JP6593032B2 - Steel for die casting mold - Google Patents

Description

  The present invention relates to steel for die casting molds, and more particularly, to steel for die casting suitable for die casting using an oil-based mold release agent or a powder mold release agent.

  Die casting refers to a casting method in which a molten metal is pressed into a mold and the molten metal is solidified in the mold. Die casting has an advantage that a casting with high dimensional accuracy can be produced in a large amount in a short time. However, in die casting, the mold is exposed to large temperature amplitude and stress amplitude during use because the mold release agent is applied to the mold surface and the molten metal is pressed into the mold at short time intervals. The Therefore, die-casting die steel is required to have heat check resistance, hardenability, and high thermal conductivity as main characteristics.

In order to solve this problem, various proposals have heretofore been made. For example, Patent Document 1 includes, in mass%, C: 0.1 to 0.6%, Si: 0.01 to 0.8%, Mn: 0.1 to 2.5%, Cu: 0.01 to 2.0%, Ni: 0.01-2.0%, Cr: 0.1-2.0%, Mo: 0.01-2.0%, (V, W, Nb, Ta): 0. 01-2.0%, Al: 0.002-0.04%, N: 0.002-0.04%, and O: 0.05% or less, with the balance being Fe and inevitable impurities And steel for metal mold | die obtained by quenching at 30 degreeC / min after 30-minute soaking | uniform-heating at 1010 to 1050 degreeC, and also tempering for 30 hours at predetermined | prescribed temperature is disclosed.
This document describes that the thermal fatigue characteristics are improved by such a composition and heat treatment.

  Generally, in order to improve the heat check resistance and hardenability of steel materials, it is necessary to add a large amount of an appropriate alloy element. On the other hand, the thermal conductivity of steel materials decreases as the amount of alloy elements increases. That is, the improvement in heat check resistance and / or hardenability is contrary to the improvement in heat conduction characteristics. Therefore, it is difficult to achieve these simultaneously.

  On the other hand, the mold release agent not only prevents seizure of the casting on the mold surface but also has a function of cooling the mold and shortening the die casting cycle. Conventionally, in die casting, a water-soluble release agent having a large cooling capacity has been used. However, when a water-soluble release agent is applied to a high-temperature mold design surface, there is a problem that a large amount of water vapor containing a release agent component is generated, and the working environment is deteriorated. Therefore, in recent years, oil-based release agents and powder release agents have been used to improve the working environment.

  When using an oil-based mold release agent or a powder mold release agent, the mold is cooled mainly by flowing cooling water through a cooling water channel provided inside the mold. As a result, the temperature amplitude of the mold design surface at the time of applying the release agent is reduced, so that the requirement for heat check resistance to the mold steel is reduced. However, even when an oil-based mold release agent or a powder mold release agent is used, the heat check resistance is excessive because the conventional mold steel is used as it is. Furthermore, there has never been an example in which a die casting steel suitable for an oil-based mold release agent or a powder mold release agent has been proposed.

JP 2008-056882 A

  The problem to be solved by the present invention is to provide a die casting steel suitable for die casting using an oil release agent or a powder release agent.

In order to solve the above-described problems, a die casting steel according to the present invention has the following configuration.
(1) The die casting steel is
0.13 ≦ C ≦ 0.25 mass%,
0.02 ≦ Si ≦ 0.35 mass%,
1.50 ≦ Mn ≦ 2.0 mass%,
P ≦ 0.3 mass%,
0.01 ≦ Cu ≦ 0.25 mass%,
0.01 ≦ Ni ≦ 0.50 mass%,
1.8 ≦ Cr ≦ 2.4 mass%,
0 ≦ Mo ≦ 0.47 mass%,
0 ≦ W ≦ 0.94 mass%,
0.30 ≦ Mo + 1 / 2W ≦ 0.47 mass%,
0.22 ≦ V ≦ 0.72 mass%,
N ≦ 0.02 mass%,
O ≦ 0.0100 mass%, and
Al ≦ 0.100 mass%
The balance consists of Fe and inevitable impurities .
(2) The die-cast die steel was tempered so that the hardness was 30 HRC or more and 40 HRC or less by quenching and tempering under conditions of quenching temperature: 900 to 1030 ° C. and tempering temperature: 550 to 620 ° C. Consists of things.
(3) When the die-casting die steel is quenched so that the quenching speed of the portion where the quenching speed is the slowest is 1.0 ° C./min or more and 2.5 ° C./min or less, the entire surface of the cross section is a bainite structure. , A martensite structure, or a mixed structure of bainite and martensite.
(4) The die cast steel has a thermal conductivity of 30 W / mK or more.

  When performing die casting using an oil-based mold release agent or a powder mold release agent, the temperature amplitude to which the mold design surface is exposed is smaller than that when a water-soluble mold release agent is used. Therefore, the addition amount of the alloy element that improves the heat check resistance can be minimized. Further, by reducing the addition amount of the alloy element that improves the heat check resistance, it is possible to achieve a component balance that emphasizes hardenability and high thermal conductivity. As a result, the hardenability is improved and a larger mold can be produced. Further, since the thermal conductivity is high, the time for one molding cycle can be shortened.

  Furthermore, since the strength and hardness do not increase more than necessary, the design surface can be processed in a pre-hardened state (that is, quenched and tempered) and can be used as it is without being heat-treated. Become. As a result, the dimensional accuracy can be improved and the production delivery time can be shortened.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Steel for die casting molds]
[1.1. component]
[1.1.1. Main constituent elements]
The die casting mold steel according to the present invention contains the following elements, with the balance being Fe and inevitable impurities. The kind of additive element, its component range, and the reason for limitation are as follows.

(1) 0.13 ≦ C ≦ 0.25 mass%:
C is an element necessary for ensuring hardness and strength, and is an element that forms a carbide by combining carbide forming elements such as Cr, Mo, W, V, and Nb. C is an element that ensures hardness by solid solution in the matrix during quenching and martensite organization. In order to obtain such an effect, the C amount needs to be 0.13 mass% or more.
On the other hand, when the amount of C is excessive, the hardness is excessively increased, and the toughness and machinability are reduced. Moreover, when the amount of solid solution in a matrix increases too much, it will become the cause of reducing thermal conductivity. Therefore, the amount of C needs to be 0.25 mass% or less.

(2) 0.02 ≦ Si ≦ 0.35 mass%:
Si is an element that mainly dissolves in the matrix, promotes precipitation of carbides, and enhances secondary hardening. In order to acquire such an effect, the amount of Si needs to be 0.02 mass% or more.
On the other hand, when the amount of Si becomes excessive, hardenability is lowered. Moreover, when the amount of solid solution in a matrix increases too much, it will become the cause of reducing thermal conductivity. Therefore, the amount of Si needs to be 0.35 mass% or less.

(3) 1.50 ≦ Mn ≦ 2.0 mass%:
Mn is an element that improves hardenability. In order to acquire such an effect, the amount of Mn needs to be 1.50 mass% or more.
On the other hand, when the amount of Mn becomes excessive, hot workability is lowered. Moreover, when the amount of solid solution in a matrix increases too much, it will become the cause of reducing thermal conductivity. Therefore, the amount of Mn needs to be 2.0 mass% or less.

(4) P ≦ 0.3 mass%:
P is inevitably contained in the steel. P segregates at the grain boundary and causes toughness to decrease. Therefore, the amount of P needs to be 0.3 mass% or less.

(5) 0.01 ≦ Cu ≦ 0.25 mass%:
Cu is an element that stabilizes austenite. In order to acquire such an effect, the amount of Cu needs to be 0.01 mass% or more.
On the other hand, when the amount of Cu becomes excessive, the hot workability is lowered. Also, retained austenite increases, causing aging of the dimensions. Therefore, the amount of Cu needs to be 0.25 mass% or less.

(6) 0.01 ≦ Ni ≦ 0.50 mass%:
Ni is an element that stabilizes austenite. In order to acquire such an effect, the amount of Ni needs to be 0.01 mass% or more.
On the other hand, when the amount of Ni becomes excessive, the amount of retained austenite increases and causes aging of the dimensions. Therefore, the amount of Ni needs to be 0.50 mass% or less.

(7) 1.8 ≦ Cr ≦ 2.4 mass%:
Cr is an element that improves hardenability. In order to obtain such an effect, the Cr amount needs to be 1.8 mass% or more.
On the other hand, when the amount of Cr becomes excessive, the amount of C solid solution of austenite at the quenching temperature decreases, and the hardness cannot be obtained. Moreover, when the amount of solid solution in a matrix increases too much, it will become the cause of reducing thermal conductivity. Therefore, the Cr amount needs to be 2.4 mass% or less.

(8) 0 ≦ Mo ≦ 0.47 mass%, 0 ≦ W ≦ 0.94 mass%,
0.30 ≦ Mo + 1 / 2W ≦ 0.47 mass%:
Mo has the effect of increasing the secondary hardening amount. W has the same effect as Mo, but the specific gravity of W is about twice that of Mo. Therefore, in order to obtain the same effect as Mo by W, W needs to be added in an amount twice that of Mo. In order to obtain such an effect, (Mo + 1 / 2W) (hereinafter referred to as “Mo equivalent”) needs to be 0.30 mass% or more.
On the other hand, when the Mo equivalent is excessive, the amount of carbide remaining at the time of quenching is excessive. Therefore, Mo equivalent needs to be 0.47 mass% or less.
One of Mo and W may be included, or both may be included.

(9) 0.11 ≦ V ≦ 0.72 mass%:
V forms carbides and has the effect of suppressing crystal grain growth. In order to obtain such an effect, the V amount needs to be 0.11 mass% or more.
On the other hand, when the amount of V becomes excessive, coarse carbides are formed and the impact value is lowered. Therefore, the V amount needs to be 0.72 mass% or less.

(10) N ≦ 0.02 mass%:
N is an interstitial element and contributes to an increase in the hardness of the martensite structure. Compared to C of the same interstitial element, γ stabilization ability is strong. Moreover, it contributes to the improvement of corrosion resistance in a solid solution state.
On the other hand, when the amount of N becomes excessive, the nitrogen gas ejection limit is exceeded due to the concentration of nitrogen during solidification, and voids are generated in the ingot. Moreover, when the amount of solid solution in a matrix increases too much, it will become the cause of reducing thermal conductivity. Therefore, the N amount needs to be 0.02 mass% or less.

(11) O ≦ 0.0100 mass%:
O is an element inevitably contained in the molten steel. When the amount of O becomes excessive, coarse oxides are formed with Al and Si to become inclusions, and toughness is reduced. Therefore, the amount of O needs to be 0.0100 mass% or less.

(12) Al ≦ 0.100 mass%:
Al is an element inevitably included as a deoxidizing element. When the amount of Al becomes excessive, coarse oxides are formed in the steel to become inclusions, and the impact value and the like are lowered. Therefore, the amount of Al needs to be 0.100 mass% or less.

[1.1.2. Sub-constituent elements]
The die casting steel according to the present invention may further contain one or more of the following elements in addition to the main constituent elements described above. The kind of additive element, its component range, and the reason for limitation are as follows.

(13) 0.001 ≦ Nb ≦ 0.30 mass%:
(14) 0.001 ≦ Ta ≦ 0.30 mass%:
(15) 0.001 ≦ Ti ≦ 0.20 mass%:
(16) 0.001 ≦ Zr ≦ 0.30 mass%:
Nb, Ta, Ti, and Zr all form carbides and nitrides, and have the effect of preventing crystal grain coarsening during quenching. In order to obtain such an effect, the content of these elements is preferably not less than the above lower limit value.
On the other hand, when the content of these elements is excessive, coarse carbides and nitrides are formed, and the impact value is lowered. Therefore, the content of these elements is preferably not more than the above upper limit value.
Note that the die casting steel may contain any one of these elements, or may contain two or more.

(17) 0.005 ≦ S ≦ 0.10 mass%:
S is inevitably contained in the steel. Moreover, S may be positively added in order to improve machinability. In order to obtain such an effect, the amount of S needs to be 0.005 mass% or more.
On the other hand, when the amount of S becomes excessive, hot workability decreases. Therefore, the amount of S needs to be 0.10 mass% or less.

(18) 0.01 ≦ Se + Te ≦ 0.15 mass%:
Both Se and Te improve machinability. In order to obtain such an effect, the total amount of Se and Te is preferably 0.01 mass% or more.
On the other hand, when the total amount of Se and Te is excessive, hot workability is reduced. Therefore, the total amount of Se and Te is preferably 0.15 mass% or less.
One of Se and Te may be included, or both may be included.

(19) 0.01 ≦ Pb + 2Bi ≦ 0.15 mass%:
Both Pb and Bi improve machinability. In order to obtain such an effect, Pb + 2Bi is preferably 0.01% by mass or more.
On the other hand, when Pb + 2Bi becomes excessive, hot workability deteriorates. Therefore, Pb + 2Bi is preferably 0.15 mass% or less.
One of Pb and Bi may be included, or both may be included. In addition, Bi has the same effect as Pb in an amount 1/2 that of Pb. This is because Bi has a lower melting point and lower density than Pb, so that the weight required for the same volume is reduced.
In addition, the die casting steel may contain one of S, (Se, Te), and (Pb, Bi), or may contain two or more.

[1.2. Hardness]
The die-casting die steel according to the present invention minimizes the content of elements that improve heat check resistance, so that the strength and hardness do not become higher than necessary. Therefore, the design surface can be processed in a pre-hardened state (that is, in a quenched / tempered state), and can be used as a mold as it is without being heat-treated. As a result, the dimensional accuracy can be improved and the production delivery time can be shortened.
Specifically, by optimizing the composition and quenching / tempering conditions, the hardness can be tempered to be 30 HRC or more and 40 HRC or less.

[1.3. Organization]
The die-casting die steel according to the present invention minimizes the content of elements that improve the heat check resistance, so that the hardenability can be increased without sacrificing thermal conductivity. Therefore, even if it is a raw material which has a complicated shape, the whole surface of a cross section can be made into a desired structure | tissue by hardening and tempering.
Specifically, by optimizing the composition, even when quenching so that the quenching speed of the part where the quenching speed becomes the slowest is 1.0 ° C./min to 2.5 ° C./min, The entire cross-sectional surface of the member can be a bainite structure, a martensite structure, or a mixed structure of bainite and martensite.

[1.4. Thermal conductivity]
The die-casting die steel according to the present invention minimizes the content of elements that improve the heat check resistance, so that the thermal conductivity can be increased without sacrificing the hardenability. Specifically, the thermal conductivity can be increased to 30 W / mK or more by optimizing the composition.

[1.5. Application]
As described above, the die casting steel according to the present invention has a component balance that emphasizes hardenability and high thermal conductivity by minimizing the amount of alloy elements added to improve heat check resistance. . Therefore, the die casting steel according to the present invention is particularly suitable for die casting using an oil-based mold release agent or a powder mold release agent.

[2. Action]
When die casting is performed using an oil-based mold release agent or a powder mold release agent, the temperature amplitude to which the mold design surface is exposed is smaller than when a water-soluble mold release agent is used. Therefore, the addition amount of the alloy element that improves the heat check resistance can be minimized. Further, by reducing the addition amount of the alloy element that improves the heat check resistance, it is possible to achieve a component balance that emphasizes hardenability and high thermal conductivity. As a result, the hardenability is improved and a larger mold can be produced. Further, since the thermal conductivity is high, the time for one molding cycle can be shortened.

  Furthermore, since the strength and hardness do not increase more than necessary, the design surface can be processed in a pre-hardened state (that is, quenched and tempered) and can be used as it is without being heat-treated. Become. As a result, the dimensional accuracy can be improved and the production delivery time can be shortened.

(Examples 1-5, Reference Example 6, Examples 7-11, Reference Examples 12-14, Examples 15-20, Comparative Examples 1-3)
[1. Preparation of sample]
Raw materials having chemical components shown in Table 1 were melted in a vacuum induction furnace to obtain 50 kg of ingot. Next, the ingot was hot forged to obtain a 60 mm square bar. After hot forging, the bar was annealed.

[2. Test method]
[2.1. Hardness]
A cubic block having a side of 10 mm was cut out from the bar after annealing, and quenched and tempered. The quenching temperature was 900 to 1030 ° C. depending on the composition. Moreover, tempering temperature was 550-620 degreeC according to the composition.
The measurement surface and the ground contact surface of the test piece after tempering were polished up to # 400, and the hardness was measured by Rockwell C scale.

[2.2. Thermal conductivity]
A test piece of φ10 × 1 mm was cut out from the bar after annealing, and quenched and tempered. Using the test piece after tempering, the thermal conductivity was measured by a laser flash method.

[2.3. Hardenability]
A test piece of φ4 mm × 10 mm was produced from the bar after annealing. After holding the test piece at the quenching temperature, the cooling rate was changed to quench the test piece. The quenching temperature was 900 to 1030 ° C. depending on the composition.
The test piece after quenching was cut longitudinally to confirm the hardness and structure of the center of the test piece. From the relationship between the cooling rate and the structure, the slowest cooling rate at which a bainite structure can be obtained was obtained and used as an index of hardenability.

[2.4. Cycle time]
A test piece of φ60 × 50 mm was cut out from the tempered rod, and quenched and tempered. Next, a thermocouple was attached to the surface of the test piece after tempering. And after heating the temperature of a test piece surface from 50 degreeC to 250 degreeC by high frequency heating, the test piece was water-cooled and the time (cycle time) for the temperature of a test piece surface to fall to 50 degreeC was measured.

[3. result]
Table 2 shows the quenching temperature, tempering temperature, hardness, thermal conductivity, cycle time, and hardenability. Table 2 shows the following.

(1) Comparative Examples 1 to 3 are hot die steels that are often used in the past. Since it is necessary to balance hardness, hardenability, and thermal conductivity, the steel grade characteristics are not specific to thermal conductivity and hardenability.
(2) Examples 1-5, Reference Example 6, Examples 7-11, Reference Examples 12-14, and Examples 15-20 have comparable hardenability compared to Comparative Examples 1-3 (ie, The bainite structure is obtained at a slower cooling rate), and the hardness is low and the thermal conductivity is high. Furthermore, the cycle times of Examples 1 to 5, Reference Example 6, Examples 7 to 11, Reference Examples 12 to 14, and Examples 15 to 20 are about 2 seconds shorter than those of Comparative Examples 1 to 3. From Table 2 , Examples 1 to 5, Reference Example 6, Examples 7 to 11, Reference Examples 12 to 14, and Examples 15 to 20 are die casting molds using an oil-based release agent or a powder release agent. It turns out that it is suitable.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The die casting steel according to the present invention can be used for a die used for die casting of an Al alloy or an Mg alloy.

Claims (4)

Die-cast die steel with the following configuration.
(1) The die casting steel is
0.13 ≦ C ≦ 0.25 mass%,
0.02 ≦ Si ≦ 0.35 mass%,
1.50 ≦ Mn ≦ 2.0 mass%,
P ≦ 0.3 mass%,
0.01 ≦ Cu ≦ 0.25 mass%,
0.01 ≦ Ni ≦ 0.50 mass%,
1.8 ≦ Cr ≦ 2.4 mass%,
0 ≦ Mo ≦ 0.47 mass%,
0 ≦ W ≦ 0.94 mass%,
0.30 ≦ Mo + 1 / 2W ≦ 0.47 mass%,
0.22 ≦ V ≦ 0.72 mass%,
N ≦ 0.02 mass%,
O ≦ 0.0100 mass%, and
Al ≦ 0.100 mass%
The balance consists of Fe and inevitable impurities .
(2) The die casting mold steel was tempered so that the hardness was 30 HRC or more and 40 HRC or less by quenching and tempering under the conditions of quenching temperature: 900 to 1030 ° C. and tempering temperature: 550 to 620 ° C. Consists of things.
(3) When the die-casting die steel is quenched so that the quenching speed of the portion where the quenching speed is the slowest is 1.0 ° C./min or more and 2.5 ° C./min or less, the entire surface of the cross section is a bainite structure. , A martensite structure, or a mixed structure of bainite and martensite.
(4) The die cast steel has a thermal conductivity of 30 W / mK or more. 0.001 ≦ Nb ≦ 0.30 mass%,
0.001 ≦ Ta ≦ 0.30 mass%,
0.001 ≦ Ti ≦ 0.20 mass%, and
0.001 ≦ Zr ≦ 0.30 mass%
The die casting steel according to claim 1, further comprising at least one element selected from the group consisting of: 0.005 ≦ S ≦ 0.10 mass%,
0.01 ≦ Se + Te ≦ 0.15 mass%, and
0.01 ≦ Pb + 2Bi ≦ 0.15 mass%
The die casting steel according to claim 1 or 2, further comprising at least one element selected from the group consisting of: The steel for die-casting molds according to any one of claims 1 to 3 , which is used for a mold for performing die-casting using an oil-based mold release agent or a powder mold release agent.