JP5644166B2 - Carbon nitrided steel with excellent surface fatigue strength of hydrogen brittle type - Google Patents

Description

  The present invention relates to a carbonitrided steel excellent in surface fatigue strength of a hydrogen embrittlement type.

  In recent years, gears used in automobiles and industrial equipment, new transmission mechanism CVT (Continuously Variable Transmission), bearing parts and other parts that are subject to surface fatigue load are used with higher performance and higher speed. In addition, the types of lubricating oils used including CVT are diversified, and under such circumstances, there is a problem of causing early peeling due to a different peeling form.

In these parts subjected to surface fatigue load, in recent years, under severe load conditions such as high vibration, high load, sudden acceleration / deceleration, etc., and when special lubricating oil and water mixing conditions are combined, the normal rolling fatigue life is exceeded. However, there is a problem in that early peeling with a significantly short life occurs.
The cause of this early peeling is considered to be that hydrogen is generated on the rolling surface during the rolling process and enters the inside to cause hydrogen embrittlement, resulting in a significant reduction in peeling life.

For example, in an alternator bearing of an automobile, an early stage accompanied with a tissue change of a tree-like white layer different from a white band having an inclination (30 ° band, 80 ° band) due to a hertzian stress field which is a conventional tissue change. Peeling may occur.
This is because the oil film thickness becomes insufficient under severe load conditions such as high vibration, high load, sudden acceleration / deceleration, etc., resulting in metal contact in part, and the lubricating oil decomposes and hydrogen is generated on the rolling surface. This is thought to be because hydrogen brittle exfoliation occurred due to the penetration of the inside.
Alternator bearings have dealt with this early peeling by changing the lubricating oil.

  However, the same early peeling phenomenon due to hydrogen is caused by the severe and diversified use conditions of parts subjected to surface fatigue load, and the generated parts and environmental conditions are widened, and can be dealt with by simply changing the lubricating oil. It is disappearing.

  Under such circumstances, the present applicant has developed a V-type carbide by adding vanadium (hereinafter referred to as V) in Patent Document 1 below, thereby using a hydrogen trapping technique to improve hydrogen embrittlement resistance. A carbon high chromium bearing steel is disclosed.

  Furthermore, in Patent Document 2 below, by reducing the initial carbon content of steel materials and adding V and molybdenum (hereinafter referred to as Mo) in combination, it is excellent in hydrogen fatigue type surface fatigue strength and can be applied to a wide range of parts such as gears and CVT parts. Is disclosed.

  However, the same early peeling phenomenon caused by hydrogen has occurred even when CVT traction oil is used. High-speed rotation and high load of parts subjected to surface fatigue load, severe use conditions, and lubrication oil Due to diversification, etc., parts and environmental conditions that cause early peeling due to hydrogen embrittlement tend to increase, and the surface fatigue failure of hydrogen brittle type has not yet been sufficiently prevented. There was a need to develop better materials.

JP 2006-213981 A JP 2008-280583 A

  In the background of the above circumstances, the present invention provides a carbonitrided steel having excellent surface fatigue strength even when hydrogen brittle exfoliation occurs depending on use conditions by performing carbonitriding on the basis of case-hardened steel. It was made for the purpose of providing.

Thus, the carbonitrided steel of claim 1 is C: 0.10 to 0.40%, Si: 0.05 to 0.35%, Mn: 0.80 to 1.50%, P: 0.030% or less, S: 0.030% or less, Cr: A carbonitrided and tempered steel with a composition of 1.50 to 3.00%, Al: 0.050% or less, O: 0.0015% or less, N: 0.025% or less, Mn + Cr: 2.50 to 4.00%, balance Fe and inevitable components The surface C concentration after tempering is 0.80 to 1.50% by mass, the surface layer N concentration is 0.10 to 1.00% by mass, the surface hardness is HRC 58 or more and less than 64, and among the nitrides dispersed and precipitated on the surface layer, the particle size is 300 nm. wherein the number ratio of the total nitride MnSiN ₂ total is CrN and Mn nitrides is Cr nitrides below is and the number of 70% or more 10 ⁴ / mm ² or more.

  Further, according to claim 2, the mass% of Mo: 0.50% or less, Ni: less than 0.50%, Nb: 0.100% or less, Ti: 0.500% or less in mass%, Furthermore, it is characterized by containing.

The phenomenon of reduction in material strength due to hydrogen embrittlement has been known for some time as described above. For example, in springs and bolt parts, diffusible hydrogen entering from an exposed environment due to decomposition of water or the like causes delayed destruction.
Spring and bolt steel with excellent delayed fracture resistance Precipitating many fine carbides such as V, Ti and Nb, trapping diffusible hydrogen, and suppressing diffusion of hydrogen to grain boundaries and stress concentration parts Has been developed.
In this case, it is considered that the carbide that works as a hydrogen trap is a fine carbide having a thickness of several nanometers in which alloy elements such as V, Ti, and Nb dissolved during quenching are coherently precipitated by high-temperature tempering at 450 ° C. or higher.
However, high surface tempering at 450 ° C. or higher reduces the surface hardness, so that high surface fatigue strength cannot be obtained.

Therefore, as described above, the present applicant generates a large number of V-type carbides of 500 nm or less existing in an insoluble state at the time of quenching by adding V in the high carbon high chromium bearing steel. Thus, Patent Document 1 discloses that a decrease in life due to hydrogen embrittlement under tensile and compression fatigue conditions can be suppressed without performing high-temperature tempering at 450 ° C. or higher.
However, in the one disclosed in Patent Document 1, many coarse V-based carbides remain in the process of melting and casting. This is because the high carbon chromium steel disclosed in Patent Document 1 has a high initial carbon content.

  In this way, a large amount of coarse V-based carbides remaining in the process of melting and casting has a low effect as a hydrogen trap site. Therefore, the applicant further suppresses the initial carbon content to 0.40% or less, thereby dissolving and casting. In this process, the formation of coarse V-type carbides is suppressed, and the ratio of the number of fine V-type carbides having a particle size of less than 100 nm in the V-type carbides is 80% or more. It has been found that the life can be further improved, and Patent Document 2 discloses this.

  In this Patent Document 2, hydrogen embrittlement type surface fatigue fracture is caused by hydrogen being trapped preferentially at grain boundaries, interfacial slippage being promoted by hydrogen, and generation and propagation of intergranular cracks. It clarifies that addition of Mo is effective for grain boundary strengthening, and discloses a case-hardening steel in which V and Mo are added in combination.

However, the environment of surface fatigue strength of the hydrogen embrittlement type has become severe, and this has not been completely prevented yet.
Therefore, as a result of researches on various materials and heat treatment conditions, the present inventor has found that the life improvement effect of the carbonitrided material is greater than that of the carburized material.
And based on this knowledge, as a result of studying the material composition and heat treatment conditions for maximizing the strength improvement effect in carbonitriding, the following knowledge was obtained.

  Specifically, there are appropriate conditions for the surface layer C concentration and the surface layer N concentration for obtaining a long life in hydrogen embrittlement type rolling fatigue, and for the surface layer C concentration, the C concentration of the outermost layer is lowered by nitriding. Therefore, 0.80 to 1.50%, which is higher than normal carburizing, is suitable. On the other hand, if the surface layer N concentration is too high, coarse nitrides are formed, so 0.10 to 1.00% is suitable. did.

In the carbonitriding process, the generation of coarse nitrides of 1 μm or more is suppressed and a large number of fine nitrides are distributed. Specifically, among the nitrides generated on the surface layer, fine nitrides having a particle size of less than 300 nm are 10 ^4. It has been clarified that it is necessary to improve the surface fatigue life of the hydrogen embrittlement type to be present at least 1 piece / mm ² .
This is presumably because the life improvement mechanism is a hydrogen trap by fine nitride having a particle size of less than 300 nm, particularly fine nitride having a particle size of about 100 to 200 nm.

Further, as the types of nitrides, Cr nitride as Cr nitride and MnSiN _{2 as} Mn nitride are effective as hydrogen trap sites, and by making the total number ratio of CrN and MnSiN ₂ 70% or more, hydrogen trap It was found that the effect can be obtained sufficiently.

In a steel containing Si, a nitride of Si alone can be produced as a nitride, but this Si nitride produces a coarse nitride of 1 μm or more. In order to suppress this, it is effective that the nitride of Si is a composite nitride of Mn and Si.
In this case, in order to make all the Si-based nitrides into Si and Mn composite nitride MnSiN ₂ , the Si addition amount should be 0.5 times or less of the Mn addition amount from the atomic weight ratio, particularly 0.25 times or less. It was also found that it is desirable to suppress the addition of Si as much as possible in order to increase the hydrogen embrittlement sensitivity of steel.

On the other hand, the addition of V, which was effective in the carburized material, is not effective in carbonitriding or rather shortens the life of the steel. This is presumably because the formation of nitride by the addition of V increases the surface hardness, and as a result, the sensitivity to hydrogen embrittlement increases.
Therefore, it has been clarified that in the carbonitriding process, the steel to which V is not added has an excellent life.

The improvement effect of the hydrogen embrittlement type surface fatigue strength by the nitride thus obtained is sufficiently larger than the improvement effect by the V-based carbide described above.
This is considered because the hydrogen trap effect by the fine nitride is larger than that by the V-based carbide.
Incidentally, FIG. 1 shows a comparison of the hydrogen release profiles in the temperature-programmed desorption hydrogen analysis of the V-non-added carbonitriding material and the V-added carburizing material, that is, the hydrogen release rate (release amount) as the temperature rises.

In FIG. 1, the non-V-added carbonitrided material has a higher hydrogen release rate, that is, the release amount at each temperature than that of the V-added carburized material, and in particular, the hydrogen release rate on the high temperature side near 180 ° C. increases. Yes.
This is because the desorption activation energy of hydrogen trapped in the nitride is higher than the desorption activation energy of hydrogen trapped in the V-type carbide, that is, the trapping force of hydrogen by nitride is that of the V-type carbide. It is considered that this is a factor that the effect of nitride is great for improving the surface fatigue strength of the hydrogen embrittlement type.
This is considered to be because hydrogen traps by nitrides of 300 nm or less, particularly nitrides of 100 to 300 nm, work effectively compared to V-based carbides having a particle size of less than 100 nm.
The carbonitrided steel of the present invention has been developed based on the above knowledge.

The non-V-added carbonitriding material in FIG. 1 is an example of an invention using steel No. 7 in Table 2 described later, and the non-V-added carburizing material uses steel No. 7 in Table 1. Further, the V-added carburized material is a steel obtained by adding 0.4% V to steel type No. 7 and carburized under the following conditions. Specifically, a methanol dropping type cracking gas is used, carburized at a carbon potential of 0.8%, a temperature of 910 ° C., quenched at 830 ° C., and tempered at 160 ° C.
V-based carbides present in the V-added carburized material have an average particle size of about 60 nm. The nitride present in the surface layer of the non-V-added carbonitrided material has a particle size of about 100 to 200 nm, which is slightly large.
Measurement was performed using a test piece having a diameter of 3 mm and a length of 30 mm, and performing thermal desorption hydrogen analysis using a gas chromatograph within 10 minutes after hydrogen charging. Here, the heating rate was 100 ° C./h, and the analysis was performed from room temperature to 600 ° C. For hydrogen charging, an electrolytic solution in which 3 g of ammonium thiocyanate was dissolved in 1 L of a 3% sodium chloride solution was used, and cathode charging was performed at a current density of 0.1 mA / cm ² for 24 hours.

Next, the reason for addition and limitation of each chemical component of the carbonitrided steel excellent in surface fatigue strength of the hydrogen embrittlement type of the present invention will be described below.
C: 0.10 to 0.40%
C is an essential element for securing the core strength as a rolling bearing, and in order to maintain the hardness after a predetermined heat treatment, it is necessary to contain 0.10% or more, so the lower limit of C content is 0.10% Stipulated.
However, when the C content exceeds 0.40%, the upper limit of the C content is set to 0.40% in order to reduce the productivity of forging and turning.

Si: 0.05-0.35%
Si is used as a deoxidizer in the production of steel. Si is also added in an amount of 0.05% or more in order to improve the strength and rolling fatigue life of the steel. On the other hand, the addition of Si decreases the toughness of the steel and decreases the hot workability and increases the sensitivity to hydrogen embrittlement. If added over 0.35%, the rolling fatigue life of the hydrogen embrittlement type decreases, so the upper limit was made 0.35%.

Mn: 0.80 to 1.50%
Mn is an important additive element in the present invention. Mn forms nitrides by carbonitriding and acts as a hydrogen trap site, improving the hydrogen embrittlement surface fatigue strength. Mn is an element used for deoxidation when manufacturing steel, and is an element that improves hardenability. For these reasons, it is necessary to contain 0.80% or more of Mn.
However, if Mn is contained in a large amount exceeding 1.50%, the machinability is significantly lowered. Therefore, the upper limit of the Mn content is defined as 1.50%.
A preferable content of Mn is 0.90% or more.

P: 0.030% or less P segregates at the austenite grain boundaries of the steel, leading to a decrease in toughness and rolling fatigue life. In particular, 0.030% was made the upper limit of the P content in order to greatly reduce the grain boundary strength, which is a characteristic of hydrogen embrittlement type rolling fatigue.

S: 0.030% or less S is harmful because it impairs the hot workability of steel, forms non-metallic inclusions in the steel, reduces toughness and rolling life, and reduces hydrogen embrittlement rolling fatigue strength. Although it is desirable to reduce it as much as possible, since S also has an effect of improving the machinability, 0.030% was made the upper limit of S.

Cr: 1.50 to 3.00%
Cr is an important additive element in the present invention. Cr forms nitrides by carbonitriding and acts as a hydrogen trap site, improving the hydrogen embrittlement surface fatigue strength. Cr is added to improve hardenability, secure hardness by carbides and improve life.
In order to obtain a predetermined carbonitride, addition of 1.50% or more is necessary, so the lower limit value of the Cr content is defined as 1.50%.
However, if it is contained in a large amount exceeding 3.00%, the carburizing property is deteriorated, and a large carbonitride is formed to reduce the rolling fatigue life. Therefore, the upper limit of the Cr content is set to 3.00%.
A preferable content of Cr is 1.80 to 2.50%.

Al: 0.050% or less
Al is used as a deoxidizer during the production of steel, but it is desirable to reduce it because it produces hard non-metallic inclusions and reduces the rolling fatigue life. When a large amount of Al is contained in excess of 0.050%, a significant decrease in rolling fatigue life is observed, so the upper limit of Al content was 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%.

O: 0.0015% or less N: 0.025% or less O and N form oxides and nitrides in the steel and become the starting point of fatigue failure as non-metallic inclusions, reducing the rolling fatigue life. O: 0.0015%, N: 0.025% was made the upper limit of each element.

Mn + Cr: 2.50 to 4.00%
Even when Mn and Cr are added alone, the surface fatigue strength of the hydrogen embrittlement type is improved. However, in order to obtain a sufficient effect, it is necessary to add both appropriately and in combination. When the content of Mn + Cr is 2.50% or less, the effect of improving hydrogen embrittlement cannot be obtained sufficiently, so the lower limit was set to 2.50%. On the other hand, if the Mn + Cr content exceeds 4.00%, the manufacturability such as forging and turning properties deteriorates, so the upper limit was made 4.00%.
A preferable content of Mn + Cr is 2.80 to 3.50%.

Surface hardness: HRC 58 or more and less than 64 A correlation is observed 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 life is increased, so the surface hardness after tempering is set to HRC58 or more.
On the other hand, when the surface hardness is increased, the sensitivity to hydrogen embrittlement is increased, and when the surface hardness is HRC64 or higher, the surface fatigue strength of the hydrogen embrittlement type is significantly decreased. In terms of Hv hardness, this corresponds to about 650 Hv or more and less than 800 Hv.

Number ratio of CrN and MnSiN ₂ having a particle size of less than 300 nm: 70% or more of all nitrides and 10 ⁴ pieces / mm ² or more of the nitrides Among the nitrides, the nitride effective for hydrogen trap is CrN And MnSiN ₂ which is Mn nitride. If the number ratio of CrN and MnSiN ₂ is less than 70% of the total number of nitrides, the effect of hydrogen trap is reduced and the effect of improving the hydrogen embrittlement surface fatigue strength cannot be obtained sufficiently. Therefore, the number ratio of CrN and MnSiN ₂ is set to 70% or more.
Nitride has an effect of suppressing hydrogen embrittlement type surface fatigue peeling by trapping hydrogen. In order to obtain the effect, it is necessary to deposit a large number of fine nitrides. When the number of nitrides generated is small, or when a large number of nitrides having a particle size of 300 nm or more are generated and the number of fine nitrides having a particle size of less than 300 nm is less than 10 ⁴ / mm ² , hydrogen embrittlement type surface fatigue strength due to hydrogen traps The improvement effect of decreases rapidly. For this reason, 10 ⁴ pieces / mm ² or more of nitrides having a particle size of less than 300 nm are contained.

Surface layer C concentration: 0.80 to 1.50%
C is an essential element for securing strength as a rolling bearing, and since it is necessary to contain 0.80% or more in order to maintain the hardness after a predetermined heat treatment, the lower limit of the C content is limited to 0.80%. . However, when the C content exceeds 1.50%, it has been found that large carbides are produced and the rolling fatigue life is reduced, so the upper limit of the C content is 1.50%.

Surface layer N concentration: 0.10 to 1.00%
N improves the rolling life by improving the softening resistance of the steel. In addition, by forming fine nitride on the surface layer, it works as a hydrogen trap site and improves hydrogen embrittlement resistance. In order to obtain these effects, it is necessary to contain 0.10% or more of N, so the lower limit was made 0.10%. However, if the amount of N exceeds 1.00%, the surface hardness is reduced due to the formation of residual γ, and a predetermined surface hardness cannot be obtained. Therefore, the upper limit of the amount of N is set to 1.00%.

Mo: 0.50% or less
Mo improves the surface fatigue strength of the hydrogen embrittlement type by suppressing grain boundary fracture. Further, Mo improves the hardenability of steel and has the effect of suppressing the decrease in hardness during tempering by being dissolved in carbide. However, if the content exceeds 0.50%, the cost of the steel increases, and hot workability and machinability deteriorate. Therefore, the upper limit of Mo is set to 0.50%.

Ni: Less than 0.50%
Ni suppresses structural changes during the rolling fatigue process and improves the rolling fatigue life. In addition, the addition of Ni is effective in improving toughness and corrosion resistance. However, if it is contained in a large amount of 0.50% or more, a large amount of retained austenite is generated during quenching of the steel, the predetermined hardness cannot be obtained, and the cost of the steel material increases, so the upper limit of Ni content is 0.50%. Less than.

Nb: 0.100% or less
The carbide of Nb is also fine and effectively acts as a hydrogen trap site, thereby improving the surface fatigue strength of the hydrogen embrittlement type. Nb suppresses the coarsening of crystal grains. And it is effective in improving hydrogen embrittlement resistance by refining crystal grains. However, even if Nb is contained in a large amount exceeding 0.100%, the effect is saturated, so the upper limit of Nb content is set to 0.100%.

Ti: 0.500% or less
Ti carbide is also fine, and effectively works as a hydrogen trap site, thereby improving the surface fatigue strength of the hydrogen embrittlement type. However, Ti forms oxides and nitrides in the steel and becomes a starting point for fatigue failure as non-metallic inclusions, and lowers the rolling fatigue life, so Ti: 0.500% was made the upper limit.

  According to the present invention as described above, a carbonitriding steel having excellent surface fatigue strength can be provided by performing carbonitriding treatment even when hydrogen embrittlement type surface fatigue delamination occurs depending on use conditions. .

It is the figure which showed the hydrogen release rate-temperature profile comparison of V non-addition carbonitriding material, V addition carburizing material, and V non-addition carburizing material. It is the figure which showed an example in the temperature, holding time, carbon potential ammonia concentration, and cooling conditions in a carbonitriding quenching tempering process. It is the figure which showed the scanning electron microscope observation of the nitride, and the example of EDX analysis. It is the figure which showed the example of observation by the transmission electron microscope of the nitride of a carbonitriding material. It is explanatory drawing of the method of a rolling fatigue test. It is explanatory drawing of the method of a 2-cylinder rolling fatigue test.

Next, embodiments of the present invention will be described below.
The chemical components shown in Table 1 (-in the columns of Ni and Mo in Table 1 indicate impurity levels, and the balance is Fe in Table 1) are melted in a 50 kg vacuum melting furnace and heated. A steel bar having a diameter of 28 mm was produced by forging.
Then, it heated to 920 degreeC as a normalization process, and it air-cooled after hold | maintaining for 2 hours.
Further, as a spheroidizing annealing treatment, it was heated to 760 ° C., held for 3 hours, cooled to 650 ° C. at −15 ° C./hour, and then air-cooled to obtain materials for each test.
A test piece having a diameter of 25 mm and a length of 100 mm was cut out from the material and treated under various carbonitriding conditions. Carbonitriding is performed under various carbonitriding conditions (carbonitriding temperature, carbonitriding time, carbon potential, ammonia concentration) in a mixed atmosphere in which ammonia gas is added to carburizing gas (here RX gas), and quenching and tempering. Processed.

When the N concentration in the austenite increases, the martensite transformation start temperature (Ms point) decreases, the amount of retained austenite after quenching increases, and the surface hardness is insufficient. In order to increase the temperature, secondary quenching was performed at 840 ° C.
FIG. 2 shows an example of the carbonitriding conditions used.
Moreover, the intermediate annealing which hold | maintains at 650 degreeC for 1 hour was performed before secondary quenching as needed.
In FIG. 2, CP represents carbon potential, OQ represents oil quenching, and AC represents air cooling.

After performing the above carbonitriding quenching and tempering treatment, the outer periphery was ground to a depth of 0.2 mm, and then the Rockwell hardness (conforming to JIS Z2245) was determined with an average of 5 points.
Thereafter, the longitudinal section of the test piece was embedded and polished, and the C and N concentration in the surface layer portion was analyzed by EPMA.
Here, the surface layer C and N concentrations were the maximum values (peak values) of the C and N concentrations from the outermost surface to a depth of 10 μm.

Furthermore, electrolytic polishing is performed to produce a thin film, and surface nitrides are mapped using a transmission electron microscope to identify all nitrides having a particle size of 10 nm or more and less than 300 nm in a certain region, and the area of the observation region The number density (pieces / mm ² ) of fine nitrides having a particle diameter of less than 300 nm was determined.
Also, nitride composition analysis is performed by EDX analysis to determine whether it is Cr nitride and Mn nitride, Si nitride, Al nitride or other nitride, and the number ratio of Cr nitride and Mn nitride Asked.

FIG. 3 shows the results of observation by a scanning electron microscope (SEM) and the EDX analysis of the nitrides observed there, and Mn nitride MnSiN ₂ (FIG. 3 (A)) and Cr nitride CrN (FIG. 3). The example of EDX analysis determined as (b)) is shown.
In FIG. 3A, in addition to Fe, peaks of Mn, Si, and N appear, and therefore the particles analyzed by EDX can be identified as Mn and Si nitrides.
On the other hand, in FIG. 3 (b), Cr and N peaks appear in addition to Fe, and therefore the particles analyzed by EDX can be identified as Cr nitride.
FIG. 4 shows an example of the observation result with a transmission electron microscope.
In the figure, the portions that appear as white dots are nitride particles.
Incidentally, FIG. 3 and FIG. 4 are analysis and observation examples of the invention example of Table 2 (using steel type No. 7).

Also, a rolling fatigue test piece having a diameter of 12.3 mm and a length of 22.6 mm is roughly processed from the same material, and each steel type is subjected to carbonitriding and quenching tempering under the same carbonitriding conditions as described above, and the test surface has a diameter of 12 mm. A test piece having a length of 22 mm was prepared.
Using the electrolytic solution obtained by dissolving 3 g of ammonium thiocyanate in 1 L of 3% sodium chloride solution, the test piece was subjected to cathodic charging at a current density of 0.2 mA / cm ² for 24 hours. The rolling fatigue test was started within 10 minutes after the hydrogen charge.

Here, the rolling fatigue test was performed by the method shown in FIG.
Specifically, two balls 12 of JIS SUJ2 are pressed against the test piece 10 with a predetermined surface pressure, and the test piece 10 is driven by the drive roller 16 under the guide of the guide roller 14 and rolled. I moved it.
Here, the test conditions were a surface pressure of 5.9 GPa, and lubrication was performed by spraying turbine # 68 and applying a load speed of 46240 rpm.
Then, a 10-point test was performed under the same conditions, and an L ₁₀ life at which the cumulative failure probability of the Weibull distribution was 10% was determined and used as the evaluation life.
The surface fatigue of the hydrogen embrittlement type is considered to be caused by hydrogen penetration due to decomposition of the lubricating oil, generation of a new surface, etc. due to slippage. In a rolling fatigue test using the test piece 10 charged with cathode of hydrogen, it has been confirmed that the hydrogen brittle type early peeling phenomenon can be reproduced.

Also, after rough machining from the same material, each steel type is subjected to the same carbonitriding treatment as described above to produce a cylindrical test piece having a test surface diameter of 26 mm, and the test piece is used to perform two-cylinder rolling fatigue by the method shown in FIG. A test was conducted.
In FIG. 6, reference numeral 18 denotes a cylindrical test piece. In the method shown in FIG. 6, 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 20 is a cylinder having 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.
Specifically, using a lubricating oil that causes hydrogen embrittlement, the test was performed under test conditions (oil temperature 90 ° C., slip rate −60%, surface pressure 3 GPa, rotation speed 1500 rpm) in which hydrogen embrittlement type early rolling fatigue failure occurs. went.
Here, the slip ratio is a ratio between the peripheral speed difference between the cylindrical test piece 18 and the counterpart cylinder 20 and the peripheral speed of the test piece 18.
The test was performed at four points under the same conditions, and the average life was obtained.
Table 2 shows the results.

In the results of Table 2, all of the inventive examples have a surface hardness of HRC 58 or more and less than 64, and contain 10 ⁴ pieces / mm ² or more of fine nitrides having a particle size of less than 300 nm. The amount of surface layer C is in the range of 0.80 to 1.50%, and the amount of surface layer N is in the range of 0.10 to 1.00%.
The L ₁₀ life of rolling fatigue of the hydrogen charging material of the inventive example is excellent at 24.0-35.4 × 10 ⁷ times. Meanwhile in the comparative example, the _{L 10} life and .7 to 9.5 × 10 ⁷ times, both of which are low-life cause premature rolling fatigue fracture of hydrogen embrittlement type.
It can be seen that according to the present invention, the rolling life of the hydrogen brittle type is improved by about one order.
The average life of the two-cylinder test of the inventive example is excellent at 14.0 to 24.2 × 10 ⁶ times. On the other hand, in the comparative example, the average life is 0.2 to 6.7 × 10 ⁶ times, both of which are low life due to hydrogen embrittlement. It can be seen that according to the present invention, the rolling life of the hydrogen brittle type is improved by about one order.

In the comparative example shown in Table 2, the steel type No. 13 has a low Mn content in chemical components, and the one using No. 14 has a low Cr content. Therefore, the one using No. 15 has a Si content. This is an example of low life due to its high value.
In the comparative example, the steel type No. 1-No. 6 is an example in which the chemical composition is within the scope of the claims, but the lifetime is reduced for the following reason.
Since the carbonitriding conditions are not suitable for the steel types No. 1 and No. 2, the surface layer has less than 10 ⁴ nitrides / mm ^{2 of} less than 300 nm, which is an example of a low life.

  Similarly, No. 3 is an example in which the number of Cr and Mn nitride is less than 70%, resulting in a low life. For the same reason, those using steel type No. 4 are examples of low surface hardness and low life, and those using steel types No. 5 and 6 were carburized so that the surface layer N concentration was low, This is an example of a low life.

Claims (2)

In mass% C: 0.10 to 0.40%
Si: 0.05-0.35%
Mn: 0.80 to 1.50%
P: 0.030% or less S: 0.030% or less
Cr: 1.50 to 3.00%
Al: 0.050% or less O: 0.0015% or less N: 0.025% or less
Mn + Cr: 2.50 to 4.00%
Steel having a composition of the balance Fe and unavoidable components, which has been subjected to carbonitriding quenching and tempering treatment, and the surface hardness after tempering is 0.80 to 1.50 mass%, the surface layer N concentration is 0.10 to 1.00 mass%, and the surface hardness HRC is 58 or more and less than 64, and among the nitrides dispersed and deposited on the surface layer, the number ratio of CrN that is Cr nitride having a particle size of less than 300 nm and MnSiN ₂ that is Mn nitride is 70% or more of the total number of nitrides, and Carbonitrided steel excellent in surface fatigue strength of a hydrogen embrittlement type, characterized in that the number is 10 ⁴ pieces / mm ² or more. In Claim 1, in mass%
Mo: 0.50% or less
Ni: Less than 0.50%
Nb: 0.100% or less
Ti: 0.500% or less Carbon nitrided steel excellent in surface fatigue strength of a hydrogen embrittlement type characterized by further containing one or more of them.