CN115279932A

CN115279932A - Steel for hot working die, and method for producing same

Info

Publication number: CN115279932A
Application number: CN202180020353.6A
Authority: CN
Inventors: 山中修司; 平重贵之; 福元志保
Original assignee: Hitachi Metals Ltd
Current assignee: Proterial Ltd
Priority date: 2020-03-16
Filing date: 2021-03-16
Publication date: 2022-11-01
Anticipated expiration: 2041-03-16
Also published as: KR20220143067A; JP2021147624A; US20230099300A1; EP4123047A1; WO2021187484A1; CN115279932B

Abstract

The invention provides a die steel for hot working, a hot working die and a manufacturing method thereof, wherein the die steel can be used for manufacturing the hot working die with high hardness and high thermal conductivity. A steel for hot working dies, having the following composition: in mass%, C: 0.45-0.65%, si: 0.1-0.6%, mn: 0.1-2.5%, cr:1.0% -6.0%, mo and W (Mo + 1/2W) singly or compositely: 1.2% -3.5%, V:0.1% -0.5%, ni:0.15% -0.6%, cu: 0.1-0.6%, al:0.1 to 0.6 percent, and the balance of Fe and inevitable impurities. The present invention also provides a hot working mold having the above composition and a method for producing the same.

Description

Steel for hot working die, and method for producing same

Technical Field

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

Background

In recent years, demand for ultra-high tensile steel sheets having a tensile strength of more than 1GPa has been increasing for the purpose of weight reduction and improvement of collision safety of automobiles. However, when a steel sheet having a tensile strength of 1.2GPa or more is to be formed by cold pressing, problems such as an increase in forming load and springback and formability occur. Therefore, recently, a hot stamping (also referred to as hot pressing or hot stamping) process has received attention. In the hot stamping process, a steel sheet is heated to an austenite temperature or higher, then press-formed, held at the bottom dead center, quenched and quenched.

As an advantage of the hot stamping process, a formed product of an ultra-high tension steel sheet having a tensile strength of about 1.5GPa can be obtained by quenching by die quenching in which quenching is performed by a die. Further, there is an advantage that the moldability is excellent such that the springback is not substantially generated.

However, the hot stamping process has a problem of low productivity. That is, since it takes time to hold the bottom dead center for performing the die quenching, etc., productivity is lowered. As a countermeasure, a mold having high thermal conductivity is demanded. This is because, in the die quenching, although the die absorbs the heat of the steel sheet, the higher the thermal conductivity of the die, the shorter the time for holding the bottom dead center, and the higher the productivity. Further, a high hardness is required for improving wear resistance of a die for hot stamping, and a high hardness and a high thermal conductivity are required for a die steel for hot stamping when the die is produced.

In the field of hot forging or die casting, there is a tendency that a die steel having both high thermal conductivity and high hardness as described above is required in order to achieve a longer life of a die and a further improvement in manufacturing efficiency. In general, in order to obtain a high-hardness die, the amount of alloy of the die steel needs to be increased, but if the amount of alloy is increased, the thermal conductivity of the die decreases, and there is a trade-off relationship between hardness and thermal conductivity. Therefore, the optimum composition of the components is investigated by controlling the amount of the alloy. For example, patent documents 1 and 2 propose a composition of a die steel having both hardness and thermal conductivity. Further, patent documents 3 and 4 also disclose hot tool steels that are effective as raw materials for dies used in warm pressing, die casting, warm forging, or the like, and that have excellent thermal conductivity and excellent wear resistance.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2017-43814

Patent document 2: japanese patent laid-open publication No. 2018-24931

Patent document 3: japanese patent No. 5744300

Patent document 4: japanese patent laid-open publication No. 2017-53023

Disclosure of Invention

Problems to be solved by the invention

The die steels of patent documents 1 and 2 and the hot tool steels of patent documents 3 and 4 are useful inventions for improving hardness and thermal conductivity. However, when the die steel or the hot tool steel is used in consideration of the quenching and tempering characteristics of the die steel or the hot tool steel, or when the working surface of the die for hot stamping or the like is nitrided, the hardness may be insufficient in the case of the conventional die steel or the hot tool steel. Specifically, recently, as a die for hot stamping or the like, there has been a demand for a die steel capable of achieving a high hardness of 52HRC or more, but patent documents 1 to 3 cannot stably obtain a high hardness of 52HRC or more. The tempering temperature at which the maximum hardness of the mold steel can be obtained is usually around 575 ℃, but if the maximum hardness of the mold steel is less than 52HRC, the hardness of the mold is further reduced from 52HRC due to the nitriding treatment or the temperature rise during use.

The purpose of the present invention is to provide a hot-working die steel that can be used to produce a die that has both hardness and thermal conductivity higher than the conventional level and that can maintain the hardness, a hot-working die, and a method for manufacturing the hot-working die steel.

Means for solving the problems

In view of the above circumstances, the present inventors have found that the steel for hot working dies of the present invention can be obtained by controlling the amount of alloy so as to achieve a high hardness and a high thermal conductivity and maintain the composition of the obtained high hardness (i.e., high softening resistance). Further, the use of the above-mentioned mold steel has led to the discovery of a hot working mold which can achieve high hardness and high thermal conductivity and which is also excellent in softening resistance, and a method for producing the same.

That is, one embodiment of the present invention is a hot working die steel characterized by having a composition of components: in mass%, C: 0.45-0.65%, si: 0.1-0.6%, mn: 0.1-2.5%, cr:1.0% -6.0%, mo and W (Mo + 1/2W) singly or compositely: 1.2% -3.5%, V:0.1% -0.5%, ni:0.15% -0.6%, cu: 0.1-0.6%, al: less than 0.1 to 0.6 percent, and the balance of Fe and inevitable impurities.

Preferably, the hardness is 52HRC or more when tempered at 575 ℃.

Another embodiment of the present invention is a mold for hot working, which is characterized by having the following composition: in mass%, C: 0.45-0.65%, si: 0.1-0.6%, mn: 0.1-2.5%, cr:1.0% -6.0%, mo and W (Mo + 1/2W) singly or compositely: 1.2% -3.5%, V:0.1% -0.5%, ni:0.15% -0.6%, cu: 0.1-0.6%, al: less than 0.1 to 0.6 percent, and the balance of Fe and inevitable impurities.

Preferably, the hardness is 52HRC or more and the thermal conductivity is 25W/(mK) or more. Preferably, the work surface has a nitride layer.

Another embodiment of the present invention is a method for manufacturing a hot working die, wherein the hot working die steel is quenched and tempered at a quenching temperature of 1020 to 1080 ℃ and a tempering temperature of 540 to 620 ℃.

Preferably, the method is characterized in that after the quenching and tempering, the work surface is further subjected to nitriding treatment.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a die steel most suitable for hot working applications can be obtained. Further, by using the above-mentioned mold steel, a hot working mold having both high hardness and high thermal conductivity and capable of maintaining the above-mentioned high hardness, and a method for producing the same can be provided.

Drawings

FIG. 1 is a graph showing hardness at each tempering temperature of the die steels of the present invention and comparative examples when the die steels are quenched and tempered at 500 to 650 ℃.

FIG. 2 is a graph showing the thermal conductivity of the die steels of the present invention and comparative examples when the steels were quenched and tempered to a hardness of 45 to 52HRC.

Detailed Description

The present invention is characterized by finding that the composition of the die steel is most suitable for achieving both high hardness and high thermal conductivity of the hot working die when the hot working die is manufactured by quenching and tempering the die steel or nitriding the working surface of the die steel. In particular, it has been found that a component is most suitable for simultaneously achieving a high hardness of 52HRC or more and a high thermal conductivity of 25W/(mK) or more.

Further, the most suitable composition of the die steel was found to be the most suitable quenching and tempering conditions for achieving both high hardness and high heat conductivity. In particular, by setting the tempering temperature to the temperature range of 540 to 620 ℃ (preferably around 575 ℃), the mold steel of the present invention having the most suitable composition of components can achieve a high hardness of 52HRC or more, and therefore the mold is less likely to soften (the degree of softening is small) even in an environment where the temperature is increased during the subsequent nitriding 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 is particularly preferably applied to a hot stamping die. Hereinafter, each structural element of the present invention will be described.

The die steel for hot working according to the present invention has the following composition: in mass% (hereinafter, simply expressed as "%"), C: 0.45-0.65%, si: 0.1-0.6%, mn: 0.1-2.5%, cr:1.0% -6.0%, mo and W (Mo + 1/2W) singly or compositely: 1.2% -3.5%, V:0.1% -0.5%, ni:0.15% -0.6%, cu: 0.1-0.6%, al:0.1 to 0.6 percent, and the balance of Fe and inevitable impurities.

·C：0.45％～0.65％

C is an element that is solid-dissolved in a matrix (matrix) by quenching to increase the hardness of the mold. Further, the element is an element which forms carbide with carbide-forming elements such as Cr, mo, and V described later to improve the hardness of the mold. However, if the amount of C is too large, the toughness of the die is lowered due to coarsening of primary carbides and the like. Therefore, C is set to 0.45% to 0.65%. Preferably 0.47% or more. More preferably 0.49% or more. Further, it is preferably 0.63% or less. More preferably 0.60% or less. More preferably 0.58% or less.

·Si：0.1％～0.6％

Si is used as a deoxidizer in the melting step. The element is dissolved in the matrix to increase the hardness of the mold. However, if Si is too much, segregation tends to be increased in the steel after melting, and the solidification structure also becomes coarse, resulting in a decrease in the toughness of the die. Further, the element significantly lowers the thermal conductivity of the die after quenching and tempering. Therefore, si is set to 0.1% to 0.6%. Preferably 0.14% or more. More preferably 0.17% or more. Further, it is preferably 0.45% or less, more preferably 0.4% or less. More preferably 0.35% or less. More preferably 0.3% or less.

·Mn：0.1％～2.5％

Mn is used as a deoxidizer or a desulfurizer in the melting process. And is an element contributing to strengthening of the matrix or improvement of hardenability and toughness after quenching and tempering. However, if Mn is too large, the thermal conductivity of the mold is significantly reduced. Therefore, mn is set to 0.1% to 2.5%. Preferably 0.15% or more. Further, it is preferably 1.0% or less. More preferably 0.35% or less. More preferably 0.32% or less. More preferably 0.3% or less.

·Cr：1.0％～6.0％

Cr is an element that is dissolved in a matrix to increase hardness. Further, the element is an element which increases hardness by forming carbide, and contributes to secondary hardening at the time of tempering similarly to Mo and V described later. In particular, cr is an element that can increase temper softening resistance (the reduction ratio of hardness obtained by secondary hardening can be reduced even if the tempering temperature is increased) as compared with Mo and V. In general, when the hardness of a die is adjusted to use by quenching and tempering a die steel, it is effective to increase the tempering temperature in order to increase the thermal conductivity of the hot working die. In the present invention, by setting the Cr content to 1.0% or more, a hot working die having a hardness of 52HRC or more and a thermal conductivity of 25W/(m · K) or more can be obtained even when the tempering temperature is high. Further, a mold for hot working having a further improved thermal conductivity of 28W/(m.K) or more while maintaining the hardness can be obtained. The hardness and thermal conductivity are values measured at room temperature (normal temperature).

Further, since the nitriding property of the steel for a mold can be improved by increasing the Cr content, for example, by further nitriding the working surface of the mold after quenching and tempering, the wear resistance of the mold (hardness of the working surface) can be improved while maintaining the hardness of the mold by increasing the softening resistance.

However, if the content of Cr is too large, the alloy content of the mold steel increases, and thus it is difficult to improve the thermal conductivity of the mold. Therefore, cr is set to 1.0% to 6.0%. Preferably 1.5% or more. More preferably 2.0% or more. Further, it is preferably 5.5% or less, more preferably 4.8% or less, and further preferably less than 4.5%. In particular, when importance is attached to improving the thermal conductivity, cr may be set to 4.0% or less or 3.5% or less.

Mo and W, alone or in combination (Mo + 1/2W): 1.2 to 3.5 percent

Like Cr, mo and W are elements which increase the hardness by dissolving them in a solid solution in a matrix, and also increase the hardness by forming carbides, and are elements contributing to secondary hardening during tempering. In addition, it is also an element for improving hardenability. Since the atomic weight of W is about 2 times that of Mo, it can be defined as (Mo + 1/2W) (of course, either one or both may be added). However, if the content of Mo or W is too large, the alloy content of the die steel increases, and the thermal conductivity of the die becomes low. Therefore, mo and W are 1.2% to 3.5% based on the relational expression of the Mo equivalent of (Mo + 1/2W). Preferably 1.5% or more. More preferably 1.7% or more. More preferably 1.9% or more. Further, it is preferably 3.4% or less. More preferably 3.2% or less.

Further, in the case of the present invention, since W is an expensive element, all W may be replaced with Mo. At this time, mo:1.2% to 3.5% (the preferred range is the same). However, W may be included as an impurity.

·V：0.1％～0.5％

V is an element that increases hardness by forming carbide, similarly to Cr, and contributes to secondary hardening during tempering. However, if the amount V is too large, the amount of alloy of the die steel increases, and the thermal conductivity of the die becomes low. In particular, in the present embodiment, since the thermal conductivity tends to be low due to the influence of Ni, cu, and Al added to improve the strength characteristics of the mold as described later, it is important to limit V to 0.1% to 0.5% in order to achieve both high thermal conductivity and high hardness characteristics. Preferably 0.2% or more. Further, it is preferably 0.45% or less, more preferably 0.4% or less.

·Ni：0.15％～0.6％

Ni is an element contributing to improvement of toughness of the mold. In addition, in the present embodiment, the strength characteristics of the steel for a mold can be improved by forming an Ni — Al intermetallic compound by bonding with Al, precipitating, and performing secondary hardening. However, if the amount of Ni is too large, the amount of alloy of the die steel increases and the thermal conductivity may be significantly lowered, so Ni is 0.15% to 0.6%. Preferably 0.2% or more. Further, it is preferably 0.5% or less, more preferably 0.45% or less.

·Cu：0.1％～0.6％

Cu is also an element which forms an intermetallic compound by bonding with Al and precipitates, and which improves the strength characteristics of the steel for molds by secondary hardening, similarly to Ni. However, if the Cu content is too large, the alloy content of the die steel increases as in Ni, and the thermal conductivity of the die becomes low. Therefore, cu is set to 0.1% to 0.6%. Preferably 0.2% or more. Further, it is preferably 0.5% or less, more preferably 0.45% or less.

Al: 0.1-0.6% below

As described above, al is combined with Ni or Cu to form an intermetallic compound. If the content of Al is too low, the intermetallic compound cannot be sufficiently formed, and therefore, the strength improvement effect cannot be obtained, while if the content of Al is too high, the thermal conductivity of the mold may be significantly reduced. Therefore, al is set to 0.1% to 0.6%. Preferably 0.2% or more. Further, 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 so as not to form an intermetallic compound excessively and precipitate it. A more preferable upper limit of Ni/Al is 1.7, and a further more preferable upper limit is 1.5.

Alternatively, in the present embodiment, cu/Al is preferably 1.0 to 2.0 so as not to form an intermetallic compound excessively and precipitate it. A more preferable upper limit of Cu/Al is 1.7, and a further more preferable upper limit is 1.5.

The balance Fe and inevitable impurities

Considering that the thermal conductivity of the mold becomes low when the alloy amount of the mold steel becomes large, the remaining portion other than the above element species is preferably substantially composed of Fe. However, element species not explicitly shown here (for example, element species such as P, S, ca, mg, O (oxygen), N (nitrogen) and the like) are elements that may inevitably remain in the steel, and it is permissible to include these elements as impurities. In this case, if P is too large, P segregates to the prior austenite grain boundary during heat treatment such as tempering, and the toughness of the die deteriorates. Therefore, P is preferably limited to 0.05% or less. More preferably, the content is limited to 0.03% or less. When the amount of S is too large, hot workability is deteriorated when the steel ingot is divided into pieces. Therefore, S is preferably limited to 0.01% or less. More preferably, the content is limited to 0.008% or less.

The hot working die of the present invention having excellent hardness and thermal conductivity can be obtained by quenching and tempering the die steel having the above composition. 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 of, for example, 52HRC or more, and can impart excellent wear resistance to the die. Further, it is preferable that the hardness of the mold is 53HRC or more by adjusting the tempering temperature.

Further, in the present invention, there is no need to define the upper limit of the hardness of the mold. However, it is realistic that the die steel having the above-described composition has a peak hardness of the secondary hardening (in a range of tempering temperature of approximately 540 to 620 ℃) of about 60 HRC. The upper limit of the hardness is preferably 58HRC or less in terms of that the tempering temperature can be increased beyond the peak hardness (that is, the thermal conductivity can be increased), although the peak hardness of the secondary hardening is about 60 HRC.

In the die of the present embodiment, the thermal conductivity is 25W/(m · K) or more when the die steel having the above-described composition is quenched and tempered to adjust the hardness of the die to 52HRC. The thermal conductivity is a value measured at room temperature (normal temperature). Preferably 28W/(mK) or more. In the mold of the present invention having the thermal conductivity as described above, when the thermal conductivity is further improved, the thermal conductivity can be further improved by setting the hardness to less than 52HRC. Further, the hardness of the mold can be adjusted to more than 52HRC while having sufficient thermal conductivity. Specifically, when the hardness of the mold is 45HRC or more and 48HRC or less, the thermal conductivity is preferably 30W/(m · K) or more, more preferably 32W/(m · K) or more, and still more preferably 34W/(m · K) or more. When the mold has a hardness of 53HRC or more and 55HRC or less, the thermal conductivity is preferably 25W/(m · K) or more, and more preferably 27W/(m · K) or more.

Such a mold can be achieved by a mold steel having heat treatment characteristics that exhibit a hardness of 52HRC or more when tempered at 575 ℃. When the heat treatment characteristics are confirmed, the quenching temperature before tempering may be, for example, 1030 ℃. The steel for molds of the present invention has the above heat treatment characteristics. Thus, for example, a high hardness can be maintained in a die used in a hot stamping process (e.g., 100 to 400 ℃), and a high thermal conductivity can also be maintained.

In the case of the present invention, it is not necessary to specify the upper limit of the thermal conductivity of the mold. However, if the tempering temperature is gradually increased (for example, adjusted to a temperature exceeding 600 ℃) and the hardness of the mold is gradually decreased, it is actually about 50W/(m · K). Preferably 47W/(mK) or less. More preferably 45W/(mK) or less. Further, it is realistic that the upper limit of the thermal conductivity is about 40W/(m · K) when the mold maintains a hardness of 52HRC or more. Preferably 38W/(mK) or less.

The hot working die of the present invention preferably has a nitride layer on the working surface.

As described above, the hot working die of the present invention has both high hardness and high thermal conductivity. Further, the working surface of the die further has a nitride layer, whereby the wear resistance of the die (hardness of the working surface) can be further improved. Further, the reduction in hardness of the die body during nitriding treatment can be suppressed by the quenching and tempering properties of the die steel of the present invention. The working surface is a surface of the die which is in contact with the workpiece during hot working.

The method for manufacturing a hot working die of the present invention is a method for quenching and tempering the die steel.

When the die steel having the above-described composition is quenched and tempered, the quenching temperature may be set to, for example, approximately 1020 to 1080 ℃ depending on the target hardness or the like. Preferably 1050 ℃ or lower.

Further, by tempering the die steel quenched at the quenching temperature at a tempering temperature of, for example, 540 to 620 ℃, a die having a heat conductivity of 25W/(m.K) or more can be obtained while stably achieving a hardness of 52HRC or more. In this case, the upper limit of the tempering temperature is preferably about 600 ℃ while maintaining the hardness of 52HRC or more. More preferably 595 ℃ or lower. More preferably 590 ℃ or lower. The lower limit of the tempering temperature is preferably about 550 ℃. More preferably 555 ℃ or higher. More preferably 560 ℃ or higher.

The die steel of the present invention which satisfies the above-mentioned heat treatment characteristics can maintain a hardness of 45RHC or more even in a wide tempering temperature range of 540 to 620 ℃ based on the heat treatment characteristics which can achieve a hardness of 52HRC or more at a tempering temperature around 575 ℃ which shows the peak hardness of the secondary hardening. Further, the peak hardness can be maintained at a hardness of 52HRC or more in a wide tempering temperature range by exceeding 52HRC as high as 53HRC or more, 54HRC or more, 55HRC or more, and the like, for example. Furthermore, the thermal conductivity of 25W/(m.K) or more can be obtained in the wide tempering temperature range, and particularly the thermal conductivity can be improved at the tempering temperature of 575 ℃ or more.

The die steel of the present invention is finished into a hot working die having a predetermined hardness by quenching and tempering. During this period, the mold steel is finished into the shape of the hot working mold by various machining processes such as cutting and piercing. The sequence of the machining may be performed in a state where the hardness before the quenching and tempering is low (i.e., an annealed state). In this case, the finish may be performed after the quenching and tempering. In addition, the machining may be performed in a pre-hardened state after quenching and tempering in conjunction with the finishing.

In the method for manufacturing a hot working die according to the present invention, it is preferable that the working surface of the die after the quenching and tempering is further subjected to nitriding treatment.

As described above, by quenching and tempering the mold steel having the above-described composition, a mold having a thermal conductivity of 25W/(m · K) when the hardness is quenched and tempered to 52HRC, for example, can be obtained. Further, since the die steel having the above composition is excellent in the nitriding property, the wear resistance of the die (hardness of the working surface) can be improved by further nitriding the working surface of the die after the quenching and tempering. Further, the reduction in hardness of the die body during nitriding treatment can be suppressed by the quenching and tempering properties of the die steel of the present invention. In this case, as the conditions of the nitriding treatment, for example, various known nitriding treatment conditions such as gas nitriding treatment and salt bath nitriding treatment can be applied.

Example 1

A steel ingot of 10kg having a composition of table 1 was melted. Then, the steel ingot was heated to 1160 ℃ and hammer-forged to elongate, then left to cool, and the steel material left to cool was annealed at 870 ℃ to produce steels of nos. 1 to 6 as examples of the present invention and steels of nos. 7 to 9 as comparative examples.

[ Table 1]

※P≦0.05％、S≦0.01％

< evaluation of temper hardness >

The die steels of Nos. 1 to 9 were quenched at a quenching temperature of 1030 ℃. In this case, regarding the cooling conditions, assuming that the die steel such as the inventive steel and the comparative steel has a cooling rate at the actual size of the hot stamping die, the half-cooling time is set to 40 minutes (the half-cooling time is the time required for cooling from the quenching temperature to the temperature of (quenching temperature + room temperature)/2). Then, the quenched mold steel is tempered at a tempering temperature of 500 to 650 ℃. Tempering was performed twice and held at each temperature for 2 hours. The tempering temperature was set to 7 conditions in total in units of 25 ℃. Then, with respect to Nos. 1 to 9, the Rockwell hardness (C scale) at room temperature of the central portion thereof was measured at each tempering temperature, respectively. The results are shown in FIGS. 1 to 3.

The samples Nos. 1 to 7 according to the present invention achieved a tempering hardness of 52HRC or more in the range of the tempering temperature of 550 to 600 ℃. In contrast, the tempering hardness of comparative examples Nos. 7 to 9 was less than 52HRC in the tempering temperature range of 500 to 650 ℃.

< evaluation of thermal conductivity >

Then, the thermal conductivities of Nos. 1 to 9 were measured. The temper hardnesses of the samples in the thermal conductivity measurements were 52HRC for Nos. 1 to 6 of the present invention examples and 51HRC, 50HRC and 45HRC for Nos. 7, 8 and 9 of the comparative examples. In the measurement, a mold was first machined into a disk-shaped test piece having a diameter of 10mm × a thickness of 2mm, and the thermal diffusivity and specific heat of the test piece were measured by a laser flash method. Then, using the measured values of thermal diffusivity and specific heat, the thermal conductivity at room temperature was calculated from the following formula. The results are shown in fig. 2.

Thermal conductivity λ (W/(m · K)) = ρ · α · C_p

(ρ: room temperature density,. Alpha.: thermal diffusivity, C)_p: specific heat)

From the results of FIG. 2, it was confirmed that Nos. 1 to 6, which are examples of the present invention, achieved thermal conductivities of 25W/(m.K) or more. On the other hand, in comparative examples Nos. 7 to 9, the thermal conductivity was the same as that of the present invention, but the initial peak hardness was not 52HRC. Therefore, in the case of the mold steel as in the comparative example, when the thermal conductivity is adjusted (specifically, when the tempering temperature is increased to increase the thermal conductivity), the hardness is further decreased, and it is impossible to cope with the mold having various required characteristics. On the other hand, the die steel of the present invention example is excellent in temper softening resistance in addition to sufficient peak hardness, and therefore it was confirmed that the present invention example having both high thermal conductivity and high hardness has favorable properties for use in a hot working die.

Claims

1. A steel for a hot-working die, characterized by having the following composition: in mass%, C: 0.45-0.65%, si: 0.1-0.6%, mn: 0.1-2.5%, cr:1.0% -6.0%, mo and W (Mo + 1/2W) singly or compositely: 1.2% -3.5%, V:0.1% -0.5%, ni:0.15% -0.6%, cu: 0.1-0.6%, al:0.1 to 0.6 percent, and the balance of Fe and inevitable impurities.

2. The steel for a hot-working mold as claimed in claim 1, wherein the hardness is 52HRC or more when tempered at 575 ℃.

3. A hot working die is characterized by comprising the following components: in mass%, C: 0.45-0.65%, si: 0.1-0.6%, mn: 0.1-2.5%, cr:1.0% -6.0%, mo and W (Mo + 1/2W) singly or compositely: 1.2% -3.5%, V:0.1% -0.5%, ni:0.15% -0.6%, cu: 0.1-0.6%, al:0.1 to 0.6 percent, and the balance of Fe and inevitable impurities.

4. The mold for hot working according to claim 3, wherein the hardness is 52HRC or more and the thermal conductivity is 25W/(m.K) or more.

5. The mold for hot working according to claim 3 or 4, characterized in that the working surface has a nitride layer.

6. A method for producing a hot-working die, characterized in that the steel for a hot-working die according to claim 1 or 2 is quenched and tempered at a quenching temperature of 1020 to 1080 ℃ and a tempering temperature of 540 to 620 ℃.

7. The method of manufacturing a hot-working die as claimed in claim 6, wherein after the quenching and tempering, a nitriding treatment is further performed on the working surface.