JPH059558A - Method for modifying surface - Google Patents

Abstract

PURPOSE:To improve the adhesion, surface lubricity and erosion resistance of a nitriding layer, in a method for irradiating the surface of a substrate after nitriding treatment with a laser beam, by regulating the strength of the laser beam to specified one. CONSTITUTION:A nitriding layer 40 formed on the surface of a substrate 10 such as a tool steel is irradiated with a laser beam by irradiation power by which the matrix does not transformed into martensite and nitrides dispersed in the nitriding layer are parted. In this way, slender acicular nitrides dispersed in the diffusion layer 20 are converted into thickly and shortly spheroidized nitrides 23, and the density gradient between the nirides 23 and the matrix 24 is also relaxed. Moreover, many fine cracks which have been generated on the nitriding layer 30 disappear, by which the nitriding layer improved in surface lubricity and having high resistance to wear, particularly to erosion caused by molten metal or high temp. materials can be obtd. Thus, dies for casting, dies for extruding, dies for forming, tools or the like having a long life can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for modifying a nitride layer formed on the surface of a steel material used as a casting mold, a molding mold, a tool and the like.

[0002]

2. Description of the Related Art The surface value of die cast, extruded products, injection molded products, etc. is improved by the smooth surface thereof. This surface smoothness is affected by the surface state of the die used as the casting die and the die for molding. Further, in the casting mold, the unevenness of the surface causes the generation of oscillation marks, which shortens the life of the mold. If an extrusion die with poor smoothness is used, not only will the surface of the extruded material deteriorate, but the separation of the extruded material from the die will also deteriorate, the number of times the die will be polished will increase, and the efficiency of the extrusion operation itself will increase. Also decreases. Furthermore, in the case of an injection molding die having poor smoothness, the fluidity of the molding object in the die cavity is obstructed, and defects such as defective thickness are likely to occur particularly in the thin portion.

These molds come into contact with molten metal or molten resin at high temperature, and high pressure is applied thereto. Also, in the forging die, a large load is applied to form the material into a required shape. The machined surface of the mold exposed to such a harsh atmosphere is subject to mechanical shock, thermal shock, wear, and especially melting damage, and its smoothness is likely to be impaired.

In order to avoid deterioration of smoothness, a hardened layer having excellent wear resistance is formed by subjecting the worked surface of the mold to nitriding treatment, quenching, and the like. Further, it is also possible to irradiate the surface after nitriding with a laser beam to form a surface-hardened layer to which a hardening effect due to the irradiation is added to form a surface hardened layer.
No. 710 publication. That is, the surface of the special steel such as S45C becomes a mixed layer of nitride and martensite by the laser beam irradiation, and the surface hardness is significantly increased.

[0005]

When quenching and tempering not only special steel but also steel in general, strong resistance to mechanical wear is exhibited. However, when exposed to the atmosphere in contact with the molten metal or the atmosphere heated to a high temperature, the surface layer may soften due to the structural change of the quenched structure. In particular,
The contact with the molten metal causes the surface layer to melt and lose the smoothness of the surface.

On the other hand, the surface hardened layer formed by the nitriding treatment has fine irregularities and voids. When a large load is applied to this surface-hardened layer, fine cracks originating from irregularities, voids, etc. occur in the surface-hardened layer. As a result, the surface-hardened layer is peeled off from the base material, and the base material such as special steel is exposed and is subject to wear, melting damage, and the like. In this respect, the peeling resistance of the nitride layer is not improved even by the surface hardening method described in JP-A-54-112710, in which laser hardening is performed through the nitride layer.

The present invention has been devised in order to solve such a problem, and by adjusting the intensity of the laser beam with which the nitride layer formed on the surface of the base material is irradiated, the nitride layer on the base material is adjusted. The object of the present invention is to form a surface-hardened layer suitable for various molds by improving the adhesiveness of the above, improving the surface smoothness of the nitrided layer itself, and improving the corrosion resistance.

[0008]

In order to achieve the object, the surface modification method of the present invention is such that the matrix undergoes martensitic transformation in the nitride layer formed on the surface of the substrate by the nitriding treatment. It is characterized in that the laser beam is irradiated without irradiation with a high power density and with an irradiation power density low enough to divide the nitride dispersed in the nitride layer.

[0009]

[Operation] According to the investigations and studies by the present inventors, fine cracks are less likely to occur by adjusting the intensity of the laser beam with which the nitride layer formed on the surface of the substrate is adjusted. It has been found that the nitride layer can be modified to have a strong resistance to wear, especially to melting loss. The present invention has been completed based on this finding.

For example, when observing the surface of the special steel on which the nitrided layer is formed with a field of view of 800 times, the structure is such that nitrides are precipitated as shown in FIG. That is, the base material 10
Further, a nitride layer 40 including the diffusion layer 20 and the nitride layer 30 is formed. Needle-shaped nitrides 21 are dispersed in the diffusion layer 20. The nitride 21 grows in a slender shape, and its tip portion is sharp. Then, a clear interface is observed between the deposited nitride 21 and the matrix 22. In addition, the nitride layer 30 has many fine cracks 3
1 has occurred, and the surface itself has a large depression 32.

When the nitride layer 40 is irradiated with a laser beam whose intensity is adjusted so that the nitride layer 30 and the nitride 21 are not melted and the nitride 21 is divided, the nitride layer 40 is formed as shown in FIG. Is reformed. Nitride 23 in this state
Is thick and short, has a sharp tip disappearing and has a rounded tip. The nitride 23 and the matrix 24 are separated by an unclear interface 25. Further, the nitride layer 30 is free of the cracks 31 observed before the laser beam irradiation, and the surface smoothness is improved.

Such changes in the nitride 23 and the nitride layer 30 are presumed to be as follows. That is, the nitride 21
Are aggregated due to the progress of diffusion or decomposition by the irradiation of the laser beam, and become thick and short nitrides 23. In particular, the sharpened tip portion of the nitride 21 is concentratedly concentrated to form a rounded tip portion. In addition, diffusion progresses between the nitride 23 and the matrix 24, the concentration gradient becomes gentle, and an interface 25 with an unclear boundary is formed. Similarly, in the nitride layer 30 as well, the cracks 31 disappear due to agglomeration in the vicinity of the cracks 31, and the surface state is excellent in smoothness.

Since the spheroidized nitride 23 and the smoothed nitride layer 30 do not have sharp points or cracks which are the starting points of stress concentration, the tendency to generate cracks under high temperature and high load conditions is suppressed. To be Further, since the concentration gradient becomes gentle, microscopic peeling does not occur between the nitride 23 and the matrix 24, and the interface strength is improved. As a result, the nitrided layer 40 after the laser beam irradiation exhibits a large resistance to melting loss, abrasion, etc. due to the molten metal or high temperature material. Further, due to its excellent smoothness, when used as a casting mold, an extrusion mold, a molding mold, etc., a product having a smooth surface and high commercial value can be obtained.

Usable lasers are CO ₂ , C
There are lasers such as O and YAG. The irradiation power density of the laser beam varies depending on the moving speed of the work, the scanning speed, the beam irradiation width, etc. and cannot be uniquely determined. However, martensite transformation does not occur in the matrix and the diffusion layer 20 contains It is necessary to set the nitride 21 in such a range that it is divided. Specifically, the irradiation output 10
It is preferable to appropriately determine from the range of 0 to 900 W, the scanning speed of 10 to 1000 cm / min, and the irradiation width defocus of 0 to 250 mm in consideration of the type of work, the thickness of the nitride layer, and the like.

Here, the volume change associated with the martensitic transformation causes cracks in the surface hardened layer, embrittlement of the base material, etc., so it is necessary to avoid the martensitic transformation of the matrix. Further, in order to cause the martensitic transformation, it is necessary to maintain the matrix at a high temperature to form the austenite phase. For that purpose, it is necessary to irradiate with high energy power. When the matrix is irradiated with a high energy power that turns into a martensite phase, the nitride in the nitride layer is solid-solved in the matrix, so that the melting resistance against the molten metal is lost. Moreover, even if a part of the nitride layer on the surface remains, it is considered that the erosion resistance to the molten metal or the high temperature metal decreases depending on the degree of solid solution of the nitride.

[0016]

EXAMPLES The present invention will be specifically described below with reference to examples. Example 1: Quenched and tempered hot work tool steel (SKD6
1 tempered material) was subjected to a nitriding treatment under the following conditions. First, 57
The SKD61 refining material was immersed in a molten salt bath composed of cyanate + cyanate maintained at a high temperature of 0 to 580 ° C., treated for 2 hours, and then air-cooled or oil-cooled. When the temperature reaches near room temperature, take out the SKD61 tempered material, which is the material to be treated,
It was washed with warm water. It was then dried and finished with lap polish or abrasive paper.

On the surface of the nitriding hot-work tool steel, a nitride layer consisting of a compound layer having a thickness of 100 μm and a diffusion layer shown in FIG. 1 was formed. The surface of this hot work tool steel is output 400 W, work moving speed 150 cm / min, irradiation width defocus 50 mm (irradiation power density: 2800 w / c
It was scanned with a laser beam of m ² ). As shown in FIG. 2, the nitride layer on the surface after the laser beam irradiation was such that the nitride was spherical and the surface smoothness was improved.

An aluminum billet heated to 480 ° C. was hot-extruded using the hot-work tool steel subjected to nitriding and the hot-tool tool steel obtained by irradiating the nitrided layer with a laser beam as extrusion dies. As a result, in the mold using the hot tool steel whose nitride layer was irradiated with the laser beam, even after extruding 25 tons of aluminum billet, there were no defects such as melting loss and wear on the surface in contact with the high temperature aluminum billet. It was not detected, and no peeling of the nitride layer was observed. In contrast,
For molds using hot work tool steel that has been simply nitrided,
When 0 tons of aluminum billet is extruded, many abrasions and fine cracks occur, and the surface of the extruded material is roughened in the further extrusion work. It can be used for those with strict surface quality requirements. There wasn't.

Example 2: A hot die tool steel mold that was subjected to nitriding treatment in the same manner as in Example 1 and a hot die tool steel in which the nitride layer was irradiated with a laser beam, where the injection molten metal of the die cast die hits most intensely. It was used as a shunt placed at. Then, the aluminum alloy ADC held at the temperature of 720 ° C.
The molten metal of 10 was injection molded into a die cast mold.

When a die made of hot tool steel as-nitrided was used, the shunt was severely melted after 1000 shots, and die casting had to be stopped. On the other hand, when using a die made of hot work tool steel whose nitrided layer was irradiated with a laser beam, even after 2500 shots, only slight microcracks were observed on the surface of the shunt and It was durable enough to use.

Example 3: In order to investigate the influence of the laser beam irradiation amount on the modification of the nitrided layer, the surface of the hot work tool steel (SKD61 tempered material) nitrided under the same conditions as in Example 1 was applied. The laser beam was irradiated while changing the work moving speed and the irradiation power density. Then, as a result of examining the condition in which the change occurs in the nitrided layer, it was found that there is the relationship shown in FIG. 3 between the work moving speed and the irradiation power density. The horizontal axis in FIG. 3 represents the moving speed of the work, which corresponds to the moving speed of the beam irradiation. The vertical axis represents the irradiation power of the laser beam divided by the area of the beam spot, that is, the irradiation power density.

In the region I where the irradiation power density is low, no change is observed in the nitride layer. As in FIG. 1, the nitride layer is composed of a diffusion layer in which elongated nitrides are dispersed and a nitride layer in which many minute cracks are generated. It remained.

On the other hand, in the region II where the irradiation power density was slightly increased, spheroidization of the nitride, which is considered to be caused by the agglomeration reaction, was observed, and a nitride layer similar to that shown in FIG. 2 was obtained.
The region I and the region II can be easily discriminated by whether or not the elongated nitride dispersed in the diffusion layer is divided.
The boundary line A between the region I and the region II had a slight upward gradient as the moving speed of the work increased.

Further, when the irradiation power density was increased, the nitride layer was melted and most of the nitride flowed from the surface to become a surface layer. In this region III, only the surface hardened layer is formed by laser beam quenching, and it is not possible to expect an increase in the surface hardness derived from the nitride layer. Region II and Region II
The boundary line B with I has a larger upward slope than the boundary line A and rises in proportion to the moving speed of the work.

Therefore, it is understood that when the nitride layer is irradiated with the laser beam under the condition that the work moving speed and the radiation power density are in the region II, the elongated nitride shown in FIG. 1 is divided. When the nitride was divided, the spheroidization of the nitride and the smoothing of the nitride layer occurred at the same time, and the sharp nitride and the fine cracks of the nitride layer, which became the starting point of crack generation due to stress concentration, disappeared. As a result, Example 1 and Example 2
A surface-hardened layer having excellent wear resistance and melt resistance as described in Section 1 was obtained.

The boundary lines A and B that divide the regions I to III are
Under the conditions described above, when the work moving speed and the irradiation power density are V (cm / min) and F (w / cm ² ), respectively, they are expressed by the following equations. Boundary line A: F = V + 1500 Boundary line B: F = 3.6V + 2900 However, the boundary lines A and B vary depending on the laser beam irradiation conditions, the thickness of the nitride layer, etc., and are uniquely determined. is not. That is, the constants in the above equations are merely examples and do not restrict the present invention.

[0027]

As described above, in the present invention, the nitrided layer formed on the surface of the base material by the nitriding treatment is modified by the irradiation of the laser beam. That is, needle-like nitride having a long and sharp tip end dispersed in the diffusion layer of the nitride layer is divided into thick and short spheroidized nitride, and at the same time, a large number of the nitride layers are generated. The microcracks disappear and the surface smoothness is remarkably improved. Moreover, the concentration gradient between the nitride and the matrix dispersed in the diffusion layer becomes gentle. In this way, the nitrided layer after the laser beam treatment has no place where the stress is concentrated and is the starting point of crack generation, and exhibits the original excellent abrasion resistance, melt resistance, etc. of the nitrided layer, and has excellent peeling resistance. It will be excellent. Therefore, when it is used as a casting mold, a molding mold, a tool, etc., it is possible to obtain a product having excellent surface smoothness for a long time.

[Brief description of drawings]

FIG. 1 is a structural diagram showing a nitride layer formed on the surface of a tool steel.

FIG. 2 is a structural diagram showing a nitride layer modified by laser beam irradiation.

FIG. 3 is a graph showing a region where a modified nitride layer is obtained, as a relation between a work moving speed and a laser beam irradiation power density.

[Explanation of symbols]

10 Base Material 20 Diffusion Layer 21
Nitride 22, 24 Matrix 23 Spheroidized nitride 25 Interface between nitride 23 and matrix 24 30 Nitride layer 31 Crack 32
Depression 40 nitrided layer

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Isao Miki             1-34-1 Kambara, Kambara-cho, Anbara-gun, Shizuoka Prefecture             Nikkei Giken Co., Ltd.

Claims (2)

[Claims] 1. A laser beam is applied to a nitride layer formed on the surface of a base material by a nitriding treatment at an irradiation power density that divides the nitride dispersed in the nitride layer without undergoing martensitic transformation. A surface modification method characterized by irradiation. 2. A surface modification method, wherein the base material according to claim 1 is a steel material such as tool steel, and is a mold with which a high temperature metal comes into contact.