JP7057715B2 - Bearing steel with excellent rolling fatigue life in a hydrogen intrusion environment - Google Patents

Description

The present invention relates to a bearing steel having an excellent peeling life in an environment in which a structural change due to rolling fatigue caused by hydrogen occurs due to hydrogen invading the steel material.

In bearings for electrical equipment for automobiles and the like, there is a problem that structural changes occur due to rolling fatigue caused by hydrogen, and eventually they grow into large cracks with white structural changes and cause early damage. In addition, there is a concern about similar premature damage to bearing applications in which hydrogen is generated and invades through decomposition of lubricating oil such as bearings for offshore wind turbines. Early peeling accompanied by these structural changes is likely to become more apparent as the load increases due to the miniaturization and weight reduction of bearings and the use of low-viscosity lubricating oil to improve transmission efficiency progresses. Implementation is strongly required.

Therefore, as measures against delamination caused by these hydrogens, measures are taken from a viewpoint other than steel materials, such as generation of hydrogen on the lubricating oil side and prevention of hydrogen intrusion into steel. However, there are cases where these measures alone are insufficient or difficult to apply, and countermeasures are also required for the steel material used as the material for bearings.

In such a situation, by adding carbide-forming elements such as V, Ti, and Nb as a countermeasure on the steel material side against hydrogen generation due to decomposition of lubricating oil and early peeling due to hydrogen intrusion in the rolling environment of bearings. There is a technique for extending the life of a steel material by trapping hydrogen in a carbide (see, for example, Patent Document 1). However, the addition of these elements results in an increase in material costs. In addition, the addition of these carbide-forming elements itself is likely to generate large carbides that are sources of stress concentration, and hydrogen can be localized around the carbides and cause premature damage. Therefore, this measure alone has a long life. There is a limit to the conversion.

Further, the applicant of the present application applies to a steel having a specified composition range such as containing 2.30% or more of Cr, and further the steel after carburizing and quenching tempering or after carburizing and nitriding quenching and tempering. The total amount of Si, Mn, Cr, Ni, and Mo dissolved in the matrix component 100 to 300 μm from the outermost surface is 3.0% or more, and the amount of retained austenite (hereinafter referred to as “residual γ”) is 20. We have proposed a steel for bearings, which is characterized by having a martensite structure as the remaining structure at a value of about 50 vol%, and has an excellent whitening-resistant structure change tempering life (see Patent Document 2).

This document proposes a bearing steel that can suppress the white adipose tissue change even in an environment where hydrogen invades and the white adipose tissue change occurs due to hydrogen. However, in this technique, only the amount of residual γ is attracting attention, and other matters other than controlling the amount of residual γ, for example, the dispersed state of residual γ or the morphology of martensite structure is arbitrarily controlled. That wasn't tried. For example, as the morphology of martensite, a lath morphology, a lenticular morphology, a thin plate morphology, and the like are known, but the necessity of its control has not been considered. Therefore, in this document, it is necessary to add at least 2.30% or more of Cr in order to improve the peeling life, and there remains a problem in terms of cost and the like.

Japanese Unexamined Patent Publication No. 2008-280583 JP-A-2017-053002

The problem to be solved by the present invention is that in rolling fatigue, tissue changes occur due to rolling fatigue caused by hydrogen, and the development of the tissue changes forms large cracks accompanied by white tissue changes, resulting in early peeling. In response to the problem that arises, the morphology, size, body integration rate, and dispersed state of the residual γ and martensite, which are the constituent phases of the microstructure of the carburized, nitrided, and tempered bearing steel, are dispersed. It is an object of the present invention to provide a steel for bearings capable of extending the peeling life by controlling (the positional relationship of phases).

Now, the inventors have found that the peeling life is improved by suppressing the development of tissue changes due to rolling fatigue caused by hydrogen, based on further diligent studies. In order to improve the peeling life, the measures proposed in the past to increase the amount of residual γ are not sufficient, and the constituent phases of the microstructure of the carburized and hardened and tempered or carburized and nitriding and tempered steel for bearings. It was found that it is important to properly control the morphology, size, volume fraction, and dispersion state (positional relationship between both constituent phases) of residual γ and martensite.

In an environment where hydrogen invades and causes rolling fatigue caused by hydrogen, the inventors first generate tissue changes, which develop to form large cracks with white adipose tissue changes, resulting in premature damage. Is estimated to occur.
This tissue change appears in a needle-like shape in the early stage of fatigue, and its length is as fine as several μm or less. The location of the occurrence has been considered to be the block interface of the lath-like martensite structure and the former austenite grain boundary (former γ grain boundary). This needle-shaped tissue change splits in the process of rolling fatigue, and grows by the transmission of the crack and the connection between the cracks, and grows into a large crack with a change in white tissue. However, it is expected to bring about peeling. In other words, control of needle-shaped tissue changes is considered to be the key to prolonging the life.

Therefore, we focused on the behavior of needle-like tissue changes in the early stages of rolling fatigue and conducted intensive studies. The inventors performed carburizing, quenching and tempering after processing the skin-baked steel into a thrust-type rolling fatigue test piece shape, and while ensuring a certain amount of residual γ, the morphology of the martensite structure was lath-like martensite. The microstructure was controlled so that it was mainly a lenticular martensite structure rather than a structure.

Subsequently, after charging the test piece with hydrogen, a rolling fatigue test was carried out to investigate the relationship between the microstructure and the location of needle-like tissue changes in rolling fatigue under the influence of hydrogen. As a result, in the case of a lenticular martensite structure, needle-like structural changes occur at the peculiar crystal plane in the lenticular martensite structure, and the length tends to be shortened, so that it is difficult to shift to the transmission stage. I found out that.

Furthermore, it was found that the tip portion of the needle-shaped tissue remained in the fine and massive residual γ existing adjacent to the lenticular martensite. In other words, it was found that it is possible to prevent the cracks from growing significantly by controlling the dispersed state of the residual γ.

Based on these results, the morphology, size, body integration rate, and microstructure of residual γ and martensite as constituent phases of the microstructure of the carburized and hardened and tempered or carburized and nitrided and tempered bearing steels. The present invention has been made by finding that the peeling life of steel for bearings can be extended by controlling the dispersed state (phase positional relationship) of the constituent phases.

Therefore, the first means of the present invention for solving the above-mentioned problems is C: 0.13 to 0.35%, Si: 0.20 to 1.15%, Mn: 0.20 in mass%. Steel containing ~ 1.80%, P: 0.030% or less, S: 0.030% or less, Cr: 1.00 to 3.50%, and the balance consisting of Fe and unavoidable impurities.
Further, it is in a state of carburizing and quenching tempering or carburizing and nitriding quenching and tempering.
The range of the martensitic transformation start temperature Ms obtained by the following formula (1) is 140 ° C. or less at a depth of 100 μm from the outermost surface of the steel, and Ms at a depth of 300 μm from the outermost surface of the steel. The range of is 220 ° C or less,
The range of the index Md of the stability of the austenite structure (γ structure) obtained by the following formula (2) is 110 ° C. or less at a depth position of 100 μm from the outermost surface of the steel, and 300 μm from the outermost surface of the steel. The range of Md is 250 ° C. or less at the depth position of the steel, and the residual γ content at the depth position of 100 μm to 300 μm from the outermost surface of the steel is 25 to 50 vol%, and the lenticular martensite structure is 30 vol% or more. To be
It is a bearing steel with excellent peeling life in a hydrogen intrusion environment.

Ms (° C.) = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo ... (1)

Md (° C.) = 551-462C-9.2Si-8.1Mn-29Ni-13.7Cr-18.5Mo ... (2)

Here, the content of the element represented by mass% is substituted in the place of the element symbol of the formulas (1) and (2), and if there is an element not contained, the content of the corresponding element is set to zero. Find the value as.

In the second means of the present invention, in addition to the chemical composition of the first means described above, Ni: 0.10 to 4.00% and Mo%: 0.03 to 1.00% in mass%. , V: 0.01 to 0.30%, Nb: 0.01 to 0.20%, Ti: 0.01 to 0.20%, containing one or more selected from, the balance of Fe and Steel made of unavoidable impurities,
Further, it is in a state of carburizing and quenching tempering or carburizing and nitriding quenching and tempering.
The range of the martensitic transformation start temperature Ms obtained by the following formula (1) is 140 ° C. or less at a depth of 100 μm from the outermost surface of the steel, and Ms at a depth of 300 μm from the outermost surface of the steel. The range of is 220 ° C or less,
The range of the index Md of the stability of the γ structure obtained by the following formula (2) is 110 ° C. or less at a depth position of 100 μm from the outermost surface of the steel, and a depth position of 300 μm from the outermost surface of the steel. Then, the range of Md is 250 ° C or less,
Further, the amount of residual γ at a depth of 100 μm to 300 μm from the outermost surface of the steel is 25 to 50 vol%, and the lenticular martensite structure is 30 vol% or more.
It is a bearing steel with excellent peeling life in a hydrogen intrusion environment.

Ms (° C.) = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo ... (1)

Md (° C.) = 551-462C-9.2Si-8.1Mn-29Ni-13.7Cr-18.5Mo ... (2)

Here, the content of the element represented by mass% is substituted in the place of the element symbol of the formulas (1) and (2), and if there is an element not contained, the content of the corresponding element is set to zero. Find the value as.

By using the above means, in the bearing steel of the present invention and the bearing parts made of this steel, the range of the martensite transformation start temperature Ms of the steel after carburizing and quenching and quenching or after carburizing and nitriding and quenching and tempering is the range of the steel. The temperature is 140 ° C or lower at a depth of 100 μm from the outermost surface, and the range of Ms is 220 ° C or less at a depth of 300 μm from the outermost surface of the steel, and an index of the stability of the γ structure of the steel. The range of Md is 110 ° C. or less at a depth of 100 μm from the outermost surface of the steel, and the range of Md is 250 ° C. or less at a depth of 300 μm from the outermost surface of the steel. The amount of residual γ at a depth of 100 μm to 300 μm from the outermost surface of the steel can be 25 to 50 vol%, and the lenticular martensite structure can be 30 vol% or more.

Therefore, according to the means of the present invention, in an environment where hydrogen easily invades, it has a fatigue life more than three times that of SUJ2, which is a high carbon chrome bearing steel material specified by JIS, which is a typical material for bearings, and hydrogen. Since it has excellent life characteristics in the environment, it is effective in extending the life of parts.

This means that hydrogen invades the steel during use, simulating the actual usage environment in which structural changes occur due to rolling fatigue caused by hydrogen, and after adding test hydrogen by the cathode charging method, maximum contact is achieved. It can be confirmed even when the life until peeling is evaluated using a thrust type rolling fatigue tester with a surface pressure of 5.3 GPa, and it can be confirmed even when the life until peeling is evaluated. It has a fatigue life of 3 times or more of that of the above, and is excellent in life characteristics in a hydrogen environment. Therefore, it can be seen that it is effective in extending the life of parts.

It is a figure which shows an example of the carburizing and quenching pattern.

Prior to explaining the embodiment for carrying out the present invention, the reasons for limiting the range of the chemical components of the bearing steel described in each means of the present invention and the reasons for limiting the various types of the steel will be described below. .. The% of the chemical component is mass%, and the% of the residual γ amount and the volume fraction of lenticular martensite is vol%.

(C: 0.13 to 0.35%)
C is an element that affects the hardenability of the core portion when the steel part is hardened, or the forgeability and machinability of the steel part. If C is less than 0.13%, sufficient hardness of the core portion cannot be obtained and the strength is lowered. Therefore, it is necessary to add C of 0.13% or more. On the other hand, when C is added in an amount of more than 0.35%, the hardness of the steel material increases, which hinders workability such as machinability and forgeability. Therefore, C is 0.13 to 0.35%, preferably 0.15 to 0.30%.

(Si: 0.20 to 1.15%)
Si is an element necessary for deoxidation, and is an element that enhances the strength of the steel material in a high temperature environment, suppresses structural changes, and improves the rolling fatigue life. In order to obtain these effects sufficiently, it is necessary to add 0.20% or more of Si. Further, Si increases the stability of the residual γ by lowering Md, which is an index of the stability of the γ structure.
On the other hand, when Si is more than 1.15%, the hardness of the steel material increases, which hinders workability such as machinability and forgeability, and also causes carburizing inhibition, so that carburizing or nitriding is sufficient. Material strength cannot be obtained. Therefore, Si is 0.20 to 1.15%, preferably 0.25 to 0.80%, and more preferably 0.25 to 0.65%.

(Mn: 0.20 to 1.80%)
Mn is an element necessary for ensuring hardenability, and at the same time, it is a γ-stabilizing element. It is an element that contributes to suppression. In addition, Mn promotes the formation of lenticular martensite produced at a low temperature by lowering the martensitic transformation start temperature Ms. Further, Mn increases the stability of the residual γ by lowering Md, which is an index of the stability of the γ structure. In order to obtain these effects sufficiently, it is necessary to add Mn of 0.20% or more.
On the other hand, when Mn is more than 1.80%, the hardness of the steel material increases, which hinders processability such as machinability and forgeability, and combines with S to form MnS, thereby producing hydrogen. It acts as a starting point for the resulting tissue changes.
Therefore, Mn is set to 0.20 to 1.80%, preferably 0.45 to 1.60%, and more preferably 0.65 to 1.25%.

(P: 0.030% or less)
When P is contained in an amount of more than 0.030%, it is an element that embrittles the steel material and lowers the fatigue strength. Therefore, P is limited to 0.030% or less.

(S: 0.030% or less)
When S is contained in an amount of more than 0.030%, it is an element that inhibits the cold workability of the steel material and lowers the fatigue strength. Therefore, S is limited to 0.030% or less.

(Cr: 1.00 to 3.50%)
Cr is an element necessary for ensuring hardenability, and is an element that increases the amount of residual γ when a steel material is carburized or quenched after carburizing and nitriding, and contributes to suppression of structural changes caused by hydrogen. Further, Cr promotes the formation of lenticular martensite generated at a low temperature by lowering the martensitic transformation start temperature Ms. Further, Cr increases the stability of the residual γ by lowering Md, which is an index of the stability of the γ structure. Further, Cr is effective in forming a fine and homogeneous residual γ, and further enhances the effect of suppressing structural changes caused by hydrogen. In order to obtain these effects, it is necessary to add 1.00% or more of Cr.
On the other hand, when Cr becomes excessive, it is an element that causes carburizing inhibition by forming an oxide on the outermost surface of the steel material at the time of carburizing or nitriding, leading to deterioration of strength. Further, Cr forms coarse carbides at the time of carburizing and also becomes a starting point of white adipose tissue change caused by hydrogen around the coarse carbides, so Cr needs to be 3.50% or less.
Therefore, Cr is set to 1.00 to 3.50%, preferably 1.10 to 3.20%, and more preferably 1.30 to 2.25%.

(Ni: 0.10 to 4.00%)
Ni is an element that enhances the hardenability of steel by addition, and at the same time, because it is a γ-stabilizing element, it causes an increase in the amount of residual γ when the steel material is carburized or hardened after carburizing and nitriding. In addition, Ni promotes the formation of lenticular martensite produced at a low temperature by lowering the martensitic transformation start temperature Ms. Further, Ni enhances the stability of the residual γ by lowering Md, which is an index of the stability of the γ structure. In order to obtain these effects sufficiently, it is necessary to add 0.10% or more of Ni.
On the other hand, if Ni is added in excess, the material cost increases significantly, so it is better to add Ni up to 4.00%.
Therefore, Ni is set to 0.10 to 4.00%. Desirably, Ni is 0.25 to 2.50%.

(Mo: 0.03 to 1.00%)
Mo is an element that enhances the hardenability of the steel material by addition, and is effective in increasing the amount of residual γ, homogenizing the structure, and uniformly distributing the residual γ when the steel material is carburized or nitrided. .. In addition, Mo promotes the formation of lenticular martensite produced at a low temperature by lowering the martensitic transformation start temperature Ms. Further, Mo increases the stability of residual γ by lowering Md, which is an index of the stability of γ tissue. In order to obtain these effects sufficiently, Mo needs to be 0.03% or more.
On the other hand, if Mo is added in an excessive amount, the material cost increases significantly, and the effect of suppressing the tissue change that homogenizes the structure is saturated at 1.00%. Therefore, Mo is added at 1.00% or less.
Therefore, Mo is 0.03 to 1.00%, preferably 0.10 to 0.65%.

(V: 0.01-0.30%)
V suppresses structural changes caused by hydrogen by refining the crystal grains and reducing the average hydrogen concentration at the grain boundaries, and also forms submicron-order carbides and carbonitrides during carburizing or carburizing and nitriding. Therefore, it is an element that functions as a hydrogen trap and is effective in suppressing structural changes, and it is necessary to add 0.01% or more in order to obtain a sufficient effect. On the other hand, if it is added in an excessive amount, coarse carbides and carbonitrides are generated, which becomes a site of structural change caused by hydrogen, which adversely affects the life, so V is set to 0.30% or less. Therefore, V is set to 0.01 to 0.30%.

(Nb: 0.01 to 0.20%)
Nb suppresses the structural change caused by hydrogen by refining the crystal grains and reducing the average hydrogen concentration at the grain boundaries. In addition, it functions as a hydrogen trap by forming submicron-order carbides and carbonitrides during carburizing or carbonitride, which is effective in suppressing structural changes. In order to obtain these sufficient effects, it is necessary to add 0.01% or more of Nb. On the other hand, if it is added in an excessive amount, coarse carbides and carbonitrides are generated, which becomes a site of structural change caused by hydrogen, which adversely affects the life, so the addition of Nb should be 0.20% or less. Therefore, Nb is set to 0.01 to 0.20%.

(Ti: 0.01 to 0.20%)
Ti refines the crystal grains and reduces the hydrogen concentration at the grain boundaries, thereby suppressing the white adipose tissue change caused by hydrogen. In addition, Ti functions as a hydrogen trap by forming carbides on the order of submicrons during carburizing or nitriding, and is effective in suppressing changes in white adipose tissue. In order to obtain these effects sufficiently, it is necessary to add 0.01% or more of Ti. On the other hand, if it is added in an excessive amount, coarse carbides are generated, which becomes a site of structural change caused by hydrogen, which adversely affects the life, so the addition of Ti should be 0.20% or less. Therefore, Ti is set to 0.01 to 0.20%.

(Reason for evaluating the range of 100 μm to 300 μm from the outermost surface of carburized and hardened tempered or carburized, nitrided and hardened and tempered steel)
Within the range of 100-300 μm from the outermost surface of carburized-hardened-tempered or carburized-nitrided-hardened tempered steel, it is subject to relatively high repeated shear stresses due to rolling fatigue in the bearing environment. When hydrogen invades this region, a needle-like tissue change occurs in combination with the action of rolling fatigue. Therefore, it is important to suppress the development of tissue changes in this depth region. Therefore, in the present invention, the martensitic transformation initiation temperature Ms, the index Md of the stability of the γ structure, and the residual γ are contained within a range of 100 to 300 μm from the outermost surface of the carburized, nitrided and tempered steel. The quantity and size, and the volume fraction of the lenticular martensitic structure are specified.

(Martensitic transformation start temperature Ms at a depth of 100 μm from the outermost surface of carburized, hardened and tempered steel or carburized, nitrided, tempered and tempered steel is 140 ° C or less, and martensitic transformation at a depth of 300 μm from the outermost surface. Start temperature Ms should be 220 ° C or less)
In carburizing quenching tempering or carburizing nitriding quenching tempering, by controlling the morphology of martensite mainly to lenticular martensite, the peeling life in a hydrogen intrusion environment can be improved. For that purpose, the martensitic transformation start temperature Ms at a depth of 100 μm from the outermost surface of the carburized, hardened and tempered state or the carburized, nitrided and tempered state is set to 140 ° C. or less, and 300 μm from the outermost surface of the steel. It is preferable that the martensitic transformation start temperature Ms at the depth position of is 220 ° C. or lower. On the other hand, when it exceeds the specified Ms point for each depth position, it becomes difficult to secure the required amount of lenticular martensite, and thus it becomes difficult to improve the peeling life.

(Amount of residual γ at a depth of 100 μm to 300 μm from the outermost surface of carburized, hardened and tempered steel or carburized, nitrided, hardened and tempered steel: 25 to 50 vol%)
Residual γ functions as a trap site for hydrogen, which has the effect of suppressing the local concentration of hydrogen and slowing down the diffusion rate of hydrogen in steel, which is effective in suppressing structural changes caused by hydrogen. Is. Furthermore, by generating residual γ so as to be adjacent to the periphery of the lenticular martensite, the needle-like tissue generated by rolling fatigue in a hydrogen intrusion environment inside the lenticular martensite is cracked. It suppresses elongation and ligation, and effectively suppresses the subsequent growth into large cracks accompanied by changes in white tissue, which in turn suppresses early cracking. In order to obtain these effects, the residual γ amount needs to be 25 vol% or more.
On the other hand, if the residual γ content is more than 50 vol%, the hardness of the steel required for rolling tired parts cannot be obtained, and the dimensional stability during use is deteriorated. Therefore, the amount of residual γ is set to 50 vol% or less.
Therefore, the amount of residual γ in the range of 100 to 300 μm from the outermost surface of the steel after carburizing, nitriding, quenching and tempering is 25 to 50 vol%, more preferably the residual γ is 30 to 50 vol%, and more preferably the residual γ. Is 35 to 50 vol%.
It is preferable to disperse the residual γ having a diameter equivalent to a circle of 0.2 to 5 μm adjacent to the periphery of the lenticular martensite.

In addition, if the residual γ is easily decomposed in the process of rolling fatigue in a hydrogen intrusion environment, it adversely affects the life, so it is good that the residual γ exists stably, and therefore it is an index of the stability of the γ tissue. It is advisable to specify a certain Md as well.

(Amount of lenticular martensite at a depth of 100 μm to 300 μm from the outermost surface of carburized, hardened and tempered steel or carburized, nitrided, hardened and tempered steel: 30 vol% or more)
In rolling fatigue, the process leading to early peeling with a change in white adipose tissue caused by hydrogen is triggered by the formation of needle-like initial cracks and the formation of large internal cracks by their transmission and connection. To.
By controlling the morphology of martensite mainly to the lenticular martensite tissue, the length of the needle-like tissue change that occurs inside the martensite can be shortened as compared to the case where the lath-shaped martensite tissue is the main component. This makes it possible to improve the peeling life due to tissue changes. In order to obtain the effect, it is preferable to set the volume fraction of the lenticular martensite structure to 30 vol% or more.
The thickness of the lenticular martensite at this time is preferably 0.2 to 2.5 μm.

(Md, which is an index of the stability of the γ structure at a depth of 100 μm from the outermost surface of carburized, hardened and tempered steel or carburized, nitrided and tempered steel, is 110 ° C or less and 300 μm from the outermost surface. Md in 250 ° C or less)
In carburizing quenching tempering or carburizing nitriding quenching tempering, the residual γ is used as the second phase to precipitate an appropriate amount, and the residual γ is stably retained in rolling fatigue in a hydrogen intrusion environment to form a lenticular martensite. By keeping it adjacent to the site structure, the tempering life in a hydrogen intrusion environment can be improved. For that purpose, in the steel in the state of carburizing, quenching and tempering or in the state of carburizing and nitriding and tempering, the range of the index Md of the stability of the γ structure is 110 ° C. or less at the depth position of 100 μm from the outermost surface of the steel. Moreover, it is preferable that the range of Md is 250 ° C. or less at a depth position of 300 μm from the outermost surface of the steel. On the other hand, if it exceeds Md specified for each depth position, it becomes difficult to secure a stable required amount of residual γ, and thus the peeling life is inferior.

Next, embodiments of the invention will be described.
Example steel No. 1 satisfying the components of the present invention having the chemical composition shown in Table 1. Samples A to K, and Comparative Example Steel No. 1 which does not satisfy some of the conditions specified in the present invention. The L and M samples were each melted in a 100 kg vacuum melting furnace.
In addition, Comparative Example Steel No. L is SUJ2, which is a high carbon chrome bearing steel material specified by JIS, and is a comparative example steel No. M is SCr420, which is a chrome steel material specified by JIS.

Among these steels, Comparative Example Steel No. The steel excluding SUJ2 of L was forged to a diameter of 65 mm at 1250 ° C., held at 900 ° C. for 1 hour, then air-cooled and normalized.
Further, Comparative Example Steel No. SUJ2 of L was forged to a diameter of 65 mm at 1150 ° C., held at 900 ° C. for 1 hour, air-cooled and annealed, and then spheroidized and annealed at 800 ° C.
After that, Comparative Example Steel No. All steels except SUJ2 of L were roughly processed into thrust type rolling fatigue test pieces having an outer diameter of 60 mm, an internal total of 20 mm, and a thickness of 8.3 mm. Comparative Example Steel No. SUJ2 of L was roughly processed into a thrust type rolling fatigue test piece having an outer diameter of 60 mm, an inner diameter of 20 mm, and a thickness of 6.0 mm.
Since the embodiment is not limited to the shape of this test piece, for example, steel having the shape of a part such as a bearing may be subjected to carburizing and nitriding tempering or tempering to have the characteristics described in the present invention. For example, it belongs to the steel referred to in the present invention.

Subsequently, Example Steel No. E, No. F, and Comparative Example Steel No. Except for SUJ2 of L, Example Steel No. A to D, G to K and Comparative Example Steel No. For M, the thrust type rolling fatigue test piece was subjected to gas carburizing and quenching under the conditions of the carburizing and quenching pattern shown in FIG. 1 (carburizing temperature: 930 ° C., target carbon potential = 0.90%), and then at 180 ° C. The tempering treatment was carried out by holding for 90 minutes and cooling in the air. As shown in Table 2, these are processed No. 1-4, processing No. 7-11 and processing No. It was set to 13.

Subsequently, Example Steel No. E, No. For F, the thrust type rolling fatigue test piece was subjected to gas carburizing and quenching under the conditions of the carburizing and quenching pattern shown in FIG. 1 (carburizing temperature: 930 ° C., target carbon potential = 1.10%), and then at 180 ° C. The tempering treatment was carried out by holding for 90 minutes and cooling in the air. As shown in Table 2, these are processed No. It was set to 5 and 6.

Further, Comparative Example Steel No. For SUJ2 of L, the tempering process was carried out by holding at 840 ° C. for 30 minutes for oil cooling without carburizing, quenching, and then holding at 180 ° C. for 90 minutes for air cooling. As shown in Table 2, this is processed No. It was set to 12.

In addition, Example steel No. After the steels shown in D and F were rough-processed in the same process until the test piece was rough-processed, the conditions of the carburizing and quenching pattern as in FIG. 1 (carburizing temperature: 930 ° C., target carbon potential = 0.90%). After performing gas carburizing and quenching, sub-zero treatment with liquid nitrogen was carried out for the purpose of reducing the amount of residual γ, and then the steel was processed by holding at 180 ° C. for 90 minutes, air-cooling and tempering. No. It was set to 14 and 15.

The carburizing and quenching pattern shown in FIG. 1 is an example, and other carburizing and nitriding and quenching patterns may satisfy the scope of the present invention. Further, the secondary quenching after carburizing or carburizing nitriding may be carried out by selecting the conditions so as to satisfy the scope of the present invention.

After performing the above heat treatment, Comparative Example Steel Processing No. For all the test pieces except SUJ2 of L, the test surface was polished by 0.15 mm, and the opposite side was further polished to finish the height to 8.0 mm. Comparative Example Steel Machining No. For SUJ2 of L, the test surface was polished by 0.20 mm, and the opposite side was further polished to finish the height to 5.6 mm. In addition, these test surfaces were mirror-finished by buffing.

In order to measure the peeling life in a hydrogen intrusion environment using the thrust type rolling fatigue test piece prepared above, maximum contact is made after hydrogenating the test piece by the cathode charging method under the conditions shown in Table 3. A thrust type rolling fatigue test was conducted under the conditions that the surface pressure was 5.3 GPa, the rolling element used three 3/8 inch steel balls, and the lubrication was ISO VG10 oil bath lubrication, and rolling until peeling. The dynamic fatigue life (number of cycles, here evaluated by L ₅₀ life) was evaluated. Further, in order to calculate the martensitic transformation start temperature Ms, EPMA measurement was performed in the cross-sectional depth direction of the thrust type rolling fatigue test piece that was finish-polished after carburizing, tempering and tempering, and 100 μm and 300 μm from the surface of the carburized steel. The carbon concentration value at the position of was obtained, and the martensitic transformation start temperature Ms obtained by the following (Equation 1) was calculated.

Ms (° C.) = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo ... (1)

Similarly, the index Md of the stability of the γ tissue obtained by the following formula (2) was calculated.

Md (° C.) = 551-462C-9.2Si-8.1Mn-29Ni-13.7Cr-18.5Mo ... (2)

Here, the content of the element represented by mass% is substituted in the place of the element symbol of the formulas (1) and (2), and if there is an element not contained, the content of the corresponding element is set to zero. The value was calculated as.

Next, the measurement of the volume fraction of the lenticular martensite structure will be described. The volume fraction of the lenticular martensite structure was determined by taking an SEM photograph after mirror polishing and nital corrosion at a depth of 100 μm or 300 μm from the outermost surface of the thrust type rolling fatigue test piece, and using it. It was obtained by the point counting method. Similarly, after electrolytic polishing was performed using a thrust type rolling fatigue test piece to a depth of 100 μm or 300 μm from the outermost surface, the residual γ content was measured using an X-ray diffraction method. ..

When measuring the size of residual γ (diameter equivalent to a circle) at a predetermined depth position (within a range of 100 μm to 300 μm) from the outermost surface of the test piece and the thickness of the lenticular martensite structure, the predetermined depth is used. At this position, the processed altered layer on the surface and the strain are polished until they are eliminated as much as possible, and then the polished state is left as it is, or if necessary, the chemical corrosion is performed, and then an electron backscatter diffraction (EBSD) observation image is acquired. It can be obtained by a method of obtaining afterwards, a method of obtaining from photographs by SEM observation, or a method of combining them.
The diameter equivalent to a circle is a diameter obtained by geometrically obtaining and converting a circular shape having a regular shape corresponding to this area with respect to the obtained projected area.

Table 2 shows each processing No. The various measurement results and the evaluation results of the peeling life regarding the matters specified in the present invention are shown. In Table 2, the processing No. which is an example satisfying the range specified in the present invention. Nos. 1 to 11 are comparative examples that do not satisfy the range specified in the present invention. It is clear that the peeling life is excellent in a hydrogen intrusion environment with respect to 12 to 15.
Further, among the comparative examples, the sample No. 13 of the processing No. 13 was used. Although the component range of M satisfies the range specified in the present invention, Ms and Md according to the carbon concentration after charcoal-burning and tempering do not meet the claimed range, so that the _L50 life is inferior.

Claims (2)

By mass%, C: 0.13 to 0.35%, Si: 0.20 to 1.15%, Mn: 0.20 to 1.80%, P: 0.030% or less, S: 0.030. % Or less, Cr: 1.00 to 3.50%, and the balance is Fe and unavoidable impurities.
Further, it is in a state of carburizing and quenching tempering or carburizing and nitriding quenching and tempering.
The range of the martensitic transformation start temperature Ms obtained by the following formula (1) is 140 ° C. or less at a depth of 100 μm from the outermost surface of the steel, and Ms at a depth of 300 μm from the outermost surface of the steel. The range of is 220 ° C or less,
The range of the index Md of the stability of the γ structure obtained by the following formula (2) is 110 ° C. or less at a depth position of 100 μm from the outermost surface of the steel, and a depth position of 300 μm from the outermost surface of the steel. Then, the range of Md is 250 ° C or less,
Further, the amount of residual γ at a depth of 100 μm to 300 μm from the outermost surface of the steel is 25 to 50 vol%, and the lenticular martensite structure is 30 vol% or more.
A steel for bearings that has an excellent peeling life in a hydrogen intrusion environment.

Ms (° C.) = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo ... (1)

Md (° C.) = 551-462C-9.2Si-8.1Mn-29Ni-13.7Cr-18.5Mo ... (2)

Here, the content of the element represented by mass% is substituted in the place of the element symbol of the formulas (1) and (2), and if there is an element not contained, the content of the corresponding element is set to zero. Find the value as. In addition to the chemical composition of claim 1, in terms of mass%, Ni: 0.10 to 4.00%, Mo%: 0.03 to 1.00%, V: 0.01 to 0.30%, A steel containing one or more selected from Nb: 0.01 to 0.20% and Ti: 0.01 to 0.20%, the balance of which is Fe and unavoidable impurities.
Further, it is in a state of carburizing and quenching tempering or carburizing and nitriding quenching and tempering.
The range of the martensitic transformation start temperature Ms obtained by the following formula (1) is 140 ° C. or less at a depth of 100 μm from the outermost surface of the steel, and Ms at a depth of 300 μm from the outermost surface of the steel. The range of is 220 ° C or less,
The range of the index Md of the stability of the γ structure obtained by the following formula (2) is 110 ° C. or less at a depth position of 100 μm from the outermost surface of the steel, and a depth position of 300 μm from the outermost surface of the steel. Then, the range of Md is 250 ° C or less,
Further, the amount of residual γ at a depth of 100 μm to 300 μm from the outermost surface of the steel is 25 to 50 vol%, and the lenticular martensite structure is 30 vol% or more.
A steel for bearings that has an excellent peeling life in a hydrogen intrusion environment.

Ms (° C.) = 539-423C-30.4Mn-12.1Cr-17.7Ni-7.5Mo ... (1)

Md (° C.) = 551-462C-9.2Si-8.1Mn-29Ni-13.7Cr-18.5Mo ... (2)

Here, the content of the element represented by mass% is substituted in the place of the element symbol of the formulas (1) and (2), and if there is an element not contained, the content of the corresponding element is set to zero. Find the value as.

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