US20220316038A1 - 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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US20220316038A1
US20220316038A1 US17/616,197 US202017616197A US2022316038A1 US 20220316038 A1 US20220316038 A1 US 20220316038A1 US 202017616197 A US202017616197 A US 202017616197A US 2022316038 A1 US2022316038 A1 US 2022316038A1
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die
hardness
hot stamp
thermal conductivity
hrc
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Takayuki Hirashige
Shiho Fukumoto
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Proterial Ltd
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Hitachi Metals Ltd
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21DWORKING OR PROCESSING OF SHEET METAL OR METAL TUBES, RODS OR PROFILES WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21D22/00Shaping without cutting, by stamping, spinning, or deep-drawing
    • B21D22/02Stamping using rigid devices or tools
    • B21D22/022Stamping using rigid devices or tools by heating the blank or stamping associated with heat treatment
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06Surface hardening
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18Hardening; Quenching with or without subsequent tempering
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D1/00General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18Hardening; Quenching with or without subsequent tempering
    • C21D1/25Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/002Heat treatment of ferrous alloys containing Cr
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D7/00Modifying the physical properties of iron or steel by deformation
    • C21D7/13Modifying the physical properties of iron or steel by deformation by hot working
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/005Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273Final recrystallisation annealing
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D9/00Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/0068Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for particular articles not mentioned below
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/24Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • CCHEMISTRY; METALLURGY
    • C23COATING 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
    • C23CCOATING 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
    • C23C8/00Solid 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
    • C23C8/06Solid 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
    • C23C8/08Solid 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
    • C23C8/24Nitriding
    • C23C8/26Nitriding of ferrous surfaces
    • BPERFORMING OPERATIONS; TRANSPORTING
    • B21MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
    • B21JFORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
    • B21J13/00Details of machines for forging, pressing, or hammering
    • B21J13/02Dies or mountings therefor

Definitions

  • the present invention relates to a steel for a hot stamp die, a hot stamp die and a manufacturing method thereof.
  • An advantage of the hot stamp method is that, for example, by quenching according to die quenching, which is rapid cooling in a die, a molded article of an ultra-high tension steel sheet having a tensile strength of about 1.5 GPa is obtained.
  • another advantage is that it has excellent moldability and spring back hardly occurs.
  • the hot stamp method has a problem such as low productivity. That is, the productivity is lowered because it takes time to hold the bottom dead point for die quenching. As a countermeasure therefore, a die with high thermal conductivity is required. This is because, in die quenching, heat of the steel sheet is absorbed by the die, but the time for holding the bottom dead point becomes shorter as the thermal conductivity of the die increases, and the productivity improves.
  • hot stamp dies are required to have high hardness in order to improve the wear resistance.
  • Patent Literature 1 to 3 propose a component composition of a die steel having both hardness and thermal conductivity.
  • Patent Literature 4 also discloses hot work tool steel having excellent thermal conductivity and excellent wear resistance, which is useful as a material for a die used in hot pressing, die-casting, warm and hot forging, or the like.
  • the die steels of Patent Literature 1 to 3 and the hot work tool steel of Patent Literature 4 are useful for hot stamping.
  • the hardness may be insufficient in the case of a conventional die steel and hot work tool steel.
  • An objective of the present invention is to provide a die steel, a hot stamp die and a manufacturing method thereof through which it is possible to manufacture a die having both high hardness and high thermal conductivity suitable for a hot stamp method.
  • the inventors conducted extensive studies, and as a result, found an optimal die steel for hot stamping by controlling a content of alloying elements. Also, they found a hot stamp die that can achieve both high hardness and high thermal conductivity using the die steel and a manufacturing method thereof.
  • an aspect of the present invention is a steel for a hot stamp die having a component composition including, in 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: 1.2 to 2.6%, V: 0.4 to 0.8%, the remainder being Fe and unavoidable impurities.
  • Another aspect of the present invention is a hot stamp die having a component composition including, in 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: 1.2 to 2.6%, V: 0.4 to 0.8%, the remainder being Fe and unavoidable impurities.
  • the hardness is 45 HRC or more
  • the thermal conductivity ⁇ (W/(m ⁇ K)) satisfies the following Formula (1):
  • the hardness is 52 HRC or more.
  • the thermal conductivity ⁇ is 25 W/(m ⁇ K) or more.
  • a working surface has a nitrided layer.
  • Still another aspect of the present invention is a method of manufacturing a hot stamp die including quenching and tempering the steel for a hot stamp die at a quenching temperature of 1,020 to 1,080° C. and a tempering temperature of 500 to 625° C.
  • the tempering temperature is 540 to 600° C.
  • a working surface is additionally subjected to a nitriding treatment.
  • an optimal die steel for hot stamping is obtained.
  • FIG. 1 is a graph figure showing the hardness of examples of dies manufactured by quenching die steels of examples of the present invention and comparative examples at 1,030° C. and then tempering them at 500 to 650° C. for each tempering temperature.
  • FIG. 2 is a graph figure showing the thermal conductivity of examples of dies manufactured by quenching die steels of examples of the present invention and comparative examples at 1,030° C. and then tempering them at a hardness of 45 HRC, 50 HRC, and 55 HRC.
  • FIG. 3 is a graph figure showing the softening resistance of examples of dies manufactured by quenching die steels of examples of the present invention and comparative examples at 1,030° C. and then tempering them at a hardness of 55 HRC when they were kept at 600° C.
  • a feature of the present invention is that, when considering manufacturing a hot stamp die by quenching and tempering a die steel and by performing a nitriding treatment on its working surface, it is found that there is an optimal component composition of a die steel for achieving both high hardness and high thermal conductivity of a hot stamp die.
  • there is an optimal component composition of a die steel to achieve both a high hardness of 52 HRC or more and a high thermal conductivity of 25 W/(m ⁇ K) or more.
  • optimal quenching and tempering conditions for achieving both high hardness and high thermal conductivity have been found.
  • a steel for a hot stamp die of the present invention has a component composition including, in 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%, the remainder being Fe and unavoidable impurities.
  • C is an element that is solid-solutionized in a base (matrix) by quenching and improves the hardness of the die.
  • it is an element that forms a carbide with a carbide forming element such as Cr, Mo, and V to be described below and improves the hardness of the die.
  • the content of C is, 0.45 to 0.65%, preferably 0.47% or more, and more preferably 0.49% or more.
  • the content of C is preferably 0.63% or less, more preferably 0.60% or less, and still more preferably 0.58% or less.
  • Si is used as a deoxidizing agent in the melting process.
  • it is an element that is solid-solutionized in a base and improves the hardness of the die.
  • the content of Si is 0.1 to 0.6%, preferably 0.14% or more, and more preferably 0.17% or more.
  • the content of Si is preferably 0.45% or less, more preferably 0.4% or less, still more preferably 0.35% or less, and yet more preferably 0.3% or less.
  • Mn is used as a deoxidizing agent or a desulfurizing agent in the melting process. In addition, it is an element that contributes to strengthening the base and improving quenching properties, and toughness after quenching and tempering. However, when the content of Mn is too large, the thermal conductivity of the die significantly decreases. Therefore, the content of Mn is 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. The upper limit of Mn is more preferably 0.26% or less.
  • Cr is an element that is solid-solutionized in a base and increases the hardness. In addition, it is an element that increases the hardness by forming a carbide, and is an element that contributes to secondary hardening during tempering like Mo and V to be described below. In particular, Cr is an element that can increase the tempering softening resistance (even if the tempering temperature is raised, the rate of decrease in hardness obtained by secondary hardening can be reduced) as compared with Mo and V. Generally, the die is adjusted to have a working hardness by quenching and tempering the die steel, and thus it is effective to increase the tempering temperature in order to improve the thermal conductivity of the hot stamp die.
  • the thermal conductivity can be increased at the same time.
  • the tempering temperature is, for example, 540° C. or higher, it is possible to achieve a hardness of 52 HRC or more and a hot stamp die having a thermal conductivity of 25 W/(m ⁇ K) or more can be obtained.
  • the thermal conductivity is further improved to 28 W/(m ⁇ K) or more or 30 W/(m ⁇ K) or more while maintaining the above hardness.
  • the hardness and the thermal conductivity are values measured at room temperature (normal temperature).
  • nitriding characteristics of the die steel can be improved by increasing the content of Cr, for example, the working surface of the die after quenching and tempering is additionally subjected to a nitriding treatment, and thus it is possible to improve the wear resistance of the die (the hardness of the working surface).
  • the content of Cr is 2.5 to 6.0%, and preferably 2.8% or more, and more preferably 3.0% or more.
  • the content of Cr is preferably 5.5% or less, more preferably 4.8% or less, and still more preferably less than 4.5%.
  • the content of Cr can be 4.0% or less, or 3.5 or less.
  • Mo is an element that is solid-solutionized in a base and increases the hardness, is an element that increases the hardness by forming a carbide, and is an element that contributes to secondary hardening during tempering like Cr. In addition, it is an element that improves quenching properties.
  • the content of Mo is 1.2 to 2.6%, preferably 1.5% or more, more preferably 1.7% or more, and still more preferably 1.9% or more.
  • the content of Mo is preferably 2.5% or less, more preferably 2.3% or less, and still more preferably 2.1% or less.
  • V is an element that increases the hardness by forming a carbide, and is an element that contributes to secondary hardening during tempering like Cr.
  • the content of V is 0.4 to 0.8% and preferably 0.5% or more.
  • the content of V is preferably 0.75% or less, more preferably 0.65% or less, and still more preferably 0.6% or less.
  • the Remainder being Fe and Unavoidable Impurities
  • the remainder other than the above elemental species be substantially Fe.
  • elemental species not specified here for example, elemental species such as P, S, Cu, Al, Ca, Mg, O (oxygen), and N (nitrogen)
  • elemental species such as P, S, Cu, Al, Ca, Mg, O (oxygen), and N (nitrogen)
  • the content of P is preferably restricted to 0.05% or less, more preferably restricted to 0.03% or less.
  • the content of S is preferably restricted to 0.01% or less, and more preferably restricted to 0.008% or less.
  • Ni is useful as an elemental species that contributes to improving the toughness of the die, but its content is also preferably kept low in order to minimize a decrease in thermal conductivity of the die due to the increase of the content of alloying elements of the die steel. Therefore, 0.25% is preferably allowed as the upper limit restriction of the content of Ni.
  • the hardness of the hot stamp die of the present invention is a value measured at room temperature (normal temperature), and it is easy to achieve a sufficient hardness, for example, 45 HRC or more.
  • the hardness of the die can be preferably 52 HRC or more, and excellent wear resistance can be imparted to the die during use.
  • the hardness of the die is more preferably 53 HRC or more, and still more preferably 55 HRC or more.
  • the upper limit of the hardness of the die it is not necessary to define the upper limit of the hardness of the die.
  • the upper limit of the hardness is preferably 58 HRC or less because it exceeds the peak hardness and the tempering temperature can be raised (that is, the thermal conductivity can be increased).
  • the hardness of the die is adjusted to 45 HRC or more, and also a thermal conductivity ⁇ (W/(m ⁇ K)) that satisfies the following Formula (1) is provided.
  • H in Formula (1) is a Rockwell hardness (HRC) of the die.
  • HRC Rockwell hardness
  • the thermal conductivity is 30.5 W/(m ⁇ K) or more.
  • the thermal conductivity is 27 W/(m ⁇ K) or more.
  • “ ⁇ 0.5H+54” is preferable.
  • the thermal conductivity is a value measured at room temperature (normal temperature).
  • the die steel of the present invention satisfies the relationship of Formula (1) according to quenching and tempering, it is possible to maintain a thermal conductivity of 25 W/(m ⁇ K) or more even when the tempering hardness is in a high hardness range (for example, 52 HRC or more) in which a decrease in thermal conductivity has been a problem.
  • a high hardness range for example, 52 HRC or more
  • a 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 a low hardness range (for example, less than 52 HRC), it is possible to achieve a thermal conductivity of 30 W/(m ⁇ K) or more, and if the tempering hardness is near 45 HRC, it is possible to achieve a thermal conductivity of 32 W/(m ⁇ K) or more. Accordingly, it is possible to maintain high thermal conductivity in the die used in the hot stamp method (for example, 100 to 400° C.).
  • Such thermal conductivity can be easily achieved by increasing the tempering temperature in addition to the component composition of the die steel. For example, it is possible to adjust the thermal conductivity to 30 W/(m ⁇ K) or more by raising the tempering temperature to a temperature at which the peak hardness is obtained or higher.
  • the thermal conductivity of the die it is not necessary to specify the upper limit of the thermal conductivity of the die.
  • the thermal conductivity when the hardness of the die is less than 45 HRC exceeds 50 W/(m ⁇ K), about 50 W/(m ⁇ K) when the hardness is maintained at 45 HRC or more is realistic.
  • the thermal conductivity is preferably 47 W/(m ⁇ K) or less, and more preferably 45 W/(m ⁇ K) or less.
  • an upper limit of the thermal conductivity of about 40 W/(m ⁇ K) preferably 38 W/(m ⁇ K) or less, and more preferably 35 W/(m ⁇ K) or less is realistic. Therefore, regarding the upper limit of the thermal conductivity, a relationship between the thermal conductivity ⁇ (W/(m ⁇ K)) and the Rockwell hardness H (HRC) as shown in Formula (2) of about “ ⁇ 0.5H+70” is realistic. “ ⁇ 0.5H+66” is preferable, and “ ⁇ 0.5H+61” is more preferable.
  • the hot stamp die of the present invention preferably has a nitrided layer on the working surface.
  • the hot stamp die of the present invention has both high hardness and high thermal conductivity. Therefore, when the working surface of the die further has a nitrided layer, the wear resistance (the hardness of the working surface) of the die can be further improved.
  • the working surface is a surface of the die in contact with the steel sheet in the hot stamp.
  • the die steel is quenched and tempered.
  • the quenching temperature varies depending on the target hardness and the like, but can be, for example, about 1,020 to 1,080° C., and is preferably 1,050° C. or lower.
  • the die steel quenched at the quenching temperature is tempered, for example, at a tempering temperature of 500 to 625° C., it is possible to maintain a sufficient hardness such as 45 HRC or more.
  • the quenching temperature and the tempering temperature in this case can be selected so that the hardness and the thermal conductivity of the die after quenching and tempering satisfy the above relationship of Formula (1).
  • tempering at a high temperature is effective in maintaining sufficient hardness of the die and increasing the thermal conductivity of the die, and a hardness of 52 HRC or more can be achieved, for example, at a tempering temperature of 540° C. or higher, and thus a die having a thermal conductivity of 25 W/(m ⁇ K) or more can be obtained.
  • the upper limit of the tempering temperature is preferably about 600° C., more preferably 595° C. or lower, and still more preferably 590° C. or lower.
  • the die steel of the present invention is quenched and tempered to prepare a hot stamp die having a predetermined hardness. Therefore, during this period, the die steel is subjected to various types of machining such as cutting and drilling, and adjusted to the shape of the hot stamp die.
  • the timing of this machining can be performed when the hardness before quenching and tempering is low (that is, an annealed state). Then, in this case, finishing may be performed after quenching and tempering. In addition, in some cases, in combination with the above finishing, the above machining may be performed in a pre-hardened state after quenching and tempering are performed.
  • the working surface of the die after the quenching and tempering are performed is additionally subjected to a nitriding treatment.
  • the die steel having the above component composition when quenched and tempered, for example, it is possible to obtain a die having a hardness of 45 HRC or more and a thermal conductivity that satisfies Formula (1). Therefore, since the die steel having the above component composition has excellent nitriding characteristics, the working surface of the die after quenching and tempering are performed is additionally subjected to a nitriding treatment, and thus the wear resistance of the die (the hardness of the working surface) can be improved.
  • various known nitriding treatments for example, a gas nitriding treatment and a salt bath nitriding treatment, can be applied.
  • a 10 kg steel ingot having the component composition shown in Table 1 was melted. Then, the steel ingot was heated at 1,160° C. and extended-forged with a hammer, and then cooled, and the cooled steel material was annealed at 870° C., and thereby steels of examples Nos. 1 to 8 of the present invention and steels of comparative examples Nos. 9 to 11 were manufactured.
  • the die steels Nos. 1 to 11 were quenched at a quenching temperature of 1,030° C.
  • the half-cooling time was set to 40 minutes (half-cooling time is a time required for cooling from the quenching temperature to a temperature of (quenching temperature+room temperature)/2).
  • the quenched die steels were tempered at a tempering temperature of 500 to 650° C. . Tempering was performed twice and each temperature was maintained for 2 hours. The tempering temperature was set to a total of 7 conditions in increments of 25° C.
  • the Rockwell hardness (C scale) at room temperature in the center thereof was measured. The results are shown in FIG. 1 .
  • the examples Nos. 1 to 8 of the present invention maintained a tempering hardness of 45 HRC or more over the entire tempering temperature range of 500 to 625° C.
  • all examples had about 52 HRC or more in the tempering temperature range of 540 to 600° C. .
  • a tempering hardness of about 45 HRC or more was achieved.
  • the comparative example No. 9 also maintained a tempering hardness of 45 HRC or more in a tempering temperature range of 500 to 600° C., but No. 10 already had a tempering hardness of less than 45 HRC when the tempering temperature was 575° C. 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 was not obtained.
  • the thermal conductivity was small (not satisfying the formula ⁇ 0.5H+53) when the hardness was adjusted to a low hardness of 45 HRC and 50 HRC, and even if the hardness was increased to 52 HRC, it was found that ⁇ 0.5H+53 was not satisfied.
  • the thermal conductivity when the tempering hardness was 52 HRC was measured.
  • the measurement procedure was the same as in the case of the above Nos. 1 to 6, and 9.
  • the thermal conductivity of No. 7 was 31 W/(m ⁇ K)
  • the thermal conductivity of No. 8 was 37 W/(m ⁇ K) and a high thermal conductivity of 30 W/(m ⁇ K) or more was obtained at a hardness of 52 HRC.
  • the softening resistance of the die steel was important. Therefore, the examples Nos. 1 to 6 of the present invention and the comparative example No. 9 that were tempered at 55 HRC were kept at 600° C., and the change in the hardness was measured. The results are shown in FIG. 3 .
  • the examples Nos. 1 to 6 of the present invention since the tempering temperature could be raised, the hardness of 50 HRC or more was maintained even after being kept for 4 hours.
  • the comparative example No. 9 since the tempering temperature was low, the hardness after being kept for 4 hours was less than 50 HRC. After that, when the keeping time was longer, the difference in the hardness between the steels of the present invention and the comparative steels was larger.
  • the steels of the present invention had a large softening resistance and was effective in the hot stamp method.

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US17/616,197 2019-06-06 2020-03-11 Steel for hot stamp die, hot stamp die and manufacturing method thereof Pending US20220316038A1 (en)

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CN113939604A (zh) 2022-01-14
EP3981890A1 (fr) 2022-04-13
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KR20220002523A (ko) 2022-01-06
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JPWO2020246099A1 (fr) 2020-12-10
JP7540437B2 (ja) 2024-08-27

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