JPH1190611A - Die for die casting and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a die for die casting easy in machining and having the necessary impact strength and further, excellent in heat check resistance, etc. SOLUTION: This die for die casting is formed with a tool steel for hot working, having the hardness of HRC30-HRC45, the surface is applied with a nitriding treatment and where a nitriding hardened layer having <=50 μm thickness of and Hv 600-Hv 900 surface hardness is formed. Therefore, the hardness of the die is lower than that of a die in conventional technique, the cutting is easily executed and the wording at the time of finishing the die after heat treatment is facilitated. Further, since the tool steel for hot working has high tenacity, the necessary impact strength can be secured. Further, since the hardness of the die surface is held to Hv 600-Hv 900 with the nitriding treatment, the heat check resistant can be improved. Furthermore, since the thickness of the nitriding hardened layer is <=50 μm, the tenacity on the die surface is not lowered so much and the lowering of the impact strength can be prevented.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a die-casting die formed by heat-treating hot tool steel and a method of manufacturing the die-casting die.

[0002]

2. Description of the Related Art In a die-casting die, the filling of a high-pressure and high-temperature molten aluminum, the solidification and demolding of aluminum are performed in a short time. large. For this reason, there is a problem that heat check, which is a minute crack, occurs on the surface of the mold or wear occurs, thereby shortening the life of the mold. An effective method for reducing such heat check and wear is to increase the hardness of the mold by heat treatment. However, if the hardness of the mold is increased, the toughness is reduced and the impact strength is reduced. For this reason, there is a problem that the mold is easily broken. In order to solve this problem, a technique disclosed in Japanese Patent Application Laid-Open No. Hei 4-308059 discloses a method of forming a steel sheet containing a hot tool steel,
By adjusting the amount and adding an appropriate amount of Co, a decrease in toughness can be suppressed to some extent even if the hardness of the mold is increased to a hardness (HRC50 or more) at which heat check or the like is unlikely to occur. For this reason, a predetermined impact strength can be secured, and cracks and the like of the mold can be prevented.

[0003]

Generally, in the manufacture of a mold, a mold material is processed into a substantially mold shape with a predetermined allowance (for example, about 0.3 mm), and the mold is heat-treated. After being cured, it is finished to the final shape. However, according to the conventional mold forming method, the hardness of the mold (H
(RC50 or more), cutting becomes difficult, and it becomes very difficult to finish the mold to the final shape after heat treatment.

[0004] Therefore, claims 1 and 3 of the present invention.
In the invention described in the above, the hardness of the mold is set to a hardness that can be easily finished, and that can secure a predetermined impact strength,
The object is to improve the heat check resistance by subjecting the mold to a surface treatment. The invention described in claim 2 aims at improving the aluminum erosion resistance in addition to the object of the invention described in claim 1. A fourth aspect of the present invention has the object of further improving heat check resistance in addition to the objects of the first and third aspects of the present invention.

[0005]

The above-mentioned object is achieved by a die casting mold having the following features and a method of manufacturing the same. That is, the die casting mold according to claim 1 is formed of hot tool steel having a hardness of HRC30 to HRC45, and the surface thereof is nitrided to form a nitrided hardened layer having a thickness of 50 μm or less. , Surface hardness Hv600-Hv90
It is characterized by being 0.

According to the present invention, the mold has a hardness of HRC30 to HR.
Because it is formed from C45 hot tool steel, it has lower hardness than conventional molds, is easy to cut, and facilitates finishing when finishing the mold after heat treatment. Also,
HRC30 to HRC45 hot tool steels have high toughness and can secure the required impact strength. Furthermore, since the hardness of the mold surface is maintained at Hv600 to Hv900 by the nitriding treatment,
Heat check resistance can be improved. Also,
Since the thickness of the nitrided hardened layer on the mold surface is 50 μm or less,
The impact strength can be prevented from lowering without significantly reducing the toughness of the mold surface.

[0007] The invention described in claim 2 is the first invention.
In the die casting mold described in the above, an aluminum erosion resistant film is formed on a surface that comes into contact with the molten metal. For this reason, the aluminum erosion resistance of the mold can be improved.

In a third aspect of the present invention, there is provided a method for manufacturing a die-casting die, wherein a die is formed by processing hot tool steel, and the die is heat-treated to maintain the hardness at HRC30 to HRC45. The mold is subjected to a nitriding treatment so that the surface nitriding hardened layer of the mold has a thickness of 50 μm or less and a surface hardness of Hv600 to Hv900. That is, by carrying out the present invention, the die casting mold described in claim 1 can be manufactured, and the same operation and effect as the invention described in claim 1 can be obtained.

According to a fourth aspect of the present invention, there is provided a method of manufacturing a die-casting die according to the third aspect, wherein the nitriding treatment is performed so that a compound layer is hardly formed on the surface of the die. It is characterized by performing. According to the present invention, since a brittle compound layer is not formed on the surface of the mold, heat check resistance and the like can be further improved.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION

[First Embodiment] A die casting mold and a method for manufacturing the same according to a first embodiment of the present invention will be described below with reference to FIGS. Here, FIG. 1 is a main part process diagram in a method of manufacturing a die casting mold according to the present embodiment, and FIG. 2 is a drawing showing characteristics of the die casting mold according to the present embodiment. The die casting die according to the present embodiment includes four types of SKD61 series steels (A steel, B steel) which are hot tool steels.
(Steel, C steel, D steel). In addition,
The chemical composition (%) of steel A, steel B, steel C and steel D is shown in FIG.

First, SKD61 series steel (A
Steel, B steel, C steel, and D steel) (hereinafter referred to as base metal) are processed into a substantially die shape leaving a predetermined allowance (about 0.3 mm). Next, quenching the base material processed into the shape of the mold,
After tempering, the hardness is set to HRC30 to HRC45.
The reason for setting the heat treatment hardness of the base metal to HRC30 to HRC45 is that in order to secure the material strength necessary for the construction of the die casting
Hardness of C30 or more is required, and HRC45 or less is the result of stable impact strength (IC _C ) as shown in FIG.
≧ 40 J / cm ² ).

[0012] When the heat treatment of the base material is performed in this manner, a finishing process is performed next, and the base material is finished to the final shape of the die casting mold. At this time, the hardness of the base material is HR
Since it is set to C30 to HRC45 and is relatively soft, cutting is easy and finishing work does not take time. Next, a die casting mold (hereinafter, referred to as a mold) is subjected to a nitriding treatment. The nitriding treatment is a heat treatment for causing nitrogen to enter the surface of the steel material to harden the surface, and is performed in the following procedure.
First, the mold is housed in a vacuum chamber, and the vacuum chamber is kept at a high vacuum of 10 ^-4 torr or less at room temperature. Next, N ₂ gas is supplied into the vacuum chamber, and the temperature in the vacuum chamber is reduced to about
Heated to 400 ° C or higher. Further, CO ₂ gas, NH ₃ gas and the like are sequentially supplied into the vacuum chamber, and the gas atmosphere composition in the vacuum chamber is changed to, for example, CO ₂ ; 4
%, NH ₃ ; 10%, balance N ₂ while controlling the gas supply flow rate so that the pressure in the vacuum chamber is about 10 to
Heat and hold with rr increased to about 200 torr. And, the temperature in the vacuum chamber, for example,
With the temperature set at 520 ° C., the mold is held in the vacuum chamber for about 10 hours to perform a nitriding treatment.

When the mold is subjected to a nitriding treatment under such conditions, a nitrided hardened layer having a thickness of 50 μm or less is formed on the surface of the mold, and the surface is hardened to have a surface hardness of HV600 to HV900. Between. Here, the compound atmosphere is hardly formed on the surface of the mold after the nitriding treatment because the gas atmosphere composition in the nitriding treatment keeps the CO ₂ low at about 4%. Since the surface hardness is measured by a micro Vickers hardness tester, the term “surface micro hardness” is used instead of the term “surface hardness”. The conditions for nitriding were set so that the surface microhardness was between HV600 and HV900.
If it is smaller, the hardness is insufficient and the heat check resistance is reduced. On the other hand, if the surface microhardness is larger than HV900, the surface is too hardened, the cracking sensitivity increases, and the impact strength and heat check resistance decrease.

FIG. 6 is a graph showing the results of a heat check test performed using a test piece (SKD61-based steel) to be described later. The horizontal axis represents the surface microhardness of the test piece, and the vertical axis represents the test piece. It represents the maximum crack length that has occurred. In the figure, ○ indicates each test data. The curves in the figure schematically represent the characteristics grasped from each data. As shown in FIG. 6, when the surface microhardness is between HV600 and HV900, the maximum crack length is stably reduced, and the heat check resistance is improved.

In the above-mentioned heat check test, a cylindrical test piece having a diameter of 30 mm and a height of 20 mm was used. First,
The test piece was heated with a burner for about 9 seconds until the peak temperature (about 550 ° C.) as shown in FIG. 9 (A), then air-cooled for about 12 seconds after heating, and further water-cooled for about 9 seconds. After lowering the temperature to approximately room temperature, the water drops are removed for about 12 seconds. This was performed as one cycle for 1000 cycles, and the appearance of the test piece was observed, and the crack distribution, hardness distribution, structure change, and the like of the cross section were observed. FIG. 9 (B) shows a cross section of the test piece. Observation of a cross-sectional crack was performed at a central portion of the test piece, and measurement of a cross-sectional hardness was performed at a × portion.

From the results of the heat check test, it can be seen that when the surface microhardness of the mold is set between HV600 and HV900, the heat check resistance of the mold is improved. In addition, the conditions of the nitriding treatment described above were set so that the thickness of the nitrided hardened layer was 50 μm (= 0.05 mm) or less. This is because cracking sensitivity is increased, impact strength is reduced, and heat check resistance is also reduced. FIG. 5 is a graph showing the results of a heat check test using the same test piece (SKD61-based steel). It represents the maximum length of a crack generated in a piece. In the figure, ○ indicates each test data. The straight lines in the figure schematically represent the characteristics grasped from each data. As shown in FIG. 5, the thickness of the nitrided hardened layer is 50
In the range of μm (= 0.05 mm) or less, the maximum crack length is stably reduced, and the heat check resistance and the like are improved.
Therefore, the heat check resistance of the mold can be improved by subjecting the mold to nitriding so that the thickness of the nitrided hardened layer becomes 50 μm (= 0.05 mm) or less.

The reason for setting the nitriding treatment so that almost no compound layer is formed on the surface of the mold is that the compound layer is brittle, so that heat check resistance and impact strength are reduced. FIG. 7 shows a test piece (SKD61).
Is a graph of the results of a heat check test performed using the same type of steel, and the horizontal axis represents the thickness of the compound layer formed on the surface of the test piece, and the vertical axis represents the maximum length of a crack generated in the test piece. Represents. In the figure, ○ indicates each test data. The straight line and the curve in the figure schematically represent the characteristics grasped from each data. As shown in FIG. 7, when the thickness of the compound layer is reduced, the maximum crack length is reduced, and the heat check resistance and the like are improved. Therefore, the heat check resistance of the mold can be improved by performing the nitriding treatment so that almost no compound layer is formed on the surface of the mold.

After the mold is subjected to the nitriding treatment in this manner, the surface of the mold in contact with the molten metal is then subjected to, for example, a PVD treatment in order to secure the aluminum erosion resistance. Then, the mold is completed. Here, PVD treatment means that a hard substance such as titanium nitride is applied to the surface of steel or the like by physical vapor deposition.
This is a process for coating to a thickness of 2 μm to 10 μm, and is performed at a tempering temperature or lower. As a result, the aluminum erosion resistance of the mold can be improved without thermally deforming the mold.

FIG. 2 shows an example of characteristics of a die casting mold manufactured by using the method of manufacturing a die casting mold according to the present embodiment. When SKD61 series steel A is used as the base material and the hardness after quenching and tempering is adjusted to HRC45, the surface microhardness is HV898 and the hardening depth ( (Thickness of nitrided hardened layer) 4
The thickness was 0 μm, the thickness of the compound was ≒ 0, the impact strength was 49.1 J / cm ² , and the maximum crack length was 20 μm. In addition, PV
By performing the D treatment, the aluminum erosion resistance index was 10.0.

Here, the aluminum erosion resistance index is defined as:
Sensory evaluation of the damage state of the specimen surface by immersing the specimen in a molten aluminum alloy at 0 ° C for 2 hours using an index of 0 to 4
(1) Yes. The larger the index, the less the damage on the test piece surface. Next, the damage state of the test piece surface caused by immersing another test piece in a molten aluminum alloy at
Perform sensory evaluation (2) with an index of 4. Further, the damage state of the test piece surface by immersing the test piece in a molten aluminum alloy at 700 ° C. for 2 hours is subjected to a sensory evaluation with an index of 0 to 2 (3). Then, the total score of the sensory evaluations (1) to (3) is used as an aluminum erosion resistance index.

The test piece was, as shown in FIG.
Eight test pieces A1 to A4 and B1 to B4 are immersed in a molten aluminum alloy stored in a crucible while being set on a support (holding jig). Here, the crucible is capable of holding an aluminum alloy of about 10 kg at a predetermined temperature, and ADC12 is used as the aluminum alloy. Thus, the test piece immersed in the molten aluminum alloy for a predetermined time is taken out of the crucible, cooled, and then immersed in a caustic soda solution. Then, after the attached aluminum alloy is melted and removed, the state of damage on the surface of the test piece is observed, and the above-described sensory evaluation is performed.

Similarly, when the SKD61 series steel B is used as the base material and the hardness after quenching and tempering is adjusted to HRC45, the surface microhardness is increased by performing the nitriding treatment as described above. HV809, hardening depth (thickness of nitrided hardened layer) 3
The thickness was 0 μm, the thickness of the compound was ≒ 0, the impact strength was 56.2 J / cm ² , and the maximum crack length was 10 μm. In addition, PV
By performing the D treatment, the aluminum erosion resistance index was 10.0. In addition, when using SKD61 series C steel as the base material and adjusting the hardness after quenching and tempering to HRC45, the surface microhardness is increased by HV to HV.
726, Hardening depth (thickness of nitrided hardened layer) 40 μm, Compound thickness ≒ 0, Impact strength 69.1 J / cm ² , Maximum crack length 15μ
m. In addition, by performing PVD treatment on the surface of the mold, the aluminum erosion resistance index was 10.0.

[0023] In the case where D steel of SKD61 series steel is used as the base material and the hardness after quenching and tempering is adjusted to HRC45, the surface microhardness is reduced by nitriding.
HV646, hardening depth (thickness of nitrided hardened layer) 40 μm, compound thickness ≒ 0, impact strength 80.6 J / cm ² , maximum crack length 20
μm. In addition, by performing PVD treatment on the surface of the mold, the aluminum erosion resistance index was 10.0.

As described above, the die-casting die according to the present embodiment is formed of hot work tool steel having a hardness of HRC30 to HRC45, and therefore has a lower hardness than the die in the related art, and after the heat treatment. Processing when finishing the mold becomes easy.
In addition, hot tool steels of HRC30 to HRC45 have high toughness and can secure necessary impact strength. Further, since the hardness of the mold surface is maintained between Hv600 and Hv900 by the nitriding treatment, the heat check resistance can be improved. Further, since the thickness of the nitrided hardened layer on the surface of the mold is 50 μm or less, the toughness of the surface of the mold is not significantly reduced, and a decrease in impact strength can be prevented.

Further, since the surface in contact with the molten metal is subjected to the PVD treatment, the aluminum erosion resistance can be improved. Furthermore, since the nitriding treatment is performed so that a brittle compound layer is hardly formed on the surface of the mold, heat check resistance and the like can be further improved. In addition, in the die casting mold according to the present embodiment, PVD treatment was performed in order to improve aluminum erosion resistance, but is not limited thereto as long as the treatment can be performed at a tempering temperature of the base material or lower. For example, a discharge coating process, a plasma CVD process, plating, or the like is also possible.

[Second Embodiment] A method of manufacturing a die casting mold according to the present embodiment is a modification of the method of manufacturing a die casting mold according to the first embodiment, which is different from the method of nitriding treatment. The other steps are exactly the same as in the first embodiment. In the nitriding treatment in the present embodiment, first, the mold is housed in a vacuum chamber, and the vacuum chamber is kept at a high vacuum of 10 ^-4 torr or less at room temperature. Next, N ₂ gas is supplied into the vacuum chamber, and the vacuum chamber is heated under an inert atmosphere so that the temperature in the vacuum chamber becomes about 400 ° C. or higher. further,
Supply NH ₃ gas, RX gas, etc. sequentially into the vacuum chamber,
The gas atmosphere composition is, for example, NH ₃ ; 30%, RX gas; 30%,
While controlling the respective gas supply flow rate so as to balance N _2, heated held in a state of increased pressure in the vacuum chamber above 200 torr. Then, with the temperature in the vacuum chamber set at, for example, 520 ° C., the mold is held in the vacuum chamber for 10 hours to perform a nitriding treatment. In this way, when the predetermined processing is completed, the inside of the vacuum chamber is replaced with N ₂ gas again and then cooled.

The RX gas is a gas in which LPG and air are mixed at a high temperature and modified through a catalyst. When butane is used, the composition is as follows: CO; 24.2%, H ₂ ; 31.5%, N ₂ ; Four
4%, other CO ₂ , H ₂ O, CH ₄ etc. FIG. 10 shows an example of characteristics of a die casting mold manufactured by using the method of manufacturing a die casting mold according to the present embodiment. When SKD61 series steel A is used as the base material and the hardness after quenching and tempering is adjusted to HRC45, the surface microhardness is HV836,
Hardening depth (thickness of nitrided hardened layer) 40 μm, compound thickness 化合物
0, impact strength 48.2 J / cm ² , and maximum crack length 10 μm. In addition, by performing PVD treatment on the surface of the mold, the aluminum erosion resistance index was 10.0.

Next, when the hardness after quenching and tempering is adjusted to HRC45 using SKD61 series steel B as a base material, the surface microhardness is reduced by nitriding.
HV616, hardening depth (thickness of nitrided hardened layer) 30 μm, compound thickness ≒ 0, impact strength 56.9 J / cm ² , maximum crack length 10
μm. In addition, by performing PVD treatment on the surface of the mold, the aluminum erosion resistance index was 10.0. When using SKD61 series C steel as the base material and adjusting the hardness after quenching and tempering to HRC45, the surface microhardness is HV718, the hardening depth (nitriding Layer thickness) 30 μm, compound thickness ≒ 0, impact strength
69.2 J / cm ² , and the maximum crack length was 20 μm. Also,
By subjecting the surface of the mold to the PVD treatment, the aluminum erosion resistance index was 10.0.

[0029] When the hardness after quenching and tempering is adjusted to HRC45 using D steel of SKD61 series steel as the base material, the surface microhardness is reduced by nitriding.
HV644, hardening depth (thickness of nitrided hardened layer) 40 μm, compound thickness ≒ 0, impact strength 83.8 J / cm ² , maximum crack length 10
μm. In addition, by performing PVD treatment on the surface of the mold, the aluminum erosion resistance index was 10.0. Thus, the same operation and effect as those of the first embodiment can be obtained by the die casting mold and the method of manufacturing the same according to the present embodiment.

[0030]

According to the present invention, since the mold has a lower hardness than the mold according to the prior art, it is easy to finish the mold after the heat treatment. Therefore, the processing cost of the mold can be reduced and the processing accuracy can be improved. In addition, the impact strength required for the mold can be secured, and the heat check resistance can be improved.

[Brief description of the drawings]

FIG. 1 is a main part process diagram in a method of manufacturing a die casting mold according to a first embodiment of the present invention.

FIG. 2 is a drawing showing characteristics of a die casting mold according to the first embodiment of the present invention.

FIG. 3 is a drawing showing chemical components of a base material of a die casting mold according to the first embodiment of the present invention.

FIG. 4 is a graph showing the relationship between heat treatment hardness and impact strength of SKD61 steel.

FIG. 5 shows a test piece (SKD6) in a heat check test.
3 is a graph showing the relationship between the hardening depth (thickness of a nitrided hardened layer) and the maximum crack length of the first series steel).

FIG. 6 shows a test piece (SKD6) in a heat check test.
4 is a graph showing the relationship between the surface microhardness of No. 1 steel and the maximum crack length.

FIG. 7 shows a test piece (SKD6) in a heat check test.
4 is a graph showing the relationship between the thickness of the surface compound layer of (system 1 steel) and the maximum crack length.

FIG. 8 is a plan view showing a state of an aluminum erosion resistance test.

FIG. 9 is a heat check test method (FIG. A) and a cross-sectional view of a test piece (FIG. B).

FIG. 10 is a drawing showing characteristics of a die casting mold according to a second embodiment of the present invention.

Claims (4)

[Claims] (1) It is formed of hot tool steel having a hardness of HRC30 to HRC45, and its surface is nitrided to have a thickness of 50%.
μm or less, and the surface hardness is Hv
Die casting mold characterized by being 600 to Hv900. 2. The die-casting die according to claim 1, wherein an aluminum erosion-resistant coating is formed on a surface in contact with the molten metal. 3. A mold is formed by processing hot tool steel, and the mold is heat-treated to maintain the hardness at HRC30 to HRC45, and the thickness of the surface nitrided hardened layer of the mold is 50 μm or less. A method for manufacturing a die-casting die, characterized by nitriding the die so that the surface hardness is Hv600 to Hv900. 4. The method for manufacturing a die casting mold according to claim 3, wherein a nitriding treatment is performed so that a compound layer is hardly formed on the surface of the die.