JP6881496B2 - Parts and their manufacturing methods - Google Patents

Description

The present invention relates to parts, for example, sun gears, ring gears, shaft gears, etc. used in transmissions, pulleys for CVTs, automobile parts such as crankshafts, timing gears, cam gears, camshafts, injection nozzles, etc. used in engines, or For example, the present invention relates to a part that can be suitably used as a part for various industrial devices such as gears for a speed reducer and various shafts, has little heat treatment strain, and has excellent wear resistance and fatigue characteristics. The present invention also relates to a method for manufacturing the above-mentioned parts.

In recent years, shafts and gears used in automobiles, etc. have been required to be miniaturized due to the weight reduction of the vehicle body for energy saving, while the load has been increasing due to the increase in engine output and electrification. Therefore, improvement of durability is required. On the other hand, from the viewpoint of labor saving, omission of the manufacturing process is being considered, and it is also important that strain removal after heat treatment can be omitted.

Conventionally, as a high-strength member, a part obtained by molding a skin-baked steel such as JIS SCM420H or SNCM420H and performing a carburizing, quenching, and tempering treatment has been used. In addition, soft nitriding treatment has been used in the manufacture of parts that are required to have low strain after heat treatment.

Further, in recent years, it has been studied to manufacture a part having both strength and low strain by adopting a nitrogen quenching treatment. For example, Patent Documents 1 to 3 propose a method of controlling the hardness distribution and structure of a part by improving the conventionally performed immersion quenching method. Further, in Patent Documents 4 and 8, as a new immersion quenching method, immersion quenching using plasma is proposed.

Patent Document 5 proposes a method of controlling the nitrogen concentration in the surface layer of a component by controlling the atmosphere in the furnace at the time of immersion. Further, Patent Document 6 proposes a screw component to which quench quenching is applied.

Patent Document 7 proposes a method of subjecting and quenching a steel material having a specific composition. According to the above method, it is said that the desired hardness distribution can be obtained even if the immersion quenching time is short, and the fatigue strength is improved by controlling the structure of the surface layer.

Patent Document 9 proposes a gear having improved fatigue characteristics by carburizing, quenching, quenching and tempering a limited material to suppress heat treatment strain.

Japanese Unexamined Patent Publication No. 2009-270155 Japanese Unexamined Patent Publication No. 2010-138460 JP-A-2010-229524 Japanese Unexamined Patent Publication No. 2014-111821 Japanese Unexamined Patent Publication No. 2015-025161 Japanese Unexamined Patent Publication No. 2015-21259 Japanese Patent No. 5617747 Japanese Patent No. 5944797 Japanese Patent No. 5872863

However, in Patent Documents 1 to 4 and 8, although an improvement is attempted in the immersion quenching method, the same material as the conventional one is used. Therefore, it cannot be said that the characteristics are significantly improved as compared with the conventional product.

Further, although the wear resistance of the parts can be improved to some extent by the methods proposed in Patent Documents 5 and 6, 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 Document 7, the current required strength can be maintained by controlling the surface layer structure and hardness distribution, but it is insufficient to obtain more fatigue strength.

The carburizing and quenching treatment proposed in Patent Document 9 can reduce the strain after quenching to some extent, but it is not reduced as compared with the quenching and quenching material because the heat treatment temperature is high. In addition, in order to secure the amount of solid solution nitrogen in the surface layer to suppress tempering softening, carbon invasion interferes with carburizing, and it is not possible to dissolve a large amount of nitrogen in solid solution, which suppresses tempering softening. There is no effect.

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

The present inventors have obtained the following findings as a result of repeated diligent research toward the achievement of the above problems.

(1) Nitrogen is an element that has the effect of solid-solving and strengthening in a steel substrate. Therefore, the fatigue strength of steel parts can be improved by containing nitrogen. 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 reinforce the material. Therefore, the inclusion of nitrogen is also effective from the viewpoint of improving resistance to fatigue accompanied by wear.

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

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

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

(5) By strictly controlling the composition of the steel so as to satisfy a specific condition, the above-mentioned surface layer structure and internal structure can be realized in the quenching and quenching part.

(6) In addition, from the viewpoint of increasing the internal hardness, it is necessary to reinforce ferrite, which is a soft structure, and for that purpose, it is effective to dissolve Si in the ferrite to reinforce 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 when the component is used, and to improve the wear resistance.

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

1. 1. It is a part made of steel and has a nitriding part on the surface layer.
The steel is by mass%
C: 0.08 to 0.50%,
Si: Over 0.80%, 1.80% or less Mn: 0.30-2.50%,
P: 0.100% or less,
S: 0.100% or less,
Cr: 0.05 to 2.50%,
Al: 0.005 to 0.080%, and N: 0.0020 to 0.0300%,
It has a component composition containing the balance Fe and unavoidable impurities, and having a Z value of 30 or more as defined by the following formula (1).
The structure from the surface of the part 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 nitriding part 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%.
Note Z = 186-449C + 38Si-8Mn-4Ni + 7Cu-29Cr + 15Mo + 165V + 142Ti + 368Al + 4210B ... (1)
(However, the element symbol in the formula (1) represents the content (mass%) of each element in the steel, and if it is not contained, it is set to zero).

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

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

4. A method for manufacturing a part, wherein a steel having the component composition according to any one of 1 to 3 above is processed into a part shape, then subjected to a distilling treatment and then quenched.

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

According to the present invention, it is possible to obtain a component having less heat treatment strain and excellent wear resistance and fatigue characteristics. The parts of the present invention can be suitably used for automobiles, industrial machines, etc., and do not require strain removal after heat treatment, and are therefore extremely useful in industry.

It is a schematic diagram of the test piece used for measuring the heat treatment deformation amount in an Example. It is a schematic diagram of the test piece used for the rotary 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 soaking and quenching treatment conditions in an Example. It is a schematic diagram which shows the carburizing soaking and quenching treatment conditions in an Example.

Next, the method of 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 to the following description.

The component in one embodiment of the present invention is made of steel and has a nitrogen-impregnated portion on the surface. First, the component composition of the steel will be described. In addition, "%" in the following description shall represent "mass%" unless otherwise specified.

C: 0.08 to 0.50%
C is an element necessary for ensuring strength. If the C content is less than 0.08%, the internal hardness cannot be secured, the bending stress resistance decreases, and the fatigue strength cannot be secured. Therefore, the C content is set to 0.08% or more. On the other hand, when the C content is more than 0.50%, the residual austenite in the surface layer becomes excessive, the surface hardness decreases, the internal toughness deteriorates, and the durability against bending stress and surface pressure deteriorates. Therefore, the C content is set to 0.50% or less.

Si: More than 0.80% and less than 1.80% Si is an element that has an action of solid solution and strengthening in ferrite in addition to a deoxidizing action. Further, Si has an effect of suppressing softening due to friction and improving fatigue strength by increasing temper softening resistance. In order to obtain the above effect, the Si content is set to more than 0.80%. On the other hand, when the Si content exceeds 1.80%, the transformation temperature rises too much, so that the two-phase structure remains up to the surface layer in the nitrification temperature region, the invasion of nitrogen is delayed, and the necessary hardened layer is obtained. I can't. Therefore, the Si content is set to 1.80% or less.

Mn: 0.30-2.50%
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 it is added in excess of 2.50%, the transformation point is lowered too much, and martensite and / or bainite in the internal tissue 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 unavoidable impurity. When the P content exceeds 0.100%, P segregates at the grain boundaries and embrittles the grain boundaries, resulting in deterioration of fatigue characteristics. 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%, but reducing the P content more than necessary is disadvantageous from the viewpoint of manufacturing cost, so the P content is set to 0. It is preferably 005% or more.

S: 0.100% or less S is an element contained in steel as an unavoidable impurity, and combines with Mn in steel to form MnS. If the S content exceeds 0.100%, fatigue fracture due to the MnS starting point occurs at low strength, so that the fatigue strength deteriorates. Therefore, the S content is set to 0.100% or less. On the other hand, although the lower limit of the S content is not limited, the MnS has an action of improving machinability. Therefore, in order to form MnS in anticipation of the action, the S content is set to 0. It is preferably 003% or more.

Cr: 0.05 to 2.50%
Cr is an element that enhances hardenability and tempering resistance to softening. In order to exert both effects, the Cr content needs to be 0.05% or more. On the other hand, when 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 deteriorates, the fatigue cracks grow faster, and bending fatigue occurs. The 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 addition of 0.005% or more. Further, Al has a function of combining with N to generate AlN and suppressing coarsening of crystal grains. Therefore, the Al content is set to 0.005% or more. On the other hand, when the Al content exceeds 0.080%, coarse particles are rather generated, fatigue cracks are likely to develop, and bending fatigue strength is lowered. Therefore, the Al content is set to 0.080% or less.

N: 0.0020-0.0300%
N has the effect of combining with Al to generate AlN, 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 blow holes are generated inside the component, and the bending fatigue strength is lowered. Therefore, the N content is set to 0.0300% or less.

The component composition of steel in one embodiment of the present invention may consist of the above elements, the balance Fe, and unavoidable impurities.

Further, in another embodiment of the present invention, in order to further improve the characteristics of the steel, in addition to the above-mentioned component composition, one or two or more selected from the group consisting of Cu, Ni, and Sb are optionally contained. Can be done.

Cu: 0.50% or less Cu is an element that dissolves in steel and has the effect of increasing the strength of the steel material. However, if the Cu content exceeds 0.50%, surface defects occur during the manufacture of steel materials, and surface defects are likely to occur during the manufacture of parts, and the cost increases when the parts are manufactured after surface grinding or the like. On the contrary, it is disadvantageous. 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 the Cu content is preferably 0.02% or more from the viewpoint of obtaining a sufficient effect of adding Cu. Further, when Cu is added, it is preferable to add Ni having a high effect of suppressing the occurrence of surface defects at the same time.

Ni: 2.0% or less Ni is an element that has the effect of increasing the strength by solid solution in steel and increasing the hardenability and deepening the depth of the hardened layer on the surface. However, since Ni is expensive and excessive addition causes an increase in cost, 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 the Ni content is preferably 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 ensuring the hardness of the surface. However, even if it is added in excess of 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 in order to sufficiently obtain the effect of adding Sb, the Sb content is preferably 0.0005% or more.

Further, in another embodiment of the present invention, in order to further improve the characteristics of the steel, 1 or 2 or more selected from the group consisting of Mo, V, Ti, and B is optionally 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, if the Mo content exceeds 0.80%, the hardenability becomes too high and cracking easily occurs, and Mo is expensive, so the cost increases. 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, but in order to sufficiently obtain the hardenability improving effect of Mo, the Mo content is preferably 0.01% or more.

V: 0.200% or less V is an element having the effect of improving the hardenability of steel and increasing the temper softening resistance like Si and Cr. In addition, V also has an action of forming a carbonitride and suppressing coarsening of crystal grains. However, when the V content exceeds 0.200%, the above-mentioned effect is saturated, a sufficient effect corresponding to the increase in the content cannot be obtained, and the manufacturing cost is only increased. 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 in order to obtain the above-mentioned effect sufficiently, the V content is preferably 0.030% or more.

Ti: 0.200% or less Ti is an element having the effect of forming a fine Ti compound in steel to reduce the size of crystal grains after forging and increase the strength. However, when the Ti content exceeds 0.200%, the Ti precipitate becomes coarse, which becomes a starting point of fatigue fracture and the life is shortened. 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 in order to obtain the above-mentioned effect sufficiently, the Ti content is preferably 0.005% or more.

B: 0.0080% or less B is an element that has the effect of solid solution in steel to improve hardenability. However, if the B content exceeds 0.0080%, the effect of addition is saturated, so the B content is set to 0.0080% or less. On the other hand, the lower limit of the B content is not particularly limited, but in order to obtain the above-mentioned effect sufficiently, the B content is preferably 0.0005% or more.

[Z value]
Further, in the present invention, the composition of the steel needs to have a Z value of 30 or more as defined by the following formula (1).
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 (mass%) of each element in the steel, and if it is not contained, it is set to zero.

After quenching and quenching a part made of steel, the Z value needs to be 30 or less in order to form both the surface layer and the inside of the part into a predetermined structure described later. When the Z value is less than 30, at least one of the surface layer (nitted portion) and the inside (non-nitrated portion) does not have the desired structure, and as a result, heat treatment strain 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 contained in the formula (1), it is not necessary to limit it separately, but it can usually be 339 or less. Further, the Z value may be 300 or less, 250 or less, or 160 or less.

[Micro tissue]
Next, in the present invention, the reason for limiting the microstructure of parts (hereinafter, simply referred to as “structure”) will be described. The component of the present invention has a nitrogen-impregnated portion on the surface layer, that is, a region where nitrogen invades from the surface and has a higher nitrogen content than the inside, and the internal portion other than the nitrogen-entered portion is not invaded by nitrogen. It becomes a nitrogen-immersed part. Therefore, in the present invention, the structure in both the surface layer of the component on which the nitriding portion is formed, specifically, the region up to a depth of 100 μm from the surface of the component and the region other than the nitriding portion is controlled within a specific range. To do.

[Surface structure]
The volume fraction of retained austenite (hereinafter, also referred to as “residual γ”) is 30% or less in the structure in the region from the surface of the part to a depth of 100 μm (hereinafter, may be simply referred to as “surface structure”), and the balance. Is an organization consisting of one or both of martensite and bainite. Since the soaked part needs to be hardened, the surface structure is mainly composed of bainite and / or martensite. Residual austenite is mixed in the steel structure, but if the retained austenite remains in excess of 30% by volume, the hardness decreases. Therefore, the volume ratio of the retained austenite in the surface layer structure is set to 30% or less. And the rest of the surface tissue is one or both of martensite and bainite. Here, the volume fraction of retained austenite in the surface layer structure is a value measured by the method described in the examples.

The parts of the present invention are not carburized in addition to the carburizing treatment. When carburizing is performed, carbon invades from the surface and the carbon content of the surface layer becomes higher than that of the inside, but due to the increase in the surface carbon concentration, the solid solution amount of nitrogen in the surface layer decreases and the temper softening resistance deteriorates. To 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 carburized nitriding portion, and therefore the carbon content of the surface layer needs to be equivalent to that of the internal portion.

The component in the present invention needs to have the surface layer structure in a region from the surface to a depth of at least 100 μm. If the region having the surface layer structure is less than 100 μm from the surface, the hardness of the component surface is insufficient.

[Non-nitration tissue]
The structure other than the nitriding part (hereinafter referred to as the non-nitting part structure) shall contain 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-nitriding portion after quenching and quenching, deformation of parts can be suppressed and heat treatment strain can be reduced. In order to obtain the above effect, the area ratio of ferrite in the non-nitriding part structure is set to 5% or more. On the other hand, if the ferrite area ratio exceeds 60%, the hardness inside the component is excessively lowered, and as a result, sufficient fatigue strength cannot be obtained. Therefore, the ferrite area ratio of the non-nitriding portion is set to 60% or less.

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

The non-nitrified portion may contain structures other than ferrite, martensite, and bainite, but from the viewpoint of ensuring the characteristics of the non-nitted portion, the area ratio of the other structures is 10. It is preferably% or less, more preferably 5% or less, and even more preferably 0%. Examples of the other tissues include cementite and pearlite. The area ratio of ferrite defined in the present invention does not include the thin plate-shaped ferrite contained in the pearlite structure. Further, here, the non-nitriding portion refers to a region specified by the method described in the examples.

[Production method]
In one embodiment of the present invention, a part satisfying the above conditions can be produced by processing a steel having the above-mentioned composition to form a part shape, subjecting it to a distilling treatment, and then quenching it. .. Hereinafter, a suitable production 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 machining and forging can be preferably used. For example, after forging, it can be further machined to form a part shape. As the forging, any of hot forging, warm forging, and cold forging can be used, and a plurality of them 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.

[Nitrogen treatment]
Next, the member processed into the shape of the part is subjected to a distilling treatment. The method of the nitrification treatment is not particularly limited and may be any method, for example, it may be carried out according to a conventional method. The immersion treatment temperature is not particularly limited, but is preferably 680 to 850 ° C. The temperature at the time of immersion and the control of the atmosphere in the furnace are not limited, but for example, it is preferable to control the nitrogen potential in order to prevent the formation of a compound layer on the surface of the part. Specifically, it is preferable to monitor the atmosphere inside the furnace using a hydrogen sensor and control the flow rate of ammonia to adjust the nitrogen potential according to a conventional method. If the ferrite treatment is performed at a temperature exceeding 850 ° C. or heated to a temperature higher than 850 ° C. after the nitriding treatment, the amount of martensite or bainite structure produced in the non-ferrite-treated part structure after the subsequent quenching treatment A desired non-quenched 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 nitrification treatment, the parts are quenched from the nitrification treatment temperature. The quenching method 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 60 to 130 ° C.

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

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

Distillation and quenching parts were manufactured by the following procedure and their characteristics were evaluated.

-Preparation of test pieces First, ingods of steel having the component compositions shown in Tables 1 and 2 were melted and then hot-rolled to obtain round steel bars having a diameter of 70 mm. After normalizing the round bar steel, a heat treatment deformation amount measurement test piece, a rotary bending fatigue test piece, and a roller pitching fatigue test piece were prepared from the round bar steel. As the heat treatment deformation amount measurement test piece, a test piece having a C ring shape (outer diameter: 60 mm, inner diameter 34.8 mm, thickness: 12 mm, opening spacing: 6 mm) shown in FIG. 1 was used. As the rotary bending fatigue test piece, a test piece having a shape shown in FIG. 2 and having a parallel portion diameter of 9.6 mm was used. The parallel portion of the rotary bending fatigue test piece was provided with a notch having a depth of 0.8 mm in the direction perpendicular to the parallel portion over the entire circumference. As the roller pitching fatigue test piece, a test piece having the shape shown in FIG. 3 and having a diameter of 26 mm was used.

-Carburizing quenching, carburizing, quenching quenching Next, No. in Table 2. Each of the obtained test pieces other than 40 and 41 was subjected to nitrification treatment and quenching. The processing conditions are shown in FIG. Specifically, each test piece was heated to 800 ° C., soaked in a nitrogen atmosphere for 30 minutes, then ammonia was supplied into the furnace to perform a nitrogen immersion treatment at 800 ° C. for 300 minutes. Then, quenching was performed using oil at 60 ° C. No. Carburizing, quenching and quenching were performed on 40 test pieces. The conditions are shown in FIG. Specifically, carburizing and diffusion treatment were performed at 930 ° C. for 3 hours, then cooled to 850 ° C. for 30 minutes, and then held at 850 ° C. for 30 minutes. Ammonia was carried out by flowing into the furnace from the time when the temperature was lowered from 930 ° C to the time when it was baked. Then, it was quenched in oil at 60 ° C. No. The 41 test pieces were subjected to a quenching treatment at 900 ° C. for 300 minutes in the same manner as the quenching treatment of other materials subjected to the quenching treatment at 800 ° C., and then oil at 60 ° C. Was hardened.

-Tempering Some of the test pieces were tempered by quenching, holding at 150 ° C. for 2 hours, and then air-cooling.

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

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

(Non-nitrified tissue)
First, in order to identify the non-nitrified portion, the cross section of the nitrided test piece was polished, and the nitrogen concentration distribution in the depth direction in the cross section was investigated with an electron probe microanalyzer. As a result of the above investigation, the position in the depth direction of the non-nitriding portion, which is a portion where nitrogen does not invade and the nitrogen concentration is constant, was identified. Then, after etching the non-nitrified portion with nital, 20 fields of view were observed at a magnification of 100 times using an optical microscope. At that time, it was confirmed whether or not the main structure other than ferrite was martensite and / or bainite. The obtained image was binarized, the ratio of the area of the white portion to the total area was obtained, and the average value in 20 fields of view was taken as the area ratio of ferrite. In addition, the presence or absence of cementite and pearlite was confirmed by observing the tissue with a scanning electron microscope.

(surface hardness)
The Micro Vickers hardness at a depth of 0.05 mm from the surface of the test piece was measured at 5 points under a load of 2.94 N, and the average value was taken as the surface hardness.

(Effective curing layer depth)
The hardness of the cross section of the test piece was measured with a Micro Vickers hardness tester at intervals of 50 μm from the surface of the test piece in the depth direction under a load of 2.94 N, and the hardness distribution curve in the depth direction was obtained. In the hardness distribution curve, the depth at which the hardness was 513 HV 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 5 points under a load of 2.94 N, and the average value was taken as the internal hardness.

(Old austenite particle size in the nitrified part)
The particle size of the former austenite in the nitrified portion (former γ particle size of the nitrified portion) was investigated 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 rotary bending fatigue test, and a roller pitching fatigue test were performed. The specific test conditions were as follows.

(Measurement of heat treatment deformation amount)
Using the obtained heat treatment deformation amount measurement test piece, the absolute value of the rate of change in the opening amount of the opening before and after immersion quenching was defined as the heat treatment deformation amount. For the measurement of the opening amount, the opening amount at three points of the boundary obtained by dividing the opening into four equal parts in the width direction was measured with a micrometer, and the average value thereof was used.

(Rotary bending fatigue test)
Using the obtained rotary bending fatigue test piece, a rotary bending fatigue test was carried out at a test temperature of 20 ° C. The test was carried out 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, and a test stress was applied in increments of 10 MPa. In the above test, the stress that did not break even after being rotated 10 million times was defined as the fatigue strength in the rotary bending fatigue test.

(Roller pitching fatigue test)
A roller pitching fatigue test was performed using the obtained roller pitching fatigue test piece. A Nikko Create RP-201 type testing machine was used for the test. As the driven roller, a tempering material of steel grade SUJ2 was used, and as the lubricating oil, transmission oil at 80 ° C. was used. The test conditions were a slip rate of 40% and a rotation speed of 1500 rpm. Under the above conditions, the number of repetitions until 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 pitching fatigue test.

Further, the crowning amount on the large roller side was set to 150 mmR, the surface pressure was 2000 MPa, the rotation was performed up to 100,000 times, and the wear curve of the cross section of the contact surface of the test piece was sampled to determine the maximum wear depth with respect to the pre-test surface. ..

The measurement results are shown in Tables 3 and 4. As can be seen from this result, the examples satisfying the conditions of the present invention were excellent in wear resistance and fatigue characteristics while suppressing deformation due to heat treatment. On the other hand, in the comparative example which does not satisfy the conditions of the present invention, at least one of the heat treatment deformation amount, the wear resistance, and the fatigue property was inferior. In addition, No. In No. 42, the roller pitching test was canceled because the fatigue strength was 2000 MPa or less. In addition, No. At 54, hardenability was too high, so that cracking occurred. Therefore, the heat treatment deformation amount measurement and the fatigue test were canceled. In addition, No. 1 in which carburizing, quenching and quenching was performed. With respect to No. 40, a carburized and nitrogen-immersed portion was formed on the surface layer, and a simple nitrogen-impregnated portion (a layer in which only nitrogen was infiltrated) was not formed, so that the wear resistance and fatigue characteristics were inferior. No. 1 in which the nitrification treatment was performed at a temperature higher than the preferable temperature range of the present invention. In No. 41, the desired non-nitriding part structure was not obtained, so that the amount of heat treatment deformation increased, the surface pressure was not stable even during the roller pitching test, and the fatigue strength decreased.

Claims (5)

It is a part made of steel and has a nitriding part on the surface layer.
The steel is by mass%
C: 0.08 to 0.50%,
Si: Over 0.80%, 1.80% or less Mn: 0.30-2.50%,
P: 0.100% or less,
S: 0.100% or less,
Cr: 0.05 to 2.50%,
Al: 0.005 to 0.080%, and N: 0.0020 to 0.0300%,
It has a component composition containing the balance Fe and unavoidable impurities, and having a Z value of 30 or more as defined by the following formula (1).
The structure from the surface of the part 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 nitriding part 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%.
Note Z = 186-449C + 38Si-8Mn-4Ni + 7Cu-29Cr + 15Mo + 165V + 142Ti + 368Al + 4210B ... (1)
(However, the element symbol in the formula (1) represents the content (mass%) of each element in the steel, and if it is not contained, it is set to zero). When the component composition is mass%,
Cu: 0.50% or less,
The component according to claim 1, further containing 1 or 2 or more selected from the group consisting of Ni: 2.0% or less and Sb: 0.0050% or less. When the component composition is mass%,
Mo: 0.80% or less,
V: 0.200% or less,
The component according to claim 1 or 2, further containing 1 or 2 or more selected from the group consisting of Ti: 0.200% or less and B: 0.0080% or less. A steel having the component composition according to any one of claims 1 to 3 is processed into a part shape, then subjected to a distilling treatment and then quenched. It is made of steel and has a nitrified portion on the surface layer. It ’s a method of manufacturing parts .
The structure from the surface of the part 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 method for manufacturing a part, wherein the structure other than the nitriding 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% . The method for manufacturing a part according to claim 4, wherein after the quenching, tempering is further performed at a temperature of 150 ° C. to 350 ° C.

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