JP3294128B2 - How to make stainless steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing stainless steel. More specifically, by removing the metal oxide film formed on the surface of stainless steel, for example, semiconductors, containers for ultrapure water and the like used in the field of nuclear power, etc., suitable as a material for pipes, parts and the like The present invention relates to a method for producing stainless steel capable of providing a stainless steel having a highly corrosion-resistant surface.

[0002]

2. Description of the Related Art Stainless steel having a highly corrosion-resistant surface is widely used in the fields of foods, pharmaceuticals, precision instruments, and the like, and various types are provided according to the uses. In fields such as semiconductors and nuclear applications, particularly strict treatment and cleaning are required, so that stainless steel having a highly corrosion-resistant surface, in which elution and release of iron ions are highly prevented, has been developed. It is used as a material for containers, pipes, parts, etc. for water and the like. As stainless steel suitable for such a material, for example,
Stainless steel manufactured by the method disclosed in US Pat. Such a manufacturing method essentially goes through the following steps.

(1) Surface cleaning treatment (mechanical polishing, electrolytic polishing, etc.) (2) Formation of oxide film by oxidation treatment, and (3) Dissolution removal of oxide film By the way, stainless steel for ultrapure water or the like as described above is used. When applying such a method for manufacturing, each step must be performed with particular care so as not to adhere fine dust and the like. Therefore, the production of stainless steel used for such a purpose has been conventionally performed in a clean air environment such as a clean booth or a clean room. However, when volatile reagents are used in the manufacturing process in these relatively closed spaces, the volatile reagents fill up in a short period of time, and depending on the type of reagent, adversely affect the worker's body, Contamination of equipment and facilities in the space,
Inevitable problems such as corrosion. 2-1916
In the method of No. 3, an aqueous hydrochloric acid solution is used for dissolving and removing the oxide film in the third step, and there is a concern that the influence of the diffusion of the volatile mist may be caused.

[0004]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a method of manufacturing a stainless steel having excellent corrosion resistance, which can remove a desired degree of an oxide film without using a volatile chemical. And

[0005]

Means for Solving the Problems In view of the above prior art, the present inventors have made intensive studies to solve this problem. As a result, in the method for manufacturing stainless steel, after cleaning the surface of stainless steel, The metal oxide film formed by heat treatment in an oxidizing atmosphere is removed by an etching operation using a sulfuric acid aqueous solution, and the surface layer is substantially changed to a chromium-enriched layer that is a corrosion-resistant passive film. By doing so, they found that a method suitable for operation in an essentially closed space was provided, and completed the present invention.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS As the stainless steel to which the method of the present invention is applied, various stainless steels can be used. For example, SUS316, SUS316L, SU
S304 or SUS304L is preferably used.

[0007] The cleaning treatment is a step carried out with the intention of removing the affected layer of the stainless steel outer layer. As this cleaning step, buff polishing, electrolytic polishing, and the like are possible, and these may be combined. The electropolishing method is disclosed in Japanese Patent Application Laid-Open No. Hei 7-26613 by the present applicant.
No. 4 according to the method disclosed.

The formation of the metal oxide film after the cleaning treatment is as follows.
The heat treatment is performed in an oxidizing atmosphere. This heat treatment is preferably performed in the range of 300 to 500 ° C.
It may be carried out for 1 to 3 hours, preferably for about 2 hours. If the treatment temperature is low, the possibility of rusting is increased due to insufficient formation of an oxide film, and if the treatment temperature is high, the oxide film may become excessively thick and weak at the same time. On the other hand, if the heating time is too short, the formation of an oxide film will also be insufficient, and if the treatment is performed for a long time, the oxide film may become too thick.

By removing the oxide film thus formed as described below, the surface layer of stainless steel is substantially changed to a chromium concentration layer which is a corrosion-resistant passive film. That is, etching is performed by immersing stainless steel coated with a metal oxide film in a sulfuric acid aqueous solution.

The aqueous sulfuric acid solution is preferably a 3 to 7% by weight aqueous solution, particularly preferably 5% by weight. If the concentration of sulfuric acid is too low, it takes a long time to sufficiently dissolve and remove the film, and if high-concentration sulfuric acid is used, the surface after treatment may be rapidly removed due to rapid dissolution and removal. It becomes rough and uneven, and it is difficult to obtain a passivated film having a smooth chromium concentration layer. Further, the etching operation is preferably performed at a temperature in the range of 40 to 90 ° C., and most preferably at 60 ° C. in view of the energy required for raising and maintaining the temperature and the etching efficiency. Therefore, the etching is 5
It is the most preferred embodiment of the present invention that the reaction is performed at 60 ° C. using a weight percent aqueous sulfuric acid solution.

[0011] The etching operation depends on the sulfuric acid concentration and the processing temperature, the time required for the dissolution and removal to be achieved as required by using the fade of the iron oxide film color (gold color according to the above method) as an index, What is necessary is just to implement. For example, in a preferred embodiment of the present invention, when SUS316L stainless steel is treated, sufficient etching can be performed in about 3 hours, and a smooth chromium concentration layer can be obtained.

Hereinafter, the method for producing stainless steel of the present invention will be further described with reference to examples, but the present invention is not limited to these. Below, the% notation is
Unless otherwise specified, it represents% by weight.

[0013]

【Example】

[1] Sample preparation SUS having a thickness of 2 mm, a width of 20 mm, and a length of 20 mm
The 316L stainless steel was processed according to the following steps to obtain samples etched under various conditions.

Mechanical polishing: Using a # 600 buff,
The stainless steel surface was polished.

Electropolishing: In a phosphoric acid / sulfuric acid-based electrolytic solution,
A current of 1.5 A was passed, and electropolishing was performed at 38 ° C. for 3 minutes.

Cleaning: Ultrasonic cleaning was performed in ultrapure water at 50 ° C.

Primary pickling: Primary pickling was carried out at room temperature for 3 hours with a dilute sulfuric acid aqueous solution (0.1%) obtained by diluting concentrated sulfuric acid 1000 times.

Washing and drying: After washing with ultrapure water, drying was performed under a nitrogen stream.

Oxidation treatment: Heat treatment was performed at 425 ° C. in the air for 2 hours to form a metal oxide film.

Etching: Samples were immersed in aqueous sulfuric acid (5, 7.5 or 10% by weight) or aqueous nitric acid (5% by weight) at 25, 40 or 60 ° C. for 24 hours.

Washing and drying: The same steps as described above were performed.

[2] Evaluation on etching a. Visual gold color by the oxide film formed in the above process,
In the step, the color changes with time, and the time for decolorization was visually observed. The results obtained are shown in Table 1 below.

[0023]

[Table 1]

As is clear from Table 1, the gold color of the sample surface was decolorized within 24 hours in a sulfuric acid aqueous solution at 40 ° C. and 60 ° C., suggesting that the oxide film was dissolved and removed. On the other hand, in a 25 ° C. aqueous solution of sulfuric acid and an aqueous solution of nitric acid, no decolorization was observed even after 24 hours.

B. Iron ion concentration analysis Fe ²⁺ eluted in the etching solution in the above process
Ion concentrations were analyzed over time by phenatroline spectrophotometry as follows. The analysis was performed as described in a. The visual evaluation indicated that removal of the oxide film was suggested.
40 ° C or 60 ° C using 5 or 10% aqueous sulfuric acid
The etching was performed on the etching solution sample collected at predetermined time intervals from the etching solution subjected to the process in step.

Add 30 ml of ultrapure water to 30 ml of the sample solution and
0 ml, containing 1 ml of hydroxylammonium chloride solution (without iron, 10% by weight / volume), 5 ml of 1,10-phenanthroline solution (0.1% by weight / volume), and ammonium acetate solution (without iron, 50% by weight / volume). (% By volume) and 14 ml of ultrapure water were added to adjust the total to 100 ml. The color was allowed to develop for 20 minutes at room temperature, and then the absorbance was measured at 510 nm.

To prepare a calibration curve for the Fe ²⁺ concentration, F
e1000 (Cica-MARCK, iron standard solution for chemical test, 1000
mg Fe ²⁺ / L) diluted 1000 times and 100 times,
Iron standard solutions of 1 mg Fe ²⁺ / L and 10 mg Fe ²⁺ / L were prepared. This standard solution was added stepwise to the etching solution before use, and the same operation as above was performed on a standard sample solution of 30 ml to prepare a calibration curve of absorbance with respect to the concentration of Fe ²⁺ . Using the obtained calibration curve, the free concentration of Fe ²⁺ ions in the etching solution was quantified. F to the solution when etching is performed under each condition
The change over time of the e ²⁺ iron ion release amount is shown in FIGS.

FIGS. 1-3 show 5, 7.5 or 1 respectively.
It shows the results when the process was performed at 40 ° C. in a 0% aqueous sulfuric acid solution. As is clear from FIG.
When etching was performed at 40 ° C. in% sulfuric acid, it was found that the iron concentration in the etching solution continued to increase even after 16 hours, and that iron continued to elute very slowly. FIGS. 2 and 3 show the results of etching at 40 ° C. in 7.5 and 10% sulfuric acid aqueous solution, respectively, where the iron concentration in the etching solution increased up to about 8 hours and 4 hours, respectively. After that, it has been almost constant. Therefore, it is suggested that the elution rate of iron increases as the sulfuric acid concentration increases.

On the other hand, when the etching is performed at 60 ° C., even in a 5% sulfuric acid aqueous solution, the iron concentration reaches a constant value within about 3 hours, and the iron concentration becomes much faster than at 40 ° C. It is clear from FIG. 4 that the elution is completed.

These results and a. Comparing the visual evaluation results of the above, when using sulfuric acid aqueous solution to determine the time to complete the etching under each condition,
It is as follows.

5%, 40 ° C .: 16 hours 7.5%, 40 ° C .: 16 hours 10%, 40 ° C .: 8 hours 5%, 60 ° C .: 3 hours c. Observation of stainless steel sample surface The surface condition of each etched stainless steel sample was measured with a metal microscope (OPTIPHOTO, manufactured by Nippon Kogaku Co., Ltd.).
And compared with the state before etching.

As a result, it was suggested that the surface of the stainless steel sample after etching in 7.5 and 10% sulfuric acid aqueous solution was rough, and it was difficult to obtain a uniform passivation film. On the other hand, when a 5% sulfuric acid aqueous solution is used, even after 24 hours, 40%
No surface roughness was observed at both 60 ° C and 60 ° C,
Therefore, it was considered that a 5% sulfuric acid aqueous solution was suitable as the etching solution.

D. Elemental analysis in the depth direction of a stainless steel sample The elemental composition in the depth direction of the stainless steel before and after etching was analyzed by Auger electron spectroscopy (AES).
The etched stainless steel samples used were etched in a 10% aqueous sulfuric acid solution at 40 ° C. for 8 hours and 16 hours.

In AES, a stainless steel sample (10
× 10 × 2 mm) was subjected to argon (Ar) etching,
By using an MICROLAB310-D scanning Auger electron spectrometer manufactured by VG Scientific as an accelerating voltage of 10 keV and a sample current of 10 nA, the energy value accompanying the Auger electron emission was measured.
The ratio of the number of atoms of each element of O, Fe, Cr, Ni and C was determined every second. Ar etching has an acceleration voltage of 3 ke
V, the sample current was 400 nA, and etching was performed with Ar ⁺ ions in an etching area of about 1.5 mm ^2. At this time, the etching rate of the standard sample was about 45 ° / min (Fe). The obtained results are shown in FIGS. The sensitivity coefficient used when converting the intensity of each element into an atomic percentage is C
: 0.40, O: 0.15, Cr: 0.22, F
e: 0.18 and Ni: 0.29.

From FIG. 5 showing the elemental analysis in the depth direction of the stainless steel before etching, the oxide film (the presence of the oxide film is suggested by O atoms) obtained in the above step [1] is mainly Fe It is clear that the compound contains Cr is confirmed from around 200 seconds of Ar etching time, and Ni increases as going deeper. These observations indicate that the outermost surface of the oxide film is mainly composed of an iron oxide layer, and that a chromium-enriched layer exists below the outermost surface.

Next, in FIG. 6 showing a stainless steel sample after etching in a 10% aqueous sulfuric acid solution at 40 ° C. for 8 hours, in FIG.
The presence of the metal elements of r, Fe and Ni is recognized, indicating that the iron oxide layer existing before the etching is removed and the lower chromium concentration layer is exposed. Also, N
i is concentrated near the boundary between the oxide film layer and the bulk layer.

As shown in FIG. 7, in a 10% aqueous sulfuric acid solution at 40 ° C.
The result when etching was performed for 16 hours under the conditions described above was essentially the same as that in FIG. 6, and it was shown that the chromium-enriched layer was retained even when the etching time was further extended.

From the above results, it was suggested that when the iron oxide film coated on the stainless steel after the oxidation treatment was removed by acid etching using sulfuric acid, a chromium-enriched layer was provided as the outermost layer. It was also found that the chromium-enriched layer was not removed even after the etching was continued after the chromium-enriched layer was exposed.

E. Measurement of oxide film thickness d. The thickness of the oxide film was measured by ellipsometry after 2, 8 and 16 hours for a stainless steel sample etched in a 10% sulfuric acid aqueous solution at 40 ° C. in the same manner as described above, and compared with the thickness before etching. The ellipsometry used a DVA-316 type automatic ellipsometer manufactured by Yokomizo Optical Co., Ltd., and the polarization angle was measured by a rotation analyzer method. The wavelength of the incident light is 6328 ° (by He-Ne laser), the incident angle is set to 70 °, the polarization angle is set to 45 °, and 1 mm
Measurements were taken at the corner area.

As a reference for relative comparison, d. The oxide film thickness predicted from the O profile of the element distribution in the depth direction obtained in and the Ar etching rate was obtained. As the Ar etching rate,
The value of [[etching rate of standard sample (［/ min) × etching time (min)]] was adopted as the predicted value of the oxide film thickness.

The results obtained are shown in Table 2 below.

[0042]

[Table 2]

As is apparent from Table 2, the oxide film thickness generally corresponds to the progress of etching indicated by the amount of Fe ²⁺ eluted, and the oxide film thickness decreases as the etching progresses. The oxide film thickness after the etching was estimated to be about 65 °.

F. Overall evaluation a. To e. Table 3 below summarizes the evaluations of stainless steel obtained under each etching condition based on the results obtained in Table 1.

[0045]

[Table 3]

As described above, the following conclusions are derived from the results of the examples.

(1) The etching conditions of the stainless steel oxide film of the present invention are as follows:
Three hours is optimal.

(2) A stainless steel having a chromium-rich surface is obtained by etching, and its oxide film thickness is about 65
Å.

(3) The progress of etching represented by the amount of Fe ²⁺ eluted into the etching solution and the oxide film thickness have a corresponding relationship.

[0050]

According to the present invention, in a method for producing a stainless steel having excellent corrosion resistance, a desired degree of oxide film can be removed without using a volatile chemical solution, and a stainless steel having a chromium-rich surface is obtained. A method can be provided.
Accordingly, for example, stainless steel used for containers such as ultrapure water is manufactured in a substantially closed space such as a clean room in an environment in which adhesion of dust and the like is minimized. There is an effect that the operation can be performed.

[Brief description of the drawings]

FIG. 1 is a graph showing a change over time in the amount of iron released into an etching solution when stainless steel after forming an oxide film is etched in a 5% by weight aqueous sulfuric acid solution at 40 ° C.

FIG. 2 shows a case where a 7.5% by weight aqueous sulfuric acid solution is used.
It is a graph similar to.

FIG. 3 is a graph similar to FIG. 1 when a 10% by weight aqueous sulfuric acid solution is used.

FIG. 4 is a graph similar to FIG. 1 when etched at 60 ° C. in a 5% by weight sulfuric acid aqueous solution.

FIG. 5 is a graph showing the elemental composition in the depth direction of stainless steel after forming an oxide film.

FIG. 6 is a graph similar to FIG. 5 of stainless steel after being etched at 40 ° C. for 8 hours in a 10% by weight aqueous sulfuric acid solution.

FIG. 7 is a graph similar to FIG. 5 of stainless steel after being etched in a 10% by weight aqueous sulfuric acid solution at 40 ° C. for 16 hours.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Mitsuru Shinoda 3-35-30 Kojidai, Nishi-ku, Kobe-shi, Hyogo (56) References JP-A-62-17184 (JP, A) JP-A-8-92724 (JP) , A)

Claims (3)

(57) [Claims] 1. A method for producing stainless steel, comprising: cleaning a stainless steel surface, and then performing a heat treatment in an oxidizing atmosphere to form a metal oxide film having a content of 3 to 7% by weight. Manufacturing of stainless steel, wherein the surface layer is substantially changed to a chromium-enriched layer, which is a corrosion-resistant passive film, by being removed by performing an etching operation in a range of 40 to 90 ° C. using a sulfuric acid aqueous solution. Method. Wherein said etching, using 5 wt% aqueous sulfuric acid method according to claim 1, wherein the stainless steel carried out at 60 ° C.. 3. The method according to claim 1, wherein the stainless steel is SUS316L stainless steel.