JPH0649589A - Bearing steel for cold forming - Google Patents

Abstract

PURPOSE:To obtain a bearing steel for cold forming excellent in cold formability and rolling fatigue characteristics by specifying a composition consisting of C, Si, Mn, S, Cr, O, N, Ti, B, and Fe. CONSTITUTION:This steel is a bearing steel for cold forming having a composition consisting of, by weight, 0.8-1.2% C, <0.10% Si, <0.25% Mn, <0.007% S, 1.2-3.5% Cr, <0.0006% O, <0.0050% N, <0.0015% T, 0.0005-0.0050% B, and the balance Fe with inevitable impurities and excellent in rolling fatigue characteristics as well as in cold formability. In this steel, cold formability is improved by decreasing oxides and nitrides and ferrite is softened by decreasing Si and Mn, and further, N is fixed by increasing Cr and hardenability is improved by adding B.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-forming bearing steel, and more particularly to a bearing steel having excellent cold formability and rolling fatigue characteristics.

[0002]

2. Description of the Related Art Various bearings are used as automobile parts and machine parts.
SUJ2 of 4805 is the main subject. On the other hand, as the manufacturing process of the bearing, hot forging-spheroidizing annealing-cutting has hitherto been the mainstream, but in recent years, from the viewpoint of improving the yield, reducing the cost, etc., the bearing is manufactured by cold forming. Is becoming.

However, since SUJ2 has a low deformability and many fine cracks are generated during cold forming, a bearing steel having excellent cold formability is desired. Steels having improved cold formability have already been disclosed in JP-A-60-194047 and JP-A-2-209.
452, JP-A-2-294451, and JP-A-3-1466.
39, JP-A-3-285041, and JP-A-4-349.

[0004]

However, the techniques disclosed in JP-A-60-194047, JP-A-3-146639, JP-A-3-285041, and JP-A-4-349 improve the cold workability by reducing inclusions. However, since ferrite softening is not taken into consideration, there is a limit to the improvement of cold workability.

On the other hand, JP-A-2-209452 and JP-A-2-209452
-294451 considers the softening of ferrite due to the reduction of Si and Mn, and in both of them, the addition of B compensates for the deterioration of the hardenability due to the reduction of Si and Mn. However,
Japanese Patent Application Laid-Open No. 2-209452 has added Ti in order to obtain the effect of adding B, and thus has a problem in ensuring rolling fatigue characteristics. Further, the steel disclosed in JP-A-2-294451 is C
However, there is a problem in ensuring the rolling fatigue characteristics. As described above, the bearing steel satisfying both the cold formability and the rolling contact fatigue property has not yet been developed. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a cold-forming bearing steel that is excellent in both cold formability and rolling contact fatigue characteristics.

[0006]

The present invention provides C:% by weight.
0.8-1.2%, Si: less than 0.10%, Mn: 0.
Less than 25%, S: less than 0.007%, Cr: 1.2-
3.5%, O: less than 0.0006%, N: 0.0050
%, Ti: less than 0.0015%, B: 0.0005
Provided is a cold-forming bearing steel containing 0.0050% to the balance Fe and unavoidable impurities. The inventors of the present application have conducted the earnest research to solve the above problems, and as a result, have obtained the following findings. (A) In order to improve cold formability, it is necessary to reduce inclusions (oxides and sulfides), reduce Si and Mn, and soften ferrite.

(B) The addition of B compensates for the deterioration of the hardenability due to the reduction of Si and Mn, but the fixation of N for the effective addition of B is not performed by Ti, which is harmful to the rolling fatigue characteristics, and the high Cr content is high. Can be achieved. The present invention has been completed based on the findings of the present inventors.

[0008]

The present invention will be described in detail below. First,
The reason for limiting the composition will be described. In the following explanation,
All percentages are by weight.

[0009] C: 0.8 to 1.2% C in order to ensure the general hardness H _R C 60 or more necessary as bearing steel, requires more than 0.8%. However, if the amount exceeds 1.2%, coarse carbides are formed and the rolling fatigue characteristics are deteriorated. Therefore, the amount of C is specified in the range of 0.8 to 1.2%.

Si: Less than 0.10% Si is solid-solved in ferrite, and when the amount is 0.1% or more, the ferrite is hardened and cold formability is deteriorated.
Therefore, the amount of Si is specified to be less than 0.10%.

Mn: less than 0.25% Mn, like Si, also forms a solid solution in ferrite, and when the amount is 0.25% or more, ferrite is hardened and cold formability is deteriorated. Therefore, the amount of Mn is specified to be less than 0.25%.

S: less than 0.007% S has an effect on machinability and is necessary, but forms MnS, and if 0.007% or more, deteriorates cold formability. Therefore, the amount of S is specified to be less than 0.007%.

Cr: 1.2 to 3.5% Cr has a function of making carbide fine, improving rolling fatigue characteristics and enhancing hardenability, and further fixing N to exert an effect of adding B. There is. However, the amount is 1.
If it is less than 2%, the above-mentioned effect is small, while if it exceeds 3.5%, the carbide becomes coarse, and rolling fatigue characteristics and cold formability are deteriorated. Therefore, the Cr content is 1.2% to
Specify in the range of 3.5%.

O: less than 0.0006% O becomes an oxide inclusion, and if it is contained in an amount of 0.0006% or more, it deteriorates rolling contact fatigue characteristics and cold formability. Therefore, the amount of O is specified to be less than 0.0006%.

N: less than 0.0050% N forms nitride-based inclusions (TiN), and is 0.0050.
% Or more, rolling fatigue characteristics are deteriorated. Therefore, the amount of N is specified to be less than 0.0050%.

Ti: less than 0.0015% Ti forms a nitride-based inclusion (TiN), and 0.001
If it is 5% or more, rolling fatigue characteristics are deteriorated. Therefore, Ti
Of less than 0.0015%.

B: 0.0005 to 0.0050% B is an element necessary for compensating for the deterioration of the hardenability due to the reduction of Si and Mn, and its effect appears when it is 0.0005% or more. However, if it exceeds 0.0050%, the hot workability deteriorates. Therefore, the amount of B is 0.0005-0.0.
Specified within the range of 050%.

According to the steel having such a composition, Si, Mn
Is sufficiently small, the ferrite is softened and the cold formability is improved. In addition, the deterioration of the hardenability due to the reduction of Si and Mn is compensated by the addition of B, and the fixing of N to make the addition of B effective is not performed by Ti but by high Cr, so that the rolling fatigue characteristics are excellent. There is.

[0019]

EXAMPLES Examples of the present invention will be described below.

Steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace of 150 kg, hot forged to 70 mmφ, and normalized (holding at 880 ° C. for 1 hour and air cooling), and then spheroidized. Annealed (hold at 780 ° C for 5 hours, then at 680 ° C for 1 hour
After holding for 0 hour, furnace cooling was performed.

[0021]

[Table 1]

Diameter 14 mm, height 21 mm from this material
A cylindrical test piece having a notch depth of 0.8 mm was cut out and subjected to a compression test, and the deformability (cold formability) was evaluated by the cracking limit compression rate.

Further, the spheroidized and annealed material is subjected to quenching and tempering treatment (holding at 840 ° C. for 20 minutes, followed by oil cooling, then holding at 160 ° C. for 40 minutes and then air cooling),
After that, a disk-shaped test piece having a diameter of 60 mm and a height of 5 mm was prepared and a rolling fatigue test was conducted. The rolling life at this time was evaluated by the ratio when the rolling life of the steel of Comparative Example 1 was 1.0. For rolling fatigue, use a Mori radial endurance tester,
Load stress 4900N / mm ² I went there. The results are shown in Table 2.
Shown in.

[0024]

[Table 2]

As is clear from Table 2, all the steels of Examples 1 to 4 having the composition within the range of the present invention are Comparative Example 1 (SU.
It was confirmed that the limit compression rate (cold formability) and rolling life were better than those of the steel of J2).

On the other hand, it was confirmed that the steel of Comparative Example 2 was inferior in cold formability to the steels of the Examples because the contents of Si and N were high. It was also confirmed that the steel of Comparative Example 3 had a good cold formability, but had a low C content and thus had a rolling life inferior to that of the steel of the present invention.

[0027]

According to the present invention, there is provided a bearing steel for cold forming which is excellent not only in cold formability but also in rolling fatigue characteristics. Therefore, the present invention greatly contributes to cost reduction and yield improvement in bearing manufacturing, and has an extremely high industrial value.

Claims (1)

[Claims] 1. C: 0.8 to 1.2% by weight, Si:
Less than 0.10%, Mn: less than 0.25%, S: 0.00
Less than 7%, Cr: 1.2 to 3.5%, O: 0.0006
%, N: less than 0.0050%, Ti: 0.0015
%, B: 0.0005 to 0.0050% is contained,
A bearing steel for cold forming comprising the balance Fe and inevitable impurities.