TWI507543B - Anti-coking excellent gear - Google Patents

Description

Gear with excellent coking resistance

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a gear having excellent anti-coking property in an operating portion that produces high rotation and high slidability, and more particularly to a gear for an electric vehicle motor.

For example, in the case of a mechanical structural component used in a power transmission portion such as a gear for a transmission of an automobile, when used, it is known that the metal contact portion is peeled and damaged by the increase in contact surface pressure. Therefore, as a steel part used for such a use, various surface hardened steels, such as SCr, SCM, and SNCM, are subjected to hot-forging or cutting forming, and then subjected to surface hardening treatment such as carburizing treatment or carburizing and nitriding treatment. And, if necessary, a solid lubricating film such as molybdenum disulfide is formed on the surface of the part.

In recent years, the demand for high output and small size and weight reduction of the mechanical structure portion has increased, and the load applied to the mechanical structural component used in the power transmission portion tends to increase. Therefore, if only various types of case hardened steels such as SCr, SCM, and SNCM are surface-hardened, it is difficult to achieve the required resistance even if a solid lubricating film is formed. Pitting corrosion.

In an electric vehicle whose production capacity is gradually increased due to a decrease in environmental load in recent years, since the rotation of the motor is directly transmitted to the reduction gear, the parts are more exposed to high rotation than the gasoline-powered vehicle. In the use environment, since the lubricating oil used is lower in dynamic viscosity than the gasoline-powered vehicle, the oil film formed on the surface of the steel component constituting the power transmission portion is thin, and the environment is locally generated in a portion where the oil film is hardly formed. . In particular, with high rotation and high slidability, the oil temperature also rises, and the dynamic viscosity of the lubricating oil is liable to decrease, and the portion where the oil film disappears increases. Therefore, in such an environment, it is easy to cause abrasion due to metal contact of steel parts with each other, or it is easy to cause softening of steel parts due to temperature rise due to friction heat, and coking may occur at an early stage.

Various related technologies for steel members used in the above environments have been proposed so far. For example, Patent Document 1 discloses a rolling element for an automobile motor or an auxiliary machine, which includes C: 0.7 to 1.1%, Si: 0.3 to 0.7%, Mn: 0.3 to 0.8%, Ni: 0.5 to 1.2%, and Cr: 1.3. ~1.8%, Mo: 0.1~0.7% and V: 0.2~0.4%, the remaining part is composed of iron and impurities, from Si+Mn: 1.0% or less, Ni+Cr: 2.3% or more, Cr+Mo+V: 3.0% or less of steel is formed, and a high carbon-containing layer is formed, and the hardness of the surface layer portion is 725 to 800 Hv, and the maximum particle diameter of the carbide in the surface layer portion is 10 μm or less, and the area ratio is 7 to 20%, and the surface layer portion is compared. The amount of carbon in the interior is 0.2 to 0.4% higher, and the amount of nitrogen in the surface layer is 0.1 to 0.5%.

However, in the case where the technique is applied to a motor for an electric vehicle, in order to suppress coking in an environment where coking is more likely to occur, the watch It is not enough that the amount of nitrogen in the layer is controlled to 0.5%.

Patent Document 2 discloses a hot-worked steel material and product, which is composed of 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, and O: 0.0030. % or less, N: 0.015% or less, the remaining part is a steel sheet or a steel material of a component composed of iron and unavoidable impurities, and is heated to 800 ° C or higher, which is 50 ° C or lower lower than the solidus temperature of the steel sheet and the steel material. At a temperature of the temperature, the hot-processed steel material is cooled to room temperature, and graphite having an average particle diameter of 0.5 μm or more is precipitated at 100 parts/mm ² or more, and the metal structure is mainly composed of the ferrite and the rapid cutting property is excellent. . The technique also reveals that it is useful to appropriately add: a graphite precipitation promoting element, a ferrite iron refining element, a machinable lifting element, and a quenching promoting element.

However, in this technique, in order to ensure the toughness, the structure of the main body of the ferritic iron is not properly controlled in the surface layer of the part, and excellent anti-coking property cannot be exhibited in a high slidability environment.

On the other hand, in Patent Document 3, a steel for soft nitriding is proposed, which has a mixture of C: 0.60 to 0.85%, Si: 0.20 to 1.50%, Mn: 0.40 to 1.60%, and Cr: more than 0.30 to 1.50%, and V: 0.05~0.80%, Mo: 0.05~0.50%, and selected from the group consisting of Al: 0.020~0.100%, Ti: 0.010~0.100%, Nb: 0.010~0.100%, and B: 0.0005~0.0050%. One or two or more selected from the group consisting of one or two or more types, and/or S: 0.040 to 0.130%, Pb: 0.030 to 0.350%, and Ca: 0.0010 to 0.0100%.

However, in this technology, since it is not carried out in order to properly control carbon The spheroidization treatment does not allow the spheroidal carbide to be dispersed in the surface layer of the part, and does not exhibit good anti-coking property in a highly slidable environment.

[prior technical literature] [Patent Document]

[Patent Document 1] Japanese Patent Laid-Open Publication No. 2009-1848

[Patent Document 2] Japanese Patent Laid-Open No. Hei 11-293386

[Patent Document 3] Japanese Patent Laid-Open No. Hei 6-220579

In order to solve the problems of the prior art, the present invention has an object to provide a gear for an electric vehicle motor, which is capable of exhibiting more excellent anti-coking by using a power transmission portion of a lubricating oil having high rotation, high sliding property, and low dynamic viscosity. Sex.

The gear of the present invention which can solve the above problems includes C: more than 0.80% to 1.30% (representing "% by mass", the same chemical composition below), Si: 0.05 to 1%, and Mn: 0.1 to 1%, P: 0.05% or less, S: 0.05% or less, Cr: 0.9 to 2%, Al: 0.01 to 0.1%, and N: 0.02% or less, and the remaining portion is composed of iron and unavoidable impurities, and has: tempering At least one of the granulated iron and the tempered toughened iron, and a spheroidal carbide having a surface area ratio of 5% or less and 30% or less The deposited steel structure; and the nitrogen concentration at a depth of 20 μm from the surface is 2.0 to 6.0%.

The gear of the present invention may further contain, if necessary, (a) Mo: 0.5% or less (excluding 0%), (b) V: 0.2% or less (excluding 0%), and Ti: 0.1% or less (not One or more selected from the group consisting of 0%) and Nb: 0.2% or less (excluding 0%), (c) from Cu: 5% or less (excluding 0%), and Ni: 5% or less (not One or more selected from the group consisting of 0%) is also effective, and the characteristics of the gear can be improved more in response to the contained components.

In the gear of the present invention, a nitride layer having a concentration of iron nitride of 80% by mass or more is present in the surface layer portion having a depth of 20 μm from the surface, and the ratio of Fe ₄ N in the nitride layer is 20 in the nitride layer. More than % by mass. In the gear of the present invention, a lubricating film is formed on the surface. The gear of the present invention can be used as a gear for an electric vehicle motor.

In the present invention, the composition of the surface layer is appropriately adjusted, and the structure of the surface layer portion is made to ensure a predetermined amount of the spherical carbide area ratio and is at least one of the tempered granulated iron and the tempered toughened iron. The nitrogen concentration at a depth of 20 μm is 2.0 to 6.0%. It is excellent in anti-coking properties in a power transmission unit using a high-rotation, high-sliding, low-viscosity lubricating oil. This type of gear is very useful as an electric vehicle motor. .

Fig. 1 is a schematic explanatory view of a test piece used in the roller pitting test.

The gear excellent in anti-coking property of the present invention is characterized in that: (i) the nitrogen concentration at a depth of 20 μm from the gear surface is 2.0 to 6.0%, and (ii) the area ratio of the spherical carbide in the surface portion is more than 5%. 30% or less.

It is mainly used as a pitting corrosion damage caused by a gasoline-powered vehicle. As the oil film disappears, the metal between the gears contacts each other to generate frictional heat, which causes the gear to soften. In order to achieve a gear part excellent in pitting resistance, the surface, internal strength, or temper softening resistance of the part is improved, and the pitting resistance can be improved by high strength. However, in the electric motor motor gear, compared with the conventional gasoline-powered vehicle, the sliding speed between the tooth surfaces is several times or more, and the damage mechanism changes toward the coking condition, so that the surface, internal strength, or temper softening resistance of the part is made. In terms of improvement, we cannot find an improvement policy.

The present inventors have carefully studied the mechanism of occurrence of a coking situation in a field where the sliding speed is extremely high. As a result, it was judged that at the high temperature, high pressure, and high sliding property, the inter-atomic bonding, that is, the adhesive wear, was caused due to the abrasion. As a result of further investigation, it has been found that in order to easily cause inter-atomic bonding in an environment where adhesion wear is likely to occur, the N content (nitrogen concentration) in the surface layer of the gear is improved, which is effective compared with the nitrogenation with various added elements. It is effective to form a larger number of iron nitrides and to appropriately control the composition of the iron nitride. Further, it has been found that a predetermined amount of spheroidal carbide is dispersed in the steel, and the effect of addition and multiplication can significantly improve the anti-coking property, and the invention is completed.

The predictive mechanism for improving the anti-coking property of the present invention is as follows. In other words, as described above, (i) controlling the N content (nitrogen concentration) of the surface layer portion, it is possible to control N to a stable iron nitride composition by heat, and to suppress interatomic bonding in the metal contact portion. In the metal contact portion, in addition to the adhesive wear, the influence of the polishing wear due to the peeling of the iron piece is caused. Therefore, (ii) the method of controlling the amount of the spherical carbide in the surface layer portion can be suppressed by dispersing the hard spherical carbide. Grinding wear. According to this effect, it is found that neither (i) nor (ii) alone can significantly improve the anti-coking property, and by the addition effect of (i) and (ii), it is possible to achieve an electric vehicle motor excellent in anti-coking property. gear. The specific effects achieved by defining the requirements are as follows.

[(i) requirements]

In order to improve the anti-coking property of the gear, it is necessary to control the nitrogen concentration at a depth of 20 μm from the surface to 2.0 to 6.0%. If the nitrogen concentration in this portion is less than 2.0%, adhesion between atoms is likely to occur due to metal contact, and adhesive wear is caused. On the other hand, if the nitrogen concentration is more than 6.0%, the atomic structure of the nitride in the vicinity of the surface layer changes, and adhesive wear is likely to occur. A preferred lower limit of the nitrogen concentration is 2.2% or more (more preferably 2.5% or more), and a preferred upper limit is 5.8% or less (more preferably 5.5%). under).

[(ii) requirements]

The spheroidal carbides present in the surface layer of the part can improve the anti-coking property in a high sliding environment. The spheroidal carbides present in the surface layer also serve to assist in the concentration of nitrogen in the surface layer by nitriding. For this reason, the area ratio of the spheroidal carbide in the surface portion needs to be more than 5% (greater than 5%). When the area ratio of the spheroidal carbide is 5% or less, the concentration of nitrogen in the vicinity of the surface layer portion is insufficient, and the effect of improving the anti-coking property by the carbide itself is insufficient, so that predetermined anti-coking property cannot be obtained. On the other hand, when the area ratio of the spheroidal carbide is more than 30%, the nitrogen is too concentrated in the vicinity of the surface layer portion to change the structure of the nitride, so that the anti-coking property is deteriorated. The spheroidal carbide is precipitated in a state of being dispersed in steel by annealing, spheroidizing annealing, quenching and tempering, and the aspect ratio (short diameter/long diameter) is 0.4 or more (in the short diameter) / Long diameter ≦ 1.0 range of carbides. The composition thereof includes an Fe-C compound or a compound of carbon and an alloy element, a composite compound of an Fe-C-alloy element, and the like. A preferred lower limit of the area ratio of the spheroidal carbide is 6% or more (more preferably 7% or more), and a preferred upper limit is 28% or less (more preferably 25% or less).

The spheroidal carbides are substantially precipitated as tempered granulated iron or tempered toughened iron of the basal structure, or a composite structure thereof (structure composed of tempered granitic iron and tempered toughened iron). After the nitriding treatment, a structure other than the base structure may be formed in the steel, such as a ferrite iron or a ferrite, a tough ferrite iron, a quenched granulated iron, a quenched toughened iron, or the like. These organizations have a bad influence on the difference in characteristics of the gears and the anti-coking property, so it is hoped that they will not be generated as much as possible. The structure of ferrite iron or wave iron, tough ferrite iron, quenched granulated iron, quenched toughened iron, etc., is limited to a ratio of 5% or less, which does not cause adverse effects on the present invention. The effect is allowed.

When the spheroidizing treatment (described later) for obtaining the gear of the present invention is performed, the area ratio of the spherical carbide is constant regardless of the entire steel material. However, when nitriding treatment is performed later, iron nitride (iron nitride) is formed on the outermost surface, and spheroidal carbides cannot be seen (measured). Therefore, in the example described later, since the area ratio of the spherical carbide on the outermost surface of the product (nitride) is not reached, the area ratio of the iron nitride "the depth position of 20 μm from the surface" cannot be measured, and the value thereof is measured. The area ratio of the spheroidal carbides in the surface layer before nitriding is equal.

In order to exert the characteristics as a final product (gear part), the gear of the present invention needs to be more appropriately adjusted in its chemical composition. The reason for limiting the range of each component (element) of the chemical composition is as follows.

[C: Greater than 0.80%~1.30%]

C is an element required to form a spherical carbide having improved anti-coking property by a predetermined amount or more. It is effective in increasing the quenching hardness, maintaining the strength at room temperature and high temperature, and adding wear resistance. In order to effectively exert this effect, it is necessary to contain at least more than 0.80%. However, if the C content is excessive, it is easy to generate a large amount of carbides in the core, and the anti-coking property is deteriorated, so it is necessary to set 1.30% or less. A preferred lower limit of the C content is 0.85% or more (more preferably 0.95% or more), and a preferred upper limit is 1.25% or less (more preferably 1.15% or less).

[Si: 0.05~1%]

Si is used to increase the temper softening resistance and to suppress the decrease in hardness. In order to exert this effect, it is necessary to contain 0.05% or more. However, if the Si content is too large, the life of the mold at the time of cold forging is lowered, and the machinability is also deteriorated, so that it is required to be 1% or less. A preferred lower limit of the Si content is 0.10% or more (more preferably 0.15% or more), and a preferred upper limit is 0.8% or less (more preferably 0.5% or less).

[Mn: 0.1~1%]

Mn has an effect of improving solid solution strengthening and hardenability of the base material. In order to exert this effect, it is necessary to contain 0.1% or more. However, if the Mn content is too large, the lower oxide, that is, the MnO concentration is increased, the fatigue characteristics are deteriorated, and the workability or the machinability is remarkably lowered. Therefore, 1% or less is required. A preferred lower limit of the Mn content is 0.15% or more (more preferably 0.20% or more), and a preferred upper limit is 0.95% or less (more preferably 0.90% or less).

[P: 0.05% or less]

P segregates at the grain boundary and shortens the fatigue life, so it needs to be reduced as much as possible. In particular, if the content is more than 0.05%, the fatigue life is remarkably lowered. Therefore, the P content is 0.05% or less. P content is preferably 0.045% or less, preferably 0.040% or less.

[S: 0.05% or less]

S is an element which forms a sulfide, and if it is contained in an amount of more than 0.05%, coarse sulfide is generated, so that the fatigue life is shortened. Then, the content of S is 0.05% or less. The S content is preferably 0.045% or less, preferably 0.040% or less.

[Cr: 0.9~2%]

Cr is the use of quenching to enhance and form stable carbides, and effectively acts to enhance strength and anti-coking. In order to exert this effect, Cr needs to be contained in an amount of 0.9% or more. However, if the Cr content is too large, the carbides are coarsened, and the fatigue characteristics and the machinability are lowered. Therefore, the content thereof is required to be 2% or less. A preferred lower limit of the Cr content is 1.0% or more (more preferably 1.1% or more), and a preferred upper limit is 1.9% or less (more preferably 1.8% or less).

[Al: 0.01~0.1%]

Al acts as a deoxidizer and is used to reduce the amount of oxide inclusions and to increase the internal quality of the steel. From this viewpoint, the Al content is 0.01% or more. However, when the Al content is excessive, coarse and hard inclusions (Al ₂ O ₃ ) are generated, and the fatigue characteristics are lowered. Therefore, it is necessary to set it to 0.1% or less. A preferred lower limit of the Al content is 0.015% or more (more preferably 0.020% or more), and a preferred upper limit is 0.08% or less (more preferably 0.06% or less).

[N: 0.02% or less]

N combines with Al to form AlN, and also has an effect of refining the crystal grain size. When the N content is too large, cracks are likely to occur during rolling, and therefore it is necessary to be limited to 0.02% or less. The N content is preferably 0.018% or less, preferably 0.016% or less.

The basic components of the gear of the present invention are as described above, and the remaining portion is iron and unavoidable impurities (for example, Sb, Mg, etc.). The gear of the present invention may contain, if necessary, (a) Mo: 0.5% or less (excluding 0%), (b) V: 0.2% or less (excluding 0%), and Ti: 0.1% or less (excluding 0%) and Nb: 0.2% or less (excluding 0%), one or more selected from the group, (c) Cu: 5% or less (excluding 0%), and Ni: 5% or less (excluding One or more selected from the group consisting of 0%), and the characteristics of the gear can be improved in response to the type of the element contained. The reason for setting the preferred range of these elements is as follows.

[Mo: 0.5% or less (excluding 0%)]

Mo has an effect of remarkably improving the hardenability at the time of quenching, and is an element which effectively improves the impact strength. However, when the Mo content is too large, the material hardness is high and the machinability is poor, and since it is an expensive element, the cost is increased. Therefore, it is preferably 0.5% or less. It is preferably 0.45% or less, more preferably 0.40% or less. In order to effectively exert the effect of Mo, it is preferably 0.05% or more, more preferably 0.07% or more (preferably 0.10%). on).

[1 or more selected from the group consisting of V: 0.2% or less (excluding 0%), Ti: 0.1% or less (excluding 0%), and Nb: 0.2% or less (excluding 0%)]

V, Ti, and Nb are elements that effectively improve the anti-coking property by increasing the surface hardness of the gear. The detailed effects are as follows.

V is a combination of intrusion N and intrusion C due to soft nitridation, and fine-grained V-carbon, nitride (V-containing carbide, nitride or carbonitride) on the surface to improve surface hardness and resistance. Improved coking. However, if the V content is too large and is more than 0.2%, V carbon and nitride are likely to be coarsened, and the surface hardness is lowered to deteriorate the fatigue strength. It is preferably 0.19% or less, more preferably 0.18% or less. In order to effectively exert the effect of V, it is preferably 0.05% or more, more preferably 0.06% or more (preferably 0.07% or more).

Ti is a combination of intrusion N and intrusion C due to soft nitridation, and chromatizes fine Ti carbon and nitride (Ti-containing carbide, nitride or carbonitride) on the surface to improve surface hardness and resist Improved coking. However, if the Ti content is too large and is more than 0.1%, Ti carbon and nitride are likely to be coarsened, and the surface hardness is lowered to deteriorate the fatigue strength. It is preferably 0.09% or less, more preferably 0.08% or less. In order to effectively exert the effect of Ti, it is preferably 0.03% or more, more preferably 0.04% or more (preferably 0.05% or more).

Nb is combined with intrusion N and intrusion C due to soft nitriding On the surface, fine Nb carbon and nitride (carbide, nitride or carbonitride containing Nb) are chromatographed to increase the surface hardness and improve the anti-coking property. However, if the Nb content is too large and is more than 0.2%, Nb carbon and nitride are likely to be coarsened, and the surface hardness is lowered to deteriorate the fatigue strength. It is preferably 0.19% or less, more preferably 0.18% or less. In order to effectively exert the effect of Nb, it is preferably 0.05% or more, more preferably 0.06% or more (preferably 0.07% or more).

[1 or more selected from the group consisting of Cu: 5% or less (excluding 0%) and Ni: 5% or less (excluding 0%)]

Cu is solid-dissolved in steel, which enhances the surface layer and internal hardness, and effectively improves the anti-coking property. When it is nitriding, it precipitates finely, and it functions as hardening of a steel material. However, if the Cu content is too large, the steel material is embrittled, so that the Cu content is preferably 5% or less. It is preferably 4% or less, more preferably 3% or less.

Ni has a function of solid-solution strengthening of steel. By compounding with Cu, the precipitation hardening action of Cu is more exhibited. However, if the Ni content is too large, the effect is saturated, so that the Ni content is preferably 5% or less. It is preferably 4% or less, more preferably 3% or less.

The gear of the present invention is subjected to spheroidal annealing using a steel material having the chemical composition as described above, and then processed into a predetermined gear shape, quenched and tempered, and then subjected to nitriding treatment. In this manufacturing step, a general method can also be used before processing into a gear shape, and the gear processing is also performed by hot forging, cold forging, warm forging, etc., various forging, embossing, rolling Manufacture, or combination of cutting and grinding.

The purpose of quenching is to homogenize the structure and increase the solid solution C for precipitating the nitrided carbide. The quenching can be carried out by a general method, and it is heated to 780 ° C or higher (but if the heating temperature is too high, the Worthfield iron particles are too coarse, so the upper limit is 1100 ° C), and the quenching is as low as the Masuda iron transition temperature Ms.

After heating, the whole tissue is homogenized by holding it for about 1 to 180 minutes. If the heating time at this time is too long, the influence of decarburization or the like becomes remarkable, so the upper limit is 180 minutes. The quenching speed may be as long as the cooling rate of the fermented iron and the incomplete combustion is less, and the upper limit is not particularly set. The lower limit is set to 1 ° C /sec or more from the above viewpoint. Even if the Worstian iron remains in the quenching time, it can be decomposed during the nitriding treatment, so that no particular problem occurs. Tempering is carried out in the view of preventing cracking. In the case where the nitriding treatment is performed immediately after quenching, tempering may be omitted. The tempering treatment, for example, in the temperature range of 100 to 300 ° C for 1 to 180 minutes, prevents cracking.

The important steps in the manufacture of the gear of the present invention are (A) spheroidizing annealing treatment step and (B) nitriding treatment step. When the steel material having the chemical composition described above is subjected to a predetermined spheroidizing annealing treatment and nitriding treatment, a predetermined structure state can be obtained, and the anti-coking property can be improved.

[(A) Spheroidizing annealing treatment step]

In the spheroidizing annealing treatment step, since the scaly carbide area ratio after nitriding is controlled to a predetermined amount, it is necessary to form a spheroidized structure before. The spheroidizing annealing step may be any one of conventional methods, and examples thereof include a gradual cooling method, a reverse coating method, and a high temperature tempering method. In the present invention, in order to stably precipitate spheroidal carbides, a gradual cooling method is applied. The spheroidizing annealing treatment step is subdivided into a heating and holding step of decomposing the ferrite, leaving the spheroidized core undissolved carbide residue, and a gradual cooling step of growing the spheroidal carbide.

The above heating temperature is controlled in a temperature range of 750 to 850 °C. When the heating temperature is less than 750 ° C, the decomposition of the ferrite is insufficient, and residual iron is also present after the gradual cooling, so that it is not suitable from the viewpoint of suppressing the anti-coking property. On the other hand, when the heating temperature is higher than 850 ° C, the amount of the undissolved carbide is remarkably reduced, and the regenerated wave is generated after the gradual cooling, so that it is not suitable from the viewpoint of suppressing the anti-coking property. After heating, the undissolved carbide is used as the core of the spherical carbide by holding it for about 2 to 20 hours, and it can be controlled to an appropriate form. If the holding time after heating is less than 2 hours, the decomposition of the iron is insufficient, and if it is more than 20 hours, the undissolved carbide is remarkably reduced.

In the subsequent gradual cooling step, the cooling rate is as slow as possible from the viewpoint of growing the spheroidal carbide, and this portion hinders productivity. In the case where the cooling rate is too fast, the spheroidal carbide cannot sufficiently grow to the Ar ₁ metamorphic point, which causes the regenerated wave to be generated. From the viewpoint of simultaneously achieving the growth and productivity of the carbide, 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 equal to or less than the Ar ₁ transformation point, and is preferably about 600 to 700 ° C from the viewpoint of productivity. The cooling after the cooling stop temperature is air cooling, standing cooling, furnace cooling, etc., and can be appropriately selected in accordance with production.

[(B) Nitriding treatment step]

In the nitriding treatment step, since the purpose is to immerse N in steel, the method can be applied to any of the conventional methods. For example, gas nitriding, gas nitrocarburizing, salt bath nitriding, salt bath carbonitrile, ion nitriding, plasma nitriding, soft nitriding, gas carburizing, and the like are listed. In the present invention, as an example of the nitriding treatment, plasma nitrocarburizing treatment and plasma nitriding treatment are applied. Plasma nitrocarburizing treatment, nitriding treatment by glow discharge in a nitrogen-hydrogen-carbon mixed gas, plasma nitriding treatment, nitriding treatment by glow discharge in a nitrogen-hydrogen mixed gas . In the case where finishing such as machining is required, it may be performed before the nitriding treatment, and may be performed after the nitriding treatment in a range that does not affect the nitride layer.

(1) Nitriding treatment temperature: 350~650°C

In the present invention, a predetermined amount of spheroidal carbide is precipitated by nitriding treatment, and diffusion of N to the steel material is promoted, and excellent anti-coking property can be obtained by controlling the composition of the iron nitride which is difficult to cause interatomic bonding. . The lower limit of the treatment temperature is 350 ° C because the diffusion rate of N is lowered when the nitriding treatment temperature is too low, the treatment time is made longer, and the time for obtaining a predetermined spheroidal carbide is also prolonged. Will hinder productivity. The reason why the upper limit is 650 ° C is because the nitriding treatment temperature is too high, it is effective for the growth of carbides, promotes the diffusion of N, and the tempering of the parent phase mother material. When the temperature is too high, the internal hardness is lowered, the characteristics as a gear component are not obtained, and the diffusion reaction inside the surface layer nitrogen is also increased, so that the concentration of nitrogen in the surface layer portion is hindered. By setting the nitriding temperature in the range of 350 to 650 ° C, the characteristics of the gear components are satisfied, and excellent anti-coking properties can be exhibited even in a high sliding environment such as an electric automobile motor. A preferred lower limit of the nitriding temperature is 400 ° C or higher (more preferably 450 ° C or higher), and a preferred upper limit is 625 ° C or lower (more preferably 600 ° C or lower).

(2) Nitriding treatment time: 3~30 hours

The nitriding treatment time (holding time at the time of nitriding treatment) is a time required for the carbide to grow and to diffuse N into the steel to form the iron nitride. Usually, the temperature is time-dependent to determine the amount of carbide, the amount of diffusion of N, and the amount of iron nitride. In the present invention, the range for stably obtaining a predetermined structure is determined by temperature and time, respectively. By setting the nitriding treatment time to 3 to 30 hours, the desired structure can be obtained, and excellent anti-coking property can be exhibited even in a high sliding environment such as an electric automobile motor. In the short time of the nitriding treatment time of less than 3 hours, the spherical carbide and the iron nitride cannot be sufficiently obtained, and if the desired structure is achieved in less than 3 hours, if the temperature is too high, the mother phase is obtained. The disadvantage of softening the base metal. On the other hand, when the nitriding treatment time is longer than 30 hours, the parent phase base material is softened, and in order to avoid this, the spheroidal carbide may not be fully analyzed, and N may not be sufficiently diffused. Disadvantages in steel.

(3) Nitriding treatment environment: nitrogen concentration 30~80%

The nitrogen concentration (N ₂ fraction) of the nitriding treatment environment is excellent in anti-coking property by controlling the diffusion of N into the steel by controlling the composition of the iron nitride which is difficult to cause interatomic bonding. The N ₂ fraction in the environment is less than 30%, and N cannot be sufficiently diffused into the steel, and the required gear characteristics cannot be obtained. On the other hand, when the N ₂ fraction is more than 80% and the amount of diffusion of N is excessively increased, the composition of the iron nitride which is easy to bond between atoms changes again, so that the anti-coking property cannot be improved. A preferred lower limit of the N ₂ fraction is 35% or more (more preferably 40% or more), and a preferred upper limit is 78% or less (more preferably 75% or less).

In the gear portion subjected to the nitriding treatment, a nitride layer having a concentration of iron nitride (iron nitride) of 80% by mass or more is present in the surface layer portion having a depth of 20 μm from the surface, and the nitride layer is formed of iron nitride. The ratio of Fe ₄ N in the middle is preferably 20% by mass or more. By conforming to this requirement, the anti-coking property can be further improved. The reason for exerting this effect is as follows.

Although the adhesive wear is caused by the inter-atomic bonding caused by the metal contact, it is difficult to cause the inter-atomic bonding by increasing the ratio of the iron nitride, and the adhesion wear can be suppressed. In is formed by nitriding a surface layer of iron nitride, although _{Fe 2 ~ 3 N, Fe 4} N type and the like, and wherein the adhesive wear is likely to occur Fe ₄ N, Fe ₄ N so by increasing the The ratio can increase the anti-coking property. The concentration of iron nitride (iron nitride) in the nitrided layer is preferably 85% by mass or more, more preferably 90% by mass or more. The ratio of Fe ₄ N in the iron nitride is preferably 25% by mass or more, more preferably 30% by mass or more.

Among the steel component of the present invention, Cr, Al, and Mo are elements that are easily linked to nitrogen. When the content of these elements is increased, the diffusion rate of nitrogen is slowed down, and the amount of Fe _2~3 N is increased. These elements are added in an appropriate amount (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, Mo is preferably 0.30% or less, and more preferably 0.20%). In the following, the surface layer portion is a nitride layer having a large amount of Fe ₄ N, and adhesion wear can be prevented (for example, refer to Test Nos. 29 to 31 of Example 2 to be described later).

When the nitrocarburizing treatment is performed, the nitrogen concentration in the surface layer portion is increased, and Fe _{2 to 3} N is more easily formed than in the nitriding treatment. In order to increase the ratio of Fe ₄ N, a method of nitriding treatment is suitable (refer to Test Nos. 25 and 26 of Example 2 to be described later). By increasing the nitriding temperature (about 560 to 625 ° C, preferably about 570 to 600 ° C), nitrogen is moderately diffused, and the ratio of Fe ₄ N is increased (refer to Test Nos. 25 and 27 of Example 2 to be described later).

In the gas fraction of the nitriding treatment condition, if the N ₂ fraction is increased, Fe _2~3 N will increase, and if it is decreased, Fe ₄ N will increase. In order to secure a preferred amount of iron nitride and the amount of Fe ₄ N, it is preferable to adjust to an appropriate N ₂ fraction (about 35 to 55%, preferably about 40 to 45%) (refer to the test of Example 2 described later). No. 25 and 28).

In the gear of the present invention, in order to further improve the anti-coking property, the method of performing the lubrication film treatment on the gear surface is also effective. This lubricating film treatment suppresses metal contact, suppresses temperature rise, and suppresses occurrence of adhesive wear. Lubricating film treatment, for example, representative of copper, zinc, lead, etc. Metal oxides such as soft metals and lead oxides, sulfides such as molybdenum disulfide and tungsten disulfide, fluorides, nitrides, graphite, manganese phosphate, etc., are treated in a general manner.

The benefit of the priority is claimed in Japanese Patent Application No. 2012-082614 filed on March 30, 2012. The entire contents of the specification of Japanese Patent Application No. 2012-082614, filed on March 30, 2012, are hereby incorporated by reference.

[Examples]

The present invention will be more specifically described by the following examples, but the present invention is not limited by the following examples, and may be appropriately added and modified insofar as they are suitable for the scope of the subject matter described above, and these are included in the technology of the present invention. range.

(Example 1)

Steel blocks having various chemical compositions shown in the following Tables 1 and 2 were hot forged to a diameter of 32 mm after being produced in a dissolution furnace. The steel is heated and held for 2 to 8 hours at a temperature of 760 to 820 ° C, and then cooled to 625 to 675 ° C at a cooling rate of 10 to 200 ° C / hour, and then subjected to spheroidal annealing treatment of atmospheric cooling. Thereafter, it is processed into a predetermined shape, and quenching (840 ° C × 30 minutes), tempering (160 ° C × 120 minutes), and nitriding treatment (plasma nitrocarburizing treatment) are performed. After the heat treatment, finishing was performed, and the shape of the test piece (the shape of the roller pitting test piece) shown in Fig. 1 was processed to prepare a test piece. Plasma soft nitriding treatment conditions, using the following Scope implementation.

[Plasma soft nitriding conditions]

Nitriding temperature: 300~670°C

Hold time: 1~25 hours

N ₂ fraction: 20 to 85%, C mixed gas fraction (C ₃ H ₈ ): 1%, (remaining part: H ₂ fraction)

At this time, the spheroidizing treatment conditions and plasma nitrocarburizing treatment (hereinafter sometimes referred to simply as "nitriding treatment conditions") of each test are shown below. Tables 3 and 4 (test No. 1 to 24).

With respect to each of the obtained test pieces, the area ratio of the spheroidal carbide in the structure and the nitrogen concentration of the depth of 20 μm from the surface were measured by the following method, and the part characteristics (anti-coking) were evaluated by the following method. Sex).

[Method for Measuring Area Ratio of Spherical Carbide]

The area ratio of the spheroidal carbide in the microstructure (the main structure is tempered granulated iron and/or tempered toughened iron) is measured by scanning electron microscopy (SEM) to determine the depth of 20 μm from the surface of the test piece. . At this time, a sample obtained by nitriding the test piece was cut in a cross section, and the sample was embedded in a resin, and mirror-polished and etched. An arbitrary 9 μm × 12 μm field of view was observed at a magnification of 8000 times, and the image analysis software was used to determine the aspect ratio of the spherical carbide by using an aspect ratio of 0.4 or more as a critical value. The measurement was performed in three fields of view, and the arithmetic mean of the three fields of view was defined as the area ratio of the carbide.

[Method for Measuring Nitrogen Concentration at a Depth of 20 μm from the Surface]

The nitrogen concentration of the surface layer portion (the position at a depth of 20 μm from the surface) is obtained by cutting the test piece in a cross section, embedding it in a resin, and polishing it, using an electron microprobe analyzer (Electron Probe). Analysis by Microanalyzer: EPMA) to determine the nitrogen concentration from the surface portion toward the inside.

[Method of evaluating part characteristics]

As a method of evaluating the characteristics of the parts, a roller pitting test was carried out. For the roller pitting test, the above test piece (small roller) and a large roller (object) made of high carbon chromium bearing steel SUJ2 were used by a roller pitting tester. The test conditions were carried out at a rotation speed of 1000 rpm, a relative slip ratio of 700%, and an oil temperature of 90 ° C, and the number of revolutions before the test apparatus was stopped at the vibration generated by coking was determined. At this time, 20,000 × 10 ³ rotation was taken as the upper limit, and the life was generated as a coking. Those who did not produce coking before 10,000 × 10 ³ turns were evaluated as excellent in anti-coking property.

The results (spherical carbide area ratio, surface layer nitrogen concentration, coking production life), together with the presence or absence of a lubricating film (manganese phosphate for the lubricating film), are shown in Tables 5 and 6 below. The ">20000" displayed in the field of the coking generation life of Table 5 indicates that coking did not occur at 20,000 x 10 ³ rpm.

Based on this result, the following situation can be found. First, in Test Nos. 1 to 14, since the chemical composition and production conditions of the steel material were controlled to an appropriate range, the spherical carbide area ratio and the surface layer nitrogen concentration were controlled within the range defined by the present invention. As a result, the adhesion suppression effect is excellent, and excellent anti-coking property can be exhibited. It is understood that excellent anti-coking properties can be obtained even if a lubricating film is formed.

In Comparative Test Nos. 15 to 24, the chemical composition or manufacturing conditions of the steel were not controlled to an appropriate range, so that the anti-coking property was deteriorated. In other words, in Test No. 15, a steel grade (steel type O) having an excessive C content was used, and the surface layer nitrogen was too concentrated to deteriorate the anti-coking property. Test No. 16 is a steel grade (steel type P) with a small C content, which cannot be confirmed. The spherical area ratio is preserved to deteriorate the anti-coking property.

In Test No. 17, a steel grade (steel type Q) having a small Cr content was used, and even if the production conditions were appropriate, stable spherical carbides could not be analyzed, and the spherical carbide area ratio and the surface layer nitrogen concentration were insufficient. The coking property deteriorates. In Test No. 18, since the amount of nitrogen (N ₂ fraction) at the time of nitriding treatment was too large, the nitrogen was too concentrated in the surface layer to deteriorate the anti-coking property.

In Test No. 19, the amount of nitrogen gas (N ₂ fraction) at the time of nitriding treatment was too small, so that the nitrogen concentration in the surface layer portion could not be secured and the anti-coking property was deteriorated. In Test No. 20, since the nitriding treatment temperature was low, the spherical carbide area ratio could not be secured, and the nitrogen concentration in the surface layer portion was insufficient to deteriorate the anti-coking property.

In Test No. 21, since the nitriding treatment temperature is high, the internal diffusion of nitrogen in the surface layer portion is facilitated, and the nitrogen concentration in the surface layer portion cannot be secured, and the anti-coking property is deteriorated. In Test No. 22, since the holding time at the time of nitriding treatment was short, the nitrogen concentration in the surface layer portion could not be secured and the anti-coking property was deteriorated.

In Test No. 23, the cooling rate during the spheroidization treatment was too fast, and the regenerated wave-forming iron structure covered the entire structure. Therefore, the spherical carbide area ratio could not be secured, and the anti-coking property was deteriorated. Test No. 24 is a steel equivalent to JIS SCM420, and after the carburizing, the lubricating film treatment is performed, and the spheroidizing annealing treatment and the nitriding treatment are not performed. Therefore, the spherical carbide area ratio cannot be ensured, and the surface layer nitrogen is not secured. The concentration is also low, so the anti-coking property is extremely deteriorated.

(Example 2)

Steel blocks of various chemical compositions shown in Table 7 below (steel types N, AA, AB, AC: wherein steel type N is the same as steel type N shown in Table 1 above), hot forged to a diameter after dissolution furnace production : 32mm. The steel is heated and held for 2 to 8 hours at a temperature of 760 to 820 ° C, and then cooled to 625 to 675 ° C at a cooling rate of 10 to 200 ° C / hour, and then subjected to spheroidal annealing treatment of atmospheric cooling. Thereafter, it is processed into a predetermined shape, and quenching (840 ° C × 30 minutes), tempering (160 ° C × 120 minutes), and nitriding treatment (plasma nitrocarburizing treatment or plasma nitriding treatment) are performed. After the heat treatment, finishing was performed, and the shape of the test piece (the shape of the roller pitting test piece) shown in Fig. 1 was processed to prepare a test piece. The same roller pitting test as in Example 1 was not performed.

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

For each of the obtained test pieces, as shown in Example 1, The method was to measure the area ratio of spheroidal carbides in the structure, the nitrogen concentration at a depth of 20 μm from the surface, and the part characteristics (anti-coking property), and the nitride layer composition was evaluated by the following method.

(Measurement method of nitride layer composition)

Part of the test piece was taken out, and X-ray diffraction was performed on the surface layer portion of the test piece (the surface layer portion having a depth of 20 μm from the surface) under the measurement conditions shown below.

(Measurement conditions for X-ray diffraction)

Analysis device: 2-dimensional micro-part X-ray diffraction device "RINT-RAPID II" Co., Ltd. (Rigaku) company system

(1) Analysis conditions

Tube: Co, monochromization: using a monochromator (Kα line), tube output: 40kV-26mA, detector: imaging board (2 dimensions)

(2) Reflection method

Collimator: 300μm, ω angle (X-ray incident angle): 22°~30° swing (1°/sec), Angle (in-plane rotation): fixed, measurement time (exposure): 30 minutes

Based on the obtained X-ray diffraction profile, the compound present in the surface layer of the test piece was identified. The relative concentration of each component was obtained by the semi-quantitative method achieved by peak separation for the identified compound (component). The ratio of the surface iron nitride is a total (100% by mass) of the relative concentrations of all the components present in the surface layer of the test piece confirmed by the X-ray diffraction, and the total of Fe ₄ N and Fe _2~3 N is relatively the concentration ratio (mass%), the so-called iron nitride Fe ₄ N ratio with respect to Fe ₄ N and Fe _{2 ~ 3} N total relative concentrations, the ratio of the relative concentrations of Fe ₄ N (mass%) The value is calculated by the following formula. All the components present in the surface layer of the test piece are Fe ₄ N, Fe _2~3 N, Fe ₅ C ₂ , Fe ₃ C, γ-Fe, α-Fe, etc., and these total ratios are 100%, and A small amount (5% or less) may contain other components (for example, Fe ₂ C, Fe ₂₂ C, Fe ₇ C _{3 ,} etc.) (in the case of inclusion, these components are added to become 100%).

Iron nitride ratio (% by mass) = {relative concentration (% by mass) of Fe ₄ N in the surface layer + relative concentration (% by mass) of Fe _{2 to 3} N in the surface layer}

Fe ratio (mass%) = {Fe ₄ N relative concentration (mass%) / [Fe ₄ N is the relative concentration (% by mass) of the iron nitride of a ₄ N + Fe relative concentration of _{2 ~ 3} N (mass% )]}×100 (%)

The results are summarized as shown in Table 9 below.

As can be seen from the results, in any case, the chemical composition and manufacturing conditions of the steel are controlled to an appropriate range, so that the spherical carbide area ratio and the surface portion nitrogen concentration are controlled within the range prescribed by the present invention. As a result, the adhesion suppression effect is excellent, and excellent anti-coking property can be exhibited. Among them, especially in Test Nos. 26 to 28, 30, and 31, which are also suitable for the composition of the nitride layer, particularly excellent anti-coking property can be obtained.

[Industrial Availability]

The gear of the present invention has a predetermined chemical composition and has a structure of at least one of tempered granulated iron and tempered toughened iron, and a steel structure in which spheroidal carbides having an area ratio of 5% and 30% or less are precipitated. In addition, by setting the nitrogen concentration of the depth of 20 μm from the surface to 2.0 to 6.0%, it is possible to exhibit more excellent anti-coking property in the power transmission portion using the lubricating oil having high rotation, high sliding property, and low dynamic viscosity.

Claims (7)

A gear excellent in anti-coking property, comprising: C: more than 0.80% to 1.30% (representing "% by mass", the same for chemical composition), Si: 0.05 to 1%, Mn: 0.1 to 1% , P: 0.05% or less, S: 0.05% or less, Cr: 0.9 to 2%, Al: 0.01 to 0.1%, and N: 0.02% or less, and the remaining portion is composed of iron and unavoidable impurities, tempered Ma Tian The loose iron and/or the tempered toughened iron has a steel structure in which the spheroidal carbide having an aspect ratio (short diameter/long diameter) of 0.4 or more and 1.0 or less is precipitated at an area ratio of 5% or more and 30% or less; The nitrogen concentration of the surface at a depth of 20 μm is 2.0 to 6.0%. For example, the gear of claim 1 of the patent scope further contains: Mo: 0.5% or less (excluding 0%). For example, the gear of the first application of the patent scope includes: from V: 0.2% or less (excluding 0%), Ti: 0.1% or less (excluding 0%), and Nb: 0.2% or less (excluding 0%). One or more selected from the group consisting of. In the gear of the first aspect of the patent application, the gear further includes one or more selected from the group consisting of Cu: 5% or less (excluding 0%) and Ni: 5% or less (excluding 0%). The gear of the first aspect of the patent application, wherein the surface layer portion having a depth of 20 μm from the surface has a nitride layer having a concentration of iron nitride of 80% by mass or more, and the nitride layer is Fe ₄ in the iron nitride. The ratio of N is 20% by mass or more. For example, the gear of the first paragraph of the patent application, in which the surface is formed There is a lubricating film. For example, the gear of claim 1 of the patent scope, wherein the gear is used for an electric vehicle motor.