WO2020246099A1 - ホットスタンプ用金型用鋼、ホットスタンプ用金型およびその製造方法 - Google Patents

ホットスタンプ用金型用鋼、ホットスタンプ用金型およびその製造方法 Download PDF

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WO2020246099A1
WO2020246099A1 PCT/JP2020/010562 JP2020010562W WO2020246099A1 WO 2020246099 A1 WO2020246099 A1 WO 2020246099A1 JP 2020010562 W JP2020010562 W JP 2020010562W WO 2020246099 A1 WO2020246099 A1 WO 2020246099A1
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Prior art keywords
hardness
mold
steel
thermal conductivity
tempering
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English (en)
French (fr)
Japanese (ja)
Inventor
貴之 平重
志保 福元
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Proterial Ltd
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Hitachi Metals Ltd
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Priority to KR1020217038731A priority Critical patent/KR20220002523A/ko
Priority to EP20818816.9A priority patent/EP3981890A4/en
Priority to CN202080041017.5A priority patent/CN113939604A/zh
Priority to JP2021524675A priority patent/JP7540437B2/ja
Priority to US17/616,197 priority patent/US20220316038A1/en
Priority to KR1020247008574A priority patent/KR20240042117A/ko
Publication of WO2020246099A1 publication Critical patent/WO2020246099A1/ja
Anticipated expiration legal-status Critical
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D—WORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00—Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02—Stamping using rigid devices or tools
    • B21D22/022—Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D7/00—Modifying the physical properties of iron or steel by deformation
    • C21D7/13—Modifying the physical properties of iron or steel by deformation by hot working
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24—Nitriding
    • C23C8/26—Nitriding of ferrous surfaces
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
    • B21J13/00—Details of machines for forging, pressing, or hammering
    • B21J13/02—Dies or mountings therefor

Definitions

  • the present invention relates to steel for hot stamping dies, hot stamping dies, and a method for manufacturing the same.
  • 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 quenching in which the die is rapidly cooled. Another advantage is that it has excellent moldability, such as almost no springback.
  • the hot stamping method has a problem of low productivity. That is, the productivity is lowered because it takes time to maintain the bottom dead center for diquenching.
  • a mold with high thermal conductivity is required. This is because the heat of the steel sheet is absorbed by the mold in the die quenching, but the higher the thermal conductivity of the mold, the shorter the time for holding the bottom dead center and the higher the productivity. Further, the hot stamping die is required to have high hardness in order to improve wear resistance.
  • hot stamping die steel is required to have both high hardness and high thermal conductivity when made into a die.
  • 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?
  • the optimum composition of components is being investigated by controlling the amount of alloy.
  • Patent Documents 1 to 3 propose a composition of a mold steel having both hardness and thermal conductivity.
  • Patent Document 4 also discloses hot tool steel, which is useful as a material for dies used for hot pressing, die casting, hot forging, etc., has excellent thermal conductivity, and is also excellent in wear resistance. Has been done.
  • Patent Documents 1 to 3 and the hot tool steels of Patent Document 4 are useful for hot stamping.
  • conventional mold steel and hot steel are used.
  • the hardness may be insufficient.
  • Patent Documents 1 to 4 do not stably obtain a high hardness of 52 HRC or higher.
  • An object of the present invention is to provide a die steel suitable for a hot stamping method, which can produce a die having both high hardness and high thermal conductivity, a hot stamping die, and a method for producing the same. Is.
  • the present inventor has conducted diligent research and found the optimum mold steel for hot stamping by controlling the amount of alloy. Then, by using the above-mentioned mold steel, a mold for hot stamping capable of achieving high hardness and high thermal conductivity and a method for manufacturing the same were found.
  • one aspect of the present invention is C: 0.45 to 0.65%, Si: 0.1 to 0.6%, Mn: 0.1 to 0.3%, Cr: 2.
  • a mold for hot stamping having a component composition of 5 to 6.0%, Mo: 1.2 to 2.6%, V: 0.4 to 0.8%, balance Fe and unavoidable impurities. Steel for use.
  • 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 0.3%, Cr: 2.
  • a mold for hot stamping having a component composition of 5 to 6.0%, Mo: 1.2 to 2.6%, V: 0.4 to 0.8%, balance Fe and unavoidable impurities.
  • the hot stamping die according to claim 2 preferably having a hardness of 45 HRC or more and a thermal conductivity ⁇ (W / (m ⁇ K)) satisfying the following formula (1). Is. ⁇ ⁇ ⁇ 0.5H + 53... Equation (1)
  • H Rockwell hardness of mold (HRC) More preferably, the hardness is 52 HRC or more. At this time, more preferably, the thermal conductivity ⁇ is 25 W / (m ⁇ K) or more. Further, preferably, a nitride layer is provided on the working surface.
  • Another aspect of the present invention is to perform quenching and tempering of the above-mentioned steel for a hot stamping die at a quenching temperature of 1020 to 1080 ° C. and a tempering temperature of 500 to 625 ° C. It is a manufacturing method of.
  • the tempering temperature is 540-600 ° C.
  • the working surface is further subjected to nitriding treatment.
  • the optimum mold steel for hot stamping can be obtained. Further, by using this die steel, it is possible to provide a die for hot stamping having both high hardness and high thermal conductivity, and a method for manufacturing the same.
  • the feature of the present invention is that the hot stamping die is manufactured by quenching and tempering the die steel and nitriding the working surface thereof.
  • the optimum quenching and tempering conditions for simultaneously achieving the above-mentioned high hardness and high thermal conductivity have been identified.
  • Each constituent requirement of the present invention will be described below.
  • the hot stamping die steel 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 0.3%, Cr: 2.5 to 6.0%, Mo: 1.2 to 2.6%, V: 0.4 to 0.8%, balance Fe and unavoidable impurities Has a component composition of.
  • 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.
  • 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 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 0.3%.
  • the lower limit of Mn is preferably 0.15% or more.
  • the upper limit of Mn is preferably 0.28% or less. A more preferable upper limit of Mn is 0.26% or less.
  • Cr is an element that dissolves in the substrate to increase its hardness.
  • it is 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.
  • 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). is there.
  • 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 stamping mold, it is effective to raise the tempering temperature. is there.
  • the tempering temperature is raised (for example, even if it exceeds 600 ° C.), it is sufficient that it is 45 HRC or more. Since the hardness can be maintained, the thermal conductivity can be increased at the same time. Then, even if the tempering temperature is set to, for example, 540 ° C. or higher, a hardness of 52 HRC or higher can be achieved, and a hot stamping die having a thermal conductivity of 25 W / (m ⁇ K) or higher can be obtained. ..
  • the thermal conductivity is further improved to 28 W / (m ⁇ K) or more and 30 W / (m ⁇ K) or more.
  • the above hardness and thermal conductivity are values measured at room temperature (normal temperature).
  • the nitriding characteristics of the mold steel can be improved by increasing the Cr content, for example, the work surface of the mold after quenching and tempering can be further nitrided to perform the nitriding treatment on the mold. Abrasion resistance (hardness of work surface) can be improved.
  • Cr is set to 2.5 to 6.0%. It is preferably 2.8% or more. More preferably, it is 3.0% or more. Further, it is preferably 5.5% or less, more preferably 4.8% or less, and further 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 is an element that dissolves in the substrate to increase its hardness, and also increases its hardness by forming carbides, and is an element that contributes to secondary curing during tempering. is there. It is also an element that improves hardenability.
  • Mo is set to 1.2 to 2.6%. 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 2.5% or less. More preferably, it is 2.3% or less. More preferably, it is 2.1% or less.
  • 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 will increase, and the thermal conductivity of the mold will decrease. Therefore, V is set to 0.4 to 0.8%. It is preferably 0.5% or more. Further, it is preferably 0.75% or less, more preferably 0.65% or less, still more preferably 0.6% or less.
  • 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. ..
  • elemental species not specified here for example, elemental species such as P, S, Cu, Al, Ca, Mg, O (oxygen), N (nitrogen)
  • P is preferably regulated to 0.05% or less.
  • S is preferably regulated to 0.03% or less. If the amount of S is too large, the hot workability deteriorates when the ingot is divided. Therefore, S is preferably regulated to 0.01% or less. More preferably, it is regulated to 0.008% or less.
  • Ni is useful as an elemental species that contributes to improving the toughness of the mold, but its content is also in that it suppresses a decrease in the thermal conductivity of the mold due to an increase in the alloy amount of the mold steel. It is preferable to keep the amount low. Then, as the upper limit of regulation of the amount of Ni, preferably 0.25% is allowed.
  • the mold for hot stamping of the present invention having excellent hardness and thermal conductivity can be obtained.
  • the hardness of the hot stamping die of the present invention is a value measured at room temperature (normal temperature), and it is easy to achieve a sufficient hardness such as 45 HRC or more.
  • the tempering temperature the hardness of the mold can be preferably 52 HRC or more, and excellent wear resistance can be imparted to the mold during use.
  • the hardness of the mold is more preferably 53 HRC or more, still more preferably 55 HRC or more. In the present invention, it is not necessary to specify the upper limit of the hardness of the mold.
  • the upper limit of this hardness is preferably 58 HRC or less in that the tempering temperature can be increased beyond the above peak hardness (that is, the thermal conductivity can be increased).
  • the hardness of the mold is adjusted to 45 HRC or more, and further, the thermal conductivity ⁇ satisfying the following formula (1). It is characterized by having (W / (m ⁇ K)). ⁇ ⁇ ⁇ 0.5H + 53... Equation (1)
  • H in the formula (1) is the Rockwell hardness (HRC) of the mold.
  • HRC Rockwell hardness
  • the thermal conductivity is 30.5 W / (m ⁇ K) or more.
  • the thermal conductivity is 27 W / (m ⁇ K) or more.
  • it is preferably " ⁇ ⁇ ⁇ 0.5H + 54".
  • This thermal conductivity is a value measured at room temperature (normal temperature). Since the mold steel of the present invention satisfies the relationship of the formula (1) by quenching and tempering, even when the tempering hardness is in the high hardness range (for example, 52 HRC or more) where the decrease in thermal conductivity has been a problem. It is possible to maintain a thermal conductivity of 25 W / (m ⁇ K) or more. When the hardness of the mold is 52 HRC or more, the preferable thermal conductivity is 28 W / (m ⁇ K) or more. A more preferable thermal conductivity is 30 W / (m ⁇ K) or more.
  • the tempering hardness is in the low hardness range (for example, less than 52 HRC), it is possible to achieve a thermal conductivity of 30 W / (m ⁇ K) or more, even if the hardness is around 45 HRC. For example, it is possible to achieve a thermal conductivity of 32 W / (m ⁇ K) or more. This makes it possible to maintain high thermal conductivity in the mold used in the hot stamping method (for example, 100 to 400 ° C.). Such thermal conductivity can be easily achieved by increasing the tempering temperature in addition to the composition of the mold steel described above. For example, the thermal conductivity can be adjusted to 30 W / (m ⁇ K) or more by raising the tempering temperature to a temperature higher than the temperature at which the peak hardness can be obtained.
  • the thermal conductivity of the mold it is not necessary to specify the upper limit of the thermal conductivity of the mold.
  • the hardness of the mold decreases as the tempering temperature is raised (for example, adjusted to a temperature exceeding 600 ° C.) when the hardness of the mold falls below 45 HRC.
  • the thermal conductivity exceeds 50 W / (m ⁇ K)
  • the upper limit of the thermal conductivity is about 40 W / (m ⁇ K). It is preferably 38 W / (m ⁇ K) or less. More preferably, it is 35 W / (m ⁇ K) or less. From these upper limits of thermal conductivity, the relationship between the above-mentioned thermal conductivity ⁇ (W / (m ⁇ K)) and Rockwell hardness H (HRC) is approximately the formula “ ⁇ ⁇ ⁇ 0.5H + 70”. It is realistic that the relationship is (2). It is preferably " ⁇ ⁇ -0.5H + 66", and more preferably " ⁇ ⁇ -0.5H + 61".
  • the hot stamping die of the present invention preferably has a nitride layer on its working surface.
  • the hot stamping die of the present invention has both high hardness and high thermal conductivity.
  • the wear resistance (hardness of the working surface) of the mold can be further improved.
  • the working surface is the surface of the mold in contact with the steel plate in the hot stamp.
  • the method for manufacturing a hot stamping die of the present invention is to quench and temper the above-mentioned die steel.
  • 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.
  • tempering the mold steel hardened at this quenching temperature at a tempering temperature of, for example, 500 to 625 ° C. sufficient hardness of 45 HRC or more can be maintained.
  • the quenching temperature and tempering temperature at this time can be selected so that the hardness of the mold after quenching and tempering and the thermal conductivity satisfy the relationship of the above-mentioned formula (1). Further, tempering at a high temperature is also effective in maintaining sufficient hardness of the mold and increasing the thermal conductivity of the mold. For example, at a tempering temperature of 540 ° C. or higher, Since a hardness of 52 HRC or more can be achieved, a mold having a thermal conductivity of 25 W / (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 mold steel of the present invention is prepared into a hot stamping mold having a predetermined hardness by quenching and tempering. During this period, the die steel is adjusted to the shape of the hot stamping die 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.
  • the method for producing a hot stamping die of the present invention preferably further nitrides the working surface of the die after the above quenching and tempering.
  • a mold having a hardness of 45 HRC or more and a thermal conductivity satisfying the formula (1) can be obtained. ..
  • the mold steel having the above-mentioned composition is also excellent in nitriding characteristics, the work 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.
  • 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 8 and No. 1 which is a comparative example. Steels 9-11 were made.
  • Nos. 1 to 8 maintained a tempering hardness of 45 HRC or more over the entire tempering temperature of 500 to 625 ° C. In particular, about 52 HRC or more was obtained in the range of tempering temperature of 540 to 600 ° C. Further, even if the tempering temperature was raised to more than 600 ° C., which is effective for increasing the thermal conductivity of the mold, the tempering hardness of about 45 HRC or more was achieved.
  • No. No. 9 also maintained a tempering hardness of 45 HRC or more in the tempering temperature range of 500 to 600 ° C.
  • No. 10 had a tempering hardness of less than 45 HRC when the tempering temperature was 575 ° C.
  • No. No. 11 had a tempering hardness of 45 HRC or more in a tempering temperature range of 500 to 625 ° C., but a hardness of 50 HRC or more could not be obtained.
  • No. 1 which is an example of the present invention.
  • 1 to 6 satisfy the equation of thermal conductivity of ⁇ ⁇ -0.5H + 53 at all hardnesses of 45HRC, 50HRC, and 55HRC, and even when the hardness is increased to 52HRC, 30 W / (m ⁇ K). It can be seen that the above high thermal conductivity is maintained. Further, even at a high hardness of 55 HRC, it had a high thermal conductivity of 25 W / (m ⁇ K) or more. On the other hand, No. In No.
  • the thermal conductivity was small at the time of tempering to a low hardness of 45HRC and 50HRC (not satisfying the equation of ⁇ -0.5H + 53), and even if the hardness was increased to 52HRC, ⁇ - I heard that they are not satisfied with 0.5H + 53.
  • the thermal conductivity was measured when the tempering hardness was 52 HRC.
  • the measurement procedure is described in No. 1 described above. It is the same as the time of 1 to 6 and 9.
  • No. The thermal conductivity of No. 7 is 31 W / (m ⁇ K)
  • No. It was confirmed that the thermal conductivity of No. 8 was 37 W / (m ⁇ K), and that even with a hardness of 52 HRC, it had a high thermal conductivity of 30 W / (m ⁇ K) or more.

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