JP2013209721A - Ni-BASED ALLOY AND METHOD FOR PRODUCING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an Ni-based alloy having improved creep rupture strength, and a method for producing the same.SOLUTION: An Ni-based alloy includes, by mass, 0.007 to <0.20% C, ≤0.3% Si, ≤0.3% Mn and 19.0 to 22.0% Cr, satisfies Mo+(1/2)×W: 9.0 to 12.0% with Mo as a single element or Mo as an essential element, includes 0.5 to 1.8% Al, 1.0 to 2.0% Ti, ≤2.0% Fe, ≤0.02% Mg and either or both of ≤0.02% B and ≤0.02% Zr, and in which a value represented by Al/(Al+0.56Ti) is 0.45 to 0.70, and the balance Ni with impurities, and has the average crystal grain size of 50 to 200 μm and hardness of ≥250 HV.

Description

  The present invention relates to a Ni-based alloy suitable for application to a member exposed to a high-temperature part of a thermal power plant particularly under super-supercritical steam conditions, and a method for producing the same.

In recent years, in order to reduce CO ₂ emissions from the viewpoint of preventing global warming, higher efficiency of thermal power plants has been demanded. Currently, the steam temperature of a thermal power plant has reached 600 to 630 ° C., and the applicant of the present application has disclosed, for example, Japanese Patent Application Laid-Open No. 9-157779 as a Ni-based alloy suitable for use in a 650 ° C. class super supercritical pressure thermal power plant. In Gazette (Patent Document 1), by weight, C: 0.2% or less, Si: 1% or less, Mn: 1% or less, Cr: 10 to 24%, and one or two of Mo and W Mo + (1/2) × W: 5 to 17%, Al: 0.5 to 2%, Ti: 1 to 3%, Fe: 10% or less, and B: 0.02% or less, Zr: 0. A low thermal expansion Ni-based superalloy containing 2% or less of one or two of the balance Ni and inevitable impurities was proposed.
Further, as a proposal for reducing the microsegregation of Patent Document 1 described above, in the republished patent WO 10/038680 (Patent Document 2), C: 0.15% or less, Si: 1% or less, and Mn: 1% or less, Cr: 10 to 24%, Mo alone or Mo as essential Mo + (1/2) × W: 5 to 17%, Al: 0.5 to 1.8%, Ti: 1 to 2.5 %, Mg: 0.02% or less, and (B: 0.02% or less, Zr: 0.2% or less) or both, and further represented by Al / (Al + 0.56Ti) In the method for producing a Ni-base alloy having a value of 0.45 to 0.70 and the balance Ni and impurities, a Ni-base alloy material having the above composition obtained by vacuum melting is 1-100 at 1160-1220 ° C. Manufacture of Ni-based alloys that undergo at least one time of homogenization heat treatment A method, by homogenization heat treatment of the proposed a Ni-base superalloy according to 1 to 1.17 to segregation ratio of Mo.

JP-A-9-157779 Republished patent WO10 / 038680

The alloys of Patent Document 1 and Patent Document 2 described above have excellent high-temperature strength, good oxidation resistance, and notch sensitivity, are inexpensive and easy to manufacture, and are easy to manufacture. It is attracting attention as a heat-resistant alloy.
By the way, in a member such as a steam turbine used in the above-described 700 ° C. class super-supercritical thermal power plant, the usage environment is extremely severe, and thus higher reliability is required.
For this purpose, the Ni-based alloy proposed in Patent Document 1 or Patent Document 2 is further improved so that it can be applied to a steam turbine or the like used in a 700 ° C-class ultra supercritical thermal power plant more reliably. It is necessary to make the best use of it. More specifically, it is required to further improve the creep rupture strength as compared with the alloy of Patent Document 1 or Patent Document 2 described above.
An object of the present invention is to provide a Ni-based alloy having improved creep rupture strength and a method for producing the same.

The present inventors diligently studied a method capable of exhibiting higher creep rupture strength based on the alloys described in Patent Document 1 and Patent Document 2. In order to obtain good mechanical properties, it is important to control the metal structure, one of which is control of the crystal grain size. Focusing on the control of the crystal grain size, which depends on the C content and heat treatment conditions, the C content is optimized and the crystal grain size of a specific size is used to dramatically increase the creep rupture strength. We have found that it can be improved, and have reached the present invention.
That is, the present invention, in mass%, C: 0.007% or more and less than 0.020%, Si: 0.3% or less, Mn: 0.3% or less, Cr: 19.0-22.0%, Mo Mo or (1/2) × W: 9.0 to 12.0%, Al: 0.5 to 1.8%, Ti: 1.0 to 2.0%, Fe: 2. 0% or less, Mg: 0.02% or less, B: 0.02% or less and Zr: 0.2% or less, or both, and further expressed by Al / (Al + 0.56Ti) The value is 0.45 to 0.70, and is a Ni-based alloy composed of the balance Ni and impurities, an average crystal grain size of 50 to 200 μm, and a hardness of 250 HV or more.
In the present invention, the hot-worked material having the above composition is subjected to a solution treatment at 980 to 1150 ° C., and then the first aging treatment at 820 to 880 ° C. and the second aging treatment at 700 to 800 ° C. Is an Ni-based alloy manufacturing method in which the average grain size is 50 to 200 μm and the hardness is 250 HV or more.

  Since the Ni-based alloy of the present invention has improved creep rupture strength while maintaining good creep rupture ductility by optimizing the C content and crystal grain size, a steam turbine and a boiler using the same Pipe materials and the like can exhibit higher reliability.

The reason why each element is specified in the Ni-based alloy of the present invention is as follows. In addition, content of each element is the mass%.
C: 0.007% or more and less than 0.020% C is the most important element for obtaining excellent creep rupture strength. In order to improve the creep rupture strength, it is effective to enlarge the crystal grains by a solution heat treatment. However, when C is 0.020% or more, the effect of suppressing the growth of crystal grains is increased. Therefore, in the present invention, the upper limit of C is set to less than 0.020%. Moreover, C has an effect of strengthening the grain boundary by precipitating carbides at the grain boundary by performing an aging treatment, and suppressing creep deformation. However, if C is less than 0.007%, sufficient carbide cannot be precipitated at the grain boundaries. As a result, the grain boundary cannot be strengthened and the creep rupture strength is lowered. Therefore, the lower limit of C is defined as 0.007%.
Si: 0.3% or less Si is used as a deoxidizer during alloy melting. Moreover, Si has an effect of suppressing peeling of the oxide film. However, when it contains excessively, ductility and workability will fall, It limits to 0.3% or less. A particularly preferable upper limit of Si is less than 0.1%.
Mn: 0.3% or less Mn is used as a deoxidizing agent or a desulfurizing agent during alloy melting. Deoxidation and desulfurization are performed using Mn because O and S as inevitable impurities segregate at the grain boundaries and cause hot brittleness. However, since ductility will fall when it adds excessively, it limits to 0.3% or less. A preferable upper limit is less than 0.1%.

Cr: 19.0 to 22.0%
Cr has the effect of strengthening the grain boundary by combining with C and generating carbide at the grain boundary to improve the strength and ductility at high temperatures. In addition, it has the effect of improving the oxidation resistance and corrosion resistance of the alloy by solid solution in the base and greatly reducing notch sensitivity. To obtain this effect with certainty, 19.0 to 22.0% is required. If the Cr content is less than 19.0%, the above effect cannot be obtained with certainty. Excessive addition exceeding 22.0% can cause cracking problems during high temperature use due to an increase in the thermal expansion coefficient, and the manufacturability of the alloy. The problem that workability falls arises. For these reasons, Cr is limited to 19.0 to 22.0%.
Mo alone or Mo as an essential component Mo + (1/2) × W: 9.0 to 12.0%
Mo and W have the effect of solid-dissolving in the base to strengthen the base and lowering the thermal expansion coefficient of the alloy. Since the Ni-based alloy has a large coefficient of thermal expansion, there is a difficulty in that it is liable to cause thermal fatigue when used stably at high temperatures and lacks reliability. Since Mo is the most effective element for lowering the thermal expansion coefficient, Mo is essential, and Mo alone or Mo and W are added. In order to obtain this effect reliably, 9.0 to 12.0% of Mo + (1/2) × W is required. If the Mo + 1 / 2W amount is less than 9.0%, the above effect cannot be obtained with certainty, and if it exceeds 12.0%, the manufacturability and workability of the alloy tends to be difficult. / 2W amount is 9.0 to 12.0%.

Al: 0.5 to 1.8%
Al forms an intermetallic compound (Ni ₃ (Al, Ti)) called a gamma prime phase together with Ni and Ti, and is added to increase the high temperature strength of the alloy. If the content is less than 0.5%, the above effects cannot be obtained, and excessive addition deteriorates the manufacturability and workability of the alloy, so Al is limited to 0.5 to 1.8%. Moreover, the range of preferable Al is 1.0 to 1.7%.
Ti: 1.0-2.0%
Ti, like Ni and Al, has the effect of forming a gamma prime phase (Ni ₃ (Ti, Al)) and increasing the high temperature strength of the alloy. Since the atomic diameter of Ti is larger than that of Ni and gives elastic strain to the base, it contributes to strengthening more than Ni ₃ Al. If less than 1.0%, the above effect cannot be obtained, and if added excessively, the productivity and workability of the alloy deteriorate, so Ti is limited to 1 to 2.0%. A preferable range of Ti is 1.4 to 1.8%.
Al / (Al + 0.56Ti): 0.45-0.70
In the present invention, Al / (Al + 0.56Ti) is in the range of 0.45 to 0.70 by mass ratio. This equation represents the ratio of the number of atoms (at%) between Ai and Ti in Al and Ti.
In the present invention, the balance between Al and Ti is important. Ni ₃ Ti is large, the effect of high temperature and high strength than _Ni 3 Al, high temperature tends to brittle eta phase worse than the phase stability _Ni 3 Al at a high temperature. Therefore, by adding Ti together with Al, the gamma prime phase is precipitated in the form of (Ni ₃ (Al, Ti)) in which Al and Ti are partially substituted. (Ni ₃ (Al, Ti)) provides higher high-temperature strength than Ni ₃ Al, but the ductility is inferior. As the proportion of Al increases, the ductility improves, but conversely the strength decreases. Balance is important. In the alloy of the present invention, it is important to ensure sufficient ductility, and in order to express the ratio of Al in the gamma prime phase as a ratio of atomic weight, a numerical value of Al / (Al + 0.56Ti) was set. If this value is lower than 0.45, sufficient ductility cannot be obtained. On the other hand, since the strength is insufficient when it exceeds 0.70, the Al / (Al + 0.56Ti) value is limited to 0.45 to 0.70.

Fe: 2.0% or less Fe is not necessarily added, but it has an effect of improving the hot workability of the alloy and can be added as necessary. When Fe is added excessively, the thermal expansion coefficient of the alloy increases, and there is a problem that cracks occur when used at high temperatures. Moreover, since oxidation resistance deteriorates, it limits to 2.0% or less.
Mg: 0.02% or less Mg is used as a desulfurization material during alloy melting, and has the effect of improving the hot workability by suppressing the grain boundary segregation of S by forming a compound with S. However, if added excessively, ductility and workability deteriorate, so Mg is limited to 0.02% or less. A preferable range is 0.0005 to 0.01% or less.
B: 0.02% or less, Zr: 0.2% or less B and Zr are used for strengthening grain boundaries, and it is necessary to add either or both of B and Zr. Since B and Zr are significantly smaller in size than Ni which is an atom constituting the base, they are segregated at the crystal grain boundary and have an effect of suppressing the grain boundary sliding at a high temperature. In particular, it has the effect of greatly reducing notch sensitivity. Therefore, the effect of improving the creep rupture strength and creep rupture ductility can be obtained, but if added excessively, the oxidation resistance deteriorates, so B and Zr are limited to 0.02% or less and 0.2% or less, respectively. Preferable ranges are 0.0005 to 0.01% and 0.005 to 0.007%, respectively.

  The remaining Ni is an austenite generating element. Since the austenite phase is densely packed with atoms, the diffusion of atoms is slow even at high temperatures, and the high-temperature strength is higher than that of the ferrite phase. In addition, the austenite base has a large solid solubility limit of the alloy element, which is advantageous for precipitation of the gamma prime phase, which is the key to precipitation strengthening, and for strengthening the austenite base itself by solid solution strengthening. Since the most effective element constituting the austenite base is Ni, in the present invention, the balance is Ni. Of course, impurities are included.

Average crystal grain size: 50-200 μm
As described above, in the present invention, the C content is made low, and the crystal grain size is kept large to improve the creep rupture strength within a specific range. In creep, voids generated by sliding or movement of grain boundaries are connected and lead to fracture. Therefore, the larger the crystal grain size, the smaller the specific surface area of the grain boundary and the higher the creep rupture strength. Therefore, in order to obtain sufficient creep rupture strength, the average crystal grain size is defined as 50 μm or more. Further, if the crystal grain size becomes too large, the proof stress and tensile strength may decrease, so the upper limit of the average crystal grain size is 200 μm.

In order to obtain the crystal grain size described above, a solution treatment is performed on a hot-worked material such as hot forging.
For example, when performing hot forging, it is preferable to apply a larger strain at a high temperature of 900 to 1200 ° C. to obtain a hot forged material.
A solution treatment is performed on the aforementioned hot-worked material. The metal structure of the hot forged material is often a mixed grain structure, and this is a uniform metal structure with an average crystal grain size of 50 to 200 μm. In order to precipitate the phase and carbide, the constituent elements of these compounds are once dissolved in a matrix (base). If the solution temperature is low, the effect of the solution treatment described above cannot be expected, so the temperature is limited to 980 ° C. or higher. On the other hand, if the solution treatment temperature is too high, the crystal grains are likely to become coarser than necessary, and the control thereof is difficult.

In the present invention, an aging treatment is performed on the Ni-base alloy after the solution treatment described above.
In the aging treatment, the first aging treatment is performed at 820 to 880 ° C. and the second aging treatment is performed at 700 to 800 ° C. after the above-described solution treatment.
In the first-stage aging treatment, carbides can be precipitated at the crystal grain boundaries to strengthen the crystal grain boundaries. If the aging treatment temperature at the first stage is less than 820 ° C., the precipitation amount of carbide is small and the strengthening of the grain boundaries becomes insufficient. Moreover, since crystal grains grow when the temperature exceeds 880 ° C., the crystal grain size adjusted by the solution treatment may be coarsened. Therefore, in the present invention, the first stage aging treatment is defined as 820 to 880 ° C. The time for the first aging treatment is preferably 1 to 5 hours.
In the second aging treatment, a gamma prime phase is precipitated in the grains, and the high temperature strength of the alloy can be increased. If the second stage aging treatment temperature is less than 700 ° C., the amount of gamma prime phase deposited is small and it becomes difficult to obtain sufficient high-temperature strength. If it exceeds 800 ° C., the gamma plum phase becomes coarse and the high-temperature strength tends to decrease. Therefore, in the present invention, the second stage aging treatment is defined as 700 to 800 ° C. The time for the second stage aging treatment is preferably 1 to 50 hours.

In the present invention, the hot-worked material is subjected to a solution treatment and then subjected to an aging treatment to impart strength. Therefore, the minimum necessary hardness is 250 HV. Preferably it is 270HV or more.
As described above, in the present invention, an excellent creep rupture strength is realized by reducing C and adjusting the average crystal grain size to 50 to 200 μm, and further, Ni alloy having strength by aging treatment can do.

  A 10 kg ingot was prepared by vacuum induction melting, and Ni-based alloys having chemical components shown in Table 1 were obtained. The balance is Ni and impurities.

The Ni-based alloy shown in Table 1 was subjected to homogenization heat treatment and hot forging to obtain a hot work material. The temperature for hot forging was 1150 ° C. Thereafter, solution treatment and aging treatment were performed, and the mechanical properties were investigated. Samples were taken along the longitudinal direction of the forging. The solution treatment was performed by heating at 1066 ° C. for 4 hours and then air cooling. As the first stage aging treatment, heating was performed at 850 ° C. for 4 hours and then air cooling was performed, and as second stage aging, heating was performed at 760 ° C. for 16 hours and then air cooling was performed.
In order to evaluate the mechanical properties of these heat-treated materials, tensile tests were performed at normal temperatures, 700 ° C., and 750 ° C., and Vickers hardness measurements were performed at normal temperatures. These results are shown in Table 2.
Furthermore, a creep rupture test was performed at 700 ° C. and 750 ° C. Table 3 shows the results of a creep rupture test conducted under the conditions of a test temperature of 700 ° C., a load stress of 385 N / mm ² , a test temperature of 750 ° C., and a load stress of 242 N / mm ² . In addition, the average crystal grain size shown in Table 3 is the value after the solution treatment and the aging treatment.

From Table 2, the 0.2% proof stress, tensile strength, elongation and drawing at room temperature, 700 ° C. and 750 ° C. No. 1 alloy, no. No. 2 alloy, No. No. 3 alloy and no. All four alloys are No. in the comparative example. It is equal to or better than 11 alloys. In addition, No. Although it is slightly inferior to 12 alloy, it does not greatly deteriorate but maintains good characteristics.
On the other hand, from Table 3, the creep rupture life at 700 ° C. and 750 ° C. No. 1 alloy, no. No. 2 alloy, No. No. 3 alloy and no. No. 4 alloy is No. of the comparative example. No. 11 alloy and No. Compared to 12 alloys. Regarding the creep rupture drawing, the alloy of the present invention is No. of the comparative example. Although slightly inferior to 12, satisfactory ductility is satisfied with a fracture drawing of 30% or more. As far as satisfactory creep rupture ductility is satisfied, the most important creep rupture strength can be improved.

It can be seen that the Ni-based alloy of the present invention has improved creep rupture strength while maintaining good creep rupture ductility by optimizing the C content and crystal grain size. Therefore, the steam turbine material using this can exhibit higher reliability.

Al: 0.5 to 1.8%
Al forms an intermetallic compound (Ni ₃ (Al, Ti)) called a gamma prime phase together with Ni and Ti, and is added to increase the high temperature strength of the alloy. If the content is less than 0.5%, the above effects cannot be obtained, and excessive addition deteriorates the manufacturability and workability of the alloy, so Al is limited to 0.5 to 1.8%. Moreover, the range of preferable Al is 1.0 to 1.7%.
Ti: 1.0-2.0%
Ti, like Ni and Al, has the effect of forming a gamma prime phase (Ni ₃ (Ti, Al)) and increasing the high temperature strength of the alloy. Since the atomic diameter of Ti is larger than that of Ni and gives elastic strain to the base, it contributes to strengthening more than Ni ₃ Al. If less than 1.0%, the above effect cannot be obtained, and if added excessively, the productivity and workability of the alloy deteriorate, so Ti is limited to 1 to 2.0%. A preferable range of Ti is 1.4 to 1.8%.
Al / (Al + 0.56Ti): 0.45-0.70
In the present invention, Al / (Al + 0.56Ti) is in a range of 0.45 to 0.70 in terms of atomic %. This equation represents the ratio of the number of atoms (at%) between Ai and Ti in Al and Ti.
In the present invention, the balance between Al and Ti is important. Ni ₃ Ti is large, the effect of high temperature and high strength than _Ni 3 Al, high temperature tends to brittle eta phase worse than the phase stability _Ni 3 Al at a high temperature. Therefore, by adding Ti together with Al, the gamma prime phase is precipitated in the form of (Ni ₃ (Al, Ti)) in which Al and Ti are partially substituted. (Ni ₃ (Al, Ti)) provides higher high-temperature strength than Ni ₃ Al, but the ductility is inferior. As the proportion of Al increases, the ductility improves, but conversely the strength decreases. Balance is important. In the alloy of the present invention, it is important to ensure sufficient ductility, and in order to express the ratio of Al in the gamma prime phase as a ratio of atomic weight, a numerical value of Al / (Al + 0.56Ti) was set. If this value is lower than 0.45, sufficient ductility cannot be obtained. On the other hand, since the strength is insufficient when it exceeds 0.70, the Al / (Al + 0.56Ti) value is limited to 0.45 to 0.70.

Claims (2)

  C: 0.007% or more and less than 0.020% by mass%, Si: 0.3% or less, Mn: 0.3% or less, Cr: 19.0 to 22.0%, Mo alone or Mo as essential Mo + (1/2) × W: 9.0 to 12.0%, Al: 0.5 to 1.8%, Ti: 1.0 to 2.0%, Fe: 2.0% or less, Mg: 0.02% or less, and B: 0.02% or less and Zr: 0.2% or less, or both, and the value represented by Al / (Al + 0.56Ti) is 0.45. A Ni-based alloy characterized by being -0.70, comprising the balance Ni and impurities, having an average crystal grain size of 50-200 μm and a hardness of 250 HV or more. C: 0.007% or more and less than 0.020% by mass%, Si: 0.3% or less, Mn: 0.3% or less, Cr: 19.0 to 22.0%, Mo alone or Mo as essential Mo + (1/2) × W: 9.0 to 12.0%, Al: 0.5 to 1.8%, Ti: 1.0 to 2.0%, Fe: 2.0% or less, Mg: 0.02% or less, and B: 0.02% or less and Zr: 0.2% or less, or both, and the value represented by Al / (Al + 0.56Ti) is 0.45. The hot-worked material composed of the remaining Ni and impurities is subjected to a solution treatment at 980 to 1150 ° C., then the first aging treatment at 820 to 880 ° C. and 2 at 700 to 800 ° C. A Ni-based alloy characterized by performing an aging treatment at the stage to have an average crystal grain size of 50 to 200 μm and a hardness of 250 HV or more. Production method.