Austenitic stainless steel and surface treatment method of the same

Abstract

To provide a heating method of stainless steel in which a metal elution amount to a chemical or the like is extremely small.SOLUTION: Provided is an austenitic stainless steel, where the ration of iron atomic number on a surface is chromium atomic number or less, and the depth of CrO2 is 2.5nm or more stainless steel. In the stainless steel, an electrolytically polished stainless steel material is heated within the range of 200 to 350°C for 1 to 2 hours in the oxidative atmosphere, and then, immersed in 1 to 4 wt.% of HCl for a predetermined time to remove an iron oxide film on the surface, by which stainless steel having an extremely small metal elution amount can be obtained. By heating stainless steel from which the oxide film has been removed furthermore at 100 to 200°C for a predetermined time, stainless steel with further excellent performance can be obtained.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to austenitic stainless steel, in particular stainless steel that is resistant to corrosion by chemicals and the like and resistant to the leaching of metal ions, and to a processing method for obtaining such stainless steel. [Background technology]

[0002] Austenitic stainless steels (eg, SUS304, SUS316) are commonly used for piping, vessels, and equipment used in processes where purity of the processed materials is a critical requirement.

[0003] Such steels are also used in processes where strong solvents or other corrosive substances are present, where the presence of very pure materials, strong solvents, or corrosive substances makes it difficult to prevent corrosion of the steel and / or inhibit the dissolution of contaminants, such as various metal ions, from the steel, especially at high temperatures.

[0004] For example, isopropyl alcohol (IPA), which is used in the cleaning process of semiconductors, is currently required to have a metal ion deposition concentration of at most ppb units.

[0005] The amount of metal ions eluted into the above-mentioned chemicals, foods, and beverages depends on the resistance of the oxide film layer formed by the surface treatment of the stainless steel to the chemicals, foods, and beverages.

[0006] Japanese Patent Laid-Open Publication No. 64-31956 discloses that stainless steel material is heated in an oxygen atmosphere at 280 to 500°C for a predetermined period of time, which reduces the amount of metal eluted in pure water to a maximum of about one-quarter of that of a sample that has only been electropolished (Patent Document 1, Table 1, Sample 4).

[0007] Furthermore, Japanese Patent Laid-Open Publication No. 05-287496 discloses that stainless steel material is heated in an ozone atmosphere at temperatures ranging from 150 to 300°C for a predetermined period of time, which reduces the total amount of metal elution in pure water to up to one-fifth of that of a sample that has only been electropolished (Patent Document 2, Table 1, Samples 8 and 9).

[0008] Japanese Patent No. 6714159 discloses that stainless steel (stainless steel 304L) with a reduced proportion of chromium on the surface is heated at a predetermined temperature for a predetermined time after being electrolytically polished and washed with inorganic nitric acid.

[0009] The applicant of the present application has proposed in Japanese Patent Application Laid-Open No. 2024-012865 that electrolytically polished stainless steel be heated at 200 to 250° C. for 1 to 3 hours. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Japanese Patent Application Publication No. 64-31956 [Patent Document 2] Japanese Patent Application Publication No. 05-287496 [Patent Document 3] Patent No. 6714159 [Patent Document 4] Japanese Patent Application Publication No. 2024-012865 Summary of the Invention [Problem to be solved by the invention]

[0011] With 5G already in practical use in mobile communications and 6G now in the testing phase, the quality of the semiconductors used in these communications systems must also be adjusted to match the communication capacity of each generation. Therefore, the quality of the containers used to hold IPA used in the semiconductor manufacturing process must also be reviewed, and it is desirable to limit the amount of metal leaching into IPA to ppb units, and more preferably, to ppt units.

[0012] The contents disclosed in the above-mentioned prior patent documents 1 and 2 were developed and filed in a generation that did not have the above-mentioned strict requirements, and the following problems occur when IPA, a semiconductor cleaning solution, is added.

[0013] To reduce the amount of metal leaching from stainless steel, it is necessary to increase the thickness of the oxide film on its surface, but to ensure the thickness of this oxide film, the stainless steel must be heated to a specified temperature for a specified period of time.

[0014] However, as will be explained in detail later, when stainless steel is heated to 400°C or above, tiny white spots appear within the grains or at the grain boundaries, and corrosion progresses from these spots, naturally resulting in a large amount of metal leaching.

[0015] Therefore, with the above-mentioned prior art, it was not possible to suppress the amount of metal elution to the ppt level.

[0016] The technique disclosed in the aforementioned Patent Document 1 requires an oxygen atmosphere with a specific concentration, and equipment for supplying oxygen is required. Also, the total amount of metal elution is dissatisfied, being only about 1 / 4 of that of a sample that is only electropolished.

[0017] The technique disclosed in the aforementioned Patent Document 2 requires an ozone atmosphere with a specific concentration, but improper handling of ozone can be harmful to the health of workers. Furthermore, the total amount of metal elution is dissatisfied, being only about one-fifth of that of samples that are electropolished only.

[0018] The technology disclosed in the aforementioned Patent Document 3 involves a nitric acid treatment prior to heat treatment to reduce the proportion of chromium on the surface. However, the corrosion resistance of stainless steel is inherently achieved by chromium oxide on the surface, and even if a reduction in the amount of metal dissolved in a non-electrolyte such as isopropyl is achieved, the resistance to electrolytes is unclear because the pitting potential is not disclosed. In reality, Sample 11 of the present application (Table 2) has a low amount of metal dissolved in pure water, but its pitting potential is low and it is thought that its resistance to electrolytes is not high.

[0019] Prior art document 4 is an application filed by the applicant of the present application. The disclosure of heating electropolished stainless steel at 200-250°C for 1-3 hours has been successful in suppressing the amount of metal elution to the order of ppb (see Table 3), but the results are still insufficient to meet the demand for ppt-order elution.

[0020] The present invention has been proposed in view of the above-mentioned conventional circumstances, and aims to provide a method for surface treatment of stainless steel which minimizes the amount of metal eluted in chemicals and food and also provides excellent corrosion resistance to electrolytes. [Means for solving the problem]

[0021] The present invention is an austenitic stainless steel in which the ratio of iron atoms on the surface to the number of chromium atoms is equal to or less than the number of chromium atoms, and the depth of CrO2 (passive film) is 2.5 nm or more.

[0022] The above stainless steel can be obtained by heating electrolytically polished stainless steel material in an oxidizing atmosphere at a temperature in the range of 200 to 350°C for 1 to 2 hours, and then immersing it in 1 to 4 wt% HCl for a predetermined time to remove the iron oxide film on the surface, thereby obtaining stainless steel with an extremely low amount of metal elution.

[0023] By further heating the stainless steel from which the oxide film has been removed at 100 to 200°C for a predetermined period of time, stainless steel with even better performance can be obtained. [Effects of the Invention]

[0024] White spots, which are the nuclei of corrosion caused by chemicals and food, do not form, or even if they do form, they are extremely few in number, so the amount of metal elution is significantly reduced, and a high pitting potential can be achieved. [Brief explanation of the drawings]

[0025] [Figure 1] SEM photograph of sample 3. [Figure 2]SEM photograph of sample 4. [Figure 3] SEM photograph of sample 6. [Figure 4] SEM photograph of sample 7. [Figure 5] SEM photograph of sample 9. [Figure 6] SEM photograph of sample 10. [Figure 7] SEM photograph of sample 12. [Figure 8] SEM photograph of sample 13. [Figure 9] XPS data for sample 3. [Figure 10] XPS data for sample 4. [Figure 11] XPS data for sample 6. [Figure 12] XPS data for sample 9. [Figure 13] Pitting potential of electropolished, primary heat treated specimens. [Figure 14] Pitting potential of electropolished, primary heat treated, and HCl treated specimens. [Figure 15] Pitting potential of electropolished, primary heat-treated, HCl-treated, and secondary heat-treated specimens. DETAILED DESCRIPTION OF THE INVENTION

[0026] First, in the following explanation, the stainless steel in question is SUS316L.

[0027] After buffing and electrolytic polishing, the samples were heated for one hour at 250°C, 350°C, and 450°C (primary heat treatment). The samples were then immersed in 2.5 wt% HCl for a predetermined time, and then heated for one hour at 150°C (secondary heat treatment). Furthermore, samples were also prepared by primary heat treatment at 250°C for two hours, followed by immersion in 2.5 wt% HCl as above, and then secondary heat treatment at 150°C for one hour.

[0028] In the following description, each of Samples 1 to 13 will be as follows:

[0029] Sample 1: Electrolytically polished sample (primary heat treatment: 1 hour) Sample 2: Electrolytic polishing + primary heat treatment at 250°C Sample 3: Electrolytic polishing + primary heat treatment at 250°C + immersion in HCl Sample 4: Electrolytic polishing + primary heat treatment at 250°C + immersion in HCl + secondary heat treatment at 150°C Sample 5: Electrolytic polishing + primary heat treatment at 350°C Sample 6: Electrolytic polishing + primary heat treatment 350°C + immersion in HCl Sample 7: Electrolytic polishing + primary heat treatment at 350°C + immersion in HCl + secondary heat treatment at 150°C Sample 8: Electrolytic polishing + primary heat treatment at 450°C Sample 9: Electrolytic polishing + primary heat treatment at 450°C + immersion in HCl Sample 10: Electrolytic polishing + primary heat treatment at 450°C + immersion in HCl + secondary heat treatment at 150°C (First heat treatment: 2 hours) Sample 11: Electrolytic polishing + primary heat treatment at 250°C Sample 12: Electrolytic polishing + primary heat treatment 250°C + immersion in HCl Sample 13: Electrolytic polishing + primary heat treatment at 250°C + immersion in HCl + secondary heat treatment at 150°C In the primary heat treatment, the temperature was increased at a rate of 10°C / min until the predetermined temperature (250°C, 350°C, 450°C) was reached. The secondary heat treatment at 150°C after the HCl treatment lasted for 1 hour, and the temperature was increased at a rate of 10°C / min.

[0030] The HCl treatment is performed to remove surface iron oxides. However, because the thickness of the oxide film formed varies depending on the primary heat treatment temperature (250°C, 350°C, or 450°C), the immersion time in HCl is determined as the time until the surface iron oxide disappears. Samples 3 and 4 were immersed for 5 minutes, samples 6 and 7 for 30 minutes, and samples 9 and 10 for 40 minutes. Samples 12 and 13 were immersed for 5 minutes.

[0031] The electrolytic polishing used here was an immersion electrolytic polishing method (not the wipe electrolytic polishing used in Patent Application No. 2022-114635).

[0032] <SEM Observation> Figures 1, 3, and 5 are 1000 - fold operational electron microscope (SEM) photographs of Samples 3, 6, and 9, which are stainless steel samples subjected to electrolytic polishing, heated at 250°C for 1 hour, 350°C for 1 hour, and 450°C for 1 hour (primary heat treatment) respectively, and then immersed in 2.5 wt% HCl to remove the iron oxide film on the surface.

[0033] Figures 2, 4, and 6 are 1000 - fold operational electron microscope (SEM) photographs of Samples 4, 7, and 10, which are stainless steel samples subjected to electrolytic polishing, heated at 250°C for 1 hour, 350°C for 1 hour, and 450°C for 1 hour (primary heat treatment) respectively, then immersed in 2.5 wt% HCl to remove the iron oxide film on the surface, and further subjected to a secondary heat treatment at 150°C for 1 hour.

[0034] In Sample 9, which was subjected to primary heat treatment at 450°C (Figure 5), a number of rather large spots appear in the metal structure, and it can be seen that there are fine cracks in the structure. This is the same for Sample 10 (Figure 6), which is Sample ⑨ further subjected to secondary heat treatment. In Sample 3, which was treated with HCl after heating at 250°C (Figure 1), no spots appear and there are no cracks. The same is true for Sample 4, which was further subjected to secondary heat treatment at 150°C after the HCl treatment. In Sample 6, which was treated with HCl after primary treatment at 350°C (Figure 3), a few spots appear, but in Sample 7, which was subjected to secondary heat treatment at 150°C after the HCl treatment, the size of the said spots is slightly alleviated.

[0035] <XPS Observation> As a result of immersing in 2.5 wt% HCl for a predetermined time after primary heat treatment to remove the iron oxide film as described above, as shown in the XPS diagrams of Figures 9 - 12, CrO₂ appears on the surface. As a result, almost no iron oxide is seen on the surface of each sample (Figures 9, 11, 12), or it is about 60% of CrO₂ (Figure 10).

[0036] Also, the thickness of CrO₂ (passive film thickness) can be read from the graph.

[0037] That is, in sample 3 shown in FIG. 9, which was subjected to a primary heat treatment at 250°C and then to an HCl treatment, the thickness was 2.8 nm; in sample 4 shown in FIG. 10, which was subjected to an additional heat treatment at 150°C for one hour after the HCl treatment, the thickness was 3.5 nm; in sample 9 shown in FIG. 11, which was subjected to a primary heat treatment at 350°C and then to an HCl treatment, the thickness was 4.4 nm; and in sample 9 shown in FIG. 12, which was subjected to a primary heat treatment at 450°C and then to an HCl treatment, the thickness was 6.7 nm.

[0038] Considering only the thickness of the CrO2 oxide film, heating at 450°C is superior to heating at 250°C, but a comprehensive judgment is required taking into account the surface condition as seen in the SEM photographs above, as well as the pitting potential and amount of dissolved metal, which will be described later.

[0039] <Amount of metal elution> Table 1 shows the amount of metal dissolved from each of the above samples (samples 2 to 10) when the primary heat treatment was performed for one hour and when they were immersed in ultrapure water for one week, along with the pitting potential (sample 1 was not subjected to primary heat treatment). Table 2 shows the amount of metal dissolved from each of the above samples (samples 11 to 13) when they were immersed in ultrapure water for one week, along with the pitting potential, when they were subjected to primary heat treatment at 250°C for two hours.

[0040] [Table 1]

[0041] [Table 2]

[0042] At pitting potentials of 850 mV or higher in a 0.15 M HCl / L solution, samples 3, 4, 6, 7, 12, and 13 showed high performance, indicating that removing iron oxide with HCl after heating for a specified time in the primary heat treatment and enriching the surface with CrO2 effectively suppresses metal elution. This is evident when comparing sample 2 (not treated with HCl) with sample 3. Furthermore, comparing samples 3 and 4, and samples 12 and 13, it is clear that further heating at 150°C (secondary heat treatment) after HCl treatment further enhances the effect.

[0043] Judging from Tables 1 and 2, along with the SEM photographs, it is clear that the appropriate heating temperature for the primary heat treatment is 250°C to 350°C, and that temperatures higher than this reduce the effect. Furthermore, the heating time for the primary heat treatment should be no more than two hours at 250°C, and no more than one hour at 350°C.

[0044] <Pitting potential> Figures 13, 14, and 15 are graphs showing the actual measurements of the pitting potential of each sample in a 0.15M HCl / L solution. The pitting potentials of samples not covered here are shown in Tables 1 and 2, along with the amount of metal elution. The bath temperature was 30°C, and the sweep rate was 20mV / min.

[0045] FIG. 13 shows the pitting potentials of Samples 1, 2, 5, and 8, which were subjected to primary heat treatment but not HCl treatment. FIG. 14 shows the pitting potentials of Samples 3, 6, and 9, which were subjected to primary heat treatment and then HCl treatment. Furthermore, FIG. 15 shows the pitting potentials of Samples 4, 7, and 10, which were subjected to primary heat treatment, then HCl treatment, and then secondary heat treatment.

[0046] When comparing samples that were not treated with HCl after the primary heat treatment with samples that were treated with HCl (samples 2 and 3, samples 5 and 6, samples 8 and 9, and samples 11 and 12), the samples that were treated with HCl after heating in the primary heat treatment had a higher pitting potential, which shows the effectiveness of the HCl treatment after the primary heat treatment.

[0047] 14, the pitting potential of sample 9 heated at 450°C is significantly lower than that of sample 3 heated at 250°C as the primary heat treatment. On the other hand, when the primary heat treatment is for one hour and the samples are heated at 250°C and 350°C before being subjected to HCl treatment (samples 3 and 6), the results are almost the same.

[0048] Figure 15 shows the pitting potentials of samples 4, 7, and 10, which underwent primary heat treatment, HCl treatment, and secondary heat treatment. Samples 4 and 7, which underwent primary heat treatment at 250°C and 350°C, showed good results, but sample 10, which underwent primary heat treatment at 450°C, showed significantly inferior performance to sample 1.

[0049] Even if the primary heat treatment is at 250°C, if the HCl treatment is performed after heating for 2 hours, the pitting potential will drop slightly (Table 2).

[0050] Considering the above, it is believed that the temperature for the primary heat treatment should be in the range of 250°C to 350°C. Furthermore, the appropriate heating time is up to 2 hours at 250°C, and 1 hour or less at 350°C. From another perspective, the range of the present invention is one in which the total amount of metal elution after immersion in pure water for 2 weeks is 0.45 ppb or less and the pitting potential in a 0.15 mol HCl / L solution is 850 mV or more.

[0051] As explained above, by heating stainless steel at 250-350°C and then removing the iron oxide on the surface with HCl, or by further heating at 100-150°C, the amount of metal elution is suppressed and resistance to electrolytic and non-electrolytic chemicals is significantly improved.

[0052] Furthermore, following the example of Patent Documents 1 and 2, when the amount of elution from Sample 3 treated according to the present invention (which had the largest amount of elution among the samples of the present invention) is compared to the amount of metal elution from the sample that was only electropolished (Sample 1), it is found to be slightly less than one-sixth, which is less than that of Patent Documents 1 and 2. Patent Document 3 gives the amount of elution in IPA, which cannot be compared with the amount of elution in pure water of the present invention, and unless the pitting potential is shown, corrosion resistance to an electrolyte cannot be discussed.

Claims

1. It is an austenitic stainless steel, and the ratio of the number of iron atoms on the surface is equal to or less than the number of chromium atoms, and CrO ２ The austenitic stainless steel has a pitting depth of 2.5 nm or more and a pitting potential of 850 mV or more in a 0.15M HCl / L solution.

2. A surface treatment method for obtaining the stainless steel of claim 1, Electrolytic polishing of austenitic stainless steel; heating in an oxidizing atmosphere at a temperature in the range of 200 to 350°C for 1 to 3 hours; a step of immersing the heat-treated stainless steel in 1 to 4 wt % HCl for a predetermined time to remove the iron oxide film on the surface; The surface treatment method for austenitic stainless steel comprises:

3. 3. The method for surface treatment of austenitic stainless steel according to claim 2, wherein the stainless steel from which the oxide film has been removed is further heated at 100 to 200° C. for 0.5 to 1.5 hours.

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