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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
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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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  • C21D8/0247—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
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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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  • 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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  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C21D2201/00—Treatment for obtaining particular effects
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite

Definitions

• the present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bending properties under tension in a rolling direction.
• the steel sheet may be bent under tension. Bending under tension is often performed along the rolling direction of the steel sheet. Therefore, the steel sheets that are applied to vehicle components are also required to have excellent bending properties under tension particularly in the rolling direction.
• Patent Document 1 discloses a hot-rolled steel sheet including a microstructure including 70% or more of ferrite in terms of area ratio and pearlite, having a sheet thickness T0 of 6 to 25 mm and an average grain size of ferrite grains GC inside the sheet thickness of 5 to 15 ⁇ m, the hot-rolled steel sheet including a fine grain layer formed from a surface in a sheet thickness direction, the average grain size of ferrite grains being less than 1.0 times the average grain size GC, the fine grain layer including a specific fine grain layer in which the average grain size of ferrite grains is 0.1 to 0.4 times the average grain size GC, a predetermined formula being satisfied when the thickness of the specific fine grain layer is TF0 and the thickness of an ultrafine grain layer in which the average grain size of ferrite grains is less than 0.1 times the average grain size GC is TF1 among the fine grain layer, and the average grain size of ferrite grains in the specific fine grain layer and the ultrafine grain layer being 0.1 to 0.4 times the average grain size GC.
• Patent Document 2 discloses a high strength hot rolled steel sheet having excellent hole expansibility and weld fatigue properties, in which a random intensity ratio of a ⁇ 110 ⁇ 111> to ⁇ 110 ⁇ 001> orientation group of a sheet thickness cross section in a region from an outermost layer to a sheet thickness of 1/6 is 3.5 or less.
• Patent Documents 1 and 2 do not consider bending properties under tension in the rolling direction.
• an object of the present invention is to provide a hot-rolled steel sheet having high strength, excellent ductility and hole expandability, and excellent bending properties under tension in the rolling direction.
• the present inventors have found that it is effective to control rough rolling conditions and finish rolling conditions of hot rolling in order to preferably control the texture in the surface layer region of the hot-rolled steel sheet.
• the gist of the present invention made based on the above findings is as follows.
• FIG. 1 A view for explaining a test method of a tension bending test.
• 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.
• a peak position of ⁇ where the maximum value A is located is ⁇ A and a peak position of ⁇ where the maximum value B is located is ⁇ B
• is 10° or less.
• a texture in a region at 1/4 position from an end surface in a width direction and from a surface to a depth of 500 ⁇ m in a sheet thickness 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.
• ⁇ 2 , ⁇ , and ⁇ 1 in the crystal orientation distribution function are, for example, rotation angles in each of Bunge-Euler notation described in FIG. 4 of Light Metals 60 (2010), 12 p 666-675 .
• the maximum value A and the maximum value B and the peak positions of ⁇ at which these maximum values are located 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 from the surface to a depth of 500 ⁇ m in the sheet thickness direction and a region of 2000 ⁇ m or more at an arbitrary position in the rolling direction of the polished sample are measured at a measurement interval of 5.0 ⁇ m.
• an apparatus combining a scanning electron microscope and an EBSD analyzer and OIM Analysis (registered trademark) manufactured by TSL Solutions are used.
• the sample is analyzed by an electron back scattering diffraction (EBSD) method.
• EBSD electron back scattering diffraction
• ODF crystal orientation distribution function
• 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 1/4 + 15 mm position from the end surface in the width direction is a position advanced by 15 mm in a direction opposite to the end surface from the "w/4 position from the end surface in the width direction" when the length in the width direction is w.
• the 1/4 - 15 mm position from the end surface in the width direction is a position advanced by 15 mm in a direction of the end surface from the "w/4 position from the end surface in the width direction" when the length in the width direction is w.
• the absolute value of the difference between the maximum value A and the maximum value B at each position is obtained by performing EBSD analysis by the above-described method and calculating a crystal orientation distribution function at each of the 1/4 position from the end surface in the width direction, the 1/4 - 15 mm position from the end surface in the width direction, and the 1/4 + 15 mm position from the end surface in the width 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%.
• a microstructure at the 1/4 position from the end surface in the width direction and at the position of 1/4 depth from the surface in the sheet thickness direction is defined.
• 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 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.
• desired strength, ductility, and degree of bending properties under tension vary depending on the applied vehicle component.
• the hot-rolled steel sheet according to the present embodiment may have the above-described chemical composition, texture, and microstructure, and then may have either microstructure of a first aspect or a second aspect described below depending on the desired strength, ductility, and degree of bending properties under tension.
• 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 60% or more and more preferably 70% 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 60% or more and more preferably 70% 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 value" of each microstructure of the hot-rolled steel sheet is measured by an electron backscatter pattern (EBSP) method.
• EBSP electron backscatter pattern
• 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 around a 1/4 depth position from the surface in the sheet thickness direction of the polished sample and 400 ⁇ m or more at an arbitrary position in the rolling direction 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.
• a region where the crystal structure is fcc and a region where the crystal structure is bcc are specified from the obtained crystal orientation information, using a "Phase Map” function installed on software "OIM Analysis (registered trademark)" attached to the EBSD analyzer.
• a region where the crystal structure is bcc a region surrounded by grain boundaries having an orientation difference of 15° or more is regarded as one grain, and the average value of orientation differences between adjacent pixels in the grain is calculated to calculate the GAM value of the grain.
• 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 grains having an obtained GAM value of more than 3.0° are calculated to obtain the area ratio of each region.
• the defined grains having an equivalent circle diameter of 0.6 ⁇ m or less may have a large measurement error and thus are excluded from the measurement.
• 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 ⁇ (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.
• the bending properties under tension can be evaluated by performing a tension bending test by the method shown in FIG. 1 .
• L 0 when the length of the steel sheet before the test is L 0 and the length of the steel sheet at fracture is L MAX , L MAX /L 0 when the tension bending test has been performed in the rolling direction or the width direction is used as an index of the bending properties under tension.
• the rolling direction is arranged in the direction of L 0 , a punch is pushed down, and depression amount h when the steel sheet fractures is measured.
• the depression amount h is a stroke amount (mm) of the punch from a position at which the punch comes into contact with the steel sheet until the steel sheet fractures.
• the tension bending test in the width direction is similar to the tension bending test in the rolling direction except that the width direction is arranged in the L 0 direction.
• L MAX /L 0 when the tension bending test has been performed in the rolling direction may be 1.028 or more. In the case where the tensile strength is less than 1040 MPa, if L MAX /L 0 when the tension bending test has been performed in the rolling direction is 1.028 or more, it can be determined that the steel sheet has excellent bending properties under tension in the rolling direction.
• the tensile strength is less than 1040 MPa
• L MAX /L 0 when a tension bending test has been performed in the width direction is 1.028 or more, it can be determined that the steel sheet has excellent bending properties under tension also in the width direction.
• L MAX /L 0 when the tension bending test has been performed in the rolling direction may be 1.018 or more. In the case where the tensile strength is 1040 MPa or more, if L MAX /L 0 when the tension bending test has been performed in the rolling direction is 1.018 or more, it can be determined that the steel sheet has excellent bending properties under tension in the rolling direction.
• the tension bending test shown in FIG. 1 is performed under the following conditions.
• the pressurizing force by a blank holder may be set to such a degree that the hot-rolled steel sheet does not move.
• Initial sheet thickness t 0 of the hot-rolled steel sheet before the test is set to 1.5 mm.
• mechanical grinding is performed from one surface of the hot-rolled steel sheet to set the sheet thickness to 1.5 mm, and then the test is performed with the mechanically ground surface on the punch side.
• the test is performed without performing mechanical grinding.
• the sheet thickness of the hot-rolled steel sheet is less than 1.5 mm, the bending properties under tension are evaluated by an index of L MAX /L 0 - (1.5 - t 0 ) ⁇ 0.0242 instead of L MAX /L 0 .
• the steel sheet may have the following strength, ductility, and bending properties under tension in each aspect. 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 less. Also, in the first aspect, the uniform elongation may be 5.0% or less.
• L MAX /L 0 when the tension bending test has been performed in the rolling direction may be 1.028 or more.
• L MAX /L 0 when the tension bending test has been performed in the width direction may be 1.028 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 1080 MPa or less. In the second aspect, the uniform elongation may be 4.0% or less.
• L MAX /L 0 when the tension bending test has been performed in the rolling direction may be 1.018 or more.
• L MAX /L 0 when the tension bending test has been performed in the width direction may be 1.018 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.
• strain is applied once or multiple times to the slab having the above-described chemical composition such that the strain in the width direction is 3 to 15% in total. This makes it possible to improve uniformity of the texture while reducing unevenness of the surface layer of the slab.
• the strain may be applied after slab heating for rough rolling is performed.
• the peak positions of the maximum value A and the maximum value B in the texture of the hot-rolled steel sheet may not be controlled within a preferable range.
• 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 total strain applied in the width direction of the slab can be expressed by (1 - w 1 /w 0 ) ⁇ 100 (%), where w 0 is the length in the width direction of the slab before the first strain application, and w 1 is the length in the width direction of the slab after the last strain application.
• 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 is performed so that the difference in inlet side temperature between the path immediately preceding the final path and the final path is in a temperature range of 30°C or higher and the finishing temperature is in a temperature range of 920°C or higher.
• the difference in inlet side temperature between the path immediately preceding the final path and the final path is lower than 30°C, the peak positions of the maximum value A and the maximum value B in the texture of the hot-rolled steel sheet may not be controlled within a preferable range.
• the finishing temperature is lower than 920°C, the maximum value A and the maximum value B cannot be controlled to preferable values.
• Examples of a method of setting the difference in inlet side temperature between the path immediately preceding the final path and the final path to 30°C or higher include control by controlling the injection amount of a coolant such as water from a cooling device such as a cooling spray immediately after rolling, controlling the conveyance speed of the steel sheet during rolling, and the like.
• a coolant such as water from a cooling device such as a cooling spray immediately after rolling
• the path immediately preceding the final path is a path one stage before the final path.
• the path immediately preceding the final path refers to the path of F6.
• finishing temperature is an outlet side temperature of the final path of the finish rolling.
• 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.
• 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 2.0° or more 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.
• accelerated cooling was performed at an average cooling rate of 30°C/s or faster to the "Start temperature of slow cooling" in the table.
• the slow cooling was performed by air cooling, and the average cooling rate in the slow cooling was 20°C/s or slower.
• accelerated cooling was performed by "Average cooling rate until reaching 300°C after completion of slow cooling" in the table. After the accelerated cooling was stopped, coiling was immediately performed.
• the texture, the microstructure, the tensile strength (TS), the uniform elongation (uEl), the hole expansion ratio ( ⁇ ), and the bending properties under tension (L MAX /L 0 in the rolling direction (L direction) and L MAX /L 0 in the width direction (C direction)) were evaluated by the above-described methods.
• the absolute value of the difference between the maximum value A and the maximum value B at the 1/4 position from the end surface in the width direction, the 1/4 - 15 mm position from the end surface in the width direction, and the 1/4 + 15 mm position from the end surface in the width direction is 3.0 or less
• "OK” was written in the column of "Absolute value of difference between maximum value A and maximum value B at each of three positions in width direction of 3.0 or less” in the table.
• the absolute value was more than 3.0, "NG” was written in the column.
• TS tensile strength
• the bending properties under tension were evaluated according to the following criteria depending on the tensile strength.
• Bending properties under tension (L MAX /L 0 in rolling direction (L direction)): acceptable if 1.028 or more, unacceptable if less than 1.028.
• the hot-rolled steel sheets according to the present invention examples have high strength and excellent ductility and hole expandability, and have excellent bending properties under tension in the rolling direction.

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