JP2010125483A - Method for detecting solidified structure of steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for detecting a solidified structure of steel, even of low carbon steel in which a solute element concentration in the steel is low, and especially a carbon concentration is ≤0.01 mass%, by identifying the solidified structure by corrosion. <P>SOLUTION: In the method for detecting the solidified structure of steel, a cross-section of a sample of a steel cast piece is polished, and the sample is heated at 40-90°C; then the polished surface of the sample is contacted to corrosive liquid, and the polished surface is corroded. Additionally, the corrosive liquid is heated at 40-90°C, and then the polished surface of the sample is contacted to the corrosive liquid. Even after the sample is contacted to the corrosive liquid, the corrosive liquid is not decreased in temperature, corrosion performance when using hot corrosive liquid is maintained in high level, electrochemical corrosion using potential difference by a difference in solute concentration is developed in a short period of time, and a clear solidified structure is identified. The temperature of the corrosive liquid is preferably -10 to -5°C, or +5°C or more to the temperature of the sample. <P>COPYRIGHT: (C)2010,JPO&INPIT

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 caused by a solute concentration difference at each position in a sample. The second is a chemical corrosion method utilizing 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. Examples of the second chemical corrosion method include observation of a pearlite structure using a chemical potential difference between Fe ₃ C and ferrite, and macro corrosion using a chemical potential difference depending on the surface orientation of coarse ferrite grains. 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.

  In general, in the electrochemical corrosion method using the potential difference due to the difference in solute concentration at each position in the sample, the electrochemical corrosion proceeds as the temperature of the corrosive liquid increases, and a clear solidified structure is revealed in a short time. can do. Non-Patent Document 1 also discloses a method in which a steel sample is brought into contact with a corrosive liquid after the temperature of the corrosive liquid is maintained at a high temperature in advance. However, if the temperature of the sample is low, such as room temperature, when the sample is immersed in the corrosive liquid, the sensible heat of the corrosive liquid is taken away by the sample, the temperature of the corrosive liquid decreases, and the capacity of the corrosive liquid decreases. It was difficult to obtain a clear coagulated tissue. In particular, when the sample is large, for example, when the cross-sectional cut sample of the continuous cast slab is polished and corroded as it is, this tendency is remarkable.

  On the other hand, after polishing the cross section of the steel slab sample, if the sample temperature is heated to a temperature of 40 to 90 ° C., and then the polished surface of the sample is contacted with a corrosive liquid, the polished surface is corroded. After that, the temperature of the corrosive liquid does not decrease, the corrosive ability when using a high temperature corrosive liquid is maintained at a high level, and the electrochemical corrosion utilizing the potential difference due to the difference in solute concentration proceeds in a short time and is clear It was found that the solidified tissue could be revealed.

That is, the gist of the present invention is as follows.
(1) Solidification of steel characterized by polishing a cross section of a sample of a steel slab, heating the sample temperature to 40 to 90 ° C., and then contacting the polished surface of the sample with a corrosive liquid to corrode the polished surface Tissue detection method.
(2) The method for detecting a solidified structure of steel according to (1) above, wherein the temperature of the corrosive liquid is heated to 40 to 90 ° C., and then the polished surface of the sample is brought into contact with the corrosive liquid.
(3) When the polished surface of the sample is brought into contact with the corrosive liquid, the temperature of the corrosive liquid is −10 to −5 ° C. with respect to the temperature of the sample. Of solidification structure of steel.
(4) The method for detecting a solidified structure of steel according to (3) above, wherein the specimen is immersed in a corrosive solution with the polished surface facing upward.
(5) The method for detecting a solidified structure of steel as described in (2) above, wherein when the polished surface of the sample is brought into contact with the corrosive liquid, the temperature of the corrosive liquid is set to + 5 ° C. or higher with respect to the temperature of the sample. .
(6) The method for detecting a solidified structure of steel according to (5) above, wherein the specimen is immersed in a corrosive solution with the polished surface facing downward.
(7) The method for detecting a solidified structure of steel as described in (6) above, wherein only a part including the polished surface of the sample is immersed in a corrosive liquid.
(8) After the polished surface of the sample is brought into contact with a corrosive liquid to corrode the polished surface, the sample is washed and dried, and polishing powder is embedded in the corroded holes on the corroded polished surface, and a transparent adhesive tape is applied to the polished surface. The steel according to any one of the above (1) to (7), wherein the steel powder is adhered 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 the mount. Coagulation tissue detection method.
(9) The method for detecting a solidified structure of steel according to any one of (1) to (8) above, wherein the steel slab has a carbon content of 0.01% by mass or less.

  In the present invention, since the sample temperature is heated to 40 to 90 ° C. and the polished surface of the sample is brought into contact with the corrosive liquid, it is difficult to detect a clear solidified structure in the past, particularly the carbon concentration. Even for a low carbon steel having a mass of 0.01% by mass or less, it is possible to provide a method of revealing a solidified structure by corrosion and thereby detecting the solidified structure of the steel.

  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.

  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, the sample that has been polished is heated to a temperature of 40 to 90 ° C. Thereafter, the polished surface of the heated sample is brought into contact with a corrosive liquid to corrode the polished surface. Since the sample is heated, the temperature of the corrosive liquid does not decrease even after the corrosive liquid comes into contact with the sample, the corrosive ability when the high temperature corrosive liquid is used is maintained at a high level, and the potential difference due to the difference in solute concentration Electrochemical corrosion using can proceed in a short time and reveal a clear solidified structure. If the heating temperature of the sample is too low, the difference from the solidified structure by the conventional corrosion method in which the sample is at room temperature is not clear, but if the heating temperature of the sample is 40 ° C. or higher, the sample at normal temperature that has been conventionally performed The difference from the solidification structure when using is clear. When the sample temperature is 70 ° C. or higher, more preferable results can be obtained. On the other hand, if the sample temperature is too high and the temperature is higher than 100 ° C., the corrosive liquid boils, bubbles are generated on the corroded surface, and the solidified structure becomes mottled. By setting the sample heating temperature to 90 ° C. or less, such a problem does not occur, and it is preferable from the viewpoint of ensuring safety when handling the sample at a high temperature. The method for heating the sample may be selected from a method in which the sample is immersed in a constant temperature water bath, a method in which the sample is placed in a low-temperature heating furnace, a method in which the sample is wrapped with a cloth-like electric heater, a method in which the sample is energized and heated, etc. it can.

  In the present invention, as described above, it is preferable to heat the sample before corrosion and simultaneously perform the corrosion while maintaining the temperature of the corrosive liquid at a high temperature. By heating the temperature of the corrosive liquid to 40 to 90 ° C. and then bringing the polished surface of the sample into contact with the corrosive liquid, a solidified structure can be suitably revealed. By setting the temperature of the corrosive liquid to 40 ° C. or higher, the effect of the present invention in which the temperature of the sample is set to 40 ° C. or higher can be fully enjoyed. More preferable results can be obtained when the temperature of the corrosive liquid is 70 ° C. or higher. Moreover, the boiling of a corrosive liquid can be prevented by making the temperature of a corrosive liquid into 90 degrees C or less.

  The effect of the present invention can be exhibited even when the temperature of the sample and the etchant when the polished surface of the sample is brought into contact with the etchant is the same temperature, but the temperature of both must be different by 5 ° C. or more. A better effect can be exhibited. If the difference between the sample temperature and the temperature of the corrosive liquid is 5 ° C or more, a large thermal convection that is driven by this temperature difference is generated in the corrosive liquid, and the agitation of the corrosive liquid is naturally promoted. This is because a highly corrosive liquid is always supplied to the corroded surface, and corrosion can be performed more efficiently and a clear solidified structure can be obtained.

  When the temperature of the corrosive liquid is lower than the sample temperature, if the temperature difference is too large, the corrosive liquid temperature becomes low and the corrosive ability cannot be fully exhibited. The range is −5 ° C. If the temperature of the corrosive liquid is higher than the sample temperature, the corrosive capacity will not be insufficient even if the temperature difference is large. Therefore, the temperature of the corrosive liquid should be + 5 ° C or higher with respect to the sample temperature, and the upper limit of the temperature difference Is not provided. By setting the sample temperature in the range of 40 to 90 ° C. and selecting the temperature of the corrosive liquid from the range of 40 to 90 ° C. which is a preferable range, and additionally setting the temperature difference between the sample and the corrosive liquid within the above preferable range, An excellent effect can be obtained.

  When the difference between the corrosion liquid temperature and the sample temperature is set, there is a suitable combination depending on whether the corrosion surface is directed upward or downward when the surface (corrosion surface) for detecting the solidified structure of the sample is brought into contact with the corrosion liquid. That is, since it is important to generate thermal convection in the corrosive liquid, when the sample temperature is higher than the corrosive liquid temperature, it is desirable to immerse the sample in the corrosive liquid with the corrosive surface of the sample facing upward. Conversely, when the sample temperature is lower than the corrosive liquid temperature, it is desirable to immerse the sample in the corrosive liquid with the corroded surface of the sample facing downward.

  In the present invention, the entire sample may be immersed in the corrosive liquid, but it is more preferable to perform corrosion by immersing only a part of the sample including the polished surface (corrosive surface) of the sample in the corrosive liquid. Can be obtained. In this case, it will be immersed in a corrosive liquid with the corroded surface down. By immersing only a part of the sample in the corrosive solution with the corroded surface facing down, a current loop is formed in preference to the corroded surface, and the waste generated on the corroded surface due to corrosion is easily removed and corrosive ability is improved. It is considered that high corrosive liquid is supplied to the corroded surface and the clarity of the solidified structure is improved. The immersion depth of the corroded surface in the corrosive liquid is preferably 10 mm or less.

  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 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 50 mm or 100 mm. As a result, the corroded surface has a height of 250 mm and a width of 500 to 700 mm. For the example of the present invention, the sample was heated before being immersed in the corrosive liquid, the sample temperatures were 40 ° C., 70 ° C., and 90 ° C., and then immersed in the corrosive liquid. The sample was heated by immersing the sample in a constant temperature water bath adjusted to a predetermined temperature in advance. About the comparative example, sample temperature was 25 degreeC and it immersed in the corrosive liquid.

  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 changed in the range of 25 to 90 ° C., and the corrosion time was 60 minutes.

  In the examples shown in Table 1, an 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 the polished surface of the sample was brought into contact with the corrosive liquid, for the most part of the examples, the entire sample was completely immersed in the corrosive liquid. When the corrosive liquid temperature was equal to or higher than the sample temperature, the corroded surface was directed downward, and when the corrosive liquid temperature was lower than the sample temperature, the corroded surface was directed upward. Thereby, the thermal convection resulting from the temperature difference between the corrosive liquid temperature and the sample temperature can be effectively utilized. In addition, when the temperature of the corrosive liquid is equal to or higher than the sample temperature, at some test levels, only the part including the polished surface of the sample is immersed downward in the corrosive liquid and the other part of the sample is corroded. Corrosion was performed under the condition that the liquid was not immersed. The level written as “whole” in the “immersion depth” column of Table 1 indicates that the entire sample is immersed in the corrosive solution, and a numerical value is entered in the “immersion depth” column. The level is a case where only a part including the polished surface of the sample is immersed, and the numerical value indicates the immersion depth at that time.

  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.

  First, the case where the whole sample is immersed in a corrosive liquid among the present invention examples and comparative examples shown in Table 1 will be described.

  Comparative Examples 1 to 12 are the results of corrosion with a corrosive liquid having the above composition having a sample temperature of 25 ° C. and a corrosive liquid temperature of 25 ° C., 40 ° C., 70 ° C., and 90 ° C. Originally, in 0.1 mass% C steel, in which the concentration difference due to segregation of solute elements during solidification is relatively large, the solidified structure can be detected to some extent by increasing the temperature of the corrosive solution. Thus, when the temperature of the corrosive liquid was 90 ° C., it was possible to clearly discriminate all of the inclination and interval of the dendritic structure, the degree of internal cracking and central segregation. On the other hand, in 0.01 mass% C steel and 0.001 mass% C steel, since the concentration difference due to segregation of solute elements during solidification is relatively small, the solidification structure tends to become clear by increasing the temperature of the corrosion solution. However, as shown in Comparative Examples 4 and 8, even when the temperature of the corrosive solution is 90 ° C., the presence of internal cracks and center segregation can be confirmed, but it is unclear and almost no dendritic structure can be detected. It was. When the temperature of the corrosive solution shown in Comparative Examples 1 and 5 was 25 ° C., the dendritic structure, internal cracks, and center segregation could not be detected. When the sample temperature is 25 ° C. (normal temperature), even if the temperature of the corrosive liquid is increased in advance, the temperature of the corrosive liquid rapidly decreases due to the sensible heat of the sample, and the corrosive capacity itself decreases. It is presumed that the solubility of picric acid drastically decreased as the temperature decreased, and excessive picric acid precipitated on the sample surface, which also prevented corrosion.

  On the other hand, in Examples 1 to 57 of the present invention in which corrosion was performed after heating the sample temperature to 40 to 90 ° C. in advance, the corrosive ability was maintained at a high level as compared with Comparative Examples 1 to 12, and thus all of the solidified structures. It has been found that the intelligibility is remarkably improved. For example, in the case of 0.1 mass% C steel that has a relatively large concentration difference due to segregation of solute elements during solidification, the temperature of the corrosive liquid is reduced by corrosion after heating the sample temperature to 40 to 90 ° C. in advance. Even at 25 ° C. (normal temperature), as shown in Invention Examples 39, 47, and 54, the inclination and interval of the dendritic structure, the degree of internal cracking, and the center segregation could be clearly distinguished. When the temperature of the corrosive liquid is further increased, as shown in Invention Examples 45, 46, 49, 50, 51, 52, 53, 55, 56, and 57, the inclination and interval of the dendritic structure, internal cracks and center segregation The degree of corrosion can be distinguished very clearly, and the corrosive solution temperature at which these can be distinguished very clearly becomes lower as the sample temperature becomes higher and the workability is improved. That is, when the sample temperature is 40 ° C, the corrosive liquid temperature is 70 ° C or higher, when the sample temperature is 70 ° C, the corrosive liquid temperature is 60 ° C or higher, and when the sample temperature is 90 ° C, the corrosive liquid temperature is 40 ° C or higher. If present, the solidified tissue could be distinguished very clearly. On the other hand, even in the case of 0.01 mass% C steel or 0.001 mass% C steel having a relatively small concentration difference due to segregation of solute elements during solidification, corrosion occurs after the sample temperature is heated to 40 to 90 ° C. in advance. The tendency for the solidified structure to become clear was similar. However, the inclination and interval of the dendritic structure, the clarity of internal cracks and center segregation are lower than those of 0.1 mass% C steel, and the temperature of the corrosive solution that makes it possible to distinguish these clearly is shifted to the high temperature side. .

  Inventive Example 4 is the result of corroding 0.001% C steel with a 40 ° C. corrosive solution at a sample temperature of 40 ° C., whereas Inventive Examples 1, 2, and 3 are 0.001% C This is the result of corroding steel with a corrosive solution of 25, 30, and 35 ° C. at a sample temperature of 40 ° C., respectively. In the present inventions 2 and 3, the center segregation can be clearly distinguished from the present invention example 4, and the effect of thermal convection due to the temperature difference between the sample temperature and the corrosive liquid compensates for the decrease in the corrosive ability due to the decrease in the corrosive liquid temperature. The effect of is recognized. However, when the temperature difference is 15 ° C. as in Example 1 of the present invention, a solidified structure having almost the same clarity as Example 4 of the present invention is obtained. It seems that it became more dominant than the effect of thermal convection due to the difference. Thus, when the temperature of the corrosive liquid is lower than the sample temperature and the difference between the sample temperature and the corrosive liquid temperature is set to about 5 to 10 ° C., the corrosion is promoted as compared with the case of the same temperature. When the temperature of the corrosive liquid is higher than the sample temperature, if the difference between the sample temperature and the corrosive liquid temperature is 5 ° C or higher, good results can be obtained in the range where the temperature of the corrosive liquid is 90 ° C or lower. .

  The above-mentioned effect due to the difference between the sample temperature and the corrosive solution temperature should be observed in the same way even in the case of 0.01 mass% C steel or 0.1 mass% C steel, or when the sample temperature is increased. Can be seen from Table 1. However, when the sample temperature is 90 ° C or when the solidified structure is clearly detected, such as 0.1% C steel, the effect is relatively small. In principle, there is no doubt that the effect of promoting corrosion by thermal convection due to the difference is obtained.

  Next, in the present invention example and comparative example shown in Table 1, the case where only a part of the sample including the polished surface is immersed in the corrosive liquid will be described.

  About Example 4, 4-1, and 4-2 of this invention, conditions other than the immersion method of a sample are made equivalent. Inventive Example 4 immerses the entire sample in the corrosive liquid, while Inventive Example 4-1 immerses the polished surface of the sample to a depth of 10 mm of the corrosive liquid, and Inventive Example 4-2 The polished surface is immersed to a depth of 5 mm of the corrosive liquid, and the other part of the sample is exposed from the corrosive liquid. As is clear from the comparison between Examples 4, 4-1, and 4-2 of the present invention, as the immersion depth of the sample is decreased, the solidified tissue appears more prominently, and the solidified tissue discrimination status is better. You can see that Invention Example 7, 7-1, 7-2, Invention Example 13, 13-1, 13-2, Invention Example 23, 23-1, 23-2, Invention Example 26, 26-1, 26- 2. The same applies to Invention Examples 32, 32-1, 32-2, and Invention Examples 42, 42-1, 42-2.

  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.

Claims (9)

  A method for detecting a solidified structure of steel, comprising: polishing a cross-section of a sample of a steel slab, heating the sample temperature to 40 to 90 ° C., and then contacting the polished surface of the sample with a corrosive liquid to corrode the polished surface .   The method for detecting a solidified structure of steel according to claim 1, wherein the temperature of the corrosive liquid is heated to 40 to 90 ° C, and then the polished surface of the sample is brought into contact with the corrosive liquid.   3. The solidification structure detection of steel according to claim 1, wherein when the polished surface of the sample is brought into contact with the corrosive liquid, the temperature of the corrosive liquid is −10 to −5 ° C. with respect to the temperature of the sample. Method.   The method for detecting a solidified structure of steel according to claim 3, wherein the sample is immersed in a corrosive solution with the polished surface facing upward.   The method for detecting a solidified structure of steel according to claim 2, wherein when the polished surface of the sample is brought into contact with the corrosive liquid, the temperature of the corrosive liquid is set to + 5 ° C or higher with respect to the temperature of the sample.   6. The method for detecting a solidified structure of steel according to claim 5, wherein the sample is immersed in a corrosive solution with the polished surface facing downward.   The method for detecting a solidified structure of steel according to claim 6, wherein only a part including the polished surface of the sample is immersed in a corrosive liquid.   After contacting the polishing surface of the sample with a corrosive liquid to corrode the polishing surface, the sample is washed and dried, and then the polishing powder is embedded in the corrosion holes on the corroded polishing surface, and a transparent adhesive tape is applied to the surface of the polishing surface. The method for detecting a solidified structure of steel according to any one of claims 1 to 7, wherein after the abrasive powder in the corrosion holes is adhered to the adhesive tape, the tape is peeled off, and then the tape is attached to the mount.   The method for detecting a solidified structure of steel according to any one of claims 1 to 8, wherein the carbon content of the steel slab is 0.01 mass% or less.