KR102467393B1 - Austenitic steel sinter, austenitic steel powder and turbine member - Google Patents

Abstract

It is an object of the present invention to provide an austenitic steel sintered material, an austenitic steel powder, and a turbine member that have strength equivalent to or higher than that of Ni-based alloys and are less susceptible to the influence of oxygen. In mass%, Ni: 25 to 50%, Cr: 12 to 25%, Nb: 3 to 6%, B: 0.001 to 0.05%, Ti: 1.6% or less, W: 6% or less, Mo: 4.8% or less, An austenitic steel sintered material and an austenitic steel powder containing Zr: 0.5% or less, the remainder being Fe and unavoidable impurities are provided.

Description

Austenitic steel sinter, austenitic steel powder and turbine member

The present invention relates to an austenitic steel sintered material, an austenitic steel powder, and a turbine member.

BACKGROUND OF THE INVENTION [0002] In recent years, with the aim of increasing the efficiency of coal-fired power plants, higher steam temperatures are being promoted. Among coal-fired power plants currently in operation, a steam turbine with a steam temperature of 620°C class is operated as _a steam turbine (USC: Ultra Super Critical) with the highest steam temperature. It is thought that further high temperature will advance. Although 9Cr-based and 12Cr-based heat-resistant ferrite steels and the like have been used as high-temperature members of steam turbines so far, it is considered that their application becomes difficult with the increase in steam temperature.

As an alloy applied to high-temperature members, a Ni-based alloy having a service temperature higher than that of ferritic steel can be a candidate. The Ni-based alloy uses Al or Ti as a precipitation strengthening element to form a γ' phase that becomes a stable phase at high temperatures, and exhibits excellent strength at high temperatures. However, turbine valve casings and turbine disks are generally manufactured by a casting method. However, in the casting method, shielding from air during melting is not sufficient, and if there are many active elements (Al or Ti), these elements are oxidized.

Patent Literature 1 discloses a technique for applying an austenitic steel having both excellent strength and castability and an austenitic cast product using the same to a turbine member instead of Ni-based alloy.

Japanese Unexamined Patent Publication No. 2017-88963

Patent Literature 1 described above proposes a composition of austenitic steel in which macro defects are reduced in a large cast product, but manufacturing of a mold used for a cast product is relatively time-consuming. In particular, if it is in the form for castings having a large and complex shape, the process cost increases. Therefore, if the member can be obtained by sintering instead of casting, the manufacturability of the turbine member can be further improved.

In view of the above circumstances, an object of the present invention is to provide an austenitic steel sintered material, an austenitic steel powder, and a turbine member that have strength equal to or higher than that of Ni-based alloys and are less susceptible to the influence of oxygen.

The first aspect of the present invention for solving the above problems is, in mass%, Ni: 25 to 50%, Cr: 12 to 25%, Nb: 3 to 6%, B: 0.001 to 0.05%, Ti: 1.6% Hereinafter, W: 6% or less, Mo: 4.8% or less, Zr: 0.5% or less, the balance being an austenitic steel sintered material consisting of Fe and unavoidable impurities.

The second aspect for solving the above problems is, in mass%, Ni: 30 to 45%, Cr: 12 to 20%, Nb: 3 to 5%, B: 0.001 to 0.02%, Ti: 0.3 to 1.3%, It is an austenitic steel sintered material containing W: 5.5% or less, Mo: 2% or less, Zr: 0.3% or less, the balance being Fe and unavoidable impurities.

A third aspect for solving the above problems is, in mass%, Ni: 30 to 40%, Cr: 15 to 20%, Nb: 3.5 to 4.5%, B: 0.001 to 0.02%, Ti: 0.5 to 1.1% or less , W: 5.5% or less, Zr: 0.3% or less, the balance being an austenitic steel sintered material consisting of Fe and unavoidable impurities.

A fourth aspect for solving the above problems is a turbine member using an austenitic sintered material.

The fifth aspect of the present invention for solving the above problems is, in mass%, Ni: 25 to 50%, Cr: 12 to 25%, Nb: 3 to 6%, B: 0.001 to 0.05%, Ti: 1.6% or less , W: 6% or less, Mo: 4.8% or less, Zr: 0.5% or less, the balance being an austenitic steel powder consisting of Fe and unavoidable impurities.

A sixth aspect for solving the above problems is, in mass%, Ni: 30 to 45%, Cr: 12 to 20%, Nb: 3 to 5%, B: 0.001 to 0.02%, Ti: 0.3 to 1.3%, It is an austenitic steel powder containing W: 5.5% or less, Mo: 2% or less, Zr: 0.3% or less, the balance being Fe and unavoidable impurities.

A seventh aspect for solving the above problems is, in mass%, Ni: 30 to 40%, Cr: 15 to 20%, Nb: 3.5 to 4.5%, B: 0.001 to 0.02%, Ti: 0.5 to 1.1% or less , W: 5.5% or less, Zr: 0.3% or less, the balance being an austenitic steel powder consisting of Fe and unavoidable impurities.

A more specific configuration of the present invention is described in the claims.

According to the present invention, it is possible to provide an austenitic sintered material, an austenitic steel powder, and a turbine member that have strength equivalent to or higher than that of Ni-based alloys and are less susceptible to the influence of oxygen.

Subjects, configurations, and effects other than those described above will be clarified by the description of the following embodiments.

Figure 1a is a schematic diagram of an example of the structure of an austenitic steel sintered material obtained by sintering the austenitic steel powder of the present invention.
Figure 1b is a SEM observation photograph of an example of the structure of an austenitic steel sintered material obtained by sintering the austenitic steel powder of the present invention.
2 is a schematic diagram of an example of the structure of an austenitic steel cast material of Patent Document 1;
3 is a schematic diagram of an example of the structure of a conventional Ni-based alloy forged material.
4 is a schematic view showing an example of a turbine valve casing to which an austenitic steel sintered material obtained by sintering the austenitic steel powder of the present invention is applied.
5 is a schematic view showing an example of a turbine disk to which an austenitic steel sintered material obtained by sintering the austenitic steel powder of the present invention is applied.
Fig. 6 is a graph showing the 0.2% yield strength ratio (based on Comparative Example 4) of Examples 1 to 3 and Comparative Examples 1 to 4;
Fig. 7 is a graph showing creep resistance temperature ratios (based on Comparative Example 3) of Examples 1 to 3 and Comparative Examples 1 to 4;
Fig. 8 is a graph showing 0.2% yield strength ratio and creep resistance temperature ratio of Examples 1 and 3 and Comparative Examples 1, 3 and 4;

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail, using drawings.

[Austenitic steel powder and austenitic sintered material]

1A is a schematic diagram of an example of the structure of an austenitic steel sintered material obtained by sintering the austenitic steel powder of the present invention, and FIG. This is an observation picture. As shown in FIGS. 1A and 1B, the austenitic steel sintered material of the present invention includes an austenitic steel powder crystal 1, a grain boundary 2 existing at the boundary of an adjacent austenitic steel crystal, and a grain boundary ( 2) has a Laves phase (3) precipitated on the phase.

The average grain size of the austenitic steel powder crystal 1 is preferably 10 to 300 µm. If it is smaller than 10 μm, there is a risk that the creep strength may not be sufficient. When it is larger than 300 μm, there is a possibility that the tensile strength and fatigue strength may not be sufficient. In addition, when the total amount of grain boundaries is changed, the grain boundary coverage of the Laves phase is changed, and strength (creep strength, tensile strength, fatigue strength, etc.) may decrease. The above "particle size" can be measured on a plane when observed with an observation means such as an electron microscope. In addition, the "average particle diameter" can be a value obtained by averaging the grain diameters of a predetermined number of austenitic steel powder crystals 1 displayed in an observation photograph at a predetermined magnification.

As a comparison with the structure of the austenitic sintered steel of the present invention, the structure of the cast in Patent Document 1 and the structure of the conventional Ni-based alloy (Alloy718) will also be described. 2 is a schematic diagram of an example of the structure of an austenitic steel cast material of Patent Document 1. As shown in FIG. 2, the austenitic steel casting material has austenitic steel powder crystals 4, crystal grain boundaries 5 existing at the boundaries of adjacent austenitic steel crystals, and ura precipitated on the grain boundaries 5. It has a bath phase (6). The cast structure has few grain boundaries, and the grain size and shape of the crystals are not homogeneous. In addition, the micro segregation of the cast structure is larger than that of the sintered material structure. It is thought that the larger the size of the member, the greater the microsegregation, and there is a possibility that defects due to microsegregation or reduction in strength are likely to occur. On the other hand, since a homogeneous structure is formed in the sintered material regardless of the size of the member, microsegregation becomes difficult to occur.

3 is a schematic diagram of an example of the structure of a conventional Ni-based alloy forged material (Alloy718). As shown in FIG. 3, the Ni-based alloy forged material has a Ni-based alloy crystal 7 and a sphere grain boundary (PPB) 8 existing at the boundary between adjacent Ni-based alloy crystals, and a sphere grain boundary (PPB) ) has a delta phase 9 precipitated on (8).

As can be seen from comparison of Figs. 1 to 3, the structure of the austenitic sintered material of the present invention and the structure of the conventional austenitic steel cast material and Ni-based alloy are clearly distinguished.

Below, the composition of the austenitic steel sintered material of this invention is demonstrated. In the composition description below, "%" shall mean "mass %" unless otherwise specified.

Ni (nickel): 25 to 50%

Ni is added as an austenite phase stabilizing element. In addition, Nb and an intermetallic compound (δ phase, Ni ₃ Nb), which will be described later, are generated and precipitated in the particles, thereby contributing to strengthening of the particles. From the viewpoint of phase stability, Ni is preferably 25 to 50% (25% or more and 50% or less), more preferably 30 to 45%, and still more preferably 30 to 40%.

Cr (chromium): 12 to 25%

Cr is an element that improves oxidation resistance and steam oxidation resistance. Sufficient oxidation resistance can be obtained by adding 12% or more in consideration of the operating temperature of the steam turbine. Moreover, when more than 25% is added, intermetallic compounds, such as a (sigma) phase, precipitate, and a fall of high-temperature ductility and toughness is caused. Considering the balance of these, the amount of Cr is preferably 12 to 25%, more preferably 12 to 20%, still more preferably 15 to 20%.

Nb (niobium): 3-6%

Nb is added for stabilization of Laves phase (Fe ₂ Nb) and δ phase (Ni ₃ Nb). As shown in Fig. 2, the Laves phase 6 is mainly precipitated at the grain boundary 2 and contributes to grain boundary strengthening. The δ phase mainly precipitates in the particles and contributes to strengthening. By adding 3% or more, sufficient high-temperature creep strength can be obtained. When more than 6% is added, there is a possibility that harmful phases such as δ phases tend to precipitate. In order to obtain more sufficient strength, the amount of Nb is preferably 3 to 6%, more preferably 3 to 5%, still more preferably 3.5 to 4.5%.

B (boron): 0.001 to 0.05%

B contributes to the precipitation of the Laves phase in the grain boundary. When B is not added, it becomes difficult to precipitate the Laves phase at the grain boundary, and creep strength and creep ductility decrease. The effect of grain boundary precipitation is obtained by adding 0.001% or more. On the other hand, if the addition amount is too large, the melting point is lowered locally, and there is concern about, for example, decrease in weldability. Considering this, the amount of B is preferably 0.001 to 0.05%, and more preferably 0.001 to 0.02%.

Ti (titanium): 0 to 1.6%

Ti is an element that contributes to strengthening precipitation within particles, such as the γ" phase and the δ phase. By adding it appropriately, creep strain in the initial stage can be greatly reduced. However, if it is added excessively, it is oxidized during production. Taking this into account, Ti is preferably 1.6% or less, more preferably 0.3 to 1.3%, and still more preferably 0.5 to 1.1%.

W (tungsten): 0 to 6%

In addition to employment enhancement, W contributes to the stabilization of the Laves phase. Addition of W increases the amount of Laves phase precipitated at grain boundaries, contributing to improvement in breaking strength and ductility in creep characteristics for a long time. When it exceeds 6%, there is a possibility that harmful phases such as δ phases tend to precipitate. Considering this, W is preferably 6% or less, more preferably 5.3 to 6% or less, still more preferably 5.5 to 5.5%.

Mo (molybdenum): 0 to 4.8%

Mo contributes to the stabilization of the Laves phase in addition to the employment enhancement. Addition of Mo increases the amount of Laves phase precipitated at grain boundaries, contributing to breaking strength and ductility in creep characteristics for a long time. Considering this, Mo is preferably 0 to 4.8%, and more preferably 0 to 2% or less.

Zr (zirconium): 0 to 0.5%

The addition of Zr contributes to the precipitation of the Laves phase at the grain boundary, as well as the precipitation of the γ" phase (Ni ₃ Nb) as in B. In a short time or low temperature (less than 750°C, preferably 700°C or less) It is particularly effective. However, since the γ" phase is a metastable phase, it changes to the δ phase by maintaining it at a high temperature (particularly 750°C or higher) for a long time. Therefore, it is not necessary to add it. If the addition amount is too large, the stability of the δ phase is improved, and the γ" phase is quickly changed to the δ phase. In addition, weldability deteriorates. Considering this, Zr is preferably 0 to 0.5%, and 0 to 0.3% or less. more preferable

As described above, the austenitic sintered material of the present invention contains Nb and Ti as main reinforcing elements and does not contain Al as a reinforcing element. For this reason, it is less affected by oxidation by oxygen, etc., and strength (creep strength, tensile strength, fatigue strength, etc.) can be improved.

In addition, the sintered material has a forged structure, and the strength characteristics can be easily controlled according to the required strength of the product by controlling the grain size in heat treatment or the like.

In addition, since the mold of the sintered material is easier to manufacture than the mold of the cast material, even a complex product shape can be manufactured with high yield.

[Method of manufacturing austenitic sintered material]

Next, the manufacturing method of the austenitic steel sintered material of this invention is demonstrated. The austenitic sintered material of the present invention can be manufactured, for example, by the following process.

(1) The raw material powder or raw material alloy having the above-mentioned composition is made into an alloy powder (austenitic steel powder) having an average particle diameter of 250 µm or less by gas atomization or water atomization.

(2) The alloy powder obtained in (1) above is sintered by hot isostatic pressing (HIP). The sintering conditions are, for example, sintering temperature: 1100 to 1300°C and isostatic pressure: 50 MPa or more.

For sintering, instead of HIP, hot pressing under anisotropic pressure or metal powder injection molding (MIM) may be used. Further, after sintering, solution heat treatment (heat treatment temperature: 1100 to 1300°C) and aging heat treatment (heat treatment temperature: 1000°C or less) may be performed.

[Turbine member using austenitic sintered material]

4 is a schematic diagram showing an example of a turbine valve casing to which the austenitic steel sinter of the present invention is applied, and FIG. 5 is a schematic diagram showing an example of a turbine disk to which the austenitic steel sinter of the present invention is applied. As shown in FIG. 4, since the austenitic sintered material of the present invention has excellent strength, it is suitable for a turbine valve casing 10 or a turbine disk 11.

Example

Hereinafter, based on examples, the present invention will be described in more detail.

[Production and Evaluation of Austenitic Steel Sintered Materials]

Sintered materials of Examples 1 to 3 and Comparative Examples 1 to 2 were produced and evaluated. The compositions of Examples 1 to 3 and Comparative Examples 1 to 2 are shown in Table 1 described later. A master ingot or raw material having the composition shown in Table 1 was prepared, and an alloy powder having a particle size of 250 µm or less was produced by a gas atomization method. The obtained alloy powder was sintered by HIP (sintering temperature: 1160°C, isostatic pressure: 100 MPa) to produce sintered materials of Examples 1 to 3 and Comparative Examples 1 to 2. In Comparative Example 1, the amount of Cr was outside the range of the present invention, and in Comparative Example 2, the amount of Ni was outside the range of the present invention.

Alloy (INCONEL) 718 (forged material), which is a Ni-based alloy, as Comparative Example 3, and Alloy (INCONEL) 625 (cast material), which is a Ni-based alloy, as Comparative Example 4, were also prepared and evaluated. Compositions of Comparative Examples 3 and 4 are also listed in Table 1. "INCONEL" is a registered trademark of Hantinton Alloys Corporation.

About Examples 1-3 and Comparative Examples 1-4, 0.2% proof stress and creep resistance temperature were evaluated. The 0.2% yield strength was tested based on JIS G 0567, and the creep test was conducted based on JIS Z 22761.

6 is a graph showing the 0.2% yield strength ratio (based on Comparative Example 4) of Examples 1 to 3 and Comparative Examples 1 to 4. As shown in Fig. 6, both the sintered materials of Examples 1 and 3 showed values higher than those of Comparative Examples 1, 2 and 4, and showed a yield strength ratio of 0.2% equal to or higher than that of the conventional Comparative Example 3 (Alloy718).

7 is a graph showing creep resistance temperature ratios (based on Comparative Example 3) of Examples 1 to 3 and Comparative Examples 1 to 4. As shown in Fig. 7, any sintered material of Examples 1 to 2 also showed a higher value than Comparative Examples 1 to 3, and showed a yield strength ratio of 0.2% equal to or higher than that of the conventional Comparative Example 4 (Alloy625).

6 and 7, Example 2 has a 0.2% yield strength ratio slightly lower than Comparative Examples 2 to 4, but a creep resistance temperature higher than Comparative Examples 2 to 4, and a 0.2% yield strength ratio and creep resistance temperature When judging both together, it can be said to be superior to the comparative example.

6 and 7, although the creep resistance temperature of Example 3 is slightly lower than that of Comparative Example 4, the 0.2% yield strength ratio is much larger than that of Comparative Example 4, and both the 0.2% yield strength ratio and the creep resistance temperature are shown in FIG. 6 and FIG. 7 . When judged collectively, it can be said to be superior to the comparative example.

Fig. 8 is a graph showing the 0.2% yield strength ratio and creep resistance temperature ratio of Example 3 and Comparative Examples 1, 3, and 4; As shown in FIG. 8 , Examples 1 and 3 show larger values than Comparative Example 1 for both the 0.2% yield strength ratio and the creep resistance temperature ratio. In terms of the 0.2% yield strength ratio, Examples 1 and 3 are larger than Comparative Example 4 (Alloy625) and achieve a level equivalent to Comparative Example 3 (Alloy718). Further, with regard to the creep resistance temperature ratio, Examples 1 and 3 are larger than Comparative Example 3 (Alloy718). In particular, with regard to Example 1, a level equivalent to that of Comparative Example 4 (Alloy625) has been achieved.

In general, 0.2% yield strength and creep resistance temperature show a trade-off relationship, that is, when the 0.2% yield strength increases, the creep resistance temperature decreases, and when the creep resistance temperature increases, the 0.2% yield strength decreases. Since both Example 1 and Example 3 are located on the upper right side of the straight line connecting Comparative Example 3 and Comparative Example 4, when judging both the 0.2% yield strength ratio and the creep resistance temperature together, Comparative Example 3 And it can be said to be superior to Comparative Example 4.

As described above, according to the present invention, it has been shown that an austenitic sintered steel material and a turbine member having strength equivalent to or higher than that of Ni-based alloys and less susceptible to the influence of oxygen can be provided.

In addition, the present invention is not limited to the above embodiment, and various modified examples are included. For example, the above embodiments have been described in detail to easily understand the present invention, and are not necessarily limited to those having all the described configurations. In addition, it is possible to replace part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. In addition, it is possible to add/delete/replace a part of the configuration of each embodiment with another configuration.

1, 4: austenitic steel powder crystal
2, 5: grain boundary
3, 6: Laves statue
7: Ni-based alloy crystal
8: sphere grain boundary (PPB)
9: delta phase
10: turbine valve casing
11: Turbine disk

Claims (12)

In mass%, Ni: 25 to 50%, Cr: 12 to 25%, Nb: 3 to 6%, B: 0.001 to 0.05%, Ti: 1.6% or less, W: 6% or less, Mo: 4.8% or less, Zr: 0.5% or less, the balance being Fe and unavoidable impurities, As an austenitic steel sintered material,
The average particle diameter of the powder crystals of the austenitic steel sintered material is 10 to 300㎛, the austenitic steel sintered material. In terms of mass%, Ni: 30 to 45%, Cr: 12 to 20%, Nb: 3 to 5%, B: 0.001 to 0.02%, Ti: 0.3 to 1.3%, W: 5.5% or less, Mo: 2% or less , Zr: 0.3% or less, the balance being Fe and unavoidable impurities, an austenitic steel sintered material,
The average particle diameter of the powder crystals of the austenitic steel sintered material is 10 to 300㎛, the austenitic steel sintered material. In mass %, Ni: 30 to 40%, Cr: 15 to 20%, Nb: 3.5 to 4.5%, B: 0.001 to 0.02%, Ti: 0.5 to 1.1% or less, W: 5.5% or less, Zr: 0.3% An austenitic sintered material comprising the following, the remainder being Fe and unavoidable impurities,
The average particle diameter of the powder crystals of the austenitic steel sintered material is 10 to 300㎛, the austenitic steel sintered material. delete The austenitic steel sintered material according to any one of claims 1 to 3, wherein a Laves phase is precipitated at grain boundaries of the austenitic steel sintered material. The austenitic steel sintered material according to claim 5, wherein the Laves phase is composed of Fe ₂ Nb. A turbine member using the austenitic sintered material according to any one of claims 1 to 3. The turbine member according to claim 7, wherein the turbine member is a turbine valve casing or a turbine disk. delete delete delete delete

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