US20210054488A1 - Steel plate for hot stamping - Google Patents

Steel plate for hot stamping Download PDF

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Publication number
US20210054488A1
US20210054488A1 US17/041,223 US201917041223A US2021054488A1 US 20210054488 A1 US20210054488 A1 US 20210054488A1 US 201917041223 A US201917041223 A US 201917041223A US 2021054488 A1 US2021054488 A1 US 2021054488A1
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Prior art keywords
steel plate
less
mass
hot stamping
hardness
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US17/041,223
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Inventor
Haruka Araki
Sae Hamamoto
Tatsuya Asai
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Kobe Steel Ltd
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Kobe Steel Ltd
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Priority claimed from PCT/JP2019/011606 external-priority patent/WO2019188622A1/ja
Assigned to KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) reassignment KABUSHIKI KAISHA KOBE SEIKO SHO (KOBE STEEL, LTD.) ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ARAKI, Haruka, ASAI, TATSUYA, HAMAMOTO, SAE
Publication of US20210054488A1 publication Critical patent/US20210054488A1/en
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted 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
    • 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
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    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
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    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • 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
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    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
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    • 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
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    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0405—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing of ferrous alloys
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    • 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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0426—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
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
    • C21D8/0436—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
    • 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
    • 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
    • C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—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
    • C22C38/18—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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
    • 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with 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
    • C22C38/18—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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
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    • 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
    • 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
    • C21D2211/001—Austenite

Definitions

  • the present invention relates to a steel plate for hot stamping.
  • Non Patent Literature 1 proposes that the balance between strength and toughness of a steel plate is improved by refining the former austenite grains after hot stamping.
  • the cooling velocity inside the steel plate may decrease by an increase in the die temperature and the clearance between the die and the steel plate.
  • the cooling velocity of the steel plate is equal to or lower than the critical cooling velocity, soft phases such as ferrite and bainite precipitate and the hardness of the steel plate thus decreases.
  • the cooling velocity at a temperature equal to or lower than the Ms point decreases, auto tempering is promoted and this causes a decrease in hardness of the steel plate.
  • Non Patent Literature 2 a change in cooling velocity when changing the clearance between the die and the steel plate is examined and it has been indicated that the cooling velocity decreases to about 15° C./s when this clearance is 0.4 mm.
  • Non Patent Literature 1 there is a method in which the crystal grains of steel are refined as a general structure design technology of steel plates for hot stamping and this method makes it possible to obtain a steel plate having an excellent balance between strength and toughness.
  • a method for refining the crystal grains there is a method in which elements such as Nb, Ni, and Ti are added, but the economical efficiency of the steel plate becomes poor in this case.
  • a steel plate having refined crystal grains exhibits poor hardenability and thus lacks hardness stability.
  • An object of the present invention is to provide a steel plate for hot stamping which can provide a molded product which exhibits excellent hardness stability in addition to the balance between strength and toughness while suppressing increases in labor and cost in the hot stamping process.
  • a steel plate for hot stamping according to an aspect of the present invention contains,
  • Si 1.05% or more and 1.4% or less
  • Al 0.01% or more and 1% or less
  • N 0% or more and 0.01% or less
  • This steel plate for hot stamping exhibits excellent hardness stability in addition to a balance between strength and toughness as a following relational expression (1) is satisfied, where [C] denotes a C content, [Si] denotes a Si content, [Mn] denotes a Mn content, and [Cr] denotes a Cr content.
  • a steel plate for hot stamping which can provide a molded product which exhibits excellent hardness stability in addition to the balance between strength and toughness while suppressing increases in labor and cost in the hot stamping process.
  • FIG. 1 is a graph illustrating the relation between absorbed energy in a Charpy impact test when a flat plate is hardened using a die and hardness when hardening is performed at a cooling velocity of 10° C./s.
  • FIG. 2 is a diagram schematically illustrating a hot stamping process.
  • FIG. 3 is a schematic diagram illustrating the respective dimensions of a test piece used in a Charpy pendulum impact test.
  • FIG. 4 is a schematic diagram illustrating the respective dimensions of a test piece used in a hardness test.
  • the steel plate for hot stamping according to the present embodiment contains,
  • Si 1.05% or more and 1.4% or less
  • Al 0.01% or more and 1% or less
  • N 0% or more and 0.01% or less
  • This steel plate for hot stamping exhibits excellent hardness stability in addition to a balance between strength and toughness as a following relational expression (1) is satisfied, where [C] denotes a C content, [Si] denotes a Si content, [Mn] denotes a Mn content, and [Cr] denotes a Cr content.
  • Non Patent Literature 2 it has been expected that the cooling velocity of a normal member fluctuates in a range of 30° C./s to 10° C./s in the hot stamping process due to the clearance between the die and the steel plate and an increase in the die temperature. For this reason, the present inventors have focused on suppression of the variation in hardness even when the cooling velocity fluctuates in addition to the balance between strength and toughness, and conducted detailed investigations on the component system of steel plate for achieving this.
  • the present inventors have newly found out that the balance between strength and toughness and hardness stability can be both achieved by adjusting the balance among the contents of C, Si, Mn, and Cr so that the relational expression (1) is satisfied as well as each component composition in a steel plate satisfies the above range, and thus conceived the present invention.
  • the C content determines the strength of the steel plate after die cooling.
  • the C content is 0.25% by mass or more, preferably 0.255% by mass or more, more preferably 0.260% by mass or more.
  • the C content is 0.4% by mass or less, preferably 0.38% by mass or less, more preferably 0.36% by mass or less.
  • Si contributes to the hardness stability of the steel plate by increasing the temper softening resistance. Si also has an effect of preventing scale peeling off after die cooling when the surface of the steel plate is not plated. In order to exert these effects, the Si content is 1.05% by mass or more.
  • Si facilitates the generation of retained austenite (y) and promotes a decrease in yield strength (YS) and segregation of Mn.
  • the Si content is 1.4% by mass or less, preferably 1.35% by mass or less.
  • Mn is one of the important elements contained in the steel plate for hot stamping according to the present embodiment and contributes to an increase in the strength of the steel plate after die cooling by enhancing the hardenability of the steel plate.
  • the Mn content is preferably 0.5% by mass or more, more preferably 0.6% by mass or more, still more preferably 0.8% by mass or more.
  • the Mn content is 1.4% by mass or less, preferably 1.35% by mass or less, more preferably 1.30% by mass or less.
  • Mn is an element that is inevitably mixed into the steel plate and it is thus difficult to set the Mn content to 0% by mass.
  • Cr is one of the important elements in the steel plate for hot stamping according to the present embodiment.
  • Cr contributes to securing of the hardness at a low cooling velocity (for example, 10° C./s) as well as suppression of coarse carbide precipitation during die cooling and thus suppresses brittle fracture when shocking stress is applied to the steel plate in a low temperature environment.
  • the Cr content is 0.6% by mass or more, preferably 0.8% by mass or more, more preferably 1.05% by mass or more.
  • the Cr content is 3.0% by mass or less, preferably 2.5% by mass or less.
  • the P content is 0.03% by mass or less, preferably 0.025% by mass or less, more preferably 0.02% by mass or less.
  • P is an element that is inevitably mixed into the steel plate and it is thus difficult to set the P content to 0% by mass.
  • the S content is 0.02% by mass or less, preferably 0.018% by mass or less, more preferably 0.015% by mass or less.
  • S is an element that is inevitably mixed into the steel plate as P and it is thus difficult to set the S content to 0% by mass.
  • Al is an element that acts as a deoxidizer. In order to exert this effect, the Al content is 0.01% by mass or more, preferably 0.015% by mass or more.
  • the Al content is 1% by mass or less, preferably 0.8% by mass or less, more preferably 0.1% by mass or less.
  • the Al content here means the content of Al(sol.Al) in a solid solution state.
  • N is an element that is inevitably mixed into the steel plate.
  • the N content is 0.01% by mass or less, preferably 0.008% by mass or less, more preferably 0.005% by mass or less.
  • B is an important element for improving the hardenability of the steel plate.
  • the hardenability is enhanced and this makes it possible to stably increase the strength of the steel plate after die cooling.
  • the B content is 0.0005% by mass or more, preferably 0.0010% by mass or more, more preferably 0.0015% by mass or more.
  • the B content is 0.0050% by mass or less, preferably 0.0045% by mass or less, more preferably 0.0030% by mass or less.
  • Ti decreases the amount of BN generated in the steel plate by forming TiN. This increases the amount of solid solution B in the steel plate and makes it possible to enhance the hardenability improving effect by B.
  • the Ti content is 0.0050% by mass or more, preferably 0.010% by mass or more, more preferably 0.015% by mass or more.
  • the Ti content is 0.1% by mass or less, preferably 0.08% by mass or less, more preferably 0.06% by mass or less.
  • the steel plate for hot stamping according to the present embodiment may further contain one or more selected from the group consisting of Mo, Nb, and V or one or more selected from the group consisting of Cu and Ni in addition to the above component composition.
  • the ranges of component compositions of these will be described below. These elements are not essential elements in the steel plate for hot stamping of the present invention and may not be added.
  • Mo is an element that contributes to the improvement in hardenability of the steel plate.
  • the Mo content is preferably 0.01% by mass or more.
  • the Mo content is preferably 1.0% by mass or less.
  • Nb and V form fine carbides and have the effect of refining the structure of steel by the pinning effect. V also has a secondary hardening action by being precipitated during tempering. In order to exert these effects, the Nb and V contents are both preferably 0.0008% by mass or more.
  • the Nb and V contents are both preferably 0.1% by mass or less, more preferably 0.08% by mass or less, still more preferably 0.07% by mass or less.
  • Cu and Ni are preferably added when it is required to improve the delayed fracture properties of the member.
  • the Cu and Ni contents are each 0.5% by mass or less and it is more preferable that the sum of the Cu and Ni contents is 0.5% by mass or less.
  • the steel plate for hot stamping according to the present embodiment exhibits excellent hardness stability in addition to the balance between strength and toughness as the following relational expression (1) is satisfied by adjustment of the balance among the contents of C, Si. Mn, and Cr.
  • [C] denotes the C content (% by mass) in the steel plate for hot stamping.
  • Si denotes the Si content (% by mass) in the steel plate for hot stamping.
  • Mn denotes the Mn content (% by mass) in the steel plate for hot stamping.
  • Cr denotes the Cr content (% by mass) in the steel plate for hot stamping.
  • the steel plate for hot stamping according to the present embodiment exhibits excellent hardness stability as well as is a steel plate having an excellent balance between the strength after hardening by die cooling and the low temperature toughness.
  • the following relational expressions (2), (3), and (4) are all satisfied where, A (J/cm 2 ) denotes the absorbed energy in a Charpy impact test at ⁇ 40° C.
  • B(Hv) denotes the hardness when the steel plate for hot stamping is heated to the austenite region, then cooled to room temperature at a cooling velocity of 10° C./s, and hardened
  • C(Hv) denotes the hardness when the steel plate for hot stamping is heated to the austenite range, then cooled to room temperature at a cooling velocity of 30° C./s, and hardened.
  • the relational expression (2) is an index of the balance between the strength and toughness of the steel plate newly devised by the present inventors and is an important concept when considering the balance between the strength and toughness of the steel plate for hot stamping.
  • the present inventors have focused on the hardness when the cooling velocity is 10° C./s and the toughness after die cooling of a flat plate. In the die cooling of a flat plate, ideal cooling conditions in which a clearance is not generated between the die and the steel plate in the hot stamping process are taken into consideration.
  • the relational expression (2) it is possible to more faithfully evaluate the balance between strength and toughness when the steel plate for hot stamping is processed into a member (molded product).
  • the graph of FIG. 1 illustrates the relation between the absorbed energy A (horizontal axis) in a Charpy impact test at 10° C. when a flat plate is hardened using a die and the hardness B (vertical axis) of the steel plate when being hardened at a cooling velocity of 10° C./s.
  • the straight line (1) in this graph corresponds to the relational expression (2).
  • the horizontal axis (A) of the graph of FIG. 1 assumes the toughness at the most brittle portion of the member after die cooling.
  • the die and the steel plate are in contact with each other in an ideal state and the cooling velocity is thus high. For this reason, the strength after cooling is high but, on the other hand, the flat plate is extremely brittle.
  • this horizontal axis has a meaning as toughness at the most brittle portion when the steel plate for hot stamping is molded into a member (molded product).
  • the vertical axis (B) of the graph of FIG. 1 assumes the hardness of the most softened portion of the member after die cooling.
  • the member after die cooling has a portion that is cooled at a low cooling velocity and has a low hardness (strength).
  • the minimum cooling velocity during die cooling is about 10° C./s.
  • this vertical axis has a meaning as hardness (strength) at the most softened portion of the member (molded product) after die cooling. Consequently, by using these two axes, the toughness of the weakest portion when shocking stress is applied to the member after being molded and the strength of the weakest portion when static stress is applied to this member can be evaluated.
  • the straight line (2) is one index that indicates the hardness stability.
  • the temperature of the die may rise and a clearance may be generated between the die and the steel plate. Due to these factors, the cooling velocity of the steel plate during hardening decreases and the hardness of the steel plate after being hardened decreases as the cooling velocity decreases.
  • a steel plate in which the balance between strength and toughness is improved by the refinement of crystal grains it is usually difficult for the hardness when the steel plate is hardened in a low cooling velocity region (10° C./s) to satisfy a range of 516 Hv or more.
  • a low cooling velocity region (10° C./s)
  • the steel plate for hot stamping according to the present embodiment exhibits excellent hardness stability in addition to the balance between strength and toughness.
  • this steel plate for hot stamping has an excellent balance between strength and toughness that satisfies the relational expression (2) and can realize a hardness at a certain degree or more even when being cooled at the minimum cooling velocity of 10° C./s.
  • the relational expression (4) is another index of the hardness stability of steel plate.
  • the cooling velocity of the steel plate may decrease and the hardness of the steel plate after being hardened may become unstable.
  • the relational expression (4) is an index indicating that the difference (variation) in hardness after hardening is small between the upper and lower limits of the cooling velocity assumed in hot stamping. According to the steel plate for hot stamping of the present embodiment, it is possible to stabilize the hardness of the steel plate after being hardened to the extent to which the relational expression (4) is satisfied regardless of the temperature rise of the die and the generation of clearance between the die and the steel plate.
  • the steel plate for hot stamping of the present invention may be a base steel plate having a surface not subjected to a plating treatment or a plated steel plate having a surface subjected to a plating treatment.
  • a slab manufacturing process is performed.
  • a slab is obtained by melting steel according to a conventional method, pouring the molten steel into a mold, and performing continuous casting.
  • the component composition of the steel is adjusted during melting so that the compositions of the respective components contained in the slab satisfy the above ranges and the contents of C, Si, Mn, and Cr satisfy the relational expression (1).
  • the slab obtained in the above process is first disposed in a heating furnace, heated to a predetermined temperature (for example, 1200° C.), and held at the heating temperature for a predetermined time (for example, 30 minutes).
  • a predetermined temperature for example, 1200° C.
  • the heated slab is placed upstream of the hot rolling line. Thereafter, the slab is rolled into a steel plate having a predetermined thickness by allowing the slab to sequentially pass through between the rolls of the rolling stands of the rough rolling mill and the finishing rolling mill and allowing the slab to flow downstream. Thereafter, the steel plate after being hot-rolled is cooled to a predetermined temperature in a cooling apparatus and then wound by a coiler.
  • a cold rolling process is performed.
  • the scale (oxides of iron) generated on the surface of the steel plate in the hot rolling step is first washed off with an acid (pickling) and then the hot-rolled steel plate is further rolled so that the thickness further decreases.
  • the hot-rolled steel plate after being subjected to pickling is allowed to pass through between the rolls of the rolling stands so that the hot-rolled steel plate is further thinned.
  • the cold-rolled steel plate obtained by the above processes is the steel plate for hot stamping according to the present embodiment.
  • a steel plate for hot stamping 1 manufactured by the above processes is heated in a predetermined heating furnace 2 to a temperature equal to or higher than the austenite transformation temperature. Thereafter, the steel plate for hot stamping 1 after being heated is disposed between dies 3 and 4 and press-molded into a desired shape by the dies 3 and 4 . At this time, the steel plate for hot stamping 1 is cooled by coming into contact with the dies 3 and 4 , and hardening is performed at the same time as molding. Thereafter, the steel plate after being hardened is taken out from the dies 3 and 4 as a molded product 5 (molded member).
  • the molded product 5 has the same component composition as that of the steel plate for hot stamping 1 according to the present embodiment described above and is one in which the balance among the contents of C, Si, Mn, and Cr is adjusted so that the relational expression (1) is satisfied. Hence, the molded product 5 exhibits excellent hardness stability in addition to the balance between strength and toughness and can be utilized in various applications including members for motor vehicles.
  • a steel plate for hot stamping according to the present embodiment contains,
  • Si 0.05% or more and 1.4% or less
  • Al 0.01% or more and 1% or less
  • N 0% or more and 0.01% or less
  • This steel plate for hot stamping exhibits
  • the steel plate for hot stamping may contain, in % by mass, one or more selected from the group consisting of
  • Nb 0% or more and 0.1% or less
  • V 0% or more and 0.1% or less.
  • the steel plate for hot stamping may contain, in % by mass, one or more selected from the group consisting of
  • Ni 0% or more and 0.5% or less.
  • the present invention will be described in more detail based on examples.
  • the present invention is not limited to the following examples, it is also possible to carry out the present invention by adding changes within a range that is compatible with the above-mentioned and below-mentioned gist, and all of them are included in the technical scope of the present invention.
  • a slab was manufactured by melting steel (the remainder being iron and inevitable impurities) having the component composition shown in Nos. 1 to 17 in the following Table 1.
  • This molten slab was heated to 1200° C., held for 30 minutes, and then hot-rolled.
  • the finishing temperature was set to 900 ⁇ 20° C., and the finishing plate thickness was set to 2.8 mm.
  • the hot-rolled steel plate was cooled to a winding temperature (CT temperature) at a cooling velocity of 20° C./s to 30° C./s, held at 650° C. for 30 minutes, and then cooled in the furnace.
  • CT temperature winding temperature
  • the hot-rolled steel plate was subjected to pickling and cold-rolled so that the steel plate had a thickness of 1.4 mm.
  • the cold-rolled steel plate fabricated according to the above procedure was cut and hardened. Hardening was performed under the following conditions by a die quench method using a flat plate simulating a die (testing machine: JIS Charpy impact tester (300J)).
  • Cooling time about 15 seconds
  • a Charpy pendulum impact test was performed using the cold-rolled steel plate after being subjected to the hardening. This test was performed in conformity with JIS 2242 “Charpy impact test method for metal materials” except the dimensions of the test piece.
  • the dimensions of the test piece used in the present test are as follows. The reference numerals indicating the respective dimensions correspond to the reference numerals illustrated in FIG. 3 .
  • Test piece height h 1 10 mm ⁇ 0.05 mm
  • Test piece length L 55 mm ⁇ 0.6 mm
  • Test piece width b 1.4 mm ⁇ 0.05 mm
  • Notch bottom radius 0.25 mm ⁇ 0.025 mm
  • test piece having the above dimensions was disposed in liquid nitrogen adjusted to have a temperature of ⁇ 40° C. ⁇ 1° C. and held for at least 10 minutes. Thereafter, the test piece was taken out from the liquid nitrogen and placed on a support, and an impact was made on the test piece. At this time, the time until the impact was made after the test piece was placed on a support was set to 5 seconds or less.
  • a JIS Charpy impact tester 300J was used as a tester, and an impact blade having a radius of 2 mm was used. The number of test pieces was two, and the average value of two measured values was used for evaluation.
  • the cold-rolled steel plate fabricated according to the above procedure was processed into a test piece having a shape illustrated in FIG. 4 .
  • L 1 is 10 mm
  • L 2 is 2 mm
  • L 3 is 1.4 mm
  • L 4 is 0.7 mm
  • L 5 is 3 mm
  • L 6 is 1 mm. Hardening was performed using this test piece under the following conditions.
  • Rate of temperature rise when converting to austenite 10° C./s
  • High temperature holding held at 900° C. for 100 seconds
  • Cooling velocity constant cooling from 900° C. to room temperature at 10° C./s or 30° C./s
  • a hardness test was performed using the test piece after being subjected to the hardening in conformity with the “Vickers hardness test method” prescribed in JIS Z 2244. In this test, five points were measured at the positions to be the 1 ⁇ 4 plate thickness from the surface of the test piece at a test load of 9.8 N, and evaluation was performed using the average value of these.
  • Tables 1 and 2 present each of the component composition (% by mass), absorbed energy A (J/cm 2 ) in a Charpy impact test at ⁇ 40° C., Vickers hardness B (Hv) when the cooling velocity is 10° C./s, Vickers hardness C (Hv) when the cooling velocity is 30° C./s, difference in hardness (Hv) between a case having a cooling velocity of 30° C./s and a case having a cooling velocity of 10° C./s, value on the left side of the relational expression (1), value when the right side of the relational expression (2) is subtracted from the left side, and evaluation results on scale adhesive property for each of Nos. 1 to 17 steel plates.
  • the Si content is less than 1.05% by mass and the value of “[C]+2/9[Si]+7/9[Mn]+8/9[Cr] ⁇ 7/4” is a negative value, thus the value of “B+4A ⁇ 627” is a negative value and the balance between strength and toughness is poor.
  • the hardness B when the cooling velocity is 10° C./s is less than 516 Hv, the difference in hardness between a case having a cooling velocity of 30° C./s and a case having a cooling velocity of 10° C./s also exceeds 35 Hv, and the hardness stability is also poor.
  • the evaluation results on the scale adhesive property are also “x”.
  • the Cr content is less than 0.6% by mass and the value of “[C]+2/9[Si]+7/9[Mn]+8/9[Cr] ⁇ 7/4” is a negative value, thus the value of “B+4A ⁇ 627” is a negative value and the balance between strength and toughness is poor.
  • the hardness B when the cooling velocity is 10° C./s is less than 516 Hv, the difference in hardness between a case having a cooling velocity of 30° C./s and a case having a cooling velocity of 10° C./s also exceeds 35 Hv, and the hardness stability is also poor.

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