Method for etching steel, method for producing sample for optical microscopy observation, solution set for etching steel, and etching device

Abstract

The invention provides a method for etching steel, a method for manufacturing a sample for optical microscopy observation, a solution set for etching steel, and an etching device, wherein microstructure analysis can be carried out simply and with high precision. A steel etching method includes: a first etching step in which steel is electrolytically etched in an alkaline solution having a pH of greater than 7.0; and a second etching step in which, after the first etching step, etching is performed in which the steel is brought into contact with an acidic solution having a pH of less than 7.0. The pH value of the alkaline solution can be 13.0 or above, and the alkaline solution can be a KOH aqueous solution. The current density during electrolytic etching may be 0.5 A / cm2 or more. The steel may also have martensite and ferrite.

Description

Technical Field

[0001] The present invention relates to an etching method, a method for producing a specimen for optical microscope observation, a solution group for etching steel, and an etching apparatus that can accurately identify the phase fractions affecting material properties with high precision based on image analysis of an optical microscope photograph when observing the microstructure of steel containing martensite and ferrite, etc. Background Art

[0002] The development of oil wells in severe corrosion environments such as deep oil fields and environments containing carbon dioxide gas is actively underway. As one of the steel pipes for oil wells that can be used in such environments, there is the stainless steel containing martensite and ferrite proposed in Patent Document 1 and Patent Document 2.

[0003] Prior Art Documents

[0004] Patent Documents

[0005] Patent Document 1: Japanese Patent No. 5348354 Gazette

[0006] Patent Document 2: International Publication No. 2017 / 010036 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] Since the above stainless steel has a structure containing martensite and ferrite, characteristics of the structure such as the phase fraction affect the properties of the steel. For quality control, the phase fraction of the produced steel pipes is sometimes measured, but in the measurement of the phase fraction of mass-produced steel pipes, simplicity is required in addition to accuracy.

[0009] However, the stainless steel disclosed in Patent Document 1 and Patent Document 2 is composed of martensite, ferrite and retained austenite. Among them, the retained austenite can be relatively easily obtained by X-ray diffraction. On the other hand, regarding the separation of martensite and ferrite, in Patent Document 1, after grinding the stainless steel, it is etched with a mixed solution of aqua regia and glycerin, and photographed at 100 times magnification. For the obtained optical microscope photograph, the ferrite fraction is obtained by the point counting method according to JIS G0555. The measurement of the point counting method is to insert a glass plate with 20 grid lines in the vertical and horizontal directions into the eyepiece of the microscope, and measure the number of grid point centers occupied by the object (ferrite). In addition, at least 30 fields of view are measured for one specimen. Therefore, it takes a lot of time to obtain the ferrite fraction of one specimen. In addition, regarding the separation of martensite and ferrite, in Patent Document 2, after etching with Vilella's reagent (a mixed solution of 100 mL of ethanol, 10 mL of hydrochloric acid, and 2 g of picric acid), the microstructure is photographed with a scanning electron microscope, and the ferrite fraction is obtained using an image analysis device. In this method, an expensive analysis device is required, and since the photographing is performed under vacuum, there is a problem that the investigation of multiple specimens takes time.

[0010] Referring to the above patent documents, as a simple method for investigating the respective phase fractions of martensite and ferrite, it can be cited that an optical microscope is used in photographing and an image analysis device is used in the measurement method. For the specimens obtained by the etching methods shown in Patent Document 1 and Patent Document 2, that is, by etching with a mixed solution of aqua regia and glycerin or Vilella's reagent, optical microscope photographs are taken and image analysis is performed. It is known from this research that there are problems in terms of accuracy. Specifically, both etching methods corrode martensite, so in the optical microscope photograph, martensite becomes dark and ferrite becomes bright. However, martensite does not become dark on the entire surface, and there is a contrast difference according to the crystal orientation, and parts that are easily close to ferrite in terms of light and dark are likely to occur. Therefore, when performing image analysis, parts that are originally recognized as martensite are easily misrecognized as ferrite, and the accuracy of the obtained ferrite fraction may be reduced.

[0011] Thus, for steels such as steels having martensite and ferrite, a technique that can simply and with higher accuracy distinguish the microstructure is desired.

[0012] The present invention has been completed in view of the above problems, and an object thereof is to provide an etching method for steel, a method for producing a specimen for optical microscope observation, a solution group for etching steel, and an etching device that can simply and with high accuracy analyze the microstructure.

[0013] Means for Solving the Problems

[0014] In order to solve the above problems, the inventors of the present invention have conducted in-depth research on an etching method for stainless steel containing martensite and ferrite as an example of an etching method for steel. As a result, it has been found that, for example, by first performing electrolytic etching (electrolytic corrosion) using an aqueous KOH solution and then performing etching using an acid, martensite is uniformly corroded, and the contrast difference with ferrite can be increased.

[0015] The present invention has been completed through further research based on the above insights. That is, the gist of the present invention is as follows.

[0016] [1] An etching method for steel, comprising:

[0017] a first etching step of electrolytically etching the steel in an alkaline solution having a pH greater than 7.0; and

[0018] a second etching step of etching the steel by bringing it into contact with an acidic solution having a pH less than 7.0 after the first etching step.

[0019] [2] In the etching method for steel according to the above [1], the pH of the alkaline solution is 13.0 or more.

[0020] [3] In the etching method for steel according to the above [1] or [2], the alkaline solution is an aqueous KOH solution or an aqueous NaOH solution.

[0021] [4] In the etching method for steel according to any one of the above [1] to [3], the current density during the electrolytic etching is 0.5 A / cm 2 or more.

[0022] [5] In the etching method for steel according to any one of the above [1] to [4], the steel has martensite and ferrite.

[0023] [6] A method for producing a specimen for optical microscope observation, wherein the steel obtained by the etching method for steel according to any one of the above [1] to [5] is used as the specimen for observation with an optical microscope.

[0024] [7] A set of solutions for etching steel, which has an alkaline solution having a pH greater than 7.0 for electrolytically etching the steel and an acidic solution having a pH less than 7.0 for etching the steel.

[0025] [8] In the set of solutions for etching steel according to the above [7], the pH of the alkaline solution is 13.0 or more.

[0026] [9] In the set of solutions for etching steel according to the above [7] or [8], the alkaline solution is an aqueous KOH solution or an aqueous NaOH solution.

[0027] In the solution group for etching steel according to any one of [7] to [9] above, the steel has martensite and ferrite.

[0028]

[11] An etching apparatus, comprising the solution group for etching steel according to any one of [7] to

[10] above and an electrode.

[0029] Advantages of the Invention

[0030] According to the present invention, the analysis of the microstructure can be carried out simply and with high precision. Description of the Drawings

[0031] Figure 1 Fig. 1 shows an example of an optical microscope photograph of the microstructure of a specimen obtained by the etching method of Example 1 of the present invention.

[0032] Figure 2 Fig. 2 shows an example of an optical microscope photograph of the microstructure of a specimen obtained by the etching method of Comparative Example 1.

[0033] Figure 3 Fig. 3 shows an example of an optical microscope photograph of the microstructure of a specimen obtained by the etching method of Comparative Example 2.

[0034] Figure 4 Fig. 4 shows an example of an optical microscope photograph of the microstructure of a specimen obtained by the etching method of Example 2 of the present invention. Detailed Description of the Invention

[0035] Hereinafter, embodiments of the present invention will be described.

[0036] The steel etching method of the present invention includes: a first etching step of electrolytically etching steel in an alkaline solution having a pH greater than 7.0 and a second etching step of etching the steel by bringing it into contact with an acidic solution having a pH less than 7.0 after the first etching step. In the subsequent observation of the microstructure of the steel using a microscope (optical microscope), analysis can be carried out simply and with high precision.

[0037] The steel to be etched in the present invention is not particularly limited. As the steel structure, it can be steel having martensite and ferrite. In addition, the steel can be stainless steel. Residual austenite may also be contained in the steel structure. In the present invention, after etching, martensite and ferrite can be distinguished with high precision in the observation of the microstructure of the steel using an optical microscope or the like.

[0038] <First Etching Step>

[0039] In the first etching step, the steel is electrolytically etched in an alkaline aqueous solution having a pH greater than 7.0. Specifically, in an alkaline solution having a pH greater than 7.0, electrolytic etching is carried out by anodic polarization of the steel.

[0040] The pH of the alkaline solution is preferably 13.0 or higher. In addition, the pH in the present invention can be measured under the condition of 25 °C (hereinafter also referred to as pH(25 °C)).

[0041] It should be noted that in the present invention, the temperature of the solution (etching temperature) during the etching treatment in the first etching step and the second etching step described later is not limited to 25 °C.

[0042] In addition, as the alkaline solution, there is no particular limitation, and examples thereof include an aqueous KOH solution containing KOH and an aqueous NaOH solution containing NaOH.

[0043] The aqueous KOH solution is a solution containing KOH and water. The aqueous NaOH solution is a solution containing NaOH and water.

[0044] The aqueous KOH solution is preferably an aqueous solution having a KOH concentration of 1.0 mass% or more. In addition, the aqueous NaOH solution is preferably an aqueous solution having a NaOH concentration of 1.0 mass% or more.

[0045] KOH concentration: 1.0 mass% or more, NaOH concentration: 1.0 mass% or more.

[0046] By performing electrolytic etching (electrolytic corrosion) in an aqueous KOH solution or an aqueous NaOH solution, martensite is etched more uniformly. In order to obtain such an effect, it is preferable that the KOH concentration in the aqueous KOH solution is 1.0 mass% or more. In order to obtain such an effect, it is preferable that the NaOH concentration in the aqueous NaOH solution is 1.0 mass% or more. The concentration of the aqueous KOH solution is more preferably 10.0 mass% or more, and further preferably 20.0 mass% or more. The concentration of the aqueous NaOH solution is more preferably 10.0 mass% or more, and further preferably 20.0 mass% or more. Since it is important to contain KOH, there is no particular upper limit set, but even if it is contained in excess, the solubility of KOH is reached and the effect is saturated, so the concentration of the aqueous KOH solution is preferably 50.0 mass% or less. The concentration of the aqueous NaOH solution is also preferably 50.0 mass% or less.

[0047] Set the current density to 0.5 A / cm 2 or more.

[0048] Regarding the current density during electrolytic corrosion, since the degree of corrosion varies depending on the composition of the steel, it can be adjusted appropriately, but a suitable current density is 0.5 A / cm 2 or more. More preferably 1.5 A / cm 2 or more, and further preferably 2.5 A / cm 2 or more.

[0049] The upper limit value of the current density is not particularly limited, and the current density is preferably 20 A / cm 2 Hereinafter, it is more preferably 10 A / cm 2 Hereinafter.

[0050] Regarding the conditions during electrolytic etching (electrolytic corrosion), the conditions other than the current density are not particularly limited. For example, the etching time in the first etching process is preferably 10 seconds or more. Additionally, the etching time in the first etching process is preferably 60 seconds or less.

[0051] <Second Etching Process>

[0052] After the above-mentioned first etching process, in the second etching process, etching is performed by bringing the steel into contact with an acidic solution having a pH less than 7.0. Specifically, it is preferable to immerse the steel in the acidic solution and then perform water washing and drying.

[0053] For the acidic solution, the pH (25 °C) is less than 7.0. The pH of the acidic solution is preferably 1.0 or less.

[0054] As the acidic solution, a corrosion liquid for steel materials ([Revised 4th Edition of Metal Data Handbook], edited by The Japan Institute of Metals, Maruzen (2004)) can be used, and there is no particular limitation. Examples include nital (nitric acid 1.5 ml, alcohol (any one or more of methanol, ethanol, and amyl alcohol) 100 ml). Additionally, as the acidic solution, picral (picric acid 4 g, alcohol (methanol and / or ethanol) 100 ml) can also be cited. Additionally, as the acidic solution, sodium picrate (picric acid 2 g, sodium hydroxide 25 g, water 100 ml) can also be cited. Additionally, as the acidic solution, sodium hydroxide (20 ml of a 10 mass% aqueous solution of sodium hydroxide, hydrogen peroxide 10 ml) can also be cited. Additionally, as the acidic solution, a corrosion liquid containing 1 - 4 g of potassium ferricyanide, 10 g of sodium hydroxide, and 100 ml of water can also be cited.

[0055] Additionally, as the acidic solution, a corrosion liquid containing 6.3 g of benzoic anhydride, 20 g of sodium hydroxide, and 100 ml of water can also be cited. Additionally, as the acidic solution, a corrosion liquid containing 5 g of meta-nitrobenzene sulfonic acid and an alcohol solution can also be cited.

[0056] Additionally, as the acidic solution, hydrochloric acid - picric acid (hydrochloric acid 5 ml, picric acid 1 g, alcohol (methanol and / or ethanol) 100 g) can also be cited. Additionally, as the acidic solution, a corrosion liquid containing the following substances can also be cited: 10 ml of o-nitrophenol saturated in methanol and 20 ml of hydrochloric acid contained at a content of 20 vol% in amyl alcohol.

[0057] In addition, as an acidic solution, a 4 vol% nitroglycerin solution can also be cited. In addition, as an acidic solution, a 3 - 4 vol% nitric acid - alcohol (or water) solution can also be cited. In addition, as an acidic solution, an etching solution containing 5 g of ferric chloride, 50 ml of hydrochloric acid, and 100 ml of water can also be cited.

[0058] In addition, as an acidic solution, an etching solution containing 10 ml of nitric acid, 20 ml of hydrochloric acid, 20 ml of glycerin, and 10 ml of hydrogen peroxide solution can also be cited. In addition, as an acidic solution, an etching solution containing 10 g of ferric chloride, 30 ml of hydrochloric acid, and 120 ml of water can also be cited. In addition, as an acidic solution, an etching solution containing 30 ml of hydrochloric acid and 10 ml of nitric acid can also be cited. In addition, as an acidic solution, an etching solution containing saturated ferric chloride - hydrochloric acid and nitric acid can also be cited. In addition, as an acidic solution, an etching solution containing 4 g of copper sulfate, 20 ml of hydrochloric acid, and 20 ml of water can also be cited. In addition, as an acidic solution, an etching solution containing 5 g of copper sulfate, 100 ml of hydrochloric acid, 100 ml of ethanol, and 100 ml of water can also be cited. In addition, as an acidic solution, an etching solution containing 10 ml of nitric acid, 20 - 30 ml of hydrochloric acid, and 20 - 30 ml of glycerin can also be cited. In addition, as an acidic solution, an etching solution containing 10 g of potassium ferricyanide, 10 g of potassium hydroxide, and 100 ml of water can also be cited. In addition, as an acidic solution, an etching solution containing 10 ml of hydrochloric acid, 3 ml of nitric acid, and 100 ml of methanol can also be cited. In addition, as an acidic solution, an etching solution containing 20 ml of hydrochloric acid, 15 ml of water, 65 ml of ethanol, and 1 g of copper sulfate can also be cited. In addition, as an acidic solution, a neutral aqueous solution of potassium ferricyanide can also be cited. In addition, as an acidic solution, an etching solution containing 30 ml of hydrochloric acid, 10 ml of nitric acid, and copper chloride added to saturation can also be cited. In addition, as an acidic solution, an etching solution containing 4 g of potassium permanganate, 1 g of sodium hydroxide, and 100 ml of water can also be cited. In addition, as an acidic solution, an etching solution containing 10 g of potassium ferricyanide, 0.8 g of sodium hydroxide, and 100 ml of water can also be cited. In addition, as an acidic solution, an etching solution containing 10 g of copper chloride, 40 g of magnesium chloride, 20 ml of hydrochloric acid, and 1000 ml of ethanol can also be cited. In addition, as an acidic solution, an etching solution containing 1 g of copper chloride, 4 g of magnesium chloride, 1 ml of hydrochloric acid, 20 ml of water, and 100 ml of ethanol can also be cited. In addition, as an acidic solution, an etching solution containing 5 g of copper chloride, 40 ml of hydrochloric acid, 30 ml of water, and 25 ml of alcohol can also be cited.

[0059] In particular, as the steel to be etched in the present invention, considering that it is preferable to use steel having martensite and ferrite, stainless steel, in order to analyze the microstructure with higher precision, the acidic solution preferably contains one or more of hydrochloric acid, nitric acid, and picric acid. Specifically, as the acidic solution, aqua regia (15 ml of nitric acid, 45 ml of hydrochloric acid), Vilella's reagent (50 ml of ethanol, 2 g of picric acid, 5 ml of hydrochloric acid) can be cited.

[0060] The conditions for etching when bringing the steel into contact with the acidic solution are not particularly limited. For example, the etching time in the second etching step is preferably 10 to 90 seconds.

[0061] Taking the steel obtained by the steel etching method of the present invention described above as a specimen, by observing with an optical microscope, the microstructure of the steel can be analyzed with high precision.

[0062] For example, when the steel has martensite and ferrite, since the entire surface of the martensite is corroded and it is difficult to generate contrast within the martensite, the calculation accuracy of the ferrite fraction obtained by image analysis can be improved. Therefore, the ferrite fraction of the steel can be obtained with high precision and simply.

[0063] In addition, the present invention also provides a method for producing a specimen for optical microscope observation, which uses the steel obtained by the steel etching method of the present invention as a specimen for observation with an optical microscope.

[0064] In addition, the present invention also provides a solution set for etching steel, which has the alkaline solution and acidic solution used in the steel etching method of the present invention.

[0065] Furthermore, the present invention also provides an etching device, which includes the above solution set for etching steel and an electrode.

[0066] As the etching device, it is preferable that a current of a certain magnitude flows stably during energization. Therefore, a device such as a constant current mirror can also be used, and the current is preferably a current of a certain magnitude. In addition, in order to suppress the corrosion of the electrode by the alkaline solution or rusting during storage, stainless steel is also preferably used.

[0067] Examples

[0068] Hereinafter, the present invention will be further described based on examples. It should be noted that the present invention is not limited to the following examples.

[0069] The 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. The ingot was heated at 1250 °C for 1 hour, and a 15-mm-thick steel plate was produced by hot rolling. The steel plate was heated at 960 °C for 20 minutes and then water quenched. The quenched steel plate was tempered at 600 °C for 30 minutes and air cooled. Then, small pieces for microstructural investigation were cut out in such a way that the plane including the rolling direction and the wall thickness direction became the observation plane. The small pieces were embedded in resin and mirror-polished. For the mirror-polished specimens, etching was carried out under the following three conditions.

[0070] [Table 1]

[0071]

[0072] The remaining part other than the above composition is Fe and inevitable impurities.

[0073] [Etching condition (1) - Example 1 of the present invention]

[0074] In a 25 mass% aqueous KOH solution, anodic polarization electrolytic corrosion was carried out at a current density of 3.0 A / cm 2 for 35 seconds. After one water wash and drying, it was immersed in Vilella's reagent (50 ml of ethanol, 2 g of picric acid, 5 ml of hydrochloric acid) for 30 seconds, followed by water wash and drying. Figure 1 is an example of the microstructure photographed with a 1000-fold optical microscope afterwards.

[0075] [Etching condition (2) - Comparative Example 1 mixture of aqua regia and glycerol]

[0076] It was immersed in a mixture of aqua regia (mixing 15 ml of nitric acid and 45 ml of hydrochloric acid) and 30 ml of glycerol for 30 seconds, followed by water wash and drying. Figure 2 is an example of the microstructure photographed with a 1000-fold optical microscope afterwards.

[0077] [Etching condition (3) - Comparative Example 2 Vilella's reagent]

[0078] It was immersed in Vilella's reagent (50 ml of ethanol, 2 g of picric acid, 5 ml of hydrochloric acid) for 30 seconds, followed by water wash and drying. Figure 3 is an example of the microstructure photographed with a 1000-fold optical microscope afterwards.

[0079] [Etching condition (4) - Example 2 of the present invention]

[0080] In a 25 mass% aqueous NaOH solution, at 3.0 A / cm 2Electrolytic corrosion with anodic polarization at a current density of [current density value] for 35 seconds was carried out. After one water wash and drying, it was immersed in Vilella's reagent (50 ml of ethanol, 2 g of picric acid, 5 ml of hydrochloric acid) for 30 seconds, followed by water wash and drying. Figure 4 is an example of a microstructural image taken with an optical microscope at 1000 times magnification.

[0081] In the example of the present invention Figure 1 and Figure 4 the part of martensite with a dark contrast is generally darkened, and the contrast with ferrite with a bright contrast is large.

[0082] Figure 2 In the mixed solution of aqua regia and glycerol of Comparative Example 1 shown, the darker-contrast side is the martensite side, but it is not much corroded, and the contrast difference between martensite and ferrite is small.

[0083] In such a photograph, since the contrast difference between martensite and ferrite is small, the two cannot be separated with high precision by image analysis.

[0084] In addition, the optical microscope photograph when using the Vilella's reagent of Comparative Example 2 is as shown in Figure 3 The darker-contrast side is the martensite side. A part of the martensite has a bright contrast and is the same brightness as the ferrite part. Since a part of the martensite becomes the same brightness as the ferrite, the two cannot be separated with high precision by image analysis.

[0085] On the other hand, in the example of the present invention, since the contrast difference between martensite and ferrite is large, the two can be separated with high precision by image analysis.

Claims

1. A method for etching steel, comprising: a first etching step of electrolytically etching the steel in an alkaline solution having a pH greater than 7.0; and a second etching step of etching the steel by bringing it into contact with an acidic solution having a pH less than 7.0 after the first etching step.

2. The etching method of steel according to claim 1, wherein, The pH of the alkaline solution is 13.0 or more.

3. The etching method of steel according to claim 1 or 2, wherein, The alkaline solution is an aqueous solution of KOH or an aqueous solution of NaOH.

4. The etching method of steel according to any one of claims 1 to 3, wherein, The current density during the electrolytic etching is 0.5 A / cm 2 or more.

5. The etching method of steel according to any one of claims 1 to 4, wherein, The steel has martensite and ferrite.

6. Method for producing a specimen for optical microscope observation, wherein, The steel obtained by the method for etching steel according to any one of claims 1 to 5 is used as a specimen for observation with an optical microscope.

7. A set of solutions for etching steel, which has an alkaline solution having a pH greater than 7.0 for electrolytically etching the steel and an acidic solution having a pH less than 7.0 for etching the steel.

8. The solution set for etching steel according to claim 7, wherein, The pH of the alkaline solution is 13.0 or more.

9. The solution set for etching steel according to claim 7 or 8, wherein, The alkaline solution is an aqueous solution of KOH or an aqueous solution of NaOH.

10. The solution set for etching steel according to any one of claims 7 to 9, wherein, The steel has martensite and ferrite.

11. An etching apparatus, which comprises the set of solutions for etching steel according to any one of claims 7 to 10 and an electrode.

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