JPS5929646B2 - Manufacturing method of steel for rolling bearings - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION In recent years, demands for improved performance of mechanical structures have been increasing in the industrial world.

In other words, in addition to the demand for improving the performance of mechanical structures without changing their dimensions and shapes, there is even a demand for making them smaller than before while improving their performance.
Against this background, rolling parts such as rolling bearings and gears that are power transmission parts, which are responsible for many of the rotating parts of mechanical structures, are increasingly required to have increased load capacity and improved reliability.

The present invention was made to meet the demands of the industry, and its gist is that C: 0.7 to 1.
2%, Si 2.0%, Mn 2.0%, Cr 0
．． 5-2.5%, A7■ 0.01-1.0%, 00:
<0.0015%, N2■ (O, 015% and optionally Mo■ 0.05 to 095% and Ni:0
．． 1 Steel containing 3% to less than 160% of one or two types
After heating the carbide to a temperature of 300°C or higher to form a solid solution,
The average grain size of the carbide is 0.1 μm to 0.2 μm.
After holding for a time of m and rapidly cooling to room temperature, holding in an austenitizing region for a time such that C% in the matrix becomes 0.45 to 0.60%, and then quenching and tempering. This is a method for producing steel for rolling bearings, which suppresses the formation of plate-shaped carbides caused by rolling contact stress and has excellent rolling fatigue strength in the temperature range from room temperature to around 200°C. The lifespan of a rolling bearing (the operating time from the start of use until it becomes unusable, hereinafter the same) is determined by the internal properties of the steel for rolling bearings (hardness, amount of nonmetallic inclusions,
It is affected by the shape and distribution of carbides), the usage conditions of the rolling bearing (lubrication, load, etc.), and the manufacturing conditions of the bearing (surface roughness, etc.).

However, in any case, it is most important to improve the internal properties of bearing steel. It is well known that oxide-based non-metallic inclusions, especially Al2O3 and SlO2, act as stress concentration media in the steel and have a significant negative impact on the lifespan, as internal properties that have a large effect on the lifespan of bearing steel. ing.

It is preferable for the life of these non-metallic inclusions to be small and fine, but recent advances in steel manufacturing methods, especially vacuum degassing technology, have gradually reduced the oxygen content in steel, and it has reached its limit. It's getting closer. Therefore, it is no longer possible to expect much improvement in service life due to the reduction of nonmetallic inclusions and refinement due to the decrease in the amount of oxygen in steel.

Taking this background into consideration, the present invention examines how the dimensions of carbides in steel and the carbon content in the martensitic matrix, which are major factors other than non-metallic inclusions, have an adverse effect on life. This was done after researching how it would affect the situation.

In steel for bearings, there are several documents that indicate that the carbon content and carbide dimensions in the martensitic matrix affect the service life; Since each dimension is treated as an independent factor, the combined influence of both on the lifespan has not been clarified.

Carbides in low-alloy bearing steel with a carbon content of around 1% are F.
e, C (cementite), and its hardness is thick Hv
12OO~1500.

This hardness is considerably higher than the maximum hardness (approximately Hv85O) normally obtained by heat treating such steel. For this reason, Fe and C act as notches in the steel and have an adverse effect on the service life. In particular, the presence of large amounts of Fe and C is unfavorable to the lifespan because it acts as a notch.Also, in order to suppress the formation of plate-shaped carbides and improve the lifespan, it is necessary to finely disperse the carbides. On the other hand, if the amount of carbon in the martensitic matrix is too large, it increases brittleness and promotes the formation of "one plate-shaped carbide" described later, reducing the lifespan. Although the formation of "carbide" is suppressed, the hardness decreases and the life span is shortened.

From this point of view, it is important to optimize the amount of carbon in the martensitic matrix and to finely disperse carbides in order to improve the life. The effects of oxide inclusions, carbides, and C% in the matrix on individual lifespans have been described above. ) are mutually related in the generation of

In bearing steels that undergo rolling contact fatigue under high surface pressure,
Various microstructural changes occur beneath the transfer surface.

For example, there is something called "one butterfly" because of its shape, which is difficult to corrode during microcorrosion and is observed as white.

Particularly high surface pressure (approximately 200 kg/M4 or more in Hertzian surface pressure)
At the bottom, the above-mentioned 9-plate-shaped carbide is generated due to rolling contact stress.

The single plate-shaped carbide has a thickness of several μm and a maximum diameter of approximately 0.5 m.
It is a plate-shaped carbide that reaches m, and its hardness is that of Fe3C.
It is almost equivalent to

Therefore, when such a "plate-like carbide" is formed, it becomes a stress concentration source, cracks occur around it, and flaking occurs.

Such "plate-shaped carbides" are thought to be generated in stress concentration areas between nonmetallic inclusions under the rolling contact surface, or in stress concentration areas between inclusions and carbides.

In addition, such single plate-shaped carbides are non-existent before rolling contact fatigue, and are formed in steel by rolling contact fatigue. One method is to hold the bearing steel at a high temperature to dissolve all the carbides into solid solution, then cause bainite transformation or martensitic transformation to suppress the precipitation of carbides during cooling from high temperature, and then cool the bearing steel. After austenitization, there is a method of causing martensitic transformation to prevent the formation of coarse carbides.

Although this method does make the carbide finer, it does not necessarily improve the life. The most important reason for this is that the carbides are too fine, so they easily re-dissolve during austenitization.
It becomes difficult to control the amount of carbon in the martensite matrix, and if the amount of carbon in the matrix is too large, the martensite matrix becomes brittle, and as mentioned above, ``plate-shaped carbides'' tend to form under high surface pressure. it is conceivable that.

In order to obtain excellent service life under the above high surface pressure, it is necessary to reduce the oxide inclusions in the steel, adjust the amount of carbon in the martensite matrix, and make the carbides finer to form "plate-shaped carbides". It is necessary to suppress the generation of

The present invention takes these points into account and provides a method for obtaining an excellent service life.

The present invention refines the carbides of steel having the chemical composition shown in the claims by the following method, and
The present invention relates to a method of controlling the amount of carbon in a martensite matrix to suppress the formation of "plate-like carbides" and obtain an excellent lifespan.

That is, steel having the chemical composition shown in the claims,
(1) Heat to 1'000°C or higher to dissolve carbides.

(hereinafter referred to as solution treatment) (2) Then, rapidly cooled to a temperature range of 300 to 700 °C (hereinafter referred to as intermediate quenching)
The austenite is decomposed and then rapidly cooled to near room temperature.

(hereinafter referred to as intermediate quenching) (3) Then 300C/Min
The mixture is heated to 500° C. to 700° C. or higher at the fastest possible temperature increase rate to precipitate carbides evenly and finely.

(Hereinafter referred to as intermediate tempering.) At this time, the heating temperature and time are controlled so that the average carbide grain size is 0.1 μm or more and 0.1 μm or more.
Do not exceed 2 μm. (4) After that, normal quenching and tempering (quenching, tempering, the same shall apply hereinafter) is performed, but
At this time, the C coefficient in the matrix is 0.45 to 0.6
Adjust the heating temperature and heating holding time so that it becomes 0%.

When setting each stage of (1) to (4) above,
Various tests were carried out using the test materials shown in the table.

In addition, in Table 1, TP&2, 3, 4, 7, 10,
17 is a steel that satisfies the regulatory range of components set forth in claim 1, TPA5, l3,] 4 is a steel that also satisfies the regulatory range of components set forth in claim 2, and the others are different from the steel of the present invention in terms of composition. It is steel. The experimental results are shown in Tables 2 to 7.

Table 9 shows the results of a life test performed on the test materials shown in Table 1 by subjecting them to the heat treatment shown in FIG. It is clear that the steel of the present invention has a longer life. The reasons for setting each of the above steps 0) to (4) will be described below.

(1) Carbides in low alloy steel containing around 1% carbon are 100%
Complete solid solution will not occur unless heated to 0%C or higher (see Table 2).

(2) Rapid cooling to a temperature below 300°C may cause quench cracking.

Furthermore, if the steel is not rapidly cooled to a temperature below 700°C, austenite decomposition will be delayed and carbides will become enlarged (see Table 3).

(3) When heated at a slow temperature increase rate of 30° C./Min or less, carbides become coarse.

In order to precipitate fine and uniform carbides, the heating rate should be as fast as possible (see Table 4). Further, if the temperature is kept at a temperature of 500° C. or lower, it takes a long time for carbide to precipitate, and if the temperature is kept at a temperature of 700° C. or higher, the carbide becomes coarse.

When the carbide particle size is 0.1 μm or less, the formation of the aforementioned "plate-shaped carbide" is better suppressed, but the carbide dissolves faster during heating for quenching in the post-process, making it difficult to control the C% of the matrix. Since this becomes difficult, the carbide particle size must be 0.1 μm or more.

Further, if the carbide particle size exceeds 0.2 μm, it becomes difficult to suppress the formation of the three-plate-like carbide properties described above, and the rolling fatigue strength decreases, so the carbide particle size should not exceed 0.2 μm. (See Tables 5, 6, and 7). (4) When the C% in the matrix is less than 0.45% and more than 0.60%, the rolling life decreases.

(See Table 8). Next, Fig. 1 shows heat treatment 1 according to the present invention.
An example of the heat curve of normal heat treatment 2 is shown.

Test specimens were prepared by subjecting the test materials shown in Table 1 to both heat treatments, and life tests and measurements of carbide particle diameter, etc., were performed.

(See Table 9) Next, it will be described that the present invention can be used for rolling bearings in a semi-high temperature range (temperature range of approximately 200° C. or lower).

FIG. 2 shows the tempering curve of 'l'PJl6.4 which was subjected to normal hardening and the tempering curve when the same material was subjected to the heat treatment according to the present invention.

It is clear that the hardness of the present invention is higher at the same tempering temperature, and this shows that it can be used for bearings in the semi-high temperature range up to approximately 200°C.

Next, the reason for determining the content of each element will be described.

Basically, in connection with the method of the present invention, a range of ingredients has been identified which makes the method of the present invention particularly effective. Carbon In the present invention, in order to stably obtain the hardness necessary to obtain a satisfactory lifespan, the amount of carbon in the matrix is set to 0.4.
The content should be 5% or more, and some residual cementite is also required.

In this sense, the object of the present invention can be achieved by setting the lower limit of the carbon content in steel to 0.7%.

Therefore, the lower limit of carbon is set to 0.7. On the other hand, the amount of carbon is high《
In this case, the amount of carbon in the matrix increases during the quenching heat treatment, but in order to suppress the formation of "two-plate carbide", the amount of carbon in the matrix needs to be set at a ratio of 0.6.

Furthermore, as the amount of carbon increases, the carbides in the steel tend to grow larger, so it is not possible to increase the amount of carbon in the steel too much.

When the C% in the steel exceeds a coefficient of 1.2, the C% in the matrix
There is an increased possibility that C% exceeds 0.6%, and the carbides become gigantic, making it difficult to finely disperse the carbides using the method of the present invention. The upper limit of is set to 1.2.

Si In the present invention, Si is mainly used to adjust hardenability necessary to obtain appropriate hardness and to impart heat resistance.

For this purpose, the more Si there is, the more effective it is.
If Si exceeds a coefficient of 2.0, surface coalescence during solution treatment becomes large, so it is not preferable to add Si in a coefficient of more than 2.0. Therefore, the upper limit of Si is set to 2.0.

Mn In the present invention, Mn is mainly used to adjust the hardenability necessary to obtain appropriate hardness.

In the present invention, this objective can be achieved with a Mn content of up to 2.0%. Therefore, the upper limit of Mn is set to 2.0%.

Cr affects the shape of carbides, stability during heat treatment, and also affects hardenability.

In the present invention, Cr is contained in order to ensure carbide refinement, stability, and hardenability to suppress the formation of plate-like carbide properties.

For this purpose, it is necessary to contain at least 0.5% Cr.

Therefore, the lower limit of Cr is set to 0.5.

Furthermore, if Cr exceeds a ratio of 2.5, it becomes difficult to obtain fine and uniform carbides, which impedes the effects of the present invention, so the upper limit of Cr is set at 2.5.

AI Al is contained as a deoxidizing agent in order to remove oxygen which adversely affects rolling fatigue strength, and is also necessary to impart heat resistance.

For this purpose, the higher the Al content, the better; however, if it exceeds 1.0'%, a structure in which fine and uniform cementite is dispersed will be created.
In order to inhibit the effect of the present invention, the upper limit of Al is set to 1.
I will be in charge of 0.

Further, Al is required to be 0.01% or more as a deoxidizing agent.

Therefore, the lower limit of A7 is set to 0.01%.

Oxygen is normally present in steel as Al2O3, which becomes SiO2,
These oxides have a negative effect on lifespan.

The present invention was made with the purpose of further extending the limit of the life extension due to the reduction of these oxide inclusions by suppressing the formation of "mono-plate carbide", so it is necessary to lower the oxygen content as much as possible. And the oxygen content is 0.0015
% or less, the effect of reducing oxide inclusions is saturated, and large oxide inclusions that are closely related to the formation of plate-like carbides are reduced, making the effects of the present invention even more remarkable. Therefore, the upper limit of the oxygen content is set to 0.0015%.
N2 nitrogen combines with A7N and Ti as an impurity in steel to form TiN, etc., which has a negative effect on rolling life.

Therefore, less N2 is better for rolling life. However, in practice, some amount of N2 enters the steel during the melting and agglomeration process.

It has been found that in the steel of the present invention, the effects of the present invention are not inhibited if the content is 0.015% or less.

Therefore, the upper limit of N2 is regulated to 0.015. The reasons for limiting the chemical components regulated in claim 1 have been described above, but depending on the application, even higher hardenability may be required, and for such purposes, in addition to the above components, As shown in claim 2, it is necessary to add MO and/or Ni as follows.

M.O. MO plays an important role in adjusting hardenability.

In the present invention this objective is achieved with a content of up to 0.5 parts. Therefore, the upper limit of MO is set to 0.5chi.

Moreover, since the effect is not recognized below 0.05 ratio, the lower limit is set to 0.05 ratio. Ni In the present invention, Ni is used to adjust the hardenability, but if it contains 1% or more of Ni, carbides will not be finely and uniformly dispersed, so the upper limit of Ni is set at 1.0%.

Further, if the content is less than 0.3%, the desired effect cannot be obtained, so the lower limit is set at 0.3%. As described above, the present invention makes the carbides that act as notches in the steel as fine as possible, and makes the C% in the matrix an appropriate amount to obtain an excellent life.
It can withstand use even at semi-high temperatures up to.

[Brief explanation of the drawing]

FIG. 1 is a drawing showing a heat treatment heat curve showing the difference between the heat treatment according to the present invention and normal heat treatment, and FIG. 2 is a drawing showing a tempering hardness curve between the present invention and normal heat treatment.
Figure 3 is a micrograph showing plate-like carbides.

Claims (1)

[Claims] 1 C: 0.7 to 1.2%, Si: 2.0%, Mn:<
2.0%, Cr: 0.5-2.5%, Al: 0.01-
1.0%, O_2:<0.0015%, N_2:<0.
Steel for rolling bearings consisting of 0.15% balance Fe and other inevitably contained impurities is heated to 1000°C or higher to form a solid solution of carbide, and then rapidly cooled to a temperature range of 300°C to 700°C. After decomposing the austenite, it was rapidly cooled to around room temperature, and then heated at 30°C/min.
Heating to 500°C to 700°C at the above temperature increase rate, the average particle size of the precipitated carbide is 0.1 μm to 0.2 μm.
The temperature was maintained for a period of time such that C% in the matrix was 0.45 to 0.60%, then rapidly cooled to near room temperature, heated to an austenitizing region, and maintained at this temperature for a period of time such that C% in the matrix was 0.45 to 0.60%. A rolling bearing characterized by suppressing the formation of plate-shaped carbides caused by rolling contact stress by subsequently quenching and tempering, and having excellent rolling fatigue strength in a temperature range from room temperature to 200°C. Manufacturing method for industrial steel. 2C: 0.7-1.2%, Si:<2.0%, Mn:
<2.0%, Cr: 0.5-2.5%, Al: 0.01
~1.0%, O_2:<0.0015%, N_2:0.
015% and if necessary Mo: 0.05 to 0.015%.
5% Ni and one or two types of Ni: 0.3 to less than 1.0%, the balance being Fe and other impurities that are inevitably included, is heated to 1000°C or higher to dissolve carbides into solid solution. After that, it was rapidly cooled to a temperature range of 300°C to 700°C to decompose austenite in the temperature range, and then rapidly cooled to around room temperature, and then 30°C/mi
Heating to 500°C to 700°C at a temperature increase rate of n or more,
The average particle size of the precipitated carbide is 0.1 μm to 0.1 μm.
It was held at the temperature for a period of time such that the particle diameter was 2 μm or less, then rapidly cooled to near room temperature, heated to an austenitizing region, and held at the temperature for a period of time when the C% in the matrix was 0.45 to 0.60%. A rolling bearing characterized by suppressing the formation of plate-shaped carbides caused by rolling contact stress by subsequently quenching and tempering, and having excellent rolling fatigue strength in a temperature range from room temperature to 200°C. Manufacturing method for industrial steel.