JP6644334B2 - Mold cooling hole surface treatment method and mold - Google Patents

Description

  The present invention relates to a surface treatment method for a mold cooling hole, and a mold having a cooling hole surface-treated by the method, and more particularly, to a hot mold for introducing a coolant such as cooling water. The present invention relates to a surface treatment method capable of imparting stress corrosion cracking resistance to the surface of a cooling hole provided in a mold, and a mold having a cooling hole surface-treated by the method.

  In a hot mold in which a high-temperature material is introduced into a cavity, such as a die-casting mold in which a molten material is poured into a cavity, a cooling hole is provided in the mold to smoothly cool the mold. The cooling is performed by introducing a coolant such as cooling water into the cooling holes.

  In molds with such cooling holes, the cooling holes have recently tended to be formed close to the cavity surface for the purpose of improving cooling performance. As a result, the effect of repeated thermal stress on the cooling holes has been reduced. Stress corrosion cracking is likely to occur on the cooling hole surface in a large and corrosive environment, and there is an increased risk of large cracks in the mold and leakage of refrigerant from this as a starting point.

  Therefore, various methods for preventing such stress corrosion cracking on the surface of the mold cooling hole are being studied.

  Here, stress corrosion cracking is caused by three factors: the presence of tensile stress, the presence of a corrosive environment, and material properties, and can be avoided by removing one or more of these factors.

  Therefore, the following proposals have been made as methods for preventing stress corrosion cracking in response to these factors.

(Applying compressive residual stress)
Among the factors that cause stress corrosion cracking, compressive residual stress is applied to the surface of cooling holes to reduce tensile stress. It has been proposed to process the surface layer by shot peening (Claim 1 of Patent Document 1).

[Coating with protective film]
Also, by covering the cooling hole surface with a protective film to isolate it from corrosive environments (water, dissolved oxygen, chloride ions, etc.), corrosion on the cooling hole surface, and thus the occurrence of stress corrosion cracking, can be prevented. As an example of such protection by a protective film, it has been proposed to cover the surface of the cooling hole with an electroless plating (Ni-P plating) layer (claim 1 of Patent Document 2). .

[Combined use of applying compressive stress and forming protective film]
Furthermore, it has been proposed to use both the above-described application of the compressive residual stress and the formation of the protective film. The surface of the cooling hole is subjected to low-concentration nitriding and then coated with an electroless Ni-P plating layer. Then, it is also proposed to further perform shot peening (claims 1 and 2 of Patent Document 3).

  In the configuration described in Patent Document 3 described above, the electroless Ni-P plating layer that covers the surface of the cooling hole is formed of a metal that is less likely to corrode (has a lower ionization tendency) than Fe, a so-called “barrier type”. However, in the case of such a barrier type anticorrosion film, if a scratch or a hole is formed in the protective film, the local cell having the plating layer as a cathode and the substrate as an anode is formed, and the substrate as an anode is formed. There is a problem that corrosion (elution) of the steel is accelerated.

  In order to solve the problem of the barrier type anticorrosion film, the surface of the cooling hole is subjected to vacuum pulse nitriding, followed by peening, and furthermore, Zn or Zn alloy having a greater ionization tendency than Fe by electrolytic plating. It has also been proposed to form a sacrifice film to cover the surface of the cooling hole (Claim 1, Claim 3 and FIG. 4 of Patent Document 4).

  As described above, in the configuration in which the surface of the cooling hole is covered with the sacrificial film of Zn or Zn alloy, which has a higher ionization tendency than Fe, the sacrificial film may be damaged to the substrate or a hole may be formed to form a local battery. However, since this local battery uses the plating layer as the anode and the substrate as the cathode, Zn on the anode side corrodes first, so that the occurrence of corrosion on the substrate side can be delayed. .

(Improvement of material properties)
In order to prevent stress corrosion cracking by improving the material properties, it is necessary to select a steel type with a high nickel content, add silicon, and crack along the grain boundaries. In general, this is done by reviewing the steel type of the entire mold to a material with low susceptibility to stress corrosion cracking, such as using intergranular corrosion resistant steel for grain boundary cracking where cracks progress. By modifying the crystal structure and components near the surface of the cooling hole by local surface treatment on the surface, the material itself on the surface is modified to a property that is less likely to cause stress corrosion cracking. Has also been proposed.

Examples of such surface treatment, Patent Document 5 described subsequently herein, formed by changing the magnetite (Fe ₃ O ₄₎ by exposing 1-3 hours the surface of the cooling holes to four hundred eighty to six hundred ° C. steam It is proposed to provide a modified surface modified layer (magnetite layer) (Patent Document 5 [0027], sample 2 in column [Table 2]).

[Others]
Although it is not an invention relating to the surface treatment of the mold cooling holes and discloses a treatment relating to the prevention of stress corrosion cracking, there is no description for predicting the effect of preventing the stress corrosion cracking. Patent Document 6 describes a room temperature diffusion and permeation plating method in which an element in a composition in a metal particle is diffused to the surface of the metal product by spraying the metal particle on the surface of the metal product. This Patent Document 6 also suggests the use of tin as metal particles to be sprayed (Patent Document 6 [0084]).

  In Patent Document 6, the method described herein is referred to as a “plating method”. However, the surface layer formed by the method described in Patent Document 6 is formed on the surface of a steel product by carburizing or nitriding. Like the layer to be formed, it is a “surface modified layer” formed by diffusing and penetrating other elements into the substrate, and is formed by electroless plating or electrolytic plating as in Patent Documents 2 to 4. It is different from the “plating layer” that covers the substrate with a different component from the substrate.

JP-A-7-290222 JP-A-9-271923 JP 2009-72798 A JP 2013-159831 A JP-A-2006-204754 JP-A-8-333671

  Among the conventional methods for treating the surface of a mold cooling hole described above, the method of applying a compressive residual stress by shot peening (Patent Document 1) is based on the tensile residual stress generated by forming the cooling hole and the external force applied during use. Although it can relieve stress, it does not completely eliminate tensile stress, and is used in combination with other methods.

  As described in Patent Document 3, a barrier-type protective film is formed by electroless Ni-P plating to cope with the application of the compressive residual stress by such shot peening, and the surface of the cooling hole is exposed to a corrosive environment. (Water, dissolved oxygen, chloride ions, etc.) can be considered. However, in this configuration, if the protective film has scratches or holes that reach the substrate, the corrosion of the substrate can be further promoted by the action of the local battery. Is as described above.

  On the other hand, in the configuration described in Patent Literature 4 in which the protective film is a sacrificial film made of Zn or a Zn alloy, even if the sacrificial film has a flaw or a hole reaching the substrate, the corrosion of the substrate can be delayed. Although it is advantageous in that it can be performed, this configuration can only "delay" the corrosion of the substrate, and eventually the substrate is also corroded with the progress of the corrosion of the protective film.

  In addition, the configuration in which a sacrificial film made of Zn or a Zn alloy is formed as a protective film is advantageous in that corrosion of the substrate can be delayed when scratches or holes are formed on the substrate, but the ionization tendency is large. A sacrificial film made of a material (which is susceptible to corrosion) becomes thinner as it erodes over time and loses its protective effect, and this corrosion causes rust (white rust) on the surface of the plating layer. As a result, a decrease in the channel area may lead to a decrease in cooling efficiency.

  In order to prevent such corrosion of the sacrificial film itself, it is necessary to form a sacrificial film on the surface of the cooling hole and then perform a chemical conversion treatment on the surface of the sacrificial film. In this case, an increase in the number of processing steps leads to a longer surface treatment of the cooling holes and a higher cost.

  As described above, since the formation of the protective film imparts stress corrosion cracking resistance by isolating the substrate from the corrosive environment, if the protective film is damaged, the substrate cannot be protected from corrosion. .

  On the other hand, if at least the vicinity of the surface of the cooling hole can be modified to a material property in which stress corrosion cracking is unlikely to occur, cooling can be performed even if a plating layer formed by electrolytic plating or electroless plating is not formed as a protective film. In addition to preventing stress corrosion cracking on the surface of the hole, if such surface modification is performed as a base treatment when forming the above-mentioned protective film, even if the protective film is damaged, etc. The occurrence of cracks can be prevented.

As a method of preventing stress corrosion cracking by such surface modification, Patent Document 5 discloses that the surface structure of the cooling hole of a mold is changed to magnetite (Fe ₃ O ₄ ) to cause corrosion, and thus the occurrence of stress corrosion cracking. And that the life of the mold can be greatly extended.

However, in Patent Document 5, in order to change the surface structure of the cooling hole to magnetite (Fe ₃ O ₄ ), the surface of the cooling hole is exposed to water vapor at 480 to 600 ° C. for 1 to 3 hours. Patent Document 5 [0014]) It is more convenient if a surface modified layer having stress corrosion cracking resistance can be formed on the surface of the cooling hole with a simpler and shorter process.

  In addition, the surface modified layer of magnetite formed by the above method has relatively low hardness and low abrasion resistance, so that when the magnetite layer is lost due to friction, stress corrosion cracking resistance is lost.

  Therefore, the present invention provides a relatively simple and short-time surface modified layer in the vicinity of the surface of the cooling hole of the mold, which has been modified to a material property that is less likely to corrode and therefore less likely to cause stress corrosion cracking. By providing a surface treatment method of a mold cooling hole, which can form a surface modified layer that can hardly lose stress corrosion cracking resistance even by friction with other members, It is an object of the present invention to prevent the occurrence of stress corrosion cracking on the surface of a steel sheet and extend the life of the mold.

In order to achieve the above object, a method for surface treatment of a mold cooling hole of the present invention comprises:
By performing the diffusion process of diffusing the Sn element at least on the surface of the cooling holes formed in the mold, characterized that you form a surface modified layer matrix component and Sn of the mold are mixed (according Item 1).

  In the diffusion treatment, an injection particle made of tin and / or tin alloy particles having an average particle diameter of 10 to 100 μm is injected together with a compressed gas having an injection pressure of 0.4 to 0.8 MPa to collide with the surface of the cooling hole. This can be performed by diffusing the Sn element in the spray particles to the surface of the cooling hole (claim 2).

  In the present invention, the average particle diameter means a median diameter (d50).

When the surface of a cooling hole whose one end is a closed end is to be treated,
In the diffusion process, the ejection of the ejection granules is performed by using an ejection nozzle having a diameter smaller than the cooling hole with the tip of the ejection nozzle near the closed end of the cooling hole, for example, a distance of several mm from the inner surface of the closed end. It is preferable that the diffusion process is performed by continuing the injection of the injected particles while the injection nozzle is removed from the cooling hole, for example, while gradually pulling out the injection nozzle until the injection nozzle comes out of the cooling hole. (Claim 3).

Preferably, the surface of the cooling hole is subjected to shot peening either before the diffusion treatment or during the diffusion treatment (claim 4). In this shot peening, scale removal or the like of the surface is performed. Because of the effect, it is preferable to perform shot peening before performing the above-described diffusion processing for diffusing the Sn element.

  In this case, shot peening can be performed by injecting a shot having an average particle diameter of 20 to 149 μm together with a compressed gas having an injection pressure of 0.3 to 0.8 MPa.

When the surface of a cooling hole whose one end is a closed end is to be treated,
Also for the injection of the shot in the shot peening, the injection nozzle having a diameter smaller than that of the cooling hole is set such that the tip of the injection nozzle is close to the closed end of the cooling hole, for example, a distance of several mm from the inner surface of the closed end. It is preferable that the shot be started by inserting the shot and that the shot be continuously ejected while gradually pulling out the spray nozzle until the ejected nozzle comes out of the cooling hole (claim 6).

  In this case, it is preferable to swing the tip of the nozzle in the cooling hole (claim 7).

  In addition, the surface treatment method of the present invention described above can be performed on the surface of the cooling hole that has been subjected to nitriding or nitrocarburizing treatment (claim 8).

The mold of the present invention
In a mold with a cooling hole for introducing a coolant for cooling,
The surface of the cooling hole is formed with a surface modified layer in which Sn element is diffused and mixed with a base component of the mold (Claim 9).

  It is preferable that the surface modified layer is further provided with a compressive residual stress.

  Further, the cooling hole may include a nitrided layer or a nitrocarburized layer on the surface, and the surface modified layer may be formed in the nitrided layer or the nitrocarburized layer.

  According to the configuration of the present invention described above, the mold having the cooling holes subjected to the surface treatment by the method of the present invention exhibits the corrosion resistance by diffusing the Sn element on the surface of the cooling holes. The occurrence of stress corrosion cracking was successfully prevented by preventing the occurrence of corrosion on the cooling hole surface.

  Moreover, the surface treated by the method of the present invention exhibited excellent corrosion resistance even after frictional wear, and was excellent in wear resistance.

  Such diffusion of the Sn element is performed in a relatively short time by a relatively simple method of injecting spray particles, which are particles of tin and / or tin alloy having a predetermined particle size, at a predetermined injection pressure. It is possible.

  In the configuration in which shot peening is performed on the surface of the cooling hole, more stress corrosion cracking resistance is imparted by the synergistic effect of the above-described imparting of corrosion resistance due to the diffusion of Sn element and the imparting of compressive residual stress by shot peening. I was able to.

  Further, the injection of the spray particles in the diffusion processing and the shot injection in the shot peening are started with the injection nozzle having a smaller diameter than the cooling hole inserted into the vicinity of the closed end of the cooling hole. In the configuration in which the injection is continued while gradually pulling out, the distance between the outer circumference of the injection nozzle and the inner circumference of the cooling hole is narrow, so that the shot or injected particles that are injected are difficult to fall out of the hole and are easily clogged. Even when the surface was to be treated, shot peening and Sn element diffusion treatment could be suitably performed.

  In particular, by performing the above-described injection while oscillating the tip side of the injection nozzle in the cooling hole, it is possible to suitably prevent the occurrence of processing unevenness and the like.

  The surface treatment of the present invention can be performed on the surface of the cooling hole where nitriding or nitrocarburizing is performed, so that a synergistic effect with nitriding or nitrocarburizing can be obtained.

FIG. 3 is an explanatory view schematically showing a mold and its cooling holes. It is explanatory drawing of the injection method of the injection granule / shot with respect to the cooling hole surface which has a closed end, (A) is the whole structure, (B) is an enlarged view of the tip part of an injection nozzle, (C) is the tip of an injection nozzle. FIG. 2 is a view as viewed from the direction of arrow C shown in FIG. (A) is the contour curve of Sample 1 (Comparative Example), (B) is the contour curve of Sample 2 (Comparative Example), (C) is the contour curve of Sample 3 (Example), and (D) is the contour curve of Sample 4 (Example). It is the SEM image of the surface of the sample 1 (comparative example), (A) is 1000 times, (B) is 3000 times. It is the SEM image of the surface of the sample 2 (comparative example), (A) is 1000 times, (B) is 3000 times. It is the SEM image of the surface of the sample 3 (Example), (A) is 1000 times, (B) is 3000 times. It is the SEM image of the surface of the sample 4 (Example), (A) is 1000 times, (B) is 3000 times. EDX qualitative analysis result of sample 1 (comparative example) surface. EDX qualitative analysis result of the surface of sample 2 (comparative example). Sample 3 (Example) EDX qualitative analysis result of surface. EDX qualitative analysis result of the surface of Sample 4 (Example). Explanatory drawing of a friction wear test (ball-on-disk test). The photograph which image | photographed the surface of each sample before and after the test which shows a corrosion resistance test result. Explanatory drawing of the test piece used for the evaluation test with respect to the injection method of a shot material, and a test method.

  Hereinafter, the surface treatment method of the mold cooling hole of the present invention will be described with reference to the accompanying drawings.

[Processing target: Mold cooling hole]
The surface treatment method of the present invention treats the surface (inner wall) of a cooling hole provided for introducing a coolant such as cooling water into a hot mold such as a die casting mold.

  The steel type of the die to be processed is not particularly limited. For example, SKD-based hot die steels (SKD4, SKD5, SKD6, SKD61, SKD62, SKD7, SKD8) represented by SKD61 (JIS G4404 2006), and the like. Any steel type used for a hot die such as a hot forging die steel (SKT3, SKT4, SKT6) of the SKT series can be subjected to the surface treatment of the present invention.

  As schematically shown in FIG. 1, the cooling hole formed in such a mold is, for example, the thickness of the mold from the back side of the mold (the side opposite to the cavity surface) toward the cavity surface. A cooling hole formed at one end as a closed end, such as a cooling hole formed in a direction, and a cooling hole formed as a through hole, such as a cooling hole shown in a direction perpendicular to the thickness direction of the mold in FIG. There are various cooling holes such as holes. Then, for example, these various cooling holes are connected to each other, and the refrigerant circulates in the connected cooling holes. The surface treatment method of the present invention can be applied to any of these cooling holes.

  The size of the target cooling hole is not particularly limited, but for a cooling hole with a closed end, as the hole diameter becomes smaller, shots and spray particles are more likely to be clogged in the cooling hole, making processing difficult. It has been confirmed that the surface treatment can be performed up to a cooling hole having a diameter of 2 mm, which is the minimum size of the cooling hole formed in the mold, when the injection method described later adopted in the present invention is applied.

  The surface treatment method of the present invention can be applied to the surface of the cooling hole while cutting or the like is performed. However, when the surface of the cooling hole has irregularities such as a tool mark, In addition, when the cooling holes are formed by electric discharge machining, the discharge hardened layer that causes oxide scale and micro cracks adhered to the surface should be removed in advance. Since it is preferable to perform the polishing, the surface of the cooling hole may be subjected to a polishing process such as sandblasting in which abrasive grains are sprayed and polished before performing the process of the present invention.

  If necessary, the surface of the cooling hole may be subjected to a treatment such as nitriding or nitrocarburizing by a known method. The treatment of the present invention may be performed on the surface.

〔surface treatment〕
The surface treatment of the present invention is applied to the cooling hole surface of the mold described above.

  This surface treatment method includes at least a diffusion treatment for diffusing Sn element into at least the surface of the cooling hole formed in the mold. Preferably, in addition to the diffusion treatment, shot peening is applied to the surface of the cooling hole. It is preferable to apply a compressive residual stress.

  In the present embodiment, an example will be described in which the surface of the cooling hole is first subjected to shot peening to impart a compressive residual stress, and then the above-described diffusion process for diffusing the Sn element is performed.

  Shot peening is a type of cold working in which shots composed of roughly spherical particles of metal, ceramic, glass, etc., are sprayed and collided with the compressed gas onto the surface of the cooling holes. By performing this shot peening, An increase in surface hardness (work hardening) of the cooling holes due to the refinement of the surface structure can be obtained, and a compressive residual stress can be given.

  As the shot to be used, as described above, shots of various materials known as materials of the shot material used for shot peening, such as metal, ceramic, and glass, can be used. Use a cooling hole whose hardness is higher than the surface hardness.

  The shot may have an average particle diameter of 20 to 149 μm. The shot is injected together with a compressed gas such as compressed air having an injection pressure of 0.3 to 0.8 MPa.

  The shot peening can be performed only once, but may be performed in a plurality of times. For example, by changing the injection pressure or the particle size of the shot stepwise, cooling after the processing may be performed. It may be performed so that the surface roughness of the holes is improved.

  In the present embodiment, the above-described diffusion processing for diffusing the Sn element is performed on the surface of the cooling hole after the above-described shot peening.

  Such diffusion of the Sn element can be carried out by injecting the blast particles made of tin or a tin alloy onto the surface of the cooling hole and causing them to collide with each other. Some can be diffused to the surface of the cooling holes.

  The spray particles used for this diffusion process need to contain Sn elements to be diffused to the surface of the cooling hole, and are used not only for pure tin particles but also for tin alloy particles containing tin as a main component. Alternatively, some or all of tin particles having an oxide film formed on the surface by natural oxidation may be in the form of tin oxide.

  Unlike the shot used for shot peening described above, the shape of the spray particles used for the diffusion process is not limited to a substantially spherical shape, but can also be used for shapes having corners, and the shape is not limited. .

  The average particle size of the spray particles used is in the range of 10 to 100 μm. By spraying the spray particles together with a compressed gas such as compressed air having a spray pressure of 0.4 to 0.8 MPa, the surface of the cooling hole is formed. Sn element can be diffused.

  In the present embodiment, the case where the above-described shot peening is performed and then the diffusion process of injecting tin or tin alloy spray particles to diffuse the Sn element is described as an example. The order is not particularly limited as long as both the application of compressive residual stress and the diffusion of Sn element can be performed on the surface of the cooling hole, and the surface of the cooling hole is shot-peened after the Sn element is diffused. Alternatively, the injection of spray particles (diffusion processing) and the injection of shots (shot peening) may be performed at the same time to simultaneously diffuse the Sn element and impart compressive residual stress.

  However, since shot peening has the effect of removing scale from the surface, it is preferable to perform shot peening before performing the above-described diffusion treatment for diffusing the Sn element.

  The shot injection in the shot peening and the injection of the spray particles in the diffusion process can be performed by the following methods.

  In addition, the method of injecting shots and blasting particles described below can be applied to the blasting of abrasive grains when the surface of the cooling hole is polished by blasting of abrasive grains prior to performing the processing of the present invention. It is.

  If the cooling hole to be treated is one of the cooling holes shown in FIG. 1 which is formed as a through hole having an inlet and an outlet, a shot is taken together with a compressed gas such as compressed air from either the inlet or the outlet. Alternatively, shots or spray particles may be caused to collide with the surface of the cooling hole by introducing or spraying particles from the other side.

  When the cooling hole to be processed is one of the cooling holes shown in FIG. 1 having one end closed, as shown in FIG. 2A, the diameter is smaller than the diameter of the cooling hole, and Using an injection nozzle having an outer diameter between the surface of the cooling hole and the outer peripheral surface of the injection nozzle at least capable of forming a permissible interval for the passage of the mixed fluid of the compressed gas, the shot and the injection particles. Is inserted into the cooling hole and injected. This method is also applicable to cooling holes formed as through holes.

  In the shots and the injection of the spray particles using such an injection nozzle, the injection nozzle is cooled until the tip of the injection nozzle is spaced from the inner surface of the closed end of the cooling hole by about several mm, for example, about 3 to 5 mm. Inserting into the hole, starting the injection of shots and spray particles at this position, continuing the injection until the injection nozzle comes out of the cooling hole while gradually pulling out the injection nozzle from the cooling hole, the inner surface of the closed end Shots and spray particles can be caused to collide with the entire surface of the cooling hole including the particles.

  If the ejection nozzle is too early, the surface of the cooling hole cannot be sufficiently treated if it is too early, and if it is too late, the shot or injected particles clogged in the cooling hole and cannot be ejected. / S withdrawal speed is preferred, for example, 0.5 to 50 mm / s when spraying into a cooling hole having a diameter of 4 mm.

  During shot or injection particle injection, the tip side of the injection nozzle is swung in the cooling hole as shown by the arrows in FIGS. 2 (B) and 2 (C) to uniformly process the surface of the cooling hole. May be performed.

  When a cooling hole having a relatively large hole diameter is to be processed, it is easy to secure a gap between the outer periphery of the injection nozzle and the inner periphery of the cooling hole. It becomes difficult to secure the space between the outer circumference of the nozzle and the inner circumference of the cooling hole, and it becomes difficult for the shots and spray particles ejected to be discharged out of the cooling holes, so that the shots and spray particles easily clog in the cooling holes. Therefore, it is necessary to select an injection nozzle having an appropriate size for the diameter of the cooling hole.

  Table 1 below shows examples of the combination of the injection nozzles used for the cooling holes having a hole diameter of 8 mm or less, in particular, of the cooling holes having the closed ends, in which the injection of the injection particles is difficult.

  Table 2 below shows an example of the surface treatment of the cooling holes in the present invention including the diffusion treatment for diffusing the Sn element and shot peening, and an example of the treatment conditions in each step.

  The example shown in Table 2 shows an example of surface treatment for a mold in which cooling holes are formed by electric discharge machining. In step 1, SiC abrasive grains are sprayed, and oxidation generated on the surface of the cooling holes by electric discharge machining is performed. After performing sandblasting for removing the scale and the discharge hardened layer, shot peening is performed in step 2.

  In addition, after performing shot peening in step 2, iron is removed from the surface of the cooling hole by shot peening in step 3, and shots are injected to improve the surface roughness. Diffusion processing is performed by injecting the body to diffuse the Sn element on the cooling hole surface.

1. Evaluation by test piece The following shows the results of the evaluation of the surface condition and the evaluation test of the corrosion resistance of the test piece treated by the surface treatment method of the present invention, and the evaluation test for the injection method of the shot material used in the present application. Are shown below.

(Surface condition evaluation test)
(1) Object of the test The condition of the test piece surface after the surface treatment by the method of the present invention is confirmed based on the residual compressive stress, the surface shape, and the components.

(2) Test method Four test pieces (hardened / tempered product: width 25 mm x length 25 mm x thickness 5 mm, hardness 45HRC) made of SKD61 were obtained by performing the treatments shown in Table 3 below. X-ray residual stress measurement using cosα method, surface shape observation based on laser microscope and scanning electron microscope (SEM), and elemental analysis by energy dispersive X-ray analysis (EDX) on the surfaces of samples 1 to 4 Was done.

(3) Test results
(3-1) Residual stress measurement results Table 4 below shows the results of measuring the residual stress in the width direction (X direction) and the length direction (Y direction) for the above four types of test pieces.

  As a result of measuring the residual stress of sample 2 (soft-nitrided product), it was confirmed that a high compressive residual stress was applied, but the dispersion of the measured values was large (the standard deviation was large) and obtained. In view of the low reliability of the accuracy of the numerical values, the values are left blank in Table 4.

  From the above results, the sample (sample 3 and sample 4) treated by the method of the present invention imparted a larger compressive residual stress than the untreated (hardened / tempered) sample 1 sample. It was confirmed that it was done.

  In the test piece of sample 1, the stress value was largely different between the X direction and the Y direction. However, in the test piece treated by the method of the present invention, the stress value was almost different between the X direction and the Y direction. Therefore, it was confirmed that the compressive residual stress could be uniformly applied to the test piece surface in any direction by the method of the present invention.

(3-2) Surface shape The contour curves of samples 1 to 4 are shown in Fig. 3, and the SEM images of the surface are shown in Figs. 4 to 7, respectively.
According to the contour curves shown in FIG. 3, a large number of deep and sharp concave portions (valleys) which can be the starting points of cracks are formed on the surfaces of the sample 1 and the sample 2 which are the comparative examples. It can be seen that the surfaces of the processed samples 3 and 4 have a gentler contour as compared to the samples 1 and 2.

  The arithmetic mean tune (Spc: ISO 25178) of the peaks on the surfaces of Sample 1 (untreated product) and Sample 4 treated by the method of the present invention was measured. 0.066 mm and 1 / 4529.218 mm for Sample 4 (Example). From this result, it can be seen from the results that the peaks of the irregularities were flattened and the surface became smooth in the test piece treated by the method of the present invention. Has been confirmed.

  From the SEM image of the surface of each sample, a tool mark remains on the surface of sample 1 as a comparative example (see FIG. 4). Although the presence of fine irregularities was confirmed (see FIG. 5), the existence of such granular fine irregularities was not confirmed on the surfaces of Samples 3 and 4 treated by the method of the present invention (FIG. 6). It can be seen that the shape is less prone to cracking as compared to Samples 1 and 2.

(3-3) EDX Quantitative Analysis Results FIGS. 8 to 11 show the results of elemental analysis of the surfaces of Samples 1 to 4 by energy dispersive X-ray analysis (EDX).
Table 5 below shows the semi-quantitative EDX value of each sample surface.

  The chemical components of SKD61 in JIS G 4404 2006 are shown in Table 6 below for reference.

  8, C, Fe, Si, Mo, V, Cr, and Mn are detected from the surface of the sample 1. These components are all constituents of the SKD 61 as shown in Table 6. At the same time, the semi-quantitative value of each component (see Table 5) also substantially coincides with the composition of SKD61, which indicates that, in Sample 1, the component of SKD61, which is the base material, is detected as a surface component as it is.

  From FIG. 9, N and O are detected from the surface of the sample 2 in addition to C, Fe, Si, Mo, V, Cr, and Mn, which are the components of the base material (SKD61). It can be confirmed that N is diffused.

  From FIG. 10, Sn, in addition to C, Fe, Si, Mo, V, Cr, and Mn, which are components of the base material (SKD61), N diffused to the surface by nitrocarburizing, and Sn were detected from the surface of Sample 3. In addition, the amount of O detected is larger than that of Sample 2.

  In addition, as a result of quantification by semi-quantitative analysis, it was confirmed that Sn was contained as much as 55.1%.

  This indicates that the Sn element diffused in a large amount on the surface of the base material (SKD61) of Sample 3 by the surface treatment method of the present invention.

  Also, from the increase in O, it is considered that the surface was oxidized by performing the treatment of the present invention.

  From FIG. 11, Sn, in addition to C, Fe, Si, Mo, V, Cr, and Mn, which are components of the base material (SKD61), N diffused to the surface by nitrocarburizing, and Sn were detected from the surface of Sample 4. In addition, the amount of O detected is larger than that of Sample 2.

  The results of quantification by semi-quantitative analysis confirmed that Sn was contained as much as 28.5%.

  Thus, it can be seen that the Sn element diffused into the surface of the sample 4 in a large amount by the surface treatment method of the present invention.

  Also, from the increase in O, it is considered that the surface was oxidized by performing the treatment of the present invention.

(3-4) Measurement of element distribution by FE-EPMA Surface analysis was performed on Samples 2 and 4 above using a field emission-electron probe microanalyzer (FE-EPMA), The distribution was measured.

  Samples 2 and 4 were cut with a wire in the thickness direction, the cut surfaces were embedded in a conductive resin and mirror-polished, and the element distribution near the surface layer of the cross-section after mirror polishing was mapped by FE-EPMA.

  As a result of the measurement, in the test piece of Sample 2 (Comparative Example), a part where Fe was thin and Cr was deep was present in a dot-like manner in the internal Fe part, and Cr carbide as an alloy component was spotted in this part. It is considered that there is.

  Further, in the surface layer portion of the sample 2, it was confirmed that the concentration of N, which is considered to be affected by the compound layer accompanying the nitrocarburizing treatment, and the O was partially concentrated.

  It should be noted that Sn elements were not detected in the test piece of Sample 2 in which the injection of Sn particles was not performed.

  On the other hand, also in the internal Fe portion of the test piece of Sample 4 (Example), the point that the portion where Fe is thin and the portion where Cr is dense is present in a dot-like manner is the same as Sample 2, and the treatment of the present invention was performed. It can also be seen that the internal structure is maintained without change.

  Also, unlike the sample 2, the N concentration cannot be observed in the surface layer portion of the sample 4, but the existence of a portion where the Sn concentration has been confirmed was confirmed. Thus, the nitrogen compound layer was treated by the method of the present invention. Was removed, and it was confirmed that the Sn element was diffused into the surface of the sample by the injection of the Sn particles.

(Corrosion resistance evaluation test)
(1) Object of the test The test piece surface-treated by the method of the present invention has corrosion resistance (particularly, corrosion resistance is imparted by diffusion of Sn element), and the corrosion resistance is not lost even by the friction and wear test. Make sure that

(2) Test method Surface treatment of each of six SKD61 test pieces (hardened / tempered product; width 25 mm x length 25 mm x thickness 5 mm, hardness 45 HRC) under the conditions shown in Table 7 below After that, the surface was frictionally worn by a ball-on-disk wear machine.

  After each test piece after the friction and abrasion was washed, the state of occurrence of corrosion (red rust) after immersion in industrial water (about 25 ° C.) for 240 hours was visually observed.

The friction and wear test of the test piece surface was performed by pressing an A5052 ball (diameter: 10 mm) against the surface of the test piece with a load of 9.8 N as shown in FIG. 12, a friction radius of 6 mm and a rotation speed of 100 min ^-1 for 500 seconds. Slided.

  For immersion of each sample in industrial water, water was replaced every 50 hours to replenish dissolved oxygen in order to create an environment in which rust is likely to occur.

(3) Test Results FIG. 13 shows the surface condition of each sample before and after immersion in industrial water, and Table 8 below shows the results of the corrosion resistance evaluation based on the surface condition.

  From the above results, it was confirmed that high corrosion resistance was obtained by performing the surface treatment by the surface treatment method of the present invention.

  Note that significant rust was observed in Sample 6 to which high compressive residual stress was applied by the nitrocarburizing treatment, indicating that corrosion resistance was not obtained only by applying compressive residual stress.

  Therefore, the corrosion resistance of Samples 9 and 10 treated by the surface method of the present invention was not obtained by applying compressive residual stress by nitrocarburizing (Sample 10) or shot peening (Samples 9 and 10). It is reasonably speculated that this was obtained by the change in the material properties of the cooling hole surface due to the diffusion of.

The test piece of Sample 8 in which the surface modified layer of magnetite (Fe ₃ O ₄ ) was formed on the surface of the test piece after the nitrocarburizing treatment was also used. Like the test piece of Sample 10, the corrosion resistance of the friction surface was not lost even after the friction and wear test.

However, even also obtained by forming a surface modification layer of magnetite (Fe ₃ O _4), directly to the surface of the untreated (left-hardened), surface modification of magnetite (Fe ₃ O ₄₎ sample 7 to form a layer, magnetite layer in a portion in contact with the ball is the corrosion resistance and abrasion loss in friction test, a large amount of red rust has occurred, the surface modification layer of magnetite (Fe ₃ O ₄₎ is However, it was confirmed that although it had high corrosion resistance, it was weak to friction and abrasion without being subjected to surface strengthening treatment such as soft nitriding.

  On the other hand, in the surface treatment method of the present invention, in addition to the case where the surface treatment of the present invention is applied to the surface after the nitrocarburizing treatment (sample 10), it is directly applied to the untreated (as-quenched / tempered) surface. In any of the cases where the surface treatment of the present invention was performed (sample 9), the corrosion resistance was not lost including the portion in contact with the ball in the friction test, and nitriding (soft nitriding) was performed as a base treatment. It was confirmed that a surface treatment resistant to friction was performed even when there was no such treatment.

  The reason why such high abrasion resistance was obtained is unknown, but as shown in the results of the EDX qualitative analysis in FIGS. 10 and 11, the surface of the sample subjected to the surface treatment by the method of the present invention has oxygen. Due to the increase in the detection amount of (O), Sn (hardness 5 HV) was oxidized to become high hardness tin oxide (hardness 1650 HV at the maximum) and high wear resistance was exhibited. It is speculated that this is not the case.

(Evaluation test for shot material injection method)
(1) Object of the test It is confirmed that the method of spraying shot and / or spray particles employed in the present invention can perform processing even on a relatively small diameter cooling hole having a closed end.

(2) Test method As shown in FIG. 14, two test pieces (SKD61) each having a semicircular cross-sectional groove are overlapped so that the grooves overlap each other, and a hole having one end as a closed end is formed. Was formed, and the surface treatment of the present invention was performed on the holes.

As each test piece, a magnetite (Fe ₃ O ₄ ) layer was formed by exposing the groove forming surface to high-temperature steam (corresponding to the treatment of Patent Document 5), and the groove forming surfaces of the test pieces were overlapped. By combining, a hole having a diameter of 4 mm and a length of 290 mm was formed.

  An injection nozzle having an outer diameter of 2.0 mm, an inner diameter of 1.4 mm, and a length of 350 mm is inserted into the hole, a SiC shot (53 to 45 μm) is injected at an injection pressure of 0.7 MPa, and then Sn particles are injected. A body (50-20 μm) was injected at an injection pressure of 0.7 MPa.

  Inject the shot and spray particles until the tip of the spray nozzle is several mm away from the closed end, start spraying at this position, and gradually pull out the spray nozzle from the hole, Injection was continued until the tip came off the hole.

  After performing the above-mentioned surface treatment, the superposed test pieces were separated and the residual stresses on the surface of the hole (groove) near the closed end and on the other surface (the body of the hole) In each case, the residual stress in the longitudinal direction of the hole) was measured by X-ray residual stress measurement using the cosα method.

(3) Test results The residual stress value of the hole surface measured by the above method was −25 MPa near the closed end before the surface treatment according to the present invention and +450 MPa in the other portions before the surface treatment according to the present invention. After the surface treatment, the pressure was -585 MPa near the closed end and -831 MPa in other portions.

  When injecting shots or spray particles from a spray nozzle inserted into a cooling hole with a small diameter and having a closed end, the gap between the outer periphery of the spray nozzle and the inner wall of the cooling hole is narrow, and the shot or spray particles sprayed. As described above, it is difficult to treat the surface of the cooling hole because the cooling hole is clogged.

  However, as proposed in the surface treatment method of the present application, in the process of ejecting shots or ejected particles while pulling out the ejection nozzle, the ejection can be performed without causing such clogging, and the hole can be formed. It was confirmed that shots and blasted particles collided with the surface with energy that could impart high compressive residual stress.

  In addition, as a result of confirming the diffusion of the Sn element on the surface after the treatment, it was found that the Sn element was diffused on the surface of the cooling hole as in the test results for the samples 3 and 4 (see FIGS. 10 and 11). confirmed.

  Therefore, the injection method adopted in the surface treatment method of the present invention is an effective injection method for performing the shot peening or the diffusion treatment of the Sn element by spraying the spray particles on the surface of the cooling hole having the closed end. It can be said.

2. Evaluation by die (1) Purpose of test It is confirmed that stress corrosion cracking resistance is imparted by applying the treatment method of the present invention to cooling holes of the die.

(2) Test method Heating and cooling of the mold were repeated while cooling water was introduced into the cooling hole of the mold (SKD61) surface-treated by the method of the present invention and the untreated cooling hole. Thereafter, the surface condition inside the cooling hole was visually observed.

(3) Test results As a result of observation, it was confirmed that rust was generated on the entire surface of the cooling hole and cracks were generated on the surface of the untreated cooling hole by repeating 20,000 cycles of heating and cooling. Was.

  On the other hand, on the surface of the cooling hole subjected to the surface treatment by the method of the present invention, only slight generation of rust was confirmed even after repeating 30,000 cycles of heating and cooling. Most were kept clean without rust.

  Also, no cracks could be found on the surface of the cooling hole treated by the method of the present invention.

From the above results, it was confirmed that the surface treatment method of the present invention had an effect of imparting stress corrosion cracking resistance to the cooling hole surface of the mold.

Claims (11)

Mold cooling characterized by forming a surface modified layer in which the base component of the mold and Sn are mixed by performing a diffusion treatment for diffusing Sn element into at least the surface of the cooling hole formed in the mold. Surface treatment method for holes.   In the diffusion treatment, a sprayed particle made of tin and / or tin alloy particles having an average particle diameter of 10 to 100 μm is injected together with a compressed gas having an injection pressure of 0.4 to 0.8 MPa to impinge on the surface of the cooling hole. 2. The method according to claim 1, wherein the step is carried out by diffusing the Sn element in the spray particles into the surface of the cooling hole. The surface of the cooling hole whose one end is a closed end is treated and
The spraying of the spray particles in the diffusion process is started with a spray nozzle having a smaller diameter than the cooling hole inserted with the tip of the spray nozzle inserted near the closed end of the cooling hole, and the spray nozzle is gradually pulled out. 3. The method according to claim 2, wherein the injection of the injection particles is continued while the injection is being performed. The surface treatment method for a die cooling hole according to any one of claims 1 to 3 , wherein shot peening is performed on the surface of the cooling hole before the diffusion treatment or during the diffusion treatment.   5. The method according to claim 4, wherein the shot peening is performed by injecting a shot having an average particle diameter of 20 to 149 [mu] m together with a compressed gas having an injection pressure of 0.3 to 0.8 MPa. The surface of the cooling hole whose one end is a closed end is treated and
The injection of the shot in the shot peening is started with an injection nozzle having a smaller diameter than the cooling hole inserted with the tip of the injection nozzle inserted near the closed end of the cooling hole, and the injection nozzle is gradually withdrawn. 6. The method according to claim 5, wherein the injection of the shot is continued.   7. The method according to claim 3, wherein the tip of the nozzle is swung within the cooling hole.   The surface treatment method of a mold cooling hole according to any one of claims 1 to 7, wherein a surface of the cooling hole subjected to nitriding or nitrocarburizing is treated. In a mold with a cooling hole for introducing a coolant for cooling,
A mold wherein a surface modified layer in which Sn element is diffused and mixed with a base component of the mold is formed on a surface of the cooling hole.   The mold according to claim 9, wherein a compressive residual stress is applied to the surface-modified layer. The mold according to claim 9, wherein the cooling hole includes a nitrided layer or a nitrocarburized layer on the surface, and the surface modified layer is formed in the nitrided layer or the nitrocarburized layer.