JPH10176242A - Bearing steel excellent in quenching crack resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To minimize the degrees of segregation of Mn, Cr, and Mo in a banded segregation zone and hydrogen as an impurity and to prevent the occurrence of quenching crack by specifying the relations among the contents of contained Mn, Cr, and Mo, the degrees of segregation, and the amount of hydrogen in a bearing steel so that they satisfy specific conditions. SOLUTION: The bearing steel has a composition containing, by mass, one or more kinds among <=3.0% (not including 0%) Mn, <=3.0% (not including 0%) Cr, and <=1.0% (not including 0%) Mo. In this bearing steel, when [Mn(%)]max , [Cr(%)]max , and [Mo(%)]max represent the maximum amounts of segregation of respective elements in the longitudinal section of a rolled stock, respectively, the degrees of segregation, represented by the ratios of the contents of respective elements, are regulated so that they satisfy inequalities I, II, and III. Further, the amount of hydrogen as impurity [H(ppm)] is regulated so that is satisfies inequality IV. The reduction of these amounts of segregation is performed, e.g. by reducing the difference between the molten steel temp. before casting and the molten steel temp. in the mold at the time of casting, and applying soaking treatment to a steel ingot for a long time, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bearing steel used as a bearing for automobiles and various industrial machines, and more particularly to a steel having excellent resistance to quenching by suppressing the occurrence of cracks during quenching (hereinafter referred to as "quenching cracks"). The present invention relates to a bearing steel capable of exhibiting cracking properties.

[0002]

2. Description of the Related Art Various bearings are used in automobiles and various industrial machines.
High carbon chromium bearing steel such as SUJ2 specified in SG4805, and SCr specified in JISG4104
Alloy steel for machine structural use such as 420 has been conventionally used.

[0003] In producing bearings from the above-mentioned various material steels, after forming into a component shape by cutting, cold, warm, hot working, or the like, high carbon chromium bearing steel is quenched, while mechanical Structural alloy steels are generally carburized or carbonitrided and then quenched, followed by tempering or polishing.

[0004] However, when a bearing is manufactured in the above-described manufacturing process using the steel that has been used so far, the above-described burn cracking may occur. Since such cracks occur in the final process of bearing processing, they cannot be corrected, and must be disposed of in the finished product or in a state close to the finished product. In addition, it is necessary to perform a full inspection to find the cracked product These lead to an increase in the cost of producing the bearing.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to suppress the occurrence of burning cracks and reduce the manufacturing cost. The purpose of the present invention is to provide a bearing steel that can be used.

[0006]

[｛3 × [Mn (%)] _max + 2 × [Cr (%)] _max + [Mo (%)] _max ｝ / ｛3 × [Mn (%)] _ave + 2 × [Cr (%)] _ave + [Mo (%)] _ave ｝]

 × [H (ppm)] ≦ 2.5… (4)

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied from various angles the cause of the occurrence of burning cracks in bearing steel. As a result, it was clarified that the cracks generated in the bearing steel were concentrated in the striped segregated portion of the steel material, and the mechanism of occurrence was as follows.

First, Ms point lowering elements such as Mn, Cr, and Mo are segregated in the striped segregated portions. During quenching and cooling, the non-segregated portions undergo martensitic transformation, and the segregated portions become untransformed austenite. Become. On the other hand, although a few ppm of hydrogen are present in steel, the hydrogen solubility of austenite is much higher than that of martensite. As a result, the hydrogen is concentrated in the untransformed austenite in the segregated portion. Thereafter, when further cooled, untransformed austenite in the segregated portion undergoes martensitic transformation, and the solubility of hydrogen atoms enriched in the untransformed austenite is reduced, so that hydrogen gas is generated as hydrogen gas. Accumulate on inclusions. It is considered that when the internal pressure of the hydrogen gas exceeds the critical fracture strength, sintering cracks occur.

The present inventors have further studied based on the above research results, and as a result, have clarified the following facts. First, when a maximum segregation amount of Mn is expressed as [Mn (%)] _max in a longitudinal section of a rolled material having a Mn content of [Mn (%)] _ave , the ratio ([Mn (%) ] _max / [Mn (%)] _ave ) When the degree of segregation exceeds 3.0, quenching cracks are liable to occur. If the degree of segregation is 3.0 or less, the occurrence of sintering cracks is reduced. It has been found that it can be effectively suppressed. Cr and M
In steel containing o, the ratio ([Cr (%)] _max / [Cr (%)] _ave ) and the ratio ([Mo (%)] _max /
[Mo (%)] _ave ) is 2.50 or less,
It has been found that by setting the content to 5.0 or less, the occurrence of burning cracks can be effectively prevented. The longitudinal section of the rolled material means a section including the axis of the rolled material.

Therefore, in a bearing steel containing at least one of Mn, Cr and Mo, by satisfying one of the above-mentioned ratios (degree of segregation) according to the contained components, the occurrence of sintering cracks The effect of preventing is exhibited.

In order to reduce the amount of segregation, for example, the difference between the temperature of molten steel before casting and the temperature of molten steel in a mold during casting is reduced,
This can be achieved by performing a soaking treatment on the steel ingot for a long period of time as shown in Examples described later. For the measurement of segregation, for example, a rolled material may be cut into a length of 50 mm in the axial direction, and a longitudinal section including the axis may be subjected to surface analysis of each component with an EPMA analyzer to determine the maximum amount of segregation.

On the other hand, as described above, the direct cause of the cracking is the impurity hydrogen atom contained in the steel, but the present inventors have determined that the amount of impurity hydrogen [H (ppm)] is as follows. The maximum segregation amount [Mn (%)] _max , [Cr (%)] _max , [Mo (%)] _max and the content of each element [Mn (%)] _ave , [Cr (%)] _ave , [Mo (%)] _ave, etc., if the relationship of the above equation (4) is satisfied, that is, Mn,
It has also been found that if the content of hydrogen, which is an impurity element in steel, is reduced in accordance with the amount of segregation of Cr and Mo, burning cracks can be prevented. In order to reduce the amount of hydrogen, for example, there is a method of gradually cooling a steel ingot or a steel slab.

That is, in the present invention, the object can be achieved if at least one of the requirements defined by the above-mentioned formulas (1) to (3) or (4) is satisfied. May be satisfied, and the effect of the present invention thereby becomes more remarkable. Next, the reason for defining the chemical composition of the steel used in the present invention will be described.

Mn: 3.0% or less (excluding 0%) Mn is a deoxidizing / desulfurizing element, and improves the hardenability to increase the hardness of the surface layer and the inside to improve the rolling fatigue life. , Is an element effective in preventing surface depression. However, even if the content of Mn exceeds 3.0%, the effect is further reduced, and the segregation amount increases, and even if the above-described segregation amount reduction treatment is performed, sintering cracks easily occur. Therefore, the content of Mn needs to be 3.0% or less. Note that a more preferable content of Mn in consideration of the above addition effect is
It is in the range of 0.2 to 2.0%.

Cr: 3.0% or less (excluding 0%) Like Mn, Cr improves the hardenability, increases the hardness of the surface layer and the inside, improves the rolling fatigue life, and reduces the surface depression. It is an effective element to prevent. However, when the Cr content exceeds 3.0%, the segregation amount increases, and even if the above-described segregation amount reduction treatment is performed, quenching cracks are likely to occur,
In addition, a huge Cr carbide is easily formed, and the rolling fatigue life is rather shortened. Therefore, the content of Cr is 3.0%.
It must be: Cr considering the above-mentioned effect
Is more preferably in the range of 0.2 to 2.0%.

Mo: 1.0% or less (excluding 0%) Mo is also an element that enhances hardenability, and facilitates quenching and tempering of parts having a large mass. It is also an element effective for improving the tempering softening resistance. However, if the Mo content exceeds 1.0% and becomes excessive, the segregation amount increases, and even if the above-described segregation amount reduction treatment is performed, sintering cracks are likely to occur. From such a viewpoint, the content of Mo needs to be 1.0% or less. In addition, the more preferable content of Mo in consideration of the above addition effect is 0.08 to
The range is 0.5%.

The purpose of the bearing steel of the present invention is achieved by appropriately defining the amount of segregation of Mn, Cr and Mo described above. It is preferable to adjust the range of the other basic components normally contained as follows.

C: 0.1 to 1.2% C is an element for improving the core strength of the bearing component.
If it is less than 1%, such an effect cannot be exhibited, and if it is contained in excess of 1.2%, a giant carbide is easily formed,
This has a negative effect on rolling fatigue. For these reasons, the content of C is preferably set to about 0.1 to 1.2%.

Si: 0.01 to 2.0% Si is an element effective not only for effectively acting as a deoxidizing component at the time of melting, but also for improving quenchability and tempering softening resistance. In order to exhibit such an effect, it is preferable to contain 0.01% or more. However, even when Si is excessively contained in excess of 2.0%, the effect is saturated, and on the contrary, cold workability and corrosion resistance are reduced. This will reduce the machinability. Therefore, S
The content of i is preferably 0.01 to 2.0%.

[0020] S: 0.03% or less S is contained in most of the MnS form in the steel, but the element that improves machinability in the form of MnS, with a small O content Al ₂ When O ₃ is reduced, MnS becomes a starting point of rolling fatigue fracture and lowers rolling fatigue of the bearing. From such a viewpoint, the content of S is preferably set to 0.03% or less.

Ni: 0.25 to 5.0% Ni is an element effective for improving hardenability, and is an element for facilitating quenching and tempering treatment in parts having a large mass, and exhibits such an effect. 0.2 to make
It is preferable to contain 5% or more. However, 5.
When Ni is excessively contained in excess of 0%, cold workability and machinability are rather lowered. Therefore, the content of Ni is preferably set to 0.25 to 5.0%.

Al: 0.01-0.06% Al, like Si, also effectively acts as a deoxidizing component during melting, and is also an effective element in forming nitrides to refine austenite crystal grains. And these effects are 0.0
Effectively exhibited by adding 1% or more. However, if it is contained excessively, the austenite crystal grains are rather coarsened and the toughness is deteriorated.
It is preferable to set the following.

The above-mentioned components can be said to be basic components of the bearing steel of the present invention. The balance is iron and unavoidable impurities. However, if necessary, the following unavoidable impurities such as P, Ti, and O are described below. In this case, the characteristics of the bearing steel can be further improved.

P: 0.03% or less P is an element which lowers toughness, and its content is preferably reduced as much as possible. From such a viewpoint, the content of P is restricted to 0.03% or less. preferable.

Ti: 0.005% or less Ti is an element that combines with N to form TiN that adversely affects the rolling fatigue of the bearing, and also reduces cold workability and hot workability. From this point of view, the content is preferably reduced to 0.005% or less.

O: 0.0020% or less O combines with Al to form Al ₂ O ₃ which adversely affects the rolling fatigue of the bearing, and is an element which lowers cold workability and hot workability. Yes, it is preferable to reduce as much as possible, and from such a viewpoint, it is preferable to regulate the content to 0.0020% or less.

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 the scope of the present invention is not limited thereto. Of course, it is also possible to carry out the present invention with modifications, all of which are included in the technical scope of the present invention.

[0028]

EXAMPLES Steel materials having the chemical composition shown in Table 1 below were melted in a production furnace, and then cast into 11-ton steel ingots. After processing these steel materials under the following conditions, rolling to φ20mm,
Samples were used. (Condition 1) Soaking treatment (1350 ° C. × 10 hours) (Condition 2) Soaking treatment (1350 ° C. × 10 hours) (Condition 3) Soaking treatment (1350 ° C. × 1 hour) → Slow cooling (10 ° C.)
/ Hour) (Condition 4) Soaking treatment (1350 ° C x 1 hour)

[0029]

[Table 1]

Thereafter, the hydrogen content of each steel material was measured by the thermal conductivity method. For each steel material, a sample having a length of 50 mm was cut out from a longitudinal section including the axis, and the entire surface was subjected to surface analysis of Mn, Cr, and Mo by EPMA (beam diameter = 20 μm) to measure the amount of segregation of each element.

On the other hand, steel No. For things 1 and 5,
After spheroidizing annealing under the following conditions, other steels (Nos. 2 to 4, 6 to 8) were rolled to 30
A sample having a length of mm was cut out, cold forged at a compression ratio of 60%, and a test piece shown in FIG. 1 was prepared by machining. (Spheroidizing annealing conditions) 790 ° C x 2 hours → Cooling to 680 ° C at 20 ° C / hour, then air cooling

Further, steel No. The steels of Nos. 1 and 5 were quenched, and those of other steels (Nos. 2 to 4, 6 to 8) were carburized and quenched, and visually checked for cracks (number of samples: n = 10). . Table 2 below shows the segregation degree, the hydrogen content, the value on the left side of the above equation (4) (hereinafter referred to as “A value”), and the results of the cracking measurement of these steels.

[0033]

[Table 2]

From these results, the following can be considered.
Bearing steel satisfying the chemical composition specified in the present invention (N
o. In the cases 1 to 4), when the treatment was carried out under the condition 1, since the segregation degree and the A value were within the ranges specified in the present invention, no sintering cracks occurred. When treated under the condition 2, the A value is out of the range specified in the present invention, but since the segregation degree is in the range specified in the present invention, no sintering crack occurs. When treated under condition 3, Mn, Cr,
Although any of the segregation degrees of Mo is out of the range specified in the present invention, since the A value is 2.5 or less, no crack occurs during quenching.

On the other hand, when processing is performed under the condition 4,
Since the degree of segregation of any of Mn, Cr and Mo is out of the range specified in the present invention, and the A value exceeds 2.5, sintering cracks occur. In addition, steel No. Nos. 5, 7 and No. 5 having a large Mn content. No. 6 and No. 6 having a high Mo content. In the case of No. 8, the segregation degree of each element exceeded the range specified in the present invention even after the segregation degree reduction treatment was performed, and sintering cracks occurred.

[0036]

The present invention is configured as described above.
By appropriately defining the degree of segregation of n, Cr or Mo and the amount of hydrogen [H (ppm)], a bearing steel having excellent resistance to sintering cracking was realized.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view showing the shape of a test piece.

Claims (1)

[Claims] 1. mass%, Mn: 3.0% or less (excluding 0%), Cr: 3.0% or less (excluding 0%),
Mo: In a bearing steel containing at least one kind of 1.0% or less (not including 0%), the maximum segregation amounts of the above elements in the longitudinal section of the rolled material are respectively [Mn (%)] _max and [Cr ( %)] _max and [M
o (%)] _max and the segregation indicated by the ratio of these values and the content of each element [Mn (%)] _ave , [Cr (%)] _ave and [Mo (%)] _ave A bearing steel excellent in burn-out crack resistance, wherein the degree satisfies the following formulas (1) to (3) and / or the amount of impurity hydrogen [H (ppm)] satisfies the following formula (4). [Mn (%)] _max / [Mn (%)] _ave ≦ 3.0… (1) [Cr (%)] _max / [Cr (%)] _ave ≦ 2.50… (2) [Mo (% )] _max / [Mo (%)] _ave ≦ 5.0… (3) [{3 × [Mn (%)] _max + 2 × [Cr (%)] _max + [Mo (%)] _max ｛/ ｛ 3 × [Mn (%)] _ave + 2 × [Cr (%)] _ave + [Mo (%)] _ave [] × [H (ppm)] ≦ 2.5… (4)