JP2018150580A - Austenitic stainless steel for strong oxidizing acidic environment - Google Patents

Abstract

The present invention provides an austenitic stainless steel excellent in corrosion resistance suitable for use in a low pH strong oxidizing environment containing chromic acid. In the present invention, C: 0.07% or less, Si: 1.0 to 4.0%, Mn: 1.0% or less, P: 0.040% or less, S: 0% by mass. 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 And the balance is Fe and inevitable impurities, and the composition represented by the following formula (1) is 20 or more, and is an austenitic stainless steel having excellent corrosion resistance in a strong oxidizing acidic environment including chromic acid. 5 (Si + Cu) + Ni-5Mo Expression (1) [Selection] FIG.

Description

  The present invention relates to an austenitic stainless steel having excellent corrosion resistance in a low pH strong oxidizing plant internal environment. More particularly, the present invention relates to a highly corrosion-resistant austenitic stainless steel suitable for use in a low-pH, strong oxidizing environment containing chromic acid.

  Stainless steel is known to exhibit excellent corrosion resistance by forming a stable passive film in an oxidizing environment. Austenitic stainless steel represented by SUS304 and SUS316 is widely used in equipment and facilities of chemical plants that produce chemical substances because of its excellent corrosion resistance, workability, and weldability.

  In a plant for producing chromic anhydride, high-concentration chromic acid is processed at a high temperature by a processing apparatus such as a rotary kiln. From the viewpoint of corrosion resistance, the processing apparatus is currently AISI316Ti, which is a SUS316-based material, or NAS155N (18Cr-15.5Ni-4.1Mo-3.4Cu-0.18N), which is excellent in corrosion resistance in a low pH environment. used. However, since chromic acid is a substance having a strong oxidizability, the processing apparatus is exposed to a low pH strong oxidative environment. In a strong oxidizing acidic environment containing a high concentration of chromic acid at high temperatures, thinning due to oxidative corrosion is inevitable even with the above stainless steel. The service life of the processing equipment and equipment is short, and repair is performed by build-up welding of a Ni-based alloy at a reduced thickness portion at regular intervals.

  Stainless steel and Ni-based alloys have high corrosion resistance by forming a dense and thin passive film on the surface. However, in a strongly oxidizing acidic environment with a low pH such as pH 0.8 or lower, for example, the passive film on the surface of the material dissolves and the protective state of the substrate cannot be sufficiently maintained. A substrate exposed to a strong oxidizing acidic environment dissolves at a high rate, resulting in significant 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 thickness of the stainless steel base and Ni-based alloy repairs. It was unavoidable and it was necessary to continue repairing each time. Therefore, since the cost required for maintenance increases, a corrosion-resistant material suitable for a low-pH strong oxidizing environment is demanded.

  Conventionally, austenitic stainless steels in Patent Documents 1 and 2 have been reported as corrosion-resistant materials suitable for environments where nitric acid, which is an oxidizing acid, is present. Patent Document 1 describes nitric acid corrosion-resistant austenitic stainless steel containing, by weight%, Si: 2.0 to 4.0%, Cr: 16.0 to 25%, Ni: 9.0 to 17%. In addition, by setting the ratio of Si and Cr to 0.08 to 0.25, it is said that excellent corrosion resistance can be exhibited in a wide nitric acid concentration range from low concentration to high concentration. Patent Document 2 describes nitrate-resistant austenitic stainless steel containing, by weight%, Si: 0.5% or less, Ni: 9-15%, and Cr: 16-20%, with an S content of 0.00. It is said that tunnel corrosion in a nitric acid-containing environment can be prevented by reducing the Mn content at the same time as 002% or less.

Japanese Patent Laid-Open No. 5-287460 Japanese Patent Laid-Open 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 with high corrosion resistance in a strong oxidizing environment with low pH containing chromic acid such as a chemical plant for producing chromic anhydride has not been known. Patent Documents 1 and 2 disclose the corrosion resistance in an environment containing nitric acid, but do not disclose the corrosion resistance in a strong oxidizing acidic environment where chromic acid is present.

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

  The present inventors have found that in a low pH strong oxidizing environment containing chromic acid, Mo in the austenitic stainless steel is oxidized and eluted to lower the corrosion resistance, and further, Cr contained excessively. In the same manner, the present invention was found to affect the corrosion resistance by elution, 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 mass%,
C: 0.07% or less,
Si: 1.0-4.0%,
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: including 0.20% or less,
The balance is Fe and inevitable impurities,
The following formula (1) has a composition of 20 or more,
It is an austenitic stainless steel having excellent corrosion resistance in a strong oxidizing acidic environment containing chromic acid.
5 (Si + Cu) + Ni-5Mo Formula (1)
Here, the element symbol of Formula (1) means content (mass%) of each element.

(2) In the present invention, further, by mass, W: 1.0% or less, V: 1.0% or less, Ca: 0.01% or less, REM: 0.01% or less, or B: 0.0. The austenitic stainless steel having excellent corrosion resistance in a strongly oxidizing acidic environment as described in (1), comprising 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.

  ADVANTAGE OF THE INVENTION According to this invention, the stainless steel which shows the excellent corrosion resistance in the low pH strong oxidation environment containing chromic acid can be provided. The service life of equipment, equipment, and equipment used in a strong oxidizing acidic environment can be improved, and the repair frequency can be reduced. Therefore, it contributes to a reduction in manufacturing cost.

It is a figure which shows the relationship between Formula (1) value and 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 is in 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 to 15.0%, Cu: 4.0% or less, Mo: 1.5% or less, N: 0.20 %, The balance is Fe and inevitable impurities, and the following formula (1) has a composition of 20 or more.
5 (Si + Cu) + Ni-5Mo Formula (1)
Here, the element symbol of Formula (1) means content (mass%) of each element.

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

  Furthermore, Cr is a main component that forms a passive film and maintains corrosion resistance. However, it has been found that when Cr is excessively contained, Cr tends to ionize and elute to the outside under a low pH strong oxidizing environment containing chromic acid, thereby deteriorating the corrosion resistance of the substrate. . Therefore, the stainless steel according to the present invention was adjusted to have an optimum content of Cr.

  Hereinafter, the effect of the component contained in the stainless steel according to the present invention and the reason for limiting the content will be described. Hereinafter, the mass% of the chemical composition is simply expressed as%.

  C is a strong austenite forming element and is an element effective for improving the strength, and therefore can contain 0.01% or more. On the other hand, if added excessively, Cr carbide precipitates 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, and more preferably 0.05% or less.

  Si has an action of strengthening a passive film on the surface of a steel material in a low pH strongly oxidizing environment containing chromic acid, and is an element necessary for ensuring corrosion resistance. Therefore, Si is preferably added in an amount of 1.0% or more. On the other hand, excessive Si content decreases 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, excessive addition causes a decrease in workability, so the Mn content is preferably 1.0% or less.

  Since P impairs the hot workability of the base metal and the toughness of the weld, it is desirable to reduce the content as much as possible. However, since it is difficult to remove P by refining in the production of Cr-containing steel, excessive reduction in the P content is accompanied by an excessive cost increase in the careful selection of raw materials. 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, leading to an increase in manufacturing cost. Therefore, it is preferable to reduce S content as much as possible, and 0.030% or less is preferable.

  Ni is an element necessary for maintaining the austenite structure, and has an effect of lowering the dissolution rate of steel in a low pH strong oxidizing environment containing chromic acid, and is an element necessary for ensuring corrosion resistance. It is. If the Ni content is less than 9.0%, these effects cannot be obtained sufficiently. Therefore, the lower limit of the Ni content is preferably 9.0% or more, and 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 increases, and the hot workability of the steel may be reduced. 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 passive film, and is an element that improves corrosion resistance and oxidation resistance at high temperatures. Therefore, the Cr content needs to be 17.0% or more. Therefore, the lower limit of the Cr content is preferably 17.0% or more, and more preferably 18.0% or more. On the other hand, even if Cr is contained in a large amount, the effect of improving the corrosion resistance in a low pH environment is saturated. Moreover, since it is a ferrite forming element, it is necessary to increase the Ni content in order to secure an austenite structure, resulting in high costs. Furthermore, in a strong oxidizing environment at low pH containing chromic acid, the excessively contained Cr tends to elute to the outside, leading to an increase in corrosion weight loss. Therefore, the upper limit of the Cr content is preferably 20.0% or less, and more preferably 19.0% or less.

  Cu is an element that generally improves the corrosion resistance of austenitic stainless steel, and has the effect of reducing the dissolution rate of steel in a low-pH, strong oxidizing environment containing chromic acid, similar to Ni. Cu can be added at 0.01% or more, and more preferably 0.1% or more. On the other hand, since excessive addition of Cu inhibits pitting corrosion resistance and hot workability, the Cu content is preferably 4.0% or less, and 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 substrate. Moreover, 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 generating element and is an element having an action of increasing strength. However, excessive addition of N may cause blowholes during casting, which may impair durability. Therefore, the N content is preferably 0.20% or less, and more preferably 0.07% or less.

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

  Ca and REM are elements effective for fixing S in steel, enhancing the deformability of the grain boundary of the austenite phase, and preventing the occurrence of edge cracks in the hot-rolled steel strip. You may add at 01% or less.

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

  In addition to the addition amount for each alloy element described above, the contents of Ni, Si, Cu, and Mo are arranged by the formula (1) of “5 (Si + Cu) + Ni-5Mo”. It was found that good corrosion resistance can be obtained in a specific range with respect to corrosion resistance in a natural environment. Ni, Si, Cu, and Mo in the above formula (1) mean the content (% by 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 strong oxidizability refers to an environment having a pH of 0.8 or less and containing chromic acid. That is, the low pH is an environment having a pH of 0.8 or less, and the strong oxidation property is an environment containing an oxidizing agent typified by chromic acid. The austenitic stainless steel according to the present invention is suitable for a constituent material that requires high corrosion resistance in a chemical plant for producing chromic anhydride.

  Examples of the present invention will be described below. 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 sheets having a thickness of 3.0 mm were produced by hot rolling. The hot-rolled sheet was annealed at 1050 ° C. to 1175 ° C. for 3 minutes, and then the oxide film on the surface was removed using dry honing. Thereafter, 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, and then pickled with an aqueous solution containing hydrofluoric acid and nitric acid to austenite with a thickness of 1 mm. A stainless steel plate was prepared and used as a test material. Next, after a 30 mm × 30 mm test piece was cut out from the specimen, it was subjected to # 600 wet polishing finish 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 inevitable impurities.

(Corrosion resistance test)
The test for evaluating the corrosion resistance in a low pH strong 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 was embedded in the test solution. The container was sealed and heated at 200 ° C. for 7.5 hours in a thermostatic bath. After the heating, the test piece was taken out from the test solution and immersed in a 30% nitric acid solution at 60 ° C. to remove the 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 (×). The measurement results are shown in Table 1. Moreover, the relationship between the numerical value of Formula (1) and corrosion weight loss is shown in FIG.

  As shown in Table 1 and FIG. 1, Examples 1 to 6 included in the composition range of the present invention had little corrosion weight loss and good corrosion resistance in a strong oxidizing acidic environment containing chromic acid. On the other hand, in Comparative Examples 1 to 9, the value of the formula (1) is less than 20, and the content of each alloy element is also outside the scope of the present invention. Corrosion resistance in a strongly oxidizing acidic environment was reduced.

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

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

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

Claims (3)

% By mass
C: 0.07% or less,
Si: 1.0-4.0%,
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: including 0.20% or less,
The balance is Fe and inevitable impurities,
The following formula (1) has a composition of 20 or more,
An austenitic stainless steel with excellent corrosion resistance in a strong oxidizing acidic environment containing chromic acid.
5 (Si + Cu) + Ni-5Mo Formula (1)
Here, the element symbol of Formula (1) means content (mass%) of each element.   Further, in mass%, W: 1.0% or less, V: 1.0% or less, Ca: 0.01% or less, REM: 0.01% or less, or B: 0.01% or less An austenitic stainless steel having excellent corrosion resistance in a strongly oxidizing acidic environment containing chromic acid according to claim 1, comprising at least one kind.   The austenitic stainless steel having excellent corrosion resistance in a strong oxidizing acidic environment containing chromic acid according to claim 1 or 2, wherein the strongly oxidizing acidic environment has a pH of 0.8 or less.

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