The present invention has control the composition of the nonmetallic inclusions, regarding steel excellent in rolling fatigue characteristics. In particular, by the cluster-like oxide inclusions and REM-containing inclusions was suppressed fatigue fracture starting from the inclusions relates steel having good rolling fatigue characteristics.
BACKGROUND
[0002]
　Hardened steel, high-frequency hardening steel, various steel products such as steel for bearings are used, such as in industrial machinery and automotive parts, is also used as a rolling bearing material such as "ball bearings" and "roller bearing".
　Rolling bearing comprises for example a ball shape or a roller shape as "rolling body", convey load rolling elements contact the "inner" and "outer" ". The rolling elements and the inner ring, the steel used for rolling member such as an outer ring, which is required to have excellent rolling fatigue characteristics. Inclusions contained in the steel material, the purpose of improving the rolling contact fatigue life, it is desired that a possible fine and small quantities. The inclusions contained in the steel material, alumina (Al 2 O 3 oxides and the like), sulfides such as manganese sulfide (MnS), nitrides such as titanium nitride (TiN) has been known.
[0003]
　Alumina inclusions, the dissolved oxygen remaining in the molten steel is refined in a converter or vacuum treatment vessel, to produce combined with strong oxygen and affinity Al. Also, it ladles, etc. often built with alumina-based refractories. Thus, during deoxidation, by reaction with the molten steel and the refractory material, alumina is dissolved into the molten steel as Al, it is reoxidized, the alumina-based inclusions. Alumina inclusions clusters to form in the steel during after solidification, which causes a reduction in rolling fatigue life.
[0004]
　For alumina clusters reduction in the manufacturing method of Al-killed steel containing Al 0.005 mass% or more, in the molten steel, Ca, Mg, and was charged with an alloy of two or more and Al REM, Al of inclusions to be produced 2 O 3 is adjusted to 30 mass% to 85 mass%, it is known to produce no alumina clusters Al killed steel.
[0005]
　For example, Patent Document 1, in order to prevent the formation of alumina cluster, REM, Mg, and was added to the molten steel of two or more of Ca, a method of forming inclusions low melting point is disclosed. This method is effective in preventing the sliver flaws. However, in this method, the size of inclusions can not be reduced to a level required by steel bearings. The reason is that inclusions of a low melting point, aggregation and coalesce, because easily coarsened.
[0006]
　Moreover, REM may inclusions by spheroidizing, to improve the fatigue properties. However, if too put many intervening the number is increased, the fatigue life is rather decreased, which is one of fatigue properties. Patent Document 2, in order not to lower the fatigue life, it is disclosed that it is necessary to set the content of REM below 0.010 wt%. However, Patent Document 2 does not disclose the presence state of the mechanism and inclusions fatigue life reduction.
[0007]
　Thus, although case often achieved improvement in fatigue properties the shape of inclusions by changing from clustered into spherical case which has improved fatigue properties by modifying the cluster itself is not seen.
CITATION
Patent Document
[0008]
Patent Document 1: Japanese Laid-Open Patent Publication No. 09-263820
Patent Document 2: Japanese Laid-Open Patent Publication No. 11-279695
Summary of the Invention
Problems that the Invention is to Solve
[0009]
　In view of the problems of the prior art, and an object thereof is to provide a steel material superior in the rolling fatigue characteristics.
Means for Solving the Problems
[0010]
　The gist of the present invention is as follows.
[1]
　by
mass%,
C:
0.10% ~ 1.50%, Si: 0.01% ~ 0.80%, Mn:
0.10% ~ 1.50%, Cr: 0.02% ~
2.50%, Al: less than% ~ 0.010
0.002%,
Ce + La + Nd: 0.0001% ~ 0.0025%, Mg: 0.0005%
~ 0.0050%, O: 0.0001% ~ 0
% .0020, Ti: less than% ~ 0.005
0.000%, N: 0.0180% or
less, P: 0.030% or
less, S: 0.005% or
less, Ca: 0.0000% ~ 0.
Pasento
0010,
V: 0.00 The ～ 0.40 Pasento, Mo: 0.00 The ～ 0.60
Pasento, Cu: 0.00 The ～ 0.50 Pasento, Nb: less than 0.050 Pasento ～
0.000, Ni:
~
2.50% 0.00, Pb: 0.00 ~ 0.10%, Bi: 0.00 ~
0.10%, B: 0.0 00 to 0.0050 percent,
Balance being Fe and impurities,
and fatigue origin inclusions Ce, La, 1 or more Nd detected by the ultrasonic fatigue test, Mg, Al, and contains O, and the composition ratio of the formula and satisfies the (1), excellent steel to rolling fatigue characteristics.
(Ce% + La% + Nd % + Mg%) / Al% ≧ 0.20 ··· Equation (1)
where, in the formula (1), Ce%, La %, Nd%, Mg%, Al% , respectively, Ce fatigue origin inclusions contain, La, Nd, Mg, an atomic weight% of Al.
[2]
　by mass%, C: less than 0.10% ~ 0.45%, Cr: it characterized in that it is a 0.02 to 1.50 percent, the rolling fatigue characteristics according to [1] excellent steel to.
[3]
　by mass%, C: less than 0.45% ~ 0.90%, Cr: it characterized in that it is a 0.70 to 2.50%, the rolling fatigue characteristics according to [1] excellent steel to.
[4]
　by mass%, C: a 0.90% ~ 1.50%, Cr: it characterized in that it is a 0.70 to 2.50%, the rolling fatigue characteristics according to [1] excellent steel.
The invention's effect
[0011]
　According to the present invention, a cluster-like Al-O-type inclusions by reforming the REM-Al-Mg-O-based inclusions, by reducing the influence on the fatigue properties of the oxide inclusions, rolling it is possible to provide a superior steel to dynamic fatigue properties.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012]
It is an explanatory view of FIG. 1 rolling fatigue test pieces, (a) is a plan view, (b) a side view.
FIG. 2 is an explanatory view of an ultrasonic fatigue test piece.
3 is an explanatory view of the inclusions contained between the gauge length of the ultrasonic fatigue test piece.
FIG. 4 is an explanatory view schematically showing a state in which fatigue fracture gradually progresses.
FIG. 5 is an explanatory view of a fracture surface of the ultrasonic fatigue test piece.
6 is an example of a reflected electron composition image of fatigue origin inclusions.
DESCRIPTION OF THE INVENTION
[0013]
　The present inventors have found that in order to solve the problems of the prior art, carried out an extensive experiment and study. As a result, by adjusting the content of content and Al and S of REM, the following findings were obtained.
(1) the Al-O-type inclusions is clustered oxide by reforming the REM-Al-Mg-O-based inclusions, thereby improving the adhesion between the oxide inclusions and the matrix.
(2) in order to mix the Mg and REM based inclusions of the Al-O-type inclusions is clustered oxide, S is highly reactive with Mg and REM should be kept as much as possible content .
(3) Al-O-type inclusions is clustered oxide is coarse, because an adverse effect on the fatigue properties, but is better to suppress the Al content is as much as possible, coarse lower oxides in the non-addition is formed, modifying effect of oxide inclusions according to Mg and REM can not be obtained. Accordingly, deoxidation of Al is minimum.
[0014]
　The following describes a steel material according to the embodiment of the present invention made based on the above findings and its manufacturing method in detail.
　First, a description component composition of the steel according to the present embodiment and the reasons for limitation. Incidentally,% regarding the content of the following elements means mass%.
[0015]
　C: 0.10% ~ 1.50%
　C is to ensure the hardness, is an element improving the fatigue life. To ensure the required strength and hardness, it is necessary to include C 0.10% or more. However, the hardness when C content exceeds 1.50% is excessively increased, causing quench cracking. Therefore, C content is 0.10% 1.50%. Incidentally, C: if it is less than 0.10% to 0.45% is suitable for steel for case hardening. C: If it is less than 0.45% to 0.90% is suitable for steel for induction hardening. C: If 0.90% 1.50% are suitable steel for the entire quenching. The lower limit of the C content is preferably 0.15%. The upper limit of the C content is preferably 1.35%.
[0016]
　Si: 0.01% ~
　0.80% Si is to increase the hardenability, which is an element improving the fatigue life. To obtain this effect, the Si needs to be 0.01% or more. However, Si content exceeds 0.80% hardenability improving effect is saturated, further affect the deoxidation state, fatigue characteristics affect the formation of the oxide is reduced. Therefore, Si content is 0.01% to 0.80%. The lower limit of the Si content is preferably 0.07%. The upper limit of the Si content is preferably not more than 0.65%.
[0017]
　Mn: 0.10% ~
　1.50% Mn increases the strength by increasing hardenability, which is an element improving the fatigue life. To obtain this effect, it is necessary to include Mn 0.10% or more. However, Mn content is more than 1.50%, hardenability improving effect is saturated, rather, causing quench cracking. Therefore, Mn content is 0.10% to 1.50%. The lower limit of the Mn content is preferably 0.20%. The upper limit of the Mn content is preferably 1.20%.
[0018]
　Cr: 0.02% ~
　2.50% Cr is enhanced hardenability, it is an element improving the fatigue life. To get this effect stably, it is preferable to contain Cr 0.02% or more. However, if the Cr content exceeds 2.50%, hardenability improving effect is saturated, rather causing quench cracking. Therefore, the upper limit of the Cr content is 2.50%. The lower limit of the Cr content is preferably 0.15% or more. The upper limit of the Cr content is preferably not more than 2.00%. Cr content may define 1.90% or less, or 1.80% or less.
　In the case of using as the steel for the bearing by hardening it is, Cr: it is desirable to 0.02 to 1.50%. When used as a steel material for the bearing according to steel or entire hardening, for the bearing by induction hardening is, Cr: it is desirable to 0.70 to 2.50%.
[0019]
　Al: less than Pasento ～ 0.010 0.002 Pasento
　Al is, T. O as a deoxidizing element to reduce the (total oxygen content), it is necessary to include more than 0.002%. However, an Al content of 0.010% or more, the amount clustered alumina is increased, modification of the REM-Al-Mg-O-based inclusions by Mg and REM added is considered not sufficiently. Therefore, Al content is less than 0.010%. Al content is preferably 0.005% or more for the lower limit. Al content is preferably the upper limit is 0.008% or less.
[0020]
　+ La + Ce Nd: 0.0001% ~
　0.0025% Ce (cerium), La (lanthanum), and Nd (neodymium) is an element classified as rare earth elements. The rare earth element, atomic number 15 elements from lanthanum of 57 to 71 lutetium, and is a generic name for total 17 elements atomic numbers 21 scandium and atomic number plus the yttrium 39. Rare earth element is a strong deoxidizing element, the steel for the bearing according to the present embodiment, it plays a pivotal role. Rare earth element alloy for steel, because the Ce, La, and the three elements of Nd and has a main component, in the present invention, among the rare earth 17 elements, Ce, La, and limits the Nd. Without changes to elements other than 3 element contained in the rare earth alloy is also a potent deoxidizing element, the same effects as three elements. Ce, La, to any one of Nd may contain between 0.0001% 0.0025% may contain between 0.0001% 0.0025% of two or more in total. In the description of the present invention Ce, La, and, collectively referred to Nd and REM. REM is first reacted with oxygen in the molten steel, to produce a REM-based oxide. Then, REM based oxide with the aggregation clustering of alumina oxide in molten steel are also incorporated at the same time. Al-O-based oxide is reformed into REM-Al-Mg-O-based inclusions in doing so.
[0021]
　Function of REM in steel bearing according to the present embodiment is as follows. The size of the clustered inclusions great difference between the case of not adding to the case without the addition of REM is not observed. However, since the oxide of REM system coexist, improves the interface state between the base material, in order in particular to improve adhesion, less likely to be fracture origin have the same size, the fatigue characteristics improves.
[0022]
　In order to obtain such an effect, T. Depending on the amount of O (total oxygen content), it is necessary to contain a certain amount or more of REM.
　Result of studying these respects, REM has knowledge experimentally that contain effect is insufficient with less than 0.0001%. Therefore, the lower limit of the REM content is set to 0.0001%, preferably 0.0003% or more, and more preferably, 0.0008% or more. However, when the REM content exceeds 0.0025%, not only increases the cost, blockage of the casting nozzle are easily generated, inhibits the production of steel. Therefore, the upper limit of the content of REM is 0.0025%, preferably 0.0020%, more preferably 0.0018%.
[0023]
　Mg: 0.0005% ~
　0.0050% Mg is a strong deoxidation element similarly to Al, the steel material according to the present embodiment, it plays a pivotal role. The effect of suppressing alone is destroyed cluster oxides origin Mg is small, by the combined use of REM and Mg, than REM alone enhances the improving effect of the fatigue properties. To obtain this effect, it is necessary to contain an amount of more than 0.0005% of Mg. When Mg content is high, it believed oxide content itself can not reforming the REM-Al-Mg-O-based inclusions due to increased REM added. Therefore, Mg content is 0.0050% or less. Mg content is preferably defined as the lower limit 0.0010% or more. Mg content is preferably defines the upper limit is 0.0040%.
[0024]
　O: 0.0001% ~ 0.0020%
　O is an impurity, is an element to be removed from the steel by deoxidation. Alumina clusters not occur if it is possible to eradicate O in the steel by deoxidation, an object of the present invention is to provide not occur well. However, from the technical and cost, O of 0.0001% or more in the current steel are inevitably contained, fatigue properties of alumina clusters generated by the this may decrease. The present invention in a steel containing normal comparable oxygen and that in, improved fatigue properties as compared with the prior art. In general, O content of the steel is often 0.0005% or more. On the other hand, the O content exceeds 0.0020% oxide such as alumina large amount remains, because the fatigue life decreases, the upper limit of the O content is 0.0020%. O content is preferably 0.0015% or less.
[0025]
　Ce Fatigue origin inclusions, La, Nd, Mg, the composition ratio of Al (Ce% + La% + Nd% + Mg%) / Al%: 0.20 or more
　is a cluster-shaped oxide Al-O-based inclusions REM by modifying the -al-Mg-O-based inclusions to improve the adhesion between the oxide inclusions and the matrix, thereby improving the fatigue characteristics. This effect, Ce Fatigue origin inclusions, La, Nd, Mg, the composition ratio of Al (Ce% + La% + Nd% + Mg%) / Al% is expressed when the 0.20% or more. Therefore, the (Ce% + La% + Nd % + Mg%) / Al% 0.20 or more. To enhance the effect of the above, (Ce% + La% + Nd% + Mg%) / Al% is preferably 0.50 or more.
[0026]
　Ce% Fatigue origin inclusions, La%, Nd%, Mg%, Al%, respectively, Ce containing fatigue origin inclusions, La, Nd, Mg, against atomic sum of Al, the number of atoms of each element the ratio of the (atomic weight%). In fatigue origin inclusions, Al%, Mg%, and (Ce% + La% + Nd%) when three items is both 0.1 or more and a fatigue origin inclusions "REM-Al-Mg-O and a system inclusions ". Desirably, the fatigue origin inclusions, Mg%, and (Ce% + La% + Nd%) both is desirable to contain 1.0 or more. Note that in the calculation of the atomic weight percent ratio of the atoms and the O of O is not considered, each element forms a complex inclusions through O, the fatigue origin inclusions contain O is It is.
[0027]
　The above is the basic component composition of the steel material according to the present embodiment, the remainder being iron and impurities. Incidentally, "the balance is iron and impurities" and "impurity" in refers to the manufacture of steel industrially, ore as a raw material, scrap or the like manufacturing environment which inevitably mixed. However, the steel material according to the present embodiment, Ti is an impurity, N, P, S, and Ca, it is necessary to limit as follows.
[0028]
　Ti: less than 0.000% ~ 0.005%
　Ti is an impurity, when present in the steel, so deteriorating the fatigue characteristics to generate TiN, restricts the Ti content to less than 0.005%. Preferably limits the Ti content to 0.004% or less.
[0029]
　N: 0.0180% or less
　N is an impurity, to be present in the steel, thereby deteriorating the fatigue characteristics by forming a nitride, also deteriorates the ductility and toughness by strain aging. N content is more than 0.0180%, the fatigue properties, ductility, and becomes remarkable problems such as toughness deterioration. Therefore, to limit the upper limit of the N content to 0.0180%. Preferably limits the N content below 0.0150%. Although N is may be 0.0000%, there is industrial constraints in the reduction of N content, also it does not make sense to be too low. As a substantial lower limit of the possible N in normally costs may be 0.0020%.
[0030]
　P: 0.030% or less
　P is an impurity, when present in the steel, reduce the fatigue life segregated in the grain boundary. P content exceeds 0.030%, and the fatigue life decreases. Therefore, to limit the upper limit of the P content to 0.030% or. Preferably, to limit the P content to 0.020% or less. The lower limit of the P content may be 0.000%, but may be 0.001% as an industrial limit.
[0031]
　S: 0.005% or less
　S, when present in the steel to form a sulfide. S content is more than 0.005%, S forms sulfides in combination with REM, reducing the effective REM for modifying alumina clusters, decreases the thus fatigue life. Therefore, to limit the upper limit of the S content to 0.005%. Preferably, to limit the S content to 0.0025% or less. The lower limit of the S content may be 0.000%, but may be 0.001% as an industrial limit.
[0032]
　Ca: 0.0000% ~
　0.0010% Ca, when present in the steel, and generate coarse CaO, since the fatigue life decreases to 0.0010% of the upper limit. Ca content is preferably not more than 0.0002%, even better if 0.0000%.
[0033]
　In addition to the elements described above, it may be selectively contain the following elements. The following describes the selected element.
　Steel material according to the present embodiment further, V: 0.00% ~ 0.40% , Mo: 0.00% ~ 0.60%, Cu: 0.00% ~ 0.50%, Nb: 0. 000% ~ 0.050%, Ni: 0.00% ~ 2.50%, Pb: 0.00 ~ 0.10%, Bi: 0.00 ~ 0.10%, B: 0.0000 ~ 0. 0,050%, it may contain one or more.
[0034]
　V: 0.00% ~ 0.40% V
　combines with C and N in the steel, carbide, nitride, or carbonitride is formed, in an element contributing to strengthening and refinement of the structure of the steel is there. To get this effect stably, it is preferable to contain V 0.05% or more. V content is more preferably 0.10% or more. However, when the V content exceeds 0.40%, and the content effect because cracking occurs during hot working as well as saturated and 0.40% of the upper limit of the V content. Preferably, the V content is 0.30% or less.
[0035]
　Mo: 0.00% ~
　0.60% Mo combines with C in steel to increase the hardenability, carbide is formed and an element which contributes to the improvement of the strength of steel by precipitation strengthening. To get this effect stably, it is preferable to contain Mo at least 0.05%. Mo content is more preferably 0.10% or more. However, when the Mo content exceeds 0.60% rather so causing quench cracking, 0.60% of the upper limit of the Mo content. Mo content is preferably 0.50% or less.
[0036]
　Cu: 0.00% ~
　0.50% Cu is an element contributing to the improvement of fatigue characteristics by strengthening the base material. To get this effect stably, it is preferable to contain Cu 0.05% or more. However, when the Cu content exceeds 0.50%, since cracks generated during hot working, and 0.50% the upper limit of the Cu content. Cu content is preferably 0.35% or less.
[0037]
　Nb: less than Pasento ～ 0.050 0.000 Pasento
　Nb is an element contributing to the improvement of fatigue characteristics by strengthening the base material. To get this effect stably, it is preferable to contain Nb 0.005% or more. Nb content is more preferably 0.010% or more. However, when the Nb content is more than 0.050%, since containing effect cracking occurs during hot working as well as saturated, Nb content is less than 0.050%. Nb content is preferably 0.030% or less.
[0038]
　Ni: 0.00% ~ 2.50% or less
　Ni is an element contributing to the improvement of fatigue life by increasing the corrosion resistance. To get this effect stably, it is preferable to contain Ni 0.10% or more. Ni content is more preferably 0.30% or more. However, when the Ni content exceeds 2.50%, the machinability of the steel is lowered, and 2.50% the upper limit of the Ni content. Ni content is preferably 2.00% or less.
[0039]
　Pb: 0.00% ~
　0.10% Pb is added to increase the machinability of the steel. However, the fatigue strength become starting points of fatigue cracks and Pb content exceeds 0.10% decreases. Therefore, the upper limit of the Pb content is 0.10%. Pb content is preferably 0.06% or less.
[0040]
　Bi: 0.00% ~
　0.10% Bi is added in order to improve the machinability of steel. However, the fatigue strength become starting points of fatigue cracks and Bi content exceeds 0.10% decreases. Therefore, the upper limit of Bi content is 0.10%. Bi content is preferably 0.06% or less.
[0041]
　B: 0.0000% ~ 0.0050%
　B segregates in austenite grain boundaries, the effect of improving the toughness by increasing the grain boundary strength. However, B content austenite grains is reduced abnormal grain growth fatigue strength during heat treatment exceeds 0.0050%. Therefore, the upper limit of the B content is 0.0050%. B content is preferably not more than 0.0030% or.
[0042]
　In the steel according to the present embodiment, a cluster-like oxides of the first, stretched by rolling. However, the steel according to the present embodiment, the form and state of the interface between the base material by being a composite from alumina alone regardless of the size and REM oxides is improved modified fatigue properties.
[0043]
　It described preferred manufacturing method of the steel according to the present embodiment.
[0044]
　The method of manufacturing a steel material according to the present embodiment, when refining molten steel, the order of turning on the deoxidizer is important. In this manufacturing method, first, the deoxidation with Al, the Mg. Then, after deoxidation least 60 seconds using a REM, performs ladle refining including a vacuum degassing.
[0045]
　The addition of REM initial deoxidation to, will be secured by forming the REM-O-based oxide, it is impossible to modify the alumina or Al-Mg-O-based oxide is formed later. Therefore, the first Al deoxidation, the O contained in subsequently molten steel by addition of Mg fixed as an oxide. Followed by the addition of REM, to modify the cluster-like oxides REM-Al-Mg-O-based inclusions. The addition of REM, or the like can be used mischmetal (alloy composed of a plurality of rare earth metals), for example, the refining of the end, may be added bulk of misch metal in the molten steel.
[0046]
　Deoxidation by REM is carried out more than 60 seconds. This is the time required to form an oxide of REM system captures oxygen from the Al-Mg-O-based oxide the added REM is once formed.
[0047]
　For deoxidation, when adding Ca, easily stretched at low melting point Al-Ca-O-based inclusions are generated number. Therefore, after the Al-Ca-O-based inclusions generated number, even with the addition of REM, it is difficult to modify the composition of the inclusions. Thus, the addition or incorporation of Ca, it is necessary to suppress as much as possible.
[0048]
　As described above, in the present manufacturing method, the Al-O-based cluster-like oxides can be modified in REM-Al-Mg-O-based inclusions, which improves rolling property of steel material.
[0049]
　When using a steel material according to the present embodiment the bearing includes a generation amount of MnS, it and the amount of TiN exist independently is extremely small is ideal, need not be completely eliminated. The addition amount of S and Ti to limit as described above, not be MnS and TiN becomes coarse than clustered oxide, because not be a starting point of fatigue fracture.
[0050]
　In this manufacturing method, the slab after casting was heated to the heating temperature, 1200 ° C. ~ 1250 ° C. in a temperature range of 60 seconds or more after keeping 60 minutes or less, hot-rolling, or subjected to a hot forging the production of steel Te. The steel as a material, after cutting to shape close to the final shape, carburizing and quenching, induction hardening, by heat treatment such as whole quenching, the hardness of the surface can be a hardness suitable for bearing. Note that the steel according to the present embodiment, C: is a 0.10% - 1.50% C: if it is less than 0.10% to 0.45% is suitable for steel for case hardening , by performing carburizing quenching, the hardness of the surface, can be Vickers hardness 700 Hv (measuring load 2.94 N) or more. Also, C: if 0.45% to 1.50% by performing induction hardening, the hardness of the surface can be more than Vickers hardness 650 Hv (measuring load 2.94 N). Also, C: if 0.90% 1.50% are suitable steel bearing according entire quenching.
[0051]
　Carburizing using the steel of the present invention, induction hardening, heat treatment and rolling members such as entire hardening is excellent in fatigue characteristics. In the case of using as a rolling member, if necessary, with high hardness and high-precision processing that can means such as grinding, to finish in the final product is generally used.
Example
[0052]
　Next, a description will be given of an embodiment of the present invention, conditions in examples are an example of conditions adopted for confirming the workability and effects of the present invention, the present invention is, in this single condition example the present invention is not limited. The present invention does not depart from the gist of the present invention, as long as they achieve the object of the present invention, it is capable of adopting various conditions.
[0053]
(Example 1) Example was assumed bearing according hardening
　No. shown in Table 1 A1 ~ A16 (Invention Example), No. The steel grade of the components B1 ~ B14 (Comparative Example) was cast with 150kg vacuum furnace. The deoxidation conditions, varied as a deoxidizing conditions a ~ f shown in Table 2, were investigated the effects. In the case of adding the REM was added misch metal in the assumed yield of 40%.
　Deoxidation conditions a, b, d are all added Al, Mg, a deoxidizer in the order of REM, the deoxidation conditions a, after 90 seconds from the REM addition was tapped. In deoxidation conditions b, and tapping sure that has elapsed since REM added 500 seconds. In deoxidation conditions d, it was tapped immediately after the REM added 30 seconds. Deoxidation conditions c is, REM, Al, was added deoxidizer in the order of Mg, deoxidation time by REM was 120 seconds. Deoxidation conditions e is, Al, by adding a deoxidizer in the order of Mg performs deoxidation was not carried out deoxidation by REM addition. Deoxidation conditions f is, Al, by adding a deoxidizer in the order of REM perform deoxidation and steel out to confirm that has elapsed since REM added 90 seconds.
[0054]
　After tapping, and forging heat to the round bar of φ80, and the material for the test piece collection. The round bar, after cut in a cross section perpendicular to the longitudinal direction were taken rolling fatigue test piece shown in FIG. Specifically, the rolling contact fatigue test piece, the thickness of 6.0 mm, a disk having a diameter of 60 mm, a circular plane, and perpendicular to the longitudinal direction of the round bar is the source. The rolling fatigue test piece is the inner and outer rings of the bearing imitates. The rolling fatigue test, the circular surface of the rolling fatigue test piece corresponds to the test surface, the fatigue load by the surface is in contact with the rolling elements is applied.
[0055]
　After collection of the rolling fatigue test pieces, as load-bearing parts (test surface) is homogeneously bearing applications material equivalent to 700Hv or more hardness were tempering and carburizing and quenching. Here, the Vickers hardness was measured by measuring load 2.94N. Tempering conditions is a 1hr at 180 ℃. After tempering, the test surfaces were subjected to machining and rolling fatigue test mirror finished. Rolling fatigue test Mori thrust-type tester (contact surface pressure: 5.33gpa) was carried out. About 10 times of test results for each level (Table 3 No.1 ~ 32), using a Weibull statistics, 10% of the evaluation samples was evaluated the number of cycles to fracture as a fatigue characteristics L10.
[0056]
　Evaluation of Fatigue origin inclusions was performed by ultrasonic fatigue test using a test piece as shown in FIG. 2. Ultrasonic fatigue test piece for the same test was also taken from a round bar that became the material of the rolling fatigue test piece. Ultrasonic fatigue test piece in the longitudinal direction of the test piece was taken so that the longitudinal and vertical round bar as a material. The In collecting the ultrasonic fatigue test piece, diameter than the ultrasonic fatigue test piece shape which is scheduled taken as a large material about 0.3 mm, the further gripping portion was formed by welding a separate steel. Thereafter, the test unit, the carbon concentration of the surface layer is equal to the rolling fatigue test pieces, and, to the test unit center performs carburizing treatment long enough to allow carburizing, and later quenching, of 1hr at 180 ° C. the tempering treatment was carried out. Then finished into a predetermined ultrasonic specimen geometry. Ultrasonic fatigue test frequency 20 kHz, stress ratio -1, the stress amplitude was kept constant at 850MPa from 700 was performed until breakage. Fatigue origin inclusion of the ultrasonic fatigue test and analyze the composition using EDX (energy dispersive X-ray analysis), the previous REM atomic weight% of the total amount and Mg, the atomic weight% of Al was measured.
[0057]
　Fatigue origin inclusions is specified as follows. As shown in FIG. 3, between the ultrasonic fatigue test piece 1 of gage length L, it contains a large number of inclusions a. Among these inclusions a, by starting from the inclusions a 'generating the most stress concentration in the ultrasonic fatigue test, the fatigue fracture will occur. Most stress inclusions cause the concentration a ', the size, the stress concentration factor in the ultrasonic fatigue test or the like shape is most larger fatigue origin inclusions.
[0058]
　Figure 4 is an explanatory view schematically showing a state in which fatigue fracture gradually progresses. First, as shown in FIG. 4 (a), around the inclusions a '(fatigue origin inclusions), fatigue crack occurs in a circular shape in the longitudinal direction perpendicular to the cross section of the ultrasonic fatigue test piece 1 . Then, as shown in FIG. 4 (b), with an increase in the number of stress amplitude, spreads the fracture surface 10 in a circular shape. Further, the fracture surface 10 with an increase in the number of stress amplitude is a certain size, as shown in FIG. 4 (c), leading once to rupture.
[0059]
　Thus the fracture surface reaches the fracture ultrasonic fatigue test piece 1, as shown in FIG. 5, around the inclusions a '(fatigue origin inclusions), the circular pattern 11 called fish eye remains and Become. Circle pattern 11, corresponding to the fracture surface 10 just before the stretch breaking advances. Therefore, Ce contained in the inclusions a in the center of this circle pattern 11 '(fatigue origin inclusions), La, Nd, Mg, atomic weight% of Al was measured, (Ce% + La% + Nd% + Mg%) / seek the Al%.
[0060]
　The measurement of the origin inclusion composition, accelerating voltage 20 kV, to observe at 500 magnifications. Center of inclusions determines the field of view of the EDS measurement so that the center of the field of view. An example of a reflected electron composition image of origin inclusions used in the composition analysis shown in Fig. As shown in FIG. 6, the inclusions portion, the portion of the non-inclusion is clearly discernible. Therefore, to identify the region corresponding to the inclusions (inclusions area) within the field of view were measured chemical composition by extracting the region.
　Major axis of the inclusions is about 100 ~ 300 [mu] m, even if the entire inclusions does not fit into the field of view, using the values obtained in one field of view as the composition of the inclusions. For the above field, dwell time 0.5 .mu.s, performed elemental mapping by EDS preset 5, calculated Ce, La, Nd, Mg, atomic weight% of Al by X-ray spectra obtained from the inclusions area, inclusions composition to measure. Data acquisition and analysis of EDS, EDS analysis systems
Analysis using Station (manufactured by JEOL). Note that when atomic weight percent calculated in Table 3, but the proportion of O is not considered, fatigue origin inclusion is a complex inclusions which are formed through the O both contained O. Further, in the comparative example, fatigue origins inclusions other elements (Mn, Ti, etc.) was also the case including, Ce, La, Nd, Mg, elements other than Al are taken into account in both atomic weight% calculation Absent.
[0061]
　Table 1 shows the chemical composition of each steel type in the first embodiment. Table 2 shows the deoxidation conditions a ~ f. Table 3 shows the steel species in each level (No.1 ~ 34), and deoxidation conditions, oxides form of fatigue origin inclusions ultrasonic fatigue test, the fatigue characteristics of the composition and the rolling fatigue test (L10 life) .
　Fatigue life L10 of the present invention example, 10 7 and the cycles or more, was superior to steel type as a comparative example.
[0062]
[Table 1]

[0063]
[Table 2]

[0064]
[table 3]

[0065]
(Example 2) Example assuming the bearing by induction hardening
　No. shown in Table 4 C1 ~ C14 (present invention examples), No. The steel grade of the components of the D1 ~ D10 (Comparative Example) was cast at 150kg vacuum melting furnace and hot-forged into round bars of φ80mm in the same manner as in Example 1 (Material for specimen collection). Deoxidation conditions were as in Example 1, was performed by deoxidation conditions a ~ f shown in Table 2. After collection of the rolling fatigue test piece, and the high-frequency quenching treatment to the test unit were tempered for 1hr at 0.99 ° C.. Induction hardening treatment was performed under conditions in which the surface hardness after tempering is 650 Hv (measuring load 2.94 N) or more. Furthermore, the test surfaces were subjected to machining and rolling fatigue test mirror finished. Rolling fatigue test Mori thrust-type tester (contact surface pressure: 5.33GPa) was carried out. About 10 times of test results for each level (Table 5 No.1 ~ 28), using a Weibull statistics, 10% of the evaluation samples was evaluated the number of cycles to fracture as a fatigue characteristics L10.
[0066]
　Evaluation of Fatigue origin inclusions was performed in the same ultrasonic fatigue test as in Example 1. Heat treatment of the ultrasonic fatigue test pieces were subjected to tempering treatment 1hr thereafter 0.99 ° C. performs induction hardening processing to the test unit. Induction hardening process, after tempering was carried out under conditions to be 650 Hv (measuring load 2.94 N) or more to center from the test surface. Fatigue tests were constant frequency 20 kHz, stress ratio -1, the stress amplitude from 700 and 850 MPa, were performed until rupture. The origin inclusion of the ultrasonic fatigue test and analyze the composition using EDX (energy dispersive X-ray analysis), the previous REM atomic weight% of the total amount and Mg, the atomic weight% of Al was measured. Also in Table 5, the proportion of O to same atomic weight percent calculated as in Table 3 is not considered, in any of the embodiments, the origin inclusions contained O.
[0067]
　Table 4 shows the chemical composition of each steel type according to the second embodiment. Table 5 shows the steel species in each level (No.101 ~ 128), and deoxidation conditions, oxides form of fatigue origin inclusions ultrasonic fatigue test, the fatigue characteristics of the composition and the rolling fatigue test (L10 life) .
　The REM containing an appropriate amount, fatigue characteristics L10 of the present invention example, 10 6 and at cycles or more, was superior to steel type as a comparative example.
[0068]
[Table 4]

[0069]
[table 5]

[0070]
Example assuming a bearing according to (Embodiment 3) overall quenching
　No. shown in Table 6 E1 ~ E12 (present invention examples), No. The F1 ~ F12 steels each component (Comparative Example) was cast at 150kg vacuum melting furnace, and hot forging into round bars of φ80mm in the same manner as in Example 1 (Material for specimen collection). Deoxidation conditions were as in Example 1, was performed by deoxidation conditions a ~ f shown in Table 2. After collection of the rolling fatigue test piece, the test piece subjected to quenching treatment was heated to 850 ° C., were tempered for 1hr in subsequent 180 ° C.. Furthermore, the test surfaces were subjected to machining and rolling fatigue test mirror finished. Rolling fatigue test Mori thrust-type tester (contact surface pressure: 5.33GPa) was carried out. About 10 times of test results for each level (No.201 ~ 228 in Table 7), using a Weibull statistics, 10% of the evaluation samples was evaluated the number of cycles to fracture as a fatigue characteristics L10.
[0071]
　Evaluation of Fatigue origin inclusions was performed in the same ultrasonic fatigue test as in Example 1. Heat treatment of the ultrasonic fatigue test pieces were conducted under the same conditions as the rolling fatigue test piece. Fatigue tests were constant frequency 20 kHz, stress ratio -1, the stress amplitude from 700 and 850 MPa, were performed until rupture. The origin inclusion of the ultrasonic fatigue test and analyze the composition using EDX (energy dispersive X-ray analysis), the total amount of Al of the previous REM, in atomic weight% of Mg was measured. Also in Table 7, the proportion of O to same atomic weight percent calculated as in Table 3 is not considered, in any of the embodiments, the origin inclusions contained O.
[0072]
　Table 6 shows the chemical composition of each steel type in Embodiment 3. Table 7 shows the steel species in each level (No.201 ~ 228), and deoxidation conditions, oxides form of fatigue origin inclusions ultrasonic fatigue test, the fatigue characteristics of the composition and the rolling fatigue test (L10 life) .
[0073]
　The REM containing an appropriate amount, fatigue characteristics L10 of the present invention example, 5.0 × 10 6 is at cycles or more, was superior to steel type as a comparative example.
[0074]
[Table 6]

[0075]
[Table 7]

Industrial Applicability
[0076]
　According to the present invention, by compounding a cluster-like Al-O-type inclusions in REM-Al-Mg-O-based inclusions, it is possible to provide a superior steel fatigue properties.
DESCRIPTION OF SYMBOLS
[0077]
　1 ultrasonic fatigue test piece
　10 fracture surfaces
　11 concentric pattern
　(fisheyes) L gauge length
　a inclusions
　a 'inclusions (fatigue origin inclusion)

WE claims

[Requested item 1]
　で
mass%,
C:
0.10% ~ 1.50%, Si: 0.01% ~ 0.80%,
Mn: 0.10% ~ 1.50%, of Cr: 0.02 ~ 2.50%
%, of Al: 0.002% ~ 0.010% is not the
Man,
of Ce + La + of Nd: from 0.0001% ~ 0.0025%,
of Mg: 0.0005% ~ 0.0050%, O: from 0.0001% 0.0020% ~ ,
of Ti: 0.000 ~ 0.005% is not the Man%,
N: 0.0180% or
less, P: 0.030% or
less, S: 0.005% or
less, of Ca: 0.0000% ~ 0.0010%, V:
0.00 ~ 0.40%, of Mo: 0.00 ~ 0.60%, a Cu: 0.00 ~ 0.50%, of Nb: 0.000 ~ 0.050% is not the Man, of Ni: 0.00 2.50% ~, ~ 0.10%, B: 0.0000 0.0050%, remnants ga yoびおFe impuritiesでthou ri,

Features and fatigue origin inclusions Ce, La, 1 or more Nd detected by the ultrasonic fatigue test, Mg, Al, and contains O, and, that the composition ratio satisfies the formula (1) to excellent steel to rolling fatigue characteristics.
(Ce% + La% + Nd % + Mg%) / Al% ≧ 0.20 ··· Equation (1)
where, in the formula (1), Ce%, La %, Nd%, Mg%, Al% , respectively, Ce fatigue origin inclusions contain, La, Nd, Mg, an atomic weight% of Al.
[Requested item 2]
　By mass%, C: less than 0.10% ~ 0.45%, Cr: it characterized in that it is a 0.02 to 1.50%, excellent rolling fatigue characteristics of claim 1 steel.
[Requested item 3]
　By mass%, C: less than 0.45% ~ 0.90%, Cr: it characterized in that it is a 0.70 to 2.50%, excellent rolling fatigue characteristics of claim 1 steel.
[Requested item 4]
　By mass%, C: a 0.90% ~ 1.50%, Cr: it characterized in that it is a 0.70 to 2.50%, the steel having excellent rolling fatigue characteristics of claim 1 .