EP4105343A1 - Article moulé estampé à chaud - Google Patents

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Publication number
EP4105343A1
EP4105343A1 EP21752921.3A EP21752921A EP4105343A1 EP 4105343 A1 EP4105343 A1 EP 4105343A1 EP 21752921 A EP21752921 A EP 21752921A EP 4105343 A1 EP4105343 A1 EP 4105343A1
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EP
European Patent Office
Prior art keywords
hot
steel sheet
less
stamped product
steel
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP21752921.3A
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German (de)
English (en)
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EP4105343A4 (fr
Inventor
Jun Haga
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Nippon Steel Corp
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Nippon Steel Corp
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Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4105343A1 publication Critical patent/EP4105343A1/fr
Publication of EP4105343A4 publication Critical patent/EP4105343A4/fr
Pending legal-status Critical Current

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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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • 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
    • 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
    • C21D1/26—Methods of 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/62—Quenching devices
    • C21D1/673—Quenching devices for die quenching
    • 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
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • 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
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • 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
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • 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
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • 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
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • 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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • 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
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • 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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • 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
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • 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
    • 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
    • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
    • 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
    • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
    • 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
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • 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
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • 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
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • 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
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/12—Aluminium or alloys based thereon
    • 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
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • 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
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the present invention relates to a hot-stamped product.
  • press forming which is a step of forming vehicle body members of vehicles
  • cracks and wrinkles are more likely to be generated as the thicknesses of steel sheets to be used decrease. Therefore, excellent press formability is also required for steel sheets for a vehicle.
  • Patent Document 1 describes that a member having a tensile strength of 1400 MPa or more can be obtained by hot stamping a steel sheet having a tensile strength of 500 to 600 MPa.
  • Patent Document 2 discloses a hot stamping member having a tensile strength of 1770 to 1940 MPa and a manufacturing method thereof
  • Patent Document 3 discloses a hot stamping member having a tensile strength of 1960 to 2130 MPa and a manufacturing method thereof.
  • a steel sheet for hot stamping is heated to a two phase region of ferrite and austenite and then hot-stamped to form a composite structure of ferrite and martensite having an average grain diameter of 7 ⁇ m or less as the microstructure of a hot stamping member, thereby enhancing the ductility of the steel sheet that configures a member.
  • Patent Document 4 discloses a technique for manufacturing a hot stamping member having excellent toughness and a tensile strength of 1800 MPa or more.
  • a steel sheet for hot stamping is heated to a low-temperature region of austenite, then, hot-stamped and relatively slowly cooled in a temperature range of the Ms point or lower, thereby forming a microstructure formed of tempered martensite having a prior austenite grain size of 10 ⁇ m or less and enhancing the toughness of a member.
  • the technique disclosed in Patent Document 4 is capable of obtaining a 1800 MPa-class hot stamping member in which cracking does not occur even in a lot-temperature impact test and is thus excellent.
  • Patent Document 4 describes nothing about a member having a tensile strength of 2300 MPa or more. According to the present inventors' research, it was found that, even in a hot stamping member formed of a single-phase structure of tempered martensite as described in Patent Document 4, if the tensile strength is increased up to 2300 MPa or more, particularly in a case where the forming temperature during the hot stamping of a steel sheet is low, a local fluctuation in hardness is caused in the member, and a strong requirement for crashworthiness of these days cannot be sufficiently met. In addition, it was found that such a local fluctuation in hardness is large particularly in a case where a material steel sheet for hot stamping is a plated steel sheet.
  • An objective of the present invention is to solve the above-described problems and to provide a hot-stamped product having a portion with excellent crashworthiness and a tensile strength of 2300 MPa or more.
  • the present invention has been made to solve the above-described problems, and the gist of the present invention is the following hot-stamped product.
  • the present inventors intensively studied a method for suppressing the occurrence of cracking when a hot-stamped product having a tensile strength of 2300 MPa or more distorts due to a collision. As a result, the following findings were obtained.
  • the present inventors found that, when a steel sheet as cold rolled is used as a material steel sheet, and the steel sheet as cold rolled is heated and then hot stamping is started at a high temperature, it is possible to manufacture a hot-stamped product in which the tensile strength is 2300 MPa or more, the local fluctuation in hardness is small and the crashworthiness is excellent.
  • All or part of a steel sheet included in the hot-stamped product according to the present embodiment has a chemical composition shown below (in a case where the hot-stamped product is consisting of a steel sheet, all or part of the hot-stamped product can be said to have a chemical composition shown below).
  • the reason for limiting each element is as described below. "%” regarding contents in the following description means “mass%”.
  • numerical value ranges shown using “to” include numerical values at both ends in the ranges.
  • numerical values shown using "less than” or “more than” do not include the numerical values in the ranges.
  • the hot-stamped product includes a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa (in a case where the steel sheet included in the hot-stamped product according to the present embodiment includes a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa), at least the portion where the tensile strength becomes 2300 MPa or more needs to have the following chemical composition.
  • the chemical composition to be described below means the chemical composition of the steel sheet excluding the plating layer.
  • C is an element having an effect on an increase in the tensile strength of the steel sheet after hot-stamping (the steel sheet included in the hot-stamped product).
  • the C content is set to more than 0.40%.
  • the preferable C content is more than 0.42%, more than 0.43%, more than 0.44% or more than 0.45%.
  • the C content is set to 0.70% or less.
  • the preferable C content is 0.65% or less, 0.60% or less, 0.55% or less or 0.50% or less.
  • Si is an element that is contained in steel as an impurity and embrittles steel.
  • the Si content is set to less than 2.00%.
  • the preferable Si content is less than 1.50%, less than 1.00%, less than 0.75% or less than 0.50%.
  • the Si content is preferably set to 0.40% or less, 0.30% or less or 0.20% or less.
  • the lower limit of the Si content is not particularly limited, but an excessive decrease in the Si content increases the steelmaking cost. Therefore, the Si content is preferably set to 0.001% or more.
  • Si has an action of enhancing the hardenability of steel and thus may be actively incorporated. From the viewpoint of improving the hardenability, the Si content is preferably 0.10% or more, 0.20% or more or 0.30% or more.
  • Mn 0.01% or more and less than 0.50%
  • Mn is an element that degrades the crashworthiness of the hot-stamped product.
  • the Mn content is set to less than 0.50%.
  • the Mn content is preferably less than 0.45%, less than 0.40%, less than 0.35% or less than 0.30%.
  • Mn is an element that bonds to S, which is an impurity, to form MnS and has an action of suppressing the harmful effect of S.
  • the Mn content is set to 0.01% or more.
  • the Mn content is preferably 0.05% or more or 0.10% or more.
  • Mn is an element that improves the hardenability of steel. From the viewpoint of improving the hardenability, the Mn content is preferably 0.15% or more, 0.20% or more or 0.25% or more.
  • P is an element that is contained in steel as an impurity and embrittles steel.
  • the P content exceeds 0.200%, the adverse influence of P becomes particularly large, and furthermore, the weldability significantly deteriorates. Therefore, the P content is set to 0.200% or less.
  • the preferable P content is less than 0.100%, less than 0.050% or less than 0.020%. From the viewpoint of securing the plateability, the P content is preferably less than 0.020%, less than 0.015% or less than 0.010%.
  • the lower limit of the P content is not particularly limited, but an excessive decrease in the P content increase the steelmaking cost. Therefore, the P content may be set to 0.001% or more.
  • S is an element that is contained in steel as an impurity and embrittles steel.
  • the S content exceeds 0.0200%, the adverse influence of S becomes particularly large. Therefore, the S content is set to 0.0200% or less.
  • the preferable S content is less than 0.0050%, less than 0.0020% or less than 0.0010%.
  • the lower limit of the S content is not particularly limited, but an excessive decrease in the S content increase the steelmaking cost. Therefore, the S content may be set to 0.0001% or more.
  • Al is an element having an action of deoxidizing molten steel.
  • the sol. Al content (acid-soluble Al content) is less than 0.001%, deoxidation becomes insufficient. Therefore, the sol. Al content is set to 0.001% or more.
  • the sol. Al content is preferably 0.005% or more, 0.010% or more or 0.020% or more.
  • the sol. Al content is set to 1.000% or less.
  • the sol. Al content is preferably less than 0.500%, less than 0.100%, less than 0.060% or less than 0.040%.
  • N is an element that is contained in steel as an impurity and forms a nitride during the continuous casting of steel. Since this nitride degrades the ductility of the hot-stamped product sheet, the N content is preferably as low as possible. When the N content becomes more than 0.0200%, the adverse influence of Si becomes particularly large. Therefore, the N content is set to 0.0200% or less. The N content is preferably less than 0.0100%, less than 0.0080% or less than 0.0050%.
  • the lower limit of the N content is not particularly limited, but an excessive decrease in the N content increase the steelmaking cost. Therefore, the N content may be set to 0.0010% or more.
  • Mo is an element that improves the hardenability of steel and an effective element for securing the strength of the hot-stamped product by forming a microstructure mainly containing martensite.
  • the Mo, content is set to 0.01% or more.
  • the preferable Mo content is 0.05% or more, 0.10% or more or 0.15% or more.
  • Mo is an element that degrades the crashworthiness of the hot-stamped product.
  • the Mo content is set to less than 0.50%.
  • the Mo content is preferably less than 0.40%, less than 0.35% or less than 0.30%.
  • B is an element that improves the hardenability of steel and an effective element for forming a microstructure mainly containing martensite and securing the strength of the hot-stamped product.
  • the B content is set to 0.0002% or more.
  • the preferable B content is 0.0006% or more, 0.0010% or more or 0.0015% or more.
  • the B content is set to 0.0200% or less.
  • the preferable B content is less than 0.0050%, less than 0.0040% or less than 0.0030%.
  • the hot-stamped product according to the present embodiment may have a chemical composition including the above-described chemical components with a remainder of Fe and impurities; however, in order to improve characteristics or the like, the hot-stamped product according to the present embodiment may further contain one or more selected from the group consisting of Ti, Nb, V, Zr, Cr, W, Cu, Ni, Ca, Mg, REM and Bi. These elements (optional elements) do not need to be contained at all times, and thus the lower limits are 0%.
  • the "impurity” means a component that is mixed in from a raw material such as ore or a scrap or due to a variety of factors in manufacturing steps at the time of industrially manufacturing the steel sheet and is allowed to an extent that the hot-stamped product according to the present embodiment is not adversely affected.
  • Ti, Nb, V and Zr are elements having an action of improving the crashworthiness of the hot-stamped product by the refinement of the microstructure.
  • one or more selected from the group consisting of Ti, Nb, V and Zr may be contained as necessary.
  • the amount of each of one or more selected from the group consisting of Ti, Nb, V and Zr contained is preferably 0.001% or more, more preferably 0.005% or more and still more preferably 0.010% or more.
  • the amount of each of Ti, Nb, V and Zr exceeds 0.200%, the above-described effect is saturated, and thus the manufacturing cost of the steel sheet increases. Therefore, in the case of being contained, the amount of each of Ti, Nb, V and Zr is set to 0.200% or less.
  • the preferable Ti content is less than 0.050% or less than 0.030%
  • the preferable Nb content is less than 0.050%, less than 0.030% or less than 0.020%
  • the preferable V content is less than 0.100% or less than 0.050%
  • the preferable Zr content is less than 0.100% or less than 0.050%.
  • Cr, W, Cu and Ni are elements having an action of enhancing the hardenability of steel. Therefore, one or more selected from the group consisting of Cr, W, Cu and Ni may be contained as necessary.
  • the amount of each of one or more selected from the group consisting of Cr, W, Cu and Ni contained is preferably 0.001% or more.
  • a more preferable Cr content is 0.05% or more or 0.10% or more
  • a more preferable W content is 0.05% or more or 0.10% or more
  • a more preferable Cu content is 0.10% or more
  • a more preferable Ni content is 0.10% or more.
  • the amount of each of Cr, W, Cu and Ni exceeds 2.00%, the crashworthiness of the hot-stamped product deteriorates. Therefore, in the case of being contained, the amount of each of Cr, W, Cu and Ni is set to 2.00% or less.
  • the preferable Cr content is less than 0.50%, less than 0.40% or less than 0.30%
  • the preferable W content is less than 0.50%, less than 0.40% or less than 0.30%
  • the preferable Cu content is less than 1.00% or less than 0.50%
  • the preferable Ni content is less than 1.00% or less than 0.50%.
  • Ca, Mg and REM are elements having an action of improving the ductility of the steel sheet after hot-stamping by adjusting the shape of an inclusion. Therefore, Ca, Mg and REM may be contained as necessary. In the case of hoping to obtain the above-described effect, the amount of each of one or more selected from the group consisting of Ca, Mg and REM contained is preferably 0.0001% or more.
  • the amount of each of Ca and Mg is set to 0.0100% or less, and the REM content is set to 0.1000% or less.
  • REM refers to a total of 17 elements of Sc, Y, and lanthanoids, and the REM content means the total amount of these elements.
  • lanthanoids are added in a mischmetal form.
  • Bi is an element having an action of improving the crashworthiness of the hot-stamped product by refining a solidification structure. Therefore, Bi may be contained as necessary. In the case of hoping to obtain the above-described effect, the Bi content is preferably 0.0001% or more. A more preferable Bi content is 0.0003% or more or 0.0005% or more.
  • the Bi content is set to 0.0500% or less.
  • a more preferable Bi content is 0.0100% or less or 0.0050% or less.
  • the essential elements are contained and the remainder may be Fe and impurities or the essential elements are contained, and, furthermore, one or more of the optional elements are contained, and the remainder may be Fe and impurities.
  • microstructure of the steel sheet included in the hot-stamped product according to the present embodiment will be described. All or part of the steel sheet included in the hot-stamped product according to the present embodiment has a microstructure containing martensite in the amount shown below (in a case where the hot-stamped product is consisting of a steel sheet, all or part of the hot-stamped product can be said to have a microstructure containing martensite in an amount shown below). "%" in the following description regarding the microstructure means "vol%".
  • the hot-stamped product includes a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa, at least the portion where the tensile strength becomes 2300 MPa or more needs to have the following microstructure.
  • the microstructure to be described below means the microstructure of the steel sheet.
  • a microstructure at a 1/4 depth position of the sheet thickness from the surface of the steel sheet (the interface between the steel sheet and the plating layer in the case of having the plating layer) is specified.
  • Martensite is an important structure for increasing the tensile strength of the hot-stamped product sheet.
  • the volume percentage of martensite is set to more than 90.0%.
  • the preferable volume percentage of martensite is more than 91.0%, more than 93.0% or more than 95.0%.
  • the upper limit of the volume percentage of martensite does not need to be particularly determined; however, in order to significantly increase the volume percentage of martensite, it is necessary to excessively increase the heating temperature of the steel sheet or excessively increase the cooling rate in a hot stamping step, which significantly impairs the productivity of the hot-stamped product. Therefore, the volume percentage of martensite is preferably set to 99.0% or less or 98.0% or less.
  • tempered martensite in addition to fresh martensite that is not tempered, tempered martensite that has been tempered and contains an iron carbide is contained.
  • the remainder of the microstructure may contain ferrite, pearlite, bainite or residual austenite and may further contain a precipitate such as cementite. Since there is no need to contain ferrite, pearlite, bainite, residual austenite and the precipitate, the lower limits of the volume percentages of ferrite, pearlite, bainite, residual austenite and the precipitate are all 0%.
  • ferrite, pearlite and bainite have an action of improving the ductility of the hot-stamped product sheet, in the case of obtaining this effect, one or more selected from the group consisting of ferrite, pearlite and bainite are preferably contained.
  • the volume percentage of ferrite is preferably set to 0.5% or more or 1.0% or more, and the volume percentages of pearlite and bainite are each preferably set to 1.0% or more and each more preferably set to 2.0% or more.
  • the volume percentage of ferrite is preferably set to less than 3.0% or less than 2.0%, and the volume percentages of pearlite and bainite are each preferably set to less than 10.0% and each more preferably set to less than 5.0%.
  • Residual austenite has an action of improving the ductility of the hot-stamped product sheet.
  • the volume percentage of residual austenite is preferably set to 0.5% or more, 1.0% or more or 2.0% or more.
  • the volume percentage of residual austenite is preferably set to less than 9.0%, less than 7.0%, less than 5.0% or less than 4.0%.
  • the volume percentage of each structure is obtained as described below.
  • a test piece is collected from the hot-stamped product, the longitudinal section of the steel sheet is polished with a buffer, and then the structure is observed at a 1/4 depth position of the sheet thickness of the steel sheet in the sheet thickness direction of the steel sheet from the surface of the steel sheet (the interface between the steel sheet, which is a substrate, and a plating layer in the case of having the plating layer).
  • the hot-stamped product includes a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa
  • the test piece is collected from the portion where the tensile strength becomes 2300 MPa or more, and the structure is observed.
  • the polished surface is Nital-etched or electrolytically polished, the structure is observed using an optical microscope and a scanning electron microscope (SEM), and image analysis is carried out on an obtained structural photograph based on a luminance difference or a difference in form among iron carbides present in the phase, thereby obtaining the area ratio of each of ferrite, pearlite, bainite and tempered martensite.
  • the same observation position is LePera-etched, the structure is observed using the optical microscope and the scanning electron microscope (SEM), and image analysis is carried out on an obtained structural photograph, thereby calculating the total area ratio of residual austenite and martensite.
  • the longitudinal section is electrolytically polished, and then the area ratio of residual austenite is measured using a SEM including an electron backscatter diffraction pattern analyzer (EBSP).
  • EBSP electron backscatter diffraction pattern analyzer
  • the area ratio of each of ferrite, pearlite, bainite, tempered martensite, martensite and residual austenite is obtained.
  • the measured area ratio is regarded as the volume percentage of each structure.
  • tempered martensite can be differentiated from martensite due to the fact that iron carbides are present in tempered martensite and can be differentiated from bainite due to the fact that the iron carbides present in tempered martensite elongate in a plurality of directions.
  • All or part of the hot-stamped product according to the present embodiment has a tensile strength of 2300 MPa or more.
  • the tensile strength of all or part of the steel sheet included in the hot-stamped product according to the present embodiment is 2300 MPa or more.
  • the tensile strength of all or part of the hot-stamped product is set to 2300 MPa or more.
  • the tensile strength is preferably 2400 MPa or more or 2500 MPa or more.
  • the tensile strength of the hot-stamped product is preferably set to less than 3000 MPa or less than 2800 MPa.
  • the tensile strength is 2300 MPa or more and the yield ratio is 0.65 or more.
  • the yield ratio is more preferably 0.68 or more or 0.70 or more.
  • the upper limit of the yield ratio is not particularly limited; however, in order to significantly increase the yield ratio, it is necessary to excessively increase the reheating temperature in the reheating step to be described below, which decreases the strength of the formed article. Therefore, the yield ratio is preferably set to less than 0.90, less than 0.85 or less than 0.80.
  • the tensile strength may be 2300 MPa or more in all of the hot-stamped product (the entire formed article), but a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa may be present in a mixed form in the hot-stamped product.
  • portions with different strengths are provided, it becomes possible to control the distortion state of the hot-stamped product upon a collision.
  • a hot-stamped product having portions with different strengths can be manufactured by a method in which two or more steel sheets having different chemical compositions are joined together and then hot-stamped, a method in which the heating temperature or cooling rate after hot stamping of a steel sheet is partially changed in a hot stamping step, a method in which a reheating treatment is partially carried out on a hot-stamped product or the like.
  • the tensile strength and the yield ratio are obtained by collecting a JIS No. 13B tensile test piece along the longitudinal direction of a member and carrying out a tensile test at a tension rate of 10 mm/minute.
  • the yield ratio is obtained by dividing the yield stress of the steel sheet by the tensile strength.
  • the yield stress the 0.2% proof stress is used in a case where the steel sheet yields continuously, and a stress at an upper yield point is used in a case where the steel sheet yields discontinuously.
  • the plating layer has a small influence on the tensile strength or the yield ratio, and thus the plating layer may be present on the surface of the test piece.
  • the average value of Vickers hardness is 670 (Hv) or more and the standard deviation of the Vickers hardness is 20 (Hv) or less in a 0.18 mm 2 region, that is, a region that is 0.3 mm long in the sheet thickness direction and 0.6 mm long in a direction orthogonal to the sheet thickness direction, in which the 1/4 depth position of the sheet thickness of the steel sheet in the sheet thickness direction of the steel sheet from the surface of the steel sheet (the interface between the steel sheet, which is a substrate, and the plating layer in the case of having the plating layer) is centered, in the portion where the tensile strength is 2300 MPa or more.
  • the average value of the Vickers hardness is 670 (Hv) or more means that the tensile strength is 2300 MPa or more in the hardness measurement region, and, when the average value of the Vickers hardness is less than 670 (Hv), the strength of the formed article becomes insufficient. Therefore, the average value of the Vickers hardness in the above-described region is set to 670 (Hv) or more.
  • the average value of the Vickers hardness is preferably 695 (Hv) or more or 720 (Hv) or more.
  • the standard deviation of the Vickers hardness in the above-described region is more than 20 (Hv)
  • the standard deviation of the hardness in the above-described region is set to 20 (Hv) or less.
  • the standard deviation of the hardness is preferably 15 (Hv) or less, 12 (Hv) or less or 10 (Hv) or less.
  • the Vickers hardness of the hot-stamped product is obtained as described below.
  • a test piece is collected from the hot-stamped product, the longitudinal section of the steel sheet is polished with waterproof abrasive paper and further polished with a buffer using a diamond suspension, and then the Vickers hardness is measured at a 1/4 depth position of the sheet thickness of the steel sheet in the sheet thickness direction of the steel sheet from the surface of the steel sheet (the interface between the steel sheet and the plating layer in the case of having the plating layer).
  • the hot-stamped product includes a portion having a tensile strength of 2300 MPa or more and a portion having a tensile strength of less than 2300 MPa
  • the test piece is collected from the portion where the tensile strength is 2300 MPa or more, and the Vickers hardness is measured.
  • the Vickers hardness is measured at 45 points at predetermined intervals according to JIS Z 2244: 2009, and the arithmetic average value and the standard deviation are obtained from the obtained measurement values.
  • a micro Vickers hardness tester is used, and, as the measurement conditions, the applied load is set to 0.49 N, and the load retention time is set to 10 seconds.
  • the applied load is determined to be 0.49 N.
  • PCT International Publication No. WO 2018/151325 describes that it is important in terms of securing the crashworthiness that a variation in hardness in a cross section of the compact perpendicular to the longitudinal direction is small.
  • a variation in microhardness in the entire cross-sectional region of the compact is obtained by measuring the Vickers hardness in the central part in the sheet thickness direction at 1 mm intervals with an applied load set to 1 kgf, and it can be said that the distribution of the hardness is different from that of the hot-stamped product according to the present embodiment.
  • the hot-stamped product according to the present embodiment may have a plating layer on a surface of the steel sheet.
  • the type of plating needs to be suitable for the objective and is not particularly limited.
  • the plating layer in the hot-stamped product can be formed by carrying out hot stamping using a plated steel sheet as described below.
  • An example of the type of the plating layer includes a zinc-based plating layer or aluminum-based plating layer hot-stamped using a zinc-based plated steel sheet or an aluminum-based plated steel sheet.
  • the plating layer may be formed on one surface or may be formed on both surfaces.
  • the chemical composition of the steel sheet for hot stamping is set to be the same as the chemical composition of the hot-stamped product.
  • the steel sheet for hot stamping is a steel sheet that is manufactured without annealing after a cold rolling step and has a microstructure expanded in a rolling direction in which strain energy is high (also referred to as the steel sheet as cold rolled or full hard) or a plated steel sheet.
  • the reason for forming such a microstructure is to decrease a local fluctuation in the hardness of the hot-stamped product and to improve the crashworthiness of the formed article.
  • the steel sheet as cold rolled in which accumulated strain energy is high is preferably used since it is possible to decrease the local fluctuation in hardness by a small number of manufacturing steps.
  • the plated steel sheet is preferably used from the viewpoint of preventing the generation of scale in the manufacturing steps and, furthermore, improving the corrosion resistance of the hot-stamped product.
  • the microstructure of the steel sheet for hot stamping preferably mainly contains ferrite, pearlite and/or bainite expanded in the rolling direction.
  • the total volume percentage of ferrite expanded in the rolling direction, pearlite expanded in the rolling direction and bainite expanded in the rolling direction is more preferably more than 90.0% or more than 95.0%.
  • the microstructure preferably mainly contains ferrite, pearlite and/or bainite.
  • the volume percentage in the microstructure of the steel sheet for hot stamping can be obtained by collecting a test piece from the steel sheet for hot stamping, polishing a longitudinal section of the steel sheet parallel to the rolling direction with a buff and then observing the structure by the same method as that in the case of the hot-stamped product at the 1/4 depth position of the sheet thickness of the steel sheet in the sheet thickness direction of the steel sheet from the surface of the steel sheet (the interface between the steel sheet and the playing layer in the case of the plated steel sheet).
  • the type of the plated steel sheet is not particularly limited, and examples thereof include a hot-dip galvanized steel sheet, a galvannealed steel sheet, a hot-dip aluminum-plated steel sheet, a hot-dip Zn-Al alloy-plated steel sheet, a hot-dip Zn-Al-Mg alloy-plated steel sheet, a hot-dip Zn-Al-Mg-Si alloy-plated steel sheet and the like.
  • the plating layer may be provided on one surface of the steel sheet or may be provided on both surfaces.
  • the tensile strength is preferably more than 900 MPa.
  • the tensile strength is more preferably more than 950 MPa or more than 1000 MPa.
  • a manufacturing method of the hot-stamped product according to the present embodiment and the preferable manufacturing method of the steel sheet for hot stamping according to the present embodiment will be described.
  • the hot-stamped product according to the present embodiment can be manufactured by a manufacturing method including the following steps (I) and (II) or a manufacturing method including the following steps (i), (ii) and (iii).
  • the heating temperature is preferably set to higher than 1050°C and higher than the Ac 3 point.
  • the hot stamping start temperature can be set to higher than 1050°C, and it becomes easy to secure the crashworthiness of the hot-stamped product.
  • the Ac 3 point is a temperature at which ferrite in the microstructure disappears when the material steel sheet has been heated and can be obtained from a change in the thermal expansion of the steel sheet in the heating step.
  • the heating temperature is preferably higher than 1100°C and higher than the Ac 3 point.
  • the upper limit of the heating temperature is not particularly limited; however, when the heating temperature is too high, in a case where the steel sheet for hot stamping is the steel sheet as cold rolled, scale is excessively generated on the hot-stamped product, and the productivity of the formed article deteriorates due to the deposition of the scale in a die. In a case where the steel sheet for hot stamping is a plated steel sheet, the amount of a plating attached decreases, and the corrosion resistance of the hot-stamped product deteriorates. Therefore, the heating temperature is preferably set to 1200°C or lower or 1150°C or lower.
  • the heating rate of the steel sheet does not need to be particularly limited; however, as the heating rate is faster, the local fluctuation in hardness in the hot-stamped product decreases, and the crashworthiness improves. Therefore, the average heating rate up to 700°C is preferably set to faster than 10 °C/second, faster than 20°C/second, faster than 30 °C/second or faster than 50 °C/second. On the other hand, when the heating rate is suppressed, it is possible to suppress the formation of a coarse iron carbide in microstructure of the hot-stamped product, and the ductility of the hot-stamped product sheet can be enhanced. Therefore, the average heating rate is preferably set to slower than 150 °C/second, slower than 120°C/second or slower than 90 °C/second.
  • the heated steel sheet is taken out from a heating furnace and cooled in the atmosphere, and hot stamping is started.
  • the hot stamping start temperature is preferably higher than 1050°C.
  • the hot stamping start temperature is preferably higher than 1100°C.
  • the upper limit of the hot stamping start temperature is not particularly limited; however, in order to increase the start temperature, it is necessary to increase the heating temperature of the steel sheet in the above-described heating step. In this case, scale is excessively generated on the hot-stamped product, and the productivity of the formed article deteriorates or the corrosion resistance of the hot-stamped product deteriorates. Therefore, the start temperature is preferably set to 1200°C or lower or 1150°C or lower.
  • the formed article After forming by hot stamping, the formed article is cooled while held in the die and/or the formed article is removed from the die and cooled by an arbitrary method.
  • the average cooling rate from the hot stamping start temperature to 400°C is preferably set to 30 °C/second or faster, 60 °C/second or faster or 90 °C/second or faster.
  • the cooling stop temperature when the cooling stop temperature is low, similarly, the volume percentage of martensite is secured in the microstructure of the hot-stamped product, and the strength of the formed article increases.
  • the cooling step temperature of the cooling is preferably set to lower than 90°C or lower than 50°C.
  • the steel sheet after hot-stamping is reheated.
  • the reheating temperature is 90°C or higher, the local fluctuation in hardness in the hot-stamped product decreases, and the crashworthiness can be enhanced.
  • the reheating temperature is lower than 150°C, the softening of the steel sheet is suppressed, and the strength of the formed article is secured.
  • the precipitation of a coarse iron carbide is suppressed, and the crashworthiness improves. Therefore, the reheating temperature is preferably set to 90°C or higher and lower than 150°C.
  • the reheating temperature is more preferably set to 100°C or higher, 110°C or higher or 120°C or higher.
  • the reheating temperature is more preferably set to lower than 140°C or lower than 130°C.
  • the retention time is preferably set to five minutes or longer or 10 minutes or longer.
  • the retention time is preferably set to shorter than 20 minutes or shorter than 15 minutes.
  • the reheating step may not be carried out.
  • the fluctuation in hardness decreases when the strain energy accumulated in the steel sheet for hot stamping is high. This is because, in the steel sheet as cold rolled, processing strain accumulates during hot rolling, and thus a target standard deviation of Vickers hardness can be achieved even without reheating.
  • reheating may be carried out on a hot-stamped product not including any plating layer on a surface. In order to sufficiently obtain the effect on an increase in the yield ratio, reheating is preferably carried out under the same conditions as those in a case where a plated steel sheet is used as the steel sheet for hot stamping.
  • the steel sheet for hot stamping according to the present embodiment that is to be subjected to the manufacturing of the hot-stamped product is preferably manufactured by the following manufacturing method.
  • a manufacturing method of a slab that is subjected to the manufacturing method of the steel sheet for hot stamping according to the present embodiment is not particularly limited.
  • steel having the above-described composition (chemical composition) is melted by well-known means, then, made into a steel ingot by a continuous casting method or made into a steel ingot by an arbitrary casting method and made into a steel piece by a blooming method or the like.
  • an externally-added viscous flow such as electromagnetic stirring in molten steel in a mold.
  • the steel ingot or the steel piece may be once cooled, reheated and hot-rolled or the steel ingot in a high-temperature state after continuous casting or the steel piece in a high-temperature state after blooming may be hot-rolled as it is or after thermally insulated or supplementarily heated.
  • such a steel ingot and a steel piece will be collectively referred to as "slab" as a material for hot rolling.
  • slab heating temperature The temperature of the slab that is subjected to hot rolling (slab heating temperature) is preferably set to lower than 1250°C or more preferably set to lower than 1200°C in order to prevent the coarsening of austenite.
  • slab heating temperature When the slab heating temperature is low, since rolling becomes difficult, the slab heating temperature may be set to 1050°C or higher.
  • the heated slab is hot-rolled, thereby obtaining a hot-rolled steel sheet.
  • the hot rolling is preferably completed in a temperature range of the Ar 3 point or higher in order to refine the microstructure of the hot-rolled steel sheet by the transformation of austenite after the completion of the rolling.
  • the Ar 3 point is a temperature at which ferritic transformation from austenite starts in the microstructure when the steel sheet has been cooled and can be obtained from a change in the thermal expansion of the steel sheet during cooling.
  • a rough rolling material may be heated between the rough rolling and the finish rolling. At this time, it is desirable to suppress fluctuations in temperature throughout the entire length of the rough rolling material at the time of the start of the finish rolling to 140°C or less by heating the rough rolling material such that the temperature becomes higher at the rear end than the front end. In such a case, the uniformity of product characteristics in a coil after a coiling step improves.
  • the rough rolling material may be heated using well-known means.
  • a solenoid-type induction heating device may be provided between a roughing mill and a finishing mill, and the amount of the temperature increased by heating may be controlled based on the temperature distribution or the like of the rough rolling material in the longitudinal direction on the upstream side of this induction heating device.
  • the coiling temperature is preferably set to higher than 600°C.
  • the coiling temperature is 600°C or lower, the hot-rolled steel sheet becomes excessively hard, it becomes difficult to carry out hot rolling, and there is a case where the crashworthiness of the hot-stamped product deteriorates.
  • a more preferable coiling temperature is higher than 620°C or higher than 650°C.
  • the coiling temperature is preferably set to 750°C or lower or 700°C or lower.
  • the hot-rolled steel sheet may be annealed before a cold rolling step.
  • the steel sheet as cold rolled is used as the steel sheet for hot stamping
  • a steel sheet that has been hot-rolled and coiled is cold-rolled according to a normal method, thereby producing a cold-rolled steel sheet.
  • the cold rolling reduction (cumulative rolling reduction in the cold rolling) is preferably set to 10% or larger.
  • a more preferably cold rolling reduction is 20% or larger, 30% or larger or 40% or larger.
  • the upper limit of the cold rolling reduction does not need to be particularly limited; however, an excessive increase in the cold rolling reduction increases the load on a rolling facility and degrades the productivity, and thus the cold rolling reduction is preferably set to smaller than 70%, smaller than 60% or smaller than 50%.
  • the sheet thickness of the cold-rolled steel sheet is preferably 2.0 mm or less, more preferably 1.8 mm or less and still more preferably 1.6 mm or less.
  • flatness correction by skin pass rolling or the like or descaling by pickling or the like may be carried out according to a well-known method.
  • a treatment such as degreasing may be carried out according to a normal method.
  • the cold-rolled steel sheet is not annealed.
  • annealing is not carried out, due to the strain energy accumulated during the cold rolling, the local fluctuation in hardness in the hot-stamped product can be decreased, and the crashworthiness of the formed article improves.
  • cold rolling may not be carried out or may be carried out under the above-described conditions.
  • the steel sheet for hot stamping is cold-rolled, the microstructure is refined, and the crashworthiness of the hot-stamped product improves.
  • the lower limit of the soaking temperature in an annealing process of continuous hot-dip plating is preferably set to 600°C, 650°C or 700°C.
  • the heating rate is too slow, the soaking temperature is too high or the soaking time is too long, the microstructure of the plated steel sheet coarsens due to grain growth, and the crashworthiness of the hot-stamped product deteriorates.
  • the average heating rate up to the soaking temperature is preferably set to 1 °C/second or faster, the soaking temperature is preferably set to 800°C or lower or 760°C or lower, and the soaking time (the retention time at the soaking temperature) is preferably set to shorter than 300 seconds or shorter than 120 seconds.
  • the average heating rate up to the soaking temperature in the continuous annealing is preferably set to 1 °C/second or faster.
  • temper rolling may be carried out according to a normal method.
  • Molten steel was cast using a vacuum melting furnace to manufacture steels A to V having chemical compositions shown in Table 1.
  • the Ac 3 points in Table 1 were obtained from changes in thermal expansion when cold-rolled steel sheets having the chemical compositions of the steels A to V were heated at 8 °C/second. After the steels A to V were heated to 1200°C and retained for 60 minutes, hot rolling was carried out under hot rolling conditions shown in Table 2.
  • the column of the sheet thickness after rolling as a hot rolling condition indicates the sheet thickness of the hot-rolled steel sheet.
  • the column of the sheet thickness after rolling as a cold rolling condition indicates the sheet thickness of the cold-rolled steel sheet.
  • the "-" marks indicate that the cold rolling was not carried out. 3.
  • the "Present” marks indicate that annealing was carried out, and the "Absent” marks indicate that annealing was not carried out.
  • hot-rolled steel sheets were picked to produce base materials for cold rolling, and cold rolling was carried out under cold rolling conditions shown in Table 2 to produce cold-rolled steel sheets having a thickness of 1.4 mm.
  • some of the hot-rolled steel sheets were mechanically ground to produce hot-rolled and ground steel sheets having a thickness of 1.4 mm.
  • some of the cold-rolled steel sheets were heated up to 780°C at an average heating rate of 5 °C/second and soaked for 120 seconds using a continuous annealing simulator. Subsequently, the cold-rolled steel sheets were cooled to room temperature at an average cooling rate of 5 °C/second to produce annealed steel sheets.
  • JIS No. 13B tensile test pieces were collected from the steel sheets for hot stamping along a direction orthogonal to the rolling direction, a tensile test was carried out at a tension rate of 10 mm/minute, and the tensile strengths were obtained.
  • Table 2 shows the observation results of the microstructures of the steel sheets for hot stamping and the investigation results of the mechanical properties of the steel sheets for hot stamping.
  • Raw sheets for hot stamping that were 240 mm in width and 800 mm in length were collected from the steel sheets for hot stamping, and hat members having a shape shown in Fig. 2 were manufactured by hot stamping.
  • the raw sheets (steel sheets for hot stamping) were heated using a gas heating furnace up to the heating temperatures shown in Table 3-1 at an average heating rate up to 700°C set to 22 °C/second and retained at the temperatures for one minute.
  • the raw sheets were taken out from the heating furnace, cooled in the air, inserted into a die including a cooling apparatus to be formed into a hat shape at start temperatures shown in Table 3-1 and subsequently cooled in the die to cooling stop temperatures shown in Table 3-1.
  • some of the hat members were reheated using an electrical heating furnace under conditions shown in Table 3-1.
  • the "-" marks in Table 3-1 regarding the hot stamping conditions indicate that the reheating step was not carried out.
  • Test pieces for structural observation were collected from the vertical wall portions of the obtained hat members (hot-stamped products), the longitudinal sections of these test pieces were polished, and then the microstructures at the 1/4 depth positions of the sheet thicknesses of the steel sheets from the surfaces of the steel sheets were observed by the above-described method.
  • JIS No. 13B tensile test pieces were collected from the vertical wall portions of the hat members along the longitudinal directions of the members, a tensile test was carried out at a tension rate of 10 mm/minute, and the tensile strengths, the yield stresses and the yield ratios were obtained.
  • test pieces for hardness measurement were collected from the vertical wall portions of the hat members, the longitudinal sections of these test pieces were polished, then, Vickers hardness were measured according to JIS Z 2244: 2009 at the 1/4 depth positions of the sheet thicknesses of the steel sheets from the surfaces of the steel sheets by the above-described method at an applied load of 0.49 N, and the average values and standard deviations of the Vickers hardness were obtained.
  • closing plates that were 1.4 mm in thickness, 130 mm in width and 800 mm in length were welded to the hat members to manufacture test bodies for a three-point bend test.
  • As the closing plate a steel sheet having a tensile strength of 1553 MPa was used.
  • the 800 mm-long test body was placed on two support rolls disposed at a roll interval of 700 mm such that the closing plate faced downward, a three-point bend test was carried out at a testing rate of 2 m/second, and the maximum load, the displacement caused while the test body and an impactor came into contact with each other and then cracking began to occur in the test body and the absorbed energy until cracking began to occur were obtained.
  • the maximum load was 23.0 kN or more
  • the cracking occurrence displacement was 35 mm or more
  • the absorbed energy was 0.80 kJ or more
  • Table 3-1 and Table 3-2 show the observation results of the microstructures of the hat members, the evaluation results of the mechanical properties of the hat members and the evaluation results of the crashworthiness of the hat members.
  • underlined numerical values mean that the corresponding values are outside the ranges of the present invention. [Table 3-1] Test No.
  • the column of the start temperature as a hot stamping condition indicates the forming start temperature. 2.
  • the "-" marks in the column of the reheating temperature and the column of the retention time as hot stamping conditions indicate that the reheating treatment was not carried out. 3.
  • the "-" marks in the column of the tensile strength, the column of the yield stress and the column of the yield ratio as the mechanical properties of the hot-stamped product indicate that the tensile strength, the yield stress and the yield ratio were not measurable.
  • the column of the absorbed energy as the crashworthiness of the hot-stamped product indicates the absorbed energy until the occurrence of cracking. [Table 3-2] Test No.
  • the column of the start temperature as a hot stamping condition indicates the forming start temperature. 2.
  • the "-" marks in the column of the reheating temperature and the column of the retention time as hot stamping conditions indicate that the reheating treatment was not carried out. 3.
  • the "-" marks in the column of the tensile strength, the column of the yield stress and the column of the yield ratio as the mechanical properties of the hot-stamped product indicate that the tensile strength, the yield stress and the yield ratio were not measurable.
  • the column of the absorbed energy as the crashworthiness of the hot-stamped product indicates the absorbed energy until the occurrence of cracking.
  • the tensile strengths of the hot-stamped products were 2300 MPa or more, the average values of the Vickers hardness were 670 or more and the standard deviations of the Vickers hardness were 20 or less.
  • the maximum loads were 23.0 kN or more, the cracking occurrence displacements were 35 mm or more, the absorbed energies were 0.80 kJ or more, and favorable crashworthiness was exhibited.
  • the tensile strengths of the hot-stamped products were 2300 MPa or more, the average values of the Vickers hardness were 670 or more and the standard deviations of the Vickers hardness were 10 or less.
  • the yield ratios were 0.65 or more, in the three-point bend tests of the formed articles, the maximum loads were 23.0 kN or more, the cracking occurrence displacements were 45 mm or more, the absorbed energies were 0.95 kJ or more, and the crashworthiness was particularly favorable.
  • Test No. 2 where the steel A was used
  • Test No. 8 where the steel B was used
  • Test No. 27 where the steel M was used
  • the standard deviations of the Vickers hardness of the formed articles were more than 20, and the maximum loads, the cracking occurrence displacements and the absorbed energies were low.
  • Test No. 5 where the steel A was used
  • Test No. 11 where the steel B was used
  • Test No. 29 where the steel N was used
  • the standard deviations of the Vickers hardness of the formed articles were more than 20, and the maximum loads, the cracking occurrence displacements and the absorbed energies were low.
  • Test Nos. 3 and 4 where the steel A was used, Test Nos. 9 and 10 where the steel B was used, Test No. 24 where the steel L was used, and Test No. 33 where the steel P was used, since the forming start temperatures in the hot stamping steps were too low, the standard deviations of the Vickers hardness of the formed articles were more than 20, and the cracking occurrence displacements and the absorbed energies were low.
  • Molten steel was cast using a vacuum melting furnace to manufacture steels a to w having chemical compositions shown in Table 4.
  • the Ac 3 points in Table 4 were obtained from changes in thermal expansion when plated steel sheets having the chemical compositions of the steels a to w were heated at 8 °C/second. After the steels a to w were heated to 1200°C and retained for 60 minutes, hot rolling was carried out under hot rolling conditions shown in Table 5.
  • hot-rolled steel sheets were picked to produce base materials for cold rolling, and cold rolling was carried out under cold rolling conditions shown in Table 5 to produce cold-rolled steel sheets having a thickness of 1.4 mm.
  • some of the hot-rolled steel sheets (examples where hot rolling was not carried out) were mechanically ground to produce hot-rolled and ground steel sheets having a thickness of 1.4 mm.
  • the obtained steel sheets (cold-rolled steel sheets and hot-rolled and ground steel sheets) were heated up to the soaking temperatures of annealing shown in Table 5 at an average heating rate of 5 °C/second and soaked for 120 seconds using a hot-dip plating simulator. Subsequently, the steel sheets were cooled and immersed in a hot-dip galvanizing bath or hot-dip aluminum plating bath, and hot-dip galvanizing or hot-dip aluminum plating was carried out. After the hot-dip galvanizing, some of the material steel sheets were heated up to 520°C to carry out an alloying treatment. [Table 4] Steel Chemical composition (mass%) (remainder: Fe and impurity) Ac 3 point (°C) C Si Mn P S sol.
  • the column of the sheet thickness after rolling as a hot rolling condition indicates the sheet thickness of the hot-rolled steel sheet.
  • the column of the sheet thickness after rolling as a cold rolling condition indicates the sheet thickness of the cold-rolled steel sheet.
  • the "-" marks indicate that the cold rolling was not carried out. 3.
  • the "GI” marks indicate hot-dip galvanized steel sheets
  • the "GA” marks indicate galvannealed steel sheets
  • the "AL" marks indicate hot-dip aluminum-plated steel sheets.
  • the hot-dip galvanized steel sheets galvannealed steel sheets and hot-dip aluminum-plated steel sheets obtained as described (these steel sheets will be collectively referred to as the steel sheets for hot stamping), raw sheets for hot stamping that were 240 mm in width and 800 mm in length were collected, and hat members having a shape shown in Fig. 2 were manufactured by hot stamping.
  • the raw sheets were heated using a gas heating furnace up to the heating temperatures shown in Table 6-1 at an average heating rate up to 700°C set to 11 °C/second or faster and retained at the temperatures for one minute.
  • the raw sheets were taken out from the heating furnace, cooled in the air, inserted into a die including a cooling apparatus to be formed into a hat shape at start temperatures shown in Table 6-1 and subsequently cooled in the die to cooling stop temperatures shown in Table 6-1.
  • some of the hat members were reheated using an electrical heating furnace under conditions shown in Table 6-1.
  • the "-" marks in Table 6-1 regarding the hot stamping conditions indicate that the reheating step was not carried out.
  • Test pieces for structural observation were collected from the vertical wall portions of the obtained hat members (hot-stamped products), the longitudinal sections of these test pieces were polished, and then the microstructures at the 1/4 depth positions of the sheet thicknesses of the steel sheets, which were substrates, from the interfaces between the steel sheets, which were substrates, and the plated layers were observed by the above-described method.
  • JIS No. 13B tensile test pieces were collected from the vertical wall portions of the hat members along the longitudinal directions of the members, a tensile test was carried out at a tension rate of 10 mm/minute, and the tensile strengths, the yield stresses and the yield ratios were obtained.
  • test pieces for hardness measurement were collected from the vertical wall portions of the hat members, the longitudinal sections of these test pieces were polished, then, Vickers hardness were measured at the 1/4 depth positions of the sheet thicknesses of the steel sheets from the interfaces between the steel sheets and the plated layers by the above-described method at an applied load of 0.49 N, and the average values and standard deviations of the Vickers hardness were obtained.
  • closing plates that were 1.4 mm in thickness, 130 mm in width and 800 mm in length were welded to the hat members to manufacture test bodies for a three-point bend test.
  • As the closing plate a steel sheet having a tensile strength of 1553 MPa was used.
  • the 800 mm-long test body was placed on two support rolls disposed at a roll interval of 700 mm such that the closing plate faced downward, a three-point bend test was carried out at a testing rate of 2 m/second, and the maximum load, the displacement caused while the test body and an impactor came into contact with each other and then cracking began to occur in the test body and the absorbed energy until cracking began to occur were obtained.
  • the maximum load was 23.0 kN or more
  • the cracking occurrence displacement was 35 mm or more
  • the absorbed energy was 0.80 kJ or more
  • Table 6-1 and Table 6-2 show the observation results of the microstructures of the hat members, the evaluation results of the mechanical properties of the hat members and the evaluation results of the crashworthiness of the hat members.
  • underlined numerical values mean that the corresponding values are outside the ranges of the present invention. [Table 6-1] Test No.
  • the column of the start temperature as a hot stamping condition indicates the forming start temperature. 2.
  • the "-" marks in the column of the reheating temperature and the column of the retention time as hot stamping conditions indicate that the reheating treatment was not carried out. 3.
  • the "-" marks in the column of the tensile strength, the column of the yield stress and the column of the yield ratio as the mechanical properties of the hot-stamped product indicate that the tensile strength, the yield stress and the yield ratio were not measurable.
  • the column of the absorbed energy as the crashworthiness of the hot-stamped product indicates the absorbed energy until the occurrence of cracking. [Table 6-2] Test No.
  • the column of the start temperature as a hot stamping condition indicates the forming start temperature. 2.
  • the "-" marks in the column of the reheating temperature and the column of the retention time as hot stamping conditions indicate that the reheating treatment was not carried out. 3.
  • the "-" marks in the column of the tensile strength, the column of the yield stress and the column of the yield ratio as the mechanical properties of the hot-stamped product indicate that the tensile strength, the yield stress and the yield ratio were not measurable.
  • the column of the absorbed energy as the crashworthiness of the hot-stamped product indicates the absorbed energy until the occurrence of cracking.
  • the tensile strengths of the hot-stamped products were 2300 MPa or more, the average values of the Vickers hardness were 670 or more and the standard deviations of the Vickers hardness were 20 or less.
  • the yield ratios were 0.65 or more, in the three-point bend tests of the formed articles, the maximum loads were 23.0 kN or more, the cracking occurrence displacements were 35 mm or more, the absorbed energies were 0.80 kJ or more, and favorable crashworthiness was exhibited.
  • Test Nos. 104 and 105 where the steel a was used, Test Nos. 110 and 111 where the steel b was used, Test No. 114 where the steel c was used, Test No. 119 where the steel e was used and Test No. 130 where the steel n was used, since the forming start temperatures in the hot stamping steps were too low, the standard deviations of the Vickers hardness of the formed articles were more than 20, and the cracking occurrence displacements and the absorbed energies were low.
  • Test No. 112 where the steel b was used, Test No. 117 where the steel d was used and Test No. 132 where the steel o was used, since the reheating temperatures in the reheating steps were too low or the reheating treatments were not carried out, the standard deviations of the Vickers hardness were more than 20, the yield ratios were less than 0.65 and the maximum loads, the cracking occurrence displacements and the absorbed energies were low.

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