JPH08209311A - Cast austenitic stainless steel excellent in nitric acid resistance and surface treatment therefor - Google Patents

Abstract

PURPOSE: To produce a cast austenitic stainless steel for structural purposes, excellent in corrosion resistance under a nitric acid environment with a concentration not higher than azeotropic concentration and also having high strength and superior castability. CONSTITUTION: This steel is a cast high strength austenitic stainless steel having a steel composition which consists of, by weight, <=0.03% C, 0.3-1% Si, <=2% Mn, <=0.03% P, <=0.03% S, 19-21% Cr, 8.5-12% Ni, 0.05-0.2% N, 0.03-0.1% Nb+Ta, <=0.001% B, and the balance Fe with inevitable impurities and in which the ratio between Creq, represented by Creq=%Cr+1.5×%Si+1.4×%Mo+%Nb-4.99, and Nieq, represented by Nieq=%Ni+30×%C+0.5×%Mn+26×(%N-0.02)+2.77, is regulated to 0.9-1.2, also having superior castability, and excellent in nitric acid resistance under a nitric acid environment with a concentration not higher than azeotropic concentration. Surface treatment for this cast austenitic stainless steel excellent in nitric acid resistance can be performed by immersing this cast steel for >=30min in an aqueous solution having a temp. of 40-70 deg.C and 20-50wt.% nitric acid concentration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength austenitic stainless cast steel having good castability and exhibiting excellent nitric acid resistance in a nitric acid environment below an azeotropic concentration.

[0002]

2. Description of the Related Art Structures exposed to nitric acid such as a nitric acid production plant, a nuclear fuel reprocessing plant, and a urea plant are required to have strength and nitric acid resistance. In the presence of nitric acid, general corrosion, intergranular corrosion and crevice corrosion occur, but in the present invention, the corrosion resistance to these is collectively referred to as nitric acid resistance.
Since stainless steel exhibits excellent nitric acid resistance in the environment where nitric acid exists, SUS30 is used for the casings of impeller circulation pumps, impellers, valves, etc. in the above plants.
Austenitic stainless cast steel represented by SCS19 equivalent to 4 L is often used. For austenitic stainless steel sheets, pipes and forged materials, fine grain austenite single-phase structure can be obtained by hot working and heat treatment.
A predetermined strength is obtained. On the other hand, since cast steel is used as it is cast, the strength cannot be improved by refining the crystal grains. Therefore, in cast steel, the strength is improved by controlling the amount of precipitation of δ-ferrite during solidification.

Cast steel is often used for casting products having complicated shapes, and is required to have good castability (flowability).

JP-A-53-89819 and JP-A-5-156411 disclose cast steels having improved nitric acid resistance or intergranular corrosion resistance.

The method for producing cast steel described in Japanese Patent Laid-Open No. 53-89819 aims at making the cast structure fine by a combination of plastic working and heat treatment to improve the intergranular corrosion resistance, and at the same time, make the surface of the material uneven. It is characterized in that it enables ultrasonic flaw detection.

The cast steel described in Japanese Patent Laid-Open No. 5-156411 was developed by the inventors of the present invention. Although this cast steel is a cast steel for nitric acid environment, it is a cast steel for high-concentration nitric acid and has SiO _{2 on} the steel surface. 3.5 ~ 6 to prevent corrosion by nitric acid by forming a system film
The feature is that the amount of Si is high.

Further, JP-A-5-331670 discloses a surface treatment method for austenitic stainless steel in which Cr is enriched on the steel surface by heat treatment in a vacuum atmosphere after shot peening treatment for improving corrosion resistance.

[0008]

The cast steel described in JP-A-53-89819 has no specific components specified, and the cast steel described in JP-A-5-156411 has a medium concentration. In the nitric acid environment below, that is, in the nitric acid environment below the azeotropic concentration, it was found that there are problems in corrosion resistance and intergranular corrosion resistance.

The surface treatment method described in JP-A-5-331670 is a surface treatment method for improving corrosion resistance in high temperature steam, and it is unknown whether it is effective even in a nitric acid environment. In addition, the invention described in the publication is SUS210 material (17Cr
-4Ni-6Mn) is an invention targeted at high Mn materials, and similar enrichment of Cr cannot be expected for the cast steel of this invention.

Conventionally, there has been no investigation on cast steel which is excellent in nitric acid resistance, castability, and strength in a nitric acid environment having a nitric acid content of about 70% or less, that is, a nitric acid environment of an azeotropic concentration or less. It wasn't done.

An object of the present invention is to provide a high-strength austenitic stainless cast steel which exhibits excellent corrosion resistance when used in a nitric acid environment below the azeotropic concentration and has good castability.

[0012]

[Means for Solving the Problems] The present inventors have been keen to develop an austenitic stainless cast steel for a structure that exhibits excellent corrosion resistance in a nitric acid environment below an azeotropic concentration and has high strength and good castability. We conducted research and obtained the following findings.

That is, as a result of various studies on the factors in which the cast steel for high-concentration nitric acid developed by the present inventors does not exhibit nitric acid resistance in a nitric acid environment having an azeotropic concentration or less, as a result, the corrosion resistance in the presence of high-concentration nitric acid has an effect. When the content of Si exceeds 1%, it adversely affects the nitric acid environment below the azeotropic concentration, and B, which is an impurity that penetrates into the molten steel from the refractory during casting, deteriorates the nitric acid resistance and reduces the B content. I found what I had to do. Also, the strength of cast steel depends on Creq / Nieq,
In order to maintain high strength by controlling the value within the range of 0.9 to 1.2 and to improve the corrosion resistance against crevice corrosion of cast steel parts incorporated as a structure, nitric acid should be added before assembling as a structure. We have found that the passivation treatment by dipping in an aqueous solution is effective.

The present invention is based on such knowledge, and the gist of the present invention is "C: 0.03% by weight.
% Or less, Si: 0.3 to 1%, Mn: 2% or less, P: 0.03% or less,
S: 0.03% or less, Cr: 19-21%, Ni: 8.5-12%, N:
0.05 to 0.2%, Nb + Ta: 0.03 to 0.1%, B: 0.001% or less, having a steel composition consisting of balance Fe and unavoidable impurities, and a ratio of Creq and Nieq shown in the following formula, Creq / Nieq Is 0.9 ~
1.2 high strength cast austenitic stainless steel excellent in nitric acid resistance in a nitric acid environment with good azeotropic concentration and good castability, and this cast steel at 40 to 70 ° C.
The method for surface treatment of austenitic stainless cast steel excellent in nitric acid resistance, which comprises immersing in an aqueous solution having a nitric acid concentration of 20 to 50% by weight in the temperature range of 30 minutes or more.

Creq =% Cr + 1.5 ×% Si + 1.4 ×% Mo +% Nb-4.99 Nieq =% Ni + 30 ×% C + 0.5 ×% Mn + 26 × (% N-0.02) +2.77 ".

[0016]

[Function] The reasons for limiting the component composition, Creq / Nieq and the surface treatment method and the function in the present invention will be described in detail below. % In the description indicates weight%.

C: C is a Cr carbide which adversely affects the corrosion resistance, especially the intergranular corrosion resistance and the selective corrosion at the boundary between the ferrite phase and the austenite phase. Therefore, the C content should be low in order to suppress its precipitation. Is preferred. However, if the C content is extremely reduced, the cost will increase, so the upper limit is made 0.03%.

Si: Si is an element that improves the castability as well as a deoxidizing component at the time of melting, but if it is less than 0.3%, the effect of improving the castability is small, and if it exceeds 1%, it is below the azeotropic concentration. Since the deterioration of nitric acid resistance becomes remarkable under the environment, the range is set to 0.03 to 1%. The reason why Si deteriorates nitric acid resistance in an environment at an azeotropic concentration or less is that the oxidizing property of the environment is weak and the passivation by Si cannot be strengthened.

Like Mn: Si, it is a deoxidizing component, but addition of a large amount deteriorates nitric acid resistance, so the upper limit is made 2%.

P: P segregates at the crystal grain boundaries and lowers the intergranular corrosion resistance, so it is preferable that the content be small, but considering manufacturability and economic efficiency, the upper limit is made 0.03%.

S: S is an element that binds to Mn to form MnS and inhibits corrosion resistance, and it is desirable that the content is small, but in view of manufacturability and economy, it is made 0.03% or less.

Cr: Cr is an element effective in improving nitric acid resistance, but if it is less than 19%, its effect is small, and if it exceeds 21%, precipitation of ferrite phase is promoted, and an increase in the amount of Ni is necessary. Since it is not economical, the range is set to 19-21%.

Ni: Ni is an important element in improving nitric acid resistance by stabilizing the austenite structure,
If it is less than 8.5%, its effect is small. On the other hand, if it exceeds 11%, the cost increases and the strength decreases, so the range is made 8.5 to 11%.

N: N contributes to the stabilization of the austenite structure and improves the intergranular corrosion resistance by the combined addition of a trace amount of Nb + Ta. However, if it is less than 0.05%, its effect is small, and if it exceeds 0.2%, it is a thermal effect. Since the interworkability is impaired, the range is set to 0.05 to 0.2%.

It is an element that suppresses the grain boundary precipitation of Cr carbide and improves the intergranular corrosion resistance by the combined addition of Nb + Ta: N, but if it is less than 0.03%, the effect cannot be exhibited, and
If it exceeds 1%, the castability will deteriorate, so the range is 0.03-0.
1%

B: B is an impurity that enters the molten steel from the refractory and accelerates the precipitation of Cr carbide at the grain boundaries and deteriorates the nitric acid resistance, so it is preferable that it is low, but in consideration of economy.
0.001% or less.

Creq / Nieq: The reason for defining the Creq / Nieq value is to make the strength as high as 500 N / mm ² or more. If it is less than 0.9, the strength cannot be obtained. On the other hand, if it exceeds 1.2, the nitric acid resistance is high. Is deteriorated, so the range is set to 0.9 to 1.2. C
If the amount of r is increased, the value of Creq / Nieq increases and the strength increases, but if the amount of Cr is too large, the ferrite is connected in a mesh and the nitric acid resistance deteriorates.

(Surface treatment conditions) Nitric acid concentration: When the nitric acid concentration is less than 20%, the effect of improving the crevice corrosion resistance is not recognized, and when it exceeds 50%, the effect is saturated, so that the nitric acid concentration range is 20 to 50%. And

Surface treatment temperature: If the surface treatment temperature is less than 40 ° C., the effect of improving crevice corrosion resistance is small, and the effect increases as the temperature increases, but the upper limit was set to 70 ° C. in consideration of handling.

Immersion time: The immersion time is set to 30 minutes or more since it is necessary to soak for at least 30 minutes so that the crevice corrosion resistance can be improved.

[0031]

【Example】

(Embodiment 1) Next, the present invention will be specifically described based on embodiments.

Cast steels having the chemical composition (% by weight) shown in Table 1 were melted in the air using a high frequency melting furnace and cast into a sand mold of a closed impeller having an outer diameter of 300 mm and a blade thickness of 5 mm as a castability test. .

[0033]

[Table 1]

On the other hand, for the strength and nitric acid resistance test, it was cast into a sand mold having a thickness of 50 mm, a width of 100 mm, and a length of 250 mm, and after the die cutting, solution heat treatment was performed at 1100 ° C.

A JIS 14A tensile test piece was prepared from the cast steel after the solution heat treatment, and a nitric acid resistance test piece having a thickness of 3 mm, a width of 10 mm and a length of 40 mm was prepared.

These test pieces were subjected to boiling 65% nitric acid test and tensile test. In the boiling 65% nitric acid test, before the test
Sensitization heat treatment was performed at 650 ° C for 2 hours.

The test results are shown in Table 2. Nitric acid resistance test is JI
According to S G0573, the corrosion rate in Table 2 is boiling 65% nitric acid.
The average value is shown after repeating the immersion for 48 hours 5 times (renewing the solution once). The erosion depth is the erosion depth measured by an optical microscope on the cross section of the test piece after repeating 5 times, and shows the depth of intergranular corrosion.

The castability was evaluated by ◯ when it was completely cast into the shape of the impeller, and when it was poor in the molten metal flow and there was some casting failure, it was rated as x.

[0039]

[Table 2]

Table 3 shows the tensile test results of the cast steel after the solution treatment.

[0041]

[Table 3]

As is clear from Table 3, the cast steel of the present invention satisfying all the conditions specified in the present invention has a low corrosion rate and a small erosion depth. On the other hand, Comparative Steel No. 8 has Creq / N
Since the ieq value is small, the tensile strength is as low as 454 N / mm ² . Since No. 9 has high Si, it is not preferable in terms of corrosion degree and erosion depth. No. 7 has no Nb and Ta added, and contains a little B, so the nitric acid resistance is not preferable. In addition, No. 6 has a small amount of Nb + Ta and a small amount of N, so that the coexistence effect of both is not exhibited and the erosion depth is large.

(Example 2) A part of the molten steel having the components shown in Table 1 used in Example 1 was cast into a sand mold and, after demolding, 1100 ° C.
Solution heat treatment was performed. A crevice corrosion test piece having a thickness of 3 mm, a width of 30 mm and a length of 30 mm was prepared from the cast steel after the solution heat treatment. This test piece was wet-polished to emery paper # 600, and then the gap forming surface was subjected to surface treatment with nitric acid under the conditions shown in Table 4.

The treated test piece was sandwiched with a Teflon O-ring to form a gap, and the test piece was heated to 40 ° C. and 50% HNO ₃ for 4 hours.
A corrosion test of immersion for 8 hours was carried out. The test results are also shown in Table 4. The processing number 25 represents no processing.

[0045]

[Table 4]

From the above results, 20 to 5 at 40 to 70 ° C.
It can be seen that good crevice corrosion resistance can be obtained by immersing in 0% HNO ₃ for 30 minutes or more.

[0047]

INDUSTRIAL APPLICABILITY The austenitic stainless cast steel for nitric acid environmental structures of the present invention exhibits excellent nitric acid resistance in an environment below the azeotropic concentration, and also has excellent castability and strength. Suitable for medium concentration nitric acid environmental structures.

Claims (2)

[Claims] 1. C: 0.03% or less by weight%, Si: 0.3 to 1%,
Mn: 2% or less, P: 0.03% or less, S: 0.03% or less, Cr: 19
~ 21%, Ni: 8.5 ~ 12%, N: 0.05 ~ 0.2%, Nb + Ta: 0.
03-0.1%, B: 0.001% or less, has a steel composition consisting of balance Fe and unavoidable impurities, and has the following formula
A high-strength austenitic stainless cast steel with excellent nitric acid resistance in a nitric acid environment below azeotropic concentration with good castability, characterized by a ratio of Creq / Nieq of 0.9 to 1.2. Creq =% Cr + 1.5 ×% Si + 1.4 ×% Mo +% Nb-4.99 Nieq =% Ni + 30 ×% C + 0.5 ×% Mn + 26 × (% N-0.02) +2.77 2. The cast steel according to claim 1 in an aqueous solution having a nitric acid concentration of 20 to 50% by weight in a temperature range of 40 to 70 ° C.
A surface treatment method for austenitic stainless cast steel excellent in nitric acid resistance, characterized by dipping for 0 minute or more.