JPH0925512A - Method for producing low carbon austenitic stainless steel with excellent nitric acid resistance - Google Patents

Abstract

(57) [Summary] [Objective] To obtain a method for producing a low carbon austenitic stainless steel which exhibits excellent intergranular corrosion resistance in a high concentration nitric acid solution such as a reprocessing facility for spent nuclear fuel. [Structure] C: 0.02% or less, Si: 0.35% or less, P: 0.02% or less, Ni: 10 to 20%, Cr: 16 to 25% by weight
%, B: Austenitic stainless steel containing 10 to 50 ppm, and C: 0.02% or less, Si: 0.35% or less, P:
0.02% or less, Ni: 13 to 20%, Cr: 16 to 25%, Mo:
A nitric acid solution characterized by subjecting an austenitic stainless steel containing 1 to 3%, B: 10 to 50 ppm, to a solution treatment and then a heat treatment for 10 minutes to 2 hours in a temperature range of 550 ° C to 750 ° C. Method for producing low carbon austenitic stainless steel excellent in intergranular corrosion resistance in the atmosphere.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for producing a low carbon austenitic stainless steel which exhibits excellent intergranular corrosion resistance in a high concentration nitric acid solution such as a spent nuclear fuel reprocessing facility. .

[0002]

2. Description of the Related Art In a nuclear fuel reprocessing facility, spent nuclear fuel is dissolved and separated using high-concentration nitric acid, which prevents structural intergranular corrosion in nitric acid solutions. In order to do so, a low carbon austenitic stainless steel such as SUS304L steel (18Cr-8Ni type) or SUS316L steel (17Cr-12Ni-2Mo type) in which the amount of carbon in steel is reduced to 0.03% or less is used.

It has been reported that intergranular corrosion also occurs when such a low carbon austenitic stainless steel is used. This is due to the small amount of B contained in the steel, and it has been reported that the amount of B in the stainless steel used in nitric acid environment must be 10 ppm or less (Stainless Steel. '87 .P234. (1988)).

However, in recent years, the cost has been reduced by using as much scrap as the melting raw material as much as possible, and it is necessary to carefully select the scrap or reduce the amount of the scrap to reduce the B content in the steel to 10 ppm or less. ,
There are disadvantages such as an increase in manufacturing cost.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have investigated a treatment method for eliminating the adverse effect of B even if B is not extremely reduced as in the above concentration. The present invention provides a method for producing stainless steel which prevents the occurrence of intergranular corrosion of low carbon austenitic stainless steel due to B in a nitric acid solution and can be used for a long period of time as a structural material for nuclear fuel reprocessing equipment. The purpose is.

[0006]

Means for Solving the Problems The present inventors have conducted many experiments to achieve the above object, and as a result, 10 ppm
It has been found that even in the above stainless steel containing B, it is possible to prevent the occurrence of intergranular corrosion in a nitric acid solution by further performing heat treatment in the temperature range of 550 ° C. to 750 ° C. after solution heat treatment. is there. The gist of the present invention is as follows.

(1)% by weight, C: 0.02% or less, S
i: 0.35% or less, Mn: 2.0% or less, P: 0.0
2% or less, S: 0.03% or less, Ni: 10 to 20%,
After subjecting austenitic stainless steel containing Cr: 16 to 25%, B: 10 ppm to 50 ppm, and the balance being substantially iron at a temperature of 950 ° C to 1200 ° C, a solution heat treatment is further performed at 550 ° C. 1 in the temperature range of 750 ℃
A method for producing a low carbon austenitic stainless steel having excellent nitric acid resistance, which is characterized by performing a heat treatment for 0 minutes or more and 2 hours or less.

(2)% by weight, C: 0.02% or less, S
i: 0.35% or less, Mn: 2.0% or less, P: 0.0
2% or less, S: 0.03% or less, Ni: 13 to 20%,
Cr: 16-25%, Mo: 1-3%, B: 10 ppm-
After subjecting the austenitic stainless steel containing 50 ppm, the balance being essentially iron, to a solution heat treatment at a temperature of 950 ° C to 1200 ° C, a further 550 ° C to 75
A method for producing a low carbon austenitic stainless steel excellent in nitric acid resistance, which comprises performing a heat treatment for 10 minutes or more and 2 hours or less in a temperature range of 0 ° C.

The constituent elements and the action and the reason for limitation of the heat treatment will be described below with reference to the claims. C: C precipitates as (Cr, Fe) ₂₃ C ₆ at the grain boundary when the cooling rate after solution heat treatment is slow in the weld heat affected zone and forms a Cr deficient layer to form the grain resistance of the steel. The intercalation property deteriorates, so the content is made 0.02% or less.

Si: Usually, austenitic stainless steel contains about 0.5 to 0.6% of Si because it is subjected to secondary refining with Si as a deoxidizing agent. However, Si is a grain in a nitric acid solution. Since it causes intergranular corrosion, its content should be 0.35% or less. Mn: Mn deteriorates the hot workability when it exceeds 2.0%, so its content is made 2.0% or less.

P: P is 500 ° C. like the heat affected zone
When heated in the temperature range from 1 to 700 ° C, N
Since it is an element that precipitates as an i-P compound and deteriorates the intergranular corrosion resistance of stainless steel in a nitric acid solution, its content is 0.02% or less. S: S is an element that forms sulfides and deteriorates corrosion resistance such as pitting corrosion, so its content is 0.03% or less.

Ni: Ni is an austenite stabilizing element, and a content of at least 10% or more is necessary to stabilize the austenite structure. However, if it exceeds 20%, the grain boundary precipitation of the Ni-P compound is promoted and the intergranular corrosion resistance of the present steel is significantly deteriorated, so the content is made 20% or less.

Cr: Cr is an element essential for ensuring the corrosion resistance of stainless steel in a nitric acid solution, and its content must be at least 16% or more. However, if it exceeds 25%, δ ferrite is formed, and grain boundary precipitation of intermetallic compounds such as σ phase occurs in the weld heat affected zone, causing embrittlement of the material, so the upper limit of its content is 25%. To do.

B: B is considered to deteriorate the intergranular corrosion resistance of stainless steel in a nitric acid solution by precipitating as Cr ₂ B in the grain boundary, and its content is 10 ppm or less. It is considered necessary to reduce it. However, in the present invention, this B
It is not necessary to reduce the B content to 10 ppm or less, because the adverse effect of can be eliminated by the reheat treatment. However, if the amount of B exceeds 50 ppm, excellent intergranular corrosion resistance cannot be obtained even if re-heat treatment is carried out after solution heat treatment, so the upper limit is made 50 ppm.

Subsequently, the austenitic stainless steel adjusted to the above component concentration is heat-treated at a temperature between 950 ° C. and 1200 ° C. This is for the purpose of homogenizing the structure. In the temperature range below 950 ° C., recrystallization does not occur, so that the structure cannot be made uniform, and in the temperature range over 1200 ° C., the crystal grain size is remarkably coarsened, and both strength and toughness deteriorate, The heat treatment is performed within the above temperature range.

The cooling after the heat treatment is preferably water cooling in order to prevent grain boundary precipitation of Cr carbide and Ni-P compound. Further, in the present invention, the solution heat treatment time is not particularly specified, but this is because the time required for recrystallization changes depending on the processing conditions before the solution heat treatment. However, in most cases 5 minutes to 3
A heat treatment time of 0 minutes is often sufficient.

Further, in the present invention, the heat treatment for eliminating the adverse effect of B is performed in the temperature range of 550 ° C. to 750 ° C. for 10 hours.
Heat treatment is performed for at least minutes and no more than 2 hours. Although the mechanism by which the adverse effect of B disappears by heat treatment in such a temperature range is unknown, it is believed that the intergranular corrosion resistance is improved by changing the composition or morphology of the boride at the grain boundaries.

Therefore, if the heat treatment time is less than 10 minutes, the effect of the reforming heat treatment cannot be sufficiently exerted, and if the heat treatment time is longer than 2 hours, even if low C / low P is applied, the particles are The upper limit of the heat treatment time is set to 2 hours because Ni-P compound or Cr carbide precipitates in the boundary and deteriorates the intergranular corrosion resistance of the present steel.

Next, a description will be given according to claim 2. Claim 2
Relates to an austenitic stainless steel whose crevice corrosion resistance and pitting corrosion resistance are improved by adding Mo in addition to the above components.
The contents of the elements other than the above and the reasons for the limitation are exactly the same as in the case of claim 1.

Ni: For Ni, the content is 1
3% or more and 20% or less, and the lower limit of Ni higher than that of claim 1 is specified. This defines the intergranular precipitation of the Laves phase, which is the cause of intergranular corrosion of Mo-containing stainless steel, as Ni.
Has a delaying effect. That is,
When the Ni content is less than 13%, La in the heat affected zone of welding, etc.
This is because grain boundary precipitation of the ves phase cannot be suppressed and the intergranular corrosion resistance deteriorates.

Therefore, from the viewpoint of the effect of suppressing the Laves phase, it is desirable that the Ni content is high, but if the Ni content exceeds 20%, the Ni-P content is the same as in the case of claim 1.
The upper limit of the Ni content is 20% because it promotes the grain boundary precipitation of the compound and deteriorates the intergranular corrosion resistance.

Mo: Mo is an element effective for improving the pitting corrosion resistance and crevice corrosion resistance of stainless steel,
The content must be at least 1% or more to exert a sufficient effect. However, Mo is an element that promotes precipitation of the Laves phase, and if the content exceeds 3%, the intergranular corrosion resistance of the present steel is extremely deteriorated. Therefore, the upper limit of the content is 3%.

Next, the austenitic stainless steel adjusted to the above component concentration is subjected to solution heat treatment at a temperature of 950 ° C. to 1200 ° C., and then 550 ° C. to 75 ° C.
Heat treatment is performed at a temperature of 0 ° C. for 10 minutes or more and 2 hours or less,
The purpose and the reason for limitation of the heat treatment are exactly the same as in the case of claim 1.

[0024]

EXAMPLES Tables 1 and 2 show the chemical composition of the steels of the present invention and the comparative steels whose composition concentrations were adjusted according to claims 1 and 2, respectively, the heat treatment conditions and the boiling 65% nitric acid test method (JI).
S method: JIS G0573) shows the result of evaluating the corrosion resistance in nitric acid solution by the average corrosion rate. By adjusting the concentration of the constituents in the steel according to the present invention, and by subjecting it to solution heat treatment and reheat treatment, the B content can be increased. Without reducing
It can be seen that it exhibits excellent nitric acid resistance.

[0025]

[Table 1]

[0026]

[Table 2]

[0027]

EFFECTS OF THE INVENTION Even if the B content in steel is not reduced to 10 ppm or less, the austenitic stainless steel that has been subjected to a modified heat treatment by defining the component concentrations in the steel according to the present invention has excellent grain resistance even in a nitric acid solution. It exhibits intercalation properties and can be used for a long period of time as a structural material in highly concentrated nitric acid solutions such as nuclear fuel reprocessing facilities.

Claims (2)

[Claims] 1. By weight%, C: 0.02% or less, Si: 0.35% or less, Mn: 2.0% or less, P: 0.02% or less, S: 0.03% or less, Ni : 10 to 20%, Cr: 16 to 25%, B: 10 ppm to 50 ppm, and after the solution heat treatment of the austenitic stainless steel with the balance being substantially iron at a temperature of 950 to 1200 ° C. A method for producing a low-carbon austenitic stainless steel having excellent nitric acid resistance, which further comprises performing heat treatment in a temperature range of 550 ° C. to 750 ° C. for 10 minutes or more and 2 hours or less. 2. By weight%, C: 0.02% or less, Si: 0.35% or less, Mn: 2.0% or less, P: 0.02% or less, S: 0.03% or less, Ni : 13 to 20%, Cr: 16 to 25%, Mo: 1 to 3%, B: 10 ppm to 50 ppm and an austenitic stainless steel whose balance is substantially iron at a temperature of 950 to 1200 ° C. A method for producing a low carbon austenitic stainless steel excellent in nitric acid resistance, which is characterized by further performing a heat treatment for 10 minutes or more and 2 hours or less in a temperature range of 550 ° C. to 750 ° C. after performing a solution heat treatment.