JP4352261B2 - gear - Google Patents

Description

  The present invention relates to a gear excellent in pitching strength. More specifically, the present invention relates to a gear excellent in pitching strength subjected to surface hardening treatment such as carburizing quenching or carbonitriding quenching, such as a gear used in a transmission of an automobile or a gear in a transmission shaft.

  Conventionally, parts such as gears and shafts used in automobile transmissions are formed from alloy steels for mechanical structures such as SCM420, SCr420 and SNCM420 specified in JIS G 4053 (2003), and then carburized. After surface treatment or carbonitriding treatment, quenching or induction hardening and surface hardening (hereinafter referred to as “surface hardening treatment such as carburizing quenching, carbonitriding quenching or induction hardening” It may be generically referred to as “curing treatment.”), And then, if necessary, it has been used by tempering at a temperature of 200 ° C. or lower. However, due to recent improvements in engine output and reduction in the size and weight of parts for reducing fuel consumption, the load on a single part tends to increase remarkably. In particular, gears and gears in mission shafts have a problem of so-called “pitting fatigue” in which peeling occurs from the surface due to rolling fatigue accompanied by slippage.

  For this reason, for example, Non-Patent Document 1 shows that the effect of preventing pitching fatigue can be obtained by increasing the hardness under the contact point. In order to satisfy the high pitching strength requested by the industry, tempering is performed at low temperature by carburizing quenching or carbonitriding or after carburizing quenching or carbonitriding (hereinafter referred to as “tempering at low temperature” in this specification). In some cases, it may be referred to as “carburizing and quenching” or “carbonitriding and quenching”.) By doing so, the hardness under the contact point is significantly increased from the hardness of the base material.

  However, simply increasing the hardness under the contact point makes it difficult to obtain the desired high pitching strength. For this reason, molybdenum disulfide is not formed on the tooth surface of the gear after carburizing or carbonitriding. A coating film is formed to improve the solid lubricating effect and improve the pitching strength, and have achieved some results. However, this method cannot always cope with the recent increase in the size of parts and the increase in load due to the increase in engine torque. For this reason, there is a growing demand from industry for pitching strength improvement technology that replaces solid lubrication, such as molybdenum disulfide coating.

  Conventionally, it is known that when a pearlite structure or a bainite structure that is softer than a martensite structure is formed in the surface layer portion of a carburized or carbonitrided and quenched material, the pitching strength is significantly reduced. The pearlite structure and the bainite structure are often generated in the vicinity of the grain boundary oxide layer generated in the surface layer part, and this is because the elements such as Si, Mn, and Cr that improve hardenability are more easily oxidized than Fe. For this reason, it is considered that a deficient layer of Si, Mn and Cr is formed in the vicinity of the grain boundary oxide layer by being preferentially oxidized during the carburizing process or the carbonitriding process.

  Note that recent gears are often subjected to shot peening for the purpose of improving the root fatigue strength to impart compressive residual stress to the root. In this case, since the surface hardness increases simultaneously, the pitching strength is improved. This is because the austenite of the surface layer portion that remains without transformation when performing carburizing quenching or carbonitriding quenching (so-called “residual austenite”) causes stress-induced martensitic transformation by undergoing shot peening treatment, This is thought to be due to the introduction of new dislocations in the martensite.

  Next, Patent Literature 1 and Patent Literature 2 propose a technique for improving the pitching fatigue resistance by increasing the pitching strength.

  That is, Patent Document 1 discloses that machine structural steel is chopped or chopped and shaved, and then surface hardened by carburizing treatment and / or nitriding treatment and quenching / tempering treatment, or after the surface hardening treatment. Further, after shot peening treatment, the tooth surface is barrel-polished to a roughness (Rmax) of 0.3 μm or more and 2 μm or less, or the tooth surface is roughened (Rmax) 0 after gear cutting or gear cutting + shaving processing. A “high contact fatigue strength gear manufacturing method” has been proposed in which barrel polishing is performed to a thickness of 3 μm or more and 2 μm or less, and then surface hardening is performed by carburizing treatment and / or nitriding treatment and quenching / tempering treatment.

  Further, Patent Document 2 has a specific chemical composition, a surface compressive residual stress of 400 MPa or more, an incompletely hardened layer of 5 μm to 15 μm, a surface roughness Rmax of 4.5 μm or less, and a surface roughness distribution. The degree of strain Sk, which is an asymmetry parameter, is −1.2 ≦ Sk <−0.5, and the area ratio of retained austenite on the surface is 10% or less. "Excellent gears" have been proposed.

“The second edition of carburizing and quenching”, page 262, author: Takeshi Naito, publication date: February 26, 1999, publication place: Nikkan Kogyo Shimbun JP-A-6-246548 JP 2002-121644 A

  The techniques disclosed in Patent Documents 1 and 2 described above cannot always ensure excellent pitting fatigue resistance under recent and more severe contact conditions such as a reduction in parts and an increase in load due to an increase in engine torque. .

  For example, in the case of a final gear in which an anti-pitting fatigue characteristic is regarded as important in an automobile transmission, the slip ratio of the gear has recently become very large at 80% or more and is in a very severe contact state. Therefore, under severe contact conditions as described above, if there is a protruding “mountain” portion in the contact area, or conversely, if there is a “valley” portion of several μm, stress concentration occurs around the pitching portion. It will cause a drop in strength. For this reason, in order to increase the pitching strength, it is necessary to positively control the shape of the roughness protrusions so that the height of the roughness protrusions in the contact area is substantially uniform.

  However, in the technique proposed in Patent Document 1, the steel for machine structure is chopped or chopped and shaved, and then surface-hardened by carburizing and / or nitriding and quenching / tempering, or the above After the surface hardening treatment, after shot peening treatment, if the tooth surface is barrel-polished to a roughness (Rmax) of 0.3 μm or more and 2 μm or less, an incompletely quenched portion with low hardness generated during quenching remains. Since the subsequent shot peening process and barrel polishing process are performed, the incompletely quenched portion is preferentially processed. Then, the preferentially processed incompletely hardened portion becomes a “valley” portion having a diameter of several μm as compared with the normal portion, so that stress concentration occurs at the time of contact and the pitching strength is lowered. On the other hand, after gear cutting or gear cutting + shaving, the tooth surface is barrel-polished to a roughness (Rmax) of 0.3 μm or more and 2 μm or less, and then hardened by carburizing and / or nitriding and quenching / tempering. In this case, since the incompletely quenched portion having a low hardness generated at the time of quenching with a depth of about 15 μm remains, the pitching strength is lowered. In any of the above cases, particularly in the case of mass-produced products on an industrial scale, the pitching strength is extremely unstable.

  In the case of the technique proposed in Patent Document 2, the variation in the height of the protrusion is reduced by setting the strain Sk value, which is an asymmetry parameter of the surface roughness distribution, to −1.2 ≦ Sk <−0.5. However, since the roughness protrusions are almost flattened on the surface having the Sk value in this range, there are few places that become oil reservoirs. For this reason, under the very severe contact condition where the slip rate is 80% or more like the final gear used in the final stage of a recent automobile transmission, the damage form becomes seized. As with, it is difficult to increase the pitching strength. Furthermore, in the case of mass-produced products on an industrial scale, it is difficult to completely remove the incompletely hardened layer having a depth of about 15 μm that is generated during carburizing and quenching, and therefore the pitching strength is unstable.

  The object of the present invention is also in the case where a solid lubrication treatment such as a coating of molybdenum disulfide is not performed, such as a gear subjected to a surface hardening treatment, especially a gear used in a transmission of an automobile or a gear in a transmission shaft. It is to provide a gear subjected to surface hardening treatment having high pitching strength. In addition, the specific target of the anti-pitting fatigue characteristic in the gear of the present invention is to have a pitching strength exceeding 2000 MPa in a roller pitching test described later.

  In order to solve the above-described problems, the present inventors have made various investigations and studies on conditions under which the formation of pearlite structure and bainite structure in the surface layer portion can be stably suppressed. As a result, the following findings (a) to (d) were obtained.

  (A) Even if the size of the pearlite structure or bainite structure present in the martensite structure is a minute one having a diameter of about 10 μm, the pitching strength is greatly reduced.

  (B) In order to reduce the grain boundary oxide layer, the contents of Si, Mn and Cr may be reduced. However, even if the Si, Mn, and Cr contents are reduced, the grain boundary oxide layer cannot be completely eliminated, and this is in combination with a decrease in hardenability due to a reduction in the Si, Mn, and Cr contents. Thus, the formation of a pearlite structure and a bainite structure cannot be completely suppressed.

  (C) The pearlite structure and the bainite structure are not generated in all parts in the vicinity of the grain boundary oxide layer, but are formed in a part thereof. And the part which the pearlite structure | tissue and the bainite structure | tissue produced | generated in the vicinity of the grain boundary oxide layer has the density | concentration of Mn, Cr, and Mo lower than the part which the martensite structure | tissue produced | generated in the vicinity of the grain boundary oxide layer.

  (D) Therefore, in order to stably and reliably suppress the formation of the pearlite structure and the bainite structure in the surface layer part, it is only necessary to manage the average content in the materials of Si, Mn, Cr and Mo, which are hardenability improving elements. Insufficient amount of Si, Mn, Cr, and Mo to form martensite even in a region where the content of Si, Mn, and Cr is reduced by the grain boundary oxide layer in the negative segregation part. It must be contained.

  In addition, even when the present inventors suppressed the formation of the pearlite structure and bainite structure of the surface layer portion, because the pitching strength was sometimes low, the surface condition of the test piece and the lubrication conditions are considered to affect the pitching strength, Various studies were conducted. As a result, the following conclusions (e) to (g) were reached.

  (E) The lubrication mode between two contacting surfaces during gear operation is a severe lubrication state such as mixed lubrication or boundary lubrication that does not involve a lubricating oil film. In the above-mentioned lubrication state, since “direct contact” occurs between the “peaks” of the two surfaces that come into contact with each other, it is necessary to determine parameters that can appropriately represent the “peaks” of the surface roughness curve. There is.

  (F) Conventionally, the maximum height “Rz” used as a parameter for surface roughness is defined in JIS B 0601 (2001), but includes both “mountain” and “valley”. It is a parameter and cannot be said to express the influence of only the “mountain” part. That is, the surface whose surface is polished is a surface in which the “mountain” portion is preferentially worn, and the roughness curve has a “valley” portion that is larger than the “mountain” portion. In “Rz”, the influence of the “mountain” portion cannot be expressed directly.

  (G) On the other hand, in JIS B 0671-2 (2002), roughness parameters “Rpk”, “Rk”, and “Rvk” are used. The above “Rpk”, “Rk” and “Rvk” are expressed by separating the protruding peak height, the core level difference and the protruding valley height in the surface roughness load curve in the load moving direction, respectively. It is a thing. For this reason, in order to directly express the influence of the “mountain” portion, it is preferable to use the surface roughness load curve in the load moving direction defined in the JIS B 0671-2 (2002).

Therefore, the present inventors
(H) The protruding valley height “Rvk” represents the valley bottom in the roughness curve and is not involved in “direct contact”.

  (I) On the other hand, the protruding peak height “Rpk” represents a protrusion having a particularly high height in the roughness curve, and thus has a great influence on “direct contact”. Further, the level difference “Rk” of the core part represents a roughness distribution intermediate between “Rvk” and “Rpk”, and it is considered that a specific region of “Rk” affects “direct contact”. In order to directly express the influence of the “mountain” portion, the relationship between “Rpk” and the specific region of “Rk” was examined in more detail. As a result, the following knowledge (j) was obtained.

  (J) In a severe lubrication state without a lubricating oil film, the sum of “Rpk” and “0.5Rk” as a specific region of “Rk”, that is, “Rpk + 0.5Rk”, The influence of the part can be directly evaluated.

  Next, based on the above findings (j), the present inventors further combined the load by carburizing quenching or carbonitriding quenching conditions, surface grinding, shot peening conditions and frequency, and whether or not barrel polishing was performed. In addition to changing the value of “Rpk + 0.5Rk” in the surface roughness in the moving direction, the roller pitching test described later was performed by changing the lubrication conditions by coating with molybdenum disulfide after shot peening. As a result, the following findings (k) to (n) were obtained.

(K) In order to investigate the initial change of “Rpk + 0.5Rk” in the surface roughness in the load transfer direction of the molybdenum disulfide coated product, “Rpk + 0 before the roller pitching test and after 0.5 × 10 ⁵ repetitions. .5Rk "was measured and decreased from 8.18 μm before the test to 0.81 μm. As a result of investigation by EPMA, molybdenum disulfide was not detected on the rolling surface. From this it is clear that the molybdenum disulfide coating is removed by abrasion.

(L) By adjusting the “Rpk + 0.5Rk” in the surface roughness in the load moving direction to a range of 0.2 to 0.8 μm by grinding and polishing, in most cases, 1.0 × 10 ⁷ times After the repetition, a pitching strength equal to or higher than that of the molybdenum disulfide coated product can be secured. However, spalling breakage may occur at an early stage due to internal starting points due to non-metallic inclusions. The spalling failure means that a crack is generated from the inside of a test piece and progresses parallel to the surface and then peels.

  (M) When “Rpk + 0.5Rk” in the surface roughness in the load moving direction is less than 0.2 μm by polishing, seizure occurs on the test piece before pitching occurs.

(N) When “Rpk + 0.5Rk” in the surface roughness in the load moving direction exceeds 0.8 μm, the pitching strength is the molybdenum disulfide coating product after the repetition of 1.0 × 10 ⁷ times (initial wear investigation) The test was carried out separately from the product.) The desired pitching strength (pitching strength exceeding 2000 MPa) cannot be obtained.

  The present invention has been completed based on the above findings, and the gist thereof lies in the gears shown in the following (1) to (5).

(1) By mass%, C: 0.1 to 0.3%, Si: 0.02 to 0.6%, Mn: 0.3 to 1.5%, S: 0.003 to 0.050% , Cr: 0.2 to 2.0%, Al: 0.005 to 0.05% and N: 0.005 to 0.025%, and the balance is composed of Fe and impurities, and O ( Oxygen) is a surface-hardened gear whose base material is a steel material of 0.002% or less and P is 0.025% or less, and the surface roughness of the gear surface in the load movement direction is (1) A gear satisfying the formula.
0.2 μm ≦ Rpk + 0.5 Rk ≦ 0.8 μm (1).
Note that Rpk and Rk respectively represent the protruding peak height (μm) and the core level difference (μm) in the surface roughness load curve in the load movement direction of the gear surface.

  (2) The gear according to (1) above, wherein the base material portion contains Mo: 0.1 to 0.8% in mass% instead of part of Fe.

(3) The gear according to (1) or (2) above, wherein the value of A represented by the following formula (2) is 13 or more.
A = (1 + 0.681Si) × (1 + 3.066Mn + 0.329Mn ² ) × (1 + 2.007Cr) × (1 + 3.14Mo) (2).
In addition, the element symbol in Formula (2) represents the minimum value among the content in the mass% of the element in the area | region within the depth of 0.1 mm from the surface of a gearwheel.

  (4) The base material portion is in mass% instead of part of Fe, Nb: 0.01 to 0.08%, Ti: 0.01 to 0.20%, and V: 0.01 to 0. The gear according to (1) or (2) above, which contains one or more of 20%.

(5) The gear according to (4) above, wherein the value of A represented by the following formula (2) is 15 or more.
A = (1 + 0.681Si) × (1 + 3.066Mn + 0.329Mn ² ) × (1 + 2.007Cr) × (1 + 3.14Mo) (2).
In addition, the element symbol in Formula (2) represents the minimum value among the content in the mass% of the element in the area | region within the depth of 0.1 mm from the surface of a gearwheel.

  The “base material portion” refers to a portion that has not been cured by the surface curing treatment. Therefore, the chemical composition is the same as the chemical composition of the steel material before the surface hardening treatment, that is, the steel material.

  “Surface-treated” specifically means that the average value of Vickers hardness measured at a test force of 2.942 N from the surface to a position of depth of 500 μm at a pitch of 100 μm is 650 or more. Point to.

  Hereinafter, the inventions related to the gears (1) to (5) are referred to as “present invention (1)” to “present invention (5)”, respectively. Also, it may be collectively referred to as “the present invention”.

  Since the gear subjected to the surface hardening treatment of the present invention has excellent pitching strength and has a pitching strength exceeding 2000 MPa in a roller pitching test described later, it is suitable for a gear used in a transmission of an automobile, a gear in a mission shaft, and the like. Can be used. In addition, since the gear subjected to the surface hardening treatment can omit the solid lubrication treatment such as molybdenum disulfide coating, the effect of reducing the product cost is great.

  Hereinafter, each requirement of the present invention will be described in detail. In addition, “%” of the content of the chemical component means “mass%”.

(A) Chemical composition of base material part C: 0.1 to 0.3%
C is an essential element for securing the strength of the base material of the gear subjected to surface hardening treatment such as carburizing quenching, carbonitriding quenching and induction quenching. However, if the C content is less than 0.1%, the above effects are insufficient. On the other hand, if the C content exceeds 0.3%, the hardness of the steel after hot working including hot rolling becomes too high, and the subsequent machinability is markedly lowered. Therefore, the content of C is set to 0.1 to 0.3%.

Si: 0.02 to 0.6%
Si is an effective element for ensuring the hardenability of the base material portion of the gear subjected to surface hardening treatment such as carburizing quenching, carbonitriding quenching and induction quenching, and it is necessary to contain at least 0.02%. However, if its content exceeds 0.6%, the hardness of the steel after hot working becomes too high, and the subsequent machinability is markedly lowered. In addition, when surface hardening is performed by carburizing or carbonitriding, the grain boundary oxide layer is significantly increased during the carburizing or carbonitriding process, and the pitching strength is greatly reduced. Therefore, the Si content is set to 0.02 to 0.6%. The Si content is preferably 0.02 to 0.5%.

Mn: 0.3 to 1.5%
Mn has the effect of increasing the hardenability of the surface hardened layer. Even when the surface is hardened by carburizing or carbonitriding, the effect of increasing the hardenability of the carburized or carbonitrided layer is increased in the grain boundary oxide layer. Greater than adverse effects on For this reason, it is an element effective for increasing the pitching strength. However, if the content is less than 0.3%, the above effect is insufficient. On the other hand, if the Mn content exceeds 1.5%, not only the effect of increasing the pitching strength is saturated, but also the hardness of the steel after hot working becomes too high, and the machinability is greatly reduced. Therefore, the Mn content is set to 0.3 to 1.5%. Note that when the Mn content is 0.4% or more, the improvement of the pitching strength becomes remarkable. For this reason, it is desirable that the Mn content be 0.4 to 1.5%.

S: 0.003 to 0.050%
S combines with Mn to form MnS and has an effect of improving the machinability. However, if the content is less than 0.003%, it is difficult to obtain the above effect. On the other hand, when the S content increases, coarse MnS tends to be generated and the pitching strength tends to be lowered. In particular, when the content exceeds 0.050%, the spalling failure at the internal origin occurs. Thus, a desired pitching strength (pitching strength exceeding 2000 MPa in a roller pitching test described later) cannot be obtained. Therefore, the content of S is set to 0.003 to 0.050%. In addition, it is preferable that content of S shall be 0.003-0.025%.

Cr: 0.2 to 2.0%
Cr has the effect of increasing the hardenability of the surface hardened layer. Even when the surface is hardened by carburizing or carbonitriding, the effect of increasing the hardenability of the carburized or carbonitrided layer is increased in the grain boundary oxide layer. Greater than adverse effects on For this reason, it is an element effective for increasing the pitching strength. However, if the content is less than 0.2%, the above effect is insufficient. On the other hand, when the Cr content exceeds 2.0%, not only the effect of increasing the pitching strength is saturated, but also the hardness of the steel after hot working becomes too high, and the machinability is remarkably lowered. Therefore, the Cr content is set to 0.2 to 2.0%.

Al: 0.005 to 0.05%
Al is an element having a deoxidizing action. Further, Al is an element that is easily bonded to N to form AlN. AlN is effective for refining the crystal grains of the hardened surface layer and has the effect of increasing the pitching strength. However, when the Al content is less than 0.005%, it is difficult to obtain the above-described effect. On the other hand, when Al exceeds 0.05%, in addition to saturation of the above effects, coarse AlN is formed and spalling breakage of the internal origin occurs. Therefore, the content of Al is set to 0.005 to 0.05%.

N: 0.005 to 0.025%
N is easy to form AlN, NbN, VN and TiN by combining with Al, Nb, V and Ti. Among them, AlN, NbN and VN are effective for refining the crystal grains of the surface hardened layer and have a pitching strength. There is an effect to increase. However, if the N content is less than 0.005%, it is difficult to obtain the above effect, while if it exceeds 0.025%, the effect is saturated. Therefore, the N content is set to 0.005 to 0.025%. When the N content is 0.010% or more, the improvement of the pitching strength due to the refinement of crystal grains becomes significant. For this reason, it is desirable that the N content be 0.010 to 0.025%.

  In the present invention, the contents of O (oxygen) and P as impurity elements are limited as follows.

O (oxygen): 0.002% or less O is liable to form hard oxide inclusions by bonding with Al, and lowers the pitching strength. In particular, when the O content exceeds 0.002%, spalling failure at the internal origin occurs, and a desired pitching strength (pitching strength exceeding 2000 MPa in a roller pitching test described later) cannot be obtained. Therefore, the content of O is set to 0.002% or less. It is desirable to reduce the content of O as an impurity element as much as possible. In consideration of the cost in steelmaking, the O content is preferably 0.001% or less.

P: 0.025% or less P segregates at the grain boundary, embrittles the grain boundary, and lowers the pitching strength. In particular, when the P content exceeds 0.025%, the pitching strength is significantly reduced. Therefore, the content of P is set to 0.025% or less. It is desirable to reduce the content of P as an impurity element as much as possible. In consideration of the cost in steelmaking, the P content is preferably 0.020% or less.

  Therefore, the chemical composition of the base material portion of the gear according to the present invention (1) includes the elements from C to N in the above-mentioned range, the balance is made of Fe and impurities, and O (oxygen) in the impurities is 0. It was specified that the content was 002% or less and P was 0.025% or less.

  In addition, in the base material part of the gear according to the present invention, one or more kinds selected from at least one of the first group and the second group, which will be described later, may be used instead of a part of Fe if necessary. These elements may be added and added as optional additional elements.

  Hereinafter, the optional additive element will be described.

First group: Mo: 0.1 to 0.8%
Mo is an element effective for enhancing the hardenability and the pitching strength. However, if the content is less than 0.1%, the above effect is insufficient. On the other hand, if the Mo content exceeds 0.8%, not only the effect of increasing the pitching strength is saturated, but the hardness of the steel after hot working becomes too high, and the machinability is remarkably lowered. Therefore, the Mo content is set to 0.1 to 0.8%.

Second group: Nb: 0.01 to 0.08%, Ti: 0.01 to 0.20% and V: 0.01 to 0.20%
Nb is an element that easily forms NbC, NbN, and Nb (C, N) by combining with C or / and N. NbC, NbN, and Nb (C, N) are effective in complementing the above-described grain refinement of the surface hardened layer by AlN, and have an effect of increasing the pitching strength. In order to reliably obtain this effect, the Nb content is preferably 0.01% or more. However, even if the Nb content exceeds 0.08%, the above effects are saturated and the cost is increased. Therefore, the content of Nb when added is set to 0.01 to 0.08%.

  Ti easily binds to C to form TiC, and this TiC is effective in complementing the above-described grain refinement of the surface hardened layer by AlN and has an effect of increasing the pitching strength. In order to reliably obtain this effect, Ti is preferably contained in an amount of 0.01% or more. On the other hand, if the Ti content exceeds 0.20%, the above effects are saturated and the cost is increased, and coarse TiO2 is formed to cause spalling failure at the internal origin. Therefore, when Ti is added, the content of Ti is set to 0.01 to 0.20%.

  V is an element that is easily bonded to C and N to form VC and VN. Among the above, VN is effective in complementing the above-described grain refinement of the surface hardened layer by AlN and has an effect of increasing the pitching strength. In order to reliably obtain this effect, it is preferable that V is a content of 0.01% or more. However, even if the content of V exceeds 0.20%, the above effect is saturated and the cost is increased. Therefore, when V is added, the content of V is set to 0.01 to 0.20%.

  In addition, said Nb, Ti, and V can be added only with any 1 type or 2 or more types of composite.

  For the above reasons, the chemical composition of the base material part of the gear according to the present invention (2) is replaced with a part of Fe of the base material part of the gear according to the present invention (1), and Mo: 0.1 to 0. It was specified to contain 8%.

  Further, the chemical composition of the base material part of the gear according to the present invention (4) is replaced with a part of Fe of the base material part of the gear according to the present invention (1) or the present invention (2), Nb: 0.01 -0.08%, Ti: 0.01-0.20% and V: It specified that it contained 1 type (s) or 2 or more types in 0.01-0.20%.

  In addition, about Cu and Ni mixed as an impurity in steel, the content in particular is not prescribed | regulated.

(B) Surface roughness of the gear surface in the load movement direction The gear according to the present invention is represented by “0.2 μm ≦ Rpk + 0.5Rk ≦ 0.8 μm” in the surface roughness of the gear surface in the load movement direction (1 ) Must be satisfied.

  First, “Rpk + 0.5Rk” is focused on, for example, in the case of final gears in which the anti-pitching characteristics are important in recent automobile transmissions, the slip ratio of the gears is as high as 80% or more during gear operation. Since the lubrication mode between the two surfaces in contact with each other is in a state where no lubricating oil film is interposed, the “mountain” portions of the two surfaces in contact with each other are directly contacted, so the “mountain” portion of the surface roughness curve is appropriately evaluated. Because.

  That is, as described above, the maximum height “Rz” conventionally used as a parameter for surface roughness is defined in JIS B 0601 (2001). The surface of the polished surface is a surface where the “mountain” portion has been preferentially worn, and the roughness curve is larger in the “valley” portion than the “mountain” portion. ing. For this reason, the maximum height “Rz” cannot directly express the influence of only the “mountain” portion.

  On the other hand, out of the protruding valley height “Rvk”, the protruding peak height “Rpk” and the core level difference “Rk” which are roughness parameters defined in JIS B 0671-2 (2002) Since “Rvk” represents a valley bottom in the roughness curve, it does not participate in “direct contact”, but “Rpk” represents a particularly high protrusion in the roughness curve. Greatly affects “direct contact”. Further, the level difference “Rk” of the core portion represents a roughness distribution intermediate between “Rvk” and “Rpk”, and a specific region of “Rk” affects “direct contact”. The sum of “0.5 Rk” and “Rpk” as a specific region of “Rk”, that is, “Rpk + 0.5 Rk”, influences the “mountain” portion in “direct contact” that is a severe lubrication state without a lubricating oil film. Can be evaluated directly.

  If “Rpk + 0.5Rk” in the surface roughness in the load moving direction is less than 0.2 μm, seizure occurs. On the other hand, if the thickness exceeds 0.8 μm, the amount of wear on the contact surface increases, and a desired pitching strength (pitching strength exceeding 2000 MPa in a roller pitching test described later) cannot be obtained.

  Therefore, regarding the gear according to the present invention, “Rpk + 0.5Rk” in the surface roughness in the load moving direction of the gear surface is defined as 0.2 to 0.8 μm. A preferable range of “Rpk + 0.5Rk” is 0.2 to 0.6 μm.

  In addition, in order to set “Rpk + 0.5Rk” in the surface roughness in the load movement direction of the gear surface to 0.2 to 0.8 μm, for example, the gear shape is changed under specific conditions as shown in the examples described later. After roughing and surface hardening treatment, honing and shot peening are performed, and then barrel polishing is performed as a final finish.

(C) Chemical component in a region within a depth of 0.1 mm from the gear surface The gear according to the present invention (1) or the present invention (2) is a region within a depth of 0.1 mm from the gear surface (hereinafter referred to as “gear surface layer”). Part).) By specifically optimizing the chemical component in (2), specifically, by setting the value of A represented by the formula (2) to 13 or more, the pearlite structure and bainite structure in the surface layer part Generation can be suppressed stably and reliably.

  In the gear according to the present invention (4), by optimizing the chemical component in the gear surface layer portion, specifically, by setting the value of A represented by the above formula (2) to 15 or more, Generation of a pearlite structure and a bainite structure in the surface layer portion can be stably and reliably suppressed.

  Hereinafter, the above matters will be described in detail.

  The present inventors melted steels a to k shown in Table 1 in a 150 kg vacuum melting furnace, and then cast them into ingots using cast iron (hereinafter referred to as “normal mold”) as the mold. At the time of dissolution, careful attention was paid to the selection of raw materials so that the impurity elements were sufficiently reduced.

  Next, each ingot was held at 1200 to 1280 ° C. for 10 hours or more and subjected to a solution treatment, allowed to cool to room temperature, then heated again to 1250 ° C. and held for 1 hour, and then the finishing temperature was 900 Hot forging was performed to ˜1000 ° C. and the mixture was allowed to cool to room temperature to produce a steel bar having a diameter of 35 mm.

  Next, each of these steel bars was subjected to a heat treatment that was held at 925 ° C. for 1 hour, allowed to cool to room temperature, and further peeled to a diameter of 25 mm.

  Each steel bar having a diameter of 25 mm thus obtained was cut into a length of 300 mm, further subjected to center hole processing, and then subjected to gas carburizing and tempering under the conditions shown in No. 1 in Table 2. In Table 2, “CP” means “carbon potential”, and “NP” means “nitrogen potential”.

  Next, each steel bar was cut to a length of 150 mm, a sample having a length of 10 mm was further taken from the cut surface, and then cut through the center parallel to the axis. After embedding in a resin so that this surface becomes an observation surface and mirror polishing, each element of Si, Mn, Cr and Mo was subjected to line analysis using EPMA. In the line analysis by EPMA, the beam diameter was 1 μm, the scanning speed was 200 μm / min, the scanning length was 100 μm, and 10 points were measured perpendicularly from the surface to the axis. C is also known as an element that easily segregates, but C was not measured because it easily and uniformly diffuses when heated in the austenite region.

  From the measurement results with EPMA, the contents of Si, Mn, Cr and Mo were quantified at the positions where the respective contents of Si, Mn, Cr and Mo were the lowest. Here, since the segregation tendency of Si, Mn, Cr, and Mo is the same, the contents of Si, Mn, Cr, and Mo are quantified at positions where the respective contents of Si, Mn, Cr, and Mo are the lowest. Then, it can be evaluated as the value of A represented by the above formula (2).

  In addition, hardenability is, for example, in Inoue Kei's 131st and 132rd Nishiyama Memorial Lecture "Present and Future of Material Prediction and Control Technology of Steel Materials", pages 215 to 217 (Japan Steel Association, September 1989). (Issued on the 25th) can be estimated from the content of C and other alloy elements and the austenite grain size, and in order to improve the pitching strength aimed by the present invention, carburizing quenching, carbonitriding quenching and high frequency The hardenability of the surface layer that has been subjected to surface hardening treatment such as quenching is important. In particular, when surface hardening is performed by carburizing or carbonitriding, the general C content in the surface layer is often about 0.8%, and the austenite grain size is Nb, Ti In the case where neither of N and V is contained, the particle size number is about 9, and in the case of containing one or more of Nb, Ti and V, the particle size number is often about 11, so that any of Nb, Ti and V is Can be evaluated from the contents of Si, Mn, Cr, and Mo by distinguishing between the case of not containing Cu and the case of containing one or more of Nb, Ti, and V.

  Therefore, the present inventors adopt the value of the equation (2), that is, the value of A as the evaluation standard of the hardenability based on the above-mentioned “present state and future of material prediction / control technology of steel materials”. did.

  Table 3 shows the value of A represented by the formula (2) and the contents of Si, Mn, Cr and Mo corresponding to the value for each sample.

  In addition, the mirror-polished sample is subjected to nital corrosion, and then an area of 100 μm from the surface layer is observed using a SEM (scanning electron microscope) to investigate the presence of a bainite structure and a pearlite structure. It was.

  Table 3 also shows the survey results regarding the presence of the bainite structure and the pearlite structure. Further, FIG. 1 shows the results of the investigation on the presence of the bainite structure and the pearlite structure, organized by the value of A expressed by the above equation (2).

  In Table 3, “None” and “Present” in the “Perlite structure, bainite structure” column indicate that “Neither bainite structure nor pearlite structure exists” and “One of bainite structure and pearlite structure, respectively. Or both exist. "

  Furthermore, “O: No pearlite, no bainite” and “X: No pearlite, bainite” in FIG. 1 are respectively “no bainite structure and no pearlite structure” and “any of bainite structure and pearlite structure, respectively. It means that one or both exist. In FIG. 1, a case where none of Nb, V and Ti is included is expressed as “Nb, V and Ti are absent”, and a case where one or more of Nb, V and Ti are included is “ “Nb, V, Ti present”.

  In addition, the value of A represented by the formula (2) is simply expressed as “A value” in FIG.

  From Table 3 and FIG. 1, when none of Nb, Ti and V is included, if the value of A represented by the formula (2) is 13 or more, neither bainite structure nor pearlite structure exists. It is clear. Further, when one or more of Nb, Ti and V are included, it is clear that neither the bainite structure nor the pearlite structure exists if the value of A represented by the formula (2) is 15 or more. It is.

  Therefore, in the gear according to the present invention (3), in the gear according to the present invention (1) or the present invention (2), the value of A represented by the formula (2) is defined as 13 or more.

  Further, in the gear according to the present invention (5), in the gear according to the present invention (4), the value of A represented by the formula (2) is defined as 15 or more.

In addition, the average composition of steel, the solidification speed, the solidification form, etc. influence the value of A represented by the above-mentioned formula (2) in the steel material section. It is also affected by steelmaking facilities.

  For this reason, in order to make the value of A represented by the above formula (2) in a gear not including any of Nb, Ti, and V to be 13 or more, for example, a large cross section of 400 mm × 300 mm square by continuous casting. When producing a bloom, first, the average composition of steel is melted so that the value of A represented by the formula (2) is 20 or more. The molten steel is sufficiently cast after being electromagnetically stirred, and further subjected to homogenization heat treatment at 1200 to 1280 ° C. for 8 hours or longer to make the bloom into a square billet with a side of 200 mm or less. Is heated at 1200 to 1280 ° C. for 2 hours or more and then hot rolled so that the rolling finish temperature is 850 to 1000 ° C.

  In addition, in order to increase the value of A represented by the above formula (2) in a gear including one or more of Nb, Ti, and V to 15 or more, for example, a bloom having a large cross section of 400 mm × 300 mm square by continuous casting. First, the average composition of steel is melted so that the value of A represented by the formula (2) is 23 or more. The molten steel is sufficiently cast after being electromagnetically stirred, and further subjected to homogenization heat treatment at 1200 to 1280 ° C. for 8 hours or longer to make the bloom into a square billet with a side of 200 mm or less. Is heated at 1200 to 1280 ° C. for 2 hours or more and then hot rolled so that the rolling finish temperature is 850 to 1000 ° C.

  The “surface hardening treatment” for the gear according to the present invention may be a normal surface hardening treatment such as carburizing and quenching, carbonitriding and induction hardening.

  In the case of performing carburizing quenching or carbonitriding quenching as the “surface hardening treatment”, for example, after cutting a part, it is heated to 930 ° C. and kept in an atmosphere having a carbon potential of 1.1%, and carburizing is performed. And after the diffusion treatment, it is cooled to 850 ° C., maintained in an atmosphere with a carbon potential of 0.8%, and after adjusting the surface carbon concentration, it is immersed in oil at an oil temperature of 60 ° C. and then oil-quenched, And tempering at 120 ° C. for 2 hours.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  Steels A to P having chemical components shown in Table 4 were dissolved.

  Among the above steels, Steel A, Steel B, Steel D to F, Steel K, and Steel L were melted in a 150 kg vacuum melting furnace and cast into an ingot using a normal mold. Steel G to I and steel M were melted in a 30 kg vacuum melting furnace and cast into an ingot using a normal mold.

  Steels N to P were melted in a 3 t (ton) atmospheric melting furnace, and a 450 mm × 550 mm ingot was manufactured using a normal mold.

  Further, Steel C and Steel J were melted in a 70 t (ton) converter, subjected to RH vacuum degassing for a long time by secondary refining, and further, after sufficient electromagnetic stirring of the molten steel, was performed by continuous casting. A 400 mm × 300 mm square bloom was produced.

  Steels A to J in Table 4 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels K to P in Table 4 are steels whose chemical compositions deviate from the definition of the present invention.

  The following process was performed on the ingot and bloom obtained as described above.

  First, ingots of steel A, steel B, steel D to F, steel K and steel L melted in a 150 kg vacuum furnace are heated at 1250 ° C. for 8 hours, and then hot so that the finishing temperature is 900 to 1000 ° C. The steel bar was forged and allowed to cool to room temperature to produce a steel bar having a diameter of 35 mm. Next, each of these bar steels was subjected to a heat treatment that was held at 925 ° C. for 1 hour and allowed to cool to room temperature.

  Ingots of steel G to I and steel M melted in a 30 kg vacuum furnace were heated at 1250 ° C. for 8 hours, then hot forged to a finish temperature of 900 to 1000 ° C., allowed to cool to room temperature, and a diameter of 35 mm A steel bar was prepared. Next, each of these bar steels was subjected to a heat treatment that was held at 925 ° C. for 1 hour and allowed to cool to room temperature.

  Ingots of steel N to P melted in a 3 t (ton) atmosphere furnace were heated at 1350 ° C. for 4 hours and then rolled into a 180 mm × 180 mm square billet. Next, the above-mentioned square billet was heated at 1250 ° C. for 2 hours, then hot-rolled to a steel bar having a diameter of 35 mm at a rolling finishing temperature of 850 to 1000 ° C., allowed to cool to room temperature, and further heated at 925 ° C. A heat treatment was performed for a period of time, and the mixture was allowed to cool to room temperature.

  Further, the blooms of Steel C and Steel J were heated at 1250 ° C. for 10 hours, and then rolled into pieces of 180 mm × 180 mm square billets. Next, the square billet was heated at 1250 ° C. for 2 hours, then hot rolled into a steel bar having a diameter of 35 mm at a rolling finishing temperature of 850 to 1000 ° C., allowed to cool to room temperature, and further at 925 ° C. for 1 hour. Apply heat treatment to hold. Allowed to cool to room temperature.

  From each steel bar 35 mm in diameter of steel A to P obtained in this way, a small roller coarse for a roller pitching test comprising a test part 1 having a diameter of 26 mm and a length of 28 mm and a grip part 2 having a length of 51 mm shown in FIG. The product was cut out, carburized or carbonitrided under the conditions shown in Table 2 and FIG. 3, and oil-quenched, and then tempered at 180 ° C. for 2 hours. As already mentioned, “CP” and “NP” in Table 2 mean “carbon potential” and “nitrogen potential”, respectively.

  After the tempering process, the grip part 2 was finished, and the test part 1 was subjected to grinding, shot peening and barrel polishing to finish the surface. The grip 2 was masked so that it was not affected by shot peening or barrel polishing.

  The specific surface finishing method of the test part 1 of the small roller for roller pitching test is as shown in the following (a) to (f).

  (A) A silicon carbide type abrasive grain having a diameter of 50 to 100 μm and water are mixed, and a liquid pressure honing is performed for 20 minutes with a processing pressure of 3 MPa and a test piece rotation speed of 150 times per minute (hereinafter referred to as “rpm”). Implemented. Next, silicon carbide abrasive grains having a diameter of 5 to 50 μm are mixed with water, liquid honing is performed for 30 minutes with a processing pressure of 3 MPa and a test piece rotation speed of 150 rpm, and after a mirror finish, a shot with a diameter of 0.6 mm Shot peening is performed on a ball (steel wire) under conditions where the projection pressure is 0.35 MPa and the coverage is 300%. Finally, barrel polishing was performed for 60 minutes with a gyro barrel polishing machine using alumina abrasive grains having a diameter of 300 to 2000 μm.

  (B) A silicon carbide abrasive grain having a diameter of 50 to 100 μm and water are mixed, and a liquid honing is performed for 20 minutes at a processing pressure of 3 MPa and a test piece rotation speed of 150 rpm. Next, silicon carbide abrasive grains having a diameter of 5 to 50 μm are mixed with water, liquid honing is performed for 30 minutes with a processing pressure of 3 MPa and a test piece rotation speed of 150 rpm, and after a mirror finish, a shot with a diameter of 0.6 mm Shot peening is performed on a sphere (steel wire) at a projection pressure of 0.35 MPa and a coverage of 300%, and then a shot sphere (steel wire) having a diameter of 0.3 mm is further projected at a projection pressure of 0.35 MPa and a coverage of Shot peening is performed at 300%. Finally, barrel polishing was performed for 60 minutes with a gyro barrel polishing machine using alumina abrasive grains having a diameter of 300 to 2000 μm.

  (C) Silicon carbide abrasive grains having a diameter of 50 to 100 μm and water are mixed, and liquid honing is performed for 20 minutes at a processing pressure of 3 MPa and a test piece rotation speed of 150 rpm. Next, barrel polishing was performed for 60 minutes with a gyro barrel polishing machine using alumina abrasive grains having a diameter of 300 to 2000 μm.

  (D) The honing with a solid whetstone is implemented by changing the rotation direction of the whetstone so that the machine marks remain in the circumferential direction and the axial direction. Next, shot peening was performed on a shot ball (steel wire) with a diameter of 0.6 mm under the conditions of a projection pressure of 0.35 MPa and a coverage of 300%. Finally, barrel polishing was performed for 60 minutes with a gyro barrel polishing machine using alumina abrasive grains having a diameter of 300 to 2000 μm.

  (E) Honing with a solid grindstone is performed by changing the rotation direction of the grindstone so that the machine marks remain in the circumferential direction and the axial direction. Next, shot peening was performed on a shot sphere (steel wire) having a diameter of 0.6 mm under conditions where the projection pressure was 0.35 MPa and the coverage was 300%, and then a shot sphere (steel wire) having a diameter of 0.3 mm was further projected. Shot peening was performed under conditions of 0.35MPa pressure and 300% coverage. Finally, barrel polishing was performed for 60 minutes with a gyro barrel polishing machine using alumina abrasive grains having a diameter of 300 to 2000 μm.

  (F) Shot peening was performed on a shot ball (steel wire) having a diameter of 0.6 mm under the conditions of a projection pressure of 0.35 MPa and a coverage of 300%.

  In addition, by performing liquid honing before shot peening as described in the above (a) and (b), the influence of the rough processing of the test section 1 in the small roller rough shape product is removed, and the roughness after shot peening is reduced. Variations can be suppressed.

  Various surface roughness of the test part 1 was measured about each small roller which gave surface finish. That is, the surface roughness was measured at a speed of 0.3 mm / second and a scanning length of 3 mm in the circumferential direction of the test part 1, and the protruding peak height “Rpk” defined in JIS B 0671-2 (2002) The level difference “Rk” of the core portion and the degree of distortion “Sk” and “Rmax”, which are asymmetry parameters of the surface roughness distribution described in Patent Document 2, were obtained. For the above “Sk” and “Rmax”, the degree of distortion Rsk and the maximum height Rz defined in JIS B 0601 (2001) were used, respectively.

  “Rpk”, “Rk”, “Sk”, and “Rmax” were converted to roughness around the circumferential surface by performing least square curve correction on the surface roughness measured as described above.

  In addition, a roller pitching test was performed under the conditions shown in Table 5 using a small roller manufactured as described above and a large roller made of SUJ2 specified in JIS G 4805 (1999).

  The large roller made of SUJ2 has a diameter of 130 mm as shown in FIG. 4 and a contact portion length of 7 mm. After spheroidizing annealing, test piece processing, quenching, and tempering, silicon carbide-based abrasive grains having a diameter of 50 to 100 μm And water are mixed, the processing pressure is 3 MPa, the test piece rotation speed is 150 times per minute, and the liquid honing is performed for 20 minutes. Then, the silicon carbide abrasive grains having a diameter of 5 to 50 μm are mixed with water and the processing pressure is increased. Was prepared by performing liquid honing for 30 minutes at 3 MPa and a test piece rotation speed of 150 rpm.

The number of tests in the roller pitching test was set to 7 each, an SN diagram was prepared with the vertical axis representing the surface pressure and the horizontal axis representing the number of repetitions until failure, and the surface pressure at the number of repetitions of 1.0 × 10 ⁷ times. Was defined as the pitching strength.

As a small roller, the pitching strength when using the steel J coated with molybdenum disulfide coating on the surface finish of the test part 1 by the above method (f) is used as a reference value, and the goal is to exceed this. did. In addition, about the small roller which gave the molybdenum disulfide coating to the said steel J, a halfway stop test was implemented separately from the durability test to 1.0 * 10 < ⁷ > repetition number, and 0.5 * 10 < ⁵ > repetition number Later “Rpk” and “Rk” were also measured. Further, by analyzing the rolling surface of the sample with EPMA, it was confirmed that the concentrated surface of Mo and S was lost on the rolling surface.

  After the roller pitching test, the test part 1 of each small roller was cut out, the cross section cut through the axis parallel to the axis was mirror-polished, and Si in a part 5 mm from the end (a part not in contact with the large roller) , Mn, Cr and Mo were subjected to line analysis using EPMA. In the line analysis by EPMA, the beam diameter was 1 μm, the scanning speed was 200 μm / min, the scanning length was 100 μm, and 10 points were measured from the surface on the carburized surface side perpendicular to the axis. From the measurement results with EPMA, the contents of Si, Mn, Cr and Mo were quantified at the positions where the respective contents of Si, Mn, Cr and Mo were the lowest. Moreover, the Vickers hardness (henceforth Hv hardness) was measured from the carburized surface side surface using the said sample. The Hv hardness from the surface to a position of depth of 500 μm at a pitch of 100 μm was measured with a test force of 2.942 N, and an average value of 5 points was calculated.

  Tables 6 and 7 show the results of the above tests. Note that the numbers in the “surface hardening treatment condition” column in Tables 6 and 7 correspond to the treatment numbers in Table 2. The “surface finishing” column means that the surface finishing of the test part 1 was performed by the surface finishing methods (a) to (f). In Tables 6 and 7, “Rpk + 0.5Rk” was calculated from the surface roughness parameters “Rpk” and “Rk” defined in JIS B 0671-2 (2002). The “Hv hardness of the surface layer portion” column shows an average value of the above five Hv hardness values.

  From Table 6 and Table 7, in the case of Test No. 7, Test No. 8, Test No. 18-23 and Test No. 25-31 that deviated from the conditions specified in the present invention, the pitching strength in the roller pitching test is It is apparent that the test number 32 does not reach 2000 MPa which is the pitching strength.

  That is, since steel B and steel J whose chemical composition is within the range specified in the present invention were used, the test composition 7, test number 18 and test number 22 whose chemical composition of the base metal part was also within the range specified by the present invention. In the case of (1), surface finishing was performed by the method (c) of barrel polishing without roughening the surface by shot peening after the liquid honing, so that “Rpk + 0.5Rk” in the surface roughness in the load moving direction is 0.13 μm, 0. As low as 12 μm and 0.12 μm. For this reason, seizure frequently occurred and the pitching strength did not reach the target.

  Since Steel B and Steel J were used, in the case of Test No. 8, Test No. 19, Test No. 21, Test No. 23 and Test No. 25 in which the chemical composition of the base material is within the range specified in the present invention, after shot peening Since the surface was finished by the method (f) without grinding, “Rpk + 0.5Rk” in the surface roughness in the load moving direction was 2.52 μm, 3.31 μm, 2.30 μm, 2.84 μm and 2.22 μm, respectively. And big. For this reason, the pitching strength did not reach the target.

  In addition, since steel J was used, in the case of test number 20 in which the chemical composition of the base metal part is within the range specified in the present invention, an incompletely quenched structure is formed after carburizing and quenching at an oil temperature of 200 ° C. However, since this could not be removed by liquid honing, the surface of the carburized hardened product quenched with oil at an oil temperature of 60 ° C. was roughened by shot peening, and “Rpk + 0.5Rk” was 0.8 or more. The pitching strength did not reach the target.

  Furthermore, since steels K to P whose chemical composition deviated from the provisions of the present invention were used, among the test numbers 26 to 31 in which the chemical composition of the base material part also deviated from the prescriptions of the present invention, the test numbers 26 to 30 are internal. Spalling failure starting from non-metallic inclusions occurred, and the pitching strength did not reach the target. In Test No. 31, since the P content exceeded the definition of the present invention, the pitching strength did not reach the target.

  On the other hand, in the case of the test numbers 1 to 6, the test numbers 9 to 17, and the test number 24 that satisfy the conditions defined in the present invention, the pitching strength in the roller pitching test is the pitching strength of the test number 32 as a reference. It is clear that this is a large value exceeding 2000 MPa. In Test No. 24, the oil temperature during the oil quenching was 200 ° C., but since carbonitriding quenching was used as the surface hardening treatment, an incompletely quenched structure was not generated.

  Among the test numbers that satisfy the conditions defined in the present invention, a test in which the minimum value of A represented by the formula (2) is 13 or more when none of Nb, Ti, and V is included The pitching strengths of No. 9, Test No. 11 and Test No. 13 are even better, and when one or more of Nb, Ti and V are included, the minimum value of A represented by the above formula (2) is 15 The above test numbers 2-6, test number 12, test numbers 14-17, and test number 24 have better pitching strength.

  The present invention has been specifically described with reference to one embodiment. However, the present invention is not limited by the above embodiment, and may be implemented with appropriate modifications within a range that can be adapted to the above-described purpose. All of which are within the scope of the present invention.

  The gear subjected to the surface hardening treatment of the present invention is excellent in pitching strength and has a pitching strength exceeding 2000 MPa in a roller pitching test. Therefore, it is used for a gear used in a transmission of an automobile, a gear in a mission shaft, or the like. Can do. In addition, since the gear subjected to the surface hardening treatment can omit the solid lubrication treatment such as molybdenum disulfide coating, the effect of reducing the product cost is great.

(2) A bainite structure and a pearlite structure in the case where the minimum value of A (expressed as “A value” in the drawing) and the content of Nb, Ti, and V represented by the formula and the region of 100 μm from the surface layer are observed by SEM It is a figure shown about the influence which it has on the presence or absence of presence. It is a figure which shows the shape of the small roller used by the roller pitching test. It is a figure explaining the carburizing process or carbonitriding process implemented in the Example. It is a figure which shows the shape of the large roller used by the roller pitching test.

Explanation of symbols

1: Test section of small roller for roller pitching test,
2: Gripping part of small roller for roller pitching test

Claims (5)

In mass%, C: 0.1 to 0.3%, Si: 0.02 to 0.6%, Mn: 0.3 to 1.5%, S: 0.003 to 0.050%, Cr: 0.2 to 2.0%, Al: 0.005 to 0.05% and N: 0.005 to 0.025%, the balance is composed of Fe and impurities, O (oxygen) in the impurities is 0.002% or less, P is a surface-hardened gear whose steel base is 0.025% or less, and the surface roughness of the gear surface in the load movement direction satisfies the following formula (1) A gear characterized by that.
0.2 μm ≦ Rpk + 0.5 Rk ≦ 0.8 μm (1)
Note that Rpk and Rk respectively represent the protruding peak height (μm) and the core level difference (μm) in the surface roughness load curve in the load movement direction of the gear surface.   The gear according to claim 1, wherein the base material portion contains Mo: 0.1 to 0.8% in mass% instead of part of Fe. The gear according to claim 1 or 2, wherein the value of A represented by the following formula (2) is 13 or more.
A = (1 + 0.681Si) × (1 + 3.066Mn + 0.329Mn ² ) × (1 + 2.007Cr) × (1 + 3.14Mo) (2)
In addition, the element symbol in Formula (2) represents the minimum value among the content in the mass% of the element in the area | region within the depth of 0.1 mm from the surface of a gearwheel.   Instead of part of Fe, the base material part is in mass%, Nb: 0.01 to 0.08%, Ti: 0.01 to 0.20%, and V: 0.01 to 0.20%. The gear according to claim 1 or 2, comprising one or more of them. The gear according to claim 4, wherein the value of A represented by the following formula (2) is 15 or more.
A = (1 + 0.681Si) × (1 + 3.066Mn + 0.329Mn ² ) × (1 + 2.007Cr) × (1 + 3.14Mo) (2)
In addition, the element symbol in Formula (2) represents the minimum value among the content in the mass% of the element in the area | region within the depth of 0.1 mm from the surface of a gearwheel.

Images