EP4682280A1 - Steel member and steel sheet - Google Patents

Steel member and steel sheet

Info

Publication number
EP4682280A1
EP4682280A1 EP24770602.1A EP24770602A EP4682280A1 EP 4682280 A1 EP4682280 A1 EP 4682280A1 EP 24770602 A EP24770602 A EP 24770602A EP 4682280 A1 EP4682280 A1 EP 4682280A1
Authority
EP
European Patent Office
Prior art keywords
less
content
steel
steel sheet
steel member
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
EP24770602.1A
Other languages
German (de)
English (en)
French (fr)
Inventor
Shohei Yabu
Shinichiro TABATA
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
Original Assignee
Nippon Steel Corp
Priority date (The priority date 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 date listed.)
Filing date
Publication date
Application filed by Nippon Steel Corp filed Critical Nippon Steel Corp
Publication of EP4682280A1 publication Critical patent/EP4682280A1/en
Pending legal-status Critical Current

Links

Classifications

    • 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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • 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
    • C21D1/62—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
    • 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
    • C22C38/002—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
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • 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
    • C22C38/08—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
    • C22C38/10—Ferrous alloys, e.g. steel alloys containing cobalt
    • 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
    • C22C38/18—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
    • C22C38/30—Ferrous alloys, e.g. steel alloys containing chromium with cobalt
    • 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
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    • 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
    • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—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
    • C22C38/44—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
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
    • C22C38/40—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
    • 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
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • 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
    • C23C2/26—After-treatment
    • C23C2/28—Thermal after-treatment, e.g. treatment in oil bath
    • C23C2/29—Cooling or quenching
    • C—CHEMISTRY; METALLURGY
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    • 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
    • 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
    • C21D2211/002—Bainite
    • 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 steel member and a steel sheet.
  • ductility of the steel sheet decreases with the high-strengthening, and there is a problem in that the steel sheet is fractured at a highly processed portion in the case of being processed into a complex shape.
  • residual stress after processing causes springback and wall curvature, which also causes a problem that dimensional accuracy is deteriorated. Therefore, it is not easy to perform press forming on a steel sheet having high strength, particularly a tensile strength of 780 MPa or more, into a product having a complex shape.
  • Roll forming rather than press forming makes it easier to process a high strength steel sheet, but the application thereof is limited to components having a uniform cross section in a longitudinal direction thereof.
  • hot stamping has been adopted as a technology of press-forming a material that is difficult to form, such as a high strength steel sheet.
  • the hot stamping is a hot forming technology of heating a material to be subjected to forming and then forming the material.
  • the material is formed after being heated. Therefore, the steel is soft at the time of forming and has good formability. Accordingly, even a high strength steel sheet can be accurately formed into a complex shape. Furthermore, in the hot stamping, since quenching is performed simultaneously with forming by a press die, steel (steel member) after the forming has sufficient strength.
  • Patent Document 1 it is disclosed that it is possible to impart a tensile strength of 1,400 MPa or more to a steel member obtained by forming a steel sheet through the hot stamping.
  • Patent Document 2 discloses a press-formed article that has excellent toughness and a tensile strength of 1.8 GPa or more and is hot press-formed.
  • Patent Document 3 discloses a steel having a tensile strength as extremely high as 2.0 GPa or more, and further having good toughness and ductility.
  • Patent Document 4 discloses a steel having a tensile strength as high as 1.8 GPa or more and further having good toughness.
  • Patent Document 5 discloses a steel having a tensile strength as extremely high as 2.0 GPa or more and further having good toughness.
  • An object of the present invention is to provide a steel member having high strength and excellent hydrogen embrittlement resistance, and a steel sheet suitable as a material for the steel member.
  • the present inventors investigated influences of a steel sheet that serves as a material and a microstructure on these properties. As a result, the following findings were obtained.
  • the present inventors conducted a detailed examination to obtain a steel member having a 3 strength as high as more than 1.5 GPa after a heat treatment by increasing a C content. As a result, it was found that by setting the C content to 0.260 mass% or more, an ultrahigh strength of more than 1.5 GPa in terms of tensile strength can be obtained after a heat treatment including quenching such as hot stamping.
  • the present inventors examined a method for improving the hydrogen embrittlement resistance in a high strength steel member having a tensile strength of more than 1.5 GPa. As a result, it was found that the hydrogen embrittlement resistance can be improved by developing a specific texture.
  • the present invention has been made in view of the above findings.
  • the gist of the present invention is as follows.
  • a steel member according to an embodiment of the present invention (a steel member according to the present embodiment), a steel sheet according to an embodiment of the present invention suitable as a material thereof (a steel sheet according to the present embodiment), and manufacturing methods thereof will be described.
  • the steel member according to the present embodiment has a chemical composition, which will be described later, and a microstructure at a 1/4 depth position (a range between a 1/8 position of a thickness and a 3/8 position of the thickness in a thickness direction (in a case of including a steel sheet, a sheet thickness direction of the steel sheet) from a surface of the steel member, with respect to a 1/4 position of the thickness from the surface as a center, hereinafter, the same applies) includes, by area ratio, martensite, bainite, and tempered martensite: 90% or more in total.
  • the steel member at the 1/4 depth position, when a random intensity ratio of ⁇ 111 ⁇ 011> is denoted by I1, a random intensity ratio of ⁇ 111 ⁇ 112> is denoted by I2, a random intensity ratio of ⁇ 100 ⁇ 011> is denoted by I3, and a random intensity ratio of ⁇ 100 ⁇ 001> is denoted by I4, the steel member has a texture in which the I1, the I2, the I3, and the I4 satisfy (I1 + I3)/(I2 + I4) ⁇ 1.20.
  • the steel member according to the present embodiment may be coated on the surface.
  • the chemical composition, microstructure, texture, and the like of the steel member are a chemical composition, a microstructure, a texture, and the like of a part excluding the coating (this part is sometimes referred to as a "base steel member” or "base metal steel member”).
  • the surface of the steel member according to the present embodiment is coated, that is, the surface of the steel member according to the present embodiment serving as a reference for the 1/4 depth position is a surface of the part excluding the coating (base steel member), that is, a boundary between the base steel member and the coating.
  • a shape of the steel member according to the present embodiment is not particularly limited. That is, the steel member may be a flat sheet, or may be a formed body obtained by forming a steel sheet into a predetermined shape.
  • a hot-formed steel member is often a formed body, for example, a hot-stamping formed body.
  • a case of a formed body and a case of a flat sheet are collectively referred to as a "steel member".
  • the chemical composition of the steel member according to the present embodiment includes, by mass%: C: 0.260% to 0.700%; Si: 0% to 2.000%; Mn: 0% to 3.00%; Al: 0% to 1.000%; Nb: 0% to 0.100%; Ti: 0% to 0.200%; Cr: 0% to 1.00%; B: 0% to 0.0200%; Mo: 0% to 1.00%; W: 0% to 2.00%; Co: 0% to 1.00%; Ni: 0% to 2.00%; Cu: 0% to 2.00%; V: 0% to 1.00%; Ca: 0% to 0.200%; Mg: 0% to 0.20%; REM: 0% to 0.300%; Sb: 0% to 1.00%; Sn: 0% to 1.00%; Zr: 0% to 1.00%; As: 0% to 1.00%; Se: 0% to 1.00%; Bi: 0% to 1.00%; Ta: 0% to 1.00%; Re: 0% to 1.00%; Os: 0%
  • C is an element that enhances hardenability of steel and improves strength of a steel member obtained after a steel sheet is subjected to a heat treatment including quenching such as hot stamping (post-quenching).
  • quenching such as hot stamping (post-quenching).
  • the C content is set to 0.260% or more.
  • the C content is set to preferably 0.280% or more, and more preferably 0.310% or more.
  • the C content is preferably 0.450% or more.
  • the C content is set to 0.700% or less.
  • the C content is set to preferably 0.650% or less, and more preferably 0.600% or less.
  • Si does not have to be contained (may be 0%), but is an effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching. Therefore, Si may be contained.
  • a Si content is set to preferably 0.100% or more, and more preferably 0.350% or more.
  • the Si content in steel is more than 2.000%, a heating temperature required for austenitic transformation becomes significantly high during the heat treatment (quenching). Accordingly, there are cases where the cost required for the heat treatment increases, or ferrite remains during heating, resulting in a decrease in the strength of the steel member. Therefore, the Si content is set to 2.000% or less. The Si content is preferably set to 1.500% or less.
  • Mn does not have to be contained (may be 0%), but is a very effective element for enhancing the hardenability of the steel and stably securing the strength after quenching. Further, Mn is an element that lowers an Ac3 point and promotes lowering of a quenching treatment temperature. Therefore, Mn may be contained. In a case of obtaining the above effect, a Mn content is set to preferably 0.05% or more, and more preferably 0.15% or more or 0.40% or more.
  • the Mn content is set to 3.00% or less.
  • the Mn content is set to preferably 2.50% or less, and more preferably 1.50% or less.
  • Al is an element generally used as a steel deoxidizing agent. Therefore, Al may be contained.
  • An Al content may be 0%, but in order to obtain the above effects, the Al content is preferably set to 0.010% or more.
  • the Al content may be set to 0.020% or more or 0.030% or more as necessary.
  • the Al content is set to 1.000% or less.
  • the Al content may be set to 0.300% or less, 0.100% or less, or 0.075% or less as necessary.
  • the Al content mentioned here is the total A1 content.
  • Nb is an element that forms fine carbides, nitrides, or carbonitrides in steel and suppresses Cu hot embrittlement cracking in a hot rolling step through a grain refining effect of these precipitates.
  • the hydrogen embrittlement resistance of the steel member is improved by concentrating W of Nb-based precipitates (making a W concentration higher than a W concentration of a base steel material). Therefore, Nb may be contained.
  • a Nb content may be 0%, but in a case where the above effects are obtained, the Nb content is preferably set to 0.005% or more.
  • the Nb content is more preferably 0.010% or more.
  • the Nb content is set to 0.100% or less.
  • the Nb content is preferably 0.080% or less.
  • the Nb content may be set to 0.060% or less or 0.040% or less as necessary.
  • Ti is an element that forms fine carbides, carbonitrides, and the like together with Nb in steel, suppresses Cu hot embrittlement cracking in the hot rolling step through the grain refining effect thereof, and has an action of improving the hydrogen embrittlement resistance of the steel member.
  • Ti is an element that also forms nitrides by being preferentially bonded to N in the steel, suppresses the consumption of solute B due to precipitation of BN, and promotes an effect of improving the hardenability by B, which will be described later. Therefore, Ti may be contained.
  • a Ti content may be 0%, but in a case where the above effects are obtained, the Ti content is preferably set to 0.005% or more.
  • the Ti content is set to more preferably 0.010% or more, and even more preferably 0.015% or more.
  • the carbonitrides and the like become coarse and bending straightening cracking in the continuous casting step is promoted.
  • solute Ti inhibits the development of grain boundaries having a specific rotation angle in the steel member, which will be described later, resulting in a decrease in the hydrogen embrittlement resistance of the steel member.
  • the amount of TiC precipitated increases and C is consumed, so that the strength of the steel member after quenching decreases.
  • the Ti content is set to 0.200% or less.
  • the Ti content is preferably set to 0.080% or less.
  • the Ti content may be set to 0.060% or less or 0.040% or less as necessary.
  • Cr is an effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching. Therefore, Cr may be contained.
  • a Cr content may be 0%, but in a case where the above effects are obtained, the Cr content is set to preferably 0.01% or more, and more preferably 0.03% or more.
  • the Cr content is set to 1.00% or less.
  • the Cr content is set to preferably 0.50% or less, more preferably 0.20% or less, and even more preferably 0.15% or less.
  • B is an element having an action of enhancing the hardenability of the steel even in a small amount.
  • B is an element that strengthens grain boundaries and improves the hydrogen embrittlement resistance by being segregated at the grain boundaries, and is an element that suppresses the growth of austenite grains when the steel sheet is heated. Therefore, B may be contained.
  • the B content may be 0%, but in a case where the above effects are obtained, the B content is set to preferably 0.0005% or more, and more preferably 0.0010% or more or 0.0020% or more.
  • the B content is set to 0.0200% or less.
  • the B content is set to preferably 0.0100% or less or 0.0050% or less.
  • Mo is a very effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching.
  • a synergistic effect on the improvement in the hardenability can be obtained by including a compound of Mo and B. Therefore, Mo may be contained.
  • a Mo content may be 0%, but in a case where the above effects are obtained, the Mo content is set to preferably 0.01% or more, and more preferably 0.03% or more.
  • the Mo content when the Mo content is more than 1.00%, the above effect is saturated and the cost increase is significant. Therefore, in a case where Mo is contained, the Mo content is set to 1.00% or less. In order to reduce the cost, the Mo content is preferably set to 0.80% or less, 0.50% or less, or 0.25% or less.
  • W is an effective element for enhancing the hardenability of steel and stably securing the strength of the steel member after quenching.
  • W is an element that improves corrosion resistance in a corrosive environment.
  • W is an element that segregates to grain boundaries and is also an element that contributes to the development of the above-described texture. Therefore, W may be contained.
  • a W content is less than 0.01%, it is not possible to sufficiently obtain the effect.
  • the W content may be 0%, but in a case where the above effects are obtained, the W content is set to preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.10% or more, and still more preferably 0.20% or more.
  • the W content is set to 2.00% or less.
  • the W content is set to preferably 1.50% or less, and more preferably 1.00% or less, 0.50% or less, or 0.20% or less.
  • Co is a very effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching.
  • a synergistic effect on the improvement in the hardenability can be obtained by including a compound of Co and B. Therefore, Co may be contained.
  • a Co content may be 0%, but in a case where the above effects are obtained, the Co content is set to preferably 0.10% or more, and more preferably 0.20% or more.
  • the Co content is set to 1.00% or less.
  • the Co content is preferably set to 0.80% or less, 0.50% or less, or 0.25% or less.
  • Ni is an effective element for enhancing the hardenability of the steel and stably securing the strength of the steel member after quenching.
  • Ni is an element having an action of suppressing Cu hot embrittlement cracking in the manufacturing of a steel sheet. Therefore, Ni may be contained.
  • a Ni content may be 0%, but in a case where the above effects are obtained, the Ni content is set to preferably 0.01% or more, and more preferably 0.03% or more.
  • the Ni content is set to 2.00% or less.
  • the Ni content is set to preferably 1.00% or less, more preferably 0.50% or less, and even more preferably 0.20% or less.
  • Cu is an effective element for enhancing the hardenability of steel and stably securing the strength of the steel member after quenching.
  • Cu is an element that improves corrosion resistance in a corrosive environment. Therefore, Cu may be contained.
  • a Cu content may be 0%, but in a case where the above effects are obtained, the Cu content is preferably set to 0.01% or more. The Cu content is more preferably 0.03% or more.
  • the Cu content is set to 2.00% or less.
  • the Cu content is set to preferably 1.50% or less, and more preferably 1.00% or less, 0.80% or less, or 0.50% or less.
  • V 0% to 1.00%
  • V is an element that forms fine carbides in steel and improves the hydrogen embrittlement resistance of the steel member through a refining effect or hydrogen trapping effect of carbides thereof. Therefore, V may be contained.
  • a V content may be 0%, but in a case where the above effects are obtained, the V content is preferably set to 0.01% or more, and more preferably set to 0.05% or more.
  • the V content is set to 1.00% or less.
  • the V content is set to preferably 0.80% or less, and more preferably 0.50% or less, 0.30% or less, or 0.10% or less.
  • Ca is an element having an effect of refining inclusions in steel and enhancing the hydrogen embrittlement resistance of the steel member after quenching. Therefore, Ca may be contained.
  • a Ca content may be 0%, but in a case where the above effects are obtained, the Ca content is preferably set to 0.001 % or more, and more preferably set to 0.010% or more or 0.020% or more.
  • the Ca content is set to 0.200% or less.
  • the Ca content is set to preferably 0.100% or less, and more preferably 0.050% or less.
  • Mg is an element having an effect of refining inclusions in steel and enhancing the hydrogen embrittlement resistance after the heat treatment. Therefore, Mg may be contained.
  • a Mg content may be 0%, but in a case where the above effects are obtained, the Mg content is preferably set to 0.01% or more.
  • the Mg content is more preferably 0.02% or more.
  • the Mg content is set to 0.20% or less.
  • the Mg content is preferably 0.10% or less, and more preferably 0.05% or less.
  • REM is an element having an effect of refining inclusions in steel and improving the hydrogen embrittlement resistance of the steel member after quenching. Therefore, REM may be contained.
  • a REM content may be 0%, but in a case where the above effects are obtained, the REM content is preferably set to 0.001% or more, and more preferably set to 0.010% or more or 0.020% or more.
  • the REM content when the REM content is more than 0.300%, the effect is saturated and the cost increases. Therefore, in a case where REM is contained, the REM content is set to 0.300% or less. In order to reduce the alloying cost, the REM content is preferably set to 0.200% or less, 0.100% or less, or 0.050% or less.
  • REM refers to a total of 17 elements including Sc, Y, and lanthanoids such as La, Ce, and Nd, and the REM content means the total amount of these elements.
  • REM is added to molten steel using, for example, an Fe-Si-REM alloy, and this alloy contains, for example, Sc, Y, La, Ce, Pr, and Nd.
  • Sb is an element that improves corrosion resistance in a corrosive environment. Therefore, Sb may be contained.
  • a Sb content may be 0%, but in a case where the above effects are obtained, the Sb content is preferably set to 0.01% or more.
  • the Sb content is set to 1.00% or less.
  • the Sb content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Sn is an element that contributes to an increase in the strength of the steel member. When a Sn content is less than 0.01%, these effects are not sufficient. Therefore, the Sn content may be 0%, but in a case where Sn is contained, the Sn content is preferably set to 0.01% or more. The Sn content is set to more preferably 0.03% or more, and even more preferably 0.05% or more.
  • the Sn content is set to 1.00% or less.
  • the Sn content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Zr is an element that improves corrosion resistance in a corrosive environment. Therefore, Zr may be contained.
  • a Zr content may be 0%, but in a case where the above effects are obtained, the Zr content is preferably set to 0.01% or more.
  • the Zr content is set to 1.00% or less.
  • the Zr content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • the As content may be 0%, but in a case where the above effects are obtained, the As content is preferably set to 0.01% or more.
  • the As content is set to 1.00% or less.
  • the As content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Se is an element that improves the hydrogen embrittlement resistance. Therefore, Se may be contained.
  • a Se content may be 0%, but in a case where the above effects are obtained, the Se content is preferably set to 0.01% or more.
  • the Se content is set to 1.00% or less.
  • the Se content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Bi is an element that improves the hydrogen embrittlement resistance. Therefore, Bi may be contained.
  • the Bi content may be 0%, but in a case where the above effects are obtained, the Bi content is preferably set to 0.01% or more.
  • the Bi content when the Bi content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Bi is contained, the Bi content is set to 1.00% or less.
  • the Bi content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Ta is an element that improves the hydrogen embrittlement resistance. Therefore, Ta may be contained.
  • a Ta content may be 0%, but in a case where the above effects are obtained, the Ta content is preferably set to 0.01% or more.
  • the Ta content is set to 1.00% or less.
  • the Ta content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Re is an element that improves the hydrogen embrittlement resistance. Therefore, Re may be contained.
  • a Re content may be 0%, but in a case where the above effects are obtained, the Re content is preferably set to 0.01% or more.
  • the Re content when the Re content is more than 1.00%, the effect is saturated and the cost increases. Therefore, in a case where Re is contained, the Re content is set to 1.00% or less.
  • the Re content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Os is an element that improves the hydrogen embrittlement resistance. Therefore, Os may be contained.
  • An Os content may be 0%, but in a case where the above effects are obtained, the Os content is preferably set to 0.01% or more.
  • the Os content is set to 1.00% or less.
  • the Os content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Ir is an element that improves the hydrogen embrittlement resistance. Therefore, Ir may be contained.
  • An Ir content may be 0%, but in a case where the above effects are obtained, the Ir content is preferably set to 0.01% or more.
  • the Ir content is set to 1.00% or less.
  • the Ir content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • Tc is an element that improves the hydrogen embrittlement resistance. Therefore, Tc may be contained.
  • the Tc content may be 0%, but in a case where the above effects are obtained, the Tc content is preferably set to 0.01% or more.
  • the Tc content is set to 1.00% or less.
  • the Tc content may be set to 0.50% or less, 0.20% or less, 0.10% or less, or 0.05% or less as necessary.
  • P is an element that decreases the hydrogen embrittlement resistance of the steel member after quenching.
  • the P content is limited to 0.100% or less.
  • the P content is preferably limited to 0.050% or less or 0.020% or less.
  • the P content is preferably as small as possible, the P content may be 0%. However, from the viewpoint of cost, the P content may be set to 0.001% or more.
  • S is an element that decreases the hydrogen embrittlement resistance of the steel member after quenching.
  • the S content is limited to 0.0100% or less.
  • the S content is preferably limited to 0.0050% or less. Since the S content is preferably as small as possible, the S content may be 0%. However, from the viewpoint of cost, the S content may be set to 0.0001% or more.
  • N is an element that decreases the hydrogen embrittlement resistance of the steel member after quenching.
  • the N content is set to 0.020% or less.
  • a lower limit of the N content does not need to be particularly limited and may be 0%.
  • setting the N content to less than 0.001% leads to an increase in steelmaking cost and is economically undesirable. Therefore, the N content may be set to 0.001 % or more, 0.002% or more, 0.008% or more, or 0.010% or more.
  • O is an element that decreases the hydrogen embrittlement resistance of the steel member after quenching.
  • the O content is set to 0.010% or less.
  • a lower limit of the O content does not need to be particularly limited and may be 0%. However, setting the O content to less than 0.001% leads to an increase in steelmaking cost and is economically undesirable. Therefore, the O content may be set to 0.0001% or more, 0.002% or more, 0.0008% or more, or 0.001% or more.
  • the addition or inclusion of elements other than the above-described elements is not excluded as long as the effects of the present embodiment are achieved. However, the addition or inclusion of elements other than the above-described elements may not be permitted as necessary.
  • the remainder including elements other than the above-described elements includes at least Fe and impurities. The remainder may include only Fe and impurities.
  • the "impurities” are elements that are incorporated due to various factors including raw materials such as ore and scrap and a manufacturing process when the steel sheet is industrially manufactured, and are acceptable in a range without adversely affecting the properties of the steel member according to the present embodiment.
  • An industrial manufacturing method is a blast furnace steelmaking method or an electric furnace steelmaking method, and includes a level (impurity level) incorporated during manufacturing by any of the methods.
  • the impurities include Pb and Zn.
  • the total of the amounts of the impurities is usually 1.0% or less, the total of the amounts of the impurities may be set to 1.0% or less.
  • the total of the amounts of the impurities may be set to 0.5% or less, 0.2% or less, 0.1 % or less, or 0.05% or less as necessary.
  • raw materials that contain relatively large amounts of elements other than the above-mentioned elements may be intentionally used. Therefore, in the present embodiment, these elements are all regarded as impurity elements, regardless of whether these elements are mixed in or intentionally used. Therefore, the total of the concentrations of these elements may be set to 1.0% or less as described above.
  • the chemical composition of the steel member can be obtained by the following method.
  • the chemical composition can be obtained by performing elemental analysis on the 1/4 depth position of the steel member (a range of 1/8 to 3/8 of the thickness from the surface in the thickness direction) using a general method such as ICP-AES.
  • a general method such as ICP-AES.
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusion-thermal conductivity method
  • O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
  • the chemical composition of the steel sheet or the ladle analysis values of the molten steel may be used as the chemical composition of the steel member.
  • the microstructure at the 1/4 depth position includes, by area ratio, martensite, bainite, and tempered martensite: 90% or more in total.
  • Martensite, bainite, and tempered martensite are structures (phases) that contribute to the high-strengthening of the steel member, and it is difficult to obtain sufficient strength in the steel member when the total of the area ratios of these structures is less than 90%.
  • martensite, bainite, and tempered martensite may be collectively referred to as a hard structure.
  • the total area ratio of martensite, bainite, and tempered martensite may be 95% or more, 98% or more, or 100% (an area ratio of a remainder in microstructure excluding the hard structure may be 0%), but the remainder in microstructure may include one or more of pearlite, bainite, ferrite, cementite, and residual austenite.
  • the area ratio of each structure can be measured by the following method.
  • a test piece is collected from any position (a position avoiding an end portion in a case where the sample cannot be collected from this position) 50 mm or more away from an end surface of the steel member so that the microstructure at the 1/4 depth position can be observed in a cross section parallel to a rolling direction and parallel to the sheet thickness direction.
  • the cross section of the test piece is polished using #600 to #1500 silicon carbide paper and is thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 ⁇ m in a diluted solution such as alcohol or in pure water.
  • a diluted solution such as alcohol or in pure water.
  • the cross section of the test piece is polished at room temperature using colloidal silica containing no alkaline solution to remove strain introduced into a surface layer of the sample.
  • a region extending 200 ⁇ m in the longitudinal direction and extending from a 1/8 position of a thickness from a surface to a 3/8 position of the thickness from the surface with respect to a 1/4 position of the thickness from the surface as a center is set as an observation region. Then, the observation region is measured by an electron backscatter diffraction method at a measurement interval of 0.1 ⁇ m to obtain crystal orientation information.
  • an apparatus including a thermal field-emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
  • the degree of vacuum in the apparatus is set to 9.6 ⁇ 10 -5 Pa or less, an accelerating voltage is set to 15 kv, an irradiation current level is set to 13, and an irradiation level of an electron beam is set to 62.
  • "iron- ⁇ " and “iron-y” are set as Phases, and the measurement is performed.
  • EBSD measurement region the same region as the measurement region in EBSD (EBSD measurement region) is observed at a magnification of 1,000-fold or more using the thermal field-emission scanning electron microscope (JSM-7001F manufactured by JEOL Ltd.)
  • Vickers indentations are stamped at three points among four corners of the EBSD measurement region in a range of 100 ⁇ m or less from the four corners of the EBSD measurement region so that observation positions can be specified. Thereafter, foreign matter or the like adhering to a surface layer is polished away to leave a structure of an observed section, followed by nital etching. When the Vickers indentation is used as a mark, the same region as the EBSD measurement region can be observed.
  • the foreign matter is removed as necessary by a method such as buffing using alumina particles having a secondary particle size of 0.1 ⁇ m or less, polishing using colloidal silica not containing an alkaline solution at room temperature, or Ar ion sputtering.
  • An image obtained by the observation with the thermal field-emission scanning electron microscope (FE-SEM) is subjected to image analysis, and particles having a higher brightness than that of a primary phase structure, a particle size (circle equivalent diameter) of 0.3 ⁇ m or more and 2.0 ⁇ m or less, and an aspect ratio RI/Rs, which is a ratio between a minor axis Rs and a major axis Rl of a particle, of less than 2.5 (spherical), or plate-like particles having an aspect ratio RI/Rs of 2.5 or more are determined to be cementite.
  • FE-SEM thermal field-emission scanning electron microscope
  • the plate-like cementite may have a deformed and curved shape due to rolling.
  • the lamellar form means a form in which, for three or more of the above plate-like cementite particles, an interior angle at an intersection point where long sides of adjacent particles intersect is within 15° (including cases where the particles are parallel and do not intersect), and the closest distance between the adjacent plate-like cementite particles is 2 ⁇ m or less.
  • the crystal orientation information obtained by the EBSD measurement is used to calculate the area ratio of the residual austenite by using the "Phase Map” function provided in the software "OIM Analysis (registered trademark)" included in the EBSD analysis apparatus.
  • the area ratio a region having an fcc crystal structure is determined to be residual austenite.
  • the "Grain Average Misorientation" function provided in the software "OIM Analysis (registered trademark)” included in the EBSD analysis apparatus is used to determine whether the region is cementite, pearlite, ferrite, bainite, martensite, or tempered martensite, and an area ratio of the region is measured.
  • a boundary having a crystal misorientation of 15° or more is defined as a grain boundary (15° grain boundary)
  • a region in which a Grain Average Misorientation value (GAM value) is 3.0° or less is determined to be ferrite.
  • a region where the GAM value is more than 3.0° is determined to be martensite, bainite, or tempered martensite.
  • the EBSD measurement results and the structure image obtained by the FE-SEM observation are superimposed using the Vickers indentation as a mark, and for regions determined to be cementite and pearlite from the structure image of the FE-SEM observation, these regions are determined to be cementite and pearlite regardless of the classification of the structure determined by the "Phase Map" function and the "Grain Average Misorientation" function.
  • positions of cementite and pearlite may be specified by comparison between a grain boundary MAP using the 15° grain boundary and a position of the Vickers indentation.
  • the rolling direction can be determined by the following method.
  • a test piece is collected from any position 50 mm or more away from the end portion of the steel member (a position avoiding the end portion in a case where the test piece cannot be collected from this position) so that a cross section in the sheet thickness direction can be observed.
  • a cross section parallel to a cross section obtained by rotating the test piece around the sheet thickness direction as an axis in 5° increments within a range of 0° to 180° is observed.
  • An average value of lengths of major axes of a plurality of inclusions in each of the obtained cross sections is calculated for each cross section, and a direction parallel to a major axis direction of the inclusions in a cross section in which the average value of the lengths of the major axes of the inclusions is maximum is determined to be the rolling direction.
  • EBSD electron backscatter diffraction
  • a tensile (maximum) strength TS of the steel member according to the present embodiment is preferably more than 1,500 MPa.
  • the tensile strength is more preferably 1,800 MPa or more, and even more preferably 2,300 MPa or more.
  • the tensile strength may be set to 3,000 MPa or less or 2,700 MPa or less as necessary.
  • There is a correlation between the tensile strength and a Vickers hardness and in the present embodiment, a value obtained by multiplying the Vickers hardness by 3.33 can be regarded as the tensile (maximum) strength TS. Therefore, in the steel member according to the present embodiment, the Vickers hardness (HV1) at a test force of 9.807 N (load 1 kgf) is preferably 450 or more.
  • the Vickers hardness (HV1) is more preferably 470 or more, 510 or more, or 540 or more, and even more preferably 600 or more or 690 or more.
  • the Vickers hardness (HV1) may be set to 900 or less, 860 or less, or 820 or less as necessary.
  • the Vickers hardness can be obtained by the following method.
  • a sample is cut out from any position 50 mm or more away from the end surface of the steel member so that a cross section (thickness direction cross section) perpendicular to the surface can be observed.
  • a size of the sample depends on a measurement device, but may be set so that a size of about 10 mm can be observed in the rolling direction.
  • the cross section of the sample is polished using #600 to #1500 silicon carbide paper and thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 ⁇ m in a diluted solution such as alcohol or pure water.
  • the Vickers hardness (HV1) is obtained by measuring the hardness at 20 points in total at intervals of three or more times the indentation with a test force of 9.807 N in accordance with JIS Z 2244-1:2020 in a direction parallel to a sheet surface at the 1/4 depth position of the base steel sheet using a micro-Vickers hardness tester, and calculating an average value thereof.
  • the chemical composition, the microstructure, and the texture are controlled as described above, so that the hydrogen embrittlement resistance is excellent.
  • a part or the entirety of the surface of the steel member according to the present embodiment may have a coating.
  • the coating may be a coating primarily containing an Fe-Al-based alloy (Fe-Al-based coating) or a coating primarily containing an Fe-Zn-based alloy (Fe-Zn-based coating).
  • the coating is also referred to as a film, an alloyed plating layer, or an intermetallic compound layer.
  • the coating primarily containing an Fe-Al-based alloy is a coating containing 70 mass% or more of Fe and Al in total
  • the coating primarily containing an Fe-Zn-based alloy is a coating containing 70 mass% or more of Fe and Zn in total.
  • the coating primarily containing an Fe-Al-based alloy may further contain, in addition to Fe and Al, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, and a remainder including impurities.
  • the coating primarily containing an Fe-Zn-based alloy may further contain, in addition to Fe and Zn, Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, and a remainder including impurities.
  • a thickness of the coating is preferably 10 to 100 ⁇ m.
  • the chemical composition and the thickness of the coating can be obtained by observing a cross section with a scanning electron microscope.
  • a measurement sample is cut out from a 1/2 portion of the steel member in a longitudinal direction (a 1/2 position of a length in the longitudinal direction from a longitudinal end portion) and a 1/4 width portion (a 1/4 position of the width in the width direction from the width-directional end portion) and is observed.
  • An observation range of the microscope is set to, for example, a range of 40,000 ⁇ m 2 or more in terms of area at a magnification of 400-fold.
  • the cut sample is mechanically polished and subsequently mirror-finished.
  • the thickness of the coating is measured in any 10 visual fields, and an average value thereof is used as the thickness of the coating.
  • the thickness of the coating can be measured by measuring a thickness from an outermost surface to a position where the contrast changes. Measurement is performed at 20 points at equal intervals in an observation photograph, and a distance between the measurement points is set to 6.5 ⁇ m. During the measurement, observation is performed in five visual fields in the above-described manner, and an average value thereof is used as the thickness of the coating.
  • the amounts of Fe, Al, and Zn contained in the coating can be obtained by performing spot elemental analysis (beam diameter: 1.0 ⁇ m or less) on the observation range described above using an electron probe micro-analyzer (EPMA).
  • spot elemental analysis beam diameter: 1.0 ⁇ m or less
  • EPMA electron probe micro-analyzer
  • a total of 10 points are analyzed in the coating in 10 random visual fields, and average values thereof are regarded as the amounts of Fe, Al, and Zn contained in coating. Even in a case where an element other than Fe, Al, and Zn is contained, the amount thereof is obtained using the same method.
  • the steel sheet according to the present embodiment can be used as a material for the steel member according to the present embodiment, since the steel member according to the present embodiment can be obtained by performing a heat treatment such as hot stamping on the steel sheet.
  • the steel sheet according to the present embodiment has a predetermined chemical composition, and, at the 1/4 depth position, when a random intensity ratio of ⁇ 111 ⁇ 011> is denoted by I1, a random intensity ratio of ⁇ 111 ⁇ 112> is denoted by 12, a random intensity ratio of ⁇ 100 ⁇ 011> is denoted by I3, and a random intensity ratio of ⁇ 100 ⁇ 001> is denoted by I4, has a texture in which I1, I2, I3, and I4 satisfy Expression (1).
  • the steel sheet according to the present embodiment may be coated on the surface. Even in that case, since the coating is not a steel sheet, the chemical composition, a microstructure, the texture, and the like of the steel sheet are a chemical composition, a microstructure, a texture, and the like of a part excluding the coating (this part is sometimes referred to as a "base steel sheet” or “base metal steel sheet”).
  • the chemical composition of the steel sheet according to the present embodiment needs to be set to obtain preferable properties for the steel member after the heat treatment. However, since the chemical composition does not substantially change due to the heat treatment, the chemical composition of the steel sheet according to the present embodiment may be the same as the chemical composition of the steel member according to the present embodiment.
  • the chemical composition of the steel sheet can be obtained by the following method.
  • the chemical composition can be obtained by performing elemental analysis on the 1/4 depth position of the steel sheet (a range of 1/8 to 3/8 of the thickness from the surface in the thickness direction) using a general method such as ICP-AES.
  • a general method such as ICP-AES.
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusionthermal conductivity method
  • O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
  • the ladle analysis values of molten steel or a chemical composition of a slab at the 1/4 depth position may be used as the chemical composition of the steel sheet.
  • Pearlite contains fine lamellar cementite within a structure, and is an important structure that shares austenite nucleation sites during heating.
  • the area ratio is preferably more than 10% from the viewpoint of suppressing coarsening of prior ⁇ grains of the steel member.
  • an upper limit thereof is preferably set to 95% or less.
  • the total rolling reduction (%) in each temperature range is calculated by (start thickness - final thickness) / start thickness ⁇ 100, and is calculated for each rolling in each temperature range. Therefore, the sum of the total rolling reductions in each temperature range may simply exceed 100%.
  • the annealing is preferably performed under the conditions in which the annealing temperature (maximum attainment temperature) is 680°C to 950°C and the holding time in a temperature range of 680°C to 950°C is 5 to 1,200 seconds.
  • the holding time referred to here means the time from when the steel sheet temperature rises and reaches 680°C to when the steel sheet temperature decreases and reaches 680°C after being held at 680°C to 950°C.
  • Examples of the coating may include an Al-based coating containing Al and a Zn-based coating containing Zn.
  • plating may be performed after the steel sheet after the annealing step is cooled to room temperature and is then heated again, or hot-dip plating may be performed after performing cooling to 450°C to 750°C, which is close to a plating bath temperature, after annealing without temporarily performing cooling to room temperature.
  • this step does not need to be performed.
  • Pretreatments and post-treatments of the coating are not particularly limited, and precoating, solvent coating, an alloying treatment, temper rolling, or the like can be performed.
  • the alloying treatment for example, annealing at 450°C to 800°C can be performed.
  • temper rolling is useful for shape adjustment and the like, and can achieve, for example, a rolling reduction of 0.1% to 0.5%.
  • the steel sheet according to the present embodiment is heated to a temperature range of Ac3 to Ac3 + 300°C at an average heating rate of 1.0 to 1,000 °C/s, held in the temperature range for 60 to 600 seconds, and then cooled to a temperature range of 300°C or lower at an average cooling rate of 20 °C/s or faster.
  • the average heating rate referred to here is a value obtained by dividing a temperature difference between the surface temperature of the steel sheet at the time of the start of the heating and the holding temperature by a time difference from the start of the heating to the time when the holding temperature is reached.
  • Ac3 (°C) is calculated from Expression (4) using the amount (mass%) of each element in the chemical composition of the steel sheet (steel sheet for hot stamping).
  • Ac 3 854 ⁇ 179 ⁇ C + 44 ⁇ Si ⁇ 14 ⁇ Mn ⁇ 18 ⁇ Ni ⁇ 2 ⁇ Cr
  • the element symbols in the expression are the amounts of the elements in the steel sheet by mass%.
  • the slabs after the heating were subjected to hot rolling (rough rolling and finish rolling) to obtain hot-rolled steel sheets.
  • rough rolling was performed at a rolling reduction of 30% or more in rolling passes including a final stage of the rough rolling for the number of times shown in "Number of rolling passes at rolling reduction of 30% or more (times) " in Tables 2-1 to 2-6, and the rolling was performed so that the rolling was completed at the temperature of "Rough rolling completion temperature (°C)".
  • a sheet thickness of the rough-rolled sheet was controlled in a range of 25 to 50 mm.
  • the sheet thickness after the finish rolling was set to 4.5 to 2.0 mm.
  • Cooling to "Cooling stop temperature (°C)” was performed at "Average cooling rate (°C/s)” in Tables 2-7 to 2-12, followed by coiling into coils at "Coiling temperature (°C)", and air cooling to room temperature.
  • the obtained hot-rolled steel sheets were cold-rolled at "Rolling reduction (%)" in Tables 2-7 to 2-12 to obtain cold-rolled steel sheets.
  • some of the cold-rolled steel sheets were further annealed at "Annealing temperature (°C)" for "Annealing holding time (sec)” shown in Tables 2-7 to 2-12.
  • coating was performed by hot-dip galvanizing (GI), hot-dip galvannealing (GA), or Al plating (Al).
  • GI hot-dip galvanizing
  • GA hot-dip galvannealing
  • Al Al plating
  • the plating was performed by a known method.
  • GA heating to a temperature range of 500°C to 570°C was performed after hot-dip galvanizing to promote alloying.
  • some of the examples were subjected to temper rolling at an elongation rate of 0.2% after the annealing.
  • microstructures at the 1/4 depth positions of the obtained steel sheets were observed in the above-described manner.
  • the area ratios of ferrite and pearlite are shown in Tables 2-7 to 2-12.
  • the remainder of the microstructure was not shown in the table, but included one or more of bainite, martensite, cementite, and residual austenite.
  • the steel sheets were subjected to a heat treatment of heating to "Heating temperature (°C)” shown in Tables 3-1 to 3-6 at “Average heating rate (°C/s)", holding for “Heating holding time (sec)", and then cooling to "Cooling completion temperature (°C)” at “Average cooling rate (°C/s)” to obtain steel members.
  • the Vickers hardness was measured as an alternative index of tensile strength.
  • a sample is cut out from a position 50 mm or more away from the end surface of the steel member so that a cross section (sheet thickness cross section) perpendicular to the surface could be observed.
  • the sample was set to a size that allowed observation of 10 mm in the rolling direction.
  • the cross section of the sample was polished using #600 to #1500 silicon carbide paper and thereafter mirror-finished using a liquid obtained by dispersing a diamond powder having a particle size of 1 to 6 ⁇ m in a diluted solution such as alcohol or pure water.
  • the hardness was measured in a direction parallel to the sheet surface at the 1/4 depth position of the sheet thickness from the surface of the base steel sheet using a micro-Vickers hardness tester with a test force of 9.807 N at intervals of three or more times the indentation. A total of 20 points were measured, and the average value thereof was taken as the Vickers hardness (M HV ) of the steel member.
  • the test piece provided with the U-notches was immersed in a 3% NaCl solution, and using a galvanostat as a power source, a current density at the immersed part of the surface of the test piece was controlled to 0.1 mA/cm 2 to perform hydrogen charging for 24 hours.
  • a low strain rate tensile test was conducted on the test piece subjected to the hydrogen charging at a tensile rate of 0.0060 mm/min, and a stress (SSRT TS (MPa)) at the time of fracture was investigated.
  • the same test was conducted three times for the same manufacturing No., and an average value of the three fracture loads in such a hydrogen environment was obtained.
  • the chemical composition, the microstructure (area ratio of each phase), and the texture were within predetermined ranges, and as a result, the Vickers hardness was high (that is, the tensile strength was high) and the hydrogen embrittlement resistance was excellent.
  • the steel members of the comparative examples one or more of the chemical composition, the microstructure (area ratio of each phase), and the texture were outside the ranges of the present invention.
  • the Vickers hardness and the hydrogen embrittlement resistance of the steel member were both or either inferior.
  • the present invention it is possible to provide a steel member having a high tensile strength and excellent hydrogen embrittlement resistance, and a steel sheet which is a material for the steel member. Therefore, high industrial applicability is achieved.

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EP24770602.1A 2023-03-13 2024-03-04 Steel member and steel sheet Pending EP4682280A1 (en)

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JP2023038696 2023-03-13
PCT/JP2024/008038 WO2024190491A1 (ja) 2023-03-13 2024-03-04 鋼部材及び鋼板

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JP3389562B2 (ja) 2000-07-28 2003-03-24 アイシン高丘株式会社 車輌用衝突補強材の製造方法
JP4964488B2 (ja) * 2006-04-20 2012-06-27 新日本製鐵株式会社 プレス成形性の良好な高強度高ヤング率鋼板、溶融亜鉛めっき鋼板、合金化溶融亜鉛めっき鋼板及び鋼管、並びにそれらの製造方法
WO2007129676A1 (ja) 2006-05-10 2007-11-15 Sumitomo Metal Industries, Ltd. 熱間プレス成形鋼板部材およびその製造方法
JP5521818B2 (ja) 2010-06-21 2014-06-18 新日鐵住金株式会社 鋼材およびその製造方法
JP5454488B2 (ja) * 2011-02-17 2014-03-26 新日鐵住金株式会社 均一変形能及び局部変形能に優れた高強度冷延鋼板
JP6264082B2 (ja) * 2014-02-18 2018-01-24 新日鐵住金株式会社 熱延鋼板の製造方法
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US11180837B2 (en) * 2018-03-29 2021-11-23 Nippos Steel Corporation Hot stamped article
JP7771577B2 (ja) 2021-09-07 2025-11-18 コニカミノルタ株式会社 画像形成装置、情報処理装置、調整チャート生成方法及び調整チャート生成プログラム

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WO2024190491A1 (ja) 2024-09-19

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