JPH0790497A - Nitric acid resistant austenitic stainless steel - Google Patents

Abstract

PURPOSE:To produce stainless steel excellent in corrosion resistance to tunnel- shaped corrosion, suitable as the material used in a nitric acid-contg. environment such as the structural material for a nuclear fuel reprocessing apparatus and furthermore excellent in intergranular corrosion resistance in HAZ. CONSTITUTION:(1) This austenitic stainless steel is the one having a compsn. contg., by weight, <=0.5% Si, <=0.5% Mn, 10 to 16% Ni, 16 to 20% Cr, 2.0 to 3.0% Mo and 0.06 to 0.15% N, and the balance Fe with impurities (<=0.02% C, <=0.03% P and <=0.002% S). (2) This steel is the one obtd. by furthermore incorporating the stainless steel (1) (<=1.0% Mn and <=0.006% S) with Ca and/Ce independently or in total by 2XS(%) to 0.03%. (3) This steel is the one having the chemical compsn. in (1) and satisfying the following inequality: Ni(%)+60N (%)-4Mo(%)>=7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to nitric acid or a nitric acid resistant austenite exhibiting excellent corrosion resistance (corrosion resistance to tunnel corrosion) as a material used in an environment containing nitric acid, especially as a structural material of a nuclear fuel reprocessing apparatus. System stainless steel.

[0002]

2. Description of the Related Art Generally, austenitic stainless steel has a passive film formed on the surface in an environment containing a highly oxidizing acid such as nitric acid and exhibits excellent corrosion resistance. It is widely used as a structural material for reprocessing equipment. However, if the concentration of nitric acid becomes high, or if cerium ion (Ce ⁴⁺ ), ruthenium ion (Ru ³⁺ ), chromium ion (Cr ⁶⁺ ), etc. are mixed into nitric acid from the spent nuclear fuel, the oxidizing property becomes strong. , Becomes subject to severe corrosion accompanied by intergranular corrosion.

Further, since molybdenum (Mo) is generally effective for pitting corrosion resistance, Mo-added steel is used in a portion where pitting corrosion resistance is required. It is known that intergranular corrosion is more likely to occur than that of. On the other hand, in order to prevent precipitation of intermetallic compounds mainly composed of σ phase, which causes intergranular corrosion, the present applicant limits the content of components usually added to a predetermined range. , Especially specifying the content of carbon (C) and nitrogen (N),
C (%) + N (%) ≦ 0.15 and 120C (%) + 36
N (%) ≧ 1.36 {Cr (%) + Mo (%) + 1.5Si (%)}
We proposed an austenitic stainless steel satisfying the conditions of − {Ni (%) + 0.5Mn (%) + 11.6} (“%” means “% by weight.”). 57-
28740). Further, in Japanese Patent Laid-Open No. 4-143214,
In austenitic stainless steel (SUS 316L) to which nitrogen (N) is not added positively, it is considered that intergranular corrosion is caused by precipitation of χ phase in addition to precipitation of σ phase at the grain boundary, and solid solution occurs. Cooling rate during processing is 1-30 ℃ / sec
Is disclosed to limit the precipitation of the χ phase.

However, in the above conventional technique, C and N
In the actual operation, it is not always easy to perform the melting process by controlling the content of the other components in relation to other components, and since the cooling rate after the solution treatment is also required to have a high level of technology, the manufacturing process is difficult. There was the problem of difficulty.
Moreover, these measures are effective for suppressing intergranular corrosion, but are not necessarily effective for tunneling corrosion. It should be noted that tunnel-like corrosion is different from mere intergranular corrosion and is different from that shown in FIG.
As shown in Fig. 5, the corrosion is a corrosion in which the corrosion progresses in a tunnel shape in the rolling direction of the steel plate or the steel pipe (longitudinal direction of the paper surface of Fig. 2) accompanied by intergranular corrosion. Since it is difficult to predict the amount and the rate of progress of corrosion is high, it may lead to serious trouble of the equipment.

As described above, nitric acid or a stainless steel used in an environment containing nitric acid (hereinafter referred to as nitric acid-containing environment) has excellent corrosion resistance against the above-mentioned tunnel-like corrosion. It is necessary to establish drastic measures.

[0006]

The present invention has been made in view of such circumstances, and an object thereof is to provide an austenitic stainless steel having excellent corrosion resistance against tunnel-like corrosion in an environment containing nitric acid. It was made as.

[0007]

Means for Solving the Problems In order to solve the above-mentioned problems, the present inventors have made various investigations on the occurrence of tunnel-like corrosion by changing various components contained in stainless steel. When the content of (S) is 0.01% or more, the occurrence of tunnel-like corrosion becomes remarkable. Furthermore, when the corrosion state is examined in detail, tunnel-like corrosion is caused by the presence of MnS extended in the rolling direction during rolling. It was found that the corrosion of the crystal grain boundaries is accelerated along the portions where the MnS is formed and in the peripheral portion of MnS.

In order to prevent this tunnel-like corrosion,
Manganese (Mn) at the same time as S content 0.002% or less
It is necessary to reduce the content of Ca, and it is also effective to add calcium (Ca) or cerium (Ce). Further, in order to improve the intergranular corrosion resistance in the weld heat affected zone (hereinafter referred to as HAZ), nickel (N
It was found that the contents of i), nitrogen (N) and molybdenum (Mo) should be controlled appropriately.

The present invention has been made based on the above findings, and its gist resides in the following austenitic stainless steels.

Silicon (Si): 0.5% or less, manganese (Mn): 0.5% or less, nickel (Ni): 10 to 16%, chromium (Cr): 16 to 20%, molybdenum (Mo): 2.0 to 3.0
% And nitrogen (N): 0.06 to 0.15%, the balance consisting of iron (Fe) and inevitable impurities, carbon (C) in the impurities is 0.02% or less, phosphorus (P) is 0.03% or less, and sulfur ( S) is 0.002% or less, and is a nitrate-resistant austenitic stainless steel.

In the above stainless steel, Mn is
Nitrate-resistant austenitic stainless steel containing 1.0% or less, S of 0.006% or less, and further containing one or both of calcium (Ca) and cerium (Ce) alone or in total of 2 × S (%) to 0.03%. steel.

[0012] Having the above chemical composition, and Ni,
N and Mo contents are nitric acid resistant austenitic stainless steels satisfying the following formula (1).

Ni (%) + 60N (%)-4Mo (%) ≧ 7 (1) Having the above chemical composition, the contents of Ni, N and Mo are expressed by the above formula (1). Satisfactory nitric acid resistant austenitic stainless steel.

Although the above-mentioned stainless steel is actually a pipe, a plate, or other members having various shapes, these are collectively referred to as stainless steel here.

[0015]

The present invention (above inventions) will be described in detail below.

[0016] Conventionally, in pitting and crevice corrosion that occur in stainless steel in seawater, the starting point of corrosion is MnS contained in the steel.
In order to prevent such corrosion, it was sufficient if the S content was suppressed to about 0.01% or less. Moreover, Mn is often contained in an amount of 1% or more. From the viewpoint of weldability, the greater the S content, the easier it is to melt and the wider the allowable range of welding conditions. Therefore, the S content is often set higher unless there is a problem with pitting corrosion resistance and the like.

However, as described above, in a nitric acid-containing environment, tunnel-like corrosion occurs along the MnS extended in the rolling direction if the S content is merely set to about 0.01% or less. In order to prevent the occurrence of this tunnel-like corrosion, the content of S was reduced to 0.002% or less, the content of Mn was also reduced, and the other components were adjusted to have a predetermined content. The stainless steel of the above invention.

The effect of each component constituting the steel and the reason for limiting the content thereof will be described below.

Si is an element necessary for deoxidation and it is necessary to add Si to some extent. However, since the intergranular corrosion susceptibility of steel increases as the Si content increases, the upper limit is made 0.5%. If the content is too small, the deoxidizing effect will not be sufficient, so it is desirable to contain 0.05% or more.

Mn also has a deoxidizing action, and it is necessary to add Mn to some extent. However, if the addition amount is too large, the formation of MnS is promoted, which causes tunnel-like corrosion in a nitric acid-containing environment. Becomes Therefore, its content should be 0.5% or less. However, if the content is too small, the deoxidizing effect will not be sufficient, so it is desirable to contain 0.05% or more. In addition, as described later, when Ca or Ce is added, the upper limit of the Mn content can be increased to 1.0%.

Ni is an element necessary for stabilizing the austenite structure and suppressing intergranular corrosion. However, if the content is less than 10%, a sufficiently stable austenite structure cannot be secured, while if it exceeds 16%, the cost increases, so the content is made 10 to 16%.

Cr is an essential element for maintaining the corrosion resistance of steel. However, if the content is less than 16%, sufficient corrosion resistance cannot be ensured, while if it exceeds 20%, the Ni content must be increased in order to form an austenite structure, which causes an increase in cost. Its content is 16 to 20%.

Mo is an element necessary for improving the pitting corrosion resistance of steel. However, if the content is less than 2.0%, sufficient corrosion resistance cannot be ensured. On the other hand, if it exceeds 3.0%, intermetallic compounds precipitate and intergranular corrosion resistance deteriorates. 3.0%

N is contained in an amount of 0.06% or more in order to suppress intergranular corrosion. However, if the content exceeds 0.15%, the cleanliness of steel decreases, so the upper limit is made 0.15%.

In addition to the above-mentioned components, the stainless steel of [1] contains the balance Fe and inevitable impurities. It is important to suppress the upper limits of C, P and S as impurities.

C is the grain boundary of the HAZ when welding is performed.
Since Cr ₂₃ C ₆ is precipitated and a Cr-deficient region is generated in the vicinity thereof to deteriorate the corrosion resistance of grain boundaries, its content is 0.
02% or less.

Since P segregates at the grain boundaries and adversely affects the intergranular corrosion resistance, it is preferable that the content of P be low, and the content thereof be 0.03% or less.

S is the most important impurity element in the steel of the present invention, and its content must be suppressed to 0.002% or less in order to suppress tunnel corrosion caused by MnS. However, as described later, when Ca or Ce is added, the upper limit can be increased to 0.006%.

The above-mentioned stainless steel is a stainless steel further containing one or both of Ca and Ce.

Ca and Ce are elements having a strong binding force with S, and the addition of these elements suppresses the formation of MnS and prevents the occurrence of tunnel corrosion due to MnS extending in the rolling direction. be able to. Moreover, since segregation of S to the grain boundaries is suppressed, it is also effective in suppressing grain boundary corrosion. In order to fully exert such effects, Ca
It is necessary to add one or both of Ce and Ce alone or in total at least twice the S content, that is, at least 2 × S (%). However, if the content exceeds 0.03%, not only the effect is saturated, but also the cleanliness of the steel decreases, so the upper limit is made 0.03%.

When Ca or Ce is added, S is Ca
Since it is fixed with or Ce to suppress precipitation as MnS, Ca
The upper limit of Mn content compared to the case where Ce and Ce are not added.
From 0.5% to 1.0%, and the upper limit of S content is 0.002
% Can be increased to 0.006%. When the S content increases, it easily dissolves during welding, which is also advantageous for improving the weldability of steel.

The stainless steels mentioned above and HAZ are
Is a stainless steel with improved intergranular corrosion resistance, the steel of which is based on the above steel, and the steel of which is based on the above steel, and the contents of Ni, N and Mo are in a predetermined relationship. Controlled to meet.

The above Japanese Patent Publication No. 57-28740 also discloses a formula containing Ni, N and Mo which should be satisfied in order to prevent precipitation of the σ phase which causes intergranular corrosion. However, this is because the cause of the deterioration of corrosion resistance when heat treatment is performed at 700 ° C for 10 hours is due to the σ phase, so in order to suppress the formation of the ferrite phase that causes the precipitation of the σ phase, It shows the balance of the weights of the austenite phase forming elements.

However, according to the research results of the present inventors,
This occurs when HAZ is affected by welding heat in a relatively short time, for example, when an austenitic stainless steel containing Mo such as the steel of the present invention is heat-treated at 650 ° C for about 2 hours. Intergranular corrosion is not caused by the σ phase,
Dense hexagonal metal compound with C14 type structure (Fe, Cr, Ni ₂ M
This is due to the precipitation of o). If the contents of Ni, N and Mo among the alloying elements contained in the steel are controlled so as to satisfy the following equation (1), intergranular corrosion under such conditions will occur. Can be prevented.

Ni (%) + 60N (%)-4Mo (%) ≧ 7 (1) Further, if the contents of Ni, N and Mo are controlled as such, cooling during the solution treatment Speed 1 ~ 30 ℃
It is possible to prevent the precipitation of the χ phase described in JP-A-4-143214 and to ensure the corrosion resistance even if the water cooling treatment which is generally performed is not limited to / sec.

In the stainless steels mentioned above and
The reason why the condition (1) is satisfied is added for the above reason. The stainless steels of and have excellent corrosion resistance against tunnel-like corrosion, and also have excellent intergranular corrosion resistance of HAZ.

[0037]

[Example] 180 kg of austenitic stainless steel having the chemical composition shown in Table 1 was melted by vacuum melting, forged and hot rolled to finish a plate material with a thickness of 12 mm, then heated to 1050 ° C and water-cooled for solution treatment. A tunnel-like corrosion test and a nitric acid corrosion test were carried out using as a test material the one subjected to the solution treatment or the solution subjected to the solution treatment and further subjected to the sensitization treatment of heating at 650 ° C. for 2 hours and then air cooling.

In the tunnel-shaped corrosion test, a test piece was cut out from the test solution subjected to the solution treatment so that the end face in the rolling direction would be the surface of the test piece, and then machined to give a width of 10
mm, length 40 mm, thickness 4 mm (width 10 mm, length 40 mm is the surface of the test piece), corrosion test piece 0.02 g in 65% nitric acid
The operation of immersing in a boiling solution to which 1 / liter of Cr ⁶⁺ was added for 24 hours, renewing the solution, and immersing again for 24 hours was repeated 5 times. Next, the test piece after this corrosion test was cut from the surface in the thickness direction and then embedded in a resin, and the tunnel-like corrosion depth was measured with an optical microscope.

In the nitric acid corrosion test, a test piece of the same shape produced by the same method as that of the test piece of the tunnel corrosion test was used in the 65% boiling nitric acid. The operation of immersing for a time, renewing the liquid and then immersing again for 48 hours was repeated 5 times, and the weight loss of the test piece was measured to determine the corrosion rate. When severe intergranular corrosion occurs, the corrosion rate increases due to the loss of crystal grains.

The results of the tunnel corrosion test are shown in Table 1 and FIG. The numbers or symbols attached to each measurement point in the figure are
Each corresponds to the numbers (1 to 16 in the examples of the present invention) or symbols (AK in the comparative examples) in the leftmost column of Table 1. As shown in FIG. 1, in the Ca- and Ce-free steel, tunnel-like corrosion does not occur if the S content is 0.002% or less (Invention Example 1).
.About.6, 12 to 14), and above 0.002%, the corrosion depth becomes deeper as the S content increases (Comparative Examples A to E). Also, S
Even if the content is 0.002% or less, if the Mn content exceeds 0.5%, tunnel-like corrosion becomes large (Comparative Example F), and S
When both the content and the Mn content are higher than the content specified in the present invention, the corrosion depth becomes deeper (Comparative Example G).

An appropriate amount of one or more of Ca and Ce was contained.
In Ca and Ce-added steels, tunnel-like corrosion does not occur up to S content of 0.006% (Examples 7 to 9, 14, 15 of the present invention), and even if the Mn content exceeds 0.5%, the corrosion resistance to tunnel-like corrosion is high. It is maintained (Invention Examples 10 and 11). If the addition amounts of Ca and Ce are small (Comparative Example I) and the Mn content exceeds 1% (Comparative Example H), the occurrence of tunnel corrosion cannot be prevented.

FIG. 2 is a diagram showing an example of a metal structure of a cross section in the vicinity of the surface layer portion of the test piece after the tunnel-like corrosion test, where (a) is the invention example 2 in Table 1 and (b) is Similarly, it corresponds to Comparative Example B. As shown in this figure, tunnel steel corrosion occurred in the steel of Comparative Example B. The tunnel-like corrosion depth means the depth from the surface of the test piece to the portion where corrosion has occurred, that is, the length of d in FIG.

The results of the corrosion test in nitric acid are shown in Table 1 and FIG. The numbers or symbols attached to each measurement point in the figure are those shown in Figure 1.
In the same manner as in the above case, the numbers correspond to the numbers (1 to 16 in the examples of the present invention) or symbols (AK in the comparative examples) in the leftmost column of Table 1. As shown in FIG. 3, Ni (%) + 60N (%)-4
When the value of Mo (%) is less than 7 (invention examples 12 to 16 and comparative examples J and K), the corrosion rate increases as this value decreases, whereas when it is 7 or more (invention examples 1 to 1). In 6), the corrosion rate is extremely low.

As shown in Table 1, the steels of Examples 12 to 16 of the present invention are steels in which the contents of Ni, N, and Mo do not satisfy the above formula (1). Steels that are applicable to steel but are not included in the steel of the invention.

[0045]

[Table 1 (1)]

[0046]

[Table 1 (2)]

[0047]

INDUSTRIAL APPLICABILITY The stainless steel of the present invention has excellent corrosion resistance against tunnel corrosion and is suitable as a material used in an environment containing nitric acid, particularly as a structural material for a nuclear fuel reprocessing apparatus. A steel in which the contents of Ni, N and Mo in the steel are adjusted to satisfy a predetermined relationship is excellent in corrosion resistance against tunnel corrosion and also in intergranular corrosion resistance of HAZ.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the depth of tunnel corrosion and the S content of steel.

FIG. 2 is a copy drawing of a metal structure of a cross section in the vicinity of a surface layer portion of a test piece after a tunnel corrosion test.

FIG. 3 is a diagram showing the relationship between the value of Ni (%) + 60N (%)-4Mo (%) and the corrosion rate in nitric acid.

Claims (4)

[Claims] 1. By weight%, Si: 0.5% or less, Mn: 0.5% or less, Ni: 10 to 16%, Cr: 16 to 20%, Mo: 2.0 to 3.0% and N: 0.06 to 0.15% are contained. However, the balance consists of Fe and unavoidable impurities, C in the impurities is 0.02% or less, and P is 0.03%.
%, S is 0.002% or less, nitric acid resistant austenitic stainless steel. 2. By weight%, Si: 0.5% or less, Mn: 1.0% or less, Ni: 10 to 16%, Cr: 16 to 20%, Mo: 2.0 to 3.0% and N: 0.06 to 0.15% are contained. In addition, either or both of Ca and Ce alone or in total 2 × S
(%) To 0.03%, the balance consisting of Fe and unavoidable impurities, C in the impurities is 0.02% or less, P is 0.03% or less,
Nitrogen resistant austenitic stainless steel characterized by S of 0.006% or less. 3. By weight%, Si: 0.5% or less, Mn: 0.5% or less, Ni: 10 to 16%, Cr: 16 to 20%, Mo: 2.0 to 3.0% and N: 0.06 to 0.15% are contained. However, the balance consists of Fe and unavoidable impurities, C in the impurities is 0.02% or less, and P is 0.03%.
% Or less, S is 0.002% or less, and Ni, N and
A nitric acid resistant austenitic stainless steel characterized by satisfying the following formula (1): Mo content. Ni (%) + 60N (%)-4Mo (%) ≧ 7 ・ ・ ・ (1) 4. By weight%, Si: 0.5% or less, Mn: 1.0% or less, Ni: 10 to 16%, Cr: 16 to 20%, Mo: 2.0 to 3.0% and N: 0.06 to 0.15% are contained. In addition, either or both of Ca and Ce alone or in total 2 × S
(%) To 0.03%, the balance consisting of Fe and unavoidable impurities, C in the impurities is 0.02% or less, P is 0.03% or less,
A nitrate-resistant austenitic stainless steel characterized in that S is 0.006% or less, and the contents of Ni, N and Mo satisfy the following formula (1). Ni (%) + 60N (%)-4Mo (%) ≧ 7 ・ ・ ・ (1)