EP4696800A1 - High strength steel sheet, high strength plated steel sheet, methods for producing same, and member - Google Patents

High strength steel sheet, high strength plated steel sheet, methods for producing same, and member

Info

Publication number
EP4696800A1
EP4696800A1 EP24819120.7A EP24819120A EP4696800A1 EP 4696800 A1 EP4696800 A1 EP 4696800A1 EP 24819120 A EP24819120 A EP 24819120A EP 4696800 A1 EP4696800 A1 EP 4696800A1
Authority
EP
European Patent Office
Prior art keywords
less
steel sheet
high strength
temperature
sheet
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
EP24819120.7A
Other languages
German (de)
English (en)
French (fr)
Inventor
Hidekazu Minami
Yusuke Wada
Yuki Toji
Katsutoshi Takashima
Mai AOYAMA
Katsuya Hoshino
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 EP4696800A1 publication Critical patent/EP4696800A1/en
Pending legal-status Critical Current

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Classifications

    • 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
    • C21D1/22—Martempering
    • 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
    • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/021—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving particular fabrication steps or treatments of ingots or slabs
    • 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys 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 of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys 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
    • C22C38/002—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
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/28—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
    • C22C38/60—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
    • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
    • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts
    • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
    • C23G1/08—Iron or steel
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • 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
    • C21D2211/005—Ferrite
    • 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 disclosure relates to a high strength steel sheet and a high strength coated steel sheet that are optimal for reinforcing parts and framework structural parts of automobiles, to methods for producing these steel sheets, and to a member.
  • High strength steel sheets used for reinforcing parts and framework structural parts of automobiles are required to have excellent formability.
  • the formed parts are required to have high dimensional accuracy.
  • parts such as crash boxes have punched edges and bent portions, so that steel sheets with high stretch flangeability and bendability are suitable from the viewpoint of formability.
  • the yield ratio (YR) of steel sheets is controlled within a certain range. This allows springback of the steel sheets formed into the parts to be reduced, and the dimensional accuracy of the parts can thereby by controlled.
  • the hold time provided for preventing the occurrence of intra-steel sheet cracking in the spot weld HAZ increases.
  • a long hold time causes a reduction in productivity. Therefore, there is a demand for high strength steel sheets in which the occurrence of intra-steel sheet cracking in the spot weld HAZ can be prevented even when the hold time is short.
  • Patent Literature 1 provides a high strength steel sheet that has a tensile strength of 980 MPa or more, has excellent bendability and high LME resistance, and can be used for production of parts with high dimensional accuracy.
  • the high strength steel sheet described in Patent Literature 1 satisfies both bendability and LME resistance in a comprehensive manner and can be used for production of parts with high dimensional accuracy.
  • the high strength steel sheet described in Patent Literature 1 has a TS grade of 980 MPa, and there is room for further improvement in strength.
  • the present disclosure has been developed in view of the above circumstances, and it is an object to provide a high strength steel sheet of 1180 MPa or more that has excellent bendability and high resistance to intra-steel sheet cracking in the spot weld HAZ and can be used for production of parts with high dimensional accuracy and to provide an advantageous method for producing the high strength steel sheet.
  • the phrase "parts can be produced with high dimensional accuracy (dimensional accuracy during forming is high)” means that the yield ratio (YR) is 65% or more and 90% or less.
  • a bending test is performed using the V-block method with a bending angle of 90 degrees. Specifically, the bending test is performed on five samples, with the bending radius (R) set such that the value of R/t obtained by dividing the bending radius (R) by the sheet thickness (t) is about 4.5, i.e., 4.3 to 4.7. Next, for all the five samples, the length of a crack in a ridge portion at the apex of the bend is evaluated. When the crack length is 200 ⁇ m or less, the bendability is considered excellent.
  • the resistance to intra-steel sheet cracking in the spot weld HAZ a cross-section of a weld described in Examples is observed under an optical microscope (200X), and the resistance to intra-steel sheet cracking in the spot weld HAZ is evaluated according to the following criteria.
  • the rating is A or B
  • the resistance to intra-steel sheet cracking in the spot weld HAZ is considered excellent.
  • the rating is C
  • the resistance to intra-steel sheet cracking in the spot weld HAZ is considered poor.
  • the hold time is the period from the end of the application of the welding current to the start of release of the electrodes.
  • the present disclosure can provide a high strength steel sheet of 1180 MPa or more that has excellent bendability and high resistance to intra-steel sheet cracking in a spot weld HAZ and can be used for production of a member with high dimensional accuracy and can also provide a member.
  • % representing the content of a component element of the steel sheet means “% by mass” unless otherwise specified.
  • a numerical range represented using “to” means a range including the numerical values before and after the "to” as the lower limit and the upper limit, respectively.
  • C is one of the important basic components of the steel.
  • C is an important element that affects the area fractions of martensite and ferrite, the volume fraction of retained austenite, the standard deviation of the Vickers hardness of the surface of the steel sheet, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet. If the content of C is less than 0.030%, the area fraction of martensite decreases, and the area fraction of ferrite increases, which makes it difficult to achieve a TS of 1180 MPa or more. This also makes it difficult to achieve the desired YR. If the content of C exceeds 0.500%, the hardness distribution of martensite in the sheet width direction becomes nonuniform.
  • the content of C is 0.030% or more and 0.500% or less.
  • the content of C is preferably 0.080% or more.
  • the content of C is preferably 0.400% or less.
  • the content of C is more preferably 0.110% or more.
  • the content of C is more preferably 0.350% or less.
  • Si is one of the important basic components of the steel.
  • Si reduces the formation of carbides during annealing, facilitates the formation of retained austenite, and is an element that affects the volume fraction of retained austenite.
  • Si provides high temper softening resistance at 400°C or lower and is therefore an important element that affects the standard deviation of the Vickers hardness of the surface of the steel sheet and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet.
  • the content of Si is less than 0.01%, the distribution of the hardness of martensite in the sheet width direction becomes nonuniform.
  • the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates.
  • the content of Si exceeds 2.50%, the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction increases, so that the resistance to intra-steel sheet cracking in the spot weld HAZ deteriorates. Therefore, the content of Si is 0.01% or more and 2.50% or less.
  • the content of Si is preferably 0.20% or more.
  • the content of Si is preferably 2.00% or less.
  • the content of Si is more preferably 0.25% or more.
  • the content of Si is more preferably 1.50% or less.
  • Mn is one of the important basic components of the steel.
  • Mn is an important element that affects the area fractions of martensite and ferrite, the volume fraction of retained austenite, the standard deviation of the Vickers hardness of the surface of the steel sheet, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet.
  • the content of Mn is less than 0.10%, the area fraction of martensite decreases, and the area fraction of ferrite increase, which makes it difficult to achieve a TS of 1180 MPa or more. This also make it difficult to achieve the desired YR.
  • the content of Mn exceeds 5.00%, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. Therefore, the content of Mn is 0.10% or more and 5.00% or less.
  • the content of Mn is preferably 1.00% or more.
  • the content of Mn is preferably 4.00% or less.
  • the content of Mn is more preferably 2.00% or more.
  • the content of Mn is more preferably 3.50% or less.
  • the content of P When the content of P is excessively large, it segregates at prior-austenite grain boundaries and embrittles the grain boundaries, and the ultimate deformation ability of the steel sheet deteriorates, causing deterioration of the bendability. Therefore, the content of P must be 0.100% or less. No particular limitation is imposed on the lower limit of the content of P. However, since P is a solid solution strengthening element and increases the strength of the steel sheet, the content of P is preferably 0.001% or more. Therefore, the content of P is 0.100% or less. The content of P is preferably 0.001% or more and more preferably 0.070% or less.
  • the content of S is present as sulfides and causes deterioration of the ultimate deformation ability of the steel, so that the bendability deteriorates. Therefore, the content of S must be 0.0200% or less. No particular limitation is imposed on the lower limit of the content of S. However, in view of the limitations on the production technique, the content of S is preferably 0.0001% or more. Specifically, the content of S is 0.0200% or less. The content of S is preferably 0.0001% or more. The content of S is preferably 0.0050% or less.
  • the content of Al When the content of Al is excessively large, the A 3 transformation temperature increases, and a large amount of ferrite is contained in the microstructure, which makes it difficult to achieve the desired YR. Therefore, the content of Al must be 0.100% or less. No particular limitation is imposed on the lower limit of the content of Al. However, since Al reduces the formation of carbides and facilitates the formation of retained austenite during continuous annealing, the content of Al is preferably 0.001% or more. Specifically, the content of Al is 0.100% or less. The content of Al is preferably 0.001% or more. The content of Al is preferably 0.050% or less.
  • the content of N is present as nitrides and causes deterioration of the ultimate deformation ability of the steel sheet, so that the bendability deteriorates. Therefore, the content of N must be 0.0100% or less. No particular limitation is imposed on the lower limit of the content of N. However, in view of the limitations on the production technique, the content of N is preferably 0.0005% or more. Specifically, the content of N is 0.0100% or less. The content of N is preferably 0.0005% or more. The content of N is preferably 0.0050% or less.
  • the content of O is present as oxides and causes deterioration of the ultimate deformation ability of the steel sheet, so that the bendability deteriorates. Therefore, the content of O must be 0.0100% or less. No particular limitation is imposed on the lower limit of the content of O. However, in view of the limitations on the production technique, the content of O is preferably 0.0001% or more. Specifically, the content of O is 0.0100% or less. The content of O is preferably 0.0001% or more. The content of O is preferably 0.0050% or less.
  • Ti forms a fine carbide, nitride, or carbonitride during hot rolling or annealing and thereby increases the strength of the steel sheet.
  • the content of Ti must be 0.002% or more.
  • the content of Ti exceeds 0.200%, the amount of the carbide, nitride, or carbonitride increases, and this makes it difficult to achieve the desired YR. Therefore, the content of Ti is 0.002% or more and 0.200% or less.
  • the content of Ti is preferably 0.006% or more.
  • the content of Ti is preferably 0.100% or less.
  • the content of Ti is more preferably 0.010% or more.
  • the content of Ti is more preferably 0.050% or less.
  • x Ti , x N , and x S are contents (mole fractions) of the respective elements in the steel sheet.
  • the effective Ti mole fraction is adjusted to 0.001% or more.
  • the effective Ti mole fraction is preferably 0.040 or less. Otherwise, the amount of the carbide, nitride, or carbonitride increases, and this makes it difficult to achieve the desired YR.
  • the effective Ti mole fraction is 0.001% or more.
  • the effective Ti mole fraction is preferably 0.002% or more.
  • the effective Ti mole fraction is preferably 0.040% or less.
  • the mole fraction of a component is determined by converting its mass percentage.
  • a high strength steel sheet according to an embodiment of the present invention has a chemical composition containing the elements described above and further containing Fe and incidental impurities as the balance.
  • the high strength steel sheet according to the present embodiment of the present invention has a chemical composition containing the elements described above, with the balance being Fe and incidental impurities.
  • the incidental impurities include Zn, Pb, and As.
  • the acceptable total content of these impurities is 0.100% or less.
  • the chemical composition of the high strength steel sheet of the present disclosure may further contain, in addition to the essential components described above, in % by mass, at least one element or a combination of elements selected from Nb: 0.200% or less, V: 0.200% or less, Ta: 0.10% or less, W: 0.10% or less, B: 0.0100% or less, Cr: 1.00% or less, Mo: 1.00% or less, Ni: 1.00% or less, Co: 0.010% or less, Cu: 1.00% or less, Sn: 0.200% or less, Sb: 0.200% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, REMs: 0.0100% or less, Zr: 0.100% or less, Te: 0.100% or less, Hf: 0.10% or less, and Bi: 0.200% or less.
  • Nb forms a large number of coarse precipitates and inclusions, and this causes deterioration of the ultimate deformation ability of the steel sheet.
  • the content of Nb is more than 0.200%, the bendability deteriorates. Therefore, the content of Nb is 0.200% or less.
  • No particular limitation is imposed on the lower limit of the content of Nb.
  • the content of Nb is 0.001% or more, a fine carbide, nitride, or carbonitride is formed during hot rolling or continuous annealing, and the strength of the steel sheet thereby increases, so that the YR can be controlled within the desired range. Therefore, the content of Nb is preferably 0.001% or more.
  • the content of Nb is preferably 0.001% or more.
  • the content of Nb is preferably 0.100% or less.
  • V forms a large number of coarse precipitates and inclusions, and this causes deterioration of the ultimate deformation ability of the steel sheet.
  • the content of V is set to 0.200% or less.
  • No particular limitation is imposed on the lower limit of the content of V.
  • the content of V is 0.001% or more, a fine carbide, nitride, or carbonitride is formed during hot rolling or continuous annealing, and the strength of the steel sheet thereby increases, so that the YR can be controlled within the desired range. Therefore, the content of V is preferably 0.001% or more.
  • V When V is added, its content is set to 0.200% or less.
  • the content of V is preferably 0.001% or more.
  • the content of V is preferably 0.100% or less.
  • the contents of Ta and W are each set to 0.10% or less.
  • the contents of Ta and W are each preferably 0.01% or more.
  • the contents of Ta and W are each preferably 0.01% or more.
  • the contents of Ta and W are each preferably 0.01% or more.
  • the contents of Ta and W are each preferably 0.08% or less.
  • the content of B is preferably 0.0100% or less.
  • the content of B is more preferably 0.0003% or more.
  • B is contained, its content is set to 0.0100% or less.
  • the content of B is more preferably 0.0003% or more.
  • the content of B is still more preferably 0.0080% or less.
  • the contents of Cr, Mo, and Ni are each set to 1.00% or less.
  • the contents of Cr, Mo, and Ni are each preferably 0.01% or more.
  • the contents of Cr, Mo, and Ni are each set to 1.00% or less.
  • the contents of Cr, Mo, and Ni are each preferably 0.01% or more.
  • the contents of Cr, Mo, and Ni are each preferably 0.80% or less.
  • the content of Co is set to 0.010% or less.
  • the content of Co is preferably 0.001% or more.
  • the content of Co is preferably 0.001% or more.
  • the content of Co is preferably 0.008% or less.
  • the content of Cu is set to 1.00% or less.
  • the content of Cu is preferably 0.01% or more.
  • the content of Cu is preferably 0.80% or less.
  • the content of Sn is set to 0.200% or less.
  • the content of Sn is preferably 0.001% or more.
  • Sn when Sn is added, its content is set to 0.200% or less.
  • the content of Sn is preferably 0.001% or more.
  • the content of Sn is preferably 0.100% or less.
  • the content of Sb is set to 0.200% or less.
  • the content of Sb is preferably 0.001% or more.
  • Sb is added, its content is set to 0.200% or less.
  • the content of Sb is preferably 0.001% or more.
  • the content of Sb is preferably 0.100% or less.
  • the contents of Ca, Mg, and REMs are each more than 0.0100%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the contents of Ca, Mg, and REMs are each set to 0.0100% or less. Now, no particular limitation is imposed on the lower limits of the contents of Ca, Mg, and REMs. However, since these elements spheroidize nitrides and sulfides and improve the ultimate deformation ability of the steel sheet, the contents of Ca, Mg, and REMs are each preferably 0.0005% or more.
  • the contents of Ca, Mg, and REMs are each set to 0.0100% or less.
  • the contents of Ca, Mg, and REMs are each preferably 0.0005% or more.
  • the contents of Ca, Mg, and REMs are each preferably 0.0050% or less.
  • the REMs (rare earth metals) are a generic term for Sc, Y, and 15 elements from lanthanum (La) with an atomic number of 57 to lutetium (Lu) with an atomic number of 71, and the content of REMs is the total content of these elements.
  • the contents of Zr and Te When the contents of Zr and Te are each more than 0.100%, the number of coarse precipitates and inclusions increases, and the ultimate deformation ability of the steel sheet deteriorates, so that the bendability deteriorates. Therefore, the contents of Zr and Te must be each 0.100% or less. No particular limitation is imposed on the lower limits of the contents of Zr and Te. However, since Zr and Te are elements that spheroidize nitrides and sulfides and improve the ultimate deformation ability of the steel sheet, their contents are each more preferably 0.001% or more. Specifically, when Zr and Te are added, their contents are each set to 0.100% or less. The contents of Zr and Te are each preferably 0.001% or more. The contents of Zr and Te are each preferably 0.080% or less.
  • the content of Hf is set to 0.10% or less.
  • the content of Hf is set to preferably 0.01% or more.
  • Hf when Hf is added, its content is set to 0.10% or less.
  • the content of Hf is preferably 0.01% or more.
  • the content of Hf is preferably 0.08% or less.
  • the content of Bi is set to 0.200% or less.
  • the content of Bi is set to 0.200% or less.
  • the content of Bi is set to 0.200% or less.
  • the content of Bi is preferably 0.001% or more.
  • the content of Bi is preferably 0.100% or less.
  • the area fraction of martensite is less than 80%, the area fraction of ferrite is large, which makes it difficult to achieve a TS of 1180 MPa or more. This also makes it difficult to achieve the desired YR.
  • the hardness distribution in the sheet width direction becomes nonuniform, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates. If the area fraction of martensite exceeds 99%, a sufficient amount of ferrite and/or retained austenite effective in controlling the YR is not present in the steel microstructure, and this makes it difficult to achieve the desired YR.
  • the area fraction of martensite is set to 80% or more and 99% or less.
  • the area fraction of martensite is preferably 85% or more.
  • the area fraction of martensite is preferably 98% or less.
  • the area fraction of martensite is more preferably 87% or more.
  • the area fraction of martensite is preferably 97% or less.
  • the term "martensite” as used herein encompasses quenched martensite (fresh martensite), tempered martensite, and bainite. Now, the observation position for the area fraction of martensite is a position 1/4 of the thickness of the steel sheet as described later.
  • the steel microstructure becomes a martensite single-phase microstructure, and this makes it difficult to achieve the desired YR.
  • the sum of the area fraction of ferrite and/or the volume fraction of retained austenite exceeds 20%, the area fraction of martensite is small, and this makes it difficult to achieve a TS of 1180 MPa or more. This also makes it difficult to achieve the desired YR.
  • the sum of the area fraction of ferrite and/or the volume fraction of retained austenite is set to more than 0% and 20% or less.
  • the sum of the area fraction of ferrite and/or the volume fraction of retained austenite is preferably 1% or more.
  • the area fraction of martensite is preferably 18% or less.
  • the area fraction of martensite is more preferably 2% or more.
  • the area fraction of martensite is preferably 15% or less.
  • the term "ferrite” as used herein encompasses bainitic ferrite.
  • the observation position for the area fraction of ferrite and the volume fraction of retained austenite is a position 1/4 of the thickness of the steel sheet as described later.
  • a method for measuring the area fraction of martensite (quenched martensite, tempered martensite, and bainite) and the area fraction of ferrite (bainitic ferrite) is as follows.
  • a sample is cut from the steel sheet such that a thicknesswise cross section (L cross section) parallel to the rolling direction serves as an observation surface, and the observation surface is mirror-polished using a diamond paste. Then the resulting observation surface is etched with 3 vol.% nital solution to make the microstructure visible.
  • a position 1/4 of the thickness of the steel sheet is used as an observation position, and an SEM (Scanning Electron Microscope) is used to observe three viewing fields each having a viewing area of 17 ⁇ m ⁇ 23 ⁇ m at a magnification of 5000X under the condition of an acceleration voltage of 15 kV.
  • ferrite (bainitic ferrite) and martensite (tempered martensite, bainite, and quenched martensite).
  • the obtained values are averaged to obtain the area fractions of the microstructures.
  • ferrite (bainitic ferrite) appears as a concave microstructure and is a flat microstructure containing no carbides
  • tempered martensite and bainite appear as concave microstructures and are microstructures containing fine carbides.
  • Quenched martensite appears as convex portions and is a microstructure with fine irregularities thereinside. These microstructures can be distinguished from each other. However, it is unnecessary that quenched martensite, tempered martensite, and bainite be distinguishable from each other because their total area fraction is used to determine the area fraction of martensite.
  • a method for measuring the volume fraction of retained austenite is as follows.
  • the steel sheet is ground such that a position 1/4 of the thickness from the surface of the steel sheet (a position corresponding to 1/4 of the thickness in the depth direction from the surface of the steel sheet) serves as an observation surface, and the resulting surface is further polished by 0.1 mm by chemical polishing. Then, on the polished surface, the CoK ⁇ line from an X-ray diffractometer is used to measure the integrated reflection intensities from (200), (220), and (311) planes of fcc iron (austenite) and (200), (211), and (220) plans of bcc iron.
  • the volume fraction of austenite is determined from the intensity ratios of the integrated reflection intensities from the above planes of fcc iron (austenite) to the integrated reflection intensities from the above planes of bcc iron and is used as the volume fraction of retained austenite.
  • To control the resistance to intra-steel sheet cracking in the spot weld HAZ it is important to control the concentrations of elements at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction is reduced, the resistance to intra-steel sheet cracking in the spot weld HAZ can be improved.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction be controlled to 0.60% or less.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction is set to 0.10% or more and 0.60% or less.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction is preferably 0.15% or more.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction is preferably 0.55% or less.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction is more preferably 0.20% or more.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction is preferably 0.50% or less.
  • the unit of the concentration of Si is % by mass.
  • a method for measuring the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction is as follows.
  • a sample with a rolling direction length of 20 mm and a width direction length of 20 mm is obtained by cutting the steel sheet.
  • the surface of the high strength steel sheet is used as a measurement surface, and measurement is performed by glow discharge optical emission spectrometry (hereinafter referred to as GDS).
  • GDS glow discharge optical emission spectrometry
  • the concentration of Si is analyzed in the sheet thickness direction under the conditions of a high-frequency discharge pressure of 300 Pa, a high-frequency output power of 35 W, and a pulse frequency of 100 Hz.
  • the concentration values of Si at positions 5 ⁇ m from the surface of the steel sheet are averaged to calculate the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet.
  • the measurement data is converted to the concentration of Si by a calibration curve method.
  • this is an extremely important constituent factor of the invention.
  • it is important to control the number density of inclusions present in Mn-segregated regions on the surface of the steel sheet, i.e., in regions containing harder martensite than that in surrounding regions.
  • the bendability can be improved.
  • it is necessary that the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet be set to 5.0 pieces/mm 2 or less.
  • the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is set to 5.0 pieces/mm 2 or less.
  • the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is preferably 0.0 pieces/mm 2 or more.
  • the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is preferably 4.0 pieces/mm 2 or less.
  • a method for measuring the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is as follows.
  • a bending test is performed using the V-block method with a bending angle of 90 degrees. Specifically, the bending test is performed with R set such that the value of R/t is about 4.5, i.e., 4.3 to 4.7. Then a sample with a rolling direction length of 20 mm and a width direction length of 5 mm is obtained by cutting the test piece so as to include a crack in the ridge portion at the apex of the bend. The outer surface of the bend is used as an observation surface, and the observation surface is mirror-polished using a diamond paste. Next, measurement is performed using an electron probe microanalyzer (EPMA) (JXA-8230 manufactured by JEOL Ltd.).
  • EPMA electron probe microanalyzer
  • Mn and S are measured in three viewing fields under the conditions of acceleration voltage: 15 kV, measurement region: rolling direction length 1.2 mm ⁇ width direction length 1.0 mm, and irradiation current: 1.0 ⁇ 10 -7 A.
  • the measurement data is converted to the concentration of C by a calibration curve method.
  • Mn element mapping is performed on the obtained three viewing fields, and regions in which a large amount of Mn is detected are identified as Mn-segregated regions.
  • S element mapping is performed to identify S-concentrated portions, i.e., MnS inclusions, in the Mn-segregated regions, and the number of S-concentrated portions is evaluated.
  • the number of MnS inclusions present in the Mn-segregated regions is divided by the measurement surface area 1.2 mm 2 to calculate the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet.
  • To control the bendability it is important to ensure a uniform hardness distribution over the surface of the steel sheet.
  • the standard deviation of the Vickers hardness of the surface of the steel sheet must be 15 or less. No particular limitation is imposed on the lower limit of the standard deviation of the Vickers hardness of the surface of the steel sheet.
  • the standard deviation of the Vickers hardness of the surface of the steel sheet is preferably 0 or more.
  • the standard deviation of the Vickers hardness of the surface of the steel sheet is preferably 13 or less.
  • a method for measuring the standard deviation of the Vickers hardness of the surface of the steel sheet is as follows.
  • a bending test is performed using the V-block method with a bending angle of 90 degrees. Specifically, the bending test is performed with R set such that the value of R/t is about 4.5, i.e., 4.3 to 4.7. Then a sample with a rolling direction length of 20 mm and a width direction length of 5 mm is obtained by cutting the test piece so as to include a crack in the ridge portion at the apex of the bend. The outer surface of the bend is used as an observation surface, and the observation surface is mirror-polished using a diamond paste.
  • the Vickers hardness is measured at 11 points on the mirror-polished observation surface at 100 ⁇ m intervals in the sheet width direction using a Vickers hardness meter under the condition of a load of 100 gf.
  • the measurement points are set such that the sixth point is located at a position 500 ⁇ m from the edge of the crack in the rolling direction so as to be parallel to the crack.
  • the standard deviation is determined from the obtained results and used as the standard deviation of the Vickers hardness of the surface of the steel sheet.
  • the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet By reducing the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet, the desired bendability can be achieved. To obtain this effect, the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet must be 7 or less. No particular limitation is imposed on the lower limit of the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet. However, the lower the hardness variation frequency, the better. Even when the hardness variation frequency is 0, the effects of the disclosure can be obtained. Specifically, the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet is 7 or less. The hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet is preferably 0 or more. The hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel
  • the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet is as follows.
  • a bending test is performed using the V-block method with a bending angle of 90 degrees. Specifically, the bending test is performed with R set such that the value of R/t is about 4.5, i.e., 4.3 to 4.7. Then a sample with a rolling direction length of 20 mm and a width direction length of 5 mm is obtained by cutting the test piece so as to include a crack in the ridge portion at the apex of the bend. The outer surface of the bend is used as an observation surface, and the observation surface is mirror-polished using a diamond paste.
  • the Vickers hardness is measured at 11 points on the mirror-polished observation surface at 100 ⁇ m intervals in the sheet width direction using a Vickers hardness meter under the condition of a load of 100 gf.
  • the measurement points are set such that the sixth point is located at a position 500 ⁇ m from the edge of the crack so as to be parallel to the crack.
  • the obtained results are used to produce the distribution of the hardness measured on the surface of the steel sheet using the Vickers hardness meter in the sheet width direction.
  • the value of ⁇ (maximum hardness value Hv max ) - (minimum hardness value Hv min ) ⁇ / 2 is calculated.
  • the value of ⁇ (maximum hardness value Hv max ) - (minimum hardness value Hv min ) ⁇ / 2 is used as a reference variation value. Then, any occurrence in which the absolute value of hardness variation is larger than or equal to the reference variation value is identified. Over the hardness measurement range (length: 1100 ⁇ m), the number of occurrences in which the absolute value of hardness variation is larger than or equal to the reference variation value is counted. Specifically, each instance in which the hardness variation value is positive and larger than or equal to the reference variation value is counted as one occurrence, and each instance in which the hardness variation value is negative, and its absolute value is larger than or equal to the reference variation value is counted as one occurrence. Therefore, when one measured hardness variation value is positive and larger than or equal to the reference variation value and another one measured hardness variation value is negative with an absolute value larger than or equal to the reference variation value, the hardness variation frequency is two.
  • the steel microstructure in the present disclosure may contain remaining microstructures other than the martensite (quenched martensite, tempered martensite, and bainite), ferrite (including bainitic ferrite), and retained austenite described above. Even when pearlite, carbides such as cementite and metastable carbides, and other well-known steel sheet microstructures are contained as remaining microstructures, the effects of the disclosure are not impaired when their area fraction is 5% or less.
  • the metastable carbides include epsilon ( ⁇ ) carbide, eta ( ⁇ ) carbide, and chi ( ⁇ ) carbide.
  • the chemical composition and steel microstructure of the high strength steel sheet are as described above. No particular limitation is imposed on the thickness of the high strength steel sheet.
  • the thickness is generally 0.3 mm or more and 2.8 mm or less.
  • the high strength coated steel sheet of the disclosure is a high strength coated steel sheet produced by forming a coated layer on at least one side of the high strength steel sheet of the disclosure.
  • the coated layer may be, for example, a hot-dip coated layer or an electroplated layer.
  • the coated layer may be an alloyed coated layer.
  • the coated layer is preferably a galvanized layer.
  • the galvanized layer may contain Al and Mg.
  • the coated layer is also preferably a hot-dip zinc-aluminum-magnesium alloy coated layer (Zn-Al-Mg coated layer).
  • the content of Al is set to 1% by mass or more and 22% by mass or less and the content of Mg is set to 0.1% by mass or more and 10% by mass or less, with the balance being Zn.
  • the Zn-Al-Mg coated layer may contain, in addition to Zn, Al, and Mg, at least one selected from Si, Ni, Ce, and La in a total amount of 1% by mass or less. No particular limitation is imposed on the coating metal. In addition to the Zn coating, an Al coating etc. may be used.
  • a hot-dip galvanized layer and a hot-dip galvannealed layer generally contain Fe: 20% by mass or less and Al: 0.001% by mass or more and 1.0% by mass or less.
  • Their chemical compositions further contain one or two or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REMs in a total amount of 0% by mass or more and 3.5% by mass or less, with the balance being Zn and incidental impurities.
  • a hot-dip galvanized layer with a coating weight per side of 20 to 80 g/m 2 or a hot-dip galvannealed layer obtained by subjecting the hot-dip galvanized layer to galvannealing.
  • the content of Fe in the coated layer may be less than 7% by mass.
  • the content of Fe in the coated layer may be 7 to 20% by mass.
  • a steel slab is produced by melting a steel material having the chemical composition described above.
  • the method for melting the steel material any known melting method using a converter, an electric arc furnace, etc. may be suitably used.
  • the steel slab (slab) is produced using a continuous casting method in order to prevent macro segregation.
  • the steel slab may be produced using another method such as ingot casting or thin slab casting.
  • a conventional method may be used to cool the steel slab to room temperature and then reheat the steel slab.
  • an energy saving process such as hot charge rolling or hot direct rolling can be used without any problem.
  • the hot charge rolling or hot direct rolling the slab is not cooled. The hot slab is charged into a heating furnace, or the slab is subjected to heat retention treatment for a short time, and then the resulting slab is immediately hot-rolled.
  • the steel slab is heated to a slab heating temperature of 1150°C or higher at an average slab heating rate of 25°C/min or less in the temperature range of 900°C or higher and 1150°C or lower such that the residence time in a range of from 1100°C to the slab heating temperature is 20 minutes or longer.
  • the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower to a low value (25°C/min or less)
  • the detachment of Si from the surface of the steel sheet is facilitated, and the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction can be reduced.
  • the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower to a low value, the segregation of Mn formed during casting can be reduced, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet can be reduced.
  • the standard deviation of the Vickers hardness of the surface of the steel sheet can be reduced, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet can be reduced.
  • the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower is set to 25°C/min or less. No particular limitation is imposed on the lower limit of the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower.
  • the average slab heating rate is set to preferably 5°C/min or more.
  • the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower is set to 25°C/min or less.
  • the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower is preferably 5°C/min or more.
  • the average slab heating rate in the temperature range of 900°C or higher and 1150°C or lower is preferably 15°C/min or less.
  • the slab heating temperature is the temperature of the surface of the steel slab during slab heating.
  • the slab heating temperature By setting the slab heating temperature to a high value of 1150°C or higher, the detachment of Si from the surface of the steel sheet is facilitated, and the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction can be reduced.
  • the slab heating temperature By setting the slab heating temperature to a high value, the segregation of Mn formed during casting is reduced, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is reduced.
  • the standard deviation of the Vickers hardness of the surface of the steel sheet can be reduced, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet can be reduced.
  • the slab heating temperature is set to 1150°C or higher.
  • the slab heating temperature is set to preferably 1300°C or lower. Therefore, the slab heating temperature is 1150°C or higher.
  • the slab heating temperature is preferably 1180°C or higher.
  • the slab heating temperature is preferably 1300°C or lower.
  • the slab heating temperature is the temperature of the surface of the steel slab during slab heating.
  • the residence time in the range of from 1100°C to the slab heating temperature to a large value of 20 minutes or longer, the detachment of Si from the surface of the steel sheet can be facilitated, and the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction can be reduced.
  • the residence time in the range of from 1100°C to the slab heating temperature to a large value, the segregation of Mn formed during casting is reduced, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet is reduced.
  • the residence time in the range of from 1100°C to the slab heating temperature is set to 20 minutes or longer.
  • the residence time is set to preferably 100 minutes or shorter.
  • the residence time in the range of from 1100°C to the slab heating temperature is set to 20 minutes or longer.
  • the residence time in the range of from 1100°C to the slab heating temperature is preferably 30 minutes or longer.
  • the residence time in the range of from 1100°C to the slab heating temperature is preferably 100 minutes or shorter.
  • the slab heating temperature is the temperature of the surface of the steel slab during slab heating.
  • the slab is subjected to rough rolling under ordinary conditions to form a sheet bar.
  • the slab heating temperature is set to be low, it is preferable that the sheet bar is heated before finish rolling using, for example, a bar heater, from the viewpoint of preventing troubles during hot rolling.
  • the rolling reduction in the pass immediately preceding the final pass is set to be more than or equal to the rolling reduction in the final pass, and the rolling reduction in the pass two passes prior to the final pass is set to be more than or equal to the rolling reduction in the pass immediately preceding the final pass.
  • concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction, the standard deviation of the Vickers hardness of the surface of the steel sheet, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet can be controlled appropriately.
  • the rolling reduction in the final pass of the finish rolling is less than 9%, the grain size of austenite on the surface of the steel sheet becomes coarse during hot rolling, i.e., the crystal grain size in the annealed sheet becomes coarse, so that the diffusion of Si to the surface of the steel sheet is suppressed.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction cannot be reduced.
  • precipitation of Ti i.e., precipitation of sulfides, is reduced, so that the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet cannot be reduced.
  • the rolling reduction in the final pass exceeds 15%, the segregation of Mn formed during casting cannot be reduced.
  • the rolling reduction in the final pass of the finish rolling is set to 9% or more and 15% or less.
  • the rolling reduction in the pass immediately preceding the final pass is less than 15%, the grain size of austenite on the surface of the steel sheet becomes coarse during hot rolling, i.e., the crystal grain size in the annealed sheet becomes coarse, so that the diffusion of Si to the surface of the steel sheet is reduced.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction cannot be reduced.
  • precipitation of Ti i.e., precipitation of sulfides, is reduced, so that the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet cannot be reduced.
  • the rolling reduction in the pass immediately preceding the final pass exceeds 21%, the segregation of Mn formed during casting cannot be reduced.
  • the standard deviation of the Vickers hardness of the surface of the steel sheet cannot be reduced, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet cannot be reduced. Therefore, the rolling reduction in the pass immediately preceding the final pass in the finish rolling is set to 15% or more and 21% or less.
  • the rolling reduction in the pass two passes prior to the final pass is less than 21%, the grain size of austenite on the surface of the steel sheet becomes coarse during hot rolling, i.e., the crystal grain size in the annealed sheet becomes coarse, so that the diffusion of Si to the surface of the steel sheet is reduced.
  • the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction cannot be reduced.
  • precipitation of Ti i.e., precipitation of sulfides, is reduced, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet cannot be reduced.
  • the rolling reduction in the pass two passes prior to the final pass exceeds 27%, the segregation of Mn formed during casting cannot be reduced.
  • the standard deviation of the Vickers hardness of the surface of the steel sheet cannot be reduced, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet cannot be reduced. Therefore, the rolling reduction in the pass two passes prior to the final pass of the finish rolling is set to 21% or more and 27% or less.
  • the rolling load may increase, and the rolling reduction in a state in which austenite is not recrystallized may increase.
  • an unexpected microstructure extending in the rolling direction develops, resulting in a deterioration in the workability of the annealed sheet. Therefore, it is preferable to perform the finish rolling at a finish rolling temperature higher than or equal to the Ar 3 transformation temperature.
  • the temperature of coiling performed after hot rolling is preferably 300°C or higher and more preferably 700°C or lower in order to further improve the workability after annealing.
  • the Ar 3 transformation temperature is determined by the following formula.
  • Ar 3 transformation temperature (°C) 868 - 396 ⁇ [%C] + 24.6 ⁇ [%Si] - 68.1 ⁇ [%Mn] - 36.1 ⁇ [%Ni] - 20.7 ⁇ [%Cu] - 24.8 ⁇ [%Cr]
  • [% element symbol] represents the content (% by mass) of the element in the chemical composition and is 0 when the element is not contained.
  • rough-rolled sheets may be joined together to allow finish rolling to be performed continuously.
  • the rough-rolled sheets may be temporarily coiled.
  • part or all of the finish rolling may be performed as lubrication rolling.
  • the lubrication rolling is also effective from the viewpoint of allowing the steel sheet to have a uniform shape and uniform material properties.
  • the coefficient of friction during the lubrication rolling is preferably 0.10 or more and is preferably 0.25 or less.
  • the thus-produced hot rolled steel sheet is pickled.
  • the pickling allows removal of oxides from the surface of the steel sheet and is important for imparting excellent chemical convertibility to the high strength steel sheet used as the final product and for ensuring excellent coating quality.
  • the pickling may be performed once or repeatedly in a plurality of passes.
  • the hot rolled sheet subjected to pickling or the hot rolled sheet subjected to pickling and then to optional heat treatment is cold-rolled to obtain a cold rolled sheet.
  • the cold rolling is performed through multi-pass rolling including two or more passes such as tandem-type multi-stand rolling or reverse rolling because uniform strain can be introduced effectively, and a uniform microstructure can be obtained.
  • the bending and unbending before cold rolling are generally performed using rolls with a roll diameter of 300 to 1500 mm.
  • the area fraction of ferrite can be reduced, i.e., the sum of the area fraction of ferrite and/or the volume fraction of retained austenite can be adjusted to 20% or less.
  • the cumulative rolling reduction ratio in the cold rolling is set to 20% or more. If the cumulative rolling reduction ratio in the cold rolling exceeds 75%, the grain size of austenite generated during annealing becomes small, and the amount of retained austenite in the annealed sheet increases.
  • the cumulative rolling reduction ratio in the cold rolling is set to 20% or more and 75% or less.
  • the cumulative rolling reduction ratio in the cold rolling is preferably 25% or more.
  • the cumulative rolling reduction ratio in the cold rolling is preferably 70% or less.
  • the cumulative rolling reduction ratio in the cold rolling is more preferably 27% or more.
  • the cumulative rolling reduction ratio in the cold rolling is more preferably 60% or less.
  • the cold rolled sheet obtained as described above is subjected to an annealing step.
  • the annealing conditions are as follows.
  • the average heating rate in the temperature range of 250°C or higher and 700°C or lower By reducing the average heating rate in the temperature range of 250°C or higher and 700°C or lower, Si diffuses to the surface of the steel sheet, so that the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction can be reduced.
  • the average heating rate in the temperature range of 250°C or higher and 700°C or lower must be 100°C/s or lower. No particular limitation is imposed on the lower limit of the average heating rate in the temperature range of 250°C or higher and 700°C or lower.
  • the average heating rate is set to preferably 5°C/s or more and more preferably 10°C/s or more.
  • the average heating rate in the temperature range of 250°C or higher and 700°C or lower is set to 100°C/s or lower.
  • the average heating rate in the temperature range of 250°C or higher and 700°C or lower is preferably 5°C/s or more.
  • the average heating rate in the temperature range of 250°C or higher and 700°C or lower is preferably 75°C/s or lower.
  • the average heating rate in the temperature range of 250°C or higher and 700°C or lower is more preferably 10°C/s or more.
  • the average heating rate in the temperature range of 250°C or higher and 700°C or lower is more preferably 50°C/s or lower.
  • the average heating rate is measured based on the surface temperature of the steel sheet.
  • the annealing treatment proceeds in the ferrite-austenite two phase region, and the resulting steel sheet contains a large amount of ferrite after annealing.
  • the TS of 1180 MPa or more cannot be achieved, and this makes it difficult to achieve the desired YR.
  • the hardness distribution in the sheet width direction is nonuniform, the standard deviation of the Vickers hardness of the surface of the steel sheet exceeds 15, and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet exceeds 7, so that the bendability deteriorates.
  • the heating temperature is preferably 1000°C or lower. Specifically, the heating temperature is set to 780°C or higher. The heating temperature is more preferably 820°C or higher.
  • the heating temperature is still more preferably 830°C or higher.
  • the upper limit of the heating temperature is preferably 1000°C or lower.
  • the upper limit of the heating temperature is more preferably 980°C or lower.
  • the heating temperature is measured based on the surface temperature of the steel sheet.
  • the residence time in the temperature range of 750°C or higher and the heating temperature or lower is set to 10 seconds or longer.
  • the residence time is preferably 400 seconds or shorter.
  • the residence time in the temperature range of 750°C or higher and the heating temperature or lower is set to 10 seconds or longer.
  • the residence time in the temperature range of 750°C or higher and the heating temperature or lower is preferably 20 seconds or longer.
  • the residence time in the temperature range of 750°C or higher and the heating temperature or lower is preferably 400 seconds or shorter.
  • the residence time in the temperature range of 750°C or higher and the heating temperature or lower is more preferably 25 seconds or longer.
  • the residence time from 750°C or higher to the heating temperature is more preferably 300 seconds or shorter.
  • the detachment of Si from the surface of the steel sheet via oxygen in air is facilitated, and the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction can be reduced.
  • the detachment of Mn from the surface of the steel sheet is facilitated, and this allows a reduction in the standard deviation of the Vickers hardness of the surface of the steel sheet and a reduction in the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet.
  • the concentration of oxygen in the temperature range of 750°C or higher and the heating temperature or lower is preferably 0.5% by volume or more, more preferably 1.0% by volume or more, and still more preferably 1.5% by volume or more.
  • the concentration of oxygen in the temperature range of 750°C or higher and the heating temperature or lower is preferably 5.0% by volume or less, more preferably 4.5% by volume or less, and still more preferably 4.0% by volume or less.
  • the temperature in the range of 750°C or higher and the heating temperature or lower is based on the surface temperature of the steel sheet. Specifically, when the surface temperature of the steel sheet is 750°C or higher and the heating temperature or lower, the concentration of oxygen is adjusted within the above range.
  • the detachment of Si from the surface of the steel sheet via moisture in air can be facilitated, and the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction can be reduced.
  • the detachment of Mn from the surface of the steel sheet is facilitated, and this allows a reduction in the standard deviation of the Vickers hardness of the surface of the steel sheet and a reduction in the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet.
  • the dew point in the temperature range of 750°C or higher and the heating temperature or lower is preferably -35°C or higher, more preferably -30°C or higher, and still more preferably -25°C or higher.
  • the dew point in the temperature range of 750°C or higher and the heating temperature or lower is preferably 15°C or lower and more preferably 5°C or lower. Otherwise, the thickness of the softened surface layer after annealing increases, and the TS decreases.
  • the temperature of 750°C or higher and the heating temperature or lower is based on the surface temperature of the steel sheet. Specifically, when the surface temperature of the steel sheet is 750°C or higher and the heating temperature or lower, the dew point is adjusted within the above range.
  • the cold rolled sheet is optionally cooled.
  • the cooling rate is preferably 5°C/s or more and 30°C/s or lower.
  • the high strength steel sheet may be cooled once, and then the temperature of the steel sheet may be again increased.
  • the average cooling rate in the temperature range of 250°C or higher and 400°C or lower is 1.0°C/s or more, the amount of bainitic ferrite included after annealing can be further reduced, and the YR and bendability can be further improved.
  • the average cooling rate in the temperature range of 250°C or higher and 400°C or lower is preferably 1.0°C/s or more, more preferably 2.0°C/s or more, and still more preferably 3.0°C/s or more. No particular limitation is imposed on the upper limit of the average cooling rate in the temperature range of 250°C or higher and 400°C or lower.
  • the average cooling rate is preferably 100.0°C/s or lower and still more preferably 80.0°C/s or lower.
  • the average cooling rate is the value in the temperature range of the cooling stop temperature or higher and 400°C or lower. The average cooling rate is measured based on the surface temperature of the steel sheet.
  • a cooling method used in the temperature range of 250°C or higher and 400°C or lower may be gas jet cooling, mist cooling, water cooling, natural cooling, etc.
  • heat retention treatment is performed in the temperature range of 100°C or higher and 450°C or lower for 5 seconds or longer.
  • the YR and bendability can be adjusted within more preferred ranges.
  • the area fraction of bainitic ferrite can be further reduced, and the TS can be further improved.
  • the heat retention temperature in the cooling step is more preferably 150°C or higher and still more preferably 200°C or higher.
  • the heat retention temperature in the cooling step is more preferably 400°C or lower and still more preferably 350°C or lower.
  • the temperature in the cooling step is based on the surface temperature of the steel sheet.
  • the heat retention time at the heat retention temperature in the cooling step is preferably 5 seconds or longer, more preferably 10 seconds or longer, and still more preferably 15 seconds or longer. No particular limitation is imposed on the upper limit of the heat retention time at the heat retention temperature in the cooling step.
  • the heat retention time at the heat retention temperature in the cooling step is preferably 500 seconds or shorter and more preferably 250 seconds or shorter.
  • the cooling stop temperature is preferably 250°C or lower and more preferably 200°C or lower.
  • the cooling stop temperature is 250°C or lower, the formation of a large amount of retained austenite after annealing can be prevented, and the YR and bendability can be further improved.
  • No particular limitation is imposed on the lower limit of the cooling stop temperature.
  • the cooling stop temperature is preferably higher than or equal to room temperature. The cooling stop temperature is measured based on the surface temperature of the steel sheet.
  • the average cooling rate to 250°C or lower is preferably 1°C/s or more and more preferably 2°C/s or more.
  • the average cooling rate to 250°C or lower is preferably 1000°C/s or lower and more preferably 150°C/s or lower.
  • the cold rolled sheet may be further cooled from the cooling stop temperature to room temperature.
  • No particular limitation is imposed on the average cooling rate from the cooling stop temperature to room temperature, and any method may be used to cool the cold rolled sheet to room temperature. Examples of the cooling method that can be used include gas jet cooling, mist cooling, water cooling, and natural cooling.
  • the cold rolled sheet subjected to the annealing described above and cooled to the cooling stop temperature may be rolled.
  • the elongation rate in the rolling is preferably 0.05% or more and more preferably 0.10% or more.
  • the YR can be controlled within the desired range.
  • the elongation rate in the rolling is preferably 2.00% or less and more preferably 1.00% or less.
  • the volume fraction of retained austenite can be adjusted within a more preferred range.
  • the bendability and the degree of damage to sheared end faces in a corrosive environment can be adjusted within more preferred ranges.
  • the rolling performed after cooling to the cooling stop temperature may be performed in an apparatus connected to the continuous annealing apparatus described above (on-line) or may be performed in an apparatus separated from the continuous annealing apparatus (off-line).
  • the target elongation rate may be achieved by a single rolling pass, or a plurality of rolling passes may be performed to obtain a total elongation rate of 0.05% or more and 2.00% or less.
  • the rolling described above is generally temper rolling. However, any working method such as repeated bending using a tension leveler, or rolls may be used so long as an elongation rate comparable to that obtained by temper rolling can be obtained.
  • the high strength steel sheet may be reheated (reheating step).
  • the reheating temperature is preferably (the cooling stop temperature + 50°C) or higher, more preferably (the cooling stop temperature + 100°C) or higher, and still more preferably (the cooling stop temperature + 150°C) or higher.
  • the reheating temperature is preferably 450°C or lower, more preferably 400°C or lower, and still more preferably 380°C or lower. The reheating temperature is based on the surface temperature of the steel sheet.
  • the heat retention time at the reheating temperature is preferably 5 seconds or longer, more preferably 10 seconds or longer, and still more preferably 15 seconds or longer. No particular limitation is imposed on the upper limit of the heat retention time at the reheating temperature. However, to adjust the TS within a more preferred range, the heat retention time at the reheating temperature is preferably 500 seconds or shorter and more preferably 250 seconds or shorter.
  • the steel sheet may be cooled from the reheating temperature to room temperature.
  • No particular limitation is imposed on the cooling rate from the reheating temperature to room temperature, and any method may be used for the cooling to room temperature. Examples of the cooling method that can be used include gas jet cooling, mist cooling, water cooling, and natural cooling.
  • the high strength steel sheet When the high strength steel sheet is used as a trade product, the high strength steel sheet is generally cooled and then used as the trade product.
  • a high strength coated steel sheet can be obtained by subjecting at least one side of the high strength steel sheet produced as described above to coating treatment.
  • the coating treatment include hot-dip galvanizing treatment and galvannealing treatment performed after the hot-dip galvanizing treatment. The annealing and galvanization may be performed continuously in a single line.
  • the coated layer may be formed by electroplating such as Zn-Ni alloy electroplating, or hot-dip zinc-aluminum-magnesium alloy plating may be performed.
  • the galvanization has mainly been described above. However, no particular limitation is imposed on the type of coating metal, and the coating may be Zn coating, Al coating, etc.
  • the high strength steel sheet is immersed in a galvanizing bath at 440°C or higher and 500°C or lower to perform the hot-dip galvanizing treatment and then the coating weight is controlled by, for example, gas wiping.
  • the hot-dip galvanization is performed using a galvanizing bath with an Al content of 0.10% by mass or more and 0.23% by mass or less.
  • the temperature range when galvannealing treatment is performed after the hot-dip galvanization is preferably 470°C or higher and 600°C or lower and more preferably 470°C or higher and 560°C or lower.
  • the coating weight per side is preferably 20 to 80 g/m 2 (double-sided coating).
  • galvannealing described later is performed to adjust the concentration of Fe in the coated layer to 7 to 15% by mass.
  • the high strength steel sheet subjected to the annealing step may be subjected to the coating treatment in the temperature range of the heating temperature or lower and 400°C or higher without cooling the high strength steel sheet, or the cold rolled steel sheet may be cooled to lower than 400°C, reheated such that the temperature of the steel sheet is increased to 400°C or higher, and then subjected to the coating treatment.
  • the high strength coated steel sheet subjected to the coating treatment may be rolled.
  • the elongation rate in the rolling is preferably 0.05% or more and more preferably 0.10% or more.
  • the YR can be controlled within the desired range.
  • the elongation rate in the rolling is preferably 2.00% or less and more preferably 1.00% or less.
  • the rolling performed after the coating treatment may be performed in an apparatus connected to the continuous annealing apparatus described above (on-line) or may be performed in an apparatus separated from the continuous annealing apparatus (off-line).
  • the target elongation rate may be achieved by a single rolling pass, or a plurality of rolling passes may be performed to obtain a total elongation rate of 0.05% or more and 2.00% or less.
  • the rolling described above is generally temper rolling. However, any working method such as repeated bending using a tension leveler, or rolls may be used so long as an elongation rate comparable to that obtained by temper rolling can be obtained.
  • Reheating treatment may be performed after the rolling performed after the coating treatment.
  • the series of treatments including the annealing, hot-dip galvanization, galvannealing treatment, etc. described above is performed in a hot-dip galvanization line known as a CGL (Continuous Galvanizing Line). After the hot-dip galvanization, wiping may be performed in order to adjust the coating weight.
  • the coating conditions other than the conditions described above may be set according to a conventional procedure for hot-dip galvanization.
  • the high strength coated steel sheet When the high strength coated steel sheet is used as a trade product, the high strength coated steel sheet is generally cooled to room temperature and then used as the trade product.
  • Production conditions other than the conditions described above may be set according to a conventional procedure.
  • the member according to the present embodiment of the invention is a member prepared using the high strength steel sheet or high strength coated steel sheet according to the preceding embodiment of the invention.
  • the member according to the present embodiment of the invention is prepared, for example, by forming the high strength steel sheet or high strength coated steel sheet according to the preceding embodiment of the invention into an intended shape by, for example, cold pressing. Therefore, the member has the steel microstructure and properties of the high strength steel sheet, or the high strength coated steel sheet even after forming.
  • the member according to the present embodiment of the invention is preferably used for framework structural parts of automobiles and reinforcing parts of automobiles.
  • the high strength steel sheet according to the preceding embodiment of the invention is a high strength steel sheet of 1180 MPa or more that has excellent bendability and high resistance to intra-steel sheet cracking in the spot weld HAZ and is usable for the production of parts with high dimensional accuracy. Therefore, the member according to the present embodiment of the invention can contribute to a reduction in vehicle body weight and can be particularly preferably used for framework structural parts of automobiles or variety of members for reinforcing parts of automobiles.
  • annealing, cooling, and reheating were performed under the conditions shown in Tables 2 and 3 to thereby obtain high strength cold rolled steel sheets (CRs).
  • Some of the steel sheets were subjected to coating treatment to thereby obtain hot-dip galvanized steel sheets (GIs), hot-dip galvannealed steel sheets (GAs), and an electrogalvanized steel sheet (EG).
  • the hot-dip galvanizing bath used for the GIs was a zinc bath containing Al: 0.14 to 0.19% by mass
  • the hot-dip galvanizing bath used for the GAs was a zinc bath containing Al: 0.14% by mass.
  • the temperature of the baths was 470°C.
  • the coating weight per side for the GIs was about 45 to 72 g/m 2 (double-sided coating), and the coating weight per side for the GAs was about 45 g/m 2 (double-sided coating).
  • the concentration of Fe in the coated layer was 9% by mass or more and 12% by mass or less.
  • the content of Ni in the coated layer was 9% by mass or more and 25% by mass or less.
  • the tensile test was performed according to JIS Z 2241:2022.
  • a JIS No. 5 test specimen was cut from one of the obtained steel sheets such that the test specimen was orthogonal to the rolling direction of the steel sheet.
  • the tensile test was performed under the condition of a crosshead speed of 1.67 ⁇ 10 -1 mm/s to measure the YS and TS.
  • YR yield ratio
  • the YR was calculated using the calculation method described in formula (2) above.
  • the bending test was performed according to JIS Z 2248:2022.
  • a strip-shaped test specimen having a width of 30 mm and a length of 100 mm was cut from one of the obtained steel sheets such that a direction parallel to the rolling direction of the steel sheet was aligned with the axial direction of the bending test.
  • a 90° V-bending test was performed under the conditions of a pressing load of 100 kN and a press-holding time of 5 seconds.
  • the bending test was performed on five samples, with the bending radius (R) set such that the value of R/t obtained by dividing the bending radius (R) by the sheet thickness (t) was about 4.5, i.e., 4.3 to 4.7.
  • the length of a crack in a ridge portion at the apex of the bend was evaluated.
  • the bendability was considered excellent.
  • the crack length was evaluated by performing the measurement on the ridge portion at the apex of the bend using a digital microscope (RH-2000 manufactured by HIROX CO., LTD.) at a magnification of 40 to 160X.
  • a test specimen with a thickness of 1.4 mm, a length of 30 mm, and a width of 100 mm was cut out such that its lengthwise direction coincided with the rolling direction and placed on a hot-dip galvanized steel sheet for the test having the same size as the cut test specimen and including a hot-dip galvanized layer with a coating weight per side of 50 g/m 2 to thereby prepare a sheet set.
  • a servo motor pressing-type single-phase AC (50 Hz) resistance welder and electrodes with a tip radius of 6 mm were used to perform resistance welding while the sheet set was inclined 5° with respect to the electrodes of the resistance welder and a clearance of 1.5 mm was provided between the lower electrode and the lower steel sheet.
  • the sheet set was resistance-welded under the welding conditions of a welding pressure of 3.5 kN and a holding time of 0.16 seconds or 0.20 seconds while the welding current and the welding time were adjusted such that the nugget had a diameter of 5.9 mm, and the sheet set with a weld was thereby obtained.
  • the sheet set with the weld was cut into halves such that the weld was included in the cross sections, and one of the cross sections of the weld was observed under an optical microscope (200X).
  • the resistance to intra-steel sheet cracking in the spot weld HAZ was evaluated according to the criteria described above. The same evaluation was also performed on samples with thicknesses of 0.8 mm and 2.3 mm.
  • the area fraction of martensite, the area fraction of ferrite, the volume fraction of retained austenite, the concentration of Si at a position 5 ⁇ m from the surface of the steel sheet in the thickness direction, and the number density of MnS inclusions present in the Mn-segregated regions on the surface of the steel sheet were determined by the methods described above. Moreover, the standard deviation of the Vickers hardness of the surface of the steel sheet and the hardness variation frequency per 1100 ⁇ m in the sheet width direction on the surface of the steel sheet were determined. As for the remaining microstructures, observation was performed using the following method.
  • a sample was cut from one of the steel sheets such that a thicknesswise cross section (L cross section) parallel to the rolling direction of the steel sheet served as the observation surface, and the observation surface was mirror-polished using a diamond paste. Then the resulting observation surface was etched with 3 vol.% nital solution to make the microstructure visible. A position 1/4 of the thickness of the steel sheet was used as an observation position, and an SEM was used to observe three viewing fields each having a viewing area of 17 ⁇ m ⁇ 23 ⁇ m at a magnification of 5000X under the condition of an acceleration voltage of 15 kV. Carbides in the obtained microstructure images were identified as remaining microstructures.
  • the TS, YR, bendability, and resistance to intra-steel sheet cracking in the spot weld HAZ were excellent.
  • at least one of the TS, YR, bendability, and resistance to intra-steel sheet cracking in the spot weld HAZ was poor.
  • a high strength steel sheet of 1180 MPa or more can be obtained, which has excellent bendability and high resistance to intra-steel sheet cracking in the spot weld HAZ and can be used to produce parts with high dimensional accuracy.
  • the high strength steel sheet of the invention has high resistance to intra-steel sheet cracking in the spot weld HAZ and can therefore be used for automobile framework structural parts with different sizes and shapes while high component strength is ensured. This allows a reduction in vehicle body weight, and the fuel economy can thereby be improved, so that the high strength steel sheet is extremely valuable for industrial applications.

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