EP4350016A1 - Steel sheet, member, method for producing said steel sheet, and method for producing said member - Google Patents

Steel sheet, member, method for producing said steel sheet, and method for producing said member Download PDF

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Publication number
EP4350016A1
EP4350016A1 EP22849078.5A EP22849078A EP4350016A1 EP 4350016 A1 EP4350016 A1 EP 4350016A1 EP 22849078 A EP22849078 A EP 22849078A EP 4350016 A1 EP4350016 A1 EP 4350016A1
Authority
EP
European Patent Office
Prior art keywords
less
steel sheet
temperature
content
delayed fracture
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
EP22849078.5A
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German (de)
English (en)
French (fr)
Inventor
Taiyo ASAKAWA
Shimpei Yoshioka
Shinjiro Kaneko
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.)
JFE Steel Corp
Original Assignee
JFE 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 JFE Steel Corp filed Critical JFE Steel Corp
Publication of EP4350016A1 publication Critical patent/EP4350016A1/en
Pending legal-status Critical Current

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    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following 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
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—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
    • C22C38/06—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
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/20—Ferrous alloys, e.g. steel alloys containing chromium with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/32—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
    • C22C38/38—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
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • 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
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • 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
    • C23C2/26—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
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/34—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the shape of the material to be treated
    • C23C2/36—Elongated material
    • C23C2/40—Plates; Strips
    • 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
    • C21D2211/004—Dispersions; Precipitations
    • 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 sheet such as a high strength steel sheet for cold press forming that is to be subjected to cold press forming before being used for automobiles, etc. and also relates to a member using the steel sheet and methods for manufacturing the same.
  • steel sheets having a tensile strength TS of 1310 MPa grade or higher are being increasingly used for automobile frame members for the purpose of crash safety and weight reduction of automobiles.
  • Steel sheets having a tensile strength TS of 1470 MPa grade or higher are being increasingly used for bumpers, impact beams, etc.
  • delayed fracture may occur due to an increase in residual stress in the component or deterioration of the delayed fracture resistance of the steel sheet itself.
  • the delayed fracture is a phenomenon described below.
  • hydrogen enters a steel sheet included in the component. This causes a reduction in interatomic bonding force and local deformation, and microcracks are thereby formed. The microcracks grow, and fracture thereby occurs.
  • Patent Literature 1 discloses a steel sheet having improved delayed fracture resistance.
  • the steel sheet has a chemical composition containing, in mass %:, C: 0.13% or more and 0.40% or less, Si: 1.5% or less, Mn: 1.8% or more and 4% or less, P: 0.02% or less, S: less than 0.0010%, sol.
  • Al 0.2% or less, N: less than 0.0060%, B: 0.0003% or more and less than 0.0035%, and O: less than 0.0020% and further containing one or two of Nb: 0.002% or more and less than 0.035% and Ti: 0.002% or more and less than 0.040% such that [%Ti] + [%Nb] > 0.007 and [%Ti] ⁇ [%Nb] 2 ⁇ 7.5 ⁇ 10 -6 are satisfied, with the balance being Fe and incidental impurities.
  • the steel sheet has a steel microstructure in which the area fraction of martensite and bainite with respect to the entire microstructure is more than 90% and 100% or less, in which the average grain size of prior austenite grains is 6 to 15 um, in which the number of inclusion clusters satisfying specific conditions is reduced, and in which carbide particles having an aspect ratio of 2.0 or less and a major axis of 0.30 um or more and 2 um or less and containing Fe as a main component are present at 4000 particles / mm 2 or less.
  • the steel sheet has a thickness of 0.5 to 2.6 mm and a tensile strength of 1320 MPa or more.
  • Patent Literature 2 discloses a high strength cold rolled steel sheet having good hydrogen embrittlement resistance and good workability.
  • the steel sheet has a chemical composition containing, in mass%, C: 0.05% to 0.30%, Si: 2.0% or less (including 0%), Mn: more than 0.1% and 2.8% or less, P: 0.1% or less, S: 0.005% or less, N: 0.01% or less, and Al: 0.01 to 0.50% or less and further containing one or two or more of Nb, Ti, and Zr in a total amount of 0.01% or more such that [%C] - [%Nb] / 92.9 ⁇ 12 - [%Ti] / 47.9 ⁇ 12 - [%Zr] / 91.2 ⁇ 12 > 0.03 is satisfied, with the balance being iron and incidental impurities.
  • the steel sheet has a microstructure containing tempered martensite at an area fraction of 50% or more (including 100%) with the balance being ferrite.
  • Precipitates in tempered martensite are distributed such that the number of precipitates having an equivalent circular diameter of 1 to 10 nm is 20 or more per 1 ⁇ m 2 of the tempered martensite, that the number of precipitates having an equivalent circular diameter of 20 nm or more and containing one or two or more of Nb, Ti, and Zr is 10 or less per 1 ⁇ m 2 in the tempered martensite, and that the average grain diameter of ferrite grains surrounded by high-angle grain boundaries with a misorientation of 15° or more is 5 um or less.
  • the present invention has been made to solve the foregoing problem, and it is an object to provide a steel sheet having a tensile strength of 1470 MPa or more (TS ⁇ 1470 MPa) and good delayed fracture resistance, a member, and methods for manufacturing the same.
  • the good delayed fracture resistance means good delayed fracture resistance determined by the following evaluation.
  • the clearance is set to 13%, and the rake angle is set to 1°.
  • the bending is performed such that the corner bending radius is 10 mm and the inner apex angle of the bend is 90° (V bending).
  • the punch used has a tip radius equal to the corner bending radius R described above and has a U-shape (the round tip portion of the punch has a semicircular shape, and the barrel portion of the punch has a thickness of 2R).
  • the die used has a corner R of 30 mm. The depth when the punch pushes the steel sheet is adjusted, and the steel sheet is subjected to shaping such that the corner bending angle (the inner apex angle of the bend) is 90° (the steel sheet has a V shape).
  • the test specimen is clamped with a hydraulic jack such that the distance between flange ends of straight portions of the bent test specimen is the same as that at the time of the bending (such that an increase in the opening angle between the straight portions due to springback is canceled out), and then the test specimen in this state is fixed with the bolt.
  • the bolt is inserted into elliptical holes (minor axis: 10 mm, major axis: 15 mm) formed in advance at positions 10 mm inward from the short side edges of the strip-shaped test specimen to thereby fix the test specimen.
  • the bolted test specimen is immersed in a solution prepared by mixing a 0.1% by mass aqueous ammonium thiocyanate solution and a McIlvaine buffer solution at a ratio of 1:1 and having a pH adjusted to 8.0, and a delayed fracture resistance evaluation test is performed.
  • the temperature of the solution is 20°C, and the amount of the solution per 1 cm 3 of the surface area of the test specimen is 20 mL.
  • the inventors of the present invention have conducted extensive studies in order to solve the foregoing problem and found that, when all the following conditions are satisfied, the delayed fracture resistance can be significantly improved.
  • [B] is the content of B (% by mass).
  • a steel sheet and a member having high strength and good delayed fracture resistance and methods for manufacturing the same are provided.
  • the steel sheet of the present invention has a chemical composition containing, in mass %, C: 0.15% or more and 0.45% or less, Si: 1.5% or less, Mn: more than 1.7%, P:0.03% or less, S: less than 0.0040%, sol. Al: 0.20% or less, N: 0.005% or less, B: 0.0015% or more and 0.0100% or less, and at least one of Nb and Ti in a total amount to 0.005% or more and 0.080% or less, with the balance being Fe and incidental impurities.
  • the steel sheet has a microstructure in which the area fraction of martensite with respect to the total area of the microstructure is 95% or more and 100% or less, and prior austenite grains (which hereinafter may be referred to also as prior ⁇ grains) have an average grain diameter (prior ⁇ grain diameter) of less than 11.2 um.
  • the number density A of precipitates having an equivalent circular diameter of 500 nm or more satisfies formula (1) below.
  • [B] represents the content of B (% by mass).
  • the content of C is contained in order to improve hardenability to thereby obtain a martensite steel microstructure and in order to increase the strength of the martensite.
  • the content of C is 0.15% or more.
  • the content of C is preferably 0.20% or more and more preferably 0.27% or more.
  • An excess portion of the C added may form iron carbide or segregate at grain boundaries and may cause deterioration of delayed fracture resistance. From the above points of view, the content of C is limited to 0.45% or less.
  • the content of C is preferably 0.40% or less and more preferably 0.37% or less.
  • Si is an element for strengthening through solid solution strengthening and is contained from the viewpoint of reducing the formation of film-shaped carbides when the steel sheet is tempered in a temperature range of 200°C or higher to thereby improve the delayed fracture resistance. Si is contained also from the viewpoint of reducing segregation of Mn in a thicknesswise central portion to thereby reduce the formation of MnS. Moreover, Si is contained in order to prevent decarbonization and deboronation due to oxidation of a surface layer during annealing in a continuous annealing line (CAL). The lower limit of the content of Si is not specified. However, from the viewpoint of obtaining the above effects, it is preferable that Si is contained in an amount of 0.02% or more.
  • the content of Si is preferably 0.10% or more and more preferably 0.20% or more.
  • the content of Si is 1.5% or less (including 0%).
  • the content of Si is preferably 1.2% or less and more preferably 1.0% or less.
  • Mn is contained in an amount of more than 1.7% in order to improve the hardenability of the steel and to adjust the area fraction of martensite within the prescribed range so that the desired strength is obtained.
  • the content of Mn is 1.8% or more.
  • the upper limit of the content of Mn is not particularly specified.
  • Mn is an element that facilitates in particular the formation and coarsening of MnS in the thicknesswise central portion and is precipitated when combined with inclusion particles such as Al 2 O 3 , (Nb,Ti) (C,N), TiN, and TiS particles to thereby facilitate delayed fracture. Therefore, the content of Mn is preferably 4.0% or less.
  • the content of Mn is more preferably 3.0% or less.
  • P is an element that strengthens the steel.
  • the content of P is 0.03% or less.
  • the content of P is preferably 0.02% or less and more preferably 0.01% or less.
  • the lower limit of the content of P is not specified. However, the practical lower limit that is at present industrially achievable is 0.002%.
  • S forms coarse MnS that can serve as a starting point of delayed fracture and therefore causes deterioration of the delayed fracture resistance.
  • the content of S must be at most less than 0.0040%.
  • the content of S is preferably less than 0.0020%, more preferably 0.0010% or less, and still more preferably 0.0007% or less.
  • the lower limit of the content of S is not specified. However, the practical lower limit that is at present industrially achievable is 0.0002%.
  • the lower limit of sol. Al is not particularly specified.
  • the content of sol. Al is preferably 0.005% or more.
  • the content of sol. Al is more preferably 0.01% or more and still more preferably 0.02% or more.
  • the content of sol. Al is 0.20% or less.
  • the content of sol. Al is preferably 0.10% or less and more preferably 0.05% or less.
  • N forms precipitates such as TiN, (Nb,Ti)(C,N) in the steel and reduces the amounts of NbC, TiC, and (Nb,Ti)C that are effective in reducing the size of prior austenite grains through the formation of the precipitates.
  • These precipitates hinder adjustment to the steel microstructure required in the present invention and adversely affect the delayed fracture resistance.
  • the content of N is 0.005% or less.
  • the content of N is preferably 0.0040% or less.
  • the lower limit of the content of N is not specified. However, the practical lower limit that is at present industrially achievable is 0.0006%.
  • B is an element that improves the hardenability of the steel and has an advantage in that martensite can be formed at the prescribed area fraction even when the content of Mn is small.
  • B segregates at grain boundaries to increase the bonding force at the grain boundaries and reduces the segregation of P that causes a reduction in the grain boundary strength to thereby improve the delayed fracture resistance.
  • the amounts of Fe 23 (C,B) 6 and BN increase. These precipitates serve as a starting point of delayed fracture, and the delayed fracture resistance rather deteriorates. Therefore, to obtain the effect of improving the delayed fracture resistance by the addition of B, it is necessary to increase the amount of solute B at the grain boundaries and reduce the number of B-based precipitates simultaneously.
  • the content of B is 0.0015% or more.
  • the content of B is preferably 0.0025% or more and more preferably 0.0040% or more.
  • the content of B is 0.0100% or less.
  • the content of B is preferably 0.0090% or less and more preferably 0.0080% or less.
  • At least one of Nb and Ti in total amount of 0.005% or more and 0.080% or less
  • Nb and Ti contribute to an increase in strength through refinement of the internal structure of martensite and reduce the size of prior ⁇ grains to thereby improve the delayed fracture resistance.
  • at least one of Nb and Ti is contained in a total amount of 0.005% or more.
  • the total content of Nb and Ti is preferably 0.010% or more and more preferably 0.020% or more. When the total amount of at least one of Nb and Ti is more than 0.080%, Nb and Ti do not completely dissolve during reheating of the slab.
  • the number of precipitates such as TiN, Ti(C,N), NbN, Nb(C,N), and (Nb,Ti)(C,N) having an equivalent circular diameter of 500 nm or more increases, and the precipitates serve as starting points of delayed fracture, so that the delayed fracture resistance rather deteriorates. Therefore, the upper limit of the total content of Nb and Ti is 0.080%.
  • the total content of Nb and Ti (Ti + Nb) is preferably 0.07% or less and more preferably 0.06% or less.
  • the chemical composition of the steel sheet of the present invention contains the above components as basic components, with the balance being iron (Fe) and incidental impurities.
  • the steel sheet of the present invention has the chemical composition containing the basic components described above, with the balance being iron (Fe) and incidental impurities.
  • the chemical composition may contain one or two or more selected from the following (A) to (D).
  • Cu improves corrosion resistance in the use environment of automobiles.
  • Cu is contained, corrosion products cover the surface of the steel sheet, and the effect of preventing intrusion of hydrogen into the steel sheet is obtained.
  • Cu is an element that is mixed when scrap is used as a raw material. When mixing of Cu is permissible, recycled materials can be used as raw materials, and the manufacturing cost can be reduced. From the above point of view, it is preferable that Cu is contained in an amount of 0.01% or more. From the viewpoint of improving the delayed fracture resistance, the content of Cu is preferably 0.05% or more. The content of Cu is more preferably 0.10% or more. However, when the content of Cu is excessively large, Cu causes surface defects. Therefore, the content of Cu is preferably 1.0% or less. As described above, when Cu is contained, the content of Cu is 1.0% or less. The content of Cu is more preferably 0.50% or less and still more preferably 0.30% or less.
  • Ni also is an element that has the effect of improving the corrosion resistance. Moreover, Ni has the effect of reducing surface defects that are likely to occur when Cu is contained. Therefore, from the above point of view, it is preferable that Ni is contained in an amount of 0.01% or more.
  • the content of Ni is more preferably 0.05% or more and still more preferably 0.10% or more.
  • the content of Ni is 1.0% or less.
  • the content of Ni is more preferably 0.50% or less and still more preferably 0.30% or less.
  • Cr may be added in order to obtain the effect of improving the hardenability of the steel.
  • it is preferable that Cr is contained in an amount of 0.01% or more.
  • the content of Cr is more preferably 0.05% or more and still more preferably 0.10% or more.
  • the rate of dissolution of cementite during annealing is reduced, and undissolved cementite remains present, so that the delayed fracture resistance at sheared end faces deteriorates.
  • pitting corrosion resistance deteriorates.
  • chemical convertibility deteriorates. Therefore, when Cr is contained, the content of Cr is 1.0% or less.
  • the delayed fracture resistance, the pitting corrosion resistance, and the chemical convertibility tend to deteriorate when the content of Cr exceeds 0.2%. From the viewpoint of preventing the deterioration of these properties, the content of Cr is more preferably 0.2% or less.
  • Mo may be added in order to obtain the effect of improving the hardenability of the steel, the effect of forming fine carbides containing Mo and serving as hydrogen trapping sites, and the effect of improving the delayed fracture resistance through a reduction in the size of martensite grains.
  • Nb and Ti When large amounts of Nb and Ti are added, coarse precipitates of these elements are formed, and the delayed fracture resistance rather deteriorates.
  • the solubility limit of Mo is larger than those of Nb and Ti.
  • the microstructure can be refined with no coarse precipitates remaining therein, and a large number of fine carbides can be dispersed in the microstructure, so that the delayed fracture resistance can be improved.
  • Mo is contained in an amount of 0.01% or more.
  • the content of Mo is more preferably 0.03% or more and still more preferably 0.05% or more.
  • Mo is contained in an amount of 0.3% or more, the chemical convertibility deteriorates. Therefore, when Mo is contained, the content of Mo is less than 0.3%.
  • the content of Mo is preferably 0.2% or less.
  • V may be added in order to obtain the effect of improving the hardenability of the steel, the effect of forming fine carbides containing V and serving as hydrogen trapping sites, and the effect of improving the delayed fracture resistance through a reduction in the size of martensite grains.
  • the content of V is preferably 0.003% or more.
  • the content of V is more preferably 0.03% or more and still more preferably 0.05% or more.
  • the content of V is 0.5% or less.
  • the content of V is more preferably 0.3% or less and still more preferably 0.2% or less.
  • the content of V is yet more preferably 0.1% or less.
  • Zr contributes to an increase in strength through a reduction in the size of prior ⁇ grains and refinement of the internal structure of martensite and improves the delayed fracture resistance. Moreover, Zr forms fine Zr-based carbides and carbonitrides serving as hydrogen trapping sites to thereby increase the strength and improve the delayed fracture resistance. In addition, Zr improves the castability. From these points of view, the content of Zr is preferably 0.005% or more. The content of Zr is more preferably 0.010% or more and still more preferably 0.015% or more.
  • the content of Zr is 0.2% or less.
  • the content of Zr is more preferably 0.1% or less and still more preferably 0.04% or less.
  • W contributes to an increase in strength and an improvement in the delayed fracture resistance through the formation of fine W-based carbides and carbonitrides serving as hydrogen trapping sites. From this point of view, it is preferable that W is contained in an amount of 0.005% or more.
  • the content of W is more preferably 0.010% or more and still more preferably 0.030% or more.
  • the content of W is 0.2% or less.
  • the content of W is more preferably 0.1% or less.
  • Ca fixes S as CaS and improves the delayed fracture resistance.
  • the content of Ca is more preferably 0.0005% or more and still more preferably 0.0010% or more.
  • the content of Ca is preferably 0.0030% or less.
  • the content of Ca is more preferably 0.0025% or less and still more preferably 0.0020% or less.
  • Ce also fixes S and improves the delayed fracture resistance. To obtain this effect, it is preferable that Ce is contained in an amount of 0.0002% or more.
  • the content of Ce is more preferably 0.0003% or more and still more preferably 0.0005% or more.
  • the content of Ce is preferably 0.0030% or less. As described above, when Ce is contained, the content of Ce is 0.0030% or less.
  • the content of Ce is more preferably 0.0020% or less and still more preferably 0.0015% or less.
  • La also fixes S and improves the delayed fracture resistance.
  • the content of La is more preferably 0.0005% or more and still more preferably 0.0010% or more.
  • the content of La is preferably 0.0030% or less.
  • the content of La is more preferably 0.0020% or less and still more preferably 0.0015% or less.
  • REMs also fix S and improve the delayed fracture resistance. To obtain this effect, it is preferable that REMs are contained in an amount of 0.0002% or more.
  • the content of REMs is more preferably 0.0003% or more and still more preferably 0.0005% or more.
  • the content of REMs is preferably 0.0030% or less. As descried above, when REMs are contained, the content of REMs is 0.0030% or less.
  • the content of REMs is more preferably 0.0020% or less and still more preferably 0.0015% or less.
  • the REMs in the present invention are elements including scandium (Sc) with an atomic number of 21, yttrium (Y) with an atomic number of 39, lanthanoid elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71 except for Ce and La.
  • the concentration of REMs in the present invention is the total content of one or two or more elements selected from the REMs described above.
  • Mg fixes O as MgO and improves the delayed fracture resistance. To obtain this effect, it is preferable that Mg is contained in an amount of 0.0002% or more.
  • the content of Mg is more preferably 0.0005% or more and still more preferably 0.0010% or more.
  • the content of Mg is preferably 0.0030% or less. As described above, when Mg is contained, the content of Mg is 0.0030% or less.
  • the content of Mg is more preferably 0.0020% or less and still more preferably 0.0015% or less.
  • the content of Sb is preferably 0.002% or more.
  • the content of Sb is more preferably 0.004% or more and still more preferably 0.006% or more.
  • the content of Sb is preferably 0.1% or less. As described above, when Sb is contained, the content of Sb is 0.1% or less.
  • the content of Sb is more preferably 0.05% or less and still more preferably 0.02% or less.
  • the content of Sn is preferably 0.002% or more.
  • the content of Sn is preferably 0.003% or more.
  • the content of Sn exceeds 0.1%, the castability deteriorates, and Sn segregates at prior ⁇ grain boundaries, so that the delayed fracture resistance of the sheared end faces deteriorates. Therefore, when Sn is contained, the content of Sn is 0.1% or less.
  • the content of Sn is more preferably 0.05% or less and still more preferably 0.01% or less.
  • the element is considered to be contained as an incidental impurity.
  • the steel microstructure of the steel sheet of the present invention has the following structural features.
  • [B] represents the content of B (% by mass).
  • the area fraction of martensite in the steel microstructure is 95% or more.
  • the area fraction is more preferably 99% or more and still more preferably 100%.
  • the remaining microstructures include bainite, ferrite, and retained austenite (retained ⁇ ). Materials other than these microstructures are small numbers of carbides, sulfides, nitrides, and oxides.
  • the area fraction of the remaining microstructures is 5% or less and preferably 1% or less.
  • the martensite includes martensite that has not undergone tempering, which is the process of holding the steel sheet at about 150°C or higher for a prescribed time and includes self-tempering during the continuous cooling.
  • tempering is the process of holding the steel sheet at about 150°C or higher for a prescribed time and includes self-tempering during the continuous cooling.
  • the area fraction of the martensite may be 100% with no other microstructures.
  • the fracture surface of delayed fracture is often an intergranular fracture surface, and it is considered that the starting point of the delayed fracture and the initial crack growth path of the delayed fracture are on prior austenite grain boundaries.
  • reducing the size of the prior austenite grains is effective.
  • the reduction in the size of the prior austenite grains improves the delayed fracture resistance significantly.
  • the mechanism of the improvement may be as follows. The reduction in the size of the prior austenite grains increases the area fraction of the prior austenite grain boundaries, and the concentrations of impurity elements such as P, which are grain boundary embrittlement elements, on the prior austenite grain boundaries may decrease.
  • the reduction in the size of the prior austenite grains also contributes to an improvement in the tensile strength.
  • the average grain diameter of the prior austenite grains (the prior ⁇ grain diameter) is less than 11.2 um. This average grain diameter is preferably 10 um or less, more preferably 7.0 um or less, and still more preferably 5.0 um or less.
  • [B] represents the content of B (% by mass).
  • the inventors of the present invention have found that, when the hot-rolling conditions are controlled to reduce the number density A of precipitates having an equivalent circular diameter of 500 nm or more so that the following condition is satisfied, the improvement in the delayed fracture resistance through grain boundary strengthening by B and prevention of fracture starting from the precipitates can be achieved simultaneously.
  • the area fractions of martensite, bainite, and ferrite are measured as follows.
  • An L cross section of a steel sheet (its cross section parallel to the rolling direction and perpendicular to the surfaces of the steel sheet (the cross section is hereinafter referred to also as a vertical cross section parallel to the rolling direction)) is polished and etched with nital.
  • the cross section is observed at positions 1/4 of the thickness from a surface of the steel sheet under an SEM at a magnification of 2000X to capture microstructure photographs in four viewing areas.
  • the captured microstructure photographs are subjected to image analysis to measure the area fractions.
  • the martensite and bainite appear as gray or white microstructures under the SEM.
  • the ferrite appears as dark contrast regions under the SEM.
  • the martensite and bainite include thereinside small numbers of carbides, nitrides, sulfides, and oxides. However, since it is difficult to remove these particles, the area fractions of the martensite and bainite are the area fractions of regions including these particles.
  • the bainite has the following features. Specifically, the bainite is in the form of a plate having an aspect ratio of 2.5 or more and is a slightly black microstructure than the martensite. The width of the plate is 0.3 to 1.7 um. The distribution density of carbides having a diameter of 10 to 200 nm and present inside the bainite is 0 to 3 particles / ⁇ m 2 .
  • the retained austenite (retained ⁇ ) is measured as follows.
  • the surface layer of the steel sheet is chemically polished with oxalic acid to a depth of 200 um, and the area fraction of the retained austenite on the sheet surface is determined by an X-ray diffraction intensity method.
  • the area fraction is computed from the integrated intensities of peaks from (200) ⁇ , (211) ⁇ , (220) ⁇ , (200) ⁇ , (220) ⁇ , and (311) ⁇ diffraction planes that are measured using the Mo-K ⁇ line.
  • the average grain diameter of the prior austenite grains (the prior ⁇ grain diameter) is measured as follows.
  • An L cross section of the steel sheet (its vertical cross section parallel to the rolling direction) is polished and etched with a chemical solution capable of etching the prior ⁇ grain boundaries (for example, a saturated aqueous picric acid solution or a solution prepared by adding ferric chloride to the saturated aqueous picric acid solution).
  • the cross section is observed at positions 1/4 of the thickness from a surface of the steel sheet under an optical microscope at a magnification of 500X to capture photographs in four viewing areas. In each of the obtained photographs, 15 lines are drawn in the thickness direction at intervals of 10 um or more in actual length, and 15 lines are drawn in the rolling direction at intervals of 10 um or more in actual length.
  • the number of intersections of the grain boundaries and the lines is counted.
  • a value obtained by dividing the total line length by the number of intersections is multiplied by 1.13, and the prior ⁇ grain diameter (the average grain diameter of the prior austenite grains) can thereby be measured.
  • the number density A of the precipitates having an equivalent circular diameter of 500 nm or more is determined as follows. An L cross section of the steel sheet (its vertical cross section parallel to the rolling direction) is polished. In a region extending from a position 1/5 of the thickness of the steel sheet to a position 4/5 of the thickness, i.e., in a region extending from the position 1/5 of the thickness from a surface of the steel sheet to the position 4/5 of the thickness with the thicknesswise center positioned therebetween, images of 2 mm 2 regions are continuously captured under an SEM. The number density A is determined by counting the number of the above-described precipitates in the captured SEM photographs. The magnification of the captured images is 2000X.
  • the inclusion particle When each of the inclusion particles is subjected to component analysis, the inclusion particle is magnified by a factor of 10000 to analyze the precipitate.
  • the precipitates having an equivalent circular diameter of 500 nm or more are precipitates containing B such as Fe 23 (C,B) 6 .
  • Elemental analysis by energy dispersive X-ray spectrometry (EDS) using an acceleration voltage of 3 kV is performed to check for the presence of a peak of B. When a peak of B is observed, it is determined that the above-described precipitates are present.
  • EDS energy dispersive X-ray spectrometry
  • the equivalent circular diameter of a precipitate is the diameter of a perfect circle having the same area as the precipitate computed in an SEM photograph.
  • the deterioration of the delayed fracture resistance of a steel sheet is significant when the tensile strength of the steel sheet is 1470 MPa or more.
  • One feature of the present invention is that the delayed fracture resistance is good even when the tensile strength is 1470 MPa or more. Therefore, in the present invention, it is necessary that the tensile strength be 1470 MPa or more. From the viewpoint of reducing the weight of automobile frame members, the tensile strength is preferably 1700 MPa or more. The tensile strength of the steel sheet of the present invention may be 2100 MPa or less.
  • the tensile strength can be measured as follows. A JIS No. 5 tensile test piece is cut from a position 1/4 of the coil width such that a direction orthogonal to the rolling direction is the longitudinal direction of the test piece, and the tensile strength is measured using a tensile test according to JIS Z 2241.
  • the steel sheet of the present invention may be a steel sheet having a coated layer on a surface thereof.
  • the coated layer may be a zinc coating or may be a coating of another metal.
  • the coated layer may be a hot-dip coated layer or an electroplated layer.
  • the method for manufacturing the steel sheet of the present invention includes: heating a steel slab having the chemical composition described above such that the surface temperature of the slab is increased from 1000°C to a heat holding temperature of 1250°C or higher at an average heating rate of 10°C/minute or less and then holding the steel slab at the heat holding temperature for 30 minutes or longer; then setting a residence time at 900 to 1000°C to 20 seconds or longer and 150 seconds or shorter and performing hot finish rolling under the condition of a finish rolling temperature of 850°C or higher; performing cooling at an average cooling rate of 40°C/second or more in the range from the finish rolling temperature to 650°C; performing coiling at a coiling temperature of 650°C or lower to obtain a hot rolled steel sheet; subjecting the hot rolled steel sheet to cold rolling at a rolling reduction of 40% or more to obtain a cold rolled steel sheet; and subjecting the cold rolled steel sheet to continuous annealing including heating the cold rolled steel sheet from 400°C to an annealing temperature of 800 to 950°C at an
  • the average heating rate in the range of from 1000°C to the heat holding temperature of 1250°C or higher is 10°C/minute or less.
  • the heat holding temperature is set such that the surface temperature of the slab reaches 1250°C or higher, and the holding time is set to 30 minutes or longer. In this manner, the dissolution of Nb and Ti is facilitated, and the number of inclusions and their size are reduced.
  • the heat holding temperature is preferably 1300°C or higher.
  • the heat holding temperature is more preferably 1350°C or higher.
  • the average heating rate is "(the temperature at the end of the slab heating (the heat holding temperature) (°C) - the temperature at the start of the slab heating (°C) (1000°C)) / the heating time from the start of the heating to the end of the heating (minutes)."
  • the slab is held at 900 to 1000°C for 20 seconds or longer and 150 seconds or shorter.
  • the residence time in the temperature range of 900 to 1000°C is increased, precipitates composed mainly of BN are formed and coarsened. The precipitates formed in this temperature range do not easily dissolve by anneal heating and cause a reduction in the amount of solute B after annealing. Therefore, when the residence time exceeds 150 seconds, the amount of solute B effective in preventing delayed fracture cannot be obtained.
  • the residence time is 150 seconds or shorter and is preferably 120 seconds or shorter and more preferably 100 seconds or shorter.
  • the residence time is shorter than 20 seconds, the microstructure may be nonuniform. Therefore, the residence time is 20 seconds or longer and is preferably 30 seconds or longer and more preferably 40 seconds or longer.
  • the finish rolling temperature is set to 850°C or higher in order to reduce the precipitation of Nb, Ti, B, etc.
  • the finish rolling temperature is preferably 930°C or lower.
  • cooling is performed at an average cooling rate of 40°C/second or more in the range from the finish rolling temperature to 650°C.
  • the average cooling rate is preferably 250°C/second or lower and more preferably 200°C/second or lower.
  • the average cooling rate in the hot rolling step is "(the temperature at the start of the cooling (the finish rolling temperature) (°C) - the temperature at the end of the cooling (°C) (650°C)) / the cooling time from the start of the cooling to the end of the cooling (seconds)."
  • the steel slab After the cooling to 650°C, the steel slab is coiled while being cooled as needed.
  • the coiling temperature is set to 650°C or lower.
  • the coiling temperature is preferably 500°C or higher.
  • the rolling reduction when the rolling reduction (cold rolling reduction ratio) is 40% or more, the recrystallization behavior and the texture orientation in the subsequent continuous annealing can be stabilized.
  • the rolling reduction is less than 40%, some of austenite grains are coarsened during annealing, and the strength may decrease.
  • the cold rolling reduction is preferably 80% or less.
  • the cold-rolled steel sheet is subjected to annealing in a continuous annealing line (CAL) and optionally subjected to tempering and temper-rolling.
  • CAL continuous annealing line
  • the average heating rate at 400°C or higher is set to 1.0°C/second or more.
  • the average heating rate at 400°C or higher is preferably 1.5°C/second or more and more preferably 3.0°C/second or more.
  • the average heating rate is preferably 10°C/second or lower.
  • the average heating rate is "the annealing temperature (°C) described later - 400 (°C)) / the heating time from 400°C to the annealing temperature (minutes)."
  • the annealing is performed at a high temperature for a long time. Specifically, it is necessary to set the annealing temperature to 800°C or higher.
  • the annealing temperature is 950°C or lower.
  • the annealing temperature is more preferably 900°C or lower.
  • the soaking time (holding time) at the annealing temperature is increased, the diameter of the prior ⁇ grains increases excessively. Therefore, the soaking time is 600 seconds or shorter. The soaking time is preferably 10 seconds or longer.
  • the annealing temperature to 420°C at the first average cooling rate of 2°C/second or more.
  • the first average cooling rate is lower than 2°C/second, a large amount of ferrite is formed, and carbon is concentrated in ⁇ . In this case, the martensite is hardened, and the delayed fracture resistance deteriorates.
  • the upper limit of the first average cooling rate is preferably 100°C/second.
  • the steel sheet When the steel sheet is subjected to hot-dip galvanizing treatment, it is preferable to perform the galvanizing treatment by immersing the cold rolled steel sheet in a coating bath in a cooling process from the annealing temperature to 420°C, more specifically when cooling is performed at the first average cooling rate in the continuous annealing.
  • the steel sheet may be heated to 480°C to 600°C to perform galvannealing treatment.
  • the second average cooling rate from 420°C to the finish cooling temperature of 260°C or lower is set to 10°C/second or more.
  • the second average cooling rate is preferably 20°C/second or more and more preferably 70°C/second or more. No particular limitation is imposed on the upper limit of the second average cooling rate, but the upper limit is preferably 2000°C/second.
  • the first average cooling rate is "(the annealing temperature (°C) - 420 (°C)) / (the cooling time from the annealing temperature to 420°C (seconds))."
  • the second average cooling rate is "(420 (°C) - the finish cooling temperature (°C)) / (the cooling time from 420°C to the finish cooling temperature (seconds))."
  • the finish cooling temperature exceeds 260°C, a problem arises in that upper bainite and lower bainite are formed and the amounts of retained austenite and fresh martensite increase. Therefore, the finish cooling temperature is 260°C or lower.
  • Carbides distributed inside the martensite are carbides formed when the steel sheet is held in the low-temperature range after the quenching. It is necessary to control the formation of the carbides appropriately such that good delayed fracture resistance and a tensile strength of 1470 MPa or more (TS ⁇ 1470 MPa) can be obtained.
  • the steel sheet is held in the continuous annealing at a holding temperature of 150 to 260°C for 20 to 1500 seconds.
  • the thus-obtained steel sheet may be subjected to skin pass rolling.
  • the skin pass elongation rate is preferably 0.1% or more.
  • the skin pass elongation rate is preferably 0.6% or less.
  • the skin pass roll used is a dull roll and that the roughness Ra of the steel sheet is adjusted to 0.8 um or more. It is also preferable that the roughness Ra of the steel sheet is adjusted to 1.8 um or less.
  • the steel sheet may be subjected to hot-dip coating treatment during cooling after the soaking in the annealing or subjected to electroplating after the continuous annealing to thereby obtain a coated steel sheet.
  • the type of coating include Zn-based coatings (such as Zn-based, Zn-Ni-based, and Zn-Fe-based coatings) and Al coatings.
  • the hot-dip coating treatment is performed, the steel sheet may be immersed in a coating bath in the process of cooling the steel sheet from the annealing temperature to 420°C at the first average cooling rate of 2°C/second or more and heated to 480 to 600°C after the immersion in the coating bath to thereby perform alloying treatment. After the alloying treatment, the steel sheet may be cooled at the second average cooling rate described above and subjected to holding treatment at a holding temperature of 150 to 260°C for 20 to 1500 seconds.
  • the steel sheet may be subjected to heat treatment in the temperature range of 260°C or lower in order to reduce the amount of hydrogen that has penetrated into the steel.
  • the delayed fracture resistance of the high strength cold rolled steel sheet is significantly improved, and the application of the high strength steel sheet to a component contributes to an improvement in the strength of the component and a reduction in the weight of the component.
  • the steel sheet of the present invention has a thickness of preferably 0.5 mm or more. The thickness is preferably 2.0 mm or less.
  • the member of the present invention is prepared by subjecting the steel sheet of the present invention to at least one of forming and joining.
  • the method for manufacturing the member of the present invention includes the step of subjecting the steel sheet of the present invention to at least one of forming and joining.
  • the steel sheet of the present invention has a tensile strength of 1470 MPa or more and has good delayed fracture resistance. Therefore, the member obtained using the steel sheet of the present invention also has high strength and has better delayed fracture resistance than conventional high strength members. By using the member of the present invention, a reduction in weight can be achieved. Therefore, the member of the present invention can be preferably used, for example, for a vehicle body frame member.
  • the forming used may be any common working method such as press forming.
  • the joining used may be any of common welding such as spot or arc welding, riveting, and crimping.
  • Molten steel having a chemical composition shown in Table 1 was prepared and cast into a slab.
  • the slab was subjected to heat treatment and rolling shown in Table 2 to thereby obtain a steel sheet having a thickness of 1.4 mm.
  • a slab having one of the chemical compositions was heated such that the surface temperature of the slab was increased to a heat holding temperature shown in Table 2 at an average heating rate of 6°C/minute and then held for a heat holding time shown in Table 2. Then the slab was held at 900 to 1000°C for a residence time shown in Table 2, subjected to hot finish rolling at a finish rolling temperature of 870°C, and cooled at an average cooling rate of 50°C/second in the range from the finish rolling temperature to 650°C.
  • the resulting slab was cooled and coiled at a coiling temperature of 550°C to thereby obtain a hot rolled steel sheet, and the hot rolled steel sheet was cold rolled at a rolling reduction (cold rolling reduction) of 50% to thereby obtain a cold rolled steel sheet.
  • the cold rolled steel sheet was heated from 400°C to an annealing temperature shown in Table 2 at an average heating rate shown in Table 2 and soaked at the annealing temperature for a soaking time shown in Table 2.
  • the resulting cold rolled steel sheet was subjected to continuous annealing. Specifically, the cold rolled steel sheet was cooled from the annealing temperature (a first cooling start temperature) to 420°C (a second cooling start temperature) at a first average cooling rate shown in Table 2, cooled from 420°C (the second cooling start temperature) to a finish cooling temperature shown in Table 2 at a second average cooling rate shown in Table 2, optionally reheated, and then held at a holding temperature shown in Table 2 for a holding time shown in Table 2.
  • the steel sheet No. 12 was immersed in a hot-dip galvanizing bath at 480°C in the process of cooling to 420°C at the first average cooling rate in the continuous annealing, then heated to 540°C, and held for 15 seconds to perform galvannealing treatment to thereby obtain a hot-dip galvannealed steel sheet. Then the hot-dip galvannealed steel sheet was cooled at a second average cooling rate shown in Table 2 and subjected to holding treatment at a holding temperature shown in Table 2 for a holding time shown in Table 2.
  • the steel sheet subjected to the continuous annealing was subjected to electroplating to thereby obtain a steel sheet having a Zn-coated layer formed thereon.
  • [Table 1] Steel grade Chemical composition (% by mass) Remarks C Si Mn P S sol.
  • the metallic microstructures were quantified using the methods described above, and a tensile test and a delayed fracture resistance evaluation test were performed.
  • microstructures were measured using the following methods.
  • the area fractions of martensite, bainite, and ferrite were measured as follows. An L cross section of a steel sheet (its vertical cross section parallel to the rolling direction) was polished and etched with nital. The cross section was observed at positions 1/4 of the thickness from a surface of the steel sheet under an SEM at a magnification of 2000X to capture microstructure photographs in four viewing areas. The captured microstructure photographs were subjected to image analysis to measure the area fractions.
  • the martensite and bainite appear as gray or white microstructures under the SEM.
  • the bainite has the following features. Specifically, the bainite is in the form of a plate having an aspect ratio of 2.5 or more and is a slightly black microstructure than the martensite.
  • the width of the plate is 0.3 to 1.7 um.
  • the distribution density of carbides having a diameter of 10 to 200 nm and present inside the bainite is 0 to 3 particles / ⁇ m 2 .
  • the ferrite appears as dark contrast regions under the SEM.
  • the martensite and bainite include thereinside small numbers of carbides, nitrides, sulfides, and oxides. However, since it is difficult to remove these particles, the area fractions of the martensite and bainite are the area fractions of regions including these particles.
  • the retained austenite (retained ⁇ ) was measured as follows.
  • the surface layer of the steel sheet was chemically polished with oxalic acid to a depth of 200 um, and the area fraction of the retained austenite on the sheet surface was determined by an X-ray diffraction intensity method.
  • the area fraction was computed from the integrated intensities of peaks from (200) ⁇ , (211) ⁇ , (220) ⁇ , (200) ⁇ , (220) ⁇ , and (311) ⁇ diffraction planes that were measured using the Mo-K ⁇ line.
  • the average grain diameter of the prior austenite grains was measured as follows.
  • An L cross section of the steel sheet (its vertical cross section parallel to the rolling direction) was polished and etched with a chemical solution capable of etching the prior ⁇ grain boundaries (for example, a saturated aqueous picric acid solution or a solution prepared by adding ferric chloride to the saturated aqueous picric acid solution).
  • the cross section was observed at positions 1/4 of the thickness from a surface of the steel sheet under an optical microscope at a magnification of 500X to capture photographs in four viewing areas.
  • 15 lines were drawn in the thickness direction at intervals of 10 um or more in actual length, and 15 lines were drawn in the rolling direction at intervals of 10 um or more in actual length.
  • the number of intersections of the grain boundaries and the lines was counted. A value obtained by dividing the total line length by the number of intersections was multiplied by 1.13, and the prior ⁇ grain diameter was thereby determined.
  • the number density A of the precipitates having an equivalent circular diameter of 500 nm or more was determined as follows. An L cross section of the steel sheet (its vertical cross section parallel to the rolling direction) was polished. In a region extending from a position 1/5 of the thickness of the steel sheet to a position 4/5 of the thickness, i.e., in a region extending from the position 1/5 of the thickness from a surface of the steel sheet to the position 4/5 of the thickness with the thicknesswise center positioned therebetween, images of 2 mm 2 regions were continuously captured under an SEM. The number density A was determined by counting the number of precipitates described above in the captured SEM photographs. The magnification of the captured images is 2000X.
  • the inclusion particle When each of the inclusion particles is subjected to component analysis, the inclusion particle was magnified by a factor of 10000 to analyze the precipitate.
  • the precipitates having an equivalent circular diameter of 500 nm or more are precipitates containing B such as Fe 23 (C,B) 6 .
  • Elemental analysis by energy dispersive X-ray spectrometry (EDS) using an acceleration voltage of 3 kV was performed to check for the presence of a peak of B. When a peak of B was observed, it was determined that the above-described precipitates were present.
  • EDS energy dispersive X-ray spectrometry
  • a tensile test was performed as follows. A JIS No. 5 tensile test piece was cut from a position 1/4 of the coil width such that a direction orthogonal to the rolling direction was the longitudinal direction of the test piece, and the tensile test (according to JIS Z 2241) was performed to evaluate the YP, TS, and El.
  • the delayed fracture resistance was evaluated as follows.
  • a strip-shaped test specimen was cut from a steel sheet (coil) obtained. Specifically, the test specimen had dimensions of 100 mm in a direction orthogonal to the rolling direction and 30 mm in the rolling direction and was cut from a position 1/4 of the width of the steel sheet (coil) in the width direction of the coil. Then the evaluation was performed. To obtain the long-side end faces of the test specimen that had a length of 100 mm, the steel sheet was sheared, and the as-sheared test specimen (without machining for removing burrs) was subjected to bending such that the burrs were present on the outer peripheral side of the bend and fixed with a bolt while the shape at the time of bending was maintained.
  • the clearance was set to 13%, and the rake angle was set to 1°.
  • the bending was performed such that the corner bending radius was 10 mm and the inner apex angle of the bend was 90° (V bending).
  • the punch used had a tip radius equal to the corner bending radius R described above and had a U-shape (the round tip portion of the punch had a semicircular shape, and the barrel portion of the punch had a thickness of 2R).
  • the die used had a corner R of 30 mm.
  • the depth when the punch pushed the steel sheet was adjusted, and the steel sheet was subjected to shaping such that the corner bending angle (the inner apex angle of the bend) was 90° (the steel sheet had a V shape).
  • the test specimen was clamped with a hydraulic jack such that the distance between flange ends of straight portions of the bent test specimen was the same as that at the time of the bending (such that an increase in the opening angle between the straight portions due to springback was canceled out), and then the test specimen in this state was fixed with the bolt.
  • the bolt was inserted into elliptical holes (minor axis: 10 mm, major axis: 15 mm) formed in advance at positions 10 mm inward from the short side edges of the strip-shaped test specimen to thereby fix the test specimen.
  • the bolted test specimen was immersed in a solution prepared by mixing a 0.1% by mass aqueous ammonium thiocyanate solution and a McIlvaine buffer solution at a ratio of 1:1 and having a pH adjusted to 8.0, and a delayed fracture resistance evaluation test was performed.
  • the temperature of the solution was 20°C, and the amount of the solution per 1 cm 3 of the surface area of the test specimen was 20 mL.
  • the presence or absence of a visually observable crack (having a length of 1 mm or more) was checked after a lapse of 24 hours. A test specimen with no crack observed was considered to have good delayed fracture resistance.
  • Steel sheets within the scope of the present invention had high strength and had good delayed fracture resistance.
  • the heating temperature (slab surface temperature (SRT)) was lower than the lower limit of the value specified in the present invention. Therefore, the diameter of the prior ⁇ grains was large, and sufficient delayed fracture resistance was not obtained.

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EP22849078.5A 2021-07-28 2022-06-22 Steel sheet, member, method for producing said steel sheet, and method for producing said member Pending EP4350016A1 (en)

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PCT/JP2022/024964 WO2023008003A1 (ja) 2021-07-28 2022-06-22 鋼板、部材およびそれらの製造方法

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JP4712882B2 (ja) 2008-07-11 2011-06-29 株式会社神戸製鋼所 耐水素脆化特性および加工性に優れた高強度冷延鋼板
WO2018062380A1 (ja) 2016-09-28 2018-04-05 Ｊｆｅスチール株式会社 鋼板およびその製造方法
CN109642295B (zh) * 2016-09-28 2022-04-29 杰富意钢铁株式会社 钢板及其制造方法
KR102547460B1 (ko) * 2018-12-21 2023-06-26 제이에프이 스틸 가부시키가이샤 강판, 부재 및 이것들의 제조 방법
EP3929321B1 (en) * 2019-02-21 2023-09-27 JFE Steel Corporation Hot-pressed member, cold-rolled steel sheet for hot pressing, and manufacturing methods therefor
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