JP6043078B2 - Electric car motor gear with excellent seizure resistance - Google Patents

Description

  The present invention relates to a gear for an electric vehicle motor that is used in an operation site where high rotation and high slip occur and has excellent seizure resistance.

  For example, it is known that mechanical structural parts used in power transmission parts such as automobile transmission gears cause pitching damage in which a metal contact portion peels off due to an increase in contact surface pressure. . Therefore, as steel parts used for such applications, various case-hardened steels such as SCr, SCM, SNCM, etc. are used for forming by hot forging or cutting, followed by carburizing or carbonitriding. A surface hardening treatment is applied, and if necessary, a solid lubricating film such as molybdenum disulfide is formed on the component surface.

  However, in recent years, there is an increasing demand for higher output and smaller size and weight of the mechanical structure part, and the load on the mechanical structure parts used in these power transmission parts tends to increase more and more. For this reason, it is becoming difficult to achieve the required pitting resistance even if a solid lubricating film is formed as well as a component obtained by surface hardening treatment of various case-hardened steels such as SCr, SCM, and SNCM.

  By the way, in an electric vehicle whose production volume is expanding due to a reduction in environmental load in recent years, since the rotation of the motor is directly transmitted to the reduction gear, these parts are exposed to a higher rotation than a gasoline vehicle. . In addition, since lubricating oil with a lower kinematic viscosity in the usage environment is used than in gasoline cars, the oil film formed on the surface of the steel parts that make up these power transmission parts is thin, and in some cases, the oil film is hardly formed. It is in a local environment. In particular, the oil temperature rises and the kinematic viscosity of the lubricating oil tends to decrease with higher rotation and higher slip, and more oil film breakage occurs. Therefore, in these environments, wear due to metal contact between steel parts tends to occur, and temperature rise due to frictional heat tends to cause softening of the steel parts, so seizure occurs early.

Various techniques related to steel members used in the above environment have been proposed. For example, Patent Document 1 proposes a technique for forming hard Ti carbide on the surface during nitriding treatment in order to ensure high surface hardness for low carbon steel. It has also been disclosed that Fe ₃ P is precipitated by increasing P in the steel and the surface is precipitation strengthened. This technique places importance on increasing the surface hardness by the combined effect of nitriding and precipitation strengthening, and exhibits excellent seizure resistance under high surface pressure.

  However, in high slip environments such as gears for electric vehicle motors, although heat softening due to temperature rise can be suppressed to some extent, adhesion wear due to metal contact is not particularly taken into account, so adhesion wear easily occurs. And seize early. That is, with the technique such as Patent Document 1, seizure resistance cannot be satisfied in a high slip environment.

  Patent Document 2 proposes a technique of forming a diffusion phase in which Cr nitride is dispersed not only in the surface layer but also in the deep part by increasing the Cr content in the steel to 2% or more. Further, by increasing the Cr content, the N content can also be increased. As a result, the N content in the nitrided layer can be significantly increased to about 1 to 6% as compared with the normal nitriding treatment, and the surface layer hardness can be increased. It is also disclosed to improve the wear resistance and seizure resistance by setting the thickness to 900 Hv or higher in terms of Vickers hardness. This technique places importance on improving surface hardness and toughness of the nitrided layer on the surface, and exhibits excellent wear resistance and seizure resistance under high surface pressure.

  However, in the case of repeated collisions in a high sliding environment such as a gear for an electric vehicle motor, the nitrided layers having large surface undulations such as Cr nitrides are easily cracked during operation. If the material layer falls off, seizure tends to occur on the contrary, and variation as a gear part increases. That is, even in the technique of Patent Document 2, seizure resistance cannot be stably satisfied under a high slip environment.

  In Patent Document 3, in order to perform nitrogen diffusion treatment on the steel material surface, by adjusting the microstructure of the nitriding alloy steel containing the nitride-forming element to 10% or more of block-like non-pearlite (for example, ferrite), Technologies that can achieve both wear resistance and fatigue strength have been proposed. In this technique, the surface hardness during operation and the toughness as a part are enhanced, and excellent wear resistance and fatigue characteristics are exhibited. However, even in such a technique, adhesion wear due to metal contact cannot be suppressed under a high slip environment such as a gear for an electric vehicle motor.

  On the other hand, Patent Document 4 proposes a nitriding steel that can be applied to a part that is used for a long time under a high surface pressure, such as a gear part, by nitriding. . In this technique, after melting and performing predetermined refining, solution treatment is sufficiently performed in various heat treatments such as hot rolling, forging, quenching, and tempering, and Si, Cr, Mo, and V that effectively act during nitriding. , Ti, Al and the like are sufficiently dissolved in the matrix, and subsequent cooling prevents these elements from being precipitated as carbides or nitrides. However, this technique basically does not precipitate carbides or nitrides, and therefore cannot exhibit good seizure resistance in a high slip environment.

  In the technologies proposed so far, there is no technology that places emphasis on suppressing adhesive wear in a high-slip environment, and there is a technology that emphasizes wear resistance and seizure resistance by increasing the surface layer strength. However, the reality is that no technology has been established to take measures against repeated impacts caused by metal contact.

JP 2006-22351 A JP 2003-148488 A JP-A-7-90490 JP 2006-348321 A

  The present invention has been made to solve such problems in the prior art, and its purpose is to achieve better seizure resistance in a power transmission section in which lubricating oil with high rotation / slip and low kinematic viscosity is used. An object of the present invention is to provide a gear for an electric vehicle motor that exhibits its properties.

The electric vehicle motor gear of the present invention capable of solving the above-mentioned problems includes C: more than 0.45 to 0.80% (meaning “mass%”, the same applies to the chemical composition), Si: 0.00. 05 to 1%, Mn: 0.1 to 1%, P: 0.05% or less (not including 0%), S: 0.05% or less (not including 0%), Cr: 0.9 to 2%, Al: 0.01 to 0.1%, and N: 0.02% or less (not including 0%), V: 0.05 to 0.6%, Mo: 0. Containing at least one selected from the group consisting of 05 to 2%, Ti: 0.05 to 1% and Nb: 0.05 to 1%, the balance consisting of iron and inevitable impurities, tempered martensite and / or Carbide with an average equivalent circle diameter of 0.05 to 1.0 μm is precipitated on tempered bainite at an area ratio of 1% or more and 5% or less. Has been that the steel structure, and the balance, ferrite includes pearlite, bainitic ferrite, martensite as quenched, the tissue selected from the group consisting of as-quenched bainite at an area ratio of 5% or less, and 20μm deep from the surface It has a gist in that the nitrogen concentration is 2.0 to 6.0%. In the present invention, “average equivalent circle diameter” means an average value of diameters when converted into the same area, focusing on the size (size) of carbides.

  In the electric vehicle motor gear of the present invention, if necessary, (a) B: 0.01% or less (not including 0%), (b) Cu: 5% or less (not including 0%) and / or It is also effective to contain Ni: 5% or less (not including 0%) and the like, and the characteristics of the electric vehicle motor gear are further improved depending on the components contained. The electric vehicle motor gear of the present invention includes those having a lubricating film formed on the surface.

  In the present invention, the chemical composition is appropriately adjusted, and a tempered martensite and / or tempered bainite structure is obtained while securing a predetermined area ratio of carbide of a predetermined size, and a nitrogen concentration at a depth of 20 μm from the surface is set. Since it is set to 2.0 to 6.0%, it can exhibit better seizure resistance in a power transmission portion in which a lubricating oil having high rotation / slip and low kinematic viscosity is used. Such gears are extremely useful for electric vehicle motors.

It is a schematic explanatory drawing of the test piece used for the roller pitching test.

  The electric vehicle motor gear excellent in seizure resistance according to the present invention has (i) a nitrogen concentration of 2.0 to 6.0% at a depth of 20 μm from the gear surface, and (ii) an average equivalent circle diameter of 0. The area ratio of carbide of 0.05 to 1.0 μm is 1% or more and 5% or less, and the like.

  It has been thought that the pitching damage that occurs mainly in gasoline cars is due to the frictional heat generated by the metal-to-metal contact between the gears due to the oil film breakage, and the gears being thermally softened. In order to realize a gear part having excellent pitting resistance, it is effective to improve the surface of the part itself, the internal strength, or the temper softening resistance, and the pitting resistance has been improved by increasing the strength. However, in electric vehicle motor gears, compared to conventional gasoline cars, the slip speed between the tooth surfaces is several times more and the damage mechanism changes to seizure, so the surface of the part itself, the internal strength, Or the improvement guideline cannot be found on the extension line of improving the temper softening resistance.

  The present inventors have studied in detail the mechanism of seizure occurrence in a region where the sliding speed is extremely high. As a result, it was found that interatomic bonds, that is, adhesive wear due to wear occurring under high temperature, high pressure, and high slip, are dominant. As a result of further investigation, it is effective to increase the N content (nitrogen concentration) of the gear surface layer part in order to make it difficult to bond between atoms even in an environment where adhesion wear is likely to occur. In addition, it has been found that it is more effective to form a large number of iron nitrides and to appropriately control the composition of iron nitrides than to nitrogen compounds with various additive elements. In addition, it was also effective to disperse carbides of a predetermined size in steel, and it was found that the seizure resistance can be remarkably improved by these synergistic effects, and the invention was completed.

  The presumed mechanism for improving seizure resistance in the present invention is considered as follows. That is, controlling the nitrogen concentration in the surface layer as in (i) above can control N to a thermally more stable iron nitride composition and suppress interatomic bonding even at the metal contact portion. it can. Moreover, as in (ii) above, controlling the amount of carbide (area ratio) of a predetermined size causes the oil film to break, causing the carbide to decompose in the portion where the metals are in contact with each other and the temperature is likely to rise. Thus, a lubricating action can be imparted and metal contact can be suppressed. Such an effect cannot produce a significant improvement in seizure resistance by either (i) or (ii) alone, and is an electric having excellent seizure resistance only after the synergistic effect of (i) and (ii). An automobile motor gear can be realized. Specific actions and effects by defining these requirements are as follows.

[Requirements for (i)]
In order to improve the seizure resistance as a gear for an electric vehicle motor, it is necessary to control the nitrogen concentration at a depth of 20 μm from the surface so as to be 2.0 to 6.0%. If the nitrogen concentration in this portion is less than 2.0%, interatomic bonds due to metal contact are likely to occur, and adhesive wear will occur. On the other hand, if the nitrogen concentration exceeds 6.0%, the atomic structure of the nitride in the vicinity of the surface layer is changed, so that adhesive wear tends to occur. The preferable lower limit of the nitrogen concentration is 2.2% or more (more preferably 2.5% or more), and the preferable upper limit is 5.8% or less (more preferably 5.5% or less).

[Requirements for (ii)]
Carbide precipitated in steel can improve seizure resistance in a high slip environment. Moreover, especially in the carbide | carbonized_material which exists in a surface layer part, it also has the effect | action which assists that nitrogen is remarkably concentrated in a surface layer part. For this purpose, the area ratio of carbides needs to be 1% or more. When the area ratio of the carbide is less than 1%, the lubricating action due to the decomposition of the carbide tends to be insufficient, and predetermined seizure resistance cannot be obtained. On the other hand, when the area ratio of the carbide exceeds 5%, nitrogen is excessively concentrated in the vicinity of the surface layer portion and the structure of the iron nitride is changed, so that the seizure resistance is deteriorated.

  The carbides targeted in the present invention are those having an average equivalent-circle diameter of 0.05 to 1.0 μm. The carbides precipitated in the steel exhibit the above-described effects, but the size is also important for effectively exhibiting these effects. The smaller this size is, the more effective it is, but if the average equivalent circle diameter is smaller than 0.05 μm, the balance between the temperature rise and the decomposition of carbides, which are important for lubrication during high slippage, is lost and sufficient. The seizure resistance cannot be exhibited. On the other hand, if the size of the carbide exceeds 1.0 μm, the interface between the carbide and the matrix tends to be a starting point for fatigue failure, and seizure resistance and fatigue strength deteriorate.

  The carbide is substantially precipitated in tempered martensite or tempered bainite, or a composite structure thereof (structure composed of tempered martensite and tempered bainite). After nitriding treatment, other structures such as ferrite, pearlite, bainitic ferrite, as-quenched martensite, or as-quenched bainite may be formed in the steel. Since these structures adversely affect the characteristic variation and seizure resistance of the gear, it is desirable that these structures be not generated as much as possible. However, only when these structures are present in an area ratio of 5% or less, it does not adversely affect the operation of the present invention, and is allowed.

  In the electric vehicle motor gear of the present invention, it is necessary to appropriately adjust the chemical component composition in order to exhibit the characteristics as the final product (gear part). The reason for the range limitation by each component (element) in the chemical component composition is as follows.

[C: more than 0.45 to 0.80%]
C is an element necessary for forming a predetermined amount or more of carbide that improves seizure resistance. It is also effective for increasing the quenching hardness and maintaining the strength at room temperature and high temperature. In order to exhibit such an effect effectively, it is necessary to contain at least more than 0.45%. However, if the C content is excessive, the steel material becomes too hard and various processing becomes difficult, and the toughness as a part is also impaired, so 0.80% or less is necessary. The preferable lower limit of the C content is 0.47% or more (more preferably 0.50% or more), and the preferable upper limit is 0.70% or less (more preferably 0.65% or less).

[Si: 0.05 to 1%]
Si exhibits the effect of increasing the temper softening resistance and suppressing the decrease in hardness. In order to exhibit such an effect, it is necessary to contain 0.05% or more. However, when the Si content is excessive, the die life at the time of cold forging is reduced and the machinability is also deteriorated. The preferable lower limit of the Si content is 0.10% or more (more preferably 0.15% or more), and the preferable upper limit is 0.8% or less (more preferably 0.5% or less).

[Mn: 0.1 to 1%]
Mn has an effect of improving solid solution strengthening and hardenability of the matrix. In order to exhibit this effect, it is necessary to contain 0.1% or more. However, if the Mn content is excessive, the concentration of MnO, which is a lower oxide, is increased and the fatigue characteristics are deteriorated, and the workability and machinability are remarkably lowered. The preferable lower limit of the Mn content is 0.15% or more (more preferably 0.20% or more), and the preferable upper limit is 0.95% or less (more preferably 0.90% or less).

[P: 0.05% or less (excluding 0%)]
P segregates at the grain boundaries and shortens the fatigue life, so it is necessary to reduce it as much as possible. In particular, when the content exceeds 0.05%, the fatigue life is significantly reduced. For these reasons, the P content is set to 0.05% or less. The P content is preferably 0.045% or less, more preferably 0.040% or less.

[S: 0.05% or less (excluding 0%)]
S is an element that forms sulfides. If the content exceeds 0.05%, coarse sulfides are generated, and thus the fatigue life is shortened. Therefore, the S content is 0.05% or less. The S content is preferably 0.045% or less, and more preferably 0.040% or less.

[Cr: 0.9-2%]
Cr effectively acts to improve strength and improve seizure resistance through improving hardenability and forming stable carbides. In order to exert such effects, it is necessary to contain Cr by 0.9% or more. However, if the Cr content is excessive, the carbide is coarsened and the fatigue characteristics and machinability are reduced, so the content needs to be 2% or less. A preferable lower limit of the Cr content is 1.0% or more (more preferably 1.1% or more), and a preferable upper limit is 1.9% or less (more preferably 1.8% or less).

[Al: 0.01 to 0.1%]
Al is preferably added in an appropriate amount in order to act as a deoxidizer and to reduce the amount of oxide inclusions and enhance the internal quality of the steel material. From such a viewpoint, the Al content is set to 0.01% or more. However, if the Al content is excessive, coarse and hard inclusions (Al ₂ O ₃ ) are generated and the fatigue characteristics are deteriorated. The preferable lower limit of the Al content is 0.015% or more (more preferably 0.020% or more), and the preferable upper limit is 0.08% or less (more preferably 0.06% or less).

[N: 0.02% or less (excluding 0%)]
N combines with Al to form AlN and has the effect of refining the crystal grain size. On the other hand, if the N content is too large, cracking is likely to occur during rolling, so 0.02% or less. It is necessary to limit to. The N content is preferably 0.018% or less, and more preferably 0.016% or less.

[V: 0.05 to 0.6%, Mo: 0.05 to 2%, Ti: 0.05 to 1% and Nb: One or More Selected from the Group of 0.05 to 1%]
V, Mo, Ti and Nb are effective elements for improving the seizure resistance by improving the surface hardness of the gear. The detailed effects of these are as follows.

  V combines with C in the steel during nitriding to precipitate V carbide, thereby improving surface hardness and seizure resistance. Moreover, V carbide improves fatigue strength by improving internal hardness. In order to effectively exhibit these effects, V is preferably contained in an amount of 0.05% or more, more preferably 0.07% or more (more preferably 0.10% or more). However, if the V content becomes excessive and exceeds 0.6%, the internal strength due to precipitation strengthening becomes too high, so that the fatigue strength is deteriorated. Preferably, it is 0.55% or less, more preferably 0.50% or less.

  Mo combines with C in the steel and precipitates Mo carbides during nitriding, thereby improving surface hardness and seizure resistance. Moreover, Mo carbide improves fatigue strength by improving internal hardness. In order to exhibit such an effect effectively, it is preferable to contain Mo 0.05% or more, more preferably 0.07% or more (more preferably 0.10% or more). However, if the Mo content becomes excessive and exceeds 2%, the internal strength due to precipitation strengthening becomes too high, so that the fatigue strength is deteriorated and the toughness is also impaired in the surface layer portion, and the seizure resistance is deteriorated. Let Preferably, it is 1.8% or less, more preferably 1.6% or less.

  Ti combines with C in the steel and precipitates Ti carbide during nitriding, thereby improving surface hardness and seizure resistance. Ti carbide also improves fatigue strength by improving internal hardness. In order to effectively exhibit such an effect, Ti is preferably contained in an amount of 0.05% or more, more preferably 0.07% or more (more preferably 0.10% or more). However, if the Ti content is excessive and exceeds 1%, the internal strength due to precipitation strengthening becomes too high, so that the fatigue strength is deteriorated and the toughness is also impaired in the surface layer portion, and the seizure resistance is deteriorated. Let Preferably, it is 0.8% or less, more preferably 0.6% or less.

  Nb combines with C in the steel to precipitate Nb carbide during nitriding, thereby improving surface hardness and seizure resistance. Moreover, Nb carbide improves fatigue strength by improving internal hardness. In order to effectively exhibit such an effect, the content is preferably 0.05% or more, more preferably 0.07% or more (more preferably 0.10% or more). However, if the Nb content is excessive and exceeds 1%, the internal strength due to precipitation strengthening becomes too high, and on the contrary, the fatigue strength is deteriorated and the toughness is also impaired in the surface layer portion, and the seizure resistance is deteriorated. Let Preferably, it is 0.8% or less, more preferably 0.6% or less.

  The basic components in the electric vehicle motor gear of the present invention are as described above, and the balance is iron and inevitable impurities (for example, Sb, Mg, etc.). In the electric vehicle motor gear of the present invention, if necessary, (a) B: 0.01% or less (excluding 0%), (b) Cu: 5% or less (excluding 0%) and / or Ni: 5% or less (not including 0%) or the like may be included, and the characteristics of the electric vehicle motor gear are further improved depending on the type of element to be included. The reason for setting a preferable range of these elements is as follows.

[B: 0.01% or less (excluding 0%)]
B not only has the effect of significantly improving the hardenability, but is also effective in improving the impact strength. However, when the B content is excessive, the B compound is excessively precipitated to lower the grain boundary strength, so that the fatigue strength is deteriorated. From such a viewpoint, the content is preferably 0.01% or less. More preferably, it is 0.007% or less, More preferably, it is 0.004% or less.

[Cu: 5% or less (not including 0%) and / or Ni: 5% or less (not including 0%)]
Cu dissolves in steel and effectively acts to improve the surface layer and internal hardness and to improve seizure resistance. Moreover, it precipitates finely at the time of nitriding, and exhibits the effect | action which hardens steel materials. However, when the Cu content is excessive, the steel material is embrittled, so the Cu content is preferably 5% or less. More preferably, it is 4% or less, More preferably, it is 3% or less.

  Ni has the effect of strengthening the solid solution of the steel material. Moreover, the Cu precipitation hardening effect | action can be exhibited more by compounding with Cu. However, if the Ni content is excessive, the effect is saturated, so the Ni content is preferably 5% or less. More preferably, it is 4% or less, More preferably, it is 3% or less.

  The electric vehicle motor gear uses a steel material having the chemical composition as described above, and is appropriately subjected to heat treatment such as annealing as necessary, processed into a predetermined gear shape, quenched and tempered, and then subjected to nitriding treatment It is manufactured by. In this manufacturing process, it is only necessary to adopt a generally used method until it is processed into a gear shape, and the gear processing is also hot forging, cold forging, warm forging, various forging / forging, rolling, Alternatively, it is manufactured by cutting, grinding, or a combination of these methods.

  Quenching is performed for the purpose of increasing the solid solution C for homogenizing the structure and precipitating carbides during nitriding. Quenching may be performed by a general method. Heating to the austenite single phase region (however, if the heating temperature is too high, the austenite grains become too coarse and the upper limit is set to 1100 ° C.), and then rapidly cools to the martensite transformation start temperature Ms or less. To do.

  After heating, the entire tissue becomes uniform by holding for about 1 to 180 minutes. If the heating time at this time is too long, the effect of decarburization becomes significant, so the upper limit is 180 minutes. The rapid cooling rate is not particularly limited as long as it is faster than a cooling rate that does not cause ferrite transformation and incomplete firing. From the above viewpoint, the lower limit is 1 ° C./second or more. Tempering is performed from the viewpoint of preventing cracking. When nitriding is performed immediately after quenching, tempering can be omitted. If the tempering process is performed in a temperature range of, for example, 100 to 300 ° C. for about 1 to 180 minutes, it is possible to prevent cracking.

  An important process in manufacturing the electric vehicle motor gear of the present invention is a nitriding process. By applying a predetermined nitriding treatment to the steel material having the chemical composition described above, a predetermined structure form can be obtained and seizure resistance can be improved. Any known method may be applied to the nitriding treatment. Examples thereof include gas nitriding, gas soft nitriding, salt bath nitriding, salt bath carbonitriding, ion nitriding, plasma nitriding, taffle treatment, gas carbonitriding, and the like. In the present invention, plasma soft nitriding is applied as an example of nitriding. If finishing such as machining is required, it may be performed before nitriding or after nitriding as long as it does not affect the nitrided layer. Specific conditions in the nitriding process will be described.

(1) Nitriding temperature: 580 to 720 ° C.
In the present invention, the nitriding temperature (heating temperature) is carried out in a temperature range higher than the normal nitriding temperature (about 570 ° C.). By increasing the nitriding temperature, a predetermined amount of carbide of a predetermined size is precipitated, and the diffusion of N into the steel material is promoted, thereby controlling to an iron nitride composition in which interatomic bonds are less likely to occur. Excellent seizure resistance can be obtained. The lower limit of the processing temperature is set to 580 ° C., if the nitriding temperature is too low, N diffusion and a processing time for obtaining a predetermined carbide (holding time at the time of nitriding processing) are increased. This is because productivity is significantly reduced. The upper limit of 720 ° C. is effective for promoting the growth of carbides and diffusion of N when the nitriding temperature is too high, but the tempering of the matrix matrix proceeds so much that the internal hardness is increased. This is because the characteristic of the gear part cannot be obtained. Therefore, by setting the nitriding temperature in the range of 580 to 720 ° C., various characteristics as gear parts can be satisfied, and excellent seizure resistance can be exhibited even in a high slip environment such as an electric vehicle motor. A more preferable lower limit of the nitriding temperature is 590 ° C. or higher (more preferably 600 ° C. or higher), and a more preferable upper limit is 700 ° C. or lower (more preferably 650 ° C. or lower).

(2) Nitriding time: 1 to 20 hours The nitriding time (holding time during nitriding) is required for growing carbide and diffusing N into the steel to form iron nitride. Is. Normally, the amount of carbide, carbide size, N diffusion amount, and iron nitride amount are determined in conjunction with temperature and time, but in the present invention, the range for stably obtaining a predetermined structure is determined by temperature and time. Set each one. By setting the nitriding time to 1 to 20 hours, a desired structure can be obtained, and excellent seizure resistance can be exhibited even in a highly slipping environment such as an electric vehicle motor. If the nitriding time is shorter than 1 hour, sufficient carbide and iron nitride cannot be obtained, and in order to achieve a desired structure in less than 1 hour, if the temperature is raised too much, the matrix matrix becomes soft. There is a bad effect that will be. On the other hand, when the nitriding time exceeds 20 hours, the matrix matrix becomes soft, and if it is attempted to process at a low temperature in order to prevent this, there is a problem that carbides are not sufficiently precipitated.

(3) Nitriding atmosphere: nitrogen gas concentration 23-86%
Nitrogen gas concentration (N ₂ fraction) in the nitriding atmosphere can obtain excellent seizure resistance by diffusing N into the steel and controlling it to an iron nitride composition in which interatomic bonds are less likely to occur. it can. If the N ₂ fraction in the atmosphere is less than 23%, N cannot be sufficiently diffused into the steel, and desired gear characteristics cannot be obtained. On the other hand, if the N ₂ fraction exceeds 86% and the diffusion amount of N increases too much, the composition changes again to iron nitride that easily bonds between atoms, and therefore seizure resistance cannot be improved. A more preferable lower limit of the N ₂ fraction is 31% or more (more preferably 41% or more), and a more preferable upper limit is 81% or less (more preferably 76% or less).

(4) Extraction temperature and cooling rate after nitriding treatment: less than 500 ° C., 1 ° C./second or less After nitriding treatment under the conditions as described above, it is preferable to extract after gradually cooling to less than 500 ° C. By performing such cooling, the area ratio of carbide of a predetermined size can be secured. If the extraction temperature is 500 ° C. or higher, or the cooling rate is higher than 1 ° C./second, the area ratio of carbides of a predetermined size cannot be secured. This extraction temperature is more preferably 450 ° C. or lower, and further preferably 400 ° C. or lower. The cooling rate is more preferably 0.5 ° C./second or less, and can be realized by, for example, furnace cooling.

  In the automobile motor gear of the present invention, it is also effective to treat the gear surface with a lubricating film in order to further improve the seizure resistance. This lubricating film treatment can suppress metal contact, suppress temperature rise, and suppress the occurrence of adhesive wear. Lubricant coating treatment includes, for example, soft metals such as copper, zinc and lead, metal oxides such as lead oxide, sulfides such as molybdenum disulfide and tungsten disulfide, fluorides, nitrides, graphite, manganese phosphate, etc. Typical examples are used, and general processing types and processing methods are adopted.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Steel ingots of various chemical composition shown in Tables 1 and 2 below (in the range specified in the present invention, the component judgment “◯” and out of the range, the component judgment “x”) are shown in a melting furnace. After fabrication, hot forging to a diameter of 32 mm. Using the round bar produced in this way, it was cut into a predetermined shape. As shown in Tables 3 and 4 below, these processed materials are heated to a temperature of 850 to 1050 ° C., held for 0.5 to 5 hours, and then cooled to 50 to 200 ° C. at a cooling rate of 50 ° C./second. And about some processed materials, the tempering process of 150 degreeC x 1 hour was implemented. Thereafter, plasma soft nitriding (hereinafter simply referred to as “nitriding”) was performed under the conditions shown in Tables 3 and 4 below. After the nitriding treatment, finishing was performed, and the test piece shape (roller pitching test piece shape) shown in FIG. 1 was processed to obtain a test piece.

About each obtained test piece (test No. 1-30, 33-37), the carbide | carbonized_material size in a structure | tissue, its area rate, nitrogen concentration (surface layer nitrogen concentration) of the depth of 20 micrometers from the surface, and hardness distribution While measuring by the following method, the component characteristics (seizure resistance) were evaluated by the following method.

[Measurement method of carbide size and area ratio]
The size and area ratio of carbides in the structure (the main structure is tempered martensite and / or tempered bainite) are obtained by using a sample obtained by cutting a specimen after nitriding treatment in a cross section, embedding in a resin, mirror polishing, and etching. A position 20 μm deep from the surface was measured using a scanning electron microscope (SEM). An arbitrary 9 μm × 12 μm visual field was observed at a magnification of 8000 times, and the carbide portion was identified by image analysis software, and its size (equivalent circle diameter) and area ratio were determined. The measurement was performed for three fields, and the arithmetic average of these three fields was defined as the size and area ratio of the carbide.

[Method for measuring nitrogen concentration 20 μm deep from the surface]
The nitrogen concentration in the surface layer part (at a depth of 20 μm from the surface) was obtained by cutting the test piece in a cross section, embedding in a resin, mirror polishing, and then measuring the nitrogen concentration from the surface layer part to the inside by using an electron beam microprobe analyzer ( It was measured by analysis using an Electron Probe Microanalyzer (EPMA).

[Part characteristics evaluation method]
As an evaluation method for component characteristics, a roller pitching test was performed. The roller pitching test was performed with a roller pitching tester using the test piece (small roller: FIG. 1) and a large roller (partner material) made of high carbon chromium bearing steel SUJ2. The test conditions were as follows: rotational speed: 1000 rpm, relative slip ratio: 700%, oil temperature: 90 ° C., and the number of revolutions until the test apparatus stopped due to vibration caused by the occurrence of seizure was determined. At this time, 20000 × 10 ³ times was set as the upper limit, and the seizure generation life was assumed. Those having no seizure up to 10,000 × 10 ³ times were evaluated as having excellent seizure resistance.

[Measurement of hardness distribution]
A hardness test was conducted as a method for evaluating the component characteristics. As representative values, the hardness at a depth of 20 μm (surface layer portion) from the surface of the test piece and the D / 2 position (D is the diameter: inside) of the test piece was used. The test piece was cut in a cross section, embedded in resin, polished, and then measured for Vickers hardness Hv. At this time, using a micro Vickers hardness tester, the test load was 100 g, n = 5 times, and the average value was defined as the surface layer portion and internal hardness of the steel material. About hardness standard, it was set as the pass when the hardness of a surface layer part was 700-1000Hv, and internal hardness was 400-550Hv.

These results (carbide size, carbide area ratio, surface layer nitrogen concentration, hardness of each part, seizure life) are shown in Tables 5 and 6 below along with the presence or absence of a lubricating film (the lubricating film uses manganese phosphate). Show. Note that “> 20000” in the term of seizure life in Table 6 means that seizure did not occur even after 20000 × 10 ³ times.

From these results, it can be considered as follows. First, test no. 1, 3, 4, 7, 8, 11, 12, 14, 17-21, 23-25, 27-30, 33 , 35-37 are controlled within a range where both the chemical composition of the steel material and the manufacturing conditions are appropriate. Therefore, all of the carbide size, the carbide area ratio, and the surface layer nitrogen concentration can be controlled within the range defined in the present invention. As a result, the adhesion suppressing effect is excellent, and excellent component strength and seizure resistance can be exhibited. It can also be seen that excellent seizure resistance can be obtained even when a lubricating film is formed (characteristic determination “◯”).

  In contrast, test no. 2, 5, 6, 9, 10, 13, 15, 16, 22, 26, and 34 are not controlled within the proper range of chemical composition and manufacturing conditions of steel materials. Has deteriorated (characteristic determination “×”).

  Test No. No. 2 has a low heating temperature during the nitriding treatment, and the extraction temperature after the nitriding treatment is low, and the cooling rate until the extraction is fast, so the carbide size is small and N cannot be sufficiently impregnated. (The nitrogen concentration in the surface layer portion is reduced), the hardness in the surface layer portion and inside is low, and adhesion cannot be sufficiently suppressed, so that the seizure resistance is deteriorated. Test No. No. 5, because the heating temperature during nitriding treatment is high, the carbide size is large, the area ratio of the carbide is excessive, and the nitrogen content of the surface layer is also excessive, so the precipitation strengthening inside The seizure resistance is deteriorated because the performance is lowered and the adhesion suppressing action is deteriorated in the surface layer portion.

  Test No. No. 6 has a short holding time during nitriding treatment, and carbide cannot be sufficiently grown, and N cannot be sufficiently impregnated, and the internal hardness is appropriate due to precipitation strengthening of fine carbide. However, since the hardness of the surface layer portion is low and the adhesion at the surface layer portion cannot be sufficiently suppressed, the seizure resistance is deteriorated. Test No. No. 9 has a longer holding time during nitriding treatment, an excessive increase in the carbide area ratio due to excessive precipitation of carbides, and an excessive nitrogen concentration in the surface layer portion, although the hardness becomes appropriate by precipitation strengthening inside. Further, the hardness of the surface layer portion becomes excessively high, and further, the adhesion suppressing action of the nitride layer is deteriorated, so that the seizure resistance is deteriorated.

Test No. In No. 10, since the amount of nitrogen gas (N ₂ fraction) during nitriding is too small, the requirements for carbide are satisfied, but the nitrogen concentration in the surface layer portion cannot be ensured, and the seizure resistance is deteriorated. Test No. No. 13, since the amount of nitrogen gas (N ₂ fraction) during nitriding is too large, the requirement for carbide is satisfied, but the nitrogen concentration in the surface layer becomes excessive, and the hardness of the surface layer becomes excessively high. Moreover, since the anti-adhesion action of the nitride layer is deteriorated, the seizure resistance is deteriorated.

  Test No. No. 15 is a steel type (steel type D) having an excessive C content (component determination “x” in Table 1). Even if the production conditions are appropriate, the carbide size increases and the area of the carbide. The rate is also excessive, and the seizure resistance is deteriorated. Moreover, internal hardness is high and does not satisfy component characteristics. Test No. No. 16 is a steel type (steel type E) having an excessive V content (component determination “x” in Table 1), and even if the production conditions are appropriate, the area ratio of carbide becomes excessive, Hardness becomes excessively high, and the resistance to impact deteriorates. Further, since the nitrogen concentration in the surface layer portion is excessive and the adhesion suppressing action of the nitride layer cannot be obtained, the seizure resistance is deteriorated.

  Test No. No. 22 is a steel type (steel type K) with a low C content (component determination “x” in Table 1), and even if the production conditions are appropriate, the carbide area ratio and the nitrogen concentration in the surface layer are insufficient. In addition, since both the internal hardness and the hardness of the surface layer portion are low, and the adhesion suppressing action of the nitride layer is deteriorated, the seizure resistance is deteriorated.

  Test No. No. 26 uses a steel type with low V content (steel type O) (component determination “x” in Table 2), and even if the production conditions are appropriate, the carbide area ratio and the nitrogen concentration in the surface layer are insufficient. In addition, since both the internal hardness and the hardness of the surface layer portion are low, and the adhesion suppressing action of the nitride layer is deteriorated, the seizure resistance is deteriorated.

  Test No. No. 34 uses a steel type with low Cr content (steel type W) (component determination “×” in Table 2), and even if the production conditions are appropriate, the carbide area ratio and the nitrogen concentration in the surface layer are insufficient. In addition, since both the internal hardness and the hardness of the surface layer portion are low, and the adhesion suppressing action of the nitride layer is deteriorated, the seizure resistance is deteriorated.

Claims (4)

C: more than 0.45 to 0.80% (meaning “mass%”, the same applies to the chemical composition)
Si: 0.05 to 1%
Mn: 0.1 to 1%,
P: 0.05% or less (excluding 0%),
S: 0.05% or less (excluding 0%),
Cr: 0.9-2%,
Al: 0.01 to 0.1%, and N: 0.02% or less (excluding 0%), respectively,
V: 0.05-0.6%
Mo: 0.05-2%,
Containing one or more selected from the group consisting of Ti: 0.05 to 1% and Nb: 0.05 to 1%,
The balance consists of iron and inevitable impurities,
The tempered martensite and / or tempered bainite has a steel structure in which carbide having an average equivalent circle diameter of 0.05 to 1.0 μm is precipitated at an area ratio of 1% or more and 5% or less,
The balance includes a structure selected from the group consisting of ferrite, pearlite, bainitic ferrite, as-quenched martensite, and as-quenched bainite in an area ratio of 5% or less, and the nitrogen concentration at a depth of 20 μm from the surface is 2.0 to 2.0%. The roller pitching test is performed at a rotational speed of 1000 rpm, a relative slip rate of 700%, and an oil temperature of 90 ° C., and the test apparatus is stopped by vibration caused by seizure. A gear for an electric vehicle motor excellent in seizure resistance, wherein the number of rotations until 10000 × 10 ³ times or more .   The electric vehicle motor gear according to claim 1, further comprising B: 0.01% or less (not including 0%).   The gear for an electric vehicle motor according to claim 1 or 2, further comprising Cu: 5% or less (not including 0%) and / or Ni: 5% or less (not including 0%).   The electric vehicle motor gear according to any one of claims 1 to 3, wherein a lubricating film is formed on the surface.

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