JP2001159627A - Method for evaluating cleanliness of metal material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method which corresponds not only to enhancement in recent metallurgical technique but also to substantial improvements in the cleanliness of a metal material, such as steel or the like and high in reliability. SOLUTION: (N) sample pieces are collected from a metal material to be analyzed, and are electrolyzed to obtain residues, and these residues are elutriated to separate impurities contained in respective sample pieces and the maximum diameters aj (j=1, n) of impurities are measured with respect to (n) sample pieces and the estimated maximum diameter amax of the impurities contained in the metal material to be analyzed is calculated, to evaluate the cleanliness of the metal material to be analyzed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for evaluating the cleanliness of a metal material. More specifically, extraction of inclusions by electrolysis and elutriation for a predetermined sample piece, separation and measurement of the inclusion diameter, using a statistical method,
The present invention relates to a method for performing highly reliable cleanliness evaluation of a metal material.

[0002]

2. Description of the Related Art With the recent improvement in metallurgical technology, the cleanliness of metallic materials such as steel has been greatly improved, and non-metallic inclusions in medium to large metallic materials exceeding 20 microns have been further reduced and large. It is also getting smaller. Under such circumstances, it is very difficult to detect a large inclusion that occurs accidentally or with a very low probability.

However, oxide inclusions (eg, Al ₂ O ₃ , M
Non-metallic inclusions such as gO ・ Al ₂ O ₃ , (Ca, Mg) O ・ Al ₂ O ₃ ) and nitride-based inclusions can easily cause fatigue failure in bearing steel and carbon steel for machine structural use. Still a problem.

At present, as a standard inspection method for checking the cleanliness of a metal material, a method using an optical microscope is standard. But,
In this method, the area to be inspected is as small as about 1000 mm ^2, and as a method for evaluating the cleanliness of a metal material having high cleanliness as described above, it cannot be practically used (JIS method,
ASTM method).

Further, a method of extracting inclusions from a metal material by acid dissolution and evaluating the particle size of the inclusions with a microscope and a method of dissolving the metal material by the EB dissolution method and observing the inclusions that floated by a microscope have been proposed. (Japanese Unexamined Patent Publication No. 9-125199)
No., JP-A-9-125200), the inclusions may be dissolved in the acid, or the inclusions themselves may be melted or aggregated.
For example, it takes a long time to dissolve the acid, resulting in poor processing speed, and it is difficult to cope with a mass production process of a product.

[0006] Under such circumstances, it has been desired to develop a new technique capable of actually evaluating and guaranteeing the cleanliness of a metal material.

[0007]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a highly reliable metal material having a high degree of cleanliness in response to recent improvements in metallurgical technology and a great improvement in the cleanliness of metal materials such as steel. It does not provide an evaluation method for

[0008]

Means for Solving the Problems The present inventor has set forth the above object.
For the purpose of solving ₀
= 100mm^TwoAnd the number of samples n = 30 to 60 are collected,
Statistical analysis based on the maximum inclusion diameter that appeared in each sample
Method to determine the maximum diameter of inclusions in the metal material to be analyzed.
We've considered ways to make predictions. However, when observed under a microscope
In order to increase the speculum area in the method
There is a limit, and a large amount of metallic materials are analyzed
And the prediction of large inclusions tends to be less reliable
Could have happened. Therefore, in a method using a microscope,
A practically satisfactory evaluation of the cleanliness of metallic materials
Did not reach.

The present inventor has further conducted research, and in the method of electrolyzing a metal material, has repeatedly studied the electrolytic weight and the maximum diameter of the extracted inclusions. As a result, it is possible to use the electrolytic decomposition of metallic materials to predict the size of large inclusions that cause fatigue failure of mechanical parts, etc., and it is important that the size of the sample to be subjected to electrolysis is important. This has led to the completion of the present invention.

The present invention provides means for solving the above-mentioned problems, and is as follows. (1) A method for determining the maximum diameter of inclusions contained in a metal material to be analyzed and evaluating the cleanliness of the metal material, wherein n pieces of samples are collected from the metal material to be analyzed, and The sample pieces were electrolyzed, the residue obtained by the electrolysis was elutriated to separate the inclusions contained in each sample piece, and the maximum diameter a _j of the inclusions for each of the n sample pieces was determined. (j = 1,
n) is measured, and the estimated maximum diameter a _{max of the} inclusions contained in the metal material to be analyzed is calculated by the following equations (1) and (1 ′).
And calculating the cleanliness of the metal material to be analyzed. [Equation 1] Linear regression equation of maximum diameter a _j (j = 1, n) of inclusion and normalized variable y _j (j = 1, n) a = ty + u (1) where n = Number of inspections Normalized variable y _j = -ln [-ln {j / (n + 1)}]
(J = 1, n) t = regression coefficient u = constant [Equation 1 ′] Formula for calculating the estimated maximum diameter a _max of inclusions in the metal material to be analyzed (regression formula) a _max = t × y _max + u · (1 ′) where V ₀ = inspection reference weight (g) V = weight to be predicted (g) T (recursion period) = (V + V ₀ ) / V ₀ y _max (normalization variable) = − ln [-ln ｛(T-1) /
T｝] (2) The cleanliness of the metal material according to (1), wherein the sample piece collected from the metal material to be analyzed is collected from a portion where a solidified structure of branched columnar crystals is formed. Evaluation method. (3) The method for evaluating the cleanliness of a metal material according to (1) or (2), wherein the sample piece is electrolyzed, the residue is elutriated, and magnetic inclusions are separated to separate inclusions. (4) The method for evaluating cleanliness of a metal material according to any one of (1) to (3), wherein the metal material to be analyzed is steel. (5) ΔV so that Δa _max / ΔV ₀ is 0.5 or less.
_The method for evaluating cleanliness of a metal material according to any one of (1) to (4), wherein ₀ is set. (Δ: delta, for example, Δa _max is a difference of a _max .)

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION As a result of various studies, the present inventors have inadvertently found that inclusions in a metal material exceeding 20 microns have become much smaller and smaller in size. Alternatively, it has been concluded that it is extremely difficult to detect large inclusions that occur with a very low probability by a method based on microscopic observation. It was considered that such large inclusions did not always appear on the test surface, but rather were often hidden and not observed.

For this reason, it has been considered that it is practically difficult to determine the diameter of inclusions contained in a large amount of metal material using a method based on microscopic observation and to evaluate the cleanliness and assure quality.

The inventors of the present invention have conducted various studies, and as a result, the metal material is electrolyzed, and the residue obtained by the electrolysis is washed away with water to remove impurities (elutriation method) to separate inclusions. By using the method, it has been found that the maximum diameter of non-metallic inclusions such as oxides and nitrides in a metal material can be accurately estimated. (In the present specification, a method of electrolyzing a metal material and elutriating the residue obtained by electrolysis is sometimes simply referred to as "electrolysis / elutriation method.") By adopting the method basically, the present invention has succeeded in achieving the effect of being able to perform the inspection (in terms of weight) more than 10 ⁴ to 10 ⁵ times the conventional one. Hereinafter, embodiments of the present invention will be specifically described.

In the present invention, n sample pieces to be subjected to electrolysis are collected from the metal material to be analyzed. The n sample pieces may be collected from any part of the metal material to be analyzed. However, a predetermined inspection part is set so that the metal material to be analyzed can be appropriately evaluated according to the purpose of the evaluation. It is preferable to set and collect from the inspection site. For example, if a continuously cast steel slab is to be analyzed, the inspection site is set at the top, middle, and bottom of the continuously cast steel slab as shown in FIG. Thus, it can be set at a site where large inclusions are likely to occur. Furthermore, when setting a specific inspection site, it is preferable to collect a plurality (for example, three) of sample pieces having the same shape from the set inspection site. In this way, all parts of the metal material to be analyzed can be inspected on average. In the case of using a continuously cast steel slab as the analysis target metal material, setting the inspection site at all the top, middle, and bottom sites and collecting a sample piece will allow the initial, middle, and
It will also examine the site corresponding to the terminal stage.

As the number n of the sample pieces increases statistically, the reliability increases, but in practice, it is preferably 20 to 20.
The number is 60, particularly preferably 20 to 40. Within this range, the test can be performed quickly without a great burden on the work, and statistically reliable estimation data of the maximum diameter of the inclusion can be obtained.

Next, the size of a sample piece to be subjected to electrolysis is described. When determining the size of the sample piece, it is preferable to consider 1) the thickness of the sample piece including large inclusions, 2) the area of the sample that is not affected by the solidified structure, and the like. Specifically, when deciding the size of the sample piece, it is desirable to avoid breaking the inclusions to be extracted from the sample piece. In addition, although it depends on the manufacturing history of the metal material to be analyzed, inclusions in the metal material are often present in a solidification structure called columnar to equiaxed branched columnar crystals. Is preferably collected from a portion where a coagulated tissue is formed. That is, it is preferable that the sample piece has a size and shape that include inclusions from the solidified structure having a large amount of inclusions as described above. The size and shape of such a preferable specimen are
It can be determined as appropriate based on accumulated data and experience. V ₀ = 10 with steel as the metal material to be analyzed
To illustrate the case of 0 g, the thickness of the sample piece is:
Preferably it is 0.5 to 2 cm. More specifically, the sample piece has a length of 5 to 15 cm, a width of 2 to 5 cm, and a thickness of 0.5 to
It is preferable to form a rectangular parallelepiped of 2 cm. With such a size and shape, large inclusions of about 20 μm are included in the sample sufficiently without destruction of the shape of the inclusion during sample preparation, and the large inclusions are usually large. It is possible to evaluate the branched columnar crystal part extending from the generated columnar crystal to the equiaxed crystal. The above is a steel having a solidification structure remains cast a Considerations when sampling at the time of the sample piece, in the case of rolled steel solidification structure is destroyed, it may be a proper V _0, in particular It is not limited to the shape.

Next, in the present invention, inclusions are extracted from the collected test pieces by electrolysis and elutriation, and the diameters of the inclusions are measured to determine the maximum inclusion diameter a _j (j = 1, n). .

The electrolysis can be performed according to a usual method according to the type of the metal to be analyzed. For example, the electrolysis can be performed as follows. First, a metal material to be analyzed is suspended in an electrolytic cell and an electrolytic solution is put into the electrolytic cell. Electrolysis is performed by applying a current to the metal material to be analyzed as an anode and an iron wire mesh as a cathode. Electrolysis produces residues due to electrolysis.

Residues after the electrolysis include 1) electrolytic iron pieces dropped from the cathode, 2) carbides, 3) iron hydroxide, 4) inclusions,
Consists of Among them, iron hydroxide having the largest volume is 3). The iron hydroxide is in the form of submicron colloid and is smaller by one to two orders of magnitude than the size of the inclusions to be extracted, so that it can be easily removed by elutriation. That is, the residue after the electrolysis is transferred to a container of an appropriate size, for example, a magnetic dish, and iron hydroxide can be removed while flowing tap water. More specifically, 250
It is preferable to change the height of three magnetic dishes having a diameter of about mmφ in a step-like manner, and to perform the operation while flowing tap water into the magnetic dishes. Magnetic dish 3
The reason for using the sheets is to capture the target inclusions in the second and third stages even if the target inclusions flow out of the first stage magnetic dish due to excessive flow of tap water. That is, any number of magnetic dishes may be used, but usually two or three sheets are appropriate.

After removing the iron hydroxide, which accounts for the majority of the residue, visually or using a binocular stereo microscope,
Inclusions can be separated. However, separation by visual inspection or the like often requires the experience of an operator. Therefore, when the metal material to be analyzed is a magnetic material, the iron hydroxide is removed and then separated using a magnet,
Inclusions can be easily separated. That is, the residue after electrolysis to remove iron hydroxide, 1) electrolytic iron pieces dropped from the cathode, and 2) carbides are magnetic, and can be removed from a magnetic dish using a magnet by magnetic separation. Lastly, there are inclusions 4), which mainly include oxide inclusions and nitride diameter inclusions, depending on the type of metal material.

The inclusions separated from the inclusions separated by magnetic separation or the like are extracted using a binocular stereo microscope or the like, and the diameter of the inclusions in the separated inclusions is measured.

In this way, for each test piece, the maximum diameter of the inclusions included in the test piece is determined, and a _j (j = 1, n) for n sample pieces is determined.

Next, the maximum inclusion diameter a _j (j = 1, n) for the n test pieces is obtained in this manner, and the maximum inclusion diameter a _j (j = 1, n) is calculated by the above equations (1) and (1 ′). _Is calculated as the estimated maximum non-metallic inclusion diameter a _max .

FIG. 2 shows the relationship between the weight of the test piece, that is, the inspection reference weight V ₀ and the estimated maximum diameter a _max of the inclusions, with respect to steel. As can be derived from FIG. 2, when V ₀ is small, the value of a _max which should not be varied originally changes, and the reliability of a _max tends to be low in some cases. Therefore, it is preferable that Δa _max / ΔV ₀ is set to V ₀ to 0.5 or less, particularly preferably, may be set to V ₀ as Δa _max / ΔV ₀ is 0.05 or less.

The method for evaluating cleanliness of the present invention can be applied to metal materials, and can be particularly suitably used for steel and the like. Further, when the problem of fatigue fracture of a mechanical part is a problem, it is preferable that the predicted weight of the metal material be 0.5 kg or more. Further, the cleanliness evaluation method of the present invention is particularly useful as a cleanliness evaluation method based on the inclusion diameter of nonmetallic inclusions such as oxide-based inclusions and nitride-based inclusions.

[0026]

[Example 1]

(1) Metal Material to be Analyzed As a metal material to be analyzed, a round bar-shaped steel piece as shown in FIG. 1 of 160 t of high carbon steel for a machine structure (for a rod tube) manufactured by a continuous casting method was used. The cleanliness was evaluated by the method of the present invention as described above.

Inspection sites were set in the circles of the round bar-shaped steel slab shown in FIG. 1 and sample pieces for electrolysis were collected from each inspection site. 3 each from top and middle
A total of 30 electrolysis sample pieces were cut out from each of the pieces and the bottom portion (that is, n = 30).
Since the bottom portion generally has a lower degree of cleanliness of the steel material than the other portions, the bottom portion has more than the other portions.
Sample pieces were taken.

The shape of each sample piece is 5 cm in length × width × thickness.
It is a rectangular parallelepiped of size 3cm x 0.5cm and weighs 5
0 g (inspection reference weight V ₀ ).

(2) Extraction of inclusions by electrolysis and elutriation Next, inclusions were extracted and separated by electrolysis and elutriation to determine the diameter of the inclusions at each analysis site, and the largest nonmetallic inclusions were obtained from this. The diameter a _j (j = 1, n) was determined.

The metal material to be analyzed is 200 mm in diameter,
The suspension was suspended in a 250 mm-high electrolytic cell made of polyvinyl chloride, and a 10 to 15% ferrous chloride solution as an electrolytic solution was put in the electrolytic cell. A sample piece collected from the metal material to be analyzed was used as an anode, and a 3 mmφ iron wire mesh was arranged in the circumferential direction inside the electrolytic cell as a cathode. Then 50mA / cm
Electrolysis was performed at a current density of ² . Sample 30 in the electrolyzer
The samples were arranged in series, and 30 sample pieces were electrolyzed simultaneously.

After the electrolysis, the wire mesh of the cathode was removed, and the electrolytic solution in the electrolyzer and the residue of each sample piece were removed for 250 minutes.
It was transferred to a magnetic dish of mmφ and elutriated. Elutriation is 250mmφ
The height of three magnetic dishes was changed in a stepwise manner, and the tapping was performed while flowing. The residue after the electrolysis is composed of 1) electrolytic iron pieces dropped from the cathode, 2) carbides, 3) iron hydroxide, 4) inclusions, and the like. Although it depends on the type of steel such as the amount of carbon, the largest volume of 1) to 4) is iron hydroxide 3). This iron hydroxide is a submicron colloid, and is smaller by one to two orders of magnitude than the size of inclusions to be extracted. Therefore, the iron hydroxide can be removed under an appropriate amount of flowing water. Tap water was flowed through the electrolytic solution containing the electrolytic residue transferred to the first (first from the top) magnetic dish, and iron hydroxide was allowed to flow to the downstream side with gentle stirring with a glass rod to remove it.

After the removal of the iron hydroxide, magnetic separation was performed. As a result, the residue after the electrolysis 1) the electrolytic iron pieces dropped from the cathode and 2) the carbide can be removed because they are magnetic. The magnetic separation was performed using a bar magnet while rotating the bar magnet in flowing water so as to prevent inclusions from being mixed into the magnetically sorted material (magnetic material attached to the bar magnet).

Finally, the remaining inclusions of 4) were washed with alcohol, and then dried in a dryer. After drying, the paper was taken out, transferred to a smooth paper, the paper was slightly inclined, and the inclusions were rolled. Then, the larger inclusions rolled preferentially, and the largest inclusion diameter was measured using a stereoscopic microscope from these. Thus, the maximum inclusion diameter was measured for each of the 30 test pieces, and the maximum inclusion diameter a _j (j = 1,3
0) was determined.

(3) Estimation of the maximum diameter of inclusions in the metal material to be analyzed From the maximum diameter a _j (j = 1, 30) of the inclusions for each of the 30 sample pieces obtained as described above. The estimated maximum inclusion diameter a _max was determined.

First, the maximum diameters a of the inclusions are arranged in order from the minimum value, and defined as a ₁ , a ₂ ,..., A _{30 in} ascending order.

Here, 1, 2,.
The value obtained by calculating j twice by logarithm is the standardized variable y _{j described} in [Expression 1]. Table 1 summarizes j, a _j , and y _j .

[0037]

[Table 1]

In FIG. 3, the inclusion diameter is plotted in order from the smallest inclusion diameter (ie, a ₁ ), with the inclusion diameter plotted on the horizontal axis and the normalized variable on the vertical axis. And this ●
Is a straight line that rises to the right on the right side (this straight line is expressed by an equation, which is the equation (1) for the metal material to be analyzed in the present example).

Since the weight of each sample is constant in the electrolysis test, the standardized variable on the vertical axis indicates the weight of the sample. Speaking in Table 1, y _j = -1.2337 means 50g as inspection reference weight one minute specimens, y _j =
-1.002 is the inspection reference weight for two sample pieces, 2V ₀
Is equivalent to When it is desired to predict the maximum diameter inclusion a _max included in the region for a certain weight V,
What is necessary is just to calculate back from the value of the vertical axis | shaft corresponding to the weight V. This conversion equation is [Equation 1 '], and [Expression 1'] The value of the vertical axis corresponding to the weight V to be predicted at T (recursion period) described in the proviso may be obtained.

For example, in the method of the present invention shown in FIG. 3, Δa _max / ΔV ₀ =
If it corresponds to 0.049, the weight you want to predict is 10kg
With respect to (provided T (recursion period) of Equation 1 ′ = 201), the maximum inclusion diameter indicated by a straight line rising to the right on the right is 40.3 μm.

Although not shown in FIG. 3, the reference weight V ₀ = 2 g (Δa _max / ΔV ₀ = 1.33), the weight to be predicted is 10 kg (the proviso T (recursion period) of equation 1 ′ = 500
In contrast to 1), the maximum inclusion diameter indicated by a straight line rising to the right on the right was 34.8 μm.

On the other hand, in the microscopic method (conventional method), the inspection reference area S ₀ = 100 mm ² , and the area S to be predicted S = 30000-
50000 mm ² [Equation 1 '] proviso T (recursion period) =
300 to 500], and T = 301
When the maximum diameter of the inclusions appeared was estimated from FIG. 3, it was 16.2 μm.

Although not shown in FIG. 3, the standard weight V ₀ = 7.8 × 10 ⁻³ g (S ₀ = 100 m) was obtained by microscopy.
m ² , weight when the thickness is 10 μm), weight to be predicted 10 kg (provided T (recursion period) of equation 1 ′ = 1282)
052), the maximum diameter of the appearing inclusion is shown in FIG.
As a result, it was 31.8 μm.

(4) Evaluation of the cleanliness of the metal material to be analyzed The cleanliness of the metal material to be analyzed is evaluated by estimating the maximum inclusion diameter = a _max , the inspection reference weight V ₀ g, and the weight Vg to be predicted. Can be given. The above results are summarized as follows.

According to the method of the present invention, Δa _max / ΔV ₀ = 0.5, the evaluation of the cleanliness of the round bar-shaped lump as the metal material to be analyzed is based on the estimated maximum inclusion diameter: a _max = 40. 3μ
m, inspection reference weight V ₀ = 50 g, prediction weight V = 10
kg.

The method of the present invention, Δa _max / ΔV ₀ =
According to the method of 1.33, the evaluation of the cleanliness of the round bar-shaped lump as the metal material to be analyzed is based on the estimated maximum inclusion diameter: a _max =
The weight was 34.8 μm, the inspection reference weight V ₀ = 2 g, and the weight V for prediction was 10 kg.

On the other hand, the inclusion investigation by microscopy
The result is the inspection reference area S₀= 100mm^TwoThe area to make a prediction
S = 30000mm^TwoThe maximum inclusion diameter that appears on the surface is 16.2
μm. In addition, the reference weight V₀= 7.8 ×
10^-3g (S₀= 100mm ^TwoWhen the thickness is 10 μm
Weight), the maximum number of inclusions that appear at a
The large diameter was 31.8 μm.

The following tests were conducted to confirm the evaluation accuracy. A mechanical component that was subjected to repeated stress and required to have fatigue strength was prepared from the round bar-shaped mass used in the present invention, and a fatigue fracture test was performed. When the diameter of the inclusions observed on the fracture surface of the fractured specimen was measured, it was 40.9 μm.

Therefore, it was found that the value of the estimated maximum diameter of the present invention matched well with the actually measured value, and the prediction accuracy of the evaluation method of the present invention was extremely excellent.

Example 2 150 tons of spring steel (JIS steel type SUP10) was melted in an electric furnace. After degassing the RH, it was cast into a slab (bloom) having a cross section of 380 × 490 mm by continuous casting. Then, it was subjected to slab rolling to obtain a billet of 167 mmφ and a weight of 2 t. This was rolled and processed into a 5 mmφ valve spring. When this 2t spring was used, it broke during use. When the broken portion was examined, an inclusion of 60 μm was confirmed.

On the other hand, of the rolled material subjected to the spring, a sample was cut out from the remaining village of the rolled material stored without being subjected to the spring processing, and the sample preparation, electrolysis and Table 2 shows the results of evaluating the particle size of the inclusions by elutriation.

[0052]

[Table 2]

The maximum inclusion prediction with conventional microscopy was 18.5 μm. In contrast, according to the method of the present invention,
It was estimated that the maximum inclusion diameter that could exist in about 2 t of the rolled material subjected to this spring working was 61 μm. in this way,
It was found that it was suitable as a method for evaluating oxide-based inclusions.

[0054]

According to the present invention, it is possible to quickly and accurately evaluate the cleanliness of a large amount of metallic materials with high reliability.

Further, it contributes to the recent evaluation of cleanliness and quality assurance of metal materials such as steel, and is a very useful invention that meets the needs of the industry.

[Brief description of the drawings]

FIG. 1 shows the setting of an inspection part in a metal material to be analyzed;
It is a figure which shows an example of the collection number of the sample piece from each test | inspection part.

2 is a diagram showing an example of the relationship between the inspection standard weight V ₀ estimated maximum inclusion diameter a _max.

FIG. 3 is a diagram showing a comparison between optical microscopy (conventional method) and the method of the present invention in estimating the maximum diameter of inclusions in a metal material.

Claims (5)

[Claims] 1. A method for evaluating the cleanliness of a metal material by determining the maximum diameter of inclusions contained in the metal material to be analyzed, comprising collecting n sample pieces from the metal material to be analyzed. Electrolyzing each sample piece, elutriating the residue obtained by the electrolysis to separate inclusions contained in each sample piece,
The maximum diameter of the inclusion a _j (j
= 1, n), and the estimated maximum diameter a _max of the inclusions contained in the metal material to be analyzed is calculated by the following equations (1) and (1 ′) to determine the cleanliness of the metal material to be analyzed. A method for evaluating the cleanliness of a metal material, which is characterized by performing the evaluation. [Equation 1] Linear regression equation of maximum diameter a _j (j = 1, n) of inclusion and normalized variable y _j (j = 1, n) a = ty + u (1) where n = Number of inspections Normalized variable y _j = -ln [-ln {j / (n + 1)}]
(J = 1, n) t = regression coefficient u = constant [Equation 1 ′] Formula for calculating the estimated maximum diameter a _max of inclusions in the metal material to be analyzed (regression formula) a _max = t × y _max + u · (1 ′) where V ₀ = inspection reference weight (g) V = weight to be predicted (g) T (recursion period) = (V + V ₀ ) / V ₀ y _max (normalization variable) = − ln [-ln ｛(T-1) /
T｝] 2. The cleanliness evaluation of a metal material according to claim 1, wherein the sample piece collected from the metal material to be analyzed is collected from a portion where a solidified structure of branched columnar crystals is formed. Method. 3. A method comprising electrolyzing a sample piece, elutriating the residue, and magnetically separating the inclusions.
The method for evaluating cleanliness of a metal material according to claim 1. 4. The method for evaluating cleanliness of a metal material according to claim 1, wherein the metal material to be analyzed is steel. 5. The metal material cleanliness evaluation method according to claim 1, wherein ΔV ₀ is set so that Δa _max / ΔV ₀ is 0.5 or less.