KR20160056403A - Bearing steel having high fatigue life and manufacturing method for the same - Google Patents

Abstract

An objective of the present invention is to provide bearing steel having excellent fatigue property, and a manufacturing method thereof which improves electrodynamic fatigue life by allowing a bainite structure to exist in a specific fraction. The bearing steel having excellent fatigue property comprises: 0.9-1.1 wt% of carbon (C), 0.01-1.5 wt% of silicon (Si), 0.01-1.5 wt% of manganese (Mn), 0.5-2 wt% of chrome (Cr), and the remainder consisting of iron (Fe) and inevitable impurities. An area fraction occupied by a bainite structure in the entire micro-structure is 40-70%.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a bearing steel having excellent fatigue characteristics and a method of manufacturing the bearing steel.

More particularly, the present invention relates to a bearing steel having excellent fatigue characteristics and a manufacturing method thereof.

The final heat treatment of the conventional bearing steel is quenching and tempering, where the obtained microstructure is tempered martensite and a trace amount of retained austenite. However, as bearing environments become increasingly harsh, the demand for longevity of bearing steels has grown considerably, reaching the limits of tempered martensite to meet these demands.

Heat treatment to obtain bainite structure is frequently performed in place of quenching and tempering to obtain martensitic structure, which is generally due to the fact that the bainite structure is more resistant to toughness and crack growth than the tempered martensite structure Because it is good.

As a heat treatment technique for obtaining such a Vernite structure, 25 to 99% of the austenite structure is transformed into a bainite structure by the constant temperature transformation at a martensitic transformation starting temperature or higher, and then the bainite structure of the remaining austenite structure A method of raising the temperature to promote 100% complete transformation has been proposed. However, the bearing steel of 100% bainite structure has a problem that the strength and hardness are remarkably lower than that of the tempered martensite structure, and it is difficult to realize a desired level of fatigue life.

In addition to this, a method of obtaining 100% bainite structure by thermally transforming the inner and outer rings of the bearing components at 260 ° C. for 4 hours or at 290 ° C. for 1 to 4 hours by a heat treatment technique to obtain a bernite structure, There has been proposed a method of obtaining a 100% bainite structure by constant temperature transformation at 230 to 290 ° C for 1 to 4 hours. However, these 100% bainite structures are significantly lower in strength and hardness than the tempered martensite structure, It is difficult to realize fatigue life.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned problems, and it is an object of the present invention to provide a bearing steel capable of improving the rolling fatigue life of a bearing steel by allowing a bainite structure to exist in a specific fraction.

On the other hand, the object of the present invention is not limited to the above description. It will be understood by those of ordinary skill in the art that there is no difficulty in understanding the additional problems of the present invention.

In one aspect, the present invention provides a method of manufacturing a semiconductor device, comprising the steps of: 0.9 to 1.1 wt% carbon; 0.01 to 1.50 wt% silicon; 0.01 to 1.50 wt% manganese; 0.5 to 2.0 wt% (Fe) and other unavoidable impurities, and provides an excellent bearing fat with an area fraction occupied by the bainite structure of 40 to 70% in the entire microstructure.

At this time, it is preferable that the microstructure of the bearing steel includes martensite structure, bainite structure, and retained austenite structure.

More specifically, the area fraction occupied by the martensite structure in the entire microstructure is 15 to 55%, the area fraction occupied by the bainite structure is 40 to 70%, and the area fraction occupied by the retained austenite structure is 5 To 15%.

In another aspect, the present invention provides a method of manufacturing a semiconductor device, comprising the steps of: 0.9 to 1.1 wt% of carbon; 0.01 to 1.50 wt% of silicon; 0.01 to 1.50 wt% of manganese; 0.5 to 2.0 wt% , And the balance iron (Fe) and other unavoidable impurities; And heat treating the steel material in order to obtain a bainite structure. The present invention provides a method of manufacturing a bearing steel having excellent fatigue characteristics in which the area fraction occupied by the bainite structure in the entire microstructure of the bearing steel is 40 to 70%.

At this time, the step of heat-treating includes: (A) heating the steel material to the austenite region; (B) rapidly heating to a temperature not lower than the martensitic transformation starting temperature (Ms) after heating and keeping the temperature constant; And (C) cooling the mixture to a temperature not higher than the martensitic transformation starting temperature (Ms) after keeping the temperature at a constant temperature.

At this time, the heating temperature is preferably 800 to 900 ° C.

The heating time is preferably 30 to 60 minutes.

Also, it is preferable that the temperature for maintaining the constant temperature is 200 to 300 ° C.

It is preferable that the above-mentioned constant temperature holding time is 10 to 60 minutes.

On the other hand, the steel material may contain 0.9 to 1.1% by weight of carbon (C), 0.01 to 1.50% by weight of silicon (Si), 0.01 to 1.50% by weight of manganese (Mn) And may be prepared by casting, hot rolling and spheroidizing heat treatment of molten steel containing remaining iron (Fe) and other unavoidable impurities.

In addition, the solution of the above-mentioned problems does not list all the features of the present invention. The various features of the present invention and the advantages and effects thereof will be more fully understood by reference to the following specific embodiments.

According to the present invention, a bearing steel excellent in electric fatigue life can be provided by allowing the bainite structure to exist in a specific fraction.

1 is a photograph of a microstructure of a bearing steel having a bainite area fraction of 40 to 70% by an optical microscope.
2 is a photograph of a microstructure of a bearing steel having a bainite area fraction of more than 70% by an optical microscope.
3 is a photograph of a microstructure of a bearing steel having a bainite area fraction of less than 40% by an optical microscope.

Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Further, the embodiments of the present invention are provided to more fully explain the present invention to those skilled in the art.

One. Bearing steel

As a result of extensive research, the inventors of the present invention have found that the present inventors have found that a steel sheet having a specific composition ratio of carbon (C), silicon (Si), manganese (Mn), chromium (Cr), and iron (F) The bearing fatigue life of the bearing steel in which a bainite structure having a specific fraction of bainite structure is superior is obtained and the present invention has been completed.

Specifically, the bearing steel having excellent fatigue characteristics according to the present invention is characterized by containing 0.9 to 1.1% by weight of carbon (C), 0.01 to 1.50% by weight of silicon (Si), 0.01 to 1.50% by weight of manganese (Mn) ): 0.5 to 2.0% by weight, the balance of iron (Fe) and other unavoidable impurities, and the area fraction occupied by the bainite structure in the entire microstructure is 40 to 70%.

First, the reason for adding each component constituting the steel composition of the present invention and the appropriate content range thereof will be described in detail.

Carbon (C): 0.9 to 1.1 wt%

Carbon is a very important element to secure the strength of bearings. If the content of carbon is low, the strength and fatigue strength of the bearing are low, which makes it unsuitable as a bearing part. Therefore, the content of carbon is preferably 0.9 wt% or more, more preferably 0.95 wt% or more. On the other hand, when the carbon content is too high, the untreated large-size carbide remains, thereby lowering the fatigue strength and deteriorating the workability before quenching. Therefore, the upper limit of the carbon content is preferably 1.1 wt% More preferable.

Silicon (Si): 0.01 to 1.50 wt%

When the content of silicon is low, a problem of hardenability may occur. Therefore, it is preferably added in an amount of 0.01 wt% or more, more preferably 0.15 wt% or more. However, if the content of silicon is too high, decarburization may occur due to a site-to-carbon competition reaction, and similarly to carbon, not only the workability before quenching is deteriorated but also the center segregation increases, so that the upper limit of the silicon content is 1.50 weight %, More preferably 1.30% by weight.

Manganese (Mn): 0.01 to 1.50 wt%

Manganese is an important element for securing strength by improving the ingotability of steel. Therefore, the content of manganese is preferably 0.01% by weight or more, and more preferably 0.25% by weight or more. However, when the content of manganese is too high, not only the workability before quenching is deteriorated but also precipitates which adversely affect the center segregation and fatigue characteristics increase, so that the upper limit of the manganese content is preferably 1.50% by weight, more preferably 1.30% More preferable.

Cr (Cr): 0.5 to 2.0 wt%

Chromium is an element effective in improving the ingotability of steel to provide hardenability and refinement of steel structure, so that it is preferably added in an amount of 0.5 wt% or more, more preferably 0.7 wt% or more. However, if the content of chromium is excessive, the effect is saturated, so the upper limit of the content of chromium is preferably 2.0 wt%, more preferably 1.7 wt%.

The remainder of the present invention is iron (Fe). However, in the ordinary steel manufacturing process, impurities which are not intended from the raw material or the surrounding environment may be inevitably incorporated, so that it can not be excluded. These impurities are not specifically mentioned in this specification, as they are known to any person skilled in the art of steel making.

Next, the microstructure of the steel of the present invention will be described in detail.

In the microstructure of the steel of the present invention, the area fraction occupied by the bainite structure in the entire microstructure is 40 to 70%. In general, since the bainite structure is superior to the tempered martensite structure in resistance to toughness and crack growth, it is preferable that the area fraction occupied by the bainite structure in the entire microstructure of the bearing steel after the heat treatment for bainitization is high , The bearing steel having a high fraction of bainite structure is harder to attain the desired level of fatigue characteristics than the tempered martensite structure because the strength and hardness are remarkably reduced. Therefore, the area fraction of the preferable bainite structure is 40 to 70%.

More specifically, it is preferable that the microstructure of the steel of the present invention is a mixed structure including a martensite structure, a bainite structure, and a residual austenite structure, wherein an area fraction occupied by the martensite structure in the entire microstructure is 15 to 55 %, The area fraction occupied by the bainite structure is 40 to 70%, and the area fraction occupied by the retained austenite structure is more preferably 5 to 15%. In the case of a mixed structure having such an area fraction, the effect of the present invention can be most preferably realized. On the other hand, bainite may be partially contained in the martensite structure.

2. Manufacturing method of bearing steel

Hereinafter, a method of manufacturing a bearing steel excellent in fatigue characteristics of the present invention will be described.

The method for producing a bearing steel excellent in fatigue characteristics according to the present invention comprises the steps of: 0.9 to 1.1% by weight of carbon (C), 0.01 to 1.50% by weight of silicon (Si), 0.01 to 1.50% by weight of manganese (Mn) : 0.5 to 2.0 wt.%, Balance iron (Fe) and other unavoidable impurities; And heat treating the steel material to obtain a bainite structure.

First, the steel material is not particularly limited, and molten steel having the above-described components can be manufactured through a series of casting, hot rolling and spheroidizing heat treatment. Generally, the bearing steel is manufactured by subjecting the cast steel to a hot rolling and a spheroidizing annealing process and then a final heat treatment, which means the state before the final heat treatment after the spheroidizing heat treatment.

Meanwhile, the conditions of the hot rolling and spheroidizing heat treatment are not particularly limited, and the conditions to be applied in the production of normal bearing steel may be applied. For example, a steel having the above composition is manufactured through a series of casting, hot rolled at a finishing temperature of about 800 to 900 DEG C, and slowly cooled to obtain about 100% pearlite structure And then heated at a temperature of about 770 to 830 ° C, held for about 6 hours, and then cooled to a supercool temperature of 0.5 ° C / s or lower to produce a steel material.

Next, the heat treatment step refers to a final heat treatment after the spheroidizing heat treatment. The conventional heat treatment of the bearing steel is generally performed by quenching and tempering. In this case, the microstructure obtained is tempered martensite and a trace amount of retained austenite , And did not contain bainite. Accordingly, the present invention provides a final heat treatment for obtaining a bainite structure, and more particularly, (A) heating a steel material to austenite region; (B) rapidly heating to a temperature not lower than the martensitic transformation starting temperature (Ms) after heating and keeping the temperature constant; And (C) cooling to a temperature not higher than the martensitic transformation starting temperature (Ms) after the keeping at a constant temperature. The microstructure of the bearing steel produced through such a final heat treatment may include a certain fraction of the bainite structure.

At this time, detailed conditions for each step of the final heat treatment are as follows.

Steel heating temperature: 800 ~ 900 ℃

First, the obtained steel material is heated to the austenite region, and the heating temperature is preferably 800 to 900 占 폚. If the heating temperature exceeds 900 ° C, the spheroidized carbides are completely dissolved into the matrix, and the fatigue characteristics of the bearing steel may be deteriorated. If the heating temperature is lower than 800 ° C, the complete transformation into 100% austenite is not achieved and ferrite may remain in the final microstructure. Also, the homogenization of the austenite may not be completed and bainite or martene Problems such as inconsistency in site organization may occur. On the other hand, the rate of temperature rise to the temperature is not particularly limited, and may be, for example, about 0.5 to 2.0 DEG C / sec.

Heating time: 30 to 60 minutes

On the other hand, the heating time is preferably 30 to 60 minutes. When the heating temperature exceeds 60 minutes, the spheroidized carbides completely dissolve into the matrix or the amount of remaining spheroidized carbides is very small, and the fatigue characteristics of the bearing steel may be deteriorated. If the heating temperature is less than 30 minutes, the complete transformation to 100% austenite is not carried out and ferrite may remain in the final microstructure. Moreover, homogenization of the austenite may not be completed and bainite or martene Problems such as inconsistency in site organization may occur.

Quenching temperature / holding temperature: 200 ~ 300 ℃

After heating the steel material to the austenite region, it is quenched to a temperature equal to or higher than the martensitic transformation starting temperature (Ms) for bainite transformation. It is difficult to obtain a desired hardness value at higher temperatures, It is preferable to quench at a temperature of 200 to 300 DEG C, which is slightly higher than the martensitic transformation start temperature (Ms), and to maintain the temperature at this temperature. On the other hand, the quenching rate to the temperature is not particularly limited, and may be, for example, about 10 to 100 ° C / sec.

Constant keeping time: 10 to 60 minutes

It is preferable that the constant temperature holding time is 10 to 60 minutes. The longer the holding time for the constant-temperature transformation at the constant-temperature holding temperature, the larger the bainite fraction generated by the transformation becomes, while the longer the time, the lower the productivity. On the other hand, as described above, the preferred bainite fraction is 40 to 70%, and according to the study of the inventors of the present invention, it was found that the fraction in this range is obtained when the temperature is maintained in the constant temperature transformation temperature range for 10 to 60 minutes.

The bearing steel having excellent fatigue life can be obtained by cooling to a temperature not higher than the martensitic transformation starting temperature (Ms) after maintaining the temperature at such a condition. At this time, the cooling step is not particularly limited, and may be performed at a cooling rate of, for example, about 1 to 50 ° C / sec.

On the other hand, in the entire microstructure of the bearing steel after the heat treatment, the area fraction occupied by the bainite structure is about 40 to 70%. More specifically, the microstructure of the bearing steel after the heat treatment is a mixed structure including a martensite structure, a bainite structure, and a residual austenite structure, wherein an area fraction occupied by the martensite structure in the entire microstructure is 15 to 55% , The area fraction occupied by the bainite structure is 40 to 70%, and the area fraction occupied by the retained austenite structure is 5 to 15%.

To summarize, a method of manufacturing a bearing steel having excellent fatigue characteristics according to the present invention is characterized in that a steel material having the above composition is prepared through a series of casting, hot rolling, spheroidizing heat treatment and the like and then heated in an austenite region at 800 to 900 ° C, After heating, it is quenched to a temperature of 200 to 300 ° C which is a temperature higher than the martensitic transformation starting temperature (Ms), is kept at a constant temperature for 10 to 60 minutes, cooled at a temperature below the martensitic transformation start temperature (Ms) The microstructure of the bearing steel includes martensite structure, bainite structure, and retained austenite structure, and the area fraction occupied by the bainite structure in the entire microstructure is 40 to 70%.

Hereinafter, the present invention will be described more specifically by way of examples.

Example

A steel material having an alloy composition (% by weight) as shown in Table 1 was produced through a series of casting, hot rolling and spheroidizing heat treatment. Specifically, the steel having such an alloy composition is manufactured through a series of casting processes, followed by hot rolling at a finishing temperature of about 850 DEG C, followed by gradual cooling to make a pearlite structure of about 100% , Maintained for about 6 hours, and then cooled at a cooling rate of 0.5 DEG C / s or lower to prepare a steel material. Thereafter, heat treatment for bainitization was performed under the heat treatment conditions shown in Table 1 below to produce bearing steel.

The bainite area ratio and relative fatigue life of the bearing steels were measured and the results are also shown in Table 1 below. Relative Fatigue Life in Table 1 shows the relative ratio to the fatigue life of Condition 1. The fatigue life was measured by a rolling contact fatigue test. The test conditions were a rotational speed of 1,000 rpm and contact stress of 5,248 MPa.

Condition Alloy composition Bainite heat treatment Bay
Night
Area ratio
(%) opponent
Fatigue life C Si Mn Cr Heating temperature
(° C) Maintaining the temperature constant
(Quench) temperature
(° C) Retention time
(minute) One 0.87 0.28 0.36 1.42 850 250 60 49 1.00 2 1.04 1.62 0.41 1.61 36 1.06 3 0.99 0.30 1.68 0.98 42 0.92 4 1.11 0.84 0.75 0.41 21 0.84 5 1.01 0.26 0.34 1.44 790 250 60 38 1.08 6 820 45 1.58 7 850 62 2.17 8 910 72 1.12 9 0.95 0.07 1.44 1.81 850 190 60 35 1.15 10 220 44 1.59 11 250 54 1.65 12 280 68 1.54 13 310 77 1.01 14 0.97 0.38 0.82 0.79 850 250 5 17 0.97 15 10 43 1.84 16 30 50 2.01 17 60 61 2.08 18 90 89 1.11 19 1.03 0.65 0.62 1.28 825 275 30 53 2.77 20 1.08 1.45 0.08 0.97 875 225 45 61 1.95 21 1.00 0.95 1.29 0.52 850 250 60 48 1.68

First, in the bearing steels according to the conditions 1 to 4 in Table 1 above, the content range of the alloy composition is out of the range suggested in the present invention, and in this case, the relative fatigue life is not improved even though the bainite heat treatment is performed.

Next, the bearing steels according to the conditions 5 to 8 in Table 1 were obtained by varying the heating temperature for bainite heat treatment. When the heating temperature was 800 to 900 ° C, the bainite area ratio satisfied 40 to 70% And the fatigue life improving effect was excellent.

On the other hand, FIG. 1 shows the result of microscopic observation of the microstructure of the bearing steel according to Condition 7. 1, the bearing steel according to an embodiment of the present invention is composed of a martensite structure, a bainite structure, and a residual austenite structure, wherein the area fraction occupied by the martensite structure is about 27% The area fraction occupied by the bainite structure is about 62%, and the area fraction occupied by the retained austenite structure is about 11%.

Next, in Table 1, the bearing steels according to Conditions 9 to 13 have different bendite constant transformation temperatures. When the bainite constant temperature was maintained at 200 to 300 ° C, the bainite area ratio was 40 to 70%, and the relative fatigue life Improvement effect.

On the other hand, FIG. 2 shows the result of observing the microstructure of the bearing steel according to condition 13 with an optical microscope. As can be seen from Fig. 2, in this case, the area ratio occupied by the bainite structure exceeds 70%.

Next, in Table 1, the bearing steels according to the conditions 14 to 18 have different bainitic constant temperature transformation holding times. When the holding time is 10 to 60 minutes, the bainite area ratio satisfies 40 to 70% and the relative fatigue life The improvement effect was particularly good.

3 shows the result of observing the microstructure of the bearing steel according to condition 14 with an optical microscope. As can be seen from Fig. 3, in this case, the area ratio occupied by the bainite structure is less than 40%.

Next, the bearing steels according to the conditions 19 to 21 in Table 1 are various examples satisfying the numerical range proposed in the present invention. All of the bearing steels satisfy the bainite area ratio of 40 to 70% and the effect of improving the relative fatigue life .

As described above, it is preferable that 0.9 to 1.1 wt% of carbon (C), 0.01 to 1.50 wt% of silicon (Si), 0.01 to 1.50 wt% of manganese (Mn), 0.5 to 2.0 wt% of chromium (Cr) (Fe) and other unavoidable impurities is heated in an austenite region at 800 to 900 占 폚, quenched at a temperature of 200 to 300 占 폚 and maintained at a constant temperature for 10 to 60 minutes to obtain a steel When the bainite structure fraction is set to 40 to 70%, the service life of the bearing steel can be improved.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, It will be obvious to those of ordinary skill in the art.

Claims (10)

(Cr): 0.5 to 2.0% by weight, and the balance of iron (Fe), carbon (C): 0.9 to 1.1 wt%, silicon (Si): 0.01 to 1.50 wt%, manganese And other unavoidable impurities, and the bearing fraction of the bainite structure in the entire microstructure is 40 to 70%.
The method according to claim 1,
Wherein the microstructure of the bearing steel comprises martensite structure, bainite structure, and retained austenite structure.
3. The method of claim 2,
The area fraction occupied by the martensite structure in the entire microstructure is 15 to 55%, the area fraction occupied by the bainite structure is 40 to 70%, and the area fraction occupied by the retained austenite structure is 5 to 15% Bearing steel with excellent characteristics.
(Cr): 0.5 to 2.0% by weight, and the balance of iron (Fe), carbon (C): 0.9 to 1.1 wt%, silicon (Si): 0.01 to 1.50 wt%, manganese And other inevitable impurities; And heat treating the steel material to obtain a bainite structure,
A manufacturing method of a bearing steel excellent in fatigue characteristics in which the area fraction occupied by the bainite structure in the entire microstructure of the bearing steel is 40 to 70%.
5. The method of claim 4,
The heat treating step comprises: (A) heating the steel material to the austenite area; (B) rapidly heating to a temperature not lower than the martensitic transformation starting temperature (Ms) after heating and keeping the temperature constant; And (C) cooling at a temperature not higher than the martensitic transformation starting temperature (Ms) after maintaining the temperature at a constant temperature.
6. The method of claim 5,
Wherein the heating temperature is 800 to 900 占 폚.
6. The method of claim 5,
Wherein the heating time is 30 to 60 minutes.
6. The method of claim 5,
Wherein the constant temperature holding temperature is 200 to 300 占 폚.
6. The method of claim 5,
Wherein the constant-temperature holding time is 10 to 60 minutes.
5. The method of claim 4,
Wherein the steel material contains 0.9 to 1.1% by weight of carbon (C), 0.01 to 1.50% by weight of silicon (Si), 0.01 to 1.50% by weight of manganese (Mn) (Fe) and other unavoidable impurities is prepared by casting, hot rolling and spheroidizing heat treatment.

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