Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B24—GRINDING; POLISHING
  • B24B—MACHINES, DEVICES, OR PROCESSES FOR GRINDING OR POLISHING; DRESSING OR CONDITIONING OF ABRADING SURFACES; FEEDING OF GRINDING, POLISHING, OR LAPPING AGENTS
  • B24B27/00—Other grinding machines or devices
  • B24B27/033—Other grinding machines or devices for grinding a surface for cleaning purposes, e.g. for descaling or for grinding off flaws in the surface
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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
  • C21D1/25—Hardening, combined with annealing between 300 degrees Celsius and 600 degrees Celsius, i.e. heat refining ("Vergüten")
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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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  • 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
  • C21D8/0273—Final recrystallisation annealing
• • 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
  • C21D8/0278—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 a particular surface treatment 
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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
  • 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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  • 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
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  • 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
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • 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
  • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/28—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
  • C22C38/32—Ferrous alloys, e.g. steel alloys containing chromium with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/38—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/50—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
  • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—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
  • 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
  • C21D2211/002—Bainite
• • C—CHEMISTRY; METALLURGY
  • C21—METALLURGY OF IRON
  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/003—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
  • 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
• • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/009—Pearlite

Definitions

• the present invention relates to a steel sheet and a method for manufacturing the same.
• a high strength steel sheet is used as a steel sheet for a vehicle.
• the high strength steel sheet to be used for a component for a vehicle is required to have not only strength but also properties necessary for forming components, such as bendability.
• the steel sheet When the steel sheet is formed into a predetermined member or is subjected to a collision, the steel sheet undergoes bending deformation. In general, the bendability of the steel sheet decreases as the strength increases. Therefore, the present inventors recognized the importance of increasing bendability as formability and bendability as collision performance while maintaining high strength.
• Patent Document 1 As a method for improving bendability of a high strength steel sheet, there are a method of softening a surface layer area of a steel sheet (Patent Document 1) and a method of specifying a microstructure (Patent Document 2).
• a deformation capability is improved by forming the surface layer area of the steel sheet into a ferrite primary phase or a decarburized ferrite phase.
• a hard phase that may serve as a crack origin remains, and thus there is room for improvement in bendability in terms of both the properties described above.
• Patent Document 3 proposes a method for evaluating cracking susceptibility during a collision using a VDA bending test.
• VDA German Association of the Automotive Industry
• an object of the present invention is to provide a steel sheet having high strength, excellent bendability as formability, and excellent bendability as collision characteristics and a method for manufacturing the same.
• the gist of the present invention is as follows.
• the present invention it is possible to provide a steel sheet having high strength, excellent bendability as formability, and excellent bendability as collision characteristics, and a method for manufacturing the same.
• a steel sheet according to an embodiment of the present invention and a method for manufacturing the same will be described below.
• a range indicated by “to” includes, in principle, values at both ends thereof as a lower limit and an upper limit of the range. However, numerical values indicated as “more than” or “less than” are not included in the range.
• the steel sheet according to the present embodiment contains the following elements.
• % of an amount of each element means mass%.
• C (carbon) is an essential element for increasing strength of the steel sheet.
• the C content is set to 0.070% or more. From the viewpoint of securing ferrite and improving elongation, the C content is preferably 0.08% or more.
• the C content is set to 0.15% or less. From the viewpoint of weldability, the C content is preferably 0.14% or less.
• Si is a solid solution strengthening element and is an element effective in increasing the strength of the steel sheet.
• a Si content is set to 0.10% or more. From the viewpoint of securing a desired ferrite fraction over a wide range of annealing temperatures, the Si content is preferably 0.30% or more.
• the Si content is set to 2.00% or less.
• the Si content is preferably 1.8% or less.
• Mn manganese
• Mn manganese
• a Mn content is set to 1.00% or more. From the viewpoint of securing the strength, the Mn content is preferably 1.50% or more.
• the Mn content is set to 4.00% or less. From the viewpoint that Mn promotes co-segregation with P or S and causes a significant deterioration in workability, the Mn content is preferably 3.20% or less.
• Al is an element having a deoxidizing action on steel.
• a sol. Al content is set to 0.001% or more.
• the sol. Al content is preferably 0.005% or more.
• the sol. Al content is set to 1.500% or less.
• the sol. Al content is preferably 1.000% or less.
• P (phosphorus) is a solid solution strengthening element and is an element effective in increasing the strength of the steel sheet.
• a P content is set to 0.0010% or more.
• the P content is preferably 0.0050% or more.
• the P content is set to 0.0300% or less.
• the P content is preferably 0.0200% or less.
• S sulfur is an element that causes hot embrittlement and is also an element that inhibits the weldability and corrosion resistance.
• S content is set to 0.0200% or less.
• the S content is preferably 0.0100% or less.
• the S content is preferably low and may be 0%. However, in order to set the S content to less than 0.0001%, a manufacturing cost significantly increases. Therefore, the S content may be set to 0.0001% or more. The S content may be set to 0.0010% or more.
• N nitrogen
• nitrogen is an element that forms coarse nitrides in steel and deteriorates the bendability and hole expansibility.
• the N content is set to 0.0100% or less.
• the N content is preferably 0.0050% or less.
• the N content is preferably low and may be 0%. However, reducing the N content excessively increases a denitrification cost. Therefore, the N content may be set to 0.0005% or more from the viewpoint of economic efficiency.
• O oxygen
• Oxgen is an element that forms coarse oxides in steel and deteriorates the bendability and hole expansibility.
• O content is set to 0.0100% or less.
• the O content is preferably 0.0070% or less.
• the O content is preferably small and may be 0%. However, the O content may be set to 0.0001% or more from the viewpoint of the manufacturing cost. The O content may be set to 0.0010% or more.
• the steel sheet according to the present embodiment may contain the above elements and a remainder including Fe and impurities.
• one or more elements (optional elements) selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM shown below may be further contained. Since optional elements do not have to be contained, lower limits thereof are 0%.
• Ti titanium is an element that fixes N in steel as TiN, thereby suppressing the formation of BN, which is a factor for reducing hardenability.
• Ti is an element that refines an austenite grain size during heating and improves toughness.
• a Ti content is preferably set to 0.005% or more.
• the Ti content is more preferably set to 0.010% or more.
• the Ti content is set to 0.200% or less.
• the Ti content is preferably set to 0.050% or less.
• B is an element that segregates to austenite grain boundaries during welding, thereby strengthening the grain boundaries, and contributing to an improvement in resistance to molten metal embrittlement cracking.
• a B content is preferably set to 0.0005% or more.
• the B content is more preferably set to 0.0008% or more.
• the B content is set to 0.0100% or less.
• the B content is preferably 0.0050% or less.
• Cr chromium
• Mo molybdenum
• Ni nickel
• Cu copper
• Sn tin
• Cr, Mo, Ni, and Cu contents are each set to 1.000% or less, and a Sn content is set to 0.500% or less.
• the Cr, Mo, Ni, and Cu contents are each preferably set to 0.600% or less, and the Sn content is preferably set to 0.300% or less.
• Nb (niobium), V (vanadium), and W (tungsten) are carbide forming elements and are elements effective in increasing the strength of the steel sheet.
• a Nb content is set to 0.200% or less, and V and W contents are each set to 0.500% or less.
• the Nb content is preferably set to 0.100% or less, and the V and W contents are each preferably set to 0.300% or less.
• Ca calcium
• Mg magnesium
• Sb antimony
• Zr zirconium
• REM rare earth elements
• Bi bismuth
• Ca, Mg, Bi, Sb and Zr contents are each set to 0.0100% or less.
• a REM content is set to 0.1000% or less.
• the Ca, Mg, Bi, Sb, and Zr contents are each set to preferably 0.0080% or less, and more preferably 0.0060% or less.
• the REM content is preferably 0.0800% or less, more preferably 0.0600% or less, and even more preferably 0.0200% or less.
• REM refers to a total of 17 elements including Sc, Y, and lanthanoids, and the REM content means the total amount of these elements.
• Lanthanoids are industrially added in the form of mischmetal.
• a chemical composition of the steel sheet according to the present embodiment can be obtained by the following method.
• the chemical composition of the steel sheet described above may be measured by a general chemical composition measurement.
• the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
• ICP-AES inductively coupled plasma-atomic emission spectrometry
• sol. Al may be measured by ICP-AES using a filtrate obtained by heating and decomposing a sample with an acid.
• C and S may be measured using a combustion-infrared absorption method
• N may be measured using an inert gas fusion-thermal conductivity method
• O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
• the chemical composition may be analyzed after removing the plating layer by mechanical grinding.
• the steel sheet according to the present embodiment contains, as the chemical composition, C, Si, Mn, sol. Al, P, S, O, and N, and a remainder including Fe and impurities, or contains C, Si, Mn, sol. Al, P, S, O, and N and further contains one or more elements selected from Ti, B, Cr, Mo, Ni, Cu, Sn, Nb, V, W, Ca, Mg, Bi, Sb, Zr, and REM, and a remainder including Fe and impurities.
• a microstructure in a range of 1/8 to 3/8 of a sheet thickness from the surface of the steel sheet in a sheet thickness direction of the steel sheet, with respect to a 1/4 position of the sheet thickness of the steel sheet as a center includes, by area ratio, 0% to 60% of ferrite, 0% to 3% of residual austenite, and a remainder containing one or more selected from martensite, bainite, pearlite, and cementite.
• this "range of 1/8 to 3/8 of the sheet thickness from the surface of the steel sheet” is referred to as a "1/4 thickness position".
• the sheet thickness direction of the steel sheet in the present embodiment is a direction perpendicular to the surface of the steel sheet.
• the area ratio means a proportion of each structure to the entire microstructure in the above range.
• the remainder of the microstructure may include one or more selected from martensite, bainite, pearlite, and cementite.
• the microstructure at the 1/4 thickness position more preferably contains 0% to 30% of ferrite from the viewpoint of securing strength.
• the microstructure at the 1/4 thickness position more preferably contains 40% to 100% in total of tempered martensite and fresh martensite from the viewpoint of securing strength.
• the microstructure at the 1/4 thickness position more preferably contains 40% to 100% in total of tempered martensite and bainite from the viewpoint of securing bendability.
• the microstructure at the 1/4 thickness position preferably contains 0% to 5% in total of pearlite and cementite, and more preferably contains 0% to 3% in total of pearlite and cementite from the viewpoint of securing strength.
• the area ratios of ferrite, residual austenite, martensite (including tempered martensite and fresh martensite), and bainite included in the microstructure at the 1/4 thickness position can be measured by the following method.
• a sample is collected with a cross section parallel to a rolling direction and the sheet thickness direction of the steel sheet as an observation section, and the observation section is polished and etched with nital.
• the rolling direction in the present embodiment is parallel to a flat surface of the steel sheet and is a longitudinal direction of the steel sheet when the steel sheet is elongated by rolling. Since highly ductile non-ferrous inclusions such as MnS contained in the steel sheet are also elongated together with the steel sheet by rolling, the rolling direction also coincides with a direction in which the highly ductile non-ferrous inclusions such as MnS are elongated.
• a cutting direction in which an aspect ratio of the highly ductile non-ferrous inclusions such as MnS is the largest by observing a cross section cut on a plane perpendicular to the surface of the steel sheet may be set as the rolling direction.
• a region having a substructure within grains and having a plurality of long sides of carbides when observed with a scanning electron microscope is determined to be tempered martensite.
• the region is determined to have a substructure.
• a region where cementite is precipitated in a lamellar form is determined to be pearlite or cementite.
• a region with low brightness and no observable substructure is determined to be ferrite.
• a region with high brightness and no substructure revealed by etching is determined to be fresh martensite or residual austenite. The remainder is determined to be bainite.
• the area ratio of each phase is calculated by a point counting method to obtain the area ratio of each structure.
• the measurement is performed at 300 or more measurement points per visual field at intervals between the measurement points of 2 ⁇ m. Calculated values in each visual field are arithmetically averaged to obtain the area ratio of each structure.
• the area ratio of fresh martensite can be obtained by subtracting the area ratio of residual austenite obtained by an EBSD method, which will be described below, from the area ratio of fresh martensite or residual austenite. Then, a sum of this and the area ratio of tempered martensite calculated by the point counting method is taken as the area ratio of martensite. In a case where the area ratio of fresh martensite or residual austenite calculated by the point counting method is smaller than the area ratio of residual austenite obtained by the EBSD method described below, the area ratio of fresh martensite is set to zero.
• the area ratio of residual austenite at the 1/4 thickness position is evaluated by performing high-resolution crystal structure analysis by the EBSD method (electron backscatter diffraction method). Specifically, a sample is collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section, and the observation section is polished to a mirror finish. Furthermore, in order remove a processed layer of a surface layer, electrolytic polishing or mechanical polishing using colloidal silica is performed.
• EBSD method electron backscatter diffraction method
• crystal structure analysis is performed by the EBSD method on five visual fields, each visual field set to a size of 250 ⁇ m 2 or more, at a magnification of 5000-fold.
• a distance between evaluation points (step) is set to 0.01 to 0.20 ⁇ m.
• a ferrite fraction in a range of up to 2 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet is 95% or more.
• the surface of the steel sheet that has undergone a cold rolling step is ground and annealed, so that crystal grains are refined during annealing due to strain introduced by grinding. Accordingly, ferrite having an in-plane average grain size of 2.0 ⁇ m or less is generated in a surface layer area of the steel sheet at a ferrite fraction of 95% or more, and soft ferrite is formed in an outermost layer of the steel sheet. As a result, initiation of microcracks on the surface of the steel sheet, which is particularly observed in a 90° V-bending test, can be suppressed. In addition, propagation of such microcracks can also be suppressed.
• the ferrite fraction can be measured using the following method.
• the ferrite fraction means a proportion of a ferrite structure to the entire microstructure in a range of up to 2 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
• the ferrite fraction in a range of up to 2 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet can be measured using the following method.
• a sample is collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section, and the observation section is polished and etched with nital.
• a total of 5 visual fields, each visual field set to 250 ⁇ m 2 or more, are observed using the field-emission scanning electron microscope at a magnification of 5000-fold. Then, the area ratio of each ferrite is measured.
• a region with low brightness and no observable substructure is determined as a ferrite structure.
• the area ratio of the ferrite structure is calculated by the point counting method described above to obtain the area ratio of the ferrite structure.
• the range of up to 2 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet means a range within 2 ⁇ m along the sheet thickness direction of the steel sheet from the surface of the steel sheet toward an inside of the steel sheet.
• the in-plane average grain size of ferrite can be measured using the following method.
• a sample with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section and a sample with a cross section parallel to a sheet width direction and the sheet thickness direction of the steel sheet as an observation section are prepared.
• the observation section of each sample is polished and etched with nital.
• a surface layer of the sample in which the cross section parallel to the rolling direction and the sheet thickness direction is the observation section is photographed at a magnification of 5000-fold, and a connected photograph of a total of five visual fields, each visual field set to a connected photograph of 250 ⁇ m 2 or more obtained by moving a photographing range in the rolling direction, is acquired by the field-emission scanning electron microscope.
• a surface layer is photographed at a magnification of 5000-fold, and a connected photograph of a total of five visual fields, each visual field set to a connected photograph of 250 ⁇ m 2 or more obtained by moving a photographing range in the sheet width direction, is acquired by the field-emission scanning electron microscope.
• a connected photograph of a total of 10 visual fields obtained from the samples grain sizes of ferrite grains at depth positions of 0.5, 1.0, 1.5, and 2.0 ⁇ m from the surface of the steel sheet in the sheet thickness direction are measured.
• a straight line parallel to the surface of the steel sheet is assumed, and for ferrite grains intersected by the straight line, (length of the straight line)/(number of ferrite grains intersected by the straight line - 1) is defined as the grain size of the ferrite grain at the depth position.
• the length of the straight line that is, an observation distance is set to 50 ⁇ m or more.
• the in-plane average grain size of ferrite is obtained by arithmetically averaging all the grain sizes of the ferrite grains, that is, the grain sizes obtained from 40 straight lines in 10 visual fields, each visual field including the four depth positions.
• any direction in a sheet surface of the component may be set as a reference direction, each of a sample with a cross section parallel to a reference direction and a sheet thickness direction as an observation section and a sample with a cross section parallel to a direction orthogonal to the reference direction in the sheet surface and parallel to the sheet thickness direction as an observation section may be prepared, and the above-described measurement may be performed.
• the average grain size (in-plane average grain size) of ferrite in the measurement range at a portion intersecting the straight line parallel to the surface of the steel sheet can be evaluated.
• the ferrite grain sizes in a portion parallel to the surface of the steel sheet are fine, bending strain that occurs in a steel surface layer during bending deformation is dispersed into individual fine ferrite grains, and this dispersion suppresses localization of strain within the grains, which is likely to occur in coarse ferrite grains, whereby crack initiation is less likely to occur during bending deformation and crack propagation in the sheet thickness direction is less likely to occur. Therefore, it is important that the ferrite grain sizes on the straight line parallel to the surface of the steel sheet are in a predetermined range (2.0 ⁇ m or less) rather than cross-sectional areas of the ferrite grains being small.
• the steel sheet according to the present embodiment has such a configuration, and thus it is possible to suppress the initiation and propagation of microcracks on the surface of the steel sheet, particularly in a 90° V-bending test.
• the steel sheet according to the present embodiment has a tensile strength of 950 MPa or more.
• the tensile strength of 950 MPa or more allows preferable use as a steel sheet for a vehicle.
• the tensile strength is more preferably 980 MPa or more or 1,050 MPa or more, and even more preferably 1,100 MPa or more.
• the steel sheet according to the present embodiment more preferably has a tensile strength of less than 1,300 MPa.
• the tensile strength of less than 1,300 MPa has an advantage that elongation can be easily ensured.
• the tensile strength is set to 1,400 MPa or less.
• a transition region based on a C concentration in the sheet thickness direction of the steel sheet is more preferably 150 ⁇ m or less.
• the transition region in the present embodiment is a region where the C concentration is 20% to 90% with respect to a C concentration of a steady state portion, which will be described later.
• the transition region based on the C concentration is 150 ⁇ m or less, a change in C concentration from a low carbon region to a high carbon region from the surface layer to the inside becomes steeper compared to a case where the transition region based on the C concentration is larger than 150 ⁇ m, with the same amount of decarburization.
• the low carbon region having a sufficient thickness can be secured in the surface layer, and the initiation and propagation of microcracks in the surface layer can be suppressed.
• the transition region is 100 ⁇ m or less, the bendability can be further improved, which is more preferable.
• the C concentration is a carbon concentration in the steel sheet.
• the C concentration can be measured by a method using a GDS (glow discharge optical emission spectrometer) as described below.
• a surface of a sample is degreased and washed, and the C concentration is continuously measured from the surface of the sample. After the measurement, a reduction in thickness is measured with a micrometer, and assuming that the reduction in thickness has occurred at a constant rate, so that the C concentration at each depth can be obtained.
• a measurement time a measurement time is set such that the measurement depth of the C concentration in the steady state portion is 50 ⁇ m or more.
• the steady state portion measured by this measurement method is defined as C concentration 100%.
• the transition region in the present embodiment is a region in which the C concentration is 20% to 90% in the sheet thickness direction of the steel sheet.
• a fresh martensite fraction is more preferably 10% or less in a range of 5 to 20 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
• the microstructure in this range is important in terms of suppressing the propagation of microcracks.
• the fresh martensite fraction in this range is 10% or less, the effect of suppressing the propagation of microcracks is improved.
• the fresh martensite fraction in this range is more preferably 5% or less.
• the fresh martensite fraction means a proportion of a fresh martensite structure to the entire microstructure in a range of 5 to 20 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
• the fresh martensite fraction is obtained by calculating the proportion of fresh martensite in the microstructure in the above range, based on the area ratio obtained by the above-described measurement method of the area ratio of the microstructure.
• the range of 5 to 20 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet means a range of 5 ⁇ m or more and 20 ⁇ m or less from the surface of the steel sheet toward the inside of the steel sheet along the sheet thickness direction of the steel sheet from the surface of the steel sheet.
• a ferrite fraction is more preferably 50% or more in the range of 5 to 20 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
• the ferrite fraction in this range is 50% or more, a hard phase that serves as the origin of fracture is substantially eliminated, and ferrite having good ductility has an effect of suppressing the propagation of cracks.
• the ferrite fraction in this range is more preferably 70% or more.
• the ferrite fraction means a proportion of a ferrite structure to the entire microstructure in the range of 5 to 20 ⁇ m from the surface of the steel sheet in the sheet thickness direction of the steel sheet.
• each structure having the above-described properties is appropriately disposed in the sheet thickness direction, so that the initiation and propagation of microcracks are suppressed.
• the 90° V-bending test is a method capable of observing microcracks on the surface of the steel sheet, that is, initial microcracks, and can evaluate bending performance corresponding to the formability of the steel sheet. Suppression of the initial microcracks means that the properties in the 90° V-bending test are improved, and the formability of the steel sheet is improved.
• the VDA bending test is a method for determining a crack based on a change in load applied to a measurement target, and can evaluate bending performance corresponding to collision performance. Suppression of the propagation of microcracks means that the properties in the VDA bending test are improved, and the collision performance of the steel sheet as a steel sheet for a vehicle is improved.
• the steel sheet according to the present embodiment may have a plating layer such as a galvanized layer on the surface of the steel sheet as the base metal.
• the galvanized layer is, for example, a hot-dip galvanized layer.
• the galvanized layer means a plating layer containing 80 mass% or more of Zn. The presence of the hot-dip galvanized layer on the surface improves corrosion resistance.
• an adhesion amount of the galvanized layer is not particularly limited. However, from the viewpoint of continuous weldability, the adhesion amount is preferably set to 150 g/m 2 or less, and more preferably 100 g/m 2 or less. On the other hand, in terms of improving the corrosion resistance, the adhesion amount is preferably 20 g/m 2 or more.
• a chemical composition of the galvanized layer is not limited, and preferably contains, for example, Al: 0.1% to 2.0%, Fe: 5.0% or less, and a remainder including Zn and impurities.
• the adhesion amount and the chemical composition of the galvanized layer are obtained by the following methods.
• the plating layer is melted using hydrochloric acid containing an inhibitor, and weights before and after melting are compared to each other to obtain the adhesion amount.
• a solution obtained by the melting is quantitatively analyzed by ICP to measure the chemical composition of the plating layer.
• the position in the sheet thickness direction in the present embodiment is set as a depth from the surface of the base steel sheet (interface between an Fe phase and the plating layer) as a reference.
• the method for manufacturing the steel sheet of the present embodiment includes a cold rolling step of performing cold rolling on a steel sheet having a predetermined chemical composition, a grinding step of grinding a surface of the steel sheet subjected to the cold rolling, and an annealing step of annealing the steel sheet of which the surface is ground in the grinding step.
• a steel sheet having the following chemical composition is cold-rolled.
• the chemical composition includes, by mass%:
• Cold rolling conditions are not particularly limited, and the cold-rolled steel sheet can be manufactured by performing cold rolling on the hot-rolled steel sheet under normal conditions.
• Manufacturing conditions of the steel sheet to be subjected to the cold rolling step are not limited.
• molten steel having the above-described chemical composition is cast under normal conditions to obtain a steel piece, and then hot rolling is performed on the steel piece under normal conditions to manufacture a hot-rolled steel sheet.
• Cold rolling can be performed on the hot-rolled steel sheet.
• the surface of the steel sheet subjected to the cold rolling step is ground in the sheet thickness direction by an average of 0.1 ⁇ m or more.
• strong strain is introduced into the inside of the steel sheet. Therefore, recrystallization in the steel sheet structure surface layer is promoted during heating in the subsequent annealing step, and fine ferrite crystal grains can be obtained in the outermost layer of the steel sheet.
• the surface of the steel sheet is ground by 0.15 ⁇ m or more.
• the amount ( ⁇ m) of the steel sheet to be ground is calculated based on a change in the weight of the steel sheet before and after grinding.
• a weight loss of the steel sheet before and after grinding is divided by the area of the ground steel sheet to obtain a weight loss (g) per 1 m 2 .
• the amount of grinding ( ⁇ m) is calculated based on a value (value representing a relationship between the weight loss per 1 m 2 and the amount of grinding ( ⁇ m)) from an offline test conducted in advance under the same conditions.
• grinding is performed using a grinding brush at a predetermined rotation speed, reduction, and grinding speed.
• grinding may be performed using a D-100 grinding brush manufactured by Hotani Co., Ltd, at a rotation speed of 1,000 to 1,500 rpm, a reduction of 2.0 mm, and a grinding speed of about 100 mpm.
• the amount of grinding may be adjusted by performing the grinding a plurality of times, for example, two to ten times.
• Only one surface of the steel sheet may be ground, or both surfaces of the steel sheet may be ground. However, it is more preferable to grind both surfaces of the steel sheet in consideration of versatility of the steel sheet as a steel sheet for a vehicle.
• the annealing step includes a heating process of heating the steel sheet having the predetermined chemical composition (the same chemical composition as the steel sheet according to the present embodiment to be obtained) to a predetermined annealing temperature (highest heating temperature), a holding process of holding the heated steel sheet at the annealing temperature for a certain period of time, and a cooling process of cooling the steel sheet from the annealing temperature to a predetermined temperature.
• the steel sheet is heated to the annealing temperature.
• a heating rate is not particularly limited, and it is important to control the following atmosphere.
• a dew point is set to -15°C to 20°C.
• recrystallization is promoted by the strain introduced in the grinding step, and fine crystal grains can be obtained in the outermost layer of the steel sheet.
• a decarbonizing reaction is promoted, and the ferrite fraction of the outermost layer of the steel sheet can be set to 95% or more.
• the dew point in a furnace is lower than -15°C, a sufficient ferrite fraction cannot be obtained. Therefore, the dew point is set to -15°C or higher.
• the dew point is more preferably -10°C or higher from the viewpoint of obtaining a high ferrite fraction.
• the dew point is set to 20°C or lower.
• the steel sheet After being heated to the annealing temperature under the above conditions, the steel sheet is held at a predetermined highest heating temperature for five seconds or longer. When the holding time is shorter than five seconds, it is not possible to sufficiently secure austenite, which will later become the hard phase.
• An upper limit of the holding time is not particularly limited. However, when the holding time is too long, manufacturability of the steel sheet is impaired. Therefore, from the viewpoint of cost, the holding time is preferably shorter than 500 seconds. In addition, from the viewpoint of securing austenite sufficiently at a low cost, the holding time is more preferably about 10 to 120 seconds.
• the annealing temperature is set to 750°C or higher in order to sufficiently secure austenite, which will later become the hard phase.
• the annealing temperature is set to 1,000°C or lower.
• the annealing temperature is preferably 900°C or lower.
• the dew point may be set to -15°C to 20°C in the holding process.
• annealing may be performed without setting the dew point to -15°C to 20°C, and a step of heating the steel sheet at a dew point of -15 °C to 20 °C may be provided separately from the annealing step before or after the annealing step.
• a quenching step and a tempering step may be performed after the holding process in the annealing step. Accordingly, the fresh martensite fraction can be reduced, and the propagation of microcracks can be further suppressed.
• the tempering step in addition to the quenching step.
• the steel sheet that has been heated and held in the annealing step is cooled and quenched so that the temperature of the steel sheet reaches 300°C or lower.
• the quenching so that the temperature of the steel sheet reaches 300°C or lower, it is possible to reduce fresh martensite that serves as an origin of fracture. From the viewpoint of further reducing fresh martensite that serves as the origin of fracture, it is more preferable to perform quenching to 250°C or lower.
• An average cooling rate during quenching is set to 0.4 °C/s or faster in order to obtain a hard layer.
• the ferrite fraction of the surface layer 5 to 20 ⁇ m can be set to less than 90%. Excessive softening of the surface layer not only leads to a reduction in bending strength but also hinders securing the strength of the steel sheet.
• tempering is performed on the steel sheet cooled and held in the quenching step so that the temperature of the steel sheet reaches 150°C or higher.
• a holding time for the tempering is set to 2 seconds or longer. Although no particular upper limit is defined, it is preferable to set the holding time for the tempering to 500 seconds or shorter, since the effect is saturated.
• the quenching step is performed after the annealing step.
• the steel sheet may be once cooled to a predetermined temperature, and then heated again to perform the quenching step.
• a plating step may be performed between the annealing step and the quenching step, between the quenching step and the tempering step, or after the tempering step.
• the plating step may be performed as a part of the quenching step.
• the plating step may be performed after the tempering step.
• the above-described plating layer may be formed by electroplating.
• quenching was performed at the average cooling rate and the cooling stop temperature shown in Tables 3A and 3B, and tempering was performed at the heat treatment temperature and the heat treatment time shown in Tables 3A and 3B.
• hot-dip plating was performed under the conditions shown in Tables 3A and 3B (Kind of plating (GA: hot-dip galvannealing, GI: hot-dip galvanizing), steel sheet temperature before plating, and alloying temperature), to form a plating layer on the cold-rolled steel sheet after annealing.
• Tables 3A and 3B Kind of plating (GA: hot-dip galvannealing, GI: hot-dip galvanizing), steel sheet temperature before plating, and alloying temperature
• the ferrite fraction and the in-plane average grain size of ferrite in a range of up to 2 ⁇ m from the surface of the steel sheet were measured by the following method.
• a sample was collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section.
• the observation section was polished and then etched with nital, and a total of five visual fields, each visual field set to 250 ⁇ m 2 or more, were observed at a magnification of 5,000-fold using the field-emission scanning electron microscope. Then, the area ratio of ferrite was measured for each of the samples.
• a region with low brightness and no observable substructure was determined to be a ferrite structure.
• the area ratio of the ferrite structure was calculated by a point counting method in which measurement was performed at 300 or more measurement points per visual field. Calculated values in each visual field were arithmetically averaged to obtain the ferrite fraction (V ⁇ ).
• a sample with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet an observation section and a sample with a cross section parallel to the sheet width direction and the sheet thickness direction of the steel sheet as an observation section were prepared.
• the observation sections of the samples were polished and etched with nital.
• a total of five visual fields, each visual field set to 250 ⁇ m 2 or more, were observed at a magnification of 5,000-fold using the field-emission scanning electron microscope. ⁇
• the grain sizes in the rolling direction and in the sheet width direction of ferrite grains present at positions 0.5, 1.0, 1.5, and 2.0 ⁇ m away from the surface of the steel sheet in the sheet thickness direction were measured. All of the grain sizes of these ferrite grains were arithmetically averaged to obtain the in-plane average grain size of ferrite.
• the structure at the 1/4 thickness position was observed by the following method.
• a sample is collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section, and the observation section is polished and etched with nital.
• the area ratios of ferrite, residual austenite, martensite, bainite, pearlite, and cementite were each measured.
• the area ratio of ferrite was denoted by (V ⁇ )
• the area ratio of residual austenite was denoted by (V ⁇ )
• the area ratio of bainite was denoted by (VB)
• the area ratio of fresh martensite was denoted by (VfM)
• the area ratio of tempered martensite was denoted by (VtM).
• Pearlite and cementite are included as other structures, and total values thereof are shown.
• each phase was performed as follows. A region having a substructure within grains and having a plurality of long sides of carbides when observed with a scanning electron microscope was determined to be tempered martensite. In addition, a region where cementite is precipitated in a lamellar form was determined to be pearlite or cementite. A region with low brightness and no observable substructure was determined to be ferrite. A region with high brightness and no substructure revealed by etching was determined to be fresh martensite or residual austenite. The remainder was determined to be bainite.
• the area ratio of each phase was calculated by a point counting method to obtain the area ratio of each structure.
• measurement was performed at 300 or more measurement points per visual field. Calculated values in each visual field are arithmetically averaged to obtain the area ratio of each structure.
• the area ratio of fresh martensite was obtained by subtracting the area ratio of residual austenite obtained by the above-described EBSD method from the area ratio of fresh martensite or residual austenite. A sum of this and the area ratio of tempered martensite calculated by the point counting method was taken as the area ratio of martensite.
• the area ratio of residual austenite at the 1/4 thickness position was evaluated by performing high-resolution crystal structure analysis by the EBSD method.
• a sample was collected with a cross section parallel to the rolling direction and the sheet thickness direction of the steel sheet as an observation section, the observation section is polished to a mirror finish, and in order remove a processed layer of a surface layer, electrolytic polishing or mechanical polishing using colloidal silica was performed.
• V ⁇ ferrite fraction
• VfM fresh martensite fraction
• the C concentration of the steel sheet was measured using a glow discharge optical emission spectrometer (GD-Profiler 2 manufactured by HORIBA).
• the surface of the sample was degreased and washed, and the C concentration is continuously measured from the surface of the sample. After the measurement, a reduction in thickness was measured with a micrometer, and assuming that the reduction in thickness had occurred at a constant rate, the C concentration at each depth was obtained.
• a measurement time was set such that a steady state portion having a C concentration of 50 ⁇ m or more could be obtained.
• the steady state portion was defined as a region with a fluctuation range of ⁇ 5% after noise removal.
• a region having a C concentration of 20% to 90% in a range in the sheet thickness direction of the steel sheet was defined as the transition region.
• the tensile strength of the steel sheet was measured by the following method.
• a JIS No. 5 test piece was collected from a direction perpendicular to the rolling direction of the steel sheet, and the yield stress was measured in accordance with JIS Z 2241:2011.
• a JIS No. 5 tensile test piece was collected from a direction (width direction) perpendicular to the rolling direction and the thickness direction of the steel sheet, and a tensile test was conducted in accordance with JIS Z 2241:2011 to measure the tensile strength (TS).
• a JIS No. 5 test piece was collected from the direction perpendicular to the rolling direction of the steel sheet, and the elongation of the steel sheet was measured in accordance with JIS Z 2241:2011.
• the obtained steel sheets were subjected to the following two bending tests, and the performance in each bending test was evaluated.
• a bending test was conducted in accordance with VDA238-100, and bendability performance was evaluated by giving an evaluation point according to the bending angle of bending in the VDA standard as follows. The test piece was collected in a direction in which the bending ridge was parallel to the rolling direction.
• a 90° V-bending test was conducted in accordance with JIS Z 2248.
• a test piece had a strip shape of 30 mm ⁇ 150 mm.
• bending performance was evaluated by giving an evaluation point according to the limit r/t obtained in 90° V-bending, as follows.
• t at the limit r/t is the sheet thickness
• r is the minimum bending radius at which no crack occurs.
• test pieces in which the sum of the evaluation point from the VDA bending test and the evaluation point of the 90° V-bending test was 3 points or more were determined to have good (OK) bendability, and other test pieces were determined to have poor (NG) bendability.
• the steel sheet of the present disclosure has high strength, excellent bendability as formability, and excellent bendability as collision characteristics, and thus has high industrial applicability.

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