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
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  • 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
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties 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
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  • 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
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • 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
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  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/12—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
  • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium with vanadium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/26—Ferrous alloys, e.g. steel alloys containing chromium with niobium or tantalum
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—Ferrous alloys, e.g. steel alloys containing chromium with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—Ferrous alloys, e.g. steel alloys containing chromium with nickel with titanium or zirconium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel 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
• • 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
• • 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
  • 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
  • C21D2211/003—Cementite
• • 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
  • 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

Definitions

• the present disclosure relates to a steel sheet and a method for manufacturing the same, and, more specifically, to a high-strength steel sheet having excellent formability and a high yield ratio, and a method for manufacturing the same.
• Solid solution strengthening steel is a steel that increases yield strength by performing solid-solution strengthening on solid solution strengthening elements (Mn, Si, Cr, and the like) in a ferrite phase having excellent formability.
• Si or Cr are elements that easily form oxides on a surface of a steel sheet in continuous annealing lines or continuous hot-dip galvanized lines.
• Mn is an element that promotes a low-temperature transformation phase (bainite or martensite), and the low-temperature transformation phase may have the characteristic of lowering the yield strength. Accordingly, the solid solution strengthening steel to which a large amount of Mn, Si, and Cr is added is not an appropriate method for increasing a yield ratio of high-strength steel having a tensile strength of 610 MPa or more.
• precipitation strengthening steel using Nb, Ti, V, and the like improves yield strength by precipitating fine carbides in ferrite. Since precipitation strengthening steel increases a yield ratio without deteriorating the workability, it is a strengthening mechanism suitable for a high-strength steel sheet having a tensile strength of 610 MPa or more that have excellent crash performance and workability.
• a method of introducing unrecrystallized ferrite and utilizing the addition of Ti or Nb is disclosed in Patent Documents 1 and 2.
• Precipitation strengthening and unrecrystallized ferrite using Ti or Nb are effective in increasing yield strength without significantly increasing the tensile strength by directly strengthening the ferrite.
• Patent Documents 1 and 2 did not have an appropriate fraction of unrecrystallized ferrite, and thus did not have both excellent formability and a high yield ratio.
• Patent Document 3 included an appropriate area fraction of unrecrystallized ferrite, but the balance of tensile strength, elongation, hole expandability, and yield ratio were not appropriate.
• a steel sheet and a method for manufacturing the same.
• a high-strength steel sheet having excellent formability and a high yield ratio, and a method for manufacturing the same.
• a steel sheet comprising: by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and comprising at least one selected from Ti, Nb, and V in an amount of 0.22% or less,
• the steel sheet further may comprise at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.
• the steel sheet may comprise at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.
• the steel sheet may have a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.95.
• TS tensile strength
• YR yield ratio
• a steel sheet comprising, by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and including at least one selected from Ti, Nb, and V in an amount of 0.22% or less,
• the steel sheet may further comprise at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.
• the steel sheet may include at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.
• the steel sheet may have a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.95.
• TS tensile strength
• YR yield ratio
• a steel sheet comprising, by wt%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and including at least one selected from Ti, Nb, and V in an amount of 0.22% or less,
• the steel sheet may further include at least one selected from, by wt %, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.
• the steel sheet may include at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.
• the steel sheet may have a tensile strength (TS) of 610 MPa or more and a yield ratio (YR) of 0.8 to 0.95, and a product of a square of a tensile strength (TS) and a square root of a pore expansion ratio (HER) (TS 2 ⁇ HER) is 2.5 to 3.8 ⁇ 10 6 MPa 2 % 0.5
• a method for manufacturing a steel sheet comprising: a reheating a steel slab comprising, by wt.%, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and including at least one selected from Ti, Nb, and V in an amount of 0.22% or less;
• the steel slab may further comprise at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, and Hf: 0.2% or less.
• the steel sheet may comprise at least one selected from Ti, Nb, and V in an amount of 0.01 to 0.22%.
• the reheating may be performed at a temperature range of 1000 to 1350°C,
• the steel sheet may be cooled to a coiling temperature at an average cooling rate of 10°C/s or more after the hot-rolling.
• the method for manufacturing a steel sheet may further include: pickling the steel sheet after the heat treatment operation.
• the method for manufacturing a steel sheet may further include: plating the steel sheet after the third cooling and holding operation.
• a steel sheet and a method for manufacturing the steel sheet may be provided.
• a high-strength steel sheet having excellent formability and a high yield ratio and a method for manufacturing the same may be provided.
• a high-strength steel sheet that may be used for various purposes including automobile components and has excellent formability such as ductility and hole expandability, and a method for manufacturing the same may be provided.
• the inventors of the present disclosure has confirmed that an alloy composition and microstructure of steel may be optimized to manufacture a high-strength steel sheet having excellent formability and a high yield ratio, and have completed the present disclosure.
• % indicating the content of each element is based on weight.
• a steel sheet according to an embodiment of the present disclosure may include, in weight %, C: 0.04 to 0.25%, Si: 0 to 0.7%, Mn: 0.46 to 1.8%, Al: 0 to 0.7%, P: 0.05% or less, S: 0.03% or less, N: 0.03% or less, and a balance of Fe and other inevitable impurities, and may include at least one selected from Ti, Nb, and V at 0.22% or less.
• Carbon (C) is an essential element for forming precipitates with Ti, Nb, or V in a ferrite phase to provide strength to a steel sheet.
• content of carbon (C) is less than 0.04%, it may be difficult to secure a desired level of strength.
• carbon (C) may be comprised at the content of 0.05% or more.
• an upper limit of the content of carbon (C) may be limited to 0.24%.
• Silicon (Si) is an element that has an effect of improving strength through solid solution strengthening, and is an element that strengthens ferrite, uniformizes a structure, and improves workability. Additionally, silicon (Si) is an element necessary for deoxidation during steelmaking. When the content of silicon (Si) exceeds 0.7%, there is a concern that plating defects such as underplating may occur in a plating process and that the weldability of the steel sheet may be reduced. According to an embodiment of the present disclosure, silicon (Si) may be included in the content of 0.68% or less to further improve weldability if necessary. On the other hand, a lower limit of the content may be limited to 0.01% to strengthen ferrite and uniformize the structure.
• Manganese (Mn) is a useful element for increasing both strength and ductility. When the content of manganese (Mn) is less than 0.46%, it is difficult to secure the above-described effect. According to an embodiment of the present disclosure, manganese (Mn) may be included in the content of 0.47% or more to further improve strength and ductility if necessary. On the other hand, when the content thereof exceeds 1.8%, the formation of a low-temperature transformation phase from austenite to martensite or bainite may be promoted, which may lower a yield ratio of the steel sheet. According to an embodiment, an upper limit of the content thereof may be limited to 1.78% as needed.
• Aluminum (Al) is an element combined with oxygen in steel to perform a deoxidizing effect. Additionally, like Si, aluminum (Al) is an element that strengthens ferrite, uniformizes the structure, and improves workability. When the content of aluminum (Al) exceeds 0.7%, aluminum (Al) may cause plating defects such as underplating in a plating process and may reduce the weldability of the steel sheet. According to an embodiment of the present disclosure, an upper limit thereof may be limited to 0.68% as needed to more effectively secure plating and weldability. Meanwhile, a lower limit of the content thereof may be limited to 0.01% to strengthen ferrite and uniformize the structure.
• Phosphorus (P) is an element contained as an impurity in steel to deteriorate impact toughness. Accordingly, the content of phosphorus (P) may be controlled to be 0.05% or less. However, considering a portion thereof that is inevitably added during a manufacturing process, 0% is excluded.
• Sulfur (S) is an element contained as an impurity in steel to form MnS in a steel sheet and deteriorate ductility. Accordingly, the content of the sulfur (S) may be preferably controlled to be 0.03% or less. However, considering a portion thereof that is inevitably added during the manufacturing process, 0% is excluded.
• Nitrogen (N) is an element contained as an impurity in steel to generate nitrides during continuous casting, causing cracks in a slab. Accordingly, the content of the nitrogen (N) may be preferably controlled to be 0.03% or less. However, considering a portion thereof that is inevitably added during the manufacturing process, 0% is excluded.
• Titanium (Ti), niobium (Nb) and vanadium (V) are important elements forming precipitates of a steel sheet. Titanium (Ti), niobium (Nb) and vanadium (V) may be contained to improve the strength and impact toughness of the steel sheet. In an embodiment of the present disclosure, a sum of these contents may be 0.01% or more. When the content of at least one of titanium (Ti), niobium (Nb) and vanadium (V) exceeds 0.22%, unrecrystallized ferrite may be excessively formed due to the formation of excessive precipitates, which may cause excessive characteristic effects as well as an increase in manufacturing costs. According to an embodiment of the present disclosure, the content thereof may be limited to 0.20% or less. According to an embodiment of the present disclosure, titanium (Ti) may be 0.01 to 0.15%, niobium (Nb) may be 0.01 to 0.12%, and vanadium (V) may be 0.01 to 0.12%.
• the steel of the present disclosure may comprise a balance of iron (Fe) and unavoidable impurities in addition to the composition described above. Since unavoidable impurities may be unintentionally mixed in during a normal manufacturing process, they may not be excluded. Since these impurities are known to anyone skilled in the art of normal steel manufacturing, all of their contents are not specifically mentioned in this specification.
• the steel sheet may further include at least one selected from, by wt.%, Cr: 0.8% or less, Mo: 0.8% or less, Cu: 0.8% or less, Ni: 0.8% or less, B: 0.005% or less, Ca: 0.05% or less, Mg: 0.05% or less, REM excluding Y: 0.05% or less, W: 0.5% or less, Zr: 0.5% or less, Sb: 0.5% or less, Sn: 0.5% or less, Y: 0.2% or less, Hf: 0.2% or less.
• Molybdenum (Mo) 0.8% or less
• Chromium (Cr) and molybdenum (Mo) are elements that suppress austenite decomposition during alloying treatment and, stabilize austenite, similarly to Mn.
• content of chromium (Cr) or molybdenum (Mo) exceeds 0.8%, a low-temperature transformation phase of martensite or bainite may be promoted, which may lower a yield ratio of the steel sheet.
• Copper (Cu) and nickel (Ni) are elements that stabilize austenite and suppress corrosion. Additionally, the copper (Cu) and nickel (Ni) are concentrated on a surface of the steel sheet to prevent hydrogen from penetrating into the steel sheet, thereby suppressing hydrogen-delayed destruction. When the content of copper (Cu) or nickel (Ni) exceeds 0.8%, copper (Cu) or nickel (Ni) may cause excessive characteristic effects as well as increased manufacturing costs.
• Boron (B) is an element that improves hardenability, increases strength, and suppresses nucleation at grain boundaries. When the content of boron (B) exceeds 0.005%, not only excessive characteristic effects but also manufacturing costs may increase.
• Rare earth elements refer to a total of 17 elements, including scandium (Sc), yttrium (Y), and lanthanides.
• Rare earth elements (REM) excluding calcium (Ca), magnesium (Mg) and yttrium (Y) are elements that improve the ductility of steel sheets by spheroidizing sulfides.
• Tungsten (W) and zirconium (Zr) are elements that improve hardenability and increase the strength of the steel sheet.
• tungsten (W) or zirconium (Zr) exceeds 0.5%, tungsten (W) or zirconium (Zr) may cause excessive characteristic effects as well as increased manufacturing costs.
• Antimony (Sb) and tin (Sn) are elements that improve the plating wettability and plating adhesion of the steel sheet.
• the content of antimony (Sb) or tin (Sn) exceeds 0.5%, the brittleness of the steel sheet increases, which may cause cracks to occur during hot working or cold working.
• Yttrium (Y) and hafnium (Hf) are elements that improve the corrosion resistance of the steel sheet.
• Y yttrium
• Hf hafnium
• % indicating the fraction of the microstructure is based on an area.
• a microstructure of the steel sheet according to an embodiment of the present disclosure may include, in area %, 80 to 99% of recrystallized ferrite and 1 to 20% of cementite.
• the microstructure may be observed through a scanning electron microscope (SEM) after etching the steel with nital. After nital etching, a structure without irregularities on a surface of the specimen may be determined as ferrite, and a structure having a spherical or lamellar structure may be determined as cementite.
• SEM scanning electron microscope
• unrecrystallized ferrite containing a large amount of dislocations has a difference in crystal orientation within grains. Accordingly, after measuring the crystal orientation of ferrite using FESEM-EBSD, unrecrystallized ferrite among ferrites may be distinguished using a Kernel Average Misorientation (KAM) method. That is, the ferrite proposed in the present disclosure may denote recrystallized ferrite, and in an embodiment of the present disclosure, recrystallized ferrite may be included in an amount of 80 to 99%.
• KAM Kernel Average Misorientation
• ferrite may be included to secure appropriate strength and ductility.
• an area fraction of ferrite exceeds 99%, there may be a problem of not securing a desired strength of the steel sheet.
• Cementite may be included in an amount of 1% or more to secure the strength of the steel sheet.
• an area fraction thereof exceeds 20%, there may be a problem in securing the ductility and hole expandability of the steel sheet.
• inevitable structures may be comprised as the balance, and for example, low-temperature transformation structures such as bainite and martensite may be included.
• a steel sheet according to an embodiment of the present disclosure may be manufactured by reheating, hot rolling, coiling, heat treating, cold rolling, continuous annealing, and cooling a steel slab satisfying the alloy composition described above.
• a steel slab satisfying the alloy composition of the present disclosure may be reheated in a temperature range of 1000 to 1350°C.
• a reheating temperature is less than 1000°C, there is a concern that hot rolling may be performed in a temperature range below a finishing rolling temperature proposed by the present disclosure.
• the reheating temperature exceeds 1350°C, the steel may melt by reaching a melting point of the steel.
• the reheated steel slab may be hot rolled at a finishing rolling temperature of 800 to 1000°C.
• finishing rolling temperature When the finishing rolling temperature is less than 800°C, the high strength of the steel slab may place a great burden on the hot rolling mill. On the other hand, when the finishing rolling temperature exceeds 1000°C, there is a concern that the grains of the steel sheet after hot rolling may be coarse, which may deteriorate the properties of the high-strength steel sheet.
• the hot-rolled steel sheet may be coiled in a temperature range of 25 ⁇ 300°C.
• the cooling rate to a coiling temperature after the hot rolling is not particularly limited, but cooling may be performed at an average cooling rate of 10°C/s or more in order to further refine the grains of the steel sheet.
• a coiling temperature of the hot-rolled steel sheet may be limited to 25 to 300°C.
• the coiling temperature exceeds 300°C, it is difficult to manufacture a high-strength steel sheet having excellent balance of tensile strength, elongation, hole expandability, and yield ratio that does not include unrecrystallized ferrite by annealing and heating the cold-rolled steel sheet in a continuous annealing line or a continuous hot-dip galvanizing line.
• the temperature is less than 25°C, there is a concern that the workability may be poor and the cold-rollability may deteriorate.
• the coiled steel sheet may be heated in a temperature range of 650 to 800°C and may be held for 600 to 1,700 seconds.
• the heat treatment temperature is less than 650°C or less than 600 seconds, it may not be easy to optimize the precipitates of the heat-treated steel sheet.
• a heat treatment conditions exceed 800°C or exceed 1700 seconds, it may not be easy to form precipitates of the heat-treated steel sheet.
• the heat-treated steel sheet may be cold-rolled at a reduction ratio of 30% or more.
• a cumulative reduction ratio may be preferably 30 to 90%.
• the cumulative reduction ratio exceeds 90%, it may be difficult to perform the cold rolling in a short period of time due to the high strength of the steel sheet.
• an operation of pickling the steel sheet before the cold rolling may be further comprised.
• a pickling condition is not particularly limited, and a typical condition may be applied.
• the cold-rolled steel sheet may be primarily heated in a temperature range of 720 to 880°C and may be held for 50 seconds or more, and primarily cooled to a temperature within a range of 600 to 760°C at an average cooling rate of 1°C/s or more.
• the heating temperature is less than 720°C, there is a concern in which non-recrystallized ferrite may be generated.
• the heating temperature exceeds 880°C, a yield ratio of the steel sheet may be reduced.
• an upper limit of the holding time may be limited to 200 seconds in consideration of the durability and limitations of the production equipment and the production speed.
• a cementite fraction may exceed 20%, which may reduce the desired properties.
• the cooling end temperature exceeds 760°C, the strength and ductility may not be secured at a desired level.
• the primarily cooled steel sheet may be secondarily cooled to a temperature within a range of 520 to 620°C at an average cooling rate of 2°C/s or more and may be held for 20 seconds or more.
• the cooling end temperature is less than 520°C during the secondary cooling, there is a problem that the yield ratio, tensile strength and hole expandability of the steel sheet may not be secured at a desired level due to the low heat treatment temperature.
• the cooling end temperature exceeds 620°C, the cementite fraction may exceed 20%, which may reduce the strength and ductility of the steel sheet.
• the heat treatment time after the secondary cooling is less than 20 seconds, the heat treatment time may be insufficient, making it impossible to secure the desired level of physical properties.
• the upper limit of the holding time may be limited to 300 seconds in consideration of the durability and restrictions of the production facility and the production speed.
• the secondarily cooled and held steel sheet may be thirdly cooled to a temperature within a range of 420 to 520°C at an average cooling rate of 2°C/s or more and may then be held for 20 seconds or more.
• the cooling end temperature When the cooling end temperature is lower than 420°C during the third cooling, the desired steel sheet properties may not be secured due to the low heat treatment temperature. On the other hand, when the cooling end temperature exceeds 520°C, the yield ratio, tensile strength, and hole expandability of the steel sheet may be reduced.
• an upper limit of the holding time may be limited to 200 seconds in consideration of the durability and restrictions of the production facility and the production speed.
• the thirdly cooled and held steel sheet may be cooled to room temperature.
• air cooling may be performed as an example.
• the thirdly cooled and held steel sheet may be plated and then cooled.
• the steel sheet manufactured in the present disclosure may be plated to manufacture a plated steel sheet.
• hot-dip galvanizing, electrogalvanizing, and hot-dip galvanizing may be performed, and a plating condition is not particularly limited, but plating may be performed under normal conditions that may be applied in the same technical field.
• the steel sheet of the present disclosure manufactured in this manner has a tensile strength (TS) of 610 MPa or more, a yield ratio (YR) of 0.8 to 0.95, a product of a square of a tensile strength and a square root of the elongation (TS 2 ⁇ EL) of 1.8 to 2.3 ⁇ 10 6 MPa 2 % 0.5 , and a product of a square of the tensile strength and a square root of a hole expansion ratio (TS 2 ⁇ HER) of 2.5 to 3.8 ⁇ 10 6 MPa 2 % 0.5 , thereby securing characteristics of an excellent balance of strength, elongation, hole expansion, and a yield ratio.
• TS tensile strength
• YiR yield ratio
• TS 2 ⁇ EL yield ratio
• TS 2 ⁇ HER hole expansion ratio
• the hot-rolled steel sheet was cooled at an average cooling rate of 30°C/s, and was coiled at a coiling temperature of Table 2 to manufacture a steel sheet having a thickness of 3 mm.
• the steel sheet was heated and held according to the heat treatment conditions of Table 2.
• the heat-treated steel sheet was pickled to remove a surface scale, and then cold-rolled at a thickness of 1.5 mm. Additionally, the steel sheet was heated and held at the heating temperature described in Table 2, and cooled under the conditions of primary cooling, secondary cooling, and third cooling.
• Table 3 illustrates a microstructure and physical properties of the manufactured steel sheet observed and measured.
• a microstructure of the steel sheet was observed through a scanning electron microscope (SEM) after performing nital etching on a polished specimen cross-section with. After the nital etching, a structure without irregularities on a specimen surface was determined to be recrystallized ferrite, and a structure having a spherical or lamellar structure was determined to be cementite. Unrecrystallized ferrite including a large amount of dislocations has crystal orientation differences within the grains. Accordingly, after measuring the crystal orientation of ferrite using FESEM-EBSD, the unrecrystallized ferrite among the ferrites was distinguished using the Kernel Average Misorientation (KAM) method.
• KAM Kernel Average Misorientation
• the physical properties of the steel sheet were evaluated by a tensile test and a hole expansion test.
• a tensile test was conducted for evaluation with a test piece collected in accordance with the JIS No. 5 standard based on the 0° direction with respect to a rolling direction of a rolled plate, thus calculating a yield ratio (YR) and a product (TS 2 ⁇ EL) of a square of the tensile strength and a square root of the elongation.
• the yield ratio (YR) refers to a value obtained by dividing the yield strength (YS) by the tensile strength (TS).
• the hole expansion test was conducted by pressing and expanding a cone punch at a top angle of 60° to a punching hole having a diameter of 10mm (die internal diameter of 10.3mm, clearance of 12.5%) in a direction in which a burr of the punching hole is an outer side. Accordingly, the hole expansion ratio (HER) was calculated using the following formula. Accordingly, the product of the square of the tensile strength and the square root of the hole expansion ratio (TS 2 ⁇ HER) was calculated.
• Comparative Examples 3 and 4 are examples in which the heat treatment temperature was outside the range of the present disclosure.
• Comparative Example 3 since the heat treatment temperature was excessively high, precipitation was not easily performed. As a result, it was difficult to secure the desired properties.
• Comparative Example 4 since the heat treatment temperature was below the range of the present disclosure, precipitation optimization was not easy, and the desired strength and ductility were not be secured.
• Comparative Examples 5 and 6 are cases in which the heat treatment time was outside the range of the present disclosure. Comparative Examples 5 and 6 are cases in which the heat treatment time is excessively long or short, and since precipitation was not easily performed in Comparative Examples 5 and 6, the properties targeted by the present disclosure were not secured.
• Comparative Examples 7 and 8 are cases in which the first heating and holding temperature are outside the temperature range proposed by the present disclosure.
• Comparative Example 7 since the heating temperature was excessively high, an area fraction of recrystallized ferrite was below a value proposed by the present disclosure, and as a result, the yield ratio was inferior.
• Comparative Example 8 since the heating temperature was low, unrecrystallized ferrite was present, and the desired strength and ductility were not secured.
• Comparative Example 9 is a case in which the holding time after the first heating was insufficient, and unrecrystallized ferrite was formed. As a result, the desired strength and elongation were not secured.
• Comparative Examples 10 and 11 are cases in which the cooling end temperature proposed by the present disclosure was not satisfied during the primary cooling. In Comparative Example 10, since the cooling end temperature was exceeded during the primary cooling, the properties were inferior. In Comparative Example 11, since the cooling end temperature was significantly low during the primary cooling, so that cementite was excessively formed, and as a result, the desired strength and elongation were not secured.
• Comparative Examples 13 and 14 the cooling end temperature during the secondary cooling was outside the range of the present disclosure. In Comparative Example 13, since the cooling end temperature was exceeded during the secondary cooling, the cementite fraction was excessive, and the desired physical properties were not secured. In Comparative Example 14, the yield ratio exceeded the desired range because the secondary cooling end temperature was low, and the strength and elongation were also inferior.
• Comparative Examples 16 and 17 the cooling end temperature during the third cooling was outside the range of the present disclosure.
• the cooling end temperature was excessively high during the third cooling, and unrecrystallized ferrite was formed. As a result, the recrystallized ferrite fraction was insufficient, the yield ratio exceeded the proposed range, and the desired strength and ductility were not secured.
• Comparative Example 17 is a case in which the cooling end temperature was insufficient during the third cooling, which made it difficult to secure strength and ductility.
• Comparative Examples 19 and 20 the carbon content was outside the range of the present disclosure. Comparative Example 19 is a case in which the content of carbon was insufficient, where the yield ratio was insufficient, and the strength and elongation were reduced. Comparative Example 20 is a case in which the carbon content was excessive, where non-recrystallized ferrite was formed, and as a result, the recrystallized ferrite fraction was insufficient, and the desired physical properties were not secured.
• Comparative Example 21 did not secure the desired strength and elongation because the silicon content exceeded the range proposed by the present disclosure.
• Comparative Examples 22 and 23 are examples in which the content of manganese was outside the range of the present disclosure. Comparative Example 22 did not secure the desired strength and ductility because the content of manganese was insufficient, and in Comparative Example 23, the content of manganese was excessive, and unrecrystallized ferrite was formed, which exceeded a proposed property level.
• Comparative Example 24 did not satisfy the proposed property level of the present disclosure because the aluminum content was excessive.
• Comparative examples 25 to 28 are a case in which a total content of titanium, niobium, and vanadium exceeded the range proposed by the present invention, where unrecrystallized ferrite was formed and recrystallized ferrite was insufficient. As a result, the yield ratio exceeded a desired level, and the strength and elongation were also excessive.

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