METHOD FOR EXTRACTING MgO-CONTAINING INCLUSION IN STEEL

Abstract

To provide a method which enables an MgO-containing inclusion having little residue in steel to be accurately evaluated.SOLUTION: Solution treatment at 850°C to 1000°C for 1 hour to 3 hours, or solution treatment at 1000°C to 1250°C for 1 hour is applied to a steel sample, then the steel sample is oil-cooled, and after that, the surface portion of the sample is removed by using hydrochloric acid, the steel sample is then immersed for 48 hours to 72 hours in an electrolytic solution including ferrous chloride, potassium hydroxide and an antioxidant and having the pH of 5.0 to 5.4, and the steel sample is thus subjected to constant-current electrolysis, or the sample of steel is immersed for 48 hours to 96 hours in an electrolytic solution including ferrous chloride, potassium hydroxide and an antioxidant and having the pH of 5.4 to 6.2, and the steel sample is thus subjected to constant-current electrolysis.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a method for extracting MgO-containing inclusions in steel.

Background Art

[0002] With the recent development of high-purity steelmaking, it is necessary to reduce non-metallic inclusions and the like that are likely to cause cracking during steel plate processing. Although technologies for reducing non-metallic inclusions and the like during steel production have advanced, accurately evaluating the non-metallic inclusions and the like existing in the steel before obtaining the final product is important for ensuring the quality of the final product.

[0003] As methods for evaluating non-metallic inclusions and the like in steel, Patent Document 1 and Patent Document 2 describe methods for evaluating CaO-containing inclusions in steel.

[0004] Patent Document 1 describes that "a steel sample preliminarily subjected to a solution treatment at 800 to 1100°C for 3 to 10 minutes is subjected to constant current electrolysis in an aqueous ferrous chloride solution adjusted to pH 5 to 7 to extract CaO-containing inclusions" (see "Claims" of Cited Document 1). Patent Document 1 describes that "the composition and size of non-metallic inclusions in steel, particularly CaO-containing inclusions, can be accurately measured by the above method" (see "Effects of the Invention" in

[0043] of Cited Document 1).

[0005] Patent Document 2 describes that "a steel sample preliminarily subjected to a solution treatment at 1150 to 1350°C for 10 minutes or more is immersed in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant of 0.2 w / v% or more and having a pH of 4.5 to 6.5, and subjected to constant current electrolysis. After applying ultrasonic vibration to the obtained residue for 4 minutes or more, quantitative analysis of CaO-containing inclusions is performed" (see "Claims" of Cited Document 2). Patent Document 2 describes that "unnecessary residues can be easily separated, and CaO-containing inclusions that are chemically unstable can be extracted simply and without loss or deficiency" (see "Effects of the Invention" in

[0021] of Cited Document 2).

Prior Art Documents

Patent Document

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] In Cited Document 1 and Cited Document 2, attention is paid to CaO-containing inclusions, but depending on the components of the steel, etc., MgO-containing inclusions may be the main cause of cracking, etc. In order to evaluate MgO-containing inclusions in steel, the method for evaluating the above-described known CaO-containing inclusions was carried out. As a result, it was found that the same effects as those described in Patent Document 1 and Patent Document 2 could not be obtained. Specifically, after constant current electrolysis, a large amount of residue was present, and it sometimes took time to analyze MgO-containing inclusions. Also, MgO-containing inclusions were eroded during electrolysis, and the extraction amount of MgO-containing inclusions was sometimes small. Therefore, it was found that the method for evaluating known CaO-containing inclusions cannot accurately evaluate MgO-containing inclusions in steel.

[0008] An object of the present invention is to provide a method with less residue and capable of accurately evaluating MgO-containing inclusions when extracting MgO-containing inclusions in steel.

Means for Solving the Problems

[0009] The inventors of the present application conducted research aiming to find a method suitable for evaluating MgO-containing inclusions in steel and found the following method.

[0010] The method for extracting MgO-containing inclusions in steel disclosed in this specification involves performing a solution treatment at 850°C or higher and 1000°C or lower for 1 hour or more and 3 hours or less, or a solution treatment at 1000°C or higher and 1250°C or lower for 1 hour. After oil quenching, a steel sample with its surface removed by hydrochloric acid is immersed in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant, with a pH of 5.0 or higher and 5.4 or lower, for 48 hours or more and 72 hours or less, and subjected to constant current electrolysis, or immersed in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant, with a pH of 5.4 or higher and 6.2 or lower, for 48 hours or more and 96 hours or less, and subjected to constant current electrolysis.

Advantages of the Invention

[0011] By performing the solution treatment under the above conditions before constant current electrolysis, the layer that causes residues on the surface of the sample can be reduced. Furthermore, by performing constant current electrolysis after removing the layer that causes residues, the residues generated during electrolysis can be reduced. As a result, after constant current electrolysis, there will be less residue in the microscope image, making it easier to analyze the MgO-containing inclusions. Also, by performing constant current electrolysis under the above conditions, the MgO-containing inclusions are less likely to be dissolved, so the extraction amount of the MgO-containing inclusions can be accurately evaluated. From the above, when extracting MgO-containing inclusions in steel, there are fewer residues, and the MgO-containing inclusions in steel can be accurately evaluated.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0013] Hereinafter, preferred embodiments of the present invention will be described.

[0014] When Si (silicon) is contained in the steel, SiO 2) reacts with MgO derived from refractories to form MgO-containing inclusions containing MgO and SiO 2 . When the Si content in the steel is high, MgO-containing inclusions containing MgO and SiO 2 are likely to form. The formed MgO-containing inclusions cause cracks during steel plate processing. Such MgO-containing inclusions are MgO-containing inclusions containing 10% by mass or more of MgO and SiO 2 of 40% by mass or more. The "MgO-containing inclusions" in this specification refer to MgO-containing inclusions containing 10% by mass or more of MgO and SiO 2 of 40% by mass or more. Examples of the above MgO-containing inclusions include composite oxides of MgO and SiO 2 , and composite oxides of MgO and SiO 2 and one or more other oxides. Examples of the above MgO-containing inclusions include 2MgO·SiO 2 , MgO·SiO 2 , 2MgO·2Al 2 O 3 ·SiO 2 . Examples of the steel in which the above MgO-containing inclusions are likely to form include steel containing 0.58 to 0.63% by mass of C (carbon) and 1.4 to 2.0% by mass of Si.

[0015] The inventors of the present application have conducted research aiming to obtain a method suitable for evaluating MgO-containing inclusions in steel when the above MgO-containing inclusions are formed, and have found the following method. (1) A steel sample is subjected to a solution treatment at 850 °C or higher and 1000 °C or lower for 1 hour or more and 3 hours or less, or a solution treatment at 1000 °C or higher and 1250 °C or lower for 1 hour, and then oil-cooled. (2) After oil-cooling, the surface part of the sample is removed with hydrochloric acid. (3) The sample is immersed in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant, with a pH of 5.0 or higher and 5.4 or lower, for 48 hours or more and 72 hours or less, and subjected to constant current electrolysis, or the sample is immersed in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant, with a pH of 5.4 or higher and 6.2 or lower, for 48 hours or more and 96 hours or less, and subjected to constant current electrolysis. The reasons for defining the above method and the above conditions will be described below.

[0016] [Solution treatment and oil quenching] Before extracting non-metallic inclusions, it is known that it is very effective to subject a steel sample to solution treatment. The inventors of the present application, when evaluating MgO-containing inclusions, subjected a steel sample to solution treatment under the same conditions as the solution treatment conditions in the known evaluation method of CaO-containing inclusions described above, water-cooled it, and then performed constant current electrolysis. Water cooling is a common cooling method and is also performed in the above-described evaluation method of CaO-containing inclusions. After electrolysis, MgO-containing inclusions were analyzed using a microscope or the like. At this time, there were many residues in the microscope image, and it took a long time to analyze the MgO-containing inclusions.

[0017] The inventors of the present application considered that the solution treatment time in the known evaluation method of CaO-containing inclusions was short, and decided to increase the solution treatment time. Also, while increasing the solution treatment time, the solution treatment was carried out in a wider temperature range than before to investigate whether the residue was reduced. Furthermore, the cooling performed after the solution treatment is generally water cooling, but in water cooling, the steel is rapidly cooled. In the steel in which the above-described MgO-containing inclusions are likely to be generated, due to the composition, it becomes a martensite structure, so cracks are likely to occur due to thermal stress caused by rapid cooling. Therefore, oil cooling is also considered preferable for the purpose of reducing thermal stress. Therefore, water cooling in which the steel is rapidly cooled and oil cooling in which the steel is slowly cooled were performed to investigate a preferable method for reducing the residue. The experiments performed on these will be described below.

[0018] (Experiment 1) Steel containing 0.58 to 0.63% by mass of C (carbon), 1.4 to 2.0% by mass of Si (silicon), and 1.00 to 1.75% by mass of Cr (chromium) was cast, and after primary rolling, steel pieces of 20 mm (length) × 30 mm (width) × 5 mm (thickness) were taken. The steel pieces were subjected to solution treatment in a heating furnace in an air atmosphere, and then cooled to room temperature to prepare samples. Table 1 shows the solution treatment conditions and the cooling conditions.

[0019] In the solution treatment in the known method for evaluating CaO-containing inclusions, the treatment time is in minutes. In this experiment, the treatment time was changed to the hour unit. Also, the solution treatment was carried out over a wider temperature range than before. Further, the cooling after the solution treatment was either water cooling or oil cooling. Tap water was used for water cooling. JIS Class 1 No. 2 cold quench oil was used for oil cooling.

[0020] In this experiment, as an example, the aim was to make the steel structure a martensite structure. Fig. 1 shows a schematic diagram of an example of the cross-section of the sample. In the process of preparing the sample, an oxide layer A such as iron oxide and a structure B different from the target are formed on the surface part of the sample. The structure different from the target is a ferrite structure, a bainite structure, etc. These layers exist on the martensite structure C. The oxide layer A and the structure B different from the target cause residues.

[0021] Also, as shown in Fig. 1, cracks may occur near the surface of the sample. When the crack is large, when performing constant current electrolysis, the electrolytic solution may enter from the crack, resulting in residues. Therefore, large cracks also cause residues.

[0022] The inventors of the present application aimed to make the portion near the surface of the sample (hereinafter referred to as the "non-uniform layer") that causes residues as thin as possible in order to reduce the residues generated during constant current electrolysis. Here, the "non-uniform layer" is a portion where at least one of the oxide layer A, the structure B different from the target, and the crack shown in Fig. 1 exists. Also, by thinning the non-uniform layer and washing the sample with an acidic solution, etc., the non-uniform layer was removed before constant current electrolysis. It is considered that by removing in advance the non-uniform layer that causes residues, the residues generated during constant current electrolysis can be reduced.

[0023] From the previous experiments, when extracting MgO-containing inclusions, the thickness of the non-uniform layer was about 400 μm when there was less residue and high extraction accuracy. Therefore, aiming to make the thickness of the non-uniform layer 400 μm or less, considering variations and the like. And after making the thickness of the non-uniform layer 400 μm, it was considered that the risk of residue generation could be surely minimized by removing a portion with a thickness equivalent to 400 μm from the sample surface before constant current electrolysis. The following describes the evaluations conducted to investigate the solution heat treatment conditions and cooling conditions under which the thickness of the non-uniform layer is 400 μm or less.

[0024] Using an optical microscope and a scanning electron microscope (SEM), the cross-section of the prepared sample was observed. As shown in Fig. 1, the thickness (Ta) of the oxide layer A, the thickness (Tb) of the structure B different from the target, and the crack length (Tx) outside A and B were measured. The "crack length outside A and B" is the length of the crack in the target structure C when the crack reaches the target structure C. The "length of the crack in structure C" is the vertical length from the upper end of structure C to the tip of the crack in structure C. When the crack does not reach the target structure C, the "crack length outside A and B" is 0 (zero). When multiple cracks reach the target structure C, the crack with the longest length in structure C is taken as the "crack length outside A and B" and is shown in Table 1.

[0025] The total thickness (Ta + Tb + Tx) of the "thickness of the oxide layer A" (Ta), the "thickness of the structure B different from the target" (Tb), and the "crack length outside A and B" (Tx) was calculated, and the total thickness (Ta + Tb + Tx) was taken as the "thickness of the non-uniform layer".

[0026] When the "thickness of the non-uniform layer" exceeded 400 μm, the evaluation was marked as "×". Also, even when the "thickness of the non-uniform layer" was within 400 μm, in the steel in which the MgO-containing inclusions used in this experiment were likely to be generated, when water-cooled, there was a concern that cracks were likely to occur and become large cracks. Therefore, in the case of water-cooling, even when the "thickness of the non-uniform layer" was within 400 μm, it was judged that there was a possibility that the "thickness of the non-uniform layer" would exceed 400 μm, and the evaluation was marked as "×". Otherwise, the evaluation was marked as "〇". The results are shown in Table 1.

[0027]

Table 1

[0028] From Table 1, the evaluations of experiment numbers 1 to 8 and 16 were "×". The evaluations of experiment numbers 9 to 15 were "〇", and the non-uniform layer was within 400 μm. From the solution treatment conditions and cooling conditions of experiment numbers 1 to 15, the following was found. After solution treatment at 850 °C or higher and 1000 °C or lower for 1 hour or more and 3 hours or less, followed by oil cooling, the thickness of the non-uniform layer that causes residues becomes 400 μm or less. After solution treatment at 1000 °C or higher and 1250 °C or lower for 1 hour, followed by oil cooling, the thickness of the non-uniform layer that causes residues becomes 400 μm or less. When the solution treatment temperature is 1000 °C, from experiment numbers 11 and 15 in Table 1, whether the treatment time is 1 hour or 1 hour or more and 3 hours or less, the thickness of the non-uniform layer becomes 400 μm or less. From the above, when the solution treatment temperature is 1000 °C, it can be said that when the treatment time is 1 hour or more than 1 hour and 3 hours or less, the thickness of the non-uniform layer becomes 400 μm or less. Furthermore, it can be said that after solution treatment at 850 °C or higher and 1000 °C or lower for 1 hour or more and 3 hours or less, or after solution treatment at more than 1000 °C and 1250 °C or lower for 1 hour, followed by oil cooling, the thickness of the non-uniform layer that causes residues becomes 400 μm or less.

[0029] Among experiment numbers 9 to 15, the thickness of the non-uniform layer of experiment number 11 is the thinnest. In experiment number 11, after solution treatment at 1000 °C for 1 hour, it is then oil cooled. From this, it is considered that solution treatment at 1000 °C for 1 hour followed by oil cooling, or solution treatment at around 1000 °C for about 1 hour followed by oil cooling can make the thickness of the non-uniform layer as thin as possible close to the minimum, which is very effective.

[0030] Next, after Experiment No. 11, the thickness of the non-uniform layer in Experiment No. 15 is thin. In Experiment No. 11, after solutionizing at 1000°C for 3 hours, it is oil-cooled. From Experiments No. 11 and 15, solutionizing at 1000°C for 1 hour or more and within 3 hours, followed by oil-cooling, or solutionizing at around 1000°C for about 1 hour or more and within 3 hours, followed by oil-cooling, is effective in that it can make the thickness of the non-uniform layer thinner.

[0031] [Removal of the surface part] After making the thickness of the non-uniform layer 400 μm or less by the method described above, a portion corresponding to a thickness of 400 μm is removed from the sample surface. The surface part of the sample can be removed by dissolving it in an acidic solution. As the acidic solution, generally used hydrochloric acid can be used. Here, the "hydrochloric acid" is an aqueous hydrochloric acid solution. For example, commercially available hydrochloric acid used for dissolving iron etc. may be used. Also, hydrochloric acid with a general concentration used for dissolving iron etc. may be used.

[0032] The samples of Experiment Nos. 9 to 15, for which the evaluation in Table 1 was "〇", were immersed in hydrochloric acid with a concentration of 20% to wash the sample surface. While weighing the sample, the sample surface was washed, and by removing the portion (surface part) corresponding to a thickness of 400 μm from the sample surface before immersion, the non-uniform layer that caused the residue could be removed.

[0033] [Constant current electrolysis] In order to ensure a sufficient amount of inclusions so that the inclusions in clean steel can be accurately quantified, it is known that it is very effective to use the constant current electrolysis method, so-called slime method, using an aqueous ferrous chloride solution that can dissolve a large amount of iron matrix using a steel sample on the kg order as the electrolyte.

[0034] Also when evaluating MgO-containing inclusions, constant current electrolysis using an aqueous ferrous chloride solution as the electrolyte is performed. As the electrolyte, an electrolyte containing ferrous chloride, potassium hydroxide, and an antioxidant is used.

[0035] Potassium hydroxide is used as a basic substance to adjust the pH of the electrolyte. K (potassium) is an element not used in the steel manufacturing process. Therefore, even if the extracted inclusions contain K, it is clear that it is an element derived from the analytical reagent, and the MgO-containing inclusions can be evaluated by excluding K. When adjusting the pH of the electrolyte using potassium hydroxide, for example, the pH of the electrolyte can be adjusted by changing the concentration of the potassium hydroxide aqueous solution or the addition amount of the potassium hydroxide aqueous solution.

[0036] The antioxidant suppresses the increase in pH and prevents divalent iron ions in the electrolyte from becoming trivalent iron ions. When trivalent iron ions are present in the electrolyte, iron oxyhydroxide (FeOOH) is generated, which becomes an unnecessary residue and hinders the analysis of MgO-containing inclusions. The mechanism of stabilizing divalent iron ions by adding an antioxidant is considered to be that in an aqueous solution, divalent iron ions react with dissolved oxygen in the water to form active superoxide radicals with the participation of the antioxidant, thereby achieving the stabilization and activation of divalent iron ions.

[0037] The content of the antioxidant in the electrolyte is not particularly limited. However, in order to fully exert the above-described effects of the antioxidant, like the analysis method of known CaO-containing inclusions (

[0034] of JP-A-2010-8090), it is preferable that the concentration of the antioxidant in the electrolyte is 0.2% by mass or more. When the concentration of the antioxidant in the electrolyte is less than 0.2% by mass, there is a risk that the electrolyte is air-oxidized and iron hydroxide (FeOOH) is likely to be generated, resulting in an increase in unnecessary residues and difficulty in separating from MgO-containing inclusions. The concentration of the antioxidant in the electrolyte is preferably 1.0% by mass or more. On the other hand, even if the concentration of the antioxidant is high, there is no change in the stabilizing effect of iron ions. Therefore, the upper limit of the concentration of the antioxidant is not particularly limited from the viewpoint of the stabilizing effect of iron ions. However, when the concentration of the antioxidant is too high, the progress of electrolysis may be slowed down, so it is preferably 15% by mass or less, more preferably 10% by mass or less. As the antioxidant, L-ascorbic acid (C 6 H 8 O6 ) Examples include citric acid. L-ascorbic acid (C 6 H 8 O 6 ) and citric acid are acidic substances, and in addition to the above-described effects, they are also used for adjusting the pH of the electrolytic solution.

[0038] The concentration of ferrous chloride (FeCl 2 ) is not particularly limited. For example, an aqueous solution of ferrous chloride (FeCl 2 ) with a concentration of 10% by mass, which is common in constant current electrolysis, may be used. Since the concentration of FeCl 2 ) may be appropriately adjusted to obtain a desired pH range, the concentration of FeCl 2 ) is not particularly limited. 2

[0039] The sample is immersed in the above-described electrolytic solution, and constant current electrolysis is performed. As for the conditions of the constant current electrolysis itself, the conditions of the conventional slime method may be adopted. The sample to be immersed in the electrolytic solution is a sample that has been solutionized, cooled, and had its surface portion removed under conditions where the thickness of the non-uniform layer is 400 μm or less as found in Experiment 1.

[0040] To capture MgO-containing inclusions, for example, a filter or a mesh cloth may be used. For example, the sample may be covered with a filter or a mesh cloth. The sample may be placed in a bag made of a filter or a mesh cloth. The pore diameter of the filter and the mesh cloth is preferably 20 to 30 μm. When the pore diameter is larger than 30 μm, it is difficult to sufficiently capture MgO-containing inclusions of a size that may cause defects in the final product. When the pore diameter is smaller than 20 μm, the residues generated even after performing the above-described solutionizing treatment, cooling, and removal of the sample surface portion are likely to remain together with the MgO-containing inclusions, which may affect the subsequent analysis of the MgO-containing inclusions.

[0041] ​After constant current electrolysis, ultrasonic vibration may be applied to the MgO-containing inclusions and residues in the mesh cloth. As a result, the MgO-containing inclusions and the residues can be separated, and the residues can be decomposed and refined to pass through the mesh cloth, so that only the MgO-containing inclusions remain in the mesh cloth.

[0042] The inventors of the present application performed constant current electrolysis under the same conditions as those in the known method for evaluating CaO-containing inclusions. As a result, the MgO-containing inclusions were liable to be dissolved and eroded, and the extraction amount of the MgO-containing inclusions was sometimes small. In addition, the electrolysis did not proceed sometimes. Therefore, paying attention to the electrolysis time not specified in the known method for evaluating CaO-containing inclusions, the following simulation experiment (Experiment 2) was conducted by changing the pH of the electrolytic solution and the electrolysis time, and the conditions under which the MgO-containing inclusions can be accurately evaluated were investigated.

[0043] (Experiment 2) Examples of MgO-containing inclusions that affect the quality of the product include, for example, 2MgO·SiO 2 , MgO·SiO 2 and 2MgO·2Al 2 O 3 ·SiO 2 . In this experiment, these three MgO-containing inclusions (MgO·SiO 2 , MgO·SiO 2 and 2MgO·2Al 2 O 3 ·SiO 2 ) were used as simulated inclusion reagents, respectively, and the following evaluation tests were conducted for each simulated inclusion reagent. Table 2 shows the compositions of the three MgO-containing inclusions. [Table 2]

[0044] 0.1 g of the simulated inclusion reagent was dissolved, and the MgO concentration in the solution was measured by a high-frequency plasma emission spectrometer (ICPS-8000, manufactured by Shimadzu Corporation). This MgO concentration was defined as the "MgO concentration before the test".

[0045] Next, the apparatus shown in Fig. 2 was prepared. As shown in Fig. 2, 0.1 g (sample) of the simulated inclusion reagent 1 was set on the anode and immersed in the electrolyte 4. A SUS plate 2 was set on the cathode. The sample 1 is covered with a membrane filter 3. In this experiment, a membrane filter 3 with a pore size of 0.1 μm, which is extremely small, was used. By using a membrane filter 3 with a small pore size, almost all of the MgO-containing inclusions that were not dissolved in the electrolyte 4 can be recovered, so that the extraction amount of the MgO-containing inclusions can be accurately evaluated. The electrolyte 4 was adjusted by the following method. L-ascorbic acid, an antioxidant, was mixed into the ferrous chloride aqueous solution so that the concentration of the electrolyte became the value shown in Table 3 described later. Using an aqueous potassium hydroxide solution, the pH of the electrolyte was adjusted as shown in Table 3.

[0046] As shown in Fig. 2, constant current electrolysis was performed with the sample 1 immersed in the electrolyte 4. Table 3 shows the electrolysis conditions. The current density of the constant current electrolysis is 20 A / m 2 is.

[0047] After the constant current electrolysis, the MgO-containing inclusions and residues captured by the membrane filter 3 were filtered with filter paper. After filtration, all of the MgO-containing inclusions and residues on the filter paper were solubilized. The MgO concentration in the solution was measured with a high-frequency plasma emission analyzer (ICPS-8000, manufactured by Shimadzu Corporation). This MgO concentration was defined as the "MgO concentration after the test".

[0048] The recovery rate of the MgO-containing inclusions was calculated from the following formula. Recovery rate (%) = {(MgO concentration after the test) / (MgO concentration before the test)} × 100 The recovery rate is shown in Table 3.

[0049] The "Guidelines for Evaluating the Validity of Test Methods for Pesticides and Other Residues in Foods" (December 24, 2010, Food Safety First 1224 No. 1, https: / / www.mhlw.go.jp / web / t_doc?dataId=00tb6662&dataType=1&pageNo=1) describes a test in which the pesticide or other substance to be tested is added to a sample (blank sample) to evaluate the solubility of the pesticide or other substance. In this experiment, since the MgO-containing inclusion is solubilized and the MgO concentration in the solution is measured to evaluate the MgO recovery rate, it is considered in the same way as the above test. Therefore, referring to the trueness described in the above guidelines (which is considered to correspond to the recovery rate of this experiment), when the recovery rates of all MgO-containing inclusions shown in Table 2 and Table 3 are 70% or more, it is judged that the MgO-containing inclusion has little dissolution loss and the MgO-containing inclusion can be sufficiently extracted, and the evaluation is "〇". If there is even one MgO-containing inclusion with a recovery rate of less than 70%, the evaluation is "×". The evaluation results are shown in Table 3.

[0050]

Table 3

[0051] In Experiment Nos. 32 to 40, the recovery rate of 2MgO·SiO 2 is 90% or more and close to 100%. For MgO-containing inclusions other than 2MgO·SiO 2 (2MgO·2Al 2 O 3 ·5SiO 2 , MgO·SiO 2 ), as shown in Table 2, since the proportion of the mass of MgO in the inclusion is smaller than that of 2MgO·SiO 2 , it is qualitatively suggested that it is less likely to dissolve in the electrolyte. From this, the recovery rates of MgO-containing inclusions other than 2MgO·SiO 2 are higher than the recovery rate of 2MgO·SiO 2 or 2MgO·SiO 2It is suggested that it may be equivalent to the recovery rate. Actually, such is the case with the experimental results of experiment numbers 21 to 27, 34, 37, and 40 in which the recovery rates of three MgO-containing inclusions were determined. Therefore, in experiment numbers 32 to 40, those with a recovery rate of 2MgO·SiO 2 being 90% or more and close to 100% were considered that the recovery rates of other MgO-containing inclusions would also be about 90% or more, and the evaluation was marked as "〇". In addition, in experiment numbers 29 to 31, since the recovery rate of 2MgO·SiO 2 was less than 70%, the evaluation was marked as "×" without determining the recovery rate of MgO·SiO 2 .

[0052] From Table 3, the following was found. In experiment number 25 (electrolysis time: 96 hours) and experiment number 29 (electrolysis time: 168 hours) where the pH of the electrolyte was 3.5, the evaluation was "×". When the pH of the electrolyte was as low as 3.5, it is considered that the MgO-containing inclusions were corroded and the recovery rate of 2MgO·SiO 2 was low.

[0053] When the pH of the electrolyte was 5.0, the evaluations of experiment number 21 (electrolysis time: 48 hours) and experiment number 23 (electrolysis time: 72 hours) were "〇", but the evaluations of experiment number 26 (electrolysis time: 96 hours) and experiment number 30 (electrolysis time: 168 hours) were "×".

[0054] When the pH of the electrolyte was 5.4, all the evaluations of experiment number 32 (electrolysis time: 48 hours), experiment number 33 (electrolysis time: 72 hours), and experiment number 34 (electrolysis time: 96 hours) were "〇".

[0055] In experiment number 35 (electrolysis time: 48 hours), experiment number 36 (electrolysis time: 72 hours), and experiment number 37 (electrolysis time: 96 hours) where the pH of the electrolyte was 5.8, all the evaluations were "〇".

[0056] When the pH of the electrolytic solution was 6.0, the evaluations of Experiment No. 22 (electrolysis time: 48 hours), Experiment No. 24 (electrolysis time: 72 hours), and Experiment No. 27 (electrolysis time: 96 hours) were "〇", but the evaluation of Experiment No. 31 (electrolysis time: 168 hours) was "×".

[0057] When the pH of the electrolytic solution was 6.2, all the evaluations of Experiment No. 38 (electrolysis time: 48 hours), Experiment No. 39 (electrolysis time: 72 hours), and Experiment No. 40 (electrolysis time: 96 hours) were "〇".

[0058] In Experiment No. 28 (electrolysis time: 96 hours) where the pH of the electrolytic solution was 6.5, precipitates were generated during electrolysis, and electrolysis could not be continued. It is considered that these precipitates were formed by the reaction of iron ions and hydroxide ions in the electrolytic solution. Due to the influence of the precipitates, the sample did not dissolve, and electrolysis could not be continued. From this, it was found that when the pH of the electrolytic solution was 6.5, MgO-containing inclusions could not be extracted.

[0059] From the above experiments, the following was found. When the pH of the electrolytic solution is 5.0 or more and 5.4 or less, by setting the electrolysis time to 48 hours or more and 72 hours or less, the dissolution loss of MgO-containing inclusions can be suppressed, and more MgO-containing inclusions can be extracted. When the pH of the electrolytic solution is 5.4 or more and 6.2 or less, by setting the electrolysis time to 48 hours or more and 96 hours or less, the dissolution loss of MgO-containing inclusions can be suppressed, and more MgO-containing inclusions can be extracted. When the pH of the electrolytic solution is 5.4, from Experiment Nos. 32 to 34 in Table 3, whether the electrolysis time is 48 hours or more and 72 hours or less, 72 hours or more and 96 hours or less, or 48 hours or more and 96 hours or less, more MgO-containing inclusions can be extracted. Also, from the above, it can be said that when the pH of the electrolytic solution is 5.0 or more and less than 5.4, by setting the electrolysis time to 48 hours or more and 72 hours or less, the dissolution loss of MgO-containing inclusions can be suppressed, and more MgO-containing inclusions can be extracted.

[0060] From the above, when extracting MgO-containing inclusions in steel, (1) Apply a solution treatment to a steel sample at 850 °C or higher and 1000 °C or lower for 1 hour or longer and 3 hours or shorter, or a solution treatment at 1000 °C or higher and 1250 °C or lower for 1 hour, and then perform oil quenching. (2) After oil quenching, remove the surface part of the sample with hydrochloric acid. (3) Immerse the sample in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant, with a pH of 5.0 or higher and 5.4 or lower, for 48 hours or longer and 72 hours or shorter, and perform constant current electrolysis, or immerse the sample in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant, with a pH of 5.4 or higher and 6.2 or lower, for 48 hours or longer and 96 hours or shorter, and perform constant current electrolysis. By the above method, when extracting MgO-containing inclusions in steel, there is less residue, and the MgO-containing inclusions in steel can be accurately evaluated.

[0061] The above-described method is implemented on a steel material, for example, after primary rolling. By implementing the above method after primary rolling, the MgO-containing inclusions in steel can be accurately evaluated before obtaining the final product.

[0062] As described above, the embodiments of the present invention have been described with reference to the drawings, but the specific configuration should be considered not to be limited to these embodiments. The scope of the present invention is indicated by the claims rather than the above description, and includes all modifications within the meaning and scope equivalent to the claims.

[0063] For example, in Experiment 2 above, as MgO-containing inclusions, 2MgO·SiO 2 , MgO·SiO 2 and 2MgO·2Al 2 O 3 ·5SiO 2 were experimented with. However, the MgO-containing inclusions are not limited to these. Even when the method according to the present invention is implemented when extracting MgO-containing inclusions other than the above, there is less residue, and the MgO-containing inclusions in steel can be accurately evaluated.

Explanation of Reference Numerals

[0064] 1 Sample 2 Plate made of SUS 3 Membrane filter 4 Electrolyte

Claims

【Claim 1】 A steel sample is subjected to solution treatment at 850°C or higher and 1000°C or lower for 1 hour or more and 3 hours or less, or solution treatment at 1000°C or higher and 1250°C or lower for 1 hour, followed by oil cooling, and then the surface part is removed with hydrochloric acid. The sample is immersed in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant, with a pH of 5.0 or higher and 5.4 or lower, for 48 hours or more and 72 hours or less, and subjected to constant current electrolysis, or immersed in an electrolytic solution containing ferrous chloride, potassium hydroxide, and an antioxidant, with a pH of 5.4 or higher and 6.2 or lower, for 48 hours or more and 96 hours or less, and subjected to constant current electrolysis. A method for extracting MgO inclusions in steel, characterized by the above.

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