JP2001123277A - Method for surface treating die for diecasting and same die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a die surface treating technique by which the adhesion of a film for the surface of a die is excellent, moreover, a thick film excellent in thermal resistance, thermal impact resistance, thermal fatigue resistance, wear resistance and thermal insulating properties can be deposited, and further more, the casting characteristics (fluidity) thereof is not checked and to produce a die excellent in shape stability and suitable for diecasting. SOLUTION: As for a die for diecasting, at the time of subjecting a die made of steel to surface treatment, the surface of the die is subjected to nitriding treatment, next, the surface of the die deposited with a nitride layer is coated with a chromium-containing treating solution, is thereafter dried and is moreover baked under heating in the temperature region of 480 to 600 deg.C, by which a Cr2O3 thin film layer is deposited on the surface of the nitride layer to deposit a composite film composed of the nitride layer and the Cr2O3 thin film layer.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating a surface of a die for use in die-casting, which is used, for example, when manufacturing a die-cast part made of a magnesium alloy, and to a die for the same. The present invention can be applied to a mold used for the thixomolding method, and further can be applied to a mold for forming a die-cast part of aluminum or an alloy thereof.

[0002]

2. Description of the Related Art In general, magnesium (Mg) has the same strength as aluminum (Al), despite its lightest specific gravity (1.74) as a practical metal, as well as heat dissipation, vibration damping, and electromagnetic shielding. Is also excellent. Further, since there is an advantage that the thickness can be reduced as a die-cast member, the die-cast member has been put to practical use as a housing material for protecting electronic devices and the like.

Heretofore, as a die material for die casting for casting a molten Mg, SDK61 material excellent in high-temperature strength has been used. However, in molds made of this material, the injection of high-temperature Mg melt repeatedly causes rapid temperature rise and rapid cooling, and as a result, thermal fatigue cracks are likely to occur on the contact surface with the molten metal. As cracks increased, the cracks grew progressively, causing harmful scratches and defects to the product.

As a countermeasure against this, conventionally, TiN
For forming a thin film of TiAlN or TiAlN (for example,
No. 04822) and a method of applying a self-fluxing alloy sprayed coating (Japanese Patent Laid-Open No. 8-23258).

[0005]

However, among the surface treatment techniques proposed by the above-mentioned prior arts, the surface treatment technique by forming a TiN thin film has a higher durability than an untreated mold. Although useful for forming excellent mold surfaces, TiN itself has poor heat resistance, while TiAlN has problems with the casting properties (flow of molten metal) of the Mg alloy melt and room for improvement in the film life itself. was there. on the other hand,
In the case of technology for forming a self-fluxing alloy spray coating and treating the surface,
Since it is necessary to heat the film to a temperature of 1000 ° C. or higher after the formation of the film, there are problems such as distortion of the mold and deterioration of the material.

Accordingly, an object of the present invention is to form a thick film having excellent heat resistance, thermal shock resistance, thermal fatigue resistance, abrasion resistance, and heat insulation, in addition to excellent adhesion of the film to the mold surface. To establish a mold surface treatment technology that does not hinder casting characteristics (fluent flow) and to propose a mold that is suitable as a die-casting mold and has excellent so-called mold shape stability. is there.

[0007]

[Means for Solving the Problems] To achieve the above-mentioned object
In research, the inventors found that nitride layers and fine crystalline Cr ₂
O₃Cr consisting of particles₂O₃Compounded with laminated thin film
Is found to be effective on the mold surface.
Thus, the present invention has been developed.

That is, according to the present invention, in the surface treatment of a steel mold, the mold surface is nitrided, and then a chromium-containing treatment solution is applied to the mold surface on which a nitride layer is formed, followed by drying. Further, by heating and baking in a temperature range of 480 ° C. to 600 ° C., a Cr ₂ O ₃ thin film layer is generated on the surface of the nitride layer, and a composite film composed of the nitride layer and the Cr ₂ O ₃ thin film layer is formed. A surface treatment method for a die for die casting characterized by being formed is proposed.

Further, the present invention provides that the surface of a steel mold in contact with a molten metal is covered with a composite film composed of a lower nitride layer and an upper Cr ₂ O ₃ thin film layer. We propose a characteristic die-casting mold.

[0010] In the present invention, the above Cr₂O₃Thin film
In forming the layer, after forming a chromium-containing treatment liquid,
By repeating the operation of drying and heating and firing several times,
And Cr ₂O₃Forming a thin film layer into a laminated film; and
Chromic anhydride, chromic acid
Use ammonium or ammonium bichromate
That the above Cr₂O₃The thin film layer is made of fine crystalline Cr₂O₃particle
Thin film consisting of multiple layers
It is preferable that it is constituted.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention relates to a die casting mold for casting a Mg alloy, in particular, a die casting in which a composite film conforming to the present invention is formed on the surface of the side of the cast metal Mg or its alloy which is in contact with the molten metal. The embodiment and its operation will be described with an example applied to a metal mold. For example, as a die for Mg alloy die casting, a surface of a die made of hot tool steel represented by SKD61, in particular, a surface in contact with a molten Mg alloy is subjected to a nitriding treatment in advance. Metal nitride layers (eg, FeNx, AlNx, Cr
Nx, TiN, etc.) to form a hardened surface. The nitriding method employed in this treatment includes a gas nitriding method,
It may be either a plasma nitriding method or a salt bath nitriding method, but the thickness of such a nitride layer requires a minimum of 3μm or more, Cr ₂ O formed by the chemical treatment method described later in a thin layer than this ₃ There is a possibility that the effect of the film cannot be sufficiently exhibited.

Next, with respect to the surface of the nitride layer of the mold,
After applying a chromium-containing treatment liquid to form a coating, the coating is dried in the air, and is further heated in a temperature range of 480 ° C. to 600 ° C. by an electric furnace. By this operation, the chromium-containing treatment solution is evaporated, decomposed, and oxidized, and finally becomes crystalline Cr ₂ O ₃
A Cr ₂ O ₃ thin film composed of an aggregate of fine particles is formed and remains.

Hereinafter, this processing step will be described in more detail. In other words, looking at the process of producing Cr ₂ O ₃ , the concentration of the chromium-containing treatment solution by evaporation of water, the precipitation of soft and amorphous Cr ₂ O ₃ particles, the hard and fine crystalline Cr ₂ O ₃ particles It is thought that changes to have occurred.

If the chromium-containing treating solution is an aqueous solution of chromic anhydride, it exerts a very strong oxidizing power in the above steps and oxidizes and decomposes a part of the metal nitride. It evaporates as NOx. For this reason, the surface of the nitride layer forms fine irregularities, and the processing liquid or the soft Cr ₂ O ₃ particles in the above state or the like enter into the concave portions, and both are physically strongly connected to each other. Compared to the same processing step on the mold surface not subjected to nitriding treatment, a much thicker Cr ₂ O ₃ layer can be formed. For example, using a test piece of SKD61, one of them is subjected to an ion nitriding treatment to form a nitride layer, and then the application, heating and decomposition operations are repeated 12 times using a CrO ₃ aqueous solution having a Baume specific gravity of 1.6. 7μm thick Cr
SK that produces ₂ O ₃ laminated thin film layer but does not perform nitriding
In the case of the D61 test piece, this Cr ₂ O ₃ thin film layer was only 0.7
Only 0.8 μm thick.

FIG. 1 shows the present invention formed by such a method.
Cr suitable for the method₂O₃Thin film layer and Cr as comparative example₂O₃Thin
2 schematically shows a cross section with a film layer. Where 1
Is a SKD61 substrate, 2 is a nitrogen diffusion part (solid solution), 3 is
Nitride layer (FeNx), 4 for Cr ₂O₃It is a thin film layer. The present invention
Cr formed by a method compatible with₂O₃The film is hard from the outside
Quality and dense Cr₂O ₃Thin film-metal nitride layer-substrate (nitrogen containing
Solid solution) in order.
You can see that there is.

The chromium-containing treating solution used in the method of the present invention includes, in addition to the above-mentioned chromic anhydride [CrO ₃ ], ammonium chromate [(NH ₄ ) ₂ CrO ₄ ] and ammonium bichromate [(NH ₄ ) ₂ Cr ₂ O ₇ ] or the like can be used. Further, in the present invention, instead of applying a chemical solution, a method of forming by immersion or spraying can be applied.

The following advantages are obtained when a mold formed with the Cr ₂ O ₃ thin film layer formed as described above is used as a die casting mold of molten Mg alloy. Since the thermal conductivity of the Cr ₂ O ₃ thin film layer formed on the surface in contact with the molten metal of the mold is lower than that of metallic material, the cooling rate of the molten Mg alloy injected into the mold is reduced, and the flow is excellent. Since it is possible to maintain the performance, it is familiar with the mold and contributes to the improvement of productivity. Further, since the Cr ₂ O ₃ thin film layer is formed by heating and decomposing the chromium-containing treatment liquid, the Cr ₂ O ₃ thin film layer is formed of an aggregate of fine crystalline Cr ₂ O ₃ particles. Has the advantage that the surface of the die-cast member is beautifully finished. Such knowledge has been found as a result of putting a Cr ₂ O ₃ thin film layer formed by applying the method of the present invention to an actual die casting mold to practical use.

[0018] With respect to such a Cr ₂ O ₃ thin film layer, since if the thin film is contacted with molten Mg alloy melt, towards the Mg is thermodynamically is chemical affinity for oxygen than Cr greater, Cr ₂ O
Film ₃ should be reduced by Mg and film breakage should occur. However, according to the study of the present inventors, the effectiveness of the Cr ₂ O ₃ thin film which seems to be seemingly not satisfied in terms of equilibrium theory
(Melting metal resistance) has also been found to be sufficiently durable in some cases when considered kinetically. That is,
If such a Cr ₂ O ₃ thin film is heated to a high temperature of 480 ° C. to 600 ° C. and sufficiently crystallized, the time of die casting is because the contact time between the molten Mg alloy to be cast and the mold is short. It is considered that the reaction kinetics is such that the reduction reaction hardly occurs.

In the present invention, the Cr ₂ O ₃ thin film layer is formed by performing a series of operations of applying the chromic acid aqueous solution to the nitride layer, drying and heating and baking a plurality of times, preferably 2 times.
By repeating about 10 to about 10 times, it is preferable to form a laminated film in which the thin films are laminated in several layers and thickened, and such a Cr ₂ O ₃ thin film laminated film is used as an upper layer,
It is a preferred embodiment that the lower layer is a composite film in which the metal nitride layer is interposed.

It is preferable that the nitride layer is formed of a nitride diffusion portion in which metal nitride particles are diffused and penetrate into the surface of the base material by the nitriding treatment, and a nitride layer covering the surface.

[0021]

Example 1 In this example, an SKD61 test piece (dimension width 50 mm) was used.
× length 100 mm × thickness 5 mm)
Using a base material provided with a nitrogen diffusion layer having a thickness of 30 μm and a nitride layer having a thickness of 5 μm, a Cr layer was formed on the surface of the nitride layer having a thickness of 5 μm.
_{A 2} O ₃ thin film is formed to a thickness of 5 μm, and then
Heating was performed at 0 ° C. for 15 minutes to form a composite film. Thereafter, the test piece after the film formation was put into water at 20 ° C. and cooled. Then, the heating-cooling cycle of the above-mentioned film is set to 15 times.
A thermal shock test repeated several times was performed. As a result, no abnormality was observed in appearance. Even when the test piece was cut and its cross section was observed with an optical microscope, the film maintained a healthy state. On the other hand, in the case where the Cr ₂ O ₃ film was formed on the surface of the SKD61 test piece which was not subjected to the nitriding treatment (Comparative Example), local peeling occurred. In addition, Tokiko
Cr ₂ O containing silicon particles proposed in -14833 JP
_{With the three} films, although a thick film having a thickness of 50 to 70 μm was formed, generation of fine cracks was observed after the thermal shock test.

Example 2 Using the test film for the thermal shock test of Example 1, the temperature was raised to 630 ° C.
Heated magnesium alloy specified in JIS H 5303 (MDI
A) After immersing in a bath for 10 seconds, pull it up and blow
1 cycle of cooling operation for 1 minute
The cycle was repeated. As a result, the ion
Cr on the nitrided SKD51 specimen₂O ₃Form film
Those that have been in a healthy state after 10 cycles
-Excellent effect on thermal shock resistance due to repeated cooling
As well as against erosion by magnesium alloys
Excellent corrosion resistance was observed. In contrast,
Cr on SKD51 test piece without₂O₃Shape the film
The formed film of the comparative example had fine cracks after 6 cycles.
As well as magnesium alloy in the film from the cracks
Into the part, causing metallurgical reaction with the SKD51 substrate and sticking
A phenomenon of growing and breaking the film from the bottom was observed.

Embodiment 3 In this embodiment, when a main body of a personal computer body using a hot chamber method is manufactured using a Mg alloy (MDIA specified in JIS H 5303), the method of the present invention is applied to the surface of an ion-nitrided SKD61 mold. A 5 μm-thick composite film composed of a nitride layer and a Cr ₂ O ₃ thin film layer formed according to the above was used. As a result, the conventional SKD6 with only ion nitriding treatment
In the one mold, the cooling rate of the Mg alloy is high, so the flow of the molten metal is insufficient, the productivity is low, and there is a problem that the plate thickness tends to be uneven even in quality. Since the surface treatment mold conforming to the present invention has an excellent heat retaining effect, the flow of the molten metal is good, and a product having good quality can be efficiently produced.

[0024]

As described above, according to the present invention, after nitriding the surface of a steel mold base material, a Cr ₂ O ₃ thin film is formed thereon by a chemical method to form a composite film. In this case, the adhesion between the films is strong and the film can be formed thick. As a result, by forming a composite film of a nitride layer and a Cr ₂ O ₃ thin film layer, it is possible to improve the heat resistance, thermal shock resistance, thermal fatigue resistance, wear resistance, and heat insulation of the die for die casting, and For example, even when a molten magnesium alloy is die-cast, the flow of the molten metal can be maintained satisfactorily, which can greatly contribute to a reduction in production cost including improvement in product quality and productivity.

[Brief description of the drawings]

FIG. 1 is a schematic view showing an example of a composite film composed of a nitride layer and a Cr ₂ O ₃ thin film layer conforming to the present invention formed on the surface of a steel base material by the method of the present invention, and a film of a comparative example. It is sectional drawing shown in FIG.

[Explanation of symbols]

1 substrate 2 nitride diffusing unit 3 nitride layer 4 Cr ₂ O ₃ thin film layer

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (Reference) C23C 8/38 C23C 8/38 22/24 22/24 22/74 22/74 22/82 22/82 ( 72) Inventor Yoshio Harada 4-13-4 Fukae Kita-cho, Higashinada-ku, Kobe City, Hyogo Prefecture Inside Tokaro Corporation (72) Inventor Katsunori Ishii 1-1-1 Gyoda, Funabashi City, Chiba Prefecture Tokaro Corporation Tokyo Plant (72) Inventor Kunio Hamanaka 2-9-9 Suehirocho, Ome-shi, Tokyo Inside the Toshiba Ome Plant Co., Ltd. (72) Inventor Shigeru Masuchi 1-5 Kumano-cho, Itabashi-ku, Tokyo Tsukuba Die Cast Industrial Co. Reference) 4E093 NB08 4K026 AA02 BA08 BA11 BA12 BB03 BB10 CA16 CA20 CA21 EB11 4K028 AA02 AB06 BA02 BA12 4K044 AA02 AB10 BA15 BA18 BB03 BC01 BC05 BC07 BC11 CA12 CA16 CA62

Claims (5)

[Claims] In the surface treatment of a steel mold, the mold surface is nitrided, and then a chromium-containing treatment liquid is applied to the mold surface on which a nitride layer has been formed, followed by drying.
By heating and baking in a temperature range of 480 ° C. to 600 ° C., a Cr ₂ O ₃ thin film layer is generated on the surface of the nitride layer, and a composite film composed of the nitride layer and the Cr ₂ O ₃ thin film layer is formed. A surface treatment method for a die casting die. Strikes the formation of wherein said Cr ₂ O ₃ thin film layer, after the form the chromium-containing treatment solution, by repeating several times the operation of heating and firing and drying, the multilayer coating of the Cr ₂ O ₃ thin film layer The surface treatment method according to claim 1, wherein: 3. The surface treatment method according to claim 1, wherein chromic anhydride, ammonium chromate, or ammonium bichromate is used as the chromium-containing treatment liquid. 4. The surface of a steel mold in contact with a molten metal,
A die casting die, which is covered with a composite film including a lower nitride layer and an upper Cr ₂ O ₃ thin film layer. 5. The Cr ₂ O ₃ thin film layer comprises fine crystalline Cr
A mold according to claim 4 thin film made ₂ O ₃ particles are those composed of laminated film provided over a plurality of layers.