TW201736620A - Steel material for bearing having excellent rolling fatigue characteristics, method for manufacturing same, and bearing component - Google Patents

Abstract

This steel material for a bearing comprises a steel component containing C, Si, Mn, Cr, P, S, Al, Ti, N, and O, the remainder being iron and unavoidable impurities, oxide-based inclusions having a minor axis of 1 μm or greater which are present in the steel material containing Al2O3, SiO2, and TiO2, the remainder comprising unavoidable impurities, the total percentage by mass of Al2O3, SiO2, and TiO2 being 60% or greater, the mass ratio of TiO2 with respect to the total mass of Al2O3 and SiO2 being 0.10 to 1.50, and the percentage of composite inclusions in which TiN is present at the interface with a parent phase of the steel material among the oxide-based inclusions with respect to the total number of oxide-based inclusions being 30% or greater.

Description

Steel for bearings with excellent rolling fatigue characteristics, manufacturing method thereof, and bearing parts

The present invention relates to a steel material for a bearing excellent in rolling fatigue characteristics, a method for producing the same, and a bearing component excellent in rolling fatigue characteristics. More specifically, it is a steel material for bearings that can exhibit excellent rolling fatigue characteristics when used as a component such as a rolling element or a rail wheel in bearings used in various industrial machines and automobiles. A method of manufacturing a steel material and a bearing component obtained from the steel material for the bearing.

When bearings are used in various industrial machinery and automobiles, the rolling elements and the track trains in the bearings are subjected to high repetitive stresses. Therefore, the rolling elements of the bearing as well as the track train are required to have a long rolling fatigue life. The demand for the improvement of the rolling fatigue life is becoming stricter year by year in accordance with the high performance and light weight of industrial machinery. In order to further improve the durability of the bearing parts, the bearing steel is required to have a longer rolling fatigue life.

Conventionally, it is considered that the rolling fatigue life is among the oxide-based inclusions formed in steel, for example, a hard oxide system such as Al ₂ O ₃ which is mainly formed in Al deoxidized steel. The number density of inclusions has a deep correlation, and the rolling fatigue life can be prolonged by reducing the number density of the hard oxide-based inclusions. Therefore, in the steel making process, attempts have been made to increase the rolling fatigue life by reducing the oxygen content in the steel.

However, in recent years, with the progress of research on the relationship between rolling fatigue life and non-metallic inclusions represented by oxide-based inclusions, it has been known that the number density of oxide-based inclusions and rolling fatigue Life is not necessarily relevant. It has also been known that the rolling fatigue life is related to the size of non-metallic inclusions, for example, the square root of the area of the non-metallic inclusions, and the non-metallic inclusions are required to extend the rolling fatigue life. The practice of reducing the size is more effective than reducing the number density of non-metallic inclusions.

On the other hand, the technical solution proposed (the following Patent Documents 1, 2) does not use the conventional Al deoxidized steel, except that the Al content in the steel is reduced as much as possible, and the Si deoxidized steel is used. The composition of the produced oxide is controlled so as not to have Al ₂ O ₃ as a main component but a composition component mainly composed of SiO ₂ or CaO, and thus, a rolling process can be used to extend the non-metallic inclusions. The method of dividing the size of the non-metallic inclusions, thereby prolonging the rolling fatigue life.

The bearing steel material according to the technical solution disclosed in Patent Document 1 contains, by mass%, CaO: 10 to 60%, Al ₂ O ₃ : 20% or less, MnO: 50% or less, and MgO: 15% or less, and the rest is The oxide-based inclusions composed of SiO ₂ and an oxide-based inclusion composed of an unavoidable oxide present in the steel material, and the oxide system is present in an area of 100 mm ² at 10 places in the longitudinal direction of the steel material in the longitudinal direction. The arithmetic mean of the arithmetic mean of the maximum thickness of the inclusions and the maximum thickness of the sulfide-based inclusions is 8.5 μm or less.

The high-definition-purity Si deoxidized steel material of the invention disclosed in Patent Document 2 contains the oxide component of the oxide-based inclusion described in Patent Document 1 and contains a predetermined content of ZrO ₂ .

On the other hand, the technical solution disclosed in Patent Document 3 is a steel for spring which has a long fatigue life by controlling the formation of REM inclusions and detoxifying alumina, TiN, and MnS, and a method for producing the same. More specifically, alumina is modified into REM-Al-OS inclusions to prevent coarsening, and S is immobilized to become REM-Al-OS inclusions to suppress generation of coarse MnS, and TiN is A method of compounding into the inclusions of REM-Al-OS to reduce the number density of TiN which is detrimental to fatigue life.

In the technical solution of the steel material for bearing disclosed in Patent Document 4, TiO ₂ is contained in the oxide-based inclusion obtained by deoxidizing Si, thereby suppressing crystallization of the oxide-based inclusion, and thus, The rolling fatigue life can be improved by reducing voids at the interface between the parent phase (base phase) and the oxide-based inclusions in the steel.

[Previous Technical Literature] [Patent Literature]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2009-30145

[Patent Document 2] Japanese Patent Laid-Open Publication No. 2010-202905

[Patent Document 3] Japanese Patent Laid-Open Publication No. 2013-108171

[Patent Document 4] Japanese Patent Laid-Open Publication No. 2014-25083

Patent Document 1 does not deal with voids at the interface between the mother phase and the oxide-based inclusions in the steel, and measures to reduce the voids. Therefore, it cannot be said that sufficient rolling fatigue life can be obtained. .

Patent Document 2 does not disclose any description relating to voids caused by peeling at the interface. The original Patent Document 2 mainly focuses on the technique of miniaturizing the entire non-metallic inclusions, and even in the evaluation method of the embodiment, only the arithmetic mean of the C-series inclusions of the ASTM E45 method is used for evaluation. . Therefore, the rolling fatigue life of the steel material produced by this method is not necessarily long.

Patent Document 3, the oxide-based inclusions are composed of strong deoxidizing elements such as REM and Al, and do not mainly contain weak deoxidizing elements such as Si. Therefore, oxide-based inclusions in steel cannot be suppressed. Peeling from the interface of the parent phase.

According to Patent Document 4, since voids generated at the interface between the oxide-based inclusions of the amorphous body and the matrix phase are reduced, the rolling fatigue life can be improved regardless of the direction in which the load is applied. However, it is based on the requirement that the durability of the bearing parts be improved compared to the conventional bearing parts, and it is desirable to develop a steel material for bearings with a longer rolling fatigue life.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a steel material for a bearing excellent in rolling fatigue characteristics, a method for producing the same, and a method for producing the same Bearing parts with excellent rolling fatigue characteristics.

The steel material for bearings having excellent rolling fatigue characteristics of the present invention contains C: 0.8% or more and 1.1% or less, Si: 0.15% or more and 0.8% or less, and Mn: 0.1% or more and 1.0% by mass%. Hereinafter, Cr: 1.3% or more and 1.8% or less, P: more than 0% and 0.05% or less, S: more than 0% and 0.015% or less, Al: 0.0002% or more and 0.005% or less, and Ti: 0.0005% or more and 0.010% or less. N: 0.0030% or more and 0.010% or less, O: more than 0% and 0.0030% or less, and the rest are iron and unavoidable impurities for the bearing steel, and the oxide having a short diameter of 1 μm or more is present in the steel material. The inclusions, in mass%, contain Al ₂ O ₃ : 5% or more and 50% or less, SiO ₂ : 10% or more and 70% or less, TiO ₂ : 3% or more and 50% or less, and the rest are inevitable oxides. , the Al ₂ O _3, the SiO ₂ and the total mass percentage of TiO ₂ of not less than 60%, ₂ O ₃ and the SiO ratio of the total mass of the TiO mass ₂ with respect to the Al ₂ was 0.10 or more and 1.50 or less, Among the oxide-based inclusions, there is a composite inclusion of TiN present at the interface with the parent phase of the steel material. The number of the system is 30% or more of the total number of the oxide-based inclusions.

A method for producing a steel material for a bearing having excellent rolling fatigue characteristics according to another aspect of the present invention includes: a melting step of preparing a steel material composed of the steel component by performing a Si deoxidation treatment; and a first soaking step; Blocking step; second soaking step; hot rolling step, in the second soaking step The holding temperature is 1240 ° C or lower, and in the first soaking step and the second soaking step, the holding time at a temperature of 900 to 1240 ° C is 60 minutes or more in total.

Another aspect of the present invention is a bearing component comprising the aforementioned steel material for bearings.

The objects, features, aspects and advantages of the present invention will become more apparent

"Steel for Bearings"

First, a steel material for a bearing excellent in rolling fatigue characteristics of one aspect of the present invention will be described.

The inventors of the present invention have further reviewed the steel materials for bearings which are intended to provide a more extended rolling fatigue life after the publication of Patent Document 4 described above. Thus, it has been found that when the amount of CaO contained in the oxide-based inclusions obtained by deoxidation of Si is small, the temperature is maintained or the heating retention time is maintained depending on the heating in the heating process before the block rolling or hot rolling. The length of the rolling fatigue life of the steel for bearings has been greatly changed.

Therefore, the inventors of the present invention used oxide-based inclusions containing Al ₂ O ₃ , SiO ₂ and TiO ₂ as main components as oxide-based inclusions obtained by deoxidation of Si, and assumed that if the oxidation is performed If the interface between the inclusions of the system and the mother phase of the steel (the base phase of the steel) is TiN, the idea of extending the rolling fatigue life should be extended. The specific method is to examine the size and composition of the oxide-based inclusions and the ratio of the inclusions of TiN in the interface to the number of the oxide-based inclusions.

As a result, it has been found that a desired material can be achieved as long as the following requirements (a) to (c) are satisfied, and thus the steel material for bearings excellent in rolling fatigue characteristics of the present invention is completed.

(a) The steel component of the steel material contains C: 0.8% or more and 1.1% or less, Si: 0.15% or more and 0.8% or less, Mn: 0.1% or more and 1.0% or less, and Cr: 1.3% or more and 1.8% or less by mass%. P: higher than 0% and 0.05% or less, S: higher than 0% and 0.015% or less, Al: 0.0002% or more and 0.005% or less, Ti: 0.0005% or more and 0.010% or less, and N: 0.0030% or more and 0.010% or less. O: more than 0% and 0.0030% or less, the balance being iron and unavoidable impurities; (b) oxide-based inclusions having a short diameter of 1 μm or more present in the steel, and containing Al ₂ by mass% O ₃ : 5% or more and 50% or less, SiO ₂ : 10% or more and 70% or less, TiO ₂ : 3% or more and 50% or less, and the rest are inevitable oxides, Al ₂ O ₃ , SiO ₂ and TiO ₂ The total mass percentage is 60% or more, and the ratio of the mass of TiO ₂ to the total mass of Al ₂ O ₃ and SiO ₂ is 0.10 or more and 1.50 or less; (c) among the oxide-based inclusions, the mother phase with the steel material The number of inclusions of TiN present in the interface (sometimes referred to as composite inclusions in this specification) is 30% of the total number of oxide inclusions. On.

In the thrust rolling fatigue test described in Examples to be described later, the fatigue life L ₁₀ is 5.4 × 10 ⁷ or more.

The relationship until the completion of the present invention, and the relationship between the present invention and Patent Document 4 and Patent Document 3 will be described in detail below.

The inventors of the present invention have been continually conducting various reviews in order to provide a Si deoxidized steel material having a longer rolling fatigue life than the steel material of Patent Document 4.

It is known that oxide-based inclusions obtained by deoxidation of Si are easily amorphous, and are easily extended by hot rolling. Therefore, the steel after hot rolling causes an anisotropy in the oxide-based inclusions. As a result, an anisotropy also occurs in the rolling contact fatigue life of the steel material, which is not preferable. On the other hand, by controlling the composition of the oxide-based inclusions, it is possible to crystallize in the high temperature range when the inter-heat processing is performed, and it is possible to become a polycrystalline body. However, since the oxide-based inclusions after becoming a polycrystalline body have higher deformation resistance than the parent phase of the steel, the parent phase and the oxide-based inclusions in the steel are easily formed during the hot working or the cold working. A gap occurs in the interface. The voids that occur at the interface will have an adverse effect on the rolling fatigue life and, therefore, are not suitable.

Therefore, the inventors of the present invention have not only focused on the composition of the oxide-based inclusions obtained by deoxidation of Si, but also on the control method for reducing the occurrence of voids by controlling the formation of TiN. As a result, it was found that if a predetermined amount of TiN is formed at the interface between the oxide-based inclusion obtained by deoxidation of Si and the parent phase, the peeling at the interface can be reduced, and the peeling can be remarkably improved. Rolling fatigue life.

Regarding TiN, as shown in Patent Document 3, there have been many reports indicating that it would adversely affect fatigue life. However, all of the above reports are related to TiN generated in Al deoxidized steel as in Patent Document 3. That is, in the case of Al deoxidized steel, Al ₂ O ₃ , MgO. Since the deoxidation product such as Al ₂ O ₃ or (Ca, Al)-based oxide is formed in a solid phase state in the molten steel, it is easy to form TiN by using the deoxidation product as a generation nucleus. Further, since Al ₂ O ₃ or the like is easily aggregated in the molten steel to form coarsens, TiN formed in Al ₂ O ₃ or the like tends to be coarsened. As a result, it is considered that the composite of the deoxidation product such as Al ₂ O ₃ and TiN tends to be coarsened, which adversely affects the rolling fatigue life. In addition, it is considered that if a large amount of coarsened Al ₂ O ₃ or the like is to be coated with TiN, a large amount of TiN must be generated, and as a result, the composite body is coarsened, and the composite body is coarsened. Will have an adverse effect on rolling fatigue life. In addition, since the peeling phenomenon still occurs at the interface between the oxide-based inclusions such as Al ₂ O ₃ and the mother phase which are not covered by TiN, it is considered that the peeling phenomenon is impossible to improve the rolling fatigue life of the steel. The reason.

Therefore, in the conventional technology, many have been revealed: a technique of focusing on TiN generated at the interface between an oxide-based inclusion and a parent phase. However, as shown in Patent Document 3, all of them are made of steel materials using Al deoxidized steel as a raw material. Moreover, the solution only reveals a technique for reducing the number density of TiN which is harmful to fatigue life and making it harmless. The solution disclosed in Patent Document 3 cannot be solved. Inhibition: A peeling phenomenon that occurs at the interface between the oxide-based inclusions and the parent phase, which adversely affects the rolling fatigue life.

On the other hand, in the steel material of the present invention, Si deoxidized steel is used, and in the melting stage of the Si deoxidized steel, the generation of a deoxidized product which is easily coarsened such as Al ₂ O ₃ is suppressed. And the steel of the present invention is also a Si deoxidized steel, and in the melting stage, Al ₂ O ₃ , SiO ₂ and TiO _{2 are formed} as deoxidation products, and the total mass of Al ₂ O ₃ , SiO ₂ and TiO _{2 is obtained} . The percentage of the total amount of the deoxygenated product is 60% or more. Oxide-based inclusions composed of Al ₂ O _{3 ,} SiO ₂ and TiO ₂ , and deoxidation products (Al ₂ O ₃ , MgO.Al ₂ O ₃ , (Ca, Al)-based oxidation formed by deoxidation of Al Compared with the material, the melting point is lower, and it is less likely to aggregate in the molten steel, and tends to be less likely to be coarsened. Therefore, even in the heating stage before the hot working (for example, block rolling, block forging, hot rolling), oxide inclusions (deoxidation products generated by deoxidation of Si) are used as the generation core. TiN is formed to form a composite inclusion, and the composite inclusion is maintained in a relatively fine state. Further, it is well known that the TiN system has excellent integration with the crystal lattice of α-Fe having a crystal structure of bcc structure. Therefore, the adhesion between the composite inclusion and the matrix phase can be further improved by using TiN, and as a result, it is considered that the peeling phenomenon occurring at the interface between the composite inclusion and the parent phase can be reduced. The result is considered to be: a dramatic increase in rolling fatigue life.

In order to secure the above-mentioned composite inclusions in a predetermined amount, it is necessary to set the holding temperature and the holding time at the time of heating performed before the block rolling, block forging, hot rolling, etc., to prepare TiN, as described above. Temperature Degree, and keep the appropriate time. In order to prolong the rolling fatigue life, Patent Document 4 mainly focuses on maintaining the oxide-based inclusions obtained by deoxidizing Si in an amorphous body, and the holding temperature and the holding time during the heating are maintained. In the traditional state, no special considerations and cooperation were made at all. In addition, as a result of reviewing the rolling fatigue life in the patent document 4, the inventors of the present invention have further reviewed the holding time during the heating, which has not been particularly noticed in the past, at a temperature suitable for the formation of TiN. Compared with the conventional method, TiN is formed at the interface between the oxide-based inclusions and the matrix phase, and the adhesion between the oxide-based inclusions and the matrix phase can be improved, and voids can be reduced. It can extend the rolling fatigue life. Finally, it has been known that, for example, the holding temperature and the holding time during heating performed before the block rolling, block forging, hot rolling, and the like are maintained at a temperature suitable for the formation of TiN, and TiN will be maintained for a long time. At the interface between the oxide-based inclusions and the parent phase, the adhesion between the oxide-based inclusions and the parent phase is enhanced, and the voids are reduced, so that the rolling fatigue life can be improved, and the present invention has been completed.

Hereinafter, the requirements (a) to (c) of the steel material for bearings of the present invention will be described.

(a) About the chemical composition of steel (steel composition)

The steel material for bearings of the present invention contains, by mass%, C: 0.8% or more and 1.1% or less, Si: 0.15% or more and 0.8% or less, Mn: 0.1% or more and 1.0% or less, and Cr: 1.3% or more and 1.8. %the following, P: more than 0% and 0.05% or less, S: more than 0% and 0.015% or less, Al: 0.0002% or more and 0.005% or less, Ti: 0.0005% or more and 0.010% or less, and N: 0.0030% or more and 0.010% or less, and O; : Steels composed of steel components higher than 0% and 0.0030% or less, and the balance being iron and unavoidable impurities. Next, the reasons for limiting the range to the following are as follows.

[C: 0.8~1.1%]

C can increase the cold quenching hardness and is an essential element for maintaining the strength at room temperature and high temperature and imparting wear resistance. In order to exert such an effect, the C content is required to be 0.8% or more, preferably 0.85% or more, more preferably 0.90% or more. However, if the C content is too large, it is easy to form a large carbide in the core of the bearing, which adversely affects the rolling fatigue characteristics. Therefore, the C content is set to 1.1% or less, preferably 1.05% or less, more preferably 1.0% or less.

[Si: 0.15~0.8%]

Si can effectively function as a deoxidizing element, and also has an effect of improving hardenability and temper softening resistance and further improving hardness. In order to effectively exert such an effect, the Si content must be 0.15% or more, preferably 0.20% or more, more preferably 0.25% or more. However, if the Si content is excessive, not only the life of the mold during forging will be deteriorated, but also the cost will increase. Therefore, the Si content is set to 0.8% or less, preferably 0.7% or less, more preferably 0.6% or less.

[Mn: 0.1~1.0%]

Mn is an element which enhances the solid solution strengthening and quench hardenability of the steel mother phase. In order to effectively exert such an effect, the Mn content must be 0.1% or more, preferably 0.2% or more, more preferably 0.3% or more. However, if the Mn content is excessive, the content of the lower oxide, that is, the MnO, is increased, and the rolling fatigue characteristics are deteriorated, and the workability and the machinability are also remarkably deteriorated. Therefore, the Mn content is set to 1.0% or less, preferably 0.8% or less, more preferably 0.6% or less.

[Cr: 1.3~1.8%]

Cr is an element which can improve quench hardenability and can form strength and wear resistance by forming stable carbides, thereby improving rolling fatigue characteristics. In order to exert such an effect, the Cr content must be 1.3% or more, preferably 1.35% or more, more preferably 1.4% or more. However, when the Cr content is excessive, the carbide becomes coarse and the rolling fatigue characteristics and the machinability are deteriorated. Therefore, the Cr content is set to 1.8% or less, preferably 1.7% or less, more preferably 1.6% or less.

[P: higher than 0% and less than 0.05%]

P is an inevitable element contained in steel. If the P content is excessive, it will segregate at the crystal grain boundary and adversely affect the rolling fatigue characteristics. Therefore, the P content is set to 0.05% or less, preferably 0.03% or less, more preferably 0.02% or less. The less the P content, the better. Lower limit of P content The value is not particularly limited, and is 0.002% in terms of the content achievable on an industrial scale.

[S: above 0% and below 0.015%]

S is an inevitable element contained in steel and an element which forms a sulfide. If the S content is excessive, coarse sulfides may remain in the steel material, and rolling fatigue characteristics may be deteriorated. Therefore, the S content is set to 0.015% or less, preferably 0.007% or less, more preferably 0.005% or less. The less the S content, the better. The lower limit of the S content is not particularly limited, and is 0.0005% in terms of the content achievable on an industrial scale.

[Al: 0.0002~0.005%]

Al is a deoxidizing element, and is an element which changes the composition of the oxide-based inclusion depending on the amount of the content. When the Al content is excessive, the amount of formation of the hard oxide mainly composed of Al ₂ O ₃ increases, and the coarse oxide remains after the hot rolling, so that the rolling fatigue characteristics are deteriorated. Therefore, the Al content is set to 0.005% or less, preferably 0.002% or less, more preferably 0.0015% or less. In the present invention, since Si is used for deoxidation, it is not necessary to add Al to perform deoxidation treatment after oxidative refining like Al deoxidized steel. However, when the Al content is too small, the Al ₂ O ₃ content in the oxide becomes too small, and oxide-based inclusions having a large SiO ₂ content are formed to deteriorate rolling fatigue characteristics. In addition, in order to reduce the Al content as much as possible, it is necessary to reduce the mixing of Al. Therefore, it is not only the composition in the steel, but also the Al ₂ O ₃ content in the flux must be reduced. However, fluxes with a low Al ₂ O ₃ content are very expensive and are not economical for high carbon steel bearing steels. Therefore, the Al content is set to 0.0002% or more, preferably 0.0003% or more, more preferably 0.0005% or more.

[Ti: 0.0005~0.010%]

Ti is an element that imparts characteristics to the present invention. By adding a certain amount of Ti, TiN can be formed at the interface between the oxide-based inclusions and the parent phase, and the peeling phenomenon occurring at the interface can be reduced. As a result, rolling fatigue characteristics can be improved. Further, the concentration of TiO ₂ in the oxide-based inclusions can be controlled, and the aspect ratio (detailed later) can also be effectively acted, and the rolling fatigue characteristics can be further improved. In order to exert such an effect, the Ti content must be 0.0005% or more, preferably 0.0008% or more, more preferably 0.0011% or more. However, when the Ti content is excessive, TiN becomes coarser, and the TiO ₂ -based oxide is coarsened, and rolling fatigue characteristics are deteriorated. Therefore, the Ti content is set to 0.010% or less, preferably 0.0050% or less, more preferably 0.0030% or less.

[N: 0.0030~0.010%]

N is also an element imparted to the present invention in the same manner as the above Ti. By adding a predetermined amount of N, TiN can be formed at the interface between the oxide-based inclusions and the parent phase, and the peeling phenomenon occurring at the interface can be reduced. As a result, rolling fatigue characteristics can be improved. In order to exert such an effect, the N content must be 0.0030% or more, preferably 0.0035% or more. More preferably 0.0040% or more. However, if the N content is excessive, TiN will become coarser and the rolling fatigue characteristics will deteriorate. Therefore, the N content is set to 0.010% or less, preferably 0.008% or less, more preferably 0.007% or less.

[O: above 0% and below 0.0030%]

O is an inevitable element contained in steel. When the O content is excessive, coarse oxides are easily formed, and even after hot rolling and cold rolling, oxide-based inclusions remain coarse oxides, which adversely affects rolling fatigue characteristics. Therefore, the O content is set to 0.0030% or less, preferably 0.0025% or less, more preferably 0.0020% or less. In order to improve the rolling fatigue characteristics, the O content is as small as possible. The lower limit of the O content is not particularly limited, from the viewpoint of improving rolling fatigue characteristics, but is preferably 0.0004% or more, more preferably 0.0008% or more in consideration of economical factors and the like. Since it is desired to control the O content to less than 0.0004%, the work of removing O from the molten steel must be strictly performed, and the molten steel processing time will become long without conforming to economic effects.

[other ingredients]

The steel material for bearings of the present invention conforms to the above respective components, and the remaining components are iron and unavoidable impurities other than the above P, S and O. The unavoidable impurities include elements (for example, H, Ni, etc.) that are entrained by factors such as raw materials, materials, and manufacturing equipment.

(b) Composition of oxide-based inclusions having a short diameter of 1 μm or more

The steel material for a bearing of the present invention has an oxide-based inclusion having a short diameter of 1 μm or more, and the composition thereof contains Al ₂ O ₃ : 5% or more and 50% or less, and SiO ₂ : 10% or more. 70% or less, TiO ₂ : 3% or more and 50% or less, the rest is an unavoidable oxide, and the total mass percentage of Al ₂ O ₃ , SiO _{2 ,} and TiO ₂ is 60% or more, and the mass of TiO ₂ is relative to Al _{2 .} The ratio of the total mass of O ₃ and SiO ₂ is 0.10 or more and 1.50 or less. Next, the reasons for limiting the range to the following are as follows.

[Short diameter of oxide-based inclusions: 1 μm or more]

The rolling fatigue characteristics are considered to be: the larger the size of the oxide-based inclusions, the greater the degree of adverse effects. Therefore, in the present invention, in order to evaluate oxide-based inclusions having a large size which may adversely affect the rolling fatigue characteristics, the oxide-based inclusions having the above-described size (short diameter: 1 μm or more) are controlled.

[Al ₂ O ₃ : 5~50%]

The Al ₂ O ₃ system has an effect of lowering the liquidus temperature of the oxide mainly composed of SiO ₂ . Therefore, it is possible to suppress the coarsening of the oxide and to form TiN at the interface between the oxide-based inclusion steel and the parent phase. As a result, the rolling fatigue characteristics can be improved. Further, the Al ₂ O ₃ system has an effect of promoting crystallization of oxide-based inclusions. Therefore, it has an important function for reducing the aspect ratio of oxide-based inclusions. In order to effectively exhibit such an effect, the content of Al ₂ O ₃ in the composition of the oxide-based inclusion must be 5% or more, preferably 8% or more, more preferably 12% or more. However, if the content of Al ₂ O ₃ in the composition of the oxide-based inclusion is excessive, the crystal phase of Al ₂ O ₃ (steel jade) crystallizes in the molten steel and during solidification, or crystallizes together with MgO. Come out of MgO. Crystalline phase of Al ₂ O ₃ (spinel). These solid phases are all hard and exist in the form of coarse inclusions, and voids are easily formed during processing, and rolling fatigue characteristics are deteriorated. Therefore, the content of Al ₂ O ₃ in the composition of the oxide-based inclusion must be 50% or less, preferably 40% or less, more preferably 30% or less.

[SiO ₂ : 10~70%]

SiO ₂ has an effect of lowering the liquidus temperature of oxide-based inclusions. Therefore, it is possible to suppress the coarsening of the oxide and to have an effect of forming TiN at the interface between the oxide-based inclusion and the parent phase. As a result, rolling fatigue characteristics can be improved. In order to effectively exhibit such an effect, the content of SiO ₂ in the composition of the oxide-based inclusion must be 10% or more, preferably 15% or more, more preferably 25% or more, and still more preferably 30% or more. However, when the content of SiO ₂ in the composition of the oxide-based inclusions is excessive, the oxide becomes coarser and the rolling fatigue characteristics are deteriorated. Further, the oxide will be extended to make the aspect ratio large, and the rolling fatigue characteristics are deteriorated. Therefore, the content of SiO ₂ in the composition of the oxide-based inclusion must be 70% or less, preferably 60% or less, more preferably 45% or less.

[TiO ₂ : 3~50%]

TiO ₂ has an effect of lowering the liquidus temperature of an oxide mainly composed of SiO ₂ . Therefore, it is possible to suppress the coarsening of the oxide and to have an effect of forming TiN at the interface between the oxide-based inclusion and the parent phase. As a result, rolling fatigue characteristics can be improved. Further, TiO ₂ has an effect of promoting crystallization of oxide-based inclusions. Therefore, it has an important function for reducing the aspect ratio of oxide-based inclusions. In order to effectively exhibit such an effect, the content of TiO ₂ in the composition of the oxide-based inclusion must be 3% or more, preferably 5% or more, more preferably 10% or more, and still more preferably 20% or more. However, when the content of TiO ₂ in the composition of the oxide-based inclusions is excessive, the oxide becomes coarse and the rolling fatigue characteristics are deteriorated. Therefore, the content of TiO ₂ in the composition of the oxide-based inclusion must be 50% or less, preferably 45% or less, more preferably 40% or less.

[inevitable oxides]

The oxide-based inclusions contain Al ₂ O ₃ , SiO ₂ , and TiO ₂ , and the remaining components are inevitable oxides. The unavoidable oxide means an oxide which is inevitably contained in the production process, and examples thereof include CaO, REM ₂ O ₃ , MgO, MnO, ZrO ₂ , Na ₂ O, K ₂ O, and Li _{2 .} O, Cr ₂ O ₃ , NbO, FeO, Fe ₃ O ₄ , Fe ₂ O ₃ . However, it is also possible to contain an unavoidable oxide within a range in which the crystallization state and the aspect ratio of the oxide-based inclusions are not adversely affected and the desired characteristics can be obtained. The percentage of the total mass of the inevitable oxides to the total mass of the oxide-based inclusions is preferably 30% or less, more preferably 20% or less. For example, CaO may be contained in a range of 20% by mass or less based on the total mass of the oxide-based inclusions. Further, REM ₂ O ₃ , MgO, MnO, ZrO ₂ , Na ₂ O, K ₂ O, Li ₂ O, Cr ₂ O ₃ , NbO, FeO, Fe ₃ O ₄ , Fe ₂ O ₃ , respectively, may be in relative It is contained in a range in which the mass percentage of the total mass of the oxide-based inclusions is less than 10%. Further, in the present invention, the term "REM" refers to a total of 17 kinds of lanthanoid elements (15 elements belonging to La to Lu of Group 3 of the sixth cycle of the periodic table), Sc (钪), and Y (钇). element. Among these elements, it is preferred to contain at least one element selected from the group consisting of La, Ce and Y, and it is more preferable to contain La and/or Ce.

[Total mass percentage of Al ₂ O ₃ , SiO ₂ and TiO ₂ : 60% or more (Al ₂ O ₃ + SiO ₂ + TiO ₂ ≧ 60%)]

As described above, Al ₂ O ₃ , SiO ₂ and TiO ₂ are main components of the oxide-based inclusions in the present invention, and their respective contents are controlled, and in the present invention, it is necessary to use Al ₂ O ₃ and SiO. ₂ and the total content of TiO ₂ is suitable for control. As a result, TiN can be formed at the interface between the oxide-based inclusions and the parent phase, and the peeling phenomenon at the interface can be suppressed, and the rolling fatigue characteristics can be improved. When the total content of Al ₂ O ₃ , SiO _{2 ,} and TiO ₂ is too small, the oxide becomes coarse, and TiN cannot be sufficiently utilized to control the interface, and rolling fatigue characteristics cannot be improved. Based on this viewpoint, the total content of Al ₂ O ₃ , SiO _{2 ,} and TiO ₂ is set to 60% or more. The more the total content of Al ₂ O ₃ , SiO ₂ and TiO _{2 is} , the better, and it is preferably 65% or more, more preferably 70% or more. On the other hand, the upper limit of the total content of Al ₂ O ₃ , SiO ₂ and TiO ₂ is not particularly limited, and may be, for example, 100%.

[Ti02 mass ₂ with respect to Al ₂ O ₃ and the ratio of the total mass of SiO _2: 0.10 or more and 1.50 or less _{(1.50 ≧ TiO 2 / (Al} 2 O 3 + SiO 2) ≧ 0.10)]

As described above, Al ₂ O ₃ , SiO ₂ and TiO ₂ are main components of the oxide-based inclusions in the present invention. When the ratio of the mass of TiO _{2 to} the total mass of Al ₂ O ₃ and SiO ₂ falls within a predetermined range, TiN can be formed at the interface between the oxide-based inclusion and the parent phase and can be suppressed at the interface. Peeling phenomenon. When the ratio of the mass of TiO _{2 to} the total mass of Al ₂ O ₃ and SiO ₂ is too small, the ratio of TiO ₂ in the oxide-based inclusion is too small to become Al ₂ O ₃ and SiO _{2 .} main body. As a result, TiN cannot be formed at the interface between the oxide-based inclusions and the parent phase, and the rolling fatigue characteristics are not improved. Further, the aspect ratio of the oxide-based inclusions also becomes large. From the viewpoint of this viewpoint, the ratio of the mass of TiO ₂ to the total mass of Al ₂ O ₃ and SiO ₂ must be 0.10 or more, preferably 0.15 or more, more preferably 0.25 or more. However, when the ratio of the mass of TiO ₂ to the total mass of Al ₂ O ₃ and SiO ₂ is too large, the ratio of TiO ₂ in the oxide-based inclusion is too large to form TiN in the oxide system. At the interface between the inclusions and the parent phase, the rolling fatigue characteristics were not improved. Therefore, the ratio of the mass of TiO ₂ to the total mass of Al ₂ O ₃ and SiO ₂ must be 1.50 or less, preferably 1.30 or less, more preferably 1.00 or less.

(c) Among the oxide-based inclusions, the number of composite inclusions having TiN at the interface with the parent phase of the steel material accounts for oxygen having a short diameter of 1 μm or more. Percentage of total number of inclusions

Oxide-based inclusions having a short diameter of 1 μm or more in the steel material for bearings of the present invention, and inclusions of TiN in the interface with the mother phase (base phase) of the steel material among the oxide-based inclusions The percentage of the total number of the oxide-based inclusions (hereinafter, referred to as the ratio of the number of TiN) is 30% or more. Next, the reasons for limiting the range to the following are as follows.

The term "TiN present on the interface" means that the oxide-based inclusions (the interface with the parent phase (base phase) of the steel material) are formed as shown in the explanation column of the embodiment to be described later. The meaning of TiN. Such TiN is extremely important for improving the rolling fatigue life, and by allowing TiN to exist at the aforementioned interface, the peeling phenomenon occurring at the interface between the composite inclusion and the parent phase can be suppressed. The rolling fatigue life can be improved as a result of the suppression of the interface peeling which adversely affects the rolling fatigue life. The ratio of the number of TiNs that can produce such an effect must be 30% or more. The higher the ratio of TiN number, the better, 40% or more is better, and 50% or more is better. On the other hand, the upper limit of the ratio of the number of TiN is not particularly limited, and may be, for example, 100%.

The method of measuring the ratio of the number of TiN will be described in detail in the description of the examples to be described later.

In the steel material for a bearing of the present invention, the steel composition and the oxide composition are appropriately controlled in the above-described manner, and thus the cutting can be performed in a direction parallel to the longitudinal direction of the steel material. The average value (hereinafter, simply referred to as an aspect ratio) of the aspect ratio (longitudinal diameter/short diameter) of the oxide-based inclusions in the surface is reduced to 3.0 or less. By this, whether it is Applying any load in one direction can stably improve the rolling fatigue characteristics. The aforementioned aspect ratio is as small as possible, and is preferably 2.5 or less, and more preferably 2.0 or less.

The method of measuring the aspect ratio will be described in detail in the description of the examples to be described later.

"Manufacturing method of steel for bearings"

Next, a method of producing a steel material for a bearing excellent in rolling fatigue characteristics of another aspect of the present invention will be described.

The steel material for bearing of the present invention is a melting step of preparing a steel material composed of the steel component by performing a Si deoxidation treatment; a first soaking step; a blocking step; a second soaking step; In the rolling step, the holding temperature in the second soaking step is set to 1240° C. or lower, and the holding time at the temperature of 900 to 1240° C. in the first soaking step and the second soaking step is totaled. Set it to 60 minutes or more and make it.

When manufacturing the steel material for bearings of the present invention, it is necessary to pay special attention to the melting step, the first soaking step, and the second soaking step, so that a predetermined oxide component and the number of TiN ratios can be obtained. For the steps other than these steps, a method generally employed in the production of steel for bearings can be suitably used.

Hereinafter, the melting step, the first soaking step, and the second soaking step will be described.

[melting process]

The steel composed of the aforementioned steel component is first melted and then cast to obtain a cast piece, that is, a steel material. When the steel is melted, the deoxidation treatment is performed by adding Al instead of the deoxidation treatment by the addition of Si, and the deoxidation treatment (Si deoxidation treatment) is performed by adding Si.

A preferred melting method for obtaining the aforementioned oxide component is as follows.

At the time of melting, in order to control the content of Al ₂ O ₃ , the content of Al contained in the steel is controlled to be 0.0002 to 0.005% as described above.

The method of controlling TiO ₂ is not particularly limited. Ti may be added in such a manner that the Ti content contained in the steel can be controlled within the range of 0.0005 to 0.010% as described above, in accordance with a general method in the technical field. The method of adding Ti in this case is not particularly limited. For example, it may be adjusted by adding an iron-based alloy containing Ti, or the Ti concentration in the molten steel may be controlled by controlling the composition of the slag.

The content of SiO ₂ is indirectly controlled by controlling other oxides in the manner described above.

The preferred control method for obtaining the number ratio of the aforementioned TiN is as follows.

When the steel is melted according to a general method, the Ti content in the steel is controlled within the range of 0.0005 to 0.010%, and the N content is controlled within the range of 0.003 to 0.010% as described above. Ti, N. The method of adding Ti is not particularly limited. For example: by adding The Ti-based iron-based alloy is added for adjustment, or the Ti concentration in the molten steel can be controlled by controlling the composition of the slag. The method of adding N is also not particularly limited. For example, it can be adjusted by adding an alloy containing N, or nitrogen can be used to control the N content when aerating the molten steel, or by controlling the gas phase in contact with the molten steel. Nitrogen partial pressure.

[1st soaking process and 2nd soaking process]

The heating performed before the block rolling or the block forging, and the heating performed before the hot rolling are maintained at a temperature at which the TiN can be easily formed at the interface between the oxide-based inclusion and the steel ( At 900~1240 ° C), the time kept at this temperature is controlled for more than a certain period of time (60 minutes or more), so that after hot rolling, TiN exists in the mother phase of the oxide-based inclusions and steel (steel) The base phase of the interface.

The details are as follows.

The heating performed before the block rolling or the block forging includes a process of maintaining the specified time (the first specified time) at the specified temperature (the first specified temperature) after the temperature of the steel material reaches the specified temperature. (1st soaking step). On the other hand, the heating performed before the hot rolling includes: after the rolling material obtained by the block rolling or the block forging of the steel material is heated to reach another specified temperature, at the other specified temperature ( At the second predetermined temperature, the other predetermined time (second designated time) is maintained (the second soaking step). In the heating performed before hot rolling, in order to prevent the formation of the interface between the oxide-based inclusions and the steel The upper TiN disappears, and the second specified temperature is set to 1240 ° C or lower. Further, the holding time of the steel material and the first soaking step and the second soaking step in the second soaking step at the temperature of 900 to 1240 ° C is controlled to be 60 minutes or more in total.

Specifically, the first predetermined temperature, the second predetermined temperature, the first designated time, and the second designated time in the first soaking step and the second soaking step are controlled so as to satisfy the following conditions 1 to 3 Any of the conditions.

Condition 1: The first specified temperature is 900 to 1240 ° C, and the second specified temperature is lower than 900 ° C. In the first soaking step, the time (first specified time) at which the steel material is held at the first specified temperature is 60. Minutes or more; Condition 2: The first specified temperature is higher than 1240 ° C, the second specified temperature is 900 to 1240 ° C, and the rolling material is held at the second specified temperature in the second soaking step (second specified time) ) is 60 minutes or longer; Condition 3: Both the first specified temperature and the second specified temperature are 900 to 1240 ° C, and the time during which the steel material is held at the first specified temperature in the first soaking step (first) The total time (the second designated time) in which the rolling material is held at the second predetermined temperature in the second soaking step is 60 minutes or longer.

For example, (1) can be controlled so that the time during which the steel material is held at a temperature (900 to 1240 ° C) at which TiN is easily generated in the first soaking step is set to 60 minutes or longer, and the second time is In the thermal process, the rolled material is held at a temperature (less than 900 ° C) at which TiN is less likely to be formed. (2) It can also be controlled to: in the first soaking process, The steel material is held at a temperature (higher than 1240 ° C) that does not easily disappear even if TiN is generated, and in the second soaking step, the temperature (900 to 1240 ° C) at which TiN can be easily formed is used. The time for holding the rolled material was set to 60 minutes or longer. (3) It is also possible to control: in the first soaking step, the steel material is held at a temperature at which TiN is less likely to be generated (less than 900 ° C), and in the second soaking step, The temperature at which TiN is easily generated (900 to 1240 ° C) is set to a time for holding the rolled material for 60 minutes or more. (4) It is also possible to control the temperature at which the TiN can be easily formed (900 to 1240 ° C) in the first soaking step and the temperature at which the TiN can be easily formed in the second soaking step ( One or both of the time for holding from 900 to 1240 ° C) is set to 60 minutes or longer. (5) It is also possible to control the time during which the TiN can be easily formed in the first soaking step (900 to 1240 ° C), and the TiN can be easily formed in the second soaking step. The total of the time during which the temperature (900 to 1240 ° C) is held is set to 60 minutes or more.

The lower limit of the first specified temperature in the above condition 1 is preferably 950 °C. The upper limit of the first specified temperature in the above condition 1 is preferably 1200 ° C, more preferably 1150 ° C. The longer the first specified time in the aforementioned condition 1, the better. For example, it is preferably 80 minutes or more, more preferably 100 minutes or more, and more preferably 150 minutes or more. On the other hand, the upper limit of the first predetermined time in the above-described condition 1 is not particularly limited, but it is preferably set to 20 hours or less in consideration of factors such as production efficiency. The lower limit of the second specified temperature in the above condition 1 is preferably 700 °C.

The lower limit of the second specified temperature in the above condition 2 is preferably 950 °C. The upper limit of the second specified temperature in the above condition 2 is preferably 1200 ° C, more preferably 1150 ° C. The second specified time in the aforementioned condition 2 is as long as possible. For example, it is preferably 80 minutes or more, more preferably 100 minutes or more, and more preferably 150 minutes or more. On the other hand, the upper limit of the second predetermined time in the above-described condition 2 is not particularly limited, but it is preferably set to 20 hours or less in consideration of factors such as production efficiency. The upper limit of the first specified temperature in the above condition 2 is preferably 1300 °C.

The first predetermined temperature and the lower limit of the second specified temperature in the above condition 3 are preferably 950 ° C. The first predetermined temperature and the upper limit of the second specified temperature in the above condition 3 are preferably 1200 ° C and more preferably 1150 ° C. The total value of the first designated time and the second designated time in the above condition 3 is as long as possible. For example, the total of the first designated time and the second designated time is preferably 80 minutes or more, more preferably 100 minutes or more, and more preferably 150 minutes or more. On the other hand, the upper limit of the total of the first designated time and the second designated time in the above-described condition 3 is not particularly limited, but it is preferably set to 20 hours or less in consideration of factors such as manufacturing efficiency.

In the above-described conditions 1 to 3, each of the first predetermined time and the second specified time is different from the first specified temperature and the second specified temperature in the first soaking step and the second soaking step. Therefore, it is preferable to set the preferred first designated time in response to the first specified temperature and the preferred second designated time in response to the second specified temperature.

In addition, the maintenance in the first soaking process means: maintaining the steel material The first specified temperature includes a case where the temperature of the steel material changes to a high temperature or a low temperature with respect to the target first predetermined temperature due to the restriction of the equipment. If the specified temperature range has been specified as the first specified temperature, it means that the steel material is maintained within the established temperature range. For example, if the first specified temperature is 900 to 1240 ° C, the steel material may be maintained in the range of 900 to 1240 ° C, but it may be maintained at a specific temperature in the range of 900 to 1240 ° C (for example, 1200 ° C). ).

In addition, the holding in the second soaking step means that the rolling material is maintained at the second predetermined temperature, which is caused by the restriction of the equipment: the second specified temperature of the target, the steel material The temperature changes to a high temperature or a low temperature. If a predetermined temperature range has been specified as the second specified temperature, it means that the rolling material is maintained within the predetermined temperature range. For example, if the second specified temperature is 900 to 1240 ° C, the rolling material may be maintained in the range of 900 to 1240 ° C, but it may be maintained at a specific temperature in the range of 900 to 1240 ° C (for example, 1000 ° C). ).

Further, the steel material which has been held for the first predetermined time at the first specified temperature in the first soaking step is subjected to block rolling or block forging according to a generally common method. In addition, in the second soaking step, the rolling material after the second predetermined time is held at the second specified temperature is hot-rolled according to a generally common method, and then subjected to spheroidizing annealing, and then subjected to hot working. Or cold room processing. In this way, the steel material for bearings of the present invention can be obtained. In addition, the rolling material after the block rolling or the block forging is implemented. It can be cooled before being heated, or it can be heated without cooling.

Bearing Parts

The bearing material of the present invention obtained by such a process is subjected to cutting into a predetermined component shape, and then subjected to hardening and tempering treatment to obtain another aspect of the bearing component of the present invention. The shape at the stage of manufacturing the steel material includes a linear shape and a rod shape which can be applied to the above-described production method. The size of the intermediate product in the stage of manufacturing the steel material is appropriately selected in accordance with the size of the final product.

Examples of the bearing component include a rolling element such as a roller, a needle roller, and a ball; and a track wheel such as an outer ring or an inner ring.

As described above, the steel material for bearings having excellent rolling fatigue characteristics of the present invention contains, by mass%, C: 0.8% or more and 1.1% or less, Si: 0.15% or more and 0.8% or less, and Mn: 0.1% or more. 1.0% or less, Cr: 1.3% or more and 1.8% or less, P: more than 0% and 0.05% or less, S: more than 0% and 0.015% or less, Al: 0.0002% or more and 0.005% or less, and Ti: 0.0005% or more and 0.010 % or less, N: 0.0030% or more and 0.010% or less, O: more than 0% and 0.0030% or less, and the rest are iron and unavoidable impurities for the bearing steel, and the short diameter of the steel material is 1 μm or more. The composition of the inclusions of the system is, in mass%, Al ₂ O ₃ : 5% or more and 50% or less, SiO ₂ : 10% or more and 70% or less, and TiO ₂ : 3% or more and 50% or less, and the rest is inevitable oxides, the Al ₂ O _3, SiO _2, and the percentage of the total mass of TiO ₂ is more than 60% of the mass of TiO ₂ to the total mass of the Al ₂ O ₃ and SiO ₂ in the ratio of 0.10 Above 1.50 or less, among the oxide-based inclusions, TiN exists at the interface with the parent phase of the steel material The number-based composite inclusions account for more than 30% of the percentage of the total number of oxide-based inclusions.

According to this configuration, the composition of the steel component and the oxide-based inclusions contained in the steel is appropriately controlled, and therefore, excellent rolling fatigue characteristics can be exhibited. In particular, TiN existing at the interface between the mother phase and the oxide-based inclusions allows the two to be in close contact with each other, so that the early peeling phenomenon due to rolling fatigue can be suppressed.

Therefore, the steel material for bearings of the present invention can be suitably used as a material for a bearing component in which a load in a thrust direction is repeatedly applied, such as an outer ring or an inner ring. Further, the TiN adhesion force existing at the interface between the mother phase and the oxide-based inclusions is not different depending on the thrust direction or the radial direction, and therefore, the steel material for bearings of the present invention can also be suitably used as: Columns, needles, balls, etc. are mainly repetitively applied to the material of the radial load bearing parts.

In the steel material for bearings of the present invention, the content of CaO in the unavoidable oxide contained in the oxide-based inclusions is preferably more than 0% and less than 10% by mass. According to this configuration, TiN can be formed more at the interface between the oxide-based inclusions and the matrix phase, and excellent rolling fatigue characteristics can be obtained.

Another aspect of the present invention is a bearing steel excellent in rolling fatigue characteristics The method for producing a material includes: a melting step of obtaining a steel material composed of the steel component by performing a Si deoxidation treatment, a first soaking step, a blocking step, a second soaking step, and a hot rolling step; The holding temperature in the soaking step is 1240 ° C or lower. In the first soaking step and the second soaking step, the holding time at a temperature of 900 to 1240 ° C is 60 minutes or more in total.

More specifically, the method for producing a steel material for a bearing having excellent rolling fatigue characteristics includes a process of melting a steel material composed of the steel component by performing a Si deoxidation treatment on the molten steel and performing casting. a first soaking step of holding the steel material at a first specified temperature; and the step of rolling the steel material held at the first predetermined temperature in the first soaking step a block forming step of obtaining a rolled material by block forging; a second soaking step of holding the rolled material at a second specified temperature; and maintaining the second specified temperature in the second soaking step In the hot rolling step of hot rolling, the second predetermined temperature is 1240 ° C or lower, and at least one of the steel material and the rolling material is in the first soaking step and the second soaking step. The holding time at a temperature of 900 to 1240 ° C is 60 minutes or more in total.

According to this configuration, the steel material for bearings of the present invention can be suitably produced.

In the method for producing a steel material for a bearing of the present invention, for example, (1) the holding temperature (first specified temperature) in the first soaking step may be set to 900 to 1240 ° C, and the second soaking step may be performed. Keep the temperature (2nd) The predetermined temperature is set to be lower than 900 ° C, and the holding time in the first soaking step is set to 60 minutes or longer; (2) the holding temperature in the first soaking step (first specified temperature) may be set. The temperature is set to be higher than 1240 ° C, the holding temperature (second specified temperature) in the second soaking step is set to 900 to 1240 ° C, and the holding time in the second soaking step is set to 60 minutes or longer; 3) The holding temperature (first specified temperature) in the first soaking step and the holding temperature (second specified temperature) in the second soaking step may be set to 900 to 1240 ° C, and The total of the holding time in the first soaking step and the holding time in the second soaking step is set to 60 minutes or longer.

Another aspect of the present invention is a bearing component composed of the aforementioned steel material for bearings. According to this configuration, since the material for the bearing component is the steel material for the bearing of the present invention, the rolling fatigue characteristics of the bearing component can be stably improved regardless of the direction in which the load is applied.

According to the present invention, it is possible to provide a steel material for a bearing excellent in rolling fatigue characteristics, a method for producing the same, and a bearing component excellent in rolling fatigue characteristics.

[Examples]

Hereinafter, the present invention will be described more specifically by way of examples. In addition, the present invention is not limited by the following embodiments, and may be modified within the scope of the invention and the scope of the invention described below, and these modifications are also included in the technical scope of the present invention.

<1> Making cast pieces

A small-sized melting furnace having a capacity of 170 kg was used to melt the steel of the steel composition shown in Table 1 below (the balance being iron and unavoidable impurities). A cast piece of the steel materials No. 1 to 47 shown in Table 1 was produced from the molten steel. The size of each slab, the diameter of the upper portion is 245 mm, the diameter of the lower portion is 210 mm, and the height (length) is 480 mm.

In the above-described melting, a pouring bucket of MgO-based refractories is used. For steels of steel materials No. 1 to 46, Si is deoxidized during melting, and C, Si, Mn, and Cr are used to adjust the dissolved oxygen content in the molten steel, and then a Ti-containing source is introduced to control the Ti content. The aforementioned Ti-containing source is an Fe-Ti alloy. The N content is adjusted by controlling the partial pressure of nitrogen in the gas phase atmosphere, and adding manganese nitride to adjust the N content before adding the Ti-containing source. On the other hand, for the steel of the steel material No. 47, when the melting is performed, the deoxidation treatment is performed by adding Al.

<2>Making hot rolled products

The slabs of the steel materials No. 1 to 47 were subjected to block rolling and hot rolling to obtain a hot rolled material (round bar steel) having a diameter of 65 mm. The slab is heated to a pre-blocking furnace holding temperature as shown in Table 2 below before the slab rolling (the holding temperature in the first soaking step, that is, the first specified temperature) At this temperature, the pre-blocking furnace holding time as shown in Table 2 (the holding time in the first soaking step, that is, the first predetermined time) is maintained at 900 to 1200 ° C. At a temperature, the rolling was carried out in blocks, and then cooled to room temperature. Then, the slabs that have been subjected to the block rolling are first heated to reach the table shown in Table 2. The preheating preheating furnace maintains the temperature (the holding temperature in the second soaking step, that is, the second specified temperature), and maintains the holding time of the preheating furnace as shown in Table 2 at this temperature. (The holding time in the second soaking step, that is, the second predetermined time) is hot rolling at a temperature of 830 to 1100 °C.

<3> Preparation of test piece for measurement of composition of oxide-based inclusions and measurement of composition

The hot-rolled material of steel No. 1 to 47 is heated at 760 to 800 ° C for 2 to 8 hours, and then cooled at an average cooling rate of 10 to 15 ° C / hour (Ar ₁ metamorphic point - 60 After the temperature of °C), it is allowed to cool in the atmosphere (spheroidal annealing). In this way, a spheroidizing annealed material in which the spheroidized stellite is dispersed can be obtained. A cylindrical test piece having a diameter (D) of 60 mm and a height (length of the hot-rolled material in the rolling direction) of 30 mm was cut out from this spheroidized annealed material. The cut test piece was heated at a temperature of 840 ° C for 30 minutes, and then oil-hardened and hardened, followed by tempering at a temperature of 160 ° C for 120 minutes to prepare an oxide-based inclusion. The test piece for the measurement.

A micro sample was cut from the test piece for measuring the composition of the steel materials No. 1 to 47 produced in this manner, and was cut from a position 1/4 of the diameter D (60 mm) of the test piece. The size of the micro sample in the section including the rolling direction was 20 mm in the rolling direction and 20 mm in the direction perpendicular to the rolling direction. Further, the aforementioned cross section of the micro sample was polished.

For the polishing surface of the micro-samples of the steel materials No. 1 to 47, an electron micro-probe X-ray analysis apparatus (Electron Probe X-ray Micro Analyzer: EPMA, trade name "JXA-8500F" manufactured by JEOL Ltd. was used. The observation was carried out, and the composition of the oxide-based inclusions having a short diameter of 1 μm or more was quantitatively analyzed. The details are as follows. The observation area of the polished surface of the micro sample was set to 100 mm ² , and the composition of the central portion of the inclusion was quantitatively analyzed by wavelength dispersion of characteristic X-rays. The element to be analyzed is set to Ca, Al, Si, Ti, Ce, La, Mg, Mn, Zr, Na, K, Cr, O (oxygen), and the X of each element is obtained in advance by using a known substance. a calibration curve for the relationship between the ray intensity and the element concentration, and then quantizing the amount of the element contained in each inclusion from the X-ray intensity obtained from the inclusion as the analysis target and the calibration curve. As a result, arithmetical averaging is performed to determine the composition of the inclusions. Among the inclusions which were quantitatively analyzed in this manner, inclusions having an oxygen (O) content of 5% by mass or more were regarded as oxide-based inclusions. In this case, if a plurality of elements are observed from a single oxide-based inclusion, the ratio of the X-ray intensities indicating the presence of these elements is converted into individual oxides of the respective elements, and then the oxide is calculated. Group ingredients. In the present invention, the average value after mass conversion as the above individual oxide is regarded as the composition of the oxide. The results of the foregoing quantitative analysis are shown in Table 3. The compositional component shown in Table 3 is a composition component of an oxide-based inclusion having a short diameter of 1 μm or more (the balance being an unavoidable oxide). Ratio of the mass percentage (Al ₂ O ₃ + SiO ₂ + TiO ₂ ) to the total mass of Al ₂ O ₃ , SiO ₂ and TiO ₂ , and the mass of TiO ₂ of the total mass of Al ₂ O ₃ and SiO ₂ (TiO _{2 )} /(Al ₂ O ₃ +SiO ₂ )) is also shown in Table 3.

<4> Determination of the aspect ratio of oxide-based inclusions

The test piece for measuring the composition of the oxide-based inclusions of the steel materials No. 1 to 47 was used, and 100 arbitrary oxide-based inclusions having a short diameter of 1 μm or more were selected (the analysis target elements were Ca, Al, Si, Ti, and the like). Ce, La, Mg, Mn, Zr, Na, K, Cr, O (oxygen)), the long diameter and the short diameter of each oxide-based inclusion were measured, and the aspect ratio of each oxide-based inclusion was calculated (=long Trail / short diameter). The results were arithmetically averaged to obtain an average aspect ratio of the oxide-based inclusions. The obtained aspect ratio is shown in Table 3.

<5> Determination of the ratio of the number of TiN

First, for the observation area of the test surface of the oxide-based inclusions of the steel materials No. 1 to 47, the observation surface of the test piece of 100 mm ² was selected, and five short lines were selected using an electron beam microprobe X-ray analyzer. Oxide-based inclusions having a diameter of 1 μm or more (the elements to be analyzed are Ca, Al, Si, Ti, Ce, La, Mg, Mn, Zr, Na, K, Cr, O (oxygen), and the oxygen content is 5 mass. More than % of inclusions). The selection criteria of the five oxide-based inclusions were selected from the oxide-based inclusions existing in the observation area of 100 mm ² , and five of them were sequentially selected from the largest size. The reason why the size of the oxide-based inclusions is the largest is that the larger the size of the oxide-based inclusions, the greater the degree of adverse effect on the rolling fatigue characteristics. Further, the size of the oxide-based inclusions is compared with the value of the "long diameter x short diameter" of the oxide-based inclusions appearing in the observation surface.

Next, the oxide-based inclusions to be selected are used, and the oxide-based inclusions are thinned by a FIB method (Focused Ion Beam) to obtain a transmission electron microscope (TEM). The thickness of the observation. The apparatus used for the thinning of the oxide-based inclusions was a concentrated ion beam processing observation apparatus FB2000A manufactured by Hitachi, Ltd., and the acceleration voltage was 30 kV, and Ga metal was used as the ion emission source. Then, the exfoliated oxide-based inclusions were observed by a transmission electron microscope (TEM). The device used for the TEM observation was an electric field release type transmission electron microscope JEM-2010F manufactured by JEOL Ltd., and was used for an oxide system inclusion using an EDX (Energy Dispersive X-ray spectrometry) analysis device Vantage manufactured by Noran Co., Ltd. The interface between the object and the parent phase was subjected to EDX analysis. The analysis target element is set to: Ca, Al, Si, Ti, Ce, La, Mg, Mn, Zr, Na, K, Cr, and a phase having a Ti concentration of 30% or more is selected, and the phase is subjected to electron wire winding The identification of the shot is analyzed, and the crystal structure having a cubic crystal is judged as TiN. At this time, if TiN is present at the interface between the oxide-based inclusions and the parent phase (in other words, by the above method for judging TiN, it is confirmed that TiN is present), it is judged that there is TiN is present in the oxide-based inclusions at the interface between the oxide-based inclusions and the parent phase. Therefore, it is known that the oxides present in the above-mentioned TiN are present among the five oxide-based inclusions measured. The ratio of the number of inclusions. And the ratio (that is, the ratio of the number of TiN) is shown in Table 3.

<6>Comparison of the production of rolling rolling fatigue test piece and rolling fatigue characteristics

A test piece having a diameter of 60 mm and a thickness of 6 mm was cut out from the spheroidizing annealed material obtained in the above <3>. The cut test piece was heated at 840 ° C for 30 minutes, and then oil-hardened and then tempered at 120 ° C for 120 minutes. The test piece after the tempering treatment was subjected to mirror polishing to prepare a thrust rolling fatigue test piece having a surface roughness Ra of 0.04 μm or less. For the thrust rolling fatigue test piece of the steel materials No. 1 to 47 obtained in this manner, a thrust fatigue tester (a thrust type rolling fatigue tester "FJ-5T", manufactured by Fuji Test Machine Co., Ltd.) was used at a load speed. The thrust rolling fatigue test was carried out at 1200 rpm with three steel balls, a surface pressure of 5.24 GPa, and a stoppage of 200 million times.

As a measure of the rolling fatigue life, it is usually adopted: fatigue life L ₁₀ (the cumulative damage probability is 10%, the number of stress repetitions before the fatigue damage is reached, and sometimes referred to as "L ₁₀ life"). In detail, L ₁₀ means: the number of times the cumulative damage probability obtained by marking the test result on the Weibull probability paper is 10% before the fatigue damage is reached (please refer to the "Bearing" book, issued by Iwanami Bookstore) , Zeng Tian Fan Zong). For each steel material of the steel materials No. 1 to 47, the above-described thrust rolling fatigue test was performed using 16 samples to determine the L ₁₀ life. Next, the L ₁₀ life of each steel material of the steel materials No. 1 to 46 was determined with respect to the L ₁₀ life (1.2 × 10 ⁷ times) of the steel No. 47 of the conventional steel which was subjected to deoxidation treatment at the time of melting. The life ratio is based on the following criteria.

Not (low rolling fatigue life): L ₁₀ life is less than 5.4 × 10 ⁷ times (life ratio is less than 4.5 times)

(Excellent rolling fatigue life): L ₁₀ life is 5.4 × 10 ⁷ or more and less than 6.0 × 10 ⁷ times (life ratio is 4.5 times or more and less than 5.0 times)

Good (external rolling fatigue life): L ₁₀ life is 6.0 × 10 ⁷ or more and less than 6.5 × 10 ⁷ times (life ratio is 5.0 times or more and less than 5.4 times)

Excellent (rolling fatigue life special): L ₁₀ life is 6.5 × 10 ⁷ times or more (life ratio is 5.4 times or more)

Among the above criteria, "not available" means disqualification, and "may", "good" and "excellent" are qualified.

In addition, the life ratio (4.5 times or more) of the lowest level in the above-mentioned acceptance criteria is the test No. 11 and the test No. of Table 2 which exceed the maximum life ratio which can be obtained in the example of Patent Document 4. The life ratio of 35 (lifetime ratio is 3.8 times) is the qualification standard in the present embodiment, which is set to a higher qualification standard than Patent Document 4.

The L ₁₀ life, life ratio, and eligibility of the steel materials No. 1 to 47 are shown in Table 4. The "E+07" in Table 4 means: "×10 ⁷ ".

The steel materials No. 1 to 26 are examples conforming to the requirements (a) to (c) defined in the present invention, and all exhibit excellent rolling fatigue characteristics. Steel Nos. 1 to 26, the aspect ratio of oxide-based inclusions is also appropriately controlled.

Further, in the present embodiment, although the roll in the thrust direction is measured Dynamic fatigue characteristics, but since the aspect ratio of the oxide-based inclusions in the present embodiment is small, it can be estimated that the radial rolling fatigue life is also good.

On the other hand, the steel materials No. 27 to 47 are examples which do not satisfy at least one of the requirements (a) to (c) specified in the present invention, and do not exhibit the desired rolling fatigue characteristics.

Specifically, the steel material No. 27 is an example in which the C content in the steel is too large. Steel No. 28 is an example in which the Mn content in steel is too large. Steel No. 29 is an example of too much Cr in steel. Steel No. 30 is an example in which the Cr content in steel is too small. Steel No. 31 is an example of too much P in steel. Steel No. 32 is an example of too much S in steel. Steel No. 37 is an example of too much N in steel. Steel No. 39 is an example of too much O in steel. These L ₁₀ lifetimes and life ratios are all below the reference value and, therefore, do not exhibit the desired rolling fatigue characteristics.

Steel No. 33 is an example in which the Al content in the steel is too large. Further, in the steel material No. 33, the content of Al ₂ O ₃ in the oxide-based inclusion having a short diameter of 1 μm or more is too large, and in the oxide-based inclusion, the mass of TiO ₂ is relative to Al ₂ O ₃ and SiO. The ratio of the total mass of ₂ is too small, and the ratio of the number of TiN is too small. The L ₁₀ life and life ratio of the steel No. 33 were both lower than the reference value, and therefore, the desired rolling fatigue characteristics were not exhibited.

Steel No. 34 is an example in which the Al content in the steel is too small. Further, in the steel material No. 34, the content of Al ₂ O ₃ in the oxide-based inclusions having a short diameter of 1 μm or more is too small, and in the oxide-based inclusions, the mass of TiO ₂ is relative to Al ₂ O ₃ and SiO. The ratio of the total mass of ₂ is too large, and the ratio of the number of TiN is too small. The L ₁₀ life and life ratio of the steel No. 34 were both lower than the reference value, and therefore, the desired rolling fatigue characteristics were not exhibited.

Steel No. 35 is an example of too much Ti in steel. Further, in the steel material No. 35, the oxide-based inclusions having a short diameter of 1 μm or more have too much TiO ₂ content and too little SiO ₂ content, and in the oxide-based inclusions, the mass of TiO ₂ is relative to Al ₂ O. ₃₎ An example in which the ratio of the total mass of SiO ₂ is too large and the ratio of the number of TiN is too small. The L ₁₀ life and life ratio of the steel No. 35 were both lower than the reference value, and therefore, the desired rolling fatigue characteristics were not exhibited.

Steel No. 36 is an example in which the Al content and the Ti content in the steel are too small. Further, in the steel material No. 36, the Al ₂ O ₃ content in the oxide-based inclusions having a short diameter of 1 μm or more and the TiO ₂ content are too small and the SiO ₂ content is too large, and the quality of the TiO ₂ in the oxide-based inclusions. The ratio of the total mass of Al ₂ O ₃ and SiO ₂ is too small, and there is no example in the presence of TiN at the interface between the oxide-based inclusion and the parent phase (that is, the ratio of the number of TiN is too small). The L ₁₀ life and life ratio of the steel No. 36 were both lower than the reference value, and therefore, the desired rolling fatigue characteristics were not exhibited.

Steel No. 38 is an example in which the N content in the steel is too small and the ratio of the number of TiN is too small. The L ₁₀ life and life ratio of the steel No. 38 were lower than the reference value, and therefore, the desired rolling fatigue characteristics were not exhibited.

Steel No. 40 is an example in which the total mass of the oxide-based inclusions having a short diameter of 1 μm or more and the total mass of Al ₂ O ₃ , SiO _{2 ,} and TiO ₂ is too small and the ratio of the number of TiN is too small. Since the L _l0 life and life ratio of the steel material No. 40 are lower than the reference value, the desired rolling fatigue characteristics are not exhibited.

Steel No. 41 is an example in which the ratio of the mass of TiO ₂ to the total mass of Al ₂ O ₃ and SiO ₂ is too large and the ratio of the number of TiN is too small in an oxide-based inclusion having a short diameter of 1 μm or more. . The L ₁₀ life and life ratio of the steel No. 41 were lower than the reference value, and therefore, the desired rolling fatigue characteristics were not exhibited.

Steel No. 42 is an example in which the ratio of the mass of TiO ₂ to the total mass of Al ₂ O ₃ and SiO ₂ is too small and the ratio of the number of TiN is too small in an oxide-based inclusion having a short diameter of 1 μm or more. . The L ₁₀ life and life ratio of the steel No. 42 were lower than the reference value, and therefore, the desired rolling fatigue characteristics were not exhibited.

Steel No. 43 is an example in which the furnace holding temperature before the block rolling and the holding temperature before the hot rolling are too high, and the ratio of the number of TiN is too small. The L ₁₀ life and life ratio of the steel material No. 43 were lower than the reference value, and therefore, the desired rolling fatigue characteristics were not exhibited.

The steel materials No. 44 and 46 are examples in which the heating furnace before the block rolling keeps the temperature too high, and the holding time before the hot rolling is too short, and the ratio of the number of TiN is too small. Since the L ₁₀ life and life ratio of the steel materials No. 44 and 46 are lower than the reference value, the desired rolling fatigue characteristics are not exhibited.

The steel No. 45 is an example in which the heating furnace before the block rolling keeps the temperature too high, and the furnace before the hot rolling keeps the temperature too low and the ratio of the number of TiN is too small. The L ₁₀ life and life ratio of the steel material No. 45 is lower than the reference value, and therefore, the desired rolling fatigue characteristics are not exhibited.

Steel No. 47 is an example in which deoxidation treatment is performed by adding Al at the time of melting. In the oxide-based inclusions having a short diameter of 1 μm or more, the presence of SiO ₂ and TiO ₂ is not present. Steel No. 47 is a reference value for the L ₁₀ life and life ratio, and therefore does not exhibit the desired rolling fatigue characteristics.

The basic application of the present application is Japanese Patent Application No. 2016-043159, filed on March 7, 2016, the content of which is hereby incorporated by reference.

The present invention has been described with respect to the embodiments as described above, and the present invention may be appropriately modified and/or improved as long as it is a person skilled in the art. Therefore, if the modified form or the modified form implemented by those skilled in the art does not exceed the scope of the claims of the claims described in the patent application scope, the modified form or the modified form should be interpreted as being Included in the scope of the claim.

Claims (7)

A steel material for bearings having excellent rolling fatigue characteristics, comprising C: 0.8% or more and 1.1% or less, Si: 0.15% or more and 0.8% or less, Mn: 0.1% or more and 1.0% or less, and Cr: 1.3. % or more and 1.8% or less, P: more than 0% and 0.05% or less, S: more than 0% and 0.015% or less, Al: 0.0002% or more and 0.005% or less, Ti: 0.0005% or more and 0.010% or less, and N: 0.0030% The above-mentioned 0.010% or less, O: more than 0% and 0.0030% or less, and the remaining part is a steel material for bearings composed of steel of iron and unavoidable impurities, and an oxide system having a short diameter of 1 μm or more in the steel material The inclusions contain, by mass%, Al ₂ O ₃ : 5% or more and 50% or less, SiO ₂ : 10% or more and 70% or less, TiO ₂ : 3% or more and 50% or less, and the balance is an unavoidable oxide. the Al ₂ O _3, the SiO ₂ and the total mass percentage of TiO ₂ of not less than 60% of the mass of TiO ₂ ratio of the total mass of the Al ₂ O ₃ and the SiO ₂ with respect to 0.10 or more 1.50 or less, the Among the oxide-based inclusions, there is a composite inclusion of TiN present at the interface with the parent phase of the steel material. The number of the total number of the oxide-based inclusions is 30% or more. The steel material for a bearing having excellent rolling fatigue characteristics according to the above-mentioned claim 1, wherein the unavoidable oxide of the oxide-based inclusion contains CaO: more than 0% and less than 10% by mass%. . A method for producing a steel material for a bearing, which is used for producing a steel material for a bearing excellent in rolling fatigue characteristics according to claim 1 or 2 The method includes a melting step of obtaining a steel material composed of the steel component by performing a Si deoxidation treatment, a first soaking step, a blocking step, a second soaking step, and a hot rolling step, and the second soaking step The holding temperature at the time is 1240 ° C or lower; in the first soaking step and the second soaking step, the holding time at a temperature of 900 to 1240 ° C is 60 minutes or more in total. The method for producing a steel material for a bearing according to claim 3, wherein the holding temperature in the first soaking step is 900 to 1240 ° C; and the holding temperature in the second soaking step is lower than 900 ° C; The holding time in the soaking step is 60 minutes or longer. The method for producing a steel material for a bearing according to claim 3, wherein the holding temperature in the first soaking step is higher than 1240 ° C; and the holding temperature in the second soaking step is 900 to 1240 ° C; 2 The holding time in the soaking step is 60 minutes or longer. The method for producing a steel material for a bearing according to claim 3, wherein the holding temperature in the first soaking step and the holding temperature in the second soaking step are 900 to 1240 ° C; and the first soaking step The total of the holding time and the holding time in the second soaking step is 60 minutes or longer. A bearing component according to the bearing of claim 1 or 2 Made of steel.