JP6481652B2 - Bearing steel - Google Patents

Description

  The present invention can suppress bearing damage caused by a white structure derived from hydrogen, which is a problem in bearings for automobiles, windmills, industrial machines, etc., in particular, bearings made of general-purpose bearing steel (JIS-SUJ2). The present invention relates to a bearing steel having excellent rolling fatigue characteristics in a hydrogen environment.

  Bearings are components that require an excellent rolling fatigue life. In recent years, general-purpose bearing steel (JIS-SUJ2) has been used for bearings used in automobiles, windmills, and industrial machines. In this general-purpose bearing steel bearing, there is a problem of early peeling of the steel material used in the bearing due to the white structure derived from hydrogen. This early peeling is said to occur when the lubricating oil used in the bearing decomposes by a tribochemical reaction on the rolling surface to generate hydrogen, and this hydrogen penetrates into and accumulates in the steel.

  Regarding such white structure formation suppression, for example, Patent Document 1 discloses a method of increasing the amount of retained austenite by increasing the amount of Cr and N added. However, in the invention described in Patent Document 1, since the Cr addition amount is 4.0% or more, it is necessary to make the quenching temperature very high such as 975 ° C. or higher, and it is difficult to perform heat treatment in a general quenching furnace. Even if the heat treatment can be performed, the heat treatment is performed at a very high temperature, so that the thermal deformation is large, causing a fire crack, and the processing for correcting the shape after the heat treatment requires a great number of man-hours and costs.

  Patent Document 2 discloses a method for suppressing white structure and ensuring workability by adding Si, Cr and Mo. In this document, it is a quenching temperature that can be handled by a general heat treatment furnace, and it has excellent life characteristics in a rolling fatigue test under a hydrogen content of 0.5 mass% while maintaining workability. It is shown. However, in recent years, in order to improve the fuel consumption of automobiles and reduce the transmission load of wind power generation in order to reduce the environmental load, the viscosity of the lubricating oil has been lowered, and the lubrication state of the rolling surface has become severe. For this reason, decomposition of the lubricating oil on the rolling surface is promoted, and the amount of hydrogen entering the steel tends to increase. Therefore, there is a demand for improving the rolling fatigue life characteristics of bearing parts even under such severe environments in recent years.

Japanese Patent No.4942374 Japanese Patent No. 5803618

  The present invention solves such problems of the prior art and can suppress bearing damage caused by a white structure derived from hydrogen, particularly rolling fatigue characteristics in an environment in which the amount of hydrogen penetration into steel in recent years has increased. It aims at providing the bearing steel which is excellent in.

In order to achieve the above-mentioned object, the present inventors diligently investigated factors that influence the early peeling phenomenon due to the white tissue.
Now, the white structure is a structure changed from martensite and carbide which are base materials. In the process of changing to a white structure, first, hydrogen is concentrated at the boundary surface of the martensite block, and when a shearing stress is repeatedly applied in this state, a needle-like structure as a precursor stage is formed. Further, when hydrogen penetrates and thickens and a repeated shear stress is applied, a part of the pre-existing needle-like structures connected and aggregated to form an initial white structure. An electron micrograph of the metal structure at this stage is shown in FIG. This white structure is mainly a fine ferrite structure of several tens of nanometers. Since this structure is lower in hardness than the surrounding martensite, it undergoes stress concentration and fatigue when subjected to repeated stress. It is also the starting point of the crack. Subsequently, when the white structure is repeatedly subjected to shear stress, the region of the white structure expands starting from the initial stage white structure. When the white structure becomes larger than a certain size, the above fatigue cracks rapidly develop. And it was found that this is the mechanism of peeling of steel.

When using in a hydrogen environment, first, in order to reduce the amount of hydrogen that penetrates from the external environment and diffuses into the steel, it is possible to reduce the hydrogen diffusion rate by containing a certain amount or more of retained austenite in the martensite structure. It is valid. However, as described above, the amount of hydrogen generated in the external environment has increased in recent severe environments, so it is not sufficient to reduce the hydrogen diffusion rate. White tissue must also be suppressed.
On the other hand, in order to suppress the formation of the needle-like structure formed at the martensite block boundary surface, it is effective to completely suppress hydrogen intrusion into the steel or to eliminate the martensite block boundary surface. However, it is virtually impossible to realize these. For this reason, delaying the generation of the white tissue in the initial stage, which is one step further from the needle-shaped tissue, or suppressing the expansion of the white tissue region is very effective for suppressing early peeling. I got the knowledge.

  The white structure at the initial stage is a structure in which a part of the interspersed acicular structure is connected and aggregated to change from a martensite structure to a fine ferrite structure. Although a large amount of C is dissolved in the martensite structure, the amount of dissolved C that the ferrite structure can accept is very small. Therefore, when the white structure in the initial stage is formed, C in the martensitic structure of the portion that changes to the ferrite structure needs to be diffused around. At that time, it was newly found that when Si and Cr are contained in a certain amount or more, the resistance to C diffusion becomes large, and the formation of the white structure in the initial stage can be delayed.

  Next, in the process of expanding the area of the white structure in the initial stage, it was found that the carbides scattered around the area play a role of suppressing the expansion of the white structure. That is, as shown in FIG. 1 (b), an electron micrograph of the metal structure in the stage where the white structure expands from the stage of FIG. 1 (a), the white structure takes in surrounding carbides and widens the region. In order to continue, the captured carbide must be dissolved in the substrate and disappear. However, the white structure is composed of a ferrite structure as described above, and the amount of solid solution C that can be accepted by the ferrite structure is very small, so that the carbide is dissolved in the white structure involves a large energy resistance. Become. As described above, the present inventors have newly obtained knowledge that the presence of carbides that are difficult to dissolve in the substrate is effective for expanding the area of the white structure.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
(1) In mass%,
C: 0.55% to 1.10%,
Si: 1.0% to 2.0%,
Mn: 2.0% to 3.0%,
P: 0.02% or less,
S: 0.02% or less,
Al: 0.05% or less,
Cr: 0.8% or more and 2.4% or less,
N: not more than 0.020% and O: not more than 0.0020%, the balance having a component composition of Fe and inevitable impurities, having a metal structure containing tempered martensite, residual austenite and carbide, and the volume of the residual austenite A bearing steel having a fraction of more than 10% and 20% or less, a Cr concentration in the carbide of 6% or more, and an Mn concentration of 5% or more.

(2) The component composition is further in mass%,
Sb: 0.0050% or less,
Mo: 0.50% or less,
Ti: 0.01% or less,
Cu: 0.30% or less,
Bearing steel as described in said (1) containing 1 type (s) or 2 or more types chosen from Ni: 0.15% or less and B: 0.0010% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the bearing damage resulting from the white structure | tissue derived from hydrogen can be suppressed. In particular, the present invention has a great effect on the demand to provide a bearing steel that is excellent in rolling fatigue characteristics in an environment in which hydrogen penetration into steel has increased in recent years.

It is an example of a scanning electron microscope (SEM) photograph of a white tissue It is a figure which shows the relationship between B10 life ratio and the amount of Mn + Cr in carbide. It is a figure which shows the relationship between B10 life ratio and the amount of Mn in carbide. It is a figure which shows the relationship between B10 life ratio and Cr amount in carbide. It is a figure which shows the relationship between B10 life ratio and residual austenite amount.

Hereinafter, the bearing steel of the present invention will be described in detail. First, the reason why the component composition of the bearing steel is limited to the above range will be described. In addition, unless otherwise indicated, the "%" display regarding the following component composition means "mass%".
C: 0.55% or more and 1.10% or less C is an element important for ensuring hardness after quenching and tempering and ensuring a high rolling fatigue life of the bearing, and suppresses the expansion of the white structure region. Even in the case of forming, it is necessary to contain 0.55% or more. In order to secure a certain amount or more of retained austenite for reducing the amount of hydrogen that penetrates from the external environment and diffuses into the steel, C needs to be 0.55% or more. Preferably, it is 0.70% or more. On the other hand, if the content exceeds 1.10%, a disadvantage arises in that a large amount of retained austenite is generated and the hardness necessary to ensure a high rolling fatigue life cannot be ensured. In addition, since the steel component of the present invention contains Cr and Si, when C is added in an amount exceeding 1.10%, a large carbide [(Fe, Cr) 3C] crystallizes and does not disappear even by subsequent hot rolling, so that the rolling fatigue life is greatly deteriorated. For this reason, the amount of C is limited to 1.10% or less. Preferably, it is 0.95% or less.

Si: 1.0% to 2.0%
Si is an extremely important element for securing the rolling fatigue life when hydrogen penetrates. As described above, the martensite structure is transformed into a fine ferrite structure during the formation process of the white structure in the initial stage, and the carbides existing there are taken into the substrate and disappear, but Si is 1.0% or more. By adding, it is possible to suppress the transition from the martensite structure to the ferrite structure and the solid solution of carbide into the substrate. More preferably, it is 1.2% or more. On the other hand, when Si exceeds 2.0%, the embrittlement of the steel material becomes remarkable and the workability is remarkably deteriorated, so 2.0% was made the upper limit.

Mn: 2.0% to 3.0%
Mn is also an extremely important element for securing the rolling fatigue life when hydrogen penetrates. That is, Mn is an important element for generating and stabilizing retained austenite for reducing the amount of hydrogen that penetrates from the external environment and diffuses into the steel. In order to secure a certain amount or more of this retained austenite, it is necessary to add Mn at 2.0% or more. In addition, Mn is an element that also dissolves in carbides and suppresses the incorporated carbides from dissolving in the substrate during the process of expanding the white structure. In particular, the solid solution of carbide to the substrate can be reliably suppressed by the synergistic effect of the composite solid solution with Cr described later. In order to ensure a certain amount or more of Mn in the carbide, addition of 2.0% or more is required. Preferably, it is 2.2% or more. On the other hand, if Mn exceeds 3.0%, the amount of retained austenite becomes excessive, the required hardness cannot be ensured, and the dimensional change during product use increases. Moreover, since embrittlement of the steel material becomes remarkable, it is limited to 3.0% or less. Preferably, it is 2.8% or less.

P: 0.02% or less (including zero)
P is an impurity, and segregates at the austenite grain boundary to reduce the grain boundary strength, thereby promoting quench cracking during quenching. Therefore, it is desirable to reduce the P content as much as possible, but the content up to 0.02% is acceptable.

S: 0.02% or less S is an element useful for improving the machinability by forming MnS in steel, and is preferably added in an amount of 0.003% or more. On the other hand, if added over 0.02%, the formed MnS becomes enormous and becomes the starting point of rolling fatigue fracture, which has the adverse effect of reducing fatigue strength, so 0.02% is made the upper limit. Preferably it is 0.01% or less.

Al: 0.05% or less
Al is an element effective for deoxidation, and is an element useful for reducing oxygen. However, since Al oxide existing in steel deteriorates rolling fatigue properties, in the present invention, 0.05% or less is added. And

Cr: 0.8% to 2.4%
Cr is also an extremely important element for securing the rolling fatigue life when hydrogen penetrates, and is effective for the generation and stabilization of retained austenite. Further, Cr is an element that dissolves in the carbide as well as Mn, and suppresses the solid solution of carbide in the base material in the process of expanding the region of the white structure. As described above, the solid solution of carbide to the substrate can be remarkably suppressed by the synergistic effect of the composite solid solution with Mn. In order to ensure a certain amount or more of Cr in the carbide, addition of 0.8% or more is required. Preferably, it is 1.0% or more. On the other hand, if the content exceeds 2.4%, the amount of retained austenite becomes excessive and the hardness decreases, and the growth of giant carbide [(Fe, Cr) 3C] formed in the final solidification center during continuous casting is increased. 2.4% or less to promote and deteriorate the rolling fatigue life. Preferably it is 2.2% or less.

N: 0.020% or less N is a beneficial element that forms a nitride or carbonitride with Al and Ti and suppresses the growth of austenite grains during quenching heating. For that purpose, it is preferable to add at 0.0015% or more. Further, nitrides or carbonitrides with Al and Ti also serve as hydrogen trap sites. On the other hand, coarse nitrides and carbonitrides cause a decrease in rolling fatigue life, so N is 0.020% or less. Preferably it is 0.015% or less.

O: 0.0020% or less O exists as a hard oxide-based nonmetallic inclusion. This increase in the amount of O coarsens the size of the oxide-based nonmetallic inclusion. Since this coarse inclusion is particularly harmful to rolling fatigue conditions, it is desirable to reduce it as much as possible. For that purpose, it is necessary to reduce the amount of O to 0.0020% or less. Preferably it is 0.0010% or less.

The bearing steel excellent in rolling fatigue life characteristics in a hydrogen environment according to the present invention has the above chemical components as basic components, and the balance is iron and inevitable impurities.
Furthermore, if necessary, in addition to the above basic components, Sb: 0.0050% or less, Mo: 0.50% or less, Ti: 0.01% or less, Cu: 0.30% or less, Ni: 0.15% or less, and B: 0.0010% or less 1 type (s) or 2 or more types selected from among them can be contained. Thereby, the rolling fatigue life characteristic in a hydrogen environment can be secured more stably.

Sb: 0.0050 or less
Sb concentrates on the surface of the steel sheet and suppresses C diffusion, suppresses surface decarburization during heat treatment, and serves as a hydrogen trap site, and is an element useful for suppressing the formation of a white structure in the initial stage. . Since the effect of addition is poor at 0.0015% or less, it is preferable to add more than 0.0015% when necessary. On the other hand, even if added over 0.0050%, this effect is saturated.

Mo: 0.50% or less
Mo is an element useful for suppressing the formation of white tissue in the initial stage. If the content is less than 0.15%, the effect of addition is poor. Therefore, when necessary, it is preferably added at 0.15% or more. If the content exceeds 0.50%, this effect is saturated, so the content is made 0.50% or less from the viewpoint of cost.

Ti: 0.01% or less
Ti becomes TiN and exhibits a pinning effect in the austenite region, has an effect of suppressing grain growth, and TiN is preferably added because it also serves as a hydrogen trap site. However, if added in a large amount, TiN precipitates in a large amount, and the size of TiN is coarsened, so that the rolling fatigue characteristics are deteriorated and the steel material becomes brittle. In order to acquire the said effect, adding at 0.003% or more is suitable.

Cu: 0.30% or less
Since Cu is an element that improves hardenability, it can be added at 0.03% or more as necessary. However, if it exceeds 0.30%, the hot workability is inhibited, so 0.30% or less is added.

Ni: 0.15% or less
Ni is an element that improves hardenability and can be used to adjust hardenability. Moreover, since there exists an effect which suppresses the deterioration of the hot workability at the time of Cu addition, it can add at 0.03% or more as needed. However, Ni is an expensive element, and the steel material price increases as the amount added increases, so 0.15% or less is added.

B: 0.0010% or less B is an element that improves hardenability, and can be used to adjust hardenability. However, if the content exceeds 0.0010%, the effect is saturated, so it is preferable to add 0.0010% or less. In order to obtain the effect of improving hardenability, it is preferable to add at 0.003% or more.

Next, the structure of the bearing steel of the present invention will be described in detail.
First, the structure is a metal structure containing tempered martensite, retained austenite and carbide. Tempered martensite is necessary for increasing the hardness of steel and obtaining a good rolling fatigue life. Carbides are always present after quenching and tempering in the composition of the present invention.

In the above metal structure, the volume fraction of retained austenite is more than 10% and 20% or less, the Cr concentration in the carbide is 6% or more, the Mn concentration is 5% or more, and the total concentration of Cr and Mn is 11% or more. It is important.
[Volume fraction of retained austenite is more than 10% and less than 20%]
By containing a certain amount or more of retained austenite, the hydrogen diffusion rate in the steel can be reduced, and the amount of hydrogen that penetrates from the external environment and diffuses into the steel can be reduced. The effect is not exhibited unless the retained austenite exceeds 10% in the martensite structure. On the other hand, if the retained austenite exceeds 20%, the required hardness cannot be ensured, and the dimensional change becomes large due to the martensite formed by processing-induced transformation (TRIP) during the use of the product. As a result, abnormal noise is increased and additional stress is increased when the bearing is used. For this reason, the volume fraction of retained austenite was limited to more than 10% and not more than 20%.

[Cr concentration in carbide is 6% or more, Mn concentration is 5% or more]
As described above, the process of expanding the area of the white structure in the initial stage proceeds while the white structure takes in carbides present in the surroundings, and the incorporated carbides dissolve into the substrate and disappear. Among the above carbides, Cr and Mn composite carbides enriched with Cr and Mn can remarkably suppress diffusion (solid solution) into the substrate. Even when Cr or Mn alone is concentrated in carbides, there is an effect of suppressing the diffusion of carbides to the substrate, but composite concentration of Cr and Mn provides a synergistic effect compared to single concentration, leading to a carbide substrate. Remarkably suppresses solid solution. In particular, the synergistic effect is effective when the Cr concentration is 6% or more, the Mn concentration is 5% or more, and the Cr + Mn concentration is 11% or more. The Cr concentration and Mn concentration in the carbide can be measured by a measurement method by energy dispersive X-ray analysis (EDS) attached to a scanning electron microscope (SEM) described later.

  Thus, if the bearing steel having the composition and structure according to the present invention is obtained, a bearing steel excellent in rolling fatigue characteristics in an environment in which the expected characteristics, in particular, the amount of hydrogen penetration into the steel in recent years has increased is obtained. be able to.

The bearing steel having the component composition and structure described above can be manufactured by a known method.
That is, after being melted by a melting method such as a converter or a degassing facility, it is cast, and the resulting slab is made into a predetermined size through diffusion annealing, rolling, or forging forming process. The steel material is subjected to conventionally known spheroidizing annealing, and becomes a material for processing a bearing part. After that, the bearing steel of the present invention is obtained through processing steps such as cutting and forging.

Particularly preferable production conditions are as follows.
First, in the spheroidizing annealing, it is preferable to perform a treatment of holding at 750 to 820 ° C for about 4 to 16 hours and gradually cooling to 650 ° C at about 8 to 20 ° C / h. It is preferable that the structure in the raw material after spheroidizing annealing is ferrite and spheroidizing carbide, and the Vickers hardness is about 180 to 250.

  The pre-processed material is processed into the shape of the bearing component, and then subjected to a quenching process by quenching, and the subsequent tempering process is preferably performed at 150 to 250 ° C. for about 30 to 180 minutes. The structure after quenching and tempering is tempered martensite and carbide containing residual austenite, and the Vickers hardness is preferably about 700 to 800.

  A steel ingot (100 kg) having the component composition shown in Table 1 with the balance being Fe and inevitable impurities was vacuum-melted and subjected to diffusion annealing at 1250 ° C. for 24 hours. Next, after hot forging at 1200 ° C. to make a round bar with a diameter of 60 mm, it was kept at 950 ° C. for 1 hour, then air cooled and normalized. After that, rough machining of the thrust type rolling fatigue test with a diameter of 60mm and a thickness of 5.5mm from a round bar annealed by spheroidizing annealed at 780-820 ° C for 7 hours and slowly cooled at 15 ° C / h. Was given.

  After holding the obtained rough-processed test piece in the range of 820 to 880 ° C. for 30 minutes, oil quenching, and further tempering in the range of 170 to 220 ° C. for 2 hours, followed by surface finishing A thrust type rolling fatigue test piece of 5 mm was prepared. In addition, about steel No. 6 and 7, using the raw material of steel No. 5, the quenching temperature is set to the high temperature side and the low temperature side of the two-phase region (austenite + carbide), so that the amount of retained austenite of the present invention is reduced. Removed from range.

In order to simulate the hydrogen intrusion environment, hydrogen charging was performed on the test piece after finishing. The hydrogen charge was carried out by holding in a 20% aqueous solution of ammonium thiocyanate (NH ₄ SCN) at 50 ° C. for 24 hours. Here, when the hydrogen-charged test piece is subjected to a thrust type rolling fatigue test described later, since hydrogen escapes from the test piece during the fatigue test, the long-time evaluation is underestimated. For this reason, the hydrogen-charged test piece was subjected to a rolling fatigue test for 24 hours, and then subjected to hydrogen charging again under the same conditions, and the hydrogen charging and the rolling fatigue test were repeated every 24 hours. Under these conditions, JIS-standard high carbon chromium bearing steel type 2 (SUJ2) is used to repeatedly test with hydrogen penetrating 1 to 3 mass ppm into the steel at 600 ° C using temperature rising hydrogen analysis. It is confirmed by measuring the amount of hydrogen up to. In order to prevent the formation of corrosion products on the rolling surface during hydrogen charging, a 5 mm area around the rolling track was masked with tape.

The thrust rolling fatigue test conditions were Hertz stress 3.8 GPa, stress load rate 3600 cpm, and turbine oil (FGK turbine oil 68, manufactured by JX Nippon Oil & Energy Corporation) lubrication. It was subjected to a dynamic fatigue test. Each steel type was tested seven times and organized by Weibull plot to obtain a B10 life with a cumulative failure probability of 10%.
By dividing the rolling fatigue life B10 obtained for each steel by the value of the rolling fatigue life B10 for JIS SUJ2 equivalent steel, the degree of life improvement compared to general-purpose steel (B10 life ratio = B10 life / JIS SUJ2 equivalent) The B10 life of the steel was determined and evaluated.

  To measure the concentration in carbides, the sample after quenching and tempering is corroded with a saturated aqueous solution of picric acid to reveal carbides, and energy dispersive X-ray analysis (EDS) attached to a scanning electron microscope (SEM) is used. Then, for each sample, the carbide is extracted at random, irradiated so that the EDS electron beam fits in the carbide, the amount of Mn and Cr contained therein is measured, and the average value thereof is calculated in the carbide of the sample. The concentration was Mn and Cr.

  The above measurements and evaluation results are also shown in Table 1. 2 to 5 collectively show the relationship between the B10 life shown in Table 1 and various concentrations in carbides. From Table 1 and FIGS. 2 to 5, when all of the component composition, the Mn and Cr concentration in the carbide, and the amount of retained austenite satisfy the scope of the present invention, the amount of hydrogen penetration into the steel increased. It can be seen that it has excellent rolling fatigue characteristics. On the other hand, the comparative example in which any of the component composition, the concentration of Mn and Cr in the carbide, and the amount of retained austenite does not satisfy the conditions of the present invention is inferior in rolling fatigue characteristics in a hydrogen environment.

Claims (2)

% By mass
C: 0.55% to 1.10%,
Si: 1.0% to 2.0%,
Mn: 2.0% to 3.0%,
P: 0.02% or less,
S: 0.02% or less,
Al: 0.05% or less,
Cr: 0.8% to 2.4%,
N: 0.020% or less and O: 0.0020% or less, with the balance being a component composition of Fe and inevitable impurities, having a metal structure composed of tempered martensite, residual austenite and carbide, and the volume of the residual austenite A bearing steel having a fraction of more than 10% and 20% or less, wherein the Cr concentration of the carbide is 6% or more and the Mn concentration is 5% or more. The component composition is further in mass%,
Sb: 0.0050% or less,
Mo: 0.50% or less,
Ti: 0.01% or less,
Cu: 0.30% or less,
The bearing steel according to claim 1, containing one or more selected from Ni: 0.15% or less and B: 0.0010% or less.