JP4872846B2 - Rough shape for nitriding gear and nitriding gear - Google Patents

Description

  The present invention relates to a rough product for a nitriding gear and a nitriding gear. More specifically, the present invention relates to a nitrided gear excellent in wear resistance and pitching strength used for automobile gears and the like, and a rough product for a nitrided gear suitable for use as a material for the gear. The nitrided gear refers to a gear having a “nitrided layer” that is a hardened layer.

  Since gears used in automobile transmissions require wear resistance, bending fatigue strength, and pitching strength, surface hardening is usually performed.

  Typical surface hardening treatments include, for example, a treatment for forming a “quenched layer” as a “hardened layer” such as “carburizing quenching” and “high frequency quenching”, and “nitriding” and “soft nitriding” An example of such a “cured layer” is a process of forming a “nitride layer”.

The above-mentioned carburizing and quenching is generally a treatment in which low carbon steel is used as material steel (raw steel), C is intruded and diffused in a high temperature austenite region of Ac ₃ or higher, and then quenched. For this reason, although high surface hardness and a large hardened layer depth can be obtained, there is a problem that heat treatment deformation (heat treatment strain) becomes large because the treatment involves transformation, and high precision is required for the parts. Requires finishing such as grinding and honing after carburizing and quenching.

In addition, induction hardening is a process of rapid heating in a high-temperature austenite region of Ac ₃ or higher and then quenching and quenching, and although the adjustment of the hardened layer depth is relatively easy, as in carburizing. Medium carbon with higher C content than carburizing and quenching as material steel for obtaining desired surface hardness, hardened layer depth, and core hardness, because it is not a treatment for intruding and diffusing C. It is common to use steel. However, since medium carbon steel is harder than low carbon steel, machinability when machining into a part shape is low, and it is necessary to produce a high-frequency heating coil suitable for the shape for each part.

Compared to the cases of “carburization quenching” and “induction quenching” in which “hardened layer” is formed as “hardened layer”, “nitriding” and “softening” in which “nitrified layer” is formed as “hardened layer” In the case of “nitriding”, since the treatment is performed at a temperature of Ac ₁ point or less, no transformation is involved, and since the treatment temperature is low, the heat treatment is small. Therefore, it is often used as a surface hardening treatment for gears that require high accuracy, and in particular, high surface hardness by intruding and diffusing N and C in a temperature range of about 500 to 600 ° C. below Ac ₁ point. “Soft nitriding” in which the processing time is as short as several hours, and NH ₃ is added to RX gas (“RX gas” is a trademark of endothermic modified gas) as a processing atmosphere. Since the used gas can be used, it is frequently used as a process suitable for mass production.

However, the “nitrided layer” formed on the surface of the article to be processed when the “nitriding” or “soft nitriding” treatment is performed is the outermost porous layer (hereinafter referred to as “porous layer”) and It consists of a compound layer mainly composed of an ε-Fe _2-3 N phase composed of a dense layer below it and a diffusion layer below it.

  The compound layer formed on the surface is very brittle and easily peels from the surface in a rolling contact state. Therefore, when “nitriding” or “soft nitriding” is performed on the gear, the gear may be easily peeled off at the initial stage of meshing. And when the said compound layer peels, an unevenness | corrugation will be formed in the surface of a gearwheel and it will act as a local stress concentration location, Therefore Abnormal wear and pitching had generate | occur | produced.

  For this reason, there is a great demand for a nitrided gear excellent in wear resistance and excellent in pitting resistance and a rough product for a nitrided gear suitable for use as a material thereof.

  Therefore, in order to meet the above-described demands, for example, Patent Documents 1 to 3 propose nitriding steel and soft nitriding gears and methods for producing them.

Specifically, Patent Document 1 describes that “in a nitriding atmosphere in which the mixing ratio of N ₂ gas and H ₂ gas is 1: 2 to 40, ion nitriding treatment is performed on a steel workpiece. In the manufacturing method described above, and further in the above manufacturing method, “the object to be processed is a gear and the glow discharge glow width is 1 to 3 mm by adding Ar gas to the nitriding atmosphere. A technique of “adjusting” and a technique of “performing an article to be processed after ion nitriding” are disclosed.

  Patent Document 2 states that “C: 0.05 to 0.15 wt%, Si: 0.50 wt% or less, Mn: 1.00 wt% or less, Cr: 1.00 to 2.00 wt%, Mo: 0.90 to 1.50 wt%, Al: 0.010 to 0.100 wt%, N: 0.0070 to 0.0200 wt%, Ni: 1.00 wt% or less, V: 0 10 to 0.30% by weight, Ti: 0.10% by weight or less, Nb: 0.030% by weight or less, including one or more, and if necessary, S: 0.005 to 0 100% by weight, Pb: 0.03 to 0.35% by weight, Ca: 0.0010 to 0.0100% by weight, Te: 0.001 to 0.100% by weight, Zr: 0.01 to 0.20 A precipitation-strengthened steel containing one or more of wt%, the balance being Fe and inevitable impurities, and having a temperature of 50 A steel for nitriding treatment characterized by having a property that the Vickers hardness (HV) of the core is 30 or more higher than that before the gas nitriding treatment by gas nitriding treatment at ˜600 ° C. ” A “nitriding method” is disclosed.

  Furthermore, Patent Document 3 states that “C by weight; 0.18 to 0.23, Si; 0.15 to 0.35, Mn; 0.60 to 0.85, P; 0.03 or less, S; 0.03 or less, Cr; 0.90 to 1.20, Mo; 0.15 to 0.30, with the balance being steel composed of Fe and impurities, or C in wt%; 0.16 to 0.21 , Si; 0.15 to 0.35, Mn; 0.55 to 0.90, P; 0.03 or less, S; 0.03 or less, Cu; 0.3 or less, Ni; 0.25 or less, Cr 0.90 to 1.10, Al; 0.07 to 0.13, V; 0.10 to 0.15, and the remainder is made by subjecting the steel composed of Fe and impurities to gas soft nitriding treatment, A compound layer having a compound of N and Fe having a thickness of 2 to 12 [μm] on the tooth surface, and adjacent to the lower layer, the hardness of the steel itself is Hv50 or less. Thickness 200 [[mu] m] or more diffusion layers and a gear ", characterized in that it comprises a its" manufacturing method "is disclosed having a hardness of.

JP-A-4-141573 Japanese Patent Laid-Open No. 11-124653 JP-A-11-72159

  The technique disclosed in Patent Document 1 described above suppresses the generation of the nitrogen compound layer itself formed on the surface of the steel material, and as a result, reduces the porous layer of the outermost layer portion that is hard, brittle, and easily peeled off. This is an effective technology. However, simply reducing the porous layer by simply improving the nitriding method does not necessarily improve the pitching strength of the nitriding steel part, particularly the nitriding gear.

  The technology disclosed in Patent Document 2 requires that Mo, which is an expensive element, be contained in a large amount of 0.90 to 1.50% by weight, so that the component cost is extremely high and is inferior in terms of economy. there were. Further, although the core hardness after nitriding treatment is improved, it is difficult to say that the effect of improving the surface hardness is sufficient, and therefore, it is difficult to use as a rough product for nitriding gear, which is a material of nitriding gear. It was a thing.

  The technique disclosed in Patent Document 3 is an effective technique in terms of suppressing the formation of a porous layer formed on the outermost layer of the compound layers. However, even with the above technique, the pitching strength of the gears is not always sufficient.

  Therefore, the object of the present invention is economical because it is not necessary to contain Mo, which is an expensive element, in an amount as high as 0.90% or more, and is excellent in wear resistance and pitting resistance. It is another object of the present invention to provide a nitriding gear and a rough product for nitriding gear suitable for use as a material thereof.

  The present inventors have made various studies in order to solve the above-described problems. As a result, first, the following findings (a) to (c) were obtained.

  (A) Abrasion resistance is not necessarily improved and the occurrence of pitting cannot be suppressed merely by suppressing the formation of a porous layer formed on the outermost layer among the compound layers of the nitrided gear.

  (B) Pitching occurs when the tooth surface is plastically deformed during use of the nitriding gear. Therefore, in order to improve the pitching strength of the nitriding gear, the formation of the entire compound layer including not only the outermost porous layer but also the dense layer below it is suppressed or the entire compound layer is removed. It is necessary to increase the surface hardness of the gear. Increasing the surface hardness of the gear is also effective for improving wear resistance.

  (C) Even if the formation of the compound layer on the surface of the nitrided gear is suppressed or the compound layer is removed, if hard non-metallic inclusions appear on the surface of the gear, it may cause pitting. The starting point.

  And as a result of repeating detailed examination about said point, the knowledge of (d)-(i) shown next was acquired.

  (D) V is combined with C and N during nitriding or soft nitriding and precipitates as fine carbides, nitrides and carbonitrides to improve surface hardness and hardened layer depth. It is an indispensable element for crude products.

  (E) Al combines with N during nitriding or soft nitriding to form AlN, thereby improving the surface hardness. For this reason, it is preferable to positively contain Al in the nitrided gear coarse product.

(F) However, Al, on the other hand, forms Al ₂ O ₃ , which is a hard and coarse non-metallic inclusion, so it is necessary to regulate the content of O (oxygen) in the nitrided gear crude product.

  (G) Ti having high affinity with N combines with N in steel to form TiN, which is a hard and coarse non-metallic inclusion. For this reason, it is necessary to control the Ti content in the impurities in the coarse-shaped product for nitriding gears and to control the N content within a specific range in order to utilize AlN.

  (H) If the microstructure of the nitrided gear coarse product contains a large amount of ferrite, which is a soft phase, plastic deformation and wear occur on the tooth surfaces of the nitrided gear, which causes a decrease in pitching strength. Further, in soft ferrite, cracks in pitching and bending fatigue tend to progress, which causes a decrease in the pitching strength and bending fatigue strength of the nitrided gear. However, when the ferrite in the microstructure of the nitrided gear coarse product has an area ratio of 30% or less, a decrease in the pitching strength and bending fatigue strength of the nitrided gear is suppressed.

  (I) The maximum depth of the compound layer on the surface of the nitriding gear where stable and excellent pitching strength is obtained and the effect of improving wear resistance is 5 μm.

  The present invention has been completed based on the above findings, and the gist of the present invention resides in a rough product for a nitrided gear shown in the following (1) to (4) and a nitrided gear shown in (5).

  (1) By mass%, C: 0.10 to 0.30%, Si: more than 0.10% and 0.50% or less, Mn: 0.50 to 1.3%, S: 0.005 0.05%, Cr: 0.70 to 2.0%, Al: 0.035 to 0.10%, V: 0.01 to 0.30% and N: 0.003 to 0.015% The balance consists of Fe and impurities, and P, Ti and O (oxygen) in the impurities are P: 0.025% or less, Ti: less than 0.010% and O: less than 0.0020%, respectively. And a microstructure of bainite, martensite, or a mixed structure of bainite and martensite.

  (2) The microstructure for nitrided gears as described in (1) above, wherein the microstructure further contains 30% or less of ferrite in area ratio.

  (3) The rough shape for a nitrided gear according to (1) or (2) above, wherein the chemical composition contains, by mass%, Mo: 0.5% or less instead of part of Fe Goods.

  (4) The chemical composition is characterized by containing, in mass%, one or two of Pb: 0.3% or less and Ca: 0.005% or less instead of part of Fe. The rough shaped product for a nitriding gear according to any one of (1) to (3) above.

  (5) A nitriding gear made of the rough shaped product for nitriding gear according to any one of (1) to (4) above, wherein the depth of the compound layer forming the nitrided layer on the surface is 5 μm or less A nitriding gear characterized by being.

  As already described, “nitriding gear” refers to a gear having a “nitriding layer” that is a hardened layer.

  The “roughed product for nitriding gear” is a material of the above “nitriding gear”, and after hot forging into a predetermined gear rough shape, “normalizing-tempering” or “tempering” It refers to a material that has been subjected to a heat treatment of “entry-tempering”.

  Further, the “depth of the compound layer” of the nitriding gear refers to the depth from the surface of the compound layer, and more specifically, a magnification of 2000 so as to include the outermost surface of the nitriding gear by a scanning electron microscope. The maximum distance from the outermost surface of the nitriding gear of the “compound layer” in each visual field when 100 visual fields are observed at an arbitrary magnification. Therefore, “the depth of the surface compound layer is 5 μm or less” means that the maximum value of the maximum distance from the outermost surface in each field of view determined as described above is 5 μm or less.

  Hereinafter, the invention according to the above-described (1) to (4) coarse product for nitriding gear and the invention according to (5) the nitriding gear are referred to as “present invention (1)” to “present invention (5)”, respectively. Also, it may be collectively referred to as “the present invention”.

  The nitrided gear of the present invention is excellent in wear resistance and pitching resistance, and it is not necessary to contain Mo, which is an expensive element, in an amount as large as 0.90% or more. It can be used as an economical gear for a transmission or the like. Moreover, the rough product for nitriding gear of the present invention is suitable for use as a material for the nitriding gear.

  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 coarse product for nitriding gear:
C: 0.10 to 0.30%
C has the effect of improving the hardened layer depth and the core hardness of the nitrided gear, and is an essential element for ensuring pitching strength and bending fatigue strength, and has a content of 0.10% or more. is necessary. However, if the content of C increases, the hardness increases and machinability is reduced. In particular, when the content exceeds 0.30%, the machinability is significantly decreased as the hardness increases. Become. Therefore, the content of C is set to 0.10 to 0.30%. In addition, it is preferable that content of C shall be 0.15-0.25%.

Si: more than 0.10% and 0.50% or less Si has a deoxidizing action. Further, Si is an element that improves the temper softening resistance. It is difficult to cause softening when the tooth surface temperature rises during use of the nitriding gear, and effectively acts to improve the pitching strength. In order to obtain these effects, a content exceeding 0.10% is required. However, if the Si content is too large, the hardness increases and the machinability decreases, and in particular, if the content exceeds 0.50%, the machinability decreases with an increase in hardness. It becomes remarkable. Therefore, the Si content is more than 0.10% and 0.50% or less. The Si content is preferably 0.20 to 0.35%.

Mn: 0.50 to 1.3%
Mn is an element that is effective in improving the depth of the hardened layer and the core hardness of the nitrided gear, and has the effect of ensuring the pitching strength and bending fatigue strength. Mn also has a deoxidizing action. In order to obtain these effects, a content of 0.50% or more is necessary. However, if the content of Mn is too large, the hardness increases and machinability is reduced. In particular, if the content exceeds 1.3%, the machinability is reduced due to the increase in hardness. It becomes remarkable. Therefore, the Mn content is set to 0.50 to 1.3%. In addition, it is preferable that content of Mn shall be 0.60-1.0%.

S: 0.005-0.05%
S combines with Mn to form MnS and has the effect of improving machinability. However, if the content is less than 0.005%, it is difficult to obtain the above effect. On the other hand, if the S content exceeds 0.05%, coarse MnS is formed, and the hot forgeability decreases. Therefore, the content of S is set to 0.005 to 0.05%. In addition, it is preferable that content of S shall be 0.015-0.03%.

Cr: 0.70 to 2.0%
Cr has the effect of improving the surface hardness, the hardened layer depth, and the core hardness, and ensuring the pitching strength and bending fatigue strength of the nitrided gear. In order to obtain this effect, a content of 0.70% or more is necessary. However, if the Cr content increases, the hardness increases and machinability is reduced. In particular, if the content exceeds 2.0%, the machinability is significantly reduced as the hardness increases. Become. Therefore, the content of Cr is set to 0.70 to 2.0%. The Cr content is preferably 0.80 to 1.5%.

Al: 0.035 to 0.10%
Al is an important element having an action of combining with N during nitriding or soft nitriding to form AlN, improving surface hardness, and increasing pitching strength and bending fatigue strength. In order to obtain such effects, it is necessary to contain 0.035% or more of Al. However, when the content of Al becomes excessive, especially when it exceeds 0.10%, a large amount of hard Al ₂ O ₃ is formed, resulting in a decrease in machinability and a hardened layer depth becomes shallow. In addition, when Al ₂ O ₃ is present in the surface layer, it becomes a starting point of pitching, and therefore the pitching strength is lowered. Therefore, the content of Al is set to 0.035 to 0.10%. In addition, preferable Al content is 0.040 to 0.080%.

V: 0.01-0.30%
V combines with C and N during nitriding or soft nitriding and precipitates as fine carbides, nitrides and carbonitrides, improving surface hardness and hardened layer depth, and increasing pitching strength and bending fatigue strength It is an important element having an action. In order to acquire these effects, it is necessary to contain V 0.01% or more. However, if the content of V increases, especially when it exceeds 0.30%, coarse carbides, nitrides and carbonitrides are precipitated, leading to a decrease in toughness. Therefore, the content of V is set to 0.01 to 0.30%. A preferable V content is 0.05 to 0.20%.

N: 0.003 to 0.015%
N combines with Al at the time of nitriding or soft nitriding to form AlN, thereby improving the surface hardness and increasing the pitching strength and bending fatigue strength. In order to obtain such an effect, it is necessary to contain 0.003% or more of N. However, when the N content is excessive, coarse nitrides such as TiN are formed, leading to a decrease in toughness. In particular, when the content exceeds 0.015%, the toughness is significantly decreased. Therefore, the N content is set to 0.003 to 0.015%. A preferable N content is 0.004 to 0.008%.

  In the present invention, the contents of P, Ti and O (oxygen) in the impurities are limited to P: 0.025% or less, Ti: less than 0.010% and O: less than 0.0020%, respectively.

  This will be described below.

P: 0.025% or less P is an impurity contained in the steel, and segregates at the grain boundaries to embrittle the steel. In particular, when the content exceeds 0.025%, the degree of embrittlement becomes significant. Therefore, the content of P in the impurities is set to 0.025% or less. A more preferable content of P in the impurities is 0.020% or less.

Ti: Less than 0.010% Since Ti has a high affinity with N, it combines with N in steel to form hard and coarse non-metallic inclusions TiN, and this coarse and hard TiN is nitrided If it exists on the surface of the gear, it will be the starting point of pitching. In particular, when the Ti content is 0.010% or more, the formation of coarse and hard TiN becomes remarkable. Therefore, the content of Ti in the impurities is set to less than 0.010%. In addition, the more preferable content of Ti in the impurities is less than 0.005%.

O (oxygen): less than 0.0020% In the present invention containing 0.035 to 0.10% of Al as an essential element, Al ₂ which is a hard and coarse oxide when the content of O is large When O ₃ is formed and this coarse and hard Al ₂ O ₃ is present on the surface of the nitriding gear, it becomes a starting point of pitching. In addition, when Ca is contained, CaO which is a coarse oxide is formed, and when this CaO is also present on the surface of the nitriding gear, it becomes a starting point for occurrence of pitching. In particular, when the O content is 0.0020% or more, the formation of the Al ₂ O ₃ and CaO becomes remarkable. Therefore, the content of O in impurities is set to less than 0.0020%. A more preferable content of O in the impurities is less than 0.0015%.

  For the above reasons, the chemical composition of the nitrided gear crude product according to the present invention (1) and the present invention (2) contains the elements from C to N in the above-mentioned range, and the balance consists of Fe and impurities. It was defined that P, Ti, and O in the impurities are in the above-mentioned ranges.

In the rough shape product for nitriding gear according to the present invention, if necessary, instead of a part of Fe in the present invention (1) or the present invention (2),
First group: Mo: 0.5% or less,
Second group: Pb: 0.3% or less and Ca: 0.005% or less
One or more elements of at least one group of the above may be contained.

  That is, in order to obtain even more excellent characteristics, at least one of the elements of at least one of the first group and the second group is used as a rough material for a nitrided gear according to the present invention (1) or the present invention (2). Instead of part of Fe in the shaped product, it may be contained.

  Hereinafter, the above elements will be described.

First group: Mo: 0.5% or less Mo is an element that is effective in improving the hardened layer depth and core hardness of the nitriding gear, and has an action to ensure pitching strength and bending fatigue strength. It may be added and contained accordingly. However, even if Mo exceeding 0.5% is contained, the above-described effect is saturated, so that the cost is increased and the economic efficiency is impaired, and the machinability is lowered due to the increase in hardness. Therefore, the Mo content when added is set to 0.5% or less. When even better machinability is required, the Mo content is preferably 0.3% or less.

  In order to reliably obtain the effect of securing the above-described pitching strength and bending fatigue strength of Mo, the Mo content is preferably set to 0.05% or more. For this reason, when Mo is added, the more preferable content of Mo is 0.05 to 0.5%. In addition, it is preferable that content of Mo to add shall be 0.05-0.3%.

Second group: Pb: 0.3% or less and Ca: 0.005% or less One or two of Pb and Ca both have an effect of improving machinability. For this reason, when it is desired to obtain better machinability, it may be contained in the following range.

Pb: 0.3% or less Pb has an effect of improving machinability, and may be contained for further machinability improvement. However, even if containing Pb exceeding 0.3%, the machinability improving effect is saturated, so the cost is increased, the economic efficiency is impaired, and the toughness and strength are further reduced. Therefore, the content of Pb when added is set to 0.3% or less.

  In order to reliably obtain the effect of improving the machinability of Pb described above, the Pb content is preferably 0.05% or more. For this reason, when Pb is added, the more preferable content of Pb is 0.05 to 0.3%. When Pb is added, the more desirable Pb content is 0.05 to 0.15%.

Ca: 0.005% or less Ca has an effect of improving machinability, and therefore may be contained for further machinability improvement. However, even if Ca exceeding 0.005% is contained, the machinability improving effect is saturated, so the cost is increased, the economy is impaired, and the toughness and strength are lowered, and the oxide is coarser. CaO is formed and the pitching strength is lowered. Therefore, when Ca is added, the content of Ca is set to 0.005% or less.

  In order to reliably obtain the above-described effect of improving the machinability of Ca, the Ca content is preferably set to 0.0005% or more. For this reason, the more preferable content of Ca in the case of adding is 0.0005 to 0.005%. In addition, when adding, the more desirable content of Ca is 0.0005 to 0.003%.

  Said Pb and Ca can be contained only in any 1 type or 2 types of composite.

  For the above reason, the chemical composition of the nitrided gear crude product according to the present invention (3) is replaced with a part of Fe of the nitrided gear crude product according to the present invention (1) or the present invention (2). , Mo: Specified to contain 0.5% or less.

  The chemical composition of the nitrided gear crude product according to the present invention (4) is replaced with a part of Fe of the nitrided gear crude product according to any of the present invention (1) to the present invention (3). Thus, it is defined that one or two of Pb: 0.3% or less and Ca: 0.005% or less are contained.

(B) Microstructure of rough shaped product for nitriding gear:
The basic microstructure of the nitrided gear crude product according to the present invention is either a hard phase, that is, bainite, martensite, or a mixed structure of bainite and martensite. This is because if the microstructure of the nitrided gear coarse product contains a large amount of ferrite, which is a soft phase, plastic deformation and wear occur on the tooth surface of the nitrided gear, which causes a reduction in pitching strength. This is because cracks in pitching and bending fatigue tend to progress, causing a decrease in the pitching strength and bending fatigue strength of the nitriding gear.

  However, when the ferrite in the microstructure of the nitrided gear coarse product has an area ratio of 30% or less, a decrease in the pitching strength and bending fatigue strength of the nitrided gear is suppressed.

  For the reasons described above, the microstructure of the nitrided gear crude product according to the present invention (1) is defined as being one of bainite, martensite, or a mixed structure of bainite and martensite.

  Further, the microstructure of the nitrided gear coarse product according to the present invention (2) is one in which the microstructure of the nitrided gear coarse product according to the present invention (1) further contains 30% or less of ferrite in area ratio. It was stipulated.

  In addition, the rough-shaped product for nitrided gears having the chemical composition described in the item (A) and having the microstructure described in the item (B) is, for example, as shown in the following <1> to <4>. It can manufacture by the method of giving the heat processing shown to <5> or <5 '> through a simple method.

  <1> The steel melted in the converter is refined in a vacuum degassing facility to adjust the components and remove impurities.

In the case of melting in the atmosphere, since the molten steel comes into contact with the atmosphere and the O content in the steel increases, non-metallic inclusions such as Al ₂ O ₃ are likely to be coarsened, but by using a vacuum degassing facility, the above Al _Since coarsening of non-metallic inclusions such as ₂ O ₃ can be suppressed, it is particularly preferable to refine the molten steel in a vacuum degassing facility with a vacuum degree of 800 Pa or less (600 Torr or less).

  <2> The molten steel subjected to the component adjustment and removal of impurities in the above <1> is sufficiently subjected to electromagnetic stirring treatment, and after removing non-metallic inclusions by floating, a slab is produced by a continuous casting method, An ingot is produced by the ingot-making method.

  In the case of the ingot-making method, it is preferable to sufficiently cut and remove the feeder part of the ingot.

  <3> The slab or ingot of the above <2> is heated at a temperature of 1250 ° C. or more, homogenized, and made into a steel slab by ingot rolling, followed by steel bar rolling or wire rod rolling.

  In addition, if there is a surface flaw in the above rolled material, it will lead to a large crack when hot forging into a predetermined gear rough shape. It is preferred to care for and remove.

  <4> The rolled material of the above <3> is cut into a predetermined length and then hot-forged to obtain a rough shape of the gear. In this case, it is preferable to perform hot forging at a finishing temperature of 1100 ° C. or higher after heating at 1250 ° C. or higher for 10 minutes or longer.

  Further, when the rolled material is coiled, it is preferable to perform correction before the cutting.

  <5> The hot forged material having the rough shape of the gear in <4> is subjected to normalization as a heat treatment before nitriding at a cooling rate after normalization of 0.5 ° C./s or more.

<5 ′> As a heat treatment before nitriding, after heating and holding at a temperature of Ac ₃ point or higher, quenching is performed by quenching with oil or salt.

  In addition, when isothermal annealing is performed as a heat treatment before nitriding, the ferrite transformation temperature is maintained, so that the ferrite fraction becomes high and a microstructure with an area ratio of 30% or less of ferrite cannot be obtained.

(C) Depth of the compound layer forming the nitrided layer on the surface of the nitrided gear:
As already described, the “nitrided layer” formed on the surface of the article to be processed when “nitriding” or “soft nitriding” is performed is composed of a porous layer at the outermost layer and a dense layer below it. And a compound layer mainly composed of the ε-Fe _2-3 N phase and a diffusion layer therebelow.

  Since the compound layer formed on the surface is very brittle, when the nitriding or soft nitriding treatment is performed on the gear, it easily peels off at the initial stage of meshing of the gear, As a result, irregularities are formed on the surface of the metal, which acts as a local stress concentration point, thereby causing abnormal wear and pitching.

  Thus, the compound layer forming the nitride layer on the surface of the nitrided gear has a great influence on the abnormal wear and pitching strength of the nitrided gear. However, when the depth is 5 μm or less, stable and excellent pitching strength can be obtained and abnormal wear can be suppressed. For this reason, the nitrided gear according to the present invention (5), which is made from the rough shaped product for a nitrided gear according to any of the present invention (1) to the present invention (4), is a compound that forms a nitrided layer on the surface thereof It was defined that the depth of the layer was 5 μm or less.

  In order to reduce the depth of the compound layer forming the nitride layer on the surface of the nitriding gear to 5 μm or less, for example, nitriding treatment that does not generate a compound layer such as so-called “radical nitridation” or gas soft nitriding After the application, mechanical polishing or chemical polishing for reducing the compound layer depth may be performed.

  The processing temperature of “nitriding” and “soft nitriding” for forming the “nitriding layer” as the “hardened layer” is too low, so that it is difficult for N to diffuse, and a long processing time is required. If it is too high, the fabric (base material) is softened, leading to a decrease in pitching strength and bending fatigue strength, and wear progresses. Therefore, it is preferable that the processing temperature shall be 400-600 degreeC.

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

  Steels 1 to 7 and steels 9 to 11 having the chemical compositions shown in Table 1 were melted by a 180 kg vacuum melting furnace to produce ingots.

  Moreover, the steel 8 which has the chemical composition shown in Table 1 was melt | dissolved by the 3 ton atmospheric melting furnace, and the ingot was produced.

  Steels 1 to 5 in Table 1 are steels whose chemical compositions are within the range defined by the present invention. On the other hand, steels 6 to 11 are steels of comparative examples whose chemical compositions deviate from the conditions specified in the present invention.

  Each ingot was processed into a test piece to be subjected to roller pitching in the process outlined in FIG.

  Hereinafter, each of the processing steps of the test piece used for the roller pitching will be described in detail.

[1] Homogenization treatment:
Each ingot was charged in a soaking furnace and held at 1250 ° C. for 8 hours to homogenize.

[2] Hot forging:
Each ingot was held at the above 1250 ° C. for 8 hours and then taken out from the soaking furnace and hot forged into two types of round bars having a diameter of 30 mm, a length of 1000 mm, a diameter of 140 mm and a length of 1000 mm. The hot forging finishing temperature was 1000 ° C. or higher, and the forging was allowed to cool in the air after forging.

[3] Normalization:
For steels 1 to 4 and steels 6 to 11, each of the hot-forged round bars with a diameter of 30 mm was held at 900 ° C. for 1 hour and then allowed to cool in the atmosphere and normalized. Each 140 mm round bar was held at 900 ° C. for 3 hours, and then allowed to cool in the atmosphere and normalized.

  As for Steel 5, a hot-forged round bar with a diameter of 30 mm was cut into two, one of which was held at 900 ° C. for 1 hour, and then allowed to cool in the atmosphere and normalized. Also, a hot forged round bar having a diameter of 140 mm was cut into two, one of which was held at 900 ° C. for 3 hours, and then allowed to cool in the atmosphere and normalized.

[4] Isothermal annealing:
The following isothermal annealing was performed on the remaining part of the steel 5 cut into two in [3]. That is, after holding one of the hot-forged round bars with a diameter of 30 mm at 900 ° C. for 1 hour, it was allowed to cool in the atmosphere until the surface temperature reached 680 ° C., and then held at 680 ° C. for 1 hour. Isothermal annealing to cool in the atmosphere was performed. In addition, one of the hot forged round bars with a diameter of 140 mm was held at 900 ° C. for 3 hours, then allowed to cool in the atmosphere until the surface temperature reached 680 ° C., and then held at 680 ° C. for 3 hours. Isothermal annealing to cool in the atmosphere was performed.

[5] Quenching A:
For Steel 4 and Steel 5, the round bars with diameters of 30 mm and 140 mm subjected to normalization of [3] were cut into lengths of 150 mm and 21 mm, respectively, and both were held at 900 ° C. for 1 hour. Quenching by cooling in oil at 100 ° C was performed.

[5 '] Quenching B:
For steel 1, cut the round rods with diameters of 30 mm and 140 mm that were subjected to the normalization of [3] into two, and cut one of them into 150 mm and 21 mm in length, both at 900 ° C. After holding for 1 hour, quenching was performed by cooling in oil at 170 ° C.

[6] Tempering:
In order to remove the internal stress generated in the normalization, isothermal annealing, quenching A, and quenching B, all the materials were held at 500 ° C. for 1 hour and then allowed to cool in the atmosphere and tempered.

[7] Machining (rough machining):
A roller pitching small roller test piece shown in FIG. 2 was cut out from the center of each round bar having a tempered diameter of 30 mm in parallel with the forging axis. Moreover, the roller pitching large roller test piece shown in FIG. 3 was cut out from the center of each round bar having a diameter of 140 mm in parallel with the forging axis.

  2 and 3 are all “mm”, and the finishing symbols “▽” and “▽▽▽” in the figures are JIS B 0601 (1982). Is a “triangular symbol” indicating the surface roughness of Table 1.

  Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978).

  Similarly, “E (paper finish)” means an abbreviation of a processing method indicating “polishing” with “paper file”.

  Using a part of each roller pitching small roller test piece cut out as described above, the microstructure was examined across the central part of φ26 mm. Details of the survey method will be described later.

[8] Radical nitriding or gas soft nitriding as surface hardening treatment:
With respect to all of the remaining test pieces subjected to the microstructural examination of the roller pitching small roller test piece cut out in the above [7] and the roller pitching large roller test piece, as a surface hardening treatment, the heat pattern shown in FIG. Radical nitriding (hereinafter referred to as “condition A”) or gas soft nitriding (hereinafter referred to as “condition B”) by the heat pattern shown in FIG. 5 was performed.

[9] Surface polishing:
Part of the test piece subjected to gas soft nitriding under the condition B of [8] above is subjected to polishing by machining to adjust the depth of the compound layer forming the nitrided layer and remove the compound layer itself. went.

[10] Shot peening:
A two-stage shot peening treatment was performed on a part of the test piece from which the compound layer was removed by surface polishing in [9] above under the following conditions.

first stage:
・ Type of projection material: Round cut wire,
・ Diameter of shot material (particle size of shot grains): 0.6 mm,
Projection material hardness: Vickers hardness (hereinafter referred to as “Hv hardness”) 700,
-Arc height: 0.4 mmA,
Second stage:
・ Type of projection material: Steel beads,
・ Diameter of projection material: 0.1 mm,
・ Hardness of projection material: 700 in Hv hardness
-Arc height: 0.1 mmA.

[11] Machining (finishing):
Each test piece subjected to the above-described various treatments was finished to produce a roller pitching small roller test piece shown in FIG. 6 and a roller pitching large roller test piece shown in FIG.

  6 and 7 are all “mm”, and the finish symbols “▽” and “▽▽▽” in the above figures are the same as those in FIGS. Similarly, “Triangle symbol” indicating the surface roughness in Table 1 of JIS B 0601 (1982).

  Further, “G” added to “▽▽▽” means an abbreviation of a processing method indicating “grinding” defined in JIS B 0122 (1978).

  Furthermore, “˜” is a “waveform symbol”, which means that it is a fabric, that is, it remains a surface treated under any of the conditions [8] to [10].

  Table 2 shows, as test numbers 1 to 13, the processing steps after hot forging of the test pieces used for the roller pitching described above, that is, the steps after the normalization of [3].

  Of the test numbers set as “execution” in the surface polishing column of Table 2, for test number 5, the compound layer depth for forming the nitride layer is 3 μm, and for test number 10, nitriding is performed. Surface polishing was performed by adjusting the depth of the compound layer forming the layer to 9 μm. Furthermore, for Test No. 6 and Test No. 11, the surface was polished by 25 μm to completely remove the compound layer forming the nitride layer.

  For each of the test numbers 1 to 13 in Table 2, a micro structure investigation using a roller pitching small roller test piece before surface hardening treatment, and forming a surface nitrided layer using a finished roller pitching small roller test piece Investigation of the depth, surface hardness and effective hardened layer depth of the working compound, as well as investigation of anti-pitching properties by roller pitching test using finished roller pitching small roller test piece and roller pitching large roller test piece went.

  Moreover, the amount of surface abrasion was also investigated using the roller pitching small roller test piece which carried out the roller pitching test.

  Hereinafter, the contents of each of the above surveys will be described in detail.

<< 1 >> Microstructure investigation:
Using a roller pitching small roller test piece before surface hardening treatment, that is, a roller pitching small roller test piece that has been cut out as described in [7] above, after crossing the central portion of φ26 mm, the cut surface is covered. It was embedded in a resin so as to be a surface to be examined, mirror-polished, and then corroded with nital to examine the microstructure.

  More specifically, 100 phases were arbitrarily observed with an optical microscope at a magnification of 400 times, and the “phase” was investigated. When ferrite was included in the “phase” of the microstructure, the ratio of ferrite in each field of view was measured, and the area ratio of ferrite in the microstructure was determined from the arithmetic average in the 100 fields of view.

<< 2 >> Investigation of the depth of the compound layer forming the nitride layer on the surface:
Using the finished roller pitching small roller test piece of [11], after crossing the central part of φ26 mm, the cut surface is embedded in a resin so as to be a test surface, and then mirror-polished. The depth of the compound layer that corrodes to form a nitrided surface was investigated.

  Specifically, by using a scanning electron microscope, 100 fields of view are arbitrarily observed at a magnification of 2000 to include the outermost surface of the test piece, and the maximum distance of the “compound layer” from the outermost surface of the test piece in each field of view. The maximum value was taken as the depth of the compound layer forming the surface nitride layer.

<3> Investigation of surface hardness:
Surface hardness was investigated using the resin-embedded test piece used in the investigation of the depth of the compound layer on the surface of << 2 >>.

  Specifically, in accordance with “Vickers hardness test-test method” described in JIS Z 2244 (2003), Vickers hardness at an arbitrary 100 points at a depth of 0.03 mm from the surface of the test piece. Was measured with a test force of 0.9807 N, and the minimum value was defined as surface hardness.

<4> Investigation of effective hardened layer depth:
The effective hardened layer depth was investigated using the resin-embedded test piece used in the investigation of the compound layer depth on the surface of << 2 >>.

  Specifically, in the same manner as in the case of the surface hardness investigation of the above << 3 >>, in accordance with “Vickers hardness test-test method” described in JIS Z 2244 (2003), the surface from the surface of the test piece is centered. The depth from the surface when the test force is 0.9807 N and the Hv hardness is 420 is obtained in the direction toward the surface, and the effective hardened layer depth is determined by the minimum value obtained by measuring 100 arbitrary locations.

<< 5 >> Investigation of anti-pitching characteristics by roller pitting test:
Using the finished roller pitching small roller test piece and roller pitching large roller test piece of [11] above, the test is performed under the following test conditions, and the long side has a length of 1 mm or more when the number of repetitions is 10 ⁷ times. The maximum surface pressure at which no occurrence occurs was defined as the pitching strength. In addition, the target of this pitching strength shall be 1800 Mpa or more, and when the pitching strength is 1800 Mpa or more which is a target, it shall be excellent in a pitching-proof characteristic.

・ Slip rate: 40%
・ Rotation speed: 1000rpm
・ Lubrication: Lubricating oil for an automatic transmission with an oil temperature of 80 ° C. was jetted at a rate of 2 liters / minute to the contact portion between the roller pitching small roller test piece and the roller pitching large roller test piece.

However, the above-mentioned “slip ratio” is a value calculated by the following formula, where “V1” is the tangential speed of the roller pitching small roller test piece surface and “V2” is the tangential speed of the roller pitching large roller test piece surface Point to.
{(V2-V1) / V1} × 100.

<< 6 >> Investigation of surface wear:
The amount of surface wear after the roller pitching test of << 5 >> was investigated as follows.

That is, the stylus at any 10 positions on the φ26 mm test surface of the roller pitching small roller test piece in which the maximum surface pressure that does not generate pitching with a length of 1 mm or more at 10 ⁷ repetitions was obtained. The profile of the non-contact part, the contact part, and the non-contact part was measured continuously in the axial direction using a surface roughness meter of the formula. And in the profile obtained by said measurement, the maximum value of 10 places was defined as the amount of surface wear among the difference of the position of the valley bottom which became concave near the center of a contact part, and a non-contact part.

  The target of the surface wear amount is 10 μm or less, and when the surface wear amount is 10 μm or less, the wear resistance is excellent.

  Table 3 summarizes the results of the above investigations. In Table 3, “the depth of the compound layer forming the surface nitride layer” was expressed as “the depth of the compound layer”.

  From Table 3, the test numbers 1 to 6 that satisfy the conditions specified in the present invention are large pitching of 1800 MPa or more in spite of the small amount of 0.35% even when containing Mo. It is clear that it has strength and excellent anti-pitching characteristics, and further, the surface wear amount is 10 μm or less and is excellent in wear resistance.

  On the other hand, in the case of the test number 7 and the test number 9 of the comparative example that deviates from the conditions specified in the present invention, the pitching strength is in the case of the test number 8 and the test numbers 11 to 13 of the comparative example. Both the surface wear and the amount of surface wear do not reach the target of the present invention.

  That is, in test number 7, since the Si content of steel 6 is 0.05%, which is less than the value specified in the present invention, the pitching strength is as low as 1650 MPa, and does not reach the target of the present invention.

  In Test No. 8, the Al content of steel 7 is 0.120%, which is higher than the value specified in the present invention, and the area ratio of ferrite before the surface hardening treatment is 35%, which is higher than the value specified in the present invention. For this reason, the pitching strength is as low as 1500 MPa, and the surface wear amount is as large as 15.0 μm, and none of them reaches the target of the present invention.

  In Test No. 9, since the O content of steel 8 is 0.0030%, which is higher than the value specified in the present invention, the pitching strength is as low as 1600 MPa and does not reach the target of the present invention.

  Test No. 10 has a Ti content of steel 9 of 0.025%, which is higher than the value specified in the present invention, and further, the depth of the compound layer forming the nitrided layer on the surface is 9 μm, which is higher than the value specified in the present invention. large. For this reason, the pitching strength is as low as 1550 MPa, and the surface wear amount is as large as 12.0 μm, which does not reach the target of the present invention.

  In test number 11, the Mn content of steel 10 is 0.35%, which is less than the value specified in the present invention, and the area ratio of ferrite before the surface hardening treatment is 45%, which is higher than the value specified in the present invention. For this reason, the pitching strength is as low as 1500 MPa, and the surface wear amount is as large as 15.0 μm, and none of them reaches the target of the present invention.

  In Test No. 12, the steel 11 does not contain V, the N content is 0.0280%, which is larger than the value specified in the present invention, and the depth of the compound layer forming the nitrided layer on the surface is larger. 21 μm, which is larger than the value specified in the present invention. For this reason, the pitching strength is as low as 1500 MPa, and the surface wear amount is as large as 49.5 μm, and none of them reach the target of the present invention.

  In Test No. 13, although the chemical composition of the steel 5 is within the range specified by the present invention, the area ratio of ferrite before the surface hardening treatment is 65%, which is larger than the value specified by the present invention. For this reason, the pitching strength is as low as 1500 MPa, and the surface wear amount is as large as 25.0 μm, and none of them reach the target of the present invention.

  The nitrided gear of the present invention is excellent in wear resistance and pitching resistance, and it is not necessary to contain Mo, which is an expensive element, in an amount as large as 0.90% or more. It can be used as an economical gear for a transmission or the like. Moreover, the rough product for nitriding gear of the present invention is suitable for use as a material for the nitriding gear.

It is a figure which shows the outline | summary of the process of processing the test piece used for roller pitching from the steel ingot used in the Example. It is a figure which shows the shape as cut out from the round bar of the roller pitching small roller test piece used in the Example. It is a figure which shows the shape as cut out from the round bar of the roller pitching large roller test piece used in the Example. It is a figure which shows the heat pattern of "radical nitriding" in an Example. It is a figure which shows the heat pattern of "gas soft nitriding" in an Example. It is a figure which shows the shape of the roller pitching small roller test piece used by the roller pitching test of the Example. It is a figure which shows the shape of the roller pitching large roller test piece used by the roller pitching test of an Example.

Claims (5)

  In mass%, C: 0.10 to 0.30%, Si: more than 0.10% and 0.50% or less, Mn: 0.50 to 1.3%, S: 0.005 to 0.05 %, Cr: 0.70 to 2.0%, Al: 0.035 to 0.10%, V: 0.01 to 0.30% and N: 0.003 to 0.015%, the balance Is composed of Fe and impurities, and P, Ti and O (oxygen) in the impurities have a chemical composition of P: 0.025% or less, Ti: less than 0.010% and O: less than 0.0020%, respectively. The rough structure for nitrided gears, wherein the microstructure is any one of bainite, martensite, or a mixed structure of bainite and martensite.   The microstructure of the nitrided gear according to claim 1, wherein the microstructure further contains 30% or less of ferrite in area ratio.   The chemical composition contains Mo: 0.5% or less in terms of mass% instead of a part of Fe, and the rough shaped product for nitrided gear according to claim 1 or 2.   The chemical composition contains one or two of Pb: 0.3% or less and Ca: 0.005% or less in mass%, instead of a part of Fe. 4 to 3 for nitriding gears.   A nitrided gear made of the roughened product for a nitrided gear according to any one of claims 1 to 4, wherein the depth of the compound layer forming the nitrided layer on the surface is 5 µm or less. Nitriding gear.

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