JPH0499806A - Manufacture of boron-containing austenitic stainless steel having excellent corrosion resistance and workability with austenitic stainless steel clad on steel plate surface - Google Patents

Abstract

PURPOSE:To obtain a boron-containing austenitic stainless steel plate available to transportation and storage, etc., of spent nuclear fuel by packing a rapid solidified boron-containing austenitic stainless steel powder in an SUS304 or SUS316 can, solid-compacting to a steel slab, manufacturing a clad steel slab and executing hot-rolling treatment. CONSTITUTION:Atomized powder (wt.% of 0.3-3.0 B, <=0.08 C, 0.01-2.0 Si, <=2.0 Mn, 16.0-20.0 Cr, 8.0-15.0 Ni, <=0.2 N and the balance Fe with inevitable impurities) is charged in the SUS304(L) or SUS316(L)-made can having 2-50mm thickness to make the clad steel slab with HIP treatment, hot-extrusion, etc. This is hot-rolled and the boron-contained austenitic stainless steel plate having excellent corrosion resistance and workability is obtd. by cladding the austenitic stainless steel on the steel plate surface.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a boron-containing austenitic stainless steel with excellent corrosion resistance and workability, which is used as a nuclear material for nuclear power materials such as fuel baskets for casks for transporting and storing spent nuclear fuel. This invention relates to a method for manufacturing steel plates.

[Conventional technology]

Boron 0.3~3. Austenitic stainless steel containing Out% has excellent thermal neutron absorption properties and is used as a material for combustion baskets in casks for transporting and storing spent nuclear fuel.It is manufactured using the melting → casting → rolling process like ordinary steel. Ru. However, this steel generally has low hot ductility and is prone to edge cracking during rolling, so in the manufacturing method of this type of steel sheet, the rolling temperature is strictly limited to suppress cracking (for example, No. 8221, Japanese Unexamined Patent Publication No. 6
3-50429).

On the other hand, boron-containing austenitic stainless steel sheets are often formed into square pipes for the above-mentioned baskets. Austenitic stainless steel containing 0.1 to 3.0 wt% boron contains borides ((Fe, cr),
B) is crystallized. Borides are hard and brittle, so when processed, the boride itself tends to crack or peel off at the interface with the base steel.The coarser the crystallized boride, the larger the cracks and peeling become, and the more difficult it is to process. This results in poor ductility and toughness.

Therefore, not only edge cracks occur during rolling, but also cracks occur when bending into a square pipe at room temperature. Therefore, recently, several powder metallurgy-based manufacturing methods have been proposed as new manufacturing methods for improving the workability of boron-containing austenitic stainless steels. For example, ■Japanese Patent Application Laid-open No. 63-293139 discloses that after mixing austenitic stainless steel powder and boron nitride (BN) powder, the mixture is formed into a steel billet using hot isostatic pressing (HIP). The technique is rolling.

■In JP-A-1-172543, a surrounding layer of austenitic stainless steel powder is formed around a mixed powder of austenitic stainless steel powder and ferroboron powder, solidified by HIP to form a clad copper piece, and then hot rolled. That is what it is.

■ In JP-A-1-172544, a surrounding layer of austenitic stainless steel powder is formed around a mixed powder of austenitic stainless steel powder and NiB powder, solidified by HIP to form a clad steel billet, and then hot rolled. Actually, it is 3~.

■In JP-A-1-172545, a surrounding layer of austenitic stainless steel powder is formed around boron-containing austenitic stainless powder, solidified by HIP to form a clad steel billet, and then hot rolled. .

(1) Japanese Patent Application No. 64-182431 discloses a technique in which rapidly solidified boron-containing austenitic stainless steel powder is formed into a steel billet by HIP or hot extrusion, and then hot rolled.

[Problem that the invention seeks to solve]

These technologies provide a new manufacturing method using powder metallurgy to improve the ductility of boron-containing austenitic stainless steel sheets, but they each have the following drawbacks: have.

As is common to the technology described in ■■■, BN powder,
Ferroboron powder or NiB powder must be uniformly dispersed.

However, it is technically difficult to uniformly mix austenitic stainless steel powder and these powders.

In addition, in the technique (2), BN powder and stainless steel powder have poor wettability, and it is difficult to reach true density even if they are solidified by HIP.

Furthermore, with the technique described in ■■■, it is difficult to form a surrounding layer of another powder around the powder. Because the narrow gap (5~15
This is because if an attempt is made to fill powder into I), the powder will cause bridging with each other, creating large voids.

On the other hand, the technique (■) clearly stipulates the powder particle size, HIP conditions, and hot extrusion conditions to obtain 100% true density. It is excellent in that it does not cause cracks or the like and can improve workability. but.

As mentioned above, in boron-containing austenitic stainless steel, the amount of solid solution Cr, which is effective for corrosion resistance, decreases due to the formation of borides during solidification.

For example, in the case of SUS304 steel containing 1% boron, 5.3
%Cr is consumed for the production of polide, and the amount of solid solution Cr decreases to 13 to 15%. Therefore,
Compared to SUS304 steel without boron addition, its corrosion resistance is significantly deteriorated.

The present invention was devised in view of the above-mentioned problems of the prior art, and further develops the technology (2) above to improve not only the workability but also the corrosion resistance of boron-containing austenitic stainless steel. This is to ensure that the following results are obtained.

[Means for solving problems]

Therefore, the present invention uses a rapidly solidified boron-containing austenitic stainless steel powder as a raw material, and the material of the can to be filled with the powder is SUS304. (L) or SUS316
(L), and by solidifying and molding the can filled with this powder into a steel piece, the surface becomes SUS304 (L) or SUS
316 (L) layer, the interior of which is a clad steel piece made of boron-containing austenitic stainless steel, and then hot-rolled to obtain a boron-containing austenitic stainless steel plate with excellent corrosion resistance and workability. .

To explain the configuration of the present invention in detail, first of all, in the first invention,
B: 0.3-3.0%, C: 0.08 in weight%
% or less, Si: 0.01-2.0%, Mn: 2.
0% or less, Cr: 16.0-20.0%, Ni:
Atomized powder containing 8.0 to 15.0%, N: 0.2% or less, and the balance consisting of iron and inevitable impurities, is made of SUS304 (L) or SU with a plate thickness of 2 to 50 rm.
The basic feature is that it is placed in a square can made of S316 (L) and made into a clad steel billet by hot isostatic pressing or hot extrusion, and then hot rolled. In the second invention, the atomized powder further contains 3.0% or less of MO, and after producing a clad steel piece in the same manner, it is hot rolled.

The boron-containing austenitic stainless steel sheet obtained by the above method has excellent corrosion resistance and excellent workability at room temperature and high temperature.

The reasons for limiting each component of the above configuration will be explained below.

First, the reasons for limiting the components of the boron-containing austenitic stainless steel rapidly solidifying powder will be described.

B: In its natural state, boron contains about 20% of the isotope 10B, and B is an element with a large neutron absorption cross section. Therefore, it is the most important element in this steel plate used for neutron blocking. However, weight% (hereinafter omitted)
If it is less than 0.3%, the thermal neutron absorption performance is insufficient,
If it exceeds 3.0%, the volume fraction of boride reaches 40% or more, and hot workability and room temperature ductility and toughness are significantly deteriorated, so it is limited to 0.3 to 3.0%.

C: If carbon exceeds 0.08%, carbides are likely to be formed, which deteriorates corrosion resistance, so it was limited to 0.08% or less.

Si: Silicon needs to be added for deoxidation, but
If it is less than 0.01%, deoxidation is not sufficient, and if it exceeds 2.0%, embrittlement occurs, so it is limited to 0.01 to 2.0%.

Mn: Manganese is also an element that has a deoxidizing effect, but 2.0
If it exceeds 2.0%, the corrosion resistance will deteriorate, so it was limited to 2.0% or less.

Cr: 16.0% or more of chromium is required to maintain corrosion resistance, and if it exceeds 20.0%, the σ phase tends to precipitate and become brittle, so it was limited to 16.0 to 20.0%.

Ni: Nickel is an element necessary to make the structure austenite, and for this purpose, 8.0% or more is required.

However, since it is an expensive element, it was limited to 8.0 to 15.0%.

MO = molybdenum is an element effective in pitting corrosion resistance among corrosion resistance. Therefore, when pitting corrosion resistance is required, the same element is added, but if it exceeds 3.0%, embrittlement tends to occur due to precipitation of the σ phase, so it is limited to 3.0% or less.

N: Nitrogen is an element effective in increasing pitting corrosion resistance and strength.

However, if it exceeds 0.2%, embrittlement will be significant, so 0.2%
Limited to the following.

Next, the reason for using the atomized powder of the above components as the raw material will be described. Borides crystallize during solidification, and increasing the solidification rate tends to make them finer. However, in normal ingot casting or continuous casting processes, there is a limit to increasing the solidification rate, and significant borides tend to become finer. On the other hand, atomization is a process in which molten steel flows out from pores and high-pressure water, gas, oil, etc. are sprayed onto the molten steel flow to rapidly solidify it into powder. It is possible to easily achieve a solidification rate of more than 100 ml. It has been found that in atomized boron-containing austenitic stainless steel, fine borides (polides) with a size of 1 μm or less are uniformly distributed. Furthermore, atomization is a process that can be mass-produced. Therefore, in the present invention, it was decided to use an atomized boron-containing austenitic stainless steel copper powder as the material.

The above powder (2) is filled into a steel can (1) as shown in Figures 1 (a) and (b), vacuum degassed and sealed, and then subjected to HIP treatment. They are joined to form a so-called clad copper piece X' having a cross section as shown in FIG. Also, if the clad steel piece X' is hot rolled as it is, the third
A rad steel billet X is obtained as shown in the cross-sectional view taken along a plane perpendicular to the rolling direction.

In the present invention, as a filling container for powder (2), the plate thickness is 2 to 50 mm.
A square can (1) made of SUS304 (L) or SUS316 (L) is used.

Among them, the material of the can (1) is SUS304 (L), 5O
The reason for choosing 3316(L) is its excellent corrosion resistance,
It lies in ductility and cost. Cask basket materials are required to have corrosion resistance, but if carbon steel is used for the can (1) and a boron-containing austenitic stainless clad steel plate with a carbon steel surface is manufactured, the corrosion resistance will be extremely poor. In other words, from the viewpoint of improving corrosion resistance, the material of the can (1) needs to be stainless steel. Among these stainless steels, ferritic stainless steels are inferior in ductility and toughness compared to austenitic stainless steels, so they are excluded from the can (1) materials of the present invention.
304 (L) and SUS316 (L).

Next, the reason for limiting the shape of the can (1) will be described. Generally, when HIPing is performed, a cylindrical container is generally filled with powder (2). However, in the present invention, the shape is limited to a square shape in order to prevent edge cracking during rolling. That is, if a cylindrical can (1) is used, a cylindrical piece of steel can naturally be produced. When it is subjected to plate rolling, the free surface that is not in contact with the rolls is widened, and tensile stress acts on the free surface, so that edge cracking is more likely to occur than in a rectangular steel piece X'. Therefore,
Here, the shape of the can (1) is assumed to be square. The configuration of such a rectangular can (1) is illustrated in FIGS. 1(a) and 1(b), which include: - a box-shaped can body (1a) with an opening on the side; After filling the powder (2) into the can body (1a),
It is attached by closing it with a can lid (1b) and welding the surrounding area, vacuum deaerating the inside of the can (1) through the deaeration hole (10) provided in the can lid (lb), and sealing it by crimping. do.

Furthermore, the thickness of the can can is limited for the following reasons. In the first place, the purpose of cladding in the present invention is to improve corrosion resistance, suppress edge cracking, and prevent cracking during corner bending, but considering the reason why edge cracking is likely to occur in boron-containing austenitic stainless steel. As a result, the melting point of polide is around 1170°C, so the clad steel piece X'
When rolling is carried out, it is limited to temperatures below this temperature, and as the temperature is further lowered, ductility is rapidly lost. In particular, the area near the surface of the width end of the steel piece X' is easily cooled during rolling, and is also in the tensile stress field as described above, so edge cracking occurs.

Therefore, the range in which boron-containing austenitic stainless steel can be rolled is generally limited to a very narrow temperature range. In the case of a boron-containing austenitic stainless steel piece X' whose surface is covered with SUS304 (L) or SUS316 (L), highly ductile 5O3304 (L ), SUS316 (L
) is located, so ear cracking is suppressed. but,
As is clear from the test results described later, the effect is small when the thickness of the can (1) is less than two strokes, so the thickness was set to 2 m or more. In addition, the thicker the can (1) is, the more effective it is in suppressing edge cracking, but if the thickness exceeds 50 mm, the thickness of the can (1) is more effective.
) increases in rigidity, and during the HIP process, sufficient pressure is not applied to the corner part of the can (1), causing the powder (2) in that part to increase.
Assimilation becomes insufficient. On the other hand, even in hot extrusion, when the can thickness exceeds 50 mm, chipping at the corners becomes noticeable. Therefore, the upper limit of the thickness of can (1) is 50+
limited to nm. Further, when the steel plate X is bent and formed into a basket, the area near the surface of the steel plate X is the part where the most tensile stress is applied. SUS304 (L), SUS316 with excellent ductility
In the case of the clad steel plate X (L), cracking during the above-mentioned bending process is suppressed.

〔Example〕

Specific examples of the present invention will be described.

Clad steel plates and solid steel plates were manufactured using atomized powder and ingots having the compositions shown in Table 1 below, and according to the manufacturing methods of the present invention and conventional methods shown in Table 2 below.

-15 = -16 = As the atomized powder shown in Table 1, gas atomized powder with a mesh size of =42 or less is used. Furthermore, a 5 ton ingot was used as the material for the casting method.

Furthermore, in the HIP solid method and hot extrusion solid method shown in Table 2, the atomized powder is solidified by HIP or hot extrusion, and then the can part is ground away to form a solid steel piece of only B-8O8. , which is then hot rolled into a solid steel plate.

First, an example will be described which is the basis for limiting the thickness of the container filled with powder, which is a feature of the present invention. As the material, Nn 5 atomized powder shown in Table 1 was used. The atomized powder is made of SUS304 (L) with a thickness of 0.5 to 80 mm.
) Filled and vacuum-sealed square cans at 1150℃ x 2
HIP treatment was performed under the conditions of holding at 000 atm x 2 hours to produce a steel piece. A sample for density measurement was taken from the corner part (original powder part) of the steel piece, and the achieved density was measured. The results are shown in FIG. From the figure, it is clear that if the can thickness exceeds 50 mm, the true density will not be reached, so the upper limit of the can thickness was limited to 50 books.

In addition, the relative density in FIG. 4 is the density of the sample taken from the corner after solidification divided by the true density (=7.92 g/cc).

Further, the above HIP steel piece (crutted with SUS304 (L)) was heated to 1150°C, then continuously hot rolled at 10%/pass, and the temperature at which edge cracking occurred was investigated. The results are shown in FIG. When the can thickness is up to 2 m, no significant drop in the temperature at which edge cracking occurs is observed compared to the case of a solid steel piece with the can part removed. Therefore, the lower limit of can thickness is 2n
limited to n.

Table 3 below shows the occurrence of edge cracks, the presence or absence of cracks during bending, and the pitting corrosion potential of steel plates manufactured using the materials and processes shown in Tables 1 and 2. The bending test was a 90° close bending with a bending radius of 2t to 4t (t: thickness), and was conducted at room temperature. or,
The pitting potential was measured according to JIS GO577. The pitting potential in this table is the value corresponding to a current density of 100 μA/an2 when using a saturated agar reference electrode (V vs SCE at 100 μA/an2
) is used.

The steel plate produced by the method of the present invention has no edge cracking, can be bent at 2 in bending tests, and has a pitting potential of 0.2.
Shows a good value of v or more.

〔Effect of the invention〕

As described above, according to the present invention, boron-containing austenitic stainless steel atomized powder is made of SUS30 of an appropriate thickness.
4 (L), filled into a square can made of SUS316 (L),
Solidified by IP and hot extrusion, made into a clad steel piece, then hot rolled to surface: SUS304 (L) or SUS316
(L)/A clad steel plate made of internally di-boron austenitic stainless steel will be produced, and the clad steel plate made by this manufacturing method will be useful for suppressing edge cracking during rolling and for used nuclear fuel storage and transportation. Not only does it have remarkable effects in terms of workability, such as being able to bend it into a cask basket with a small bending radius, but it also has a remarkable improvement in corrosion resistance, which has the effect of greatly reducing corrosion problems during use.

In addition, the material must be atomized powder and SUS30
By cladding with 4(L) and 5O3316(L), the toughness is also improved and an effect on safety is expected.

The present invention dramatically improves the performance of boron-containing austenitic stainless steel, which is used for the storage and transportation of nuclear waste, for which demand is expected to increase in the future and is also of great social significance. Yes, and its social value is extremely high.

[Brief explanation of drawings]

Figures 1 (a) and (b) are schematic diagrams of a powder-filled can used in the method of the present invention, Figure 2 is a schematic cross-sectional diagram of a clad steel piece made in the manufacturing process of the method of the present invention, and Figure 3 is a schematic diagram of the powder-filled can used in the method of the present invention. A cross-sectional view of the clad steel plate according to the invention method. Figure 4 is a graph showing the relationship between the thickness of the powder-filled can and the relative density reached at the corner of the HIP steel billet. Figure 5 is the relationship between the thickness of the powder-filled can and the relative density reached during hot rolling. FIG. 2 is a graph diagram showing the relationship with the temperature at which edge cracking occurs. In the figure, (1) indicates a can, (2) indicates powder, X' indicates a clad steel piece, and X indicates a clad steel plate. Figure 1 2FIA

Claims (2)

[Claims] (1) B: 0.3-3.0%, C: 0.0 in weight%
8% or less, Si: 0.01-2.0%, Mn: 2.0%
Below, Cr: 16.0-20.0%, Ni: 8.0-1
5.0%, N: 0.2% or less, and the balance is iron and unavoidable impurities.
A steel plate with austenite on its surface, which is characterized in that it is placed in a square can made of SUS304 (L) or SUS316 (L) of mm and is made into a clad steel piece by hot isostatic pressing or hot extrusion, and then hot rolled. A method for manufacturing a boron-containing austenitic stainless steel plate that is clad with stainless steel and has excellent corrosion resistance and workability. (2) B: 0.3-3.0%, C: 0.0 in weight%
8% or less, Si: 0.01-2.0%, Mn: 2.0%
Below, Cr: 16.0-20.0%, Ni: 8.0-1
5.0%, Mo: 3.0% or less, N: 0.2% or less, and the balance is iron and unavoidable impurities. Corrosion resistance and processing with austenitic stainless steel cladding on the steel plate surface characterized by placing it in a square can made of L) and making it into a clad steel piece by hot isostatic pressing or hot extrusion, and then hot rolling. A method for manufacturing a boron-containing austenitic stainless steel sheet with excellent properties.