JP2014012870A - Carbonitriding component excellent in surface fatigue strength due to hydrogen embrittlement - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a carbonitriding component that ensures excellent surface fatigue strength even when hydrogen embrittlement peeling occurs.SOLUTION: A carbonitriding component contains, by mass%, C:0.30 to 1.30%, Si:0.05 to 0.35%, Mn:0.40 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% and the balance Fe with inevitable impurities and has a surface layer C concentration after a temper treatment of 0.80 to 1.50 mass%, a surface layer N concentration of 0.10 to 1.00 mass% and a surface hardness of HRC 58 to 64, a percentage of the number of Cr nitrides and Mn nitrides having a particle diameter of less than 300 nm to all nitrides dispersed with dispersion over surface layer of 70% or more, the number of the Cr nitrides and Mn nitrides of 10/mmor more, and a surface layer compressive residual stress of 600 MPa or more.

Description

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

  In recent years, gears used in automobiles and industrial equipment, CVT, which is a new speed change mechanism, and parts that are subject to surface fatigue load such as bearing parts have become severer in terms of use due to higher performance and higher speed. The types of lubricating oils used are also diversifying, and under such circumstances, there is a problem of causing early peeling due to a different peeling form.

For example, a bearing for a car alternator is accompanied by a change in the structure of a tree-like white layer different from a white band having an inclination (30 ° band, 80 ° band) due to the Hertzian stress field, which is a conventional structure change. Early 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 into the inside. Alternator bearings have dealt with this early peeling by changing the lubricating oil.
However, by simply changing the lubricating oil, it is becoming impossible to suppress the early peeling due to hydrogen, and there has been a demand for the development of a material with excellent hydrogen embrittlement.

The present applicant, as shown in the following Patent Document 1, has improved the hydrogen embrittlement type surface fatigue strength using a V-type carbide hydrogen trap technology by adding vanadium (hereinafter referred to as V). Steel has already been developed.
Further, as shown in Patent Document 2 below, by reducing the initial carbon content of the steel material and adding V and molybdenum (hereinafter referred to as Mo) in combination, the hydrogen embrittlement type surface fatigue strength is excellent, and gears, CVT parts, etc. Is developing case-hardened steel applicable to a wide range of parts.
Furthermore, as shown in Patent Document 3 below, a carbon steel for carbonitriding that has excellent surface fatigue strength of a hydrogen embrittlement type using a hydrogen trap of Cr-based nitride and Mo-based nitride has been developed.

JP 2006-213981 A JP 2008-280583 A JP 2011-225936 A

However, the same phenomenon of early peeling due to hydrogen occurs even when CVT traction oil is used. High-speed rotation and high-loading of parts subject to surface fatigue load, severe use conditions, and lubricating oil As a result of diversification, the parts and environmental conditions that cause early peeling due to hydrogen embrittlement tend to increase. For this reason, the surface fatigue failure of the hydrogen embrittlement type has not been sufficiently prevented, and the development of a material more excellent in the surface fatigue strength of the hydrogen embrittlement type has been demanded.
On the other hand, the case-hardened steel having a C content of about 0.2% in the steel as described in Patent Document 3 requires a long-time carburizing treatment in order to form a predetermined C concentration distribution on the surface layer. . Carbon steel with a high amount of C in the steel material can omit this long-time carburizing treatment, and therefore has a great cost merit. However, carbon steel having a high C content in the steel material has a problem that even if carbon nitride is performed by adding a predetermined amount of Cr and Mo, the surface fatigue strength of the hydrogen embrittlement type may not be improved.

  The present invention has been made in the background as described above. The purpose of the present invention is to perform carbonitriding on the basis of carbon steel with a high C content that can omit carburizing treatment for a long time. An object of the present invention is to provide a carbonitrided part capable of ensuring excellent surface fatigue strength even in the case where brittle peeling occurs.

  As a result of investigating the cause of the above problems, the present inventors have found that the carbon concentration distribution from the surface layer after carbonitriding may be small in carbon steel having a high C content in the steel material. It was thought that the surface fatigue life of the hydrogen embrittlement type decreased due to the lower stress. Therefore, the effect of compressive residual stress on the surface fatigue life of hydrogen embrittlement by carbon nitriding or by shot peening after carbonitriding using various carbon steels with high C content in steel. Was investigated.

  As a result, even when the C concentration distribution from the surface layer is small in carbon steel with a high amount of C in the steel material, by applying a surface layer compressive residual stress by shot peening, it is about the same as when the C concentration distribution from the surface layer is large In addition, it was found that the surface fatigue life of the hydrogen embrittlement type is improved, and that the surface fatigue life of the hydrogen embrittlement type is sufficient when the surface compressive residual stress is 600 MPa or more. In the above investigation, surface compressive residual stress was applied by shot peening treatment, but not limited to shot peening treatment, for example, plastic deformation of the surface layer portion by applying pressure such as coining, or hardness distribution from the surface layer by induction hardening is used. Even so, it is considered that surface compressive residual stress can be applied in the same manner as described above.

  The mechanism by which the surface compressive residual stress improves the surface fatigue life of the hydrogen embrittlement type is considered as follows. Surface fatigue peeling is greatly affected by shear stress, and peeling occurs due to the maximum shear stress generated at a depth of about 0.2 mm from the surface layer as shown in FIG. The hydrogen embrittlement type delamination occurs very early because the delamination itself is accelerated by hydrogen. Specifically, hydrogen moves to a high stress position (fracture high stress portion) due to interaction with dislocations, etc., and hydrogen accumulates at the maximum shear stress position, causing early peeling.

  As shown in FIG. 1B, when a high residual stress is applied to the outermost layer, hydrogen that has penetrated from the outer layer is trapped in the dislocations and stress field of the outer layer, and reaches the maximum shear stress depth position, which is a fracture starting point. Therefore, the surface fatigue life of the hydrogen embrittlement type is improved.

Based on the above knowledge, the carbon-nitrided part excellent in surface fatigue strength of the hydrogen embrittlement type of the present invention is mass%, C: 0.30 to 1.30%, Si: 0.05 to 0.35%. , Mn: 0.40 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 Carbonitriding parts that have been subjected to carbonitriding, quenching and tempering treatment, and comprising tempering treatment, comprising N.0.025% or less, N: 0.025% or less, Mn + Cr: 2.50 to 4.00%, the balance being Fe and inevitable impurities The surface layer C concentration 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 HRC58 or more and less than 64, and the grains with respect to all nitrides dispersed and precipitated on the surface layer The number ratio of Cr nitride and Mn nitride having a diameter of less than 300 nm is 70% or more and Number does not exceed 10 ⁴ / mm ² or more, the surface layer compressive residual stress, characterized in that at least 600 MPa. In this case, by mass%, Mo: 0.50% or less, Ni: less than 0.50%, Ti: 0.500% or less, Nb: 0.100% or less, one or more of them Furthermore, it can also be set as the structure contained. Further, the difference between the surface layer C concentration after tempering treatment and the steel material C concentration (the amount of C in the steel material (core)) may be 0.40% or more by mass%.

  According to the carbonitrided part of the present invention, as described above, the high compressive residual stress applied to the outermost layer traps hydrogen that has penetrated from the outer layer into the dislocations and stress field of the outer layer, and the maximum shear stress depth that is the starting point of fracture. Since the movement to the vertical position is suppressed, the surface fatigue strength of the hydrogen embrittlement type can be further improved as compared with the prior art.

(A) is explanatory drawing which shows the state where hydrogen accumulates in the maximum shearing stress position at the time of hydrogen embrittlement type surface fatigue peeling. (B) is explanatory drawing which shows the state by which hydrogen is trapped by the dislocation and stress field of a surface layer by compressive residual stress. Explanatory drawing which showed an example of the temperature, holding time, carbon potential, ammonia concentration, and cooling conditions in a carbonitriding quenching tempering process. Explanatory drawing of a rolling fatigue test method. Explanatory drawing of the 2 cylindrical rolling fatigue test method.

  Hereinafter, the reason for addition and limitation of each chemical component of the carbonitrided part excellent in surface fatigue strength of the hydrogen embrittlement type of the present invention will be described.

(1) C: 0.30 to 1.30%
C (steel C concentration) is added according to the core strength that is usually required. In order to obtain a surface layer C concentration of 0.80% or more without performing carburizing treatment for a long time, it is necessary to contain 0.30% or more, so the lower limit of the C content is 0.30%. Stipulated. On the other hand, when the C content exceeds 1.30%, the upper limit of the C content is set to 1.30% in order to reduce manufacturability such as forging and turning. Preferably it is 0.60 to 1.10%.

(2) Si: 0.05 to 0.35%
Si is used as a deoxidizer when manufacturing steel. Si is contained 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, Si reduces 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-brittle type decreases, so the upper limit of the Si content was set to 0.35%.

(3) Mn: 0.40 to 1.50%
Mn is an important additive element in the present invention. Mn forms Mn nitride and Mn composite nitride (for example, MnSiN ₂ etc.) by carbonitriding to serve as a hydrogen trap site and improve hydrogen embrittlement surface fatigue strength. Further, Mn is an element used for deoxidation when manufacturing steel, and is an element that improves hardenability. In order to obtain these effects, it is necessary to contain 0.40% or more of Mn. On the other hand, when Mn is contained in a large amount exceeding 1.50%, manufacturability such as forging and turning is lowered, so the upper limit of Mn content is set to 1.50%. Preferably it is 0.80 to 1.50%.

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

(5) S: 0.030% or less S impairs the hot workability of steel, forms non-metallic inclusions in the steel, reduces toughness and rolling life, and increases hydrogen embrittlement rolling fatigue strength. It is desirable to reduce as much as possible. For this reason, the upper limit of the S content is set to 0.030%. On the other hand, since S also has an effect of improving the machinability, the lower limit is preferably set to 0.010%.

(6) 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. Further, Cr is added for improving hardenability, ensuring hardness by carbide and improving rolling life. In order to obtain a predetermined carbonitride, addition of 1.50% or more is necessary, so the lower limit of the Cr content is defined as 1.50%. On the other hand, if Cr 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 Cr content is 3.00%. It was. Preferably it is 2.00 to 3.00%.

(7) Al: 0.050% or less Although Al is used as a deoxidizer during the production of steel, it is desirable to reduce it because it generates hard non-metallic inclusions and reduces the rolling fatigue life. When a large amount of Al is contained exceeding 0.050%, a significant decrease in rolling fatigue life is observed, so the upper limit of the Al content was set to 0.050%. In addition, since the cost of steel materials will raise when Al content is made less than 0.005%, it is preferable to make the minimum of Al content into 0.005%.

(8) O: 0.0015% or less, N: 0.025% or less O and N form oxides and nitrides in steel and become the starting point of fatigue failure as non-metallic inclusions, reducing the rolling fatigue life. Therefore, the upper limit of the O content is 0.0015% and the upper limit of the N content is 0.025%.

(9) Mn + Cr: 2.50 to 4.00%
Even if Mn and Cr are added alone, the surface fatigue strength of the hydrogen embrittlement type is improved, but in order to obtain a sufficient effect, it is necessary to add both appropriately and in combination. If the Mn + Cr content is less than 2.50%, a sufficient improvement effect on hydrogen embrittlement cannot be obtained, so the lower limit was made 2.50%. On the other hand, if the content of Mn + Cr exceeds 4.00%, productivity such as forging and turning decreases, so the upper limit was made 4.00%. Preferably it is 2.80 to 4.00%.

(10) Surface hardness: HRC 58 or more and less than 64 There is a correlation between the surface hardness after tempering and the rolling fatigue life, and the higher the surface hardness, the longer the rolling fatigue life. In particular, when the surface hardness after the tempering treatment is less than HRC58, the rolling fatigue life is drastically reduced and the variation in the life is increased. Therefore, the surface hardness after the tempering treatment 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 more, 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.

(11) Number ratio of Cr nitride and Mn nitride having a particle size of less than 300 nm, number: 70% or more of all nitrides, 10 ⁴ pieces / mm ² or more Among nitrides, nitrides effective for hydrogen trap are Cr CrN that is a nitride and MnSiN ₂ that is a Mn nitride. When the ratio of the number of Cr nitrides and Mn nitrides to the total nitrides is less than 70%, the effect of hydrogen traps is reduced, and the effect of improving the hydrogen embrittlement surface fatigue strength cannot be obtained. Therefore, the number ratio 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. If nitrides number generated is small and, when the fine nitrides below this size 300nm by particle diameter 300nm or more nitrides produces many of 10 less than ⁴ / mm ^2, the hydrogen embrittlement by the hydrogen trapping The effect of improving the mold surface fatigue strength decreases rapidly. For this reason, 10 ⁴ pieces / mm ² or more of nitrides having a particle diameter of less than 300 nm are included.

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

(13) 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%. On the other hand, when the N content exceeds 1.00%, the surface hardness is lowered due to the formation of retained austenite γ, and a predetermined surface hardness cannot be obtained. Therefore, the upper limit of the N content is set to 1.00%.

(14) Surface layer compressive residual stress: 600 MPa or more An increase in surface layer compressive residual stress is effective in improving the hydrogen embrittlement surface fatigue strength. This is because the compressive residual stress suppresses the generation and propagation of cracks and suppresses the diffusion of hydrogen that has entered from the surface layer to the fracture starting point. In order to obtain this effect, the surface layer compressive residual stress is required to be at least 600 MPa, so the lower limit of the surface layer compressive residual stress was set to 600 MPa. Preferably it is 700 MPa or more.

In the present invention, one or more of the following chemical components can be added.
(15) 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 the steel and has the effect of suppressing the decrease in hardness during tempering by dissolving in the carbide. On the other hand, when Mo is contained in a large amount exceeding 0.50%, the cost of the steel material is increased, and the productivity of forging, turning and the like is lowered. Therefore, the upper limit of Mo is set to 0.50%.

(16) Ni: less than 0.50% Ni suppresses structural changes in the rolling fatigue process and improves the rolling fatigue life. Further, the addition of Ni is effective in improving toughness and corrosion resistance. On the other hand, if Ni is contained in a large amount exceeding 0.50%, a large amount of retained austenite γ is generated at the time of quenching of the steel, and a predetermined hardness cannot be obtained. Was less than 0.50%.

(17) Ti: 0.500% or less Ti carbide is fine and effectively acts as a hydrogen trap site, thereby improving the surface fatigue strength of the hydrogen embrittlement type. On the other hand, Ti forms oxides and nitrides in the steel and becomes a starting point of fatigue failure as a non-metallic inclusion, and lowers the rolling fatigue life. Therefore, the upper limit of Ti content is set to 0.500%.

(18) Nb: 0.100% or less Nb carbide 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. On the other hand, 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%.

(19) Difference between surface layer C concentration and steel material C concentration after tempering: 0.40% or more The difference between surface layer C concentration and steel material C concentration after tempering determines C distribution and hardness distribution from the surface layer. Thus, as the surface C concentration is higher than the steel C concentration, higher compressive residual stress can be applied to the surface layer. Therefore, it is effective for improving the hydrogen embrittlement life. In order to obtain this effect, the difference between the surface layer C concentration and the steel material C concentration after the tempering process needs to be 0.40% or more by mass%, so the lower limit value was set to 0.40%. Preferably it is 0.50% or more.

(20) Remainder: Fe and inevitable impurities The inevitable impurities here (inevitable impurities) mean impurity-level chemical components represented by Ni and Mo in Table 1.

Examples of the present invention will be described below.
50 kg of a material having chemical components shown in Table 1 ("-" in the columns of Ni and Mo in Table 1 indicates an impurity level. The balance is Fe in Table 1) is melted by vacuum melting, and hot forged. Thus, a steel bar having a diameter of 28 mm was manufactured. Then, it heated to 920 degreeC as a normalization process, and it air-cooled after hold | maintaining for 2 hours. Furthermore, as a spheroidizing annealing treatment, it was heated to 760 ° C., held for 3 hours, then 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 (RX gas is used here) and quenched. A tempering treatment was performed. FIG. 2 shows an example of carbonitriding conditions. In FIG. 2, CP represents carbon potential, OQ represents oil quenching, and AC represents air cooling.

  A shot peening treatment was performed on each test piece subjected to the carbonitriding quenching and tempering treatment. The shot peening treatment was performed under various conditions (projection material, coverage (ratio of indentation area to total area of workpiece), projection pressure). As an example, when a projection material having a hardness of about 700 Hv and a particle size of about 0.8 mm is used and the projection pressure is 0.2 MPa, the arc height is about 0.6 mmA (the warpage of the test plate). It was.

  For each test piece subjected to shot peening treatment, the outer periphery was ground to a depth of 0.1 mm, and the Rockwell hardness (conforming to JIS Z2245) was obtained with an average of 5 points. Thereafter, the longitudinal section of the test piece was embedded and polished, and the surface layer C concentration and the surface layer N concentration in the surface layer portion were analyzed by EPMA. Here, the surface layer C concentration and the surface layer N concentration were set to the maximum value (peak value) of the C concentration and N concentration from the outermost layer to a depth of 10 μm.

Furthermore, the compressive residual stress of the circumferential direction and the axial direction of each test piece was measured using the X-ray residual stress measuring apparatus. As the measuring method, the parallel tilt method was used, and the characteristic X-ray was measured using a Cr tube with an irradiation area of 1 mm. In addition, ion milling (sample preparation method using an ion beam) is performed to produce a thin film, and nitride of the surface layer is mapped using a transmission electron microscope, and a particle size of 10 nm or more and less than 300 nm existing in a certain region. All the nitrides were identified and divided by the area of the observation region to obtain the number density (pieces / mm ² ) of fine nitrides having a particle size of less than 300 nm. Also, nitride composition analysis is performed by EDX analysis, Cr nitride, Cr composite nitride, Mn nitride, Mn composite nitride (eg MnSiN ₂ etc.), Si nitride, Si composite nitride, Al nitride, It was judged whether it was Al composite nitride or other nitrides, and the number ratio of Cr nitride, Cr composite nitride, Mn nitride and Mn composite nitride was determined.

In addition, a rolling fatigue test piece having a diameter of 12.2 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 after shot peening The test surface was ground to a diameter of 12 mm to prepare a test piece having a length of 22 mm. 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 hydrogen charging.

In the rolling fatigue test, as shown in FIGS. 3 (A) and 3 (B), two SUJ2 balls 12 are pressed against the test piece 10 with a predetermined surface pressure by using a guide roller 14. The test piece 10 is rolled by the driving roller 16 under the guide. The test conditions were a surface pressure of 5.9 GPa, and the lubrication was performed by spraying turbine # 68 and applying a load speed of 46240 rpm. Were tested at 10 points under the same conditions, the cumulative failure probability of the Weibull distribution was evaluated life seeking 10% become L ₁₀ life. The surface fatigue of the hydrogen embrittlement type is considered to be caused by intrusion of hydrogen 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.

  In addition, after rough machining from the same material, each steel type is subjected to the same carbonitriding treatment as described above, followed by shot peening treatment to produce a cylindrical test piece having a test surface diameter of 26 mm, and a two-cylinder rolling fatigue test using the test piece. Went. In the two-cylinder rolling fatigue test, as shown in FIG. 4, the mating cylinder 20 is pressed against the cylindrical test piece 18 with a predetermined surface pressure, and the test piece 18 is pushed by the motor 22 via the shaft portion 24 in this state. While rotating, the rotation of the motor 22 is transmitted to the shaft 30 via the gears 26 and 28 to rotate the counterpart cylinder 20. The counterpart cylinder 20 is made of a quenching and tempering material made of SUJ2, and is formed into a shape with a diameter of 130 mm having a crowning with a curvature radius of 150 mm in the axial direction.

  The test conditions were such that the hydrogen embrittlement type surface fatigue peeling was reproduced. Specifically, using hydrogen-brittle lubricating oil, the test was conducted under test conditions (oil temperature 90 ° C., slip rate -60%, surface pressure 3 GPa, rotation speed 1500 rpm) in which hydrogen brittle type early rolling fatigue failure occurs. went. Here, the slip ratio is a ratio of the difference between the peripheral speeds of the 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 test results.

Each of the inventive examples has a surface hardness of HRC 58 or more and less than 64, and contains 10 ⁴ pieces / mm ² or more of fine nitrides having a particle size of less than 300 nm. The surface layer C concentration is in the range of 0.80 to 1.50 mass%, and the surface layer N amount is in the range of 0.10 to 1.00 mass%.

The L ₁₀ life of rolling fatigue of the hydrogen charge material of the invention example is excellent at 31.0 × 10 ⁷ times (steel type No. 5) to 51.2 × 10 ⁷ times (steel type No. 7). On the other hand, in the comparative example, the _{L 10} life and 0.7 × 10 ⁷ times (steels No.7) ~8.5 × 10 ⁷ times (steel type No.13), both early rolling fatigue hydrogen embrittlement type Destruction occurs and has a low life. It can be seen that the rolling life of the hydrogen embrittlement type is improved by about one order according to the present invention.

Moreover, the average life of the two-cylinder test of the invention example is excellent at 25.6 × 10 ⁶ times (steel type No. 3) to 39.5 × 10 ⁶ times (steel type No. 8). On the other hand, in the comparative example, the average life is 0.2 × 10 ⁶ times (steel type No. 5) to 7.2 × 10 ⁶ times (steel type No. 12), both of which have a low life due to hydrogen embrittlement. It can be seen that the rolling life of the hydrogen embrittlement type is improved by about one order according to the present invention.

  In Table 2, since the steel types No. 12 and 14 of the comparative example have a low Cr amount, the steel type No. 13 has a low Mn + Cr amount, so both are examples of low life. Moreover, steel types No. 1 to No. 7 of the comparative examples are examples in which the chemical components are within the claimed range but have a low life due to the following reasons. That is, steel types Nos. 1, 3, 4, and 6 have a high steel material C concentration, but the difference between the surface layer C concentration and the steel material C concentration is small, and the shot peening treatment or the like was not performed, so the surface layer compressive residual stress was less than 600 MPa. This is an example of a low life.

Here, the steel type No. 4 has the surface layer C concentration and the steel material C concentration set to substantially the same magnitude, but the invention example using this steel type No. 4 and the comparative example have other than the surface layer compressive residual stress. Although the surface hardness, the surface layer C concentration, the surface layer N concentration, and the number ratio and number of Cr nitride and Mn nitride having a particle size of less than 300 nm are almost the same, the rolling of the hydrogen charge material remarkable difference occurs in the fatigue of L ₁₀ life and life expectancy of second cylindrical test. As a result, even when the steel material C concentration is high and the difference between the surface layer C concentration and the steel material C concentration is small, if the surface layer compressive residual stress of 600 MPa or more is applied by performing a shot peening process, both the above-mentioned lifetimes are improved. You can see that

Steel types No. 2 and 5 of the comparative example are examples in which the carbonitriding conditions are not suitable, and thus the surface layer has less than 10 ⁴ nitrides / mm ^{2 of} less than 300 nm, resulting in a low life. This steel type No. 2 is an example in which the surface layer N concentration satisfies an appropriate condition, but the diameter of the nitride is increased and the number of nitrides less than 300 nm is insufficient. Steel type No. 7 is an example of a low life because the surface layer C concentration is low.

  As described above, the comparative example using the steel types Nos. 1, 3, and 6 has a high steel material C concentration, but the difference between the surface layer C concentration and the steel material C concentration is small, and shot peening treatment or the like was not performed. Although the residual compressive stress did not reach 600 MPa, the carburization that increases the difference between the surface layer C concentration and the steel material C concentration without performing shot peening using these steel types No. 1, 3, 6 A nitriding treatment was performed, and a test was performed to obtain the relationship between the surface layer compressive residual stress and the above-mentioned both lifetimes. Table 3 shows the test results.

Even if shot peening treatment or the like is not performed from Table 3, when the surface layer C concentration during carbonitriding is increased and the difference between the surface layer C concentration and the steel material C concentration is 0.40% or more, the surface layer compressive residual stress is 600 MPa or more. Thus, it can be seen that the L ₁₀ life of rolling fatigue of the hydrogen charge material and the average life of the two-cylinder test are improved.

  As is clear from the above description, according to the carbonitrided component of the present invention in which the compressive residual stress at the outermost layer is increased, the surface fatigue strength of the hydrogen embrittlement type is further improved as compared with the prior art. be able to.

10,18 Specimen

Claims (3)

% By mass
C: 0.30 to 1.30%
Si: 0.05 to 0.35%,
Mn: 0.40 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%
Carbonitriding and quenching and tempering carbonitriding parts consisting of Fe and inevitable impurities, the surface C concentration after tempering being 0.80 to 1.50% by mass, and the surface layer N concentration being 0.10 to 1 The number ratio of Cr nitride and Mn nitride having a particle size of less than 300 nm to the total nitride dispersed and deposited on the surface layer is 70% or more and the number is 10 ^4. pieces / mm ² or more, carbonitriding component surface compressive residual stress is excellent in surface fatigue strength of the hydrogen embrittlement type, characterized in that at least 600 MPa. In claim 1, in mass%,
Mo: 0.50% or less,
Ni: less than 0.50%,
Ti: 0.500% or less,
Nb: 0.100% or less,
A carbon-nitrided part excellent in surface fatigue strength of a hydrogen embrittlement type, further comprising any one or more of them.   The carbonitriding excellent in surface fatigue strength of the hydrogen embrittlement type according to claim 1 or 2, wherein the difference between the surface layer C concentration and the steel material C concentration after tempering is 0.40% or more by mass%. parts.

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