JP6858600B2 - Austenitic stainless steel for strongly oxidizing acidic environments - Google Patents

Description

The present invention relates to an austenitic stainless steel having excellent corrosion resistance in a low pH strongly oxidizing plant internal environment. In particular, the present invention relates to austenitic stainless steel having high corrosion resistance suitable for use in a low pH strongly oxidizing environment containing chromic acid.

Stainless steel is known to form a stable passivation film in an oxidizing environment and exhibit excellent corrosion resistance. Austenitic stainless steels typified by SUS304 and SUS316 are excellent in corrosion resistance, workability and weldability, and are therefore widely used in equipment and facilities of chemical plants for producing chemical substances.

In plants that produce chromic anhydride, high concentrations of chromic acid are processed at high temperatures by processing equipment such as rotary kilns. From the viewpoint of corrosion resistance, the processing equipment is currently AISI316Ti, which is a SUS316-based material, and NAS155N (18Cr-15.5Ni-4.1Mo-3.4Cu-0.18N), which has excellent corrosion resistance in a low pH environment. used. However, since chromic acid is a highly oxidizing substance, the treatment equipment is exposed to a strongly oxidizing environment at a low pH. In a strongly oxidizing acidic environment containing a high concentration of chromic acid at a high temperature, thinning due to oxidative corrosion is unavoidable even with the above-mentioned stainless steel. The service life of the processing equipment and equipment is short, and repairs are performed at regular intervals by overlay welding of Ni-based alloys.

Stainless steel and Ni-based alloys have high corrosion resistance by forming a dense and thin passivation film on the surface. However, in a strongly oxidizing acidic environment where the pH is lowered and the pH is as low as 0.8 or less, the passivation film on the surface of the material is dissolved and the protective state of the base material cannot be sufficiently maintained. Substrates exposed to a strongly oxidizing acidic environment dissolve at a high rate, resulting in significant wall thinning.

Since the chemical plant for producing chromic anhydride operates in a strongly oxidizing acidic environment with a low pH of 0.8 or less, the processing equipment reduces the wall thickness in the base part of stainless steel and the repair part of Ni-based alloy. It was unavoidable, and it was necessary to continue repairs each time. Therefore, since the cost required for maintenance increases, there is a demand for a corrosion-resistant material suitable for a strongly oxidizing environment with a low pH.

Conventionally, austenitic stainless steels of Patent Documents 1 and 2 have been reported as corrosion-resistant materials suitable for an environment in which nitric acid, which is an oxidizing acid, is present. Patent Document 1 describes a nitric acid corrosion resistant austenitic stainless steel containing Si: 2.0 to 4.0%, Cr: 16.0 to 25%, and Ni: 9.0 to 17% by weight. It is said that excellent corrosion resistance can be exhibited in a wide nitric acid concentration range from low concentration to high concentration by setting the ratio of Si and Cr to 0.08 to 0.25. Patent Document 2 describes a nitrate-resistant austenitic stainless steel containing Si: 0.5% or less, Ni: 9 to 15%, and Cr: 16 to 20% by weight, and has an S content of 0. It is said that tunnel-like corrosion can be prevented in a nitric acid-containing environment by reducing the Mn content to 002% or less at the same time.

Japanese Unexamined Patent Publication No. 5-287460 Japanese Unexamined Patent Publication No. 7-90501

Conventionally, Cr, Ni, Si, and Mo are known to be effective elements for improving the corrosion resistance of stainless steel. However, stainless steel having high corrosion resistance in a low pH strongly oxidizing environment containing chromic acid, such as a chemical plant for producing chromic anhydride, has not been known conventionally. Patent Documents 1 and 2 disclose the corrosion resistance in an environment containing nitric acid, but do not disclose the corrosion resistance in a strongly oxidizing acidic environment in which chromic acid is present.

The present invention has been devised to solve the above problems, and provides an austenitic stainless steel having excellent corrosion resistance suitable for use in a strongly oxidizing environment having a low pH containing chromic acid. The purpose.

The present inventors have found that Mo in austenitic stainless steel is oxidized and eluted to reduce corrosion resistance in a low pH strongly oxidizing environment containing chromium, and further, Cr contained in excess is contained. Similarly, it was found that the austenitic stainless steel was eluted and affected the corrosion resistance, and the present invention was completed by adjusting the composition of the austenitic stainless steel. Specifically, the present invention provides the following.

(1) The present invention is based on mass%.
C: 0.07% or less,
Si: 1.0 to 4.0%,
Mn: 1.0% or less,
P: 0.040% or less,
S: 0.030% or less,
Cr: 17.0 to 20.0%,
Ni: 9.0-15.0%,
Cu: 4.0% or less,
Mo: 1.5% or less,
N: Including 0.20% or less
The rest is Fe and unavoidable impurities,
It has a composition in which the following formula (1) is 20 or more.
It is an austenitic stainless steel having excellent corrosion resistance in a strongly oxidizing acidic environment containing chromic acid.
5 (Si + Cu) + Ni-5Mo ・ ・ ・ Equation (1)
Here, the element symbol of the formula (1) means the content (mass%) of each element.

(2) In the present invention, W: 1.0% or less, V: 1.0% or less, Ca: 0.01% or less, REM: 0.01% or less, or B: 0. The austenitic stainless steel having excellent corrosion resistance in the strongly oxidizing acidic environment according to (1), which contains one or more selected from 01% or less.

The austenitic stainless steel having excellent corrosion resistance in the strongly oxidizing acidic environment according to (1) or (2), wherein the strongly oxidizing acidic environment has a pH of 0.8 or less.

According to the present invention, it is possible to provide a stainless steel containing chromic acid and exhibiting excellent corrosion resistance in a low pH strongly oxidizing environment. The life of equipment, devices and equipment used in a strongly oxidizing acidic environment can be improved, and the frequency of repairs can be reduced. Therefore, it contributes to the reduction of manufacturing cost.

It is a figure which shows the relationship between the value of the formula (1) and the corrosion weight loss in an Example.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following description.

The austenitic stainless steel according to the present invention has a mass% of C: 0.07% or less, Si: 1.0 to 4.0%, Mn: 1.0% or less, P: 0.040% or less, S. : 0.030% or less, Cr: 17.0 to 20.0%, Ni: 9.0 to 15.0%, Cu: 4.0% or less, Mo: 1.5% or less, N: 0.20 % Or less, the balance is Fe and unavoidable impurities, and the following formula (1) has a composition of 20 or more.
5 (Si + Cu) + Ni-5Mo ・ ・ ・ Equation (1)
Here, the element symbol of the formula (1) means the content (mass%) of each element.

In consideration of corrosion resistance in a strongly oxidizing environment at low pH, acid resistance was improved by adding Si, Cu and Ni. Further, since Mo is an element that generally improves corrosion resistance, it has been contained in a large amount in conventional highly corrosion-resistant stainless steels and Ni-based alloys. However, it has been found that Mo contained in the base material tends to be oxidized, ionized and eluted in a strongly oxidizing environment at a low pH containing chromic acid, which lowers the corrosion resistance of the base material. Therefore, the stainless steel according to the present invention has a reduced Mo content.

Further, Cr is a main component that forms a passivation film and maintains corrosion resistance. However, it has been found that when Cr is excessively contained, in a low pH strongly oxidizing environment containing chromic acid, Cr tends to be ionized and eluted to the outside, which deteriorates the corrosion resistance of the base material. .. Therefore, the stainless steel according to the present invention was adjusted to have an optimum Cr content.

Hereinafter, the action and effect of the components contained in the stainless steel according to the present invention and the reasons for limiting the content will be described. Hereinafter, the mass% of the chemical composition is simply indicated by%.

Since C is a strong austenite-forming element and an element effective for improving the strength, it can contain 0.01% or more. On the other hand, if it is added excessively, Cr carbides are precipitated at the grain boundaries by heating at 400 ° C. or higher, which causes intergranular corrosion. Therefore, the upper limit of the C content is preferably 0.07% or less, more preferably 0.05% or less.

Si has an action of strengthening the passivation film on the surface of the steel material in a low pH strongly oxidizing environment containing chromic acid, and is an element necessary for ensuring corrosion resistance. Therefore, it is preferable to add 1.0% or more of Si. On the other hand, excessive Si content reduces workability and toughness. Therefore, the Si content is preferably 4.0% or less.

Mn is an austenite-forming element and can contain 0.1% or more. On the other hand, the Mn content is preferably 1.0% or less because excessive addition causes a decrease in processability.

Since P impairs the hot workability of the base metal and the toughness of the welded portion, it is desirable to reduce the content as much as possible. However, since it is difficult to remove P by refining in the melting of Cr-containing steel, excessive cost increase is required in careful selection of raw materials in order to minimize the P content. Therefore, the P content is allowed to be 0.040% or less.

S is an element that lowers the hot workability of austenitic stainless steel, and also causes surface defects due to MnS, resulting in an increase in manufacturing cost. Therefore, the S content is preferably reduced as much as possible, preferably 0.030% or less.

Ni is an element necessary for maintaining the austenite structure and also has an action of lowering the dissolution rate of steel in a strongly oxidizing environment of low pH containing chromic acid, and is an element necessary for ensuring corrosion resistance. Is. If the Ni content is less than 9.0%, these effects cannot be sufficiently obtained. Therefore, the lower limit of the Ni content is preferably 9.0% or more, more preferably 11.0% or more and 12.0% or more. On the other hand, if the Ni content exceeds 15.0%, the raw material cost of the steel material may increase and the hot workability of the steel may decrease. Therefore, the upper limit of the Ni content is preferably 15.0% or less, more preferably 14.0% or less and 13.0% or less.

Cr is a main constituent element of the passivation film, and is an element that improves corrosion resistance and oxidation resistance at high temperatures. Therefore, it is necessary to add 17.0% or more of Cr content. Therefore, the lower limit of the Cr content is preferably 17.0% or more, more preferably 18.0% or more. On the other hand, even if a large amount of Cr is contained, the effect of improving corrosion resistance in a low pH environment is saturated. Further, since it is a ferrite-forming element, it is necessary to increase the Ni content in order to secure the austenite structure, which increases the cost. Further, in a low pH strongly oxidizing environment containing chromic acid, the tendency of excessively contained Cr to elute to the outside increases, which leads to an increase in corrosion weight loss. Therefore, the upper limit of the Cr content is preferably 20.0% or less, more preferably 19.0% or less.

Cu is an element that generally improves the corrosion resistance of austenite-based stainless steel, and like Ni, has the effect of lowering the dissolution rate of steel in a low-pH, strongly oxidizing environment containing dichromic acid. Cu can be added in an amount of 0.01% or more, more preferably 0.1% or more. On the other hand, excessive addition of Cu impairs pitting corrosion resistance and hot workability, so the Cu content is preferably 4.0% or less, more preferably 3.5% or less.

Mo is an effective element for improving the corrosion resistance level. However, in a low pH strongly oxidizing environment containing chromic acid, Mo tends to be oxidized and ionized, which lowers the corrosion resistance of the base material. In addition, excessive content of Mo leads to an increase in raw material costs. Therefore, the Mo content is preferably 1.5% or less.

N is an austenite-producing element and an element having an action of increasing strength. However, excessive addition of N causes blowholes during casting, which may impair durability. Therefore, the N content is preferably 0.20% or less, more preferably 0.07% or less.

W and V are carbide-forming elements, and have the effect of fixing C as a carbide to improve intergranular corrosion resistance. It is also an element that is effective in improving high-temperature strength. Therefore, W or V may be added in an amount of 1.0% or less.

Ca and REM are elements that fix S in steel, enhance the deformability of the grain boundaries of the austenite phase, and prevent the occurrence of edge cracks in the hot-rolled steel strip. It may be added at 01% or less.

Since B is an element effective for improving hot workability, it may be added in an amount of 0.01% or less.

In addition to the amount added for each alloy element described above, the contents of Ni, Si, Cu, and Mo can be arranged by the formula (1) of "5 (Si + Cu) + Ni-5Mo". Regarding the corrosion resistance in the sexual environment, it was found that good corrosion resistance can be obtained in a specific range. Ni, Si, Cu, and Mo in the above formula (1) mean the content (mass%) of each element. Specifically, when the numerical value of the above formula (1) is 20 or more, good corrosion resistance is exhibited, and 30 or more and 35 or more are more preferable.

As used herein, low pH strongly oxidizing means an environment having a pH of 0.8 or less and containing chromic acid. That is, a low pH is an environment having a pH of 0.8 or less, and a strong oxidizing property is an environment containing an oxidizing agent typified by chromic acid. The austenitic stainless steel according to the present invention is suitable as a constituent material that requires high corrosion resistance in a chemical plant for producing chromic anhydride.

Hereinafter, examples of the present invention will be described. The present invention is not limited to the following description.

Stainless steel having the chemical composition shown in Table 1 was melted and hot-rolled to prepare a hot-rolled plate having a plate thickness of 3.0 mm. The hot-rolled plate was annealed at 1050 ° C to 1175 ° C for 3 minutes, and then the oxide film on the surface was removed by dry honing. Then, it is cold-rolled to a plate thickness of 1.0 mm, subjected to finish annealing at 1000 ° C. to 1150 ° C. for 1 minute, pickled with an aqueous solution of a mixture of hydrofluoric acid and nitric acid, and austenitic stainless steel having a thickness of 1 mm. A stainless steel plate was produced and used as a test material. Next, a 30 mm × 30 mm test piece was cut out from the test material, subjected to a wet polishing finish of # 600, and subjected to a corrosion resistance test as Examples 1 to 6 and Comparative Examples 1 to 9. In the steel composition (% by mass) shown in Table 1, the balance is Fe and unavoidable impurities.

(Corrosion resistance test)
The test for evaluating the corrosion resistance in a low pH strongly oxidizing environment containing chromic acid was carried out by the following procedure. After measuring the weight of the test piece, the test piece was placed in a container containing a test solution in which 50% by mass of dichromic acid and 50% by mass of sulfuric acid were mixed, and the test piece was embedded in the test solution. The container was sealed and heated at 200 ° C. for 7.5 hours in a constant temperature bath. After the heating was completed, the test piece was taken out from the test solution and immersed in a 30% nitric acid solution at 60 ° C. to remove scale (corrosion product) on the surface of the test piece. Next, the weight of the test piece was measured, and the corrosion weight loss (mg / cm ² ) was measured from the difference from the weight before immersion.

When the corrosion weight loss was ^{less than 0.30 mg / cm 2} , the corrosion resistance was evaluated as good (◯), and ^{when it was 0.30 mg / cm 2} or more, the corrosion resistance was evaluated as poor (x). The measurement results are shown in Table 1. Further, FIG. 1 shows the relationship between the numerical value of the formula (1) and the corrosion weight loss.

As shown in Table 1 and FIG. 1, Examples 1, 3 and 4 included in the composition range of the present invention had a small amount of corrosion loss and good corrosion resistance in a strongly oxidizing acidic environment containing chromic acid. On the other hand, in Comparative Examples 1 to 9, the value of the formula (1) was less than 20, and the content of each alloying element was also outside the scope of the present invention. Corrosion resistance in a strongly oxidizing acidic environment including was reduced.

In Comparative Example 1, Si was less than 1.0% and Mn was more than 1.0%. In Comparative Example 2, Mo further exceeded 1.5%. In Comparative Example 3, Si was less than 0.10%, Ni was more than 15.0%, and Mo was more than 1.5%. In Comparative Example 4, Si, Ni, Cr and the like were within the range of the present invention, whereas Mo was more than 1.5%.

In Comparative Examples 5 to 9, the contents of Si, Ni, Cr, and Mo were outside the range of the present invention, Si was less than 1.0%, and Mo was more than 1.5%. Further, in Comparative Examples 5 and 6, Ni was less than 9.0% and Cr was more than 20.0%. In Comparative Examples 7 and 8, Ni exceeded 15.0% and Cr exceeded 20.0%. Further, Comparative Example 9 corresponds to a Ni-based alloy in which Cr exceeds 20.0% and Ni exceeds 15.0%.

From the above, it was confirmed that the austenitic stainless steel having the constitution of the present invention has excellent corrosion resistance in a strongly oxidizing acidic environment containing chromic acid.

Claims (2)

By mass%
C: 0.07% or less,
Si: 1.0 to 4.0%,
Mn: 1.0% or less,
P: 0.040% or less,
S: 0.030% or less,
Cr: 17.0 to 20.0%,
Ni: 9.0-15.0%,
Cu: 4.0% or less,
Mo: 1.5% or less,
N: Including 0.20% or less
The rest is Fe and unavoidable impurities,
It has a composition in which the following formula (1) is 35 or more.
Austenitic stainless steel with excellent corrosion resistance in strongly oxidizing acidic environments containing chromic acid.
5 (Si + Cu) + Ni-5Mo ・ ・ ・ Equation (1)
Here, the element symbol of the formula (1) means the content (mass%) of each element. The austenitic stainless steel having excellent corrosion resistance in the strongly oxidizing acidic environment containing chromic acid according to claim 1, wherein the strongly oxidizing acidic environment has a pH of 0.8 or less.

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