JP2012153919A - Steel for die-casting die and the die-casting die - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide steel for a die-casting die capable of easily forming a relatively thick compound layer and hardly causing a seizure, and a die-casting die using the same.SOLUTION: The steel for the die-casting die contains 0.25≤C≤0.50 mass%, 0.0005≤Si≤0.30 mass%, 0.40≤Mn≤2.00 mass%, 1.50≤Cr≤3.00 mass%, Mo≤2.00 mass%, V≤0.60 mass%, W≤3.00 mass%, and Al≤3.00 mass%, with the balance being Fe and inevitable impurities, has nitrogen content (=[Si]+[Cr]+[Mo]/2+[V]+[W]+[Al], where [ ] means a content of each element (mass%).) of 6.00 mass% or less, and is used in a state where a compound layer with a thickness of 10 μm or more containing Fe-N compound as a main phase is formed on at least a surface in contact with molten metal. The die-casting die using the same is also provided.

Description

  The present invention relates to a die casting mold steel and a die casting mold, and more particularly to a die casting mold steel having excellent seizure resistance and a die casting mold using the same.

  Die casting refers to a method in which molten metal is pressed into a mold to obtain a cast product. Die casting has advantages such as (1) high dimensional accuracy of the product, (2) a smooth surface of the product, and (3) easy mass production of products having the same shape. In general, hot tool steel (for example, JIS SKD61) having excellent mechanical properties at high temperatures is used for a die casting die.

However, since the surface of the die casting die is repeatedly heated by contact with the molten metal and cooled by the application of the release agent, cracks due to thermal fatigue are likely to occur on the die surface. Cracks formed on the surface of the mold are transferred to the product as they are, which causes the surface quality of the product to deteriorate.
In particular, in Al die casting, since the molten metal and the mold are likely to react, as the number of casting shots increases, Al tends to adhere to the mold surface. When so-called “seizure” occurs in which Al adheres strongly to the mold, it becomes difficult to take out the product. Further, when taking out the product, the burned-in portion is forcibly torn off. As a result, the surface quality and dimensional accuracy of the product decrease, and the product defect rate increases.

In order to solve this problem, various proposals have heretofore been made.
For example, in Patent Document 1, in mass%, C: 0.15 to 0.55%, Si: 0.01 to 0.5%, Mn: 0.01 to 2.0%, Cr: 0.3 -1.5%, Mo: 0.8-2.0%, V + W: 0.05-0.5%, Cu: 0.01-2.0%, and Ni: 0.01-2.0 A tool steel is disclosed in which the remainder is made up of Fe and inevitable impurities.
In the same document, the optimization of the alloy composition increases the softening resistance and thermal conductivity, and the fact that cracks due to thermal fatigue are less likely to occur even when such tool steel is applied to a low-pressure casting mold. Is described.

In Patent Document 2, by weight, C: 0.1 to 0.3%, Si: 1.0% or less, Mn: 3.0% or less, S: 0.01 to 0.05%, Cr: Die-casting gold consisting of 1.0 to 3.0%, Mo and W alone or in combination (Mo + 1 / 2W): 2.0% or less, V: 0.01 to 1.0%, balance Fe and inevitable impurities Mold steel is disclosed.
This document describes that the heat crack resistance is improved by minimizing the addition of free-cutting elements, and that the heat crack life is further improved by nitriding.

In Patent Document 3, C: 0.10 to 0.35%, Si: <0.80%, Mn: ≦ 3.0%, Cr: 2.0 to less than 7.0% by weight%, / 2W + Mo: 0.3 to 5.0%, V: <0.5%, N: more than 0.05 to 0.50%, C + N: 0.20 to 0.60%, O: ≦ 0.0100% , P: ≦ 0.050%, Al: ≦ 0.050% is satisfied, and a hot tool steel whose balance is substantially made of Fe is disclosed.
This document describes that by increasing the amount of N, reducing the amount of V, and limiting the amount of C + N to a certain range, the Al corrosion resistance is improved without impairing the heat check resistance. Has been.

In Patent Document 4, C: 0.30 to 0.55%, Si: 1.20% or less, Mn: 0.1 to 1.5%, Cr: 2.55 to 6.50% by weight% 1 and 2 of W and Mo are composed of 1.0 to 4.5% at 1/2 W + Mo, V: 0.2 to 1.5%, the balance Fe and unavoidable impurities, and at least a nitride layer on the work surface A mold for hot working is disclosed in which the hardness inside the nitride layer from the surface of the nitride layer is HV800 or less, and the depth of the hardened layer higher than the hardness of the base is 100 μm or less.
This document describes that when a nitride layer is formed on the mold surface, the heat crack resistance is improved by optimizing the hardness gradient of the nitride layer.

Patent Document 5 discloses a die casting mold in which a surface hardened layer having at least one of chromium nitride and chromium carbonitride is formed on the surface.
This document describes that when a Cr-based hardened surface layer is formed on the mold surface, wear and erosion are remarkably reduced.

Furthermore, Patent Document 6 describes, in mass%, C: 0.15 to 0.55%, Si: 0.02 to 0.60%, Mo: 0.30 to 4.00%, and V: 0. A steel for molds containing 0.03 to 1.00%, further containing a predetermined amount of Cr, Mn, Ni and Cu, the balance being Fe and inevitable impurities is disclosed.
This document describes that when the contents of Cr, Mn, Ni, and Cu are within a predetermined range, the hardenability is improved and the thermal conductivity can be increased.

If seizure occurs in die casting, the device must be stopped and seizure must be removed. Since casting is not possible during seizure removal, productivity is reduced. In other words, seizure is a serious trouble that not only increases the defective rate of castings but also decreases productivity.
In order to avoid seizure, it is effective to form a nitride layer (compound layer) on the mold surface by nitriding. However, even if nitriding is performed on the surface of a mold made of a conventional material, the compound layer formed on the surface is extremely thin. Therefore, at the site where the flow rate of the molten metal is large, the compound layer disappears early due to wear, and the effect of avoiding seizure disappears.
Furthermore, die casting molds require not only good seizure resistance, but also good hardenability to obtain the required hardness and high thermal conductivity to reduce thermal stress that causes heat cracks. It is done. However, there has never been an example in which a material having both seizure resistance, hardenability, and high thermal conductivity has been proposed.

JP 2009-13465 A JP 2000-297351 A JP 2004-19001 A JP-A-6-88166 Japanese Patent Laid-Open No. 2001-25856 JP 2009-242820 A

The problem to be solved by the present invention is to provide a die-casting die steel that can easily form a relatively thick compound layer and hardly cause seizure, and a die-casting die using the same. It is in.
Another problem to be solved by the present invention is to provide a die casting mold steel excellent in hardenability and high thermal conductivity in addition to seizure resistance, and a die casting mold using the same. .

In order to solve the above-described problems, the steel for die casting molds according to the present invention is summarized as having the following configuration.
(1) The die casting steel is
0.25 ≦ C ≦ 0.50 mass%,
0.005 ≦ Si ≦ 0.30 mass%,
0.40 ≦ Mn ≦ 2.00 mass%,
1.50 ≦ Cr ≦ 3.00 mass%,
Mo ≦ 2.00 mass%,
V ≦ 0.60 mass%,
W ≦ 3.00 mass%, and
Including Al ≦ 3.00 mass%,
The balance consists of Fe and inevitable impurities.
(2) The die casting mold steel has a nitrogen holding amount represented by the formula (a) of 6.00 mass% or less.
Nitrogen holding amount = [Si] + [Cr] + [Mo] / 2 + [V] + [W] + [Al] (a)
However, [] is the content of each element (mass%).
(3) The die-casting die steel is used in a state in which a compound layer having a thickness of 10 μm or more having an Fe—N compound as a main phase is formed on at least a surface in contact with the molten metal.

The die casting die according to the present invention is
The die casting steel according to the present invention is used on at least the surface in contact with the molten metal,
The gist is that a compound layer having a thickness of 10 μm or more having an Fe—N-based compound as a main phase is formed at least on the surface in contact with the molten metal.

When the steel that has been quenched and tempered is nitrided, nitrogen diffuses from the surface toward the inside, and a diffusion layer in which nitrogen is dissolved is formed. When the diffusion of nitrogen further progresses and the solid solution amount of nitrogen exceeds the limit, Fe—N-based compounds such as γ′-Fe ₄ N and ε-Fe _2-3 N are used as the main phase on the steel surface. A compound layer is formed.
At this time, if the steel contains a relatively large amount of elements that easily form nitrides such as Si, Cr, Mo, V, W, and Al (for example, in the case of JIS SKD61), the steel diffuses. Nitrogen is preferentially consumed in the formation of these non-ferrous nitrides. As a result, it becomes difficult to form a thick compound layer made of Fe-N compounds on the surface of the steel.
On the other hand, if the amount of nitride-forming elements such as Si in the steel is limited to a predetermined amount or less, the amount of nitrogen consumed to form non-ferrous nitrides decreases. As a result, a relatively thick compound layer can be formed on the steel surface by nitriding.

Furthermore, the compound layer having a Fe—N-based compound as a main phase has low reactivity with molten metal (particularly, molten metal made of Al or an Al alloy). Therefore, when steel containing a predetermined component and having a relatively thick compound layer formed on the surface is applied to a die casting die, seizure of the molten metal can be suppressed. In addition, since the compound layer is formed thick, the effect of avoiding seizure can be maintained for a long time even at a portion where the flow rate of the molten metal is high.
Furthermore, when the component elements are optimized, hardenability and thermal conductivity are improved.

It is a figure which shows the relationship between the compound layer thickness and the mold release resistance after 500 shots. It is a figure which shows the relationship between nitrogen holding amount and a compound layer thickness. It is a figure which shows the relationship between Cr amount and compound layer thickness, and the relationship between Cr amount and hardenability (slow cooling material impact value). FIG. 4A and FIG. 4B are external appearance photographs after 500 shots of the nesting pins produced in Example 1 and Comparative Example 1, respectively. FIG. 4C and FIG. 4D are cross-sectional photographs immediately after the nitriding treatment of the nested pins produced in Example 1 and Comparative Example 1, respectively.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Steel for die casting molds]
[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) 0.25 ≦ C ≦ 0.50 mass%.
C contributes to the improvement of the hardness of martensite by quenching and tempering, and the improvement of strength and hardness by precipitation hardening of alloy carbides. In order to acquire such an effect, C content needs to be 0.25 mass% or more. The C content is more preferably 0.32 mass% or more.
On the other hand, when the C content is excessive, the supersaturated solid solution amount of C in martensite becomes excessive, and toughness and thermal conductivity are reduced. Therefore, the C content needs to be 0.50 mass% or less. The C content is more preferably 0.40 mass% or less.

(2) 0.005 ≦ Si ≦ 0.30 mass%.
Si dissolves in the ground iron and contributes to solid solution hardening. If the Si content is too small, sufficient softening resistance cannot be obtained. Therefore, the Si content needs to be 0.005 mass% or more. The Si content is more preferably 0.10 mass% or more.
In order to ensure softening resistance, the higher the Si content, the better. However, when the Si content is excessive, the influence on the thermal conductivity becomes very large. Further, since Si consumes nitrogen that has entered during nitridation to form a nitride, it prevents the growth of a compound layer containing a Fe—N-based compound as a main phase. Therefore, the Si content needs to be 0.30 mass% or less. The Si content is more preferably 0.15 mass% or less.

(3) 0.40 ≦ Mn ≦ 2.00 mass%.
Mn contributes to improvement of hardenability. In order to acquire such an effect, Mn content needs to be 0.40 mass% or more. The Mn content is more preferably 0.60 mass% or more.
On the other hand, when the Mn content is excessive, softening resistance and thermal conductivity are deteriorated. In addition, a long time is required for the spheroidizing annealing treatment, and the productivity is lowered. Therefore, the Mn content needs to be 2.00 mass% or less. The Mn content is more preferably 1.0 mass% or less.

(4) 1.50 ≦ Cr ≦ 3.00 mass%.
Cr not only improves hardenability, but is also useful as an element that forms carbides and increases the strength of steel. In order to acquire such an effect, Cr content needs to be 1.50 mass% or more.
On the other hand, when the Cr content is excessive, softening resistance, high temperature strength and thermal conductivity are lowered. Moreover, Cr is easier to form nitride CrN than other alloy elements. For this reason, excessive Cr consumes a large amount of nitrogen that has entered during nitriding, which significantly hinders the growth of the compound layer. Therefore, the Cr content needs to be 3.00 mass% or less. The Cr content is more preferably 1.80 mass% or less.

(5) Mo ≦ 2.00 mass%.
Mo is an element effective for improving the high-temperature strength. In order to obtain such an effect, the Mo content is preferably 0.30 mass% or more.
On the other hand, when the Mo content is excessive, fracture toughness is reduced. In addition, Mo consumes nitrogen that has entered during nitridation to form Mo ₂ N, which hinders the growth of the compound layer. Therefore, the Mo content needs to be 2.00 mass% or less. The Mo content is more preferably 1.35 mass% or less.

(6) V ≦ 0.60 mass%.
V is an element that greatly affects the toughness, and suppresses the coarsening of crystal grains during quenching to prevent a decrease in impact resistance. In order to obtain such an effect, the V content is preferably 0.10 mass% or more.
On the other hand, when the V content is excessive, coarse crystallized VC that cannot be completely dissolved by soaking remains, and the fatigue strength and impact resistance are greatly reduced. In addition, V consumes nitrogen that has entered during nitridation for the formation of VN, thus preventing the growth of the compound layer. Therefore, the V content needs to be 0.60 mass% or less. The V content is more preferably 0.30 mass% or less.

(7) W ≦ 3.00 mass%.
W is useful as an element that not only improves hardenability but also forms carbides to increase the strength of steel. In particular, the effect of increasing the softening resistance is great.
On the other hand, when the W content is excessive, not only saturation of characteristics and an increase in material cost are caused, but coarse carbides are liable to be produced during casting. Coarse carbide is likely to be a starting point of fracture, and causes a decrease in fatigue strength and impact value. Further, W consumes nitrogen that has entered during nitridation for the formation of WN, thus preventing the growth of the compound layer. Therefore, the W content needs to be 3.00 mass% or less. The W content is more preferably 0.90 mass% or less.

(8) Al ≦ 3.00 mass%.
Al is an element that forms AlN and prevents coarsening of crystal grains during quenching. It forms and precipitates intermetallic compounds, contributing to dispersion strengthening.
On the other hand, when the Al content is excessive, not only saturation of characteristics and an increase in manufacturing cost are caused, but also alumina as an inclusion is increased. Alumina, which is an inclusion, tends to be a starting point of fracture, and causes a decrease in fatigue strength and impact value. Furthermore, since Al consumes nitrogen that has entered during nitriding for the formation of AlN, the growth of the compound layer is hindered. Therefore, the Al content needs to be 3.00 mass% or less. The Al content is more preferably 0.01 mass% or less.

[1.2. Sub-constituent elements]
The die casting steel according to the present invention may further include one or more sub-constituent elements as described below in addition to the main constituent elements described above. The types of additive elements and the reasons for limiting them are as follows.

(9) Ni ≦ 0.50 mass%.
Ni contributes to the improvement of hardenability and impact resistance.
On the other hand, if the Ni content is excessive, it takes a long time for annealing. Therefore, the Ni content is preferably 0.50 mass% or less.

(10) Co ≦ 2.00 mass%.
Co contributes to the improvement of high temperature strength by solid solution strengthening.
On the other hand, if the Co content is excessive, Co is an expensive element, which causes an increase in manufacturing cost. Therefore, the Co content is preferably 2.00 mass% or less. The Co content is more preferably 1.00 mass% or less.

[1.3. Nitrogen holding amount]
In the die-casting die steel according to the present invention, in addition to each component element being in the above range, the nitrogen holding amount represented by the formula (a) needs to be in a predetermined range.
Nitrogen holding amount = [Si] + [Cr] + [Mo] / 2 + [V] + [W] + [Al] (a)
However, [] is the content of each element (mass%).
The nitrogen retention amount is an index of the amount of nitrogen that has entered the steel and stored as non-ferrous nitrides. It represents that the compound layer which uses a Fe-N type compound as a main phase is easy to be formed, so that there is little nitrogen retention. In order to enable formation of a compound layer having a thickness of 10 μm or more within a practical nitriding time, the nitrogen holding amount needs to be 6.00 mass% or less. The nitrogen holding amount is more preferably 5.00 mass% or less, and further preferably 4.00 mass% or less.

[1.4. Characteristic]
Optimizing the component elements and nitrogen retention not only facilitates the formation of a relatively thick compound layer, but also improves thermal conductivity and hardenability.
Specifically, when the component elements and the nitrogen holding amount are optimized, a die-casting steel having a thermal conductivity at room temperature of 30 (W / m / K) or more can be obtained.
Further, when the component elements and nitrogen holding amount are optimized, a die casting steel having a hardenability (slow cooling material impact value) of 15 J / cm ² or more can be obtained.

[1.5. Application]
The steel according to the present invention is particularly suitable as a material for a die casting die. When using the steel which concerns on this invention for the metal mold | die for die-casting, it is used in the state which formed the compound layer with a thickness of 10 micrometers or more which has a Fe-N type compound as a main phase at least on the surface which contacts a molten metal.
Here, the “compound layer” refers to a layer having an Fe—N-based compound formed on the surface of steel as a main phase.
“Fe—N compound” refers to a compound composed of Fe and N (for example, γ′-Fe ₄ N, ε-Fe _2-3 N, etc.).
“Using an Fe—N compound as a main phase” means that the proportion of the Fe—N compound contained in the compound layer is 50 vol% or more. Usually, the compound layer contains not only Fe—N compounds but also non-ferrous nitrides produced by nitriding such as Si and Cr.

[2. Die casting mold]
The die casting die according to the present invention is
The die casting steel according to the present invention is used on at least the surface in contact with the molten metal,
A compound layer having a thickness of 10 μm or more having an Fe—N-based compound as a main phase is formed at least on a surface in contact with the molten metal.

  In the present invention, the term “die casting die” includes, for example, a cast pin, an extrusion pin, a core, a nest, a sprue core, and the like. Among these, inserts, cores, cored pins, etc. installed in the vicinity of the gate are particularly suitable for the use of the steel according to the present invention because they are in direct and strong contact with the molten metal and easily seize. is there.

The die casting die according to the present invention may use the steel according to the present invention for all of the components, but it is sufficient that the steel according to the present invention is used at least on the surface in contact with the molten metal. The steel according to the present invention is particularly preferably used for a portion (for example, a nesting pin) where the flow rate of the molten metal is large.
Further, the compound layer may be formed on the entire surface of the component using steel according to the present invention, but it is sufficient that the compound layer is formed at least on the surface in contact with the molten metal.

[3. Manufacturing method of die casting steel]
The die casting steel according to the present invention can be produced by melting and casting raw materials blended so as to have a predetermined composition. The melting / casting method is not particularly limited, and various methods can be used depending on the purpose. Usually, after melting and casting, a homogenized heat treatment is performed in order to solidify the crystallized coarse carbide.

[4. Die casting mold manufacturing method]
The die casting die according to the present invention is
(1) Processing the homogenized heat-treated steel into a predetermined shape,
(2) Quenching and tempering the processed material,
(3) Finishing the surface of the material,
(4) nitriding the material surface;
Can be manufactured.

  Nitriding is performed to form a compound layer on the surface. In order to form a relatively thick compound layer within a realistic processing time, the nitriding method is preferably a method in which the introduction rate of nitrogen into the steel is high. Examples of such nitriding methods include salt bath soft nitriding, gas soft nitriding, gas nitrosulphurizing, and the like. Plasma nitriding and ion nitriding are not suitable for the method of nitriding steel according to the present invention because the introduction rate of nitrogen into the steel is slow.

The nitriding temperature is not particularly limited, and an optimum temperature is selected according to the nitriding method. In general, the higher the nitriding temperature, the faster the nitrogen introduction rate, and the easier it is for the compound layer to form. Specifically, the nitriding temperature is preferably 520 ° C. or higher.
On the other hand, if the nitriding temperature is too high, the base material may be softened or deformed. Therefore, the nitriding temperature is preferably 600 ° C. or lower. The nitriding temperature is more preferably 580 ° C. or lower.

[5. Action of Die Casting Die Steel and Die Casting Die]
When the steel that has been quenched and tempered is nitrided, nitrogen diffuses from the surface toward the inside, and a diffusion layer in which nitrogen is dissolved is formed. When the diffusion of nitrogen further progresses and the solid solution amount of nitrogen exceeds the limit, Fe—N-based compounds such as γ′-Fe ₄ N and ε-Fe _2-3 N are used as the main phase on the steel surface. A compound layer is formed.
At this time, if the steel contains a relatively large amount of elements that easily form nitrides such as Si, Cr, Mo, V, W, and Al (for example, in the case of JIS SKD61), the steel diffuses. Nitrogen is preferentially consumed in the formation of these non-ferrous nitrides. As a result, it becomes difficult to form a thick compound layer made of Fe-N compounds on the surface of the steel.
On the other hand, if the amount of nitride-forming elements such as Si in the steel is limited to a predetermined amount or less, the amount of nitrogen consumed to form non-ferrous nitrides decreases. As a result, a relatively thick compound layer can be formed on the steel surface by nitriding.

Furthermore, the compound layer having a Fe—N-based compound as a main phase has low reactivity with molten metal (particularly, molten metal made of Al or an Al alloy). Therefore, when steel containing a predetermined component and having a relatively thick compound layer formed on the surface is applied to a die casting die, seizure of the molten metal can be suppressed. In addition, since the compound layer is formed thick, the effect of avoiding seizure can be maintained for a long time even at a portion where the flow rate of the molten metal is high.
Furthermore, when the component elements are optimized, hardenability and thermal conductivity are improved.

(Examples 1-22, Comparative Examples 1-8)
[1. Preparation of sample]
An alloy having the composition shown in Table 1 was melted in a high frequency induction furnace to obtain a 50 kg ingot. The melted ingot was subjected to homogenization heat treatment at 1280 ° C. for 24 hours, normalizing at 1020 ° C., and spheroidizing annealing at 870 ° C. after hot forging, and then cutting into a nested pin shape. The cut material was further quenched and tempered. The quenching conditions were 1030 ° C. The tempering conditions were 590 to 630 ° C. and twice or more.
Next, the material after quenching and tempering was subjected to finish cutting and then subjected to nitriding treatment (compound forming treatment). The nitriding treatment was performed by salt bath soft nitriding (550 ° C. × 3 hr) or gas soft nitriding (550 ° C. × 3-4 hr).

[2. Test method]
[2.1. Thermal conductivity]
A sample of φ10 × 2 mm was cut out from the material after forging. Using this sample, the thermal conductivity was measured by a laser flash method.
[2.2. Compound layer thickness]
The cross section of the material after the nitriding treatment was observed with a microscope, the thickness of the compound layer was measured at five randomly selected locations, and the average value was calculated.

[2.3. Release resistance]
Using a mold with the insert pin inserted, 500 shots of die casting of an Al-based alloy (ADC12) were performed. The molten metal temperature was 700 ° C. Moreover, the cycle time of one shot was 30 to 60 seconds, and the mold release agent was sprayed on the mold design surface for 2 seconds for each shot. When releasing the cast product (that is, when removing the tip of the insert pin buried in the cast product), the load applied to the extruding push pin was measured with a load cell. The average value of the mold release load of the last 5 shots of 500 shots was calculated, and this was defined as “mold release resistance”.
[2.4. Hardenability (slow cooling material impact value)]
A JIS Charpy impact test piece No. 3 test piece (2 mmU notch) was cut out from the material after the spheroidizing annealing. In order to simulate quenching of the center of the large material, the test piece was heated and held at 1030 ° C. for 1 hour and then cooled to 550 ° C. at 20 ° C./min and from 550 ° C. to 150 ° C. at 0.3 ° C./min. Then, finish processing was performed and the impact value was measured by the Charpy impact test.

[3. result]
Table 1 shows the results. Table 1 also shows the alloy composition and the compound forming method. Table 1 shows the following.
(1) In the case of Comparative Examples 1 to 8, the compound layer thickness was 10 μm or less. This is presumably because the nitrogen retention amount exceeds 6.0 mass%. On the other hand, in Examples 1-22, the compound layer thickness was 10 micrometers or more in any case.
(2) In the case of Comparative Examples 1 and 2, the thermal conductivity at room temperature was 20 W / m / K or less. This is considered to be due to containing excessive Si and Cr. In contrast, in Examples 1 to 22, the thermal conductivity at room temperature was 30 W / m / K or more.
(3) In the case of Comparative Examples 1 to 8, the release resistance after 500 shots is over 200N. This is considered because the compound layer thickness is too thin. On the other hand, in Examples 1-22, the release resistance after 500 shots was 100 N or less.

FIG. 1 shows the relationship between the compound layer thickness and the mold release resistance after 500 shots. FIG. 2 shows the relationship between the amount of nitrogen retained and the thickness of the compound layer. 1 and 2 are plots of the results in Table 1.
From FIG. 1 and FIG.
(1) When the compound layer thickness is less than 10 μm, the release resistance after 500 shots increases rapidly.
(2) In order to form a compound layer of 10 μm or more under predetermined nitriding conditions, the nitrogen holding amount needs to be 6 mass% or less.
I understand that.

FIG. 3 shows the relationship between the Cr amount and the compound layer thickness, and the relationship between the Cr amount and hardenability (slow cooling material impact value). From FIG. 3, when the Cr amount is 1.5 to 3.0 mass%, the compound layer thickness is 10 μm or more and the hardenability is improved (the slow cooling material impact value is 15 J / cm ² or more). I understand. This relationship is as follows: C: 0.4 mass%, Si: 0.1 mass%, Mn: 0.45 mass%, Mo: 1.2 mass%, V: 0.5 mass%, W: 0.01 mass%, Al: The alloy composition was fixed at 0.001 mass%, and the test result was obtained by changing the Cr amount.

4 (a) and 4 (b) show appearance photographs after 500 shots of the nesting pins produced in Example 1 and Comparative Example 1, respectively. FIG. 4C and FIG. 4D show cross-sectional photographs immediately after the nitriding treatment of the nested pins produced in Example 1 and Comparative Example 1, respectively.
From FIG.
(1) The nested pin of Example 1 has a thicker compound layer than Comparative Example 1.
(2) The nesting pin of Example 1 has less seizure of Al molten metal compared to Comparative Example 1,
I understand that.

  The embodiment of the present invention has been described in detail above, but the present invention is not limited to the above embodiment, 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 as a material of a die used for die casting such as an Al alloy.

Claims (5)

Die-cast die steel with the following configuration.
(1) The die casting steel is
0.25 ≦ C ≦ 0.50 mass%,
0.005 ≦ Si ≦ 0.30 mass%,
0.40 ≦ Mn ≦ 2.00 mass%,
1.50 ≦ Cr ≦ 3.00 mass%,
Mo ≦ 2.00 mass%,
V ≦ 0.60 mass%,
W ≦ 3.00 mass%, and
Including Al ≦ 3.00 mass%,
The balance consists of Fe and inevitable impurities.
(2) The die casting mold steel has a nitrogen holding amount represented by the formula (a) of 6.00 mass% or less.
Nitrogen holding amount = [Si] + [Cr] + [Mo] / 2 + [V] + [W] + [Al] (a)
However, [] is the content of each element (mass%).
(3) The die-casting die steel is used in a state in which a compound layer having a thickness of 10 μm or more having an Fe—N compound as a main phase is formed on at least a surface in contact with the molten metal.   The steel for die-casting molds according to claim 1, further comprising Ni ≦ 0.50 mass%.   The steel for die-casting molds according to claim 1 or 2, further comprising Co ≦ 2.00 mass%.   4. The die casting steel according to claim 1, wherein the thermal conductivity at room temperature is 30 (W / m / K) or more. The die casting steel according to any one of claims 1 to 4 is used at least on a surface in contact with the molten metal,
A die casting mold in which a compound layer having a thickness of 10 μm or more having an Fe—N compound as a main phase is formed on at least a surface in contact with the molten metal.

Images