KR20210024083A - Sintered austenitic steel, austenitic steel powder and turbine member - Google Patents

Abstract

It is an object of the present invention to provide a sintered austenitic steel material, austenitic steel powder, and a turbine member having a strength equal to or higher than that of a Ni-based alloy and less susceptible to the influence of oxygen. By 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: Provides an austenitic steel sintered material and austenitic steel powder containing 0.5% or less, and the balance consisting of Fe and unavoidable impurities.

Description

Sintered austenitic steel, austenitic steel powder and turbine member

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

Nowadays, with the aim of increasing the efficiency of coal-fired power plants, high-temperature steam temperature has been increased. Among the coal-fired power plants currently in operation, the steam temperature of 620°C is being operated as the highest steam turbine (USC: Ultra Super Critical), but in order to suppress _{CO 2 emissions,} It is thought that the higher temperature will proceed. Until now, 9Cr-based and 12Cr-based heat-resistant ferritic steels and the like have been used as high-temperature members of steam turbines, but it is considered that their application becomes difficult as the steam temperature rises.

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

In place of the Ni-based alloy, Patent Document 1 discloses a technique of applying an austenitic steel having excellent strength and castability compatible and an austenitic steel cast product using the same to a turbine member.

Japanese Patent Publication No. 2017-88963

The above-described Patent Document 1 proposes a composition of an austenitic steel in which macro defects in a large cast product are reduced, but manufacturing a mold used for a cast product is relatively labor-intensive. Particularly, when the shape is made for a cast product 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 having a strength equal to or higher than that of a Ni-based alloy and less susceptible to the influence of oxygen.

The first aspect of the present invention for solving the above problems is, by 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 are included, and the balance is an austenitic steel sintered material composed of Fe and unavoidable impurities.

The second aspect for solving the above problems is by 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, and the balance is an austenitic steel sintered material composed of Fe and unavoidable impurities.

The third aspect for solving the above problem is, by 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, and the balance is an austenitic steel sintered material composed of Fe and unavoidable impurities.

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

The fifth aspect of the present invention for solving the above problem is, by 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, and the balance is an austenitic steel powder composed of Fe and unavoidable impurities.

A sixth aspect for solving the above problems is by 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, and the balance is an austenitic steel powder composed of Fe and unavoidable impurities.

The seventh aspect for solving the above problems is by 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, and the balance is an austenitic steel powder composed 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 steel sintered material, austenitic steel powder, and a turbine member that have a strength equal to or higher than that of a Ni-based alloy and are less susceptible to the influence of oxygen.

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

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

Hereinafter, the present invention will be described in detail using the drawings.

[Austenitic steel powder and sintered austenitic steel]

1A is a schematic diagram of an example of the structure of an austenitic steel sintered material obtained by sintering an austenitic steel powder of the present invention, and FIG. This is an observation picture. 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 a boundary between an austenitic steel crystal, and a grain boundary ( 2) It has the Laves phase (3) deposited on the phase.

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

As a comparison with the structure of the austenitic steel sintered material of the present invention, the casting structure of Patent Document 1 and the structure of a conventional Ni-based alloy (Alloy718) are also described. FIG. 2 is a schematic diagram of an example of the structure of the cast austenitic steel material of Patent Document 1. FIG. As shown in Fig. 2, the austenitic steel cast material has a grain boundary 5 existing at the boundary between the austenitic steel powder crystal 4, the adjacent austenitic steel crystal, and the la precipitated on the grain boundary 5. It has a bass shape (6). The cast structure has few grain boundaries, and the grain size and shape of the crystal are not homogeneous. In addition, the micro-segregation of the cast structure is larger than that of the sintered material. It is considered that the larger the member is, the larger the micro-segregation is, and there is a concern that the occurrence of defects due to the micro-segregation and a decrease in strength are liable to occur. On the other hand, in the sintered material, since a homogeneous structure is formed regardless of the size of the member, micro-segregation is less likely to occur.

3 is a schematic diagram of an example of a structure of a conventional Ni-based alloy forging material (Alloy718). As shown in Fig. 3, the Ni-based alloy forged material has a spherical grain boundary (PPB) 8 and a spherical grain boundary (PPB) present at the boundary between the Ni-based alloy crystal 7 and the adjacent Ni-based alloy crystal. ) It has a delta phase (9) deposited on (8).

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

Hereinafter, the composition of the austenitic steel sintered material of the present invention will be described. In the following composition description, "%" shall mean "mass%" unless otherwise specified.

Ni (nickel): 25 to 50%

Ni is added as an austenite phase stabilizing element. Further, Nb and an intermetallic compound (δ-phase, Ni ₃ Nb) described later are generated and precipitated in the particles, thereby contributing to the reinforcement in 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 (chrome): 12 to 25%

Cr is an element that improves oxidation resistance and water vapor oxidation resistance. In consideration of the operating temperature of the steam turbine, by adding 12% or more, sufficient oxidation resistance can be obtained. In addition, when more than 25% is added, intermetallic compounds such as σ phase are precipitated, resulting in a decrease in high temperature ductility and toughness. Considering these balances, the Cr amount is preferably 12 to 25%, more preferably 12 to 20%, and even more preferably 15 to 20%.

Nb (niobium): 3 to 6%

Nb is added for stabilization of the Laves phase (Fe ₂ Nb) and the δ phase (Ni _{3 Nb).} As shown in FIG. 2, the Laves phase 6 mainly precipitates in the grain boundary 2, and contributes to the strengthening of the grain boundary. The δ phase mainly precipitates in the particles and contributes to reinforcement. By adding 3% or more, sufficient high temperature creep strength can be obtained. If more than 6% is added, there is a possibility that harmful phases such as δ phase are liable to precipitate. In order to obtain more sufficient strength, the amount of Nb is preferably 3 to 6%, more preferably 3 to 5%, and even more preferably 3.5 to 4.5%.

B (boron): 0.001 to 0.05%

B contributes to the precipitation of the Laves phase at the grain boundary. When B is not added, the grain boundary raves phase becomes difficult to precipitate, and the creep strength and creep ductility are deteriorated. When 0.001% or more is added, the effect of grain boundary precipitation is obtained. On the other hand, when the addition amount is too large, the melting point is locally lowered, and for example, there is a concern about a decrease in weldability. Taking this into consideration, the amount B is preferably 0.001 to 0.05%, more preferably 0.001 to 0.02%.

Ti (titanium): 0 to 1.6%

Ti is an element that contributes to the in-granular precipitation strengthening, such as a γ" phase or a δ phase. By appropriately adding it, it is possible to significantly reduce the creep strain in the initial stage. However, if it is added excessively, oxidation during production can be achieved. In consideration of this, Ti is preferably 1.6% or less, more preferably 0.3 to 1.3%, and even more preferably 0.5 to 1.1%.

W (tungsten): 0 to 6%

In addition to strengthening employment, W contributes to the stabilization of the Laves Awards. The addition of W increases the amount of the Laves phase precipitated at the grain boundary, and can contribute to the improvement of breaking strength and ductility in creep characteristics over a long period of time. If it exceeds 6%, there is a possibility that harmful phases such as a δ phase are liable to precipitate. In consideration of this, 6% or less of W is preferable, 5.3 to 6% or less is more preferable, and 5.5 to 5.5% is still more preferable.

Mo (molybdenum): 0 to 4.8%

In addition to strengthening employment, Mo contributes to the stabilization of the Laves phase. The addition of Mo increases the amount of Laves phase precipitated at the grain boundaries, and can contribute to breaking strength and ductility in creep characteristics over a long period of time. In view of this, Mo is preferably 0 to 4.8%, and more preferably 0 to 2% or less.

Zr (zirconium): 0 to 0.5%

As with B, the addition of Zr contributes to the precipitation of the raves phase at the grain boundary, and also contributes to the precipitation of the γ" phase (Ni ₃ Nb). For a short time or at a 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 (especially 750°C or higher) for a long time. Therefore, it is not necessary to add. If the addition amount is too large, the stability of the δ phase is improved, and the γ" phase changes rapidly to the δ phase. Further, the weldability deteriorates. Taking this into account, Zr is preferably 0 to 0.5%, and 0 to 0.3% or less. It is more preferable.

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

Further, the sintered material has a forged structure, and by controlling the crystal grain size by heat treatment or the like, it is possible to easily control the strength characteristics according to the required strength of the product.

Further, 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 produced with high yield.

[Method of manufacturing austenite steel sintered material]

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

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

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

Instead of HIP, sintering may use hot pressing under anisotropic pressure or a metal powder injection molding method (MIM). 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 sintered austenitic steel]

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

Example

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

[Production and evaluation of sintered austenitic steel]

The sintered materials of Examples 1 to 3 and Comparative Examples 1 to 2 were prepared and evaluated. The compositions of Examples 1 to 3 and Comparative Examples 1 to 2 are shown in Table 1 to be described later. A master ingot or raw material having the composition shown in Table 1 was prepared, and an alloy powder having a particle diameter 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 prepare 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.

As Comparative Example 3, Alloy (INCONEL) 718 (forging material) as a Ni-based alloy and Alloy (INCONEL) 625 (casting material) as Ni-based alloy as Comparative Example 4 were prepared and evaluated. The composition of Comparative Example 3 and Comparative Example 4 is also indicated in Table 1. "INCONEL" is a registered trademark of Hantinton Alliance Corporation.

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

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

7 is a graph showing the creep content temperature ratio (based on Comparative Example 3) of Examples 1 to 3 and Comparative Examples 1 to 4; As shown in FIG. 7, any of the sintered materials of Examples 1 to 2 exhibited a higher value than that of Comparative Examples 1 to 3, and exhibited a 0.2% proof strength ratio equal to or greater than that of the conventional Comparative Example 4 (Alloy625).

6 and 7, in Example 2, the 0.2% proof strength ratio is slightly lower than that of Comparative Examples 2 to 4, but the creep content temperature is larger than that of Comparative Examples 2 to 4, and the 0.2% proof strength ratio and the creep content temperature are When both are combined and judged, it can be said that it is superior to the comparative example.

In addition, from Figs. 6 and 7, in Example 3, the creep content temperature is slightly lower than that of Comparative Example 4, but the 0.2% proof strength ratio is much larger than that of Comparative Example 4, and both of the 0.2% proof strength ratio and the creep contents temperature It can be said that it is superior to the comparative example when it is judged collectively.

8 is a graph showing 0.2% proof strength ratio and creep content temperature ratio of Example 3 and Comparative Examples 1, 3, and 4; As shown in Fig. 8, in Examples 1 and 3, both the 0.2% proof strength ratio and the creep content temperature ratio are larger than those of Comparative Example 1. As shown in FIG. In addition, with respect to the 0.2% proof strength ratio, Examples 1 and 3 were larger than Comparative Example 4 (Alloy625), and achieved a level equivalent to that of Comparative Example 3 (Alloy718). In addition, regarding the creep content temperature ratio, Examples 1 and 3 are larger than those of Comparative Example 3 (Alloy718). In particular, with respect to Example 1, a level equivalent to that of Comparative Example 4 (Alloy625) was achieved.

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

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

In addition, the present invention is not limited to the above-described embodiments, and various modifications are included. For example, the above-described embodiments have been described in detail to facilitate understanding of the present invention, and are not necessarily limited to 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 a certain embodiment. In addition, it is possible to add, delete, or replace other configurations with respect to some of the configurations of each embodiment.

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

Claims (12)

By 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: An austenitic steel sintered material containing 0.5% or less, and the balance being made of Fe and unavoidable impurities. By 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: An austenitic steel sintered material containing 0.3% or less, and the balance being made of Fe and inevitable impurities. By 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 steel sintered material including the following, the balance being made of Fe and unavoidable impurities. The austenitic steel sintered material according to any one of claims 1 to 3, wherein the austenite steel sintered material has an average particle diameter of 10 to 300 µm. The austenitic steel sintered material according to any one of claims 1 to 3, wherein a Laves phase is precipitated at the grain boundaries of the austenite steel sintered material. The austenitic steel sintered material according to claim 5, wherein the Laves phase _{is made of Fe 2 Nb.} A turbine member using the austenitic steel 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. By 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: An austenitic steel powder containing 0.5% or less, and the balance consisting of Fe and unavoidable impurities. By 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, and the balance is made of Fe and inevitable impurities, austenitic steel powder. By 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% Austenitic steel powder including the following, the balance consisting of Fe and unavoidable impurities. The austenitic steel powder according to any one of claims 9 to 11, wherein the austenitic steel powder has an average particle diameter of 250 µm or less.

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