JP2001342516A - Highly clean steel and production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly clean steel which can meet a requested characteristic for harsh environment in using machine parts, without using a high-cost remelting method. SOLUTION: A method for producing a highly clean steel includes producing an ingot, after pouring a molten steel melted in an arc melting furnace or a converter into a ladle, refining it in the ladle for a short time of 60 minutes or less, and degassing it for a long time of 25 minutes or more, particularly degassing for a long time setting the reflux amount of the molten steel eight times or more of the all molten steel with a reflux type of vacuum degassing device.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to steel for machine parts requiring fatigue strength, fatigue life and quietness, particularly steel for rolling bearings, steel for constant velocity joints, steel for gears, and for toroidal type continuously variable transmissions. The present invention relates to high cleanliness steel used as steel, steel for machine structural use for cold forging, tool steel, spring steel and the like, and a method for producing the same.

[0002]

2. Description of the Related Art It is important that steel used for machine parts requiring fatigue strength and fatigue life has high cleanliness (the amount of nonmetallic inclusions in the steel is small). The manufacturing processes of these high cleanliness steels include oxidizing refining of molten steel by an arc melting furnace or a converter, reduction refining by a ladle refining furnace (LF), and recirculating vacuum degassing (RH) by a recirculating vacuum degasser (RH). Processing), casting of a steel ingot by continuous casting or general ingot-forming, processing of a steel ingot by forging, and a process of a product steel material by heat treatment. In this process, scrap is heated and melted by an arc, or hot metal is put into a converter, oxidized and refined, and transferred to a ladle refining furnace. The temperature at the time of transfer is set to a high temperature of about 30 ° C. or more and less than 100 ° C. than the melting point of the steel. In a transferred ladle refining furnace, a deoxidizing alloy such as Al, Mn, and Si is charged, and reductive refining of deoxidation and desulfurization by a desulfurizing agent is performed to adjust alloy components. Generally, the longer the processing time, the longer the effect is considered to be more effective, and the processing time is generally longer than 60 minutes, and the processing temperature is generally 50 ° C. higher than the melting point. In the RH treatment, degassing is performed by vacuum degassing while circulating in a circulating vacuum degassing tank to perform deoxygenation and dehydrogenation.
About 6 times. The RH-treated molten metal is transferred to a tundish and continuously cast and cast into a bloom, billet, slab, or the like, or the molten metal is poured directly from a ladle into a steel ingot mold and cast into a steel ingot. , Rolls or forges blooms, billets, slabs, or ingots and heat-treats them as steel.

[0003] In addition, when steel with a particularly high degree of cleanliness is required, a steel ingot cast after the above process is used as a raw material.
It is manufactured by a vacuum remelting method or an electroslag remelting method.

[0004]

By the way, in recent years, the harsh environment in which mechanical parts are used, the required characteristics of steel materials have become more severe, and steel materials with higher cleanliness have been demanded. For such a request, usually the above ~
It is difficult to cope with the production by the above manufacturing process. Further, in order to meet such demands, steel materials are produced by the above-described vacuum remelting method or electroslag remelting method, but there is a problem that the production cost is extremely increased.

[0005] The present invention has been made in view of the above situation, and an object of the present invention is to provide a highly clean steel material without using a remelting method in order to avoid an extreme increase in cost.

[0006]

Means for Solving the Problems The inventors of the present invention have made intensive studies on the manufacturing process of high cleanliness steel for the above purpose, and as a result, have found that the following processes can greatly improve cleanliness. It is a thing.

The means of the present invention for solving the above-mentioned problems will be described below. Conventionally, in a process having a refining furnace such as an arc melting furnace or a converter, the arc melting furnace or the converter is mainly used for melting and oxidizing refining, and the reduction period (deoxidation) is performed in a ladle refining furnace. However, in the invention of claim 1, molten steel produced in an arc melting furnace or a converter is transferred to a ladle for refining, followed by reflux vacuum degassing, and then casting to form an ingot. In the production process of the steel to be produced, the refining in the ladle is 60 minutes or less, preferably 45 minutes or less, more preferably 45 minutes or less and 25 minutes or more, and in the subsequent degassing, usually using a reflux vacuum degassing apparatus. It is performed in less than 25 minutes with the annular flow rate of the molten steel being 5 times or more of the total molten steel, but in the present invention, the annular flow rate of the molten steel is 8 times or more, preferably 10 times or more of the total molten steel in the reflux vacuum degassing apparatus. More desirably 15 times or more and degassing for 25 minutes A process for producing a high cleanliness steel which is characterized in that on.

According to the second aspect of the present invention, when the molten steel is transferred to the ladle refining furnace, it is transferred at a temperature higher than the melting point of the molten steel by 100 ° C. or more, preferably 120 ° C. or more, more preferably 150 ° C. or more. A method for producing high cleanliness steel according to claim 1, characterized in that:

According to a third aspect of the present invention, there is provided a high-cleanliness steel manufactured by the method according to the first or second aspect.

According to the invention of claim 4, the oxygen content in the steel is 10 ppm or less, and preferably, the C content of the steel component is C <C.
At 0.6% by mass, 8 ppm or less, particularly preferably C ≧
The high cleanliness steel according to the means of claim 3, wherein the content is 6 ppm or less at 0.6% by mass.

According to the fifth aspect of the present invention, an oxide inclusion having a size of 20 μm or more, which is detected by dissolving a steel material in an acid, for example, A
The oxide-based inclusion having a l ₂ O ₃ content of 50% or more is 40 or less, preferably 30 or less, and more preferably 20 or less per 100 g of steel material. High cleanliness steel.

In the invention of claim 6, for example, the measurement of the maximum inclusion diameter in a steel material surface of 100 mm ^{2 is performed} as a test condition by 30 minutes.
3000 performed at locations and calculated by extreme value statistics
The predicted value of the maximum inclusion diameter at 0 mm ² is 60 μm or less, preferably 40 μm or less, more preferably 25 μm or less.
m or less, and the high cleanliness steel according to claim 3.

[0013]

Embodiments of the present invention will be described below. The method for producing high cleanliness steel according to claim 1 is as follows.
Process.

[0014] The molten steel is oxidized and refined by an arc melting furnace or a converter to obtain a predetermined component and temperature, and then transferred to a ladle refining furnace. Reduction smelting and component adjustment of molten steel transferred to a ladle refining furnace are performed by ladle refining. At this time, in the ladle refining furnace,
1.5 to 5.0 N. from the ladle bottom. The refining time in a ladle refining furnace, which is generally considered to be more effective when the stirring gas is blown at 1 / min / t and the forced stirring is performed for longer than 60 minutes, is 60 minutes or less, preferably 45 minutes or less, and more preferably. Is 25 minutes or more and 45 minutes or less. The molten steel after the refining and refining and the component adjustment are recirculated by a recirculating vacuum degassing device to perform degassing, and the components are finally adjusted. At this time, the ring flow rate of the molten steel in the recirculating vacuum degassing apparatus, which is generally less than 25 minutes in degassing and has a circulation rate of about 5 times that of the total molten steel, is defined as the total molten steel in the present invention. And 8 times or more, preferably 10 times or more, more preferably 15 times or more, and degassing is performed for a longer time of 25 minutes or more. This step is the most important step of the present invention, and the time required for ladle refining, in which refining is performed while heating is minimized, is a degassing step in which heating is not performed. The slag on the upper surface of the molten steel is sufficiently calmed down because the nozzle is immersed in the molten steel and only the molten steel is refluxed. For this reason, the entrapment of the oxide from the slag into the molten steel is less than in the reduction stage process of the ladle-making noodle furnace. With this equipment, sufficient flotation time for oxide-based inclusions is ensured to prevent an increase in the oxygen content due to contamination from the refractory or slag inside the ladle refining furnace and to prevent large inclusions of about 30 μm or more. Preventing the generation of products makes it possible to produce steel with a high degree of cleanliness. The molten steel whose components have been finally adjusted is cast into an ingot. After the ingot is pressed and forged into a product shape, necessary heat treatment is applied to produce a product steel material.

According to a second aspect of the present invention, there is provided a method for producing high cleanliness steel.
When transferring to a ladle refining furnace in the above manufacturing processes, the transfer temperature of the molten steel is usually set to 50 ° C. from the melting point of the molten steel.
However, in the present invention, the melting point of the molten steel is 100 ° C. or more, preferably 120 ° C. or more, more preferably 150 ° C. or more.
It should be higher than ℃. This is to sufficiently dissolve the metal that has adhered to the surroundings of the ladle refining furnace into the molten steel, and also to sufficiently raise the slag, so that the metal and slag are peeled off during the ladle refining, and The purpose is to prevent mixing and increase in the oxygen content.

According to a third aspect of the present invention, there is provided a highly clean steel manufactured by the means described in the first or second aspect.

The steel according to claim 4 is the high cleanliness steel according to claim 3, wherein the oxygen content is 10 ppm or less, preferably 8 ppm or less when the C content of the steel component is C <0.6% by mass,
Particularly desirable is a high cleanliness steel characterized by being 6 ppm or less when C ≧ 0.6 mass%, and particularly excellent in rolling fatigue life. It is generally known that the rolling fatigue life is improved by reducing the oxygen content, but among the steels manufactured by the method of the present invention, the oxygen content is 10 ppm or less, and preferably, the C content of the steel component is C <C. 8p at 0.6% by mass
pm or less, particularly preferably 6 pp at C ≧ 0.6% by mass.
m or less, a particularly excellent rolling fatigue life can be stably obtained.

According to a fifth aspect of the present invention, there is provided a steel containing an oxide-based inclusion having a size of 20 μm or more, such as Al ₂ O ₃ , which is detected by dissolving a steel material with an acid among the high cleanliness steels of the _{third aspect} . A rolling fatigue life, in which the number of oxide-based inclusions having a ratio of 50% or more is 40 or less, preferably 30 or less, more preferably 20 or less per 100 g of steel material;
High cleanliness steel with excellent fatigue strength. This method of evaluating a steel material reflects both the oxygen content and the maximum inclusion diameter in a predetermined volume. In addition, with respect to fatigue strength, fatigue life, and quietness, in steels having the same oxygen content, oxide inclusions having a certain size are harmful, and especially oxide inclusions having a size of 20 μm or more are harmful. It is. Therefore, among the steels manufactured by the method of the present invention, oxide-based inclusions having a size of 20 μm or more detected by dissolving a steel material in an acid, for example, having an Al ₂ O ₃ content of 50% or more, Is steel material 100
A steel having 40 or less, preferably 30 or less, and particularly preferably 20 or less per g is a high cleanliness steel having both excellent rolling fatigue life and fatigue strength and excellent silence.

In the steel according to the sixth aspect, among the high cleanliness steels according to the third aspect, the maximum inclusion diameter in a steel material cross section of 100 mm ² is measured at 30 locations, and the maximum inclusion diameter at 30,000 mm ² calculated by extreme value statistics is obtained. The predicted value of the maximum inclusion diameter is 60
μm or less, preferably 40 μm or less, more preferably 2 μm or less.
It is a high cleanliness steel that is particularly resistant to fatigue due to rotational bending fatigue strength and cyclic stress, which is characterized by being 5 μm or less.
It is known that the strength or fatigue limit for repeated stress largely depends on the maximum inclusion diameter in a given volume.
As disclosed in Japanese Patent Application Laid-Open No. H11-194121 filed by the present applicant, as a representative test example, measurement of the maximum inclusion diameter in a steel material cross section of 100 mm ^{2 was} performed at 30 locations, and the maximum inclusion diameter was calculated by extreme value statistics. 30000mm ²
The high cleanliness steel having a predicted maximum inclusion diameter of 60 μm or less, desirably 40 μm or less, and more desirably 25 μm or less, in particular, can stably obtain particularly excellent fatigue strength. The oxygen content is 10 ppm or less, desirably 8 ppm or less when the C content of the steel component is C <0.6 mass%, particularly desirably 6 ppm or less when C ≧ 0.6 mass%, and the maximum inclusion diameter. The steel produced according to the present invention, having a predicted value of 60 μm or less, preferably 40 μm or less, more preferably 25 μm or less, is a high cleanliness steel having both excellent rolling fatigue life and fatigue strength. By the way, acid dissolution is a very time-consuming and laborious operation.This method is capable of observing a certain area under a microscope without dissolving the steel material and statistically predicting the maximum value of the diameter of inclusions.
In particular, it is known that the maximum diameter of inclusions present at a site where there is a risk of fracture is a major factor in determining the strength of fatigue caused by repeated stress of tension and compression, and this method can statistically predict this. Is advantageous.

[0020]

EXAMPLE A molten steel oxidized and refined in an arc melting furnace was transferred to a ladle refining furnace, and the ladle was refined in a ladle refining furnace for a short time of 60 minutes or less. After degassing with a recirculation type vacuum degassing device at a circulation flow rate of at least 8 times that of the total molten steel, a 10-charge product of JIS SUJ2 steel and SCM435 manufactured in the ingot manufacturing process by casting Amount of oxygen contained in steel, maximum inclusion diameter predicted value based on extreme value statistics, L ₁₀ based on thrust type rolling life test
The service life was investigated. The maximum inclusion size predicted value Test pieces were cut out from φ65 forged material, carried out for 30 pieces of observation 100 mm ^2, predicted maximum inclusion size in 30,000 mm ² by extremes statistics. Thrust-type rolling fatigue life test using a test piece of φ60 × φ20 × 8.3T subjected to carburizing quenching and tempering, the maximum Hertzian stress Pmax: were tested with 4900MPa conditions was calculated L ₁₀ life.

In Table 1, following the oxidizing refining by the arc melting furnace or the converter which is the embodiment of claim 1 with 10 charges of SUJ2 steel, it is transferred to a ladle refining furnace to reduce the refining time to 60 minutes or less. Pot refining, followed by degassing (referred to as "short LF long RH or LF short RH length" in this specification) for 25 minutes or more by a reflux type vacuum degassing apparatus. An operation example is shown.

[0022]

[Table 1]

In Table 2, following the oxidation refining by the arc melting furnace or the converter according to the first embodiment of the present invention in which the SCM435 steel is charged 10 times, the SCM435 steel is transferred to a ladle refining furnace to have a refining time of 60.
The following is an example of a short-time LF long-time RH operation in which ladle refining is performed in a fraction of a minute or less, and then degassing is performed for 25 minutes or more by a reflux type vacuum degassing apparatus.

[0024]

[Table 2]

Table 3 shows the temperature of the molten steel to be tapped when it is transferred to a ladle refining furnace following the oxidation refining by an arc melting furnace or a converter, which is an embodiment of claim 2, with 10 charges of SUJ2 steel. At a high temperature of 100 ° C. or more from the melting point of molten steel (hereinafter referred to as “high-temperature tapping”), transferred to a ladle refining furnace, refining time is 60 minutes or less, and ladle refining is performed. An operation example of short-time LF long-time RH + high-temperature tapping in which degassing is performed for 25 minutes or more by a reflux-type vacuum degasser is shown.

[0026]

[Table 3]

Table 4 shows that the temperature of the molten steel to be tapped at the time of transfer to the ladle refining furnace following the oxidizing refining by the arc melting furnace or the converter according to the embodiment of claim 2 in which the SCM435 steel is charged 10 times. At a high temperature of 100 ° C or more from the melting point of the molten steel, transferred to a ladle refining furnace to refine the ladle with a refining time of 60 minutes or less, and then degassed by a reflux vacuum degassing device for 25 minutes or more. The operation example of the short-time LF long-time RH + high-temperature tapping in which the heat treatment was performed is shown.

[0028]

[Table 4]

Table 5 shows an example of operation of the conventional SUJ2 in comparison with the present invention, and Table 6 shows an example of operation of the conventional SCM435.

[0030]

[Table 5]

[0031]

[Table 6]

As can be seen from Tables 1 to 4, the molten steel produced in the arc melting furnace or the converter according to the present invention is transferred to a ladle refining furnace, and a short time of about 60 minutes or less is obtained. Pot refining is performed, and the molten steel is further circulated to the recirculating vacuum degassing apparatus to increase the RH rotation amount (that is, a multiple of the circulating flow rate to the total molten steel amount) to perform a long-term degassing of 25 minutes or more. The LF long-time RH treatment, and the combination of the short-time LF long-time RH treatment and the high tapping temperature of the melting point + 100 ° C. or higher, which is higher than the normal operation, are the steel types SUJ2 and SCM435. In both cases, the amount of oxygen contained in the product is small, and the number of inclusions of 20 μm or more is significantly reduced. Table 1 shows the quality of cleanliness.
As shown in Table 4, in Examples of the present invention, ○ is excellent or ◎ is very good, and both are excellent high cleanliness steels. On the other hand, in the conventional example, as shown in Tables 5 and 6, all of them are not good, and cannot be said to be cleanliness steel.

In the above, the molten steel oxidized and refined in the arc melting furnace or the converter is charged with oxygen in each charge when (transfer temperature of molten steel to ladle refining furnace) − (melting point of molten steel) = T _SH Both the quantity and the maximum inclusion diameter predicted value are reduced by increasing T _SH , and the cleanliness is improved. In addition, in each charge, the refining time and oxygen amount in the ladle refining furnace,
According to the relationship between the maximum inclusion diameter predicted value, the refining time is 60 minutes or less, for example, a short time of about 25 minutes, the oxygen content and the maximum inclusion diameter predicted value are sufficiently reduced. The predicted diameter is rather large. That is, it is considered that the erosion of the refractory of the ladle refining furnace increases with time, and the slag system loses its equilibrium due to oxidation and the like due to contact with the atmosphere, and the dissolved oxygen deviates from the minimum level. Furthermore, in the relationship between the annular flow rate with respect to the total molten steel amount, the oxygen amount, and the maximum inclusion diameter predicted value in the reflux type vacuum degassing apparatus, the higher the annular flow rate, that is, the longer the degassing time, the higher the cleanliness. The effect is high, 15
Almost saturated at more than twice.

[0034] Then, oxygen content, reducing the maximum inclusion size estimated value, it was confirmed that the L ₁₀ life is improved. From this, it was clarified that the steel produced by the method of the present invention, which can reduce the oxygen content and the maximum inclusion diameter predicted value, has excellent fatigue strength such as rolling fatigue life.

FIG. 1 shows that, in the processing of molten steel of SUJ2 steel, a molten steel obtained by oxidizing and refining molten steel by an arc melting furnace or a converter is transferred to a ladle refining furnace, and then ladle refining is performed for a short time. The method of the present invention in which degassing is carried out for a long time, and the molten steel obtained by oxidizing and refining molten steel by an arc melting furnace or a converter is transferred to a ladle refining furnace to perform ladle refining for a long time. The oxygen content in the product of each of the 10 charge examples in the case of the conventional method which is performed for a short time is shown. In addition,
In FIGS. 1, 3 and 5, A ₁ indicates the short-time LF long-time RH according to the first aspect of the invention, and A ₂ indicates the high-temperature tapping and the short-time LF long-time according to the second aspect of the invention. An example in which RH is performed is shown, and an example in which RH is performed is shown in the related art.

FIG. 2 shows that, in the treatment of molten steel of SCM435 steel, the molten steel obtained by oxidizing and refining the molten steel by an arc melting furnace or a converter is transferred to a ladle refining furnace, and the ladle is refined for a long time. The oxygen content in the product of each of 10 charge examples in the case of the conventional method in which degassing is performed for a short time is shown. In FIGS. 1, 3 and 5, A ₁ indicates the short-time LF long-time RH according to the first aspect of the present invention.
₂ shows what said hot tapping and short LF RH long an invention of claim 2, slave indicates the due prior art.

FIG. 3 shows a method according to claim 1 or 2 of the present invention for the treatment of molten steel of SUJ2 steel, and the method for a long time.
F and 1 in the case of the conventional method of performing short-time RH, respectively.
The maximum predicted inclusion diameter according to extreme value statistics in the product of the 0 charge example is shown.

FIG. 4 is a graph showing the results of the method according to the first and second aspects of the present invention and the conventional method of performing long-time LF and short-time RH in the treatment of molten steel of SCM435 steel. 2 shows the maximum predicted inclusion diameter according to the extreme value statistics of FIG.

FIG. 5 shows a method according to claim 1 or 2 of the present invention for the treatment of molten steel of SUJ2 steel,
F and 1 in the case of the conventional method of performing short-time RH, respectively.
0 indicating the L ₁₀ life by thrust-type rolling fatigue life test of a product charge examples.

FIG. 6 shows the results of the method according to claim 1 or 2 of the present invention and the conventional method of performing long-time LF and short-time RH in the treatment of molten steel of SCM435 steel. shows the L ₁₀ life by thrust-type rolling fatigue life test.

As a result, for both SUJ2 steel and SCM435 steel, the molten steel was oxidized and refined by an arc melting furnace or a converter, and the molten steel was transferred to a ladle refining furnace to perform ladle refining for a short time. By the method of performing degassing for a long time by circulating in the reflux type vacuum degassing apparatus, both the oxygen content in the product and the predicted value of the maximum inclusion diameter are greatly reduced, and the cleanliness is greatly improved by the method of the present invention. that it is L ₁₀ life significantly improved by the rolling life test was confirmed. Furthermore, when the respective processing methods are sequentially weighted from the short-time LF long-time RH according to the first aspect to the high-temperature tapping + short-time LF-long duration RH according to the second aspect, oxygen content, the L ₁₀ life both by the maximum inclusion size estimated value thrust type rolling life test, it is found to be greatly improved.

[0042]

As described above, the practice of the present invention enables the use of a very expensive re-dissolving method without using a re-dissolving method.
It is possible to provide a large quantity of steel materials with extremely high cleanliness, and steel for machine parts that require fatigue strength, fatigue life and quietness, especially rolling bearing steel, constant velocity joint steel, gear steel, toroidal High cleanliness steel used as a mold continuously variable transmission steel, cold forging mechanical structure steel, tool steel, spring steel, and the like, and a method for producing the same can be provided, and the present invention has an unprecedented superior effect.

[Brief description of the drawings]

FIG. 1 shows a short time LF in the processing of molten steel of SUJ2 steel.
Process and RH process for a long time,
The oxygen content in the product of each of the 10 charge examples in the case of the conventional method of the F treatment and the short-time RH treatment is shown.

FIG. 2 shows a 10-charge example of a method of the present invention in which a short-time LF treatment and a long-time RH treatment are performed and a conventional method of a long-time LF treatment and a short-time RH treatment in the treatment of molten steel of SCM435 steel. 2 shows the oxygen content in the product.

FIG. 3 shows a short time LF in the processing of molten steel of SUJ2 steel.
Process and RH process for a long time,
The maximum predicted inclusion diameter based on extreme value statistics in products of 10 charge examples in the case of the conventional method of the F treatment and the short-time RH treatment is shown.

FIG. 4 shows 10 charging examples of the method of the present invention in which short-time LF processing and long-time RH processing are performed and the conventional method of long-time LF processing and short-time RH processing in the processing of molten steel of SCM435 steel, respectively. 2 shows the maximum predicted inclusion diameter by extreme value statistics in the product of Example 1.

FIG. 5 shows a short time LF in the processing of molten steel of SUJ2 steel.
Process and RH process for a long time,
Shows the L ₁₀ life by thrust-type rolling life test product each 10 charge examples of the conventional example of the method of F processing and short RH treatment.

FIG. 6 shows a 10-charge example of the method of the present invention in which a short-time LF process and a long-time RH process are performed and a conventional method of a long-time LF process and a short-time RH process in the treatment of molten steel of SCM435 steel. shows the L ₁₀ life by thrust-type rolling life test products.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Kiyoshi Kawakami 3007 character, Nakajima character, Shima, Ward, Himeji City, Hyogo Prefecture Inside (72) Inventor Shuhei Kitano 3007 character, Nakashima character, Shima, Ward, Himeji City, Hyogo Prefecture Sanyo F-term (reference) in Special Steel Co., Ltd. 4K013 AA07 BA08 BA14 CC02 CE01 CF12 CF13 DA03 DA05 DA12 DA13 FA01 FA02

Claims (6)

[Claims] 1. A steel for producing an ingot by transferring molten steel produced in an arc melting furnace or a converter into a ladle refining furnace for refining, followed by reflux vacuum degassing, and then casting. In the manufacturing process, the refining in the ladle refining furnace is performed for 60 minutes or less, and the degassing is performed for 25 minutes or more by setting the ring flow rate of the molten steel by the recirculating vacuum degassing device to 8 times or more of the total molten steel. Degree steel manufacturing method. 2. The high cleanliness according to claim 1, wherein when transferring the molten steel to the ladle refining furnace, the temperature of the molten steel to be transferred is higher than the melting point of the molten steel by 100 ° C. or more. Degree steel manufacturing method. 3. A high-cleanliness steel produced by the production method according to claim 1. 4. The high cleanliness steel according to claim 3, wherein the oxygen content in the steel is 10 ppm or less. 5. The high cleanliness steel according to claim 3, wherein the number of oxide-based inclusions having a size of 20 μm or more and detected by dissolving the steel in an acid is 40 or less per 100 g of the steel. 6. 30,000 m calculated by extreme value statistics
4. The high cleanliness steel according to claim 3, wherein the predicted value of the maximum inclusion diameter at m ² is 60 μm or less.