JP2018197371A - Bearing steel and bearing component - Google Patents

Abstract

The present invention provides a bearing steel that can provide a bearing component that has both a retained austenite amount and strength and is excellent in rolling fatigue life. The bearing steel according to the present invention is, in mass%, C: 0.40 to 1.00%, Si: 0.75 to 3.00%, Mn: 0.30 to 2.00%, P : 0.015% or less, S: 0.015% or less, Cr: 0.10 to 1.60%, V: more than 0.10% to 1.00%, Al: 0.010 to 0.500%, And N: 0.015% or less, the balance being Fe and impurities, Ms defined by formula (1) is 100 to 220, and Nf defined by formula (2) is 0.5 or less Have a chemical composition of Ms = 539-423C-30Mn-11Si-12Cr-7Mo-18Ni-18Cu (1) Nf = (N / 14) / (Al / 27 + Ti / 46 + Nb / 93) (2) Here, the equations (1) and ( The element content (mass%) of the corresponding element is substituted for the element symbol in 2). [Selection figure] None

Description

  The present invention relates to a bearing steel and a bearing component manufactured using the bearing steel.

  Bearing parts are manufactured by subjecting bearing steel, which is a material, to hot working or cold working to form a part shape, and heat treatment such as quenching and tempering. For example, a rolling bearing part is manufactured as follows in order to achieve a long life. A high-chromium bearing steel SUJ2 or alloy steel SCM420 specified in the JIS standard is subjected to quenching or carburizing to produce an intermediate product having a high carbon martensite structure as a surface layer. Rolling bearing parts are manufactured by tempering intermediate products.

  When quenching a high carbon steel, the martensitic transformation may not be completed, and untransformed retained austenite may remain in part. In recent years, the knowledge that this retained austenite contributes to the improvement of the rolling fatigue life of bearing parts has been obtained.

  Techniques for improving rolling fatigue life by utilizing retained austenite are disclosed in JP-A-7-278752 (Patent Document 1), JP-A-2007-231332 (Patent Document 2), and JP-A-2007-100126 ( Patent Document 3) and Japanese Patent Application Laid-Open No. 2015-030899 (Patent Document 4).

  The bearing member disclosed in Patent Document 1 contains C: 0.5 to 1.5%, V: 0.05 to 1.0%, and O: 0.0020% or less by weight%, with the balance remaining. It has a component composition consisting of Fe and inevitable impurities. In the steel structure of the bearing member, the amount of retained austenite in the steel is 10 to 35% in volume ratio. Patent Document 1 describes that this bearing member is excellent in the delay characteristic of microstructure change due to repeated stress loading.

  The steel member disclosed in Patent Document 2 is mass%, C: 0.7 to 1.1%, Si: 0.5 to 2.0%, Mn: 0.4 to 2.5%, Cr: It contains 1.6 to 5.0%, Mo: less than 0.1 to 0.5%, and Al: 0.010 to 0.050%, and suppresses the mixing of Sb as an impurity to less than 0.0015%. The balance has a component composition of Fe and inevitable impurities. The steel member is further subjected to quenching and tempering. The steel structure of the steel member has a residual cementite particle size of 0.05 to 1.5 μm, a prior austenite particle size of 30 μm or less, and a residual austenite amount of 25% by volume in a portion from the surface to a depth of 5 mm. Is less than. Patent Document 2 describes that this steel member is excellent in rolling fatigue.

  The rolling member disclosed in Patent Document 3 is mass%, C: 0.6 to 1.2%, Si: 0.15 to 1%, Mn: 0.3 to 1.5%, Cr: 0 0.1 to 2%, V: 0.1 to 2%. In the rolling member, vanadium carbide having a particle size of 50 to 300 nm is further dispersed in the steel, and the average particle size of the prior austenite crystal grains is 12 μm or less. A nitrogen-enriched layer is formed on the surface layer portion of the rolling member. The amount of retained austenite in the surface layer portion of the rolling member is 20 to 40% by volume. The rolling surface of the rolling member has a hardness of 59 HRC or higher. Patent Document 3 describes that this rolling member has excellent rolling fatigue life.

  The steel for bearings disclosed in Patent Document 4 is mass%, C: 0.4 to 1.0%, Si: 0.75 to 3.0%, Mn: 0.55 to 3.0%, Al : 0.005 to 0.50%, P: 0.015% or less, S: 0.015% or less, the remainder has a chemical composition consisting of Fe and inevitable impurities. The bearing steel further has a martensitic transformation start temperature Ms (= 539-423 [C%]-30 [Mn%]-11 [Si%]) of 100 to 220 ° C. Patent Document 4 describes that this steel for bearings is used as a material for bearings and can extend the life of the bearings.

Japanese Patent Laid-Open No. 7-278752 JP 2007-231332 A JP 2007-100126 A Japanese Patent Laying-Open No. 2015-030899

  However, the bearing member disclosed in Patent Document 1 contains a large amount of V, Mo, and Ni elements. Therefore, the raw material cost is expensive. The steel member disclosed in Patent Document 2 has a complicated and long heat treatment process. The rolling member disclosed in Patent Document 3 performs secondary quenching after performing carbonitriding. As described above, the techniques disclosed in Patent Document 2 and Patent Document 3 have a complicated heat treatment process and low productivity. In the steel for a bearing of patent document 4, a rolling fatigue life may be insufficient.

  The objective of this invention is providing the steel for bearings from which the bearing components excellent in intensity | strength and rolling fatigue life are obtained.

The steel for bearing according to the present invention is mass%, C: 0.40 to 1.00%, Si: 0.75 to 3.00%, Mn: 0.30 to 2.00%, P: 0.015. %: S: 0.015% or less, Cr: 0.10 to 1.60%, V: more than 0.10% to 1.00%, Al: 0.010 to 0.500%, N: 0.00. 015% or less, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 3.00%, Ti: 0 to 0.100%, and Nb: 0 to 0.100% And the balance is Fe and impurities, Ms defined by the formula (1) is 100 to 220, and Nf defined by the formula (2) is 0.5 or less.
Ms = 539-423C-30Mn-11Si-12Cr-7Mo-18Ni-18Cu (1)
Nf = (N / 14) / (Al / 27 + Ti / 46 + Nb / 93) (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2).

The bearing component according to the present invention has the above chemical composition. The Vickers hardness of the surface of the bearing component is 670 Hv or more. The structure of the bearing component contains 5-40% residual austenite in volume fraction, and the phase having the maximum volume fraction among the remaining phases is tempered martensite. In the steel of the bearing component, the number density of residual austenite grains having a circle-equivalent diameter of 0.2 to 2.0 μm is 10/100 μm ² or more. Further, the number density of carbides having a major axis of 150 to 300 nm in the steel of the bearing component is 50 pieces / 100 μm ² or more.

  With the bearing steel according to the present invention, a bearing component having excellent strength and rolling fatigue life can be obtained.

  Hereinafter, embodiments of the present invention will be described in detail.

  The present inventors investigated and examined the amount of retained austenite for increasing the rolling fatigue life of the bearing parts and the strength of the bearing parts, and obtained the following knowledge.

(A) About retained austenite As described above, it is effective to increase the amount of retained austenite of a bearing component in order to increase the rolling fatigue life of the bearing component. However, if the amount of retained austenite is too high, the strength of the steel decreases. If the amount of retained austenite is too high, the retained austenite grains are further coarsened. In the environment used as a bearing, coarse retained austenite is easily transformed into martensite by stress. In this case, the rolling fatigue life of the bearing component is reduced.

  Therefore, the present inventors examined the amount and particle size of retained austenite. As a result, the following knowledge was obtained.

(A) The amount of retained austenite after quenching can be controlled by adjusting the chemical composition of the bearing steel and adjusting Ms defined by the formula (1).
Ms = 539-423C-30Mn-11Si-12Cr-7Mo-18Ni-18Cu (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). If the corresponding element is not contained, “0” is assigned to the element symbol.

  Ms represents the martensitic transformation start temperature (° C.). If Ms is less than 100 ° C, the start of martensitic transformation during quenching is too slow. Therefore, the amount of retained austenite after quenching is too large. On the other hand, if Ms exceeds 220 ° C., martensitic transformation at the time of quenching starts too early. Therefore, the amount of retained austenite after quenching is too small. If Ms is 100 to 220 ° C, an appropriate amount of retained austenite can be obtained after quenching.

  (B) The grain size of the retained austenite crystal grains (hereinafter also referred to as “residual austenite grains”) after tempering can be refined by tempering at a high temperature. However, when tempering is carried out at 200 ° C. or higher, residual austenite is transformed into tempered martensite. In this case, rolling fatigue characteristics are deteriorated.

  Therefore, the present inventors examined the tempering temperature and the amount of retained austenite after tempering. As a result, by increasing the silicon (Si) content, the reduction in retained austenite can be suppressed even if the tempering temperature is increased to 250 to 400 ° C., and further, fine retained austenite grains can be dispersed in the steel. The present inventors have found out. Regarding the mechanism by which fine retained austenite is generated by increasing the Si content, the present inventors have found that supersaturated carbon (C) in martensite diffuses into retained austenite, and the transformation start temperature to martensite locally. I think this is because of a decline.

  As a result of further investigation based on the above knowledge, when the Si content of the bearing steel is 0.75% or more by mass%, an appropriate amount of retained austenite and an appropriate amount of fine retained austenite grains can be obtained.

(B) Strength of bearing parts As described above, residual austenite is finely dispersed in steel by increasing the Si content and performing tempering at a high temperature of 250 to 400 ° C. However, retained austenite has lower strength than tempered martensite. Therefore, the strength of the bearing component is reduced, and the rolling fatigue strength of the bearing component is reduced.

  Accordingly, the present inventors have further investigated the enhancement of the strength of bearing components during high temperature tempering. Usually, high temperature tempering reduces the strength of the steel. As a result, rolling fatigue strength also decreases. However, the present inventors have found that the strength of steel may be increased by performing high temperature tempering under certain conditions. Specifically, under certain conditions, composite carbide of vanadium (V) and chromium (Cr) (hereinafter also simply referred to as “composite carbide”) is precipitated by high-temperature tempering, and the strength of the bearing component is increased. In the bearing steel of the present invention, a carbide having a major axis of 150 to 300 nm in the L cross section corresponds to a composite carbide.

  In order to precipitate the composite carbide by high temperature tempering, it is effective to increase the contents of V and Cr. The precipitation temperature of V carbide exceeds 400 ° C. That is, V carbides are not easily precipitated by tempering at 250 to 400 ° C. On the other hand, when Cr is contained together with V, composite carbide precipitates in the steel. The precipitation temperature of this composite carbide is about 250-400 degreeC. Therefore, even if the tempering temperature is 250 to 400 ° C., composite carbide that contributes to high strength is deposited. Therefore, in order to precipitate the composite carbide, the V content of the bearing steel is more than 0.10% to 1.60% by mass%, and the Cr content of the bearing steel is 0.10% by mass%. That's it.

  In order to increase the amount of precipitation of composite carbides in bearing parts, it is effective to increase the amount of solute V in the steel before tempering. Therefore, the present inventors have focused on the heat treatment in the quenching process and studied a method for increasing the amount of solute V. As a result, the present inventors obtained the following knowledge.

  (A) In order to increase the amount of solute V, it is effective to increase the quenching temperature. If the quenching temperature is low, V is difficult to dissolve sufficiently in the steel. As a result, the precipitation amount of the composite carbide of the bearing component is reduced.

  (B) In order to increase the amount of solute V, it is effective to reduce the amount of solute nitrogen (N) in the bearing steel. V has a higher affinity for N than C. Therefore, V tends to form nitrides and carbonitrides as compared with carbides. V nitride and V carbonitride are thermally stable and do not dissolve and remain after quenching and tempering. That is, when the amount of solute N in steel is large during quenching, the amount of solute V decreases. Therefore, the precipitation amount of composite carbide decreases.

  Therefore, the present inventors have studied a method for reducing the amount of solute N in bearing steel. As a result, it has been found that the amount of solute N in bearing steel can be reduced by adjusting the contents of aluminum (Al), titanium (Ti), and niobium (Nb), which are easier to form nitrides than V. It was. Specifically, the amount of solute N can be reduced by adjusting the chemical composition of the bearing steel and adjusting Nf defined by the following formula (2).

Nf = (N / 14) / (Al / 27 + Ti / 46 + Nb / 93) (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2). If the corresponding element is not contained, “0” is assigned to the element symbol.

  Nf is a material amount ratio of N to Al, Ti, and Nb that easily form nitrides. If Nf is 0.5 or less, a part of N in the steel is fixed as a nitride by Al, Ti, or Nb, so the amount of solute N in the bearing steel is reduced. As a result, the precipitation amount of the composite carbide of the bearing component increases, and the strength of the bearing component increases. On the other hand, if Nf exceeds 0.5, the solid solution N amount of the bearing steel is not sufficiently reduced. Therefore, V nitride and V carbonitride are generated. As a result, the precipitation amount of the composite carbide of the bearing part is reduced, and the strength of the bearing part is reduced. Therefore, Nf is 0.5 or less.

The steel for bearings according to the present invention completed based on the above knowledge is in mass%, C: 0.40 to 1.00%, Si: 0.75 to 3.00%, Mn: 0.30 to 2. 00%, P: 0.015% or less, S: 0.015% or less, Cr: 0.10 to 1.60%, V: more than 0.10% to 1.00%, Al: 0.010 to 0 500%, N: 0.015% or less, Mo: 0 to 1.00%, Cu: 0 to 1.00%, Ni: 0 to 3.00%, Ti: 0 to 0.100%, and Nb: 0 to 0.100% is contained, the balance is Fe and impurities, Ms defined by the formula (1) is 100 to 220, and Nf defined by the formula (2) is 0.5 or less Have a chemical composition of
Ms = 539-423C-30Mn-11Si-12Cr-7Mo-18Ni-18Cu (1)
Nf = (N / 14) / (Al / 27 + Ti / 46 + Nb / 93) (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2).

  The chemical composition is one or two selected from the group consisting of Mo: 0.10 to 1.00%, Cu: 0.10 to 1.00%, and Ni: 0.10 to 3.00% It may contain seeds or more.

  The said chemical composition may contain 1 or more types selected from the group which consists of Ti: 0.010-0.100% and Nb: 0.010-0.100%.

The bearing component according to the present invention has the above chemical composition. The Vickers hardness of the surface of the bearing component is 670 Hv or more. The structure of the bearing component contains 5-40% residual austenite in volume fraction, and the phase having the maximum volume fraction among the remaining phases is tempered martensite. In the steel of the bearing component, the number density of residual austenite grains having a circle-equivalent diameter of 0.2 to 2.0 μm is 10/100 μm ² or more. Further, the number density of carbides having a major axis of 150 to 300 nm in the steel of the bearing component is 50 pieces / 100 μm ² or more.

  Hereinafter, the steel for bearings and bearing parts of the present invention will be described in detail. “%” Regarding an element means mass% unless otherwise specified.

[Bearing steel]
[Chemical composition]
The chemical composition of the bearing steel according to the present invention contains the following elements:

C: 0.40 to 1.00%
Carbon (C) increases the strength of the steel. C further increases the amount of retained austenite after quenching. As a result, the rolling fatigue life of the bearing component is increased. If the C content is too low, these effects cannot be obtained. On the other hand, if the C content is too high, the amount of retained austenite becomes too high and the strength of the steel decreases. As a result, the rolling fatigue life of the bearing component is reduced. Therefore, the C content is 0.40 to 1.00%. The minimum with preferable C content is 0.50%, More preferably, it is 0.60%. The upper limit with preferable C content is 0.95%, More preferably, it is 0.90%.

Si: 0.75 to 3.00%
Silicon (Si) increases the strength of the steel. Si further suppresses the decrease in the amount of retained austenite during high-temperature tempering and refines the retained austenite grains. As a result, the rolling fatigue life of the bearing component is increased. Further, Si suppresses the production amount of steel cementite and increases the production amount of composite carbide. As a result, the strength of the steel is increased. If the Si content is too low, these effects cannot be obtained. On the other hand, if the Si content is too high, the steel becomes brittle. Therefore, the Si content is 0.75 to 3.00%. The minimum with preferable Si content is 1.00%, More preferably, it is 1.20%. The upper limit with preferable Si content is 2.50%, More preferably, it is 2.30%.

Mn: 0.30 to 2.00%
Manganese (Mn) increases the hardenability of the steel. Mn further increases the amount of retained austenite after quenching. As a result, the rolling fatigue life of the bearing component is increased. If the Mn content is too low, these effects cannot be obtained. On the other hand, if the Mn content is too high, the amount of retained austenite after quenching becomes too high and the strength of the steel decreases. As a result, the rolling fatigue life of the bearing component is reduced. Therefore, the Mn content is 0.30 to 2.00%. The minimum with preferable Mn content is 0.45%, More preferably, it is 0.65%. The upper limit with preferable Mn content is 1.50%, More preferably, it is 1.20%.

P: 0.015% or less Phosphorus (P) is an impurity. P embrittles the steel. Therefore, the P content is 0.015% or less. The upper limit with preferable P content is 0.012%, More preferably, it is 0.008%. The P content is preferably as low as possible.

S: 0.015% or less Sulfur (S) is an impurity. S embrittles the steel. Therefore, the S content is 0.015% or less. The upper limit with preferable S content is 0.012%, More preferably, it is 0.008%. The S content is preferably as low as possible.

Cr: 0.10 to 1.60%
Chromium (Cr) increases the hardenability of the steel. Cr further increases the amount of retained austenite after quenching. As a result, the rolling fatigue life of the bearing component is increased. Cr further forms a composite carbide with V, increasing the strength of the steel. If the Cr content is too low, these effects cannot be obtained. On the other hand, if the Cr content is too high, coarse precipitates that do not dissolve at the time of quenching are generated. As a result, the rolling fatigue life of the bearing component is reduced. Therefore, the Cr content is 0.10 to 1.60%. The minimum with preferable Cr content is 0.40%, More preferably, it is 0.60%. The upper limit with preferable Cr content is 1.40%, More preferably, it is 1.20%.

V: Over 0.10% to 1.00%
Vanadium (V) increases the hardenability of the steel. V further increases the amount of retained austenite after quenching. As a result, the rolling fatigue life of the bearing component is increased. V further forms a composite carbide with Cr and increases the strength of the steel. If the V content is too low, these effects cannot be obtained. On the other hand, if the V content is too high, coarse precipitates that do not dissolve at the time of quenching are generated. As a result, the rolling fatigue life of the bearing component is reduced. Therefore, the V content is more than 0.10% to 1.00%. The minimum with preferable V content is 0.14%, More preferably, it is 0.20%. The upper limit with preferable V content is 0.60%, More preferably, it is 0.40%.

Al: 0.010-0.500%
Aluminum (Al) deoxidizes steel. Al further combines with nitrogen (N) in the steel to form AlN. Therefore, the amount of solute N in the steel is reduced, and composite carbide is easily formed. As a result, the strength of the steel is increased and the rolling fatigue strength of the bearing component is increased. If the Al content is too low, these effects cannot be obtained. On the other hand, if the Al content is too high, coarse oxide inclusions are formed and the steel becomes brittle. Therefore, the Al content is 0.010 to 0.500%. The minimum with preferable Al content is 0.015%, More preferably, it is 0.018%. The upper limit with preferable Al content is 0.10%, More preferably, it is 0.05%. As used herein, “Al” content means “acid-soluble Al”, that is, the content of “sol. Al”.

N: 0.015% or less Nitrogen (N) is inevitably contained. N combines with Al and V to form nitrides and carbonitrides in the steel. V nitride and V carbonitride are thermally stable and hardly dissolve in austenite during quenching. Therefore, V nitride and V carbonitride inhibit the formation of composite carbide. As a result, the strength of the steel is reduced and the rolling fatigue life of the bearing parts is reduced. Therefore, the N content is 0.015% or less. The upper limit with preferable N content is 0.0080%, More preferably, it is 0.0060%. The N content is preferably as low as possible.

  The balance of the chemical composition of the bearing steel according to the present invention consists of Fe and impurities. Here, the impurities are mixed from ore as a raw material, scrap, or production environment when industrially producing bearing steel, and do not adversely affect the bearing steel of the present invention. It means what is allowed in the range.

[Arbitrary elements]
The chemical composition of the above-described bearing steel may further include one or more selected from the group consisting of Mo, Cu, and Ni instead of a part of Fe. All of these elements are arbitrary elements. All of these elements increase the hardenability of the steel and increase the amount of retained austenite after quenching.

Mo: 0 to 1.00%
Molybdenum (Mo) is an optional element and may not be contained. When contained, Mo increases the hardenability of the steel. Mo further increases the amount of retained austenite after quenching. As a result, the rolling fatigue life of the bearing component is increased. If Mo is contained even a little, these effects can be obtained to some extent. On the other hand, if the Mo content is too high, the amount of retained austenite after quenching becomes too high, and the strength of the steel decreases. As a result, the rolling fatigue strength of the bearing component is reduced. Therefore, the Mo content is 0 to 1.00%. The minimum with preferable Mo content is 0.10%, More preferably, it is 0.20%. The upper limit with preferable Mo content is 0.80%, More preferably, it is 0.60%.

Cu: 0 to 1.00%
Copper (Cu) is an optional element and may not be contained. When contained, Cu increases the hardenability of the steel. Cu further increases the amount of retained austenite after quenching. As a result, the rolling fatigue life of the bearing component is increased. If Cu is contained even a little, these effects can be obtained to some extent. However, if the Cu content is too high, the amount of retained austenite after quenching becomes too high and the strength of the steel decreases. As a result, the rolling fatigue strength of the bearing component is reduced. Therefore, the Cu content is 0 to 1.00%. The minimum with preferable Cu content is 0.10%, More preferably, it is 0.20%. The upper limit with preferable Cu content is 0.80%, More preferably, it is 0.60%.

Ni: 0 to 3.00%
Nickel (Ni) is an optional element and may not be contained. When contained, Ni increases the hardenability of the steel. Ni further increases the amount of retained austenite after quenching. As a result, the rolling fatigue life of the bearing component is increased. If Ni is contained even a little, these effects can be obtained to some extent. However, if the Ni content is too high, the amount of retained austenite after quenching becomes too high and the strength of the steel decreases. As a result, the rolling fatigue strength of the bearing component is reduced. Therefore, the Ni content is 0 to 3.00%. The minimum with preferable Ni content is 0.10%, More preferably, it is 0.20%, More preferably, it is 0.40%. The upper limit with preferable Ni content is 2.40%, More preferably, it is 1.60%.

  The chemical composition of the above-described bearing steel may further include one or more selected from the group consisting of Ti and Nb instead of a part of Fe. All of these elements are optional elements, which refine the austenite crystal grains and increase the strength of the steel.

Ti: 0 to 0.100%
Titanium (Ti) is an optional element and may not be contained. When contained, Ti refines austenite crystal grains by the pinning effect and increases the strength of the steel. As a result, the rolling fatigue life of the bearing parts is increased. Ti further combines with N in the steel to form TiN. Therefore, the production amount of composite carbide increases. As a result, the strength of the steel is increased and the rolling fatigue life of the bearing component is increased. If Ti is contained even a little, these effects can be obtained to some extent. However, if the Ti content is too high, the steel becomes brittle. Therefore, the Ti content is 0 to 0.100%. The minimum with preferable Ti content is 0.010%, More preferably, it is 0.030%. The upper limit with preferable Ti content is 0.070%, More preferably, it is 0.050%.

Nb: 0 to 0.100%
Niobium (Nb) is an optional element and may not be contained. When contained, Nb refines austenite crystal grains by the pinning effect and increases the strength of the steel. As a result, the rolling fatigue life of the bearing component is increased. Nb further combines with N in the steel to form NbN. Therefore, the production amount of composite carbide increases. As a result, the strength of the steel is increased and the rolling fatigue life of the bearing component is increased. If Nb is contained even a little, these effects can be obtained to some extent. However, if the Nb content is too high, the steel becomes brittle. Therefore, the Nb content is 0 to 0.100%. The minimum with preferable Nb content is 0.010%, More preferably, it is 0.030%. The upper limit with preferable Nb content is 0.070%, More preferably, it is 0.050%.

[About Ms]
In the chemical composition of the bearing steel of the present invention, Ms defined by the formula (1) is 100 to 220.
Ms = 539-423C-30Mn-11Si-12Cr-7Mo-18Ni-18Cu (1)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (1). If the corresponding element is not contained, “0” is assigned to the element symbol.

  Ms represents a martensitic transformation start temperature (° C.). If Ms is too low, the start of martensitic transformation during quenching is too slow. Therefore, in the steel after quenching, the amount of retained austenite is too much, and the strength of the steel is reduced. As a result, the rolling fatigue strength of the bearing component is reduced. If the amount of retained austenite is too large, the retained austenite becomes coarser. Coarse retained austenite is likely to be transformed into martensite by stress under the usage environment of the bearing component. As a result, the rolling fatigue life of the bearing component is reduced.

  On the other hand, if Ms is too high, the start of martensitic transformation during quenching is too early. Therefore, the amount of retained austenite after quenching is too small. As a result, the rolling fatigue life of the bearing component is reduced. Therefore, Ms is 100-220. A preferable lower limit of Ms is 120, and more preferably 140. The upper limit with preferable Ms is 200, More preferably, it is 180.

[About Nf]
In the chemical composition of the bearing steel of the present invention, Nf defined by the formula (2) is 0.5 or less.
Nf = (N / 14) / (Al / 27 + Ti / 46 + Nb / 93) (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbol in the formula (2). If the corresponding element is not contained, “0” is assigned to the element symbol.

  Nf is a material amount ratio of N to Al, Ti, and Nb that easily form nitrides. If Nf is 0.5 or less, N in the steel is fixed as a nitride by Al, Ti, or Nb. Therefore, V nitride and V carbonitride are hardly formed, and the amount of composite carbide generated in the bearing component is increased. As a result, the strength of the steel is increased and the rolling fatigue life of the bearing component is increased. On the other hand, if Nf exceeds 0.5, N in the steel is not fixed as nitride, V nitride and V carbonitride are generated, and the amount of composite carbide generated in the bearing component is reduced. As a result, the strength of the steel is reduced and the rolling fatigue life of the bearing parts is reduced. Therefore, Nf is 0.5 or less. A preferable upper limit of Nf is 0.4. Nf is preferably as low as possible.

[Production method]
An example of the above-described bearing steel and a method for manufacturing the bearing component will be described.

[Production method of bearing steel]
The bearing steel manufacturing method according to the present invention includes a casting process and a hot working process.

[Casting process]
In the casting process, molten steel having the above-described chemical composition, Ms of 100 to 220, and Nf of 0.5 or less is manufactured by a well-known method. The produced molten steel is used to form a slab (slab or bloom) by a continuous casting method. You may make into a steel ingot (ingot) by the ingot-making method using the manufactured molten steel.

[Hot working process]
In the hot working step, one or more hot workings are performed on the slab or steel ingot produced in the casting step to produce a steel bar, a wire rod, or a steel material for a bearing having a desired shape. Hot working is, for example, hot rolling. In the case where a plurality of hot workings are performed, for example, rough rolling and finish rolling are performed. In rough rolling, for example, a slab or a steel ingot is made into a steel slab (billet) by split rolling. In finish rolling, for example, a continuous rolling mill in which a plurality of rolling stands are arranged in a line is used. Each rolling stand is provided with a plurality of rolling rolls. A hole mold is formed in each rolling roll. A billet is hot-rolled using a continuous rolling mill to produce a steel for bearings such as a steel bar or a wire rod. In the above description, hot rolling has been described as an example of hot working. However, as hot working, a steel bar or wire rod steel may be manufactured by hot forging.

  You may implement a normalization process (normalization process) and a spheroidizing annealing process with respect to the manufactured steel material for bearings as needed. The steel for bearings is manufactured by the above process.

[Bearing parts]
The bearing component according to the present invention is manufactured using the bearing steel described above. Hereinafter, the bearing component of the present invention will be described.

[Production method of bearing parts]
The bearing component manufacturing method of the present invention includes an intermediate product forming step, a quenching step, and a tempering step.

[Intermediate product forming process]
First, an intermediate product is formed using steel for bearings. For example, hot forging is performed on the bearing steel material to produce a coarse intermediate product. Further, the intermediate product is machined to make the intermediate product into a predetermined shape. Machining is, for example, cutting or drilling.

[Quenching process]
Quenching is performed on the molded intermediate product. The heating temperature of the quenching process is 900 ° C. or higher. If the heating temperature of the quenching treatment is 900 ° C. or higher, the solid solution amount of V and Cr increases. As a result, the precipitation amount of the composite carbide increases and the strength of the steel increases. A preferable heating temperature for the quenching treatment is 930 ° C. or higher. Subsequently, it is rapidly cooled and a quenching process is performed. The quenching process is, for example, oil cooling. The quenching process may be water cooling.

[Tempering process]
Tempering is performed on the intermediate product after quenching. The heating temperature in the tempering process is 250 to 400 ° C. By tempering at a temperature higher than usual, the precipitation amount of the composite carbide increases and the strength of the bearing component increases. By tempering at a high temperature, the retained austenite grains can be further refined. These effects cannot be obtained if the heating temperature of the tempering treatment is less than 250 ° C. On the other hand, if the heating temperature of the tempering treatment exceeds 400 ° C., the retained austenite is transformed into tempered martensite. In this case, the rolling fatigue strength of the bearing component is reduced. Therefore, the heating temperature of the tempering process is 250 to 400 ° C. The minimum of the heating temperature of a preferable tempering process is 280 degreeC. The upper limit of the heating temperature for the preferred tempering treatment is 360 ° C.

  A bearing part is manufactured by the above process. Bearing parts manufactured using the steel for bearings of the present invention are excellent in rolling fatigue life.

[Hardness of bearing parts]
The Vickers hardness of the surface of the bearing component of the present invention is 670 Hv or more. High loads are applied to the bearing parts. Therefore, high strength is required for the bearing parts. Therefore, the Vickers hardness of the bearing component of the present invention is 670 Hv or more. The Vickers hardness of the bearing part can be adjusted by the tempering temperature. The lower limit of the preferred Vickers hardness is 700 Hv. The upper limit of the preferred Vickers hardness is 800 Hv, more preferably 750 Hv.

  Vickers hardness can be measured by the following method. Take a specimen from any location on the surface of the bearing part. A Vickers hardness test based on JIS Z 2244 (2009) is performed 5 times on the collected specimen. The test force is 98.07N. The average of the obtained Vickers hardness is defined as the Vickers hardness (Hv) of the bearing part.

[Microstructure of bearing parts]
The microstructure of the bearing component of the present invention contains 5.0-40% residual austenite in volume fraction. Among the remaining phases, the phase with the highest volume fraction is tempered martensite. Furthermore, in the microstructure, the number density of residual austenite grains having a circle-equivalent diameter of 0.2 to 2.0 μm is 10/100 μm ² or more. The equivalent circle diameter means the diameter of a circle when the area of each retained austenite grain is converted into a circle having the same area.

[Residual austenite volume fraction]
As described above, the retained austenite increases the rolling fatigue life of the bearing. Residual austenite is most abundant in steel immediately after quenching. The amount of retained austenite immediately after quenching is determined by Ms. Thereafter, part of the material is transformed into tempered martensite by tempering.

  In this invention, Si content is raised and the fall of the amount of retained austenite at the time of tempering is suppressed. If the volume fraction of retained austenite in the bearing part is less than 5%, the effect of increasing the rolling fatigue life of the bearing part cannot be obtained. On the other hand, if the volume fraction of retained austenite in the bearing component exceeds 40%, the strength of the bearing component decreases and the rolling fatigue life of the bearing component decreases. Therefore, the volume fraction of retained austenite in the bearing component is 5 to 40%. A preferred lower limit for the volume fraction of retained austenite is 8.0%. A preferable upper limit of the volume fraction of retained austenite is 30%.

  The volume fraction of retained austenite is measured by the following method. Mirror polishing is performed on the L cross section of the bearing part (cross section parallel to the rolling direction and the rolling direction). The mirror-polished surface is irradiated with X-rays, and the amount of retained austenite is calculated from the intensity difference between the diffraction peaks of the BCC phase and the FCC phase. Specifically, XRD (X-ray diffraction) measurement by the θ / 2θ method using CrKα rays is performed by the two-peak method. Then, the residual austenite volume fraction (%) is calculated based on the measured intensity ratio and the theoretical intensity ratio of the (211) peak of BCC and the (220) peak of FCC.

  Of the remaining phases of the microstructure of the bearing component of the present invention, the phase having the largest volume fraction is tempered martensite. More specifically, 70% or more of the remaining volume of the microstructure is composed of tempered martensite. Tempered martensite has higher toughness than quenched martensite. Therefore, cracks and chipping can be prevented when used as a bearing. Therefore, the remainder of the microstructure of the present invention is mainly tempered martensite.

  The volume fraction of tempered martensite is measured by the following method. Mirror polishing is performed on the L cross section of the bearing part (cross section parallel to the rolling direction and the rolling direction). The mirror-polished surface is corroded with 3% nital (3% nitric acid-ethanol solution) to reveal a microstructure. Five visual fields are observed (photographed) with a 500 × optical microscope, and a microstructure image of each visual field is generated. Based on the contrast difference, residual austenite, tempered martensite, and other phases are identified from the generated microstructure image. After identification, the area of tempered martensite is obtained, and the area ratio of tempered martensite is obtained from the area of each visual field. The average of the area ratio of tempered martensite in each visual field is defined as the volume fraction (%) of tempered martensite.

[Number density of fine retained austenite grains]
As described above, the finely dispersed residual austenite grains are difficult to transform into martensite due to stress. However, residual austenite grains having an equivalent circle diameter exceeding 2.0 μm are easily transformed into martensite by stress-induced transformation. Therefore, during use as a bearing component, the retained austenite gradually decreases, and the rolling fatigue life decreases. Accordingly, if the retained austenite has an equivalent circle diameter of 2.0 μm or less, the rolling fatigue life of the bearing part can be increased. The residual austenite grains having a circle equivalent diameter of 2.0 μm or less can be objectively counted as long as the circle equivalent diameter is 0.2 μm or more. Accordingly, retained austenite having an equivalent circle diameter of 0.2 to 2.0 μm is an object of the present invention.

Furthermore, if a large number of fine retained austenite grains having an equivalent circle diameter of 0.2 to 2.0 μm are dispersed in the steel, the rolling fatigue life of the bearing component can be increased. If the number density of retained austenite grains having a circle-equivalent diameter of 0.2 to 2.0 μm is less than 10/100 μm ² , this effect cannot be obtained. Therefore, the number density of residual austenite grains having a circle-equivalent diameter of 0.2 to 2.0 μm is 10/100 μm ² or more. The preferred lower limit of the number density of retained austenite grains having a circle-equivalent diameter of 0.2 to 2.0 μm for further increasing the rolling fatigue life is 25/100 μm ² . If the volume fraction of retained austenite does not exceed 40%, it is desirable that the number density of retained austenite grains be higher.

The number density of residual austenite grains having a circle-equivalent diameter of 0.2 to 2.0 μm is measured by the following method. The bearing parts whose microstructure has been revealed by the above-described method are observed using a scanning electron microscope (SEM). Specifically, 10 fields of view of 100 μm ² are observed (photographed) from the surface of the test piece, and an SEM image of each field is generated. The generated SEM image is subjected to image processing (binarization processing), and a retained austenite structure having a circle-equivalent diameter of 0.2 to 2.0 μm is specified. The average of the number density obtained in each field of view is obtained and defined as the number density (number / 100 μm ² ) of retained austenite grains having a circle-equivalent diameter of 0.2 to 2.0 μm.

[About composite carbide]
In the bearing component of the present invention, the number density of carbides having a major axis of 150 to 300 nm in steel is 50/100 μm ² or more. In the steel of the bearing component of the present invention, V and Cr composite carbides, cementite, and other carbides are included as carbides. The major axis of the composite carbide is 300 nm or less. On the other hand, the long diameter of cementite exceeds 300 nm. Further, the major axis of other carbides is less than 150 nm. By the way, in SEM observation described later, a carbide having a major axis of 150 nm or more can be reliably confirmed. Therefore, a carbide having a major axis of 150 to 300 nm that can be confirmed by SEM observation means a composite carbide. Therefore, in the present invention, the number density of carbides having a major axis of 150 to 300 nm is used as an index of steel strength. Specifically, if the number density of carbides having a major axis of 150 to 300 nm in the steel is 50/100 μm ² or more, the composite carbide is sufficiently precipitated in the steel, so that the strength of the steel can be increased. .

If the number of carbides having a major axis of 150 to 300 nm is less than 50/100 μm ² , the strength of the bearing component is insufficient. Therefore, in the bearing component of the present invention, the number density of carbide having a major axis of 150 to 300 nm is 50 pieces / 100 μm ² or more in steel. A preferred number density of carbides is 70/100 μm ² .

The number density of carbide is measured by the following method. In the above-mentioned SEM image (SEM image of the L cross section), the carbide is distinguishable because the contrast is different from the tempered martensite matrix. The average of the number density calculated | required in each visual field about the carbide | carbonized_material of long diameter 150-300 nm is calculated | required, and it defines as the number density (piece / 100 micrometer < ² >) of the long diameter 150-300 nm carbide | carbonized_material. The major axis is defined as the largest straight line among the straight lines connecting any two points on the interface with the carbide matrix.

  160 kg of molten steel having the chemical composition shown in Table 1 was melted by vacuum melting to produce an ingot.

  With reference to Table 1, the chemical composition of the test numbers 1-18 was appropriate, Ms satisfy | filled 100-220, and Nf satisfy | filled 0.50 or less. On the other hand, in the test numbers 19 to 27, the chemical composition was inappropriate, Ms did not satisfy 100 to 220, or Nf exceeded 0.50.

  The ingot of each test number was hot forged to produce a round bar (steel material for bearing) having a diameter of 70 mm.

  A soaking treatment was performed in which the manufactured round bar was held at 1250 ° C. for 2 hours. The steel material after the soaking was held at 1000 ° C. for 30 minutes, and then subjected to a normal treatment that was gradually cooled.

[Manufacture of pseudo bearing parts (Mori-type thrust rolling fatigue test piece)]
The round bar after the normal treatment was machined to produce a coarse intermediate product of a ring-shaped forest thrust rolling fatigue test piece having a diameter of 58 mm.

  Quenching and tempering were performed on the produced intermediate product. Specifically, the intermediate product was heated at 950 ° C. for 30 minutes, and then quenched with oil at 60 ° C. Further, the intermediate product after quenching was heated at 250 to 400 ° C. for 30 minutes, and a tempering treatment was performed so that the surface Vickers hardness became 700 ± 30 Hv. The intermediate product after the tempering treatment was ground and polished to produce a ring-shaped forest thrust rolling fatigue test piece having a diameter of 58 mm, simulating a bearing component.

[Evaluation test]
The following microstructure observation and rolling fatigue test were performed on the Mori type thrust rolling fatigue test piece.

[Microstructure observation]
Microstructure observation was carried out by the following method. The volume fraction (%) of retained austenite was measured using the XRD2 peak method described above for the rolling fatigue test pieces of each test number. Subsequently, the equivalent circle diameter (μm) of each retained austenite grain was measured by SEM observation by the above-described method. Furthermore, residual austenite grains having an equivalent circle diameter of 0.2 to 2.0 μm were specified, and the number density (pieces / 100 μm ² ) was measured. These results are shown in Table 2.

[Number density measurement of carbides]
With the above-mentioned method, the major axis (nm) of the carbide was measured using SEM observation. Furthermore, the carbide | carbonized_material of 150-300 nm of long diameters was specified, and the number density (piece / 100 micrometer < ² >) was measured. The results are shown in Table 2.

[Rolling fatigue test]
The rolling fatigue test was carried out by the following method. A rolling fatigue test was carried out by combining a rolling fatigue test piece of each test number, a thrust bearing race of nominal number # 51305 as an upper race, and three steel balls. Specifically, a test with a durability of 200 × 10 ⁶ was repeated 10 times in a state where the test load was 400 kgf, the maximum surface pressure was 5.23 GPa, the rotation speed was 1200 rpm, and the lubricating oil was immersed in Crisef H8. The rolling fatigue life L ₁₀ (× 10 ⁶ cycles) was determined from the Weibull plot. The results are shown in Table 2.

[Evaluation results]
With reference to Tables 1 and 2, the chemical compositions of test numbers 1 to 18 were appropriate, Ms satisfied 100 to 220, and Nf satisfied 0.50 or less. Therefore, the volume fraction of retained austenite (%), the number density of retained austenite (pieces / 100 μm ² ), and the number density of carbides (pieces / 100 μm ² ) were within the scope of the present invention. As a result, the rolling fatigue life L ₁₀ (× 10 ⁶ cycles) was 50 or more, indicating an excellent rolling fatigue life.

  On the other hand, the Si content of Test No. 19 was too low. Therefore, the volume fraction of retained austenite was too low. The number density of retained austenite was too low. Furthermore, the number density of carbides was too low. As a result, the rolling fatigue life was low.

  Ms of test number 20 was too high. Therefore, the volume fraction of retained austenite was too low. Furthermore, the number density of retained austenite was too low. As a result, the rolling fatigue life was low.

  Ms of test number 21 was too low. Therefore, the volume fraction of retained austenite was too high. As a result, the rolling fatigue life was low.

  The Cr content of test number 22 was too low. Therefore, the number density of carbides was too low. As a result, the rolling fatigue life was low.

  The Cr content of test number 23 was too high. As a result, the rolling fatigue life was low.

  The V content of test number 24 was too low. Therefore, the number density of carbides was too low. As a result, the rolling fatigue life was low.

  The V content of test number 25 was too high. As a result, the rolling fatigue life was low.

  The Al content of test number 26 was too low. Furthermore, Nf was too high. Therefore, the number density of carbides was too low. As a result, the rolling fatigue life was low.

  Nf of test number 27 was too high. Therefore, the number density of carbides was too low. As a result, the rolling fatigue life was low.

  The embodiment of the present invention has been described above. However, the above-described embodiment is merely an example for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiment, and can be carried out by appropriately changing the above-described embodiment without departing from the spirit thereof.

  The steel for bearings according to the present invention can be widely applied as steel for bearing members, and is particularly suitable as steel for rolling bearing members.

Claims (4)

% By mass
C: 0.40 to 1.00%,
Si: 0.75 to 3.00%,
Mn: 0.30 to 2.00%
P: 0.015% or less,
S: 0.015% or less,
Cr: 0.10 to 1.60%,
V: more than 0.10% to 1.00%,
Al: 0.010 to 0.500%,
N: 0.015% or less,
Mo: 0 to 1.00%,
Cu: 0 to 1.00%,
Ni: 0 to 3.00%,
Ti: 0 to 0.100%, and
Nb: 0 to 0.100% is contained, the balance consists of Fe and impurities,
Ms defined by the formula (1) is 100 to 220,
A bearing steel having a chemical composition in which Nf defined by the formula (2) is 0.5 or less.
Ms = 539-423C-30Mn-11Si-12Cr-7Mo-18Ni-18Cu (1)
Nf = (N / 14) / (Al / 27 + Ti / 46 + Nb / 93) (2)
Here, the content (mass%) of the corresponding element is substituted for the element symbols in the formulas (1) and (2). The bearing steel according to claim 1,
The chemical composition is
Mo: 0.10 to 1.00%,
Cu: 0.10 to 1.00%, and
Ni: Steel for bearings containing one or more selected from the group consisting of 0.10 to 3.00%. The bearing steel according to claim 1 or 2,
The chemical composition is
Ti: 0.010 to 0.100%, and
Nb: Steel for bearings containing at least one selected from the group consisting of 0.010 to 0.100%. Bearing parts,
Having the chemical composition according to any one of claims 1 to 3,
The surface of the bearing part has a Vickers hardness of 670 Hv or more,
The structure of the bearing component contains 5-40% residual austenite in volume fraction, and the phase having the largest volume fraction among the remaining phases is tempered martensite,
The number density of the residual austenite grains having an equivalent circle diameter of 0.2 to 2.0 μm is 10/100 μm ² or more,
The bearing part whose number density of the carbide | carbonized_material which has a major axis of 150-300 nm is 50 piece / 100 micrometer < ² > or more.