US20250101553A1 - Hot-rolled steel sheet - Google Patents

Hot-rolled steel sheet Download PDF

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US20250101553A1
US20250101553A1 US18/728,338 US202318728338A US2025101553A1 US 20250101553 A1 US20250101553 A1 US 20250101553A1 US 202318728338 A US202318728338 A US 202318728338A US 2025101553 A1 US2025101553 A1 US 2025101553A1
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hot
steel sheet
rolling
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Hideto HIROSHIMA
Hiroshi Shuto
Kazumasa TSUTSUI
Yukiko Kobayashi
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Nippon Steel Corp
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Nippon Steel Corp
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Assigned to NIPPON STEEL CORPORATION reassignment NIPPON STEEL CORPORATION ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: HIROSHIMA, Hideto, KOBAYASHI, YUKIKO, SHUTO, HIROSHI, TSUTSUI, Kazumasa
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips of ferrous alloys
    • 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 by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/008—Ferrous alloys, e.g. steel alloys containing tin
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/08—Ferrous alloys, e.g. steel alloys containing nickel
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/14—Ferrous alloys, e.g. steel alloys containing titanium or zirconium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/16—Ferrous alloys, e.g. steel alloys containing copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/38—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
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with 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
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/54—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
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
    • C—CHEMISTRY; METALLURGY
    • 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
    • C21D2211/005—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
    • C21D2211/009—Pearlite

Definitions

  • the present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is formed into various shapes by press working or the like to be used, and particularly relates to a hot-rolled steel sheet that has high strength and excellent ductility, fatigue property and shearing property.
  • vehicle members are formed by press forming, and the press-formed blank sheet is often manufactured by highly productive shearing working.
  • a blank sheet manufactured by shearing working needs to be excellent in terms of the end surface accuracy after shearing working. For example, when a secondary sheared surface consisting of a sheared surface, a fractured surface, and a sheared surface is generated in the appearance of the end surface after shearing working (sheared end surface), the accuracy of the sheared end surface significantly deteriorates.
  • Patent Document I discloses a high-strength steel sheet having excellent ductility and stretch flangeability and having a tensile strength of 980 MPa or more, in which a second phase consisting of residual austenite and/or martensite is finely dispersed in crystal grains.
  • Patent Document 2 discloses a technique for controlling burr height after punching by controlling a ratio d s /d b of the ferrite grain size d s of the surface layer to the ferrite grain d b of an inside to 0.95 or less.
  • Patent Documents 1 and 2 are all techniques for improving either ductility or an end surface property after shearing working. However, Patent Documents 1 and 2 do not refer to a technique for achieving both of the properties.
  • hot-rolled steel sheet having high strength may be required to have better fatigue property.
  • the present invention has been made in view of the above problems of the related art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent ductility, fatigue property and shearing property.
  • the gist of the present invention made based on the above findings is as follows.
  • the hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.
  • FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet according to a present invention example.
  • FIG. 2 is an example of a sheared end surface of a hot-rolled steel sheet according to a comparative example.
  • the hot-rolled steel sheet according to the present embodiment includes, in terms of mass %, C: 0.050% to 0.250%, Si: 0.05% to 3.00%, Mn: 1.00% to 4.00%, one or two or more of Ti, Nb, and V: 0.060% to 0.500% in total, sol.
  • the C content is set to 0.250% or less.
  • the C content is preferably 0.200% or less, 0.180% or less or 0.150% or less.
  • Si has an action of improving the ductility of the hot-rolled steel sheet by promoting the formation of ferrite and has an action of increasing the strength of the hot-rolled steel sheet by the solid solution strengthening of ferrite.
  • Si has an action of making steel sound by deoxidation (suppressing the occurrence of a defect such as a blowhole in steel).
  • the Si content is set to 0.05% or more.
  • the Si content is preferably 0.50% or more and more preferably 0.80% or more.
  • the Si content is set to 3.00% or less.
  • the Si content is preferably 2.50% or less, and more preferably 2.00% or less or 1.50% or less.
  • the sol. Al content is more than 2.000%, the above effects are saturated, which is not economically preferable, and thus the sol. Al content is set to 2.000% or less.
  • the sol. Al content is preferably 1.500% v or less, more preferably 1.000% or less, and still more preferably 0.500% or less.
  • N is an element that is contained in steel as an impurity and has an action of degrading the ductility of the hot-rolled steel sheet.
  • the N content is set to 0.1000% or less.
  • the N content is preferably 0.0800% or less, more preferably 0.0700% or less, and still more preferably 0.0100% or less or 0.0050% or less.
  • the lower limit of the N content does not need to be particularly specified, but may be 0%.
  • the N content is preferably set to 0.0010% or more and more preferably set to 0.0020% or more to promote the precipitation of a carbonitride.
  • Cu has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of being precipitated as a carbide in steel at a low temperature to increase the strength of the hot-rolled steel sheet.
  • the Cu content is preferably set to 0.01% or more and more preferably set to 0.05% or more.
  • the Cu content is set to 2.00% or less.
  • the Cu content is preferably 1.50% or less and more preferably 1.00% or less.
  • the Cr content is preferably set to 0.01% or more and more preferably set to 0.05% or more.
  • the Cr content is set to 2.00% or less.
  • Mo has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of being precipitated as a carbide in steel to increase the strength of the hot-rolled steel sheet.
  • the Mo content is preferably set to 0.01% or more and more preferably set to 0.02% or more.
  • the Mo content is set to 1.00% or less.
  • the Mo content is preferably 0.50% or less and more preferably 0.20% or less.
  • B has an action of enhancing the hardenability of the hot-rolled steel sheet.
  • the B content is preferably set to 0.0001% or more and more preferably set to 0.0002% or more.
  • the B content is set to 0.0100% or less.
  • the B content is preferably 0.0050% or less.
  • All of Ca, Mg, and REM have an action of enhancing the ductility of the hot-rolled steel sheet by adjusting the shape of inclusions in steel to a preferable shape.
  • Bi has an action of enhancing the ductility of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or two or more of these elements may be contained. In order to more reliably obtain the effect by the action, it is preferable that any one or more of Ca, Mg, REM, and Bi are set to 0.0005% or more.
  • the Ca content or Mg content is more than 0.0200% or when the REM content is more than 0.1000%, an inclusion is excessively formed in steel, and thus the ductility of the hot-rolled steel sheet may be conversely degraded in some cases.
  • the Bi content is set to more than 0.020%, the above effect by the action is saturated, which is not economically preferable. Therefore, the Ca content and the Mg content are set to 0.0200% or less, the REM content is set to 0.1000% or less, and the Bi content is set to 0.020% or less.
  • the Bi content is preferably 0.010% or less.
  • REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids
  • the REM content refers to the total amount of these elements.
  • the lanthanoids are industrially added in the form of misch metal.
  • One or two or more of Zr, Co, Zn, or W 0% to 1.00% in total
  • the present inventors have confirmed that, even when a total of 1.00% or less of these elements are contained, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. Therefore, one or two or more of Zr, Co, Zn, or W may be contained in a total of 1.00% or less.
  • the present inventors have confirmed that, even when a small amount of Sn is contained, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. However, when a large amount of Sn is contained, a defect may be generated during hot rolling, and thus the Sn content is set to 0.05% or less.
  • the chemical composition of the above hot-rolled steel sheet may be measured by a general analytical method.
  • ICP-AES inductively coupled plasma-atomic emission spectrometry
  • sol. Al may be measured by the ICP-AES using a filtrate after a sample is decomposed with an acid by heating.
  • C and S may be measured by using a combustion-infrared absorption method
  • N may be measured by using the inert gas melting-thermal conductivity method
  • O may be measured using an inert gas melting-non-dispersive infrared absorption method.
  • residual austenite is less than 3.0%
  • ferrite is 15.0% or more and less than 60.0%
  • pearlite is less than 5.0%
  • an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm
  • an average number density of the alloy carbides in the ferrite is 3.5 ⁇ 10 16 /cm 3 or more
  • the E value that indicates the periodicity of the microstructure is 10.7 or more
  • the I value that indicates the uniformity of the microstructure is 1.020 or more
  • the standard deviation of the Mn concentration is 0.60 mass % or less.
  • the hot-rolled steel sheet according to the present embodiment has the above microstructure, high strength and excellent ductility, fatigue property and shearing property can be obtained.
  • the microstructural ratios, the average sphere equivalent radius and the average number density of the alloy carbides, the E value, the I value, and the standard deviation of the Mn concentration in the microstructure at a depth of 1 ⁇ 4 of the sheet thickness from the surface (a region between a depth of 1 ⁇ 4 of the sheet thickness from the surface and a depth of 3 ⁇ 4 of the sheet thickness from the surface) and the center position in the sheet width direction in a cross section parallel to the rolling direction are specified.
  • the reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet.
  • Residual austenite is a microstructure that is present as a face-centered cubic lattice even at room temperature. Residual austenite has an action of enhancing the ductility of the hot-rolled steel sheet by transformation-induced plasticity (TRIP). On the other hand, residual austenite transforms into high-carbon martensite during shearing working, which inhibits the stable occurrence of cracking and causes the formation of a secondary sheared surface. When the area ratio of the residual austenite is 3.0% or more, the action is actualized, and the shearing property of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the residual austenite is set to less than 3.0%. The area ratio of the residual austenite is preferably less than 1.5% and more preferably less than 1.0%. Since residual austenite is preferably as little as possible, the area ratio of the residual austenite may be 0%.
  • the measurement method of the area ratio of the residual austenite there are methods by X-ray diffraction, EBSP (electron back scattering diffraction pattern) analysis, and magnetic measurement and the like.
  • the area ratio of the residual austenite is measured by X-ray diffraction.
  • the integrated intensities of a total of 6 peaks of ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and ⁇ (220) are obtained in the cross section parallel to the rolling direction at a depth of 1 ⁇ 4 of the sheet thickness (a region between a depth of 1 ⁇ 8 of the sheet thickness from the surface to a depth of 3 ⁇ 8 of the sheet thickness from the surface) and the center position in the sheet width direction of the hot-rolled steel sheet using Co-K ⁇ rays, and the volume ratio of the residual austenite is obtained by calculation using the strength averaging method.
  • the obtained volume ratio of the residual austenite is regarded as an area ratio of the residual austenite.
  • Ferrite is a structure formed when fcc transforms into bcc at a relatively high temperature. Ferrite has a high work hardening rate and thus has an action of enhancing the strength-ductility balance of the hot-rolled steel sheet.
  • the area ratio of the ferrite is set to 15.0% or more.
  • the area ratio of the ferrite is preferably 20.0% or more, more preferably 25.0% or more, and still more preferably 30.0% or more.
  • the area ratio of the ferrite is set to less than 60.0%.
  • the area ratio of the ferrite is preferably 50.0% or less and more preferably 45.0% or less.
  • Pearlite is a lamellar microstructure in which cementite is precipitated in layers between ferrite and is a soft microstructure as compared with bainite and martensite.
  • the area ratio of the pearlite is 5.0% or more, carbon is consumed by cementite that is contained in pearlite, and the strengths of martensite and bainite, which are the remainder in microstructure, decrease, and a desired strength cannot be obtained. Therefore, the area ratio of the pearlite is set to less than 5.0%.
  • the area ratio of the pearlite is preferably 3.0% or less.
  • the area ratio of the pearlite is preferably reduced as much as possible, and the area ratio of the pearlite is more preferably 0%.
  • the steel sheet according to the present embodiment contains a full hard structure consisting of one or two or more of bainite, martensite, and tempered martensite in a total area ratio of 32.0% or more and less than 85.0% as the remainder in microstructure other than residual austenite, ferrite, and pearlite.
  • Measurement of the area ratios of the microstructure is conducted by the following method.
  • a sheet thickness cross section parallel to the rolling direction is mirror-finished and, furthermore, polished at room temperature with colloidal silica not containing an alkaline solution for 8 minutes, thereby removing strain introduced into the surface layer of a sample.
  • a region with a length of 50 ⁇ m and at a 1 ⁇ 4 depth position of the sheet thickness from the surface is measured by electron backscatter diffraction at a measurement interval of 0.1 ⁇ m to obtain crystal orientation information.
  • an EBSD analyzer configured of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used.
  • the degree of vacuum inside the EBSD analyzer is set to 9.6 ⁇ 10 ⁇ 5 Pa or less
  • the acceleration voltage is set to 15 kV
  • the irradiation current level is set to 13
  • the electron beam irradiation level is set to 62.
  • a reflected electron image is photographed at the same visual field.
  • crystal grains where ferrite and cementite are precipitated in layers are specified from the reflected electron image, and the area ratio of the crystal grains is calculated, thereby obtaining the area ratio of pearlite.
  • regions where the grain average misorientation value is 1.0° or less are determined as ferrite using a “Grain Average Misorientation” function installed in software “OIM Analysis (registered trademark)” included in the EBSD analyzer.
  • the Grain Tolerance Angle is set to 15°, the area ratio of the region determined as the ferrite is obtained, thereby obtaining the area ratio of the ferrite.
  • the hot-rolled steel sheet according to the present embodiment has excellent fatigue property since the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite is preferably controlled.
  • the average sphere equivalent radius of alloy carbides in the ferrite is less than 0.5 mu, the strength against repeated deformation of ferrite cannot be increased and a desired fatigue property cannot be obtained. Therefore, the average sphere equivalent radius of alloy carbides in the ferrite is set to 0.5 nm or more.
  • the average sphere equivalent radius of alloy carbides in the ferrite is set to less than 5.0 nm.
  • the average sphere equivalent radius of alloy carbides in the ferrite is preferably 4.0 nm or less, 3.0 nm or less or 2.0 nm or less, and more preferably less than 1.5 nm.
  • the average number density of the alloy carbides in the ferrite is set to 3.5 ⁇ 10 16 /cm 3 or more.
  • the average number density of the alloy carbides in the ferrite is preferably 5.0 ⁇ 10 16 /cm 3 or more, 10.0 ⁇ 10 16 /cm 3 or more or 20.0 ⁇ 10 16 /cm 3 or more.
  • the upper limit of the average number density of the alloy carbides in the ferrite is particularly limited, the more the better.
  • the average number density of the alloy carbides in the ferrite may set to 1.0 ⁇ 10 19 /cm 3 or less.
  • the alloy carbides refer to carbides containing one or two or more of Ti, Nb, Mo, and V.
  • a sphere equivalent radius and a number density of alloy carbides in ferrite are measured by three-dimensional atom probe.
  • the laser wavelength ( ⁇ ) is set to 355 nm
  • the laser power is set to 30 pJ
  • the temperature of the needle-shaped test piece is set to 50K.
  • the device used for three-dimensional atom probe measurement is not particularly limited.
  • the three-dimensional atom probe measuring device is, for example, LEAP4000XHR manufactured by AMETEK Corporation.
  • a sample is taken using an FIB (focused ion beam) device.
  • FIB focused ion beam
  • the equivalent sphere radius and number density of fine precipitates ranging from less than I nm to several tens of nanometers in equivalent sphere radius can be accurately measured.
  • the number density of precipitates can be obtained by dividing the number of precipitates included in the area measured with the three-dimensional atom probe by the volume of the measurement area at precipitates identified as alloy carbides by the method described below.
  • the total volume of precipitates in the measurement area is obtained by dividing the total number of atoms of alloying elements (Ti, Nb, Mo, V, C) contained in all the precipitates in the measurement area by the atomic density of the alloy carbide.
  • the volume of precipitates is obtained by dividing the total volume of precipitates by the number of precipitates. From the obtained volume of precipitates, the spherical equivalent radius is calculated assuming that the precipitate is spherical.
  • the average number density and the average sphere equivalent radius are obtained by performing the above-described method on five or more of measurement data having a measurement area volume of 30000 nm 3 or more.
  • the region where Ga introduced during FIB processing is less than 0.025 at % is defined as the observation region, and the region where Ga is mixed in at 0.025 at % or more is excluded from the measurement area.
  • the amount of Ga in the longitudinal direction of the needle sample can be confirmed using the ID Concentration Profile function of the data analysis software IVAS 3.6.14 (manufactured by CAMECA Instruments Inc.).

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