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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
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  • 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
  • C21D8/0247—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
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • 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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  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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  • 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
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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  • 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
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  • 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
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/002—Bainite
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  • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/021—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving particular fabrication steps or treatments of ingots or slabs

Definitions

• the present invention relates to a hot-rolled steel sheet.
• Patent Document 1 discloses a high strength steel sheet in which a microstructure is substantially a two-phase microstructure of ferrite and bainite, and a carbide containing Ti and Mo is dispersed and precipitated in a ferrite phase.
• Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2003-321725
• Patent Document 1 does not consider ductility and hole expandability.
• the vehicle suspension part as described above is manufactured by forming a hot-rolled steel sheet in a plurality of steps. Therefore, a hot-rolled steel sheet applied to a vehicle suspension part is required to have excellent bendability even after being subjected to a certain degree of prestrain in a pre-step.
• composite deformation including compression is generated as prestrain in forming in a plurality of steps, development of irregularities on a surface layer of the hot-rolled steel sheet increases, which increases the risk of fracture in subsequent bending forming.
• the present inventors have found that such a problem does not occur in a cold-rolled steel sheet having a small surface roughness, and is a problem unique to a hot-rolled steel sheet.
• An object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bendability after prestrain application.
• the gist of the present invention is as follows.
• FIG. 1 is a view for explaining a method of forming a hat component.
• the chemical composition of the hot-rolled steel sheet according to the present embodiment includes, in mass%, C: 0.045 to 0.120%, Si: 0 to 3.00%, Mn: 1.20 to 2.60%, Ti: 0.020 to 0.180%, Al: 0.010 to 0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, and a remainder: Fe and impurities.
• the C is an element necessary for obtaining a desired tensile strength of the hot-rolled steel sheet.
• the C content is set to 0.045% or more.
• the C content is preferably 0.050% or more, more preferably 0.060% or more, and still more preferably 0.080% or more.
• the C content is set to 0.120% or less.
• the C content is preferably 0.110% or less and more preferably 0.100% or less.
• the Si is an element that improves the tensile strength of the hot-rolled steel sheet by solid solution strengthening.
• the hot-rolled steel sheet according to the present embodiment ensures sufficient tensile strength even without containing Si. Therefore, the Si content may be 0%.
• the Si content is preferably 0.01% or more and more preferably 0.03% or more.
• the Si content is set to 3.00% or less.
• the Si content is preferably 2.50% or less and more preferably 1.50% or less.
• the strength, elongation and hole expandability of the hot-rolled steel sheet can be realized in a high balance by setting the Si content to 0 to 3.00%.
• Mn is an element necessary for improving the strength of the hot-rolled steel sheet.
• the Mn content is set to 1.20% or more.
• the Mn content is preferably 1.40% or more and more preferably 1.60% or more.
• the Mn content is set to 2.60% or less.
• the Mn content is preferably 2.30% or less and more preferably 2.20% or less.
• Ti is an element that increases the strength of the hot-rolled steel sheet by forming a fine nitride in steel.
• the Ti content is set to 0.020% or more.
• the Ti content is preferably 0.050% or more and more preferably 0.080% or more.
• the Ti content is set to 0.180% or less.
• the Ti content is preferably 0.160% or less and more preferably 0.150% or less.
• Al is an element that acts as a deoxidizer and improves the cleanliness of the steel.
• the Al content is set to 0.010% or more.
• the Al content is preferably 0.020% or more and more preferably 0.030% or more.
• the Al content is set to 0.400% or less.
• the Al content is preferably 0.300% or less, more preferably 0.200% or less, and still more preferably 0.100% or less.
• the P content is an element that segregates at grain boundaries in steel and promotes embrittlement of the grain boundaries.
• the P content is set to 0.080% or less.
• the P content is preferably 0.020% or less and more preferably 0.010% or less.
• the P content is preferably as low as possible, and is preferably 0%. However, when the P content is excessively reduced, P removal cost significantly increases, and thus the P content may be 0.001% or more.
• S is an element that embrittles slabs by being present as a sulfide.
• S is also an element that degrades the workability of the hot-rolled steel sheet.
• the S content is set to 0.0100% or less.
• the S content is preferably 0.0080% or less and more preferably 0.0050% or less.
• the S content is preferably as low as possible, and is preferably 0%. However, when the S content is excessively reduced, S removal cost significantly increases, and thus the S content may be 0.0005% or more.
• N is an element that forms a coarse nitride in steel and deteriorates the hole expandability of the hot-rolled steel sheet.
• the N content is set to 0.0050% or less.
• the N content is preferably 0.0040% or less and more preferably 0.0035% or less.
• the N content is preferably as low as possible, and is preferably 0%. However, when the N content is excessively reduced, N removal cost significantly increases, and thus the N content may be 0.0005% or more.
• O is an element that forms an oxide and lowers the workability of the hot-rolled steel sheet.
• the O content is set to 0.010% or less.
• the O content is preferably 0.008% or less and more preferably 0.006% or less.
• the O content is preferably as low as possible, and is preferably 0%. However, when the O content is excessively reduced, O removal cost significantly increases, and thus the O content may be 0.001% or more.
• the remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and impurities.
• the impurities mean substances that are mixed from ore as a raw material, a scrap, a manufacturing environment, or the like, or substances acceptable within a range not adversely affecting the hot-rolled steel sheet according to the present embodiment.
• the chemical composition of the hot-rolled steel sheet according to the present embodiment may contain the following optional elements instead of a part of Fe.
• the lower limit of the content when optional elements are not contained is 0%.
• Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb is also an element that increases the strength of the hot-rolled steel sheet by forming a fine carbide. In order to reliably obtain these effects, the Nb content is preferably set to 0.001% or more. The Nb content is more preferably 0.010% or more and still more preferably 0.030% or more.
• the Nb content is set to 0.100% or less.
• the Nb content is preferably 0.080% or less and more preferably 0.060% or less.
• V is an element that increases the strength of the hot-rolled steel sheet by forming a fine carbide in steel.
• the V content is preferably 0.001% or more.
• the V content is more preferably 0.050% or more and still more preferably 0.100% or more.
• the V content is set to 1.000% or less.
• the V content is preferably 0.500% or less and more preferably 0.300% or less.
• the Cu has an action of enhancing the hardenability of the hot-rolled steel sheet, and an action of increasing the strength of the hot-rolled steel sheet by being precipitated as a carbide in steel at a low temperature.
• the Cu content is preferably set to 0.001% or more.
• the Cu content is more preferably 0.050% or more and still more preferably 0.100% or more.
• the Cu content is set to 1.000% or less.
• the Cu content is preferably 0.500% or less and more preferably 0.300% or less.
• the Cr content is an element exhibiting an effect similar to that of Mn.
• the Cr content is preferably set to 0.001% or more.
• the Cr content is more preferably 0.050% or more and still more preferably 0.100% or more.
• the Cr content is set to 2.000% or less. From the viewpoint of reducing the alloy cost, the Cr content is preferably 1.000% or less and more preferably 0.500% or less.
• Mo is an element that increases the strength of the hot-rolled steel sheet by forming a fine carbide in steel.
• the Mo content is preferably set to 0.001% or more.
• the Mo content is more preferably 0.050% or more and still more preferably 0.100% or more.
• the Mo content is set to 3.000% or less.
• the Mo content is preferably 2.000% or less and more preferably 1.000% or less.
• Ni is an element that enhances hardenability of the hot-rolled steel sheet.
• Ni has an action of effectively suppressing intergranular cracking of the slab caused by Cu.
• the Ni content is preferably set to 0.001% or more.
• the Ni content is more preferably 0.050% or more and still more preferably 0.100% or more.
• the Ni content is set to 0.500% or less. From the viewpoint of reducing the alloy cost, the Ni content is preferably 0.300% or less and more preferably 0.200% or less.
• the B is an element that increases the strength of the hot-rolled steel sheet.
• the B content is preferably set to 0.0001% or more.
• the B content is more preferably 0.0005% or more and still more preferably 0.0010% or more.
• the B content is set to 0.0100% or less.
• the B content is preferably 0.0070% or less and more preferably 0.0050% or less.
• Ca is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferable shape.
• the Ca content is preferably set to 0.0001% or more.
• the Ca content is preferably 0.0010% or more and more preferably 0.0050% or more.
• the Ca content is set to 0.0500% or less.
• the Ca content is preferably 0.0300% or less and more preferably 0.0100% or less.
• Mg is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferable shape.
• the Mg content is preferably set to 0.0001% or more.
• the Mg content is preferably 0.0010% or more and more preferably 0.0020% or more.
• the Mg content is set to 0.0500% or less.
• the Mg content is preferably 0.0300% or less and more preferably 0.0100% or less.
• the REM is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by controlling the shape of inclusions to a preferable shape.
• the REM content is preferably set to 0.001% or more.
• the REM content is preferably 0.003% or more and more preferably 0.005% or more.
• the REM content is set to 0.100% or less.
• the REM content is preferably 0.050% or less and more preferably 0.030% or less.
• REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoid
• the content of REM refers to the total content of these elements.
• Lanthanoid is industrially added in a form of misch metal.
• Bi is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by refining the solidified structure.
• the Bi content is preferably set to 0.001% or more.
• the Bi content is preferably 0.002% or more and more preferably 0.003% or more.
• the Bi content is set to 0.100% or less.
• the Bi content is preferably 0.050% or less and more preferably 0.030% or less.
• Ta 0.001 to 0.100%
• Ta is an element that increases the strength of the hot-rolled steel sheet by forming a fine carbide in steel.
• the Ta content is preferably set to 0.001% or more.
• the Ta content is preferably 0.005% or more and still more preferably 0.010% or more.
• the Ta content is set to 0.100% or less.
• the Ta content is preferably 0.080% or less and more preferably 0.050% or less.
• the Zr is an element that increases the strength of the hot-rolled steel sheet by solid solution strengthening.
• the Zr content is preferably set to 0.001% or more.
• the Zr content is more preferably 0.005% or more and still more preferably 0.010% or more.
• the Zr content is set to set to 0.500% or less.
• the Zr content is preferably 0.300% or less and more preferably 0.100% or less.
• Co is an element that increases the strength of the hot-rolled steel sheet by solid solution strengthening.
• the Co content is preferably set to 0.001% or more.
• the Co content is more preferably 0.005% or more and still more preferably 0.010% or more.
• the Co content is set to 3.000% or less.
• the Co content is preferably 1.000% or less and more preferably 0.500% or less.
• Zn is an element that increases the strength of the hot-rolled steel sheet by solid solution strengthening.
• the Zn content is preferably set to 0.001% or more.
• the Zn content is preferably 0.005% or more and still more preferably 0.010% or more.
• the Zn content is set to 0.200% or less.
• the Zn content is preferably 0.150% or less and more preferably 0.100% or less.
• the W is an element that increases the strength of the hot-rolled steel sheet by solid solution strengthening.
• the W content is preferably set to 0.001% or more.
• the W content is more preferably 0.005% or more and still more preferably 0.010% or more.
• the W content is set to 0.200% or less.
• the W content is preferably 0.150% or less and more preferably 0.100% or less.
• Sb is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by suppressing generation of an oxide serving as a starting point of fracture.
• the Sb content is preferably set to 0.001% or more.
• the Sb content is more preferably 0.005% or more and still more preferably 0.10% or more.
• the Sb content is set to 0.500% or less.
• the Sb content is preferably 0.300% or less and more preferably 0.100% or less.
• the As content is preferably set to 0.001% or more.
• the As content is preferably 0.005% or more and still more preferably 0.010% or more.
• the As content is set to 0.050% or less.
• the As content is preferably 0.040% or less and more preferably 0.030% or less.
• Sn is an element that enhances the ductility and hole expandability of the hot-rolled steel sheet by suppressing generation of an oxide serving as a starting point of fracture.
• the Sn content is preferably set to 0.001% or more.
• the Sn content is preferably 0.005% or more and still more preferably 0.010% or more.
• the Sn content is set to 0.050% or less.
• the Sn content is preferably 0.040% or less and more preferably 0.030% or less.
• the chemical composition of the hot-rolled steel sheet described above may be analyzed using a spark discharge optical emission spectrometer or the like.
• C and S adopt values identified by burning in an oxygen stream using a gas component analyzer or the like and measuring by an infrared absorption method.
• N adopts a value identified by melting a test piece collected from a steel sheet in a helium gas flow and measuring the melted test piece by a thermal conductivity method.
• the chemical composition may be analyzed after the plating layer is removed by mechanical grinding or the like as necessary.
• GAM S /GAM I which is a ratio of GAM I , which is an area average value of GAM values of grains at a position of 1/4 depth from a surface in a sheet thickness direction, to GAM S , which is an area average value of GAM values of grains in a region from the surface to a depth of 200 ⁇ m in the sheet thickness direction, is 0.70 to 1.05, and in a microstructure at the position of 1/4 depth from the surface in the sheet thickness direction, the area ratio of a region having a GAM value of more than 0.6° is 50% or more, and the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is less than 15%, and a standard deviation of the area average value of the GAM values of grains in a region from the surface to a depth of 200 ⁇ m in the sheet thickness direction is 0.25 to 0.65°.
• a microstructure at 1/4 position from an end surface in a width direction is defined.
• the 1/4 position from the end surface in the width direction is a w/4 position from the end surface in the width direction when the length in the width direction is w.
• the "x/y position (here, x and y are natural numbers satisfying x ⁇ y.) from an end surface” means a position moved from the end surface in the width direction of the steel sheet toward the central part of the steel sheet by a distance of x/y of the sheet width in the width direction.
• the "1/4 position from the end surface” means a position at a distance of 0.25 m from the end surface in the width direction of the steel sheet.
• the "sheet thickness x/y position (here, x and y are natural numbers satisfying x ⁇ y.)" means a position moved from the surface (sheet surface) in the sheet thickness direction of the steel sheet toward the central part of the steel sheet by a distance (depth) of x/y of a sheet thickness t in the sheet thickness direction.
• the "sheet thickness 1/8 position” means a position at a depth of 0.25 mm from the surface in the sheet thickness direction of the steel sheet.
• the "surface of the steel sheet” means an interface between the steel sheet and the coating
• the "sheet thickness t” means the sheet thickness of the steel sheet (base metal) excluding the coating.
• GAM S /GAM I 0.70 to 1.05
• GAM Garin Average Misorientation
• EBSP electron backscatter pattern
• a grain having a small GAM value improves ductility of the hot-rolled steel sheet, but reduces strength.
• the strength depends on the average properties of the hot-rolled steel sheet in the sheet thickness direction, whereas bendability depends on the properties of the sheet surface. Therefore, the present inventors have found that the strength and bendability of the hot-rolled steel sheet after prestrain application can be improved by arranging grains having a small GAM value and excellent ductility in a surface layer region of the hot-rolled steel sheet.
• GAM S /GAM I which is a ratio of GAM I , which is an area average value of GAM values of grains at a position of 1/4 depth from a surface in a sheet thickness direction (hereinafter, it may be referred to as an internal region)
• GAMs which is an area average value of GAM values of grains in a region from the surface to a depth of 200 ⁇ m in the sheet thickness direction (hereinafter, it may be referred to as a surface layer region)
• GAM S /GAM I is set to 0.70 or more.
• GAM S /GAM I is preferably set to 0.80 or more.
• GAM S /GAM I when GAM S /GAM I is more than 1.05, the GAM value of grain in the surface layer region becomes too large, and desired bendability cannot be obtained after prestrain application. Therefore, GAM S /GAM I is set to 1.05 or less. GAM S /GAM I is preferably set to 0.95 or less.
• Standard deviation of area average value of GAM values of grains in surface layer region 0.25 to 0.65°
• the fracture in the hot-rolled steel sheet occurs mainly in grains having a large GAM value.
• the grains having a small GAM value are preferentially deformed at the time of prestrain, and deformation of the grains having a large GAM value is suppressed, so that it is possible to suppress the fracture of the grains having a large GAM value at the time of bending forming after prestrain application.
• the standard deviation of the area average value of the GAM values in the region from the surface to a depth of 200 ⁇ m in the sheet thickness direction (the surface layer region) is less than 0.25°, the deformation of the grains having a large GAM value cannot be suppressed, and desired bendability cannot be obtained after prestrain application. Therefore, the standard deviation of the area average value of the GAM values of grains in the surface layer region is set to 0.25° or more.
• the standard deviation of the area average value of the GAM values of grains in the surface layer region is preferably set to 0.35° or more.
• the standard deviation of the area average value of the GAM values of grains in the surface layer region is set to 0.65° or less.
• the standard deviation of the area average value of the GAM values of grains in the surface layer region is preferably set to 0.55° or less.
• the GAM values of grains in the internal region and the surface layer region are measured by the following method.
• a sample is collected so that a microstructure of a cross section with the width direction as a normal direction (the sheet thickness direction ⁇ a cross section in the rolling direction) can be observed at the 1/4 position from the end surface in the width direction of the hot-rolled steel sheet.
• the size of the sample may be, for example, a rectangular parallelepiped having a total thickness in the sheet thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction, depending on the measuring device.
• the observed section of the sample is mirror-polished, and then polished using colloidal silica containing no alkaline solution at room temperature for 8 minutes to remove strain introduced into the surface of the sample.
• a region of 200 ⁇ m in the sheet thickness direction around a 1/4 depth position from the surface in the sheet thickness direction and 400 ⁇ m or more at an arbitrary position in the rolling direction (a rectangular region having a center at a 1/4 depth position in the sheet thickness direction, the rectangular region having a length of 200 ⁇ m (short side) in the sheet thickness direction and a length of 400 ⁇ m or more (long side) in the rolling direction) of the polished sample is measured at a measurement interval of 0.2 ⁇ m to obtain crystal orientation information.
• an EBSD analyzer including a thermal field emission scanning electron microscope (JSM-7001F, manufactured by JEOL Ltd.) and an EBSD detector (HIKARI detector, manufactured by TSL Solutions Ltd.) is used.
• the degree of vacuum in the EBSD analyzer is 9.6 ⁇ 10 -5 Pa or less
• the acceleration voltage is 15 kV
• the irradiation current level is 13
• the irradiation level of the electron beam is 62.
• GAM I which is an area average value of GAM values of grains in the internal region, is obtained by performing calculation for all grains using the obtained GAM value, the area of grains measured in the EBSD analysis image, and the following formula (1).
• GAM i represents the GAM value of the i-th grain
• a i represents the area of the i-th grain
• n represents the number of grains included in the measurement range.
• GAM S which is an area average value of GAM values of grains in the surface layer region, is obtained by performing measurement in the same manner for a region of 200 ⁇ m in the sheet thickness direction around a depth position of 100 ⁇ m from the surface in the sheet thickness direction and 400 ⁇ m or more at an arbitrary position in the rolling direction of the polished sample.
• the standard deviation of the area average value of the GAM values of grains in the surface layer region is obtained.
• GAM i the GAM value of the i-th grain
• a i the area of the i-th grain
• n the number of grains included in the measurement range.
• the rolling direction of the hot-rolled steel sheet is determined by the following method.
• a test piece is collected so that a cross section parallel to the sheet surface of the hot-rolled steel sheet can be observed.
• a cross section at which the distance from the surface is 1/4 position of the sheet thickness is finished by mirror polishing, and then observed using an optical microscope.
• the observation range is set to 500 ⁇ m ⁇ 500 ⁇ m or more, and a direction parallel to the elongation direction of the grains is determined as the rolling direction.
• a direction orthogonal to the determined rolling direction is determined as the width direction of the hot-rolled steel sheet.
• the area ratio of a region having a GAM value of more than 0.6° is set to 50% or more.
• the area ratio of a region having a GAM value of more than 0.6° is preferably 80% or more, more preferably 90% or more, and still more preferably 95% or more.
• the area ratio of a region having a GAM value of more than 0.6° may be 100%.
• the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is 15% or more, a desired strength may not be obtained or a desired hole expandability may not be obtained in the hot-rolled steel sheet. Therefore, the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is less than 15%.
• the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite is preferably 10% or less and more preferably 5% or less.
• the sum of the area ratio of a region having a GAM value of more than 3.0° and the area ratio of residual austenite may be 0% or 1% or more.
• the hot-rolled steel sheet according to the present embodiment may have the above-described chemical composition and metallographic structure, and then may have either microstructure of a first or second aspect described below depending on the desired strength, ductility, and degree of bendability after prestrain application.
• the first aspect is a microstructure relatively suitable in a case where it is required to achieve both strength and ductility at higher levels.
• by setting the area ratio of a region having a GAM value of more than 0.6° and less than 2.0° to 50% or more it is possible to achieve both strength and ductility at higher levels in the hot-rolled steel sheet.
• the area ratio of a region having a GAM value of more than 0.6° and less than 2.0° is preferably 70% or more and more preferably 85% or more.
• the area ratio of a region having a GAM value of more than 0.6° and less than 2.0° may be 100%.
• the region having a GAM value of 2.0° or more and the region having a GAM value of 0.6° or less at a total area ratio of 0 to 50% may be included as the remainder in microstructure other than the region having a GAM value of more than 0.6° and less than 2.0°.
• the second aspect is a microstructure relatively suitable in a case where higher strength is required.
• the area ratio of a region having a GAM value of 2.0° or more is preferably 70% or more and more preferably 85% or more.
• the area ratio of a region having a GAM value of 2.0° or more may be 100%.
• the region having a GAM value of less than 2.0° at an area ratio of 0 to 50% may be included as the remainder in microstructure other than the region having a GAM value of 2.0° or more.
• the area ratio of a region having a GAM value of more than 0.6°, the area ratio of a region having a GAM value of more than 0.6° and less than 2.0°, the area ratio of a region having a GAM value of 2.0° or more, and the area ratio of a region having a GAM value of more than 3.0° are measured by the following methods.
• the GAM values of grains in the internal region are calculated in the same manner as in measuring the GAM values of grains in the internal region described above.
• the area ratio of grains having an obtained GAM value of more than 0.6°, the area ratio of grains having an obtained GAM value of more than 0.6° and less than 2.0°, the area ratio of grains having an obtained GAM value of 2.0° or more, and the area ratio of a region having an obtained GAM value of more than 3.0° are calculated to obtain the area ratio of each region.
• the area ratio of residual austenite is measured by the following method.
• a sample is collected so that a microstructure in a region of 1 mm or more at an arbitrary position in the rolling direction and 1 mm or more around a 1/4 position from the end surface in the width direction can be observed in the cross section at a 1/4 position from the surface in the sheet thickness direction of the hot-rolled steel sheet.
• the integrated intensity of a total of six peaks ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and y(220) of the sample is determined using a Co-K ⁇ ray.
• the volume percentage of residual austenite is calculated from the integrated intensity using an intensity average method. The obtained volume percentage of residual austenite is regarded as the area ratio of residual austenite.
• the tensile strength may be 940 MPa or more. By setting the tensile strength to 940 MPa or more, contribution to weight reduction of a vehicle body can be increased, and the hot-rolled steel sheet can be suitably applied to a vehicle component.
• the upper limit of the tensile strength is not particularly limited, but may be 1400 MPa or less from the viewpoint of suppressing die wear.
• the uniform elongation may be 3.0% or more. By setting the uniform elongation to 3.0% or more, the hot-rolled steel sheet can be suitably applied to a vehicle component.
• the upper limit of the uniform elongation is not particularly limited, but may be 10.0% or less.
• the tensile strength and the uniform elongation are measured by performing a tensile test in accordance with JIS Z 2241:2022 using a No. 5 test piece of JIS Z 2241:2022.
• the tensile test piece is collected at the center position in the width direction, and the direction perpendicular to the rolling direction and the sheet thickness direction (width direction) is taken as the longitudinal direction.
• a minute test piece having the width direction as the longitudinal direction can be substituted as a test piece for measuring the tensile strength.
• the hole expansion ratio may be 40% or more. By setting the hole expansion ratio to 40% or more, the hot-rolled steel sheet can be suitably applied to a vehicle component.
• the upper limit of the hole expansion ratio is not particularly limited, but may be 80% or less.
• the hole expansion ratio is measured by performing a hole expansion test in accordance with JIS Z 2256:2020.
• Bendability after prestrain application is evaluated by performing a bending test on the hot-rolled steel sheet after draw bending process.
• the draw bending process is performed, for example, by forming a hat component 10 with a forming height of 60 mm under the conditions shown in FIG. 1 .
• the test piece is collected from the hot-rolled steel sheet such that the longitudinal direction of the test piece is the width direction of the hot-rolled steel sheet and the size is 240 mm ⁇ 60 mm.
• Conditions for the draw bending process are as follows: the width of a punch 1 is 75 mm, the corner R of the punch 1 is "sheet thickness ⁇ 5 (mm)", the corner R of a die 2 is “sheet thickness ⁇ 3.125 mm”, the clearance between the punch 1 and the die 2 is “sheet thickness + 0.9 mm”, and the blank holder force (BHF) is "sheet thickness ⁇ 6.25 (ton)".
• the conditions shown in FIG. 1 are conditions when the sheet thickness is 1.6 mm.
• the steel sheet comes into contact with the punch 1 while being subjected to bending and unbending deformation, so that it is possible to reproduce a recessed part formed in a flat-R portion in the vicinity of the vertical wall portion of the vehicle suspension part.
• a test piece is collected from the vertical wall portion 11 of the hat component 10 such that the die 2 side is outside the bending and the stroke direction D of the punch 1 is in the bending axis direction.
• a bending test is performed under the following conditions based on the VDA standard (VDA238-100) defined by the Verband der Automobilindustrie.
• the hot-rolled steel sheet has excellent bendability after prestrain application.
• the hot-rolled steel sheet has excellent bendability after prestrain application.
• the surface on the punch side is ground to set the sheet thickness to 1.6 mm, and then the bending test is performed.
• the maximum bending angle obtained by the following formula is adopted.
• ⁇ 0 represents the maximum bending angle obtained by the bending test
• t represents the sheet thickness
• uEL represents uniform elongation.
• Maximum bending angle in the case of 1.6 mm or less ⁇ 0 - 13.852 ⁇ (1 - t/1.6) ⁇ (uEL + 0.22) 0.292
• each aspect may have the following strength, ductility, and bendability after prestrain application. Since desired hole expandability is equivalent in either aspect, description thereof is omitted.
• the tensile strength may be 940 MPa or more, and the uniform elongation may be 4.0% or more. In the first aspect, the tensile strength may be 980 MPa or more. Also, in the first aspect, the uniform elongation may be 5.0% or more.
• the maximum bending angle obtained by the bending test described above may be 70° or more.
• the tensile strength may be 1040 MPa or more, and the uniform elongation may be 3.0% or more. In the second aspect, the tensile strength may be 1140 MPa or more. Also, in the second aspect, the uniform elongation may be 4.0% or more.
• the maximum bending angle obtained by the bending test described above may be 50° or more.
• the hot-rolled steel sheet according to the present embodiment may be a surface-treated steel sheet by providing a plating layer on the surface for the purpose of improving corrosion resistance and the like.
• the plating layer may be an electroplating layer or a hot-dip plating layer.
• the electroplating layer include electro-galvanizing and electric Zn-Ni alloy plating.
• the hot-dip plating layer include hot-dip galvanizing, alloying hot-dip galvanizing, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating.
• the plating adhesion amount is not particularly limited, and may be the same as in the conventional plating layer.
• it is also possible to further enhance corrosion resistance by performing an appropriate chemical conversion treatment (for example, application and drying of a silicate-based chromium-free chemical treatment solution) after plating.
• the hot-rolled steel sheet according to the present embodiment can be stably manufactured.
• the temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet.
• Steps (1) to (3) described below are steps common in the first and second aspects.
• steps (4) and (5) correspond to the first aspect
• step (6) corresponds to the second aspect.
• the width direction of the slab is a direction orthogonal to the conveyance direction of the slab and the sheet thickness direction, and the conveyance direction of the slab corresponds to the rolling direction in a later step.
• the number of times of applying strain in the width direction is two or more.
• descaling is preferably performed every time strain is applied in the width direction of the slab, that is, descaling is preferably performed every time before multiple times of strain application.
• Descaling is preferably performed by injecting water.
• the strain may be applied after slab heating for rough rolling is performed.
• Examples of a method of applying strain in the width direction of the slab include a method of applying strain in the width direction (pressing down in the width direction) to the slab by passing the slab between rolls installed so that a rotary shaft is perpendicular to a sheet surface of the slab and the conveyance direction.
• the slab to which strain is applied is not particularly limited except for having the above-described chemical composition.
• a slab manufactured by melting molten steel having the above chemical composition using a converter, an electric furnace, or the like and a continuous casting method can be used.
• a continuous casting method an ingot-making method, a thin slab casting method, or the like may be adopted.
• the heating temperature may be set to a temperature range of 1100°C to 1300°C.
• the conditions for rough rolling are not particularly limited, and the rough rolling can be, for example, a step of performing rolling a plurality of times at a temperature of 1100°C or higher to set the sheet thickness to 30 to 60 mm.
• finish rolling step it is preferable to perform finish rolling such that a difference between a finish rolling start temperature (inlet side temperature) and a finishing temperature (outlet side temperature) is 60°C or higher and lower than 120°C, and the finishing temperature is 950°C or lower.
• the lower limit of the finishing temperature is not particularly limited, and may be appropriately determined according to the rolling load limitation of the equipment. In order to suppress a rapid increase in load, the finishing temperature can be, for example, 850°C or higher.
• the steel sheet After completion of the finish rolling, the steel sheet is accelerated cooled to a temperature range of 580°C to 680°C at an average cooling rate of 30°C/s or faster, and slowly cooled (air-cooled) in this temperature range for 2.0 seconds or longer.
• the area ratio of a region having a GAM value of more than 0.6° and less than 2.0° can be increased by slow cooling (air cooling) in a temperature range of 580°C to 680°C for 2.0 seconds or longer.
• the slow cooling (air cooling) in the present embodiment refers to cooling with an average cooling rate of 20°C/s or slower.
• accelerated cooling is performed to 300°C at an average cooling rate of 30°C/s or faster.
• slow cooling air cooling
• accelerated cooling is performed to 300°C at an average cooling rate of 30°C/s or faster, whereby a desired microstructure can be obtained.
• the steel sheet After being accelerated cooled to 300°C, the steel sheet may be air cooled to room temperature, or may be coiled into a coil shape and then water-cooled.
• accelerated cooling is performed to 300°C at an average cooling rate of 30°C/s or faster.
• the area ratio of a region having a GAM value of more than 0.6° can be increased.
• the steel sheet After being accelerated cooled to 300°C, the steel sheet may be air cooled to room temperature, or may be coiled into a coil shape and then water-cooled.
• the average cooling rate in the present embodiment is a value obtained by dividing a temperature difference between a start point and an end point of a set range by an elapsed time from the start point to the end point.
• strain was applied two or more times in the width direction.
• the difference between a temperature at the time of the first strain application in the width direction and a temperature at the time of the final strain application in the width direction is described in the column of "Temperature difference at time of strain application in width direction" in the table.
• the metallographic structure For the obtained hot-rolled steel sheet, the metallographic structure, the tensile strength (TS), the uniform elongation (uEl), the hole expansion ratio ( ⁇ ), and the bendability after prestrain application were evaluated by the above-described methods.
• TS tensile strength
• the bendability (maximum bending angle) after prestrain application was evaluated according to the following criteria depending on the tensile strength.
• the hot-rolled steel sheets according to the present invention examples have high strength and excellent ductility and hole expandability, and have excellent bendability after prestrain application.
• a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bendability after prestrain application is provided.

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