JP2014122385A - Forging member and steam turbine rotor using the same, steam turbine moving blade, boiler piping, boiler tube and steam turbine bolt using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a large forging member having homogeneity of the grain structure and excellent high-temperature strength.SOLUTION: A forging member comprises 0.005-0.030 mass% C, 0.10 mass% or less Si, 0.10 mass% or less Mn, 15-21 mass% Cr, 25 mass% or less Co and 1.0-1.5 mass% Al, 5.0-17 mass% Mo and W, specified as the formula Mo+0.5W, specified amounts of Ti, Ta and Nb and remaining Ni and unavoidable impurities.

Description

  The present invention relates to a forged member and a high-temperature component used in a gas turbine, a steam turbine plant, and the like such as a steam turbine rotor, a steam turbine rotor blade, a boiler pipe, a boiler tube, and a steam turbine bolt using the forged member.

  Up to now, steel materials have been used as high-temperature components for steam turbine plants, but studies are underway to apply Ni-based alloys with excellent high-temperature strength for the purpose of increasing power generation efficiency by improving steam temperature. It has been.

  Although Ni-based alloys are excellent in high-temperature strength, the problem is that the productivity of large members is inferior to that of steel materials, and the development of Ni-based alloys that are excellent in the productivity of large members has been underway. Defects associated with gravity segregation (macrosegregation), which is a problem when manufacturing large ingots, are caused by floating segregation acceleration elements (Al, Ti, W) and sedimentation segregation acceleration elements (Nb, Ta, Mo). It has been clarified that the addition can be suppressed in a balanced manner, and an alloy design based on this finding has been made.

  Patent document 1 is mentioned as an alloy excellent in the segregation characteristic. In Patent Document 1, the amounts of addition of floating segregation acceleration elements (Al, Ti) and sedimentation segregation acceleration elements (Mo) are balanced, and gravity segregation does not occur. A lump (850 mm or more) can be produced.

International Publication No. 2009/028671

  In the alloy described in Patent Document 1, a large amount of Mo is added for the purpose of increasing the high-temperature strength. However, since Mo concentrates in the liquid phase during solidification, the concentration is different between the dendrite center portion where the solidification starts and the dendrite boundary portion, and the dendritic boundary portion has a higher concentration. Since Mo has a low diffusion rate, variations in the concentration of Mo remain even after the ingot is forged.

  Further, in the case of the alloy described in Patent Document 1, a large amount of C can be added. However, since C also concentrates in the liquid phase during solidification, at the dendrite center and the dendrite boundary where solidification starts, The density is different, and the density is higher at the dendrite boundary. C has a faster diffusion rate than Mo, but becomes stable when bonded to Mo and forms carbides. Since the concentration of C is higher at the dendrite boundary than at the center of the dendrite, the amount of carbide precipitated at the dendrite boundary is greater than at the dendrite center.

  As described above, when the technique of Patent Document 1 is used, macro segregation hardly occurs, but when a large steel ingot is produced, the Mo concentration and the precipitation amount of carbides vary. Since the Mo concentration is high in the portion that is the dendrite boundary portion, the diffusion is slower than the portion that is the dendrite center portion. In addition, since the amount of precipitation of carbide is large, the growth rate when recrystallizing and growing crystal grains after forging is slower than that in the portion that was the center of the dendrite.

  Thereby, in the part which was a dendrite center part, a crystal grain becomes large and a mixed grain structure is formed. In the portion where the crystal grains are fine, the creep strength is weak, and in the portion where the creep strength is coarse, the fatigue strength is weak. Therefore, in the mixed grain structure, both the creep strength and the fatigue strength are lowered. Further, since the Mo concentration becomes inhomogeneous, the solid solution strengthening becomes inhomogeneous and a weak portion is generated as compared with the homogeneous case, so that the tensile strength, fatigue strength, etc. are reduced as compared with the homogeneous case.

  An object of the present invention is to obtain a large forged member having a homogeneous crystal grain structure and high strength at high temperatures.

  The forged member of the present invention comprises 0.005 to 0.030 mass% C, 0.10 mass% or less Si, 0.10 mass% or less Mn, 15 to 21 mass% Cr, or 25 mass% or less. Co and 1.0 to 1.5% by mass of Al, Mo and W in an amount defined by the formula “Mo + 0.5W”, 5.0 to 17% by mass, and predetermined amounts of Ti, Ta and Nb The balance is made of Ni and inevitable impurities.

  According to the present invention, it is possible to provide a large forged product having uniform crystal grains and a solid solution strengthening amount and excellent in high temperature strength.

It is an expanded sectional view which shows the structure | tissue of the large sized forging material produced by C1 which is a comparative example. It is a graph which shows distribution of the solid solution strengthening element in a large forged material. It is a graph which shows the relationship between C amount and the crystal grain size ratio of the large forged material of an Example. It is a graph which shows the relationship between W / Mo ratio and the grain size ratio of the large forged material of an Example. It is a graph which shows the creep test result of a large forge simulation material.

  By setting it as the component range shown next, the dispersion | variation in said mixed grain structure and solid solution strengthening can be suppressed, and the outstanding mechanical characteristic is acquired also with a large forged product.

  In patent document 1, although the component range of C is 0.1 mass% or less, in order to suppress the dispersion | variation in a mixed grain structure and solid solution strengthening, it is set as 0.005-0.030 mass%. desirable. The most preferred range is 0.005 to 0.025 mass%.

  The Cr content is preferably added in an amount of 15% by mass or more in order to maintain corrosion resistance. However, if it is added too much, a harmful phase is precipitated and the strength characteristics are deteriorated. That is, a preferable range is 15% by mass to 21% by mass. Furthermore, the most preferable range is 19% by mass to 21% by mass.

  Co is a solid solution strengthening element and is preferably added. However, when excessively added, the γ ′ phase, which is a precipitation strengthening phase, becomes unstable, so it is desirable that the amount be 25% by mass or less (excluding 0% by mass).

  Mo and W are solid solution strengthening elements, and the high temperature strength is improved by adding them. However, when it is added too much, a harmful phase precipitates and the material becomes brittle. A preferable range is 5.0 to 17% by mass of Mo and W in an amount defined by the formula “Mo + 0.5W”.

  W shows segregation characteristics opposite to those of Mo, and the concentration is high at the center of the dendrite and the concentration is low at the dendrite boundary. For this reason, when W and Mo are added in a well-balanced manner, the Mo concentration is high in the portion where the W concentration is low, and the W concentration is high in the portion where the Mo concentration is low. As a result, the solid solution strengthening and the diffusion rate become uniform, the mixed grain structure can be suppressed, and a decrease in strength due to variations in solid solution strengthening can also be suppressed. For this purpose, it is effective to set W / Mo, which is the ratio of the contents of W and Mo, to 0.5 to 1.5.

  Al, Ti, Nb, and Ta are elements that stabilize the γ 'phase. However, if added too much, the γ' phase is excessively precipitated and ductility is lowered. Therefore, the preferable addition range of Al is 1.0 to 1.5 mass%, the preferable addition range of Ti is 1.0 to 2.0 mass%, and the preferable addition range of Ta is 0.1 It is -0.6 mass%, and the preferable addition range of Nb is 0.1-0.6 mass%. The most preferable range of “Ti + Ta + Nb”, which is the sum of Ti, Ta, and Nb, is 1.45 to 2% by mass. Si and Mn are elements that embrittle the material, and each of them is desirably 0.1% by mass or less.

  Hereinafter, description will be made using Examples and Comparative Examples.

  Table 1 summarizes the chemical components of the test materials.

  Among these, for C1 as a comparative example and LCW3 as an example, an ingot having a diameter of 750 mm was produced by high-frequency vacuum melting and electroslag remelting, and a 300 mmφ forging material (about 3 tons) was produced by hot forging. . After the forging material was melted at 1150 ° C. for 2 hours, the structure was observed at the center of the cross section.

  FIG. 1 shows the organization of C1.

  From this figure, it can be seen that in C1, a band-like structure composed of a fine grain part and a coarse grain part is formed.

  On the other hand, in the LCW3, although not shown, a clear band structure was not observed.

  FIG. 2 shows the results of energy dispersive X-ray analysis (EDX-ray analysis) in a cross-sectional structure. The horizontal axis indicates a value obtained by standardizing the position from the coarse grain part (center) to the fine grain part (center). The vertical axis represents a value of Mo + 0.5W, that is, a value obtained by adding the Mo content on the mass basis and 0.5 times the W content.

  As shown in the figure, in the case of C1, which is a comparative example (conventional example), the value of Mo + 0.5W varies greatly depending on the position. In other words, the value of Mo + 0.5W is smaller in the coarse grain part than in the fine grain part. On the other hand, in the case of LCW3 which is an example, the value of Mo + 0.5W is almost constant regardless of the position.

  From this result, it can be seen that according to the present invention, homogenization of the structure and homogenization of the solid solution strengthening amount can be achieved.

  Next, a test material simulating a large steel ingot was produced using a lateral unidirectional solidification furnace. The segregation index of the simulated material is in the range of about 10-1. Here, the segregation index 10 simulates a 1 ton class forging, and the segregation index 1 simulates a 10 ton class forging. A test material having a segregation index of about 1 was hot forged, and then the structure was observed.

  FIG. 3 is a graph showing the relationship between the amount of C and the grain size ratio of the large forged material of the example. The horizontal axis indicates the carbon content (C amount) based on mass. The vertical axis represents the crystal grain size ratio, which is the ratio between the grain size in the coarse grain part and the grain size in the fine grain part. The trial numbers of the examples and comparative examples are described so as to correspond to the respective data (black circles) in the figure.

  From this figure, it can be seen that the smaller the amount of C, the smaller the crystal grain size ratio.

  When the amount of C is large, it becomes extremely mixed and the crystal grain size ratio increases. On the other hand, when the C content is 0.030% by mass or less, the entire forged material has a homogeneous structure, and the crystal grain size ratio is close to 1.

  A small crystal grain size ratio corresponds to a state where no clear band structure is observed. Therefore, the C content is desirably 0.030% by mass or less, and more desirably 0.025% by mass or less.

  FIG. 4 shows the relationship between W / Mo and the crystal grain size ratio. The horizontal axis shows the value of W / Mo, that is, the ratio between the W content and the Mo content on a mass basis. The vertical axis represents the crystal grain size ratio. The trial numbers of the examples and comparative examples are described so as to correspond to the respective data (black circles) in the figure.

  From this figure, it can be seen that when W / Mo is 1, the crystal grain size ratio is the smallest. It can also be seen that when W / Mo is in the range of 0.5 to 1.5, the crystal grain size ratio is 3 or less, resulting in a relatively homogeneous structure.

FIG. 5 shows the creep test result of the large steel ingot simulation material. The horizontal axis indicates the segregation index. The vertical axis represents the creep rupture time. Here, the segregation index is a product of (cooling rate) and (solidification rate) ^1.1, and is an index corresponding to the size of the forging material, and increases as the diameter of the forging material decreases.

  As shown in this figure, in the case of C3 which is a comparative example (conventional example), the creep rupture time becomes shorter as the segregation index becomes smaller (the diameter of the forged material becomes larger). That is, the creep strength decreases. On the other hand, in the case of LC1 and WLC3 which are examples, even when the segregation index is small, the creep rupture time is not shortened, and the decrease in creep strength is slight.

  From the above results, according to the present invention, it is possible to obtain a large forged member having a uniform crystal grain structure and high strength at high temperatures.

Claims (8)

  0.005-0.030 mass% C, 0.10 mass% or less Si, 0.10 mass% or less Mn, 15-21 mass% Cr, 25 mass% or less Co, and 1.0- 1.5% by mass of Al, 5.0 to 17% by mass of Mo and W in an amount defined by the formula “Mo + 0.5W”, and the total content of Ti, Ta and Nb is 1.45-2 0.0% by mass, the balance being Ni and inevitable impurities.   The forged member according to claim 1, wherein the content ratio of W and Mo is 0.5 to 1.5 on a mass basis.   0.005-0.030 mass% C, 0.10 mass% or less Si, 0.10 mass% or less Mn, 15-21 mass% Cr, 25 mass% or less Co, and 1.0- 1.5% by mass of Al, 5.0 to 17% by mass of Mo and W in an amount specified by the formula “Mo + 0.5W”, 1.0 to 2.0% by mass of Ti, 0.1% It contains ~ 0.6 mass% Ta and 0.1-0.6 mass% Nb, the balance is made of Ni and inevitable impurities, and the content ratio of W and Mo is 0. A forged member characterized by being 5-1.5.   The steam turbine rotor using the forged member as described in any one of Claims 1-3.   A steam turbine rotor blade comprising the forged member according to claim 1.   Boiler piping using the forged member according to any one of claims 1 to 3.   The boiler tube using the forged member as described in any one of Claims 1-3.   A steam turbine bolt using the forged member according to any one of claims 1 to 3.

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