500 ° C. or less, and did not reach the target value of the 1350 MPa class.

  Sample 3 of 17-4PH steel, which is the same precipitation hardening stainless steel as the steel of the present invention, satisfied the target value of 1350 MPa class with a tensile strength of 1350 MPa or more and Charpy absorbed energy of 20 J or more by aging at about 480 ° C. However, the tensile strength of 1500 MPa class was not reached. Sample 4, which is a commercially available maraging steel, achieved a target value of 1350 MPa class and a target value of 1500 MPa class for tensile strength and Charpy absorbed energy by aging at about 480 to 550 ° C.

  Next, a delayed cracking test was performed on samples 5 to 10 of the present invention steel and samples 1 to 4 of other steel types. The delayed crack test evaluated whether or not a crack occurred in a sample piece after 500 hours in water at a temperature of 80 ° C. and a dissolved oxygen concentration of 8.0 ppm. The results are shown in FIGS. FIG. 9 is a graph showing the change in the limit tensile strength (delayed crack initiation limit strength) at which cracks are generated in the delayed crack test with respect to the Cr + Mo amounts of Samples 1-6. FIG. 10 is a graph showing changes in the delayed crack initiation limit strength with respect to the Al amount of Sample 5 and Samples 7-10. 9 and 10, black marks indicate the occurrence of cracks, and white marks indicate that no cracks have occurred.

  As shown in FIG. 9, in the other steel types Sample 1, Sample 2, and Sample 4, delayed cracking occurred at a strength level of 1350 MPa class or higher. On the other hand, delayed cracking did not occur in sample 1 of another steel type and sample 5 of the steel of the present invention even in the 1350 MPa class. However, delayed cracking occurred when a strength of 1500 MPa class was required. Sample 6 of the steel of the present invention showed no delayed cracking at the target 1350 MPa class, and further, no delayed cracking occurred at the 1500 MPa class. Thus, delayed cracking is more likely to occur as the tensile strength is higher, and the delayed crack initiation limit strength generally has a good correlation with the Cr + Mo amount, which is a parameter indicating corrosion resistance. Therefore, the higher the Cr + Mo amount, the more delayed cracking occurs. It turns out that it is hard to generate. That is, from FIG. 9, by making the Cr + Mo amount 15.5% by weight or more, the occurrence of delayed cracking can be prevented even in the 1500 MPa class.

  Further, from FIG. 10 comparing the delayed cracking characteristics when the Al amount is increased, the inventive steel samples 7 to 10 having an Al amount exceeding 1.35% are all delayed cracking at a tensile strength of 1500 MPa class. Did not occur. As shown in Table 1, although the Cr + Mo content of these samples was as low as about 14.5%, the occurrence of delayed cracking could be prevented even at the 1500 MPa class. That is, when the Al content is high and the aging temperature is 550 ° C. or more in the overaging region and the tensile strength is 1500 MPa or more, the total amount of Cr and Mo is less than 15.5%, which is good even in the low range. Delayed cracking characteristics can be satisfied.

  Furthermore, the detection test of (delta) ferrite phase precipitation and residual austenite phase precipitation was done about sample 3 of other steel types, samples 5-11 of this invention steel, and samples 12 and 13 of comparative steel. In the detection test, the structure of the sample after the heat treatment was observed with an optical microscope. As a result, although the precipitation of δ ferrite phase exceeding 1% was detected in Sample 12, the precipitation amount of δ ferrite phase in other samples was as small as 1% or less. Further, although precipitation of residual austenite phase was detected in Sample 13, precipitation of residual austenite phase was not detected in other samples. For Sample 3 and Samples 5 to 13, the influence of the Cr equivalent and Ni equivalent on the structure (Schaeffler state diagram) is shown in FIG. As shown in FIG. 11, when the Cr equivalent is less than 28.0 and the Ni equivalent is less than 10.5, precipitation of both the δ ferrite phase and the retained austenite phase can be avoided.

  Table 2 summarizes the above results. As shown in Table 2, Samples 5 to 11 which are steels of the present invention satisfied all target values of 1350 MPa class. In addition, the sample 6 which made Cr + Mo amount 15.5 wt% or more also satisfied the 1500 MPa class target value. Samples 7 to 10 having an Al content higher than 1.35 wt% also satisfied the target value of 1500 MPa class.

  The procedure for producing the 45-inch blade length blade (for 3600 rpm steam turbine) shown in FIG. 1 using the steel having the chemical composition of Sample 6 shown in Example 1 will be described below. First, the steel having the chemical composition of sample 6 was melted by vacuum induction and then remelted by vacuum arc, and a round bar-shaped material having a diameter of about 200 mm was produced by hot forging. After that, rough ground forging was performed in a skewer shape having different diameters depending on the thickness of each part such as a blade root, and after heating to a high temperature and forming a near net shape by die forging, heat treatment was performed. The heat treatment was performed by 925 ° C. × 2 h heating, forced air cooling and solution treatment so as to obtain a tensile strength of 1350 MPa or more, and 550 ° C. × 4 h heating after air cooling and aging treatment. Finishing was performed by strain relief, polishing, and machining, and 45-inch blades were produced.

  Test pieces were collected from each part of the manufactured blade (blade tip, blade center, blade root) and subjected to a tensile test and a Charpy impact test at room temperature (20 ° C.). The results are shown in Table 3. The test pieces at any part satisfied the target values of tensile strength of 1350 MPa or more and Charpy absorbed energy of 20 J or more. Further, no precipitation of δ ferrite phase was observed, and the structure was a martensite single-phase microstructure, and it was confirmed that the 45-inch blade had sound properties.

  The precipitation hardening martensitic steel according to the present invention has high tensile strength of 1350 MPa or higher, room temperature Charpy absorption energy of 20 J or higher, high toughness, and satisfactory corrosion resistance. In addition to being used for moving blades, it can also be applied to blades of gas turbine compressors and chemical plant compressors.

It is a schematic diagram which shows an example of a 45-inch class long blade. It is sectional drawing which shows an example of an integrated low-pressure turbine rotor. It is sectional drawing which shows an example of a welding type low pressure turbine rotor. It is sectional drawing which shows an example of a shrink fitting low-pressure turbine rotor. It is a graph showing the change of the tensile strength with respect to aging and tempering temperature. It is a graph showing the change of the absorbed energy with respect to aging and tempering temperature. It is a graph showing the change of the tensile strength with respect to Al amount. It is a graph showing the relationship between tensile strength and absorbed energy. It is a graph showing the change of the delayed crack generation limit strength with respect to the amount of Cr + Mo. It is a graph showing the change of the delayed crack generation limit strength with respect to the amount of Al. It is a graph showing the influence (schaeffler state figure) of Cr equivalent and Ni equivalent which acts on a structure | tissue.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Long blade 2 Shroud 3 Stub 4 Blade root 5 Erosion shield 20 Integrated rotor 30 Welded rotor 40 Shrink fit rotor

Claims (10)

By mass ratio , 12.25 to 14.25% Cr, 7.5 to 8.5% Ni, 1.0 to 2.5% Mo, 0.05% or less C, 0 .2% or less of Si, 0.4% or less of Mn, 0.03% or less of P, 0.005% or less of S, 0.008% or less of N, 0.90 to 1. and 35% of Al, and Nb and Ta your maximum is 0.01%, consists of a balance of Fe and inevitably mixed impurities, the sum of Cr and Mo is at 14.25 to 16.75% Oh Ri, 910 ° C. to 940 after the solution treatment at ° C., 510 ° C. to 530 precipitation hardenable martensitic steel subjected to aging treatment at ° C.. Cr equivalent = [Cr] +2 [Si] +1.5 [Mo] +5.5 [Al] +1.75 [Nb] +1.5 [Ti],
Ni equivalent = [Ni] +30 [C] +0.5 [Mn] +25 [N] +0.3 [Cu]
The square brackets in the above formula indicate the mass ratio (%) of the element, the Cr equivalent is less than 28.0, and the Ni equivalent is less than 10.5. steel. By mass ratio, 12.25 to 14.25% Cr, 7.5 to 8.5% Ni, 1.0 to 2.5% Mo, 0.05% or less C, 0 .2% or less of Si, 0.4% or less of Mn, 0.03% or less of P, 0.005% or less of S, 0.008% or less of N, 0.90 to 2. It consists of 25% Al, Nb and Ta whose upper limit of the total is 0.01%, the remaining Fe and impurities inevitably mixed, and the total of Cr and Mo is 15.5 to 16.75%. A precipitation hardening martensitic steel. By mass ratio, 12.25 to 14.25% Cr, 7.5 to 8.5% Ni, 1.0 to 2.5% Mo, 0.05% or less C, 0 Less than 2% Si, 0.4% or less Mn, 0.03% or less P, 0.005% or less S, 0.008% or less N, more than 1.35% And 2.25% or less of Al, Nb and Ta whose upper limit of the total is 0.01%, the remaining Fe and impurities inevitably mixed, and the total of Cr and Mo is 14.25 to 25%. Precipitation hardening type martensitic steel which is 16.75% . By mass ratio , 12.25 to 14.25% Cr, 7.5 to 8.5% Ni, 1.0 to 2.5% Mo, 0.05% or less C, 0 .2% or less of Si, 0.4% or less of Mn, 0.03% or less of P, 0.005% or less of S, 0.008% or less of N, 0.90 to 1. and 35% of Al, and Nb and Ta your maximum is 0.01%, consists of a balance of Fe and inevitably mixed impurities, the sum of Cr and Mo is at 14.25 to 16.75% A method for producing precipitation hardening martensitic steel, comprising subjecting a steel piece to solution treatment at 910 ° C to 940 ° C and then aging treatment at 510 ° C to 530 ° C. By mass ratio , 12.25 to 14.25% Cr, 7.5 to 8.5% Ni, 1.0 to 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, more than 1.35% Te consists of a 2.25% or less of Al, and Nb and Ta your maximum is 0.01%, and the balance of Fe and inevitably mixed impurities, the sum of Cr and Mo is from 14.25 to 16 A method for producing a precipitation hardening martensitic steel, comprising subjecting a slab of .75% to solution treatment at 910 ° C. to 940 ° C. and then aging treatment at 550 ° C. to 600 ° C. Cr equivalent = [Cr] +2 [Si] +1.5 [Mo] +5.5 [Al] +1.75 [Nb] +1.5 [Ti],
Ni equivalent = [Ni] +30 [C] +0.5 [Mn] +25 [N] +0.3 [Cu]
The square brackets in the above formula indicate the mass ratio (%) of the element, the Cr equivalent is less than 28.0, and the Ni equivalent is less than 10.5, Precipitation hardening type according to claim 5 or 6 Method for producing martensitic steel.   The method for producing a precipitation hardening martensitic steel according to claim 5 or 7, wherein a total of Cr and Mo is 15.5 to 16.75%.   A turbine rotor blade using the martensitic steel according to claim 1. A steam turbine comprising the turbine rotor blade according to claim 9.

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