JP2002332542A - High toughness steel for bearing - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an inexpensive high toughness steel for a bearing which has excellent toughness and rolling fatigue strength. SOLUTION: The high toughness steel for a bearing has a composition containing, by weight, 0.6 to 1.3% C, <=3.0% Si, 0.2 to 1.5% Mn, <=0.03% P, <=0.03% S, 0.3 to 5.0% Cr, <=0.050% Al, <=0.0015% O, <=0.015% N, 0.0005 to 0.0050% B and 0.005 to 0.10% Ti, and in which the ratio of the Ti content to the N content, i.e., Ti/N is 3.42 to 8.0, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high toughness bearing steel, and more particularly to a high toughness bearing steel excellent in toughness and rolling fatigue strength.

[0002]

2. Description of the Related Art Conventionally, JIS SUJ2 has been used as bearing steel.
(C: 0.95 to 1.10%, Si: 0.15 to 0.3
5%, Mn: 0.5% or less, P: 0.025% or less,
S: 0.025% or less, Cr: 1.30 to 1.60%,
Although a large amount of residual Fe and unavoidable impurities has been used, there is a problem that the toughness is low due to high quenching and tempering hardness. For this reason, carburized bearing steel having a low core hardness and excellent toughness is used for bearing components requiring high cracking strength, such as large bearing components.

[0003] However, when carburized bearing steel is used, carburizing must be carried out, which raises the cost and the processing time. Therefore, there is a need for bearing steel that does not harden the core even after soaking. However, cored quenching requires the reduction of quenching elements, so high toughness with excellent rolling fatigue strength is required. It was difficult to manufacture steel for bearings. In other words, when the quenching element is reduced, the surface hardness decreases, and the rolling fatigue strength tends to be equal to or lower than that of the conventional steel. Therefore, it is difficult to develop a high toughness bearing steel having excellent rolling fatigue strength. was there.

[0004]

An object of the present invention is to provide an inexpensive high toughness bearing steel which is excellent in toughness and rolling fatigue strength.

[0005]

In order to solve the above-mentioned problems, the present inventors have intensively studied a bearing steel which is inexpensive, has excellent rolling fatigue strength, and has high toughness even when subjected to hardening. However, when a small amount of B is added based on SUJ2, B segregates preferentially at the grain boundaries to prevent segregation of P, thereby improving toughness.
That is, the impact fracture that governs toughness is mainly a grain boundary fracture, and B is preferentially segregated at the grain boundary to lower the grain boundary strength. By segregating in the field, segregation of P is prevented, so that the toughness is improved. If B combines with N to form BN, this effect cannot be obtained, so N is fixed with Ti, which has stronger bonding than B. It has been found that it is necessary to improve the grain boundary strength and toughness of Cr, Ni and Mo by making crystal grains fine. The present invention has been made based on these findings.

That is, in the high toughness bearing steel of the present invention, C: 0.6 to 1.3%, Si: 3.0% or less, M
n: 0.2-1.5%, P: 0.03% or less, S: 0.
03% or less, Cr: 0.3 to 5.0%, Al: 0.05
0% or less, O: 0.0015% or less, N: 0.015%
Hereinafter, B: 0.0005 to 0.0050% and Ti:
0.005 to 0.10%, and optionally Ni:
0.1 to 3.0%, Mo: 0.05 to 0.25%, and V: One or more of 0.05 to 1.0%, and the content ratio of Ti and N Ti / N of 3.42-8.
0, with the balance being Fe and unavoidable impurities.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the high toughness bearing steel of the present invention will be described in detail. First, the reason why the component composition of the high toughness bearing steel of the present invention is specified as described above will be described. C: 0.6 to 1.3% C is an element contained for improving the hardness and strength of steel. In order to obtain the required hardness and strength, the content must be 0.60% or more. However, if the content is too high, the content becomes too hard and the toughness is reduced. Therefore, the upper limit of the content is set to 1.3.

Si: 3.0% or less Si is an element to be contained as a deoxidizing agent during smelting and to improve the strength, rolling life and hardenability of steel. When the content increases, the toughness is reduced and the hot workability is also reduced. Therefore, the upper limit of the content is set to 3.0%, preferably 1.7%.

Mn: 0.2 to 1.5% Mn is an element to be contained for Mn as a deoxidizing agent at the time of smelting as well as Si and for greatly improving the hardenability of steel. is there. 0.20 to obtain these effects
However, if the content is too high, the hot workability is lowered, and the steel becomes too hard and the toughness is lowered. Therefore, the upper limit of the content is set to 1.50%, preferably 1.2%. I do.

P: not more than 0.03% P is an impurity and segregates at the grain boundary to lower the grain boundary strength and toughness. Therefore, the content of P is preferably small, but if the content is reduced more than necessary, the cost increases. Therefore, its content is made 0.03% or less, preferably 0.02% or less. S: 0.03% or less S is an impurity, which lowers the toughness and the hot workability. Therefore, the content of S is preferably smaller, but if the content is reduced more than necessary, the cost is increased.
Its content is reduced to 0.03% or less.

Cr: 0.3-5.0% Cr improves the hardenability of steel and also improves the tempering softening resistance, and is therefore an element to be contained therein.
In order to obtain these functions and effects, it is necessary to contain 0.3% or more, preferably 1.0% or more. However, if the content is too large, the hot workability is reduced, and the steel becomes too hard and the toughness is reduced. , The upper limit of the content is 5.0%, preferably 2.7%.

Al: 0.050% or less Al is an element to be contained as a deoxidizing agent at the time of smelting and to improve the toughness by refining the crystal grains. Since the effect is saturated and the toughness is reduced, the upper limit of the content is 0.050.
%, Preferably 0.030%. O: 0.0015% or less O is an impurity and forms Al ₂ O ₃ in steel to lower the rolling fatigue strength. Therefore, it is preferable that the content is small, but if the content is reduced more than necessary, the cost is reduced. To increase the content, the content is made 0.0015% or less.

N: 0.015% or less N is an impurity and forms BN by combining with B to reduce the effect of containing B. Therefore, the content is preferably as small as possible, but 0.015% If the content is less than the above, the effect is small, so the content is made 0.015% or less.

B: 0.0005 to 0.0050% B segregates preferentially at the grain boundaries in steel to prevent segregation of P, and the segregation of P at the grain boundaries lowers the grain boundary strength. Is an element contained for that purpose. To obtain the effect, 0.000 as solid solution B is used.
Although it is necessary to contain 5% or more, if the content is too large, hot workability is remarkably reduced and cracks are generated in a rolling step or the like. Therefore, the upper limit of the content is made 0.0050%.

Ti: 0.005 to 0.10%, Ti /
N: 3.42 to 8.0 Ti binds to B and reduces the above-described effects of B.
Is an element to be included because TiN is generated by combining with Ti. In order to obtain the effect, it is necessary to contain 0.005% or more, and the Ti / N ratio is set to 3.42.
However, if the content is too large, large inclusions of TiN are generated, which deteriorates the fatigue properties and promotes cracking during forging. To 0.10% and the Ti / N ratio to 8.0
To

Ni: 0.1 to 3.0% Ni refines crystal grains to increase toughness, improves hardenability, and reduces the formation of white structure and carbide structure during the rolling fatigue process. Since the effect of suppressing the rolling contact fatigue life can be expected by the suppressing action, it is an element to be contained for them. In order to obtain these effects, it is necessary to contain 0.1% or more. However, if the content is increased, the effect is saturated and the cost is increased. Therefore, the upper limit of the content is set to 3.0%.

Mo: 0.05 to 0.25% Mo is an element that is contained for reducing the size of crystal grains to increase toughness and improving hardenability. 0.05 to obtain these effects
%, The effect is saturated when the content increases, so the upper limit of the content is set to 0.25%.
To V: 0.05 to 1.0% V is an element to be contained for forming carbides to increase the wear resistance of the bearing. To obtain the effect, it is necessary to contain 0.05% or more. However, if the content is increased, giant carbide is generated and the toughness is reduced. Therefore, the upper limit of the content is set to 1.0%.

The production of the high toughness bearing steel of the present invention can be carried out by a usual melting method without any particular problem similarly to the above SUJ2.

[0019]

Embodiments of the present invention will be described below. Table 1 below
The steel of the component composition shown in the following is melted by vacuum melting of 150 kg,
A steel bar having a diameter of 32 mm was manufactured by hot forging. Thereafter, it was heated to 920 ° C. as a normalizing treatment, kept for 2 hours, air-cooled, and further subjected to a spheroidizing annealing treatment under the conditions shown in FIG. 1 to obtain a test material for each test. A test piece having a length of 25 mm was cut out from these test materials, quenched and tempered under the conditions shown in FIG. I asked. Since the surface hardness after heat treatment affects the impact value, the tempering temperature was adjusted so that the hardness after tempering was about 61 to 62 HRC.

In addition, 10.5 mm × 10.
5mm, 55mm long Charpy test piece is cut out,
After quenching and tempering under the conditions shown in FIG. 2, a test piece having a size of 10 mm × 10 mm and a length of 55 mm was finished by machining. Similarly, the tempering temperature was adjusted so that the surface hardness after the heat treatment was about 61 to 62 HRC.
In addition, the notch portion is high hardness and low toughness in any of the test materials, and no difference appears in the JIS No. 3 test piece.
A 10R notch with a curvature of m and a depth of 2 mm was used. The test was carried out using a Charpy tester to measure the impact value at room temperature, n number 3
The average value was calculated. The results are shown in Table 2 below.

[0021]

[Table 1]

Further, the same material is used to obtain a diameter of 12 mm and a length of 22 mm.
A cylindrical test specimen of mm was cut out and subjected to a quenching and tempering treatment under the conditions shown in FIG. Similarly, the tempering temperature was adjusted so that the surface hardness after the heat treatment was about 61 to 62 HRC. Thereafter, the surface was polished by machining, and a radial rolling test was performed under the conditions of room temperature and surface pressure of 5.9 GPa, with n being about 10. The results are shown in Table 2 below.

[0023]

[Table 2]

As is clear from Table 2, B and Ti
Each of the inventive examples in which was contained did not differ in surface hardness from the comparative example containing no B even in the solute quenching, but had a high impact value. In particular, N with low P content
o. 12, No. 6 with high Si content, Mo included
No. 7, No. 8 containing V, No. 9 and No. 10 containing Ni, and No. 11 having an increased Cr content showed further improvement in toughness. In each of the examples of the present invention, reduction of the segregation amount of P at the grain boundary was confirmed, and it is considered that the toughness was improved by segregation of B at the grain boundary.

On the other hand, the rolling L ₁₀ and L ₅₀ in the normal temperature rolling life representing the fatigue strength are both No.1~5 and P content lower No. 12 of the present invention embodiment, comparative example the same level However, No. 6 and No. 1 with increased Si or Cr content
Nos. 7 to 10 containing 1, Mo, V or Ni have a longer life than the comparative example, and the S
It was about 2.3 times or more that of No. 1 of the comparative example of UJ2.

[0026]

According to the present invention, the above-described configuration enables
An excellent effect is obtained that high-toughness bearing steel having excellent toughness can be provided at low cost without lowering the rolling fatigue strength.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of spheroidizing annealing treatment conditions performed in Examples.

FIG. 2 is an explanatory diagram of quenching and tempering processing conditions performed in Examples.

Claims (2)

[Claims] C .: 0.6 to 1% by weight (the same applies hereinafter)
1.3%, Si: 3.0% or less, Mn: 0.2 to 1.5
%, P: 0.03% or less, S: 0.03% or less, Cr:
0.3-5.0%, Al: 0.050% or less, O: 0.
0015% or less, N: 0.015% or less, B: 0.00
0.05 to 0.0050% and Ti: 0.005 to 0.10
% And the content ratio of Ti and N, Ti / N, is 3.42.
-8.0, the balance being Fe and unavoidable impurities, a high toughness bearing steel. 2. C: 0.6-1.3%, Si: 3.0%
Hereinafter, Mn: 0.2 to 1.5%, P: 0.03% or less,
S: 0.03% or less, Cr: 0.3 to 5.0%, Al:
0.050% or less, O: 0.0015% or less, N: 0.
015% or less, B: 0.0005 to 0.0050%, and Ti: 0.005 to 0.10%.
1 to 3.0%, Mo: 0.05 to 0.25% and V:
Including one or more of 0.05 to 1.0%,
High toughness bearing steel, characterized in that the ratio Ti / N of Ti and N is 3.42 to 8.0, and the balance consists of Fe and unavoidable impurities.