KR20050103978A - Steel having finely dispersed inclusions - Google Patents

Abstract

It is an object of the invention to provide a steel with good fatigue life and acoustic properties by eliminating the deleterious effects of oxide-based inclusions. The objective is obtained by the next river. Rare earth metal (REM) in an amount satisfying Formula (1); A steel having at least a portion comprising a number of REM containing oxides satisfying formula (2); The concentration of Al ₂ O ₃ of the REM-containing inclusions is 30% by mass or less (including 0%); Equation (1); 30 <REM (ppm) − (TO (ppm) × 280/48) <50, formula (2); The number of REM-containing inclusions / total inclusions> 0.8, said inclusions of formula (2) have an equivalent diameter of at least 1 μm.

Description

Steel with finely dispersed inclusions {STEEL HAVING FINELY DISPERSED INCLUSIONS}

 The present invention claims priority to Japanese Application No. 2003-068541, filed in Japan on March 13, 2003, which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION The present invention relates to steels with finely dispersed oxidic inclusions and oxysulfide inclusions, and more particularly to steels having good fatigue life and acoustic properties achieved by eliminating the deleterious effects of oxide-based inclusions. By inclusions is meant oxide inclusions, sulfide inclusions, oxysulfide inclusions, nitride inclusions, and combinations thereof. Steel means molten steel, cast slab, cast iron, cast steel and other semi-finished and finished steel products, and is not limited in use.

In recent years, the quality of steel products is required to be more stringent and diverse. There is a need for a high cleanliness steel with harmlessly produced inclusions.

The molten steel smelted in a converter or vacuum treatment vessel contains a large amount of dissolved oxygen. This excess oxygen normally reacts with aluminum having a strong affinity for oxygen to form an alumina (Al ₂ O ₃ ) inclusion. Since the Si or Mn is mostly made of Al ₂ O ₃ based refractory materials may be added to react with oxygen, the redeul (ladles) using reducing the Al ₂ O ₃ is melted steel reaction to dissociate Al is present in the molten steel do. The dissociated Al is eluted into the molten steel and then reoxidized to form Al ₂ O ₃ again.

These Al ₂ O ₃ based inclusions are cured and agglomerated to form coarse Al ₂ O ₃ clumps, which include (a) snapping of wires such as tire cords; (b) impairment of rolling fatigue properties in steel bars, such as bearing steel; And (c) causing the can to crack during the can forming process, such as drawn and ironing (DI) cans made from thin steel sheets. In particular, it is desired to reduce the size of inclusions as much as possible to improve the fatigue life of bearing steel.

Products containing bearing steel components include consumer electronics such as VCRs or CD players, general equipment such as measuring or medical instruments, office automatic equipment and electronic devices such as hard disk drives. In these products, the bearing steel is used under light loads, but the sound or vibration generated in the moving parts made of the bearing steel is required to be as small as possible. Among other things, small parts made of bearing steel for hard disk drives have stringent requirements to not produce noise and vibration. The main cause for the production of sound and / or vibration is likely to be hard inclusions such as Ti carbide nitrides or Al ₂ O ₃ based inclusions on the bearing surface.

In addition to reducing the total amount of Al ₂ O ₃ based inclusions of hard base metals in order to improve cleanliness, it is important to render them inclusions and reduce their size (finely) to make them harmless. Inclusions can be made harmless by subjecting the inclusions present in the initial stages of the process to deoxygenation of the molten steel to subsequent harmless products in composition, shape and size.

In order to reduce and eliminate Al ₂ O ₃ based inclusions, (1) prevent the reoxidation of Al by blocking air and / or slag reforming with reducing non-oxide inclusions mixed by slag cutting, (2) Attempts have been made to reduce deoxygenated products by applying secondary smelting apparatus such as RH type vacuum degassing apparatus or powder injection apparatus.

Typically, Al ₂ O ₃ inclusions suppress Al ₂ O ₃ roughening through solidification by reforming Al ₂ O ₃ to spinel (MgO · Al ₂ O ₃ ) or MgO by adding Mg alloy to molten steel. It is produced harmlessly (see, for example, Japanese Patent Application Laid-Open No. 193-311225). Regarding the production of Al weakened steel containing Al by 0.005% or more by mass, a method for producing Al killed steel without lumps is known, wherein 2 selected from Ca, Mg and REM (rare earth metal) An alloy consisting of more than one component and Al is added to the molten steel and the Al ₂ O ₃ content of the final inclusion is limited to 30 to 85 mass% (see, for example, Japanese Patent Application Laid-Open No. 1997-263820).

However, the aforementioned method for removing Al ₂ O ₃ based inclusions is no longer sufficient to meet the required high quality level. In addition, the following problems are known for certain methods for making the inclusion finer by adding Mg and / or REM. In reducing the size of inclusions using Mg, the Mg added is easily vaporized (boiled) because the temperature of the molten steel to which Mg is added is higher than the boiling point (1070 ° C.) of the metal Mg. Even if Mg is added in the form of a mixture with Si, Al or Fe-Si, the Mg activity still remains at 1. Thus, the vaporization problem remains the same as with Mg alone. When alloys such as Al-Mg or Si-Mg are used, the Mg loss due to vaporization can be reduced to some extent. However, after being dissolved in the molten steel, it is difficult to prevent Mg from vaporizing as in the case where Mg alone is added, leading to low yields.

Mg vaporizes more rapidly when added in vacuo compared to when added under normal pressure. Therefore, Mg addition should be made under the normal pressure set after vacuum smelting is performed.

In addition, as shown in Table 1, the spinel formed in the reforming step is not as hard as the initial Al ₂ O ₃ , but is still hard. Thus, the requirements of low vibration and acoustic properties for small bearing steel products cannot be sufficiently met.

The prior art for refining inclusions (reducing size) by adding REM has the following problems. In the case of REM addition disclosed in Japanese Patent Application Laid-Open No. 1997-263820, the addition of two or more components selected from Ca, Mg and REM to avoid the formation of Al ₂ O ₃ agglomerates results in the formation of compound inclusions having a low melting point. Brings about. This may be useful in preventing sliver defects but cannot reduce the size of inclusions to the level required for bearing steel. This is because the low melting point inclusions usually solidify to become coarse.

In addition, with respect to the miniaturization method using conventional REM addition, it is known that REM can be added to control the shape of the inclusions, since the REM can produce a spherical shape of the inclusions providing a good fatigue life. The amount of REM to be added should be 0.010% by mass or less because addition of 0.010% by mass or more of REM increases the amount of inclusions that reduce fatigue life (see Japanese Patent Application Laid-Open No. 1999-279695). However, there is no analysis and suggestion on the state and mechanism of the current composition of inclusions.

REM is an element that forms a bond with oxygen (O) to form a REM oxide, and also attempts to form a sulfide by forming a bond with sulfur. Thus, if there are more REMs than necessary to react with all the oxygen, it is believed that excessive REMs will form sulfides that increase the size of the inclusions, thereby providing a deleterious effect on fatigue life characteristics. It is important to strictly control the composition of the inclusions by adjusting the amount of REM added to control the inclusion size. In other words, the formation of coarse sulfides will be prevented by balancing the O content and the REM addition amount to avoid excessive REM amount. Sulphides will also have to be refined (decreasing in size) as sulfides also affect fatigue life characteristics. As mentioned above, the S content should be low so that the size of the inclusion structure is reduced by addition, since REM readily forms sulfides. These technical concepts are not disclosed in Japanese Patent Application Laid-Open No. 1999-279695.

Japanese Patent Application Laid-Open No. 2000-45048 describes the acoustic properties of small bearing steel products used in consumer electronics such as VCRs or CD players, general equipment such as measuring or medical instruments, office automatic equipment and electronic devices such as hard disk drives. Disclosed is a bearing steel containing 7 ppm or less of O and 7 ppm or less of Ti to reduce the amount and size of Ti-based inclusions in order to improve the efficiency. Japanese Patent Application Laid-Open No. 1996-312651 discloses a rolling bearing steel having a surface hardness of at least HRC57 prepared by quenching or tempering to a high temperature of 350 ° C. or higher and containing no residual austenite (0%) in the carbonitride layer. To start. However, even if the absolute amount of the oxide-based inclusions is reduced by lowering TO (total oxygen), further improvement of the acoustic characteristics cannot be expected because the oxide-based inclusions are hard due to Al ₂ O ₃ . Al ₂ O ₃ can be reformed into fine spinels by Mg addition, which is Al ₂ O ₃ as shown in Table 1 It is not as hard as it is, but is still hard and does not provide adequate improvement. In contrast, REM based inclusions are soft as shown in Table 1. Therefore, reforming the hard Al ₂ O ₃ based inclusions into soft REM based inclusions by adding REM is useful for improving acoustic characteristics.

Figure 1 shows the effect of the value [REM (ppm)-T.O (ppm) x 280/48) on the size of the inclusions.

2A shows a stable region for oxides of Ce, oxysulfides of Ce and sulfides of Ce.

2B shows a stable region for oxides of La, oxysulfides of La, and sulfides of La.

3 shows the effect of a REM source on the size of inclusions.

4 shows the effect of [S] on the size of inclusions.

It is an object of the invention to provide a steel with good fatigue life characteristics and good acoustic / vibration properties by miniaturizing and dispersing oxide and oxysulfide inclusions. This steel is referred to as inclusion finely dispersed steel.

The term "REM containing oxysulfide" is defined as follows. REM can form oxides as described above and can also easily form sulfides. Thus, if S is present, it is possible for REM to combine with both S and O to form a REM oxysulfide whose stoichiometric composition is represented by RE ₂ O ₂ S, where RE represents REM.

After the experiments and tests were made, the conditions for achieving the purpose were obtained for the amount of REM added, inclusion composition and steel component to enable the formation of fine inclusions. A brief summary follows.

Item (1): a REM whose quantity satisfies Expression (1); The number is a steel having at least a portion comprising a REM containing inclusion that satisfies formula (2); The concentration of Al ₂ O ₃ contained in the REM-containing inclusions amounts to 30% by mass;

Formula (1)

30 <REM (ppm)-(T.O (ppm) × 280/48) <50;

Formula (2)

Number of REM containing inclusions / total number of inclusions> 0.8;

Here, the inclusion of Formula (2) has an equivalent diameter of 1 µm or more.

Item (2): The steel described in item (1) above, wherein the REM is at least one selected from the group consisting of Ce, La, Nd and Pr.

Item (3): The steel described in item (1) or (2) above, wherein the steel corresponds to at least one of the following four conditions.

Al content = 0.05 mass%;

T.O. Content = 0.005 mass%;

S content = 0.003 mass%;

Ti content = 0.001 mass%;

Here, the steel has improved fatigue life and / or acoustic properties by miniaturizing inclusions using the conditions described above.

Item (4): the steel described in item (3) above; Where the steel is the bearing steel.

Item (5): the steel described in item (4) above; Steel is used here for small bearing steel elements for hard disks or audiovisual equipment.

In item (1), steel is defined as having "at least a portion" to meet the requirements of equations (1) and (2). In the manufacture of steel, it is generally known that the composition of the surface region (s) may differ from the composition of the core region (s) of the steel. By indicating that the original steel has at least a portion that meets the requirements of equations (1) and (2), the original steel will have area (s) that do not satisfy both equations (1) and (2). It is envisioned that it may have at least one region that may satisfy both formulas (1) and (2).

A more detailed description of the various aspects of the embodiments of the invention shown in the accompanying drawings will now be made. It is understood that these drawings are merely illustrative of exemplary embodiments of the invention and thus do not limit the scope of the invention in any way, and the various features of these exemplary embodiments, together with additional specificity and detail, through the use of the accompanying drawings. Will be described and described.

The components and compositions of the steels of any non-limiting example of the invention are described below. The concentration of each component is expressed in percent by mass.

Al content

As mentioned above, the steel of the invention is produced in the course of the conversion of inclusions, such as oxides, from Al ₂ O ₃ to REM oxides or REM oxysulfides by the addition of REM. Al is not essential for all types of steels, but is a deoxygenating element for reducing TO and is a useful component for controlling the crystal grain size. However, after the Al content reaches 0.05%, there is no addition effect on the crystal grain size as well as Al ₂ O ₃ cannot be converted into REM oxide or REM oxysulfide, which makes it difficult to achieve the object of the invention. do. Therefore, the upper limit of Al content should be 0.05%. Applicants theorize that Al ₂ O ₃ is in a more stable thermodynamic state than REM oxide or REM oxysulfide at high Al concentrations. Thus, no REM oxide or REM oxysulfide is formed.

T.O. (total oxygen) content

As used herein, the term "TO content" is generally equivalent to the amount of dissolved oxygen that forms an oxide (primarily Al ₂ O ₃ ) in the steel. Thus, the higher the TO content, the more Al ₂ O ₃ to be reformed in the steel. If the TO content exceeds 0.0050%, the Al ₂ O ₃ amount is too large to allow conversion of the total Al ₂ O ₃ amount to the REM oxide or REM oxysulfide by adding REM, i.e., a predetermined amount of Si ₂ O ₃ remains in the river. Therefore, TO content is 0.0050% or less.

REM is a strong deoxidizing element is added to react with Al ₂ O ₃ in the steel to form REM oxide by reacting with oxygen in Al ₂ O _3. As a result, if an appropriate amount of REM proportional to the amount of Al ₂ O ₃ , that is, TO content is not added, unreacted Al ₂ O ₃ remains. The test on this case derived the existence of a relationship between the REM content and the TO content as shown in the following equation (1).

Equation (1): -30 <REM (ppm)-(T.O (ppm) × 280/48) <50

This equation (1) is explained based on FIG. Fig. 1 shows (REM (ppm)-(TO (ppm) x 280 /) using Dmax, 30000 (µm) at an inclusion content of 0.005% S (hereafter, [S]) and 0.002% [S]. It is shown how it is affected by the value of 48).

The vertical axis, “dmax, 30000 (μm)” represents the maximum size of inclusions present in an area of 30000 mm ² expected by extreme statistics (Beretta et al., Published in 2001, which is hereby incorporated by reference in its entirety). And Material Report B, 32B, pp. 517-523). The evaluation procedure by limit statistics is as follows.

i) 16 specimens of 10 × 10 mm (100 mm ² ) steel are prepared for microscopic testing;

ii) the maximum size inclusions of each of the 16 samples are listed and sized;

iii) The maximum size present at 30000 mm ² is estimated from 16 maximum size data using extreme statistical processing.

First, after the two types of molten bearing steels containing 8 ppm T.O. in which [S] is 0.005% and 0.002%, respectively, a misch metal is added to REM. The micronized behavior of the inclusions is tested. It is assessed how the inclusion size is affected by the amount of REM added. Samples for microscopic testing are taken from steel ingots, the maximum size of inclusions is evaluated by extreme statistics and the relationship between the maximum size and the amount of REM added is tested. In Fig. 1, the solid line represents the case of 0.005% [S], and the dotted line represents the case of 0.002% [S].

The final inclusion is stably found in the range {REM (ppm)-(T.O. (ppm) x 280/48)} between -30 and 50.

In the case of 0.002% [S], the inclusion becomes finer. Incidentally, the composition of the inclusion satisfies the following formula (2), in which the amount of Al ₂ O ₃ is 30 mass% or less. That is, to prevent Al ₂ O ₃ from remaining unreacted, and the oxide is -30 as defined in the formula (1) (REM (ppm)-(TO (ppm) × 280/48)) -30 It is possible to switch to REM oxide, which is intended by keeping it at from 50 to 50. If added, the value (REM (ppm)-(TO (ppm) x 280/48) exceeds 50, the formation of sulfides is enhanced, resulting in the formation of coarse sulfides, which lowers the fatigue life. If ppm)-(TO (ppm) x 280/48) stays below -30, the formation of REM oxide or REM oxysulfide is small to achieve the object of the invention.

Coefficient of T.O. (ppm) in equation (1), ie 280/48

The main components of the micrometals are Ce, La, Nd and Pr. The atomic weight of Ce and the atomic weight of O are 16. Assuming that the REM oxide formed is RE2O3, a coefficient of "280/48" is thus used to maintain the REM (stoichiometric) balance at the O content.

particle Diameter  Oxide based on 1 μm or more and Oxysulfide Inclusions Number of  Reasons to limit percentages

In the steel smelting process, steel has inevitable inclusions unlike REM oxide based and REM oxysulfide based inclusions. For example, a large amount of high oxidation slag flows out of a decarbonate smelting converter. If the degree of oxidation does not decrease in the second smelting process, the molten steel is reoxidized with slag, which results in increased Al ₂ O ₃ based inclusions. In addition, if the air seal is incomplete after the REM source is added, there will be no REM residue to dissolve into the molten steel since all REMs are used. Therefore, Al ₂ O ₃ -based inclusions increase due to oxidation by air. By addition, since the slag basicity is high in the second smelting process, Ca is fed from the slag through an equilibrium reaction between the slag and the molten steel. This leads to the formation of CaO rich inclusions that cannot be reformed by REM. If the effect of inclusion refinement by adding REM cannot be expected as in the case described, it is important to completely remove the reoxidation. Accordingly, if the number of REM-containing inclusions and other inclusions (including sulfides and nitrides) is less than 20% of the total inclusions, i.e., if Equation (2) described below is satisfied, then a high percentage of inclusions will be refined and reliably sprayed. Fatigue life is improved by addition.

Equation (2) = REM Inclusions amount Of Inclusions  all amount  > 0.8

Here's how to check the number of inclusions.

Measurements are made using an analysis instrument consisting of a combination of an X-ray microanalyzer and a computer using the following steps:

i) designation of the measuring area of the steel sample;

One area of the viewing area is defined as 0.5 mm x 0.5 mm and 5 areas of the viewing area per sample are taken;

ii) electron beam survey

The beam diameter is 0.5 μm and the beam is irradiated 1000 times in the “X” direction and 1000 times in the “Y” direction for each of the five viewing zones for urea analysis.

iii) identification of inclusions;

Information from factor analysis by electronic beam survey was processed by the computer to identify inclusions.

iv) identification of REM-containing inclusions;

Information from urea analysis by electron beam irradiation was processed by a computer, and if the inclusions contained REM, the inclusions were identified as REM containing inclusions and the composition was evaluated quantitatively.

v) number of identification

The equivalent diameters of the inclusion particles of iii) and iv) were calculated to check the size of the inclusions and the number of inclusions of iii) and iv) present in the five 0.5 mm x 0.5 mm field of view was counted.

The reason for limiting the amount of Al ₂ O ₃ of inclusions to 30% or less is as follows. If 30% or more of Al ₂ O ₃ is present, it has been found that inclusions are harder than REM oxide and REM oxysulfide when less than 30% of Al ₂ O ₃ is used. This has a negative effect on fatigue life and acoustic properties. From this viewpoint, the upper limit of the amount of Al ₂ O ₃ of the inclusions is 30%. Preferably, Al ₂ O ₃ is less than 30 mass%, more preferably Al ₂ O ₃ is 0 to about 29 mass% in the REM-containing inclusions.

Using Micro Metals as a REM Source

REM represents 17 elements of the third group in the periodic table, namely Sc (atomic number 21), Y (39), La, Ce. Rare earth metals (rare earth elements) are common names for lanthanides 57-71 of Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. These are treated together in batches with the same behavior in the literature, but each of them actually has a different tendency to form oxides, oxysulfides and sulfides. For example, as shown in FIGS. 2A and 2B, La and Ce each have different types of compounds to be stable at the same O or S concentrations.

It was found that the addition of micrometals whose main components are Ce, La, Nd and Pr (the sum of four elements occupy more than 98%) can refine the inclusion more stably than the addition of a single element (see FIG. 3). .

High S content promotes the formation of REM sulfides. Although no REM sulfide is formed, S is present in excess of the amount required in the stoichiometric composition of the REM oxysulfide, which forms coarse inclusions. In this respect, as shown in Fig. 4, a low content of S is preferred. For example, coarse sulfides are not formed and good quality materials are obtained with an S content of 0.003% or less. When the Ti content exceeds 0.001%, the shape of the hard TiN increases dramatically, having a negative effect on the fatigue life, acoustic characteristics, and vibration characteristics. Therefore, the Ti content should be 0.001% or less.

An Al content of 0.05% or less (including 0%), T.O. or less of 0.005%, in addition to the components used in the original steel products, including at least one of the following: Content, S content up to 0.003% or Ti content up to 0.001%; At least one of the following reinforcing components may be added specifically for bearing steel or small bearing steel products; That is, Si content of 0.01 to 0.4%, Mn content of 0.1 to 0.5% and Cr content of 0.01 to 1.5%.

The C concentration is not particularly limited in the invention.

In the present invention, the carbon concentration is not particularly limited. However, if the C concentration exceeds 1.2%, the REM forms carbides with C, which decreases the reforming efficiency of Al ₂ O ₃ , and if the C concentration is less than 0.005%, the amount of Al ₂ O ₃ initially present is large. In order to reduce the efficiency of reforming Al ₂ O ₃ , the C concentration is preferably in the range of 0.005% to 1.2%.

The production method for the steel of the invention is not limited to any particular method. The base molten steel can be prepared in a converter or simply in a blast furnace with an electric furnace. Other components may be added to the base molten steel as needed. The method and apparatus for the addition is not limited to the specific one. The addition of the components may be by free fall, and the components may be blown into the mixture using an inert gas or some other method. By addition, the manner in which the steel ingot is formed in the molten steel and the ingot is rolled is not excluded.

REM sources are added after vacuum smelting such as RH. For example, micrometals stored (in blocks) in a hopper disposed outside the top of the vacuum chamber are added to the surface of the molten metal in the vacuum chamber in the final stage of Ruhrstahl-Hausen (RH) treatment.

Yes

(Example 1)

After the pig iron from the blast furnace was subjected to dephosphorization and desulfurization, 270 tonnes of pig iron were transferred to the converter for coral blowing, and a base molten steel for bearing steel with a certain amount of S, P, and S was prepared. . Al, Si, Mn, and Cr were added to the prepared base molten steel while the base molten steel was discharged into the sewage basin and subjected to the sewage basin furnace (LF) and RE vacuum degassing treatment. In the LF treatment, the high oxidation slag discharged from the converter was reduced to low iron oxide and MnO, and CaO was added to increase the CaO / SiO2 ratio, thereby reducing the components causing reoxidation. Al ₂ O ₃ content was adjusted to achieve a slag composition with high inclusion absorption capacity. In the coke treatment, dehydrogenation and removal of inclusions were made. In addition, a predetermined amount of REM stored in a hopper disposed outside the top of the RH vacuum chamber was added to a subsequent step of the RH treatment step. The micrometals shown in Table 2 were used as REMs with an average particle size of 35 to 45 mm.

Steel wrought iron is made of molten steel prepared above in a continuous casting process. Steel ingots are rolled to form steel bars of bearing steel (diameter 65 mmφ) whose chemical composition is shown in Table 3.

For inclusions in steel products (river baram), REM containing inclusions account for a high percentage of inclusions that are very fine in size. The maximum inclusion size in 30000mm ² area is extremum statistics were evaluated with the (base ^{area:: 100mm 2, n = 16} , evaluated area of 30000mm ^2), a result indicating a good size was obtained as shown in Figure 3 . In addition, rolling fatigue tests (see Technical Report, "Development of High Temperature and Long-Life Bearing Steels (STJ2)" by Hiromas Tanaka et al., Incorporated herein by reference in their entirety, Vol. 68, May 2000, pages 51-57) Showed good results as shown in Table 3. The concentrations of the components of the REM shown in Table 3 for each corresponding sample are described in Table 4.

(Comparative Example 1)

The bearing steels shown in Table 3 were produced in the same manner as in Example 1. However, in Comparative Example 1, the following conditions were adopted; REM was not added to the final step in the RH treatment step as in Example 1; REM addition was made using the same technique as in Example 1 but the amount of REM added was above or below the appropriate upper limit of the amount of REM; The air was not completely sealed after the RH process was applied to increase the percentage of REM containing inclusions outside the appropriate range defined by the invention. The results of the inclusion size and rolling fatigue test of the bearing steels are shown in Table 3 as a comparative example, which is worse than the results of Example 1.

(Example 2)

The bearing steels described in Table 5 were manufactured in the same manner as in Example 1.

The number of REM containing inclusions accounts for a large percentage of the number of inclusions in bearing steel products, and the size is very fine. In (30000mm ² base ^{area:: 100mm 2, n = 16} , evaluated area), the preferred size of the table 5 with respect to the rolled steel bar (diameter: 65mmφ), the largest inclusion size of 30000mm ² area was evaluated by using the extremum Statistics The results are shown as achieved as shown. The steel product was rolled into wire rods (100 mmφ) and then made of small bearing steel. The acoustic and vibration characteristics were then tested and found to be good. The REM compositions shown in Table 5 are described in Table 6.

(Comparative Example 2)

The bearing steels shown in Table 5 were produced in the same manner as in Example 2. However, in Comparative Example 2, the following conditions were adopted; REM was not added to the final step in the RH treatment step; REM addition was made using the same addition method as in Example 1, but the amount of REM added was above or below the lower limit of the original REM amount; The air was not completely sealed after the RH process was applied to increase the percentage of REM containing inclusions outside the appropriate range defined by the invention; Ti was added so that the Ti content exceeded the concentration range of the invention. The results of the inclusion size and the acoustic / vibration characteristics of the bearing steel are shown in Table 5 as a comparative example, which is worse than the results of Example 2.

Accordingly, the present invention relates to steel with fine dispersion of REM oxide and REM oxysulfide inclusions, which can provide a steel having good fatigue life characteristics and good volume / vibration characteristics by eliminating the harmful effects of oxide inclusions. It turned out.

Claims (18)

A steel having at least a portion of a rare earth metal (REM) satisfying formula (1) and a number of REM containing inclusions satisfying formula (2), The concentration of Al ₂ O ₃ in the REM-containing inclusions is 0-30 mass%, Formula (1) -30 <REM (ppm)-(T.O. (ppm) × 280/48) <50 Formula (2) Number of inclusions with REM / total number of inclusions> 0.8 The inclusions of formula (2) are steels having an equivalent diameter of 1 µm or more.  The steel of claim 1, wherein the REM is at least one selected from the group consisting of Ce, La, Nd, and Pr. The steel of claim 2, wherein the REM comprises at least Ce, La, Nd, and Pr. The steel according to claim 1 or 2, wherein the steel satisfies at least one of the following four conditions. a) 0-0.05 mass% Al content, b) total oxygen (T.O.) content = 0.005 mass%, c) S content = 0.003 mass%, d) Ti content = 0.001 mass%. The steel of claim 4, wherein the steel has a content of 0-0.05 mass%. The steel of claim 4, wherein the steel has a total oxygen (T.O.) content = 0.005 mass%. The steel of claim 4, wherein the steel has an S content = 0.003 mass%. The steel according to claim 4, wherein the steel has a Ti content of 0.001 mass%. The steel according to claim 1 or 2, wherein the steel has a rolling fatigue life (L ₁₀ ) of 3.2 times greater than the rolling fatigue life (L ₁₀ ) of essentially the same steel prepared without added REM. 10. Steel according to claim 9, wherein the steel has a rolling fatigue life (L ₁₀ ) 7.6 times greater than the rolling fatigue life (L ₁₀ ) of essentially the same steel prepared without added REM. 10. The steel of claim 9, wherein the steel has a rolling fatigue life (L ₁₀ ) that is 3.2 times greater than 9.2 times less than the rolling fatigue life (L ₁₀ ) of essentially the same steel prepared without added REM. The steel of claim 1, wherein the steel does not have added calcium or magnesium. The steel of claim 4, wherein the steel is a bearing steel. The steel of claim 13, wherein the bearing steel is shaped for use in a hard disk drive or audiovisual installation. Steel manufacturing method, The method comprises the steps of preparing a molten steel, Adding rare earth metal (REM) and oxygen to the molten steel; Casting the molten steel into the steel, The steel has at least a portion comprising a rare earth metal (REM) satisfying formula (1) and a number of REM containing oxides satisfying formula (2), wherein the concentration of Al ₂ O ₃ of the REM containing inclusions is 0-30 mass%, Formula (1) -30 <REM (ppm)-(T.O. (ppm) × 280/48) <50 Formula (2) Number of inclusions with REM / total number of inclusions> 0.8 Wherein said inclusion of formula (2) has an equivalent diameter of at least 1 μm. The method of claim 15, wherein the REM is added during an RH vacuum degassing treatment of the molten steel. The method of claim 16, wherein the aluminum is added to the molten steel prior to REM addition. The method of claim 16, wherein the air is essentially completely sealed after the RH treatment.