JP2019167551A - Steel component excellent in rolling fatigue characteristics - Google Patents

Abstract

To provide a steel component having high surface hardness exceeding 800 HV, and capable of exhibiting excellent rolling fatigue characteristics even under an environment where a foreign matter such as abrasion powder may be mixed.SOLUTION: A steel component includes C:0.50 to 1.20%, Si:0.20 to 1.50%, Mn:0.20 to 1.50%, Cr:5.00 to 9.00%, Mo:0 to 1.00% (including 0%), V:0 to 1.00% (including 0%), and the balance made of Fe and inevitable impurities, and has a surface hardened layer. The surface characteristics satisfy (a) a retained austenite amount of 20 to 50% by area rate, (b) the major axis length of the maximum nitride of 15 μm or smaller, (c) the number of carbon nitride having a circle equivalent diameter of 0.1 to 2.0 μm of 400,000/mmor larger, (d) Cs of 1.5 to 3.4%, (e) Ns of 0.5% or smaller, and (f) the surface hardness exceeding 800 HV. Furthermore, the formula 1: 4×([Cs]+[Ns])-([Cr]+[Mo]+[V])<3, 6 is satisfied.SELECTED DRAWING: None

Description

  The present invention relates to a steel part having excellent rolling fatigue characteristics.

  For example, pinion shafts and bearing races (inner and outer rings) used in automatic transmission (AT) units are steel parts that receive spherical or conical rollers (rolling elements) and have high rolling fatigue. Characteristics are required. The oil in the AT unit contains foreign matters such as gears and other wear particles. If foreign particles such as wear particles get caught between the rolling elements such as rollers and the steel parts, The surface of the steel part may be plastically deformed at the biting position. The plastically deformed position has an irregular shape, and stress is concentrated when a rolling element such as a roller rolls at that position, which may lead to cracking or breakage.

  In order to enhance the rolling fatigue characteristics, it is effective to make the surface hardness sufficiently high to exceed 800 HV as a material characteristic, but when used in an environment where so-called foreign matter such as wear powder may be generated, That alone is not sufficient, and it is necessary to take measures to suppress stress concentration caused by the plastic deformation described above.

  As a technique which tried to ensure the conventional high rolling fatigue characteristic, there exist some which were described in patent documents 1-3 regarding the bearing ring of a bearing component, for example. However, the rolling fatigue characteristics of the bearing components described in these documents are not necessarily sufficient.

  First, the bearing ring for rolling bearing described in Patent Document 1 emphasizes toughness, and the upper limit of the surface hardness is 64 HRC or less (about 800 HV or less), and the basic characteristics do not reach the level required in the present application. It is.

  Next, from the description of the examples, the rolling bearing described in Patent Document 2 not only has a surface hardness of 64 HRC or less as in Patent Document 1, but also carburized and carburized as described in Patent Document 1. No study has been made on the improvement of service life by surface hardening treatment such as nitrogen.

  In addition, the rolling bearing of Patent Document 3 has no description of hardness in the examples of the bearing rings shown in Table 6, the amount of retained austenite is fixed at 10%, and the surface hardening treatment is performed when the Cr content exceeds 5%. The optimization of conditions for achieving a long service life is insufficient.

JP 2003-343577 A JP 2001-221238 A JP 2008-255399 A

  The present invention has been made in view of such a background, has a high surface hardness exceeding 800 HV, and exhibits excellent rolling fatigue characteristics even in an environment where foreign matter such as wear powder can be mixed. The present invention aims to provide a steel part having excellent rolling fatigue characteristics.

One embodiment of the present invention has a mass ratio of C: 0.50 to 1.20%, Si: 0.20 to 1.50%, Mn: 0.20 to 1.50%, Cr: 5.00 9.00%, Mo: 0 to 1.00% (including 0%), V: 0 to 1.00% (including 0%), with the balance being Fe and inevitable impurities,
Having a hardened surface layer including a carburized layer or a carburized and nitrocarburized layer;
As surface characteristics, the following (a) to (f) are provided,
(A) The amount of retained austenite is 20 to 50% in area ratio,
(B) The long axis length of the maximum carbonitride is 15 μm or less,
(C) The number of carbonitrides having an equivalent circle diameter of 0.1 to 2.0 μm is 400,000 pieces / mm ² or more,
(D) Cs (C concentration) is 1.5 to 3.4%,
(E) Ns (N concentration) is 0.5% or less,
(F) the surface hardness exceeds 800 HV, and
The steel parts satisfy the following formula 1 and have excellent rolling fatigue characteristics.
Formula 1: 4 × ([Cs] + [Ns]) − ([Cr] + [Mo] + [V]) <3.6,
(In the formula, [Cs] is the value of Cs in (d) above, [Ns] is the value of Ns in (e), [Cr], [Mo] and [V] are the values of Cr, Mo and V (The content (% by mass) is indicated.)

  The steel part has the chemical composition described above, has the above-described (a) to (f) as surface characteristics, and satisfies Formula 1. By having all of these requirements listed, while having a very high surface hardness, in a state where high pressure is loaded with rolling elements such as rollers in an environment where foreign matter such as wear powder is contained in the lubricating oil. Even if it is used in contact, it is possible to ensure an excellent life.

  When a foreign object is bitten, the surface is deformed, which causes a decrease in the life. To suppress the influence of the decrease in the life due to the biting of the foreign object, it occurs when the foreign object is bitten. It is important to control the surface shape (plastic deformation shape) so that the stress concentration is easily suppressed. For this purpose, it is effective to disperse the soft retained austenite (γ) phase in the portion of the surface layer where foreign matter biting occurs. This is because an appropriate amount of retained austenite phase can be dispersed to make the plastic deformation shape when the foreign object is bitten into a relatively smooth contour shape, which can be suppressed to a shape that can suppress stress concentration. is there.

  However, since having a soft retained austenite phase leads to a decrease in surface hardness, maintaining the surface hardness and controlling the plastic deformation shape by biting foreign matter due to dispersion of the retained austenite phase are contradictory requirements. In particular, when the hardness is over 800 HV, it is very difficult to propose specific measures for achieving both.

  Here, increasing the surface hardness is an indispensable requirement for obtaining high rolling fatigue characteristics, but after appropriately dispersing the relatively soft retained austenite phase for plastic deformation shape control. It is necessary to find a condition capable of ensuring high hardness as a whole by appropriately dispersing high-hardness carbonitrides.

  If the amount of carbonitride dispersed is increased, the hardness will improve, but if the amount of carbonitride is increased, they will become coarser. If coarse carbonitride is present, this becomes the starting point of stress concentration during use of the steel part (when stress is applied), which may cause cracking and breakage and reduce rolling fatigue characteristics. Therefore, when dispersing carbonitride, it is necessary to increase the total amount of carbonitride without increasing coarse carbonitride, and optimization of conditions for depositing a large amount of fine carbonitride is necessary. It becomes important.

  The present inventors propose a steel part that satisfies the above-described configuration, thereby realizing the appropriate dispersion of fine carbonitrides while appropriately dispersing the retained austenite phase, and thus the contradictory requirements described above. Was successfully established. As a result, it was possible to obtain a steel part having a high surface hardness exceeding 800 HV and capable of exhibiting excellent rolling fatigue characteristics even in an environment where foreign matter such as wear powder can be mixed.

<Chemical component>
The steel parts having excellent rolling fatigue characteristics are, by mass ratio, C: 0.50 to 1.20%, Si: 0.20 to 1.50%, Mn: 0.20 to 1.50%, Cr : 5.00 to 9.00%, Mo: 0 to 1.00% (including 0%), V: 0 to 1.00% (including 0%), the balance from Fe and inevitable impurities Become. The reasons for limiting the content ranges of these elements are as follows.

C: 0.50 to 1.20%
C (carbon) is an element necessary for improving the hardness after quenching and obtaining internal hardness for ensuring strength. When the C content is less than 0.50%, the internal hardness after quenching is lowered, and the bending fatigue strength may be lowered. On the other hand, when it exceeds 1.20%, coarse carbon Nitride is likely to be generated, which causes a decrease in rolling fatigue strength and bending fatigue strength, and there is a concern that workability may also be reduced.

Si: 0.20 to 1.50%,
Si (silicon) is an element indispensable as a deoxidizer during steelmaking, and is an element that suppresses the formation of carbonitrides during tempering and improves tempering softening resistance. When the Si content is less than 0.20%, it is difficult to secure a target level with respect to temper softening resistance, and a surface hardness exceeding 800 HV may not be obtained. If it exceeds 1, the hardness after annealing increases, and the workability to a predetermined shape may be reduced.

Mn: 0.20 to 1.50%,
Mn (manganese) is an element that acts as a deoxidizer during steelmaking and is an element that is effective in improving hardenability. If the Mn content is less than 0.20%, the hardenability may decrease and the hardness may decrease. On the other hand, if it exceeds 1.50%, the hardness after annealing may decrease. There is a risk that the processability is lowered.

Cr: 5.00 to 9.00%,
Cr (chromium) is an element that enhances hardenability and facilitates formation of fine carbonitrides. If the Cr content is less than 5.00%, carbonitrides may not be sufficiently formed and the surface hardness may decrease. On the other hand, if it exceeds 9.00%, coarse carbon Nitride is generated, and rolling fatigue strength and bending fatigue strength may be reduced.

Mo: 0 to 1.00% (including 0%),
Since Mo (molybdenum) is an optional additive element, it is not always necessary to add it. However, Mo is an element having a stronger affinity with C than Cr, and is important for increasing the amount of surface fine carbonitride after heat treatment and increasing hardness in order to facilitate the formation of fine carbonitride. It is an element. Therefore, it is preferable to contain an appropriate amount. When Mo is contained, the above effect can be obtained even with a small amount, but it is preferably 0.05% or more, and more preferably 0.10% or more. On the other hand, when the Mo content exceeds 1.00%, the effect is saturated and the cost is increased.
In addition, when manufacturing scrap with an electric furnace as a raw material, Mo is normally contained as an impurity in the range below 0.05%.

V: 0 to 1.00% (including 0%),
V (vanadium) is an optional additive element and thus does not necessarily have to be added. However, V is an element having an extremely strong affinity with C, and is an element important for increasing the amount of surface fine carbonitride after heat treatment and increasing the hardness in order to easily generate fine carbonitride. Therefore, it is preferable to add. When V is contained, the above effect can be obtained even in a small amount, but it is preferably 0.01% or more, more preferably 0.05% or more, still more preferably 0.10% or more. It is good. On the other hand, when the V content exceeds 1.00%, the effect is saturated and the cost is increased.

<Surface characteristics>
(A) The amount of retained austenite is 20 to 50% in area ratio,
As described above, the retained austenite on the surface is effective for controlling the plastic deformation shape at the time of foreign object biting to a shape in which stress concentration is difficult. When the amount of retained austenite observed from the surface is less than 20% in terms of area ratio, the above-described plastic deformation shape control effect at the time of foreign object biting may not be sufficiently obtained, whereas, when it exceeds 50% In addition, since the ratio of the soft retained austenite phase is too large, even if the effect due to the large amount of fine carbonitride is precipitated, the hardness exceeding 800 HV cannot be secured, and the rolling fatigue strength improving effect may be reduced. is there.

(B) The long axis length of the maximum carbonitride is 15 μm or less,
In order to improve the rolling fatigue strength, it is effective to improve the surface hardness by dispersing fine carbonitrides. As the carbonitride, in addition to Cr-based carbonitride, when Mo or V is added, Mo-based carbonitride or V-based carbonitride also exhibits the same effect. Of these carbonitrides, the required number of fine ones is as will be described later, but even if the fine carbonitrides are increased to improve hardness, if coarse carbonitrides are present, they will be stress concentrated. This has an adverse effect on improving rolling fatigue strength. Therefore, it is important to control the long axis length of the maximum carbonitride observed by surface observation to be 15 μm or less. In particular, when the long axis length of the maximum carbonitride exceeds 15 μm, it is necessary to avoid the adverse effect due to stress concentration, which increases the risk of shortening the life.

(C) The number of carbonitrides having an equivalent circle diameter of 0.1 to 2.0 μm is 400,000 pieces / mm ² or more,
As described above, improvement in hardness by dispersion of fine carbonitrides is indispensable for improving rolling fatigue strength. In surface observation, the size of the fine carbonitride effective for improving the hardness is a carbonitride having an equivalent circle diameter of 0.1 to 2.0 μm. Therefore, the number of fine carbonitrides of this size The effect is sufficiently exhibited by the presence of at least 400,000 pieces / mm ² . On the other hand, when the number of fine carbonitrides is less than 400,000 pieces / mm ² , there is a possibility that the effect of improving the hardness when securing the above-mentioned amount of retained austenite cannot be obtained sufficiently.

(D) Cs (C concentration) is 1.5 to 3.4%,
Cs indicates the C (carbon) concentration measured on the surface. In order to ensure the carbon concentration within this range, for example, a high-concentration gas carburizing process or a high-concentration gas carburizing / nitriding process may be performed as described later. Then, the above-described fine carbonitride is precipitated and coarse carbonitriding by carburizing or carburizing and nitriding while controlling Cs so as to be in the above range with respect to the steel composed of the above components. Steel parts in which the production of objects is suppressed can be obtained. When Cs exceeds 3.4%, coarse carbonitrides are produced, and the coarse carbonitrides may become a starting point of fracture, while when less than 1.5%, It is difficult to secure a sufficient amount of carbonitride to obtain a thickness of more than 800 HV.

(E) Ns (N concentration) is 0.5% or less,
Ns indicates the N (nitrogen) concentration measured on the surface, and when nitriding is performed, nitrogen can be diffused on the surface and rolling fatigue characteristics can be improved. However, when the N concentration is too high, the coarsening of the carbonitride is promoted, and the rolling fatigue characteristics may be deteriorated. Therefore, in order to prevent coarsening of the carbonitride, it is important to set Ns to 0.5% or less. In addition, since N is an element contained as an impurity in steel, it is contained in a small amount even when the nitriding treatment is not performed.

(F) the surface hardness exceeds 800 HV,
Improving the surface hardness is a basic requirement for improving rolling fatigue characteristics. In the steel part in this application, since it aims at obtaining the characteristic more than the past, it is essential to exceed 800HV.

  In addition, the surface in a surface characteristic means the outermost surface in a final steel part. That is, when the surface is machined after carburizing or carburizing and nitriding, it means the outermost surface after the processing.

<Formula 1>
Further, in addition to satisfying the above-mentioned component range, surface C concentration and N concentration conditions, the formula 1: 4 × ([Cs] + [Ns]) − ([Cr] + [Mo] + [V ]) <3.6 is effective in preventing the coarsening of carbonitrides more reliably and obtaining excellent rolling fatigue characteristics. When the value on the left side of Equation 1 is 3.6 or more, the possibility of formation of coarse carbonitrides remains, but by making it less than 3.6, the production of coarse carbonitrides is more reliably achieved. Can be prevented.
As described above, [Cs] in the formula is the value of Cs in (d), [Ns] is the value of Ns in (e), [Cr], [Mo] and [V] are Cr, The content rate (mass%) of Mo and V is shown.

<Internal characteristics rather than surface hardened layer>
In the steel part, preferably, the amount of retained austenite is 10% or less in terms of area ratio as an internal characteristic, that is, a characteristic at a cross-sectional position that is not affected by the surface hardening treatment inside the surface hardened layer. It is preferable to satisfy the requirements of.

  Residual austenite has the property of gradually martensitic transformation over time and expanding due to phase transformation. Therefore, if the amount of retained austenite in the interior is large, the amount of deformation becomes large, and there is a possibility that a malfunction will occur. Therefore, it is preferable to control the amount of retained austenite inside as described above.

<Manufacturing method>
Steel parts in the present application are manufactured by forming at least hot working to form rough parts, subjecting the rough parts to annealing treatment and rough cutting processing, surface hardening treatment, and then finishing cutting processing. can do. And the said surface hardening process can employ | adopt a high concentration gas carburizing process, a high concentration gas carburizing nitriding process, a high concentration vacuum carburizing process, a high concentration vacuum carburizing nitriding process etc., for example.

The high-concentration gas carburizing process is a process in which carburizing gas is introduced under a relatively high condition of carbon potential (Cp) of 0.8 to 1.6 to generate fine carbonitrides in the matrix. In the high-concentration gas carburizing and nitriding treatment, similarly to the high-concentration gas carburizing treatment, while introducing the carburizing gas under a relatively high condition where the carbon potential (Cp) is 0.8 to 1.6, ammonia (NH ₃ ) Gas is also introduced and both carburizing and nitriding are performed to generate fine carbonitrides in the matrix as described above.

  As processing temperature in these processes, it is preferable to set it as 880-960 degreeC. If the heating temperature during carburizing or carburizing and nitrocarburizing treatment exceeds 960 ° C, the carburizing abnormal layer may be deepened and the strength may decrease. On the other hand, if it is less than 880 ° C, C or C and N Penetration / diffusion is slow, and there is a possibility that sufficient hardness improvement effect and curing depth cannot be obtained.

  Further, after the high-concentration gas carburizing process or the high-concentration gas carburizing and nitriding process, a quenching process is continuously performed. The temperature before quenching in the quenching treatment (hereinafter referred to as quenching temperature) is preferably 850 to 930 ° C. When the quenching temperature exceeds 930 ° C., deformation during quenching may be excessively large. On the other hand, when the quenching temperature is less than 850 ° C., the generated carbonitride tends to be coarsened, and coarse carbonitride is generated. As a result, the rolling fatigue strength may decrease. In addition, as a rapid cooling method at the time of a quenching process, the oil cooling thrown in in 40-130 degreeC oil can be employ | adopted, for example. Moreover, it is preferable to perform the tempering process hold | maintained at about 150 degreeC after a quenching process.

  In addition, when shifting from the temperature during high-concentration gas carburizing treatment or high-concentration gas carburizing and nitriding treatment (hereinafter referred to as “treatment temperature”) to the quenching temperature, the carbonitriding is performed by lowering the temperature at a certain cooling rate or higher. The product can be produced more finely. And the higher the treatment temperature and the lower the quenching temperature, the greater the amount of carbonitride produced. That is, the amount of fine carbonitride increases as the difference between the treatment temperature and the quenching temperature increases. For this reason, the temperature difference between the treatment temperature and the quenching temperature is preferably within a range in which the formation of coarse carbonitrides can be suppressed, preferably 10 ° C. or more, preferably 20 ° C. or more. More preferably, the temperature is 30 ° C. or higher.

  Examples of steel parts having excellent rolling fatigue characteristics will be described together with comparative examples. First, a plurality of steel types having chemical components shown in Table 1 (Test Nos. 1 to 31 (Test Nos. 1 to 19 are examples, Test Nos. 20 to 30 are comparative examples, and Test No. 31 is a conventional steel SUJ2). Here, steel less than 0.05% of Mo is not positively added, and indicates the amount contained as an impurity.

  These steels are melted in an electric furnace to produce a steel ingot, and the steel ingot is forged and processed into a bar material before roughing. And after hold | maintaining this bar at 780-1080 degree C for 4 hours, after cooling to 770-880 degree C over 3 hours, gradually cooling to 600 degree C over 12 hours 40 minutes, and then air-cooling spheroidizing annealing treatment Went. In addition, since optimal conditions differ with components, the processing temperature is changed for every test material. Thereafter, a disk-shaped rough processed product having a diameter of 45 mm × 12 mm was prepared.

  The rough processed product is subjected to high-concentration gas carburizing and nitriding treatment (test Nos. 16-18, 23, and 29 are high-concentration gas carburizing treatment) and quenching / tempering treatment under the conditions described later, and then finished. As a result, the surface was cut by 0.1 mm to obtain a test piece. The surface after cutting is a position corresponding to the surface of the actual part. The surface characteristics were measured on the finished surface of the test piece. Further, the internal characteristics were measured on a surface cut to 2 mm from the surface, which was a position where it was confirmed that the high concentration gas carburizing / nitrogenizing treatment or the high concentration gas carburizing treatment was not affected.

  In addition, Test No. For No. 21, in order to clarify the difference in the effect due to the presence or absence of the high-concentration gas carburizing and nitriding treatment, together with the conventional steel SUJ2, a quenching and tempering treatment (850 ° C. × 30 minutes) which is a heat treatment usually performed on the conventional steel Oil cooling after heating → 150 ° C. for 1 hour followed by air cooling).

  In addition, in order to clarify that the characteristic range of the surface specified in the present invention is important for some test materials, some of the test materials were intentionally adjusted for carburizing and nitriding treatment, It processed so that it might become out of the range of the conditions of invention, and compared with the Example of this invention.

High concentration gas carbonitriding treatment, a carburizing gas while introducing the conditions carbon potential (Cp) is 0.8 to 1.6%, and the desired Ns (%) NH ₃ under conditions to obtain Conditions for holding for 4 to 12 hours at the temperature described in the column of “Carburizing and Nitrogen Treatment Temperature” in Tables 1 and 2 while introducing (Test Nos. 16 to 18, 23, and 29 are no introduction of NH ₃ ) I went there.

  The quenching and tempering treatment after the high-concentration gas carburizing and nitriding treatment, the temperature is directly lowered in a short time (about 15 minutes) from the “treatment temperature” to the temperature described in the “quenching temperature” column of Tables 1 and 2. It was kept at that temperature for 30 minutes and quenched in oil at 40 to 130 ° C. Then, the tempering process hold | maintained at 150 degreeC for 1 hour was performed. Table 1 and Table 2 also show the temperature difference when the temperature is lowered from the “treatment temperature” to the “quenching temperature”.

<(A) Surface retained austenite amount and internal retained austenite amount>
The amount of retained austenite (γ) on the surface and inside is as follows: a micro part X-ray residual stress device (PSPC) (manufactured by Rigaku), tube: Cr tube, X-ray: Kα1, acceleration voltage 20 kV, irradiation current: 40 mA. The value measured in the condition setting was used.

<(B) Maximum carbonitride major axis length>
Using an optical microscope, observe the surface under the conditions of magnification: × 1000, measurement range: about 0.0045 mm ² , number of fields of view: 10 fields of view, and select the longest axis in the observation area The value was taken as the maximum carbonitride major axis length.

<(C) Number of fine carbonitrides>
SEM was used and observed at a magnification of × 10000, and the number of circle-equivalent diameters of 0.1 to 2.0 μm was counted by image processing. The index is 400,000 or more / mm ² .

<(D) Cs (%) and (e) Ns (%)>
The surface carbon concentration and the surface nitrogen concentration were measured using EPMA (X-ray microanalyzer) (only Cs (%) was measured for a test material not subjected to nitriding treatment).

<(F) Surface hardness>
The surface hardness was measured on the cut surface of the test piece 1 using a Vickers hardness tester under a load of 20 kgf.

<B10 life characteristics and life ratio>
The B10 life characteristics (rolling fatigue life characteristics) test was performed using a Mori-type thrust type rolling fatigue tester. Maximum contact surface pressure: 5.3 GPa, rotation speed: 1500 rpm, lubricating oil: machine oil # 30, ball size 3 In addition to the conditions of / 8 inch, the number of balls of 3, and the temperature: room temperature, high-speed steel powder (hardness of 730 HVV, particle size of 100 to 150 μm) was mixed as foreign matter. For the evaluation of rolling fatigue life, the B10 life defined as 90% probability of not breaking by Weibull analysis was obtained, and the ratio to this was obtained as the life ratio based on the result of conventional steel SUJ2 (Test No. 31). . And the case where 4 times or more of lifetime was acquired with respect to conventional steel SUJ2 was judged as the pass.

  These evaluation results are shown in Tables 1 and 2.

As can be seen from Table 1, test No. 1 to 19 have the above-described specific chemical component composition, satisfy Formula 1, and satisfy all of the surface characteristics (a) to (f).
Thereby, it was excellent in B10 life, and exhibited life characteristics (rolling fatigue characteristics) four times or more superior to that of the conventional steel (test No. 31) in spite of all severe conditions having foreign matters. Here, among the examples, the test No. 6 to 8 show the results of changing the temperature difference with the same component in order to see the influence of the temperature difference value. The larger the temperature difference, the more the number of fine carbonitrides increases, resulting in an increase in surface hardness. B10 life was found to be improved.

  On the other hand, test No. of the comparative example. In Nos. 20 to 30, at least one of the chemical components, Formula 1, and surface characteristics (a) to (f) deviated from the desired range, and none of the life characteristics was improved to be comparable to the examples.

  Test No. 20 has no problem in chemical composition, including the formula 1, but after carbonization nitriding treatment, after quenching without lowering the temperature, fine carbonitrides are not sufficiently produced, and as a result Since the effect of improving the surface hardness due to the formation of the nitride was reduced and the surface hardness was less than 800 HV, the life characteristics as in Examples were not obtained.

  Test No. 21 has no problem with chemical components including Formula 1, but in order to clarify the effect of the surface hardening treatment, the high-concentration carburizing and nitriding treatment is not performed, and the quenching and tempering treatment normally performed in the conventional steel SUJ2 As a natural result, (a) the amount of retained austenite on the surface, (c) the number of fine carbonitrides, (d) Cs, (f) the surface hardness does not satisfy the conditions of the present invention. The life characteristics were inferior to those of steel.

  Test No. No. 22 has no problem with individual chemical components, but the formula 1 is not satisfied, and the surface nitrogen concentration Ns after the carbonitriding process is too high, so the carbonitride is coarsened, and the life characteristics are higher than that of the conventional steel. The result was inferior.

  Test No. 23, there is no problem with the chemical composition including the value of Formula 1, but the surface carbon concentration Cs after the carburizing treatment was too low, and as a result, the number of fine carbonitrides was less than the predetermined number. Thus, the life characteristics as in Examples were not obtained.

  Test No. No. 24 had a sufficient surface hardness because the C content in the chemical component was too high, and as a result, a coarse carbonitride was produced, so that the maximum carbonitride major axis length did not satisfy the conditions of the present invention. However, the life characteristics were inferior to conventional steel.

  Test No. For No. 25, the target (f) surface hardness could not be satisfied due to the low Si content, and the life characteristics as in Examples were not obtained.

  Test No. No. 26 was affected by the Mn content being too low in the chemical component, and (f) the surface hardness could not satisfy the target value, and the life characteristics as in Examples were not obtained.

  Test No. No. 27 is the effect that the Cr content in the chemical component is too low, and (c) the number of fine carbonitrides was not generated as intended, and (f) the surface hardness did not exceed 800 HV. Not so long life characteristics were obtained.

  Test No. 28 is a component in which the Cr content in the chemical component is too high and the carbonitride tends to coarsen, and (b) the maximum carbonitride major axis length did not satisfy the conditions of the present invention, While having hardness, the life characteristics were inferior to conventional steel.

  Test No. 29, although there is no problem in chemical composition including Formula 1, the surface carbon concentration Cs became too high after the carbonitriding process, so that the precipitated carbonitride was coarsened, and (b) the maximum carbonitride major axis length However, the conditions of the present invention could not be satisfied, and the life characteristics were inferior to those of the conventional steel while having a very high surface hardness.

  Test No. 30 has no problem in chemical composition including Formula 1, but the amount of residual γ on the surface became too high after carburizing and nitriding treatment, and the surface hardness of over 800 HV could not be obtained. The above lifetime could not be secured.

Claims (2)

By mass ratio, C: 0.50 to 1.20%, Si: 0.20 to 1.50%, Mn: 0.20 to 1.50%, Cr: 5.00 to 9.00%, Mo: 0 to 1.00% (including 0%), V: 0 to 1.00% (including 0%), the balance consisting of Fe and inevitable impurities,
Having a hardened surface layer including a carburized layer or a carburized and nitrocarburized layer;
As surface characteristics, the following (a) to (f) are provided,
(A) The amount of retained austenite is 20 to 50% in area ratio,
(B) The long axis length of the maximum carbonitride is 15 μm or less,
(C) The number of carbonitrides having an equivalent circle diameter of 0.1 to 2.0 μm is 400,000 pieces / mm ² or more,
(D) Cs (C concentration) is 1.5 to 3.4%,
(E) Ns (N concentration) is 0.5% or less,
(F) the surface hardness exceeds 800 HV, and
A steel part that satisfies the following formula 1 and has excellent rolling fatigue characteristics.
Formula 1: 4 × ([Cs] + [Ns]) − ([Cr] + [Mo] + [V]) <3.6,
(In the formula, [Cs] is the value of Cs in (d) above, [Ns] is the value of Ns in (e), [Cr], [Mo] and [V] are the values of Cr, Mo and V (The content (% by mass) is indicated.)   The steel part excellent in rolling fatigue characteristics according to claim 1, wherein the amount of retained austenite inside the hardened surface layer is 10% or less in terms of area ratio.