JP7264117B2 - Steel part and its manufacturing method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steel part having a long life, particularly against contact fatigue, which is used as a material for machine structures used in the fields of construction machinery and automobiles. In the field of construction and industrial machinery, the steel parts of the present invention include, for example, gears for travel reduction gears (gears such as planetary gears and sun gears), gears for large reduction gears, valve plates for hydraulic pumps, nuts for ball screws, and cyclone reduction gears. In the automobile field, various bearings, engine piston pins, camshafts and timing gears, transmission gears (missing gears, rings gears, sun gears, planetary gears, etc.), and drive system differential bevel gears, tripods, inners, balls, and the like. In addition to the fields of construction and industrial machinery and automobiles, there are bearings and reduction gears for wind power generators in the field of electrical equipment.

Carburizing, quenching, and tempering are well known as heat treatments for improving the fatigue resistance of steel, so-called fatigue characteristics. This carburizing, quenching and tempering is applied to various steel parts including gears for automobiles. Improving fatigue properties is one of the persistent demands for materials because it enables the miniaturization of parts and leads to weight reduction of vehicles.

Here, as means for increasing the anti-breakage fatigue strength of parts, a technique of using both carburizing treatment and shot peening treatment has been proposed. In this technology, by applying shot peening to carburized steel parts, it is possible to impart compressive residual stress and improve fatigue properties. For cutting tools and die materials for forging, the formation of a hard film by chemical vapor deposition (CVD) or physical vapor deposition (PVD) also has the effect of improving the surface fatigue strength.

For example, in Patent Document 1, a steel part excellent in surface fatigue strength and bending fatigue strength is processed in the order of a chemical surface hardening treatment process in which carbon is impregnated on the surface of carburizing steel, a shot peening process, and a hard coating treatment process. is proposed.

Further, Patent Document 2 proposes a high-strength gear in which a coating containing iron nitride and/or iron carbonitride is applied to the meshing surface of the gear after carburizing or carbonitriding. Further, Patent Document 3 proposes a method of manufacturing high contact pressure resistant parts in which steel for machine structural use is used to form a hard coating after carburizing or carbonitriding treatment, followed by quenching and tempering.

Japanese Patent Application Laid-Open No. 2005-23399 Japanese Patent Application Laid-Open No. 2002-286115 Japanese Patent Application Laid-Open No. 2000-8121

In recent years, the demand for weight reduction of various parts has been increasing more and more for environmental friendliness. In order to meet this demand, parts are required to have higher fatigue strength. In addition, for contact-type parts such as gears, the viscosity of lubricating oil is being reduced in order to reduce energy loss (friction loss) due to friction. Lowering the viscosity of lubricating oil increases the coefficient of friction and is a factor in reducing the fatigue life. Therefore, it is becoming necessary to improve the contact fatigue characteristics of steel parts in particular. Against this background, conventional carburized steel parts often do not have sufficient fatigue properties.

In the technique described in Patent Literature 1, surface impressions are formed on the surface of the steel sheet due to shot peening because the hard coating treatment process follows the shot peening process. When a hard coating is formed on such a surface, the adhesion between the coating and the steel is insufficient, and the coating peels off early due to contact fatigue, resulting in deterioration of fatigue characteristics.

In the technique described in Patent Document 2, in order to provide a coating containing iron nitride and/or iron carbonitride on the meshing surface of a gear, these iron nitrides and/or iron carbonitrides vacancies called porosity may occur. Such a defect becomes a starting point of fatigue fracture and deteriorates fatigue properties.

In the technique described in Patent Document 3, after carburizing or carbonitriding steel for machine structural use, a hard coating is formed and then quenched and tempered. During quenching of this steel, heat treatment strain is introduced with martensite transformation. This causes a difference in the amount of expansion and contraction between the hard coating and the steel material, and as a result, peeling of the coating from the steel material is unavoidable. Furthermore, in the steel for machine structural use described in Patent Document 3, since Al is not added, oxide-based inclusions are coarsely dispersed, resulting in a decrease in contact fatigue characteristics.

The present invention has been developed in view of the above circumstances, and an object of the present invention is to provide a steel part having particularly excellent durability against contact-type fatigue (hereinafter also referred to as contact fatigue characteristics).

In order to achieve the above object, the inventors have extensively studied the effects of hard coatings on the fatigue properties of steel parts that have been carburized or carbonitrided. The inventors have found that the life against contact fatigue can be effectively improved by applying a hard coating that is not a nitride, and have completed the present invention.
That is, the gist of the present invention is as follows.

1. C: 0.10 to 0.35% by mass,
Si: 0.01 to 0.70% by mass,
Mn: 0.30-1.50% by mass,
P: 0.1% by mass or less,
S: 0.5% by mass or less,
Cr: 0.30 to 2.00% by mass,
A steel containing Al: 0.010 to 0.080% by mass and N: 0.0250% by mass or less, the balance being Fe and unavoidable impurities, and a structure having a total area ratio of martensite and retained austenite of 90% or more. and a hard coating present in close contact with the surface of the steel portion,
Having a hardened layer of a carburized layer or a carbonitrided layer on the steel portion immediately below the hard coating,
The ten-point average roughness of the interface between the steel portion immediately below the hard coating and the hard coating is 1 to 5 μm,
The hard film is any one of titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), chromium nitride (CrN), vanadium carbide (VC) and hard carbon film (diamond-like carbon) One or more steel parts.

2. 2. The steel part according to 1 above, wherein the ratio Hf/Hs between the Vickers hardness Hs of the hardened layer and the Vickers hardness Hf of the hard coating is 2.0 or more and 10.0 or less.

3. 3. The steel component according to 1 or 2 above, wherein the hard coating is a hard carbon coating, and the ratio Hf/Hs between the Vickers hardness Hs of the hardened layer and the Vickers hardness Hf of the hard coating is 3.8 or more and 10.0 or less. .

4. 4. The steel part according to any one of 1 to 3, wherein the hard coating has a thickness of 0.2 μm or more and 25.0 μm or less.

5. The component composition further includes:
Mo: less than 0.30% by mass,
Cu: 1% by mass or less,
5. The steel part according to any one of 1 to 4 above, containing one or more selected from Ni: 1% by mass or less and B: 0.01% by mass or less.

6. The component composition further includes:
Ti: 0.1% by mass or less,
Nb: 0.1% by mass or less,
V: 0.1% by mass or less,
Hf: 0.1% by mass or less,
Ta: 0.1% by mass or less and
6. The steel part according to any one of 1 to 5 above, containing at least one selected from Se: 0.3% by mass or less.
7. The component composition further includes:
Sn: 0.1% by mass or less and
7. The steel part according to any one of 1 to 6 above, containing one or two of Sb: 0.1% by mass or less.

8. The component composition further includes:
Pb: 0.3% by mass or less and
Bi: 0.3% by mass or less,
8. The steel component according to any one of 1 to 7 above, containing any one or two of

9. A steel material having the chemical composition described in any one of 1, 5 to 8 above is processed into a desired shape by forging and cutting or a combination thereof, and then carburized or carbonitrided to obtain a carburized layer or carburized. A hardened layer consisting of a nitride layer is formed, and titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN) is formed on the hardened layer having a surface roughness (ten-point average roughness) of 1 to 5 μm. ), chromium nitride (CrN), vanadium carbide (VC) and hard carbon film (diamond-like carbon).

INDUSTRIAL APPLICABILITY According to the present invention, as a result of improving the fatigue properties of steel parts, it is possible to reduce the size and weight of the parts, which is very useful industrially.

The steel component of the present invention will be described in detail.
First, a steel component is a component made of steel, ie, has a steel portion. The reason for limiting the chemical composition of the steel portion will be explained for each component element.
C: 0.10-0.35% by mass
C is preferably 0.10% by mass or more in order to increase the hardness of the central portion by quenching after the carburizing heat treatment. On the other hand, if the C content exceeds 0.35% by mass, the toughness of the core after quenching is lowered, so the C content is made 0.35% by mass or less. It is preferably in the range of 0.13-0.27% by mass, more preferably in the range of 0.15-0.25%.

Si: 0.01-0.70% by mass
Si is required as a deoxidizing agent and is added in an amount of at least 0.01% by mass. On the other hand, excessive addition of Si promotes preferential oxidation of Si in the surface layer of the carburized layer or the carbonitrided layer, resulting in the formation of a grain boundary oxidized layer. In PVD or CVD treatment, such a grain boundary oxide layer impairs the adhesion between the coating and the steel material. To avoid this, the upper limit of Si is set at 0.70% by mass. It is preferably 0.05 to 0.65% by mass, more preferably 0.10 to 0.35% by mass.

Mn: 0.30-1.50% by mass
Mn is an element effective in improving hardenability and is added in an amount of at least 0.30% by mass. On the other hand, excessive addition of Mn causes an increase in deformation resistance due to solid solution strengthening, so the upper limit is made 1.50% by mass. It is preferably 0.40 to 1.30% by mass, more preferably 0.70 to 1.20% by mass.

P: 0.1% by mass or less P segregates at grain boundaries and lowers toughness, so the lower the content, the better, but up to 0.1% by mass is permissible. Preferably, it is 0.02% by mass or less. There is no problem even if the lower limit is not particularly limited, but wasteful reduction of P increases the refining time and increases the refining cost, so it is preferably 0.003% by mass or more.

S: 0.5% by mass or less S exists as sulfide inclusions and is an element effective in improving machinability. and The lower limit of S content is not particularly limited, but it is preferable to set it to 0.003% by mass or more because excessively reducing the S content increases the refining cost. It is more preferably 0.004 to 0.300% by mass, further preferably 0.005 to 0.090% by mass.

Cr: 0.30-2.00% by mass
Cr is an element that contributes to the improvement of hardenability and temper softening resistance, and is also useful in promoting the spheroidization of carbides. On the other hand, if it exceeds 2.00% by mass, it promotes excessive carburization and formation of retained austenite, adversely affecting fatigue strength. Therefore, the Cr content should be in the range of 0.30 to 2.00% by mass. It is preferably in the range of 0.7-1.9% by mass, more preferably 0.80-1.24% by mass.

Al: 0.010-0.080% by mass
Al is an element that forms oxides and is effective in deoxidizing, and has the effect of suppressing the formation of coarse oxide-based inclusions. poor. However, excessive addition leads to an increase in inclusions, increases the starting points of fatigue fracture, and causes low fatigue strength, so the upper limit is made 0.080% by mass. It is preferably 0.015 to 0.080% by mass, more preferably 0.015 to 0.060% by mass. Further, improvement of hardenability by solid solution B in combination with B is also effective for improvement of fatigue strength.

N: 0.0250 mass % or less N combines with Al to form nitride (AlN). Such AlN precipitates finely and has the effect of refining crystal grains during carburizing heating and improving fatigue properties. However, excessive addition causes surface cracks in steel slabs after casting, so the upper limit is 0.0250% by mass. Although the lower limit is not particularly limited, it is preferably 0.0010% or more because an excessive reduction in N increases the refining cost. It is more preferably 0.0015 to 0.0180% by mass, further preferably 0.0020 to 0.0150% by mass.
The balance other than the elements explained above is Fe and unavoidable impurities.

Although the preferred basic ingredients of the present invention have been described above, in the present invention, the following ingredients can be added as appropriate.
Mo: less than 0.30% by mass,
Cu: 1% by mass or less,
One or more selected from Ni: 1% by mass or less and B: 0.01% by mass or less

Mo: less than 0.30% by mass
Mo contributes to the improvement of hardenability and resistance to temper softening, and further exhibits the effect of reducing abnormal carburization layers, and is a useful element, so it may be added. However, if the content is 0.30% by mass or more, there is a concern that the hardenability will be excessive, the hardness after rolling will increase, and the forgeability and machinability will deteriorate. Therefore, it is preferable to limit the Mo content to less than 0.30% by mass. In order to bring out the above-mentioned effects of Mo such as improvement of hardenability, resistance to temper softening, and reduction of abnormal carburization layer, Mo is preferably contained in an amount of 0.01% by mass or more. Furthermore, it is preferably in the range of 0.03 to 0.25% by mass. More preferably 0.05 to 0.22% by mass.

Cu: 1% by mass or less
Cu is an element that contributes to the improvement of hardenability. In order to obtain this effect, Cu is preferably contained in an amount of 0.01% by mass or more. On the other hand, if the Cu content exceeds 1% by mass, the surface texture of the rolled material may become rough and may remain as flaws. Therefore, it is preferable to limit the amount of Cu to a range of 1% by mass or less. More preferably, it is in the range of 0.015-0.500% by mass. More preferably, it is 0.03 to 0.30% by mass.

Ni: 1% by mass or less
Ni is an element that contributes to improving hardenability and is useful for improving toughness. In order to obtain these effects, Ni is preferably contained in an amount of 0.01% by mass or more. On the other hand, even if the content exceeds 1% by mass, the above effects are saturated. Therefore, it is preferable to limit the Ni content to a range of 1% by mass or less. More preferably, it is in the range of 0.015-0.500% by mass. More preferably, it is 0.03 to 0.30% by mass.

B: 0.01% by mass or less B segregates at grain boundaries and suppresses diffusion type transformation, which is effective for improving hardenability.In addition, it strengthens grain boundaries and suppresses the occurrence and propagation of fatigue cracks. It also has the effect of improving fatigue strength. In order to obtain this effect of B, it is preferable to contain B in an amount of 0.0003% by mass or more. On the other hand, if the B content exceeds 0.01% by mass, the toughness decreases, so the B content is preferably limited to a range of 0.01% by mass or less. More preferably, it ranges from 0.0005 to 0.0050% by mass. More preferably, it is 0.0007 to 0.0020% by mass.

Furthermore, if necessary, each component shown below can be added as appropriate.
Nb: 0.1% by mass or less,
Ti: 0.1% by mass or less,
V: 0.1% by mass or less,
Hf: 0.1% by mass or less,
Ta: 0.1% by mass or less and
Se: One or more selected from 0.3% by mass or less

Nb: 0.1% by mass or less
The addition of Nb has the effect of suppressing grain coarsening during carburizing and improving fatigue properties. However, even if it is added in excess of 0.1% by mass, the effect is saturated and economically disadvantageous, so the Nb content is preferably 0.1% by mass or less. More preferably, it is 0.005 to 0.080% by mass.
More preferably, it is 0.01 to 0.06% by mass.

Ti: 0.1% by mass or less
Addition of Ti has the effect of suppressing surface cracks after casting. However, even if it is added in excess of 0.1% by mass, the effect is saturated and becomes economically disadvantageous, so the Ti content is preferably 0.1% by mass or less. More preferably, it is 0.005 to 0.080% by mass. More preferably, it is 0.01 to 0.06% by mass.

V: 0.1 mass % or less V forms VC in the steel and suppresses coarsening of the austenite grain size during carburizing heat treatment due to the pinning effect. In order to obtain this effect of V, it is preferable to contain V in an amount of at least 0.003% by mass or more. On the other hand, even if it is added in excess of 0.1% by mass, the effect of preventing grain coarsening is saturated, but the alloy cost only increases. Therefore, the V content is preferably 0.1% by mass or less. More preferably, it is 0.005 to 0.080% by mass. More preferably, it is 0.01 to 0.06% by mass.

Hf: 0.1% by mass or less
Hf forms carbides in steel and suppresses coarsening of austenite grains during carburizing heat treatment by a pinning effect. To obtain this effect of Hf, it is preferred to add Hf in an amount of at least 0.003% by weight. On the other hand, if the Hf content exceeds 0.1% by mass, coarse precipitates are formed during casting solidification, which may lead to a decrease in the ability to suppress grain coarsening and deterioration in fatigue strength. It is preferable to More preferably, it is 0.005 to 0.060% by mass. More preferably, it is 0.01 to 0.05% by mass.

Ta: 0.1% by mass or less
Ta forms carbides in steel and suppresses coarsening of austenite grains during carburizing heat treatment by a pinning effect. In order to obtain this effect of Ta, it is preferable to add Ta in an amount of at least 0.003% by mass. On the other hand, if it is added in excess of 0.1% by mass, cracks are likely to occur during casting solidification, and there is a concern that flaws may remain even after rolling and forging. Therefore, the Ta content is preferably 0.1% by mass or less. . More preferably, it is 0.005 to 0.060% by mass. More preferably, it is 0.01 to 0.05% by mass.

Se: 0.3% by mass or less
Se combines with Mn and Cu and disperses as precipitates in steel. Se precipitates are stably present in the carburizing heat treatment temperature range with almost no precipitate growth, and have a high austenite grain size pinning effect. Therefore, the addition of Se is effective in preventing coarsening of crystal grains, and in order to obtain this effect, it is preferable to add Se in an amount of at least 0.001% by mass or more. On the other hand, even if it is added in excess of 0.3% by mass, the effect of preventing coarsening of crystal grains is saturated. Therefore, the Se content is preferably 0.3% by mass. More preferably, it is 0.005 to 0.100% by mass. More preferably, it is 0.008 to 0.090% by mass.

Furthermore, if necessary, each component shown below can be added as appropriate.
Sn: 0.1% by mass or less and
Sb: Contains one or more selected from 0.1% by mass or less
Sb: 0.1% by mass or less
Sb is an element effective for suppressing decarburization of the steel material surface and preventing a decrease in surface hardness. In order to exhibit this effect, Sb is preferably contained in an amount of 0.0003% by mass or more. On the other hand, since excessive addition deteriorates the forgeability, the Sb content is preferably 0.1% by mass or less. More preferably 0.001 to 0.050% by mass, still more preferably 0.0015 to 0.0350% by mass.

Sn: 0.1% by mass or less
Sn is an effective element for improving the corrosion resistance of the steel material surface. From the viewpoint of improving corrosion resistance, Sn is preferably contained in an amount of 0.003% by mass or more. On the other hand, since excessive addition deteriorates the forgeability, the Sn content is preferably 0.1% by mass or less. It is more preferably 0.0010 to 0.0500% by mass, still more preferably 0.0015 to 0.0350% by mass.

Furthermore, if necessary, each component shown below can be added as appropriate.
Pb: 0.3% by mass or less and
Bi: Contains one or more selected from 0.3% by mass or less
Pb and Bi have the effect of making chips finer during cutting, and addition of these elements is effective for improving chip disposability. In order to obtain this effect, Pb and Bi are preferably added in an amount of 0.01% by mass or more. However, even if these elements are excessively added, the effect of improving the chip disposability is saturated, which is economically disadvantageous. Therefore, in order to suppress an increase in alloy cost, the upper limits of the amounts of Pb and Bi are each set to 0.3% by mass. More preferable Pb content and Bi content are respectively 0.01 to 0.20% by mass, more preferably 0.01 to 0.10% by mass.

The steel part in the steel part of the present invention has, in addition to the chemical composition described above, a structure in which the total area ratio of martensite and retained austenite is 90% or more, and a hard material existing in close contact with the surface of the steel part. It has a coating. Furthermore, a hardened layer of a carburized layer or a carbonitrided layer is formed directly under the hard coating of the steel portion, that is, the portion where the hard coating of the steel portion adheres, and the steel portion directly under the hard coating and the hard coating are formed. The ten-point average roughness of the interface is 1 to 5 μm.
Each of these requirements will be described below.

[Steel structure]
It is essential that the steel parts of the present invention have a structure in which the total area ratio of martensite and retained austenite is 90% or more. This is because it is necessary to ensure the strength of steel. The total area ratio of martensite and retained austenite is preferably 95% or more, and may be 100%. The structure of the remainder is not particularly limited.

[Hardened layer]
The surface of the steel part of the steel part, that is, the part of the steel part to which the hard coating is to be applied, is a hardened layer formed by a carburized layer or a carbonitrided layer. This is because fatigue strength is improved by hardening the surface of steel parts by carburizing or carbonitriding.

Here, the carburized layer or carbonitrided layer is obtained by a general carburizing treatment or carbonitriding treatment.

In order to ensure fatigue strength, it is preferable that the hardened layer formed of the above-described carburized layer or carbonitrided layer has a depth of 0.3 mm or more at which the Vickers hardness is 550 or more.

[Hard coating]
Furthermore, it is essential to have a hard coating other than iron carbides and iron nitrides on the hardened layer. When a hard coating of iron carbide and iron nitride is formed on the hardened layer, porosity is generated in the hard coating, making it easier for the hard coating to peel off, thereby deteriorating fatigue characteristics. When a hard coating other than iron carbide and iron nitride is provided on the hardened layer, the hard coating does not become porous, and the effect of improving the contact fatigue life of the hard coating can be obtained.

Hard coatings other than iron carbide and iron nitride include titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), chromium nitride (CrN), vanadium carbide (VC), hard carbon A film (diamond-like carbon: DLC) is suitable. Since these hard coatings themselves have high strength and also have a lubricating action, the contact fatigue life can be improved by exhibiting this lubricating action.

The thickness of the hard coating is preferably 0.2 μm or more and 25.0 μm or less. That is, a hard coating thickness of 0.2 μm or more is sufficient to improve the contact fatigue life. On the other hand, an excessive increase in the film thickness lengthens the time required for the film forming process, which is disadvantageous in terms of cost. Therefore, the upper limit of the film thickness is preferably 25 μm.

[Interface between the steel portion immediately below the hard coating and the hard coating]
The surface roughness at the interface between the steel portion immediately below the hard coating and the hard coating should be 1 to 5 μm in ten-point mean roughness. That is, if the surface roughness of the object on which the hard coating is to be formed becomes excessively small, for example, by subjecting the hard coating to precise polishing after the carburizing heat treatment, the adhesion between the coating and the object decreases. On the other hand, when the surface roughness of the object becomes excessively large, the contact fatigue life tends to decrease. Therefore, the surface roughness of the object before the application of the hard coating should be 1 to 5 μm in terms of ten-point mean roughness.

The surface roughness of the object before application of the hard coating means the surface roughness of the object after application of the hard coating, that is, the surface roughness of the interface between the steel portion having the hard coating and the hard coating in the part having the coating. As described above, the fact that the surface roughness (ten-point average roughness) of the object before coating is 1 to 5 μm means that in the steel parts of the present invention, the cross section is observed using a scanning electron microscope. Then, the contour profile of the boundary between the steel part and the hard coating is obtained by image analysis and used as a roughness curve. The average sum of valley depths up to the fifth in order (ten-point average roughness) is 1 to 5 μm.

Also, the ratio Hf/Hs between the Vickers hardness Hs of the hardened layer and the Vickers hardness Hf of the hard coating (hereinafter simply referred to as hardness ratio Hf/Hs) is preferably 2.0 or more and 10.0 or less. If the hardness ratio Hf/Hs is less than 2.0, the hard coating does not contribute to improving the contact fatigue life. On the other hand, in order for the hardness ratio Hf/Hs to exceed 10.0, the Vickers hardness Hs of the hardened layer must be extremely low, or the Vickers hardness Hf of the hard coating must be high. Considering that the Vickers hardness of the hardened layer is 550 or more as described above from the viewpoint of ensuring fatigue strength, and the hardness of the various hard coatings described above, the realistic Hf/Hs is in the range of 2.0 to 10.0. is. When the hard film is a hard carbon film (diamond-like carbon: DLC), Hf/Hs is in the range of 3.8-7.0.

[Production method]
The above-described steel parts are produced by forging, cutting, or a combination of these processes into a steel material into a desired shape, followed by carburizing or carbonitriding to form a hardened layer consisting of a carburized layer or a carbonitrided layer, It can be produced by forming a hard coating on the cured layer by PVD treatment or CVD treatment.

[Carburizing treatment or carbonitriding treatment]
First, the carburizing process involves quenching at a temperature of 830° C. or higher in a carburizing atmosphere with a carbon potential of 0.5 to 1.2% and cooling in oil. After that, tempering treatment at 120° C. or higher may be performed.

In the carbonitriding process, the steel is quenched at a temperature of 750° C. or higher in an atmosphere with a carbon potential of 0.5 to 1.2% and a nitrogen potential of 0.1 to 1.2%, and then cooled in oil. After that, tempering treatment at 120° C. or higher may be performed.

Since the PVD treatment or CVD treatment following the carburizing treatment or carbonitriding treatment is performed in a high temperature range as described later, tempering after quenching in carburizing or carbonitriding may be omitted.

In addition, PVD treatment or CVD treatment is applied after carburizing treatment or carbonitriding treatment, and the surface roughness (ten-point average roughness) of the target part to be subjected to PVD treatment or CVD treatment is 1 to 5 μm as described above. . The surface roughness (ten-point average roughness) here can be adjusted by mechanical grinding or polishing.

In particular, the above-described treatment temperature in PVD treatment and CVD treatment is preferably 700° C. or lower in order to suppress softening of the steel material. The temperature is more preferably 600°C or lower, and most preferably 500°C or lower.

Quenching after PVD treatment or CVD treatment should be avoided because it reduces the adhesion between the hard coating and the steel material.

Hereinafter, the configuration and effects of the present invention will be described more specifically according to examples. It should be noted that the present invention is not limited by the following examples, and can be modified as appropriate within the scope of the gist of the present invention, and any of these are included in the technical scope of the present invention. be

A steel having the chemical composition shown in Table 1 was melted and formed into a round bar with a diameter of 30 mm by hot rolling. A roller pitting test piece was taken from the obtained steel bar. Here, the surface roughness of the surface to be tested of the roller pitching test piece was adjusted by changing the machining conditions when collecting the roller pitching test piece. Using steel of the same steel No. as a test material, multiple test pieces that were machined under the same conditions were collected, and for some of these, the cross-section was observed using a scanning electron microscope. The contour profile of the boundary between the steel and the coating is obtained by image analysis and used as a roughness curve. The average sum of valley depths up to the fifth (ten-point average roughness) was determined. The test pieces other than those used for surface roughness measurement were subjected to carburizing heat treatment at 930° C. for 3 hours.

That is, the carburizing heat treatment is carried out at 930°C for 2 hours at a carburizing stage carbon potential of 1.0% by mass and 1 hour at a diffusion stage carbon potential of 0.8% by mass, for a total of 3 hours. cooled with After that, a tempering treatment was performed by holding at 180° C. for 1 hour and then air cooling. After this tempering, hard coatings were formed under various conditions shown in Table 2. Here, the ten-point average roughness of the boundary between the hard coating and the steel part having the coating after forming the hard coating is the same as the surface roughness (ten-point average roughness) immediately after processing to the roller pinching test piece. It is confirmed that it is a value. That is, it has been confirmed that the change in the ten-point average roughness before and after the carburizing heat treatment is extremely small and can be ignored. For comparison, under certain conditions, after tempering, polishing (precision processing) of 0.1 mm was performed, and then the ten-point average roughness was measured before the hard coating was applied.

For comparison, the case where no hard coating is applied, the case where the hard coating is quenched at 850°C after the hard coating is applied and then the steel is held at 180°C for 1 hour and then air-cooled is tempered, and the case where the hard coating is formed without carburizing treatment is carried out. bottom.

A roller pitting test was performed on each test piece with and without a hard coating thus obtained. The roller pitting test was conducted at a slip ratio of 40%, a test speed of 3000 rpm, and a low-viscosity oil (kinematic viscosity at 80°C of 7 mm/s) at an oil temperature of 80°C. In addition, the mating roller was made of SUJ2 refining material and crowned R150. The test surface pressure was 3.3 GPa, and five tests were performed, and the upper limit of the number of repetitions was set to 1×10 ⁶ times and discontinued. The number of test pieces in which pitting (peeling) occurred up to the upper limit number of repetitions was evaluated as fatigue characteristics. Table 3 shows the evaluation results.

In addition, in order to evaluate the adhesion of the hard coating, the test piece with the hard coating was further tested one by one at a surface pressure of ^3.3 GPa. Image analysis was performed to determine the peeled area ratio of the hard coating. In this test, the fatigue life is sufficiently long if all five are undamaged.
Steel components with long life fatigue properties are obtained according to the present invention, as shown in Table 3 for the results of the roller pitting test.

Claims (8)

C: 0.10 to 0.35% by mass,
Si: 0.01 to 0.70% by mass,
Mn: 0.30-1.50% by mass,
P: 0.1% by mass or less,
S: 0.5% by mass or less,
Cr: 0.30 to 2.00% by mass,
Al: 0.010 to 0.080% by mass and N: 0.0250% by mass or less, the balance being the composition of Fe and unavoidable impurities, and a structure with a total area ratio of martensite and retained austenite of 90% or more , After the steel material is processed into a desired shape by forging, cutting, or a combination thereof, it is subjected to carburizing or carbonitriding to form a hardened layer consisting of a carburized layer or a carbonitrided layer. Titanium carbide (TiC), titanium nitride (TiN), titanium carbonitride (TiCN), chromium nitride (CrN), vanadium carbide (VC) and Form a hard coating of any one or more of hard carbon films (diamond-like carbon), and quench and temper after forming the hard coating
A method for manufacturing a steel part without any treatment . 2. The method of manufacturing a steel part according to claim 1, wherein the ratio Hf/Hs of the Vickers hardness Hs of the hardened layer to the Vickers hardness Hf of the hard coating is 2.0 or more and 10.0 or less. 3. The steel according to claim 1 or 2, wherein the hard coating is a hard carbon coating, and the ratio Hf/Hs between the Vickers hardness Hs of the hardened layer and the Vickers hardness Hf of the hard coating is 3.8 or more and 10.0 or less. How the parts are made . 4. The method of manufacturing a steel part according to claim 1, wherein the hard coating has a thickness of 0.2 [mu]m or more and 25.0 [mu]m or less. The component composition further includes:
Mo: less than 0.30% by mass,
Cu: 1% by mass or less,
5. The method for manufacturing a steel part according to any one of claims 1 to 4, wherein one or more selected from Ni: 1% by mass or less and B: 0.01% by mass or less are contained. The component composition further includes:
Ti: 0.1% by mass or less,
Nb: 0.1% by mass or less,
V: 0.1% by mass or less,
Hf: 0.1% by mass or less,
Ta: 0.1% by mass or less and
Se: The method for manufacturing a steel part according to any one of claims 1 to 5, wherein Se contains at least one selected from 0.3% by mass or less. The component composition further includes:
Sn: 0.1% by mass or less and
7. The method for manufacturing a steel part according to any one of claims 1 to 6, wherein Sb: 0.1% by mass or less of either one or two of them is contained. The component composition further includes:
Pb: 0.3% by mass or less and
Bi: 0.3% by mass or less,
8. The method for manufacturing a steel component according to any one of claims 1 to 7, containing any one or two of