JP2020033636A - Component and manufacturing method therefor - Google Patents

Abstract

To provide a component less in heat treatment strain and excellent in abrasion resistance and fatigue characteristic, and a manufacturing method therefor.SOLUTION: A component consists of a prescribed constituent, has a component composition with a Z value defined by the following (1) formula of 30 or more, in which a structure by depth of 100 μm from a surface of the component has volume percentage of retained austenite of 30% or less, the balance consists of one or both of martensite and bainite, a structure other than the nitrided part contains ferrite 5 to 60 area%, and one or both of martensite and bainite of 40 to 95 area% as total. Z=186-449C+38Si-8Mn-4 Ni+7Cu-29Cr+15Mo+165V+142Ti+368Al+4210B (1).SELECTED DRAWING: None

Description

  The present invention relates to parts, for example, sun gears, ring gears, shaft gears and the like used in transmissions, pulleys for CVT, and automobile parts such as crankshafts, timing gears, cam gears, camshafts, injection nozzles used in engines, or For example, the present invention relates to a component which can be suitably used as a component for various industrial devices such as gears for reduction gears and various shafts, has a small heat treatment distortion, and has excellent wear resistance and fatigue characteristics. The present invention also relates to a method for manufacturing the component.

  In recent years, shafts and gears used in automobiles and the like have been required to be reduced in size in accordance with the weight reduction of the vehicle body for energy saving, but the load has been increased due to the increase in engine output and electrification. Therefore, improvement in durability is required. On the other hand, from the viewpoint of labor saving, omitting the manufacturing process is being studied, and it is also important that distortion removal after heat treatment can be omitted.

Conventionally, as a high-strength member, a part obtained by forming case-hardened steel such as JIS SCM420H or SNCM420H and performing carburizing, quenching and tempering has been used. In addition, nitrocarburizing treatment has been used for the production of components that require a small distortion after heat treatment.

  Furthermore, in recent years, it has been studied to adopt a nitriding and quenching process to produce a component that achieves both strength and low distortion. For example, Patent Literatures 1 to 3 propose a method of controlling the hardness distribution and the structure of a part by improving a conventionally practiced nitriding and quenching method. Patent Literatures 4 and 8 propose nitriding and quenching using plasma as a new method of nitriding and quenching.

  Patent Literature 5 proposes a method of controlling the nitrogen concentration in the surface layer of a component by controlling the atmosphere in a furnace during nitriding. Patent Document 6 proposes a screw component to which nitriding and quenching is applied.

  Patent Document 7 proposes a method of performing nitriding and quenching on steel having a specific component composition. According to the method, a target hardness distribution can be obtained even if the time of nitriding and quenching is short, and the fatigue strength is improved by controlling the structure of the surface layer.

  Patent Literature 9 proposes a gear in which a limited material is carburized, quenched and tempered and tempered to suppress heat treatment distortion and thereby improve fatigue characteristics.

JP 2009-270155 A JP 2010-138460 A JP 2010-229524 A JP 2014-111821 A JP-A-2005-025161 JP 2015-212559 A Japanese Patent No. 5617747 Japanese Patent No. 5944797 Japanese Patent No. 5872863

  However, in Patent Literatures 1 to 4 and 8, although an attempt is made to improve the nitriding and quenching method, the same material as in the related art is used. Therefore, it is hard to say that the characteristics are greatly improved as compared with the conventional product.

  Further, according to the methods proposed in Patent Literatures 5 and 6, although the wear resistance of components can be improved to some extent, it cannot be said that the wear resistance is significantly improved. In addition, the above method cannot improve other fatigue characteristics.

  According to the method proposed in Patent Literature 7, the current required strength can be maintained by controlling the surface layer structure and the hardness distribution, but it is insufficient to obtain a higher fatigue strength.

  In the carburizing and nitriding treatment proposed in Patent Literature 9, the strain after quenching can be reduced to some extent, but because of the high heat treatment temperature, it is not reduced as compared with the quenched and quenched material. In addition, in order to secure the amount of nitrogen dissolved in the surface layer in order to suppress tempering softening, if carburization is performed, the intrusion of carbon is hindered, so that a large amount of nitrogen cannot be dissolved, so that tempering softening is suppressed. No effect.

  The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide a component having a small heat treatment distortion, and having excellent wear resistance and fatigue characteristics, and a method for manufacturing the same.

  The present inventors have earnestly studied to achieve the above object, and have obtained the following findings.

(1) Nitrogen is an element having a function of strengthening by dissolving in a steel base. Therefore, by including nitrogen, the fatigue strength of the steel component can be improved. Further, the nitrogen dissolved in the base material of the component forms fine nitrides when the component generates heat during use, and the nitride is dispersed to strengthen the material. Therefore, the inclusion of nitrogen is effective from the viewpoint of improving the resistance to fatigue accompanied by wear.

(2) For this reason, it is effective to perform the quenching and quenching treatment in order to improve the wear resistance and the fatigue strength of the part. However, in order to improve the fatigue strength of the quenched and quenched part, the surface of the part is hardened. In addition to increasing the hardness and increasing the thickness of the hardened layer, it is necessary to increase the hardness in the non-nitrided region. Therefore, from the viewpoint of increasing the fatigue strength, it is desirable that not only the surface layer of the component but also the internal (non-nitrided region) structure be martensite or bainite.

(3) However, when the hardness in the non-nitriding region is improved, the distortion (heat treatment distortion) at the time of quenching increases. The heat treatment distortion is caused by volume expansion that occurs when austenite is transformed into martensite or bainite during quenching. Therefore, in order to suppress expansion during quenching and reduce heat treatment distortion, it is necessary not to transform the internal structure or to reduce the amount of transformation.

(4) As described above, reduction of heat treatment strain and improvement of fatigue strength are contradictory issues. However, when the surface layer structure is martensite and / or bainite and the internal structure is a structure containing ferrite in addition to martensite and / or bainite, it is possible to achieve both a reduction in heat treatment strain and an improvement in fatigue strength. it can.

(5) By strictly controlling the component composition of the steel to satisfy specific conditions, the above-described surface layer structure and internal structure can be realized in the quenched and quenched part.

(6) In addition, from the viewpoint of increasing the internal hardness, it is necessary to strengthen ferrite which is a soft structure, and for that purpose, it is effective to form a solid solution of Si in ferrite and strengthen it. Further, Si has an effect of suppressing softening due to tempering. Therefore, by containing a predetermined amount of Si, it is possible to prevent the surface layer portion from being softened by frictional heat during use of the component, and to improve wear resistance.

  The present invention has been made based on the above findings, and the gist configuration thereof is as follows.

1. A part made of steel and having a nitriding portion on a surface layer,
The steel, in mass%,
C: 0.08 to 0.50%,
Si: more than 0.80%, 1.80% or less Mn: 0.30 to 2.50%,
P: 0.100% or less,
S: 0.100% or less,
Cr: 0.05-2.50%,
Al: 0.005 to 0.080%, and N: 0.0020 to 0.0300%,
And a component composition comprising the balance of Fe and unavoidable impurities, and having a Z value defined by the following formula (1) of 30 or more,
The structure from the surface of the component to a depth of 100 µm has a volume fraction of retained austenite of 30% or less, and the balance consists of one or both of martensite and bainite;
A component in which the structure other than the nitrided portion contains ferrite in an area ratio of 5 to 60% and one or both of martensite and bainite in a total area ratio of 40 to 95%.
Z = 186-449C + 38Si-8Mn-4Ni + 7Cu-29Cr + 15Mo + 165V + 142Ti + 368Al + 4210B (1)
(However, the element symbol in the formula (1) indicates the content (% by mass) of each element in the steel, and is zero when not contained.)

2. The component composition is represented by mass%,
Cu: 0.50% or less,
2. The component according to the above 1, further comprising one or more selected from the group consisting of Ni: 2.0% or less and Sb: 0.0050% or less.

3. The component composition is represented by mass%,
Mo: 0.80% or less,
V: 0.200% or less,
The component according to 1 or 2, further containing one or more selected from the group consisting of Ti: 0.200% or less and B: 0.0080% or less.

4. A method for producing a part, comprising: processing a steel having the component composition described in any one of the above items 1 to 3 to form a part, performing a nitriding treatment, and then quenching.

5. 5. The method for manufacturing a component according to the above item 4, wherein after the quenching, tempering is further performed at a temperature of 150C to 350C.

  ADVANTAGE OF THE INVENTION According to this invention, the heat treatment distortion is small and the component excellent in wear resistance and fatigue characteristics can be obtained. The component of the present invention can be suitably used for automobiles, industrial machines, and the like, and does not require distortion removal after heat treatment, and is therefore extremely useful in industry.

It is a schematic diagram of the test piece used for the measurement of the heat processing deformation amount in an Example. It is a schematic diagram of the test piece used for the rotating bending fatigue test in an Example. It is a schematic diagram of the test piece used for the roller pitching test in an Example. It is a schematic diagram which shows the nitriding-hardening process conditions in an Example. It is a schematic diagram which shows the carburizing and quenching and quenching processing conditions in an Example.

  Next, a method for carrying out the present invention will be specifically described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited by the following description.

  The component in one embodiment of the present invention is made of steel and has a nitrided portion on the surface. First, the composition of the steel will be described. In the following description, “%” means “% by mass” unless otherwise specified.

C: 0.08 to 0.50%
C is an element necessary for securing the strength. When the C content is less than 0.08%, the internal hardness cannot be secured, the bending stress resistance is reduced, and the fatigue strength cannot be secured. Therefore, the C content is set to 0.08% or more. On the other hand, if the C content is more than 0.50%, the amount of retained austenite in the surface layer becomes excessive and the surface hardness is reduced, and the internal toughness is deteriorated, so that the durability against bending stress and surface pressure is deteriorated. Therefore, the C content is set to 0.50% or less.

Si: more than 0.80% and 1.80% or less Si is an element having a function of strengthening by solid solution in ferrite in addition to a deoxidizing function. Further, Si has the effect of suppressing the softening due to friction and increasing the fatigue strength by increasing the tempering softening resistance. In order to obtain the above effect, the Si content is set to more than 0.80%. On the other hand, if the Si content exceeds 1.80%, the transformation temperature rises too much, so that in the nitriding temperature region, a two-phase structure is maintained up to the surface layer, and the penetration of nitrogen is slowed down, and a necessary hardened layer is obtained. Can not be. Therefore, the Si content is set to 1.80% or less.

Mn: 0.30 to 2.50%
Mn is an element that enhances hardenability. In order to ensure hardenability, the Mn content is set to 0.30% or more. On the other hand, if added in excess of 2.50%, the transformation point is too low, and the martensite and / or bainite in the internal structure becomes excessive. As a result, the heat treatment deformation becomes too large. Therefore, the Mn content is set to 2.50% or less. The Mn content is preferably 2.00% or less.

P: 0.100% or less P is an element contained in steel as an inevitable impurity. If the P content exceeds 0.100%, P segregates at the grain boundaries, embrittles the grain boundaries, and as a result, fatigue properties deteriorate. Therefore, the P content is set to 0.100% or less. On the other hand, the lower limit of the P content is not limited, and may be 0%. However, if the P content is reduced more than necessary, it is disadvantageous from the viewpoint of manufacturing cost. It is preferably at least 005%.

S: 0.100% or less S is an element contained in steel as an inevitable impurity, and combines with Mn in steel to form MnS. If the S content exceeds 0.100%, the fatigue fracture due to the MnS starting point occurs at low strength, and the fatigue strength deteriorates. Therefore, the S content is set to 0.100% or less. On the other hand, the lower limit of the S content is not limited. However, since the MnS has an action of improving machinability, in order to form MnS with the expectation of the action, the S content is set to 0.1%. It is preferably set to 003% or more.

Cr: 0.05-2.50%
Cr is an element that improves the hardenability and also increases the tempering softening resistance. In order to exert both effects, the Cr content needs to be 0.05% or more. On the other hand, if the Cr content exceeds 2.50%, the effect of increasing the softening resistance is saturated, and the hardenability becomes too high, so that the internal toughness is deteriorated, and the fatigue crack progresses rapidly, resulting in bending fatigue. Strength decreases. Therefore, the Cr content is set to 2.50% or less.

Al: 0.005 to 0.080%
Al is an element effective for deoxidation, and its effect is exhibited by adding 0.005% or more. Further, Al combines with N to generate AlN, and has a function of suppressing the coarsening of crystal grains. Therefore, the Al content is set to 0.005% or more. On the other hand, if the Al content exceeds 0.080%, coarse grains are rather generated, and fatigue cracks are more likely to develop, and the bending fatigue strength decreases. Therefore, the Al content is set to 0.080% or less.

N: 0.0020 to 0.0300%
N combines with Al to form AlN, and has the effect of suppressing coarsening of crystal grains and improving fatigue strength. To obtain the above effect, the N content is set to 0.0020% or more. On the other hand, when the N content exceeds 0.0300%, not only the effect is saturated, but also defects such as blowholes are generated inside the component, and the bending fatigue strength is reduced. Therefore, the N content is set to 0.0300% or less.

  In one embodiment of the present invention, the composition of the steel may be composed of the above elements, the balance of Fe and inevitable impurities.

  In another embodiment of the present invention, in order to further improve the properties of the steel, one or more selected from the group consisting of Cu, Ni, and Sb are arbitrarily contained in addition to the above component composition. Can be.

Cu: 0.50% or less Cu is an element that forms a solid solution in steel and has the effect of increasing the strength of a steel material. However, if the Cu content exceeds 0.50%, surface defects occur during the production of steel materials, and surface defects tend to occur during the production of parts. In addition, if parts are produced after surface grinding or the like, the cost increases. It is disadvantageous on the contrary. Therefore, when Cu is contained, the Cu content is set to 0.50% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.02% or more from the viewpoint of sufficiently obtaining the effect of adding Cu. In addition, when adding Cu, it is preferable to simultaneously add Ni having a high effect of suppressing the generation of surface flaws.

Ni: 2.0% or less Ni is an element that has the effect of increasing the hardenability and increasing the depth of the hardened layer on the surface while increasing the strength by forming a solid solution in steel. However, Ni is expensive, and excessive addition causes an increase in cost. Therefore, the Ni content is set to 2.0% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but it is preferable that the Ni content be 0.02% or more in order to sufficiently obtain the effect of adding Ni.

Sb: 0.0050% or less Sb is an element having an effect of suppressing decarburization of the surface of a steel material and securing hardness of the surface. However, even if added over 0.0050%, the effect is saturated, so the Sb content is set to 0.0050% or less. On the other hand, the lower limit of the Sb content is not particularly limited, but the Sb content is preferably 0.0005% or more in order to sufficiently obtain the effect of adding Sb.

  In another embodiment of the present invention, in order to further improve the properties of steel, one or more selected from the group consisting of Mo, V, Ti, and B are arbitrarily contained in addition to the above component composition. can do.

Mo: 0.80% or less Mo is an element having an effect of improving hardenability. However, when the Mo content exceeds 0.80%, the quenchability becomes too high and quenching cracks easily occur, and the cost increases because Mo is expensive. Therefore, the Mo content is set to 0.80% or less. On the other hand, the lower limit of the Mo content is not particularly limited. However, in order to sufficiently obtain the effect of improving the hardenability by Mo, the Mo content is preferably 0.01% or more.

V: 0.200% or less V is an element that has the effect of improving the hardenability of steel and increasing the temper softening resistance like Si and Cr. V also has the effect of forming carbonitrides and suppressing the coarsening of crystal grains. However, if the V content exceeds 0.200%, the above effect is saturated, and a sufficient effect corresponding to increasing the V content cannot be obtained, and only the manufacturing cost increases. Therefore, the V content is set to 0.200% or less. On the other hand, the lower limit of the V content is not particularly limited, but it is preferable that the V content be 0.030% or more in order to sufficiently obtain the above effects.

Ti: 0.200% or less Ti is an element having an effect of generating a fine Ti compound in steel to reduce crystal grains after forging to increase strength. However, if the Ti content exceeds 0.200%, the Ti precipitates become coarse and serve as starting points for fatigue failure, resulting in a shortened life. Therefore, the Ti content is set to 0.200% or less. On the other hand, the lower limit of the Ti content is not particularly limited, but it is preferable that the Ti content be 0.005% or more in order to sufficiently obtain the above effects.

B: 0.0080% or less B is an element having the effect of increasing the hardenability by forming a solid solution in steel. However, if the B content exceeds 0.0080%, the effect of the addition is saturated, so the B content is set to 0.0080% or less. On the other hand, although the lower limit of the B content is not particularly limited, it is preferable that the B content be 0.0005% or more in order to sufficiently obtain the above effects.

[Z value]
Furthermore, in the present invention, it is necessary that the component composition of the steel be a composition such that the Z value defined by the following formula (1) is 30 or more.
Z = 186-449C + 38Si-8Mn-4Ni + 7Cu-29Cr + 15Mo + 165V + 142Ti + 368Al + 4210B (1)
However, the element symbol in the above formula (1) represents the content (% by mass) of each element in the steel, and is zero when not contained.

  The Z value must be 30 or less in order to make both the surface layer and the inside of the steel part have a predetermined structure described later after the steel part is nitrided and quenched. When the Z value is less than 30, at least one of the surface layer (nitrided portion) and the inner portion (non-nitrided portion) does not have a desired structure, and as a result, heat treatment distortion is reduced and excellent wear resistance is obtained. And fatigue strength cannot be compatible.

  On the other hand, since the upper limit of the Z value is substantially defined by the upper limit of the content of each element included in the formula (1), it is not necessary to separately limit it, but it can be usually 339 or less. Further, the Z value may be 300 or less, 250 or less, or 160 or less.

[Microstructure]
Next, the reason why the microstructure (hereinafter, simply referred to as “structure”) of a part is limited in the present invention will be described. The component of the present invention has a nitriding portion in the surface layer, that is, a region in which nitrogen enters from the surface and has a higher nitrogen content than the inside, and the inside portion other than the nitriding portion has no nitrogen therein. It becomes a nitriding part. Therefore, in the present invention, the structures in the surface layer of the part where the nitrided part is formed, specifically, both the region from the surface of the part to a depth of 100 μm and the region other than the nitrided part are controlled to a specific range. I do.

[Surface organization]
The structure in the region from the surface of the component to a depth of 100 μm (hereinafter, may be simply referred to as “surface structure”) has a volume fraction of retained austenite (hereinafter, also referred to as “residual γ”) of 30% or less, Is a structure composed of one or both of martensite and bainite. Since the nitrided portion needs to have a high hardness, the surface layer structure is mainly composed of bainite and / or martensite. Although retained austenite is mixed in the steel structure, if the residual austenite exceeds 30% by volume, the hardness decreases, so the volume fraction of the retained austenite in the surface layer structure is set to 30% or less. The remainder of the surface structure is one or both of martensite and bainite. Here, the volume ratio of retained austenite in the surface layer structure is a value measured by the method described in Examples.

  Note that, in the component of the present invention, carburizing treatment is not performed in addition to nitriding treatment. When carburizing is performed, carbon penetrates from the surface and the carbon content of the surface layer becomes higher than that of the inside, but the increase in surface carbon concentration decreases the amount of nitrogen dissolved in the surface layer, deteriorating temper softening resistance. I do. Therefore, it is assumed that the parts of the present invention have not been carburized. That is, the nitriding portion of the surface layer in the component of the present invention does not include the carburizing and nitriding portion, and therefore, the carbon content of the surface layer needs to be equal to that of the inner portion.

  In the component of the present invention, it is necessary that a region from the surface to a depth of at least 100 μm has the above surface structure. If the area having the surface layer structure is less than 100 μm from the surface, the hardness of the component surface is insufficient.

[Non-nitrided tissue]
The structure other than the nitrided portion (hereinafter referred to as non-nitrided portion structure) contains ferrite in an area ratio of 5 to 60% and one or both of martensite and bainite in a total area ratio of 40 to 95%. By allowing a certain amount of ferrite to be present in the structure of the non-nitrided part after the quenching and quenching, the deformation of the part can be suppressed and the heat treatment distortion can be reduced. To obtain the above effect, the area ratio of ferrite in the non-nitrided portion structure is set to 5% or more. On the other hand, if the ferrite area ratio exceeds 60%, the hardness inside the part is excessively reduced, and as a result, it is not possible to obtain a sufficient fatigue strength. Therefore, the ferrite area ratio of the non-nitrided portion is set to 60% or less.

  The remainder other than ferrite in the non-nitrided structure is mainly martensite and / or bainite from the viewpoint of securing internal hardness and improving fatigue strength. Specifically, the total area ratio of one or both of martensite and bainite is set to 40 to 95%.

  The non-nitrided portion may contain other structures other than ferrite, martensite, and bainite, but from the viewpoint of securing the characteristics of the non-nitrided portion, the area ratio of the other structure is 10%. %, Preferably 5% or less, more preferably 0%. Examples of the other structure include cementite and pearlite. It should be noted that the area ratio of ferrite specified in the present invention does not include thin plate-like ferrite contained in the pearlite structure. Here, the non-nitrided portion indicates a region specified by the method described in the embodiment.

[Production method]
In one embodiment of the present invention, a part satisfying the above conditions can be manufactured by processing a steel having the above-described component composition into a part shape, performing a nitriding treatment, and then performing quenching. . Hereinafter, a preferred manufacturing method will be described.

[processing]
The above processing is not particularly limited and can be performed by any method as long as it can process steel. As the processing method, one or both of mechanical processing and forging can be suitably used. For example, after forging is performed, machining may be further performed to obtain a part shape. As the forging, any of hot forging, warm forging, and cold forging can be used, and a plurality of forgings can be used in combination. The hot forging is preferably performed after heating to a temperature of 1100 to 1250 ° C. Further, the warm forging is preferably performed at 400 to 950 ° C.

[Nitriding treatment]
Next, the member processed into the component shape is subjected to a nitriding treatment. The method of the nitriding treatment is not particularly limited, and may be any method. For example, it may be performed according to a conventional method. The nitriding temperature is not particularly limited, but is preferably 680 to 850 ° C. The temperature at the time of nitriding and the control of the furnace atmosphere are not limited. For example, it is preferable to control the nitrogen potential in order to prevent the formation of a compound layer on the component surface. Specifically, it is preferable to monitor the atmosphere in the furnace using a hydrogen sensor and control the flow rate of ammonia to adjust the nitrogen potential according to a conventional method. When the nitriding treatment is performed at a temperature exceeding 850 ° C., or when the temperature is increased to a temperature exceeding 850 ° C. after the nitriding treatment, the formation amount of martensite or bainite in the non-nitriding treated portion after the subsequent quenching treatment is increased. And a desired non-nitrided portion structure, that is, a structure having a ferrite area ratio of 5% or more, cannot be obtained, and deformation of parts cannot be suppressed.

[Quenching]
After the nitriding treatment, the parts are quenched from the nitriding temperature. The method of quenching is not particularly limited, and any method such as oil quenching or water quenching can be used. In the case of oil quenching, the temperature of the oil is preferably set to 60 to 130 ° C.

[Tempering]
After quenching, it is preferable to further perform tempering. The tempering temperature in the tempering is preferably 150 to 350 ° C.

[Post-processing]
One or more post-treatments such as induction quenching and tempering, peening, plating, and coating are combined with the components manufactured in the above process for the purpose of further improving durability. Can be done. As the peening process, for example, shot peening can be used. As the plating treatment, for example, electroplating can be used. Further, as the coating treatment, any method such as a coating treatment by physical vapor deposition (PVD) or chemical vapor deposition (CVD) can be used. As the coating, for example, diamond-like carbon is preferably used.

  Nitric quenched parts were manufactured according to the following procedure, and the characteristics were evaluated.

Preparation of Test Piece First, a steel ingot having the component composition shown in Tables 1 and 2 was melted and then hot-rolled into a round bar steel having a diameter of 70 mm. After normalizing the round steel bars, heat treatment deformation measurement specimens, rotating bending fatigue specimens, and roller pitting fatigue specimens were prepared from the round steel bars. As the test piece for measuring the amount of deformation of the heat treatment, a C-shaped test piece (outer diameter: 60 mm, inner diameter: 34.8 mm, thickness: 12 mm, opening interval: 6 mm) shown in FIG. 1 was used. As the rotating bending fatigue test piece, a test piece having a parallel part diameter of 9.6 mm having a shape shown in FIG. 2 was used. A notch with a depth of 0.8 mm in a direction perpendicular to the parallel portion was provided in the parallel portion of the rotating bending fatigue test piece over the entire circumference. As the roller pitting fatigue test piece, a test piece having a shape shown in FIG. 3 and having a diameter of 26 mm was used.

・ Carburizing and quenching and quenching Each of the obtained test pieces other than 40 and 41 was subjected to nitriding treatment and quenching. FIG. 4 shows the processing conditions. Specifically, each test piece was heated to 800 ° C., soaked in a nitrogen atmosphere for 30 minutes, and then supplied with ammonia in a furnace and subjected to a nitriding treatment at 800 ° C. for 300 minutes. Thereafter, quenching was performed using oil at 60 ° C. No. Carburizing and quenching and quenching were performed on 40 test pieces. The conditions are shown in FIG. Specifically, carburizing and diffusion treatments were performed at 930 ° C. for 3 hours, then cooled to 850 ° C. in 30 minutes, and then kept at 850 ° C. for 30 minutes. From the time when the temperature was lowered from 930 ° C. to the time of quenching, nitriding was performed by flowing ammonia into the furnace. Then, it was quenched in oil at 60 ° C. No. The test piece No. 41 was subjected to a nitriding treatment at a temperature of 900 ° C., a nitriding treatment at 800 ° C. for 300 minutes in the same manner as the nitriding treatment of other materials, and then an oil at 60 ° C. Was quenched.

Tempering About some test pieces, after the above-mentioned quenching, tempering was further performed by holding at 150 ° C. for 2 hours and then air-cooling.

  For each of the roller pitting fatigue test pieces obtained as described above, the surface layer structure, the non-nitrided portion structure, the surface hardness, the effective hardened layer depth, the internal hardness, and the nitrified portion old austenite particle size were measured by the following procedure. did.

(Surface organization)
Electropolishing is performed in the depth direction from the surface layer portion, and the volume ratio of retained austenite at each of the 0 μm, 25 μm, 50 μm, 75 μm, and 100 μm depth positions is measured using X-rays, and the maximum value is determined as the residual volume in the surface layer structure. The volume fraction of austenite was used. After the nital etching, the structure was observed with an optical microscope to confirm the presence or absence of pearlite and cementite.

(Non-nitrided tissue)
First, in order to identify a non-nitrided portion, a cross section of a test piece which was nitrided was polished, and a nitrogen concentration distribution in the depth direction in the cross section was examined with an electron beam microanalyzer. As a result of the investigation, the position in the depth direction of the non-nitrided portion, which is the portion where nitrogen has not entered and the nitrogen concentration is constant, was specified. Then, after etching the non-nitrided portion with nital, 20 visual fields were observed at 100 times magnification using an optical microscope. At that time, it was confirmed whether the main structure other than ferrite was martensite and / or bainite. The obtained image was subjected to a binarization process, the ratio of the area of the white portion to the entire area was determined, and the average value in 20 visual fields was defined as the area ratio of ferrite. The structure was observed with a scanning electron microscope to confirm the presence or absence of cementite and pearlite.

(surface hardness)
The micro Vickers hardness at a depth of 0.05 mm from the surface of the test piece was measured at five points under a load of 2.94 N, and the average value was defined as the surface hardness.

(Effective hardened layer depth)
The hardness of the cross section of the test piece was measured from the surface of the test piece at 50 μm intervals in the depth direction at a load of 2.94 N using a micro Vickers hardness meter, and a hardness distribution curve in the depth direction was obtained. The depth at which the hardness was 513 HV in the hardness distribution curve was determined as the effective hardened layer depth.

(Internal hardness)
The micro Vickers hardness at the same position where the non-nitrided tissue was observed was measured at five points under a load of 2.94 N, and the average value was taken as the internal hardness.

(Former austenite grain size in the nitrided part)
The prior austenite grain size in the nitrided part (former γ grain size in the nitrided part) was measured at a position of 50 μm in the depth direction from the surface of the test piece in the cross section of the test piece.

  Further, using each test piece, a heat treatment deformation amount measurement, a rotational bending fatigue test, and a roller pitting fatigue test were performed. Specific test conditions were as follows.

(Measurement of heat treatment deformation)
Using the obtained heat treatment deformation amount measurement test piece, the absolute value of the rate of change of the opening amount of the opening before and after nitrocarburizing was defined as the heat treatment deformation amount. The opening amount was measured by using a micrometer to measure the opening amount at three locations on the boundary where the opening was divided into four equal parts in the width direction, and the average value was used.

(Rotating bending fatigue test)
Using the obtained rotating bending fatigue test piece, a rotating bending fatigue test was performed at a test temperature of 20 ° C. The test was performed using an Ono-type rotary bending fatigue tester manufactured by Shimadzu Corporation at a test temperature of 20 ° C. and a rotation speed of 3000 rpm while applying a test stress in increments of 10 MPa. In the above test, the stress that did not break even after rotating 10 million times was defined as the fatigue strength in the rotating bending fatigue test.

(Roller pitting fatigue test)
A roller pitting fatigue test was performed using the obtained roller pitting fatigue test piece. For the test, a RP-201 type tester manufactured by Nikko Create was used. A tempered material of steel grade SUJ2 was used as the driven roller, and a transmission oil at 80 ° C. was used as the lubricating oil. The test conditions were a slip ratio of 40% and a rotation speed of 1500 rpm. Under the above conditions, the number of repetitions up to breakage was examined by changing the surface pressure, and the maximum surface pressure that did not break after 10 million repetitions was defined as the fatigue strength in the roller pitting fatigue test.

  Further, the crowning amount on the side of the large roller was set to 150 mmR, the surface was rotated up to 100,000 times at a surface pressure of 2000 MPa, and the wear curve of the cross section of the contact surface of the test piece was collected to determine the maximum wear depth for the surface before the test. .

  Tables 3 and 4 show the measurement results. As can be seen from the results, the examples satisfying the conditions of the present invention suppressed deformation due to heat treatment and were excellent in wear resistance and fatigue properties. On the other hand, in Comparative Examples not satisfying the conditions of the present invention, at least one of the heat treatment deformation, the wear resistance, and the fatigue properties was inferior. In addition, No. In No. 42, the roller pitting test was stopped because the fatigue strength was 2000 MPa or less. In addition, No. In No. 54, quenching cracking occurred because the hardenability was too high. Therefore, the heat treatment deformation measurement and the fatigue test were discontinued. In addition, No. 1 in which carburizing and quenching was performed. As for No. 40, the carburized carbonitrid portion was formed in the surface layer and the mere nitriding portion (the layer into which only nitrogen was infiltrated) was not formed, so that the abrasion resistance and the fatigue characteristics were inferior. No. 1 which was subjected to the nitriding treatment at a temperature higher than the preferred temperature range of the present invention. In No. 41, a desired non-nitrided portion structure was not obtained, so that the amount of heat treatment deformation increased, and the surface pressure was not stable even during the roller pitting test, resulting in a decrease in fatigue strength.

Claims (5)

A part made of steel and having a nitriding portion on a surface layer,
The steel, in mass%,
C: 0.08 to 0.50%,
Si: more than 0.80%, 1.80% or less Mn: 0.30 to 2.50%,
P: 0.100% or less,
S: 0.100% or less,
Cr: 0.05-2.50%,
Al: 0.005 to 0.080%, and N: 0.0020 to 0.0300%,
And a component composition comprising the balance of Fe and unavoidable impurities, and having a Z value defined by the following formula (1) of 30 or more,
The structure from the surface of the component to a depth of 100 µm has a volume fraction of retained austenite of 30% or less, and the balance consists of one or both of martensite and bainite;
A component in which the structure other than the nitrided portion contains ferrite in an area ratio of 5 to 60% and one or both of martensite and bainite in a total area ratio of 40 to 95%.
Z = 186-449C + 38Si-8Mn-4Ni + 7Cu-29Cr + 15Mo + 165V + 142Ti + 368Al + 4210B (1)
(However, the element symbol in the formula (1) indicates the content (% by mass) of each element in the steel, and is zero when not contained.) The component composition is represented by mass%,
Cu: 0.50% or less,
The component according to claim 1, further comprising one or more selected from the group consisting of Ni: 2.0% or less and Sb: 0.0050% or less. The component composition is represented by mass%,
Mo: 0.80% or less,
V: 0.200% or less,
3. The component according to claim 1, further comprising one or more selected from the group consisting of Ti: 0.200% or less, and B: 0.0080% or less. 4.   A method for producing a part, comprising: after processing a steel having the component composition according to any one of claims 1 to 3 into a part shape, performing a nitriding treatment and then quenching.   The method for manufacturing a component according to claim 4, wherein after the quenching, tempering is further performed at a temperature of 150C to 350C.