JP2010008090A - Analysis method of cao-containing inclusion in steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method capable of performing accurately quantitative analysis of a CaO-containing inclusion in a steel. <P>SOLUTION: A steel sample subjected beforehand to solution treatment at 1,150-1,350°C as long as 10 minutes or longer is dipped into an electrolytic solution containing ferrous chloride, potassium hydroxide and 0.2 w/v% or more antioxidant and having pH of 4.5-6.5, and constant-current electrolysis is performed, and an acquired residue is subjected to ultrasonic vibration as long as 4 minutes or longer, and then quantitative analysis of the CaO-containing inclusion is performed. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a useful analysis method for CaO-containing inclusions present in steel samples.

  With the recent demand for high cleanliness steel, there is a need to reduce CaO-containing inclusions that are likely to cause cracks during steel plate processing, for example. Although techniques for reducing CaO-containing inclusions at the time of manufacture are also progressing, if the trace amount of CaO-containing inclusions in steel still present cannot be accurately evaluated, the extent of CaO-containing inclusion reduction by the above technique, and It is difficult to confirm the improvement effect of various characteristics due to inclusion reduction, and it is difficult to guarantee the quality of the final product.

  Therefore, in order to reduce CaO-containing inclusions that are harmful to the product, an evaluation technique for confirming the improvement effect is required together with the improvement of the manufacturing process.

  Conventionally, in the analysis of non-metallic inclusions such as CaO-containing inclusions, representative methods for extracting the inclusions include acid decomposition method, halogen dissolution method, non-aqueous solvent electrolysis method and constant current electrolysis method. (Slime method).

  The acid decomposition method is a method in which the iron matrix of a steel sample is dissolved in an aqueous solution of sulfuric acid, nitric acid or a mixed acid thereof heated to about 85 to 90 ° C., and the composition and size of inclusions remaining as a residue are measured. is there. This method is relatively easy to operate, and since the amount of carbides and iron hydroxide present in the residue with the inclusions is small, it is relatively easy to observe and measure the inclusions with a microscope, an X-ray spectroscopic analyzer, or the like. It has the feature of being easy.

However, although the above acid decomposition method is suitable for quantification of chemically stable inclusions such as Al ₂ O ₃ , a part of it is applied to inclusions containing CaO which is unstable to acids. Or since all will melt | dissolve at the time of extraction, there exists a problem that the composition and size of a CaO containing inclusion cannot be grasped | ascertained accurately.

  Examples of the halogen dissolution method include an iodine-methanol method and a bromine-methanol method. Examples of the nonaqueous solvent electrolysis method include a method using a nonaqueous solvent such as acetylacetone-tetramethylammonium chloride-methanol system or methyl salicylate-tetramethylammonium chloride-methanol system.

  Since the halogen dissolution method and the nonaqueous solvent electrolysis method are unlikely to cause inclusions to dissolve in the solvent and change its composition and size as in the case of the acid decomposition method, -It has excellent extraction accuracy in that it can be extracted with almost no defects. However, halogens such as iodine-methanol and non-aqueous solvents such as acetylacetone and methyl salicylate have a very low solubility of iron ions, so that a large amount of steel samples cannot be dissolved. The amount dissolved is only about 1 to 5 g. Therefore, when the above method is applied to clean steel, it is difficult to ensure the amount of inclusions that can be accurately quantified, which is not suitable for evaluation of inclusions in clean steel.

An example of the constant current electrolysis method is a slime method using an aqueous solvent. In this method, ferrous chloride (FeCl ₂ ) aqueous solution is used as an electrolytic solution to dissolve the iron matrix in the steel sample, and the inclusions remaining as residues are evaluated using a microscope, an X-ray spectroscopic analyzer, or the like. Is to do. The feature of this method is that, unlike the halogen dissolution method and the non-aqueous solvent electrolysis method, it is possible to use a steel sample in the order of kg and extract chemically unstable inclusions such as CaO-containing inclusions. Therefore, reliable data can be obtained even with clean steel with a small amount of inclusions in the steel.

  However, in the conventional slime method, the chemically unstable inclusions can be extracted, but at the same time, a large amount of iron hydroxide or carbide remains as a residue, and subsequent quantitative analysis of the inclusions becomes difficult. There was a problem such as. Although the slime method is suitable for the inclusion analysis of clean steel with a small amount of inclusions, it has not been used effectively until now because of the above-mentioned problems. It is a fact.

  Various techniques for improving the slime method have been proposed so far. For example, Patent Document 1 discloses a method capable of accurately performing quantitative analysis and / or particle size distribution measurement of non-metallic inclusions in steel, particularly CaO-containing inclusions. Specifically, it is obtained by subjecting a steel sample previously subjected to solution treatment at 800 ° C. to 1100 ° C. for 3 to 10 minutes to constant current electrolysis in a ferrous chloride aqueous solution adjusted to pH 5 to 7. The inclusion of CaO-containing inclusions for quantitative analysis and / or particle size distribution measurement is described.

  In Patent Document 2, as a method for extracting nonmetallic inclusions used for measurement of the composition and / or particle size of nonmetallic inclusions by laser excitation-ICP analysis, 800 μl of the metallic sample is dissolved prior to dissolution of the metal sample. It shows that constant current electrolysis is performed in an aqueous ferrous chloride solution after a solution treatment at 3 ° C. or higher for 3 minutes or more.

  However, the solution treatment performed by these methods has a temperature of about 800 ° C. to 1100 ° C., and particularly when high carbon steel is targeted, solution treatment becomes insufficient. A huge carbide or the like remains as a residue, and inclusions cannot be quantitatively analyzed. Further, the solution treatment time is about 3 to 10 minutes. However, the solution treatment is not sufficient in this time, and the above-mentioned problems may occur. Furthermore, there are cases where inclusions and unnecessary residues are mixed as residues after constant-current electrolysis, but the above method has not been studied until the separation of inclusions and unnecessary residues.

  In Patent Document 3, the influence of carbides that inhibit the quantification of inclusions is suppressed to a minimum, and chemically unstable CaO-containing inclusions are extracted without loss or loss, so that CaO is contained in clean steel. As a method for accurately quantifying inclusions, a metal material previously subjected to solution treatment at a temperature of 800 ° C. or higher is subjected to constant current electrolysis in an aqueous ferrous chloride solution or a halogen organic solvent, and then acid-treated with a weak acid aqueous solution. It is shown that the CaO-containing inclusions obtained by doing so are subjected to quantitative analysis and / or particle size distribution measurement.

  In this method, weak acid treatment is used as a method for separating inclusions and residues. However, it is impossible to completely separate both, and it requires labor and time. Difficult to apply to. Furthermore, it is not shown about the concrete method of pH control at the time of constant current electrolysis.

  Patent Document 4 discloses an oxide system in steel by an electrolytic method for the purpose of shortening the purification treatment time after electrolytic extraction of oxide inclusions in steel and improving the extraction rate of oxide inclusions. Inclusion extraction methods have been proposed. Specifically, the pH of the electrolytic solution used for constant current electrolysis is controlled to 5.5 to 7.2, and the amount of iron hydroxide in the solution is reduced by continuously supplying and discharging the electrolytic solution, A method for extracting oxide inclusions is shown. However, since sodium chloride is used in this method, for example, when analyzing Na in inclusions containing Na derived from powder or the like used in the steel manufacturing process, whether Na is derived from the above powder or an analytical reagent. There is a problem that it is unclear. Also in this method, a method for separating inclusions and unnecessary residues after constant current electrolysis is not shown.

Patent Document 5 describes that inclusion residue removal by ultrasonic vibration is performed after electrolysis of a steel sample. However, the above method relates to a non-aqueous solvent constant current electrolysis method, and as described above, a large amount of steel samples cannot be dissolved. When applied to clean steel, the amount of inclusions that can be accurately quantified It is difficult to secure. The ultrasonic vibration is 180 seconds at the longest, but this is not sufficient. Furthermore, although the solution treatment is carried out at 700 ° C. for 20 hours, there is a problem that the solution treatment of the high carbon steel does not proceed sufficiently at such a low temperature.
JP 2002-340885 A JP 2004-198144 A JP 2004-198145 A JP-A-63-115047 Japanese Examined Patent Publication No. 53-44836

  The present invention has been made in view of the above-described problems, and the object of the present invention is to analyze a CaO-containing inclusion in a high carbon steel as well as a low medium carbon steel. To provide a method that can extract the above chemically unstable inclusions without loss or loss and accurately analyze them, minimizing the effects of unwanted residues such as carbides that inhibit the determination of It is in.

  The method for analyzing CaO-containing inclusions in steel according to the present invention includes ferrous chloride, potassium hydroxide and 0.2 w / v% or more antioxidant, and has a pH of 4.5 to 6.5. A steel sample previously subjected to a solution treatment for 10 minutes or more at 1150 to 1350 ° C. is immersed in a certain electrolytic solution and subjected to constant current electrolysis, and ultrasonic vibration is applied to the obtained residue for 4 minutes or more. It is characterized by quantitative analysis of inclusions.

The CaO-containing inclusion is a complex oxide of CaO and at least one oxide selected from the group consisting of Al ₂ O ₃ , SiO ₂ , MnO, MgO, Na ₂ O, and FeO (for example, CaO.Al ₂ O ₃ , CaO.SiO ₂ , CaO.Al ₂ O ₃ .SiO _2, etc.), and the proportion of CaO is 5% by mass or more.

  The present invention is configured as described above. When analyzing CaO-containing inclusions in steel, solution treatment is performed under specified conditions, and ferrous chloride is mixed with KOH and an antioxidant at a specified concentration. Then, after performing constant-current electrolysis with an electrolytic solution having a pH within a specified range, by applying ultrasonic vibration to the obtained residue for a predetermined time or more, unnecessary residues can be easily separated, and chemically insoluble. Stable CaO-containing inclusions can be extracted easily and without loss or loss. By realizing such an analysis method, it is possible to provide a useful method as a means for evaluating inclusions during product development, as well as ensuring the quality of CaO-containing inclusions in steel products to the end user.

Under the circumstances as described above, the present inventors aim to accurately quantify CaO-containing inclusions in steel that is high carbon and clean, and the influence of unnecessary residues that hinder the determination of inclusions. In order to realize a method for analyzing CaO-containing inclusions in steel, which can be extracted without damaging and losing chemically unstable CaO-containing inclusions. as a result,
(I) During the inclusion extraction, a solution treatment of the steel sample is performed under specified conditions,
(Ii) Constant current electrolysis (hereinafter referred to as electricity) using an electrolytic solution containing ferrous chloride, potassium hydroxide and 0.2 w / v% or more antioxidant and having a pH of 4.5 to 6.5. Decomposition), and further,
(Iii) After constant current electrolysis, if ultrasonic vibration is applied to the obtained residue for a predetermined time or longer,
It was found that quantitative analysis of CaO-containing inclusions can be performed with high accuracy. The reason why each condition is specified will be described in detail below.

  In order to minimize the adverse effects of carbides on the inclusion analysis, it is very effective to subject the steel sample to a solution treatment when extracting the inclusions. Especially in the case of high carbon steel, there is a possibility that a large amount of carbide may remain as a residue together with CaO-containing inclusions. Therefore, this solution treatment sufficiently decomposed the carbide to cause solid solution of carbon, and constant current electrolysis was performed. It is necessary to sufficiently reduce unnecessary residues such as carbides remaining later.

  In the present invention, from such a viewpoint, the temperature of the solution treatment is set to 1150 ° C. or higher. Below 1150 ° C., particularly in the case of high carbon steel, a large amount of unnecessary residues such as giant carbides remain after constant current electrolysis, and separation from inclusions becomes very difficult. Preferably it is set to 1200 ° C. or higher. On the other hand, when the temperature of the solution treatment is too high, iron oxide is precipitated, and the iron oxide remains together with the CaO-containing inclusions during extraction, so that the inclusions cannot be favorably observed with a microscope. Therefore, the solution treatment is performed at a temperature of 1350 ° C. or lower, preferably 1300 ° C. or lower.

  Even when the solution treatment is performed at an appropriate temperature, if the treatment time is too short, the carbide cannot be sufficiently decomposed to dissolve the carbon into solid solution. It becomes difficult. Therefore, in the present invention, the solution treatment time is 10 minutes or longer, preferably 15 minutes or longer.

  To secure sufficient inclusions to accurately quantify inclusions in clean steel, as described above, ferrous chloride that can dissolve a large amount of iron matrix using a steel sample in the order of kg. It is very effective to use a constant current electrolysis method using an aqueous solution as an electrolyte, so-called slime method.

  In the present invention, the above-described constant current electrolysis method (slime method) is applied as a method for extracting CaO-containing inclusions, and detailed conditions of the above constant-current electrolysis method are required to accurately perform quantitative analysis of the CaO-containing inclusions. The following were also examined.

  FIG. 1 is a graph showing the relationship between the pH of the electrolytic solution used in the constant current electrolysis method and the CaO recovery rate obtained in the examples described later, and the results of the examples are organized. From this, it can be seen that when the pH of the electrolyte is below 4.5 and the acidity of the electrolyte is increased, the CaO recovery rate is significantly reduced. This means that the CaO-containing inclusions in the steel are dissolved in the electrolytic solution, which is an acidic solution, and the CaO-containing inclusions in the steel cannot be accurately determined. In the present invention, by causing the pH of the electrolytic solution to be 4.5 or more, the above-described dissolution of CaO-containing inclusions is prevented.

  On the other hand, FIG. 2 is a graph showing the relationship between the pH of the electrolytic solution and the dissolution rate of the sample (steel sample). The results of Examples described later are arranged. From FIG. 2, the pH is 6.5. It can be seen that the dissolution rate of the steel sample is significantly reduced when the temperature exceeds. If the pH is too high in this way, the steel sample is not sufficiently electrolyzed, resulting in a problem that the analysis of inclusions is delayed or the analysis itself cannot be performed. Therefore, in this invention, pH of electrolyte solution shall be 6.5 or less.

  In the present invention, the pH adjustment of the electrolytic solution using ferrous chloride is performed using KOH and an antioxidant.

When NaOH is used as a basic substance for pH adjustment, Na ⁺ ions are present in the electrolyte. When the sample is dissolved with this electrolytic solution, Na is contained in the inclusions, and Na is detected when analyzing the inclusions, that is, Na-containing inclusions despite being derived from the analytical reagent. It will be judged. On the other hand, in the steel manufacturing process, Na is partly used in powder used as a lubricant or the like, and Na-containing inclusions may exist as inclusions derived from the powder or the like. When NaOH is used as the basic substance, it becomes unclear whether the Na contained in the Na-containing inclusions is derived from the powder or the like used in the production process or from the analytical reagent.

  Therefore, in the present invention, K (potassium) is an element that is not used in the steel manufacturing process, and even if K is contained in the extracted inclusions, it is clear that the element is derived from an analytical reagent, Since it can be evaluated by excluding this, KOH was used as a basic substance for pH adjustment. In the present invention, the pH of the electrolytic solution is adjusted by changing the concentration of the KOH aqueous solution to be added.

  In the present invention, an antioxidant is further used to suppress an increase in pH and prevent divalent iron ions in the electrolyte from becoming trivalent iron ions. If trivalent iron ions are present in the electrolytic solution, iron oxyhydroxide (FeOOH) is generated, which becomes an unnecessary residue and hinders the analysis of inclusions. The stabilization mechanism of divalent iron ions by the addition of this antioxidant is that the divalent iron ions react with dissolved oxygen in water to form active superoxide radicals in the aqueous solution due to the involvement of antioxidants. For this reason, it is thought that stabilization and activation of a divalent iron ion are achieved.

  In order to sufficiently exhibit the above-described effects, the concentration of the antioxidant in the electrolytic solution needs to be 0.2 w / v% or more. If it is less than 0.2 w / v%, the electrolytic solution is oxidized by air and iron hydroxide (FeOOH) is likely to be generated, and there is a possibility that unnecessary residues increase and separation from inclusions becomes difficult. The concentration of the antioxidant in the electrolytic solution is preferably 1.0 w / v% or more. On the other hand, even if the concentration of the antioxidant is high, the effect of stabilizing the iron ions is not changed. Therefore, the upper limit of the concentration of the antioxidant is not particularly limited from the viewpoint of the effect of stabilizing iron ions, but if the concentration of the antioxidant is too high, the progress of electrolysis may be slow, so / V% or less, and more preferably 10 w / v% or less. Examples of the antioxidant include ascorbic acid and citric acid. In the present invention, it is preferable to use ascorbic acid.

Constant current electrolysis (electrolysis) is performed by immersing a steel sample in the electrolyte solution adjusted to pH. In the constant current electrolysis, a ferrous chloride (FeCl ₂ ) aqueous solution having a concentration of 10 w / v% is generally used. In the present invention, the pH is defined as described above, and the defined pH range. The concentration of FeCl ₂ to be used may be adjusted as appropriate so that the concentration is not particularly limited. Moreover, the conditions of the conventional slime method should just be employ | adopted about the conditions of constant current electrolysis itself.

  What is necessary is just to capture | acquire the residue (inclusion and unnecessary residue) obtained by the constant current electrolysis of the said steel sample, for example with the mesh cloth installed in the bath as shown in the Example mentioned later.

  The opening of the mesh cloth is preferably 20 to 40 μm. If it is larger than this, it is not possible to sufficiently secure CaO-containing inclusions of a size that causes defects in the final product, and it is difficult to accurately grasp the relationship between the inclusions and fatigue characteristics. Also, if it is smaller than this, the carbides that are still not decomposed even if solution treatment is performed under the above-mentioned appropriate conditions are likely to remain as unnecessary residues together with the CaO-containing inclusions, and the subsequent determination of the CaO-containing inclusions Unfavorable because it has an adverse effect.

  After the constant current electrolysis, ultrasonic vibration is applied to the residue in the mesh cloth. Even if the steel sample is melted by performing the electrolysis, inclusions and unnecessary residues such as carbides are mixed in the mesh cloth, and it becomes difficult to analyze the inclusions as they are. In the present invention, by applying ultrasonic vibration to the residue in the mesh cloth, the inclusion and the unnecessary residue such as carbide are separated, and the unnecessary residue is decomposed and refined to pass through the mesh cloth. So that only inclusions remain on the surface. Thus, in order to separate unnecessary residues such as inclusions and carbides and to decompose and refine the unnecessary residues such as carbides, it is necessary to apply ultrasonic vibration for 4 minutes or more. Preferably it is 8 minutes or more, More preferably, it is 12 minutes or more. On the other hand, the upper limit of the time for applying ultrasonic vibration is not particularly limited from the viewpoint of decomposition and refinement of the unnecessary residue, but is preferably 30 minutes or less from the viewpoint of the stability of inclusions. In the present invention, inclusions and unnecessary residues can be efficiently separated in a short time by applying ultrasonic vibration to the obtained residue as described above.

  The inclusions in the mesh cloth thus obtained are subjected to, for example, SEM (Scanning Electron Microscope) / EDX (Energy Dispersive X-ray spectrometer, energy dispersive fluorescence X) as in the examples described later. (Line analysis) and the like, and quantitative analysis of CaO-containing inclusions can be performed.

  By adopting the above method, various features of the slime method can be enjoyed. As described above, until now, the slime method using low carbon steel represented by thin steel plate has less influence of unnecessary residue, so it was possible to extract inclusions relatively easily. Analysis of inclusions was difficult because a large amount of unnecessary residue was generated. However, according to the present invention, not only when applied to low-medium carbon steel, but also when analyzing CaO-containing inclusions in clean steel with high carbon content (particularly containing 0.8% by mass or more of carbon). In addition, the amount of unnecessary residue deposited during electrolytic extraction can be suppressed to an unaffected level, and the CaO-containing inclusions in the steel can be accurately analyzed without causing erosion and loss.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. It is also possible to implement, and they are all included in the technical scope of the present invention.

  A high carbon steel (bearing steel) containing 1.0% by mass of C and 1.4% by mass of Cr is melted and cast to obtain a billet, and then a plate shape having a shape of 5 mm × 50 mm × 150 mm from the billet Steel pieces were collected.

  Then, this plate-shaped steel piece was subjected to a solution treatment (temperature and time are as shown in Table 1) in the air, and then cooled with water to prepare a sample.

And the said sample was immersed in electrolyte solution, and the iron matrix etc. were decomposed | disassembled by the constant current electrolysis method (current density: 200 A / m < ² >). The constant current electrolysis was performed with an apparatus schematically shown in FIG. As shown in FIG. 3, a sample 1 was set on the anode, and a SUS plate 2 was set on the cathode. Further, 1000 mL of a 10 w / v% ferrous chloride aqueous solution and 1000 mL of various concentrations of KOH aqueous solution shown in Table 1 were mixed as the electrolytic solution 3, ascorbic acid was used as an antioxidant, and the concentration in the electrolytic solution was shown in Table 1. The pH of the electrolytic solution was adjusted as shown in Table 1 by mixing so that the value was reached.

  The electrolysis was performed using the 10 samples, and a total of about 1 kg of high carbon steel was melted.

By dissolving the sample by electrolysis, a residue [inclusion (CaO · Al ₂ O ₃ ) with a short diameter of 30 μm or more is placed in a bag of mesh cloth (N-420T, manufactured by Nylon Kogyo Co., Ltd.) 4 having an opening of 30 μm. , Al ₂ O ₃ , MgO · Al ₂ O ₃ , SiO _{2, and the} like) and unnecessary residue (carbide, hydroxide such as iron hydroxide, iron oxide, etc.)] 5 were collected. The opening of the mesh cloth is not particularly defined. However, since the size of the inclusion that affects the rolling fatigue life is about 30 μm or more, a mesh cloth of this size was used for the opening.

  After the constant current electrolysis, ultrasonic vibration was given to the residue for the time shown in Table 1. As shown in Table 1, the frequency for applying ultrasonic vibration was generally 25 Hz.

  Then, the residue containing CaO-containing inclusions obtained by applying the ultrasonic vibration is subjected to SEM / EDX analysis (the apparatus uses “JXA-8000 series” manufactured by JEOL Ltd.), and the minor axis is The number of inclusions containing CaO of 30 μm or more was determined and converted to the number per 1 kg of sample. The results are shown in Table 1.

The SEM image photograph (magnification: 1500 times) of the CaO containing inclusion extracted by the prescribed method and its composition are shown in FIG. CaO-containing inclusions of FIG. 4, the CaO 17.5 wt%, _Al 2 _{O 3} to it is understood that CaO · _Al 2 _{O 3} containing 79.5 wt%.

  According to the present invention, such CaO-containing inclusions can be obtained without damage, and the CaO-containing inclusions can be analyzed accurately.

  The following evaluation was also performed using the sample (high carbon steel).

<Dissolution rate of sample>
For the purpose of confirming the correlation between the pH of the electrolytic solution and the amount of sample dissolution, the sample dissolution rate when using electrolytic solutions of various pHs was determined.

The sample obtained as described above is immersed in each electrolytic solution shown in Table 1 and subjected to constant current electrolysis for 6 hours, and the sample dissolution rate is measured by measuring the mass of the sample before electrolysis and the mass of the sample after electrolysis. Was determined from the following formula (1). The results are shown in Table 1.
Sample dissolution rate (%) = [(mass of sample before electrolysis−mass of sample after electrolysis)
/ (Mass of sample before electrolysis)] × 100 (1)

<Area ratio of unnecessary residue in mesh cloth>
If a large amount of unnecessary residue remains in the mesh cloth, the CaO-containing inclusion is covered with the unnecessary residue as schematically shown in FIG. Cannot be analyzed accurately. Therefore, as described above, the sample was subjected to constant current electrolysis, electrolyzed, and then the ratio (area ratio) of unnecessary residues in the mesh cloth was obtained and evaluated. The results are shown in Table 1.

  In addition, the SEM image photograph (magnification: 200 times) of the carbide | carbonized_material which is an unnecessary residue is shown in FIG. 6 (the number of the vicinity of the inclusion in FIG. 6 is attached on observation). Inclusions such as CaO-containing inclusions are about 30 to 50 μm, whereas the carbides have a large size with a major axis exceeding 300 μm, and the presence of the unnecessary residue adversely affects the analysis of inclusions as shown in FIG. Therefore, it can be seen that the processing is important.

Further, in this example, synthetic slag prepared from a reagent (CaO · 2Al ₂ O ₃ having a CaO concentration of 21.6% by mass, an average particle size of 47 μm, hereinafter referred to as “synthetic slag”) was used for the following evaluation. Also went.

<Evaluation of CaO recovery rate>
In order to investigate whether or not the CaO-containing inclusions were melted in the electrolytic solution, the CaO recovery rate when each electrolytic solution shown in Table 1 was used was also evaluated. Specifically, the synthetic slag was added to each electrolyte solution shown in Table 1, and allowed to stand for 1 day after stirring without passing an electric current. And after that, it filtered with the filter paper (5 types C), and analyzed the CaO density | concentration of the synthetic slag on this filter paper. And CaO collection | recovery rate was calculated | required from following formula (2). The results are shown in Table 1.
CaO recovery rate (%) = [CaO concentration (mass%) / 21.6] × 100 (2)

<Investigation of Na concentration>
This was carried out for the purpose of confirming whether or not Na was detected by component analysis of the synthetic slag (slag sample not containing Na) when NaOH was used as the basic substance. Specifically, in the same manner as the evaluation of the CaO recovery rate, the synthetic slag is converted into each electrolytic solution shown in Table 1 (an electrolytic solution using NaOH as a basic substance, or an electrolytic solution using KOH as a basic substance). ) And allowed to stand for 1 day after stirring without passing current. And after that, it filtered with the filter paper (5 types C), and analyzed Na concentration (mass%) in the synthetic slag on this filter paper. The results are shown in Table 1.

  It can be considered from Table 1 as follows (note that the following No. indicates the experiment No. in Table 1). No. Nos. 17 to 22 show that the method specified in the present invention is carried out, and therefore, chemically unstable CaO-containing inclusions can be analyzed with high accuracy.

  In contrast, no. Nos. 1 to 16 did not satisfy at least one of the requirements defined in the present invention, and thus undesirable results such that a large amount of unnecessary residues remained and analysis of CaO-containing inclusions became difficult.

  Specifically, no. In 1 and 2, since the solution treatment temperature was too low, a large amount of unnecessary residues such as carbide remained, and the CaO-containing inclusions could not be analyzed with high accuracy.

  No. In 3 and 4, since the solution treatment temperature was too high, iron oxides were precipitated and unnecessary residues increased. In this case as well, CaO-containing inclusions could not be analyzed with high accuracy.

  No. In 5 and 6, since the solution treatment time is too short, C and the like cannot be sufficiently dissolved, and a large amount of unnecessary residues such as carbides remain, and CaO-containing inclusions cannot be analyzed with high accuracy. It was.

  No. Examples 7 and 8 are examples in which NaOH is used to adjust the pH of the electrolytic solution. When an electrolytic solution containing Na is used in this way, Na-containing inclusions derived from lubricant powder used in the steel manufacturing process are removed. It has a problem that it cannot be analyzed accurately.

  No. In 9 and 10, since the pH of the electrolyte was too low, that is, the acidity was strong, the CaO-containing inclusions were dissolved, and the CaO-containing inclusions could not be analyzed with high accuracy.

  No. In 11 and 12, since the pH of the electrolytic solution was too high, that is, the basicity was strong, it was difficult to dissolve the steel sample itself.

  No. In 13 and 14, since the concentration of the antioxidant is too low, the electrolytic solution is oxidized in the air and iron hydroxide (FeOOH) is generated, unnecessary residues increase, and CaO-containing inclusions can be analyzed accurately. could not.

  No. In 15 and 16, since the time for applying ultrasonic vibration was too short, a large amount of unnecessary residue remained in this case, and as a result, the CaO-containing inclusions could not be analyzed accurately.

It is a graph which shows the relationship between pH of electrolyte solution, and a CaO recovery rate. It is a graph which shows the relationship between pH of electrolyte solution, and a sample dissolution rate. It is a model side view of the constant current electrolysis apparatus used in the Example. The SEM image photograph (magnification: 1500 times) of the CaO containing inclusion extracted in the Example, and its composition are shown. It is the figure which showed typically the state with which the unnecessary residue remains in a mesh cloth in large quantities. It is a SEM image photograph (magnification: 200 times) of the carbide | carbonized_material (unnecessary residue) which generate | occur | produced in the Example.

Explanation of symbols

1 Sample 2 SUS plate 3 Electrolytic solution 4 Mesh cloth 5 Residue with minor axis of 30 μm or more (inclusions and unnecessary residues)

Claims (1)

To an electrolytic solution containing ferrous chloride, potassium hydroxide and 0.2 w / v% or more antioxidant and having a pH of 4.5 to 6.5,
A steel sample previously subjected to a solution treatment for 10 minutes or more at 1150 to 1350 ° C. is immersed and subjected to constant current electrolysis, and ultrasonic vibration is applied to the obtained residue for 4 minutes or more, followed by quantification of CaO-containing inclusions. An analysis method for CaO-containing inclusions in steel, wherein analysis is performed.