Steel having finely dispersed inclusions
An object of the invention is to provide a steel having good fatigue life characteristics and acoustic characteristics by eliminating the harmful effects of oxide based inclusions. The object is attained by the following steel. A steel having at least a part comprising: REM of which amount meets formula (1); and REM-containing inclusions of which number meets formula (2); wherein the concentration of Al203 in the REM-containing inclusions is 30 mass % or less (including 0%); Formula (1); −30<REM(ppm)−(T.O(ppm)×280/48)<50, Formula (2); number of REM-containing inclusions/total number of inclusions >0.8, wherein the inclusions of Formula (2) having an equivalent diameter of 1 μm or more.
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The present application claims priority to Japanese Application 2003-068541, filed in Japan on Mar. 13, 2003 and which is herein incorporated by reference in its entirety.FIELD OF THE INVENTION
This invention relates to a steel having finely dispersed oxidized inclusions and oxysulfide inclusions, and particularly to a steel having good fatigue life characteristic and acoustic characteristics obtained by eliminating a harmful effect of oxide based inclusions. The inclusions refer to an oxide inclusion, a sulfide inclusion, an oxysulfide inclusion, a nitride inclusion and a combination thereof. The steel refers to a molten steel, a cast slab, cast bloom, cast billet and other half finished products and final steel products and is not limited in use.DESCRIPTION OF THE RELATED ART
Recently it is required that the quality for steel products is more strict and diversified. A steel with a high degree of cleanliness having inclusions which are made harmless is demanded.
Molten steel refined in a converter or in a vacuum treating vessel contains a large quantity of dissolved oxygen. This excess of oxygen is usually reacted with aluminum which has strong affinity for oxygen to form alumina (Al2O3) inclusions. Even though Si or Mn is added to react with the oxygen, Al is present in the molten steel because the ladles used are mostly made of Al2O3 based refractory materials with which molten steel reacts to reduce and dissociate Al2O3. The dissociated Al is eluted into the molten steel and then re-oxidized to form Al2O3 again.
These Al2O3 based inclusions are hard and coagulate to form coarse Al2O3 clusters, which cause: (a) snapping of wires such as tire cords; (b) an impairment in the rolling fatigue characteristics in steel bars such as in bearing steel; and (c) a crack in the can during the can-forming process such as drawn and ironing (DI) can made of thin steel sheet. Reducing the size of the inclusion as much as possible has been demanded, in particular, to improve the fatigue life of bearing steels.
Products incorporating bearing steel parts include electric appliances such as VCR or CD player, general equipment such as measuring instruments or medical instruments, office automation equipment, and electronic devices such as hard disk drives. In these products the bearing steel is used under a light load, however, the sound or vibration generated from moving parts made of bearing steel is required to be as low as possible. Among others, miniature parts made of bearing steel for hard disk drives have strict requirements to make no noise and no vibration. The main cause for the generation of sound and/or vibration is likely to be hard inclusions such as Ti carbonitride or Al2O3 based inclusions which are at the surface of the bearing.
It is important to re-form Al2O3 based inclusions and to make them harmless by reducing their size (fining), in addition to reducing the total amount of Al2O3 based inclusions of hard non-metal to upgrade the cleanliness. The inclusions can be made harmless by conforming the inclusions existing at the initial stage of the process following deoxidization treatment of the refined molten steel, in composition, shape and size, into harmless products.
In order to reduce and eliminate Al2O3 base inclusions attempts have been made to: (1) inhibit re-oxidation of Al by slag reformation and/or shutting off the air along with reducing mixed-in non-oxide inclusions by slag-cutting, and (2) reducing deoxidation products by applying a secondary refining apparatus such as a RH-type vacuum degassing device or powder injection device.
Conventionally, Al2O3 based inclusions are made harmless by inhibiting the coarsening of Al2O3 through coagulation by reforming Al2O3 into spinel (MgO Al2O3) or MgO by adding Mg alloy to molten steel. (For example, see JP H05-311225 A). With regard to the manufacturing of Al killed steel containing 0.005% or more of Al by mass, a method for manufacturing Al killed steel without clusters is known wherein the alloy consisting of Al and more than two ingredients selected from Ca, Mg and REM (rare earth metal) are added into molten steel and the content of Al2O3 in the resulting inclusions is limited to 30-85 mass %. (For example, see JP H09-263820 A).
However, the above-mentioned methods for eliminating Al2O3 based inclusions are not sufficient to meet the high quality level now in demand. Also, with respect to the specific method for making inclusions finer by adding Mg and/or REM, the following problems are known. In decreasing the size of the inclusions by using Mg, the Mg which is added easily vaporizes (boils away) since the temperature of molten steel to which Mg is added is higher than the boiling point of metal Mg (1070° C.). Even if Mg is added in the form of a mixture with Si, Al or Fe—Si, then the Mg activity still remains 1. Therefore, the vaporization problem remains the same as with Mg alone. In the case of using alloys such as Al—Mg or Si—Mg, the Mg loss due to vaporization can be reduced to some extent. However, after being dissolved into the molten steel, it is difficult to prevent Mg from vaporizing as in the case of when Mg alone added, which leads to low yields.
Mg vaporizes more quickly when it is added in a vacuum as compared to when it is added under normal pressure. Therefore, the Mg addition has to be made under normal pressure which is set up after vacuum refining has been performed.
Further as shown in Table 1, a spinel formed in a reforming step is not as hard as the initial Al203, but it is still hard. Therefore, the requirements of low vibration and the acoustic characteristics for small bearing steel products still can not be sufficiently met.
Conventional technology for fining (size-reducing) inclusions by adding REM has the following problems. In the case of REM addition disclosed in JP H09-263820 A, the addition of more than two ingredients selected from Ca, Mg and REM for avoiding the formation of the Al2O3 clusters, results in the formation of a compound inclusion having a low melting point. This may be helpful to prevent sliver defects but it can not reduce the size of the inclusion to the level required for a bearing steel. This is because low melting point inclusions usually coagulate to became coarse.
Further, as for the fining method using conventional REM addition, it is known that REM can be added to control the shape of inclusions, since REM is capable of making the shape of the inclusions spherical which provides better fatigue life. An amount of the REM to be added should be 0.010 mass t or less, since the REM addition of more than 0.010 mass % increases the amount of inclusions which lowers the fatigue life. (See JP H11-279695 A). However there is no analysis and no suggestion about the mechanism and the state of existing composition of inclusions.
REM is an element which makes a bond with oxygen (O) to form an REM oxide and also tends to form a sulfide by forming a bond with sulfur. Therefore, it is thought that if there is more REM present than is necessary to react with all of the O, then the excess REM will form a sulfide which provides a harmful effect to the fatigue life characteristics by increasing the size of the inclusions. It is important to strictly control the composition of the inclusions by adjusting the amount of REM added in order to control the inclusion size. In other words, formation of coarse sulfide should be prevented by balancing the amount of REM addition with the content of O to avoid an excess amount of REM. The sulfide also should be fined (reduced in size) as the sulfide also influences the fatigue life characteristics. As mentioned above, the content of S should be low to further reduce the size of the inclusion structures, since REM easily forms a sulfide. These technical ideas are not disclosed in JP H11-279695 A.
JP 2000-45048 A discloses a bearing steel containing Ti of 7 ppm or less and 0 of 7 ppm or less for reducing the amount of Ti based inclusions and the size thereof so as to improve acoustic characteristics of small bearing steel products which are used for electric appliances such as a VCR's or CD players, general equipment such as measuring instruments or medical instruments, office automation equipment, and electronic devices such as hard disk drives. JP H08-312651 A discloses a rolling bearing steel containing no residual austenite (0%) in the carbonitrided layer and having a surface hardness of HRC57 or more which is made by tempering at high temperatures of 350° C. or more after quenching or carbonitriding. However, further improvement in the acoustic characteristics can not be expected even if an absolute amount of oxide based inclusions is reduced by lowering T.O. (total oxygen), since oxide based inclusions are hard because of the Al2O3. Al2O3 can be reformed into a fine spinel by Mg addition, which is not as hard as Al2O3 as shown in Table 1, but is still hard and does not provide adequate improvement. To the contrary, REM based inclusions are soft as shown in Table 1. Therefore, it is useful for improvement in the acoustic characteristics to reform hard Al2O3 based inclusions into soft REM based inclusions by adding REM.BRIEF DESCRIPTION OF THE DRAWINGS
A more particular description of various aspects of the embodiments of the invention illustrated in the appended drawings will now be rendered. Understanding that such drawings depict only exemplary embodiments of the invention, and are not therefore to be considered limiting of the scope of the invention in any way, various features of such exemplary embodiments will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:
An object of the invention is to provide a steel having good fatigue life characteristics and good acoustic/vibrational characteristics by fining and dispersing oxide based and oxysulfide based inclusions. This steel is referred to as inclusion-fined-dispersed steel.
The term “REM-containing oxysulfide” is defined as follows. REM forms oxides as mentioned above and also is capable of easily forming sulfides. Therefore if S exists, it is possible that REM couples with both S and O to form an REM oxysulfide, the stoichiometric composition of which is represented as RE2O2S, where RE represents REM.
After experiments and examinations were made, conditions for attaining the object were obtained with respect to the REM addition amount, inclusion composition and steel ingredients, which enable the formation of fine inclusions. A brief summary is as follows.
Item (1): A steel having at least a part comprising:
REM of which amount meets formula (1); and REM-containing inclusions of which number meets formula (2); wherein the concentration of Al2O3 included in the REM-containing inclusions is up to 30 mass %:
−30<REM(ppm)−(T.O(ppm)×280/48)<50; Formula (1)
number of REM-containing inclusions/total number of inclusions>0.8; Formula (2)
wherein the inclusions of Formula (2) have an equivalent diameter of 1 μm or more.
Item (2): A steel described in Item (1) above, wherein REM is at least one selected from the group consisting of Ce, La, Nd and Pr.
Item (3): A steel described in Items (1) or (2) above, wherein the steel meets at least one of following four conditions:
content of Al=0.05 mass %;
content of T.O.=0.005 mass %;
content of S=0.003 mass %; and
content of Ti=0.001 mass %,
wherein the steel has improved fatigue life and/or acoustic characteristics by fining inclusions using the above described conditions.
Item (4): A steel described in Item (3) above, wherein the steel is a bearing steel.
Item (5): A steel described in Item (4) above, wherein the steel is used for a small bearing steel component for hard disk or audiovisual equipment.
In Item (1), the steel is defined as having “at least a part” which meets the requirements of Formulas (1) and (2). It is generally known that in the manufacture of steel, the composition of the surface region(s) can differ from the composition of the core region(s) of the steel. By indicating that the inventive steel has at least a part which meets the requirements of Formulas (1) and (2), it is envisioned that the inventive steel may have region(s) that do not satisfy both Formulas (1) and (2), but must have at least one region which satisfies both Formulas (1) and (2).DETAILED DESCRIPTION OF THE INVENTION
Ingredients and compositions of steel of certain non-limiting embodiments of the invention are explained below. The concentration of each ingredient is represented by mass percent.
Content of Al
As described above, a steel of the invention is made in a process wherein inclusions such as oxides are converted from Al2O3 into REM oxide or REM oxysulfide by adding REM. Al is not an essential element to all types of steel, but is a useful ingredient for adjusting the crystal grain size and as a deoxidizing element for reducing T.O. However, after the Al content reaches 0.05%, not only is there no additional affect on crystal grain size, but the Al2O3 can not be converted into REM oxide or REM oxysulfide, which makes it difficult to attain an object of the invention. Therefore, an upper limit of Al content should be 0.05%. Applicant theorizes that Al2O3 is in a more stable thermodynamic state than REM oxide or REM oxysulfide when the concentration of Al is high. Therefore, the REM oxide or REM oxysulfide is not formed.
Content of T.O. (Total Oxygen)
As used herein, the term “Content of T.O.” is substantially the same as the amount of dissolved oxygen which forms an oxide (mainly Al2O3) in the steel. Thus, the higher the T.O. content becomes, the more Al2O3 to be reformed in the steel becomes. It was found that when the T.O. content exceeds 0.0050%, the Al2O3 amount becomes too high to allow for conversion of the entire Al2O3 amount into REM oxide or REM oxysulfide by adding REM, i.e., some amount of Al2O3 is left in the steel. Therefore, the T.O. content is 0.0050% or less.
REM is a strong deoxidizing element which is added to react with Al2O3 in the steel to form REM oxide by reacting with the oxygen in the Al2O3. Consequently, if an appropriate amount of REM in proportion to the amount of Al2O3, i.e., content of T.O. is not added, unreacted Al2O3 is left. Further examination on this matter brought out that there is a relationship between the REM content and T.O. content as shown in the following formula (1).
This formula (1) is explained based on
The vertical axis, “dmax, 30000 (μm)” represents the maximum size of inclusions existing in the area of 30000 mm2 which is estimated by extreme value statistics (see Beretta et al., Metallurgical and Materials Transactions B. vol. 32B, pp. 517-523, 2001, herein incorporated by reference in its entirety). The procedure for the estimation by extreme value statistics is as follows:
i) Sixteen (16) samples of 10×10 mM (100 mm2) steel are prepared for microscopic examination;
ii) Maximum size inclusion of each of 16 samples is specified and the size is measured; and
iii) Maximum size existing in 30000 mm2 is estimated from 16 maximum size data by using the extreme value statistics processing.
First, two kinds of molten bearing steel containing 8 ppm T.O. of which [S] is 0.005% and 0.002% respectively are prepared, then misch metal is added as REM. The behavior of the inclusion's being fined is examined. How the inclusion size is affected by the amount of added REM is estimated. Samples for microscopic examination are taken from a steel ingot, the maximum size of the inclusion is estimated by the extreme value statistics and the relationship between the maximum size and the amount of added REM is examined. In
The resulting inclusions are stably fined in the range where the value of (REM(ppm)−(T.O.(ppm)×280/48)) is between −30 and 50.
In the case of [S] 0.002%, the inclusion becomes finer. Also, the compositions of inclusions satisfy formula (2) which is later described and an amount of Al2O3 is 30 mass % or less. That is, it is possible to prevent Al2O3 from remaining unreacted and to convert the oxide to REM oxide which is intended, by keeping the value of (REM(ppm)−(T.O.(ppm)×280/48)) from −30 to 50 as defined in formula (1). If the addition is made so that the value (REM(ppm)−(T.O.(ppm)×280/48)) exceeds 50, formation of sulfide is promoted and coarse sulfide is formed, which lowers the fatigue life. If the addition is made so that the value (REM(ppm)−(T.O.(ppm)×280/48)) stays less than −30, the formation of REM oxide or REM oxysulfide is too low to attain the object of the invention.
Coefficient of T.O. (ppm) in Formula (1), i.e., 280/48
The main ingredients of misch metal are Ce, La, Nd and Pr. The atomic mass of Ce is 140 and that of 0 is 16. It is assumed that the REM oxide formed is RE2O 3, therefore the coefficient “280/48” is used to maintain the (stoichiometric) balance of REM to O content.
Reason for Limiting Percentage (Ratio) of the Number of Oxide Based and Oxysulfide Based Inclusions of which Grain Diameters are 1 μm or More.
In the process of steel refining, the steel has unavoidable inclusions other than REM oxide based and REM oxysulfide based inclusions. For example, in the case where a great deal of high oxidation degree slag flows out from a decarburizing refining converter. If the oxidation degree is not lowered in a secondary refining process, the molten steel will re-oxidize with the slag, which results in increased Al2O3 based inclusions. Also, if air sealing is incomplete after the REM source has been added, there will be no REM remaining to dissolve into the molten steel, since all the REM has been used. Therefore, Al2O3 based inclusions increase because of re-oxidization by air. Further, if a slag basicity is high at the secondary refining process, Ca is fed from the slag via an equilibrium reaction between the slag and the molten steel. This causes formation of CaO-rich inclusions which can not be reformed by REM. In the case where the inclusion fining effect by REM addition can not be expected as in the case mentioned above, eliminating the re-oxidation completely is important. Thus, it is found that if the number of inclusions (comprising sulfides and nitrides) other than REM-containing inclusions is less than 20% of the total inclusions, i.e., formula (2) described below is met, a high percentage of inclusions can be fined and stably dispersed so that fatigue life is further improved.
Formula (2)=Number of REM-containing inclusions/total number of inclusions>0.8
The following is the method for checking the number of inclusions.
The measurement is made using an analytical instrument consisting of a combination of a X-ray micro-analyzer and computer using the following steps:
i) Designation of measurement area of steel sample, one area of viewing field is defined as 0.5 mm×0.5 mm and five areas of the viewing field per sample are taken;
ii) Electron beam irradiation,
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 fields to make an elemental analysis;
iii) Identification of inclusion,
Information from the elemental analysis by electron beam irradiation was processed by computer to identify the inclusions;
iv) Identification of REM-containing inclusion,
Information from the elemental analysis by electron beam irradiation was processed by computer, and if an inclusion contained an REM, the inclusion was identified as an REM-containing inclusion, and the composition was quantitatively estimated; and
v) Identification of the number,
Equivalent diameter of the inclusion grains of above iii) and iv) were calculated to check the size of the inclusions and the number of inclusions of above iii) and iv) existing in five 0.5 mm×0.5 mm viewing fields were counted.
The reason for limiting the amount of Al2O3 in the inclusions to 30% or less is as follows. It was found that if more than 30% Al2O3 is present, the inclusion becomes harder compared to REM oxide and REM oxysulfide when less than 30% Al2O3 is used. This brings negative effects to fatigue life and acoustic characteristics. In view of this, the upper limit of the Al2O3 amount in the inclusion is 30%. Preferably, Al2O3 is less than 30 mass % and more preferably Al2O3 is 0 to about 29 mass % in the REM containing inclusions.
Using Misch Metal as the REM Source.
REM represents rare earth metal (rare earth element) which is a generic term for 17 elements belonging to group III in the periodic table, i.e., Sc (atomic number 21), Y (39) and lanthanoid (57-71) of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Although they have been lumped together as having the same behavior in the literature, actually each of them has a different propensity for forming an oxide, oxysulfide and sulfide. For example, as shown in
It was found that addition of misch metal, of which the main ingredients are Ce, La, Nd and Pr (summation of the 4 elements accounts for more than 98%), can fine the inclusions more stably than the addition of a single element. (see
High S content promotes the formation of REM sulfide. Even if REM sulfide is not formed, S exists more than the amount in the stoichiometric composition of REM oxysulfide requires, which forms coarse inclusions. In view of this, as shown in
In addition to the ingredients used in the inventive steel product, which include at least one of following; Al content of 0.05% or less (includes 0%), T.O. content of 0.005% or less, S content of 0.003% or less or Ti content of 0.001% or less; it is envisioned that at least one of the following reinforcement ingredients can be added specially for bearing steel or small bearing steel products, i.e., Si of 0.01-0.4% of content, Mn of 0.1-0.5% of content and Cr of 0.01-1.5% of content.
C Concentration is not Particularly Limited in the Invention.
In the present invention, the carbon concentration is not particularly limited. However, it is preferred that the C concentration ranges from 0.005 to 1.2% because if the C concentration exceeds 1.2%, the REM will form a carbide with C, which lowers the efficiency of reforming Al2O3, and if the C concentration is less than 0.005%, the amount of initially existing Al2O3 is large, which lowers the efficiency of the reforming Al2O3.
The manufacturing method for the steel of the invention is not limited to a specific method. The base molten steel can be prepared in a blast furnace with a converter or simply with an electric furnace. Other ingredients can be added to the base molten steel according to need. The method and apparatus for the addition are not limited to specific ones. The addition of ingredients can be made by free falling, and the ingredients can be blown into the mixture using inert gas or by some other method. Furthermore, the method where the steel ingot is formed from molten steel and the ingot is rolled are not precluded.
The REM source is added after vacuum refining, such as RH. For example, misch metal (in block), stored in a hopper located outside on top of the vacuum chamber, is added to the surface of molten steel in the vacuum chamber at the final stage of Ruhrstahl-Hausen (RH) treatment.EXAMPLES Example 1
Pig iron discharged from blast furnace had dephosphorization and desulfurization treatment, then 270 tons of the pig iron was transported into the converter for oxygen blowing, and the base molten steel for bearing steel which has prescribed contents of C, 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 a ladle and having Ladle Furnace (LF) and RH vacuum degassing treatment. At LF treatment, a high oxidation degree slag discharged from the converter was reduced to a lower content of iron oxide and MnO, and CaO was added to increase the CaO/SiO2 ratio, by which ingredients causing re-oxidization was reduced. The content of Al2O3 was adjusted to obtain a slag composition which has a high capability of absorbing inclusions. At the RH treatment, dehydrogenation and elimination of inclusions were made. Furthermore, a predetermined amount of REM stored in a hopper located outside on top of the RH vacuum chamber was added in a late stage of the RH treatment step. Misch metal shown in Table 2 was used as REM, having an average particle size of 35-45 mm.
A steel bloom is manufactured from the molten steel prepared above by a continuous casting process. The steel bloom is rolled to form a steel bar of bearing steel (diameter 65 mmφ) of which chemical composition is shown in Table 3.
aRolling fatigue test results are based on Comparative Example 1 wherein Comparative Example 1 has a value set at 1.0.
bComparative Samples prepared in the procedure of Comparative Example 1
cCr: 1.40 ˜ 1.44%, P: 0.0010 ˜ 0.0015%
With respect to inclusions in the steel product (steel bar), REM-containing inclusions accounted for a large percentage of inclusions which were very fine in size. The maximum inclusion size in a 30000 mm 2 area was estimated by using extreme value statistics (base area: 100 mm2, n=16, estimation area: 30000 mm2) and the results indicate a good size was obtained as shown in Table 3. Also the rolling fatigue test (see “Development of High Temperature, Long Life Bearing Steel (STJ2)”, Hiromasa Tanaka et al., Technical Review, No. 68, May 2000, pages 51-57, which is herein incorporated by reference in its entirety) gave good results as shown in Table 3. The concentration of the components of REM shown in Table 3 for each corresponding sample is described in Table 4.
A bearing steel shown in Table 3 was manufactured in the same way as in example 1. However, in the comparative example 1, the following conditions are adopted: the REM was not added at the final stage in the RH treatment step as in Example 1; the REM addition was made using the same technique as in example 1, but the added amount of REM was either more than the upper limit of proper REM amount or less than lower limit; and the air was not completely sealed after the RH process was applied to increase the percentage of REM-containing inclusions to be outside the proper range defined by the invention. Results of the inclusion size and rolling fatigue test of the bearing steel are shown in Table 3 as Comparative Examples, which are inferior to the results of Example 1.Example 2
A bearing steel described in Table 5 was manufactured in the same way as in Example 1.
aComparative Samples prepared using the method of Comparative Example 2
bCr: 1.40 ˜ 1.44%, P: 0.0010 ˜ 0.0015%
The number of REM-containing inclusions accounted for large percentage of the number of inclusions in the bearing steel product and the size was very fine. With respect to rolled steel bar (diameter 65 mmφ), the maximum inclusion size in a 30000 mm2 area was estimated by using extreme value statistics (base area: 100 mm2, n=16, estimation area: 30000 mm2 and the result indicates that a good size was obtained as shown in Table 5. The steel product was rolled into wire rod (10 mmφ) and then made into a miniature bearing steel. The acoustic and vibrational characteristics were then examined and found to be good. The REM composition shown in Table 5 is described in Table 6.
A bearing steel shown in Table 5 was manufactured in the same way as in Example 2. However, in the Comparative Example 2, the following conditions are adopted: the REM is not added at the final stage in the RH treatment step; the REM addition was made using the same adding method as in Example 1, but the addition amount of REM was either more than the upper limit of the inventive REM amount or less than the lower limit; the air was not completely sealed after the RH process was applied to increase the percentage of REM-containing inclusions to be outside the proper range defined by the invention; and Ti was added so that the content of Ti exceeds the concentration range of the invention. The results of inclusion size and acoustic/vibrational characteristics of the bearing steel are shown in Table 5 as Comparative Samples, which are inferior to the results of Example 2.
Thus, it is found that this invention relates to a steel having finely dispersed REM based oxide and REM oxysulfide inclusions, and can provide a steel having good fatigue life characteristics and good acoustic/vibrational characteristics by eliminating the harmful effect of oxide based inclusions.
1. A steel having at least a part comprising:
- rare earth metal (REM) in an amount meeting formula (1); and REM-containing inclusions in a number meeting formula (2); wherein a concentration of Al2O3 in the REM-containing inclusions is 0-30 mass %;
- −30<REM(ppm)−(T.O.(ppm)×280/48)<50 formula (1) number of REM-containing inclusions/total number of inclusions>0.8 formula (2)
- wherein the inclusions in formula (2) have an equivalent diameter of 1 μm or more.
2. The steel according to claim 1, wherein REM is at least one selected from the group consisting of Ce, La, Nd and Pr.
3. The steel according to claim 2, wherein REM comprises at least Ce, La, Nd and Pr.
4. The steel according to claim 1 or 2, wherein the steel meets at least one of following four conditions:
- a) content of Al 0-0.05 mass %;
- b) content of total oxygen (T.O.)=0.005 mass %;
- c) content of S=0.003 mass %; and
- d) content of Ti=0.001 mass %.
5. The steel according to claim 4, wherein the steel has a content of Al 0-0.05 mass %.
6. The steel according to claim 4, wherein the steel has a content of total oxygen (T.O.)=0.005 mass %.
7. The steel according to claim 4, wherein the steel has a content of S=0.003 mass %.
8. The steel according to claim 4, wherein the steel has a content of Ti=0.001 mass %.
9. The steel according to claim 1 or 2, wherein the steel has a rolling fatigue life (L10) which is greater than 3.2 times the L10 of essentially the same steel which has been prepared without added REM.
10. The steel according to claim 9, wherein the steel has a rolling fatigue life (L10) which is at least 7.6 times greater than the L10 of essentially the same steel which has been prepared without added REM.
11. The steel according to claim 9, wherein the steel has a rolling fatigue life (L10) which is greater than 3.2 up to and including 9.2 times the L10 of essentially the same steel which has been prepared without added REM.
12. The steel according to claim 1, wherein the steel has no added calcium or magnesium.
13. The steel according to claim 4, wherein the steel is a bearing steel.
14. The steel according to claim 13, wherein the bearing steel is shaped for use in a hard disk drive or audiovisual equipment.
15. A method for manufacturing a steel:
- said method comprising the steps of: preparing a molten steel, adding rare earth metal (REM) and oxygen to the molten steel, casting the molten steel into said steel; wherein the steel has at least a part comprising REM in an amount meeting formula (1); and REM-containing inclusions in a number meeting formula (2); wherein a concentration of Al2O3 in the REM-containing inclusions is 0-30 mass %; −30<REM(ppm)−(T.O.(ppm)×280/48)<50 formula (1) number of REM-containing inclusions/total number of inclusions>0.8 formula (2) wherein the inclusions in formula (2) have an equivalent diameter of 1 μm or more.
16. The method according to claim 15, wherein the REM is added during RH vacuum degassing treatment of the molten steel.
17. The method according to claim 16, wherein the aluminum is added to the molten steel prior to addition of REM.
18. The method according to claim 16, wherein air is essentially completely sealed after RH treatment.
International Classification: C22C 38/00 (20060101);