US20260022442A1 - Hot-rolled steel sheet - Google Patents

Hot-rolled steel sheet

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Publication number
US20260022442A1
US20260022442A1 US19/106,480 US202319106480A US2026022442A1 US 20260022442 A1 US20260022442 A1 US 20260022442A1 US 202319106480 A US202319106480 A US 202319106480A US 2026022442 A1 US2026022442 A1 US 2026022442A1
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present
less
hot
steel sheet
rolling
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US19/106,480
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English (en)
Inventor
Hideto HIROSHIMA
Hiroshi Shuto
Yukiko Kobayashi
Kazumasa TSUTSUI
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Nippon Steel Corp
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Nippon Steel Corp
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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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0205—
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—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
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0263—Modifying the physical properties of ferrous metals or ferrous alloys by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • 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
    • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
    • 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
    • C22C38/06—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
    • C22C38/18—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
    • C22C38/58—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
    • C21D2211/004—Dispersions; Precipitations
    • 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 1 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 ds/db of the ferrite grain size ds of the surface layer to the ferrite grain db of an inside to 0.95 or less.
  • Patent Document 1 Japanese Unexamined Patent Application, First Publication No. 2005-179703
  • Patent Document 2 Japanese Unexamined Patent Application, First Publication No. H10-168544
  • Non-Patent Document 1 J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2016) 584-596
  • Non-Patent Document 2 D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546
  • Non-Patent Document 3 K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485
  • 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 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.
  • C increases the area ratio of a hard phase and increases the strength of ferrite by bonding to a precipitation hardening element such as Ti, Nb, or V.
  • a precipitation hardening element such as Ti, Nb, or V.
  • the C content is set to 0.050% or more.
  • the C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably more than 0.070%, 0.75% or more or 0.080% or more.
  • 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.
  • Mn has an action of suppressing ferritic transformation to enhance strength of the hot-rolled steel sheet.
  • the Mn content is set to 1.00% or more.
  • the Mn content is preferably 1.30% or more and more preferably 1.50% or more.
  • the Mn content is set to 4.00% or less.
  • the Mn content is preferably 3.50% or less and more preferably 3.00% or less or 2.50% or less.
  • Ti, Nb, and V are elements that are finely precipitated in steel as a carbide and a nitride and improve the strength of steel by precipitation hardening. Furthermore, these elements are essential elements to obtain a desired fatigue property. When the total amount of Ti, Nb, and V is less than 0.060%, these effects cannot be obtained. Therefore, the total amount of Ti, Nb, and V is set to 0.060% or more. Not all of Ti, Nb, and V need to be contained, and any one thereof may be contained, and the amount thereof may be 0.060% or more. The total amount of Ti, Nb, and V is preferably 0.080% or more, more preferably 0.100% or more, and still more preferably 0.120% or more.
  • the total amount of Ti, Nb, and V exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the total amount of Ti, Nb, and V is set to 0.500% or less.
  • the total amount of Ti, Nb, and V is preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.200% or less.
  • Al has an action of making steel sound by deoxidizing and has an action of enhancing the ductility of the hot-rolled steel sheet by promoting the formation of ferrite.
  • the sol. Al content is set to 0.001% or more.
  • the sol. Al content is preferably 0.010% or more, and more preferably 0.020% or more or 0.030% or more.
  • 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 0.400% or less, more preferably 0.300% or less, and still more preferably 0.250% or less.
  • the sol. Al means acid-soluble Al and refers to solid solution Al present in steel in a solid solution state.
  • P has an action of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, P may be positively contained.
  • P is an element that is easily segregated, and, when the P content exceeds 0.100%, the deterioration of ductility attributed to boundary segregation becomes significant. Therefore, the P content is set to 0.100% or less.
  • the P content is preferably 0.030% or less.
  • the lower limit of the P content does not need to be particularly specified, but the P content is preferably set to 0.001% from the viewpoint of the refining cost.
  • the S content forms a sulfide-based inclusion in steel to degrade the ductility of the hot-rolled steel sheet.
  • the S content is set to 0.0300% or less.
  • the S content is preferably 0.0050% or less.
  • the lower limit of the S content does not need to be particularly specified, but the S content is preferably set to 0.0001% from the viewpoint of the refining cost.
  • N 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 0.0050% or less.
  • the lower limit of the N content does not need to be particularly specified, but 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 in a case where one or two or more of Ti, Nb, and V are contained to further refine the microstructure.
  • the O content is set to 0.0100% or less.
  • the O content is preferably 0.0080% or less and more preferably 0.0055% or less, and still more preferably 0.0050% or less.
  • the O content may be set to 0.0005% or more or 0.0010% or more to disperse a large number of fine oxides when molten steel is deoxidized.
  • the remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and an impurity.
  • the impurities mean substances that are incorporated from ore as a raw material, a scrap, manufacturing environment, or the like and/or substances that are permitted to an extent that the hot-rolled steel sheet according to the present embodiment is not adversely affected.
  • the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements.
  • the optional elements are not contained, the lower limit of the content thereof is 0%.
  • the optional elements will be described in detail.
  • 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.
  • Ni has an action of enhancing the hardenability of the hot-rolled steel sheet.
  • Ni has an action of effectively suppressing the grain boundary cracking of the slab caused by Cu.
  • the Ni content is preferably set to 0.02% or more. Since Ni is an expensive element, it is not economically preferable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.
  • the As content is preferably set to 0.001% or more.
  • the As content is set to 0.100% or less.
  • 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.
  • the chemical composition is analyzed after the plating layer or the coating film is removed by mechanical grinding or the like as necessary.
  • 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 10.0 nm
  • an average number density of the alloy carbides in the ferrite is 0.10 ⁇ 10 16 pieces/cm 3 or more and less than 1.45 ⁇ 10 16 pieces/cm 3
  • 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 region of a position of 1 ⁇ 4 from the surface in a sheet thickness direction of the hot-rolled steel sheet and a position of 1 ⁇ 4 from an end surface in a direction (a sheet width direction) perpendicular to a rolling direction and the sheet thickness direction (the rolling direction is an arbitrary position) are specified.
  • the reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet.
  • the “surface” refers to the interface of the plating layer or the coating film and 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 area ratio of the residual austenite by X-ray diffraction in the present embodiment, first, in a cross section at a position of 1 ⁇ 4 from the surface in the sheet thickness direction of the hot-rolled steel sheet, a sample is collected so that the microstructure at a region of 1 mm or more in an arbitrary position of the rolling direction and 1 mm or more centered in a position of 1 ⁇ 4 from the end surface in the sheet width direction can be observed.
  • the integrated intensities of a total of 6 peaks of ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and ⁇ (220) are obtained using Co-K ⁇ rays.
  • the volume ratio of the residual austenite is obtained using the strength averaging method from the integrated intensities, and the obtained volume ratio 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.
  • 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. In order to improve the stretch flangeability of the steel sheet, the area ratio of the pearlite is preferably reduced as much as possible, and the area ratio of the pearlite is still 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 other than residual austenite is conducted by the following method. First, in a position of 1 ⁇ 4 from the end surface in the sheet width direction of the hot-rolled steel sheet, a sample is collected so that the microstructure in a region of a position of 1 ⁇ 4 from the surface in the sheet thickness direction in a cross section parallel to the rolling direction can be observed.
  • the size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
  • the observed cross section of the sample is polished at room temperature with colloidal silica not containing an alkaline solution for 8 minutes, thereby removing strain introduced into the surface layer of the sample.
  • crystal orientation information is obtained by a measurement using electron backscatter diffraction at a measurement interval of 0.1 ⁇ m.
  • 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, and 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 area ratio of a remainder of the microstructure is obtained by subtracting the area ratios of residual austenite, pearlite, and ferrite from 100%.
  • the remainder of the microstructure can be estimated to be a hard structure consisting of one or two or more of bainite, martensite, and tempered martensite from the chemical composition of the hot-rolled steel sheet and the manufacturing conditions.
  • the rolling direction of the hot-rolled steel sheet is determined by the following method.
  • a test piece is collected so that a sheet thickness cross section of the hot-rolled steel sheet can be observed.
  • the sheet thickness cross section of the collected test piece is observed using an optical microscope after mirror polishing and corrosion with a picric acid-saturated aqueous solution.
  • An observation range is set to an entire thickness of the sheet thickness, and an extending direction of grains is determined.
  • an angle difference between the extending direction and the sheet thickness direction is set to ⁇ .
  • a surface at a position of 1 ⁇ 4 of the sheet thickness that is perpendicular to the sheet thickness direction and parallel to the above-mentioned extending direction is polished similar to the sheet thickness cross section, and the extending direction of grains is determined.
  • an angle difference between the sheet thickness cross section and the extending direction is set to ⁇ .
  • the direction with ⁇ and ⁇ which are obtained from the extending direction of the grains in the above-mentioned two cross sections, as deflection angles is determined as the rolling direction.
  • the “Analyze Particles” function of the image analysis software “ImageJ” can be used to obtain the extending direction of the grains by setting the circularity to 0.7 or less.
  • Average Sphere Equivalent Radius of Alloy Carbides in Ferrite 0.5 nm or More and Less than 10.0 nm
  • 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 nm, the strength against repeated deformation of ferrite cannot be increased and a desired fatigue property cannot be obtained.
  • 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 preferably 1.0 nm or more.
  • the average sphere equivalent radius of alloy carbides in the ferrite is 10.0 nm or more, the strength of ferrite cannot be sufficiently increased. cracks from the cutting edge of the shearing tool occur very early in the shearing process and a fractured surface is formed due to the hardness difference between grains, and then a sheared surface is formed again. As a result, since secondary sheared surface are more likely to be formed, and a desired shearing property cannot be obtained in the hot-rolled steel sheet. Therefore, the average sphere equivalent radius of alloy carbides in the ferrite is set to less than 10.0 nm.
  • the average sphere equivalent radius of alloy carbides in the ferrite is preferably 8.0 nm or less, 6.0 nm or less or 4.0 nm or less, and more preferably less than 2.0 nm.
  • Average Number Density of Alloy Carbides in Ferrite 0.10 ⁇ 10 16 Pieces/cm 3 or More and Less than 1.45 ⁇ 10 16 Pieces/cm 3
  • the average number density of the alloy carbides in the ferrite is set to 0.10 ⁇ 10 16 pieces/cm 3 or more and less than 1.45 ⁇ 10 16 pieces/cm 3 .
  • the average number density of the alloy carbides in the ferrite is preferably 0.50 ⁇ 10 16 pieces/cm 3 or more, and more preferably 1.00 ⁇ 10 16 pieces/cm 3 or more.
  • the average number density of the alloy carbides in the ferrite is preferably 1.40 ⁇ 10 16 pieces/cm 3 or less, more preferably 1.20 ⁇ 10 16 pieces/cm 3 or less, and still more preferably 1.10 ⁇ 10 16 pieces/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 1 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, and 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 nm3 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 1D Concentration Profile function of the data analysis software IVAS 3.6.14 (manufactured by CAMECA Instruments Inc.).
  • the generation of a secondary sheared surface is suppressed by controlling the E (Entropy) value that indicates the periodicity of the microstructure and the I (inverse difference normalized) value that indicates the uniformity of the microstructure.
  • the E value represents the periodicity of the microstructure.
  • the E value decreases.
  • the E value since there is a need to make the microstructure poorly periodic. it is necessary to increase the E value.
  • the E value is less than 10.7, a secondary sheared surface is likely to be generated. From periodically arranged structures as starting points, cracking occurs from the cutting edge of a shearing tool in an extremely early stage of shearing working to form a fractured surface, and then a sheared surface is formed again. It is presumed that this makes it likely for a secondary sheared surface to be generated.
  • the E value is set to 10.7 or more.
  • the E value is preferably 10.8 or more and more preferably 11.0 or more.
  • the E value is preferably as high as possible, and the upper limit is not particularly specified and may be set to 13.0 or less, 12.5 or less, or 12.0 or less.
  • the I value represents the uniformity of the microstructure and increases as the area of a region having certain brightness increases.
  • a high I value means that the uniformity of the microstructure is high.
  • the I value is set to 1.020 or more.
  • the I value is preferably 1.025 or more and more preferably 1.030 or more.
  • the I value is preferably as high as possible, and the upper limit is not particularly specified and may be set to 1.200 or less, 1.150 or less, or 1.100 or less.
  • the E Value and the I Value can be Obtained by the Following Method.
  • the photographing region of a SEM image photographed for calculating the E value and the I value is set to, in a cross section parallel to the rolling direction at a position of 1 ⁇ 4 from the end surface in the sheet width direction, 200 ⁇ m ⁇ 200 ⁇ m centered in a position of 1 ⁇ 4 from the surface in the sheet thickness direction, and the number of the observation fields is set to 5.
  • the SEM image is photographed using an SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation with a tungsten emitter and an acceleration voltage of 1.5 kV. Based on the above settings, the SEM image is output at a magnification of 1000 times and a gray scale of 256 gradations.
  • Non-Patent Document 3 On an image obtained by cutting out the obtained SEM image into a 880 ⁇ 880-pixel region, a smoothing treatment described in Non-Patent Document 3, in which the contrast-enhanced limit magnification is set to 2.0 and the tile grid size is 8 ⁇ 8 is performed.
  • the smoothed SEM image is rotated counterclockwise from 0 degrees to 179 degrees in increments of 1 degree. excluding 90 degrees, and an image is created at each angle, thereby obtaining a total of 179 images.
  • the frequency values of brightness between adjacent pixels are sampled in a matrix form using the gray level co-occurrence matrices method (the GLCM method) described in Non-Patent Document 1.
  • P(i,j) in the following formula (1) and formula (2) is the gray level co-occurrence matrix
  • the value at the ith row and jth column of the matrix P is expressed as P(i,j).
  • the following formula (1) can be modified to the following formula (1′)
  • the following formula (2) can be modified to the following formula (2′).
  • the value at the ith row and the jth column of the matrix P is expressed as Pij.
  • the standard deviation of the Mn concentration of the hot-rolled steel sheet according to the present embodiment is 0.60 mass % or less. This makes it possible to uniformly disperse the hard phase and makes it possible to prevent the occurrence of cracking from the cutting edge of the shearing tool in an extremely early stage of shearing working. As a result, the generation of a secondary sheared surface can be suppressed.
  • the standard deviation of the Mn concentration is preferably 0.50 mass % or less and more preferably 0.47 mass % or less.
  • the value of the lower limit of the standard deviation of the Mn concentration is desirably as small as possible from the viewpoint of suppressing excessively large burrs, but the substantial lower limit is 0.10 mass % due to restrictions in the manufacturing process.
  • a sample is collected so that a region of 1 ⁇ 4 from the surface in the sheet thickness direction in a cross section parallel to the rolling direction can be observed.
  • the size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction.
  • the standard deviation of the Mn concentration is measured using an electron probe micro analyzer (EPMA).
  • the acceleration voltage is set to 15 kV and the magnification is set to 5000 times, and the distribution image of 40000 or more points in a range that is 20 ⁇ m in the rolling direction and 20 ⁇ m in the sheet thickness direction are measured with the measurement interval of to 0.1 ⁇ m.
  • the standard deviation is calculated based on the Mn concentrations obtained from all of the measurement points, thereby obtaining the standard deviation of the Mn concentration.
  • test piece is a No. 5 test piece of JIS Z 2241:2011.
  • the sampling position of the test piece may be set to a position of 1 ⁇ 4 from the end surface in the sheet width direction of the hot-rolled steel sheet, and the sheet width direction may be set to the longitudinal direction.
  • the tensile (maximum) strength is preferably 980 MPa or more.
  • the tensile strength is more preferably 1000 MPa or more.
  • the upper limit does not need to be particularly limited and may be set to 1780 MPa from the viewpoint of suppressing the wearing of a die.
  • the total elongation is preferably set to 10.0% or more, and the product of the tensile strength and the total elongation (TS ⁇ El) is preferably set to 13000 MPa ⁇ % or more.
  • the total elongation is more preferably set to 11.0% or more and still more preferably set to 13.0% or more.
  • the product of the tensile strength and the total elongation is more preferably set to 14000 MPa ⁇ % or more and still more preferably 15000 MPa ⁇ % or more.
  • the hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like and thereby made into a surface-treated steel sheet.
  • the plating layer may be an electro plating layer or a hot-dip plating layer.
  • the electro plating layer include electrogalvanizing, electro Zn—Ni alloy plating, and the like.
  • the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, hot-dip Zn—Al—Mg—Si alloy plating, and the like.
  • the plating adhesion amount is not particularly limited and may be the same as before.
  • a suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure is as follows.
  • the hot-rolled steel sheet In order to obtain the hot-rolled steel sheet according to the present embodiment, it is effective to perform hot rolling after heating a slab under predetermined conditions, perform accelerated cooling to a predetermined temperature range, then, slowly cool the slab, and control the cooling history until coiling.
  • 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.
  • stress refers to tension that is loaded in the rolling direction of the steel sheet. The stress can be controlled by adjusting the rotation speed of the rolling stand and the coiling device, and can be determined by dividing the measured load in the rolling direction by the cross-sectional area of the passing sheet.
  • a hot-rolled steel sheet with high strength, excellent ductility, fatigue property and shearing property can be stably manufactured by adopting the above manufacturing method. That is, when the slab heating conditions and the hot rolling conditions are appropriately controlled, the reduction of Mn segregation and equiaxed austenite before transformation are achieved, and, in cooperation with the cooling conditions after the hot rolling to be described below, a hot-rolled steel sheet having a desired microstructure can be stably manufactured.
  • the slab that is subjected to hot rolling a slab obtained by continuous casting, a slab obtained by casting and blooming, or the like can be used, and, if necessary, it is possible to use the above slabs after hot working or cold working.
  • the slab that is subjected to hot rolling is preferably retained in a temperature range of 700° C. to 850° C. for 900 seconds or longer during slab heating, then, further heated, and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer.
  • the steel sheet temperature may be fluctuated or be maintained constant in this temperature range.
  • the steel sheet temperature may be fluctuated or be maintained constant in the temperature range of 1100° C. or higher.
  • Mn is distributed between ferrite and austenite, and Mn can be diffused into the ferrite region by extending the transformation time. Accordingly, the Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. Therefore, it is preferable to retain the slab in the temperature range of 700° C. to 850° C. for 900 seconds or longer. In addition, by retention the slab in the temperature range of 1100° C. or higher for 6000 seconds or longer, the Mn concentration can be significantly reduced.
  • the hot rolling it is preferable to use a reverse mill or a tandem mill for multi-pass rolling. Particularly, from the viewpoint of industrial productivity and the viewpoint of stress loading on the steel sheet during the rolling, at least the final two stands are more preferably hot rolling in which a tandem mill is used.
  • the hot rolling is performed so that the sheet thickness reduction is 90% or more in total in a temperature range of 850° C. to 1100° C.
  • mainly recrystallized austenite grains are refined, and accumulation of strain energy into the unrecrystallized austenite grains is promoted.
  • the recrystallization of austenite is promoted, and the atomic diffusion of Mn is promoted, which makes it possible to reduce the standard deviation of the Mn concentration. Therefore, it is preferable to perform the hot rolling so that the sheet thickness reduction is 90% or more in total in the temperature range of 850° C. to 1100° C.
  • the sheet thickness reduction in total in the temperature range of 850° C. to 1100° C. can be expressed as ⁇ (t0 ⁇ t1)/t0 ⁇ 100 (%) where an inlet sheet thickness before the rolling of the first rolling in this temperature range is t0 and an outlet sheet thickness after the rolling of the final stand in this temperature range is t1.
  • Rolling at the one stand before the final stand is preferably performed at a temperature range of 900° C. or higher and lower than 1010° C., and the stress that is loaded to the steel sheet after the rolling at the one stand before the final stand of hot rolling and before the rolling at the final stand is preferably set to 170 kPa or more. These make it possible to reduce the number of crystal grains having a ⁇ 110 ⁇ 001> crystal orientation in the recrystallized austenite after the rolling at the one stand before the final stand. Since ⁇ 110 ⁇ 001> is a crystal orientation that is difficult to recrystallize, recrystallization by the rolling of the final stand can be effectively promoted by suppressing the formation of this crystal orientation.
  • the band-like structure of the hot-rolled steel sheet is improved, the periodicity of the microstructure is reduced, and the E value increases.
  • the stress that is loaded to the steel sheet is less than 170 kPa, it may be impossible to obtain a desired E value.
  • the stress that is loaded to the steel sheet is more preferably 190 kPa or more. Note that the stress that is loaded to the steel sheet refers to tension in the longitudinal direction of the steel sheet, and can be controlled by adjusting the roll rotation speed during tandem rolling.
  • the upper limit of the stress that is loaded to the steel sheet is not particularly limited, but may be 350 kPa or lower.
  • the rolling reduction at the final stand of the hot rolling is set to 8% or more and the finishing temperature Tf is set to 900° C. or higher.
  • the rolling reduction at the final stand of the hot rolling is set to 8% or more, it is possible to promote recrystallization caused by the final stand rolling. As a result, the band-like structure of the hot-rolled steel sheet is improved, the periodicity of the microstructure is reduced, and the E value increases.
  • the finishing temperature Tf is set to 900° C. or higher, it is possible to suppress an excessive increase in the number of ferrite nucleation sites in austenite.
  • the formation of ferrite in the final structure is suppressed, and a desired strength of the hot-rolled steel sheet can be obtained.
  • Tf is set to lower than 1010° C.
  • the upper limit of the rolling reduction at the final stand of the hot rolling is not particularly limited, but can be 30% or lower, 20% or lower, and preferably set to 15% or lower.
  • the sheet thickness reduction is 5% or more and less than 8% in total at a temperature range of 840° C. or higher and lower than 900° C. This can make it possible to control the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite in desired amounts.
  • Light rolling may be performed, for example, at the final stand of the finishing mill, or by introducing new rolling equipment between the finishing mill and the cooling bed.
  • the sheet thickness reduction in total in light rolling can be expressed as ⁇ (t0 ⁇ t1)/t0 ⁇ 100 (%) where an inlet sheet thickness before the first rolling in the light rolling is t0 and an outlet sheet thickness after the rolling of the final stand in the light rolling.
  • Stress that is loaded to the steel sheet after the rolling at the final stand of the hot rolling and before the first rolling of the light rolling, and stress that is loaded to the steel sheet after the rolling of the final stand of the light rolling and until the steel sheet is cooled to 800° C. is preferably set to less than 200 kPa respectively.
  • the stresses that are loaded to the steel sheet at the above positions are set to less than 200 kPa, the recrystallization of austenite preferentially proceeds in the rolling direction, and an increase in the periodicity of the microstructure can be suppressed. As a result, a desired E value can be obtained.
  • the stresses that are loaded to the steel sheet at the above positions are more preferably 180 kPa or less respectively.
  • accelerated cooling is preferably performed to a temperature range of lower than 680° C. at the average cooling rate of 50° C./s or faster after the light rolling.
  • the average number density of the alloy carbides in the ferrite can be set to less than 1.45 ⁇ 10 16 pieces/cm 3 .
  • the average cooling rate in the temperature range of 600° C. or higher and lower than 680° C. to 50° C./s or faster, excessive generation of pearlite can be suppressed.
  • the average cooling rate referred to herein is a value obtained by dividing the temperature drop width of the steel sheet from the start of accelerated cooling (when introducing the steel sheet into cooling equipment) to the completion of accelerated cooling (when deriving the steel sheet from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.
  • the upper limit of the cooling rate is not particularly specified, but when the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, the average cooling rate is preferably 300° C./s or slower.
  • the cooling stop temperature of the accelerated cooling is preferably set to 600° C. or higher.
  • cooling with a high average cooling rate may be performed after completion of the light rolling, for example, by injecting cooling water onto the surface of the steel sheet.
  • the average cooling rate referred to herein refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the accelerated cooling to the stop temperature of the slow cooling by the time required from the stop of the accelerated cooling to the stop of the slow cooling.
  • the slow cooling time is preferably 3.0 seconds or longer.
  • the upper limit of the slow cooling time is determined by the equipment layout and may be set to approximately shorter than 10.0 seconds.
  • the lower limit of the average cooling rate of the slow cooling is not particularly provided and may be set to 0° C./s or faster since heating the steel sheet without cooling accompanies a huge equipment investment.
  • the average cooling rate from the cooling stop temperature of the slow cooling to the coiling temperature is preferably set to 50° C./s or faster.
  • the primary phase structure can be made full hard, and the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite can be controlled in a desired amount.
  • the average cooling rate referred to herein refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the slow cooling where the average cooling rate is slower than 5° C./s to the coiling temperature by the time required from the stop of the slow cooling where the average cooling rate is slower than 5° C./s to coiling.
  • the coiling temperature is preferably set to 350° C. or lower.
  • the amount of an iron carbide precipitated is reduced, and the variation in the hardness distribution in the hard phase can be reduced. As a result, it is possible to obtain a desired I value.
  • the average cooling rate of slow cooling was set to slower than 5° C./s.
  • the measurement lower limit of the coiling temperature shown in Table 4A and Table 4B is 50° C.
  • the actual coiling temperatures of the examples with a value of 50° C. are 50° C. or lower.
  • the rolling at the one stand before the final stand of hot rolling was performed at a temperature of 900° C. or higher and lower than 1010° C.
  • the area ratio of the microstructure, the E value, the I value, the standard deviation of the Mn concentration, the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite, the tensile strength TS, and the total elongation El of each the obtained hot-rolled steel sheets were obtained by the above methods.
  • the fatigue property was evaluated by performing the plane bending fatigue test. The obtained measurement results are shown in Table 5A to Table 6B.
  • the hot-rolled steel sheet was determined as having high strength and excellent ductility, and being successful. In a case where any one was not satisfied, the hot-rolled steel sheet was determined as not having high strength and excellent ductility, and not being successful.
  • the shearing property of the hot-rolled steel sheet was evaluated by a punching test.
  • FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet according to the present invention example
  • FIG. 2 is an example of a sheared end surface of a hot-rolled steel sheet according to a comparative example. In FIG. 1 .
  • the sheared end surface is a sheared end surface with a shear droop, a sheared surface, a fractured surface, and a burr.
  • the sheared end surface is a sheared end surface with a shear droop, a sheared surface, a fractured surface, a sheared surface, a fractured surface, and a burr.
  • the shear droop is an R-like smooth surface region
  • the sheared surface is the region of a punched end surface separated by shear deformation
  • the fractured surface is the region of a punched end surface separated by a crack initiated from the vicinity of the cutting edge
  • a burr is a surface having projections protruding from the lower surface of the hot-rolled steel sheet.
  • a sheared surface, a fractured surface, and a sheared surface as shown in FIG. 2 appeared on two surfaces perpendicular to the rolling direction and two surfaces parallel to the rolling direction in the obtained sheared end surface.
  • a secondary sheared surface was determined to be formed, 4 surfaces for each punched hole, that is, a total of 12 surfaces were observed, and, in a case where there was no surface on which a secondary sheared surface appeared, the hot-rolled steel sheet was determined as having excellent shearing property and being successful.
  • the hot-rolled steel sheet was determined as not having excellent shearing property and not being successful.
  • the examples that were determined as being successful were mentioned with “Good” in the column of shearing property in Tables, and the examples that were determined as not being successful were mentioned with “NG” in Tables.
  • the hot-rolled steel sheets according to the present invention examples have high strength and excellent ductility, fatigue property and shearing property;

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