WO2021187484A1

WO2021187484A1 - Steel for hot working die, die for hot working, and manufacturing method for same

Info

Publication number: WO2021187484A1
Application number: PCT/JP2021/010616
Authority: WO
Inventors: 修司山中; 貴之平重; 志保福元
Original assignee: 日立金属株式会社
Priority date: 2020-03-16
Filing date: 2021-03-16
Publication date: 2021-09-23
Also published as: KR20220143067A; EP4123047A1; US20230099300A1; CN115279932A; JP2021147624A; CN115279932B

Abstract

Provided are: a steel that is for a die and that enables production of a die being for hot working and having both high hardness and high thermal conductivity; a die for hot working; and a manufacturing method for the same.　The steel for a hot working die has a compositional makeup containing, in mass%, 0.45-0.65% of C, 0.1-0.6% of Si, 0.1-2.5% of Mn, 1.0-6.0% of Cr, 1.2-3.5% of (Mo+1/2W) where Mo and W are contained independently or in combination, 0.1-0.5% of V, 0.15-0.6% of Ni, 0.1-0.6% of Cu, and 0.1-0.6% of Al, the balance being Fe and inevitable impurities. Further, this die for hot working has said compositional makeup, and this manufacturing method is for manufacturing said die for hot working.

Description

Steel for hot working dies, hot working dies and their manufacturing methods

The present invention relates to steel for hot working dies, hot working dies, and a method for manufacturing the same.

In recent years, there has been an increasing need for ultra-high-strength steel sheets with a tensile strength of over 1 GPa for the purpose of reducing the weight of automobiles and improving collision safety. However, when an attempt is made to form a steel sheet having a tensile strength of 1.2 GPa or more by a cold press, problems such as an increase in forming load and springback, and formability occur. Therefore, recently, the hot stamping (also called hot stamping or hot stamping) method has attracted attention. In the hot stamping method, the steel sheet is heated to an austenite temperature or higher, press-formed, the mold is held at bottom dead center, and the steel sheet is rapidly cooled and quenched.

An advantage of the hot stamping method is that a molded product of an ultra-high-strength steel plate having a tensile strength of about 1.5 GPa can be obtained by quenching by die quenching in which the mold is rapidly cooled. Another advantage is that it has excellent moldability, such as almost no springback.
However, the hot stamping method has a problem of low productivity. That is, since it takes time to maintain the bottom dead center for diquenching, the productivity is lowered. As a countermeasure, a mold with high thermal conductivity is required. This is because in diquenching, the heat of the steel sheet is absorbed by the mold, but the higher the thermal conductivity of the mold, the shorter the time for holding the bottom dead point and the higher the productivity. In addition, hot stamping dies are required to have high hardness in order to improve wear resistance, and hot stamping die steel must have both high hardness and high thermal conductivity when made into a die. Is required.

Also in the fields of hot forging and die casting, there is a tendency that die steel having both high thermal conductivity and high hardness as described above is required in order to achieve a longer die life and further improvement in manufacturing efficiency. It is in. Generally, in order to obtain a mold with high hardness, it is necessary to increase the alloy amount of the mold steel, but there is a problem that the thermal conductivity of the mold decreases as the alloy amount increases, so what is the hardness and thermal conductivity? There is a trade-off relationship. Therefore, the optimum composition of components is being investigated by controlling the amount of alloy. For example, Patent Document 1 and Patent Document 2 propose a composition of a mold steel having both hardness and thermal conductivity. Further, Patent Document 3 and Patent Document 4 are also useful as a material for dies used for hot pressing, die casting, hot forging, etc., and have excellent thermal conductivity and wear resistance. It is disclosed about tool steel.

Japanese Unexamined Patent Publication No. 2017-43814 Japanese Unexamined Patent Publication No. 2018-24931 Japanese Patent No. 5744300 JP-A-2017-53023

The mold steels of Patent Documents 1 and 2 and the hot tool steels of Patent Documents 3 and 4 are useful inventions capable of increasing hardness and thermal conductivity. However, considering the quenching and tempering characteristics of mold steel and hot tool steel, and the fact that the work surface of the mold for hot stamping, etc. is nitrided and used, conventional mold steel and In the case of hot tool steel, the hardness may be insufficient. Specifically, recently, there has been a demand for mold steel capable of achieving a hardness of 52 HRC or higher as a mold for hot stamping, etc., but Patent Documents 1 to 3 indicate that a high hardness of 52 HRC or higher is stable. I can't get it. Further, the tempering temperature at which the maximum hardness of the mold steel can be obtained is generally around 575 ° C., but if the maximum hardness of the mold steel is less than 52 HRC, due to nitriding treatment or temperature rise during use, The hardness of the mold is further reduced from less than 52 HRC.
An object of the present invention is a die steel for hot working, which has a hardness and a thermal conductivity at a higher level than before, and can produce a die in which the hardness is maintained, and a steel for hot working. It is to provide a mold and a method for manufacturing the same.

In view of such circumstances, the present inventor has found a component composition capable of controlling the amount of alloy, achieving high hardness and high thermal conductivity, and maintaining the achieved high hardness (that is, having a large softening resistance). The steel for hot working dies of the present invention has been reached. Then, by using the above-mentioned steel for dies, a mold for hot working, which can achieve high hardness and high thermal conductivity and has excellent softening resistance, and a manufacturing method thereof have been found.

That is, one aspect of the present invention is, in terms of mass%, C: 0.45 to 0.65%, Si: 0.1 to 0.6%, Mn: 0.1 to 2.5%, Cr: 1. 0 to 6.0%, Mo and W alone or in combination (Mo + 1 / 2W): 1.2 to 3.5%, V: 0.1 to 0.5%, Ni: 0.15 to 0.6 %, Cu: 0.1 to 0.6%, Al: 0.1 to 0.6% or less, and a steel for hot working molds having a component composition of the balance Fe and unavoidable impurities. be.
Preferably, when tempered at 575 ° C, the hardness is 52 HRC or higher.

Another aspect of the present invention is, in mass%, C: 0.45 to 0.65%, Si: 0.1 to 0.6%, Mn: 0.1 to 2.5%, Cr: 1. 0 to 6.0%, Mo and W alone or in combination (Mo + 1 / 2W): 1.2 to 3.5%, V: 0.1 to 0.5%, Ni: 0.15 to 0.6 %, Cu: 0.1 to 0.6%, Al: 0.1 to 0.6% or less, and a hot working mold having a component composition of the balance Fe and unavoidable impurities.
Preferably, the hardness is 52 HRC or more and the thermal conductivity is 25 W / (m · K) or more. Preferably, the working surface has a nitride layer.

Another aspect of the present invention is for hot working, wherein the above-mentioned steel for a hot working die is hardened and tempered at a quenching temperature of 1020 to 1080 ° C. and a tempering temperature of 540 to 620 ° C. This is a mold manufacturing method.
Preferably, after the quenching and tempering, the working surface is further subjected to nitriding treatment.

According to the present invention, the optimum mold steel for hot working can be obtained. Further, by using this die steel, it is possible to provide a die for hot working, which has both high hardness and high thermal conductivity and maintains the high hardness, and a method for manufacturing the same.

It is a graph which shows the hardness for each tempering temperature by tempering the mold steel of this invention example and comparative example at 500 to 650 ° C. after quenching. FIG. 5 is a graph showing the thermal conductivity of the mold steels of the examples of the present invention and the comparative examples after being hardened and tempered to a hardness of 45 to 52 HRC.

The feature of the present invention is that the hot working die is manufactured by quenching and tempering the die steel, or by performing nitriding treatment on the working surface thereof. It has been found that there is an optimum composition of steel for dies to simultaneously achieve high hardness and high thermal conductivity of dies for quenching. In particular, it has been found that there is an optimum component for simultaneously achieving a high hardness of 52 HRC or higher and a high thermal conductivity of 25 W / (m · K) or higher.
Further, in the composition of the optimum die steel, the optimum quenching and tempering conditions for achieving high hardness and high thermal conductivity at the same time have been identified. In particular, by setting the tempering temperature in the temperature range of 540 to 620 ° C. (preferably around 575 ° C.), the mold steel of the present invention having the optimum composition can achieve a high hardness of 52 HRC or more. The mold is difficult to soften (the degree of softening is small) even in a temperature rising environment during treatment or use.
The hot working die of the present invention can be applied to, for example, a hot forging die, a die casting die, a hot extrusion die, and a hot stamping die, and particularly for a hot stamping die. It is preferable to apply. Hereinafter, each constituent requirement of the present invention will be described.

The steel for hot working dies of the present invention has a mass% (hereinafter, simply referred to as "%"), C: 0.45 to 0.65%, Si: 0.1 to 0.6%. , Mn: 0.1 to 2.5%, Cr: 1.0 to 6.0%, Mo and W alone or in combination (Mo + 1 / 2W): 1.2 to 3.5%, V: 0. 1 to 0.5%, Ni: 0.15 to 0.6%, Cu: 0.1 to 0.6%, Al: 0.1 to 0.6%, balance Fe and unavoidable impurity composition Have.

・ C: 0.45 to 0.65%
C is an element that is solid-solved in the substrate (matrix) by quenching to improve the hardness of the mold. Further, it is an element that improves the hardness of the mold by forming carbides with carbide-forming elements such as Cr, Mo, and V, which will be described later. However, if the amount of C is too large, the toughness of the mold is lowered due to the coarsening of the primary carbide and the like. Therefore, C is set to 0.45 to 0.65%. It is preferably 0.47% or more. More preferably, it is 0.49% or more. Further, it is preferably 0.63% or less. More preferably, it is 0.60% or less. More preferably, it is 0.58% or less.

・ Si: 0.1-0.6%
Si is used as a deoxidizer in the melting process. It is an element that dissolves in the substrate to improve the hardness of the mold. However, if the amount of Si is too large, the segregation tendency in the steel becomes stronger after melting, and the solidified structure becomes coarse, which leads to a decrease in the toughness of the mold. It is an element that significantly lowers the thermal conductivity of the mold after quenching and tempering. Therefore, Si is set to 0.1 to 0.6%. It is preferably 0.14% or more. More preferably, it is 0.17% or more. Further, it is preferably 0.45% or less, and more preferably 0.4% or less. More preferably, it is 0.35% or less. Even more preferably, it is 0.3% or less.

-Mn: 0.1 to 2.5%
Mn is used as a deoxidizing agent or a desulfurizing agent in the melting process. It is an element that contributes to strengthening the substrate and improving hardenability and toughness after quenching and tempering. However, if the amount of Mn is too large, the thermal conductivity of the mold is significantly lowered. Therefore, Mn is set to 0.1 to 2.5%. It is preferably 0.15% or more. Further, it is preferably 1.0% or less. More preferably, it is 0.35% or less. More preferably, it is 0.32% or less. Even more preferably, it is 0.3% or less.

-Cr: 1.0 to 6.0%
Cr is an element that dissolves in the substrate to increase its hardness. It is also an element that increases hardness by forming carbides, and like Mo and V described later, it is an element that contributes to secondary curing during tempering. In particular, Cr is an element that can increase the tempering softening resistance as compared with Mo and V (even if the tempering temperature is raised, the rate of decrease in hardness obtained by secondary curing can be reduced). be. Normally, the mold is adjusted to the working hardness by quenching and tempering the mold steel, but in order to increase the thermal conductivity of the hot working mold, it is effective to raise the tempering temperature. Is. In the present invention, by setting the Cr content to 1.0% or more, a hardness of 52 HRC or more can be achieved even when the tempering temperature is high, and the thermal conductivity is 25 W / (m · K). ) The above hot working dies can be obtained. Then, while maintaining the above hardness, it is also possible to obtain a hot working die in which the thermal conductivity is further improved to 28 W / (m · K) or more. The above hardness and thermal conductivity are values measured at room temperature (normal temperature).
Further, by increasing the Cr content, the nitriding characteristics of the mold steel can be improved. Therefore, for example, the working surface of the mold after quenching and tempering can be further nitrided to soften resistance. It is possible to improve the abrasion resistance (hardness of the work surface) of the mold while maintaining the hardness of the mold by improving the above.

However, if the Cr content is too high, it becomes difficult to increase the thermal conductivity of the mold due to the fact that the alloy content of the mold steel increases. Therefore, Cr is set to 1.0 to 6.0%. It is preferably 1.5% or more. More preferably, it is 2.0% or more. Further, it is preferably 5.5% or less, more preferably 4.8% or less, still more preferably less than 4.5%. Then, when it is particularly important to improve the thermal conductivity, Cr can be set to 4.0% or less or 3.5% or less.

-Mo and W alone or in combination (Mo + 1 / 2W): 1.2-3.5%
Like Cr, Mo and W are elements that dissolve in the substrate to increase hardness, and also elements that increase hardness by forming carbides, which contribute to secondary hardening during tempering. It is an element. It is also an element that improves hardenability. Since W has an atomic weight about twice that of Mo, it can be specified by (Mo + 1 / 2W) (of course, only one of them may be added, or both may be added). However, if the content of Mo or W is too large, the amount of alloy in the mold steel will increase, and the thermal conductivity of the mold will decrease. Therefore, Mo and W are 1.2 to 3.5% in the relational expression of the Mo equivalent of (Mo + 1 / 2W). It is preferably 1.5% or more. More preferably, it is 1.7% or more. More preferably, it is 1.9% or more. Further, it is preferably 3.4% or less. More preferably, it is 3.2% or less.
In the case of the present invention, since W is an expensive element, all of W can be replaced with Mo. At this time, Mo: 1.2 to 3.5% (the same applies to the preferable range). However, W may be contained as an impurity.

・ V: 0.1-0.5%
Like Cr, V is an element that increases hardness by forming carbides, and is an element that contributes to secondary curing during tempering. However, if the amount of V is too large, the amount of alloy in the mold steel increases, and the thermal conductivity of the mold becomes low. In particular, in the present embodiment, as will be described later, the thermal conductivity tends to be low due to the effect of adding Ni, Cu, and Al in order to improve the strength characteristics of the mold, so V is 0.1 to 0. Limiting to 5.5% is important to achieve both high thermal conductivity and high hardness properties. It is preferably 0.2% or more. Further, it is preferably 0.45% or less, and more preferably 0.4% or less.

・ Ni: 0.15 to 0.6%
Ni is an element that contributes to improving the toughness of the mold. Further, in the present embodiment, the strength characteristics of the mold steel can be improved by combining with Al to form and precipitate a Ni—Al intermetallic compound and secondary curing. However, if the amount of Ni is too large, the thermal conductivity may be significantly lowered due to the increase in the alloy amount of the mold steel, so the Ni is set to 0.15 to 0.6%. Preferably, it is 0.2% or more. Further, it is preferably 0.5% or less, and more preferably 0.45% or less.

・ Cu: 0.1-0.6%
Like Ni, Cu is an element that can improve the strength characteristics of mold steel by combining with Al to form and precipitate intermetallic compounds and secondary hardening. However, if the amount of Cu is too large, the amount of alloy in the mold steel will be large as in Ni, and the thermal conductivity of the mold will be low. Therefore, Cu is set to 0.1 to 0.6%. It is preferably 0.2% or more. Further, it is preferably 0.5% or less, and more preferably 0.45% or less.

-Al: 0.1 to 0.6% or less As described above, Al combines with Ni and Cu to form an intermetallic compound. If the Al content is too low, the intermetallic compound is not sufficiently formed and the strength improving effect cannot be obtained. On the other hand, if the Al content is too high, the thermal conductivity of the mold may be significantly reduced. There is. Therefore, Al is set to 0.1 to 0.6%. It is preferably 0.2% or more. Also. It is preferably 0.5% or less, and more preferably 0.4% or less.

Further, in the present embodiment, Ni / Al is preferably 1.0 to 2.0 in order to form and precipitate the intermetallic compound in just proportion. The more preferable upper limit of Ni / Al is 1.7, and the more preferable upper limit is 1.5.
Further, in the present embodiment, it is preferable that Cu / Al is 1.0 to 2.0 in order to form and precipitate the intermetallic compound without excess or deficiency. The more preferable upper limit of Cu / Al is 1.7, and the more preferable upper limit is 1.5.

-Remaining Fe and unavoidable impurities Considering that the thermal conductivity of the mold decreases as the amount of alloy in the mold steel increases, it is preferable that the balance other than the above elemental species is substantially Fe. .. However, elemental species not specified here (for example, elemental species such as P, S, Ca, Mg, O (oxygen), N (nitrogen)) are elements that may inevitably remain in steel. , It is permissible to include these elements as impurities. At this time, if the amount of P is too large, it segregates at the old austenite grain boundaries during heat treatment such as tempering, and the toughness of the mold deteriorates. Therefore, P is preferably regulated to 0.05% or less. More preferably, it is regulated to 0.03% or less. If the amount of S is too large, the hot workability deteriorates when the steel ingot is agglomerated. Therefore, S is preferably regulated to 0.01% or less. More preferably, it is regulated to 0.008% or less.

By quenching and tempering the die steel having the above-mentioned composition, it is possible to obtain the hot working die of the present invention having excellent hardness and thermal conductivity. The hardness of the hot working die of the present invention is a value measured at room temperature (normal temperature), and can achieve a sufficient hardness such as 52 HRC or more, which imparts excellent wear resistance to the die. be able to. Then, by adjusting the tempering temperature, the hardness of the mold can be preferably 53 HRC or more.
In the present invention, it is not necessary to specify the upper limit of the hardness of the mold. However, in the case of a mold steel having the above-mentioned composition, it is realistic that it is about 60 HRC from the peak hardness of the secondary hardening (generally in the range of tempering temperature of 540 to 620 ° C.). .. The upper limit of this hardness should be 58 HRC or less regardless of the peak hardness of the secondary hardening being about 60 HRC, that is, the tempering temperature can be raised beyond the above peak hardness. (That is, it is preferable in that the thermal conductivity can be increased).

The mold of the present embodiment has a thermal conductivity of 25 W / (m · K) when the hardness of the mold is adjusted to 52 HRC by quenching and tempering the mold steel having the above-mentioned composition. That is all. This thermal conductivity is a value measured at room temperature (normal temperature). It is preferably 28 W / (m · K) or more. The mold of the present invention having the above-mentioned thermal conductivity can further increase the thermal conductivity by making the hardness less than 52 HRC when it is desired to increase the thermal conductivity. It is also possible to adjust the hardness of the mold to more than 52 HRC while having sufficient thermal conductivity. Specifically, when the hardness of the mold is 45 HRC or more and 48 HRC or less, the thermal conductivity is preferably 30 W / (m · K) or more, and 32 W / (m · K) or more. More preferably, it is 34 W / (m · K) or more. Further, when the hardness of the mold is 53 HRC or more and 55 HRC or less, the thermal conductivity is preferably 25 W / (m · K) or more, and more preferably 27 W / (m · K) or more.
Such a mold can be achieved by a mold steel having a heat treatment property showing a hardness of 52 HRC or more when tempered at 575 ° C. When confirming this heat treatment characteristic, the quenching temperature before tempering can be, for example, 1030 ° C. The mold steel of the present invention has the above heat treatment characteristics. As a result, high hardness can be maintained and high thermal conductivity can be maintained in the mold used in, for example, the hot stamping method (for example, 100 to 400 ° C.).

In the case of the present invention, it is not necessary to specify the upper limit of the thermal conductivity of the mold. However, considering that the hardness of the mold decreases as the tempering temperature is raised (for example, adjusted to a temperature exceeding 600 ° C.), it is about 50 W / (m · K). Is realistic. It is preferably 47 W / (m · K) or less. More preferably, it is 45 W / (m · K) or less. When the mold maintains a hardness of 52 HRC or higher, it is realistic that the upper limit of the thermal conductivity is about 40 W / (m · K). It is preferably 38 W / (m · K) or less.

The hot working die of the present invention preferably has a nitride layer on its working surface.
As described above, the hot working die of the present invention has both high hardness and high thermal conductivity. Then, when the working surface of the mold further has a nitrided layer, the wear resistance (hardness of the working surface) of the mold can be further improved. Further, due to the quenching and tempering characteristics of the mold steel of the present invention, it is possible to suppress a decrease in hardness of the mold body during the nitriding treatment. The working surface is the surface of the mold that comes into contact with the work material being hot-worked.

The method for producing a hot working die of the present invention is to quench and temper the above-mentioned die steel.
When quenching and tempering a mold steel having the above-mentioned composition, the quenching temperature varies depending on the target hardness and the like, but can be, for example, approximately 1020 to 1080 ° C. It is preferably 1050 ° C. or lower.
Then, the mold steel hardened at this quenching temperature is tempered at a tempering temperature of, for example, 540 to 620 ° C., thereby stably achieving a hardness of 52 HRC or more and a thermal conductivity of 25 W. A mold of / (m · K) or more can be obtained. At this time, in order to maintain a hardness of 52 HRC or higher, the upper limit of the tempering temperature is preferably about 600 ° C. More preferably, it is 595 ° C. or lower. More preferably, it is 590 ° C. or lower. The lower limit of the tempering temperature is preferably about 550 ° C. More preferably, it is 555 ° C. or higher. More preferably, it is 560 ° C. or higher.
By using the heat treatment characteristics that can achieve a hardness of 52 HRC or higher at a tempering temperature of around 575 ° C, which indicates the peak hardness of secondary hardening, as the standard, the mold steel of the present invention that satisfies the heat treatment characteristics criteria is 540 to Hardness of 45 HRC or higher can be maintained even in a wide tempering temperature range of 620 ° C. When the peak hardness is higher than 52 HRC, for example, 53 HRC or higher, 54 HRC or higher, 55 HRC or higher, the hardness of 52 HRC or higher can be maintained in a wide tempering temperature range. Then, a thermal conductivity of 25 W / (m · K) or more can be obtained in this wide tempering temperature range, and the thermal conductivity can be improved particularly at a tempering temperature of 575 ° C. or higher.

The mold steel of the present invention is prepared into a hot working mold having a predetermined hardness by quenching and tempering. During this period, the mold steel is adjusted to the shape of the hot working mold by various machining such as cutting and drilling. The timing of this machining can be performed in a state where the hardness before quenching and tempering is low (that is, in an annealed state). Then, in this case, finishing processing may be performed after quenching and tempering. In some cases, the above machining may be performed in the pre-hardened state after quenching and tempering in combination with the above finishing process.

In the method for producing a hot working die of the present invention, preferably, the working surface of the die after the above quenching and tempering is further subjected to nitriding treatment.
As described above, by quenching and tempering the mold steel having the above-mentioned composition, for example, gold having a thermal conductivity of 25 W / (m · K) when the hardness is adjusted to 52 HRC. You can get the mold. Since the mold steel having the above-mentioned composition is also excellent in nitriding characteristics, the working surface of the mold after this quenching and tempering is further subjected to nitriding treatment to form the mold. Abrasion resistance (hardness of work surface) can be improved. Further, due to the quenching and tempering characteristics of the mold steel of the present invention, it is possible to suppress a decrease in hardness of the mold body during the nitriding treatment. At this time, various known nitriding treatments such as gas nitriding treatment and salt bath nitriding treatment can be applied to the conditions of the nitriding treatment.

A 10 kg steel ingot having the component composition shown in Table 1 was melted. Then, the ingot was heated to 1160 ° C., forged by a hammer, and then allowed to cool, and the steel material after the cooling was allowed to be annealed at 870 ° C. to obtain No. 1 of the present invention. Steels 1 to 6 and No. 1 which is a comparative example. 7-9 steels were made.

<Evaluation of tempering hardness>
No. The mold steels 1 to 9 were quenched at a quenching temperature of 1030 ° C. At this time, the cooling condition was set to a semi-cooling time of 40 minutes, assuming a cooling rate when the die steel such as the steel of the present invention and the comparative steel is the size of the actual hot stamping die (half). The cooling time is the time required for cooling from the quenching temperature to the temperature of (quenching temperature + room temperature) / 2. Then, the hardened steel for the mold was tempered at a tempering temperature of 500 to 650 ° C. Tempering was performed twice and kept at each temperature for 2 hours. The tempering temperature was set to a total of 7 conditions in increments of 25 ° C. And No. For each of 1 to 9, the Rockwell hardness (C scale) at room temperature was measured at the center of each tempering temperature. The results are shown in FIGS. 1 to 3.

No. which is an example of the present invention. 1 to 7 achieved a tempering hardness of 52 HRC or more within the range of the tempering temperature of 550 to 600 ° C. On the other hand, No. In each of 7 to 9, the tempering hardness was lower than 52HRC in the tempering temperature range of 500 to 650 ° C.

<Evaluation of thermal conductivity>
Then, No. The thermal conductivity of 1 to 9 was measured. The tempering hardness of the sample at the time of measuring the thermal conductivity is No. 1 which is an example of the present invention. 1 to 6 are 52HRC, No. 1 which is a comparative example. 7, 8 and 9 were 51 HRC, 50 HRC and 45 HRC, respectively. First, the mold was processed into a disk-shaped test piece having a diameter of 10 mm and a thickness of 2 mm, and the thermal diffusivity and specific heat of this test piece were measured by a laser flash method. Then, using the measured thermal diffusivity and specific heat values, the thermal conductivity at room temperature was calculated from the following formula. The results are shown in FIG.
Thermal conductivity λ (W / (m ・ K)) = ρ ・ α ・ C _p
(Ρ: Room temperature density, α: Thermal diffusivity, C _p : Specific heat)

From the result of FIG. 2, No. 1 which is an example of the present invention. It was confirmed that 1 to 6 achieved a thermal conductivity of 25 W / (m · K) or more. On the other hand, No. The thermal conductivity of 7 to 9 is the same as that of the present invention, but the peak hardness does not reach 52 HRC in the first place. For this reason, in the case of mold steel as in the comparative example, the hardness is further reduced when the thermal conductivity is to be adjusted (specifically, when the tempering temperature is increased in order to increase the thermal conductivity). Therefore, it is not possible to handle molds with various required characteristics. On the other hand, the mold steel of the present invention example has excellent temper softening resistance in addition to sufficient peak hardness. Therefore, the mold steel of the present invention example has higher thermal conductivity and high hardness characteristics. It was confirmed that it has advantageous characteristics for use in die processing.

Claims

By mass%, C: 0.45 to 0.65%, Si: 0.1 to 0.6%, Mn: 0.1 to 2.5%, Cr: 1.0 to 6.0%, Mo and W alone or in combination (Mo + 1 / 2W): 1.2 to 3.5%, V: 0.1 to 0.5%, Ni: 0.15 to 0.6%, Cu: 0.1 to 0 A steel for a mold for hot working, which has a component composition of 0.6%, Al: 0.1 to 0.6%, a balance Fe and unavoidable impurities.
The steel for hot working dies according to claim 1, wherein the hardness is 52 HRC or more when tempered at 575 ° C.
By mass%, C: 0.45 to 0.65%, Si: 0.1 to 0.6%, Mn: 0.1 to 2.5%, Cr: 1.0 to 6.0%, Mo and W alone or in combination (Mo + 1 / 2W): 1.2 to 3.5%, V: 0.1 to 0.5%, Ni: 0.15 to 0.6%, Cu: 0.1 to 0 A mold for hot working, characterized by having a component composition of 0.6%, Al: 0.1 to 0.6%, a balance Fe and unavoidable impurities.
The hot working die according to claim 3, wherein the die has a hardness of 52 HRC or more and a thermal conductivity of 25 W / (m · K) or more.
The hot working die according to claim 3 or 4, wherein the work surface has a nitride layer.
A hot working die according to claim 1 or 2, wherein the steel for hot working is hardened and tempered at a quenching temperature of 1020 to 1080 ° C. and a tempering temperature of 540 to 620 ° C. Production method.
The method for manufacturing a hot working die according to claim 6, wherein the work surface is further subjected to a nitriding treatment after the quenching and tempering.