WO2017170435A1 - Environment-resistant bearing steel excellent in producibility and resistance to hydrogen embrittlement - Google Patents

Abstract

The present invention provides an environment-resistant bearing steel excellent in terms of producibility and resistance to hydrogen embrittlement, the bearing steel being characterized by having a composition which contains, in terms of mass%, 0.5-1.0% C, up to 0.1% Si, 0.4-1.5% Mn, up to 0.03% P, up to 0.03% S, 1.5-3.5% Cr, up to 0.050% Al, up to 0.0015% O, up to 0.003% Ti, and up to 0.015% N, with the remainder comprising Fe and unavoidable impurities. The bearing steel is further characterized in that after spheroidizing annealing, the steel has a hardness of 92 HRB or less and that after carbonitriding, the steel has a surface-layer N concentration of 0.1-1.0%, a surface-layer C concentration of 0.8-1.5%, and a surface-layer hardness of 58 or greater but less than 64 in terms of HRC, the number density of coarse nitride grains of CrN or MnSiN₂ which have grain diameters of 2 µm or larger being 10³ grains/mm² or less. The bearing steel is characterized in that fine nitride grains have been dispersed and precipitated.

Description

Environmentally resistant bearing steel with excellent manufacturability and hydrogen embrittlement resistance

The present invention relates to an environmentally resistant bearing steel that has excellent manufacturability (carburizing time and workability), suppresses a decrease in life due to hydrogen embrittlement peeling, and has a long life. Conventionally, bearing parts such as automobiles, industrial equipment, etc., under severe load conditions such as high vibration, high load, sudden acceleration / deceleration, etc. and combined with specific lubricating oil and water mixing conditions, etc. There is a problem that short-lived early peeling occurs, and the cause of this early peeling is that hydrogen is generated on the rolling surface in the rolling process, and when it enters the inside, hydrogen embrittlement occurs and the peeling life is significantly reduced. It is considered. The present invention provides means for solving such problems.

In recent years, bearing parts used in automobiles and industrial equipment have become severer in terms of performance with higher performance and higher speed. There are various types of lubricating oil including a new transmission CVT (continuous variable transmission), and there is a case in which early peeling due to a peeling form different from the conventional one occurs.

For example, in an automobile alternator bearing, early peeling may occur with a change in the structure of a dendritic white layer along a grain boundary different from a white band that is a conventional change in structure. This is thought to be because hydrogen embrittlement peeling occurred due to the decomposition of the lubricating oil under severe load conditions of high vibration and high load, generation of hydrogen on the rolling surface, and penetration into the interior.
On the other hand, in the alternator bearing, this early peeling has been prevented by changing the lubricating oil. However, due to the harsh and diversified use conditions of bearing parts, there is a tendency to increase the conditions under which hydrogen embrittlement delamination occurs, a problem that has hardly become a problem in rolling fatigue failure of conventional bearing steel. It is becoming impossible to suppress by itself. For this reason, a material having a long life against hydrogen embrittlement delamination in such rolling fatigue has been demanded.

The material strength reduction phenomenon due to hydrogen embrittlement has been known for some time. For example, in springs and bolt parts, diffusible hydrogen entering from the exposure environment due to decomposition of water or the like causes delayed destruction. As steel for springs and bolts with excellent delayed fracture resistance, steel with a large amount of fine carbonitrides precipitated, trapping diffusible hydrogen, and suppressing diffusion of hydrogen to grain boundaries and stress concentration parts is used. It has been. As already disclosed in Patent Document 1, the present inventor has studied the combination of various alloy elements based on SUJ2, and as a result, by adding V, a fine V-based carbide of about several tens to several hundreds of nanometers is added. It has been found that a decrease in life due to hydrogen embrittlement under repeated fatigue conditions is suppressed.

Also, as disclosed in Patent Document 2, by forming a Cr oxide film with Cr-added bearing steel, hydrogen penetration can be suppressed and the hydrogen embrittlement life can be extended.

As further disclosed in Patent Document 3, hydrogen embrittlement is achieved by hydrogen trapping of fine nitrides such as CrN and MnSiN ₂ deposited on the surface layer by carbonitriding the case-hardened steel to which Cr, Mn, etc. are added as disclosed in Patent Document 3. It is necessary to extend the service life.

However, when carburizing and nitriding a case-hardened steel such as SCM440, a long-time carburizing process is required to obtain a predetermined surface C concentration and C concentration depth distribution, and productivity is lowered. In order to shorten the carburizing time and increase the productivity, it has been required to perform carbonitriding on bearing steel represented by SUJ2 having a high initial C concentration.

However, just adding an alloying element such as Cr or Mn to bearing steel represented by SUJ2 increases the material hardness and decreases the workability such as machinability and cold forging.

Based on the above, using bearing steel that can be carbonitrided in a short time, adding alloy elements such as Cr and Mn in order to prolong the hydrogen embrittlement life, and ensuring the desired workability Development of steel with reduced hardness has been demanded.

Japanese Unexamined Patent Publication No. 2006-213981 Japanese Laid-Open Patent Publication No. 5-26244 Japanese Unexamined Patent Publication No. 2011-225936

The present invention has been made in the background as described above, and an object of the present invention is an alloy element (composition ratio of chemical components) based on bearing steel represented by SUJ2 for shortening the carbonitriding time. Optimized and carbonitrided to have excellent rolling fatigue life and work such as cold forgeability and machinability even when used under atmospheric conditions where hydrogen brittle exfoliation has occurred in the past The object is to provide an environmentally resistant bearing steel having excellent properties.

The improvement of the hydrogen embrittlement life by carbonitriding is due to the hydrogen trap of fine nitride deposited on the surface layer. The fine nitride is produced by CrN and MnSiN _2, and it is effective to increase the amount of Cr and Mn for the improvement.
However, just increasing the amount of Cr and Mn in the bearing steel increases the material hardness after spheroidizing annealing to 93HRB or higher. In order to reduce the material hardness, it is effective to lower the C concentration. However, if the C concentration is lowered too much, the carbonitriding time becomes longer and the productivity is lowered.
On the other hand, it has been found that Si reduction is effective in reducing hardness and increases the number of formed nitrides to improve hydrogen embrittlement resistance. This is presumably because the total number of nitrides is increased and the toughness of the mother layer is improved by changing the generated nitride from MnSiN ₂ to CrN by reducing Si.

The inventor of the present application conducted various tests and found a component range of C amount and Si amount capable of achieving both manufacturability (carburizing time and workability) and hydrogen embrittlement resistance.
Moreover, although hardenability fell by Si amount reduction, it discovered that hydrogen embrittlement resistance was improved and hardenability could be supplemented by adding Mn amount.

That is, in the bearing steel of the present invention, the alloy element content is expressed by mass%, C: 0.5 to 1.0%, Si: 0.1% or less, Mn: 0.4 to 1.5%, P: 0.03% or less, S: 0.03% or less, Cr: 1.5 to 3.5%, Al: 0.050% or less, O: 0.0015% or less, Ti: 0.003% or less , N: 0.015% or less, balance Fe and inevitable impurities composition, hardness after spheroidizing annealing is 92 HRB or less, surface layer N concentration after carbonitriding 0.1-1.0%, surface layer The number density of coarse CrN or MnSiN ₂ nitride having a C concentration of 0.8 to 1.5%, a surface hardness of 58 to less than 64, and a particle size of 2 μm or more is 10 ³ pieces / mm ² or less, It is an environmentally resistant bearing steel excellent in manufacturability and hydrogen embrittlement resistance, characterized in that fine nitrides are dispersed and precipitated.

In addition to the alloy elements described above, the bearing steel of the present invention includes V: 0.05 to 2.0%, Ni: 0.1 to 3.0%, Mo: 0.05 to 2.0%. It is preferable to further include seeds or two or more kinds.
That is, the bearing steel of the present invention is expressed by mass%, C: 0.5 to 1.0%, Si: 0.1% or less, Mn: 0.4 to 1.5%, P: 0.03% S: 0.03% or less, Cr: 1.5 to 3.5%, Al: 0.050% or less, O: 0.0015% or less, Ti: 0.003% or less, N: 0.015 %: V: 0.05 to 2.0%, Ni: 0.1 to 3.0%, Mo: 0.05 to 2.0%, and further including one or more of them, The composition is preferably composed of the balance Fe and inevitable impurities.

According to the present invention, it is possible to provide an environmentally resistant bearing steel that has excellent manufacturability (carburizing time and workability), suppresses a decrease in life due to hydrogen embrittlement peeling, and has a long life.

It is the figure which showed an example of the carbonitriding conditions in an Example. It is explanatory drawing of the method of a 2-cylinder rolling fatigue test.

The bearing steel of the present invention will be described.
The bearing steel of the present invention is a steel material composed of a chemical component (composition) described later, and has excellent manufacturability and hydrogen embrittlement resistance by performing at least two treatments of spheroidizing annealing and carbonitriding and quenching and tempering. Can be used as environmentally resistant bearing steel.
The bearing steel of the present invention is a steel material having a specific chemical composition (composition) described later, and when spheroidizing annealing is performed, the hardness immediately after that is 92 HRB or less, and carbonitriding was performed. In this case, the surface layer N concentration immediately after that is 0.1 to 1.0%, the surface layer C concentration is 0.8 to 1.5%, the surface layer hardness is HRC 58 or more and less than 64, and the grain size is 2 μm or more. If the number density of the nitride of CrN or MnSiN ₂ is 10 ³ pieces / mm ² or less, the steel material after performing two treatments, spheroidizing annealing treatment and carbonitriding quenching and tempering treatment, Alternatively, it may be a steel material before performing at least one of these two treatments.
Any of these corresponds to the bearing steel of the present invention.

The reason for limiting the chemical composition of the environmentally resistant bearing steel of the present invention will be described. Hereinafter, unless otherwise specified, “%” means “mass%”.

C Regarding the content of C (0.5 to 1.0%), C is an essential element for securing strength as a rolling bearing. However, if the amount of C is less than 0.5%, a long time carburizing treatment is required to obtain the surface C concentration and C concentration depth distribution necessary for maintaining the strength, and the productivity is lowered. The lower limit of the amount was limited to 0.5%. However, when the C content exceeds 1.0%, the material hardness after spheroidizing annealing is increased, and it is found that workability such as cold forgeability and machinability is deteriorated. Therefore, the upper limit of the C amount is set to 1.0%.

With respect to the Si content (0.1% or less), Si is used as a deoxidizing agent when producing steel. However, adding over 0.1% increases the material hardness after spheroidizing annealing, lowers the machinability and cold forgeability, and the nitride form after carbonitriding changes from CrN to MnSiN ₂ By changing, the total number of nitrides serving as hydrogen trap sites is reduced and the hydrogen embrittlement resistance is also lowered. Therefore, the upper limit is set to 0.1%.

Regarding the Mn content (0.4 to 1.5%), Mn is an element used for deoxidation when manufacturing steel. Mn is an element that improves hardenability, and complements the decrease in hardenability due to the suppression of the amount of C and Si for cold forgeability and machinability, and MnSiN that becomes a hydrogen trap site after carbonitriding _The hydrogen embrittlement resistance can be improved by finely increasing ₂ to precipitate 2. However, in order to obtain this effect, an Mn amount of 0.4% or more is required, so the lower limit of Mn is 0.4%. did. However, if Mn is contained in a large amount exceeding 1.5%, the material hardness after spheroidizing annealing is increased and the machinability and cold forgeability are lowered, so the upper limit of Mn content is 1.5%. Limited to.

Regarding the P content (0.03% or less), P segregates at the austenite grain boundaries of the steel, leading to a decrease in toughness and rolling fatigue life, so 0.03% was made the upper limit of the P content.

Regarding S content (0.03% or less), S impairs hot workability of steel and forms non-metallic inclusions in the steel to reduce toughness and rolling life. Although it is desirable to reduce it, since S also has the effect of improving the machinability, 0.03% was made the upper limit of S.

Regarding the Cr content (1.5 to 3.5%), Cr is added to improve hardenability, ensure hardness by carbides and improve life. Furthermore, by depositing CrN after carbonitriding, it becomes a hydrogen trap site and can improve hydrogen embrittlement resistance. In order to obtain this effect, addition of 1.5% or more is necessary, so the lower limit value of the Cr content is limited to 1.5%. However, if the content exceeds 3.5%, the material hardness after spheroidizing annealing is increased, and the machinability and cold forgeability are reduced, and a large carbonitride is formed, which has a rolling fatigue life. Since the reduction occurs, the upper limit of the Cr content is set to 3.5%.

Regarding Al content (0.050% or less), Al is used as a deoxidizer during steel production, but it is reduced to produce hard non-metallic inclusions and reduce rolling fatigue life. Is desirable. When the Al content exceeds 0.050%, a significant decrease in rolling fatigue life is observed. Therefore, the upper limit of the Al content is set to 0.050%.
In order to reduce the Al content to less than 0.005%, the steel manufacturing cost increases, so the lower limit of the Al content is preferably set to 0.005%.

Regarding Ti content (0.003% or less), O (oxygen) content (0.0015% or less), and N content (0.015% or less), Ti, O and N are oxidized in steel. Ti, 0.003%, O: 0.0015%, and N: 0.015% are included in each element in order to reduce fatigue life as non-metallic inclusions by forming oxides and nitrides. The upper limit.

For V content (0.05 to 2.0%), V precipitates fine V-based carbides with a particle size of several hundred nanometers or less, and suppresses hydrogen embrittlement delamination by trapping diffusible hydrogen in steel. There is an effect to. In order to obtain this effect, the V content is preferably 0.05% or more. However, if the content exceeds 2.0%, the workability such as machinability and forgeability deteriorates, so the upper limit of the V content is preferably 2.0%.

Regarding the Ni content (0.1-3.0%), Ni suppresses the structural change during the rolling fatigue process and improves the rolling fatigue life. Further, the addition of Ni is effective in improving toughness and corrosion resistance. In order to obtain these effects, the Ni content is preferably 0.1% or more. However, if it is contained in a large amount exceeding 3.0%, a large amount of retained austenite is generated at the time of quenching the steel, and the predetermined hardness cannot be obtained, and the cost of the steel material may increase, so the Ni content Is preferably 3.0%.

Regarding the Mo content (0.05 to 2.0%), Mo has the effect of improving the hardenability of the steel and suppressing the decrease in hardness during tempering by dissolving in the carbide. In order to obtain this effect, the Mo content is preferably 0.05% or more. However, if the content exceeds 2.0%, the cost of the steel material is increased, and hot workability and machinability may be reduced. Therefore, the upper limit value of Mo is preferably 2.0%. .

As described above, the bearing steel of the present invention is a steel material containing C, Si, Mn, P, S, Cr, Al, O, Ti, and N at a specific ratio, and one or more of V, Ni, and Mo It is preferable that it contains 2 or more types, and the balance may be Fe and inevitable impurities.
Examples of inevitable impurities that can be contained in Fe include conventionally known components. For example, Cu etc. are mentioned. The smaller the content of inevitable impurities, the better.

Next, the surface hardness and CrN or MnSiN ₂ nitride of the environmental bearing steel of the present invention will be mentioned.

<Surface hardness HRC 58 or more and less than 64>
There is a correlation between the surface hardness after tempering and the rolling fatigue life, and the higher the surface hardness, the longer the rolling fatigue life. In particular, when the surface hardness after tempering is HRC58 or less, the rolling fatigue life is abruptly reduced and the variation in the life is increased, so the surface hardness after tempering is set to 58 HRC or more. Moreover, the reason for setting it to less than HRC64 is that when the surface hardness is increased, the sensitivity to hydrogen embrittlement is increased, and the rolling fatigue life is significantly reduced by hydrogen embrittlement delamination.
In addition, the measuring method of surface hardness (HRC) is mentioned later.

<When the number of coarse CrN or MnSiN ₂ nitride having a particle size of 2 μm or more is 10 ³ pieces / mm ² or less>
In order to improve the hydrogen embrittlement surface fatigue strength, it is necessary to precipitate a large number of fine nitrides. That is, of the nitrides, nitrides effective for hydrogen trapping are fine Cr nitrides (eg, CrN) having a particle size of 300 nm or less, and composite nitrides of Mn and Si (eg, MnSiN ₂ ). However, increasing the surface layer N concentration and the alloy element facilitates the formation of coarse nitrides having a large particle size, which causes a decrease in strength. If the number ratio of coarse CrN or MnSiN ₂ nitride having a particle size of 2 μm or more exceeds 10 ³ pieces / mm ² , the hydrogen embrittlement type surface fatigue strength is significantly reduced. The upper limit of the number ratio was 10 ³ pieces / mm ² .
A method for measuring the number of coarse CrN or MnSiN ₂ nitride having a particle diameter of 2 μm or more will be described later.

<Surface layer C concentration (surface layer carbon concentration): 0.8 to 1.5%>
The surface layer C is an essential element for securing the strength as a rolling bearing, and the surface layer C concentration of 0.8% or more is necessary to maintain the hardness after a predetermined heat treatment. The lower limit was defined as 0.80%. On the other hand, when the surface layer C concentration exceeds 1.5%, it was found that large carbides are generated and the rolling fatigue life is reduced, so the upper limit of the surface layer C concentration was 1.5%.
In addition, the measuring method of surface layer C density | concentration is mentioned later.

<Surface layer N concentration (surface layer nitrogen concentration): 0.1 to 1.0%>
The surface layer N works as a hydrogen trap site by generating fine nitride in the surface layer, and improves hydrogen embrittlement resistance. It also improves the rolling life by improving the softening resistance of the steel. In order to obtain these effects, the surface layer N concentration needs to be 0.1% or more, so the lower limit of the surface layer N concentration was set to 0.1%. On the other hand, when the surface layer N concentration exceeds 1.0%, the surface hardness is lowered due to the formation of retained austenite, and a predetermined surface hardness cannot be obtained. Therefore, the upper limit of the surface layer N concentration is set to 1.0%.
A method for measuring the surface layer N concentration will be described later.

The method for producing the bearing steel of the present invention is not particularly limited. The bearing steel of the present invention can be obtained by adjusting the raw materials so as to contain the specific chemical component (composition) as described above, and dissolving and solidifying by a conventionally known method.

In addition, the raw material is adjusted so that the bearing steel of the present invention contains the specific chemical component (composition) as described above, melted and solidified by a conventionally known method, and then rolled and subjected to spheroidizing annealing treatment. By performing the carbonitriding quenching and tempering process, the bearing steel for environment resistance having excellent manufacturability and hydrogen embrittlement resistance is obtained.
Here, the rolling is preferably hot rolling and low temperature rolling. For example, it is preferable to perform hot rolling at 1000 to 1200 ° C. and then cold rolling at 700 to 900 ° C.
In addition, the spheroidizing annealing treatment includes heating to 700 to 800 ° C., holding for 1 to 10 hours, cooling to 450 to 700 ° C. at −5 to −30 ° C./hour, and then air cooling. Is done.
The carbonitriding quenching and tempering process is exemplified by the process shown in FIG.

Hereinafter, examples of the present invention will be described. Bar material with diameters of 32 mm and 65 mm by hot forging at a forging temperature of 1200 ° C. and a final temperature of 900 ° C. after melting the materials of chemical components shown in Table 1 by vacuum melting of 150 kg and holding at a heating temperature of 1200 ° C. for 3 hours. Manufactured. Thereafter, it is heated to 920 ° C. as a normalizing treatment, held for 2 hours, then air-cooled, further heated to 760 ° C. as a spheroidizing annealing treatment, held for 3 hours, and then 650 ° C. at −15 ° C./hour. After cooling to room temperature, it was air-cooled and used as a material for each test (hereinafter also referred to as “spheroidizing annealing material”).

Here, the spheroidizing hardness was measured. The spheroidizing annealing hardness is a hardness test piece in which the cross section of the spheroidizing annealing material having a diameter of 32 mm (surface perpendicular to the longitudinal axis of the steel bar) is exposed, and is ½ of the cross section The HRB hardness (JIS Z2245) was obtained from the radius part with a Rockwell hardness tester by an average of 4 points.
The measurement results are shown in the column of “spheroidizing annealing hardness (HRB)” in Table 2.

Next, a test piece having a cross-sectional diameter of 25 mm and a length of 100 mm was cut out from a spheroidized annealed material having a diameter of 32 mm and treated under various carbonitriding conditions. The carbonitriding treatment was performed under various carbonitriding conditions (carburizing temperature, carburizing time, carbon potential, ammonia concentration) in a mixed atmosphere in which ammonia gas was added to carburizing gas, followed by quenching and tempering treatment. FIG. 1 is an example of the carbonitriding conditions used.
In Table 1, No. 5 and no. As for the steel No. 12, when the N concentration in austenite increases, the martensite transformation start temperature (Ms point) decreases, the amount of retained austenite after quenching increases, and the surface hardness may be insufficient. The intermediate annealing which hold | maintains at 1 degreeC for 1 hour was performed, and the secondary quenching was performed at 840 degreeC.

The test piece obtained by performing the carbonitriding quenching and tempering treatment as described above was ground at a depth of 0.15 mm, and the Rockwell hardness (conforming to JIS Z2245) was determined for the outer peripheral portion with an average of 5 points.
The results are shown in the column of “surface hardness (HRC)” in Table 2.

Thereafter, the test piece obtained by performing the carbonitriding quenching and tempering treatment as described above was embedded in the resin so that the cross section (surface perpendicular to the longitudinal axis of the steel bar) was exposed, and the cross section was polished and finished. The C and N concentrations in the surface layer were analyzed by EPMA.
The results are shown in the columns of “surface layer C concentration” and “surface layer N concentration” in Table 2.
Here, the surface layer C and N concentrations were the maximum values (peak values) of the C and N concentrations from the outermost layer to a depth of 10 μm.

Further, for the test piece, the number of nitrides having a particle diameter of 2 μm or more existing in the depth region from the surface layer to a depth of 100 μm was measured using FE-SEM and elemental analysis (EDX). The number density (pieces / mm ² ) of coarse nitrides having a particle size of 2 μm or more was determined.
The results are shown in Table 2.

Further, after obtaining a cylindrical roughing test piece having a cross-sectional diameter of 26.3 mm from a spheroidized annealing material having a diameter of 32 mm, the above-mentioned carbonitriding treatment was performed on each test piece, and thereafter 0.15 mm of the surface was reduced. Roughing to finish grinding was performed to produce a cylindrical test piece with a test surface diameter of 26 mm. And about this cylindrical test piece, the 2-cylinder rolling fatigue test was done using the roller pitching testing machine (made by Nikko Create) shown in FIG. In FIG. 2, reference numeral 18 denotes a cylindrical test piece. In the method shown in FIG. 2, a mating cylinder 20 made of a tempered tempering material of JIS SUJ2 is pressed against the test piece 18 with a predetermined surface pressure. The test piece 18 was rotated through 24 and the rotation of the motor 22 was transmitted to the shaft 30 through the gears 26 and 28 to rotate the counterpart cylinder 20.
Here, the counterpart cylinder is a SUJ2 quenching and tempering material, and the shape is a cylinder with a diameter of 130 mm having a crowning with a curvature radius of 150 mm in the axial direction. The test conditions were such that the hydrogen embrittlement type surface fatigue peeling was reproduced. The test was performed under the test conditions (oil temperature 90 ° C., slip rate −60%, surface pressure 3 Gpa, rotation speed 1500 rpm) in which a hydrogen embrittlement type lubricating oil was used and an early rolling fatigue failure of the hydrogen embrittlement type occurred. Here, the slip ratio is a ratio of the difference between the peripheral speeds of the test cylinder and the counterpart cylinder and the peripheral speed of the test cylinder. The test was performed at four points under the same conditions, and the average life was obtained.
Table 2 shows the test results.

As shown in Table 2, the steels of the present invention corresponding to the present invention each have a surface hardness (HRC) of 58 or more and less than 64, the surface layer C content in the range of 0.8 to 1.5%, and the surface layer N content. Is in the range of 0.1 to 1.0%, and the number of coarse nitrides having a particle size of 2 μm or more is 10 ³ pieces / mm ² or less.
Further, the average life of the two-cylinder test of the steel of the present invention is excellent at 10.6 to 19.3 × 10 ⁶ times. On the other hand, in the comparative steel, the steel type No. The average lifespans of 13, 15, 1, and 2 are 0.5 to 4.7 × 10 ⁶ times, which are all low due to hydrogen embrittlement.

In addition, steel type No. in the comparative steel. Nos. 1 and 2 have the chemical composition No. Although it is the same as 1 and 2, it is an example in which the surface layer C, N concentration or surface layer hardness after carbonitriding is out of the range. On the other hand, steel type No. Nos. 14 and 16 have a long life of 10.4 to 12.1 × 10 ⁶ times, but the material hardness after spheroidizing annealing is as high as 93,94 HRB and is inferior in manufacturability.
Among the comparative steels in Table 2, the steel type No. No. 13 is an example in which Si is a high chemical component and coarse nitrides are produced, resulting in a short life.
No. No. 14 is an example in which the material hardness is increased due to the high amount of C.
No. No. 15 is an example in which the lifetime is reduced due to the low amount of Mn.
No. 16 is an example in which the Cr content is high and the material hardness is high.

Among the comparative examples, steel type No. In the examples using Nos. 1 and 2, the chemical composition is the steel type No. in the steel of the present invention. Although it is the same as 1 and 2, it is an example in which the lifetime is reduced for the following reason.
Steel type no. No. 1 is an example in which the carbon potential at the time of carbonitriding is low (Cp = about 0.7%), so the surface layer C concentration is low, the surface layer hardness is lowered, and the life is shortened.
Steel type no. No. 2 is an example in which nitriding is not performed and the surface layer N concentration is low, resulting in a short life.

According to the present invention, it is possible to provide an environmentally resistant bearing steel that has excellent manufacturability (carburizing time and workability), suppresses a decrease in life due to hydrogen embrittlement peeling, and has a long life.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on March 30, 2016 (Japanese Patent Application No. 2016-069716), the contents of which are incorporated herein by reference.

Claims (2)

1. In mass% display,
   C: 0.5 to 1.0%
   Si: 0.1% or less Mn: 0.4 to 1.5%
   P: 0.03% or less S: 0.03% or less Cr: 1.5 to 3.5%
   Al: 0.050% or less O: 0.0015% or less Ti: 0.003% or less N: 0.015% or less The composition is composed of the balance Fe and inevitable impurities, and the hardness after spheroidizing annealing is 92HRB. Hereinafter, coarse CrN or MnSiN having a surface layer N concentration of 0.1 to 1.0% after carbonitriding, a surface layer C concentration of 0.8 to 1.5%, a surface layer hardness of HRC 58 or more and less than 64, and a particle size of 2 μm or more. _{2 is} a bearing steel for environment resistance excellent in manufacturability and resistance to hydrogen embrittlement, wherein the number density of nitrides of ₂ is 10 ³ pieces / mm ² or less, and fine nitrides are dispersed and precipitated.
2. V: 0.05-2.0%
   Ni: 0.1-3.0%
   Mo: 0.05-2.0%
   The environmentally resistant bearing steel excellent in manufacturability and hydrogen embrittlement resistance according to claim 1, further comprising one or more of them.

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