JPH0362783B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field This invention is applicable to nuclear reactor constituent materials subjected to neutron irradiation, such as metal materials used as reactor vessel materials such as fast breeder reactors and light water reactors, particularly metal materials containing trace amounts of boron (B), such as The present invention relates to a method for producing metal materials such as carbon steel, low chromium molybdenum (Cr-Mo) steel, ferritic high chromium steel, stainless steel, and superalloys. Conventional technology As is well known, examples of low carbon steel for nuclear reactor pressure vessels include ASTM standard A533 B class 1 steel,
A508 class 3 steel etc. are used. On the other hand, low
Cr-Mo steel, ferritic high chromium steel, and ferritic stainless steel are cheaper than austenitic stainless steel, and have better properties than austenitic stainless steel, so they are used as steel for nuclear reactors. In particular, application to fast breeder reactors or nuclear fusion reactors is being considered. Furthermore, since superalloys such as Inconel and Incoloy have excellent heat resistance and oxidation resistance, their application to nuclear reactor materials, particularly nuclear fusion reactors, is being considered. On the other hand, austenitic stainless steel has been traditionally used as various constituent materials in nuclear reactors due to its excellent high-temperature strength and corrosion resistance, and is particularly used as a reactor vessel material for fast breeder reactors and light water reactors that are subjected to thermal neutron irradiation. Its use is also planned. By the way, regarding austenitic stainless steel, for example, "Kinen Giho" No. 50 or JP-A-53
-As disclosed in Publication No. 88499 etc., B
It is known that by adding , it is possible to refine and stabilize carbides, suppress grain boundary precipitation of carbides, strengthen grain boundaries, and improve strength, ductility, and workability. . Problems to be Solved by the Invention As mentioned above, the addition of B to austenitic stainless steel is effective from the viewpoint of strengthening grain boundaries, but on the other hand, there are the following problems. In other words, B generally consists of two types of isotopes ¹⁰ B,
It is composed of ¹¹ B, and its natural abundance is ¹⁰ B.
19.6% and ¹¹ B account for about 80.4%, but among these isotopes, ¹⁰ B has a particularly large absorption of thermal neutrons, and therefore austenitic stainless steel containing B is used in nuclear reactor vessel materials that are exposed to thermal neutron irradiation. When using a relatively light thermal neutron irradiation of about 10 ¹⁷ n/cm ² , ^{a 10} B (n, α) ⁷ Li nuclear reaction occurs and ¹⁰ B collapses, resulting in the generation of He gas.
The He promotes the occurrence and propagation of sleep cracks,
This causes creep embrittlement. Furthermore, even in austenitic stainless steels that do not actively add B, austenitic stainless steels obtained through normal steelmaking processes contain at least several ppm of B; Even in this case, if thermal neutron irradiation is applied, creep embrittlement may occur due to the ¹⁰ B(n, α) ⁷ Li reaction, as described above. Furthermore, metal materials other than austenitic stainless steel, such as carbon steel as mentioned above, low Cr-
Even in Mo steel, ferritic high chromium steel, ferritic stainless steel, super alloy,
Even when not actively added, the metal material obtained after the smelting process usually contains at least a few
It contains approximately ppm of B as an impurity, and if B is actively added, even more B is contained. In these cases, as already explained for austenitic stainless steel, ¹⁰ B decays due to ¹⁰ B (n, α) ⁷ Li nuclear reaction,
As a result, He gas is generated, which causes creep embrittlement. This invention was made against the background of the above-mentioned circumstances.
Metal materials that simply contain impurities,
Provided is a method for manufacturing a metal material that can effectively prevent the occurrence of creep embrittlement caused by ¹⁰ B when a metal material containing B is used as a reactor constituent material that is subjected to thermal neutron irradiation. The purpose is to Measures to solve the problem As already mentioned, the cause of creep embrittlement due to thermal neutron irradiation is especially among B isotopes.
¹⁰ B reaction, that is, ¹⁰ B(n, α) ⁷ Li reaction, He
^11B , on the other hand, is stable and does not cause He-producing nuclear reactions. Therefore, in this invention, stable ¹¹ B is actively added in advance in the manufacturing process of metal materials such as carbon steel, low Cr-Mo steel, ferritic high chromium steel, ferritic stainless steel, austenitic stainless steel, or superalloy. The amount ratio of ¹⁰ B out of the total B (= ¹⁰ B + ¹¹ B) contained in the molten metal, that is, ¹⁰ B/( ¹⁰ B + ¹¹ B), is lowered by adding B to the molten metal. By doing this, you can achieve your goal
Although the absolute amount (= ¹⁰ B + ¹¹ B) remains the same, it has become possible to obtain a metal material with a low amount ratio of ¹⁰ B, which causes problems with thermal neutron irradiation. That is, in the method for producing a metal material for a nuclear reactor of the present invention, when producing a boron-containing metal material for a nuclear reactor used in an environment subjected to thermal neutron irradiation, the amount ratio of B isotopes ¹¹ B to ¹¹ B is determined in advance. /( ¹⁰ B
+ ¹¹ B) is characterized by adding a boron-containing raw material higher than the natural abundance ratio, and then smelting to remove B, thereby reducing the ¹⁰ B content in the metal material. . Detailed Description for Carrying Out the Invention In the method of the present invention, the amount ratio of ¹¹ B, that is, ¹¹ B/(
Boron-containing raw materials with a value of ¹⁰ B + ¹¹ B) higher than the natural abundance ratio are actively added. In other words, the natural abundance ratios of B isotopes ¹⁰ B and ¹¹ B are ¹⁰ B: 19.6% and ¹¹ B: 80.4%, so boron containing B contains more than 80.4% ¹¹ B. A boron-containing raw material such as Fe-B alloy, boron oxide (B ₂ O ₃ ), boric acid (H ₃ BO ₃ ) is added. However, in the actual formulation, Fe−B
B isotope ¹¹ B in boron-containing raw materials such as alloys
The amount ratio is desirably 90% or more in terms of manufacturing costs and the effect of preventing creep embrittlement after thermal neutron irradiation. In other words, more than 90% of ¹¹ B
It is desirable to add a boron-containing feedstock containing B comprised by less than 10% ¹⁰ B. Such a boron-containing raw material may be added in the smelting process of the target metal material until B is removed. i.e. generally carbon steel, low
Smelting of Cr-Mo steel, high chromium ferritic steel, stainless steel, or superalloys involves smelting with hot metal pretreatment, then rough decarburization in a converter or electric furnace, and then VOD. Furnace or RH
It is usually produced by interstitial decarburization in a degassing tank or even an AOD furnace, and in some cases, only the steps after the electric furnace of the above steps are used. In these cases, deborium progresses together with decarburization, so the boron-containing raw material is added during pretreatment of hot metal or during rough decarburization in a converter or electric furnace, or added to smelting raw materials or alloys before the start of rough decarburization. It may be added together with the raw materials, or in some cases may be added to the molten metal before vacuum decarburization. In this way, the quantity ratio of ¹¹ B is ¹¹ B/( ¹⁰ B + ¹¹ B)
By actively adding a boron-containing raw material containing B that has a higher abundance ratio than the natural abundance ratio, ¹¹ B/( ¹⁰ B+
¹¹ The ratio of B) is also higher than the natural abundance ratio. Here, the amount of the boron-containing raw material to be added should be determined based on the B removal rate in de-smelting when no effect of B addition is expected in the final product (that is, when B remains only as an impurity). In addition, if the effect of adding B such as grain boundary strengthening as mentioned above is expected for the final product to be obtained, it should be determined according to the target remaining amount of B and the B removal rate in the B removal smelting. All you have to do is set it. In de-B smelting, as is clear from the examples described later, although the total amount of B remaining in the molten metal decreases, the isotopes ^11B , 11B,
¹⁰ The ratio of B remains unchanged. Therefore, after de-B smelting, the total amount of B has decreased to the required concentration, and the amount ratio of ¹¹ B in the residual B is ¹¹ B/( ¹⁰ B + ¹¹ B)
It is possible to obtain metal materials with a higher abundance ratio than the natural abundance ratio. In other words, this metallic material contains isotopes ¹¹ B and ¹⁰ B in residual B, which react with thermal neutron irradiation and cause creep embrittlement.
The amount ratio of ¹⁰ B/( ¹⁰ B + ¹¹ B) is lower than the natural abundance ^ratio , so compared to conventional metal materials with the same total residual B content, the content of ¹⁰ B is lower. As a result, the risk of creep embrittlement due to thermal neutron irradiation can be reduced compared to conventional metal materials. In addition, when smelting steel such as stainless steel and low carbon steel, for the purpose of deoxidizing etc. during smelting.
FeSi (ferrosilicon), FeMn (ferromanganese), SiMn (silicon manganese), etc. are often added, but in this case, these additive raw materials usually also unavoidably contain B.
Considering the amount of B achieved after B removal smelting, it is desirable to use additive raw materials with as little B content as possible. Furthermore, it goes without saying that B-free smelting is not limited to one-time smelting, but may be repeated two or more times, thereby further reducing the absolute amount of ¹⁰ B. . Examples Example 1 First, an example of the present invention applied to the production of austenitic stainless steel will be described together with a comparative example. Weight% C0.05%, Si0.50%, Mn1.00%,
P0.020%, S0.006%, Ni9.0%, Cr18.3%,
In manufacturing SUS 304 steel consisting of 0.0005% B, the balance being Fe and unavoidable impurities, the on-site manufacturing conditions were simulated by rough decarburization in a converter and vacuum decarburization that also serves as de-B smelting in a VOD furnace. B was removed using a small experimental smelting furnace under the following three conditions (A), (B), and (C).
carried out smelting. In each case, the B content as an impurity in the molten iron before B-removal smelting is 10 ppm. (A) B-removal smelting was performed to B5ppm without adding any boron-containing raw materials (Comparative Example 1). (B) 80%Fe-20%B alloy with the natural abundance ratio (approximately 80%) of ^11B /( ^10B + ^11B ) is smelted to remove B to 5ppm by adding 10ppm of B. (Comparative Example 2). (C) 80% Fe− where the value of ¹¹ B/( ¹⁰ B + ¹¹ B) is 98%, which is higher than the natural abundance ratio (approximately 80%).
Adding 20% B alloy to 40 ppm B amount,
B smelting was carried out to remove B to 5 ppm (Example 1a of the present invention). (D) The same 80% Fe-20B alloy as used in (C) above was added with a B content of 50 ppm, and B-removal smelting was performed to B45 ppm (Example 1b of the present invention). In either case, samples were taken at any time from the start to the end of the B removal smelting process, and the total B amount and ¹⁰ B/( ¹⁰ B
⁺¹¹ B) was measured. The results are shown in FIGS. 1 and 2. Figure 2 particularly shows the cases of Comparative Example 1 in which no boron-containing raw material was added and Comparative Example 2 in which a boron-containing raw material was added in which the amount ratio of ^10B was the same as the natural abundance ratio, and the case of Comparative Example 1 The amount of B only decreased from 10 ppm of impurities before the start of de-B smelting to 5 ppm, and the value of ¹⁰ B/( ¹⁰ B + ¹¹ B) remained almost constant at the natural abundance ratio of 0.2 during the smelting period. Ta. In addition, in the case of Comparative Example 2, the amount of B was 20 ppm at the start of smelting due to the addition of B before smelting to remove B, and it finally decreased to 5 ppm, but the amount of B added was ¹¹ B/( ¹⁰ B
Since the value of ⁺¹¹ B) is the natural abundance ratio, the value of ¹⁰ B/( ¹⁰ B+ ¹¹ B) remained constant at approximately 0.2 during the smelting period. On the other hand, in Figure 1, the amount ratio of ¹¹ B is ¹¹ B/( ¹⁰ B+
¹¹ This shows the case of Examples 1a and 1b of the present invention in which a boron-containing raw material containing B was added, where the value of B) was 98%, which is much higher than the natural abundance ratio,
In this case, due to the addition of the boron-containing raw material, the ¹⁰ B quantity ratio ¹⁰ B/( ¹⁰ B + ¹¹ B) at the start of B removal becomes 0.05, and ^{the 10} B quantity ratio remains almost constant throughout the entire B removal smelting period thereafter. and finally the quantity ratio of ¹⁰ B becomes
0.05, much less austenitic stainless steel than in Comparative Examples 1 and 2 could be obtained. In Example 1 above, de-B smelting was shown as a laboratory example, but this does not particularly limit the smelting method. , electric furnace, VOD furnace, AOD furnace or
Of course, all methods are applicable, such as de-B smelting using an RH degassing tank or the like. Furthermore, in the above Example 1, 80% was used as the boron-containing raw material to reduce the amount ratio of ^10B .
Although Fe-20%B alloy is melted, it goes without saying that similar effects can be obtained by using other boron-containing raw materials such as ferrobonne. Example 2 Next, an example in which the method of the present invention was applied to the production of low carbon steel will be described together with a comparative example. When smelting low carbon steel with the target component compositions shown in A and B in Table 1 according to the conventional method, a boron raw material enriched with ¹¹ B than the natural abundance ratio is added at the final stage of smelting, and then B-free smelting was carried out. Table 2 shows the amount of ¹⁰ B in the hot metal before de-B smelting, the increased amount of B in the molten steel due to the added boron raw material, and the amount of B in the added boron raw material.
¹¹ B amount ratio, that is, ¹¹ B/( ¹⁰ B + ¹¹ B), ¹⁰ B amount immediately after boron raw material addition, total B amount after B removal smelting,
^{The 10} B amount ratio, ie, ¹⁰ B/( ¹⁰ B + ¹¹ B), and the ¹⁰ B amount are also shown. In Table 2, No. 5 and
No. 7 is a comparative example in which no boron raw material was added according to the conventional method, and No. 6 is a comparative example in which a boron raw material having a natural abundance ratio of ¹¹ B was added. As is clear from Table 2, according to the method of the present invention, the total amount of B in the steel after B-removal smelting is ^as follows: Although it is almost the same as the comparative example in which ¹⁰ B is added, the ratio of ¹⁰ B amount is significantly reduced, and the absolute amount of ¹⁰ B is also significantly reduced. It turns out that it is effective in reducing Example 3 Next, an example in which the method of the present invention was applied to the production of low Cr-Mo steel and ferritic high chromium steel will be described together with a comparative example. When smelting steel with the target composition shown in C, D, and E in Table 3 according to the conventional method, a boron raw material enriched with ¹¹ B than the natural abundance ratio is added at the final stage of smelting. Post-de-B smelting was carried out. Departed from B in Table 4
The amount of ¹⁰ B in molten iron before smelting, the increased amount of B in molten steel due to added boron raw material, and the amount of B in molten steel added to it.
¹¹ B amount ratio, that is, ¹¹ B/( ¹⁰ B + ¹¹ B), ¹⁰ B amount immediately after boron raw material addition, total B amount after B removal smelting,
^{The 10} B amount ratio, ie, ¹⁰ B/( ¹⁰ B + ¹¹ B), and the ¹⁰ B amount are also shown. In addition, No. 7 and No. 9 in Table 4
And No. 10 is a comparative example in which no boron raw material was added according to the conventional method, and No. 8 is a comparative example in which a boron raw material having a natural abundance ratio of ¹¹ B was added. As is clear from Table 4, according to the method of the present invention, the total amount of B in the steel after B-removal smelting is ^as follows: Although it is almost the same as the comparative example in which 10B is added, the ratio of ^10B amount is significantly decreased, and the absolute amount of ^10B is also significantly decreased. high chromium steel
It can be seen that it is also effective in reducing ^10B . Example 4 Further, an example in which the method of the present invention was applied to the production of a superalloy will be described together with a comparative example. Incoloy 800H with the target composition shown in Table 5
In smelting alloys and Inconel 652 alloys,
A boron raw material enriched with ¹¹ B than the natural abundance ratio at the final stage of smelting was added, and then B-free smelting was performed. 6th
The table shows the ratio of the amount of ¹⁰ B in the molten alloy before B removal, the increased amount of B in the molten steel due to the added boron raw material, and the amount of ¹¹ B in the added boron raw material, that is, ¹¹ B / ( ¹⁰ B +
^11B ), ^10B amount immediately after boron raw material addition, total B amount after B removal smelting, ^10B amount ratio, that is, ^10B /( ^10B +
¹¹ B) and ¹⁰ B amounts are also shown. In Table 6, No. 5 and No. 7 are comparative examples in which no boron raw material was added according to the conventional method, and No. 6 is a comparative example in which a boron raw material with a natural abundance ratio of ¹¹ B was added. This is an example. As is clear from Table 6, according to the method of the present invention, the total amount of B in the alloy after B-removal smelting is as ^follows : Although it is almost the same as the comparative example in which ¹⁰ B was added, the ratio of the amount of ^{10 B has decreased significantly, and the absolute amount of 10 B} has also decreased significantly. It turns out that it is also effective. In each of Examples 1 to 4 above, austenitic stainless steel, low Cr-Mo steel, ferritic chromium steel, and superalloy were shown, but other metal materials such as ferritic stainless steel and martensitic stainless steel were used. It goes without saying that the present invention is also effective when applied to steel.

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[Table] Effects of the Invention As is clear from the above examples, according to the method of the present invention, the abundance of ¹⁰ B, which causes the nuclear reaction that causes creep embrittlement when exposed to thermal neutron irradiation, is reduced. It is possible to obtain metal materials with lower cost than conventional metal materials. In other words, according to the method of the present invention, B-containing metal materials with little risk of creep embrittlement even when subjected to thermal neutron irradiation can be produced. This makes it possible to apply metal materials with grain boundary reinforcement, etc., and at the same time, it also becomes possible to prevent the risk of creep embrittlement due to thermal neutron irradiation even for metal materials that contain B only as an impurity.

[Brief explanation of drawings]

Fig. 1 is a graph showing the change in the amount of B in molten iron and ¹⁰ B/( ¹⁰ B + ¹¹ B) in B-removal smelting in Example 1 of the present invention, and Fig. 2 is a graph of Comparative Example 1 using the conventional method.
and a graph showing changes in the amount of B in molten iron and ¹⁰ B/( ¹⁰ B+ ¹¹ B) in B-removal smelting in Comparative Example 2.

Claims (1)

[Claims] 1. When smelting the metal material for the furnace used in a neutron irradiation environment, the isotope of B is prepared in advance.
A metal material for a nuclear reactor with a low ¹⁰ B content, characterized by adding a boron-containing raw material with a ¹¹ B content ratio of ¹¹ B/( ¹⁰ B + ¹¹ B) higher than the natural abundance ratio, and then smelting it to remove B. manufacturing method.