JPH045744B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a 12Cr alloy steel having excellent toughness and high-temperature strength, and particularly to a 12Cr alloy steel suitable as a material for high-temperature and high-pressure turbines. [Prior art] Current steam turbines have a maximum steam temperature of 566°C and a maximum steam pressure of 246 atg-, and the rotor shaft materials are 1Cr-1Mo-1/4V steel (ASTM470- class 8) and 11Cr-1Mo-V-Nb- N steel (Japanese Patent Application Laid-Open No. 56-116858) is used. However, recently, the cost of fossil fuels such as oil and coal continues to rise, and it has become important to improve the power generation efficiency of thermal power plants that use these fossil fuels. In order to increase power generation efficiency, it is necessary to increase the steam temperature or pressure of the steam turbine. As materials for these high-temperature, high-pressure (high-efficiency) turbines, the aforementioned materials currently used as turbine rotor materials lack creep rupture strength, and materials with higher strength are required. On the other hand, although Ni-based alloys and Co-based alloys are excellent in terms of creep rupture strength, these alloys have the drawbacks of high cost and poor workability (forging, cutting). [Object of the Invention] The object of the present invention is to provide a 12Cr alloy steel that has high creep rupture strength and does not impair toughness,
The object of the present invention is to provide a 12Cr alloy steel that is particularly suitable as a material for high-efficiency turbines with steam temperatures up to 600°C. [Summary of the Invention] In general, there is a contradictory phenomenon that increasing toughness lowers creep rupture strength, but the present inventors have
It has been found that creep rupture strength and toughness can be simultaneously improved by adding appropriate amounts of rare earth elements and/or calcium to 12Cr alloy steel to create a substantially fully tempered martensitic structure. The present invention was obtained based on the above findings, and contains 0.3% by weight or less of rare earth elements and
This is a 12Cr-based alloy steel containing at least 0.01% by weight of calcium and having a substantially fully tempered martensitic structure, which has excellent toughness and high-temperature strength. The 12Cr alloy steel of the present invention has a weight ratio of Cr: 8 to
13%, Mo: 0.75-1.75%, V: 0.05-0.5%,
Nb: 0.02-0.15%, N: 0.025-0.1%, C: 0.05
~0.25%, Si: 0.60% or less, Ni: 1.5% or less,
Contains Mn: 1.5% or less, W: 0.1 to 0.5%, and the balance is Fe
and rare earth elements and/or impurities.
Alternatively, by adding calcium, a heat-resistant steel with excellent creep rupture strength and toughness required as a rotor material for high-efficiency steam turbines can be obtained. 0.05% or more of C is required to obtain high tensile strength, but if it exceeds 0.25%, the structure will become unstable when exposed to high temperatures for a long time, reducing the long-term creep rupture strength. Limited to ~0.25%. Particularly preferred is 0.1 to 0.2%. Nb is an element that is very effective in increasing high-temperature strength at 0.02% or more, but when it exceeds 0.15%, coarse Nb carbides are formed in large steel ingots, and it also lowers the C concentration in the matrix, causing It has the disadvantage of reducing strength and precipitating δ ferrite which reduces fatigue strength, so it is necessary to suppress it to 0.15% or less. In particular, 0.04 to 0.12% is preferable. N is effective in improving creep rupture strength and preventing the formation of δ ferrite, but if it is less than 0.025%, the effect is not sufficient, and if it exceeds 0.1%, the toughness is significantly reduced. Particularly preferred is 0.03 to 0.07%. Cr improves high temperature strength, but when it exceeds 13%, δ
It causes the formation of ferrite, and if it is less than 8%, corrosion resistance against high temperature and high pressure steam will be insufficient. Particularly preferred is 10 to 11.5%. Although V has the effect of increasing creep rupture strength,
If it is less than 0.05%, the effect is insufficient, and if it exceeds 0.5%, δ ferrite is produced and the fatigue strength is reduced. In particular, 0.1 to 0.3% is preferable. Mo improves creep strength through solid solution strengthening and precipitation hardening, but if it is less than 0.75%, the effect is small, and if it exceeds 1.75%, it produces δ ferrite and reduces toughness and creep rupture strength. especially
1.0-1.5% is preferred. Ni is a very effective element for increasing toughness and preventing the formation of δ ferrite.
Addition of more than % is not preferable because it lowers the creep rupture strength. Particularly preferred is 0.2 to 0.8%. Mn is added as a deoxidizing agent, and its effect can be achieved by adding a small amount, and adding a large amount exceeding 1.5% lowers the creep rupture strength. especially,
0.5-1% is preferred. Although Si is also added as a deoxidizing agent, Si deoxidation is not necessary according to steel manufacturing techniques such as the vacuum C deoxidation method. In addition, by lowering Si, 0.6% is effective to prevent δ ferrite precipitation and improve toughness.
It is necessary to keep it below. When adding, especially
It is preferably 0.25% or less. Even a small amount of W significantly increases high temperature strength. If it is less than 0.1%, the effect will be small, and if it exceeds 0.65%, the strength will decrease rapidly. On the other hand, if W exceeds 0.5%, the toughness will be significantly lowered, so it is preferably less than 0.5% for members requiring toughness. In particular, 0.2
~0.45% is preferred. Addition of 0.3% by weight or less of rare earth elements and 0.01% by weight or less of calcium to the above-mentioned 12Cr alloy steel improves toughness and creep rupture strength. Toughness and creep rupture strength improve as the content of rare earth elements increases, but the effect is saturated even if it exceeds 0.3% by weight, so from the economic point of view,
It is necessary to keep it below % by weight. Calcium also has the same effect as rare earth elements, but the effect is saturated even when the amount of calcium added exceeds 0.01% by weight.
Furthermore, in the present invention, toughness and creep rupture strength are also improved by the combined addition of rare earth elements and calcium. The 12Cr alloy steel of the present invention consists essentially of a completely tempered martensitic structure. This alloy forms δ ferrite depending on its composition, so it is essentially δ
High toughness and creep rupture strength cannot be obtained unless the composition is such that ferrite is not formed. δ ferrite can be controlled by the chromium equivalent. Chromium equivalent = -40 x C% -30 x N% -2 x Mn
% −4×Ni%+Cr%+6×Si%+4×Mo% +1.5×W%+11×V%+5×Nb% The 12Cr alloy steel of the present invention is used in large steel ingots such as steam turbine rotors (50~ 100 tons), the chromium equivalent is preferably 10.5 or less because δ ferrite is likely to be generated due to component segregation. Furthermore, if the chromium equivalent is too small, the creep rupture strength will be low, so it is preferably 6.5 or more. Especially 7
~10 is preferred. Conventional rare earth element-added steel contains harmful δ, which reduces fatigue strength and toughness when the Cr equivalent is 9 or more.
Although ferrite precipitates, it was confirmed that δ ferrite does not precipitate in steel to which rare earth elements have been added, even when the Cr equivalent is 9 or more, even if the Cr equivalent is 9 or more. However, even in steel containing rare earth elements, when the Cr equivalent is 11 or more, δ ferrite precipitates. EXAMPLES OF THE INVENTION Table 1 shows the chemical composition of representative samples. In the table,
0.3% of Mitsushi Metal (a rare earth element alloy whose main component is La-Ce) was added to Samples Nos. 4 to 7 before casting. Samples No. 5 and No. 6 are examples according to the present invention. Sample No. 8 is an example in which Ca was added,
Sample No. 9 is an example in which Ca and rare earth elements were added in combination.

[Table] After casting a sample with the composition shown in Table 1, the steel ingot is
It was heated to 1150℃ and forged to become an experimental sample. The heat treatment of the sample simulates the conditions at the center of a large rotor, heating to 1050℃ and cooling at 100℃/h to 565℃.
Two-stage tempering was performed: furnace cooling at ℃×20 hours and furnace cooling at 665℃×45 hours. Although some δ ferrite was precipitated in sample No. 7, the other samples had substantially completely tempered martensitic structures. Tensile test specimens, creep rupture test specimens, and 2 mm V-notched pierced impact test specimens were taken from the heat-treated samples in the direction perpendicular to the forging and tested. The results are shown in Table 2.

〔Effect of the invention〕

The toughness and creep rupture strength of the steel of the present invention at 593°C are extremely high and fully satisfy the mechanical properties required for a high-efficiency turbine rotor. F (593℃)
It is suitable as a rotor for high-efficiency turbines.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between absorbed energy and Cr equivalent in an impact test at 20℃, Figure 2 is a diagram showing the relationship between 593
℃, 10 ⁵ h A diagram showing the relationship between creep rupture strength and Cr equivalent. Figure 3 is a diagram showing the relationship between absorbed energy and the amount of rare earth element added. Figure 4 is a diagram showing the relationship between absorbed energy and the amount of calcium added. FIG.

Claims (1)

[Claims] 1 Weight ratio: Cr8~13%, Mo0.75~1.75%,
V0.05~0.5%, Nb0.02~0.15%, N0.025~0.1%,
C0.05~0.25%, Si0.60% or less, Ni1.5% or less,
Mn 1.5% or less, W 0.1~0.5%, rare earth elements 0.3%
12Cr system with excellent toughness and high-temperature strength, characterized by containing at least one element below and 0.01% or less of calcium, with the remainder being substantially Fe, and having a substantially fully tempered martensitic structure. Alloy steel.