KR102744440B1 - ferritic alloys with hierarchical B2-NiAl precipitates and manufacturing method thereof - Google Patents

Abstract

The present invention relates to a ferrite alloy including hierarchical NiAl precipitates, which contain, in wt%, Cr 8 to 12, Ni 8 to 12, Al 5 to 8, V more than 0 and less than 15, Mo 2.5 to 4, Zr 0.2 to 0.3, B 0.004 to 0.006, and the remainder Fe and unavoidable impurities.

Description

{Ferritic alloys with hierarchical NiAl precipitates and manufacturing method thereof}

The present invention relates to a ferrite alloy in which V is added to an Fe-Cr-Ni-Al ferrite alloy to form hierarchical precipitates within the alloy.

Fe-Cr-Ni-Al ferritic alloys are alloys designed to maintain strength in high temperature and high pressure environments above 800 K. In particular, they have high compressive strength in high temperature environments and excellent creep resistance, so they are widely used in aircraft engines, energy, and plant industries.

For this reason, ferrite alloys with excellent high-temperature strength and creep resistance are being introduced, such as Korean registered patent 10-1833404, Japanese published patent 2011-068919, and Japanese registered patent 5769204.

However, ferrite alloys have an unstable microstructure, so when used in a high-temperature environment for a long time, precipitates become coarser and mechanical properties decrease. For this reason, a ferrite alloy with a stabilized microstructure and fine precipitates even in a high-temperature environment and a method for manufacturing the same are required.

(0001) Republic of Korea Patent Publication No. 10-1833404 (February 22, 2018) (0002) Japanese Patent Publication No. 2011-068919 (April 7, 2011) (0003) Japanese Patent Publication No. 5769204 (July 3, 2015)

(0001) Microstructural evolution of single Ni2TiAl or hierarchical NiAl/Ni2TiAl precipitates in Fe-Ni-Al-Cr-Ti ferritic alloys during thermal treatment for elevated-temperature applications, GianSong et al., Acta Materialia, Volume 127, 1 April 2017, Pages 1-16)

In order to solve the above problems, the present invention relates to a ferrite alloy in which V and Zr are added and heat treatment is performed after ingot production to form hierarchical precipitates within the alloy.

One embodiment of the present invention for achieving the above object relates to a ferrite alloy including a hierarchical NiAl precipitate, which includes, in wt%, Cr 8 to 12, Ni 8 to 12, Al 5 to 8, V more than 0 and 15 or less, Mo 2.5 to 4, Zr 0.2 to 0.3, B 0.004 to 0.006, and the remainder Fe and unavoidable impurities.

In the above embodiment, the ferrite alloy can form a B2 phase precipitate (B2-NiAl) composed of NiAl.

In the above embodiment, the precipitate may be precipitated in the form of a hierarchical structure in which a B2-NiAl phase and a Zr-rich phase are mixed.

In the above embodiment, when the ferrite alloy contains 4 to 12 V, a precipitate having an average particle size of 50 nm or less can be formed.

In the above embodiment, the ferrite alloy may have a compressive yield strength of 220 to 450 MPa when compressed at a high temperature of 800 to 1,100 K.

In the above embodiment, when the ferrite alloy contains 8 to 12 V, a precipitate having an average particle size of 45 nm or less can be formed.

In the above embodiment, the ferrite alloy may have a compressive yield strength of 280 to 450 MPa when compressed at a high temperature of 800 to 1,100 K.

Another embodiment of the present invention for achieving the above object relates to a method for producing a ferrite alloy including hierarchical NiAl precipitates, the step of a) preparing raw materials composed of, in wt%, 8 to 12 Cr, 8 to 12 Ni, 5 to 8 Al, more than 0 and 15 or less V, 2.5 to 4 Mo, 0.2 to 0.3 Zr, 0.004 to 0.006 B, and the remainder being Fe and unavoidable impurities, b) manufacturing an ingot by vacuum arc melting the raw materials, c) homogenizing the ingot, and d) aging the homogenized ingot.

In the above embodiment, the c) homogenization step can be performed at 1,400 K to 1,600 K for 0.1 to 1 hour.

In the above embodiment, the d) aging treatment step can be performed at 1,400 K to 1,600 K for 0.1 to 1 hour.

The present invention can refine NiAl precipitates by including vanadium (V) in an Fe-Cr-Ni-Al ferrite alloy.

In addition, the present invention additionally includes Zr in a Fe-Cr-Ni-Al ferrite alloy, and can form a precipitate of a hierarchical structure composed of two phases of NiAl and Zr-rich by heat treatment.

Through this, the present invention can improve the strength of the alloy in a high temperature environment of 800K or higher and stabilize precipitates to improve the lifespan of the alloy.

Figure 1 is a drawing for explaining the precipitation hardening mechanism of a ferritic alloy including hierarchical NiAl precipitates according to the addition of vanadium (V).
FIG. 2 is a drawing for explaining a method for manufacturing a ferrite alloy including hierarchical NiAl precipitates according to an embodiment of the present invention.
Figure 3 is an XRD graph of a ferrite alloy according to vanadium (V) content.
Figure 4 is a photograph of a cross-section of a ferrite alloy according to vanadium (V) content taken using SEM-EDS.
Figure 5 is a graph showing the average particle size and volume fraction of a ferrite alloy according to the vanadium (V) content.
FIG. 6 is a drawing for comparing the Vickers hardness at room temperature of a ferrite alloy according to an embodiment of the present invention.
FIG. 7 is a drawing for comparing the compressive yield strength at 973 K of a ferrite alloy according to an embodiment of the present invention.

Hereinafter, a ferrite alloy including hierarchical NiAl precipitates according to the present invention will be described in detail. The drawings introduced below are provided as examples so that those skilled in the art can sufficiently convey the spirit of the present invention. Therefore, the present invention is not limited to the drawings presented below and may be embodied in other forms, and the drawings presented below may be illustrated in an exaggerated manner to clarify the spirit of the present invention. At this time, if there is no other definition in the technical and scientific terms used, they have the meaning commonly understood by those skilled in the art to which this invention belongs, and in the following description and the attached drawings, a description of well-known functions and configurations that may unnecessarily obscure the gist of the present invention will be omitted.

The present invention relates to a ferritic alloy comprising hierarchical NiAl precipitates, specifically, the alloy may comprise, in wt%, 8 to 12 Cr, 8 to 12 Ni, 5 to 8 Al, more than 0 and 15 or less V, 2.5 to 4 Mo, 0.2 to 0.3 Zr, 0.004 to 0.006 B, and the remainder Fe and unavoidable impurities.

According to an embodiment, the present invention can precipitate an intermetallic compound composed of NiAl during a casting and heat treatment process by adding aluminum (Al) to an Fe-Cr-Ni ferrite alloy. In other words, the present invention can form a B2 phase precipitate composed of NiAl in a base material having a BCC lattice structure.

The B2 phase is a small, deformed phase based on the BCC structure, and is a regular phase with a BCC crystal structure similar to that of the parent material, ferrite. Since the B2 phase has a lattice constant similar to that of the parent material, ferrite, it can secure high-temperature stability by forming a coherent relationship with the parent phase. In addition, it can contribute to improving mechanical properties at high temperatures.

That is, the present invention can complement the existing BCC lattice structure ferrite alloy, which has excellent strength at low temperatures but rapidly decreases in strength at high temperatures, by precipitating a B2 phase NiAl intermetallic compound in a BCC ferrite matrix. Hereinafter, a B2 phase precipitate composed of NiAl is defined as a B2-NiAl precipitate.

In addition, the present invention can reduce the lattice mismatch (δ) between the parent material and B2-NiAl precipitates by adding vanadium (V) to the Fe-Cr-Ni-Al ferrite alloy, thereby reducing the average grain size of the NiAl precipitates. Specifically, since V has a larger lattice constant (lattice parameters) than Fe and a higher solubility with Fe, when V is included in the alloy, the lattice constant (α _p ) of the B2-NiAl precipitates and the lattice constant (α _m ) of the parent material in the following relationship 1 can be increased to an appropriate range, thereby reducing the difference between the two parameters. As a result, the lattice mismatch (δ) moves closer to 0, thereby reducing the nucleation energy required for the generation of B2-NiAl nuclei. Through this, the present invention can induce more NiAl precipitates to be formed in the parent material, and can form even finer B2-NiAl precipitates.

[Relationship 1]

δ = (2(α _p - α _m ))/((α _p +α _m ))

(In the above relational expression 1, δ represents the lattice misfit, α _p represents the lattice constant of the B2-NiAl precipitate, and α _m represents the lattice constant of the parent material.)

In other words, the present invention can improve mechanical properties by reducing the nucleation energy of B2-NiAl precipitates by adding V, and as a result, refining the average particle size of B2-NiAl precipitates to 30 to 70 nm.

Figure 1 is a drawing for explaining the precipitation hardening mechanism of a ferritic alloy including hierarchical NiAl precipitates according to the addition of vanadium (V).

As another feature of the present invention, the present invention can form a Zr-rich phase in which zirconium (Zr) is locally concentrated in the B2-NiAl precipitate by performing a homogenization treatment at 1,400 K to 1,600 K for 0.1 to 1 hour and an aging treatment at 900 to 1,200 K for 3 to 8 hours on a ferrite alloy having the composition described above. The Zr-rich phase can be precipitated in a spherical shape having a size of several nanometers. According to an embodiment, the Zr-rich phase can have a composition of Fe ₂₃ Zr ₆ , but is not limited thereto, and it is obvious that the Zr-rich phase can be formed with another composition including Zr.

Specifically, the Zr_rich phase is a phase that is locally formed within the B2 precipitation phase and can be formed during the heat treatment and aging process. Referring to Fig. 1, it can be confirmed that the Zr-rich phase is formed within the ferrite alloy. However, when V is contained in less than 10 wt% in the ferrite alloy, the Zr-rich phase is located in the orowan region (the red line region on the right in Fig. 1) and does not contribute to strengthening or has a minimal influence. However, when V is contained in more than 10 wt% in the ferrite alloy, the Zr-rich phase is located at the intersection of the shearing region (the black region on the left in Fig. 1) and the orowan region, so that strengthening due to shearing and strengthening due to orowan occur simultaneously. As a result, significant precipitation hardening can be implemented due to the Zr-rich phase, so that the strength can be additionally improved.

In other words, the present invention can form a hierarchical structure (or layered structure) in which a B2-NiAl phase and a Zr-rich phase are mixed, rather than a single structure of B2-NiAl, by performing the homogenization treatment and the aging treatment steps in the base material. This goes beyond the B2-NiAl phase and the Zr-rich phase simply forming a composite phase in the alloy, and simultaneously implements strengthening effects due to shearing and strengthening due to Orowan, thereby implementing an improvement in strength that cannot be expected from a typical composite phase.

That is, in the present invention, the hierarchical structure simply means a composite phase in which a B2-NiAl phase and a Zr-rich phase are mixed, but upon closer examination, it can mean a composite phase in which the strength is further enhanced by simultaneously implementing the strengthening effects due to shearing and Orowan due to the B2-NiAl phase and the Zr-rich phase.

Due to these characteristics, the precipitates formed in a hierarchical structure can enhance the properties at high temperatures compared to when they exist as a B2-NiAl single phase. For example, the ferritic alloy manufactured according to an embodiment of the present invention can realize a compressive yield strength of 220 to 450 MPa when compressed at a high temperature of 800 to 1,100 K.

The ferrite alloy including hierarchical NiAl precipitates according to an embodiment of the present invention has been described above. Hereinafter, the composition of the ferrite alloy including hierarchical NiAl precipitates according to an embodiment of the present invention will be described.

The above Cr is contained in an amount of 8 to 12 wt%.

The Cr above is a ferrite stabilizing element and can determine the lattice structure of the base material. If the Cr above is included in the alloy at less than 8 wt%, the alloy may lack ferrite stabilizing elements and an FCC structure may be formed. However, if the Cr above exceeds 12 wt%, intergranular corrosion may be caused and the unit cost increases. For this reason, the Cr above may be included at 8 to 12 wt%.

The above Ni is contained in an amount of 8 to 12 wt%.

The above Ni can improve the corrosion resistance of the ferrite alloy, and can induce precipitate hardening of the ferrite alloy by being precipitated as B2-NiAl. If the Ni is contained in an amount less than 8 wt%, it is difficult to implement the above-described effect, and if the Ni exceeds 12 wt%, the B2-NiAl precipitate may become coarse, thereby reducing the strength. For this reason, the Ni may be contained in an amount of 8 to 12 wt%, more preferably 9 to 11 wt%, and even more preferably 9.8 to 10.2 wt%.

The above Al is contained in an amount of 5 to 8 wt%.

The above Al, like the above Ni, can be precipitated as B2-NiAl to enhance the strength of the ferrite alloy. If the above Al is less than 5 wt%, the precipitate is not sufficiently formed, and conversely, if the above Al exceeds 8 wt%, the B2-NiAl precipitate may become coarse and the strength may be reduced. For this reason, the above Al may be included in an amount of 5 to 8 wt%, more preferably 5.5 to 7 wt%, and even more preferably 6.2 to 6.8 wt%.

The above V is contained in an amount of more than 0 and less than or equal to 15 wt%.

The above V can reduce the lattice mismatch of the ferrite alloy by adjusting the lattice constant of the parent material and the precipitate within an appropriate range, and can further promote the precipitation hardening of the ferrite alloy by reducing the nucleation energy of B2-NiAl. If the above V is not included, the role described above cannot be performed. In addition, when the above V is included in an amount of 10 wt% or more, the precipitation mechanism of the Zr-rich phase moves from the Orowan region to the intersection of the shearing region and the Orowan region, so that the strengthening due to shearing and the strengthening due to Orowan can work simultaneously. As a result, the strength can be additionally improved due to the Zr-rich phase.

On the contrary, when the V exceeds 15 wt%, the composition of the Fe base phase may relatively decrease, so that the crystal structure of the parent material may change from the BCC structure to the L2 ₁ structure. The L2 ₁ structure is a deformation structure based on BCC, but has very strong brittleness at room temperature, so that the compressive strain at room temperature may decrease rapidly. In addition, marketability may be reduced due to the high price of V. For this reason, the V may be included in an amount of more than 0 and less than or equal to 15 wt%, more preferably 3.5 to 13 wt%, even more preferably 7.5 to 13 wt%, and even more preferably 10 to 13 wt%.

The above Zr is contained in an amount of 0.2 to 0.3 wt%.

The above Zr can be precipitated as a Zr-rich phase having a size of several nanometers by going through the homogenization treatment and aging treatment steps described later. Even more preferably, the Zr-rich phase can be precipitated in a spherical shape in the B2-NiAl precipitate to form a hierarchical structure. For this purpose, the Zr can be included in an amount of 0.2 to 0.3 wt%, more preferably 0.23 to 0.28 wt%.

In addition, the above ferrite alloy contains 2.5 to 4 wt% of Mo and 0.004 to 0.006 wt% of B, so as to control the degree of precipitation of precipitates and the mechanical properties of the alloy.

The ferrite alloy including hierarchical NiAl precipitates according to an embodiment of the present invention has been described above. Hereinafter, a method for manufacturing a ferrite alloy including hierarchical NiAl precipitates according to an embodiment of the present invention will be described.

FIG. 2 is a drawing for explaining a method for manufacturing a ferrite alloy including hierarchical NiAl precipitates according to an embodiment of the present invention.

Referring to FIG. 2, the method for manufacturing a ferrite alloy including the hierarchical NiAl precipitate may include the steps of a) preparing raw materials, b) vacuum arc melting the raw materials to manufacture an ingot, c) homogenizing at 1,400 K to 1,600 K for 0.1 to 1 hour, and d) aging at 900 to 1,200 K for 3 to 8 hours.

In the above step a), a raw material having a purity of 99.95% or more can be prepared. The raw material may contain, in wt%, 8 to 12 Cr, 8 to 12 Ni, 5 to 8 Al, more than 0 and 15 or less V, 2.5 to 4 Mo, 0.2 to 0.3 Zr, 0.004 to 0.006 B, and the remainder Fe and unavoidable impurities.

In the above step b), the raw materials prepared in the above step a) can be vacuum arc melted to manufacture an ingot.

According to an embodiment, the step b) may be performed in an argon atmosphere having a purity of 99.9% or higher, more preferably in an argon atmosphere having a purity of 99.99% or higher (4N), and even more preferably in an argon atmosphere having a purity of 99.999% or higher (5N). By using high-purity argon as in the above-mentioned range, the amount of impurities included in the ingot can be reduced, thereby increasing the strength of the alloy.

According to an embodiment, the ingot can be re-melted more than 10 times and solidified in a water-cooled copper furnace to improve the homogeneity of the composition.

Thereafter, the ingot manufactured in the step b) can be heated at a temperature selected from 1,400 K to 1,600 K for 0.1 to 1 hour, and the heated ingot can be maintained at a temperature selected from 900 to 1,200 K for 3 to 8 hours. Hereinafter, the step of heating the ingot can be defined as c) a homogenization treatment step, and the step of maintaining the heated ingot at 900 to 1,200 K for a predetermined time after the homogenization treatment can be defined as d) an aging treatment step.

Specifically, in the step c), the ingot manufactured in the step b) may be loaded into a quartz tube and then loaded into a vacuum chamber. Thereafter, the vacuum chamber may be evacuated to 10 ^-2 torr, the quartz tube may be sealed, and in a vacuum state, the ingot may be heated at a temperature selected from 1,400 K to 1,600 K for 0.1 to 1 hour to perform homogenization treatment.

Finally, the step d) above can be performed by cooling the homogenized test piece to room temperature in the air, then heating it again to a temperature selected from 900 to 1,200 K, and maintaining it for 3 to 8 hours.

That is, the present invention can reduce the lattice mismatch between the parent material and the precipitate and reduce the nucleation energy of the B2-NiAl precipitate through steps a) and b). Thereafter, the average grain size of the B2-NiAl precipitate in the ferrite alloy can be refined to 30 to 70 nm through steps c) and d) while containing 0.1 to 4 wt% of Zr in the raw material, and a Zr-rich phase having a nanoparticle size can be formed inside the B2-NiAl precipitate to form a hierarchical NiAl precipitate.

Through this, the present invention can manufacture a ferrite alloy including hierarchical NiAl precipitates having a Vickers hardness of 480 Hv or more at room temperature and a compressive yield strength of 220 to 450 MPa when compressed at a high temperature of 800 to 1,100 K.

Hereinafter, the ferrite alloy including the hierarchical NiAl precipitate according to the present invention will be described in more detail through examples. However, the following examples are only a reference for describing the present invention in detail, and the present invention is not limited thereto and may be implemented in various forms.

Also, unless otherwise defined, all technical and scientific terms have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description herein is only for the purpose of effectively describing specific embodiments and is not intended to limit the present invention. In addition, the unit of additives not specifically described in the specification may be weight percent.

[Examples 1 to 3, Comparative Examples 1 to 3]

Cr, Ni, Al, Mo, Zr, B, V and Fe having a purity of 99.95% were prepared with the compositions shown in Table 1 below. The above metals were melted by vacuum arc in a high-purity argon (Ar) atmosphere of 5N (99.999%) to produce an ingot.

Next, the steps of melting and cooling the ingot were repeated eight times. At this time, the cooling was performed using a water-cooled copper hearth.

Next, the ingot was processed into a cylindrical test piece with a diameter of 3 mm and a length of 50 mm by vacuum suction casting.

Thereafter, the above test specimen was subjected to homogenization treatment at 1,473 K for 0.5 hour, and the heated test specimen, which was cooled to room temperature through air cooling after the homogenization treatment, was subjected to aging treatment at 1,037 K for 5 hours.

Cr Ni Al V Mo Zr B Fe Example 1 (4V) 10 10 6.5 4 3.4 0.25 0.005 Bal. Example 2 (8V) 10 10 6.5 8 3.4 0.25 0.005 Bal. Example 3 (12V) 10 10 6.5 12 3.4 0.25 0.005 Bal. Comparative Example 1 (0V) 10 10 6.5 - 3.4 0.25 0.005 Bal. Comparative Example 2 (V16) 10 10 6.5 16 3.4 0.25 0.005 Bal. Comparative Example 3 (V20) 10 10 6.5 20 3.4 0.25 0.005 Bal.

* The unit of composition in Table 1 above is weight%.

A. Analysis of precipitate refinement

Fig. 3 is an XRD graph of a ferrite alloy according to the vanadium (V) content, Fig. 4 is a photograph of a cross-section of a ferrite alloy according to the vanadium (V) content taken by SEM-EDS, and Fig. 5 is a graph showing the average grain size and volume fraction of a ferrite alloy according to the vanadium (V) content.

At this time, SEM used TESCAN Mira LMH equipped with VISULAS YAG of ZEISS, and EDS used BRUKER X-Flash4010. XRD measurement was confirmed at Cu-Kα radiation (λ = 1.54187 Å) using BRUKER AXS D8 ADVANCE.

Referring to FIG. 3, it can be confirmed that in the ferrite alloy according to the embodiment of the present invention, both the BCC peak and the B2 preak shift toward a lower angle as V is added. This means that the lattice constants of the base material (Fe) having the BCC phase and the B2-NiAl precipitate having the B2 phase increase. As a result of actually measuring the lattice constant, it was confirmed that the lattice constants of the BCC phase of Examples 1 to 3 and Comparative Example 1 changed as shown in Table 2, the lattice constant of the B2 phase and the average lattice constant of the BCC phase and the lattice mismatch (δ) at that time changed as shown in Table 3 below.

V
(weight%) (110)
BCC (Å) (200)
BCC (Å) (211)
BCC (Å) (Average)
BCC (Å) Comparative Example 1 0 2.8769 ± 0.0001 2.8811 ± 0.0001 2.8835 ± 0.0002 2.8805 ± 0.0001 Example 1 4 2.8822 ± 0.0001 2.8858 ± 0.0001 2.8871 ± 0.0001 2.8850 ± 0.0001 Example 2 8 2.8905 ± 0.0001 2.8891 ± 0.0002 2.8925 ± 0.0001 2.8907 ± 0.0001 Example 3 12 2.8930 ± 0.0001 2.8940 ± 0.0001 2.8952 ± 0.0001 2.8941 ± 0.0001

V
(weight%) (100)
B2 (Å) (Average)
BCC (Å) Misfit
(δ) Comparative Example 1 0 2.8717 ± 0.0003 2.8805 ± 0.0001 -0.00305 Example 1 4 2.8785 ± 0.0003 2.8850 ± 0.0001 -0.00227 Example 2 8 2.8867 ± 0.0003 2.8907 ± 0.0001 -0.00139 Example 3 12 2.8927 ± 0.0011 2.8941 ± 0.0001 -0.00047

That is, referring to Tables 2 and 3 above, as V in the ferrite alloy increases, both the lattice constant of B2 and the lattice constant of BCC increase to an appropriate size, and the lattice mismatch (δ) approaches 0, and more preferably can increase to -0.003 or more.

However, when V in the ferrite alloy exceeds 15 wt%, the lattice structure of the parent material is transformed from BCC to L2 ₁ lattice structure, making it vulnerable to room temperature brittleness. The change in physical properties due to the L2 ₁ structure will be described later.

Referring to FIGS. 4 and 5, it can be confirmed that Comparative Example 1 (FIG. 3-a) not including vanadium has a relatively coarse B2-NiAl precipitate size compared to Examples 1 (FIG. 3-b), 2 (FIG. 3-c), and 3 (FIG. 3-d) including vanadium, and that the average particle size of the B2-NiAl precipitate decreases as the V content increases. Specific data in FIG. 5 are as shown in Table 4 below.

V (weight%) Mean radius (nm) Volume fraction (%) Comparative Example 1 0 56 17.23 ± 0.90 Example 1 4 49 16.82 ± 1.62 Example 2 8 40 18.58 ± 0.82 Example 3 12 30 18.05 ± 1.79

That is, the present invention can refine the average grain size of precipitates to 70 nm or less by including V in a ferrite alloy in an amount of more than 0 and less than or equal to 15 wt%, and more preferably, can reduce the average grain size of precipitates to 50 nm or less by including V in an amount of 4 to 12 wt%. Even more preferably, can reduce the average grain size of precipitates to 45 nm or less by including V in an amount of 8 to 12 wt%, and can reduce the average grain size of precipitates to 35 nm or less by including V in an amount of 10 to 12 wt%.

B. Microstructural analysis of precipitates

FIG. 6 is a photograph of an SAED pattern of a ferrite alloy manufactured according to an embodiment (12V) of the present invention, and FIG. 7 is a photograph of an SAED pattern of a ferrite alloy manufactured according to a comparative example (20V) of the present invention.

In order to observe the hierarchical structure according to the composition of V, selected-area-electron-diffraction (SAED) patterns were captured for Example 4 (V12) containing 10 to 12 wt% of V and Comparative Example 3 (V20) containing more than 15 wt% of V.

Referring to (a) of FIG. 6, Example 4 containing 10 wt% or more of V and 15 wt% or less confirmed that B2 phase precipitates (red circle portion in FIG. 6) and fine precipitates of several nanometers in size with the Fd3m structure (yellow circle portion in FIG. 6) were formed in a hierarchical structure between the precipitates. In particular, referring to (b) and (c) of FIG. 6, which enlarge the B2 phase precipitates in more detail, it can be seen that a large number of fine precipitates were formed between the B2 phase precipitates.

As a result of composition analysis, it was confirmed that the precipitate of the B2 phase was NiAl, and the fine precipitate of several nanometers in size was Zr-rich precipitate. That is, it can be confirmed that the present invention formed a B2 phase precipitate (B2-NiAl) composed of NiAl and a Zr-rich phase of the Fd3m structure in a hierarchical structure.

On the other hand, referring to Fig. 7, the precipitate was transformed into a pattern indicating that it was an L2 ₁ structure rather than a BCC structure. This means that when V in the ferrite alloy exceeds 15 wt%, the precipitate of the existing BCC structure was transformed into an L2 ₁ structure.

C. Yield strength and compressive strain analysis

FIG. 8 is a drawing for comparing Vickers hardness at room temperature of ferrite alloys according to embodiments of the present invention, FIG. 9 is a graph for comparing compressive yield strength at 973 K of ferrite alloys according to embodiments of the present invention, and FIG. 10 is a graph for comparing compressive yield strength at room temperature (300 K).

At this time, the Vickers hardness was measured based on the time when a load of 1 kg was applied for 5 seconds to a cylindrical test piece of 3 mm in diameter and 50 mm in length by an indenter with an interfacial angle of 136° at room temperature, and the average of measurements made 5 or more times was obtained. The compressive strength was measured based on the time when compressed at 1 × 10 ^-3 s ^-1 at 973 K and 300 K.

The results of the Vickers hardness at room temperature and the compressive stress test at 973 K for Examples 1 to 3 and Comparative Examples 1 to 3 are shown in FIGS. 8 and 9 and Table 5 below.

V
(weight%) Heat treatment Vickers hardness (Hv at R.T.) Yield strength (MPa at 973K) experimental value theoretical value Example 1 4 O 481 ± 7 238 222 Example 2 8 O 480 ± 7 284 287 Example 3 12 O 479 ± 5 401 340 Comparative Example 1 0 O 447 ± 5 210 203

Referring to Table 5 above and FIGS. 8 and 9, it can be confirmed that when V is added to a ferrite alloy in an amount of 2 to 15 wt%, the Vickers hardness at room temperature is 450 Hv or higher, more preferably 480 Hv or higher. In addition, it can be seen that when compressed at a high temperature of 800 to 1,100 K, the alloy has a compressive yield strength of 220 to 450 MPa.

Specifically, it can be seen that as the V content increases, the compressive yield strength at high temperatures increases to over 220 MPa. When 8 to 12 V is included, the compressive yield strength at high temperatures increases to over 280 MPa, and when the V content is 10 to 12 wt%, it significantly increases to over 400 MPa.

This is because, as explained above, as the V content increases, the size of the B2-NiAl precipitates becomes finer. In particular, when the V content is 10 to 15 wt%, the compressive yield strength at high temperatures can increase even more significantly due to the hierarchical structure of the precipitates in which the B2-NiAl phase and the Zr-rich phase are mixed.

That is, in the above Example 3, by performing homogenization treatment and aging treatment, the B2-NiAl phase and the Zr-rich phase were formed in a hierarchical structure, and the yield strength at 800 K or higher was improved to 400 MPa or more.

Referring to Fig. 10, it was confirmed that the compressive strain at room temperature rapidly decreased when the V exceeded 15 wt%. This is because, as explained above, when V exceeds 15 wt%, the crystal structure of the precipitate changed from the existing BCC structure to the L21 structure. Although the L21 structure is also a deformation structure based on BCC, unlike B2, it has very low ductility at room temperature and very strong brittleness, making it unsuitable for use. In fact, the examples in which V was included less than 15 wt% had a compressive strain of 30% or more at room temperature, but Comparative Example 2 in which V was included 16 wt% showed a significant decrease to 16.9%, and Comparative Example 3 in which V was included 20 wt% showed a significant decrease to 12.2%. The yield strength and room temperature compressive strain in Fig. 10 are summarized in Table 6 below.

V
(weight%) Yield strength
(MPa at 300K) Compressive Strain
(% at 300K) Example 1 4 1,098 30 Example 2 8 1,062 30 Example 3 12 1,057 30 Comparative Example 1 0 1,044 30 Comparative Example 2 16 1,102 16.9 Comparative Example 3 20 1,110 12.2

For this reason, it is preferable that the V is contained in an amount of more than 0 and less than or equal to 15 wt%, more preferably 3.5 to 13 wt%, still more preferably 7.5 to 13 wt%, and still more preferably 10 to 13 wt%.

As described above, the present invention can provide a ferrite alloy including, in wt%, 8 to 12 Cr, 8 to 12 Ni, 5 to 8 Al, more than 0 and 15 or less V, 2.5 to 4 Mo, 0.2 to 0.3 Zr, 0.004 to 0.006 B, and the remainder Fe and inevitable impurities.

In this process, the present invention can precipitate an intermetallic compound composed of NiAl during the casting and heat treatment processes by adding 5 to 8 wt% of aluminum (Al). As a result, the present invention can form a B2 phase precipitate composed of NiAl in a base material having a BCC lattice structure.

In addition, the present invention can further improve the mechanical properties by making the average particle size of B2-NiAl precipitates fine to 30 to 70 nm by including vanadium (V) exceeding 0 wt%. However, if the vanadium (V) exceeds 15 wt%, the crystal structure of the parent material may change from the BCC structure to the L21 structure, so that the compressive strain at room temperature may decrease to less than 30%. For this reason, it is preferable that vanadium (V) is included in an amount exceeding 0 wt% and 15 wt% or less of V.

Through this, the present invention can manufacture a ferrite alloy including hierarchical NiAl precipitates having a Vickers hardness of 480 Hv or higher and a compressive yield strength of 220 to 450 MPa when compressed at a high temperature of 800 to 1,100 K.

According to another embodiment of the present invention, the present invention can form a Zr-rich phase in which zirconium (Zr) is locally concentrated within the B2-NiAl precipitate by performing a homogenization treatment at 400 K to 1,600 K for 0.1 to 1 hour and an aging treatment at 900 to 1,200 K for 3 to 8 hours in a state in which the vanadium (V) is contained at 10 wt% or more. As a result, the B2-NiAl precipitate and the Zr-rich phase form a hierarchical structure, so that a ferrite alloy having a yield strength of 400 MPa or more in a high-temperature environment of 800 to 1,100 K can be manufactured.

Although the present invention has been described through specific matters and limited examples as above, these have been provided only to help a more general understanding of the present invention, and the present invention is not limited to the above examples, and those skilled in the art to which the present invention pertains can make various modifications and variations from this description.

Therefore, the idea of the present invention should not be limited to the described embodiments, and all things that are equivalent or equivalent to the claims described below as well as the claims are included in the scope of the idea of the present invention.

Claims (10)

Containing, in wt%, Cr 8 to 12, Ni 8 to 12, Al 5 to 8, V more than 0 and 15 or less, Mo 2.5 to 4, Zr 0.2 to 0.3, B 0.004 to 0.006, and the remainder Fe and unavoidable impurities,
Contains B2 phase precipitates (B2-NiAl) composed of NiAl,
A ferritic alloy comprising hierarchical NiAl precipitates, wherein the precipitates have a hierarchical structure in which a B2-NiAl phase and a Zr-rich phase are mixed. delete delete In paragraph 1,
A ferritic alloy comprising hierarchical NiAl precipitates, wherein the above B2 phase precipitates have an average grain size of 50 nm or less. In paragraph 4,
The above ferritic alloy is a ferritic alloy including hierarchical NiAl precipitates having a compressive yield strength of 220 to 450 MPa at 800 to 1,100 K. In paragraph 1,
The above ferritic alloy is a ferritic alloy including hierarchical NiAl precipitates, which contain 8 to 12 V. In paragraph 6,
The above ferritic alloy is a ferritic alloy including hierarchical NiAl precipitates, having a compressive yield strength of 280 to 450 MPa at 800 to 1,100 K. a) a step of preparing raw materials composed of, in wt%, Cr 8 to 12, Ni 8 to 12, Al 5 to 8, V more than 0 and 15 or less, Mo 2.5 to 4, Zr 0.2 to 0.3, B 0.004 to 0.006, and the remainder Fe and unavoidable impurities;
b) A step of manufacturing an ingot by melting the above raw materials using a vacuum arc.
c) a step of homogenizing the ingot; and
d) a step of aging the homogenized ingot;
The above c) homogenization step is performed at 1,400 K to 1,600 K for 0.1 to 1 hour,
A method for producing a ferritic alloy including hierarchical NiAl precipitation, wherein the above d) aging treatment step is performed at 900 K to 1,200 K for 3 to 8 hours.
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