JP5821512B2 - NITRIDED COMPONENT AND MANUFACTURING METHOD THEREOF - Google Patents

Description

  The present invention relates to a nitrided part and a manufacturing method thereof.

  Conventionally, in the field of machine structural parts, for example, parts such as gears, continuously variable transmission ring belts, and coil springs are used. Since this type of component is often used in an environment in which repeated stress is applied, it needs to have high fatigue characteristics.

  As a method for improving the fatigue characteristics of the component, a method of improving the fatigue characteristics by applying compressive residual stress to the component surface by surface treatment is known. Recently, further improvement in fatigue characteristics has been demanded, and as a technique for that purpose, proposals have been made to perform shot peening in addition to surface treatment represented by nitriding or carburizing and quenching (Patent Document 1, etc.) See).

Japanese Patent Laid-Open No. 5-339763

  However, when shot peening is performed in addition to the nitriding treatment, the construction time is required, so that the productivity is lowered and the cost of nitrided parts is greatly increased. It is conceivable to improve fatigue strength by increasing the strength of nitrided parts, but in this case as well, cost increases due to the addition of expensive alloy elements for increasing the strength and the accompanying decrease in manufacturability. Easy to invite.

  The present invention has been made in view of such a background, and an object of the present invention is to provide a nitrided part capable of improving fatigue characteristics without performing expensive shot peening, and a method for manufacturing the same. is there.

One aspect of the present invention is a steel part having a surface hardened layer formed by hardening the surface,
The surface hardened layer includes a decarburized layer having a carbon concentration lower than the C content inside the component, and a nitride layer having a nitrogen concentration higher than the N content inside the component,
The chemical composition inside the parts is mass%,
C: 0.15% or more and less than 0.5%,
One or more selected from Cr: 6.0% or less, V: 2.5% or less, Mo: 3.0% or less, and Al: 1.5% or less,
N content is 0.03% or less,
The nitriding coefficient N1 defined by the equation (0.08 × [% Cr] + 0.29 × [% V] + 0.15 × [% Mo] + 0.65 × [% Al]) / [% C] is 1.0 or more,
The surface hardened layer is defined by a formula: _(C-C 1) / C decarburization rate is defined by a formula: 0.30 or more on, _{and, N 2 / (C-C} 1 +0.2) The surface nitrogen concentration coefficient Ns is 1.0 or more (where C ₁ is the average value (% by mass) of the carbon concentration on the surface hardened layer surface, and N2 is the nitrogen concentration on the surface hardened layer surface). The average value (mass%) of the nitrided component (claim 1).

Another aspect of the present invention is a decarburization step of forming a decarburization layer on the surface of a steel nitriding member as a material,
Nitriding the surface of the nitriding member that has undergone the decarburization step to form a nitride layer, and at least a nitridation step to form a surface hardened layer including the decarburization layer and the nitride layer,
The chemical component of the nitriding member as the material is mass%,
C: 0.15% or more and less than 0.5%,
One or more selected from Cr: 6.0% or less, V: 2.5% or less, Mo: 3.0% or less, and Al: 1.5% or less,
N content is 0.03% or less,
The nitriding coefficient N1 defined by the equation (0.08 × [% Cr] + 0.29 × [% V] + 0.15 × [% Mo] + 0.65 × [% Al]) / [% C] is 1.0 or more,
In the decarburization step , the decarburization rate defined by the formula ( C-C ₁ ) / C is set to 0.30 or more, and the nitriding step is N 2 / (C-C ₁ +0.2. ) Is subjected to nitriding treatment so that the surface nitrogen concentration coefficient Ns defined by the formula of 1.0 is not less than 1.0 (where C ₁ is the average value (mass%) of the carbon concentration on the surface hardened layer surface. N2 is an average value (mass%) of the nitrogen concentration on the surface hardened layer surface) ( nitriding component manufacturing method).

Conventionally, parts that require high fatigue strength are subjected to hot working or the like to be molded into a predetermined part shape. The fatigue strength required as a part could not be obtained because of a significant decrease. Further, when carbides are precipitated in the matrix in order to ensure wear resistance, the wear resistance may be deteriorated due to decarburization of the part surface. Therefore, in terms of the manufacturing method, the surface of the decarburized part needs to be removed to the inner layer of the part that has not been decarburized by cutting or the like, which has been a factor of reducing productivity.
As a method of suppressing this, there is a method such as performing pretreatment that does not decarburize the surface such as bright annealing, but productivity has been reduced compared to removing the decarburized layer, and the cost has to be high Generally, it was removed by cutting or the like. Further, after removing the decarburized part surface by cutting or the like, nitriding treatment is performed to form a nitrided layer at a predetermined site, thereby forming a hardened surface layer and ensuring fatigue strength.
However, the present inventors focused on the relationship between the decarburized layer and the nitriding treatment, and as a result of intensive research, the decarburized layer treated as a defective part contrary to the conventional expectation has an adverse effect such as a decrease in fatigue strength. On the contrary, it has been found that the fatigue strength of the surface of the nitrided part can be improved by nitriding the decarburized layer under predetermined conditions. Further, it has been found that, in the prior art, it is not necessary to perform cutting or the like, which has been indispensable for removing the decarburized layer existing on the surface of the component before nitriding.

  The nitrided part includes a decarburized layer having a carbon concentration lower than the C content inside the part and a nitrided layer having a nitrogen concentration higher than the N content inside the part to form a hardened surface layer. The chemical component inside the component contains a specific component, and the nitriding coefficient N1 defined above is 1.0 or more. Further, the surface hardened layer has a decarburization rate specified above of 0.30 or more and a surface nitrogen concentration coefficient Ns specified above of 1.0 or more. Therefore, fatigue characteristics can be improved without expensive shot peening.

  The method of manufacturing the nitrided part includes a decarburization step of forming a decarburization layer on the surface of a steel nitriding member as a material, and nitriding the surface of the nitriding member after the decarburization step to form a nitrided layer. And a nitriding step for forming a hardened surface layer including a decarburized layer and a nitrided layer. At this time, a nitriding member having the specific chemical component and having a nitriding coefficient N1 defined above of 1.0 or more is used as a material. In the decarburization step, the decarburization rate specified above is 0.30 or more. Further, in the nitriding step, nitriding is performed so that the surface nitrogen concentration coefficient Ns defined above becomes 1.0 or more.

  In general, structural steel contains some C for the purpose of strengthening the substrate. Since C is an essential element for securing strength and toughness, it is difficult to think of reducing the amount of C from steel because it usually causes a decrease in strength. However, by deliberately forming the decarburized layer as in the above-described method for manufacturing a nitrided part, it is possible to obtain a nitrided part capable of improving fatigue characteristics without expensive shot peening.

  This is presumably due to the following reasons. That is, in the method for manufacturing a nitrided part, a nitriding member as a material is made of steel having the specific chemical component described above and having a nitriding coefficient N1 defined above of 1.0 or more. Steel contains a specific amount of C for the purpose of strengthening the substrate. When the matrix in the steel is martensite and C is in a solid solution state, the solid solution / diffusion of N is likely to be inhibited during the nitriding treatment. In addition, when an element capable of forming a nitride such as Cr, V, or Mo is combined with C to form a carbide, the amount of nitride that can be precipitated by N that has entered and diffused is reduced. However, as shown in the method for manufacturing a nitrided part, the surface of the nitriding member on which the decarburized layer is formed is nitrided on the same condition by nitriding the surface of the nitriding member on which the decarburized layer is formed. Compared to the case, more N can be contained in the surface of the obtained nitrided part. That is, during the nitriding treatment, an amount of N exceeding the normal nitriding limit (nitriding limit when no decarburized layer is formed) is introduced to the component surface. Therefore, the amount of nitride on the surface of the component after nitriding increases, thereby ensuring a high strain amount. Thereby, it is thought that the manufacturing method of the said nitrided part can give a high compressive residual stress to the surface of the nitrided part after nitriding treatment. As a result, it is considered that the obtained nitrided part can exhibit excellent fatigue characteristics.

The nitride component and the manufacturing method thereof will be described.
First, the nitrided part will be described. The nitrided part is a steel part having a hardened surface formed by hardening the surface. The surface hardened layer includes a decarburized layer having a carbon concentration lower than the C content inside the component, and a nitride layer having a nitrogen concentration higher than the N content inside the component. Here, the inside of the component refers to a matrix portion (base material portion) existing inside the decarburized layer and the nitrided layer. The decarburized layer is a region existing inward from the part surface, and the carbon concentration is continuous when taking a carbon concentration distribution (% by mass) with respect to the depth from the part surface to the inside. After the increase, the portion until the carbon concentration reaches a certain concentration in the carbon concentration distribution where the concentration becomes a certain concentration (that is, the carbon concentration inside the part) is meant. The nitride layer is a region existing inward from the part surface, and the nitrogen concentration is continuous when taking a nitrogen concentration distribution (mass%) with respect to the depth from the part surface to the inside. This is the portion of the nitrogen concentration distribution in which the concentration is constant (that is, the nitrogen concentration inside the part) before the nitrogen concentration reaches a certain concentration. The depth of the decarburized layer from the component surface and the depth of the nitrided layer from the component surface may be substantially the same, or one of them may be deep. Moreover, there is no restriction | limiting in the depth of a decarburization layer and a nitride layer, and the effect of this invention will be acquired if both exist simultaneously.

  Here, as described above, the chemical components inside the component are in mass% and contain C: 0.15% or more and less than 0.5%, Cr: 6.0% or less, V: 2.5% or less, One or two or more selected from Mo: 3.0% or less and Al: 1.5% or less are contained, N content is 0.03% or less, (0.08 × [% Cr] The nitriding coefficient N1 defined by the formula of + 0.29 × [% V] + 0.15 × [% Mo] + 0.65 × [% Al]) / [% C] is 1.0 or more. Below, the reason for limitation of each component range etc. are demonstrated.

C: 0.15% or more and less than 0.5% C is an essential element for securing strength and toughness. Furthermore, it is desirable to provide a difference between the carbon concentration in the decarburized layer and the C content inside the part. Therefore, the lower limit value for the C content is 0.15%. The lower limit of the C content is preferably 0.20%, more preferably 0.30%. On the other hand, when C is contained excessively, coarse carbides are generated and ductility and toughness are lowered. Therefore, the upper limit of C content is less than 0.5%. In addition, welding may be performed when a nitriding member as a raw material is formed into a predetermined shape at the time of manufacturing a nitrided part. If C is excessively contained, weldability may not be ensured. The upper limit value of the C content is preferably 0.45%.

One or more selected from Cr: 6.0% or less, V: 2.5% or less, Mo: 3.0% or less, and Al: 1.5% or less Cr: 6.0% or less Cr is In addition to being effective for improving the surface hardness by nitriding treatment, it is an element effective for imparting compressive residual stress on the part surface. On the other hand, when Cr is excessively contained, a passive film is easily formed on the surface of the component, and the nitriding property is drastically lowered. In addition, the carbide stabilizing effect promotes the growth of the carbide and causes a decrease in strength, making it difficult to control the depth of the decarburized layer. Therefore, the upper limit value of the Cr content is set to 6.0%. The upper limit of the Cr content is preferably 5.0%.

V: 2.5% or less V is an element that is effective for improving the surface hardness by nitriding even in a relatively small amount and is effective for imparting compressive residual stress on the surface of the component. However, if the V content becomes too large, the effect is saturated, and coarse carbides may be generated to reduce strength and toughness. Furthermore, it becomes difficult to control the depth of the decarburized layer. Therefore, the upper limit of V content is set to 2.5%. The upper limit value of the V content is preferably 2.0%, more preferably 1.0%.

Mo: 3.0% or less Mo is an element effective not only for improving the surface hardness by nitriding but also for imparting compressive residual stress on the part surface. However, even if the Mo content is too large, the effect is saturated and the cost is increased. Moreover, the carbide tends to exist stably in the temperature range at the time of the decarburization treatment, and it becomes difficult to control the depth of the decarburization layer. Therefore, the upper limit of the Mo content is 3.0% or less. The upper limit of the Mo content is preferably 2.0%.

Al: 1.5% or less Al is an element that is effective for improving the surface hardness by nitriding even in a relatively small amount and is effective for imparting a compressive residual stress on the surface of the component. However, the effect is saturated even if the Al content becomes too large. Furthermore, not only the manufacturability of steel is extremely deteriorated, but also the strength is reduced. Therefore, the upper limit of Al content is 1.5% or less. The upper limit of the Al content is preferably 1.0%.

N content: 0.03% or less When N content is limited to 0.03% or less, nitrides contained in steel can be reduced before nitriding, and surface hardness by nitriding is ensured. It becomes easy to do. The upper limit of the N content is preferably 0.015%.

The nitriding coefficient N1 defined by the equation (0.08 × [% Cr] + 0.29 × [% V] + 0.15 × [% Mo] + 0.65 × [% Al]) / [% C] is 1.0 or more In the above formula, [% X] means the content (mass%) of the X component. If the amount of carbon contained in the substrate is greater than the amount of nitrogen contained in the nitride that can be precipitated by nitriding, alloy carbides will precipitate in advance at the precipitation sites that can precipitate as nitride, reducing the precipitation efficiency of the nitride. The surface compressive residual stress cannot be applied efficiently. Therefore, it is necessary to satisfy the above formula. If the nitriding coefficient N1 is less than 1.0, the amount of carbon becomes excessive, and the effect of nitriding cannot be obtained sufficiently. Therefore, the compressive residual stress on the part surface is not sufficiently improved, and sufficient fatigue characteristics cannot be obtained. The nitriding coefficient N1 is preferably 1.05 or more, more preferably 1.10 or more, and further preferably 1.20 or more. The upper limit of the nitriding coefficient N1 is not particularly limited in order to ensure the N content that can be precipitated as alloy nitride by nitriding as compared with the C content contained in the substrate. The nitriding coefficient N1 depends on the amount of alloying elements that form nitrides, and if these alloying elements are contained in a large amount, the cost of parts is increased. Therefore, it is preferably 3.0 or less, more preferably 2.5 or less. More preferably, it is 2.0 or less.

  In the chemical component, other elements can be added in addition to the above elements as long as the above-described effects can be obtained. As the most basic chemical components, the remainder other than the above elements can be basically made of Fe and inevitable impurities.

  In addition to the above-described component elements, the chemical components include, as optional elements, Si: 1.5% or less, Mn: 1.5% or less, Ni: 4.0% or less, W: 5.0% or less, and B: One or two or more selected from 0.03% or less can be contained (claim 2). The reason for the limitation is as follows.

Si: 1.5% or less Si is an element effective as a deoxidizer during melting. However, if the Si content is too large, the ductility is remarkably lowered, so the upper limit is made 1.5%. The upper limit of the Si content is preferably 0.5%.

Mn: 1.5% or less Mn is an element effective as a deoxidizer during melting. However, if the Mn content is too large, the ductility is lowered, so the upper limit is made 1.5%. The upper limit of the Mn content is preferably 1.0%.

Ni: 4.0% or less Ni is an element effective for improving hardenability. It is also an element effective in suppressing the formation of carbides and can contribute to the improvement of strength and toughness due to the reduction of grain boundary carbides. However, if Ni is contained excessively, the productivity is deteriorated and the obtained effect is saturated. Furthermore, it is an expensive element and causes an increase in cost. Therefore, the upper limit of Ni content is set to 4.0%. The upper limit of the Ni content is preferably 1.0%.

W: 5.0% or less W is an element effective for improving the functionality such as wear resistance of parts by forming carbides. However, excessive addition causes a decrease in strength and toughness due to coarsening of the carbide, and the obtained effect is saturated. Furthermore, it is an expensive element and causes a significant cost increase. Therefore, the upper limit value of W content is 5.0%. The upper limit value of the W content is preferably 2.0%.

B: 0.03% or less When the B content is 0.03% or less, borides contained in the steel can be reduced before nitriding, and the surface hardness by nitriding can be easily secured. . The upper limit of the B content is preferably 0.005%.

Oite to the nitride portions article, C ₁ is the average value of the carbon concentration of the surface hardened layer surface (wt%) Ru der. Specifically , C ₁ is an average value of each measurement value obtained by measuring the carbon concentration at a pitch of 1 μm in an area of 50 μm × 50 μm of the surface hardened layer using EPMA (Electron Probe Micro Analyzer).

Upper Symbol surface hardened layer is decarburization rate is 0.30 or more, which is defined by a formula: _{(C-C 1) / C} . When the decarburization rate is less than 0.30, the amount of decarburization is small, so that the effect of increasing the nitrogen amount by nitriding cannot be expected, and the compressive residual stress on the part surface cannot be sufficiently improved. As a result, it becomes difficult to improve the fatigue characteristics. The lower limit value of the decarburization rate is preferably 0.45, more preferably 0.60. The upper limit of the decarburization rate is not particularly limited because it is more efficient to completely decarburize, but from the viewpoints of stabilization of quality variations and manufacturability due to part shapes and processing conditions Therefore, it is preferably 0.95 or less, more preferably 0.90 or less, and still more preferably 0.85 or less.

Oite to the nitride portions article, N2 is the average value of the nitrogen concentration in the surface hardened layer surface (wt%) Ru der. Specifically , N2 is an average value of each measurement value obtained by measuring the nitrogen concentration at a pitch of 1 μm in a 50 μm × 50 μm surface area of the surface hardened layer using the above-mentioned EPMA.

Upper Symbol surface hardened layer is _{N2 / (C-C 1 +0.2} ) surface nitrogen concentration coefficient Ns defined by a formula of 1.0 or more. When Ns is less than 1.0, the effect of increasing the amount of nitrogen by nitriding is not sufficiently obtained as compared with the case where the decarburized layer is not formed, and the effect of improving the compressive residual stress on the part surface cannot be obtained. . As a result, it becomes difficult to improve the fatigue characteristics. The lower limit value of Ns is preferably 1.2, and more preferably 1.4. The upper limit of Ns is not particularly limited because the effect of increasing the amount of nitrogen by nitriding is better, but the fragile iron nitride layer referred to as a white layer or a compound layer is formed by nitriding. It may be generated on the surface and the fatigue strength may decrease. Therefore, the upper limit of Ns is preferably 3.5 or less, and more preferably 2.0 or less.

  Next, a method for manufacturing the nitride component will be described. The method for manufacturing a nitrided part includes at least a decarburizing step and a nitriding step.

  The decarburization step is a step of forming a decarburization layer on the surface of a steel nitriding member as a material. In the decarburization step, the decarburization layer can use the decarburization layer generated on the surface of the member when the steel nitriding member as a material is formed into a predetermined shape. Preferably, the decarburization step may be a step of forming a decarburization layer by performing a decarburization process on a surface of a steel nitriding member as a material. This is because the decarburization layer is surely formed by positively decarburizing the surface of the nitriding member.

  In the decarburization step, the shape of the nitriding member as the material is not particularly limited. Specific examples of the shape include a gear shape, a crankshaft shape, and a bolt shape. The nitriding member as a raw material may be, for example, a steel ingot that has been hot worked (hot forging, hot rolling, etc.) so as to have a required shape, and then cold worked (if necessary) It can be prepared by forming by cold forging, cold rolling or the like. Moreover, joining, cutting | disconnection, cutting, grinding, heat processing, such as hardening and tempering, etc. can be performed with respect to a molded article as needed. In addition, in the case of steel parts having a surface hardened layer with a hardened surface, especially when manufacturing small parts, it is difficult to form a decarburized layer by cooling after hot working. A decarburized layer can be reliably formed by performing the charcoal treatment.

  The chemical components of the nitriding member as the material are as described above. Regarding the reason for limiting each component range in the chemical components of the nitriding member, the nitriding coefficient N1, and the like, the description of the chemical components inside the components described in the description of the nitrided components applies mutatis mutandis.

  In addition to the above elements, other elements can be added to the chemical component of the nitriding member. As the most basic chemical components, the remainder other than the above elements can be basically made of Fe and inevitable impurities.

  In the chemical component of the nitriding member, in addition to the above component elements, as optional elements, Si: 1.5% or less, Mn: 1.5% or less, Ni: 4.0% or less, W: 5.0% One or two or more selected from the following and B: 0.03% or less can be contained (claim 5). The explanation of the chemical components inside the parts described in the explanation of the nitrided parts applies mutatis mutandis to the reasons for limiting the ranges of these components in the chemical components of the nitriding member.

The decarburization step, (C-C 1) / C decarburization rate is defined by a formula: is 0.30 or more (however, C ₁ is the average value of the carbon concentration of the surface hardened layer surface (mass %)) . Preferably, the decarburization process is performed so that the decarburization rate is 0.30 or more. Incidentally, the or measurement method C _1, the preferred range, and the like of the decarburization rate, mutatis mutandis description described in the explanation of the nitride component.

  As a decarburization method, for example, a nitriding member as a material to be treated is kept in an oxidizing atmosphere such as an air atmosphere, preferably at 300 ° C. to 600 ° C. for 1 minute to 10 minutes, and more preferably, Examples of the method include air cooling and quenching after holding at 400 to 500 ° C. for 1 to 10 minutes.

  Next, the nitriding step is a step of forming a nitrided layer by nitriding the surface of the nitriding member that has undergone the decarburizing step, and forming a hardened surface layer including the decarburized layer and the nitrided layer. That is, in this step, a nitrided layer is formed by nitriding treatment so as to overlap a part or all of the decarburized layer formed on the surface layer of the nitriding member by decarburizing treatment or the like, and a hardened surface layer is formed.

In the nitriding step, nitriding is performed so that the surface nitrogen concentration coefficient Ns defined by the formula of N 2 / (C−C ₁ +0.2) is 1.0 or more (provided that C ₁ is Average value (mass%) of carbon concentration on the surface hardened layer surface, N2 is an average value (mass%) of nitrogen concentration on the surface hardened layer surface . The measurement method and the C 1, _N2, for the preferred range, and the like of Ns, mutatis mutandis description described in the explanation of the nitride component.

As a nitriding treatment method, for example, ion nitriding treatment is performed by using a batch furnace capable of controlling the gas atmosphere, and maintaining at a temperature of 400 ° C. to 550 ° C. for 1 hour to 40 hours in a mixed gas atmosphere of H ₂ and N _2. Can be illustrated. Further, for example, gas nitriding treatment using a batch furnace capable of controlling the gas atmosphere and holding at a temperature of 400 ° C. to 550 ° C. for 1 hour to 40 hours in a mixed gas atmosphere of NH ₃ and N ₂ and / or H ₂ It can be illustrated.

  The manufacturing method of the nitrided part basically includes at least the above-described steps. In addition, for example, a heat treatment step for heat-treating the nitrided part that has undergone the decarburization step can be arbitrarily added between the decarburization step and the nitridation step. Specific examples of the heat treatment include annealing and quenching treatment. These can be carried out by one or a combination of two or more. It is also possible to repeatedly perform the same kind of heat treatment while changing the heat treatment conditions. When annealing is performed, if distortion occurs in the decarburization process, it is easy to remove the influence. In addition, even when quenching is performed, if distortion occurs in the decarburization process as in annealing, it is easy to remove the influence.

  In addition, for example, a deformation process is optionally added between the decarburization process and the nitridation process so that all or part of the surface of the nitrided part that has undergone the decarburization process undergoes plastic deformation due to tension or compression. can do. In this case, the tensile residual stress on the surface of the nitrided part that can be generated by the decarburization step can be reduced or shifted to the compressive residual stress. Therefore, it becomes easy to contribute to the improvement of the compressive residual stress on the surface of the nitrided part after the nitriding process.

  In addition, after the nitriding step, in order to give further compressive residual stress to the surface of the part, or to give compressive residual stress to a region deeper than the region to which compressive residual stress is given by nitriding, It is possible to arbitrarily add a process such as a coating process for imparting oil lubricity and wear resistance, machining for the purpose of improving dimensional accuracy, and the like. In addition, since the method for manufacturing a nitrided part includes the steps described above, a nitrided part having excellent fatigue strength can be obtained basically without performing shot peening. In some cases, it may be desired to give higher fatigue strength than is possible. Therefore, the case where the shot peening process is additionally performed as described above is not excluded.

In the method for manufacturing a nitrided part, the improvement rate of the compressive residual stress on the surface defined by the formula σ ₁ / σ ₀ is 1.1 or more (provided that σ ₀ does not go through a decarburization step). The compressive residual stress of the surface of the nitrided part obtained by nitriding the surface of the nitriding member, σ ₁ is the compressive residual of the surface of the nitrided part obtained by nitriding the surface of the nitriding member that has undergone the decarburization process Stress) is preferred (claim 4).

In this case, it can be confirmed that the fatigue strength is clearly improved. However, if the σ ₁ / σ ₀ is 1.0 or more, a sufficiently clear effect can be expected even when the test variation is taken into consideration. The upper limit value of σ ₁ / σ ₀ is not particularly limited from the viewpoint of improving fatigue strength.

  Below, samples of nitrided parts with different manufacturing conditions were prepared and experiments were conducted to investigate their characteristics. Based on this experimental example, the nitrided part and the manufacturing method thereof according to the example will be described in detail.

(Experimental example)
<Preparation of sample>
Steel ingots having the respective chemical composition shown in Table 1 were prepared by melting in a 30 kg VIM melting furnace. Next, the steel ingot was subjected to a surface grinding process for surface grinding, and then formed into a plate shape (plate thickness 5 mm) by hot forging and cold rolling. Thereby, a plate-shaped nitriding member (plate thickness 5 mm × width 10 mm × length 600 mm) as a material was prepared.

  The surface of the nitriding member as the prepared material was decarburized. At this time, as shown in Table 2, the decarburization treatment condition is that each nitriding member is held in an air atmosphere (in an oxidizing atmosphere) at 300 ° C. to 600 ° C. for 1 minute to 10 minutes, and then air-cooled. Condition.

Next, heat treatment using a vacuum heat treatment furnace was performed on some of the nitriding members that had undergone the decarburization step. The reason why the heat treatment is performed is that there is an advantage that it becomes easy to remove the influence of heat treatment strain that may be generated by the decarburization process. However, the heat treatment may depend on the part shape and decarburization processing conditions, and can be appropriately performed as necessary. In this example, heat treatment was performed when Sample 5, Sample 14, and Sample 20 were produced. The remaining samples were subjected to nitriding treatment without performing heat treatment after decarburization treatment. At this time, as shown in Table 2, the heat treatment conditions were as follows. For sample 5, the nitriding member that had been decarburized was held in an N ₂ gas atmosphere at 950 ° C. for 5 minutes, and then N ₂ gas N ₂ gas atmosphere quenching, further tempered by forcibly cooled by, after holding for 10 minutes at 500 ° C., and the condition of cooling at a N ₂ gas atmosphere. For sample 14 and sample 20, as shown in Table 2, in the member for nitriding the decarburization treatment is performed N ₂ gas atmosphere, was held for 10 minutes to 600 ° C., cooled at an N ₂ gas atmosphere The condition was as follows.

Next, ion nitriding treatment was performed on the surface of the nitriding member subjected to decarburization treatment (nitriding member after decarburization treatment or nitriding member after decarburization treatment and heat treatment). At this time, the condition of the ion nitriding treatment is that the mixed gas atmosphere of H ₂ and N ₂ is maintained at a temperature of 400 ° C. to 550 ° C. for 2 hours so that an iron nitride layer called a compound layer is not generated. Condition.

  Thus, Sample 1 to Sample 26 were produced.

<Decarburization rate>
The decarburization rate was calculated as follows. Specifically, the surface carbon concentration was measured at a pitch of 1 μm in an area of 50 μm × 50 μm of the sample surface after nitriding using EPMA (Electron Probe Micro Analyzer). The average value of each measured value was calculated, and this was defined as the carbon concentration C ₁ (mass%) of the sample surface. Then, it was calculated formula _(C-C 1) / decarburization rate than C. In addition, C content of the member for nitriding used as a raw material was used for C (mass%) in the formula. The C content inside the sample that does not include the decarburized layer and the nitrided layer is substantially the same as the C content of the nitriding member used as the material.

<Surface nitrogen concentration coefficient>
The surface nitrogen concentration coefficient was calculated as follows. Specifically, using the above-mentioned EPMA, the nitrogen concentration of the surface was measured at a pitch of 1 μm in a 50 μm × 50 μm region of the sample surface after nitriding. The average value of each measured value was calculated, and this was used as the nitrogen concentration N2 (mass%) on the sample surface. Then, the surface nitrogen concentration coefficient Ns was calculated from the formula N2 / (C−C ₁ +0.2).

<Improvement rate of surface compressive residual stress>
About the obtained nitrided component, the improvement rate of the compressive residual stress of the component surface was calculated | required. Specifically, Reference Sample 1 to Reference Sample 26 were manufactured in the same manner except that the decarburization treatment was not performed when Sample 1 to Sample 26 were manufactured. Subsequently, the compressive residual stress in the plate surface of each sample and each reference sample was measured. Specifically, a Cr tube is used as the tube, and a parallel tilt method is applied to the analysis method. X-rays are applied to the plate surfaces of each sample and each reference sample, and the compressive residual stress σ _{1 on} each sample surface, The compressive residual stress σ ₀ on the surface of each reference sample was measured. And the improvement rate of compressive residual stress was computed from Formula (sigma) ₁ / (sigma) ₀ .

<Fatigue properties>
For fatigue tests prepared without performing decarburization treatment as in the case of the above-mentioned respective reference samples (plate thickness 5 mm × width 10 mm × length 600 mm). A three-point bending fatigue test was performed using a 10-t hydraulic servo press. Under the present circumstances, the conditions of the said fatigue test were performed on the conditions of the distance between fulcrums of 40 mm, and the single swing of 50 Hz with a load of 0.1 to 5 kN. “A” indicates that the fatigue life is significantly improved by carrying out the decarburization treatment, “B” indicates that the fatigue life is slightly improved, and “C” indicates that the fatigue life is not changed or decreased. .

  Table 2 summarizes the manufacturing conditions and various characteristics of Sample 1 to Sample 26.

  Table 2 shows the following. That is, since the C content of the steel material used as a raw material is 0.5% or more and the nitriding coefficient N1 is less than 1, Sample 18 and Sample 19 have an excessive amount of carbon, and the effect of nitriding treatment is sufficient. I can't get it. Therefore, although the decarburization rate and the surface nitrogen concentration coefficient Ns satisfy the regulations, the compressive residual stress is not sufficiently improved, and as a result, sufficient fatigue characteristics cannot be obtained.

  In Samples 20 to 23, the steel material used as the material has an excessive C content, and any of the Cr, V, Mo, and Al contents is excessive. Therefore, although the decarburization rate and the surface nitrogen concentration coefficient Ns satisfy the regulations, coarse carbides are easily generated, and this becomes a fatigue fracture starting point and sufficient fatigue characteristics cannot be obtained.

  Samples 24 to 26 are in an appropriate range for the chemical composition of the steel used as the material, and the decarburization rate is in a range that sufficiently satisfies the regulations, but does not satisfy the surface nitrogen concentration coefficient Ns. The compressive residual stress is not sufficiently improved, and as a result, sufficient fatigue characteristics cannot be obtained.

  On the other hand, although the sample 15-sample 17 exists in the appropriate range about the chemical component of the steel materials used as a raw material, it is given by the lower limit side of the range in which the decarburization rate by a decarburization process is prescribed | regulated. However, since the nitrogen concentration on the sample surface by nitriding treatment is high, the compressive residual stress is not sufficiently improved, but the fatigue characteristics are slightly improved.

  Samples 1 to 14 are made of steel having the above-mentioned specific chemical composition as a material, and the decarburization rate by decarburization treatment is applied in the range of more than 0.45, and nitriding is performed after this decarburization treatment. Processed and manufactured. Therefore, according to the samples 1 to 14, the fatigue characteristics can be improved even if expensive shot peening is not performed. In addition, it is effective not at steel containing expensive alloy elements (for example, maraging steel) but at the level of alloy carbon steel that is generally used as steel for machine structural use. As a result, it contributes to cost reduction. Can do.

  Further, as can be seen by comparing Sample 1 to Sample 14, by making the decarburization rate at the time of decarburization treatment 0.5 or more, further 0.6 or more, the effect of improving the compressive residual stress is increased, and fatigue is reduced. It turns out that it becomes easy to contribute to the improvement of a characteristic.

  Furthermore, it can be seen that if Si, Mn, Ni, W, N, and B are less than the specified amount, the effect of improving the compressive residual stress is obtained, and the fatigue characteristics are improved.

  Although the embodiments have been described above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention.

Claims (6)

A steel part having a hardened surface formed by hardening the surface,
The surface hardened layer includes a decarburized layer having a carbon concentration lower than the C content inside the component, and a nitride layer having a nitrogen concentration higher than the N content inside the component,
The chemical composition inside the parts is mass%,
C: 0.15% or more and less than 0.5%,
One or more selected from Cr: 6.0% or less, V: 2.5% or less, Mo: 3.0% or less, and Al: 1.5% or less,
N content is 0.03% or less,
The nitriding coefficient N1 defined by the equation (0.08 × [% Cr] + 0.29 × [% V] + 0.15 × [% Mo] + 0.65 × [% Al]) / [% C] is 1.0 or more,
The surface hardened layer is defined by a formula: _(C-C 1) / C decarburization rate is defined by a formula: 0.30 or more on, _{and, N 2 / (C-C} 1 +0.2) The surface nitrogen concentration coefficient Ns is 1.0 or more (where C ₁ is the average value (% by mass) of the carbon concentration on the surface hardened layer surface, and N2 is the nitrogen concentration on the surface hardened layer surface). Nitride parts characterized by an average value of (mass%) . The nitrided part according to claim 1,
The chemical component is selected from mass%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 4.0% or less, W: 5.0% or less, and B: 0.03% or less. A nitrided part containing one or more of the above. A decarburization step of forming a decarburization layer on the surface of a steel nitriding member as a material;
Nitriding the surface of the nitriding member that has undergone the decarburization step to form a nitride layer, and at least a nitridation step to form a surface hardened layer including the decarburization layer and the nitride layer,
The chemical component of the nitriding member as the material is mass%,
C: 0.15% or more and less than 0.5%,
One or more selected from Cr: 6.0% or less, V: 2.5% or less, Mo: 3.0% or less, and Al: 1.5% or less,
N content is 0.03% or less,
The nitriding coefficient N1 defined by the equation (0.08 × [% Cr] + 0.29 × [% V] + 0.15 × [% Mo] + 0.65 × [% Al]) / [% C] is 1.0 or more,
In the decarburization step , the decarburization rate defined by the formula ( C-C ₁ ) / C is set to 0.30 or more, and the nitriding step is N 2 / (C-C ₁ +0.2. ) Is subjected to nitriding treatment so that the surface nitrogen concentration coefficient Ns defined by the formula of 1.0 is not less than 1.0 (where C ₁ is the average value (mass%) of the carbon concentration on the surface hardened layer surface. N2 is an average value (% by mass) of nitrogen concentration on the surface hardened layer surface) . A method for producing a nitrided part according to claim 3 ,
The improvement rate of the compressive residual stress of the surface defined by the equation of σ ₁ / σ ₀ is 1.1 or more (provided that σ ₀ is the surface of the nitriding member without passing through the decarburization step. The compressive residual stress on the surface of the nitrided part obtained by nitriding treatment, σ ₁ is the compressive residual stress on the surface of the nitrided part obtained by nitriding the surface of the nitriding member that has undergone the decarburization step) A method for producing a nitrided component. A method for producing a nitrided part according to claim 3 or 4,
The chemical component is selected from mass%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 4.0% or less, W: 5.0% or less, and B: 0.03% or less. A method for producing a nitrided part, comprising one or more of the above. A method for manufacturing a nitride component according to any one of claims 3 to 5,
The said decarburization process forms a decarburization layer by performing a decarburization process on the surface of the steel nitriding member as said raw material, The manufacturing method of the nitriding component characterized by the above-mentioned.