JPH08253879A - Method for revealing small inclination grain boundary in steel material and corroding the same - Google Patents

Abstract

PURPOSE: To easily enable the simultaneous discrimination of small inclination grain boundaries, small inclination magnetic fields and large inclination grain boundaries in a steel structure by grinding a steel material, thereafter priviously corroding the same by hydrochloric acid, executing water washing and thereafter treating the same by a corroding soln. having a specified compsn. CONSTITUTION: A steel material is ground, is thereafter preliminarily corroded by 0.03 to 0.1 normal hydrochloric acid to remove away oxides or the like on the surface, and water washing and cleaning are executed. Next, it is immersed in a soln. obtd. by mixing 0.1 to 0.2 normal oxalic acid, an aq. of hydrogen peroxide contg., by weight, 30 to 40% hydrogen peroxide, 0.1 to 0.2 normal sulfuric acid and methanol or ethanol in such a manner that the ratios of oxalic acid: aq. of hydrogen peroxide: sulfuric acid: methanol or ethanol are regulated to (45 to 55):(45 to 55):(7 to 18): 1 for 3 to 20sec to corrode the ground surface of the steel material. By observing the corroded face by an optical microscope and a scanning type electron microscope, small inclination grain boundaries, small inclination magnetic fields and large inclination grain boundaries in the structure can simultaneously be discriminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The method of the present invention is a corrosion method that clearly reveals a small tilt grain boundary of a steel material and a corrosion method that can easily distinguish between a small tilt grain boundary and a large tilt grain boundary on the same sample. It is about.

[0002]

2. Description of the Related Art A crystal grain boundary found in a metal structure of a metal material is a boundary between two crystal grains in a metal structure having different crystal orientations. The crystal grain size and crystal grain morphology are extremely important information for clarifying the mechanical properties of metals. Until now, the polished mirror surface of metal materials has been corroded with acid, and observation with an optical microscope or scanning electron microscope or observation of thin film metal materials with a transmission electron microscope has been performed by various methods. I've been told.

For example, in a material such as a ferrous material (ferrite (α) austenite (γ) two-phase rolled material), dislocations introduced in crystal grains move and collect, and nital corrosion which is a usual corrosive method of steel material. In the case of Picral corrosion, a grain boundary between crystal grains having a small difference in crystal orientation of 5 to 15 ° which is not revealed, that is, a so-called small tilt grain boundary may be formed. On the other hand, the difference in crystal orientation between the separated crystal grains is called a large-angle grain boundary when the grain boundary is larger than about 15 °.

Sub-grains separated by small-angle grain boundaries have different contributions to the materials from ordinary crystal grains separated by large-angle boundaries, and are hardly revealed by ordinary corrosion methods. In order to precisely investigate the controlling factors of the material such as the two-phase rolled material, it is necessary to adjust the thin film sample of the transmission electron microscope and the special scanning electron microscope using the electron beam with a small beam diameter. It was necessary to detect the low-angle grain boundaries by the crystal orientation analysis.

Further, as a simple method for identifying a low-angle grain boundary, Metalography 14 (198)
1) p. 61 and Metal progress
75 (1958) p. A method of observing a polished surface of a steel material by observing it by a so-called Marshall's reagent, which is shown in the literature such as 96, has been adopted. In these methods, the corroded surface was not uniformly corroded, and unevenness and air bubbles were easily generated, so that the small tilt grain boundaries were slightly unclear.

[0006] These conventional thin-film samples can be observed with a transmission electron microscope, and the polished mirror surface of a steel material sample can be observed by being corroded by Marshall's reagent. The identification method is troublesome, and the small-angle grain boundaries that are exposed are somewhat unclear, and the corrosion state is not stable, so there are problems such as bubbles and uneven corrosion on the corroded surface of the same sample. there were.

Further, since the grain size delimited by the low-angle grain boundary and the grain size delimited by the high-angle grain boundary have different contributions to the mechanical properties, some properties such as fatigue properties and deformation behavior of steel materials are The survey needs to distinguish and identify this. The conventional method of observing a thin film with a transmission electron microscope is very troublesome because it requires a diffraction pattern analysis for each crystal grain. Also, when observing a corroded surface using Marshall's reagent, a large inclination grain boundary and a small inclination angle are observed. Since the grain boundaries are similarly corroded and indistinguishable on one sample, there is a problem that comparison with nital corrosion is necessary.

[0008]

The problems to be solved by the present invention have been impossible in the prior arts (1) Clear and clear grain boundaries with small tilt angles in which no bubbles or corrosion unevenness occur on the corroded surface. (2) It is an object of the present invention to provide a simple observation method capable of simultaneously discriminating a high-angle grain boundary and a low-angle grain boundary on one sample, which is impossible by the conventional method and the observation method.

[0009]

Means for Solving the Problems The present invention has been made to solve the above problems. The means are: (1) a steel material is polished and pre-corroded with 0.03 to 0.1N hydrochloric acid. After that, it is washed with water to form a hydrogen peroxide solution containing 0.1 to 0.2N oxalic acid and 30 to 40% by weight of hydrogen peroxide, and 0.1 to 0.2N sulfuric acid and methanol or methanol. 3% by volume of the alternative ethanol in a mixed solution of oxalic acid: hydrogen peroxide: sulfuric acid: methanol or ethanol = 45-55: 45-55: 7-18: 1 (volume% ratio). A corrosion method in which a low-angle grain boundary of a steel material is clearly revealed by immersion for 20 seconds.

(2) The steel material is polished, and first, the portion of the crystal grain surrounded by the high-angle grain boundaries is corroded to reduce the thickness more strongly, and then the sample surface is subjected to 0.03 to 0.1N hydrochloric acid. After corroding to remove the film, it is washed with water and washed with 0.1 to 0.
2% oxalic acid and hydrogen peroxide containing 30-40% by weight of hydrogen peroxide, 0.1-0.2N sulfuric acid and methanol or ethanol replacing methanol at a volume% ratio of oxalic acid: peroxide Hydrogen oxide water: sulfuric acid: methanol or ethanol = 45 to 55:45 to 55: 7 to 18: 1 (volume% ratio). This is a corrosion method that clearly distinguishes the low-angle grain boundaries on the same sample.

[0011]

Next, the function of each procedure in the method of the present invention and the reason for defining each condition of claim items will be described. (1) The reason for defining each condition of claim 1 of the present invention, and the reason for preliminarily corroding a polished steel material with 0.03-0.1N hydrochloric acid will be described.

The reason for pre-corrosion with 0.03 to 0.1N hydrochloric acid is to remove a film of oxide or the like on the surface of the polished surface which is left after polishing. The reason why the normality of hydrochloric acid is set to 0.03 to 0.1 is that below this, corrosion due to hydrochloric acid is too slow to remove the film on the surface, and above this, the progress of corrosion is too rapid. This is because the later corrosion will not be good. The reason for subsequent washing with water is that hydrochloric acid does not excessively corrode the surface of the steel material sample. Further continuously, a hydrogen peroxide solution containing 0.1 to 0.2 normal oxalic acid and 30 to 40% by weight of hydrogen peroxide,
0.1-0.2 N sulfuric acid and methanol or ethanol in place of methanol in a volume ratio of oxalic acid: hydrogen peroxide: sulfuric acid: methanol or ethanol = 45-55: 45-55: 7-18: 1 ( The reason for immersing in a mixed solution mixed for 3 to 20 seconds will be described.

Unless hydrogen peroxide solution having a hydrogen peroxide concentration of 30% or more is added to this mixed solution, the phenomenon that only the grain boundaries are corroded does not occur. The reason for this is that in a hydrogen peroxide solution with a lower concentration than this, the portion of the crystal grains surrounded by the grain boundaries is oxidized by the hydrogen peroxide solution to protect the film, and the effect of stalling the corrosion by oxalic acid and sulfuric acid. Is not likely to occur enough. 40% hydrogen peroxide
When hydrogen peroxide water having the above concentration is added, corrosion tends to be unclear. This is considered to be because, in a hydrogen peroxide solution having a high concentration, a thick film is formed even at the grain boundary portion due to oxidation, and corrosion of the grain boundary is less likely to occur. It was also found that 0.1-0.2 normal oxalic acid and 0.1-0.2 normal sulfuric acid are particularly effective in selectively corroding grain boundaries. By using these at the same time,
It is a relatively weak acid that limits the locations of the grain boundaries on steel samples covered with a hydrogen peroxide solution on the grain boundaries with many selective lattice defects due to the effect of sulfuric acid reduction. It seems that oxalic acid has an effect of moderately corroding the grain boundaries.

The range of the volume% ratio of each simple substance in the mixed solution was determined by repeating the experiment to find a range in which the effect of selectively corroding the grain boundaries was remarkable. As a result, the high-angle grain boundaries and the low-angle grain boundaries can be corroded into distinct groove shapes. If the content of sulfuric acid is 0.1 N or less, the effect of selectively corroding the grain boundaries is not clearly recognized, and the corroded surface becomes unclear. Further, when a sulfuric acid having a concentration of 0.2 N or more is mixed, the film inside the crystal grains surrounded by the grain boundaries is also corroded, and the portions other than the grain boundaries are corroded. The reason why the normality of oxalic acid is set to 0.1 to 0.2 is that the grain boundary cannot be clearly corroded below this, and the grain boundary corrosion is too strong and the corroded surface is not good above this. Because.

Next, the reason for adding methanol or ethanol instead of methanol will be described. The inventors of the present invention have proposed a conventional corrosion method using Marshall's reagent.
It was found that the appearance of the low-angle grain boundaries is somewhat unclear, and it is effective to improve the wettability on the polished surface of the steel material in order to reveal the low-angle grain boundaries clearly and evenly. . However, with reagents that are often used as surfactants, such as aqueous solutions of linear alkyl sulfate sodium salt,
It was also found that it cannot be used at the same time as an acid mixture of oxalic acid and sulfuric acid because smoke is generated by the reaction between sulfuric acid and an alkyl group in the corrosive liquid and corrosion does not proceed. However, as a result of investigating the use of various liquids in the research, it was found that the addition of an alcohol having a relatively small molecular weight compared to a straight chain alkyl sulfate sodium salt aqueous solution such as methanol or ethanol improves the wettability. Clarified that it is effective. Methanol or ethanol is mixed to ensure even surface corrosion. If this addition of methanol or ethanol is omitted, bubbles and corrosion unevenness are likely to occur on the corroded surface, making it difficult to stably corrode the observation surface. If the volume ratio is more than the specified value, the concentration of the acid becomes thin, and if it is less than this, the effect of improving the wettability becomes low.

Next, for the reason that each condition of claim 2 of the present invention is defined, it is necessary to perform the corrosion of the polished steel material so that the portion of the crystal grain surrounded by the high-angle grain boundary is more strongly thinned. There is a reason for this. Corrosion that strongly reduces the thickness of the crystal grains surrounded by the high-angle grain boundaries is, for example, 7 to 20 in 0.1 to 0.2 normal nitric acid ethanol solution.
Corrosion method of soaking for about 2 seconds (so-called Nital corrosion method)
Alternatively, a corrosion method of dipping in an aqueous solution of 2,4,6-trinitrophenol of 0.1 to 0.2 normal for about 7 to 20 seconds (so-called picric acid corrosion method), or 0.1 to 0.2 normal A portion of crystal grains surrounded by a high-angle grain boundary of a metal structure of a steel material, such as a corrosion method of dipping in an ethanol solution of 2,4,6-trinitrophenol for about 7 to 20 seconds (so-called Picral corrosion method) Is more strongly corroded, and refers to all of the corrosion methods that have the effect of reducing the thickness of the crystal grain portion in a concave shape with respect to the high-angle grain boundary.

The inventors of the present invention observed and studied the state of the corroded surface in the conventional corrosion method, and found that the corrosion methods such as the so-called Nital corrosion method, picric acid corrosion method, and picral corrosion method have a large inclination grain of the steel material structure. It can be found that the grain boundaries surrounded by the boundaries are corroded more strongly, and the grain boundaries of the large-angle boundaries are concavely thinned to clarify the large-angle boundaries. It was Furthermore, it was found that with these corrosion methods, only the large-angle grain boundaries were clear, and the small-angle grain boundaries were hardly exposed. On the other hand, it was found that the large-angle grain boundary and the small-angle grain boundary both corrode in a groove shape in the corrosion method described in claim 1. The inventors of the present invention more strongly corrode the part of the crystal grain surrounded by the high-angle grain boundary of the steel material structure by any of these corrosion methods, and reduce the part of the crystal grain concave to the high-angle grain boundary. Then, the high-angle grain boundary is also corroded by the corrosion method described in claim 1, and the low-angle grain boundary of the crystal grain portion (concave portion) surrounded by the high-angle grain boundary is corroded like a groove. A corrosion method has been developed in which a large-angle grain boundary and a small-angle grain boundary can be simultaneously distinguished and observed by performing the above-described corrosion, and this method is defined as claim 2 of the present invention.

Next, the reason why corrosion is performed to remove the film on the surface of the sample with 0.03 to 0.1N hydrochloric acid will be described. The reason for corroding with 0.03 to 0.1N hydrochloric acid to remove the film on the surface of the sample is to remove the film due to oxides and the like on the surface of the polished surface by leaving it after polishing.
The reason for washing with water after corroding with 0.03 to 0.1N hydrochloric acid is to prevent the surface of the steel material sample from being corroded excessively by hydrochloric acid. Furthermore, 0.1 to 0.2 normal oxalic acid and weight%
Hydrogen peroxide solution containing 30 to 40% hydrogen peroxide in a ratio,
0.1-0.2 N sulfuric acid and methanol or ethanol in place of methanol in a volume ratio of oxalic acid: hydrogen peroxide: sulfuric acid: methanol or ethanol = 45-55: 45-55: 7-18: 1 ( The reason for immersing in a mixed solution mixed for 3 to 20 seconds will be described.

Unless hydrogen peroxide solution having a concentration of hydrogen peroxide of 30% or more is added to this mixed solution, the small tilt angle in the portion of the crystal grain which is surrounded by the large tilt grain boundary and is thinned in a concave shape. The phenomenon that only the grain boundaries are corroded does not occur. Also, when hydrogen peroxide solution with a concentration of hydrogen peroxide of 40% or more is added,
Corrosion of low-angle grain boundaries tends to be unclear. Also,
0.1-0.2 normal oxalic acid and 0.1-0.2 normal sulfuric acid,
In particular, it is a corrosive solution effective for selectively corroding grain boundaries.
As a result, it is possible to corrode the small-angle grain boundaries in the recessed grain boundaries surrounded by the large-angle grain boundaries into a groove shape. The range of the volume% ratio of each simple substance in the mixed solution is
By repeating the experiment, the remarkable range of the effect of selectively corroding the grain boundaries was determined and determined. Methanol or ethanol is mixed to evenly corrode the surface. If this addition of methanol or ethanol is omitted, bubbles and corrosion unevenness are likely to occur on the corroded surface, making it difficult to stably corrode the observation surface. By carrying out all the steps defined in claim 2 of the present invention according to the specified order, the portion of the crystal grain surrounded by the high-angle grain boundaries can be identified by the wall thickness reduction due to the concave corrosion, and at the same time, The low-angle grain boundaries are identifiable as corroded troughs in the portion of the crystal grain surrounded by the high-angle grain boundaries.

[0020]

EXAMPLES Next, examples of the method of the present invention will be described. Table 1
The steel grades of the components shown in 1 above were used as the test steel. Two-phase region processing with each of the sample steels A to J shown in Table 1 (sample steel D,
E is heated in the austenite region at 1100 ° C and then allowed to cool,
By the actual rolling by the rolling equipment of the factory, 30% of the two-phase area cumulative processing at 650 ° C. was performed, and then it was allowed to cool. A to C, F to J
The test steel of No. 2 is a cylindrical small test piece of 8 mmφ x 12 mm height, heated at 1100 ° C, and then cooled at 0.2 ° C / s at 650 ° C
After 30% under a pressure of 1 pass, it was cooled to room temperature at 0.2 ° C / s. ) Was performed to prepare a sample having a large tilt grain boundary and a small tilt grain boundary at the same time.

[0021]

[Table 1]

Table 2 shows the corrosion method corresponding to claim 1 of the present invention, and Table 3 shows the results of corrosion of the test steels prepared above by the comparative method. Similarly, Table 4 shows the corrosion method corresponding to claim 2 of the present invention, and Table 5 shows the results of corrosion by the comparative method. Since all of Examples 1 to 10 in Table 2 are in accordance with the method of the present invention, the developed structure could very clearly express the low-angle grain boundaries. However, in Comparative Example 1 of Table 3, 0.05N
Preliminary corrosion with hydrochloric acid was omitted, and the removal of the film was not sufficient, so the low-angle grain boundaries were unclear. Comparative Example 2
No water washing after pre-corrosion was omitted, uneven corrosion occurred and the structure was partially blurred. Comparative Example 3,
In Nos. 4, 9 and 10, the mixing ratio of the mixed corrosive liquid deviated from the regulation, and the acid corrosion was so strong that the grain boundaries were bleeding. Comparative Examples 5, 7 and 8 are also examples in which the mixing ratio of the mixed corrosive liquid deviates from the regulation, but the corrosive power of the acid is weak and the corrosion of the grain boundaries was unclear. Comparative Example 4 was an example in which alcohol was not mixed, and pits were formed on the corroded surface.

[0023]

[Table 2]

[0024]

[Table 3]

Incidentally, FIG. 1 is a photograph of the corroded surface of the steel sample obtained in Example 4 of Table 2 (scanning electron microscope secondary electron image),
2 and FIG. 2, the same portion of the same sample was further repolished, and then corroded with 0.15N nitric acid ethanol solution (nital corrosion) for 15 seconds. The corroded surface was taken at the same magnification as in FIG. 2 (scanning type). Electron microscope secondary electron image) is shown.
From these, it can be seen that the grain boundaries of the crystals are corroded in a groove shape, and that the portion surrounded by the crystal grain boundaries is corroded by thinning in the conventional nital corrosion method. Also, it can be seen that it is possible to reveal a finer structure that could not be revealed by the Nital method.

[0026]

[Table 4]

[0027]

[Table 5]

The samples corroded in Examples 1 to 10 in Table 4 were able to discriminate between large-angle tilt boundaries and small-angle tilt boundaries, and had extremely clear corrosion surfaces. However, Comparative Example 1 in Table 5
The sample No. 1 omits preliminary corrosion with 0.05 N hydrochloric acid, and the film is not sufficiently removed, so that the large-angle tilt boundaries are clearly recognized, but the small-angle tilt boundaries are unclear. In Comparative Example 2, washing with water after pre-corrosion was omitted, and corrosion unevenness occurred at the low-angle grain boundaries, and the structure was partially unclear. In Comparative Example 3, the mixing ratio of the mixed corrosive liquid deviated from the regulation, and the acid corrosion was so strong that the small-angle tilt grain boundary was bleeding. Comparative Example 5 is also an example in which the mixing ratio of the mixed corrosive liquid deviates from the regulation, but it is an example in which the corrosive force of acid is weak, and the corrosion at the low-angle tilt grain boundary was unclear. Comparative Example 4 was an example in which alcohol was not mixed, and pits were formed on the corroded surface. Comparative Examples 6 to 10 are examples in which the small-angle grain boundaries could not be clarified by the subsequent corrosion because the corrosion for revealing the large-angle grain boundaries was performed with the acid having a high normality. .

Incidentally, FIG. 3 is a photograph of the corroded surface corroded for 15 seconds with 0.15N ethanolic ethanol solution (nital corrosion) after the corroded surface of the steel sample was further polished (scanning electron microscope secondary electron image). ), And the same portion of the same sample shown in FIG.
The photograph (scanning electron microscope secondary electron image) of the corroded surface obtained in 1. is shown. From these, it can be seen that, in addition to the convex large-angle grain boundaries revealed by the conventional Nital corrosion method, the small-angle grain boundaries are corroded in a groove shape and can be distinguished at the same time.

[0030]

EFFECT OF THE INVENTION The corrosion method of the present invention is a method of revealing the metallographic structure of a steel material, and by observing the corroded surface with an optical microscope and a scanning electron microscope, it is possible to obtain a small tilt angle boundary and a small tilt angle boundary. Corrosion method that can easily identify tilt angle boundaries on the same sample by uniformly and clearly revealing small tilt boundaries of steel materials and simultaneously identifying large tilt boundaries and small tilt boundaries It is possible to obtain a plane, which makes it possible to use a transmission electron microscope (TEM), etc. as in the past, without using a method such as crystal orientation analysis that requires special skill, and simply and in a short time. It is now possible to confirm small-angle tilt boundaries and simultaneously identify large-angle tilt boundaries and small-angle tilt boundaries with an inexpensive and easy-to-use device such as an optical microscope.

[Brief description of drawings]

FIG. 1 is a photograph showing a corroded surface of a steel sample metal structure of Example 4 corresponding to claim 1 of the present invention, which was taken as a secondary electron image by a scanning electron microscope.

FIG. 2 is a photograph of a corroded surface of a steel sample metal structure obtained by a conventional nital corrosion method, which is taken as a secondary electron image by a scanning electron microscope.

FIG. 3 is a photograph of a corroded surface of a metal structure of a steel sample obtained by a conventional nital corrosion method, which is taken as a scanning electron microscope secondary electron image.

FIG. 4 is a photograph of a corroded surface of a steel microstructure of a steel sample of Example 4 corresponding to claim 2 of the present invention, which was taken as a scanning electron microscope secondary electron image.

Front page continuation (72) Inventor Tadanori Yakushinji 1st Nishinosu, Oita-shi, Oita Pref. Nippon Steel Co., Ltd. Oita Works (72) Inventor Kazuo Saeki 1st Nishinosu, Oita-shi, Oita New Nippon Steel Oita Steel Co., Ltd. (72) Minor Fujiwara Minoru Fujiwara, Oita City, Oita Prefecture 1 Nishinosu, Nippon Steel Co., Ltd. Oita Steel Co., Ltd. (72) Inventor, Shuichi Oita City, Oita Prefecture Nishinosu 1 No. 1 Nippon Steel Oita Steel Co., Ltd. (72) Inventor Toshinaga Hasegawa 1st Nishinosu, Oita City, Oita Prefecture Shin Nippon Steel Co., Ltd. Oita Steel Co., Ltd.

Claims (2)

[Claims] 1. A steel material is polished to 0.03 to 0.
After pre-corrosion with 1N hydrochloric acid, wash with water and 0.1-0.2
A mixture of hydrogen peroxide solution containing normal oxalic acid and 30 to 40% by weight of hydrogen peroxide, 0.1 to 0.2 normal sulfuric acid and methanol or ethanol instead of methanol in the following volume ratio. A low-angle grain boundary-exposing corrosion method for a steel material, comprising immersing in a solution for 3 to 20 seconds. Oxalic acid: Hydrogen peroxide water: Sulfuric acid: Methanol or ethanol = 45-55: 45-55: 7-18: 1 (volume% ratio) 2. A steel material is polished, and first, corrosion is performed to reduce the thickness of the crystal grains surrounded by the high-angle grain boundaries to expose the high-angle grain boundaries, and then 0.03 to 0 is added. 1. After corroding with 1N hydrochloric acid to remove the sample surface film, wash with water,
A volume of hydrogen peroxide solution containing 0.1 to 0.2 normal oxalic acid and 30 to 40% by weight of hydrogen peroxide, 0.1 to 0.2 normal sulfuric acid and methanol or ethanol replacing methanol is used in the following volume. % Of 3 to 3 in the mixed mixed solution.
A low-angle tilt grain boundary emergence corrosion method for a steel material, characterized by immersing for 20 seconds. Oxalic acid: Hydrogen peroxide water: Sulfuric acid: Methanol or ethanol = 45-55: 45-55: 7-18: 1 (volume% ratio)