JP2022170056A - steel - Google Patents

Abstract

To provide a steel having excellent hot workability, and having an excellent surface starting exfoliation service life and excellent shape stability when bearing parts are manufactured.SOLUTION: A steel which includes a decarbonized layer, and a steel core part inner than the decarbonized layer is such that: a chemical composition of the steel core part comprises by mass%, C:0.80-1.10%, Si:0.15-0.50%, Mn:0.30-0.70%, Cr:1.30-1.60%, Mo:0.10-0.50%, V:0.12-0.50%, Al:0.005-0.050%, P:0.020% or less, S:0.010% or less, N:0.0150% or less, and O:0.0015% or less; the balance comprises Fe and impurities; formula (1) and formula (2) described in the specifications are satisfied; and a depth of the decarbonized layer and a diameter of a cross section perpendicular to the longitudinal direction of the steel satisfy formula (3) described in the specification.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present disclosure relates to steel materials, and more particularly to steel materials that are used as materials for bearing components.

Bearing steel and case hardened steel have been used as steel materials for bearing parts. Bearing steels are represented by SUJ3 and SUJ5 specified in JIS G 4805 (2019). Case hardening steel is represented by SNCM815 defined in JIS G 4053 (2016).

When a bearing component is manufactured using these steel materials, the manufacturing method is as follows. Hot working (hot forging) and spheroidizing annealing are performed on the steel material. After the spheroidizing annealing, the steel material is subjected to cutting or the like to manufacture an intermediate product having a desired shape. The intermediate product is subjected to heat treatment to adjust the hardness of the steel and the microstructure of the steel. Through the above steps, a bearing component having desired bearing performance (surface-originating flaking life, etc.) is manufactured using the above-described steel material as a raw material.

As described above, heat treatment is performed in the manufacturing process of bearing components in order to improve bearing performance. Heat treatment includes, for example, quenching and tempering, carburizing, nitriding, and carbo-nitriding. As used herein, carburizing treatment means carburizing quenching and tempering. Further, in this specification, nitriding treatment means nitriding quenching and tempering. Further, in this specification, the term carbo-nitriding treatment means carbo-nitriding quenching and tempering. Hardened layers such as a quenched layer, a carburized layer, a nitrified layer, and a carbo-nitrided layer are formed on the surface layer of the steel material by the heat treatment. These hardened layers improve the bearing performance such as the surface initiated flaking life.

Techniques for improving the bearing performance of bearing components have been proposed in Japanese Unexamined Patent Application Publication No. 8-49057 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2008-280583 (Patent Document 2).

The rolling bearing proposed in Patent Document 1 consists of a bearing ring and a rolling element, and at least one of the bearing ring and the rolling element contains, by weight %, C: 0.1 to 0.7%, Cr: 0.5 to 3.0%, Mn: 0.3-1.2%, Si: 0.3-1.5%, Mo: 3% or less, V: 0.8-2.0% do. Products manufactured by carburizing or carbonitriding the material have a surface carbon concentration of 0.8 to 1.5% by weight and a surface V/C concentration ratio of 1 to 2.5. satisfy the relationship As a result, VC-type carbide precipitates on the surface of the product. Patent Document 1 discloses that this rolling bearing has high wear resistance, which is one of the bearing performances.

The case-hardened steel proposed in Patent Document 2 has, in mass%, C: 0.1 to 0.4%, Si: 0.5% or less, Mn: 1.5% or less, P: 0.03% or less. , S: 0.03% or less, Cr: 0.3 to 2.5%, Mo: 0.1 to 2.0%, V: 0.1 to 2.0%, Al: 0.050% or less, It contains O: 0.0015% or less, N: 0.025% or less, V+Mo: 0.4 to 3.0%, and the balance is Fe and impurities. This case-hardening steel further has a surface layer C concentration of 0.6 to 1.2% after tempering, a surface hardness of HRC 58 to less than 64, and, among V-based carbides dispersedly precipitated on the surface layer, the grain size The number ratio of fine V-based carbides of less than 100 nm is 80% or more. In a bearing component or the like manufactured from this case-hardening steel, the finely dispersed V-based carbides serve as trap sites for hydrogen, increasing hydrogen embrittlement resistance. Patent document 2 discloses that as a result, the surface fatigue life, which is one of the bearing performances, is increased.

JP-A-8-49057 JP 2008-280583 A

Bearing parts include medium-sized and large-sized bearing parts used in mining machines and construction machines, and small-sized bearing parts used in automobiles and the like. Small bearing parts for automotive applications are, for example, bearing parts applied to drive parts such as transmissions. Small bearing parts for automobiles are often used in an environment where lubricating oil circulates. At this time, abrasion powder derived from the gear parts around the bearing parts may mix with the circulating lubricating oil and enter the bearing as foreign matter. Therefore, small bearing parts are required to have an excellent surface-initiated flaking life even in a state in which such foreign matter is mixed. However, in Patent Literatures 1 and 2, no consideration is given to the life of surface originating flaking in a state in which foreign matter is mixed in the bearing component.

In recent years, in order to reduce frictional resistance and torque transmission resistance during use, attempts have been made to reduce the viscosity and amount of lubricating oil used in small bearing parts for automobiles. ing. On the other hand, when the viscosity of the lubricating oil is lowered and/or when the amount of the lubricating oil used is reduced, the film thickness of the lubricating oil on the contact surfaces between the bearing parts is reduced. As a result, the pressure applied to the bearing component (hereinafter also referred to as surface pressure) tends to increase.

On the other hand, if high surface pressure is repeatedly applied to the bearing component, the bearing component may deform. Specifically, for example, when the amount of deformation of the rolling surface of the bearing component increases, the gap between the rolling elements (balls, rollers) and the inner and outer rings increases. In this case, the bearing component cannot rotatably support a rotating member (such as a shaft) at an accurate position. As a result, the bearing component and members supported by the bearing component vibrate, causing noise. For this reason, bearing components are required to have properties (hereinafter also referred to as shape stability) that are resistant to deformation and change in shape even when high surface pressure is repeatedly applied during use. However, in Patent Documents 1 and 2, the shape stability of the bearing component is not considered.

Further, the bearing component is manufactured by subjecting the steel material to hot forging, as described above. Therefore, the steel material used as the raw material for the bearing parts is required to have a characteristic that cracks are unlikely to occur during hot forging (hereinafter also referred to as hot workability). However, in Patent Documents 1 and 2, the hot workability of steel materials is not examined.

The object of the present disclosure is to provide a bearing component that has excellent hot workability, and when a bearing component is manufactured, has an excellent surface-originating flaking life even when foreign matter is mixed in the bearing component, and high surface pressure is repeatedly applied to the bearing component. To provide a steel material having excellent shape stability even if it is imparted.

The steel according to the present disclosure is
a decarburized layer;
and a steel core inside the decarburized layer,
The chemical composition of the steel core is, in mass %,
C: 0.80 to 1.10%,
Si: 0.15 to 0.50%,
Mn: 0.30-0.70%,
Cr: 1.30-1.60%,
Mo: 0.10-0.50%,
V: 0.12 to 0.50%,
Al: 0.005 to 0.050%,
P: 0.020% or less,
S: 0.010% or less,
N: 0.0150% or less,
O: 0.0015% or less, and
The balance consists of Fe and impurities, and satisfies formulas (1) and (2),
The depth of the decarburized layer and the diameter of the cross section perpendicular to the longitudinal direction of the steel material satisfy expression (3).
1.40<Cr+0.1Mo+0.25(V+Nb+Ti)≦1.70 (1)
15Si+35Mn+15Ni+20Cr≦55 (2)
d/D<0.010 (3)
Here, the content of the corresponding element is substituted for each element symbol in the formulas (1) and (2) in mass %. If the corresponding element is not contained, "0" is substituted for the element symbol. The depth of the decarburized layer in mm is substituted for d in the formula (3), and the diameter of the cross section perpendicular to the longitudinal direction of the steel material is substituted for D in mm.

The steel material according to the present disclosure has excellent hot workability, and when a bearing component is manufactured, it has an excellent surface-originating flaking life even when foreign matter is mixed in, and an excellent surface-induced flaking life even when high surface pressure is repeatedly applied. It has shape stability.

FIG. 1 is a schematic diagram of a thrust-type rolling contact fatigue tester.

The inventors of the present invention have improved the hot workability of the steel material that is the raw material of the bearing parts, and when the bearing parts are manufactured from the steel material, the surface originating flaking life is increased even if foreign matter is mixed in the bearing parts, and the bearing Investigations and studies were conducted on methods for improving shape stability even when high surface pressure is repeatedly applied to parts.

First, the present inventors focused on the chemical composition to improve the hot workability of the steel material, increase the life of surface-originating flaking even when foreign matter is mixed in the bearing parts, and repeatedly apply high surface pressure to the bearing parts. We investigated a method to improve the shape stability even if it is As a result, the present inventors found that the chemical composition is, in mass%, C: 0.80 to 1.10%, Si: 0.15 to 0.50%, Mn: 0.30 to 0.70%, Cr: 1.30-1.60%, Mo: 0.10-0.50%, V: 0.12-0.50%, Al: 0.005-0.050%, P: 0.020% Below, S: 0.010% or less, N: 0.0150% or less, O: 0.0015% or less, Ca: 0-0.0050%, Cu: 0-0.50%, Ni: 0-0. 50%, Nb: 0 to 0.050%, Ti: 0 to 0.050% or less, and the balance being Fe and impurities, the steel material has excellent hot workability and prevents foreign matter from entering the bearing parts. It was thought that it could have an excellent surface-initiated flaking life even if it is mixed in, and that it may have excellent shape stability even if high surface pressure is repeatedly applied to the bearing part.

However, as a result of investigations by the present inventors, it was found that even if the content of each element in the chemical composition satisfies the above range, the above properties (hot workability of the steel material, the performance of bearing parts) It was found that there are cases where sufficient surface-originating peeling life and shape stability are not obtained. Therefore, the present inventors conducted further studies. As a result, the present inventors obtained the following findings.

[Regarding formula (1)]
As described above, in the case of small bearing parts for automotive applications, foreign matter may enter the bearing parts. If a foreign object gets into a bearing component, it is caught between the raceway surface of the bearing ring of the bearing component and the rolling surface of the rolling element during use, and dents are likely to be formed on the raceway surface and/or the rolling surface. Become. Furthermore, stress concentration is likely to occur in the raised portion formed around the indentation. As a result, fatigue fracture is likely to occur on the raceway surface and/or the rolling contact surface. As described above, when foreign matter is mixed in the bearing component, the life of the bearing component due to surface-initiated flaking tends to decrease.

Furthermore, as described above, when steel materials are used as raw materials to manufacture bearing parts, the steel materials (intermediate products) subjected to hot forging and cutting are subjected to heat treatment (quenching and tempering, carburizing, carbonitriding, or immersion). nitrogen treatment) is carried out. Here, when a steel material (intermediate product) having the chemical composition described above is subjected to heat treatment, precipitates are dispersed in the surface layer of the intermediate product. If a large number of precipitates are dispersed on the surface layer of the bearing component, the surface layer of the bearing component can be further hardened by precipitation strengthening. As a result, the surface initiated flaking life of the bearing component may be further increased. Therefore, the present inventors focused on the precipitates on the surface layer of the bearing component after heat treatment, and investigated a method for increasing the surface-initiated flaking life of the bearing component.

Of the chemical compositions described above, V, Nb, and Ti are carbides, nitrides, and carbonitrides among the precipitates (hereinafter, carbides, nitrides, and carbonitrides are collectively referred to as “carbonitrides, etc.” ). Specifically, V, Nb, and Ti form V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof. In other words, if a large number of V carbonitrides, Nb carbonitrides, Ti carbonitrides, and composite carbonitrides of these are precipitated on the surface layer of the bearing component by heat treatment, surface-originating flaking of the bearing component will occur. Life expectancy may be further increased.

On the other hand, when producing a steel material having the chemical composition described above, coarse precipitates may be formed in the steel material. If coarse precipitates are formed in the steel material, the coarse precipitates will also remain in bearing components manufactured using the steel material as a raw material. Coarse precipitates tend to act as starting points for fatigue fracture during use of the bearing component. In other words, when coarse precipitates are formed in the steel material, the life of the bearing component due to surface-initiated flaking is reduced. In short, it is not sufficient simply to deposit a large number of precipitates, but fine V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof, etc. are formed into bearings by heat treatment. It is thought that by forming a large number of these on the component, the life of the surface-initiated flaking can be increased even if foreign matter is mixed in the bearing component.

Therefore, the present inventors performed a heat treatment on the intermediate product having the above-described chemical composition, and added fine V carbonitrides, Nb carbonitrides, Ti carbonitrides, etc., to the bearing parts. It was considered effective to increase these precipitation nucleation sites in order to form composite carbonitrides and the like. Specifically, among the chemical compositions described above, Cr and Mo increase precipitation nucleation sites for forming the above-described carbonitrides and the like during heat treatment. As a result, the formation of the above-described carbonitrides and the like is promoted in the bearing component. On the other hand, if the contents of V, Nb, Ti, Cr, and Mo are too high, a large number of the above-mentioned carbonitrides and the like are formed during the production of the steel material that is the raw material for the bearing parts, and may become coarse in the steel material. . Therefore, in order to finely disperse the above-mentioned carbonitrides and the like in the bearing component after heat treatment and further increase the life of the surface-initiated flaking of the bearing component, V, Nb, Ti, Cr and The study by the present inventors has revealed that the Mo content should be appropriately controlled.

Based on the above findings, the present inventors investigated the relationship between the V, Nb, Ti, Cr, and Mo contents of steel materials and the surface-initiated flaking life of bearing components. As a result, the present inventors found that, in a steel material in which the content of each element in the chemical composition is within the above range, if the content of V, Nb, Ti, Cr and Mo satisfies the following formula (1): On the premise that the formula (2) described below is satisfied, it was found that a sufficient surface-originating flaking life can be obtained when used as a bearing component.
1.40<Cr+0.1Mo+0.25(V+Nb+Ti)≦1.70 (1)
Here, the content of the corresponding element is substituted for each element symbol in formula (1) in mass %. If the corresponding element is not contained, "0" is substituted for the element symbol.

[Regarding formula (2)]
As described above, bearing components may be repeatedly subjected to high surface pressure. Bearing components to which high surface pressure is repeatedly applied are likely to be deformed. As a result, the bearing component and members supported by the bearing component vibrate, causing noise. Therefore, bearing components are required to have properties (shape stability) that are resistant to deformation and change in shape even when high surface pressure is repeatedly applied during use.

Therefore, the present inventors investigated the factors that affect the shape stability of bearing components manufactured using steel as a raw material. Here, as described above, the bearing component is manufactured by heat-treating an intermediate product having the chemical composition described above. In other words, there is a high possibility that the core portion of the bearing component undergoes a microstructure transformation due to the heat treatment. Therefore, the present inventors focused on the microstructure in the component core of the bearing component and investigated the relationship with the shape stability of the bearing component. As a result, it was found that if retained austenite is contained in the component core of the bearing component after heat treatment, the bearing component is likely to deform due to repeated application of high surface pressure.

Based on the above findings, the present inventors investigated elements that particularly affect the formation of retained austenite when the intermediate product having the chemical composition described above is heat-treated. As a result, it was found that among the chemical compositions described above, the contents of Si, Mn, Cr and Ni in particular affect the amount of retained austenite produced in the core of the bearing component. Therefore, the inventors further investigated the relationship between the Si, Mn, Cr and Ni contents and the shape stability of the bearing component. As a result, if the content of each element in the chemical composition is within the above range and the Si, Mn, Cr and Ni contents satisfy the following formula (2), on the premise that formula (1) is satisfied, It was found that the bearing component not only has excellent surface-initiated flaking life, but also improves the shape stability of the bearing component.
15Si+35Mn+15Ni+20Cr≦55 (2)
Here, the content of the corresponding element is substituted for each element symbol in formula (2) in mass%.

[Regarding formula (3)]
As described above, the bearing component is manufactured by performing hot forging. However, when a steel material having the above chemical composition is subjected to hot forging to produce an intermediate product, cracks (hot forging cracks) may occur on the surface of the intermediate product. On the other hand, it is presumed that the steel material having the chemical composition described above is able to maintain the hot workability from the viewpoint of the chemical composition. In other words, the steel material having the chemical composition described above may have deteriorated hot workability due to factors other than the chemical composition. Therefore, the present inventors have made detailed studies on factors other than the chemical composition among the factors that reduce the hot workability. As a result, when minute surface flaws are formed on the surface of the steel material that is the raw material for the bearing parts, the surface flaws act as surface flaws, and hot forging cracks originating from the surface flaws occur. became clear.

Accordingly, the present inventors have made detailed studies on means for suppressing the formation of surface flaws in steel materials. Specifically, the present inventors investigated the C concentration in the peripheral portion of the surface flaw of the steel material having the chemical composition described above. As a result, it was found that in the steel material having the chemical composition described above, the C concentration was lowered in the peripheral portions of the surface flaws. From this, the present inventors considered that a decarburized layer may be formed on the surface layer of the steel material.

When manufacturing a steel material having the above chemical composition, a decarburized layer may be formed on the surface. A decarburized layer means a layer with a reduced C content, which is formed when C is oxidized on the surface of a steel material and escapes as gas. Since the decarburized layer has a reduced C content, the hardness is low. That is, if a decarburized layer is formed on the surface layer, surface flaws are likely to be formed on the surface layer of the steel material due to contact between the steel material and a conveying device or the like in a high temperature state. In particular, if the decarburized layer is formed deeply, surface flaws are likely to be formed deeply.

That is, it is thought that forming a thin decarburized layer on the surface of the steel material having the above-described chemical composition makes it difficult for deep surface flaws to form, and makes it difficult for cracks due to surface flaws to occur during hot forging. As a result of detailed studies by the present inventors, it was found that the formation of surface flaws, which are the origin of hot forging cracks, depends not only on the depth of the decarburized layer of the steel material, but also on the diameter of the cross section perpendicular to the longitudinal direction of the steel material. revealed to be affected. That is, by adjusting the relationship between the depth of the decarburized layer of the steel material and the diameter of the cross section perpendicular to the longitudinal direction of the steel material, it is possible to suppress the occurrence of cracks caused by surface defects during hot forging.

Therefore, the present inventors focused on the depth of the decarburized layer and the diameter of the cross section perpendicular to the longitudinal direction of the steel material having the above-described chemical composition, and investigated in detail the relationship between the hot workability of the steel material. investigated. As a result, the present inventors have found that the steel material having the above-described chemical composition has a decarburized layer and a steel core part inside the decarburized layer, and the depth of the decarburized layer of the steel material and the longitudinal direction of the steel material If the diameter of the cross section perpendicular to the It was found that not only the shape stability can be improved, but also the hot workability of the steel material can be improved.
d/D<0.010 (3)
Here, the depth of the decarburized layer in mm is substituted for d in the formula (3), and the diameter of the cross section perpendicular to the longitudinal direction of the steel material is substituted for D in mm.

Define F3=d/D. F3 is an index indicating the susceptibility of cracks to occur in steel during hot forging. As F3 increases, cracks are more likely to occur during hot forging. That is, the larger F3 is, the lower the hot workability of the steel material. Therefore, in the steel according to this embodiment, the depth of the decarburized layer formed on the surface is made smaller than the diameter of the cross section perpendicular to the longitudinal direction of the steel. In particular, when F3 is 0.010 or more, the hot workability of the steel is remarkably lowered. Therefore, the steel material according to the present embodiment has F3 less than 0.010 on the premise that it has the chemical composition described above that satisfies the formulas (1) and (2). As a result, the steel material according to this embodiment has excellent hot workability.

The gist of the steel material according to the present embodiment completed based on the above knowledge is as follows.

[1]
is steel,
a decarburized layer;
and a steel core inside the decarburized layer,
The chemical composition of the steel core is, in mass %,
C: 0.80 to 1.10%,
Si: 0.15 to 0.50%,
Mn: 0.30-0.70%,
Cr: 1.30-1.60%,
Mo: 0.10-0.50%,
V: 0.12 to 0.50%,
Al: 0.005 to 0.050%,
P: 0.020% or less,
S: 0.010% or less,
N: 0.0150% or less,
O: 0.0015% or less, and
The balance consists of Fe and impurities, and satisfies formulas (1) and (2),
The depth of the decarburized layer and the diameter of the cross section perpendicular to the longitudinal direction of the steel satisfy formula (3),
steel.
1.40<Cr+0.1Mo+0.25(V+Nb+Ti)≦1.70 (1)
15Si+35Mn+15Ni+20Cr≦55 (2)
d/D<0.010 (3)
Here, the content of the corresponding element is substituted for each element symbol in the formulas (1) and (2) in mass%. If the corresponding element is not contained, "0" is substituted for the element symbol. The depth of the decarburized layer in mm is substituted for d in the formula (3), and the diameter of the cross section perpendicular to the longitudinal direction of the steel material is substituted for D in mm.

[2]
The steel material according to [1],
The chemical composition of the steel core further includes, instead of part of Fe,
Ca: 0.0050% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Nb: 0.050% or less, and
Ti: 0.050% or less, containing one or more selected from the group consisting of
steel.

The steel material of this embodiment will be described in detail below. "%" for elements means % by weight unless otherwise specified.

[Structure of steel]
The steel material according to this embodiment includes a decarburized layer and a steel material core inside the decarburized layer. The steel material of the present embodiment is suitable as a material for bearing components manufactured by heat treatment. The heat treatment referred to here includes quenching and tempering, nitriding, quenching and tempering.

[Chemical Composition of Steel Core]
The chemical composition of the steel core of this embodiment contains the following elements.

C: 0.80-1.10%
Carbon (C) increases the strength of steel. If the C content is less than 0.80%, even if the content of other elements is within the range of the present embodiment, it is not possible to obtain the core hardness required for the heat-treated bearing component. On the other hand, if the C content exceeds 1.10%, coarse carbides tend to form during production, and it is difficult to dissolve the carbides during the subsequent heat treatment. Since this carbide is coarse, it reduces the life of the bearing part due to flaking originating from the surface. Therefore, the C content is 0.80-1.10%. A preferred lower limit for the C content is 0.85%, more preferably 0.90%. A preferable upper limit of the C content is 1.07%, more preferably 1.05%.

Si: 0.15-0.50%
Silicon (Si) increases the strength of the steel material, and increases the life of the bearing component, when the steel material is used as the bearing component, for surface-initiated flaking. If the Si content is less than 0.15%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Si content exceeds 0.50%, the amount of retained austenite in the core portion of the component increases and the shape stability decreases even if the content of other elements is within the range of the present embodiment. If the Si content exceeds 0.50%, the decarburization depth becomes deeper. In this case, cracking during hot forging of steel cannot be suppressed. Therefore, the Si content is 0.15-0.50%. A preferred lower limit for the Si content is 0.18%, more preferably 0.20%. A preferable upper limit of the Si content is 0.45%, more preferably 0.40%, and still more preferably 0.35%.

Mn: 0.30-0.70%
Manganese (Mn) enhances the hardenability of steel and enhances the life of surface-initiated flaking of a bearing component when the steel material is used as the bearing component. If the Mn content is less than 0.30%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mn content exceeds 0.70%, the amount of retained austenite in the core of the component increases and the shape stability decreases even if the contents of other elements are within the ranges of the present embodiment. Therefore, the Mn content is 0.30-0.70%. A preferred lower limit for the Mn content is 0.32%, more preferably 0.35%. A preferable upper limit of the Mn content is 0.65%, more preferably 0.60%.

Cr: 1.30-1.60%
Chromium (Cr) increases the hardenability of steel. Cr further promotes the formation of fine precipitates of V, Nb, and Ti during heat treatment when the steel material is used for bearing parts. As a result, the surface originating flaking life of the bearing component is increased. If the Cr content is less than 1.30%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Cr content exceeds 1.60%, the stability of cementite increases and coarse cementite tends to form. This coarse cementite tends to be the origin of surface-initiated flaking. As a result, the surface initiated flaking life of the bearing component is reduced. Therefore, the Cr content is 1.30-1.60%. A preferred lower limit for the Cr content is 1.33, more preferably 1.35%. A preferable upper limit of the Cr content is 1.55%, more preferably 1.50%.

Mo: 0.10-0.50%
Molybdenum (Mo) increases the hardenability of steel. Mo also promotes the formation of fine precipitates of V, Nb, and Ti during heat treatment when the steel material is used as a bearing component. As a result, the surface originating flaking life of the bearing component is increased. If the Mo content is less than 0.10%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Mo content exceeds 0.50%, Mo carbide (Mo ₂ C) is stabilized and remains even at high temperatures. As a result, a sufficient number of precipitation sites for fine precipitates such as V carbonitrides, Nb carbonitrides, and Ti carbonitrides cannot be obtained. Therefore, the Mo content is 0.10-0.50%. A preferred lower limit for the Mo content is 0.13%, more preferably 0.15%. A preferable upper limit of the Mo content is 0.48%, more preferably 0.46%, and still more preferably 0.44%.

V: 0.12-0.50%
Vanadium (V) increases the hardenability of steel. Furthermore, when steel is used as a bearing component, V forms fine V carbonitrides and the like in the bearing component. As a result, the surface originating flaking life of the bearing component is increased. If the V content is less than 0.12%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the V content exceeds 0.50%, coarse V carbonitrides and the like are formed even if the content of other elements is within the range of the present embodiment. Coarse V carbonitrides and the like remain in the bearing component after the heat treatment, and become starting points of fatigue fracture. As a result, the surface initiated flaking life of the bearing component is reduced. Therefore, the V content is 0.12-0.50%. A preferable lower limit of the V content is 0.15%, more preferably 0.17%. A preferable upper limit of the V content is 0.48%, more preferably 0.45%.

Al: 0.005-0.050%
Aluminum (Al) deoxidizes steel. Al further forms AlN, which suppresses coarsening of austenite grains in the steel material during heating during quenching during the manufacturing process of the bearing component. As a result, the surface initiated flaking life of the bearing component is increased. If the Al content is less than 0.005%, the above effect cannot be sufficiently obtained even if the content of other elements is within the range of the present embodiment. On the other hand, if the Al content exceeds 0.050%, coarse alumina-based oxides are formed even if the content of other elements is within the range of the present embodiment. A coarse alumina-based oxide becomes a starting point of fatigue. As a result, the surface originating flaking life of the bearing component is reduced. Therefore, the Al content is 0.005-0.050%. A preferable lower limit of the Al content is 0.010%, more preferably 0.013%, and still more preferably 0.015%. A preferable upper limit of the Al content is 0.045%, more preferably 0.040%. In this embodiment, the Al content means the total Al content.

P: 0.020% or less Phosphorus (P) is an unavoidable impurity. That is, the P content is over 0%. P segregates at grain boundaries. As a result, the grain boundary strength is lowered. If the P content exceeds 0.020%, even if the content of other elements is within the range of the present embodiment, excessive segregation of P occurs at the grain boundaries and the grain boundary strength significantly decreases. As a result, when steel is used as the bearing component, the surface originating flaking life of the bearing component is reduced. Therefore, the P content is 0.020% or less. A preferred upper limit of the P content is 0.015%, and a more preferred upper limit is 0.012%. The lower the P content is, the better. However, excessive reduction of the P content raises production costs. Therefore, considering normal industrial production, the preferable lower limit of the P content is 0.001%, more preferably 0.002%, and still more preferably 0.004%.

S: 0.010% or less Sulfur (S) is an unavoidable impurity. That is, the S content is over 0%. S forms sulfide inclusions. Coarse sulfide-based inclusions serve as starting points for fatigue in bearing components. If the S content exceeds 0.010%, even if the content of other elements is within the range of the present embodiment, coarse sulfide-based inclusions are formed in the bearing part when the steel material is used as the bearing part. remain. As a result, the surface initiated flaking life of the bearing component is reduced. Therefore, the S content is 0.010% or less. A preferable upper limit of the S content is 0.008%, more preferably 0.006%. It is preferable that the S content is as low as possible. However, excessive reduction of the S content raises manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the S content is 0.001%, more preferably 0.002%.

N: 0.0150% or less Nitrogen (N) is an unavoidable impurity. That is, the N content is over 0%. N forms a solid solution in the steel material and lowers the hot workability of the steel material. If the N content exceeds 0.0150%, the hot workability of the steel is remarkably lowered even if the content of other elements is within the range of the present embodiment. Therefore, the N content is 0.0150% or less. A preferable upper limit of the N content is 0.0140%, more preferably 0.0130%. N content is preferably as low as possible. However, excessive reduction of the N content raises manufacturing costs. Therefore, considering normal industrial production, the preferred lower limit of the N content is 0.0001%, more preferably 0.0030%, and still more preferably 0.0050%.

O (oxygen): 0.0015% or less Oxygen (O) is an impurity that is inevitably contained. That is, the O content is over 0%. O combines with other elements in steel to form coarse oxides. Coarse oxides serve as starting points for fatigue of bearing components. If the O content exceeds 0.0015%, even if the content of other elements is within the range of the present embodiment, when the steel material is used as a bearing part, coarse particles such as alumina and TiO ₂ are contained in the bearing part. Oxides remain. As a result, the surface initiated flaking life of the bearing component is reduced. Therefore, the O content is 0.0015% or less. A preferable upper limit of the O content is 0.0013%, more preferably 0.0011%. It is preferable that the O content is as low as possible. However, excessive reduction of O content raises production costs. Therefore, considering normal industrial production, the preferred lower limit of the O content is 0.0001%, more preferably 0.0005%.

The remainder of the chemical composition of the steel according to this embodiment consists of Fe and impurities. Here, the impurities are those that are mixed from ore, scrap, or the manufacturing environment as raw materials when the steel material is industrially manufactured, and are within a range that does not adversely affect the steel material of the present embodiment. means acceptable.

[Regarding optional elements]
The chemical composition of the steel material of the present embodiment further replaces part of Fe with Ca: 0.0050% or less, Cu: 0.50% or less, Ni: 0.50% or less, Nb: 0.050% or less , and Ti: 0.050% or less. These elements are optional elements, and all of them increase the life of the bearing part against surface-initiated flaking when the steel material is used as the bearing part.

Ca: 0.0050% or less Calcium (Ca) may not be contained. That is, the Ca content may be 0%. When it is contained, Ca forms a solid solution in the sulfide-based inclusions in the steel material and spheroidizes the sulfide-based inclusions. When contained, Ca further enhances the deformation resistance of sulfide inclusions at high temperatures and maintains the spheroidization of sulfide inclusions even after hot working. When it is contained, Ca further dissolves in alumina-based oxides and suppresses agglomeration of alumina-based oxides in molten steel. Therefore, the formation of coarse alumina-based oxides is suppressed. As a result, when the steel material is used as the bearing component, the life of the surface-originating flaking of the bearing component is increased. If even a little Ca is contained, the above effect can be obtained to some extent. However, if the Ca content exceeds 0.0050%, coarse oxides are formed even if the content of other elements is within the range of the present embodiment. In this case, when a steel material is used as a bearing component, the surface originating flaking life of the bearing component is reduced. Therefore, the Ca content is between 0 and 0.0050%, and when included, the Ca content is 0.0050% or less. That is, if included, the Ca content is greater than 0 to 0.0050%. The lower limit of the Ca content is preferably 0.0001%, more preferably 0.0003%, still more preferably 0.0005%. A preferable upper limit of the Ca content is 0.0045%, more preferably 0.0040%.

Cu: 0.50% or less Cu is an optional element and may not be contained. That is, the Cu content may be 0%. When contained, Cu enhances the hardenability of the steel material and enhances the strength of the steel material. As a result, when the steel material is used as the bearing component, the surface originating flaking life of the bearing component increases. If even a small amount of Cu is contained, the above effects can be obtained to some extent. However, if the Cu content exceeds 0.50%, the nitriding property is lowered during the surface hardening heat treatment. Therefore, the Cu content is 0-0.50%, and if included, the Cu content is 0.50% or less. That is, if included, the Cu content is greater than 0 to 0.50%. A preferred lower limit for the Cu content is 0.01%, more preferably 0.02%. A preferable upper limit of the Cu content is 0.30%, more preferably 0.20%.

Ni: 0.50% or less Nickel (Ni) is an optional element and may not be contained. That is, the Ni content may be 0%. When contained, Ni enhances the hardenability of the steel material and enhances the strength of the steel material. As a result, when the steel material is used as the bearing component, the surface originating flaking life of the bearing component increases. If Ni is contained even in a small amount, the above effect can be obtained to some extent. However, if the Ni content exceeds 0.50%, the nitriding property is lowered during the surface hardening heat treatment. Therefore, the Ni content is 0-0.50%, and if included, the Ni content is 0.50% or less. That is, if included, the Ni content is greater than 0 to 0.50%. A preferable lower limit of the Ni content is 0.01%, more preferably 0.02%. A preferable upper limit of the Ni content is 0.30%, more preferably 0.20%.

Nb: 0.050% or less Niobium (Nb) is an optional element and may not be contained. That is, the Nb content may be 0%. When contained, Nb combines with C and N in steel to form Nb carbonitrides and the like. Nb carbonitride and the like suppress coarsening of crystal grains and increase the strength of the bearing component. As a result, the surface initiated flaking life of the bearing component is increased. If even a small amount of Nb is contained, the above effect can be obtained to some extent. However, if the Nb content exceeds 0.050%, Nb carbonitrides and the like become coarse. In this case, even if the contents of the other elements are within the range of the present embodiment, the surface originating flaking life of the bearing component is reduced. Therefore, the Nb content is 0-0.050%, and if included, the Nb content is 0.050% or less. That is, if included, the Nb content is greater than 0 to 0.050%. A preferable lower limit of the Nb content is 0.0001%, more preferably 0.0050%, and still more preferably 0.010%. A preferable upper limit of the Nb content is 0.045%, more preferably 0.040%.

Ti: 0.050% or less Titanium (Ti) is an optional element and may not be contained. That is, the Ti content may be 0%. When contained, Ti combines with C and N in steel to form Ti carbonitrides and the like. These precipitates suppress the coarsening of crystal grains and increase the strength of the bearing component. As a result, the surface initiated flaking life of the bearing component is increased. If even a small amount of Ti is contained, the above effect can be obtained to some extent. However, if the Ti content exceeds 0.050%, Ti carbonitrides and the like become coarse. Also, when a large amount of oxygen is contained, coarse Ti oxide (TiO ₂ ) is generated. In these cases, even if the contents of the other elements are within the range of the present embodiment, the life of the bearing component due to surface-initiated flaking is reduced. Therefore, the Ti content is 0-0.050%, and if included, the Ti content is 0.050% or less. That is, if included, the Ti content is greater than 0 to 0.050%. A preferable lower limit of the Ti content is 0.001%, more preferably 0.005%, and still more preferably 0.010%. A preferable upper limit of the Ti content is 0.045%, more preferably 0.040%.

[Regarding formulas (1) to (2)]
Further, the chemical composition of the steel material of the present embodiment has a content of each element within the above range and satisfies the following formulas (1) and (2).
1.40<Cr+0.1Mo+0.25(V+Nb+Ti)≦1.70 (1)
15Si+35Mn+15Ni+20Cr≦55 (2)
Here, the content of the corresponding element is substituted for each element symbol in the formulas (1) and (2) in mass%. If the corresponding element is not contained, "0" is substituted for the element symbol.

[Regarding formula (1)]
Define F1=Cr+0.1Mo+0.25(V+Nb+Ti). F1 is a precipitation nucleation site of fine V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and these composite carbonitrides, etc. during heat treatment when manufacturing bearing parts from steel materials. is an indicator. As described above, Cr and Mo promote the formation of precipitation nucleation sites for V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof. Specifically, Cr is V carbonitride, etc., Nb carbonitride, etc., Ti carbonitride, etc., and in a temperature range lower than the temperature range where these composite carbonitrides, etc. are generated, Fe such as cementite It produces sintered carbides or Cr carbides. Mo converts Mo carbide (Mo ₂ C) in a temperature range lower than the temperature range in which V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and these composite carbonitrides, etc. are generated. Generate. As the temperature rises, Fe-based carbides, Cr carbides, and Mo carbides form a solid solution to form V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof. becomes a precipitation nucleation site for

If F1 is 1.40 or less, each element content is within the range of the present embodiment, and even if the formula (2) is satisfied, any one of V, Nb, Ti, Cr and Mo is insufficient. ing. In this case, precipitation nucleation sites for V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof are insufficient. Alternatively, the V, Nb and Ti contents necessary for producing V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and these composite carbonitrides, etc. are Cr and Mo contents insufficient for Therefore, during the heat treatment, fine V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof, etc. are not sufficiently formed. As a result, it is not possible to obtain a sufficient surface-originating flaking life in the bearing component.

On the other hand, if F1 is greater than 1.70, precipitation nucleation sites are excessively generated even if the content of each element is within the range of the present embodiment and the formula (2) is satisfied. In this case, V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof, etc. are excessively generated in the steel material. As a result, when a bearing component is manufactured using steel as a raw material, V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and these composite carbonitrides, etc., are produced during heating during hot working. coarsens without solid solution. Coarse V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof, etc. remain in the bearing component after the heat treatment process. As a result, the surface initiated flaking life of the bearing component is reduced.

If F1 is higher than 1.40 and 1.70 or less, the steel material and In the bearing component, coarsening of V carbonitrides, Nb carbonitrides, Ti carbonitrides, and composite carbonitrides thereof is suppressed. Further, fine V carbonitrides, etc., Nb carbonitrides, etc., Ti carbonitrides, etc., and composite carbonitrides thereof, etc. are sufficiently formed on the surface layer of the bearing component. As a result, the bearing component has an excellent surface-initiated exfoliation life even when foreign matter is mixed in the bearing component.

A preferred lower limit for F1 is 1.41, more preferably 1.43, and even more preferably 1.45. The upper limit of F1 is preferably less than 1.70, more preferably 1.67, still more preferably 1.65, still more preferably 1.63. Note that the value of F1 is a value obtained by rounding off to the third decimal place.

[Regarding formula (2)]
Define F2=15Si+35Mn+15Ni+20Cr. F2 is an index of the shape stability during use of a bearing component made of steel. Among the elements in the chemical composition described above, the content of Si, Mn, Cr, and Ni in particular is the amount of retained austenite produced in bearing parts obtained by hot working, cutting, and heat-treating a steel material. affect.

If F2 is greater than 55, the bearing component after heat treatment contains a large amount of retained austenite even if the content of each element is within the range of the present embodiment and the formula (1) is satisfied. This reduces the shape stability of the bearing component during use. If F2 is 55 or less, the content of each element is within the range of the present embodiment, and the content of retained austenite is reduced in the core portion of the bearing component on the premise that the formula (1) is satisfied. can do. As a result, the bearing component has excellent shape stability even when high surface pressure is repeatedly applied to the bearing component.

A preferred upper limit for F2 is 55, more preferably 54, and even more preferably 53. The lower limit of F2 is not particularly limited. However, considering the lower limit of the content of each element in the chemical composition of this embodiment, the lower limit of F2 is preferably 39, more preferably 41, and even more preferably 43. Note that the value of F2 is a value obtained by rounding off to the first decimal place.

[Regarding the microstructure of the steel core]
In the steel material according to this embodiment, the microstructure of the steel material core is not particularly limited. The steel material according to the present embodiment has excellent hot workability, and when a bearing component is manufactured, it has an excellent surface-initiated flaking life even when foreign matter is mixed in, and is excellent even when high surface pressure is repeatedly applied. and shape stability. In other words, the steel material according to the present embodiment has properties required for the steel material during hot forging or after hot forging. Normally, during hot forging, the steel is heated to A _{c 3} point or higher, and the microstructure of the steel material transforms into austenite during hot forging. Therefore, the steel material according to the present embodiment has the above effects regardless of the microstructure of the steel material core.

[Decarburized layer]
As described above, the steel material according to the present embodiment includes a decarburized layer and a steel material core inside the decarburized layer. The decarburized layer forming the steel material according to the present embodiment is a layer with a reduced C content formed by oxidizing the surface portion of the steel material. Since the decarburized layer has a reduced C content, the hardness is low. That is, if a decarburized layer is formed on the surface layer, surface flaws are likely to be formed on the surface layer of the steel material due to contact between the steel material and a conveying device or the like in a high temperature state. In particular, if the decarburized layer is formed deeply, surface flaws are likely to be formed deeply.

[Regarding formula (3)]
In the steel material of the present embodiment, the chemical composition of the steel core satisfies the above-described ranges and also satisfies the formulas (1) and (2), and the depth of the decarburized layer of the steel material and the depth of the steel material perpendicular to the longitudinal direction of the steel material and the diameter of the cross section satisfy the formula (3).
d/D<0.010 (3)
Here, the depth of the decarburized layer in mm is substituted for d in the formula (3), and the diameter of the cross section perpendicular to the longitudinal direction of the steel material is substituted for D in mm.

F3 (=d/D) is an index that indicates the likelihood of cracking when hot forging a steel material. That is, F3 is an index showing the hot workability of steel. When F3 is 0.010 or more, the hot workability of the steel is remarkably lowered. Therefore, the steel material according to the present embodiment has F3 less than 0.010 on the premise that it has the chemical composition described above that satisfies the formulas (1) and (2). As a result, the steel material according to this embodiment has excellent hot workability.

A preferable upper limit of F3 is 0.009, more preferably 0.008, and still more preferably 0.006. A smaller F3 is preferable. Therefore, the lower limit of F3 is not particularly limited. That is, the lower limit of F3 may be 0.000. Note that the value of F3 is a value obtained by rounding off to the fourth decimal place.

[Method for measuring depth d of decarburized layer]
In this embodiment, the depth d of the decarburized layer can be obtained by the following method. A test piece is taken from the steel material according to the present embodiment. Specifically, a cross section perpendicular to the longitudinal direction of the steel material is used as an observation surface, and a test piece including the observation surface and the outer surface is sampled. The position where the test piece is taken is not particularly limited, and for example, it is taken from the position 300 mm from the end of the steel material. The size of the test piece is not particularly limited, and is, for example, 10 mm long, 10 mm wide, and 10 mm high. The observation surface of the test piece is polished to a mirror finish.

The carbon concentration distribution of the mirror-polished test piece is investigated using EPMA (Electron Probe Micro Analyzer). The measurement point is a position including the outer surface from the inner side toward the outer surface on the mirror-polished observation surface. The measurement length is not particularly limited, and is 2 mm, for example. In EPMA, the beam diameter is 1 μm and the scanning speed is 100 μm/min. A decarburized region is defined as a region in which the carbon concentration in the surface layer of the steel is 30% or more lower than the average carbon concentration in the core of the steel. The depth of the defined decarburized region is obtained and defined as "decarburized layer depth d".

[Uses of steel]
The steel material of the present embodiment is suitable as a material for bearing components, as described above. The steel material of this embodiment is particularly suitable for small bearing parts for automotive applications. Steel, which is used as a raw material for small bearing parts, is required to have characteristics (hot workability) that make it difficult for cracks to occur during hot forging. In addition, small bearing parts for automobiles are required to have excellent surface-initiated flaking life even in the presence of foreign matter. Small bearing parts for automobiles are also required to have characteristics (shape stability) that do not easily deform or change shape even when high surface pressure is repeatedly applied during use. The steel material according to this embodiment has excellent hot workability, and when small bearing parts for automobiles are manufactured, even when foreign matter is mixed in the bearing parts, the steel material has an excellent surface-originating flaking life and It has excellent shape stability even when high surface pressure is repeatedly applied. Therefore, the steel material according to this embodiment is suitable as a material for small bearing parts for automobiles. It is of course possible to use the steel material according to this embodiment as a material for medium-sized and large-sized bearing components.

[Manufacturing method of steel]
An example of the method for manufacturing the steel material of the present embodiment will be described. The steel material manufacturing method described below is an example for manufacturing the steel material of the present embodiment. Therefore, the steel material having the above configuration may be manufactured by a manufacturing method other than the manufacturing method described below. However, the manufacturing method described below is a preferred example of the steel material manufacturing method of the present embodiment.

An example of the steel material manufacturing method of the present embodiment includes a material preparation step of preparing a material and a hot working process of hot working the material to manufacture the steel material. Each step will be described below.

[Material preparation process]
In the material preparation step, the material for the steel material of the present embodiment is prepared. Specifically, molten steel is produced in which the content of each element in the chemical composition is within the range of the present embodiment and which satisfies the formulas (1) and (2). A refining method is not particularly limited, and a well-known method may be used. For example, molten iron produced by a known method is subjected to refining (primary refining) in a converter. Well-known secondary refining is performed on the molten steel tapped from the converter. In the secondary refining, alloying elements are added to the molten steel to adjust the composition, and molten steel having a chemical composition in which the content of each element is within the range of the present embodiment and which satisfies the formulas (1) and (2) is obtained. manufacture.

Using the molten steel produced by the refining method described above, a raw material is produced by a well-known casting method. For example, an ingot is manufactured by an ingot casting method using molten steel. Alternatively, a bloom or a billet may be produced by continuous casting using molten steel. A raw material (ingot, bloom or billet) is manufactured by the above method.

[Hot working process]
In the hot working process, the material (ingot, bloom or billet) prepared in the material preparing process is hot worked to manufacture the steel material of the present embodiment. Steel materials are steel bars or wire rods. The hot working process includes a blooming rolling process and a finish rolling process.

In the hot working process, after heating the raw material, hot working (blooming rolling and finish rolling) is performed. The material is heated by holding the material in a heating furnace (heat treatment furnace). Here, in the steel material manufacturing method of the present embodiment, the depth of the decarburized layer may be affected by the heating in the hot working process. Specifically, when the heating temperature is high and the heating time is long, a deep decarburized layer tends to be formed. In this case, furthermore, if the decarburized layer is formed deeply during heating and holding in the hot working step, surface flaws are likely to be formed during hot working. Surface flaws formed during hot working can become starting points for hot forging cracks. Therefore, it is preferable to be able to reduce the depth of the decarburized layer during heating and holding in the hot working process. The blooming rolling process and the finish rolling process will be described below.

[Blooming rolling process]
In the blooming process, the raw material is bloomed to produce a billet. When a continuous rolling mill is arranged downstream of the blooming mill, the billet after blooming is further hot-rolled using the continuous rolling mill to produce a smaller billet. may In a continuous rolling mill, horizontal stands with a pair of horizontal rolls and vertical stands with a pair of vertical rolls are alternately arranged in a row. As described above, in the blooming process, the raw material is manufactured into billets by using a blooming mill or by using a blooming mill and a continuous rolling mill.

In this embodiment, the heating temperature in the heating furnace in the blooming step is preferably 1150 to 1300°C. Further, in the present embodiment, the holding time in the heating furnace in the blooming step is preferably 2 to 12 hours. If the holding time is too long, a deep decarburized layer is formed on the surface of the billet. The decarburized layer thus formed remains on the surface of the steel material. That is, if the holding time is too long, the depth of the decarburized layer of the steel material becomes too deep with respect to the diameter of the cross section perpendicular to the longitudinal direction of the manufactured steel material. As a result, the manufactured steel does not satisfy the formula (3). On the other hand, if the holding time is too short, the billet may not be sufficiently soaked. Therefore, in the blooming rolling process according to the present embodiment, the holding time is set to 2 to 12 hours.

[Finish rolling process]
In the finish rolling process, the billet produced in the blooming process is subjected to finish rolling to produce a steel material (steel bar or wire rod). Finish rolling is performed using a continuous rolling mill. Preferably, in the present embodiment, when the heating temperature in the heating furnace in the finish rolling step is T° C. and the holding time in the heating furnace is t hours, the following conditions are satisfied.
T (°C): 1000-1300 (°C)
t (time): 1 to 10 (time)
-50t+1250≤T≤-50t+1500 (A)

Define T _L =−50t+1250. Define T _H =−50t+1500. If the heating temperature T in the heating furnace in the finish rolling step is lower than _TL , the billet may not be sufficiently soaked even if T and t satisfy the above ranges. On the other hand, if the heating temperature T in the heating furnace in the finish rolling process is higher than _TH , even if T and t satisfy the above ranges, a decarburized layer may be formed deeply on the surface of the billet. . In other words, the depth of the decarburized layer of the steel material becomes too deep with respect to the diameter of the cross section perpendicular to the longitudinal direction of the manufactured steel material, and the manufactured steel material may not satisfy the formula (3). Therefore, in the finish rolling process according to the present embodiment, the heating temperature T in the heating furnace satisfies 1000 to 1300 ° C., the holding time t in the heating furnace satisfies 1 to 10 hours, and T and t satisfy the formula ( A) is preferably satisfied.

The steel material after the finish rolling process is cooled. In the finish rolling process according to this embodiment, the cooling rate is not particularly limited. For example, the cooling in the finish rolling step may be standing cooling.

Through the above-described manufacturing process, the steel material of the present embodiment having the above-described configuration can be manufactured. In addition, well-known normalizing treatment or well-known spheroidizing annealing treatment may be performed on the steel material after the hot working process.

[Regarding bearing parts]
By bearing part is meant a part of a rolling bearing. Bearing parts are, for example, bearing rings, washer, rolling elements and the like. The bearing ring may be an inner ring or an outer ring, and the washer may be a shaft washer, a housing washer, a center washer, or an alignment housing washer. The bearing rings and bearing washers are not particularly limited as long as they are members having raceway surfaces. The rolling elements may be balls or rollers. Rollers are, for example, cylindrical rollers, bar rollers, needle rollers, tapered rollers, convex rollers, and the like.

The steel material of this embodiment is suitable as a material for bearing components. A bearing component using the steel material of this embodiment is manufactured by a well-known manufacturing method. For example, a bearing component made of the steel material of the present embodiment is manufactured by the following manufacturing method.

A method of manufacturing a bearing component includes, for example, a hot working process, a spheroidizing annealing process, a cutting process, and a heat treatment process. In the hot working step, hot working is performed on the steel material of the present embodiment. Hot working is, for example, well-known hot forging. In the hot working step, the steel is worked after being heated to _Ac3 point or higher. Therefore, the microstructure of the steel is reset during heating during the hot working process. The heating temperature is a well-known temperature, for example, 1000-1300.degree. The steel material after hot working is air-cooled.

A well-known spheroidizing annealing process is performed on the steel material after the hot working process. Then, the steel material after the spheroidizing annealing process is subjected to a cutting process to manufacture an intermediate product having a predetermined shape. During this cutting process, the steel material is required to have high machinability. In the cutting process, well-known cutting is performed. An intermediate product is manufactured by the above steps.

A heat treatment process is performed on the intermediate product after the cutting process. In the present embodiment, "heat treatment" means quenching and tempering, carburizing and quenching and tempering, carbonitriding and quenching and tempering, or nitriding and quenching and tempering, as described above. Any heat treatment may be performed under well-known conditions. Quenching conditions, carburizing and quenching conditions, carbo-nitriding and quenching conditions, nitriding and quenching conditions, and tempering conditions are appropriately adjusted to determine the surface hardness and core hardness of the bearing parts, the surface carbon concentration of the bearing parts, and the bearing It is a technical matter well known to those skilled in the art that the surface nitrogen concentration of parts can be appropriately adjusted. The intermediate product undergoes the well-known heat treatment described above to produce the bearing component.

A bearing component made of the steel material of the present embodiment is excellent in surface-originating flaking life. The bearing component made of the steel material of this embodiment is further excellent in shape stability during use.

In the manufacturing process of the bearing component, the intermediate product after the heat treatment process may be subjected to finishing (cutting, polishing, etc.). By finishing, the surface roughness of the intermediate product after the surface hardening treatment can be adjusted. A finishing method may be implemented by a well-known method.

[Regarding the microstructure of the core part of the bearing part]
However, in the present embodiment, when a steel material is used as a bearing component, cutting is performed on the steel material that has been hot forged and then spheroidized to produce an intermediate product. The intermediate product is subjected to heat treatment (quenching and tempering, carburizing, carbonitriding, nitriding, etc.). Therefore, during hot forging and heat treatment, the steel material is heated to an austenite region or an austenite-cementite two-phase region, and then rapidly cooled. Therefore, the microstructure of the component core of the bearing component according to this embodiment is a martensite single-phase structure or a mixed structure of martensite and cementite.

2 tons of molten steel having the chemical composition shown in Table 1 was produced by vacuum melting. From the chemical composition of the molten steel of each test number and the definitions of F1 (=Cr + 0.1Mo + 0.25 (V + Nb + Ti)) and F2 (= 15Si + 35Mn + 15Ni + 20Cr) described above, the values of F1 and F2 for each test number were obtained. Table 1 shows the obtained F1 and F2 values.

"-" in Table 1 means that the content of the corresponding element was at the impurity level. For example, it means that the Nb and Ti contents of Steel A were 0% rounded to the fourth decimal place. It means that the Ca content of steel A was 0% rounded to the fifth decimal place. The Cu and Ni contents of Steel C are meant to be 0% rounded to the third decimal place. It means that the Mo and V contents of Steel AE were 0% rounded to the third decimal place. Steel AE was molten steel having a chemical composition corresponding to SUJ2 defined in JIS G 4805 (2019), which is a conventional steel material, as a comparative reference steel material.

Molten steel in Table 1 was cast to produce an ingot as a raw material. A hot working process was performed on an ingot as a raw material. Specifically, the ingots of each test number were heated and bloomed by a blooming mill to produce billets. The cross section perpendicular to the longitudinal direction of the billet of each test number was a rectangle of 160 mm x 160 mm. The manufactured billet was allowed to cool to room temperature. Table 2 shows the heating conditions in the blooming process. The heating conditions in the blooming rolling process and the heating conditions in the finish rolling process, which will be described later, which were performed on the ingots of each test number are shown in the "Manufacturing conditions" column of Table 3.

Finish rolling was performed on the manufactured billet of each test number to manufacture a steel material of each test number. Specifically, the billet of each test number was heated and hot-rolled using a continuous rolling mill to produce a steel material (round bar) of each test number having a diameter D (mm). Table 2 shows the heating conditions in the finish rolling process. Furthermore, Table 2 shows T _L (=-50t+1250) and T _H (=-50t+1500) under each condition of the finish rolling process. Table 3 shows the diameter D (mm) of the obtained steel material of each test number. The round bar after hot rolling was allowed to cool to room temperature. A steel material (round bar) of each test number was manufactured by the manufacturing process described above.

[Evaluation test]
A steel surface layer decarburization depth evaluation test, a hot workability evaluation test, a surface originating exfoliation life evaluation test, and a shape stability evaluation test were performed on the steel materials manufactured by the above manufacturing processes. The surface-originating flaking life evaluation test and the shape stability evaluation test were performed only on steel materials with test numbers 1 to 34 (round bars with a diameter of 80 mm), and steel materials with test numbers 35 and 36 (round bars with a diameter of 40 mm). bar) was not performed.

[Steel material surface layer decarburization depth evaluation test]
The depth of the decarburized layer in the surface layer of the steel material (80 mm diameter or 40 mm diameter round bar) of each test number was evaluated. Specifically, the cross section obtained by crossing the steel material of each test number at a position of 300 mm from the end face was defined as the observation surface. From the crossed steel material of each test number, four test pieces including the observation surface and the outer surface were cut out every 90 degrees. The size of the test piece was 10 mm long, 10 mm wide and 10 mm high. One surface of the test piece was a curved surface including the outer surface, and the other five surfaces were flat surfaces. The observation surface of the cut test piece was polished to a mirror surface.

The carbon concentration distribution was investigated using EPMA on the mirror-polished test piece. The measurement point was a position including the outer surface from the inner side toward the outer surface on the mirror-polished observation surface. The measured length was about 2 mm. In EPMA, the beam diameter was 1 μm and the scanning speed was 100 μm/min. A decarburized region was defined as a region in which the carbon concentration in the surface layer of the steel material was 30% or more lower than the average carbon concentration in the core of the steel material. The depth of the defined decarburized region was determined for each of the four test pieces of each test number, and the arithmetic mean value was taken as the "decarburized layer depth d". F3 was obtained from the obtained depth d (mm) of the decarburized layer for each test number and the diameter D (that is, 80 mm or 40 mm) of the cross section perpendicular to the longitudinal direction of the steel material. Table 3 shows the obtained F3 for each test number.

[Hot workability evaluation test]
A test piece for a hot workability evaluation test was taken from the steel material of each test number. A test piece was obtained by cutting the steel material in a direction perpendicular to the longitudinal direction. The length of the test piece was 2.5 times the diameter D of the cross section perpendicular to the longitudinal direction. Specifically, the steel material of each test number (80 mm diameter or 40 mm diameter round bar) is cut in a direction perpendicular to the longitudinal direction, and a test piece with a diameter of 80 mm and a length of 200 mm, or a test piece with a diameter of 40 mm and a length of 100 mm Ten strips were taken for each test number. After heating the test piece of each test number to 1000° C. and holding it for 60 minutes, it was quickly subjected to 90% hot compression in the axial direction to form a disk shape. After air-cooling the disk-shaped test piece of each test number to room temperature, the presence or absence of cracks on the surface was visually confirmed. If the number of test pieces in which cracks were confirmed was 0 out of 10, it was judged that the hot workability was excellent (indicated by "E" in the "hot workability" column in Table 3). On the other hand, if the number of test pieces in which cracks were confirmed was 1 or more out of 10, it was determined that the hot workability was insufficient (indicated by "NA" in the "hot workability" column in Table 3) ).

[Surface origin peeling life evaluation test]
A heat treatment simulating hot working was performed on the steel material of each test number. In the heat treatment, the steel material was held at 1000° C. for 1 hour and then allowed to cool (air-cooled) to room temperature. Well-known spheroidizing annealing was performed on the steel materials after standing to cool. Specifically, in the spheroidizing annealing, the steel material was held at 760°C for 8 hours. After that, it was slowly cooled to 600°C at a cooling rate of 10 to 15°C/hour. After that, the steel material was allowed to cool (air-cooled) to room temperature. A steel material (round bar) with a diameter of 80 mm after spheroidizing annealing is subjected to cutting (peripheral turning) and milling to obtain a disk-shaped rough test piece (intermediate product) with a diameter of 60 mm and a thickness of 6 mm. Simulated crude specimens) were taken. Ten rough test pieces were taken from each test number steel material.

Rough specimens of each test number were held at 800° C. for 5 hours to perform nitriding treatment. Ammonia gas was used for the nitriding treatment, and the flow rate was adjusted so that the surface nitrogen concentration of the crude test piece was 0.4%. After that, quenching was performed by cooling with oil at 60°C. The crude test piece after oil cooling was tempered at a tempering temperature of 150°C for a holding time of 120 minutes.

Each test piece after the heat treatment was flat-polished on a circular surface with a diameter of 60 mm by 30 μm on each side to prepare a test piece as a simulated bearing component. One surface of the flat-polished circular surface of each test piece was lapped to obtain a rolling contact fatigue test piece.

A rolling contact fatigue test was performed using a thrust-type rolling contact fatigue tester. Specifically, FIG. 1 is a schematic diagram of a thrust-type rolling contact fatigue tester. As shown in FIG. 1, a rolling contact fatigue test piece 100 was immersed in lubricating oil 102 . Three steel balls 101 were placed on the rolling contact fatigue test piece 100, and the jig 103 was rotated around the central axis C1 while pressing the three steel balls 101 against the rolling contact fatigue test piece 100. . The maximum contact surface pressure during the test was set to 4.0 GPa, and the repetition rate was set to 2500 cpm (cycle per minute). Lubricating oil 102 used in the test was mixed with high-speed steel gas atomized powder classified to have a Vickers hardness of 750 (HV) and a particle size of 100 to 180 μm as foreign matter. The amount of the gas atomized powder mixed was set to 0.02% by mass with respect to the lubricating oil 102 . The steel ball 101 used in the test had a diameter of 9.525 mm and was made of SUJ2 refining material specified in JIS G 4805 (2019).

The results of the rolling contact fatigue test were plotted on Weibull probability paper, and the L10 life indicating a 10% failure probability was defined as the "surface-originating flaking life." In a severe use environment (this test) in which foreign matter is mixed in, it was determined that if the L10 life is 1.0 × 10 ⁷ or more, the surface-originating flaking life is excellent (in the “surface-originating flaking life” column in Table 3, labeled with an “E”). On the other hand, when the L10 life was less than 1.0×10 ⁷ , it was judged that the surface originating flaking life was short (indicated by “NA” in the “surface originating flaking life” column in Table 3).

[Shape stability evaluation test]
For the rolling contact fatigue test piece after the rolling contact fatigue test when the L10 life was 1.0 × 10 ⁷ or more in the rolling contact fatigue test, the surface of the test piece was measured using a stylus type roughness meter. A cross-sectional curve was measured across the rolling and non-rolling portions at the same time. Then, the difference ΔH (μm) between the average height of the non-rolling portion and the average height of the rolling portion was calculated. The rolling part means the part where the test piece and the steel ball are in contact in the rolling contact fatigue test. A non-rolling portion means a portion where the test piece and the steel ball are not in contact in the rolling contact fatigue test. The cross-sectional curve was measured at four points for each test piece. The arithmetic average value of the differences ΔH at four points was defined as the deformation amount of the rolling part. If the amount of deformation of the rolling part was less than 25 μm, it was judged that the shape stability was excellent (indicated by “E” in the “shape stability” column in Table 3). On the other hand, if the amount of deformation of the rolling part was 25 μm or more, it was determined that the shape stability was insufficient (indicated by “NA” in the “shape stability” column in Table 3).

[Test results]
Table 3 shows the test results. With reference to Tables 1 to 3, the steel materials of test numbers 1 to 6 all had appropriate chemical compositions and satisfied formulas (1) and (2). Furthermore, the manufacturing method was also the preferred manufacturing method described in the specification. As a result, F3 was less than 0.010, satisfying formula (3). As a result, it showed excellent hot workability in the hot workability evaluation test. As a result, in the surface-initiated exfoliation life evaluation test, it exhibited an excellent surface-initiated exfoliation life. As a result, it also showed excellent shape stability in the shape stability evaluation test.

The steel material of test number 35 had an appropriate chemical composition and satisfied the formulas (1) and (2). Furthermore, the manufacturing method was also the preferred manufacturing method described in the specification. As a result, F3 was less than 0.010, satisfying formula (3). As a result, it showed excellent hot workability in the hot workability evaluation test.

On the other hand, the steel material of test number 7 had too low a C content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 8 had too high a C content. Furthermore, F2 exceeded 55 and did not satisfy equation (2). As a result, it did not exhibit excellent shape stability in the shape stability evaluation test.

The steel material of test number 9 had too low a Si content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 10 had too high Si content. Furthermore, F2 exceeded 55 and did not satisfy equation (2). Furthermore, F3 was 0.010 or more, and the formula (3) was not satisfied. As a result, it did not exhibit excellent hot workability in the hot workability evaluation test. As a result, it did not exhibit excellent shape stability in the shape stability evaluation test.

The steel material of test number 11 had too low Mn content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 12 had too high Mn content. Furthermore, F2 exceeded 55 and did not satisfy equation (2). As a result, it did not exhibit excellent shape stability in the shape stability evaluation test.

The steel material of test number 13 had too low a Cr content. Furthermore, F1 was 1.40 or less, and the formula (1) was not satisfied. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 14 had too high a Cr content. Furthermore, F1 exceeded 1.70 and did not satisfy formula (1). Furthermore, F2 exceeded 55 and did not satisfy equation (2). As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test. As a result, it did not exhibit excellent shape stability in the shape stability evaluation test.

The steel material of test number 15 had too low Mo content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 16 had too high Mo content. Furthermore, F1 exceeded 1.70 and did not satisfy formula (1). As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 17 had too low a V content. Furthermore, F1 was 1.40 or less, and the formula (1) was not satisfied. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 18 had too high a V content. Furthermore, F1 exceeded 1.70 and did not satisfy formula (1). As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 19 had too high an Al content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 20 had too high an S content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 21 had too high a Cu content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 22 had too high a Ni content. Furthermore, F2 exceeded 55 and did not satisfy equation (2). As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 23 had too high N content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 24 had too high a Ca content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 25 had too high a Nb content. Furthermore, F1 exceeded 1.70 and did not satisfy formula (1). As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 26 had too high a Ti content. Furthermore, F1 exceeded 1.70 and did not satisfy formula (1). As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 27 had too high an O content. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 28 had too low a V content. Furthermore, F1 was 1.40 or less, and the formula (1) was not satisfied. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 29 exceeded 1.70 in F1 and did not satisfy the formula (1). As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 30 exceeded F2 of 55 and did not satisfy the formula (2). As a result, it did not exhibit excellent shape stability in the shape stability evaluation test.

The steel material of test number 31 had a chemical composition corresponding to SUJ2 defined in JIS G 4805 (2019), which is a conventional steel material. Therefore, Mo content and V content were too low. As a result, it did not exhibit an excellent surface-initiated exfoliation life in the surface-initiated exfoliation life evaluation test.

The steel material of test number 32 had too long a holding time in the blooming process. As a result, F3 was 0.010 or more, and the formula (3) was not satisfied. As a result, it did not exhibit excellent hot workability in the hot workability evaluation test.

For the steel material of test number 33, the heating temperature T in the finish rolling process was higher than _TH . As a result, F3 was 0.010 or more, and the formula (3) was not satisfied. As a result, it did not exhibit excellent hot workability in the hot workability evaluation test.

The steel material of test number 34 exceeded F2 of 55 and did not satisfy the formula (2). As a result, it did not exhibit excellent shape stability in the shape stability evaluation test.

For the steel material of test number 36, the heating temperature T in the finish rolling process was higher than _TH . As a result, F3 was 0.010 or more, and the formula (3) was not satisfied. As a result, it did not exhibit excellent hot workability in the hot workability evaluation test.

The embodiments of the present disclosure have been described above. However, the above-described embodiments are merely examples for implementing the present disclosure. Therefore, the present disclosure is not limited to the above-described embodiments, and the above-described embodiments can be modified as appropriate without departing from the scope of the present disclosure.

Claims (2)

is steel,
a decarburized layer;
and a steel core inside the decarburized layer,
The chemical composition of the steel core is, in mass %,
C: 0.80 to 1.10%,
Si: 0.15 to 0.50%,
Mn: 0.30-0.70%,
Cr: 1.30-1.60%,
Mo: 0.10-0.50%,
V: 0.12 to 0.50%,
Al: 0.005 to 0.050%,
P: 0.020% or less,
S: 0.010% or less,
N: 0.0150% or less,
O: 0.0015% or less, and
The balance consists of Fe and impurities, and satisfies formulas (1) and (2),
The depth of the decarburized layer and the diameter of the cross section perpendicular to the longitudinal direction of the steel satisfy formula (3),
steel.
1.40<Cr+0.1Mo+0.25(V+Nb+Ti)≦1.70 (1)
15Si+35Mn+15Ni+20Cr≦55 (2)
d/D<0.010 (3)
Here, the content of the corresponding element is substituted for each element symbol in the formulas (1) and (2) in mass %. If the corresponding element is not contained, "0" is substituted for the element symbol. The depth of the decarburized layer in mm is substituted for d in the formula (3), and the diameter of the cross section perpendicular to the longitudinal direction of the steel material is substituted for D in mm. The steel material according to claim 1,
The chemical composition of the steel core further includes, instead of part of Fe,
Ca: 0.0050% or less,
Cu: 0.50% or less,
Ni: 0.50% or less,
Nb: 0.050% or less, and
Ti: 0.050% or less, containing one or more selected from the group consisting of
steel.

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