Steel etching method and method for preparing specimens for optical microscope observation

Abstract

Provided are a steel etching method that enables a microstructure to be analyzed conveniently and with good precision, a method for preparing a sample for optical microscope observation, a steel etching solution set, and an etching device.　Provided is a steel etching method that includes a first etching step for performing electrolytic etching of steel in a basic solution having a pH greater than 7.0, and a second etching step for performing etching in which the steel is brought into contact with an acidic solution having a pH less than 7.0 after the first etching step. The pH of the basic solution may be 13.0 or higher, and the basic solution may be a KOH aqueous solution. The electric current density during the electrolytic etching can be set to 0.5 A / cm2 or greater. The steel may have martensite and ferrite.

Description

[Technical Field]

[0001] The present invention relates to an etching method that enables accurate identification of the fraction of each phase that affects material properties from image analysis of optical microscope photographs when observing the microstructure of steel or the like that contains martensite and ferrite, a method for preparing a sample for optical microscope observation, a steel etching solution set, and an etching apparatus. [Background technology]

[0002] The development of oil wells in severely corrosive environments, such as deep oil fields and environments containing carbon dioxide, is actively underway. One type of oil well steel pipe that can be used in such environments is stainless steel containing martensite and ferrite, as proposed in Patent Documents 1 and 2. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 5348354 [Patent Document 2] International Publication No. 2017 / 010036 Summary of the Invention [Problem to be solved by the invention]

[0004] Since the aforementioned stainless steel has a structure containing martensite and ferrite, structural characteristics such as the phase fraction affect the properties of the steel. For quality control, the phase fraction of produced steel pipes is sometimes measured, but for mass-produced steel pipes, measurement of the phase fraction requires not only accuracy but also simplicity.

[0005] The stainless steels disclosed in Patent Documents 1 and 2 consist of martensite, ferrite, and retained austenite. Of these, retained austenite can be determined relatively easily by X-ray diffraction. Regarding the separation of martensite and ferrite, Patent Document 1 describes polishing a stainless steel sample, etching it with a mixed solution of aqua regia and glycerin, and then determining the ferrite fraction using the point counting method in accordance with JIS G0555 on optical microscope photographs taken at 100x magnification. The point counting method involves inserting a glass plate with 20 grid lines in each direction into the microscope's eyepiece and counting the number of grid points occupied by the target (ferrite). Furthermore, at least 30 fields of view must be measured per specimen. Therefore, determining the ferrite fraction of a specimen requires a significant amount of time. Regarding the separation of martensite and ferrite, Patent Document 2 describes etching with Virrella's reagent (a mixture of 100 mL of ethanol, 10 mL of hydrochloric acid, and 2 g of picric acid), then imaging the structure with a scanning electron microscope and determining the ferrite fraction using an image analyzer. This method requires expensive analytical equipment, and has the drawback of taking images under vacuum, which makes it time-consuming to investigate a large number of specimens.

[0006] The above patent documents suggest that a simple method for investigating the martensite and ferrite phase fractions involves the use of an optical microscope for photography and an image analyzer for measurement. We investigated the use of optical microscope photographs and image analysis of samples obtained by etching using a mixture of aqua regia and glycerin or Virrella's reagent, the corrosion methods described in Patent Documents 1 and 2, but found problems with accuracy. Specifically, both etching methods corrode martensite, resulting in dark martensite and bright ferrite in optical microscope photographs. However, martensite does not appear dark across its entire surface; instead, contrast differences occur depending on the crystal orientation, resulting in areas where the light and dark areas are close to ferrite. Therefore, when performing image analysis, areas that are actually recognized as martensite are easily mistaken for ferrite, potentially reducing the accuracy of the resulting ferrite fraction.

[0007] Thus, there has been a demand for a technique that allows for easy and more accurate discrimination of the microstructure of steels such as steels having martensite and ferrite.

[0008] The present invention has been made in view of the above-mentioned problems, and aims to provide a steel etching method, a method for preparing a sample for optical microscope observation, a steel etching solution set, and an etching apparatus that enable simple and accurate analysis of the microstructure. [Means for solving the problem]

[0009] In order to solve the above problems, the present inventors have conducted extensive research into an etching method for stainless steel containing martensite and ferrite, as an example of a method for etching steel. As a result, they have found that, for example, by first performing electrolytic etching (electrolytic corrosion) using a KOH aqueous solution and then performing etching using an acid, the martensite can be uniformly corroded and the contrast with the ferrite can be increased.

[0010] The present invention was completed based on these findings and further investigations. That is, the gist of the present invention is as follows. [1] A first etching step in which steel is electrolytically etched in a basic solution having a pH greater than 7.0; After the first etching step, a second etching step is performed in which the steel is brought into contact with an acidic solution having a pH of less than 7.0. [2] The method for etching steel according to [1] above, wherein the pH of the basic solution is 13.0 or higher. [3] The method for etching steel according to [1] or [2] above, wherein the basic solution is an aqueous KOH solution or an aqueous NaOH solution. [4] The current density during the electrolytic etching is 0.5 A / cm 2 The method for etching steel according to any one of the above [1] to [3]. [5] The method for etching steel according to any one of [1] to [4] above, wherein the steel has martensite and ferrite. [6] A method for preparing a sample for observation under an optical microscope, in which the steel obtained by the method for etching steel according to any one of [1] to [5] above is used as a sample for observation under an optical microscope. [7] A basic solution for electrolytic etching of steel, the basic solution having a pH greater than 7.0; and an acidic solution having a pH of less than 7.0 for etching the steel. [8] The steel etching solution set according to [7], wherein the pH of the basic solution is 13.0 or higher. [9] The steel etching solution set according to [7] or [8], wherein the basic solution is a KOH aqueous solution or a NaOH aqueous solution.

[10] The steel etching solution set according to any one of [7] to [9] above, wherein the steel has martensite and ferrite.

[11] A steel etching solution set according to any one of [7] to

[10] above, An etching apparatus comprising: [Effects of the Invention]

[0011] According to the present invention, it is possible to carry out a microstructure analysis simply and accurately. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 shows an example of an optical microscope photograph of the microstructure of a sample obtained by the etching method of Example 1 of the present invention. [Figure 2] FIG. 2 shows an example of an optical microscope photograph of the microstructure of the sample obtained by the etching method of Comparative Example 1. [Figure 3] FIG. 3 shows an example of an optical microscope photograph of the microstructure of the sample obtained by the etching method of Comparative Example 2. [Figure 4] FIG. 4 shows an example of an optical microscope photograph of the microstructure of a sample obtained by the etching method of Example 2 of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, an embodiment of the present invention will be described. The steel etching method of the present invention comprises a first etching step in which steel is electrolytically etched in a basic solution having a pH greater than 7.0, and a second etching step in which, after the first etching step, the steel is brought into contact with an acidic solution having a pH less than 7.0, thereby enabling simple and accurate analysis in the subsequent observation of the steel microstructure using a microscope (optical microscope).

[0014] The steel to be etched in the present invention is not particularly limited, but may be a steel having martensite and ferrite as a steel structure. This steel may also be a stainless steel. The steel structure may contain retained austenite. In the present invention, after etching, martensite and ferrite can be accurately distinguished by observing the microstructure of the steel using an optical microscope or the like.

[0015] <First etching step> In the first etching step, the steel is subjected to electrolytic etching in a basic aqueous solution having a pH of more than 7.0. Specifically, the steel is subjected to electrolytic etching by anodic polarization in a basic solution having a pH of more than 7.0. The pH of the basic solution is preferably 13.0 or higher. The pH in the present invention can be measured under conditions of 25° C. (hereinafter, also referred to as pH (25° C.)). In the present invention, the temperature of the solution (etching temperature) when performing the etching treatment in the first etching step and the second etching step described below is not limited to 25°C. The basic solution is not particularly limited, but examples thereof include an aqueous KOH solution containing KOH and an aqueous NaOH solution containing NaOH. A KOH solution is a solution containing KOH and water. A NaOH solution is a solution containing NaOH and water. The KOH aqueous solution is preferably an aqueous solution having a KOH concentration of 1.0 mass % or more, and the NaOH aqueous solution is preferably an aqueous solution having a NaOH concentration of 1.0 mass % or more.

[0016] KOH concentration: 1.0% by mass or more, NaOH concentration: 1.0% by mass or more By performing electrolytic etching (electrolytic corrosion) in a KOH aqueous solution or a NaOH aqueous solution, martensite is etched more uniformly. To achieve this effect, it is preferable that the KOH concentration in the KOH aqueous solution be 1.0 mass% or more. To achieve this effect, it is preferable that the NaOH concentration in the NaOH aqueous solution be 1.0 mass% or more. The concentration of the KOH aqueous solution is more preferably 10.0 mass% or more, and even more preferably 20.0 mass% or more. The concentration of the NaOH aqueous solution is more preferably 10.0 mass% or more, and even more preferably 20.0 mass% or more. Since it is important that KOH is contained, no upper limit is set. However, even if an excessive amount is contained, the solubility of KOH is reached and the effect is saturated, so the concentration of the KOH aqueous solution is preferably 50.0 mass% or less. The concentration of the NaOH aqueous solution is also preferably 50.0 mass% or less.

[0017] Current density: 0.5A / cm 2 End The current density during electrolytic corrosion can be adjusted as needed, as the degree of corrosion varies depending on the steel composition. A suitable current density is 0.5 A / cm 2 More preferably, it is 1.5 A / cm 2 More preferably, 2.5 A / cm 2 That's all. The upper limit of the current density is not particularly limited, but the current density is preferably 20 A / cm 2 Preferably, it is less than 10 A / cm 2 More preferably, it is:

[0018] The conditions for electrolytic etching (electrolytic corrosion) are not particularly limited, except for the current density, but for example, the etching time in the first etching step is preferably 10 seconds or more, and the etching time in the first etching step is preferably 60 seconds or less.

[0019] <Second etching step> After the first etching step, the second etching step involves etching the steel by bringing it into contact with an acidic solution having a pH of less than 7.0. Specifically, it is preferable to immerse the steel in the acidic solution, then rinse with water and dry it. The acidic solution has a pH (at 25° C.) of less than 7.0. The pH of the acidic solution is preferably 1.0 or less.

[0020] The acidic solution can be a corrosion solution for steel materials (Metal Data Book, 4th Revised Edition, edited by the Japan Institute of Metals, Maruzen (2004)). Examples of the acidic solution include, but are not limited to, nital (1.5 ml of nitric acid, 100 ml of alcohol (one or more of methanol, ethanol, and amyl alcohol)). Other examples of the acidic solution include picral (4 g of picric acid, 100 ml of alcohol (methanol and / or ethanol)). Other examples of the acidic solution include sodium picrate (2 g of picric acid, 25 g of caustic soda, 100 ml of water). Other examples of the acidic solution include caustic soda (20 ml of a 10% by weight aqueous solution of caustic soda, 10 ml of hydrogen peroxide). Other examples of the acidic solution include a corrosion solution containing 1 to 4 g of ferrous iron, 10 g of caustic soda, and 100 ml of water. Other examples of acidic solutions include a corrosive solution containing 6.3 g of benzoic anhydride, 20 g of caustic soda, and 100 ml of water, and a corrosive solution containing 5 g of meta-nitrobenzenesulfonic acid and an alcohol solution. Other examples of acidic solutions include hydrochloric acid-picric acid (5 ml of hydrochloric acid, 1 g of picric acid, and 100 g of alcohol (methanol and / or ethanol)). Other examples of acidic solutions include a corrosive solution containing 10 ml of orthonitrophenol saturated with methanol and 20 ml of 20 vol% hydrochloric acid in amyl alcohol. Other examples of acidic solutions include a 4 vol% glycerin nitric acid solution, a 3 to 4 vol% nitric acid-alcohol (or water) solution, and a corrosive solution containing 5 g of ferric chloride, 50 ml of hydrochloric acid, and 100 ml of water. Another example of an acidic solution is a etchant containing 10 ml of nitric acid, 20 ml of hydrochloric acid, 20 ml of glycerin, and 10 ml of hydrogen peroxide. Another example is a etchant containing 10 g of ferric chloride, 30 ml of hydrochloric acid, and 120 ml of water. Another example is a etchant containing 30 ml of hydrochloric acid and 10 ml of nitric acid. Another example is a etchant containing saturated ferric chloride-hydrochloric acid and nitric acid. Another example is a etchant containing 4 g of copper sulfate, 20 ml of hydrochloric acid, and 20 ml of water. Another example is a etchant containing 5 g of copper sulfate, 100 ml of hydrochloric acid, 100 ml of ethanol, and 100 ml of water. Another example is a etchant containing 10 ml of nitric acid, 20 to 30 ml of hydrochloric acid, and 20 to 30 ml of glycerin. Other examples of acidic solutions include a etchant containing 10 g of ferrous iron, 10 g of caustic potassium, and 100 ml of water. Other examples of acidic solutions include a etchant containing 10 ml of hydrochloric acid, 3 ml of nitric acid, and 100 ml of methanol. Other examples of acidic solutions include a etchant containing 20 ml of hydrochloric acid, 15 ml of water, 65 ml of ethanol, and 1 g of copper sulfate. Other examples of acidic solutions include a neutral aqueous solution of ferrous iron. Other examples of acidic solutions include a etchant containing 30 ml of hydrochloric acid, 10 ml of nitric acid, and cupric chloride added until saturated. Other examples of acidic solutions include a etchant containing 4 g of potassium permanganate, 1 g of caustic soda, and 100 ml of water. Other examples of acidic solutions include a etchant containing 10 g of ferrous iron, 0.8 g of caustic soda, and 100 ml of water. Other examples of acidic solutions include a etchant containing 10 g of cupric chloride, 40 g of magnesium chloride, 20 ml of hydrochloric acid, and 1000 ml of ethanol. Other examples of acidic solutions include a corrosive solution containing 1 g of cupric chloride, 4 g of magnesium chloride, 1 ml of hydrochloric acid, 20 ml of water, and 100 ml of ethanol, and a corrosive solution containing 5 g of cupric chloride, 40 ml of hydrochloric acid, 30 ml of water, and 25 ml of alcohol.

[0021] In particular, considering that steels containing martensite and ferrite and stainless steels are preferably used as the steels to be etched in the present invention, the acidic solution preferably contains one or more of hydrochloric acid, nitric acid, and picric acid in order to perform more accurate microstructural analysis.Specific examples of the acidic solution include aqua regia (15 ml of nitric acid, 45 ml of hydrochloric acid) and Villela's reagent (50 ml of ethanol, 2 g of picric acid, and 5 ml of hydrochloric acid).

[0022] The conditions for etching by bringing steel into contact with an acidic solution are not particularly limited, but for example, the etching time in the second etching step is preferably 10 to 90 seconds.

[0023] The steel obtained by the steel etching method of the present invention as described above is used as a sample and observed under an optical microscope, thereby enabling the microstructure of the steel to be analyzed with high precision. For example, when steel contains martensite and ferrite, the entire martensite is corroded, which reduces the contrast within the martensite, improving the accuracy of calculation of the ferrite fraction by image analysis. Thus, the ferrite fraction of steel can be determined easily and with high accuracy.

[0024] The present invention also provides a method for preparing a sample for optical microscope observation, in which the steel obtained by the above-described method for etching steel of the present invention is used as a sample for observation under an optical microscope.

[0025] The present invention also provides a steel etching solution set comprising a basic solution and an acidic solution used in the steel etching method of the present invention described above. Furthermore, the present invention also provides an etching apparatus comprising the above-described steel etching solution set and an electrode. As the etching device, it is preferable that a constant current can be stably applied during the energization, so a device such as a galvanostat may be used, and the current is preferably constant. Furthermore, it is preferable to use stainless steel for the electrodes to prevent corrosion in basic solutions and rust during storage. [Example]

[0026] The present invention will be further described below based on examples, but the present invention is not limited to the following examples.

[0027] Molten steel with the chemical composition shown in Table 1 was melted in a vacuum high-frequency melting furnace to produce a 50 kg ingot. This ingot was heated at 1250°C for 1 hour and hot-rolled to produce a 15 mm thick steel plate. This steel plate was heated at 960°C for 20 minutes and then water-quenched. After quenching, the steel plate was tempered by heating at 600°C for 30 minutes and air-cooled. Small pieces were then cut out for microstructure investigation so that the surface including the rolling direction and thickness direction would be the observation surface. The small pieces were embedded in resin and mirror-polished. The mirror-polished sample was then etched under the following three conditions.

[0028] [Table 1]

[0029] [Etching Condition (1) - Example 1 of the Invention] 3.0 A / cm in a 25% by mass KOH aqueous solution 2 The specimen was subjected to electrolytic corrosion with anodic polarization at a current density of 1000x for 35 seconds. After rinsing with water and drying, it was immersed in Virrella's reagent (50 ml of ethanol, 2 g of picric acid, and 5 ml of hydrochloric acid) for 30 seconds, rinsed with water, and dried. Figure 1 shows an example of the microstructure photographed with an optical microscope at 1000x magnification.

[0030] [Etching Condition (2) - Comparative Example 1: Aqua Regia and Glycerin Mixture] The specimen was immersed in a mixture of aqua regia (15 ml of nitric acid and 45 ml of hydrochloric acid) and 30 ml of glycerin for 30 seconds, then rinsed with water and dried. Figure 2 shows an example of the microstructure photographed with an optical microscope at 1000x magnification.

[0031] [Etching Condition (3) - Comparative Example 2: Vilela Reagent] The specimen was immersed in Villela's reagent (ethanol 50 ml, picric acid 2 g, hydrochloric acid 5 ml) for 30 seconds, rinsed with water, and dried. Figure 3 shows an example of the microstructure photographed with an optical microscope at 1000x magnification.

[0032] [Etching Condition (4) - Example 2 of the Invention] 3.0A / cm in a 25% by mass NaOH aqueous solution 2 The specimen was subjected to electrolytic corrosion with anodic polarization at a current density of 1000x for 35 seconds. After rinsing with water and drying, it was immersed in Virrella's reagent (50 ml of ethanol, 2 g of picric acid, and 5 ml of hydrochloric acid) for 30 seconds, rinsed with water, and dried again. Figure 4 shows an example of the microstructure photographed with an optical microscope at 1000x magnification.

[0033] In Figs. 1 and 4 of the examples of the present invention, the martensite portions, which are dark in contrast, are generally dark, and the contrast with the bright ferrite is large.

[0034] In the mixed solution of aqua regia and glycerin of Comparative Example 1 shown in FIG. 2, the darker side in contrast is the martensite side, but it is not corroded much and the difference in contrast between martensite and ferrite is small. In such photographs, the contrast difference between martensite and ferrite is small, making it impossible to accurately separate the two by image analysis.

[0035] FIG. 3 shows an optical microscope photograph of Comparative Example 2, in which the darker contrast is the martensite side. A portion of the martensite is bright in contrast, and is the same brightness as the ferrite. Because a portion of the martensite is the same brightness as the ferrite, it is not possible to accurately separate the two by image analysis.

[0036] On the other hand, in the example of the present invention, the contrast difference between martensite and ferrite was large, so that the two could be separated with high precision by image analysis.

Claims

1. a first etching step in which the steel is electrolytically etched in a basic solution having a pH greater than 7.0; and a second etching step of etching the steel by contacting the steel with an acidic solution having a pH of less than 7.0 after the first etching step, The current density during the electrolytic etching is 0.5 A / cm ２ That is all, A method for etching steel, wherein the steel is a stainless steel having martensite and ferrite.

2. 2. The method of claim 1, wherein the pH of the basic solution is 13.0 or greater.

3. 2. The method of claim 1, wherein the basic solution is an aqueous KOH solution or an aqueous NaOH solution.

4. 3. The method of claim 2, wherein the basic solution is an aqueous KOH solution or an aqueous NaOH solution.

5. 2. The method of etching steel according to claim 1, wherein the acidic solution in the second etching step is aqua regia or Villela's reagent.

6. The time for the electrolytic etching in the first etching step is 10 seconds or more and 60 seconds or less, The steel etching method according to claim 1 , wherein the etching time in the second etching step is 10 seconds or more and 90 seconds or less.

7. A method for preparing a sample for observation under an optical microscope, comprising using the steel obtained by the method for etching steel according to any one of claims 1 to 6 as a sample for observation under an optical microscope.

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