JPH09165643A - Bearing steel excellent in rolling fatigue characteristic - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a bearing steel excellent in rolling fatigue characteristics by specifying the chemical components in a steel and prescribing the area ratio of carbides with a specified size or above present in the center part of the longitudinal section of a rolled stock. SOLUTION: This bearing steel satisfies the following requirements: by mass, 0.6 to 1.2% C, 0.2 to 1.5% Mn, <=2.0% (including zero) Si, 0.005 to 0.06% Al, <=0.03% (including zero) P and S, <=0.005% (including zero) Ti, <=0.0020% (including zero) O, and the balance Fe with inevitable impurities. In this way, its rolling fatigue characteristics can be improved, and, for moreover securing this, the following is important: the longitudinal sectional area of carbides with >=2μm thickness appearing in the center region within 1/8.D (D; the width of the longitudinal section) respectively to one side from the axial line including the axial center of the longitudinal section in the center line of the longitudinal section passing through the axial center of a linear or bar-shaped rolling stock is suppressed to <=0.3% to the above longitudinal sectional area.

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 a bearing steel used for the above-mentioned applications, SU conventionally specified in JIS G 4805 has been used.
High carbon chromium bearing steels such as J2 have been used primarily. However, since these bearing steels have a high carbon content, elements such as C cause center segregation during casting, and even if slabbing or hot rolling is performed thereafter, the segregated portion becomes a huge carbide as shown in FIG. May appear. And, in the case where such a huge carbide is generated, the huge carbide remains even after being processed into a bearing component through a process such as spheroidizing annealing, quenching and tempering, which serves as a stress concentration source. It has been empirically confirmed that the rolling fatigue life is shortened.

Therefore, in order to eliminate the center segregation that occurs during the casting of high carbon chromium bearing steel and the giant carbide that occurs in the center segregation portion, for example, in Japanese Patent Laid-Open No. 3-75312,
A method has been proposed in which the casting material is rolled into a billet and then subjected to soaking treatment. Further, in Japanese Unexamined Patent Publication (Kokai) No. 104819/1990, a method of performing reduction in a state in which the inside is not solidified when producing high carbon chromium bearing steel by continuous casting, and in Japanese Unexamined Patent Publication No. 59-137164, continuous casting is used. 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 and the carbon content C _{2 in the} surface layer to be 1.2 or less. doing.

These methods are significant in that they aim to reduce or eliminate the above-mentioned giant carbides that have been qualitatively confirmed to affect the rolling fatigue life. Quantification of how much the rolling fatigue life of carbon chrome bearing steel is adversely affected, and how much the presence of the huge carbides can be suppressed to surely increase the rolling fatigue life. Relationship has not been clarified.

[0005]

The present invention has been made in view of the above circumstances, and its purpose is to quantitatively clarify the influence of a large amount of carbide on the rolling fatigue life, An object of the present invention is to provide a bearing steel that can surely exhibit excellent rolling fatigue characteristics.

[0006]

The composition of the bearing steel according to the present invention which is capable of solving the above-mentioned problems and has excellent rolling contact fatigue characteristics is as follows: C: 0.6 to 1.2% Mn: 0.2 to 1.5% Si: 2.0% or less (including 0%) Al: 0.005-0.06% 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%) Remainder: Fe and unavoidable impurities requirements, and in a linear or rod-shaped rolled material At the center line of the vertical section passing through the axis, the thickness that appears in the center region including the axis of the vertical section and within 1/8 · D (D represents the width of the vertical section) on each side from the axis The total cross-sectional area of carbides with a length of 2 μm or more is 0.3% with respect to the vertical cross-sectional area.
Its features are as follows.

The bearing steel according to the present invention further comprises, as other elements, Cr: 2.0% or less (not including 0%), Ni:
2.0% or less (0% is not included), Mo: 1.0% or less (0% is not included), Cu: 1.0% or less (0% is not included), 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%), rolling fatigue characteristics can be further improved, and further, as another element, Pb: 0.1% or less (0% is not included), Ca: 0.01% or less (0% is not included), T
e: 0.1% or less (excluding 0%), Bi: 0.1%
Machinability can be enhanced by incorporating at least one selected from the group consisting of the following (not including 0%).

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 relation of the following formula (1). By increasing the hardenability by rolling, 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 Represents the mass% of

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for defining the chemical composition of the steel material used in the present invention and the reasons for defining the area ratio of carbides present in the central portion of the longitudinal section of the linear or rod-shaped rolled material will be described below. The details will be described. First of all, the reasons for defining the chemical composition of steel materials will be clarified.

C: 0.6 to 1.2% C is a minimum hardness required for the bearing steel after quenching and tempering, that is, HRc58 or more is secured, and the bearing has rolling fatigue characteristics. It is an element that is indispensable as a strengthening element for ensuring the characteristics and must be contained at least 0.6% or more, preferably 0.7% or more. However, if the C content is too high, giant carbides are likely to be generated in the core portion, which adversely affects the rolling fatigue characteristics, so at most 1.2% or less, preferably 1.1% or less. Should be kept to.

Si: 2.0% or less (including 0%) Si acts effectively as a deoxidizing element,
It has the effect of increasing the temper softening resistance to increase the hardness of the core portion, and these effects are effectively exhibited by containing about 0.03% or more. However, these effects saturate at 2.0%, and if contained more than that, cold workability and machinability will be adversely affected, so 2.0% or less, and more preferably 1.0% or less. There must be.

Mn: 0.2 to 1.5% Mn effectively acts as a deoxidizing / desulfurizing agent, and also enhances the hardenability to enhance the hardness of the surface layer and the core and prevents the surface from sinking and rolling. It is an element that is indispensable for improving the dynamic fatigue life, and must be contained in an amount of 0.2% or more in order to exert these effects effectively. However, these effects are saturated at 1.5%, and if they are contained more than that, cold workability and machinability are adversely affected.
It must be kept below 5%. The more preferable content of Mn is in the range of 0.2 to 1.2%.

Al: 0.005-0.06% Al not only acts effectively as a deoxidizing element, but also has the action of forming nitrides and refining austenite crystal grains to enhance toughness. Must be contained in an amount of 0.005% or more to effectively exhibit the above. However, if the amount of Al becomes too large, the austenite crystal grains become rather coarse and the toughness deteriorates. Therefore, it 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%) P becomes a non-metallic inclusion and adversely affects toughness, so the content should be suppressed as low as possible, and the adverse effect is practically almost impossible. The upper limit is 0.03%, which is not a problem. It is more preferably 0.015% or less.

S: 0.03% or less (including 0%) S is an element that exists in the form of MnS in most of the steel and contributes to the improvement of machinability, but in a steel material with a low oxygen content. Has a significant adverse effect on rolling contact fatigue life, so it must be kept to 0.03% or less. It is more preferably 0.015% or less.

Ti: 0.005% (Including 0%) Ti not only has a negative effect on rolling contact fatigue characteristics by forming TiN by combining with N that is mixed in a trace amount in steel.
It is a harmful element that also impairs cold workability and hot workability, and in order to avoid these obstacles, it must be suppressed to 0.005% or less, more preferably 0.004% or less.

O: 0.0020% or less (including 0%) O combines with Al and causes deterioration of rolling contact fatigue characteristics A
Since it forms l ₂ O ₃ -based inclusions and adversely affects the cold workability and hot workability, it must be suppressed to 0.0020% or less, more preferably 0.0015% or less.

The balance components of the bearing steel according to the present invention are Fe and unavoidable impurities, but the properties as the bearing steel are further improved by adding an appropriate amount of the following elements as other elements if necessary. It is possible.

Cr: 2.0% or less (not including 0%),
Ni: 2.0% or less (not including 0%), Mo: 1.0
% Or less (not including 0%), Cu: 1.0% or less (0%
, V: 0.3% or less (0% is not included),
Nb: from the group consisting of 0.1% or less under (0%)
At least one of these selected elements is a synonymous element in that all of these elements contribute to the improvement of rolling fatigue life. That is, Cr, Ni, M
Both o and Cu act as hardenability improving elements, enhance the hardness of the surface layer and the core, and contribute to the improvement of rolling contact fatigue characteristics and the suppression of surface depression, and V and Nb are C in steel.
Combined with N and N to form carbonitrides, which refines the crystal grains and contributes to the improvement of rolling contact fatigue life. Of these, Cr and N
i and Mo act effectively as a hardenability improving element to facilitate quenching and tempering of a component having a large mass. Further, Cu has the effect of enhancing the corrosion resistance in addition to the improvement of the hardenability, and V and Nb also exert the effect of enhancing the toughness by the grain refining effect. These effects are 0.2% or more for Cr and 0.
25% or more, 0.08% or more for Mo, 0.25% for Cu
As described above, it is effectively exhibited by containing 0.01% or more of V and 0.01% or more of Nb.

However, if the Cr content exceeds 2.0%, a huge Cr carbide tends to be formed, and if 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 and Cu content of 1.
If it exceeds 0%, cold workability and machinability are deteriorated, and red hot embrittlement is promoted to easily cause hot work cracking.
Each should be kept below the upper limit. Further, the effect of addition of V is saturated at 0.3% and the effect of addition of Nb is saturated at 0.1%, so any further addition is economically useless.

Pb: 0.1% or less (not including 0%),
Ca: 0.01% or less (not including 0%), Te: 0.
1% or less (not including 0%), Bi: 0.1% or less ( 0
At least one selected from the group consisting of
Species All of these elements act as machinability improving elements, but their effects saturate near their upper limit values, and if the content is too high, the rolling fatigue life will be adversely affected. It must be kept below the upper limit.

Further, in the present invention, in addition to the type of each of the above-mentioned contained elements and the respective contents thereof, C, M among the above-mentioned contained elements as an influencing factor that governs the hardness after quenching and tempering.
It is preferable that the contents of n, Cr, Ni, and Mo are adjusted so that the relation of the formula (1) is satisfied, because rolling fatigue characteristics can be further improved. In particular,
If the steel material satisfying the relation of the above formula (1) is quenched (preferably 830 to 870 ° C) and tempered (preferably 140 to 180 ° C) in an appropriate temperature range, HRc of 60 or more in the state after quenching and tempering The resulting bearing steel has excellent rolling fatigue characteristics.

The bearing steel of the present invention satisfies the requirements of the above-mentioned composition of ingredients to secure excellent quenching and tempering properties and to secure the hardness of the surface layer portion and the core portion, while controlling the amount of carbides formed. However, in order to make sure that the rolling fatigue life is improved, in addition to the requirements for these component compositions, rolling rolled into a linear or rod shape is required. Material or the center of the vertical section including the axis of the drawn material (specifically, as shown in Fig. 2, a line separated from the axis by 1/8 × D on each side with respect to the width D of the vertical section). It is extremely important to suppress the total cross-sectional area of the carbide having a thickness of 2 μm or more existing in the sandwiched area) to 0.3% or less of the vertical cross-sectional area.

Incidentally, FIG. 3 shows the influence of the total sectional area ratio of the carbides having a thickness of 2 μm or more existing in the central portion of the longitudinal section on the rolling contact fatigue life from many experimental data including the examples described later. It is a graph arranged and shown, and as is clear from this figure, there is a slight tendency for the rolling fatigue life to decrease until the area ratio of carbides having a thickness of 2 μm or more is around 0.2%. It can be confirmed that when the value exceeds 0.3%, the rolling fatigue life is drastically reduced. The reason why such a tendency is obtained is that when the carbide is a huge one having a thickness of 2 μm or more, stress concentration easily occurs in that portion,
It is considered that when the area ratio exceeds 0.3%, the rolling fatigue life is remarkably reduced.

As a means for suppressing the area ratio of carbides of 2 μm or more existing in the central portion of the longitudinal section of the axial center portion to 0.3% or less as described above, for example, when a continuous casting method is adopted. Is a method of suppressing center segregation during solidification by applying a large diameter roll reduction with a solid fraction of 0.2 to 0.7 during cooling and solidification, or 115
A preferred method is a method of soaking at a temperature of 0 ° C. or higher for 10 hours or longer. In addition, when the usual ingot making method is adopted, the slab is 2
A preferred method is a method of soaking for 0 hour or more.

[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 by using a 150 kg vacuum furnace, and then a slab was manufactured by an ingot making method.
65mm diameter after soaking at 200 ℃ for 10 hours
Was hot forged, and subsequently spheroidized and annealed under the following conditions. (Nos. 2, 3 and 16) 790 ° C x 2 hours → furnace cooling to 680 ° C at 20 ° C / Hr →
Then air cooling (other) 760 ° C x 2 hours → furnace cooling at 20 ° C / Hr to 680 ° C →
Then air cooled

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

For each annealed material, as shown in FIG. 4, a disk having a thickness of 60 mm and a thickness of 5 mm was cut out from the longitudinal section including the axis, and the surface roughness was 0.04 μmRa or less by lapping after quenching and tempering. Then, a rolling fatigue test was conducted under the following conditions (N = 20). Table 2 shows the results. (Rolling fatigue test) Surface pressure: 527 kgf / mm ² Rotational speed: 1000 rpm Steel ball number: 6 Lubricating oil: Turbine # 68

[0030]

[Table 1]

[0031]

[Table 2]

From Tables 1 and 2, the following can be considered. No. Examples 1 to 16 are examples in which the composition of the steel material satisfies all the specified requirements of the present invention and the area ratio of carbides 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, it can be seen that those having a calculated value of the formula (1) of 1.05 or more have a relatively higher rolling contact fatigue life than those having a calculated value of less than 1.05.

For these, No. Nos. 17 to 24 are comparative examples lacking any of the requirements defined in the present invention, and there is a problem in the performance as the bearing steel as described below. No. 17: Giant carbides are not recognized, but since the amount of C is small and the value of the formula (1) is extremely low, satisfactory rolling fatigue characteristics cannot be obtained due to insufficient hardness. No. 18: Since the amount of C is too large, huge carbides having an area ratio exceeding the specified amount are generated in the central 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 huge carbides is observed in the central portion, and rolling fatigue life is good, but cold workability and machinability are extremely high. It is bad and not suitable for practical use. No. 21, 22, 23, 24: The area ratio of the giant carbide itself satisfies the requirements of the present invention, but P, S,
This is a comparative example in which the contents of Ti and O are too large, and the rolling fatigue life of each is remarkably inferior.

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

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

Melting using an actual machine → 30 by continuous casting
Production of 0 mm x 430 mm slab (implementation of electromagnetic stirring in the mold and pressure reduction of large diameter roll in the range of solid fraction 0.3 to 0.7) → 1200 ° C x 10 hours soaking treatment → hot rolling to 65 mm diameter Forging → 790 ℃ × 2 hours heating → 680 ℃
Furnace cooling → air cooling at 20 ° C / hour. Melting with an actual machine → 7-ton ingot ingot → 1200 ° C × 1
Soaking treatment for 0 hours → hot forging to 65 mm diameter → 7
90 ° C × 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 carbides of 2 μm or more in the central portion of the longitudinal section was measured in the same manner as in Example 1 and a rolling fatigue test was conducted. The results are shown in Table 3, and even in the case of steels having the same composition, the rolling contact fatigue characteristics remarkably change depending on the area ratio of carbides of 2 μm or more in the central portion of the longitudinal section, and the area ratio is 0.3%. It can be seen that the rolling fatigue life of the comparative examples exceeding the range is very poor. In these experiments, it is considered that the difference in cooling rate caused by the difference in casting scale has a large influence on the area ratio of the giant carbide, and the smaller the casting scale and the faster the cooling rate, the greater the generation of giant carbide. It's getting harder.

[0039]

[Table 3]

No. When the relationship between the deviation angle from the center of the test piece and the occurrence frequency of peeling observed in the test piece after the rolling fatigue test was examined for the test materials using the steel types of No. 2 and FIG. Sample material) and Fig. 6
The results shown in (Test material) were obtained.

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

[0042]

EFFECTS OF THE INVENTION The present invention is constituted as described above, and specifies the chemical composition of the steel material and also defines the area ratio of carbides of a specific size or more present in the central portion 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), it has become possible to provide a bearing steel having excellent rolling fatigue characteristics.

[Brief description of the drawings]

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

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

FIG. 3 is a graph showing a relationship between an area ratio of carbide having a thickness of 2 μm or more existing in a central portion of a longitudinal section and 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 deviation angle from the center of a rolling fatigue test piece (comparative material) and the occurrence frequency of peeling.

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

Claims (4)

[Claims] 1. C: 0.6 to 1.2% (hereinafter referred to as mass% unless otherwise specified) Mn: 0.2 to 1.5% Si: 2.0% or less (including 0%) Al: 0.005-0.06% 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%) balance: Fe and unavoidable impurities are satisfied, and at the center line of the longitudinal section passing through the axis of the linear or rod-shaped rolled material, the axis of the longitudinal section The total cross-sectional area of carbides having a thickness of 2 μm or more appearing in the central region within 1/8 · D (D represents the width of the longitudinal section) on one side from the axial center line with respect to the longitudinal section area. 0.3%
Bearing steel with excellent rolling fatigue characteristics, characterized in that 2. A steel material containing Cr: 2.0 as another element.
% Or less (not including 0%), Ni: 2.0% or less (0%
, Mo: 1.0% or less (0% is not included), Cu: 1.0% or less (0% is not included), V:
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 (not including 0%) and Nb: 0.1% or less (not including 0%). . 3. A steel material containing Pb: 0.1 as another element.
% Or less (not including 0%), Ca: 0.01% or less (0
%), Te: 0.1% or less (0% is not included), Bi: 0.1% or less (0% is not included), at least one selected from the group consisting of. The bearing steel according to claim 1 or 2. 4. The content of alloying elements contained in the steel material is
Claimed steels 1 to 3 satisfying the relationship of the following formula (1)
Bearing steel according to any one of 1. [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 Represents the mass% of