JP2548654B2 - Etching solution for complex structure steel and etching method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an etching solution and method for etching the structure of a steel material having a composite structure in a distinguishable manner for each structure and exposing the crystal grain boundaries.

[0002]

2. Description of the Related Art Microstructures, which are one of the factors that control the properties of steel, are usually microstructure photographs and microstructure measurements (area ratio of each microstructure,
Grain size, grain length, etc.), hardness according to structure, etc. Conventionally, the above-mentioned investigation was carried out by the structure revealed by a 5% nitric acid alcohol etchant. However, with this corrosive liquid, when there were structures such as pearlite, bainite, and martensite other than ferrite, it was difficult to clearly distinguish them. Therefore, in the case of measuring tissue by mechanical image processing, the tissue is quantified by mechanical image processing (image processing by a computer etc.) after coloring and coloring each tissue according to human judgment, and this work takes a lot of time. It was impossible to do this work even if the organization could not appear.
As means for solving this problem, JP-A-59-219
In Japanese Patent No. 473, a method is proposed in which ferrite, pearlite, bainite, and martensite are color-coded and identifiable by a color etching solution and a color etching method. According to this method, mechanical image processing for texture measurement can be performed directly from the corrosion sample, and texture measurement can be efficiently performed.

In recent years, a composite structure steel sheet having retained austenite has been developed as a steel sheet that meets the contradictory needs of reducing the weight and improving the formability of automobiles. The composite structure steel sheet has a composite structure composed of ferrite, bainite, and retained austenite, and has high strength but high ductility due to the strain-induced plasticity effect of retained austenite. In the composite structure steel sheet, the space factor, the form, and the location of the retained austenite are important for obtaining high ductility. However, it has been impossible to reveal the residual austenite by using a general corrosive liquid or a corrosive method that has been conventionally known. In addition, the above-mentioned JP-A-59-2194.
Although ferrite and bainite can be revealed even by using the color etching solution and the color etching method in Japanese Patent Laid-Open No. 73-73, it is impossible to reveal the retained austenite. The position was not clarified by the above-mentioned etching method, so that it was impossible to directly perform mechanical image processing for texture measurement from the corroded sample.

As a method for clarifying the space factor, morphology and existing position of the retained austenite by a method other than etching, there are a method using X-ray diffraction and a method using an electron microscope. In the method using X-ray diffraction, the residual austenite space factor can be calculated from the X-ray diffraction intensity. However, the morphology and location of austenite are not required. With the method using an electron microscope, it is possible to know the existing position and morphology of retained austenite, but it requires a high-level observation technique. Further, although this method can only observe a very small area at one time, it is difficult to obtain the space factor because it is necessary to observe in a wider range to obtain the space factor of the retained austenite. Therefore, there has been a need for an etching solution and an etching method that can easily clarify the space factor, the form, and the location of the retained austenite without requiring advanced technology.

[0005]

DISCLOSURE OF THE INVENTION The present invention makes it possible to easily clarify the space factor, morphology and existing position of retained austenite without requiring a high level technique, and to perform mechanical measurement for direct microstructural measurement from a corroded sample. It is an object to enable image processing.

[0006]

In order to solve the above-mentioned problems, the present invention uses 2 to 100 ml of picric acid per 100 ml of water.
4 g, sodium thiosulfate 0.1 to 1 g, citric acid 0.
2-1g, sodium pyrosulfite 0.1-1g are mixed and dissolved, and nitric acid is added and mixed so that the nitric acid concentration is 1-2.5% by volume.
Means,

2 to 4 picric acid per 100 ml of water
g, sodium thiosulfate 0.1 to 1 g, citric acid 0.2
-1 g and 0.1-1 g of sodium pyrosulfite are mixed and dissolved, and the steel material is immersed for 12 to 20 seconds in an etching solution in which nitric acid is added and mixed so that the nitric acid concentration is 1 to 2.5% by volume, followed by washing with water. The second method is an etching method for a composite structure steel material, which is characterized in that

2 to 4 picric acid per 100 ml of water
g, sodium thiosulfate 0.1 to 1 g, citric acid 0.2
-1 g and 0.1-1 g of sodium pyrosulfite are mixed and dissolved, and the steel material is immersed for 12 to 20 seconds in an etching solution in which nitric acid is added and mixed so that the nitric acid concentration is 1 to 2.5% by volume, followed by washing with water. And then dried, and further, after immersing the steel material in an etching solution in which picric acid alcohol and an aqueous solution of sodium pyrosulfite are mixed for 5 to 10 seconds, washing with water for 2 to 3 seconds and drying. A third method is an etching method for steel materials.

[0009]

In order to achieve the above-mentioned object, the present inventors have improved the color etching solution and the color etching method in JP-A-59-219473 so that residual austenite is also revealed and classified. To develop a possible etching solution and etching method, as a result of etching with a solution in which various chemicals are mixed with the color etching solution, it is possible to reveal residual austenite when sodium pyrosulfite is mixed. I got the knowledge of. Further, in order to clearly reveal the structures of ferrite, bainite, and retained austenite, various amounts of chemicals were mixed and tested, and as a result, it was found that picric acid 2 was added to 100 ml of water.
~ 4 g, sodium thiosulfate 0.1-1 g, citric acid 0.2-1 g, and sodium pyrosulfite 0.1-1 g are mixed and dissolved, and nitric acid is added so that the nitric acid concentration is 1 to 2.5% by volume. It was found that the mixed etching solution was able to best reveal the retained austenite, the ferrite and bainite were relatively well discernible, and each grain boundary was also capable of being revealed. Further, it has been found that the best method for etching is to immerse the steel material in the etching solution for 12 to 20 seconds, wash it with water, and then dry it.

Even with the above method, it is sufficiently possible to distinguish retained austenite from ferrite and bainite, but since retained austenite and ferrite have similar colors, it may be difficult to distinguish between them. Therefore, the inventors have further decided to increase the color difference between the retained austenite and the ferrite by coloring the ferrite so that the two can be easily distinguished from each other. Paying attention to the point that a generally known Repeller solution colors ferrite, first, after etching the steel material by the above method, immersing the steel material in the Repeller solution for 5 to 10 seconds and then washing with water for 2 to 3 seconds, By drying, the retained austenite and the ferrite can be more clearly distinguished, and the ferrite grain boundaries are also more clearly defined. Incidentally, the Repeller solution generally has a ratio of 4% picric acid alcohol in which 4 g of picric acid is dissolved in 100 ml of ethyl alcohol and 1% sodium pyrosulfite aqueous solution in which 1 g of sodium pyrosulfite is dissolved in 100 ml of water. It means a liquid mixed so as to be ~ 2.

[0011]

Example As the test steel material, a steel sheet having the composition shown in Table 1 having a composite structure of retained austenite, ferrite and bainite was used. Table 2 shows the composition of each etching solution, etching conditions such as immersion time and water washing time, and the definition of each structure after etching. For each of the steel materials A to C, after the emery polishing and the alumina polishing, the first stage corrosion was performed under the conditions shown in Table 2, followed by washing with water and drying, and the second stage corrosion was performed under the conditions shown in Table 2. Then, the surface of the steel material was photographed with an optical microscope to determine the sharpness of each tissue.

[0012]

[Table 1]

[0013]

[Table 2]

As shown in Table 2, No. 1 of the present invention example. 1
Each tissue can be clearly discerned from ~ 6. FIG. 1 shows the above No. 1 for steel type A. The schematic diagram of the structure photograph when etching by the etching method of 4 was shown. From this, the retained austenite (white), the ferrite (gray), and the bainite (black) can be clearly identified, and the grain boundaries of the ferrite also appear clearly. On the other hand, in Comparative Example No. 1 in which the composition of the etching solution in the first step was out of alignment. Nos. 1 to 10 of Comparative Example with the first-stage immersion time deviated. Nos. 11 and 12 have a poor degree of identification of any structure or grain boundary, which causes a problem in practical use.

[0015]

EFFECTS OF THE INVENTION According to the present invention, the structure of a composite steel material can be clarified without the need for advanced technology, and the space factor, shape, existing position, etc. of retained austenite can be clearly known. Moreover, mechanical image processing for direct microstructural measurement can be performed directly from the corroded sample, which not only saves a great deal of labor, time and processing cost, but also greatly enhances the development speed of new steel material, which has an industrial effect. Is very large.

[Brief description of drawings]

FIG. 1 shows a steel type A of Table 1 and an invention example No. 2 of Table 2. 4 is a schematic view of a microstructure photograph of a sample etched by the etching method of FIG.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junji Haji 1 Nishinosu, Oita, Oita, Oita Pref., Nippon Steel Co., Ltd. Oita Works (72) Tadanori Yakushinji, Nishinosu, Oita, Oita Pref. Nippon Steel Co., Ltd. Oita Works

Claims (3)

(57) [Claims] 1. Picric acid 2 to 4 per 100 ml of water
g, sodium thiosulfate 0.1 to 1 g, citric acid 0.2
-1 g and sodium pyrosulfite 0.1-1 g are mixed and dissolved, and nitric acid is added and mixed so that the nitric acid concentration is 1 to 2.5% by volume. 2. Picric acid 2 to 4 per 100 ml of water
g, sodium thiosulfate 0.1 to 1 g, citric acid 0.2
-1 g and 0.1-1 g of sodium pyrosulfite are mixed and dissolved, and the steel material is immersed for 12 to 20 seconds in an etching solution in which nitric acid is added and mixed so that the nitric acid concentration is 1 to 2.5% by volume, followed by washing with water. A method for etching a steel material having a composite structure, comprising the steps of drying after drying. 3. Picric acid 2 to 4 per 100 ml of water
g, sodium thiosulfate 0.1 to 1 g, citric acid 0.2
-1 g and 0.1-1 g of sodium pyrosulfite are mixed and dissolved, and the steel material is immersed for 12 to 20 seconds in an etching solution in which nitric acid is added and mixed so that the nitric acid concentration is 1 to 2.5% by volume, followed by washing with water. And then dried, and further, after immersing the steel material in an etching solution in which picric acid alcohol and an aqueous solution of sodium pyrosulfite are mixed for 5 to 10 seconds, washing with water for 2 to 3 seconds and drying. Steel material etching method.