JP5448432B2 - Method for detecting solidification structure of steel - Google Patents

Description

  The present invention relates to a method for detecting a solidified structure of steel, and more particularly to a method for detecting a solidified structure of steel capable of revealing a solidified structure even in a low carbon steel having a carbon content of 0.01% by mass or less. is there.

  In the steel manufacturing process, detecting the solidification structure of the cast slab, which is a steel material after casting, evaluates internal defects such as macro segregation such as crack occurrence and center segregation of the slab, and guarantees quality in the subsequent process. Important to do. In addition, it is important to judge the presence or absence of abnormalities in the casting process and casting equipment from the occurrence of these internal defects in the slab, and to correct and maintain it in an appropriate state to prevent internal defects from occurring. . Furthermore, it is important to estimate the flow state of the internal molten steel during solidification and the cooling rate of the slab from the inclination and interval of the dendritic structure called dendrites in order to optimize the operating conditions.

The solidified structure of the slab can be observed by polishing the sample cross section of the slab, bringing the polished surface into contact with a corrosive liquid, and revealing the solidified structure. The manifestation of steel structure due to corrosion is roughly divided into two in principle. The first is an electrochemical corrosion method using a potential difference resulting from a solute concentration difference at each position in a sample. The second is a chemical corrosion method using a chemical potential difference of crystal grains depending on phases having different chemical potentials or crystal orientations on the surface. The first electrochemical corrosion method is used, for example, to detect dendritic structures, internal cracks, and center segregation using concentration differences due to segregation of solute elements during solidification. The second chemical corrosion method includes observation of a pearlite structure using the chemical potential difference between Fe ₃ C and ferrite, macro corrosion using the chemical potential difference depending on the surface orientation of coarse ferrite grains, and the like. Therefore, in order to reveal and detect the solidified structure of the slab by corrosion, it is necessary to suppress the second chemical corrosion and cause the first electrochemical corrosion.

  As a method for revealing the solidified structure of a slab, a method of corroding the surface of a sample using a corrosive liquid containing picric acid as a main component is generally implemented (Non-patent Document 1). In addition, an etch print method has been proposed as a method for recording the revealed solidified structure (Patent Documents 1 to 4). The etch print method is a method in which the polished surface of the sample is brought into contact with a corrosive solution to corrode the polished surface, and then the sample is washed and dried, and abrasive powder is embedded in the corrosive holes on the corroded polished surface, and the surface is transparent. In this method, an adhesive tape is applied, and the abrasive powder in the corrosion holes is adhered to the transparent adhesive tape, then the tape is peeled off, and then the tape is attached to a white mount. The abrasive powder embedded in the corrosion holes is transferred to the tape, and the tape is affixed on the mount, whereby a solidified structure is revealed on the mount.

Japanese Patent Publication No. 64-2212 JP 61-170581 A Japanese Patent Laid-Open No. 1-227943 JP-A-7-198565 Japan Iron and Steel Institute, 3rd edition Steel Handbook I Basics, page 205

  Regarding the method for revealing the solidification structure of the slab using the corrosive liquid mainly composed of picric acid described in Non-Patent Document 1, if the solute element concentration in the steel is not so low, solidification is in progress. Since the concentration difference due to segregation of solute elements is not small, a clear solidified structure can be revealed. On the other hand, the low solute element concentration in the steel, especially in the low carbon steel with a carbon concentration of 0.01% by mass or less, the concentration difference due to segregation of the solute element during solidification is small, so that the solidified structure is clearly revealed. It turned out to be difficult.

  The present invention has a low solute element concentration in steel, and it has been difficult to detect a solidified structure with a conventional structure, particularly low carbon steel having a carbon concentration of 0.01% by mass or less. It is an object of the present invention to provide a method of revealing a solidified structure by means of this and thereby detecting the solidified structure of steel.

  Conventionally, when the polished surface of a sample is corroded to reveal a solidified structure, the entire sample has been immersed in a corrosive liquid. Therefore, corrosion proceeds not only on the polished surface that reveals corrosion but also on other surfaces of the sample. On the other hand, by subjecting the surface other than the polished surface (corrosive surface) of the sample to electrical insulation treatment, the corrosion progresses only on the polished surface, so that it is difficult to reveal the solidified structure in the past, especially the carbon concentration. It was clarified that a solidified structure can be revealed even in a low carbon steel having a content of 0.01% by mass or less.

This invention is made | formed based on the said knowledge, The place made into the summary is as follows.
(1) A steel slab sample having a cross-section is polished, a surface other than the polished surface of the sample is electrically insulated , and a polished surface having a height of 200 to 500 mm and a width of 300 to 2100 mm is corroded. Coagulated tissue detection method.
(2) After corroding the polished surface of the sample, the sample is washed and dried, the abrasive powder is embedded in the corroded surface of the corroded polished surface, a transparent adhesive tape is applied to the surface of the polished surface, and the transparent adhesive tape is in the corroded hole. 2. The method for detecting a solidified structure of steel according to claim 1, wherein after the adhesive powder is adhered, the tape is peeled off, and then the tape is stuck on the mount.
(3) The method for detecting a solidified structure of steel according to claim 1 or 2, wherein the carbon content of the steel slab is 0.01% by mass or less.

  The present invention can detect a solidified structure that has been conventionally clear by polishing a cross section of a sample of a steel slab, electrically insulating a surface other than the polished surface of the sample, and corroding only the polished surface. Even for difficult varieties, particularly low carbon steel having a carbon concentration of 0.01% by mass or less, a solidified structure is revealed by corrosion, whereby the solidified structure of the steel can be detected.

  A sample is cut out from the slab to detect the solidified structure. Next, polishing is performed on the cross section of the sample where the solidified structure is to be detected. The polishing condition is preferably a finished surface of about # 240 to # 1000 after the cross section to be polished is planed and rough polished. The size of the sample is preferably such that the surface to be polished and corroded (corrosion surface) has a height of 200 to 500 mm, a width of 300 to 2100 mm, and a thickness of 50 to 200 mm. By setting it as such a size, it is possible to reveal the solidified structure within a range in which the sample can be easily handled and using a wide surface as a corroded surface.

  In the present invention, as shown in FIGS. 1A and 1B, the cross-section of the sample 1 of the steel slab, which is corroded, is polished to form the polished surface 2, and then the sample other than the polished surface 2 of the sample is used. The surface is electrically insulated to obtain an electrically insulated surface 3. The electrical insulation treatment can be performed by applying a resin electrical insulation paint to the surface of the sample. For example, a commercially available epoxy resin insulating lacquer or insulating varnish can be used as the resin electrical insulating paint.

  For example, an aqueous solution containing 20 g / liter of picric acid, 5 g / liter of cupric chloride, and 20 g / liter of a surfactant can be used as a corrosive liquid that reveals a solidified structure of steel. As the surfactant, for example, a commercially available product under the trade name Rypon F can be used.

  As described above, the sample obtained by electrically insulating the surface other than the polished surface is immersed in the corrosive liquid 4 in the corrosive liquid tank 5 as shown in FIG. Corrosion does not proceed on the surface other than the polished surface 2, and corrosion proceeds only on the polished surface 2. This makes it possible to reveal the solidified structure by corrosion even for varieties that have conventionally been difficult to detect a clear solidified structure, particularly low carbon steel having a carbon concentration of 0.01% by mass or less. The coagulated tissue can be detected.

  As described above, a surface other than the polished surface of the sample is electrically insulated and immersed in a corrosive solution, and the polished surface (corroded surface 2) is corroded, so that a current loop is formed in preference to the corroded surface 2. It is considered that the clarity of the solidified structure has been improved.

  In the steel solidification structure detection method targeted by the present invention, electrochemical corrosion using a potential difference due to a solute concentration difference at each position in a sample is efficiently promoted. The reason is that the electrochemical corrosion effective for detecting the solidified structure is a corrosion using a potential difference due to a difference in solute concentration at each position in the sample, and a current loop is formed at a portion where the solute concentration is high and a portion where the solute concentration is low. When a local battery reaction occurs, a solid and solid solidified structure is revealed.

At this time, when the sample is immersed in the corrosive liquid without electrically insulating the surface other than the polished surface of the sample as in the prior art, the surface other than the surface where the solidified structure is to be exposed (corrosion surface) is in a conductive state. Since current flows in addition to the corroded surface, the density of the solidified structure is blurred on the corroded surface.
On the other hand, in the method for detecting a solidified structure of steel according to the present invention, only the surface where the solidified structure is to be exposed is brought into direct contact with the corrosive liquid, and the local cell reaction, that is, the electrochemistry only on the surface where the solidified structure is to be exposed In order to efficiently promote the corrosion, the conventional solidification structure has been revealed, such as steel types with a relatively small concentration difference due to segregation of solute elements during solidification, especially low carbon steel with a carbon concentration of 0.01 mass% or less. A clear solidified structure can be revealed even with difficult steel types.

  After exposing the solidified structure to the corroded surface, record the solidified structure. It is good also as taking a direct photograph of the corroded surface which revealed the solidification structure | tissue by corrosion. More preferably, an etch print method can be used. In this method, the polished surface of the sample is brought into contact with a corrosive liquid to corrode the polished surface, then the sample is washed and dried, and abrasive powder is embedded in the corroded holes on the corroded polished surface, and the transparent adhesive tape is applied to the polished surface. Is applied, and the abrasive powder in the corrosion holes is adhered to the transparent adhesive tape, and then the tape is peeled off, and then the tape is attached to a white mount. The abrasive powder embedded in the corrosion holes is transferred to the tape, and the tape is affixed on the mount, so that the density of the abrasive powder transferred to the tape corresponds to the solidified structure. It is revealed in

  The method for detecting a solidified structure of steel according to the present invention can be applied to a wide range of steel components to reveal a solidified structure. In particular, it is also possible to detect a solidified structure by using the present invention for a component system in which it is difficult to reveal a solidified structure by a conventional method, that is, a low carbon steel having a carbon content of 0.01% by mass or less. This is preferable.

  The electrical insulation treatment on the surface other than the solidified tissue detection surface, which is an important constituent element of the present invention, can achieve the same effect as the application of the insulating paint even if it is performed by a method such as surrounding with an insulating tape. As the insulating tape, for example, a commercially available vinyl insulating tape or a polyester base insulating tape can be used. Further, the composition of the corrosive liquid is not limited to the above composition, and the same effect can be obtained if the corrosive liquid acts as an electrochemical corrosive liquid using a potential difference due to a solute concentration difference at each position in the sample. It is done.

  The present invention was applied using an ultra-low carbon steel for automobiles having a carbon concentration of 0.001 mass%, a low-carbon steel sheet for cold rolling of 0.01 mass%, and a medium carbon steel sheet for thick plates of 0.1 mass%. The size of the sample cut out from the slab was the full height in the slab, the half width in the width direction, and the thickness was 100 mm. As a result, the corroded surface was 250 mm high and 600 mm wide.

  The corroded surface of the sample was polished, and in the example of the present invention, the surface other than the corroded surface was electrically insulated by applying an electrically insulating paint or enclosing it with an electrically insulating tape. A commercially available epoxy resin insulation lacquer was used as the electrical insulation paint. A commercially available vinyl insulating tape was used as the electric insulating tape. A comparative example was also prepared in which surfaces other than the corroded surface were not electrically insulated.

  An aqueous solution containing 20 g / liter of picric acid, 5 g / liter of cupric chloride and 20 g / liter of surfactant was used as the corrosive liquid. As the surfactant, a commercial product under the trade name Raipon F was used. The temperature of the corrosive liquid was 25 ° C. The prepared sample was immersed in the corrosive solution with the corroded surface (polished surface) facing up to perform corrosion. The corrosion time was 60 minutes.

  The etch print method was used as a method for recording the solidified structure after corrosion. In this method, the polished surface of the sample is brought into contact with a corrosive liquid to corrode the polished surface, then the sample is washed and dried, and abrasive powder is embedded in the corroded holes on the corroded polished surface, and the transparent adhesive tape is applied to the polished surface Is applied, and the abrasive powder in the corrosion holes is adhered to the transparent adhesive tape, and then the tape is peeled off, and then the tape is attached to a white mount.

  Test conditions and evaluation results are shown in Table 1.

  When detecting the solidified structure, the center segregation, internal cracking, and dendritic structure were detected. ◎: Extremely clear, ◯: Clear, Δ: Existence could be confirmed but unclear, x: Existence itself could not be identified. The degree of detection of the solidified structure becomes difficult in the order of center segregation → internal crack → dendritic structure.

  In Comparative Examples 1 to 3 in which the electrical insulation treatment other than the corroded surface was not performed, the solidified structure was unclear. In Comparative Example 3 having a carbon concentration of 0.1% by mass, since the solute concentration difference due to solidification segregation is relatively large, the center segregation was detected to some extent clearly. However, the presence of internal cracks, dendritic gradients and intervals Although it can be confirmed, it was unclear. In Comparative Example 1 having a carbon concentration of 0.001% by mass and Comparative Example 2 having a carbon concentration of 0.01% by mass, the difference in solute concentration due to solidification segregation is relatively small. The existence itself could not be identified from cracks and center segregation.

  On the other hand, in Examples 1 to 6 of the present invention in which an insulating paint was applied to the surface other than the corroded surface, or an insulating tape was applied, the clarity of the solidified structure was greatly improved. In 0.1% C steel, as shown in Invention Examples 2, 3, 5 and 6, the inclination and interval of the dendritic structure, internal cracks, and center segregation can be clearly identified. Further, as shown in Examples 1 and 4 of the present invention, 0.001% C steel, which is most difficult to produce a solidified structure, the inclination and interval of the dendritic structure are somewhat unclear, but internal cracks and center segregation can be clearly distinguished. Improved to level.

  The present invention is not limited to the above-described embodiment, and can be changed without changing the gist of the present invention. For example, some or all of the above-described embodiments and modifications are possible. The method of detecting the solidification structure of steel of the present invention by combining the above is also included in the scope of the right of the present invention.

  As described above, the present invention is a steel type in which the concentration difference due to segregation of solute elements during solidification, in which the solidification structure is difficult to detect by a simple process, in particular, the carbon concentration is 0.01% by mass or less. Since the solidification structure of low carbon steel can be detected clearly, it is extremely useful in industry.

It is a figure which shows an example of the solidification structure | tissue detection method of the steel of this invention, (a) is the figure which looked at the sample which formed the electrical insulation processing surface from the grinding | polishing surface side, (b) formed the electrical insulation processing surface similarly It is the figure which looked at the sample from the opposite side of the polish side, and (c) is a figure showing the situation where the sample in which the electric insulation processing side was formed is immersed in a corrosive liquid.

Explanation of symbols

1 Sample 2 Polished surface (corrosion surface)
3 Electrical insulation treatment surface 4 Corrosion liquid 5 Corrosion liquid tank

Claims (3)

Solidified structure detection of steel characterized by polishing a cross-section of a steel slab sample, electrically insulating a surface other than the polished surface of the sample, and corroding a polished surface having a height of 200 to 500 mm and a width of 300 to 2100 mm Method.   After corroding the polished surface of the sample, the sample is washed and dried, the abrasive powder is embedded in the corroded holes on the corroded polished surface, a transparent adhesive tape is applied to the polished surface, and the abrasive powder in the corroded holes is attached to the transparent adhesive tape. 2. The method for detecting a solidified structure of steel according to claim 1, wherein the tape is peeled off after adhering the adhesive, and then the tape is stuck on the mount.   The method for detecting the solidified structure of steel according to claim 1 or 2, wherein the carbon content of the steel slab is 0.01 mass% or less.

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