JPH06220566A - Molybdenum-base alloy minimal in anisotropy and its production - Google Patents

Abstract

PURPOSE:To provide a molybdenum-base alloy having fine and uniform crystalline grain size, having high strength, free from anisotropy of ductility, and used for structural material for nuclear power use, die for high temp. work, heat resistant structural material, material for heating unit, and mandrel material. CONSTITUTION:At the time of forging a molybdenum-base alloy having a composition consisting of 0.20-0.60% Ti, 0.05-0.20% Zr, 0.01-0.40% C, and the balance molybdenum with inevitable impurities at a temp. in the region between the recrystallization temp. and 500 deg.C, a metal having a deformation resistance in this temp. region lower than that of the molybdenum-base alloy, such as SUS304, SUS314, and SCM420H, is heated up to the temp. region and allowed to exist between a forging tool and the molybdenum-base alloy. By this method, the molybdenum-base alloy where the ratio between the major axis (a) and the minor axis (b) of crystalline grain is regulated to 1.0-1.5 and which has a fine and uniform structure of <=15mum crystalline grain size and is minimal in anisotropy can be produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molybdenum-based alloy having small anisotropy and a method for producing the same. The molybdenum-based alloy with improved strength and ductility anisotropy according to the present invention,
For example, structural materials for nuclear power, molds for high temperature molding, 80
Heat-resistant structural members that are exposed to high-temperature combustion gas at 0 ° C or higher, structural members that require high-temperature impact crack resistance, heating element materials, mandrel materials for high alloy pipes, and die materials for high temperature forging Can be used for etc. Further, the manufacturing method can be applied to not only molybdenum-based alloys but also high-melting-point metals such as Ta alloys, Nb alloys and W alloys.

[0002]

2. Description of the Related Art Conventionally, molybdenum-based alloys are metals with a high melting point of about 2600 ° C.
It has been developed and used as a heat-resistant alloy that can be used as a structural material at temperatures above 00 ° C.

Manufacturing methods of molybdenum-based alloys can be roughly classified into a melting method and a powder molding method.

First, in the manufacturing method by the melting method, since molybdenum is a refractory metal, electron beam melting (EB melting),
It is manufactured by a method such as vacuum arc melting (VAR melting). However, the molybdenum-based alloy produced by the melting method has a crystal grain size of several tens.
Coarse crystal grains of about 0 μm are formed. The coarsening of crystal grains leads to a decrease in strength and variations due to the concentration of stress in the alloy at the crystal grain boundaries.

Therefore, as a method for producing a molybdenum-based alloy, a powder molding method capable of reducing the crystal grain size is widely used today. In that case, since the molybdenum-based alloy has a high melting point, it is usually sintered in a hydrogen atmosphere at 1700 to 2300 ° C. However, even if this sintering is performed, the bulk density can only be increased to about 98% to 99%. Therefore, in order to increase the density, high-temperature processing (rolling, extrusion, etc.) is further performed at 1000 ° C to 1400 ° C to bring the bulk density to almost 100%.

However, the molybdenum-based alloy produced by such a method exhibits strength and ductility anisotropy. It is known that a molybdenum-based alloy is recrystallized by heat treatment at 1400 ° C or higher, and although ductility anisotropy can be improved by performing such recrystallization treatment, this time it can be recrystallized. It is known that the crystal grains become coarse and the ductility and strength are significantly reduced.

However, no method has been reported to date for improving the anisotropy of strength and ductility of a molybdenum-based alloy other than the method of modifying the material by utilizing the recrystallization phenomenon. The following various problems can be considered as the cause of the molybdenum-based alloy exhibiting the anisotropy of strength and ductility.

It is considered that the stretched crystal grains are the cause since the crystal grains extend in the processing direction and show stretched grains due to the high temperature processing which is indispensable for increasing the density. Body-centered cubic metal (bcc metal, same hereafter)
0) Fiber texture [(110) Fiber Texture: In the processing axis direction
The strong texture of (110)] suggests the influence of texture. In addition to this, adverse effects of impurity elements and precipitates existing at the grain boundaries may be considered.

However, the essential cause of the anisotropy of the molybdenum-based alloy has not yet been elucidated, and the range of its application is extremely limited due to rapid cracking and the like which are still considered to be due to the anisotropy of ductility and strength. Is the reality.

The problem of characteristic anisotropy associated with such processing is also found in, for example, the creep strength of an oxide dispersion alloy (ODS alloy) for boilers. The texture generated in such an alloy is disclosed to exhibit strength anisotropy such as creep strength, as shown in JP-A-3-146617 and JP-A-3-243202. There is. In order to improve the anisotropy, adopt a method of removing texture,
Two methods are conceivable: adopting a processing method that does not form a texture in the manufacturing process.

In the above publication, a spinning finishing method and a unidirectional rolling method are proposed as processing methods which do not allow a fiber texture. However, in a molybdenum-based alloy, the high-temperature deformation resistance is very high, and the high-temperature deformation behavior is completely different, so the method disclosed in this publication cannot improve the anisotropy. Further, these publications also suggest that it is impossible to uniformly remove the texture once formed by strong working by working again due to problems such as dead metal formation.

Next, as a texture removing method utilizing the recrystallization phenomenon, Japanese Patent Application Laid-Open No. 54-146206 and Japanese Patent Publication No. 58-360 are disclosed.
There is a method shown in Japanese Patent Publication No. 43. But the melting point is 2600
With a molybdenum-based alloy of ℃, the recrystallization temperature is about 1400 ℃.
As described above, recrystallization causes remarkable coarsening of crystal grains, resulting in a significant decrease in strength. Therefore, it is understood that the improvement of anisotropy by recrystallization is not effective.

[0013]

Thus, at present, due to the anisotropy of strength and ductility, it is problematic to show rapid fracture and cracking of molybdenum-based alloys by rapid cooling from a high temperature of 800 ° C. or higher. Has become. Therefore, it is required to establish a manufacturing method that improves the anisotropy of ductility.

As described above, although there is no prior art for improving the anisotropy of strength and ductility by subjecting a molybdenum-based alloy raw material to strong working, in general alloys, recrystallization treatment is used. Conventionally, various methods for improving the anisotropy of strength and ductility have been proposed. But,
Recrystallization of a molybdenum-based alloy causes coarsening of crystal grains, resulting in a significant decrease in strength. Therefore, the ductility anisotropy improving method utilizing recrystallization cannot be applied.

The object of the present invention is to provide a structural material for nuclear power having a uniform and fine crystal grain size, high strength and no anisotropy of strength and ductility, a mold for high temperature molding, a heat-resistant structural member, It is an object to provide a molybdenum-based alloy used for a heating element material and a mandrel material, and a method for producing the same. A further specific object of the present invention is to provide a molybdenum-based alloy in which the texture is uniformly removed to eliminate the anisotropy of strength and ductility, and a method for producing the same.

[0016]

[Means for Solving the Problems] Regarding the low ductility and ductile anisotropy of molybdenum-based alloys, the following findings were obtained as a result of studying a method of improving the plastic anisotropy and grain boundary strength peculiar to bcc metal. Obtained.

That is, the ductility of a molybdenum-based alloy, which is a bcc metal, is largely dependent on the (110) fiber texture and crystal-stretched grain structure formed by high temperature extrusion, which is essential for powder molding. When such a stretched grain structure is formed, the ductility in the direction perpendicular to the hot extrusion axis (T direction) is significantly reduced or does not show ductility. As a result of a tensile test in the T direction at room temperature, it was found that the fracture morphology shows intragranular cleavage fracture, and therefore the anisotropy of ductility can be reduced by removing the above-mentioned fiber texture and drawn grain texture.

Therefore, it is considered that upsetting deformation from the extrusion axial direction is effective for these improvements, but the fiber texture of the molybdenum-based alloy is obtained by conventional hot pressing and forging from the extrusion axial direction (L direction). When trying to remove the stretched crystal grains, the high temperature deformation resistance is extremely large, so that uniform deformation becomes extremely difficult, resulting in non-uniform properties in the material (
It was found that the tensile strength, elongation, and ductility are partially nonuniform.

The cause is that, in the case of a normal upsetting press, the deformation of the raw material prevents the lateral movement of the raw material along the upsetting surface due to the frictional force between the tool surface and the raw material. For,
Deformation becomes non-uniform. Due to such uneven deformation, the side surface of the material bulges like a barrel, and the lattice lines on the cross section are complicated.
Shows uneven distortion. For this reason, there are zero or very small areas of plastic deformation called dead zones or dead metals near the material side surface and the center of the upper and lower tool surfaces. As a result, since the portion that has undergone plastic deformation and the portion that has not undergone plastic deformation exist in the raw material, the strength in the raw material becomes non-uniform. Further, it is impossible to uniformly remove the fiber texture and the crystal-stretched grain structure, which are the causes of the ductility and strength anisotropy, existing in the matrix.

Therefore, the inventors of the present invention have made various studies, and as a result, a spacer having a lower deformation resistance than that of a molybdenum-based alloy, such as a spacer, is formed between a press forging material as a raw material and a press forging table as a forging tool. , It was found that by heating and sandwiching a SUS304 steel spacer, the material can be uniformly deformed, and the fiber texture and crystal grains can be reduced.

As a result of a more detailed examination, the major axis a and the minor axis of the crystal grains were changed depending on the combination of specific upsetting conditions and alloy composition.
The ratio of b (a / b) is 1.0 to 1.5, and the average particle size is 15μ.
It was found that uniform fine particles of m or less can be realized, and as a result, high strength and a dramatic improvement in the anisotropy of strength and ductility can be achieved, and the present invention was completed.

In the present invention, the weight ratio of Ti: 0.20
% To 0.60%, Zr: 0.05% to 0.20%, C: 0.01% to 0.40%
With the balance consisting of molybdenum and unavoidable impurities, the ratio (a / b) of the major axis a to the minor axis b of the crystal grains is 1.0 to 1.5, and the crystal grain size is 15 μm or less and has a fine and uniform structure. This is a molybdenum-based alloy having small strength and ductility anisotropy.

From another aspect, the present invention provides, by weight ratio, Ti: 0.20% to 0.60%, Zr: 0.05% to 0.20%, C:
A method for producing a molybdenum-based alloy having a small anisotropy in which a molybdenum-based alloy material containing 0.01% to 0.40% and the balance being molybdenum and unavoidable impurities is forged into a predetermined shape. The recrystallization temperature below 500 ℃
Performed in the above temperature range, the deformation resistance at the forging temperature between the forging tool and the molybdenum-based alloy is smaller than the deformation resistance of the molybdenum-based alloy, for example, SUS304, SU
The method for producing a molybdenum-based alloy described above is characterized in that S314, SCM420H and the like are interposed and the metal is preheated to a temperature in the temperature range.

Here, the intervening form of the metal is not limited to the case where the metal is simply sandwiched between the forging tool and the molybdenum-based alloy that is the workpiece, but the case where it is provided so as to surround the entire molybdenum-based alloy. It may be. Many other forms are possible. In any case, there is no particular limitation as long as it acts between the forging tool and the molybdenum-based alloy so as to prevent a sudden temperature drop and enable uniform deformation.

Similarly, regarding the kind of the metal to be interposed, it is sufficient that the forging temperature, that is, the deformation resistance during the actual working is smaller than that of the molybdenum-based alloy. Although the above-mentioned raw material is generally a sintered material or a hot-worked material after sintering, it may be a molten material.

[0026]

Next, the specific operation of the forming method according to the present invention and the operation and effect thereof will be described, and the reason why the alloy composition and the treatment method are limited as described above in the present invention will be described in detail. In the following description, the present invention will be described by taking upset forging as an example, but the present invention is similarly applied to die forging, tap forging, and hot forging.

FIG. 1 is a schematic explanatory view showing an operation example of an upsetting press method according to the present invention. FIG. 1 (a) shows a form before installation and FIG. 1 (b) shows a form after installation. Show. First,
As shown in Fig. 1 (a), spacers with low deformation resistance are used to improve the ductility and strength anisotropy of molybdenum-based alloys.
Are sandwiched between the forging tools 12 and 12 and the blank 14 respectively. In this way, the upsetting press table, that is, the spacer heated between 500 ° C and the recrystallization temperature of Mo between the forging tool and the molybdenum-based alloy raw material heated to the forging temperature (500 ° C to recrystallization temperature). It is sandwiched and pressed or forged. By setting the upsetting amount to 10 to 50%, it becomes possible to uniformly remove the fiber texture and the crystal grain structure in the forged material.

Since the molybdenum-based alloy has a very high melting point of 2600 ° C., it is known as a heat-resistant material and exhibits a very high deformation resistance at a temperature of 500 ° C. or higher for upset forging. Therefore, the spacer is characterized by using a material having a lower deformation resistance than the molybdenum-based alloy within the forging temperature range.

This spacer has the following two characteristic effects. The spacer heated to a high temperature has a role to prevent the temperature of the molybdenum-based alloy from rapidly decreasing due to the contact of the forged tool with the molybdenum-based alloy heated to a high temperature. If the temperature of the end surface of the molybdenum-based alloy drops sharply, the forged material cannot be forged uniformly. Since the spacer heated to a high temperature is inferior in compressive strength to the molybdenum-based alloy at the time of upsetting, it is more susceptible to compressive deformation due to upset forging, and thus spreads to the outer peripheral portion than the molybdenum-based alloy.

Therefore, the spacer heated to a high temperature exerts an effect of spreading the molybdenum-based alloy to the outer peripheral portion at the contact portion with the molybdenum-based alloy at the time of installation. The shape of the molybdenum-based alloy is uniformly deformed by this pushing and spreading effect. Here, the forging temperature is limited to 500 ℃ or more and the recrystallization temperature or less because the molybdenum-based alloy must be heated to 500 ℃ or more to cause brittle fracture cracking at the grain boundaries during upset forging, so 500 ℃ or more. Must be heated to. On the other hand, when the temperature is higher than the recrystallization temperature, grain growth becomes remarkable and the coarsening of the crystal grains cannot be prevented. Therefore, the cast material is heated to the recrystallization temperature or lower.

In the high temperature upsetting press forging without using a usual spacer, when the above-mentioned fiber texture and crystal stretched grain structure are removed, the forging and uneven deformation between the press table and the upsetting material are observed. However, an undeformed portion is formed in the upsetting material, and it is difficult to uniformly remove and remove the fiber texture and the crystal stretched texture. Examples of the metal used as the spacer in the present invention include austenitic stainless steels such as SUS304 and SUS314, but other metals can be used as long as they have predetermined characteristics.

The molybdenum-based alloy of the present invention has a weight ratio of Ti: 0.20% to 0.60%, Zr: 0.05% to 0.20%, and C: 0.
Includes 01% to 0.40%. Here, the reason for adding each element is shown.

Ti: Ti is compounded for the purpose of finely dispersing as TiC in molybdenum, and for increasing the high temperature strength by the pinning effect due to the fine dispersion effect of TiC. In order to exert such effect
Add 0.20% or more of Ti. However, if Ti is added in a large amount exceeding this composition range such as Ti: more than 0.60%, Ti becomes Ti
Since it does not disperse as C but exists as an inclusion of a coarse Ti-based compound of several μm or more and reduces the strength, Ti must be suppressed within the above composition.

Further, the addition of Ti has a function of preventing grain boundary cracking due to oxygen segregation of several hundred ppm which is an unavoidable impurity in the molybdenum grain boundary. This is because Ti combines with oxygen and TiO ₂
And prevents the segregation of oxygen to the molybdenum grain boundaries,
TiO ₂ finely disperses, plays a role of a pinning effect, and has a function of promoting strength improvement. Such an effect is exhibited within the above composition range.

Zr: 0.05% or more is added in order to improve high temperature strength and high temperature ductility. Zr, like Ti, forms and disperses ZrC, thereby having the function of improving the high-temperature strength and the important function of forming a solid solution to improve the high-temperature ductility. That is, Zr is about 600 ° C in molybdenum.
6.7 wt% solid solution was possible and did not bind to C
Zr plays a role of helping high temperature deformation by forming a solid solution in crystal grains. Therefore, Zr is added in an amount of 0.20% by weight or less that can be dissolved in molybdenum at room temperature.

Thus, the addition of Zr has the function of promoting upsetting during high temperature upsetting, but if Zr is added in excess of this composition ratio, it tends to become a starting point of fracture at high temperatures and has a low melting point.
Zr phase of hcp structure (closest hexagonal lattice) precipitates and lowers high temperature strength.

C: Add 0.01% or more to form carbides with Ti and Zr and disperse them in molybdenum. Excessive C in the grain boundaries acts to reduce the grain boundary strength, so the amount of C added must be kept within the range where carbides can be formed with Ti and Zr. Therefore, the weight ratio is 0.01% to 0.40% C. Next, the function and effect of the present invention will be described more specifically by way of examples.

[0038]

【Example】

(Example 1) As a typical molybdenum-based alloy (0.5% Ti-
0.08% Zr-0.02% C-Mo) was sintered at 2000 ° C. and then subjected to conventional hot extrusion. The forging material according to the present invention was subjected to the thus obtained material. SUS3 as spacer material
Using 04, upsetting press (upsetting rate 40%) was performed in the temperature range of 1000 ° C to 1200 ° C, and tensile test pieces were sampled from each part of the upsetting material and tested. The results are shown in Fig. 2 (a) and Fig. 2 (b).
Are shown together.

[Conditions of Tensile Test] Temperature: Room temperature Test piece: Diameter 6.0 mm (parallel portion length GL = 20 mm) Strain rate: 0.5 × 10 ^-3 [S ^-1 ] Test item: Tensile strength, elongation Fig. 2 (a 2) and FIG. 2B show the results in the T direction and the L direction, respectively, there is no significant difference in tensile strength and elongation depending on the sampling location of the test piece, and it can be said that it is homogeneous inside the upsetting material. .

Further, the results of measuring the presence or absence of (110) fiber texture (extrusion axis direction) peculiar to bcc metal before and after upsetting are shown in pole figures in FIGS. 3 (a) and 3 (b), respectively. As is clear from the results of these pole figures, it is possible to uniformly remove the (110) fiber texture by the upsetting according to the present invention.

It was also found that the crystal grain structure can be uniformly removed. As a result of further investigation of these results, it is possible to eliminate the dead zone or dead metal (plastic deformation is zero or very small part) formed on the side surface of the upsetting material and the center of the upper and lower upsetting bases.
It has been found that compressive deformation can be uniformly applied to all parts.

(Example 2) In the same manner as in Example 1, the molybdenum-based alloy having the composition shown in Table 1 was subjected to upset forging of the hot extruded material. The results are summarized in Table 1 together with the treatment conditions. In all the alloys, the elongation ratios in the L and T directions are 70% to 130%.

As described above, it was found that the anisotropy of elongation can be significantly reduced as compared with that after extrusion. The material homogeneity in Table 1 shows the elongation when tensile test pieces are sampled from the alloy billet at various locations in the upset forging axis and in the direction perpendicular to the forging axis and the tensile test is performed 20 times under the above-mentioned tensile test conditions. The value is shown in the range of 0 to 80%, the case where variation occurs is indicated by x, and the case where the elongation value falls within the range of 10 to 30% is indicated as o. For comparison, the case where the material uniformity is not satisfied due to the large number of coarse inclusions is shown. In addition, in the examples of the present invention, the characteristics before performing the upsetting method according to the present invention (in the state of being hot extruded) are shown for comparison, but it is understood that the uniformity inside the billet material is not satisfied.

[0044]

[Table 1]

[0045]

When a molybdenum-based alloy is used as various heat-resistant structural materials, any material exhibiting ductility and strength anisotropy will rapidly break and crack in the cold. However, a material (S
(110) The high temperature upsetting press and forging method according to the present invention sandwiching (for example, US304 steel) can improve the (110) fiber texture and drawn grain structure and produce a material having no anisotropy of ductility and strength. Is possible.

Further, the molybdenum-based alloy having improved ductility anisotropy can prevent thermal stress cracking, which often occurs in normal molybdenum-based alloys, when rapid cooling occurs at high temperatures of 800 ° C. or higher. As described above, by improving the ductility and the anisotropy of strength according to the present invention, the applications can be expanded to the fields where the applications of molybdenum-based alloys as heat-resistant structural materials have been limited so far.

[Brief description of drawings]

1 (a) and 1 (b) are views of an upsetting press performed by inserting a spacer according to the present invention, respectively.
It is a schematic explanatory drawing of the form after installation.

2 (a) is a graph showing tensile strength and elongation in the T direction after forging press, and FIG. 2 (b) is a graph showing tensile strength and elongation in the L direction, respectively.

FIG. 3 shows a comparison between the pole figure after hot extrusion and the pole figure of an upset forged material of this extruded material. FIG. 3 (a) shows the pole figure before upset (after extrusion) (110), Figure 3 (b) shows the pole figure after installation (11
0).

Claims (2)

[Claims] 1. By weight ratio, Ti: 0.20% to 0.60%, Zr: 0.05% to 0.20%, C: 0.01%
~ 0.40%, the balance consisting of molybdenum and unavoidable impurities, the ratio (a / b) of major axis a to minor axis b of crystal grains is 1.0 to 1.5, and the crystal grain size is 15 μm or less. A molybdenum-based alloy having a structure and low anisotropy in strength and ductility. 2. By weight ratio, Ti: 0.20% to 0.60%, Zr: 0.05% to 0.20%, C: 0.01%
A method of manufacturing a molybdenum-based alloy that has a low anisotropy and is made into a prescribed shape by forging a molybdenum-based alloy matrix that contains ~ 0.40% and the balance is molybdenum and unavoidable impurities. It is carried out in a temperature range not higher than the crystallization temperature and not lower than 500 ° C, and a metal whose deformation resistance at the forging temperature is smaller than that of the molybdenum-based alloy is interposed between the forging tool and the molybdenum-based alloy, and the metal is preliminarily added The method for producing a molybdenum-based alloy according to claim 1, wherein heating is performed to a temperature in the temperature range.