JP6790610B2 - Mold steel and molding tools - Google Patents

Description

The present invention relates to a mold steel and a molding tool using the same. A molding tool is composed of a mold or a combination of mold parts alone or in combination. Molding tools are used for die casting, injection molding of plastics, rubber processing, various castings, warm forging, hot forging, hot stamping and the like. These molding tools have a portion that comes into contact with an object to be molded whose temperature is higher than room temperature.

Molds used for die casting, injection molding, hot to warm plastic working, etc. are usually manufactured by quenching and tempering the material and processing it into a predetermined shape by die carving or the like. Further, when performing hot to warm processing using such a mold, the mold receives a large heat cycle and a large load. Therefore, the material used for this type of mold is required to be excellent in toughness, high temperature strength, wear resistance, crack resistance, heat check resistance and the like. However, in general, it is difficult to improve a plurality of characteristics of mold steel at the same time.

Therefore, in order to solve this problem, various proposals have been made conventionally.
For example, Patent Document 1 states that C: 0.1 to 0.6, Si: 0.01 to 0.8, Mn: 0.1 to 2.5, Cu: 0.01 to 2.0 in mass%. , Ni: 0.01 to 2.0, Cr: 0.1 to 2.0, Mo: 0.01 to 2.0, V, W, Nb and Ta, one or more of them in total: Mold steel containing 0.01 to 2.0, Al: 0.002 to 0.04, N: 0.002 to 0.04, O: 0.005 or less, and the balance consisting of Fe and unavoidable impurities. Is disclosed.
It is described in the same document that heat treatment of such a material under predetermined conditions enhances thermal fatigue characteristics and softening resistance, whereby heat check and water cooling hole cracking can be suppressed. ..

Patent Document 2 describes in terms of mass%, C: 0.2 to 0.6%, Si: 0.01 to 1.5%, Mn: 0.1 to 2.0%, Cu: 0.01 to 2 .0%, Ni: 0.01-2.0%, Cr: 0.1-8.0%, Mo: 0.01-5.0%, one or two of V, W, Nb, and Ta Total of seeds and above: 0.01-2.0%, Al: 0.002-0.04%, and N: 0.002-0.04%, with the balance being Fe and unavoidable impurities. Mold steel is disclosed.
According to the document, such a material has good hardenability, and by heat-treating it under predetermined conditions, a required impact value can be obtained and the life of the mold can be extended. It is described that there is, and that cutting is easy.

Patent Document 3 describes C: 0.15 to 0.55% by mass, Si: 0.01 to 2.0% by mass, Mn: 0.01 to 2.5% by mass, Cu: 0.01 to 2. Select from the group consisting of 0% by mass, Ni: 0.01 to 2.0% by mass, Cr: 0.01 to 2.5% by mass, Mo: 0.01 to 3.0% by mass, and V and W. A mold steel containing at least one total amount: 0.01 to 1.0% by mass and having a balance of Fe and unavoidable impurities is disclosed.
The document describes that by heat-treating such a material under predetermined conditions, the softening resistance is increased and the wear resistance is also improved.

Patent Document 4 describes C: 0.26 to 0.55% by weight, Cr: less than 2% by weight, Mo: 0 to 10% by weight, W: 0 to 15% by weight (however, the content of W and Mo). Is 1.8 to 15% by weight in total), (Ti, Zr, Hf, Nb, Ta): 0 to 3% by weight, V: 0 to 4% by weight, Co: 0 to 6% by weight, Si: 0 to 0 A tool steel containing 1.6% by weight, Mn: 0 to 2% by weight, Ni: 0 to 2.99% by weight, and S: 0 to 1% by weight, and the balance of which is iron and unavoidable impurities is disclosed. ing.
The document describes that such a composition results in higher thermal conductivity than conventional tool steel.

Further, Patent Document 5 states that 0.35 <C ≦ 0.50, 0.01 ≦ Si <0.19, 1.50 <Mn <1.78, 2.00 <Cr <3.05 in mass%. , 0.51 <Mo <1.25, 0.30 <V <0.80, and 0.004 ≤ N ≤ 0.040, and the balance is Fe and unavoidable impurities. Has been done.
The document describes that the thermal conductivity of the mold can be increased by adopting such a composition.

A molding tool composed of a mold or a combination of mold parts alone or in combination has a portion that comes into contact with an object to be molded having a temperature higher than room temperature, and is therefore exposed to a thermal cycle of temperature rise and fall during use. Depending on the application, high pressure may be applied. In order to withstand this harsh heat cycle, dies and mold parts are used in a hardened and tempered state. The heating conditions at the time of quenching depend on the composition of the steel material, the application, the size of the mold, and the like, but in many cases, the temperature is maintained at about 1 to 3 hours at 1030 ° C.
On the other hand, industrially, "mixed loading" in which a large mold and a small mold are heated together at the time of quenching is common. However, in the case of mixed loading, if the heating conditions at the time of quenching are adjusted to the large mold, the small mold is excessively heated and the crystal grains become coarse.

Also, in recent years, when using high thermal conductivity steel (thermal conductivity λ: 24-27 [W / m / K]) with excellent cooling efficiency for die casting in order to shorten the die casting cycle time and reduce mold damage. Is increasing. In order to increase the thermal conductivity of the high thermal conductivity steel, the Cr content of the general hot die steel is significantly lower than that of the general hot die steel (about 5%).
On the other hand, since low Cr steel has a small amount of carbides remaining during quenching, it is necessary to lower the quenching temperature in order to prevent coarsening of crystal grains during quenching. However, when a plurality of molds are manufactured at the same time, there is a problem that mixed loading cannot be performed when the quenching temperature of some molds is different from the quenching temperature of other molds.

Further, steels having a thermal conductivity λ of more than 42 [W / m / K] by reducing Cr to 0.5 mass% or less are also known. However, such steels have low high temperature strength and corrosion resistance and are not recommended for use in mold parts exposed to temperature cycles.

That is, for mold steel exposed to temperature cycles,
(A) The required high-temperature strength and corrosion resistance can be ensured.
(B) It is possible to improve the productivity of quenching (that is, mixed loading).
(C) It has a high thermal conductivity that can shorten the cycle time and reduce the seizure of the mold, and
(D) At the time of quenching, it is required to be able to produce fine austenite capable of preventing the mold from cracking.
However, there has never been a proposal for a steel that simultaneously meets such requirements.

Japanese Unexamined Patent Publication No. 2008-056982 Japanese Unexamined Patent Publication No. 2008-120132 JP-A-2008-169411 Japanese Patent Publication No. 2010-500471 Japanese Unexamined Patent Publication No. 2011-094168

The problems to be solved by the present invention are excellent high temperature strength and corrosion resistance, high quenching productivity, high thermal conductivity, and mold steel capable of producing fine austenite crystal grains at the time of quenching. An object of the present invention is to provide a mold and a molding tool composed of mold parts using the mold.

In order to solve the above problems, it is a gist that the molding tool according to the present invention has the following configurations (1) and (2) .
(1) The molding tool is
It is composed of a mold or a combination of mold parts alone or in combination, and includes a part that comes into direct contact with an object to be molded whose temperature is higher than room temperature.
(2) At least one of the mold and the mold parts comprises the molding tool alone or in combination.
0.40 ≤ C <0.55 mass%,
0.003 ≤ Si <0.300 mass%,
0.70 <Mn <1.80 mass%,
0.80 ≤ Cr <2.00 mass%,
0.003 ≤ Cu <1.200 mass%,
0.003 ≤ Ni <1.380 mass%,
0.500 <Mo <3.500 mass%,
0.64 ≤ V <1.20 mass%, and
0.0002 ≤ N <0.1200 mass%
The balance consists of Fe and unavoidable impurities.
0.550 <Cu + Ni + Mo <3.600 mass%
Consists of mold steel that meets the requirements
Hardness is more than 33HRC and less than 57HRC,
The old austenite crystal grain size number at the time of quenching is 5 or more,
The thermal conductivity λ at 25 ° C. measured by the laser flash method is more than 27.0 [W / m / K].

The mold steel according to the present invention is
0.40 ≤ C <0.55 mass%,
0.003 ≤ Si <0.300 mass%,
0.70 <Mn <1.80 mass%,
0.80 ≤ Cr <2.00 mass%,
0.003 ≤ Cu <1.200 mass%,
0.003 ≤ Ni <1.380 mass%,
0.500 <Mo <3.500 mass%,
0.64 ≤ V <1.20 mass%, and
0.0002 ≤ N <0.1200 mass%
The balance consists of Fe and unavoidable impurities.
0.550 <Cu + Ni + Mo <3.600 mass%
The gist is to satisfy.

In the present invention
(A) In order to ensure tempering hardness, the amounts of C, Mo and V are optimized.
(B) In order to ensure high thermal conductivity, the amounts of Si, Cr and Mn are optimized and
(C) In order to ensure hardenability, the amounts of Cr and Mn were optimized.
Further, in the present invention, in order to refine the old austenite crystal grains, a pinning effect and a solute drag effect are positively used in combination.
That is,
(D) Optimize the amount of C, V, and N related to VC particles that suppress the movement of grain boundaries by a pinning effect.
(E) The amounts of Cu, Ni, and Mo, which are solute elements that suppress the movement of grain boundaries by the solute drag effect, were optimized.
As a result, the mold steel according to the present invention is excellent in high temperature strength and corrosion resistance, has high quenching productivity, has high thermal conductivity, and can generate fine austenite crystal grains at the time of quenching.

It is a schematic diagram of the transition of the furnace temperature and the mold temperature at the time of heating of the mixed loading. FIG. 2A is a microstructure photograph of a Cu-free steel (steel A) hardened after heating at 1030 ° C. × 5 Hr. FIG. 2B is a microstructure photograph of Cu-added steel (steel B) hardened after heating at 1030 ° C. × 5 Hr. It is a figure which shows the relationship between the amount of (Cu + Ni + Mo) and the γ crystal particle size number at the time of quenching. It is a figure which shows the relationship between the amount of V and the γ crystal particle size number at the time of quenching.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Mold steel]
The mold steel according to the present invention contains the following elements, and the balance is composed of Fe and unavoidable impurities. The types of additive elements, their component ranges, and the reasons for their limitation are as follows.

[1.1. Main constituent elements]
(1) 0.38 <C <0.55 mass%:
When the quenching speed is slow and the tempering temperature is high, the smaller the amount of C, the more difficult it is to stably obtain a hardness exceeding 33 HRC. Therefore, the amount of C needs to be more than 0.38 mass%.
On the other hand, when the amount of C becomes excessive, coarse carbides increase, which becomes the starting point of cracks and reduces toughness. Also, retained austenite increases, which becomes coarse bainite during tempering, resulting in reduced toughness. Further, if the amount of C is excessive, the weldability is lowered. In addition, the maximum hardness becomes too high, which makes machining difficult. Therefore, the amount of C needs to be less than 0.55 mass%. The amount of C is preferably less than 0.54 mass%.

(2) 0.003 ≤ Si <0.300 mass%:
Generally, the smaller the amount of Si, the higher the thermal conductivity. However, even if the amount of Si is reduced more than necessary, the effect of improving the thermal conductivity tends to be saturated, and it is more difficult to obtain the effect of increasing the thermal conductivity. Further, if the amount of Si is too small, the machinability during machining is significantly deteriorated. Furthermore, reducing the amount of Si more than necessary leads to a significant cost increase, although it cannot be said that it is impossible due to careful selection of raw materials and optimization of refining. Therefore, the amount of Si needs to be 0.003 mass% or more. The amount of Si is preferably 0.005 mass% or more, more preferably 0.007 mass% or more.

On the other hand, when the amount of Si becomes excessive, the decrease in thermal conductivity becomes large. Further, since the mold steel according to the present invention has a relatively large amount of V, V-based carbides are likely to crystallize during casting, and it is necessary to dissolve them in a subsequent heat treatment. However, if the amount of Si is excessive, this V-based crystallized carbide tends to become large, and it becomes difficult to dissolve it in solid solution. The V-based crystallized carbide remaining without solid solution is harmful because it becomes a starting point of fracture during use as a mold. Further, if the amount of Si is excessive, a problem that segregation of other elements becomes remarkable during casting tends to occur. Therefore, the amount of Si needs to be less than 0.300 mass%. The amount of Si is preferably less than 0.230 mass%, more preferably less than 0.190 mass%.

(3) 0.70 <Mn <1.80 mass%:
The mold steel according to the present invention has a relatively small amount of Cr. Therefore, if the amount of Mn is small, the hardenability is insufficient, and the toughness is lowered due to the mixing of bainite. Therefore, the amount of Mn needs to be more than 0.70 mass%. The amount of Mn is preferably more than 0.75 mass%, more preferably more than 0.87 mass%.
On the other hand, when the amount of Mn is excessive, the thermal conductivity is greatly reduced. Further, if the amount of Mn is excessive, segregation becomes remarkable at the time of casting. Therefore, the amount of Mn needs to be less than 1.80 mass%. The amount of Mn is preferably less than 1.78 mass%, more preferably less than 1.76 mass%.

(4) 0.80 ≦ Cr <2.00 mass%:
If the amount of Cr is small, the hardenability is insufficient. Further, when the amount of Cr is small, the corrosion resistance becomes extremely poor. Therefore, the amount of Cr needs to be 0.80 mass% or more. The amount of Cr is preferably more than 0.85 mass%, more preferably more than 0.90 mass%.
On the other hand, when the amount of Cr becomes excessive, the decrease in thermal conductivity becomes large. Therefore, the amount of Cr needs to be less than 2.00 mass%. The amount of Cr is more preferably less than 1.9 mass%.

(5) 0.003 ≤ Cu <1.200 mass%:
When the amount of Cu is small, the solute drag effect that suppresses the movement of γ grain boundaries during quenching becomes poor, and therefore the effect of suppressing the coarsening of crystal grains (the crystal grain size number becomes smaller) cannot be obtained. .. Further, when the amount of Cu is small, (a) the effect of improving hardenability is poor, (b) weather resistance as a steel containing Cr-Cu-Ni is difficult to be exhibited, and (c) hardness is increased by age hardening. Problems such as poor effect of increasing and (d) small effect of improving machinability occur. Furthermore, reducing the amount of Cu more than necessary is not impossible by applying the Cu removal technology by careful selection of raw materials and refining researched in various fields, but it causes a significant cost increase. Therefore, the amount of Cu needs to be 0.003 mass% or more. The amount of Cu is preferably 0.004 mass% or more, and more preferably 0.005 mass% or more.

On the other hand, when the amount of Cu becomes excessive, (a) cracks during hot working become apparent, (b) thermal conductivity decreases, (c) cost increases become remarkable, and (d) machinability. Problems such as an improvement effect and increasing hardness due to age hardening approaching saturation occur. Therefore, the amount of Cu needs to be less than 1.200 mass%. The amount of Cu is preferably less than 1.170 mass%, more preferably less than 1.150 mass%, still more preferably 0.7 mass% or less. When the amount of Cu is 0.7 mass% or less, the dragging effect can be greatly exhibited, and the annealing property and the excessive decrease in thermal conductivity can be avoided.

(6) 0.003 ≤ Ni <1.380 mass%:
Since Ni has a large dragging effect like Cu, it can be added for the purpose of maintaining fine particles during quenching. On the other hand, Cu may impair hot workability, whereas Ni not only does not impair hot workability, but also has the effect of recovering the deterioration of hot workability due to the addition of Cu.

However, when the amount of Ni is small, (a) the dragging effect becomes poor, (b) the hardenability improving effect becomes small, and (c) the weather resistance as a steel containing Cr-Cu-Ni becomes difficult to develop. , Etc. occur. Further, Ni has an effect of increasing the strength by combining with Al in the presence of Al to form an intermetallic compound, but when the amount of Ni is small, such an effect becomes poor. Furthermore, reducing Ni more than necessary is not impossible due to careful selection of raw materials, but it causes a significant cost increase. Therefore, the amount of Ni needs to be 0.003 mass% or more. The amount of Ni is preferably 0.004 mass% or more, and more preferably 0.005 mass% or more.

On the other hand, when the amount of Ni becomes excessive, (a) the effect of recovering the deterioration of hot workability due to the addition of Cu is saturated, (b) the decrease in thermal conductivity becomes remarkable, and (c) the metal bonded to Al. Problems such as a significant decrease in toughness due to precipitation of intermetallic compounds, a significant decrease in (d) segregation, and difficulty in homogenizing the properties occur. Therefore, the amount of Ni needs to be less than 1.380 mass%. The amount of Ni is preferably less than 1.250 mass%, more preferably less than 1.150 mass%, still more preferably 0.7 mass% or less. When the amount of Ni is 0.7 mass% or less, it is possible to avoid an excessive decrease in annealing property and thermal conductivity while exhibiting a large drag effect.

When Cu is contained in a certain amount or more and the hot workability is extremely poor, the amount of Ni is preferably 0.3 to 1.2 times the amount of Cu.
On the other hand, even when Cu is contained, it is not always necessary to increase the amount of Ni to 0.3 to 1.2 times the amount of Cu when cracking can be reduced by optimizing the processing temperature and processing method. ..

(7) 0.500 <Mo <3.500 mass%:
Since Mo has a relatively large dragging effect like Cu and Ni, it can be added for the purpose of maintaining fine particles during quenching. Mo also has an advantage that it does not impair hot workability like Cu. When the amount of Mo is small, (a) the dragging effect is small, (b) the contribution of secondary curing is small, and when the tempering temperature is high, it becomes difficult to stably obtain a hardness exceeding 33 HRC (c). ) The combined addition with Cr causes a problem that the effect of improving the corrosion resistance is small. Therefore, the amount of Mo needs to be more than 0.500 mass%. The amount of Mo is preferably more than 0.530 mass%, more preferably more than 0.560 mass%.

On the other hand, when the amount of Mo is excessive, problems such as (a) a decrease in fracture toughness and (b) a significant increase in material cost occur. Therefore, the amount of Mo needs to be less than 3.500 mass%. The amount of Mo is preferably less than 3.400 mass%, more preferably less than 3.300 mass%.

(8) 0.55 <V <1.20 mass%:
In order to maintain fine particles during quenching, it is necessary to combine the dragging effect of solid solution elements and the pinning effect of dispersed particles. It is preferable to optimize the V amount in consideration of the C amount so that the VC of the dispersed particles becomes an appropriate amount. When the amount of V is small, the amount of VC is small, so that the effect of suppressing the coarsening of γ crystal grains (the crystal grain size number becomes small) is poor. Therefore, the amount of V needs to be more than 0.55 mass%. The amount of V is preferably more than 0.56 mass%, more preferably more than 0.57 mass%, still more preferably more than 0.7 mass%. When the amount of V is more than 0.7 mass%, the crystal grains are very fine and have a particle size number of 8 or more, which is preferable.

On the other hand, even if V is added more than necessary, the effect of maintaining fine crystal grains is saturated. Further, when the amount of V becomes excessive, coarse crystallized carbides (those that precipitate during solidification) increase, which becomes the starting point of cracks, and thus the toughness decreases. Further, as the amount of V increases, the cost increases significantly. Therefore, the amount of V needs to be less than 1.20 mass%. The amount of V is preferably less than 1.16 mass%, more preferably less than 1.13 mass%.
In the present invention, in addition to containing other elements in the specified range, the amount of V and the amount of (Cu + Ni + Mo) are in an unprecedented range, and the dragging effect of the solid solution element and the pinning effect of the dispersed particles are positive. The feature is that it is used together with.

(9) 0.0002 ≦ N <0.1200 mass%:
N also affects the amount of dispersed particles VC. The larger the amount of N, the higher the solid solution temperature of VC. Therefore, even if the amounts of C and V are the same, the residual VC at the time of quenching increases.
When the amount of N is small, the amount of VC particles at the time of quenching becomes excessively small. Therefore, the effect of suppressing the coarsening of γ crystal grains (the grain size number becomes smaller) is poor. Further, N has an effect of forming AlN particles in the presence of Al to assist in preventing grain coarsening, but when the amount of N is small, such an effect is small. Therefore, the amount of N needs to be 0.0002 mass% or more. The amount of N is preferably more than 0.0010 mass%, more preferably more than 0.0030 mass%.

On the other hand, when the amount of N becomes excessive, the refining time and cost required for adding N increase, which leads to an increase in material cost. Furthermore, when the amount of N becomes excessive, coarse nitrides, carbonitrides, or carbides increase, which becomes the starting point of cracks, and thus the toughness decreases. Therefore, the amount of N needs to be less than 0.1200 mass%. The amount of N is preferably less than 0.1000 mass%, more preferably less than 0.0800 mass%.

(10) Inevitable impurities:
The mold steel according to the present invention is an unavoidable impurity.
P ≤ 0.05 mass%,
S ≤ 0.003 mass%,
Al ≤ 0.10 mass%,
W ≤ 0.30 mass%,
O ≤ 0.01 mass%,
Co ≤ 0.10 mass%,
Nb ≤ 0.004 mass%,
Ta ≤ 0.004 mass%,
Ti ≤ 0.004 mass%,
Zr ≤ 0.004 mass%,
B ≤ 0.0001 mass%,
Ca ≤ 0.0005 mass%,
Se ≤ 0.03 mass%,
Te ≤ 0.005 mass%,
Bi ≤ 0.01 mass%,
Pb ≤ 0.03 mass%,
Mg ≤ 0.02 mass% or
REM ≤ 0.10 mass%,
May be included.

The mold steel according to the present invention may contain one or more of the above-mentioned elements. When the content of the element is not more than or equal to the above upper limit, the element behaves as an unavoidable impurity.
On the other hand, some of the above elements may be contained in excess of the above upper limit value. In this case, the effects described later can be obtained depending on the type and content of the element.

[1.2. Ingredient balance]
The mold steel according to the present invention is characterized in that, in addition to containing the above elements, the total amount of Cu, Ni and Mo satisfies the relationship of the following formula (a).
0.550 <Cu + Ni + Mo <3.600 mass% ... (a)

The amount of Cu + Ni + Mo is important as an index of the drag effect. If the total amount of these elements is small, a sufficient drag effect cannot be obtained. Therefore, the total amount of these elements needs to be greater than 0.550 mass%. The total amount is preferably more than 0.600 mass%, more preferably more than 0.700 mass%.
On the other hand, if the total amount of these elements becomes excessive, it causes the manifestation of cracks during hot working, the decrease in thermal conductivity, the decrease in toughness due to excessive precipitation of intermetallic compounds, and the decrease in fracture toughness. Therefore, the total amount of these elements needs to be less than 3.600 mass%. The total amount is preferably less than 3.550 mass%, more preferably less than 3.500 mass%, still more preferably 2.000 mass% or less. When the amount of Cu + Ni + Mo is 2.0 mass% or less, high thermal conductivity can be maintained. These elements, especially Cu and Ni, have a great effect of lowering the thermal conductivity. Therefore, in order to ensure that the crystal grains are fine and have a high thermal conductivity, first, the pinning effect is mainly used as the high V, and the drag effect is assisted to obtain fine grains. Next, high thermal conductivity is obtained by not excessively increasing Cu + Ni + Mo.

[1.3. Sub-components]
The mold steel according to the present invention may further contain one or more of the following elements in addition to the above-mentioned main constituent elements. The types of additive elements, their component ranges, and the reasons for their limitation are as follows.

(1) 0.30 <W ≦ 5.00 mass%:
(2) 0.10 <Co ≦ 4.00 mass%:
In the present invention, the total amount of Mn and Cr is smaller than that of SKD61, which is a general-purpose steel for die casting dies, and therefore the hardenability is not so high. Therefore, when the quenching speed is slow and the tempering is performed at a high temperature, it is difficult to secure a hardness exceeding 33 HRC. In such a case, W or Co may be selectively added to ensure the strength. W increases the strength by precipitation of carbides. Co increases the strength by solid solution to the base metal, and at the same time, contributes to precipitation hardening through the change of carbide morphology.

In addition, these elements dissolve in γ during quenching and exert a relatively large drag effect. Addition of W or Co is effective in order to stably obtain fine γ crystal grains by combining the pinning effect of VC particles and the dragging effect of solute atoms. In order to obtain such an effect, the W amount and the Co amount are preferably amounts exceeding the above lower limit values, respectively.
On the other hand, excessive amounts of these elements lead to saturation of properties and significant cost increase. Therefore, the amount of W and the amount of Co are preferably not more than the above upper limit values, respectively.
The mold steel may contain either W or Co, or may contain both.

(3) 0.004 <Nb ≤ 0.100 mass%:
(4) 0.004 <Ta ≤ 0.100 mass%:
(5) 0.004 <Ti ≤ 0.100 mass%:
(6) 0.004 <Zr ≦ 0.100 mass%:
When the quenching heating temperature becomes high or the quenching time becomes long due to an unexpected equipment trouble or the like, there is a concern that the crystal grains may become coarse even if it is the basic component of the mold steel according to the present invention. To. In preparation for such a case, Nb, Ta, Ti, and / or Zr may be selectively added. When these elements are added, these elements form fine precipitates. Since the fine precipitate suppresses the movement of the γ grain boundary (pinning effect), the fine austenite structure can be maintained. In order to obtain such an effect, the amount of each of these elements preferably exceeds the above lower limit value.

On the other hand, if the amount of these elements is excessive, carbides, nitrides, or oxides are excessively formed, resulting in a decrease in toughness. Therefore, the amount of each of these elements is preferably not more than the above upper limit value.
The mold steel may contain any one of these elements, or may contain two or more of these elements.

(7) 0.10 <Al ≤ 1.50 mass%:
Al has an effect of suppressing the growth of γ crystal grains (pinning effect) by combining with N to form AlN. In addition, Al has a high affinity for N and accelerates the invasion of N into steel. Therefore, when the steel material containing Al is nitrided, the surface hardness tends to increase. It is effective to use a steel material containing Al for the mold to be nitrided for higher wear resistance. In order to obtain such an effect, the amount of Al is preferably more than 0.10 mass%.
On the other hand, if the amount of Al is excessive, the thermal conductivity and toughness are lowered. Therefore, the amount of Al is preferably 1.50 mass% or less.
Even if the amount of Al is an impurity level (0.10 mass% or less), the above effect may be exhibited depending on the amount of N.

(8) 0.0001 <B ≤ 0.0050 mass%:
Addition of B is effective as a measure for improving hardenability. However, when B forms BN, the effect of improving hardenability is lost, so it is necessary for B to exist alone in the steel. Specifically, the nitride may be formed with an element having a stronger affinity for N than B to suppress the bond between B and N. Examples of such elements include the above-mentioned Nb, Ta, Ti, Zr and the like. These elements have the effect of fixing N even if they are present at the impurity level (0.004 mass% or less), but depending on the amount of N, an amount exceeding the impurity level may be added. Even if a part of B is combined with N in steel to form BN, if excess B is present alone in steel, it enhances hardenability.

B is also effective in improving machinability. In order to improve machinability, BN may be formed. BN is similar in nature to graphite, reducing cutting resistance and at the same time improving chip crushability. Further, when B and BN are present in the steel, hardenability and machinability are improved at the same time.
In order to obtain such an effect, the amount of B is preferably more than 0.0001 mass%.
On the other hand, if the amount of B is excessive, the hardenability is rather lowered. Therefore, the amount of B is preferably 0.0050 mass% or less.

(9) 0.003 <S ≤ 0.050 mass%:
(10) 0.0005 <Ca ≦ 0.2000 mass%:
(11) 0.03 <Se ≦ 0.50 mass%:
(12) 0.005 <Te ≤ 0.100 mass%:
(13) 0.01 <Bi ≤ 0.50 mass%:
(14) 0.03 <Pb ≦ 0.50 mass%:
It is also effective to selectively add S, Ca, Se, Te, Bi, or Pb to improve the machinability. In order to obtain such an effect, the amount of each of these elements preferably exceeds the above lower limit value.
On the other hand, if the amount of these elements is excessive, not only the effect of improving machinability is saturated, but also the hot workability is deteriorated, and the impact value and the mirror polishing property are deteriorated. Therefore, the amount of each of these elements is preferably not more than the above upper limit value.
The mold steel may contain any one of these elements, or may contain two or more of these elements.

[1.4. Characteristic]
When the mold steel according to the present invention is heat-treated under appropriate conditions,
Hardness is more than 33HRC and less than 57HRC
The old austenite crystal grain size number at the time of quenching is 5 or more, and
The thermal conductivity λ at 25 ° C. measured by the laser flash method exceeds 27.0 [W / m / K].

[1.4.1. Hardness]
The mold is required to be hard to wear and hard to be deformed. Therefore, the mold needs to be hard. If the hardness exceeds 33 HRC, problems of wear and deformation are unlikely to occur even if it is applied to various applications. The hardness is more preferably 35 HRC or more.
On the other hand, if the hardness is too high, not only the finishing machining of the mold becomes very difficult, but also large cracks and chips are likely to occur during use as the mold. Therefore, the hardness needs to be 57 HRC or less. The hardness is more preferably 56 HRC or less.
This point is the same for the mold parts, and the hardness thereof is preferably within the above range.

[1.4.2. Former austenite crystal grain size number]
In order to prevent cracking and chipping of the mold, it is better to increase the austenite crystal grain number at the time of quenching (fine austenite crystal grains). If the crystal grain size number is small, cracks are likely to grow, and cracks and chips are likely to occur. Therefore, the austenite crystal particle size number at the time of quenching needs to be 5 or more. The austenite crystal particle size number is more preferably 5.5 or more. When the production conditions are optimized, the crystal grain size number becomes 6 or more, or 6.5 or more.
This point is the same for the mold parts, and the old austenite crystal particle size number is preferably within the above range.

[1.4.3. Thermal conductivity]
In order to cool the product quickly and reduce mold damage (seizure, cracking, wear) by lowering the temperature of the mold and reducing thermal stress, it is necessary to increase the thermal conductivity of the mold. The thermal conductivity λ of general-purpose steel used for die casting or the like at 25 ° C. is 23.0 to 24.0 [W / m / K]. Even in steel having high thermal conductivity, λ is 27.0 [W / m / K] or less, which is insufficient. The thermal conductivity λ must be greater than 27.0 [W / m / K] in order to cool the product quickly and reduce mold damage. The thermal conductivity λ is more preferably more than 27.5 [W / m / K]. When the production conditions are optimized, the thermal conductivity becomes 28.0 [W / m / K] or more, or 28.5 [W / m / K] or more.
This point is the same for the mold parts, and the thermal conductivity thereof is preferably within the above range.
In the present invention, the "thermal conductivity" means a value at 25 ° C. measured by a laser flash method.

[2. Molding tool]
The molding tool according to the present invention has the following configurations.
(1) The molding tool is
It is composed of a mold or a combination of mold parts alone or in combination, and includes a part that comes into direct contact with an object to be molded whose temperature is higher than room temperature.
(2) At least one of the mold and the mold parts is made of the mold steel according to the present invention.
(3) At least one of the mold and the mold parts
Hardness is more than 33HRC and less than 57HRC,
The old austenite crystal grain size number at the time of quenching is 5 or more,
The thermal conductivity λ at 25 ° C. measured by the laser flash method is more than 27.0 [W / m / K].

[2.1. Use]
The molding tool according to the present invention is used for processing an object to be molded whose temperature is higher than room temperature. Examples of such processing include die casting, injection molding of plastic, rubber processing, various castings, warm forging, hot forging, and hot stamping.

[2.2. Definition]
In the present invention, the term "molding tool" is used.
(A) A mold having a part that comes into direct contact with an object whose temperature is higher than room temperature, and
(B) Refers to a mold component having a portion that comes into direct contact with an object to be molded having a temperature higher than room temperature, alone or in combination, and playing a role of molding the object to be molded into a predetermined shape.
In the present invention, the "mold" refers to a molding tool other than a mold part and a part (for example, a mold fastener) having no part in direct contact with an object to be molded. For example, in the case of die casting, there are molds on the movable side and the fixed side, respectively. Some molds are called cavities, cores or nests. In the present invention, the nesting is treated as a mold part described later.
In the present invention, the "mold part" refers to a part that plays a role of molding an object to be molded having a temperature higher than room temperature into a predetermined shape by itself or in combination with the mold. Therefore, for example, bolts and nuts for fastening the mold are not included in the "mold parts" referred to in the present invention. One of the objects of the present invention is to have high thermal conductivity, and to quickly cool die-cast, hot-stamped, and injection-molded products. Therefore, the subject of the present invention is a mold component having a portion that comes into contact with a molten metal, a heated steel plate, or a molten resin.
For example, in the case of die casting molding tools, mold parts include a plunger tip, a spool bush, a spool core (branch element), a ejection pin, a chill vent, and a nest. These mold parts are sometimes referred to as molds in a broad concept.

The object to be molded may be a melt or a semi-melt, or may be a solid. Further, the temperature of the object to be molded differs depending on the use of the molding tool.
For example, in the case of die casting, the temperature of the object to be molded (molten metal) is usually 580 to 750 ° C. in a melting furnace. In the case of injection molding of plastic, the temperature of the work piece (molten plastic) is usually 70 to 400 ° C. in a kneader. In the case of rubber processing, the temperature of the object to be molded (unvulcanized rubber) is usually 50 to 250 ° C. In the case of warm forging, the heating temperature of the workpiece (steel material) is usually 150 to 800 ° C. In the case of hot forging, the heating temperature of the object to be molded (steel material) is usually 800 to 1350 ° C. In the case of hot stamping, the heating temperature of the object to be molded (steel plate) is usually 800 to 1050 ° C.

[2.3. Mold steel]
In the molding tool according to the present invention, all or part of the mold and the mold parts are made of the mold steel according to the present invention. The details of the composition of the mold steel and the characteristics (hardness, former austenite particle size number, thermal conductivity) obtained after appropriate heat treatment are as described above, and thus the description thereof will be omitted.

[3. Action]
[3.1. Required characteristics]
In the following, a die casting mold or its parts will be described as an example. The die casting mold is used in a quenched and tempered state. The heating conditions for quenching are often a quenching temperature: 1030 ° C. and a holding time at the quenching temperature: 1 to 3 hours.
During quenching and heating, the die-casting steel may have a single-phase austenite, but generally has a mixed structure of austenite and residual carbides. After that, austenite is transformed into a martensite-based tissue by cooling, and in combination with tempering, hardness and toughness are imparted. This is because the mold needs to have hardness to ensure erosion resistance and toughness to ensure crack resistance.

Here, considering the toughness, it is desirable that the austenite crystal particle size number at the time of quenching is large (the austenite crystal particle size is small). The reason for this is that the finer the crystal grains, the more difficult it is for cracks to propagate, and the greater the effect of suppressing cracks in the mold.
The austenite crystal particle size number at the time of quenching is determined by the heating temperature and the holding time. The austenite grain size number becomes large (crystal grains become fine) when the heating temperature is low and the holding time is short. Therefore, in quenching, care should be taken so that the heating temperature does not become excessively high and the holding time does not become excessively long.

In order to prevent coarsening of crystal grains, a method of dispersing residual carbides in austenite may be adopted. In this case, the steel is a component-based steel in which the amount of C and the amount of carbide-forming elements are optimized. The residual carbide has the effect of pinning the movement of the austenite grain boundaries (pinning effect), and as a result, the coarsening of the austenite grain is prevented (a large grain size number is maintained).

Here, in quenching, "mixed loading" in which a large mold and a small mold are heated together is common. The reason for mixed loading is that if the molds are processed one by one, the productivity will not increase and the cost will increase. FIG. 1 shows a schematic diagram of changes in the furnace temperature and the mold temperature during mixed loading heating.
As described above, the heating time at the quenching temperature needs to be about 1 to 3 hours. At the time of mixed loading, a large mold gives a holding time of the furnace temperature so that this condition is satisfied. Then, the small mold whose temperature rises rapidly will be held at a maximum of about 5 hours, and the crystal grains will be coarsened (the crystal grain size number will be small).

In recent years, in order to shorten the cycle time of die casting and reduce mold damage, the use of high thermal conductivity steel having excellent cooling efficiency for die casting has increased. The thermal conductivity λ of SKD61, which is a general-purpose steel for die casting dies, at 25 ° C. is 23.0 to 24.0 [W / m / K], whereas the thermal conductivity of steel, which is considered to be high thermal conductivity, is high. λ is 24.0 to 27.0 [W / m / K]. In order to increase the thermal conductivity of such steel, the Cr content of such steel is significantly lower than that of general hot die steel (about 5%).

However, such steels have little or almost no carbides remaining during quenching. Therefore, in order to prevent coarsening of crystal grains during quenching, it is necessary to lower the quenching temperature to less than 1020 ° C. Then, only the steel mold has a different quenching temperature from the others, so it must be hardened individually. That is, since only one steel mold is charged into a large furnace for heat treatment, it becomes difficult to create an opportunity for heat treatment, and the productivity becomes very low and the cost increases.

In addition, some steels have a thermal conductivity λ of more than 42.0 [W / m / K] because they contain almost no Cr (Cr ≦ 0.5%). However, such steels have low high temperature strength and corrosion resistance and are not recommended for use in die casting dies.

Summarizing the above, the amount of Cr is low (0.5% <Cr << 5%), the austenite crystal particle size number is 5 or more even if the temperature is maintained at 1030 ° C. for 5 hours, and 25 when quenching and tempering is performed from that state. If the thermal conductivity at ° C. exceeds 27.0 [W / m / K] and there is a steel having high temperature strength and corrosion resistance that can withstand practical use, the following three points can be realized at the same time.
(1) Improvement of hardenability productivity (can be mixedly loaded in a large mold for quenching at 1030 ° C).
(2) Shortening the cycle time of die casting and reducing seizure of molds (high thermal conductivity).
(3) Prevention of cracking of die casting mold (fine austenite during quenching).
However, at present, such steel does not exist. There is a strong need in the industry for high thermal conductivity steel that does not easily coarsen during quenching.

[3.2. Ingredient optimization]
The steel that realizes the above is the present invention. The amounts of Cr, Mo and V were optimized to ensure tempering hardness. Further, in order to maintain high thermal conductivity, the amounts of Si, Cr and Mn were optimized. Further, in order to ensure hardenability, the amounts of Cr and Mn were optimized.

In addition, in order to make the austenite crystal grains finer during quenching (increasing the grain size number), the amount of C, V, and N related to the VC particles that suppress the grain boundary movement of the crystal grains by the pinning effect can be determined. Optimized. In particular, the amount of V is important.
Further, in order to make the austenite crystal grains finer at the time of quenching, the amounts of Cu, Ni and Mo, which are solute elements that suppress the movement of the grain boundaries by the solute drag effect, were optimized. In particular, the amount of (Cu + Ni + Mo) is important.
A major feature of the present invention is that the pinning effect and the dragging effect are positively used in combination, and the amount of V and the amount of (Cu + Ni + Mo) are in an unprecedented balance.

When a large amount of Cu is added, cracks during hot working are likely to become apparent. In order to prevent it, the addition of Ni is effective. However, the addition of Ni is limited to an amount that does not significantly reduce the thermal conductivity of the mold.

The mold steel according to the present invention has an austenite crystal grain size number of 5 or more even when quenched at 1030 ° C. for 5 hours. Therefore, the toughness after quenching and tempering is high, and cracking of the mold can be prevented.
Further, since the mold steel according to the present invention has a thermal conductivity exceeding 27.0 [W / m / K] after quenching and tempering, it is possible to shorten the cycle time of die casting and reduce seizure.
Further, since a maximum hardness of 57 HRC can be obtained after quenching and tempering, it is resistant to wear due to die casting injection. High hardness is preferable because high wear resistance can be obtained even when applied to a hot stamping die.

Since the mold steel according to the present invention contains Cr, it also has corrosion resistance to withstand practical use. Therefore, as compared with steel containing almost no Cr (Cr ≦ 0.5%), rust is less likely to occur during storage of the material or use as a mold.

[3.3. Combined drag effect and pinning effect]
Steel materials to which Cu is intentionally added already exist, but the purpose of adding Cu is to increase hardness and improve machinability. The present invention is decisively different from the conventional Cu-added steel in that it focuses on the strong dragging effect of Cu.
FIG. 2A shows a microstructure photograph of Cu-free steel (steel A) hardened after heating at 1030 ° C. × 5 Hr. FIG. 2B shows a microstructure photograph of the Cu-added steel (steel B) hardened after heating at 1030 ° C. × 5 Hr. The composition of steel A is 0.04C-0.05Si-1.58Mn-1.93Cr-1.10Mo-0.81V-0.020N. Steel B is obtained by adding 1.120 mass% Cu to steel A.

FIG. 2 is a comparison of the structures in which the old γ grain boundaries appeared after quenching. Quenching temperature: At 1030 ° C., VC particles are dispersed in steel A and steel B, and the amount of VC particles is almost the same in steel A and steel B. Therefore, the growth of γ particles is suppressed by the pinning effect of the crystal grain boundaries by the VC particles. However, as schematically shown in FIG. 1, when the holding at 1030 ° C. reaches 5 hours, the pinning effect is weakened due to the decrease due to the solid solution of the VC particles, and the driving force for the extremely large grain boundary movement is stopped. You will not be able to. As a result, it can be seen from FIG. 2 that the γ particle size of the steel A was 100 to 200 μm (2 to 4 in terms of crystal particle size number).

On the other hand, the γ crystal grain size of steel B is about 15 μm (crystal grain size number is about 9). At 1030 ° C, 1.120% Cu is all solid solution in γ. This solid-dissolved Cu suppressed the movement of the γ grain boundaries by the "solute drag effect", and together with the pinning of the γ grain boundaries by the VC particles, maintained a very fine γ structure. Can be judged.
As described above, the solid-dissolved Cu has a strong dragging effect, and when combined with the pinning effect of the dispersed particles, a stable fine grain structure can be obtained. This is the greatest feature of the present invention.

(Reference Example 1, Examples 2 to 6, Reference Examples 7 to 8, Examples 9 to 12, Reference Example 13, Example 14, Reference Example 15, Example 16, Reference Example 17, Examples 18 to 19, Reference Example 20, Examples 21 to 25, Comparative Examples 1 to 5)
[1. Preparation of sample]
The molten steel having the components shown in Table 1 was cast into a 50 kg ingot and then homogenized at 1240 ° C. Then, it was finished into a rod shape having a rectangular cross section of 60 mm × 45 mm by hot forging.
Subsequently, normalizing was performed by heating to 1020 ° C. and quenching, and tempering was performed by heating to 630 ° C. Further, after heating the steel bar to 820 to 900 ° C., the steel bar was controlledly cooled to 600 ° C. at 15 ° C./Hr, allowed to cool to 100 ° C. or lower, and subsequently annealed to 630 ° C. A test piece was cut out from this steel bar and used for various investigations.

Comparative Example 1 is a general-purpose steel JIS SKD61 for a die casting die. Comparative Example 2 is also a hot die steel, but is a commercially available brand steel. Comparative Examples 3 and 4 are JIS SNCM439 and JIS SCM435, respectively. Comparative Example 5 is a brand steel commercially available as a high thermal conductivity steel.

[2. Test method]
[2.1. Crystal grain size]
A small block of 15 mm × 15 mm × 25 mm cut out from the annealed steel bar was used as a test piece. This block was heated to 1030 ° C. and held for 5 hours, and then cooled at a rate of 50 ° C./min for martensitic transformation. Then, the old austenite grain boundaries before transformation were revealed with a corrosive liquid, and the grain size numbers were evaluated.

[2.2. Hardness]
After evaluating the crystal grain size number, the small block was heated and held at a general tempering temperature of 580 to 630 ° C., and tempering to 47HRC, which is a typical hardness of a die casting die, was attempted. After tempering, Rockwell hardness was measured.
[2.3. Thermal conductivity]
A small disk-shaped test piece having a diameter of 10 mm and a thickness of 2 mm was prepared from a small tempered block. The thermal conductivity λ [W / m / K] of this test piece at 25 ° C. was measured by a laser flash method.

[3. result]
[3.1. Crystal grain size number]
Table 2 shows the crystal grain size numbers. FIG. 3 shows the relationship between the amount of (Cu + Ni + Mo) and the γ crystal grain size number at the time of quenching. FIG. 4 shows the relationship between the amount of V and the γ crystal grain size number at the time of quenching.
The crystal particle size numbers of Comparative Example 1 and Comparative Example 2 are as large as about 10, and the austenite at the time of quenching is very fine. In Comparative Example 3, since both the amount of V and the amount of (Cu + Ni + Mo) are small, the crystal particle size number is about 2, which is a coarse grain.
In Comparative Example 4 and Comparative Example 5, since the hardenability was poor, ferrite was precipitated. The amount of ferrite is larger in Comparative Example 5. When ferrite precipitates at the austenite grain boundaries, the old austenite grain boundaries diffuse, making it difficult to distinguish. Therefore, the austenite crystal particle size numbers before transformation in Comparative Example 4 and Comparative Example 5 in which ferrite is precipitated are reference values. However, the crystal particle size number was clearly smaller than 5, and it was judged to be about 2.

On the other hand , Reference Example 1, Examples 2 to 6, Reference Examples 7 to 8, Examples 9 to 12, Reference Example 13, Example 14, Reference Example 15, Example 16, Reference Example 17, and Examples 18 to 18 to 19. The crystal particle size numbers of Reference Example 20 and Examples 21 to 25 stably exceed 5. As shown in FIGS. 3 and 4, the larger the amount of (Cu + Ni + Mo) and the amount of V, the larger the crystal grain size number tends to be, and the drag effect (FIG. 3) and the pinning effect (FIG. 4) are superimposed. I understand.

[3.2. Hardness]
Table 3 shows the hardness after tempering. In Comparative Example 4, ferrite was precipitated during quenching and the softening resistance was low, so that the hardness was about 27 HRC, and the hardness required for the mold: 33 HRC or more could not be secured. Comparative Example 5 also had a low hardness (<20HRC) that could not be measured by HRC because a large amount of ferrite was precipitated during quenching. It can be seen that it is practically impossible to use Comparative Example 4 and Comparative Example 5 for die casting mold parts from the viewpoint of hardenability and softening resistance.
Comparative Example 1 and Comparative Example 2 were only used for die casting dies, and could be tempered to 47HRC without any problem. In addition , Reference Example 1, Examples 2 to 6, Reference Examples 7 to 8, Examples 9 to 12, Reference Example 13, Example 14, Reference Example 15, Example 16, Reference Example 17, Examples 18 to 19, It was confirmed that all of Reference Example 20 and Examples 21 to 25 could be tempered to 47HRC and could be applied to a die casting mold from the viewpoint of hardenability and softening resistance.

[3.3. Thermal conductivity]
Table 4 shows the thermal conductivity of the materials shown in Table 3. In Comparative Example 1, since there are many Si and Cr, the thermal conductivity is the lowest. Comparative Example 2 has a higher thermal conductivity than Comparative Example 1 due to the extremely low Si, but remains at λ ≦ 27.0 due to the large amount of Cr. Comparative Examples 3 to 5 have a high thermal conductivity of λ> 27.0 because they have low Si and low Cr.

[3.4. Summary of evaluation]
Table 5 summarizes the above survey results. The austenite crystal particle size number when heated at 1030 ° C. × 5 Hr, the hardness in the quenching and tempering state, and the thermal conductivity are summarized. In Comparative Example 4 and Comparative Example 5, the tempering hardness required for the mold: 33HRC or more could not be obtained. The other steels could be tempered to 47 HRC except for Comparative Example 3. In Table 5, "○" means that the goal is achieved and is good, and "x" means that the goal is not achieved and is inferior.

In Comparative Examples 1 to 5, there is an "x" in any of the items. Comparative Example 1 and Comparative Example 2 have low thermal conductivity. In Comparative Examples 3 to 5, the crystal grain size number is small (the crystal grain is large). In Comparative Examples 1 and 2 having low thermal conductivity, it is difficult to reduce damage to the die and to quickly cool the product when the die is cast.
In Comparative Examples 3 to 5, there is a concern that a large crack may occur when the die-casting die is used. Further, since Comparative Examples 4 and 5 have low hardenability, it is difficult to apply them to the die casting mold itself.

On the other hand , Reference Example 1, Examples 2 to 6, Reference Examples 7 to 8, Examples 9 to 12, Reference Example 13, Example 14, Reference Example 15, Example 16, Reference Example 17, and Examples 18 to 18. 19, Reference Example 20, and Examples 21 to 25 have fine austenite crystal grains having a particle size of 5 or more at the time of quenching, and have a high thermal conductivity of more than 27 [W / m / K] in a tempered state of 47 HRC. Actually , Reference Example 1, Examples 2 to 6, Reference Examples 7 to 8, Examples 9 to 12, Reference Example 13, Example 14, Reference Example 15, Example 16, Reference Example 17, Examples 18 to 19, When Reference Example 20 and Examples 21 to 25 are applied to the die casting mold, it is expected that the following three points can be realized at the same time.
(1) Improvement of hardenability productivity (can be mixedly loaded in a large mold for quenching at 1030 ° C).
(2) Shortening the cycle time of die casting and reducing seizure of molds (high thermal conductivity).
(3) Prevention of cracking of die casting mold (fine austenite during quenching).

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 gist of the present invention.

The mold steel according to the present invention is suitable for die casting dies or parts thereof because the austenite crystal grains at the time of quenching are hard to be coarsened and high hardness and high thermal conductivity can be obtained after tempering. When the mold steel according to the present invention is applied to a die casting mold or its parts, cracking or seizure of the mold or its parts can be suppressed, and the die casting cycle time can be shortened.

Further, even if it is applied to a mold for injection molding plastic or its parts, the same effect as in the case of die casting can be obtained.
When applied to a hot forging, subhot forging, or hot forging die, overheating of the die surface can be suppressed by high thermal conductivity, and high temperature strength and toughness are sufficient, so that wear and cracks can be reduced.
Even when applied to hot stamping (also called hot press or press quenching), which is a method for forming high-strength steel sheets, the effects of high cycle and suppression of mold wear and cracking can be obtained due to high thermal conductivity.

Further, it is also effective to combine the mold steel according to the present invention with surface modification (shot blasting, sand blasting, nitriding, PVD, CVD, plating, nitriding, etc.).
It is also possible to make the mold steel according to the present invention into a rod shape or a linear shape and use it as a welding repair material for the mold or its parts. Alternatively, it can be applied to a mold or a part thereof manufactured by laminating molding of a plate or powder. In this case, it is not necessary to form the entire mold or its parts by laminating, and a part of the mold or its parts may be formed by laminating. Further, if a complicated internal cooling circuit is provided in the laminated molded portion, the effect of the high thermal conductivity of the mold steel according to the present invention can be further exerted.

Claims (14)

A molding tool having the following configurations (1) and (2) .
(1) The molding tool is
It is composed of a mold or a combination of mold parts alone or in combination, and includes a part that comes into direct contact with an object to be molded whose temperature is higher than room temperature.
(2) At least one of the mold and the mold parts comprises the molding tool alone or in combination.
0.40 ≤ C <0.55 mass%,
0.003 ≤ Si <0.300 mass%,
0.70 <Mn <1.80 mass%,
0.80 ≤ Cr <2.00 mass%,
0.003 ≤ Cu <1.200 mass%,
0.003 ≤ Ni <1.380 mass%,
0.500 <Mo <3.500 mass%,
0.64 ≤ V <1.20 mass%, and
0.0002 ≤ N <0.1200 mass%
The balance consists of Fe and unavoidable impurities.
0.550 <Cu + Ni + Mo <3.600 mass%
Consists of mold steel that meets the requirements
Hardness is more than 33HRC and less than 57HRC,
The old austenite crystal grain size number at the time of quenching is 5 or more,
The thermal conductivity λ at 25 ° C. measured by the laser flash method is more than 27.0 [W / m / K]. The mold steel is
0.30 <W ≤ 5.00 mass% and / or
0.10 <Co ≤ 4.00 mass%
The molding tool according to claim 1, further comprising. The mold steel is
0.004 <Nb ≤ 0.100 mass%,
0.004 <Ta ≤ 0.100 mass%,
0.004 <Ti ≤ 0.100 mass%, and
0.004 <Zr ≤ 0.100 mass%
The molding tool according to claim 1 or 2, further comprising any one or more elements selected from the group consisting of. The mold steel is
0.10 <Al ≤ 1.50 mass%
The molding tool according to any one of claims 1 to 3, further comprising. The mold steel is
0.0001 <B ≤ 0.0050 mass%
The molding tool according to any one of claims 1 to 4, further comprising. The mold steel is
0.003 <S ≤ 0.050 mass%,
0.0005 <Ca ≤ 0.2000 mass%,
0.03 <Se ≤ 0.50 mass%,
0.005 <Te ≤ 0.100 mass%,
0.01 <Bi ≤ 0.50 mass%, and
0.03 <Pb ≤ 0.50 mass%
The molding tool according to any one of claims 1 to 5, further comprising any one or more elements selected from the group consisting of. The molding tool according to any one of claims 1 to 6, wherein the mold component includes a plunger tip, a spool bush, a spool core, a punching pin, a chill vent, or a nest. 0.40 ≤ C <0.55 mass%,
0.003 ≤ Si <0.300 mass%,
0.70 <Mn <1.80 mass%,
0.80 ≤ Cr <2.00 mass%,
0.003 ≤ Cu <1.200 mass%,
0.003 ≤ Ni <1.380 mass%,
0.500 <Mo <3.500 mass%,
0.64 ≤ V <1.20 mass%, and
0.0002 ≤ N <0.1200 mass%
The balance consists of Fe and unavoidable impurities.
0.550 <Cu + Ni + Mo <3.600 mass%
Mold steel that meets the requirements. Hardness is more than 33HRC and less than 57HRC,
The old austenite crystal grain size number at the time of quenching is 5 or more,
The steel for molds according to claim 8, wherein the thermal conductivity λ at 25 ° C. measured by a laser flash method is more than 27.0 [W / m / K]. 0.30 <W ≤ 5.00 mass% and / or
0.10 <Co ≤ 4.00 mass%
The mold steel according to claim 8 or 9, further comprising. 0.004 <Nb ≤ 0.100 mass%,
0.004 <Ta ≤ 0.100 mass%,
0.004 <Ti ≤ 0.100 mass%, and
0.004 <Zr ≤ 0.100 mass%
The mold steel according to any one of claims 8 to 10, further comprising any one or more elements selected from the group consisting of. 0.10 <Al ≤ 1.50 mass%
The mold steel according to any one of claims 8 to 11, further comprising. 0.0001 <B ≤ 0.0050 mass%
The mold steel according to any one of claims 8 to 12, further comprising. 0.003 <S ≤ 0.050 mass%,
0.0005 <Ca ≤ 0.2000 mass%,
0.03 <Se ≤ 0.50 mass%,
0.005 <Te ≤ 0.100 mass%,
0.01 <Bi ≤ 0.50 mass%, and
0.03 <Pb ≤ 0.50 mass%
The mold steel according to any one of claims 8 to 13, further comprising any one or more elements selected from the group consisting of.

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