JP7206825B2 - Inclusion evaluation method - Google Patents

Description

The present invention particularly relates to an inclusion evaluation method for slag-based inclusions.

Conventionally, when inclusions are contained in a steel sheet, there is a possibility that defects may occur during product molding, so it is desired to remove inclusions as much as possible. In particular, thin steel products are prone to cracking due to inclusions, so there is a great need to remove inclusions.

Inclusions can be broadly classified into two types. The first is inclusions mainly composed of alumina (hereinafter referred to as alumina-based inclusions), which are produced in the process of deoxidizing molten steel or reoxidizing molten steel. The second is inclusions mainly composed of slag (hereinafter referred to as slag-based inclusions), which are included in the molten steel due to the inclusion of slag floating above the ladle or tundish. In particular, since slag-based inclusions are not crushed during rolling and remain in the steel sheet as coarse inclusions, they tend to cause defects such as cracks during product molding.

Methods for analyzing inclusions conventionally performed include, for example, the slime method described in Non-Patent Document 1, observation of inclusions with an optical microscope, acid decomposition method, and the like. In addition, Patent Document 1 discloses a method of extracting inclusions by electrolyzing a steel sample that has been solution treated in advance at 800 to 1000 ° C. in a ferrous chloride aqueous solution adjusted to pH 5 to 7 by constant current electrolysis. It is Furthermore, in Patent Document 2, after the inclusion is extracted and separated from the steel, illumination is applied from one direction to the inclusion to be measured, and the image analysis of the silhouette of the inclusion particle formed by the transmitted light is performed. A method for rapidly evaluating the particle size, shape, number, etc. of inclusions is disclosed.

JP-A-2002-340885 JP-A-10-185902

Yoshio Yoshida, Yoshiko Funahashi, "Extraction and sizing of large non-metallic inclusions in steel by slime method", Tetsu to Hagane, 61 (1975) No. 10, p. 171-182 Timothy J. Drake, Ph.D., Application Specialist ASPEX Corporation, Delmont, PA USA, "Metal Quality AnalyzerTM Comparison with Optical Microscopic Analysis of Steel", [online], Kyokuto Boeki Co., Ltd., [September 28, 2018] date search], Internet (URL: http://www.kbk.co.jp/files/2015/0771/7234/MQA_.pdf0.pdf)

In recent years, steel with a high degree of cleanliness is required, and inclusions in molten steel are decreasing due to improvements in production technology and the like. Along with this, there is a demand for a method capable of quickly and accurately analyzing and evaluating inclusions even if the amount of inclusions is very small.

Assuming the rapid evaluation of inclusions in molten steel, observation with an optical microscope is relatively easy to measure because it does not involve a special solution or melting. However, since it is a planar observation, it is not suitable for evaluating inclusions such as slag-based inclusions that are contained in molten steel in a very small amount. In addition, in the slime method and the acid decomposition method, inclusions are extracted by dissolving a certain volume, so even slag-based inclusions contained in molten steel in a very small amount can be evaluated. However, in these methods, it is necessary to melt the sample once and separate the inclusions, and it takes about one week to complete the analysis, which makes it impossible to perform the analysis and evaluation quickly.

The method described in Patent Document 1 requires time for electrolysis. The method described in Patent Document 2 also describes an acid dissolution method, a halogen dissolution method, an electrolysis method, and the like as extraction and separation methods, but all of these methods dissolve iron and have the problem of requiring time. be.

SUMMARY OF THE INVENTION An object of the present invention is to provide an inclusion evaluation method capable of quickly and accurately evaluating slag inclusions.

The present invention is as follows.
(1)
The equivalent circle diameter and the Ca concentration (% by mass) of Al--Ca inclusions having a particle size of 1 μm or more in the metal sample are measured with an electron beam analyzer, and the equivalent sphere volume calculated from the measured equivalent circle diameter and the above A method for evaluating inclusions, wherein the amount of slag-based inclusions in the metal sample is evaluated based on the Ca concentration.
(2)
The inclusion evaluation method according to (1) above, wherein the amount of the slag-based inclusions is evaluated based on the sum of the product of the sphere-equivalent volume and the Ca concentration.

According to the present invention, slag-based inclusions can be evaluated quickly and accurately.

Shows the relationship between the sum of the volume of Al--Ca inclusions x the Ca concentration calculated based on the inclusion diameter obtained by MQA analysis and the Ca concentration of the Al--Ca inclusions, and the actual amount of slag-based inclusions. It is a diagram.

The present invention will be described below with reference to the drawings. In recent years, inclusions in molten steel have decreased due to improvements in production technology and the like. In particular, slag-based inclusions (inclusions mainly composed of CaO) are very few in number compared to alumina-based inclusions (inclusions mainly composed of Al ₂ O ₃ ). is difficult.

Therefore, the inventors of the present invention made extensive studies and focused on alumina-based inclusions, which are present in a relatively large amount in molten steel. Since slag-based inclusions contain Ca, the presence of slag-based inclusions causes Ca to diffuse in molten steel. Then, the diffused Ca reacts with alumina-based inclusions, which are relatively abundant in the molten steel, and changes into Al--Ca-based inclusions. Therefore, the present inventors focused on the fact that the amount of Ca contained in Al—Ca inclusions increases as the number of slag inclusions increases, and the amount of slag inclusions can be estimated by evaluating these. I found

Al-Ca compounds are contained in molten steel more than slag compounds, so even if the measurement range is narrow, analysis can be performed with a certain degree of accuracy. Analysis of system compounds can be performed rapidly. In this embodiment, for example, MQA (Metal Quality Analyzer: manufactured by ASPEX Corporation) is used as an electron beam measuring device. MQA can simultaneously and rapidly measure the diameter and elemental composition of inclusions. Details of MQA are disclosed in Non-Patent Document 2, for example.

First, the present inventors considered that the amount of Ca contained in Al—Ca inclusions increases as the amount of slag inclusions increases. We investigated the relationship with the analysis results. FIG. 1 is a diagram showing the relationship between the amount of Ca in Al--Ca inclusions determined by MQA and the actual amount of slag inclusions. The horizontal axis of FIG. 1 represents the sum of (the volume of Al—Ca inclusions×the concentration of Ca in the inclusions) calculated based on the MQA measurement results; the amount of Ca contained in the alumina inclusions, The vertical axis in FIG. 1 represents the “amount of slag-based inclusions” analyzed by the slime method described in Non-Patent Document 1. From the results shown in FIG. 1, it can be seen that the amount of slag inclusions tends to increase as the amount of Ca in the alumina inclusions increases. It was confirmed that the amount of

Next, the details of the inclusion evaluation method according to the embodiment of the present invention will be described. First, a sample to be analyzed is taken from a tundish or a ladle after secondary refining. The sample size is, for example, a columnar shape of about φ25 mm×25 mm. Then, the collected molten steel sample is immediately cooled and analyzed by MQA after cooling is completed. For the analysis, a sample piece obtained by slicing a portion 10 mm or more away from the upper end of the collected sample is used. This is to eliminate the effects of contamination and contamination that occur on the top of the sample.

In MQA, the sample is dynamically scanned using an electron microscope with a field of view of eg 20×20 mm. Then, the brightness of the backscattered signal is recorded at each point, and if the signal brightness at that position is below the reference, inclusions (in this embodiment, Al—Ca inclusions) are assumed to be present, and the Al—Ca system Start a sequence for analyzing inclusions. Then, detailed data is collected at the position where the Al--Ca inclusion exists, and the size (diameter) of the Al--Ca inclusion is specified. The diameter specified here is the equivalent circle diameter. Once the size of the Al--Ca inclusions has been identified, an energy dispersive X-ray spectrum (200 to 500 milliseconds) is obtained to analyze the concentration of each element in the Al--Ca inclusions. The Al--Ca-based inclusions to be specified should have a diameter of 1 μm or more, depending on the resolution of the electron microscope.

After the analysis of the diameter and Ca concentration of the Al-Ca inclusions by MQA is completed, the diameter and Ca concentration of each inclusion are obtained, and the sphere equivalent volume and Ca concentration of the Al-Ca inclusions for each inclusion are calculated. Calculate the product of The sphere equivalent volume is calculated based on the measured circle equivalent diameter. Then, the products of the sphere-equivalent volume and the Ca concentration are calculated for all Al--Ca-based inclusions, the sum of these values is calculated, and the amount of slag-based inclusions n _sl ( g/kg).

In formula (1), r represents the radius (μm) when the Al—Ca inclusions are converted to a circle, and [Ca] represents the Ca concentration (% by mass) in the Al—Ca inclusions. . Further, a is a constant of proportionality, and the relationship between the amount of Ca contained in the Al--Ca inclusions and the slag inclusions is obtained and determined in advance as shown in FIG. In this embodiment, the constant of proportionality a=41.

As described above, in the present embodiment, the amount of slag-based inclusions is evaluated from the formula (1) by analyzing the Al—Ca-based inclusions, so the slag-based inclusions can be evaluated quickly and accurately. be able to.

In the present embodiment, MQA has been described as an example of an electron beam measuring device for analyzing Al—Ca inclusions, but if the diameter and Ca concentration of Al—Ca inclusions can be analyzed quickly, Other electron beam measurement devices may be used.

In the process from analyzing the diameter and Ca concentration of Al--Ca inclusions to estimating the amount of slag inclusions, any calculation may be performed. For example, an operator confirms the diameter and Ca concentration of Al-Ca inclusions by MQA, inputs the numerical values into another personal computer (PC), and the CPU of the PC uses calculation software etc. The quantity (estimated value) of the substance may be calculated, and the information on the diameter and Ca concentration of the Al-Ca inclusions may be transmitted from the MQA to the PC, and the PC receives this information and transmits it to the PC. The amount (estimated value) of slag-based inclusions may be calculated by the CPU. Further, the operator may confirm the diameter and Ca concentration of the Al--Ca inclusions by MQA, and calculate the amount (estimated value) of the slag inclusions using a calculator or the like.

Next, an example of the present invention will be described, but this condition is an example of conditions for confirming the feasibility of the present invention, and the present invention is not limited to the description of this example. . The present invention can be implemented in various ways to achieve the objects of the present invention without departing from the gist of the present invention.

First, a sample (cylindrical shape of about φ25 mm×25 mm) was taken from a boat-shaped tundish having a long nozzle in the center and immersion nozzles on both sides. Samples were taken from the vicinity of the center of the melt depth around the long nozzle. Then, the collected sample was immediately water-cooled, and after the completion of cooling, it was analyzed by MQA. A sample piece obtained by slicing a 10 mm position from the lower end of the collected sample was used for the analysis. The MQA analysis was performed with a field of view of 20×20 mm, and the composition and grain size of Al—Ca inclusions with a grain size of ≧1 μm were analyzed.

Subsequently, the product of the volume of the Al--Ca inclusions and the Ca concentration was calculated from the obtained analysis results, and the sum was calculated. The volume of inclusions (equivalent sphere volume) was calculated by calculating the equivalent circle diameter from the particle size obtained from the above analysis. Then, the amount of slag-based inclusions was estimated based on the above formula (1). In addition, the calculation was performed with the constant of proportionality a=41.

On the other hand, for comparison, an analysis of slag-based inclusions by the slime method was also carried out in parallel. Analysis by the slime method involves cutting and removing the top 10 mm of a cylindrical sample of about φ25 mm x 25 mm, electrolyzing it for about 16 hours, and elutriating the resulting electrolytic residue sample of about 16 to 18 g. separated things. The sample was taken at the same timing as the sample used for MQA. Table 1 below shows a comparison between the amount of slag-based inclusions estimated by the method of the present invention and the amount of slag-based inclusions directly analyzed by the slime method.

As shown in Table 1, there is no significant difference between the amount of slag inclusions estimated from the Ca concentration of Al—Ca inclusions and the amount of slag inclusions directly analyzed by the slime method. We were able to quickly and accurately evaluate the quantity of an object.

Claims (2)

The circle-equivalent diameter and Ca concentration (% by mass) of Al—Ca-based inclusions with a grain size of 1 μm or more in the metal sample are measured by an electron beam analyzer, and the sphere-equivalent volume calculated from the measured circle-equivalent diameter and the above-mentioned A method for evaluating inclusions, wherein the amount of slag-based inclusions in the metal sample is evaluated based on the Ca concentration. 2. The inclusion evaluation method according to claim 1, wherein the amount of said slag-based inclusions is evaluated based on the sum of products of said spherical equivalent volume and said Ca concentration.

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