WO2020246099A1 - Steel for hot stamp die, hot stamp die and manufacturing method thereof - Google Patents

Steel for hot stamp die, hot stamp die and manufacturing method thereof Download PDF

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Publication number
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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PCT/JP2020/010562
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French (fr)
Japanese (ja)
Inventor
貴之 平重
志保 福元
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日立金属株式会社
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Application filed by 日立金属株式会社 filed Critical 日立金属株式会社
Priority to US17/616,197 priority Critical patent/US20220316038A1/en
Priority to KR1020217038731A priority patent/KR20220002523A/en
Priority to JP2021524675A priority patent/JPWO2020246099A1/ja
Priority to EP20818816.9A priority patent/EP3981890A4/en
Priority to CN202080041017.5A priority patent/CN113939604A/en
Priority to KR1020247008574A priority patent/KR20240042117A/en
Publication of WO2020246099A1 publication Critical patent/WO2020246099A1/en

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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 by deformation combined with, or followed by, heat treatment
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties 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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Abstract

A die steel which enables manufacturing a hot stamp die that has both high hardness and high thermal conductivity, a hot stamp die, and a manufacturing method thereof are provided.　This steel for a hot stamp die has a component composition, in mass% of 0.45-0.65% C, 0.1-0.6% Si, 0.1-0.3% Mn, 2.5-6.0% Cr, 1.2-2.6% Mo, and 0.4-0.8% V, the remainder being Fe and unavoidable impurities. Further, this hot stamp die has the aforementioned component composition, and the manufacturing method is for manufacturing said hot stamp die.

Description

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Steel for hot stamping dies, dies for hot stamping and their manufacturing methods
　本発明は、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に関するものである。 The present invention relates to steel for hot stamping dies, hot stamping dies, and a method for manufacturing the same.
　近年、自動車の軽量化と衝突安全性向上を目的に、引張強さが１ＧＰａを超える超高張力鋼板のニーズが高まっている。しかし、引張強さが１．２ＧＰａ以上の鋼板を冷間プレスで成形しようとすると、成形荷重やスプリングバックの増大、成形性などの問題が発生する。そこで、最近ではホットスタンプ（ホットプレス、もしくはホットスタンピングとも称する）工法が注目されている。ホットスタンプ工法では、鋼板をオーステナイト温度以上に加熱後、プレス成形し、金型を下死点で保持し急冷して焼入れする。 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 a steel sheet having a tensile strength of 1.2 GPa or more is to be formed 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.
　ホットスタンプ工法の利点として、金型で急冷するダイクエンチングによる焼入れによって、１．５ＧＰａ程度の引張強さを持つ超高張力鋼板の成形品が得られることが挙げられる。また、スプリングバックがほとんど生じないなど成形性が優れているという利点も挙げられる。
　しかし、ホットスタンプ工法は生産性が低いという問題がある。つまり、ダイクエンチングのための下死点保持などに時間が必要となるため、生産性が低くなる。その対策として、高熱伝導率の金型が求められている。これは、ダイクエンチングでは鋼板の熱を金型に吸収させているが、金型の熱伝導率が高いほど、下死点保持の時間が短縮されて生産性が高くなるからである。
　また、ホットスタンプ用金型では、耐摩耗性を高めるために高硬度が求められている。 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.
However, 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. As a countermeasure, 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.
　したがって、ホットスタンプ用金型用鋼では、金型にしたときに高硬度と高熱伝導率とを合わせ持つことが求められる。一般に、高硬度の金型を得るには金型用鋼の合金量を増やす必要があるが、合金量が多くなると金型の熱伝導率が下がるという問題があり、硬度と熱伝導率とはトレードオフの関係にある。そこで、合金量を制御することで最適な成分組成が検討されている。例えば、特許文献１～３では、硬さと熱伝導率とを合わせもつ金型用鋼の成分組成が提案されている。また特許文献４にも、温熱間プレス、ダイカスト、又は温熱間鍛造等に使用される金型の素材として有用であり、熱伝導率が優れており耐摩耗性にも優れる熱間工具鋼について開示されている。 Therefore, hot stamping die steel is required to have both high hardness and high thermal conductivity when made into a die. 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 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.
特開２０１７－４３８１４号公報Japanese Unexamined Patent Publication No. 2017-43814 特開２０１７－５３０２３号公報JP-A-2017-53023 特開２０１８－２４９３１号公報Japanese Unexamined Patent Publication No. 2018-24931 特許第５７４４３００号公報Japanese Patent No. 5744300
　特許文献１～３の金型用鋼、および特許文献４の熱間工具鋼は、ホットスタンプ用として有用である。しかし、金型用鋼や熱間工具鋼の焼入れ焼戻し特性や、ホットスタンプ用金型の作業面が窒化処理されて使用されること等を考えたときに、従来の金型用鋼や熱間工具鋼の場合、硬度が不足する場合があった。具体的には、最近ではホットスタンプ用鋼として５２ＨＲＣ以上の高硬度化が求められてきているが、特許文献１～４では５２ＨＲＣ以上の高硬度は安定的に得られない。
　本発明の目的は、ホットスタンプ工法に適した、高硬度と高熱伝導率とを合わせ持つ金型を作製することができる金型用鋼と、ホットスタンプ用金型およびその製造方法を提供することである。 The mold steels of Patent Documents 1 to 3 and the hot tool steels of Patent Document 4 are useful for hot stamping. However, considering the quenching and quenching characteristics of mold steel and hot tool steel, and the fact that the work surface of the hot stamping mold is nitrided and used, conventional mold steel and hot steel are used. In the case of tool steel, the hardness may be insufficient. Specifically, recently, there has been a demand for a steel for hot stamping having a hardness of 52 HRC or higher, but 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.
　かかる実状に鑑み、本発明者は鋭意研究を行った結果、合金量を制御することでホットスタンプ用に最適な金型用鋼を見いだした。そして、上記の金型用鋼を用いることで、高硬度および高熱伝導率を達成できるホットスタンプ用金型と、その製造方法を見いだした。 In view of this situation, 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.
　すなわち、本発明の一態様は、質量％で、Ｃ：０．４５～０．６５％、Ｓｉ：０．１～０．６％、Ｍｎ：０．１～０．３％、Ｃｒ：２．５～６．０％、Ｍｏ：１．２～２．６％、Ｖ：０．４～０．８％、残部Ｆｅおよび不可避的不純物の成分組成を有することを特徴とするホットスタンプ用金型用鋼である。 That is, 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.
　本発明の他の一態様は、質量％で、Ｃ：０．４５～０．６５％、Ｓｉ：０．１～０．６％、Ｍｎ：０．１～０．３％、Ｃｒ：２．５～６．０％、Ｍｏ：１．２～２．６％、Ｖ：０．４～０．８％、残部Ｆｅおよび不可避的不純物の成分組成を有することを特徴とするホットスタンプ用金型である。
　好ましくは、硬さが４５ＨＲＣ以上、かつ熱伝導率λ（Ｗ／（ｍ・Ｋ））が下記の式（１）を満足することを特徴とする、請求項２に記載のホットスタンプ用金型である。
λ≧－０．５Ｈ＋５３　…式（１）
Ｈ：金型のロックウェル硬さ（ＨＲＣ）
　より好ましくは、硬さが５２ＨＲＣ以上である。そして、このとき、さらに好ましくは、熱伝導率λが２５Ｗ／（ｍ・Ｋ）以上である。
　また、好ましくは、作業面に窒化層を有する。 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. Is.
The hot stamping die according to claim 2, preferably having a hardness of 45 HRC or more and a thermal conductivity λ (W / (m · K)) satisfying the following formula (1). Is.
λ ≧ −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 λ is 25 W / (m · K) or more.
Further, preferably, a nitride layer is provided on the working surface.
　本発明の他の一態様は、上記のホットスタンプ用金型用鋼に、１０２０～１０８０℃の焼入れ温度および５００～６２５℃の焼戻し温度による焼入れ焼戻しを行うことを特徴とするホットスタンプ用金型の製造方法である。
　好ましくは、上記の焼戻し温度が５４０～６００℃である。
　好ましくは、前記焼入れ焼戻しを行った後に、さらに、作業面に窒化処理を行うことを特徴とする。 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.
Preferably, the tempering temperature is 540-600 ° C.
Preferably, after the quenching and tempering, the working surface is further subjected to nitriding treatment.
　本発明によれば、ホットスタンプ用に最適な金型用鋼が得られる。また、この金型用鋼を用いることで、高硬度と高熱伝導率とを併せ持つホットスタンプ用金型と、その製造方法を提供できる。 According to the present invention, 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.
本発明例および比較例の金型用鋼を１０３０℃から焼入れ後、５００～６５０℃で焼戻して作製した金型の一例について、その焼戻し温度毎の硬度を示すグラフ図である。It is a graph which shows the hardness for each tempering temperature about an example of the mold manufactured by quenching the mold steel of this invention example and comparative example from 1030 degreeC, and then tempering at 500 to 650 ° C. 本発明例および比較例の金型用鋼を１０３０℃から焼入れ後、４５ＨＲＣ、５０ＨＲＣ、５５ＨＲＣの硬さに焼戻して作製した金型の一例について、その熱伝導率を示すグラフ図である。It is a graph which shows the thermal conductivity of an example of a mold produced by quenching the mold steel of the present invention example and the comparative example from 1030 ° C. and then tempering to a hardness of 45HRC, 50HRC, 55HRC. 本発明例および比較例の金型用鋼を１０３０℃から焼入れ後、５５ＨＲＣの硬さに焼戻して作製した金型の一例について、６００℃で保持した場合の軟化抵抗を示すグラフ図である。It is a graph which shows the softening resistance when the mold steel of this invention example and comparative example was hardened from 1030 degreeC, and then tempered to the hardness of 55HRC, and held at 600 degreeC.
　本発明の特徴は、ホットスタンプ用金型が、金型用鋼に焼入れ焼戻しを行って作製されることや、その作業面に窒化処理を行って作製されることを考えたときに、ホットスタンプ用金型の高硬度と高熱伝導率とを同時に達成するのに最適な金型用鋼の成分組成があることをつきとめたところにある。特に５２ＨＲＣ以上の高硬度と、２５Ｗ／（ｍ・Ｋ）以上の高熱伝導率を同時に達成するのに最適な金型用鋼の成分組成があることをつきとめたところにある。また、上記の高硬度と高熱伝導率とを同時に達成するのに最適な焼入れ焼戻し条件をつきとめたところにある。以下に、本発明の各構成要件について説明する。 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. We have found that there is an optimum composition of mold steel to achieve high hardness and high thermal conductivity of the mold at the same time. In particular, it has been found that there is an optimum composition of mold steel for simultaneously achieving a high hardness of 52 HRC or higher and a high thermal conductivity of 25 W / (m · K) or higher. In addition, 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.
　本発明のホットスタンプ用金型用鋼は、質量％（以下、単に「％」と表記する。）で、Ｃ：０．４５～０．６５％、Ｓｉ：０．１～０．６％、Ｍｎ：０．１～０．３％、Ｃｒ：２．５～６．０％、Ｍｏ：１．２～２．６％、Ｖ：０．４～０．８％、残部Ｆｅおよび不可避的不純物の成分組成を有する。 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：０．４５～０．６５％
　Ｃは、焼入れにより素地（マトリックス）に固溶して、金型の硬さを向上させる元素である。また、後述するＣｒやＭｏ、Ｖなどの炭化物形成元素と炭化物を形成して、金型の硬さを向上させる元素である。しかし、Ｃ量が多すぎると、一次炭化物の粗大化などにより、金型の靭性が低下する。よって、Ｃは、０．４５～０．６５％とする。好ましくは０．４７％以上である。より好ましくは０．４９％以上である。また、好ましくは０．６３％以下である。より好ましくは０．６０％以下である。さらに好ましくは０．５８％以下である。 ・ 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は、溶製工程で脱酸剤として使用される。そして、素地に固溶して金型の硬さを向上させる元素である。しかし、Ｓｉが多すぎると、溶製後において鋼中の偏析傾向が強まり、また凝固組織も粗大になって、金型の靭性低下につながる。そして、焼入れ焼戻し後の金型の熱伝導率を著しく下げる元素である。よって、Ｓｉは、０．１～０．６％とする。好ましくは０．１４％以上である。より好ましくは０．１７％以上である。また、好ましくは０．４５％以下であり、より好ましくは０．４％以下である。さらに好ましくは０．３５％以下である。よりさらに好ましくは０．３％以下である。 ・ 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：０．１～０．３％
　Ｍｎは、溶製工程で脱酸剤や脱硫剤として使用される。そして、素地の強化や、焼入れ性、焼入れ焼戻し後の靭性の向上に寄与する元素である。しかし、Ｍｎが多すぎると、金型の熱伝導率が著しく低下する。よって、Ｍｎは、０．１～０．３％とする。好ましいＭｎの下限は０．１５％以上である。また、好ましいＭｎの上限は０．２８％以下である。より好ましいＭｎの上限は０．２６％以下である。 -Mn: 0.1 to 0.3%
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：２．５～６．０％
　Ｃｒは、素地に固溶して硬さを上昇させる元素である。また、炭化物を形成することでも硬さを上昇させる元素であり、後述するＭｏ、Ｖと同様、焼戻し時における二次硬化に寄与する元素である。特にＣｒは、Ｍｏ、Ｖに比べて、焼戻し軟化抵抗を大きくすることができる（焼戻し温度を高くしても、二次硬化で得られた硬さの低下割合を小さくすることができる）元素である。通常、金型は、金型用鋼に焼入れ焼戻しを行って使用硬さに調整されるところ、ホットスタンプ用金型の熱伝導率を高めるためには、焼戻し温度を高くするのが効果的である。そして、本発明においては、Ｃｒの含有量を２．５％以上とすることで、焼戻し温度を高くしてしても（例えば、６００℃を超えても）、４５ＨＲＣ以上といったような、十分な硬度を維持することができるので、同時に、熱伝導率を高くすることもできる。そして、焼戻し温度を、例えば、５４０℃以上にしても、５２ＨＲＣ以上の硬さを達成できて、かつ、熱伝導率が２５Ｗ／（ｍ・Ｋ）以上のホットスタンプ用金型を得ることができる。そして、上記の硬さを維持した上で、熱伝導率が、さらに、２８Ｗ／（ｍ・Ｋ）以上や、３０Ｗ／（ｍ・Ｋ）以上にまで向上されたホットスタンプ用金型を得ることができる。なお、上記の硬度および熱伝導率は、室温（常温）で測定したときの値である。
　また、Ｃｒの含有量を高くすることで、金型用鋼の窒化特性を向上させることができるので、例えば、焼入れ焼戻し後の金型の作業面に、さらに窒化処理を行うことで、金型の耐摩耗性（作業面の硬さ）を向上させることができる。 -Cr: 2.5-6.0%
Cr is an element that dissolves in the substrate to increase its hardness. In addition, 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. 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). is there. 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 stamping mold, it is effective to raise the tempering temperature. is there. Then, in the present invention, by setting the Cr content to 2.5% or more, even if 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. .. Then, while maintaining the above hardness, it is possible to obtain a hot stamping die in which the thermal conductivity is further improved to 28 W / (m · K) or more and 30 W / (m · K) or more. Can be done. The above hardness and thermal conductivity are values measured at room temperature (normal temperature).
Further, since 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.
　但し、Ｃｒの含有量が多すぎると、金型用鋼の合金量が多くなるということ自体によって、金型の熱伝導率を高くするのが難しくなる。よって、Ｃｒは、２．５～６．０％とする。好ましくは２．８％以上である。より好ましくは３．０％以上である。また、好ましくは５．５％以下であり、より好ましくは４．８％以下、さらに好ましくは４．５％未満である。そして、特に熱伝導率の向上を重視したい場合、Ｃｒは、４．０％以下や、３．５以下にすることもできる。 However, if the Cr content is too high, the amount of alloy in the mold steel increases, which makes it difficult to increase the thermal conductivity of the mold. Therefore, 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：１．２～２．６％
　Ｍｏは、Ｃｒと同様、素地に固溶して硬さを上昇させる元素であり、また、炭化物を形成することでも硬さを上昇させる元素であり、焼戻し時における二次硬化に寄与する元素である。また、焼入れ性を向上させる元素でもある。但し、Ｍｏ量が多すぎると、金型用鋼の合金量が多くなるということ自体によって、金型の熱伝導率が低くなる。よって、Ｍｏは、１．２～２．６％とする。好ましくは１．５％以上である。より好ましくは１．７％以上である。さらに好ましくは１．９％以上である。また、好ましくは２．５％以下である。より好ましくは２．３％以下である。さらに好ましくは２．１％以下である。 ・ Mo: 1.2-2.6%
Like Cr, 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. However, if the amount of Mo is too large, the amount of alloy in the mold steel will be large, and the thermal conductivity of the mold will be low. Therefore, 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：０．４～０．８％
　Ｖは、Ｃｒと同様、炭化物を形成することでも硬さを上昇させる元素であり、焼戻し時における二次硬化に寄与する元素である。但し、Ｖ量が多すぎると、金型用鋼の合金量が多くなるということ自体によって、金型の熱伝導率が低くなる。よって、Ｖは、０．４～０．８％とする。好ましくは０．５％以上である。また、好ましくは０．７５％以下であり、より好ましくは０．６５％以下、さらに好ましくは０．６％以下である。 ・ V: 0.4-0.8%
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 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.
・残部Ｆｅおよび不可避的不純物
　金型用鋼の合金量が多くなると、金型の熱伝導率が低くなることを考えれば、上記の元素種以外の残部は、実質的にＦｅでなることが好ましい。但し、ここに明示しない元素種（例えば、Ｐ、Ｓ、Ｃｕ、Ａｌ、Ｃａ、Ｍｇ、Ｏ（酸素）、Ｎ（窒素）等の元素種）は、不可避的に鋼中に残留する可能性がある元素であり、これらの元素を不純物として含むことは許容される。このとき、Ｐは、多すぎると、焼戻しなどの熱処理時に旧オーステナイト粒界に偏析して、金型の靭性が劣化する。よって、Ｐは、０．０５％以下に規制することが好ましい。より好ましくは０．０３％以下に規制する。そして、Ｓは、多すぎると、鋼塊を分塊するときなどにおいて熱間加工性が劣化する。よって、Ｓは、０．０１％以下に規制することが好ましい。より好ましくは０．００８％以下に規制する。
　また、Ｎｉは、金型の靭性向上に寄与する元素種として有用ではあるが、金型用鋼の合金量の増加による金型の熱伝導率の低下を抑制する点で、やはり、その含有量を低く抑えることが好ましい。そして、Ｎｉ量の規制上限として、好ましくは０．２５％が許容される。 -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, Cu, Al, Ca, Mg, O (oxygen), N (nitrogen)) may inevitably remain in the steel. It is an element, and 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 ingot is divided. Therefore, S is preferably regulated to 0.01% or less. More preferably, it is regulated to 0.008% or less.
In addition, 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.
　上記の成分組成を有した金型用鋼に焼入れ焼戻しを行うことで、硬さと熱伝導率性に優れた本発明のホットスタンプ用金型を得ることができる。本発明のホットスタンプ用金型の硬さは、室温（常温）で測定した値で、例えば４５ＨＲＣ以上といった、十分な硬度を達成することが容易である。そして、焼戻し温度を調整することで、金型の硬さを、好ましくは５２ＨＲＣ以上にすることができて、使用時における金型に優れた耐摩耗性を付与することができる。金型の硬さは、より好ましくは５３ＨＲＣ以上であり、さらに好ましくは５５ＨＲＣ以上である。
　なお、本発明において、金型の硬さの上限を規定することは要しない。但し、上記の成分組成を有した金型用鋼の場合、その二次硬化のピーク硬さ（概ね５００～６００℃の焼戻し温度の範囲にある）から、６０ＨＲＣ程度であることが現実的である。そして、この硬さの上限について、５８ＨＲＣ以下とすることが、上記のピーク硬さを超えて焼戻し温度を高くできる点で（すなわち、熱伝導率を高くできる点で）、好ましい。 By quenching and tempering the mold steel having the above-mentioned component composition, 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. Then, by adjusting 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. 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 500 to 600 ° C.). .. 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).
　そして、上記の成分組成を有した金型用鋼に焼入れ焼戻しを行うことで、金型の硬さを４５ＨＲＣ以上に調整した上で、さらに、下記の式（１）を満足する熱伝導率λ（Ｗ／（ｍ・Ｋ））を有することを特徴とする。
λ≧－０．５Ｈ＋５３　…式（１）
　ここで、式（１）のＨは金型のロックウェル硬さ（ＨＲＣ）である。例えば、本実施形態の金型の硬さが４５ＨＲＣの場合、熱伝導率は３０．５Ｗ／（ｍ・Ｋ）以上である。また、金型の硬さが５２ＨＲＣの場合、熱伝導率は２７Ｗ／（ｍ・Ｋ）以上である。そして、好ましくは「λ≧－０．５Ｈ＋５４」である。なお、この熱伝導率は、室温（常温）で測定した値である。
　本発明の金型用鋼は、焼入れ焼戻しによって式（１）の関係を満足することから、熱伝導率の低下が課題であった高硬度域（例えば５２ＨＲＣ以上）の焼戻し硬さのときでも、２５Ｗ／（ｍ・Ｋ）以上の熱伝導率を維持することができる。金型の硬さが５２ＨＲＣ以上のとき、好ましい熱伝導率は２８Ｗ／（ｍ・Ｋ）以上である。より好ましい熱伝導率は３０Ｗ／（ｍ・Ｋ）以上である。そして、低硬度域（例えば５２ＨＲＣ未満）の焼戻し硬さのときであれば、３０Ｗ／（ｍ・Ｋ）以上の熱伝導率の達成も可能であるし、それこそ、４５ＨＲＣ付近の硬さであれば、３２Ｗ／（ｍ・Ｋ）以上の熱伝導率の達成も可能である。このことによって、ホットスタンプ工法に使用中（例えば、１００～４００℃）の金型で高い熱伝導率を維持することができる。
　このような熱伝導率は、上記の金型用鋼の成分組成に加えて、焼戻し温度を高くすることで、達成が容易である。例えば、焼戻し温度をピーク硬さが得られる温度以上に高くすることで、熱伝導率３０Ｗ／（ｍ・Ｋ）以上に調整することが可能である。 Then, by quenching and tempering the mold steel having the above component composition, the hardness of the mold is adjusted to 45 HRC or more, and further, the thermal conductivity λ satisfying the following formula (1). It is characterized by having (W / (m · K)).
λ ≧ −0.5H + 53… Equation (1)
Here, H in the formula (1) is the Rockwell hardness (HRC) of the mold. For example, when the hardness of the mold of the present embodiment is 45 HRC, the thermal conductivity is 30.5 W / (m · K) or more. When the hardness of the mold is 52 HRC, the thermal conductivity is 27 W / (m · K) or more. And it is preferably "λ ≧ −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. If 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.
　本発明の場合、金型の熱伝導率の上限を特定する必要はない。但し、焼戻し温度を高くしていって（例えば、６００℃を超える温度に調整して）、金型の硬さが低下していくことを考えれば、金型の硬さが４５ＨＲＣを下回るときの熱伝導率が５０Ｗ／（ｍ・Ｋ）を上回ることから、４５ＨＲＣ以上の硬さを維持しているときで、凡そ５０Ｗ／（ｍ・Ｋ）程度であることが現実的である。好ましくは４７Ｗ／（ｍ・Ｋ）以下である。より好ましくは４５Ｗ／（ｍ・Ｋ）以下である。そして、金型が５２ＨＲＣ以上の硬さを維持しているときであれば、熱伝導率の上限は凡そ４０Ｗ／（ｍ・Ｋ）程度であることが現実的である。好ましくは３８Ｗ／（ｍ・Ｋ）以下である。より好ましくは３５Ｗ／（ｍ・Ｋ）以下である。そして、これら熱伝導率の上限より、上述した熱伝導率λ（Ｗ／（ｍ・Ｋ））とロックウェル硬さＨ（ＨＲＣ）との関係は、凡そ「λ≦－０．５Ｈ＋７０」の式（２）の関係であることが現実的である。好ましくは、「λ≦－０．５Ｈ＋６６」であり、より好ましくは、「λ≦－０．５Ｈ＋６１」である。 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.), when the hardness of the mold falls below 45 HRC. Since the thermal conductivity exceeds 50 W / (m · K), it is realistic that the hardness is about 50 W / (m · K) when the hardness of 45 HRC or more is maintained. 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. 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 λ (W / (m · K)) and Rockwell hardness H (HRC) is approximately the formula “λ ≦ −0.5H + 70”. It is realistic that the relationship is (2). It is preferably "λ ≦ -0.5H + 66", and more preferably "λ ≦ -0.5H + 61".
　本発明のホットスタンプ用金型は、好ましくは、その作業面に窒化層を有するものである。
　上述の通り、本発明のホットスタンプ用金型は、高硬度および高熱伝導率を合わせ持ったものである。そして、この金型の作業面が、さらに窒化層を有することで、金型の耐摩耗性（作業面の硬さ）を、さらに向上させることができる。なお、作業面とは、ホットスタンプ中の鋼板と接する金型の面のことである。 The hot stamping die of the present invention preferably has a nitride layer on its working surface.
As described above, the hot stamping die of the present invention has both high hardness and high thermal conductivity. Further, 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. The working surface is the surface of the mold in contact with the steel plate in the hot stamp.
　本発明のホットスタンプ用金型の製造方法は、上記の金型用鋼に、焼入れ焼戻しを行うものである。
　上記の成分組成を有した金型用鋼に焼入れ焼戻しを行うとき、焼入れ温度は、狙い硬さ等によって異なるが、例えば、概ね１０２０～１０８０℃とすることができる。好ましくは１０５０℃以下である。
　そして、この焼入れ温度による焼入れを行った金型用鋼に、例えば、５００～６２５℃の焼戻し温度による焼戻しを行うことで、４５ＨＲＣ以上の十分な硬さを維持することができる。このときの焼入れ温度および焼戻し温度は、焼入れ焼戻し後の金型の硬さと熱伝導率とが、上述した式（１）の関係を満たすように選択することができる。
　そして、高温による焼戻しを行うことでも、十分な金型の硬さを維持して、かつ、金型の熱伝導率を高くすることに効果的であり、例えば、５４０℃以上の焼戻し温度で、５２ＨＲＣ以上の硬さを達成できるから、熱伝導率が２５Ｗ／（ｍ・Ｋ）以上の金型を得ることができる。このとき、５２ＨＲＣ以上の硬さを維持する上で、焼戻し温度の上限は６００℃程度とすることが好ましい。より好ましくは５９５℃以下である。さらに好ましくは５９０℃以下である。 The method for manufacturing a hot stamping 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 component 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, by 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.
　本発明の金型用鋼は、焼入れ焼戻しによって所定の硬さを有したホットスタンプ用金型に整えられる。そして、この間で、金型用鋼は、切削や穿孔といった各種の機械加工等によって、ホットスタンプ用金型の形状に整えられる。この機械加工のタイミングは、焼入れ焼戻し前の硬さが低い状態（つまり、焼鈍状態）で行うことができる。そして、この場合、焼入れ焼戻し後に仕上げ加工を行ってもよい。また、場合によっては、上記の仕上げ加工も合わせて、焼入れ焼戻しを行った後のプリハードン状態で、上記の機械加工を行ってもよい。 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.
　本発明のホットスタンプ用金型の製造方法は、好ましくは、上記の焼入れ焼戻しを行った後の金型の作業面に、さらに、窒化処理を行うものである。
　上述の通り、上記の成分組成を有した金型用鋼に焼入れ焼戻しを行うことで、例えば、硬さが４５ＨＲＣ以上、かつ熱伝導率が式（１）を満足する金型を得ることができる。そして、上記の成分組成を有した金型用鋼は、窒化特性にも優れているので、この焼入れ焼戻しを行った後の金型の作業面に、さらに、窒化処理を行うことで、金型の耐摩耗性（作業面の硬さ）を向上させることができる。このとき、窒化処理の条件には、例えば、ガス窒化処理や塩浴窒化処理といった、既知の各種窒化処理のものを適用することができる。 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.
As described above, by quenching and tempering the mold steel having the above-mentioned composition, for example, a mold having a hardness of 45 HRC or more and a thermal conductivity satisfying the formula (1) can be obtained. .. Since 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. 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.
　表１の成分組成を有する、１０ｋｇの鋼塊を溶製した。そして、この鋼塊を１１６０℃に加熱してハンマー鍛伸した後に放冷し、この放冷後の鋼材に８７０℃の焼鈍処理を行って、本発明例であるＮｏ．１～８の鋼、および比較例であるＮｏ．９～１１の鋼を作製した。 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.
Figure JPOXMLDOC01-appb-T000001
Figure JPOXMLDOC01-appb-T000001
＜焼戻し硬さの評価＞
　Ｎｏ．１～１１の金型用鋼に、１０３０℃の焼入れ温度による焼入れを実施した。このとき、冷却条件は、本発明鋼および比較鋼といった金型用鋼が実際のホットスタンプ用金型の大きさであるときの冷却速度を想定して、半冷時間を４０分とした（半冷時間とは、焼入れ温度から、（焼入れ温度＋室温）／２の温度までの冷却に要する時間のことである）。そして、この焼入れ後の金型用鋼に、５００～６５０℃の焼戻し温度による焼戻しを行った。焼戻しは２回実施し、それぞれの温度で２時間保持した。焼戻し温度は、２５℃刻みの、計７条件とした。そして、Ｎｏ．１～１１のそれぞれについて、焼戻し温度毎に、その中心部の室温におけるロックウェル硬さ（Ｃスケール）を測定した。結果を図１に示す。 <Evaluation of tempering hardness>
No. The mold steels 1 to 11 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 11, the Rockwell hardness (C scale) at room temperature was measured at the center of each tempering temperature. The results are shown in FIG.
　本発明例であるＮｏ．１～８は、５００～６２５℃の焼戻し温度の全般に亘って、４５ＨＲＣ以上の焼戻し硬さを維持した。そして特に、いずれも５４０～６００℃の焼戻し温度の範囲で概ね５２ＨＲＣ以上が得られた。また、焼戻し温度を、金型の熱伝導率を高めるのに効果的とされる、６００℃超に高めても、概ね４５ＨＲＣ以上の焼戻し硬さを達成した。
　これに対して、比較例であるＮｏ．９も、５００～６００℃の焼戻し温度範囲では４５ＨＲＣ以上の焼戻し硬さを維持したが、Ｎｏ．１０は、焼戻し温度が５７５℃の時点で、既に、焼戻し硬さが４５ＨＲＣを下回っていた。Ｎｏ．１１は焼戻し温度が５００～６２５℃の範囲で焼戻し硬さが４５ＨＲＣ以上であったが、５０ＨＲＣ以上の硬度は得られなかった。 No. which is an example of the present invention. 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.
On the other hand, 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.
＜熱伝導率の評価＞
　上記の＜焼戻し硬さの評価＞の結果を踏まえて、Ｎｏ．１～６、９について焼戻し硬さが４５ＨＲＣ、５０ＨＲＣ、５５ＨＲＣのときの熱伝導率を測定した。測定要領は、まず、金型を直径１０ｍｍ×厚さ２ｍｍの円盤状の試験片に加工して、この試験片の熱拡散率および比熱をレーザーフラッシュ法により測定した。そして、この測定した熱拡散率および比熱の値を用いて、下記の式（３）より室温における熱伝導率を算出した。結果を図２に示す。
　　熱伝導率λ（Ｗ／（ｍ・Ｋ））＝ρ・α・Ｃｐ　…式（３）
　　　（ρ：室温密度、α：熱拡散率、Ｃｐ：比熱） <Evaluation of thermal conductivity>
Based on the result of <Evaluation of tempering hardness> above, No. The thermal conductivity of 1 to 6 and 9 when the tempering hardness was 45HRC, 50HRC, or 55HRC was measured. The measurement procedure was as follows: 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 (3). The results are shown in FIG.
Thermal conductivity λ (W / (m ・ K)) = ρ ・ α ・ C p ... Equation (3)
(Ρ: Room temperature density, α: Thermal diffusivity, C p : Specific heat)
　図２の結果より、本発明例であるＮｏ．１～６は、４５ＨＲＣ、５０ＨＲＣ、５５ＨＲＣの全ての硬さにおいて、熱伝導率がλ≧－０．５Ｈ＋５３の式を満足し、硬さを５２ＨＲＣに高めたときでも、３０Ｗ／（ｍ・Ｋ）以上の高い熱伝導率を維持していることが伺えた。また、硬さ５５ＨＲＣという高硬度においても２５Ｗ／（ｍ・Ｋ）以上の高い熱伝導率を有していた。
　これに対して、比較例であるＮｏ．９では、４５ＨＲＣおよび５０ＨＲＣの低い硬さに調質した時点で熱伝導率が小さく（λ≧－０．５Ｈ＋５３の式を満足しておらず）、硬さを５２ＨＲＣに高めても、λ≧－０．５Ｈ＋５３を満足しないことが伺えた。 From the result of FIG. 2, No. 1 which is an example of the present invention. 1 to 6 satisfy the equation of thermal conductivity of λ ≧ -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. 9, the thermal conductivity was small at the time of tempering to a low hardness of 45HRC and 50HRC (not satisfying the equation of λ≥-0.5H + 53), and even if the hardness was increased to 52HRC, λ≥- I heard that they are not satisfied with 0.5H + 53.
　Ｎｏ、７、８の試料については、焼戻し硬さが５２ＨＲＣのときの熱伝導率を測定した。測定要領は上述したＮｏ．１～６、９の時と同様である。その結果、Ｎｏ．７の熱伝導率は３１Ｗ／（ｍ・Ｋ）、Ｎｏ．８の熱伝導率は３７Ｗ／（ｍ・Ｋ）と、硬さ５２ＨＲＣでも３０Ｗ／（ｍ・Ｋ）以上の高い熱伝導率を有していることを確認した。 For the samples No. 7 and 8, 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. As a result, 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.
＜軟化抵抗の評価＞
　ホットスタンプ工法での金型は高温で使用されるため、金型用鋼の軟化抵抗が重要になってくる。そこで、本発明例Ｎｏ．１～６および比較例Ｎｏ．９を５５ＨＲＣに焼戻した状態で６００℃に保持し、硬度の変化を測定した。結果を図３に示す。本発明例であるＮｏ．１～６では、焼戻し温度を高くできたことから、４時間保持した後でも５０ＨＲＣ以上の硬さを維持した。一方で、比較例Ｎｏ．９では、焼戻し温度が低かったことから、４時間保持後の硬さが５０ＨＲＣを下回った。そして、これ以降、保持時間が長くなるにつれ、本発明鋼と比較鋼との硬さの差は大きくなった。本発明鋼では軟化抵抗が大きく、ホットスタンプ工法において有効である。 <Evaluation of softening resistance>
Since the mold in the hot stamping method is used at a high temperature, the softening resistance of the mold steel becomes important. Therefore, the present invention example No. 1 to 6 and Comparative Example No. 9 was kept at 600 ° C. in a state of being tempered to 55 HRC, and the change in hardness was measured. The results are shown in FIG. No. which is an example of the present invention. In Nos. 1 to 6, since the tempering temperature could be raised, the hardness of 50 HRC or more was maintained even after holding for 4 hours. On the other hand, Comparative Example No. In No. 9, since the tempering temperature was low, the hardness after holding for 4 hours was less than 50 HRC. After that, as the holding time became longer, the difference in hardness between the steel of the present invention and the comparative steel became larger. The steel of the present invention has a large softening resistance and is effective in the hot stamping method.

Claims (9)

  1. 質量％で、Ｃ：０．４５～０．６５％、Ｓｉ：０．１～０．６％、Ｍｎ：０．１～０．３％、Ｃｒ：２．５～６．０％、Ｍｏ：１．２～２．６％、Ｖ：０．４～０．８％、残部Ｆｅおよび不可避的不純物の成分組成を有することを特徴とするホットスタンプ用金型用鋼。 By mass%, 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: A steel for dies for hot stamping, which has a component composition of 1.2 to 2.6%, V: 0.4 to 0.8%, balance Fe and unavoidable impurities.
  2. 質量％で、Ｃ：０．４５～０．６５％、Ｓｉ：０．１～０．６％、Ｍｎ：０．１～０．３％、Ｃｒ：２．５～６．０％、Ｍｏ：１．２～２．６％、Ｖ：０．４～０．８％、残部Ｆｅおよび不可避的不純物の成分組成を有することを特徴とするホットスタンプ用金型。 By mass%, 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: A hot stamping die having a component composition of 1.2 to 2.6%, V: 0.4 to 0.8%, balance Fe and unavoidable impurities.
  3. 硬さが４５ＨＲＣ以上、かつ熱伝導率λ（Ｗ／（ｍ・Ｋ））が下記の式（１）を満足することを特徴とする、請求項２に記載のホットスタンプ用金型。
    λ≧－０．５Ｈ＋５３　…式（１）
    Ｈ：金型のロックウェル硬さ（ＨＲＣ）     The hot stamping die according to claim 2, wherein the hardness is 45 HRC or more, and the thermal conductivity λ (W / (m · K)) satisfies the following formula (1).
    λ ≧ −0.5H + 53… Equation (1)
    H: Rockwell hardness of mold (HRC)
  4. 硬さが５２ＨＲＣ以上であることを特徴とする、請求項３に記載のホットスタンプ用金型。 The hot stamping die according to claim 3, wherein the hardness is 52 HRC or more.
  5. 熱伝導率λが２５Ｗ／（ｍ・Ｋ）以上であることを特徴とする、請求項４に記載のホットスタンプ用金型。 The hot stamping die according to claim 4, wherein the thermal conductivity λ is 25 W / (m · K) or more.
  6. 作業面に窒化層を有することを特徴とする請求項２ないし５のいずれかに記載のホットスタンプ用金型。 The hot stamping die according to any one of claims 2 to 5, wherein the work surface has a nitrided layer.
  7. 請求項１に記載のホットスタンプ用金型用鋼に、１０２０～１０８０℃の焼入れ温度および５００～６２５℃の焼戻し温度による焼入れ焼戻しを行うことを特徴とするホットスタンプ用金型の製造方法。 A method for producing a hot stamping die, which comprises performing quenching and tempering of the hot stamping die steel according to claim 1 at a quenching temperature of 1020 to 1080 ° C. and a tempering temperature of 500 to 625 ° C.
  8. 前記焼戻し温度を５４０～６００℃とすることを特徴とする請求項７に記載のホットスタンプ用金型の製造方法。 The method for manufacturing a hot stamping die according to claim 7, wherein the tempering temperature is 540 to 600 ° C.
  9. 前記焼入れ焼戻しを行った後に、さらに、作業面に窒化処理を行うことを特徴とする請求項７または８に記載のホットスタンプ用金型の製造方法。 The method for manufacturing a hot stamping die according to claim 7 or 8, wherein the work surface is further subjected to nitriding treatment after the quenching and tempering.
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