JPH07100849B2 - Method for producing high corrosion resistant stainless steel having metallic Ti surface layer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial field of application) The present invention relates to a high corrosion resistance used as a material for a nuclear fuel cladding tube for a fast breeder reactor or a transportation pipe or a storage container of ultra-high purity gas for a semiconductor manufacturing apparatus. TECHNICAL FIELD The present invention relates to a method for producing stainless steel, particularly highly corrosion-resistant stainless steel having a metal Ti surface layer.

(Prior Art) In a nuclear fuel cladding tube for a fast breeder reactor (FBR), when oxide fuel (U0 ₂ -PuO ₂ ) burns, Cs (cesium), I (iodine), Te (fission products) are produced as fission products. Tellurium) etc. are generated. Of these fission products, I and Te are elements that corrode stainless steel, but since the amount of Cs produced is much larger than the amount of I or Te produced, the corrosive action of I or Te is diminished. Not a big problem. However, in an environment with a large amount of O (oxygen), Cs and O combine with each other to form an oxide, and the activity of I and Te increases as the activity of Cs decreases, and I and Te and stainless steel increase. Fe, Ni, which are the constituent elements of steel,
Corrosion (high temperature corrosion) occurs due to the reaction with Cr. The oxygen sources are oxygen contained in the He gas filled in the nuclear fuel cladding tube and oxygen released from the oxide fuel.

On the other hand, in a gas transport pipe or storage container for semiconductor manufacturing equipment, for example, an argon gas for a carrier or an ultra-high purity gas such as a strong oxidizing nitrogen trifluoride gas for etching, oxygen (O ₂ ) And hydrogen (H ₂ ) and the release of oxygen (O ₂ ), hydrogen (H ₂ ) and water (H ₂ O) adsorbed on the inner surfaces of transport pipes and storage vessels, ultra high purity gas. Is polluted.

In order to prevent high temperature corrosion of the nuclear fuel cladding tube or contamination of ultra-high purity gas as described above, in the nuclear fuel cladding tube, an oxygen getter material is used in the tube, and also in the gas transportation piping and storage container. May contain oxygen, hydrogen and water getter materials to trap oxygen, hydrogen and water. However, thin long pipes such as cladding pipes and gas transportation pipes (for example, outer diameter 6.5 mm × wall thickness 0.47 mm × length 3
It is extremely difficult to coat the inner surface of (000 mm) with Ti (Zr), Nb, etc., which has a strong getter effect because it has a strong bond with H (hydrogen) and O (oxygen) by CVD (chemical vapor deposition) or plating. It is difficult and has not been put to practical use.

Further, even if Ti is pressure-bonded to the inner surface of these tubes by the clad method, it is difficult to obtain an ultrathin thin tube with good adhesion because the ductility of stainless steel and Ti is different.

(Problems to be Solved by the Invention) The present invention relates to a nuclear fuel cladding tube for a fast breeder reactor, which has good corrosion resistance to high-temperature corrosion caused by the above fission products, and an ultrahigh-purity gas transportation pipe. An object of the present invention is to provide a method for producing a stainless steel having a metal Ti surface layer having a good effect of suppressing the release of oxygen, hydrogen and water in a storage container.

(Means for Solving the Problems) The present inventor firstly formed a stainless steel containing Ti on the surface by heating it in a low oxygen potential atmosphere.
A method for forming a Ti oxide coating was proposed (Japanese Patent Application No. 2-
No. 113023). This stainless steel has excellent corrosion resistance against corrosion in the atmosphere or high temperature water. And
As a result of further studies on the heat treatment in this low oxygen potential atmosphere, as a result of Ti-containing low carbon-low nitrogen
When the Fe-Ni-Cr alloy is heated at 600 to 1050 ° C in an extremely low oxygen potential atmosphere with a lower oxygen concentration, Ti does not become carbonitrides or oxides, and the alloy remains in the metallic state. The phenomenon that the surface layer is thickened was found. That is, the metallic Ti layer can be formed on the surface of the Fe-Ni-Cr alloy without using the coating or clad method.

The present invention has been made based on this finding, and the gist thereof is "a stainless steel containing 0.2 to 1.0% by weight of Ti and 9 to 18% of Cr, and C in impurities being 0.03% or less. For steel, vacuum degree of 10 ^-7 Torr or more or inert gas (however,
(Excluding nitrogen) has a high corrosion resistance stainless steel characterized by forming a metallic Ti layer on the surface by heating at 600 to 1050 ° C for 2 minutes to 10 hours in an extremely low oxygen potential atmosphere exceeding 99.999%. Manufacturing method ”(hereinafter,“% ”)
Means all% by weight).

The stainless steel to be the subject of the present invention is a stainless steel containing Ti and Cr in the above-mentioned composition range, and representative examples thereof include SUS 304, SUS 316, SUS 410Ti, and SUS XM8.
The shape may be a tube, a plate, a rod, or the like.

The extremely low oxygen potential atmosphere has a vacuum degree of 10 ^−7.
Inert gas with a Torr or higher or a purity higher than 99.999% 10
0%, in other words, an atmosphere in which the concentration of the inert gas exceeds 99.999%.

As the inert gas, argon (Ar), helium (H
e), neon (Ne), etc. are applicable, but nitrogen (N ₂ ) is excluded for the reason described later.

(Function) Various conditions specified in the method of the present invention will be described below.

First, the reason why the Cr content of the stainless steel used in the method of the present invention is set to 9 to 18% is that the materials for nuclear power and semiconductor manufacturing equipment must have high corrosion resistance and oxidation resistance. If the content of Cr, which is the element that controls the, is less than 9%, the required corrosion resistance and oxidation resistance cannot be obtained,
On the other hand, if it exceeds 18%, it tends to become brittle (475 ° C brittleness) when used for a long time.

Ti is an essential element for forming a metal Ti layer as a getter of oxygen, hydrogen and water on the surface of stainless steel. However, if the Ti content is less than 0.2%, it is not possible to obtain a Ti layer (thickness of at least 0.3 μm is required) that is thick enough to trap (trap) oxygen, hydrogen and moisture, while 1.0% %, The effect of trapping oxygen, hydrogen and water will be saturated, and
Intermetallic compounds such as Ni ₃ Ti are produced in a large amount and reduce toughness. Therefore, the Ti content is set to 0.2 to 1.0%.

Stainless steel used in the method of the present invention, in addition to the above components
Synthetic components such as Ni, Si, Mn, and Mo may be contained in an amount that is contained in ordinary stainless steel.

However, as an impurity, it is important to suppress the upper limit of C, and when C exceeds 0.03%, a surface layer mainly composed of TiC is formed by preferentially bonding with Ti, and a metal acting as a getter of oxygen, hydrogen, and water is formed. Ti is reduced.

As for N, it does not matter as long as it is contained in the usual melting method, but if it is too much, TiN
Since the amount of metallic Ti formed is reduced, the content is 0.10.
% Or less is desirable.

Next, the atmosphere at the time of heating should be an atmosphere with an extremely low oxygen potential in which the degree of vacuum is 10 ^-7 Torr or more or the concentration of the inert gas excluding nitrogen exceeds 99.999%.
Is to concentrate on the surface of stainless steel to form a metal Ti layer. When the oxygen potential is higher than this, Ti concentrated on the surface of stainless steel reacts with oxygen in the atmosphere to form stable Ti oxide (TiO ₂ ), which does not act as a getter of oxygen, hydrogen and water. Nitrogen is removed from the inert gas because, like N in steel, Ti concentrated on the surface reacts with nitrogen to form TiN, which does not combine with oxygen.

The thickness of the metal Ti layer must be at least 0.3 μm to function as a getter of oxygen, hydrogen and water.

The heating temperature in the above low oxygen potential atmosphere is 60
If the temperature is lower than 0 ° C, the diffusion rate of Ti contained in stainless steel to the steel surface is very slow, and heating for a long time is required, which is not practical. Further, if the heating temperature exceeds 1050 ° C, the crystal grains become coarse and the mechanical properties (particularly strength) of the material deteriorate. Therefore, the heating temperature is 600 to 1050 ° C.

The heating time may be appropriately set between 2 minutes and 10 hours. If it is less than 2 minutes, it is difficult to secure a surface thickness of 0.3 μm or more. On the other hand, if it is heated for a long time of more than 10 hours, a TiO ₂ layer is formed on the outermost surface of the metal Ti, which forms hydrogen and oxygen. It becomes a non-adsorbable layer that is hard to bind, and the trap effect of oxygen, hydrogen, and water disappears.
Therefore, the heating time is 2 minutes to 10 hours.

The metallic Ti layer formed on the surface of stainless steel under the above conditions does not depend on the coating or clad method, but utilizes the fact that Ti is diffused on the steel surface and the surface layer is modified. Yes, excellent adhesion and no fear of peeling.

(Example) A and B2 test materials shown in Table 1 were melted by a vacuum melting method, and after hot working, hot rolling and cold rolling were performed to obtain a thickness of 2 mm.
The plate material of was produced.

Then, the surface of these plate materials was ground to 1000 with emery paper and subjected to heat treatment under the conditions (heating atmosphere, heating temperature, heating time) shown in Table 2, and the thickness of the produced metal Ti layer was adjusted. The measurement, the corrosion test, and the desorption amount of adsorbed molecules were measured.

The thickness of the Ti layer was obtained by analyzing the Ti concentration in the plate thickness direction from the surface of the test material using an ion microanalyzer.

Corrosion tests were carried out under the oxygen atmosphere assuming a combustion atmosphere in a fast breeder reactor.
2 ppm thickness in a gas containing 3 ppm, Cs 10 gram atom / cc and I (iodine) 1 gram atom / cc, with the balance being He.
A strip-shaped test piece having a width of 10 mm and a length of 40 mm was exposed at 550 ° C. for 1000 hours, and then the corrosion weight loss was measured.

The desorption amount of adsorbed molecules was measured by heating the test piece to 800 ° C for 30 minutes in a container with a vacuum degree of 10 ^-12 Torr, and measuring the H ₂ O and H ₂ molecules released from the steel surface by gas chromatography. It was performed by quantifying.

The test results are shown in Table 2. In the same table, the corrosion weight loss is converted to the erosion depth, and the desorption amount of adsorbed molecules is expressed as the number of molecules per cm ² .

From the results shown in Table 2, a metal Ti layer of 0.5 μm was formed on the surface of the test material.
It can be seen that the examples of the present invention (Nos. 1 to 10) formed above do not cause any corrosion, and have excellent corrosion resistance against high temperature corrosion caused by fission products. Also, the desorption amount of adsorbed molecules is extremely small on the order of 10 ¹² to 10 ¹³ molecules / cm ² ,
It can be seen that the gas pollution prevention effect is excellent.

On the other hand, when the prescribed heat treatment is not applied (No. 11
And 12), or even if the heat treatment is performed under a condition that deviates from the low oxygen potential atmosphere specified by the method of the present invention (No. 13 to 18), no metal Ti layer is present on the surface. Or, even if it is present, it does not reach a sufficient thickness, and it is clear that the corrosion resistance is poor. Also, the desorption amount of adsorbed molecules is extremely large on the order of 10 ¹⁵ to 10 ¹⁶ molecules / cm ² , and it is clear that the effect of preventing gas contamination is poor.

(Effects of the Invention) The stainless steel having the metallic Ti surface layer produced by the method of the present invention has excellent corrosion resistance against corrosion caused by fission products generated in nuclear fuel cladding tubes for fast breeder reactors. . In addition, the release of oxygen and hydrogen from the steel that occurs in ultra-high purity gas transportation pipes for semiconductor manufacturing equipment, and the gas pollution caused by the release of oxygen, hydrogen and moisture adsorbed on the inner surface of transportation pipes, etc. On the other hand, it exhibits excellent anti-pollution performance. This steel can be easily manufactured by subjecting stainless steel having a predetermined composition to heat treatment in an extremely low oxygen potential atmosphere.

Claims (1)

[Claims] 1. By weight%, 0.2-1.0% Ti and 9-18%
Ultra-low oxygen potential of stainless steel containing Cr of less than 0.03% in impurities and having a vacuum degree of 10 ^-7 Torr or more or an inert gas (excluding nitrogen) concentration exceeding 99.999%. A method for producing high corrosion resistant stainless steel, which comprises heating at 600 to 1050 ° C. for 2 minutes to 10 hours in an atmosphere to form a metal Ti layer on the surface thereof.