JP3007834B2 - Bearing steel with excellent rolling fatigue characteristics - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bearing steel having excellent rolling fatigue characteristics for bearings such as ball bearings and roller bearings used in automobiles and various industrial machines.

[0002]

2. Description of the Related Art As bearing steel used for the above-mentioned applications, there has been conventionally used a SU as defined in JIS G 4805.
High carbon chromium bearing steels such as J2 have been mainly used. However, since these bearing steels are high carbon, elements such as C cause segregation at the center during casting, and even after lumping or hot rolling, the segregated portions become large carbides as shown in FIG. May appear. In the case where such a large carbide is generated, the huge carbide remains even after being processed into a bearing part through processes such as spheroidizing annealing, quenching and tempering, and this becomes a stress concentration source. It has been empirically confirmed that the rolling fatigue life is reduced.

[0003] In order to eliminate the center segregation generated during casting of high carbon chromium bearing steel and the giant carbide generated in the center segregation portion, for example, Japanese Patent Application Laid-Open No. 3-75312 discloses
A method of rolling a cast material into a billet and then performing a soaking process has been proposed. Japanese Patent Application Laid-Open No. 3-104819 discloses a method in which high carbon chromium bearing steel is produced by continuous casting while the internal portion is unsolidified. At this time, a method is proposed in which electromagnetic stirring is performed to control the ratio (C ₁ / C ₂ ) of the carbon content C _{1 in the} center of the slab to the carbon content C _{2 in the} surface layer to be 1.2 or less. are doing.

[0004] These methods are significant in that they aim to reduce or eliminate the above-mentioned giant carbide which has been qualitatively confirmed to affect the rolling fatigue life, but the giant carbide has a high effect. Quantitative analysis of how much the rolling fatigue life of carbon chromium bearing steel is adversely affected, and how much the abundance of the giant carbide can be suppressed to reliably increase the rolling fatigue life. The relationship is not clarified.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and its object is to quantitatively clarify the effect of the amount of a large carbide on rolling contact fatigue life. An object of the present invention is to provide a bearing steel capable of reliably exhibiting excellent rolling fatigue characteristics.

[0006]

Means for Solving the Problems The constitution of the bearing steel having excellent rolling fatigue characteristics according to the present invention which can solve the above problems is as follows: C: 0.6 to 1.2% Mn: 0.2 to 1.5% Si: 2.0% or less (including 0%) Al: 0.005 to 0.06% P: 0.03% or less (including 0%) S: 0.03% or less (0% Ti: 0.005% or less (including 0%) O: 0.0020% or less (including 0%) Balance: Fe and unavoidable impurities Thickness appearing in the center line of the longitudinal section passing through the axis, including the axis of the longitudinal section, in a central region within 1/8 · D (D represents the width of the longitudinal section) on each side from the axis. The total cross-sectional area of carbide having a thickness of 2 μm or more is 0.3% of the vertical cross-sectional area.
It has the following features.

[0007] The bearing steel according to the present invention further comprises, as other elements, Cr: 2.0% or less (excluding 0%), Ni:
2.0% or less (excluding 0%), Mo: 1.0% or less (excluding 0%), Cu: 1.0% or less (excluding 0%), V: 0.3% or less (Not including 0%), Nb:
By including at least one selected from the group consisting of 0.1% or less (not including 0%), the rolling fatigue characteristics can be further improved, and as another element, Pb: 0.1% or less (excluding 0%), Ca: 0.01% or less (excluding 0%), T
e: 0.1% or less (excluding 0%), Bi: 0.1%
By containing at least one selected from the group consisting of the following (not including 0%), machinability can be enhanced.

Further, among the alloying elements contained in the steel material, the elements of C, Mn, Cr, Ni, and Mo are adjusted so that their contents satisfy the relationship of the following equation (1). By increasing the hardenability by heating, the rolling fatigue characteristics can be further improved. [C] ^1/2 +0.12 × [Mn] + 0.11 × [Cr] + 0.05 × [Ni] + 0.03 × [Mo] ≧ 1.05 ... (1) (where [element] is each element in the steel material Represents the mass% of

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for defining the chemical composition of the steel used in the present invention and the reason for defining the area ratio of the carbide existing in the center of the longitudinal section of a linear or rod-shaped rolled material are described below. This will be described in detail. First, the reasons for determining the chemical composition of steel are clarified.

C: 0.6 to 1.2% C secures HRc 58 or more, which is the minimum hardness required for bearing steel after quenching and tempering, and provides bearings with rolling fatigue characteristics and the like. It is an indispensable element as a strengthening element for securing characteristics, and must be contained at least 0.6% or more, and preferably 0.7% or more. However, if the C content is too large, a large carbide is likely to be formed in the core portion, which adversely affects the rolling fatigue characteristics, so that at most 1.2% or less, preferably 1.1% or less. Should be kept to a minimum.

Si: 2.0% or less (including 0%) Si not only effectively acts as a deacidifying element, but also
It has the effect of increasing the temper softening resistance to increase the core hardness, and these effects are effectively exhibited by containing about 0.03% or more. However, their effect is saturated at 2.0%, and if contained more than that, adverse effects appear on the cold workability and machinability, so that the content is suppressed to 2.0% or less, more preferably 1.0% or less. There must be.

Mn: 0.2 to 1.5% Mn not only functions effectively as a deoxidizing / desulfurizing agent, but also enhances the hardenability to increase the hardness of the surface layer and the core portion, thereby preventing the surface from being depressed and rolling. It is an indispensable element for improving the dynamic fatigue life, and must be contained in an amount of 0.2% or more in order to effectively exert those effects. However, these effects are saturated at 1.5%, and if contained more than 1.5%, the cold workability and machinability will be adversely affected.
It must be kept below 5%. The more preferred content of Mn is in the range of 0.2 to 1.2%.

Al: 0.005 to 0.06% Al not only functions effectively as a deacidifying element, but also has a function of forming nitrides to refine austenite crystal grains and increase toughness. Must be contained in an amount of not less than 0.005% in order to exhibit the effect effectively. However, if the amount of Al is too large, austenite crystal grains are rather coarsened and the toughness is deteriorated. Therefore, the content must be suppressed to 0.06% or less. The more preferable content of Al is in the range of 0.01 to 0.04%.

P: 0.03% or less (including 0%) Since P becomes a non-metallic inclusion and adversely affects toughness, its content should be kept as low as possible, and its adverse effect is practically negligible. The upper limit is 0.03% that does not cause a problem. More preferably, it is 0.015% or less.

S: 0.03% or less (including 0%) S is an element which exists mostly in the form of MnS in steel and contributes to the improvement of machinability, but in steel materials having a small oxygen content, Has a significant adverse effect on the rolling fatigue life, and must be suppressed to 0.03% or less. More preferably, it is 0.015% or less.

Ti: 0.005% (including 0%) Ti combines with N slightly mixed in steel to form TiN, which not only adversely affects rolling fatigue characteristics, but also
It is a harmful element that also impairs cold workability and hot workability, and must be suppressed to 0.005% or less, more preferably 0.004% or less to avoid such obstacles.

O: 0.0020% or less (including 0%) O combines with Al to cause deterioration of rolling contact fatigue characteristics.
l to form a ₂ O ₃ inclusions, and because also adversely affect cold workability and hot workability, 0.0020% or less, more preferably to be suppressed to 0.0015% or less.

The remaining components of the bearing steel according to the present invention are Fe and unavoidable impurities. However, if necessary, the following elements can be added in appropriate amounts to further improve the characteristics of the bearing steel. It is possible.

Cr: 2.0% or less (excluding 0%),
Ni: 2.0% or less (excluding 0%), Mo: 1.0
% Or less (not including 0%), Cu: 1.0% or less (0%
), V: 0.3% or less (excluding 0%),
Nb: from the group consisting of 0.1% or less under (0%)
At least one of these elements is the same element in that all of these elements contribute to the improvement of the rolling fatigue life. That is, Cr, Ni, M
Both o and Cu act as hardenability improving elements, increase the hardness of the surface layer and the core, and contribute to the improvement of rolling fatigue characteristics and the suppression of surface depression, and V and Nb represent C in steel.
In combination with N and N, a carbonitride is generated, and the crystal grains are refined to contribute to the improvement of rolling fatigue life. Of these, Cr, N
i and Mo effectively act in facilitating quenching and tempering in parts having a large mass by acting as hardenability improving elements. Further, Cu has an effect of increasing corrosion resistance in addition to the improvement of hardenability, and V and Nb also exert an effect of increasing toughness by the above-mentioned crystal grain refining action. These effects are at least 0.2% with Cr and 0.1% with Ni.
25% or more, 0.08% or more of Mo, 0.25% of Cu
As described above, the effect is effectively exhibited by containing 0.01% or more of V and 0.01% or more of Nb.

However, when the Cr content exceeds 2.0%, huge Cr carbides are easily formed, and when the Ni content exceeds 2.0% or the Mo content exceeds 1.0%, cold workability and In addition to poor machinability, a large amount of retained austenite is formed after quenching and tempering, resulting in poor dimensional stability.
If it exceeds 0%, cold workability and machinability are reduced, and red hot brittleness is promoted and hot work cracks are easily generated.
Each should be kept below the upper limit. Further, the effect of adding V is saturated at 0.3%, and the effect of adding Vb is saturated at 0.1%, so that further addition is economically useless.

Pb: 0.1% or less (excluding 0%),
Ca: 0.01% or less (excluding 0%), Te: 0.
1% or less (excluding 0%), Bi: 0.1% or less ( 0 %
% Excluding) selected from the group consisting of
Seed Both of these elements act as a machinability improving element, but their effect is saturated in the vicinity of the upper limit of each because the too much content the rolling fatigue life is adversely emerge, respectively Must be kept below the upper limit.

Further, in the present invention, in addition to the type and content of each of the above-mentioned contained elements, as an influential factor which governs the hardness after quenching / tempering, C, M among the above contained elements are considered.
A composition in which the contents of n, Cr, Ni, and Mo are adjusted so as to satisfy the relationship of the above formula (1) is preferable because the rolling fatigue characteristics can be further improved. In particular,
If quenching (preferably 830 to 870 ° C) and tempering (preferably 140 to 180 ° C) a steel material satisfying the relationship of the above formula (1) in an appropriate temperature range, HRc 60 or more in a state after quenching and tempering And provides a bearing steel with more excellent rolling fatigue characteristics.

The bearing steel of the present invention satisfies the above requirements for the component composition, thereby ensuring excellent quenching / tempering properties and ensuring the hardness of the surface layer and the core, and at the same time increasing the amount of carbides generated. Although the rolling fatigue characteristics are improved and rolling fatigue life is definitely improved, in addition to the requirements of these component compositions, rolling in a linear or rod shape is required. The center of the longitudinal section including the axis of the material or the drawn material (specifically, as shown in FIG. 2, each line is separated by 1/8 × D on one side with respect to the width D of the longitudinal section from the axis. It is extremely important that the total cross-sectional area of carbides having a thickness of 2 μm or more existing in the region between them is suppressed to 0.3% or less with respect to the vertical cross-sectional area.

FIG. 3 shows the effect of the total cross-sectional area ratio occupied by the carbide having a thickness of 2 μm or more existing in the center of the longitudinal section on the rolling contact fatigue life based on many experimental data including the examples described later. As is clear from this figure, the tendency of the rolling fatigue life to decrease is small until the area ratio of the carbide having a thickness of 2 μm or more reaches about 0.2%. It can be confirmed that when the value exceeds 0.3%, the rolling fatigue life sharply decreases. The reason why such a tendency was obtained is that when the carbide is a huge one having a thickness of 2 μm or more, stress concentration tends to occur in that part,
It is considered that when the area ratio exceeds 0.3%, the rolling fatigue life is remarkably reduced, and this appears.

As means for suppressing the area ratio of carbides of 2 μm or more existing at the center of the longitudinal section of the axial center portion to 0.3% or less as described above, for example, when adopting a continuous casting method, Is a method of suppressing the center segregation at the time of solidification by applying a large-diameter roll reduction at a solid phase ratio of 0.2 to 0.7 at the time of cooling and solidification, or further reducing the slab to 115
A preferred example is a method of soaking at a temperature of 0 ° C. or more for 10 hours or more. When the usual ingot making method is adopted, the slab is kept at a temperature of 1150 ° C. or more for 2 minutes.
A method of performing soaking treatment for 0 hour or more is exemplified as a preferable method.

[0026]

EXAMPLES Next, the structure and operation and effect of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and can be adapted to the spirit of the preceding and following examples. Of course, the present invention can be implemented with modifications within the scope, and all of them are included in the technical scope of the present invention.

Example A steel material having the composition shown in Table 1 was melted using a 150 kg vacuum furnace, and then cast pieces were produced by an ingot method.
65mm in diameter after soaking at 200 ° C for 10 hours
Was hot forged and subsequently subjected to a spheroidizing annealing treatment under the following conditions. (No. 2, 3 and 16) 790 ° C x 2 hours → Furnace cooling at 20 ° C / Hr up to 680 ° C →
After that, air cooling (others) 760 ° C x 2 hours → Furnace cooling to 680 ° C at 20 ° C / Hr →
Then air cooling

For each of the obtained annealing materials, a sample having a length of 10 mm from a range sandwiched by lines (see FIG. 2) separated by 1/8 × diameter on one side from the center line of the longitudinal section including the axis. Were cut out, the entire surface to be inspected was observed with a 400 × optical microscope, and the area ratio of carbide having a thickness of 2 μm or more was determined using an image analyzer.

As shown in FIG. 4, a disk having a thickness of 60 mm and a thickness of 5 mm was cut out from a longitudinal section including the axis as shown in FIG. 4, and quenching and tempering were performed, followed by lapping to reduce the surface roughness to 0.04 μm Ra or less. After that, a rolling fatigue test was performed under the following conditions (N = 20). Table 2 shows the results. (Rolling fatigue test) Surface pressure: 527 kgf / mm ² Number of rotations: 1000 rpm Number of steel balls: 6 Lubricating oil: turbine # 68

[0030]

[Table 1]

[0031]

[Table 2]

The following can be considered from Tables 1 and 2. No. Examples 1 to 16 are examples in which the component composition of the steel material satisfies all of the specified requirements of the present invention, and the area ratio of carbide having a thickness of 2 µm or more present in the central region of the longitudinal section is 0.3% or less, All have excellent rolling fatigue life. Among them, those having a calculated value of the formula (1) of 1.05 or more have relatively higher rolling fatigue life than those having a value of less than 1.05.

On the other hand, no. Nos. 17 to 24 are comparative examples lacking any of the requirements defined in the present invention, and have problems in performance as bearing steel as described below. No. 17: No giant carbide was recognized, but the content of C was small and the value of equation (1) was extremely low, so that satisfactory rolling fatigue characteristics could not be obtained due to insufficient hardness. No. 18: Since the amount of C is too large, a large carbide having an area ratio exceeding the specified amount is generated in the center portion, and satisfactory rolling fatigue characteristics cannot be obtained.

No. 19, 20: Comparative examples in which the amount of Si or Mn exceeds the specified range, no formation of giant carbide at the center and good rolling fatigue life, but very poor cold workability and machinability. Bad for practical use. No. 21, 22, 23, 24: Although the area ratio of the giant carbide itself satisfies the requirements of the present invention, P, S,
These are comparative examples in which the contents of Ti and O are too large, and both have significantly poor rolling fatigue lives.

Example 2 Of the steel types shown in Table 1 above, No. After selecting 2, 3 and performing casting, soaking, hot forging, and spheroidizing annealing, respectively, under the following conditions, the area ratio of the giant carbide in the center was measured in the same manner as in Example 1 above. And a rolling fatigue test.

Melting with a 10 kgf vacuum furnace → ingot making → 1
Soaking treatment at 200 ° C x 10 hours → hot forging to a diameter of 65mm → heating at 790 ° C x 2 hours → 20 ° C to 680 ° C
/ Hour furnace cooling → air cooling. Melting with 150kgf vacuum furnace → ingot making → 1200 ° C x
10 hours soaking → hot forging to 65mm diameter →
Heating at 790 ° C for 2 hours → Furnace cooling to 680 ° C at 20 ° C / hour → Air cooling.

Melting using an actual machine → 30 by continuous casting
Production of a slab of 0 mm x 430 mm (implementation of electromagnetic stirring in the mold and under large-diameter roll pressure in the region of solid phase ratio of 0.3 to 0.7) → 1200 ° C x 10 hours soaking treatment → hot to 65 mm diameter Forging → 790 ℃ × 2 hours heating → 680 ℃
Furnace cooling at 20 ° C / hour until air cooling. Melting with actual machine → ingot of 7 ton steel ingot → 1200 ° C x 1
0 hour soaking process → hot forging to 65mm diameter → 7
90 ° C x 2 hours heating → Furnace cooling to 680 ° C at 20 ° C / hour → Air cooling.

With respect to the obtained annealed material, the area ratio of carbide of 2 μm or more at the center of the longitudinal section was measured in the same manner as in Example 1 and a rolling fatigue test was performed. The results are as shown in Table 3. Even in the case of steels having the same composition, the rolling fatigue characteristics are significantly changed depending on the area ratio of carbide of 2 μm or more at the center of the longitudinal section, and the area ratio is reduced by 0.3%. It can be seen that the rolling fatigue life of the comparative example exceeding this is very bad. In these experiments, it is considered that the difference in cooling rate caused by the difference in casting scale has a large effect on the area ratio of the giant carbide, and the larger the casting scale is, the faster the cooling rate is. It has become difficult.

[0039]

[Table 3]

In addition, No. The relationship between the angle of deviation from the center of the test piece and the frequency of occurrence of peeling observed in the test piece after the rolling fatigue test was examined for the test materials using the steel type No. 2 as described above. Test material) and Fig. 6
The results shown in (test material) were obtained.

As is clear from these figures, FIG.
(where the carbide area ratio of μm or more is 0.3% or less) in the range from the center (offset angle from the center = 0 degree) to the surface layer (offset angle from the center = 90 degrees) of the test material. In FIG. 6 (where the area ratio of carbides of 2 μm or more exceeds 0.3%), a portion where the angle of deviation from the center is 0 to 10 degrees, whereas a portion where the frequency of peeling is high is observed at random. In other words, it can be confirmed that the frequency of occurrence of peeling is extremely high in the center region), and that peeling due to rolling fatigue is extremely likely to occur at the axial center of the rolled material.

[0042]

The present invention is configured as described above, specifies the chemical composition of the steel material, and specifies the area ratio of carbide having a specific size or more existing in the center of the longitudinal section of the rolled material,
Alternatively, by further adjusting the component composition of the steel material so as to satisfy the relationship of the above formula (1), a bearing steel having excellent rolling fatigue characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a micrograph showing a longitudinal cross-sectional structure of a bearing steel;
An example of a giant carbide appearing in the longitudinal section is shown.

FIG. 2 is an explanatory diagram showing an observation region for obtaining an area ratio of a giant carbide.

FIG. 3 is a graph showing the relationship between the area ratio occupied by carbide having a thickness of 2 μm or more existing at the center of the longitudinal section and the rolling fatigue life.

FIG. 4 is an explanatory view showing a sampling position of a rolling fatigue test piece.

FIG. 5 is a graph showing the relationship between the angle of deviation from the center of a rolling fatigue test piece (comparative material) and the frequency of occurrence of peeling.

FIG. 6 is a graph showing the relationship between the angle of deviation from the center of a rolling fatigue test specimen (material of the invention) and the frequency of occurrence of peeling.

Continuation of the front page (56) References JP-A-3-285041 (JP, A) JP-A-1-129952 (JP, A) JP-A-2-125841 (JP, A)

Claims (4)

(57) [Claims] 1. C: 0.6 to 1.2% (hereinafter, mass% unless otherwise specified) Mn: 0.2 to 1.5% Si: 2.0% or less (including 0%) Al: 0.005 to 0.06% P: 0.03% or less (including 0%) S: 0.03% or less (including 0%) Ti: 0.005% or less (including 0%) O : 0.0020% or less (including 0%) Remainder: Fe and unavoidable impurities are satisfied, and the center of the longitudinal section passing through the axis of the linear or rod-shaped rolled material is the axis of the longitudinal section. The total cross-sectional area of carbides having a thickness of 2 μm or more and appearing in a central region within 1/8 · D (D represents the width of the vertical section) on each side from the axis is 0.3%
A bearing steel having excellent rolling fatigue characteristics, characterized in that: 2. The steel according to claim 1, wherein the other element is Cr: 2.0
% Or less (excluding 0%), Ni: 2.0% or less (0%
, Mo: 1.0% or less (excluding 0%), Cu: 1.0% or less (excluding 0%), V:
2. The bearing steel according to claim 1, wherein the bearing steel contains at least one selected from the group consisting of 0.3% or less (excluding 0%) and Nb: 0.1% or less (excluding 0%). . 3. The steel according to claim 1, wherein Pb: 0.1 as another element.
% Or less (not including 0%), Ca: 0.01% or less (0%
%), Te: 0.1% or less (excluding 0%), Bi: 0.1% or less (excluding 0%). The bearing steel according to claim 1. 4. The content of an alloy element contained in a steel material is as follows:
Claims 1 to 3 satisfy the relationship of the following formula (1).
The bearing steel according to any one of the above. [C] ^1/2 +0.12 × [Mn] + 0.11 × [Cr] + 0.05 × [Ni] + 0.03 × [Mo] ≧ 1.05 ... (1) (where [element] is each element in the steel material Represents the mass% of