JP2014043620A - METHOD OF MANUFACTURING INGOT OF Ni-BASED SUPERALLOY - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of manufacturing an ingot of a Ni-based superalloy by ESR method or VAR method, capable of preventing occurrence of defects such as freckle defect or segregation.SOLUTION: A method of manufacturing an ingot 1 of a Ni-based superalloy containing, by mass%, Ni:45 to 60%, Cr:15 to 30%, Mo:1 to 10%, Ti:5% or less, Nb and Ta:total 10% or less and Al:0.2 to 5%, and further Bi:10 to 100 mass ppm by ESR method or VAR method includes using a consumable electrode 2 not containing Bi in a base material and adding Bi when the consumable electrode 2 is dissolved.

Description

  The present invention relates to a method for producing a Ni-base superalloy ingot, and more particularly to a method for producing an ingot with uniform internal quality.

The Ni-base superalloy has excellent corrosion resistance and heat resistance, and maintains strength even at high temperatures. This is because precipitation strengthening by coherent precipitation of the γ ′ phase (Ni ₃ (Al, Ti)) and the γ ″ phase (Ni ₃ Nb) in the γ phase which is the parent phase, and elements such as Cr and Mo Therefore, this superalloy is attracting attention as a material that can be used in high-temperature environments exceeding 700 ° C and severe corrosive environments such as aircraft engines, gas turbines, and oil well equipment. Collecting.

  Furthermore, since this type of alloy is applied to applications requiring safety as described above, the level of quality required for the ingot has been tightened, and along with the increase in size of products. Larger ingots are also required.

  However, since Ni-base superalloys contain a large amount of alloy elements such as Nb and Mo for the purpose of precipitation of the strengthening phase, macrosegregation is more likely to occur compared to alloys with a low alloy element content. It is said that it is difficult to increase the size of the lump.

  Therefore, the ingot of the Ni-base superalloy is an electroslag melting method (ESR (Electro-Slag Remelting) method) or a vacuum arc remelting method (VAR (Vacuum Arc Remelting)), which is a remelting process in which macrosegregation hardly occurs. Method) or a combination of these methods.

  When producing an ingot by these remelting processes, a molten metal pool exists at the top of a columnar ingot formed by solidification, and the molten metal solidifies from the bottom of the pool, while from the upper side of the pool. The molten metal is replenished and ingot formation proceeds.

  In this process, macrosegregation hardly occurs, but defects caused by segregation called Freckle defects may occur depending on conditions such as cooling conditions, alloy composition, and ingot shape.

  Freckle defects are difficult to remove even if heat treatment is performed on the ingot in a later step, and remain as segregation inside the ingot. For this reason, the quality of a product manufactured from the ingot is reduced, which is a serious problem that the required quality cannot be met.

  FIG. 1 is a longitudinal sectional view of a general ingot obtained by the ESR method. In the ESR method, an ingot is manufactured using a water-cooled copper mold. Therefore, dendrites 11 grow on the ingot 1 from the surface toward the inside. Also, inside the ingot 1, depending on the depth of the molten metal pool at the time of manufacture, a Freckle defect 12 is generated in the vicinity of R / 2 of the ingot where the curvature of the pool is large and the interval between the dendrite arms tends to be widened. Cheap. Further, when the ingot is large, Freckle defects are likely to occur near the surface of the slab in addition to R / 2 when the content of heavy elements is high. Here, “R / 2” refers to a position where the distance from the central axis of a cylindrical ingot having a radius R is R / 2 (the same applies hereinafter).

  Freckle defects are segregation caused by concentration of Nb, Mo, W, and other alloy elements, and are a kind of macrosegregation that develops in a streak shape. Further, the freckle defect is generated by a mechanism similar to ghost segregation found in an ingot produced by a normal ingot casting method (a method in which a molten metal is poured into a mold and cast), and is a kind of channel type segregation.

  Here, the generation mechanism of the freckle defect can be explained as follows.

  When solidification proceeds at the solid-liquid interface of the alloy during the production of the ingot, the solute is redistributed, and depending on the alloying elements contained, light elements such as Al, Ti, Si, or Nb between the dendrite arms , Metal liquid mass (micro segregation) in which heavy elements such as Mo and W are concentrated (aggregated) is generated in the bulk (base material) of the molten metal.

  Since microsegregation in which light elements are aggregated has a lower density than bulk, it floats by buoyancy. On the other hand, the microsegregation in which heavy elements are aggregated has a higher density than the bulk, and thus settles due to gravity.

  Although these microsegregations are stationary between dendritic arms formed in the shape of dendrites at the beginning, they are moved slightly by buoyancy or gravity and then merged (aggregated) with other microsegregations. Grows into a macroscopic segregation aggregate to increase volume. The large segregation produced in this way is frozen as the solidification progresses, and the segregation line that remains inside the ingot is the Freckle defect.

  Freckel defects are more likely to occur as the content of light elements or heavy elements in the molten metal is higher due to the generation mechanism. In addition, when the dendrite structure, which is a solidified structure, is coarse, the individual microsegregation that occurs first is large and the microsegregation easily moves, so it easily aggregates and increases in volume, and the Freckle defects become coarser. Cheap.

  As described above, the level of quality required for a Ni-base superalloy ingot is stricter, and high quality without defects is required. In order to meet this requirement, it is necessary to reduce freckle defects at the ingot casting stage.

  Considering the generation mechanism, it is considered that the generation of freckle defects can be suppressed by miniaturizing the dendrite structure. It is known that the dendrite structure can be refined by increasing the cooling rate during casting. As a method for increasing the cooling rate, for example, there is a method for reducing the diameter of the ingot. In this case, however, there is a problem that the size of the product is limited.

  Patent Document 1 describes a method of performing the ESR method with the purpose of increasing the cooling rate at the time of casting and supplying He gas having excellent thermal conductivity as a cooling gas from the lower part of the ingot. However, when the ingot is large, the effect of making the cooling gas excellent in thermal conductivity can be obtained only in the vicinity of the surface portion of the ingot, so the vicinity of R / 2 of the ingot where freckle defects are likely to occur. The cooling rate cannot be changed greatly. For this reason, it is considered that the method described in this document has a small effect of suppressing the Freckle defect.

  In Patent Document 2, as a method for reducing the occurrence of streak-like segregation during the production of an ingot of a Ni-base superalloy, the distribution coefficient of W having a large density difference from Ni is made close to 1 by adding Co. A method of reducing the density difference between the bulk of the molten metal and the microsegregation in which the alloy elements are concentrated is described. Although this method is effective in suppressing streak-like segregation in a Ni-base superalloy containing W, the examples show that if the casting material is changed, the alloy must be redesigned. In addition, there is a problem that mechanical characteristics must be evaluated again.

  Patent Document 3 discloses a method for controlling the shape of a molten metal pool so that the inclination of a solid-liquid interface becomes gentle as a method of manufacturing an ingot of a Ni-base superalloy having no freckle defect by a VAR method or an ESR method. Have been described. In general, it is known from the relationship between the mold and the angle of the dendrite growth direction of the ingot that the side of the pool should be parallel to the mold as much as possible, and that the smaller the pool, the less likely that Freckle defects will occur. Yes. However, to achieve these conditions, it is necessary to slow down the dissolution rate. If the dissolution rate is slow, problems such as deterioration of the casting surface and ingot quality due to coarsening of the solidified structure may occur due to thickening and non-uniformity of the slag shell, and further the production volume. The problem of lowering of the size also occurs.

  In addition, in the Patent Document 4, the inventors have added a very small amount of Bi as a very effective method for suppressing the occurrence of ghost segregation when casting a steel ingot by the ordinary ingot forming method. We proposed a method to reduce the flow of micro-segregation, which causes segregation, by reducing the distance between dendrite arms. However, this method is applied when casting a steel ingot by a normal ingot forming method, and when applied to a casting method by an ESR method or a VAR method, a local high temperature region is used because arc heating is used. It is considered that the yield of Bi is poor and the same effect cannot be obtained.

  By the way, Patent Document 5 describes a method for producing free-cutting steel containing Bi as an element replacing Pb. This document describes that an alloy wire or granular lump containing 5 to 70% by mass of Bi is added to the molten steel injection flow during casting, and the yield of Bi is particularly good in the case of a wire. It is described that. However, the method described in the same document is also applied when casting a steel ingot by the ordinary ingot forming method similarly to the method described in Patent Document 4, and is applied to the casting method by the ESR method or the VAR method. In such a case, it is considered that the same effect cannot be obtained.

JP-A-9-29420 International Publication No. 2009/102028 JP 2003-164946 A JP 2012-35286 A JP 2002-363683 A

  As described above, when a large ingot of a Ni-base superalloy is manufactured by the ESR method or the VAR method, it is difficult to suppress the occurrence of freckle defects by applying the methods described in Patent Documents 1 to 5. is there.

  The present invention has been made in view of this problem, and it is possible to suppress generation of defects such as freckle defects and segregation, and to obtain a large ingot having excellent quality, which is Ni by ESR or VAR. It aims at providing the manufacturing method of the ingot of a base superalloy.

  As a result of studying a method for suppressing the occurrence of freckle defects in an ingot of a Ni-base superalloy manufactured by the ESR method or the VAR method, the present inventors have found that it is effective to contain a predetermined amount of Bi. I found out. This is because the dendrite structure is refined and the movement of microsegregation is inhibited, as in the case of the steel ingot.

  Further, as a result of examining the process of adding Bi, it is not added when casting the consumable electrode used in the ESR method or VAR method, but added when casting the ingot by melting the consumable electrode in the ESR method or VAR method. Thus, it was found that Bi can be added with a high yield. Details of these studies will be described later.

  This invention is made | formed based on these knowledge, The summary exists in the manufacturing method of the ingot of the Ni base superalloy shown to following (1)-(5).

(1) In mass%, Ni: 45-60%, Cr: 15-30%, Mo: 1-10%, Ti: 5% or less, Nb and Ta: 10% or less in total, and Al: 0.2 A consumable electrode that contains a Ni-based superalloy ingot containing 5 to 5% and Bi: 10 to 100 ppm by mass by the ESR method or the VAR method, and does not contain Bi in the base material. A method for producing a Ni-base superalloy ingot, wherein Bi is added when the consumable electrode is melted.

(2) The method for producing a Ni-base superalloy ingot according to (1), wherein a wire containing Bi is welded to a side surface of the base material of the consumable electrode, and the consumable electrode is used.

(3) The method for producing an ingot of a Ni-based superalloy according to (2), wherein the wire is a Ni-Bi alloy.

(4) The method for producing a Ni-base superalloy ingot according to (2) or (3), wherein the wire is coated with iron.

(5) Production of ingot of Ni-base superalloy according to any one of (1) to (4), wherein Bi content of ingot is 40% or more of Bi addition amount Method.

  In the following description, “metal” includes both pure metals and alloys. “Mass%” and “mass ppm” for the component compositions of the Ni-base superalloy and Ni—Bi alloy are also simply expressed as “%” and “ppm”, respectively.

  According to the method for producing a Ni-base superalloy ingot of the present invention, a large ingot having few defects such as Freckle defects and segregation can be obtained. A product manufactured using this ingot as a raw material can be used stably in a high temperature environment or a severe corrosive environment. In addition, since Bi can be added with a high yield, defects can be reliably reduced.

It is a longitudinal cross-sectional view of the general ingot obtained by ESR method. It is a schematic diagram which shows an example of the state at the time of casting an ingot by the ESR method in the manufacturing method of the ingot of the Ni base superalloy of this invention. It is a figure which shows the relationship between Bi content rate and a primary dendrite arm space | interval.

  The ingot of the Ni-base superalloy according to the present invention is manufactured by Ni: 45-60%, Cr: 15-30%, Mo: 1-10%, Ti: 5% or less, Nb and Ta: 10% in total The following is a method for producing an ingot of a Ni-based superalloy containing Al: 0.2 to 5% and further containing Bi: 10 to 100 ppm by the ESR method or the VAR method. A consumable electrode that does not contain is added, and Bi is added when the consumable electrode is dissolved.

  Below, the reason for having prescribed | regulated the manufacturing method of the ingot of the Ni base superalloy of this invention as mentioned above and the preferable aspect of this invention are demonstrated.

1. Production of Ingot by ESR Method FIG. 2 is a schematic diagram showing an example of a state when casting an ingot by the ESR method in the method for producing an ingot of the Ni-base superalloy according to the present invention. As shown in the figure, in the ESR method, a cylindrical consumable electrode 2 that is a material of the ingot 1 has a stub 4 connected to its upper end by welding, and descends as the stub 4 is lowered by a lifting mechanism (not shown). To do. At that time, a molten slag 7 is held in a mold (water-cooled copper mold) 6 in the chamber 5, and when the consumable electrode 2 is immersed in the molten slag 7, an electric current is applied to the molten slag 7. Flows and the molten slag 7 generates heat. The consumable electrode 2 is sequentially dissolved from the lower end by Joule heat of the molten slag 7. The melted consumable electrode 2 becomes droplets and settles in the molten slag 7 and is stacked and solidified while being stored as a pool of the molten metal 3 in the mold 6. In this way, the consumable electrode 2 is sequentially melted to the upper end, and the molten metal 3 is sequentially solidified in the mold 6, whereby the Ni-base superalloy ingot 1 is obtained. Also by the VAR method, a Ni-base superalloy ingot can be obtained from the consumable electrode in substantially the same process as the ESR method described above.

2. In the present invention, in order to contain a predetermined amount of Bi in the ingot 1 obtained by the ESR method or VAR method, Bi is contained in the molten metal 3 in the pool in the casting process by the ESR method or VAR method. It is necessary to let

  Two methods are conceivable: (1) a method of containing Bi in the consumable electrode 2 and (2) a method of adding Bi when the consumable electrode is dissolved by the ESR method or the VAR method.

  As a method of containing Bi in the consumable electrode 2 of (1), the pretreatment stage of casting by the ESR method or the VAR method, that is, the vacuum induction melting method (VIM (Vacuum Induction Melting) method) or the normal ingot forming method is used. A method of adding Bi to the molten metal when producing the electrode 2 can be mentioned. However, in this method, Bi is volatilized from the molten metal twice during the production of the consumable electrode 2 and the casting stage by the ESR method or the VAR method, and Bi has a low melting point and is likely to volatilize from the molten metal. There is a problem that it is bad and inefficient. Therefore, in order to obtain a desired Bi content in the ingot 1 obtained by this method, Bi must be added to the molten metal in anticipation of the volatilization amount for two times when the consumable electrode 2 is produced. In addition, as shown in Examples described later, when an ingot is manufactured by an ESR method or a VAR method using a consumable electrode containing Bi, there is also a problem that the yield of Bi is very poor.

  In the method of adding Bi when the consumable electrode is melted by the ESR method or the VAR method of (2), when Bi is not added to the consumable electrode 2, the opportunity for Bi to volatilize from the molten metal is only once. Become. Therefore, the yield of Bi is better in the method (2) than in the method (1). In the present invention, a consumable electrode to which Bi is not added is used as a base material, and Bi is added when the consumable electrode is dissolved by the ESR method or the VAR method.

  As a method of adding Bi when the consumable electrode is dissolved by the ESR method or the VAR method, (2-1) a method of adding pure Bi granules or a granule of an alloy containing Bi, and (2-2) pure Bi. And a method of adding an alloy wire containing Bi.

  In the method of adding the granule of (2-1), when it is continuously added, undissolved residue may be generated, or it may stay in the upper part of the melt 3 of the boule and may not be uniformly dissolved in the melt 3. In these cases May cause a non-uniform structure in the resulting ingot 1.

  In the method of adding the wire of (2-2), the Bi content of the molten metal 3 can be made constant by matching the descending speed of the wire with the descending speed of the consumable electrode 2, and the ingot 1 having a uniform structure can be obtained. Can be obtained.

  A wire may be provided and fed to the molten metal 3 by a supply device, or may be welded along the axial direction to the side surface of the base material 2a of the consumable electrode 2. FIG. 2 shows a state in which a plurality of wires 2b are welded to the side surface of the base material 2a along the axial direction. In addition to this, one or more holes penetrating in the axial direction may be provided in the base material 2a, and the wire 2b may be passed through the holes.

  When the wire 2b is welded to the base material 2a of the consumable electrode 2 or is passed through a hole provided in the base material 2a and disposed in the axial direction of the consumable electrode 2, that is, in the descending direction, there is no need to newly provide a supply device. There is no need to adjust the descent speed of the wire. Therefore, it is advantageous when there is no sufficient space around the mold 6 and it is difficult to provide a supply device.

  The wire 2b is preferably an alloy wire containing Bi rather than a pure Bi wire. The temperature of the molten metal 3 is 1600 ° C. or higher near the interface with the molten slag 7, whereas pure Bi has a boiling point of 1564 ° C. For this reason, when pure Bi wire is added, Bi volatilizes during casting, and Bi may not be effectively retained in the molten metal 3.

  On the other hand, an alloy containing Bi, for example, an alloy of Ni and Bi (hereinafter referred to as “Ni-Bi alloy”) has a melting point (liquidus temperature) of about 1300 ° C., which is higher than 271 ° C. of pure Bi, Is also considered to be higher than pure Bi. For this reason, when a Ni-Bi alloy wire is added, Bi is less likely to volatilize during casting, so that Bi can be effectively retained in the molten metal 3 and the yield of Bi can be improved. In the Ni—Bi alloy, the Bi content is preferably set to 20 to 70% in which Bi becomes a liquid phase in the molten metal 3.

  Further, in the ESR method, it is necessary to pass the wire 2b without melting the layer of the high-temperature molten slag 7 and to add Bi to the molten metal 3. For this reason, it is preferable to coat the wire 2b of pure Bi or Ni—Bi alloy with iron. When the wire 2b covered with iron is welded to the side surface of the base material 2a of the consumable electrode 2 as shown in FIG. 2, even if the wire 2b is heated, heat is extracted to the base material 2a by the heat conduction of the wire 2b itself. Therefore, even if it is inserted into the molten slag 7, it does not melt immediately, and after melting it settles down due to the difference in specific gravity with the molten slag 7, and flows into the molten metal 3 of the pool. Therefore, by using a wire coated with iron, the yield of Bi can be improved as compared with the case where a wire not coated with iron is used, and Bi can be stably added.

  The above-described method for incorporating Bi into the ingot can also be applied to the production of an ingot of a Ni-base superalloy using an electron beam melting furnace.

3. Component composition of Ni-base superalloy and reason for limitation Ni: 45-60%
Ni is an element effective for stabilizing the parent phase γ phase (austenite) and improving the heat resistance and high-temperature strength of the Ni-base superalloy, and contributes to increasing the strength of the Ni-base superalloy. It is an element indispensable for precipitating the phase and the γ ″ phase from the γ phase. It also has the effect of preventing the formation of harmful precipitates such as the σ phase that degrade the mechanical properties of the Ni-base superalloy. If the rate is less than 45%, the γ phase may become unstable, and even if Ni is contained in excess of 60%, these effects are small and the Ni-base superalloy is only expensive. Therefore, the Ni content is 45 to 60%.

Cr: 15-30%
Cr is an element effective for improving the oxidation resistance and high temperature strength of the Ni-base superalloy. When the Cr content is less than 15%, the oxidation resistance of the Ni-base superalloy is poor, and when it exceeds 30%, the toughness is lowered. Therefore, the Cr content is 15 to 30%.

Mo: 1-10%
Mo is an element effective for improving the high-temperature tensile properties, creep properties, and creep fatigue properties of the Ni-base superalloy by dissolving in the γ phase as a parent phase. Further, it has the effect of suppressing the growth of crystal grains of the Ni-base superalloy by forming carbide with C. However, when the Mo content is excessive, the hot workability of the Ni-base superalloy is reduced and the embrittlement phase is precipitated. As an effective range for these reasons, the Mo content is set to 1 to 10%.

Ti: 5% or less Ti is an element that contributes to improving the high-temperature strength of the Ni-base superalloy by being dissolved in the γ ′ phase, which is a precipitation strengthening phase. Further, it has an effect of suppressing the coarsening of crystal grains of the Ni-base superalloy by forming carbide with C. However, when the Ti content is excessive, the embrittlement phase is precipitated. Therefore, the Ti content is set to 5% or less.

Nb and Ta: 10% or less in total Nb and Ta are precipitation strengthening elements, and are included as necessary in order to precipitate a γ ″ phase that contributes to increasing the strength of the Ni-based superalloy. When the content of Ta is excessive, intermetallic compound phases such as a Laves phase and a σ phase are formed, which is not preferable, so the content of Nb and Ta is 10% or less in total.

Al: 0.1 to 5%
Al constitutes the γ ′ phase, which is the main precipitation strengthening phase of the Ni-base superalloy, and is an element effective for improving the high-temperature strength of the Ni-base superalloy. However, if the Al content is less than 0.1%, a sufficient effect of improving the high-temperature strength cannot be obtained. Further, when the Al content is excessive, coarse aggregation of the γ 'phase at the crystal grain boundaries is caused, and the mechanical properties of the Ni-base superalloy are greatly deteriorated. As an effective range for these reasons, the Al content is set to 0.1 to 5%.

Bi: 10 to 100 ppm
Since Nb and Mo are heavy elements, when the content is within the above range, a sedimentation type freckle defect is likely to occur in the ingot of the Ni-base superalloy. However, the occurrence of freckle defects can be suppressed by casting by the ESR method or the VAR method and setting the Bi content to 10 ppm or more. On the other hand, when the Bi content exceeds 100 ppm, the Ni-based superalloy becomes brittle during hot working, although it is minute. Therefore, the Bi content is set to 10 to 100 ppm.

  The balance other than the above components is Fe and impurities.

  As a result of casting by the ESR method or VAR method, the Ni-base superalloy ingot having the above composition has a fine dendrite structure as compared with the case where it does not contain Bi, so that the Freckle defects are completely suppressed or segregation is prevented. Suppressed and has high internal quality. A product manufactured using this ingot as a raw material can be used stably in a high temperature environment or a severe corrosive environment.

4). Effect of containing Bi The present inventors refined the dendrite structure by adding Bi to the molten metal in the course of casting by the ESR method or VAR method, and adding a trace amount (10 ppm or more) of Bi to the ingot. It was found by the following unidirectional solidification test that it was possible to suppress the occurrence of freckle defects. The reason why the unidirectional solidification is adopted is that the cooling rate can be arbitrarily set.

4-1. Test conditions A cylindrical ingot having a diameter of 15 mm and a height of 50 mm was cast by a general ingot casting method. At that time, Bi is added to the molten metal in which the consumable electrode is melted to form a pool, and ingots having Bi contents of 0 ppm, 4 ppm, 11 ppm, 21 ppm, 25 ppm, 32 ppm and 34 ppm are produced, and Bi is added. No ingot containing no Bi was prepared. The cooling rate was set to 5 to 15 ° C./min in accordance with the cooling rate of the practical ingot.

  For each of the ingots obtained, the distance between about 10 primary dendrite arms extending approximately parallel to the axial direction in the longitudinal section passing through the center was measured, and the arithmetic average value was used as the primary dendrite arm distance of each ingot. It was.

4-2. Test Results FIG. 3 is a diagram showing the relationship between Bi content and primary dendrite arm spacing. In the figure, the primary dendrite arm interval (d) is shown on the vertical axis as the ratio (d / d _B ) to the primary dendrite arm interval (d _B ) of the ingot without Bi. From this figure, it can be seen that the higher the Bi content, the narrower the primary dendrite arm spacing of the Ni-base superalloy becomes, and the finer the dendrite structure. This is considered to be because Bi is an element having an effect of lowering the solid-liquid interface energy of the Ni-base superalloy, and is effective in reducing the primary dendrite arm interval even if its content is very small. As described above, if the Bi content is 10 ppm or more, it is effective in suppressing Freckle segregation.

5. As described above, the Freckle defect is formed due to the movement of the liquid mass (micro segregation) of the metal enriched with the alloy element from between the dendrite arms. Therefore, it is possible to suppress the occurrence of freckle defects by suppressing the movement of the microsegregation.

The present inventors paid attention to the Ra number, which is known as a measure of the occurrence of freckle defects and shows the relationship between the buoyancy of the concentrated liquid mass and the passage resistance. The Ra number is a dimensionless number of convection flows in the temperature field, and is the product of the Pr number (Prandtl number: Prandtl number) and the Gr number (Grashof number: Grashof number), and is represented by the following equation (1).
Ra = Pr · Gr = gβ (T _s −T _∞ ) L ³ / να (1)
Here, g [m / s ² ]: gravity acceleration, β [1 / K]: body expansion coefficient, Ts [K]: object surface temperature, T _∞ [K]: fluid temperature, ν [m ² / s ]: Kinematic viscosity coefficient, α [m ² / s]: Thermal diffusivity, L [m]: Representative length.

  The number of Ra is physically considered to be the ratio of the settling force due to the density difference that is the flow driving force to the flow resistance force, and is proportional to the cube of the representative length L as shown in the above equation (1). When considering the criticality of occurrence of freckle defects, the representative length L in Ra number should be the magnitude of microsegregation between dendrite arms. In this case, since the microsegregation fills the space between the dendrite arms at the initial stage of generation, the size of the microsegregation can be set as the primary dendrite arm interval. Therefore, it can be said that the number of Ra is proportional to the cube of the primary dendrite arm interval.

  As described above, the freckle defect is likely to be coarser as the dendrite structure is coarser. Therefore, it is considered that the freckle defect is more likely to occur as the Ra number is larger. Even if the reduction of the primary dendrite arm interval by adding a small amount of Bi to the ingot of the Ni-base superalloy is relatively small, the Ra number is proportional to the cube of the primary dendrite arm interval. Inclusion of Bi is effective in reducing the number of Ra and is very effective in suppressing the occurrence of freckle defects.

  In order to confirm the effect of the method for producing an ingot of a Ni-base superalloy ingot according to the present invention, the following tests were conducted and the results were evaluated.

1. Test Method Using the apparatus shown in FIG. 2, a consumable electrode was melted by an ESR method to produce a Ni-base superalloy ingot. The production conditions were as shown in Table 1 and the following.

Consumable electrode (base material)
Composition: 54% Ni-18% Cr-3% Mo-5% Nb-1% Ti (the balance being Fe and impurities)
Liquidus temperature: 1336 ° C
Solidus temperature: 1260 ° C
Diameter: 430mm
Produced ingot Diameter: 650mm
Length: 1100mm

  Table 1 shows the addition method of Bi and the addition rate of Bi. The addition method of Bi was as follows.

  In Invention Examples 1 and 2 and Comparative Examples 3 and 4, as shown in FIG. 2, a wire made of pure Bi having a diameter of 10 mm is disposed on the side surface of the base material of the consumable electrode in the axial direction, that is, along the descending direction of the consumable electrode. And welded. In Invention Examples 3 to 5, a wire made of 30% Ni-70% Bi having a diameter of 10 mm and covered with iron having a thickness of 3 mm was welded along the axial direction to the side surface of the base material of the consumable electrode. As a result, in Invention Examples 1 to 5 and Comparative Examples 3 and 4, Bi was added to the molten metal pool formed in the mold.

  In Comparative Example 1, Bi was not added. In Comparative Example 2, Bi was added to the molten metal when producing a consumable electrode by the VIM method, and a consumable electrode containing Bi was used when producing an ingot by the ESR method. Therefore, the composition of the consumable electrode of Comparative Example 2 was obtained by adding 30 ppm Bi to the above composition. Except for Comparative Example 2, Bi was not added to the consumable electrode itself or its base material.

  The addition ratio of Bi is the ratio of Bi in the entire consumable electrode composed of the base material and the wire in Invention Examples 1 to 5 and Comparative Examples 3 and 4 in which the wire is welded to the side surface of the base material of the consumable electrode. In Comparative Example 2 in which Bi is contained, the content of Bi in the consumable electrode.

  As can be seen from Table 1, this test was performed under different conditions: (1) Bi addition rate and content, (2) Bi addition method, and (3) Bi used for Bi addition. Inventive Examples 1 to 5 all satisfied the provisions of the present invention. None of Comparative Examples 1 to 4 satisfied the Bi content of the present invention.

2. Evaluation Items The evaluation items were the Bi content of the manufactured ingot, the yield of Bi, the rate of change of the primary dendrite arm interval, Ra / Ra ₀ , and the occurrence status of the Freckle defect.

  The Bi content of the ingot is a value measured by performing gas analysis using a chip collected from the ingot produced under each of the above conditions as a sample. The yield of Bi was defined as the ratio of the Bi content to the added amount of Bi.

The change rate of the primary dendrite arm interval is determined by the primary dendrite arm interval (d) of the ingot produced under the above conditions and the primary dendrite arm interval (d ₀ ) of the ingot produced under the conditions of Comparative Example 1. Was used as (d−d ₀ ) / d ₀ .

The primary dendrite arm spacing (d, d ₀ ) was determined by etching a longitudinal cross-section specimen taken from the ingot under the following conditions to a position corresponding to R / 2 of the ingot (in the radial direction from the center axis of the ingot). At a position of 100 mm), a plurality of intervals were measured, and the arithmetic average value was obtained. The longitudinal section test piece was 50 mm in width including the surface layer of the ingot, 120 mm in length, and 8 mm in thickness. The reason why R / 2 is selected as the measurement location is that Freckle defects are more likely to appear than the surface of the slab.
Etching conditions Etching solution: Oxalic acid Etching method: Electric field etching Liquid temperature: Room temperature Etching time: 60 to 180 seconds

Ra in Ra / Ra ₀ is the Ra number for the ingot produced under the above conditions, and Ra ₀ is the Ra number for the ingot produced under the conditions of Comparative Example 1.

  The state of occurrence of freckle defects was visually observed on a longitudinal section test piece used for measuring the primary dendrite arm interval. In Table 1, the case where the occurrence of the Freckle defect was not confirmed by visual observation was ○ (good), the case where the occurrence of a minor Freckle defect was confirmed was △ (possible), and the occurrence of a coarse Freckle defect was confirmed. X (impossible).

3. Test results Table 1 shows the results of the above evaluation items together with the addition method of Bi. As can be seen by comparing Invention Examples 1 and 2 and Comparative Examples 3 and 4, when Bi was added using a wire made of pure Bi, the yield of Bi was 39 to 39, regardless of the Bi addition rate. It was as good as 45%. Moreover, when the wire which consists of a Ni-Bi alloy coat | covered with iron like Example 3-5 of this invention was used, the yield of Bi was still better with 51 to 63%. When a consumable electrode containing Bi was used as in Comparative Example 2, the yield was 16%, which was significantly inferior to the case where a wire was used.

  As can be seen from the change rate of the primary dendrite arm interval, the higher the Bi content, the narrower the primary dendrite arm interval and the finer the dendrite structure. This is the same as the result shown in FIG.

Also, high Bi content, as dendrite arm spacing is narrow, the value of Ra / Ra ₀ had become significantly smaller. From this, it can be judged that the higher the Bi content is, the greater the effect of suppressing the occurrence of the Freckle defect. Actually, the evaluation of the occurrence of freckle defects was x when the Bi content was 2.2% or less, whereas the evaluation was Δ if it was 4.0 ppm or more, and 11.7 ppm or more. The evaluation was ○.

  That is, even if the Bi content is as small as 4.0 ppm, it is slightly effective in reducing the primary dendrite arm interval, and if it is 10 ppm or more, it can be said that there is a sufficient effect in suppressing the occurrence of Freckle defects. .

  According to the method for producing a Ni-base superalloy ingot of the present invention, a large ingot having few defects such as Freckle defects and segregation can be obtained. A product manufactured using this ingot as a raw material can be used stably in a high temperature environment or a severe corrosive environment. In addition, since Bi can be added with a high yield, defects can be reliably reduced.

1: ingot, 2: consumable electrode, 2a: base material, 2b: wire 3: molten metal,
4: stub, 5: chamber, 6: mold, 7: molten slag, 11: dendrite,
12: Freckle defect

Claims (5)

In mass%, Ni: 45-60%, Cr: 15-30%, Mo: 1-10%, Ti: 5% or less, Nb and Ta: 10% or less in total, and Al: 0.2-5% A Ni-based superalloy ingot containing Bi: 10 to 100 ppm by mass is produced by an ESR method or a VAR method,
A method for producing a Ni-base superalloy ingot, wherein a consumable electrode containing no Bi is used in a base material, and Bi is added when the consumable electrode is melted.   The method for producing a Ni-base superalloy ingot according to claim 1, wherein a wire containing Bi is welded to a side surface of the base material of the consumable electrode, and the consumable electrode is used.   The method for producing a Ni-base superalloy ingot according to claim 2, wherein the wire is a Ni-Bi alloy.   The method for producing a Ni-base superalloy ingot according to claim 2 or 3, wherein the wire is coated with iron.   The method for producing a Ni-base superalloy ingot according to any one of claims 1 to 4, wherein the Bi content of the ingot is 40% or more of the Bi addition amount.