JPH0394044A - Intermetallic compound precipitation strengthened-type high strength powder high cr steel - Google Patents

Abstract

PURPOSE:To manufacture the high strength powder high Cr steel excellent in ductility and toughness by preparing an intermetallic compound precipitation strengthened-type high strength powder high Cr steel contg. specified ratios of C, Cr, Mo and Ni and having a ferrite single phase structure. CONSTITUTION:An intermetallic compound precipitation strengthened-type high strength powder high Cr steel contg., by weight, <=0.03% C and 9 to 18% Cr, furthermore contg. 6 to 15% Mo, moreover contg., at need, <=2% Ni and the balance Fe with inevitable impurities and having a ferrite single phase structure is prepd. In this way, the high strength powder high Cr steel showing excellent capacity as the one for a fast return reactor fuel cladding pipe or the like can be obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Fields] The present invention is applicable to high strength. The present invention relates to an intermetallic compound precipitation-strengthened high-strength powder high-Cr steel that has excellent ductility and toughness and is particularly suitable for fast reactor fuel cladding tubes.

[Problems to be Solved by Prior Art and Inventions] In the fast reactors of Practical 111, it is desired to extend the replacement period of core fuel as long as possible in order to improve economic efficiency, and it is desirable to extend the replacement period of core fuel as much as possible. To achieve this goal, there is a need to develop fuel cladding materials that can withstand long-term use.

Generally, the life of fuel cladding is mainly controlled by creep strength and swelling resistance against neutron irradiation.In the case of fast reactors in the early stages of development, 316 stainless steel etc. are used as fuel cladding material. Austenitic steels have been used in the past, but although austenitic steels have excellent creep strength, they swell to a large extent, making it difficult to use them to extend the lifespan of firewood.

On the other hand, ferritic steel is compared to austenite 11 steel,
Because of its extremely low swelling, it is viewed as a desirable coating material for long life. However, in general,
Since ferritic steel (1 Austenite 1/Series J) has a low creep strength, improving this point is an important issue in commercializing ferritic steel as a fuel cladding material and extending the life of the fuel. It becomes.

Conventionally, methods for improving the creep strength of ferritic steel include: ■ Precipitation strengthening method using carbonitrides, ■ Precipitation strengthening method using intermetallic compounds, ■ Strengthening method by predispersing oxide particles, or these methods. Strengthening methods that combine ■ or ■ are being considered. Among these, intermetallic precipitation strengthened steel produced by ■ has superior creep rupture strength compared to carbonitride precipitation strengthened steel produced by It has features such as superior workability and weldability compared to dispersion strengthened steel. However, this type of material has the disadvantage that intermetallic compounds precipitate during use, deteriorating ductility and toughness.

Also, normal dissolution. Materials manufactured in the forging process (hereinafter referred to as ingot materials) have large crystal grains and poor ductility and toughness. Ductility. When 2% or more of Ni is added to improve toughness, austenite is partially formed during irradiation, causing swelling. Therefore, we developed a material made by solidifying and molding alloy powder (hereinafter referred to as P
/M material), the crystal grains become finer and the ductility and toughness are improved without adding a large amount of Ni.

The present invention eliminates the drawbacks of the ferritic steel involved in the above-mentioned case, can reduce embrittlement during use, has excellent creep rupture strength, and has excellent workability. The object of the present invention is to provide an intermetallic compound precipitation-strengthened steel that has excellent weldability.

[Means for solving the problem] The composition of the high-strength powder high Cr #4 according to the present invention that can achieve the above object is as follows: C≦0.03% and Cr:9
~18%, and further contains Mo: 6~15%. Ni≦2
%. The main feature is that the remainder consists of Fe and unavoidable impurities, and the structure is an intermetallic compound precipitation strengthened steel with a single layer of ferrite.

[Function] The present invention first takes advantage of the fact that the heat-resistant steel according to the above proposal is a ferritic steel, and precipitation of intermetallic compounds occurs during use, and solidifies and molds the alloy powder to refine the crystal grains. An attempt was made to improve ductility and toughness.

FIG. 1.2 shows an optical micrograph as a substitute for a drawing showing the metal structure of an ingot material and a P/M material embedded in solution IA at 1020°C. As is clear from this figure, P/
It can be seen that the crystal grains of the M material are significantly finer than those of the ingot material. Improvement of ductility and toughness is achieved by Mo, Ti, P,
Although it may be thought that segregation of Mn and Ni occurs, the analysis results by EPMA show that no segregation of Mo, Ti, P, Mn, and Ni is observed, and it is considered that the improvement in ductility and toughness is due to the refinement of crystal grains.

The present invention will be explained in more detail below.

C: The lower the C content, the better the toughness of the ferrite, so the lower the C content, the better. In addition, if the C content exceeds 0.03%, the impact value, elongation, and area of area will decrease significantly.
The upper limit of the amount of C is 0.03%.

Cr: Preventing rust from occurring during the manufacturing or assembly of fuel cladding tubes is an important maintenance issue, and for this purpose,
The more Crfif there is, the more rust generation is reduced. In addition, an increase in the amount of Cr lowers the solid solubility limit of Fe2Mo, so the precipitated F
The amount of e2Mo increases and the creep rupture strength is improved. Considering the corrosion resistance of Cr, the lower limit of the Cr content is set to 9%. On the other hand, if the amount of Cr is too large, the solid solubility limit of Mo will also be significantly lowered, making it easier to segregate.

M O: When Mo is added, the creep rupture strength due to the precipitation of Fe2MO increases significantly. However, if a large amount of Mo is added, the toughness decreases and even P/M material becomes brittle, so the upper limit is set at 15%.

On the other hand, if the Mo content is 6% or less, it is difficult to obtain sufficient strength, so this is set as the lower limit.

Ni: Ni is effective in improving ductility and toughness, but 2% is
Since the addition of Ni (1) causes swelling due to partial formation of austenite during high temperature and irradiation, the addition amount of Ni is set to 2% or more.

Although the above elements are essential elements, other elements normally contained as impurities in high Cr steel are also allowed within the impurity range. For example, Ti. The content of Nb or the like in an amount of 1% or less is used for fixing C or refining crystal grains, but this does not affect the wood quality of the present invention.

[Example] After filling an extruded capsule with steel having a composition of 12% Cr, 8% Mo, and 0.01% C, and performing deaeration and sealing treatment,
It did not extrude into a 30φ bar at 00℃. After that, 1050℃
Hot rolled at 10mm to make a plate with a thickness of 10mm.
Solution treatment is performed with water cooling at 0°C. An ingot material with the same composition as this material was made and its ductility and toughness were compared using Charby impact tests and tensile tests. The results show that the transition temperature of the P/M material is 33°C, and the transition i quality of the ingot material is 99°C, and the toughness of P/M or A is clearly better.

Example 2 12% Cr with 3% MO, 7%, 8%. Ingot yield with 10% addition and 12Cr% with MO of 7%, 8%, 1
P/M materials containing 2% and 15% were prepared and subjected to a tensile test. Figure 3 shows the elongation results obtained in the tensile test. M
It can be seen that the P/M material clearly has superior ductility when the O content is 6% or more.

12%Cr, x%Mo, 0.01%Cr, 2.0%Ni
An alloy powder with the same composition was prepared using the above method, and its creep rupture strength was measured. Figure 4 shows the results of creep rupture strength at 650°C for 1000 hours.

It can be seen that when Moi is 6% or more, sufficient creep rupture strength is obtained.

Example 4 12%Cr, 7%Mo, Q. Ql%C, x%Ni(0<
An ingot material having a composition of x<2) was prepared and subjected to a Charby impact test. The results are not shown in FIG. It can be seen that the transition temperature decreases with the amount of Ni, and the toughness is improved by Ni. This tendency also applies to P/M materials. 1
2%Cr, 8%Mo, 0.01%C, 2.0% and O%
A P/M material having a composition of Ni was prepared and subjected to a Charby impact test. The results are shown in FIG. It can be seen that N1 improves toughness.

[Effects of the Invention] The invention is constructed as described above, and by specifying the respective contents of CCr, Mo and Ni, specifying the crystal structure as a single ferrite phase, and making it a powder solidification molding type, An intermetallic compound precipitation-strengthened type that maintains the excellent swelling resistance of ferritic steel, improves ductility and toughness by refining grains, and has high strength and excellent performance for fast reactor fuel cladding tubes, etc. It is now possible to provide high Cr steel materials.

[Brief explanation of drawings]

Figures 1 and 2 are photomicrographs substituted for drawings showing the cross-sectional metallographic structure of ingot material and P/M material, and Figure 3 is MO of high Cr steel.
A graph showing the relationship between content and elongation rate, Figure 4 is a graph showing the relationship between MO content and creep rupture strength of high Cr copper,
Figure 5.6 is a graph showing the relationship between N1 content and transition temperature.

Claims (2)

[Claims] (1) In weight% (the same applies below), C≦0.03% and Cr
: 9 to 18%, and further contains Mo: 6 to 15%, with the balance consisting of Fe and inevitable impurities, and the structure is a single ferrite phase. Powdered high Cr steel. (2) An intermetallic compound precipitation-strengthened high-strength powder high-Cr steel, characterized in that the steel of (1) above contains Ni≦2% as other components and has a single-phase ferrite structure.