WO2017043446A1 - Steel for molds and molding tool - Google Patents

Abstract

This steel for molds contains 0.35 mass% < C < 0.55 mass%, 0.003 mass% ≤ Si < 0.300 mass%, 0.30 mass% < Mn < 1.50 mass%, 2.00 mass% ≤ Cr < 3.50 mass%, 0.003 mass% ≤ Cu < 1.200 mass%, 0.003 mass% ≤ Ni < 1.380 mass%, 0.50 mass% < Mo < 3.29 mass%, 0.55 mass% < V < 1.13 mass% and 0.0002 mass% ≤ N < 0.1200 mass%, with the balance made up of Fe and unavoidable impurities; and this steel for molds satisfies 0.55 mass% < (Cu + Ni + Mo) < 3.29 mass%. A molding tool according to the present invention comprises a mold and/or a mold component that is formed of this steel for molds.

Description

Mold steel and tools

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

Molds used for die casting, injection molding, hot-to-warm plastic processing, etc. are usually manufactured by quenching and tempering the raw material and processing it into a predetermined shape by die-sculpting. In addition, when performing a hot to warm process using such a mold, the mold is subjected to 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 properties at the same time in mold steel.

In order to solve this problem, various proposals have heretofore been made.
For example, in Patent Document 1, 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, one or more of V, W, Nb and Ta in total: Mold steel comprising 0.01 to 2.0, Al: 0.002 to 0.04, N: 0.002 to 0.04, O: 0.005 or less, the balance being Fe and inevitable impurities Is disclosed.
This document describes that heat fatigue characteristics and softening resistance are increased by heat-treating such materials under predetermined conditions, thereby suppressing heat check and water-cooled hole cracking. .

In Patent Document 2, by mass, C: 0.2 to 0.6%, Si: 0.01 to 1.5%, Mn: 0.1 to 2.0%, Cu: 0.01 to 2 0.0%, Ni: 0.01 to 2.0%, Cr: 0.1 to 8.0%, Mo: 0.01 to 5.0%, one or two of V, W, Nb and Ta Total of seeds or more: 0.01 to 2.0%, Al: 0.002 to 0.04%, and N: 0.002 to 0.04%, with the balance being Fe and inevitable impurities Mold steel is disclosed.
According to the document, such a material has good hardenability, and by heat-treating it under a predetermined condition, a required impact value can be obtained, and the mold life can be increased. It is described that it is easy to cut.

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

In Patent Document 4, C: 0.26 to 0.55 wt%, Cr: less than 2 wt%, Mo: 0 to 10 wt%, W: 0 to 15 wt% (however, the contents of W and Mo Is 1.8 to 15 wt% in total), (Ti, Zr, Hf, Nb, Ta): 0 to 3 wt%, V: 0 to 4 wt%, Co: 0 to 6 wt%, Si: 0 to Disclosed is a tool steel comprising 1.6 wt%, Mn: 0-2 wt%, Ni: 0-2.99 wt%, and S: 0-1 wt% with the balance being iron and inevitable impurities ing.
This document describes that the thermal conductivity is higher than that of conventional tool steel by using such a composition.

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

Since a molding tool constituted by a mold or a mold part alone or in combination has a portion that comes into contact with an object to be molded having a temperature higher than room temperature, it is 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 severe thermal cycle, molds and mold parts are used in a quenched and tempered state. The heating conditions during quenching depend on the steel composition, application, mold size, etc., but are often maintained at 1030 ° C. for about 1 to 3 hours.
On the other hand, industrially, “mixed loading” in which a large mold and a small mold are heated together during quenching is common. However, in the case of mixed loading, if the heating conditions during quenching are matched with a large mold, the small mold is excessively heated and the crystal grains become coarse.

In recent years, high heat conductivity steel (heat conductivity λ: 24-27 [W / m / K]) with excellent cooling efficiency is die-cast to reduce die casting cycle time, reduce seizure, and reduce heat check. More and more cases are used. In order to increase the thermal conductivity of the high thermal conductivity steel, the Cr content is significantly lower than the Cr content (about 5%) of general hot die steel.
On the other hand, since low Cr steel has a small amount of carbide remaining during quenching, it is necessary to lower the quenching temperature in order to prevent crystal grain coarsening during quenching. However, when a plurality of molds are manufactured at the same time, there is a problem that they cannot be mixed when the quenching temperature of some molds is different from the quenching temperature of other molds.

Moreover, when there is little Cr content, especially when there is much content of Mn and Mo, it will become difficult to anneal. In other words, a long heat treatment is required for softening to a hardness that can be machined, leading to an increase in cost.
Furthermore, steel whose thermal conductivity λ exceeds 42 [W / m / K] by making Cr 0.5 mass% or less is also known. However, such steels have low high temperature strength and corrosion resistance and are not recommended for use in mold parts that are subjected to temperature cycling.

That is, for mold steels exposed to temperature cycles,
(A) The necessary high temperature strength and corrosion resistance can be ensured,
(B) It is possible to reduce the cost of the material (that is, good annealing and easy heat treatment for softening),
(C) It is possible to improve quenching productivity (ie, mixed loading),
(D) having a high thermal conductivity that can reduce cycle time, mold seizure and heat check, and
(E) At the time of quenching, it is required that fine austenite crystal grains that can prevent cracking of the mold can be maintained (the coarsening of crystal grains can be prevented).
However, there has been no example in which a steel that satisfies such a requirement has been proposed.

Japanese Unexamined Patent Publication No. 2008-056882 Japanese Unexamined Patent Publication No. 2008-121032 Japanese Unexamined Patent Publication No. 2008-169411 Japanese National Table 2010-500471 Japanese Unexamined Patent Publication No. 2011-094168

The problems to be solved by the present invention are excellent in high-temperature strength and corrosion resistance, good in annealing, high in quenching productivity, high in thermal conductivity, and capable of generating fine austenite crystal grains during quenching. An object of the present invention is to provide a mold tool composed of mold steel, a mold using the mold steel, and a mold part.

In order to solve the above-described problems, the forming tool according to the present invention is summarized as having the following configuration.
(1) The molding tool is
It is composed of a mold or a mold part alone or in combination, and includes a portion that is in direct contact with a workpiece whose temperature is higher than room temperature.
(2) At least one of the mold and the mold part is:
0.35 <C <0.55 mass%,
0.003 ≦ Si <0.300 mass%,
0.30 <Mn <1.50 mass%,
2.00 ≦ Cr <3.50 mass%,
0.003 ≦ Cu <1.200 mass%,
0.003 ≦ Ni <1.380 mass%,
0.50 <Mo <3.29 mass%,
0.55 <V <1.13 mass%, and
0.0002 ≦ N <0.1200 mass%
And the balance consists of Fe and inevitable impurities,
0.55 <Cu + Ni + Mo <3.29 mass%
Made of mold steel that satisfies
Hardness is 33HRC more than 57HRC,
Old austenite grain size number at the time of quenching is 5 or more,
The thermal conductivity λ at 25 ° C. measured using a laser flash method is more than 27.0 [W / m / K].

The mold steel according to the present invention is:
0.35 <C <0.55 mass%,
0.003 ≦ Si <0.300 mass%,
0.30 <Mn <1.50 mass%,
2.00 ≦ Cr <3.50 mass%,
0.003 ≦ Cu <1.200 mass%,
0.003 ≦ Ni <1.380 mass%,
0.50 <Mo <3.29 mass%,
0.55 <V <1.13 mass%, and
0.0002 ≦ N <0.1200 mass%
And the balance consists of Fe and inevitable impurities,
0.55 <Cu + Ni + Mo <3.29 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) The amount of Cr and Mn was optimized in order to ensure hardenability and annealing.
Furthermore, in the present invention, in order to refine the prior austenite crystal grains, the pinning effect and the dragging effect were positively used together.
That is,
(D) optimizing the amount of C, V, and N related to VC particles that suppress the movement of grain boundaries by the pinning effect;
(E) The amounts of Cu, Ni, and Mo, which are solid solution elements that suppress the movement of crystal grain boundaries by the drag effect, are optimized.
As a result, the mold steel according to the present invention has excellent high temperature strength and corrosion resistance, good annealing, high quenching productivity, high thermal conductivity, and fine austenite grains during quenching. Can be generated.

It is a schematic diagram of transition of the furnace temperature and mold temperature at the time of mixed heating. It is a figure which shows the relationship between Cr amount and the Vickers hardness of an annealing material. It is a figure which shows the relationship between V amount and (gamma) grain size number at the time of hardening. It is a figure which shows the relationship between the amount of (Cu + Ni + Mo) and the gamma grain size number at the time of quenching.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Steel for molds]
The 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.

[1.1. Main constituent elements]
(1) 0.35 <C <0.55 mass%:
In the case where the quenching speed is slow and the tempering temperature is high, as the amount of C decreases, it becomes difficult to stably obtain a hardness exceeding 33 HRC. Therefore, the amount of C needs to be more than 0.35 mass%. The amount of C is preferably more than 0.36 mass%, more preferably more than 0.37 mass%.
On the other hand, when the amount of C becomes excessive, coarse carbides increase, which becomes a starting point of cracks and lowers toughness. Also, the retained austenite increases, which becomes coarse bainite during tempering, so that the toughness decreases. Furthermore, when the amount of C becomes excessive, the weldability decreases. In addition, the maximum hardness becomes too high, making 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%:
In general, 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 remarkably deteriorated. Furthermore, although reducing the amount of Si more than necessary is not impossible by careful selection of raw materials and optimization of refining, it causes a significant cost increase. Therefore, the amount of Si needs to be 0.003 mass% or more. The amount of Si is preferably 0.005 mass% or more, and more preferably 0.007 mass% or more.

On the other hand, when the amount of Si becomes excessive, the decrease in thermal conductivity increases. In addition, since the steel for molds according to the present invention has a relatively large amount of V, V-based carbides are likely to crystallize during casting, and this must be dissolved in the subsequent heat treatment. However, if the amount of Si is excessive, the V-based crystallized carbide tends to be large and difficult to be dissolved. The V-type crystallized carbide remaining without being dissolved is harmful since it becomes a starting point of destruction during use as a mold. Furthermore, when the amount of Si becomes excessive, the problem that segregation of other elements becomes remarkable at the time of 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) 0.30 <Mn <1.50 mass%:
If the amount of Mn is small, the hardenability is insufficient and the toughness is reduced due to the mixing of bainite. Therefore, the amount of Mn needs to be more than 0.30 mass%. The amount of Mn is preferably more than 0.35 mass%, more preferably more than 0.40 mass%.
On the other hand, when the amount of Mn becomes excessive, the annealing property is extremely deteriorated, and the heat treatment for softening becomes complicated and takes a long time, thereby increasing the manufacturing cost. The deterioration of the annealing property due to the increase in Mn is remarkable in the case of low Cr, high Cu, high Ni, and high Mo. Moreover, when the amount of Mn becomes excessive, the thermal conductivity is greatly lowered. Therefore, the amount of Mn needs to be less than 1.50 mass%. The amount of Mn is preferably less than 1.35 mass%, more preferably less than 1.25 mass%.

(4) 2.00 ≦ Cr <3.50 mass%:
When the amount of Cr is small, the hardenability is insufficient, the corrosion resistance is extremely deteriorated, and the annealability is extremely deteriorated. Therefore, the Cr amount needs to be 2.00 mass% or more. The amount of Cr is preferably more than 2.05 mass%, more preferably more than 2.15 mass%, more preferably more than 3.03 mass%. When the Cr amount exceeds 3.03 mass%, the drag effect of Cu, Ni, Mo, etc. is large, but even when there are many elements that degrade the annealability, the annealability can be ensured.
On the other hand, when the amount of Cr becomes excessive, the decrease in thermal conductivity increases. Therefore, the Cr amount needs to be less than 3.50 mass%. The amount of Cr is preferably less than 3.45 mass%, more preferably less than 3.40 mass%.

(5) 0.003 ≦ Cu <1.200 mass%:
When the amount of Cu is small, the drag effect that suppresses the movement of the γ grain boundary during quenching is poor, and thus the effect of suppressing the coarsening of the crystal grains (the crystal grain size number becomes small) cannot be obtained. . Further, when the amount of Cu is small, (a) the effect of improving the hardenability is poor, (b) the weather resistance as a steel containing Cr—Cu—Ni is hardly exhibited, and (c) the hardness is increased by age hardening. Problems such as poor effect of increasing, (d) small effect of improving machinability, and the like arise. Further, it is not impossible to reduce the amount of Cu more than necessary by applying a Cu removal technique by careful selection of raw materials and refining that has been studied in various fields, but it causes a significant increase in cost. 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 obvious, (b) thermal conductivity decreases, (c) cost increases become significant, (d) machinability. Problems such as improvement and higher hardness due to age hardening also approach saturation. 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%, and even more preferably 0.7 mass% or less. When the amount of Cu is 0.7 mass% or less, it is possible to avoid an excessive decrease in annealing and thermal conductivity while exhibiting a great drag effect.

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

However, when the amount of Ni decreases, (a) the drag effect becomes poor, (b) the effect of improving the hardenability decreases, and (c) the weather resistance as a steel containing Cr—Cu—Ni is hardly exhibited. Problems arise. Further, Ni has an effect of increasing the strength by bonding with Al when Al is present, and the effect of increasing the strength is poor when the amount of Ni is small. Further, it is not impossible to reduce Ni more than necessary by careful selection of raw materials, but it causes a significant increase in cost. 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 significant, (c) the metal bonded to Al Problems such as significant decrease in toughness due to precipitation of intermetallic compounds, (d) significant segregation, and difficulty in homogenizing characteristics. 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%, and even more preferably 0.7 mass% or less. When the amount of Ni is 0.7 mass% or less, an excessive decrease in annealing and thermal conductivity can be avoided while a drag effect is greatly expressed.

In addition, when it contains a certain amount or more of Cu 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 if Cu is contained, if the cracking can be reduced by optimizing the processing temperature, processing method, etc., the amount of Ni is not necessarily 0.3 to 1.2 times the amount of Cu. .

(7) 0.50 <Mo <3.29 mass%:
Since Mo has a relatively large drag effect like Cu and Ni, it can be added for the purpose of maintaining fine grains during quenching. Mo also has an advantage that hot workability is not impaired like Cu. When the amount of Mo is small, (a) the drag effect is small, (b) the contribution of secondary curing is small, and when the tempering temperature is high, it is difficult to stably obtain a hardness exceeding 33 HRC. ) Problems such as a small effect of improving the corrosion resistance by the combined addition with Cr occur. Therefore, the Mo amount needs to be more than 0.50 mass%. The amount of Mo is preferably more than 0.53 mass%, more preferably more than 0.56 mass%.

On the other hand, when the amount of Mo becomes excessive, problems such as (a) a decrease in fracture toughness and (b) a significant increase in material cost occur. Therefore, the Mo amount needs to be less than 3.29 mass%. The amount of Mo is preferably less than 3.27 mass%, more preferably less than 3.25 mass%.

(8) 0.55 <V <1.13 mass%:
In order to maintain fine particles during quenching, it is necessary to use both the drag 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 the γ crystal grains (the crystal grain size number becomes small) is poor. Therefore, the V amount 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%.

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 is excessive, coarse crystallized carbides (those that precipitate during solidification) increase, which becomes the starting point of cracks, and the toughness decreases. Further, the cost increases as the amount of V increases. Therefore, the V amount needs to be less than 1.13 mass%. The amount of V is preferably less than 1.11 mass%, more preferably less than 1.09 mass%.
In addition to the inclusion of other elements within the specified range, the present invention has a V amount and (Cu + Ni + Mo) amount that do not exist in the past, and positively provides a drag effect of solid solution elements and a pinning effect of dispersed particles. The feature is that it is used together.

(9) 0.0002 ≦ N <0.1200 mass%:
N also affects the amount of dispersed particles VC. As the amount of N increases, the solid solution temperature of VC increases. Therefore, even if the amount of C and V is the same, the residual VC at the time of quenching increases.
When the amount of N is small, VC particles at the time of quenching are excessively reduced. Therefore, the effect of suppressing the coarsening of the γ crystal grains (decreasing the crystal grain size number) is poor. In addition, N has an effect of assisting in preventing coarsening of the crystal grains by forming AlN particles when Al is present, but such an effect is small when the amount of N is small. Therefore, the N amount 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, if the amount of N becomes excessive, the refining time and cost required for N addition increase, leading to an increase in material cost. Further, when the amount of N becomes excessive, coarse nitrides, carbonitrides, or carbides increase, which becomes a starting point of cracks, so that toughness decreases. Therefore, the N amount 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 inevitable 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 elements as described above. When the content of the element is not more than the above upper limit value, the element behaves as an inevitable impurity.
On the other hand, a part of the element may be contained exceeding the upper limit. In this case, the effects described below are obtained depending on the type and content of the element.

[1.2. Ingredient balance]
In addition to containing the above elements, the mold steel according to the present invention is characterized in that the total amount of Cu, Ni and Mo satisfies the relationship of the following formula (a).
0.55 <Cu + Ni + Mo <3.29 mass% (a)

As an index of the drag effect, the amount of Cu + Ni + Mo is important. 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 more than 0.55 mass%. The total amount is preferably greater than 0.60 mass%, more preferably greater than 0.70 mass%.
On the other hand, if the total amount of these elements is excessive, it may cause cracking during hot working, decrease in thermal conductivity, decrease in toughness due to excessive precipitation of intermetallic compounds, decrease in fracture toughness, and the like. Therefore, the total amount of these elements needs to be less than 3.29 mass%. The total amount is preferably less than 3.28 mass%, more preferably less than 3.27 mass%.

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

(1) 0.30 <W ≦ 5.00 mass%:
(2) 0.10 <Co ≦ 4.00 mass%:
In the present invention, since the total amount of Mn and Cr is small as compared with SKD61, which is a general-purpose steel of a die casting mold, the hardenability is not so high. For this reason, when the quenching speed is slow and tempering is performed at a high temperature, it is difficult to ensure hardness exceeding 33 HRC. In such a case, W or Co may be selectively added to ensure strength. W increases the strength by precipitation of carbides. Co increases strength by solid solution in the base material, and at the same time contributes to precipitation hardening through a change in the form of carbide.

Further, these elements are dissolved in γ during quenching and exhibit a relatively large drag effect. Addition of W or Co is effective to obtain a stable and fine γ crystal grain by combining the pinning effect of VC particles and the drag effect of solute atoms. In order to obtain such an effect, the amount of W and the amount of Co each preferably exceed the lower limit values described above.
On the other hand, if the amount of these elements is excessive, characteristic saturation and significant cost increase are caused. Therefore, the W amount and the Co amount are each preferably not more than the above upper limit value.
In addition, either one of W or Co may be contained in the steel for metal mold | die, or both may be contained.

(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%:
If the quenching heating temperature becomes high or the quenching time becomes long due to unexpected equipment troubles, etc., there is a concern about the coarsening of crystal grains even if it is a basic component of the mold steel according to the present invention. The 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. The fine precipitates suppress the movement of the γ grain boundary (pinning effect), so that a fine austenite structure can be maintained. In order to obtain such an effect, the amount of these elements is preferably an amount exceeding the above lower limit value.

On the other hand, when the amount of these elements is excessive, carbides, nitrides, or oxides are excessively generated, resulting in a decrease in toughness. Therefore, the amount 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.

(7) 0.10 <Al ≦ 1.50 mass%:
Al combines with N to form AlN and has the effect of suppressing the growth of γ crystal grains (pinning effect). Al has a high affinity with N and accelerates the penetration of N into the steel. For this reason, when a steel material containing Al is subjected to nitriding treatment, the surface hardness tends to increase. It is effective to use a steel material containing Al for a mold for nitriding 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, when the amount of Al becomes excessive, thermal conductivity and toughness are reduced. Therefore, the amount of Al is preferably 1.50 mass% or less.
Even if the Al amount is an impurity level (0.10 mass% or less), the above effect may be exhibited depending on the N amount.

(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 the hardenability is lost, so it is necessary to make B exist alone in the steel. Specifically, a nitride may be formed with an element having an affinity for N that is greater than that of B to suppress the bond between B and N. Examples of such elements include Nb, Ta, Ti, and Zr described above. These elements have an effect of fixing N even when present at an 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 surplus B exists alone in the 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 and lowers cutting resistance while improving chip friability. Furthermore, 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, when the amount of B becomes excessive, the hardenability is 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%:
For improving machinability, it is also effective to selectively add S, Ca, Se, Te, Bi, or Pb. In order to obtain such an effect, the amount of these elements is preferably an amount exceeding the above lower limit value.
On the other hand, when the amount of these elements is excessive, not only the machinability improving effect is saturated, but also hot workability is deteriorated, impact value and mirror polishability are reduced. Therefore, the amount of these elements is preferably not more than the above upper limit value.
In addition, any 1 type of these elements may be contained in the steel for metal mold | die, or 2 or more types may be contained.

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

[1.4.1. Hardness]
The mold is required to be difficult to wear or deform. Therefore, the mold needs to have hardness. If the hardness exceeds 33 HRC, the problem of wear or deformation hardly occurs even when applied to various uses. The hardness is more preferably 35 HRC or more.
On the other hand, if the hardness is too high, not only finish machining of the mold becomes very difficult but also large cracks and chips are likely to occur during use as a 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 is preferably within the above range.

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

[1.4.3. Thermal conductivity]
In order to reduce product damage (seizure, cracking, wear) by cooling the product quickly or reducing the temperature of the die or reducing thermal stress, it is necessary to increase the thermal conductivity of the die. The thermal conductivity λ at 25 ° C. of general-purpose steel used for die casting is 23.0 to 24.0 [W / m / K]. Even in steel with high thermal conductivity, λ is 27.0 [W / m / K] or less, which is insufficient. In order to cool the product quickly or reduce mold damage, the thermal conductivity λ needs to exceed 27.0 [W / m / K]. The thermal conductivity λ is more preferably more than 27.5 [W / m / K]. When the manufacturing conditions are optimized, the thermal conductivity is 28.0 [W / m / K] or more.
This is the same for mold parts, and the thermal conductivity is preferably within the above range.
In the present invention, “thermal conductivity” refers to a value at 25 ° C. measured using a laser flash method.

[2. Molding tool]
The forming tool according to the present invention has the following configuration.
(1) The molding tool is
It is composed of a mold or a mold part alone or in combination, and includes a portion that is in direct contact with a workpiece whose temperature is higher than room temperature.
(2) At least one of the mold and the mold part is made of mold steel according to the present invention.
(3) At least one of the mold and the mold part is:
Hardness is 33HRC more than 57HRC,
Old austenite grain size number at the time of quenching is 5 or more,
The thermal conductivity λ at 25 ° C. measured using a laser flash method is more than 27.0 [W / m / K].

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

[2.2. Definition]
In the present invention, "molding tool"
(A) a mold having a portion in direct contact with a workpiece having a temperature higher than room temperature; and
(B) It is configured by a single or a combination of mold parts having a portion that is in direct contact with a molding whose temperature is higher than room temperature, and plays a role of molding the molding into a predetermined shape.
In the present invention, the “mold” refers to a part other than a mold part, a mold part, and a part (for example, a mold fastener) that does not have a portion in direct contact with a molding object. 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, and inserts. In the present invention, the nesting is handled as a mold part to be described later.
In the present invention, the “mold component” refers to a component that plays a role of molding a workpiece having a temperature higher than room temperature into a predetermined shape, alone or in combination with the mold. Therefore, for example, bolts and nuts for fastening the mold are not included in the “mold part” in the present invention. The present invention is characterized by high thermal conductivity, and one of the objects is to quickly cool a die cast, hot stamp or injection molded product. Therefore, a mold part having a portion in contact with a molten metal, a heated steel plate, or a molten resin is an application target of the present invention.
For example, in the case of a die-casting tool, mold parts include a plunger tip, a spool bush, a spool core (a diverter), a shot pin, a chill vent, and a nest.

The molding may be a melt or a semi-melt, and may be a solid. Moreover, the temperature of a to-be-molded object changes with uses of a 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 plastic injection molding, the temperature of the workpiece (molten plastic) is usually 70 to 400 ° C. in a kneader. In the case of rubber processing, the temperature of the molding (unvulcanized rubber) is usually 50 to 250 ° C. In the case of warm forging, the heating temperature of the molding (steel material) is usually 150 to 800 ° C. In the case of hot forging, the heating temperature of the molding (steel material) is usually 800 to 1350 ° C. In the case of hot stamping, the heating temperature of the molded product (steel plate) is usually 800 to 1050 ° C.

[2.3. Steel for molds]
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 properties (hardness, prior austenite grain size number, thermal conductivity) obtained after appropriate heat treatment are as described above, and thus the description thereof is omitted.

[3. Action]
[3.1. Required characteristics]
Below, a die-casting die or its component is demonstrated to an example. The die casting mold is used in a quenching and tempering state. The heating conditions for quenching are often quenching temperature: 1030 ° C. and holding time at quenching temperature: 1 to 3 Hr.
At the time of quenching and heating, the die-casting steel sometimes becomes an austenite single phase, but generally has a mixed structure of austenite and residual carbide. Thereafter, austenite is transformed into a structure mainly composed of martensite by cooling, and hardness and toughness are imparted by combination with tempering. This is because the mold requires hardness to ensure erosion resistance and toughness to ensure crack resistance.

Here, considering toughness, it is desirable that the austenite grain size number during quenching is large (the austenite crystal grain size is small). This is because cracks are less likely to propagate when the crystal grains are finer, and the effect of suppressing cracks in the mold is greater.
The austenite grain 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. For this reason, care is taken in quenching 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 technique of dispersing residual carbides in austenite may be employed. In this case, it is set as the steel of the component system which optimized C amount and the amount of carbide forming elements. Residual carbide has an effect of suppressing the movement of austenite grain boundaries by pinning (pinning effect), and as a result, coarsening of austenite crystal grains 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 generally used. The reason for the mixed loading is that if the molds are processed one by one, the productivity is not increased and the cost is increased. FIG. 1 shows a schematic diagram of changes in furnace temperature and mold temperature during mixed heating.
As described above, the heating time at the quenching temperature requires about 1 to 3 hours. At the time of mixed loading, a holding time of the furnace temperature is given so that a large mold meets this condition. If it does so, the small metal mold | die with a quick temperature rise will receive holding | maintenance of 5Hr at the longest, and a crystal grain will coarsen (crystal grain size number becomes small).

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

However, such steel has little or almost no carbide remaining when quenched at 1030 ° C. Therefore, in order to prevent crystal grain coarsening during quenching (austenite grain size number ≧ 5), it is necessary to lower the quenching temperature to less than 1020 ° C. Then, since only the steel mold has a different quenching temperature, it must be individually quenched. That is, only one steel mold is charged in a large furnace and heat-treated, resulting in a very low productivity.

When the Cr content is low, annealing becomes difficult particularly when the content of Mn and Mo is high. In other words, a long heat treatment is required for softening to a hardness that can be machined, leading to an increase in cost.
Some steels have a thermal conductivity λ exceeding 42.0 [W / m / K] by containing almost no Cr (Cr ≦ 0.5%). However, such steels are not recommended for use in die casting molds due to their low high temperature strength and corrosion resistance.

To summarize the above, it has corrosion resistance (2% ≦ Cr << 5%) that can withstand practical use, has good annealing properties, and has an austenite grain size number of 5 or more even when held at 1030 ° C. for 5 hours. If the steel has a thermal conductivity exceeding 27.0 [W / m / K] at 25 ° C. and high temperature strength that can withstand practical use, the following four points can be realized simultaneously.
(1) Cost reduction of the material (good hardenability and easy softening heat treatment).
(2) Productivity improvement of hardenability (can be mixed for quenching at 1030 ° C. for large molds).
(3) Reduction of die casting cycle time, die seizure and heat check (high thermal conductivity).
(4) Prevention of cracking of die casting mold (fine austenite during quenching).
However, at present, such steel does not exist. There is a strong industry need for high thermal conductivity steel that is difficult to coarsen during quenching.

[3.2. Ingredient optimization]
Steel that realizes the above is the present invention. In order to ensure the tempering hardness, the amounts of Cr, Mo and V were optimized. In order to maintain high thermal conductivity, the amounts of Si, Cr and Mn were optimized. Moreover, in order to ensure hardenability and annealing, the amount of Cr and Mn was optimized.

Further, in order to make the austenite crystal grains fine during quenching (increase the crystal grain size number), the amounts of C, V and N related to VC particles which suppress the grain boundary movement of crystal grains by a pinning effect are set. Optimized. In particular, the amount of V is important.
Furthermore, in order to make the austenite crystal grains fine at the time of quenching, the amounts of Cu, Ni, and Mo, which are solid solution elements that suppress the movement of crystal grain boundaries by the 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 drag effect are positively used together, and the amount of V and the amount of (Cu + Ni + Mo) are in an unprecedented balance.

In addition, when adding much Cu, the crack at the time of hot processing tends to become obvious. In order to prevent this, the addition of Ni is effective. However, the addition of Ni is limited to an amount that does not significantly reduce the thermal conductivity when the mold is formed.

The steel for molds according to the present invention has an austenite 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.
Moreover, since the steel for molds according to the present invention has a thermal conductivity exceeding 27.0 [W / m / K] after quenching and tempering, it is possible to realize a reduction in die casting cycle time and seizure.
Furthermore, since a maximum hardness of 57 HRC is obtained after quenching and tempering, it is also 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 mold.

Since the steel for molds according to the present invention contains Cr, it has corrosion resistance that can withstand practical use. Therefore, rust hardly occurs during storage of materials and use as a mold, compared with steel containing almost no Cr (Cr ≦ 0.5%).
Steel materials to which Cu is intentionally added already exist, but the purpose of the addition of Cu is to increase hardness and improve machinability. In this invention, the point which paid its attention to the strong drag effect of Cu differs decisively from the conventional Cu addition steel.

(Examples 1 to 30, Comparative Examples 1 to 5)
[1. Preparation of sample]
Molten steel having the components shown in Table 1 was cast into a 50 kg ingot, and then homogenized at 1240 ° C. And it finished in the rod shape of a rectangular cross section of 60 mm x 45 mm by hot forging.
Subsequently, normalizing by heating to 1020 ° C. and quenching and tempering by heating to 630 ° C. were performed. Further, after the steel bar was heated to 820 to 900 ° C., it was controlled and cooled down 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 the steel bar softened in this way and used for various investigations.

In addition, the comparative example 1 is the general-purpose steel JIS SKD61 of 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 marketed as a high thermal conductivity steel.

[2. Test method]
[2.1. Annealing]
A small block of 15 mm × 15 mm × 25 mm cut out from the bar after annealing was used as a test piece. For this block,
(A) Simulating hot working, heating to 1240 ° C. and holding for 0.5 Hr, then cooling to room temperature,
(B) As normalization, after heating to 1020 ° C. and holding for 2 hours, cooling to room temperature,
(C) As tempering, the sample was heated to 670 ° C. and held for 6 hours, and then cooled to room temperature.
These series of heat treatments are in accordance with the steps before annealing in actual production.
The test piece subjected to such pretreatment was heated to 870 ° C., kept at 2 Hr, cooled to 580 ° C. at 15 ° C./Hr, and thereafter annealed to cool to room temperature. After annealing, the Vickers hardness was measured.

[2.2. Crystal grain size]
A small block of 15 mm × 15 mm × 25 mm cut out from the steel bar after annealing was used as a test piece. The block was heated to 1030 ° C. and held for 5 hours, and then cooled at a rate of 50 ° C./min to cause martensitic transformation. Thereafter, prior austenite grain boundaries before transformation were revealed with a corrosive solution, and the grain size number was evaluated.

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

[3. result]
[3.1. Annealing]
[3.1.1. Comparison between Example and Comparative Example]
Table 2 shows the Vickers hardness after annealing. In order to perform machining easily, the hardness of the annealed material is preferably less than 280 HV. Comparative Example 2 with a large amount of Mn and Mo is 304 HV, Cr is small, and Comparative Example 3 with a large amount of C, Mn and Ni is 321 HV, which is hard. These steels are expected to be difficult to machine even for annealed materials. All the other comparative examples are less than 280HV.
On the other hand, Examples 1 to 30 were all softened to 210 to 276 HV. It was confirmed that Examples 1 to 30 were sufficiently softened by a normal annealing process.

[3.1.2. Effect of Cr content on annealing properties]
From the viewpoint of machinability to mold shape, it is desirable that the hardness of the annealed material is low. Therefore, with respect to a steel material in which the Cr content is changed using 0.40C-0.08Si-1.05Mn-0.18Cu-0.09Ni-1.01Mo-0.62V-0.19N as a basic component, Annealing was performed. FIG. 2 shows the relationship between the Cr content and the Vickers hardness of the annealed material.
When Cr <2.00 mass%, the hardness is 280 HV or more, and the hardness is remarkably increased (the annealing property is poor). Generally, a hardness range of less than 280 HV is necessary for efficient machining. Therefore, in the steel of Cr <2.00 mass%, the annealing cooling rate is reduced for softening, or additional heating is required after annealing. As a result, the processing takes a long time and causes an increase in cost. When Cr> 2.15 mass%, the load becomes 250 HV or less, and the machining load is considerably reduced.

[3.2. Grain size number]
[3.2.1. Comparison between Example and Comparative Example]
Table 3 shows the crystal grain size numbers. The crystal grain size number of Comparative Example 1 having a large amount of C, Cr and V is as large as about 10. In Comparative Example 2, although C and V are not so much, since the amount of Cr and Mo is large, the crystal grain size number is as large as about 7. In Comparative Example 3, since both the V amount and the (Cr + Ni + Mo) amount are small, the crystal grain size number is about 2 and coarse particles. In Comparative Examples 4 and 5, since the hardenability was poor, ferrite precipitated. The amount of ferrite is greater in Comparative Example 5. When ferrite precipitates at austenite grain boundaries, the prior austenite grain boundaries diffuse and become difficult to distinguish. For this reason, the austenite grain size before the transformation of Comparative Examples 4 and 5 in which ferrite is precipitated is a reference value. However, it was clearly judged that the crystal grain size number was smaller than 5 and about 2.

On the other hand, the crystal grain size numbers of Examples 1 to 30 stably exceed 5. This is because C, V, and N are optimized to ensure the amount of VC dispersed in the matrix during quenching, and Cu, Ni, and Mo are optimized to ensure the amount of alloy that dissolves in the matrix during quenching. It is. That is, a large grain size number was realized by superimposing the pinning effect and the drag effect.

[3.2.2. Effect of V amount on grain size number]
0.43C-0.07Si-0.10Cu-0.12Ni-0.81Mn-2.96Cr-1.12Mo-0.021N was used as a basic component, and the crystal grain size number when the V amount was changed was examined. . FIG. 3 shows the relationship between the V amount and the γ grain size number during quenching. From FIG. 3, it can be seen that when 0.55 mass% <V, a grain size number of 5 or more is stably obtained.

[3.2.3. Effect of (Cu + Ni + Mo) amount on grain size number]
The grain size number was examined when 0.40C-0.09Si-0.78Mn-2.99Cr-0.61V-0.020N was the basic component and the amount of (Cu + Ni + Mo) was changed. FIG. 4 shows the relationship between the amount of (Cu + Ni + Mo) and the γ grain size number during quenching. From FIG. 4, it can be seen that when 0.55 mass% <Cu + Ni + Mo, the grain size number 5 or more is stably obtained.

[3.3. Hardness]
Table 4 shows the hardness after tempering. In Comparative Example 4, ferrite was precipitated during quenching and softening resistance was low, so that it was about 27 HRC, and the hardness required for the mold: more than 33 HRC could not be secured. Comparative Example 5 also had a low hardness (<20 HRC) that cannot be measured by HRC because a large amount of ferrite precipitated during quenching. It can be seen that using Comparative Example 4 and Comparative Example 5 for die-cast mold parts is virtually impossible from the viewpoint of hardenability and softening resistance.
Comparative Example 1 and Comparative Example 2 were only used for die casting molds and could be tempered to 47HRC without problems. In addition, it was confirmed that all of Examples 1 to 30 could be tempered to 47 HRC and could be applied to a die casting mold from the viewpoint of hardenability and softening resistance.

[3.4. Thermal conductivity]
Table 5 shows the thermal conductivity of the materials shown in Table 4. Since the comparative example 1 has many Si and Cr, it has the lowest thermal conductivity. In Comparative Example 2, since Si is not extremely large, the thermal conductivity is higher than that of Comparative Example 1, but because of the large amount of Cr, λ ≦ 27.0 remains. Since Comparative Examples 3 to 5 are low Si and low Cr, they have high thermal conductivity of λ> 27.0.

[3.5. Summary of evaluation]
Table 6 summarizes the above survey results. The annealing properties, the austenite grain size number when heated at 1030 ° C. × 5 Hr, the hardness in the quenched and tempered state, and the thermal conductivity are summarized. In Comparative Examples 4 and 5, the tempering hardness required for the mold: more than 33 HRC could not be obtained. Other steels could be tempered to 47HRC except for Comparative Example 3. In Table 6, “◯” means that the target has been achieved and is good, and “×” means that the target has not been reached and is inferior.

In Comparative Examples 1 to 5, there is “x” in any item. Comparative Example 1 and Comparative Example 2 have low thermal conductivity. Comparative Examples 2 and 3 have poor annealing properties. Comparative Examples 3 to 5 have a small crystal grain size number (large crystal grains). In Comparative Examples 1 and 2 with low thermal conductivity, it is difficult to reduce damage to the mold and quickly cool the product when the die-cast mold is formed.
In Comparative Examples 3 to 5, there is a concern that large cracks may occur when die casting molds are obtained. Moreover, since Comparative Examples 4 and 5 have low hardenability, it is difficult to apply them to a die casting mold.

On the other hand, in Examples 1 to 30, the austenite crystal grains at the time of quenching are as fine as a grain size number of 5 or more, and have a high thermal conductivity exceeding 27 [W / m / K] in a tempered state of 47 HRC. Actually, when Examples 1 to 20 are applied to a die casting mold, it is expected that the following four points can be realized simultaneously.
(1) Cost reduction of the material (good annealing).
(2) Productivity improvement of hardenability (can be mixed for quenching at 1030 ° C. for large molds).
(3) Reduction of die casting cycle time, die seizure and heat check (high thermal conductivity).
(4) Prevention of cracking of die casting mold (fine austenite during quenching).

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

The mold steel according to the present invention is suitable for die casting molds or parts thereof because austenite crystal grains are hard to be coarsened during quenching and high hardness and high thermal conductivity are obtained after tempering. When the die steel according to the present invention is applied to a die casting die or a part thereof, it is possible to suppress cracking or seizure of the die or the part thereof and to shorten the die casting cycle time.

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

Furthermore, 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.).
The steel for molds according to the present invention can be made into a rod shape or a line shape and used as a welding repair material for the mold or its parts. Or it is also applicable to the metal mold | die manufactured by the lamination molding of a board or powder, or its components. In this case, it is not necessary to laminate-mold the entire mold or its parts, and a part of the mold or its parts may be formed by additive modeling. Moreover, if a complicated internal cooling circuit is provided in the layered and formed site, the effect of high thermal conductivity of the mold steel according to the present invention can be further exerted.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.
This application is based on a Japanese patent application filed on September 11, 2015 (Japanese Patent Application No. 2015-180193) and a Japanese patent application filed on July 27, 2016 (Japanese Patent Application No. 2016-147774). Incorporated herein by reference.

Claims (14)

1. A molding tool having the following configuration.
   (1) The molding tool is
   It is composed of a mold or a mold part alone or in combination, and includes a portion that is in direct contact with a workpiece whose temperature is higher than room temperature.
   (2) At least one of the mold and the mold part is:
   0.35 <C <0.55 mass%,
   0.003 ≦ Si <0.300 mass%,
   0.30 <Mn <1.50 mass%,
   2.00 ≦ Cr <3.50 mass%,
   0.003 ≦ Cu <1.200 mass%,
   0.003 ≦ Ni <1.380 mass%,
   0.50 <Mo <3.29 mass%,
   0.55 <V <1.13 mass%, and
   0.0002 ≦ N <0.1200 mass%
   And the balance consists of Fe and inevitable impurities,
   0.55 <Cu + Ni + Mo <3.29 mass%
   Made of mold steel that satisfies
   Hardness is 33HRC more than 57HRC,
   Old austenite grain size number at the time of quenching is 5 or more,
   The thermal conductivity λ at 25 ° C. measured using a laser flash method is more than 27.0 [W / m / K].
2. 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:
3. 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:
4. The mold steel is
   0.10 <Al ≦ 1.50 mass%
   The molding tool according to any one of claims 1 to 3, further comprising:
5. The mold steel is
   0.0001 <B ≦ 0.0050 mass%
   The molding tool according to any one of claims 1 to 4, further comprising:
6. 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 at least one element selected from the group consisting of:
7. The molding tool according to any one of claims 1 to 6, wherein the mold part includes a plunger tip, a spool bush, a spool core, a shot pin, a chill vent, or a nest.
8. 0.35 <C <0.55 mass%,
   0.003 ≦ Si <0.300 mass%,
   0.30 <Mn <1.50 mass%,
   2.00 ≦ Cr <3.50 mass%,
   0.003 ≦ Cu <1.200 mass%,
   0.003 ≦ Ni <1.380 mass%,
   0.50 <Mo <3.29 mass%,
   0.55 <V <1.13 mass%, and
   0.0002 ≦ N <0.1200 mass%
   And the balance consists of Fe and inevitable impurities,
   0.55 <Cu + Ni + Mo <3.29 mass%
   Meets mold steel.
9. Hardness is 33HRC more than 57HRC,
   Old austenite grain size number at the time of quenching is 5 or more,
   The mold steel according to claim 8, wherein the thermal conductivity λ at 25 ° C. measured using a laser flash method is more than 27.0 [W / m / K].
10. 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:
11. 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 at least one element selected from the group consisting of:
12. 0.10 <Al ≦ 1.50 mass%
    The steel for mold according to any one of claims 8 to 11, further comprising:
13. 0.0001 <B ≦ 0.0050 mass%
    The mold steel according to any one of claims 8 to 12, further comprising:
14. 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 at least one element selected from the group consisting of:

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