JP6111121B2 - Gears with excellent seizure resistance - Google Patents

Description

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

  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, in Patent Document 1, C: 0.7 to 1.1%, Si: 0.3 to 0.7%, Mn: 0.3 to 0.8%, Ni: 0.5 to 1.2% , Cr: 1.3-1.8%, Mo: 0.1-0.7% and V: 0.2-0.4%, consisting of the balance iron and impurities, Si + Mn: 1.0% Hereinafter, it is composed of steel with Ni + Cr: 2.3% or more and Cr + Mo + V: 3.0% or less, a high carbon content layer is formed, the hardness of the surface layer portion is 725 to 800 Hv, and the maximum particle size of the carbide of the surface layer portion Is 10 μm or less, the area ratio is 7 to 20%, the surface layer part has a carbon content higher by 0.2 to 0.4% than the inside, and the nitrogen content of the surface layer part is 0.1 to 0.5%. -Rolling members for auxiliary machinery are disclosed.

  However, when this technology is applied to a motor for an electric vehicle, it is an environment in which seizure is more likely to occur. Therefore, in order to suppress seizure, the amount of nitrogen in the surface layer is only about 0.5%. Is not enough.

In Patent Document 2, C: 0.80 to 1.70%, Si: 0.70 to 2.50%, Mn: less than 0.30%, P: 0.050% or less, S: 0.050% or less , O: 0.0030% or less, N: 0.015% or less, steel slab or steel material composed of the balance iron and inevitable impurities, 800 ° C. or higher, 50 ° C. lower than the solidus temperature of the steel slab or steel material Heated to a temperature below the temperature, hot worked, cooled to room temperature, precipitated graphite with an average particle size of 0.5μm or more on the obtained hot worked steel material, more than 100 pieces / mm ² , the metal structure was mainly pearlite A hot-worked steel material and a product excellent in free-cutting property are disclosed. This technique also discloses that it is useful to appropriately add a graphite precipitation promoting element, a pearlite refinement element, a machinability improving element, a quenching promoting element, and the like as necessary.

  However, this technique has a pearlite-based structure in order to ensure toughness, and carbides are not properly controlled on the surface layer of the component, and good seizure resistance cannot be exhibited in a high slip environment.

  On the other hand, in Patent Document 3, C: 0.60 to 0.85%, Si: 0.20 to 1.50%, Mn: 0.40 to 1.60%, Cr: more than 0.30 to 1.50 %, V: 0.05-0.80%, Mo: 0.05-0.50%, and further, if necessary, Al: 0.020-0.100%, Ti: 0.010-0.100 %, Nb: 0.010 to 0.100%, and B: 0.0005 to 0.0050%, one or more selected from the group consisting of, and / or S: 0.040 to 0.130% , Pb: 0.030 to 0.350%, and Ca: 0.0010 to 0.0100%, and one or more selected from the group consisting of 0.0010 to 0.0100% have been proposed.

  However, in this technique, since the spheroidizing treatment for appropriately controlling the carbide is not performed, the spherical carbide is not dispersed in the surface layer of the component, and good seizure resistance cannot be exhibited in a high slip environment.

JP 2009-1848 A JP-A-11-293386 Japanese Patent Laid-Open No. 6-220579

  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. It is to provide a gear useful for an electric vehicle motor.

  The gears of the present invention that have solved the above problems are: C: more than 0.80% to 1.30% (meaning “mass%”, the same applies to the chemical composition), Si: 0.05-1 %, 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 (excluding 0%), respectively, the balance consisting of iron and inevitable impurities, tempered martensite and / or tempered bainite, It has a steel structure in which spherical carbides with an area ratio exceeding 5% and 30% or less are precipitated, and has a gist in that the nitrogen concentration at a depth of 20 μm from the surface is 2.0 to 6.0%. is there.

  In the gear of the present invention, if necessary, (a) Mo: 0.5% or less (not including 0%), (b) V: 0.2% or less (not including 0%), Ti: 0.0. One or more selected from the group consisting of 1% or less (excluding 0%) and Nb: 0.2% or less (not including 0%), (c) Cu: 5% or less (not including 0%) It is also effective to include Ni: 5% or less (not including 0%), etc., and the characteristics of the gear are further improved depending on the components contained.

In the gear of the present invention, a nitride layer having an iron nitride concentration of 80% by mass or more exists in the surface layer portion from the surface to a depth of 20 μm, and the nitride layer is a ratio of Fe ₄ N in the iron nitride. Is preferably 20% by mass or more. The gear of the present invention includes those having a lubricating film formed on the surface. The gear of the present invention is useful for an electric vehicle motor.

  In the present invention, the chemical component composition is appropriately adjusted, and the structure in the surface layer portion is a tempered martensite and / or tempered bainite structure while securing a predetermined amount of spherical carbide area ratio, and nitrogen at a depth of 20 μm from the surface. Since the concentration is set to 2.0 to 6.0%, it is possible to exhibit better seizure resistance in a power transmission section 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 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 area ratio of spherical carbide in the surface layer portion of 5. More than 30% and 30% or less.

  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 effective to disperse a predetermined amount of spherical carbide 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. In other words, controlling the N content (nitrogen concentration) of the surface layer as in (i) above can control N to a thermally more stable iron nitride composition, and the interatomic bond even at the metal contact portion. Can be suppressed. However, in the metal contact area, in addition to the adhesive wear, the influence of the abrasive wear due to the peeled iron pieces is also taken into account, so controlling the amount of spherical carbide in the surface layer as shown in (ii) disperses the hard spherical carbide By doing so, abrasive wear can be suppressed. Such an effect cannot produce a significant improvement in seizure resistance by either (i) or (ii) alone, and is a gear excellent in seizure resistance only because of the synergistic effect of (i) and (ii). Can be realized. Specific actions and effects by defining these requirements are as follows.

[Requirements for (i)]
In order to improve seizure resistance as a gear, it is necessary to control the nitrogen concentration at a depth of 20 μm from the surface 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)]
Spherical carbides present in the component surface layer can improve seizure resistance in a high slip environment. In addition, the spherical carbide existing in the surface layer portion also has an action of assisting the nitrogen concentration in the surface layer portion by nitriding. For this purpose, the area ratio of the spherical carbide in the surface layer portion needs to be more than 5% (more than 5%). When the area ratio of the spherical carbide is 5% or less, not only the concentration of nitrogen in the vicinity of the surface layer is insufficient, but also the effect of improving the seizure resistance by the carbide itself is insufficient, so that a predetermined seizure resistance can be obtained. Absent. On the other hand, when the area ratio of the spherical carbide exceeds 30%, nitrogen is excessively concentrated in the vicinity of the surface layer portion and the structure of the nitride is changed, so that the seizure resistance is deteriorated. Here, the spherical carbide is precipitated in a state dispersed in steel by annealing, spheroidizing annealing, quenching and tempering, and the aspect ratio (minor axis / major axis) is 0.4 or more (minor axis / major axis ≦ 1. (In the range of 0). The composition includes an Fe—C compound, a compound of carbon and an alloy element, a composite compound of Fe—C—alloy element, and the like. The preferable lower limit of the area ratio of the spherical carbide is 6% or more (more preferably 7% or more), and the preferable upper limit is 28% or less (more preferably 25% or less).

  The spherical carbide is precipitated in tempered martensite or tempered bainite, which is substantially a matrix structure, or a composite structure thereof (structure composed of tempered martensite and tempered bainite). After the nitriding treatment, a structure other than these base 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, since the structure of ferrite, pearlite, bainitic ferrite, as-quenched martensite, as-quenched bainite and the like are present in a ratio of 5% or less in area ratio, it does not adversely affect the action of the present invention, Permissible.

  When the spheroidizing treatment (described later) for obtaining the gear of the present invention is performed, the area ratio of the spherical carbide becomes constant throughout the steel material regardless of the place. However, when nitriding is performed thereafter, iron nitride (iron nitride) is formed on the outermost surface, and spherical carbide is not visible (cannot be measured). For this reason, in the following examples, since the area ratio of the spherical carbide on the outermost surface of the product (nitrided product) cannot be achieved, the area ratio at the “20 μm depth position from the surface” where iron nitride cannot be obtained is measured. It is considered that the value is equivalent to the area ratio of the spherical carbide in the surface layer before nitriding.

  In the 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 0.80% to 1.30%]
C is an element necessary for forming a predetermined amount or more of a spherical carbide that improves seizure resistance. It is also effective in increasing the quenching hardness and maintaining the strength at room temperature and high temperature to impart wear resistance. In order to exhibit such an effect effectively, it is necessary to contain more than 0.80% at least. However, if the C content is excessive, giant carbides are likely to be formed in the core and the seizure resistance is deteriorated, so it is necessary to be 1.30% or less. The preferable lower limit of the C content is 0.85% or more (more preferably 0.95% or more), and the preferable upper limit is 1.25% or less (more preferably 1.15% 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 carbides become coarse and the fatigue characteristics and machinability are deteriorated, 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.

  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 gear of the present invention, if necessary, (a) Mo: 0.5% or less (not including 0%), (b) V: 0.2% or less (not including 0%), Ti: 0.0. One or more selected from the group consisting of 1% or less (excluding 0%) and Nb: 0.2% or less (not including 0%), (c) Cu: 5% or less (not including 0%) And / or Ni: 5% or less (not including 0%), etc. may be included, and the characteristics of the 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.

[Mo: 0.5% or less (excluding 0%)]
Mo is an element effective for improving the impact strength in addition to the effect of significantly improving the hardenability during quenching. However, if the Mo content is excessive, the material hardness increases and machinability becomes poor. Further, since it is an expensive element, it causes an increase in cost, so it is preferably 0.5% or less. . More preferably, it is 0.45% or less, More preferably, it is 0.40% or less. In order to effectively exhibit the effect of Mo, 0.05% or more is preferably contained, more preferably 0.07% or more (more preferably 0.10% or more).

[V: 0.2% or less (not including 0%), Ti: 0.1% or less (not including 0%) and Nb: 0.2% or less (not including 0%) One or more
V, 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 improves surface hardness by combining with intrusion N and intrusion C due to soft nitriding to deposit fine V carbon / nitride (carbide, nitride or carbonitride containing V) on the surface layer. And improve seizure resistance. However, if the V content becomes excessive and exceeds 0.2%, the V charcoal / nitride tends to be coarsened, and the surface hardness is lowered and the fatigue strength is deteriorated. More preferably, it is 0.19% or less, More preferably, it is 0.18% or less. In order to effectively exhibit the effect of V, the content is preferably 0.05% or more, more preferably 0.06% or more (more preferably 0.07% or more).

  Ti combines with intrusion N and intrusion C due to soft nitriding to improve surface hardness by precipitating fine Ti charcoal / nitride (Ti-containing carbide, nitride or carbonitride) on the surface layer. Improves seizure resistance. However, if the Ti content becomes excessive and exceeds 0.1%, Ti charcoal / nitride tends to be coarsened, and the surface hardness is lowered and the fatigue strength is deteriorated. More preferably, it is 0.09% or less, More preferably, it is 0.08% or less. In order to effectively exhibit the effect of Ti, the content is preferably 0.03% or more, more preferably 0.04% or more (more preferably 0.05% or more).

  Nb combines with intrusion N and intrusion C by soft nitriding to improve surface hardness by precipitating fine Nb carbon / nitride (carbide, nitride or carbonitride containing Nb) on the surface layer. Improves seizure resistance. However, if the Nb content becomes excessive and exceeds 0.2%, the Nb charcoal / nitride tends to be coarsened, reducing the surface hardness and deteriorating the fatigue strength. More preferably, it is 0.19% or less, More preferably, it is 0.18% or less. In order to effectively exhibit the effect of Nb, it is preferably contained in an amount of 0.05% or more, more preferably 0.06% or more (more preferably 0.07% or more).

[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 gear of the present invention is manufactured by using a steel material having the above-described chemical composition, spheroidizing annealing, processing into a predetermined gear shape, quenching and tempering, and then performing nitriding treatment. 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, and is heated to 780 ° C. or higher (however, if the heating temperature is too high, the austenite grains are too coarsened, and the upper limit is set to 1100 ° C.), and rapidly cooled to the martensite transformation start temperature Ms or less .

  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. Even if residual austenite remains at the time of quenching, it can be decomposed during the nitriding treatment, so that no particular problem occurs. 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.

  The important steps in producing the gear of the present invention are (A) spheroidizing annealing step and (B) nitriding step. By applying a predetermined spheroidizing annealing treatment and a 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.

[(A) Spheroidizing annealing treatment step]
In the spheroidizing annealing process, it is necessary to form a spheroidized structure in advance in order to control the spherical carbide area ratio after nitriding to a predetermined amount. Any known method may be applied to the spheroidizing annealing step, and examples thereof include a slow cooling method, a repeating method, and a high temperature tempering method. In the present invention, a slow cooling method was applied in order to precipitate spherical carbide stably. The spheroidizing annealing process is subdivided into a heating / holding process in which pearlite is decomposed to leave undissolved carbides that become spheroidizing nuclei and a slow cooling process in which spherical carbides are grown.

  Said heating temperature is controlled by the temperature range of 750-850 degreeC. When the heating temperature is less than 750 ° C., the decomposition of pearlite is insufficient, and residual pearlite is present even after slow cooling, which is not appropriate from the viewpoint of suppressing seizure resistance. On the other hand, when this heating temperature exceeds 850 ° C., undissolved carbides are remarkably reduced and regenerated pearlite is generated after slow cooling, which is not appropriate from the viewpoint of suppressing seizure resistance. By holding for about 2 to 20 hours after heating, the undissolved carbide can be controlled to be an appropriate shape as the core of the spherical carbide. When the holding time after heating is shorter than 2 hours, the decomposition of pearlite is insufficient, and when it exceeds 20 hours, undissolved carbides are remarkably reduced.

In the subsequent slow cooling process, from the viewpoint of growing spherical carbides, the slower the cooling rate, the better. However, the productivity is inhibited accordingly. On the other hand, when the cooling rate is too high, spherical carbides cannot be sufficiently grown up to the Ar ₁ transformation point, leading to generation of regenerated pearlite. From the viewpoint of achieving both carbide growth and productivity, an appropriate cooling rate range is about 1 to 50 ° C./hour (preferably about 5 to 30 ° C./hour). The cooling stop temperature may be not higher than the Ar ₁ transformation point, but is preferably about 600 to 700 ° C. from the viewpoint of productivity. Cooling after the cooling stop temperature may be appropriately selected according to production, such as air cooling, natural cooling, furnace cooling, and the like.

[(B) Nitriding process]
In the nitriding treatment step, the purpose is to impregnate N into the steel, so that any known method may be applied. 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 treatment and plasma nitriding treatment are applied as an example of nitriding treatment. The plasma soft nitriding treatment is nitriding by glow discharge in a nitrogen-hydrogen-carbon mixed gas, and the plasma nitriding treatment is nitriding by glow discharging in a nitrogen-hydrogen mixed gas. . When 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.

(1) Nitriding temperature: 350-650 ° C
In the present invention, a predetermined amount of spherical carbide is precipitated in the nitriding treatment, the diffusion of N into the steel material is promoted, and the iron nitride composition is controlled so that interatomic bonds are less likely to occur. Can be obtained. The lower limit of the processing temperature is set to 350 ° C., when the nitriding temperature is too low, the diffusion rate of N decreases, the processing time becomes longer, and the time for obtaining a predetermined spherical carbide is also longer. This is because the production time is hindered. The upper limit of 650 ° C. is effective for promoting the growth of carbide and diffusion of N when the nitriding temperature is too high, but the tempering of the matrix matrix has progressed too much and the internal hardness is low. This is because the characteristics as a gear part cannot be obtained and the diffusion reaction of the surface layer nitrogen into the interior is increased, thereby preventing the concentration of nitrogen in the surface layer portion. Therefore, by setting the nitriding temperature in the range of 350 to 650 ° C., various characteristics as a gear part 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 400 ° C. or higher (more preferably 450 ° C. or higher), and a more preferable upper limit is 625 ° C. or lower (more preferably 600 ° C. or lower).

(2) Nitriding time: 3 to 30 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, the amount of N diffusion, and the amount of iron nitride are determined in conjunction with temperature and time, but in the present invention, ranges for obtaining a predetermined structure stably are set by temperature and time, respectively. . By setting the nitriding treatment time to 3 to 30 hours, a desired structure can be obtained, and excellent seizure resistance can be exhibited even in a high slip environment such as an electric vehicle motor. If the nitriding time is shorter than 3 hours, sufficient spherical carbide and iron nitride cannot be obtained, and in order to achieve the desired structure in less than 3 hours, if the temperature is raised too high, the matrix matrix There is a harmful effect of softening. On the other hand, even when the nitriding time exceeds 30 hours, the matrix matrix becomes soft, and when trying to treat at low temperature to prevent this, spherical carbide does not precipitate sufficiently, and N also diffuses sufficiently in the steel. There is no evil.

(3) Nitriding atmosphere: nitrogen gas concentration 30-80%
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 30%, 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 80% and the diffusion amount of N is excessively increased, the composition changes again to iron nitride that is easy to bond between atoms, and therefore seizure resistance cannot be improved. A more preferable lower limit of the N ₂ fraction is 35% or more (more preferably 40% or more), and a more preferable upper limit is 78% or less (more preferably 75% or less).

In the electric vehicle motor gear subjected to the nitriding treatment as described above, a nitride layer having an iron nitride (iron nitride) concentration of 80% by mass or more exists in the surface layer portion from the surface to a depth of 20 μm. In the nitride layer, the proportion of Fe ₄ N in iron nitride is preferably 20% by mass or more. Satisfaction resistance is further improved by satisfying these requirements. The reason why such an effect is exhibited can be considered as follows.

Adhesion wear occurs due to interatomic bonds due to metal contact, but by increasing the proportion of iron nitride, interatomic bonds are less likely to occur and adhesion wear can be suppressed. Further, since the iron nitride produced in the surface layer by nitriding there are types such as _Fe 2~3 N, Fe ₄ N or the like, which hardly causes the most adhesive wear in this is Fe ₄ N, The seizure resistance is further improved by increasing the proportion of Fe ₄ N. The concentration of iron nitride (iron nitride) in the nitride layer is more preferably 85% by mass or more, and still more preferably 90% by mass or more. Moreover, the ratio of Fe ₄ N in iron nitride is more preferably 25% by mass or more, and further preferably 30% by mass or more.

Among the steel components of the present invention, Cr, Al and Mo are elements that are easily combined with nitrogen. When the content of these elements increases, the diffusion rate of nitrogen _decreases , and the amount of Fe2-3N increases. Appropriate amounts of these elements added (Cr is preferably 1.3% or less, more preferably 1.2% or less, Al is preferably 0.040% or less, more preferably 0.030% or less, and Mo is preferably 0 or less. .30% or less, and more preferably 0.20% or less), the surface layer portion becomes a nitride layer containing a large amount of Fe ₄ N, and adhesion wear can be prevented (for example, test No. 29 in Example 2 described later). ~ 31).

Further, when the soft nitriding treatment is performed, the nitrogen concentration in the surface layer portion is increased, and Fe ₂ to ₃ N is easily formed as compared with the nitriding treatment. In order to increase the proportion of Fe ₄ N, it is more suitable to perform nitriding (see Experiment Nos. 25 and 26 in Example 2 described later). By increasing the nitriding temperature (about 560 to 625 ° C., preferably about 570 to 600 ° C.), nitrogen diffuses moderately and the proportion of Fe ₄ N increases (test No. 2 described later in Example 2). 25 and 27).

When the N ₂ fraction is increased in the gas fraction under the nitriding conditions, Fe _{2 to 3} N increases, and when it decreases, Fe ₄ N increases. In order to secure a preferable amount of iron nitride and Fe ₄ N, it is preferable to adjust to an appropriate N ₂ fraction (about 35 to 55%, preferably about 40 to 45%) (see Example 2 below). See Test Nos. 25 and 28).

  In the 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.

Example 1
Steel ingots having various chemical composition compositions shown in Tables 1 and 2 below were produced in a melting furnace and then hot forged to a diameter of 32 mm. After these steel materials are heated and held in a temperature range of 760 to 820 ° C. for 2 to 8 hours and then cooled to 625 to 675 ° C. at a cooling rate of 10 to 200 ° C./hour, then spheroidizing annealing is performed. Processing was carried out. Thereafter, it was processed into a predetermined shape and subjected to quenching (840 ° C. × 30 minutes), tempering (160 ° C. × 120 minutes), and nitriding treatment (plasma soft nitriding treatment). After the heat 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. The conditions for plasma soft nitriding were performed in the following range.
[Plasma soft nitriding conditions]
Nitriding temperature: 300-670 ° C
Retention time: 1 to 25 hours N ₂ fraction: 20 to 85%, C mixed gas fraction (C ₃ H ₈ ): 1% (remainder: H ₂ fraction)

  At this time, the spheroidizing treatment conditions and plasma soft nitriding treatment conditions (hereinafter sometimes simply referred to as “nitriding conditions”) in each test are shown in Tables 3 and 4 below (Test Nos. 1 to 24).

  About each obtained test piece, while measuring the area ratio of the spherical carbide | carbonized_material in a structure | tissue, and the nitrogen concentration of 20 micrometers depth from the surface by the following method, component characteristics (seizure resistance) were evaluated by the following method.

[Measurement method of area ratio of spherical carbide]
The area ratio of the spherical carbide in the structure (the main structure is tempered martensite and / or tempered bainite) was measured using a scanning electron microscope (SEM) at a position 20 μm deep from the surface of the test piece. At this time, a sample obtained by cutting the test piece after nitriding in a cross section, embedding in a resin, mirror polishing, and etching was used. An arbitrary 9 μm × 12 μm visual field was observed at a magnification of 8000 times, and an aspect ratio of 0.4 or more was used as a threshold with image analysis software to identify a spherical carbide portion, and the area ratio was determined. The measurement was performed for 3 fields of view, and the arithmetic average of these 3 fields of view was defined as the area ratio of carbide.

[Method for measuring nitrogen concentration 20 μm deep from the surface]
The nitrogen concentration in the surface layer portion (20 μm depth position from the surface) was measured by cutting the test piece in a cross section, embedding it in a resin, polishing, and then polishing the nitrogen concentration from the surface layer portion to the inside. It was measured by analysis using a 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 by a roller pitching tester using the test piece (small roller) and a large roller (counter 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.

These results (spherical carbide area ratio, surface layer nitrogen concentration, seizure generation life) are shown in Tables 5 and 6 below together with the presence or absence of a lubricating coating (the lubricating coating uses manganese phosphate). Note that “> 20,000” in the term of seizure life in Table 5 means that seizure did not occur even after 20000 × 10 ³ times.

  From these results, it can be considered as follows. First, test no. 1-14, since both the chemical composition of the steel material and the production conditions are controlled within an appropriate range, both the spherical 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 seizure resistance can be exhibited. It can also be seen that excellent seizure resistance can be obtained even if a lubricating film is formed.

  In contrast, test no. In Nos. 15 to 24, the chemical component composition and manufacturing conditions of the steel material are not controlled within an appropriate range, and therefore, seizure resistance is deteriorated in all cases. That is, test no. No. 15 uses a steel type having excessive C content (steel type O), and nitrogen is excessively concentrated on the surface layer, and the seizure resistance is deteriorated. Test No. No. 16 uses a steel type with a low C content (steel type P), the spherical carbide area ratio cannot be secured, and seizure resistance is deteriorated.

Test No. No. 17 is a steel type with a low Cr content (steel type Q). Even if the production conditions are appropriate, stable spherical carbide does not sufficiently precipitate, and the spherical carbide area ratio and surface layer nitrogen concentration are low. Insufficient seizure resistance has deteriorated. Test No. No. 18 has an excessive amount of nitrogen gas (N ₂ fraction) during nitriding, so that nitrogen is excessively concentrated on the surface layer and the seizure resistance is deteriorated.

Test No. In No. 19, since the amount of nitrogen gas (N ₂ fraction) during nitriding is too small, the nitrogen concentration in the surface layer portion cannot be secured, and the seizure resistance is deteriorated. Test No. No. 20, since the nitriding temperature is low, the spherical carbide area ratio cannot be secured, the surface layer nitrogen concentration is insufficient, and the seizure resistance is deteriorated.

  Test No. No. 21 has a high nitriding temperature, so that the internal diffusion of nitrogen in the surface layer portion is easy to proceed, the nitrogen concentration in the surface layer portion cannot be secured, and the seizure resistance is deteriorated. Test No. No. 22 has a short retention time during the nitriding treatment, so that the nitrogen concentration in the surface layer portion can be secured, and the seizure resistance is deteriorated.

  Test No. In No. 23, the cooling rate during the spheroidizing treatment was too fast, and the regenerated pearlite structure was formed over the entire structure, so that the spherical carbide area ratio could not be ensured, and the seizure resistance deteriorated. Test No. No. 24 is obtained by carburizing a JIS SCM420 equivalent steel and then carrying out a lubricating film treatment. However, since a spheroidizing annealing treatment and a nitriding treatment were not performed, a spherical carbide area ratio could not be secured, and nitrogen in the surface layer portion was not obtained. Since the concentration is also lowered, the seizure resistance is extremely deteriorated.

(Example 2)
Steel ingots (steel types N, AA, AB, AC: among these, steel type N is the same as steel type N shown in Table 1) shown in Table 7 below were prepared in a melting furnace, and the diameter: Hot forged to 32 mm. After these steel materials are heated and held in a temperature range of 760 to 820 ° C. for 2 to 8 hours and then cooled to 625 to 675 ° C. at a cooling rate of 10 to 200 ° C./hour, then spheroidizing annealing is performed. Processing was carried out. Thereafter, it was processed into a predetermined shape and subjected to quenching (840 ° C. × 30 minutes), tempering (160 ° C. × 120 minutes), and nitriding treatment (plasma soft nitriding treatment or plasma nitriding treatment). After the heat 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. A roller pitching test was conducted in the same manner as in Example 1.

  At this time, spheroidizing treatment conditions and nitriding treatment conditions (plasma soft nitriding treatment or plasma nitriding treatment) in each test are shown in Table 8 below (Test Nos. 25 to 31).

  About each obtained test piece, while measuring the area ratio of the spherical carbide | carbonized_material in a structure | tissue, the nitrogen density | concentration of 20 micrometers depth from the surface, and a component characteristic (seizure resistance) by the method shown in Example 1, nitride layer The composition was evaluated by the following method.

(Measurement method of nitride layer composition)
A part of the test piece was taken out, and X-ray diffraction was performed under the following measurement conditions in the surface portion of the test piece (surface layer portion from the surface to a depth of 20 μm).

(Measurement conditions for X-ray diffraction)
Analyzer: Two-dimensional micro X-ray diffractometer "RINT-RAPID II" manufactured by Rigaku Corporation (1) Analysis conditions Tube: Co, monochromatization: use of monochromator (Kα ray), tube output: 40 kV- 26 mA, detector: imaging plate (two-dimensional)
(2) Reflection method Collimator: φ300 μm, ω angle (X-ray incident angle): 22 ° -30 ° swing (1 ° / second),
φ angle (in-plane rotation): fixed, measurement time (exposure): 30 minutes

From the obtained X-ray diffraction profile, a compound present on the surface layer of the specimen was identified. For the identified compound (component), the relative concentration of each component was obtained by a semi-quantitative method using peak separation. The ratio of iron nitride in the surface layer is the total relative of Fe ₄ N and Fe _{2 to 3} N with respect to the total relative concentration (100% by mass) of all the components present in the surface layer of the specimen, which was confirmed as a result of X-ray diffraction. the concentration ratio of a (mass%), the ratio of the Fe ₄ N on nitride in the iron, the proportion of Fe ₄ N relative concentrations to the total relative concentration of Fe ₄ N and Fe _{2 to 3} N (wt%) The value is calculated by the following formula. Note that all components present on the surface layer of the specimen are Fe ₄ N, Fe _{2 to 3} N, Fe ₅ C ₂ , Fe ₃ C, γ-Fe, α-Fe, etc. The total ratio is 100%, but other components (for example, Fe ₂ C, Fe ₂₂ C, Fe ₇ C ₃ etc.) may be contained in a small amount (5% or less). The total of these components is 100%).
Iron nitride ratio (% by mass) = {relative concentration of Fe ₄ N in the surface layer (% by mass) + relative concentration of Fe _{2 to 3} N in the surface layer (% by mass)}
Ratio of Fe ₄ N in iron nitride (mass%) = {relative concentration of Fe ₄ N (mass%) / [relative concentration of Fe ₄ N (mass%) + relative concentration of Fe _{2 to 3} N (mass%) ] X 100 (%)

  The results are collectively shown in Table 9 below.

  As is apparent from the results, in both cases, the chemical composition of the steel material and the production conditions are both controlled within an appropriate range, so that both the spherical carbide area ratio and the surface layer nitrogen concentration are specified in the present invention. Can be controlled within a range. As a result, the adhesion suppressing effect is excellent and excellent seizure resistance can be exhibited. Of these, in particular, test no. Nos. 26 to 28, 30, and 31 satisfy the preferable requirements for the composition of the nitride layer, and it is understood that particularly excellent seizure resistance is obtained.

Claims (7)

C: more than 0.80% to 1.30% (meaning “mass%”, chemical composition is the same hereinafter), Si: 0.05 to 1%, Mn: 0.1 to 1%, P: 0.00. 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 0.02% or less (excluding 0%) respectively, and the balance consists of iron and inevitable impurities, and spherical carbides with an area ratio of more than 5% and less than 30% are precipitated on tempered martensite and / or tempered bainite. has been that the steel structure, the nitrogen concentration at 20μm depth from and surface Ri 2.0 to 6.0% der,
Using a roller pitching test piece (small roller) in FIG. 1 and a large roller (mating material) made of high carbon chrome bearing steel SUJ2, rotation speed: 1000 rpm, relative slip ratio: 700 %, Oil temperature: When the roller pitching test is performed under the conditions of 90 ° C. and the number of rotations until the test apparatus stops due to vibration caused by the occurrence of seizure, seizure occurs up to 10,000 × 10 ³ times. A gear with excellent seizure resistance, characterized by not   The gear according to claim 1, further comprising Mo: 0.5% or less (not including 0%).   Furthermore, V: 0.2% or less (excluding 0%), Ti: 0.1% or less (not including 0%), and Nb: 0.2% or less (not including 0%) The gear according to claim 1 or 2 containing one or more selected.   The gear according to any one of claims 1 to 3, further containing Cu: 5% or less (excluding 0%) and / or Ni: 5% or less (excluding 0%). In the surface layer portion from the surface to a depth of 20 μm, there is a nitride layer having an iron nitride concentration of 80% by mass or more, and the nitride layer has a ratio of Fe ₄ N in the iron nitride of 20% by mass or more. The gear according to any one of claims 1 to 4.   The gear according to any one of claims 1 to 5, wherein a lubricating film is formed on the surface.   The gear according to any one of claims 1 to 6, which is used for an electric vehicle motor.

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