JPH05271853A - Intermetallic compound precipitation hardening high strength p/m high cr steel - Google Patents

Abstract

PURPOSE:To provide a high strength P/M (powder metallurgy) high Cr steel where the coarsening of crystalline grains is prevented and ductility and toughness are improved by means of fine crystalline grains and also an ultrasonic flaw detection is applicable. CONSTITUTION:This steel is an intermetallic compound precipitation strengthening steel which has a composition consisting of <=0.1% C, 9-18% Cr, 6-15% Mo, 0.01-0.1% Y2O3, and the balance Fe with inevitable impurities or containing, together with the above, <=2% Ni, and also has a structure composed of ferritic single phase.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intermetallic compound precipitation strengthened high strength powder high Cr steel which is excellent in strength, ductility and toughness and is particularly suitable for fuel cladding tubes for fast reactors.

[0002]

2. Description of the Related Art In a fast reactor in a practical period, it is desired to extend the replacement period of core fuel as long as possible in order to improve the economical efficiency. Development of fuel cladding material is desired.

In general, the life of a fuel cladding tube is mainly governed by creep strength and swelling resistance to neutron irradiation. In fast reactors in the early stages of development, austenitic steels such as SUS316 stainless steel were used as the fuel cladding material, but austenitic steels have excellent creep strength, but on the other hand they have large swaying. It is difficult to extend the life of the.

On the other hand, since ferritic steel has a remarkably smaller swelling than austenitic steel, it is regarded as a promising cladding material for long life. However, since ferritic steels generally have lower creep strength than austenitic steels, improving this point is important for practical application of ferritic steels as fuel cladding materials and for prolonging the life of fuels. Is becoming

Conventionally, as a method for improving the creep strength of ferritic steel, a precipitation strengthening method using carbonitride,
A precipitation strengthening method using an intermetallic compound, a strengthening method by previously dispersing oxide particles, or a strengthening method by a combination thereof or the like has been studied. Of these, the intermetallic compound precipitation-strengthened steel by means of which the creep rupture strength superior to that of the carbonitride precipitation-strengthened steel by means of is obtained, and because the precipitates can be processed in the form of a solid solution, the oxide dispersion strengthening by It has features such as better workability and better weldability than the shape steel. However, this type of material has a drawback that an intermetallic compound is precipitated during use and deteriorates ductility and toughness.

In the material produced by the usual melting and forging process (hereinafter referred to as ingot material), the crystal grains have large ductility,
The toughness is further deteriorated. On the other hand, when a material obtained by solidifying and molding alloy powder (hereinafter referred to as P / M material) is used, the crystal grains become finer, and the ductility and toughness are improved as compared with the ingot material.

By the way, in the cladding tube, an ultrasonic flaw detection test is required to detect minute defects, but in order to carry out this inspection, crystal grains must be made finer than ASTM No. 7. Even in the current ingot materials and P / M materials, since this steel is a ferrite single phase, it is difficult to make finer grains than ASTM No. 7 because the crystal grains are likely to grow during processing and heat treatment.

[0008]

The present invention has been made to solve the above-mentioned drawbacks of ferritic steels, and its purpose is to prevent coarsening of crystal grains and to obtain fine crystal grains to obtain ductility and toughness. It is intended to provide a high strength powder high Cr steel capable of applying an ultrasonic flaw detection test while improving the above.

[0009]

The high-strength powder high-Cr steel according to the present invention, which has been able to achieve the above objects, has a composition of C
≦ 0.1% and Cr: 9-18%, and Mo:
6 to 15% and Y ₂ O ₃ : 0.01 to 0.1%, or Ni ≦ 2% together with these, the balance consisting of Fe and unavoidable impurities, and the structure of which is a ferrite single phase intermetallic compound precipitation strengthening The point is that it is made of shaped steel.

[0010]

The inventors of the present invention first achieved high strength by utilizing the fact that the heat-resistant steel according to the above proposal is ferritic steel and that precipitation of intermetallic compounds occurs during use, and at the same time, the mechanical alloying method is used. (Hereinafter, referred to as MA method), a small amount of oxide particles are mixed and dispersed in the alloy powder, and the powder (hereinafter, referred to as MA method).
(Hereinafter referred to as powder) was solidified and molded to make the crystal grains finer, and it was attempted to improve ductility and toughness (hereinafter, the obtained steel is referred to as MA material).

FIG. 1 shows 12Cr-6 to 15Mo-0.05Y.
It is a graph which shows the tensile property in room temperature of MA material obtained from ₂ O ₃ MA powder. For comparison, FIG. 1 also shows the tensile properties of the ingot material containing predetermined Mo.
As is clear from this figure, in the MA material, the Mo content is 6
It shows an elongation of 20 to 30% in the range of -15%. On the other hand, in the ingot material, almost no elongation is observed when the Mo content is in the above range. That is, such low ductility is inferior in workability, and it is difficult to manufacture a thin-walled small-diameter tube such as a cladding tube. In addition, there is a risk of unstable destruction during use.

FIGS. 2 (1) to 2 (4) are drawing-substitute optical micrographs showing the metal structures of various steel materials solution-treated at 1040 ° C. As is clear from this figure, it can be seen that the crystal grains of the MA material are extremely fine. The reasons for limiting each component in the present invention are as follows.

C: 0.1% or less Since the lower the C content, the better the toughness of the ferrite, the C content should be as low as possible. However, when mechanical alloying is performed, it is unavoidable that C is mixed from a metal steel ball or a tank, and the upper limit of C is set to 0.1% so that ductility and toughness are not significantly impaired.

Cr: 9-18% It is an important maintenance problem that rust does not occur during the manufacture or assembly of the fuel cladding tube.
The larger the amount of Cr, the less rust generation is, which is advantageous. Considering the corrosion resistance of Cr, the lower limit of the Cr content is 9%. on the other hand,
If the amount of Cr is too large, the solid solubility limit of Mo is also greatly reduced, and segregation is likely to occur. Therefore, the upper limit is 18%.

Mo: 6 to 15% When Mo is added, the creep rupture strength due to precipitation of Fe ₂ Mo is significantly increased. However, if a large amount of Mo is added, the toughness decreases and the solution treatment becomes difficult, so the upper limit is made 15%. On the other hand, if the amount of Mo is less than 6%, it is difficult to obtain sufficient strength, so 6% is made the lower limit.

Y ₂ O ₃ : 0.01 to 0.1% If Y ₂ O ₃ particles are 0.01% or more, it is possible to suppress the growth of crystal grains and make the crystal grains extremely fine. However, when Y ₂ O ₃ particles exceeding 0.1% are mixed, although the strength is significantly increased by the dispersion of Y ₂ O ₃ particles, recrystallization is less likely to occur during the heat treatment after processing and the workability deteriorates. .. Moreover, it becomes difficult to apply a usual welding method. Therefore, the upper limit of the amount of Y ₂ O ₃ is set to 0.1% and the lower limit is set to 0.01%.

The present invention is intended to achieve high strength by precipitation strengthening of an intermetallic compound and to suppress coarsening of crystal grains, which is a drawback of ferritic single phase steel, by dispersing Y ₂ O ₃ particles.

In the present invention, the above elements are essential elements, but other elements contained as impurities in ordinary high Cr steel are also allowed within the impurity range. For example, 1 for Ti, Nb, etc.
The content of not more than% is used for fixing C or refining crystal grains, but addition of these does not impair the essence of the present invention.

The addition of P, B, Zr, etc., which is believed to change the formation of intermetallic compounds and the morphology of grain boundary precipitates, is generally well known, but the inclusion of these elements also constitutes the essence of the present invention. It does not invade. It is generally well known that addition of Ni improves toughness and ductility.
The addition of Ni described below is also effective from the viewpoint of improving the properties of the steel of the present invention. Further, in addition to Y ₂ O _3, addition of ZrO ₂ or TiO ₂ may be considered as the type of oxide particles for refining crystal grains, but this does not impair the essence of the present invention.

The present invention will be described in more detail with reference to the following examples, but the following examples are not intended to limit the present invention, and any design changes that may be made based on the spirit of the preceding and following paragraphs are intended for the present invention. It is included in the technical scope.

[0021]

EXAMPLES Example 1 As an ingot material, 12% Cr, 8% Mo, 0.01%
C, balance Fe, and steel of unavoidable impurities were manufactured by vacuum melting, and P / M material was hot extruded (extrusion ratio 5, extrusion temperature 1100 ° C.) by packing ingot powder produced by Ar gas atomization method into capsules. ) Was used for solidification.

The MA material is the above-mentioned atomized powder and Y ₂ O ₃
The particles (average particle size 180Å) uniformly mixed by the MA method were packed in capsules and solidified by hot extrusion (extrusion ratio 5, extrusion temperature 1100 ° C). The MA method is performed by using a 1D type attritor manufactured by Mitsui Miike Kakoki Co., Ltd., with an agitator rotation speed of 300 rpm and a processing time of 2
It was carried out for 4 hours under the condition of powder / ball weight ratio = 1/15. These materials were subjected to solution treatment at 1040 ° C. for 1 hour with water cooling, and the metal structure thereof was observed. The result is
This is as shown in FIG. Ingot material and P / M
Comparing the materials, the P / M material clearly has finer crystal grains, but the MA material is even finer, and it is so fine that it is almost impossible to distinguish the crystal grains in the optical micrograph. ..

Example 2 12% Cr, 8% Mo, 0.01% C steel and 12% Cr,
With respect to 8% Mo, 0.01% C, 2% Ni, the balance Fe, and inevitable impurities of steel, three materials, an ingot material, a P / M material, and an MA material, which were manufactured by the same manufacturing method as in Example 1, were prepared. A Charpy impact test was conducted to evaluate the toughness. The results are shown in Table 1. As is clear from Table 1, the transition temperature is also reduced in accordance with the refinement of crystal grains. Further, the addition of Ni also works to improve the toughness.

[0024]

[Table 1]

[0025] Example 3 12% Cr, 6% ~15 % Mo, and 0.01% C, balance Fe and unavoidable impurities steel, using Y ₂ O ₃ particles, Y ₂ O ₃ is 0.
A tensile test was performed on the sample manufactured by the MA method in accordance with the method of Example 1 so that the content became 05%. The result is as shown in FIG. The ingot material shown as a comparative example almost brittlely fractures when Mo is 6% or more, while the MA material has an elongation of 20% or more.

Example 4 Using Y ₂ O ₃ particles and steel containing 12% Cr, 8% Mo, 0.01% C, balance Fe and unavoidable impurities, Y ₂ O ₃ is 0.05% to 0.1%.
A tensile test was conducted on the sample manufactured by the MA method according to the method of Example 1 so that the content would be 10%. The result is shown in FIG. As is clear from FIG. 3, up to 0.10% Y ₂ O ₃ is 20
It shows an elongation of at least%, and is considered to have sufficient workability.

Example 5 Using 12% Cr, 8% Mo, 0.01% C, steel with balance Fe and unavoidable impurities, and Y ₂ O ₃ particles, Y ₂ O ₃ was adjusted to 0.05% and 0.10%, respectively. The MA material of the present invention was manufactured according to the method of Example 1. The sample was cold-worked at 50% and heat-treated at 1050 ° C. and 1200 ° C. for recrystallization. This recrystallized structure is shown in FIGS. 4 (1) and 4 (2). As is clear from Fig. 4, the crystal grains are N even at 1200 ° C.
It is finer than o.7, and it can be seen that coarsening of crystal grains is suppressed.

[0028]

The present invention is configured as described above,
By specifying the respective contents of C, Cr, Mo and Y ₂ O ₃ , specifying the crystal structure to ferrite single phase, and adopting the powder solidification molding type, the excellent swaying resistance of ferrite steel is maintained. At the same time, it is possible to provide an intermetallic compound precipitation-strengthened high-Cr steel material which has finer crystal grains to improve ductility and toughness and has high strength and excellent performance as a material for fuel cladding tubes for fast reactors. Became.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between Mo content and elongation of ingot materials and MA materials.

FIG. 2 is a drawing-substitute photomicrograph showing the metal structures of an ingot material, a P / M material, and an MA material.

FIG. 3 is a graph showing the relationship between the Y ₂ O ₃ content and the elongation of high Cr steel.

FIG. 4 is a drawing-substitute optical micrograph showing a recrystallized metal structure of 12Cr-8Mo-0.05Y ₂ O ₃ steel after cold working and heat treatment.

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[Procedure amendment]

[Submission date] March 15, 1993

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be corrected] Brief description of the drawing

[Correction method] Change

[Correction content]

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between Mo content and elongation of ingot materials and MA materials.

FIG. 2 is a drawing-substitute micrograph (× 10) showing the metal structures of an ingot material and a P / M material.

FIG. 3 is a drawing-substitute micrograph (× 100) showing the metal structures of P / M material and MA material.

FIG. 4 is a graph showing the relationship between the Y ₂ O ₃ content and the elongation of high Cr steel.

FIG. 5 is a drawing-substitute optical micrograph showing a recrystallized metal structure of 12Cr-8Mo-0.05Y ₂ O ₃ steel after cold working and heat treatment.

[Procedure Amendment 2]

[Document name to be corrected] Drawing

[Correction target item name] All drawings

[Correction method] Change

[Correction content]

[Figure 1]

[Fig. 2]

[Figure 3]

[Figure 4]

[Figure 5]

Claims (2)

[Claims] 1. By weight (the same applies hereinafter), C ≦ 0.1% and Cr: 9-18% and Mo: 6-15%
And Y ₂ O ₃ : 0.01 to 0.1%, the balance being Fe and unavoidable impurities, and the structure being a ferrite single phase, intermetallic compound precipitation strengthened high strength powder high Cr
steel. 2. C ≦ 0.1%, Ni ≦ 2% and Cr: 9 to
18% and Mo: 6-15% and Y
₂ O ₃ : 0.01-0.1%, the balance consisting of Fe and unavoidable impurities, and the structure being a ferrite single phase, intermetallic compound precipitation strengthened high strength powder high Cr steel.