US20240384378A1 - Hot-rolled steel sheet - Google Patents

Hot-rolled steel sheet Download PDF

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US20240384378A1
US20240384378A1 US18/689,095 US202218689095A US2024384378A1 US 20240384378 A1 US20240384378 A1 US 20240384378A1 US 202218689095 A US202218689095 A US 202218689095A US 2024384378 A1 US2024384378 A1 US 2024384378A1
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hot
steel sheet
present
rolled steel
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Inventor
Hiroshi Shuto
Kazumasa TSUTSUI
Shunsuke Kobayashi
Akifumi SAKAKIBARA
Jun Ando
Toshiki Sugiyama
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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: ANDO, JUN, KOBAYASHI, SHUNSUKE, SAKAKIBARA, Akifumi, SHUTO, HIROSHI, SUGIYAMA, TOSHIKI, 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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    • C21D6/00—Heat treatment of ferrous alloys
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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
    • 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
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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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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/34—Ferrous alloys, e.g. steel alloys containing chromium with more than 1.5% by weight of silicon
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
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    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
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    • 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
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
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    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
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    • 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 used by being formed into various shapes by press working or the like, and particularly to a hot-rolled steel sheet that has a high strength, a high limit fracture sheet thickness reduction ratio, and excellent ductility and shearing property.
  • the limit fracture sheet thickness reduction ratio is a value that is obtained from the sheet thickness of a tensile test piece before breaking and the minimum value of the sheet thickness of the tensile test piece after breaking. It is not preferable that the limit fracture sheet thickness reduction ratio be low since breaking is likely to occur in an early stage when tensile strain is applied during press forming.
  • Vehicle members are formed by press forming, and the press-formed blank sheets are 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.
  • sheared end surface when a secondary sheared surface consisting of a sheared surface, a fractured surface, and a sheared surface again 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 hot-rolled steel sheet as a material for a cold-rolled steel sheet that has excellent surface properties after press working and in which the Mn segregation degree and the P segregation degree are controlled at a center portion of the sheet thickness.
  • Patent Document 1 the limit fracture sheet thickness reduction ratio and shearing property of the hot-rolled steel sheet are not considered.
  • the present invention is contrived in view of the above-described circumstances, and an object of the present invention is to provide a hot-rolled steel sheet that has a high strength, a high limit fracture sheet thickness reduction ratio, and excellent ductility and shearing property.
  • the gist of the present invention is as follows.
  • a hot-rolled steel sheet according to an aspect of the present invention containing, as a chemical composition, by mass %:
  • an average crystal grain size of a surface layer may be less than 3.0 ⁇ m.
  • the chemical composition may contain, by mass %, one or two or more selected from the group consisting of
  • a hot-rolled steel sheet that has a high strength, a high limit fracture sheet thickness reduction ratio, and excellent ductility and shearing property.
  • a hot-rolled steel sheet that has the above various properties and, furthermore, suppresses the occurrence of inside bend cracking, that is, has excellent inside bend cracking resistance.
  • the hot-rolled steel sheet according to the 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.
  • 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 even more preferably 0.080% or more or 0.090% or more.
  • the C content is set to 0.250% or less.
  • the C content is preferably 0.200% or less, 0.150% or less, or 0.120% or less.
  • Si acts to promote the formation of ferrite, thereby improving the ductility of a hot-rolled steel sheet, and to solid solution strengthen ferrite, thereby increasing the strength of the hot-rolled steel sheet.
  • Si acts to achieve soundness of steel by deoxidation (suppressing the occurrence of defects such as blowholes in the 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, 1.00% or more, 1.20% or more, or 1.40% or more.
  • the Si content is set to 3.00% or less.
  • the Si content is preferably 2.70% or less, and more preferably 2.50% or less, 2.20% or less, 2.00% or less, or 1.80% or less.
  • Mn acts to suppress ferritic transformation, thereby increasing the 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 or 1.80% or more.
  • the Mn content is set to 4.00% or less.
  • the Mn content is preferably 3.70% or less or 3.50% or less, and more preferably 3.20% or less, 3.00% or less, or 2.60% or less.
  • Each element symbol in Expression (A) represents the amount of the corresponding element by mass %, and 0% is substituted in a case where the corresponding element is not contained.
  • 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.
  • the total amount of Ti, Nb, and V is set to 0.060% or more. That is, the value of the middle side of Expression (A) is set to 0.060% or more.
  • the lower limits of the Ti content, the Nb content, and the V content are each 0%.
  • Each of the lower limits of the Ti content, the Nb content, and the V content may be 0.001%, 0.010%, 0.030%, or 0.050%.
  • the total amount of Ti, Nb, and V is preferably 0.080% or more, and more preferably 0.100% or more.
  • each of the Ti content, the Nb content, and the V content is set to 0.500% or less, and the total amount of Ti, Nb, and V is set to 0.500% or less. That is, the value of the middle side of Expression (A) is set to 0.500% or less.
  • Each of the Ti content, the Nb content, and the V content is preferably 0.400% or less or 0.300% or less, more preferably 0.250% or less, and even more preferably 0.200% or less or 0.100% or less.
  • the total amount of Ti, Nb, and V is preferably 0.300% or less, more preferably 0.250% or less, and even more preferably 0.200% or less.
  • the sol. Al acts to deoxidize steel, thereby achieving soundness of the steel, and to promote the formation of ferrite, thereby increasing the ductility of the hot-rolled steel sheet.
  • the sol. Al content is set to 0.001% or more.
  • the sol. Al content is preferably 0.010% or more, 0.030% or more, or 0.050% or more, and more preferably 0.080% or more, 0.100% or more, or 0.150% or more.
  • the sol. Al content is set to 2.000% or less.
  • the sol. Al content is preferably 1.700% or less or 1.500% or less, more preferably 1.300% or less, and even more preferably 1.000% or less.
  • the sol. Al means acid-soluble Al and refers to solid solution Al present in steel in a solid solution state.
  • P is an element that is generally contained as an impurity, but acts to increase the strength of the hot-rolled steel sheet by solid solution strengthening.
  • the lower limit of the P content is 0%, but P may be positively contained.
  • P is an element that is easily segregated. In a case where the P content is more than 0.100%, the ductility and limit fracture sheet thickness reduction ratio of the hot-rolled steel sheet attributed to boundary segregation are significantly decreased. Therefore, the P content is set to 0.100% or less.
  • the P content is preferably 0.050% or less, 0.030% or less, 0.020% or less, or 0.015% or less.
  • the lower limit of the P content does not need to be particularly specified, and the lower limit of the P content is 0%. From the viewpoint of the refining cost, the lower limit of the P content may be 0.001%, 0.003%, or 0.005%.
  • S is an element that is contained as an impurity and forms a sulfide-based inclusion in steel, thereby decreasing the ductility and limit fracture sheet thickness reduction ratio of the hot-rolled steel sheet.
  • the S content is set to 0.0300% or less.
  • the S content is preferably 0.0100% or less, 0.0070% or less, or 0.0050% or less.
  • the lower limit of the S content is 0%, but may be 0.0001%, 0.0005%, 0.0010%, or 0.0020% from the viewpoint of the refining cost.
  • N is an element that is contained in steel as an impurity and acts to decrease the ductility and limit fracture sheet thickness reduction ratio 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 or 0.0300% or less, and even more preferably 0.0150% or less or 0.0100% or less.
  • the lower limit of the N content is 0%.
  • the N content is preferably set to 0.0010% or more, and more preferably set to 0.0015% or more or 0.0020% or more in order to promote the precipitation of a carbonitride.
  • the O content is set to 0.0100% or less.
  • the O content is preferably 0.0080% or less, and more preferably 0.0050% or less or 0.0030% or less.
  • the lower limit of the O content is 0%, but in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more or 0.0010% or more.
  • the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements instead of a portion of Fe.
  • the optional elements are not contained, the lower limit of the content thereof is 0%.
  • the optional elements will be described in detail.
  • All of Cu, Cr, Mo, Ni, and B act to increase the hardenability of the hot-rolled steel sheet.
  • Cu and Mo act to increase the strength of the hot-rolled steel sheet by being precipitated as a carbide in the steel.
  • Ni acts to effectively suppress the intergranular cracking of a slab caused by Cu. Therefore, one or two or more of these elements may be contained.
  • the Cu acts to increase the hardenability of the hot-rolled steel sheet and to increase the strength of the hot-rolled steel sheet by being precipitated as a carbide in the steel at a low temperature.
  • 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, 0.70% or less, or 0.50% 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.
  • the Cr content is preferably 1.50% or less, and more preferably 1.00% or less, 0.70% or less, or 0.50% or less.
  • Mo acts to increase the hardenability of the hot-rolled steel sheet and to increase the strength of the hot-rolled steel sheet by being precipitated as a carbide in the steel.
  • the Mo content is preferably set to 0.01% or more, and more preferably set to 0.02% or more.
  • the Mo content is 1.00% or less.
  • the O content is preferably 0.50% or less, and more preferably 0.20% or less or 0.10% or less.
  • Ni acts to increase the hardenability of the hot-rolled steel sheet.
  • Ni acts to effectively suppress the intergranular cracking of a slab caused by Cu.
  • the Ni content is preferably set to 0.01% 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 Ni content is preferably 1.50% or less, and more preferably 1.00% or less, 0.70% or less, or 0.50% or less.
  • B acts to increase 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 or 0.0025% or less.
  • All of Ca, Mg, and REM act to increase the ductility of the hot-rolled steel sheet by adjusting the shape of inclusions in steel to a preferable shape.
  • Bi acts to increase 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.
  • the amount of any one or more of Ca, Mg, REM, and Bi is preferably set to 0.0001% or more. However, in a case where the Ca content or Mg content is more than 0.0200% or 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 decreased.
  • the Bi content be set to more than 0.0200% since the effects of the actions are saturated. 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 0.0200% or less.
  • the Ca content, Mg content, and Bi content are preferably 0.0100% or less, and more preferably 0.0070% or less or 0.0040% or less.
  • the REM content is preferably 0.0070% or less or 0.0040% or less.
  • the As content is preferably set to 0.001% or more. Meanwhile, since the above effects are saturated even in a case where a large amount of As is contained, the As content is set to 0.100% or less.
  • REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements.
  • Lanthanoids are industrially added in the form of misch metal.
  • Each element symbol in Expression (B) represents the amount of the corresponding element by mass %, and 0% is substituted in a case where the corresponding element is not contained.
  • Zr, Co, Zn, and W the present inventors have confirmed that, even in a case where these elements are contained in an amount of 1.00% or less in total, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. Therefore, one or two or more of Zr, Co, Zn, and W may be contained in an amount of 1.00% or less in total. That is, the value of the left side of Expression (B) may be set to 1.00% or less, 0.50% or less, 0.10% or less, or 0.05% or less. Each of the Zr content, the Co content, the Zn content, the W content, and the Sn content may be set to 0.50% or less, 0.10% or less, or 0.05% or less.
  • each of Zr, Co, Zn, and W may be 0%.
  • each of the Zr content, the Co content, the Zn content, and the W content may be 0.01% or more.
  • the present inventors have confirmed that, even in a case where Sn is contained in a small amount, the effects of the hot-rolled steel sheet according to the present embodiment are not impaired. However, in a case where 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. Since Sn does not have to be contained, the Sn content may be 0%. In order to increase the corrosion resistance of the hot-rolled steel sheet, the Sn content may be 0.01% or more.
  • the remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may consist of Fe and impurities.
  • the impurities mean substances incorporated from ore as a raw material, a scrap, manufacturing environment, or the like and/or substances permitted to an extent that the hot-rolled steel sheet according to the present embodiment is not adversely affected.
  • the chemical composition of the above hot-rolled steel sheet may be measured by a general analytical method.
  • the chemical composition may be measured using inductively coupled plasma-atomic emission spectrometry (ICP-AES).
  • ICP-AES inductively coupled plasma-atomic emission spectrometry
  • sol. Al may be measured by ICP-AES using a filtrate obtained by heating and decomposing a sample with an acid.
  • C and S may be measured using a combustion-infrared absorption method
  • N may be measured using an inert gas fusion-thermal conductivity method
  • O may be measured using an inert gas fusion-non-dispersive infrared absorption method.
  • the chemical composition may be analyzed after removing the plating layer by mechanical grinding or the like, as necessary.
  • the hot-rolled steel sheet according to the present embodiment has a microstructure including, by area %, residual austenite of less than 3.0%, ferrite of 15.0% or more and less than 60.0%, and pearlite of less than 5.0%, in which an entropy value represented by Expression (1), obtained by analyzing an SEM image of the microstructure using a gray-level co-occurrence matrix method, is 10.7 or more, an inverse difference normalized value represented by Expression (2) is 1.020 or more, a cluster shade value represented by Expression (3) is ⁇ 8.0 ⁇ 10 5 to 8.0 ⁇ 10 5 , and a standard deviation of an Mn concentration is 0.60 mass % or less.
  • the hot-rolled steel sheet according to the present embodiment can obtain excellent ductility and shearing property while having a high strength and a high limit fracture sheet thickness reduction ratio.
  • the microstructural fraction, entropy value, inverse difference normalized value, cluster shade value, and standard deviation of the Mn concentration are specified in the microstructure in a cross section parallel to the rolling direction at a depth position 1 ⁇ 4 of the sheet thickness away from the surface (a region ranging from 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) at a center position in the sheet width direction.
  • the reason therefor is that the microstructures at the position indicate typical microstructures of the steel sheet.
  • the surface mentioned here refers to the interface between a plating layer and the steel sheet in a case where the hot-rolled steel sheet is provided with the plating layer.
  • Residual austenite is a microstructure that is present as a face-centered cubic lattice even at room temperature. Residual austenite acts to increase the ductility of the hot-rolled steel sheet by transformation-induced plasticity (TRIP). Meanwhile, 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 and the decrease of a limit fracture sheet thickness reduction ratio. In a case where 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 area ratio of the residual austenite is measured by X-ray diffraction that makes it relatively easy to obtain accurate measurement results and is hardly affected by polishing, since it is less susceptible to polishing (when affected by polishing, the residual austenite may be converted into another phase such as martensite, so that the true area ratio may not be measured).
  • the integrated intensities of a total of 6 peaks of ⁇ (110), ⁇ (200), ⁇ (211), ⁇ (111), ⁇ (200), and ⁇ (220) are obtained in a sheet thickness-directional cross section parallel to the rolling direction at a 1 ⁇ 4 depth position of the sheet thickness (a region ranging from 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) at a center position in the sheet width direction of the hot-rolled steel sheet using Co-K ⁇ rays, and the volume percentage of the residual austenite is obtained by calculation using the strength averaging method.
  • the obtained volume percentage of the residual austenite is regarded as the area ratio of the residual austenite.
  • Ferrite is a structure formed when fcc transforms into bcc at a relatively high temperature. Since ferrite has a high work hardening rate, the ferrite acts to increase the balance between the strength and ductility of the hot-rolled steel sheet. In order to obtain the above action, 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 even 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 or 40.0% 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, the strengths of martensite and bainite, that 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, 2.0% or less, or 1.0% 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 hot-rolled steel sheet according to the present embodiment contains a hard structure consisting of one or two or more of bainite, martensite, and tempered martensite in a total area ratio of more than 32.0% and 85.0% or less as the remainder in microstructure other than residual austenite, ferrite, and pearlite.
  • the lower limit of the total area ratio of the remainder in microstructure may be 36.0%, 40.0%, 44.0%, 48.0%, 52.0%, or 55.0%, and the upper limit thereof may be 82.0%, 78.0%, 74.0%, 70.0%, or 66.0%.
  • the remainder in microstructure other than residual austenite, ferrite, and pearlite may include one or two or more of bainite, martensite, and tempered martensite.
  • the measurement of the area ratio of the microstructure is conducted by the following method.
  • a sheet thickness-directional 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 50 ⁇ m-long region at a depth position 1 ⁇ 4 of the sheet thickness away from the surface (a region ranging from 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) at a center position in the sheet width direction is measured by an electron backscatter diffraction method at a measurement interval of 0.1 ⁇ m to obtain crystal orientation information.
  • an EBSD analyzer composed 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.
  • the observation area is set to 40,000 ⁇ m 2 .
  • a reflected electron image is captured in the same visual field.
  • crystal grains in which ferrite and cementite are precipitated in layers are specified from the reflected electron image and the area ratio of the crystal grains is calculated to obtain an 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 mounted in software “OIM Analysis (registered trademark)” included in the EBSD analyzer.
  • the grain tolerance angle is set to 15° and the area ratio of the regions determined as ferrite is obtained to obtain an area ratio of ferrite.
  • the area ratio of regions except the regions determined as pearlite or ferrite is measured to obtain an area ratio of the remainder in microstructure (that is, bainite, martensite, and tempered martensite).
  • these area ratios can be measured by the following method. Specifically, with respect to the remainder regions, when the maximum value of “Grain Average IQ” of the ferrite regions is represented by I ⁇ , regions where “Grain Average IQ” becomes more than I ⁇ /2 are extracted (determined) as bainite, and regions where “Grain Average IQ” becomes I ⁇ /2 or less are extracted (determined) as “martensite or tempered martensite”.
  • the area ratio of the regions extracted (determined) as bainite is calculated to obtain the area ratio of bainite.
  • the area ratio of the regions extracted (determined) as martensite or tempered martensite is calculated to obtain the total area ratio of martensite and tempered martensite.
  • the total of the area ratios of the structures obtained by the measurement may not be 100.0%.
  • the area ratios of the structures are converted so that the total of the area ratios of the structures reaches 100.0%.
  • the area ratio of each structure is multiplied by “100.0/103.0” to obtain the area ratio of each structure.
  • Entropy Value 10.7 or More
  • Inverse Difference Normalized Value 1.020 or More
  • the generation of a secondary sheared surface is suppressed by controlling the entropy value (E value) representing the periodicity of the microstructure and the inverse difference normalized value (I value) representing the uniformity of the microstructure.
  • the E value represents the periodicity of the microstructure.
  • the E value decreases.
  • the E value is less than 10.7, a secondary sheared surface is likely to be generated. From periodically arranged structures as initiation 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 less than 1.020, due to an influence of the hardness distribution attributed to precipitates in crystal grains and an element concentration difference, 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.
  • 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.
  • Cluster Shade Value ⁇ 8.0 ⁇ 10 5 to 8.0 ⁇ 10 5
  • the Cluster shade value represents the degree of strain of the microstructure.
  • the CS value becomes a positive value in a case where there are many points having higher brightness than an average value of brightness in an image obtained by photographing the microstructure, and the CS value becomes a negative value in a case where there are many points having lower brightness than the average value.
  • the brightness is high at places where the surface unevenness of a target to be observed is large, and the brightness is low at places where the unevenness is small.
  • the surface unevenness of the target to be observed is greatly affected by the grain size and the strength distribution in the microstructure.
  • the CS value is large, and in a case where the variation in strength is small or the structural unit is large, the CS value is small.
  • the CS value is set to ⁇ 8.0 ⁇ 10 5 or more.
  • the CS value is preferably ⁇ 7.5 ⁇ 10 5 or more, and more preferably ⁇ 7.0 ⁇ 10 5 or more.
  • the CS value is set to 8.0 ⁇ 10 5 or less.
  • the CS value is preferably 7.5 ⁇ 10 5 or less, and more preferably 7.0 ⁇ 10 5 or less.
  • the E value, the I value, and the CS value can be obtained by the following method.
  • a region where an SEM image (a secondary electron image of a scanning electron microscope) is captured to calculate the E value, the I value, and the CS value is set in a sheet thickness-directional cross section parallel to the rolling direction at a depth position 1 ⁇ 4 of the sheet thickness away from the surface (a region ranging from 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) at a center position in the sheet width direction.
  • the SEM image is captured 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 1,000 times in 256 grayscale levels.
  • Non-Patent Document 3 On an image obtained by cutting out the obtained SEM image into an 880 ⁇ 880-pixel region (the observation region is 160 ⁇ m ⁇ 160 ⁇ m in actual size), 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 GLCM method described in Non-Patent Document 1.
  • P(i, j) in Expressions (1) to (5) is a gray-level co-occurrence matrix, and the value at the i-th row in the j-th column of the matrix P is expressed as P(i, j).
  • the calculation is performed using the 256 ⁇ 256 matrixes P as described above, and thus in a case where it is desired to emphasize this point, Expressions (1) to (5) can be corrected to Expressions (1′) to (5′).
  • L in Expression (2) is the number of grayscale levels (quantization levels of grayscale) that can be taken in the SEM image.
  • L is 256.
  • i and j in Expressions (2) and (3) are natural numbers of 1 to L, and ⁇ x and ⁇ y in Expression (3) are each represented by Expressions (4) and (5).
  • the standard deviation of the Mn concentration at a depth position 1 ⁇ 4 of the sheet thickness away from the surface (a region ranging from 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) of the hot-rolled steel sheet according to the present embodiment at a center position in the sheet width direction is 0.60 mass % or less.
  • the standard deviation of the Mn concentration is preferably 0.55 mass % or less or 0.50 mass % or less, and more preferably 0.47 mass % or less or 0.45 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.
  • the lower limit thereof may be set to 0.20 mass % or 0.28 mass % as necessary.
  • a depth position 1 ⁇ 4 of the sheet thickness away from the surface (a region ranging from 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) at a center position in the sheet width direction is measured with an electron probe microanalyzer (EPMA) to measure the standard deviation of the Mn concentration.
  • EPMA electron probe microanalyzer
  • the acceleration voltage is set to 15 kV
  • the magnification is set to 5,000 times
  • the distribution image of a range that is 20 ⁇ m long in the sample rolling direction and 20 ⁇ m long in the sample sheet thickness direction is measured.
  • the measurement interval is set to 0.1 ⁇ m, and the Mn concentrations are measured at 40,000 or more points.
  • 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.
  • Inside bend cracking can be suppressed in the hot-rolled steel sheet by making the crystal grain size of the surface layer fine.
  • the mechanism of inside bend cracking is presumed as follows. At the time of bending, compressive stress is generated in the inside bend. In the beginning, the working proceeds while the entire inside bend is uniformly deformed; however, as the amount of the working increases, only uniform deformation is no longer sufficient to carry deformation, and the deformation proceeds as strain concentrates locally (generation of a shear deformation band). As this shear deformation band further grows, cracking occurs along the shear band from the surface of the inside bend and propagate.
  • the reason for the inside bend cracking to be more likely to occur in association with high-strengthening is presumed to be that deterioration of work hardening capability in association with high-strengthening makes it difficult for uniform deformation to proceed and makes it easy for bias of deformation to be caused, which generates a shear deformation band in an early stage of the working (or under loose working conditions).
  • the present inventors found from studies that inside bend cracking becomes significant in steel sheets having a 980 MPa or more-grade tensile strength. In addition, the present inventors found that, as the crystal grain size of the surface layer of the hot-rolled steel sheet becomes finer, local strain concentration is further suppressed, and it becomes more unlikely that inside bend cracking occurs.
  • the average crystal grain size of the surface layer of the hot-rolled steel sheet is preferably set to less than 3.0 ⁇ m. Therefore, in the present embodiment, the average crystal grain size of the surface layer may be set to less than 3.0 ⁇ m.
  • the average crystal grain size of the surface layer is more preferably 2.7 ⁇ m or less or 2.5 ⁇ m or less.
  • the lower limit of the average crystal grain size of the surface layer region is not particularly specified and may be set to 0.5 ⁇ m or 1.0 ⁇ m.
  • the surface layer is a region ranging from the surface of the hot-rolled steel sheet to a position at a depth of 50 ⁇ m from the surface.
  • the surface mentioned here refers to the interface between a plating layer and the steel sheet in a case where the hot-rolled steel sheet is provided with the plating layer.
  • the crystal grain size of the surface layer is measured using an EBSP-OIM (electron back scatter diffraction pattern-orientation image microscopy) method.
  • the EBSP-OIM method is performed using a device obtained by combining a scanning electron microscope and an EBSP analyzer and OIM Analysis (registered trademark) manufactured by AMETEK, Inc.
  • the analyzable area of the EBSP-OIM method is a region that can be observed with the SEM.
  • the EBSP-OIM method makes it possible to analyze a region with a minimum resolution of 20 nm, which varies depending on the resolution of the SEM.
  • analysis is performed in at least 5 visual fields at a magnification of 1,200 times and a region of 40 ⁇ m ⁇ 30 ⁇ m.
  • a place where an angle difference between adjacent measurement points is 5° or more is defined as a crystal grain boundary, and an area-averaged crystal grain size is calculated.
  • the obtained area-averaged crystal grain size is regarded as the average crystal grain size of the surface layer.
  • Residual austenite is not a structure formed by phase transformation at 600° C. or lower and has no dislocation accumulation effect. Accordingly, in the present measurement method (the method of measuring the average crystal grain size of the surface layer), residual austenite is not regarded as a target to be analyzed. In a case where the area ratio of residual austenite is 0%, there is no need to exclude the residual austenite from the target to be analyzed. However, in a case where there is a possibility of affecting the measurement of the average crystal grain size of the surface layer, residual austenite having an fcc crystal structure is excluded for measurement from the target to be analyzed in the EBSP-OIM method.
  • the tensile strength properties are evaluated according to JIS Z 2241:2011.
  • a test piece a No. 5 test piece of JIS Z 2241:2011 is used.
  • a position where a tensile test piece is collected a 1 ⁇ 4 portion extending from the end portion in the sheet width direction may be set, and a direction perpendicular to the rolling direction may be set as a longitudinal direction.
  • the tensile strength (TS) is 980 MPa or more.
  • the tensile strength is preferably 1,000 MPa or more. In a case where the tensile strength is less than 980 MPa, an applicable component is limited, and the contribution to vehicle body weight reduction is small.
  • the upper limit does not need to be particularly limited and may be set to 1,780 MPa from the viewpoint of suppressing the wearing of a die.
  • the total elongation of the hot-rolled steel sheet according to the present embodiment 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 13,000 MPa ⁇ % or more.
  • the total elongation is more preferably set to 11.0% or more, and even more preferably set to 13.0% or more.
  • the product of the tensile strength and the total elongation is more preferably set to 14,000 MPa ⁇ % or more, and even more preferably 15,000 MPa ⁇ % or more.
  • the upper limit of the product of the tensile strength and the total elongation does not need to be set and may be set to 22,000 MPa ⁇ % or 18,000 MPa ⁇ %.
  • the upper limit of the total elongation does not need to be set and may be set to 30.0%, 25.0%, or 22.0%.
  • the sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be set to 0.5 to 8.0 mm. In a case where the sheet thickness of the hot-rolled steel sheet is less than 0.5 mm, it may become difficult to secure the rolling finishing temperature and the rolling force may become excessive, which may make hot rolling difficult. Therefore, the sheet thickness of the hot-rolled steel sheet according to the present embodiment may be set to 0.5 mm or more.
  • the sheet thickness is preferably 1.2 mm or more, 1.4 mm or more, or 1.8 mm or more. Meanwhile, in a case where the sheet thickness is more than 8.0 mm, it becomes difficult to refine the microstructure, and it may be difficult to obtain the above-described microstructure. Therefore, the sheet thickness may be set to 8.0 mm or less.
  • the sheet thickness is preferably 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.
  • 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.
  • electro plating layer electrogalvanizing, electro Zn—Ni alloy plating, and the like are exemplary examples.
  • hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminizing, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, hot-dip Zn—Al—Mg—Si alloy plating, and the like are exemplary examples.
  • the plating adhesion amount is not particularly limited and may be the same as before.
  • a suitable manufacturing method of the hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure is as follows.
  • the temperature of a slab and the temperature of a 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.
  • a slab is held in a temperature range of 700° C. to 850° C. for 900 seconds or longer, then, further heated, and held in a temperature range of 1,100° C. or higher for 6,000 seconds or longer.
  • Hot rolling is performed so that the sheet thickness is reduced by a total of 90% or more in a temperature range of 850° C. to 1,100° C.
  • the rolling reduction at the final stage of the hot rolling is set to 8% or more, and the hot rolling is finished so that the rolling finishing temperature Tf becomes 900° C. or higher and lower than 1,010° C.
  • Stress that is loaded to the steel sheet after the final stage rolling of the hot rolling and until the steel sheet is cooled to 800° C. is set to less than 200 kPa.
  • the steel sheet is cooled to a temperature range of the hot rolling finishing temperature Tf ⁇ 50° C. or lower within 1 second after the finishing of the hot rolling, and then accelerated cooling is performed to a temperature range of 600° C. to 730° C. at an average cooling rate of 50° C./s or faster.
  • the cooling to the temperature range of the hot rolling finishing temperature Tf ⁇ 50° C. or lower within 1 second after the finishing of the hot rolling is a more preferable cooling condition.
  • Cooling is performed so that the average cooling rate in a temperature range of the coiling temperature to 450° C. is 50° C./s or faster.
  • Coiling is performed in a temperature range of 350° C. or lower.
  • a hot-rolled steel sheet having a microstructure that has a high strength, a high limit fracture sheet thickness reduction ratio, and excellent ductility and shearing property can be stably manufactured by employing the above-described manufacturing method. That is, by appropriately controlling the slab heating conditions and the hot rolling conditions, 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 later, a hot-rolled steel sheet having a desired microstructure can be stably manufactured.
  • a slab obtained by continuous casting, a slab obtained by casting and blooming, and the like can be used as the slab to be subjected to hot rolling.
  • a slab obtained by additionally performing hot working or cold working on the above-described slab can be used as necessary.
  • the slab to be subjected to hot rolling is preferably held in a temperature range of 700° C. to 850° C. for 900 seconds or longer during slab heating, then, further heated, and held in a temperature range of 1,100° C. or higher for 6,000 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 a temperature range of 1,100° 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. In addition, by holding the steel sheet in the temperature range of 1,100° C. or higher for 6,000 seconds or longer, it is possible to make the austenite grains during slab heating uniform.
  • 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 of stress loading on the steel sheet during rolling, at least the final two stages are more preferably hot rolling in which a tandem mill is used.
  • recrystallized austenite grains are mainly refined, and accumulation of strain energy into 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 is reduced by a total of 90% or more in a temperature range of 850° C. to 1,100° C.
  • the sheet thickness reduction in a temperature range of 850° C. to 1,100° C. can be expressed as ⁇ (t 0 ⁇ t 1 )/t 0 ⁇ 100 (%) where to represents an inlet sheet thickness before the first rolling in the rolling in the above temperature range and t 1 represents an outlet sheet thickness after the final stage rolling in the rolling in the above temperature range.
  • the stress that is loaded to the steel sheet after rolling one stage before the final stage of hot rolling and before the final stage rolling is preferably set to 170 kPa or more. This makes it possible to reduce the number of crystal grains having a ⁇ h110 ⁇ 001> crystal orientation in the recrystallized austenite after the rolling one stage before the final stage. Since recrystallization is difficult to occur in the ⁇ h110 ⁇ 001> crystal orientation, recrystallization by the final stage rolling can be effectively promoted by suppressing the formation of this crystal orientation. 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 stress that is loaded to the steel sheet is less than 170 kPa, it may be impossible to achieve a desired E value.
  • the stress that is loaded to the steel sheet is more preferably 190 kPa or more.
  • the stress that is loaded to the steel sheet can be controlled by adjusting the roll rotation speed during tandem rolling, and can be obtained by dividing the load in the rolling direction measured in a rolling stand by the cross-sectional area of the passing sheet.
  • Hot Rolling Finishing Temperature Tf 900° C. or Higher and Lower than 1,010° C.
  • the rolling reduction at the final stage of the hot rolling is set to 8% or more and the hot rolling finishing temperature Tf is set to 900° C. or higher.
  • the hot rolling finishing temperature Tf is set to 900° C. or higher.
  • the formation of ferrite in the final structure is suppressed, and a high-strength hot-rolled steel sheet can be obtained.
  • the hot rolling finishing temperature Tf is set to lower than 1,010° C., the coarsening of the austenite grain size can be suppressed, and a desired E value can be obtained due to the reduced periodicity of the microstructure.
  • Stress that is loaded to the steel sheet after the final stage rolling of the hot rolling and until the steel sheet is cooled to 800° C. is preferably set to less than 200 kPa.
  • the stress that is loaded to the steel sheet is more preferably 180 kPa or less.
  • the stress that is loaded in the rolling direction of the steel sheet can be controlled by adjusting the rotation speeds of the rolling stand and the coiling device, and can be obtained by dividing the measured load in the rolling direction by the cross-sectional area of the passing sheet.
  • the steel sheet is more preferably cooled by 50° C. or more within 1 second after the finishing of the hot rolling.
  • cooling may be performed at a fast average cooling rate immediately after the finishing of the hot rolling. For example, cooling water may be sprayed to the surface of the steel sheet.
  • the average cooling rate mentioned here refers to a value obtained by dividing the temperature drop width of the steel sheet from the start of the accelerated cooling (when introducing the steel sheet into cooling equipment) to the finishing of the accelerated cooling (when deriving the steel sheet from the cooling equipment) by the time required from the start of the accelerated cooling to the finishing of the accelerated cooling.
  • the upper limit of the cooling rate is not particularly specified, but in a case where the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, the cooling rate is preferably 300° C./s or lower.
  • the cooling stop temperature of the accelerated cooling may be set to 600° C. or higher in order to perform slow cooling to be described later.
  • the average cooling rate mentioned here 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 shorter than approximately 10.0 seconds.
  • the lower limit of the average cooling rate of the slow cooling is not particularly set, raising the temperature without cooling may require a large investment in equipment. Therefore, the lower limit may be set to 0° C./s or faster.
  • Cooling After End of Slow Cooling, Cooling being Performed so that Average Cooling Rate in Temperature Range of 450° C. to 600° C. is 30° C./s or Faster and Slower than 50° C./s
  • the average cooling rate in a temperature range of 450° C. to 600° C. is 30° C./s or faster and slower than 50° C./s.
  • a desired CS value can be obtained.
  • the average cooling rate is faster than 50° C./s, a flat lath-like structure having low brightness is likely to be formed, and the CS value becomes less than ⁇ 8.0 ⁇ 10 5 .
  • the CS value becomes more than 8.0 ⁇ 10 5 .
  • the average cooling rate mentioned here 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 cooling stop temperature of the cooling where the average cooling rate is 30° C./s or faster and slower than 50° C./s by the time required from the stop of the slow cooling where the average cooling rate is slower than 5° C./s to the stop of the cooling where the average cooling rate is 30° C./s or faster and slower than 50° C./s.
  • the average cooling rate in a temperature range of the coiling temperature to 450° C. is preferably set to 50° C./s or faster.
  • the primary phase structure can be made hard.
  • the average cooling rate mentioned here refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the cooling where the average cooling rate is 30° C./s or faster and slower than 50° C./s to the coiling temperature by the time required from the stop of the cooling where the average cooling rate is 30° C./s or faster and slower than 50° C./s to the coiling.
  • the coiling temperature is set to 350° C. or lower. In a case where the coiling temperature is set to 350° C. or lower, the amount of an iron carbide precipitated is reduced, and the variation in hardness distribution in the hard phase can be reduced. As a result, the I value can be increased and the generation of a secondary sheared surface can be suppressed.
  • 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 4 is 50° C.
  • the actual coiling temperatures in the examples written as 50° C. are 50° C. or lower.
  • the area ratio of the microstructure, the E value, the I value, the CS value, the standard deviation of the Mn concentration, the average crystal grain size of the surface layer, the tensile strength TS, and the total elongation El of each of the obtained hot-rolled steel sheets were obtained by the above methods.
  • the obtained measurement results are shown in Tables 5 and 6.
  • the remainder in microstructure was one or two or more of bainite, martensite, and tempered martensite.
  • the hot-rolled steel sheet was considered to have a high strength and excellent ductility, and determined as acceptable. In a case where any one was not satisfied, the hot-rolled steel sheet was not considered to have a high strength and excellent ductility, and determined as unacceptable.
  • the limit fracture sheet thickness reduction ratio of the hot-rolled steel sheet was evaluated by a tensile test.
  • the tensile test was performed by the same method as in the evaluation of the tensile properties.
  • the value of (t 1 ⁇ t 2 ) ⁇ 100/t 1 was calculated, where t 1 represents the sheet thickness before the tensile test and t 2 represents the minimum value of the sheet thickness at a center portion in the width direction of the tensile test piece after fracture, to obtain the limit fracture sheet thickness reduction ratio.
  • the tensile test was performed five times, and the average value was calculated by taking the mean of three values, excluding the maximum limit fracture sheet thickness reduction ratio and the minimum limit fracture sheet thickness reduction ratio.
  • the hot-rolled steel sheet was considered to have a high limit fracture sheet thickness reduction ratio, and determined as acceptable. Meanwhile, in a case where the limit fracture sheet thickness reduction ratio was less than 60.0%, the hot-rolled steel sheet was not considered to have a high limit fracture sheet thickness reduction ratio, and determined as unacceptable.
  • 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 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.
  • FIG. 1 shows a sheared end surface with a shear droop, a sheared surface, a fractured surface, and a burr.
  • FIG. 2 shows 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 a region of a punched end surface separated by shear deformation
  • the fractured surface is a region of a punched end surface separated by cracking occurring 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.
  • the inside bend cracking resistance was evaluated by the following bending test.
  • a 100 mm ⁇ 30 mm strip-shaped test piece was cut out from a 1 ⁇ 2 position in the width direction of the hot-rolled steel sheet to obtain a bending test piece.
  • a test was performed according to the V-block method of JIS Z 2248:2006 (the bending angle ⁇ is) 90° for both bending where the bending ridge was parallel to the rolling direction (L direction) (L-axis bending) and bending where the bending ridge was parallel to a direction perpendicular to the rolling direction (C direction) (C-axis bending).
  • the inside bend cracking resistance was investigated.
  • a value obtained by dividing the average value of the minimum bend radii in the L axis and in the C axis by the sheet thickness was regarded as the limit bending R/t and used as an index value of inside bend cracking resistance.
  • R/t was 2.5 or less, the hot-rolled steel sheet was determined to be excellent in inside bend cracking resistance.
  • the hot-rolled steel sheets according to the present invention examples have excellent ductility and shearing property while having a high strength and a high limit fracture sheet thickness reduction ratio.
  • the hot-rolled steel sheets in which the average crystal grain size of the surface layer is less than 3.0 ⁇ m have the above various properties and further have excellent inside bend cracking resistance.
  • the hot-rolled steel sheets according to the comparative examples deteriorate in any one or more of strength, ductility, limit fracture sheet thickness reduction ratio, and shearing property.
  • a hot-rolled steel sheet that has a high strength, a high limit fracture sheet thickness reduction ratio, and excellent ductility and shearing property.
  • the hot-rolled steel sheet according to the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.

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US20230257845A1 (en) * 2020-08-27 2023-08-17 Nippon Steel Corporation Hot-rolled steel sheet
US12522884B2 (en) 2020-08-27 2026-01-13 Nippon Steel Corporation Hot-rolled steel sheet
US12534785B2 (en) 2020-08-27 2026-01-27 Nippon Steel Corporation Hot-rolled steel sheet

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US11313009B2 (en) * 2017-07-07 2022-04-26 Nippon Steel Corporation Hot-rolled steel sheet and method for manufacturing same
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US20230257845A1 (en) * 2020-08-27 2023-08-17 Nippon Steel Corporation Hot-rolled steel sheet
US12522884B2 (en) 2020-08-27 2026-01-13 Nippon Steel Corporation Hot-rolled steel sheet
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