JP6416735B2 - Nitride component manufacturing method and nitride component - Google Patents

Description

  The present invention relates to a nitrided part manufacturing method and a nitrided part, and more particularly to a nitrided part manufacturing method and a nitrided part excellent in surface peel resistance.

  Conventionally, nitriding treatment by a salt bath method, a gas method or the like is widely performed as a surface modification method for steel parts such as a hot working die or tool.

  Unlike other surface modification methods such as carburizing and induction quenching, nitriding treatment is not accompanied by transformation, and the treatment temperature is low, so heat treatment distortion and dimensional change are small, and the surface is a compound layer made of iron nitride. This can be expected to improve wear resistance, seizure resistance, and corrosion resistance.

  Further, in the nitriding treatment, due to the addition of compressive residual stress caused by the compound layer that is an iron nitride layer and the nitriding diffusion layer immediately below (hereinafter simply referred to as a diffusion layer) and the increase in hardness due to the penetration of nitrogen in the diffusion layer, The fatigue strength can be improved.

In particular, in steel parts used in dies and tools such as hot forging dies under severe use conditions, it is desirable to form a nitride layer including a diffusion layer deeper from the part surface in order to extend the life of the parts.
However, in such a case, there arises a problem that carbide precipitates on the grain boundary (former austenite crystal grain boundary) of the diffusion layer, and the precipitation carbide easily causes separation on the nitriding surface.
Carbide precipitates on the grain boundaries during the nitriding treatment because C in the carbide previously existing near the surface layer is replaced with N that has entered during the nitriding treatment, and the C is reprecipitated as carbides at the grain boundaries. Presumed to be.
For example, FIG. 2A shows an actual example of a carbide (micrograph showing the microstructure after nitriding) of the carbide precipitated at the grain boundary of the diffusion layer by nitriding. As shown in this figure, most of the precipitated carbide precipitates continuously while bending along the grain boundary.
In addition, this precipitated carbide tends to precipitate parallel to the nitrided surface, and when a large number of precipitated carbides are formed so as to continue along the grain boundary, peeling is likely to occur on the plane parallel to the nitrided surface, which is nitrided. This is one of the causes of shortening the service life of parts.

  As a method for suppressing the generation of this precipitated carbide, it is effective to suppress the amount of N entering the steel part surface by shortening the treatment time in the nitriding treatment or lowering the treatment temperature, for example. However, in this case, the formed nitride layer becomes shallow, and the effect of the nitriding process itself is reduced.

In addition, as a method for suppressing carbonitride in the nitride layer, there is one described in Patent Document 1 below.
In this Patent Document 1, an invention about “nitrided tool, mold and manufacturing method thereof” is shown, in which a steel part before nitriding is subjected to decarburization, and then nitriding is performed, thereby nitriding The point which raises hardness and suppresses the production | generation and growth of a weak carbonitride in a nitride layer is disclosed.

JP 2004-332029 A

However, when the present inventors performed salt bath nitriding treatment on a steel part that had been previously decarburized in order to suppress the above-described precipitated carbide formed by nitriding treatment, the precipitated carbide in the diffusion layer was reduced. Although there was an inhibitory effect, surface peeling in the vicinity of the outermost layer, which is different from that caused by precipitated carbides, was promoted, which was insufficient for improving the life of parts such as molds.
The present invention has been made for the purpose of providing a nitrided part manufacturing method and a nitrided part capable of producing a nitrided part excellent in surface peel resistance, against the background described above.

  Thus, claim 1 relates to a method for manufacturing a nitrided part, which is a method for manufacturing a nitrided part made of a steel material containing 0.10 to 0.70% by mass of C, and heating the part made of the steel material. Then, a decarburization treatment is performed to form a decarburization layer on the surface layer portion of the part, and after the decarburization process, a surface removal process is performed to remove the surface of the decarburization layer, so that the surface C concentration of the part is 0.10 to 0 .25% by mass, followed by a salt bath nitriding treatment.

A second aspect of the present invention relates to a case in which the precipitated carbide having a length of 5 μm or more precipitated on the prior austenite grain boundaries in the nitride layer is composed of two sides bent along the grain boundaries in the first aspect. It is counted as 1.5 pieces when it is composed of 3 sides that bend further, 2 when it is composed of 4 sides that bend, and 0.5 when it is composed of only 1 side without bending. When counted , the number of the precipitated carbides is less than 10 within a range of 100 μm depth and 200 μm width from the surface of the nitride layer.

Claim 3 relates to a nitrided part, which is a nitrided part made of a steel material containing 0.10 to 0.70 mass% of C, and has a surface C concentration of 0.10 to 0.25 mass on the surface layer of the part. % And a decarburized layer, a nitride layer is formed,
The case where the precipitated carbide of 5 μm or more deposited on the prior austenite grain boundary in the nitride layer is composed of two sides bent along the grain boundary is counted as one and further bent 3 When the number of sides of the nitrided layer is 1.5, the case of 4 sides that are bent is counted as 2, and the number of one side without bending is counted as 0.5 . The number of the precipitated carbides is less than 10 in a range of 100 μm depth and 200 μm width from the surface.

  A fourth aspect is characterized in that, in the third aspect, the nitrided part is used as a mold or a tool.

As described above, the present invention performs the decarburization treatment before the salt bath nitriding treatment, and previously forms the decarburized layer in the low C region in the component surface layer portion, thereby precipitating carbide in the diffusion layer in the salt bath nitriding treatment. This is intended to suppress the occurrence of.
The precipitation carbide generated at the crystal grain boundary of the diffusion layer described above becomes more prominent as the C concentration in the part surface layer part is higher and as the amount of N entering the part surface layer part during nitriding is larger. Therefore, in the present invention, by reducing the C concentration in the surface layer portion in advance by decarburization treatment, the carbides precipitated on the grain boundaries are suppressed without reducing the amount of N entering the component surface layer portion during nitriding treatment. be able to.
As a result of confirmation by the present inventors, it is possible to suppress precipitated carbides on the grain boundaries by setting the C concentration on the part surface to 0.25% or less, more desirably 0.20% or less in the decarburization treatment. It is effective for.

According to the present invention, by securing a thick diffusion layer, the hardness is increased and the fatigue strength is increased, and the surface peeling resistance is also improved by suppressing precipitated carbides on the grain boundaries in the diffusion layer. Can do.
Even when a low C region is formed in the component surface layer by decarburization, a sufficient value can be secured for the surface of the component by nitriding.

However, surface peeling that occurs in a nitrided part that has been subjected to salt bath nitriding treatment may be caused by precipitated carbides or other (not caused by precipitated carbides).
When the present inventors investigated the cause of surface peeling not caused by precipitated carbides, salt bath nitriding treatment was performed in a state where there was a region containing almost no C (similar to the ferrite phase) on the component surface by decarburization treatment. When applied, a structure that tends to cause separation different from the normal structure is generated in the vicinity of the outermost layer, and further, when the surface removal treatment is performed after the decarburization treatment and the C concentration on the part surface is set to 0.10% or more, It has been found that generation of a tissue that easily peels can be suppressed.

The present invention is based on these findings, and by removing the surface portion containing almost no C produced by the decarburization treatment before the salt bath nitriding treatment, the C concentration on the component surface is reduced to 0.10-0. .25%, and then a salt bath nitriding treatment is performed.
According to the present invention, by optimizing the C concentration on the surface of the component before the salt bath nitriding treatment, it is possible to suppress the occurrence of surface peeling in the nitride layer which causes a decrease in the lifetime of the component. Can be expected to extend significantly.

In addition, the manufacturing method of this invention makes object the components which consist of steel materials containing C: 0.10-0.70% by mass%.
For steel materials with a C content of less than 0.10%, the C concentration is lower than the surface C concentration of 0.10 to 0.25% specified by the present invention, and the C content exceeds 0.70%. Therefore, since the gradient of the C concentration gradient from the surface to the depth direction generated by the decarburization process is large, it is difficult to keep the surface C concentration of the component in the range of 0.10 to 0.25% by the surface removal process. .
Therefore, in the present invention, the C content of the target steel material is set to 0.10 to 0.70%.

In the present invention, in particular, a case where the precipitated carbide having a length of 5 μm or more precipitated on the prior austenite grain boundary in the nitride layer is constituted by two sides bent along the grain boundary is counted as one, Further, when 1.5 is configured with 3 sides bent, 2 when configured with 4 sides bent, and 0.5 when only 1 side without bending is counted , Desirably, the number of precipitated carbides is less than 10 within a range of a depth of 100 μm and a width of 200 μm from the surface of the nitride layer.
As described above, when a large amount of precipitated carbide precipitates at the crystal grain boundaries in the nitride layer, surface peeling occurs and the life of the component is greatly reduced.

When the present inventors investigated the relationship between the number of precipitated carbides generated and the life of the forging die, the relationship shown in FIG. 1 was recognized.
This figure investigates the number of shots where peeling occurs at the tip using a forging punch that has been nitrided, and the number of precipitated carbides in the range of 100 μm deep and 200 μm wide from the surface of the nitrided layer. In this example, the number of shots in a punch in which no occurrence occurs is defined as 100, and the life of each punch (number of shots at which peeling starts) is expressed as an index.
According to the figure, when the number of precipitated carbides generated exceeds 10, the mold life index is significantly reduced.
That is, according to the manufacturing method of the first aspect, it is possible to satisfactorily prevent peeling due to the precipitated carbide by setting the number of generated carbides to 10 or less.

Here, the number of precipitated carbides generated was counted as one when the precipitated carbides having a length of 5 μm or more precipitated on the grain boundaries were composed of two sides bent in the middle.
The method of measuring the number of precipitated carbides generated is defined in this way because precipitated carbides are often composed of two sides that bend, and the form that continues while bending is often harmful to peeling. This is because it was proved in the survey.

Claim 3 relates to a nitrided part, and a nitrided layer is formed on a surface layer portion of the part together with a decarburized layer having a surface C concentration of 0.10 to 0.25% by mass, and prior austenite crystal grains in the nitrided layer are formed. The case where the precipitated carbide having a length of 5 μm or more deposited on the boundary is constituted by two sides that are bent in the middle is counted as one, and 1.5 cases that are further constituted by three sides that are bent, When the number of the bent sides is two and the number of only one side that is not bent is counted as 0.5 , the precipitated carbide is deposited in the range of 100 μm deep and 200 μm wide from the surface of the nitride layer. The number is less than 10. Such a nitrided part can be suitably manufactured by the manufacturing method of claim 1.

The nitrided part of the present invention is suitable for application to a mold or a tool that requires particularly high fatigue strength and crack resistance (claim 4).
Here, as the mold or tool, not only the die body used for hot forging punches and dies, die-casting dies, aluminum extrusion dies, etc., but also cores, pins, etc. that are assembled and used. In addition to mold parts, those for applications such as hot rolls, and jigs for processing metal, glass, resin, etc. are also included.

  According to the present invention as described above, there are provided a nitrided part manufacturing method and a nitrided part that can suppress the occurrence of precipitated carbide in the nitrided layer and can manufacture a nitrided part having excellent surface peeling resistance. be able to.

It is the figure which showed the relationship between the number of the precipitation carbide | carbonized_material produced on the crystal grain boundary, and a metal mold | die lifetime. It is explanatory drawing of the photograph of the microstructure of the comparative example 1, and the measuring method of the number of precipitation carbide | carbonized_materials. It is explanatory drawing of the peeling test in an Example. 4 is a photograph of the microstructure in Example 4.

Next, embodiments of the present invention will be described in detail.
In the present embodiment, the nitrided part is manufactured through each process of machining, decarburization, quenching and tempering, surface removal, and nitriding.
As the steel material to be used, for example, an alloy tool steel such as JIS SKD61 is suitable, but the steel material containing 0.10 to 0.70% of C can be used if necessary. .
First, a steel material is processed into a predetermined shape in a machining process.

(Decarburization treatment)
Next, the machined steel part is decarburized.
In the decarburization process, the steel part is heated, and a decarburized layer composed of a low C region is formed on the surface layer of the part.
The decarburization treatment conditions (heating temperature, holding time, etc.) can be appropriately determined depending on the components of the steel material used, but the heating temperature for the decarburization treatment is preferably Ac3 to 1050 ° C. By heating with Ac3 or more, the steel structure can be made austenite, and decarburization of the surface layer portion can be promoted.
In addition to heating in the air, the decarburization treatment can be performed in a state where the atmosphere is controlled to be weakly decarburized.

(Quenching and tempering treatment)
Next, the steel part is made into a structure mainly composed of martensite by quenching and tempering treatment, and the required hardness and toughness are imparted to each part.
In this example, the decarburization process is performed separately from the quenching and tempering process, but it is also possible to perform the decarburization process using the heating at the time of quenching.

(Surface removal treatment)
The region of the surface of the steel part produced by the decarburization process is almost free of C, and can be specifically performed by cutting or hard media injection.
Although the depth to be deleted is not particularly specified, the thickness of the layer with a C concentration of less than 0.10% formed on the surface layer is predicted based on the steel type and decarburization treatment conditions, and the part surface is removed. The C concentration of 0.10 to 0.25%.
Note that the surface removal treatment does not necessarily have to be performed on the entire surface of the component, and can be performed only in a specific range where surface peel resistance is particularly required.

(Salt bath nitriding)
After the surface removal treatment, a salt bath nitriding treatment is performed.
As the molten salt bath, nitriding is performed using a salt bath mainly composed of NaCN, KCNO, CaCN ₂ , NaCNO or the like. A mixed salt bath with NaCl, Na ₂ CO ₃ or the like can also be used.
In the salt bath nitriding treatment, it is desirable to immerse 1 to 10 hours at a treatment temperature of 500 to 600 ° C., and the nitrided layer (including the diffusion layer) is 50 μm or more.

As a test material, a rolled material having the chemical composition shown in Table 1 was used. The test material was forged into a Φ150 mm round bar and then air-cooled. Further, a steel part of Φ140 mm × L300 mm was produced by machining.
Thereafter, the steel parts were subjected to quenching and tempering treatment and surface removal treatment according to the conditions described in Table 2, followed by salt bath nitriding treatment.

In this example, the decarburization process is performed using the heating at the time of quenching, so the heating conditions at the time of quenching are performed under the conditions shown in Table 2, and then the steel parts are placed on a turntable and rotated from two directions with a blower. .
Thereafter, tempering was performed under conditions of 580 ° C. × 2 h, a test piece was collected from the steel part, and the surface C concentration after decarburization was measured.

The surface removal treatment is performed by either cutting (K method) or hard media injection (L method) as shown in Table 2, and specimens are collected from the steel parts after the surface removal treatment, and the surface C before nitriding Concentration was measured.
Thereafter, salt bath nitriding treatment was performed using a salt bath containing potassium cyanate (KCNO) as a main component under the condition of 550 ° C. × 10 Hr, and subjected to structure observation, measurement of the number of precipitated carbides, hardness test, and peeling test.
Here, the measurement of the surface C concentration, the measurement of the number of precipitated carbides, the base material hardness test, and the peeling test were performed as follows.

<Measurement of surface C concentration>
In accordance with JIS G 0558, the specimen was cut, embedded, and polished in the cross-sectional direction, and the C concentration was linearly analyzed from the surface to the inside by electron beam microanalysis (EPMA). A carbon concentration transition curve was prepared until the surface C concentration was obtained, and the surface C concentration was determined from the curve.

<Measurement of the number of precipitated carbides>
After the Nital corrosion of the cross-section of the nitrided part, using the microstructure photograph (magnification 400 times) as shown in FIG. 2 (A), the precipitation carbide generated in the grain boundary within the range of 100 μm depth and 200 μm width from the surface layer Measure the number.
As shown in FIG. 2B, the number of precipitated carbides consists of two sides that precipitate on the grain boundaries and have a length L of 5 μm or more and bend along the grain boundaries. The case is counted as one, and the case where three bent sides are connected is counted as 1.5, and the case where four bent sides are connected is counted as two. In addition, only one side without bending is counted as 0.5.
In addition, even when the precipitated carbide is gently curved, a prior austenite grain boundary is assumed, and if it extends beyond the nodal point with the adjacent grain boundary, it is regarded as a bend.
The measurement is performed for two visual fields, and the average value is obtained. In Table 2, when the number of precipitated carbides is less than 0-10, A, when less than 10-25, B, when less than 25-100, C, In the case of 100 or more, D was used.

<Base material hardness test>
The hardness of the base material was determined by performing a hardness test from the surface to the depth of 2000 μm on the cut surface of the test piece collected from the nitrided part.
The hardness test was performed in accordance with JIS Z 2244 with a load of 1.961 N using a Vickers hardness tester. In addition, measurement is performed at five locations, and an average value is used.

<Peel test>
A test piece having a size of 20 × 20 × 20 mm was prepared from the obtained nitrided part, and as shown in FIG. 3 (A), the nitrided surface was 1/16 with a load of 1471N (150 Kgf) using a Rockwell hardness meter. An indenter made of an inch steel ball is pressed to create two indentations on the test surface, and the state of cracks generated at the edges of the indentation is observed.
Then, the portion where cracks are most prominent is compared with the grade sample shown in FIG.

Next, as shown in FIG. 3B, a photograph of the cross section of the indentation portion is taken (magnification 200 times), and the number of cracks generated in the nitride layer in the range of 100 μm from the surface indicated by the dotted line in the drawing is counted. Cross-sectional observation was performed on two indentations, and the crack level was determined from the number of cracks in the cross-section of the indentation where many cracks were found.
Specifically, if the number of cracks is 0 to 5, the crack level is 1, and if the number of cracks is 6 or more, the crack level is 2.

  The evaluation of the peel test shown in Table 2 was performed based on the product value (any one of 1 to 6) of the crack grade (1, 2, 3) and the crack level (1, 2). Specifically, when the value of crack grade × crack level is 1-2, it is A, 3-4 is B, and 5-6 is C.

In the results of Table 2, in Comparative Example 1 and Comparative Example 4, the decarburization treatment was not performed before the nitriding treatment, and the surface C concentration before the nitriding treatment was 0.40% in Comparative Example 1 and 0. 0 in Comparative Example 4. 55%, which is higher than the upper limit of 0.25% of the present invention. For this reason, many precipitation carbide | carbonized_materials are produced | generated in the nitriding process (the number of precipitation carbide | carbonized_materials in Table 2 is D in the comparative example 1, and C is the comparative example 4), and the result of a peeling test is also C.
2A is a microstructural photograph after the nitriding treatment of Comparative Example 1. FIG. It can be seen from this image that a large number of precipitated carbides are generated.

  Comparative Example 2 and Comparative Example 3 are examples in which the surface removal treatment was not performed after the decarburization treatment. In these examples, the surface C concentration before nitriding is lower than the lower limit of 0.10% of the present invention. In this case, exfoliation not attributable to precipitated carbides was likely to occur, and as a result, the exfoliation test results were B in Comparative Example 2 and C in Comparative Example 3 where the surface C concentration was even lower.

  Comparative Example 5 is an example in which the decarburization treatment was not performed as in Comparative Examples 1 and 4. Since this comparative example 5 uses a relatively low C (0.18%) steel type, the value of the number of precipitated carbides is kept low and is good. However, in Comparative Example 5, surface peeling not caused by precipitated carbide occurred, and the result of the peeling test was C. Since the results of the peel tests of Examples 1 and 4 that were subjected to decarburization treatment and surface removal treatment at almost the same surface C concentration as in Comparative Example 5 were good, in order to improve the surface peel resistance, the removal was performed. It seems effective to obtain a predetermined surface C concentration through charcoal treatment and surface removal treatment.

  In Comparative Example 6, decarburization treatment and surface removal treatment were performed on a high C (1.03%) steel type. In the high C steel grade as in this example, since the gradient of the C concentration gradient from the surface after decarburization to the depth direction is large, even if the surface C concentration is 0.25%, C the amount is just precipitated carbide for rapidly increases is likely to occur. In this example, the number of precipitated carbides was B, and the peel test was C.

On the other hand, in Examples 1 to 7 that satisfy the conditions of the present invention, good results were obtained with A for all the results of the number of precipitated carbides and the peel test.
FIG. 4 shows the microstructure of Example 4. From this image, it can be seen that the number of precipitated carbides in Example 4 is well suppressed.

  Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the spirit of the present invention.

Claims (4)

A method for producing a nitrided part made of a steel material containing 0.10 to 0.70% by mass of C,
Heating the component made of the steel material, and performing a decarburization process to form a decarburized layer on the surface layer of the component;
A surface removal treatment for removing the surface of the decarburized layer after the decarburization treatment is performed, and the surface C concentration of the component is made 0.10 to 0.25% by mass,
Thereafter, a salt bath nitriding treatment is performed, and a method for manufacturing a nitrided part is provided. In claim 1, the case where the precipitated carbide having a length of 5 μm or more precipitated on the prior austenite grain boundaries in the nitride layer is composed of two sides bent along the grain boundaries is counted as one. Further, when 1.5 is configured with 3 sides bent, 2 when configured with 4 sides bent, and 0.5 when only 1 side without bending is counted , A method for producing a nitrided part, comprising less than 10 precipitated carbides in a range of a depth of 100 μm and a width of 200 μm from the surface of the nitride layer. A nitrided part made of a steel material containing 0.10 to 0.70 mass% of C,
A nitride layer is formed on the surface layer of the component together with a decarburized layer having a surface C concentration of 0.10 to 0.25% by mass,
The case where the precipitated carbide of 5 μm or more deposited on the prior austenite grain boundary in the nitride layer is composed of two sides bent along the grain boundary is counted as one and further bent 3 When the number of sides of the nitrided layer is 1.5, the case of 4 sides that are bent is counted as 2, and the number of one side without bending is counted as 0.5 . A nitrided part characterized in that the number of precipitated carbides is less than 10 in a range of 100 μm in depth and 200 μm in width from the surface.   4. The nitrided part according to claim 3, wherein the nitrided part is used as a mold or a tool.